thermodynamic principles in ball milling in powder metallurgy

principles of powder metallurgy and the powder metallurgy process - bright hub engineering

principles of powder metallurgy and the powder metallurgy process - bright hub engineering

Powder technology is the science for the manufacture of parts from metal powders by compaction and heating that creates ahomogeneous mass. Heating is executed in a furnace and is called sintering. The temperature at which sintering is performed is lower than the melting point of the powdered material. Sintering consists of diffusion in solid state by which particles of compacted powder are bonded together. This is the basic working principle of powder technology.The powder metallurgy technique finds use in various industries and manufacturing processes. It has become very popular in a very short span of time because of its efficiency, durability and reliable output. Some of them are mentioned here.Applications of Powder Metallurgy in IndustriesThe powder metallurgy technique finds use in various industries and manufacturing processes. It has become very popular in a very short span of time because of its efficiency and reliable output. Some of them are mentioned here.Manufacturing metal bonded diamond tools and materials.Manufacturing power tools and modern home appliances.Aerospace and Automobile Industry have a huge scope for power metallurgy for making large equipment and machine parts.Manufacturing of friction materials, refractory metals, switch materials and electric contact materials.Production and processing of metals with high melting points like Tungsten and Molybdenum that are used in electronics industry.Parts with irregular curves or recesses that are hard to machine can be manufactured using the powder metallurgy techniques. It is suitable for high-volume and mass production with practically negligible wastage of the manufacturing material. The process of secondary machining is virtually eliminated or reduced to negligible extent by the technique of powder metallurgy and it helps in improving efficiency by a huge margin. Cams, sprockets, pawls, iron bearings, sintered bronze, ratchets and carbide tool tips are the most commonly manufactured items with the help of powder technology.Tools produced using powder metallurgy techniques have high porosity and they are very good at soaking oil. Compaction of constituent materials means strong internal structure resulting in a strong bonding between the material constituents and thus producing a durable machine equipment or tool. Longer life and strong structure are two major characteristics of tools or equipment produced using this technique.Processes of Powder TechnologyThe processes involved in powder metallurgy are the following:Blending and Mixing: This is carried out to achieve uniformity of the product manufactured. Distribution of properly sized particles is attained by mixing elementary powder with alloy powders to obtain a homogeneous mixture. Lubricants are also mixed with powders to minimize the wear of dies and reduce friction between the surfaces of dies and particles of powder during compaction. Mixing time will depend upon the results desired, and over-mixing should be prevented, or otherwise the size of particles will be decreased and they will be hardened.Pressing: The cavity of the die is filled with a specified quantity of blended powder, necessary pressure is applied, and then the compacted part is ejected. Pressing is performed at room temperature, while the pressure is dependent upon the material, properties of the powder used, and the density required of the compaction. Friction between the powder and the wall of the die opposes application of a proper pressure that decreases with depth and thus causes uneven density in the compact. Thus the ratio of length and diameter is kept low to prevent substantial variations in density.Sintering: Changes occur during sintering, including changes in size, configuration, and the nature of pores. Commonly used atmospheres for sintering are hydrogen, carbon monoxide, and ammonia. Sintering operation ensures that powder particles are bonded strongly and that better alloying is achieved.Properties of Metal PowdersProperties of metal powder depend upon the process employed for its production. Therefore, it is essential to determine the physical and chemical properties of powders to prevent variations in the desired characteristics of the compactions. Significant properties of metal powders are:Chemical composition that is determined by chemical analysis.Shape of particles that is affected by methods employed for production of powder.Particle size influences the properties of flow and density of powder metal. It can be measured by a microscope, sieve, or by sedimentation.Distribution of particle size has a significant effect on physical properties of powder, and can be determined by sieving test.Flowability is the relative ease of the flow of powder through an orifice.Bulk density can be measured by filling a pot whose volume is known with powder, and then obtaining the weight of the powder.Other properties include compressibility, compatibility, sintering ability, and specific surface.ReferencesModule III Powder Metallurgy, SNGCE Digital RepositorySintering of a Powder Compact, Va Tech ClassesP. Ramakrishnan, History of Powder Metallurgy (PDF)ImagesSilicon Powder by Silane, WikipediaSintering, http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif

homogeneous mass. Heating is executed in a furnace and is called sintering. The temperature at which sintering is performed is lower than the melting point of the powdered material. Sintering consists of diffusion in solid state by which particles of compacted powder are bonded together. This is the basic working principle of powder technology.

The powder metallurgy technique finds use in various industries and manufacturing processes. It has become very popular in a very short span of time because of its efficiency, durability and reliable output. Some of them are mentioned here.

Applications of Powder Metallurgy in IndustriesThe powder metallurgy technique finds use in various industries and manufacturing processes. It has become very popular in a very short span of time because of its efficiency and reliable output. Some of them are mentioned here.Manufacturing metal bonded diamond tools and materials.Manufacturing power tools and modern home appliances.Aerospace and Automobile Industry have a huge scope for power metallurgy for making large equipment and machine parts.Manufacturing of friction materials, refractory metals, switch materials and electric contact materials.Production and processing of metals with high melting points like Tungsten and Molybdenum that are used in electronics industry.Parts with irregular curves or recesses that are hard to machine can be manufactured using the powder metallurgy techniques. It is suitable for high-volume and mass production with practically negligible wastage of the manufacturing material. The process of secondary machining is virtually eliminated or reduced to negligible extent by the technique of powder metallurgy and it helps in improving efficiency by a huge margin. Cams, sprockets, pawls, iron bearings, sintered bronze, ratchets and carbide tool tips are the most commonly manufactured items with the help of powder technology.Tools produced using powder metallurgy techniques have high porosity and they are very good at soaking oil. Compaction of constituent materials means strong internal structure resulting in a strong bonding between the material constituents and thus producing a durable machine equipment or tool. Longer life and strong structure are two major characteristics of tools or equipment produced using this technique.Processes of Powder TechnologyThe processes involved in powder metallurgy are the following:Blending and Mixing: This is carried out to achieve uniformity of the product manufactured. Distribution of properly sized particles is attained by mixing elementary powder with alloy powders to obtain a homogeneous mixture. Lubricants are also mixed with powders to minimize the wear of dies and reduce friction between the surfaces of dies and particles of powder during compaction. Mixing time will depend upon the results desired, and over-mixing should be prevented, or otherwise the size of particles will be decreased and they will be hardened.Pressing: The cavity of the die is filled with a specified quantity of blended powder, necessary pressure is applied, and then the compacted part is ejected. Pressing is performed at room temperature, while the pressure is dependent upon the material, properties of the powder used, and the density required of the compaction. Friction between the powder and the wall of the die opposes application of a proper pressure that decreases with depth and thus causes uneven density in the compact. Thus the ratio of length and diameter is kept low to prevent substantial variations in density.Sintering: Changes occur during sintering, including changes in size, configuration, and the nature of pores. Commonly used atmospheres for sintering are hydrogen, carbon monoxide, and ammonia. Sintering operation ensures that powder particles are bonded strongly and that better alloying is achieved.Properties of Metal PowdersProperties of metal powder depend upon the process employed for its production. Therefore, it is essential to determine the physical and chemical properties of powders to prevent variations in the desired characteristics of the compactions. Significant properties of metal powders are:Chemical composition that is determined by chemical analysis.Shape of particles that is affected by methods employed for production of powder.Particle size influences the properties of flow and density of powder metal. It can be measured by a microscope, sieve, or by sedimentation.Distribution of particle size has a significant effect on physical properties of powder, and can be determined by sieving test.Flowability is the relative ease of the flow of powder through an orifice.Bulk density can be measured by filling a pot whose volume is known with powder, and then obtaining the weight of the powder.Other properties include compressibility, compatibility, sintering ability, and specific surface.ReferencesModule III Powder Metallurgy, SNGCE Digital RepositorySintering of a Powder Compact, Va Tech ClassesP. Ramakrishnan, History of Powder Metallurgy (PDF)ImagesSilicon Powder by Silane, WikipediaSintering, http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif

The powder metallurgy technique finds use in various industries and manufacturing processes. It has become very popular in a very short span of time because of its efficiency and reliable output. Some of them are mentioned here.Manufacturing metal bonded diamond tools and materials.Manufacturing power tools and modern home appliances.Aerospace and Automobile Industry have a huge scope for power metallurgy for making large equipment and machine parts.Manufacturing of friction materials, refractory metals, switch materials and electric contact materials.Production and processing of metals with high melting points like Tungsten and Molybdenum that are used in electronics industry.Parts with irregular curves or recesses that are hard to machine can be manufactured using the powder metallurgy techniques. It is suitable for high-volume and mass production with practically negligible wastage of the manufacturing material. The process of secondary machining is virtually eliminated or reduced to negligible extent by the technique of powder metallurgy and it helps in improving efficiency by a huge margin. Cams, sprockets, pawls, iron bearings, sintered bronze, ratchets and carbide tool tips are the most commonly manufactured items with the help of powder technology.Tools produced using powder metallurgy techniques have high porosity and they are very good at soaking oil. Compaction of constituent materials means strong internal structure resulting in a strong bonding between the material constituents and thus producing a durable machine equipment or tool. Longer life and strong structure are two major characteristics of tools or equipment produced using this technique.Processes of Powder TechnologyThe processes involved in powder metallurgy are the following:Blending and Mixing: This is carried out to achieve uniformity of the product manufactured. Distribution of properly sized particles is attained by mixing elementary powder with alloy powders to obtain a homogeneous mixture. Lubricants are also mixed with powders to minimize the wear of dies and reduce friction between the surfaces of dies and particles of powder during compaction. Mixing time will depend upon the results desired, and over-mixing should be prevented, or otherwise the size of particles will be decreased and they will be hardened.Pressing: The cavity of the die is filled with a specified quantity of blended powder, necessary pressure is applied, and then the compacted part is ejected. Pressing is performed at room temperature, while the pressure is dependent upon the material, properties of the powder used, and the density required of the compaction. Friction between the powder and the wall of the die opposes application of a proper pressure that decreases with depth and thus causes uneven density in the compact. Thus the ratio of length and diameter is kept low to prevent substantial variations in density.Sintering: Changes occur during sintering, including changes in size, configuration, and the nature of pores. Commonly used atmospheres for sintering are hydrogen, carbon monoxide, and ammonia. Sintering operation ensures that powder particles are bonded strongly and that better alloying is achieved.Properties of Metal PowdersProperties of metal powder depend upon the process employed for its production. Therefore, it is essential to determine the physical and chemical properties of powders to prevent variations in the desired characteristics of the compactions. Significant properties of metal powders are:Chemical composition that is determined by chemical analysis.Shape of particles that is affected by methods employed for production of powder.Particle size influences the properties of flow and density of powder metal. It can be measured by a microscope, sieve, or by sedimentation.Distribution of particle size has a significant effect on physical properties of powder, and can be determined by sieving test.Flowability is the relative ease of the flow of powder through an orifice.Bulk density can be measured by filling a pot whose volume is known with powder, and then obtaining the weight of the powder.Other properties include compressibility, compatibility, sintering ability, and specific surface.ReferencesModule III Powder Metallurgy, SNGCE Digital RepositorySintering of a Powder Compact, Va Tech ClassesP. Ramakrishnan, History of Powder Metallurgy (PDF)ImagesSilicon Powder by Silane, WikipediaSintering, http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif

Parts with irregular curves or recesses that are hard to machine can be manufactured using the powder metallurgy techniques. It is suitable for high-volume and mass production with practically negligible wastage of the manufacturing material. The process of secondary machining is virtually eliminated or reduced to negligible extent by the technique of powder metallurgy and it helps in improving efficiency by a huge margin. Cams, sprockets, pawls, iron bearings, sintered bronze, ratchets and carbide tool tips are the most commonly manufactured items with the help of powder technology.Tools produced using powder metallurgy techniques have high porosity and they are very good at soaking oil. Compaction of constituent materials means strong internal structure resulting in a strong bonding between the material constituents and thus producing a durable machine equipment or tool. Longer life and strong structure are two major characteristics of tools or equipment produced using this technique.Processes of Powder TechnologyThe processes involved in powder metallurgy are the following:Blending and Mixing: This is carried out to achieve uniformity of the product manufactured. Distribution of properly sized particles is attained by mixing elementary powder with alloy powders to obtain a homogeneous mixture. Lubricants are also mixed with powders to minimize the wear of dies and reduce friction between the surfaces of dies and particles of powder during compaction. Mixing time will depend upon the results desired, and over-mixing should be prevented, or otherwise the size of particles will be decreased and they will be hardened.Pressing: The cavity of the die is filled with a specified quantity of blended powder, necessary pressure is applied, and then the compacted part is ejected. Pressing is performed at room temperature, while the pressure is dependent upon the material, properties of the powder used, and the density required of the compaction. Friction between the powder and the wall of the die opposes application of a proper pressure that decreases with depth and thus causes uneven density in the compact. Thus the ratio of length and diameter is kept low to prevent substantial variations in density.Sintering: Changes occur during sintering, including changes in size, configuration, and the nature of pores. Commonly used atmospheres for sintering are hydrogen, carbon monoxide, and ammonia. Sintering operation ensures that powder particles are bonded strongly and that better alloying is achieved.Properties of Metal PowdersProperties of metal powder depend upon the process employed for its production. Therefore, it is essential to determine the physical and chemical properties of powders to prevent variations in the desired characteristics of the compactions. Significant properties of metal powders are:Chemical composition that is determined by chemical analysis.Shape of particles that is affected by methods employed for production of powder.Particle size influences the properties of flow and density of powder metal. It can be measured by a microscope, sieve, or by sedimentation.Distribution of particle size has a significant effect on physical properties of powder, and can be determined by sieving test.Flowability is the relative ease of the flow of powder through an orifice.Bulk density can be measured by filling a pot whose volume is known with powder, and then obtaining the weight of the powder.Other properties include compressibility, compatibility, sintering ability, and specific surface.ReferencesModule III Powder Metallurgy, SNGCE Digital RepositorySintering of a Powder Compact, Va Tech ClassesP. Ramakrishnan, History of Powder Metallurgy (PDF)ImagesSilicon Powder by Silane, WikipediaSintering, http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif

Tools produced using powder metallurgy techniques have high porosity and they are very good at soaking oil. Compaction of constituent materials means strong internal structure resulting in a strong bonding between the material constituents and thus producing a durable machine equipment or tool. Longer life and strong structure are two major characteristics of tools or equipment produced using this technique.

Blending and Mixing: This is carried out to achieve uniformity of the product manufactured. Distribution of properly sized particles is attained by mixing elementary powder with alloy powders to obtain a homogeneous mixture. Lubricants are also mixed with powders to minimize the wear of dies and reduce friction between the surfaces of dies and particles of powder during compaction. Mixing time will depend upon the results desired, and over-mixing should be prevented, or otherwise the size of particles will be decreased and they will be hardened.Pressing: The cavity of the die is filled with a specified quantity of blended powder, necessary pressure is applied, and then the compacted part is ejected. Pressing is performed at room temperature, while the pressure is dependent upon the material, properties of the powder used, and the density required of the compaction. Friction between the powder and the wall of the die opposes application of a proper pressure that decreases with depth and thus causes uneven density in the compact. Thus the ratio of length and diameter is kept low to prevent substantial variations in density.Sintering: Changes occur during sintering, including changes in size, configuration, and the nature of pores. Commonly used atmospheres for sintering are hydrogen, carbon monoxide, and ammonia. Sintering operation ensures that powder particles are bonded strongly and that better alloying is achieved.Properties of Metal PowdersProperties of metal powder depend upon the process employed for its production. Therefore, it is essential to determine the physical and chemical properties of powders to prevent variations in the desired characteristics of the compactions. Significant properties of metal powders are:Chemical composition that is determined by chemical analysis.Shape of particles that is affected by methods employed for production of powder.Particle size influences the properties of flow and density of powder metal. It can be measured by a microscope, sieve, or by sedimentation.Distribution of particle size has a significant effect on physical properties of powder, and can be determined by sieving test.Flowability is the relative ease of the flow of powder through an orifice.Bulk density can be measured by filling a pot whose volume is known with powder, and then obtaining the weight of the powder.Other properties include compressibility, compatibility, sintering ability, and specific surface.ReferencesModule III Powder Metallurgy, SNGCE Digital RepositorySintering of a Powder Compact, Va Tech ClassesP. Ramakrishnan, History of Powder Metallurgy (PDF)ImagesSilicon Powder by Silane, WikipediaSintering, http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif

Pressing: The cavity of the die is filled with a specified quantity of blended powder, necessary pressure is applied, and then the compacted part is ejected. Pressing is performed at room temperature, while the pressure is dependent upon the material, properties of the powder used, and the density required of the compaction. Friction between the powder and the wall of the die opposes application of a proper pressure that decreases with depth and thus causes uneven density in the compact. Thus the ratio of length and diameter is kept low to prevent substantial variations in density.Sintering: Changes occur during sintering, including changes in size, configuration, and the nature of pores. Commonly used atmospheres for sintering are hydrogen, carbon monoxide, and ammonia. Sintering operation ensures that powder particles are bonded strongly and that better alloying is achieved.Properties of Metal PowdersProperties of metal powder depend upon the process employed for its production. Therefore, it is essential to determine the physical and chemical properties of powders to prevent variations in the desired characteristics of the compactions. Significant properties of metal powders are:Chemical composition that is determined by chemical analysis.Shape of particles that is affected by methods employed for production of powder.Particle size influences the properties of flow and density of powder metal. It can be measured by a microscope, sieve, or by sedimentation.Distribution of particle size has a significant effect on physical properties of powder, and can be determined by sieving test.Flowability is the relative ease of the flow of powder through an orifice.Bulk density can be measured by filling a pot whose volume is known with powder, and then obtaining the weight of the powder.Other properties include compressibility, compatibility, sintering ability, and specific surface.ReferencesModule III Powder Metallurgy, SNGCE Digital RepositorySintering of a Powder Compact, Va Tech ClassesP. Ramakrishnan, History of Powder Metallurgy (PDF)ImagesSilicon Powder by Silane, WikipediaSintering, http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif

Sintering: Changes occur during sintering, including changes in size, configuration, and the nature of pores. Commonly used atmospheres for sintering are hydrogen, carbon monoxide, and ammonia. Sintering operation ensures that powder particles are bonded strongly and that better alloying is achieved.Properties of Metal PowdersProperties of metal powder depend upon the process employed for its production. Therefore, it is essential to determine the physical and chemical properties of powders to prevent variations in the desired characteristics of the compactions. Significant properties of metal powders are:Chemical composition that is determined by chemical analysis.Shape of particles that is affected by methods employed for production of powder.Particle size influences the properties of flow and density of powder metal. It can be measured by a microscope, sieve, or by sedimentation.Distribution of particle size has a significant effect on physical properties of powder, and can be determined by sieving test.Flowability is the relative ease of the flow of powder through an orifice.Bulk density can be measured by filling a pot whose volume is known with powder, and then obtaining the weight of the powder.Other properties include compressibility, compatibility, sintering ability, and specific surface.ReferencesModule III Powder Metallurgy, SNGCE Digital RepositorySintering of a Powder Compact, Va Tech ClassesP. Ramakrishnan, History of Powder Metallurgy (PDF)ImagesSilicon Powder by Silane, WikipediaSintering, http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif

Properties of metal powder depend upon the process employed for its production. Therefore, it is essential to determine the physical and chemical properties of powders to prevent variations in the desired characteristics of the compactions. Significant properties of metal powders are:

Module III Powder Metallurgy, SNGCE Digital RepositorySintering of a Powder Compact, Va Tech ClassesP. Ramakrishnan, History of Powder Metallurgy (PDF)ImagesSilicon Powder by Silane, WikipediaSintering, http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif

Sintering of a Powder Compact, Va Tech ClassesP. Ramakrishnan, History of Powder Metallurgy (PDF)ImagesSilicon Powder by Silane, WikipediaSintering, http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/apps/sinter1.gif

milling energy - an overview | sciencedirect topics

milling energy - an overview | sciencedirect topics

Milling media possess significant influencing performance on the ball-milling process in terms of the milling energy and size of the end-product. The sort of materials used for manufacturing of the milling media (balls and vials) is important and crucial due to the impact of the milling balls on the inner walls of the milling vials. Selection of the milling media materials including shapes and sizes depends upon several factors, some of them are interrelated. In general, when a material is to be ball-milled there are certain critical factors, which have to be taken into account. In general, the milling media should fulfill two major requirements: (i) they should have large surface area to provide suitable contact with the material being milled; and (ii) they should be as heavy as possible to have sufficient energy required for size reduction of the powder particles. Some of these principal factors that govern the selections of the milling media are discussed below.

Hardnessthe hardness of a powder material is considered to be the most important characteristic to realize when deciding on what type of milling media to choose. The harder the milling media the better the milling efficiency. Using milling media made of hard materials such as hardened steel, tungsten carbide, agate, and zirconia leads to maximize the milling efficiency and thus minimize the milling time required to get fine and homogenous powder particles. However, using such hard milling media may lead also to contaminate the milled powders with foreign materials that come from the balls wear during the milling process. Table 3.2 presents the Vickers hardness values of selected materials used for manufacturing of the milling media.

Specific gravitythe specific gravity of the milling media also plays a very important role in the milling process. It is well known that balls with high density and large diameters would give excellent results due to the expected generation of high impact forces applied on the milled powders. In general, the balls should enjoy more dense values when compared with the ball-milled powders. The densities values of some selected materials used for producing the milling media are listed in Table 3.2.

Brittlenessanother important factor that governs the selection of the milling media is the brittleness of the milled powders. Brittleness is the degree to which a material will easily break. Almost all of ceramics, including metal oxides and metal carbides are brittle. Breaking down the brittle materials can be performed successfully by selection of proper type of the milling media based on their properties that are presented in Table 3.2. In general, materials that are enjoying ductility such as metals and some metal alloys cannot easily be ball-milled.

Balls sizesthe efficiency of ball milling depends on the surface area of the milling media that are used in the process. The milling process is affected by number of contact points between the balls and the powder particles. Hence, the angle of nip presents a very important factor so that the ball sizes must be carefully chosen in relation to the largest and hardest particles of the feed powders. In general, the capacity of a ball mill increases by decreasing the ball diameters. Moreover, the rate of ball-to-ball contacts per unit time increases with decreasing the ball diameters because the number of balls in the mill increases. It is predicted that using different ball sizes can lead to higher collision energy that will help on improving of the milling process [9]. Thus, balls should be as small as possible and the charge the balls should be graded such that the largest balls are just heavy enough to grind the largest and hardest particles in the feed, whereas the small balls are responsible for powder refining.

The dynamics of the balls of different sizes during attrition of Cu-15 vol.% Nb powders have been studied by Cook and Courtney, using cinematographic techniques [10]. In their experiments, an attritor with a clear tank was specially designed to study media dynamics. A high-speed video camera was used to record the motion of the balls with different diameters inside the attrition, while its impeller rotated at different speeds. The metallographic examinations of the samples obtained after ball milling for 216h using balls with 7.64-mm-diameter and a mixture of 7.64-mm-diameter and 4.76-mm balls are shown in Figures 3.6 and 3.7, respectively [10]. Obviously, after 16h of ball milling using a mixture of 7.64-mm-diameter and 4.76-mm balls (Figure 3.7(c)), the powder has finer particle size in comparison to that of Figure 3.6(c) indicates that a greater particle fracture rate can be obtained when milling with differently sized balls. Thus, relative particle fracture rates are increased during the fracture-dominant stage when differently sized balls were employed [10].

Recently, Vaezi et al. [11] have reached to the same conclusion but with a different binary system (Cu-50% Fe) when they ball-milled the starting material powders with high-energy planetary type, using different ball sizes. They conclude that using a mixture of 5- and 10-mm balls leads to enhance the milling kinetics.

Shape of the milling mediaas discussed in Chapter 2, the shape of the milling media can be balls, rods, or barrels. In general, impact, shear stresses, and their combination (impact+shear stresses) leads to the extreme mechanisms in any ball-milling or rod-milling systems. In the tumbling and ball mills and roller mills, use the rotation and friction of the vial shell to transfer energy to the milling media. The energy that is required to break the particles in the mill comes from the rotational energy that is supplied by the rotator drums, which are directly connected to the drive motor. Thus, the internal frictional forces and ball-powder-ball collisions lead the media to rise before gravity forces make them fall. Based on the motion of the milling media (balls), the ball mill can be classified into three zones: (i) shearing zone, where the balls are lifted due to the rotated vial action and frictional forces, (ii) cataracting zone, where the balls are falling due to the gravity, and (iii) inactive zone, where the balls are not moving (Figure 3.8). Each impact event is considered to deliver a finite amount of energy to the charge which in turn is distributed unequally to each particle that is in the neighborhood of the impacting media particles and which can therefore receive a fraction of the energy that is dissipated in the impact event.

However, balls have a greater surface area per unit weight than rods that make them better suited for fine particle-size reduction; rods have several advantages, which can be summarized as follows:Milling rods do not require cascading as do ball charges, thus enabling rod mills to be operated at lower peripheral speeds than ball mills;The existing void spaces in rod charge is less when compared with the ball charge;Rods offer great milling contact between the powder charges per surface;The action within the rods causes the energy of rods to be directed to the largest-sized particles of the powders;Since there is no collision between the rods during the rod-milling process, the volume fractions of contamination that are usually introduced by the milling tools to the powders are less.

In the recent years, a different type of milling medium called Cylpebs, which was developed for different shapes, has appeared in the market (developed by Doering International, Powerpebs by the Donhad and Millpebs by the Wheelabrator Allevard Enterprise are available in the market). These milling media have barrel- or cylindrical-like shapes of length equaling diameter, and all the edges being radiuses (Figure 3.9). It is claimed by the manufacturer (Doering International; www.donhad.com.au) that for a given charge volume, the bulk density of Cylpebs is 9% greater than steel balls, 12% greater than cast-balls. Moreover, the surface area of Cylpebs is 14.5 % greater than balls of equal weight. The density and surface area combination, deliver 25% greater grinding capacity in the mill charge. The grinding performance of the Cylpebs should then be correspondingly higher compared with the steel balls [12]. Because of their cylindrical geometry, Cylpebs have greater surface area and higher bulk density compared with balls of similar mass and size, as presented in Table 3.3. The milling mechanism using Cylpebs is considered to be a combination between ball milling and rod milling. Beside the point contact action similar to the balls, Cylpebs have other milling actions resulting from line contact along the cylindrical section and area contact between the end faces on the powder particles, being similar to the mechanism of rods. The line contact and area contact increase the tendency for milling to take place preferentially on the larger powder particles, as schematically shown in Figure 3.10. Once the large particles are caught on the line or between the face areas, this prevents the smaller particles from being broken further, which is similar to the rod mill practice [12].

Figure 3.9. Doering Cylpebs are slightly tapered cylindrical grinding media with length equaling diameter, and all the edges being radiuses. The largest Cylpebs available in the market are of the size 85mm85mm, and the smallest ones are 8mm8mm. Because of their geometry, Cylpebs have greater surface area than balls of the same mass.

It is worth mentioning here that the milling vial should not be completely filled with the powder and milling media charges. There should be enough space that allows the milling media to move around freely in the vial that provides alloying and particle size reduction. Suryanarayana [13] has suggested that 50% of the vial volume should be left free, as schematically presented in Figure 3.11.

Figure 3.11. Schematic representation of the cross-sectional view for a milling vial filled with powders charge and balls. The volume of the total charge (powders+balls) should be in the range between 40% and 50% of the total volume of the vial.

Since the dmin attained in a metal during milling is expected to depend on its mechanical properties, it is suspected that neither the nature of the mill nor the milling energy will have any effect on the minimum grain size achieved. It was reported (Galdeano et al., 2001) that there was no significant effect of milling intensity on nanostructure formation in a Cu-Fe-Co powder blend. But, in another investigation, it was shown that dmin was about 5nm when the TiNi intermetallic powder was milled in a high-energy SPEX shaker mill, but only about 15nm in the less-energetic Invicta vibratory ball mill (Yamada and Koch, 1993). Similar trends were noted for variations in the ball-to-powder weight ratios (BPR). The grain size of the niobium metal milled in the Invicta vibratory mill was about 262nm at a BPR of 5:1, but was only 181 at a BPR of 10:1 (Koch, 1993). Similar results were also reported for the pure metal Cu: 255nm for a BPR of 5:1 and 201 for a BPR of 10:1. Further, the kinetics of achieving this dmin value could also depend on the milling energy, although no such studies have been reported so far.

It was also reported that during nanocrystal formation, the average crystal size increased and the internal lattice strain decreased at higher milling intensities owing to the enhanced thermal effects (Kuhrt et al., 1993). In accordance with this argument, the grain size of Si milled at a high energy of 500kJg1 was 25nm, while that milled at a low energy of 20kJ g1 was only 4nm (Streleski et al., 2002).

The aim of this project is to affect white root fungi by genetic or other means in order that they may eliminate part of the lignin out of wood chips while the chips are still stored. The biological pulp obtained in this manner can then be submitted to the normal mechanical processing or boiling. The consumption of milling energy as well as chemicals is hereby expected to be less than usual.

New fungi suitable for biological pulp production will be developed and tested with regard to int. al. optimum growth conditions and ability to delignify and defiber various types of wood. Studies will include tests concerning controlled fiber separation as well as microscopic examinations.

As the name suggests, the ball milling method consists of balls and a mill chamber. Therefore, a ball mill contains a stainless steel container and many small iron, hardened steel, silicon carbide, or tungsten carbide balls are made to rotate inside a mill. The powder of a material is taken inside the steel container. This powder will be made into nanosize using the ball-milling technique. A magnet is placed outside the container to provide the pulling force to the material and this magnetic force increases the milling energy when milling container or chamber rotates the metal balls. Ball milling is a mechanical process and thus all the structural and chemical changes are produced by mechanical energy.100 Baek etal.101 recently proposed that edge-selectively functionalized graphene nanoplatelets (EFGnPs) as metal-free electrocatalysts for ORR can be large-scaled prepared by ball-milling method. The EFGnPs were obtained simply by dry ball-milling graphite in the presence of hydrogen, carbon dioxide, sulfur trioxide, or carbon dioxide/sulfur trioxide mixture. The resultant sulfonic acid- (SGnP) and carboxylic acid/sulfonic acid- (CSGnP) functionalized GnPs were found to show a superior ORR performance to commercially available platinum-based electrocatalyst in an alkaline electrolyte. It was also found that the edge polar nature of the newly prepared EFGnPs without heteroatom doping into their basal plane played an important role in regulating the ORR efficiency.

Mechanochemical synthesis involves high-energy milling techniques and is generally carried out under controlled atmospheres. Nanocomposite powders of oxide, nonoxide, and mixed oxide/nonoxide materials can be prepared using this method. The major drawbacks of this synthesis method are: (1) discrete nanoparticles in the finest size range cannot be prepared; and (2) contamination of the product by the milling media.

More or less any ceramic composite powder can be synthesized by mechanical mixing of the constituent phases. The main factors that determine the properties of the resultant nanocomposite products are the type of raw materials, purity, the particle size, size distribution, and degree of agglomeration. Maintaining purity of the powders is essential for avoiding the formation of a secondary phase during sintering. Wet ball or attrition milling techniques can be used for the synthesis of homogeneous powder mixture. Al2O3/SiC composites are widely prepared by this conventional powder mixing route by using ball milling [70]. However, the disadvantage in the milling step is that it may induce certain pollution derived from the milling media.

In this mechanical method of production of nanomaterials, which works on the principle of impact, the size reduction is achieved through the impact caused when the balls drop from the top of the chamber containing the source material.

A ball mill consists of a hollow cylindrical chamber (Fig. 6.2) which rotates about a horizontal axis, and the chamber is partially filled with small balls made of steel, tungsten carbide, zirconia, agate, alumina, or silicon nitride having diameter generally 10mm. The inner surface area of the chamber is lined with an abrasion-resistant material like manganese, steel, or rubber. The magnet, placed outside the chamber, provides the pulling force to the grinding material, and by changing the magnetic force, the milling energy can be varied as desired. The ball milling process is carried out for approximately 100150h to obtain uniform-sized fine powder. In high-energy ball milling, vacuum or a specific gaseous atmosphere is maintained inside the chamber. High-energy mills are classified into attrition ball mills, planetary ball mills, vibrating ball mills, and low-energy tumbling mills. In high-energy ball milling, formation of ceramic nano-reinforcement by in situ reaction is possible.

It is an inexpensive and easy process which enables industrial scale productivity. As grinding is done in a closed chamber, dust, or contamination from the surroundings is avoided. This technique can be used to prepare dry as well as wet nanopowders. Composition of the grinding material can be varied as desired. Even though this method has several advantages, there are some disadvantages. The major disadvantage is that the shape of the produced nanoparticles is not regular. Moreover, energy consumption is relatively high, which reduces the production efficiency. This technique is suitable for the fabrication of several nanocomposites, which include Co- and Cu-based nanomaterials, Ni-NiO nanocomposites, and nanocomposites of Ti,C [71].

Mechanical pretreatment refers to chipping, milling, and grinding of lignocellulose and is almost always the first step in all lignocellulose processes. Size reduction of biomass for easier processing and transport is the simplest form of mechanical pretreatment. Size reduction also improves the available surface area of lignocellulose that comes in contact with reactants in the process. Knife mills, hammer mill, and disc mill are some common types of size reduction instruments. The size of feedstock after chipping is in the range of 1030mm, and it can be reduced to 26mm by milling and grinding. The relationship between final size and milling energy is not linear, and further reduction of size is energetically demanding (51). Type of feedstock, moisture content, and starting milling size also influence the energy demand. Knife and hammer mills are more economical with energy consumption between 1 and 130kWht1 in comparison to disc and ball mills (52).

Extensive milling of lignocellulose also alters its physical structure and enhances its reactivity (53,54). Cellulose undergoes the most extensive change as milling disrupts the long-range ordered structure and reduces its crystallinity, making it more susceptible to chemical attacks (55). Amorphous cellulose produced by milling treatment undergoes hydrolysis at a lower temperature and exhibits higher rate of hydrolysis (56). Milling also makes lignin and hemicellulose susceptible to dissolution to facilitate their separation. The simplicity of mechanical pretreatment is attractive as it does not use solvents or corrosive chemicals. However, the high-energy requirement for milling is a major limiting factor for large-scale applications.

Mechanocatalysis, involving combined milling of lignocellulose and catalysts, is an attractive method to increase the solubility and reactivity. Combining cellulose with solid acid or small amount of mineral acid catalysts and milling it in a ball mill produced water-soluble oligomers that can be easily hydrolyzed to glucose under mild condition (5759). Acid catalyst directly depolymerized cellulose to yield soluble oligomers. NMR analysis revealed that repolymerization of fragments occurred to form -1,6 branched oligomers (59) (Fig. 7). These branched oligomers showed high reactivity for hydrolysis and hydrolytic hydrogenation reactions.

Fig. 7. Structure of branched oligomers formed by mechanocatalytic depolymerization of cellulose (A) and (B) 1H NMR of the anomeric region of branched oligomers showing cellulosic -1,4 linkages and newly formed -1,6 linkages along with and reducing ends.

Mechanocatalysis of lignocellulose produces a composite of depolymerized material that can be reacted directly to produce 5-hydroxymethylfurfural and furfural (60). Deep depolymerization of lignocellulose produces a water-soluble composite that yields sugars after hydrolysis at mild reaction conditions. The residual lignin is obtained as solid material after hydrolysis and can be recovered by simple filtration (61). Depolymerization of lignocellulose is catalyzed by the presence of acids, and influence of radicals was negligible as the presence of lignin, a radical scavenger, did not reduce the rate of depolymerization (62). It can be argued that mechanocatalysis is not merely pretreatment method as the original lignocellulose structure is chemically transformed to a large extent. Energy requirement for mechanocatalysis is analogous to mechanical pretreatment. However, the advantage of deep depolymerization reduces the cost for subsequent processing of lignocellulose. Energy requirement at gram-scale operation is 200MWht1 that can be reduced to 9.6MWht1 at kilogram scale (63). Therefore, mechanocatalysis can be feasible at large-scale operation for lignocellulose depolymerization.

The temperature rise during milling is mainly due to ball-to-ball, ball-to-powder, and ball-to-wall collisions as well as frictional effects. The overall temperature rise of the powders during milling can be due to more than one cause. First, the intense mechanical deformation due to kinetic energy of the grinding media (balls) raises the temperature of the powder. Thus, the higher the energy (milling speed, relative velocity of the balls, time of milling, size of the grinding balls, ball-to-powder weight ratio, etc.), the higher the temperature rise. Second, it is possible that exothermic processes occurring during the milling process cause the powder particles to ignite and generate additional heat [13].

It has been reported that >90% (different values have been quoted by different investigators) of the mechanical energy imparted to the powders during milling is transformed into heat [14], raising the temperature of the powder.

Bhattacharya and Arzt [15] calculated the contact temperature of the powder compact surfaces, considering that head-on collisions occur during collisions between two grinding media. Therefore, the calculated value will be the upper bound of the temperature rise. In arriving at these values, the authors have assumed that the time of impact, , could be approximated using Hertz's theory of elastic impact [16,17], and the energy flux at the compact surface is uniform over the entire contact area and is constant for times less than . They have also assumed in their calculations only a small fraction, , of the kinetic energy

where m is the mass and v is the relative velocity of the balls, is utilized in plastically deforming the powder, that is, the expended plastic energy, Up=E. By taking the appropriate heat transfer conditions into consideration, the authors [15] calculated the temperature rise, T, as

and Q represents the average quantity of heat arising from the total deformation process over a time interval , is the heat yield transfer into the powder compact, is the thermal conductivity of the powder, t0 is the thickness of the powder sheet between two colliding balls at a moment of maximum impacting force (Fig. 2.7), r0 is the radius of the powder compact, and is the thermal diffusivity.

Such as, assuming that niobium powder is milled using stainless steel balls and that the powder compact has a thickness of t0=100m and a radius r0=263m, the maximal temperature rise T reached at the time of contact surface at a moment of maximum impacting force calculated according to [15] at =0.03 by v=6m/s and 8m/s is 496 and 669K, respectively, and at =0.09 by v=6m/s and 8m/s is 941 and 1460K, respectively.

The temperature rises calculated and estimated above using theoretical models on the basis of heat build up owing to kinetic energy of the collisions of balls [15] or microstructural changes [18] are not accurate or reliable due to the assumptions made, some of which may not be substantiated. Therefore, only experimental investigations may give reasonable and accurate values for the rise in temperature. However, experimental measurements are not easy and they measure only the bulk temperature of the wall of a mill, not the instantaneous temperature at the time of impact.

Some investigators have reported very large temperature rises. The experimentally measured bulk temperature rise in different alloy systems and milling conditions is in the range 323489K [17], and more commonly it is about 373390K. A maximal temperature of 373488K was recorded [19] by milling Ni-base superalloy in an attritor. It should be realized that this is the macroscopic temperature rise, even though it is recognized that local (microscopic) temperatures can be very high, often exceeding the melting points of some component metals [20].

Example equilibrium phase diagrams and calculated Gibbs energy curves for the systems AlMn, CrCo, CuFe, FeNi, TaAl and AlTi are presented in Fig. II.20.3, Fig. II.20.4, Fig. II.20.5, Fig. II.20.6, Fig. II.20.7 and Fig. II.20.8 respectively. All Gibbs energy curves have been calculated, using assessed thermodynamic data from the SGTE Solution Database [002SGT], for a temperature of 200 C a typical ball-milling temperature. For each system, the point of intersection of the solvent-phase Gibbs energy curve with the curve for the primary precipitating phase defines the calculated metastable solubility limit (denoted in the diagrams by a dashed line to the composition axis).

II.20.3. The calculated equilibrium phase diagram for the AlMn system. (b) Calculated Gibbs energy curves for the fcc, cubic A13 and complex body-centred cubic (cbcc) phases of the AlMn system at 200 C with component Gibbs energies changed by 5000 J mol1 (stoichiometric compounds and the Al8 Mn5 D810 phase, with complex crystallography, are omitted).

II.20.4. (a) The calculated equilibrium phase diagram for the CrCo system. (b) Calculated Gibbs energy curves for the body-centred cubic (bcc), fcc and hexagonal close-packed (hcp) phases of the CrCo system at 200 C with component Gibbs energies changed by 5000 J mol1 (the phase, with complex crystallography, is omitted).

II.20.5. (a) The calculated equilibrium phase diagram for the CuFe system. (b) Calculated Gibbs energy curves for the fcc and bcc phases for the CuFe system at 200 C with component Gibbs energies changed by 5000 J mol1.

II.20.6. (a) The calculated equilibrium phase diagram for the FeNi system. (b) Calculated Gibbs energy curves for the fcc and bcc phases of the FeNi system at 200 C with component Gibbs energies changed by 5000 J mol1.

II.20.7. (a) The calculated equilibrium phase diagram for the TaAl system. (b) Calculated Gibbs energy curves for the bcc and fcc phases of the TaAl system at 200 C with component Gibbs energies changed by 5000 J mol1 (stoichiometric compounds and the phase, with complex crystallography, are omitted).

II.20.8. (a) The calculated equilibrium phase diagram for the AlTi system. (b) Calculated Gibbs energy curves for the fcc, bcc and hcp phases of the AlTi system at 200 C with component Gibbs energies changed by 5000 J mol1 (stoichiometric compounds and phases with complex crystallography are omitted).

A summary of experimental and calculated results for the above systems is presented in Table II.20.1. It can be seen from this table that results from experimental studies vary widely, although observed solubilities are in all cases significantly greater than the equilibrium values. It is likely that the various parameters associated with the different experimental ball-milling processes are in part responsible for the differing results. The selected constant milling energy (5000 J mol1) used to amend the Gibbs energies of the different powder components may, therefore, also not be a suitable value in all cases. Nevertheless, the general agreement between experimental and calculated metastable solubility boundaries, using the calculation principles listed above, is surprisingly good. An example is the CrCo system, in which the equilibrium solubility of Co in Cr is 4.9 at.% at 600 C. After mechanical alloying, the solubility from experimental measurements [001Sur] increases very significantly to 30 or 40 at.% according to two different studies. The metastable solubility limit from thermodynamic calculation, using assessed data for the system [002SGT] is 35 at.%.

Table II.20.1. Comparison of predicted and observed solid solubilities (RT, room temperature). Equilibrium solubility values and precipitated phases are taken from the Pauling File [002Vil] and solubilities after mechanical alloying from the work by Suryanarayana [001Sur]

A difficulty in making a complete comparison of calculated solubilities with the experimental results is that workers have tended to place emphasis on the observed extended solubilities and in nearly all cases provide very little information on the phases precipitating from the solvent solution. Nevertheless, in many systems and experiments, the extent of the experimentally observed solubilities is such that the compositions of intermetallic compound phases observed in the equilibrium diagram are exceeded, which supports the proposition that phases with more complex crystallographic structure are difficult to produce when starting from the pure powder components in mechanical alloying. Examples are the AlMn system for which Mn solubilities greater than the Mn concentrations of the phases Al12Mn and Al6Mn have been measured, the AlTi system, for which Ti solubilities beyond the 25 and 33 at.% Ti compositions of the phases Al3Ti and Al2Ti have been reported, and the TaAl system, with Al solubilities greater than the 2739 at.% Al range of the phase. There are some systems which, with careful systematic experimentation, could provide a sensitive test of the calculation principles used here. For example, in the CuSi system, in the Cu-rich range, not only are a number of stoichiometric compound phases found, but also a bcc and an hcp phase in addition to the fcc Cu-rich solid solution (Fig. II.20.9).

The Gibbs energies of the single-phase fcc, hcp and bcc structures have very similar values as shown by the calculated curves for a temperature of 200 C, presented in Fig. II.20.10. In this plot, no contribution to the Gibbs energies of the pure components has been made. It can be seen that the Gibbs energies of the fcc and hcp phases have very similar values and that, if no compound phase precipitates to form a two-phase structure in the alloyed material, then the hcp phase can be expected to form at compositions with xSi 0.085 up to a composition with xSi 0.175, when the bcc phase becomes stable.

At the still higher temperature of 800 C, the points of intersection of the hcp and bcc curves with the fcc curve are close to being superimposed (Fig. II.20.12). This is consistent with the temperature of the fcchcpbcc three-phase equilibrium in Fig. II.20.9. The phase formed from the fcc phase at 800 C could be hcp or bcc, depending on just very small energy changes.

All the above figures, and in particular Fig. II.20.10, Fig. II.20.11 and II.20.12, clearly demonstrate that the amount of energy imparted by the alloying process, as well as the temperature achieved during milling of the powdered components, can have a significant influence on the Gibbs energies of the potentially forming phases. The relative values of these energies will, in turn, determine which of the phases form under given conditions.

The milling process is a relatively simple route to produce ensembles of MNPs. In some cases it has been used to produce new materials from different starting compounds (mechanical alloying) and has been frequently used to produce metallic granular alloys, in which the main aim is to create ensembles of MNPs embedded in a diamagnetic matrix (M-m; M=magnetic metal, and m=nonmagnetic metal) which can give rise to interesting properties such as giant magnetoresistance, but we will not elaborate on this. The main issues can be consulted in Refs. [13, 14].

We are also not presenting here the metallurgical origin of the production of alloys by milling. The main factors for mechanical alloying have been reviewed in detail in Refs. [15, 16] and are discussed as a combination of flattening, cold welding, fracture and rewelding. These processes are influenced by the ductile/fragile nature of the components [17].

If we were to underline the major advantages of using mechanical milling, we can identify: (i) the use of simple and easily affordable equipment, (ii) large variety in the production of MNPs, (iii) production of enormous quantities of MNPs, (iv) controllable particle sizes and reasonable distribution of particle sizes, and (v) in metals, control of lattice strain. The first point is supported by the low cost of the miller in comparison to other methods, although the container may become expensive and worn out with time. Point (ii) is clearly another advantage, the variety of alloys, and compounds to be produced is extremely wide [16] and different compositions are easy to be explored. The third point is important because frequently other routes that is: chemical multilayers are sometimes restricted to small reaction yields and nanometric thicknesses, respectively. In consequence, the quality of the sample may be better but the production of large quantities may render a successful technological potential via the industrial rescaling of the milling process. The next advantage is related to the fact that the production of MNPs with a determined average size (D) is easy to achieve and, for example, in RE alloy MNPs, this is achieved with low milling times. The distribution of particle sizes is not very wide although it cannot reach the narrowness obtained by some chemical routes. The reproducibility is quite high and particle sizes of between 5 and 50nm are very easy to reach. The last point (v) is related to the milling process in metals. A measurable increase of lattice strain, concomitant with the particle size decrease is always observed after milling. This possibility of promoting an increase of atomic disorder may be useful to show alterations in the magnetic intraparticle and interparticle coupling [18]. Now we will describe the mechanical milling process and the steps and parameters to be taken into account.

The mechanical milling process is a high-energy impact process, which can be performed in different mills, typically in planetary and shaker mills, with the use of balls within containers. Planetary mills consist of a number of cylindrical containers sitting on a spinning platform (see Fig. 1.6). The planetary movements involve both the horizontal rotation around the center of the base and that around the container axis. The shaker provides the milling energy thanks to the oscillating movements of a container at high velocity. In general, the planetary mill gives more flexibility for the production of samples and more quantity, whereas shaker mills are more efficient.

There are several parameters affecting the production of MNPs by mechanical milling and they are mostly based on empirical results rather than established theories and calculations. The use of a particular kind of mill already introduces particular milling parameters, which need to be taken into account. However, the common variables to control are: (i) the material of the container, (ii) ball-to-powder weight ratio, (iii) milling speed/frequency, and (iv) the milling time. These parameters are intermixed so a combination of selected values is what finally guarantees success. There are other parameters that will also require a particular comment: milling atmosphere, temperature, and eventual milling medium.

The material (and volume) of the containers must be selected for the specific mill. In planetary mills, containers of 125mL are a good compromise between the cost and the quantity of the material to be prepared. Most common vials are made of stainless steel, tungsten carbide (CW), and zirconia (ZrO2). The advantage of using stainless steel containers is their low cost, but they can contaminate the samples with Fe or Cr, which change the magnetic properties. They are also weaker than the other two types of containers, so to achieve the reduction to the nanoscale a larger milling time (t) is expected. If possible, it is very convenient to select CW containers where the total absence of magnetic components and the hardness assure an appropriate compositional control of the resulting MNPs, with a reduced milling time. In very particular processes, it would be desirable to use the expensive ZrO2 containers. It is clear that the balls should be of the same material as the container. Both containers and balls wear with time and should be substituted after a few processes, although this depends greatly on the nature of the material that is being prepared. As a matter of fact, SEM-EDX results of the milled MNPs are key prompting for a container and ball replacement, if undesired impurities are found.

The milling velocity can also be tuned. In planetary mills, a maximum rotation speed of 200rpm is usually enough. To use always the same rotation speed, and to control the value of t, it is a sensible decision to reduce the number of variables in mechanical milling. Generally speaking the increase of velocity will tend to reduce the amount of time needed to achieve the nanocrystalline state of the MNPs.

The precise number for the ball-to-sample ratio is somehow diffuse but values around 12:1 are good and give flexibility if a portion of the sample is extracted for an intermediate state analysis (see below).

The milling time (t) can be as low as a few minutes, to >300h. It needs to be stated whether the milling is performed continuously or, on the contrary, there are intercalated stops where the sample is resting (530min, approximately). In modern planetary mills, it is possible to inverse the rotation sense after that period. Going back to the optimization of the synthesis process, it is true that the mass within the vial is strictly modified, but it can be readjusted to maintain constant the value of the ball-to-powder weight ratio. In most of the analysis, the milling time is the main control parameter. It is very likely that the nature of the alloy itself is more important than the milling time. For example, some alloys become nanometric only after a few hours of milling [19], whereas others require much longer times [18,20].

There are other less studied factors. For example, an inert atmosphere (Ar 99.99%, for instance) must be used when dealing with RE-based alloys or pure Fe, as they are oxygen avid and easily form oxides. This is easily achieved in O-ring sealed containers if we follow the precaution to fill in them with the starting crushed pellets in a glove box. An alternative could be to insert the mill in a glove box itself; this seems feasible realistically only if shaker mills (e.g., SPEX) are used. In connection with the milling environment, it is possible to use oleic acid or oleylamine during the milling. This will also reduce the eventual oxidation but, more importantly, it helps to add a surfactant layer around the particles. Naturally longer t-values expose the MNPs to a more probable surface oxidizing layer. Another factor is the temperature of the containers. The mechanical milling process involves effectively an increase in the vial (container) temperature. Depending on our needs, this can be convenient or not: the increasing temperature favors the change of structure at the nanoscale resulting in a common increase in D and (in metals especially) a decrease in strain. In consequence, temperature can be a control variable, but direct temperature control (either by a coolant or furnace) is discouraging. Precisely, our experience recommends the use of stop periods so that the containers are allowed to cool down to avoid undesired recrystallizations.

In short, the high-energy mechanical milling procedure is a powerful technique to produce nanoparticles of different natures in enormous quantities. The latter is not only an important point for industrial rescaling of the production, but also to study the magnetic arrangement at a microscopic scale. The process is simple, cost effective, and the quality of the samples is generally enough for magnetic studies.

Mechanical pretreatment of biomass aims to enhance the digestibility of biomass. Coarse size reduction, cutting, shredding, chipping, grinding, or milling are among the different mechanical methods that can be used to decrease particle size, increase accessible specific surface area, increase pore size of particles and the number of contact points, and reduce the DP or crystallinity of the cellulose, although different fractions (cellulose, hemicellulose, and lignin) will not be separate.

Commonly, starting materials are presized during harvesting or preconditioning, using methods such as shredding, forage cutting, or chipping to sizes of about 1050mm. This is the minimum pretreatment needed prior to biomass processing. The size of material can be further reduced to 0.22mm by milling or grinding through different machines: vibratory ball mills, hammers, knifes, balls, discs, colloids, and extruders.

Chipping is used to reduce heat and mass transfer limitations. Grinding and milling are more effective at reducing the particle size and cellulose crystallinity (Dumas etal., 2015). Vibratory ball milling is more effective than ordinary ball milling in reducing cellulose crystallinity. Disk milling, which produces fibers, is more efficient in enhancing cellulose hydrolysis than hammer milling. The type and duration of milling, as well as the kind of biomass, determine the digestibility of the biomass (Hideno etal., 2013; Zakaria etal., 2015).

This method has some disadvantages in terms of the energy demand. The final particle sized desired and the biomass characteristics determine the energy requirements for mechanical comminution, which is usually very high. There is a critical size below which further reduction will not affect the biomass treatment significantly (Dumas etal., 2015; Liu, 2015). Initial moisture content and biomass composition have shown to be important parameters impacting the specific energy requirement (Barakat etal., 2015). In this sense, mechanical comminution is not considered as an attractive option in biomass pretreatment.

The combination of biological or chemical treatment prior to mechanical diminution has confirmed the feasibility of reduction of the energy consumption of mechanical processes (Fougere etal., 2015; Motte etal., 2015). Studies show that milling after chemical pretreatment can significantly reduce milling energy consumption, cost of solid liquid separation, and liquid to solid ratio, and does not result in the production of fermentation inhibitors (Zhu etal., 2009).

Extrusion pretreatment is a physical pretreatment in which materials are exposed to mixing, heating, and shearing, suffering physical and chemical modification. The shear forces applied in the extrusion process serve to remove the softened surface regions, exposing the interior to chemical and/or thermal action and therefore improving the cellulose conversion (Mood etal., 2013).

Factors such as extruder temperature, screw speed, feedstock particle size, and moisture content have been investigated to determine their influence in energy requirements of the pretreatment (Karunanithy and Muthukumarappan, 2011a).

Main advantages of this method include short residence time, moderate temperature, no formation of inhibitors such as furfural or 5-hydroxymethylfurfural (HMF), no need of washing step, no solid loss, rapid mixing, feasibility of scale-up, and possibilities of continuous operation (Karunanithy and Muthukumarappan, 2011b,c).

Recent studies about alkali-combined extrusion pretreatment indicated that combined pretreatment increased the number of pores in the biomass structure, giving improved sugar yields (Zhang etal., 2012).

LHW processes are biomass pretreatments based on the use of pressure to keep water at high temperatures (160240C). It is also referred in the literature as autohydrolysis, hydrothermolysis, hydrothermal pretreatment, aqueous fractionation liquefaction or extraction, solvolysis, aquasolv, steam pretreatment, or water prehydrolysis (Mosier etal., 2005).

The reaction is initiated by the hydro ions [H3O+] generated from the dissociation of water molecules. This process changes the biomass structure, resulting in the hydrolysis of hemicellulose and removal of a small portion of the lignin, which makes the cellulose more accessible for further hydrolysis while avoiding the formation of fermentation inhibitors that occurs at higher temperatures (Vallejos etal., 2015).

It is important to maintain the pH between 4 and 7 during the pretreatment because at this pH, the dissolved hemicellulose exists mainly in oligomeric form, and the formation of monosaccharides and the subsequent degradation products that further catalyze hydrolysis of cellulosic material are minimized.

Pretreatment of some biomass feedstock can be carried out under mild conditions (140180C), but for most raw biomass, it needs to be performed at higher temperatures (up to 190230C). The water and the biomass (18%) are brought in contact up to 5min at most severe conditions. At this high temperature, sugar degradation may increase significantly. Between 40% and 60% of the total biomass can be dissolved in the process with the removal of 422% of the cellulose, 3560% of the lignin and the majority of the hemicellulose (Rogalinski etal., 2008).

The process itself is simplified. The use of lower temperatures with minimization of degradation products eliminates the need for a final washing step or neutralization because the pretreatment solvent here is water. Neither sludge handling nor acid recycling result.

The biomass glucan content is not modified. The physicochemical modification caused by treatment on lignin and cellulose facilitates the further separation of different fractions. Hemicelluloses can be converted into hemicellulosic sugars at good yields with low byproduct generation.

The main disadvantage of LWH pretreatment is related to the downstream processing. The amount of solubilized product is higher, while the concentration of these products is lower compared to other pretreatments. High energy is demanded due to the large volumes of water involved.

The recovery of hemicellulose from solution is impeded by high-lignin solubilization. A catalyst such as an acid can be added making the process similar to dilute acid pretreatment. However, degradation of sugars can result in undesirable inhibitory products. During pretreatment, the pH and pKa of water is affected by temperature, so KOH can be used to maintain the pH above 5 and below 7 to minimize the formation of monosaccharides that are degraded to fermentation inhibitors (Mosier etal., 2005).

Pyrolysis is a thermal pretreatment of lignocellulosic biomass where raw material is heated in an inert atmosphere at temperatures between 350 and 650C. It is usually employed to enhance the energy density of fuels produced from biomass. Nitrogen is the commonly used carrier gas to provide a nonoxidizing atmosphere. Torrefaction takes place at similar conditions to those of pyrolysis but at lower temperatures of 200300C, due to which, it is also called mild pyrolysis.

Cellulose decomposes rapidly to gaseous products and residual char when biomass is heated above 300C. At lower temperatures, decomposition is much slower, and resulting products are less volatile. Under severe conditions, hemicellulose is almost depleted completely, and cellulose is oxidized to a great extent. Lignin is the most difficult component to be degraded, and thus, its removal is very low under torrefaction conditions.

After thermal pretreatment, the properties of biomass are improved to a great extent. Main benefits from torrefaction are more uniform properties in the biomass, which include improved grindability and reactivity, higher energy density, lower atomic O/C and H/C ratios and moisture content, and higher hydrophobicity (Chen etal., 2015b).

Recent studies have demonstrated that torrefaction pretreatment causes mechanical disruption of biomass fibers, resulting in their size reduction as well as high solid product yield (Das and Sarmah, 2015).

Torrefaction and pyrolysis have been studied as pretreatment processes for biomass to fuel conversion. Pretreatment by torrefaction was found to be far more attractive than pyrolysis (Kumar etal., 2009).

Freeze/thaw pretreatment is a novel approach for physical pretreatment of biomass. In this process, biomass is frozen in a conventional freezer at temperature below 20C for a certain period of time (between 2 and 24h) and then immediately thawed in hot water or at room temperature (Chang etal., 2011).

Treatment with freezing/thawing could be an efficient alternative for pretreatment of lignocellulosic biomass due to significantly increasing the enzyme digestibility of substrates (Smichi etal., 2015).

Only a few studies have been carried out, but despite the high cost involved, its attractive characteristics, i.e. lower negative environmental impact, application of less dangerous chemicals, and high effectiveness, make freeze/thaw process a promising pretreatment in biomass processing.

Some authors have suggested treatments involving the use of gamma rays that give a larger surface area and lower crystallinity by cleaving the -1,4 glycosidic bonds. Gamma irradiation after sulfuric acid pretreatment on wheat straw showed a great influence on enzymatic hydrolysis, owing to disruption of cellulose crystallinity, removal of hemicelluloses, and structural modification of lignin polymers (Hong etal., 2014). This method would be very expensive on a large scale with huge environmental and safety concerns.

Microwave irradiation could be an alternative to the conventional heating in order to modify the structure of cellulose, degrade and partially remove lignin and hemicelluloses, and enhance the enzymatic susceptibility of reducing sugars. The advantages of this method include short process time, high uniformity and selectivity, and less energy input than conventional heating. Microwave-assisted pretreatment has demonstrated the improvement in enzymatic hydrolysis of corn straw and rice husk (Diaz etal., 2015).

Pulsed electric field (PEF) pretreatment is a physical pretreatment of lignocellulosic biomass that involves the application of a short burst of high voltage to a sample (biomass) situated between two electrodes. The sample can be either placed or transported between the electrodes, and the electric discharge is applied in the form of pulses. High intensity electric field produced structural changes in the cell membrane, resulting in an increase in mass permeability and mechanical rupture (Kumar etal., 2009). The creation of permanent pores in the cell walls facilitate the entry of acids (in the case of chemical treatments) or enzymes (biological processes) used to break down the cellulose into its constituent sugars, and thus PEF increases the hydrolysis rate.

Most important factors in PEF pretreatment include electric field strength, which is usually above 1kV/cm (520kV/cm), number of pulses, and treatment time, in the microsecond range. Major benefits of PEF pretreatment are that it can be carried out at ambient conditions, energy requirement is low, and the process itself is not very complex as it does not involve moving parts.

Ultrasonic pretreatment (USP) of lignocellulosic biomass has been studied at laboratory scale, although it is a well-established technique for industrial wastewater treatment. It promotes the pretreatment and conversion process through cavitation phenomenon. Ultrasonic energy allows destruction of the lignocellulosic structure and fractionation of biomass components, with increased yields of sugars, bioethanol, and gas products. Sonication promotes hydrolysis and leads to reduced reaction time, lower reaction temperature, and less amounts of solvents (Luo etal., 2014).

Experiments carried out on a model compound (carboxymethyl cellulose) showed that reaction time was dramatically increased (Imai etal., 2004). Other studies on lignocellulosic biomass in combination with hydrogen peroxide for bioethanol production gave higher yields of cellulose recovery and delignification (Ramadoss and Muthukumar, 2016).

ball-milling properties and sintering behavior of fe-tm2o3 mixture powders - sciencedirect

ball-milling properties and sintering behavior of fe-tm2o3 mixture powders - sciencedirect

Fe-based Tm2O3 and Fe-based TmFeO3 composites were first designed and synthesized.The evolution behavior of Fe-Tm2O3 powders was studied during ball milling.The sintering behavior of the bulks prepared by Fe-Tm2O3 powders were analyzed.Mechanical properties and thermal expansion of the bulks were investigated.The difference between Fe-based Tm2O3 and Fe-based TmFeO3 composites was discussed.

Aimed at the existing problems of current neutron absorbers that were used in control rods to safely operate a nuclear reactor, new type of Fe-based Tm2O3 composites and Fe-based TmFeO3 composites, which were synthesized from the Fe-25.68wt.%Tm2O3 mixture powders by ball milling, cold isostatic pressing and sintering, were designed as next generation neutron absorbers in the present work. During ball milling, Tm2O3 was fined, nano-crystallized, amorphized and then dissolved into Fe crystal lattice to form Fe (Tm, O) nanocrystalline solid solution. The effect of sintering temperature and oxygen content at sintering environment on the phases of the sintered bulks were researched, and the corresponding mechanism was discussed. Nanoscale TmFeO3 and Tm2O3 particles were distributed in the sintered bulk matrix. The microhardness, compression strength, and the coefficients of thermal expansion of the bulks sintered at different conditions were measured and discussed. The coefficient of linear expansion and compressive strength increased with increasing ball-milling time. The microhardness increased with increasing sintering temperature when the temperature was lower than 1200C. Meanwhile, the microhardness and compressive strength of Fe-based Tm2O3 composites were larger than that of Fe-based TmFeO3 composites, opposite for the coefficients of thermal expansion.

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