high energy ball milling it is a ball milling process where a powder mixture placed in the ball mill is subjected to high energy

high energy ball milling process for nanomaterial synthesis

high energy ball milling process for nanomaterial synthesis

It is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collision from the balls. This process was developed by Benjamin and his coworkers at the International Nickel Company in the late of 1960. It was found that this method, termed mechanical alloying, could successfully produce fine, uniform dispersions of oxide particles (Al2O3, Y2O3, ThO2) in nickel-base superalloys that could not be made by more conventional powder metallurgy methods. Their innovation has changed the traditional method in which production of materials is carried out by high temperature synthesis. Besides materials synthesis, high-energy ball milling is a way of modifying the conditions in which chemical reactions usually take place either by changing the reactivity of as-milled solids (mechanical activation increasing reaction rates, lowering reaction temperature of the ground powders)or by inducing chemical reactions during milling (mechanochemistry). It is, furthermore, a way of inducing phase transformations in starting powders whose particles have all the same chemical composition: amorphization or polymorphic transformations of compounds, disordering of ordered alloys, etc.

The alloying process can be carried out using different apparatus, namely, attritor, planetary mill or a horizontal ball mill. However, the principles of these operations are same for all the techniques. Since the powders are cold welded and fractured during mechanical alloying, it is critical to establish a balance between the two processes in order to alloy successfully. Planetary ball mill is a most frequently used system for mechanical alloying since only a very small amount of powder is required. Therefore, the system is particularly suitable for research purpose in the laboratory. The ball mill system consists of one turn disc (turn table) and two or four bowls. The turn disc rotates in one direction while the bowls rotate in the opposite direction. The centrifugal forces, created by the rotation of the bowl around its own axis together with the rotation of the turn disc, are applied to the powder mixture and milling balls in the bowl. The powder mixture is fractured and cold welded under high energy impact.

The figure below shows the motions of the balls and the powder. Since the rotation directions of the bowl and turn disc are opposite, the centrifugal forces are alternately synchronized. Thus friction resulted from the hardened milling balls and the powder mixture being ground alternately rolling on the inner wall of the bowl and striking the opposite wall. The impact energy of the milling balls in the normal direction attains a value of up to 40 times higher than that due to gravitational acceleration. Hence, the planetary ball mill can be used for high-speed milling.

During the high-energy ball milling process, the powder particles are subjected to high energetic impact. Microstructurally, the mechanical alloying process can be divided into four stages: (a) initial stage, (b) intermediate stage, (c) final stage, and (d) completion stage.

(a) At the initial stage of ball milling, the powder particles are flattened by the compressive forces due to the collision of the balls. Micro-forging leads to changes in the shapes of individual particles, or cluster of particles being impacted repeatedly by the milling balls with high kinetic energy. However, such deformation of the powders shows no net change in mass.

(b) At the intermediate stage of the mechanical alloying process, significant changes occur in comparison with those in the initial stage. Cold welding is now significant. The intimate mixture of the powder constituents decreases the diffusion distance to the micrometer range. Fracturing and cold welding are the dominant milling processes at this stage. Although some dissolution may take place, the chemical composition of the alloyed powder is still not homogeneous.

(c) At the final stage of the mechanical alloying process, considerable refinement and reduction in particle size is evident. The microstructure of the particle also appears to be more homogenous in microscopic scale than those at the initial and intermediate stages. True alloys may have already been formed.

(d) At the completion stage of the mechanical alloying process, the powder particles possess an extremely deformed metastable structure. At this stage, the lamellae are no longer resolvable by optical microscopy. Further mechanical alloying beyond this stage cannot physically improve the dispersoid distribution. Real alloy with composition similar to the starting constituents is thus formed.

Theoretical considerations and explorations of planetary milling process have been broadly studied in order to better understand and inteprate the concept. Joisels work is the first report to study the shock kinematics of a satellite milling machine. This work focused on the determination of the milling parameters that were optimized for shock energy. The various parameters were determined geometrically and the theoretical predictions were examined experimentally using a specifically designed planetary mill. Schilz et al. reported that from a macroscopical point of view, the geometry of the mill and the ratio of angular velocities of the planetary to the system wheel played crucial roles in the milling performance. For a particular ductile-brittle MgSi system, the milling efficiency of the planetary ball was found to be heavily influenced by the ratio of the angular velocity of the planetary wheel to that of the system wheel as well as the amount of sample load. Mio et al. studied the effect of rotational direction and rotation-to-revolution speed ratio in planetary ball milling. Some more theoretical issues and kinematic modeling of the planetary ball mill were reported later in related references. Because mechanical alloying of materials are complex processes which depend on many factors, for instance on physical and chemical parameters such as the precise dynamical conditions, temperature, nature of the grinding atmosphere, chemical composition of the powder mixtures, chemical nature of the grinding tools, etc., some theoretical problems, like predicting nonequilibrium phase transitions under milling, are still in debate.

For all nanocrystalline materials prepared by high-energy ball milling synthesis route, surface and interface contamination is a major concern. In particular, mechanical attributed contamination by the milling tools (Fe or WC) as well as ambient gas (trace impurities such as O2, N2 in rare gases) can be problems for high-energy ball milling. However, using optimized milling speed and milling time may effectively reduce the contamination. Moreover, ductile materials can form a thin coating layer on the milling tools that reduces contamination tremendously. Atmospheric contamination can be minimized or eliminated by sealing the vial with a flexible O ring after the powder has been loaded in an inert gas glove box. Small experimental ball mills can also be completely enclosed in an inert gas glove box. As a consequence, the contamination with Fe-based wear debris can be reduced to less than 12 at.% and oxygen and nitrogen contamination to less than 300 ppm. Besides the contamination, long processing time, no control on particle morphology, agglomerates, and residual strain in the crystallized phase are the other disadvantages of high-energy ball milling process.

Notwithstanding the drawbacks, high-energy ball milling process has attracted much attention and inspired numerous research interests because of its promising results, various applications and potential scientific values. The synthesis of nanostructured metal oxides for gas detection is one of the most promising applications of high-energy ball milling. Some significant works have been reported in recent years. Jiang et al. prepared metastable a-Fe2O3MO2 (M: Ti and Sn) solid solutions by high-energy milling for C2H5OH detection. The 85 mol% a-Fe2O3SnO2 sample milled for 110 hours showed the highest sensitivity among all the samples studied. The best sensitivity to 1000 ppm C2H5OH in air at an operating temperature of 250 C was about 20. Zhang et al. synthesized FeSbO4 for LPG detection. They found that there were two-step solid-state reactions occurring in the raw powders during the ball milling:

The response and recovery times of their sensor were less than one second. The sensitivity to 1000 ppm C2H5OH at an operating temperature of 375 C was about 45. Diguez et al. employed precipitation method to prepare nanocrystalline SnO2 and planetary milling to grind the obtained powder for NO2 detection. They found that the grinding procedure of the precursor and/or of the oxide had critical effect on the resistance in air. As a result, the gas sensing properties to NO2 had been considerably improved. Cukrov et al. and Kersen et al. synthesized SnO2 powders by mechanochemical processing for O2 and H2S sensing applications, respectively. The O2 sensor exhibited stable, repeatable and reproducible electrical response to O2. More recently, Yamazoes group reported the sensing properties of SnO2Co3O4 composites to CO and H2. A series of SnO2Co3O4 thick films containing 0100% Co3O4 in mass were prepared from the component oxides through mixing by ball-milling for 24 h, screen-printing and sintering at 700 C for 3 h. The composite films were found to exhibit n- or p-type response to CO and H2 depending on the Co3O4 contents in the composites. The n-type response was exhibited at 200 C or above by SnO2-rich composites (Co3O4 content up to 5 mass%). The sensor response to both CO and H2 was significantly enhanced by the addition of small amounts of Co3O4 to SnO2, and the response at 250 C achieved a sharp maximum at 1 mass% Co3O4. The p-type response was obtained at 200 C or below by the composites containing 25100 mass% Co3O4. The sensitivity as well as selectivity to CO over H2 could thus be increased by the addition of SnO2 to Co3O4.

Besides the above mentioned researches, significant efforts on the synthesis of nanostructured metal oxide with high-energy ball milling method for gas sensing have been actively pursued by the authors of this chapter. In our research, we use the high-energy ball milling technique to synthesize various nanometer powders with an average particle size down to several nm, including nano-sized a-Fe2O3 based solid solutions mixed with varied mole percentages of SnO2, ZrO2 and TiO2 separately for ethanol gas sensing application, stabilized ZrO2 based and TiO2 based solid solutions mixed with different mole percentages of a-Fe2O3 and synthesized SrTiO3 for oxygen gas sensing. The synthesized powders were characterized with XRD, TEM, SEM, XPS, and DTA. Their sensing properties were systematically investigated and sensing mechanisms were explored and discussed as well.

high-energy ball milling - an overview | sciencedirect topics

high-energy ball milling - an overview | sciencedirect topics

High-energy ball milling is a ball milling process in which a powder mixture placed in a ball mill is subjected to high-energy collisions from the balls. High-energy ball milling, also called mechanical alloying, can successfully produce fine, uniform dispersions of oxide particles in nickel-base super alloys that cannot be made by conventional powder metallurgy methods. High-energy ball milling is a way of modifying the conditions in which chemical reactions usually take place, either by changing the reactivity of as-milled solids or by inducing chemical reactions during milling [20].

High-energy ball milling is a mechanical deformation process that is frequently used for producing nanocrystalline metals or alloys in powder form. This technique belongs to the comminution or attrition approach introduced in Chapter 1. In the high-energy ball milling process, coarse-grained structures undergo disassociation as the result of severe cyclic deformation induced by milling with stiff balls in a high-energy shaker mill [8,9]. This process has been successfully used to produce metals with minimum particle sizes from 4 to 26nm. The high-energy ball milling technique is simple and has high potential to scale up to produce tonnage quantities of materials [8]. However, a serious problem of this technique is the contamination from milling media (balls and vial) and/or atmosphere. Therefore, a number of improvements, including the usages of surfactants, alloy-coated milling media, and protective atmospheres, have been developed to alleviate the contamination problem [8].

The fine powder (in nano or submicron sizes) produced from ball milling can be consolidated to bulk form for large-scale applications such as hip implants and bone screws. Usually, the fine powders are compacted and sintered together via methods like hot isostatic pressing and explosive compaction under the temperatures or conditions that suppress grain growth and maintain nanocrystalline microstructure [8,10]. Bulk metallic materials produced by this approach have achieved the theoretical densities of nanocrystalline materials and greatly improved mechanical properties compared to their conventional, micron-grained counterparts.

High-energy ball milling is effective in getting well-dispersed slurry.79 The preparation procedure is summarized in Fig.24.2. First, commercially available PZT powders (APC 850) were high-energy ball milled to get the desired particle size. Secondly, a selected dispersant was added to the milled powders to get the surface-modified powders. The smaller the powder, the more important this procedure. Afterwards, PZT precursor solution was added to these surface-modified powders and mixed by further ball milling. Finally, the resultant uniform slurry was ready for further processing, such as spin coating, tape casting, screen printing and molding. The recipe for the slurry, including the concentration of xerogel solution and powder to solution mass ratio, depends on the further processing method employed. For our convenience, the recipes for the slurry were given four numbers with regard to the above two important parameters. For example, in 3025, the first two numbers represent the concentration of the xerogel solution9 in weight percent, i.e. 30wt%, and the last two numbers represent the mass ratio of the added PZT powder to xerogel solution, namely 2 to 5.

High-energy ball milling, also called mechanical attrition, can be used to reduce the grain size of materials from many micrometers to 220nm (see Mechanical Alloying). This is a result of the cold-working process creating large-angle grain boundaries. Most of the reduction in grain size occurs rapidly, but the process slows, and long times are required to reach the smallest sizes. This process has the advantage of being relatively inexpensive and can be easily scaled up to produce large quantities of material. Usually, to maximize the energy of collision, high-mass hard-steel or WC balls are used. Contamination by materials removed by the balls is a major concern. Severe mechanical deformation and plastic deformation at high strain rates (103104s1) occurs during the process. Initially, shear bands are formed consisting of a high density of dislocations. Later these dislocations annihilate and recombine as small-angle grain boundaries forming nanometer-sized grains. Finally, the orientation of these nanometer-sized grains is randomized.

The range of solubility of multicomponent systems is greatly increased by mechanical attrition. Mechanical attrition can also produce metastable materials. If the milling is done in the presence of O2 or N2, oxides or nitrides can be formed.

High-energy ball milling, a predominantly mechanical process, nevertheless results in significant structural and chemical changes in the material. Nonequilibrium synthesis of materials at low temperatures via ball milling is possible through a combination of multiple processes, which occur during milling. These processes include thermal shock, high-speed plastic deformation, mechanical grinding and fracturing, cold welding, and intimate mixing [9].

BNNTs were typically synthesized by the prolonged (approximately 150 h) high-energy milling of pure boron or h-BN powder using stainless-steel milling vessels and hardened steel balls in a pressurized (2.3 103 Torr) NH3 atmosphere. The milled material was then annealed at high temperature (>1000 C) in an N2 atmosphere for 10 h. It was found that large quantities of BNNTs can be synthesized using this method. The yield of the BNNTs depended on the duration of the milling treatment [11]. It was proposed that nanotube formation by this method was caused by two different mechanisms. The first mechanism being the nitridation of B nanoparticles in the NH3 atmosphere, which in turn served as nucleation sites for the formation of BNNTs. The second mechanism proposed was that the Fe (and other metals such as Cr and Ni) from the milling process was incorporated into the B powder during high-energy milling and that the metal particles then served as catalysts for BNNT growth [11]. In order for these two mechanisms to operate effectively, it is necessary that both the ball milling and annealing steps be carried out for long times. Other variations of this technique have been reported including the use of tungsten carbide (WC) balls, and a mixture of NiB and alumina [5,12]. Even though the yield of BNNTs can be very high using this method, the resultant nanotubes can suffer from contamination and structural defects. Figure 8.6 shows a micrograph of BNNTs synthesized using this technique.

Figure 8.6. Transmission electron microscope image of BNNTs synthesized by the ball milling process. The growth of the nanotubes from the milled material is clearly evident. The largest nanotube imaged has a bamboo-like morphology.

High energy ball milling can lead to glass formation from elemental powder mixtures as well as by amorphization of intermetallic compound powders. Solid state amorphization by high energy milling has been demonstrated in a number of Ti- and Zr-based and other alloy systems such as NiTi, CuTi, AlGeNb, SnNb, NiZr, CuZr, CoZr and FeZr. The process of ball milling is illustrated in Figure 3.56. Powder particles are severely deformed, fractured and mutually cold welded during collisions of the balls. The repeated fracturing and cold welding of powder particles result in the formation of a layered structure in which the layer thickness keeps decreasing with milling time. A part of the mechanical energy accumulates within these powder particles in the form of excess lattice defects which facilitate interdiffusion between the layers. The continuous reduction in the diffusion distance and the enhancement in the diffusivity with increasing milling time tend to bring about chemical homogeneity of the powder particles by enriching each layer with the other species being milled together. The sequence of the events that occur during milling can be followed by taking out samples from the ball mill at several intervals and by analysing these powder samples in respect of their chemical composition and structure. Let us describe one such experiment in which elemental powders of Zr and Al were milled in an attritor under an Ar atmosphere.

Elemental powders of Zr and Al of 99.5 purity, when milled in an attritor using 5 mm diameter balls of zirconia as the milling media and keeping the ball to powder weight ratio at 10:1, showed a progressive structural change as revealed in XRD patterns (Figure 3.57(a) and (b)). Diffraction peaks associated with the individual elemental species remained distinct upto 5 h of milling at a constant milling speed of 550 rpm. All particles and the balls appeared very shiny in the initial stages. With increasing milling time, the particles lost their lustre, the 111 and 200 peaks of fcc Al gradually shrunk and the three adjacent low-angle peaks of hcp -Zr, corresponding to1010, 0002 and1011, became broader. After about 15 h of milling, XRD showed only -Zr peaks which shifted towards the high angle side, implying a decrease in the lattice parameters resulting from the enrichment of the -Zr phase with Al. After 20 h of milling, all Bragg peaks except one broad peak close to the{1010} peak disappeared. Powders milled for 25 h showed an extra reflection corresponding to a lattice spacing of 5.4 nm, which matches closely to a superlattice reflection of a metastable D019 (Zr3Al) phase. On further milling, the powders transformed into an amorphous phase. The sequence of structural evolution could be described as -Zr + Al -Zr (Al) solid solution + Al nanocrystalline solid solution + localized amorphous phase Zr3Al (D019) + -Zr (Al) solid solution + amorphous phase bulk amorphous phase.

Figure 3.57. XRD patterns showing a progressive structural change for different times when elemental powders of Zr and Al of 99.5 purity were milled in an attritor using 5 mm diameter balls of zirconia with a ball to powder weight ratio of 10:1.

The mechanism of solid state amorphization during mechanical alloying has been studied on the basis of experimental observations made on several alloy systems. One of the probable mechanisms, based on local melting followed by rapid solidification, has not found acceptance as evidence of melting could not be seen in experiments. The example of ball milling of elemental Zr and Al powders has demonstrated that the amorphisation process is preceded by the enrichment of the -Zr phase to a level of approximately 15 at.% Al. The solute concentration progressively changes during milling. The various stages encountered in the course of amorphization can be explained in terms of schematic free energy versus concentration plots for the , the metastable D019, and the amorphous phases (Figure 3.58). With increasing degrees of Al enrichment, the free energy of the interface region gradually moves along the path 1-2 (Figure 3.58). Once the concentration crosses the point 2, it becomes thermodynamically feasible to nucleate the Zr3Al phase which has the metastable D019 structure. Although the equilibrium Zr3Al phase has the L12 structure, it has been shown (Mukhopadhyay et al. 1979) that the metastable D019 structure is kinetically favoured during the early stages of precipitation from the -phase. This is not unexpected as the hcp structure and the D019 structure (which is an ordered derivative of the former) follow a one-to-one lattice correspondence and exhibit perfect lattice registry.

Figure 3.58. Schematic free energy concentration plots in ZrAl system for the , the metastable D019 and the amorphous phases illustrating the various stages encountered in the course of amorphization.

With further Al enrichment, as the concentration crosses the point 3, nucleation of the amorphous phase becomes possible. It is to be emphasized that the change in composition occurs gradually from the interface to the core of the particles, with the result that the amorphous phase starts appearing at interfaces while the core remains crystalline. As the Al concentration in the powder particles crosses point 4, each particle can turn amorphous by a polymorphic process. The observed sequence of solid state amorphization in the case of ball milling of elemental Zr and Al powders suggests the occurrence of amorphization by a lattice instability mechanism which is brought about by solute enrichment of the -phase beyond a certain limit (point 4 in Figure 3.58).

The synthesis of materials by high-energy ball milling of powders was first developed by John Benjamin (1970) and his coworkers at the International Nickel Company in the late 1960s [42,43]. It was found that this method, called mechanical alloying, could successfully produce fine and uniform dispersions of oxide particles (Al2O3, Y2O3, ThO2) in nickel-base superalloys which could not be made by conventional powder metallurgy methods.

It is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collision from the balls. Fig. I.7 shows the motions of the balls and the powder. Since the rotation directions of the bowl and balls are opposite, the centrifugal forces are alternately synchronized. Thus, friction resulted from the hardened milling balls and the powder mixture being ground alternately rolling on the inner wall of the bowl and striking the opposite wall. The impact energy of the milling balls in the normal direction attains a value of up to 40 times higher than that due to gravitational acceleration. Hence, the planetary ball mill can be used for high-speed milling [44].

During the high-energy ball milling process, the powder particles are subjected to high energetic impact. Microstructurally, the mechanical alloying process can be divided into four stages: (1) initial stage, (2) intermediate stage, (3) final stage, and (4) completion stage [44].

At the initial stage of ball milling, the powder particles are flattened by the compressive forces caused by the impact of the balls. Microforging leads to changes in the shapes of individual particles, or clusters of particles being repeatedly impacted by the balls with high kinetic energy. However, such deformation of the powders shows no net change in mass.

At the intermediate stage of the mechanical alloying process, a significant change occurs as compared to the initial stage. Cold welding becomes significant. The intimate mixture of the powder constituents decreases the diffusion distance to the micrometer range. Fracturing and cold welding are the dominant milling processes at this stage. Although some dissolution may take place, the chemical composition of the alloyed powder is still not homogeneous.

At the final stage of the mechanical alloying process, more refinement and reduction in particle size becomes evident. The microstructure of the particle also appears to be more homogeneous in microscopic scale than those at the initial and intermediate stages. True alloys may have already been formed.

At the completion stage of the mechanical alloying process, the powder particles possess an extremely deformed metastable structure. At this stage, the lamellae are no longer resolvable by optical microscopy. Further mechanical alloying beyond this stage cannot physically improve the dispersoid distribution. Real alloy with a composition similar to the starting constituents is thus formed [44].

MA in high-energy ball milling equipment is accomplished by processing an initial powder charge usually comprising a mixture of elemental, ceramic (e.g., yttria for ODS alloys), and master alloy powders, all supplied in strictly controlled size ranges. Master alloy powders are used in order to reduce in situ oxidation of highly reactive species, such as aluminum or titanium alloy additions during processing. The milling medium normally used in commercial systems is a charge of hardened steel balls, typically 2cm in diameter. The ball-to-powder weight ratio is chosen carefully for each mill and powder charge combination, but is typically around 10:1 for commercial systems. Given the enormous surface area, both of the initial powders and the fresh powder surfaces generated during MA processing, control of the milling atmosphere and its purity is essential to avoid undue alloy contamination. The principal protective atmospheres employed during commercial milling of MA powders are usually either argon or hydrogen and this protection generally extends both to pre- and post-MA powder handling. Both the purity of these gas atmospheres and the integrity of gas seals on the milling equipment are essential to control contamination, particularly when processing reactive species. For example, levels of oxide contamination in Ni3Al can double with just a few hours of milling in impure argon. Occasionally, however, deliberate doping of the milling environment has been used to facilitate alloying additions during processing.

The central event during MA is the ballpowderball collision within the milling medium during processing. It is repetition of these high-energy collisions which leads eventually to MA of the powder charge. Intimate mixing and eventual MA of the powder charge occurs in a series of identifiable, more or less discrete stages during processing (e.g., Gilman and Benjamin 1983). For ductileductile or ductilebrittle combinations of starting powders, MA initially proceeds by the flattening and work hardening of ductile powders and fragmenting of brittle constituents, which is followed by extensive cold welding between powder particles, formation of lamellar structures, and coarsening of the powder particle size distribution. Brittle powder fragments are trapped at cold weld interfaces between the evolving lamellas of the ductile constituents and thus, while continuing to comminute, become dispersed. With continued milling a balance, which is dependent on processing parameters and the composition of the constituents, is established between further cold welding and powder particle fracture, leading to relatively stable powder particle sizes.

This balance between welding and fracture is accompanied both by further decreases in lamella spacings and by folding and mixing-in of lamella fragments to produce microstructures typical of MA (Fig. 1). For ODS alloys, powder constituents are milled to the stage where light microscopy examination reveals that lamella spacings have decreased to below the resolution limit (1m). For typical levels of oxide addition (e.g., 0.5wt.% yttria) this criterion ensures average dispersoid interparticle spacings of <0.5m (Fig. 2). In other systems, milling can progress until true alloying occurs. Surprisingly, MA can also be achieved between essentially brittle powder constituents. The mechanisms by which this occurs are less well understood than in systems incorporating at least one ductile powder component. Nevertheless, granular as opposed to interlamellar mixtures of brittle powder constituents do evolve, typically with smaller, harder fragments progressively incorporated to a very fine scale within the less hard constituents, e.g., aluminanickel oxide. Moreover, MA of these brittle constituents can progress to true alloying, as has been demonstrated using lattice parameter measurements on Si28 at.% Ge progressively milled from constituent powders (Davis and Koch 1987).

Figure 1. Polished and etched metallographic section of ODM 751 FeCrAl alloy powders in the fully MA condition, showing the folded lamellar structures typical of material processed by high-energy ball milling (courtesy of D.M. Jaeger).

Figure 2. Transmission electron microscope image showing alignment of a fine-scale dispersion of oxide particles in extruded ODS alloy PM2000. The arrow shows the extrusion direction (courtesy of Y.L. Chen).

Milling of very ductile metals such as aluminum and tin has to be carefully controlled to avoid complete agglomeration of the ductile phase rather than the balance between cold welding and fracture that leads to MA. This is normally achieved by adding precise amounts of organic compounds termed process control agents (PCAs) to the milling environment. Typically waxes or solvents, these compounds that interfere with cold welding progressively break down during milling to become incorporated within the final MA powders (e.g., in aluminum alloys) as fine-scale distributions of carbides or oxides. Similar restrictions to the proclivity for cold welding in ductile powders can be achieved without use of PCAs by milling at low temperatures, e.g., below 100C for aluminum.

The processing equipment used to effect MA by high-energy ball milling of powders originated in mining and conventional powder metallurgy industries. The range of high-energy ball milling equipment divides, approximately, into two categories: small, high-energy laboratory devices, and larger facilities capable of milling commercial quantities of powder. The former category includes SPEX shaker mills and planetary ball mills. Both devices are capable of rapidly effecting MA, but in quantities of powders up to no more than a few tens of grams. SPEX mills vibrate at up to 1200rpm in three orthogonal directions to achieve ball velocities approaching 5ms1. Planetary mills incorporate a rotating base plate upon which are mounted counter-rotating, smaller-radius vials containing the ball/powder charge. The kinetic energy imparted to the ball charge in the planetary mill depends on the base plate and vial radii and angular velocities. Attritor or Szigvari ball mills, depending on their size, can be used either for laboratory or commercial ball milling applications and incorporate a rotating vertical shaft with attached horizontal impellors which stirs a container housing the ball and powder charge. These devices can process batches of up to several kilograms or more of powder through the significant differential movement the impellors generate between the ball and powder charge. Balls can either cascade or tumble when leaving the mill wall during attritor processing, depending on the ball charge and impellor velocity.

The largest commercial devices applied to MA are horizontal ball mills. When these devices exceed several meters in diameter they impart sufficient kinetic energy through ball impacts to effect MA and can process over 1000kg of powder per batch in larger units. Balls either cascade or tumble during processing in these mills depending on rotational speed (see Fig. 3). The time taken to achieve MA scales approximately inversely to the size of the milling equipment used. Hence, milling which might take minutes to accomplish in a SPEX mill could take hours in an attritor or days in a horizontal ball mill. All of these processing routes, however, have very low energy conversion efficiency, in that only a small fraction of the milling energy expended effects microstructural change contributing to the MA process.

Figure 3. Configuration of a horizontal ball mill, showing the release of the powder and ball charge (at angular position ) from the inner wall of the mill rotating with angular velocity (after Lu et al. 1995).

It is worth noting that during MA, powder particles also coat (condition) the ball milling medium, which means that, to avoid cross-contamination of commercial alloys, the repeat use of ball charges is restricted to compositionally similar batches of raw materials.

Mechanical means, such as high-energy ball milling, ultrasonic or jet milling, and others, can have powder prepared into nanoparticles. This is an example of a top-down approach, which is suitable for refractory metals or materials beyond the use of chemical reactions. The disadvantages include the difficulties in classification according to the particle size and serious surface contamination.

Bombarding a metal surface with high-energy balls makes it possible to turn the surface structure into nanoscale; this can improve the abrasion and corrosion resistance of the processed material. Meanwhile, the surface is identical to the bulk material, and thus it does not peel off like nanocoating material. The main mechanism of this method is to produce a large number of defects and dislocations, which further develop into dislocation walls, and thus cut the large crystals into nanocrystalline grains (Figure 5.11).

MCP is normally a dry, high-energy ball milling technique and has been employed to produce a variety of commercially useful and scientifically interesting materials. The formation of an amorphous phase by mechanical grinding of a Y-Co intermetallic compound in 1981 (Ermakov et al., 1981) and its formation in the Ni-Nb system by ball milling of blended elemental powder mixtures (Koch et al., 1983) brought about the recognition that this technique is a potential non-equilibrium processing technique. Beginning in the mid-1980s, a number of investigations have been carried out to synthesize a variety of equilibrium and non-equilibrium phases including supersaturated solid solutions, crystalline and quasicrystalline intermediate phases, and amorphous alloys. Additionally, it has been recognized that powder mixtures can be mechanically activated to induce chemical reactions, at room temperature or at least at much lower temperatures than normally required, to produce pure metals, nanocomposites and a variety of commercially useful materials. Efforts have also been under way since the early 1990s to understand the process fundamentals of MA through modeling studies. Because of all these special attributes, this simple but effective processing technique has been applied to metals, ceramics, polymers and composite materials. The attributes of mechanochemical processing are listed below. However, in the present chapter, the focus will be on the synthesis of nanocrystalline metal particles.

Inducement of chemical (displacement) reactions at low temperatures for (a) Mineral and Waste processing, (b) Metals refining, (c) Combustion reactions, and (d) Production of discrete ultrafine particles

Nanocrystalline materials are single- or multi-phase polycrystalline solids with a grain size of the order of a few nanometers (1nm=109m=10), typically 1100nm in at least one dimension. Since the grain sizes are so small, a significant volume of the microstructure in nanocrystalline materials is composed of interfaces, mainly grain boundaries. That is, a large volume fraction of the atoms resides in the grain boundaries. Consequently, nanocrystalline materials exhibit properties that are significantly different from, and often an improvement on, their conventional coarse-grained polycrystalline counterparts. Compared to the material with a more conventional grain size, that is, larger than a few micrometers, nanocrystalline materials show increased strength, high hardness, extremely high diffusion rates and consequently reduced sintering times for powder compaction, and improved deformation characteristics. Several excellent reviews are available giving details on different aspects of processings, properties, and applications of these materials (Gleiter, 1989; Suryanarayana, 1995a, 2005).

ball milling - an overview | sciencedirect topics

ball milling - an overview | sciencedirect topics

Ball milling is often used not only for grinding powders but also for oxides or nanocomposite synthesis and/or structure/phase composition optimization [14,41]. Mechanical activation by ball milling is known to increase the material reactivity and uniformity of spatial distribution of elements [63]. Thus, postsynthesis processing of the materials by ball milling can help with the problem of minor admixture forming during cooling under air after high-temperature sintering due to phase instability.

Ball milling technique, using mechanical alloying and mechanical milling approaches were proposed to the word wide in the 8th decade of the last century for preparing a wide spectrum of powder materials and their alloys. In fact, ball milling process is not new and dates back to more than 150 years. It has been used in size comminutions of ore, mineral dressing, preparing talc powders and many other applications. It might be interesting for us to have a look at the history and development of ball milling and the corresponding products. The photo shows the STEM-BF image of a Cu-based alloy nanoparticle prepared by mechanical alloying (After El-Eskandarany, unpublished work, 2014).

Ball milling, a shear-force dominant process where the particle size goes on reducing by impact and attrition mainly consists of metallic balls (generally Zirconia (ZrO2) or steel balls), acting as grinding media and rotating shell to create centrifugal force. In this process, graphite (precursor) was breakdown by randomly striking with grinding media in the rotating shell to create shear and compression force which helps to overcome the weak Vander Waal's interaction between the graphite layers and results in their splintering. Fig. 4A schematic illustrates ball milling process for graphene preparation. Initially, because of large size of graphite, compressive force dominates and as the graphite gets fragmented, shear force cleaves graphite to produce graphene. However, excessive compression force may damage the crystalline properties of graphene and hence needs to be minimized by controlling the milling parameters e.g. milling duration, milling revolution per minute (rpm), ball-to-graphite/powder ratio (B/P), initial graphite weight, ball diameter. High quality graphene can be achieved under low milling speed; though it will increase the processing time which is highly undesirable for large scale production.

Fig. 4. (A) Schematic illustration of graphene preparation via ball milling. SEM images of bulk graphite (B), GSs/E-H (C) GSs/K (D); (E) and (F) are the respective TEM images; (G) Raman spectra of bulk graphite versus GSs exfoliated via wet milling in E-H and K.

Milling of graphite layers can be instigated in two states: (i) dry ball milling (DBM) and (ii) wet ball milling (WBM). WBM process requires surfactant/solvent such as N,N Dimethylformamide (DMF) [22], N-methylpyrrolidone (NMP) [26], deionized (DI) water [27], potassium acetate [28], 2-ethylhexanol (E-H) [29] and kerosene (K) [29] etc. and is comparatively simpler as compared with DBM. Fig. 4BD show the scanning electron microscopy (SEM) images of bulk graphite, graphene sheets (GSs) prepared in E-H (GSs/E-H) and K (GSs/K), respectively; the corresponding transmission electron microscopy (TEM) images and the Raman spectra are shown in Fig. 4EG, respectively [29].

Compared to this, DBM requires several milling agents e.g. sodium chloride (NaCl) [30], Melamine (Na2SO4) [31,32] etc., along with the metal balls to reduce the stress induced in graphite microstructures, and hence require additional purification for exfoliant's removal. Na2SO4 can be easily washed away by hot water [19] while ammonia-borane (NH3BH3), another exfoliant used to weaken the Vander Waal's bonding between graphite layers can be using ethanol [33]. Table 1 list few ball milling processes carried out using various milling agent (in case of DBM) and solvents (WBM) under different milling conditions.

Ball milling of graphite with appropriate stabilizers is another mode of exfoliation in liquid phase.21 Graphite is ground under high sheer rates with millimeter-sized metal balls causing exfoliation to graphene (Fig. 2.5), under wet or dry conditions. For instance, this method can be employed to produce nearly 50g of graphene in the absence of any oxidant.22 Graphite (50g) was ground in the ball mill with oxalic acid (20g) in this method for 20 hours, but, the separation of unexfoliated fraction was not discussed.22 Similarly, solvent-free graphite exfoliations were carried out under dry milling conditions using KOH,23 ammonia borane,24 and so on. The list of graphite exfoliations performed using ball milling is given in Table 2.2. However, the metallic impurities from the machinery used for ball milling are a major disadvantage of this method for certain applications.25

Reactive ball-milling (RBM) technique has been considered as a powerful tool for fabrication of metallic nitrides and hydrides via room temperature ball milling. The flowchart shows the mechanism of gas-solid reaction through RBM that was proposed by El-Eskandarany. In his model, the starting metallic powders are subjected to dramatic shear and impact forces that are generated by the ball-milling media. The powders are, therefore, disintegrated into smaller particles, and very clean or fresh oxygen-free active surfaces of the powders are created. The reactive milling atmosphere (nitrogen or hydrogen gases) was gettered and absorbed completely by the first atomically clean surfaces of the metallic ball-milled powders to react in a same manner as a gas-solid reaction owing to the mechanically induced reactive milling.

Ball milling is a grinding method that grinds nanotubes into extremely fine powders. During the ball milling process, the collision between the tiny rigid balls in a concealed container will generate localized high pressure. Usually, ceramic, flint pebbles and stainless steel are used.25 In order to further improve the quality of dispersion and introduce functional groups onto the nanotube surface, selected chemicals can be included in the container during the process. The factors that affect the quality of dispersion include the milling time, rotational speed, size of balls and balls/ nanotube amount ratio. Under certain processing conditions, the particles can be ground to as small as 100nm. This process has been employed to transform carbon nanotubes into smaller nanoparticles, to generate highly curved or closed shell carbon nanostructures from graphite, to enhance the saturation of lithium composition in SWCNTs, to modify the morphologies of cup-stacked carbon nanotubes and to generate different carbon nanoparticles from graphitic carbon for hydrogen storage application.25 Even though ball milling is easy to operate and suitable for powder polymers or monomers, process-induced damage on the nanotubes can occur.

Ball milling is a way to exfoliate graphite using lateral force, as opposed to the Scotch Tape or sonication that mainly use normal force. Ball mills, like the three roll machine, are a common occurrence in industry, for the production of fine particles. During the ball milling process, there are two factors that contribute to the exfoliation. The main factor contributing is the shear force applied by the balls. Using only shear force, one can produce large graphene flakes. The secondary factor is the collisions that occur during milling. Harsh collisions can break these large flakes and can potentially disrupt the crystal structure resulting in a more amorphous mass. So in order to create good-quality, high-area graphene, the collisions have to be minimized.

The ball-milling process is common in grinding machines as well as in reactors where various functional materials can be created by mechanochemical synthesis. A simple milling process reduces both CO2 generation and energy consumption during materials production. Herein a novel mechanochemical approach 1-3) to produce sophisticated carbon nanomaterials is reported. It is demonstrated that unique carbon nanostructures including carbon nanotubes and carbon onions are synthesized by high-speed ball-milling of steel balls. It is considered that the gas-phase reaction takes place around the surface of steel balls under local high temperatures induced by the collision-friction energy in ball-milling process, which results in phase separated unique carbon nanomaterials.

Conventional ball milling is a traditional powder-processing technique, which is mainly used for reducing particle sizes and for the mixing of different materials. The technique is widely used in mineral, pharmaceutical, and ceramic industries, as well as scientific laboratories. The HEBM technique discussed in this chapter is a new technique developed initially for producing new metastable materials, which cannot be produced using thermal equilibrium processes, and thus is very different from conventional ball milling technique. HEBM was first reported by Benjamin [38] in the 1960s. So far, a large range of new materials has been synthesized using HEBM. For example, oxide-dispersion-strengthened alloys are synthesized using a powerful high-energy ball mill (attritor) because conventional ball mills could not provide sufficient grinding energy [38]. Intensive research in the synthesis of new metastable materials by HEBM was stimulated by the pioneering work in the amorphization of the Ni-Nb alloys conducted by Kock et al. in 1983 [39]. Since then, a wide spectrum of metastable materials has been produced, including nanocrystalline [40], nanocomposite [41], nanoporous phases [42], supersaturated solid solutions [43], and amorphous alloys [44]. These new phase transformations induced by HEBM are generally referred as mechanical alloying (MA). At the same time, it was found that at room temperature, HEBM can activate chemical reactions which are normally only possible at high temperatures [45]. This is called reactive milling or mechano-chemistry. Reactive ball milling has produced a large range of nanosized oxides [46], nitrides [47], hydrides [48], and carbide [49] particles.

The major differences between conventional ball milling and the HEBM are listed in the Table 1. The impact energy of HEBM is typically 1000 times higher than the conventional ball milling energy. The dominant events in the conventional ball milling are particle fracturing and size reductions, which correspond to, actually, only the first stage of the HEBM. A longer milling time is therefore generally required for HEBM. In addition to milling energy, the controls of milling atmosphere and temperature are crucial in order to create the desired structural changes or chemical reactions. This table shows that HEBM can cover most work normally performed by conventional ball milling, however, conventional ball milling equipment cannot be used to conduct any HEBM work.

Different types of high-energy ball mills have been developed, including the Spex vibrating mill, planetary ball mill, high-energy rotating mill, and attritors [50]. In the nanotube synthesis, two types of HEBM mills have been used: a vibrating ball mill and a rotating ball mill. The vibrating-frame grinder (Pulverisette O, Fritsch) is shown in Fig. 1a. This mill uses only one large ball (diameter of 50 mm) and the media of the ball and vial can be stainless steel or ceramic tungsten carbide (WC). The milling chamber, as illustrated in Fig. 1b, is sealed with an O-ring so that the atmosphere can be changed via a valve. The pressure is monitored with an attached gauge during milling.

where Mb is the mass of the milling ball, Vmax the maximum velocity of the vial,/the impact frequency, and Mp the mass of powder. The milling intensity is a very important parameter to MA and reactive ball milling. For example, a full amorphization of a crystalline NiZr alloy can only be achieved with a milling intensity above an intensity threshold of 510 ms2 [52]. The amorphization process during ball milling can be seen from the images of transmission electron microscopy (TEM) in Fig. 2a, which were taken from samples milled for different lengths of time. The TEM images show that the size and number of NiZr crystals decrease with increasing milling time, and a full amorphization is achieved after milling for 165 h. The corresponding diffraction patterns in Fig. 2b confirm this gradual amorphization process. However, when milling below the intensity threshold, a mixture of nanocrystalline and amorphous phases is produced. This intensity threshold depends on milling temperature and alloy composition [52].

Figure 2. (a) Dark-field TEM image of Ni10Zr7 alloy milled for 0.5, 23, 73, and 165 h in the vibrating ball mill with a milling intensity of 940 ms2. (b) Corresponding electron diffraction patterns [52].

Fig. 3 shows a rotating steel mill and a schematic representation of milling action inside the milling chamber. The mill has a rotating horizontal cell loaded with several hardened steel balls. As the cell rotates, the balls drop onto the powder that is being ground. An external magnet is placed close to the cell to increase milling energy [53]. Different milling actions and intensities can be realized by adjusting the cell rotation rate and magnet position.

The atmosphere inside the chamber can be controlled, and adequate gas has to be selected for different milling experiments. For example, during the ball milling of pure Zr powder in the atmosphere of ammonia (NH3), a series of chemical reactions occur between Zr and NH3 [54,55]. The X-ray diffraction (XRD) patterns in Fig. 4 show the following reaction sequence as a function of milling time:

The mechanism of a HEBM process is quite complicated. During the HEBM, material particles are repeatedly flattened, fractured, and welded. Every time two steel balls collide or one ball hits the chamber wall, they trap some particles between their surfaces. Such high-energy impacts severely deform the particles and create atomically fresh, new surfaces, as well as a high density of dislocations and other structural defects [44]. A high defect density induced by HEBM can accelerate the diffusion process [56]. Alternatively, the deformation and fracturing of particles causes continuous size reduction and can lead to reduction in diffusion distances. This can at least reduce the reaction temperatures significantly, even if the reactions do not occur at room temperature [57,58]. Since newly created surfaces are most often very reactive and readily oxidize in air, the HEBM has to be conducted in an inert atmosphere. It is now recognized that the HEBM, along with other non-equilibrium techniques such as rapid quenching, irradiation/ion-implantation, plasma processing, and gas deposition, can produce a series of metastable and nanostructured materials, which are usually difficult to prepare using melting or conventional powder metallurgy methods [59,60]. In the next section, detailed structural and morphological changes of graphite during HEBM will be presented.

Ball milling and ultrasonication were used to reduce the particle size and distribution. During ball milling the weight (grams) ratio of balls-to-clay particles was 100:2.5 and the milling operation was run for 24 hours. The effect of different types of balls on particle size reduction and narrowing particle size distribution was studied. The milled particles were dispersed in xylene to disaggregate the clumps. Again, ultrasonication was done on milled samples in xylene. An investigation on the amplitude (80% and 90%), pulsation rate (5 s on and 5 s off, 8 s on and 4 s off) and time (15 min, 1 h and 4 h) of the ultrasonication process was done with respect to particle size distribution and the optimum conditions in our laboratory were determined. A particle size analyzer was used to characterize the nanoparticles based on the principles of laser diffraction and morphological studies.

optimization of the high energy ball-milling: modeling and parametric study - sciencedirect

optimization of the high energy ball-milling: modeling and parametric study - sciencedirect

It has been investigated the effect of not so discussed milling parameters on the milling efficiencyThe effects of the milling parameters on the performance of the mill are simultaneously investigatedThe weight loss of the powder can greatly enhance the performance of millingThe excessive rise in the ball size distribution is associated with adverse outcomes in efficiencyThe balls made of WC compared to those of Si3N4 and steel have a more positive effect on efficiency

In the present study, the effect of not so much discussed milling parameters such as vial to plate spinning rate, ball size distribution and type of balls on the performance (energy) of the high energy ball milling has been investigated for the first time. Furthermore, different scenarios that lead to an increase in the BPR such as the powder weight loss, the increase of diameter and the number of balls are analyzed and their effects on the efficiency of the milling are discussed. The important point is that contrary to the previous studies in which the milling parameters were independently investigated, in this research the effects of the milling parameters on the performance of the mill are simultaneously investigated. The results showed that the powder weight loss can greatly enhance the performance of milling, while the increase of the number of balls at high BPR ratio, has a quite negative effect on the milling performance. Besides, excessive rise in the ball size distribution is associated with adverse outcomes in the milling efficiency, especially when the number of balls increases. Furthermore, the balls made of tungsten carbide compared to those of silicon nitride and steel have a more positive effect on the milling efficiency especially when the weight of powder is reduced. A mathematical model is also introduced to analyze the effect of milling parameters on the milling energy and the obtained results are compared with the experimental ones. According to the results obtained, if the vial to plate spinning rate is 1.2, the mill has a better performance however, by increasing the diameter of the balls, this ratio is changed to 1.4.

high-entropy (hftatinbzr)c and (hftatinbmo)c carbides fabricated through reactive high-energy ball milling and spark plasma sintering - sciencedirect

high-entropy (hftatinbzr)c and (hftatinbmo)c carbides fabricated through reactive high-energy ball milling and spark plasma sintering - sciencedirect

Powders of high-entropy Hf0.2Ta0.2Ti0.2Nb0.2Zr0.2C (HECZr) and Hf0.2Ta0.2Ti0.2Nb0.2Mo0.2C (HECMo) carbides were fabricated through the reactive high-energy ball milling (R-HEBM) of metal and graphite particles. It was found that 60min of R-HEBM is adequate to achieve a full conversion of the initial precursors into a FCC solid solution for both compositions. The HECZr powder possesses a unimodal particle size distribution (40% d1m, 95% d10m), and the HECMo powder features a bimodal distribution with a slightly larger particle size overall (30% d1m, 80% d10m). Bulk high-entropy ceramics with a minor presence of an oxide phase were fabricated through the spark plasma sintering of these high-entropy powders at 2000C with a 10min dwelling time. The HECZr ceramics possess a relative density of up to 94.8%, hardness of 25.73.5GPa, Young's modulus of 47337GPa, and thermal conductivity of 5.60.1W/mK. HECMo ceramics with a relative density of up to 93.8%, hardness of 23.82.7GPa, Young's modulus of 54448GPa, and thermal conductivity of 5.90.2W/mK were also fabricated. A comparison of the properties of the HECs produced in this study and those previously reported is also provided.

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