Method of nanomaterials synthesis – mechanical, gas phase and physical vapor deposition – laser and chemical vaporization

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Method of nanomaterials synthesis – mechanical, gas phase and physical vapor deposition – laser and chemical vaporization

Mechanical methods

·         High-energy ball milling/ mechanical attrition/mechanical alloying

·         Coarse-grained materials in the form of powders crushed mechanically in rotating drums by hard steel or tungsten carbide balls.

1.      Mechanical alloying or mechanical milling

·         Different components can be mechanically alloyed together by cold welding to produce nanostructured alloys.

Types of mills

·         Tumbler mills-large scale industrial applications

·         Attrition mills-small-industry scale (<100kg)

·         Vibratory mills-research miller (<250g)

·         Planetary mills

Disadvantage

·         Contamination by reaction with the atmosphere/ by wear of the milling medium

·         Consolidate the powder product after synthesis without coarsening the nanostructure

2. Mechanochemical processing

·         Mechanically activated exchange reactions

·         Low-cost synthesis of a ceramic and metallic nanopowders, including the production of UV absorbers such as ZnO and TiO₂ for the cosmetics industry.

Physical vapour deposition

·         Conversion of solid material into a gaseous phase by physical processes is then cooled & re-deposited on a substrate.

·         To fabricate thin films, multilayers, nanotubes, nano filaments or nanometre-sized particles

1. Thermal vaporization      

·         Simplest deposition system consists of an evaporation source and the substrate located in a vacuum chamber.

Disadvantage

·         Compounds may undergo chemical reactions, such as pyrolysis, decomposition & dissociation

2. Laser vaporization

·         In order to obtain the high power density required in many cases, pulsed laser beams are generally employed (laser ablation).

·         Absorption characteristics of the material to be evaporated determine the laser wavelength to be used.

·         One of the great advantages that laser ablation offers is the control of the vapor composition

·         deposition of complex films including complex metal oxides such as high temperature superconductor films

Disadvantages

·         Complex system design

·         Not always possible to find desired laser wavelength for evaporation

·         Low energy conversion efficiency.

3. Other vaporization techniques

·         Electron beam evaporation

·         For evaporation of electrically conductive source. The advantages of electron beam evaporation include a wide range of controlled evaporation rate due to a high power density and low contamination.

·         Arc evaporation is another method commonly used for evaporation of conductive source.

Plasma-assisted deposition processes

Magnetron sputtering

·            Creation of a plasma by the application of a large DC potential between two parallel plates

 DC glow discharge

·            ionization of gas atoms by electrons emitted from a heated filament. The gas ions in the plasma are then accelerated to produce a directed ion beam

Molecular Beam Epitaxy (MBE)

·            essentially an ultra-high-precision, ultraclean evaporator, combined with a set of in-situ tools, such as RHEED (Reflection high-energy electron diffraction)  and Auger spectroscopy, for characterization of the deposited layers during growth

Inert Gas Condensation

·            material, often a metal, is evaporated from a temperature-controlled crucible into a low-pressure, inert gas environment – ultra-high vacuum (UHV) inert gas condensation.

·            The metal vapor cools through collisions with the inert gas species, becomes supersaturated & then nucleates homogeneously; the particle size is usually in the range 1–100 nm and can be controlled by varying the inert gas pressure.

·            Particles can be collected on a cold finger cooled by liquid nitrogen, scraped off and compacted to produce a dense nanomaterial.

Comparison of Evaporation and Sputtering

Parameters	Evaporation	Sputtering
Deposition pressure	uses low pressures typically ranging from to torr	requires a relatively high pressure typically of -100torr
Atoms or molecules in evaporation chamber prior to arrival at the growth surface	do not collide with each other	collide with each other
evaporation	Thermodynamic equilibrium	Not in thermodynamic equilibrium
growth surface	not activated	Constantly under electron bombardment (highly energetic).
evaporated films	large grains	smaller grains- better adhesion to the substrates
Fractionation of multi-component systems	a serious challenge	the composition of the target and the film can be the same

Pulsed laser deposition

During the deposition process, a pulsed laser beam (typically 30 ns pulses with energy in the range of 0.1-1 J and a frequency of 1-20 Hz) is focused onto the target in a vacuum chamber. Lasers that are commonly used include ArF, KrF excimer lasers, and Nd:YAG laser. It is generally recognized that the shorter the wavelength, the more effective the ablation. Accordingly, excimer lasers have become the standard ones. When the laser energy density (energy per unit area at the target surface) is above a threshold value, the target (bulk ceramic of crystal) is evaporated, forming a plasma plume. The plume is normal to the target surface and collected on a suitably positioned and heated substrate. Deposition parameters have to be optimized to achieve high quality epitaxial films. These include substrate temperature, laser energy density and frequency, target-to-substrate distance, base pressure, deposition gas pressure, etc. During oxide deposition, oxygen must be introduced into the chamber in order to assisting the formation of the desired phase and film composition.

Advantages

i.        A very wide range of materials, including oxides, metal, semiconductors and even polymers, can be grown (Versatile.)

ii.      Ability to maintain target composition in the deposited thin films.

iii.    The energy associated with the high ionic content in laser ablation plumes (typically of the order of 10% and rising with increasing incident laser power density) and high particle velocities (of the order of 10⁶ cm.s^-1) appear to aid crystal growth and lower the substrate temperature required for epitaxy.

iv.    Clean, low cost & capable of producing simply by switching between several different targets.

Disadvantages

i.        The ablation plume cross section is generally small (in the order of cm²) due to a limited laser spot size. This, in turn, limits the sample size that can be prepared by PLD. In addition, this also brings difficulty to controlling thickness uniformity across the sample: This problem can be overcome, to some extent, by scanning the laser beam on a larger size target.

ii.      The plume of ablated material is highly forward directed, which causes poor conformal step coverage. It also makes thickness monitoring difficult.

iii.    Finally, there is an intrinsic “splashing” associated with laser ablation itself, which produces droplets or big particles of the target material on the substrate surface. From an industrial perspective, this is particularly serious as it will result in device failure.

1.0       Vapor Phase Deposition

Vapor phase deposition can be used to fabricate thin films, multilayers, nanotubes, nano filaments or nanometre-sized particles. The general techniques can be classified broadly as either physical vapor deposition (PVD) or chemical vapor deposition (CVD).

PVD involves the conversion of solid material into a gaseous phase by physical processes; this material is then cooled and re-deposited on a substrate with perhaps some modification, such as reaction with a gas. Examples of PVD conversion processes include thermal evaporation (such as resistive or electron beam heating or even flame synthesis), laser ablation or pulsed laser deposition (where a short nanosecond pulse from a laser is focused onto the surface of a bulk target), spark erosion and sputtering (the removal of a target material by bombardment with atoms or ions).

1.1       Thermal Evaporation

Evaporation is arguably the simplest deposition method, and has been proven particularly useful for the deposition of elemental films. Although formation of thin films by evaporation was known about 150 years ago, it acquired a wide range of applications over 50 years when the industrial scale vacuum techniques were developed. A typical evaporation system is schematically shown in Figure 1. The system consists of an evaporation source that vaporizes the desired material and a substrate is located at an appropriate distance facing the evaporation source. Both the source and the substrate are located in a vacuum chamber. The substrate can be heated or electrically biased or rotated during deposition. The desired vapor pressure of source material can be generated by simply heating the source to elevated temperatures, and the concentration of the growth species in the gas phase can be easily controlled by varying the source temperature and the flux of the carrier gas.

However, evaporation of compounds is more complicated, since compounds may undergo chemical reactions, such as pyrolysis, decomposition and dissociation, and the resulting vapor composition often differs from the source composition during evaporation at elevated temperatures. When a mixture of elements or compounds is used as a source for the growth of a complex film, the chemical composition of the vapor phase is most likely to be different from that in the source. Adjusting the composition or molar ratio of the constituents in the source may help. However, the composition of the source would change as the evaporation proceeds, since one element may evaporate much faster than another resulting in the depletion of the first element. As a result, the composition in the vapor phase will change. For a multicomponent system, the chemical composition of evaporated film is likely to be different from that of the source and varies with thickness. Therefore it is in general difficult to deposit complex films using evaporation method. Deposition of thin films by evaporation is carried out in a low pressure (torr); atoms and molecules in the vapor phase do not collide with each other prior to arrival at the growth surface, since the mean free path is very large as compared to the source-to-substrate distance. The transport of atoms or molecules from the source to the growth substrate is straightforward along the line of sight, and therefore the conformal coverage is relatively poor and a uniform film over a large area is difficult to obtain. Some special arrangements have been developed to overcome such a shortfall; these include (i) using multiple sources instead of single point source, (ii) rotating the substrates, (iii) loading both source and substrate on the surface of a sphere, and (iv) combination of all the above.

In addition to evaporation of source by resistance heat, other techniques have been developed and have attracted increasing attention and gained more popularity. For example, laser beams have been used to evaporate the material. Absorption characteristics of the material to be evaporated determine the laser wavelength to be used. In order to obtain the high power density required in many cases, pulsed laser beams are generally employed. Such a deposition process is often referred to as laser ablation. Laser ablation has proven to be an effective technique for the deposition of complex films including complex metal oxides such as high temperature superconductor films. One of the great advantages that laser ablation offers is the control of the vapor composition. In principle, the composition of the vapor phase can be controlled as that in the source. The disadvantages of laser ablation include the complex system design, not always possible to find desired laser wavelength for evaporation, and the low energy conversion efficiency. Electron beam evaporation is another technique, but it is limited to the case that the source is electrically conductive. The advantages of electron beam evaporation include a wide range of controlled evaporation rate due to a high power density and low contamination. Arc evaporation is another method commonly used for evaporation of conductive source.

1.2.      Pulsed Laser Deposition (PLD)

The schematic diagram of a basic PLD system is shown in Figure 2. During the deposition process, a pulsed laser beam (typically 30 ns pulses with energy in the range of 0.1-1 J and a frequency of 1-20 Hz) is focused onto the target in a vacuum chamber. Lasers that are commonly used include ArF, KrF excimer lasers, and Nd:YAG laser. It is generally recognized that the shorter the wavelength, the more effective the ablation. Accordingly, excimer lasers have become the standard ones. When the laser energy density (energy per unit area at the target surface) is above a threshold value, the target (bulk ceramic of crystal) is evaporated, forming a plasma plume. The plume is normal to the target surface and collected on a suitably positioned and heated substrate. Deposition parameters have to be optimized to achieve high quality epitaxial films. These include substrate temperature, laser energy density and frequency, target-to-substrate distance, base pressure, deposition gas pressure, etc. During oxide deposition, oxygen must be introduced into the chamber in order to assisting the formation of the desired phase and film composition.

There are a number of advantages of PLD over other film deposition methods, these include:

v.      The biggest advantage is that it is versatile. A very wide range of materials, including oxides, metal, semiconductors and even polymers, can be grown by PLD. All that is required is a target of the desired composition. It is unlike Molecular Beam Epitaxy (MBE) and Chemical Vapor Deposition (CVD), where different source of precursors are required for each element of the desired compound.

vi.    It has the ability to maintain target composition in the deposited thin films. Because of the very short duration and high energy of the laser pulse, target material plumes instantly toward the substrate: every component of the phase has a similar deposition rate. This makes optimization of the deposition process much easier.

vii.  The energy associated with the high ionic content in laser ablation plumes (typically of the order of 10% and rising with increasing incident laser power density) and high particle velocities (of the order of 10⁶ cm.s^-1) appear to aid crystal growth and lower the substrate temperature required for epitaxy.

viii.Other advantages are that PLD is clean, low cost, and capable of producing simply by switching between several different targets.

There are also a number of disadvantages to PLD. These include:

iv.    The ablation plume cross section is generally small (in the order of cm²) due to a limited laser spot size. This, in turn, limits the sample size that can be prepared by PLD. In addition, this also brings difficulty to controlling thickness uniformity across the sample: This problem can be overcome, to some extent, by scanning the laser beam on a larger size target.

v.      The plume of ablated material is highly forward directed, which causes poor conformal step coverage. It also makes thickness monitoring difficult.

vi.    Finally, there is an intrinsic “splashing” associated with laser ablation itself, which produces droplets or big particles of the target material on the substrate surface. From an industrial perspective, this is particularly serious as it will result in device failure.

Because of all the properties of PLD, it is mainly used for the investigation of new materials in the research environment.

1.3.      Vapor Phase Expansion

One example of a PVD technique is vapor phase expansion, which relies on the expansion of a high-pressure vapor phase through a jet into a low-pressure ambient background to produce super saturation of the vapor. Flow rates can approach supersonic speeds and the process can lead to the nucleation of extremely small clusters, ranging from a few atoms upwards, which can be analyzed using mass spectrometry. Although the quantities of such clusters are low, these can be of great use for studying the physics of small, usually metallic, clusters, which often are composed of magic numbers of atoms due to their very stable geometric and electronic configurations. A schematic diagram of such a system is shown in Figure 3.

2.0       Plasma-assisted deposition processes

The use of plasmas (i.e., ionized gases) during vapor deposition allows access to substantially different chemical and physical processes and also higher-purity final materials relative to the conventional PVD and CVD processes described above. There are several different types of plasma deposition reactor for plasma-assisted PVD.

2.1       Magnetron sputtering

Magnetron sputtering involves the creation of a plasma by the application of a large DC potential between two parallel plates (Figure 4). A static magnetic field is applied near a sputtering target and confines the plasma to the vicinity of the target. Ions from the high-density plasma sputter material, predominantly in the form of neutral atoms, from the target onto a substrate. A further benefit of the magnetic field is that it prevents secondary electrons produced by the target from impinging on the substrate and causing heating or damage. The deposition rates produced by magnetrons are high enough (~1 mm/min) to be industrially viable; multiple targets can be rotated so as to produce a multilayered coating on the substrate.

2.2.      DC glow discharge

DC glow discharge involves the ionization of gas atoms by electrons emitted from a heated filament. The gas ions in the plasma are then accelerated to produce a directed ion beam. If the gas is a reactive precursor gas, this ion beam is used to deposit directly onto a substrate; alternatively an inert gas may be used and the ion beam allowed to strike a target material which sputters neutral atoms onto a neighboring substrate (Figure 5).

2.3.      Molecular Beam Epitaxy (MBE)

A molecular beam epitaxy (MBE) machine is essentially an ultra-high-precision, ultraclean evaporator, combined with a set of in-situ tools, such as RHEED and Auger spectroscopy, for characterization of the deposited layers during growth. The reactor consists of an ultra-high-vacuum chamber (typically better than ~5 x 10^-14 atmospheric pressure) of approximately 1.5m diameter (Figure 6). MBE is a growth technique in which epitaxial, single atomic layers (~0.2 – 0.3 nm) are grown on a heated substrate under UHV conditions, using either atomic or molecular beams evaporated from effusion sources with openings directed towards a heated substrate usually consisting of a thin (~0.5 mm) wafer cut from a bulk single crystal. The sources can be either solid or gaseous and an MBE machine will typically have an array of multiple sources, which can be shuttered to allow layered, alternating heterostructures to be produced. Semiconductor quantum wells, superlattices and quantum wires and metallic or magnetic multilayers for spin valve structures are deposited using this technique.

Standard MBE uses elements in a very pure form as solid sources contained within a number of Knudsen cells. In operation the cells are heated to the temperature at which the elements evaporate, producing beams of atoms which leave the cells. The beams intersect at the substrate and deposit the appropriate semiconductor, atomic layer by atomic layer. The substrate is rotated to ensure even growth over its surface. By operating mechanical shutters in front of the cells, it is possible to control which semiconductor or metal is deposited. For example, opening the Ga and As cell shutters results in the growth of GaAs. Shutting the Ga cell and opening the Al cell switches the growth to AlAs. As the shutters can be switched rapidly, in comparison to the rate at which material is deposited, it is possible to grow very thin layers exhibiting very sharp interfaces. Other effusion cells contain elements required for doping, and it is possible to monitor the growth by observing the electron diffraction pattern produced by the surface. MBE can also be performed using gaseous sources, and this is often termed chemical beam epitaxy (CBE). When the sources are metallorganic compounds, the process is known as metallorganic MBE (MOMBE).

2.4.      Inert Gas Condensation

Another PVD process is direct gas phase condensation. Here a material, often a metal, is evaporated from a temperature-controlled crucible into a low-pressure, inert gas environment – ultra-high vacuum (UHV) inert gas condensation. The metal vapor cools through collisions with the inert gas species, becomes supersaturated and then nucleates homogeneously; the particle size is usually in the range 1–100 nm and can be controlled by varying the inert gas pressure. Particles can be collected on a cold finger cooled by liquid nitrogen, scraped off and compacted to produce a dense nanomaterial.

The inert gas evaporation–condensation (IGC) technique, in which nanoparticles are formed via the evaporation of a metallic source in an inert gas, has been widely used in the synthesis of ultrafine metal particles since the 1930s. A similar method has been used in the manufacture of carbon black, an ink pigment, since ancient times. The technique employed now for the formation of nano powders, in reality, differs from that used to produce carbon and lampblack primarily in the choice of atmospheric composition and pressure and in the use of a chemically reactive source. Thus, although the technology is old, the application to the production of truly nano scaled powders is relatively recent. A schematic representation of a typical experimental apparatus for the production of nano powders by IGC is shown in Figure 7. In its basic form, the method consists of evaporating a metallic source, using resistive heating (although radio frequency heating or use of an electron or laser beam as the heating source are all equally effective methods) inside a chamber which has been previously evacuated to about 10^-7 torr and backfilled with inert gas to a low pressure. The metal vapor migrates from the hot source into the cooler inert gas by a combination of convective flow and diffusion and the evaporated atoms collide with the gas atoms within the chamber, thus losing kinetic energy. Ultimately, the particles are collected for subsequent consolidation (i.e., compaction and sintering), usually by deposition on a cold surface. Most applications of the inert gas condensation technique carry this approach to extremes by cooling the substrate with liquid nitrogen to enhance the deposition efficiency. Particles collected in this manner are highly concentrated on the deposition substrate. While the particles deposited on the substrate have complex aggregate morphology, the structure tends to be classified in terms of the size of the crystallites that make up these larger structures. The scraping and compaction processes take place within the clean environment to ensure powder surface cleanliness (i.e., to reduce oxide formation) and to minimize problems associated with trapped gas.

Although the IGC process generates a very narrow particle size distribution of 3D primary crystallites, typically a few nanometres in diameter, the exact size of the crystallites is very dependent on the type of carrier gas used, its pressure, the evaporation temperature and the distance between source and collecting position. Reducing the gas pressure and lowering the evaporation rate of the source and employing a light gas in the chamber produces a finer particle size. This effect is amplified by using He, which has a very high thermal conductivity. Higher evaporation temperature, inert gas pressure and inert gas molecular weight favors the formation of larger particles.

2.5.      Comparison of Evaporation and Sputtering

Some major differences between evaporation and sputtering are briefly summarized below:

1. The deposition pressure differs noticeably. Evaporation uses low pressures typically ranging from to torr, whereas sputtering requires a relatively high pressure typically of -100torr. Atoms or molecules in evaporation chamber do not collide with each other, whereas the atoms and molecules in sputtering do collide with each other prior to arrival at the growth surface.
2. The evaporation is a process describable by thermodynamical equilibrium, whereas sputtering is not.
3. The growth surface is not activated in evaporation, whereas the growth surface in sputtering is constantly under electron bombardment and thus is highly energetic.
4. The evaporated films consist of large grains, whereas the sputtered films consist of smaller grains with better adhesion to the substrates.
5. Fractionation of multi-component systems is a serious challenge in evaporation, whereas the composition of the target and the film can be the same.

3.0.      Mechanical methods

One nanofabrication process of major industrial importance is high-energy ball milling, also known as mechanical attrition or mechanical alloying. As shown in Figure 8, coarse-grained materials (usually metals but also more recently ceramics and polymers) in the form of powders are crushed mechanically in rotating drums by hard steel or tungsten carbide balls, usually under controlled atmospheric conditions to prevent unwanted reactions such as oxidation. This repeated deformation can cause large reductions in grain size via the formation and organization of grain boundaries within the powder particles. Different components can be mechanically alloyed together by cold welding to produce nanostructured alloys. A nanometre dispersion of one phase in another can also be achieved. Microstructures and phases produced in this way can often be thermodynamically metastable. The technique can be operated at a large scale, hence the industrial interest. Generally any form of mechanical deformation under shear conditions and high strain rates can lead to the formation of nanostructures, since energy is being continuously pumped into crystalline structures to create lattice defects. The severe plastic deformation that occurs during machining, cold rolling, drawing, cyclic deformation or sliding wear has also been reported to form nanostructured material.

3.1.      Mechanical alloying or mechanical milling

Mechanical alloying (MA) or mechanical milling (MM) is a dry, high-energy ball milling technique. Strictly speaking, the term mechanical alloying is restricted to the formation of alloys or mixtures by mechanical means whereas mechanical milling is intended to describe the process of milling powders to reduce the particle size or for the refinement of structure. Since the original development of MA, as a way of incorporating oxide particles into nickel-based super alloys intended for application in gas turbines, it has been used in the preparation of a very wide range of materials from oxides to amorphous alloys and latterly, as MM, in the synthesis of nano-structured metals and alloys from atomized powders. The technique is simple. Powders, typically 50 mm in diameter, are placed with hardened steel, ceramic or tungsten carbide (WC) balls in a sealed container and shaken or violently agitated. A high energy input is achieved by using a high frequency and small amplitude of vibration during milling; the collision time under these conditions is generally estimated to be of the order of 2 ms. There is a rise in the system temperature associated with such violent deformation but this is actually quite modest, typically 100–200 K.

Various ball mills are used to produce MA/MM powders, including tumbler mills, attritor mills, vibratory mills and planetary mills. They differ in capacity and milling efficiency but the process is effectively the same in each case. The simplicity of the apparatus allows it to be scaled up to the production of tonne quantities with relative ease and has the added advantage that both the starting materials and the product materials are in the form of relatively coarse (~50 mm) powders.

Problems inherent to the technique, such as contamination by reaction with the atmosphere or by wear of the milling medium and also the need to consolidate the powder product after synthesis without coarsening the nanostructure, are often used as arguments against implementing it as a method of nanostructured material production. Such contamination can, however, be minimized by limiting the milling time or by using a protective milling atmosphere. Careful control of the processing conditions can also limit contamination by wear debris arising from the milling media to ~ 1 - 2 at% and oxygen and nitrogen levels for MA/MM materials can be less than 300 ppm. This, argue the proponents of the technique, is a higher purity of product than that achieved for materials synthesized by means such as IGC. In addition, the particle size of the product (~50 mm) means that there are fewer problems associated with porosity when consolidating than for true nano powders.

In the case of nanostructured metals produced by mechanical milling, the structure is generated by the creation of a deformation substructure. During the initial stages of the milling process, a high dislocation density, r_d, is generated within each of the grains of the starting material as a result of repeated deformation by the milling medium. At a certain level of strain these dislocations start to annihilate and recombine, a process referred to as recovery, thereby reducing the overall dislocation density. The dislocations form loose tangles or networks which then evolve with increasing strain to produce low-angle grain boundaries (LAGBs) that separate individual sub-grains. Each sub-grain shows a small orientation difference or mis-orientation with respect to its neighbors. As the deformation proceeds, this mis-orientation increases by incorporation of further dislocations into the boundaries, causing them to gradually change in character, finally becoming high-angle grain boundaries (HAGBs). A high initial density of dislocations is therefore required to facilitate grain refinement by ball milling. It is through this process of continuous grain subdivision and dislocation accumulation in the boundaries that the observed three to four orders of magnitude reduction in grain size is achieved.

There is, however, a natural limit to the number of dislocations which may be present in a material. This limit is reached when the number of dislocations being produced by deformation and the number being annihilated by recovery are equal.

In metallic systems the grain size decreases with milling time, reaching a minimum grain size <d>_min, characteristic for each metal. The minimum grain size that may be achieved during room temperature milling is found to vary roughly as the inverse of the absolute melting point, T_m, and also to be a function of the crystal structure of the metal. This is illustrated by Figure 9, which shows the variation of  <d>_min with T_m for face-centred cubic (fcc) metals.

The present explanation for this behavior is that, in metals with low melting points, the dislocation density is limited by recovery. Recovery is a thermally activated process, the activation energy for which is close to that for self-diffusion and it therefore scales with the alloy melting point. In low melting point metals, recovery can be significant at room temperature and it is considered that it is this process which limits the final grain size rather than the deformation supplied by the mill. In contrast, for metals of high melting point such as the refractory metals, there will be almost no recovery at the milling temperature. The minimum grain size in this case is therefore limited by the stress required to generate and propagate new dislocations rather than the rate of recovery.

3.2.      Mechanochemical processing

Mechanically activated exchange reactions have also been examined as possible methods for the synthesis of ultrafine and nanoscale powders. In this process, chemical precursors undergo reaction, either during milling or subsequent low-temperature heat treatment, to form a nanocrystalline composite consisting of ultrafine particles embedded within a salt matrix. The ultrafine powder is then recovered by removing the salt through a simple washing procedure. This process is termed mechanochemical synthesis and, to date, it has been successfully applied to the preparation of a very wide range of nanomaterials, including transition metals, magnetic intermetallics, sulfide semiconductors and oxide ceramics.

The technique is essentially equivalent to that for MA. In this process, however, the starting powders are typically in the form of reactant compounds. These are milled to form an intimate mixture of the reactants which either react during milling or during a subsequent low-temperature heat treatment, to yield a product and a by-product phase. Generally speaking, the product is finer if the reaction can be made to occur in a controlled manner during heat treatment.

The severe microstructural refinement imparted by the high-energy milling step, however, has the effect of decreasing the effective diffusion distance between the reactant phases. The net effect is that the chemical reactivity of the mixtures is increased, and if the energetic of the reaction is favorable, this will occur during the milling operation after a given milling time has elapsed.

To facilitate greater control of the reaction a diluent phase, usually the same compound as the reaction by-product is often added to the milling mixture. This leads to a reduction in the overall frequency of reactant–reactant collisions during milling, thereby reducing the reaction rate and the rate of heat generation. The diluents also increases the diffusion distance between crystallites of the product phase, such that coarsening of the nanocrystallites during the heat treatment stage will be inhibited by its presence.

As with mechanical attrition, the process can be readily scaled up to the production of commercially viable quantities of nanopowder. Furthermore, the simultaneous formation of ultrafine particles with an intervening salt matrix suggests that agglomerate formation can more readily be avoided than is possible with other synthesis techniques since the salt matrix inherently separates the particles from each other during processing.

The mechanochemical synthesis technique therefore allows significant control to be exercised over the characteristics of the final washed powder. The method has significant potential for the low-cost synthesis of a wide range of ceramic and metallic nanopowders, including the production of UV absorbers such as ZnO and TiO₂ for the cosmetics industry