Mechanochemical synthesis of metallic–ceramic composite powders

9 Mechanochemical synthesis of metallic– ceramic composite powders K. W I E C Z O R E K - C I U R O WA, Cracow University of Technology, Poland

Abstract: The chapter discusses several variants of mechanosyntheses for composite powder formation of metal alloy matrices with ceramic particles. The necessity of following progress in mechanochemical processes using different analytical methods is shown. Based on the results of experimental studies on Cu-Al/Al2O3 and Ni-Al/Al2O3 nanocomposites powder formation through mechanochemical synthesis, the method of transforming combustive processes to progressive ones by applying hydroxosalts instead of metal oxides as precursors of composites is described. Key words: metallic–ceramic composites, high-energy ball milling, metallothermic reactions, combustive self-propagating reactions, progressive reactions, soft mechanochemistry, matrices of Cu–Al and Ni–Al alloys.

9.1

Introduction

Technical progress is dependent on the development of new materials. Recently a lot of attention has been given to technologies which create and test functional materials, both constructive and tools. Materials properties, especially on the nanometric scale, are specific, comparing well with poly- or monocrystalline ones for materials with the same chemical composition (e.g. Fernández-Bertran, 1999; Ivanov and Suryanarayana, 2000; Suryanarayana, 2001a; Sˇepelak, 2002; Kelsall et al., 2005; Balázˇ, 2008). From a practical point of view, the desired features of nanocrystalline powders can be intensified by creating multiphase composites from them. Therefore, bulk composites are engineering materials made from two or more components with significantly different physical and/or chemical properties that remain separate and distinct on the macroscopic level within the finished structures. One of the components is a matrix and the other provides structural support. Three types of composite categories can be defined based on the characteristics of the matrix: • • •

metallic matrix composites (MMCs) ceramic matrix composites (CMCs) polymer matrix composites (PMCs). 193 © Woodhead Publishing Limited, 2010

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Application of the relevant composite powders to produce the final bulk product or any other composite product is one of the most attractive features in their fabrication. These composite powders need to be consolidated at elevated temperatures by thermomechanical processing and then their microstructure stability and microhardness should be determined in order to estimate the optimal parameters for the consolidation process. Actual metal–ceramic composite powders usually incorporate light alloys such as Al, Mg and Ti. Other metals commonly used are Cu, Ni, Fe, Co, Zn and Mo. Metals, ceramics or polymers can be used to support MMC powders. Metal matrix reinforced phases can exist as dispersed particles of (Kaczmar, 2000; Koch, 2001; Agrawal and Sun, 2004): • metal oxides (ZrO2, Al2O3, ThO2,Cr2O3) • carbides (SiC, TaC, WC, B4C) • nitrides (Si3N4, TaN, ZrN, TiN), • borides (TaB2, ZrB2, TiB2, WB). Cermetals are comprised of metallic and ceramic components, which combine the physical properties of metals and ceramics. The properties of metals include: • • •

ductility tensile strength thermal and electrical conductivity.

Ceramics possess physical properties such as: • • •

high melting point chemical stability resistance to oxidation.

Milling of soft metals/alloys and hard ceramic materials leads to the formation of valuable composites powders with important new features. Changing the ratio of the cermetal components produces final composite products with widely different properties. These materials have much higher mechanical, temperature and aggressive media resistance properties compared to ceramics properties. The hardness of cermetals increases with decreasing particle size (Chang et al., 1999; Zhang et al., 2000; Portnoy et al., 2002).

9.2

Composite powder formation: bottom-up and top-down techniques

Composite powders can be produced by two techniques: bottom-up and top-down. The first method is based on building nanostructures atom-by-atom, layer-by-layer. These chemical processes could take place in liquid, solid as well as in gas phases (Tjong and Chen, 2004). Examples of these are the © Woodhead Publishing Limited, 2010

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sol–gel method, melt spinning–melt quenching (MQ), chemical vapour deposition (CVD), physical vapour deposition (PVD), plasma ablation, laser pyrolysis, and so on. Unfortunately, there are often some disadvantages, for example multi-stage processes, the high cost of the alkoxide precursors, solvent evaporation and necessity for thermal treatment at high temperatures to provide coarse-grained products. The top-down technique begins with macrostructured materials and uses mechanical, chemical or other forms of energy to ‘break’ them into smaller pieces. However, it is very important to develop methods which minimize damage to the environment. One promising candidate is mechanochemistry, often referred to as ‘green’ processes. Mechanical energy can be easily explored for chemical syntheses of new functional materials like composite powders. No doubt, this method is fast, economical and gives high purity products, which can often take nanostructure forms (Heegn et al., 2003). In mechanochemical syntheses, the course of solid state reactions can be effectively controlled and regulated by choosing the precursors of syntheses and/ or milling variables.

9.2.1 Mechanical treatment Mechanical treatment of solids uses mechanical energy through high-energy ball milling, abrasion, fracture or welding to produce new materials or generate products with desired features, for example powder mixtures consisting of metal and ceramics (Courtney, 1994; Boldyrev et al., 1996). Thus, mechanical treatment of reactants can form metallic–ceramic composite powder particles. The structure quality, that is whether such composite particles are micro or nanosized depends on the type of reagents and milling conditions.

9.2.2 Mechanochemical synthesis: mechanical activation, mechanical alloying, reactive milling Mechanochemical synthesis includes mechanical alloying together with reactive milling which follows from mechanical activation of reagents. This is also done by high-energy ball milling (Murty and Ranganathan, 1998; Suryanarayana, 2001; Suryanarayana et al., 2001; Avvakumov et al., 2001; Boldyrev, 2006; Smolyakov et al., 2007; Balázˇ, 2008) which is a more complex but very effective process of metallic–ceramic composites formation. There are several variants of mechanochemical syntheses, which depend on the nature of the initial reagents nature (e.g. two different metals with ceramics, metal oxide with active metal or metal salt with active metal, and others). Mechanical activation is responsible for enhancing the reactivity of solids by enlarging the surface area or accelerating the reaction by correct mixing © Woodhead Publishing Limited, 2010

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of the reagents. In the processes of plastic deformation and fracture and friction during ball collisions, the impact energy is converted into other forms of energy. These induce structural defects, broken bonds and other forms of excess energy. These accumulate and a new, active state of the substances is produced. Consequently, the chemical reactivity of the solids increases significantly (e.g. Chen et al., 1997). Mechanical alloying is the process in which mixtures of powders are milled to achieve alloying at the atomic level. During this process, when high-energy impulses are used, metallic powders can form alloys like different solid solutions, intermetallic phases, mixtures of metals or amorphic materials with properties that are very often different from those found when using traditional methods (McCormick et al., 1989; Maurice and Courtney, 1990; Schwarz, 1996; Urakaev and Boldyrev, 2000). It is important to note that any form of metal alloy which comprises composite components, has better physical and mechanical properties than pure metals. For example, consolidated materials based on intermetallics of nickel, iron, titanium with aluminium are treated as functional materials that have specific physical properties and as constructive materials that have unique mechanical features such as structural stability at high temperature. Especially important are their high melting temperature, high mechanical resistance and low density. These properties allow them to be used in the automobile industry (turbo compressor rotors, valves, combustion chamber and exhauster details), in the aircraft industry, in nuclear power and in metallurgy. However, the low yield point at room temperature (what causes difficulties in mechanical treatment) is one of the main hindrances to using them more widely. Modification of features of the intermetallic phase can be made, for example by adding ceramic particles into their structure. Inserting ceramic particles (type: Al2O3, ZrO2 or TiC) into the intermetallic matrix (Ni, Al) leads to improvements in some mechanical properties like hardness and abrasion resistance at high temperatures and a significant rise of utility features, for example an increase in the corrosion temperature and in oxidation and erosion resistance. The mechanisms of metal alloy formation by mechanical treatment of metal powders are categorized into three different systems depending on the mechanical features: ductile–ductile, ductile–brittle and brittle–brittle. In the first case in the early stages of milling, the components become flatted to platelets by cold welding. In the next stage, they are cold-welded together and form a composite lamellar geometry of the constituent metals. Further true alloying occurs at the atomic level until the homogeneous structure of the powders is attained. The mechanisms of metallic–ceramics composites formation fall into the ductile–brittle components category because the brittle oxide particles are dispersed in a ductile metallic matrix. The ceramics reagent is closely spaced

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along the interlamellar spacing. With further milling, the ductile powder of alloys particles becomes work hardened and the lamellae become convoluted and refined. With continued milling the lamellae are further refined and the brittle particles are uniformly dispersed in the ductile metallic matrix. The ductile components are flattened by a micro-forging process while the brittle ones are fragmented. One can assume that the brittle particles of the materials are dispersed in the ductile matrix. However, if both milled materials are brittle, this phenomenon was not observed. A typical example of ductile–ductile microstructures is Cu/Al–Al2O3 composite powder formed during milling of the Cu-hydroxocarbonate and Al mixture, described in Section 9.5 (Wieczorek-Ciurowa et al., 2000, 2003a, 2003b). The microstructure of the products after the first stage of the mechanochemical synthesis is shown in Fig. 9.1 which presents the SEM microphotograph. The material reveals a lamellar microstructure. Based on quantitative energy dispersive X-ray elemental analysis (EDX) (Table 9.1) it is possible to estimate that mechanically alloyed composite particles consist of Cu–Al intermetallic phases as a matrix, and aluminium oxide.

+2

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+4

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9.1 SEM microphotograph (BSE image) illustrating a lamellar structure of Cu–Al/Al2O3 composite powder following aluminothermic reaction with Cu2(OH)2CO3 at the first stage of reactive milling. The darker network corresponds to a higher amount of Al2O3, the brighter one to a higher content of Cu–Al intermetallics (SEM Hitachi S-4700 with EDX Noran Vantage) (Wieczorek-Ciurowa et al., 2005).

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Table 9.1 Results of the EDX elemental microanalysis of the composite powder synthesized by milling a Cu2(OH)2CO3–Al2O3 mixture, from the corresponding SEM microphotograph (BSE image) in Fig. 9.1 (Wieczorek-Ciurowa et al., 2005) Concentration/mass (%) Point

Cu

O

Al

BSE image

1 2 3 4 5

32.7 39.0 47.2 38.4 23.3

30.1 25.5 22.2 25.3 32.6

37.2 35.5 30.6 35.5 44.1

Dark grey Grey Light grey Grey Dark grey

The darker network corresponds to the higher amount of Al2O3 and the brighter phase to the higher amount of metallic phases. The final form of Cu-Al/Al2O3 composite particles shown in Fig. 9.2 reveals its homogeneous microstructure. Reactive milling of solids relates to the process in which chemical reactions occur. This is a stage of mechanochemical synthesis that can occur in two different kinetic ways: • a reaction which develops slowly with each collision, which results in a gradual transformation of the substrates; • a self-propagating reaction initiated when the reaction enthalpy is sufficiently high. These kinetics models depend on the nature, type of reactants and their properties (e.g. hardness), thermodynamics of the reactions and technical parameters of milling (Butyagin, 2000, 2003; Takacs et al., 2001; Takacs and Mandal, 2001; Takacs, 2002; Lyakhov et al., 2008). The first type of mechanical syntheses concerns reactions, which proceed more slowly up to the point at which processes become a function of the milling time. If ignition does not occur, the collisions between milled material and the grinding medium contribute to comminution, mixing and defect formation. The formation of the final product occurs gradually. Figure 9.3 shows the first and final step of mechanochemical synthesis of Cu–Al/Al2O3 composite powder as a function of temperature and pressure inside the mill (Wieczorek-Ciurowa et al., 2007b). The second type requires a critical time for ignition of the reaction. When the temperature of the vial was recorded during milling, it was observed that initially the temperature increased slowly with time. After a certain period of milling the temperature increased abruptly, which confirmed the fact that ignition has occurred. The time at which a sudden increase in temperature occurs is referred to as the ignition time. Beyond this time, the

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AI2O3

Cu–AI

5.00 µm

9.2 SEM microphotograph (BSE image) illustrating a high level of structure homogeneity in a Cu–Al/Al2O3 composite powder following aluminothermic reaction with Cu2(OH)2CO3 at the final stage of reactive milling. The darker spots are finely dispersed Al2O3 particles whose average size is no larger then 1 μm. The brighter spots correspond to the higher content of Cu–Al intermetallics (see Fig. 9.1) (SEM Hitachi S-4700 with EDX Noran Vantage) (Wieczorek-Ciurowa and Gamrat, 2007b).

reaction takes place within seconds (see Fig. 9.4). The example in the figure is an illustration of Cu–Al/Al2O3 composite powder formation in the CuO– Al system (Wieczorek-Ciurowa and Gamrat, 2007a; Wieczorek-Ciurowa et al., 2007a). The ignition temperature is a function of the enthalpy change and the microstructure parameters of the particles, for example the interfacial area between the reactants. Continuing milling is necessary to form the correct desired structure of synthesis products. Generally, a self-propagating high-temperature synthesis (SHS) reaction consists of three stages: component activation, chemical reaction initiation and a synthesis process in bulk with product formation. Schaffer and McCormick (1992) stated that SHS reactions occur when the temperature created by the colliding balls in a ball mill is higher than initiation temperature, which depends on enthalpy changes in the process. The decreasing of this temperature can be the effect of mechanical activation by diminution of particle size and increasing the surface between interfacial contacts. The explanation of this phenomenon is as follows. It was assumed that after a period of comminution, mixing and activation, agglomerates begin

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9.3 Variations in temperature and pressure (GTM results) during mechanosynthesis of Cu–Al/Al2O3 composite powder from Cu2(OH)2CO3–Al mixture: (a) beginning of mechanical treatment and (b) final steps of mechanosynthesis (planetary ball mill Fritsch GmbH Pulverisette 6) (Wieczorek-Ciurowa et al., 2007b).

to form and increase in size. The reaction starts in a single agglomerate or in the powder layer coating a milling ball or the wall of the vial. One reaction front propagates into other parts of the powder layer. The powder layer can be attached to the surface of a milling ball or the inner wall of the container. When a ball hits this layer, part of the kinetic energy is

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9.4 Variations in temperature and pressure (GTM results) during selfpropagating high-temperature synthesis of Cu–Al/Al2O3 composite powder from a CuO–Al mixture. The milling conditions are the same as used for a Cu2(OH)2CO3–Al mixture treatment (see Fig. 9.3) (planetary ball mill Fritsch GmbH Pulverisette 6) (Wieczorek-Ciurowa et al., 2007a).

transferred to the powder as heat and a local increase of temperature can occur. The stresses inside the powder are not uniform but are concentrated at a few points. This results in the formation of ‘hot spots’, where the reaction can start even if the average temperature of the powder is not sufficient to initiate a reaction front. Intimate contact between the reactant phases is an essential requirement for self-propagating synthesis. This condition is easy to achieve when mechanical activation is conducted in a system of ductile-brittle substances. The temperature of initiation of a metallothermic reaction can be recognized by differential thermal analysis (DTA) (strong exoeffects). This is illustrated by the DTA curve (Fig. 9.5) for the product of the CuO–Al powder after the first step of mechanochemical synthesis. The CuO formed by the mechanical decomposition of Cu2(OH)2CO3 reacts with Al forming copper in a combustible manner at about 600°C. This procedure can be considered a simulation of a mechanochemical reaction (WieczorekCiurowa and Gamrat, 2007a). An example of self-propagation reactions are processes occurring during milling aluminium and magnetite powders in argon, given by Botta et al. (2000) as follows:

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Exo

Endo

Aluminothermic reduction of CuO

Eutectic reaction CuAI2 + AI→L

Decomposition of Cu2(OH)2CO3 100

300

500

700

Temperature (°C)

9.5 DTA curve (non-oxidizing atmosphere) of a Cu2(OH)2CO3–Al mixture after the first step in mechanical treatment. The product contains an undecomposed part of Cu2(OH)2CO3 and unreacted Al. This also represents a simulation of the reactions in the system that may occur under mechanical action (SDT 2960 TA Instrument) (Wieczorek-Ciurowa et al., 2005).

3Fe3O4 + 8Al → 9Fe + 4Al2O3

[9.1]

There are two different effects which are due to the time period of mechanical activation. After 30 min milling, a decrease in the starting reactant crystallinity could be observed as well as a small amount of product, whereas after 37 min milling time the aluminothermic reaction was completed. Traditionally, this reaction would be thermally initiated during high-temperature treatment. However, under mechanical activation conditions a ‘flash point’ is the direct consequence of previous energy accumulation in the crystalline structure of powder particles. In the case of milling time less than 30 min a certain amount of active product precursor forms from the energy accumulated in the solid phase. The precursor releases this energy by reacting at lower temperatures during thermal treatment. This effect is close to the one observed in the TiO2–Al system, but in this case, the aluminothermic reaction does not occur during milling. This phenomenon of solid activation, because of milling, can be seen in syntheses other than in metallic–ceramic composites ones. The kinetics of mechanochemical reactions depends not only on the reactant properties but also on the synthesis conditions. The synthesis can be explosive or occur in a controlled steady way. Changing the conditions of the activation process can prevent the reaction taking place at all. As well as the ball-to-powder ratio (BPR), the type of ball material is also

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Table 9.2 Examples of induction time intervals for selected SHS mechanochemical reactions occurring under different milling conditions (literature data)

Reaction

Type of mill

ZnO + Mg → Zn + MgO

Vibratory

7:1

45

2FeO + Ti → 2Fe + TiO2 8CuO + 3Fe → 4Cu2O + Fe3O4 2CuO + Zn → Cu2O + ZnO Cu2S + Fe → 2Cu + FeS

Vibratory 8:1 Planetary 30 : 1 Vibratory – Vibratory 8:1

20 153 44 –

BPR

Induction time (min) Reference Yang and McCormick, 1993 Takacs, 2002 Shen et al., 1992 Takacs, 2002 McCormick et al., 1989

important. Moreover, too low heat conductivity of the milling medium can hinder formation of hot spots in which the reaction is initiated. Table 9.2 gives induction times (in minutes) for SHS reactions carried out under the given conditions of mechanochemical syntheses. On the other hand, in order to control explosive effects during selfpropagating reactions, that is to suppress their kinetics, it is possible for example to use a process control agent (PCA) such as toluene which does not stop the reduction reaction but allows it to run in a controlled manner (Murty and Ranganathan, 1998; Takacs et al., 2001; Takacs and Mandal, 2001; Takacs, 2002). The mechanochemical syntheses of metallic–ceramic nanocomposite powders carried out in a non-explosive way based on copper–aluminium and nickel–aluminium alloys reinforced by aluminium oxide using their salts instead of oxides are described in Sections 9.5 and 9.6. For comparison, the explosive reactions in metal oxide with active metal systems are also presented.

9.3

Monitoring mechanochemical processes

A comprehensive study of the physical and chemical processes that occur during mechanical treatment by high-energy ball milling appears only to be possible if a reliable identification of solids and the quantitative phase analysis of activated products is made. Because of the complexity of mechanochemical reactions, the nature of the solids obtained closely relates to the milling conditions and so they should be well defined. Moreover, it is very important to determine the factors that influence the activation effects. Another difficulty arises from the fact that the reactions are composed of many successive stages which vary in different

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cases. The experimental methods required to identify and characterize materials synthesized mechanochemically involve not only techniques which are applicable to solids, but also others, more particularly adapted to the nanostructured character of the milling products. Thus, different types of analytical methods must be applied (e.g. Wieczorek-Ciurowa et al., 2000, 2001; Balázˇ, 2008). The best way to study the kinetics of mechanochemical transformations would be to analyse continuously the milled products in situ. However, until now this has been a difficult task. The only method that has been used in some laboratories is the gas pressure–temperature measuring system (GTM), to acquire in situ data during planetary ball milling, for example Fritsch GmbH mills, which enable indirectly control and provision of synthesis of chemical compounds by selection of appropriate parameters for milling (http://www.fritsch.com). Recording from the beginning of the milling process is especially useful when gaseous products evolve from the system caused by mechanical decomposition of the compounds. It is also useful when the self-propagating high-temperature syntheses occur in an explosive manner owing to local overheating, for example by two impacting balls (e.g. Murty and Ranganathan, 1998; Kwon et al., 2002). In such cases applying less intensive milling could be considered, for example by using lower values of rotation per minute (rpm) and/or ball-to-powder mass ratio and/or the size and material of grinding balls, as well as controlling the milling atmosphere and the type of control agent. Optimization of the milling parameters chosen experimentally must be taken into account when selecting mechanochemical syntheses conditions because not all these variables are completely independent. Solid phase characterization is realized ex situ when a sample of milled powder is picked up from the vial after a defined milling period. Thus, in the case of a material in crystalline form, the X-ray diffraction (XRD) method is advantageous; it is fast and allows the progress of the syntheses to be followed. However, this is not convenient after some stages of milling where the mechanical treatment caused comminution and prolonged activation destroyed the crystallinity of the solid powder particles leading to their complete or partial amorphization and simultaneously to consumption of the initial mixture components, making them undetectable by XRD because the amounts were too small. Thermal analysis (TA) with registration of thermogravimetric (TG/ DTG) and differential thermal analysis (DTA) curves facilitates the determination of an amount of undecomposed and/or unreacted reagents, oxidation/reduction processes and mechanically induced physical transformations in/or between the reagents. The example shown in Fig. 9.5 allows us readily to see that the analysed sample consists of mechanically undecomposed salt, that is Cu–hydroxocarbonate with the remaining amount

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being aluminium. The endothermic effect, at 350°C reveals that the Cu2(OH)2CO3–Al mixture decomposed into CuO, which was then reduced by Al to metallic Cu generating a large amount of heat at about 600°C. This heat may accelerate the alloying of Cu with Al, for example into intermetallic phases. The effect at 520°C can be related to the eutectoid transition of CuAl2 with Al →L at 528°C, according to the phase diagram of the binary Cu–Al system (Massalski, 1992), because the sample used in this thermoanalytical experiment was a product of the early stage of salt–metal mixture milling. In addition to the above-mentioned analytical techniques for solids, versatile scanning and transmission electron microscopy (SEM and TEM) with backscattered electron imaging and quantitative energy dispersive X-ray elemental microanalysis (EDS) are very helpful in estimating the microstructure of synthesized composite particles for a relatively coarse powder and for fine powders. By combining the grey tone levels with the results of EDS some compounds can be identified and localized (Yakowitz, 1975; Wieczorek-Ciurowa et al., 2003c). To illustrate the utility of transmission electron microscopy in estimating the microstructures of mechanochemically synthesized composite powders, Fig. 9.6 shows a set of TEM microphotographs of Cu2(OH)2CO3–Al powder after mechanochemical synthesis showing nanocrystalline copper. Electron diffraction patterns unquestionably confirm that the detected Cu, CuAl2 phases are in the nanocrystalline form. Moreover, it is worthwhile adding methods such as neutron diffraction, magic angle spinning solid-state nuclear magnetic resonance (MAS NMR), Fourier transform infrared (FT-IR), ultraviolet (UV) or X-ray photoelectron spectroscopy (XPS). All these techniques have been described in standard textbooks on solid state chemistry and they are not described in detail here. To determine the real phase constitution with characterization of milled powders, a combination of some methods is necessary.

9.4

Examples of applied high-energy milling in the synthesis of selected metallic–ceramic composite powders

Mechanochemical synthesis is almost an ideal method for preparing nanosized metal matrix composite powders because of its simplicity and the possibility of forming composite powder particles with a uniform distribution of grain sizes. Moreover, such in situ routes of synthesis (Sections 9.5 and 9.6) result in the production of powdered materials that have microstructures that are more homogeneous than those synthesized using conventional ex situ techniques (Section 9.7).

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(b)

250 nm

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1-10 002 2-20

9.6 Set of TEM microphotographs of the Cu2(OH)2CO3–Al system after mechanochemical synthesis showing nanocrystalline copper: (a) bright field, (b) dark field, (c) electron diffraction patterns (TEM Philips CM 20 with EDX) (Wieczorek-Ciurowa and Gamrat, 2007a).

In metallic–ceramic composite powder formation, the preparation of metals and alloys by reducing their salts or oxides with more reactive metals is commonly known as metallothermic reduction. This reaction is expressed in general by the equation: MAX + MB > MA + MBX

[9.2]

where a metal MA is reduced by a more reactive metal MB (reductant) to the pure metal MA. MAX and MBX are oxides, chlorides, sulphides and other salts. Metallothermic reactions are characterized by a large negative freeenergy change and therefore they are thermodynamically feasible at room temperature (see for example reactions [9.4] and [9.5]. Moreover, the

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mechanical activation of reagents significantly increases the synthesis kinetics. A salt such as MA hydroxocarbonate can be used as a ‘source’ of MAO, as described in Sections 9.5 and 9.6.

9.5

Copper-based composite powders with Al2O3

The formation of a copper-based composite powder with aluminium is a consequence of many complex, simultaneous and subsequent chemical reactions that occur in the milling of a Cu2(OH)2CO3 and Al mixture. They are schematically reported below (equations [9.3]–[9.5]) together with the enthalpy values (ΔH298/kJ mol−1; Barin et al., 1977): •

mechanochemical decomposition of salt: +82; Cu2(OH)2CO3(S) → 2CuO(S) + CO2(G) + H2O(G)

•

[9.3]

aluminothermic reduction of CuO to Cu with Al2O3 formation: −1179; 3CuO(S) + 2Al0(S) → 3Cu0(S) + Al2O3(S)

[9.4]

• mechanical alloying of Cu and Al (in the presence of Al2O3): −8 (Cu9Al4) and −4 (CuAl2); Cu0(S) + Al2O3(S) + Al0(S) → Cu0−Al0 + Al2O3

[9.5]

(S) indicates solid and subscript (G) indicates gas. The product is a metallic–ceramic composite powder. Cu0–Al0 alloy can exist as intermetallics and/or a solid solution. The formation enthalpy of oxide MBX should be higher than that for MAX (see Equation [9.2]). The values for Cu, Ni and Al oxides are given in Table 9.3. From a thermodynamic point of view, the aluminothermic reactions in the systems studied are significantly favoured. In Sections 9.5.1 and 9.5.2, the in situ formation of a nanostructural metallic–ceramics nanocomposite Cu–Al/Al2O3 powder is described starting from two different mixtures, Cu2(OH)2CO3 and CuO with Al, respectively (Wieczorek-Ciurowa et al., 2002a; Wieczorek-Ciurowa and Gamrat, 2007a,b; Wieczorek-Ciurowa et al., 2007a,b; 2008). Composite powders were synthesized by high-energy ball milling in a laboratory planetary mill with vial and balls made of hardened steel. The component systems were prepared as physical mixtures in which the amount of Al was calculated assuming the formation Cu9Al4 with Al2O3 and CuAl2 with Al2O3 as components of the synthesis products. Among the intermetallics in the binary Al–Cu phase diagram (Hansen, 1958) are CuAl2, Cu9Al4, Cu(Al) and Al(Cu) solid solutions. It was found that if Cu9Al4 was the expected

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High-energy ball milling Table 9.3 Formation enthalpy values of selected metal oxides (Kubashewski et al., 1993) Oxide

−ΔH (kJ mol−1)

Al2O3 NiO Cu2O CuO

1675.7 239.7 173.2 161.9

phase, mechanosynthesis in both reagents used brings about the formation of a composite material consisting of Cu(Al) solid solution and Al2O3. The amount of Al dissolved into Cu matrix is considerably higher for the CuO–Al system than for Cu2(OH)2CO3–Al. This is probably caused by the different kinetics of mechanochemical reactions in these two systems. According to Avvakumov et al. (2001), compounds containing groups of atoms with oxygen and/or hydrogen (like acidic or basic salts, hydrates) take part in reactions evolving water and/or volatile compounds. These compounds are characterized by 3–4 times lower hardness in comparison to their anhydrous oxides, probably providing the mechanochemical process with lower mechanical loading. Processes taking place with these types of substances are known as soft mechanochemical reactions. Mechanochemical syntheses where the mixtures are provided with stoichiometric proportions of reagents to form CuAl2 result in the formation of two intermetallic phases, CuAl2 and Cu9Al4, as well as Al2O3. It is evident that in the systems with CuO as a reagent for aluminothermic reaction CuAl2 appears after only 1 hour of milling. This indicates the rapid alloying process of both metals: copper and aluminium. The experimental results are described in detail in Sections 9.5.1 and 9.5.2.

9.5.1 Effects of mechanosynthesis in the Cu2(OH)2CO3–Al reagent system Figure 9.7 shows the XRD patterns (Philips X’Pert diffractometer Cu Kα) of the Cu2(OH)2CO3–Al system mechanically treated at different time intervals (Pulverissette 6 Fritsch GmbH). The results indicate that the initial components of the mixture studied still exist in the system after up to 10 hours of mechanical treatment. However, their intensities reduced significantly in the fourth hour of milling. This indicates that Cu2(OH)2CO3 started to decompose and metallothermic reduction of CuO with Al proceeded gradually with formation of metallic copper. Furthermore, a broad diffrac-

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9.7 X-ray diffraction patterns of the Cu2(OH)2CO3–Al system after different mechanochemical treatment times in a Pulverisette 6 Fritsch GmbH mill (intensities are on the same scale) (Philips X’Pert diffractometer, CuKα) (Wieczorek-Ciurowa et al., 2007b).

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tion peak appears in the range of 2θ = 42° to 45° after 10 hours of milling. Its intensity increases significantly with a lengthening of the milling time. The shape of this X-ray indicates its complexity and the deconvolution confirms the presence of copper in addition to part of the Al2O3 phase. After 15 hours and 20 hours of milling the position of the Cu(1 1 1) peak shifted from 2θ = 43.32° to 43.25°, respectively, while Al peaks completely disappeared. This may suggest that a solid solution of Cu(Al) has been formed as a result of mechanical alloying of two metals, Cu and Al. This is confirmed by an increase of metallic copper lattice parameter from 3.6151 Å to 3.6203 Å (Pearson, 1958). According to Vegard’s law, which states that the lattice parameter of the solid solution is a function of the amount of solute and using parameters for pure Cu and Al, the contents of the Al solute in the Cu matrix for the mechanically treated Cu2(OH)2CO3–Al system are estimated on a level of about 5%. From a thermodynamic point of view, the reactions occurring in the tested system are highly exothermic, for example the enthalpy of aluminothermic reduction of CuO with Al equals −1179 kJ mol−1. However, in the hydroxosalt–active metal system, processes in the mill occur in a controlled manner. Coming back to Fig. 9.3 illustrating the temperature and pressure in a vial during milling process it can be seen that in the first hours of milling the temperature and pressure increase simultaneously. The rise in pressure is due to mechanical decomposition of Cu2(OH)2CO3 to CuO with H2O and CO2 evolving, while the temperature increase is caused by the friction and impact of the balls. When Cu–hydroxocarbonate is completely decomposed, any rise in pressure is observed, whereas only a transitory rise in temperature occurs (Fig. 9.3b). On the T versus milling time curve, one can observe several small rises in the gas temperature. However, all of them take place below 40°C. This indicates that the aluminothermic reduction occurs locally, in a repeatable fashion. Based on these data we can conclude that in this case mechanochemical synthesis takes place in a controlled way, gradually, so CuO is reduced by Al in many steps proceeding locally, in isolated parts of treated powders. This process of aluminothermic reduction may be explained by the fact that the occurrence of a highly exothermic reaction via combustion depends not only on thermodynamics but also on other factors such as the nature of reactants, their mechanical properties, crystalline structures and their stability during milling. The thermoanalytical curves, TG–DTA, presented in Fig. 9.8 (WieczorekCiurowa and Gamrat, 2006) for an untreated mixture of Cu2(OH)2CO3 with Al, and after milling for 20 hours, respectively, indicate that Cu–hydroxocarbonate completely decomposes during mechanosynthesis and aluminium is consumed because of the lack of an endo effect at 640°C. Moreover, any sharp exothermic effect caused by the reaction of CuO with Al is

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9.8 TG–DTA curves of the Cu2(OH)2CO3–Al system after mechanochemical synthesis for: (a) 0 hours, (b) 20 hours (SDT 2960 TA Instrument) (Wieczorek-Ciurowa and Gamrat, 2006).

detected on the DTA curve. This means that the aluminothermic reaction is completed during that time. It is interesting that there is no endothermic peak before 450°C, which normally corresponds to the transformation of Cu9Al4 into a CuAl2 phase. This is also confirmed by the results of XRD patterns, indicating that this phase did not form.

9.5.2 Effects of mechanosynthesis in the CuO–Al reagent system Mechanosynthesis in a copper oxide–aluminium system under the same conditions as in the case where Cu2(OH)2CO3 was used as a source of CuO caused, within the first hour of milling, the formation of two intermetallic phases, CuAl2 and Cu9Al4 (Fig. 9.9) (Wieczorek-Ciurowa et al., 2007a, 2008). Lengthening the milling time results in the disappearance of both intermetallics, although with different kinetics. The first, CuAl2, decays

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9.9 X-ray diffraction patterns for a CuO–Al system after different mechanochemical treatment times in a Pulverisette 6 Fritsch GmbH mill (Philips X’Pert diffractometer, CuKα, intensities are on the same scale) (Wieczorek-Ciurowa et al., 2008).

after 1 hour of milling, while Cu9Al4 remains in the system up to 2 hours of milling. Further milling caused the consumption of aluminium from these phases to form a Cu(Al) solid solution. This is confirmed by the shift of the Cu(1 1 1) peak from 2θ = 42.57° to 43.33° observed in the system treated for 20 hours. The calculated lattice parameter of this phase is equal to 3.6564 Å. Comparison of this parameter with its value for pure Cu suggests expansion of the copper lattice by aluminium substitution, forming a solid solution of Al in the Cu matrix. In fact, the content of the Al solute was estimated to be at a level close to 10%. Conventionally Al dissolves in Cu at the temperature of eutectoid transformation (565°C) at 9.4%.

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An increase in the temperature in the milling vial registered by the GTM system indicates that the combustion process occurs during the first hour of milling (see Fig. 9.4). In this case, the reaction between CuO and Al is completed, confirmed by the absence of peaks corresponding to the initial components. Considering phase evolution in the CuO–Al mixed powders, one can suggest that intermetallic phases are formed only as intermediate products under the applied conditions. During further milling they transform to the Cu(Al) solid solution, that is the expected Cu9Al4 phase was not formed in final products. The characteristics of the milling product microstructure were provided using a scanning electron microscopy. Figure 9.10 shows the typical morphology of composite powder after milling for 20 hours. EDS analysis showed that the dark grey phase is Al2O3 while the bright grey one is a Cu(Al) matrix. The microphotograph confirmed that alumina particles are evenly embedded in the Cu–Al matrix, in size range from 100 nm to 500 nm. This indicates that milling the CuO–Al system with a stoichiometry close to Cu9Al4 brings about formation of metal matrix composite powder particles in which Cu(Al) forms a matrix while alumina grains act as its reinforcement.

AI2O3

Cu(AI)

10.0 µm

9.10 SEM microphotograph (BSE image) illustrating that Al2O3 particles (ranging from 100 nm to 500 nm) are embedded in the Cu–Al matrix formed during reactive milling of the CuO–Al system. The dark spots correspond to Al2O3, the brighter grey ones to Cu(Al) (SEM Hitachi S-4700 with EDX Noran Vantage) (Wieczorek-Ciurowa et al., 2007a).

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9.6

Nickel-based composite powders with Al2O3

Formation in situ of composite powders of nickel–aluminium metals strengthened by alumina is very important from both a theoretical and a practical point of view. Analogous to the mechanochemical synthesis of Cu–Al/Al2O3 composite described in Section 9.5, which is based on the type of reactants, three modifications of the process are possible. Nickel in the form of its salt, oxide or simply elemental metal with aluminium can be used. Because nickel and its alloys have good plastic fatigue features but are not sufficiently resistant to high temperature oxidation and abrasive wear, intermetallics such as NiAl and Ni3Al are a better choice for a composite matrix. They are resistant to very high temperature, corrosion, mechanical and abrasive actions and they have a relatively low density. However, their low intergranular cohesion is responsible for their great brittleness. For this reason, a ceramic component is often present in the composite. In Sections 9.6.1 and 9.6.2, in situ mechanochemical syntheses of Ni–Al/ Al2O3 are described, starting from two different mixtures, Ni2(OH)2CO3. xH2O or NiO with Al, respectively (Wieczorek-Ciurowa and Oleszak, 2008). The milling procedure was similar to the one described in Section 9.5. The component systems were prepared as physical mixtures in which the amount of Al was calculated assuming that NiAl with Al2O3 or Ni3Al with Al2O3 formed as final products of the synthesis. Among the intermetallics in the binary Ni–Al phase diagram (Hansen, 1958), the main ones are Ni3Al, NiAl, Ni2Al3, NiAl3 and solid solutions. It was found that if Ni3Al was the expected phase, mechanosynthesis brings about the formation of composite powder particles consisting of Ni3Al matrix and Al2O3. In mechanochemical syntheses in the NiO–Al system where reagent proportions were calculated for the NiAl phase, the expected NiAl and additionally the Ni3Al intermetallic phase were formed as well as Al2O3. However during the mechanical treatment of Ni2(OH)2CO3⋅xH2O with aluminium, the alloying process does not form NiAl but mainly proceeds to Ni3Al formation. Presumably, the reason for this is the different nature and structure of the nickel salt compared with copper salt. Ni-hydroxocarbonate belongs to hydrated salts with low crystallinity (Wieczorek-Ciurowa et al., 2002b; Wieczorek-Ciurowa and Gamrat, 2005). Moreover, crystalline water stabilizes the structure of the nickel salt, making its mechanical decomposition more difficult, in effect slowing down the delivery of NiO in the aluminothermic reaction [9.7]. The examples of two kinds of salts presented here, Ni2(OH)2CO3⋅xH2O, almost amorphous, and Cu2(OH)2CO3 with high crystallinity showed that crystalline structures are more vulnerable to mechanical treatment because their activity becomes higher, for example creating structural defects and new surfaces, and undergoing mass transfer (mixing).

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Special attention should be paid to the important part being played by water vapour in the milling system of Ni2(OH)2CO3.xH2O–Al during mechanical decomposition of salt. The compound contains both crystalline water (x = about three molecules) and OH groups. The evolved water reacting rapidly (ΔH298 = −949 kJ mol−1) with mechanically activated aluminium powder, see Equation [9.6] decreases the amount of Al needed to synthesise the intermetallics (see Section 9.6.1). 2Al + 6H2O → 2Al2O3 + 3H2

[9.6]

Too low a concentration of Al in the desired NiAl phase of the Ni2(OH)2CO3. xH2O–Al system gave Ni3Al intermetallics in the composite powder and a phase with a lower amount of aluminium. Similar to the formation of copper–aluminium matrix/Al2O3 composite particles which are the result of strong exothermic reactions, formation of composite particles of nickel–aluminium/Al2O3 is also typical of a selfpropagating high-temperature synthesis (with a combustible nature). The enthalpy changes for reactions [9.7] and [9.8] are negative (Barin et al., 1977; Kubaschewski et al., 1993; Takacs, 2002): 3NiO + 2Al → Ni + Al2O3, ΔH298 = −955 kJ mol−1

[9.7]

xNi + Al → NixAl where x = 1 or 3, ΔH298 = −117 kJ mol−1 (NiAl) and −153 kJ mol−1 (Ni3Al)

[9.8]

9.6.1 Effects of mechanosynthesis in the Ni2(OH)2CO3.xH2O–Al reagent system The XRD patterns of the Ni2(OH)2CO3.xH2O–Al systems with reagent proportions calculated for NiAl or Ni3Al intermetallics mechanically treated for 20 hours are shown in Fig. 9.11. It is seen that instead of the desired NiAl, the Ni3Al phase is formed with traces of NiAl. Furthermore, up to the fourth hour of milling, reagents such as NiO, Ni and Al still remain and after 10 hours of milling the reaction was still incomplete, about 15% salt was undecomposed and mechanical alloying of Ni3Al was in its first stage. The results of thermal analysis measurements also confirmed these effects (Wieczorek-Ciurowa and Oleszak, 2008). After 20 hours of milling the composite powder is fully formed. Figure 9.11(b) shows the synthesis of Ni3Al/Al2O3 composite powder formed after 20 hours of milling in the system with reactant proportions for the Ni3Al phase. However, after up to 4 hours of mechanical treatment, Ni and Al were present and after 10 hours milling, NiO was still detectable. It is assumed that the slower decomposition kinetics of the hydrate of nickel

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9.11 X-ray diffraction patterns (Philips X’Pert diffractometer, CuKα) for a Ni2(OH)2CO3.xH2O–Al system after 20 hours of mechanochemical treatment (Pulverisette 6 Fritsch GmbH mill) with reagent proportions calculated for intermetallic phase formation of (a) NiAl and (b) Ni3Al (Wieczorek-Ciurowa and Oleszak, 2008).

hydroxocarbonate in comparison to anhydrous Cu2(OH)2CO3 is responsible for this behaviour.

9.6.2 Effects of mechanosynthesis in the NiO–Al reagent system The effect of 20 hours mechanochemical synthesis in the NiO–Al reagent system was to produce a NiAl phase with Al2O3, when the ratio of the masses of the reagents was used to provide NiAl stoichiometry (Fig. 9.12a). A small amount of Ni3Al was still present. The composite powder Ni3Al/Al2O3 is formed explosively in the NiO–Al reagent system (Fig. 9.12b). The SEM microphotograph of the product is shown in Fig. 9.13.

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9.12 X-ray diffraction patterns (Philips X’Pert diffractometer, CuKα) of the NiO–Al system after 20 hours mechanochemical treatment (Pulverisette 6 Fritsch GmbH mill) with reagent proportions calculated for intermetallic phase formation of (a) NiAl and (b) Ni3Al (WieczorekCiurowa and Oleszak, 2008).

9.7

Other possible variants of the synthesis of metal matrix–ceramic composites in Cu–Al–O and Ni–Al–O elemental systems using mechanical treatment ex situ and in situ

It has already been reported that copper and nickel as well as other transition metals and intermetallics with these metals are important matrices in composite materials with fine dispersion of Al2O3 particles because of their applications. For instance, cermetals such as Ni–Al/Al2O3 are candidates for military and civil aero engines as exhaust nozzle materials. This type of composite is used in dentistry as a substance for fillings, in prosthetics, as

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100 µm

9.13 SEM microphotograph illustrating the product structure of a combustion aluminothermic reaction during mechanical treatment of a NiO–Al mixture (SEM Hitachi S-4700) (Wieczorek-Ciurowa and Oleszak, 2008).

vacuum tube coatings for solar hot water systems, transistors, capacitors, and so on (see, e.g. Ying and Zhang, 2000a,b; 2003; Grahle and Arzt, 1997; Morsi, 2001; Portnoy et al., 2002). To produce these materials combustive reactions need to be suppressed during mechanochemical synthesis. Ying and Zhang (2000a, b; 2003) show that this can be achieved utilizing a technique in which the Al powder is first diluted by Cu and then the resultant Cu–Al alloy powder is reacted with CuO. The dilution was done through mechanical alloying of Al and Cu into a Cu(Al) solid solution or Cu–Al intermetallics compounds (depending on the amount of Al needed in the mixture). In the next step, powders of Cu–Al alloy and CuO were milled to form a composite structured powder. Finally, this composite powder was heated (<450°C) causing the reaction between CuO and Cu(Al) or Cu9Al4. Subsequent heating at a temperature below 800°C led to formation of Cu and Al2O3. Mechanical treatment enhances the kinetics of both reactions by refining the composite particles structure. Another technique, also a combination of oxidation and mechanical alloying, has been used to produce Cu–Al2O3, metal matrix–ceramic composite powders. With this method, Cu powder is first partially oxidized and then the powder obtained is mechanically alloyed with Al powder to facili-

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tate the aluminothermic reaction of Al2O3 particles formation (Bobrova and Besterci, 1994). Particulate reinforced Cu/Al2O3 metal matrix–ceramics composites can be mechanochemically synthesized by using several methods which include mixing of Cu melt and Al2O3 powder followed by calcination and internal oxidation of Cu–Al alloy powders (Shi and Wang, 1998). The calcination process is limited (Liang et al., 2004) because the Al2O3 particle size has to be large enough to allow effective milling, while internal oxidation can only produce composites with a low volume fraction of Al2O3 particles, such as oxide dispersion strengthened alloys. Oxidation was used in conjunction with mechanical alloying (Ogbuji, 2004). In this process, Cu–Al alloy powder was milled under an oxidizing atmosphere to produce the composite powder. The advantage of this process is that very small oxide particles can be achieved. Another approach that avoids too high activities of the reactants has been made by Venugopal et al. (2005) who introduced toluene as the milling medium (PCA) in copper–alumina composite synthesis by reactive milling of CuO/Cu2O and Al. In fact, the transformation of a combustible reaction to a progressive one yields nanocomposite particles of Cu and Al2O3 with both components with a crystallite size in the range of about 20 nm. Hwang and Lee (2005) demonstrated that Cu/Al2O3 nanocomposite powders with various vol% of alumina as a reinforcement phase have been successfully produced by mechanochemical synthesis in a high-energy attritor mill using mixtures of Cu, Cu2O and Al powder components. It is important to note that in this case the heat generated during milling was removed by a cooling tube coil attached to the outside of the milling chamber. Moreover, in every stoichiometric reaction, excess Cu was added to the system. The role of excess Cu powder was not only as a diluent to control the concentration of Al2O3 but also as a thermal conductor to remove heat from the system during milling.

9.8

Conclusions

•

Metallic–ceramic nano-structured composite powders can be produced in situ simply and successfully at room temperature through a mechanochemical route using high-energy ball milling. The subsequent consolidation produces nanocomposites with the desired microstructure and mechanical characteristics. • Despite a number of studies having been carried out in different laboratories to synthesize nanocomposite powders, direct comparison and prediction of the course of the process and/or product properties is still very difficult (even when the precursors have a similar character). Using different kinds of mills and procedures for milling influences the input

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of mechanical energy to the treated material. Moreover, until now, the mechanisms and kinetics of mechanochemical syntheses in solid–solid systems are not fully understood. Therefore it seems to be reasonable to carry out experimental studies to determine the progress of reactions and quality of milling products at various milling stages in relation to the milling conditions and to collect the data in order to formulate a theory of mechanochemical processes. • In the case of composite mechanosyntheses with a metal matrix it is especially important to inhibit self-propagating explosive reactions and to transform to the progressive ones by wider use of soft mechanochemistry (acidic, basic, hydrated salts) or, for example, different PCA. Based on the results of two practical examples of syntheses, the transformation of combustion reactions to progressive reactions is shown by applying copper and nickel hydroxocarbonates as composite precursors (even if different in nature) instead of metal oxides, CuO and NiO.

9.9

Acknowledgements

The author would like to thank Professor Yu. G. Shirokov of the Ivanovo State University of Chemistry and Technology, Russian Federation, who in 1995 inspired the study of mechanochemical processes. The Polish Ministry of Science and Higher Education (Projects No. PB 4 T09B 07024 and PB 1 T09B 023 30) and the State Committee for Scientific Research (Grants CUT/C-1/DS) provided financial support (2002–2008).

9.10

References

agrawal p and sun c t (2004), ‘Fracture in metal-ceramic composites’, Comp Sci Technol, 64, 1167–78. avvakumov e g, senna m and kosova n (2001), Soft Mechanochemical Synthesis, A Basis for New Chemical Technologies, Kluwer Academic Publishers, Boston. balázˇ p (2008), Mechanochemistry in Nanoscience and Minerals Engineering, Springer, Berlin. barin i, knacke o and kubaschewski o (1977), Thermochemical Properties of Inorganic Substances, Supplement, Springer, Berlin. boldyrev v v (2006), ‘Mechanochemistry and mechanical activation of solids’, Russian Chem Rev, 75 (3), 177–89. boldyrev v v, pavlov s v and goldberg e l (1996), ‘Interrelation between fine grinding and mechanical activation’, Int J Mineral Process, 44–5, 181–5. bobrova e and besterci m (1994), ‘Processing of Cu–Al2O3 metal matrix nanocomposite materials by high-energy ball milling’, Powder Metall Sci Tech, 6, 7. botta p m, aglietti e f and porto lopez j m (2000), ‘Thermal and phase evolution of mechanochemical reactions in the Al–Fe3O4 system’, Thermochim Acta, 363, 143–7.

© Woodhead Publishing Limited, 2010

Metallic–ceramic composite powders

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butyagin p yu (2000), ‘Mechanochemical synthesis: mechanical and chemical factors’, J Mater Synth Process, 8 (3/4), 205–11. butyagin p yu (2003), ‘Diffusion and deformation models of mechanochemical synthesis’, Colloid J, 65 (5), 648–51. chang s t, tuan w h, you h c and lin i c (1999), ‘Effect of surface grinding on the strength of NiAl and Al2O3/NiAl composites’, Mater Chem Phys, 59, 220–4. chen y, hwang t, marsh m and williams j s (1997), ‘Study on mechanism of mechanical activation’, Mater Sci Eng, A226–8, 95–8. courtney t h (1994), ‘Modelling of mechanical milling and mechanical alloying’, Rev in Particulate Mater, 2, 63–116. fernández-bertran j f (1999), ‘Mechanochemistry: an overview’, Pure Appl Chem, 71 (4), 581–6. grahle p and arzt e (1997), ‘Microstructural development in dispersion strengthened NiAl produced by mechanical alloying and secondary recrystallization’, Acta Mater, 45, 201–11. hansen h (1958), Constitution of Binary Alloys, McGraw-Hill, New York. heegn h, birkeneder f and kmptner a (2003), ‘Mechanical activation of precursors for nanocrystalline materials’, Cryst Res Technol, 38 (1), 7–20. http://www.fritsch.com Laboratory planetary mill Pulverisette 6 (Fritsch GmbH, Germany). hwang s j and lee j-h (2005), ‘Mechanochemical synthesis of Cu–Al2O3 nanocomposites’, Mater Sci Eng, A405, 140–6. ivanov e and suryanarayana c (2000), ‘Materials and process design through mechanochemical routes’, J Mater Synth Process, 8 (3/4), 235–44. kaczmar j w (2000), ‘Production and application of metal matrix composite materials’, J Mater Process Technol, 106, 58–67. kelsall r w, hamley j w and geoghegan m (eds) (2005), Nanoscale Science and Technology, J Wiley & Sons, New York. koch c c (2001), Nanostructured Materials Processing, Properties and Potential Applications, Wiliam Andrew, Norwich, New York. kubashewski o, alcock c b and spencer p j (1993), Materials Thermochemistry, 6th edition, Pergamon Press, Boston. kwon y-s, gerasimov k b and yoon s-k (2002), ‘Ball temperatures during mechanical alloying in planetary mills’, J Alloys Comp, 346, 276–81. liang s, fan z, xu l and fang l (2004), ‘Kinetic analysis on Al2O3/Cu composite prepared by mechanical activation and internal oxidation’, Composites, A35, 1441–6. lyakhov n z, vityaz p a, grigoryeva t f, talako t l, barinova a p, vorsina i a, letzko a i and cherepanova s v (2008), ‘Nanocomposites intermetallics/oxides, produced by MA SHS’, Rev Adv Mater Sci, 18, 326–8. massalski t b (ed.) (1992), Binary Alloy Phase Diagrams, 1, 2nd edition, ASM Int., Ohio, USA. maurice d r and courtney t h (1990), ‘The physics of mechanical alloying: A first report’, Metallurgical Trans A, 21A, 289. mccormick p g, wharton v n and schaffer g b (1989), Physical Chemistry of Powder Metals Production and Processing, TMS, Warrendale, PA. morsi k (2001), ‘Review: reaction synthesis processing of Ni-Al intermetallic materials’, Mater Sci Eng, A299, 1–15.

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murty b s and ranganathan s (1998), ‘Novel materials synthesis by mechanical alloying/milling’, Int Mater Rev, 43, 101–41. ogbuji l u (2004), ‘The oxidation behavior of an ODS copper alloy Cu-Al2O3’. Oxidation of metals, 62 (3/4), 141–51. pearson w b (1958), A Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, London. portnoy v k, blinov a m, tomilin i a, kuznetsov v n and kulik t (2002), ‘Formation of nickel aluminides by mechanical alloying and thermodynamics of interaction’, J Alloys Comp, 336, 196–201. schaffer g b and mccormick p g (1992), ‘The direct synthesis of metals and alloys by mechanical alloying’, Mater Sci Forum, 88–90, 779–86. schwarz r b (1996), ‘Introduction to the viewpoint set on: mechanical alloying’, Scripta Mater, 34 (1), 1–4. sˇ epelak v (2002), ‘Nanocrystalline materials prepared by homogeneous and heterogeneous mechanochemical reactions’, Ann Chim Sci Mater, 27 (6), 61–76. shen t d, wang k y and quan m x (1992), ‘Solid state displacement reaction of Fe and CuO induced by mechanical alloying’, Mater Sci Eng, A151, 189–95. shi z and wang d (1998), ‘Processing of Cu-Al2O3 metal matrix nanocomposite materials by using high energy ball milling’, J Mater Sci Lett, 17, 477–81. smolyakov v k, lapshin o v and boldyrev v v (2007), ‘Macroscopic theory of mechanochemical synthesis in heterogeneous systems’. Int J Self-Propagating HighTemp Synth, 16 (1), 1–11. suryanarayana c (2001), ‘Mechanical alloying and milling’, Progress Mat Sci, 46, 1–184. suryanarayana c, ivanov e and boldyrev v v (2001), ‘The science and technology of mechanical alloying’, Mater Sci Eng, A304–6, 151–8. takacs l (2002), ‘Self-sustaining reactions induced by ball milling’, Progr Mater Sci, 47, 355–414 (and literature cited herein). takacs l and mandal s k (2001), ‘Preparation of some metal phosphides by ball milling’, Mater Sci Eng, A304–6, 429–33. takacs l, sojka v and balázˇ p (2001), ‘The effect of mechanical activation on highly exothermic powder mixtures’, Solid State Ionics, 141–2, 641–7. tjong s c and chen h (2004), ‘Nanocrystalline materials and coating’, Mater Sci Eng, R45, 1–88. urakaev f kh and boldyrev v v (2000), ‘Mechanism and kinetics of mechanochemical processes in comminuting devices: 2. Applications of the theory. Experiment’, Powder Technol, 107, 197–206. venugopal t, prasad rao k and murty b s (2005), ‘Synthesis of copper-alumina nanocomposite by reactive milling’, Mater Sci Eng, A393, 382–6. wieczorek-ciurowa k and gamrat k (2005), ‘NiAl/Ni3Al-Al2O3 composite formation by reactive ball milling’, J Therm Anal Calorim, 82 (3), 719–24. wieczorek-ciurowa k and gamrat k (2006), ‘Mechanochemical synthesis of nanocrystalline cermets’, Pol J Chem Technol, 8 (3), 51–3. wieczorek-ciurowa k and gamrat k (2007a), ‘Some aspects of mechanochemical reactions’, Mater Sci Pol, 25 (1), 219–32. wieczorek-ciurowa k and gamrat k (2007b), ‘Mechanochemical syntheses as an example of green processes’, J Therm Anal Calorim, 88 (1), 213–17. wieczorek-ciurowa k and oleszak d (2008), ‘Mechanochemical synthesis of Ni– Al/Al2O3 and Cu–Al/Al2O3 nanocomposite powder’, 15th International Sympo-

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sium on Metastable and NanoMaterials, ISMANAM2008, Buenos Aires, 26–30 June 2008, Argentina, Book of Abstracts. wieczorek-ciurowa k, shirokov ju g and paryło m (2000), ‘Use of thermogravimetry to assess the effect of mechanical activation of selected inorganic salts’, J Therm Anal Calorim, 60, 59–65. wieczorek-ciurowa k, paryło m, shirokov ju g, gamrat k (2001), ‘The characteristics of mechanically activated mixtures of copper hydroxocarbonate with aluminium’, J Therm Anal Calorim, 65, 359–66. wieczorek-ciurowa k, gamrat k, paryło m and shirokov ju g (2002a), ‘Alloy formation in mechanically activated mixtures hydroxocarbonate with Al0’, J Therm Anal Calorim, 70, 165–72. wieczorek-ciurowa k, gamrat k, paryło m and shirokov ju g (2002b), ‘The influence of aluminium and aluminium oxide on the effects of mechanical activation of nickel hydroxocarbonate’, J Therm Anal Calorim, 69, 237–43. wieczorek-ciurowa k, gamrat k and shirokov ju g (2003a), ‘Chemical reactions during high energy ball milling of the Cu2(OH)2CO3–Al system’, Solid State Ionics, 164, 193–8. wieczorek-ciurowa k, gamrat k and shirokov ju g (2003b),’Mechanism of mechanochemical reactions in malachite–active metal systems’, Thermochim Acta, 400, 221–5. wieczorek-ciurowa k, gamrat k, fela k and sawłowicz z (2003c), ‘Structural and morphological changes induced by mechanical activation in copper hydroxocarbonate–aluminium system’, Pol J Chem Technol, 5, 72–4. wieczorek-ciurowa k, gamrat k and sawłowicz z (2005), ‘Characteristics of CuAl2-Cu9Al4/Al2O3 nanocomposites synthesized by mechanical treatment’, J Therm Anal Calorim, 80 (3), 619–23. wieczorek-ciurowa k, oleszak d and gamrat k (2007a), ‘Mechanochemical synthesis of Cu–Al/Al2O3 composite in CuO–Al system under different conditions’, Chem for Sustainable Development, 15, 255–8. wieczorek-ciurowa k, oleszak d and gamrat k (2007b), ‘Mechanosynthesis and process characterization of some nanostructured intermetallics–ceramics composites’, J Alloys Comp, 434–5, 501–4. wieczorek-ciurowa k, oleszak d and gamrat k (2008), ‘Cu-Al/Al2O3 cermet synthesized by reactive ball milling of CuO-Al system’, Rev Adv Mater Sci, 18, 248–52. yakowitz h (1975), ‘Methods of quantitative X-ray analysis used in electron probe microanalysis and scanning electron microscopy’, In: Practical Scanning Electron Microscopy, Goldstein J I and Yakowitz H (eds), Plenum Press, New York. yang h and mccormick p g (1993), ‘Combustion reaction of zinc oxide with magnesium during milling’, J Solid State Chem, 107, 258–63. ying d y and zhang d l (2000a), ‘Processing of Cu-Al2O3 metal matrix nanocomposite materials by using high energy ball milling’, Mater Sci Eng, A286, 152–6. ying d y and zhang d l (2000b), ‘Solid-state reactions between Cu and Al during mechanical alloying and heat treatment’. J Alloys Comp, 311, 275–82. ying d y and zhang d l (2003), ‘Solid state reactions between CuO and Cu(Al) or Cu9Al4 in mechanically milled composite powders’, Mater Sci Eng, A361, 321–30. zhang f, kaczmarek w a, lu l and lai m o (2000), ‘Formation of Al–TiN metal matrix composite via mechanochemical route’, Scripta Mater, 43, 1097–102.

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