Mat Prod L13

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11.3 Prod. of Powders by Solid-Solid and Gas-Solid Reactions A number of powders can be prepared by direct reaction in the solid state or by gas reactions. Examples are the fabrication of silicon carbide and silicon nitride. 11.3.1 Silicon Carbide Production Silicon Carbide (SiC) is highly wear resistant and also has good mechanical properties, including high temperature strength and thermal shock resistance. SiC can be prepared by the direction reaction of silica sand (quartz) and carbon (coke). This method, known as the Acheson process. The mixture of silica and carbon, is heated in an electric furnace. The heating is accomplished by an electroded carbon core placed centrally in the furnace. The mixture of reactants is placed around this core. The mixture is then heated to reach a maximum temperature of approximately 2700oC, after which the temperature is gradually lowered. (The furnace is about 20 m long, 3 m wide, holding over 100 tons of material at a time, using as much as 5 MW or electricity) 507 E20300 Production and Processing of Materials  – Metals and Ceramics Lecture 13 References: * Hayes, 1993 * Evans & De Jonghe, 1991


The overall reaction is: SiO2 (s) + 3C (s)  SiC (s) + 2CO (g) (1) (Ellingham diagram shows that this reaction should proceed at temp. over 1500C.) Significant side reactions occur, such as: SiO2 (s) + 3C (s)  SiO (g) + CO (g) (2) SiO escapes from reacting mass and oxidizes to form SiO2 on contact with air. As SiO and CO escape from reacting mass and cools, SiO2 and C form by the reverse of Equation (2). Acheson process is therefore one that produces a great deal of fine dust. Sawdust (~7%) is often added to the mixture to provide the pores through which the CO escapes. Common salts (chlorides, ~3%) are added to react with impurities. The reacted mass around the central carbon/graphite core is then removed by chipping and raking and the SiC washed with reagents such as hydrofluoric acid, which removes residual silica. (Downside: poor energy efficiency) (due to decompose of SiC)


For demanding applications such as high-temperature structural SiC ceramics, better powders are needed and modifications have been made to suit small-scale operation. A more refined approach to the Acheson process is the reaction of silica vapor with carbon black. Silica is kept in a crucible below a bed of carbon black that is sitting on a grid. At about 1600C the silica will start to evaporate and react with the carbon bed to produce fine silicon carbide powder. Silicon carbide powders have also been produced by thermal decomposition of such vapors as CH3SiCl3 in hydrogen. 11.3.2 Silicon Nitride Production Silicon nitride (Si3N4) is light, hard, and has low thermal expansion coefficient. It has high mechanical strength, fracture toughness, and it is resistant to deformation at room temperature as well as at elevated temperatures. The most common applications are for engine parts, gas turbines, and ultra high-temperature applications. Particularly important is its corrosion resistance. Most Si3N4 is produced by the reaction of N2 and Si in the temperature range 1100 to 1400C: 3Si + 2N2 = Si3N4 The silicon is in the form of a packed bed of metallurgical-grade silicon (produced in a submerged arc furnace). Si3N4 occurs as ,  or amorphous Si3N4. The  and amorphous forms sinter more readily and reaction conditions are selected to minimize the amount of -Si3N4 produced.


Purer silicon nitride is produced by the gas-phase reaction of ammonia with silicon tetrachloride. The overall reaction is: 3SiCl4 + 16NH3 = Si3N4 + 12NH4Cl Silicon nitride can also be prepared by the reaction of silane and ammonia in the gas phase. Ammonia has to be used because nitrogen is too unreactive. Direct combustion of these gases can be done, for example, in a burner that is similar to a regular acetylene-oxygen torch. One ends up with large amount of hydrogen and voluminous quantities of smoke, consisting of very fine particles of silicon nitride. These have to be passed through a good cyclone collector to be captured. A variant is the reaction of silane with ammonia under irradiation of a laser. The incident laser radiation is chosen so that its frequency matches the stretching frequency of the bonds in one or more of the reactants. That bond is thereby stimulated directly to undergo reaction, producing a smoke of silicon nitride particles with a mean diameter of around 50 nm. 11.3.3 Production of Other Ceramic Powders at High Temperature Many other ceramic powders can be produced by high-temperature gas-phase or gas-solid reactions.


(1) Production of boron nitride by reaction of boron trichloride with ammonia: (2) Production of titanium diboride by reaction of titanium tetrachloride with boron trichloride and hydrogen: (3) Production of tungsten carbide by reaction of tungsten hexachloride with methane and hydrogen: (4) Fine alumina (Al2O3) powders have been prepared commercially by the thermal decomposition of ammonium alum: (Note that high quality fine alumina powders can also be produced by the Bayer process or by hydrolysis of the aluminum-based alkoxide.


The Bayer Process The Bayer process is the principal industrial means of producing alumina (later used in the Hall-Heroult process for producing aluminum). Bauxite, the most important ore of aluminum, contains only 40-60% alumina, the rest being a mixture of silica, various iron oxides, and titanium oxide. The alumina must be purified before it can be refined to aluminum metal. In the Bayer process, bauxite is washed with a hot solution of sodium hydroxide, NaOH, at 250°C. This converts the alumina to aluminum hydroxide, Al(OH)3, which dissolves in the hydroxide solution according to the chemical equation: The other components of bauxite do not dissolve and can be filtered out as solid impurities. Next, the hydroxide solution is cooled, and the aluminum hydroxide dissolved in it precipitates out as a white, fluffy solid. When then heated to 1050°C (calcinded in a kiln), the aluminum hydroxide decomposes to alumina, giving off water vapor in the process: (A similar process is used to obtain ZrO2 (zirconia) from impure zirconia.)


Block diagram of the Bayer-Hall process for producing aluminum.


(5) Production of magnesium oxide from concentrated seawater (i.e. NaCl removed by evaporation) and calcined dolomite (CaOMgO). 11.3.4 Production of Metal Powder The methods using powered metals for manufacturing of various objects are often referred to as P/M technology. The use of P/M methods rather than casting permits the fabrication of parts with much finer and more uniform microstructures. This is because cast parts are prone to the segregation, and such problem can be avoided in P/M processing. Three major types of metal powder production are in use: (1) atomization of molten metal (i.e. “spraying” of the liquid metal to form powder), (2) chemical methods, and (3) electrolysis with non-adhesive cathode.


Production of metal powders by various atomization


When oxidation of the sprayed metal is a problem, the spray can be produced in vacuum or under inert gas with quenching into an oil bath. An example of metal powders fabricated by atomization of molten metal is shown on the right. High-purity powders with a low oxygen content can be produced this way, with particle sizes in the range 1 m to a few tens of micrometers. SEM of an atomized rhenium powder Gaseous reduction of metal oxides has been used to produce powders of iron, cobalt, and nickel. The reducing gases are either hydrogen or carbon monoxide. In general, the lower the reaction temperature, the finer the resultant powder. Metal powders, particularly nickel and iron, can be produced by thermal decomposition of carbonyls. Carbonyls are highly toxic gases produced by reaction of the metal with carbon monoxide at temperatures of a few tens of degrees and at elevated carbon monoxide pressures. The carbonyls can then be decomposed at temperatures of a few hundred degrees, regenerating the carbon monoxide. The figure shown in the next slide shows a heated surface in contact with a gas containing a metal carbonyl. (Consideration of reaction kinetics!!!)


At low surface temperatures: Decomposition takes place at the heated surface, coating it with metal. Reaction controlled by the chemical steps at the surface. Concentration gradient across the boundary layer is small. At high surface temperatures: Kinetics of the surface reactions are accelerated. Concentration of carbonyl at the surface is low. Reaction controlled by the speed of mass transfer. At even higher surface temperatures: Heat transfer into the gas from the surface becomes significant. Carbonyl molecules decompose before reaching the surface. The metal is formed around nuclei produced away from the surface (i.e. a powder results). Manipulation of the substrate temperature therefore enables the chemistry of carbonyl decomposition to be exploited to produce coatings or powders!


The P/M process can be outlined in four main steps as: (1) powder Mixing, (2) compaction, (3) sintering, and (4) sizing. The criteria determining whether P/M technology is to be used in manu-facturing a definite part can be summarized as: 1. When it is necessary to have some ingredients kept as mixtures within a metallic matrix, or when alloying is difficult or impossible because of different melting temperatures (e.g. W/Cu contacts). 2. When porosity in the part is necessary (e.g. filters, self lubricating bearings). 3. When production quantity is high enough so that it is more economical to use P/M for its production speed, less or almost no material loss. 4. For many other reasons like complex part shape, cost efficiency, ability to engineer microstructures and properties, and net shape production etc.


Part 12: Sintering of Powder Compacts Sintering commonly refers to processes involved in the heat treatment of powder compacts at elevated temperatures, usually at T > 0.5Tm [K], in the temperature range where diffusional mass transport is appreciable. It is a process in which a compact of a crystalline or non-crystalline powder is heat treated to form a single coherent solid of required density. (Control the sintering  Control the micro-structure  Control the properties) 12.1 Thermodynamics of Sintering The driving force for sintering is a decrease in the surface free energy of powdered compacts, by replacing solid-vapour interfaces (of surface energy SV) with solid-solid interfaces (of surface energy SS), where SS < SV. Thermodynamically, then, sintering is an irreversible process in which a free energy decrease is brought about by a decrease in surface area. At a typical specific surface S of ceramic powders S = 1-10 m2/g and SV = 1 - 2 J/m2,  surface energy of SV = 1-20 J/g (sufficient enough to drive the sintering processes?). References: * Hayes, 1993 * Evans & De Jonghe, 1991 * Barsoum, 2003 * UBC MMAT382/482


The change of system energy dE due to sintering is therefore composed of the increase due to the creation of new grain boundary areas, dASS > 0, and due to the removal of vapour-solid interfaces, dASV < 0. The necessary global thermodynamic condition for the sintering to proceed is: When the powder compact is sintered, its density increases. Often the density of a practical compact, expressed as a fraction of the maximum achievable compact density (i.e. when all pores are eliminated), increases linearly with the logarithm of time below density of about 90-95% of the theoretical maximum.


Finally we have a continuous assemblage of grains, intertwined with individual pores that continue to shrink  larger and larger pore spacings. Pore may become isolated, and growing grains may engulf them so that these pores become separated from grain boundary  nearly impossible to remove by continued heating; very undesirable! Internal free surface is eliminated, while at the same time grain growth occurs.


Note that the global decrease of system free energy is a necessary, but not sufficient, condition for sintering. The sintering process will proceed only if driven by the local differences in chemical potential, pressure or tension due to differences in curvatures between the grain and the neck. The increase in strength of the component comes about by the formation of solid sate bonds between the particles. These bonds may form by a number of mechanisms, each of which involves transport of material within or via the different phases which are present in the component. These mechanisms and driving forces for material transport in each case are summarised in the table below.


12.2 Mechanisms of Sintering (1) Vapour phase sintering is important in relatively few materials systems since the vapour pressures of inorganic and metallic materials are relatively low even at quite high temperatures. The driving force for the transport of material is the difference in the vapour pressure above surfaces of different curvature. The equilibrium vapour pressure of any species is greater over a surface having a positive radius of curvature than over a surface with a negative radius of curvature. The net result of this phenomenon is that material will evaporate from the surface with the positive curvature and deposit or re-condense at the surface with the negative curvature. The point of negative curvature occurs at the point or neck where adjacent particles are in contact. As transport proceeds the neck which represents the solid sate bond between the particles becomes thicker, and hence the strength of the component is increased.


The vapour transport mechanism results in a change in the shape of the pores. It does not, however, reduce the overall porosity of the component. (2) Material transport by solid state diffusion can take place by atom or vacancy movement along surfaces, grain boundaries, dislocations or through the bulk solid. Surface diffusion will only change the shape of the pores, however, all other diffusion paths will result in densification and as a result shrinkage of the component. The driving forces for diffusion include the differences in free energy or chemical potential between the free surfaces of particles and the points of contact between two adjacent particles, between surface shapes, and between materials of different chemical composition. The rate of diffusion of atoms in solids are relatively slow even when enhanced by the use of high temperatures. Typically temperatures in excess of 0.8Tm are necessary for significant volume diffusion in the solid state to occur.


A necessary condition for densification to occur is that the grain boundary energy be less than twice the solid/vapour surface energy.


To achieve densification and to eliminate pores in the material it is necessary to rely on mechanisms which will move the particles closer together, i.e. so that the centre to centre distance will be decreased. Note that solid state diffusion along grain boundaries is a densification mechanism, however, grain coarsening is not. Grain coarsening merely moves matter from one grain to the next, without causing the grain centers to move closer together. During grain coarsening, both the pores and grains get larger with time. Grain coarsening during sintering can be avoided by addition of fine insoluble second phase particles which retard grain boundary migration. During sintering, two mechanisms are usually in competition


Finer-particle-size powder can be sintered more rapidly and at a lower temperature than coarser powder (the uniformity of particle packing, the particle shape and the particle size distribution can disturb this law). Agglomerates, gas entrapment and particle segregation (i.e. settling during slip casting) are common sources of non-uniform packing of particles. (3) Liquid phase sintering, where sintering rates are increased substantially by the presence of a liquid phase during all or part of the sintering treatment (see Figure shown in the next slide). Not only are the rates of mass transport in liquid phases significantly greater than in solids, but particle rearrangement can also occur by vicious flow (i.e. better packing can be achieved). The driving forces for diffusion in liquid phase systems are the same as for solids with the addition of capillary pressure which is present when the liquid phase wets the solids. When the liquid thoroughly wets the solid particles, the liquid in the narrow channels between the particles results in substantial capillary pressure, which aids densification by rearranging the particles to achieve better packing and by increasing the contact pressure between particles. The presence of a liquid phase during sintering may be achieved in a number of ways. In many instances, supposedly solid state sintering proceeds in the presence of previously undetected small amounts of liquid (perhaps introduced as impurities during the powder preparation stage, such as silicates in oxide ceramics Al2O3, ZrO2).


It may be possible that in the system under treatment a liquid is formed directly from one of the components of the mixture or as a result of reaction between components. The presence and extent of liquid formation is then determined by the composition of the mixture and the reaction temperature. It may be possible to make additions in the form of sintering aids to assist sintering. These additions gradually dissolve in the matrix (matrix material dissolved in it) to assist in material transport as sintering proceeds. Densification behaviour of compacts of nominally 1 m alumina powders sintered for 4 hours as a function of temperature using three different techniques (Reed).


Phase diagrams are useful in predicting and understanding liquid phase sintering providing information on melting temperatures, liquid compositions, the equilibrium proportions of liquid and solid, solid solutions and polymorphic transformations. Activated Sintering - A process by which the rate of sintering is significantly increased by means other than changing time or temperature, e.g., addition of a constituent to the powder, or the atmosphere, or thermal cycling.


Note that if the proportion of liquid phase is too high, then the component will distort or warp during sintering treatment. Rule of thumb: in practice there is little to be gained in increasing the volume fraction of liquid phase to a value greater than that of the original porosity of the material. Sintering reactions can also be influenced by the prevailing bulk gas atmosphere. In oxide-containing systems the oxygen potential can affect the stable equilibrium phases which form, the liquidus and solidus temperatures, the ranges of solid and liquid solution of the various phases, the mass transport properties of the phases present and the final physical properties of the components. (4) Reactive liquid sintering is also referred to as transient liquid sintering. A liquid is present during sintering to provide the same types of densification driving forces as discussed for liquid phase sintering, but the liquid either changes composition or disappears as the sintering process progresses or after it is completed. Since the liquid phase is consumed in the reaction, the resulting material can have extremely good-high temperature properties. One means of achieving reactive liquid sintering is to select starting powders or additives that go through a series of chemical combinations or reactions before final stable compound is formed, with one or more of the intermediate compounds being liquid and the final compound being solid. Another is to use starting powders that will form a solid solution at equilibrium but will pass through a liquid stage before equilibrium is reached.


Coarsening Revisiting the transport mechanisms


12.3 Three Stages of Sintering (1) During the initial stage, the interparticle contact area increases by neck growth, and the relative density increases from about 60 to 65 percent. (rearrangement; neck formation; pore volume shrinks) (2) The intermediate stage is characterized by continuous pore channels that are coincident with three-grain edges. During this stage, the relative density increases from 65 to about 90 percent by having matter diffuse toward, and vacancies away from the pore channels. (neck grow; slow grain growth; high shrinkage; continuous pore shrinks; lengthening of grain boundaries) (3) The final stage begins when the pore phase is eventually pinched off and is characterized by the absence of a continuous pore channel. Individual pores are either of lenticular shape, if they reside on the grain boundaries, or rounded, if they reside within a grain. An important characteristic of this stage is the increase in pore and grain boundary mobilities, which have to be controlled if the theoretical density is to be achieved (i.e. we don’t want grain coarsening!). (grain growth; discontinuous pores; closed porosity, elimination of pores at grain boundaries)


A large number of equations describing the variation of porosity P/P0 (or shrinkage L/L0) versus time t have been proposed. All of these equations use simplified assumptions of idealised models, or are based purely on empirical observations. These equations have usually forms of: P / P0 = (a + bt)c P - P0 = d ln(t/t0) or (L / L0 )n = at (where: a,b,c,n = constants) A remarkable feature of most of these equations is that, using curve-fitting procedures, a nearly perfect fit can be obtained with experimental data on sintering. However, little or no insight into sintering mechanisms is obtained and incorrect conclusions are drawn when physical meanings are attributed to the fitted constants. This is because real systems deviate significantly from idealised models: particles are irregular in size and shape. particles are non-uniformly distributed in space and rearrange during sintering. necks grow asymmetrically. different sintering stages (neck formation, neck growth, rearrangement, grain growth with or without shrinkage, closed porosity sintering) are controlled by different mechanisms which interact and overlap in a complex way. 12.4 Empirical Modelling of Sintering

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