F.F. Lange et al materials Science and Engineering A195(1995)145-150 The problem of packing powders within three- pressurized, entrapped gas to the surface. Such gas dimensional preforms was solved by Jamet et al. [29] diffusion is controlled by its solubility at higher pres using pressure filtration. Our adaptation is shown in sures Fig. 2. In this process the consolidated layer builds up The flow of liquid into a porous medium by differ- within the preform, fixed to a filter. Powders can be ential pressure AP is described by Darcy's law[33, 34] packed within the preform provided that three condi- tions are satisfied (25)J. First, the particles must be small h-2KAP /2 nough to flow through the preform channels and smaller yet(R <0.05)to achieve high packing densities for the reasons described above. Second, the particles where h is the distance of liquid intruded within a n the slurry must be repulsive( flocced slurries clog the period t, K is the permeability of the porous body and hannels). Third, repulsive surface forces must exist n is the viscosity of liquid. Gas diffusion obeys Fick's between the preform material and the particles High pressures are desirable because of the para- h=(2DBP /ri/2 bolic kinetics of pressure filtration and the low per- meability of highly packed, submicron powders. where De is the diffusion coefficient of the gas within During filtration, both the reinforcement material and the liquid, B is Henry's constant and Pi is the pressure the surrounding powder are compressed. Both relieve of the entrapped gas. Although both phenomena are their stored strain when the pressure is removed. Since concurrent, the flow of liquid due to differential pres each has different strain recoveries, stresses arise, sure initially dominates. Once the gas within the which can damage these bodies, as shown in Fig. 2(b). compact is sufficiently compressed, gas diffusion Bodies formed from dispersed slurries still flow after becomes dominant onsolidation and can dissipate stresses [26]. The When the intruded precursor is converted to an recovery is time dependent[26]. Thus, the rheology of inorganic during heat treatment, the void space is the consolidated body must be understood and con- partially filled with pyrolyzed precursor without trolled to avoid processing damage induced by the shrinkage of the powder. The kinetics of subsequent reinforcements [301 cycles depends on the permeability of the pyrolyzed precursor, which in turn depends on microstructural development during the heat treatment subsequent to pyrolysis Surface cracks can form within the powder 4. Infiltration physics and kinetics compact during either precursor drying or pyrolys They can be avoided by strengthening the powder The infiltration of a dry, porous medium containing compact by forming small necks between touching gas occurs by two different mechanisms [31, 32]. First, particles by evaporation-condensation [5]. Moreover, capillary plus applied pressures cause a wetting liquid precursor molecules concentrate near the surface as to flow into a granular medium; flow will diminish and the solvent is removed by drying [5]. This can be pre- then stop when the pressure of the entrapped gas vented by gelling the precursor prior to drying. For causes the differential pressure to approach zero. example, a Zr acetate is gelled by soaking the infiltrated Second, gas can diffuse through the liquid from the bodies in aqueous NH OH [5] Iloeetd slur Stift Bod Rcisforernt Prior 嬗 ceas●latl Powdr 88 a) reinforcement preforms by pressure filtration. When the pressure is removed, the re or less strain than the powder compact. (b) Differential strain can induce damage in theF.F. Lange et al. / Materials Science and Engineering A195 (1995) 145-150 147 The problem of packing powders within three￾dimensional preforms was solved by Jamet et al. [29] using pressure filtration. Our adaptation is shown in Fig. 2. In this process the consolidated layer builds up within the preform, fixed to a filter. Powders can be packed within the preform provided that three condi￾tions are satisfied [25]. First, the particles must be small enough to flow through the preform channels and smaller yet (R < 0.05) to achieve high packing densities for the reasons described above. Second, the particles in the slurry must be repulsive (flocced slurries clog the channels). Third, repulsive surface forces must exist between the preform material and the particles. High pressures are desirable because of the para￾bolic kinetics of pressure filtration and the low per￾meability of highly packed, submicron powders. During filtration, both the reinforcement material and the surrounding powder are compressed. Both relieve their stored strain when the pressure is removed. Since each has different strain recoveries, stresses arise, which can damage these bodies, as shown in Fig. 2(b). Bodies formed from dispersed slurries still flow after consolidation and can dissipate stresses [26]. The recovery is time dependent [26]. Thus, the rheology of the consolidated body must be understood and con￾trolled to avoid processing damage induced by the reinforcements [30]. 4. Infiltration physics and kinetics The infiltration of a dry, porous medium containing gas occurs by two different mechanisms [31,32]. First, capillary plus applied pressures cause a wetting liquid to flow into a granular medium; flow will diminish and then stop when the pressure of the entrapped gas causes the differential pressure to approach zero. Second, gas can diffuse through the liquid from the pressurized, entrapped gas to the surface. Such gas diffusion is controlled by its solubility at higher pres￾sures. The flow of liquid into a porous medium by differ￾ential pressure AP is described by Darcy's law [33,34]: = -- t 1/2 (2) where h is the distance of liquid intruded within a period t, K is the permeability of the porous body and r/is the viscosity of liquid. Gas diffusion obeys Fick's law [35]: h =(2OgflPi)l/ztff2 (3) where Dg is the diffusion coefficient of the gas within the liquid, fl is Henry's constant and Pi is the pressure of the entrapped gas. Although both phenomena are concurrent, the flow of liquid due to differential pres￾sure initially dominates. Once the gas within the compact is sufficiently compressed, gas diffusion becomes dominant. When the intruded precursor is converted to an inorganic during heat treatment, the void space is partially filled with pyrolyzed precursor without shrinkage of the powder. The kinetics of subsequent cycles depends on the permeability of the pyrolyzed precursor, which in turn depends on microstructural development during the heat treatment subsequent to pyrolysis. Surface cracks can form within the powder compact during either precursor drying or pyrolysis. They can be avoided by strengthening the powder compact by forming small necks between touching particles by evaporation-condensation [5]. Moreover, precursor molecules concentrate near the surface as the solvent is removed by drying [5]. This can be pre￾vented by gelling the precursor prior to drying. For example, a Zr acetate is gelled by soaking the infiltrated bodies in aqueous NH4OH [5]. [ Slurry __ . St ¢i~ o rot ~.~:~t l~tor'~. Co~ ol.i~td l~ow~v StJt! 3 o<l.y~ Di.+~,trtl Slur~/' W Bo~y ~1o w'~ Ilo I) ~m.lti~ (a) (b) Fig. 2. (a) Powder can be packed within reinforcement preforms by pressure filtration. When the pressure is removed, the reinforcement material can recover either more or less strain than the powder compact. (b) Differential strain can induce damage in the powder compact unless stresses are dissipated by body flow