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Fabrication of stabilized ZrO2 by hot petroleum drying method
138
CERAMICS
modified form of the rate constant in the Ginstling and Brounsthein model has been shown to describe the sodium carbonate - silica reaction of dried samples. This equation is: (a) K = 1 - 2x13 - (1 ~ x)2/3
INTERNATIONAL.
Vol
11, n. 4.1985
noticeable but that of Mg was not. The thickness of the segregation region for both interfaces was 600 to 800 A. The spheroidization of tubular voids into pores at the grainboundary was controlled by surface diffusion. Discussion of the fabrication process of the bicrystal in terms of initial sintering kinetics concluded that the process was controlled by the surface diffusion.
were K=
t is the time, D is the diffusion coefficient
of the rate controlling species, C, is the concentration gradient, k, and kz are constants, R, is the sodium carbonate particle size, f?sioz is the silica particle size, and M and Q are the total volumes of the sodium carbonate and silica, respectively. Surface diffusion was shown to cover all silica particles rapidly and supply Na’ and O= ions to the reaction interface. The activation energy of the process was found to be 30 k-Cal/mole. The presence of water vapor caused the diffusion rate to increase rapidly and the rate controlling step became chemical processes which occurred at the phase boundary. This process was determined to have an activation energy of 35 kcallmole.
SYNTHESIS
OF MULLITE
BY A FREEZE-DRY
EFFECTS OF STRAIN ENERGY ON PRECIPITATE MORPHOLOGY IN MgO A.F. Henriksen and W.D. Kingery Differences between the morphology of MgFe204 precipitates, which form octahedra with { 111) habit planes and grow into dendritic forms propagating along < lOO> ; MgCr204 and MgA1204, which assume plate-like morphology with a { lOO} habit plane; and Sc203, which precipitates as randomly oriented platelets, are discussed in terms of the differences in strain energy associated with exsolution of the phases in MgO. The strain energy effects, which occur both during nucleation and subsequent growth, provide a rational explanation of the observed behavior.
PROCESS
T.A. Wheat, E. M. H. Sallam and A.C. D. Chaklader
FABRICATION OF STABILIZED Zr02 BY HOT PETROLEUM DRYING METHOD
A freeze-drying technique has been developed to synthesize fine-grained stoichiometric mullite (3A1203*2Si02) by doping a silica sol with an appropriate quantity of a one molar Al2(SO& solution followed by spray freezing in liquid N2 and freeze drying the mixture. The freeze-dried powder when calcined at 1400°C produced a single-phase stoichiometric mullite as determined by X-ray diffraction. The final product was very friable and could be easily broken down to a mean particle size of - 7 pm (equivalent spherical diameter).
Su-II Pyun A series of MgO- and CaO- partially and fully-stabilized Zr02 bodies were produced from sulfate and acetate powders using a wet chemical drying method (<
EFFECT OF CALCINATION TEMPERATURE ON LATTICE DEFECTS IN IMPURE MAGNESIUM OXIDE S.A. Abdel-Hady,
N.A. Ahmed, A.R. El-Dib and M.S. Farag
HOT PRESSING
OF LITHIA-STABILIZED
The crystallite size and the strain of powdered MgO prepared by decomposing magnesium carbonate in the range 600-1300% were measured in [loo] and [ill] crystallographic directions by analyzing the X-ray line broadening. The Warren and Averbach multiple order method was used. The results showed that the crystallite size and the strain are anisotropic. The lattice parameter was calculated from the centre of gravity of the k,,, line profile of all planes using Wagner’s extrapolation method. The changes in the density of the powdered samples and the changes in the unit cell constant showed that the Schottky defects are probably present at low firing temperatures and there are adsorbed contaminant agents on the primary surfaces of the crystallites. The impurities affected the properties of the polycrystalline material. Their effects were very evident in the magnetic susceptibility values, in the sudden increase of the crystallite size above 7000% and in the high value of the strain at that temperature.
A. V. Virkar, T.D. Ketcham and R.S. Gordon
OBSERVATIONS BICRYSTALS
TRANSPARENT HOT-PRESSED I:HOT PRESSING OF ALUMINA
0. Maruyama
ON THE GRAIN-BOUNDARY
OF Al203
and W. Komatsu
The grain-boundary thickness of alumina bicrystals, fabricated without the aid of sintering additives and pressure, was measured with an optical microscope. The thickness was 500 to 550 A and independent of the tilt angle. Impurities segregated at the grain-boundary and the surface of the same sample was studied with an ion microanalyser. The segregation of Ca and Si at both interfaces was
@“-ALUMINA
Fine-grained \<5wm), dense (3.26 - 3.27 g/cm3), and strong (>200 MN/m ) polycrystalline @‘-alumina (lithia-stabilized) ceramics can be fabricated by vacuum hot-pressing at temperatures 2 1330% and pressures between 28 and 41 MN/m2. By suitable low temperature annealing (< 1400%), a fine-grained, high strength P”-alumina body can be fabricated with a low resistance (~5 ohm-cm at 300%) to sodium ion conduction. The relatively high sodium ion resistivities (6-11 ohm-cm) obtained directly after hotpressing are caused by an incomplete phase conversion to 0”-alumina during densification and not to a small grain size. Low temperature annealing can promote phase conversion and yet prevent any significant grain growth. The high strength ceramics produced by hot-pressing can serve as the standard to be achieved in conventional sintering.
ALUMINA
J. G. J. Peelen The densification of alumina to nearly full density by continuous hot pressing is described. The influence of the atmosphere on hot pressing is discussed as well as the influence of the process parameters temperature, pressure and transit rate. The microstructure of hot-pressed alumina is examined and compared with that of normally sintered alumina. X-ray diffraction is applied to investigate a possible
CERAMICS
modified form of the rate constant in the Ginstling and Brounsthein model has been shown to describe the sodium carbonate - silica reaction of dried samples. This equation is: (a) K = 1 - 2x13 - (1 ~ x)2/3
INTERNATIONAL.
Vol
11, n. 4.1985
noticeable but that of Mg was not. The thickness of the segregation region for both interfaces was 600 to 800 A. The spheroidization of tubular voids into pores at the grainboundary was controlled by surface diffusion. Discussion of the fabrication process of the bicrystal in terms of initial sintering kinetics concluded that the process was controlled by the surface diffusion.
were K=
t is the time, D is the diffusion coefficient
of the rate controlling species, C, is the concentration gradient, k, and kz are constants, R, is the sodium carbonate particle size, f?sioz is the silica particle size, and M and Q are the total volumes of the sodium carbonate and silica, respectively. Surface diffusion was shown to cover all silica particles rapidly and supply Na’ and O= ions to the reaction interface. The activation energy of the process was found to be 30 k-Cal/mole. The presence of water vapor caused the diffusion rate to increase rapidly and the rate controlling step became chemical processes which occurred at the phase boundary. This process was determined to have an activation energy of 35 kcallmole.
SYNTHESIS
OF MULLITE
BY A FREEZE-DRY
EFFECTS OF STRAIN ENERGY ON PRECIPITATE MORPHOLOGY IN MgO A.F. Henriksen and W.D. Kingery Differences between the morphology of MgFe204 precipitates, which form octahedra with { 111) habit planes and grow into dendritic forms propagating along < lOO> ; MgCr204 and MgA1204, which assume plate-like morphology with a { lOO} habit plane; and Sc203, which precipitates as randomly oriented platelets, are discussed in terms of the differences in strain energy associated with exsolution of the phases in MgO. The strain energy effects, which occur both during nucleation and subsequent growth, provide a rational explanation of the observed behavior.
PROCESS
T.A. Wheat, E. M. H. Sallam and A.C. D. Chaklader
FABRICATION OF STABILIZED Zr02 BY HOT PETROLEUM DRYING METHOD
A freeze-drying technique has been developed to synthesize fine-grained stoichiometric mullite (3A1203*2Si02) by doping a silica sol with an appropriate quantity of a one molar Al2(SO& solution followed by spray freezing in liquid N2 and freeze drying the mixture. The freeze-dried powder when calcined at 1400°C produced a single-phase stoichiometric mullite as determined by X-ray diffraction. The final product was very friable and could be easily broken down to a mean particle size of - 7 pm (equivalent spherical diameter).
Su-II Pyun A series of MgO- and CaO- partially and fully-stabilized Zr02 bodies were produced from sulfate and acetate powders using a wet chemical drying method (<
EFFECT OF CALCINATION TEMPERATURE ON LATTICE DEFECTS IN IMPURE MAGNESIUM OXIDE S.A. Abdel-Hady,
N.A. Ahmed, A.R. El-Dib and M.S. Farag
HOT PRESSING
OF LITHIA-STABILIZED
The crystallite size and the strain of powdered MgO prepared by decomposing magnesium carbonate in the range 600-1300% were measured in [loo] and [ill] crystallographic directions by analyzing the X-ray line broadening. The Warren and Averbach multiple order method was used. The results showed that the crystallite size and the strain are anisotropic. The lattice parameter was calculated from the centre of gravity of the k,,, line profile of all planes using Wagner’s extrapolation method. The changes in the density of the powdered samples and the changes in the unit cell constant showed that the Schottky defects are probably present at low firing temperatures and there are adsorbed contaminant agents on the primary surfaces of the crystallites. The impurities affected the properties of the polycrystalline material. Their effects were very evident in the magnetic susceptibility values, in the sudden increase of the crystallite size above 7000% and in the high value of the strain at that temperature.
A. V. Virkar, T.D. Ketcham and R.S. Gordon
OBSERVATIONS BICRYSTALS
TRANSPARENT HOT-PRESSED I:HOT PRESSING OF ALUMINA
0. Maruyama
ON THE GRAIN-BOUNDARY
OF Al203
and W. Komatsu
The grain-boundary thickness of alumina bicrystals, fabricated without the aid of sintering additives and pressure, was measured with an optical microscope. The thickness was 500 to 550 A and independent of the tilt angle. Impurities segregated at the grain-boundary and the surface of the same sample was studied with an ion microanalyser. The segregation of Ca and Si at both interfaces was
@“-ALUMINA
Fine-grained \<5wm), dense (3.26 - 3.27 g/cm3), and strong (>200 MN/m ) polycrystalline @‘-alumina (lithia-stabilized) ceramics can be fabricated by vacuum hot-pressing at temperatures 2 1330% and pressures between 28 and 41 MN/m2. By suitable low temperature annealing (< 1400%), a fine-grained, high strength P”-alumina body can be fabricated with a low resistance (~5 ohm-cm at 300%) to sodium ion conduction. The relatively high sodium ion resistivities (6-11 ohm-cm) obtained directly after hotpressing are caused by an incomplete phase conversion to 0”-alumina during densification and not to a small grain size. Low temperature annealing can promote phase conversion and yet prevent any significant grain growth. The high strength ceramics produced by hot-pressing can serve as the standard to be achieved in conventional sintering.
ALUMINA
J. G. J. Peelen The densification of alumina to nearly full density by continuous hot pressing is described. The influence of the atmosphere on hot pressing is discussed as well as the influence of the process parameters temperature, pressure and transit rate. The microstructure of hot-pressed alumina is examined and compared with that of normally sintered alumina. X-ray diffraction is applied to investigate a possible