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Mechanisms of hot pressing of magnesium oxide powders
CERAMICS
INTERNATIONAL,
139
Vol 11, n 4.1985
crystallographic texture of the grains in hot-pressed alumina. The optical properties of the material are treated in a second paper.
DEVELOPMENT OF POROUS SINTERED-CERAMIC SEPARATORS FOR APPLICATION IN A Li-AI/LiCIKCllFeS BATTERY G. Bandyopadhyay,
MECHANISMS OF HOT PRESSING OXIDE POWDERS
OF MAGNESIUM
R. Pampuch Estimation of the rate controlling mechanisms of mass transport during the initial, intermediate and a portion of the final stage of hot pressing of powder compacts by an adequate application of the theory of creep of non-porous polycrystals at elevated temperature is first discussed and their significance for understanding of the relations between powder characteristics and their densification during hot pressing stressed. Review of recent developments of the creep theory indicates that if intraparticle (intracrystallite) i.e. dislocation mechanisms are acting to a negligible extent only, the steady-state strain rate E, should be controlled entirely either by boundary reactions (emission/absorption of point defects at sources and sinks, e.g. at boundary line-defects) or by diffusion between sources and sinks (Nabarro-Herring and/or Coble creep). An analysis has been made in these terms of d observed with well characterised MgO powders, having a rather uniform crystallite size, during hot pressing at 775975K under loads of PA=60-295 MPa. Two types of powders, obtained by thermal decomposition of Mg(OH)* at different temperatures, have been, studied, namely i. powders constituted by well-annealed fine crystallites (d =28-56 nm) showing no lattice microstrains, and ii. powders constituted by fine crystallites (25-45 nm) showing appreciable microstrains of the lattice and, consequently, a high density of the line defects. Experimental determination of the stress sensitivity, n, particle sensitivity, m, and true activation energy for creep, Q,, has shown that the data correspond very closely to is 0~ PA2/d in the first, and to & 0: PAId in the second case, respectively. The first situation is expected for strain rates controlled by boundary reactions while the second for the ones controlled by Coble creep, respectively. The different rate-controlling mechanisms enable to explain rationally the different densifications obtained under comparable conditions of hot pressing with the two types of MgO powders.
J. T. Dusek and T.M. Galvin
The procedure for fabrication of porous sintered-ceramic separators and the technical feasibility of using such separators in Li-Al/molten LiCI-KCIIFeS battery cells have been investigated. Processing techniques have been developed to fabricate - 1.5-2.5 mm thick, -35-600/o porous, flat, sintered Yz03 and MgO separator plates with sufficient strength to allow handling prior to and during cell assembly. These separators performed successfully in laboratory-scale cells for up to - 2000 h and 283 cycles, indicating that the concept of a sintered separator, is viable for Li-AIIFeS batteries. The particularly attractive features of these separators are potentially low cost, prefabricated form that allows easy cell assembly, and small pore size (average diameter 0.5-l .O pm) which provides good particle retention. The test results from the sintered-separator cells indicate that Y20J is probably unsuitable for long-term performance in Li-AIIFeS cells because of its reaction with the positive active material. This is in agreement with the recently reported data on cells with YzOl felt and powder separators. Sintered MgO separators, however, showed good chemical and mechanical stability in the cell environment.
PHASE EQUILIBRIA G. Schneider,
IN THE Si, Al, Be/C, N SYSTEM
L.J. Gauckler and G. Petzow
Phase equilibria studies in parts of the system Si, Al, Be/C, N were carried out at 1760 and 1860°C. In the carbide system Al&-Sic-Be2C seven new compounds were found and characterized by their chemical composition and crystal symmetry. In the nitride system AIN-SilN4-BejN2 a solid solution between AIN and BeSiN was found to exist besides the already known compounds. The carbonitride system showed solid solution between a-AlSiCq and wAI&~N. Furthermore the compound AIN*SiC*AL,C was found to exist. CONTRIBUTION TO THE SYSTEM EUROPIUM-BORON-CARBON K.A. Schwetz, M. Hoerle and J. Bauer
SUBMICRON Ti02-i!rOz POWDERS PRODUCED BY VAPOR PHASE REACTION OF Tic&-ZrCbO2 SYSTEM Y. Suyama, M. Tanaka and A. Kato Submicron Ti02-Zr02 powders were prepared by the vapor phase reaction of the TiC14-ZrC&-O2 system. The phases appeared were Ti02 ss, Zr02 ss and ZrTi04 ss. The major crystal forms of TiOz and ZrO2 were rutile and tetragonal system, respectively. Pure TiOz was predominantly anatase and consisted of single crystalline particles with square and plate-like shape. As ZrC4 concentration increased, the particle size decreased, the content and lattice constants of rutile increased and particles became polycrystalline. The compound ZrTi04 was formed most abundantly as the ratio ZrC4/TiClo approached 1 and the effective reaction temperature was raised. It was found that TiCI accelerates the oxidation of ZrCh and ZrC& retards that of TiCI, in the vapor phase reaction of the TiCId-ZrCh-O2 system. It was concluded that the formation process of TiOz-Zr02 particles consists of the preferential nucleation of TiO2 and the growth by codeposition of Ti02 and Zr02.
GRAIN GROWTH SUBSTRATES
DURING SINTERING
A. Bellosi and P. Vincenzini Short communication
OF ALUMINA
By means of chemical preparation techniques and x-ray analysis the ternary system ELI-B-C was studied, with special emphasis on the technologically important sections EuBh-C and EuB6-B&. It was possible to prove the existance of a hexaboride-carbon solid solution EuB&, (0 IX 5 0.25) a ternary phase EuB2C2, the europium carbides EuC, (~~EuG), EuC2, as well as another carbide of the approximate composition Eu2C1.
POST-SINTERING NITRIDE A. Giachello
OF REACTION-BONDED
SILICON
and P. Popper
Reaction-bonded silicon nitride, RBSN, may be used as the starting material for a post-densification process carried out at about 1800°C in nitrogen at atmospheric pressure. The sintering aids used are similar to those employed in hot pressing and pressureless sintering (e.g. MgO, Y203). They may be added to the silicon powder before nitridation or, in the case of MgO, by diffusing the vapour into preformed RBSN. High densities (98.5% theoretical) have been achieved accompanied by a shrinkage of only 6.2%. The strength at R.T. was about 1000 MN/m ; the strength at H.T. depends on devitrification. The process thus offers complex components to be formed by injection moulding, with strength values after sintering similar to those of hot-pressed silicon nitride.
INTERNATIONAL,
139
Vol 11, n 4.1985
crystallographic texture of the grains in hot-pressed alumina. The optical properties of the material are treated in a second paper.
DEVELOPMENT OF POROUS SINTERED-CERAMIC SEPARATORS FOR APPLICATION IN A Li-AI/LiCIKCllFeS BATTERY G. Bandyopadhyay,
MECHANISMS OF HOT PRESSING OXIDE POWDERS
OF MAGNESIUM
R. Pampuch Estimation of the rate controlling mechanisms of mass transport during the initial, intermediate and a portion of the final stage of hot pressing of powder compacts by an adequate application of the theory of creep of non-porous polycrystals at elevated temperature is first discussed and their significance for understanding of the relations between powder characteristics and their densification during hot pressing stressed. Review of recent developments of the creep theory indicates that if intraparticle (intracrystallite) i.e. dislocation mechanisms are acting to a negligible extent only, the steady-state strain rate E, should be controlled entirely either by boundary reactions (emission/absorption of point defects at sources and sinks, e.g. at boundary line-defects) or by diffusion between sources and sinks (Nabarro-Herring and/or Coble creep). An analysis has been made in these terms of d observed with well characterised MgO powders, having a rather uniform crystallite size, during hot pressing at 775975K under loads of PA=60-295 MPa. Two types of powders, obtained by thermal decomposition of Mg(OH)* at different temperatures, have been, studied, namely i. powders constituted by well-annealed fine crystallites (d =28-56 nm) showing no lattice microstrains, and ii. powders constituted by fine crystallites (25-45 nm) showing appreciable microstrains of the lattice and, consequently, a high density of the line defects. Experimental determination of the stress sensitivity, n, particle sensitivity, m, and true activation energy for creep, Q,, has shown that the data correspond very closely to is 0~ PA2/d in the first, and to & 0: PAId in the second case, respectively. The first situation is expected for strain rates controlled by boundary reactions while the second for the ones controlled by Coble creep, respectively. The different rate-controlling mechanisms enable to explain rationally the different densifications obtained under comparable conditions of hot pressing with the two types of MgO powders.
J. T. Dusek and T.M. Galvin
The procedure for fabrication of porous sintered-ceramic separators and the technical feasibility of using such separators in Li-Al/molten LiCI-KCIIFeS battery cells have been investigated. Processing techniques have been developed to fabricate - 1.5-2.5 mm thick, -35-600/o porous, flat, sintered Yz03 and MgO separator plates with sufficient strength to allow handling prior to and during cell assembly. These separators performed successfully in laboratory-scale cells for up to - 2000 h and 283 cycles, indicating that the concept of a sintered separator, is viable for Li-AIIFeS batteries. The particularly attractive features of these separators are potentially low cost, prefabricated form that allows easy cell assembly, and small pore size (average diameter 0.5-l .O pm) which provides good particle retention. The test results from the sintered-separator cells indicate that Y20J is probably unsuitable for long-term performance in Li-AIIFeS cells because of its reaction with the positive active material. This is in agreement with the recently reported data on cells with YzOl felt and powder separators. Sintered MgO separators, however, showed good chemical and mechanical stability in the cell environment.
PHASE EQUILIBRIA G. Schneider,
IN THE Si, Al, Be/C, N SYSTEM
L.J. Gauckler and G. Petzow
Phase equilibria studies in parts of the system Si, Al, Be/C, N were carried out at 1760 and 1860°C. In the carbide system Al&-Sic-Be2C seven new compounds were found and characterized by their chemical composition and crystal symmetry. In the nitride system AIN-SilN4-BejN2 a solid solution between AIN and BeSiN was found to exist besides the already known compounds. The carbonitride system showed solid solution between a-AlSiCq and wAI&~N. Furthermore the compound AIN*SiC*AL,C was found to exist. CONTRIBUTION TO THE SYSTEM EUROPIUM-BORON-CARBON K.A. Schwetz, M. Hoerle and J. Bauer
SUBMICRON Ti02-i!rOz POWDERS PRODUCED BY VAPOR PHASE REACTION OF Tic&-ZrCbO2 SYSTEM Y. Suyama, M. Tanaka and A. Kato Submicron Ti02-Zr02 powders were prepared by the vapor phase reaction of the TiC14-ZrC&-O2 system. The phases appeared were Ti02 ss, Zr02 ss and ZrTi04 ss. The major crystal forms of TiOz and ZrO2 were rutile and tetragonal system, respectively. Pure TiOz was predominantly anatase and consisted of single crystalline particles with square and plate-like shape. As ZrC4 concentration increased, the particle size decreased, the content and lattice constants of rutile increased and particles became polycrystalline. The compound ZrTi04 was formed most abundantly as the ratio ZrC4/TiClo approached 1 and the effective reaction temperature was raised. It was found that TiCI accelerates the oxidation of ZrCh and ZrC& retards that of TiCI, in the vapor phase reaction of the TiCId-ZrCh-O2 system. It was concluded that the formation process of TiOz-Zr02 particles consists of the preferential nucleation of TiO2 and the growth by codeposition of Ti02 and Zr02.
GRAIN GROWTH SUBSTRATES
DURING SINTERING
A. Bellosi and P. Vincenzini Short communication
OF ALUMINA
By means of chemical preparation techniques and x-ray analysis the ternary system ELI-B-C was studied, with special emphasis on the technologically important sections EuBh-C and EuB6-B&. It was possible to prove the existance of a hexaboride-carbon solid solution EuB&, (0 IX 5 0.25) a ternary phase EuB2C2, the europium carbides EuC, (~~EuG), EuC2, as well as another carbide of the approximate composition Eu2C1.
POST-SINTERING NITRIDE A. Giachello
OF REACTION-BONDED
SILICON
and P. Popper
Reaction-bonded silicon nitride, RBSN, may be used as the starting material for a post-densification process carried out at about 1800°C in nitrogen at atmospheric pressure. The sintering aids used are similar to those employed in hot pressing and pressureless sintering (e.g. MgO, Y203). They may be added to the silicon powder before nitridation or, in the case of MgO, by diffusing the vapour into preformed RBSN. High densities (98.5% theoretical) have been achieved accompanied by a shrinkage of only 6.2%. The strength at R.T. was about 1000 MN/m ; the strength at H.T. depends on devitrification. The process thus offers complex components to be formed by injection moulding, with strength values after sintering similar to those of hot-pressed silicon nitride.