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Transparent hot-pressed alumina II: Transparent versus translucent alumina
140
CERAMICS
TRANSPARENT HOT-PRESSED ALUMINA II: TRANSPARENT VERSUS TRANSLUCENT
ALUMINA
CHARACTERIZATION HAFNIA
INTERNATIONAL,
OF METASTABLE
Vol. 11, n. 4. 1985
TETRAGONAL
J. G.J. Pee/en
0. Hunter, Jr., R. W. Scheidecker
The optical properties of very dense hot-pressed alumina are compared with those of normally sintered alumina with equal pore volume. The influence of the pore size on the in-line and diffuse transmission of light through these materials is considered. The concept of transparency and translucency is critically discussed and it is demonstrated that alumina powder can be hot pressed to form a really transparent material. Relations between the degree of translucency and the microstructure and the preparation conditions of dense alumina are given.
Conditions for retaining metastable tetragonal HfOz at room temperature were determined. Mechanical properties were measured as a function both of the fraction of retained metastable HfOz and of the porosity. The metastable phase was retained by 1) thermal decomposition of pure Hf(OH)4 or HfOCl2 and 2) hot pressing of HfOz + 1% Er20J. The critical size for retaining pure tetragonal Hf02 at room temperature in a Stress free condition in 100 A, whereas the critical size for the 1% Er203 hot pressed composition is significantly larger. The elastic moduli were found to vary linearly with porosity and showed a small effect due to the metastable phase presence.
EFFECT OF TITANATE ADDITIONS TEMPERATURE OF Al203
and Setsuo Tojo
ON SINTERING
G. Wroblewska
PREPARATION AND PROPERTIES OF NEW OXYGEN ION CONDUCTORS FOR USE AT LOW TEMPERATURES
The effect of small additions of magnesium titanate, calcium titanate, strontium titanate, and barium titanate on the sintering temperature of aluminum oxide was investigated. The dependence of linear shrinkage and density on sintering temperature and dopant concentration and type was studied. X-ray analysis, microscopic examination, and electron microprobe analysis indicate that sintering occurred through a liquid phase and that Ti4+ diffused into the Al203 grains. When CaTiOJ, SrTi03, and BaTi03 were added, the compounds 3CaO*Ah03, Sr0*6A120j, and Ba0=6A1203 were formed. The sintering temperature was reduced to 1500°C by adding less than 2 mol% of magnesium, calcium, strontium, or barium titanate.
K. Keizer, M. J. Verkerk and A. J. Burggraaf
ON THE FRACTURE STRENGTH ALUMINA SUBSTRATES
OF TAPE-CASTED
C. Fiori, G. Fusaroli, A. Krajewski and P. Vincenzini
Two new systems, Bi20j-Er203 and Zr02-Y20J (CaO)-B&O3 were investigated. The first system has a homogeneous cubic, fluorite phase between 17.5 and 45.5 mol% Er203 and can be sintered to densities near 95% at 1200 K. At temperatures between 700 K and 1000 K the highest Value of the a.c. oxygen ion conduction in this system is twice as much as the highest value found in the literature. In the second system concentrations of l-3 mole/, B&O3 act as an excellent sintering aid for ZrOz-Y203 and ZrO2-CaO samples which can be sintered to densities higher than 95% at temperatures of 1350 K. During this procedure a liquid Zr02-Bi20, phase exists from which Biz03 partly evaporates with increasing sintering time. The oxygen ion conduction is little affected by the Bi203-rich second phase. The influence of annealing procedures up to 1570 K on the conduction in the ZrOz-Yz03-Biz03 system is small despite weight losses up to 4%.
Short communication. CHARACTERISTICS AND SINTERING ZIRCONIA ULTRAFINE POWDERS
BEHAVIOUR
OF
EFFECTOFDATASCATTERONAPPARENTTHERMAL STRESS FAILURE MODE OF BRITTLE CERAMICS
K. Haberko
G. Ziegler and D. P. H. Hasselman
Coprecipitation from water solution of chlorides was used to obtain yttria-stabilized zirconia powders. It was shown that precalcination processing of the coprecipitated gel greatly affects the microstructure of the resultant powder as well as its cold compaction behaviour and hence its sinterability. Relative densities of + 98% theoretical have been achieved at sintering temperature as low as 1300°C.
Short communication.
GRAIN BOUNDARY POLYCRYSTALLINE BY STEM T. Mitamura,
SEGREGATION OF IRON IN MAGNESIUM OXIDE OBSERVED
E. L. Hall, W.D. Kingery and J. B. Vandersande
Utilizing X-ray microanalysis in samples studied with scanning transmission electron microscopy, substantial grain boundary segregation of Fe is found in grain boundaries in polycrystalline MgO. Samples studied contained between 540 and 5400 cation ppm Fe. Grain boundary segregation in the region within 200 A of the boundary was found in all cases, with the amount of segregation increasing with increasing Fe content. Samples slow-cooled from 1500°C showed an increased amount of segregation attributed to continued diffusion during cooling. Samples quenched from llOO°C showed greater segregation than those quenched from 1500°C, consistent with space charge theory. In some samples precipitates were observed which are correlated with a low-temperature phase diagram for the solidus in the iron oxide-magnesium oxide system. In addition to Fe, grain boundary segregation of Ca and Si impurities in the samples was also observed.
SELF-DIFFUSION CARBIDE
IN ALPHA
AND BETA SILICON
J.D. Hong, M.H. Hon and RF. Davis The self-diffusion coefficients of 14C and “Si have been measured for lattice transport in high purity and N-doped pSiC single crystals and in high purity polycrystalline CVD pSIC in the temperature range of 2123-2573 K. Grain boundary diffusion of 14C has also been determined in the p-Sic material. The results of these studies reveal a vacancy mechanism wherein 14C diffuses considerably faster than “Si in both materials. Furthermore, D, in the N-doped single crystals is smaller than for the undoped materials, while the opposite is true for the “Si transport in these crystals. Changes in the concentration of the charged C and Si vacancies are reasoned to be the underlying cause of this last phenomena. A discussion of the effect of Si evaporation and its effect upon the value of the diffusion coefficient is also presented.
CERAMICS
TRANSPARENT HOT-PRESSED ALUMINA II: TRANSPARENT VERSUS TRANSLUCENT
ALUMINA
CHARACTERIZATION HAFNIA
INTERNATIONAL,
OF METASTABLE
Vol. 11, n. 4. 1985
TETRAGONAL
J. G.J. Pee/en
0. Hunter, Jr., R. W. Scheidecker
The optical properties of very dense hot-pressed alumina are compared with those of normally sintered alumina with equal pore volume. The influence of the pore size on the in-line and diffuse transmission of light through these materials is considered. The concept of transparency and translucency is critically discussed and it is demonstrated that alumina powder can be hot pressed to form a really transparent material. Relations between the degree of translucency and the microstructure and the preparation conditions of dense alumina are given.
Conditions for retaining metastable tetragonal HfOz at room temperature were determined. Mechanical properties were measured as a function both of the fraction of retained metastable HfOz and of the porosity. The metastable phase was retained by 1) thermal decomposition of pure Hf(OH)4 or HfOCl2 and 2) hot pressing of HfOz + 1% Er20J. The critical size for retaining pure tetragonal Hf02 at room temperature in a Stress free condition in 100 A, whereas the critical size for the 1% Er203 hot pressed composition is significantly larger. The elastic moduli were found to vary linearly with porosity and showed a small effect due to the metastable phase presence.
EFFECT OF TITANATE ADDITIONS TEMPERATURE OF Al203
and Setsuo Tojo
ON SINTERING
G. Wroblewska
PREPARATION AND PROPERTIES OF NEW OXYGEN ION CONDUCTORS FOR USE AT LOW TEMPERATURES
The effect of small additions of magnesium titanate, calcium titanate, strontium titanate, and barium titanate on the sintering temperature of aluminum oxide was investigated. The dependence of linear shrinkage and density on sintering temperature and dopant concentration and type was studied. X-ray analysis, microscopic examination, and electron microprobe analysis indicate that sintering occurred through a liquid phase and that Ti4+ diffused into the Al203 grains. When CaTiOJ, SrTi03, and BaTi03 were added, the compounds 3CaO*Ah03, Sr0*6A120j, and Ba0=6A1203 were formed. The sintering temperature was reduced to 1500°C by adding less than 2 mol% of magnesium, calcium, strontium, or barium titanate.
K. Keizer, M. J. Verkerk and A. J. Burggraaf
ON THE FRACTURE STRENGTH ALUMINA SUBSTRATES
OF TAPE-CASTED
C. Fiori, G. Fusaroli, A. Krajewski and P. Vincenzini
Two new systems, Bi20j-Er203 and Zr02-Y20J (CaO)-B&O3 were investigated. The first system has a homogeneous cubic, fluorite phase between 17.5 and 45.5 mol% Er203 and can be sintered to densities near 95% at 1200 K. At temperatures between 700 K and 1000 K the highest Value of the a.c. oxygen ion conduction in this system is twice as much as the highest value found in the literature. In the second system concentrations of l-3 mole/, B&O3 act as an excellent sintering aid for ZrOz-Y203 and ZrO2-CaO samples which can be sintered to densities higher than 95% at temperatures of 1350 K. During this procedure a liquid Zr02-Bi20, phase exists from which Biz03 partly evaporates with increasing sintering time. The oxygen ion conduction is little affected by the Bi203-rich second phase. The influence of annealing procedures up to 1570 K on the conduction in the ZrOz-Yz03-Biz03 system is small despite weight losses up to 4%.
Short communication. CHARACTERISTICS AND SINTERING ZIRCONIA ULTRAFINE POWDERS
BEHAVIOUR
OF
EFFECTOFDATASCATTERONAPPARENTTHERMAL STRESS FAILURE MODE OF BRITTLE CERAMICS
K. Haberko
G. Ziegler and D. P. H. Hasselman
Coprecipitation from water solution of chlorides was used to obtain yttria-stabilized zirconia powders. It was shown that precalcination processing of the coprecipitated gel greatly affects the microstructure of the resultant powder as well as its cold compaction behaviour and hence its sinterability. Relative densities of + 98% theoretical have been achieved at sintering temperature as low as 1300°C.
Short communication.
GRAIN BOUNDARY POLYCRYSTALLINE BY STEM T. Mitamura,
SEGREGATION OF IRON IN MAGNESIUM OXIDE OBSERVED
E. L. Hall, W.D. Kingery and J. B. Vandersande
Utilizing X-ray microanalysis in samples studied with scanning transmission electron microscopy, substantial grain boundary segregation of Fe is found in grain boundaries in polycrystalline MgO. Samples studied contained between 540 and 5400 cation ppm Fe. Grain boundary segregation in the region within 200 A of the boundary was found in all cases, with the amount of segregation increasing with increasing Fe content. Samples slow-cooled from 1500°C showed an increased amount of segregation attributed to continued diffusion during cooling. Samples quenched from llOO°C showed greater segregation than those quenched from 1500°C, consistent with space charge theory. In some samples precipitates were observed which are correlated with a low-temperature phase diagram for the solidus in the iron oxide-magnesium oxide system. In addition to Fe, grain boundary segregation of Ca and Si impurities in the samples was also observed.
SELF-DIFFUSION CARBIDE
IN ALPHA
AND BETA SILICON
J.D. Hong, M.H. Hon and RF. Davis The self-diffusion coefficients of 14C and “Si have been measured for lattice transport in high purity and N-doped pSiC single crystals and in high purity polycrystalline CVD pSIC in the temperature range of 2123-2573 K. Grain boundary diffusion of 14C has also been determined in the p-Sic material. The results of these studies reveal a vacancy mechanism wherein 14C diffuses considerably faster than “Si in both materials. Furthermore, D, in the N-doped single crystals is smaller than for the undoped materials, while the opposite is true for the “Si transport in these crystals. Changes in the concentration of the charged C and Si vacancies are reasoned to be the underlying cause of this last phenomena. A discussion of the effect of Si evaporation and its effect upon the value of the diffusion coefficient is also presented.