摘要:

Unfired spinel (MgAl2O4) compacts and sintered materials with small hard agglomerates (<5 μm) were studied using small‐angle neutron scattering (SANS) techniques. The SANS results were compared with those from mercury porosimetry and gas adsorption. The results from green‐state samples are consistent with interconnected “ink‐bottle”‐type porosity. In the latter stages of densification the average void size is significantly larger than that found in the unfired compact. The presence of the hard agglomerates affects the observed SANS scattering much more in the partially densified samples than in the unfired compacts. It was demonstrated that the use of multiple SANS techniques to study large voids (<0.1 μm) and large pore fractions (45%) is a useful, sensitive, nondestructive diagnostic probe for the evaluation of porosity dur

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05752.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The peak toughness of a commercial magnesia‐partially‐stabilized zirconia was strongly dependent on a postprocessing, postspecimen preparation, and modest annealing (1000°C for 20 min), increasing it from ∼9 to almost ∼18 MP a·m1/2. Further study of this surprising phenomenon revealed that increases in toughness occurred for annealing temperatures as low as 275°C. The low toughness of the as‐received materials is due to a post‐manufacture room‐t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05766.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The effect of stress on pore closure was studied by hot isostatic pressing and hot forging of sintered Al2O3, cubic ZrO2, and finer‐grained Al2O3/ZrO2(cubic) composite materials containing pores remnant of monosized plastic spheres. For the temperature‐time conditions explored, plastic deformation was the dominant mechanism for pore shape change and closure. Namely, only pores in the highly deformable two‐phase material were observed to either change shape (under uniaxial forging) or shrink (during hot isostatic pressing). Large plastic strains (0.3 to 0.6, dependent on location) were required to produce pore closure under forging conditions. At temperatures where the two‐phase materials were sufficiently deformable to affect pore closure, pores close to the surface would puncture as the thin web of material separating the pore from the surface was deformed into the pore. These results are discussed in terms of the common practice of finishing by hot isostatic pressing of sintered ma

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05753.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The transpiration method was used to study the evaporation of Sb2O3from a glass melt with the composition 70SiO2·15K2O·15CaO·MgO (in wt%) at 1200° to 1300°C. The glass contained about 0.9 wt% Sb2O3. Assuming the monomer Sb2O3is the species of evaporation, the saturation vapor pressures could be calculated with ΔHV=218±20 kJ·mol−1and ΔSV=128±10

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05768.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Microwave ceramic resonators composed of BaO‐TiO2‐WO3were developed. The effect of WO3addition on the system of BaO·xTiO2·(1+x)yWO3(x=4 and 4.5, y=0 to 0.04) was studied. The ceramics of this system are composed of crystallines including Ba2Ti9O20, BaTi4O9, BaWO4, and TiO2. At y=0.02, the BaO·4TiO2·0.1WO3ceramic was found to have excellent microwave properties such as ε=35, Q=8400 at 6 GHz, and nearly 0 ppm/

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05769.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Dense samples in the TiO2‐SnO2system were fabricated by pressureless sintering in air using commercial powders as well as powders obtained by coprecipitation. Phase equilibria in the system were examined using X‐ray diffraction on samples that were annealed for long periods of time. Samples of near equi‐molar composition made with controlled amounts of Al and Ta (separately) as dopants were annealed inside the coherent spinodal. X‐ray diffraction and electron microscopy were the principal characterization tools used to follow the kinetics of phase transformation. The composition modulations occurred along the [001] direction consistent with prior work. During the early stages of the decomposition process, interlamellar surfaces were coherent. At later stages, strain mismatch was accommodated by interface dislocations. The kinetics of decomposition were strongly influenced by the type and the amount of dopant. Specifically, trivalent aluminum enhanced the decomposition while pentavalent tantalum suppressed it. The kinetics in a sample doped with 0.5 mol% Al2O3were more than 3 orders of magnitude more rapid than a sample doped with 1.0 mol% Ta2O5. These observations are rationalized on the premise that cation interstitial mobility is greater than cation vacancy m

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05754.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The effect of aging upon the dielectric properties of MnO‐doped lead magnesium niobate‐lead titantate (0.9PMN·0.1PT) ceramics has been measured for samples aged at room temperature (25°C) both under zero and under high (22 kV/cm) dc fields. In the 0.9PMN·0.1PT composition, 25°C is above the limit for stable remanent polarization, P=0, in the absence of an external field; nonetheless, aging under a high dc bias at 25°C, then cooling to low temperature under zero bias produces a stable remanence at lower temperature and a loss of the dispersive character of the micropolar state. It is suggested that dipolar defects induced by MnO doping in the PMN‐PT can slowly reorient to stabilize the existing domain structure acting like a local bias field. Evidence is presented to show the stabilization of both random (unpoled) and ordered (poled) micropolar states. By comparing the remanent polarization induced by cooling after aging under dc field with that produced by straight dc poling of deaged samples, it appears that the defect ordering is macroscopically equivalent to a field of order 2 kV/cm. This field is in rough accord with the field to be expected from the known defect concentration, when the defects a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05770.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Transparent lanthana‐doped yttria fabricated by transient solid second‐phase sintering under wet hydrogen typically has a broad absorption band with a peak at 3.08 μm. The absorption band shift observed in samples treated in wet deuterium indicated that the 3.08‐μm absorption was due to OH−ions. The diffusion rates of hydrogen defects in lanthana‐doped yttria were determined in the temperature range from 1000° to 1400°C. The changes in the concentrations of OH−ions upon anneals were determined by measuring infrared absorbance at 3.08 μm. The diffusion coefficient is 1.3 × 10−7, 9.9 × 10−7, and 4.1 × 10−6cm2/s at 1000°, 1200°, and 1400°C, respectively, with an activation energy of 140 kJ/mol. Annealing in a controlled oxygen partial‐pressure environment can remove the OH−absorption band and bring the total absorption in the 3‐ to 5‐μm

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05771.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Reaction kinetics in a coarse equimolar powder mixture were slow enough to allow for the different stages to be identified, notably in the lower and higher temperature ranges, respectively. In the former (T≤ 1600 K), Al2TiO5nucleation was hindered by the strain energy contribution to the overall driving force. The setting up of metastable layer sequences Al2TiO5/TiO2/Al2O3was found to occur generally during subsequent growth. The high Al mobility in the TiO2provided a rapid aluminum transport from the metastable Al2O3/TiO2interface to the TiO2/Al2TiO5front. At temperatures above ∼1700 K the Al2O3/TiO2interface was very rapidly sealed off by Al2TiO5formation. Reactant transport across the Al2TiO5was slow because of the low mobilities in the product phase. Therefore, much lower product growth velocities were observed at higher temperatures than at lower temperatu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05755.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The thermal stresses are calculated during quenching of short glass cylinders for materials used in solid‐state laser devices. The stresses are determined numerically, and they are parameterized by the Biot number which characterizes cooling by convection. The solution is compared to the stresses for flat, penny‐shaped and for long, rod‐shaped cylinders. The validity of the steady‐state thermal boundary conditions is di

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1988.tb05772.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY