摘要:

An 8‐wt%‐scandia silicon nitride with an elongated grain structure was fabricated. The material exhibited high fracture toughness (∼ 7 MPa · m1/2) and a risingR‐curve as measured by the indentation strength technique. The “toughening” exponentmwas found to bem∼ 0.1. The high fracture toughness andR‐curve behavior was attributed mainly to bridging of the crack faces by the elongated grains. The crack closure (bridging) stress distribution in the wake region of the crack tip was estimated as afunction of crack size from theR‐curve data, with an arbitrarily assumed dis

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04217.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Crystallization in a Ba‐containing magnesium aluminosilicate (Ba‐MAS) glass‐ceramic was studied using transmission electron microscopy. Ba‐MAS slabs were hot‐pressed at 1000°C to form dense glassy bodies, which were crystallized by further heating in air or argon between 850° and 1300°C. Heterogeneous nucleation of a metastable highquartz solid solution (μ‐cordierite) occurred at crystallization temperatures below 1100°C, followed by dendritic growth; the interdendrite phase was Ba‐rich silicate glass. The high‐quartz solid solution generally acted as a precursor for the nucleation of the stableβ‐phase, which was the final crystallization product. Surprisingly, slower crystallization kinetics were observed in argon than in air. Further morphological changes at the higher temperatures and longer annealing, times resulted from a Rayleigh type instability, which led to breakup of the interdendrite silicate rods, and simultaneous polygo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04218.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The nonuniform crystal structure of SiC whiskers complicates the interpretation of X‐ray texture data collected from whisker‐reinforced composites. A simple model for assessing whisker textures in alumina matrix composites which is able to account for the mixed polytype structure of SiC whiskers is discussed. Synchrotron X‐ray radiation was used to quantitatively measure SiC whisker texture in hot‐pressed and hot‐forged Al2O3–SiC whisker composites. Pole figures of the 2.51 Å reflection ({111} cubic, (001) hexagonal) were obtained and compared to pole figures calculated from hypothetical whisker axis disribution describing the whisker orientation distribution within the axisymmetric composites. The X‐ray data suggest that the degree of reinforcement phase texture in hot‐worked specimens is dependent upon final specimen strain. The data also demonstrate that for conventionally hot‐pressed composites the whiskers are angularly distributed about the compression plane, having an average orientation which does not approximate a two‐dimensionally

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04219.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Aluminum titanate was prepared by two different methods. A monophasic gel was obtained by cohydrolysis and polycondensation reactions of chelated aluminumsec‐butoxide and titanium isopropoxide. A diphasic gel was obtained from boehmite and anatase. Effects of homogeneous dispersion of magnesium oxide and iron oxide incorporated as alkoxides in the monophasic gel and also the effects of isostructural seed on the crystallization and densification of aluminum titanate obtained from monophasic and diphasic gels were investigated. Seed particles of MgTi2O5facilitated lowering of crystallization temperature and enhanced densification of aluminum titanate. Microstructure studies indicated that almost‐microcrack‐free sintered bodies with smaller grains were formed from the seeded monophasic and diphasic

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04220.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Fatigue of ferroelectric polarization and the electric field induced strain is investigated in lead lanthanum zirconate titanate ceramics. Fatigue rate increases as the coercive field or the hysteretic nature of the electric field induced polarization of the ferroelectric ceramics increases. The ferroelectric polarization lost during fatigue can be partly restored by decreasing the frequency and increasing the amplitude of the applied ac electric field. The ferroelectric fatigue is modeled phenomenologically by an increase of the energy barrier for the ferroelectric switching and atomistically by a coupling of the spontaneous polarization with defects and/or space charge in the orientation of ferroelectric fatigue.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04221.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Oxygen vacancies can be introduced into zirconia solid solution ZrO2–MOu(u= 1 and 1.5) to maintain electroneutrality. Recently, the local structures around Zr4+and M2u+ions in ZrO2–MOusolid solutions have been studied through EXAFS spectroscopy, diffuse scattering analysis, and single‐crystal structure analysis. The present study constructs an ion‐packing model for zirconia solid solutions based on some defect cluster models. The decrease of cell volume with the occurrence of vacancies is assumed to be expressed by decreasing the coordination number (CN) of cations around the vacancy. The distribution of CNs in a solid solution was calculated from a certain defect cluster model. The average interatomic distances, the average CN, and the short‐range order parameters were calculated using this distribution of CNs. The local structures calculated from the model were compared with experimental data in the systems ZrO2–MO1.5(M = Y, Gd, Yb, and Ca). In the ZrO2–YO1.5system, ther(s–O) interatomic distance, where s represents Zr4+or Y3+and O represents O2−, decreased with Y content and therefore vacancy content. The probability of finding Y3+around a vacancy increases with increasing yttria content from a comparison of the calculated results with the ones from recent EXAFS studies. The present model can qualitatively explain compositional and size dependences of the dopant on various

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04222.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Lattice parameter data of cubic phases and cube roots of unit cell volumes of tetragonal phases in homogeneous ZrO2‐containing solid solutions were compiled to examine the validity of Vegard's law. Except for ZrO2–CeO2and ZrO2–UO2systems, the data for cubic phases were expressed by the equationd=asX+b, whered,as,X, andbdenote the lattice parameter, a constant depending on dopant species, the dopant content, and a constant independent of dopant species, respectively. For tetragonal phases, the cube roots of unit‐cell volumes could be fitted by a similar equation except for the data in the ZrO2–MO2systems (M = Ge and U). The constantaswas calculated using an ion‐packing model and was independent of the defect cluster models. The calculatedasis close to the experimentally observed one, although the former is slightly smaller than the latter in the ZrO2–MOusystems (u= 1 and 1.5). This difference was ascribed to the lack of consideration of the ionic distortions from the ideal sites of the fluorite

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04223.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A new type of silicon carbide having rodlike grains has been fabricated by hot‐pressing or hot‐pressing followed by hot isostatic pressing using SiC whiskers as the raw powder. Preliminary relationships between sintering conditions and microstructure and fracture toughness of the processed ceramics were established. Ceramics with relative density greater than 98% were prepared at temperatures of 2050°C or greater. The fracture toughness of these ceramics was better than that of ordinary SiC, and its maximum value was 7.3 MPa · m1/2. Grain pullout, grain bridging, and crack deflection were considered to be the main operative mechanisms which led to improved fractrue toug

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04224.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The behavior of the impurities iron, cobalt, yttrium, and cerium is determined via radiotracer techniques for the preparation of high‐purity lanthanum fluoride. The behavior of nickel and copper during the coprecipitation of a lanthanum nitrate solution is determined by graphite furnace atomic absorption spectrometric (GFAAS) analysis. There is no commercially available radiotracer for neodymium, a key impurity associated with absorption losses in fluoride glasses. However, the chemical behavior of neodymium and that of yttrium are very similar and, therefore, it is resonable to assume that the behavior of yttrium throughout the processing is indicative of the behavior of neodymium. The concentrations of impurities in lanthanum nitrate, carbonate, and fluoride are estimated using the radiotracer and GFAAS data for each processing step. Results indicate that while high‐purity lanthanum carbonate can be prepared, any impurities present in the lanthanum carbonate will be carried quantitatively into lanthanum fluoride upon hydrofluorination. The data also indicate that lanthanum nitrate can be prepared in higher purity than lanthanum carbonate; however, lanthanum fluoride prepared from the nitrate forms a very wet precipitate which is extremely difficult to filter and dry. The presence of excess water in lanthanum fluoride results in high levels of oxide contamination in lanthanum fluoride as well as the fluoride glass. Preparation of low‐oxide lanthanum fluoride is required because high oxide levels in fluoride glasses cause increased scattering. Therefore, from a fluoride glass point of view, the preferred synthetic route for the preparation of high‐purity lanthanum fluoride is to first purify lanthanum nitrate, then convert this to the carbonate, which is then converted to lanthanum fluoride. The estimated impurity concentrations in lanthanum fluoride derived from lanthanum carbonate are as follows: Fe, 89 ng/g; Co,<3 ng/g; Ni,<12 ng/g; Cu, 6 ng/g; Y, 62 ng/g; Ce, 248 ng/g. The loss contributed to a ZBLAN fiber incorporating this lanthanum fluoride is calculated to be 0.082 dB/km; here the concentration of neodymium is assumed to be equal to that of yttrium. These data indicate that the lanthanum fluoride prepared by the current method will not meet the ultimate requirements for ultra‐low‐loss fluor

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04225.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The coarsening kinetics of a magnesia‐partially‐stabilized zirconia with and without ternary additions of calcia, titania, or yttria at the 2 mol% level have been studied as a function of aging time at 1420°C. Addition of titania was found to markedly increase the coarsening rate and it is thought that this is due to a higher precipitate volume fraction in this system. Both calcia and yttria decreased the coarsening kinetics by a similar amount, probably due to both lower precipitate volume fractions and lower interdiffusion coefficients. The precipitate “transformability” was found to vary among the four systems examined, with calcia and yttria acting to stabilize tetragonal precipitates to larger sizes; titania was found to act as a destabilizer. Coherency stresses are important in determining the precipitate morphology and distribution in these alloys but we were unable to ascertain what effect these stresses had on the coarsening rates of the differen

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04226.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY