摘要:

The structures of time–temperature–transformation diagrams of glasses which crystallize by combined homogeneous and heterogeneous crystallization mechanisms are examined. Considerations are given to the factors which might produce more than one extremum in such diagrams. Specific nucleation and growth models are used, and the influence of the parameters which appear in the nucleation and growth rate expressions upon the structure of the diagrams is evalua

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05441.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si2N2O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures.α‐Si3N4is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si2N2O is approximately 19 GPa, comparable with the hardness of Si3N4hot isostatically pressed with 2.5 wt% Y2O3, while the fracture toughness level is around 3.1 MPa. m1/2, being approximately 0.8 MPa.m1/2lower. The three‐point bending strength increased with HIP temperature from approximately 300 t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05442.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Fracture toughness behavior, stress–strain behavior, and flaw resistance of pressureless‐sintered Si3N4‐BN ceramics are investigated. The results are discussed with respect to the reported thermal shock behavior of these composites. Although the materials behave linear‐elastic and exhibit noR‐curve behavior, their flaw resistance is different from that of other linear‐elastic materials. Whereas the critical thermal shock temperature difference (ΔTc) is enhanced by adding BN, the content of BN has no influence on the strength loss during severe th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05443.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The compressive creep behavior of single‐crystal BeO was studied under inert atmosphere. The steady‐state creep behavior was highly anisotropic because of easily activated basal slip. Crystals compressed along a [1101] axis deformed at temperatures as low as 650°C, whereas [1100]‐aligned crystals showed comparable strain rates at temperatures 1650°C. The measured activation energies and stress exponents for creep were 371 kJ/mol and 4.65 for the [1101] alignment and 496 kJ/mol and 3.39 for the [1100]alignment, respectively. The precise mechanisms controlling creep behavior were not determined. Only one semiquantitative data point was obtained for the [0001] alignment, which indicates that the creep rate at 100 MPa and 1750°C is less than that forc‐axi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05444.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Variation of the amount and the composition of the glassy phase in high‐purity Al2O3‐SiO2shell molds and the crystallization of the glassy phase have been studied. The results indicate that the two main modes of transformation from a glassy phase to a crystalline phase during firing are (1) formation of mullite through a chemical reaction and (2) devitrification of the glassy phase that comes from the original materials. The transformation is mainly influenced by theS/(A + S) value (AandSare the weight percentages of Al2O3and SiO2, respectively), the Na2O content, and the firing temperature. LowS/(A+S), low Na2O, and high firing temperature are favorable to the transformat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05445.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The mechanisms of formation and decomposition of the ferroelectric perovskite PZN [Pb(Zn1/3Nb2/3)O3] were examined. The formation of perovskite PZN from the excess molten PbO environment is characterized by an inititial rapid formation of pyrochlore phase, followed by a subsequent reaction between the intermediate pyrochlore phase, ZnO, and liquid PbO to produce perovskite phase. The pure perovskite PZN crystal prepared by the PbO flux method is thermodynamically unstable over a wide range of temperature (600° to 1400°C), yielding pyrochlore phase and PbO as the decomposition products. The decomposition reaction of perovskite PZN proceeded uniformly with a spatial homogeneity throughout the specimen. The estimated activation energy of the decomposition reaction is approximatley 18 kcal/mo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05446.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The synthesis of chemically stabilized cristobalite (CSC) by a wet‐chemical route is described. CSC is a form of silica (containing small amounts of other oxides) which is similar in many ways to β‐cristobalite, but does not invert to α‐cristobalite upon cooling. The effects of changing the dopant levels and various synthetic parameters on the phases formed in the CaO‐Al2O3‐SiO2system have been investigated. Stabilization of cristobalite with a number of different dopant combinations has been attempted, with mixed results. The use of wet‐chemical techniques has led to the synthesis of apparently phase‐pure CSC in a composition region where traditional solid‐state synthetic techniques yield a mixtur

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05447.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The modified sol‐gel method using ethylenediaminetetraacetic acid as a chelating agent to prepare high‐Tcsuperconducting Bi‐Pb‐Sr‐Ca‐Cu oxides is reported. Results revealed that the desirable Bi‐based compounds can be acquired by such a process with a high percentage of the 110 K phase and aTc(zero) of 108 K. On the other hand, a new method is proposed to quantitatively evaluate the volume fraction of the 110 K phase in Bi‐based superconducting materials, mainly composed of 85 and 110 K phases. That is a lever‐rule type of relation,V= (Y– 29.07°)/ (28.81°– 29.07°), whereVis the volume fraction of the 110 K phase andY= 2θfor the X‐ray diffraction peak emerging at about 29° with a CuKα radiation source and 28.81°Y29.07°. The volume fraction of the 2223 phase estimated in this way is more reliable than that estimated by measuring the area ratios of XRD peaks and it is expected to be even more reliable than those derived from conventional method

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05448.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Gas‐pressure sintering of α‐Si3N4was carried out at 1850 ° to 2000°C in 980‐kPa N2. The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α‐Si3N4powder was similar to that in the materials from β‐Si3N4powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α‐to‐β phase transformation. The fracture toughness of gaspressure‐sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas‐pressu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05449.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05450.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY