摘要:

Raman spectroscopy has been used to examine the connection between room‐temperature creep and stress‐induced phase transformations in TS‐Grade Mg‐PSZ. The in‐situ phase measurements obtained show that the tetragonal phase transforms to both the orthorhombic and monoclinic structures under stress. The effect of precipitate orientation is indicated by the variation of creep strain with monoclinic volume

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04227.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Zeolite formation in Class F fly ash blended cement pastes is under investigation. A Na–P type zeolite and Zeolite Y were synthesized from Class F fly ash and NaOH solution after 2 days of aging at room temperature and 6 days of curing at 80°C. However, no zeolites formed when KOH was used. In additional experiments, a Na–P type zeolite, Zeolite Y, and chabazite developed in cement pastes blended with Class F fly ash and NaOH solution which had been aged 2 days at room temperature and then cured 6 days at temperatures ranging from room temperature to 90°C. Seeding the pastes with natural zeolites was also investi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04228.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Microencapsulation of ceramic powders using metalloorganic stearate soaps was investigated as an economical means to increase solid‐state reactivity of multicomponent mixtures. The specific system investigated was lead magnesium niobate (PMN); however, the process may be applicable to a wide range of other compositions. The physical and chemical characteristics of the unfired powder mixtures and reactivity during subsequent calcination were studied as a function of batch composition and mixing method. Batch composition was varied by molar substitution of magnesium stearate for magnesium carbonate. Mixing method was investigated by comparing a dry‐mixing technique developed for particle coating (mechanofusion) with conventional wet ball milling. Both mixing processes resulted in surface coating of the ceramic particles by the stearate soap. In addition, the mechanofusion process produced densely packed spherical granules of coated particles (multicored microcapsules) in the 50‐ to 200‐μm range. Solid‐state reactivity was measured in terms of perovskite phase yield, increased yields being indicative of a more reactive mixture. The highest perovskite yields (95 to 98 vol%) were achieved at 100 mol% substitution of magnesium stearate for magnesium carbonate, independent of mixing method. However, when magnesium stearate was only partially substituted for magnesium carbonate, the mechanofusion process produced consistently higher perovskite yields than did ball milling. Compared to conventional mixed‐oxide processing, the increased reactivity of the microencapsulated mixtures can be attributed to higher chemical activity of the metallo‐organic precursor, finer scale of mixing achieved by particle coating, and a further reduction in segregation scale due to the dense intragranule packing of multicore

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04229.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Methods of controlling the size and morphology of particles precipitated from the hydrolysis and condensation of tetraethyl orthotitanate (TEOT) in aqueous ethanol solutions through the addition of electrolytes are described. The precipitates display great sensitivity to agitation where, above a critical shear rate, agglomerated morphologies are observed. For reactions carried out at shear rates less than the critical value, narrow size distribution, spherical titania particles are formed under conditions where the particles have electrophoretic mobilities greater than 0.2 × 10−8m2· V−1· s−1for 0.05M<[TEOT]<0.2Mand 0.15M

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04230.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Laser‐induced internal cracks in LiF single crystals were annealed and healed at various temperatures in the absence of environmental contamination. The thermal healing process consists of three distinct stages: (a) pinching off of the crack into radially directed cylindrical pores within the rim of the crack front and within inner regions close to 〈110〉 directions, (b) ovulation of the 〈110〉 pores into rows of isolated, square‐shaped pores and, simultaneously, regression of the plate‐shaped subcracks situated near 〈100〉 directions, and (c) shrinkage of the isolated pores. A modified Arrhenius relationship between the rate of subcrack shrinkage and the annealing temperature was obtained. The apparent activation energy was found to be 2.0 eV (192.7 kJ/mol), suggesting that the healing process is controlled by

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04231.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The reaction between SiC and SiO2has been studied in the temperature range 1400–1600 K. A Knudsen cell in conjunction with a vacuum microbalance and a high‐temperature mass spectrometer was used for this study. Two systems were studied—1:1 SiC (2 wt% excess carbon) and SiO2; and 1:1:1 SiC, carbon, and SiO2. In both cases the excess carbon forms additional SiC within the Knudsen cell and adjusts to the direct reaction of stoichiometric SiC and SiO2to form SiO(g) and CO(g) in approximately a 3:1 ratio. These results are interpreted in terms of the SiC‐O stability

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04232.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Grain boundary sliding (GBS) is thought to be the principal driving force for the nucleation, growth, and coalescence of grain boundary cavities during compressive creep of polycrystalline ceramics. It has been shown theoretically that stochastic GBS gives rise to continuous cavity nucleation and transient cavity growth and coalescence, eventually leading to crack formation and failure. This paper will show through experimental measurements, using stereoimaging techniques, that GBS is in fact stochastic. Also, mode II GBS, in‐plane grain rotation, and in‐grain shear displacements, strains, and strain rate measurements during creep of Lucalox Al2O3will be presented. These displacements, measured on a machine vision system, will be presented in terms of the surrounding microstructural constraint and their lack of angular relation to the compressive load a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04233.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

It has been shown that the monoclinicβ‐phase of dicalcium silicate (Ca2SiO4) can be stabilized against transformation to the orthorhombicγ‐phase by physical rather than chemical factors. Stabilization was studied in different types of microstructures fabricated under various processing conditions such as different powder or grain sizes, chemical additives, cooling kinetics, or high‐temperature annealing treatments. The observations can be explained in terms of a critical particle size effect controlling nucleation of the transformation. Rapid quenching through the high‐temperature hexagonal (α) to orthorhombic (a′H) transformation at 1425°C, which is accompanied by a −4.7% volume decrease, causes periodic fracture ofβ‐twins due to accumulated strains. Chemical doping with K2O or Al2O3promotes the formation of amorphous phases which mold themselves aroundβ‐Ca2SiO4grains. Annealing treatments cause crystallization of the glass and subsequent transfo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04234.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The effects of processing parameters on the microstructure and mechanical behavior of a SiC‐fiber‐reinforced ZrTiO4matrix composite were evaluated through a controlled study. The microstructure was analyzed using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Mechanical behavior was characterized by strength and toughness measurements, which were correlated with the microstructure of the composite. The optimized processing schedule developed included incorporation of CO‐heat‐treated BN‐coated fibers, composite calcination at 530°C, and consolidation via hot‐pressing at 1270°C and 17.25 MPa applied pressure in an atmosphere of CO at an overpressure of 111.5 kPa (1.1 atm). Use of this processing schedule improved in situ fiber strength and modified the fiber/matrix interfacial microstructure to ameliorate its sliding and debonding resistance, leading to a composite with average strength over 1 GPa and a toughness

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04235.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Ceramic varistors based on ZnO with lead zinc borosilicate glass instead of Bi2O3were prepared. The effect of sintering conditions on the electrical properties was studied by sintering samples at various temperatures and cooling them at different rates. The sample sintered at 1250°C for 1 h, then furnace cooled, possessed the best electrical properties, as characterized by the highest nonlinear coefficient, lowest leakage current, and lowest degradation. The microstructure and crystal structure of the glass phase of ZnO–glass varistors were examined by means of scanning electron microscopy, transmission electron microscopy, and powder X‐ray diffractometry. The glass phase was originally amorphous, but crystallized as an intergranular layer in the sintered and furnace‐cooled samples. This crystallized phase was a zinc borate phase (5ZnO·2B2O3), which was identified by X‐ray diffractometry, transmission electron microscopy, and Auger electron spectroscopy. The zinc borate phase at the grain boundary of ZnO–glass samples enhanced the nonohmic characteristics of the cerami

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04236.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY