摘要:

Sintering of a fine‐grained polycrystalline diamond compact with grains less than 1 μm in size was successfully carried out by making a laminate on a WC/Co powder compact under sintering conditions of 5.8 GPa and 1430° to 1480°C for 30 min, in which small amounts of poly(ethylene glycol) and fine powder of cubic boron nitride were added to the starting diamond powder. The former played the role of preventing agglomeration of the diamond powder and the latter of suppressing abnormal grain growth during sintering. Microstructural observation of the polycrystalline diamond showed that in the regions near the WC/Co layer a comparatively large amount of Co metal was present between diamond grains, but in other regions the amount of Co decreased, and the diamond grains were seen to be bonded strongly. The Vickers hardness of the polycrystalline diamond was 55 ± 5 GPa with 19.6 N

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07288.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The fracture toughness behavior during crack growth (KR‐curve behavior) of duplex ceramics is investigated. Different types ofKR‐curves can be distinguished depending on the microstructural designs of these materials which are characterized by the volume fraction and size of the dispersed pressure zones, and by their effective volume expansion. According to theirKR‐curve behavior, duplex ceramics can be subdivided into two groups consisting of “short‐range” and “long‐range” toughened materials. The experimental results are discussed regarding the appearance of different toughening mechanisms which are documented by crack path micrographs. An unusual toughening effect, a “crackbranching chain reaction,” is documented by in situ observations. The critical stress to nucleate the observed process zone development is calculated and compared with the internal stress intensity factorKiwhich has been previously proposed for these materials and with

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07289.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The thermal‐stress fracture behavior of duplex ceramics is investigated by quenching in water and in oil. Comparison with the matrix materials shows that the critical quenching temperature difference, ΔTc, is not or is only slightly reduced, even for duplex ceramics of significantly reduced strength. In sintered composites, thermal‐stress‐induced microcracking within pressure zones and crack initiation at pressure zone–matrix interfacial defects develop before ΔTcis reached. The effects are accompanied by a gradual reduction in strength. At ΔTc, critical crack propagation occurs. The retained strength after thermal shock of duplex ceramics is significantly improved compared with the respective matrix materials. This behavior can be related reasonably well with theKR‐cu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07290.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

A model involving ionic double layers at the surface has been constructed for magnesia‐doped sapphire based on earlier models which were developed for cubic halides. The model takes into account the presence of electrostatic potentials, isolated point defects, defect complexes, and special surface sites which can act as sources and sinks for ions. Equations have been set up for the various defect concentrations, and Poisson's equation has been solved numerically to give depth profiles for defects and corresponding electric fields. The calculations suggest that Mg2+ions can segregate both to the free surface and to the space charge region. The effective (or Langmuir) enthalpy of segregation depends not only on the actual binding energies of the dopant ion, but also on other parameters such as the density of special surface sites. Over the temperature range studied, the variation of the calculated surface magnesium concentration with temperature is found to be approximately Arrhenius in nature, as was observed in segregation experiment

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07291.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Conditions for carbothermal synthesis of α‐Si3N4are presented with special emphasis on the reaction temperature, C:SiO2ratio, and precursor mixing. With pure precursors, the conversion temperature is 1500° to 1550°C. An excess of C is necessary for complete conversion, and a simple sol–gelmixing technique provides excellent intermixing of the precursors. Copious flow of N2gas throughout the reactor bed is essential if pure Si3N4is to be produced; small concentrations of CO and O2promote SiC and Si2N2O, respec

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07292.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Pure, single‐phase stoichiometric Pb(Fe0.5Nb0.5)O3(PFN) powders were successfully formed by molten salt synthesis using mixtures of NaCl and KCl salts. Lower temperatures and shorter times (1/2 h at 800°C) were needed for singlephase PFN formation from molten salts relative to those required for solid‐phase methods (4 h at 1000°C). A systematic study indicating the effects of process parameters, such as temperature, time, and amount of flux with respect to starting oxides, on the PFN formation mechanism and its resulting powder characteristics is reported. The particle size increased with increasing synthesis temperature; the rate of increase was greatest above 900°C, which is close to the melting point of PbO. PFN powders formed by molten salt synthesis are spheroidal, free from aggregates, and sintered to good densities at temperatures as low as

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07293.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The formation mechanisms, phase stability, impurity incorporation, and morphological characteristics of lead niobates obtained by molten salt synthesis were studied by X‐ray diffraction analysis, scanning electron microscopy, and energy dispersive analysis. Results indicate that PbO is the fastdissolving component of the constituent oxides and that the formation of lead niobates initiates at the surface of Nb2O5powders. Reaction rates were faster in molten salts when compared to those of solid‐state synthesis. Significant morphological differences were observed between the powders prepared by both solid‐state and molten salt synthesis. At low PbO/Nb2O5molar ratios, reaction between starting powders and molten salts resulted in the formation of NaNbO3, along with potassium incorporation into lead niobates. Orthorhombic PbNb2O6, monoclinic Pb2Nb2O7, and fcc Pb3Nb3O8phases were stabilized using molten salt synthesis, while only rhombohedral PbNb2O6and Pb2Nb2O7, and tetragonal Pb3Nb2O8, were obtained by solid‐state synthesis. Molten salt compositions containing PbO/Nb2O5ratios of 2 and 3 resulted in the stabilization of phases different from those obtained using solid‐state synthesis. For a PbO/Nb2O5ratio equal to 3, Pb5Nb4O13observed in solidstate synthesis was not found in the products of molten salt

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07294.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The failure probability of ceramic components in multiaxial stress states can be predicted from the results of uniaxial tests, if a suitable fracture criterion for multiaxial loading is known. In the paper it is investigated how the selected failure criterion influences the predicted distribution of the fracture stress of a component. Several acceptable failure criteria are found by comparing the results of four‐point‐bend tests performed with aluminum nitride with the results of concentric ring‐on‐rin

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07295.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Preparation of potassium niobate from metal alkoxides was investigated. Potassium‐niobium ethoxide, KNb(OC2H5)6, and potassium‐niobium propoxide, KNb(OC3H7)6, were synthesized and subsequently hydrolyzed using several water concentrations (0.75 to 6.0 mol of water/(mol of alkoxide)). Rapid precipitation of potassium‐deficient particles occurred when higher concentrations of water were used. This resulted in the formation of a multiphase material after calcination, as X‐ray diffraction showed the presence of both KNbO3and potassium‐deficient oxide phases(s). Single‐phase KNbO3could be prepared by two methods: (1) hydrolysis of KNb(OC3H7)6/propanol solutions using 1 mol of water (per mole of propoxide) added as a water/propanol solution and (2) hydrolysis of KNb(OC2H5)6/ethanol solutions using 1 mol of water (per mole of ethoxide) added as a water/methanol solution. The latter method provided advantages of low calcination temperature for the formation of single‐phase KNbO3and low weight loss afte

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07296.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The crystallization kinetics of a 40Li2O · 60SiO2(mol%) glass were studied using DTA and both isothermal and nonisothermal DSC techniques as a function of (1) particle size of the glass powder, (2) crucible material, e.g., alumina or platinum, (3) furnace atmosphere, e.g., nitrogen, oxygen, or argon, and (4) surface pretreatment of the glass powder with deionized water, HCl, or HF. The kinetic parameters for crystallization, namely, the activation energy,E, frequency factor,v, and Avrami exponent,n, for a particular sample particle size are in close agreement for all three techniques.Eandvboth decrease whilenincreases (from 0.9 to 3.0) as the particle size of the glass powder increases from 40 to ∼250 μm, whereupon these parameters become fairly constant for larger particles. Neither the furnace atmosphere nor crucible material had a significant effect onE,v, orn.Washing the glass with deionized water, HCl, or HF before the measurements decreased the crystallization temperature by 25° to 30°C.Eandvremain unchanged by this treatment, butndecreased from 3 to 1.6 for a sample of particle size

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb07297.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY