摘要:

Viscosity of Simple Soda‐Silicate Glasses, 500° to 1400°CComparison of the results given by English with those of Washburn, Shelton and Libman, indicates a discrepancy in the absolute values of log10viscosity amounting to 0.6, those of Washburnet al., being relatively too high. If correction for this is made, the isothermal curves of log10viscosity as a function of soda content are smooth up to 50% Na2O, showing no inflection. The observations as a function of temperatureTare all represented within accidental error by an equation of the type log10η=−A+B× 103/ (T−T0) where all three constants vary regularly with the composition.Change of Viscosity of Glass (6SiO2, 2Na2O) due to Molecular Substitution of CaO, MgO and Al2O3for Na2OThe effect is clearly brought out by plotting (from the results of English) the change of log10ndue to the substitution as a function of temperature. The curves each show a sharp bend at a temperature between 840° and 1050°C, which is designated the aggregation temperatureTa. If we divide these curves by the corresponding percentage substituted, we get curves for each oxide which are straight and parallel below the aggregation temperatures, the slopes (increase of change of log10nper 100°C) being −0.056 (CaO), −0.055 (MgO), −0.018 (Al2O3) per per cent oxide substituted. For substitution of 1/2 molecule the slopes are −0.325 (CaO), −0.23 (MgO) and −0.18 (Al2O3) per 100°. At the aggregation temperature the change of log10nper per cent is a minimum, 0.03 to 0.06 for CaO, 0.12 for MgO, 0.07 for Al2O3.Evidence of Aggregation in Glasses, from Viscosity Measurements. The sharp bends in the plots of change of log10ndue to substitution of an oxide for Na2O, suggest the beginning of molecular aggregation at these temperatures. These aggregation temperatures are close to the devitrification temperatures, but the effect on the viscosity curves cannot be due to actual devitrification since it does not change with time. Taking the aggregation temperatures as equal to devitrification temperatures, additional isotherms are roughly sketched into theequilibrium triangle of the system Na2O─CaO─SiO2.Change of Viscosity of Glass (4SiO2, 2Na2O) due to Substitution of B2O3for SiO2The change of log10n(from the results of English) is plotted as a function of temperature, and also the change of log10nper per cent B2O3. The curves are more complex than

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05536.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.tb05537.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The sintering sequence of Y2O3‐Al2O3‐doped Si3N4was investigated with respect to the relationship between densification, α→β transformation, and microstructural development. Quenching studies were performed to reveal these interactions during a complete sintering cycle. Isothermal studies were conducted to examine the sintering kinetics and compared to Kingery's liquid‐phase sintering model. The bulk density increased to ≥90% of theoretical density with only minor transformation taking place. Major transformation occurred in a late sintering stage and was accompanied by the development of elongated grains. The kinetic order of the densification process, taking into account an appropriate correction, was larger than any of the rate exponents predicted by the Kingery model, indicating that other single or mixed mechanisms

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05538.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Several positive temperature coefficient (PTC) thermistors made of barium titanate with an excess of titania and containing additives such as yttria, and eventually silica, have been prepared following two different routes. The electrical properties of the ceramic samples have been studied at room temperature, i.e., below the transition temperature, using complex impedance spectroscopy. The latter proved to be very useful to measure separately the grain and grain‐boundary resistivities which have been followed as a function of the yttrium concentration. They behave very similarly and go through a minimum for the same composition. From both electrical resistivity measurements and local chemical analysis, it is inferred that the average dopant concentration in the grains is lower than the nominal content in the starting powders. An overall interpretation is given, emphasizing the importance of liquid‐phase sinter

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05539.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A novel shear‐thickening phenomenon has been observed in superplastic silicon nitrides compression tested between 1500° and 1600°C. Liquid‐enhanced creep of SiAlONs undergoes a transition from Newtonian behavior to shear‐thickening behavior at a characteristic stress, with the strain rate sensitivity increasing from unity to around 2. The transition stress is always around 20 MPa, even though the Newtonian flow stress is very sensitive to temperature, grain size, and phase composition. Rheopexic hysteresis, manifested as a slow stress relaxation to a steady‐state value after a strain rate decrease, was also observed in the shear‐thickening regime. We attribute the cause for shear thickening to a repulsive force between initially wetted SiAlON grains, which form a “dry” and “rigid” bridge in between when pressed above a characteristic stress, possibly due to the contact of the residue Stern layers on the opposing grain/liquid interfaces. A micromechanical model, which takes into account the stress variation among differently oriented grain boundaries, has been formulated to assess the effect of “rigid” grain boundaries. A continual stochastic rearrangement of grain configurations and a relatively thick Stern layer are suggested as the necessary prerequisites for shear thickening i

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05540.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The microstructure of silicon carbide whiskers synthesized by carbothermal reduction of silicon nitride has been studied using transmission electron microscopy. All of the whiskers examined are single crystals, and grow in the (111) crystallographic direction. Two different forms of stacking faults and microtwins were observed; in one the planar defects are normal to the whisker growth direction, and the other has the defect planes at an angle of about 70° to the growth axis, while both forms of the defects are on the [111] closed‐packed planes. Without the addition of catalyst, droplets containing metallic impurities were not found at the tips of the whiskers synthesized by the present process. A core and outer regions were observed in the single‐crystal whiskers, which may be evidence that the whiskers were formed by a two‐stage mec

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05541.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Mullite, an extremely creep‐resistant ceramic, has been fabricated using a novel processing/forming approach taking advantage of superplastic transitional phases. Starting with a mixture of alumina, silica, and a small amount of lithia additive (0.8 wt%), a processing window of about 50°C around 1350°C has been found within which the material can be densified and superplastically deformed with negligible mullitization. The lithia additive promotes a transient lithium aluminosilicate glassy phase that greatly enhances sintering and deformation. The superplastic premullite maintains a nearly constant grain size during deformation between 1250° and 1400°C, over a strain rate from 6 × 10−7to 10−1s−1, and has unusually high activation energy values in the range of 1150 to 2086 kJ/mol. An increase in the transient glassy phase content due to the increased matrix dissolution at higher temperatures contributes in part to this anomaly. The mullite work pieces thus shaped become creep resistant again after a postforming annealing/mullitization treatment which decreases the creep rate by 6 orders of magnitude. The mechanical properties (hardness, toughness, and strength) of the finished mullite are compared to those of conventionally proce

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05542.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Changes in lattice parameter in high‐purity polycrystalline (2Hδ) aluminum nitride due to oxygen contamination have been examined by convergent beam electron diffraction. The operative lattice distortion was determined by computer simulation to be a continuous increase inc/aratio commensurate with constant volume as impurity concentration increases and a fine break in hexagonal symmetry. These results are consistent with the distortions associated with planar polytypoid structures at higher oxygen leve

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05543.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Continuous α‐Fe2O3films grown on bulk (0001)Al2O2substrates by low‐pressure chemical vapor deposition have been studied by transmission electron microscopy and the observations compared to those obtained from discontinuous films at an earlier stage of the growth process. Plan‐view specimens revealed significant thermal stress in the continuous films, while cross‐sectional specimens showed that cracking occurs in thicker films. The free surface of the film and the film/substrate interface appeared sharp and flat, apart from growth ledges and steps. Weak‐beam imaging revealed a hexagonal misfit dislocation network consisting of perfect edge dislocations. Fine structure in the selected‐area diffraction patterns which corroborates these observations is also discussed. The misfit network of partial dislocations previously observed in the discontinuous films was not observed for the continuous films, indicating an effect of film thickness, growth rate, or surface preparation on the Fe2O3/(0001)Al2O3interfa

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05544.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

High‐strength bioactive ceramics, MgO‐CaO‐SiO2‐P2O5glass‐ceramic composites toughened by zirconia, were prepared by sinter‐hot isostatic pressing (sinter‐HIPing) to achieve easier mass production and higher reliability. Raw materials for preparing a dense presintered body were investigated to obtain almost complete densification by sinter‐HIPing. It was found that the densification of a presintered body was influenced by residual glassy phases in the crystallized glass particles. By using a controlled crystallized glass powder and a zirconia powder as raw materials, a presintered body with a relative density higher than 94% was prepared, and then it was densified to near its theoretical density by sinter‐HIPing. This bioceramic exhibited an extremely high bending strength of 400 to 1000 MPa and fracture toughness of 3 to 5 MPa.m1/2for 30 to 80

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05545.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY