Oxidation behaviour of Β-sialon fabricated from α-Si3N4 and aluminium iso-propoxide

-sialon with z=0.5 was fabricated by hot pressing of a spray-dried mixture of -Si₃N₄ and aluminium iso-propoxide solution. The oxidation behaviour of this -sialon was investigated, comparing it with commercial -sialon containing Y₂O₃ as a sintering aid. Oxidation tests were carried out at 1200 and 1400C for 25 to 200 h in air. The oxide layer of aluminium isopropoxide-derived -sialon was thin, dense, smooth and homogeneous without bubbles and cracks. The strength after oxidation at 1400C for 200 h was about 800 MN m^–2, almost the same value as before oxidation. The oxide layer of Y₂O₃-doped -sialon was thick and inhomogeneous, containing many bubbles, cracks and grown needle-like crystallites (Y₂Si₂O₇). The strength after oxidation at 1200C for 200 h fell to 1/2(440 MN m^–2) because of pit formation in the oxide layer, and at 1400C for 200 h fell to 1/4(200 MN m^–2) because of severe swelling and flaking of the oxide layer. The high oxidation resistance of aluminium iso-propoxide derived -sialon was mainly due to its homogeneous microstructure and freedom from foreign constituents such as Y₂O₃.     

Accuracy of temperature measurements in determining the thermal conductivity of substances by stationary methods in the range of moderate temperatures

An empirical comparison is made of the accuracy of platinum-rhodium-platinum and Chromel-Alumel thermocouples in determining the thermal conductivity of substances.Notation T, t temperature - temperature difference - y thermocouple readings - A_i parameters of approximating equation - sensitivity of thermocouple - sensitivity found from the generalized function - _st standard values of sensitivity - S₀ standard deviation of sensitivity for a given series - maximum deviation of from in different series - I, II indices indicating that the values pertain to platinum-rhodium —platinum and Chromel —Alumel thermocouples, respectively Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 2, pp. 306–310, August, 1980.     

α-β phase transformation of silicon nitride — computer simulation

The simulation model of - silicon nitride phase transformation was developed for the case when the crystallization of the phase is the rate controlling step. The results gained on the base of the present model indicate that the temperature and total free surface area of silicon nitride phase present in the firing body limit the rate of transformation and total amount of transformed silicon nitride phase. These results are acceptable from the point of view of experimental experience and support the applicability of the presented model.     

Investigation of the backscatter coefficient of Β radiations from an aluminum backing in a 2π geometry

Conclusions It follows from the investigation of backscatter of radiations (at maximum energies of 10–290 fJ, i.e., of 0.155–1.711 MeV) from an aluminum backing with a 2 geometry that for weightless samples the intensity of the saturation backscatter is virtually independent of the -radiation energy, whereas for weightable samples it increases with a rising energy of particles.The coefficients thus obtained for -radiators with a finite active-layer thickness, which include the effects of self-scatter and self-absorption of output radiation over an angle of 2, can be used for approximate computation of the activity of radiators with a known active-layer thickness from the measured external output of -particles (and vice versa).     

Development of microstructure during the fabrication of Si3N4 by nitridation and pressureless sintering of Si:Si3N4 compacts

The technique for the fabrication of Si₃N₄ which was investigated involves the nitridation of Si:Si₃N₄ powder compacts containing additions of sintering aids (e.g. Y₂O₃ and Al₂O₃) followed by pressureless sintering. The development of microstructure during fabrication by this method has been followed by X-ray diffraction and analytical electron microscopy. As well as being important for the sintering process, it was found that the sintering aids promote nitridation through reaction with the surface silica on the powder particles. During nitridation extremely fine grained Si₃N₄ forms at silicon powder particle surfaces and at tunnel walls extending into the interior of these powder particles. Secondary crystalline phases which form during nitridation are eliminated from the microstructure during sintering. The- to-Si₃N₄ phase transformation is completed early in the sintering process, but despite this the fully sintered product contains fine-Si₃N₄ grains. The grains are surrounded by a thin intergranular amorphous film.     

The effect of silicon on the structure and mechanical properties of an α+Β titanium alloy

Titanium 4 wt% Al-4 wt% Mo-2 wt% Sn containing 0, 0.25 and 0.5 wt% Si has been solution-treated in the + phase field at 900 C. The microstructures obtained at room temperature after cooling from 900 C at various rates have been determined using transmission electron microscopy and the partitioning of the elements between the phases has been established using X-ray energy dispersive analysis on the thin foils. The degree of partitioning increases with decreasing cooling rate: aluminium partitions to the -phase, molybdenum and silicon to the -phase and tin remains uniformly distributed. Silicon is found to inhibit the partitioning of molybdenum: this has a profound effect on the stability of the -phase and the resultant microstructure. In quenched material containing transformed , substantial age hardening can be obtained in the range 350 to 600 C and is associated with precipitation within the orthorhombic martensite phase, possibly occurring via a spinodal mechanism. Silicon has little effect on the microstructure of air-cooled samples but contributes to high-temperature strength via dynamic strain ageing.     

Phase transformation and densification during pressureless sintering of Si3N4 with MgO and Y3Al5O12 additives

The - transformation of Si₃N₄ during liquid-phase sintering appears to be controlled by the growth of the -Si₃N₄ grains in the direction perpendicular to thec-axis in the case of MgO additive. The diffusion through the liquid is the rate-controlling step in the case of the Y₄Al₅O₁₂ additive. The density of the sintered body at the solid skeleton stage was influenced by the change in the - transformation rate and/or by a change of the transformation mechanism. The indirect proportionality between the -phase content in the starting powder and the density at the solid skeleton stage was found. The microstructure of the sintered body is influenced by both the -phase content in the starting powder and the chemical composition of the additive. Fine, uniform microstructure with a high aspect ratio of -grains is obtained, when the -phase content in the starting powder is as small as possible and when the - transformation is controlled by grain growth.     

Sintering process of Y2O3 and Al2O3-doped Si3N4

The sintering process of Y₂O₃- and Al₂O₃-doped Si₃N₄ has been investigated by dilatometry and microstructural observations. The densification progressed through three processes. The bulk density increased to 85% theoretical without the formation of -Si₃N₄ in the initial process. The densification once terminated after the second process. The / transformation of Si₃N₄ and the related formation of prismatic grains reduced the densification rate in the second process, although the grain size and the aspect ratio were very small. The final process was the densification of -Si₃N₄, where the fibrous grains grew remarkably. The kinetic order for the densification of -Si₃N₄ indicated a diffusion-rate controlling mechanism with the activation energy of 244 kJ mol^–1 (<1450 ° C) and 193 kJ mol^–1 (>1450 ° C). The influence of heating rate on the grain growth was characterized by a parameter derived from kinetic parameters. The relationships between grain growth and densification behaviour have also been discussed.     

Dielectrical properties of u.v. cured acrylated polysiloxane films

The dielectric properties of films obtained through u.v. curing of an acrylated polysiloxane resin were evaluated in the temperature range from –136 to 24C. Different relaxation processes (, , , ), characterized by decreasing dielectric relaxation intensities, were evidenced and quantitatively analysed: the process was attributed to the glass transition process on the basis of the dynamic mechanical spectrum of the film and of its behaviour at different temperatures. The other processes (, , ), weaker than the process, could be attributed to local dipolar motions present, even at low temperatures, in the polymer network.     

Densification of zirconia-containing sialon composites by Sm2O3

Sm₂O₃ was used as an additive for ZrO₂/O-sialon composites and showed remarkably good densification behaviour on sintering. Sm₂O₃ appeared to behave as a densification rather than a stabilization additive and the low eutectic temperature in this system facilitated densification before zircon dissociation. An important advantage is that the final sintering temperature is then determined by the minimum temperature at which full conversion from -Si₃N₄ to O-sialon can be achieved. At this temperature (1500–1550 °C), grain growth of zirconia is minimal, and this allows control of small grain size for the zirconia particles.     

Surface nucleation and cellular growth kinetics of cordierite glass ceramics containing 3 mol % Y2O3-ZrO2

Cordierite-based glass ceramics of the 2MgO2Al₂O₃5SiO₂ composition with t-ZrO₂ (3 mol% Y₂O₃-ZrO₂) and P₂O₅ addition, was heat-treated isothermally and isochronically for crystallization studies. Major crystalline phases incurred by the heat treatment were t-ZrO₂ and -cordierite. Surface nucleation predominated when edge and corner nucleation in these samples were suppressed regardless of their radii of curvature. Crystallization began with the formation of -quartz S.S. and its transformation to -cordierite was followed by prolonged heating. Cellular growth of -cordierite on the surface of the quenched glass plates, gave a linear kinetics. The activation energy for cellular growth was 410 kJ mol^–1.     

Structure and Magnetism of the Composite Crystal Ca0.824CuO2

We have studied the magnetic state from a viewpoint of crystallographic features of the 1-D chain compound Ca_0.824CuO₂. A possible spin-hole arrangement in the magnetically coexisting state was determined by analyzing the local structural distortion in the CuO₂ chain by means of a modulated-crystal-structure analysis. The essential periodic sequence expected is (: up- and down-spin, : hole), which can be regarded as a kind of spin-1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain.     

Effect of selected impurities on the high temperature mechanical properties of hot-pressed silicon nitride

The effect of impurities on the high temperature mechanical properties of hot-pressed silicon nitride has been determined. Selected impurity additions were made to both relatively pure -phase and -phase silicon nitride starting powders. These powders were hot-pressed to full density using 5 wt % MgO as the pressive additive. The silicon nitride hot-pressed from the -phase powder exhibited higher strength at both 25 and 1400 C than that fabricated from the -phase powder. The impurity additions had no effect on the room temperature mechanical properties. The CaO additions had the most significant effect on the high temperature mechanical properties. In both the material hot-pressed from the -phase and -phase powders, increasing CaO additions severely reduced the high temperature strength and increased the amount of non-elastic deformation observed prior to fracture. Although alkali additions (Na₂CO₃, Li₂CO₃, K₂CO₃) also tended to have the same effects as the CaO, the high volatility of these compounds resulted in a much reduced concentration in the hot-pressed material, thus minimizing somewhat their tendency to enhance the high temperature strength degradation. The Fe₂O₃ and Al₂O₃ had no apparent effect on the high temperature mechanical properties.     

Thermodynamic properties of the superconductorsBaPb 0.7 Bi 0.3 O 3 andBa 0.7 K 0.3 BiO 3 using Eliashberg theory

We present results of a thermodynamic analysis for the superconductors compounds BaPb_0.7Bi_0.3O₃ and Ba_0.7K_0.3BiO₃. The physical quantities are calculated making use of the Eliashberg theory and the electron-phonon spectra ²()F() as calculated by Shirai et al. For the superconductor BaPb_0.7Bi_0.3O₃, several models of the ²()F() were studied looking for a better agreement with experimental data. The best fit is achieved with a simple constant scaling (C = 1.25) of the Shirai's spectra. The functional derivative of the deviation function D(t) with respect to changes in ²()F() is also calculated.     

The setting of gypsum plaster

The - and -forms of calcium sulphate hemihydrate, CaSO₄·1/2H₂O, were characterized using thermal, X-ray diffraction, optical and electron microscopy techniques. The differences between the two forms arose from their mode of production, resulting in different crystal size and habit, and not from different crystal structures. The hydration of CaSO₄·1/2H₂O to CaSO₄·2H₂O was investigated using calorimetry, phase analysis and the changes in electrical resistance of the plaster slurries. The reaction occurred in three steps: dissolution, nucleation and precipitation, and finally completion due to depletion of reactants. The differences in reactivity of the - and -forms with water again appeared to be due to their different physical states.     

Effect of BaO and SiO2 addition on PTCR BaTiO3 ceramics

The influence of Ba-excess and liquid phase sintering with SiO₂ on the electrical conduction and microstructure in PTCR BaTiO₃ has been investigated. Dense (95–96%), small grain (5–10 m) PTCR materials were obtained in Ba-excess (Ba/Ti=1.006) BaTiO₃. The materials exhibit low room temperature resistivity _RT (10⁰–10² ·cm) and high PTCR respond (more than 5 orders). Solid state sintering was found to inhabit the semiconducting and PTCR behavior in Ba-excess materials. Liquid phase sintering, using SiO₂ in the Ba-excess BaTiO₃, resulted in low _RT and significant PTCR response. Through domain observation, interior Polaron deficient zones were found in samples which exhibit limited liquid phase sintering, leading to non-uniform directional domains and low charge carrier mobility. Proper control of the SiO₂ concentration was found critical for obtaining uniform directional domain microstructures and low _RT.     

Investigation of lithium,magnesium-aluminosilicate solid solutions crystallized in some cordierite-zircon ceramics containing lithium

The crystallization of Li,Mg-aluminosilicate solid solutions in lithium-bearing zircon and/or cordierite-based ceramics is studied as a function of firing temperature up to 1500 C. X-ray diffraction, electron microprobe analysis and microscopic techniques were used for the identification of the studied phases. Keatite solid solution (ss) coexists with zircon and/or cordierite at temperatures up to 1300 C. On raising the temperature to 1400–1500 C,-quartz ss is crystallized at the expense of both keatite and cordierite with the precipitation of spinel on cooling from such high temperatures. In bodies containing zircon, the dissociation of zircon at 1400–1500 C into its constituents epitaxially enhances the nucleation of-quartz ss crystallization.     

Microwave and conventional sintering of rapidly solidified Al2O3-ZrO2 powders

The Al₂O₃-ZrO₂ eutectic composition was rapidly solidified, forming amorphous and crystalline structures. The as-quenched material was crushed and pressed into pellets which were sintered conventionally or with microwaves. Conventional and microwave sintering at temperatures up to 1600 °C resulted in a microstructure where 100–200 nm ZrO₂ grains were present intergranularly in the -Al₂O₃ grains. Larger ZrO₂ grains (1 m) were found intergranularly. The as-quenched lamellar structure spheroidized during sintering at high temperatures. Boron contamination of the powders resulted in more homogeneous and dense as-fired samples but promoted the ZrO₂ tetragonal-to-monoclinic transformation, which was attributed to increased grain boundary diffusivity. Conventional sintering at low temperatures resulted in the formation of rods of an Al₂O₃-rich phase which grew from a low-melting B₂O₃-rich liquid.     

Surface layers in polycrystalline sodium \-alumina

X-ray powder diffractometry using CrK radiation has revealed the existence of surface layers 5m deep in samples of high-density polycrystalline sodium -alumina. The surface layer is prominent by virtue of its crystal lattice distortion in which the c-axis is elongated by 1%. Its presence is due to ion exchange between Na⁺ and H₃O⁺ that occurs on exposure to air.     

Phase transformation and microstructural changes of Si3N4 during sintering

Changes of density, the - phase transformation, and composition of grains and grain boundaries during sintering of Si₃N₄ with various sintering conditions using additives of Y₂O₃ and Al₂O₃ were investigated. The phase determination of individual Si₃N₄ grains was performed by convergent beam electron diffraction. The relations between densification and transformation were divided into two groups, depending on the additive compositions. Aluminium dissolution into Si₃N₄ grains occurred mostly during - transformation process. The concentrations of aluminium and oxygen in the grain boundaries decreased as the - transformation progressed.