Liquid-phase sintering of chemically unstable silicon carbide-lithium/magnesium aluminosilicate-titania composites

The fully dense 55 wt % SiC-45 wt % (LAS/MAS/TiO₂) composite consisting of SiC filler particles and glassy matrix has been prepared by liquid-phase sintering in the presence of carbon, using the heating rate of 800C min^–1, the maximum sintering temperature 1600C and the nitrogen overpressure of 8×10^–5 Pa applied at a maximum particle mobility stage. The total liquid-phase sintering time did not exceed 4 min. The bloating effect was always observed in the carbon-free atmosphere during liquid-phase sintering.     

Effects of heat treatments on the creep properties of a hot pressed silicon nitride ceramic

Effects of heat treatment in an argon atmosphere at high temperatures for varying times on the creep properties of a Y₂O₃-Al₂O₃ (8-2 wt%) doped hot pressed silicon nitride (HPSN) ceramic were investigated. It was observed from the creep measurements that higher temperature, i.e. 1360C, and longer time, i.e. 8 h, heat treatment in an argon atmosphere improved the creep properties, (e.g. secondary creep rate) of this material. Heat treatment at a lower temperature of 1300C and for a shorter time of 4 h did not change the creep behaviour. Improvement of the creep properties was related to the crystallization of an amorphous grain boundary phase by heat treatment. Secondary creep rate parameters of the as-received material: stress exponent, n (2.95–3.08) and activation energy, Q (634–818 kJ mol^S–1), were in the range of values found by other investigators for various hot pressed silicon nitride ceramics.     

Fabrication and properties of alumino-borosilicate glass-ceramic systems

In current microelectronics packaging applications, low-temperature fired substrates with low dielectric constant are required. Formulations of SiO₂, B₂O₃, Al₂O₃, and CaO have been used as substrate materials which can be sintered as low as 1000C in air. The electrical behaviour, thermal expansion coefficient, and mechanical property of the fabricated substrate materials are evaluated. The as-sintered substrates possess the following characteristics: low dielectric constant of 4–5 at 1 MHz; a loss factor smaller than 0.2% at 1 MHz; and a thermal expansion of 3.57 × 10^–6C^–1 which is very close to that of silicon (3.5 × 10^–6C^–1).     

Fracture toughness of concretes at high temperature

The fracture toughness of ordinary and refractory concretes in the range of 20–1300C was investigated, and the stress intensity factor, K _Ic, on three-point bent specimens (according to ASTM E-399 recommendation) determined. With an increase in testing temperature, the stress intensity factor decreases for both concretes. The values of K _Ic at 20C for both concretes are comparable, being equal to 0.64 MNm^–3/2 for ordinary concrete, and 0.72 MNm^–3/2 for refractory concrete, respectively. At 1100C, K _Ic has a value of 0.043 MNm^–3/2 for ordinary concrete, and for the refractory concrete at 1300C, K _Ic=0.34 MNm^–3/2. The method presented for predicting the behaviour of concrete at high temperature may be used in engineering practice.     

Temperature dependence of the piezoelectric,mechanical, and dielectric characteristics of rubber-elastic piezoelectric composites

New rubber-elastic piezoelectric composite materials are investigated by a resonance technique in the temperature range from –60C to +80C. A slight change in the bulk piezoelectric parameters is noted in the interval 5–70C g _v = 60–75 mVm/N). A theoretical physical model is proposed for calculating the piezoelectric characteristics. A piezoelectric cable and large-area piezoelectric sheets have been developed using the new materials.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 54–57, June, 1995.     

Stress relaxation in zirconium carbide. Report 2. Mechanisms of stress relaxation. The relationship of the processes of creep and relaxation

Mechanisms of stress relaxation in ZrC_1.00 (d=6–35 .m) in the area of the ductile-to-brittle transition temperature (1600–2200C) are considered. It is established that in the 1800–2000C range for ZrC_1.00 with grain sizes of 6 and 14–35 m, respectively, stress relaxation occurs as the result of pure grain boundary slip while at higher temperatures by intragranular cross slip. It is shown that creep in the steady stage and stress relaxation in ZrC_1.00 with grain sizes of 6–35 m are controlled by different physical processes, which makes impossible obtaining for these materials of data on stress relaxation by conversion with use of information on steady creep.Translated from Problemy Prochnosti, No. 2, pp. 55–60, February, 1994.     

The kinetics of hot-pressing for undoped and donor-doped BaTiO3 ceramics

The kinetics of hot-pressing for undoped and donor-doped BaTiO₃ of high purity have been studied at 1050 C and at 2.3 to 27.6 MPa in an oxidizing atmosphere. The stress exponent and grain size exponent of the densification rate are indicative of control of densification by a grain boundary diffusion mechanism. The activation enthalpy (400 kJ mol^–1) for the densification process has been measured over the range of 1050 to 1 200 C. No significant influence of the donor is observed on the densification of BaTiO₃ until the late stages of sintering.     

Creep rupture studies of two alumina-based ceramic fibres

Creep rupture tests were performed in air on two polycrystalline oxide fibres (Al₂O₃, Al₂O₃-ZrO₂) using both filament bundles and single filaments. Tests were performed at applied stresses ranging from 50–150 MPa over the temperature range 1150–1250 °C. Under these conditions, creep rates for the alumina-zirconia fibre ranged from 4.12 × 10^–8–7.70 × 10^–6s^–1. At a given applied stress, at 1200°C, creep rates for the alumina fibre were 2–10 times greater than those of the alumina-zirconia fibre. Stress exponents for both fibres ranged from 1.2–2.8, while the apparent activation energy for creep of bundles of the alumina-zirconia fibre was determined to be 648 ± 100kJmol^–1. For the alumina-zirconia fibre, the two test methods yielded similar steady-state creep rates, but the rupture times were generally found to be longer for bundles than for single filaments. The steady-state creep behaviour of these alumina-based fibres is consistent with an interface-reaction-controlled diffusion-controlling mechanism.     

Preparation of lithium aluminosilicate glass-ceramic monolith from metal alkoxide solution

Glass-ceramic monoliths with a composition of Li₂O·Al₂O₃·4SiO₂ have been synthesized by the sol-gel technique using metal alkoxides as starting materials. Heating dried gel monoliths of about 36 mm diameter and height to appropriate temperatures gave glass-ceramic monoliths of about 20 mm diameter and height without the occurrence of cracks, while a melt-derived glass body of the same composition was cracked or softened on heating. The glass-ceramic monoliths obtained by heating gel monoliths at 1000 C for 40 h were a porous body, whose bulk density and porosity were 1.62 g cm^–3 and 34%, respectively. On heating gel monoliths, -eucryptite crystals were first precipitated around 750 C, followed by precipitation of -spodumene crystals at 830 C. At higher temperatures the latter grew at the expense of the former phase. The crystallized specimens exhibited very low thermal expansion coefficient ranging from –13-12×10^–7 C^–1 at temperatures from room temperature to 700 C depending on the heat-treatment temperature of gels, indicating that a porous glass-ceramic monolith with ultra-low thermal expansion can be prepared using the present sol-gel method.     

Thermal expansion of Li2O-ZnO-Al2O3-SiO2 glasses and corresponding glass-ceramics

The thermal expansion behaviour of some glasses and glass-ceramics within the system Li₂ZnSiO₄-LiAlSi₂O₆ is described. The effect of TiO₂ and ZrO₂ additions is also evaluated. The expansion coefficient () of the glasses increases with an increase of the Li₂ZnSiO₄ component in the glass composition. TiO₂ and ZrO₂ were found to decrease the thermal expansion of the glasses investigated. The dilatometric transition and softening points of the glasses showed the reverse behaviour. The thermal expansion of the glass-ceramics exhibited a wide range, depending upon the type and relative proportions of the crystalline phases present. The values of the glasses ranged between 73.6 and 97.4×10^–7C^–1 in the temperature range 20–450C and those for the crystalline products ranged from 36.1 and 102.6 × 10^–7 in the temperature range 20–450C.     

Plastic deformation of polycrystalline zirconium carbide

The compressive yield strength of arc melted, polycrystalline zirconium carbide has been found to vary from 77 kg mm² at 1200C to 19 kg mm² at 1800C. Yield drops were observed with plastic strain-rates greater than 3×10^–3sec^–1 but not with slower strainrates. Strain-rate change experiments yielded values for the strain-rate sensitivity parameterm which range from 6.5 to 1500C to 3.8 at 1800C, and the productm ^*(T) was found to decrease linearly with increasing temperature. The deformation rate results are consistent with the Kelly-Rowcliffe model in which the diffusion of carbon assists the motion of dislocations.     

Methanation reactivity of carbon deposited directly from CO2 on to the oxygen-deficient magnetite

The methanation reactivity of surface carbon deposited from CO₂ on the oxygen-deficient magnetite was studied by the isothermal methanation reaction and the temperature programmed surface reaction (TPSR). In the methanation reaction with H₂ gas, in a closed system at 150–200C, active carbon, which was not observed by the TPSR measurements, was found. About 20% out of the deposited carbon was in the form of atomic carbon and readily converted into CH₄ (5 min) above 150C with H₂, and about 80% at 300C. At 350C these atomic carbons were transformed into polymerized carbons, which was less reactive for methanation.     

Thermal expansion of phenolic resin and phenolic-fibre composites

A large number of thermal expansion measurements in the temperature range 20 to 300C are presented for the Monsanto phenolic resin SC-1008 as a function of heating rate, position in cured block, curing treatment and repeated heating cycles. Thermal expansion measurements in those directions where the resin dominates are also reported up to 400C for composite systems consisting of the phenolic resin reinforced with 60% (volume fraction) of either continuous unidirectional silica or bidirectional carbon (rayon precursor) fibres. The large variation in thermal expansion measurements is used to show that current curing treatments for the materials do not yield a resin with predictable expansion response. Annealing at a higher temperature (>200 C) than the maximum used in the curing treatments is shown to stabilize the phenolic resin SC-1008. Then, the mean linear coefficient of thermal expansion for the pure resin up to 100 C is within the range (55±5)×10^–6C^–1.     

Barium titanate fabricated from fur-fibres

Long-fibrous barium titanate (BaTiO₃) particles were prepared by a hydrothermal reaction of potassium titanate hydrate (2K₂O·11TiO₂·3H₂O) and barium hydroxide (Ba(OH)₂). Effects of preparation conditions on crystal structure and powder morphology were examined. Fur-fibres of K₂O·4TiO₂, 1–10 mm long and 1–100 m in diameter, were obtained by heating a mixture of K₂CO₃ and TiO₂ powders at 1000 C for 100 h. Keeping the fur-fibres in ion-exchanged water for 4 days gave fur-fibres of 2K₂O·11TiO₂·3H₂O). Long-fibrous BaTiO₃, with fibres 100m–1mm long and 1–10 m in diameter, was obtained by a hydrothermal reaction of the hydrate and Ba(OH)₂ (Ba/Ti ratio of 1) at 150 C for 24 h. As-prepared long-fibrous BaTiO₃ was composed of fine crystallites (average size about 270 nm) of cubic phase. The cubic phase and morphology of fur-fibres were maintained up to 1250 C, but heat treatment at 1300 C brought about a growth of crystallites to a few micrometers and a phase transformation to tetragonal phase. It was found that the hydrothermal reaction was effective in producing crystalline BaTiO₃ powder at a low temperature of 150 C.     

Fabrication and characterization of polymer-derived Si2N2O-ZrO2 nanocomposite ceramics

Amorphous Si-Zr-N-O powders, obtained by nitridation in an NH₃ flow of zirconium modified polycarbosilane, have been sintered to full density by hot pressing at 1500C. The resulting ceramic shows an extremely fine-grained microstructure composed of Si₂N₂O and ZrO₂ crystallites 20–30 nm in diameter. Thermal stability measured in air appears excellent up to 1300C for 48 h. Mechanical characterization pointed out good values of flexural strength (330 MPa), fracture toughness (4.1 MPam^0.5) and Weibull modulus.     

Mössbauer studies of O4+-bombarded Nb3Sn

Mössbauer spectra have been taken for Nb₃Sn films on Hastelloy substrate bombarded with 25-MeV O⁴⁺ ions at fluences of 9.6 × 10¹⁵ and 4.0 × 10¹⁶ cm^–2, and annealed at temperatures up to 1050 C. The bombarded samples are found to contain highly damaged Nb₃Sn and Nb₆Sn₅, and show superconducting transitions at 7.3 and 3.3 K, respectively. Annealing at higher temperatures does not restore the properties of the samples to those known for undamaged Nb₃Sn films. The results rather indicate the presence of Nb₃Sn and also Nb₆Sn₅ even after annealing at 850 C. Annealing at 1050 C seems to produce only regions of heavily damaged Nb₃Sn.On leave from North Dakota State University, Fargo, North Dakota.     

Effects of oxidation of Cr3C2 particulate-reinforced alumina composites on microstructure and mechanical properties

Al₂O₃ can be strengthened and toughened by incorporating Cr₃C₂ particles through hotpressing. For instance, an Al₂O₃-10 vol% Cr₃C₂ composite exhibits fracture strength and toughness of 600 MPa and 5.5 Mpa m^0.5, respectively. An annealing treatment in air from 1000–1200C may further substantially strengthen the same composite to give _f = 800 MPa andK _IC = 9.5 MPam^0.5. Possible oxidation reactions and toughening mechanisms are discussed in terms of oxygen diffusion, the formation of micropores beneath the exposed surface, as well as the fracture mode.     

Corrosion of silicon nitride ceramics under hydrothermal conditions

Corrosion behaviour of Si₃N₄ ceramics containing Y₂O₃, Al₂O₃ and AIN as sintering aids was investigated under hydrothermal conditions at 200–300 C and saturated vapour pressures of water for 1–10 days. Hydrothermal corrosion resulted in the dissolution of the Si₃N₄ matrix and the formation of a product layer consisting of the original grain-boundary phases and hydrated silica. The dissolution rate of Si₃N₄ ceramics decreased with decreasing crystallinity of the grain-boundary phase. The dissolution rate could be adequately described by a parabolic plot in the initial stage of the reaction. The apparent activation energies were 83.5–108 kJ mol^–1, and the bending strength of the corroded samples decreased from 600 to 400 MPa in the initial stage of the reaction upto a weight loss of 0.004 g cm^–2, and then was almost constant up to a weight loss of 0.012 g cm^–2.     

Effect of nickel on microwave dielectric properties of Ba(Mg1/3Ta2/3)O3

The dielectric and physical properties of the complex perovskite Ba(Mg_1/3Ta_2/3)O₃ system in which magnesium was substituted for nickel from 0.03–0.67 mol%, were investigated in the temperature range 20–110C, and the frequency range 10.5–14.5 GHz. As the nickel content was increased, the dielectric constant, the degree of ordering, and the unloaded Q decreased. The temperature dependence of the dielectric constant and the temperature coefficient of resonant frequency of the specimens annealed at 1500C for 20 h were found to be greater than those of the specimens sintered at 1650C for 2 h. These results are due to the increase in the density, the increase in grain size, and the lattice distortion.