Magnetic Co2Y ferrite, Ba2Co2Fe12O22 fibres produced by a blow spun process

Gel fibres of Co2Y,Ba2Co2Fe12O22, were blow spun from an aqueous inorganic sol and calcined at temperatures of up to 1200°C. The ceramic fibres were shown by X-ray diffraction to form crystalline Co2Y at 1000°C, and surface area and porosity measurements indicated an unusually high degree of sintering at this temperature. The fibres also demonstrated a small grain size of 1–3 m across the hexagonal plane and 0.1–0.3 m thickness at 1000°C. This only increased to 3 m in diameter and 1 m thickness even at temperature up to 1200°C. The fibrous nature combined with the improved microstructures could be an important factor in improving the magnetic properties of this material.     

Thermal stability and its improvement of the alumina membrane top-layers prepared by sol-gel methods

The thermal stability of unsupported alumina membrane top-layers was studied by determining the pore structure (mainly pore size) change of alumina gels, prepared by sol-gel methods, after sintering at different temperatures ranging from 450 to 1200 °C. The average pore size of the pure alumina membranes and PVA-added membranes increased sharply after sintering at temperatures higher than 1000 °C. Addition of 3% lanthanum, either by mixing lanthanum nitrate in the alumina sol or impregnating lanthanum nitrate into calcined alumina gel, followed by a second heat treatment, can considerably stabilize the pore structure of the alumina membrane top-layers. The pore diameter for the lanthanum-doped membranes was stabilized within 25 nm after sintering at 1200 °C for 30 h, about one-sixth of that for the pure alumina membranes after sintering at 1200 °C for 30 h. The substantial increase in the pore size for the pure alumina membranes at the sintering temperature of 1000 to 1200 °C was accompanied by the phase transformation from -to -alumina. The addition of lanthanum can raise this phase transformation temperature by about 200 °C.     

Effects of alkali-metal ions on the thermal decomposition of ammonium metavanadate supported on alumina

The thermal decomposition of ammonium metavandate (AMV) supported on -alumina (11 mol) and treated with alkali-metal ions was investigated using differential thermal analysis (DTA), thermogravimetric analysis (TGA), infrared (i.r.) absorption spectroscopy and X-ray diffraction (XRD). The results revealed that the presence of alkali-metal ions did little to affect the thermal decomposition processes of AMV supported on alumina. However, these ions enhanced creation of V⁴⁺ ions by heating the samples in air at 400 °C. The oxides produced underwent a solid-solid interaction at 500–700 °C, forming alkali-metal vanadates. The formation of these spinels was found to be strongly dependent on the degree of diffusion and the concentration of the alkali-metal ions. Moreover, the addition of a higher concentration of alkali-metal ions inhibits the formation of aluminium vanadate while enhancing the crystallinity of - to -alumina.     

Characterization of hydroxyapatite powders prepared by ultrasonic spray-pyrolysis technique

The hydroxyapatite (HAp) powder was prepared by the ultrasonic spray-pyrolysis technique; the characterization of the resulting powders was performed. Five kinds of the starting solutions with the Ca/P ratio of 1.67 were prepared by mixing Ca(NO₃)₂, (NH₄)₂HPO₄ and HNO₃; the concentrations of Ca²⁺ and PO₄ ^3– were in the ranges of 0.10 to 0.90 mol·dm^–3 and 0.06 to 0.54 mol·dm^–3, respectively. These solutions were sprayed into the heating zone to prepare the HAp powders. The heating zone was composed of two electric furnaces; the lower furnace was used for the evaporation of the solvent from the droplets (300–500°C) and the upper furnace for the pyrolysis of the precipitated metal salts (750–900°C). The easily sinterable HAp powder was prepared by spray-pyrolysing the solution with Ca²⁺ (0.50 mol·dm^–3) and PO₄ ^3– (0.30 mol·dm^–3) at the temperatures of 800°C (the upper furnaces) and 400°C (the lower furnaces). The resulting powder was composed of the spherical particles with diameters of 1 m or below. Even without the calcination and grinding operations, the relative densities of the compacts fired at 1150 and 1200°C for 5 h attained maxima 95%. The microstructure of the sintered compacts appeared to be uniform; the average grain size was 3 m. The activation energies for the grain growth of the sintered HAp compacts were 120 to 147 kJ · mol^–1 · K^–1.     

High temperature bending creep of a Sm-α-β Sialon composite

The four-point bending creep behavior of a Sm-- Sialon composite, in which Sm-melilite solid solution (denoted as M) was designed as intergranular phase, was investigated in the temperature range 1260–1350°C and stresses between 85 and 290 MPa. At temperatures less than 1300°C, the stress exponents were measured to be 1.2–1.5, and the creep activation energy was 708 kJ mol^–1, the dominant creep mechanism was identified as diffusion coupled with grain boundary sliding. At temperatures above 1300°C, the stress exponents were determined to be 2.3–2.4, and creep activation energy was 507 kJ mol ^–1, the dominant creep mechanism was suggested to be diffusion cavity growth at sliding grain boundaries. Creep test at 1350°C for pre-oxidation sample showed a pure diffusion mechanism, because of a stress exponent of 1. N^3– diffusing along grain boundaries was believed to be the rate controlling mechanism for diffusion creep. The oxidation and Sialon phase transformation were analyzed and their effect on creep was evaluated.     

Mechanical properties of β-Si3N4 whisker reinforced α′-SiAlON ceramics

The effects of -Si₃N₄ whisker additions on the mechanical properties of -SiAlON ceramics were studied. The room temperature fracture toughness and fracture strength of the composites increased with increasing whisker content, and were 6.5 MPa m^1/2 and 900 MPa, respectively, for the addition of 30 vol% whiskers. Although creep resistance of the composites was not enhanced at 1200°C, the whisker additions were observed to be beneficial in reducing the oxidation induced slow crack growth of -SiAlON that occurred at 1300 °C, and thereby, improved the creep resistance of the composites at 1300°C.ORNL Postdoctoral Fellow, Oak Ridge Institute of Science and Technology, Oak Ridge Associated University.     

High-temperature flow behaviour and concurrent microstructural evolution in an Al-24 wt% Cu alloy

Tensile specimens of an Al-24 wt% Cu alloy of grain sizes in the range 7.6–20.6 m were deformed at 400–540 °C using constant initial strain rates ranging from 5×10^–6 to 2×10^–2 s^–1. Initially the stress-strain (-) curves show work hardening which is followed by strain softening at higher strain rates and lower temperatures. At lower strain rates and higher temperatures, on the other hand, continues to increase with strain or tends to be independent of strain. Grain growth and cavitation occur to varying extents depending on strain rate and test temperature. While the grain growth can account for the work hardening at higher temperatures as well as at lower strain rates, it fails to do so at higher strain rates. The strain softening is associated with cavitation. The presence of non-steady-state flow influences the parameters of the constitutive relation to varying extents.     

Mössbauer study on the magnetic state of iron particles in Fe-N and Fe-Zr-N soft magnetic thin films

The magnetic state of -Fe particles and the behaviour of nitrogen and zirconium during annealing in Fe₉₆N₄ and Fe_85.6Zr_7.6N_6.8 magnetic thin films have been studied by conversion electron Mössbauer spectroscopy for ⁵⁷Fe. The crystalline phases present in the Fe-N annealed films were -Fe and -Fe₄N, and those in the Fe-Zr-N annealed films were -Fe and ZrN. In the Fe-N films annealed below 300°C, about 60% nitrogen is incorporated interstitially into -Fe and the rest is used for the formation of -Fe₄N. In the Fe-N film annealed at 500°C, almost all nitrogen participates in the formation of -Fe₄N, leading to the grain growth of -Fe particles and an increase in coercive force. The values (291–325 kOe) of internal magnetic field of iron sites in -Fe in the Fe-Zr-N films are much smaller than that (333 kOe) of the iron site in pure -Fe. Even if the Fe-Zr-N films were annealed at 500–700°C, some zirconium and nitrogen is still incorporated substitutionally and interstitially into -Fe, respectively. In particular, the substitutional zirconium depresses the grain growth of -Fe particles, perhaps due to a chemical interaction between zirconium and iron.     

Thermal conductivity and phase evolution of plasma-sprayed multilayer coatings

Multilayer coatings were prepared using small-particle plasma spray to investigate the effect of interfaces on thermal conductivity and phase stability. Monolithic and multilayer alumina and yttria partially-stabilized zirconia coatings, with 0, 3, 20, and 40 interfaces in 200–380 m thick coatings were studied. Thermal conductivity was determined for the temperature range 25 °C to 1200 °C using the laser flash method and differential scanning calorimetry. Thermal conductivity of the multilayer coatings was accurately modeled by a series heat transfer equation, indicating that interfacial resistance plays a negligible role in heat transfer in the direction perpendicular to the coating plane. Powder X-ray diffraction results indicate that identical phase transitions occur in all the coatings. Independent of coating microstructure (i.e. layer thickness), as-sprayed -Al₂O₃ transforms to -Al₂O₃ after 100 hours at 1200°C; as-sprayed metastable t–ZrO₂ converts to a mixture of t–ZrO₂ and c–ZrO₂ after 100 hours at 1300 °C. Thus, the results indicate that the interfaces do not aid in stabilizing the as-sprayed phases after prolonged severe heat treatments.     

Temperature effects on the properties of Ge thin films

The effects of substrate temperature (T_s) on the properties of vacuum evaporated p-type Ge thin films have been investigated for 25s<400°C. Increase in the substrate temperature improves the crystallinity and increases the grain size resulting a gradual change from amorphous to polycrystalline structure which was attained above a substrate temperature of 225°C. Low resistive (1×10^–2 ohm-cm) and high mobility (280 cm²/V·s) films were obtained at T_s=400°C. It has been observed that the conduction mechanism in polycrystalline films was dominated successively by hopping, tunneling and thermionic emission as the sample temperature was increased from 40 to 400 K. In amorphous samples, conduction was described in terms of different hopping mechanisms.     

An analytical investigation of the longitudinal temperature profile developing during the cooling of a cryogenic pipeline

Analytical expressions are obtained for the longitudinal temperature profiles of the wall and the stream of cryogen during the cooling of a cryogenic pipeline. A comparison of the calculated data with experiment gives their good agreement.Notation T temperature, °K - density, kg/m³ - heat-transfer coefficient between wall and stream, W/m²·°K - perimeter wetted by stream, m - c heat capacity, J/kg·°K - F cross-sectional area, m² - G flow rate of cryogen, kg/sec - t time, sec - x longitudinal coordinate, m - coefficient of thermal conductivity of cryogen, W/m·°K - coefficient of dynamic viscosity, m²/sec - Pr Prandtl number - dimensionless time - dimensionless longitudinal coordinate - dimensionless longitudinal coordinate in the moving coordinate system - width of zone of heat exchange - dimensionless temperature - P pressure of cryogen, N/m² - R gas constant, J/kg·°K - dimensionality of temperature - v₁ dimensionless velocity of movement of steady temperature profile - ¯c_w integral-mean heat capacity of wall, J/kg·°K - a b, m, constant coefficients in the approximating equations. Indices: 0, initial value - w wall - g cryogen - r relative value Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 41, No. 3, pp. 524–531, September, 1981.     

Characterization of the spinel phase in a diphasic mullite gel using dynamic X-ray diffraction

Dynamic X-ray diffraction (DXRD) has been used in an effort to identify the specific phase changes which are responsible for observed thermal events at 980 °C in mullite gel precursors. Specifically, changes in the evolution of the common and strongest diffraction peak (d = 0.139 nm) corresponding to both transient alumina phases and the Al-Si spinel were followed in order to descriminate between these two phases. Results which compare the DXRD results for a diphasic mullite gel and a boehmite gel are presented and suggest that the Al-Si spinel phase forms at 980 °C in diphasic gels along with - and/or -Al₂O₃. These results are corroborated by separate TEM measurements which indicate the presence of both phases in samples quenched from 1000 °C.     

Hollow Alumina Microspheres From Boehmite Sols

Hollow alumina microspheres were obtained from emulsified boehmite sols by an ion extraction method. The viscosity and the equivalent alumina content of the sols were found to affect the characteristics of the derived microspheres. High-viscosity sols produced broken microspheres. A temperature of about 400–500 °C for boehmite to -Al2O3 transformation in the gel microspheres was observed by differential thermal analysis and X-ray diffraction. Complete crystallization of the gel microspheres to -Al2O3 occurred at 1200 °C. A tentative mechanism for the formation of the hollow microspheres is presented.     

Sintering behaviour of the diamond-super invar alloy system at high temperature and pressure

In the diamond-super invar alloy system, effect of the grain size of the starting diamond powder on the sintering behaviour was investigated at 5.8 G Pa and 1300 to 1500 ° C for 30 to 60 min. Abnormal grain growth was often observed in the sintered diamond using fine diamond powder, but a homogeneous sintered diamond was easily synthesized using 5 to 10m diamond. By the rigorous control of sintering conditions, a 3m grain size homogeneous sintered diamond was synthesized under conditions of 5.8 G Pa and 1350 ° C for 30 min. The Vickers hardness and thermal resistance of the fine grained sintered diamond was examined.     

Preparation and properties of some alumina-chrome refractories

Preparative procedures, grain growth, densification and change in corundum lattice dimensions in sintered compacts made from chromium (III) oxide/aluminium (III) oxide mixtures were evaluated in a study of some alumina-chrome refractories. High-energy milling using A17 reactive alumina gave densification at low temperature and rapid grain growth, also a low variability in modulus of rupture. X-ray diffraction studies confirmed solid solution formation, the corundum lattice dimensions decreasing with 7 wt% chromium (III) oxide, other compositions giving lattice expansion. The modulus of rupture at 1150 °C for a series of ethyl silicate-bonded alumina-chrome refractories containing 5 to 12 wt% chromium (III) oxide was also minimal at 7 wt% chromium (III) oxide. The composition of this series is typical of ethyl silicate-bonded alumina-chrome refractories used in the steel industry.     

Transient processes of heat exchange in an air-plasma jet

The results of an experimental investigation of transient processes of nonsteady heat exchange in a high-temperature air stream are presented.Notation R radius of thermocouple junction, m - a thermal diffusivity, m²/sec - _th thickness of thermal boundary layer, m - thermal conductivity of gas, J/m·sec·deg - heat-transfer coefficient, W/m²·deg Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 41, No. 3, pp. 520–523, September, 1981.     

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as ₂ phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (₂) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich ₂ phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles and-Co matrix, such that (0 1 1)₂ ¶ (1 1 1) _-Co and [1 1]₂ ¶ [1 0] _-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.     

Thermal conductivity of argon and argon-neon mixtures at high temperature

The thermal conductivity of neon and argon-neon mixtures is studied in the temperature range 400–1500°K. This is the first recording of such data above 793°K.Notation T_g true temperature drop in gas layer °K - ¯T temperature, °K, Q, effective thermal flux, W - Q_t, Qr thermal flux transmitted by thermal conductivity and radiation, respectively, W - T_sh correction for temperature shift, % - thermal conductivity of gas mixture, W/m·°K - x_i a molar concentration of neon - _i, _i, _ij, _ij potential function parameters for inter molecular interaction of homogeneous and inhomogeneous molecules - slope of exponential repulsion term Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 5, pp. 848–856, November, 1977.     

Numerical calculation of a generalized thermal model of radio-electronic apparatus

A method is proposed for numerical calculation of the temperature field of a generalized model of electronic equipment with high component density.Notation x,y,z,x,y spatial coordinates, m - time, sec - L_x, L_v, L_z dimensions of heated zone, m - _x, _y, _z effective thermal-conductivity coefficients of heated zone, W/m·deg - ₂ thermal conductivity of chassis, W/m·deg - a _z thermal diffusivity of heated zone along z axis, m²/sec - c₁ effective specific heat of heated zone, J/kg·deg - ₁ effective density of heated zone, kg/m³ - c₃, ₃, c₂, ₂ thermophysical characteristics of cooling agent and chassis, J/kg·deg·kg/m³ - q_v(x, ), q(x, y) volume heat-source distribution, W/m³ - q_s (x) surface heat-source distribution, W/m² - p number of cooling agent channels - Fo Fourier number - Bi Biot number - Uⁱ coolant velocity in i-th channel, m/sec - T₁(x, ), T₂(x, ), T₃(x, ) temperature distribution of heated zone, chassis, and coolant, °K - T₃₀, T₁₀(x), T₂₀(x) initial temperatures, °K - T_3in coolant temperature at input to channel, °K - T_T(x) effective temperature distribution of heat loss elements, °K - T_C temperature of external medium, °K - dimensionless heated zone temperature - _v(x) local volume heat exchange coefficient, W/m³·deg - ₁₂(x), _1C(x), _1T(x) heat liberation coefficients - W/m²·sec; ₂₁(x, y), _2c(x, y), _2T(x, y) volume heat-exchange coefficients of chassis with heated zone, medium, and cooling elements, W/m³·deg Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 5, pp. 876–882, May, 1981.     

Sinter forging of zirconia toughened alumina

Sinter forging experiments have been carried out on powder compacts of zirconia toughened alumina (ZTA) Ceramics Alumina-15 wt% zirconia was prepared by a gel precipitation method and calcined at temperatures of 900 or 1100°C. Full densification of ZTA ceramics was obtained within 15 min at 1400°C and 40 MPa. A homogeneous microstructure can be observed with an alumina grain size of 0.7 m and a zirconia grain size of 0.2 m. Almost no textural evolution occurred in the microstructure. During sinter forging the densification behaviour of the compacts was improved by an effective shear strain, for which values of more than 100% could be obtained. As a result of the shear deformation the densification of ZTA in the alumina phase stage shifted to lower temperature. During pressureless sintering the to alumina transformation temperature was dependent of the preceding calcination temperature, while during sinter forging this phase transformation was independent of calcination temperature and took place at a lower temperature.