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.     

An improvement in processing of hydroxyapatite ceramics

Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in softening particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min^–1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.     

Some properties of mullite powders prepared by chemical vapour deposition

The sinterability of mullite (3Al₂O₃·2SiO₂) powder prepared by chemical vapour deposition was examined to improve the conditions for fabricating dense mullite ceramics. The starting powder contained not only mullite, but also a small amount of -Al₂O₃ (Al-Si spinel) and amorphous material. Although the compressed powder was fired at a temperature between 1550 and 1700 °C for 1, 3 and 5 h, the relative densities of the sintered compacts were limited to 90%: (i) due to the creation of pores/microcracks during the solid state reaction (1100–1350 °C), and (ii) due to restriction on the rearrangement of grains because the amount of liquid phase (1550–1700 °C) was insufficient. Calcination of the starting powder was effective for preparation of easily sinterable powder with homogeneous composition. When the compact formed by compressing the calcined powder at 1400 °C for 1 h was fired at 1650 °C for 3 h, the relative density was raised up to 97.2%; moreover, mullite was the only phase detected from the sintered compact. The sintered compact was composed of polyhedral grains with sizes of 1–2 m and elongated grains with long axes of 6 m.     

Gelatin manipulation of latent macropores formation in brushite cement

Macroporous brushite cement was prepared from a mixture of -tricalcium phosphate (-TCP) and monocalcium phosphate monohydrate (MCPM) using gelatin powder as a latent templates. In a setting reaction coexisting with gelatin, closed packed, open-pore structure with 100–200 m macropores are obtained after immersion of the set cement into PBS buffer (pH 7.4) at 37 °C for 1–4 weeks. The macroporous brushite cement has compressive strength of 15 MPa originally, which reducing to 5.5 MPa with macropore formation gradually in comparison to that of cancellous bone (5–10 MPa).     

Interfacial bond strength of electrophoretically deposited hydroxyapatite coatings on metals

Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method 10Ca(NO₃)₂ + 6(NH₄)₂HPO₄ + 8NH₄OH), could produce dense coatings when sintered at 875–1000°C. Single EPD coatings cracked during sintering owing to the 15–18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating filled in the valleys in the cracks of the first coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be 12 MPa on a titanium substrate and 22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since -316L (20.5 m mK^-1) > -HAp (14 m mK^-1), resulting in residual compressive stresses in the coating, whereas -titanium (10.3 m mK^-1) < -HAp, resulting in residual tensile stresses in the coating. © 1999 Kluwer Academic Publishers     

Duplex spinel-ZrO2 ceramics

Duplex spinel-ZrO₂ ceramic composites were produced by an emulsion-hot kerosene drying technique. The sintered duplex spinel-ZrO₂ ceramics which had the composition of 55 wt% Al₂O₃-20 wt% ZrO₂-25 wt% MgO, consisted of a spinel matrix, whose grain size was in the range of 1.5 to 2.0 m, and uniformly dispersed zirconia agglomerates having grain sizes ranging from 1.0 to 2.0 m. Zirconia agglomerates began to appear at a temperature of 1500 °C and the duplex spinel-ZrO₂ structure was formed with the weight ratio of Al₂O₃/MgO being within 1.67 to 2.20 and the amount of ZrO₂ addition being within 5 to 25 wt %. The relative density, fracture toughness, flexural strength, and critical temperature difference of the spinel-ZrO₂ composite were 97.8%, 1.98 MPam^0.5, 390 MPa, and 275 °C, respectively.     

Preparation of PLZT powders from several aqueous solutions

Fine, soft agglomerate and chemically homogeneous PLZT powders were prepared from nitrate, chloride and alkoxide precursors. The preparation is based on a coprecipitation method in which the aqueous clear solution of multicomponent systems is reacted with ammonia gas at the liquid surface. As-dried powder characteristics — the microstructure of agglomerates, colour and X-ray diffraction structure — were similar in all cases; however, the properties of calcined powders and of two-stage sintered pellet were affected not only by precursors, but also by postprecipitation processing: (a) washing with organic solvent, (b) drying and ageing, (c) calcining. A novel lattice behaviour, i.e. decomposition and recovery of perovskite structure of calcined powders regardless of precursors, was found over a wide temperature range from room temperature to 1150° C. Transparent ceramics were fabricated by sintering the acetone washed-aged powder derived from TiO(NO₃)₂ precursor.     

Effect of β-TCP granularity on setting time and strength of calcium phosphate hydraulic cements

The effects of -tricalcium phosphate (-TCP) granularity on the properties of calcium phosphate hydraulic cements (CPHC) have been investigated. A model system based on mixtures of (-TCP) and aqueous solution of orthophosphoric acid has been used. Powders with different shapes (irregular agglomerates or spheres) and sizes (d ₅₀=7 to 130 m) were prepared from two different calcium phosphate sources: Ca-deficient hydroxyapatite (DAP) or hydroxyapatite dicalcium phosphate mixtures (HAP-DCP). The cements exhibited setting times (ST) ranging from 65 to 510 s; they are mainly affected by the specific surface area (S _BET) of the -TCP powders, longer ST corresponding to lower S _BET. In general, lower S _BET and shorter ST values were obtained with HAP-DCP powders. Diametral tensile strengths (DTS) ranging from 3.5 to 10.4 MPa were obtained. The results show that DTS is affected in a complex way by the experimental variables. In general, DTS is higher with irregular agglomerates compared to spheres (+1.2 MPa), while better results are obtained with HAP-DCP powders (+0.9 MPa). The dependence of DTS on particle size is variable according to powder source and shape. The highest DTS (10.4 MPa) was obtained with irregular agglomerates prepared from HAP-DCP mixtures (d ₅₀=16 m), and the lowest (3.5 MPa), with irregular agglomerates prepared with DAP (d ₅₀=123 m). It can be concluded from this work that specific CPHC formulations can give quite different cement properties, such as setting time and ultimate mechanical strength, depending on the characteristics of the raw materials used. In the case of -TCP based cements, the granularity of the starting cement powder, including particle size, shape and specific surface area, is of crucial importance and should be specified when the performances of different formulations are to be compared.     

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.     

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.     

Chemical Transformations of γ-Al(OH)3during Closed-System Heat Treatment

The chemical transformations of -Al(OH)₃during closed-system heat treatment in a self-generated gaseous atmosphere (water vapor) were studied. At t 200°C, -Al(OH)₃was found to convert into -AlOOH, which, in turn, converts into the equilibrium phase -Al₂O₃at t 400°C. The processes underlying the effect of water vapor on the kinetics and mechanisms of the chemical transformations of -Al(OH)₃and -AlOOH in a closed system are discussed.     

Synthesis of titanium nitride by a spark-discharge method in liquid ammonia

Titanium nitride powders were synthesized by the spark-discharge method in liquid ammonia at — 78 to 130 °C and 3.5–10.5 kV discharge voltage using titanium pellets as the starting materials. Titanium nitride possessing nitrogen defect, TiN_1–x (x0.5), was obtained as the main product, together with small amounts of -Ti alloyed with nitrogen. The increase in temperature of the liquid ammonia resulted in an increase in the titanium nitride content in the product but a decrease in the powder production rate. By calcining the mixed powders of TiN_1–x and -Ti in a nitrogen atmosphere around 1200 °C, stoichiometric TiN was obtained as single phase.     

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.     

Crystalline transformation of Ti-B-O,Al-Ti-B-O,Si-Ti-B-O and Al-Si-Ti-B-O by thermal treatments

The thermal transformations of Ti-B-O, Al-Ti-B-O, Si-Ti-B-O and Al-Si-Ti-B-O have been investigated using the methods of thermal analysis and X-ray powder diffraction. The materials are a crystalline series of TiO₂ with partial replacement of titanium by the elements, aluminium, boron and silicon. The anatase form of the materials was transformed to the rutile form at 520680 °C for Ti-B-O, 880950 °C for Al-Ti-B-O, 10801280 °C for Si-Ti-B-O and 11801330 °C for Al-Si-Ti-B-O. The rate constant for the anatase-rutile transformation of Ti-B-O was 6.908×10^–3 min^–1 under isothermal conditions at 680 °C. Analysis of the kinetic data obtained by differential thermal analysis (DTA) gave the activation energy for transformation of anatase into rutile as 663.7 Kcal mol^–1 for Al-Ti-B-O. The lattice parameters for the compounds studied at various temperatures were calculated by least-squares fitting of the X-ray powder diffraction data.     

Solid state phase transformation of BaB2O4 during the isothermal annealing process

Solid-state phase transformation of BaB₂O₄ during the isothermal annealing process for both to and to were investigated using a platinum crucible. For the -phase crystal at the -phase stable temperature (> 925 °C), the phase transforms to the phase perfectly below the melting temperature of 1100 °C. Meanwhile, for the -phase crystal at the -phase stable temperature (< 925 °C), the phase transforms to the phase perfectly above 800 °C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.     

Ageing characteristics of dendritic and non-dendritic (stir-cast) Zn-Al alloy (ZA-27)

The dimensional changes of dendritic and non-dendritic (stir-cast) Zn-Al alloy (ZA-27) were investigated during ageing at temperatures in the range 20–245 °C. The linear expansion of both dendritic and non-dendritic samples increased rapidly with ageing time after about 24 h at 95 °C. An initial normalization treatment led to a large initial growth of the alloy which increased further after prolonged ageing times ( 10⁴h). Accelerated ageing tests in the range 75–250 °C showed that increasing the ageing temperature decreased the long term linear expansion of the alloy. In comparing the behaviour of the two materials, the stir-cast material grew less than the dendritic alloy. Extensive SEM and TEM done on as-stir-cast and aged samples showed that the main changes during ageing occurred in the (FCC) lamellae of the +(Zn) eutectoid, the phase at the primary _particles/_eutectoid interface and in the interparticle areas. In the lamellae and at the interface, zinc precipitated whilst the phase precipitated in the interparticle areas. It is considered that the occurrence of this latter phase, rich in copper, is responsible for the growth of stir-cast material during ageing.     

Density Response and Equilibration in a Pure Fluid Near the Liquid-Vapor Critical Point: 3He

We report measurements of the local density response inside a quasi 1-D horizontal ³ He fluid layer to a step-like change T of the boundary temperature, where |T| 80 K and much smaller than |T – T_c| where T_c is the critical temperature. These experiments used a new cell design, described in the text, and were carried out along the critical isochore both above and below T_c. The observed temporal and spatial density response (t, z) and its equilibration time are described adequately by the relations developed from the thermodynamic theory of Onuki and Ferrell. We verify that over the temperature range of low stratification, where computer simulations and closed-form calculations can be compared, they are in exact agreement. The systematic differences of experimental results from predictions can be accounted for by the departure of the cell from the ideal 1-D geometry. The much larger disagreement between the experimental and predicted equilibration time scale in earlier experiments is also explained. Finally, deviations from linearity observed in the density response for steps |T| larger than 90 K are reported and the implications of such nonlinearity for the (t, z) profile and especially the effective relaxation time _eff are analyzed. We also discuss the predicted onset of convection near T_c for the conditions in our experiment. In the Appendix, the likely sources for systematic deviations in the density response function for the experimental cell from calculations in the ideal 1-D geometry are presented and their effects calculated. The so-obtained response function Z_F(, z) is compared with previously published data.     

Formation of Friction Layers and Wear Resistance of Steels under Conditions of Boundary Friction

We studied the influence of the lubricant compositions Grafitol with 10% graphite (1), Aerol containing 17% MoS₂ (2), Limol containing 60% MoS₂ (3), Limol + 10% chlorine-paraffin (4), and Limol + 10% copper powder (5) on the wear rate and formation of the fine structure of surface friction layers of structural steels. We established a correlation between the tribological characteristics of steels and lubricants. The abrasive wear of 40KhFA steel was minimum if it was lubricated with Limol + 10% copper powder. In this case, its wear was smaller by a factor of 10, 2, 1.25, and 7.25 as compared with lubricant compositions 1–4, respectively. In the course of minimum wear of 40KhFA steel, in the surface friction layers, we observed the minimum values of second-kind distortions (a/a) and of the true size of domains of X-ray coherent scattering (D) as well as the minimum difference between the crystal lattice constants (a) of steel before and after friction.     

Microstructural study of two LAS-type glass-ceramics and their parent glass

The two glass-ceramics studied here derive from the complex system (MgO,ZnO,Li₂O)-Al₂O₃-SiO₂ and are obtained by controlled devitrification of the same parent glass. Although they have the same chemical composition, one is a -quartz (or -eucryptite) type while the other one is a -spodumene glass-ceramic. A detailed microstructural analysis of these materials has been performed at different scales by several complementary characterization methods (SEM, TEM, DTA, XRD and FTIR). This extensive study has shown the great microstructure difference (grain distribution, grain size, nature of vitreous and crystalline phases) between these two glass-ceramics obtained from the same parent glass.     

Dual-phase magnesia-zirconia ceramics with strength retention at elevated temperatures

Two-phase polycrystalline ceramics containing MgO and ZrO₂ were fabricated by pressureless sintering powder compacts in air to near theoretical density. MnO was added as a densification aid in most compositions. For samples fabricated with 20 vol% ZrO₂ and 80 vol% MgO (which actually contained 23 vol% ZrO₂(ss) after sintering because some of the MgO dissolved in zirconia), densities in excess of 98% theoretical were achieved at temperatures as low as about 1250° C. However, most of the samples were typically sintered at 1420±10° C. The grain sizes of the two phases, ZrO₂(ss) and MgO(ss), were of the order of 1.4m. Thermal etching of the specimens showed the presence of very uniform sized domains (approximately 240 nm in size) in zirconia grains. Some samples were also fabricated in which 8 mol% CaO was added in order to stabilize the high-temperature cubic polymorph of zirconia to room temperature. The grain sizes of the two phases in this composition were also of the order of 1.4m. No domains were observed in zirconia grains in CaO-doped samples. Fracture strength was measured as a function of volume fraction of zirconia. Strength values in excess of 500 MPa have been measured on samples fabricated with 40 vol% zirconia (the amount of zirconia (ss) is 43 vol%). Samples of similar composition but with CaO doping exhibited strength of the order of 300 MPa despite an essentially identical grain size and density. Fracture toughness of samples containing CaO was 3.0 MPa m^1/2 while that of the samples without CaO was 5.2 MPam^1/2. No monoclinic phase was observed on either the fracture or the ground surfaces of CaO-doped and undoped samples. Fracture strength and toughness, measured as a function of temperature up to 1000° C, were found to be nearly independent of temperature. The temperature independence of the strength suggests that strengthening and toughening in this material does not occur by transformation toughening.