Physicomechanical and thermophysical properties of SiC-based ceramics

Fine-grain SiC-based ceramics have been produced via infiltration of molten silicon into preforms fabricated from SiC and graphite powders, with a phenol-formaldehyde resin as a binder. The materials thus prepared have a density of 2.70–3.15 g/cm³, dynamic modulus of elasticity from 200 to 400 GPa, compressive strength from 800 to 1900 MPa, bending strength from 150 to 315 MPa, thermal expansion coefficient (KTE) of 4.1 × 10⁻⁶ K⁻¹, and thermal conductivity of 140–150 W/(m K). Their properties are compared to those of known silicon carbide materials fabricated by other processes. The results indicate that the density and physicomechanical properties of the silicon carbide ceramics depend little on the fabrication process and are determined primarily by the SiC content. Increasing the SiC content from 20 to 99.5 wt % increases the density of the ceramics from 2.2 to 3.15 g/cm³ and leads to an exponential rise in their physicomechanical parameters: an increase in modulus of elasticity from 95 to 430 GPa, in compressive strength from 120 to 4200 MPa, and in bending strength from 70 to 410 MPa. The thermal conductivity of the ceramics depends very little on the fabrication process, falling in the range 100–150 W/(m K) over the entire range of SiC concentrations. Their KTE decreases slightly, from 4.3 × 10⁻⁶ to 2.4 × 10⁻⁶ K⁻¹, as the SiC content increases to 99–100 wt %.     

Thermal-Conductivity Measurements and Predictions for Ni–Cr Solid Solution Alloys

Thermal conductivities of Ni–Cr solid solution alloys have been measured to develop a prediction equation for thermal conductivities as functions of temperature and chemical composition. Samples used were Ni–x at% Cr (0 ≤ x ≤ 22) and commercial alloys of Nichrome Nos. 1 and 2. Thermal conductivity measurements were carried out using the transient hot-strip method over a temperature range from 293 K to 1273 K. The thermal conductivities of the alloys increased with increasing temperature and decreased with increasing Cr concentration at constant temperature. The Smith–Palmer equation has been examined to relate the thermal conductivities of the alloys to the electrical resistivities. The thermal conductivity and electrical-resistivity data, respectively, in the present work and in the literature have confirmed that the Smith–Palmer equation applies to Ni–Cr solid solutions and Nichrome alloys. On the basis of this equation, the thermal conductivity of Ni–Cr solid solution alloys has been expressed as a function of temperature and chemical composition. This analysis has also been applied to Ni–Fe and Cu–Ni solid solution alloys.     

High temperature thermoelectric properties and figure-of-merit of sintered Nd2−xCexCuO4

Thermoelectric properties such as the Seebeck coefficient, electrical resistivity, and thermal conductivity are measured in the temperature range of 300– 673 K on Nd2−xCexCuO4 (x = 0–0.1) sintered bodies in order to estimate the figure-of-merit for thermoelectric energy conversion. The temperature dependences of both the Seebeck coefficient and electrical resistivity indicated n-type semiconducting behaviour. The thermal conductivities whose value decreased with increasing temperature were in the range of 3.7–7.5 Wm-1K-1. The maxima of the power factor and the figure-of-merit estimated from data measured at 320 K on a sample of a composition of x = 0.05 were 9.2×10-5 Wm-1K-2 and 1.7×10-5 K-1, respectively. The limitation of the power factor is discussed based on the measured Seebeck coefficient and electrical conductivity data. The thermal conductivity could be separated into a small electron component and a large phonon component by applying the Wiedemann–Franz law. This suggests the possibility of improving the figure-of-merit to some extent by a reduction of the phonon thermal conductivity. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electromagnetic interference shielding of carbon nanotube/ethylene vinyl acetate composites

Single-walled carbon nanotube (SWCNT) and ethylene vinyl acetate (EVA) composites were synthesized in an internal mixer by melt mixing. The electrical conductivity as well as electromagnetic interference (EMI) shielding effectiveness (SE) over the X-band (8–12 GHz) and microwave (200–2,000 MHz) frequency ranges of these composites were investigated. It was observed that the electrical conductivity of composites increases with increasing SWCNT loading. A percolation threshold of about 3.5 wt.% was obtained and the electrical conductivity of EVA was increased by ten orders of magnitude, from 10⁻¹⁴ to 10⁻⁴ Ω⁻¹ cm⁻¹. The effect of sample thickness on SE was investigated. The correlation between SE and conductivity of the composites is discussed. The experimental data showed that the SE of the composites containing higher carbon nanotube loadings (above 10 wt.%) could be used as an EMI shielding material and lower SWCNT loadings could be used for the dissipation of electrostatic charge.     

The effect of Si and microstructure evolution on the thermal expansion properties of Fe–42Ni–Si alloy strips

An alloying element of 0–1.5 wt.% Si was added to an Fe–42%Ni system, and alloy strips were fabricated using a melt drag casting process. The effects of the Si and annealing treatments on the thermal expansion properties of Fe–42Ni alloy were investigated. The addition of Si enlarged the coexisting temperature region of the solid–liquid phase and reduced the melting point, which improved the formability of the alloy strip. An alloy containing 0.6 wt.% Si had a lower thermal expansion coefficient than any other alloy in the temperature range from 20 to 350 °C. The grain size increased with the rolling reduction ratio and annealing temperature, which caused an increase in magnetostriction and consequently a decrease in the thermal expansion coefficient of the strip. The alloy strip containing 1.5 wt.% Si had a higher thermal expansion coefficient than the alloy containing 0.6 wt.% Si because of grain refining caused by the precipitation of Ni₃Fe.     

Dependence of electrical and thermal conductivity on temperature in directionally solidified Sn–3.5 wt% Ag eutectic alloy

Sn–3.5 wt% Ag alloy was directionally solidified upward with a constant growth rate (V = 16.5 μm/s) and a temperature gradient (G = 3.3 K/mm) in a Bridgman-type growth apparatus. The variations of electrical resistivity (ρ) with temperature in the range of 293–476 K for the directionally solidified Sn–3.5 wt% Ag eutectic alloy was measured. The measurements indicate that the electrical resistivity of the directionally solidified Sn–Ag eutectic solder increases with increasing temperature. The variations of thermal conductivity of solid phases versus temperature for the same alloy was determined from the Wiedemann-Franz and Smith-Palmer equations by using the measured values of electrical conductivity. From the graphs of electrical resistivity and thermal conductivity versus temperature, the temperature coefficient of electrical resistivity (α _TCR ) and the temperature coefficient of thermal conductivity (α _TCT ) for the same alloy were obtained. According to experimental results, the electrical and thermal conductivity of Sn–Ag eutectic solder linearly decrease with increasing the temperature. The enthalpy of fusion (ΔH) and the change of specific heat (ΔC _P ) during the transformation at the studied alloy were determined from heating curve during the transformation from eutectic solid to eutectic liquid by means of differential scanning calorimeter (DSC).     

Porous yttria-stabilized zirconia ceramics with ultra-low thermal conductivity. Part II: temperature dependence of thermophysical properties

This study describes the experimental results of thermal diffusivity, specific heat at constant pressure, and thermal conductivity of porous 8 mol% yttria-stabilized zirconia (YSZ) ceramics in a temperature range from room temperature to 1,400 °C. It is a follow-up study of the earlier report titled by “Porous YSZ ceramics with ultra-low thermal conductivity”, which focused on the room-temperature thermal conductivity. The thermal diffusivity of porous YSZ ceramics decreased with the increase of the measurement temperature up to 600–1,000 °C, followed by an increasing trend with increasing temperature. The specific heat did not exhibit any significant dependence on sintering temperature and agreed with literature data. The thermal conductivity of the porous YSZ ceramics showed an insensitive tendency of change with measurement temperature. The thermal conductivity fell in groups by the sintering temperature level. This investigation also discussed an appropriate sintering temperature of porous YSZ ceramics, which had both low thermal conductivity and high strength required by the practical service.     

Thermal Conductivity of Thermoplastics Reinforced with Natural Fibers

With restrictions for environmental protection being strengthened, thermoplastics reinforced with natural fibers such as jute, kenaf, flax, etc., have replaced automotive interior materials such as chemical plastics. In this study, the thermal conductivity of several kinds of thermoplastic composites in the form of board composed of 48.5 mass% polypropylene (PP) and 48.5 mass% natural fiber (NF), and reinforced with 3.0 mass% maleated polypropylene (MAPP) and 0.3 mass% silane as the coupling agents, were measured at temperatures of −10, 10, and 30°C, using a heat flow meter apparatus. The results show that the thermal conductivity is in the range of 0.05–0.07 W · m⁻¹ · K⁻¹, and the thermal conductivity increased about 10–15% by adding MAPP and about 10–25% by soaking in a silane aqueous solution. The tensile strength was also measured, and the result shows similar trends as the thermal conductivity.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

The use of an explicit method of identifying objects to solve problems of nonstationary heat conduction

The theoretical principles of an explicit method of identifying multidimensional objects with nonstationary thermal conductivity are described. The solution of problems of measuring nonstationary heat flux and thermal conductivity in the range λ = 0.03–800 W/(m·K), the thermal conductivity of one of the materials of a double-layer system, the temperature dependence of the thermal conductivity, and the combined “thermal conductivity and volume heat capacity” are presented. The results of investigations on thermal models are given. __________ Translated from Izmeritel’naya Tekhnika, No. 6, pp. 32–38, June, 2008.     

Liquid thermal conductivity of binary mixtures of difluoromethane (R32) and pentafluoroethane (R125)

The thermal conductivities of refrigerant mixtures of difluoromethane (R32) and pentafluoroethane (R125) in the liquid phase are presented. The thermal conductivities were measured with the transient hot-wire method with one bare platinum wire. The experiments were conducted in the temperature range of 233–323 K and in the pressure range of 2–20 MPa. An empirical equation to describe the thermal conductivity of a near-azeotropic mixture, R32+R125, is provided based on the measured 168 thermal conductivity data as a function of temperature and pressure. The dependence of thermal conductivity on the composition at different temperatures and pressures is also presented. The uncertainty of our measurements is estimated to be ±2%. Paper dedicated to Professor Edward A. Mason.     

A new class of instruments: Multivalued measures of thermal conductivity

New multivalued measures of thermal conductivity are described, which are calibrators of thermal conductivity and which, depending on the controlling signal, can reproduce the unit in the 0.02–0.2 W/(m·K) range with an overall standard uncertainty of not greater than 1%. __________ Translated from Izmeritel’naya Tekhnika, No. 4, pp. 50–52, April, 2006.     

Dielectric properties and structural dynamics of melt compounded hot-pressed poly(ethylene oxide)–organophilic montmorillonite clay nanocomposite films

The dielectric properties of melt compounded hot-pressed nanocomposite films consisting of a poly(ethylene oxide) (PEO) and organophilic montmorillonite (OMMT) clay surface modified with trimethyl stearyl ammonium as filler with increasing amount up to 20 wt.% OMMT were investigated in a frequency range of 20 Hz–1 MHz at 30 °C. The predominance of OMMT exfoliated structures in PEO–OMMT nanocomposites were recognized by a decrease of the real part of complex dielectric function. OMMT concentration dependent dielectric and electric modulus relaxation times have revealed that the interactions compatibility between PEO molecules and dispersed OMMT nano-platelets in PEO matrix governs the PEO segmental dynamics. A.C. conductivity of these nanocomposites increases by two orders of magnitude in the experimental frequency range.     

Experimental study of thermal conductivity of the R-407C refrigerant in the vapor phase

Thermal conductivity of the gaseous R-407C refrigerant was investigated by the coaxial cylinders method within the temperature range of 303–425 K and the pressure range of 0.7–2.1 MPa. Approximating dependence of thermal conductivity on pressure and temperature was obtained. Thermal conductivity on dew line and in ideal gas state was calculated. The comparison is performed of the data obtained with those available in the literature.     

Measurement of liquid tin heat transfer coefficients within the temperature range of 506–1170 K

The thermal conductivity and thermal diffusivity coefficients of liquid tin within the temperature range of 506–1170 K were measured by the laser flash technique. The measurement errors for the heat transfer coefficients were equal to ±(2.5–3.5)%. Approximation equations and the reference data tables were obtained for the temperature dependency of the properties. The measurement results were compared with the available literature data. The Lorentz number temperature dependence was calculated up to 1000 K.     

Thermal conductivity of Sn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Nd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S single crystals

The thermal conductivity of Sn_{1 − x} Nd _x S (x = 0, 0.001, 0.002) single crystals has been measured in the temperature range 80–840 K. The results have been used to evaluate the electronic and lattice components of the thermal conductivity. Both the electronic and lattice components are shown to decrease with increasing temperature and to increase with neodymium content.     

Effective thermal conductivity and thermal diffusivity of catalysts and their supports as functions of temperature in various gaseous media and in vacuum. II. Thermal conductivity and thermal diffusivity of granulated porous catalysts as functions of internal and external factors

It is established in studies of the thermal conductivity λ and thermal diffusivity a of granulated porous catalysts and their supports that in gaseous mixtures λ and a for a catalyst load is determined by the properties of the main component of the mixture, and an increase in λ and a of catalysts under investigation is observed in the region of stresses that disintegrate the granules. The quantities λ and a of a load of a catalysts and support increase with increasing thermal conductivity of the filling gas, granule dimensions, concentration, and thermal conductivity of metallic additives. The values of λ and a of a catalyst load decrease linearly with increasing total volume of pores and specific surface. K. Dzhuraev Pedagogical University, Dushanbe, Tadjikistan. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 69, No. 1, pp. 141–150, January–February 1996.     

Titanium diboride copper-matrix composites

Copper-matrix titanium diboride platelet (3–5 μm) composites containing 15–60 vol% TiB2, were fabricated by powder metallurgy, using copper-coated TiB2 (60 vol% TiB2) and various amounts of copper powder. The porosity was ≤0.5% when TiB2 was ≤48 vol%. Above 48 vol% TiB2, the porosity increased abruptly with increasing TiB2 content, reaching 6.7% at 60 vol% TiB2. As a result, the hardness and compressive yield strength dropped precipitously with increasing TiB2 volume fraction beyond 48%. At 48 vol% TiB2, the thermal conductivity was 176 W m-1°C-1, the electrical resistivity was 3.42× 10-6Ωcm, the coefficient of thermal expansion (CTE) was 10.2×10-6°C-1, the compressive yield strength was 659 MPa, and the Brinell hardness was 218. For composites made by conventional powder metallurgy, using a mixture of TiB2 platelets (not coated) and copper powder, the porosity was ≤1.8% when TiB2 was at ≤42 vol%; above 42 vol% TiB2, the porosity increased abruptly and the hardness and compressive yield strength decreased abruptly. The electrical resistivity and thermal conductivity were also affected by the porosity, but less so than the mechanical properties. Composites made using copper-coated TiB2 exhibited lower electrical resistivity, higher thermal conductivity, lower CTE, higher compressive yield strength, greater hardness, greater abrasive wear resistance, greater scratch resistance and lower porosity than the corresponding composites made from uncoated TiB2. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermophysical Properties of Sn–Ag–Cu Based Pb-Free Solders

Lead–tin (Pb–Sn) alloys are the dominant solders used for electronic packaging because of their low cost and superior properties required for interconnecting electronic components. However, increasing environmental and health concerns over the toxicity of lead, combined with global legislation to limit the use of Pb in manufactured products, have led to extensive research and development studies of lead-free solders. The Sn–Ag–Cu ternary eutectic alloy is considered to be one of the promising alternatives. Except for thermal properties, much research on several properties of Sn–Ag–Cu alloy has been performed. In this study, five Sn–xAg–0.5Cu alloys with variations of Ag content x of 1.0 mass%, 2.5 mass%, 3.0 mass%, 3.5 mass%, and 4.0 mass% were prepared, and their thermal diffusivity and specific heat were measured from room temperature to 150 °C, and the thermal conductivity was calculated using the measured thermal diffusivity, specific heat, and density values. Also, the linear thermal expansion was measured from room temperature to 170 °C. The results show that Sn–3.5Ag–0.5Cu is the best candidate because it has a maximum thermal conductivity and a low thermal expansion, which are the ideal conditions to be a proper packaging alloy for effective cooling and thermostability.     

Thermoluminescence dosimetric properties of beryllium oxide

Beryllium oxide (BeO) displays strong thermoluminescence (TL) together with tissue - equivalent properties which underline its application as a TL dosimeter. In the dosimetry of X- and γ-rays some of the advantages of BeO over other TL materials are its commercial availability, low cost, chemical inertness, non-toxicity (as a ceramic), high sensitivity to ionizing radiations, good reproducibility of response, low fading, absence of low-temperature peaks and moderate energy dependence. Various authors have reported glow curves of BeO TL phosphor, whose dominant dosimetric peak lies between about 160 and 200 °C. The position of this peak, however, depends upon the type of the radiation used for exciting the phosphor. Although fading of TL is nominal when kept in the dark, the γ-exposed BeO phosphors fade faster when exposed to ambient light. When exposed to γ-radiation, these phosphors exhibit linearity from a minimum of about 1 mrad (1 rad =10-2 gray) up to approximately 10 rad, above which there is supralinear behaviour, and the concentration of impurity ions in BeO is reported to expand the linearity region. Ceramic samples have been reported to exhibit a roughly flat response when exposed to X-rays of 30–115 keV and γ-rays of 60Co. Because their response to thermal neutrons is negligible compared to the γ-response, the use of BeO has been suggested to measure the γ-component in the (n, γ) mixed fields. This revised version was published online in November 2006 with corrections to the Cover Date.