On the thermal conductivity of Cu–Zr/diamond composites

Interfaces and close proximity between the diamond and the metal matrix are very important for their thermal conductance performance. Matrix-alloying is a useful approach to greatly enhance the interfacial bonding and thermal conductivity. In this study, the copper–diamond (Cu/Dia) composites with addition of 0.8, 1.2 and 2.4 wt.% zirconium (Zr) are prepared to investigate the influence of minor addition of Zr on the microstructure and thermal conductivity of the composites. The thermal conductivity of the composites is analyzed both experimentally and theoretically. It is demonstrated that moderate interfacial modification due to the Zr added is beneficial to improve the thermal conductivity of the Cu/Dia composites.     

Preparation of high thermal conductivity copper–diamond composites using molybdenum carbide-coated diamond particles

Molybdenum carbide (Mo₂C) coatings on diamond particles were proposed to improve the interfacial bonding between diamond particles and copper. The Mo₂C-coated diamond particles were prepared by molten salts method and the copper–diamond composites were obtained by vacuum pressure infiltration of Mo₂C-coated diamond particles with pure copper. The structures of the coatings and composites were investigated using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results indicated that the Mo₂C coatings effectively improved the wettability between diamond particles and copper matrix, and Mo₂C intermediate layers were proved to decrease the interfacial thermal resistance of composites. The thermal conductivity of the composite reached 608 Wm^?1 K^?1 with 65 vol.% Mo₂C-coated diamond, which was much higher than that with uncoated diamond. The greatly enhanced thermal conductivity is ascribed to the 1-μm-thick Mo₂C coatings. Mo₂C coatings on diamond particles are proved to be an effective way to enhance the thermal conductivities of copper–diamond composites.     

Thermal conductivity of wood-derived graphite and copper–graphite composites produced via electrodeposition

The thermal conductivity of wood-derived graphite and graphite/copper composites was studied both experimentally and using finite element analysis. The unique, naturally-derived, anisotropic porosity inherent to wood-derived carbon makes standard porosity-based approximations for thermal conductivity poor estimators. For this reason, a finite element technique which uses sample microstructure as model input was utilized to determine the conductivity of the carbon phase independent of porosity. Similar modeling techniques were also applied to carbon/copper composite microstructures and predicted conductivities compared well to those determined via experiment.     

Heat treatment method for making high strength and conductivity Cu–Cr–Zr alloy

Abstract

In this paper, the heat treatment called ‘a two-step aging process’ with the feature of the first step aging at a low temperature for a long time and the second step aging at a higher temperature for a short time has been proposed. Applying this process, the Cu–Cr–Zr alloy possessing both high strength and high conductivity can be acquired due to the formation of numerous tiny particles precipitated fully out from Cu matrix.     

Effect of copper content on the thermal conductivity and thermal expansion of Al–Cu/diamond composites

Al–Cu matrix composites reinforced with diamond particles (Al–Cu/diamond composites) have been produced by a squeeze casting method. Cu content added to Al matrix was varied from 0 to 3.0 wt.% to detect the effect on thermal conductivity and thermal expansion behavior of the resultant Al–Cu/diamond composites. The measured thermal conductivity for the Al–Cu/diamond composites increased from 210 to 330 W/m/K with increasing Cu content from 0 to 3.0 wt.%. Accordingly, the coefficient of thermal expansion (CTE) was tailored from 13 × 10⁻⁶ to 6 × 10⁻⁶/K, which is compatible with the CTE of semiconductors in electronic packaging applications. The enhanced thermal conductivity and reduced coefficient of thermal expansion were ascribed to strong interface bonding in the Al–Cu/diamond composites. Cu addition has lowered the melting point and resulted in the formation of Al₂Cu phase in Al matrix. This is the underlying mechanism responsible for the strengthening of Al–Cu/diamond interface. The results show that Cu alloying is an effective approach to promoting interface bonding between Al and diamond.     

Mechanical properties of a diamond–copper composite with high thermal conductivity

Using pressureless infiltration of copper into a bed of coarse (180 μm) diamond particles pre-coated with tungsten, a composite with a thermal conductivity of 720 W/(m K) was prepared. The bending strength and compression strength of the composite were measured as 380 MPa. As measured by sound velocity, the Young's modulus of the composite was 310 GPa. Model calculations of the thermal conductivity, the strength and elastic constants of the copper–diamond composite were carried out, depending on the size and volume fraction of filler particles. The coincidence of the values of bending strength and compressive strength and the relatively high deformation at failure (a few percent) characterize the fabricated diamond–copper composite as ductile. The properties of the composite are compared to the known analogues — metal matrix composites with a high thermal conductivity having a high content of filler particles (~ 60 vol.%). In strength and ductility our composite is superior to diamond–metal composites with a coarse filler; in thermal conductivity it surpasses composites of SiC–Al, W–Cu and WC–Cu, and dispersion-strengthened copper.     

Ultrafine-grained Ti–Nb–Ta–Zr alloy produced by ECAP at room temperature

To ascertain the influence of severe plastic deformation (SPD) on a Ti–Nb–Ta–Zr (TNTZ) alloy, we studied the room temperature mechanical behavior and microstructural evolution of an ultrafine-grained (UFG) Ti–36Nb–2Ta–3Zr (wt%) alloy prepared via equal-channel angular pressing (ECAP) of the as-hot-extruded alloy. The tensile behavior, phase composition, grain size, preferred orientation, and dislocation density of the UFG alloy, processed under different conditions, were analyzed and discussed. Compared to the as-hot-extruded alloy, the ECAP-processed TNTZ alloy (3 passes) exhibited approximately 40 and 88 % increase in average ultimate strength and yield strength, respectively. Moreover, as the number of ECAP passes increased from 3 to 6, the TNTZ alloy exhibited not only the expected increase in ultimate and yield strength values, but also a slight increase in elongation. Our results suggest that the deformation mechanisms that govern the behavior of the as-hot-extruded coarse grained (CG) TNTZ alloy during ECAP involve a combination of stress-induced martensitic transformation and dislocation activity. In the case of the ECAP-processed UFG TNTZ alloy, the deformation mechanism is proposed to involve two components: first, dislocation activity induced by the strain field imposed during ECAP; and second, the formation of α″ martensite phase during the early stages of ECAP which eventually transforms into β phase during continued deformation. We propose that the deformation mechanism governing the room temperature behavior of the TNTZ alloy strongly depends on the grain size of the β phase.     

Studies on the mechanical properties of steel–TiB2 composites obtained by high pressure sintering

The currently used composites produced by classical sintering methods are characterised by numerous limitations due to the difficulties in combining different materials with extreme properties. One of the ways to overcome these limitations is in the use of modern sintering methods, including the high pressure-high temperature process. This study describes the composite materials based on 316L austenitic steel reinforced with titanium diboride and examines the effect of sintering conditions on the mechanical properties and microstructure of sintered composites. It has been found that the key parameter in the manufacture of composites with optimal properties is the sintering time and temperature, while martensitic transformation taking place in the composite matrix can be controlled by the properly selected pressure applied during the sintering process.     

Enhanced thermal conductivity of SiCp/PS composites by electrospinning–hot press technique

Silicon carbide particle/polystyrene (SiCp/PS) electrospun mats are firstly prepared by electrospinning technology, then to be fabricated the corresponding thermally conductive SiCp/PS composites by the method of “laminating-hot press”. The mass fraction of SiCp and laminating mode of SiCp/PS electrospun mats affecting on the thermal conductivities, dielectric and thermal properties of the composites are investigated. The addition of 32.8 vol% SiCp improves the thermally conductive coefficient λ of pure PS from 0.182 to 0.566 W/m K and thermal diffusivity of pure PS from 0.169 to 0.376 mm²/s, whereas the dielectric constant values still remain at relatively low levels. The thermal stabilities of the SiCp/PS composites are increased with the increasing addition of SiCp. For a given SiCp loading, the SiCp/PS composites from warp–weft arrangement of SiCp/PS electrospun mats possess relative higher thermally conductive coefficient λ and dielectric constant values than those of SiCp/PS composites from warp–warp arrangement of SiCp/PS electrospun mats.     

Improved pressure–volume–temperature method for estimation of cryogenic liquid volume

One of the most important issues in a liquid propellant rocket is to measure the amount of remaining liquid propellant under low gravity environment during space mission. This paper presents the results of experiment and analysis of a pressure–volume–temperature (PVT) method which is a gauging method for low gravity environment. The experiment is conducted using 7.4 l tank for liquid nitrogen with various liquid-fill levels. To maximize the accuracy of a PVT method with minimum hardware, the technique of a helium injection with low mass flow rate is applied to maintain stable temperature profile in the ullage volume. The PVT analysis considering both pressurant and cryogen as a binary mixture is suggested. At high liquid-fill levels of 72–80%, the accuracy from the conventional PVT analysis is within 4.6%. At low fill levels of 27–30%, the gauging error is within 3.4% by mixture analysis of a PVT method with specific low mass flow rate of a helium injection. It is concluded that the proper mass flow rate of a helium injection and PVT analyses are crucial to enhance the accuracy of the PVT method with regard to various liquid-fill levels.     

Cu–Ti–C alloy with high strength and high electrical conductivity prepared by two-step ball-milling processes

This study reports a novel Cu–Ti–C alloy prepared using two-step ball-milling processes. Furthermore, the effects of the addition of both C and Ti to pure copper on the electrical conductivity and strength of the developed alloy were investigated in detail. In particular, the addition of both C and Ti in equal molar amounts to the copper matrix yielded an alloy with high strength without sacrificing the electrical conductivity. The addition of C both decreased the amount of titanium in the alloy matrix in the Cu–Ti alloy and led to the formation of homogeneously distributed nanocarbide precipitates in the matrix, which together contributed to the improved properties of the alloy.     

Ceramic–metal composites produced by laser surface treatment

Abstract

The injection of SiC particles (150 μm size) into laser surface melted commercial purity titanium, Ti–6Al–4V (wt-%) alloy, and Ti–2·5Cu (wt-%) alloy has been investigated using 1·75 kW laser power, 5 mm beam diameter, 0·15 g s^?l powder flowrate and traverse speeds ranging from 7 to 20 mm ^?l. Partial dissolution of SiC occurred and fine dendrites of TiC nucleated at the particle/matrix interfaces and also within the matrix. Silicon enrichment of the matrix and a eutectic constituent were observed. The microhardness of the melted zone was increased to 600–650 HV (500 g).

MST/964     

Thermal conductivity of polystyrene–aluminum nitride composite

The thermal conductivity of polymer composites having a matrix of polystyrene (PS) containing aluminum nitride (AlN) reinforcement has been investigated under a special dispersion state of filler in the composites: aluminum nitride filler particles surrounding polystyrene matrix particles. Data for the thermal conductivity of the composites are discussed as a function of composition parameters (aluminum nitride concentration, polystyrene particle size) and temperature. It is found that the thermal conductivity of composites is higher for a polystyrene particle size of 2 mm than that for a particle size of 0.15 mm. The thermal conductivity of the composite is five times that of pure polystyrene at about 20% volume fraction of AlN for the composite containing 2 mm polystyrene particle size. The relationship between thermal conductivity of composites and AlN filler concentrations has been compared with the predictions of two theoretical models from the literature.     

Formation of non–equilibrium alloys by high pressure melt quenching

Melt quenching under high pressure can promote the formation of metastable materials. High pressure accelerates the amorphization of Cu₆₀Ti₄₀ and Cd₄₃Sb₅₇.For an alloy systems having volume expansion after solidification, the higher the applied pressure, the lower the melting point and the higher the amorphization temperature, which promotes the formationof metallic glass. High pressure also enhances nucleation and suppresses grain growth, so solidification under high pressure can refine the crystal grains to form nanocrystalline alloys, such as Ti₆₀Cu₄₀,Cu₇₀Si₃₀ and Pd₇₈Si₁₆Cu₆ alloys.     

Effect of temperature on dielectric and electrical properties of Co–Zr doped barium hexaferrites prepared by sol–gel method

In the present research, temperature dependence of dielectric properties of cobalt–zirconium substituted barium hexaferrites, fabricated using citric acid sol gel method, has been reported. The dielectric constant, loss tangent and A.C. conductivity were investigated on the circular pellets in temperature range 30–350 °C and frequency range 10 kHz–1 MHz using impedance analyzer. This paper also presents impedance (Z*) and electric modulus (M*) analysis of all the samples. The single semi-circular arcs, observed in impedance Nyquist plots, suggest the dominance of grain boundaries in the conduction process. Dielectric constant and dielectric loss tangent show very small variation up to 200–250 °C temperature and abrupt increase afterwards up to 350 °C. Thus, these ferrites can be successfully implemented in the practical applications like capacitors, microwave devices etc. up to 250 °C, without any significant change in properties.     

Properties of coatings of the Al–Cr–Fe–Co–Ni–Cu–V high entropy alloy produced by the magnetron sputtering

It has been found that coatings from an Al–Fe–Co–Ni–Cu–Cr–V high entropy equiatomic alloy produced by the magnetron sputtering have nanocrystalline microstructures, are textured, and present a solid two-phase solution, which crystallizes in the bcc (a = 2.91 Å) and fcc (a = 3.65 Å) phases. The ion bombardment of a growing coating caused by the bias voltage (0–(–200) V), which has been applied to the substrate, decreases the growth rate of a condensate and affects its composition and structure. It has been shown that the composition of coatings deposited without an ion bombardment coincides with the target composition, whereas an increase of the ion bombardment intensity leads to the depletion of the coating composition in Al, Cu, and Ni and increase the microhardness. The anisotropy of the coating produced has been revealed.     

Synthesis and high temperature oxidation of Mo–Si–B–O pseudo in situ composites

In this paper, the new concept of ‘pseudo in situ composites’ is introduced into artificial composites for ultra-high temperature applications, composed of five phases, Mo, Mo₃Si, Mo₅Si₃, Mo₅SiB₂ and SiO₂. Among these phases, Mo and Mo₅Si₃ are not thermodynamically stable with each other, but they are locally equilibrated in the composites due to the formation of Mo₃Si as their reactant. Using the spark plasma sintering (SPS) technique, the Mo–Si–B–O pseudo in situ composites are successfully synthesized from Mo₃Si/Mo₅Si₃/Mo₅SiB₂ in situ composite powder, Mo and/or SiO₂ powders. The consolidated compacts are sound and fully dense, indicating that the SPS is a promising technique to synthesize the Mo–Si–B–O pseudo in situ composites. High temperature oxidation properties of the composites were examined up to 1673 K. The temperature range is divided into three with respect to the oxidation behavior; i.e. (I) below 1000 K, (II) between 1000 and 1400 K, and (III) above 1400 K. In the range II, the oxidation resistance of the composites is significantly improved by SiO₂ addition. In the range III, the oxidation resistance of the composites is good enough even at 1673 K in spite of the existence of Mo, displaying high potential for ultra-high temperature applications.     

Preparation of nanocrystalline CuTi by quenching the melt under high pressure

A new method of preparing a bulk nanocrystalline alloy by means of quenching a melt under high pressure has been developed. Using this method, a bulk CuTi alloy with 10–20 nm crystallites was synthesized. The structures and grain sizes of the alloy were examined by means of X-ray diffraction and TEM. We know of no precedent for using this method to directly prepare nanocrystalline alloys. The interfaces within the bulk alloys are very clean, and there is no porosity. The mechanism for nanometer-sized crystallite formation by this method is discussed.