Chengchang Jia

Thermal Properties of Carbon Nanotube–Copper Composites for Thermal Management Applications

Carbon nanotube–copper (CNT/Cu) composites have been successfully synthesized by means of a novel particles-compositing process followed by spark plasma sintering (SPS) technique. The thermal conductivity of the composites was measured by a laser flash technique and theoretical analyzed using an effective medium approach. The experimental results showed that the thermal conductivity unusually decreased after the incorporation of CNTs. Theoretical analyses revealed that the interfacial thermal resistance between the CNTs and the Cu matrix plays a crucial role in determining the thermal conductivity of bulk composites, and only small interfacial thermal resistance can induce a significant degradation in thermal conductivity for CNT/Cu composites. The influence of sintering condition on the thermal conductivity depended on the combined effects of multiple factors, i.e. porosity, CNTs distribution and CNT kinks or twists. The composites sintered at 600°C for 5 min under 50 MPa showed the maximum thermal conductivity. CNT/Cu composites are considered to be a promising material for thermal management applications.

Thermal conductivity and microstructure of Al/diamond composites with Ti-coated diamond particles consolidated by spark plasma sintering

Metal/diamond composites have been considered as the new generation of thermal management material. The critical challenge to obtain composites with high thermal conductivity (TC) is to improve the interfacial bonding between the matrix and diamond. In the present study, a titanium coating was plated on the surface of diamond particles via vacuum evaporation–deposition, and Al/diamond composites were consolidated by spark plasma sintering (SPS) technique. The TC and microstructure of composites, respectively, with coated and uncoated diamond particles are compared and discussed. The results show that the Ti coating can significantly increase the wetting property between Al and diamond, leading to a strong interfacial bonding. The diffusion of Ti into the matrix and the formation of TiC are detected at the Al–diamond interface. The properties of composites, respectively, with coated and uncoated diamond exhibit different trends with increasing sintering temperature or diamond volume fraction. Compared with composites with uncoated particles, the Al/Ti–diamond composites obtained the much higher relative density and TC as high as 491 W/mK. Based on the comparison between the experimental and theoretical values, it is found that the thermal conductivities of Al/Ti–diamond composites have reached or surpassed the theoretical calculations with the particle volume fraction not more than 50%.

Experimental and modeling study of the thermal conductivity of SiCp/Al composites with bimodal size distribution

The thermal conductivity of SiCp/Al composites with high volume fractions of 46 to 68% has been investigated. The composites were fabricated by pressureless infiltrating liquid aluminum into SiC preforms with monomodal and bimodal size distributions. The density measurement indicates that a small amount of pores is presented for the composites approaching their maximum volume fractions. An analytical model with an explicit expression is proposed for describing the thermal conductive behavior of the composites with multimodal-reinforced mixtures in terms of an effective medium approach taking into account the porosity effect. Predictions of the developed effective medium expression reveal good correspondence with the experimental results, and explore how each of the considered factors (i.e., particle size ratio, volume fraction ratio, and porosity) can have a significant effect on the thermal conductivity of the composites with bimodal mixtures.

Fabrication and thermal conductivity of copper matrix composites reinforced by tungsten-coated carbon nanotubes

Carbon nanotubes (CNTs) were coated by tungsten using metal organic chemical vapor deposition. Magnetic stirring was employed to disperse the W-coated CNTs (W-CNTs) in a Cu matrix, and then, the mixed powders were consolidated by spark plasma sintering. The W-CNTs obtained a uniform dispersion within the Cu matrix when the W-CNT content was less than 5.0vol%, but high content of W-CNTs (10vol%) resulted in the presence of clusters. The W-CNT/Cu composites containing low content of W-CNTs (<5.0vol%) exhibited a higher thermal conductivity than the sintered pure Cu, while the CNT/Cu composites exhibited no increase in thermal conductivity after the incorporation of uncoated CNTs. The W-CNT content was found to play a crucial role in determining the thermal conductivity of the W-CNT/Cu composites. The thermal conductivity of the W-CNT/Cu composites increased first and then decreased with the W-CNT content increasing. When the W-CNT content was 2.5vol%, the W-CNT/Cu composite obtained the maximum value of thermal conductivity. The thermal resistance of the (W-CNT)-Cu interface was predicted in terms of Maxwell-Garnett effective medium approximation, and its calculated value was about 3.0×10−9 m2·K·W−1.

Microstructure and thermal conductivity of Cu–B/diamond composites

This article considers the potential of boron as matrix-alloying element and gives perspectives about which content of boron is favorable to maximize the interfacial bonding and thermal conductivity of copper/diamond composites. The thermal conductivity of Cu–B/diamond composites is investigated both experimentally and theoretically. The thermal conductivity measurements show that the optimum boron content at 0.8 wt% has provided highest thermal conductivity of 538 W/mK, increases 190% compared to that of copper/diamond composite without boron addition. Theoretical models are used to understand the underlying thermal conductivity enhancement mechanisms of matrix alloying. It is found that the Hasselman–Johnson model combined with a modified Debye model considering the carbide thermal resistance can provide a satisfactory agreement to the experimental data.

Selective interfacial bonding and thermal conductivity of diamond/Cu-alloy composites prepared by HPHT technique

Cu-based and Cu-alloy-based diamond composites were made by high-pressure-high-temperature (HPHT) sintering with the aim of maximizing the thermal conductivity of the composites. Improvements in interfacial bonding strength and thermo-physical properties of the composites were achieved using an atomized copper alloy with minor additions of Co, Cr, B, and Ti. The thermal conductivity (TC) obtained exhibited as high as 688 W·m−1·K−1, but also as low as 325 W·m−1·K−1. A large variation in TC can be rationalized by the discrepancy of diamond-matrix interfacial bonding. It was found from fractography that preferential bonding between diamond and the Cu-alloy matrix occurred only on the diamond {100} faces. EDS analysis and Raman spectra suggested that selective interfacial bonding may be attributed to amorphous carbon increasing the wettability between diamond and the Cu-alloy matrix. Amorphous carbon was found to significantly affect the TC of the composite by interface modification.

Effect of solution cooling rate on the γ′ precipitation behaviors of a Ni-base P/M superalloy

Abstract The effect of cooling rate on the cooling γ′ precipitation behaviors was investigated in a Ni-base powder/metallurgy (P/M) superalloy (FGH4096). The empirical equations were established between the cooling rate and the average sizes of secondary and tertiary γ′ precipitates within grains and tertiary γ′ precipitates at grain boundaries, as well as the apparent width of grain boundaries. The results show that the average sizes of secondary or tertiary γ′ precipitates are inversely correlated with the cooling rate. The shape of secondary γ′ precipitates within grains changes from butterfly-like to spherical with the increase of cooling rate, but all the tertiary γ′ precipitates formed are spherical in shape. It is also found that tertiary γ′ may be precipitated in the latter part of the cooling cycle only if the cooling rate is not faster than 4.3°C/s, and the apparent width of grain boundaries decreases linearly with the increase of cooling rate.

Fabrication of micro gear wheels by micropowder injection molding

The micropowder injection molding technology was investigated to fabricate the microsized gear wheels on a conventional injection molding machine. The feedstock comprised of carbonyl ferrum powder and a wax-based thermoplastic binder. Microinjection molding was fulfilled at about 423 K under 100 MPa. The heating system was applied to the die to improve the fluidity of the feedstock and subsequently the cooling system was used to enhance the strength of the green compacts after injection by decreasing the temperature of the die. The gear wheels were realized successfully with their addendum circle diameter ranging from 800 to 200 μm and with the center hole as small as 60μm.

Effect of sintering parameters on the microstructure and thermal conductivity of diamond/Cu composites prepared by high pressure and high temperature infiltration

Pure Cu composites reinforced with diamond particles were fabricated by a high pressure and high temperature (HPHT) infiltration technique. Their microstructural evolution and thermal conductivity were presented as a function of sintering parameters (temperature, pressure, and time). The improvement in interfacial bonding strength and the maximum thermal conductivity of 750 W/(m·K) were achieved at the optimal sintering parameters of 1200°C, 6 GPa and 10 min. It is found that the thermal conductivity of the composites depends strongly on sintering pressure. When the sintering pressure is above 6 GPa, the diamond skeleton is detected, which greatly contributes to the excellent thermal conductivity.

On CTE of SPS consolidated SiCp/Al composites with various particle size distributions

Abstract The coefficient of thermal expansion (CTE) of spark plasma sintering consolidated SiCp/Al composites with various size distributions was investigated with the combination of experimental measurements and modelling analyses. The CTE of the composites decreased with increasing particle volume fraction, and large particles played a major role in the decline of CTE. The measured CTE lay between the predictions of Kerner model and Schapery lower bound, but the possible formation of percolating particle network and the influence of matrix plasticisation led to the slight deviation of the experimental values from model predictions. A CTE peak appeared for all the composites with increasing temperature to about 250–300°C due to the action of matrix plasticisation filling the microvoids in the composites. The composites with mixed particles of substantially different sizes were prone to concentrate thermal stresses on large particles, which induced an early appearance of matrix plastic deformation that can result in a comparably low CTE peak temperature.