International Journal of Refractory Metals and Hard Materials
Microstructure and sintering behavior of W–Cu nanocomposite powder produced by thermo-chemical process
Introduction
W–Cu composites have been used as heavy duty electrical contacts and heat sink materials for high power microelectronic devices [1], [2]. Generally, W–Cu composites are fabricated by the infiltration process. However, this technique can be only used for fabricating the composites with low W composition due to the limitation of copper composition because the composition of copper infiltrant strongly depends on the porosity and structure of the tungsten skeleton. Therefore, the conventional sintering process has been recognized as the only way to achieve full density of W–Cu composite with high W composition. However, it is generally known that the W–Cu hardly reaches full density without the addition of a small amount of sintering aids such as Co and Ni [3], [4], [5]. Unfortunately, the sintering aids cause the deterioration of thermo-physical properties such as the thermal conductivity of composite materials [6]. For this reason, full density processing of high W composition W–Cu composite without the use of sintering aids are most desirable.
From this viewpoint, some attempts have been made to obtain nano-sized particles and homogeneous mixing of the W–Cu powder. It has been reported that W–Cu nanocomposite powder can be fabricated by mechanical alloying [7] and a mechano-chemical process [8]. However, these techniques, especially mechanical alloying process, are problematic due to contamination during the process. Recently, the thermo-chemical process has been suggested in order to solve the contamination problem [9]. The thermo-chemical method consists of spray drying and calcination of salts as well as subsequent hydrogen reduction of calcined oxide powder. This process is advantageous since the contamination due to mechanical milling process can be avoided effectively.
To produce an optimum W–Cu composites with high W content from the nanocomposite powder, the microstructure and sintering process of the powder should be carefully controlled. This is because W–W sintering, which is negligible in coarse powder, can occur easily in the initial sintering process of nanocomposite powder during heat up [10]. Such an activated W sintering behavior in initial sintering stage leads to formation of strong W skeleton structure which suppresses subsequent liquid phase sintering of W–Cu.
In this respect, the understanding of microstructure development specially during heat-up sintering of W–Cu nanocomposite powder is essential to optimization of production of thermo-chemical process of W–Cu nanocomposite powder. From this point of view, the microstructure and sintering behavior of W–Cu nanocomposite powder produced by the thermo-chemical process were investigated in terms of the characteristics of nanocomposite powders and the influence of residual oxide phase.
Section snippets
Experimental
The starting solution was prepared by dissolving ammonium metatungstate, (NH4)6(H2W12O40) · 4H2O, in an aqueous solution of copper nitrate, (Cu(NO3)2·3H2O), to obtain a final composition of W–20wt.%Cu (W–20Cu). Spray drying was performed using a rotary atomizer with a solution feed rate of 20 ml/min and an atomizer rotating speed of 11,000 rev/min in a hot air stream (250 °C). A precursor powder obtained by spray drying was burnt out at 750 °C for 1 h in air to remove organic salt components and
Spray dried oxide powder
Fig. 1 shows an SEM micrograph of spray dried oxide powder with the composition of W–20wt.%Cu. It is seen that the oxide powder is comprised of spherical agglomerates in the size range of 10–50 μm. Fig. 2 shows the XRD pattern of the oxide powder which reveals that the oxide powder consists of WO3 and copper tungstate (CuWO4) phases.
It has been reported that the microstructure of oxide agglomerate substantially affects hydrogen reduction behavior, as a consequence determining the structure of
Conclusions
The microstructure and sintering behavior of W–20wt.%Cu nanocomposite powder produced by a thermo-chemical process were investigated. Spray dried W–Cu oxide powder showed spherical agglomerate with a hollow structure and a different Cu phase distribution according to the radius. The hollow structure of the oxide powder was changed into fine W–Cu nanocomposite agglomerates during hydrogen reduction process. The nanocomposite W–20Cu powder consisted of W–Cu composite particles less than 200 nm in
Acknowledgements
The authors gratefully acknowledge the financial support of the Dual Use Technology Program of the Ministry of Science and Technology of Korea.
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