Nanosized Copper Tungsten Alloy from Ammonium Paratungstate

Copper tungsten alloy (W-Cu) is the pseudo-alloy of tungsten and copper, which exhibits some excellent properties such as a high melt point of 3042℃.  Owing to its good thermal and electrical conductivity, low thermal expansion, being non-magnetic, good performance under high temperature and vacuum. It has been widely applied in industry such as electrodes for resistance welding, arc contacts and vacuum contacts in high/medium voltage breakers or vacuum interrupters, electrodes in electric spark erosion cutting machines, heat sinks and heat spreaders for passive cooling of electronic devices, electronic packaging materials, and radio base station components

Homogeneous structure of W-Cu alloy is a major factor to its higher properties. However, high densification and homogeneous structure of the alloy is not easy to reach as different melting points and densities of the two materials. The size of the grain is another important factor, a nano size could largely enhance its performance.  In order to overcome these disadvantages, a thermal plasma route was used to produce core-shell W-Cu nanoparticles with enhanced sintering activity for high-performance alloy fabrication. Ammonium paratungstate (APT) and copper nitrate were used as the tungsten and copper source.

The synthesis process can be concluded as below:

Ammonium paratungstate (APT) and copper nitrate with theoretical Cu weight ratio of 20% were pre-mixed and used as precursor to prepare W-Cu composite powders, and the experiments were conducted on a radio frequency thermal plasma system (30 KW, 4 MHz) using argon as center gas and sheath gas. The details of the setup were illustrated in our previously work. In a typical synthesis process, the starting materials were axially injected into thermal flame with a feeding rate of 10 g/min using hydrogen as carrier gas (200 ml/min), and underwent chemical reaction and crystal growth with flowing gas in reactor. Finally, the obtained products were collected at the bottom of collector. The as-synthesized W-Cu nanopowders were then pressed into mould (ϕ = 10 mm) under the pressure of 400 MPa and keeping for 5 min. Subsequently, the obtained W-Cu samples (ϕ = 10 mm) were heated to the required temperature in a tube furnace  under flowing hydrogen atmosphere, and then naturally cooled down to room temperature.

The morphologies of the obtained composite powders and sintered specimens were characterized by scanning electron microscopy and transmission electron microscopy

To sum up, a thermal plasma route was used to produce core-shell W-Cu nanoparticles with ammonium paratungstate and copper nitrate. The grain size of the alloy is 30 nm and shell thickness is 3 nm. As synthesized nanopowders merit the homogeneous elemental distribution and short diffusion distance for mass transport, and responsible for the sharply density increase during liquid sintering stage compared to pure W and mixed W-Cu nanoparticles. As such, the homogeneous alloys with nearly full densification of 99.31% and grain size of 260 nm were obtained at low temperature of 1100 °C, and in turn dramatically improved its mechanical and electrical conductivity properties due to the synergistic strengthening effect derived from the enhanced density and suppressed grain size.

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