Tungsten Copper Electrode — Chemical Co-precipitation

Tungsten copper is a kind of psudoalloy, which is composed of W with body-centered cubic structure (bcc) and Cu with face-centered cubic (fcc) structure and they are immiscible or not forming intermetallic compound. It not only has high hardness, high strength, low coefficient of thermal expansion and excellent wear resistance and corrosion resistance of W, but also has excellent electrical conductivity, thermal conductivity and plasticity, which can be widely used in some electrical contact materials or electrode materials. Generally, the basic process is: mixing → forming → infiltration, sintering → post-processing.

But tungsten copper electrode fabricated by conventional process, regardless of liquid-phase sintering or solid-phase sintering, the relative density of tungsten copper products produced will lower than 98% because of the wetting angle and the insoluble W in liquid Cu. Although the heating process and re-pressing or re-sintering can remarkably improve this situation, the cost is increasing and the overall efficiency is decreasing. Furthermore, in the sintering process will happen inevitably growth of tungsten grains, which would make it difficult to meet the performance of the market demands tungsten copper electrode material now.

In order to improve the sintering density of tungsten copper electrode and other tungsten copper composite materials, relevant researchers use activated sintering by adding additives. However, the activator can significantly improve the density of tungsten-copper alloy, it will also have some effects on the electrical and thermal conductivity, which is not suitable for the occasions has high demands on electrical and thermal conductivity. So chemical co-precipitation of copper tungsten nano composite powder prepared by reduction combined with hydrogen, by pressing and sintering process to obtain a high-performance compact tungsten copper alloy electrodes become a new hotspot.

The definition of chemical co-precipitation is that in solution mixing materials with different chemical compositions were prepared precursor precipitate mixture added a suitable precipitating agent, then the precipitate is dried or calcined, to thereby obtain the corresponding powder particles. It has two advantages, one is a nano-powder materials can be obtained directly by chemical composition homogeneous solution in a variety of chemical reactions, and the other is easy to prepare small particle size and distribution of nano-powder materials.

The experiment use ammonium tungstate, copper sulfate pentahydrate and concentrated nitric acid as raw materials. The specific operation of concentrated nitric acid in a copper nitrate solution was added with stirring, and then the mixed solution was added a solution of ammonium tungstate, a magnetic stirrer in the chemical co-precipitation reaction; After 1h the reaction solution was removed and the precipitate was placed in a muffle furnace firing 2h, to obtain tungsten copper composite oxide powder; after reduction by hydrogen obtained tungsten copper composite powder; then the tungsten-copper composite powder is extruded to form a green with a certain density and strength, ultra-fine grain was finally sintered tungsten copper alloy products.

Tungsten copper powder is polygon and the granularity is between 30nm – 50nm. Cu into a network-like structure of the W particles are bonded together in an approximation polygon and evenly distributed. This is because the raw material for preparing tungsten-copper composite powders of tungsten and copper salt solution, the distribution of the particles in the solution itself more evenly in the strong magnetic stirring effect, will make the precursor powder to retain the basic solution mixed state molecular level. And calcination and hydrogen reduction process in low temperature effectively inhibited the growth W grains. In addition, the initial structure of the powder is the oxides of tungsten and copper, their mutual isolation is not conductive to the growth of tungsten oxide particles, but is beneficial for evenly dispersed tungsten copper composite powder. From the fracture morphology, W grain evenly distributed in the Cu phase, forming larger dimples around, Cu phase has continuous web-like distribution along with the W grains. From the structure, with the increase in the sintering temperature, tissue distribution is more uniform, the porosity is also decreased. From the properties of alloy, Tungsten-copper composite compacts during sintering two-phase distribution, effective at power-conduction electron-average running speed increases, the conductivity also increases; complete copper network-like structure decrease the contact area of coarse W grains, which is difficult to form necking and beneficial for thermal conduction.