Compared with traditional powder metallurgy (PM), tungsten copper electrode nano composite material including powder milling, compacting and sintering process, but due to the characteristics of nano particles, there are some differences between traditional powder metallurgy and nano tungsten powder preparation.

1. Phase transfer synthesis

According to the principle of phase transfer synthesis, uses unique ion exchange technology to produce ultra fine ammonium wolframate grains. And then put it to a common rotary tube furnace calcining and reduction at four tube furnace, with a high purity hydrogen -70 ℃ and powder passivation techniques to prepare nano scale tungsten particles. The specific area BET of this kind of nano tungsten powder is 6-10㎡/g (the granularity of BET is 30-50nm), the granularity of nano tungsten powder is 40-70nm, the chemical composition and properties meets a criterion of national standard.

2. Electroreduction

It uses tungsten or tungsten scraps as anode for electrolysis to produce methoxy-tungsten in methanol, which can be easily purified by sublimation and crystallization. Then uses hydrogen (H₂) to reduce methoxy-tungsten and fabricate high purity tungsten powder at 1000℃. In addition, it can use CO reduction to produce high purity tungsten carbide powder and fabricate tungsten powder in regular size by increasing reduction temperature.

3. Hybrid plasma

Thermal plasma plays an important role in the preparation of ultrafine powder by ultra-high temperature and high speed quenching. The ideal quenching rate is higher than 10℃_n^-1, DC plasma quenching rate is prompt and higher than 10℃_n^-1. Therefore, In the quenching plume of super-saturation quickly produce nucleation, which produce ultra-fine powder. By using the mixed plasma obtained from tungsten metal and ultrafine tungsten powder, the average particle size of 10μm can be obtained.

4. Laser irradiation

Related researchers from Japan develops a kind of system that uses low energy Nd : YAG as laser device to manufacture refractory metal nano grain, such as WTA, Mo. According to laser irradiation causes the liquid sputtered tungsten target to generate nano-particles of tungsten, tungsten nano particle size depends on the density and pressure of the laser energy, under different conditions can produce two types of particles, namely the large size of nano tungsten particles and small nano-size particles of tungsten.

Tungsten copper material is a kind of composite material that composed of tungsten and copper, which not only has high hardness and high density, but also has excellent electrical and thermal conductivity, electric erosive resistance and welding resistance. It is the best choice in electrode and contact materials, especially in EDM (electrical discharge machining), ECM (Electrochemical Machining), circuit breaker, vacuum contact and high voltage switch has an extensive application prospect. Generally, the comprehensive property of tungsten copper is determined by the proportion of tungsten and copper.

On one hand, tungsten (W) has high density, high hardness, high strength and good chemical stability, the hardness of tungsten copper electrode will remarkably increase with the increasing amount of tungsten, especially the tensile strength at high temperature. It can practically embody that the wastage of tungsten copper electrode is lower and the stability of machining is improved. On the other hand, owing to the increasing amount of tungsten, the content of Cu will decrease, which weakens the thermal and electrical conductivity of tungsten copper electrode. On the contrary, the less tungsten that is the content of Cu becomes a majority, the plasticity, thermal and electrical conductivity, sweat cooling and thermal shock resistance of tungsten copper electrode will be correspondingly improved.

In addition, there is a big difference between the melting point of tungsten and copper (W-3410℃, Cu-1084.5℃). It belongs to completely mutual insolubility system and only infiltrates with each other at high temperature so that it is not suitable for common casting process and requires powder metallurgy (PM) with copper infiltration (Tungsten skeleton infiltrated by copper liquid and the pores filled with copper at the high temperature higher than the melting point of copper).

Recent years, more and more tungsten copper electrode manufacturing processes are appearing, which improve the traditional process to strengthen some properties and push forward the better applications of tungsten copper electrode according to the different emphasizes of electrode (such as intercepting level, electric erosive resistance, welding resistance and so on). This paper concentrates on two kinds of process: one is research in nano tungsten copper fabricating, which produces nano grains through improving the porosity of tungsten skeleton infiltrated; another one is based on the traditional powder metallurgy (PM), add rare earth or rare earth oxide grain before copper infiltration. Both of two kinds of process have some advantages and disadvantages.

Compared with traditional powder metallurgy (PM), tungsten copper electrode nano composite material including powder milling, compacting and sintering process, but due to the characteristics of nano particles, there are some differences between traditional powder metallurgy and nano copper powder preparation.

3. Mechanochemistry

It combined both high energy ball milling and chemical reaction. High energy ball milling has many advantages, such as simple operation, high efficiency, especially suitable for the metal with high melting point, mutually insoluble solid solution system, micro intermetallic compound fabricating and micro metal ceramics composite materials. However, the distribution of grain produced in this way is uneven and it will easily bring in other impurities in the process of ball milling. The researchers uses Mechanochemistry, which smash the mixture of CuN and Na powder and produces Cu and NaCl micro nanocrystaline mixture by solid substitution reaction. Washing the mixture to remove the milled sodium chloride, ultrafine copper powder can be obtained. It can not be neglected that adding NaCl will avoid the mixture burning, and the copper powder will be finer (20-50μm of granularity).

4. γ Ray irradiation - hydrothermal crystallization

The method mainly in the experimental stage, some scholars use Co γ ray irradiation source strong metal ultrafine particles prepared using γ-ray radiation - hydrothermal crystallization method jointly received an average particle diameter of about 50μm of copper nano particles.

5. Liquid phase reduction

In liquid or very close to the liquid state, the raw material can be synthesized directly metal nano powders. The use of inorganic metal compounds in the alcohols (ethylene glycol, diethylene glycol, etc.) in the heating reduction can be prepared mono disperse copper nano particles. The researchers used hydrazine hydrate reduced copper pickled copper, while adding polyethylene polymer protective agent pyrrolidone (PVP) help stabilize the grain to prevent agglomeration, use magnesium Mg as the catalyst reaction, copper obtained a considerable part of condensed matter. However, the methods used hydrazine hydrate is highly toxic, is not suitable to promote production use.

6. Self suspended directional flow

Metal material melts in high frequency power supply heating to the metal droplets in countercurrent sensor electromagnetic field generated by spherical droplets of molten metal suspended in studio and continues to be heated. Meanwhile, the inert gas as a cooling medium is always a constant flow rate passing molten metal droplet surface. When the spherical droplet reaches a certain temperature, the surface of the metal vapor produced in the cooling stream of inert gas is too cold nucleation and subsequently grow up (including a collision with the merger between the particles), the final formation of the desired metal nano particles.

According to the mechanism of one solid phase sintering, we can build a compact model as follow (the show map of the change of the powder during the solid phase):

Viewed from the diagram, the particle with deep color, cladded internal is tungsten (W), and the external particles in white is copper (Cu), the particles contacts by point. In the solid-phase state, the shrinkage of powder takes up 1/3 shrinkage of total volume. Generally, tungsten copper W-Cu powder does not shrink in the solid-phase state. However, if the temperature rises up to or exceeds copper recrystallization temperature during the copper powder sintering, it will remarkably sintering in solid-phase.

In addition, tungsten grain of tungsten copper nano composite powder is disperse at the beginning, basically Cu phase contacts with each other. Therefore, some related researchers propose that Cu solid-phase sintering at the temperature lower than copper liquid is the main mechanism of sintering densification, and the internal tungsten grains cladded in Cu blocks out the sintering process to some extent.

Picture a to picture c shows that due to the diffusing and flowing effect of copper liquid, the contact area of particles is expanding, the internal gas is extruded so that the compact densification. Tungsten particles close to each other and in contact gradually under the action of the flow of fluid passing copper will eventually gather and grow up under the influence of interfacial tension.

Picture a is the initial status of compact, which powder is disperse and the contact of particles is point with small contact area, the small tungsten grain inside of Cu phase is separated by the larger Cu grain; Picture b shows that with the increasing sintering temperature gradually, the tension of diffusion of surface atoms and surface tension force it to flow to the contact, the contact area expands, the pores shrinks and the internal tungsten grain cladded by Cu phase get close, gathers and grows up by the flowing copper liquid and interfacial energy; Picture c, with the temperature increasing further, the contact area is becoming larger, the pore is shrinking to ball-shaped, tungsten grain is gathering and composed of the larger grain, which blocks out copper solid-phase sintering further.