Tungsten Copper Heat Sink Plasma Spraying (2/2)

The researchers use two kinds of spray gun (internal and external powder feed) to observe the structure and properties of tungsten copper heat sink. It can be found that in the internal powder feeding conditions, W content of tungsten copper composite material is higher than the content of external powder feed. This is due to internal feed powder is powder being ejected before the gun are all located in the plasma flame, part of the W particle is melted, melted and un-melted W particles, and the molten copper solution together is sprayed into the substrate, so that easy obtain raw material powder composition approximating tungsten copper composite material; while in the process of external powder feed, he trajectory of the powder particles are not entirely consistent with the plasma flame, which can only make a fraction of smaller particle size of the W particles melt, and a lot without melting of W particles are difficult to be deposited, so the W collected rate decrease.

Besides, at internal powder feeding conditions, only a small amount of cuprous oxide can be detected, which indicates the impact of power in copper oxidation is not obvious. This is due to the internal powder feeding only a very small amount of oxygen is brought into a powder particle flow, thereby largely avoiding the oxidation of copper; by contrast, in the same power, a large number of external powders feed oxygen into the powder particles flow and exacerbates the copper oxide. With the power increase, the plasma flame temperature also increases, the oxidation of copper increased significantly. Overall, plasma spraying uses plasma arc, which compared with free arc has fine arc, high current density, and high degree of gas ionization. So it has many advantages, such as high temperature, high energy concentration, excellent arc stability, which is the ideal material for tungsten copper heat sink.

Tungsten Copper Heat Sink Plasma Spraying

With the rapid development of science and technology, integrated in the microelectronics industry sectors related electronic products have become more sophisticated, the corresponding energy consumption also increased. So it places higher demands on heat sink properties, which not only required high density and strength, but also has excellent electrical conductivity, thermal conductivity and low coefficient of thermal expansion. Tungsten copper is composed of W with high hardness, high density, high strength, high melting point and low coefficient of thermal expansion and Cu with perfect electrical and thermal conductivity so that it can be an ideal material for heat sink. Furthermore, the properties of tungsten copper heat sink can be improved by the adjustment of W and Cu content.

Since there is a great difference in physical and chemical properties between W and Cu, which has 2000℃ difference in melting point. Both of them do not form a compound or solution, so it is a kind of typical pseudo alloy. Therefore, it is difficult to achieve by common pressed and sintered, the infiltration is the best way. But infiltration is easy to form pores certain deficiencies and to bring some difficulties to the subsequent processing. Plasma spraying is a strengthening of the material surface and surface-modified new multi-purpose precision spraying techniques. It uses plasma arc drove by DC as the heat source and has the following characteristics:

1. Ultra-high temperature characteristics, suitable for spraying refractory material;

2. High particle injection rate, the coating is dense and has higher bonding strength;

3. In the process of spraying, inert gas used as the shielding gas so that the coating material difficult to be oxidized.

Plasma spraying technique can make the surface of the substrate with a high temperature oxidation, thermal insulation, anti-friction, wear and corrosion, insulation, radiation protection and sealing properties, which can heat metal, alloy and ceramics to molten or semi-molten state and spray to the surface after pretreatment at high speed to form a firmly attached layer. In addition, plasma spraying also can be used in medical field, the coating layer of a few microns in artificial bone surface coating can achieve enhanced affinity for artificial bone and the strengthening effect.

Tungsten Copper Heat Sink Material Hermeticity (2/2)

Merely rely on raising the temperature of the tungsten blanks shrink to increase the density, which is difficult to control precisely. Such as the deviation of heat matching, it is the major influencing factor of conventional infiltration hermeticity. Some researchers try to ultrafine tungsten powder added to certain activators, such as nickel (Ni), cobalt (Co), iron (Fe), etc., and then mixed with copper powder. After pressing and activated sintering, the relative density of tungsten copper can reach 98 % or more, which can be a good solution to tungsten copper heat sink material hermeticity issue. But it added activator of Ni, Co, Fe and other elements will be Cu and W-phase and phase mutually dissolved, and magnetic Fe, Cu content changes will directly affect the conductivity and the thermal conductivity of the material, so not suitable in the heat sink industry applications.

Therefore, on this basis, the researchers conducted a process optimization, the tungsten powder mixed with a small amount of copper powder, copper powder can effectively preserve this part of the green bodies connected pores, when carried out at high temperature and pressure infiltration of liquid copper can be fully effective filled tungsten skeleton. This section is also known as copper induction of copper, its main role lies in two main aspects, one is to enhance the strength of tungsten blanks, and the other is infiltration process hermeticity guarantee.

Tungsten powder has high hardness, poor plasticity, generally under pressure prone to "arch effect" between the powder. Once the pressure is too large, blank-arch bridge will make a release force is rupture and delaminated, and thus the relative density of tungsten powder can only reach about 60%. But copper powder has good plasticity, which can effectively destroy the "arch effect" between the tungsten powder and plastically deformed. Both of them can fill each other, engage with each other, thereby increasing the density and strength of tungsten copper green. On the other hand, since the initial tungsten skeleton have a uniform distribution of copper, the part of copper changed into liquid and complement with the copper liquid out side the skeleton at 1350℃.

Tungsten Copper Heat Sink Material Hermeticity

Based on high density, high strength, excellent chemical stability and arc ablation resistance, tungsten copper material also has perfect thermal and electrical conductivity, which can be widely used in EDM electrodes, electrical contacts, military industry and heat sink materials. However, tungsten copper electrodes and contacts used for electrical machining has different requirements of properties with tungsten copper used in heat sink. As heat sink materials, tungsten copper not only be supposed to have excellent thermal conductivity, but also should have excellent hermeticity to ensure the equipment working stability. Since any porosity and leakage will lead to the operation failure of the microwave component.

In the early 1980s, tungsten copper has been chosen as heat sink materials, which due to it has the similar coefficient of thermal expansion to some ceramic, and both of them after brazing can not only guarantee the thermal matching, but also form a good thermal channel. Theoretically, W and Cu has big difference in physical and chemical properties, which the difference of melting point of them are as much as 2000℃. And they can not form a solid solution or a compound so that it is only suitable for using powder metallurgy process. And the best process is infiltration, tungsten powder is compacted to a certain density of blank, after sintering and shrinking at high temperature forms tungsten skeleton with a certain density and then at a temperature above the melting point of copper was infiltrated tungsten skeleton.

Since the high hardness and poor plasticity of tungsten powder, using a general manufacturing method infiltrated tungsten copper alloy will be some of the pores, the relative density can only reach about 60%. Densification at high temperatures also have some closing pores in copper infiltration process is not sufficiently filled with tungsten skeleton, reducing the product's tightness. In order to reduce the porosity of the relevant parameters need to be improved so that it can meet the requirement of hermeticity.

Tungsten Copper Sheet Properties

Tungsten copper sheet properties refer to phase analysis, micro-structure observation and other properties detection. The phase analysis can be specifically divided into DTA (Differential Thermal Analysis), XRD phase analysis (X-ray Diffraction), component test and so on. DTA uses differential thermal analyzer, the principle is to compare by not undergo any chemical reactions and physical changes at a certain temperature stable substance as a reference with an equal amount of measured (tungsten copper sheet) in the same environment under constant variable temperature conditions. Any physical and chemical changes on the measured and it is located in the same environment as compared to the standard temperature will be a temporary increase or decrease. The XRD phase analysis uses copper K radiation target, adjust current, voltage, and scanning rate, the measured characteristic X lines of tungsten copper sheet specimen major constituent phases.

For tungsten copper material, component test usually uses dilute nitric acid - hydrofluoric acid solution, which due to W will precipitate in nitric acid medium in the form of acid and separated from the Cu. Next ammonium hydroxide precipitate was dissolved tungstate, ammonium tungstate determination of tungsten burning gravimetric method; then inductively coupled plasma atomic emission spectrometry filtrate the residual tungsten, both of which add up to the total amount of tungsten. The process of micro-structure of tungsten copper sheet detecting is Sandpaper milling → Washing → Polishing (Al2O3) → Washing → Alcohol Cleaning → Drying → Detecting Sample.

The performance tests include density test, hardness test (Vickers hardness HV), resistivity (electrical conductivity) test, thermal conductivity test, coefficient of thermal expansion test and so on. Density test uses a classic method of drainage method of Archimedes; micro hardness is measured by the length of indentation of diamond indenter pressed into the surface of the sample; the resistivity usually uses bridge method; Thermal conductivity is the first on the surface of tungsten copper sheet sample sprayed on the surface of the toner in order to prevent the reflection of incident light, then at a certain temperature using the flash method thermal analyzer heating element xenon lamp emits a pulse beat in the sample surface, warmed by the infrared detector measures the situation was thermal diffusivity.

Nano Tungsten Copper Electrode Research

The granularity and uniformity of tungsten and copper powder has an effect on tungsten copper electrode sintering, and the decreasing granularity will improve the comprehensive properties of materials, such as the density, the hardness, the electric and thermal conductivity. Theoretically, powder metallurgy (PM) uses metal powder (or metallic oxide powder) as raw materials, after compacting and sintering, the granularity of powder is coarser. Compared with PM, nano tungsten copper has higher surface energy, which atoms move by higher interfacial energy, shrink the small pores in sintering process and in case of the diffusion of pores. Therefore, nano tungsten copper research is beneficial for the densification at lower temperature.

Although the process of nano tungsten copper is similar to the traditional PM, which can be divided into preparation, compacting and sintering, due to the characteristic of nano particle, there are some differences between them. Overall, the most widely used process of nano tungsten copper main are Sol-Gel method, Metal Alloying (MA), Mechano-Thermo-chemical Process, spray drying and so on.

Metal Alloying (MA), uses high energy ball milling for tungsten copper mixed powder grinding, achieves the nano powder (the granularity is about 20nm-30nm). Then compacts the blank by the nano powder and sintering in hydrogen atmosphere at 1000℃, nano tungsten copper with high related density can be achieved.

The so-called sol - gel (Sol-Gel) is the use of compounds with high chemical composition as the precursor in the liquid phase under these uniformly mixed materials, hydrolysis and condensation reactions to form a stable transparent sol in solution sol particles by aging between slow polymerization to form a three-dimensional network structure of the gel, the gel network between full solvent loses fluidity to form a gel. After drying, sintering and solidifying, sol can be produced with molecular or nano substructure. At the academic front, some researchers succeed in preparing MoW and CuW high-performance powder. However, this kind of process has some shortcomings, such as the content of water vapor and impurities is difficult to control in the process of hydrogen reduction, which these problems will influence the later sintering process.

Mechano-Thermo-Chemical process, uses ammonium paratungstate and copper nitrate as raw materials to prepare a mixed solution, and then spray drying the mixed solution to obtain a metal powder mixture through a rotary atomizer and the subsequent drying process of the precursor, the precursor of combustion, the formation of tungsten copper W-Cu composite oxide powder, then milled oxide powder and two-step hydrogen reduction, thereby preparing nano-tungsten-copper composite powder.