Porous tungsten with controllable porosity by low temperature sintering
Porous tungsten as a high current density cathode is one of the important applications of tungsten. Its porous framework is an important part of reserve cathode. The framework should be impregnated with electron emission compounds. After each emission from the surface, new materials should be fed into the surface pores through the open pore channels, which requires very uniform porosity. However, the conventional sintering method of tungsten is carried out in high temperature (≥ 2 000°C) and strong reducing atmosphere, so the porosity is difficult to control. For this reason, British scholar C. seleuk et al. Developed a new method of low temperature reaction sintering, which can well control the micro pore structure. The principle of the method is that the metal oxide reacts with low melting point aluminum to release heat for tungsten sintering. The cost of this reaction sintering method is low, and the parts with high quality microstructure can be produced.
Porous tungsten blanks were prepared by tungsten powder with average particle size of 8 μ m and 18.4 μm and aluminum powder with average particle size of 6 μm. The method is that the tungsten powder is oxidized in air at 200-500°C to obtain an oxide film on the surface of the powder particles, and then the oxidized powder is mixed with aluminum (≤ Lu%) and milled for 20 min. aluminum powder is coated on the surface of tungsten oxide. Molding and isostatic pressing (pressure greater than 275 MPa) were used respectively. The compacts were sintered in ammonia decomposition atmosphere (75% H2, 25% N2) or pure hydrogen atmosphere. Firstly, the billet was heated to 550°C at a heating rate of 10 K / min for 15 min, and then heated to 1 150°C at a rate of 20 K / min for sintering for 1 h. finally, the compacts were cooled in the furnace in hydrogen atmosphere. According to the size and mass of the sample, the relative density of the sample before and after sintering is 60% and 80%, respectively. For comparison, porous tungsten samples were also prepared by conventional sintering method (sintering temperature ≥ 2 000°C). The results of SEM, XRD and microhardness of the two samples are as follows.
(1) The density distribution of porous tungsten produced by conventional sintering method is not uniform, especially the density difference between the center and the periphery of the sample is larger. However, the density distribution of the reaction sintered sample is very uniform whether it is molded or cold isostatic pressing. The microhardness of the surrounding and central area of the cross section of the sample is the same, which indicates that the porosity inside and outside the sample is the same. The ratio of microhardness to density of reaction sintered samples is lower than that of conventional sintered samples, which may be due to the existence of residual oxides in reaction sintered samples.
(2) The heat energy of reaction sintering process consists of two parts: a large amount of internal heat released by reduction reaction and external heat from furnace heating. The bonding of tungsten powder mainly depends on the internal heat released by thermite reaction. The reaction of Wo with aluminum gives rise to a temperature of over 4 000°C, which indicates that this reaction should be a high temperature self propagating synthesis process. Once the ignition reaction is ignited, the reaction will propagate until the reaction is completed. In the process of tungsten reaction sintering, the reductant and aluminum start fire at the same time, so the tungsten powder particles are easy to bond and deform at high temperature, resulting in high bonding strength. Aluminum with low melting point forms a liquid flow between tungsten particles and reacts along the channel, and the reaction heat enters into the system. The sintering is completed together with external heating.
（3）Thermal analysis shows that the exothermic reaction is about 550 ~ 1150°C. the metal thermal reaction starts at about 660 °C, which provides heating source for the system just after aluminum melting. The melting temperature of aluminum is lower than that of aluminum (662°C) due to solid phase diffusion during heating. At about 800°C, an endothermic phase transition occurs, and the temperature fluctuation of the phase transition is about 100 C (780 ~ 880°C). The phase transformation may be due to the sublimation of WO3 and the reduction of WO3 to low valence oxides and the existence of oxygen to solid oxides outside the system.
（4）The heat energy of reaction sintering process consists of two parts: a large amount of internal heat released by reduction reaction and external heat from furnace heating. The bonding of tungsten powder mainly depends on the internal heat released by thermite reaction. The reaction of Wo with aluminum gives rise to a temperature of over 4 000°C, which indicates that this reaction should be a high temperature self propagating synthesis process. Once the ignition reaction is ignited, the reaction will propagate until the reaction is completed. In the process of tungsten reaction sintering, the reductant and aluminum start fire at the same time, so the tungsten powder particles are easy to bond and deform at high temperature, resulting in high bonding strength. Aluminum with low melting point forms liquid flow between tungsten particles and reacts along the channel. The reaction heat enters into the system and completes sintering together with external heat.
(6) Thermal analysis shows that the exothermic reaction is about 550 ~ 1150°C. the metal thermal reaction starts at about 660°C, which provides heating source for the system just after aluminum melting. The melting temperature of aluminum is lower than that of aluminum (662°C) due to solid phase diffusion during heating. At about 800 C, an endothermic phase transition occurs, and the temperature fluctuation of the phase transition is about 100 C (780 ~ 880°C). The phase transition may be the sublimation of WO3, accompanied by reduction to low valence oxides and oxygen to the solid outside the system.
