Since the ductility of tungsten is very sensitive to most of the impurities, purification is important. Therefore, special care must be taken so that, during sintering, evaporation can take place to the desired extent (i.e., as long as there is an open porosity). If the ingot densifies too quickly, impurities can be trapped. Due to the higher sintering temperature, direct sintering is more effective in cleaning than indirect sintering.
The sintering of tungsten can be enhanced by the addition of small amounts of alloying elements (0.5-1%), such as Ni or Pd. This phenomenon is called“activated sintering.” It is explained by the enhancement of grain-boundary diffusion in tungsten due to the presence of the respective element in the grain boundary, High densities (up to 99%) can be obtained even at 1100℃ (at this temperature, tungsten compacts are commonly presintered). However, since the alloys so produced are rather brittle, activated sintering has never been used in industry.
The sintering of doped tungsten is a peculiar case in sintering of tungsten. This includes dispersion-strengthened materials such as thoriated tungsten or tungsten with additions of CeO2, La2O3, and ZrO2 as well as NS (non-sag) tungsten used for lamp filaments.
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