Sizes of some high temperature materials such as Alumina, Titanium oxide, Titanium nitride and Iron nanoparticles synthesized by Thermal plasma technique show sizes below 10 nm but very recently we have started working on tungsten nanoparticles and we have seen that sizes are very much bigger than those aforesaid materials i.e., above 50 nanometers. Why is this so?
Tungsten is a metal and metals have very different sintering properties (grain growth) than non-metallic compounds. There is a complex theory of dislocation mobility that explains this, but from practice, along known that the metals have significantly lower sintering temperature than oxides in relation to their melting points. With a melting temperature of 3422°C tungsten can be sintered even at 1500 to 1600°C (less than half of the melting point). Oxides, such as Al2O3, can not be sintered at the half of the melting temperature (1000°C). He needs about 80% of the melting temperature. Nitrides and carbides need for sintering even higher relative temperatures as oxides.
First of all thanks for your prompt reply.
But how can we relate particle size with sintering properties?
A material can only be sintered when its particles coalesce: several small particles grow to a large one. This happens because the entropy of a large crystal or particles is greater as entropy of several small crystals and particles (expand of entropy is the driving force of sintering and grain growth). But there are energetic barriers that allow sintering or grain growth for materials only from certain temperatures. For metals, these energetic barriren are smaller than for oxides and yet smaller than for carbides and nitrides.
Thank you very much Sir.
Can you provide me with some reference papers for the same?
Moreover, We have also observed cauliflower like structures in tungsten (photograph attached), We are wondering, what may the possible reason for the growth mechanism of such structures?
Regards
Literature I have not, but the question of cauliflower structure is simple:
all nanopowders tend to agglomerate. The smaller particles the stronger the electrostatic forces that hold them together. Agglomerates of nanoparticles may be millimeter in size and see all about the same.
Sorry meddling your conversation, but Trinayan, are you asking about SYNTHESIZE or SINTERIZE? I guess there's a misunderstanding here.
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