Negative TCR materials

Greetings, I'm on a search for negative temperature coefficient suitable UHV material for electron amplification (gain) using high aspect ratio row
and column geometry; i.e., six micron pores on eight micron centers 390 microns thick. Searches to date with experimentation have used semiconductors which are positive TCR. Present experiments use copper oxide or silver sulphide. Any suggestions welcome. Thank you, boer
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