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لانتان
Sauer and Flaschen1 presented the first report on the existence of a positive temperature coefficient region in the resistance-temperature characteristics of barium titanate containing small quantities of lanthanum.
A solid solution of barium- strontium titanate was also doped with lanthanum to give a maximum temperature coefficient of+9% centred around 25CC with the PTC region commencing at - 50CC and extending to 110°C.
The substitution of lanthanum in the two compositions was shown to provide a valency controlled n-type semiconductor in a manner similar to that found for • NTC oxides and the anomalous PTC region was associated with cubic-to- tetragonal crystallographic phase transformation in the semiconductor.
These two alternatives can be illustrated by lanthanum substitution for barium and tantalum substitution for titanium as follows:
A similar effect was also observed for lanthanum substitution by Jonker.26 At the higher levels of substitution the blue colouration due to Ti3" disappears again and the material is white.
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