Diamond Thermistors

So, 61 years ago, started a story in Electronics Weekly’s edition of December 21st 1960.

The story continues:

The Diamond Research Laboratory in Johannesburg is engaged on a programme of research into the properties of such diamonds with a view to possible industrial applications.

Semiconducting diamond is mined in very small quantities in a very pure state, the principal impurities being aluminium and silicon.

The material is usually in the form of  pale blue, transparent  single crystals which exhibit p-type conductivity.

The material has a bulk resistivity which is generally in the range 50 to 500 ohm-cm. and the change of resistance with temperature is of the order of 4 to 5 per cent per degree centigrade at room temperature.

The diamond material whicb has been used to make thermistors for experimental purposes has been in the form of small rectangular blocks or slivers about 2 millimetres in length.

Small rods of semiconducting diamond have alse been prepared by making use of a process similar to one used commercially for the production of gramophone styli.

Metal contact leads (nickel or platinum)may be fixed to the specimen by heating the contacts to about 1,000’C and using alloys which will wet the diamond surface (eg. Cu-Ag-Ti) to form a bond at the contacts. In this way, ohmic contacts have been made.

The stability of the fnished thermistor promises to be good. In contrast to oxide thermistors, which are widely used, the diamond is a homogeneous single crystal with good mechanical properties and is capable of being used in high-pressure environments.

In addition,diamond is very resistant to chemical attack and is capuble of withstanding high temperatures.

Diamond may be heated to 500 degC in air and to about 1,200 degC in an inert atmosphere witbout sustaining damage.

In some applications, use may be made of the high thermal conductivity of the material (at room temperature, the thermal difusivity of diamond is greater than that of copper).

There are,howerer,two major obstacdes to the widespread use of devices made from semiconducting diamond.

At present we have no convenient  method for the control of  the the impurity conent in diamond.

However,recent work has shown that considenable modifcation of the properties of semiconducting diamond is possible by electron irradiation and subsequent heat treatment.

Preliminary research on the electrical properties of semiconducting malerial after irradiation with high cnergy (l to 2 MeV) electrons has shown that the bulk resistivity can be increased in a controlled manner and, more important, the temperature coefficient of resistance is increased over the. whole temperature range and particularly at high temperatures.

The long-term stability of the irradiated material still remains to be determined. The major difficulty is that semiconducting diamond is extremely rare.

For this reason this Laboratory has embarked on a programme of research into the possibility of diffusing impurity elements into the more readily available non-conducting diamond in an attempt to produce semiconducting properties.

This would make feasible a variety of interesting solid state devices capable of operating under extreme conditions.


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