Patent GB-930994-A (GB)
Title: Improvements in or relating to thallium-tellurium semi-conductor materials and bodies
IPC: C30B1/02
Filed: 1959-07-22
Abstract: <PICT:0930994/III/1> A semi conductor material consists mainly of thallium telluride, Tl2Te3, or an isomorphous mixed Tl2Te3 crystal, in which, while retaining the Tl2Te3 structure as identified by X-ray diffraction data, part of the thallium and/or the tellurium is replaced by another element.  For example gallium or indium may partly replace the thallium whilst sulphur or selenium may partly replace the tellurium; the thallium may also be partly replaced by bismuth lead or mercury, and significant impurity atoms such as copper, silver or halogen such as iodine may be incorporated to modify the conductivity type. The thallium telluride material is prepared by heating the components or compounds supplying them in the desired ratio at a temperature below the decomposition temperature of the thallium telluride Tl2Te3, such temperature generally being about 238 DEG C.  Heating at 180 DEG  to 238 DEG C. is preferred and the heating may be effected in a non-oxidizing atmosphere or in vacuo. The components or compounds forming the thallium telluride may be heated together in a powder state or they may be shaped to form a body by compression before the heating; furthermore the first heating may be above the decomposition temperature and even to cause melting, after which the resultant material may be powdered and subsequently heated at the temperature below the decomposition temperature until the desired thallium telluride is formed.  The thallium telluride product may be subjected locally to a further heat treatment which may be above or below the decomposition temperature, for example to provide it with contacts and it is preferred subsequently to heat it below the decomposition temperature for a time sufficient to reform the desired Tl2Te3.  The contact material is preferably bismuth. The thallium telluride product is suitable for use as a semi-conductor material for example in thermo electric devices such as Peltier effect refrigerating devices.  The conductivity may be from 4,5 X 10-2 to 3,1 X 10-2 at room temperature and 4,0 X 10-7 and 1,7 X 10-10 at 180 DEG C.  The Hall constant at room temperature may range from 1,7 X 102 to 7,8 X 102 whilst at 180 DEG C. the Hall constant is from 3 X 104 to 4 X 105; the thermo-e.m.f. may be up to 800 microvolts per degree and the thermal conductivity is approximately 6,20-3. According to examples (2) thallium telluride Tl Te was ground to a powder and mixed with sufficient tellurium to produce Tl2Te3 and then compressed under 1 ton/cm2.  The bodies were heated in vacuum or in hydrogen or argon at 200 DEG C. for eight days when X-ray examination indicated that Tl2Te3 had been formed. (3) mixed crystals of formula Tl1,85-In0,05Te3 are formed by milling thallium in an argon atmosphere, adding the requisite amounts of tellurium and indium and sealing in a tube under vacuum.  The tube was heated at 500 DEG C., cooled to 100 DEG C. and thereafter heated at 200 DEG C. for 100 hours to form Tl2Te3; selenium may replace the indium.  Examples 5 and 6 indicate the effect of excess selenium and excess tellurium (7)1-5 mol. per cent of PbTe were added to a composition equivalent to Tl2Te3 which was then ground, compressed and heated at 210 DEG C. for ten days to form a mixed crystal having the characteristic Tl2Te3 X-ray diffraction pattern.  A further example illustrates the effect of the addition of 0,5 atomic per cent of iodine or 0,5 atomic per cent of copper the electrical conductivity in each case being considerably increased.  A Peltier effect refrigerator is shown in Fig. 4, where two limbs 6 and 7 have a substance of high thermal and electrical conductivity, for example copper and silver, connected thereacross; current is supplied and taken off through contacts 9 and 10 and attached to the semi-conductor bodies 6 and 7 by a bismuth alloy 11.  At least one of the limbs 6 or 7 may consist of the Tl2Te3 semi-conductor material.