GB630307A - Improvements in or relating to electric resistances - Google Patents

Abstract

630,307. Non-metal resistances; temperature variable resistances. PHILIPS LAMPS, Ltd. March 13, 1946, No. 7862. Convention date, March 16, 1945. [Class 37] A sintered resistor consists of mixed crystals of Fe3O4 with a spinel of the type 2R<11>O.R<IV>O2, for example Mg2TiO4,ZnTiO4,Ni2TiO4,Zn2SnO4 ; in the formula R<11> and R<IV> represent respectively divalent and tetravalent elements. In the manufacture of such a resistor, the sintering temperature is 1300‹ C. The sintering process may be carried out in an atmosphere of pure hydrogen without the Fe3O4 formed from Fe2O3 being reduced further to FeO. The content of Fe3O4 is controlled according to the composition of the initial mixture and the conditions in the sintering and cooling processes. Resistors having a low content of Fe3O4 and therefore a high negative temperature coefficient may be produced which are stable at high operating temperatures, i.e. the spinel phase, is uniform and does not separate out into a second phase when the resistor in use develops a temperature of 500‹ C. In resistors of low negative temperature coefficient and therefore a high content of Fe3O4, a second phase may, however, be present; this is introduced by adding an insulating compound such as MgO and ZrO2 which does not dissolve into the spinel phase. In one example the starting mixture comprises Fe2O3-NiO-TiO2; this material is ground with alcohol in a ball-mill for 16 hours and then worked with a solution of 20 per cent methyl metacrylate in methyl-glycol acetate to form a kneadable mass which is pressed into rods of 4 mms. diameter. The rods are sintered at 1300‹ C. for 1 hour in an atmosphere of pure nitrogen and then cooled rapidly in a stream of nitrogen. These resistors are 22,000 ohms per c.m. at 22‹ C. and 50 ohms/c.m. at 332‹ C. In another example a composition of Fe2O3- ZnO-TiO2 is ground with ethyl acetate for 16 hours and then with the use of 10 per cent nitro.cellulose solution in ethyl glycol tubes are extruded 6.3 mms. long and having an external diameter of 8 mms. and an internal diameter of 4.5 mms. The tubes are heated at 1320‹ C. in a continuous furnace of 30 c.m.s. length at a travelling speed of 1 c.m. per minute whilst using a stream of pure nitrogen (oxygen content about 0.1 per cent) flowing at 5 litres per minute. These resistors are 1600 ohms at room temperature and 81. ohms at 110‹ C.