Patent GB-618966-A (GB)
Title: Improvements in or relating to electric thermally sensitive resistance elements
IPC: H01C7/04
Filed: 1946-06-14
Abstract: 618,966. Temperature-variable resistances ; resistance coatings and deposits ; couplings. STANDARD TELEPHONES & CABLES, Ltd., SHEPARD, S. C., and WOLFSON, H. June 14, 1946, No. 17999. [Classes 37 and 38 (i)] A thermistor comprises a metal plate having an insulating coating on one edge, a thin film of thermistor material on this coating, and two electrodes in contact with the film over separated areas thereof. The thermistor shown may be used as a sensitive bolometer for infra-red spectroscopy. A nickel base plate 41, which may be 0.625 inches long, 0.1 inch wide and 0.008 inch thick, has one edge 42 ground flat and polished. The base plate is then cleaned and oxidised at 1050‹ C. for 2 hours. The coating suspension is made by ball rolling a mixture of 80 grammes of manganese oxide, 20 grammes of nickel oxide and 25 grammes of copper oxide in 200 c.c. of methylated spirit. After filtering and drying, the mixture is heated for 1 hour, at 1300‹ C. and is then crushed and ground and sieved through a 200 mesh sieve. 20 grammes of the sieved oxide mixture is then ball milled for 24 hours with 30 c.c. of amyl acetate, 30 c.c. of methylated spirit, and 1 gramme of polyvinyl butyral. This suspension is coated on the polished edge 42 by application with a glass rod. The applied suspension is allowed to dry and the process repeated until a film of the required thickness is obtained. After coating, the plate is fired for 2 minutes at 1000‹ C. and silver paste electrodes 43, 44 are applied with a pointed glass rod on top of the film and partly over the adjacent face of the plate. The electrodes 43, 44 may be spaced about 3/16 inch apart. Platinum lead wires 47, 48 which may be 0.002 inch diameter, are each prepared by melting the end to form a small ball, which is then flattened to a disc. The disc ends are then applied to the paste electrodes with additional paste, and the plate is fired for 5 minutes at 700‹ C. Specifications 540,844, 541,922, 545,679, 555,563 and 557,541 are referred to. The Provisional Specification describes other constructions, in one of which a rectangular base-plate or sheet of oxidised nickel about 0.02 inch thick has a thin film about 0.0005 inch thick of thermistor material consisting of a mixture of manganese, nickel and copper oxides in the proportion of 4 : 1 : 1 deposited over one face. A suspension of these oxides is formed in a 2¢ per cent nitrocellulose amyl acetate solution by ball milling and is sprayed on to one side of the oxidised nickel plate. The coated plate is then heated in air at 1100‹ C. to sinter and combine the oxides so as to form a continuous film which adheres firmly to the base-plate. The electrodes of the thermistor are formed by painting narrow strips 4, 5 of paste on the surface of the layer 3 adjacent to two opposite edges of the base-plate. This is then oven dried and the finished plate is finally heated in air at 800‹ C. For high resistances the thermistor material may consist of a mixture of manganese and nickel oxides only, in the proportions of 4 : 1. The base-plate may be mounted on a copper block to promote dissipation of heat. The baseplate may alternatively be formed of ferrous metal or of a pressed sintered block of a metal oxide such as nickel oxide. Other oxides such as uranium dioxide, iron oxide, vanadium pentoxide, &c. may be used for the oxide film. The oxides may be formed into suspensions in nitrocellulose solutions or in water using metal nitrates as binders. Other binders such as starch, glue, synthetic resins, &c. may be used. Suspensions in the form of pastes may be used for painting. Several separate parallel resistance tracks may be carried by the same oxidised base-plate. The tracks may be formed by spraying the plate through a mesh, or by coating it all over and removing the unwanted material. A thermistor may be arranged with the resistance track through the thickness of the film. An oxidised metal tube may have a thermistor film spread over the whole or part of the outer surface. For use as an indirectly heated thermistor a heater may be inserted into the tube, or a heating coil may be wound on the outside of the tube and insulated from the film and electrodes. The electrodes may be formed by winding several turns of platinum or other metal wire round the tube and coating the thermistor film over the tube and wires. Such wires may be used to support the thermistor. Another form of multi-track thermistor comprises a nickel grid made of a number of wires or a grid may be punched out of nickel sheet. To obtain a narrow track an oxidised wire may be coated and laid on an oxidised sheet. After heat treatment the coating partly combines with the oxide on the sheet, thus giving a narrow resistance between the wire and the sheet. Tubes may be coated inside and outside with resistance material. Materials with widely differing temperature coefficients may be used. One film may be used as a heater for an indirectly heated thermistor. Thin strips of metal may be coated on both sides.