Patent GB-900771-A (GB)
Title: Resistors and methods for the manufacture thereof
IPC: H01C7/02
Filed: 1958-07-04
Abstract: 900,771. Temperature dependent resistors. SIEMENS & HALSKE A.G. July 14, 1958 [July 15, 1957], No. 22479/58. Class 37. A resistor with a positive temperature coefficient over a certain temperature range comprises a ferro-electric substance with a Curie temperature below the upper limit of the range to which conductivity is imparted in such manner that the spacing between the donor and conductivity bands or between the acceptor and valence bands is less than half the width of the prohibited zone between the valence and conductivity bands, and in which the contribution to the conductivity of these donors or acceptors is great compared with the intrinsic conductivity of the substance or conductivity due to other donors and acceptors in said temperature range. The resistor is produced by sintering a number of crystallites and is subsequently electroformed with a series of high voltage pulses to eliminate its voltage dependence. It may be made of a perovskite consisting of mixed oxides of bi- and tetravalent elements, e.g. BaTiO 3 , (Ba, Sr) TiO 3 , (Ba, Pb) TiO 3  substituted by trivalent or quinquevalent elements such as niobium, lanthanum and tungsten, of a ferro-electric perovskite consisting of mixed oxides of mono- and quinquevalent elements, e.g. NaNbO 3 , KTaO 3 , or (Na, K) TaO 3  doped with bivalent or hexavalent elements such as barium, strontium, molybdenum and tungsten, or of the ferro-electric Cd 2 Nb 2 O 7  similarly substituted. A suitable material is made by heating a powdered mixture of 100.2 moles of titanium dioxide, 72 moles barium carbonate, 28 moles strontium carbonate and 0.2 moles of lanthanum oxide to react them. The product is then powdered, pressed to the desired shape and sintered at below 1300‹ C. in air or oxygen, after which metal electrode coatings are applied to which current leads are affixed. 0.1 mole of tungsten oxide WO 3  or 1.15 moles of niobium oxide Nb 2 O 5  may be used in place of the lanthanum oxide. In an alternative method the mixed titanate is formed in a preliminary heating and the substituting oxide incorporated during the sintering process. For instance 0.2 moles of lanthanum oxide may be added to powdered barium titanate and sintered at 1270-1300‹ C. The sintering temperature is critical since it determines the manner in which the substituted atoms are incorporated in the crystal lattice, and the crystal size which determines the conductivity of the material. The positive temperature coefficient is attributed to the fact that the activation energy of the impurity centres, which determines the number of free charge carriers at a given temperature is inversely proportional to the square of the effective polarisibility. In a ferro-electric in which the spacing between the donor or acceptor and the conductivity or valence band is small the fall of the effective polarisibility with rising temperature above the Curie point causes n, which is proportional to e(-E/kT), to rise with temperature. Consequently, if n represents a large fraction of the total number of carriers the material will have a positive temperature coefficient. Specifications 900,772, 900,773 and German Specification 660,971 are referred to.