Patent GB-747893-A (GB)
Title: Improvements relating to electro-conductive coatings on high-temperature-resistant substrates
IPC: H01B1/08
Filed: 1954-04-01
Abstract: 747,893. Electric resistance and semiconductor devices. BRITISH THOMSONHOUSTON CO., Ltd. April 1, 1954 [April 2, 1953], No. 9614/54. Class 37. [Also in Groups XI and XL (a)] An electric resistance device comprises a hightemperature resistant support or substrate such as glass and an electrically conducting iridized coating thereon comprising zinc oxide with a trivalent activator. A method of producing the resistance device may comprise treating a vitreous support to a temperature close to that of its softening point and spraying the heated support with a water solution of an acetate, formate or chloride of zinc and a water-soluble salt of the activator(s). It is important that the selected zinc salt decomposes to the oxide very rapidly on striking the glass or other support and acetate is stated to produce the best film from the point of view of clearness and high conductivity. In explaining the invention it is stated that the trivalent activator atoms produce conductivity in otherwise pure zinc oxide by inducing some zinc atoms to be monovalent instead of divalent. The ionic radius of the trivalent activator atoms may approximate that of the divalent zinc ions and allow coexistence with divalent zinc ions in the zinc oxide lattice. The Specification discusses the use of (i) indium, scandium yttrium and thallium as single activators; and (ii) activator mixtures comprising indium mixed with gallium, nickel, cobalt, iron or chromium; in these mixtures indium is the primary activator. Aluminium is also mentioned as a possible activator but its use is not discussed specifically. The indium/gallium mixture is very effective in increasing conductivity of zinc oxide because relative to the Zn + 2 ion the oversize of the In + 3 ion is compensated by the undersize of the Ga + 3 ion. The indium and gallium may be present in an atomic ratio of 3 to 2 and added to zinc in a total atomic ratio relative to zinc greater than 2 per cent and less than 20 per cent. In the use of indium as single activator the indium may be present in an atomic ratio relative to zinc less than 20 per cent. In a manufacturing example using indium as single activator a film was formed by spraying a solution of zinc acetate to which indium chloride was added, onto soft lime glass at a temperature of 620‹ C. A number of test panels were formed in which the atomic percentage of indium relative to zinc was varied over the range from 1 to 10 per cent. A decided maximum in conductivity was observed at 4 per cent atomic weight of indium relative to zinc. In a second manufacturing example indium and gallium (atomic ratio 3 to 2) were added as nitrates to zinc acetate and the solution sprayed onto a glass panel at a temperature of 620‹ C.; the coated glass panel was then annealed for three hours at a temperature of 250‹ C. The highest conductivity was obtained with a solution in which the atomic percentage of gallium plus indium was 8 per cent of the zinc. The support on which the resistance coating is formed may be selected from hard boro-silicate glasses, ceramics, quartz and quartz-lime glasses, oxidation resistant metals, and such high temperature-stable crystals as quartz, corundum, rutile, diamond and silicon carbide. The invention has applications including de-iceable windshields for aircraft, heater elements for bread toasters and room heaters, conductive coatings for glass envelopes of lamps-and other electric discharge devices, also as the transparent and electrically conductive layer in electroluminescent cells and panels.