GB768056A - Barium titanate composition - Google Patents

Abstract

<PICT:0768056/III/1> A monocrystalline tetragonal composition composition comprising barium titanate and from 1 to 10 per cent of an oxidic compound of strontium or calcium, calculated as oxide, is synthetically prepared by crystallizing in the form of a boule a fused mixture of finely divided barium titanate and the oxidic compound, suitably by introducing the mixture into a stream of oxygen, surrounding the stream of oxygen with a stream of hydrogen and forming a flame having a central oxygen cone in which the mixture is melted and crystallized in the form of a boule adjacent the end of the oxygen cone. The so formed crystal may be subsequently annealed by heating at 650 DEG to 1550 DEG C. in an oxidizing atmosphere. Suitable strontium and calcium compounds are the oxides, carbonates and titanates, which may be added to the barium values either during or after preparation of the barium titanate. The starting materials may be pure or impure and may contain natural or added colouring or modifying agents, normally in amounts not exceeding a few tenths of a per cent. The preferred apparatus is shown in the Figure where three concentric nozzles 1, 2 and 3 feed respectively the powdered titanate mixture with oxygen, additional oxygen, and hydrogen. A boule 4 is formed on a pedestal 5 of refractory material such as firebrick or zirconia which is lowered as the boule grows so that the molten top of boule 4 is at or near the point of the conical flame produced. The growth of the boule may be initiated by a seed crystal. Suitable flame temperatures are between 1600 DEG and 1725 DEG C. and a starting material of ultimate unit particle size of less than 1 micron is preferred. The monocrystalline tetragonal titanate composition obtained has an average index of refraction at 25 DEG C. of 2.40, a reciprocal relative dispersion below 20 and a bi-refrigence of 0.055 at 25 DEG C. At 25 DEG C. and low field strength the dielectric constant parallel to the C axis is 160 and parallel to the A axis, 100,000. The spontaneous polarization at 25 DEG C. is 25 microcoulombs per square centimeter, the coercive field is 500 volts per centimeter and the Curie temperature is from 80 to 120 DEG C. Specification 664,889 is referred to.ALSO:A monocrystalline tetragonal titanate composition comprising barium titanate and from 1 to 10 per cent of an oxidic compound of strontium or calcium, calculated as oxide, is synthetically prepared by crystallizing in the form of a boule a fused mixture of finely divided barium titanate and the oxidic compound, suitably by introducing the mixture into a stream of oxygen, surrounding the stream of oxygen with a stream of hydrogen and forming a flame having a central oxygen cone in which the mixture is melted and crystallized in the form of a boule adjacent the end of the oxygen cone. The so formed crystal may be subsequently annealed by heating at 650 DEG to 1550 DEG C. in an oxidizing atmosphere. Suitable strontium and calcium compounds are the oxides, carbonates and titanates, which may be added to the barium values either during or after preparation of the barium titanate. The starting materials may be pure or impure and may contain natural or added colouring or modifying agents, normally in amounts not exceeding a few tenths of a per cent. Specification 664,889 [Group III] is referred to.