Hitachi Metals, Ltd.

Kazunori Haratoh

Kiyoshi Furushima

Hideshige Matsuo

A protective tube for a thermocouple for measuring the temperature of molten metal, comprising two layers, an inner layer being made of a silicon nitride or sialon ceramic having a bending strength of 50 kg/mm.sup.2 or more, a density of 90% or more based on a theoretical density and a thermal shock temperature .DELTA.T of 400.degree. C. or more, and an outer layer being formed on the silicon nitride or sialon ceramic and containing BN and SiO.sub.2. The silicon nitride of sialon ceramic comprises 70 weight % or more of Si.sub.3 N.sub.4 having 65 weight % or more of an .alpha.-phase content, 20 weight % or less of one or more oxides of elements of Group IIIa of the Periodic Table, 20 weight % or less of Al.sub.2 O.sub.3, and optionally 15 weight % or less of AlN or AlN solid solution, and the outer layer is composed mainly of BN-SiO.sub.2 -Al.sub.2 O.sub.3 -Y.sub.2 O.sub.3. This protective tube is produced by coating ceramic powder comprising BN and SiO.sub.2 before sintering. This protective tube can enjoy an extremely long life.

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Kitakyushu, Japan

Kazunori Haratoh