Transfer tube

Claim 1

A transfer tube useful for transfer of molten metal comprised of a hollow high density tube having directly bonded only to its outer surface wall leaving no significant portion thereof exposed a continuous low density shell, said high density tube being comprised of polycrystalline ceramic oxide with a density of at least about 90% of its theoretical density, said high density tube having a passageway extending through its length with a cross-sectional area at least sufficient for transfer of molten metal therethrough, said shell being comprised of ceramic oxide with at least about 75 weight % of said shell being polycrystalline, said shell being comprised of a plurality of sequential layers directly bonded to each other, said sequential layers being comprised of at least two primary layers and at least one intermediate secondary layer disposed between said primary layers, the ceramic oxide grains in said primary layers having an average size which is significantly smaller than the average size of the ceramic oxide grains in said intermediate secondary layer, said low density shell ranging in density from about 40% to about 80% of its theoretical density, said low density shell having a thermal expansion coefficient within about ±25% of the thermal expansion coefficient of said high density tube, said low density shell having a thermal conductivity ranging from about 10% to about 90% lower than that of said high density tube.