Shrink-free ceramic and method and raw batch for the manufacture thereof

A shrink-free ceramic raw batch for forming into a shaped raw body and then firing to a fired body substantially the same in size and shape as the raw body, said batch consisting essentially of at least 50% by weight particulate aluminum oxide, at least 5% by weight particulate magnesium oxide, from about 5% to 25% by weight glass frit and from about 10% to 15% by weight silicone resin, the amount of aluminum oxide being at least twice that stoichiometrically required to react with all of the magnesium oxide to form magnesium aluminate spinel in situ during firing.

A method for making a ceramic body comprising the steps of:

• preparing a raw batch consisting essentially of at least 50% by weight particulate aluminum oxide, at least 5% by weight particulate magnesium oxide, from about 5% to 25% by weight glass frit and from about 10% to 15% by weight silicone resin containing at least 50% by weight SiO, the amount of aluminum oxide being at least twice that stoichiometrically required to react with all of the magnesium oxide to form magnesium aluminate spinel;
• forming the raw batch into a self-sustaining compact having a shape and size substantially identical to the shape and size of the body desired to be made and
• firing said compact in an oxidizing atmosphere at a temperature sufficient to decompose the silicone resin and to cause the SiO of the silicone resin to convert to SiO.sub.2 and then further firing the compact at a temperature sufficient to cause at least most of the magnesium oxide to react with some of the aluminum oxide to form magnesium aluminate spinel, and to cause formation of an interstitial glass phase, the formation of said magnesium aluminate spinel during said firing causing expansion which compensates for shrinkage which would otherwise occur.

A pressed self-sustaining raw substantially shrink-free ceramic body for use as a pre-form for transfer molding and for subsequent firing to a fired body, said raw body consisting essentially of a uniform mixture containing at least 50% by weight aluminum oxide, at least 5% by weight magnesium oxide, from about 5% to 25% by weight glass frit and from about 10% to 15% by weight silicone resin containing at least 50% by weight SiO, the amount of aluminum oxide being at least twice that stoichiometrically required to react with all of the magnesium oxide to form magnesium aluminate spinel during said firing, said aluminum oxide, magnesium oxide and glass frit being in particulate form and bonded together by said silicone resin, the particle size range of the aggregate of the aluminum oxide, magnesium oxide and glass frit being from submicron to about 200 mesh Tyler.