Low pressure sintering of boron nitride using low thermal expansion static sintering molds

Claim 2

In a method for manufacturing a sintered and shaped body consisting essentially of boron nitride and having an improved transverse strength wherein a powder consisting essentially of boron nitride is subjected to cold-pressure molding under high pressure to obtain a solid molded boron nitride article of the desired shape and said molded article is sintered at a temperature of about 1500* to 2,100*C., the improvement which comprises placing said thus molded boron nitride article within a sintering mold which is made of a material having a linear thermal expansion coefficient smaller than that of said boron nitride molded article and which has dimensions such that the clearance between said molded article and the mold is small enough that upon heating to the sintering temperature said boron nitride article will expand to contact the walls of said mold, heat-sintering said boron nitride article thus confined in a mold by placing the mold confining said boron nitride article into a furnace and then heating said mold to a temperature ranging between about 1500* and 2100*C., while no outer pressure is applied to said mold, said mold thereby restricting the expansion of said article whereby sintering is effected under only that pressure resulting from the restricting mold.

Claim 3

A method according to claim 1, wherein the high pressure employed in the step of cold-pressure molding is about 2 to 3 tons per sq.cm.

Claim 4

A method according to claim 1, wherein the mold is made of graphite.

Claim 5

A method according to claim 1, wherein the clearance existing between said mold and said article is less than 5 percent of any dimension of said molded article in the direction of the clearance.

Claim 6

A method according to claim 1 in which the particle size of the boron nitride powder is up to 1 micron.

Claim 7

In a method for manufacturing a sintered and shaped boron nitride body wherein a mixture of boron nitride powder and an organic binder is subjected to cold-pressure molding under a high pressure to obtain a solid molded article of the desired shape and then said molded article is sintered at a temperature of about 1500* to 2100*C., the improvement whereby a shaped boron nitride body having improved transverse strength is obtained which comprises after shaping said boron nitride article by cold pressure molding, placing said thus molded boron nitride article within a sintering mold which is made of a material having a linear thermal expansion coefficient smaller than that of said boron nitride molded article and which has dimensions such that the clearance between said molded article and the mold is small enough that upon heating to the sintering temperature said boron nitride article will expand to contact the walls of said mold, heat-sintering said boron nitride article thus confined in a mold by placing the mold confining said boron nitride article into a furnace and then heating said mold to a temperature ranging between about 1500* and 2100*C. while no outer pressure is applied to said mold, said mold thereby restricting the expansion of said article whereby sintering is effected under only that pressure resulting from the restricting mold.

Claim 8

In a method for manufacturing a sintered and shaped boron nitride body wherein a mixture of boron nitride powder and an organic binder is subjected to cold-pressure molding under a high pressure to obtain a solid molded article of the desired shape and then said molded article is sintered at a temperature of about 1500* to 2100*C., the improvement whereby a shaped boron nitride body having improved transverse strength is obtained which comprises after shaping said boron nitride article by cold pressure molding, placing said thus molded boron nitride article within a sintering mold which is made of a material having a linear thermal expansion coefficient smaller than that of said boron nitride molded article and which has dimensions such that the clearance between said molded article and the mold is small enough that upon heating to the sintering temperature said boron nitride article will expand to contact the walls of said mold, heat-sintering said boron nitride article thus confined in a mold by placing the mold confining said boron nitride article into a furnace and then heating said mold to a temperature ranging between about 1500* and 2100*C. while no outer pressure is applied to said mold, said mold thereby restricting the expansion of said article whereby sintering is effected under only that pressure resulting from the restricting mold.

Claim 9

A method according to claim 6, wherein the clearance existing between said mold and said article is less than 5% of any dimension of said molded article in the direction of the clearance.

Claim 10

A method according to claim 6 in which said binder consists of polyethylene glycol.

Claim 11

A method according to claim 6 in which said binder consists of polyethylene glycol which is present in an amount of about 2.5 parts by weight per 100 parts by weight of boron nitride.

Claim 12

A method according to claim 6, wherein said binder consists of polyethylene glycol in an amount of about 2.5 parts by weight per 100 parts by weight of boron nitride, and the mixture of boron nitride powder and binder is prepared by drying a mixture of boron nitride powder and poly-ethylene glycol dissolved in a solvent for the binder.

Claim 13

In a process for manufacturing a sintered and shaped boron nitride body by cold-pressure molding boron nitride powder to form a shaped article and then sintering said article at a temperature of about 1500* to 2100*C., the improvement which comprises after shaping said boron nitride article by cold-pressure molding, placing said thus-molded boron nitride article within a sintering mold which is made of a material having a linear thermal expansion coefficient smaller than that of said boron nitride molded article and which has dimensions such that the clearance between said molded article and the mold is small enough that upon heating to the sintering temperature said the boron nitride article will expand to contact the walls of said mold and then heat-sintering said boron nitride article thUs confined in a mold by placing the mold confining said boron nitride article into a furnace and then heating said mold to a temperature ranging between about 1500* and 2100*C., while no outer pressure is applied to said mold, said mold thereby restricting the expansion of said article.

Claim 14

In a process for manufacturing a sintered and shaped boron nitride body by cold-pressure molding boron nitride powder to form a shaped article and then sintering said article at a temperature of about 1500* to 2100*C., the improvement which comprises after shaping said boron nitride article by cold-pressure molding, placing said thus-molded boron nitride article within a sintering mold which is made of a material having a linear thermal expansion coefficient smaller than that of said boron nitride molded article and which has dimensions such that the clearance between said molded article and the mold is small enough that upon heating to the sintering temperature said the boron nitride article will expand to contact the walls of said mold and then heat-sintering said boron nitride article thUs confined in a mold by placing the mold confining said boron nitride article into a furnace and then heating said mold to a temperature ranging between about 1500* and 2100*C., while no outer pressure is applied to said mold, said mold thereby restricting the expansion of said article.

Claim 15

A method according to claim 11, wherein the mold is made of graphite.