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1. (WO1990006906) PARTICULES CERAMIQUES A ENROBAGE UNIFORME
Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

WHAT IS CLAIMED IS:

1. A ceramic particle comprising an inorganic nitride or carbide having an average diameter less than about 2 microns and having deposited thereon a substantially uniform surface layer coating of a metal compound selected from metal hydroxides and metal oxides.

2. The particle of Claim 1 wherein the inorganic nitride or carbide is comprised of a nitride selected from the group consisting of silicon nitride, boron nitride, titanium nitride, and aluminum nitride.

3. The particle of Claim 1 wherein the inorganic nitride or carbide consists essentially of silicon nitride.

4. The particle of Claim 1 wherein the inorganic nitride or carbide is comprised of a carbide selected from the group consisting of silicon carbide, boron carbide, and titanium carbide.

5. The particle of Claim 1 wherein the inorganic nitride or carbide consists essentially of silicon carbide.

6. The particle of Claim 1 wherein the metal compound is a metal hydroxide which is hydrolized to form the metal oxide.

7. The particle of Claim 6 wherein the metal oxide is selected from the group consisting of yttria, magnesia, zirconia, ceria, hafnia, and alumina.

8. The particle of Claim 1 wherein the metal compound is deposited from an aqueous solution.

9. The particle of Claim 1 wherein the coating is less than about 10 angstroms thick on average.

10. The particle of Claim 1 wherein the metal compound is selected from yttrium hydroxide and yttrium oxide.

11. The particle of Claim 10 wherein the yttrium hydroxide or yttrium oxide is present in an amount of about 0.5 to about 1.2 percent by weight of the particle.

12. The particle of Claim 1 in the substantial absence of any free particles of a metal oxide.

13. The particle of Claim 1 wherein the metal compound represents from about 0.5 to about 10 percent of the weight of the particle.

14. The particle of Claim 1 wherein the metal compound is deposited by a surface precipitation procedure from an aqueous solution which comprises ions of the metal.

15. The particle of Claim 14 wherein the surface precipitation procedure is performed under a procedure which precludes the formation of free particles of the metal compound.

16. The particle of Claim 15 wherein said precluding procedure comprises maintaining the pH of the aqueous solution at least about 1.5 pH units below the hydrolysis constant of the metal ion until at least about 90 weight percent of the metal ions have been deposited upon the silicon nitride particles.

17. A method of preparing the particle of Claim 1 in the substantial absence of free particles of metal hydroxide and metal oxide which comprises (i) placing nitride or carbide particles having average diameters less than about 2 microns into a solution comprising water and a surfactant which reacts with the nitride or carbide particles at a faster rate than the water, (ii) adding thereto an aqueous solution which comprises ions of the metal, which will form the metal compound, and (iii) increasing the pH of the solution of the metal and the nitride or carbide to deposit the metal hydroxide on the surface of the ceramic nitride or carbide particles, wherein the pH of the silicon nitride solution is increased to no higher than about 1.5 pH units below the hydrolysis constant of the metal ion until at least about 90 weight percent of the metal ions have been deposited upon the silicon nitride particles.

18. The method of Claim 17 comprising the additional step (iv) of increasing the pH of the solution to within about 0.5 pH units of the hydrolysis constant of the metal ion.

19. The method of Claim 17 wherein the pH in (iii) is maintained at least about 2 pH units below the hydrolysis constant of the metal ion.

20. A process for the preparation of suspensions of finely divided particles of a first solid which is predominantly an inorganic nitride or carbide in a fluid suspension medium which is predominantly water and with which surfaces of said first solid are chemically reactive, comprising the steps of:
(a) mixing together particles of said first solid having a first average size, said suspension medium, and a surfactant that is (i) soluble in said suspension medium and (ii) capable of adsorbing on freshly created surfaces of said first solid at a faster rate than the suspension medium can react therewith so that the surfactant at least retards the reaction that would otherwise occur between said first solid and said dispersion medium; and
(b) comminuting the solid particles in the mixture formed in step (a) to particles of a second average size which is smaller than said first average size.

21. The process of Claim 20 wherein the inorganic nitride or carbide is comprised of a nitride selected from the group consisting of silicon nitride, boron nitride, titanium nitride.

and aluminum nitride.

22. The process of Claim 20 wherein the inorganic nitride or carbide consists essentially of silicon.nitride.

23. The process of Claim 20 wherein the inorganic nitride or carbide is comprised of a carbide selected from the group consisting of silicon carbide, boron carbide, and titanium carbide.

24. The process of Claim 20 wherein the inorganic nitride or carbide consists essentially of silicon carbide.

25. The process of Claim 20 wherein the surfactant is selected from the group consisting essentially of amino-function-al zircoaluminate surfactants having an inorganic polymer backbone, silanes, titanium alkoxides, aluminum alkoxides, zirconium alkoxides, and iridium-based surfactants.

26. The process of Claim 25, wherein said surfactant consists essentially of the reaction product of a chelated aluminum moiety, an organofunctional ligand, and a zirconium oxyhalide, the organofunctional ligand being complexed with and chemically bound to the chelated aluminum moiety, the aluminum moiety having the formula Al2(OR10)aAt)Bc, wherein each of A and B is hydroxy or halide and each of a, b, and c are integers such that 2a + b + c = 6, and OR^ is (a) an alpha, beta or an alpha, gamma glycol group in which -j^ is an alkyl group having about 1 to about 6 carbon atoms or (b) an alpha hydroxy carboxylic acid residue of the formula



wherein R3 is H or an alkyl group having about 1 to about 4 carbon atoms; said organofunctional ligand being (a) an alkyl, alkenyl, alkynl, or aralkyl carboxylic acid having about 2 to about 36 carbon atoms, (b) an aminofunctional carboxylic acid having about 2 to about 18 carbon atoms, (c) a dibasic carboxylic acid having about 2 to about 18 carbon atoms, (d) an acid an-ydride of dibasic acid having about 2 to about 18 carbon atoms, (e) a mercapto functional carboxylic acid having about 2 to about 18 carbon atoms, or (f) an epoxy functional carboxylic acid having about 2 to about 18 carbon atoms; said zirconium oxyhalide moiety having the formula ZrA^Bg, wherein A and B are as above defined and d and e are integers such that d + e = 4; the molar ratio of chelated aluminum moiety to zirconium oxyhalide moiety being about 1.5 to about 10; and the molar ratio of organofunctional ligand to total metal being about 0.05 to about 2.

27. The process of Claim 20 comprising said second average particle size being less than about 2 microns and an additional step of filtering a dispersion of said first solid in said dispersion medium through a filter having an average opening size of about 2 microns or less and recovering in the filtrate at least 90% of the first solid content.

28. A process for depositing a first solid material upon the surface of a particle of a second solid material suspended in a dispersion medium, comprising the steps of:
(i) providing in solution in said dispersion medium at least two distinct reagent materials that are capable of reaction to form said first solid material; and
(ii) establishing an increased chemical activity of at least one of said reagents in the part of said dispersion medium in the vicinity of the surface of said second solid material, compared with the bulk of said dispersion medium,
whereby reaction occurs between said reagents to deposit said first solid upon the surface of said second solid while substantially avoiding precipitation of said first solid from the bulk of said dispersion medium.

29. The process of Claim 28 wherein said second solid is predominantly silicon nitride and said first solid is a densification aid metal oxide for silicon nitride or a precursor of such a densification aid.

30. The process of Claim 29 wherein said densification aid precursor is predominantly a rare earth hydroxide.

31. The process of Claim 29 wherein the deposition while avoiding precipitation of the rare earth hydroxide from the bulk is produced by maintaining the pH of the bulk solution at least about 1.5 pH units below the hydrolysis constant of an ion of the metal until at least 90 weight percent of the metal ion has deposited on the silicon nitride.

32. The process of Claim 29 wherein said sintering aid precursor consists essentially of yttrium hydroxide.

33. A solid material consisting essentially of a plurality of particles, each of said particles comprising a core predominantly of a ceramic material selected from silicon nitride and silicon carbide and a surface layer predominantly of one or more rare earth hydroxides or oxides, said solid material being substantially free from particles consisting essentially of rare earth hydroxides or oxides with no silicon nitride or silicon carbide content.

34. The solid material of Claim 33 wherein said particle core consists essentially of silicon nitride having a purity of at least about 96% and an alpha phase content of at least about 90%.

35. The solid material of Claim 33, wherein said particle surface layers consist essentially of yttria or a precursor of yttria and have a thickness such that about 0.5 to about 5 weight % of the solid is yttria or a stoichiometric equivalent thereto .

36. A solid material comprising a plurality of particles, each of said particles comprising (i) a core predominantly of silicon nitride or silicon carbide and (ii) a substantially uniform surface layer predominantly of one or more rare earth hydroxides or oxides, said solid material having been prepared by a process which precludes the simultaneous preparation of particles consisting essentially of rare earth hydroxides or oxides without any silicon nitride or silicon carbide content.

37. The solid material of Claim 36 further comprising particles consisting essentially of rare earth hydroxides or oxides with no silicon nitride content.

38. The solid material of Claim 36 wherein the rare earth hydroxide or oxide is yttria or precursor of yttria and is present in an amount of about 0.5 to about 1.2 weight percent.