Production of sintered three-dimensional ceramic bodies

1. A freeze-forging method for producing a net-shape transparent sintered three-dimensional magnesium aluminate spinel body of a desired complex geometry, comprising the steps of:

(a) providing a ready-to-sinter spinel powder consisting of a nanomixture of magnesium aluminate spinel nanoparticles and a uniformly distributed controlled concentration of nanoparticles of an inorganic sintering aid, wherein said nanomixture has been formed by a process including induced precipitation of said inorganic sintering aid nanoparticles from a dispersion of said spinel nanoparticles in an aqueous solution of said inorganic sintering aid;

(b) providing a nonaqueous liquefied vehicle comprising a sublimable organic binding agent and having a solidification temperature of from about room temperature to below about 200° C.;

(c) mixing the spinel powder with the liquefied vehicle to form a homogeneous pseudoplastic spinel mix comprising said spinel powder dispersed in said liquefied vehicle;

(d) reducing the temperature of said spinel mix to below the solidification temperature of said vehicle to thereby freeze the spinel mix;

(e) crushing the frozen spinel mix into powdered form;

(f) molding the powdered frozen spinel mix into a net-shaped green body preform of said desired geometry by cold forging the powdered frozen spinel mix in a mold of said desired geometry, said cold forging being carried out at or near room temperature at a pressure of at least about 1 Ksi;

(g) densifying said green body preform by hot pressing or pressureless sintering into a sintered spinel body of said desired geometry and substantially free of said vehicle and said sintering aid; and

(h) subjecting said sintered spinel body to further densification via hot isostatic pressing to thereby substantially eliminate any residual porosity in said sintered body.

2. The method of claim 1 , wherein said inorganic sintering aid is LiF, the LiF nanoparticles are 20-100 nm in size, and the spinel nanoparticles are 10-2000 nm in size.

3. The method of claim 1 , wherein said inorganic sintering aid is LiF, and said controlled concentration is within the range of from about 0.2 to about 2.0 weight percent.

4. The method of claim 3 , wherein said controlled concentration is within the range of from about 0.3 to about 1.25 weight percent.

5. The method of claim 4 , wherein said controlled concentration is within the range of from about 0.35 to about 0.75 weight percent.

6. The method of claim 3 , wherein the densifying step is carried out by hot pressing of said green body preform, and the hot pressing profile comprises LiF liquefaction at 950° C., followed by LiF sublimation at 1300° C. to 1450° C., followed by sintering at 1600° C. to 1800° C. under a ram pressure of from 500 to 5000 psi.

7. The method of claim 6 , wherein the hot isostatic pressing is carried out at 1600° C. to 1750° C. at a pressure of 27 to 30 Ksi for 2 to 5 hours.

8. The method of claim 3 , wherein the densifying step is carried out by pressureless sintering of said green body preform, and the pressureless sintering profile comprises LiF liquefaction at 950° C., followed by LiF sublimation at 1300° C. to 1450° C., followed by sintering at 1650° C. to 1900° C. for 2 to 3 hours.

9. The method of claim 8 , wherein the hot isostatic pressing is carried out at 1600° C. to 1750° C. at a pressure of 27 to 30 Ksi for 2 to 5 hours.

10. The method of claim 1 , wherein the solidification temperature of said vehicle is within the range of from about 30° C. to about 60° C.

11. The method of claim 1 , wherein said binding agent is selected from the group consisting of camphor, naphthalene, camphor-naphthalene mixtures and camphene.

12. The method of claim 11 , wherein said binding agent is a mixture of from about 55 to about 80 weight percent camphor and from about 45 to about 20 weight percent naphthalene.

13. The method of claim 12 , wherein said binding agent is a close to eutectic mixture of about 60 weight percent camphor and about 40 weight percent naphthalene.

14. The method of claim 1 , wherein said spinel powder and said vehicle are each preheated to a temperature above the solidification temperature of the vehicle prior to the mixing step.

15. The method of claim 14 , wherein the preheating is to a temperature of from about 10° C. to about 40° C. above the solidification temperature of the vehicle.

16. The method of claim 15 , wherein the mixing step is carried out while simultaneously drip-feeding said vehicle onto said spinel powder.

17. The method of claim 1 , wherein said spinel mix comprises from about 70 to about 97 weight percent of said spinel powder and from about 30 to about 3 weight percent of said vehicle.

18. The method of claim 1 , wherein the temperature reducing step to freeze the spinel mix is to a temperature below 0° C.

19. The method of claim 1 , wherein the crushing step is carried out until the frozen spinel mix has been reduced to a size finer than about 30 mesh.

20. The method of claim 1 , wherein said cold forging is carried out at a pressure of from about 1.5 to about 15 Ksi.

21. The method of claim 20 , wherein said cold forging is carried out at a pressure of from about 2 to about 10 Ksi.

22. The method of claim 1 , wherein said cold forging is carried out in two stages of from about 2 to about 5 minutes each stage.

23. The method of claim 1 , wherein the densifying step is carried out by hot pressing of said green body preform loaded in a graphite die lined with upper and lower 3D contoured grafoil.

24. The method of claim 1 , wherein said sintered spinel body is in the shape of a dome with an aperture as large as 180°.

25. The method of claim 24 , wherein said aperture is in the range of from 50° to 170°.