METAL OXIDE CERAMIC MATERIAL, PRECURSORS, PREPARATION AND USE THEREOF

Abstract

The present invention relates to a green body, a pre-ceramic body and a ceramic body based on metal oxide particles, in particular zirconium oxide. The present invention also relates to the method of producing said materials and to the use thereof, in particular in the field of dentistry.

Claims

1. A method for preparing pre-ceramic body P2, comprising the following steps: (a1) providing a dispersion of crystalline metal oxide particles having a mean size of 40 nm or less; (b1) introducing the dispersion into a mould and forming a wet body by pressure filtration of the dispersion in the mould; (b1′) optionally, pressing the wet body by pressure filtration; (b2) demoulding the wet body under conditions of relative humidity of more than 80%; (b3) optionally, when the wet body has a density gradient, removing the part of the wet body at the end of filtration; (c1) forming a green body P1 by drying the wet body under conditions of relative humidity of more than or equal to 90%; (c1′) optionally, shaping the green body P1; (d1) optionally, debinding the green body; and (e1) forming a pre-ceramic body P2 by pre-sintering green body P1 of one of steps (c1), (c1′) or (d1), at a temperature of between 400° C. and 800° C.

2. The method according to claim 1, wherein the crystalline metal oxide particles in step (a1) are ZrO.sub.2 particles having a mean size of between 3 nm and 25 nm, the dispersion in step (a1) comprises 0.4 to 1.5% by weight of binder, the pressure in step (b1) is between 5 bar and 50 bar, and the green body P1 in step (c1) has a thickness of at least 5 mm.

3. The method according to claim 1 wherein the method comprises: one step (b1′) of pressing the wet body by pressure filtration, and one step (b3) consisting of removing the part of the wet body at the end of filtration when the wet body has a density gradient.

4. The method according to claim 1 wherein the method comprises: one step (c1′) of shaping the green body P1, and one step (d1) of unbinding the green body.

5. The method according to claim 1 wherein the method comprises: one step (b1′) of pressing the wet body by pressure filtration, one step (b3) consisting of removing the part of the wet body at the end of filtration when the wet body has a density gradient, and one step (c1′) of shaping the green body P1.

6. The method according to claim 1 wherein the method comprises: one step (b1′) of pressing the wet body by pressure filtration, one step (c1′) of shaping the green body P1, and one step (d1) of unbinding the green body.

7. The method according to claim 1 wherein the method comprises: one step (b1′) of pressing the wet body by pressure filtration, one step (b3) consisting of removing the part of the wet body at the end of filtration when the wet body has a density gradient, and one step (d1) of unbinding the green body.

8. The method according to claim 1 wherein the method comprises: one step (b1′) of pressing the wet body by pressure filtration, one step (b3) consisting of removing the part of the wet body at the end of filtration when the wet body has a density gradient, one step (c1′) of shaping the green body P1, and one step (d1) of debinding the green body.

9. The method according to claim 1 wherein the method comprises: one step (b1′) of pressing the wet body by pressure filtration, and one step (d1) of debinding the green body.

10. A pre-ceramic body P2z of zirconium oxide having: a mean grain size of less than 45 nm, a density of between 52% and 68%, relative to the theoretical density in the absence of pores, a hardness of more than 150 HV, a mechanical biaxial bending strength of at least 25 MPa, and a mean pore size of between 2 nm and 15 nm.

11. The pre-ceramic body according to claim 10, wherein pre-ceramic body P2z has a thickness of more than or equal to 5 mm and wherein the zirconium oxide is doped with 1 mol % to 15 mol % of a dopant selected from yttrium oxide, cerium oxide or a mixture of these two oxides.

12. (canceled)

13. A method for the preparation of a ceramic body P3, comprising the following steps: (a1) providing a dispersion of crystalline metal oxide particles having a mean size of 40 nm or less; (b1) introducing the dispersion into a mould and forming a wet body by pressure filtering the dispersion into the mould; (b1′) optionally, pressing the wet body, by pressure filtration; (b2) demoulding the wet body under conditions of relative humidity of more than 80%; (b3) optionally, when the wet body has a density gradient, removing the part of the wet body at the end of filtration; (c1) forming a green body P1 by drying the wet body under conditions of relative humidity higher than 90%; (c1′) optionally, shaping the green body P1; (d1) optionally, unbinding the green body P1; (e1) optionally, forming a pre-ceramic body P2 by pre-sintering green body P1 of one of steps (c1), (c1′) or (d1), at a temperature between 400° C. and 800° C.; (e2) optionally, shaping pre-ceramic body P2; and (f1) forming a ceramic body P3 by sintering green body P1 of one of steps (c1), (c1′) or (d1) or by sintering pre-ceramic body P2 of step (e1) or (e2), sintering being performed at a temperature of between 900° C. and 1300° C.

14. A method for the preparation of a green body, comprising the following steps: (a1) providing a dispersion of crystalline metal oxide particles having a mean size of 40 nm or less; (b1) introducing the dispersion into a mould and forming a wet body by pressure filtering the dispersion into the mould; (b1′) optionally, pressing the wet body by pressure filtration; (b2) unmoulding the wet body under conditions of relative humidity of more than 80%; (b3) optionally, when the wet body has a density gradient, removing the part of the wet body at the end of filtration; (c1) forming a green body P1 by drying the wet body under conditions of relative humidity higher than 90%, and (c1′) optionally, shaping the green body P1.

15. The method according to claim 13 wherein the dispersion in step (a1) comprises a binder and a dispersing agent, and in that the pressurised filtration in step (b1) is performed by applying a pressure of between 5 bar and 50 bar, and wherein the crystalline metal oxide particles are zirconium oxide particles, doped with 1 mol % to 15 mol % of a dopant chosen from Yttrium oxide, cerim oxide or a mixture of these two oxides.

16. (canceled)

17. A ceramic body P3z of crystalline zirconium oxide having: a mean grain size of less than 200 nm, a density of more than 99%, a mechanical biaxial bending strength of at least 600 MPa, and a mean pore size of less than or equal to 20 nm.

18. The ceramic body P3 according to claim 17, wherein the zirconium oxide is doped with 1.5 mol % to 2.5 mol % of yttrium oxide and has a mean mechanical biaxial bending strength of at least 2000 Mpa and an opalescence of between 9 and 23.

19. (canceled)

20. The ceramic body P3z according to claim 17, wherein the ceramic body is based on zirconium oxide doped with 3.5 mol % to 6.5 mol % of yttrium oxide and has an opalescence of between 16 and 22.

21. The ceramic body P3z according to claim 17 wherein the ceramic body is based on zirconium oxide doped with at least 2.5 mol % yttria, and has a transmittance of at least 47% and a direct transmittance value of at least 22% at 780 nm, for a thickness of 1 mm.

22. (canceled)

23. A green body comprising crystalline zirconium oxide particles having a mean size of 3 nm to 25 nm, pores having a mean size of between 2 nm and 6 nm, and being free of cracks of more than 50 μm, having a density of between 45% and 60% relative to the theoretical density, and a thickness of more than or equal to 5 mm.

24. The pre-ceramic body P2z according to claim 10, wherein the pre-ceramic body P2z has a concentration gradient of a colouring agent or a colouring agent precursor, and a lanthanum oxide concentration of less than 0.1 mol %.

25. (canceled)

26. (canceled)

Description

DESCRIPTION OF THE FIGURES

[0247] FIG. 1 illustrates TEM images of the particles of the dispersions (a): D1, (b): D2 and (c): D3.

[0248] FIG. 2 illustrates the micro-structure observed, e.g., INV-1_P3.

[0249] FIG. 3 illustrates the micro-structure observed, e.g., INV-22_P3.

[0250] FIG. 4 illustrates the micro-structure observed, e.g., INV-11_P3.

[0251] FIG. 5 illustrates the micro-structure observed, e.g., INV-5_P3 and the size of one of the pores.

[0252] FIG. 6 illustrates the appearance of ceramic materials of zirconia according to the prior art and of ceramic zirconia according to the invention, observed by reflection (upper line) and by transmission (lower line) under the same lighting conditions and for a thickness of 1±0.05 mm: (a): zirconia 3 mol % yttria (TZ-3YSB-E-Tosoh), (b) zirconia 5.3 mol % yttria (Zpex Smile-Tosoh), (c) zirconia according to Example INV-1_P3, (d) zirconia according to Example INV-22_P3, (e) zirconia according to Example INV-11_P3.

[0253] FIG. 7 illustrates pre-ceramic body P2 according to Example INV-12_P1 (left) and according to Example INV-15_P1 (Tight)

EXAMPLES OF EMBODIMENTS OF THE INVENTION

[0254] A plurality of examples has been produced to illustrate methods PC1, PC2 and PC3, and also to illustrate bodies P1, P1z, P2, P2z, P3 and P3z.

[0255] The steps implemented to prepare these bodies are as follows: [0256] (a1) providing a dispersion of crystalline metal oxide particles; [0257] (b1) introducing the dispersion into a mould and forming a wet body by pressure filtration of the dispersion in the mould; [0258] (b1′) optionally, pressing the wet body by pressure filtration; [0259] (b2) demoulding the wet body; [0260] (b3) optionally, removing the part of the wet body at the end of filtration, and optionally, the part at the beginning of filtration; [0261] (c1) forming a green body by drying the wet body; [0262] (c1′) optionally, shaping the green body; [0263] (d1) optionally, debinding the green body; [0264] (e1) optionally, forming a pre-ceramic body by pre-sintering the green body; [0265] (e2) optionally, shaping the pre-ceramic body; [0266] (f1) forming a ceramic body by sintering the pre-ceramic body.

[0267] According to certain examples, steps (d1), (e1) and (f1) are performed simultaneously.

[0268] In such a case, the green body is sintered directly, the debinding (d1) and pre-sintering (e1) steps being performed during sintering heat treatment (f1). In this case, the green body and the pre-ceramic body are therefore not isolated.

[0269] 1/Dispersions Used

[0270] Typically, step (a1) consists of preparing a dispersion according to the following embodiment corresponding to dispersion D1 in Table 6:

[0271] A dispersion is prepared by sonication from 80 g of particles of ZrO.sub.2 doped with 3.35 mol % Y.sub.2O.sub.3 and 8 nm in mean size (by number) in dispersion in water with the presence of a dispersant (40% by weight of particles+dispersant).

[0272] The metal oxide particles are advantageously prepared by hydrothermal treatment according to the protocol described in U.S. Pat. No. 8,337,788 or patent application FR 1872183. Where appropriate, the protocol is modified to incorporate the Y.sub.2O.sub.3, by means of the addition of a Y.sub.2O.sub.3 precursor, e.g., YCl.sub.3, before the hydrothermal treatment. Once the particles are prepared by hydrothermal treatment, a dispersant (triammonium citrate, TAC) is added through stirring and sonication. The pH is adjusted by adding ammonia, then the dispersion is purified by dilution and concentration cycles. The concentration is performed by centrifugation when the dispersion is not stable and by tangential filtration when it has good colloidal stability. After purification, the dispersion is stable overtime. The dispersion is then concentrated to 40% by weight by tangential filtration.

[0273] This protocol is adjusted according to the data in Table 6 for preparing dispersions D2 to D22. Where appropriate, the synthesis protocol is modified to incorporate a dopant, by adding a Y.sub.2O.sub.3 or CeO.sub.2 precursor before the hydrothermal treatment.

[0274] However, in dispersion D6, the dispersion resulting from the hydrothermal treatment is concentrated by centrifugation. The supernatant is removed and the functionalisation is performed by diluting the pellet with isopropanol in the presence of MEEEA. The dispersion is then purified by dilution by tangential filtration and concentrated by the same technique. In dispersion D5, the pH is adjusted by adding nitric acid.

TABLE-US-00006 TABLE 6 Characteristics of the dispersion in step (a1) Dispersion Particles Dispersant Solvent pH D1 YSZ-3.35 - 8 nm TAC (3.7) Water 8.5 D2 YSZ-3.35 - 11 nm TAC (3.5) Water 8.5 D3 YSZ-3.20 - 5 nm TAC (6.2) Water 8.5 D4 YSZ-3.50 - 20 nm TAC (5.5) Water 8.0 D5 YSZ-3.35 - 8 nm MEEEA (3.5) Water 3 D6 YSZ-3.35 - 8 nm MEEEA (8) Isopropanol — D7 YSZ-1.63 - 11 nm TAC (3.8) Water 8.5 D8 YSZ-2.20 - 10 nm TAC (3.9) Water 8.5 D9 YSZ-10.1 - 11 nm TAC (4.2) Water 8.5 D10 YSZ-3.73 - 8 nm TAC (3.7) Water 8.5 D11 YSZ-4.10 - 11 nm TAC (3.7) Water 8.5 D12 YSZ-5.11 - 11 nm TAC (3.6) Water 8.5 D13 YSZ-6.12 - 11 nm TAC (4.1) Water 8.5 D14 YSZ-8.13 - 11 nm TAC (4.1) Water 8.5 D15 YSZ-mix1 TAC (4.4) Water 8.5 D16 CeZ-8.9 - 8 nm TAC (5.4) Water 9 D17 CeZ-10 - 8 nm TAC (5.1) Water 9 D18 CeZ-11.1 - 8 nm TAC (5.1) Water 9 D19 CeZ-12.2 - 8 nm TAC (5.4) Water 9 D20 CeZ-13.31 - 8 nm TAC (5.4) Water 9 D21 CeZ-mix1 TAC (—) Water 8.5 D22 CeZ-mix2 TAC (—) Water 8.5 YSZ-3.35 - 8 nm: particles of ZrO.sub.2 doped with 3.35 mol % Y.sub.2O.sub.3 and having a size of 8 nm YSZ-mix1: mixture, having a ratio of 80/20 by weight, of particles of ZrO.sub.2 doped with 6.12 mol % Y.sub.2O.sub.3 of 8 nm and particles of ZrO.sub.2 doped with 2.20 mol % Y.sub.2O.sub.3 of 11 nm CeZ-mix1: mixture, having a ratio of 20/80 by weight, of particles of ZrO.sub.2 doped with 13.31 mol % CeO.sub.2 of 8 nm (dispersion D20) and particles of ZrO.sub.2 doped with 5.11 mol % Y.sub.2O.sub.3 of 11 nm (dispersion D12) CeZ-mix2: mixture, having a ratio of 20/80 by weight, of particles of ZrO.sub.2 doped with 13.31 mol % CeO.sub.2 of 8 nm (dispersion D20) and particles of ZrO.sub.2 doped with 3.35 mol % Y.sub.2O.sub.3 of 11 nm (dispersion D2) TAC (3.7): triammonium citrate, used at 3.7% by weight relative to the weight of the particles TAC (—): % by weight of triammonium citrate = weighted mean of the constituents of the CeZ-mix1 and CeZ-mix2 mixtures MEEEA: [2-(2-methoxyethoxy)ethoxy] acetic acid

[0275] 2/Preparing Green Bodies by Pressure Filtration by Means of a Fluid

[0276] 2.1/a Plurality of Green Bodies were Prepared by Modifying Steps (a1) to (c1) according to the parameters of Table 7.

[0277] In Example INV-1_P1, a solution of 0.72 g (0.9% by weight relative to the weight of the particles) of binder in 27.85 g of deionised water is added to 200 g of the dispersion D1, drop by drop for 30 minutes through stirring. The dispersion, containing 35% by weight of particles, is kept under sonication for 2 hours.

[0278] This dispersion is then poured into 8 identical moulds in assembly. The assembly rests on a rigid porous support in a vertical filtration system, in which the filtration is performed from top to bottom and the filtered solvent is discharged downwards through the porous support. Each mould used during step (b1) (and optionally (b1′)) is cylindrical in shape (20 mm in diameter). A 100 μm thick polycarbonate filter with a pore size of 100 mm to 200 nm (Nucleopore™ Whatman) is placed between the porous support and the dispersion. The system is closed so as to be hermetically sealed and connected to a pressurised gas circuit (argon, air or nitrogen). The pressure is gradually increased (10 minutes) until the value indicated in Table 7 is reached, and the dispersion is filtered under pressure. The pressure filtration is therefore performed by means of a fluid, in this case a gas. However, identical results may be obtained by means of a piston when the dispersion is excessive.

[0279] The pressure is gradually released (10 minutes). The still-closed filtration system is transferred to an environment having a relative humidity of more than 80%, the excess dispersion that has not been filtered is recovered and each wet body is demoulded by means of a polytetrafluoroethylene (PTFE) cylinder by simple pressure. The bodies prepared are in the form of a block, have the consistency of a rigid wet body in their lower part (part at the beginning of filtration), and the consistency of a soft gel in their part at the end of filtration.

[0280] During step (b3), the part at the end of filtration is removed by means of a spatula over a thickness of approximately 2 mm. The part at the beginning of filtration is removed over a thickness of 1 mm. The body is deposited on a support (PTFE grid) and inserted into a climatic chamber already at a relative humidity of 95% and at 30° C. The relative humidity is maintained at 95±3% during the first 24 hours, then gradually decreased with a gradient of 0.41%/h during days 2, 3, 4 and at 1%/h during day 5, to reach the final value of 40%. The green body obtained, in the form of a block, is free of cracks, has a translucent and opalescent appearance (orange if observed by transmission, bluish if observed by reflection) and has a visible deformation relative to the initial shape. This deformation is characterised by the presence of a curvature at the upper and lower surfaces, the upper surface being slightly concave and the lower surface slightly convex.

[0281] This protocol is adjusted to prepare green bodies according to the data in Table 7 using the dispersions presented in Table 6. For each example, an amount of excess dispersion is introduced into the mould and filtration is performed for 15 minutes to 24 hours depending on the thickness intended, so as not to filter the entire dispersion. When the drying time is longer, the relative humidity reduction gradients are adjusted proportionally to the time.

TABLE-US-00007 TABLE 7 Green bodies prepared from metal oxide particle dispersions by fluid pressure filtration (b1) (b2) (c1) P1 (a1) Pressure Wet Drying Density Thickness Disp. P1 Binder (bar) body (b3) (days) (%) Cracks (mm) D1.sup.(a) INV-1_P1 0.9 20 CH + GM Yes 5 51 No 6 D1.sup.(a) INV-2_P1 0.9 5 CH + GM Yes 5 47 No ≥3 mm D1.sup.(a) INV-3_P1 0.9 10 CH + GM Yes 5 — No 5 D2.sup.(a) INV-4_P1 0.9 20 CH + GM Yes 6 54 No 6 D2.sup.(b) INV-5_P1 0.9 20 CH + GM No 5 53 No 2.5 D2.sup.(a) INV-6_P1 0.9 45 CH + GM Yes 5 — No — D3.sup.(c) INV-7_P1 0.9 20 CH + GM Yes 6 60 No 5 D4.sup.(a) INV-8_P1 0.9 20 CH + GM Yes 5 — No 5 D6.sup.(d) INV-9_P1 0 20 CH + GM No 20  — No 2.5 D7.sup.(a) INV-10_P1 0.9 20 CH + GM Yes 5 — No 5 D14.sup.(a) INV-11_P1 0.9 20 CH + GM Yes 5 — No 5 D1.sup.(a) CE1 0.4 20 CH + GM Yes 6 65 Yes 6 D1.sup.(a) CE2 0.9 80 CH + GM Yes 6 — Yes 6 D1.sup.(a) CE3 0 80 CH + GM No 6 67 Yes 6 D1.sup.(a) CE4 0 40 CH + GM Yes 6 66 Yes 6 D2.sup.(a) CE5 0 3 No body NA NA NA NA NA formed D2.sup.(a) CE6 0 20 CH + GM Yes 4 53 Yes 6 D2.sup.(a) CE7 0 20 CH + GM No 6 54 Yes 6 D2.sup.(a) CE8 0 20 CH + GM Yes 6 — Yes 8 CH + GM: wet body + soft gel Dispersions at .sup.(a)35% or .sup.(b)40% or .sup.(c)30% or .sup.(d)51% by weight of particles relative to the total weight of the dispersion. PVA (polyvinyl alcohol) in % by weight relative to the weight of the particles Density: expressed in % relative to the theoretical density calculated according to the composition NA: not applicable

[0282] In general, when demoulding (b2) is performed at less than 80% relative humidity, the green body has cracks of more than 500 μm.

[0283] When demoulding (b2) is performed at more than 80% relative humidity, and when drying (c1) is performed at less than 90%, the green body has cracks of more than 500 μm.

[0284] The examples show that a green body free of cracks may only be obtained for a combination of parameters, in particular, a sufficient quantity of binder and a specific filtration pressure range. Under optimum conditions, step (b3) may prove necessary to minimise the density gradient and avoid cracking when the green body is at least 3 mm to 4 mm thick (INV-4_P1 and INV-5_P1).

[0285] According to CE1, CE4 and CE6 to CE8, the presence of 0% or 0.4% by weight of binder is not sufficient to form a green body free of cracks, but in the case of a thickness of at least 5 mm.

[0286] According to CE2 and CE3, the filtration pressure is too high (80 bar) to avoid cracks being formed, in the presence or not of binder, for a green body at least 5 mm thick.

[0287] In CE5, there is no formation of a solid wet body because the pressure is not high enough to form a wet body.

[0288] Green body INV-7_P1 has a mean pore size (BJH) of 3.4 nm and a specific surface area (BET) of 140 m.sup.2/g.

[0289] Green body INV-4_P1 has a mean pore size (BJH) of 4.9 nm and a specific surface area (BET) of 114 m.sup.2/g.

[0290] Green body INV-1_P1 has a mean pore size (BJH) of 4.2 nm and a specific surface area (BET) of 117 m.sup.2/g.

[0291] 2.2/Binders distinct from the PVA were also used to form the green body. These examples are given in Table 8.

TABLE-US-00008 TABLE 8 Green bodies prepared from dispersions of metal oxide particles by pressure filtration by means of a piston (b2) (c1) P1 (a1) (b1) Wet Drying Thickness Disp. P1 Binder.sup.(d) Pressure body (b3) (days) Cracks (mm) D2.sup.(b) INV-21_P1 1 PVP 20 CH + GM Yes 5 No 4 D5.sup.(c) INV-22_P1 — 40 CH + GM No 5 No 3 D1.sup.(a) INV-23_P1 0.4PVP + 20 CH + GM Yes 5 No 5 0.4 PEG D1.sup.(a) INV-24_P1 0.75 PVA + 20 CH + GM Yes 5 No 5 0.8 glycerol Dispersions at .sup.(a)30% or .sup.(b)35% or .sup.(c)44% by weight of particles .sup.(d)in % by weight relative to the weight of the dispersion CH + GM: wet body + soft gel PVP: polyvinyl pyrrolidone PEG: polyethylene glycol

[0292] 3/Preparing Green Bodies by Centrifugation

[0293] 56 ml of the D2 dispersion were separated into 4 equal parts, and acetic acid was added to modify the pH. The amount of acetic acid varies from one sample to another to obtain dispersions having a pH of between 5.5 and 8.5. The pH of the 4 dispersions after adding was 8.5, 7.5, 6.5 and 5.5. The 4 dispersions were centrifuged, at 30,000 g for 10 minutes, in cylindrical centrifugation pots having a volume of 50 ml, so as to separate the particles that form a solid precipitate, and the dispersion solvent (supernatant). The supernatant is then removed and the precipitate is dried by a drying cycle similar to that presented in Example INV-4_P1. The tests result in green bodies having numerous cracks. These green bodies are not suitable for forming a pre-ceramic or ceramic material having a thickness of several mm.

[0294] The cracked green bodies (thickness <5 mm) were recovered and subjected to debinding/sintering treatment as in Example INV-3_P3. The cracked pieces obtained had a translucent appearance and a density of between 99% and 99.8% relative to the theoretical density.

[0295] This method makes it possible to obtain good densification of the cracked bodies after sintering, but does not make it possible to obtain green body P1 or P1z that is free of cracks.

[0296] 4/Preparing Green Bodies by Gel Casting

[0297] 100 ml of the dispersion D2 were prepared, but without binder. The dispersion was separated into 5 equal parts and heated to 80° C. through stirring in a closed container. Incremental amounts of gelling agent (purified agar) of between 0.2% and 1.2% by weight relative to the mass of the particles were introduced progressively into the dispersion through stirring. The dispersion was then degassed under vacuum and poured into silicone moulds 30 mm in diameter and cooled to ambient temperature, forming a solid gel. The gel was then dried according to a drying cycle similar to that presented in Example INV-4_P1. After drying, the green bodies were observed. In the case of low gelling agent contents (less than 0.8% by weight), the gel did not retain the cylindrical shape of the mould. In the case of higher gelling agent contents (between 0.8% and 1.2% by weight), a green body of cylindrical shape without apparent cracks was obtained. All the green bodies were subjected to debinding/sintering treatment as in Example INV-3_P3. The green bodies free of cracks made it possible to obtain a sintered body with a density of 92% to 96% of the theoretical density and a partially translucent appearance. Analysis of a polished section of these sintered bodies has made it possible to show the presence of numerous residual pores with a size of between 20 nm and 100 nm as well as numerous macropores with a size of between 5 μm and 100 μm. The cracked green bodies made it possible to obtain small-sized sintered bodies and with a density of between 96.0% and 97.6% of the theoretical density, with a reduced presence of both pore families (20 nm to 100 nm and 5 μm to 100 μm).

[0298] These tests result in green bodies having a fairly high density and numerous cracks, or green bodies having a low density and macroporous defects.

[0299] These green bodies are not suitable for forming a pre-ceramic or ceramic material having a thickness of several mm.

[0300] 5/Preparing Bodies by Extrusion and Micro-Extrusion

[0301] 30 ml of the dispersion D2 were prepared, but without binder. The dispersion was then reconcentrated by osmotic compression according to the following steps:

[0302] 800 ml of a 20% by weight solution of PEG8000 (M=8000 g/mol) in water were prepared and then adjusted to pH 8.5 by adding a 30% by weight aqueous ammonia solution.

[0303] The dispersion was transferred into a dialysis membrane in the form of a tube closed at the ends (Spectra/Por supplied by Spectnumlab) with a nominal cut-off threshold of molecular weight 12-14 kD. This dialysis membrane was then placed in the PEG8000 solution previously prepared. The dispersion was thus dialysed against this PEG8000 solution to extract a portion of the water present in the zirconia dispersion and thus concentrate it.

[0304] After approximately 9 hours of dialysis, a colloidal paste of nanoparticles of zirconia at 66% by weight in water was recovered. The colloidal paste was then homogenised and deaerated using a planetary mixer with asymmetric double axes under vacuum (Speedmixer DAC150.1 FVZ-K, Hauschild Engineering). The rheological properties of this paste show a shear-thinning behaviour (viscosity decreasing with the shear rate). Measurements by oscillatory rotational rheometry in plane-plane geometry at a frequency of 1 Hz have shown that this paste behaves like a solid at rest, with an elastic (or conservation) modulus G′ of approximately 6×10.sup.5 Pa, and a viscous (or loss) modulus of approximately 3×10.sup.3 Pa, having a yield point of approximately 6500 Pa. These properties make it possible to shape the paste by extrusion or by additive manufacturing by means of microextrusion.

[0305] A 5 cm.sup.3 tubular cartridge was filled with this paste and then centrifuged using a planetary mixer with asymmetrical double axes to extract the air which may remain trapped between the paste and the wall of the cartridge. A cylinder (12 mm in diameter and 16 mm in height) was produced by microextrusion of the paste in filament form through a 250 μm diameter nozzle screwed onto the syringe. The microextrusion was controlled by means of a piston with a controlled speed of movement. The cartridge-piston assembly is mounted on a printing machine which may control its movement in the three spatial directions and perform microextrusion in an environment with a relative humidity of more than 95%. A 10 cm.sup.3 cartridge was filled and centrifuged in a similar manner, then the end part of the cartridge was cut and a cylinder (18 mm in diameter and 25 mm in height) was manufactured by extruding the contents of the cartridge onto the same support used for drying, in Example INV-4_P1. The two objects were dried according to a drying cycle similar to that in Example INV-4_P1. Both green bodies had no cracks and had a translucent appearance. Both green bodies were debonded and pre-sintered according to the protocol used in Example INV-2_P2, forming two pre-ceramic bodies. The body obtained by extrusion had a macroscopic crack, whereas the body obtained by microextrusion had no cracks. Unlike the pre-ceramic bodies in Examples INV-1_P2 to INV-11_P2, both bodies had lost their translucent appearance.

[0306] Discs 2 mm thick of both pre-ceramic bodies were subjected to the sintering conditions in Example INV-19_P3. After sintering, the discs in both cases had a translucent appearance in their outer part, over a thickness of approximately 3 mm, and an opaque appearance in their inner part. MEB observations after vibratory polishing revealed a microstructure having a mean grain size of 130 nm, a mean pore size of less than 20 nm in the translucent outer part, and a mean pore size of more than 50 nm in the opaque inner part.

[0307] This technique does not make it possible, starting from a dispersion of nanoparticles with a size of less than 40 nm, to obtain both green bodies free of cracks, a good densification of the material without the presence of a densification gradient and the absence of nanopores with a mean size of more than 20 nm in the sintered piece.

[0308] These green bodies are not suitable for forming a pre-ceramic or ceramic material having a thickness of several mm.

[0309] 5B/Preparing Bodies by Vacuum Filtration

[0310] A plaster mould (20 mm in diameter and 10 mm in height) was filled with 20 ml of dispersion D2, and placed under vacuum between 1 mbar and 10 mbar. After 24 hours of operation, the mould was emptied of the still fluid dispersion. An extremely thin layer (<1 mm) of zirconia had been deposited on the surface of the plaster. The layer cracked into several parts during drying performed according to the conditions in Example INV-4_P1.

[0311] This technique does not make it possible to obtain a green body with a satisfactory thickness starting from a dispersion of nanoparticles with a size of less than 40 nm.

[0312] 6/Preparing Green Bodies by Double Pressure Filtration

[0313] This section shows that pressing step (b1′) may prove to be essential to form a green body of at least 5 mm thick and free of cracks. Step (b3) may also be performed, but it is not necessary when pressing is performed by means of a piston.

[0314] 6.1/Filtrations (b1) and (b′) by Means of a Dispersion

[0315] Green bodies prepared from dispersions of metal oxide particles by pressure filtration by means of a fluid (b1+b1′).

[0316] In Example INV-12_P1, filtration (b1) is performed by means of a first dispersion D2, at a concentration of 35% by weight, used to form the green body at 20 bar. When the desired wet body thickness is reached (72 hours), the pressure is returned to atmospheric pressure, the filtration device is opened and the remaining dispersion is removed and replaced by a second dispersion, D1, at a concentration of 25% by weight, to perform a new filtration cycle for 12 hours. During the second cycle, which corresponds to pressing step (b1′), the tiltration rate of dispersion D1 is lower than that of dispersion D2. Under these conditions, the body formed from the first dispersion D2 is compacted and the density gradient is reduced. The following steps are performed as in Example INV-1_P1. During demoulding, step (b3) is performed to remove the upper part of the wet body, of variable thickness according to the dispersion used and the pressing time, corresponding to the part formed with the second dispersion. After drying (6 days), green body P1 in the form of a cylinder with a thickness of 18.7 mm is free of cracks and has a very slight curvature at the upper and lower surfaces and a very slight inclination of the lateral surface.

[0317] In all the examples in Table 9, the pressing step (b1′) is performed for a period equal to 10% to 25% of the filtration period in step (b)

TABLE-US-00009 TABLE 9 Green bodies prepared from dispersions of metal oxide particles by double pressure filtration by means of a dispersion of particles. (b1) (b1′) (c1) P1 (a1) Pressure (b1′) Pressure Drying Thickness Disp. P1 Binder (bar) Disp.2 (bar) (days) (b3) Cracks (mm) D2.sup.(a) INV-12_P1 0.9 20 D1.sup.(b) 20 6 Yes No 18.7 D12.sup.(a) INV-13_P1 0.9 20 D1.sup.(b) 20 6 Yes No 15 D1.sup.(a) INV-19_P1 2 20 D1.sup.(b) 40 6 Yes No 15 D2.sup.(a) INV-20_P1 1.8 20 D1.sup.(b) 20 6 Yes No 22 Binder: PVA in % by weight relative to the weight of the dispersion Dispersions at .sup.(a)35% or .sup.(b)25% or .sup.(c)30% by weight of particles

[0318] The green bodies in the examples in Table 9, in the form of a cylinder, are very slightly deformed after drying and are free of cracks.

[0319] 6.2/Filtration (b1) and (b1′) by Means of a Piston

[0320] Green bodies were also prepared from dispersions of metal oxide particles by double pressure filtration by means of a piston. In this case, the pressure is applied by means of a PTFE piston, with a diameter equal to the diameter of the mould and provided with an O-ring to guarantee sealing, directly in contact with the dispersion. The force exerted by the piston is controlled to ensure a constant pressure during step (b1).

[0321] In Example INV-14_P1, 30 ml of dispersion D1 are prepared with the addition of 0.9% by weight of PVA binder as in Example INV-1_P1. The resulting dispersion is diluted to 30% by weight. 6 ml of the dispersion are poured into a cylindrical mould 20 mm in diameter, in a vertical filtration system similar to that in Example INV-1_P1. A PTFE piston is inserted into the upper part of the mould and brought into contact with the dispersion. A force corresponding to a pressure of 20 bar is then applied. The entire dispersion is filtered in step (b1), which lasts 3 hours. The force is maintained during pressing step (b1′) lasting 1 hour. The force was then withdrawn and the body is demoulded under the same conditions as, e.g., INV-1_P1, by exerting a slight pressure on the piston. The wet body obtained has the consistency of a rigid body over its entire thickness. Step (b3) is not performed. Drying is then performed as in Example INV-12_P1 for 6 days. The resulting green body, translucent and opalescent, is free of cracks. It does not have any curvature at the upper and lower surfaces, and retains the block shape obtained after step (b2).

[0322] The examples of Table 10 are performed by the same technique, by varying the initial dispersion and the durations of steps (b1) and (b1′) to obtain different thicknesses for body P1. In all the examples, the same binder is used, as well as the same drying protocol.

TABLE-US-00010 TABLE 10 Green bodies prepared from dispersions of metal oxide particles by double pressure filtration by means of a piston. (b2) (b1) Cracks (c1) − P1 Pressure (b1) (b1′) after Thickness Density Disp. P1 (bar) Duration Duration (b2) Cracks Gradient (mm) (%) D1.sup.(c) 6 ml INV-14_P1 20 3 h 1 h No No No 4 51.5 D2.sup.(a) 16 ml INV-15_P1 20 48 h 15 h No No No 12 49.3 D2.sup.(a) 29 ml INV-16_P1 20 98 h 26 h No No No 21 48.4 D3.sup.(c) 6 ml INV-17_P1 20 3.5 h 1.5 h No No No 4 55.4 D18.sup.(d) 29 mL INV-18_P1 20 65 h 17 h No No No 22 53.2

[0323] In Table 10, the dispersions comprise .sup.(a) 35% or .sup.(b) 25% or .sup.(c) 30% or .sup.(d) 38% by weight of particles. The term “Gradient” indicates the visual observation of a curvature of the lower and upper surfaces of the green body, associated with a difference in diameter. This curvature indicates the presence of a density gradient in the wet body. As already indicated, the curvature of the lower and/or upper surfaces indicates the presence of a deformation relative to the initial shape, namely obtaining a slightly concave upper surface and/or a slightly convex lower surface. The density is indicated in % relative to the theoretical density.

[0324] Bodies P1 in Example INV-15_P1 were analysed by XRD (diffractometer mod. Bruker D8 Advance) and the Scherrer method was applied to calculate the size of the crystallites. The size of the crystallites is 10.11 nm. The diffraction pattern has only peaks corresponding to the quadratic/cubic phase.

[0325] 7/Preparing Pre-Ceramic Bodies

[0326] Pre-ceramic bodies were prepared by combined heat treatment of debinding-pre-sintering from green bodies according to the data of Table 11. When the green bodies had a curvature, they were flattened manually by means of a silicon carbide polishing disc of grain 640 in the absence of water or lubricant.

[0327] The green bodies were deposited in porous alumina containers and inserted into a conventional muffle-type furnace with ceramic heating bodies (mod. Nabertherm L 9/11 BO). The heat treatment applied was as follows: [0328] 0.1° C./min from ambient temperature up to 200° C. [0329] plateau at 200° C. for 3 hours 25-0.2° C./min from 200° C. to 400° C. [0330] plateau at 400° C. for 3 hours [0331] 0.2° C./min at 400° C. at the pre-ceramic body formation temperature [0332] plateau at the pre-ceramic body formation temperature for the indicated time [0333] cooling to 0.5° C./min up to ambient temperature.

TABLE-US-00011 TABLE 11 Preparing pre-ceramic bodies P2 Specific P2 (e1) Pore surface Thickness Duration Density Mech. size area Disp. P1 P2 (mm) (° C.) (h) (%) Hardness res. (BJH) (BET) D2 INV-4_P1 INV-1_P2 6 500 1 59.5 — — 6.5 67 D1 INV-1_P1 INV-2_P2 6 550 1 56.9 150 25 — — D1 INV-1_P1 INV-3_P2 6 600 1 57.9 164 29 6.5 68 D1 INV-1_P1 INV-4_P2 6 650 1 59.6 180 36 — — D1 INV-1_P1 INV-5_P2 5.5 800 1 66.1 — — 8.6   26.5 D3 INV-7_P1 INV-6_P2 5 550 1 62.9 — — — — D2 INV-12_P1 INV-7_P2 20 800 1 67.8 — — — — D1 INV-14_P1 INV-8_P2 4 500 1 56.1 — — 5.0 120  D2 INV-15_P1 INV-9_P2 12 600 1 62.0 — — 7.3 57 D2 INV-16_P1 INV-10_P2 20 750 1 65.9 210 43 — — D18 INV-18_P1 INV-11_P2 — 550 3 57.1 — — — — D1 INV-19_P1 CE9 15 550 1 No P2 formation Mech. res.: mechanical biaxial bending strength in MPa Hardness: Vickers HV1 hardness measured with a load of 1 kgf (in Vickers units) Pore size: size of the interconnected porosity in nm (BJH method) Specific surface area: in m.sup.2/g (BET method) Density: in % relative to the theoretical density CE9 shows that, when the body is more than 5 mm thick, it is preferable to limit the quantity of binder to avoid cracking. Bodies P2 of Examples INV-2_P2 and INV-9_P2 were analysed by XRD (diffractometer mod. Bruker D8 Advance). The diffraction patterns have only peaks corresponding to the quadratic/cubic phase. The size of the crystallites measured by the Scherrer method is 10.11 nm for INV-2_P2 and 12.06 nm for INV-3_P2.

[0334] A section of body P2 in Example INV-2_P2 was polished by ionic polishing using an ionic polisher with Argon ion beams (mod. Ilion II—Gatan) and observed by SEM. The observations revealed that the size measured by XRD corresponds approximately to the size of the grains that comprise the microstructure of the pre-ceramic body.

[0335] 8/Shaping Pre-Ceramic Bodies by CFAO

[0336] The pre-ceramic bodies were bonded on a support compatible with a dental CAD/CAM system of the Cerec (Denstply Sirona) type, represented by a multi-axis milling machine, and machining tests were performed with protocols typical of dental machining. One of these protocols is represented by milling, using a tool with a defined cutting geometry, commonly used in machining zirconia blocks, available on the market with a defined maximum forward speed. One second protocol is represented by “grinding”, using an abrasive tip tool, commonly used in machining glass-ceramic dental blocks, available on the market, with a defined maximum speed.

[0337] The examples in Table 12 summarise the results of machining on pre-ceramic bodies pre-sintered at different temperatures, according to one of the two protocols, with or without water cooling. In the case where there are no cracks or chipping on the body after step (c1), the pre-ceramic body is retained and declared compatible with the dental CAD/CAM.

TABLE-US-00012 TABLE 12 Compatibility of pre-ceramic bodies with shaping techniques (e1) (c1′) Cooling CAD/ Pre- Protocol with CAM P1 P2 sintering.sup.(d) % speed.sup.(c) water.sup.(a) accounting INV-15_P1 INV-12_P2 500 G - 100% Yes Yes INV-4_P1 INV-13_P2 550 G - 100% Yes Yes INV-16_P1 INV-14_P2 600 G - 100% Yes Yes INV-4_P2 INV-15_P2 650 G - 100% Yes Yes INV-1_P1 INV-16_P2 650 G - 100% Yes Yes INV-16_P1 INV-17_P2 700 G - 100% Yes Yes INV-12_P1 CE17 850 G - 100% Yes No, chipping INV-12_P1 CE18 550 F.sup.(b) - 50%.sup.  Yes No, chipping INV-12_P1 CE19 550  G - 80% No No, chipping + cracks .sup.(a)cooling during CAD/CAM shaping .sup.(b)milling .sup.(c)% of maximum speed according to protocol; G = grinding; F = milling .sup.(d)pre-sintering temperature in ° C.

[0338] In the specific case of CAD/CAM machining, Example CE17 shows that an excessive pre-sintering temperature (850° C.) leads to cracks being formed during machining. Examples EC18 and EC19 show that machining conditions, in particular, the speed or absence of cooling, may also generate cracks. The person skilled in the art will be able to adapt the machining conditions (speed and cooling) according to their general knowledge.

[0339] 9/Preparing Ceramic Bodies

[0340] 9.1/Ceramic bodies were prepared from green bodies, without isolating bodies from the intermediate debinding (d1) and pre-sintering (e1) steps. To minimise the machining steps on the sintered material, the thickness of the green bodies was reduced by manual machining to 2 mm by means of a silicon carbide polishing disc of grain 640, in the absence of water or lubricant.

[0341] The heat treatment is performed in a high-temperature furnace of the muffle type with heating elements made of MoSi.sub.2 (mod. Nabertherm LHT 03/17 D). The treatment is performed in a manner identical to Example INV-4_P2 up to 650° C., then a gradient of 3° C./min is applied up to the sintering temperature, then the temperature is maintained for a plateau time. The sintered body is then cooled at a rate of 50° C./min to ambient temperature.

[0342] After sintering, the ceramic bodies were pre-polished with diamond pre-polishing discs (MD-Piano, Struers) of grain 120 to 1200, mounted on a polishing machine, to reduce their thickness. Then, they were polished with diamond dispersions by means of polishing discs until a mirror finish was obtained. The resulting discs, 1.2±0.2 mm thick, were characterised in terms of mechanical properties (hardness, mechanical strength) and microstructure (grain size measured by SEM after vibratory polishing). Discs with a thickness of 1.00±0.05 mm, prepared in a similar manner, were characterised in terms of optical properties (value b*, CR, OP, TP).

[0343] The results of the characterizations are reported in Tables 14v and 15. In the examples where body P1 is not indicated, body P1 is obtained in a manner identical to Example INV-4_P1, except that each dispersion is different.

[0344] In Examples INV-2_P3, INV-4_P3, INV-5_P3, bodies are sintered by the “two-step sintering” method, with a short plateau at a higher temperature followed by a longer plateau at a lower temperature.

TABLE-US-00013 TABLE 13 Ceramic bodies P3 Vickers Grain (f1) Density Hardness Mech. size Disp. P1 P3 Sintering (%) (GPa) res. (nm) D1 INV-20_P1 INV-1_P3 2 hours at 1150° C. 99.9 — — 117 D1 INV-20_P1 INV-2_P3 2 minutes at 1200° C. + 99.8 12.7 103 10 hours at 1000° C. D2 INV-21_P1 INV-3_P3 2 hours at 1150° C. 99.9 — — 131 D2 INV-21_P1 INV-4_P3 1100° C. 60 min + — 13.0 2620  120 1000° C. 10 hours D2 INV-21_P1 INV-5_P3 18 minutes at 1250° C. + 99.9 14   2215  145 10 hours at 1000° C. D2 INV-21P1 INV-6_P3 2 hours at 1200° C. 99.9 13.7 1900  134 D4 INV-8_P1 INV-7_P3 2 hours at 1150° C. 99.8 — — 137 D6 INV-9_P1 INV-8_P3 2 hours at 1150° C. 99.9 — — 124 D7 INV-10_P1 INV-9_P3 2 hours at 1150° C. — 12.2 2550  151 D12 INV-13_P1 INV-10_P3 1 hour at 1200° C. 99.9 14.4 794 136 D14 INV-11_P1 INV-11_P3 1 hours at 1200° C. 99.9 14.3 680 133 D16 — INV-12_P3 2 hours at 1150° C. 99.8 — 595 113 D17 — INV-13_P3 2 hours at 1150° C. 99.7 — 780 111 D18 — INV-14_P3 2 hours at 1150° C. 99.7 — 910 115 D19 — INV-15_P3 2 hours at 1150° C. 99.9 — 952 126 D20 — INV-16_P3 2 hours at 1150° C. 99.8 — 923 115 D21 — INV-17_P3 2 hours at 1175° C. 99.8 13.9 740 129 D22 — INV-18_P3 2 hours at 1150° C. 99.9 13.6 840 122 D10 — INV-19_P3 2 hours at 1150° C. 99.8 13.9 1075  128 D9 — INV-20_P3 3 hours at 1225° C. 99.9 — — 145 D8 — INV-21_P3 2 hours at 1150° C. — 13.0 2360  129 D13 — INV-22_P3 1 hour at 1200° C. 99.9 14.2 731 136 D15 — INV-23_P3 2 hours at 1175° C. 99.9 14   675 134 D11 — INV-24_P3 2 hours at 1175° C. 99.8 14.1 890 113 D2 INV-20_P1 CE21 2 hours at 1150° C. 97.3 — — — Sintering: sintering temperature and plateau time Mech. res.: mean mechanical biaxial bending strength in MPa Hardness: Vickers HV10 mean hardness measured in GPa The mean pore size of bodies INV-1_P3 to INV-24_P3 is less than 15 nm. EC21 (1.8% binder) shows that, for bodies more than 5 mm thick, it is preferable to limit the binder quantity. In fact, in the presence of an excessive quantity of binder, it is possible for the densification during sintering to be altered and for it to be impossible to obtain a nanocrystalline ceramic material with a density of more than 99%.

TABLE-US-00014 TABLE 14 Transmittance values of the ceramic materials TFT 555 nm- RIT 555 nm- 600 nm- 600 nm- Disp. P1 P3 L*, a*, b* CR OP TP 780 nm 780 nm D1 INV-23_P1 INV-1_P3 94, −1.1, −0.8 0.47 17   26.1 43.4-45.8-56.0) 6.9-11.9-35.2) D1 INV-23_P1 INV-2_P3 90, −1.1, 9.1 0.62 20.3 20.7 — — D2 INV-21_P1 INV-3_P3 94.1, −1.2, 0.4 0.48 14.6 25.7 42.9-44.8-53.4) 4.4-7.7-23.1) D2 INV-21_P1 INV-4_P3 91.4, −1.2, 8.4 0.59 19.2 22.2 — — D2 INV-21_P1 INV-5_P3 — — — — — — D2 INV-21_P1 INV-6_P3 90.6, −1.2, 8.7 0.53 17   24.1 — — D4 INV-8_P1 INV-7_P3 — — — — — — D6 INV-9_P1 INV-8_P3 — — — — — — D7 INV-10_P1 INV-9_P3 93.4, −0.7, 8.9 0.67 14.9 17.3 31.3-34.2-39.4) — D12 INV-13_P1 INV-10_P3 88.1, −1.2, 9.1 0.52 20.6 25.5 39.6-42.8-53.3) 4.9-9.3-23.6) D14 INV-11_P1 INV-11_P3 91.3, −0.7, 6.2 0.28  9.5 38   60.4-62.1-66.8) 38.6-41.0-48.1) D16 — INV-12_P3 — — — — 7-15-30) — D17 — INV-13_P3 — — — — 9-17-32) — D18 — INV-14_P3 — — — — 9-17-33) — D19 — INV-15_P3 — — — — 10-19-37 — D20 — INV-16_P3 — — — — 11-19-36 — D21 — INV-17_P3 — — — — 43.4-47.6-58.6) 14.7-20.1-38.2) D22 — INV-18_P3 — — — — 30.3-34.7-45.1) 0.49-1.2-9.5) D10 — INV-19_P3 — — — — — — D9 — INV-20_P3 95.3, −0.6, 4.2 0.23 14   43.9 64.6-66-69.7) 40.2-42.7-49.9) D8 — INV-21_P3 87, 1.8, 15.3 0.71 21.2 17.3 28.6-32.1-39.8) — D13 — INV-22_P3 92.2, −0.8, 5.4 0.31 18.3 37   52.9-56.3-64.3) 22.1-25.7-35.4) D15 — INV-23_P3 88.1, −0.8, 8.3 0.44 20.7 28.9 45.8-49.5-59.8) 15.8-20.8-36.3) D11 — INV-24_P3 88.3, −1.2, 8.8 0.50 16.8 25.2 39.8-42.5-49.7) — D2 INV-20_P1 CE21 — — — — 0-0-0 0-0-0 OP: opalescence TP: translucency parameter CR: contrast ratio

[0345] The contrast ratio corresponds to the “contrast ratio”-CR, determined from the luminance values (Y) of body P3 or P3z, measured according to the calorimetric reference system CIE 1931 (described in the standard ISO 11664-1), when the body is placed in front of a white background (Yw) or a black background (Yb), according to the following equation:

CR=Yb/Yw

[0346] The contrast ratio is a measure of the opacity of the body. In the field of dentistry, it is often used to determine the “translucency” according to the following equation:

translucency=1−CR.

[0347] The translucency parameter corresponds to the “translucency parameter”-TP of bodies P3 or P3z. The TP is determined by the difference between the colour measured in reflection mode when the body is placed in front of a white background (indices W) and the colour measured in reflection mode when the body is placed in front of a black background (indices B). The TP is calculated using the L*, a *, b* colour coordinates defined above, according to the following equation:

TP=[(L*.sub.W−L*.sub.B).sup.2+(a*.sub.W−a*.sub.B).sup.2+(b*.sub.W−b*.sub.B).sup.2].sup.1/2

[0348] As already indicated, “RIT” designates the value of direct transmittance (real in-line transmittance) while “TFT” designates the total transmittance (total-forward transmittance). These values are measured at ambient temperature, for a thickness of 1 mm.

[0349] All the examples according to the invention reported have optimum densification during step (f1). The method used makes it possible to obtain mechanical strengths of more than the materials of the prior art formed by a single component (doped zirconia) and of similar composition.

[0350] In Examples INV-9_P3 and INV-21_P3, resistances of more than 2.5 GPa and 2 GPa are obtained, respectively.

[0351] The optical properties of the materials obtained are superior to those of materials based on zirconia having a larger grain size (non-nanometric microstructures).

[0352] In terms of identical doping in yttria, the transmittance decreases when the value b* increases. According to the examples having a b* value of more than 2, transmittance is less than the maximum value indicated in Table 3 because of the presence of dyes which have been added to increase the b* value to obtain a colour which approximates the natural colour of dental enamel.

[0353] In Example INV-22_P3, good optical properties and an acceptable strength for dental applications are obtained.

[0354] Example INV-1_P3 has the best transmittance results for a composition of 3.35 mol % yttria, and a mechanical strength close to 2 Gpa.

[0355] Example INV-11_P3 has a high transmittance which is certainly unattainable for a zirconia-based material prepared according to a method which differs from that according to the invention and which is obtained by conventional sintering and with a grain size of less than 200 nm. Furthermore, this material has a mechanical strength of more than 650 MPa, allowing it to be used in non-dental applications.

[0356] 9.2/Ceramic bodies were prepared from pre-ceramic bodies, with an intermediate pre-sintering step (e1).

TABLE-US-00015 TABLE 15 Ceramic bodies prepared after an intermediate pre-sintering step (e1) P3 Dureté (f1) Vickers Rés. Disp. P1 P2 P3 Frittage Densité (GPa) méc. D2 INV-4_P1 INV-13_P2 INV-25_P3 2 h à 1150° C. 99.8 13.3 1980 D3 INV-7_PI INV-6_P2 INV-26_P3 2 h à 1150° C. 99.9 12.8 1350 P3 TFT 555 nm- RIT 555 nm- Taille de 600 nm- 600 mm- P3 grain (nm) L*, a*, b* CR OP TP 780 nm 780 nm INV-25_P3 131 — — — — — — INV-26_P3 90 92.2, −1.4, 4.2 0.55 18.2 23.1 41.9-44.7-54.7 4.9-8.5-24.1 The total duration of the heat treatment applied in step (f1), e.g., INV-25_P3 and INV-26_P3 is between 7.5 hours and 10 hours, with a plateau at 1150° C. for 2 hours.

[0357] 10/Preparing Colour Bodies or Having a Colour Gradient or a Composition Gradient

[0358] 10.1/Preparing a Body Having a Colour Gradient: Example A

[0359] Two dispersions of iron oxide nanoparticles Fe.sub.3O.sub.4 at basic pH are prepared as follows:

[0360] In a beaker, 7.9 g FeCl.sub.2*4H.sub.2O are dissolved in a solution containing 6.3 g HCl (1.5 M in water) and 36.3 g H.sub.2O. The resulting solution is introduced into a solution containing 21.4 g of FeCl.sub.3*6H.sub.2O dissolved in 875 g of water. 75 ml of ammonia (8.6 M in water) are added at ambient temperature and by stirring vigorously to allow the coprecipitation of the FeII and FeIII ions and then the formation of magnetite Fe.sub.3O.sub.4 nanoparticles. The nanoparticles obtained, having a diameter of 8 nm (TEM), are collected by means of a magnet and peptised in 200 ml of an acid solution (2M HNO.sub.3 in water). After stirring for 15 minutes, they are collected again by means of a magnet and redispersed in 500 ml of water, giving an acidic dispersion. The particles have a hydrodynamic diameter of less than 20 nm (DLS).

[0361] To disperse the nanoparticles at basic pH, the dispersion is separated into two parts: [0362] M1: the nanoparticles are collected by means of a magnet and the supernatant is removed. The nanoparticles are redispersed in an aqueous solution containing 1 g of citric acid per g of magnetite formed. After flocculating, the particles are collected by means of a magnet. The supernatant is removed. The particles are then redispersed in 100 ml of water basified with 1 ml of NH.sub.4OH (8.6 M in water). The resulting dispersion M1 contains 2.83% by weight of magnetite particles. [0363] M2: the nanoparticles are collected by means of a magnet and the supernatant is removed. The nanoparticles are dispersed in an aqueous solution containing 1.5 g of TMAOH (tetramethylammonium hydroxide) per g of magnetite. The solution is stirred for 18 hours. Then, the nanoparticles are collected by means of a magnet and dispersed in 100 ml of water. The resulting dispersion M2 contains 2.27% by weight of magnetite particles.

[0364] The two dispersions M1 and M2 are very dark brown in colour.

[0365] 400 ppm (by weight) of dispersion M1, measured in ppm of iron oxide equivalent Fe.sub.2O.sub.3, are introduced into 18 ml of the dispersion in Example INV-15_P1, the addition being performed drop by drop through stirring. Stirring was then maintained for 15 minutes. The dispersion changes to a cream colour after adding the M1 dispersion.

[0366] From the dispersion obtained, a green body 16 mm in diameter and 13 mm thick is obtained according to steps (b1) to (c1) in Example INV-15_P1. Throughout the filtration, a magnet (20 mm in diameter and 10 mm thick) is placed at a distance of 43 mm below the dispersion/filtration support interface, in a vertical filtration system in which the filtration is performed from top to bottom and the filtered solvent is discharged downwards. The magnet used is a Neodymium-Fer-Boron type magnet of quality 42 having a residual magnetic flux density (Br) of between 12900 Gauss to 13200 Gauss, a coercive field bHc of between 10.8 kOe and 12.0 kOe and an overall energy density of 40 MGOe to 42 MGOe. After step (c1), a green body of light brown colour free of cracks is obtained, then a plate 2 mm thick is cut from the green body, in the direction of filtration, by milling. Two pieces 2 mm thick are also obtained from the upper and lower parts of the green body. The 3 plates are then heat-treated with a debinding/sintering treatment according to step (f1) in Example INV-1_P3, and the colour of the ceramic bodies obtained is compared. After reducing the thickness to 1 mm and after polishing, the three pieces are the same very pale-yellow colour. A colour gradient is not formed.

[0367] 10.2/Preparing a Body Having a Colour Gradient: Example B

[0368] 400 ppm (by weight) of dispersion M1 and 400 ppm of dispersion M2 are introduced into 18 ml of dispersion in Example INV-15_P1, similar to Example A. The dispersion then has a homogeneous brown colour. The distance between the magnet and dispersion/filtration support interface is maintained at 43 mm. The green body obtained is then cut out and the pieces are heat-treated as in Example A. The three pieces have the same pale-yellow colour, which is clearly darker than in Example A. No colour gradient is formed. The transmittance is lower than in Example A.

[0369] 10.3/Preparing a Body Having a Colour Gradient: Example C

[0370] 800 ppm (by weight) of the M1 dispersion are introduced into 18 ml of the dispersion in Example INV-15_P1, similarly to Example A. The dispersion then changes to a brown colour. The distance between the magnet and the dispersion/filtration support interface is, this time, reduced to 5 mm during filtration. The green body obtained is then cut and the pieces are heat-treated as in Example A. The resulting piece of the upper part has a white colour as for a piece without the addition of dye, the resulting piece of the lower part has a yellow colour that is clearly darker than Example B. On the piece cut in the vertical direction, a colour gradient is visible in the first 5 mm of the piece from the lower part. A colour gradient thus formed over a thickness of 5 mm of the green body.

[0371] 10.4/Preparing a Body Having a Colour Gradient: Example D

[0372] 800 ppm (by weight) of dispersion M1 and 400 ppm of dispersion M2 are introduced into 18 ml of dispersion in Example INV-15_P1, similar to Example A. The changes to a homogeneous brown colour. The distance between the magnet and the dispersion/filtration support interface is maintained at 5 mm during filtration as in Example C. The green body obtained is then cut out and the pieces are heat treated as in Example A. The piece resulting from the upper pail has a very pale-yellow colour as in Example A. The piece resulting from the lower part has a yellow colour as in the lower part of Example C. On the piece cut out in the vertical direction, a colour gradient is visible throughout the piece. A colour gradient thus formed over the entire thickness of the green body.

TABLE-US-00016 TABLE 16 Characteristics of Examples A to D MI M2 Distance magnet- Formation of Example (PPrn) (PPin) filtration interface a gradient A 400 0 43 no B 400 400 43 no C 800 0 5 Yes over 5 mm of thickness D 800 500 5 Yes, over all the thickness