NANOCRYSTALLINE ZIRCONIA AND METHODS OF PROCESSING THEREOF

Abstract

Zirconia dental ceramics exhibiting opalescence and having a grain size in the range of 10 nm to 300 nm, a density of at least 99.5% of theoretical density, a visible light transmittance at or higher than 45% at 560 nm, and a strength of at least 800 MPa.

Claims

1. A method of manufacturing an opalescent zirconia dental article comprising: providing a well-dispersed suspension of zirconia nanoparticles having an average particle size of less than 20 nm; forming the suspension into a shape of the dental article or a blank to produce a wet zirconia green body; drying the wet green body in a controlled humidity atmosphere to produce a zirconia green body; heating the zirconia green body to provide a zirconia brown body, wherein the zirconia green body is shaped before heating, or the zirconia brown body is shaped after heating; sintering the zirconia brown body at a temperature below or equal to 1200 C. to provide an opalescent zirconia sintered body; wherein a resulting grain size of the sintered dental article is between 10 and 300 nm and an average grain size is between 40 nm and 150 nm.

2. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the heating step comprises heating up the zirconia green body at a temperature in the range of from 500 to 700 C. to remove any organic residuals to form a zirconia brown body.

3. The method of manufacturing an opalescent zirconia dental article of claim 1, further comprising pre-sintering the brown body at a temperature up to 850 C. prior to sintering.

4. The method of manufacturing an opalescent zirconia dental article of claim 3, wherein the pre-sintering step and the heating step can be combined into one step.

5. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the step of forming the suspension into a shape comprises an isotropically enlarged, uniform shape.

6. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the dried green body or brown body is shaped by CAD/CAM, LPIM or dental heat-pressing.

7. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the zirconia nanoparticles have an average particle size less than 15 nm.

8. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the well-dispersed suspension of zirconia nanoparticles comprises a solids volume percent of particles in the range of 10 to 50 vol. %.

9. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the well-dispersed suspension further comprises a dispersant in an amount of not more than 10 wt. % of total solids in the suspension.

10. The method of manufacturing an opalescent zirconia dental article of claim 9, wherein the dispersant comprises poly(ethyleneimine), 2-[2-(2-methoxyethoxy)ethoxy] acetic acid, or 2-(2-methoxyethoxy)acetic acid.

11. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the well-dispersed suspension is further de-agglomerated by attrition milling.

12. The method of manufacturing an opalescent zirconia dental article of claim 11, wherein the suspension is further refined by centrifuging instead of, prior to, or after attrition milling.

13. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein sintering is conducted in conventional dental furnaces, high temperature furnaces, microwave dental furnaces or hybrid furnaces.

14. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the sintering temperature is below or equal to 1150 C.

15. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the sintering temperature is below or equal to 1125 C.

16. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein forming the suspension into blanks or the dental article comprises centrifugal casting, drop-casting, gel-casting, injection molding, slip casting, filter-pressing and/or electrophoretic deposition (EPD).

17. The method of manufacturing an opalescent zirconia dental article of claim 1, wherein the well-dispersed suspensions comprises a liquid medium selected from the group consisting of water, ethanol, methanol, toluene, dimethylformamide, or mixtures thereof.

18. A method of manufacturing an opalescent zirconia dental article comprising: providing a well-dispersed suspension of zirconia nanoparticles having an average particle size of less than 20 nm; forming the suspension into a shape of the dental article or a blank to produce a wet zirconia green body; drying the wet green body in a controlled humidity atmosphere to produce a zirconia green body; heating the zirconia green body to provide a zirconia brown body, wherein the zirconia green body is shaped before heating, or the zirconia brown body is shaped after heating; sintering the zirconia brown body at a temperature below or equal to 1200 C. to provide an opalescent zirconia sintered body; wherein the majority of the pores are greater than 25 nm at a density of at least 99.5% theoretical density.

19. The method of manufacturing a zirconia dental article of claim 18, wherein the majority of the pores are greater than 30 nm at a density of at least 99.5% theoretical density.

20. A suspension for forming a zirconia dental article comprising: well-dispersed zirconia nanoparticles having an average particle size of less than 20 nm; a solids volume percent of particles in the range of 10 to 50 vol. %; wherein the resulting grain size of the of the zirconia dental article is between 10 and 300 nm and an average grain size is between 40 nm and 150 nm; and wherein the zirconia dental article is opalescent.

21. The suspension for forming a zirconia dental article of claim 20, wherein the solids volume percent of particles is at least 14 vol %.

22. The suspension for forming a zirconia dental article of claim 20, wherein the solids volume percent of particles is at least 16 vol %.

23. The suspension for forming a zirconia dental article of claim 20, wherein the solids volume percent of particles is at least 18 vol %.

24. The suspension for forming a zirconia dental article of claim 20, having a viscosity of less than 100 cP at 25 C.

25. The suspension for forming a zirconia dental article of claim 24, having a viscosity of less than 30 cP at 25 C.

26. The suspension for forming a zirconia dental article of claim 25, having a viscosity of less than 15 cP at 25 C.

27. The suspension for forming a zirconia dental article of claim 23, wherein the well-dispersed suspension is further de-agglomerated by attrition milling.

28. A green body for forming a zirconia dental article comprising: zirconia nanoparticles having an average particle size of less than 20 nm; wherein the resulting grain size of the of the zirconia dental article is between 10 and 300 nm and average grain size is between 40 nm and 150 nm; and wherein the zirconia dental article is opalescent.

29. The green body of claim 28, wherein the green body comprises a transmittance of 58% for a 2 mm thickness at 560 nm.

30. A method of manufacturing an opalescent zirconia dental article comprising providing a zirconia green blank having zirconia nanoparticles having an average particle size of less than 20 nm; shaping the zirconia green blank by CAD/CAM, LPIM, or dental heat-pressing, or heating the zirconia green blank to form a brown blank and shaping the brown blank by CAD/CAM machining; sintering the shaped zirconia green blank or brown blank at a temperature below or equal to 1200 C. to provide an opalescent zirconia sintered body; wherein the resulting grain size of the sintered dental article is between 10 and 300 nm and average grain size is between 40 nm and 150 nm.

31. The method of manufacturing an opalescent zirconia dental article of claim 30, wherein the step of heating the zirconia green blank to form a brown blank comprises pre-sintering.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Embodiments of the present invention will be more fully understood and appreciated by the following Detailed Description in conjunction with the accompanying drawings, in which:

[0060] FIG. 1 shows the spectral (wavelength) dependence of light transmittance within visible light range of 400-700 nm for a variety of dental materials including the current state of the art commercial translucent zirconia brands fabricated from Zpex and Zpex Smile powders made by Tosoh (Japan).

[0061] FIG. 2 shows transition of tetragonal nanozirconia material of this invention from nearly transparent green to translucent fully dense stage.

[0062] FIGS. 3A and 3B compare light transmittance and opalescence of the nanozirconia materials of the present invention in green, brown and fully dense condition to commercial dental zirconia materials in a fully dense condition.

[0063] FIG. 4 shows a generic flowchart of the processing method of the present invention.

[0064] FIG. 5 shows a flowchart of an embodiment of the process in accordance with the present invention.

[0065] FIG. 6 shows a veneer made from fully dense nanozirconia of the present invention exhibiting clearly visible opalescence.

[0066] FIG. 7 shows a microstructure of 99.9% dense opalescent nanozirconia body with average grain size of 136 nm sintered in a conventional dental furnace in accordance with the present invention as described in Example 1A.

[0067] FIG. 8 shows a microstructure of 99.9% dense opalescent nanozirconia body with average grain size of 112 nm sintered in a conventional dental furnace in accordance with the present invention as described in Example 1C.

[0068] FIG. 9 shows a microstructure of 99.9% dense opalescent nanozirconia body with average grain size of 108 nm sintered in conventional dental furnace in accordance with the present invention as described in Example 2A with a pore of at least 35 nm marked in the SEM micrograph.

[0069] FIG. 10 shows a microstructure of 99.9% dense opalescent nanozirconia body with average grain size of 91 nm sintered in a hybrid microwave furnace in accordance with the present invention as described in Example 4B.

[0070] FIGS. 11A, 11B and 11C show fracture surfaces of some of nanozirconia materials of the present invention at various magnifications illustrating typical grain size range and occasional nano-pores with sizes ranging from 30 nm to 100 nm.

[0071] FIG. 12 shows the transition from transparent to opaque nanozirconia bodies made from organic solvent based suspension of ZrO.sub.2 nanoparticles without Y2O3 or any other tetragonal phase stabilizer.

[0072] FIG. 13A shows particle size distribution of nanozirconia suspension concentrated to 17 vol % from 4.5 vol % suspension prior to (1) and after attrition-milling (2).

[0073] FIG. 13B shows particle size of as-received 17 vol % nano-zirconia suspension prior to (1) and after attrition-milling (2).

DETAILED DESCRIPTION

[0074] It was surprisingly found that within a certain range of processing conditions and starting particle sizes the resulting nanozirconia bodies are opalescent in green, brown (or pre-sintered) and, most importantly, in fully dense condition. Opalescent nanozirconia bodies can be also nearly transparent or highly translucent in all stages of the processing and result in fully dense bodies (at 99.5% dense) that in addition to high light transmittance also comprise high strength (800 MPa and even in excess of 2 GPa) and sinterable at temperatures below 1200 C. in conventional dental furnaces which is especially important for dental restorative applications. The materials of the present invention are especially useful for full contour restorations combining strength of zirconia with aesthetics of glass-ceramics benchmarks. Dental restorations comprising opalescent nanozirconia can be shaped by machining/milling, injection molding, dental heat-pressing, electrophoretic deposition, gel-casting and other dental technologies or technologies used in industry at large for shaping high-performance ceramics. Specifically, CAD/CAM blanks can be formed by slip-casting (coarser nanoparticulates only), centrifugal casting, drop-casting, injection molding, filter-pressing and electrophoretic deposition (EPD).

[0075] It is specific pore size distribution and/or grain size distribution that are believed to render predominantly single phase tetragonal zirconia of this invention both highly translucent and opalescent. We can speculate that in order to generate opalescence in a fully dense nanozirconia, at least a portion, preferably a major portion of scattering species (e.g. tetragonal grains with anisotropic refractive index and occasional nano-pores) form some kind of optical sub-lattice and have a characteristic size or diameter within a specific, fairly narrow range. Within this range the scattering species are large enough to cause adequate scattering of blue light yet small enough to not cause much scattering of yellow-red light, which can be explained by the Rayleigh scattering model. Rayleigh approximation is generally applicable to scattering species much less than wavelength of light or specifically for birefringence effects when tetragonal grain size is at least an order of magnitude less than wavelength of visible light. Mie model is not restricted by grain size. Both models coincide when the grain size is less than 50 nm. Maximized opalescence will be achieved when present scattering species are about or just below the sizes transitional between the Rayleigh and the Mie models (where they start to diverge). It can be further speculated that once their size exceeds the transitional range, the opalescence effect will largely disappear as the less wavelength-dependent Mie scattering mechanism is operational. This upper size limit for opalescence is dictated by differences in refractive index between the pores and the tetragonal zirconia matrix and/or between different crystallographic orientations in a crystal lattice of individual nanozirconia crystallites. In addition, another critical factor that imposes an upper limit on the size of scattering species (mostly grains since residual porosity is minimal) is high translucence required for aesthetic dental ceramics. Also shading of nanozirconia invariably further lowers overall visible light transmittance imposing further constraints on grain size distribution to achieve the same light transmittance. Typically light transmittance of shaded zirconia is about 5-10% lower than light transmittance of unshaded or naturally colored zirconia.

[0076] Opalescence and other physical properties of the materials of the present invention can be quantified within the following ranges:

TABLE-US-00001 Property Broad Range Preferred Range Phase composition and Predominantly tetragonal YTZP (yttria-stabilized chemistry zirconia with less than 15% tetragonal zirconia monoclinic and cubic phase polycrystal) with 0-3 mol % combined. Y.sub.2O.sub.3 Opalescence Visually opalescent with OP values preferably above OP values above 9 12 Nearly transparent or Light transmittance higher Preferably light highly translucent in than 45% at wavelength of transmittance higher than shaded or unshaded 560 nm or even in the whole 50% at wavelength of 560 (natural) condition spectral range of 560 nm to nm or even in the whole 700 nm for unshaded or spectral range of 560 nm to naturally colored 700 nm for unshaded or nanozirconia; and higher naturally colored than 35% at 560 nm or nanozirconia; and higher even in the whole spectral than 40% at 560 nm or range of 560 nm to 700 nm even in the whole spectral for shaded nanozirconia range of 560 nm to 700 nm intentionally doped with for shaded nanozirconia coloring ions (to match intentionally doped with internal or external shade coloring ions (to match standards approximating internal or external shade tooth colors) standards approximating tooth colors). Overall grain size range At least 95% of grains by All grains are from 10 nm in fully sintered volume are from 10 nm to to 300 nm in size (or condition 300 nm in size (or diameter), diameter) or 20 nm to 250 nm in size (diameter) Average grain size From 40 nm to 150 nm, Preferably from 50 to 100 measured according to nm, and most preferably ASTM E112 (or EN from 50 to 80 nm. 623-3) test method Density/residual porosity Pore size mostly larger than Most preferably that in fully sintered 30 nm wherein density is porosity is less than 0.1% condition higher than 99.5%. (density 99.9% of theoretical density) Flexural strength ISO 6872 flexural strength Preferably 1200 MPa at least 800 MPa or higher flexural strength; and most preferably 2 GPa flexural strength Sinterable at Sinterable at temperatures <1200 Sinterable at temperatures 1150 temperatures <1200 C. C. using conventional dental furnaces C. using conventional dental furnaces without application of or microwave dental furnaces or microwave dental furnaces external pressure (pressureless sintering) Shaped by CAD/CAM, Preferred way is machining of partially sintered blanks EPD, LPIM, dental heat- formed by slip-casting (limited use - for coarser pressing (like glass nanoparticulates only), centrifugal casting, drop-casting, ceramic ingots) similar to gel-casting, injection molding, filter-pressing and LPIM and gel-casting electrophoretic deposition (EPD) using RP molds

[0077] To further illustrate the advantageous properties listed in the table above, FIGS. 3A and 3B compare light transmittance and opalescence of the nanozirconia materials of the present invention to commercial dental zirconia materials. In one preferred embodiment, the process schematically shown in FIG. 4 will result in green or pre-sintered (brown) millable blanks that can be further processed into dental articles such as dental restorations (blanks, full-contour FPDs (fixed partial dentures), bridges, implant bridges, multi-unit frameworks, abutments, crowns, partial crowns, veneers, inlays, onlays, orthodontic retainers, space maintainers, tooth replacement appliances, splints, dentures, posts, teeth, jackets, facings, facets, implants, cylinders, and connectors) using commercially available dental CAD/CAM systems. In the alternative embodiments, dental articles can be formed directly from suspension by EPD, gel-casting in the enlarged molds formed by rapid-prototyping (RP). In another alternative embodiment, nanoparticulates of the present invention can be provided as feed-stock for injection molding. In the latter case the enlarged molds for low-pressure injection molding (LPIM) can be formed by RP. RP is useful to form molds that are enlarged to compensate for isotropic sintering shrinkage of the materials of the present invention when they are sintered from green or pre-sintered state to a full density.

[0078] It is important to note that highly translucent tetragonal nanozirconia bodies were produced from two types of nanozirconia suspensions spanning the wide range of processing scenarios as shown in the flow chart in FIG. 4. Organic based Pixelligent (Pixeligent Technologies, Baltimore, Md.) nanozirconia suspensions (0% Y.sub.2O.sub.3) with solid loading of 14 vol % and aqueous based MEL (MEL Chemicals, Flemington, N.J.) suspension of 3Y-TZP (3 mole % Y.sub.2O.sub.3) with solid loading of 5 vol %.

EXAMPLES

[0079] The non-limiting examples illustrating some of the embodiments and features of the present invention are further elucidated in FIGS. 6-13. Commercially available nanozirconia suspensions were received from various manufacturers. The most useful suspensions preferably comprise well-dispersed nanoparticles with average primary particle size of 20 nm and preferably 15 nm. In certain cases nanosuspensions comprising partially agglomerated and/or associated nanoparticles can be also used with average particulate size up to 40-80 nm. The latter will require attrition milling to deagglomerate and commune nanoparticles to the required size range. The starting zirconia concentration is usually low, e.g. 5 vol %, but concentrated suspensions are also available from some manufacturers (see FIG. 13B). These concentrated suspensions may contain proprietary dispersants. The liquid medium of the suspension is preferably water, and can also be organic solvents, e.g. ethanol, methanol, toluene, dimethylformamide, etc. or mixtures of such. The suspension was stabilized by addition of dispersants and adjustment of pH. A dispersant used to stabilize nanosuspensions in the examples below was one of the following: Poly(ethyleneimine), 2-[2-(2-Methoxyethoxy)ethoxy] acetic acid, or 2-(2-Methoxyethoxy)acetic acid. The amount of dispersants by weight of solid zirconia was no more than 10% (e.g., from 0.5 wt % up to 10 wt %). The pH values of suspension were in the range of 2 to 13. Centrifuging and/or attrition milling may be applied to remove and/or break the agglomerated/aggregated portion of solids prior to or after stabilizing the suspensions. In some cases, binders may be added to the suspension in order to enhance the strength of the cast. The suspensions were then concentrated by evaporating off the solvents at elevated temperature with or without vacuum assistance. After concentration, the suspension will be above 10 vol %, e.g. preferably at least 14 vol %, preferably 16%, most preferably 18 vol %, and up to 50 vol % depending on requirements of forming methods. After concentration, the viscosity (measured at 25 C.) of concentrated suspensions prior to casting was well below 100 cP and in most cases below 30 cP, most preferably viscosity should be at or below 15 cP as this level of viscosity produced best casting results. Attrition milling may also be used during or after the concentrating process primarily to break down agglomerates and aggregates and sometimes to reduce particle size.

[0080] The concentrated zirconia suspensions with desired solid loadings were then used to cast zirconia green bodies. The forming methods include: slip-casting, gel-casting, electrophoretic deposition, drop-casting, filter pressing, injection molding, and centrifugal casting as well as other known applicable forming methods. After casting, the green bodies were dried in a temperature, pressure, and humidity controlled environment to ensure forming crack-free articles. The drying conditions are usually dictated by the dimensions of the articles: e.g. thicker articles require longer drying time to prevent cracking. After drying, green bodies were at least 35%, preferably 45%, more preferably over 50% of theoretical density. Dried green bodies were burnt out to remove the organic species including dispersants, binders, and any other additives. The peak burn-out temperature was no higher than 700 C., preferably from 500 C. to 600 C. Optional pre-sintering can be carried out at temperatures up to 850 C. After burn out, the articles, so-called brown bodies, were then sintered at temperatures lower than 1200 C. to reach full density. Sintering can be carried out in dental furnaces, traditional high temperature furnaces, or hybrid microwave furnaces. Density of the sintered articles was measured by the Archimedes method using water as the immersion medium. Relative density, calculated using a theoretical density value of 6.08 g/cm.sup.3, is usually 99.5% in fully sintered articles in the current invention.

[0081] The fully sintered samples were then ground to 1.0 mm for optical property measurement. Transmittance and reflectance were measured by a Konica Minolta Spectrophotometer CM-3610d, according to the CIELAB color scale in the reflectance and transmittance mode relative to the standard illuminant D65. The aperture diameter was 11 mm for reflectance measurement, and 20 mm for transmittance measurement. Measurements were repeated five times for each specimen and the values were averaged to get the final reading. The transmittance of green bodies through 1 mm thickness was at least 50% at 560 nm, and was at least 45% for the brown bodies.

[0082] Opalescence parameter was calculated as:

OP=[(CIEa.sub.T*CIEa.sub.R*).sup.2+(CIEb.sub.T*CIEb.sub.R*).sup.2].sup.1/2,

whereas (CIEa.sub.T*CIEa.sub.R*) is the difference between transmission and reflectance modes in red-green coordinate, a* of CIE L*a*b* color space; (CIEb.sub.T*CIEb.sub.R*) is the difference between transmission and reflectance modes in yellow-blue color coordinate, b* of CIE L*a*b* color space.

[0083] The biaxial flexural strength measurements were performed by an MTS Q Test machine on disk samples with a thickness of 1.20.2 mm according to ISO6872-2008. Sintered samples were also polished, thermally etched and imaged under Zeiss Sigma Field Emission scanning electron microscope (SEM). Average grain size was calculated by the intercept method according to ASTM E112-12.

Example 1

[0084] 2 kg of 5 vol % aqueous suspension of yttria (3 mol %) stabilized zirconia nanoparticulate was received from Mel Chemicals (Flemington, N.J.). This suspension was de-agglomerated by centrifuging at 7000 rpm for 40 minutes. The suspension was then stabilized by adding 2% dispersants by weight of solid zirconia. The pH of such stabilized suspension was 2.5. This suspension was concentrated from 5 vol % to 18 vol % of solid loading with an Ika RV10 vacuum evaporator at 40 C. and 40 mbar for about 4 hours. Cylindrical PTFE molds of from 18 mm to 32 mm in diameter and 10 mm in height were prepared, and the zirconia suspension was poured into the molds. 5 to 15 g of slurry was applied to each mold depending on the desired final thickness. Then molds with suspension were put into an environmental chamber for curing and drying. For the first 72120 hours, the humidity was above 85% and temperature was about 25 C. The drying time was determined by the thickness of the samples. The thicker samples took a longer time to dry without generating cracks. Then environmental humidity decreased gradually to about 20%, where final water content in the green bodies reached less than 4 wt %. The as-formed green bodies were 49% of theoretical density. Transmittance was 58% for 2 mm thick green body at 560 nm. Dried green bodies were burned out by heating at a rate of 0.5 C./min to 550 C. and holding for 2 hours. The brown bodies, of 1.8 mm thick, had transmittance of 49% at 560 nm. The brown bodies were then sintered in a dental furnace (Programat P500, Ivoclar Vivadent AG.) at a ramp rate of 10 C./min to 1150 C., held for 2 hours, and then cooled naturally in air. After sintering, the disk samples were from 12 to 23 mm in diameter and 1.5 mm in thickness, with relative density of 99.98%. Probably due to contamination by Fe, Ni or Cr from the stainless steel equipment used in manufacturing of the starting nanozirconia suspensions, all fully sintered samples in Example 1 to Example 6 appeared tinted, i.e., noticeably yellow-brownish in color with a hue that resembles the natural tooth color.

[0085] The samples were then ground down to thickness of 1.0 mm for transmittance and reflectance measurements. The transmittance of such tinted samples was 37.7%, and opalescence factor was 13.6. An SEM image of a polished and thermally etched cross-section is shown in FIG. 7, and the average grain size is 136 nm. The biaxial flexural strength is 2108386 MPa.

[0086] In the following parallel experiments, all processing conditions remained identical, except that the binder burn out and/or sintering conditions were modified.

[0087] For Example 1B, sintering was carried out at 1125 C. for 2 hours.

[0088] In example 1C to 1F, a 2-step sintering method was adapted, by heating the samples to a higher temperature (e.g. 1125 C., 1150 C.) for very short time (e.g. 6 seconds), and then quickly dropping to lower temperature (e.g. 1075 C., 1050 C.) and holding for a prolonged period of time.

[0089] In Example 1C, the sample was heated from room temperature to 1125 C. at 10 C./min rate and held at 1125 C. for 6 seconds; then it was cooled down to 1075 C. quickly and held at 1075 C. for 20 hours. An SEM image of a polished and thermally etched cross-section is shown in FIG. 8, and the average grain size is 112 nm. Biaxial flexural strength is 1983356 MPa.

[0090] In example 1D, the sample was heated from room temperature to 1150 C. at 10 C./min rate and held at 1150 C. for 6 seconds; then it was cooled down to 1075 C. quickly and held at 1075 C. for 20 hours. Biaxial flexural strength is 2087454 MPa.

[0091] In example 1E, the sample was heated from room temperature to 1125 C. at 10 C./min rate and held at 1125 C. for 6 seconds; then it was cooled down to 1075 C. quickly and held at 1075 C. for 15 hours.

[0092] In example 1F, the sample was heated from room temperature to 1125 C. at 10 C./min rate and held at 1125 C. for 10 seconds; then it was cooled down to 1075 C. quickly and held at 1075 C. for 20 hours.

[0093] In another parallel experiment, the binder burn-out conditions were altered. Example 1G was processed at all identical conditions as Example 1C, except the peak burn out temperature was raised from 550 C. to 700 C.

[0094] Results on density, biaxial flexural strength, grain size, light transmittance, and opalescence measurements are summarized in Table 1 below.

TABLE-US-00002 TABLE 1 Biaxial Solid Flexural Average Light Loading Relative Strength Grain size Transmission Opalescence Example Dispersant (vol %) Sintering Density % (MPa) (nm) @560 nm Color Factor 1A 2% 18 1150/2 h 99.98 2108 386 136 38 yellow- 14 brownish, tooth like hue 1B 2% 18 1125/2 h 99.96 114 38 yellow- 14 brownish, tooth like hue 1C 2% 18 1125/6 s- 99.95 1983 356 112 40 yellow- 15 1075/20 h brownish, tooth like hue 1D 2% 18 1150/6 s- 99.90 2087 454 39 yellow- 1075/20 h brownish, tooth like hue 1E 2% 18 1125/6 s- 99.91 39 yellow- 14 1075/15 h brownish, tooth like hue 1F 2% 18 1125/10 s- 99.92 38 yellow- 15 1075/20 h brownish, tooth like hue 1G 2% 18 1125/6 s- 99.92 39 yellow- 13 1075/20 h brownish, tooth like hue 2A 2% 18 1100/4 h 99.94 108 yellow- brownish, tooth like hue 2B 2% 18 1125/2 h 99.94 38 yellow- brownish, tooth like hue 2C 2% 18 1100/3 h 99.96 39 yellow- 14 brownish, tooth like hue 2D (2 + 3)% 18 1125/2 h 99.90 yellow- brownish, tooth like hue 2E 4% 18 1125/2 h 99.92 119 yellow- brownish, tooth like hue 3A 2% 14 1150/2 h 99.92 131 37 yellow- brownish, tooth like hue 3B 2% 14 1125/6 s- 99.91 107 39 yellow- 1075/20 h brownish, tooth like hue 4A 2% 18 1125C/2 h 99.86 yellow- brownish, tooth like hue 4B 2% 18 1125/6 s- 99.92 91 yellow- 1075/20 h brownish, tooth like hue 5 2% 18 1150/2 h 99.50 yellow- brownish, tooth like hue 6 2% 18 1150/2 h 99.90 yellow- brownish, tooth like hue

Example 2

[0095] The suspension preparation and concentration steps were identical to Example 1A. After concentration and prior to casting, an addition step, attrition milling, was carried out using Netzsch MiniCer attrition mill. The concentrated suspension was milled with 200, 100, or 50 m of yttria stabilized zirconia beads at 3000 rpm rotation speed. After attrition milling, the suspension was cast into PTFE molds, dried, and burned out in the same procedures as in Example 1A.

[0096] For Example 2A, the attrition milling time was 1 hours, and the brown bodies were sintered at 1100 C. for 4 hours.

[0097] For Example 2B, the attrition milling time was 1.5 hours, and the brown bodies were sintered at 1125 C. for 2 hours.

[0098] For Example 2C, the attrition milling time was 1.5 hours, and the brown bodies were sintered at 1100 C. for 3 hours.

[0099] For Example 2D, after original attrition milling for 1.5 hours at 3000 rpm in the attrition mill, an additional 3 wt % (according to the weight of zirconia) of additives was added to the suspension. Attrition milling continued another 1 hour. The suspension was cast into molds, dried, and burned out in same procedures as in Example 1A. The sample was then sintered at 1125 C. for 2 hours.

[0100] For Example 2E, the suspension and preparation steps were identical to Example 1A except that 4 wt % of dispersant was used. After concentration, attrition milling was performed for 3 hours. The samples were sintered at 1125 C. for 2 hours.

[0101] Density, optical properties, and grain size were measured and reported in Table 1. SEM image of Example 2A is shown in FIG. 9, where a 35 nm diameter pore was observed. All samples are visually opalescent.

Example 3

[0102] In the stabilization step, a different dispersant of 2 wt % was used in comparison to Example 1A, and the suspension was concentrated to 14 vol %. After concentration, the suspension was cast into the molds. Drying and burning out were carried out at identical procedures as Example 1A.

[0103] For Example 3A, the sample was heated to 1150 C. at 10 C./min and held for 2 hours.

[0104] For Example 3B, the sample was heated to 1125 C. with 10 C./min rate and held at 1125 C. for 10 seconds; then it was cooled down to 1075 C. quickly and held at 1075 C. for 20 hours.

[0105] Density, optical properties, and grain size were measured and reported in Table 1. All samples were visually opalescent.

Example 4

[0106] The suspension stabilization, concentration, and processing conditions are identical as Example 1A except that the brown bodies were sintered in a microwave assisted high temperature furnace, MRF 16/22, Carbolite, Hope Valley, UK.

[0107] In Example 4A, the sample was heated at 10 C./min to 1125 C. in IR sensor controlled mode, with microwave on after 700 C. in auto mode. Then the sample dwelled at 1125 C. under 500 W microwave for 2 hours. The sample was cooled down naturally.

[0108] In Example 4B, the sample was heated at 10 C./min to 1125 C. in IR sensor controlled mode for 6s, and then held at 1075 C. for 20 h. During heating, the microwave started at 700 C. in auto mode, and during dwelling the microwave was manually set at 200 W.

[0109] Density and grain size were measured and reported in Table 1. FIG. 10 shows the microstructure of Example 4B with average grain size of 91 nm and density of 99.92%. All such sintered samples are visually opalescent.

Example 5

[0110] 500 g of 5 vol % aqueous suspension of 3 mol % yttria stabilized zirconia nanoparticulate was received from Mel Chemicals (Flemington, N.J.). This suspension was stabilized by addition of 3 wt % dispersants by weight of solid zirconia. The stabilized suspension was concentrated from 5 vol % to 18 vol % in a glass beaker by heating while stirring at 50 C. for 14 hours in a water bath with a hot plate. Slip casting was carried out using plaster molds, prepared by casting cylinders of 32 mm in diameter, and 30 mm in height with USG No. 1 Pottery Plaster. The cylinders were wrapped with plastic paper for holding the slurries before consolidation. 5 to 15 g of concentrated slurry was poured into each mold depending on the desired final thickness. After the slurry was consolidated, the plastic paper was removed, and the consolidated parts were removed from the plaster and put into a drying box for curing and drying under controlled humidity (identical to Example 1A). After drying, the green bodies were burned out at a rate of 0.5 C./min to 700 C. and held for 2 hours. Brown bodies were sintered in a dental furnace (Programat P500, Ivoclar Vivadent AG.) by heating at a rate of 10 C./min to 1150 C. and held for 2 hours.

[0111] The relative density of the so-formed articles was measured to be 99.50%. All such formed articles were visually opalescent.

Example 6

[0112] The suspension was stabilized, concentrated and de-agglomerated in the identical steps as illustrated in Example 1A. 40 ml suspension was then transferred to a PTFE centrifuge vessel and centrifuged at 11000 rpm for 40 min by Legend XT Centrifuge, ThermoScientific. Afterwards, the supernatant was carefully removed by pipetting. The dense bottom part stayed in the PTFE vessel and was subjected to drying for 15 days. After the part was dried completely, it was removed from the mold and burned out at 700 C. for 2 hours. The so-formed brown body was ground into a realistically shaped veneer with an enlargement factor of 1.25 and sintered. Sintering was carried out in Programat P500 dental furnace at 1150 C. for 2 hours, and the density was measured to be 99.90%. The so-formed veneer was polished to a glossy finish with thickness between 0.3-1.5 mm. It appears opalescent as shown in FIG. 6.

Example 7

[0113] An organic solvent based nanozirconia suspension (0% Y.sub.2O.sub.3) was received from Pixelligent Technologies (Baltimore, Md.). The concentration of as-received suspension was 14.0 vol % with an average particle size of 5 to 8 nm in a toluene solution. This suspension was concentrated by slowly evaporating the solvent under ambient conditions in a PTFE tube. After the part was completely dried, it was then removed from the tube and subjected to burn out at 550 C. for 2 hours. Both green and brown bodies were transparent. Sintering was carried out at temperatures from 900 C. to 1100 C. for 1 hour. The phase and grain size was measured and calculated by grazing incidence X-ray diffraction and SEM, and the results are listed in Table 2. Some opalescence can only be observed in samples sintered at 1000 C. and 1050 C. There is no tint observed for any of the sintered bodies; they appeared basically colorless. The highest density for sintered bodies was 98.3%, and all samples showed severe cracking after heat treatment. Results on visual appearance, density, grain size and phase composition are listed in Table 2 below.

TABLE-US-00003 TABLE 2 Sintering temp C. 900 950 1000 1050 1100 Appearance (see FIG. 12) Translucent Translucent Window Window with some with some Transparent Transparent opalescence opalescence Opaque Density (%) n/a 98.3 0.2 97.8 0.2 95.5 0.1 NA Grain size na na 35 40 90 estimated from SEM (nm) Grain Size 7 13 18 22 18 from XRD (nm) Phases Tetragonal phase Monoclinic phase > 90

[0114] Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the claims which follow.