U.S. patent application number 16/798550 was filed with the patent office on 2020-07-16 for nanocrystalline zirconia and methods of processing thereof.
The applicant listed for this patent is Ivoclar Vivadent, Inc.. Invention is credited to Dmitri Brodkin, Ajmal Khan, Ling Tang, Anna B. Verano, Yijun Wang.
Application Number | 20200222287 16/798550 |
Document ID | / |
Family ID | 51177155 |
Filed Date | 2020-07-16 |
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United States Patent
Application |
20200222287 |
Kind Code |
A1 |
Brodkin; Dmitri ; et
al. |
July 16, 2020 |
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.
Inventors: |
Brodkin; Dmitri;
(Livingston, NJ) ; Wang; Yijun; (Basking Ridge,
NJ) ; Tang; Ling; (Berkeley Heights, NJ) ;
Khan; Ajmal; (Princeton, NJ) ; Verano; Anna B.;
(Hillsborough, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ivoclar Vivadent, Inc. |
Amherst |
NY |
US |
|
|
Family ID: |
51177155 |
Appl. No.: |
16/798550 |
Filed: |
February 24, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15866909 |
Jan 10, 2018 |
10610460 |
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16798550 |
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14891756 |
Nov 17, 2015 |
10004668 |
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PCT/US2014/042140 |
Jun 12, 2014 |
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15866909 |
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61840055 |
Jun 27, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61C 13/083 20130101;
A61K 6/818 20200101; A61K 6/17 20200101; A61C 8/0012 20130101; A61C
13/00 20130101; A61C 5/70 20170201; A61K 6/822 20200101; A61C 7/08
20130101 |
International
Class: |
A61K 6/818 20060101
A61K006/818; A61K 6/17 20060101 A61K006/17; A61K 6/822 20060101
A61K006/822; A61C 13/083 20060101 A61C013/083; A61C 7/08 20060101
A61C007/08; A61C 8/00 20060101 A61C008/00; A61C 13/00 20060101
A61C013/00; A61C 5/70 20060101 A61C005/70 |
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.degree. 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.degree. 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.degree. 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.degree. C.
15. The method of manufacturing an opalescent zirconia dental
article of claim 1, wherein the sintering temperature is below or
equal to 1125.degree. 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.degree. 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.degree. C.
25. The suspension for forming a zirconia dental article of claim
24, having a viscosity of less than 30 cP at 25.degree. C.
26. The suspension for forming a zirconia dental article of claim
25, having a viscosity of less than 15 cP at 25.degree. 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.degree. 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
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to and is a divisional
application of U.S. application Ser. No. 15/866,909, filed on Jan.
10, 2018, which claims priority to and is a divisional application
of U.S. application Ser. No. 14/891,756, filed on Nov. 17, 2015,
now U.S. Pat. No. 10,004,668, which is the National Stage
application of International Patent Application No.
PCT/US2014/042140 filed on Jun. 12, 2014, which claims priority to
U.S. Application No. 61/840,055, filed Jun. 27, 2013, entitled
Nanocrystalline Zirconia And Methods Of Processing Thereof, all the
disclosures of which are incorporated herein by reference in their
entirety.
TECHNICAL FIELD
[0002] The present invention is directed to dental restorations
comprising nanozirconia and methods of processing thereof, and more
particularly to nanozirconia dental ceramics combining translucency
that matches glass-ceramics, opalescence mimicking natural
dentition and high strength characteristic of tetragonal
zirconia.
BACKGROUND
[0003] Currently, the best commercially available full contour
(monolithic) zirconia dental ceramic materials are aesthetically
inferior to lithium disilicate or leucite-based glass ceramic
materials like IPS e.max or IPS Empress due to lower translucency
and lack of opalescence. Better light transmittance and opalescence
are required to better mimic natural dentition. Human enamel has
varying "anisotropic" translucency which introduces many optical
effects that are difficult to replicate with ceramic material.
Opalescence is one optical characteristic of natural enamel that
can create a highly complex visual display. To date, only glass
ceramic materials come close to duplicating such optical complexity
of natural dentition including opalescence. At the same time
glass-ceramic materials are not as strong as zirconia materials
hence limiting their clinical use to single- and multi-unit
restorations and cases without bruxism.
[0004] U.S. Pat. No. 8,309,015, which is hereby incorporated by
reference in its entirety, is directed to a method of processing
tetragonal nanozirconia with grain sizes under 100 nm. The sintered
body is claimed to only contain pores smaller than about 25 nm. The
method is lacking bulk shape consolidation technology and does not
address, mention or discuss opalescence. Rather, the requirements
set forth in the patent and claims include the diameter of any
pores which are present in the translucent zirconia sintered body
to be not more than about 25 nm, which as believed, would preclude
this material from being in the desired opalescent range as taught
in the present invention and also is unrealistic for any practical
bulk shape consolidation technology yielding dental articles via
pressureless sintering.
[0005] U.S. Pat. No. 8,598,058, which is hereby incorporated by
reference in its entirety, is directed to a method of processing
nanozirconia articles with grain sizes under 200 nm and pore size
under 50 nm comprising from about 0.5% to about 5.0% lanthanum
oxide claimed to be essential to achieve the claimed properties.
Again this patent does not address, mention or discuss opalescence
despite showing sintered bodies illuminated with incident light
whereby opalescence would be obvious if present.
[0006] U.S. Pat. Nos. 7,655,586 and 7,806,694, both hereby
incorporated by reference in their entirety, are directed to a
dental article and fabrication methods comprising: a single
component yttria-stabilized tetragonal zirconia ceramic material
having grains of average grain size exceeding 100 nanometers and
not exceeding about 400 nanometers, wherein the ceramic material is
fabricated of particulate material consisting essentially of
ceramic crystallites with an average size of less than about 20 nm;
wherein the particulate material is sintered without application of
external pressure at a temperature less than about 1300.degree. C.
to a full density wherein the final pore size does not exceed the
size of the ceramic crystallite size; and wherein the ceramic
material exhibits at least 30% relative transmission of visible
light when measured through a thickness of about 0.3 to about 0.5
mm. Again the requirements set forth in the patents and claims
limit the diameter of pores and achievable grain size distributions
which are present in the translucent zirconia sintered body, which
as believed would preclude this material from being opalescent.
[0007] The following patents and published applications, directed
to zirconia ceramics or processing methods, are hereby incorporated
by reference in their entirety: U.S. Pat. Nos. 6,787,080,
7,655,586, 7,806,694 U.S. Pat. Nos. 7,833,621, 7,674,523,
7,429,422, 7,241,437, 6,376,590, 6,869,501, 8,298,329, 7,989,504,
8,425,809, 8,216,439, 8,309,015, 7,538,055, 4,758,541,
US20110027742, US20120058883, US20100003630, US20090274993,
US20090294357, US20090115084, US20110230340, US20090004098,
US20100075170, US20040222098, and US20130313738. Among them U.S.
Pat. No. 8,298,329 and US20130313738 describe translucent
nano-crystalline dental ceramics and a process of fabrication of
the same by slip-casting or powder compaction.
[0008] The following publications are directed to processing and
properties of zirconia or transparent alumina ceramics.
[0009] Adam, J., et al. "Milling of Zirconia Nanoparticles in a
Stirred Media Mill", J. Am. Ceram. Soc., 91 [9] 2836-2843
(2008)
[0010] Alaniz, J. E., et al. "Optical Properties of Transparent
Nanocrystalline Yttria Stabilized Zirconia", Opt. Mater., 32, 62-68
(2009)
[0011] Anselmi-Tamburini, etc al. "Transparent Nanometric Cubic and
Tetragonal Zirconia Obtained by High-Pressure Pulsed Electric
Current Sintering", Adv. Funct. Mater. 17, 3267-3273 (2007)
[0012] Apetz, R., et al. "Transparent Alumina: A Light Scattering
Model", J. Am. Ceram. Soc., 86 [3], 480-486 (2003)
[0013] Binner, J., et al. "Processing of Bulk Nanostructured
Ceramics", J. Eur. Ceram. Soc. 28, 1329-1339 (2008)
[0014] Binner, J. et al. "Compositional Effects in Nanostructured
Yttria Partially Stabilized Zirconia" Int. J. Appl. Ceram. Tec., 8,
766-782 (2011)
[0015] Casolco, S. R. et al. "Transparent/translucent
polycrystalline nanostructured yttria stabilized zirconia with
varying colors" Scripta Mater. 58 [6], 516-519 (2007)
[0016] Garcia, et al. "Structural, Electronic, and Optical
Properties of ZrO2 from Ab Initio Calculations", J. Appl. Phys.,
100 [1], 104103 (2006)
[0017] Klimke, et al. "Transparent Tetragonal Yttria-Stabilized
Zirconia Ceramics" J. Am. Ceram. Soc., 94 [6] 1850-1858 (2011)
[0018] Knapp, K. "Understanding Zirconia Crown Esthetics and
Optical Properties", Inclusive Magazine, (2011)
[0019] Rignanese, et al, "First-principles Study of the Dynamical
and Dielectric Properties of Tetragonal Zirconia" Phys. Rev. B, 64
[13], 134301 (2001)
[0020] Srdic, V. V., et al. "Sintering Behavior of Nanocrystalline
Zirconia Prepared by Chemical Vapor Synthesis" J. Am. Ceram. Soc.
83 [4], 729-736 (2000)
[0021] Srdic, V. V., et al. "Sintering Behavior of Nanocrystalline
Zirconia Doped with Alumina Prepared by Chemical Vapor Synthesis"
J. Am. Ceram. Soc. 83 [8], 1853-1860 (2000)
[0022] Trunec, et al. "Compaction and Presureless Sintering of
Zirconia Nanoparticles" J. Am. Ceram. Soc. 90 [9] 2735-2740
(2007)
[0023] Vladimir V. Srdic', Markus Winterer, and Horst Hahn.
"Sintering Behavior of Nanocrystalline Zirconia Prepared by
Chemical Vapor Synthesis". J. Am. Ceram. Soc., 83 [4] 729-36
(2000)
[0024] Most or all of the above-listed patents and publications
describe a variety of properties of tetragonal nanozirconia
materials and processing methods thereof. All of these sources
appear to describe sintering with application of external pressure
such as HIP or SPS.
[0025] Light transmission at about 550-560 nm is commonly accepted
to compare light transmittance of dental materials, especially
dental zirconia materials, which is related to the color
resolution/sensitivity of photopic vision of human eyes. In humans,
photopic vision allows color perception, mediated by cone cells in
the retina. The human eye uses three types of cones to sense light
in three bands of color. The biological pigments of the cones have
maximum absorption values at wavelengths of about 420 nm
(bluish-violet), 534 nm (Bluish-Green), and 564 nm
(Yellowish-Green). Their sensitivity ranges overlap to provide
vision throughout the visible spectrum from about 400 nm to about
700 nm. Colors are perceived when the cones are stimulated, and the
color perceived depends on how much each type of cone is
stimulated. The eye is most sensitive to green light (555 nm)
because green stimulates two of the three kinds of cones almost
equally; hence light transmission at 560 nm is used as a basis for
characterization of highly translucent zirconia materials of the
present invention.
[0026] Opalescence is one of the important optical characteristics
of natural dentition that is critical to replicate in aesthetic
dental restorative material in order to fabricate life-like dental
restorations. This esthetic requirement is often referred to as the
"vitality of a restoration". It is a well-known optical effect
resulting in a bluish appearance in reflected color and an
orange/brown appearance in transmitted color. The opalescent
property is generally associated with scattering of the shorter
wavelengths of the visible spectrum, by inclusions of the second
phase(s) having a different refractive index from the matrix phase.
In human teeth, opalescence of natural enamel is related to light
scattering and dispersion produced by complex spatial organization
of enamel's elemental constituents--hydroxyapatite nanocrystals.
Hydroxyapatite crystallites forming human enamel are arranged in
bundles or sheets forming rods (bundles) and interrods (sheets),
which are organized in a honeycomb-like structure. The average
crystal size is 160 nm long and 20-40 nm wide. As light travels
through the enamel, the rods scatter and transmit the shorter
wavelength light, rendering the enamel opalescent.
[0027] The degree of opalescence can be quantified by a
colorimetric spectrophotometry measurement with a CIE standard. For
example, Lee et al. (see references below) use "Opalescence
Parameter" (OP) as a measure of opalescence. Kobashigawa et. al.
(U.S. Pat. No. 6,232,367) use the same basic formula, but termed it
"Chromaticity Difference". The opalescence parameter (OP or
"Chromaticity Difference") is calculated according to the following
formula:
OP=[(CIEa.sub.T*-CIEa.sub.R*).sup.2+(CIEb.sub.T*-CIEb.sub.R*).sup.2].sup-
.1/2,
wherein (CIEa.sub.T*-CIEa.sub.R*) is the difference between
transmission and reflectance modes in red-green coordinate a*;
(CIEb.sub.T*-CIEb.sub.R*) is the difference between transmission
and reflectance modes in yellow-blue color coordinate b*. Using
this formula, OP of the commercially available current state of the
art "translucent" zirconia is calculated to be in the range from
about 5 to about 7. These commercial materials are clearly not
opalescent. According to literature data, it is believed that
materials with low OP values are not opalescent. The measured OP
range for clearly opalescent human enamel was 19.8-27.6. According
to Kobashigawa, for matching the vitality of natural teeth, the OP
value should be at least 9, and preferably higher, so that the
opalescence effect is clearly observed. On the other hand it is not
useful to match high OP values of human enamel "just by numbers"
since the restoration will not blend well with the adjacent teeth
in the patient's mouth.
[0028] The following publications are directed to mechanisms of
opalescence in natural or synthetic materials.
[0029] Cho, M.-S. et al. "Opalescence of all-ceramic core and
veneer materials", Dental Materials, 25, 695-702, (2009)
[0030] Egen, M. et al. "Artificial Opals as Effect Pigments in
Clear-Coatings", Macromol. Mater. Eng. 289, 158-163, (2004)
[0031] Lee, Y.-K., et al. "Measurement of Opalescence of Resin
Composites", Dental Materials 21, 1068-1074, (2005)
[0032] Lee, Y.-K., et al. "Changes in Opalescence and Fluorescence
Properties of Resin Composites after Accelerated Aging", Dental
Materials 22, 653-660, (2006)
[0033] Lee, Y.-K., "Influence of Scattering/Absorption
Characteristics on the Color of Resin Composites" Dental Materials
23, 124-131, (2007)
[0034] Lee, Y.-K., "Measurement of Opalescence of Tooth Enamel",
Journal of Dentistry 35, 690-694, (2007)
[0035] Kobashigawa, A. I. et al., "Opalescent Fillers for Dental
Restorative Composites", U.S. Pat. No. 6,232,367 B1, (2001)
[0036] Peelen. J. G. J. et al. "Light Scattering by Pores in
Polycrystalline Materials: Transmission Properties of Alumina",
Journal of Applied Physics, 45, 216-220, (1974)
[0037] Primus, C. M., et al. "Opalescence of Dental Porcelain
Enamels" Quintessence International, 33, 439-449, (2002)
[0038] Yu, B., et al. "Difference in Opalescence of Restorative
Materials by the Illuminant", Dental Materials 25, 1014-1021,
(2009)
[0039] White et al., Biological Organization of Hydroxyapatite
Crystallites into a Fibrous Continuum Toughens and Controls
Anisotropy in Human Enamel, J Dent Res 80(1): 321-326, (2001).
[0040] It would be extremely beneficial to have high translucency
of glass ceramics combined with high strength of tetragonal
zirconia and opalescence mimicking natural dentition in the same
dental restorative material sinterable below 1200.degree. C., which
can be processed into a full contour zirconia restoration using
conventional techniques and equipment such as dental CAD/CAM
systems, dental pressing furnaces and dental furnaces. Other
techniques and equipment successfully used in other areas of
technology for mass production of near-net shaped parts and
components can be also used.
SUMMARY
[0041] These and other features are achieved by nanozirconia bodies
of the present invention. In one embodiment, certain ranges of
processing conditions are utilized to produce nanozirconia bodies
that are opalescent in green, brown (pre-sintered) and fully dense
condition as shown in FIG. 2. Opalescent nanozirconia bodies can be
also nearly transparent or highly translucent in all stages of
processing (visible light transmittance at or higher than 45% and
preferably higher than 50% at 560 nm for 1 mm samples) and result
in fully dense tetragonal zirconia bodies (at least 99.5% or higher
density and preferably .gtoreq.99.9% dense) that in addition to
high light transmittance also comprise high strength (at least 800
MPa or higher strength and preferably .gtoreq.1200 MPa strength)
and sinterability at temperatures below 1200.degree. C. in
conventional dental furnaces which is especially important for
dental restorative applications.
[0042] 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.TM.
and Zpex.TM. Smile powders made by Tosoh (Japan). Light
transmittance of Zpex.TM. and Zpex.TM. Smile made materials
measured at 560 nm, the wavelength of visible light of
aforementioned "maximal physiological significance," is 39% and
46%, respectfully for 1 mm samples. The difference in light
transmittance between Zpex.TM. and Zpex.TM. Smile samples is
related to their Yttria (Y.sub.2O.sub.3) content and resulting
phase composition: while Zpex.TM.-made zirconia comprising 3 mole %
of Y.sub.2O.sub.3 is tetragonal, Zpex Smile made zirconia
(.about.5.3 mole % of Y.sub.2O.sub.3) is comprising both tetragonal
and cubic phases, hence it is more translucent but only half as
strong as tetragonal zirconia (.about.1200 MPa vs.about.600 MPa,
respectfully). Both materials as well as other commercial zirconia
materials are clearly not opalescent.
[0043] By comparing curves presented in FIG. 1 it becomes apparent
that opalescent nanozirconia materials of the present invention
have steeper spectral transmittance curves as measured in
transmittance mode by a conventional visible light
spectrophotometer equipped with an integrating sphere. This is
consistent with the fact that being opalescent, nanozirconia
materials of the present invention scatter blue light, i.e. shorter
wavelengths, preferentially, while allowing yellowish red light,
i.e. longer wavelengths, to transmit through with limited
scattering. Thus, it allows us to define their advantageous light
transmittance properties as being higher than 45% and preferably
higher than 50% in the whole spectral range of 560 nm to 700 nm for
unshaded or "naturally colored" nanozirconia and higher than 35%
and preferably higher than 40% in the whole spectral range of 560
nm to 700 nm for shaded nanozirconia intentionally doped with
coloring ions such as Fe, Cr, Ni, Co, Er, Mn and other ions/oxides
listed in U.S. Pat. Nos. 6,713,421 and 8,178,012 which are hereby
incorporated by reference in their entirety. Typically, light
transmittance of shaded zirconia is 5-10% lower than light
transmittance of unshaded or "naturally colored" zirconia.
[0044] In tetragonal nanozirconia of the present invention, it is
believed that opalescence comes from the interaction of visible
light with the specific crystal structure and grain/pore size
distributions. In particular, we speculate that scattering mainly
occurs due to the existence of residual pores and/or grain size
dependent birefringence and the associated differences in
refractive index between pores and tetragonal zirconia matrix or
between different crystallographic orientations in a crystal
lattice of individual nanozirconia crystallites. In this complex
optical phenomenon or combination of optical phenomena resulting in
opalescence, both total porosity and pore size distribution will
affect the pore related scattering in all stages of nanozirconia
processing from green to brown to sintered bodies; while
contribution of birefringence intrinsic to tetragonal zirconia is
dependent on the grain size distribution in partially or fully
sintered bodies. Normally the pore and grain sizes in well-formed
nanozirconia compacts are of the same scale and increasing
concurrently with densification and grain growth. The desired level
of opalescence exists only for specific combination of porosity,
and pore/grain size distributions. Selective scattering of only the
short wavelengths of visible light is the key to achieve a
combination of optical opalescence and a high level of
translucence. It can be speculated that one of the applicable
scattering models is Rayleigh scattering, in which the size of
scattering species are much smaller than the incident wavelength,
the intensity of scattering (I) is strongly dependent on
wavelength, and the scattered intensity on both forward and
backward directions are equal for a specific wavelength. According
to Rayleigh scattering theory, the fact that scattering
cross-section .sigma..sub.s is proportional to .lamda..sup.-4,
where .lamda. is the wavelength of the incident light explains why
the shorter (blue) wavelengths are scattered more strongly than
longer (red) wavelengths. For example, the same nanoscale
scattering center/site would scatter a wavelength at 430 nm (in the
blue range) by a factor of 6 times more efficiently compared to a
wavelength of 680 nm (in the red range). As a result, an observer
will find that the samples appear bluish in color when observing
from the same side of the light source while yellowish and reddish
when observing from the opposite side of the light source. This
unique characteristic of nanozirconia materials of the present
invention occurs only for specific processing methods and starting
materials described below resulting in such specific grain and pore
size distributions during a transition from a transparent to a
translucent stage within the overall grain size range of 10 nm to
300 nm and final pore size mostly larger than 25 nm, and preferably
larger than 30 nm with total porosity being less than 0.5% and
preferably less than 0.1% (in the fully dense nanozirconia bodies).
The average grain size in translucent opalescent zirconia of the
present invention as measured according to ASTM E112-12 test method
is from 40 nm to 150 nm, preferably from 50 to 100 nm, and most
preferably from 50 to 80 nm.
[0045] The materials of the present invention are especially useful
for full contour restorations combining the strength of zirconia
with aesthetics of glass-ceramics benchmarks.
[0046] In various embodiments, dental restorations comprising
opalescent nanozirconia can be shaped by milling, injection
molding, electrophoretic deposition, gel-casting etc.
[0047] Opalescent nanozirconia dental restorations of the present
invention comprise the following key features:
[0048] Opalescent with OP values above 9 and preferably above
12.
[0049] Nearly transparent or highly translucent in shaded or
unshaded (natural) condition: Light transmittance of at least 45%
and preferably higher than 50% at a wavelength of 560 nm or even in
the whole spectral range of 560 nm to 700 nm for unshaded or
"naturally colored" nanozirconia for 1 mm samples; and higher than
35% and preferably higher than 40% at 560 nm or even in the whole
spectral range of 560 nm to 700 nm for shaded nanozirconia
intentionally doped with coloring ions for 1 mm samples.
[0050] Predominantly tetragonal, i.e., major phase is tetragonal
zirconia (less than 10% cubic) and preferably YTZP, i.e., Yttria
Stabilized Tetragonal Zirconia Polycrystal with Y.sub.2O.sub.3
content within the range from 0 to 3 mole %.
[0051] Grain size within overall range from 10 nm to 300 nm, or 20
nm to 250 nm, in fully sintered condition as confirmed by analysis
of fracture surfaces (see representative fracture surface in FIGS.
11A, 11B and 11C).
[0052] The average grain size in translucent opalescent zirconia of
the present invention as measured according to ASTM E112 (or EN
623-3) test method is from 40 nm to 150 nm, preferably from 50 to
100 nm, and most preferably from 50 to 80 nm.
[0053] Pore size mostly larger than 25 nm, preferably 30 nm when
density is higher than 99.5%. Most preferably that porosity is less
than 0.1% (density .gtoreq.99.9% of theoretical density) for
maximal visible light transmittance.
[0054] Strong--ISO 6872 flexural strength at least 800 MPa or
higher, and preferably .gtoreq.1200 MPa strength; and most
preferably .gtoreq.2 GPa strength.
[0055] Sinterable at temperatures <1200.degree. C. using
conventional dental furnaces or microwave dental furnaces.
[0056] Shaped by CAD/CAM, EPD, LPIM, dental heat-pressing (like
glass ceramic ingots) similar to LPIM and gel-casting using RP
molds.
[0057] The zirconia may include a stabilizing additive selected
from Y, Ce, Mg, or mixtures thereof, or other known stabilizing
additive.
[0058] The numbers and ranges in the specification and claims can
cover values obtained by applying the regular rules of rounding
and/or up to +1-5%.
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.TM.
and Zpex.TM. 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 .about.17 vol % from 4.5 vol %
suspension prior to (1) and after attrition-milling (2).
[0073] FIG. 13B shows particle size of as-received .about.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 .gtoreq.99.5% dense) that in addition to
high light transmittance also comprise high strength (.gtoreq.800
MPa and even in excess of 2 GPa) and sinterable at temperatures
below 1200.degree. 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
.gtoreq.99.9% of theoretical density) Flexural strength ISO 6872
flexural strength Preferably .gtoreq.1200 MPa at least 800 MPa or
higher flexural strength; and most preferably .gtoreq.2 GPa
flexural strength Sinterable at Sinterable at temperatures
<1200.degree. Sinterable at temperatures .ltoreq.1150.degree.
temperatures <1200.degree. 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 .about.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 .about.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 .ltoreq.20 nm and preferably
.ltoreq.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.degree.
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.degree. C., preferably from 500.degree. C. to
600.degree. C. Optional pre-sintering can be carried out at
temperatures up to 850.degree. C. After burn out, the articles,
so-called "brown" bodies, were then sintered at temperatures lower
than 1200.degree. 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 .gtoreq.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
1.2.+-.0.2 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.degree. 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 72.about.120 hours, the humidity was above
85% and temperature was about 25.degree. 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 .about.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.degree.
C./min to 550.degree. 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.degree. C./min to
1150.degree. 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 2108.+-.386 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.degree. 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.degree.
C., 1150.degree. C.) for very short time (e.g. 6 seconds), and then
quickly dropping to lower temperature (e.g. 1075.degree. C.,
1050.degree. C.) and holding for a prolonged period of time.
[0089] In Example 1C, the sample was heated from room temperature
to 1125.degree. C. at 10.degree. C./min rate and held at
1125.degree. C. for 6 seconds; then it was cooled down to
1075.degree. C. quickly and held at 1075.degree. 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 1983.+-.356 MPa.
[0090] In example 1D, the sample was heated from room temperature
to 1150.degree. C. at 10.degree. C./min rate and held at
1150.degree. C. for 6 seconds; then it was cooled down to
1075.degree. C. quickly and held at 1075.degree. C. for 20 hours.
Biaxial flexural strength is 2087.+-.454 MPa.
[0091] In example 1E, the sample was heated from room temperature
to 1125.degree. C. at 10.degree. C./min rate and held at
1125.degree. C. for 6 seconds; then it was cooled down to
1075.degree. C. quickly and held at 1075.degree. C. for 15
hours.
[0092] In example 1F, the sample was heated from room temperature
to 1125.degree. C. at 10.degree. C./min rate and held at
1125.degree. C. for 10 seconds; then it was cooled down to
1075.degree. C. quickly and held at 1075.degree. 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.degree. C. to 700.degree. 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 .mu.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.degree. C. for 4 hours.
[0097] For Example 2B, the attrition milling time was 1.5 hours,
and the brown bodies were sintered at 1125.degree. C. for 2
hours.
[0098] For Example 2C, the attrition milling time was 1.5 hours,
and the brown bodies were sintered at 1100.degree. 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.degree. 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.degree. 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 .about.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.degree. C. at
10.degree. C./min and held for 2 hours.
[0104] For Example 3B, the sample was heated to 1125.degree. C.
with 10.degree. C./min rate and held at 1125.degree. C. for 10
seconds; then it was cooled down to 1075.degree. C. quickly and
held at 1075.degree. 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.degree. C./min to
1125.degree. C. in IR sensor controlled mode, with microwave on
after 700.degree. C. in auto mode. Then the sample dwelled at
1125.degree. C. under 500 W microwave for 2 hours. The sample was
cooled down naturally.
[0108] In Example 4B, the sample was heated at 10.degree. C./min to
1125.degree. C. in IR sensor controlled mode for 6s, and then held
at 1075.degree. C. for 20 h. During heating, the microwave started
at 700.degree. 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.degree. 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.degree. C./min to 700.degree. 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.degree. C./min to 1150.degree. 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.degree. 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.degree. 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.degree. C. for 2 hours. Both green and
brown bodies were transparent. Sintering was carried out at
temperatures from 900.degree. C. to 1100.degree. 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.degree. C. and 1050.degree. 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 .degree. 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.
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