U.S. patent application number 10/857482 was filed with the patent office on 2010-02-25 for dental restorations using nanocrystalline materials and methods of manufacture.
Invention is credited to Dmitri Brodkin, Moisey y. Gamarnik.
Application Number | 20100047743 10/857482 |
Document ID | / |
Family ID | 41446428 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100047743 |
Kind Code |
A1 |
Brodkin; Dmitri ; et
al. |
February 25, 2010 |
DENTAL RESTORATIONS USING NANOCRYSTALLINE MATERIALS AND METHODS OF
MANUFACTURE
Abstract
Dental articles are produced using relatively low sintering
temperatures to achieve high density dental articles exhibiting
strengths equal to and greater than about 700 MPa. Ceramic powders
comprised of nanoparticulate crystallites are used to manufacture
dental articles. The ceramic powders may include sintering agents,
binders and other similar additives to aid in the processing of the
ceramic powder into a dental article. The ceramic powders may be
processed into dental articles using various methods including, but
not limited to, injection molding, gel-casting, slip casting, or
electroforming, hand, cad/camming and other various rapid
prototyping methods. The ceramic powder may be formed into a
suspension, pellet, feedstock material or a pre-sintered blank
prior to forming into the dental article.
Inventors: |
Brodkin; Dmitri;
(Livingston, NJ) ; Gamarnik; Moisey y.;
(Warminster, PA) |
Correspondence
Address: |
BOND, SCHOENECK & KING, PLLC
ONE LINCOLN CENTER
SYRACUSE
NY
13202-1355
US
|
Family ID: |
41446428 |
Appl. No.: |
10/857482 |
Filed: |
May 28, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60474166 |
May 29, 2003 |
|
|
|
Current U.S.
Class: |
433/222.1 ;
264/16; 433/215; 977/773; 977/902 |
Current CPC
Class: |
C04B 2235/5436 20130101;
C04B 2235/6027 20130101; C04B 2235/3206 20130101; C04B 2235/6022
20130101; B33Y 70/00 20141201; B82Y 30/00 20130101; C04B 2235/9653
20130101; C04B 2235/3232 20130101; C04B 2235/3262 20130101; C04B
2235/5409 20130101; C04B 2235/6023 20130101; C04B 2235/94 20130101;
A61C 13/0006 20130101; C04B 2235/612 20130101; C01P 2004/64
20130101; B22F 3/22 20130101; C04B 2235/77 20130101; C04B 2235/3418
20130101; C04B 2235/3251 20130101; C04B 2235/549 20130101; B33Y
80/00 20141201; C04B 2235/3229 20130101; B22F 3/225 20130101; C04B
2235/5472 20130101; C04B 2235/3246 20130101; C04B 2235/6565
20130101; A61C 13/083 20130101; C04B 2235/6026 20130101; A61C
13/081 20130101; C04B 2235/3225 20130101; C04B 2235/5454 20130101;
C04B 2235/668 20130101; C01G 25/00 20130101; C04B 2235/785
20130101; C04B 35/632 20130101; C04B 2235/667 20130101; C04B
2235/9615 20130101; B22F 3/14 20130101; B22F 3/12 20130101; C04B
2235/3217 20130101; C04B 2235/3279 20130101; C04B 2235/5427
20130101; C04B 2235/3241 20130101; C04B 2235/604 20130101; C04B
2235/6562 20130101; C04B 35/486 20130101; C04B 35/62695 20130101;
C04B 35/645 20130101 |
Class at
Publication: |
433/222.1 ;
264/16; 433/215; 977/773; 977/902 |
International
Class: |
A61C 5/09 20060101
A61C005/09; A61C 13/00 20060101 A61C013/00; A61C 5/00 20060101
A61C005/00 |
Claims
1-42. (canceled)
43. A dental article 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 theoretical density of
the ceramic material is known; 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.;
wherein the ceramic material has a final density exceeding 95% and
less than 97% of the theoretical density of the ceramic material;
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.
44. The dental article of claim 43 wherein the sintering of the
particulate material at temperatures less than about 1300.degree.
C. comprises furnace sintering or microwave sintering to densities
exceeding 95% of the theoretical density.
45. The dental article of claim 43 wherein the sintering of the
particulate material at temperatures less than about 1300.degree.
C. comprises sintering at a temperature less than about
1200.degree. C. to densities exceeding 95% of the theoretical
density.
46. The dental article of claim 43 wherein the sintering of the
particulate material at temperatures less than about 1300.degree.
C. comprises sintering at temperature less than about 1100.degree.
C. to densities exceeding 95% of the theoretical density.
47. (canceled)
48. The dental article of claim 43 wherein the flexural strength is
equal to or greater than about 800 MPa.
49. The dental article claim 43 wherein the particulate material is
in the form of free-flowing powder, pre-sintered blanks, a
suspension, slurry, slip, gel, pellets or feedstock.
50. The dental article of claim 43 wherein the dental article is
formed by injection molding, heat pressing, hot pressing,
gel-casting, slip casting, slurry casting, pressure casting, direct
coagulation, colloidal spray deposition, dipping, or electroforming
the particulate material.
51. The dental article of claim 43 wherein the dental article is
selected from the group consisting of orthodontic retainers,
bridges, space maintainers, tooth replacement appliances, splints,
crowns, partial crowns, dentures, posts, teeth, jackets, inlays,
onlays, facings, veneers, facets, implants, cylinders, abutments
and connectors.
52. The dental article of claim 43 wherein the ceramic material
further comprises alumina, titania, silica, magnesia, ceria or
mixtures thereof in solid solution.
53. (canceled)
54. The dental article of claim 43 wherein forming the particulate
material into the dental article is performed by hand, by CAD/CAM
methods or by rapid prototyping methods.
55. The dental article of claim 43 wherein forming of the
particulate material into the dental article is performed using
replicas, molds, dies, substrates, or shells produced by rapid
prototyping methods.
56-73. (canceled)
74. (canceled)
75. The dental article of claim 74 wherein the ceramic material
exhibits at least 30% relative transmission of visible light when
measured through a thickness of about 0.4 mm.
76. The dental article of claim 43 wherein the ceramic material
further comprises a sintering aid, a grain growth inhibitor or a
combination thereof.
77. The dental article of claim 76 wherein the sintering aid
comprises Si, Al, Mg, Zr, Ce, Ta, oxides thereof or a combination
thereof.
78. The dental article of claim 76, wherein the grain growth
inhibitor comprises Cr, Ti, Ni, Mn, oxides thereof or a combination
thereof.
79. A dental article 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 theoretical density of
the ceramic material is known; 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.;
wherein the ceramic material has a density exceeding 95% of the
theoretical density of the ceramic material; wherein the final
grain size is about 10-20 times larger than the starting
crystallite size and the final pore size does not exceed the size
of the starting crystallite size; and wherein the ceramic material
exhibits at least 30% relative transmission of visible light when
measured through a thickness of less than 0.5 mm.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Application No.
60/474,166 filed May 29, 2003, entitled Methods of Fabricating
Dental Restorations Using Nanocrystalline Materials, which is
hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates to nanocrystalline ceramic powders
especially useful for fabricating dental restorations and methods
of fabricating dental restorations using ceramic powders comprising
nanoparticles.
BACKGROUND OF THE INVENTION
[0003] Techniques of fabricating all-ceramic dental restorations by
hand and methods using commercial high-tech systems such as CAD/CAM
systems each have their limitations and target different segments
of the dental laboratory market. There are two main challenges
restricting widespread use of high-strength ceramic materials for
cost-effective fabrication of dental restorations and both
challenges are related to the sintering step of the operation.
High-strength ceramic materials are crystalline materials formed
from powder and require high temperatures for sintering that result
in substantial shrinkage. Any technique enabling use of these
materials for dental restorations should offer ways to (1)
compensate for shrinkage and (2) provide a furnace capable of
reaching the temperatures necessary to sinter the material to
nearly full density.
[0004] A technique reportedly providing the highest strength for
manually produced copings, the Vita.RTM. In-Ceram.TM. method
(developed by VITA Zahnfabrik), has been advertised as yielding a
material with flexural strength of about 500 MPa or even higher.
This technique has not become popular mostly due to esthetic
limitations and a tedious multi-step fabrication procedure that
includes a glass infiltration step. This glass infiltration
technique is one way to circumvent the above-mentioned challenges.
Vita.RTM. In-Ceram.TM. copings are slip-cast on a gypsum die and
soft-sintered with negligible shrinkage. The final glass
infiltration step does not require a special furnace. The resulting
product is a fully dense restoration having undergone no shrinkage.
Nonetheless, the presence of a glass phase in the glass-infiltrated
ceramics makes it inferior to corresponding crystalline ceramics in
mechanical strength and chemical durability.
[0005] Currently available CAD/CAM systems are capable of
compensating for shrinkage by milling enlarged shapes. Moreover,
high-temperature isotropic sintering results in fully dense and
accurate final shapes. However, CAD/CAM systems and procedures are
expensive and not affordable by small labs. For example, two of the
most recently developed commercial state-of-the art CAD/CAM
systems, the LAVA.TM. system (available from 3M ESPE) and the
CERCON.RTM. system (available from Dentsply/Degussa), require the
purchase of a scanner, milling machine and high-temperature
sintering furnace and are currently priced in the range of
approximately $60,000-$180,000. Both of the aforementioned CAD/CAM
systems employ soft-sintered zirconia blocks. The blocks are milled
to enlarged shapes and subsequently sintered to full density. Both
systems are advertised as yielding materials having a flexural
strength of about 900 MPa or higher.
[0006] Glass-ceramic materials obviate the need to compensate for
shrinkage and high temperature sintering. They can be hand-built on
a refractory die and sintered at fairly low temperatures to assure
accuracy of the final shape. One example of such a material is an
OPC.TM. Low Wear (available from Pentron Laboratory Technologies,
LLC) porcelain jacket crown (PJC). Glass-ceramic materials can also
be injection molded into a refractory investment mold formed by the
lost wax technique. Examples of commercially available materials
used in this process include OPC.RTM. porcelain, and OPC.RTM.
3G.TM. porcelain, IPS Empress.RTM. porcelain and Empress 2.TM.
porcelain. The physical mechanism underlying the high
processability/formability of these glass-ceramics is the viscous
flow of its glass component. The glass-ceramic materials listed
above (Optec.TM., OPC.RTM. and OPC.RTM. 3G.TM., Empress.RTM. and
Empress2.TM. materials) have from about 40% to about 60% of a glass
phase which serves as a matrix in which from about 40% (e.g.,
Optec) to about 60% of crystals (e.g., Empress2) are embedded.
These crystals are grown in-situ by crystallization heat-treatment
of the parent glass. Alternatively, in a method described by
Hoffman in U.S. Pat. Nos. 5,916,498, 5,849,068 and 6,126,732, in
order to improve processability of the material, up to 50% glass is
added to the crystalline ceramic powder. As a result, the reported
flexure strength is limited to less than 600 MPa. By introducing a
glass phase into the microstructure, strength is compromised to
gain better processability.
[0007] Sintering of glass-ceramic powders is a relatively fast
process compared to sintering of crystalline ceramic powders due to
the viscous flow mechanism of the former, which is associated with
higher densification rates, but the presence of the residual glass
phase limits the strength of the final product. Another benefit of
the viscous flow mechanism is that the glass ceramic conforms to
the shape of the die during sintering without cracking. On the
other hand, crystalline ceramics can be much stronger than glass
ceramics, but crystalline ceramics sinter by a solid-state
diffusion mechanism that is intrinsically slow creating
inhomogeneous shrinkage, generating significant sintering stresses
that may result in associated cracking. Liquid phase sintering
induced by the addition of sintering aids greatly enhances
sinterability of crystalline ceramics by promoting particle
rearrangement and solution-precipitation mechanisms but such
mechanisms do not achieve all the advantages of the viscous flow
mechanism.
[0008] At the same time many experimental and theoretical studies
reveal a decrease of the melting temperature of nanometallic
particles in comparison with the melting temperature of
conventional bulk metals. Its magnitude depends mostly on particle
size and crystal structure as well as particle surface conditions
and the host matrix environment such as the presence of impurities,
level of agglomeration, coating, deposition substrate and the like.
Usually, melting is associated with a pre-melting process resulting
in a change in shape of the nanoparticles followed by the formation
of a liquid skin on the melting nanoparticles. The liquid skin
thickness increases during melting gradually consuming the solid
particle core. Transmission electron microscopy studies, such as
the one discussed in "Shape Transformation and Surface Melting of
Cubic and Tetrahedral Platinum Nanocrystals" by Z. L. Wang, J. M.
Petroski, T. C. Green and M. A. El-Sayed, J. Phys. Chem. 102, (32)
6145-6151 (1998), have established that 8 nanometer platinum
nanoparticles begin to melt at about 6000 to about 650.degree. C.,
which is a much lower temperature than the melting point of bulk
platinum at 1769.degree. C. At about 500.degree. C., cubic
particles change their shape to a spherical shape with surface
melting occurring at about 600.degree. C. to about 650.degree. C.
The molten layer surrounding solid cores of platinum nanocrystals
is about 1 nm in thickness at 600.degree. C. and the thickness
increases with temperature as the nanoparticles continue to melt.
The "melting point depression" abbreviated as MPD is a
thermodynamically driven phenomenon and can be explained by a
drastic increase in the surface area/volume ratio in
nano-particulate materials and the corresponding increase in their
specific surface energy. This leads to a size-related dependence of
melting temperature that is roughly close to 1/d functionality,
where d is the mean particle size, and contains surface tension
coefficients, latent heat of melting and the molten skin thickness
as parameters.
[0009] Table 1 presents some experimental data illustrating the
difference in melting temperatures for nanoparticles and the
corresponding bulk metals and semiconductors.
TABLE-US-00001 TABLE 1 Melting temperatures of the selected
nanomaterials Nano-Material Bulk Melting Nano- Nano- Melting
Melting Point Particle Particle Temperature Temperature Depression
Melting/Surface Material Shape size, nm .degree. C. .degree. C.
.degree. C. T.sub.Mnano/T.sub.Mbulk Melting Ref. Pt cubic 8 650
1769 1100 0.37 surface melting [1] Au spherical 4 650 1057 400 0.61
melting [2] Ag spherical 7 470 961 490 0.49 melting [2] Pd wire
Diameter 4.6 300 1445 1100 0.21 melting [3] Length 202 Sn spherical
10 153 232 80 0.66 melting [4] CdS spherical 4 630 1678 1080 0.38
melting [5] Ge wire Diameter 55 650 930 280 0.70 surface melting
[6] Length 1000 from ends [1] "Shape Transformation and Surface
Melting of Cubic and Tetrahedral Platinum Nanocrystals," Z. L.
Wang, J. M. Petroski, T. C. Green and M. A. El-Sayed, J. Phys.
Chem. 102, (32), 6145-6151 (1998). [2] "Size-Dependent Melting
Temperature of Individual Nanometer-Sized Metallic Clusters," T.
Castro, R. Reifenberger, E. Choi and R. P. Andres, Phys. Rev., B 42
(13), 8548-8556 (1990). [3] "Size Controlled Synthesis of Pd
Nanowires Using a Mesoporous Silica Template Via Chemical Vapor
Infiltration," K-B Lee, S-M Lee, and J. Cheou, Adv. Mate., 13 (7),
517-520, (2001). [4] "Size-Dependent Melting Properties of Small
Tin Particles: Nanocalorimetric Measurement," S. L. Lai, J. Y. Guo,
V. Petrova, G. Ramanath and L. H. Allen, Phys. Rev. Lett., 77(1),
99-102, (1996). [5] "Melting in Semiconductor Nanocrystals," A. N.
Goldstein, C. M. Echer and A. P. Alivisatos, Science, 256,
1425-1427, (1992). [6] "Melting and Welding Semiconductor Nanowires
in Nanotubes," Y. Wu and P. Yang, Adv. Mater., 13 (7), 520-523,
(2001).
[0010] Onset of surface melting occurs usually at temperatures even
lower than the temperature at which the entire nanoparticle melts.
It can be speculated that the "molten shells" of the pre-melted
nanoparticles work as "a lubricant" inducing higher mobility of the
particles and higher diffusion rates and hence facilitating
densification at temperatures much lower than 0.6 of the melting
point (T.sub.m).
[0011] It can be further speculated that thermodynamic
considerations explaining the mechanism of MPD described above
should hold for ceramic nanoparticles as well. Nevertheless, the
MPD effect is not very well studied in ceramics for obvious
reasons--even the depressed melting point anticipated for ceramic
nanoparticles will still be very high making it extremely difficult
to conduct observations similar to those for metals and
semiconductors described above in Table 1.
[0012] For example, the melting point (T.sub.M) for pure alumina
and zirconia are 2050.degree. C. and 2700.degree. C., respectively,
and therefore the MPD effect of the order of 0.5 T.sub.M will
result in melting temperatures for nano-alumina and nano-zirconia
particles of about 1025.degree. C. and 1350.degree. C. However,
there are some indirect indications that MPD does occur in
nanoceramics such as extremely low sintering temperatures for
nanopowders as reported in R. A. Kimel, Aqueous Synthesis and
Processing of Nanosized Yttria Tetragonally Stabilized Zirconia,
Ph.D. Thesis, The Pennsylvania State University, the Graduate
School, the College of Earth and Mineral Sciences, (2002) and in G.
Skandan, H. Hahn, M. Roddy and W. R. Cannon, "Ultrafine-Grained
Dense Monoclinic and Tetragonal Zirconia," J. Am. Ceram. Soc., vol.
77, no. 7, pp. 1706-10 (1994), which are both hereby incorporated
by reference.
[0013] These studies reported onset of densification at
surprisingly low temperatures of about 0.3 T.sub.M as well as a
surprising and unique ability of nanoceramics to be translucent at
fairly high levels of porosity.
[0014] Some studies reported extreme difficulty in sintering
nanoceramics to full density due to rapid grain growth. For
example, Skandan et al. (cited above) observed that grains grew 15
times the initial particle size in the case of nano-zirconia. The
other major obstacle encountered with the use of nanoparticles in
the fabrication of dental articles is related to difficulties in
the consolidating of bulk shapes using conventional methods like
powder compaction and slip-casting. It the scope of the present
invention to utilize the advantages of nanoparticulate ceramics
while successfully overcoming the obstacles currently hampering use
of such nanoparticulates as dental ceramics.
[0015] It is desirable to provide dental ceramics having low
sintering temperatures and high strengths. It would be beneficial
to provide dental ceramics having sintering temperatures that are
low enough to be sintered in existing dental furnaces, yet
maintaining high strength and translucency. It is most desirable to
provide processing techniques for dental ceramics that result in
fully densified dental ceramics.
SUMMARY OF THE INVENTION
[0016] These and other objects and advantages are accomplished by
the ceramic powders of the present invention which are manufactured
into dental articles. The ceramic powders may include sintering
agents, binders and other similar additives to aid in the
processing of the ceramic powder into a dental article. The ceramic
powders are comprised of nanoparticulate crystallites. The ceramic
powders may be processed into the dental article using various
methods including, but not limited to, injection molding,
gel-casting, slip casting, or electroforming, hand forming,
cad/camming and other various rapid prototyping methods. The
ceramic powder may be formed into a suspension, pellet, feedstock
material or a pre-sintered blank prior to forming into the dental
article.
[0017] Dental articles are produced using relatively low sintering
temperatures to achieve high density dental articles exhibiting
strengths equal to and greater than about 700 MPa.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Features of the present invention are disclosed in the
accompanying drawings, wherein similar reference characters denote
similar elements throughout the several views, and wherein:
[0019] FIG. 1 is a schematic diagram showing the structure of
particles described herein;
[0020] FIG. 2 is a schematic diagram showing particle size
distribution of ceramic powders;
[0021] FIG. 3 is a graph showing density versus sintering
temperature of nano-sized and conventional three mole percent
yttria-stabilized zirconia;
[0022] FIG. 4 is a schematic diagram of sample geometry and an
orthogonal coordinate system used for sintering shrinkage
measurements;
[0023] FIG. 5 is a graph showing shrinkage of outside diameter
L.sub.x versus shrinkage of the height of cups made out of
conventional zirconia powder and nanosized zirconia powder (NP-2);
and
[0024] FIG. 6 is a graph showing shrinkage of outside diameter
L.sub.y versus shrinkage of the height of cups made out of
conventional zirconia powder and nanosized zirconia powder
(NP-2).
DETAILED DESCRIPTION OF THE INVENTION
[0025] This invention provides particulate materials that can be
processed into dental restorations using both the most
sophisticated state-of-the art technologies such as solid free form
manufacturing (SFF) methods as set forth in U.S. Pat. No.
6,322,728, and copending commonly owned U.S. patent application
Ser. Nos. 09/972,351 (US 2002/00335458), 10/053,430 (US
2002/0125592) and 09/946,413 (US 2002/0064745), all of which are
hereby incorporated by reference, as well as manual techniques
similar to classic methods of building porcelain jacket crowns on a
refractory die (e.g. OPC.TM. Low Wear porcelain jacket crowns made
from powder or jacket crowns made using tape-cast ceramic sheets as
described in U.S. Pat. No. 5,975,905, which is hereby incorporated
by reference) or slip casting a ceramic slip onto a porous
die/mold.
[0026] The ceramic particulate materials of the present invention
have complex hierarchical architecture with three levels of
structural organization: nano-, micro-, and macro-level as shown in
FIG. 1. On the nano-level (.ltoreq.20 nm), the structure is based
on nano-crystallites depicted at 10 as elemental building blocks.
On the micro-level (0.1-20 microns), the structure is formed from
polycrystalline particles or agglomerates comprised of clusters of
nanocrystallites depicted at 12. On a macro-level (20-200 microns),
polycrystalline particles are agglomerated into granules depicted
at 14. These granules 14 are made by spray-drying or fluidized bed
agglomerating polycrystalline particles comprised of nanoparticles
of one or more kinds of materials including, but not limited to,
metallic and ceramic materials which may be fully or partially
calcined or still in the form of organic/inorganic precursors.
Sintering aids are depicted interstially at 16. The advantage of
using nanoparticles is their drastically different sintering
behavior associated with MPD. In much of the scientific literature,
such as in, "Melting of Isolated Tin Nanoparticles" by T. Bachels,
H-J Guntherodt, and R. Schafer, Phy. Rev. Lett., 85, (6),
1250-1253, (2000), which is hereby incorporated by reference, this
effect is also referred to as "size dependence of melting
temperature" in nano-materials. As a result of this mechanism,
sintering of nanoparticles is speculated to be aided by the
occurrence of surface pre-melting and hence, controlled by
capillary forces. Beneficial utilization of capillary forces
through the hierarchical architecture of ceramic powders comprising
nanoparticles is an essential feature of this invention. The
hierarchical architecture of nanocrystalline powders of this
invention is specifically engineered to 1) aid consolidation of
particulates into green shape; 2) take advantage of capillary
effects during sintering (i.e. liquid phase sintering and surface
melting of nanoparticles) to maximize particle rearrangement,
enhance sintering kinetics and lower the sintering temperature; and
3) control the size, distribution and morphology of the residual
porosity.
[0027] Examples of metallic powders useful herein include, but are
not limited to Si, Al, Mg, Zr, Y, Ce, Ta and mixtures thereof.
These metals are primarily chosen because they oxidize easily and
form glass-forming oxides SiO.sub.2, Al.sub.2O.sub.3, MgO,
ZrO.sub.2, Y.sub.2O.sub.3, CeO.sub.2 and Ta.sub.2O.sub.5 that
facilitate liquid-phase formation during sintering. Most of these
oxides are currently used as sintering aids or dopants in the
manufacture of high-performance ceramics such as alumina, zirconia,
silicon nitride and SIALON ceramics. The advantage of adding these
elements in metallic rather than oxide form is that as nanophase
metal particles they are extremely reactive, have high enthalpy of
oxidation, i.e., generate highly localized heat upon oxidation, and
provide good coupling for microwave energy.
[0028] Examples of ceramic nanocrystalline powders useful herein
include, but are not limited to, oxide ceramics such as various
forms and modifications of zirconia, alumina, titania, silica,
magnesia, yttria, ceria and solid solutions or mixtures
thereof.
[0029] The metallic powders and ceramic nanocrystalline powders of
the present invention have sintering temperatures lower than about
1300.degree. C. and preferably lower than about 1200.degree. C. and
most preferably not exceeding about 1100.degree. C. Sintering
temperatures of lower than 1300.degree. C. are the most economical
since sintering can be carried out in the most common
resistance-heated furnaces having metallic heating elements. Each
dental lab has at least one burn-out furnace with a maximum
continuous operating temperature of at least 1100.degree. C. and a
porcelain furnace with a maximum operating temperature of about
1200.degree. C. The essential feature of this invention is that
these powders can be processed into dental restorations using both
the most sophisticated state-of-the-art technologies such as Solid
Free Form Manufacturing (SFF) methods, also known as Rapid
Prototyping (RP), as well as manual techniques similar to classic
methods of building porcelain jacket crowns on a refractory die, or
jacket crowns made using tape-cast sheets or slip casting a ceramic
slip on a porous mold. Examples of SFF/RP methods include
stereo-lithography (SLA) and photo-stereo-lithography including
Digital Light Processing (DLP) and Rapid Micro Product Development
(RMPD) mask technique, selective laser sintering (SLS), ballistic
particle manufacturing (BPM), fusion deposition modeling (FDM),
multi-jet modeling (MJM) and three-dimensional printing (3DP).
[0030] Particle size distribution of these powders made of granules
and polycrystalline particles is designed to improve handling for
making restorations by hand or to optimize powder bed or feed-stock
characteristics for specific SFF methods used. Engineered
particulate materials of the present invention have complex
hierarchical architecture with three levels of structural
organization: nano-, micro-, and macro-level as shown in FIG.
1.
[0031] On the macro-level the particulates are formed into a nearly
spherical shape with a diameter from about 10 microns to about 500
microns, more preferably from about 20 to about 200 microns. These
spherical particles should preferably be solid, not hollow. The
size distribution of powders useful herein should be optimized for
the specific forming technique. For example, for building by hand,
the particle size distribution should be bimodal with the fraction
of finer particles fitting in the interstitials between the
fraction of coarser particles. The ratio of the mean diameter of
coarser particles (D.sub.c) to the mean diameter of finer particles
(D.sub.f), D.sub.cD.sub.f, should be more than 3 and preferably
more than 6, whereas the relative amount of finer particles is from
about 10 wt % to about 25 wt %. An example of particle size
distribution is shown in FIG. 2. Curve 20 depicts particle size
distribution of nanophase powder facilitating manual build-up and
curve 22 depicts typical volume based distribution for dental
porcelain with good "handling."
[0032] In addition to using nanocrystalline powders to induce low
sintering temperatures, sintering aids and binders/additives may be
mixed with the nanocrystalline powders to further facilitate the
action of capillary forces to aid in powder consolidation and
sintering. Binders used herein may include any known binder used in
conventional powder processing methods and may be compounds or
mixtures of compounds activated by heat, light, or other types of
radiation or by chemical reaction. Examples of binders/additives
include, but are not limited to, polymeric binders, plasticizers,
surfactants and dispersants such as polyacrylic binders, polyvinyl
alcohol (PVA) binders, polyvinyl butyral binders, stearic and oleic
acids, silanes, and various natural and synthetic waxes such as
paraffin wax, polyethylene wax, carnauba wax, and bee's wax.
[0033] In accordance with a method herein, ceramic powder
comprising nanocrystals is mixed with a metallic sintering aid
comprising metallic micro- and/or macro-size particles. Other
sintering aids and binders may also be added to the mixture. During
mixing, the nanocrystalline powders become coated with the
additives using any of the available coating/agglomeration
techniques including, but not limited to, spray drying, fluidized
bed agglomeration methods, dry and wet milling and mechanical
alloying. In one of the preferred embodiments of the present
invention, the additives comprise metallic particulates. In another
embodiment, the additives comprise ceramic nanophases and/or
nanocrystallites such as grain growth inhibitors. These sintering
aids/additives facilitate the thermal sintering/densification
process.
[0034] After mixing, the mixture is then sintered at a temperature
of less than about 1300.degree. C., preferably at a temperature of
less than about 1200.degree. C. and most preferably in the range of
temperatures from about 800.degree. C. to about 1150.degree. C.
[0035] In yet another embodiment, microwave processing is used to
densify the particulate by sintering through the absorption of
microwave energy.
[0036] Processes occurring during the melting of a material
comprising nanoparticles as described above are somewhat similar to
the processes occurring during liquid phase sintering as described
in Fundamentals of Ceramics, M. Barsoum, McGraw Hill (1997). In
both cases, the defining factor is the presence of liquid and
therefore the entire process is controlled by capillary forces. In
contrast however, during liquid phase sintering the liquid phase is
formed due to the addition of sintering aids, and during the
sintering of nanoparticles, formation of the liquid skin on
nanoparticles occurs by the mechanism of surface melting, intrinsic
to nanophase materials. This invention takes advantage of both
intrinsic liquid formation (due to surface melting of
nanoparticles) and extrinisic liquid formation (due to sintering
aids); The nanocrystalline structure of the polycrystalline
particles are combined with sintering aids when they are
agglomerated into granules. The granules themselves are coated with
a coating comprising sintering aids and agents to aid capillary
forces during sintering as well as in the forming of the shape. For
example, handling of the powder/liquid paste-like mixture for
manual wet-condensing of ceramic powder on a refractory die is
primarily controlled by capillary forces.
[0037] Particulate material comprised of nanoparticles may behave
in ways similar to glass-ceramic material due to a very significant
fraction of relatively disordered material on grain boundaries. In
addition, extremely high specific surface energy associated with
nanoparticles greatly increases the driving force for
densification. A high fraction of grain boundaries substantially
alters sintering behavior of nano-sized ceramics compared to that
of conventional micron-sized ceramics. Surface melting of
nanoparticles, resulting in liquid skin formation around
nanoparticles, induces and promotes the mechanisms of sintering
previously associated with liquid phase sintering such as particle
rearrangement and solution-precipitation. At the same time,
grain-boundary diffusion sintering mechanisms are greatly enhanced
due to the enormous surface area of the nanoparticles. Normally,
the presence of agglomerates inhibits densification during
solid-state sintering, however, with added mechanisms of liquid
phase sintering and surface melting of nanoparticles, deliberate
granulation of powder is an essential feature of this invention
that facilitates beneficial capillary effects during green shape
fabrication and sintering.
[0038] It is expected that hand-built restorations will have some
residual porosity after final sintering, however, the architecture
of the powder is designed to minimize this residual porosity and
spatially coordinate it to minimize its adverse effect on
mechanical properties. It is now recognized that porosity is
practically an unavoidable element of microstructure and porous
ceramics are not necessarily weak, as stated in "Fracture Energy of
an Aligned Porous Silicon Nitride," by Y. Inagaki, T. Ohji, S.
Kanzaki and Y. Shigekaki, J. Am. Ceram. Soc., 83 (7), 1807-1809,
(2000). Porosity as an engineered element of the microstructure of
the materials of the present invention can be controlled and
spatially organized through engineered hierarchy of the starting
particulate material. It is well known in the art that the critical
flaw size causing brittle fracture of ceramics often scales with
the particle size of the starting powder. In materials of the
present invention, the powder is preferably spherical in shape
promoting better flowability of the powder. In the powder herein,
the pore size scales with the diameter of interstitial sites formed
between the particles of the powder providing that the powder was
carefully condensed or compacted and attained the maximum green
density of the compact. The pore size and spatial distribution will
be defined by the size and spatial distribution of interstitials
between the particles. For example, for a spherical powder with
particle size distribution shown in curve 1 of FIG. 2 the largest
pore diameter will be defined by the size of the largest
interstitial between the smallest spherical particles, which in
this case is about 20 .mu.m. The largest, octahedral interstitial
in close packed arrangement of 20 .mu.m spheres will be ( 3
-1).mu.m.times.20=0.732.times.20 .mu.m=14.64 .mu.m.
[0039] The equation .sigma.=K.sub.IC/(Ya.sup.1/2) calculates the
strength based on the value of fracture toughness and the critical
flaw size, where [0040] K.sub.IC is the fracture toughness; [0041]
Y is the geometric factor; [0042] .sigma. is the fracture strength;
[0043] is the square root; and [0044] 2a is the equivalent crack
length associated with the critical flaw.
[0045] For a yttria-stabilized tetragonal zirconia polycrystals
(YTZP) material with K.sub.IC of 6-9 MPam.sup.1/2, and a geometric
factor (Y) of 2, strength will most likely be well in excess of 700
MPa.
TABLE-US-00002 TABLE 2 Largest Pore Strength, K.sub.IC, MPa
m.sup.1/2 Size, .mu.m Y* (1.84-2.46) MPa 6 15 2 775 9 15 2 1162
[0046] Nanocrystalline particulate ceramic materials of this
invention are supplied as free-flowing powder, pre-sintered blanks,
feed-stock (for injection molding) and suspensions for slip-casting
or electroforming and using fabrication techniques described below
to provide materials with flexure strength of at least 700 MPa and
exceeding 1 GPa which is more than enough even for multi-unit
posterior restorations and cantilever bridges.
[0047] The following examples illustrate the invention.
Example 1
[0048] Commercially available TZ-3Y-E (which is a yttria-stabilized
tetragonal zirconia powder) powder manufactured by TOSOH
Corporation (Japan) with a crystallite size of about 30 nm can be
sintered to full density at temperatures as low as 1350.degree. C.,
which is 150.degree. C. lower than the sintering temperature for
conventional yttria-stabilized tetragonal zirconia polycrystals
(YTZP) powder. Onset of sintering normally occurs at about 0.6 of
the melting point (Tm) and the MPD effect described above results
in a corresponding decrease in sintering temperature for nanopowder
compared to conventional micron-size ceramic. If the size of the
crystallites is reduced three times to about 10 nm, the anticipated
reduction in the sintering temperature will be about 450.degree.
C., i.e., YTZP powder comprised of 10 nm nanoparticles is
sinterable at about 1050.degree. C. Thus, this powder may be
sintered in a regular burn-out furnace with the maximum operating
temperature of 1100.degree. C. Example 2 below further illustrates
the viability of sintering nanosized zirconia powders to nearly
full density at temperatures lower than 1300.degree. C. and
preferably lower than 1100.degree. C.
Example 2
Low Temperature Sinterability of Nano-Zirconia Powders
[0049] Two commercially available nano-sized3 mol % yttria
stabilized zirconia powders were obtained from NanoProducts
Corporation (Longmont, Colo. 80504, USA). The physical
characteristics of these powders are listed in Table 3. Also listed
are the properties of TZ-3YS-E powder available from Tosoh
Corporation (Tokyo, Japan), a conventional so-called "easy
sintering" 3 mol % yttria-stabilized tetragonal zirconia powder
that was used for baseline comparison. The nano-powders were mixed
with 5-10 wt. % PVA binder (Elvanol 50-42, Dupont) with a mortar
and pestle, and then sieved through an 80 mesh screen resulting in
free-flowing, pressable powder. The conventional zirconia powder
was pressable to begin with, as it contained binder.
[0050] The powders were pressed into pellets using a double action
die and then vacuum bagged and cold isostatically pressed (CIPed)
at 400 MPa to remove any green density gradients. The resulting
pellets were approximately 3 grams in weight and measured about 12
mm in diameter and about 7 mm in height. The green density of
conventional and nanosized zirconia pellets were approximately the
same, 54.+-.0.2% and 52.6.+-.1.2%, respectively. The pellets were
then burned out by heating at a rate of 2.degree. C./m to
700.degree. C. and holding for 2 hours. The pellets were
subsequently sintered at 1200-1300.degree. C. for 2 hours with a
heating and cooling rate of 4.degree. C./m. Density of the sintered
pellets was measured by the Archimedes method using water as the
immersion medium. The percent theoretical density was calculated
using a theoretical density value of 6.05 g/cm.sup.3. These results
are shown in FIG. 3 and demonstrate that at temperatures below
1250.degree. C., the nano-sized zirconia samples exhibit enhanced
sintering behavior at lower temperatures versus the conventional
zirconia sample. For example at 1225.degree. C. the NP-2 pellets
densified to 94.1+0.2% which compares to 86.5+0.2% for the
conventional zirconia sample. This improved sintering behavior is
attributed to the smaller crystallite and particle size of the NP-1
powder (see Table 3). This sintering enhancement due to smaller
crystallites/particles is also reflected by the data at
1200.degree. C., which shows a progressive increase in density from
the conventional zirconia sample (74.5.+-.0.3%) to the
nanparticulate zirconia samples, NP-1 (83.1.+-.0.1%) and NP-2
(85.8.+-.0.1%). To reduce sintering temperatures below 1000.degree.
C. the average particle size should be reduced to below about 8
microns as demonstrated by Kimel and Skandan et al. (cited
above).
TABLE-US-00003 TABLE 3 Crystallite Particle Specific Surface Powder
Size (nm) Size (nm) Area (m.sup.2/g) NP-1; NanoProducts Corp. 10.2
17.2 59.6 (Longmont, CO, USA) product number ZR3N3063 NP-2;
NanoProducts Corp. 8.8 15.3 67.1 (Longmont, CO, USA) product number
ZR3N3269 Conventional 3Y-Zirconia; 36 90 7 .+-. 2 Tosoh product
TZ-3YS-E
[0051] Example 3 further illustrates that some of these nanopowders
can be sintered isotropically using simplified cup shape
geometry.
Example 3
[0052] Using the methods described in example 1, green cylindrical
shaped bodies with dimensions of .about.12 mm diameter by .about.12
mm height were formed out of the NP-2 and conventional zirconia
powders. A 5 mm diameter.times.8 mm deep hole was machined into the
green bodies yielding a "cup" geometry. This geometry was chosen
since it simulated a dental coping. After recording the orthogonal
dimensions, L.sub.(x,y,Z) where x and y refer to the diameters
taken at a 90.degree. rotation to each other and z refers to the
height as illustrated in FIG. 4, the green bodies were burned out
by heating at a rate of 2.degree. C./m to 700.degree. C. and
holding for 2 hours, and subsequently sintering at 1225.degree. C.
for 2 h with a heating and cooling rate of 4.degree. C./m. The
dimensions of the sintered bodies were recorded and the sintering
shrinkage was calculated. These results are shown in FIGS. 5 and 6.
These data show that greater shrinkages were achieved for the
nanosized zirconia powder versus the conventional zirconia, which
agrees with the sintering results shown in Example 2. Additionally,
as manifested by the data points falling on the line of isotropic
shrinkage represented by the dashed lines, these results
demonstrate that like the conventional zirconia the nanosized
zirconia also densifies and shrinks isotropically during
sintering.
Example 4
[0053] Commercially available nano-sized alumina powder AL3N3197
obtained from NanoProducts Corporation (Longmont, Colo. 80504, USA)
was premixed into NP-2 zirconia nano-powder used in Examples 2 and
3 above. The physical characteristics of alumina nano-powder are
compared to NP-2 below:
TABLE-US-00004 TABLE 4 Crystal- Par- Specific Powders from lite
ticle Surface NanoProducts Corp. Size Size Area (Longmont, CO, USA)
Composition (nm) (nm) (m.sup.2/g) NP-3 (product # AL3N3197), 99.9%
Al2O3 4.3 8.6 175.2 Al2O3 nanopowder NP-2 (product # ZR3N3269)
94.7% ZrO2 + 8.8 15.3 67.1 YTZP nanopowder 5.3% Y2O3
[0054] The blend of 0.5 wt % of NP-3 and 99.5 wt % of NP-2
nano-powders was mixed with 5-10 wt % PVA binder (Elvanol 50-42,
Dupont) with a mortar and pestle, and then sieved through an 80
mesh screen resulting in free-flowing, pressable powder. The powder
was vacuum bagged and cold isostatically pressed at 400 MPa to
produce green billets of about 1-2 inches in diameter and about
5-10 inches in length. Following the cold isostatically pressing
step, the outer layer of the billets were removed by turning to
eliminate any green density gradients that may have existed in the
outer layer. The billet was further sectioned into shorter
cylinders of about 30 mm in diameter and about 50-60 mm in height.
The cylinders were then debinderized and pre-sintered to about 50%
theoretical density in a two step firing cycle comprising heating
at the rate of 1.degree. C./minute to about 700.degree. C. and
holding for about 2 hours at this temperature followed by a 2 hour
hold at about 900.degree. C. The attained bisque densities and the
anticipated Bisque-to-Final linear shrinkages were calculated for
each individual block based on diameter and height measurements
before and after pre-sintering.
[0055] The pre-sintered cylinders are subsequently used to mill the
enlarged frameworks for dental restorations. Each framework is
enlarged based on the linear shrinkage factor calculated for each
individual pre-sintered cylinder from which the framework is
milled. The milled frameworks are sintered at 1250.degree. C. for 4
hours with a heating rate of 2.degree. C./minute to densities
exceeding about 95% theoretical density as determined by the
Archimedes method. The isotropic shrinkage in the frameworks is
confirmed by fitting frameworks on the original master model. Some
of the frameworks were layered by 3G Porcelain to demonstrate the
finishing steps typical in fabrication of aesthetic al-ceramic
dental restoration.
[0056] The examples above demonstrate that the selected nanosized
powders exhibiting sinterability below 1300.degree. C. were
consolidated into shapes that were sintered isotropically, i.e.
without distortion wherein
L.sub.x=L.sub.y=L.sub.z=L.sub.Diameter=L.sub.Height, where L is the
linear shrinkage. It was observed that sintering of the compacts of
the nanopowders capable of isotropic sintering results in nearly
fully densified articles with the average grain size noticeably
larger than 100 nm. Two major difficulties in processing dental
articles using nanopowders were revealed: (1) the consolidation of
bulk shapes by conventional methods such as powder compaction and
slip-casting; and (2) sintering to achieve densities in excess of
95%.
[0057] To overcome the first processing obstacle mentioned above,
solid free form manufacturing methods such as rapid prototyping or
solid imaging are utilized indirectly in combination with other
processing techniques such as injection molding/heat-pressing,
various coating or deposition techniques such as gel casting, slip
casting, slurry casting, pressure infiltration, dipping, colloidal
spray deposition, direct coagulation as described in U.S. Pat. Nos.
5,667,548, 5,788,891 and 5,948,335, which are hereby incorporated
by reference, and electroforming or electrophoretic deposition
techniques. While SFF methods are used to fabricate enlarged
substrates, dies and molds, any of the above listed techniques can
be used to form nanoparticulate materials of these invention into
green shapes conforming to these substrates or molds. The
electroforming is a preferred method since it utilizes suspensions
which are particularly beneficial for the nanomaterials herein
described. Many of the nanoparticulate materials described herein
are more readily obtained as well-dispersed suspensions rather than
free-flowing powders. Example 5 illustrates electroforming as the
preferred method of depositing ceramic nanoparticulates onto
enlarged dies produced by one of solid free form manufacturing
methods. Yet another preferred technique is low-pressure injection
molding into negative molds of the rapid prototyped models, or
alternatively existing heat pressing equipment can be used for
pressing into refractory investment molds produced by lost wax
technique. Example 6 illustrates low-pressure injection molding as
another preferred method of forming ceramic nanoparticulates using
enlarged molds produced by one of SFF methods.
[0058] To alleviate the second processing obstacle mentioned above,
the nanopowders herein are agglomerated with sintering aids such as
Si, Al, Mg, Zr, Y, Ce, Ta and mixtures thereof, and grain growth
inhibitors such as Cr, Ti, Ni, Mn and mixtures thereof. Depending
on the subsequent processing steps, these additions can be added in
their elemental (metallic) form or in the form of oxides, salts,
organometallic compounds, or other precursor compounds, in the form
of colloids, powders and specifically nanopowders. To further lower
the melting temperature, the inclusion of the above-mentioned
additives as nanopowders or precursor compounds, is most
preferred.
Example 5
[0059] An optical scanner, ZFN D-21, available from ZFN
(Zahntechnisches Fraszentrum Nord GmbH & Co. KG, Warin,
Germany) is utilized to scan master models (dies) made from
impressions comprising preparations for bridges and crowns.
Three-dimensional CAD software provided with a ZFN D-21 scanner is
used to design frameworks and copings corresponding to these master
models (dies). 3D CAD files (solid models) of these frameworks and
copings are enlarged using the linear shrinkage coefficient
corresponding to the anticipated sintering shrinkage of the
nanozirconia materials of the present invention, saved as
stereolithography (.STL) files and transferred to a computer
interfaced with an RP (Rapid Prototyping) machine such as
Perfactory.RTM. Mini available from Envision Technologies GmbH
(Marl, Germany). This machine utilizes a photostereolithography
process also known as digital light processing (DLP) to build
three-dimensional objects from a light curable resin. Fifteen units
are built at the same time layer by layer with an individual layer
thickness of about 50 microns. Individual units are separated,
attached to copper wire electrodes and coated with conductive
silver paint (silver lacquer) available from Gramm GmbH or Wieland
Dental+Technik GmbH & Co., KG (Pforzheim/Germany). AN
electroforming unit, such as AGC.RTM. Micro Plus (Wieland
Dental+Technik GmbH & Co. KG), is used to deposit a dense layer
of yttria-stabilized zirconia polycrystals (YTZP) from an ethanol
based suspension as described in Example 3 of U.S. Pat. No.
6,059,949, which is hereby incorporated by reference. An
electroforming suspension is prepared by suspending NP-2 zirconia
powder available from NanoProducts Corp. (Longmont, Colo.) in pure
ethanol with addition of 0.05% vol. acetyl acetone dispersant and
0.1% vol. of 5% wt. PVB (polyvinyl butyral binder) in pure ethanol.
Alternatively, an ethanol-based suspension is prepared from an
aqueous suspension comprising tetragonal nano-zirconia particles of
about 8 nm average particle size. First, aqueous suspensions of
YTZP having a crystal size of about 8 nm are prepared via
precipitation from homogeneous solutions using complexation
chemistry techniques. Zirconium and yttrium salts,
ZrO(NO.sub.3).sub.2.xH.sub.2O (zirconyl nitrate, Aldrich Chem.,
Milwaukee, Wis.) and Y(NO.sub.3).sub.2.6H.sub.2O (yttrium nitrate
hexahydrate), Aldrich Chem., Milwaukee, Wis.) are each dissolved in
CO.sub.2-free deionized water in the appropriate amounts to achieve
0.5 M solutions of each. These are then mixed, in the appropriate
ratio to yield the desired mol. % of Y.sub.2O.sub.3 in the final
powder, with the complexing agent bicine (www.sigmaaldrich.com)
(2:1 bicine:Zr (mol)). The pH of this feed solution is adjusted to
about 13 by additions of TEAOH (Tetraethylammonium Hydroxide,
Aldrich Chem., Milwaukee, Wis.), and the solution is then put in a
teflon-lined hydrothermal vessel (Parr Instrument Company, Moline,
Ill.), which is heated to 200.degree. C. for 8 hours to
hydrothermally synthesize YTZP crystals of about 8 nm in size.
[0060] Aqueous suspensions are converted into alcohol-based
suspensions by centrifuging and then redispersing in ethanol. The
average thickness of the electrophoretic coating is about 0.5-0.6
mm. Following electroforming of the powders onto the substrates,
sintering is carried out in a Deltech furnace using a two-step
firing cycle comprising heating rate of 1.degree. C./min to about
450.degree. C., holding at this temperature for 2 hours to remove
organics, further heating at a rate of 1.degree. C./min. to
900.degree. C.-1100.degree. C. and holding at this temperature for
about 2 hours. Densities in excess of 90% of theoretical density
can be achieved.
Example 6
Low-Pressure Injection Molding (LPIM) with Peltsman Unit
[0061] An optical scanner, ZFN D-21, available from ZFN
(Zahntechnisches Fraszentrum Nord GmbH & Co. KG (Warin,
Germany) is utilized to scan master models (dies) made from
impressions comprising preparations for bridges and crowns. 3D CAD
software provided with a ZFN D-21 scanner is used to design
frameworks and copings corresponding to these master models (dies).
3D CAD files (solid models) of these frameworks and copings are
enlarged using the linear shrinkage coefficient corresponding to
the anticipated sintering shrinkage of the nanozirconia materials
of the present invention, saved as stereolithography (.STL) files
and transferred to a computer interfaced with an RP (Rapid
Prototyping) machine such as Perfactory.RTM. Mini available from
Envision Technologies GmbH (Marl, Germany). This machine utilizes
photostereolithography process also known as digital light
processing (DLP) to build 3D objects from a light curable resin.
Fifteen units are built at the same time layer by layer with an
individual layer thickness of about 50 microns. Individual units
are separated and molded in a liquid silicone rubber (Silastic.RTM.
M RTV Silicone Rubber from Dow Corning Corporation) which is
casiable and easily demolded after curing to produce negative molds
for low-pressure injection molding. It should be noted that instead
of using silicone negative molds, the molds for LPIM can be
designed and fabricated directly using the Perfactory.RTM. Mini RP
machine and the supplied software.
[0062] Feedstock containing nanosized zirconia for injection
molding is prepared from a binder comprised of 75 wt % of paraffin
wax (melting point of 49'-52.degree. C.), 10 wt % of polyethylene
wax (melting point of 80.degree.-90.degree. C.), 10% of carnauba
wax (melting point of 80.degree.-87.degree. C.), 2 wt % of stearic
acid (melting point of 75.degree. C.) and 3 wt % of oleic acid
(melting point of 16.degree. C.) readily available from a number of
suppliers. Nanosized zirconia having a crystallite size of about 19
nm and particle size of about 15 nm (available as Product Number
ZR3N3269 from NanoProducts Corp., Longmont, Colo. 80504, USA) is
used. The mixing is done directly in a low pressure molding (LPM)
machine, (Model MIGL-33 available from Peltsman Corporation,
Minneapolis, Minn.) at a temperature of 90.degree. C. The feedstock
mixture is comprised of about fifty percent (50%) by volume of a
binder. Once the feed stock mixture is thoroughly mixed it is
injected into the cavity of the silicone rubber molds at a pressure
of approximately 0.4 MPa and a temperature of approximately
90.degree. C. The injection-molded green part is then demolded from
the silicone mold, which is done easily due to elasticity of the
silicone. Green densities of approximately 50% were achieved. The
green bodies were debinderized and sintered to nearly full density
as described above.
Example 7
Injection Molding with Autopress
[0063] Feedstock containing nanosized zirconia for injection
molding is prepared from a binder comprised of paraffin wax, with
minor proportions of polyethylene wax, carnauba wax, stearic and
oleic acids using the same formulation as used in Example 6.
Nanosized zirconia having a crystallite size of about 19 nm and a
particle size of about 15 nm (available as Product Number ZR3N3269
from NanoProducts Corp., Longmont, Colo. 80504, USA) is used. The
mixing is done in a KitchenAid Professional 5 mixer (St. Joseph,
Mich.) in a bowl continuously heated to 90.degree. C., which is
above the melting point of the binder. Heating is achieved using a
high temperature heat tape available from McMaster-Carr (New
Brunswick, N.J.). The heat tape is wrapped around the mixer bowl to
provide heat to the bowl. After cooling to room temperature, the
resulting mix is crushed into powder to a 60 mesh (250 .mu.m)
particle size using a mortar and pestle. This powder is then ready
for injection into the cavity of a mold. Additionally, the mix can
be cast into pellets by pouring into a metal "clam-shell" mold,
while still in the molten state.
[0064] Previously acquired stereolithography (*.STL) files of
bridge frameworks and crown copings were sent to microTEC,
Bismarckstrasse 142 b 47057 Duisburg, Germany) for production of
the enlarged replicas using RMPD.RTM.-mask technology via toll
rapid prototyping service available through microTEC's website. The
replicas were fabricated in a layer thickness of twenty five
microns (25 .mu.m) from photo-curable resin.
[0065] The resulting replicas are invested in Universal.TM.
Refractory Investment (available from Pentron.RTM. Laboratory
Technologies, LLC, Wallingford, Conn.). After the investment has
hardened, the resin replicas inside are eliminated by placing it
into a preheated furnace thereby burning off the resin, resulting
in a mold cavity for forming the dental article. The injection
molding feedstock, in free-flowing granule or pellet form, as
described above, is then placed into the investment mold assembly,
which is then transferred into the pressing unit. It is pressed
into the investment ring using an AutoPressPlus.RTM. (Pentron.RTM.
Laboratory Technologies, LLC, Wallingford, Conn.). having an
external alumina plunger Pressing is done at approximately
90.degree. C., and after cooling the pressed green part is then
carefully divested by sand-blasting with glass beads at a pressure
of 15 psi and the plunger and mold are disposed of. Green densities
of approximately 50% were achieved. The green bodies were
debinderized and sintered to nearly full density as described
above.
[0066] It should be noted that in all the cases described in
Examples 1-7 it was observed that while the ceramic portion of the
starting powder, suspension or feedstock consists of crystallites
with average sizes of less than 20 nm, the sintered dental articles
have average grain sizes within the range from about 100 nm to
about 450 nm. It is the nature of the materials of the present
invention to exhibit substantial coarsening concurrent with
densification wherein the final grain size is about 10-20 times
larger than the starting crystallite size.
[0067] Though not within the scope of the present invention which
is directed towards sintering ceramic dental articles comprising
nanopowders to nearly full density, nevertheless, it should be
noted that the injection molding technology described in Examples 6
and 7 can be used to produce dental articles even if access to RP
machines is not available. In the latter case, if is not possible
to make enlarged replicas and green bodies fabricated therefrom as
described in Examples 6 and 7, the articles will have to be
presintered without shrinkage and glass infiltrated as described in
U.S. Pat. Nos. 4,772,436 and 5,910,273, which are hereby
incorporated by reference. In the case of YTZP zirconia cores, 3 G
porcelain can be used for both glass infiltration and esthetic
layering of the resulting glass-infiltrated cores.
[0068] While various descriptions of the present invention are
described above, it should be understood that the various features
can be used singly or in any combination thereof. Therefore, this
invention is not to be limited to only the specifically preferred
embodiments depicted herein.
[0069] Further, it should be understood that variations and
modifications within the spirit and scope of the invention may
occur to those skilled in the art to which the invention pertains.
Accordingly, all expedient modifications readily attainable by one
versed in the art from the disclosure set forth herein that are
within the scope and spirit of the present invention are to be
included as further embodiments of the present invention. The scope
of the present invention is accordingly defined as set forth in the
appended claims.
* * * * *