U.S. patent application number 12/551822 was filed with the patent office on 2010-04-01 for optical material and optical element.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Kentaro Doguchi, Michio Endo, Shigeru Fujino, Kohei Nakata.
Application Number | 20100081561 12/551822 |
Document ID | / |
Family ID | 42058079 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100081561 |
Kind Code |
A1 |
Nakata; Kohei ; et
al. |
April 1, 2010 |
OPTICAL MATERIAL AND OPTICAL ELEMENT
Abstract
Provided is an optical element, which is formed by
vacuum-sintering a molded body of ceramic particles having an
average particle diameter of 1 .mu.m or more and 10 .mu.m or less
and including Ln.sub.xAl.sub.yO.sub.[x+y].times.1.5 (Ln represents
a rare-earth element, x represents 1.ltoreq.x.ltoreq.10, and y
represents 1.ltoreq.y.ltoreq.5). Ln preferably includes at least
one kind selected from La, Gd, Yb, and Lu. The optical element
preferably has a refractive index of 1.85 or more and 2.06 .mu.m or
less, and an Abbe number of 48 or more and 65 or less. The optical
element having optical properties of high refractive index and low
dispersibility is obtained.
Inventors: |
Nakata; Kohei;
(Utsunomiya-shi, JP) ; Endo; Michio;
(Utsunomiya-shi, JP) ; Doguchi; Kentaro;
(Utsunomiya-shi, JP) ; Fujino; Shigeru;
(Kasuga-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
1290 Avenue of the Americas
NEW YORK
NY
10104-3800
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
42058079 |
Appl. No.: |
12/551822 |
Filed: |
September 1, 2009 |
Current U.S.
Class: |
501/152 |
Current CPC
Class: |
C04B 35/62897 20130101;
C04B 35/62665 20130101; C04B 2235/528 20130101; C04B 2235/5436
20130101; Y10T 428/252 20150115; C04B 2235/3224 20130101; C04B
35/44 20130101; C04B 2235/3225 20130101; C04B 2235/9646 20130101;
C04B 2235/6567 20130101; C04B 2235/6581 20130101; C04B 2235/656
20130101; C04B 35/62813 20130101; C04B 2235/3227 20130101; G02B
1/00 20130101; C04B 2235/604 20130101; C04B 35/62884 20130101; Y10T
428/2993 20150115 |
Class at
Publication: |
501/152 |
International
Class: |
C04B 35/50 20060101
C04B035/50 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2008 |
JP |
2008-255080 |
Claims
1. An optical element, comprising a vacuum-sintered body comprising
a plurality of ceramic particles, wherein the ceramic particles
each comprise Ln.sub.xAl.sub.yO.sub.[x+y].times.1.5, where Ln
represents a rare-earth element, x represents 1.ltoreq.x.ltoreq.10,
and y represents 1.ltoreq.y.ltoreq.5, and have an average particle
diameter of 1 .mu.m or more and 10 .mu.m or less.
2. The optical element according to claim 1, wherein Ln comprises
at least one kind selected from La, Gd, Yb, and Lu.
3. The optical element according to claim 1, which has a refractive
index of 1.85 or more and 2.06 .mu.m or less, and an Abbe number of
48 or more and 65 or less.
4. An optical element, comprising a vacuum-sintered body comprising
a plurality of particles each having a two-layer structure
comprising a ceramic particle and a coating layer, wherein the
ceramic particle comprises Ln.sub.xAl.sub.yO.sub.[x+y].times.1.5,
where Ln represents a rare-earth element, x represents
1.ltoreq.x.ltoreq.10, and y represents 1.ltoreq.y.ltoreq.5, and has
an average particle diameter of 1 .mu.m or more and 10 .mu.m or
less; and the coating layer comprises a ceramic having a lower
sintering temperature than a sintering temperature of the ceramic
particle.
5. The optical element according to claim 4, wherein the coating
layer comprises glass.
6. The optical element according to claim 4, wherein Ln comprises
at least one kind selected from Y, La, Gd, Yb, and Lu.
7. The optical element according to claim 4, which has a refractive
index of 1.85 or more and 2.06 .mu.m or less, and an Abbe number of
48 or more and 65 or less.
8. The optical element according to claim 4, wherein the particle
having a two-layer structure has a spherical shape.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an optical element, in
particular, a highly accurate optical element used for a lens or
the like.
[0003] 2. Description of the Related Art
[0004] In recent years, the production of cameras including digital
cameras has been increasing, and hence, there have been demanded
optical lenses having higher performance. In particular, for
enhancing the optical performance of a camera or the like, there is
required an optical material having high refractive index and low
dispersibility.
[0005] The high refractive index and low dispersibility, which are
absent in the conventional optical glass, can be realized by using
a crystal material, and in order to use the crystal material as a
material which has a good transmittance and is suitable for optical
applications, there has been a method involving using a
single-crystal material or a method involving sintering crystal
particles and using the resultant.
[0006] On the other hand, there have been problems that the
single-crystal material is extremely expensive, and it is difficult
to obtain a material which has a large diameter and is suitable for
an optical lens. That is, in the method involving sintering crystal
particles, when the particle diameter is large, a large grain
boundary occurs during sintering to cause decrease in a
transmittance as an optical lens, and a defect occurs on a lens
surface due to a grain boundary at the time of processing the
material into a lens shape, and hence, it has been difficult to
obtain a good optical lens.
[0007] In Japanese Patent Application Laid-Open No. H06-056514,
there is disclosed, as crystal particles each having a small
particle diameter, an example of light transmissive ceramics having
a crystal particle diameter of 100 nm or less. However, in the
process of sintering crystal particles each having a diameter of
100 nm or less, the handling thereof was extremely difficult due to
the small bulk density at the time of forming a preliminary molded
body before sintering. On the other hand, of the ceramics having
optical properties of high refractive index and low dispersibility,
there are many substances which have high sintering temperature and
are hence accompanied by difficulties during the process of
sintering.
SUMMARY OF THE INVENTION
[0008] The present invention has been accomplished in view of the
problems of the above-mentioned related art, and it is, therefore,
an object of the present invention to provide an optical element
having optical properties of high refractive index and low
dispersibility.
[0009] A first optical element for solving the above-mentioned
problems is formed by vacuum-sintering a molded body of ceramic
particles having an average particle diameter of 1 .mu.m or more
and 10 .mu.m or less and comprising
Ln.sub.xAl.sub.yO.sub.[x+y].times.1.5 (Ln represents a rare-earth
element, x represents 1.ltoreq.x.ltoreq.10, and y represents
1.ltoreq.y.ltoreq.5).
[0010] A second optical element for solving the above-mentioned
problems is formed by vacuum-sintering a molded body of particles
having a two-layer structure, the particles being formed by
coating, on surfaces of ceramic particles having an average
particle diameter of 1 .mu.m or more and 10 .mu.m or less and
comprising Ln.sub.xAl.sub.yO.sub.[x+y].times.1.5 (Ln represents a
rare-earth element, x represents 1.ltoreq.x.ltoreq.10, and y
represents 1.ltoreq.y.ltoreq.5), a coating layer comprising a
ceramic having a lower sintering temperature than a sintering
temperature of the ceramic particles.
[0011] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawing.
BRIEF DESCRIPTION OF THE DRAWING
[0012] The FIGURE is a cross-sectional view illustrating an optical
element according to the present invention.
DESCRIPTION OF THE INVENTION
[0013] The optical element of the present invention is formed by
vacuum-sintering, by using specific ceramic particles, crystal
particles at a lower temperature than a temperature during an
ordinary sintering process, and has optical properties of high
refractive index and low dispersibility without any defects. The
optical element of the present invention can be applied to a lens
and a prism used for various optical systems.
First Embodiment
[0014] An optical element according to Example 1 of the present
invention is formed by vacuum-sintering a molded body of ceramic
particles having an average particle diameter of 1 .mu.m or more
and 10 .mu.m or less and including
Ln.sub.xAl.sub.yO.sub.[x+y].times.1.5 (Ln represents a rare-earth
element, x represents 1.ltoreq.x.ltoreq.10, and y represents
1.ltoreq.y.ltoreq.5).
[0015] The above Ln includes rare-earth elements, and specific
examples thereof include Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,
Ho, Er, Tm, Yb, and Lu. Of those rare-earth elements, it is
particularly preferred that Ln be at least one kind or more
selected from La, Gd, Yb, and Lu.
[0016] Furthermore, the average particle diameter of the ceramic
particles is desirably 1 .mu.m or more and 10 .mu.m or less. When
the average particle diameter is less than 0.1 .mu.m, the particles
are too fine and partial agglomeration occurs, and hence, it is
difficult to sufficiently densify the particles at the time of
pressurizing, and the optical element after sintering has remaining
bubbles, and hence, the resultant element is unsuitable to be used
as an optical element. When the average particle diameter exceeds
10 .mu.m, a void is easily formed at the time of pressurizing and a
grain boundary easily occurs in the crystal obtained after
sintering, and hence, detachment of the particles occurs during
polishing, so that an optical element having a satisfactory surface
cannot be obtained.
[0017] The shape of the ceramic particle is preferably spherical.
As the particle changes from a spherical shape to an irregular
shape, a void is more easily formed at the time of pressurizing and
a grain boundary more easily occurs in the crystal obtained after
sintering, and hence, a satisfactory optical element cannot be
obtained. Incidentally, the spherical shape preferably has the
following relationship: longitudinal diameter of cross-sectional
shape of sphere/transverse diameter of cross-sectional shape of
sphere=1.+-.0.1. Furthermore, the optical element preferably has a
refractive index of 1.8 or more and has transmissivity.
[0018] A method of producing the optical element of the present
invention is as follows. First, spherical ceramic particles having
an average particle diameter of 1 .mu.m or more and 10 .mu.m or
less are prepared by a method such as plasma melting. Next, the
spherical ceramic particles are subjected to casting, dry molding,
or wet molding to thereby prepare a preliminary molded body.
Furthermore, the preliminary molded body is sintered under a
vacuum, followed by grinding and polishing processes, and as a
result, an optical element for an optical lens or the like is
obtained.
[0019] When the molded body of ceramic particles is
vacuum-sintered, air bubbles present in the voids between the
ceramic particles can be removed and the quality of the optical
element can be maintained, and hence, the optical element is a
preferred mode. In particular, in the present invention, crystal
particles are vacuum-sintered at a lower temperature than the
temperature during an ordinary sintering process, and hence, an
optical element having optical properties of high refractive index
and low dispersibility without any defects can be obtained. The
degree of vacuum in vacuum-sintering is preferably 0.1 Pa or
less.
Example 1
[0020] First, La.sub.2O.sub.3, Gd.sub.2O.sub.3, Yb.sub.2O.sub.3,
Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were oxide raw materials
each having a purity of 99.9% or more were prepared. The raw
materials were each adjusted to have a proportion in such a manner
that there can be formed ceramic particles having the compounds
shown in Sample Nos. 1 to 8 in Table 1, and the oxide raw materials
were mixed.
[0021] The raw materials were introduced into thermal plasma, and
were heated and melted, followed by cooling, by a thermal plasma
method for obtaining fine particles, and as a result, spherical
particles having an average particle diameter of 1 .mu.m were
obtained. At that time, the heating temperature was set at
1,500.degree. C. or higher and 3,200.degree. C. or lower. When the
heating temperature was lower than 1,500.degree. C., the melting
was not performed sufficiently and the spherical particles were not
obtained. When the heating temperature exceeded 3,200.degree. C.,
the volatilization of the raw materials occurred, and there were
obtained only the particles in spherical shape each having a small
particle diameter.
[0022] The spherical particles were subjected to dry molding under
a pressure of 9,800,000 Pa (100 kgf) to 196,000,000 Pa (2,000 kgf),
and as a result, a preliminary molded body having a diameter of 20
mm and a thickness of 2 mm was obtained. The preliminary molded
body was sintered at a temperature range of 1,100.degree. C. or
higher and 1,500.degree. C. or lower shown in the table below under
a vacuum of 10.sup.-1 Pa or lower. Incidentally, the time taken for
sintering was set to 6 hours or more and 24 hours or less. The
obtained sintered body was ground and polished, to thereby obtain
an optical element having a thickness of 1 mm as shown in the
FIGURE.
[0023] Table 1 shows the measurement results of the refractive
index and the Abbe number of the obtained optical element.
Incidentally, the Abbe number is a value which represents the
dispersibility. Each optical element had optical properties of high
refractive index and low dispersibility. Furthermore, when the
surface thereof was observed with an optical microscope, detachment
of surface particles or a flaw on the surface caused in the
polishing step was absent, and hence, the satisfactory optical
element was obtained.
[0024] (Measurement Method)
[0025] (1) Refractive Index
[0026] The refractive index is represented by a value (nd)
determined by measuring a refractive index at a wavelength of 587
nm using a Pulfrich refractometer (trade name "KPR-2000";
manufactured by Shimadzu Device Corporation).
[0027] (2) Abbe Number
[0028] The Abbe number .upsilon.d is represented by a value
determined by measuring refractive indices nd, nF, and nC at
wavelengths of 587 nm, 486 nm, and 656 nm using a Pulfrich
refractometer, and then performing calculation using the equation:
.upsilon.d=(nd-1)/(nF-nC).
TABLE-US-00001 TABLE 1 Sample No. 1 No. 2 No. 3 No. 4 Ceramic
LaAlO.sub.3 La.sub.0.5Gd.sub.0.5AlO.sub.3 GdAlO.sub.3
Yb.sub.3Al.sub.5O.sub.12 particles Refractive 2.06 2.03 2.02 2.00
index Abbe number 56 52 50 48 Sintering 1,200.degree. C.
1,150.degree. C. 1,100.degree. C. 1,350.degree. C. temperature
Sample No 5 No. 6 No. 7 Ceramic Lu.sub.3Al.sub.5O.sub.12
La.sub.3Al.sub.5O.sub.12 La.sub.10Al.sub.4O.sub.21 particles
Refractive 1.94 1.85 2.08 index Abbe number 65 58 54 Sintering
1,150.degree. C. 1,250.degree. C. 1,200.degree. C. temperature
Example 2
[0029] La.sub.2O.sub.3, Gd.sub.2O.sub.3, Yb.sub.2O.sub.3,
Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were oxide raw materials
each having a purity of 99.9% or more were prepared. The raw
materials were each adjusted to have a proportion in such a manner
that there can be formed ceramic particles having the compounds
shown in Sample Nos. 1 to 9 in Table 1, and the oxide raw materials
were mixed.
[0030] The raw materials were introduced into thermal plasma, and
were heated and melted, followed by cooling, by a thermal plasma
method for obtaining fine particles, and as a result, spherical
particles having an average particle diameter of 3 .mu.m were
obtained. At that time, the heating temperature was set at
1,500.degree. C. or higher and 3,000.degree. C. or lower. When the
heating temperature was lower than 1,500.degree. C., the melting
was not performed sufficiently and the spherical particles were not
obtained. When the heating temperature exceeded 3,000.degree. C.,
the volatilization of the raw materials occurred, and there were
obtained only the particles in spherical shape each having a small
particle diameter.
[0031] The spherical particles were sintered by dry molding under a
vacuum in the same manner as in Example 1, and the obtained
sintered body was ground and polished, to thereby obtain a sample
having a thickness of 1 mm.
[0032] The refractive index and the Abbe number of the obtained
optical element were optically measured. The results were the same
as those shown in Table 1, and the optical element had optical
properties of a high refractive index and a low dispersibility.
Furthermore, when the surface thereof was observed with an optical
microscope, detachment of surface particles or a flaw on the
surface caused in the polishing step was absent, and hence, the
satisfactory optical element was obtained.
Example 3
[0033] La.sub.2O.sub.3, Gd.sub.2O.sub.3, Yb.sub.2O.sub.3,
Lu.sub.2O.sub.3 and Al.sub.2O.sub.3 which were oxide raw materials
each having a purity of 99.9% or more were prepared. The raw
materials were each adjusted to have a proportion in such a manner
that there can be formed ceramic particles having the compounds
shown in Sample Nos. 1 to 9 in Table 1, and the oxide raw materials
were mixed.
[0034] The raw materials were introduced into thermal plasma, and
were heated and melted, followed by cooling, by a thermal plasma
method for obtaining fine particles, and as a result, spherical
particles having an average particle diameter of 10 .mu.m were
obtained. At that time, the heating temperature was set at
1,500.degree. C. or higher and 3,000.degree. C. or lower. When the
heating temperature was lower than 1,500.degree. C., the melting
was not performed sufficiently and the spherical particles were not
obtained. When the heating temperature exceeded 3,000.degree. C.,
the volatilization of the raw materials occurred, and there were
obtained only the particles in spherical shape each having a small
particle diameter.
[0035] The spherical particles were sintered by dry molding under a
vacuum in the same manner as in Example 1, and the obtained
sintered body was ground and polished, to thereby obtain a sample
having a thickness of 1 mm.
[0036] The refractive index and the Abbe number of the obtained
optical element were optically measured. The results were the same
as those shown in Table 1, and the optical element had optical
properties of a high refractive index and a low dispersibility.
Furthermore, when the surface thereof was observed with an optical
microscope, detachment of surface particles or a flaw on the
surface caused in the polishing step was absent, and hence, the
satisfactory optical element was obtained.
Comparative Example 1
[0037] La.sub.2O.sub.3, Gd.sub.2O.sub.3, Yb.sub.2O.sub.3,
Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were oxide raw materials
each having a purity of 99.9% or more were prepared. The raw
materials were each adjusted to have a proportion in such a manner
that there can be formed the compounds shown in Sample Nos. 1 to 8
in Table 1, and the oxide raw materials were mixed.
[0038] The raw materials were introduced into thermal plasma, and
were heated and melted, followed by cooling, by a thermal plasma
method for obtaining fine particles, and as a result, spherical
particles having an average particle diameter of 0.1 .mu.m were
obtained. At that time, the heating temperature was set at
3,500.degree. C. or higher.
[0039] The spherical particles were subjected to dry molding under
a pressure of 9,800,000 Pa (100 kgf) to 196,000,000 Pa (2,000 kgf),
and as a result, a preliminary molded body having a diameter of 20
mm and a thickness of 2 mm was obtained. The preliminary molded
body was sintered at a temperature range of 1,100.degree. C. or
higher and 1,500.degree. C. or lower shown in the table below under
a vacuum of 10.sup.-1 Pa or lower. Incidentally, the time taken for
sintering was set to 6 hours or more and 24 hours or less. The
obtained sintered body was ground and polished, to thereby obtain a
sample having a thickness of 1 mm.
[0040] When the obtained optical element was observed with an
optical microscope, there were a large number of bubbles formed in
the element, and hence, the obtained optical element was unsuitable
to be used as an optical element.
Comparative Example 2
[0041] La.sub.2O.sub.3, Gd.sub.2O.sub.3, Yb.sub.2O.sub.3,
Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were oxide raw materials
each having a purity of 99.9% or more were prepared. The raw
materials were each adjusted to have a proportion in such a manner
that there can be formed the compounds shown in Sample Nos. 1 to 7
in Table 1, and the oxide raw materials were mixed.
[0042] The raw materials were introduced into thermal plasma, and
were heated and melted, followed by cooling, by a thermal plasma
method for obtaining fine particles, and as a result, spherical
particles having an average particle diameter of 100 .mu.m were
obtained. At that time, the heating temperature was set at
1,500.degree. C. or higher and 3,200.degree. C. or lower.
[0043] The spherical particles were subjected to dry molding under
a pressure of 9,800,000 Pa (100 kgf) to 196,000,000 Pa (2,000 kgf),
and as a result, a preliminary molded body having a diameter of 20
mm and a thickness of 2 mm was obtained. The preliminary molded
body was sintered at a temperature range of 1,100.degree. C. or
higher and 1,500.degree. C. or lower shown in the table below under
a vacuum of 10.sup.-1 Pa or lower. Incidentally, the time taken for
sintering was set to 6 hours or more and 24 hours or less. The
obtained sintered body was ground and polished, to thereby obtain a
sample having a thickness of 1 mm.
[0044] When the obtained optical element was observed with an
optical microscope, there were a large number of detachments of
surface particles and flaws on the surface caused in the polishing
step, and hence, the obtained optical element was unsuitable to be
used as an optical element.
Second Embodiment
[0045] An optical element according to Example 2 of the present
invention is formed by vacuum-sintering a molded body of particles
each having a two-layer structure, the particles being formed by
coating surfaces of ceramic particles having an average particle
diameter of 1 .mu.m or more and 10 .mu.m or less and including
Ln.sub.xAl.sub.yO.sub.[x+y].times.1.5 (Ln represents a rare-earth
element, x represents 1.ltoreq.x.ltoreq.10, and y represents
1.ltoreq.y.ltoreq.5) with a coating layer including a ceramic
having a lower sintering temperature than a sintering temperature
of the ceramic particles.
[0046] The optical element according to Example 2 of the present
invention has a feature in that there are used the particles each
having a two-layer structure formed by coating the surfaces of
ceramic particles with a layer formed of a ceramic having a lower
sintering temperature than a sintering temperature of the ceramic
particles. The ceramic particles used here are the same as the
ceramic particles used for the first optical element.
[0047] It is preferred that the ceramic having a lower sintering
temperature than a sintering temperature of the ceramic particles
be crystal or glass.
[0048] Furthermore, it is preferred that the particles having the
two-layer structure have spherical shapes. Furthermore, it is
preferred that the coating layer which coats the layer formed of a
ceramic having a lower sintering temperature than the sintering
temperature of the ceramic particles have a thickness of 0.1 .mu.m
or more and 1 .mu.m or less. Furthermore, as a method of forming
the coating layer on the surface of the ceramic particles, a plasma
melting method is employed.
[0049] A method of producing the optical element of the present
invention is as follows. First, spherical ceramic particles having
an average particle diameter of 1 .mu.m or more and 10 .mu.m or
less are prepared by a method such as plasma melting. Next, the
spherical ceramic particles are subjected to casting, dry molding,
or wet molding to thereby prepare a preliminary molded body.
[0050] Next, particles each having a two-layer structure and a
spherical shape, in which the surfaces of the spherical ceramic
particles having an average particle diameter of 1 .mu.m or more
and 10 .mu.m or less is coated with a coating layer, are prepared
by a method such as plasma melting. Still further, the particles
each having a two-layer structure are subjected to casting, dry
molding, or wet molding to thereby prepare a preliminary molded
body.
[0051] Finally, the preliminary molded body is sintered under a
vacuum, followed by grinding and polishing steps, and as a result,
an optical element such as an optical lens is obtained.
Incidentally, the vacuum-sintering method used here is the same as
the vacuum-sintering method used for the first optical element.
Example 4
[0052] Y.sub.2O.sub.3, La.sub.2O.sub.3, Gd.sub.2O.sub.3,
Yb.sub.2O.sub.3, Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were
oxide raw materials each having a purity of 99.9% were prepared.
The raw materials were each adjusted to have a proportion in such a
manner that there can be formed ceramic particles having the
compounds shown in Sample Nos. 11 to 19 in Table 2, and the oxide
raw materials were mixed.
[0053] The raw materials were introduced into a central part of
thermal plasma, and a 1:1 mixture of Gd.sub.2O.sub.3 and
Al.sub.2O.sub.3 was introduced into a peripheral part thereof, and
simultaneously, they were heated and melted, followed by cooling,
by a thermal plasma method for obtaining fine particles, to thereby
obtain spherical particles having an average particle diameter of 1
.mu.m and an average thickness of GdAlO.sub.3 as a coating layer of
0.1 .mu.m. At that time, the heating temperature was set to
1,500.degree. C. or higher and 3,200.degree. C. or lower. When the
heating temperature was lower than 1,500.degree. C., the melting
was not performed sufficiently and the spherical particles were not
obtained. When the heating temperature exceeded 3,200.degree. C.,
the volatilization of the raw materials occurred, and there were
obtained only the particles in spherical shape each having a small
particle diameter.
[0054] The spherical particles were subjected to dry molding under
a pressure of 9,800,000 Pa (100 kgf) to 196,000,000 Pa (2,000 kgf),
and as a result, a preliminary molded body having a diameter of 20
mm and a thickness of 2 mm was obtained. The preliminary molded
body was sintered at 1,100.degree. C. under a vacuum of 10.sup.-1
Pa or lower. Incidentally, the sintering temperature was
1,100.degree. C. and the time taken for sintering was set to 6
hours or more and 24 hours or less. The obtained sintered body was
ground and polished, to thereby obtain an optical element having a
thickness of 1 mm.
[0055] Table 2 shows the measurement results of the refractive
index and the Abbe number of the obtained optical element. Each
optical element had optical properties of a high refractive index
and a low dispersibility. Furthermore, when the surface thereof was
observed with an optical microscope, detachment of surface
particles or a flaw on the surface caused in the polishing step was
absent, and hence, the satisfactory optical element was
obtained.
[0056] As shown in Table 2, when GdAlO.sub.3 having a sintering
temperature of 1,100.degree. C. was provided as a coating layer on
the periphery of each of the ceramic particles of LaAlO.sub.3,
La.sub.0.5Gd.sub.0.5AlO.sub.3, Yb.sub.3Al.sub.5O.sub.12,
Lu.sub.3Al.sub.5O.sub.12, La.sub.3Al.sub.5O.sub.12,
Y.sub.4Al.sub.2O.sub.9, and La.sub.10Al.sub.4O.sub.21, each having
a sintering temperature of 1,150.degree. C. to 1,500.degree. C.,
the sintering temperature of each of the two-layered particles
could be lowered to 1,100.degree. C.
TABLE-US-00002 TABLE 2 Sample No. 11 No. 12 No. 13 No. 14 No. 15
Ceramic Y.sub.3Al.sub.5O.sub.12 LaAlO.sub.3
La.sub.0.5Gd.sub.0.5AlO.sub.3 GdAlO.sub.3 Yb.sub.3Al.sub.5O.sub.12
particles Coating layer GdAlO.sub.3 GdAlO.sub.3 GdAlO.sub.3
GdAlO.sub.3 GdAlO.sub.3 Refractive index 1.83 2.06 2.03 2.02 2.00
Abbe number 56 56 52 50 48 Sintering 1,500.degree. C. 1,200.degree.
C. 1,150.degree. C. 1,100.degree. C. 1,350.degree. C. temperature
Sample No. 16 No. 17 No. 18 No. 19 Ceramic Lu.sub.3Al.sub.5O.sub.12
La.sub.3Al.sub.5O.sub.12 Y.sub.4Al.sub.2O.sub.9
La.sub.10Al.sub.4O.sub.21 particles Coating layer GdAlO.sub.3
GdAlO.sub.3 GdAlO.sub.3 GdAlO.sub.3 Refractive index 1.94 1.85 1.93
2.08 Abbe number 65 58 54 54 Sintering 1,150.degree. C.
1,250.degree. C. 1,500.degree. C. 1,200.degree. C. temperature
[0057] The sintering temperatures of the samples shown in the above
table are all 1,100.degree. C.
Example 5
[0058] Y.sub.2O.sub.3, La.sub.2O.sub.3, Gd.sub.2O.sub.3,
Yb.sub.2O.sub.3, Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were
oxide raw materials each having a purity of 99.9% were prepared.
The raw materials were each adjusted to have a proportion in such a
manner that there can be formed ceramic particles having the
compounds shown in Sample Nos. 11 to 19 in Table 2, and the oxide
raw materials were mixed.
[0059] The raw materials were introduced into a central part of
thermal plasma, and a 1:1 mixture of Gd.sub.2O.sub.3 and
Al.sub.2O.sub.3 was introduced into a peripheral part thereof, and
simultaneously, they were heated and melted, followed by cooling,
by a thermal plasma method for obtaining fine particles, to thereby
obtain spherical particles having an average particle diameter of 3
.mu.m and an average thickness of GdAlO.sub.3 as a coating layer of
0.3 .mu.m. At that time, the heating temperature was set to
1,500.degree. C. or higher and 3,000.degree. C. or lower. When the
heating temperature was lower than 1,500.degree. C., the melting
was not performed sufficiently and the spherical particles were not
obtained. When the heating temperature exceeded 3,000.degree. C.,
the volatilization of the raw materials occurred, and there were
obtained only the particles in spherical shapes each having a small
particle diameter.
[0060] The spherical particles were subjected to dry molding under
a pressure of 9,800,000 Pa (100 kgf) to 196,000,000 Pa (2,000 kgf),
and as a result, a preliminary molded body having a diameter of 20
mm and a thickness of 2 mm was obtained. The preliminary molded
body was sintered at 1,100.degree. C. under a vacuum of 10.sup.-1
Pa or lower. Incidentally, the time taken for sintering was set to
6 hours or more and 24 hours or less. The obtained sintered body
was ground and polished, to thereby obtain an optical element
having a thickness of 1 mm.
[0061] The refractive index and the Abbe number of the obtained
optical element were optically measured, and, in the same manner as
the results shown in Table 2, the optical element had optical
properties of a high refractive index and a low dispersibility.
Furthermore, when the surface thereof was observed with an optical
microscope, detachment of surface particles or a flaw on the
surface caused in the polishing step was absent, and hence, the
satisfactory optical element was obtained.
[0062] When GdAlO.sub.3 having a sintering temperature of
1,100.degree. C. was provided as a coating layer on the periphery
of each of Y.sub.3Al.sub.5O.sub.12, LaAlO.sub.3,
La.sub.0.5Gd.sub.0.5AlO.sub.3, Yb.sub.3Al.sub.5O.sub.12,
Lu.sub.3Al.sub.5O.sub.12, La.sub.3Al.sub.5O.sub.12,
Y.sub.4Al.sub.2O.sub.9, and La.sub.10Al.sub.4O.sub.21, each having
a sintering temperature of 1,150.degree. C. to 1,500.degree. C.,
the sintering temperature of each of the two-layered particles
could be lowered to 1,100.degree. C.
Example 6
[0063] Y.sub.2O.sub.3, La.sub.2O.sub.3, Gd.sub.2O.sub.3,
Yb.sub.2O.sub.3, Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were
oxide raw materials each having a purity of 99.9% were prepared.
The raw materials were each adjusted to have a proportion in such a
manner that there can be formed ceramic particles having the
compounds shown in Sample Nos. 11 to 19 in Table 2, and the oxide
raw materials were mixed.
[0064] The raw materials were introduced into a central part of
thermal plasma, and a 1:1 mixture of Gd.sub.2O.sub.3 and
Al.sub.2O.sub.3 was introduced into a peripheral part thereof, and
simultaneously, they were heated and melted, followed by cooling,
by a thermal plasma method for obtaining fine particles, to thereby
obtain spherical particles having an average particle diameter of
10 .mu.m and an average thickness of GdAlO.sub.3 as a coating layer
of 1 .mu.m. At that time, the heating temperature was set to
1,500.degree. C. or higher and 3,000.degree. C. or lower. When the
heating temperature was lower than 1,500.degree. C., the melting
was not performed sufficiently and the spherical particles were not
obtained. When the heating temperature exceeded 3,000.degree. C.,
the volatilization of the raw materials occurred, and there were
obtained only the particles in spherical shapes each having a small
particle diameter.
[0065] The spherical particles were subjected to dry molding under
a pressure of 9,800,000 Pa (100 kgf) to 196,000,000 Pa (2,000 kgf),
and as a result, a preliminary molded body having a diameter of 20
mm and a thickness of 2 mm was obtained. The preliminary molded
body was sintered at 1,100.degree. C. under a vacuum of 10.sup.-1
Pa or lower. Incidentally, the time taken for sintering was set to
6 hours or more and 24 hours or less. The obtained sintered body
was ground and polished, to thereby obtain an optical element
having a thickness of 1 mm.
[0066] The refractive index and the Abbe number of the obtained
optical element were optically measured, and, in the same manner as
the results shown in Table 2, the optical element had optical
properties of a high refractive index and a low dispersibility.
Furthermore, when the surface thereof was observed with an optical
microscope, detachment of surface particles or a flaw on the
surface caused in the polishing process was absent, and hence, the
satisfactory optical element was obtained.
[0067] When GdAlO.sub.3 having a sintering temperature of
1,100.degree. C. was provided as a coating layer on the periphery
of each of Y.sub.3A.sub.5O.sub.12, LaAlO.sub.3,
La.sub.0.5Gd.sub.0.5AlO.sub.3, Yb.sub.3Al.sub.5O.sub.12,
Lu.sub.3Al.sub.5O.sub.12, La.sub.3Al.sub.5O.sub.12,
Y.sub.4Al.sub.2O.sub.9, and La.sub.10Al.sub.4O.sub.21, each having
a sintering temperature of 1,150.degree. C. to 1,500.degree. C.,
the sintering temperature of each of the two-layered particles
could be lowered to 1,100.degree. C.
Example 7
[0068] Y.sub.2O.sub.3, La.sub.2O.sub.3, Gd.sub.2O.sub.3,
Yb.sub.2O.sub.3, Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were
oxide raw materials each having a purity of 99.9% were prepared.
The raw materials were each adjusted to have a proportion in such a
manner that there can be formed ceramic particles having the
compounds shown in Sample Nos. 11 to 19 in Table 2, and the oxide
raw materials were mixed.
[0069] Furthermore, a glass having a composition as shown in Table
3 and properties as shown in Table 4 was prepared as a coating
layer. The raw materials were introduced into a central part of
thermal plasma, and the glass having the composition as shown in
Table 3 and the properties as shown in Table 4 was introduced into
a peripheral part, and simultaneously, they were heated and melted,
followed by cooling, by a thermal plasma method for obtaining fine
particles, to thereby obtain spherical particles having an average
particle diameter of 10 .mu.m and an average thickness of the glass
as a coating layer of 1 .mu.m. At that time, the heating
temperature was set to 1,500.degree. C. or higher and 3,000.degree.
C. or lower. When the heating temperature was lower than
1,500.degree. C., the melting was not performed sufficiently and
the spherical particles were not obtained. When the heating
temperature exceeded 3,000.degree. C., the volatilization of the
raw materials occurred, and there were obtained only the particles
in spherical shape each having a small particle diameter.
[0070] The spherical particles were subjected to dry molding under
a pressure of 9,800,000 Pa (100 kgf) to 196,000,000 Pa (2,000 kgf),
and as a result, a preliminary molded body having a diameter of 20
mm and a thickness of 5 mm was obtained.
[0071] The preliminary molded body was sintered at a temperature of
750.degree. C. or higher under a vacuum of 10.sup.-1 Pa or lower.
The time taken for sintering was set to 1 hour or more and 4 hours
or less. The obtained sintered body was ground and polished, to
thereby obtain a sample having a thickness of 3 mm.
[0072] The refractive index and the Abbe number of the obtained
optical element were optically measured, and, in the same manner as
the results shown in Table 2, the optical element had optical
properties of a high refractive index and a low dispersibility.
Furthermore, when the surface thereof was observed with an optical
microscope, detachment of surface particles or a flaw on the
surface caused in the polishing step was absent, and hence, the
satisfactory optical element was obtained.
[0073] When the glass having a glass transition temperature of
600.degree. C. was provided as a coating layer on the periphery of
the ceramic having a high sintering temperature, the sintering
temperature of each of the two-layered particles could be lowered
to 750.degree. C.
TABLE-US-00003 TABLE 3 Glass composition (weight %) B.sub.2O.sub.3
Ga.sub.2O.sub.3 Gd.sub.2O.sub.3 La.sub.2O.sub.3 Li.sub.2O
Nb.sub.2O.sub.5 WO.sub.3 SiO.sub.2 Ta.sub.2O.sub.5 ZnO ZrO.sub.2
15.0 6.0 7.0 35.0 1.0 2.0 1.5 5.0 17.0 5.0 5.5
TABLE-US-00004 TABLE 4 Refractive index Abbe number Glass
transition point 1.843 40.7 600
Comparative Example 3
[0074] Y.sub.2O.sub.3, La.sub.2O.sub.3, Gd.sub.2O.sub.3,
Yb.sub.2O.sub.3, Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were
oxide raw materials each having a purity of 99.9% or more were
prepared. The raw materials were each adjusted to have a proportion
in such a manner that there can be formed ceramic particles having
the compounds shown in Sample Nos. 11 to 19 in Table 2, and the
oxide raw materials were mixed.
[0075] The raw materials were introduced into a central part of
thermal plasma, and a 1:1 mixture of Gd.sub.2O.sub.3 and
Al.sub.2O.sub.3 was introduced into a peripheral part, and
simultaneously, they were heated and melted, followed by cooling,
by a thermal plasma method for obtaining fine particles, to thereby
obtain spherical particles having an average particle diameter of
0.1 .mu.m and an average thickness of GdAlO.sub.3 as a coating
layer of 0.01 .mu.m. At that time, the heating temperature was set
to 3,500.degree. C. or higher.
[0076] The spherical particles were subjected to dry molding under
a pressure of 9,800,000 Pa (100 kgf) to 196,000,000 Pa (2,000 kgf),
and as a result, a preliminary molded body having a diameter of 20
mm and a thickness of 2 mm was obtained. The preliminary molded
body was sintered at a temperature ranging from 1,100.degree. C. or
higher to 1,500.degree. C. or lower shown in the table below under
a vacuum of 10.sup.-1 Pa or lower. Incidentally, the time taken for
sintering was set to 6 hours or more and 24 hours or less. The
obtained sintered body was ground and polished, to thereby obtain a
sample having a thickness of 1 mm.
[0077] When the obtained optical element was observed with an
optical microscope, there were a large number of bubbles formed in
the element, and hence, the obtained optical element was unsuitable
to be used as an optical element.
Comparative Example 4
[0078] Y.sub.2O.sub.3, La.sub.2O.sub.3, Gd.sub.2O.sub.3,
Yb.sub.2O.sub.3, Lu.sub.2O.sub.3, and Al.sub.2O.sub.3 which were
oxide raw materials each having a purity of 99.9% or more were
prepared. The raw materials were each adjusted to have a proportion
in such a manner that there can be formed ceramic particles having
the compounds shown in Sample Nos. 11 to 19 in Table 2, and the
oxide raw materials were mixed.
[0079] The raw materials were introduced into a central part of
thermal plasma, and the glass having the composition as shown in
Table 3 and the properties as shown in Table 4 was introduced into
a peripheral part, and simultaneously, they were heated and melted,
followed by cooling, by a thermal plasma method for obtaining fine
particles, to thereby obtain spherical particles having an average
particle diameter of 100 .mu.m and an average thickness of the
glass as a coating layer of 10 .mu.m.
[0080] The spherical particles were subjected to dry molding under
a pressure of 9,800,000 Pa (100 kgf) to 196,000,000 Pa (2,000 kgf),
and as a result, a preliminary molded body having a diameter of 20
mm and a thickness of 5 mm was obtained.
[0081] The preliminary molded body was sintered at a temperature of
750.degree. C. or higher under a vacuum of 10.sup.-1 Pa or lower.
The time taken for sintering was set to 1 hour or more and 4 hours
or less. The obtained sintered body was ground and polished, to
thereby obtain a sample having a thickness of 3 mm.
[0082] The surface of the obtained optical element was observed
with an optical microscope, there were a large number of
detachments of surface particles and flaws on the surface caused in
the polishing step, and hence, the obtained optical element was
unsuitable to be used as an optical element.
[0083] In the present invention, although the optical physical
properties of the central part are different from the optical
physical properties of the coating layer, the thickness of the
coating layer was one tenth of the diameter of the particle and the
cross-sectional area ratio was 1:100, and hence, the optical
physical properties of the obtained optical element were
approximately equal to those of the central ceramic.
[0084] The present invention is not limited to the above examples.
For instance, as raw materials, there may be used a composite oxide
such as La.sub.3Al.sub.5O.sub.12, and besides oxides, there may be
also used carbonates and nitrates. The production of the
preliminary molded body can be also performed by casting or wet
molding. In the production, a small amount of an organic binder may
be added thereto.
[0085] As the rare-earth elements to be used, in addition to Y, La,
Gd, Yb, and Lu, there can also be used Ce, Pr, Nd, Pm, Sm, Eu, Gd,
Tb, Dy, Ho, Er, and Tm.
[0086] Furthermore, in place of the two-stage processes of dry
molding and vacuum heating, hot isostatic pressing (HIP) can be
performed to thereby shorten the time taken for heating to 3 to 24
hours.
[0087] A preliminary molded body and a sintered body each having a
diameter of 20 mm or more and a thickness of mm or more can be also
produced. Furthermore, there could be produced a preliminary molded
body and a sintered body each having a size of a diameter of 20 mm
or more and a thickness of 5 mm or more. In addition, as a coating
layer, there can be also used glass having a composition other than
that of the examples of the present invention.
[0088] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0089] This application claims the benefit of Japanese Patent
Application No. 2008-255080, filed Sep. 30, 2008, which is hereby
incorporated by reference herein in its entirety.
* * * * *