U.S. patent application number 09/783602 was filed with the patent office on 2001-10-04 for method for preparation of sintered body of rare earth oxide.
Invention is credited to Kaneyoshi, Masami.
Application Number | 20010027159 09/783602 |
Document ID | / |
Family ID | 18582384 |
Filed Date | 2001-10-04 |
United States Patent
Application |
20010027159 |
Kind Code |
A1 |
Kaneyoshi, Masami |
October 4, 2001 |
Method for preparation of sintered body of rare earth oxide
Abstract
Disclosed is a method for the preparation of a high-quality
sintered body of a rare earth oxide or a composite oxide of a rare
earth oxide and an adjuvant oxide such as aluminum oxide. The
method comprises shaping a rare earth oxide powder characterized by
specified particle diameter distribution values of D.sub.50 and
D.sub.90 and a specified specific surface area or a powder blend of
the rare earth oxide and adjuvant oxide into a powder compact and
subjecting the powder compact to a sintering heat treatment at a
specified sintering temperature by increasing and decreasing the
temperature up to and from the sintering temperature each at a rate
not exceeding a specified upper limit.
Inventors: |
Kaneyoshi, Masami;
(Fukui-ken, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
18582384 |
Appl. No.: |
09/783602 |
Filed: |
February 15, 2001 |
Current U.S.
Class: |
501/152 |
Current CPC
Class: |
C04B 35/44 20130101;
C04B 35/50 20130101 |
Class at
Publication: |
501/152 |
International
Class: |
C04B 035/50 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 7, 2000 |
JP |
2000-62361 |
Claims
What is claimed is:
1. A method for the preparation of a sintered body of a rare earth
oxide which comprises the steps of: (a) shaping a powder of the
rare earth oxide, of which the D.sub.50 value of the particle
diameter distribution does not exceed 2.0 .mu.m, the D.sub.90 value
of the particle diameter distribution does not exceed 3.0 .mu.m and
the specific surface area is in the range from 5 to 20 m.sup.2/g,
into a powder compact; and (b) subjecting the powder compact to a
heat treatment for sintering at a sintering temperature of
1000.degree. C. or higher for at least 1 hour, in which the rate of
temperature elevation in the range from 500.degree. C. to the
sintering temperature does not exceed 500.degree. C. per hour and
the rate of temperature decrease from the sintering temperature
does not exceed 600.degree. C. per hour.
2. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the D.sub.50 value of
the particle diameter distribution of the rare earth oxide powder
does not exceed 1.5 .mu.m.
3. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the D.sub.50 value of
the particle diameter distribution of the rare earth oxide powder
is in the range from 0.9 to 1.3 .mu.m.
4. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the D.sub. value of the
particle diameter distribution of the rare earth oxide powder does
not exceed 2.7 .mu.m.
5. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the D.sub.90 value of
the particle diameter distribution of the rare earth oxide powder
is in the range from 1.9 to 2.3 .mu.m.
6. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the specific surface
area of the rare earth oxide powder is in the range from 7 to 18
m.sup.2/g.
7. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the specific surface
area of the rare earth oxide powder is in the range from 10 to 15
m.sup.2/g.
8. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the D'.sub.50 value of
the pore diameter distribution of the rare earth oxide powder does
not exceed 20 nm.
9. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the sintering
temperature is in the range from 1200 to 1900.degree. C.
10. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the sintering
temperature is in the range from 1400 to 1800.degree. C.
11. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which shaping of the rare
earth oxide powder into a powder compact is conducted by
hydrostatic compression.
12. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 11 in which the pressure of
hydrostatic compression is at least 100 MPa.
13. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 11 in which the pressure of
hydrostatic compression is at least 150 MPa.
14. The method for the preparation of a sintered body of a rare
earth oxide as claimed in claim 1 in which the rate of temperature
elevation is in the range from 150 to 400.degree. C. per hour.
15. A method for the preparation of a sintered body of a rare earth
oxide-based composite oxide with a n adjuvant oxide which comprises
the steps of: (a) blending a powder of the rare earth oxide, of
which the D.sub.50 value of the particle diameter distribution does
not exceed 2.0 .mu.m, the D.sub.90 value of the particle diameter
distribution does not exceed 3.0 .mu.m and the specific surface
area is in the range from 5 to 20 m.sup.2/g, and an adjuvant oxide
of an element selected from the group consisting of magnesium,
aluminum, silicon, titanium, vanadium, chromium, manganese, iron,
cobalt, nickel, copper, zinc, gallium, germanium, zirconium,
niobium, molybdenum, indium, tin, hafnium, tantalum and tungsten to
give a powder blend; (b) shaping the powder blend into a powder
compact; and (c) subjecting the powder compact to a heat treatment
for sintering at a sintering temperature of 1000.degree. C. or
higher for at least 1 hour, in which the rate of temperature
elevation in the range from 500.degree. C. to the sintering
temperature does not exceed 500.degree. C. per hour and the rate of
temperature decrease from the sintering temperature does not exceed
600.degree. C. per hour.
16. The method for the preparation of a sintered body of a rare
earth oxide-based composite oxide with an adjuvant oxide as claimed
in claim 15 in which the amount of the rare earth oxide powder in
the powder blend is at leasr 40% by weight based on the total
amount of the powder blend.
17. The method for the preparation of a sintered body of a rare
earth oxide-based composite oxide with an adjuvant oxide as claimed
in claim 15 in which the adjuvant oxide is selected from the group
consisting of the oxides of iron, aluminum, silicon, titanium,
gallium and zirconium.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method for the
preparation of a sintered body of a rare earth oxide. More
particularly, the invention relates to a method for the preparation
of a sintered body of a rare earth oxide having a large sintering
density and a small average crystallite diameter. The invention
also relates to a method for the preparation of an oxide mixture
mainly consisting of a rare earth oxide with a non-rare earth
adjuvant oxide, which crystallographically consists of a single
phase.
[0002] As is well known, sintered bodies of a rare earth oxide
generally have an outstandingly high corrosion resistance against
halogen gases or halogen-containing gases and melt of a metal or
alloy. By virtue of this unique property, applications in various
fields can be expected for the articles of a sintered rare earth
oxide body. Besides, sintered bodies of a rare earth oxide are each
a potential material in the applications as a dielectric material,
magnetic material, optical functional material and so on. For
example, yttrium aluminum garnet, referred to as YAG hereinafter,
having a chemical composition of Y.sub.3Al.sub.5O.sub.12 as a
composite oxide of yttrium and aluminum belongs
crystallographically to the cubic system having isotropy and a
high-density sintered body of YAG, which is nothing other than a
polycrystalline body, may exhibit high transmissivity to visible
light close to that of a single crystal or glassy body of YAG.
Further, sintered bodies of a composite oxide consisting of a rare
earth oxide, such as oxides of yttrium, dysprosium and terbium, and
iron oxide have an application as a material of magnetooptical
devices.
[0003] It is important in most applications of a sintered body of
rare earth oxides in order to exhibit the inherently high
performance that the sintered body has a sintering density, i.e.
the actual density of the sintered body relative to the true
density of the oxide, as close to the true density of the oxide as
possible and that the sintered body consists of a
crystallographically single phase. These desirable characteristics
of a sintered body of a rare earth oxide largely depend on the
physical properties of the starting oxide particles and it is
generally a very difficult matter to obtain a sintered body of a
rare earth oxide even by undertaking improvements and optimization
of the process conditions for the preparation of a sintered
body.
SUMMARY OF THE INVENTION
[0004] The present invention accordingly has an object, in view of
the above described problems and difficulties in the prior art
methods for the preparation of a sintered body of a rare earth
oxide or an oxide mixture mainly composed of a rare earth oxide or
a rare earth oxide-based oxide mixture, to provide a novel and
reliable method for the preparation of a sintered body of a rare
earth oxide or a rare earth oxide-based oxide mixture having a
large sintering density and consisting of a crystallographically
single phase with a small average crystal-lite diameter.
[0005] Thus, the method of the present invention for the
preparation of a sintered body of a rare earth oxide comprises the
steps of:
[0006] (a) molding a powder of a rare earth oxide, of which the
D.sub.50 value of the particle diameter distribution does not
exceed 2.0 .mu.m, the D.sub..pi.value of the particle diameter
distribution does not exceed 3.0 .mu.m and the specific surface
area is in the range from 5 to 20 m.sup.2/g, into a powder compact;
and
[0007] (b) subjecting the powder compact to a heat treatment for
sintering at a temperature of 1000.degree. C. or higher, in which
the rate of temperature elevation does not exceed 500.degree. C.
per hour and the rate of temperature decrease does not exceed
600.degree. C. per hour.
[0008] It is preferable in the above defined inventive method that
the D'.sub.50 value of the pore diameter distribution of the rare
earth oxide particles does not exceed 20 nm.
[0009] It is further optional that the starting rare earth oxide
powder defined above is admixed with an adjuvant oxide of an
element selected from the group consisting of magnesium, aluminum,
silicon, titanium, vanadium, chromium, manganese, iron, cobalt,
nickel, copper, zinc, gallium, germanium, zirconium, niobium,
molybdenum, indium, tin, hafnium, tantalum and tungsten in a
limited proportion so as to give a sintered body of a rare
earth-based composite oxide. When a sintered body is prepared from
an oxide mixture of a rare earth oxide and one or more of these
adjuvant oxides, it is desirable that the amount of the rare earth
oxide is at least 40% by weight based on the total amount of the
oxide mixture.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0010] The above defined present invention has been completed as a
result of the extensive investigations undertaken by the inventor
on the relationship between the physical properties of the rare
earth oxide powder as the starting material and the physical
properties of a sintered body of the rare earth oxide obtained from
the oxide powder. The investigations have led to a discovery that a
sintered body of a rare earth oxide having the most desirable
properties can be obtained when the starting rare earth oxide
powder has optimum values of the parameters including the average
particle diameter, particle diameter distribution, specific surface
area and pore diameter distribution.
[0011] Thus, the starting rare earth oxide powder, which is
compression-molded into a powder compact in step (a) of the
inventive method, should have the particle diameter distribution
values D.sub.50 and D.sub.90, which can be determined by the laser
diffraction method, not exceeding 2.0 .mu.m and 3.0 .mu.m,
respectively, and a specific surface area in the range from 5
m.sup.2/g to 20 m.sup.2/g. The above mentioned particle diameter
distribution value expressed by the symbol D.sub.n is defined in
such a way that the particles having a particle diameter not
exceeding D.sub.n, e.g., in .mu.m unit, constitute n% by weight of
the whole powder.
[0012] The rare earth elements forming a rare earth oxide, to which
the inventive preparation method is applicable, include yttrium and
the elements having an atomic number in the range from 57 to 71 on
the Periodic Table. Each of the rare earth oxides, excepting the
oxides of cerium, praseodymium and terbium, has a chemical
composition of the formula R.sub.2O.sub.3, in which R is the rare
earth element, while the oxides of the above mentioned cerium,
praseodymium and terbium are expressed usually by the formulas of
CeO.sub.2, Pr.sub.6O.sub.11 and Tb.sub.4O.sub.7, respectively.
These rare earth oxide powders can be used either singly or as a
blend of two kinds or more according to need. The method of the
present invention is applicable more successfully to the oxides of
yttrium, gadolinium, dysprosium, holmium, erbium, thulium,
ytterbium and lutetium or, in particular, to yttrium oxide among
the above mentioned rare earth oxides.
[0013] As is mentioned before, the rare earth oxide powder as the
starting material in the inventive method must satisfy the
requirements for the granulometric parameters including the
particle diameter distribution values, specific surface area and,
desirably, pore diameter distribution. When the particle diameter
distribution of a powder is measured by the laser diffraction
method, the center value of distribution and presence of coarse
particles must be taken into consideration.
[0014] The D.sub.50 value of the starting rare earth oxide powder
according to the above given definition should not exceed 2.0 .mu.m
or, preferably, should not exceed 1.5 .mu.m or, more preferably,
should be in the range from 0.9 to 1.3 .mu.m. When the D.sub.50
value of the starting rare earth oxide powder is too large, the
process of sintering cannot proceed as desired not to give a high
sintering density of the sintered body unless the sintering
temperature is unduly increased. When a sintered body of a rare
earth-based composite oxide with an adjuvant oxide is to be
prepared, in addition, the reaction between the oxides can hardly
be complete with coarse rare earth oxide particles not to give a
crystallographically uniform sintered body which eventually
comprises undesired oxide phases and the unreacted starting oxide
phases.
[0015] The D.sub.90 value of the starting rare earth oxide powder
according to the above given definition should not exceed 3.0 .mu.m
or, preferably, should not exceed 2.7 .mu.m or, more preferably,
should be in the range from 1.9 to 2.3 .mu.m. When the D.sub.90
value is too large, the disadvantage caused thereby is similar to
that caused with a too large D.sub.50 value mentioned above. In
addition, the average crystallite diameter cannot be small enough
with a large variation of the diameters adversely affecting the
mechanical strengths of the sintered body. The crystallite diameter
of a sintered body can be determined from an electron-microscopic
photograph of a section of the sintered body along with an
electron-microscopic examination of the surface of the section.
[0016] The specific surface area of the rare earth oxide powder can
be determined by the so-called BET method by measuring the volume
of nitrogen gas adsorbed on the unit amount of the powder at the
boiling point of liquid nitrogen, i.e. -196.degree. C. The starting
rare earth oxide powder should have a specific surface area in the
range from 5 to 20 m.sup.2/g or, preferably, from 7 to 18 m.sup.2/g
or, more preferably, from 10 to 15 m.sup.2/g. When the specific
surface area of the starting rare earth oxide powder is too small,
the oxide particles have low reactivity so that the process of
sintering cannot proceed as desired and the reaction with the
adjuvant oxide particles can hardly proceed. When the specific
surface area is too large, on the other hand, local unevenness of
sintering is sometimes unavoidable with partial oversintering and
the crystallite diameter in the sintered body cannot be fine enough
with a large variation or unevenness sometimes leaving closed pores
at the grain boundaries in addition to the problem that a
single-phase sintered body can hardly be obtained in a sintered
body of a rare earth oxide-based composite oxide with an adjuvant
oxide.
[0017] A further granulometric parameter to be determined of the
starting rare earth oxide powder is the pore diameter distribution
value D'.sub.50 which should not exceed 20 nm or, preferably,
should be in the range from 10 to 20 nm. When the D'.sub.50 value
of the starting rare earth oxide powder is too large, the sintering
behavior of the oxide powder is adversely affected. The pore
diameter distribution value D'.sub.50 is defined in such a way that
50% of the overall pore volume of the particles is occupied by the
pores of which the pore diameter does not exceed D'.sub.50 (nm).
The D'.sub.50 value can be determined by the so-called BJH method
from the adsorption and desorption behavior of nitrogen gas on and
from the oxide powder under varied pressures.
[0018] Following is a description of steps of the inventive method
for the preparation of a sintered body of a rare earth oxide or of
a rare earth oxide-based composite oxide. When the target product
is a sintered body of a rare earth oxide-based composite oxide, the
first step is mixing of the rare earth oxide powder and an adjuvant
oxide powder as intimately as possible by using a suitable
powder-mixing tool such as mortars and pestles or, preferably, by
using a powder-mixing machine such as a ball mill either as a
dry-blending process or as a wet-blending process with admixture of
water or an organic solvent or, preferably, water from the
standpoint of safety and environmental pollution. When the
wet-process powder mixing is undertaken, the wetting water or
solvent is usually removed from the wet mixture or slurry of the
powders by evaporation to give a dried powder blend.
[0019] The first of the essential steps in the inventive method is
molding of a rare earth oxide powder or, when the above described
powder blending has been undertaken, the powder blend into a powder
compact by a suitable molding method such as compression molding in
a metal mold, so-called slip casting using a mold of a
liquid-absorbent material such as gypsum and hydrostatic
compression method. When the slip casting method is undertaken with
a powder blend, it is of course that the wet powder blend or slurry
obtained by the wet-process mixing need not be dried into a dried
powder blend. The method of hydrostatic compression is advantageous
because the density of the powder compact obtained by this method
is high as compared with the other molding methods consequently
resulting in a small shrinkage of the powder compact in the
subsequent step of sintering. The pressure of the hydrostatic
compression medium in the hydrostatic compression molding method
should be at least 100 MPa or, desirably, at least 150 MPa.
[0020] The step to follow the above described molding step of the
rare earth oxide powder or a powder blend is a heat treatment of
the powder compact for sintering which is conducted in an electric
furnace under an atmosphere of the atmospheric air, non-reactive
gas or reducing gas or in vacuum depending on the types of the
desired sintered bodies. Most conveniently, the atmosphere for
sintering can be the atmospheric air when the sintered body to be
obtained is that of a single rare earth oxide or of a rare earth
oxide-based composite oxide including rare earth aluminum garnets
R.sub.3Al.sub.5O.sub.12 such as YAG, rare earth iron garnets
R.sub.3Fe.sub.5O.sub.12 such as yttrium iron garnet (YIG), yttrium
titanate Y.sub.2Ti.sub.2O.sub.7 of the pyrochlore type and so
on.
[0021] The highest temperature to be reached in the heat treatment
for sintering, referred to as the sintering temperature
hereinafter, is, though dependent on the types of the sintered body
to be obtained, 1000.degree. C. or higher or, preferably, in the
range from 1200 to 1900.degree. C. or, more preferably, in the
range from 1400 to 1800.degree. C. When the sintering temperature
is too low, sintering of the powder compact cannot proceed
completely as a matter of course while sintering at a too high
temperature is unavoidably accompanied by a disadvantage due to
premature degradation of the heater elements installed in the
electric furnace by the evaporation of vaporizable constituents
therein.
[0022] The length of time for keeping the powder compact under
sintering at the sintering temperature is desirably at least one
hour in order to accomplish complete sintering of the body. It is
preferable in the heat treatment that the rate of temperature
elevation to reach the sintering temperature does not exceed
500.degree. C. per hour or, preferably, is in the range from 150 to
400.degree. C. per hour, desirably, at least in the temperature
range from 500.degree. C. to the sintering temperature. When the
rate of temperature elevation is too high, the sintered body may
eventually suffer a defect such as cracks and chippings while a
rate of temperature elevation smaller than 150.degree. C. per hour
has no particular advantages thereby rather with an economical
disadvantage due to a decrease in the productivity. The cooling
rate of the body after the heat treatment at the sintering
temperature down to room temperature or, desirably, at least down
to 500.degree. C. should not exceed 600.degree. C. per hour. When
the sintered body is cooled down too rapidly, the sintered body
eventually suffers defects of deformation, cracks and
chippings.
[0023] In the following, the present invention is described in more
detail by way of examples and comparative examples, which, however,
never limit the scope of the invention in any way.
[0024] In the experiments described below, characterization of the
rare earth oxide powders was made for the items including the
particle diameter distribution, specific surface area and pore
diameter distribution according to the testing procedures given
below.
[0025] The particle diameter distribution of the oxide powders was
measured by the laser diffraction method by using an instrument
therefor (Model Microtrac FRA 9220, manufactured by Leeds &
Northrup Co.) to give the values of D.sub.10, D.sub.50 and D.sub.
.mu.m units. The BET specific surface area and the pore diameter
distribution of the oxide powders were determined by the BJH method
using an instrument for gas adsorption and desorption measurements
(Model Coulter SA3100, manufactured by Coulter Electronics Co.) to
give the specific surface area in m.sup.2/g units and to give the
values of D'.sub.50 in nm units.
[0026] Experiment 1
[0027] Powders of yttrium oxide were taken from four different lots
A, B, C and D of yttrium oxide products and they were subjected to
characterization as mentioned above to give the results summarized
in Table 1 below.
1 TABLE 1 Particle diameter Specific Pore diameter Yttrium
distribution, .mu.m surface distribution, oxide, lot D.sub.10
D.sub.50 D.sub.90 area, m.sup.2/g D'.sub.50 , nm A 0.69 1.10 2.15
13.2 17.3 B 0.94 1.74 4.13 39.1 31.7 C 0.65 1.16 3.22 12.1 22.8 D
1.67 3.51 6.27 7.9 38.7
[0028] A 100 g portion taken from each of these yttrium oxide
powders A to D was introduced into a rubber mold having an inner
diameter of 50 mm and tightly sealed therein with a rubber stopper
to be subjected to hydrostatic molding in a hydrostatic press under
a pressure of 200 MPa. The thus hydrostatically molded powder
compacts taken out of the rubber mold were subjected to a sintering
heat treatment in an electric furnace under an atmosphere of air at
a sintering temperature of 1700.degree. C. for 4 hours. The rate of
temperature elevation up to this sintering temperature was
300.degree. C. per hour and the cooling rate from 1700.degree. C.
down to room temperature was also 300.degree. C. per hour.
[0029] The thus obtained sintered bodies of yttrium oxide by uaing
the yttrium oxide powders A, B, C and D, referred to as the
sintered bodies 1A, 1 B, 1 C and 1 D, respectively, were electron
microscopically examined for the surface. The average crystallite
diameters were determined on the electron microscopic photographs
to give the results shown in Table 2 below. The sintered bodies
were subjected to the measurement of the density to give the
results in Table 2 as a relative density in % which is the ratio of
the sintering density to the theoretical density 5.03 g/cm.sup.3 of
yttrium oxide.
2TABLE 2 Sintered Relative Average crystallite body density, %
diameter, nm 1A 99.7 9.2 1B 99.5 26 1C 99.4 19 1D 98.2 14
[0030] The results of Table 2 indicate that the sintered body 1A
among the four had the highest sintering density and smallest
average crystallite diameter. In fact, the sintered body 1A had the
highest mechanical strength and heat-shock resistance and was free
from occurrence of any noticeable cracks or fissures on the
surface.
[0031] Experiment 2
[0032] Powder blends of yttrium oxide and aluminum oxide in a molar
ratio of 3:5 corresponding to the chemical composition of YAG were
prepared each by ball-milling 57.06 g of one of the yttrium oxide
powders A to D used in Experiment 1 and 42.94 g of an aluminum
oxide powder having a D.sub.50 value of about 0.3 .mu.m and a
D.sub.90 value of about 1.1 .mu.m (Taimicron TM-DA, a product by
Taimei Chemical Co.) with addition of 100 ml of water for 3 hours
in an alumina pot containing alumina balls of about 5 mm diameter
followed by removal of the alumina balls from the slurry by
screening and drying of the slurry to give a dried cake of the
powder blend which was lightly disintegrated with a mortar and
pestle into a powder to serve as the base material for the
preparation of sintered bodies.
[0033] These powder blends were each subjected to hydrostatic
molding and a sintering heat treatment in substantially the same
manner as in Experiment 1 except that the sintering temperature was
1600.degree. C. instead of 1700.degree. C. The thus obtained
sintered bodies are referred to hereinafter as the sintered bodies
2A, 2B, 2C and 2D corresponding to the yttrium oxide powders A, B,
C and D, respectively.
[0034] Each of the sintered bodies 2A to 2D was subjected to the
measurement of the density by the in-water weighing method to give
the relative density in %, which was the ratio of the sintering
density of the sintered body to the theoretical density 4.55
g/cm.sup.3 of YAG, as shown in Table 3 below.
[0035] According to the results of the powder X-ray diffractometry
undertaken with the sintered bodies, the sole or major constituent
phase of the sintered bodies was YAG which was, in some samples,
accompanied by other minor phases including the phase of
YAlO.sub.3, referred to as YAP hereinafter, the phase of
Y.sub.4Al.sub.2O.sub.9, referred to as YAM hereinafter, and the
phase of yttrium oxide Y.sub.2O.sub.3.
[0036] A 15 mm square and 3 mm thick plate specimen was taken by
cutting each of the sintered bodies 2A to 2D with polishing of the
square surfaces. A quantitative X-ray diffractometric analysis was
undertaken with the thus finished square specimens to determine the
contents of the yttrium atoms in the respective crystallographic
phases from the intensities of the diffraction peaks by making
reference to authentic standard specimens to give the results shown
in Table 3 below for the phases of YAG, YAP, YAM and
Y.sub.2O.sub.3.
[0037] The above prepared surface-polished square specimens were
subjected to the measurement of light transmissivity for light of
550 nm wavelength on a spectrophotometer. The results in %
transmission are shown in Table 3 below.
3TABLE 3 % Yttrium atoms contained in the Light Relative Sintered
phase of transmission, density, body YAG YAP YAM Y.sub.2O.sub.3 % %
2A 100 0 0 0 68 99.8 2B 91 6 1 1 49 99.9 2C 94 3 0 0 44 99.7 2D 87
7 2 2 36 99.8
[0038] The sintered body 2A is characteristic in consisting of a
single crystallographic phase of YAG and having a high light
transmissivity as compared with the other sintered bodies so that
this material could be employed as a material of windows of special
lamps.
[0039] The results summarized in Tables 1 to 3 clearly support the
conclusion leading to the present invention that the particle
diameter distribution values and the specific surface area of the
rare earth oxide powder are the most important granulometric
parameters which determine the quality of the sintered bodies of
the rare earth oxides and rare earth oxide-based composite
oxides.
* * * * *