U.S. patent application number 12/071785 was filed with the patent office on 2008-07-03 for method of milling cerium compound by means of ball mill.
This patent application is currently assigned to Nissan Chemical Industries, Ltd.. Invention is credited to Isao Ota, Noriyuki Takakuma, Kenji Tanimoto, Gen Yamada.
Application Number | 20080156908 12/071785 |
Document ID | / |
Family ID | 30112349 |
Filed Date | 2008-07-03 |
United States Patent
Application |
20080156908 |
Kind Code |
A1 |
Ota; Isao ; et al. |
July 3, 2008 |
Method of milling cerium compound by means of ball mill
Abstract
A method of milling cerium compound by means of a ball mill
using a milling medium, characterized in that ratio H.sub.b/r of
radius r of a cylindrical ball mill container and depth H.sub.b of
the milling medium in the ball mill container disposed horizontally
ranges from 1.2 to 1.9, and the ball mill container is rotated at a
rotational speed which is 50% or less of critical rotational speed
N.sub.c=299/r.sup.1/2 of the ball mill container converted from the
radius r expressed in centimeter. The milling method can be carried
out in a wet or dry process, and the cerium compound is preferably
cerium oxide. The method can be also applied for producing a cerium
compound slurry.
Inventors: |
Ota; Isao; (Nei-gun, JP)
; Tanimoto; Kenji; (Nei-gun, JP) ; Yamada;
Gen; (Nei-gun, JP) ; Takakuma; Noriyuki;
(Nei-gun, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
Nissan Chemical Industries,
Ltd.
Tokyo
JP
|
Family ID: |
30112349 |
Appl. No.: |
12/071785 |
Filed: |
February 26, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10518769 |
Dec 21, 2004 |
|
|
|
PCT/JP03/08475 |
Jul 3, 2003 |
|
|
|
12071785 |
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Current U.S.
Class: |
241/15 |
Current CPC
Class: |
B02C 17/20 20130101;
B02C 17/00 20130101; C09K 3/1409 20130101 |
Class at
Publication: |
241/15 |
International
Class: |
B02C 17/20 20060101
B02C017/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 4, 2002 |
JP |
2002-195742 |
Claims
1. A method of producing a slurry of cerium compound from an
aqueous or organic solvent medium containing cerium compound by
means of a ball mill using a milling medium, characterized in that
ratio H.sub.b/r of radius r of a cylindrical ball mill container
and depth H.sub.b of the milling medium in the ball mill container
disposed horizontally ranges from 1.2 to 1.9, and the ball mill
container is rotated at a rotational speed which is 50% or less of
critical rotational speed N.sub.c=299/r.sup.1/2 of the ball mill
container using the radius r expressed in centimeter.
Description
[0001] This is a Division of application Ser. No. 10/518,769 filed
Dec. 21, 2004, which in turn is a National Phase Application of
International Application No. PCT/JP03/008475, filed Jul. 3, 2003.
The disclosure of the prior applications is hereby incorporated by
reference herein in its entirety.
TECHNICAL FIELD
[0002] The present invention relates to a method appropriate for
milling cerium oxide particles by means of a ball mill.
BACKGROUND ART
[0003] Physical factors dominating milling effect of ball mill
milling equipments include dimension (radius r) and rotational
speed rpm in regard to a ball mill container. In regard to beads,
amount of filled beads (this is expressed in ratio H.sub.b/r of
depth H.sub.b of filled beads to radius r (cm) of the ball mill
container, or the ratio of the beads to the internal volume of the
container), or material, diameter and shape (spherical,
cylindrical, etc.) of the beads may be mentioned. Among these
physical factors, it is known that consumption power becomes
maximum and the best milling efficiency is obtained in case where
the amount of filled beads which is expressed in H.sub.b/r is 1.0
(corresponds to 50% based on the internal volume of the ball mill
container).
[0004] However, in case where the amount of filled beads is as
little as 30% or less (H.sub.b/r of 0.6 or less), the balls start
to slide along the inner wall of the container to cause remarkable
damages to the inner wall. Therefore, in the actual production
process, the amount of beads is generally kept to one third to half
of the total volume of the ball mill container (H.sub.b/r of 0.66
to 1.0).
[0005] In the milling by a ball mill, the balls are gradually
lifted highly in the rotational direction with the movement of the
mill, and the ball is involved in a snowslide motion together with
a plenty of balls when the balls are lifted at the position where
there is no support below the balls. Consequently, the balls slide
and fall on the surface of the balls and fall below the mill while
they collide here and there (snowslide phenomenon).
[0006] When the rotational speed is increased, the balls come to
fall like a waterfall in the space filled with vapor, rather than
the snowslide phenomenon (waterfall phenomenon).
[0007] When the rotational speed is further increased, the mill
comes to be rotated while the balls are adhered to the inner wall
of the mill due to centrifugal force (adhesion phenomenon/adhesion
state).
[0008] It is clear that no dispersion is achieved in the adhesion
state (the balls do not move relatively with the mill). In
addition, in the state of the waterfall phenomenon, the balls and
the inner wall of the mill have many damages, and dispersion is
insufficient. Therefore, these phenomena are undesirable states,
and the dispersion of pigments is carried out very efficiently in
only the state of snowslide phenomenon which is regarded as an
ideal state.
[0009] In regard to the rotational speed of the container, it is
stated that the optimum rotational speed
N.sub.0=(203-0.60r)/r.sup.1/2 wherein the unit of r is cm
(RPM.sub.0=(37-3.3r)/r.sup.1/2 wherein the unit of r is feet) at
the point of which the snowslide phenomenon occurs is an ideal
state in the milling by a ball mill (see, for example "Paint Flow
and Pigment Dispersion" written by Temple V. Patton, translation
supervised by Kenji Ueki., Kyoritsu Shuppan Co., Ltd., 1971, pp.
202-222). This publication states that the above-mentioned equation
expressing the optimum rotational speed N.sub.0 at the point of
which the snowslide phenomenon occurs is obtained in case where the
critical rotational speed N.sub.c=60
g.sup.1/22.pi.r.sup.1/2=299/r.sup.1/2, and is derived from
N.sub.0=(0.68-0.22r)N.sub.c (rpm.sub.0=(0.68-0.06r)rpm.sub.c
wherein the unit of r is feet). In addition, the publication states
that the actual production process is generally carried out in the
amount of filled beads and the rotational speed of the container as
mentioned above.
[0010] In addition, it is stated that the milling of aluminum
hydroxide powder is carried out in a ball mill made of stainless
steel having a diameter of 78 mm to 199 mm by means of steel beads
having a diameter of 10.2 mm (see, for example, "Chemical
Equipment" written by Sumiya Kano, Hiroshi Mio and Fumiyoshi Saito,
2001, No. 9, pp. 50-54). This publication reports the test results
in which the milling condition is as follows: bead-filling rate of
20 to 80% and number of revolutions of 0.6 to 1.3 time the critical
rotational speed. As a result of it, it is stated that milling rate
becomes maximum when the bead-filling rate is 40 to 80% and number
of revolutions is 80% of the critical rotational number, and the
milling rate is increased with an increase in bead diameter, and
the milling rate is lowered when the bead-filling rate is beyond
60%.
[0011] In the meanwhile, cerium oxide particles are widely used as
polishing agent for substrates containing silica as main component,
and recently there is a strong demand for cerium oxide polishing
agent by which a polished face with a high quality can be obtained
without surface defects such as scratch. On the other hand, it is
also required strongly to maintain a high removal rate so as not to
decrease the productivity. Therefor unmilled large particles
causing scratch and over-milled fine particles causing a lowering
in removal rate must be reduced in the number in cerium oxide
particles to the utmost. That is, it is required a production
method by which the particle size distribution of cerium oxide
particles can be controlled in order to make it further sharp.
[0012] Cerium oxide particles have been finely divided by milling
with ball mill using a milling medium such as partially stabilized
zirconia oxide beads or alumina beads. However, as these beads are
very hard for cerium oxide and milling condition which is generally
achieved for milling it is too vigorous, particle size distribution
of cerium oxide fine particles becomes very broad.
[0013] The present invention resolves this problem and provides a
milling method for obtaining cerium oxide particles with a narrow
particle size distribution. The cerium oxide particles obtained
according to the present invention have a narrow particle size
distribution. Therefore, in case where it is used for polishing, it
provides a polished face with a high quality without lowering in
removal rate, and thus it makes possible to improve the production
efficiency and lower the cost.
DISCLOSURE OF INVENTION
[0014] The present invention includes the following aspects: [0015]
as a first aspect, a method of milling cerium compound by means of
a ball mill using a milling medium, characterized in that ratio
H.sub.b/r of radius r of a cylindrical ball mill container and
depth H.sub.b of the milling medium in the ball mill container
disposed horizontally ranges from 1.2 to 1.9, and the ball mill
container is rotated at a rotational speed which is 50% or less of
critical rotational speed N.sub.c=299/r.sup.1/2 of the ball mill
container converted from the radius r expressed in centimeter;
[0016] as a second aspect, the method of milling cerium compound as
set forth in the first aspect, wherein the milling of the cerium
compound is carried out in wet process or dry process; [0017] as a
third aspect, the method of milling cerium compound as set forth in
the first aspect, wherein the cerium compound is cerium oxide;
[0018] as a fourth aspect, the method of milling cerium compound as
set forth in the first aspect, wherein the ball mill container is
rotated at a rotational speed which is 10% or more of N.sub.c;
[0019] as a fifth aspect, the method of milling cerium compound as
set forth in the first aspect, wherein the radius r of the ball
mill container is 5 to 50 cm; [0020] as a sixth aspect, the method
of milling cerium compound as set forth in the first aspect,
wherein the milling medium is partially stabilized zirconia ball;
[0021] as a seventh aspect, the method of milling cerium as set
forth in the first aspect, wherein the milling medium has a
diameter of 0.3 to 25 mm; [0022] as an eighth aspect, the method of
milling cerium compound as set forth in the first aspect, wherein
zirconium is used in an amount of 100 ppm to 10000 ppm based on the
cerium compound in terms of cerium (IV) oxide; [0023] as a ninth
aspect, the method of milling cerium compound as set forth in the
first aspect, wherein a water-soluble alkaline silicate is added,
pH of a slurry containing the cerium compound is adjusted to 8 to
13, and then a wet milling is carried out to obtain cerium compound
covered with amorphous silica; [0024] as a tenth aspect, the method
of milling cerium compound as set forth in the ninth aspect,
wherein the water-soluble alkaline silicate is lithium silicate,
sodium silicate, potassium silicate or quaternary ammonium
hydroxide silicate; and [0025] as an eleventh aspect, a method of
producing a slurry of cerium compound from an aqueous or organic
solvent medium containing cerium compound by means of a ball mill
using a milling medium, characterized in that ratio H.sub.b/r of
radius r of a cylindrical ball mill container and depth H.sub.b of
the milling medium in the ball mill container disposed horizontally
ranges from 1.2 to 1.9, and the ball mill container is rotated at a
rotational speed which is 50% or less of critical rotational speed
N.sub.c=299/r.sup.1/2 of the ball mill container using the radius r
expressed in centimeter.
BEST MODE FOR CARRYING OUT THE INVENTION
[0026] The present invention relates to a method of milling cerium
compound by means of a ball mill using a milling medium,
characterized in that ratio H.sub.b/r of radius r of a cylindrical
ball mill container and depth H.sub.b of the milling medium in the
ball mill container disposed horizontally ranges from 1.2 to 1.9,
and the ball mill container is rotated at a rotational speed which
is 50% or less of critical rotational speed N.sub.c=299/r.sup.1/2
of the ball mill container converted from the radius r expressed in
centimeter.
[0027] The present invention may be carried out by milling powdery
cerium compound in dry process, or milling an aqueous or organic
solvent medium containing cerium compound in wet process.
[0028] That is, in the wet process, a slurry of cerium compound can
be produced according to a method of producing a slurry of cerium
compound from an aqueous or organic solvent medium containing
cerium compound by means of a ball mill using a milling medium,
wherein ratio H.sub.b/r of radius r of a cylindrical ball mill
container and depth H.sub.b of the milling medium in the ball mill
container disposed horizontally ranges from 1.2 to 1.9, and the
ball mill container is rotated at a rotational speed which is 50%
or less of critical rotational speed N.sub.c of the ball mill
container using the radius r expressed in centimeter.
[0029] In the present invention, cerium oxide is preferably used as
cerium compound. Cerium oxides to be placed in a ball mill
container with a polishing medium are cerium oxide particles with a
particle diameter of 0.1 .mu.m or more, preferably 0.1 to 100 .mu.m
obtained by calcining commercially available cerium carbonate in a
shape of hexagonal plate of several to ten-odd .mu.m at 400 to
1200.degree. C. In addition, commercially available cerium oxide
powders with a mean particle diameter of 1 .mu.m or less or several
.mu.m can be also used.
[0030] In the meanwhile, cerium compounds are not limited to cerium
oxides, and water-insoluble cerium compound such as cerium
carbonate can be used.
[0031] As the potential of beads risen up by rotation of a ball
mill container becomes high with an increase in the radius of the
ball mill container, and the striking energy due to free fall
thereof becomes high, fine particles are apt to be obtained by
over-milling. When cerium compound, for example relatively soft
material such as cerium oxide is milled with a relatively hard
medium such as zirconia, the range of the above-mentioned radius r
is important. The ball mill container used in the present invention
preferably has the radius r ranging from 5 to 25 cm.
[0032] The amount of filled beads is set in such a way that ratio
H.sub.b/r of depth H.sub.b of the filled beads to radius r of the
ball mill container ranges from 1.2 to 1.9 (63 to 97% based on the
inner volume), which is a higher value than case where general
milling by means of a ball mill (for example, H.sub.b/r ranges from
0.63 to 1.0, 33 to 50% based on the inner volume) is carried out.
This makes it possible to mill in a condition which does not occur
a situation where snowslide phenomenon is repeated, wherein the
situation is regarded as an ideal condition in general
powder-milling.
[0033] When H.sub.b/r is set within the range from 1.2 to 1.9,
material to be milled (cerium compound in a dry-milling, an aqueous
or organic solvent slurry containing cerium compound in
wet-milling) that is placed with a milling medium in a ball mill,
is placed in an amount of the milling medium: the material to be
milled of 1:0.5 to 1:1.2 in volume ratio. When the milling medium
and the material to be milled are placed in this ratio in a ball
mill container, the combined volume of both amounts to 65 to 99.5%
based on the total volume. The slurry to be milled is a slurry
containing cerium compound in an aqueous or organic solvent in a
solid concentration of 1 to 70% by weight.
[0034] In addition, the rotational speed of the ball mill container
is 50% or less of critical rotational speed, and 80% or less of the
optimum rotational speed N.sub.O=(203-0.60r)/r.sup.1/2 that occurs
a snowslide phenomenon by which dispersion is efficiently achieved.
Thus, the present invention excludes a condition occurring a
situation where snowslide phenomenon of beads is repeated, wherein
the situation is regarded as an ideal condition in general
powder-milling.
[0035] In the present invention, milling is achieved within the
range of 10% to 50% of the critical rotational speed N.sub.c. The
rotational speed correspond to a rotational speed ranging from 20%
to 80% of the optimum rotational speed
N.sub.o=(203-0.60r)/r.sup.1/2 at which a snowslide phenomenon
occurs. As mentioned above, the present invention provides cerium
compounds, particularly cerium oxide particles having a narrow
particle size distribution by selecting a condition out of the
milling condition which it is generally regarded that milling is
achieved in the highest effect. Further, the wet milling can
provide cerium oxide slurry.
[0036] As mentioned above, the milling of cerium compound in the
present invention utilizes milling media having small particle
diameter and is carried out in a low rotational speed of a ball
mill, compared with the optimum milling condition that is normally
applied for particles. This make it possible to narrow the particle
size distribution of cerium compound, particularly cerium oxide
when it is milled.
[0037] In a process using a sand grinder or an attritor in which
beads are compulsorily rotated with an arm or disc, the milling is
carried out in the condition of ratio H.sub.b/r of depth H.sub.b of
the filled beads to radius r of the ball mill container ranging
from 1.2 to 1.9 (63 to 97% based on the inner volume). However, it
is difficult to avoid partial over-milling due to a compulsory
rotation of the milling media. Therefore, a large amount of fine
particles are produced, and it is difficult to obtain cerium oxide
particles with a sharp particle size distribution.
[0038] When the radius r of the ball mill container is over 50 cm,
the potential energy of beads risen up thereby becomes high, and
the striking energy thereof becomes high due to free fall.
Therefore, it is not preferable as over-milling occurs and the
particle size distribution of the resulting milled particles
becomes broad. On the other hand, when the radius r of the
container is less than 5 cm, it is not preferable as milled amount
per batch is too small and the cost becomes very high.
Consequently, the radius r of the container preferably ranges from
5 cm to 50 cm, more preferably from 10 cm to 40 cm.
[0039] When the ratio H.sub.b/r of depth H.sub.b of filled beads to
radius r of a cylindrical ball mill container is over 1.9 (97%
based on the inner volume), it is not economical as milling speed
is markedly lowered. The ratio H.sub.b/r of depth H.sub.b of filled
beads to radius r of a cylindrical ball mill container is
preferably 1.2 to 1.9 (the amount of filled beads is 63 to 97%
based on the inner volume), further it is more preferable that
H.sub.b/r is 1.2 to 1.7.
[0040] The material of beads is preferably partially stabilized
zirconia, alumina, mulite or silica, which is harder than cerium
oxide. Among them, partially stabilized zirconia that little beads
are worn out is most preferable.
[0041] The size of beads is preferably 0.3 to 25 mm.phi.. When the
size of beads is less than 0.3 mm.phi., its own weight of beads
becomes too light, and milling efficiency is markedly lowered. On
the other hand, when the size of beads is more than 25 mm.phi., the
striking energy of beads each other becomes high, and over-milling
occurs locally and fine particles are easily produced.
[0042] In case where milling is achieved by using partially
stabilized zirconia, it is not possible to avoid contamination of
zirconium element in a slurry of cerium compound after milling.
When the cerium compound is cerium (IV) oxide, zirconium element is
contaminated in an amount of 100 ppm to 10000 ppm based on cerium
(IV) oxide. But the element is present in the shape of zirconia
fine particle, the element itself can be utilized as polishing
agent.
[0043] The method of milling cerium compound according to the
present invention, particularly the method for producing cerium
oxide particles can be applied for wet milling or dry milling.
[0044] In the wet process, acid such as nitric acid, hydrochloric
acid, acetic acid or the like can be used as a water-soluble
dispersant. In the meantime, the wet milling for a long time causes
a rise in pH of an acid slurry, the pH approaches 5 that is the
isoelectric point of cerium (IV) oxide. Therefore, the slurry is
liable to be aggregated and lowered in grindability.
[0045] Thus, in the process of wet milling in the present
invention, a water-soluble alkaline dispersant containing silica is
added to cover cerium (IV) oxide particles with amorphous silica,
and the resulting slurry is adjusted to pH 8-13 that is higher than
the isoelectric point of cerium (IV) oxide. Thereby, cerium (IV)
oxide particles are charged negatively, and the slurry is always
kept in a dispersed state, and homogeneous wet milling can be
carried out for a long time. The water-soluble alkaline dispersant
containing silica includes a water-soluble alkaline silicate or
silica sol, such as lithium silicate, sodium silicate, potassium
silicate, quaternary ammonium hydroxide silicate, and can be added
in an amount of 0.001 to 1 in a weight ratio of
(SiO.sub.2)/(CeO.sub.2).
[0046] The material of the ball mill container according to the
present invention includes metal such as stainless steel, iron or
the like, ceramics such as alumina, mulite or the like, resin such
as nylon, polyethylene, polypropylene, engineering plastics or the
like. Containers made of resin are preferable taking contamination
of impurities on milling or hardness of material into account.
[0047] Cerium compounds obtained according to the present invention
have the particle diameter measured by centrifugal sedimentation
method ranging from 50 to 600 nm, and have a low rate of large
particles over 400 nm in the whole particles compared with those of
the prior milling method. Further, the cerium compounds have also a
low rate of fine particles less than 30 nm in the whole particles.
Consequently, the present invention can provide cerium compound
particles with a narrow particle size distribution.
[0048] In case where milling is carried out in the wet process,
cerium compound slurry that contains cerium compound with the
above-mentioned particle diameter and particle size distribution in
concentration of 10 to 60% by weight and that has pH of 3 to 11 is
obtained by milling cerium compound in concentration of 10 to 60%
by weight with an aqueous medium of pH 3-11 for 1 to 72 hours.
Particularly, it is useful for producing cerium oxide slurry from
an aqueous medium containing cerium oxide.
EXAMPLES
[0049] Hereinafter, the present invention is described based on
examples. The analytical methods adopted in the examples are as
follows.
(1) pH Measurement
[0050] A pH meter (manufactured by To a DKK Ltd., HM-30S) was used
for pH measurement.
(2) Conductivity Measurement
[0051] A conductivity meter (manufactured by To a DKK Ltd., CM-30S)
was used for conductivity measurement.
(3) Measurement of Particle Diameter by Centrifugal Sedimentation
Method
[0052] A mean particle diameter of D50 was measured with a particle
diameter measurement apparatus by centrifugal sedimentation method
(manufactured by Shimadzu Corporation, CP-3), and it was regarded
as a particle diameter based on centrifugal sedimentation
method.
(4) Measurement of Particle Diameter by Laser Diffraction
Method
[0053] A mean particle diameter of D50 was measured with a particle
diameter measurement apparatus by laser diffraction method
(manufactured by Malvern Instruments Ltd., Mastersizer 2000), and
it was regarded as a mean particle diameter based on laser
diffraction method. (5) Particle Diameter Determined from Specific
Surface Area Measured by Gas Adsorption Method A sample obtained by
drying a cerium oxide aqueous slurry in a prescribed condition was
subjected to a specific surface area analyzer by nitrogen
adsorption (manufactured by Quantachrome Instruments, Monosorb Type
MS-16) to measure the specific surface area Sw (m.sup.2/g), and a
particle diameter in terms of spherical particle (particle diameter
calculated through BET method) was determined.
(6) Measurement Method of Amount of Small Particles
[0054] In 50 ml centrifugal tube, 37 g of milled slurry obtained by
diluting to 17% by weight of solid content with pure water was
placed, the tube was centrifuged at 3000 rpm (G=1000) for 10
minutes, and then 22.5 g of supernatant was taken, and dried at
110.degree. C. to obtain powder. An amount of small particles was
determined by dividing the weight of the resulting powder by the
weight of solid content in the slurry prior to centrifugation. The
small particles were those less than 30 nm according to an
observation with transmission electron microscope.
(7) Measurement Method of BET Method-Based Particle Diameter of
Large Particles
[0055] In 100 ml glass sedimentation tube, 115 g of milled slurry
obtained by diluting to 15% by weight of solid content with pure
water was placed, and after one day, 2 ml of slurry was recovered
from the bottom. After drying the recovered slurry in a prescribed
condition, the specific surface area was measured similarly to the
procedure in (4) and the particle diameter based on BET method was
calculated, and it was regarded as particle diameter calculated
through BET method (BET method-based particle diameter) of large
particles. (8) Observation with Scanning Electron Microscope An
electron microscopic photograph of a sample to be observed was
taken with a scanning electron microscope (manufactured by JEOL
Ltd., FE-SEM S-4100), and the resulting photograph was
observed.
(9) Measurement of Powder X-Ray Diffraction
[0056] A X-ray diffraction apparatus (manufactured by JEOL Ltd.,
JEOL JDX-8200T) was used for measurement of powder X-ray
diffraction.
(10) Measurement of Isoelectric Point of Cerium (IV) Oxide
[0057] A slurry containing cerium (IV) oxide in 1% by weight was
prepared, and the isoelectric point thereof was measured with
Zetasizer HS 3000 (manufactured by Malvern Instruments Ltd.)
(11) Measurement of Removal Rate of Thermal Oxidation Layer
[0058] Film thickness of thermal oxidation layer was measured with
a film thickness analyzer NanoSpec (manufactured Nanometrics
Incorporated) before and after polishing, and removal rate was
determined.
Example 1
[0059] 150 kg of commercially available cerium oxide having
bar-shaped particles of 0.2 to 3 .mu.m with an observation by a
scanning electron microscope, mean particle diameter based on laser
diffraction of 3.2 .mu.m and a specific surface area based on BET
method of 128 m.sup.2/g was calcined in 1 m.sup.3 gas calcination
furnace at 1100.degree. C. for 5 hours to obtain yellow-white
powder. The resulting powder was measured with X-ray diffraction
apparatus and main peaks were detected at diffraction angle
2.theta.=28.6.degree., 47.5.degree. and 56.4.degree. which were
consistent with characteristic peaks of cubic system crystalline
cerium oxide described in ASTM card 34-394. An observation with a
scanning electron microscope revealed that the calcined cerium
oxide powder was aggregated particles having a primary particle
diameter of 150 to 300 nm. In addition, the specific surface
thereof was 2.8 m.sup.2/g.
[0060] Partially stabilized zirconia beads of 1 mm.phi. were placed
in an amount of 59 kg in a polyethylene container having a
dimension of radius 15 cm.times.length 34 cm (in this point,
H.sub.b/r=1.4, amount of filled beads was 71%), and further 5.9 kg
of the cerium oxide powder obtained by calcination at 1100.degree.
C., 11.8 kg of pure water and 47 g of 10% nitric acid were placed
therein. Then, milling was carried out at a rotational speed of 30
rpm corresponding to 39% of the critical rotational speed of this
container N.sub.C=77 rpm for 18 hours. This afforded a cerium (IV)
oxide aqueous slurry having solid content concentration of 33% by
weight, pH 5.9 and conductivity of 318 .mu.m/S. The powder obtained
by drying this slurry at 300.degree. C. had specific surface area
of 7.1 m.sup.2/g and BET method-based particle diameter of 117 nm.
In addition, the particle diameter thereof was 100 to 300 nm with
an observation by a scanning electron microscope, and the mean
particle diameter was 260 nm according to centrifugal sedimentation
method. Further, the proportion of small particles less than 30 nm
was 1.5% and the BET method-based particle diameter of large
particles was 140 nm. The proportion (%) that the particle diameter
of the resulting particles fell within the mean particle diameter
according to laser diffraction method .+-.30% was 66% in the whole
particles. In addition, zirconium element was contained in 1300 ppm
based on cerium (IV) oxide.
Example 2
[0061] Zirconia beads of 1 mm.phi. were placed in an amount of 135
kg in a ball mill container having polyethylene lining with a
dimension of radius 15 cm.times.length 73 cm (in this point,
H.sub.b/r=1.4, amount of filled beads was 70%), and further 13.5 kg
of the cerium oxide powder obtained by calcination at 1100.degree.
C. in Example 1, 27.0 kg of pure water and 107 g of 10% nitric acid
were placed therein. Then, milling was carried out at a rotational
speed of 35 rpm corresponding to 45% of the critical rotational
speed of this container N.sub.C=77 rpm for 16 hours. This afforded
a cerium (IV) oxide aqueous slurry having solid content
concentration of 33% by weight, pH 5.8 and conductivity of 350
.mu.m/S. The powder obtained by drying this slurry at 300.degree.
C. had specific surface area of 7.3 m.sup.2/g and BET method-based
particle diameter of 114 nm. In addition, the particle diameter
thereof was 100 to 300 nm with an observation by a scanning
electron microscope, and the mean particle diameter was 280 nm
according to centrifugal sedimentation method. Further, the
proportion of small particles less than 30 nm was 1.3% and the BET
method-based particle diameter of large particles was 138 nm. The
proportion (%) that the particle diameter of the resulting
particles fell within the mean particle diameter according to laser
diffraction method .+-.30% was 63% in the whole particles. In
addition, zirconium element was contained in 1200 ppm based on
cerium (IV) oxide.
Example 3
[0062] Commercially available cerium carbonate powder having purity
of 99.9% (mean particle diameter based on laser diffraction method
of 38 .mu.m) was calcined in an amount of 1600 g in an electric
furnace at 350.degree. C. for 5 hours, and then the temperature of
the furnace was risen to 900.degree. C. followed by calcination at
900.degree. C. for 15 hours to obtain 800 g of yellow-white powder.
The resulting powder was measured with X-ray diffraction apparatus
and main peaks were detected at diffraction angle
2.theta.=28.6.degree., 47.5.degree. and 56.4.degree. which were
consistent with characteristic peaks of cubic system crystalline
cerium oxide described in ASTM card 34-394. An observation with a
scanning electron microscope revealed that the calcined cerium
oxide powder was aggregated particles having a primary particle
diameter of 100 to 200 nm. In addition, the specific surface
thereof was 4.6 m.sup.2/g. The isoelectric point of the cerium (IV)
oxide was pH=5.
[0063] To a mixed aqueous solution of 20 g of commercially
available 25% tetramethylammonium hydroxide and 165 g of pure
water, 21 g of 95% tetraethoxysilane was added with stirring by
disper to obtain tetramethylammonium hydroxide silicate aqueous
solution being an alkaline silicate having pH of 12.8, conductivity
of 8110 .mu.m/S and SiO.sub.2 concentration of 2.9% by weight.
[0064] Partially stabilized zirconia beads of 1 mm.phi. were placed
in an amount of 6 kg in a polyethylene container having a dimension
of radius 6.5 cm.times.length 23 cm (in this point, H.sub.b/r=1.2,
amount of filled beads was 60%), and further 578 g of the resulting
cerium oxide powder, 372 g of pure water and 206 g of
tetramethylammonium hydroxide silicate aqueous solution
corresponding to weight ratio (SiO.sub.2)/(CeO.sub.2) of 0.01 were
placed therein. Then, milling was carried out at a rotational speed
of 60 rpm corresponding to 50% of the critical rotational speed of
this container N.sub.C=120 rpm for 32 hours. After milling,
beads-separation was carried out with pure water to obtain a cerium
(IV) oxide aqueous slurry (A-1) having solid content concentration
of 20% by weight, pH 11.9 and conductivity of 1734 .mu.m/S. The
resulting cerium (IV) oxide had the isoelectric point of pH 3.8.
The powder obtained by drying this slurry at 300.degree. C. had
specific surface area of 15.2 m.sup.2/g and BET method-based
particle diameter of 55 nm. In addition, the particle diameter
thereof was 100 to 200 nm with an observation by a scanning
electron microscope, and the mean particle diameter was 113 nm
according to laser diffraction method. The proportion (%) that the
particle diameter of the resulting particles fell within the mean
particle diameter according to laser diffraction method .+-.30% was
59% in the whole particles. The proportion of small particles less
than 30 nm was 7.9% and the BET method-based particle diameter of
large particles was 70 nm. In addition, zirconium element was
contained in 2760 ppm based on cerium (IV) oxide.
Comparative Example 1
[0065] Zirconia beads of 1 mm.phi. were placed in an amount of 25.1
kg in a polyethylene container having a dimension of radius 15
cm.times.length 34 cm (in this point, H.sub.b/r=0.66, amount of
filled beads was 30%), and further 2.5 kg of the cerium oxide
powder obtained in Example 1, 5.0 kg of pure water and 20 g of 10%
nitric acid were placed therein. Then, milling was carried out at a
rotational speed of 30 rpm corresponding to 39% of the critical
rotational speed of this container N.sub.C=77 rpm for 12 hours.
This afforded a cerium (IV) oxide aqueous slurry having solid
content concentration of 33% by weight, pH 5.9 and conductivity of
318 .mu.m/S. The powder obtained by drying this slurry at
300.degree. C. had specific surface area of 7.4 m.sup.2/g and BET
method-based particle diameter of 113 nm. In addition, the particle
diameter thereof was 30 to 300 nm with an observation by a scanning
electron microscope, and the mean particle diameter was 290 nm
according to centrifugal sedimentation method. Further, the
proportion of small particles less than 30 nm was 2.5% and the BET
method-based particle diameter of large particles was 163 nm. The
proportion (%) that the particle diameter of the resulting
particles fell within the mean particle diameter according to laser
diffraction method .+-.30% was 41% in the whole particles.
Comparative Example 2
[0066] Zirconia beads of 1 mm.phi. were placed in an amount of 169
kg in a nylon container having a dimension of radius 37
cm.times.length 73 cm (in this point, H.sub.b/r=0.42, amount of
filled beads was 15%), and further 16.7 kg of the cerium oxide
powder obtained in Example 1, 33.8 kg of pure water and 134 g of
10% nitric acid were placed therein. Then, milling was carried out
at a rotational speed of 12 rpm corresponding to 25% of the
critical rotational speed of this container N.sub.C=49 rpm for 13
hours. This afforded a cerium (IV) oxide aqueous slurry having
solid content concentration of 33% by weight, pH 5.5 and
conductivity of 248 .mu.m/S. The powder obtained by drying this
slurry at 300.degree. C. had specific surface area of 7.2 m.sup.2/g
and BET method-based particle diameter of 116 nm. In addition, the
particle diameter thereof was 25 to 300 nm with an observation by a
scanning electron microscope, and the mean particle diameter was
290 nm according to centrifugal sedimentation method. Further, the
proportion of small particles less than 30 nm was 3.0% and the BET
method-based particle diameter of large particles was 168 nm. The
proportion (%) that the particle diameter of the resulting
particles fell within the mean particle diameter according to laser
diffraction method .+-.30% was 39% in the whole particles.
Comparative Example 3
[0067] Zirconia beads of 1 mm.phi. were placed in an amount of 135
kg in a nylon container having a dimension of radius 15
cm.times.length 73 cm (in this point, H.sub.b/r=1.4, amount of
filled beads was 70%), and further 13.5 kg of the cerium oxide
powder obtained in Example 1, 27.0 kg of pure water and 107 g of
10% nitric acid were placed therein. Then, milling was carried out
at a rotational speed of 45 rpm corresponding to 58% of the
critical rotational speed of this container N.sub.C=77 rpm for 12
hours. This afforded a cerium (IV) oxide aqueous slurry having
solid content concentration of 33% by weight, pH 6.3 and
conductivity of 92 .mu.m/S. The powder obtained by drying this
slurry at 300.degree. C. had specific surface area of 7.2 m.sup.2/g
and BET method-based particle diameter of 116 nm. In addition, the
particle diameter thereof was 30 to 300 nm with an observation by a
scanning electron microscope, and the mean particle diameter was
340 nm according to centrifugal sedimentation method. Further, the
proportion of small particles less than 30 nm was 2.3% and the BET
method-based particle diameter of large particles was 160 nm. The
proportion (%) that the particle diameter of the resulting
particles fell within the mean particle diameter according to laser
diffraction method .+-.30% was 45% in the whole particles.
Comparative Example 4
[0068] Partially stabilized zirconia beads of 1 mm.phi. were placed
in an amount of 6 kg in a polyethylene container having a dimension
of radius 6.5 cm.times.length 23 cm (in this point, H.sub.b/r=1.2,
amount of filled beads was 60%), and further 578 g of the cerium
oxide powder obtained by calcining in a similar condition as that
of Example 3, 372 g of pure water and 206 g of tetramethylammonium
hydroxide silicate aqueous solution prepared in Example 4
corresponding to weight ratio (SiO.sub.2)/(CeO.sub.2) of 0.01 were
placed therein. Then, milling was carried out at a rotational speed
of 90 rpm corresponding to 75% of the critical rotational speed of
this container N.sub.C=120 rpm for 16 hours. After milling, bead
separation was carried out with pure water to obtain a cerium (IV)
oxide aqueous slurry (B-1) having solid content concentration of
20% by weight, pH 11.3 and conductivity of 1725 .mu.m/S. The powder
obtained by drying this slurry at 300.degree. C. had specific
surface area of 15.0 m.sup.2/g and BET method-based particle
diameter of 56 nm. In addition, the particle diameter thereof was
30 to 300 nm with an observation by a scanning electron microscope,
and the mean particle diameter was 113 nm according to laser
diffraction method. The proportion (%) that the particle diameter
of the resulting particles fell within the mean particle diameter
according to laser diffraction method .+-.30% was 43% in the whole
particles. The proportion of small particles less than 30 nm was
8.8% and the BET method-based particle diameter of large particles
was 74 nm. In addition, zirconium element was contained in 2900 ppm
based on cerium (IV) oxide.
TABLE-US-00001 TABLE 1 Item (I) (II) (III) (IV) (V) (VI) (VII)
Example 1 15 1.4 30 117 1.5 140 66 Example 2 15 1.4 35 114 1.3 138
63 Example 3 6.5 1.2 60 55 7.9 70 59 Comparative Example 1 15 0.66
30 113 2.5 163 41 Comparative Example 2 37 0.42 12 116 3.0 168 39
Comparative Example 3 15 1.4 45 116 2.3 160 45 Comparative Example
4 6.5 1.2 90 56 8.8 74 43
[0069] In table 1, item (I) is radius (cm) of ball mill container,
item (II) is H.sub.b/r ratio, item (III) is rotational speed (rpm),
item (IV) is BET method-based particle diameter (nm) of cerium
oxide aqueous slurry, item (V) is proportion (%) of small particles
less than 30 nm in the whole particles, item (VI) is BET
method-based particle diameter of large particles, and item (VII)
is proportion (%) in the whole particles that the particle diameter
of the resulting particles fell within the mean particle diameter
.+-.30%.
[0070] To aqueous sols (A-1, B-1) obtained in Example 3 and
Comparative Example 4, ammonium polyacrylate was added in a
concentration of 100% by weight based on cerium (IV) oxide, and
then polishing compositions (a-1, b-1) were prepared by diluting
the resulting mixture with pure water in a manner that the solid
content of cerium (IV) oxide would be 1% by weight.
[0071] Polishing by means of the prepared polishing compositions
was carried out as follows:
Polishing machine: a machine manufactured by Techno Rise
Corporation; Polishing pad: a polishing pad IC-1000 made of closed
formed polyurethane resin (manufactured by Rodel Nitta Company);
Material to be polished: thermal oxidation layer on 4-inch silicon
wafer; Number of revolutions: 60 rpm; Polishing pressure: 500
g/cm.sup.2; and Polishing time: 2 minutes.
[0072] The assessment of polished faces shown in Table 2 were
carried out with an optical microscope, in which the case where
fine defects were observed was indicated by symbol (.DELTA.) and
the case where no defect was observed was indicated by symbol
(.circleincircle.).
TABLE-US-00002 TABLE 2 Removal rate (.ANG./min) Polished Face a-1
800 .circleincircle. b-1 750 .DELTA.
[0073] It can be pointed out that cerium oxide aqueous slurries in
Examples 1 to 2 and Comparative Examples 1 to 3 shown in Table 1
have BET method-based particle diameter ranging form 113 to 117 nm
which is approximately equal one another. However, the comparison
between Examples 1 to 2 and Comparative Examples 1 to 2 reveals the
followings. Comparative Examples 1 and 2 having a law ratio
H.sub.b/r of depth H.sub.b of filled beads to radius r of the ball
mill container (a law filling rate of beads) contain small
particles less than 30 nm in a high rate in the whole particles,
and the large particles thereof have a large BET method-based
particle diameter and therefore they contain a large amount of
large particles. Thus, it is understood that Comparative Examples 1
and 2 have a broader particle size distribution than Examples 1 and
2.
[0074] In addition, Comparative Example 3 in which a rotational
speed of the ball mill container was adjusted to a high value has a
high rate of small particles less than 30 nm in the whole particles
and a large BET method-based particle diameter of large particles.
Therefore, it is understood that Comparative Example 3 has a broad
particle size distribution.
[0075] Example 3 containing tetramethylammonium hydroxide silicate
aqueous solution as dispersant has a higher rate (%) in the whole
particles of particles falling within mean particle diameter
according to laser diffraction method .+-.30% than Comparative
Example 4 in which a rotational speed of the ball mill container
was adjusted to a high value. Therefore, it is understood that
Example 3 has a narrow particle size distribution. In addition, it
is understood that Example 3 contains has a narrow particle size
distribution also from the facts that it contains small particles
less than 30 nm in a low rate in the whole particles and has a
small BET method-based particle diameter of large particles.
Further, as shown in Table 2, it is understood from the comparison
in polishing characteristics between Example 3 and Comparative
Example 4 that Example 3 has a higher removal rate and provides a
better quality of polished face.
[0076] Although a relationship between removal rate and smoothness
of polished face is generally in an opposite manner, particles in
cerium compound slurry obtained according to the present invention
contain small particles less than 30 nm in a law rate of 10% or
less in the whole particles and particles falling within mean
particle diameter according to laser diffraction method .+-.30% in
a high rate (%) of 50% or more in the whole particles, thereby the
present invention makes it possible to provide a high removal rate
and a good smoothness.
INDUSTRIAL APPLICABILITY
[0077] The present invention relates to a method of milling cerium
(IV) oxide particles. The milling method of the present invention
provides cerium oxide particles that contain a small amount of fine
particles and large particles and have a sharp particle size
distribution. Therefore, in case where the cerium oxide particle
obtained according to the present invention are used as polishing
agent for substrates containing silica as a main component, such as
rock crystal, quartz glass for photomask, glass hard disk or
oxidation layer of semiconductor devices, polished faces with a
high accuracy and smoothness can be efficiently obtained with a
high polishing speed and little scratch.
[0078] Further, in case where an aqueous sol containing cerium (IV)
oxide particle covered with amorphous silica is used for
particularly polishing substrates containing silica as a main
component, such as rock crystal, quartz glass for photomask, glass
hard disk or oxidation layer of semiconductor devices, it is hard
to produce residues and a good polished surface can be
obtained.
* * * * *