Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE OF THE INVENTION
IMPROVED METHOD FOR PRODUCING ~LUMINUM NITRIDE
POWDER
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the art of aluminum
nitride and more particularly, to a method for producing
aluminum nitride fine powder which is high in purity and
has good sinterability.
2. Description of the Prior Art
Aluminum nitride is now expected to have wide
utility in various fields including not only the field
of high temperature heat-resistant materials based on
good mechanical characteristics and chemical durability,
but also the field of heat-radiating materials in the
semiconductive art because of its high heat
conductivity, good electric insulting property and low
dielectric constant. In most cases, aluminum nitride
has been employed as a sintered product although it may
be utllized in the;form of a thin film.
It is known that the sin-tering properties and
characteristics of an aluminum nitride sintered product
is greatly influenced by the characteristics of a
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starting aluminum nitride powder and the type and manner
of sintering agent dispersed in -the powder. More
particularly, the aluminum nitride powder should be
highly pure as well as fine and uniform in size. A
suitable sintering agent should be uniformly dispersed
in the aluminum nitride powder.
In general~ an aluminum powder is produced by
a direct nitriding method of metallic aluminum or a
reducing and nitriding method of alumina. With the
direct nitriding method, it is very difficult to obtain
an aluminum nitride powder which is fine and uniform in
size and has high purity. In addition, it is also
difficult to uniformly disperse a sintering agent in the
aluminum nitride powder. If, in the reducing and
nitriding method, starting alumina used is fine, uniform
and highly pure, it is more likely to obtain an aluminum
nitride powder of a slightly better quality than in the
case of the direct nitriding method. Nevertheless, such
a powder is far from a desired powder.
Several improved methods of obtaining a highly
pure and fine a]uminum nitride powder with a uniform
size have been proposéd, for example, in Japanese
Laid-open Patent Application No. 61-6105 and Japanese
Patent Publication No. 61-2685. Such methods include a
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method which comprises adding water to a dispersion of
aluminum alkoxide and carbon, causing the alkoxide to be
hydrolyzed thereby obtaining a mixture of a~ uminum
hydroxide and carbon, and a method in which an alkali is
added to an aqueous solu-tion containing a water-soluble
aluminum salt and carbon to obtain a mixture of aluminum
hydroxide and carbon by neutralizing precipitation.
The mixture obtained by these methods are more
uniform than the mixture of alu~ina and carbon. When
these mix-tures are sintered in a non-oxidizing
a-tmosphere containing nitrogen, there can be obtained an
aluminum nitride powder which is more uniform and finer
than the powder from the mixture of alumina and carbon.
However, these improved methods are aIso
disadvantageous in that when water or an alkali is added
for the hydrolysis or precipitation by neutralization, a
precipitate is locally formed and immediately coagulates
in situ. This results in formation of a number of
aluminum hydroxide aggregates which are free of any
carbon fine powder and have a size of not less than 1
micron. Accordingly, the resultant mixture of aluminum
hydroxide and carbon is not satisfactory with respect to
the uniformity. This leads to a tendency toward the
irregularity in size of the aluminum nitride obtained by
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sintering the non-uniform mixture in a non-oxidizing
atmosphere containing nitrogen. After sintering, t~le
alumina is liable to remain non-nitrided with a high
possibility of forming coarse grains of aluminum nitride
having a size of from 1 to 5 microns.
SUMMARY C)F THE INVENTION
It is therefore an object of the present
invention to provide a method for producing aluminum
nitride powder which is highly pure and fine in size and
has good sinterability.
It is another object of the invention to
provide a method for producing aluminum nitride powder
by the use of a uniform dispersion of boehmite or pseudo
boehmite and a carbon source material whereby a fine
aluminum nitride powder can be obtained with a narrow
distribution in size.
It is a fur-ther objec-t of the invention to
provide a method for producing aluminum nitride powder
by the use of a specific type of carbon source material
in the orm of a mixture of a water-soluble organic
carbon source and a solid carbon powder whereby the size
distribution is more improved.
It is yet another object of the invention to
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provide a method for producing aluminum nitride powder
in which an alpha-alumina powder i9 mixed with a mixture
of a boehmite sol and a carbon source material, so that
the resultant aluminum nitride powder has a further
improved distribution in size with a reduced amount of
carbon.
According to one embodiment o~ -the present
invention, there is provided a method for producing
aluminum nitride powder which comprises adding a
boehmite powder to an aqueous medium, adjusting the
resulting dispersion to a pH of 1.2 to 4.5 to obtain a
boehmite sol, mixing the boehmite sol with a carbon
source material, drying the mixture, s.intering the dried
mixture in a non-oxidizing atmosphere containing a
nitrogen gas, and decarbonizing the sintered product.
In the above method, the drying may be effected after
gelation of the mixture of the boehmite sol and the
carbon source material.
In a preferred embodiment of the invention,
the carbon source material used in the above method is a
mixture of the carbon obtained from an organic carbon
source material and a solid carbon powder at a mixing
ratio by weight of from 0.05:1 to 0.5:1, and the mixing
ratio by weight of the total of the carbon derived from
the organic carbon source material and the solid carbon
powder and the boehmite powder is in the range of from
0.4:1 to 3:1. The use of the different carbon sources
is more effective in improving the nature of the
aluminum nitride powder.
In both embodiments described above, i-t is
preferable to further add a predetermined amount of
alpha-alumina powder to the mixture of the boehmite sol
and the carbon source material or materials in order to
realize a narrow size distribution of a final aluminum
nitride powder with a reduced amount of residual carbon
in the powder. It will be noted that the term of
"boehmite" used herein is intended to mean not only
boehmite, but also pseudo boehmite as will be described
hereina~ter.
BRIEF DESCRIPq~ION OF THE DRAWINGS
Fig.l is a graph showing the relation between
a cumulative size distribution and a particle size for
different aluminum nitride powders obtained under
different conditions according to one embodiment of the
invention and also under conditions for comparison;
Fig . 2 is similar to Fig.l and shows a graph
showing criticality of a preferred embodiment of the
i5
invention; and
Fig.3 is similar to Figs.l and 2 and shows a
graph showing criticality of another preferred
embodiment of the invention using alpha--alumina powder.
DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION
The method according to a broad embodiment of
the invention comprises the following steps.
(1) Boehmite or pseudo boehmite which is very
readily dispersable in water is added to an aqueous
medium such as water and is adjusted in pH to a range of
1.2 to 4.5, by which a boehmite sol in which the
boehmite is stably dispersed is obtained. The particle
size of the boehmite in the sol is not larger that 500
angstroms. The pseudo boehmite has an X-ray diffraction
peak at the same position as boehmite crystals but the
peak width is broader and the crystallinity is poorer
than those of boehmite crystals. The dispersability of
the pseudo boehmite in water is similar to the
dispersability of the boehmite crystals.
(2) A fine powder of a carbon source material
is added to the boehmite sol and uniformly mixed, and is
solidified while keeping the uniform state of the
mixture. Thereafter, the solidified mixture is dried
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and sintered in a non-oxidizing atmosphere containing
nitrogen.
(3) In the above step (2), for the
solidification of the mixture while keeping the uniform
state of the dispersion of the boehmite sol and the
carbon source material, the water is removed such as by
evaporation, thereby causing the boehmite sol and the
carbon to be gelled in coexistence. The gel is dried
and sintered in the non-oxidizing atmosphere con-taining
nitrogen.
(4) In the above step (2) or (3), the fine
powder of the carbon source material may be more readily
dispersed by suspending the fine powder in water by the
use of a suitable dispersant. The resultant suspension
is preferably added to the boehmite sol.
(5)In the above step (2), (3~ or (4), it is
preferred that at least one compound serving as a
sintering agent at the time of sintering of aluminum
nitride is added to the mixture of the boehmite sol and
the carbon source materialO
The sintered product is finally subjected to
decarbonization treatment to obtain a final aluminum
nitride powder.
The above procedures of the invention are
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described in more detail.
The boehmite or pseudo boehmite powder is
first added to an aqueous medium such as water/ whose pH
is adjusted to a range of 1.2 to 4.5 by addition of an
acid such as ni-tric acid, thereby permitting the
boehmite sol to be optimized in dispersion state.
Subsequently, a carbon fine powder or a carbon source
material capable of yielding carbon fine powder at high
temperatures is added to the boehmite sol and mixed.
While the uniform state of the mixture is kept, it is
solidified and dried. The dried mixture is sintered in
a non-oxidizing atmosphere containing nitrogen at a
temperature of, for example, from 1350 to 1700C to
obtain aluminum nitride powder.
The mixing with the carbon source material is
facilitated by the following procedures.
(a) The fine carbon powder is added to the
boehmite sol along with water and kneaded in a suitable
kneader such as a ball mill.
(b) The fine carbon powder and a suitable
dispersant are added to the boehmite sol along with a
suitable amount of water, and kneaùed.
(c) The fine carbon powder and a suitable
dispersant added to water and kneaded to obtain a good
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dispersion, which is mixed with the boehmite sol.
(d) A water-soluble resin is used as the
carbon source material.
The dispersion of the boehmite sol and the
carbon source material obtained in this manner is
solidified while keeping the uniform state of the
mixture.
The solidification is feasible as follows.
(a) During the kneading over a lony time, the
boehmite sol gradually forms a gel, so that the mixture
gradually increases in viscosity and is finally
.solidified.
(b) While kneading, the mixture is heated to
cause the water to be evaporated, thereby forming a gel.
(c) An additive such as an acid, an alkali,
various ions or a polymeric coagulant is added to the
mixture to facilitate the gelation.
In the practice of the invention, a sintering
agent may be added to an aluminum nitride powder and
uniformly dispersed in the powder.: At least one
compound known as an effective sintering agent for~
aluminum nitrlde, preferably a water-soluble compound
such as Y(N03)3 or the like, is added to the mixture oi
the boehmite sol and the carbon source materi.al and
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solidified while keeping the uniform mixing state as
described above.
The reason why the pH is defined in the range
of from 1.2 to 4.5 during the preparation of the
boehmite sol i5 as follows. At a pH less than 1.2, the
gelation of the boehmite sol proceeds rapidly, makin~ it
almost impossible to uniformly mix it with a carbon
source material. Over 4.5, the boehmite powder is no-t
dispersed in water and remains as aggregates having a
size larger than 1 micron, thus making uniform mixing
with a carbon source material or the dispersion thereof.
The carbon sources useful in the present
invention are preferably carbon black, graphite, or
various resins capable of yielding a high carbon rate
when sintered at high temperatures.
According to the method of the invention, an
aluminum nitride powder having a smal] size and a narrow
distribution in si~e can be reproducibly obtained in
which few particles having a size larger than 1 micron
are contained.
The method of the invention is completely
different from known aluminum nitrlde preparation
methods such as disclosed, for example, in Japanese
Laid-open Patent Application No. 61-6105 and Japanese
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Patent Publication No. 61-26485 in the fo]lowing
respects and can produce aluminu~ nitride powder
reproducibly.
(1) Since a highly dispersable boehmite powder
has once been dispersed in water as having a size not
larger than 500 angstroms with a narrow distribution in
size, boehmite particles are surrounded with fine powder
of a carbon source material after mixing with the fine
powder.
(2) The above mixture is solidified in a
uniformly mixed state, dried and sintered.
For these reasons, the growth of grains by
combination of boehmite particles or formation of coarse
aluminum nitride particles can be prevented according to
the method of the invention.
The final aluminum nitride powder obtained by
the invention can be made high in purity using a high
purity of starting boehmite. For instance, it is
possible to suppress a total or overall content of metal
ion impurities to a level of not larger than 200 ppm.
Moreover, since a fine boehmite powder and a carbon
source material are mixed uniformly, impurities such as
Fe2O3, SiO2 and the like are readily reduced and removed
by vaporization when the mixture i5 nitrided by
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3365
reduction. Thus, the final product becomes higher in
purity.
As a matter of course, the remaining carbon in
the sintered product is removed by decarbonization in an
o~idizing a-tmosphere as is known in the art. This does
not require any specific techniques and is not described
in detail herein.
A preferred embodiment of the invention i5
then described with respect to a carbon source material.
In the above embodiment, carbon black, graphite or
various resins capable of yielding carbon at high
temperatures are used as the carbon source material. A
narrower distribution in size is obtained by using, as
the carbon source material, a specific combination of
two types of carbon sources. Irhis is described below.
In this embodiment, carbon obtained from a
water-soluble organic carbon-containing material and a
solid carbon powder are used in combination. The total
carbon content relative to the boehmite powder is
preferably defined in a predetermined range. More
particularly, the mixing ratio by weight of -the carbon
from water-soluble organic carbon source material and a
solid carbon powder is preferably in the range of from
0.05:1 to 0.5:1. The total carbon content is
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preferably controlled to be at a ratio, to the boehmite
powder, of 0.4:1 to 3:1. These carbon source materials
are mixed wi-th a boehmite powder and solidified, dried
and sintered in the manner as described with respect to
the foregoin~ embodiment.
The solid carbon powder may be carbon black,
graphite or the like and should preferably have a size
of not larger than 5 mlcrons. The water-soluble organic
carbon source materials should not influence the
dispersability of the boehmite sol and can yield a high
residual carbon conten-t at high temperatures. Examples
of the materials include various water-soluble resins
such as polyvinyl alcohol, polyacrylic acid, polyamide
and the like, lingninsulphonic acid and the like.
The reason why the mixing ratio by weight
between the carbon from a water-soluble organic carbon
source and a solid carbon powder is preferably defined
to be in the range of from 0.05:1 to 0.5:1 is as
follows.
In order to permit the boehmite to be reduced
and nitrided sufficiently, it is necessary that carbon
source materials and the boehmite sol should be mixed as
uniformly as possible so as to cover individual boehmite
particles with the carbon particles. Otherwise, the
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boehmite would partially or local]y remain non-nitrided
by reduction as alumina or would be apt to form an
irregular siæe of a final aluminum nitride powder.
For uniform dispersion of carbon, it is
favorable to use a wa-ter-soluble organic carbon source
material. However, if this type of ma-terial is used in
large amounts, the bonding between carbon and carbon
atoms becomes very firm during -the sintering. This
results in forrnation of very hard lumps of aluminum
nitride and carbon obtained after the nitriding by
reduction of the boehmite.
When the sin-tered product comprising aluminum
nitride and carbon is decarbonized in an oxidizing
atmosphere while suppressing the aluminum ni~ride powder
from oxidation, it is important to break the lumps into
pieces to an extent as fine as possible. If the lumps
are too hard, the breaklng operation undesirably takes a
long time, during which impurities may inevitably
incorporate. In addition, a residual carbon content
will increase.
~ For these reasons, if a mixing ratio of the
carbon from an organic carbon source material and a
solid carbon powder is less than 0.5:1, the effect of
the organic carbon source material does not become so
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significant. On the contrary, when the ratio exceeds
0.5:1, the lumps of aluminum nitride and carbon obtained
after the sintering in a non-oxidizing atmosphere tends
to become hard. Thus, the ratio is preferably in the
range of from 0.05 to 0.5.
The reason why the total carbon content and
the boehmite powder is preferably defined at a mixing
ratio by weight of 0.4:1 to 3.0:1 is as follows.
If the mixing ratio is less than 0.4, alumina
tends to remain after nitriding by reduction with a
relatively large size of the aluminum nitride powder.
Over 3.0, some problem may be involved in the production
efficiency for the aluminum nltride powder though not
vital.
As described hereinbefore, the aluminum
nitride powder obtained according to the embodiments of
the invention is fine in size and has a narrow
distribution in lize. In some cases, it has been
experienced that the aluminum nitride obtained by the
method involves relatively coarse particles only in a
very small number. Presumably, this is because when
boehmite powder is heated, it onc~ turns into
alpha-alumina, during which abnormal growth in size
sometimes take place. Accordingly, even though very
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fine boehmite powder is used as a starting material,
there is the possibility of forming coarse
alpha-alumina. This is completely preven-ted by addition
of alpha-alumina powder to the mixture of the boehmite
sol and a carbon source material in the embodiments
described before. The alpha-alumina is added in an
amount of from 0.01 to 50 wt% of the boehmite powder
used.
The alpha-alumina may be added to the mixture
in the form of a powder or after preparation of an
alumina slurry. It is preferred that the alumina is
added after preparation of an alumina slurry having a pH
of from 1.~ to 4.5, by which the alumina powder can be
sufficiently dispersed.
As described above, when boehmite powder is
heated for sinteringj it turns into an alpha-alumina
phase, during which abnormal growth of grains may take
place. When a fine alpha-aIumina powder is added, the
added alumina serves as a nucleus-forming site when the
boehmite powder is converted into alpha-alumina, thus
preventing the abnormal growth.
Alumina includes not only alpha-alumina, but
also meta-stable gamma-alumina. However, the
meta-stable alum~na has not the action as the
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nucleus forming site. In addition, when gamma-alumina
is converted into an alpha-alumina phase, it undergoes
abnormal growth similar to -the boehmite powder.
The amount of alpha-alumina is in the range of
from 0.01 to 50 wt% of the boehmite powder. If the
amount is less than 0.01 wt~, the number of the
nucleus-forming sites are too small to suppress the
abrupt grow-th of coarse particles involved in the
conversion. Over 50 wt~, merits in use of boehmite
which has good dispersability and a small size are
sacrificed. The use of alpha-alumina in such large
amounts would not substantially differ from known
methods using alpha-alumina as a starting material.
The size o the alpha-alumina additive is not
critical but is preferable to be as fine as possible.
This is because the alumina powder serves as
nucleus-forming sites when boehmite is converted into
alpha-alumina for which a multitude of the sites should
be brought into the reaction system to cause fine
aluminum nitride powder to be formed. Moreover, the
alpha-alumina additive itself is nitrided by reduction
with the carbon and nitrogen, thereby forming an
aluminum nitride powder, so that a smaller size is more
favorable. If possible, the alpha-alumina additive
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should preferably have a smaller size than an aluminum
nitride powder to be produced. The selection in size of
the alpha-alumina powder permits preparation of an
aluminum nitride powder having a desirecl size and a
narrow size distribution.
The addition of the alpha-alumina additive is
advantageous in that the abnormal growth is suppressed
and thus a number of carbon particles are prevented from
trapping in the formed àlpha-alumina particles. This
facilita-tes removal of the carbon by oxidation, thus
leading to a reduced content of residual carbon in a
final aluminum nitride powder product.
The present invention is more particularly
described by way of examples.
Example 1
Mixtures used to produce aluminum nitride were
prepared by the following procedures (1) through (11),
sintered in a nitrogen atmosphere at 1450C for 5 hours
and decarbonized at 650C for 3 hours to obtain aluminum
nitride powders. It will be noted that the procedures
(1) to (4) are for the invention, the procedures (5) to
(10) are for comparison, and the procedure (11) is for a
known method.
(1) Carbon and water were added to a 20 wt%
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boehmite soL dispersed at a pH of 3.0 (boehmite
powder/carbon ratio by weight of 1), followed by
kneading in a pot mill and drying by heating.
(2) Carbon, water and a carboxylic acid
dispersant were added to a 20 wt% boehmite sol dispersed
at a pH of 3.0 (boehmite powder/carbon ratio by weight
of 1), followed by kneading in a pot mill and drying by
heating.
(3) A 20 wt% boehmite sol dispersed at a pH of
3.0 was prepared. Water and a dispersant were added to
carbon and kneaded for 10 hours in a ball mill to obtain
a good dispersion having a carbon content of 25%.
The boehmi~te sol and the carbon dispersion
were kneaded in a pot mill for 10 hours, and heated
while agitating to remove the water for gelation, and
dried.
(4) A 20 wt% boehmite soI was prepared by
dispersion at a pH of 3Ø Water and a dispersant were
added to carbon, to which, prior to kneading in a pot
mill, Y(NO3)3.6H2O was added in an amount of 3 wt%,
calculated as Y2O3, based on an aluminum nitride powder
after sintering. Then, the procedure of ~3) was
repeated for gelation and drying.
(5) A dispersion of 20 wt% of boehmite was
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adjus-ted in pH to 1, whereupon abrupt gelation took
place, making a difficulty in kneading with carbon.
(6) Carbon and water were added to 20 wt~
boehmite sol dispersed at a pH of 5.0 (boehmite
powder/carbon ratio by weight of 1), followed by
kneading in a pot mill for 10 hours and drying by
heating.
(7) The procedure (1) was repeated except that
bayerite (Al(OH)3) was used instead of the boehmite.
(8) The procedure (1) was repeated except that
alpha-alumina (alpha-A1203) having a primary particle
size of about 0.3 microns was used instead of the
boehmite.
(9) Carbon and water were added to an aqueous
20 wt~ of solution of aluminum nitrate (aluminum
hydroxide/carbon ratio by weight of 1), followed by
kneading in a hot mill for 10 hours in the same manner
as in (1). An aqueous ammonia solution was added to the
mixture to form a neutralized precipitate of aluminum
hydroxide (mainly composed of bayerite) containing the
fine carbon powder and dried.
(10) Carbon was suspended in an ethanol
solution of aluminum isopropoxide (aluminum
hydroxide/carbon ratio by weight of I), followed by
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kneadiny in a pot mill for 10 hours in the same manner
as in (1). Wa-ter was added to the mixture for
hydrolysis of the propoxide to cause precipitation of
aluminum hydroxide (mainly composed of bayerite)
containing the carbon fine powder, and dried.
(11) 3 wt% of Y203 having an average size of 1
micron was added to the aluminum nitride powder prepared
in (3), followed by kneading in a pot mill for 10 hours
and drying.
The size distribution of the 9 aluminum
nitride powders obtained from the mixtures except for
the mixture (5) among (1) to (10) is shown in Fig. 1.
Fig. 1 reveals that the powders (1) to (4) of the
invention have a narrow size distribution and
particularly, (3) has a very narrow size distribution
with an average size of 0.7 microns. The powders (6)
and (7) for comparison have wide size distribution
because the dispersability of the boehmite or bayerite
is very poor and coarse particles are contained in large
amounts. In (1) to (7), all the powders are made of an
aluminum nitride single phase with a content of oxygen
of not larger than 0.3 wt~. The powders from (8), (9)
and (10) for comparison have the size distribution
between those of (1) to (4) of the invention and (6),
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~7) for comparisonl but alpha-alumina was found to be
left in an amount of from 2 to 5 wt% after sintering.
The aluminum ni-tride powders obtained from (4)
of the invention in which the sintering agent was
uniformly dispersed in the powder and from (11) of the
known method were, respectively, pelletized by press
molding for comparison of sinterability. After
sintering 1800C x 3 hours, the density of (4) was 99.3%
and the density of (11) was 98.1~, with the pellets of
(4) having a better appearance. The pellets of (11~
were found to have a substantial degree of irregular
sintering.
Example 2
Mixtures for preparing aluminum nitride were
prepared with formulation indicated in (21) to (29) of
Table 1. These mixtures were each sintered in a
; nitrogen atmosphere at 1450C for S hours, and
decarbonized at 650C for 3 hours to obtain an aluminum
nitride powder.
The formulations (23), (24), (25) and (29) are
used to indicate criticality with respect to the mixing
ratio of different types of carbon sources and the total
carbon content. The other formula-tions ~21), (22),
(26), (27) and (28) are usable ln the practice of the
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inven-tion but outside the preferable range.
All the mixtures were prepared according to
the following procedure.
A 20 wt% boehmite sol dispersed a-t a pH of 3.0
was prepared. Separately, a dispersant and water were
added to solid carbon and knea~ed in a pot mill for 10
hours to obtain a dispersant of 15 wt~ of carbon. The
sol and the dispersion were mixed together, to which
predetermined amounts of a water-soluble organic carbon
source material were added. After the addition, each
mixture was heated while agitating to remove the water
and gelled, and dried.
The size distribution of the results of nine
aluminum nitride powders (21) to (29) is shown in Fig.2.
The contents of A12O3 and carbon after the nitriding
reaction and the decarbonization are shown in Table 2.
It will be noted that numerals ~21j to (29) in the
figure and table, respectively, correspond to (21) -to
(29) in Table 1.
From Fig. 2 and Table 2, it will be seen that
the nitride powders of (21) and (22) are relatively
large in size and have larger contents of A12O3 in the
aluminum nitride powders than those of (23), (24), (25)
and (29) according to the preferred embodiment.
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Further, the final aluminum nitride powders of (21) and
(22) tend to suffer a larger lot~to-lot varia-tion with
respect to the size distribution and the A12O3 conten-t.
In contrast, the aluminum nitride powders
(23), (24), (25) and (29) within a preferred mode of the
invention are finer in size, narrower in size
distribution, and smaller in lot-to-lot variation with
reduced contents of A12O3 and carbon.
The nitride powders (26) and (27) are somewhat
inferior in nature of the lamps obtained after the
nitriding reaction and are more liable to be
contaminated with impurities when broken into pieces.
Even when an alumina milling device i9 used, the content
of alumina increases as will be apparent from Table 2.
With the nitride powders (26), (27), a carbon
content after the decarbonization is larger.
The nitride~powder (28) is inferior to the
powders of the preferred embodiment with respect to the
A1203 content and the size of the aluminum nitride
powder.
From the above, it will be appreciated that
the mixing ratio between the carbon derived from an
organic material and a solid carbon powder is preferably
in the range of from 0.05:1 to 0.5:1. With regard to
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the ratio between the -total carbon content and the
content of boehmite, it was confirmed tha-t the ratio is
preferably in the range of 0.4:1 -to 3.0:1.
Table 1 Mixing ratio of Carbon Derived From Organic
Materials and Ratio of Total Carbon and
Boehmite
____ ___________________________ _ _ _ ______ ___ ___
No. 21 22 23 24 25 26 27 28 29
_____ ____
Mixing Ratio By Weight of Derived Carbon and Solid
Carbon Powder:
0 0.03 0.07 0.20 0.40 0.60 0.80 0.20 0.20
Ratio By Weight of Total Carbon Content and Boehmite
Content~
; 1 ~ 0 1 ~ 0 l o O 1 ~ 0 1 ~ 0 1 ~ 0 1 ~ 0 0 ~ 3 0 ~ 5
_______ __ _____ _______ ______________________ __
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Table 2 Contents of A12O3 and Carbon After ~eductive
Nitriding reaction and Decarbonization (wt~)
_______________.________________________________________
No. 21 22 23 24 25 26 27 28 29
____________________ ______________.____ ~_____________
Content of Residual Alumina After Reductive Nitriding
Reaction (amount relative to aluminum nitride):
0.7 0.7 0.5 0.5 0.5 0.5 0.5 5.0 0.5
Content of Residual Alumina After Decarbonization
(amount relative to aluminum nitride)*:
1.0 1.0 0.8 0.8 0.8 1.3 1.5 7.0 0.8
Content of Residual Carbon After Decarbonization (amount
relative to aluminum nitride)*:
0.08 0.08 0.09 0.09 0.10 0.3 0.5 0.09 0~09
~ -- ______
; * Decarbonized after milling in aIumina milling machine.
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: Example 3
Mixtures for preparing aluminum nitride were
prepared as having formulations (31) to (3~9) indicated
in Table 3, sintered in a nitrogen atmosphere at 1600C
f~or 5 hours, and decarbonized at 650C for 3 hours,
~ thereby obtaining aluminum nitride powders. The content
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of alumina additive is varied from 0 to 100~, in which
the formulations (38) and (39) are outside the range of
the invention. The other formulations are within the
scope of the invention although some are not in a
preferred range.
The mixtures were all prepared substantially
according to the ollowing procedure.
~ boehmite sol having a concentration of 20
wt% and dispersed in water at a pH of 3.0 was prepared.
Separately, a dispersant and water were added to a solid
carbon powder and kneaded in a pot mill for 10 hours to
obtain a dispersion having a carbon content of 15 wt%.
The sol and the dispersion were mixed so that
the ratio by weight of the carbon and the boehmite was
1. Thereafter, a dispersion of 10 wt% of alpha-alumina
powder (having an average size of 0.3 microns) dispersed
in water at a pH of 3.0 was added in a predetermined
amount, followed by heating to remove the water for
gelation and drying.
The test was effected under the respective
cond1tions five times.
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Table 3
_______________________ ____ _ ______ ____________ ___
No. 31 32 33 34 35 36 37 38 39
_.______________________ ____ _ _ ________________ ____
alpha-alumina:
0 0.001 0.02 0.1 1 10 ~0 60 100
_______________________ ______________________________
The size distribution of the nine aluminum
nitride powders of (31) to (39~ is shown in Fig. 3.
From the figure and through scanning electron
microscopic obs~rvation, it was confirmed that the
nitride powders of (31), (32), (38) and (39) are wide in
size distribution with a relatively large average
particle size and that particles abnormally grown up to
a size not less than 5 microns are found especially in
(38) and (39).
~ With the nitride powders obtained from (33),
(34), (35), (36) and (37), the particle size i9 smaller
with a narrower size distribution. No abnormally grown
particles were found with a small scattering in size
between test lots.
In view of the above results, the content of
alpha-alumina is preferably in the range of from 0.01 to
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50 wt~ based of the boehmite powder although less
amounts are usable in the practice of the invention.
However, larger amounts are not within the scope of the
invention for the reasons stated hereinbefore.
The aluminum nitride powders obtained
according to the invention has specific utility in the
field of high temperature structural materials and IC
boards.
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