Silica particles and process for producing the same

Silica particles having an average particle size of 5 to 100 .mu.m, a BET specific surface area of not more than 20 m.sup.2 /g and a pore volume of not more than 0.1 ml/g, each of which has on its surface a plurality of protuberances with a smooth configuration , a diameter at half the protuberance height of the said protuberance being 0.2 to 5.0 .mu.m and a height of the said protuberance being 0.2 to 4.0 .mu.m, and a process for producing the silica particles, which comprises (A) reacting hydrosilicofluoric acid, ammonium silicofluoride or a mixture thereof with ammonia in an aqueous medium to form a silica slurry, (B) separating a silica from the silica slurry, and (C) calcining, after optionally drying, the separated silica at a temperature of not less than 500.degree. C.

BACKGROUND OF THE INVENTION 
The present invention relates to silica particles having protuberances in 
their surfaces, which particles are to be incorporated in resin or rubber, 
and a process for producing the silica particles. The silica particles 
according to the present invention are useful as a filler for 
semiconductor sealant resin compositions, polyesters, engineering plastics 
and silicone rubber. 
Various kinds of inorganic fillers, especially silica, are popularly used 
for the purpose of improving the properties of high polymeric materials. 
For instance, in IC sealants, a silica filler in a ratio of about 70 wt % 
are blended with the matrix resin such as epoxy resin, polyphenylene 
sulfide, polyimide, etc., for the purpose of adjusting the coefficient of 
thermal expansion. With a recent tendency toward higher integration of 
semiconductors and oversizing of chips, a request is rising for 
high-purifying of sealant, lower stress and improvement of crack 
resistance. However, with the present state of art, these requirements are 
not yet fulfilled sufficiently. 
The conventional silica filler has been obtained by the following methods: 
(1) melt pulverization method of natural quartz, 
(2) calcination and pulverization method of silica gel prepared by using 
sodium silicate, 
(3) decomposition method of silicon tetrachloride by oxyhydrogen flames, 
and 
(4) sol-gel method of alkoxysilane. 
These methods, however, have the problems set forth below. 
The method (1) has a problem in stable supply of natural quartz of good 
quality. It is also impossible with this method to reduce the content of 
the radioactive elements such as uranium, thorium, etc., which could cause 
erroneous operation (soft errors) of IC's. Silica produced by the method 
(2) is unusable as a silica sealant for highly integrated semiconductor 
devices because of probability of contamination with metal components such 
as sodium, aluminum, etc., or ionic impurities from sodium silicate used 
as a starting material or a neutralizing agent. The method (3) has 
problems of high production cost and difficulty in controlling particle 
size and shape. It is also impossible with this method to avoid inclusion 
of chlorine ions as an impurity. The method (4), although capable of 
producing high-purity silica by the purifying-treatment of the raw 
material, is complicated in production process, so that the produced 
silica becomes costly and its use is limited. 
As regards the shape of silica filler, there has been mostly used square 
silica produced by the molten silica pulverization method. A composition 
containing the square silica, however, is poor in fluidity and its use 
tends to cause "hard errors" such as deformation of wiring or cracking of 
package due to stress of the sharp ends of the particles, so that recently 
spherical silica has come to be used usually for the improvement of 
fluidity and dispersion of stress. However such spherical silica still has 
the problems that it is relatively costly and that adhesion to resin is 
deteriorated, resulting in reduced strength of the molded package, since 
the silica particle surface is smoothened as a result of melting or 
sintering during the heat treatment in the production process. 
As prior arts regarding silica particles having specific shapes, Japanese 
Patent Application Laid-Open (KOKAI) No. 182212/88 discloses the perfectly 
circular silica particles having a jagged surface, and U.S. Pat. No. No. 
4,010,242 discloses a method of producing fine particles by using 
colloidal particles of 5 to 500 m.mu. as a starting material. 
The silica particles proposed hitherto had the various problems in 
preparation and practical application, and accordingly the development of 
new silica particles free from the said technical problems such as 
concentration of stress, low fluidity and unsatisfactory adhesion to 
resin, which are observed in use of the conventional square or spherical 
silica as a filler has been required. 
As a result of the present inventors, extensive and intensified studies for 
solving the said problems, it has been found that by (A) reacting 
hydrosilicofluoric acid, ammonium silicofluoride or a mixture thereof with 
ammonia in an aqueous medium to form a silica slurry, (B) separating a 
silica from the said slurry by means of a solid/liquid separation and (C) 
calcining, after optionally drying, the separated silica at a temperature 
of not less than 500.degree. C., the silica particles having a specific 
surface configuration can be obtained, which are excellently suited for 
use as a filler. The present invention was achieved on the basis of this 
finding. 
SUMMARY OF THE INVENTION 
In a first aspect of the present invention, there are provided silica 
particles having an average particle size of 5 to 100 .mu.m, a BET 
specific surface area of not more than 20 m.sup.2 /g and a pore volume of 
not more than 0.1 ml/g, each of the particles having on its surface a 
plurality of smoothly configured protuberances, a diameter at half the 
protuberance height of the said protuberance being 0.2 to 5.0 .mu.m and a 
height of the said protuberance being 0.2 to 4.0 .mu.m. 
In a second aspect of the present invention, there is provided a process 
for producing the silica particles having an average particle size of 5 to 
100 .mu.m, a BET specific surface area of not more than 20 m.sup.2 /g and 
a pore volume of not more than 0.1 ml/g, each of the particles having on 
its surface a plurality of smoothly configured protuberances, a diameter 
at half the protuberance height of the said protuberance being 0.2 to 5.0 
.mu.m and a height of the said protuberance being 0.2 to 4.0 .mu.m, which 
process comprises the steps of (a) reacting hydrosilicofluoric acid, 
ammonium silicofluoride or a mixture thereof with ammonia in an aqueous 
medium to form a silica slurry; (b) separating silica from the said slurry 
by means of a solid/liquid separation; and (c) drying optionally, and 
calcining the separated silica at a temperature of not less than 
500.degree. C. 
In a third aspect of the present invention, there is provided a composition 
comprising (i) silica particles having an average particle size of 5 to 
100 .mu.m, a BET specific surface area of not more than 20 m.sup.2 /g and 
a pore volume of not more than 0.1 ml/g, each of the particles having on 
its surface a plurality of protuberances, a diameter at half the 
protuberance height of the said protuberance being 0.2 to 5.0 .mu.m and a 
height of the said protuberance being 0.2 to 4.0 .mu.m, and (ii) a 
macromolecular material.

DETAILED DESCRIPTION OF THE INVENTION 
The average particle size of the silica particles according to the present 
invention is in the range of 5 to 100 .mu.m, preferably 5 to 50 .mu.m. 
When the average particle size is less than 5 .mu.m, a composition 
containing the particles is deteriorated in fluidity and moldability, and 
when the average particle size exceeds 100 .mu.m, it becomes impossible to 
fill silica to a high density in a composite and mechanical strength of 
the composite is reduced. Although the average particle size is in the 
range of 5 to 100 .mu.m, it is also undesirable, for the same reason, that 
fine silica particles of less than 5 .mu.m in diameter or coarse silica 
particles exceeding 100 .mu.m in diameter be contained at a high 
percentage. 
The specific surface area, as measured by the BET method, of the silica 
particles according to the present invention is not more than 20 m.sup.2 
/g, preferably not more than 10 m.sup.2 /g. When the BET specific surface 
area is more than 20 m.sup.2 /g, there arise the adverse effects by the 
high content of fine particles, and also mechanical strength of the silica 
particles themselves is reduced. In use of the silica particles of the 
present invention as a filler for an IC sealant, the BET specific surface 
are of the said particles is preferably less than 10 m.sup.2 /g, more 
preferably less than 5 m.sup.2 /g. 
The pore volume of the silica particles of the present invention is not 
more than 0.1 ml/g, preferably not more than 0.05 ml/g. When it exceeds 
0.1 ml/g, the mechanical strength of the particles themselves is reduced, 
and also air tends to be trapped in the pores, thereby causing 
air-release, or formation of voids in the course of kneading with a resin 
or in practical use of the composite. This is the cause of an unfavorable 
influence on quality of the composite or the products. 
The protuberances in the surfaces of the silica particles according to the 
present invention have a smooth configuration, in other words a generally 
roundish configuration. The term "smooth configuration" used herein is to 
be understood to mean that there exists no sharp or spicular part on the 
particle surface. A height of such a protuberance is 0.2 to 4.0 .mu.m, 
preferably 0.2 to 3.0 .mu.m, in the case that the height is measured from 
the bottom of the recess between the adjoining protuberances. Also, a 
diameter of such a protuberance is 0.2 to 5.0 .mu.m, preferably 0.2 to 3.0 
.mu.m at the height level which is half of the height of the protuberance 
(at half the protuberance height). It is also preferable that the height 
of the protuberance is 1.0 to 0.6-fold of the diameter. 
An average number of protuberances per one silica particle is ordinarily 
not less than 100, preferably not less than 500, more preferably not less 
than 1000. 
The height of the protuberances is preferably not more than 30%, more 
preferably not more than 10%, based on the average particle size of the 
particles. 
The protuberances on the silica particle surface according to the present 
invention contribute to the improvement of adhesion between silica 
particles and matrix resin, resulting in enhanced mechanical strength of 
the resin composite. 
When the diameter at half the protuberance height is less than 0.2 .mu.m, 
the adhesion improving effect of the protuberances does not become 
conspicuous, and when the said diameter exceeds 5 .mu.m, the number of the 
protuberances which are allowed to exist is lessened and also the contact 
area with matrix resin is reduced, so that the adhesion improving effect 
does not become conspicuous in comparison with the conventional spherical 
silica. It is also to be noted that appropriate roundness of the 
protuberances on the particle surface is able to disperse stress as 
opposed to the conventional pulverized square silica particles in which 
stress is concentrated at the end portions. 
The silica particles of the present invention having the said protuberances 
of a specific configuration on the surface thereof and also possessing the 
said properties can be obtained typically by reacting hydrosilicofluoric 
acid and/or an ammonium salt thereof with ammonia in an aqueous medium, 
separating a silica and calcining the thus separated silica. Also, in case 
of using the starting materials, it is possible to produce the silica 
particles which have not only the said specific configuration and 
properties but also high purity as these particles are substantially free 
of metallic impurities. 
The production process of the silica particles according to the present 
invention comprises the following steps: 
(A) To react hydrosilicofluoric acid and/or an ammonium salt thereof with 
ammonia in an aqueous medium to form a silica slurry; 
(B) To separate a silica from the slurry by means of a solid/liquid 
separation; and 
(C) To dry separated silica optionally, and to calcine the resultant 
silica. 
As hydrosilicofluoric acid used as a starting material in the step (A), 
there can be used the commercially available reagents or the by-product 
formed in the wet preparation process of phosphoric acid. It can be also 
produced by aqueous liquid absorption of a gas containing silicon 
tetrafluoride. This hydrosilicofluoric acid is used in the form an aqueous 
solution with a concentration of usually about 5 to 40%. 
Ammonium salt of hydrosilicofluoric acid, namely ammonium silicofluoride 
can be easily obtained by adding gaseous or aqueous ammonia to 
hydrosilicofluoric acid. It is also obtainable by ammonium fluoride liquid 
absorption of a gas containing silicon tetrafluoride. 
In the present invention, hydrosilicofluoric acid and ammonium 
silicofluoride can be used either singly or in the form of a mixture. 
Ammonia may be used either in a gaseous state or in the form of an aqueous 
solution. 
The purity of the starting reactants can be properly selected according to 
the use of the produced silica. For obtaining the silica particles with 
high purity, hydrosilicofluoric acid is purified either directly by 
distillation or through aqueous liquid absorption of a silicon 
tetrachloride-containing gas by acid decomposition, while ammonium 
silicofluoride is purified by recrystallization or other suitable methods. 
Also, if necessary, both ammonia of a high purity and an aqueous medium 
which contains little metal impurity may be used. 
As an aqueous medium, water and an inert aqueous organic solvent which 
reacts with neither a raw material nor a reaction product may be used. 
Water is preferable in the handling. 
The reaction of hydrosilicofluoric acid and/or an ammonium salt thereof 
with ammonia is as shown below by reaction formulae: 
EQU H.sub.2 SiF.sub.6 +2NH.sub.3 .fwdarw.(NH.sub.4).sub.2 SiF.sub.6 
EQU (NH.sub.4).sub.2 SiF.sub.6 +4NH.sub.3 +2H.sub.2 O.fwdarw.SiO.sub.2 
+6NH.sub.4 F 
The molar ratio of ammonia to hydrosilicofluoric acid is 6 
stoichiometrically, but usually it is in the range of 3 to 10, preferably 
4 to 8. When the said molar ratio is low, the reaction rate is lowered 
correspondingly and pH of the final reaction slurry is on the acid side or 
around neutrality. When the said molar ratio is high, the reaction rate is 
raised and pH of the final reaction slurry is on the basic side. The pH of 
the reaction system is an important factor for controlling the shape of 
silica particles or their properties. 
The molar ratio of ammonia to ammonium silicofluoride is 4 
stoichiometrically, but usually it is in the range of 1 to 8, preferably 2 
to 6. 
The reaction may be carried out either batch-wise or continuous. As for the 
way of addition of the starting materials, the following methods are 
usable: 
(1) Ammonia is added into an aqueous medium containing hydrosilicofluoric 
acid and/or an ammonium salt thereof. 
(2) Contrary to method (1), hydrosilicofluoric and and/or an ammonium salt 
thereof is (are) added into an aqueous medium containing ammonia. 
(3) Hydrosilicofluoric acid and/or an ammonium salt thereof and ammonia are 
added simultaneously into the aqueous medium. 
(4) The above methods are used in combination. 
When the method (1) is used, silica is produced in a pH region on the acid 
side in the early phase of reaction, and although the result is variable 
according to the other reaction conditions, there are generally obtained 
the silica particles which are small in size (height and diameter) of 
protuberances on the surface thereof, relatively compact and fine. 
According to the method (2), silica is produced in a high pH region and 
there is a tendency toward production of coarse silica particles with 
relatively large-sized protuberances on the particle surface. 
When using the method (3), the silica particles can be grown under a fixed 
pH condition by controlling the starting material feed rate to the 
reaction system. 
In the production of the silica particles according to the present 
invention, the method (3), namely simultaneous addition of the component 
materials, is preferred. Especially, continuous supply of the two starting 
component materials in the presence of seed silica occurs the growth of 
silica particles and there can be easily obtained the silica particles 
having the said protuberances on the surface. The seed silica may be 
produced according to the method (1). 
The reaction of hydrosilicofluoric acid and/or an ammonium salt thereof 
with ammonia in an aqueous medium is usually started at a temperature in 
the range of 0.degree. to 100.degree. C., typically at around room 
temperature, and the reaction is conducted on accompanied by a rise of 
temperature of about 10.degree. to 50.degree. C. by heat of the reaction. 
In some cases, the reaction can be conducted at a high temperature above 
80.degree. C. 
The above reaction is ordinarily carried out under normal pressure or 
therearound, but it may be conducted under pressure at a temperature above 
100.degree. C. or under reduced pressure at a temperature below 80.degree. 
C. The rise of reaction temperature has a tendency to enlarge the particle 
size of the produced silica particles or the protuberance diameter. 
The reaction is ordinarily carried out in the range of pH 4 to 11, 
preferably 5 to 10. 
Reaction time can not be specified as it is variable depending on the 
reaction style, starting material addition method and other reaction 
conditions. But, usually, it is in the range from one minute to 10 hours, 
preferably from 10 minutes to 5 hours. After the reaction, the reaction 
mixture may be subjected to aging. 
The silica concentration in the reaction system is usually in the range of 
0.5 to 18 wt %, preferably 1 to 10 wt %, more preferably 2 to 7 wt %. A 
too low silica concentration results in a poor productivity of the 
objective silica particles, while a too high silica concentration makes it 
difficult to control the reaction. 
In carrying out the above reaction according to the present invention, the 
reaction system may contain, beside the starting materials, an additive or 
additives such as various types of salts (ammonium fluoride, ammonium 
chloride, ammonium sulfate, etc.), polyvinyl alcohol, binder such as 
cellulose, surfactant, etc. 
Thus, in the step (A) of the process according to the present invention, a 
silica slurry is obtained by carrying out the above-described reaction by 
properly selecting the said reaction conditions such as the kind and molar 
ratio of the starting materials, reaction style, adding method, pH, 
temperature, silica concentration, additive(s), etc., and the slurry thus 
obtained is subjected to the next step (B). 
Since the silica slurry formed by the above reaction coexists with 
by-produced ammonium fluoride solution, in the step (B) the reaction 
product is subjected to a solid/liquid separation such as filtration under 
the reduced pressure, filtration under the pressure, centrifugation, etc., 
to form silica cakes. Since the obtained silica cakes contain a small 
quantity of ammonium fluoride due to adhesion of the mother liquor, the 
said cakes are washed with water or an organic solvent to remove the said 
ammonium fluoride. It is also possible to remove the coexisting ammonia by 
conducting a suitable treatment such as an acid washing or a hot water 
extraction. 
The water and acid used for washing or the said treatment need to be 
substantially free of impurities (especially metals). 
In the step (C), the said wet silica is calcined, after drying if 
necessary, to obtain the silica particles of the present invention having 
the said configuration. 
The drying can be effected by commonly employed a drying method such as 
hot-air drying, vacuum drying, spray drying, flash drying and fluidized 
drying at a temperature in the range of 50.degree. to 200.degree. C. 
The calcination is performed at a high temperature above 500.degree. C., 
preferably above 900.degree. C., but use of an excessively high 
temperature is undesirable for maintaining the specified shape and 
properties of the silica particles obtained. Usually the calcining 
temperature is below the melting point of silica, preferably below 
1,500.degree. C. 
Due to the product prepared by reacting hydrosilicofluoric acid and/or an 
ammonium salt thereof with ammonia in an aqueous medium, the silica 
particles obtained according to the present invention can be worked into a 
high-purity product with high added-value as a filler for resin, rubber 
and other materials, and the metallic impurity in the composite can be 
reduced to a level below 1,000 ppm, preferably below 100 ppm, more 
preferably below 10 ppm, expressed as the total content of metal oxides. 
In use of the silica particles of the present invention as a material for 
electrical parts, it is preferable that the particles are substantially 
free of alkaline metals such as sodium and potassium. Also, the silica 
particles produced according to the present invention can meet the 
requirement of low .alpha.-ray emission in use as an IC sealant since 
according to the production process of the present invention, there can be 
formed the silica particles with a content of radioactive elements such as 
uranium and thorium below 10 ppb, preferably below 1 ppb, more preferably 
below 0.5 ppb, by purification treatments in the stage of raw material. 
The silica particles of the present invention are expected to find a wide 
scope of practical use, but they are particularly useful as a filler for 
improving the properties of macromolecular materials such as resins and 
rubbers by making use of the said specific shape and properties of the 
particles. Especially, the silica particles are useful as a reinforcing 
agent or an adjuster of the coefficient of the thermal expansion of the 
macromolecular material. 
The amount of the silica particles used depends on the kinds of the 
macromolecular materials and purposes of the use, and it is not specified, 
but the amount of the silica particles is ordinarily 1 to 95%, preferably 
5 to 90% by weight based on the total amount of the macromolecular 
material and the silica particles. 
As the resin for the matrix of the composite of the present invention, 
there can be used thermoplastic resins such as polyvinyl chloride, 
polyethylene, polypropylene, polytetrafluoroethylene, polystyrene, 
poly(meth)acrylic esters, polycarbonate, polyester, polyamide, 
polysulfone, polyphenylene sulfide, liquid crystal polymers, etc., and 
thermosetting resins such as phenol resin, urea resin, melamine resin, 
epoxy resin, polyimide, unsaturated polyester resin, etc. 
As the rubber therefor, natural rubber, synthetic rubber, silicone rubber, 
etc. may be exemplified. 
The composite can be obtained by molding a composition comprising the 
silica particles and the macromolecular material by the conventional 
method. 
The composition may be obtained by mixing the silica particles and a powder 
of the macromolecular material or by kneading the silica particles and the 
macromolecular material and pulverizing. The composition is excellent in 
fluidity and moldability. 
The composite or the composition composed of the silica particles of the 
present invention and the resin or the rubber may contain an additive such 
as a coupling agent and a release agent, in ordinarily usable amount. 
Also, a surface treatment, for example, a treatment with a coupling agent 
such as a silicone compound or titanium compound, and surface graft 
polymerization treatment may be conducted on the surface of the silica 
particles for the purposes of further improvement of adhesion and 
compatibility between matrix resin and silica particles, and prevention of 
intrusion (absorption) of moisture from the outside of the system to 
enhance performance of the silica particles. 
Especially, a composite comprising the silica particles and a epoxy resin 
is useful as an IC sealant. 
As described above, since the silica particles of the present invention has 
the high purity, and the specific surface configuration, such silica 
particles according to the present invention are free from the 
concentration of stress, and have a sufficient adhesion to the resin. 
Further, the silica particles are useful as a raw material of the 
composite which is excellent in their properties such as a bending 
strength. [Examples] 
The present invention is described in further detail below with reference 
to the non-limitative examples thereof. 
Properties of silica particles were evaluated according to the followings: 
(1) Average particle size 
Measured by a laser diffraction particle size distribution analyzer. 
(2) Specific surface area 
Measured according to BET method with using nitrogen. 
(3) Pore volume 
Measured according to a nitrogen-adsorption method. 
(4) Height and diameter of a protuberance 
A height and a diameter of a protuberance were measured on a electron 
micrograph. 
(5) The contents of metal impurities 
From a sample, a silica component was removed as a volatile component by 
hydrofluoric acid treatment. The residue was dissolved by a dilute acid, 
the contents of metal impurities were determined according to ICP 
(Inductivity Coupled Plasma) emission spectroscopic analysis. 
(6) Ignition loss 
After a sample which was previously dried at 105.degree. C. was heated at 
950.degree. C. for 1 hour, a loss in weight was measured. 
EXAMPLE 1 
Into a 20-liter reactor provided with a stirrer (stirring reactor), 3 kg of 
25% ammonia water was supplied. Then, 11 kg of 20% hydrosilicofluoric acid 
solution and 4.5 kg of 25% ammonia water were added thereto simultaneously 
at room temperature at the flow rates of 0.37 kg/hr and 0.15 kg/hr, 
respectively, by using constant delivery pumps. The resultantly formed 
silica slurry was centrifuged. The ammonium fluoride solution was removed 
as mother liquor and the silica cakes were washed with 100 liters of water 
and 5 liters of methanol. The washed silica cakes were subjected to 
12-hour hot-air drying at 105.degree. C. and then calcined in an electric 
muffle furnace at 1,200.degree. C. for 2 hours to obtain 0.9 kg of 
calcined silica particles A. 
These silica particles, as observed from their scanning electron 
micrographs (see FIGS. 1 and 2), had a plurality of roundish protuberances 
forming ruggedness over the whole particle surface. 
Protuberances were observed, which had the heights of 0.3 to 2.0 .mu.m and 
the diameters at half the protuberance heights of 0.3 to 2.0 .mu.m. 
The average particle size was 25.8 .mu.m. The particles also had a BET 
specific surface area of 2.5 m.sup.2 /g and a pore volume of 0.01 ml/g. 
The content of metallic impurities calculated in terms of contents of 
oxides was as follows (unit: ppm): 
______________________________________ 
A1.sub.2 O.sub.3 
Fe.sub.2 O.sub.3 
CaO NaO.sub.2 
0.6 3.3 3.0 0.3 
______________________________________ 
A blend of the silica particles A and the following materials was kneaded 
by heated rolls, then cooled and pulverized to obtain an epoxy resin 
composition. 
______________________________________ 
Blend formulation 
______________________________________ 
Cresol novolak epoxy resin 
19 (parts by weight) 
Phenol novolak resin 8 
c-glycidoxypropyltrimethoxy- 
2 
silane (coupling agent) 
Releasing agent 1 
Silica particles A 70 
______________________________________ 
In addition, the same composition as above was prepared except that square 
silica particles (average particle size: 25 .mu.m) produced by the 
crushing method was used instead of the silica particle A. 
The composition containing the silica particle A was excellent in fluidity, 
compared with the composition containing square silica particles (average 
particle size: 25 .mu.m) produced by the crushing method. 
EXAMPLE 2 
Into a 10-liter stirring reactor, 3.6 kg of purified 20% hydrosilicofluoric 
acid solution was supplied, followed by addition of 2.65 kg of 25% ammonia 
water over a period of 30 minutes. The reaction temperature rose from 
20.degree. C. to 57.degree. C. and fine silica particles were produced. 
The resultantly formed silica slurry was homogenized by stirring. 1.25 kg 
of this silica slurry was collected and transferred into an another 
10-liter reactor. Into this seed silica slurry were added simultaneously 
3.6 kg of purified 20% hydrosilicofluoric acid solution and 2.45 kg of 25% 
ammonia water over a period of one hour. The temperature of the reaction 
solution rose from 21.degree. C. to 38.degree. C. There was obtained 7.1 
kg of primary reaction silica slurry. 
Into the other 10-liter reactor, 1.25 kg of this primary reaction silica 
slurry was transferred, and hydrosilicofluoric acid solution and ammonia 
water were added thereto simultaneously under the same conditions as in 
the case of the primary reaction to obtain 7.2 kg of secondary reaction 
silica slurry. 
This secondary reaction silica slurry was subjected to a solid/liquid 
separating operation by using a vacuum filter to remove the ammonium 
fluoride solution (mother liquor) containing excess ammonia, and the 
silica cakes were washed with a large amount of pure water and then 
hot-air dried at 120.degree. C. 
The dried silica cakes were then calcined at 1,250.degree. C. for 3 hours 
to obtain 0.33 kg of silica particles B of the present invention. 
The impurity content (ppm) expressed as contents of metal oxides was as 
follows: 
______________________________________ 
A1.sub.2 O.sub.3 
Fe.sub.2 O.sub.3 
CaO NaO.sub.2 
0.3 0.6 0.2 &lt;0.1 
______________________________________ 
The contents (ppb) of the radioactive elements were as follows: 
EQU U&lt;1.0 Th&lt;1.0 
These silica particles B as observed from their scanning electron 
micrographs (see FIGS. 3 and 4) had a plurality of roundish protuberances 
over the whole particle surface. Protuberances were observed, which had 
the heights of 0.5 to 3.0 .mu.m and the diameters at half the protuberance 
heights of 0.5 to 3.0 .mu.m. The average particle size was 26.0 .mu.m. 
These particles also had a BET specific surface area of 0.6 m.sup.2 /g and 
a pore volume of 0.003 ml/g. 
In the same way as Example 1, compositions were prepared except that the 
silica particles B and spherical silica particles having an average 
particle size of 25 .mu.m were respectively used. 
Each of the compositions was cast into a heated mold to obtain a hardened 
body (composite). The bending strength of each of the composites was 
measured. The bending strength of the composite containing the silica 
particles B was larger by 30% than that of the composite containing the 
spherical silica particles having an average particle size of 25 .mu.m. 
EXAMPLE 3 
Into a 20-liter stirring reactor, 7.5 kg of 25% ammonia water was supplied, 
and then 11 kg of purified 20% hydrosilicofluoric acid solution was added 
thereto over a period of 60 minutes to form a silica slurry. This silica 
slurry was filtered under a reduced pressure, and the formed silica cakes 
were washed with 150 liters of water and one liter of methanol and then 
hot-air dried at 105.degree. C. to obtain dry silica particles C'. These 
dry silica particles C' were calcined in an electric furnace at 
1,200.degree. C. for 2 hours to obtain 0.9 kg of silica particles C of the 
present invention. 
These silica particles C, as observed from their scanning electron 
micrographs (see FIGS. 5 and 6), had a plurality of roundish protuberances 
forming a continuum of concavities and convexities over the whole particle 
surface. Protuberances were observed, which had the heights of 0.2 to 1.0 
.mu.m and the diameters at half the protuberance heights of 0.2 to 1.0 
.mu.m. 
The property data of said silica particles C and C' are shown below. 
______________________________________ 
Particles C 
Particles C' 
______________________________________ 
Average particle size (.mu.m) 
30.5 --*.sup.1 
BET specific surface are (m.sup.2 /g) 
1.8 314 
Pore volume (ml/g) 0.005 0.16 
Ignition loss (wt %) 
0.01 6.9 
______________________________________ 
*.sup.1 unmeasurable because of break of particles 
The impurity content (ppm) expressed as the contents of metal oxides was as 
shown below: 
______________________________________ 
A1.sub.2 O.sub.3 
Fe.sub.2 O.sub.3 
CaO NaO.sub.2 
0.5 0.2 0.5 &lt;0.1 
______________________________________ 
In the same way as Example 2, compositions and composites were prepared 
except that the silica particles C and the silica particles C' were 
respectively used. 
In the case that the silica particles C were used, fluidity, bending 
strength and packing property are excellent. But, in the case that the 
silica particles C' were used, there occurred air-release and break of the 
particles during kneading with the resin and the fluidity was inferior.