HK1061008B - Spherical alumina particles and production process thereof - Google Patents

Description

Alumina particles and method for producing the same

Cross reference to related patent applications

This application is an application filed under 35u.s.c.111(a) and was filed under 35 u.s.c.119(e) (1) for the application date of provisional application No. 60/296,753 filed under 35u.s.c.111 (b) on 6/11/2001.

Technical Field

The present invention relates to roundish alumina particles and an industrially economical method for producing alumina particles, which are particularly suitable for use as, for example, sealing materials for electronic parts; a filler; lapping materials and aggregates incorporated into refractory, glass, ceramic or composites thereof, and which are substantially abrasion-free and have good flow properties. The invention also relates to roundish alumina particles prepared by the method and a high thermal conductive rubber/plastic composition containing the alumina particles.

Background

In recent years, high integration and high density of electronic parts have increased power consumption per chip. Therefore, in the development of electronic components, effective removal of generated heat is a critical issue in order to suppress temperature rise of the electronic components. In view of the above, alumina having excellent thermal conductivity, particularly silicon carbide (α -alumina), has become a candidate for heat-dissipating gaskets, base materials of insulating sealing materials for fixing semiconductors and semiconductor device parts, and the like; improved aluminas have been used in many fields.

Among such corundum particles, JP-A HEI 5-294613 discloses ｃA non-disintegrated spherical corundum particle which is prepared by: to a ground alumina product such as fused alumina or sintered alumina, aluminum hydroxide and any known additives for crystallization promoters are added simultaneously, and the mixture is then fired.

A thermal spraying method has also been known in which alumina produced by the bayer process is atomized into a high-temperature plasma or oxy-hydrogen flame for melting and quenching, thereby producing round particles. The thermal spraying method requires a large amount of heat energy per unit and is therefore uneconomical. Further, although the alumina thus prepared mainly contains α -alumina, it contains by-products such as δ -alumina and has low thermal conductivity. Therefore, such an alumina product is not preferable.

The ground electrofused or sintered alumina product is known as corundum particles. However, these corundum particles have an indeterminate shape with a sharp cross-section, which, during their mixing into the rubber/plastic, causes great wear on the kneader, the mould, etc. Therefore, these corundum particles are not preferred.

According to the method disclosed in the above-mentioned document, it is possible to produce circular emery particles having no fracture planes and an average particle diameter of 5 to 35 μm. However, this method has some problems in producing large quantities of such corundum particles on a large scale at low cost.

The method disclosed in the above document includes: one or more substances selected from halogen compounds, boron compounds and alumina hydrates are added to the ground fused alumina and/or sintered alumina product of a predetermined particle size, and the resulting mixture is heated at 1,000 to 1,550 ℃. When using these methods, the fired product is strongly consolidated in the firing vessel to form an aggregate.

The above documents also disclose the addition of alumina hydrate to reduce the hardness of the aggregate. However, the effect is not satisfactory. Particularly when a large calcination vessel is used for mass production of a large amount of corundum particles, the resulting calcined aggregate becomes comparable to the size of the calcination vessel. When the aggregate is crushed and ground, multi-step crushing must be performed, resulting in very high costs.

Furthermore, since the fired product strongly adheres to the inner surface of the firing vessel, it is difficult to eliminate the fired product, requiring additional treatment such as application of mechanical stress for removal. In this case, a very large stress is applied to the firing vessel, which may result in breakage of the vessel itself. Therefore, these methods are not satisfactory from the economical point of view.

In order to solve the problems of the conventional methods described above, the present inventors have conducted extensive studies and found that: in the production method of spherical carborundum particles, the mixed composition is granulated before heat treatment and the resulting granules are fired, thereby solving these problems. The present invention has been completed based on these findings.

Disclosure of the invention

The invention provides a production method of roundish alumina particles, which comprises the following steps: granulating a composition comprising at least one of ground electrofused alumina and sintered alumina products having an average particle size of 5 to 35 μm and at least one member selected from the group consisting of halogen compounds, boron compounds and alumina hydrates to obtain a granulated product; heating the granulated product at 1,000 to 1,600 ℃; the heated product is then comminuted.

The method may further comprise the step of washing the heated product with an acid before or after the pulverizing step.

The above method may further comprise the step of washing the acid-washed product with pure water and then drying the water-washed product.

In the above method, the drying step is performed using a reduced-pressure drying apparatus.

In any of the above methods, the mean particle size of the fused alumina and the sintered alumina is 10 μm to 25 μm.

In any of the above methods, the halogen compound is at least one selected from AlF₃、NaF、CaF₂、MgF₂And Na₃AlF₆The substance of (1).

In any of the above methods, the boron compound is at least one selected from B₂O₃、H₃BO₃、mNa₂O·nB₂O₃Wherein each of m and n is an integer, and a fluoroborate compound.

In any of the above methods, the alumina hydrate is at least one selected from the group consisting of aluminum hydroxide, alumina gel, amorphous aluminum hydroxide, and partially hydrated aluminum compounds.

In any of the above methods, each of the fused alumina, sintered alumina and alumina hydrate has an alpha-ray index of 0.01c/cm²Hours or less.

In any of the above processes, the granulated product has a particle size of 1 to 30mm Φ.

Any of the above methods further comprises the step of adding at least one of water and an aqueous organic solution in an amount of 5 to 50 mass% of alumina hydrate during the granulation.

The present invention also provides roundish alumina particles produced by the above production method.

The roundish alumina particles thus produced had an average particle diameter of 5 to 35 μm.

The above roundish alumina particles had an alpha-ray index of 0.01C/cm²Hours or less.

The present invention also provides a highly thermal conductive rubber compound containing roundish alumina particles.

The invention also provides a high thermal conductivity plastic mixture containing roundish alumina particles.

Best Mode for Carrying Out The Invention

The present invention is described in detail below.

The invention provides a production method of roundish alumina particles, which comprises the following steps: granulating a composition comprising at least one electrofused and/or sintered alumina having an average particle diameter of 5 to 35 μm and at least one substance selected from the group consisting of halogen compounds, boron compounds and alumina hydrates to obtain a granulated product; heating the granulated product at 1,000 to 1,600 ℃; the heated product is then comminuted.

The coarse alumina particles used as the raw material in the present invention may be a ground product of fused alumina or a ground product of sintered alumina. In either case, the ground product is produced by any known method. The particle size distribution of the electric melting alumina or the sintering alumina is as follows: the average particle diameter is 5 to 35 μm, preferably 10 to 25 μm, and the maximum particle diameter is not more than 150 μm, more preferably 75 μm or less.

When particles having an average particle diameter of less than 5 μm are used, the roundish alumina particles can be produced by a known method, and thus the method of the present invention need not be used.

In order to increase the circularity of the coarse alumina particles used as a raw material, alumina hydrate used as a circularity enhancer is previously added to fused alumina and/or sintered alumina as necessary, followed by heating.

Examples of alumina hydrates include: aluminum hydroxide such as gibbsite, bayerite, boehmite, and diaspore; amorphous aluminum hydroxide such as alumina gel and pseudo-boehmite; and partially hydrated aluminum compounds such as surface partially hydrated alumina (alumina). Among these, aluminum hydroxide, alumina gel and alumina fine particles having high thermal reactivity are particularly preferable. From the economical viewpoint, aluminum hydroxide (gibbsite) produced by the bayer process is preferred, and aluminum hydroxide (gibbsite) having an average particle diameter of 10 μm or less is most preferred.

The inventors have observed a quite surprising phenomenon: the above roundness enhancer synergistically acts on the alumina coarse particles together with other additives mentioned below (added as needed) and selectively acts on (or is absorbed by) the irregularly sharp cross-section, thereby obtaining the rounded alumina coarse particles.

The content of the above roundness enhancer is not particularly limited, since the content varies depending on the particle size distribution of the fused alumina or sintered alumina ground product or the like. For example, when aluminum hydroxide is added, the content is preferably in the range of 5 to 300 mass% of fused alumina and/or sintered alumina (calculated as reduced to alumina). The content is more preferably in the range of 50 to 150 mass%. When the above content is less than 5 mass%, the adhesion of the aggregate increases, and when the content exceeds 300 mass%, excess aluminum hydroxide is released and migrates into the product as alumina particles.

As for other additives added as needed in the granulation step before the heat treatment, known compounds used as an alumina crystal growth promoter may be used alone or in combination. Preferred crystal growth promoters are those preferably composed of at least one selected from AlF₃、NaF、CaF₂、Na₃AlF₆And MgF₂And/or at least one halogen compound introduced from the group B₂O₃、H₃BO₃、mNa₂O·nB₂O₃(wherein each of m and n is an integer) and a fluoroborate compound.

Of these, particularly preferred are fluorine compounds and boron compounds, and combinations of fluoroborate compounds. Although the amount of the additive to be added is not limited and varies depending on the heating temperature, the residence time in the heating furnace and the type of the heating furnace, the effective amount of the additive to be added is preferably 0.1 to 4% by mass, particularly preferably 1 to 3% by mass, based on the total amount of the alumina component.

Mixing a composition comprising a ground fused alumina and/or sintered alumina product together with at least one member selected from the group consisting of halogen compounds, boron compounds and alumina hydrates, granulating and finally heating.

In the above-described production method, low α -ray index roundish alumina particles can be produced from materials such as fused alumina, sintered alumina, and aluminum hydroxide, all of which contain trace amounts of radioactive elements such as uranium and thorium oxide. In other words, the fused alumina, sintered alumina and aluminum hydroxide used in the present invention preferably have an α -ray index of 0.01C/cm²Hours or less.

When the alpha-ray index is low (0.01 c/cm)²Hour) used as a filler for resin sealing materials for highly integrated circuits, large scale integrated circuits and very large scale integrated circuits, the particles are particularly useful for preventing an operational failure (i.e., software error) of a memory device caused by α -rays.

The round alumina produced by the present invention is in the form of corundum coarse particles, although some cross-sections remain, there is no problem of abrasion to the kneader or the shaped mold into which it is mixed into rubber/plastic.

The mixing method is not particularly limited, and any conventional powder mixing method can be used as long as it can uniformly mix the components.

Examples of the mixing device used in the above method include: rocking mixers, Nauter (Nauter) mixers, helical mixers, V-shaped mixers, and Hershel mixers. In addition to these apparatuses, a pulverizer such as a ball mill and a vibration ball mill may be used.

While mixing, a solvent (medium) such as water or alcohol may also be added. Subsequently, the mixed composition thus prepared was subjected to granulation. The shape of the resulting pellets is not necessarily completely spherical, and the shape is not particularly limited as long as at least a certain volume of space is present between the pellets charged into the firing vessel.

Specifically, the packing density of the granulated product in the roasting container is preferably in the range of 0.25 to 0.50 times the theoretical density of the granulated product, more preferably 0.25 to 0.30 times the theoretical density, and when the packing density exceeds 0.50 times, the fired product forms a strongly solidified aggregate which is integrally adhered to the roasting container, which is a problem. When the packing density is less than 0.25 times, the fired product can be rapidly disintegrated into individual pellets. However, since the packing density is very low, the firing efficiency is not satisfactory, resulting in being uneconomical.

Small sized particulates are preferred so that the fired product is easily pulverized. However, too small a particle size is not preferable because the packing density will deviate from the above range. Therefore, the particle size is appropriately determined according to the size of the roasting container. The particle size is generally 1 to 30mm phi (projected circle equivalent diameter), preferably 5 to 20mm phi. The projected circle equivalent diameter as used herein refers to the Heywood diameter described in the literature of Funtai Kogaku Binran (edited by the japanese powder processing association) and the like.

The granulation method is not particularly limited as long as the method can produce the above-mentioned granules. Examples of granulation equipment include stirred granulators, fluid bed granulators, and compression granulators. From the economical viewpoint, for example, a pan granulator is preferable.

During granulation, a liquid medium such as water or alcohol, or an organic binder solution such as a polyvinyl alcohol (PVA) solution or a polyacrylic resin solution may be added. Such an organic binder is preferably added because the granulated product produced has appropriate strength, thereby preventing a phenomenon such as decomposition of the granulated product during handling thereof. If the granulated product does not need to have particularly high strength, then addition of water is most preferred from an economic point of view.

The amount of the above-mentioned liquid medium or organic binder added in the granulation process is appropriately selected depending on the particle diameter of the alumina coarse particles used, and when aluminum hydroxide is added, depending on the particle diameter thereof, the amount of aluminum hydroxide added, and the like. The content is preferably 5 to 50 mass%, most preferably 25 to 40 mass% of the amount of aluminum hydroxide added. A content exceeding 50 mass% is not preferable because the composition is entirely fluidized and cannot be completely granulated. A content of less than 5 mass% is also not preferable because the granulated product is greatly reduced in strength, causes crumbling and cannot ensure formation of granules.

Subsequently, the granulated product is heat-treated. In the heat treatment step, the above organic binder is burned off.

There is no limitation on the type of heating furnace used in the heat treatment, and known apparatuses such as a single kiln, a tunnel kiln, and a rotary kiln may be used. Heating temperatures of 1,000 c or higher can achieve the objects of the present invention. The heat treatment temperature is particularly preferably in the range of 1,350 ℃ to 1,600 ℃ inclusive. When the temperature is increased to a temperature higher than 1,600 ℃, the adhesion between the particles increases, thereby affecting the crushing into primary particles.

The residence time required in the heating furnace, which varies depending on the heating temperature, is 30 minutes or more, preferably about 1 to 3 hours. The crude alumina particles produced by the above process form secondary polymeric particles. Thus, the particles are pulverized by a known pulverizing apparatus such as a ball mill, a vibration ball mill or a jet mill, thereby producing roundish alumina particles of a target particle size distribution.

When it is desired to incorporate alumina particles containing no ionic impurities in an integrated circuit sealing material-like material, water washing, acid washing, alkali washing, etc. may be carried out before and/or after the pulverization.

The washing method is not particularly limited. However, since the spherical alumina particles produced by the above methodGenerally containing ionic impurities, mainly containing elements such as F, B and Na, so a preferred washing method comprises: spherical alumina particles are suspended in an acidic solution, a solid is separated from the suspension after a predetermined time, the solid is washed with pure water or a similar liquid, and then dried. When using monobasic acids such as HCl or HNO₃At times, the filtration rate may be slowed. Therefore, among the acids used, polybasic acids are preferably used, and citric acid, phosphoric acid and sulfuric acid are particularly preferred from the economical viewpoint.

The acid concentration is not particularly limited as long as the pH of the slurry concentration falls within the acidic range. Generally, the concentration is from 0.01N to 5N, preferably from 0.1 to 1N. Concentrations less than 0.01N are not preferred because the pH of the slurry will fall within the alkaline range. Concentrations greater than 1N are also not preferred because such concentrations are uneconomical and the added acid remains in the alumina.

The ratio of the amount of the powder to the amount of the liquid (i.e., slurry concentration) is not particularly limited, and is suitably determined in accordance with the stirring performance, shape, etc. of the reactor. Generally, the ratio is 50 to 1,000 g/l, preferably 200 to 800 g/l, and more preferably 300 to 600 g/l. When the ratio (solid content) is higher than 1,000 g/l, a large amount of precipitation or the like occurs. On the contrary, a ratio (solid content) of less than 50 g/l does not achieve high efficiency.

The acid treatment temperature is not particularly limited, and the treatment is generally carried out at 60 ℃ or higher, preferably 80 ℃ or higher. Temperatures below 60 ℃ are not preferred because the content of impurities removed by the extraction process is reduced and the filtration rate is reduced.

The filtration method is not particularly limited. Examples of filtering devices that may be used include: vacuum filters such as drum filters, horizontal filters and horizontal belt filters; pressure filters such as pressure drum filters, leaf filters and filter presses; press filters such as belt presses and screw presses; centrifugal sedimentation filters such as screw settlers; and a centrifugal filter.

The amount of the washing water is appropriately determined according to the required impurity content level. Generally, the amount of water used per kg of alumina is 1 liter or more, preferably 1 to 3 liters. The use of less than 1 liter of washing water is not preferred because impurities still remain. The use of more than 3 liters is uneconomical.

The method for drying the washed particle cake is not particularly limited, and any ordinary dryer can be used as long as the residual water amount is reduced to 0.1 mass% or less. Dryers that can be used for drying the powder may be used, examples of which include a box dryer, a tunnel dryer, a belt dryer, a fluidized dryer, and a paddle dryer.

It is preferred to carry out the drying at a temperature as low as possible. This is because when the powder is dried at high temperature, the diffusion rate of alkali ions contained in the powder increases, thereby migrating to the surface of the powder. The most preferred dryer is a vibrating fluidized dryer which can be heated under reduced pressure.

The roundish alumina particles produced by the method of the present invention are preferably incorporated into rubber or plastic, thereby providing a high thermal conductivity rubber composition and a high thermal conductivity plastic composition. Particularly, the content thereof is preferably 80% by mass or more.

In the present invention, there is no particular limitation on the type of plastic (resin) constituting the above-mentioned high-thermal-conductivity plastic composition, and any known resin can be used. Examples thereof include unsaturated polyester resins, acrylic resins, vinyl ester resins, epoxy resins, xylene-formaldehyde resins, guanamine resins, diaryl phthalate resins, phenol resins, furan resins, polyimide resins, melamine resins, and urea resins. Examples of preferred resins include: unsaturated polyester resins, acrylic resins, vinyl ester resins, and epoxy resins.

In the present invention, there is no particular limitation on the type of the rubber material (for example, rubber component) constituting the above-mentioned high thermal conductive rubber composition, and any known rubber material can be used.

The present invention is described in more detail below by way of examples and comparative examples, which should not be construed as limiting the invention.

In the following examples and comparative examples, the loading density of each granulated product was measured by a method in which the granulated product was gently put into a measuring cylinder, the apparent density thereof was calculated from the volume and weight thereof, and the ratio of the apparent density thus obtained to the theoretical density calculated from the ratio of the product composition was used.

Examples 1 to 5 and comparative example 1

2.5 parts by mass of anhydrous aluminum fluoride (product of Morita chemical industries, Ltd.), 2.5 parts by mass of boric acid (product of borax, USA) and 50 parts by mass of aluminum hydroxide (product of Showa Denko K.K) having an average particle diameter of 1 μm were added to 100 parts by mass of a commercially available sintered alumina powder crushed product having an average particle diameter of 25 μm and a maximum particle diameter of 75 μm. All of them were mixed by a V-shaped mixer. The mixture was granulated by a pan granulator under the conditions shown in table 1 below. The loading density of the granulated product was measured. The results are also shown in table 1 below.

Next, the granulated product was placed in a heat-resistant roasting container made of corundum-mullite material, heated in a tunnel kiln at a maximum temperature of 1,380 ℃ and held for 3 hours. For each product, the ratio of the number of heat-resistant roasting containers that broke when the content in each container was removed to ten heat-resistant roasting containers was calculated (this ratio is referred to as the percentage of damage to the sagger). The results are also shown in table 1 below. The relative hardness of the fired products (represented by the five relative grades shown in table 1) was compared with each other. Each of the coarsely pulverized products was pulverized by a ball mill for a period of time shown in table 1, and then the particle size distribution of the thus-pulverized product was measured by a micro-diameter particle size analyzer.

Comparative example 2

The process of example 1 was repeated except that the aluminum hydroxide addition and granulation steps were omitted. The results are also shown in table 1 below.

Comparative example 3

The procedure of comparative example 1 was repeated except that 300 parts by mass of aluminum hydroxide was added. The results are also shown in table 1 below.

Examples 6 to 12

The alumina particles produced in example 5 were washed under the conditions shown in table 2 below. A horizontal belt filter was used as the filtering apparatus. The filtration rate was obtained by measuring the filtration time required for rinsing and filtering a predetermined amount of slurry using a Buchner funnel (used for measurement purposes), and judged in five grades, (1) to (5), (1) very fast, (5) very slow. A vibrating fluidized dryer was used only in example 10, and a box dryer was used in addition to example 10. 20 g of each powder was suspended in 10 ml of purified water to prepare a slurry, and the conductivity was measured at room temperature. An extract was prepared by extraction with hot water at 100 ℃ for 2 hours, and the content of ionic impurities was obtained by measuring the impurity content of the extract.

Example 13

The alpha-ray index is 0.01C/cm²An hour or less commercially available low-alpha-ray alumina (a product of Showa Denko K.K) prepared by electric melting, pulverizing and classifying under conditions preventing contamination with radioactive elements, thereby producing a powder having an average particle diameter of 30 μm and an alpha-ray index of 0.005c/cm²Coarse particles of electrically fused alumina in hours. The conventional method of example 1, including calcination and pulverization, was repeated except that 250 g of the above-produced alumina particles were used instead of the alumina particles of example 1, to obtainTo produce granules. The particle size of the granules was measured to be 1 to 5 mm and the packing density was measured to be 0.31 (relative ratio). The burned product has hardness which is easy to break by hand. After 15 minutes of pulverization, the alpha-ray index is 0.004C/cm²Roundish alumina particles having an average particle diameter of 33 μm.

Example 14

The procedure of example 8 was repeated except that the low- α -ray alumina prepared in example 13 was used, and rinsing was performed under the conditions of example 8 shown in table 2. The conductivity of the product was found to be 4. mu.s/cm, Na⁺Index 5ppm, and SO₄ ^2-The index was 7 ppm.

Industrial applicability

According to the method of the invention, the round alumina coarse particles can be produced on a large scale at low cost. The roundish alumina particles produced by the method of the present invention have excellent fluidity and substantially no problem of abrasion to machines and instruments.

Claims (11)

1. A method for producing roundish alumina particles, comprising the steps of: granulating a composition comprising a ground product of at least one of fused alumina and sintered alumina having an average particle diameter of 5 to 35 μm, and at least one selected from the group consisting of halogen compounds, boron compounds and alumina hydrates, to obtain a granulated product; heating the granulated product at 1,000 to 1,600 ℃; the heated product is then comminuted.

2. The method of claim 1, further comprising the steps of: washing the heated product with an acid before or after the pulverizing step.

3. The method of claim 2, further comprising the steps of: the pickled product was rinsed with pure water, and then the water-rinsed product was dried.

4. The method according to claim 3, wherein the drying step is performed using a reduced pressure drying apparatus.

5. A process according to any one of claims 1 to 4, wherein the mean particle size of the electrofused alumina and sintered alumina is from 10 μm to 25 μm.

6. The process according to any one of claims 1 to 4, wherein the halogen compound is at least one selected from AlF₃、NaF、CaF₂、MgF₂And Na₃AlF₆The substance of (1).

7. The process according to any one of claims 1 to 4, wherein the boron compound is at least one member selected from the group consisting of B₂O₃、H₃BO₃、mNa₂O·nB₂O₃Wherein each of m and n is an integer, and a fluoroborate compound.

8. The method according to any one of claims 1 to 4, wherein the alumina hydrate is at least one member selected from the group consisting of aluminum hydroxide, alumina gel, amorphous aluminum hydroxide and partially hydrated aluminum compounds.

9. A process according to any one of claims 1 to 4, wherein each of the electrofused alumina, sintered alumina and alumina hydrate has an alpha-ray index of 0.01c/cm²Hours or less.

10. A process according to any one of claims 1 to 4, wherein the granulated product has a particle size of from 1 to 30mm Φ.

11. The method according to any of claims 1-4, further comprising the steps of: at least one of water and an organic aqueous solution in an amount of 5 to 50 mass% based on the alumina hydrate is added in the granulation step.