JP7011525B2 - Method for manufacturing magnesium oxide powder, composite material, and magnesium oxide powder - Google Patents

Description

The present invention relates to a magnesium oxide powder, a composite material, and a method for producing a magnesium oxide powder.

Magnesium oxide is a material having high thermal conductivity (45 to 60 W / m · k) and excellent electrical insulation, and is industrially used as a filler for heat dissipation parts for semiconductors. For example, Patent Document 1 describes that columnar magnesium oxide particles produced from basic magnesium sulfate are used as a thermally conductive filler.

Magnesium oxide may also be used as a filler material for resins. The filler material for resin efficiently transfers heat by forming a heat conduction path by contacting adjacent fillers in the resin matrix.

In the particle shape of the filler material, since the needle-shaped particles have a large aspect ratio, the heat conduction efficiency of the resin composition is improved in the longitudinal direction of the filler material. That is, the thermal conductivity can be improved with a small amount of addition as compared with spherical particles having a low aspect ratio.

Japanese Unexamined Patent Publication No. 2014-214222

In order to add the filler material to the resin to achieve high thermal conductivity, it is necessary to increase the amount of the filler material added. However, in the case of needle-shaped particles, voids are formed in the resin and the filler material due to the increase in the amount of the particles added, which causes a decrease in thermal conductivity, tensile strength and bending strength of the resin, and rigidity. Further, since the needle-shaped particles are bulky, the packing density when added to the resin tends to be low, the amount added to the resin is limited, and it is difficult to significantly improve the thermal conductivity.

The present invention has been made in view of such circumstances, and by using magnesium oxide powder as a mixed material of acicular particles and spherical particles, it can be used as a resin and a filler material even if the amount added to the resin is increased. It is an object of the present invention to provide a magnesium oxide powder which is difficult to form voids and can improve thermal conductivity while suppressing deterioration of mechanical properties, a composite material to which the magnesium oxide powder is added, and a method for producing magnesium oxide powder. do.

(1) In order to achieve the above object, the magnesium oxide powder of the present invention is a magnesium oxide powder used as a filler material for a resin, and has an average fiber length of 10 μm or more and 50 μm or less and an average fiber diameter of 0.2 μm or more. It is composed of needle-shaped magnesium oxide having an average particle size of 2 μm or less and spherical magnesium oxide having an average particle size of 2 μm or more and 10 μm or less. When the particle size is B, the value of A / B is 0.5 or less.

As a result, even if the amount of magnesium oxide powder added to the resin is increased, voids (voids) are less likely to be formed in the resin and the filler material. In addition, since deterioration of mechanical properties can be suppressed when the amount added is large, the amount added to the resin can be increased as compared with the case of only needle-shaped particles or only spherical particles, and thermal conductivity can be increased. The rate can be improved.

(2) Further, the composite material of the present invention is a composite material containing a filler mainly composed of magnesium oxide powder and a resin, and the magnesium oxide powder described in (1) above is 40 vol% with respect to the volume of the resin. It is characterized in that it is dispersed in an amount of 80 vol% or less. This makes it possible to improve the thermal conductivity while suppressing the deterioration of the mechanical properties of the composite material.

(3) Further, the composite material of the present invention is characterized in that it has a tensile strength of 15 MPa or more, an elastic modulus of 2000 MPa or more, and a thermal conductivity of 1.0 W / (m · K) or more. This makes it possible to construct a composite material having high tensile strength, elastic modulus, and thermal conductivity.

(4) Further, the method for producing magnesium oxide powder of the present invention is a method for producing magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide, and the magnesium oxide raw material powder is mixed with water. In the step of producing a slurry at a concentration of 5 wt% or more and 15 wt% or less, and in the slurry so that the molar ratio of H ₂ SO ₄ / MgO to the magnesium oxide raw material powder is 0.2 or more and 0.7 or less. It is obtained by a step of adding sulfuric acid, a step of hydrothermally synthesizing the magnesium oxide raw material powder, the sulfuric acid and the water in the slurry by increasing the temperature and pressure of the slurry to which the sulfuric acid is added, and hydrothermally synthesizing the magnesium oxide raw material powder. It is characterized by including a step of suction-filtering the slurry to dry the residue of the filtration, and a step of calcining the dried residue and thermally decomposing it to produce magnesium oxide powder. This makes it possible to produce magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide.

(5) Further, in the method for producing magnesium oxide powder of the present invention, the magnesium oxide raw material powder has an average particle size of 5 μm or more and 30 μm or less, D10 of 1 μm or more, D90 of 80 μm or less, and a purity of 90 wt% or more. It is characterized by. This makes it possible to easily produce high-purity magnesium oxide powder in which needle-shaped magnesium oxide and spherical magnesium oxide are uniformly mixed.

According to the present invention, even if the amount of magnesium oxide powder added to the resin is increased, voids (voids) are less likely to be formed in the resin and the filler material. In addition, since deterioration of mechanical properties can be suppressed when the amount added is large, the amount added to the resin can be increased as compared with the case of only needle-shaped particles or only spherical particles, and thermal conductivity can be increased. The rate can be improved.

It is a conceptual diagram which shows the magnesium oxide powder of this invention. It is a flowchart which shows the manufacturing method of the magnesium oxide powder of this invention. It is a graph which shows the particle size distribution of a raw material. It is a graph which shows the XRD measurement result of a product. It is an optical micrograph of a product. It is a table which shows the manufacturing condition and result of an Example and a comparative example. (A) to (c) are tables showing the tensile strength, elastic modulus, and thermal conductivity of the composite material, respectively.

Next, an embodiment of the present invention will be described with reference to the drawings.

[Composition of magnesium oxide powder]
FIG. 1 is a conceptual diagram showing the magnesium oxide powder of the present invention. The magnesium oxide powder 10 is a magnesium oxide powder used as a filler material for a resin, and is composed of needle-shaped magnesium oxide 20 and spherical magnesium oxide 30. The needle-shaped magnesium oxide 20 has an average fiber length of 10 μm or more and 50 μm or less, and an average fiber diameter of 0.2 μm or more and 2 μm or less. The spherical magnesium oxide 30 has an average particle size of 2 μm or more and 10 μm or less. When the average fiber diameter of the needle-shaped magnesium oxide 20 is A and the average particle size of the spherical magnesium oxide 30 is B, the value of A / B is 0.5 or less.

By being composed of such needle-shaped magnesium oxide 20 and spherical magnesium oxide 30, even if the amount of magnesium oxide powder 10 added to the resin is increased, the spherical particles enter into the gaps between the needle-shaped particles. , Voids (voids) are less likely to be formed in the resin and filler material. In addition, when the amount added is large, deterioration of mechanical properties such as tensile strength, bending strength, and rigidity of the resin can be suppressed. The amount added can be increased, and the thermal conductivity can be improved.

The average fiber length, average fiber diameter, and average particle size of the spherical magnesium oxide 30 of the needle-shaped magnesium oxide 20 are measured as follows. First, SEM photographs of the sample are taken a plurality of times at a magnification of 2000 times, and the maximum diameter and the minimum diameter of all the particles in the measurement area of the captured image are measured. Next, particles having an aspect ratio (ratio of maximum diameter to minimum diameter) of 1.5 or more are defined as needle-shaped particles, and particles having an aspect ratio of less than 1.5 are defined as spherical particles. The average of the maximum diameter and the average of the minimum diameters of the needle-shaped particles are defined as the average fiber length and the average fiber diameter of the needle-shaped magnesium oxide 20. Further, the average of the maximum diameter and the minimum diameter of the spherical particles is taken as the average particle size of the spherical magnesium oxide 30. The measurement is performed until the number of measured particles is 20 or more.

In the magnesium oxide powder 10, the ratio of the number of needle-shaped magnesium oxide 20 measured by the above method is preferably 50% or more and 80% or less. Further, the magnesium oxide powder 10 preferably has an MgO content of 90 wt% or more.

[Composition of composite material]
A composite material in which the magnesium oxide powder as described above is mixed with a resin as a filler will be described. The composite material is formed by dispersing the filler in the resin. Further, as the resin used for the composite material, polypropylene (PP), polyethylene (PE), epoxy or the like is used. By dispersing magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide in these resins, spherical magnesium oxide enters the gaps between needle-shaped magnesium oxide, and the resin and filler material. Voids (voids) are less likely to be formed in the magnesium oxide. In addition, the filling rate can be increased.

The magnesium oxide powder contained in the composite material is 40 vol% or more and 80 vol% or less with respect to the volume of the resin. This is because if it is smaller than 40 vol%, the filling rate is low and the thermal conductivity is not so high, and if it is larger than 80 vol%, the tensile strength and bending strength may be low.

[Manufacturing method of magnesium oxide powder]
FIG. 2 is a flowchart showing a method for producing magnesium oxide powder. A method for producing magnesium oxide powder will be described with reference to FIG. 2. First, magnesium oxide (MgO) raw material powder is prepared. Mineral magnesium oxide can be used as the raw material powder. For example, a hydrated portion of lightly baked magnesia obtained by firing a mineral containing magnesium carbonate or magnesium hydroxide as a main component at 550 to 1400 ° C. can be used. Examples of minerals containing magnesium carbonate as a main component include magnesite, dolomite and the like.

Next, the magnesium oxide raw material powder is mixed with water to generate a slurry (step P1). Distilled water can be used as the water to be mixed. The magnesium oxide raw material powder preferably has an average particle size (D50) of 5 μm or more and 30 μm or less, D10 of 1 μm or more, and D90 of 80 μm or less. When a raw material powder having an average particle size (D50) of less than 5 μm or a D10 of less than 1 μm is used, reprecipitation may be suppressed when the MgO particles are dissolved in sulfuric acid, and the yield may be significantly reduced. Further, if a raw material powder having a D90 of more than 80 μm is used, the raw material MgO may not be sufficiently dissolved and the raw material MgO particles may remain. The magnesium oxide raw material powder preferably has a purity of 90 wt% or more. Further, the magnesium oxide raw material powder is preferably spherical particles.

The slurry is produced at a concentration of 5 wt% or more and 15 wt% or less. When the slurry concentration is less than 5 wt%, the sulfuric acid concentration (ratio of sulfuric acid and water) in the slurry becomes low, and the reaction between sulfuric acid and MgO becomes insufficient. In addition, the yield of magnesium oxide powder decreases. When the slurry concentration is larger than 15 wt%, the water content is insufficient and the slurry becomes thickened, so that the reaction becomes non-uniform. The slurry concentration is preferably 10 wt% or more and 15 wt% or less.

Next, sulfuric acid is added to the magnesium oxide raw material powder so that the molar ratio of H ₂ SO ₄ / MgO is 0.2 or more and 0.7 or less (step P2). If the amount of sulfuric acid added is less than 0.2 in terms of the molar ratio, the dissolution of MgO as a raw material becomes insufficient, and MgO particles remain. When the amount of sulfuric acid added is larger than 0.7, the reprecipitation of the dissolved MgO particles is suppressed, and the yield is significantly reduced. The amount of sulfuric acid added is preferably such that the molar ratio of H ₂ SO ₄ / MgO to the magnesium oxide raw material powder is 0.2 or more and 0.4 or less. The slurry concentration and the amount of sulfuric acid added are adjusted including the amount of water contained in the sulfuric acid.

By adjusting the slurry concentration and the amount of sulfuric acid added as described above, magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide can be easily produced.

Next, the slurry to which sulfuric acid is added is heated to a high temperature and high pressure while stirring, and the magnesium oxide raw material powder, sulfuric acid, and water are hydrothermally synthesized in the slurry to hydrothermally synthesize basic magnesium sulfate whisker (י _4.3 H _2O・ 5Mg (OH). ) ₂ ) is generated (step P3). In the step of increasing the temperature and pressure, it is preferable to keep the slurry pressurized at 0.80 MPa or more for 1 hour or more while heating the slurry to 150 ° C. or more. This makes it possible to sufficiently proceed with the hydrothermal synthesis of the raw material powder. By hydrothermal synthesis of the raw material powder, MgO of the raw material is dissolved in sulfuric acid and reprecipitated to form needle-shaped י _4.3 H ₂ O / 5Mg (OH) ₂ .

Next, the slurry obtained by hydrothermal synthesis is suction-filtered to dry the filtration residue (step P4). As the dried residue, hydroxide 4.3H ₂ O ・_{5Mg (OH) 2 of} the intermediate skeleton is obtained (step P5). Then, the dried residue is calcined, the intermediate skeleton is thermally decomposed, and heat dehydration and desulfuration are performed (step P6).

In the firing step, it is preferable to heat the residue to a temperature of 950 ° C. or higher and 1300 ° C. or lower. As a result, the dehydration and desulfation reactions of the intermediate skeleton can be promoted, the melting thereof can be suppressed, and magnesium oxide powder can be produced.

In this way, magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide can be produced (step P7). By hydrothermally synthesizing magnesium oxide raw material powder and sulfuric acid and thermally decomposing the obtained intermediate skeleton, magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide can be produced at low cost and in a short time. Can be generated.

[Examples, comparative examples]
In order to verify the above-mentioned production method and the characteristics of the product, magnesium oxide powder was produced by changing the slurry concentration and the sulfuric acid concentration using a plurality of types of magnesium oxide powder raw materials. FIG. 6 is a table showing the manufacturing conditions of Examples and Comparative Examples. In each of the examples and comparative examples, magnesium oxide raw material powder having a different particle size distribution and a purity of 90% was used as a starting material. Further, in each of the examples and comparative examples, the temperature of hydrothermal synthesis was 140 to 180 ° C., the pressure was 0.8 to 1.0 MPa, the holding time was 0.5 to 3 hours, the firing temperature was 950 ° C., and the firing time. Was produced under different conditions for 1 hour.

(XRD measurement)
In producing Example 1, X-ray diffraction (XRD) measurement was performed on the magnesium oxide product using a powder X-ray diffractometer (manufactured by Brucker). FIG. 4 is a graph showing the XRD measurement results of the product. As shown in FIG. 4, a peak peculiar to magnesium oxide appeared on the X-ray diffraction profile of the product.

(Measurement of particle size distribution)
The magnesium oxide raw material powders used in Examples 1, 2 and 9 and Comparative Examples 1 and 8 were measured for particle size distribution using a Microtrack particle size analyzer (manufactured by Nikkiso Co., Ltd.) by a laser diffraction / scattering method. .. FIG. 3 is a graph of the particle size distribution of the raw material used in Example 2.

The magnesium oxide raw material powder of Example 1 had an average particle size (D50) of 5 μm, D10 of 1 μm, and D90 of 20 μm. The magnesium oxide raw material powder of Example 2 had an average particle size of 23 μm, D10 of 3 μm, and D90 of 63 μm. The magnesium oxide raw material powder of Example 9 had an average particle size of 30 μm, D10 of 5 μm, and D90 of 80 μm. The magnesium oxide raw material powder of Comparative Example 1 had an average particle size of 3 μm, D10 of 0.5 μm, and D90 of 15 μm. The magnesium oxide raw material powder of Comparative Example 8 had an average particle size of 35 μm, D10 of 5 μm, and D90 of 85 μm. In Examples 3 to 8 and Comparative Examples 2 to 7, the same magnesium oxide raw material powder as in Example 2 was used.

(Measurement of particle morphology)
The raw materials and products of Example 4 were observed by SEM using a scanning electron microscope (manufactured by JEOL Ltd.), and particle morphology was measured. The particle morphology of the magnesium oxide powder samples of the other examples and comparative examples was also measured.

As shown in FIG. 5, the product magnesium oxide powder of Example 4 was composed of needle-shaped magnesium oxide and spherical magnesium oxide. The needle-shaped magnesium oxide had an average fiber length of 30 μm and an average fiber diameter (A) of 0.5 μm. The spherical magnesium oxide had an average particle size (B) of 5 μm. Since A was 0.5 μm and B was 5 μm, the value of A / B was 0.1. In other examples, it was confirmed that the A / B value was 0.5 or less.

(Preferable manufacturing conditions)
(1) Slurry concentration FIG. 6 is a table showing the production conditions, yields and amount of needle-shaped MgO contained in the products of Examples and Comparative Examples. Comparative Example 2, Examples 2, 5, 8 and Comparative Example 7 were prepared by using the same magnesium oxide raw material powder, changing the slurry concentration, and making other conditions the same. In Examples 2, 5 and 8, magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide was obtained. In Comparative Example 2, the yield was significantly reduced. It is considered that this is because the concentration of sulfuric acid (ratio of sulfuric acid and water) in the slurry was low because the concentration of the slurry was low, and the reaction between sulfuric acid and MgO was insufficient. Comparative Example 7 contained partially coarse spherical particles. It is considered that this is because the slurry concentration was high, the water content was insufficient, and the slurry became thickened, so that the reaction became non-uniform.

Therefore, it was found that the slurry concentration is preferably in the range of 5 wt% or more and 15 wt% or less. Further, since the yields of Examples 5 and 8 were superior to those of Example 2, it was found that the slurry concentration was more preferably in the range of 10 wt% or more and 15 wt% or less.

The yield is ⊚ when the weight of the obtained magnesium oxide powder is 80 wt% or more with respect to the weight of the magnesium oxide raw material powder, ○ when the weight is 60 wt% or more and less than 80 wt%, and ○ when the weight is less than 60 wt%. It is represented by ×. The ratio of the number of needle-shaped magnesium oxides was indicated by ◯ when it was 50% or more and 80% or less, and indicated by × when it was less than 50%.

(2) Sulfuric acid addition amount In Comparative Example 3, Examples 3, 5, 7 and Comparative Example 6, the same magnesium oxide raw material powder was used, and the molar ratio of H ₂ SO ₄ / Mg (sulfuric acid addition to Mg of the raw material MgO) was used. The amount) was changed, and the other conditions were the same. In Examples 3, 5 and 7, magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide was obtained. In Comparative Example 3, the yield was not a problem, but the amount of needle-shaped MgO was reduced. It is considered that this is because the amount of sulfuric acid added was small, so that the raw material MgO was not sufficiently dissolved and the amount of reprecipitation was insufficient, and the raw material MgO particles remained. In Comparative Example 6, the yield was significantly reduced. It is considered that this is because the amount of sulfuric acid added was too large and the reprecipitation of the dissolved MgO particles was suppressed.

Therefore, it was found that the amount of sulfuric acid added is preferably 0.2 or more and 0.7 or less in terms of the molar ratio of H ₂ SO ₄ / MgO to MgO of the magnesium oxide raw material powder. Further, since the yields of Examples 3 and 5 were superior to those of Example 7, it was found that 0.2 or more and 0.4 or less were more preferable.

(3) Particle Size Distribution of Raw Material Powder Comparative Example 1, Examples 1, 5, 9 and Comparative Example 8 were prepared using magnesium oxide raw material powders having different particle size distributions under the same conditions. In Examples 1, 5 and 9, magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide was obtained. In Comparative Example 1, the yield was significantly reduced. It is considered that this is because the average particle size and D10 of the raw material were small, so that the reprecipitation of the raw material MgO particles after the dissolution of sulfuric acid was suppressed. Comparative Example 8 contained a large amount of coarse spherical particles. It is considered that this is because the average particle size and D90 were large, so that the raw material MgO particles were insufficiently dissolved and the raw material spherical particles remained. Therefore, it was found that the magnesium oxide raw material powder having an average particle size (D50) of 5 μm or more and 30 μm or less, D10 of 1 μm or more, and D90 of 80 μm or less is suitable.

(4) Other manufacturing conditions From the hydrothermal synthesis conditions (temperature, pressure and holding time) of Comparative Examples 4, 5 and Examples 4, 5 and 6, the temperature at the time of hydrothermal synthesis was 150 ° C. or higher, and the pressure was 0. It was found that it was preferably larger than 8 MPa and the holding time was 1 hour or more.

(5) Conclusion Based on the above, the amount of magnesium oxide raw material powder with a concentration of 5 wt% or more and 15 wt% or less and the amount of sulfuric acid added to MgO is adjusted to 0.2 or more and 0.7 or less in terms of the molar ratio of H ₂ SO ₄ / MgO. It was demonstrated that magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide can be stably obtained by synthesizing and drying and firing the obtained intermediate skeleton.

(Characteristics of composite material)
Next, a composite material sample was prepared by adding and dispersing the magnesium oxide powder of Example 4 at a ratio of 35 vol% to 85 vol% with respect to polypropylene (Novatec manufactured by Japan Polypropylene Corporation) (hereinafter abbreviated as PP). Further, although the spherical magnesium oxide powder and the needle-shaped magnesium powder are mixed, the A / B is 0.6, and the magnesium oxide powder of Comparative Example 4 which is larger than 0.5 is 35 vol with respect to PP. A composite material sample added and dispersed at a ratio of% to 85 vol% was prepared. Further, needle-shaped magnesium oxide powder was added at a ratio of 35 vol% to 85 vol% with respect to PP, and a dispersed composite sample and spherical magnesium oxide powder were added at a ratio of 35 vol% to 85 vol% with respect to PP. A dispersed composite sample was prepared. These composite samples were prepared and their tensile strength, elastic modulus, and thermal conductivity were measured.

A universal material testing machine manufactured by Instron was used to measure the tensile strength and elastic modulus. The thermal conductivity was measured by a flash method using an LFA manufactured by NETZSCH. 7 (a) to 7 (c) are tables showing the results of tensile strength, elastic modulus, and thermal conductivity of the composite material of each sample, respectively. In the following description, the composite material using the magnesium oxide powder of Example 4 is sample 1, the composite material using the magnesium oxide powder of Comparative Example 4 is the sample 2, and the composite material using only the needle-shaped magnesium oxide powder is used. Sample 3 is a composite material using only spherical magnesium oxide powder as sample 4.

(Tensile strength)
Sample 1 (magnesium oxide powder of the present invention) had a slightly lower tensile strength than Sample 3 (needle-shaped particles only) when the addition rate to PP was small. When the addition rate was large, the decrease in tensile strength was suppressed to be low as compared with Sample 3. The tensile strength of sample 4 (spherical particles only) was reduced. Sample 2 (magnesium oxide powder having an A / B of greater than 0.5) had less decrease in tensile strength than Sample 4.

(Elastic modulus)
The elastic modulus of sample 1 was slightly smaller than that of sample 3 when the addition rate to PP was small. Sample 3 showed a maximum value at an addition rate of 70 vol%, and the elastic modulus decreased at 80 vol%, whereas sample 1 showed an elastic modulus increased to an addition rate of 80 vol% and showed a maximum value. This maximum value was larger than the maximum value of sample 3. Sample 2 showed a maximum value at an addition rate of 80 vol%. This maximum value was larger than the maximum value of sample 4, but smaller than the maximum value of sample 3.

(Thermal conductivity)
In sample 1, when the addition rate to PP was increased, the thermal conductivity increased accordingly. Both Sample 2 and Sample 4 showed a maximum value at an addition rate of 70 vol%. Sample 3 showed a maximum value at an addition rate of 40 vol%. This maximum value was larger than the maximum value of Sample 2 and Sample 4, but smaller than the value when the addition rate of Sample 1 was 70 vol% or more.

Since the A / B of the sample 2 was larger than 0.5, it is considered that the filling may be insufficient when the addition rate is increased. It is considered that the physical characteristics of the sample 3 deteriorated due to the formation of voids when the addition rate was increased.

From the above, magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide is suitable as a filler material to be dispersed in a resin. Such a composite material in which magnesium oxide powder is dispersed in a resin has improved thermal conductivity and elastic modulus, and a decrease in tensile strength can be suppressed to a low level. Further, the magnesium oxide powder dispersed in the resin is preferably 40 vol% or more and 80 vol% or less with respect to the volume of the resin.

10 Magnesium oxide powder 20 Needle-shaped magnesium oxide 30 Spherical magnesium oxide

Claims (5)

Magnesium oxide powder used as a filler material for resins.
Needle-shaped magnesium oxide having an average fiber length of 10 μm or more and 50 μm or less and an average fiber diameter of 0.2 μm or more and 2 μm or less.
It is composed of spherical magnesium oxide having an average particle size of 2 μm or more and 10 μm or less.
A magnesium oxide powder having an A / B value of 0.5 or less, where A is the average fiber diameter of the needle-shaped magnesium oxide and B is the average particle size of the spherical magnesium oxide. It is a composite material containing a filler mainly composed of magnesium oxide powder and a resin.
The composite material according to claim 1, wherein the magnesium oxide powder is dispersed in an amount of 40 vol% or more and 80 vol% or less with respect to the volume of the resin. The composite material according to claim 2, wherein the composite material has a tensile strength of 15 MPa or more, an elastic modulus of 2000 MPa or more, and a thermal conductivity of 1.0 W / (m · K) or more. A method for producing magnesium oxide powder composed of needle-shaped magnesium oxide and spherical magnesium oxide.
A step of mixing magnesium oxide raw material powder with water to form a slurry at a concentration of 5 wt% or more and 15 wt% or less, and
A step of adding sulfuric acid to the slurry so that the molar ratio of H ₂ SO ₄ / MgO to the magnesium oxide raw material powder is 0.2 or more and 0.7 or less.
A step of hydrothermally synthesizing the magnesium oxide raw material powder, the sulfuric acid, and the water in the slurry by increasing the temperature and pressure of the slurry to which the sulfuric acid is added.
The step of suction-filtering the slurry obtained by hydrothermal synthesis and drying the residue of the filtration,
A method for producing magnesium oxide powder, which comprises a step of firing the dried residue and thermally decomposing it to produce magnesium oxide powder. The method for producing magnesium oxide powder according to claim 4, wherein the magnesium oxide raw material powder has an average particle size of 5 μm or more and 30 μm or less, D10 of 1 μm or more, D90 of 80 μm or less, and a purity of 90 wt% or more.

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