Magnesium oxide powder, production method of the same, mixed powder, and resin composition

Abstract

To provide a magnesium oxide powder capable of achieving both low viscosity and high thermal conductivity when filled in a resin, a production method of the same, a mixed powder including the magnesium oxide powder, and a resin composition including the magnesium oxide powder.SOLUTION: A magnesium oxide powder includes a particle group in which, when the magnesium oxide powder is observed by a scanning electron microscope under specific conditions, a ratio (R) of a total area (B) of open pores existing in all particles to a total area (A) of all particles existing in an observation image is 9.0% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to magnesium oxide powder, a method for producing the same, a mixed powder, and a resin composition.

Background Art

[0002] In recent years, miniaturization and high performance of electric devices have been progressing. Along with the miniaturization and high performance of electric devices, the mounting density of electronic components constituting the electric devices has been increasing, and the need to effectively release heat generated from the electronic components has been increasing.

[0003] In addition, in power device applications such as electric vehicles with low environmental impact, a high voltage may be applied to the electronic components, or a large current may flow. In this case, since a high amount of heat is generated, in order to cope with this, the demand for more effectively releasing heat has been increasing compared to the conventional situation.

[0004] As a technology for meeting such requirements, a resin composition containing an inorganic powder with thermal conductivity as a filler is used. As the inorganic powder with thermal conductivity, for example, magnesium oxide powder is known, and Patent Document 1 discloses specific spherical magnesium oxide particles.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, a resin composition containing only magnesium oxide powder as a filler has a problem of high compound viscosity. Therefore, from the viewpoint of suppressing the thickening of the resin composition, etc., magnesium oxide powder and other inorganic powders may be combined and filled into the resin. In this case, depending on the inorganic powder to be combined, there may be problems such as a decrease in thermal conductivity, or when an inorganic powder with high hardness is combined, the molding machine or kneader may wear. Therefore, even when only magnesium oxide powder is blended into the resin as a filler, there is a demand for magnesium oxide powder that does not increase the compound viscosity.

[0007] The present invention has been made in view of the above problems, and an object thereof is to provide a magnesium oxide powder capable of achieving both low viscosity and high thermal conductivity when filled in a resin, a method for producing the same, a mixed powder containing the magnesium oxide powder, and a resin composition containing the magnesium oxide powder.

Means for Solving the Problems

[0008] As a result of intensive studies, the inventors of the present application have found that the above problems can be solved by magnesium oxide powder having the following configuration. [1] Magnesium oxide powder, When the magnesium oxide powder is observed by a scanning electron microscope under the following conditions, A magnesium oxide powder containing a particle group in which the ratio (R) of the total area (B) of open pores present in all the particles to the total area (A) of all the particles present in the observation image is 9.0% or less. (Condition) The magnesium oxide powder is embedded in an epoxy resin and then cured. After that, processing is performed with a cross-section processing device to create a sample of the magnesium oxide powder. An image obtained by observing a cross-section of the sample with a scanning electron microscope at a magnification of 500 times is analyzed with image analysis software to calculate the total area (A) of all the particles present in the observed image. Similarly, the total area (B) of the open pores communicating with the particle surface among all the particles in the observed image is calculated. Then, the ratio (R) of the total area (B) of the open pores to the total area (A) is determined.

Effect of the Invention

[0009] According to the present invention, it is possible to provide magnesium oxide powder capable of achieving both low viscosity and high thermal conductivity when filled in a resin, a method for producing the same, a mixed powder containing the magnesium oxide powder, and a resin composition containing the magnesium oxide powder.

Brief Description of the Drawings

[0010]

Figure 1

Modes for Carrying Out the Invention

[0011] Hereinafter, an embodiment of the present disclosure will be described in detail. However, the scope of the present disclosure is not limited to the embodiment described here, and various modifications can be made without departing from the spirit of the present disclosure. Each aspect disclosed in this specification can be combined with any other features disclosed in this specification. Also, when a plurality of upper limit values and lower limit values are described for a specific parameter, any upper limit value and lower limit value among these can be combined to form a suitable numerical range. Further, the lower limit value and / or the upper limit value of the numerical range described in the present disclosure can be replaced with a numerical value within that numerical range and shown in the examples. The expression "1 to 10" indicating a numerical range means "1 or more and 10 or less". When a specific description given for one embodiment also applies to other embodiments, the description may be omitted in other embodiments.

[0012] Each configuration and their combinations, etc. in each embodiment are examples, and within the scope not departing from the gist of the present disclosure, addition, omission, substitution, and other changes of the configuration can be made as appropriate. The present disclosure is not limited by the embodiments.

[0013] Each aspect disclosed in this specification can be combined with any other features disclosed in this specification. In this specification, "powder" means an aggregate of a plurality of particles.

[0014] [Magnesium Oxide Powder] The first embodiment in the present disclosure is magnesium oxide powder. The first embodiment is magnesium oxide powder, and when the magnesium oxide powder is observed by a scanning electron microscope under the following conditions, the ratio (R) of the total area (B) of the open pores present in all the particles to the total area (A) of all the particles present in the observation image is 9.0% or less. The magnesium oxide powder includes a particle group. (Conditions) The magnesium oxide powder is embedded in an epoxy resin and then cured. Thereafter, processing is performed with a cross-section processing device to create a sample of the magnesium oxide powder. An image obtained by observing a cross-section of the sample with a scanning electron microscope at a magnification of 500 times is analyzed with image analysis software, and the total area (A) of all the particles present in the observed image is calculated. Similarly, the total area (B) of the open pores communicating with the particle surface in all the particles in the observed image is calculated. Thereafter, the ratio (R) of the total area (B) of the open pores to the total area (A) is determined.

[0015] An example of a more specific method for preparing a sample of magnesium oxide powder is described below. First, after performing pretreatment under the following conditions, observation and image processing with a scanning electron microscope are carried out. ·Pretreatment Magnesium oxide powder (0.1 g) is mixed with a two-component heat-curable epoxy resin (for example, manufactured by Gatan, product name "G2") (resin: hardener = 10:1). Thereafter, vacuum impregnation (0.1 MPa, approximately 20 minutes) is performed with a vacuum impregnation device (for example, manufactured by Bühler ITW Japan Co., Ltd., product name "Cast N1000"). The epoxy resin containing the vacuum-impregnated magnesium oxide powder (hereinafter referred to as "sample liquid") is poured into an embedding plate. The embedding plate is heated on a hot plate at 110°C for 2 hours to cure the sample liquid. The cured sample liquid (hereinafter referred to as "cured sample") is attached to a cross-section processing device (ion milling device (for example, manufactured by Hitachi High-Tech Corporation, product name "IM4000PLUS")), and broad ion beam processing (acceleration voltage: 4 kV, processing time: 5 hours) is performed to create a cross-section sample.

[0016] ·Observation with a scanning electron microscope Fix the cross-sectional sample on the observation sample stage of a scanning electron microscope (hereinafter referred to as "SEM"), and attach the sample stage to the SEM observation holder. Then, introduce the SEM observation holder into a field emission type SEM (for example, manufactured by JEOL Ltd., product name "JSM-7001F") to perform SEM observation (acceleration voltage: 15 kV, W.D.: 10 mm). Observe any particle group of the cross-sectional sample at 500 times magnification to acquire an image.

[0017] ·Image processing Analyze the observation image obtained above with image analysis software (for example, manufactured by MEDIA CYBERNETICS, product name "Image Pro Premier"). First, binarize the observation image with the image analysis software. At this time, the particles (magnesium oxide particles) in the cross-sectional sample are thinly highlighted, and the epoxy resin is represented in a dark color. Further, among the voids inside the particles, the portion impregnated with the epoxy resin is represented in the same dark color as the epoxy resin, and the portion not impregnated with the epoxy resin is represented in a state where the edge is surrounded by a white highlight. That is, in the present embodiment, in the voids existing in the particle group in the observation image, the void portion represented in the same dark color as the epoxy resin is determined as "open pores", and the void surrounded by a white highlight is determined as "closed pores". In the present embodiment, in the particle group existing in the observation image, the interface with the epoxy resin of the portion corresponding to the particles is surrounded by the image analysis software, and the total area (A) is calculated. Further, among the voids inside each particle, the interface between the portion represented in the same dark color as the epoxy resin (open pores) and the particles is surrounded by the image analysis software, and the total area (B) is calculated. Further, calculate the ratio (R) of the open pores in the particle group as (B) / (A)×100 (%).

[0018] As described above, the "open pores" in the magnesium oxide powder according to the first embodiment (hereinafter, may be simply referred to as "powder") are voids that communicate from the surface of the magnesium oxide particles to the inside of the particles, and when the magnesium oxide powder is embedded with an epoxy resin under the above conditions, the epoxy resin can penetrate into the voids (that is, the voids represented by the same dark color as the epoxy resin in the image analysis process). On the other hand, the voids existing inside the magnesium oxide particles, when the magnesium oxide powder is embedded with an epoxy resin under the above conditions, the voids into which the epoxy resin cannot penetrate (that is, the voids whose edges are surrounded by white highlights in the image analysis process) are defined as "closed pores". In the present disclosure, the "particles" contained in the magnesium oxide powder mean magnesium oxide particles. That is, the above-mentioned "particle group" means a particle group of magnesium oxide.

[0019] As a result of studying by focusing on the voids inside the magnesium oxide particles, the inventors of the present application found that if the ratio (R) of the area of the open pores (hereinafter, may be referred to as "ratio (R)") in the particle group analyzed by image analysis under specific conditions is equal to or less than a certain value, when the powder is filled in the resin, both low viscosity and high thermal conductivity can be achieved. Such a powder has a low tendency to increase in viscosity even when only the magnesium oxide powder is filled in the resin, and a resin composition with a low viscosity can be prepared. The magnesium oxide powder capable of achieving such a ratio (R) of open pores also has a small absolute number of open pores existing inside the particles. The powder containing a particle group with the above-mentioned ratio (R) of open pores of 9.0% or less can be achieved, for example, by preparing magnesium oxide powder by any method and then firing the magnesium oxide powder at a temperature of 1500 °C or higher for 2 hours or more to reduce the number of open pores existing inside the magnesium oxide particles.

[0020] FIG. 1 is a photograph obtained by subjecting an arbitrary particle group 100 of the magnesium oxide powder of Example 1 to image processing under the above conditions. The particle group 100 in FIG. 1 contains a plurality of magnesium oxide particles. Here, the magnesium oxide particles 10-11, and the open pores 20-21 and closed pores 30 contained in the magnesium oxide particles 10-11 will be described. In FIG. 1, magnesium oxide particles 10 to 11 are covered with an epoxy resin 40. Inside the magnesium oxide particle 10, open pores 20 and 21, which are shown in the same dark color as the epoxy resin 40, are included. On the other hand, the magnesium oxide particle 11 includes a closed pore 30 whose periphery is white-edged. Both the open pore 21 and the closed pore 30 are large voids existing inside the particle. However, since the epoxy resin 40 is impregnated in the magnesium oxide particle 10, it can be seen that it is an open pore. On the other hand, in the magnesium oxide particle 11, since the epoxy resin 40 is not impregnated, it is a closed pore. Conventionally, a large void located exactly at the center of the magnesium oxide particle has been considered to be not an open pore (i.e., a pore communicating to the particle surface), but to be derived from the unevenness of the particle surface and the shape of the particle. The inventors of the present application have found that a large void inside the magnesium oxide particle may become an open pore, and by controlling the ratio of the open pores existing in the entire particle group including the large open pores to a certain value or less, even when only magnesium oxide powder is blended with a resin, a resin composition with a lower viscosity can be prepared.

[0021] In one embodiment, from the viewpoint of easily obtaining a resin composition having both low viscosity and high thermal conductivity when filled in a resin, the ratio (R) is preferably 7.5% or less, more preferably 6.0% or less, and even more preferably 5.0% or less. In addition, according to the study by the inventors of the present application, it has been found that when the ratio (R) of the open pores is too low, the bulk density of the filler becomes small and the contact between the fillers in the compound decreases, which may cause a decrease in the thermal conductivity. Therefore, from the viewpoint of easily achieving a higher thermal conductivity, the ratio (R) is preferably 0.1% or more. That is, the ratio (R) of the open pores in the particle group is preferably 0.1 to 9.0%, more preferably 0.1 to 7.5%, and even more preferably 0.1 to 5.0% from the viewpoint of easily achieving both low viscosity and high thermal conductivity.

[0022] <Content ratio of the particle group> In the powder according to the first embodiment, "including a particle group with an open pore ratio (R) of 9.0% or less" means that when any part of the sample cross-section obtained from the powder according to the first embodiment is observed by SEM, one or more images including a particle group with an open pore ratio (R) of 9.0% or less are obtained. In a preferred embodiment, SEM observation and image processing are performed 10 times by changing the location of the sample cross-section, and the average value (R ave. ) of the open pore ratio of the particle group in each image is 9.0% or less, which is magnesium oxide powder.

[0023] In one embodiment, the content of the particle group with a ratio (R) of 9.0% or less in the magnesium oxide powder is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. The content of the particle group may be 100%. By including 80% or more of the particle group with an open pore ratio (R) of 9.0% or less in the powder, when the powder according to the first embodiment is filled into a resin, a resin composition with a lower viscosity is more likely to be obtained. Note that the "content of the particle group with a ratio (R) of 9.0% or less" in the powder is calculated from the number of particle groups with a ratio (R) of 9.0% or less by calculating the ratio (R) for any 10 particle groups in the sample cross-section. That is, when there are 5 particle groups with a ratio (R) of 9.0% or less, the content of the particle group in the powder is 50%. Similarly, when the ratio (R) of all 10 arbitrary particle groups is 9.0% or less, the content of the particle group with a ratio (R) of 9.0% or less in the powder is 100%.

[0024] <Median diameter (D50)> The median diameter (D50) (hereinafter, may also be referred to as "D50") of the magnesium oxide powder according to the first embodiment is preferably 30 to 150 μm, and more preferably 40 to 140 μm. The D50 of the powder according to the first embodiment refers to the median diameter (D50) corresponding to a cumulative value of 50% in the volume-based particle size distribution by the laser diffraction light scattering method. The cumulative particle size distribution is represented by a distribution curve with the particle diameter (μm) on the horizontal axis and the cumulative value (%) on the vertical axis. Specifically, it can be measured under the following conditions. (Measurement Conditions for Median Diameter) The median diameter (D50) of the powder is determined by measuring the volume-based particle size distribution using a laser diffraction particle size distribution analyzer (for example, LS 13 320 manufactured by Beckman Coulter, Inc., or MT3300EXII manufactured by MicrotracBEL Corp.). First, put 50 cm 3 of pure water and 0.1 g of the powder into a glass beaker, and perform a dispersion treatment for 60 seconds using an ultrasonic homogenizer (for example, Smurt NR-50M (titanium alloy chip φ3 (NS-50M-MT3)) manufactured by Microtech Nition Co., Ltd.). Drop by drop, add the dispersion of the powder subjected to the dispersion treatment to the laser diffraction particle size distribution analyzer using a dropper. After adding a predetermined amount, perform the measurement 30 seconds later. The refractive index of water is assumed to be 1.33, and the refractive index of the powder is assumed to be 1.74. From the obtained particle size distribution, the median diameter (D50) at a cumulative frequency of 50% is determined.

[0025] <Specific Surface Area (BET)> The specific surface area (BET) of the powder according to the first embodiment is preferably 0.1 m 2 / g or less, more preferably less than 0.1 m 2 / g, and even more preferably 0.09 m 2 / g or less. The powder according to the first embodiment contains a particle group with a ratio (R) of 9.0% or less, and further has a smaller absolute number of open pores, so the specific surface area (BET) of the powder tends to be small. Such a powder easily achieves low viscosity and high thermal conductivity when filled in a resin. The specific surface area (BET) can be measured by the following method. (Measurement Method for Specific Surface Area (BET)) Fill 1 g of the powder into the measurement cell of a fully automatic specific surface area measurement device (for example, Macsorb (registered trademark) HM model-1201 manufactured by Mountech Co., Ltd., BET one-point method) to measure the specific surface area. The degassing conditions before measurement can be 200 °C for 10 minutes. Also, helium can be used as the carrier gas and nitrogen (mixed concentration: 30.5%) can be used as the adsorption gas, respectively.

[0026] <Average circularity> The average circularity of the powder according to the first embodiment may be 0.75 or more, 0.80 or more, 0.85 or more, or 0.90 or more. If the average circularity of the powder is 0.75 or more, the fillability into the resin tends to be good, and a resin composition with a lower viscosity is likely to be obtained. The average circularity of the powder can be measured by the following method. (Method for measuring average circularity) Fix the powder with carbon tape and perform osmium coating. Then, using a scanning electron microscope (for example, manufactured by JEOL Ltd., product name "JSM-7001F SHL), photograph the particles constituting the powder at a magnification of 100 to 500 times, and using an image analyzer (for example, manufactured by Nippon Roper Co., Ltd., product name "Image-Pro Premier Ver.9.3), calculate the projected area (Ap) and projected perimeter (L) of the particles, and then calculate the circularity from the following formula (1). Calculate the circularity for any 200 particles and take the average value as the average circularity. Circularity = 4πAp / L 2 ···(1)

[0027] <Apparent density> In one embodiment, the apparent density of the powder is preferably 3.2 g / mL or more and less than 3.6 g / mL, more preferably 3.25 to 3.5 g / mL, and even more preferably 3.3 to 3.4 g / mL. The powder according to the first embodiment tends to have a relatively low apparent density due to an increase in the void content (generation of closed pores) caused by the occlusion of open pores. Such a powder is likely to obtain the effects of reducing viscosity due to a decrease in open pores and improving thermal conductivity due to an increase in bulk. The apparent density of the powder can be measured by a dry density meter using helium.

[0028] The magnesium oxide powder according to the first embodiment also has the feature that the number of open pores existing inside each particle is small. In one embodiment, in a particle group where the ratio (R) of open pores is 9.0% or less, the average value of the number of open pores existing in each particle of the particle group is preferably 10 or less. Note that the average value is a value calculated by counting the number of open pores for each particle after performing image processing under the above-described conditions, obtaining the total number, and dividing the total number by the number of particles existing in the image.

[0029] [Method for manufacturing magnesium oxide powder] The second embodiment in the present disclosure is a method for manufacturing the magnesium oxide powder according to the first embodiment. The manufacturing method according to the second embodiment includes firing the raw material magnesium oxide powder at 1500°C or higher for 2 hours or more to reduce the number of open pores inside the magnesium oxide particles contained in the raw material magnesium oxide powder (step (i)). Hereinafter, the raw material magnesium oxide powder will be described in detail as the "raw material powder".

[0030] [Raw material powder] As the raw material powder, magnesium oxide powder prepared by any method can be used. In one embodiment, the raw material powder may be magnesium oxide powder obtained by firing magnesium hydroxide powder. Specifically, after firing magnesium hydroxide powder to obtain magnesium oxide, the magnesium oxide powder is dispersed in a liquid and spray-dried. The fired magnesium oxide granules obtained thereby can be used as the raw material powder.

[0031] The median diameter (D50) of the raw material powder is not particularly limited. In one embodiment, when adjusting the median diameter (D50) of the finally obtained magnesium oxide powder to the range of 30 to 150 μm, it is preferable to use the raw material powder having the median diameter (D50) in the same range. Note that the median diameter (D50) of the raw material powder can also be measured by the same method as the above-described magnesium oxide powder.

[0032] The average circularity of the raw material powder is not particularly limited. From the viewpoint of facilitating the securing of fluidity by setting the average circularity of the finally obtained magnesium oxide powder to 0.75 or more, the average circularity of the raw material powder may be 0.80 or more. The average circularity of the raw material powder can also be measured by the same method as that for the aforementioned magnesium oxide powder.

[0033] (Step (i)) The production method according to the second embodiment includes firing the raw material powder at 1500°C or higher for 2 hours or longer to reduce the number of open pores inside the magnesium oxide particles contained in the raw material powder. The firing temperature of the raw material powder is preferably 1500 to 1650°C, more preferably 1500 to 1600°C, from the viewpoint that the number of open pores is more likely to be reduced and a powder containing a particle group with a ratio (R) of 9.0% or less is likely to be obtained. Also, from the above viewpoint, the firing time is preferably 2 to 8 hours, more preferably 2 to 6 hours. The inventors of the present application have found that by firing the raw material powder at the above firing temperature and firing time, the number of open pores inside the magnesium oxide particles contained in the raw material powder can be effectively reduced. The magnesium oxide powder obtained in such a manner has a small number of open pores inside the particles, and as a result, becomes a powder containing a particle group with an open pore ratio (R) of 9.0% or less.

[0034] In one embodiment, it is preferable that reducing the number of open pores inside the magnesium oxide particles (step (i)) includes reducing the open pore ratio (r1) of the particle group contained in the raw material powder (hereinafter, may also be referred to as "ratio (r1)") by 10% or more. Here, the open pore ratio (r1) is the ratio of the total area (b1) of the open pores existing in all the particles to the total area (a1) of all the particles existing in the observation image when the raw material powder before firing is observed by a scanning electron microscope under the above conditions. Also, the reduction rate of the open pore ratio (r1) can be calculated from the following formula. Reduction rate of open pore ratio (r1) = (1 - (ratio (R) / ratio (r1))) × 100 (%) In one embodiment, step (i) may include reducing the ratio (r1) of the open pores to 88% or less, or may include reducing the ratio to 86% or less.

[0035] The method for firing the raw material powder is not particularly limited, and for example, an electric furnace, a gas furnace, or the like can be used. Further, the firing is preferably performed in an air atmosphere.

[0036] According to the manufacturing method according to the second embodiment, magnesium oxide powder containing a group of magnesium oxide particles with few open pores can be manufactured without depending on the method for preparing the raw material powder. For example, in Patent Document 1, when manufacturing magnesium oxide powder, by containing a certain amount of boron or iron, the voids inside the magnesium oxide particles are reduced. However, in this method, it is necessary to control the addition amounts of boric acid and iron so that the boron and iron contents in the finally obtained magnesium oxide powder become constant values. Further, there is also a problem that the thermal conductivity of the magnesium oxide powder itself decreases due to the inclusion of boron and iron. In the case of the manufacturing method according to the second embodiment, since it is a method including reducing the number of open pores inside the particles by firing the raw material powder prepared by any method at 1500 °C or higher for 2 hours or longer, the manufacturing steps are simpler. Further, since the addition of impurities is not particularly required, magnesium oxide powder having a higher thermal conductivity can be obtained. Furthermore, in the conventional method, since the porosity inside the magnesium oxide particles is adjusted in the manufacturing process of the raw material powder, it is difficult to control the median diameter of the finally obtained magnesium oxide powder, and the ratio of the porosity tends to increase as the median diameter increases. According to the manufacturing method according to the second embodiment, by changing the median diameter of the raw material powder, magnesium oxide powder containing a group of particles having various median diameters and a small ratio (R) of open pores can be prepared. Further, even for magnesium oxide powder having a large median diameter (for example, magnesium oxide powder having a median diameter (D50) exceeding 100 μm), powder having a small ratio (R) of open pores can be prepared.

[0037] The proportions of impurities such as boron and iron contained in the finally obtained magnesium oxide powder are not particularly limited. As described above, from the viewpoint that the lower the impurities, the easier it is to obtain good thermal conductivity, the boron content in the magnesium oxide powder may be less than 600 ppm. Similarly, the iron content in the magnesium oxide powder may be less than 500 ppm. The contents of impurities such as boron and iron in the magnesium oxide powder may be adjusted during the preparation of the raw material powder. Further, magnesium oxide powder with the content of impurities such as boron and iron within the above range may be used as the raw material powder.

[0038] Further, the manufacturing method according to the second embodiment may include a step of classifying the magnesium oxide powder after reducing the number of the open pores in order to obtain magnesium oxide powder having a desired median diameter (D50) and particle size distribution. Examples of the classification method include classification by a sieve, as well as liquid cyclone, air classification, and the like.

[0039] Further, the manufacturing method according to the second embodiment may include performing surface treatment on the obtained magnesium oxide powder after reducing the number of the open pores from the viewpoint of improving the moisture resistance of the finally obtained magnesium oxide powder. The surface treatment is not particularly limited, but for example, colloidal silica, silane-based coupling agent, titania sol, titanate-based coupling agent, phosphorus compound, alumina sol, aluminate-based coupling agent, zirconium-based coupling agent, etc. can be used. These surface treatment agents may be used alone or in combination of two or more.

[0040] [Mixed powder] The third embodiment in the present disclosure is a mixed powder. The mixed powder according to the third embodiment includes the magnesium oxide powder according to the first embodiment and inorganic powder other than the magnesium oxide powder. In the mixed powder according to the third embodiment, the inorganic powder combined with the magnesium oxide powder according to the first embodiment is not particularly limited, and any inorganic powder can be adopted. For example, powders of oxides such as titanium oxide powder, silica powder, zirconia powder, alumina powder, and magnesium oxide powder other than the magnesium oxide powder of the first embodiment; powders of sulfides such as barium sulfate powder, iron sulfate powder, and copper sulfate powder; powders of hydroxides such as aluminum hydroxide powder and magnesium hydroxide powder; powders of nitrides such as boron nitride powder, aluminum nitride powder, and silicon nitride powder; powders of carbides such as silicon carbide powder; powders of minerals such as kaolinite powder, talc powder, natural mica powder, and synthetic mica powder, etc. These may be included alone or in combination of two or more. Further, as these inorganic powders, those surface-treated with the aforementioned surface treatment agent may be used as necessary.

[0041] The magnesium oxide powder according to the first embodiment can prepare a resin composition having low viscosity and high thermal conductivity even when only the powder is blended in the resin. Therefore, it is also possible to set a high proportion of the magnesium oxide powder in the mixed powder according to the third embodiment. In one embodiment, the proportion of the magnesium oxide powder in the total mass of the mixed powder may be in the range of 1 to 99% by mass.

[0042] In one embodiment, the mixed powder may be a mixed powder of the magnesium oxide powder according to the first embodiment and a magnesium oxide powder other than the magnesium oxide powder (that is, a magnesium oxide powder containing a particle group having an open pore ratio (R) of more than 9.0%). The magnesium oxide powder according to the first embodiment can achieve low viscosity and high thermal conductivity when filled in the resin. Therefore, the powder according to the first embodiment may be combined with a general magnesium oxide powder as a viscosity adjusting material and / or a thermal conductivity adjusting material to form a mixed powder. In that case, the proportion of the powder according to the first embodiment in the mixed powder may be adjusted in the range of 1 to 99% by mass.

[0043] The mixed powder can contain magnesium oxide powder according to the first embodiment and any components other than the inorganic powder. Examples of the optional components include resin powder, organic-inorganic hybrid powder, and the like.

[0044] [Resin composition] The fourth embodiment in the present disclosure is a resin composition. The resin composition according to the fourth embodiment includes magnesium oxide powder according to the first embodiment and at least one resin selected from a thermoplastic resin and a thermosetting resin. The resin composition according to the fourth embodiment can achieve both low viscosity and high thermal conductivity.

[0045] The content of the powder in the resin composition is not particularly limited and can be appropriately adjusted according to the purpose. Note that even when only the powder according to the first embodiment is filled in the resin, a resin composition with low viscosity can be prepared. Therefore, the blending amount of the powder in the resin composition can be arbitrarily adjusted so as to obtain desired physical properties. For example, when the resin composition is used for insulating materials or high heat dissipation members, the powder may be blended in the range of 1 to 99% by mass, more preferably in the range of 10 to 90% by mass, based on the total mass of the resin composition.

[0046] [Resin] The resin composition according to the fourth embodiment contains at least one resin selected from a thermoplastic resin and a thermosetting resin. More specifically, for example, polyethylene resin; polypropylene resin; epoxy resin; silicone resin; phenol resin; melamine resin; urea resin; unsaturated polyester resin; fluororesin; polyamide resins such as polyimide resin, polyamideimide resin, polyetherimide resin, etc.; polyester resins such as polybutylene terephthalate resin, polyethylene terephthalate resin, etc.; polyphenylene sulfide resin; wholly aromatic polyester resin; polysulfone resin; liquid crystal polymer resin; polyethersulfone resin; polycarbonate resin; maleimide-modified resin; ABS resin; AAS (acrylonitrile-acrylic rubber-styrene) resin; AES (acrylonitrile-ethylene-propylene-diene rubber-styrene) resin; hydrocarbon-based elastomer resin; polyphenylene ether resin; aromatic polyene resin, etc. These may be used alone or in combination of two or more.

[0047] In the resin composition according to the fourth embodiment, a curing agent, a curing accelerator, a release agent, a coupling agent, a colorant, a flame retardant, an ion scavenger, etc. may be blended within a range not inhibiting the effects of the present invention.

[0048] In one embodiment, the thermal conductivity of the resin composition containing only the powder according to the first embodiment, measured under the following conditions, may be 3.0 to 6.0 W / m·K, or may be 3.2 W / m·K or more and less than 4.0 W / mK. Even when only the powder according to the first embodiment is blended with the resin, a resin composition having high thermal conductivity can be prepared. (Measurement conditions for thermal conductivity) Magnesium oxide powder and spherical alumina powder (a mixture of products named "DAW-10", "DAW-03", and "ASFP-40" manufactured by Denka Co., Ltd. (DAW-10):(DAW-03):(ASFP-40)=3:25:15 (volume ratio)) are mixed at a volume ratio of 57:43 to produce a powder. The mixed powder is added to an epoxy resin (for example, "JER828" manufactured by Mitsubishi Chemical Corporation) so that the filling amount is 70 vol%, and a rotation and revolution vacuum mixer (for example, "Avatori Ren Tarou (registered trademark) ARV-310P" manufactured by Shin Kee Co., Ltd.) is used to disperse the mixed powder and remove air bubbles. The obtained mixture is poured into a silicon mold of 2 cm × 2 cm × 0.6 cm, and using a hot press machine, it is pressed according to a schedule of 3 kN, 80 °C for 1 hour, 5 kN, 150 °C for 1 hour, and 7 kN, 200 °C for 0.5 hour to produce an epoxy resin molded body containing the mixed powder. The obtained epoxy resin molded body sample is processed into a disc-shaped sample with φ = 10 mm and a thickness of 1 mm to be a test piece for evaluation. After further blackening the surface of the evaluation test piece using carbon spray (for example, "Graphite" manufactured by Nitchi Japan Co., Ltd.), the thermal diffusivity is measured using a thermal diffusivity measuring instrument (for example, "LFA447 Nanoflash" manufactured by Nitchi Japan Co., Ltd.) to obtain the thermal diffusivity. At this time, the measurement sample temperature is 25 °C and the Xe lamp applied voltage is 247 V. From the obtained thermal diffusivity, the thermal conductivity is obtained. The thermal conductivity H (unit [W / mK]) is calculated from the formula H = T × D × C using the measured values of the thermal diffusivity T (unit [m 2 / s]), density D (unit [kg / m 3 ), and specific heat capacity C (unit [J / kg·K]). The density D is measured by the gravity and volume measurement of the sample, and for the specific heat capacity C, the specific heat capacities of magnesium oxide powder and resin are 940 [J / kg·K] (magnesium oxide powder) and 1250 [J / kg·K] (epoxy resin) respectively, and the specific heat capacity of spherical alumina is 770 [J / kg·K].

[0049] In one embodiment, the viscosity of the resin composition containing only the powder according to the first embodiment, measured under the following conditions, is preferably less than 900 Pa·s / 25°C, more preferably 800 Pa·s / 25°C or less, and even more preferably 600 Pa·s / 25°C or less. The powder according to the first embodiment can prepare a resin composition with a low viscosity even when only the powder is blended with the resin. (Measurement conditions for viscosity) Measure the viscosity of a resin composition consisting of 60 vol% of bisphenol A type liquid epoxy resin (epoxy equivalent: 184 - 194) and 40 vol% of powder at a shear rate of 1.0 / s, plate shape: circular flat plate (10 mm φ), sample thickness: 1 mm, and temperature: 25 ± 1°C using a rotational rheometer.

[0050] (Manufacturing method of resin composition) The method for manufacturing the resin composition according to the fourth embodiment is not particularly limited and can be manufactured by stirring, dissolving, mixing, and dispersing predetermined amounts of each material. The apparatuses for mixing, stirring, dispersing, etc. of these mixtures are not particularly limited, but a Lycra machine equipped with a stirring and heating device, a three-roll mill, a ball mill, a planetary mixer, etc. can be used. Also, these apparatuses may be used in appropriate combinations.

[0051] As described above, the resin composition containing magnesium oxide powder according to the first embodiment can achieve both high thermal conductivity and low viscosity. Such a resin composition is excellent in processability and workability due to its low viscosity.

[0052] Another embodiment in the present disclosure is a method for reducing the number of open pores inside magnesium oxide particles contained in the magnesium oxide powder, which includes firing the magnesium oxide powder at 1500°C or higher for 2 hours or longer. In the above embodiment, the firing is preferably performed such that for the ratio of open pores (1) of the group of magnesium oxide particles contained in the magnesium oxide powder before firing and the ratio of open pores (2) of the group of magnesium oxide particles contained in the magnesium oxide powder after firing, the value of ((1 - ratio (2) / ratio (1)) × 100 (%)) is 10% or more.

Example

[0053] Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited by the following description.

[0054] [Example 1 and Comparative Example 1] Magnesium oxide powder was prepared under the following conditions to obtain raw material powder 1. Magnesium oxide powder was obtained by firing magnesium hydroxide powder in an electric furnace at 1000 ° C for 1 hour under atmospheric pressure. Distilled water was added to the magnesium oxide powder to obtain a 60 mass% magnesium oxide powder aqueous dispersion. Granulation was performed by the spray drying method using this dispersion to obtain granular raw material powder 1 of magnesium oxide. The raw material powder 1 was fired at 1500 ° C for 4 hours to obtain the magnesium oxide powder of Example 1. The ratio of open pores (R), median diameter (D50), specific surface area (BET), apparent density, and average circularity of the raw material powder 1 (Comparative Example 1) and the fired magnesium oxide powder (Example 1) were measured under the following conditions. In addition, the viscosity and thermal conductivity of the resin composition containing magnesium oxide powder were evaluated under the following conditions. The results are shown in Table 1.

[0055] <Ratio of open pores (R)> First, after performing pretreatment under the following conditions, observation and image processing were carried out using a scanning electron microscope. ·Pretreatment Magnesium oxide powder (0.1 g) was mixed with a two-component heat-curable epoxy resin (manufactured by Gatan, product name "G2") (resin: hardener = 10: 1). Then, vacuum impregnation (0.1 MPa, about 20 minutes) was performed using a vacuum impregnation device (manufactured by Buehler ITW Japan Co., Ltd., product name "Cast N1000"). The epoxy resin containing the vacuum-impregnated magnesium oxide powder (hereinafter referred to as "sample solution") was poured into an embedding plate. The embedding plate was heated on a hot plate at 110 ° C for 2 hours to cure the sample solution. The cured sample solution (hereinafter referred to as "cured sample") was attached to a cross-section processing device (ion milling device (e.g., Hitachi High-Tech Corporation, product name "IM4000PLUS")), and broad ion beam processing (acceleration voltage: 4 kV, processing time: 5 hours) was performed to create a cross-section sample.

[0056] ·Observation with a scanning electron microscope The cross-section sample was fixed to the sample stage for SEM observation, and the sample stage was attached to the holder for SEM observation. Then, the holder for SEM observation was introduced into a field emission type SEM (manufactured by JEOL Ltd., product name "JSM-7001F") for SEM observation (acceleration voltage: 15 kV, W.D.: 10 mm). An arbitrary particle group of the cross-section sample was observed at 500 times magnification to acquire an image.

[0057] ·Image processing The observation image obtained above was analyzed using image analysis software (manufactured by MEDIA CYBERNETICS, product name "Image Pro Premier"). First, the observation image was binarized using the image analysis software. In the voids existing in the particle group within the observation image, the void part represented in the same dark color as the epoxy resin was determined as "open pore", and the void surrounded by a white highlight at the edge was determined as "closed pore". Furthermore, in the particle group existing in the observation image, the interface between the part corresponding to the particles and the epoxy resin was surrounded by the image analysis software, and the total area (A) was calculated. Furthermore, among the voids inside each particle, the interface between the part represented in the same dark color as the epoxy resin (open pore) and the particle was surrounded by the image analysis software, and the total area (B) was calculated. Furthermore, the ratio (R) of open pores in the particle group was obtained as (B) / (A)×100 (%).

[0058] <Method for measuring median diameter> The median diameter (D50) of magnesium oxide powder was determined by volume-based particle size distribution measurement using a laser diffraction particle size distribution analyzer (manufactured by Beckman Coulter, Inc., product name "LS 13 320"). First, 50 cm was placed in a glass beaker 3Pure water and 0.1 g of the powder were placed, and dispersion treatment was carried out for 60 seconds using an ultrasonic homogenizer (manufactured by Microtech Nition Co., Ltd., product name "Smurt NR-50M (titanium alloy chip φ3 (NS-50M-MT3))"). The dispersion of the powder after the dispersion treatment was added drop by drop with a dropper to the laser diffraction particle size distribution measuring device, and measurement was carried out 30 seconds after adding a predetermined amount. The refractive index of water was set to 1.33, and the refractive index of the powder was set to 1.74. From the obtained particle size distribution, the median diameter (D50) at a cumulative frequency of 50% was determined.

[0059] <Measurement method of specific surface area (BET)> 1 g of the powder was filled into the measurement cell of a fully automatic specific surface area measuring device (manufactured by Mountech Co., Ltd., product name "Macsorb (registered trademark) HM model-1201", BET one-point method) to measure the specific surface area. The degassing conditions before measurement were 200 °C for 10 minutes. Helium was used as the carrier gas and nitrogen (mixed concentration: 30.5%) was used as the adsorption gas.

[0060] <Measurement method of average circularity> The powder was fixed with carbon tape and osmium coating was performed. Then, using a scanning electron microscope (manufactured by JEOL Ltd., product name "JSM-7001F SHL"), particles constituting the powder were photographed at magnifications of 100 to 500 times, and using an image analysis device (manufactured by Nippon Roper Co., Ltd., product name "Image-Pro Premier Ver.9.3"), the projected area (Ap) and projected perimeter (L) of the particles were calculated, and then the circularity was calculated from the following formula (1). The circularity was calculated for any 200 particles and the average value was taken as the average circularity. Circularity = 4πAp / L 2 ···(1)

[0061] <Measurement method of apparent density> The apparent density of the powder was measured using a dry density meter (manufactured by Shimadzu Corporation, product name "Accupic II 1340"). 8 cc of the powder was placed in the measurement cell (10 cc), and the powder density was measured at room temperature (23 °C) by the gas displacement method (using helium).

[0062] <Method for Measuring Viscosity of Resin Composition> 60% by volume of bisphenol A liquid epoxy resin (epoxy equivalent: 184 - 194, manufactured by Mitsubishi Chemical Corporation, product name "JER828") and 40% by volume of powder were mixed and kneaded using a planetary mixer (manufactured by Shinki Co., Ltd., product name "Avatori Ren Taro AR - 250", rotation speed 2000 rpm) to prepare a resin composition. Next, the shear viscosity at 25°C of the obtained resin composition was measured using a rotational rheometer (manufactured by Anton Paar GmbH, product name "MCR - 302") under the following conditions. Plate shape: Circular flat plate with a diameter of 10 mm Sample thickness: 1 mm Temperature: 25 ± 1°C Speed: 1.0 / s

[0063] <Method for Measuring Thermal Conductivity of Resin Composition> Magnesium oxide powder and spherical alumina powder (a mixture of products manufactured by Denka Co., Ltd., product names "DAW-10", "DAW-03", and "ASFP-40". (DAW-10):(DAW-03):(ASFP-40) = 3:25:15 (volume ratio)) were mixed at a volume ratio of 57:43 to prepare a powder. This mixed powder was added to a liquid epoxy (manufactured by Mitsubishi Chemical Corporation, product name "JER828") with a curing agent (4,4-Diaminodiphenylmethane) added so that the filling amount was 70 vol%, and a rotation-revolution vacuum mixer (manufactured by Shinki Co., Ltd., product name "Awatori Ren Tarou ARV-310P") was used to disperse the powder and remove bubbles. The obtained mixture was poured into a 2 cm × 2 cm × 0.6 cm silicon mold, and a heating press machine was used to press it according to a schedule of 3 kN, 80 °C for 1 hour, 5 kN, 150 °C for 1 hour, and 7 kN, 200 °C for 0.5 hour to obtain an epoxy resin molded body containing the mixed powder. The obtained epoxy resin molded body sample was processed into a disk-shaped sample with φ = 10 mm and a thickness of 1 mm as a test piece for evaluation. The surface of the said evaluation test piece was blackened using carbon spray (manufactured by Nitchi Japan Co., Ltd., product name "Graphite"), and then the thermal diffusivity was measured using a thermal diffusivity measuring instrument (manufactured by Nitchi Japan Co., Ltd., product name "LFA447 Nanoflash"). At this time, the measurement sample temperature was 25 °C and the Xe lamp applied voltage was 247 V. Based on the obtained thermal diffusivity, the thermal conductivity was determined. The thermal conductivity H (unit [W / mK]) was calculated from the measurement values of the thermal diffusivity T (unit [m 2 / s]), density D (unit [kg / m 3 ), and specific heat capacity C (unit [J / kg·K]) using the calculation formula H = T × D × C. The density D was measured by the gravity and volume measurement of the sample, and for the specific heat capacity C, the specific heat capacities of magnesium oxide powder and resin were set to 940 [J / kg·K] (magnesium oxide powder) and 1250 [J / kg·K] (epoxy resin) respectively, and the specific heat capacity of spherical alumina was set to 770 [J / kg·K].

[0064] [Example 2] The raw material powder 1 was heated at 1600 °C for 4 hours to obtain the magnesium oxide powder of Example 2. The ratio of open pores (R), median diameter (D50), specific surface area (BET), apparent density, and average circularity of the obtained magnesium oxide powder were measured under the same conditions as in Example 1. Also, the viscosity and thermal conductivity of the resin composition containing the magnesium oxide powder were evaluated under the same conditions as in Example 1. The results are shown in Table 1.

[0065] [Example 3 and Comparative Example 2] Magnesium oxide powder was prepared under the following conditions to obtain raw material powder 2. Anhydrous magnesium chloride was dissolved in distilled water to obtain an aqueous magnesium chloride solution of about 4.0 M. Magnesium hydroxide formed by adding a 25% aqueous sodium hydroxide solution to the aqueous magnesium chloride solution was recovered by filtration and washing with water. The recovered powder dried by air drying was calcined in an electric furnace at 800 °C for 1 hour in an air atmosphere to obtain magnesium oxide powder. Distilled water was added to the magnesium oxide powder to obtain a 60 mass% aqueous magnesium oxide powder dispersion. Granulation was performed by the spray drying method using this dispersion to obtain granular raw material powder 2 of magnesium oxide. The raw material powder 2 was calcined at 1600 °C for 4 hours to obtain the magnesium oxide powder of Example 3. Also, the raw material powder 2 was calcined at 1500 °C for 1 hour to obtain the magnesium oxide powder of Comparative Example 2. The ratio of open pores (R), median diameter (D50), specific surface area (BET), apparent density, and average circularity of the magnesium oxide powders of Comparative Example 2 and Example 3 were measured under the same conditions as in Example 1. Also, the viscosity and thermal conductivity of the resin composition containing the magnesium oxide powder were evaluated under the same conditions as in Example 1. The results are shown in Table 1.

[0066]

Table 1

[0067] As shown in Table 1, in Examples 1 to 3 which are magnesium oxide powders of the first embodiment, resin compositions with lower viscosities could be prepared compared to the powder of Comparative Example 1. Also, the content of the particle group with a ratio (R) of 9.0% or less in the magnesium oxide powders of Examples 1 to 3 was 100%. Furthermore, it was found that by firing the raw material powder prepared by any method at 1500°C or higher for 2 hours or more according to the manufacturing method according to the second embodiment, the proportion of open pores inside the particles in the raw material powder was reduced, and the powder according to the first embodiment was obtained. Note that the ratio (R) of the open pores of the raw material powder 1 of Comparative Example 1 corresponds to the above ratio (r1) (the ratio of the open pores of the raw material powder before firing). From the comparison between Comparative Example 1 and Examples 1 to 2, it was confirmed that the manufacturing method according to the second embodiment is a method capable of reducing the ratio (r1) of open pores by 10% or more. Also, from the comparison between Example 3 and Comparative Example 2, it was also confirmed that the manufacturing method according to the second embodiment is a method capable of effectively reducing the ratio (r1) of open pores of the raw material powder. Note that although the powders of Examples 1 to 3 have a lower thermal conductivity than the powder of Comparative Example 1, since the resin compositions containing these powders of Examples 1 to 3 have a low viscosity, it is possible to adjust the thermal conductivity by increasing the proportion of the powder in the resin composition. According to the study by the inventors of the present application, the viscosity of the resin composition containing 50% by mass of the powder of the example was about 800 Pa·s / 25°C, and the thermal conductivity was 5.2 W / mK. From the above results, it was confirmed that magnesium oxide powder, its mixed powder, and a resin composition containing the magnesium oxide powder, which can achieve both high thermal conductivity and low viscosity when filled in a resin as a filler, can be provided.

[0068] A non-limiting list of exemplary embodiments of the present disclosure and combinations of exemplary embodiments is given below. [1] A magnesium oxide powder, when the magnesium oxide powder is observed by a scanning electron microscope under the following conditions, a magnesium oxide powder containing a particle group in which the ratio (R) of the total area (B) of the open pores existing in all the particles to the total area (A) of all the particles existing in the observation image is 9.0% or less. (Condition) The magnesium oxide powder is embedded in an epoxy resin and then cured. Thereafter, processing is performed with a cross-section processing device to create a sample of the magnesium oxide powder. An image obtained by observing a cross-section of the sample with a scanning electron microscope at a magnification of 500 times is analyzed with image analysis software, and the total area (A) of all the particles present in the observed image is calculated. Similarly, the total area (B) of the open pores communicating with the particle surface in all the particles in the observed image is calculated. Thereafter, the ratio (R) of the total area (B) of the open pores to the total area (A) is determined. [2] The magnesium oxide powder according to [1], wherein the ratio of the particle group in which the ratio (R) of the open pores in the magnesium oxide powder is 9.0% or less is 80% or more. [3] The magnesium oxide powder according to [1] or [2], wherein the median diameter (D50) of the magnesium oxide powder is 30 to 150 μm. [4] The magnesium oxide powder according to any one of [1] to [3], wherein the average circularity of the magnesium oxide powder is 0.75 or more. [5] The specific surface area (BET) of the magnesium oxide powder is 0.1 m 2 / g or less, and the magnesium oxide powder according to any one of [1] to [4]. [6] The magnesium oxide powder according to any one of [1] to [5], wherein the ratio (R) of the open pores in the particle group is 0.1% or more. [7] A method for producing the magnesium oxide powder according to any one of [1] to [6], comprising firing the raw material magnesium oxide powder at 1500 °C or higher for 2 hours or more to reduce the number of open pores inside the magnesium oxide particles contained in the raw material magnesium oxide powder. [8] Reducing the number of open pores inside the magnesium oxide particles includes reducing the ratio (r1) of the open pores in the particle group contained in the raw material magnesium oxide powder by 10% or more. The production method according to [7], wherein the ratio (r1) of the open pores is the ratio of the total area (b1) of the open pores present in all the particles to the total area (a1) of all the particles present in the observation image when the magnesium oxide powder of the raw material is observed by a scanning electron microscope under the above conditions. [9] A mixed powder containing the magnesium oxide powder according to any one of [1] to [6] and an inorganic powder other than the magnesium oxide powder.

[10] A resin composition containing the magnesium oxide powder according to any one of [1] to [6] and at least one resin selected from a thermoplastic resin and a thermosetting resin.

Explanation of reference numerals

[0069] 10, 11: Magnesium oxide particles 20, 21: Open pores 30: Closed pores 40: Epoxy resin 100: Particle group

Claims

1. Magnesium oxide powder, when the magnesium oxide powder is observed with a scanning electron microscope under the following conditions, a magnesium oxide powder containing a particle group in which the ratio (R) of the total area (B) of open pores present in all the particles to the total area (A) of all the particles present in the observation image is 9.0% or less. (Conditions) The magnesium oxide powder is embedded in an epoxy resin and then cured. Thereafter, processing is performed with a cross-section processing device to prepare a sample of the magnesium oxide powder. An image obtained by observing a cross-section of the sample with a scanning electron microscope at a magnification of 500 times is analyzed with image analysis software to calculate the total area (A) of all the particles present in the observation image. Similarly, the total area (B) of open pores communicating with the particle surface in all the particles in the observation image is calculated. Thereafter, the ratio (R) of the total area (B) of the open pores to the total area (A) is determined.

2. The magnesium oxide powder according to claim 1, wherein the ratio of the particle group in which the ratio (R) of the open pores in the magnesium oxide powder is 9.0% or less is 80% or more.

3. The magnesium oxide powder according to claim 1 or 2, wherein the median diameter (D50) of the magnesium oxide powder is 30 to 150 μm.

4. The magnesium oxide powder according to claim 1 or 2, wherein the average circularity of the magnesium oxide powder is 0.75 or more.

5. The specific surface area (BET) of the magnesium oxide powder is 0.1 m ２ / g or less, the magnesium oxide powder according to claim 1 or 2.

6. The magnesium oxide powder according to claim 1 or 2, wherein the ratio (R) of the open pores in the particle group is 0.1% or more.

7. A method for producing the magnesium oxide powder according to claim 1 or 2, comprising firing the raw material magnesium oxide powder at 1500 °C or higher for 2 hours or more to reduce the number of open pores inside the magnesium oxide particles contained in the raw material magnesium oxide powder.

8. Reducing the number of open pores inside the magnesium oxide particles includes reducing the ratio (r1) of the open pores in the particle group contained in the raw material magnesium oxide powder by 10% or more, wherein the ratio (r1) of the open pores is the ratio of the total area (b1) of the open pores present in all the particles to the total area (a1) of all the particles present in the observation image when the raw material magnesium oxide powder is observed with a scanning electron microscope under the above conditions. The manufacturing method according to claim 7.

9. A mixed powder comprising the magnesium oxide powder according to claim 1 or 2 and an inorganic powder other than the magnesium oxide powder.

10. A resin composition comprising the magnesium oxide powder according to claim 1 or 2 and at least one resin selected from a thermoplastic resin and a thermosetting resin.

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