Silicone composition

Abstract

The silicone composition has a thermal conductivity of 4.0 W / m.K or more and less than 7.0 W / m.K in a hot plate method in accordance with ISO 22007-2, and a viscosity at 25 DEG C of 5-800 Pa * s when measured using a spiral viscometer at a rotational speed of 10 rpm, maintains fluidity and handling properties even when filled with a large amount of a thermally conductive filler in order to impart excellent insulating properties and thermal conductivity, and has excellent thermal conductivity. The heat dissipation performance is excellent, and the durability and reliability under high-temperature or high-temperature and high-humidity conditions are excellent.

Description

Technical Field

[0001] This invention relates to organosilicon compositions. Background Technology

[0002] Heat is generated during the use of numerous electronic components, and therefore, it is essential to remove this heat to ensure that these components function properly. In particular, integrated circuit components such as CPUs used in personal computers generate significantly more heat due to their increasingly high operating frequencies, making heat management a crucial issue.

[0003] Various methods have been proposed as means of removing this heat. In particular, for electronic components that generate a lot of heat, methods have been proposed to dissipate heat by inserting thermally conductive materials such as thermally conductive grease compositions or thermally conductive sheets between the electronic components and heat sinks or other components. However, these methods are not yet sufficient for heat dissipation in areas with large differences in size and thickness (see Patent Document 1).

[0004] In addition, as such a thermally conductive material, a heat-dissipating grease composition based on silicone oil and mixed with zinc oxide and aluminum oxide powder has been proposed, but its heat resistance at 200°C does not meet the requirements (see Patent Documents 2 and 3).

[0005] To improve thermal conductivity, as a thermally conductive material using aluminum nitride powder, Patent Document 1 proposes a thixotropic thermally conductive material composed of at least one selected from liquid silicone carrier, silica fiber, dendritic zinc oxide, flake aluminum nitride, and flake boron nitride. Patent Document 4 proposes a silicone grease composition obtained by blending spherical hexagonal aluminum nitride powder with a certain particle size range into a specific organopolysiloxane. Patent Document 5 proposes a thermally conductive silicone grease composition combining fine and coarse aluminum nitride powder. Patent Document 6 proposes a thermally conductive silicone grease composition combining aluminum nitride powder and zinc oxide powder. Patent Document 7 proposes a thermally conductive grease composition using aluminum nitride powder surface-treated with an organosilane, but none of these meet the requirements for durability and reliability. Patent document 8 proposes a thermally conductive silicone composition comprising silicone resin, diamond, zinc oxide and dispersant, but its heat resistance properties do not meet the requirements.

[0006] Furthermore, metals are materials with high thermal conductivity, allowing them to be used in areas where insulation of electronic components is not required. Patent Document 9 proposes a thermally conductive grease composition obtained by mixing aluminum powder with a base oil such as silicone oil, but this is insufficient due to its lack of insulation. All current thermally conductive materials and grease compositions are inadequate for heat dissipation in integrated circuit components such as CPUs.

[0007] According to Maxwell and Braggman's theory, if the volume fraction of the thermally conductive filler is below 0.6, the thermal conductivity of the material obtained by blending the thermally conductive filler into silicone oil is almost independent of the thermal conductivity of that filler. However, when the volume fraction exceeds 0.6, it begins to affect the thermal conductivity of the filler. That is, to improve the thermal conductivity of a thermally conductive grease composition, it is important to first consider how much thermally conductive filler to use and how to use fillers with high thermal conductivity. However, due to excessive filling, the fluidity of the thermally conductive grease composition decreases, and its workability (spreadability, screen printability) deteriorates, leading to practical unusability. Furthermore, due to reduced fluidity, it cannot follow the fine irregularities of electronic components and heat sink surfaces, resulting in increased contact thermal resistance.

[0008] To date, in order to achieve high-volume filling and obtain thermally conductive materials with good flowability, research has been conducted on the formulation of alkoxy-containing organopolysiloxanes in thermally conductive materials by treating the surface of thermally conductive fillers to significantly improve dispersibility (see Patent Documents 10 and 11). However, these treatment agents have the disadvantage of denaturation due to hydrolysis and other factors under high temperature and humidity, which induces performance degradation of the thermally conductive materials.

[0009] Therefore, a thermally conductive silicone grease composition was proposed that suppresses the performance degradation of thermally conductive materials even under high temperature and high humidity conditions. Although the average particle size of the thermally conductive filler is limited, there are no limitations on the shape, hydroxyl content, or coarseness of the thermally conductive filler. In particular, it does not meet the requirements for applications that require insulation (Patent Document 12).

[0010] In Japanese Patent No. 5755977 (Patent Document 13), a high thermal conductivity resin composition was proposed, which specifies the average sphericity, the amount of hydroxyl groups, and the average particle size of spherical alumina powder limited to 10 to 50 μm, and the average particle size of alumina powder limited to 0.3 to 1 μm, and specifies the mixing ratio and volume ratio of each alumina. Although it is stated that the average particle size of the spherical alumina powder is up to 50 μm, there is no limitation on the range or content of coarse particle size. When the high thermal conductivity resin composition is to be applied to a film with a diameter of 50 μm or less, there is a problem of insufficient thermal resistance.

[0011] Furthermore, while Japanese Re-publication No. 2002-092693 (Patent Document 14) proposed a thermally conductive organosilicon composition using alumina powder with an average particle size of 0.1 to 100 μm, it did not specify the specific thermal conductivity or viscosity. Moreover, it proposed a thermally conductive organosilicon composition using a mixture of spherical alumina powder with an average particle size of 5 to 50 μm (excluding 5 μm) and spherical or irregularly shaped alumina powder with an average particle size of 0.1 to 5 μm, specifying the mixing ratio and weight ratio of each alumina, but it did not specify the average sphericality or hydroxyl content of the large-particle-size spherical alumina, nor did it specify the range or content of coarse particle size. Like Patent Document 14, it also suffers from insufficient thermal resistance.

[0012] Therefore, in Japanese Patent No. 6866877 (Patent Document 15), an organosilicon composition containing α-alumina powder having a hexagonal densest lattice crystal structure composed of polyhedra with defined average particle size and coarse particle content was found to have reduced contact thermal resistance and improved thermal resistance, but it is desirable to further improve thermal conductivity.

[0013] Existing technical documents

[0014] Patent documents

[0015] Patent Document 1: Japanese Patent Application Publication No. 56-28264

[0016] Patent Document 2: Japanese Patent Publication No. 52-33272

[0017] Patent Document 3: Japanese Patent Publication No. 59-52195

[0018] Patent Document 4: Japanese Patent Application Publication No. 2-153995

[0019] Patent Document 5: Japanese Patent Application Publication No. 3-14873

[0020] Patent Document 6: Japanese Patent Application Publication No. 10-110179

[0021] Patent Document 7: Japanese Patent Application Publication No. 2000-63872

[0022] Patent Document 8: Japanese Patent Application Publication No. 2002-30217

[0023] Patent Document 9: Japanese Patent Application Publication No. 2000-63873

[0024] Patent Document 10: Japanese Patent Application Publication No. 2004-262972

[0025] Patent Document 11: Japanese Patent Application Publication No. 2005-162975

[0026] Patent Document 12: Japanese Patent No. 4933094

[0027] Patent Document 13: Japanese Patent No. 5755977

[0028] Patent Document 14: Japanese Re-publication No. 2002-092693

[0029] Patent Document 15: Japanese Patent No. 6866877 Summary of the Invention

[0030] The problem that the invention aims to solve

[0031] The present invention was made in view of the above-mentioned actual situation, and its object is to provide an organosilicon composition that maintains good flowability, good processability, excellent heat dissipation performance, and excellent durability and reliability under high temperature or high temperature and high humidity conditions, even when a large amount of thermally conductive filler is filled in order to impart excellent insulation and thermal conductivity.

[0032] Methods for solving problems

[0033] To achieve the above objectives, the inventors conducted in-depth research and discovered that an organosilicon composition containing polyhedral α-alumina with a defined average particle size and coarse particle content exhibits excellent insulation and thermal conductivity, as well as good flowability, resulting in good processability. Furthermore, by following fine irregularities and reducing contact thermal resistance, an organosilicon composition with excellent heat dissipation performance and low thermal resistance is obtained. In addition, it exhibits excellent durability under high temperature or high temperature and high humidity conditions, and it was found that it can improve the reliability during actual installation, thus completing the present invention.

[0034] Therefore, the present invention provides the following organosilicon composition.

[0035] 1. An organosilicon composition comprising:

[0036] (A) The kinematic viscosity at 25°C is 10–10000 mm³, expressed by the following general formula (1). 2 / s of organopolysiloxane: 50-100 parts by weight:

[0037] [Chemistry 1]

[0038]

[0039] (where R is in the formula) 1 R is an unsubstituted or substituted monovalent hydrocarbon group. 2 Independently, it is an alkyl, alkoxyalkyl, alkenyl, or acyl group, where a is an integer from 5 to 100 and b is an integer from 1 to 3.

[0040] (B) The kinematic viscosity at 25°C, expressed by the following average composition formula (2), is 10–100,000 mm³. 2 / s of organopolysiloxane: 0-50 parts by weight:

[0041] R 3 c SiO (4-c) / 2 (2)

[0042] (where R is in the formula) 3 (A monovalent hydrocarbon group consisting of 1 to 18 unsubstituted or substituted carbon atoms, with c ranging from 1.8 to 2.2.)

[0043] (Where, the total amount of component (A) and component (B) is equal to 100 parts by mass.)

[0044] (C) α-alumina powder, which is α-alumina powder having a hexagonal close-packed lattice crystal structure composed of octahedrons or more, wherein the maximum particle size parallel to the hexagonal lattice planes is defined as D, and the particle size perpendicular to the hexagonal lattice planes is defined as H, the α-alumina powder has a particle shape with a D / H ratio of 0.3 or more and 30 or less, an average particle size of 7 to 30 μm, and in a laser diffraction-type particle size distribution, the proportion of coarse particles larger than 50 μm is less than 1% by mass of all particles, and the purity is 99% or more.

[0045] (D) Zinc oxide powder with an average particle size of 0.01 μm or larger but less than 3 μm, and the proportion of coarse particles larger than 10 μm in the laser diffraction particle size distribution is less than 1% by mass of the total (D) component.

[0046] The mixing ratio of component (C) to component (D) is expressed as a mass ratio of 5:5 to 9.5:0.5, and the total mixing amount of (C) and (D) is 75% to 85% by volume.

[0047] The thermal conductivity of the zinc oxide powder, measured by the hot plate method according to ISO 22007-2, is greater than 4.0 W / m·K and less than 7.0 W / m·K, and the viscosity at 25°C, measured by a spiral viscometer at a speed of 10 rpm, is 5–800 Pa·s.

[0048] 2. The organosilicon composition according to claim 1 has a thermal resistance of 11 mm at 25°C, as measured by laser flash method. 2 • Below K / W.

[0049] 3. The organosilicon composition according to 1 or 2, after being placed at 130°C / 85% RH atmosphere for 96 hours, has a thermal resistance of 11 mm at 25°C as measured by laser flash method. 2 • Below K / W.

[0050] 4. The organosilicon composition according to any one of 1 to 3, after thermal degradation at 200°C for 100 hours, has a viscosity at 25°C of less than 1000 Pa·s when measured using a spiral viscometer at a rotation speed of 10 rpm.

[0051] 5. The organosilicon composition according to any one of 1 to 4, further comprising (E) a volatile solvent that can disperse or dissolve the components (A) and (B): less than 100 parts by mass relative to the total amount of components (A) and (B).

[0052] 6. The organosilicon composition according to any one of 1 to 5, further comprising (F) an alkoxysilane represented by the following general formula (3): 0.1 to 50 parts by mass relative to 100 parts by mass of the aggregate of components (A) and (B), wherein components (C) and (D) have been surface-treated with component (F).

[0053] R 4 d R 5 e Si(OR 6 ) 4-d-e (3)

[0054] (where R is in the formula) 4 Each is an alkyl group having 9 to 15 carbon atoms, R 5 Each is an independent monovalent hydrocarbon group, either unsubstituted or substituted, having 1 to 8 carbon atoms, R 6 Each is an alkyl group having 1 to 6 carbon atoms, where d is an integer from 1 to 3 and e is an integer from 0 to 2, and d+e is an integer from 1 to 3.

[0055] 7. The organosilicon composition according to any one of 1 to 6, having a volume resistivity of 1 × 10⁻⁶. 9 Ω·cm or higher.

[0056] The effects of the invention

[0057] The silicone composition of the present invention possesses both insulating properties and good thermal conductivity, thereby maintaining good flowability and resulting in excellent workability. Furthermore, due to the excellent fit between the heat-generating electronic component and the heat-dissipating component, contact thermal resistance is reduced, making low thermal resistance possible. That is, by placing the silicone composition of the present invention between the heat-generating electronic component and the heat-dissipating component, heat generated from the heat-generating electronic component can be efficiently dissipated to the heat-dissipating component. Furthermore, the silicone composition of the present invention exhibits excellent durability under high temperature or high temperature and humidity conditions. For example, when used in the heat dissipation of general power supplies, electronic devices, etc., and in the heat dissipation of integrated circuit components such as LSIs and CPUs used in electronic devices such as personal computers and DVD drives, it can impart very good reliability. Therefore, by employing the low thermal resistance silicone composition of the present invention, the safety and lifespan of heat-generating electronic components and electronic devices using them can be significantly improved. Detailed Implementation

[0058] The present invention will now be described in detail.

[0059] [(A) ingredient]

[0060] (A) The components are derived from the following general formula (1):

[0061] [Chemistry 2]

[0062]

[0063] (where R is in the formula) 1 R is an unsubstituted or substituted monovalent hydrocarbon group. 2 Independently alkyl, alkoxyalkyl, alkenyl, or acyl, where a is an integer from 5 to 100 and b is an integer from 1 to 3. This indicates a kinematic viscosity of 10 to 10000 mm³ at 25°C. 2 / s of organopolysiloxanes can be used alone or in combination of two or more.

[0064] In order to obtain a thermally conductive silicone composition, even if the thermally conductive fillers of components (C) and (D) are filled in large quantities in the composition, component (A) maintains the fluidity of the composition and imparts good processability to the composition.

[0065] The above R 1The group is an unsubstituted or substituted monovalent hydrocarbon group, preferably with 1 to 18 carbon atoms, more preferably 1 to 10. Specifically, examples include straight-chain alkyl groups, branched-chain alkyl groups, cyclic alkyl groups, alkenyl groups, aryl groups, aralkyl groups, and haloalkyl groups. Examples of straight-chain alkyl groups include methyl, ethyl, propyl, hexyl, and octyl. Examples of branched-chain alkyl groups include isopropyl, isobutyl, tert-butyl, and 2-ethylhexyl. Examples of cyclic alkyl groups include cyclopentyl and cyclohexyl. Examples of alkenyl groups include vinyl and allyl. Examples of aryl groups include phenyl and tolyl. Examples of aralkyl groups include 2-phenylethyl and 2-methyl-2-phenylethyl. Examples of haloalkyl groups include 3,3,3-trifluoropropyl, 2-(nonafluorobutyl)ethyl, and 2-(heptadecylfluorooctyl)ethyl. 1 Methyl or phenyl is preferred.

[0066] The above R 2 It is independently alkyl, alkoxyalkyl, alkenyl, or acyl. The number of carbon atoms is preferably 1 to 18, more preferably 1 to 10. Examples of alkyl groups include those related to R. 1 Examples of alkyl groups include straight-chain alkyl groups, branched-chain alkyl groups, cyclic alkyl groups, etc. As alkenyl groups, examples such as those related to R can be listed. 1 The groups listed are the same as those used in the examples. Examples of alkoxyalkyl groups include methoxyethyl and methoxypropyl. Examples of acyl groups include acetyl and octanoyl. R 2 Alkyl groups are preferred, and methyl or ethyl groups are particularly preferred.

[0067] a is an integer from 5 to 100, preferably an integer from 5 to 50, and more preferably an integer from 5 to 30. b is an integer from 1 to 3, preferably 3.

[0068] (A) The kinematic viscosity of component A at 25°C is 10–10000 mm. 2 / s, preferably 10~5000mm 2 / s. If the kinematic viscosity is less than 10 mm. 2 If the kinematic viscosity exceeds 10000 mJ / s, oil seepage will occur from the resulting silicone composition. 2 If the viscosity is / s, the resulting silicone composition lacks fluidity. It should be noted that in this invention, the kinematic viscosity of component (A) is the value obtained at 25°C using an Orthocrites viscometer.

[0069] The amount of component (A) is in the range of 50 to 100 parts by mass, preferably 55 to 95 parts by mass. However, the total amount of component (A) and component (B) described later is 100 parts by mass. If the amount of component (A) is within this range, the silicone composition can easily maintain good flowability and workability. In addition, it is easy to fill a large amount of the thermally conductive fillers of components (C) and (D) described later into the silicone composition. Furthermore, if component (A) is less than 50 parts by mass, it is no longer possible to fill a large amount of the thermally conductive fillers of components (C) and (D) into the composition.

[0070] The following examples can be cited as preferred examples of component (A).

[0071] [Chemistry 3]

[0072]

[0073] [(B) Component]

[0074] The (B) component of this invention is composed of the following average composition formula (2):

[0075] R 3 c SiO (4-c) / 2 (2)

[0076] (where R is in the formula) 3 (A monovalent hydrocarbon group consisting of 1 to 18 unsubstituted or substituted carbon atoms, with c ranging from 1.8 to 2.2.)

[0077] The kinematic viscosity at 25°C is expressed as 10–100,000 mm³. 2 / s of organopolysiloxanes can be used alone or in combination of two or more. Component (B) is used for the purpose of imparting properties such as viscosity modifiers and adhesiveness enhancers to the organosilicon compositions of the present invention.

[0078] The above R 3 It is independently a monovalent hydrocarbon group with 1 to 18 carbon atoms, either unsubstituted or substituted, preferably with 1 to 10 carbon atoms. As R 3 Examples of such compounds include alkyl groups such as methyl, ethyl, propyl, hexyl, octyl, decyl, dodecyl, tetradecyl, hexadecyl, and octadecyl; cycloalkyl groups such as cyclopentyl and cyclohexyl; alkenyl groups such as vinyl and allyl; aryl groups such as phenyl and tolyl; aralkyl groups such as 2-phenylethyl and 2-methyl-2-phenylethyl; and halogenated hydrocarbon groups such as 3,3,3-trifluoropropyl, 2-(perfluorobutyl)ethyl, 2-(perfluorooctyl)ethyl, and p-chlorophenyl. Among these, methyl, phenyl, and alkyl groups with 6 to 18 carbon atoms are preferred, with methyl and phenyl being more preferred.

[0079] As a silicone composition with low thermal resistance, from the viewpoint of the viscosity required by the composition of the present invention, the above-mentioned c is a number from 1.8 to 2.2, preferably a number from 1.9 to 2.1.

[0080] (B) The kinematic viscosity of component B at 25°C is 10–100,000 mm³. 2 / s, preferably 10 to 10000 mm 2 / s. If the kinematic viscosity is less than 10 mm. 2 If the kinematic viscosity exceeds 100,000 mJ / s, liquid separation and oil seepage from the resulting silicone composition will occur. 2 If the viscosity is / s, the fluidity of the resulting silicone composition becomes poor, thus resulting in poor workability. It should be noted that the kinematic viscosity of component (B) is also a value obtained at 25°C using an Orthocrites viscometer.

[0081] Specific examples of component (B) include the following instances.

[0082] [Chemistry 4]

[0083]

[0084] The amount of component (B) is 0 to 50 parts by mass, more preferably 5 to 45 parts by mass (wherein the total amount of components (A) and (B) is 100 parts by mass). Sometimes component (B) is not mixed. If the amount of mixing is within this range, the silicone composition can easily maintain good flowability and workability, and it is also easy to fill a large amount of the thermally conductive fillers of components (C) and (D) described later into the composition. On the other hand, if the amount of component (B) exceeds 50 parts by mass, it is no longer possible to fill a large amount of the thermally conductive fillers of components (C) and (D) into the silicone composition.

[0085] [(C) Component]

[0086] The (C) component of this invention is α-alumina powder, which is α-alumina powder having a hexagonal close-packed lattice crystal structure composed of polyhedra with eight or more faces. When the maximum particle size parallel to the hexagonal lattice plane of the hexagonal close-packed lattice is set as D, and the particle size perpendicular to the hexagonal lattice plane is set as H, it has a particle shape with a D / H ratio of 0.3 or more and 30 or less, an average particle size of 7 to 30 μm, and the proportion of coarse particles with a diameter of 50 μm or more in the laser diffraction-type particle size distribution is less than 1% by mass of all particles, and the purity is 99% or more. It can be used alone or in combination of two or more.

[0087] (C) The alumina is preferably α-alumina, which has a hexagonal close-packed crystal structure composed of polyhedra with eight or more faces, preferably octahedral to icosahedral, having a crystal structure composed of essentially eight and / or sixteen faces. The crystal structure can be confirmed using the image diffraction apparatus described below.

[0088] Regarding the shape of the particles in component (C) of this invention, when the maximum particle size parallel to the hexagonal lattice plane of the α-alumina powder having a hexagonal close-packed lattice structure is defined as D, and the particle size perpendicular to the hexagonal lattice plane is defined as H, the D / H ratio is in the range of 0.3 or more and 30 or less. The D / H ratio can be measured by importing a particle image taken using a scanning electron microscope into an image analysis device, such as the JEOL-manufactured product "JSM-7500F", as described below. That is, the maximum particle size parallel to the hexagonal lattice plane of the particle is measured from the photograph as D, and the particle size perpendicular to the hexagonal lattice plane is measured as H. The D / H ratio of any 10 particles obtained in this way is calculated, and their average value is defined as D / H. In this invention, the D / H ratio is preferably in the range of 0.3 to 5. If the D / H ratio is less than 0.3, the filling performance in the resin deteriorates, the contact between particles decreases, and the thermal resistance increases due to the increase in the contact thermal resistance between particles. In addition, if the D / H ratio of component (C) exceeds 30, the contact between particles becomes significant, the surface roughness of the organosilicon composition increases, the interfacial thermal resistance increases, and the thermal resistance rises.

[0089] The average particle size (primary and / or secondary particle size) of component (C) is 7–30 μm, preferably 10–25 μm, on a volume basis. If the average particle size is within this range, the bulk density of component (C) tends to increase, and the specific surface area tends to decrease, thus making it easy to fill the silicone composition of the present invention with a large quantity of component (C). If the average particle size is less than 7 μm, achieving the thermal conductivity required in the present invention becomes difficult. On the other hand, if the average particle size exceeds 30 μm, oil separation may easily occur, thus leading to a deterioration in thermal resistance over time.

[0090] If the proportion of coarse particles larger than 50 μm in the laser diffraction-type particle size distribution is less than 1% by mass of the total (C) component, it is possible to achieve both the required thermal resistance and high thermal conductivity. On the other hand, if the proportion of coarse particles larger than 50 μm exceeds 1% by mass of the total (C) component, a thickness of less than 60 μm may not be achieved, and the desired thermal resistance may not be attainable. The preferred proportion of such coarse particles is less than 0.5% by mass of the total (C) component.

[0091] It should be noted that the average particle size of component (C) in this invention, based on volume, can be measured, for example, using a "Laser Diffraction Particle Size Distribution Measurement Apparatus SALD-2300" manufactured by Shimadzu Corporation. For the evaluation sample, 50 mL of pure water and 5 g of the thermally conductive powder to be measured are added to a glass beaker, stirred with a spatula, and then dispersed using an ultrasonic cleaner for 10 minutes. The solution of the dispersed thermally conductive material powder is added drop by drop to the sampling section of the apparatus using a pipette, and allowed to stabilize until absorbance can be measured. Measurement is performed at the point when absorbance has stabilized. In the laser diffraction particle size distribution measurement apparatus, the particle size distribution is calculated from the data of the intensity distribution of diffracted / scattered light from the particles detected by a sensor. The average particle size is obtained by multiplying the measured particle size value by the relative particle amount (difference%) and dividing by the total relative particle amount (100%). It should be noted that the average particle size is the average diameter of the particles. For example, the proportion of coarse particles larger than 50 μm in component (C) can be easily determined from the overall particle size distribution. It should be noted that, without impairing the effects of the present invention, two or more types with different average particle sizes can be used in combination.

[0092] The purity of component (C) is 99% or higher, preferably 99.5% or higher. If the purity is less than this value, it leads to an increase in thermal resistance. It should be noted that in this invention, the purity of component (C) is a value determined using atomic absorption spectrophotometry based on JIS K 1410. It should be noted that the mixing ratio of component (C) is as described below.

[0093] [(D) component]

[0094] (D) is zinc oxide powder containing coarse particles with an average particle size of 0.01 μm or more but less than 3 μm, and a laser diffraction particle size distribution of 10 μm or more, at a proportion of less than 1% by mass of the total. The zinc oxide powder of component (D) functions as a thermally conductive filler in the organosilicon composition of the present invention. Component (D) can be used alone or in combination of two or more.

[0095] The average particle size of component (D), on a volume basis, is 0.01 μm or more and less than 3 μm, preferably 0.01 to 2 μm, more preferably 0.01 to 1 μm, and even more preferably 0.01 to 0.5 μm. If the average particle size is within this range, the bulk density of component (D) tends to increase, and the specific surface area tends to decrease, thus facilitating the abundant filling of component (D) in the silicone composition of the present invention. Furthermore, when the average particle size is less than 0.01 μm, the filling properties in the resin deteriorate, and the viscosity increases significantly. On the other hand, if the average particle size exceeds 3 μm, oil separation is easily achieved.

[0096] In the laser diffraction-type particle size distribution, the proportion of coarse particles larger than 10 μm in component (D) is 1% by mass or less, preferably 0.2% by mass or less. By setting it within this range, both the required thermal resistance and high thermal conductivity can be achieved. On the other hand, if the proportion of coarse particles larger than 10 μm exceeds 1% by mass, a thickness of 20 μm or less may not be achieved, and the required thermal resistance can no longer be obtained. It should be noted that the method for determining the average particle size and the proportion of coarse particles in component (D) is the same as that for component (C).

[0097] The shape of component (D) can be spherical, irregular, or a mixture thereof. In component (D) of the present invention, shapes other than spherical are irregular, such as rod-shaped, needle-shaped, or disc-shaped, and are not particularly limited as long as they do not impair the effect of the present invention. Component (D) can be only spherical or irregular in shape, or a combination thereof. It should be noted that the term "spherical" for component (D) means that the average sphericity is preferably 0.8 or more, more preferably 0.9 or more.

[0098] It should be noted that the average sphericity of component (D) can be measured by importing the particle image taken using a scanning electron microscope into an image analysis device, such as the JEOL JSM-7500F, as described below. That is, the projected area (X) and perimeter (Z) of the particle are measured from the photograph. If the area of ​​the true circle corresponding to the perimeter (Z) is set as (Y), then the sphericity of the particle can be expressed as X / Y. Therefore, assuming a true circle with the same perimeter (Z) as the sample particle, then Z = 2πr and Y = πr. 2 (where r is the radius.) This becomes Y = π × (Z / 2π). 2 The sphericity of each particle can be expressed as sphericity = X / Y = X × 4π / Z 2 Calculate the sphericity of any 100 particles obtained in this way, and set their average value as the mean sphericity.

[0099] Furthermore, the purity of component (D) is preferably 99.5% or higher, and from the viewpoint of impurities such as Pb and Cd, it is particularly preferred to be 99.8% or higher. The method for determining purity is the same as that used for component (C).

[0100] The mixing ratio of component (C) to component (D) is expressed as a mass ratio and is 5:5 to 9.5:0.5, preferably 6:4 to 9:1. If the mass ratio of component (C) is less than 5, the filling performance of the filler deteriorates. Conversely, if the mass ratio of component (C) exceeds 9.5, the filler is difficult to fill densely, and the thermal conductivity decreases.

[0101] The total mixing amount of (C) and (D) in the silicone composition of the present invention, i.e., the total content of the thermally conductive filler, is 75-85% by volume of the total composition, preferably 76-84% by volume. If the total mixing amount of (C) and (D) is less than 75% by volume, the thermal conductivity of the silicone composition becomes insufficient; conversely, if it exceeds 85% by volume, the filling of the thermally conductive filler becomes difficult. In the present invention, even with such a high total mixing amount of (C) and (D), the desired effect can be obtained.

[0102] [(E) component]

[0103] In the compositions of the present invention, a volatile solvent capable of dispersing or dissolving components (A) and (B) can be further incorporated as component (E). When the present invention includes component (F) (described later) in addition to components (A) and (B), a volatile solvent capable of dispersing or dissolving component (F) is preferred. Component (E) is not particularly limited as long as it can dissolve or disperse components (A) and (B), and sometimes component (F). Component (E) can be used alone or in combination of two or more.

[0104] The thermal conductivity of thermally conductive silicone compositions is essentially related to the filling rate of the thermally conductive filler; therefore, the greater the amount of thermally conductive filler, the higher the thermal conductivity. However, increasing the filling amount of thermally conductive filler naturally increases the viscosity of the silicone composition itself, and also enhances its expansion under shear stress. In particular, during screen printing, if strong expansion occurs within the thermally conductive silicone composition when it is extruded, the flowability of the composition is temporarily and severely inhibited, preventing it from passing through the screen mask and the screen itself, sometimes resulting in extremely poor coatability. Thus, currently, it is difficult to easily and uniformly apply a thin layer of high thermally conductive silicone compositions with a large amount of thermally conductive filler to heat sinks, etc., during screen printing. Even when the silicone composition of the present invention contains thermally conductive fillers of components (C) and (D) at a high filling rate, the viscosity tends to decrease rapidly and expansion becomes less noticeable when the volatile solvent containing component (E) is present. Therefore, the coatability is easily improved, and it can be easily coated onto heat sinks or similar applications using screen printing. After coating, the contained component (E) is easily volatilized at room temperature or by active heating. Therefore, in the present invention, a low-thermal-resistance silicone composition with a large amount of thermally conductive filler can be easily and uniformly thinly applied to heat sinks or similar applications using screen printing.

[0105] The boiling point of component (E) is preferably in the range of 80 to 260°C. If the boiling point is within this range, it is easy to prevent the rapid volatilization of component (E) from the silicone composition during the coating process, thus easily suppressing the increase in viscosity of the silicone composition and ensuring sufficient coatability. In addition, after the silicone composition is coated, since component (E) is not easily retained in the silicone composition, the heat dissipation characteristics are easily improved.

[0106] Specific examples of component (E) include toluene, xylene, acetone, methyl ethyl ketone, cyclohexane, n-hexane, n-heptane, butanol, isopropanol (IPA), and isoparaffin solvents. Among these, isoparaffin solvents are preferred from the perspectives of safety, health, and workability, and isoparaffin solvents with boiling points of 80 to 260°C are particularly preferred.

[0107] When component (E) is added to the composition of the present invention, its mixing amount is preferably 100 parts by mass or less, more preferably 75 parts by mass or less, relative to 100 parts by mass of the total of components (A) and (B). If the mixing amount is within this range, the rapid precipitation of components (C) and (D) is easily suppressed, thus improving the shelf life of the silicone composition. There is no particular limitation on the lower limit of the mixing amount of component (E), but from the viewpoint of the coatability of the silicone composition of the present invention, it is preferably 1 part by mass or more, more preferably 5 parts by mass or more.

[0108] [(F)INGREDIENT]

[0109] In the compositions of the present invention, (F) alkoxysilane can be further added. The (F) component is composed of the following general formula (3):

[0110] R 4 d R 5 e Si(OR 6 ) 4-d-e (3)

[0111] (where R is in the formula) 4 Each is an alkyl group having 9 to 15 carbon atoms, R 5 Each is an independent monovalent hydrocarbon group, either unsubstituted or substituted, having 1 to 8 carbon atoms, R 6 Each is an alkyl group with 1 to 6 carbon atoms, where d is an integer from 1 to 3 and e is an integer from 0 to 2; however, d+e is an integer from 1 to 3.

[0112] The indicated alkoxysilane. (F) Components can be used alone or in combination of two or more.

[0113] Component (F) is also a wetting agent and an additive to prevent the degradation of component (A) under high temperature and high humidity conditions. By treating the surfaces of the thermally conductive fillers of components (C) and (D) with component (F), the wettability of component (A) to components (C) and (D) can be further improved. As a result, component (F) assists in the substantial filling of components (C) and (D). In addition, by using component (A) in combination, component (F) acts in a way that inhibits the contact between water vapor and component (A) under high temperature and high humidity conditions. As a result, component (F) prevents the performance of the organosilicon composition of the present invention from deteriorating due to degradation of component (A) under conditions such as hydrolysis under high temperature and high humidity conditions. Component (F) can be used alone or in combination with two or more components.

[0114] The above R 4 Each alkyl group is independently composed of 9 to 15 carbon atoms. Specific examples include nonyl, decyl, dodecyl, tetradecyl, and pentadecyl. If the number of carbon atoms is less than 9, the wettability with thermally conductive fillers (components (C) and (D)) tends to be insufficient. If the number of carbon atoms is greater than 15, component (F) tends to solidify at room temperature, making its processing inconvenient. Furthermore, the heat resistance and flame retardancy of the resulting composition may sometimes be reduced.

[0115] The above R 5 Each group is an independent monovalent hydrocarbon group with 1 to 8 carbon atoms, either unsubstituted or substituted, and can be saturated or unsaturated. Specific examples include alkyl groups such as methyl, ethyl, propyl, hexyl, and octyl; cycloalkyl groups such as cyclopentyl and cyclohexyl; alkenyl groups such as vinyl and allyl; aryl groups such as phenyl and tolyl; aralkyl groups such as 2-phenylethyl and 2-methyl-2-phenylethyl; and haloalkyl groups such as 3,3,3-trifluoropropyl, 2-(nonafluorobutyl)ethyl, and p-chlorophenyl, with methyl and ethyl groups being particularly preferred.

[0116] The above R 6 Each of the alkyl groups is an alkyl group having 1 to 6 carbon atoms. Examples of such alkyl groups include methyl, ethyl, propyl, butyl, pentyl, and hexyl, with methyl and ethyl being particularly preferred.

[0117] The above d is an integer from 1 to 3, preferably 1. The above e is an integer from 0 to 2. However, d+e is an integer from 1 to 3.

[0118] Specific examples of component (F) include the following.

[0119] C 10 H 21 Si(OCH3)3、

[0120] C 10 H 21 Si(OC2H5)3,

[0121] C 12 H 25 Si(OCH3)3、

[0122] C 12 H 25 Si(OC2H5)3,

[0123] C 10 H 21 Si(CH3)(OCH3)2、

[0124] C 10 H 21 Si(C6H5)(OCH3)2、

[0125] C 10 H 21 Si(CH3)(OC2H5)2、

[0126] C 10 H 21 Si(CH=CH2)(OCH3)2、

[0127] C 10 H 21 Si(CH2CH2CF3)(OCH3)2

[0128] When component (F) is incorporated, its amount is preferably 0.1 to 50 parts by mass, more preferably 1 to 20 parts by mass, relative to a total of 100 parts by mass of components (A) and (B). If the amount is within this range, the wetting effect and the resistance to high temperature and humidity can be easily increased, making it economical. On the other hand, since component (F) is slightly volatile, if the silicone composition containing component (F) is placed in an open system, component (F) may evaporate from the silicone composition, causing the composition to slowly harden. If the amount of component (F) is within this range, this phenomenon can be easily prevented.

[0129] As a surface treatment method for components (C) and (D) using component (F), various methods can be employed, including dry methods such as spraying with fluid nozzles, stirring with shear force, ball mills, and mixers, as well as wet methods such as aqueous or organic solvent systems. In the case of stirring methods, it is important to perform the treatment without damaging the spherical alumina powder. The system temperature in dry methods, or the drying temperature after treatment, should be appropriately determined within the non-volatile and non-decomposing range of the surface treatment agent, typically between 80 and 180°C. Alternatively, a method can be used where components (C) and (D) are heated and mixed together with components (A) and (B), then cooled, and finally mixed with component (F).

[0130] [Other Additives]

[0131] In the organosilicon composition of the present invention, conventionally used additives, fillers, etc., can be added as optional components without impairing the effects of the present invention. Specifically, fluorinated organosilicon surfactants; carbon black, titanium dioxide, red iron oxide, etc., as colorants; platinum catalysts, iron oxides, titanium oxide, cerium oxide, etc., as flame retardant imparting agents; metal hydroxides, etc., can be added. Furthermore, as a thermally conductive filler and an anti-precipitation agent at high temperatures, precipitated silica, calcined silica, or other micro-powdered silica, thixotropic agents, etc., can also be optionally added. The mixing amounts of these are appropriately selected within the normal range and without impairing the effects of the present invention.

[0132] [Preparation of the composition]

[0133] The organosilicon composition of the present invention is prepared by mixing the above-mentioned components using mixing equipment such as a kneader (mixing machine), gantry mixer, or planetary mixer. The resulting organosilicon composition exhibits significantly improved thermal conductivity and excellent workability, durability, and reliability.

[0134] Thermal conductivity

[0135] The thermal conductivity of the silicone composition of the present invention at 25°C, according to the hot plate method of ISO 22007-2, is 4.0 W / m·K or higher and less than 7.0 W / m·K, preferably 4.5 to 6.5 W / m·K. By achieving a thermal conductivity of less than 7 W / m·K, the coatability of the silicone composition is further improved. In the measurement of the thermal conductivity of the composition of the present invention, a thermal conductivity measuring device such as the "TPS2500S" manufactured by Kyoto Electronics Co., Ltd. can be used.

[0136] [Viscosity]

[0137] The viscosity of the silicone composition of the present invention at 25°C, measured using a spiral viscometer at 10 rpm, is 5–800 Pa·s, preferably 5–750 Pa·s, and more preferably 5–500 Pa·s. If the viscosity is within this range, the fluidity of the resulting silicone composition is easily improved, thus enhancing workability such as dispensability and screen printability, and facilitating thin coating of the composition onto a substrate. The viscosity of the silicone composition of the present invention can be measured using a spiral viscometer, such as the one manufactured by Malcom Co., Ltd. under the trade name "Type PC-10AA".

[0138] After the silicone composition of the present invention undergoes thermal degradation in a dryer at 200°C for 100 hours, the viscosity, measured at 25°C in the same manner as described above, is preferably 1000 Pa·s or less, more preferably 700 Pa·s or less, and even more preferably 500 Pa·s or less. By exhibiting thixotropic properties in this non-curing form, the reliability of exothermic electronic components can be ensured.

[0139] [Thermal Resistance]

[0140] The thermal resistance of the organosilicon composition of the present invention at 25°C, measured by laser flash method, is preferably 11 mm. 2 • For K / W and below, 10mm is preferred. 2 • K / W or less. With the configuration of this invention, such a low thermal resistance silicone composition can be obtained.

[0141] The thermal resistance of the organosilicon composition of the present invention at 25°C, measured by laser flash method, after being placed at 130°C / 85% RH atmosphere for 96 hours is preferably 11 mm. 2 • For K / W and below, 10mm is preferred. 2 • K / W or less. If the thermal resistivity is within this range, the silicone composition of the present invention can efficiently dissipate the heat generated by the heat-generating body to the heat dissipation component, even when applied to a heat-generating body with high heat output. It should be noted that the thermal resistance can be determined using the laser flash method according to ASTM E 1461.

[0142] [Volume Resistivity]

[0143] The organosilicon composition of the present invention preferably has a volume resistivity of 1 × 10⁻⁶, as measured according to JIS K 6911. 9 Ω·cm or higher, more preferably 1×10 10 Ω·cm or higher. If within this range, the organosilicon composition of the present invention can further ensure insulation.

[0144] [Uses of Organosilicon Compositions]

[0145] The organosilicon composition of the present invention is coated onto a heat-generating body and a heat-dissipating body. Examples of heat-generating bodies include general power supplies; electronic devices such as power transistors, power modules, thermistors, thermocouples, and temperature sensors; and heat-dissipating electronic components such as LSIs and CPUs. Examples of heat-dissipating bodies include heat sinks, heat radiators, and heat pipes and heat dissipation plates. Coating can be performed, for example, by screen printing. Screen printing can be performed, for example, using a metal mask or a screen. By coating the composition of the present invention between the heat-generating body and the heat-dissipating body, heat can be transferred efficiently from the heat-generating body to the heat-dissipating body, thus effectively removing heat from the heat-generating body.

[0146] Example

[0147] The following examples and comparative examples illustrate the invention in detail, but the invention is not limited to the following examples.

[0148] [Examples 1-6, Comparative Examples 1-4]

[0149] To prepare the organosilicon composition of the present invention, the following components were prepared. The kinematic viscosity is the value obtained at 25°C using an Orthocriteur viscometer.

[0150] (A)Ingredients

[0151] A-1: Expressed by the following formula, specific gravity (25℃) is 0.97, kinematic viscosity is 30 mm. 2 / s of organopolysiloxanes

[0152] [Chemistry 5]

[0153]

[0154] (B) Ingredients

[0155] B-1: Organopolysiloxane B-1: As shown in the following formula with c=2.0 in the average composition (2), specific gravity (25℃) is 0.97, and kinematic viscosity is 500 mm. 2 / s of organopolysiloxanes

[0156] [Chemistry 6]

[0157]

[0158] (C) Alumina powder (specific gravity 3.98),

[0159] [Table 1]

[0160]

[0161] The average particle size shown here is a volume reference value calculated from the total particle size distribution obtained from a laser diffraction-type particle size distribution, measured using a Shimadzu SALD-2300 laser diffraction particle size distribution measuring device. Additionally, it represents the proportion of coarse particles with a coarse particle content of 50 μm or more relative to the total particle size distribution obtained from the laser diffraction-type particle size distribution.

[0162] (D) Zinc oxide powder

[0163] (D-1) Irregularly shaped zinc oxide powder (average particle size 0.27 μm, coarse particles larger than 10 μm content 0.1% by mass, specific gravity 5.67)

[0164] The average particle size shown here was measured using a Shimadzu SALD-2300 laser diffraction particle size distribution measuring device, and is a volume reference value calculated from the total particle size distribution obtained from the laser diffraction particle size distribution. Additionally, the proportion of coarse particles with a coarse particle content of 10 μm relative to the total particle size distribution obtained from the laser diffraction particle size distribution is also shown.

[0165] (E) A volatile solvent (specific gravity 0.79) capable of dispersing or dissolving components (A-1), (B-1), and (F-1).

[0166] E-1: Isoparaffin solvent, boiling point 210-254℃; ISOSOL 400 (trade name, manufactured by ENEOS Co., Ltd.)

[0167] (F) Alkoxysilane

[0168] F-1: An alkoxysilane represented by the following formula (specific gravity 0.90),

[0169] C 10 H 21 Si(OCH3)3

[0170] [Manufacturing Method]

[0171] Compositions (A) to (D), and components (E) and (F) used only as needed, were mixed as described below to obtain the compositions of Examples 1 to 5 and Comparative Examples 1 to 4. Specifically, components (A) to (D) were measured in a 5-liter planetary mixer (manufactured by Inoue Manufacturing Co., Ltd.) at the composition ratios (parts by mass) shown in Tables 2 and 3, and mixed under reduced pressure at 150°C for 1 hour and 30 mmHg or less. The resulting mixture was then cooled and mixed to room temperature. When components (E) and (F) were added, components (E) and (F) were added to the cooled mixture at the mixing amounts shown in Table 2, and mixed to make it homogeneous.

[0172] [Experimental Methods]

[0173] The properties of the obtained organosilicon compositions were determined using the following experimental methods. The results are shown in Tables 2 and 3.

[0174] [Viscosity]

[0175] After the obtained organosilicon composition was placed in a constant temperature room at 25°C for 24 hours, the viscosity at 10 rpm was measured using a viscometer (trade name: spiral viscometer PC-10AA, manufactured by MALCOM Co., Ltd.).

[0176] [Viscosity after thermal degradation]

[0177] The obtained organosilicon composition was subjected to heat degradation in a dryer at 200°C for 100 hours, and then placed in a constant temperature chamber at 25°C for 24 hours before being measured in the same manner as described above.

[0178] Thermal conductivity

[0179] Based on the hot plate method according to ISO 22007-2.

[0180] Two samples of the obtained silicone composition were prepared and wrapped in kitchen plastic wrap in a way that prevented air bubbles from entering. The samples were clamped to the sensor of a thermal conductivity meter (trade name: TPS-2500S) manufactured by Kyoto Electronics Industry Co., Ltd., and the thermal conductivity of the composition at 25°C was measured.

[0181] [Volume Resistivity]

[0182] To determine the volume resistivity based on JIS K 6911 using the double ring electrode method, a test piece with a sample thickness of 1 mm was prepared, and the volume resistivity was measured after 1 minute with 500 V applied between the electrodes.

[0183] [Preparation of test pieces for thickness and thermal resistance determination]

[0184] A test piece was prepared by clamping a 40μm thick composition between two circular aluminum plates with a diameter of 12.6mm and a thickness of 1mm and applying a pressure of 0.15MPa for 60 minutes at 25°C.

[0185] [Thickness Measurement]

[0186] The thickness of the test piece was measured using a micrometer (manufactured by Mitutoyo Corporation), and the thickness of the composition was calculated by subtracting the thickness of the two pre-measured aluminum plates.

[0187] [Determination of thermal resistance]

[0188] Using the above-mentioned test piece, the thermal resistance (unit: mm) of the composition was measured at 25°C using a laser flash thermal resistance measuring instrument (Netzsch xenon flash analyzer; LFA447 NanoFlash) based on the laser flash method. 2 ·K / W).

[0189] [Determination of thermal resistance after placement under high temperature and high humidity]

[0190] After the thermal resistance measurement, the test piece was placed at 130°C / 85% RH for 96 hours, and the thermal resistance of the composition was measured again using the same thermal resistance measuring instrument (unit: mm). 2 ·K / W).

[0191] [Table 2]

[0192]

[0193] [Table 3]

[0194]

Claims

1. An organosilicon composition containing: (A) The kinematic viscosity at 25°C is 10–10000 mm³, expressed by the following general formula (1). 2 / s of organopolysiloxane: 50-100 parts by weight: [Chemistry 1] In the formula, R 1 R is an unsubstituted or substituted monovalent hydrocarbon group. 2 Independently, it is an alkyl, alkoxyalkyl, alkenyl, or acyl group, where a is an integer from 5 to 100 and b is an integer from 1 to 3. (B) The kinematic viscosity at 25°C, expressed by the following average composition formula (2), is 10–100,000 mm³. 2 / s of organopolysiloxane: 0-50 parts by weight: R 3 c SiO (4-c) / 2 (2) In the formula, R 3 Independently, it is a monovalent hydrocarbon group with 1 to 18 unsubstituted or substituted carbon atoms, and the carbon number is 1.8 to 2.

2. in, The total amount of component (A) and component (B) is 100 parts by mass. (C) α-alumina powder, which is α-alumina powder having a hexagonal close-packed lattice crystal structure composed of octahedrons or more, wherein the maximum particle size parallel to the hexagonal lattice planes is defined as D, and the particle size perpendicular to the hexagonal lattice planes is defined as H, the α-alumina powder has a particle shape with a D / H ratio of 0.3 or more and 30 or less, an average particle size of 7 to 30 μm, and in a laser diffraction-type particle size distribution, the proportion of coarse particles larger than 50 μm is less than 1% by mass of all particles, and the purity is 99% or more. (D) Zinc oxide powder with an average particle size of 0.01 μm or larger but less than 3 μm, and the proportion of coarse particles larger than 10 μm in the laser diffraction particle size distribution is less than 1% by mass of the total (D) component. The mixing ratio of component (C) to component (D) is expressed as a mass ratio of 5:5 to 9.5:0.5, and the total mixing amount of (C) and (D) is 75% to 85% by volume. The thermal conductivity of the zinc oxide powder, measured by the hot plate method according to ISO 22007-2, is greater than 4.0 W / m·K and less than 7.0 W / m·K, and the viscosity at 25°C, measured by a spiral viscometer at a speed of 10 rpm, is 5–800 Pa·s.

2. The organosilicon composition according to claim 1, wherein its thermal resistance at 25°C, measured by laser flash method, is 11 mm. 2 • Below K / W.

3. The organosilicon composition according to claim 1, after being placed at 130°C / 85% RH atmosphere for 96 hours, has a thermal resistance of 11 mm at 25°C as measured by laser flash method. 2 • Below K / W.

4. The organosilicon composition according to claim 1, after thermal degradation at 200°C for 100 hours, has a viscosity at 25°C of less than 1000 Pa·s when measured using a spiral viscometer at a rotation speed of 10 rpm.

5. The organosilicon composition according to claim 1, further comprising (E) a volatile solvent capable of dispersing or dissolving components (A) and (B): less than 100 parts by mass relative to the total amount of components (A) and (B).

6. The organosilicon composition according to claim 1, further comprising (F) an alkoxysilane represented by the following general formula (3): 0.1 to 50 parts by mass relative to 100 parts by mass of the combined amount of components (A) and (B), wherein components (C) and (D) are surface-treated with component (F): R 4 d R 5 e Si(OR 6 ) 4-d-e (3) In the formula, R 4 Each is an alkyl group having 9 to 15 carbon atoms, R 5 Each is an independent monovalent hydrocarbon group, either unsubstituted or substituted, having 1 to 8 carbon atoms, R 6 Each is an alkyl group having 1 to 6 carbon atoms, d is an integer from 1 to 3, and e is an integer from 0 to 2, wherein, d+e is an integer from 1 to 3.

7. The organosilicon composition according to any one of claims 1 to 6, wherein the volume resistivity is 1 × 10⁻⁶. 9 Ω·cm or higher.

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