Cubic boron nitride polycrystalline material and heat sinks using it

Abstract

This invention relates to a cubic boron nitride polycrystalline material, which contains 99.5% by volume or more of cubic boron nitride, and the cubic boron nitride polycrystalline material has a thermal conductivity of 300 W / mK or more, a carbon content of 100 ppm to 1000 ppm by mass, and the cubic boron nitride polycrystalline material comprises a plurality of grains, the median grain size d50 of the circle equivalent diameter of the plurality of grains being 0.9 μm to 10 μm.

Description

Technical Field

[0001] This invention relates to a cubic boron nitride polycrystalline material and a heat sink using the same. Prior Technology

[0002] As an environmental issue, the demand for solar and wind power generation, electric vehicles, and other technologies is constantly expanding in order to reduce greenhouse gas emissions. These systems utilize power semiconductors. The materials used in power semiconductors are shifting from the currently mainstream silicon (Si) to higher-performance silicon carbide (SiC) or gallium nitride (GaN). These power semiconductors employ heat sinks to facilitate heat dissipation.

[0003] Cubic boron nitride (hereinafter, also referred to as cBN) has a thermal conductivity second only to diamond. Furthermore, cubic boron nitride has a coefficient of thermal expansion close to that of power semiconductors, thus making it an insulating material. Therefore, cubic boron nitride is suitable as a material for heat sinks. Previously, heat sinks utilized cBN sintered bodies formed by sintering cBN particles using a binder. However, the binder became a major cause of the decrease in thermal conductivity.

[0004] To solve this problem, the following method is being developed: without using an adhesive, hexagonal boron nitride is directly converted into cubic boron nitride under ultra-high temperature and pressure, and then sintered to obtain a sintered cubic boron nitride body without an adhesive (Japanese Patent Application Publication No. 2004-250278 (Patent Document 1), Japanese Patent Application Publication No. Hei 11-246271 (Patent Document 2), Japanese Patent Application Publication No. 2014-34487 (Patent Document 3)). [Previous Technical Documents] [Patent Literature]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2004-250278 [Patent Document 2] Japanese Patent Application Publication No. 11-246271 [Patent Document 3] Japanese Patent Application Publication No. 2014-34487 Summary of the Invention

[0006] This invention relates to a cubic boron nitride polycrystalline material, which contains at least 99.5% by volume cubic boron nitride. The thermal conductivity of the aforementioned cubic boron nitride polycrystalline material is above 300 W / mK. The carbon content of the aforementioned cubic boron nitride polycrystalline materials, based on a mass standard, is between 100 ppm and 1000 ppm. The aforementioned cubic boron nitride polycrystalline material contains a plurality of grains. The median grain size d50 of the above plurality of grains is between 0.9 μm and 10 μm.

[0007] The present invention is a heat sink using the above-mentioned cubic boron nitride polycrystalline material. Implementation

[0008] [The problem that this invention aims to solve] From the perspective of extending the lifespan of power semiconductors, a heat sink with high thermal conductivity and high bending resistance is required.

[0009] Therefore, the object of the present invention is to provide a cubic boron nitride polycrystalline material that, when used as a material for a heat sink, can improve the thermal conductivity and flexural strength of the heat sink.

[0010] [Effects of the Invention] According to the present invention, a cubic boron nitride polycrystalline material with high thermal conductivity and high bending strength can be provided, as well as a heat sink with high thermal conductivity and high bending strength.

[0011] [Description of Embodiments of the Invention] First, the embodiments of the present invention will be described. (1) This invention relates to a cubic boron nitride polycrystalline material, which contains 99.5% by volume or more cubic boron nitride, and The thermal conductivity of the aforementioned cubic boron nitride polycrystalline material is above 300 W / mK. The carbon content of the aforementioned cubic boron nitride polycrystalline materials, based on a mass standard, is between 100 ppm and 1000 ppm. The aforementioned cubic boron nitride polycrystalline material contains a plurality of grains. The median grain size d50 of the above plurality of grains is between 0.9 μm and 10 μm.

[0012] The cubic boron nitride polycrystalline material of this invention has high thermal conductivity and high bending strength.

[0013] (2) Preferably, the dislocation density of the cubic boron nitride is 4.5 × 10¹⁵ / m² or less. This further improves the thermal conductivity of the cubic boron nitride polycrystalline material.

[0014] (3) Preferably, the oxygen content of the cubic boron nitride polycrystalline material is below 700 ppm by mass. This improves the thermal conductivity and flexural strength of the cubic boron nitride polycrystalline material.

[0015] (4) Preferably, the total content of alkali metal elements and alkaline earth metal elements in the cubic boron nitride polycrystal is less than 10 ppm by mass. This improves the thermal conductivity and flexural strength of the cubic boron nitride polycrystal.

[0016] (5) Preferably, the thermal conductivity of the cubic boron nitride polycrystalline material is 500 W / mK or higher. This further improves the thermal conductivity of the cubic boron nitride polycrystalline material.

[0017] (6) Preferably, in the X-ray diffraction spectrum of the cubic boron nitride polycrystalline material described above, the ratio of the X-ray diffraction intensity I(220) of the (220) plane to the X-ray diffraction intensity I(111) of the (111) plane, I(220) / I(111), is 0.10 or more and 0.30 or less. This reduces the anisotropy of thermal conductivity and thermal expansion within the cubic boron nitride polycrystalline material. Therefore, when this cubic boron nitride polycrystalline material is used in a heat sink, the heat sink's lifespan for thermal cycling is improved.

[0018] (7) Preferably, the above-mentioned dislocation density is calculated using the modified Williamson-Hall method and the modified Warren-Averbach method.

[0019] (8) Preferably, the above-mentioned misalignment density is measured using radioactive light as an X-ray source.

[0020] (9) This invention relates to a heat sink using the aforementioned cubic boron nitride polycrystalline material. The heat sink of this invention has high thermal conductivity and high bending strength.

[0021] [Details of the embodiments of the present invention] The following describes specific examples of the cubic boron nitride polycrystalline material and heat sink of the present invention.

[0022] In this specification, the expression "A~B" refers to the upper and lower limits of the range (i.e., above A and below B). When A does not specify a unit but only B specifies a unit, the unit of A is the same as the unit of B.

[0023] [Implementation Method 1: Cubic Boron Nitride Polycrystalline] One embodiment of the present invention (hereinafter also referred to as "this embodiment") is a cubic boron nitride polycrystalline material containing 99.5% by volume or more cubic boron nitride. The thermal conductivity of this cubic boron nitride polycrystalline material is above 300 W / mK. The carbon content of this cubic boron nitride polycrystalline material, based on a mass standard, is between 100 ppm and 1000 ppm. This cubic boron nitride polycrystalline material contains a plurality of grains. The median grain size d50 of the circular equivalent diameter of these plurality of grains is between 0.9 μm and 10 μm.

[0024] The cubic boron nitride polycrystalline material of this invention is a sintered body, but since sintered bodies usually mean that they contain binders, the term "polycrystalline" is used in this invention.

[0025] The cubic boron nitride polycrystalline material of this embodiment possesses high thermal conductivity and high flexural strength. The reasons for this are speculated to be as described in (i) to (iii) below.

[0026] (i) The cubic boron nitride polycrystalline material of this embodiment contains 99.5% by volume or more of cubic boron nitride with high thermal conductivity. Therefore, the cubic boron nitride polycrystalline material can also have high thermal conductivity.

[0027] (ii) The carbon content of the cubic boron nitride polycrystalline material of this embodiment is between 100 ppm and 1000 ppm by mass. In the cubic boron nitride polycrystalline material, carbon is incorporated into the cubic boron nitride particles (hereinafter also referred to as cBN particles). Specifically, the carbon exists between the cubic boron nitride lattice or exists through substitution with boron or nitrogen in the cubic boron nitride, and a portion exists in the form of BCN compounds, presumably carbonitrides such as BC₂N or BCN, or diamond. It is presumed that this suppresses the formation of oxides that hinder the bonding between cBN particles. Therefore, the bonding force between cBN particles is improved, and the cubic boron nitride polycrystalline material can have high thermal conductivity and high flexural strength.

[0028] (iii) The median grain size d50 of the plurality of grains contained in the cubic boron nitride polycrystalline material of this embodiment is 0.9 μm to 10 μm. The larger the grain size, the higher the thermal conductivity tends to be. Therefore, this cubic boron nitride polycrystalline material can possess high thermal conductivity.

[0029] <Composition> The cubic boron nitride polycrystalline material of this embodiment contains 99.5% by volume or more cubic boron nitride. Therefore, the cubic boron nitride polycrystalline material can possess high thermal conductivity.

[0030] The cubic boron nitride polycrystalline material has a cubic boron nitride content of 99.5% by volume or higher, preferably 99.6% by volume or higher, more preferably 99.8% by volume or higher, further preferably 99.9% by volume or higher, and most preferably 100% by volume. The cubic boron nitride polycrystalline material has a cubic boron nitride content of 99.5% by volume or higher but less than 100% by volume, preferably 99.6% by volume or higher but less than 100% by volume, more preferably 99.8% by volume or higher but less than 100% by volume, further preferably 99.9% by volume or higher but less than 100% by volume, and most preferably 100% by volume.

[0031] Within the scope of demonstrating the effects of this embodiment, the cubic boron nitride polycrystalline material may include, in addition to cubic boron nitride, one, two, or all of hexagonal boron nitride, compressed hexagonal boron nitride, and wurtzite boron nitride. In this case, the total content of hexagonal boron nitride, compressed hexagonal boron nitride, and wurtzite boron nitride in the cubic boron nitride polycrystalline material may be set to 0.5% by volume or less. Here, "compressed hexagonal boron nitride" refers to hexagonal boron nitride with a crystal structure similar to that of ordinary hexagonal boron nitride, and whose face spacing along the c-axis is smaller than that of ordinary hexagonal boron nitride (0.333 nm).

[0032] The total content of hexagonal boron nitride, compressed hexagonal boron nitride, and wurtzite boron nitride in cubic boron nitride polycrystalline materials is preferably 0% to 0.5% by volume, more preferably 0% to 0.4% by volume, even more preferably 0% to 0.2% by volume, further preferably 0% to 0.1% by volume, and most preferably 0% by volume. That is, the cubic boron nitride polycrystalline material preferably contains no hexagonal boron nitride, compressed hexagonal boron nitride, or wurtzite boron nitride.

[0033] The content of hexagonal boron nitride in cubic boron nitride polycrystals is preferably 0% to 0.5% by volume, more preferably 0% to 0.4% by volume, even more preferably 0% to 0.2% by volume, further preferably 0% to 0.1% by volume, and most preferably 0% by volume. That is, cubic boron nitride polycrystals are preferably free of hexagonal boron nitride.

[0034] The content of compressed hexagonal boron nitride in cubic boron nitride polycrystalline materials is preferably 0% to 0.5% by volume, more preferably 0% to 0.4% by volume, even more preferably 0% to 0.2% by volume, further preferably 0% to 0.1% by volume, and most preferably 0% by volume. That is, cubic boron nitride polycrystalline materials are preferably free of compressed hexagonal boron nitride.

[0035] The content of wurtzite-type boron nitride in cubic boron nitride polycrystals is preferably 0% to 0.5% by volume, more preferably 0% to 0.4% by volume, even more preferably 0% to 0.2% by volume, further preferably 0% to 0.1% by volume, and most preferably 0% by volume. That is, cubic boron nitride polycrystals are preferably free of wurtzite-type boron nitride.

[0036] The content (volume %) of cubic boron nitride, hexagonal boron nitride, compressed hexagonal boron nitride, and wurtzite boron nitride polycrystalline materials can be determined by X-ray diffraction. The specific determination method is as follows.

[0037] Cubic boron nitride polycrystalline material was cut using diamond electrodeposition wire, and the cut surface was used as the observation surface. X-ray diffraction spectra of the cut surface of the cubic boron nitride polycrystalline material were obtained using an X-ray diffraction apparatus (Rigaku's "MiniFlex600"). The conditions of the X-ray diffraction apparatus at this time are described below.

[0038] Characteristic X-rays: Cu-Kα (wavelength 1.54 Å); Tube voltage: 45 kV; Tube current: 40 mA; Filter: Multilayer mirror; Optical system: lumped method; X-ray diffraction method: θ~2θ method.

[0039] In the obtained X-ray diffraction spectrum, the following peak intensities A, B, C, and D are measured.

[0040] Peak intensity A: Peak intensity of compressed hexagonal boron nitride near the diffraction angle 2θ = 28.5° after removing the background.

[0041] Crest intensity B: Crest intensity of wurtzite boron nitride after removing background from the area near the diffraction angle 2θ = 40.8°.

[0042] Peak intensity C: Peak intensity of cubic boron nitride after background removal, near the diffraction angle 2θ = 43.5°.

[0043] Peak intensity D: Peak intensity of hexagonal boron nitride near the diffraction angle 2θ = 26.8° after removing the background.

[0044] The content of compressed hexagonal boron nitride was obtained by calculating the value of peak intensity A / (peak intensity A + peak intensity B + peak intensity C + peak intensity D). The content of wurtzite-type boron nitride was obtained by calculating the value of peak intensity B / (peak intensity A + peak intensity B + peak intensity C + peak intensity D). The content of cubic boron nitride was obtained by calculating the value of peak intensity C / (peak intensity A + peak intensity B + peak intensity C + peak intensity D). The content of hexagonal boron nitride was obtained by calculating the value of peak intensity D / (peak intensity A + peak intensity B + peak intensity C + peak intensity D).

[0045] Since compressed hexagonal boron nitride, wurtzite boron nitride, cubic boron nitride, and hexagonal boron nitride all have the same degree of electron weighting, the above-mentioned X-ray peak intensity ratio can be regarded as the volume ratio in cubic boron nitride polycrystals.

[0046] ≪Carbon content≫ The carbon content of the cubic boron nitride polycrystalline material in this embodiment is between 100 ppm and 1000 ppm by mass. This improves the bonding strength between cBN particles, enabling the cubic boron nitride polycrystalline material to possess high thermal conductivity and high flexural strength.

[0047] The lower limit of the carbon content of cubic boron nitride polycrystalline material, based on a mass basis, is preferably 100 ppm or more, more preferably 200 ppm or more, and further preferably 300 ppm or more. The upper limit of the carbon content of cubic boron nitride polycrystalline material, based on a mass basis, is preferably 1000 ppm or less, more preferably 900 ppm or less, and further preferably 800 ppm or less. The carbon content of cubic boron nitride polycrystalline material, based on a mass basis, is 100 ppm or more but less than 1000 ppm, preferably 200 ppm or more but less than 900 ppm, and more preferably 300 ppm or more but less than 800 ppm.

[0048] The carbon content of cubic boron nitride polycrystals was determined using secondary ion mass spectrometry (SIMS). For example, a CAMECA IMS-7f instrument (manufactured by AMETEK) was used, with the primary ion species set to Cs+, the primary accelerating voltage set to 15.0 kV, and the detection region set to... Carbon content was determined at 30 μm. The oxygen content, alkali metal and alkaline earth metal content, and other unavoidable impurities in cubic boron nitride polycrystals, described later, were also determined using the same method.

[0049] ≪Oxygen content≫ The oxygen content of the cubic boron nitride polycrystalline material in this embodiment is preferably below 700 ppm by mass. This improves the hardness, strength, thermal conductivity, and flexural strength of the cubic boron nitride polycrystalline material. The reason for this is speculated as follows.

[0050] The cubic boron nitride polycrystalline system of this embodiment is obtained by heating and pressurizing hexagonal boron nitride as a raw material, thereby undergoing phase transfer from wurtzite-type boron nitride to cubic boron nitride. If oxygen is present in the hexagonal boron nitride raw material, there is a tendency for this oxygen to hinder phase transfer. When the oxygen content of the cubic boron nitride polycrystalline material is below 700 ppm by mass, the aforementioned phase transfer hindrance is less likely to occur, and the conversion rate from hexagonal boron nitride to cubic boron nitride becomes higher. Therefore, it is speculated that the obtained cubic boron nitride polycrystalline material has a higher cubic boron nitride content, and the thermal conductivity of the cubic boron nitride polycrystalline material is improved.

[0051] In cubic boron nitride polycrystalline materials, if oxygen is present at the grain boundaries of the cubic boron nitride particles, there is a tendency for the interparticle bonding force to decrease. It is speculated that when the oxygen content in the cubic boron nitride polycrystalline material is below 700 ppm, the decrease in interparticle bonding force is suppressed, and the hardness, strength, thermal conductivity, and flexural strength of the cubic boron nitride polycrystalline material are improved.

[0052] The upper limit of oxygen content in cubic boron nitride polycrystalline materials, based on mass, is preferably below 700 ppm, more preferably below 500 ppm, and even more preferably below 300 ppm. The lower the oxygen content, the better. Therefore, the lower limit of oxygen content in cubic boron nitride polycrystalline materials is preferably above 0 ppm. The oxygen content in cubic boron nitride polycrystalline materials is preferably between 0 ppm and 700 ppm, more preferably between 0 ppm and 500 ppm, and further preferably between 0 ppm and 300 ppm.

[0053] ≪Total content of alkali metals and alkaline earth group metals≫ The total content of alkali metals and alkaline earth metals in cubic boron nitride polycrystals is preferably below 10 ppm by mass. Examples of alkali metals include lithium (Li), sodium (Na), and potassium (K). Examples of alkaline earth metals include calcium (Ca), magnesium (Mg), strontium (Sr), and barium (Ba). This improves the hardness, strength, thermal conductivity, and flexural strength of cubic boron nitride polycrystals. The reasons for this are speculated as follows.

[0054] Alkali metals and alkaline earth metals tend to decrease the bonding force between cubic boron nitride particles. It is speculated that if the total content of alkali metals and alkaline earth metals in cubic boron nitride polycrystals is below 10 ppm by mass, the decrease in bonding force between cubic boron nitride particles will be suppressed, and the hardness, strength, thermal conductivity, and flexural strength of the cubic boron nitride polycrystal will be improved.

[0055] The upper limit of the total content of alkali metals and alkaline earth metals in cubic boron nitride polycrystals, based on a mass basis, is preferably below 10 ppm, more preferably below 5 ppm, and further preferably below 3 ppm. The lower the total content of alkali metals and alkaline earth metals in cubic boron nitride polycrystals, the better. Therefore, the lower limit of the total content of alkali metals and alkaline earth metals in cubic boron nitride polycrystals, based on a mass basis, is preferably above 0 ppm. The total content of alkali metals and alkaline earth metals in cubic boron nitride polycrystals, based on a mass basis, is preferably above 0 ppm and below 10 ppm, more preferably above 0 ppm and below 5 ppm, and further preferably above 0 ppm and below 3 ppm.

[0056] ≪Unavoidable Impurities≫ Within the scope of the effects of this invention, cubic boron nitride polycrystalline materials may also contain unavoidable impurities. Examples of unavoidable impurities include hydrogen, silicon (Si), and aluminum (Al). The content of unavoidable impurities in cubic boron nitride polycrystalline materials is preferably 0% by mass or more and 0.1% by mass or less.

[0057] <Dislocation Density> In this embodiment, the dislocation density of cubic boron nitride is preferably 4.5 × 10¹⁵ / m² or less. This improves the thermal conductivity of the cubic boron nitride polycrystalline material.

[0058] The upper limit of the dislocation density of cubic boron nitride is preferably 4.5 × 10¹⁵ / m² or less, more preferably 4.0 × 10¹⁵ / m² or less, and even more preferably 3.5 × 10¹⁵ / m² or less. The lower limit of the dislocation density of cubic boron nitride is not particularly limited, but from a manufacturing point of view, it can be set to 1 × 10¹⁵ / m² or more. The dislocation density of cubic boron nitride is preferably 1 × 10¹⁵ / m² or more and 4.5 × 10¹⁵ / m² or less, more preferably 1 × 10¹⁵ / m² or more and 4.0 × 10¹⁵ / m² or less, and even more preferably 1 × 10¹⁵ / m² or more and 3.5 × 10¹⁵ / m² or less.

[0059] In this specification, the dislocation density of cubic boron nitride is calculated using the following procedure. A sample containing a cubic boron nitride polycrystalline material is prepared. The sample has a viewing surface size of 2.0 mm × 2.0 mm and a thickness of 1.0 mm. The viewing surface of the sample is ground using a diamond grinding stone.

[0060] X-ray diffraction measurements were performed on the observation surface of the sample under the following conditions to obtain the spectral profiles of diffraction peaks from the main orientations (111), (200), (220), (311), (400), and (331) of cubic boron nitride.

[0061] (X-ray diffraction measurement conditions) X-ray source: radiation light Device requirements: Detector NaI (fluorescence cutoff achieved using a suitable ROI (Region of Interest)). Energy: 18 keV (wavelength: 0.6888 Å) Spectroscopic crystal: Si(111) Entrance slit: 5 mm wide × 0.5 mm high Light-receiving slit: Double slit (3 mm width × 0.5 mm height) Mirror: Platinum-coated mirror Angle of incidence: 2.5 mrad Scanning method: 2θ~θ scan Peaks were measured for cubic boron nitride at (111), (200), (220), (311), (400), and (331). Peaks with surface indices were excluded when the contour could not be obtained based on texture and orientation.

[0062] Measurement conditions: Set at least 9 measurement points within the half-width. Set the peak intensity to at least 2000 counts. Since the lower edge of the peak is also used for analysis, the measurement range is set to approximately 10 times the half-width.

[0063] The spectral profile obtained by the above-described X-ray diffraction measurement comprises both the true extension caused by physical quantities such as the non-uniform strain of the sample and the extension generated by the device. To determine the non-uniform strain or crystallite size, the component generated by the device is removed from the measured spectral profile to obtain the true spectral profile. The true spectral profile is obtained by fitting the obtained spectral profile and the spectral profile generated by the device using a pseudo-Voigt function, and then subtracting the spectral profile generated by the device. LaB 6 is used as a standard sample for removing the diffraction extension generated by the device. Furthermore, when using radiation with high parallelism, the diffraction extension generated by the device can be considered as zero.

[0064] The error density is calculated by analyzing the obtained true spectral profiles using the modified Williamson-Hall method and the modified Warren-Averbach method. The modified Williamson-Hall method and the modified Warren-Averbach method are well-known analytical methods for determining error density of spectral profiles.

[0065] The modified Williamson-Hall method is shown in equation (I) below.

[0066] [Number 1]

[0067] (In the above formula (I), ΔK represents the half-width of the spectral profile, D represents the crystallite size, M represents the configuration parameter, b represents the Burgers vector, ρ represents the dislocation density, K represents the scattering vector, O(K 2C) represents the higher-order term of K 2C, and C represents the average value of the contrast factor). In the above formula (I), C is represented by the following formula (II).

[0068] C=C h00[1-q(h 2k 2+h 2l 2+k 2l 2) / (h 2+k 2+l 2) 2] (II) In equation (II) above, the contrast factor C h00 and the coefficient q related to the contrast factor in spiral dislocation and edge dislocation are calculated using the calculation code ANIZC, with the slip system as... <110> {111}, elastic stiffness C11 is 8.44 GPa, C12 is 1.9 GPa, and C44 is 4.83 GPa. The helical misalignment of the contrast factor Ch00 is 0.203, and the edge misalignment is 0.212. The coefficient q, which is related to the contrast factor, has a helical misalignment of 1.65 and an edge misalignment of 0.58. Furthermore, the helical misalignment ratio is fixed at 0.5, and the edge misalignment ratio is fixed at 0.5.

[0069] Furthermore, the relationship between misalignment and non-uniform strain is established by using a contrast factor C, which makes the following equation (III) hold true.

[0070] <ε(L) 2>=(ρCb 2 / 4π)ln(R e / L) (III) (In equation (III) above, Re represents the effective radius of the misalignment) Based on the relationship in equation (III) above and the Warren-Averbach equation, it can be expressed as equation (IV) below, and as a modified Warren-Averbach method, the mislocation density ρ and crystallite size can be determined.

[0071] lnA(L) = lnAS(L) - (πL 2ρb 2 / 2)ln(R e / L)(K 2C) + O(K 2C) 2(IV) (In the above formula (IV), A(L) represents the Fourier series, AS(L) represents the Fourier series related to the crystallite size, and L represents the Fourier length) Details of the modified Williamson-Hall method and the modified Warren-Averbach method are described in "T. Ungar and A. Borbely, "The effect of dislocation contrast on x-ray line broadening: A new approach to line profile analysis" Appl. Phys. Lett., vol.69, no.21, p.3173, 1996." and "T. Ungar, S. Ott, P. Sanders, A. Borbely, J. Weertman, "Dislocations, grain size and planar faults in nanostructured copper determined by high resolution X-ray diffraction and a new procedure of peak profile analysis" Acta Mater., vol.46, no.10, pp.3693-3699, 1998.".

[0072] Thermal conductivity The lower limit of the thermal conductivity of the cubic boron nitride polycrystalline material in this embodiment is 300 W / mK or higher, preferably 500 W / mK or higher, and more preferably 600 W / mK or higher. The upper limit of the thermal conductivity of the cubic boron nitride polycrystalline material is not particularly limited; for example, it can be set to 2000 W / mK or lower. The thermal conductivity of the cubic boron nitride polycrystalline material is preferably 300 W / mK or higher and 2000 W / mK or lower, more preferably 500 W / mK or higher and 2000 W / mK or lower, and even more preferably 600 W / mK or higher and 2000 W / mK or lower.

[0073] In this specification, the thermal conductivity of the cubic boron nitride sintered body is obtained by measuring the thermal diffusivity using the laser flash method with a xenon flash lamp and converting the thermal diffusivity into thermal conductivity. The measurement temperature is set to 25 °C. The measurement device can use "LFA467 Hyper Flash" (trademark) manufactured by NETZSCH. When performing the above conversion, the following conditions (a) to (c) are adopted. (a) Thermal conductivity = thermal diffusivity × specific heat × density; (b) The density of cubic boron nitride is 3.45 g / cm3, and the specific heat is 0.6 J / mol; (c) The content rate of cubic boron nitride in the cubic boron nitride polycrystal is regarded as 100% by volume.

[0074] <Ratio I(220) / I(111) in the X-ray diffraction spectrum> In the X-ray diffraction spectrum of the cubic boron nitride polycrystal of the present embodiment, the ratio I(220) / I(111) of the X-ray diffraction intensity I(220) of the (220) plane to the X-ray diffraction intensity I(111) of the (111) plane is preferably 0.10 or more and 0.30 or less. Thereby, the cubic boron nitride polycrystal is isotropic, and the anisotropy of heat conduction and thermal expansion in the cubic boron nitride polycrystal becomes small. Therefore, when the cubic boron nitride polycrystal is used for a heat sink, the life of the heat sink against thermal cycling is improved.

[0075] The lower limit of the above ratio I(220) / I(111) is preferably 0.10 or more, more preferably 0.15 or more, and still more preferably 0.18 or more. The upper limit of the above ratio I(220) / I(111) is preferably 0.30 or less, more preferably 0.28 or less, and still more preferably 0.25 or less. The above ratio I(220) / I(111) is preferably 0.10 or more and 0.30 or less, more preferably 0.15 or more and 0.28 or less, and still more preferably 0.18 or more and 0.25 or less.

[0076] In this specification, the above ratio I(220) / I(111) is measured by the following procedure. The cubic boron nitride polycrystal is cut by electroplating wire with a diamond grindstone, and the cut surface is used as the observation surface. The X-ray diffraction spectrum of the cut surface of the cubic boron nitride polycrystal is obtained using an X-ray diffractometer ("MiniFlex600" (trade name) manufactured by Rigaku). The conditions of the X-ray diffractometer at this time are as described below.

[0077] Characteristic X-rays: Cu-Kα (wavelength 1.54 Å) Tube voltage: 45 kV Tube current: 40 mA Filter: Multilayer mirror Optical system: lumped method X-ray diffraction method: θ~2θ method

[0078] Based on the obtained X-ray diffraction spectrum, the ratio of the X-ray diffraction intensity I(220) of the (220) plane to the X-ray diffraction intensity I(111) of the (111) plane is I(220) / I(111).

[0079] <Average grain size> The cubic boron nitride polycrystalline material of this embodiment comprises a plurality of grains, the median diameter d50 (hereinafter also referred to as "d50") of which is 0.9 μm to 10 μm. In this specification, the median diameter d50 of the plurality of grains refers to the diameter of 50% of the cumulative number of grains. This improves the thermal conductivity of the cubic boron nitride polycrystalline material.

[0080] The lower limit of the d50 of the circular equivalent diameter of the aforementioned plurality of grains is preferably 0.9 μm or more, more preferably 2.0 μm or more, and even more preferably 3.0 μm or more. The upper limit of the d50 of the circular equivalent diameter of the aforementioned plurality of grains, from a manufacturing point of view, is preferably 10 μm or less. The d50 of the circular equivalent diameter of the aforementioned plurality of grains is 0.9 μm or more and 10 μm or less, preferably 2.0 μm or more and 10 μm or less, and more preferably 3.0 μm or more and 10 μm or less.

[0081] The d90 (hereinafter also referred to as "d90") of the aforementioned plurality of grains is preferably 1.0 μm or more and 15 μm or less. In this specification, the d90 of the circumference of the plurality of grains refers to the circumference of 90% of the cumulative number of grains. This improves the thermal conductivity of cubic boron nitride polycrystalline materials.

[0082] The lower limit of the circular equivalent diameter (d90) of the aforementioned plurality of grains is preferably 1.0 μm or more, more preferably 3.0 μm or more, and even more preferably 4.0 μm or more. The upper limit of the circular equivalent diameter (d90) of the aforementioned plurality of grains, from a manufacturing point of view, is preferably 15 μm or less. The circular equivalent diameter (d90) of the aforementioned plurality of grains is preferably 1.0 μm or more and 15 μm or less, more preferably 3.0 μm or more and 15 μm or less, and even more preferably 4.0 μm or more and 15 μm or less.

[0083] In this specification, the circumferential diameters d50 and d90 of the plurality of grains contained in a cubic boron nitride polycrystal are determined by the following procedure.

[0084] Cubic boron nitride polycrystalline material was cut using a diamond abrasive electrodeposition wire, and the cut surfaces were then polished. The polished surfaces were observed using a scanning electron microscope (SEM) (JSM-7500F, manufactured by Nippon Electronics Co., Ltd., trademark), and SEM images were obtained. The measurement field size was set to 12 μm × 15 μm, and the magnification was set to 10000x. Five measurement fields were arbitrarily set.

[0085] For SEM images of five measurement fields, the distribution of the grain diameter was determined using image processing software (Win Roof ver. 7.4.5) with the grain boundaries of the observed grains separated within the measurement field.

[0086] Within each measurement field of view, based on the distribution of the corresponding circular diameters of the grains, the total number of grains within that field of view is used as the denominator to calculate the d50 of the corresponding circular diameter of the grains. Within each of the five measurement fields of view, the d50 of the corresponding circular diameter of the grains is calculated, and their average value is determined. This average value corresponds to the d50 of the corresponding circular diameters of the plurality of grains contained in the cubic boron nitride polycrystal.

[0087] Within each measurement field of view, based on the distribution of the corresponding circular diameters of the grains, the total number of grains within that field of view is used as the denominator to calculate the d90 of the corresponding circular diameter of the grains. Within each of the five measurement fields of view, the d90 of the corresponding circular diameter of the grains is calculated, and their average value is determined. This average value corresponds to the d90 of the corresponding circular diameters of the plurality of grains contained in the cubic boron nitride polycrystal.

[0088] In the measurements conducted by the applicant, as long as the median particle size d50 and d90 of the same sample are measured, even if the selected location of the measurement field of view in the cubic boron nitride polycrystalline material is changed and the calculation is repeated multiple times, the measurement results are almost identical, confirming that the measurement results will not change arbitrarily even if the measurement field of view is arbitrarily set.

[0089] <Manufacturing Method of Cubic Boron Nitride Polycrystalline> The cubic boron nitride polycrystalline material of this embodiment can be fabricated using, for example, the following method.

[0090] ≪Raw Material Preparation Steps≫ Hexagonal boron nitride powder was prepared as raw material. The graphite content in this hexagonal boron nitride powder was 50-1000 ppm by mass, and the combined content of alkali metals and alkaline earth metals was less than 10 ppm by mass. The median particle size d50 of the hexagonal boron nitride powder was 5 μm. The hexagonal boron nitride powder was placed in a carbon crucible and heat-treated at 2100°C (pressure 0.1 MPa) for 10-120 minutes. This process transferred a portion of the carbon from the crucible raw material to the hexagonal boron nitride powder. Furthermore, it reduced the amount of oxygen present as an impurity in the hexagonal boron nitride powder. Moreover, in previous methods for manufacturing cubic boron nitride polycrystalline materials, carbon was considered an impurity. Therefore, there was no technical concept of including carbon in the raw material in those manufacturing methods, and the use of carbon crucibles was not employed by the industry. The method newly discovered by the inventors is to use a carbon crucible to contain carbon in the raw material during the heat treatment of hexagonal boron nitride powder.

[0091] ≪Sintering Steps≫ Next, the heat-treated hexagonal boron nitride powder is sealed in a molybdenum-made sealed chamber. The hexagonal boron nitride powder is then heated and pressurized using a six-directional pressurized multi-anvil high-pressure generator or an eight-directional pressurized Kawai-type multi-anvil high-pressure generator until the temperature reaches 2500-2700°C and the pressure exceeds 10 GPa, thereby obtaining a cubic boron nitride sintered body. During heating and pressurization, the temperature and pressure are adjusted such that the heating and pressurization path passes through the stable region of wurtzite-type boron nitride, and the entry temperature into the stable region of wurtzite-type boron nitride reaches 800°C or higher. Here, the stable region of wurtzite-type boron nitride is the region that simultaneously satisfies Equation 1 and Equation 2 below when the temperature is set to T (°C) and the pressure is set to P (GPa). Equation 1: P ≧ -0.0037T + 11.301 Equation 2: P≦-0.085T+117

[0092] According to the heating and pressurization path described above, hexagonal boron nitride powder is converted into wurtzite-type boron nitride in an environment conducive to atomic diffusion, and then into cubic boron nitride. Therefore, in the obtained cubic boron nitride polycrystal, lattice defects are reduced, the dislocation density of cubic boron nitride decreases, and thermal conductivity increases.

[0093] The holding time under the aforementioned conditions of temperature 2500~2700℃ and pressure 10 GPa or higher is preferably 10 minutes to 30 minutes. This reduces the dislocation density of cubic boron nitride, increases the particle size of cubic boron nitride particles, and improves the thermal conductivity of the cubic boron nitride polycrystalline material.

[0094] [Implementation Method 2: Radiator] The heat sink in this embodiment uses the cubic boron nitride polycrystalline material described in Embodiment 1 above. This heat sink has high thermal conductivity and high bending strength. Therefore, this heat sink can suppress the decrease in thermal conductivity that occurs during use, thus extending the lifespan of the semiconductor.

[0095] [Postscript 1] The cubic boron nitride polycrystalline material of the present invention preferably comprises 99.5% to 100% cubic boron nitride and a total of 0% to 0.5% hexagonal boron nitride, compressed hexagonal boron nitride, and wurtzite boron nitride. The carbon content of the aforementioned cubic boron nitride polycrystalline material, based on mass, is between 100 ppm and 1000 ppm.

[0096] [Postscript 2] The cubic boron nitride polycrystalline material of the present invention preferably comprises 99.5% to 100% cubic boron nitride and a total of 0% to 0.5% hexagonal boron nitride, compressed hexagonal boron nitride, and wurtzite boron nitride. The carbon content of the aforementioned cubic boron nitride polycrystalline materials, based on a mass standard, is between 100 ppm and 1000 ppm. The oxygen content of the above-mentioned cubic boron nitride polycrystalline materials, based on mass, is between 0 ppm and 700 ppm. [Example]

[0097] The present implementation will be further described in detail through examples. However, the present implementation is not limited to these examples.

[0098] Fabrication of cubic boron nitride polycrystalline materials ≪Raw Material Preparation Steps≫ Hexagonal boron nitride powder was prepared as the raw material for the cubic boron nitride polycrystalline samples. In samples 1-4, 6, 7, and 9-12, the graphite content in the hexagonal boron nitride powder was 50 ppm by mass, the total content of alkali metals and alkaline earth metals was less than 10 ppm by mass, and the median particle size d50 of the equivalent diameter was 5 μm. In samples 5 and 8, the graphite content in the hexagonal boron nitride powder was 50 ppm by mass, the total content of alkali metals and alkaline earth metals exceeded 10 ppm by mass, and the median particle size d50 of the equivalent diameter was 5 μm.

[0099] The hexagonal boron nitride powder was placed in a carbon crucible and heat-treated at 2100°C under a nitrogen atmosphere (pressure 0.1 MPa). The heat treatment time is as recorded in the "Time (minutes)" column of "Raw Material Preparation Steps" in Table 1. For example, Sample 1 was heat-treated at 2100°C for 10 minutes.

[0100] ≪Sintering Steps≫ Next, the heat-treated hexagonal boron nitride powder was sealed in a molybdenum-made sealed chamber. The hexagonal boron nitride powder was then heated and pressurized using a high-pressure generator to obtain cubic boron nitride polycrystalline samples. Samples 1-10 and 12 were produced using a hexagonal pressurization multi-anvil high-pressure generator. Sample 11 was produced using a belt-type high-pressure generator. The heating and pressurization were performed in multiple stages, as shown in the "Sintering Steps" section of Table 1, starting from the "Start Point": "Stage 1," "Stage 2," "Stage 3," and "Stage 4." The "-" markings in the "Stage 1," "Stage 2," "Stage 3," and "Stage 4" columns of Table 1 indicate that heating and pressurization for that stage was not performed. The holding time in the final stage is shown in the "Final Stage Holding Time" column of Table 1. Here, "Final Stage Holding Time" refers to the holding time at the final pressure and temperature reached during the sintering process.

[0101] For example, regarding sample 1, the starting point for heating and pressurization is 25°C and 0 GPa. In the first stage, the temperature is maintained at 0 GPa and heated to 1000°C. In the second stage, the temperature is maintained at 1000°C and pressurized to 10 GPa. In the third stage, the temperature is maintained at 10 GPa and heated to 2600°C. The pressure (10 GPa) and temperature (2600°C) reached in the third stage are maintained for 20 minutes.

[0102] [Table 1] Table 1 Sample number Raw material preparation steps Sintering steps temperature time Starting point Phase 1 Phase 2 Phase 3 Phase 4 Final stage holding time temperature pressure temperature pressure temperature pressure temperature pressure temperature pressure (°C) (minute) (°C) (GPa) (°C) (GPa) (°C) (GPa) (°C) (GPa) (°C) (GPa) (minute) 1 2100 10 25 0 1000 0 1000 10 2600 10 - - 20 2 2100 60 25 0 900 0 900 10 2500 10 - - 20 3 2100 20 25 0 800 0 800 10 2550 10 - - 10 4 2100 5 25 0 800 0 800 10 2550 10 - - 20 5 2100 5 25 0 500 0 500 10 2500 10 - - 20 6 2100 20 25 0 25 5 1100 5 1100 10 2700 10 20 7 2100 20 25 0 0 10 2200 10 - - - - 20 8 2100 240 25 0 900 0 900 10 2500 10 - - 20 9 2100 2 25 0 900 0 900 10 2500 10 - - 20 10 2100 120 25 0 900 0 900 10 2500 10 - - 20 11 2100 2 25 0 25 6.5 2350 6.5 - - - - 20 12 2100 20 25 0 1000 0 1000 7.5 2600 7.5 - - 30

[0103] [Measurement] For each cubic boron nitride polycrystalline sample, the content of cubic boron nitride, carbon content, oxygen content, total content of alkali metals and alkaline earth metals, thermal conductivity, dislocation density of cubic boron nitride, ratio I(220) / I(111) in X-ray diffraction spectrum, d50, d90 of grains and flexural strength were measured.

[0104] <Composition> The content ratio (volume %) of cubic boron nitride in the cubic boron nitride polycrystal of each sample was measured by X-ray diffraction. The specific measurement method is as described in Embodiment 1. The results are shown in the column of "cBN content ratio" in Table 2 under "cubic boron nitride polycrystal".

[0105] <Total content ratio of carbon content ratio, oxygen content ratio, alkali metals and alkaline earth metals> The carbon content ratio, oxygen content ratio, and total content ratio of alkali metals and alkaline earth metals in the cubic boron nitride polycrystal of each sample were measured. The specific measurement method is as described in Embodiment 1. The results are shown in the columns of "carbon content ratio", "oxygen content ratio", and "alkali metal / alkaline earth metal content ratio" in Table 2 under "cubic boron nitride polycrystal".

[0106] <Thermal conductivity> The thermal conductivity of the cubic boron nitride polycrystal of each sample was measured. The specific measurement method is as described in Embodiment 1. The results are shown in the column of "thermal conductivity" in Table 2 under "cubic boron nitride polycrystal".

[0107] <Dislocation density> The dislocation density of cubic boron nitride in the cubic boron nitride of each sample was measured. The specific measurement method is as described in Embodiment 1. The results are shown in the column of "dislocation density" in Table 2 under "cubic boron nitride polycrystal".

[0108] <Ratio I(220) / I(111) in X-ray diffraction spectrum> The ratio I(220) / I(111) in the X-ray diffraction spectrum of the cubic boron nitride polycrystal of each sample was measured. The specific measurement method is as described in Embodiment 1. The results are shown in the column of "I(220) / I(111)" in Table 2 under "cubic boron nitride polycrystal".

[0109] <d50 and d90 of the equivalent circle diameter of grains> The d50 and d90 of the equivalent circle diameter of multiple grains contained in the cubic boron nitride polycrystal of each sample were measured. The specific measurement method is as described in Embodiment 1. The results are shown in the columns of "d50" and "d90" in the "grains" section of Table 2 under "cubic boron nitride polycrystal".

[0110] <Flexural strength> The flexural strength of cubic boron nitride polycrystalline samples was determined. The method is as follows: 6 mm × 3 mm × 0.5 mm specimens were cut from the cubic boron nitride polycrystalline sample and subjected to a three-point bending test using a silicon carbide (SiC) fixture. The test temperature was set to room temperature (25°C), and the span length (distance between support points) was set to 4 mm. Furthermore, although the three-point bending test was basically based on the method of JIS R1601:2008, "Room Temperature Bending Strength Test Method for Precision Ceramics," it was conducted on a scaled-down basis due to limitations in specimen size. The results are shown in the "Flexural Strength" column of "Cubic Boron Nitride Polycrystalline" in Table 2.

[0111] [Table 2] Table 2 Sample number cubic boron nitride polycrystalline grains thermal conductivity Dislocation density cBN content carbon content Oxygen content Alkali metal / alkaline earth metal content I (220) / I (111) flexural strength d50 d90 μm μm W / mK ×10 15 / m 2 volume% ppm ppm ppm GPa 1 3.5 4.9 510 3.8 99.9 100 500 5 0.21 1.0 2 1.0 1.4 358 4.4 99.8 1000 500 4 0.13 1.0 3 0.9 1.2 305 4.7 99.9 300 500 8 0.23 1.1 4 0.9 1.2 295 4.6 99.9 80 900 8 0.20 0.7 5 0.6 0.7 265 5.1 99.9 80 800 13 0.22 0.8 6 9.0 11.0 700 3.1 99.9 300 500 7 0.28 1.0 7 0.2 0.3 190 8.1 99.8 300 500 7 0.22 1.5 8 0.7 0.9 289 4.8 99.4 5000 2000 20 0.22 0.8 9 1.0 1.3 350 4.5 99.9 50 500 8 0.21 0.7 10 1.0 1.4 350 4.5 99.7 2000 500 8 0.19 0.7 11 4.0 5.5 510 8.5 95.5 50 500 8 0.03 0.4 12 3.8 5.2 520 3.9 99.5 300 500 5 0.23 0.9

[0112] [Discussion] The cubic boron nitride polycrystalline samples 1-3, 6, and 12 are examples. The cubic boron nitride polycrystalline samples 4, 5, and 7-11 are comparative examples. It was confirmed that the cubic boron nitride polycrystalline samples 1-3, 6, and 12 all have a thermal conductivity of 300 W / mK or higher, exhibiting high thermal conductivity, and a flexural strength of 0.9 GPa or higher, exhibiting high flexural strength.

[0113] The cubic boron nitride polycrystalline samples 4 and 5 exhibited lower thermal conductivity and flexural strength compared to the cubic boron nitride polycrystalline sample in the examples. This is presumably due to the lower carbon content (80 ppm), resulting in weaker interparticle bonding in the cBN particles.

[0114] The cubic boron nitride polycrystalline sample 7 exhibits lower thermal conductivity compared to the cubic boron nitride polycrystalline sample in the examples. This is presumably due to the lower final sintering temperature of 2200°C, resulting in smaller grain size. Furthermore, it is presumably due to the lower penetration temperature (approximately 35°C) into the stable region of wurtzite-type boron nitride during the heating and pressurization process, leading to a higher dislocation density.

[0115] The cubic boron nitride polycrystalline sample 8 exhibited lower thermal conductivity and flexural strength compared to the cubic boron nitride polycrystalline sample in the examples. This is presumably due to an excessive carbon content of 5000 ppm, leading to excessive precipitation of low-strength carbon at the grain boundaries. Furthermore, it is presumably due to a lower cBN content of 99.4% by volume.

[0116] The cubic boron nitride polycrystalline sample 9 exhibited lower flexural strength compared to the cubic boron nitride polycrystalline sample in the examples. This is presumably due to the lower carbon content of 50 ppm, resulting in weaker interparticle bonding in the cBN particles.

[0117] The cubic boron nitride polycrystalline sample 10 exhibited lower flexural strength compared to the cubic boron nitride polycrystalline sample of the examples. This is presumably due to an excessive carbon content of 2000 ppm, leading to excessive precipitation of low-strength carbon at the grain boundaries.

[0118] The cubic boron nitride polycrystalline sample 11 exhibits lower flexural strength compared to the cubic boron nitride polycrystalline sample of the examples. The cubic boron nitride polycrystalline sample 11 contains 95.5% by volume of cBN and 4.5% by volume of hexagonal boron nitride. Therefore, the decrease in flexural strength is presumed.

[0119] The embodiments and examples of the present invention have been described above, but it is expected from the outset that the above embodiments and examples may be appropriately combined or varied. The embodiments and examples disclosed herein should be considered illustrative in all respects and not restrictive. The scope of this invention is not defined by the above-described embodiments and examples, but by the claims, and is intended to include all modifications within the scope and meaning of the claims.

Claims

1. A cubic boron nitride polycrystalline material comprising 99.5% by volume or more of cubic boron nitride, wherein the thermal conductivity of the cubic boron nitride polycrystalline material is 300 W / mK or more, the carbon content of the cubic boron nitride polycrystalline material is 100 ppm to 1000 ppm by mass, the cubic boron nitride polycrystalline material comprises a plurality of grains, the median grain size d50 of the circular equivalent diameter of the plurality of grains is 0.9 μm to 10 μm, and the dislocation density of the cubic boron nitride is 4.5 × 10¹⁵ / m² or less.

2. The cubic boron nitride polycrystalline material as claimed in claim 1, wherein the oxygen content of the cubic boron nitride polycrystalline material is less than 700 ppm by mass.

3. For the cubic boron nitride polycrystalline material of claim 1 or 2, the total content of alkali metal elements and alkaline earth metal elements in the cubic boron nitride polycrystalline material is less than 10 ppm on a mass basis.

4. The cubic boron nitride polycrystalline material as claimed in claim 1 or 2, wherein the thermal conductivity of the cubic boron nitride polycrystalline material is 500 W / mK or higher.

5. The cubic boron nitride polycrystal of claim 1 or 2, wherein in the X-ray diffraction spectrum of the cubic boron nitride polycrystal, the ratio of the X-ray diffraction intensity I(220) of the (220) plane to the X-ray diffraction intensity I(111) of the (111) plane, I(220) / I(111), is 0.10 or more and 0.30 or less.

6. The cubic boron nitride polycrystalline material as claimed in claim 1, wherein the dislocation density is calculated using the modified Williamson-Hall method and the modified Warren-Averbach method.

7. The cubic boron nitride polycrystalline material as claimed in claim 1, wherein the dislocation density is measured using radioactive light as an X-ray source.

8. A heat sink using a cubic boron nitride polycrystalline material as claimed in any one of claims 1 to 7.