Silicon nitride-based sintered compact and silicon nitride-based heat dissipation substrate

Abstract

To provide a silicon nitride-based sintered compact and a silicon nitride-based heat dissipation substrate that achieve increased strength while maintaining high thermal conductivity.SOLUTION: A silicon nitride-based sintered compact is mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles, and contains a compound including silicon (Si), group 4 elements (Ti, Zr, Hf), and iron (Fe), the total content of the compound being 0.20 to 10 vol.%. This constitution can provide a silicon nitride-based sintered compact that achieves increased strength while maintaining high thermal conductivity.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a silicon nitride sintered body and a silicon nitride heat dissipation substrate.

Background Art

[0002] Since silicon nitride has high thermal conductivity and strength, it has been attracting attention as an insulating heat dissipation substrate for power modules used in electric vehicles (EVs) and hybrid vehicles (HVs). Conventionally, aluminum nitride has been widely used as an insulating heat dissipation substrate material. However, in the case of power modules for high currents such as EVs, the temperature rises to about 250°C, and a large thermal stress is generated in the substrate due to the difference in thermal expansion from metals such as copper joined thereto. As a result, cracks and fractures occurred in the low-strength aluminum nitride. Therefore, although its thermal conductivity is inferior to that of aluminum nitride, silicon nitride, which has high thermal conductivity among general insulating ceramics and further has higher strength, is being increasingly adopted.

[0003] Many industrially obtained silicon nitride raw materials are produced by the direct nitridation method. Although this method is characterized by being relatively inexpensive, since there is a pulverization process, the amount of iron contamination is large. Since iron has conductivity, it has an adverse effect on heat dissipation substrates that require insulation. To address such problems, there are techniques such as removing in advance the amount of impurities including iron in the silicon nitride raw material by special acid treatment (Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The silicon nitride raw material subjected to acid treatment is relatively expensive, and furthermore, since the oxide layer on the silicon nitride surface is also removed, the sinterability decreases, causing an increase in the process load due to the high firing temperature and a decrease in strength due to grain growth of the sintered body. On the other hand, in silicon nitride without acid treatment, iron components remain and react with silicon nitride during the firing process to form iron silicide. Since this iron silicide is insulating, it does not adversely affect the insulation as a substrate, but it has low wettability with silicon nitride and causes a defect, resulting in a decrease in strength.

[0006] For these reasons, even in a silicon nitride sintered body using a silicon nitride raw material containing iron derived from the raw material or equipment, a silicon nitride sintered body suitable as an insulating heat dissipation substrate for a power device, which maintains high thermal conductivity and reduces the risk of causing a decrease in strength, has been desired.

[0007] As a result of intensive research, the present inventors have discovered that if it is a compound containing silicon, a Group 4 element, and iron, the wettability with silicon nitride is improved, it is difficult to become a fracture origin, and a high-strength silicon nitride-based sintered body can be obtained, and thus completed the present invention.

[0008] The present invention has been made in view of such circumstances, and an object thereof is to provide a silicon nitride-based sintered body and a silicon nitride-based heat dissipation substrate that maintain high thermal conductivity and have high strength.

Means for Solving the Problems

[0009] (1) To achieve the above object, the silicon nitride-based sintered body of the present invention takes the following means. That is, the silicon nitride-based sintered body of an application example of the present invention is a silicon nitride-based sintered body mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles, and contains a compound containing silicon (Si), a Group 4 element (Ti, Zr, Hf), and iron (Fe), and contains a total of 0.20 to 10 vol% of the compound.

[0010] (2) Further, in the silicon nitride-based sintered body of the application example of (1) above, the average particle size of the particles of the compound is 8.0 μm or less.

[0011] (3) Further, in the silicon nitride sintered body of the application example of (1) above, the Group 4 element is zirconium, and the average particle diameter of the particles of the compound is 8.0 μm or less.

[0012] (4) Further, in the silicon nitride sintered body of the application example of (3) above, when the atomic ratio of zirconium is X, the atomic ratio of iron is Y, and the atomic ratio of silicon is Z, the compound contains a compound satisfying 5 ≦ X ≦ 20, 3 ≦ Y ≦ 15, 70 ≦ Z ≦ 90, and X + Y + Z = 100.

[0013] (5) Further, in the silicon nitride sintered body of any one of the application examples of (1) to (4) above, it further contains a total of 1.0 to 7.5 wt% of rare earth elements and a total of 0.25 to 2.0 wt% of alkali metal elements and alkaline earth metal elements.

[0014] (6) Further, the silicon nitride heat dissipation substrate of the application example of the present invention is made of the silicon nitride sintered body according to any one of (1) to (5) above.

[0015] (7) Further, in the silicon nitride heat dissipation substrate of the application example of (6) above, the flexural strength is 670 MPa or more.

[0016] (8) Further, in the silicon nitride heat dissipation substrate of the application example of (6) or (7) above, the thermal conductivity is 85 W / mK or more.

[0017] (9) Further, in the silicon nitride heat dissipation substrate of any one of the application examples of (6) to (8) above, the thickness in the direction perpendicular to one main surface of the silicon nitride heat dissipation substrate is 220 μm or more and 690 μm or less.

Advantages of the Invention

[0018] According to the silicon nitride sintered body or the silicon nitride heat dissipation substrate of the present invention, it is possible to obtain a silicon nitride sintered body or a silicon nitride heat dissipation substrate with high strength while maintaining high thermal conductivity.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0020] Next, embodiments of the present invention will be described with reference to the drawings. For ease of understanding of the description, the same reference numerals are assigned to the same components in each drawing, and duplicate descriptions are omitted. In the configuration diagrams, the sizes of the respective components are conceptually represented and do not necessarily represent the actual dimensional ratios.

[0021] [Embodiment] [Configuration of Silicon Nitride Sintered Body] First, a silicon nitride-based sintered body according to an embodiment of the present invention will be described. The silicon nitride-based sintered body according to an embodiment of the present invention is a silicon nitride-based sintered body mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles. The silicon nitride-based particles refer to silicon nitride particles or sialon. However, since sialon has a lower thermal conductivity than silicon nitride, it is preferable to set the upper limit to the amount generated within the following range of the Al content. That is, the Al content is preferably 0.1 wt% or less. Mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles means that impurities other than Al and the elements described later may be contained in an amount of 0.5 wt% or less.

[0022] The silicon nitride-based sintered body contains a compound containing silicon (Si), a Group 4 element (Ti, Zr, Hf), and iron (Fe). Further, the silicon nitride-based sintered body contains the compound in a total amount of 0.20 to 10 vol%.

[0023] By forming a compound containing silicon, a Group 4 element, and iron during firing, iron elements derived from the silicon nitride raw material that can serve as fracture initiation points are incorporated, contributing to increased strength. Furthermore, since the amount of iron elements dissolved in the silicon nitride particles also decreases, it is effective for improving thermal conductivity. If the compound is within the above range, high thermal conductivity and high strength can be achieved simultaneously. If the content of the compound is less than this range, residual pores may increase and the strength may decrease. If the content of the compound is more than this range, the area of the compound region with relatively low thermal conductivity increases, so the thermal conductivity may decrease.

[0024] The content of the compound containing silicon, a Group 4 element, and iron can be determined by SEM (Scanning Electron Microscope) observation. Specifically, for the polished surface of the silicon nitride sintered body, five locations are randomly selected and observed at a magnification of 2000 times in a field of view of 120 μm × 90 μm. Next, the areas of each particle recognized as a compound containing silicon, a Group 4 element, and iron are summed up. Then, by summing up the obtained areas, the content can be determined from the area ratio to the area of the screen. Since the compound is randomly generated in the sintered body, the area ratio of the compound in the SEM image of the cross-section of the silicon nitride sintered body can be regarded as the volume ratio in the silicon nitride sintered body. The calculation of the content may use image analysis software such as Winroof. Incidentally, the compound containing silicon, a Group 4 element, and iron can be identified from the results of XRF (X-ray Fluorescence) and XRD (X-ray Diffraction).

[0025] The average particle size of the particles of the compound containing silicon, a Group 4 element, and iron is preferably 8.0 μm or less. If the average particle size of the particles of the compound is within the above particle size range, a high-strength silicon nitride sintered body can be obtained. When the particle size is larger than this range, it becomes a fracture initiation point and at the same time the compound localizes, so the residual pores may increase and the strength may decrease.

[0026] The average particle size of the particles of the compound containing silicon, a Group 4 element, and iron can be determined by SEM observation. Specifically, for the polished surface of the silicon nitride sintered body, five locations are randomly selected, and a field of view of 120 μm × 90 μm is observed at a magnification of 2000 times. Then, the equivalent circle diameter is determined from the area of each particle recognized as the compound containing silicon, a Group 4 element, and iron, and the average particle size can be determined by averaging these. The calculation of the average particle size may use image analysis software.

[0027] The Group 4 element constituting the compound containing silicon, a Group 4 element, and iron is preferably zirconium. When the Group 4 element is zirconium, the compound containing silicon, a Group 4 element, and iron is likely to be formed. The average particle size of the particles of the compound is preferably 8.0 μm or less.

[0028] When the Group 4 element constituting the compound containing silicon, a Group 4 element, and iron is zirconium, when the atomic ratio of zirconium is X, the atomic ratio of iron is Y, and the atomic ratio of silicon is Z, the compound preferably contains a compound satisfying 5 ≦ X ≦ 20, 3 ≦ Y ≦ 15, 70 ≦ Z ≦ 90, and X + Y + Z = 100. When X, Y, and Z are within the above ranges, the compound containing silicon, a Group 4 element, and iron is more likely to be formed.

[0029] The silicon nitride sintered body preferably contains a total of 1.0 to 7.5 wt% of rare earth elements. If the amount of rare earth elements is less than this range, the sinterability may decrease and pores may remain, and the strength of the silicon nitride sintered body may be low. If the amount of rare earth elements is more than this range, the amount of the grain boundary phase may increase, and the thermal conductivity of the silicon nitride sintered body may be low. It is considered that the same effect can be obtained for any rare earth element contained. The rare earth elements can be, for example, yttrium (Y), lanthanum (La), erbium (Er), ytterbium (Yb).

[0030] The silicon nitride sintered body preferably contains a total of 0.25 to 2.0 wt% of alkali metal elements and alkaline earth metal elements. If the alkali metal elements and alkaline earth metal elements are less than this range, the sinterability decreases and pores remain, and the strength of the silicon nitride sintered body may be reduced. If the alkali metal elements and alkaline earth metal elements are more than this range, the amount of the grain boundary phase increases, and the thermal conductivity of the silicon nitride sintered body may be reduced.

[0031] Due to these characteristics, it is possible to obtain a silicon nitride sintered body with higher strength while maintaining high thermal conductivity.

[0032] [Configuration of Silicon Nitride Heat Dissipation Substrate] FIG. 1 is a schematic perspective view showing an example of a silicon nitride heat dissipation substrate according to an embodiment of the present invention. The silicon nitride heat dissipation substrate 10 of the present invention is made of the above-mentioned silicon nitride sintered body. Thereby, while maintaining high thermal conductivity, it is possible to increase the strength and reduce the risk of problems occurring in the circuit board using this. The silicon nitride heat dissipation substrate 10 of the present invention can be suitably used as a heat dissipation substrate for a circuit board for power devices. The silicon nitride heat dissipation substrate 10 is formed in a flat plate shape, for example.

[0033] The silicon nitride heat dissipation substrate 10 preferably has a flexural strength of 670 MPa or more. Thereby, the risk of the silicon nitride heat dissipation substrate 10 being damaged can be reduced.

[0034] The flexural strength can be measured as follows. In accordance with ISO23242, the silicon nitride heat dissipation substrate is processed into a certain thickness × 12 × 25 mm. Then, the flexural strength can be measured by three-point bending with a span of 15 mm. ISO23242 can be applied to ceramic thin plates with a thickness of 0.2 mm to 1.0 mm.

[0035] The silicon nitride heat dissipation substrate 10 preferably has a thermal conductivity of 85 W / mK or more. Thereby, the performance as a heat dissipation substrate can be sufficiently exhibited.

[0036] The thermal conductivity can be measured and calculated as follows. First, the silicon nitride heat dissipation substrate 10 is processed into a size of 0.32 mm×□17 mm, and the thermal diffusivity is measured by the two-dimensional method using a laser flash. Also, the density of the silicon nitride heat dissipation substrate 10 is measured by a method conforming to JIS R1634. The specific heat value is constant at 0.68 cm 2 / sec. Then, the thermal conductivity can be calculated from the measured values of the thermal diffusivity and density by (thermal conductivity)=(density)×(specific heat)×(thermal diffusivity).

[0037] The thickness in the direction perpendicular to one main surface of the silicon nitride heat dissipation substrate 10 is preferably 220 μm or more and 690 μm or less. Thereby, the balance between the strength and heat dissipation of the silicon nitride heat dissipation substrate 10 can be improved. If the thickness is smaller than this range, the strength of the substrate may be low. Also, if the thickness is larger than this range, the heat dissipation performance may decrease.

[0038] [Configuration of Power Device] FIG. 2 is a schematic cross-sectional view showing an example of a power device using the silicon nitride heat dissipation substrate according to an embodiment of the present invention. The power device 100 includes a circuit board 20, a power semiconductor 30, a heat sink 40, and a heat dissipation member 50.

[0039] On one main surface of the circuit board 20, a circuit layer 12 is formed, and on the other main surface facing the one main surface, a conductor layer 14 is formed. The circuit layer 12 and the conductor layer 14 are preferably made of metal, and more preferably made of a metal having copper as a main component. The circuit layer 12 and the conductor layer 14 are joined to the silicon nitride heat dissipation substrate 10 directly or using a joining member such as a brazing material.

[0040] A power semiconductor 30 is mounted on the upper side of the circuit layer 12 of the circuit board 20. The power semiconductor 30 and the circuit layer 12 may be joined using solder 22 or the like. The power semiconductor 30 may be, for example, a semiconductor through which a large current for an EV flows and which is likely to become hot. Since the silicon nitride-based heat dissipation substrate 10 of the present invention has high strength while maintaining high thermal conductivity, even when a large thermal stress is generated in the silicon nitride-based heat dissipation substrate 10 due to the difference in thermal expansion between the silicon nitride-based heat dissipation substrate 10 and the metal joined thereto due to temperature rise, cracks and fractures are less likely to occur.

[0041] A heat sink 40 is joined to the lower side of the conductor layer 14 of the circuit board 20. The heat sink 40 and the conductor layer 14 may be joined using solder 22 or the like. The surface of the heat sink 40 facing the surface joined to the conductor layer 14 is in contact with a heat dissipation member 50 via grease 42. The heat sink 40 is preferably made of metal, and more preferably made of a metal having copper as a main component. The heat dissipation member 50 has heat dissipation fins formed thereon. The heat dissipation member 50 is preferably made of metal, and more preferably made of a metal having copper or aluminum as a main component.

[0042] [Manufacturing Method of Silicon Nitride Sintered Body and Silicon Nitride-based Heat Dissipation Substrate] An example of the manufacturing method of the above-mentioned silicon nitride sintered body and silicon nitride-based heat dissipation substrate is shown below. First, necessary ones are selected from the raw material powders of the silicon nitride sintered body and weighed so as to have a target composition. The raw material powders of the silicon nitride sintered body may be oxides, carbonates, hydroxides, nitrides, etc. of each element contained in the silicon nitride sintered body. The raw material powders of the silicon nitride sintered body include, in addition to silicon nitride, for example, magnesium carbonate, calcium carbonate, yttrium oxide, ytterbium oxide, erbium oxide, lanthanum oxide, zirconium oxide, etc.

[0043] Ethanol is added to these raw material powders and wet-mixed and pulverized in a ball mill for, for example, 6 to 60 hours to obtain a slurry. The slurry is dried by evaporation or using a spray dryer or the like to obtain a mixed powder.

[0044] Next, the mixed powder is filled into a mold, and uniaxial pressing is performed, for example, at a pressure of 30 MPa to form it into a desired shape. Then, CIP treatment (cold isostatic pressing treatment) is performed at a pressure of 150 MPa, for example, to obtain a formed body. The obtained formed body (CIP pressed body) is placed in a silicon carbide mold with its interior coated with BN, for example, and fired by holding it at a maximum temperature of 1800°C to 1900°C for 5 hours to 20 hours in a nitrogen atmosphere of 9 atmospheres to obtain a silicon nitride sintered body.

[0045] When the silicon nitride sintered body is used as a silicon nitride heat dissipation substrate, the outer shape is processed so as to have a predetermined shape and thickness. The processing can be performed by, for example, grinding, polishing, blasting treatment, etc.

[0046] By such a manufacturing method, a silicon nitride sintered body or a silicon nitride heat dissipation substrate with high strength while maintaining high thermal conductivity can be manufactured.

[0047] [Examples, Comparative Examples] (Sample 1) With respect to 100 wt% of silicon nitride powder (average particle size 1.4 μm), 2.0 wt% of magnesium carbonate powder (average particle size 2.5 μm) and 7.0 wt% of yttrium oxide powder (average particle size 1.0 μm) were weighed by external division. Next, using the weighed raw material powders, ball milling was performed to obtain a mixed slurry. For ball milling, the raw material powders and ethanol were put into a resin pot, and ball stones made of YSZ (Y 2 O 3 partially stabilized zirconia) were used, and pulverization and mixing were performed at 60 rpm for 12 hours. The obtained mixed slurry was dried by evaporation to obtain a mixed powder.

[0048] Powder pressing forming of the obtained mixed powder was performed by uniaxial pressing and CIP to produce a formed body. First, after filling the mixed powder into a dedicated mold, preliminary forming was performed by uniaxial pressing at a pressure of 30 MPa. Next, the preliminary formed body was evacuated and put into a dedicated bag, and CIP forming was performed at a pressure of 150 MPa.

[0049] The obtained formed body was fired. The sintering method was atmospheric firing under a gas pressure of 9 atmospheres of nitrogen, and it was held at a maximum temperature of 1900 °C for 5 hours. As the mold, a silicon carbide mold with an inner surface coated with BN was used. In this way, a silicon nitride sintered body of Sample 1 was produced.

[0050] (Sample 2) The silicon nitride sintered body of Sample 2 was produced under the same conditions as the silicon nitride sintered body of Sample 1, except that the mixing time when preparing the mixed slurry was 24 hours.

[0051] (Sample 3) The silicon nitride sintered body of Sample 3 was produced under the same conditions as the silicon nitride sintered body of Sample 1, except that the mixing time when preparing the mixed slurry was 36 hours.

[0052] (Sample 4) The silicon nitride sintered body of Sample 4 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the addition amount of magnesium carbonate powder was 1.0 wt% and the addition amount of yttrium oxide powder was 4.0 wt%.

[0053] (Sample 5) The silicon nitride sintered body of Sample 5 was produced under the same conditions as the silicon nitride sintered body of Sample 4, except that the addition amount of magnesium carbonate powder was 1.5 wt%.

[0054] (Sample 6) The silicon nitride sintered body of Sample 6 was produced under the same conditions as the silicon nitride sintered body of Sample 4, except that the addition amount of magnesium carbonate powder was 2.0 wt%.

[0055] (Sample 7) The silicon nitride sintered body of Sample 7 was produced under the same conditions as the silicon nitride sintered body of Sample 4, except that the addition amount of magnesium carbonate powder was 4.0 wt%.

[0056] (Sample 8) The silicon nitride sintered body of Sample 8 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the addition amount of yttrium oxide powder was 11.0 wt%.

[0057] (Sample 9) The silicon nitride sintered body of Sample 9 was produced under the same conditions as the silicon nitride sintered body of Sample 8, except that the yttrium oxide powder was changed to yttrium oxide powder (average particle size 1.2 μm).

[0058] (Sample 10) The silicon nitride sintered body of Sample 10 was produced under the same conditions as the silicon nitride sintered body of Sample 8, except that the yttrium oxide powder was changed to erbium oxide powder (average particle size 1.4 μm).

[0059] (Sample 11) The silicon nitride sintered body of Sample 11 was produced under the same conditions as the silicon nitride sintered body of Sample 8, except that the yttrium oxide powder was changed to lanthanum oxide powder (average particle size 1.0 μm).

[0060] (Sample 12) The silicon nitride sintered body of Sample 12 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that 2.5 wt% of calcium carbonate powder (average particle size 2.5 μm) was further added.

[0061] (Sample 13) The silicon nitride sintered body of Sample 13 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the ball mill media for preparing the mixed slurry was changed from YSZ to silicon nitride.

[0062] (Sample 14) The silicon nitride sintered body of Sample 14 was produced under the same conditions as the silicon nitride sintered body of Sample 13, except that 1.0 wt% of zirconium oxide powder (average particle size 1.0 μm) was further added.

[0063] (Sample 15) The silicon nitride sintered body of Sample 15 was produced under the same conditions as the silicon nitride sintered body of Sample 1, except that the mixing time when preparing the mixed slurry was 60 hours.

[0064] (Sample 16) The silicon nitride sintered body of Sample 16 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the holding time at the maximum temperature during firing was 20 hours.

[0065] (Sample 17) The silicon nitride sintered body of Sample 17 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the addition amount of yttrium oxide powder was 0.5 wt%.

[0066] (Sample 18) The silicon nitride sintered body of Sample 18 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the addition amount of yttrium oxide powder was 13.0 wt%.

[0067] (Sample 19) The silicon nitride sintered body of Sample 19 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the addition amount of magnesium carbonate powder was 0.3 wt%.

[0068] (Sample 20) The silicon nitride sintered body of Sample 20 was produced under the same conditions as the silicon nitride sintered body of Sample 2, except that the addition amount of magnesium carbonate powder was 8.0 wt%.

[0069] [Various Measurements] The sintered bodies of each obtained sample were evaluated by the following measurements and the like.

[0070] (Measurement of Density) The density of the sintered body of each sample was measured by a method conforming to JIS R1634.

[0071] (Measurement of Elemental Amount) The types and amounts of constituent elements of the sintered body of each sample were measured by fluorescence X-ray analysis (XRF).

[0072] (Identification of Constituent Phases) The constituent phases of the sintered bodies of each sample were identified by XRD analysis. Also, from the results of XRF and XRD, the composition of the compound containing silicon, Group 4 elements, and iron and the contents of other elements were calculated.

[0073] (Measurement of Content and Particle Size) By SEM observation, the content of the compound containing silicon, Group 4 elements, and iron and the average particle size were determined. Specifically, for the polished surface of the silicon nitride-based sintered body, five locations were randomly selected, and a field of view of 120 μm × 90 μm was observed at a magnification of 2000 times. Then, the content was determined from the area of each particle recognized as the compound containing silicon, Group 4 elements, and iron. Also, the equivalent circle diameter was determined from the area of each particle, and the average particle size was determined by averaging them.

[0074] (Calculation of Thermal Conductivity) The sintered body of each sample was processed into 0.32 mm × □17 mm, and the thermal diffusivity was measured by the two-dimensional method using a laser flash. Also, the specific heat value was kept constant at 0.68 cm 2 / sec. Then, the thermal conductivity was calculated from the thermal diffusivity measured by the two-dimensional method and the above density value using (thermal conductivity) = (density) × (specific heat) × (thermal diffusivity).

[0075] (Measurement of Bending Strength) In accordance with ISO 23242, the sintered body of each sample was processed into a thickness of 0.32 × 12 × 25 mm, and the bending strength was measured by three-point bending with a span of 15 mm.

[0076] (Measurement of Fracture Toughness) In accordance with JIS R1607, the measurement surface was mirror-polished, and the fracture toughness value was calculated by the IF method. The formula of Miyoshi was used for the calculation.

[0077] (Results) Figure 3 is a table showing the elemental content of each sample and the characteristics of the grain boundary phase. Note that the Fe and Si in each sample are derived from the raw materials or the equipment used in the manufacturing process. Also, the Group 4 element (Zr) in each sample except Samples 13 and 14 is derived from the YSZ grinding balls used in the manufacturing process. The Group 4 element (Zr) in Sample 14 is derived from the raw materials.

[0078] Figure 4 is a table showing the characteristics of each sample. Samples 1 to 12, 16, 17, and 19 all had a thermal conductivity of 85 W / mK or more and a flexural strength of 670 MPa or more, resulting in high values for both thermal conductivity and flexural strength.

[0079] Sample 13 had a low flexural strength. This is presumably because silicon nitride was used for the grinding balls and no external addition of Group 4 element compounds was made, resulting in no Group 4 elements being contained at all and no compound containing silicon, Group 4 elements, and iron being formed. Sample 14 had a low flexural strength. This is presumably because silicon nitride was used for the grinding balls and the external addition amount of Group 4 element compounds was also small, resulting in too little content of the compound containing silicon, Group 4 elements, and iron.

[0080] Sample 15 had a thermal conductivity within the allowable range but a slightly low value, and a low flexural strength. This is presumably because the mixing time when preparing the mixed slurry was too long, resulting in a large amount of Group 4 elements from the grinding balls being mixed in and too much content of the compound containing silicon, Group 4 elements, and iron. Therefore, it was found that sufficient attention must be paid to the mixing time when allowing Group 4 elements to be mixed in by the wear of the grinding balls.

[0081] Sample 16 had a flexural strength within the allowable range but a slightly low value. This is presumably because the particle size of the compound containing silicon, Group 4 elements, and iron became larger than 8 μm.

[0082] Sample 17 had a flexural strength within the allowable range but a slightly low value. This is presumably because the content of rare earth elements was too small. Sample 18 had a low thermal conductivity. This is presumably because the content of rare earth elements was too large.

[0083] Although the bending strength of Specimen 19 was within the allowable range, it was slightly low. This is presumably because the contents of alkali metal and alkaline earth metal elements were too small. The thermal conductivity of Specimen 16 was low. This is presumably because the contents of alkali metal and alkaline earth metal elements were too large.

[0084] From the above results, it was confirmed that the silicon nitride sintered body and the silicon nitride heat dissipation substrate of the present invention can be enhanced in strength while maintaining a high thermal conductivity.

[0085] The present invention is not limited to the above-described embodiments, and it goes without saying that the present invention extends to various modifications and equivalents included in the spirit and scope of the present invention. Further, the structures, shapes, numbers, positions, sizes, etc. of the components shown in each drawing are for convenience of explanation and can be changed as appropriate.

Explanation of Reference Numerals

[0086] 10 Silicon nitride heat dissipation substrate 12 Circuit layer 14 Conductor layer 20 Circuit board 22 Solder 30 Power semiconductor 40 Heat sink 42 Grease 50 Heat dissipation member 100 Power device

Claims

1. A silicon nitride-based sintered body mainly composed of silicon nitride-based particles and a grain boundary phase surrounding the silicon nitride-based particles, containing a compound containing silicon (Si), a Group 4 element (Ti, Zr, Hf), and iron (Fe), wherein the total content of the compound is 0.20 to 10 vol%, characterized in that it is a silicon nitride-based sintered body.

2. The silicon nitride-based sintered body according to Claim 1, characterized in that the average particle diameter of the particles of the compound is 8.0 μm or less.

3. The silicon nitride-based sintered body according to Claim 1, characterized in that the Group 4 element is zirconium, and the average particle diameter of the particles of the compound is 8.0 μm or less.

4. When the atomic ratio of zirconium is X, the atomic ratio of iron is Y, and the atomic ratio of silicon is Z, the compound contains a compound satisfying 5 ≤ X ≤ 20, 3 ≤ Y ≤ 15, 70 ≤ Z ≤ 90, and X + Y + Z = 100, characterized in that it is a silicon nitride-based sintered body according to Claim 3.

5. The silicon nitride-based sintered body according to any one of Claims 1 to 4, further characterized by containing a total of 1.0 to 7.5 wt% of rare earth elements and a total of 0.25 to 2.0 wt% of alkali metal elements and alkaline earth metal elements.

6. A silicon nitride-based heat dissipation substrate, characterized in that it is composed of the silicon nitride-based sintered body according to Claim 1 or Claim 2.

7. The silicon nitride-based heat dissipation substrate according to Claim 6, characterized in that the flexural strength is 670 MPa or more.

8. The silicon nitride-based heat dissipation substrate according to Claim 6, characterized in that the thermal conductivity is 85 W / mK or more.

9. The silicon nitride-based heat dissipation substrate according to Claim 6, characterized in that the thickness in the direction perpendicular to one main surface of the silicon nitride-based heat dissipation substrate is 220 μm or more and 690 μm or less.

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