Method for producing silicon nitride sintered body

Abstract

To provide a method for producing a silicon nitride sintered body having high thermal conductivity.SOLUTION: There is provided a method for producing a silicon nitride sintered body, which comprises at least (a) a step of mixing a Si powder and a Mg raw material powder to obtain a mixed powder; (b) a step of forming the mixed powder into a slurry to obtain a sheet-shaped molded body; and (c) a step of sintering the molded body by heating in a nitrogen atmosphere to obtain a silicon nitride sintered body, wherein the step (c) comprises: (c1) a step of holding the molded body at a temperature T1 of 1300°C or more and 1800°C or less for 0.5 hours or more during the period from the start of heating to reaching the maximum temperature; and (c2) a step of holding the molded body at a temperature of T2 of 1300°C or more and 1800°C or less, which is higher than the temperature T1, for 0.5 hours or more.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present application relates to a method for producing a silicon nitride sintered body. [Background technology]

[0002] In recent years, high-power electronic circuits have come to be used in a variety of applications, such as power modules for efficiently driving motors used in electric vehicles and hybrid cars, and LED drive circuits. Since these circuits use electronic components that generate heat due to high power, insulating substrates with high heat dissipation properties are used. Ceramic substrates are generally used for such insulating substrates because large substrates can be manufactured relatively inexpensively. In particular, silicon nitride sintered substrates have excellent mechanical strength. For example, Patent Document 1 discloses a silicon nitride sintered substrate with excellent thermal conductivity. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-197226 Summary of the Invention [Problem to be solved by the invention]

[0004] There is a demand for silicon nitride substrates with higher thermal conductivity. The present invention provides a method for producing a silicon nitride sintered body with high thermal conductivity. [Means for solving the problem]

[0005] A method for producing a silicon nitride sintered body according to an embodiment of the present disclosure includes at least step (a) of mixing Si powder and Mg raw material powder to obtain a mixed powder, step (b) of forming a slurry of the mixed powder to obtain a sheet-like compact, and step (c) of heating and sintering the compact in a nitrogen atmosphere to obtain a silicon nitride sintered body, wherein step (c) includes, during the period from the start of heating to the maximum temperature, step (c1) of holding the compact at a temperature T1 of 1300°C or higher and 1800°C or lower for 0.5 hours or longer, and step (c2) of holding the compact at a temperature T2 of 1300°C or higher and 1800°C or lower, which is higher than temperature T1, for 0.5 hours or longer. [Effects of the Invention]

[0006] According to an embodiment of the present disclosure, a method for producing a silicon nitride sintered body having high thermal conductivity is provided. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a transmission electron microscope image showing an example of the silicon nitride sintered body of this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing the structure of the silicon nitride sintered body shown in FIG. [Figure 3] FIG. 3 is a schematic diagram showing the concentration profile of the constituent elements along the line L in FIG. [Figure 4] FIG. 4 is a flowchart showing the method for producing a silicon nitride sintered body according to this embodiment. [Figure 5] FIG. 5 is a schematic diagram showing the temperature profile of the heat treatment carried out during sintering in the method for producing a silicon nitride sintered body according to this embodiment. [Figure 6] FIG. 6 shows an example of the results of composition analysis of the grain boundary phase of Samples 8, 12, and 13 of the example. [Figure 7] FIG. 7(a) is an HAADF image of a silicon nitride particle of sample 8, and (b) shows the concentration profile of the constituent elements measured by EDS at the arrow in the image shown in (a). [Figure 8]FIG. 8(a) is an HAADF image of a silicon nitride particle of sample 12, and (b) shows the concentration profile of the constituent elements measured by EDS at the arrow in the image shown in (a). [Figure 9] FIG. 9(a) is an HAADF image of a silicon nitride particle of sample 13, and (b) shows the concentration profile of the constituent elements by EDS at the arrow in the image shown in (a). DETAILED DESCRIPTION OF THE INVENTION

[0008] The theoretical thermal conductivity of silicon nitride single crystal is believed to be 200 W / m·K or higher, as shown in Patent Document 1, for example (thermal conductivity is measured in units of W (numerator) and m·K (denominator)). However, achieving a comparable thermal conductivity with silicon nitride sintered bodies is difficult. To obtain silicon nitride sintered bodies, sintering aids must be added, and the elements used as sintering aids affect the improvement of thermal conductivity. The present inventors conducted detailed studies of raw materials, sintering aids, and sintering conditions, as well as the structure and element distribution within silicon nitride sintered bodies with high thermal conductivity. As a result, they have devised a silicon nitride sintered body with a thermal conductivity of 130 W / m·K or higher and sufficient mechanical strength, and a method for manufacturing the same. Below, embodiments of the silicon nitride sintered body, circuit board, and method for manufacturing the silicon nitride sintered body of the present disclosure are described with reference to the drawings.

[0009] (Silicon nitride sintered body) Fig. 1 is a transmission electron microscope (TEM) image showing an example of a silicon nitride sintered body of this embodiment, and Fig. 2 is a schematic diagram showing the structure of the silicon nitride sintered body 10 shown in Fig. 1. The silicon nitride sintered body 10 is composed of a plurality of silicon nitride particles 11 and a grain boundary phase 12.

[0010] The silicon nitride particles 11 are the main phase in the silicon nitride sintered body 10. The silicon nitride particles 11 are entirely single crystals, with nitrogen and silicon atoms regularly arranged. When analyzed by, for example, energy dispersive X-ray spectroscopy (EDS) using a transmission electron microscope, the silicon nitride particles 11 contain elements other than nitrogen and silicon. Specifically, the silicon nitride particles 11 contain O and Mg. The silicon nitride particles 11 may further contain RE. Here, the RE is at least one element selected from rare earth elements. More specifically, the RE is at least one element selected from Y, La, Ce, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, and Hf. It is preferable that the RE contain at least Y. The Mg and RE are derived from a sintering aid. O is derived from the sintering aid, is inevitably contained in the raw material Si powder, or is inevitably incorporated during the production of the silicon nitride sintered body 10.

[0011] The silicon nitride particle 11 includes an inner region 11A and an outer region 11B. The outer region 11B is located outside the inner region 11A so as to cover the inner region 11A, and is located on the outer edge of the silicon nitride particle 11. As will be described later, the outer region 11B is in contact with the grain boundary phase 12.

[0012] The outer region 11B contains at least one of Mg and O at a higher concentration than the inner region 11A. The outer region 11B may further contain RE at a higher concentration than the inner region 11A. The concentrations of Mg and O can be determined by scanning the inner region 11A and the outer region 11B perpendicularly to the boundary between the inner region 11A and the outer region 11B using EDS. The boundary between the outer region 11B and the inner region 11A can be determined by the position where the X-ray intensities of Mg and O begin to increase. FIG. 3 is a schematic diagram showing the X-ray intensity distribution from the start point a to b of the line segment L in FIG. 2 when the section of the line segment L is analyzed using EDS. As shown in FIG. 3, the intensities of Mg and O are almost constant in the inner region 11A. However, as the concentration of at least one of Mg and O increases at a certain point near the grain boundary phase 12, the X-ray intensity also increases. The boundary between the outer region 11B and the inner region 11A is determined at the position where the curve of X-ray intensity rises. The position of the rise may be determined, for example, by analysis software provided in a transmission electron microscope capable of analysis by energy dispersive X-ray spectroscopy (EDS).

[0013] The thickness of the outer region 11B is 2.5 nm or less. The thickness of the outer region 11B is preferably 2.2 nm or less, and more preferably 1.6 nm or less. The smaller the thickness of the outer region 11B, the more preferable. The lower limit of the thickness of the outer region 11B is, for example, about 0.5 nm.

[0014] The outer region 11B contains at least one of Mg and O, and therefore has a lower thermal conductivity than the inner region 11A. Therefore, it is preferable that the thickness of the outer region 11B is small. The Mg and O in the outer region 11B are related to the concentrations of Mg and O in the grain boundary phase 12, as will be described later.

[0015] The O content in the inner region 11A is 0.05 mass% or less. Furthermore, component analysis by EDS revealed that both Mg and RE were below the detection limit, and the inner region 11A is highly pure and in a state close to single crystal. As will be described later, the content of a specific element in the inner region 11A is expressed as a mass ratio relative to the total detected elements determined by EDS using a transmission electron microscope. For example, a plurality of silicon nitride particles 11 are arbitrarily selected, and for each, measurements are taken at any two locations within the inner region 11A to determine the average of the contents obtained (the sum of the component amounts at the two locations is divided by 2). The obtained average value is then calculated by averaging the obtained average value for a plurality of silicon nitride particles 11.

[0016] The inner region 11A exhibits high thermal conductivity because it contains almost no Mg or RE derived from the sintering aids. However, the inner region 11A contains a small amount of O, which can reduce the ideal thermal conductivity of silicon nitride, so a low O content is preferable.

[0017] The silicon nitride particle 11 preferably contains one or more fine particles 11C in the inner region 11A. The fine particles 11C contain Mg, O, and RE. They also contain Si and N. The fine particles 11C are amorphous and have a particle size of 100 nm or less. The particle size of the fine particles 11C is the maximum length of the particle, determined by acquiring a transmission electron microscope image or observing the image.

[0018] The fine particles 11C are an amorphous phase composed of Si, N, O, Mg, and RE. Preferably, the silicon nitride particles 11 contain 5 fine particles 11C or more per μm in a direct magnification image or observation image of a transmission electron microscope at 10,000 times or more.

[0019] In the inner region 11A of the silicon nitride particle 11, the fine particles 11C contain at least one element of O, Mg, and RE in a larger amount than the fine particles 11C in the inner region 11A located outside the fine particles 11C. In other words, elements other than Si and N are concentrated to form the fine particles 11C, thereby increasing the purity of the silicon nitride in the part of the inner region 11A other than the fine particles 11C, and improving the thermal conductivity.

[0020] The grain boundary phase 12 is an amorphous phase formed between multiple silicon nitride particles 11. The grain boundary phase 12 includes two-particle grain boundaries 12A and grain boundary triple junctions 12B. The two-particle grain boundaries 12A are located in the region sandwiched between two adjacent silicon nitride particles 11 and are in contact with the two silicon nitride particles 11. The grain boundary triple junctions 12B are located between and are in contact with three or more silicon nitride particles 11 among the multiple silicon nitride particles. The grain boundary triple junctions 12B are connected to the two-particle grain boundaries 12A.

[0021] The grain boundary phase 12 contains Mg and RE at higher concentrations than the silicon nitride particles 11. The ratio Mg / RE of Mg to RE at the two-particle grain boundary 12A of the grain boundary phase 12 satisfies the following formula (1): For example, quantitative analysis of the element Mg and the element RE in mass units is performed by EDS analysis, and Mg / RE is calculated in mass%. Mg / RE≦0.7 (1)

[0022] More preferably, the ratio Mg / RE of Mg to RE at the two-particle grain boundary 12A satisfies the following formula (1'). Mg / RE≦0.5 (1')

[0023] Furthermore, the ratio Mg / RE of Mg to RE at the grain boundary triple junction 12B satisfies the following formula (2). Mg / RE≦1 (2)

[0024] More preferably, the ratio Mg / RE of Mg to RE at the grain boundary triple junction 12B satisfies the following formula (2'). Mg / RE≦0.5 (2')

[0025] Mg / RE is related to thermal history. During the production of silicon nitride sintered body 10, Si, N, O, Mg, and RE constitute a liquid phase. As silicon nitride particles 11 grow from the liquid phase, the concentrations of Mg and RE in the remaining liquid phase increase. The remaining liquid phase ultimately forms grain boundary phase 12. As a result, Mg and RE are present in high concentrations in grain boundary phase 12. Furthermore, while grain boundary phase 12 has a uniform composition overall in the liquid state, during the cooling process, two-particle grain boundaries 12A are formed first, followed by the formation of grain boundary triple junctions 12B. At this time, Mg and RE migrate from two-particle grain boundaries 12A to grain boundary triple junctions 12B.

[0026] Mg and RE are included in sintering aids. Since the melting point of Mg is around 650°C, Mg forms a liquid phase at a relatively low temperature. Since rare earth metals have a relatively high melting point, RE forms a liquid phase at a relatively high temperature. Also, Mg vaporizes at around 1400°C, while RE vaporizes at a higher temperature. For example, Y vaporizes at around 1800°C.

[0027] When producing a silicon nitride sintered body, Mg is generally added in larger amounts than RE in terms of oxide, but for the reasons mentioned above, Mg evaporates in larger amounts than RE. According to the results of the inventors' studies, the silicon nitride sintered body 10 has high thermal conductivity when the two-particle grain boundary 12A satisfies formula (1) and the grain boundary triple point 12B satisfies formula (2).

[0028] Furthermore, the proportion of Mg in the entire silicon nitride sintered body 10 is preferably 0.2 mass% or less. As mentioned above, the amount of Mg added decreases as Mg evaporates during the production of the sintered body, but if the amount of evaporated Mg is small, the above-mentioned ratio Mg / RE in the grain boundary phase does not become sufficiently small. Furthermore, because rare earth metals have an affinity for oxygen, the presence of high concentrations of rare earth metals in the grain boundary phase facilitates the migration of oxygen from silicon nitride particles to grain boundaries. However, if the total amount of Mg is large, Mg inhibits this oxygen migration, and it is thought that the reduction of the O content in the silicon nitride particles does not proceed sufficiently.

[0029] The second-particle grain boundary 12A has a large area of ​​contact with the silicon nitride particle 11, and it is thought that the Mg concentration at the second-particle grain boundary 12A affects the thickness of the outer region 11B of the silicon nitride particle 11. From this perspective, it is preferable that the Mg content at the second-particle grain boundary 12A is low. Specifically, the Mg content at the second-particle grain boundary 12A is preferably 8 mass% or less. The thickness of the second-particle grain boundary 12A is, for example, 0.6 nm or more and 1.6 nm or less.

[0030] The silicon nitride sintered body 10 has a thermal conductivity of 130 W / m·K or more. The silicon nitride sintered body 10 preferably has a thermal conductivity of 140 W / m·K or more, and more preferably has a thermal conductivity of 150 W / m·K or more. The silicon nitride sintered body 10 also has a strength of 500 MPa or more. Here, the strength is a value measured in a three-point bending test according to JIS C6481. The silicon nitride sintered body 10 also has a relative density of 98% or more.

[0031] According to the silicon nitride sintered body 10 of this embodiment, the O content in the silicon nitride particles is 0.05 mass% or less, thereby increasing the purity of the silicon nitride particles 11 and increasing the thermal conductivity of the silicon nitride particles 11. Furthermore, the ratio of Mg to RE at the grain boundary triple junction (Mg / RE) satisfies formula (2), and the ratio of Mg to RE at the two-particle grain boundary (Mg / RE) satisfies formula (1), thereby reducing the Mg concentration in the grain boundary phase. This allows the thickness of the outer region 11B of the silicon nitride particles 11 to be reduced. This increases the thermal conductivity of the silicon nitride sintered body 10. Specifically, it is possible to achieve a silicon nitride sintered body with a thermal conductivity of 130 W / m·K or more. If the thickness of the outer region 11B of the silicon nitride particles 11 is 1.8 nm or less, the thermal conductivity between the silicon nitride particles 11 can be increased. Furthermore, by including fine particles containing high concentrations of Mg and RE in the silicon nitride particles 11, the purity of the inner region 11A of the silicon nitride particles 11 can be increased, resulting in higher thermal conductivity. Specifically, it is possible to realize a silicon nitride sintered body having a thermal conductivity of 150 W / m K or more.

[0032] (circuit board) The circuit board of this embodiment includes a silicon nitride substrate made of the silicon nitride sintered body of this embodiment and a conductive pattern disposed on at least one of two opposing main surfaces of the silicon nitride substrate. The circuit board has, for example, a square or rectangular shape with a side length of 3 cm to 10 cm. The thickness of the circuit board is, for example, 0.3 mm to 3.0 mm.

[0033] The circuit board of this embodiment includes a silicon nitride substrate made of the above-mentioned silicon nitride sintered body, and therefore has excellent thermal conductivity and high bending strength. For example, it has a thermal resistance of 0.2°C / W or less. It also has a durability of 2000 or more cycles in a thermal cycling test.

[0034] (Method for producing sintered silicon nitride) The method for producing the silicon nitride sintered body of this embodiment will now be described. Fig. 4 is a flowchart showing the method for producing the silicon nitride sintered body of this embodiment.

[0035] (1) Mixing process (a) (S1) First, raw material powders are prepared. Si powder is prepared as the silicon nitride raw material. Mg raw material powder and RE raw material powder are also prepared as sintering aids. Let x be the number of moles of Si3N4 obtained when all of the Si powder is nitrided, y be the number of moles of the RE raw material powder converted to the trivalent oxide RE2O3, and z be the number of moles of the Mg raw material powder converted to MgO. When x + y + z is 100 mol%, the RE raw material powder is prepared in a ratio of 0.5 mol% to less than 2 mol%, and the Mg raw material powder is prepared in a ratio of 8 mol% to less than 15 mol%, and then mixed. The remaining proportion of Si powder is 83 mol% to less than 91.5 mol%.

[0036] In this embodiment, Si powder is prepared as the raw material for silicon nitride. Nitriding the silicon using Si powder and N2 gas allows for a relative reduction in the amount of oxygen derived from the raw material. If the proportion of Si powder is less than 83 mol%, the bending strength and thermal conductivity of the resulting silicon nitride sintered body will be too low. On the other hand, if the proportion of Si powder is 91.5 mol% or more, the sintering aid will be insufficient, making it difficult to obtain a dense silicon nitride sintered body.

[0037] The Mg raw material powder contains MgSiN2 as the main component. The Mg raw material powder preferably contains 87 mass% or more of MgSiN2, and more preferably 90 mass% or more of MgSiN2. By using MgSiN2 instead of MgO, the amount of oxygen derived from the raw materials can be reduced, and the amount of O in the silicon nitride sintered body can be reduced.

[0038] If the proportion of Mg raw material powder is less than 8 mol%, it becomes difficult to obtain a dense silicon nitride sintered body. Furthermore, if the proportion of Mg raw material powder is 15 mol% or more, the grain boundary phase increases, and the Mg / RE ratio in the grain boundary phase increases, which tends to reduce the thermal conductivity of the silicon nitride sintered body. Furthermore, if the proportion of MgSiN2 in the Mg raw material powder is less than 87 mass%, as mentioned above, the O content of the silicon nitride sintered body increases, making it difficult to achieve high thermal conductivity.

[0039] As described above, the RE raw material powder is a raw material powder containing at least one element selected from rare earth elements. More specifically, the RE is at least one element selected from Y, La, Ce, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, and Hf. From the viewpoint of increasing the density of the silicon nitride sintered body, it is preferable that the RE contains Y. The RE may be in the form of an oxide or a compound other than oxygen. For example, the RE raw material powder may be a powder of YO, YN, or the like.

[0040] If the proportion of RE raw material powder is less than 0.5 mol%, it becomes difficult to obtain a dense silicon nitride sintered body, and if the proportion of RE raw material powder is 2 mol% or more, the grain boundary phase increases, and the thermal conductivity of the silicon nitride sintered body tends to decrease.

[0041] The Si powder, Mg raw material powder, and RE raw material powder are weighed so as to obtain the above-mentioned raw material powder ratios, and are mixed with a plasticizer (e.g., a phthalic acid-based plasticizer), an organic binder (e.g., polyvinyl butyral, polyisobutyl acrylate), and an organic solvent (e.g., ethyl alcohol, butyl alcohol) in a ball mill or the like to prepare a slurry containing the raw materials. The solid content of the slurry is preferably 30% by mass or more and 70% by mass or less.

[0042] (2) Molding process (b) (S2) After degassing and thickening the slurry, a sheet-like compact (green sheet) is formed, for example, by the doctor blade method. The thickness of the compact is appropriately set taking into consideration the thickness of the silicon nitride sintered body to be formed and the sintering shrinkage rate. The compact formed by the doctor blade method is usually a long strip, so it is punched or cut to the desired shape and size.

[0043] After the molding step, the molded body may be degreased by heating it to 400 to 800° C. in order to remove the organic binder and plasticizer.

[0044] (3) Sintering process (c) (S3) The compact is introduced into a heating furnace and heated in a nitrogen atmosphere to sinter the raw materials, producing a silicon nitride sintered body. Figure 5 shows a schematic example of the temperature profile T of the heating furnace during the sintering process and the pressure profile P of the nitrogen gas introduced into the heating furnace. As explained below, the preferred holding time for each step varies depending on the furnace temperature. Therefore, the time axes in Figure 5 are not necessarily spaced equally. Therefore, the slopes of the lines in the temperature profile T (solid line) and the pressure profile P (dashed line) do not necessarily accurately reflect the rate of temperature increase / decrease and the rate of pressure increase / decrease. Furthermore, two lines with the same slope in the temperature profile T and the pressure profile P do not necessarily represent the same rate of temperature increase or pressure increase.

[0045] The sintering step (c) includes a temperature rising period D1, a sintering period D2, and a cooling period D3. The temperature rising period D1 is the period from the start of heating until the maximum temperature in the temperature profile T is reached.

[0046] The temperature rise period D1 includes a step of nitriding the raw material Si powder with N2 gas to convert it into Si3N4 particles, as shown in reaction formula (3). 3Si+2N2 → Si3N4 (3)

[0047] The sintering period D2 is a period during which the maximum temperature is maintained. The cooling period D3 is a period during which the temperature of the heating furnace is lowered to room temperature after the sintering period D2 ends. Here, the maximum temperature refers to a temperature of 1800°C or higher.

[0048] The temperature inside the heating furnace in the sintering step can be, for example, the temperature measured by a radiation thermometer through an observation window provided in the heating furnace, measuring a target such as a compact inside the furnace. Specifically, the heating furnace can be provided with a sintering container in which the compact is placed, a carbon cylindrical wall on the inner periphery of which the sintering container is placed, and a target located near the outer periphery of the cylindrical wall.

[0049] (3-1) Temperature rise period D1 The temperature rise period D1 includes a decarbonization period d1, a nitriding process d2, a first temperature holding period d3, and a second temperature holding period d4. The first temperature holding period d3 and the second temperature holding period d4 are nitridation periods for nitriding Si.

[0050] (i) Decarbonization period d1 First, the ambient temperature in the heating furnace is raised from room temperature to the temperature range of the decarbonization period d1. The heating rate is, for example, 60°C / hr. Once the ambient temperature in the heating furnace reaches a temperature between 900°C and 1300°C, it is maintained within this temperature range for 0.5 hours to 2 hours. The atmosphere in the heating furnace is preferably under reduced pressure, i.e., lower than 1 atmosphere. For example, a pressure of 80 Pa or less is preferred. If carbon remains during sintering, voids are likely to form in the silicon nitride sintered body. For this reason, the carbon in the compact is removed by maintaining the compact under reduced pressure.

[0051] If the ambient temperature is lower than 900°C, carbon may not be sufficiently removed. If the temperature is higher than 1300°C, the sintering aid may also be removed. It is more preferable that the ambient temperature in the heating furnace be 1000°C or higher and 1250°C or lower.

[0052] (ii) Nitriding process d2 After the decarbonization period is over, the compact is slowly heated from the temperature at the end of the decarbonization period to the temperature at the start of the first temperature holding period d3, which is the nitriding period. Specifically, the heating temperature is increased at a rate of 1°C / min or less to increase the temperature of the compact.

[0053] The temperature increase rate in the nitriding step d2 is more preferably 0.5°C / min or less. In order to produce a silicon nitride sintered body under practical conditions, the temperature increase rate is preferably 0.1°C / min or more.

[0054] During the nitriding step d2 and subsequent periods, the heating furnace is preferably filled with a nitrogen atmosphere. Specifically, nitrogen or a mixed gas containing nitrogen as the main component and an inert gas such as argon, or a mixed gas containing nitrogen gas with approximately 3% or less of hydrogen, can be used. The pressure inside the heating furnace is preferably between 1 and 20 atmospheres. Although FIG. 5 shows the pressure of the nitrogen atmosphere as constant during the nitriding step d2 and subsequent periods, it may vary within the pressure range described above.

[0055] (iii) a first temperature holding period d3 and a second temperature holding period d4 During the first temperature holding period d3, the temperature of the atmosphere inside the heating furnace is held at a temperature T1 of 1300° C. to 1800° C. The holding time is preferably 0.5 hours to 10 hours.

[0056] Thereafter, in a second temperature holding period d4, the temperature of the atmosphere in the heating furnace is held at a temperature T2 higher than temperature T1, 1300° C. or higher and 1800° C. or lower. The holding time is preferably 0.5 hours or higher and 10 hours or lower.

[0057] Although not shown, thereafter, as a third temperature holding period d5, the temperature of the atmosphere in the heating furnace may be held at a temperature T3 higher than temperature T2, that is, at 1300° C. or more and 1800° C. or less. The holding time is preferably 0.5 hours or more and 10 hours or less.

[0058] For example, the temperature T1 is 1300°C or higher and 1500°C or lower, and the temperature T2 is 1500°C or higher and 1700°C or lower. As an example, the temperature T1 is 1400°C, and the temperature T2 is 1600°C. The temperature T3 is 1800°C.

[0059] These processes nitride the Si powder to produce silicon nitride. By nitriding the Si powder at two temperatures, deformation such as waviness of the sheet-shaped compact can be suppressed even when the compact size is large.

[0060] (3-2) Sintering period D2 After the nitriding period is over, the ambient temperature in the heating furnace is set to a temperature higher than 1800°C, and the compact is sintered. The ambient temperature is preferably between 1800°C and 2000°C, more preferably between 1800°C and 1950°C, and even more preferably between 1800°C and 1900°C. If the ambient temperature exceeds 2000°C, the vaporization of the sintering aid and the decomposition of silicon nitride become intense, making it difficult to obtain a dense silicon nitride sintered body. As long as the ambient temperature is between 1800°C and 2000°C, the ambient temperature does not have to be constant and can be changed (for example, gradually increased).

[0061] The retention time is from 1 hour to 30 hours, preferably from 2 hours to 20 hours, and more preferably from 5 hours to 15 hours.

[0062] Holding the compact at the above-mentioned temperatures promotes grain growth of the silicon nitride crystals, and Mg, RE, and O are removed from the silicon nitride crystals and migrate to the grain boundary phase, thereby increasing the purity of the silicon nitride crystals. In addition, some of the Mg, RE, and O within the silicon nitride crystals form amorphous fine particles within the silicon nitride crystals, thereby increasing the purity of the silicon nitride crystals.

[0063] (3-3) Cooling period D3 After the sintering period D2, the ambient temperature in the heating furnace is gradually lowered to approximately room temperature. During the cooling period D3, the liquid phase between the resulting silicon nitride particles is cooled and solidified as a grain boundary phase, thereby fixing the position of the main phase of the resulting silicon nitride particles. To rapidly solidify the liquid phase and maintain uniformity in the grain boundary phase distribution, the cooling rate is preferably 100°C / hr or higher, more preferably 300°C / hr or higher, and most preferably 500°C / hr or higher. For practical purposes, a cooling rate of 500°C / hr or higher and 600°C / hr or lower is preferred. Cooling at such a rate suppresses crystallization of the solidifying sintering aid, forms a grain boundary phase primarily composed of a glass phase, and increases the bending strength of the silicon nitride sintered body. By lowering the ambient temperature to 1200°C, the grain boundary phase is fixed and the entire silicon nitride sintered body becomes solid. Therefore, the cooling rate at temperatures lower than that is not particularly limited.

[0064] According to the method for producing sintered silicon nitride of this embodiment, by using Si powder as the Si source and a Mg raw material powder containing 87 mass% or more of MgSiN2, it is possible to reduce the amount of O contained during production and further reduce the amount of O in the sintered silicon nitride. Furthermore, by maintaining the compact at two temperature stages during the nitriding of the Si powder, it is believed that this contributes to the precipitation of amorphous fine particles with concentrated impurities within the silicon nitride particles.

[0065] (Example) The following describes the results of producing silicon nitride sintered bodies under various conditions and examining their properties.

[0066] 1. Manufacturing of silicon nitride sintered body [Samples 1-15] Si powder, YO powder (YO in Table 1) which is the RE raw material powder, and Mg raw material powder containing MgSiN and MgO (Mg compound in Table 1) were prepared. The number of moles of SiN obtained when all the Si powder is nitrided is defined as x, the number of moles of YO powder as y, and the number of moles of Mg raw material powder converted to MgO as z. YO powder and Mg raw material powder were weighed out in the proportions shown in Table 1, and Si powder was weighed out in a proportion equivalent to the number of moles converted to SiN, calculated as x = 100 - yz. The proportion of MgSiN in the Mg raw material powder is as shown in Table 1.

[0067] These powders were mixed to form a slurry, and a sheet-shaped compact (green sheet) was produced by the doctor blade method.

[0068] The compacts were placed in a heating furnace and sintered under the conditions shown in Table 2 to obtain ceramic sintered compacts of Samples 1 to 15.

[0069] [Table 1]

[0070] [Table 2]

[0071] 2. Characteristic Measurement (1) The relative density, thermal conductivity, and three-point bending strength were measured for the prepared samples 1 to 15. The equipment and conditions used for the measurements are as follows: 1) Relative density The density of the substrate material obtained by Archimedes' method is divided by 3.24 g / cm3 and expressed as a percentage. Board size: 30mm x 30mm 2) Thermal conductivity The substrate was processed to 5 mm x 5 mm and measured by the laser flash method. Device: Netzsch Thermal diffusivity measurement device: LFA 467 Hyper Flash Thermal conductivity is calculated as specific heat x density x thermal diffusivity. The specific heat was assumed to be constant at 0.68 kJ / (kg·K), and the density and thermal diffusivity obtained from the measurements were inserted into the above equation for calculation. 3) Three-point bending strength JIS Support distance: 20mm Board size: 30mm x 25mm x 0.32mmt Crosshead speed: 0.5 mm / min Equipment: Shimadzu Autograph AGS-100NX

[0072] (2) The prepared samples 1 to 15 were observed using a TEM to check for the presence or absence of fine particles within the silicon nitride particles. The amount of oxygen in the center of the silicon nitride particles was measured using EDS. The ratio of Mg to Y at the grain boundary between two particles and at the grain boundary triple junction was also examined. Furthermore, EDS line analysis was used to obtain the concentration profiles of Mg and O at the grain boundary between two adjacent particles, and the depth of Mg and O, which is the thickness of the outer region (i.e., the length from the interface between the silicon nitride particle and the grain boundary phase to the interior of the silicon nitride particle), was measured. The equipment and conditions used for the measurements were as follows: 1) Sample preparation Thinned using ion milling Equipment: PIPS (precision ion milling equipment): Gatan Model 691 2) Observation Equipment: Transmission electron microscope (TEM): JEOL JEM-ARM200F Observation conditions: Accelerating voltage: 200 kV, Electron diffraction: Camera length: 80 cm STEM (HAADF) mode: 5C, quantitative analysis: 30Lsec TEM-EDX conditions: Line analysis: 256Pix, 1.0msec / Pix STEM: scanning transmission electron microscope HAADF image: HAADF-STEM (High-Angle Annular Dark Field Scanning TEM) images are obtained by scanning a finely focused electron beam over a sample and detecting the high-angle scattered electrons among the transmitted electrons with an annular detector. Heavy elements appear dark in STEM images and bright in HAADF-STEM images. Since contrast proportional to the atomic weight (Z) is obtained, they are also called Z-contrast images. 3) Elemental analysis The oxygen content of the silicon nitride particles was measured by crushing the sintered body in the following manner, removing the grain boundary phase of the sintered body, and then measuring the oxygen content of the silicon nitride particles, which are the main phase. (a) Sintered body crushing The sintered body was pulverized in a vibration mill to a powder of 75 μm or less. (b) Removal of grain boundary phase by acid treatment Hydrofluoric acid was added to remove the grain boundary phase, and then the sample was washed with distilled water. The supernatant was removed, and the sample was repeatedly washed by shaking with distilled water and centrifuged. After the washing was completed, the neutrality was confirmed using pH test paper. Ethanol was then added, and the sample was then washed by shaking with ethanol (99.5%) and centrifuged, after which the silicon nitride particles were extracted by vacuum heating and drying. (c) Measurement Oxygen analysis and quantification by peak separation were performed using a LECO TCH-600 with a temperature rise of 8°C / s and a sample amount of approximately 20 mg. Oxygen peaks appear on both the low-temperature and high-temperature sides, but the low-temperature side represents oxygen on the particle surface, while the high-temperature side represents oxygen inside the particle. Therefore, only the peak on the high-temperature side was approximated to a normal distribution, and the oxygen amount was calculated from its area.

[0073] Sintered bodies were prepared under similar conditions, and the amounts of Y and Mg components (RE) in the entire sintered body were determined by pulverizing the sintered body as described above and quantitatively analyzing the amounts of Y and Mg by ICP analysis.

[0074] 3. Results and Discussion (1) Sintering aid ratio As shown in sample 5 in Table 1, when the Y content is less than 0.5 mol%, the relative density of the resulting sintered body is less than 98%. This is thought to be because a lack of sintering aid prevents uniform silicon nitride grain growth, preventing a dense sintered body. Furthermore, as shown in sample 6, when the Y content is 2 mol% or more, the thermal conductivity is less than 130 W / m K. This is thought to be because the excess sintering aid, with its high boiling point, causes a large amount of Y to remain in the grain boundary phase of the resulting sintered body, thickening the grain boundary phase and increasing the Y concentration in the grain boundary phase, thereby reducing the thermal conductivity of the grain boundary phase.

[0075] Similarly, as shown in sample 7, when the Mg content is less than 8 mol%, the relative density of the resulting sintered body is less than 98%. This is thought to be due to a lack of sintering aid, which prevented the production of a dense sintered body. Furthermore, as shown in sample 8, when the Mg content is 15 mol% or more, the thermal conductivity is less than 130 W / m·K. This is thought to be because a large amount of Mg remains in the grain boundary phase of the resulting sintered body, and the high Mg concentration in the grain boundary phase reduces the thermal conductivity of the grain boundary phase. Furthermore, as the Mg content increases, the amount of MgO, a raw material powder other than MgSiN2, contained in the Mg raw material powder also increases. This is thought to result in an increase in the amount of O remaining in the sintered body, reducing the thermal conductivity.

[0076] As shown in sample 9, even if the proportion of Mg raw material powder is less than 15 mol%, if the proportion of MgSiN2 is less than 87 mass%, the thermal conductivity is 130 W / m K or less. It is thought that the amount of MgO, a raw material powder other than MgSiN2, contained in the Mg raw material powder also increases, increasing the amount of O remaining in the sintered compact and reducing the thermal conductivity.

[0077] (2) Oxygen content in silicon nitride particles As shown in samples 1–4 and 10–15, when the oxygen content in the silicon nitride particles is 0.05 mass% or less, thermal conductivities of 130 W / m·K or more are obtained. In particular, samples 13–15, with a low oxygen content of 0.02 mass%, achieve thermal conductivities of 150 W / m·K or more. In contrast, as shown in samples 5, 8, and 9, when the oxygen content in the silicon nitride particles is greater than 0.05 mass%, thermal conductivities are below 130 W / m·K. Thus, the oxygen content of silicon nitride particles is related to thermal conductivity, with a tendency for lower oxygen content to increase conductivity. In samples 6 and 7, the oxygen content in the silicon nitride particles is 0.05 mass% or less, but the thermal conductivities are lower than 130 W / m·K. This is thought to be due to the fact that the Y content is outside the preferred range, resulting in reduced thermal conductivity for the reasons mentioned above.

[0078] (3) Amount of Mg in the sintered body As shown in samples 1 to 4 and 10 to 15, when the Mg content in the entire sintered body is 0.20 mass% or less, a thermal conductivity of 130 W / m K or more is obtained. In contrast, as shown in samples 5 to 9, when the Mg content in the entire sintered body is more than 0.20 mass%, the thermal conductivity is below 130 W / m K. This is thought to be because the Mg content in the sintered body is not low enough, and therefore Mg inhibits the ability of Y to attract oxygen in the grain boundary phase.

[0079] (4) Mg / Y ratio of grain boundary phase As shown in sample 8, when the Mg / Y ratio at the grain boundary triple junction is greater than 1, the thermal conductivity is less than 100 W / m K. This is thought to be because the presence of an excess of Mg in the raw material powder results in a high proportion of Mg in the grain boundary phase, which reduces the thermal conductivity of the grain boundary phase and prevents heat conduction between adjacent silicon nitride particles.

[0080] Figure 6 shows the composition ratios of the constituent elements at the grain boundary triple junction and two-particle grain boundary for samples 8, 12, and 13. During the production of sintered silicon nitride, when a liquid phase containing large amounts of Mg and Y is cooled, it is thought that the two-particle grain boundary is formed first, followed by the grain boundary triple junction. At this time, Mg, Y, and O are thought to migrate from the two-particle grain boundary to the grain boundary triple junction, so the amounts of Mg, Y, and O at the grain boundary triple junction are greater than at the two-particle grain boundary. Furthermore, sample 8 contains a large amount of Mg at the grain boundary triple junction due to the large amount of Mg raw material powder added.

[0081] (5) Depth of Mg and O As shown in sample 8, when the Mg depth exceeds 2.5 nm, that is, when the thickness of the outer region of the silicon nitride particle exceeds 2.5 nm, the thermal conductivity is less than 100 W / m K. This is thought to be because the inclusion of Mg thickens the outer region, which has low thermal conductivity, making it difficult for the outer region to conduct heat from adjacent silicon nitride particles to the inner region.

[0082] Figure 7(a) is a high-angle annular dark-field (HAADF) image of a silicon nitride particle in sample 8, and Figure 7(b) shows the concentration profile of the constituent elements measured by EDS at the arrow in the image shown in (a). Figures 8(a), (b) and 9(a), (b) show the HAADF images and concentration profiles of the constituent elements of sample 12 and sample 13, respectively.

[0083] These figures show that the concentrations of Mg and O are nearly constant within the inner region, but the curves of these concentration profiles rise at the boundary with the outer region, where the concentrations of Mg and O become higher. Furthermore, if the concentrations of Mg and O at the grain boundary between two particles increase, the concentrations of Mg and O in the outer region of the silicon nitride particle also increase, which is thought to shift the position of the boundary between the outer and inner regions and increase the thickness of the outer region.

[0084] (6) Fine particles in silicon nitride particles No fine particles were observed within the silicon nitride particles in samples 1 to 12, but five or more fine particles (five or more particles) were observed in samples 13 to 15. Since the thermal conductivity of samples 13 to 15 was 150 W / m K or higher, it is thought that the generation of fine particles containing Mg, Y, and O caused these elements within the silicon nitride particles to gather within the particles, increasing the purity of the silicon nitride in areas other than the fine particles and also increasing the thermal conductivity.

[0085] From the above embodiments, it can be seen that a silicon nitride sintered body with a thermal conductivity of 130 W / m K or higher can be produced by using raw materials prepared so that the RE raw material powder is 0.5 mol% or more but less than 2 mol%, the Mg raw material powder is 8 mol% or more but less than 15 mol%, and the Si powder is 83 mol% or more but less than 91.5 mol% in terms of Si3N4, totaling 100 mol%. It can also be seen that the thermal conductivity of the silicon nitride sintered body can be 130 W / m K or higher when the oxygen content in the silicon nitride particles is 0.05 mass% or less, the ratio of Mg to RE at the grain boundary triple junction (Mg / RE) is 1 or less, the ratio of Mg to RE at the grain boundary between two particles (Mg / RE) is 0.7 or less, and the thickness of the outer region of the silicon nitride particle is 2.5 nm or less. Furthermore, it can be seen that a thermal conductivity of 150 W / m·K or more can be achieved by having the amount of oxygen in the silicon nitride particles be 0.02 mass% or less, the thickness of the outer region of the silicon nitride particles (e.g., corresponding to the "depth of Mg and O" in Table 1) be 1.6 nm or less, and at least one of the silicon nitride particles containing fine particles having a particle size of 100 nm or less that contain Mg, O, and RE.

[0086] (7) Temperature profile of the sintering process Although the temperature rise rate during the annealing period for Samples 1 and 10-12 was less than 1°C / min, they achieved thermal conductivities of 132 W / m·K or higher. This is thought to be because the slow temperature rise rate allowed the silicon nitriding reaction to proceed more slowly during the annealing period, resulting in the formation of silicon nitride before the sintering period. Samples 13-15 also achieved thermal conductivities of 150 W / m·K or higher. This is thought to be because sintering at higher sintering temperatures allowed for a lower oxygen content within the silicon nitride particles and a lower magnesium content in the sintered body as a whole, thereby reducing the oxygen content within the silicon nitride particles and the magnesium content at the grain boundaries. [Industrial Applicability]

[0087] The silicon nitride sintered body, circuit board, and method for producing the silicon nitride sintered body of the present disclosure can be used for a variety of applications, and are particularly suitable for use in circuit boards for a variety of applications that have high thermal conductivity and high strength. [Explanation of symbols]

[0088] 10. Sintered silicon nitride 11 Silicon nitride particles 11A Inner area 11B Outer area 11C fine particles 12 Grain boundary phase 12A 2-grain grain boundary 12B Grain boundary triple point

Claims

1. A step (a) of mixing at least Si powder and Mg raw material powder to obtain a mixed powder; (b) forming a slurry from the mixed powder to obtain a sheet-shaped compact; (c) a step of sintering the compact by heating in a nitrogen atmosphere to obtain a silicon nitride sintered body; Including, In the step (c), during the period from the start of heating to the maximum temperature being reached, A step (c1) of holding the molded body at a temperature T1 of 1300 ° C. or higher and 1800 ° C. or lower for 0.5 hours or longer; a step (c2) of holding the compact at a temperature T2 that is 1300°C or higher and 1800°C or lower and is higher than the temperature T1 for 0.5 hours or longer; Including, A method for producing sintered silicon nitride.

2. The mixed powder is a Si powder obtained by nitriding the Si powder. ３ N ４ The mole number x of RE raw material powder is converted into trivalent oxide RE. ２ O ３ where y is the number of moles of the RE raw material powder when converted to MgO, and z is the number of moles of the Mg raw material powder when converted to MgO, and x + y + z is 100 mol%, the number of moles of the RE raw material powder y is 0.5 mol% or more and less than 2 mol%, and the number of moles of the Mg raw material powder z is 8 mol% or more and less than 15 mol%, A method for producing the silicon nitride sintered body according to claim 1.

3. The step (c) includes, before the step (c1), A step (c3) of holding the compact at a temperature of 900°C or higher and lower than 1300°C for 0.5 hours or longer in an atmosphere lower than 1 atmosphere, 3. A method for producing the silicon nitride sintered body according to claim 1 or 2.

4. The heating temperature is increased at a rate of 1° C. / min or less from the end of the step (c3) to the start of the step (c1). The method for producing the silicon nitride sintered body according to claim 3.

5. The silicon nitride sintered body contains silicon nitride particles, The silicon nitride particles have an O content of 0.05 mass% or less. A method for producing the silicon nitride sintered body according to any one of claims 1 to 4.

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