Sintered moulding based on silicon nitride

Abstract

Sintered moulding based on silicon nitride, comprising beta-Si3N4, and 0.8% by mass to 2.5% by mass of magnesium, 2.5% by mass to 4.0% by mass of yttrium and 0.6% by mass to 3.6% by mass of zirconium, the % by mass ratio Mg + Zr / Y being between 0.3 and 2.0 and the sintered moulding having a theoretical final density >98%, a strength >650 MPa and a thermal conductivity >25 mm^2 / s, and a method for producing the sintered moulding of the invention.

Description

[0001] Sintered molded bodies based on silicon nitride

[0002] The invention relates to a sintered body based on silicon nitride, which, in addition to beta-SiSn4, contains portions of yttrium oxide (Y2O3), magnesium oxide (MgO) and zirconium dioxide (ZrO2), as well as a process for producing the same, and the use of the sintered body in power electronics.

[0003] Sintered bodies based on silicon nitride are well known and used in a wide variety of applications. One area of ​​application for such sintered bodies is substrates for power electronics. Such substrates feature an electrically conductive metallization layer and electrically insulating properties, such as ceramics made of AlN, Al2O3, ZTA (zirconium dioxide reinforced aluminum oxide), or BeO.

[0004] In the field of power electronics, substrates made of Al2O3, ZTA, and AlN are primarily used. Beryllium oxide is interesting for special applications due to the toxicity of its dusts. AlN has a high thermal conductivity of 180–220 W / mK, but its flexural strength of approximately 400 MPa is too low for many applications. Furthermore, many AlN substrates exhibit poor thermal shock resistance, which has a particularly negative impact on the service life of metal-coated ceramic circuit boards, such as DOB (direct copper bonded) or AMB (active metal brazing) substrates. Al2O3 substrates are inexpensive but have the disadvantage of a lower thermal conductivity of 19–30 W / mK compared to AlN. Even substrates made of ZTA, with strengths of about 600 MPa and thermal conductivities of 20 to 26 W / mK, have significantly higher strengths compared to AIN substrates, but also significantly lower thermal conductivities.

[0005] Components based on silicon nitride achieve high mechanical strength, even at elevated temperatures, high fracture toughness, and high thermal shock resistance. Y2O3 and Al2O3 are usually added to conventional SisN^ ceramic components as sintering additives. However, these components exhibit low thermal conductivities of less than 30 W / mK. The production of components made of SisN4 (silicon nitride) is generally cost-intensive; in particular, shaping with the aim of achieving the highest possible packing density of the inorganic raw materials represents a challenge for the economical production process of silicon nitride substrates. Conventional processes such as axial or isostatic pressing are challenging for large areas with simultaneously low substrate thickness. Therefore, conventional silicon nitride sintered bodies are not suitable for electronic materials, such asSemiconductor substrates are unsuitable because they offer little advantage for applications in power electronics compared to well-known materials such as ZTA, AIN and AI2O3.

[0006] When considering the sintering processes, the disadvantages of hot pressing are that only simple shapes are possible and only a few components can be sintered per sintering cycle. This process is primarily used for research purposes and for small series with simple geometries. In the HIP process (hot isostatic pressing), temperatures of up to 2000°C and high nitrogen partial pressures of up to 2000 bar are common. This allows for more complex shapes, but the sintering costs are very high.

[0007] The present invention is based on the object of providing a method for producing a sintered compact that eliminates the aforementioned disadvantages. Furthermore, the method aims to obtain a sintered compact based on silicon nitride.

[0008] Furthermore, the method for producing a sintered body is to be provided that makes it possible to produce thinner substrates than when using other ceramic materials such as AIN or AI2O3.

[0009] A further object is to provide a method and a sintered molded body that enables a weight reduction of the power electronic component and a reduction of the thermal resistance.

[0010] A further object is to provide a method and a sintered molded body as an electrically insulating substrate body for the field of power electronics.

[0011] A further task is to provide a sintered molded body that also enables a longer service life of the power electronics components manufactured from it.

[0012] These objects could be achieved by a sintered molded body according to claims 1 to 7 and a method according to claims 8 to 14, as well as the use of the sintered body according to claim 15. The subclaims have preferred embodiments.

[0013] A first embodiment of the sintered molded body according to the invention comprises, in addition to beta-SisN4, 0.6 wt.%-3.6 wt.% zirconium Zr, preferably 0.7 wt.%-3.0 wt.% zirconium, more preferably 0.8 wt.%-2.5 wt.% zirconium, even more preferably 0.9 wt.%-2.0 wt.%, even more preferably 0.9 wt.%-1.5 wt.%, even more preferably 0.9 wt.%-1.3 wt.%, most preferably 0.9 wt.%-1.1 wt.%

[0014] The quantities of sintering additives relate to the total mass of the silicon nitride sintered molded body. In a preferred second embodiment, the sintered molded body according to the invention comprises, in addition to the features of embodiment 1, additionally 0.8 mass% to 2.5 mass% magnesium Mg and 2.5 mass% to 4.0 mass% yttrium Y, preferably 0.9 mass% to 2.2 mass% magnesium Mg and 2.5 mass% to 3.8 mass% yttrium, more preferably 0.9 mass% to 2.2 mass% magnesium Mg, and 2.5 mass% to 3.8 mass% yttrium Y. In a third preferred embodiment, the sintered molded body according to the invention additionally or alongside the features of the previous embodiment preferably comprises 0.9 mass% to 2.2 mass% magnesium Mg, 2.5 mass% to 3.8 mass% % yttrium Y and 0.8 mass % to 2.5 mass % zirconium Zr, more preferably 0.9 mass % to 2.2 mass % magnesium Mg, 2.5 mass % to 3.8 mass % yttrium Y and 0.9 mass % to 2.0 mass % zirconium Zr.

[0015] If the sintered body comprises zirconium which is added to the slip as abrasion from ZrO2 grinding balls during the grinding process, this is the fourth preferred embodiment, which can be implemented alongside or in addition to the previous embodiments.

[0016] In a fifth preferred embodiment, the sintered molded body according to the invention comprises, in addition to or in addition to the features of the previous embodiments, the feature that the zirconium is added to the slip exclusively as abrasion from ZrO2 grinding balls during the grinding process.

[0017] In a further embodiment, magnesium (Mg) is preferably completely amorphous in the grain boundary phase of the silicon nitride sintered body according to the invention. Yttrium (Y) and zirconium (Zr) are also amorphous or partially crystalline in the grain boundary phase of the silicon nitride sintered body according to the invention, but in a preferred embodiment, they at least partially form a further crystalline phase comprising Y 2 Zr 2 O 7 , Y 5 Zr 4 O 12 , and / or Y 8 Zr 9 O 11 . This ensures, on the one hand, that no cubic mixed phase Zr (C, N) forms on the surface during sintering of the individual foils, which drastically reduces the dielectric strength of the silicon nitride sintered body and renders the sintered body unsuitable for use as an insulating substrate for power electronics.On the other hand, the additional crystalline phase increases the thermal conductivity compared to substrates with the purely amorphous sintering additives yttrium (Y) and zirconium (Zr).

[0018] In a preferred embodiment, the silicon nitride sintered body according to the invention comprises only beta-silicon nitride in addition to the sintering additives.

[0019] In a preferred embodiment, the sintered molded body has a mass % ratio of Mg + Zr / Y between 0.3 and 2.0, preferably between 0.4 and 1.75, particularly preferably between 0.5 and 1.5. If the sintered molded body according to the invention contains sintering additives in a mass % ratio of Mg + Zr / Y of < 0.3, the sintering activity of the raw material mixture is too low to produce a silicon nitride sintered body according to the invention with high strength using economical processes. This is due to the fact that the final density of the sintered body is too low in relation to the theoretical final density. If the sintered molded body according to the invention contains sintering additives in a mass % ratio of Mg + Zr / Y of > 2.0, low strengths and / or low thermal conductivity are achieved because the high Mg content leads to the formation of an increased proportion of amorphous grain boundary phase.

[0020] In one embodiment, the silicon nitride sintered molded body has a final density of > 98.0%, preferably > 98.5%, particularly preferably > 99.0% of the theoretical final density in order to realize the properties according to the invention such as high strength and high thermal conductivity.

[0021] The silicon nitride sintered body according to the invention is, in one embodiment, a beta-SisN component with a thermal conductivity > 25 mm A 2 / s, preferably > 30 mm A 2 / s, particularly preferably > 35 mm A 2 / s, flexural strength > 650MPa, preferably > 750MPa, particularly preferably > 800MPa, an electrical breakdown strength > 20kV / mm, preferably > 25kV / mm, particularly preferably > 30kV / mm and a high volume resistivity p1 between 2.0x10 A 15Qcm and 6.0x10 A 15Qcm at 25°C and 1000V and a specific volume resistance p2 between 3.5x10 A 13Qcm and 8.0x10 A13Qcm at 250°C and 1000V, and a ratio p2 / p1 < 0.2. The values ​​p2 and p1, in particular p2 / p1, ensure that the sintered body functions as an electrical insulator in the temperature range between 25°C and 250°C.

[0022] According to the invention, the sintered molded body is produced by a film casting process.

[0023] A first embodiment of a method for producing a sintered molded body based on silicon nitride, comprising the following process steps,

[0024] Preparation of a slip from organic auxiliaries, as well as beta-SisN^ yttrium (Y) and magnesium (Mg), mixed grinding of the slip in a mill with grinding balls based on zirconium dioxide, tape casting of the ground slip,

[0025] - Heat-treating the foils to form a sintered body, wherein the sintered body contains 89.9-96.1 wt.% beta-Si3N4, 2.5-4 wt.% Y calculated as Y2O3, 0.8-2.5 wt.% Mg calculated as MgO, and 0.6-3.6 wt.% Zr calculated as ZrO2. Preferably, the sintered body contains 0.8 wt.% to 2.5 wt.% magnesium Mg, 2.5

[0026] Ma.% to 4.0 Ma.% yttrium Y and 0.6 Ma.% to 3.6 Ma.% zirconium Zr, preferably 0.9 Ma.% to 2.2 Ma.% magnesium Mg, 2.5 Ma.% to 3.8 Ma.%.% yttrium Y and 0.8 Ma.% to 2.5 Ma.% zirconium Zr, more preferably 0.9 Ma.% to 2.2 Ma.% magnesium Mg, 2.5 Ma.% to 3.8 Ma.%.% yttrium Y and 0.9 Ma.% to 2.0 Ma.% zirconium Zr.

[0027] The slip for the tape casting process is produced from organic and inorganic components. The inorganic components, i.e., silicon nitride and the sintering additives Mg and Y, are mixed with the organic components, such as dispersants and / or plasticizers, and blended with at least one solvent. The resulting slip is homogenized and deagglomerated in a mill using grinding media.

[0028] In order to produce the sintered body, a raw material composition is used which, in addition to silicon nitride (SisN^), also includes portions of yttrium oxide (Y2O3), magnesium oxide (MgO) and / or magnesium hydroxide (Mg(OH)2) and / or magnesium carbonate (MgCO5) as inorganic components.

[0029] Tape casting is a suitable process for component dimensions of substrates, i.e., thin, flat formats with side lengths of, for example, 5-14 cm x 5-19 cm and a thickness of 0.1-1 mm. One way to increase sintering activity is to increase the proportion of sintering additives, especially magnesium.

[0030] In order to increase the sintering activity and to obtain dense (>98% of the theoretical final density) substrates by means of tape casting at low sintering temperatures (<1900°C) and under low nitrogen partial pressure (<50 bar) despite the low packing density of the inorganic components, a slip comprising the organic components and SiL, Y2O3, and at least one Mg compound, preferably selected from MgO, Mg(OH)2, and / or MgCO3, is used according to the invention.

[0031] In a preferred embodiment, the slip comprises the following inorganic components:

[0032] Silicon nitride raw material (mixture of alpha- and beta-SiSn^, 0.8 wt.% to 2.5 wt.% magnesium Mg (calculated as MgO), 2.5 wt.% to 4 wt.% yttrium Y (calculated as Y2O3),

[0033] The inorganic constituents particularly preferably make up 100%, i.e. the slip contains no further inorganic constituents apart from the aforementioned inorganic constituents. The slip is preferably ground in a mill before film casting with the addition of grinding media, preferably with the addition of ceramic grinding media, particularly preferably with the addition of grinding media made of silicon nitride and / or zirconium dioxide, and especially preferably with the addition of grinding media made of zirconium dioxide. Grinding media made of zirconium oxide are particularly suitable because they have high fracture toughness and high density. The grinding media for grinding in a drum mill are preferably spherical, egg-shaped or cylindrical and, in a spherical and / or egg-shaped configuration, have an average diameter of between 5 mm and 25 mm, preferably between 7.5 mm and 20 mm, particularly preferably between 10 mm and 15 mm.In a cylindrical embodiment, the grinding media have a length between 8 mm and 30 mm, preferably between 10 mm and 23 mm, particularly preferably between 12 mm and 20 mm, and a diameter between 5 mm and 25 mm, preferably between 7.5 mm and 20 mm, particularly preferably between 10 mm and 15 mm. Spherical grinding media are preferred, among other things, due to the lower risk of the grinding media breaking during the grinding process. The grinding media for grinding in a stirred ball mill are preferably spherical and have an average diameter between 0.3 mm and 1.5 mm, preferably between 0.5 mm and 1.2 mm, particularly preferably between 0.6 mm and 1.00 mm.

[0034] The use of ZrO2 grinding media ensures that the Zr is distributed very finely and homogeneously in the slurry, thereby improving the sintering activity.

[0035] In a preferred second embodiment of the method, the method comprises, in addition to or in addition to the features of embodiment 1, the feature that the slip does not contain any zirconium or zirconium dioxide before grinding or zirconium or zirconium dioxide is added to the slip before grinding.

[0036] In a particularly preferred embodiment of the silicon nitride sintered body according to the invention, the introduced Zr originates exclusively from the abrasion of the ZrO2 grinding media. This allows for the production of silicon nitride-based substrates with excellent properties despite the low green density of the inorganic raw materials.

[0037] In a preferred third embodiment of the process, the process comprises, in addition to or in addition to the features of the preceding embodiments of the process, the feature that the slip comprises at least one solvent, preferably at least one organic solvent, and / or at least one dispersant and / or at least one binder, preferably at least one organic binder, as organic auxiliaries. The inorganic raw materials are dispersed in an organic solvent or an organic solvent mixture (e.g. alcohol / ketone, alcohol / aromatic). As further organic components, further components such as dispersants (e.g. fatty acid esters or polyacrylates), binders (e.g. polyvinyl butyrals, polyvinyl alcohols) and plasticizers (e.g. phthalates, benzoic acid, carboxylic acid esters) can be added to the raw material mixture.

[0038] In a preferred fourth embodiment of the method, the method comprises, in addition to or in addition to the features of the above embodiments of the method, the feature that the mixed grinding of the slip is milled before the film casting in an attritor, a stirred ball mill, ball mill or drum mill, preferably in an attritor and / or a stirred ball mill, particularly preferably in a stirred ball mill.

[0039] One aspect of the slip production process is the homogeneous distribution of the individual components. To achieve this, in a preferred embodiment, the raw material mixture is milled in an attritor, a stirred ball mill, a ball mill, an attritor, or a drum mill, preferably in an attritor and / or a stirred ball mill, particularly preferably in a stirred ball mill, particularly preferably in a drum mill.

[0040] In one embodiment, the raw material mixture is milled in a drum mill for 20 to 120 hours, preferably for 30 to 90 hours, particularly preferably for 40 to 75 hours, at a speed of 15 rpm to 50 rpm, preferably from 20 rpm to 40 rpm, particularly preferably from 22 rpm to 35 rpm. In a further embodiment, the raw material mixture is milled in a stirred ball mill with a mill filling level with grinding media between 40 vol% and 65 vol%, particularly preferably between 45 vol% and 60 vol% and to a specific energy input between 0.02 kWh / kg and 0.2 kWh / kg and for a period of 2 to 40 hours, preferably from 4 to 30 hours, particularly preferably from 6 to 20 hours. This ensures sufficient homogeneity of the sintering additives and avoids an inhomogeneous distribution of the sintering activity within the green body. This ensures that the properties of the sintered body, such asDensity, strength or thermal conductivity have comparable values ​​across the entire component and / or there is no distortion during the sintering process and uneven sintered bodies are obtained.

[0041] In addition, ZrC grinding media have a higher density than silicon nitride grinding balls. This allows more grinding energy to be introduced into the slurry in the same time period under comparable grinding parameters. The grinding time can therefore be shortened when using ZrC>2 grinding media compared to silicon nitride grinding media. In a preferred embodiment, grinding balls based on zirconium dioxide containing up to 10 wt% hafnium oxide and between 2 and 8 wt% yttrium oxide are used. The grinding media have a density of >5.8 g / cm 3Since zirconium usually occurs naturally together with hafnium, the extracted zirconium dioxide also contains up to 10 wt.% hafnium oxide. Although separation is industrially possible, the additional process step increases the cost of synthesizing HfO2-free ZrO2. Since Hf and Zr have a similar property profile, the sintered body according to the invention is not negatively affected by a proportion of up to 10 wt.% HfO2 in the grinding media, but the grinding process is more economical than when using HfO2-free grinding media. Yttrium oxide stabilizes the zirconium dioxide in its tetragonal phase, which is metastable at room temperature. This improves the fracture strength compared to unstabilized zirconium dioxide, which reduces the risk of fractures of the grinding media during the grinding process. A density of the grinding media >5.8 g / cm 3also reduces the risk of breakage of the grinding media during the grinding process, as yttrium stabilized ZrO2 with a density < 5.8g / cm 3 have too high a porosity.

[0042] When using a drum mill, the weight ratio between the slurry and the grinding media is preferably between 0.3 and 0.5, particularly preferably between 0.35 and 0.45. The solids content, which describes the percentage of inorganic raw materials in the slurry, is between 40% and 60%, preferably between 44% and 58%, particularly preferably between 48% and 54%. This achieves a grinding effect sufficient for the deagglomeration and homogenization of the inorganic and organic raw materials. In addition, the abrasion generated during grinding and the grain size of the inorganic components in the slurry remain constant, provided the grinding media used, the grinding time, and the ratio between slurry and grinding media remain constant. This makes it possible to produce substrates with very good properties based on silicon nitride despite the low green density of the inorganic raw materials.

[0043] During the grinding or homogenization process, impurities are created in the slurry due to abrasion of the grinding media and / or mill lining. Therefore, in a preferred embodiment, care must be taken to avoid the generation of further impurities that could negatively impact the manufacturing process of the silicon nitride sintered bodies and / or the properties of the final silicon nitride sintered body. Carbon-containing materials such as silicon carbide, for example, negatively influence the sintering activity of the green sheets and ensure that a lower theoretical final density is achieved after sintering. Therefore, materials containing silicon carbide should be avoided.

[0044] Conventional silicon nitride grinding media contain Al2O3 as a sintering additive, which is incorporated into the crystal lattice of the beta-silicon nitride phase of the silicon nitride sintered body in the form of impurities from grinding media abrasion during the sintering process. This leads to a reduction in the thermal conductivity of the resulting silicon nitride sintered body. Preference is given to using grinding media that neither negatively impact the production process nor the properties of the silicon nitride sintered bodies according to the invention, such as native silicon nitride grinding media. Native means that the grinding media contain the same additives as the silicon nitride sintered bodies according to the invention. Particular preference is given to using grinding media that positively influence the production process and / or the properties of the silicon nitride sintered bodies according to the invention, such as the use of zirconium dioxide grinding media.In a particularly preferred embodiment, the zirconium is added only in the form of grinding media abrasion, i.e. no zirconium is added to the raw material composition, but rather introduced into the slurry through grinding media abrasion during slip production. The grinding media abrasion from grinding media made of zirconium dioxide has a positive effect on the sintering activity of the green body, as it is distributed very finely and as homogeneously as possible throughout the green body. This results in less distortion of the sintered bodies during the sintering process, which has a positive effect on the properties, in particular their homogeneous distribution across the entire sintered body. To ensure that sufficient zirconium dioxide is introduced into the slurry as grinding media abrasion, in one embodiment the slurry is ground in an agitator ball mill, ball mill or drum mill, with cylindrical, spherical or egg-shaped grinding media made of zirconium oxide as described above.

[0045] Preferably thin films, which preferably have a green thickness of 0.15 mm to 1.5 mm, are formed from the slip in order to obtain corresponding green parts, i.e. not yet sintered films, for further processing. For film casting, in one embodiment the slip is poured into a reservoir onto a carrier material, preferably behind an upstream doctor blade, the so-called pre-blade. A doctor blade, a so-called doctor blade, is attached in the drawing direction. The gap between the doctor blade and the carrier material determines the thickness of the cast film. The slip applied to the carrier material is then dried in a drying zone and the solvent is evaporated. In this way, a dry green film is obtained on the carrier material which has sufficient green strength to be machined in this state. For example, required shapes can be punched out.The green films produced in this way are cut to the desired length and width and, if necessary, provided with a release agent such as boron nitride to prevent mutual adhesion.

[0046] In order to keep the diffusion paths of the inorganic raw material particles as short as possible during the sintering process, the foil-cast green foil in one embodiment has a green density of the inorganic raw materials > 1.40 g / cm 3 , preferably > 1.45 g / cm 3 , particularly preferably > 1.50 g / cm 3 Films with a green density < 1.40 g / cm 3 can no longer be sufficiently compacted with the preferred type and quantity of sintering additives in the described manufacturing process, i.e. not to a theoretical final density >98%.

[0047] After applying the release agent, the green sheets are placed in a sintering crucible and placed in a furnace. There, an initial heat treatment is carried out for 10 to 60 hours, preferably at temperatures between 400°C and 650°C, to remove the organic additives contained in the green body or green sheet. After the heat treatment, the weight loss of the green bodies is determined to ensure that preferably >98%, particularly >99%, of the organic material has been removed. If more than 2% of the organic material remains in the green body, the carbon produced during the sintering process negatively affects the sintering activity of the green body.

[0048] This is followed by the sintering step, in which the green foils are sintered into dense silicon nitride sintered bodies. Sintering is preferably carried out by gas pressure sintering. In a preferred embodiment, gas pressure sintering is carried out at low nitrogen partial pressures between 10 bar and 100 bar, preferably between 10 bar and 75 bar, particularly preferably between 10 bar and 50 bar, and at temperatures of 1800°C to 2000°C, preferably between 1825°C and 1975°C, particularly preferably between 1850°C and 1950°C. It is also possible to sinter the foils using other state-of-the-art sintering processes, such as hot pressing or hot isostatic pressing.

[0049] The sintered molded bodies according to the invention are then cut to the desired length and width using a laser process. Finally, in a preferred embodiment, a surface treatment is carried out to clean the sintered surfaces of release agent and to smooth the surface. Preferred processes for this purpose include brushing, grinding, honing, polishing, and / or blasting the surface. This removes process-related impurities on the surface and silicon nitride crystallites protruding from the surface, thus reducing surface roughness.

[0050] In order to increase the sintering activity during gas pressure sintering at nitrogen partial pressures between 10 bar and 100 bar, preferably between 10 bar and 75 bar, particularly preferably between 10 bar and 50 bar and at temperatures of 1800°C to 2000°C, preferably between 1825°C and 1975°C, particularly preferably between 1850°C and 1950°C, in a preferred embodiment the inorganic constituents of the raw material mixture are used with the following fineness: an average grain size (d50) between 0.5 pm and 1.5 pm and particularly preferably with a low coarse fraction (d90) between 2.5 pm and 3.5 pm. In this range the inorganic constituents have a sufficient fineness to be essentially dissolved in the liquid phase during sintering. This accelerates the sintering process. A small average grain size (<0.5 pm) of the raw material mixture means that an increased proportion of organic additives is required to stabilize the tape casting slip.This leads to a lower green density relative to the inorganic components of the raw material mixture. The sintering activity of the raw material mixture is negatively affected, resulting in a theoretical final density of <98%. An average grain size of the raw material mixture >1.5 μm leads to lower sintering activity, as the raw material components dissolve more slowly in the liquid phase. As a result, only a theoretical final density of <98% is observed under the selected sintering conditions.

[0051] The sintered body according to the invention is used as an electrically insulating substrate body, for example in the field of power electronics, since it has a high thermal conductivity >25mm A2 / s and a high electrical volume resistance of >1.0x1014Ωcm at 25°C and 1000V and >1.1x1013Ωcm at 250°C and 1000V, and an electrical breakdown strength of >20kV / mm, preferably >25kV / mm, particularly preferably >30kV / mm. A breakdown strength of <20 kV / mm reduces the electrically insulating properties of the sintered body and increases the risk of a short circuit when used in the field of power electronics. Due to the high flexural strength of >650MPa, preferably >750MPa, particularly preferably >800 MPa of the sintered body made of silicon nitride according to the invention, thinner substrates can be produced from this sintered body than when using other ceramic materials such as AIN or Al2O3. This enables a reduction in the weight of the power electronic component and a reduction in the thermal resistance.

[0052] The very good thermal shock resistance of the sintered molded body according to the invention also enables a longer service life of the power electronics components manufactured from it.

[0053] Inorganic raw material offset

[0054] Inorganic raw material blending describes the composition of a mixture of inorganic raw materials. The inorganic raw materials are silicon nitride (Si3N4) and sintering additives. Sintering additives can be magnesium (Mg), yttrium (Y), and zirconium (Zr), as well as their compounds.

[0055] Grain boundary phase

[0056] The grain boundary phase is the phase in the microstructure of the silicon nitride sintered body. It consists of the sintering additives and the SiO2 contained in the silicon nitride raw material, as well as impurities contained in the raw materials or introduced during the manufacturing process. It is in this phase that the beta-silicon nitride needles are embedded. The grain boundary phase can be amorphous or partially crystalline.

[0057] slip

[0058] A slip is a dispersed mixture of organic auxiliaries and inorganic raw materials in a solvent or solvent mixture.

[0059] Organic aids

[0060] Organic auxiliaries include dispersants, binders and solvents.

[0061] The green body is called a film after the slip has been poured and the solvent or solvent mixture has been removed.

[0062] Sintered molded bodies

[0063] The sintered body is created from the film through a single- or multi-stage heat treatment. The sintered body contains no organic additives.

[0064] Methods

[0065] The properties of the sintered molded bodies according to the invention were determined as described below.

[0066] Green density of the film (GD(F))

[0067] The green density of the film (GD(F)) refers to the calculated density of the green body comprising the organic and inorganic components. To calculate the green density of the inorganic raw materials (GD(A)), the volumetric proportion of organic additives is subtracted from GD(F).

[0068] Flexural strength

[0069] The flexural strength was determined according to DIN EN 843, but the sample thickness was between 0.25 mm and 0.4 mm. Flexural strength specimens were tested according to ISO 14704, and the 3-point flexural strength was determined. The green density was measured geometrically using the following formula:

[0070] D = mA /

[0071] Where: D - density, m - mass, V - volume

[0072] Mass percentages (Ma. %)

[0073] The mass percentages (mass %) of the inorganic components are calculated based on the total mass of the sintered body. Any Mg present is assumed to be MgO, Y as Y2O3, and Zr as ZrO2, and are used as the basis for all calculations.

[0074] The final density of the sintered bodies was determined based on the Archimedes principle according to DIN EN ISO 1183-1.

[0075] T emperature conductivity

[0076] To measure the thermal diffusivity, the samples were coated with carbon as described in DIN EN 821-1 and analyzed using the laser flash method.

[0077] In the case of silicon nitride sintered bodies, measuring thermal diffusivity is preferred over calculating thermal conductivity because the value can be determined directly from the sintered bodies. Thermal conductivity is defined as:

[0078] A = a*D*c

[0079] Where: A - thermal conductivity; a - thermal diffusivity; c - specific heat capacity.

[0080] The specific heat capacity can be determined either from a value calculated from the additives and beta-silicon nitride used, taking into account the initial weight, or from a value measured using, for example, differential scanning calorimetry or differential thermal analysis on a separate sintered body that differs geometrically from the sintered bodies according to the invention. However, both methods are subject to error, since with the calculated value, the phases in the sintered body are no longer present in the form of the reactants. For example, Y2O3 is no longer present as crystalline sesquioxide. New amorphous and / or crystalline phases are formed in the sintered body, which have specific heat capacities that differ from those of the reactants.Since both the measurement of the specific resistance is subject to measurement error and the separate sintered bodies for analysis have a different surface to volume ratio than the substrate sintered bodies to be examined, the calculation of the specific heat capacity results in different values ​​depending on the method selected.

[0081] Theoretical final density

[0082] The theoretical final density is a density for the sintered body, calculated from the densities of the sintering additives and the density of beta-silicon nitride. The densities of the sintering additives and of beta-silicon nitride are taken into account according to their mass percentages. Since new phases with densities different from those of the reactants can form in the sintered body, the theoretical final density can also be >100%. Examples

[0083] The sintered body according to the invention, as well as its production and use, are explained in more detail below using examples. Examples 1 and 2 are comparative examples in which the ZrO2 was introduced into the slurry as a weighted portion with the other materials. In Examples 3 to 7 according to the invention, no ZrO2 was weighed in. The ZrO2 is introduced into the slurry exclusively during the grinding process via the abrasion of ZrO2 grinding balls.

[0084] To produce the examples shown in Table 1, the following sintering additives were used: yttrium oxide (Y2O3) with a purity >99.0 wt. % Y2O3, a specific surface area between 10.0 m 2 / g and 16.0m 2 / g, an average grain size d50 between 0.6pm and 0.9pm, and a grain width distribution (d90-d10 / d50) between 0.5 and 3.0, magnesium oxide (MgO) with a purity >99.5wt. % MgO and a specific surface area between 5.0m 2 / g and 9.0m 2 / g and zirconium dioxide (ZrO2) with a specific surface area between 11.0m 2 / g and 17.0m 2 / g and an average grain size between 0.03pm and 0.05pm. The silicon nitride raw material used has an average grain size d50 between 1.5pm and 1.9pm, as well as a grain width distribution (d90-d10) / d50 between 1.9 and 2.8, and a specific surface area between 4.0m 2 / g and 5.0m 2 / g, a purity of >98.5 wt.% Si3N4, and an a-Si3N4 content of >85 wt.%. The oxygen content in the silicon nitride raw material is <1.0 wt.% and the Al content <0.1 wt.% to limit the loss of thermal conductivity due to impurities in the silicon nitride raw material.

[0085] The quantities of sintering additives refer to 100% by mass of the inorganic raw materials; the remaining portion up to 100% by mass of the inorganic raw materials is made up with Si3N4 raw material.

[0086] Example 1

[0087] To produce Example No. 1 from Table 1, the inorganic raw materials were dispersed and deagglomerated in water using tetraethylammonium hydroxide as a dispersant in a stirred ball mill or an attritor and with the aid of Si3N4 grinding media to produce a homogeneous slurry with a solids content between 48 and 54%. Between 6% and 10% by mass of organic additives (pressing aids, binders, plasticizers) were added to the slurry. The slurry was then dried in a spray tower at temperatures between 200°C and 260°C, and a spray granulate suitable for axial pressing was produced. The spray granulate was placed in a mold and compressed at 1250 bar to 2500 bar to form a green body with a green density of the inorganic raw materials of 1.79 ± 0.01 g / cm 3 pressed.

[0088] Sintered molded bodies according to Example No. 1 were further processed after sintering by grinding to form bending fracture bars and test specimens for measuring thermal conductivity

[0089] Example 2

[0090] To prepare Example 2, the inorganic raw materials were homogenized and deagglomerated in a drum mill in 35 wt.% to 40 wt.% of an organic solvent mixture of aromatics / alcohol, in this case toluene / ethanol, using 0.7 wt.% to 1.00 wt.% of a dispersant, in this case a fatty acid ester, together with 7.0 wt.% to 8.0 wt.% of the organic binder consisting of polyvinyl butyrals. The solvent mixture can contain up to 2.5 wt.% water. In Example 2 from Table 1, grinding media made of Al2O3 were used. Thin films with a green thickness of approximately 0.4 mm were cast from the slurry. The green density of the inorganic raw materials was between 1.45 g / cm 3 and 1.50g / cm 3After the solvent had dried, the green sheets were punched into shape, coated with a release agent such as boron nitride, and stacked into the sintering crucibles.

[0091] To remove the organic additives from the green bodies produced according to the example, an initial heat treatment, known as debinding, was carried out between 500 and 600 °C for 20 to 40 hours. The weight loss of the green bodies was then determined to ensure that > ​​98% of the organic material had been removed. If more organic material remains in the green bodies, this leads to reduced sintering activity, as the organic additives converted to carbon during sintering negatively affect the liquid-phase sintering of silicon nitride-based materials.

[0092] After the initial heat treatment, the green bodies were sintered. These were sintered at temperatures between 1850°C and 2000°C for 1.5 to 5 hours under a nitrogen pressure of 10 to 100 bar. After cooling, the sintered substrates were removed from the sintering crucibles, cleaned, and laser-cut to the desired length and width.

[0093] Examples 3-7

[0094] To produce Examples 3 to 7 from Table 1, the inorganic raw materials were homogenized and deagglomerated in a drum mill in 35% to 40% by mass of an organic solvent mixture of aromatics / alcohol, in this case toluene / ethanol, using 0.7% to 1.00% by mass of a dispersant, in this case a fatty acid ester, together with 7.0% to 8.0% by mass of the organic binder consisting of polyvinyl butyrals. The solvent mixture can contain up to 2.5% by mass of water. ZrO2 grinding media were used for homogenization; the ZrO2 content in these examples results from grinding media abrasion; no additional ZrO2 was added as a powdered raw material. The amount of ZrO2 was determined on the sintered bodies by X-ray fluorescence analysis according to DIN ISO 12677. Thin films with a green thickness of approximately 0.4 mm were cast from the slurry. The green density of the inorganic raw materials was between 1.45 g / cm 3and 1.50g / cm 3 After the solvent had dried, the green sheets were punched into shape, coated with a release agent such as boron nitride, and stacked into the sintering crucibles.

[0095] To remove the organic additives from the green bodies produced as described above, an initial heat treatment, known as debinding, was also carried out at temperatures between 500 and 600 °C for 20 to 40 hours. The weight loss of the green bodies was then determined to ensure that > ​​98% of the organic material had been removed. If more organic material remains in the green bodies, this leads to reduced sintering activity, as the organic additives converted to carbon during sintering negatively affect the liquid-phase sintering of silicon nitride-based materials.

[0096] After the initial heat treatment, the green bodies were sintered. These were sintered at temperatures between 1850°C and 2000°C for 1.5 to 5 hours under a nitrogen pressure of 10 to 100 bar. After cooling, the sintered substrates were removed from the sintering crucibles, cleaned, and laser-cut to the desired length and width.

[0097] Tabe le 1: Properties of the sintered bodies

[0098] Looking at the results in Table 1, it is clear that the axially pressed samples from Example 1 have very good thermal conductivity but poor flexural strength. Example 2 has average thermal conductivity but very poor flexural strength. These two examples are unsuitable for use as sintered compacts as electrically insulating substrates for power electronics. Example 3, which has a similarly high ZrO2 content to Example 2, has comparable thermal conductivity but, surprisingly, a much higher flexural strength, making it suitable for use as a sintered compact as an electrically insulating substrate for power electronics.The same applies to examples 4 and 7. However, the sintered bodies of examples 5 and 6 are excellently suited for use as sintered bodies as electrically insulating substrate bodies for the field of power electronics. They have an extremely high flexural strength and at the same time very high thermal conductivities.

[0099] The use of ZrO2 grinding media also ensures that Zr is distributed very finely and homogeneously in the slurry, thereby improving sintering activity. In a particularly preferred embodiment of the silicon nitride sintered body according to the invention, the introduced Zr originates exclusively from the abrasion of the ZrO2 grinding media.

[0100] Examples with properties of the different mills

[0101] Table 2: Properties of various mills

[0102] One aspect in the production of the slip is the homogeneous distribution of the individual components. Table 2 shows an overview of the advantages and disadvantages of the mill types used in a preferred embodiment. The more + a mill type has in a category, the more economical its use is in the production of the sintered body according to the invention. In addition, higher strengths can be achieved when using the preferred mill types attritor and agitated ball mill in the sintered body compared to the use of a ball mill or drum mill. The minimum grinding time required for a sintered body according to the invention is shorter with an attritor and agitated ball mill than with a ball or drum mill. In addition, the homogeneous distribution of the components is improved when using an attritor and agitated ball mill compared to a ball and drum mill.Since the grinding chamber of ball mills, drum mills, and attritors is enclosed, this limits the possible batch size of the slurry. In a stirred ball mill, the slurry is pumped from an external container into the grinding chamber, ground there, and discharged back into an external container. This limits the batch size by the size of the external container(s), allowing significantly larger batch sizes compared to other mill types, making the grinding process step more economical.

Claims

Patent claims 1. Sintered body based on beta-SisN^ and 0.6 Ma%-3.6 Ma% zirconium Zr.

2. Sintered body according to claim 1, characterized in that the sintered body comprises 0.8Ma% to 2.5Ma% magnesium Mg, 2.5Ma% to 4.0Ma% yttrium Y and 0.6Ma% to 3.6Ma% zirconium Zr.

3. Sintered molded body according to one of the preceding claims, characterized in that the zirconium is added to the slip as abrasion from ZrC>2 grinding balls during the grinding process.

4. Sintered body according to one of the preceding claims, characterized in that the zirconium is added to the slip exclusively as abrasion from ZrC>2 grinding balls during the grinding process.

5. Sintered molded body according to one of the preceding claims, characterized in that the masses of silicon nitride, yttrium, magnesium and zirconium amount to 100 mass %.

6. Sintered molded body according to one of the preceding claims, characterized in that the sintered molded body has a final density >98.0%, preferably >98.5%, particularly preferably >99.0% of the theoretical final density.

7. Sintered body according to one of the preceding claims, characterized in that the sintered body has a flexural strength > 650 MPa, preferably > 750 MPa, particularly preferably > 800 MPa and / or the sintered body has a thermal conductivity > 25 mm A 2 / s, preferably >30 mm A 2 / s, especially preferred >35mm A 2 / s.

8. A process for producing a sintered molded body based on silicon nitride, comprising the following process steps, Preparation of a slip from organic auxiliaries, such as beta-SiSl^, yttrium (Y) and magnesium (Mg), Mixing and grinding the slurry in a mill with grinding balls based on zirconium dioxide, Tape casting of the ground slip, - Heat treatment of the foils to form a sintered body, wherein the sintered body contains 89.9-96.1 Ma% beta-Si3N4, 2.5-4 Ma% Y calculated as Y2O3, 0.8-2.5 Ma% Mg calculated as MgO and 0.6-3.6 Ma% Zr calculated as ZrO2.

9. A process for producing a sintered body according to claim 8, wherein the slip does not contain zirconium before grinding.

10. A process for producing a sintered molded body according to claim 8 to 9, wherein the slip comprises at least one solvent, preferably at least one organic solvent, and / or at least one dispersant and / or at least one binder, preferably at least one organic binder, as organic auxiliaries.

11. The method according to any one of claims 8 to 10, wherein the mixed grinding of the slip is milled before the film casting in an attritor, a stirred ball mill, ball mill or drum mill, preferably in an attritor and / or a stirred ball mill, particularly preferably in a stirred ball mill.

12. The method according to any one of claims 8 to 11, wherein the slip is ground for a period of 2 to 40 hours, preferably 4 to 30 hours, particularly preferably 6 to 20 hours, wherein the solids content of the slip during grinding is between 70% and 90% and the filling level of the mill with grinding balls is between 40 and 65%, particularly preferably between 45 and 60% and the specific energy input of the grinding is between 0.02 kWh / kg and 0.2 kWh / kg, wherein the grinding balls used are based on zirconium dioxide and contain up to 10 mass% hafnium oxide and between 2 and 8 mass% yttrium oxide, and have a density >5.8 g / cm 3and have a diameter between 0.3 and 1.5 mm.

13. A process for producing a sintered molded body according to one of claims 8 to 12, characterized in that the green body of the film casting process has a green density of the inorganic raw materials > 1.40 g / cm 3 , preferably > 1.45 g / cm 3 , particularly preferably > 1.50 g / cm 3 has.

14. A process according to any one of claims 8 to 13, wherein the obtained Sintered body with a density of > 3.20g / cm 3 , a flexural strength > 650MPa, preferably > 750MPa, particularly preferably > 800MPa, a Thermal conductivity > 25mm A 2 / s, preferably > 30 mm A 2 / s, particularly preferably > 35 mm A 2 / s, a dielectric strength > 20kV / mm, preferably > 25kV / mm, particularly preferably > 30kV / mm and a high volume resistivity p1 between 2.0x10 A 15Qcm and 6.0x10 A15Qcm at 25°C and 1000V and a specific volume resistance p2 between 3.5x10 A 13Qcm and 8.0x10 A 13Qcm at 250°C and 1000V, and a ratio p2 / p1 < 0.

2.

15. Use of a sintered molded body according to one of claims 1 to 7 as an electrically insulating substrate body in power electronics.

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