Thermally conductive casting resin and method for manufacturing an electrotechnical assembly

Abstract

A thermally conductive casting resin comprises at least one resin and filler particles of different size classes. The filler particles exhibit a trimodal size distribution with a first volume fraction V1 of first particles with sizes from a first size range around a first mean particle size PG1, a second volume fraction V2 of second particles with sizes from a second size range around a second mean particle size PG2 > PG1, and a third volume fraction V3 of third particles with sizes from a third size range around a third mean particle size PG3 > PG2, wherein (i) at least 95% of all filler particles belong to one of the three particle types, such that the condition 95% ≤ (V1 + V2 + V3) ≤ 100% applies; (ii) for size ratios the conditions 3 ≤ PG 2 / PG 1 ≤ 5 and 4 ≤ PG 3 / PG 2 ≤ 8 apply and (iii) for volume ratios the conditions 20% ≤ V1 ≤ 30% , 15% ≤ V2 ≤ 25% ; 50% ≤ V3 ≤ 60% apply.

Description

SCOPE OF APPLICATION AND STATE OF THE ART

[0001] The invention relates to a thermally conductive casting resin comprising at least one resin and electrically non-conductive filler particles of different size classes, and to a method for manufacturing an electrotechnical assembly, in particular a stator unit of an electric machine, in which the thermally conductive casting resin is used.

[0002] A generic thermally conductive casting resin is a casting resin mixture based on resin with highly thermally conductive, electrically insulating filler particles that ensure thermal conductivity, and preferably with one or more additives that support the mixture in its properties.

[0003] Electrotechnical assemblies, such as chokes, transformers, and electric motors, as well as power electronics and batteries, can be significantly more efficient by potting them with thermally conductive resins. This is because effective heat dissipation allows for a significant reduction in the operating temperatures of critical areas. This creates power reserves that can be used, for example, to deliver more power until critical temperatures are reached. If required, the power density relative to the installation volume can also be increased. Smaller assemblies with the same performance are usually more cost-effective and have a lower carbon footprint. One development goal is to create highly flowable resins with high thermal conductivity that can wet thermally critical components and efficiently transfer heat to cooling surfaces.

[0004] Numerous thermally conductive casting resin systems are already available on the market. Huntsman Advanced Materials' ARATHERM™ epoxy casting systems are recommended for the production of motor stators, generators, actuators, modules, sensors, and devices requiring high thermal conductivity. These resin systems can be processed using various casting techniques, such as vacuum casting, vacuum casting, gravity casting, or automatic pressure gelation (APG). ARATHERM™ materials are designed to offer high thermal conductivity, excellent thermal stability, and superior electrical properties. Aratherm® CW 2731 is specified with a thermal conductivity (measured according to ISO 8894-1) in the range of 3.0 W / m·K.

[0005] The CoolTherm EP-3500 epoxy system from Lord-Parker Corp. is a two-component system offered for encapsulating motor stators, transformer coils, and equipment requiring a rigid, highly thermally conductive material with a low coefficient of thermal expansion. The CoolTherm® EP-3500 epoxy system is specified to have a thermal conductivity of 3.3 W / m·K at room temperature in its cured state.

[0006] From DE 10 2013 205 117 A1 a potting compound with a resin component is known, wherein the potting compound includes a filler component, wherein the proportion of the filler component to the total weight of the potting compound is 50 wt.% to 95 wt.%.

[0007] From DE 10 2019 204 191 A1 a composition of a casting resin is known, wherein a tetramodality of microfillers with mean particle sizes graded in 10s is present.

[0008] From US 2015 / 0 252 217 A1, an underlay composition is known which, in the uncured state, has a flow rate of less than about ten minutes over a distance of about two centimeters at a temperature of about 90 °C and an adhesive joint thickness of about 50 micrometers or less, and has a volume thermal conductivity greater than about 0.8 W / mK in the cured state.

[0009] WO 2004 / 065 469 A1 discloses a casting resin system in which a filler with a multimodal distribution is present. TASK AND SOLUTION

[0010] Against this background, the invention aims to provide a thermally conductive casting resin that can be reliably processed using conventional casting methods and ensures efficient heat dissipation in electrotechnical assemblies. In particular, the casting resin should have excellent flow properties. Further desirable properties include good wetting of components and good penetration into narrow joints and crevices to achieve optimal impregnation and a self-leveling surface. A further objective is to provide a method for manufacturing an electrotechnical assembly that enables performance improvements in the encapsulated assembly.

[0011] To solve these problems, the invention provides a thermally conductive casting resin with the features of claim 1 or 13. Furthermore, a method for manufacturing an electrotechnical assembly with the features of claim 16 is provided. Preferred embodiments are specified in the dependent claims. The wording of all claims is made clear by reference to the content of the description.

[0012] A thermally conductive casting resin according to the claimed invention comprises at least one resin and filler particles of different size classes. Preferably, at least one additive is also included.

[0013] The term "resin" here refers in a narrower sense to artificial or natural resins and in a broader sense to curable polymeric liquids, which include resins and other substances.

[0014] The resin is preferably an epoxy resin. Epoxy resins are synthetic resins that contain epoxy groups. They are reactive resins that react with a hardener to form a thermosetting plastic. The hardener is the reactant and, together with the resin, forms a macromolecular polyether with epoxy groups. The epoxy resin forms the liquid resin base of preferred casting resins. Instead of an epoxy resin, another resin can also be used, such as a polyester, silicone, acrylic, or polyurethane (PU) resin. Many substances that have a liquid consistency and harden thermally or through the reaction of two or more components are suitable. The filler particles could also be incorporated into a hot melt, i.e., a thermoplastic that is thermally melted and then hardens upon cooling. Additives are helpful but not essential.

[0015] Synthetic resins are poor thermal conductors. Pure epoxies, or other binders mentioned above, often have a thermal conductivity on the order of 0.2 W / m·K. To achieve the desired thermal conductivity of the composition, the casting resin contains filler particles made of one or more materials with relatively high thermal conductivity. These filler particles should be electrically non-conductive to obtain a potting compound with high electrical insulation. Some mineral or ceramic filler particles can meet these requirements. One or more additives enhance the properties of the thermally conductive casting resin. For example, the wetting of the filler particles with epoxy resin can be improved using suitable additives.

[0016] Depending on their type, fillers can have thermal conductivities ranging from 20 to 60 W / m·K or higher. The filler particles should be distributed as evenly as possible within the epoxy resin base.

[0017] According to one formulation of the claimed invention, the filler particles within the resin base should consist predominantly or exclusively of three different size classes, each with a relatively narrow particle size distribution. According to another formulation of the claimed invention, the filler particles of a filler particle type should have a trimodal size distribution with a first volume fraction V1 of first particles with sizes from a first size range around a first mean particle size PG1, a second volume fraction V2 of second particles with sizes from a second size range around a second mean particle size PG2 > PG1, and a third volume fraction V3 of third particles with sizes from a third size range around a third mean particle size PG3 > PG2.The particle size classes can also be described relative to each other as "large", "medium", and "small", with the first filler particles having relatively small sizes, the second filler particles having medium sizes, and the third filler particles having relatively large sizes. According to the invention, several conditions must be met simultaneously.

[0018] According to a first condition, at least 95% of all filler particles must belong to one of the three particle types, such that the condition 0.95 ≤ (V1+V2+V3) ≤ 1 applies. This condition states, among other things, that the size distribution should be relatively pure trimodal, with no or only relatively few filler particles that do not belong to one of the three particle size intervals.

[0019] The inventor further determined that the advantageous properties of the casting resin are essentially only sufficiently achieved when certain size ratios are present according to a second condition. According to the invention, the conditions 3 ≤ PG2 / PG1 ≤ 5 and 4 ≤ PG3 / PG2 ≤ 8 must apply simultaneously.

[0020] A further condition (third condition) concerns the volume ratios of the individual filler particles. According to the invention, the following conditions must simultaneously apply to the volume ratios: 20% ≤ V1 ≤ 30%, 15% ≤ V2 ≤ 25%; 50% ≤ V3 ≤ 60%.

[0021] According to the inventor's findings, if these conditions are met in the encapsulated state, a spatial distribution of the filler particles can be achieved in which many of the largest particles (third-size filler particles) are in direct contact with each other, resulting in a high number of thermal bridges with direct contact between immediately adjacent thermally conductive filler particles. Simultaneously, the tetragonal pockets formed between the largest particles are filled essentially without blocking the direct thermal bridges between the largest particles. Furthermore, the middle fraction, i.e., the particles with sizes from the second size range (second-size filler particles), is also able to effectively fill the three-dimensional gaps between the largest particles and create further thermal bridges through direct contact with neighboring filler particles.Applying this principle allows for the highest possible packing density to be achieved, thereby generating a correspondingly large number of heat paths through the contact of individual particles. The packing density can potentially exceed 3.2 g / ml.

[0022] Despite the high fill level, a sufficiently low viscosity for potting can be achieved.

[0023] Thermally conductive casting resins according to the claimed invention can thus meet the diverse and sometimes conflicting requirements for thermally conductive casting resins to a particularly high degree. The casting resin is highly flowable during casting and can therefore largely or completely wet all adjacent parts, even in narrow and irregular gaps, and completely fill a volume without forming voids. After casting and curing, the solidified casting compound exhibits high thermal conductivity. High thermal conductivities are achieved, among other things, by a high volume fraction of filler particles with high thermal conductivity.

[0024] In its hardened state, the potting compound should not crack even under extreme temperature fluctuations (thermal shock resistance). Crack resistance can be achieved in embodiments of the claimed invention by ensuring that the cured potting compound has a coefficient of thermal expansion similar to that of the potted components. Finally, many applications require high electrical insulation properties under all operating conditions. Thermally conductive casting resins are available within the scope of the invention that can meet such complex requirements.

[0025] Particularly good results can often be achieved if the size ratios meet the following condition: PG1=X; PG2= X·4.5 + / -0.5; PG3=PG2·6.5+ / -0.5.

[0026] In this representation, the relationship is as follows: PG1 = X; PG2 = X times 4-5; PG3 = PG2 times 6-7. The size of PG1 is initially undefined. Depending on the size of PG1, the sizes of PG2 and PG3 are determined. The model is therefore scalable within certain limits. The simple formula here starts with the smallest selected particle.

[0027] If, for example, the first average particle size PG1 is chosen to be 1 µm, then the corresponding particle sizes PG2 and PG3 have values ​​of 4.5 µm and (4.5 × 6.5) µm ≈ 29 µm, respectively (each with tolerances). Coarser or finer particle sizes can also be used in appropriate ratios, for example, as follows: PG1 = 4 µm; PG2 ≈ 18 µm; PG3 ≈ 117 µm or PG1 = 0.5 µm; PG2 ≈ 2.3 µm and PG3 ≈ 15 µm, etc.

[0028] According to further development, with regard to absolute particle sizes, it has proven advantageous, particularly for the preferred application of potting components for electric motors or other comparable applications, if the first size range comprises particle sizes from 2 µm to 5 µm. Alternatively or additionally, the second size range should preferably comprise particle sizes from 10 µm to 20 µm. Alternatively or additionally, the third size range should comprise particle sizes from 70 µm to 120 µm. Significant deviations from these particle sizes and particle size ratios may result in a drastic reduction in the number of direct contacts between thermally conductive filler particles, making it impossible to achieve the desired high thermal conductivity of the casting resin.

[0029] For other applications, such as potting heart pumps or precision motors, the most favorable first mean particle sizes can be around 1 µm to 2 µm or in the submicrometer range, i.e., less than 1 µm, e.g., in the range of 0.5 µm to 0.9 µm. The corresponding second and third mean particle sizes would then be significantly larger than 1 micrometer.

[0030] The three size ranges mentioned above, PG1 ≈ 2-5 µm, PG2 ≈ 10-20 µm, and PG3 ≈ 70-120 µm, should be understood as possible sizes. It would also be possible to combine three particle sizes in the nanometer range, provided they are in the same ratio to each other. However, the general principle is that as particle size increases, the surface area decreases relative to the displaced volume. The surface area to be wetted is therefore crucial for flowability. The more binder is bound by surface wetting, the less "free liquid" is available to maintain flowability. Combinations of larger particle sizes are therefore better due to their smaller surface area relative to the displaced volume. The particle size in application is determined, among other things, by the geometries of the components to be filled or impregnated.

[0031] The advantageous size ratios stipulate, among other things, that such casting resins should contain no or the smallest possible proportion of particles of certain intermediate sizes. According to a further development, the filler particles of the third size class are, on average, at least four, five, or six times larger than those of the filler particles of the second size class. Thus, for example, with the preferred particle size ratios, there is a relatively wide range of "unacceptable" or missing particle sizes between the medium particles (PG2) and the large particles (PG3). The difference between the medium particle sizes PG2 and PG3 can, for example, be a factor of 6 to 7. These intermediate sizes between PG2 and PG3 should ideally be absent from the casting resin or present only in a very small volume fraction.

[0032] As mentioned above, for example, in casting resins used to pot components for electric motors or other comparable applications, the first size range can comprise particle sizes from 2 µm to 5 µm. It is then preferably provided that the filler particles contain a negligible number of filler particles from an intermediate size range between medium and large particles, particularly with intermediate sizes from approximately 30 µm to approximately 60 µm. For the preferred particle sizes used in potting electric motors and similar applications, a relatively wide range of particle sizes between approximately 30 µm and approximately 70 µm is therefore possible, which should ideally be absent from the casting resin or present only in a very small volume fraction.

[0033] With reference to a first mean particle size PG1 ≈ X, the “gap” in the particle size spectrum can, for example, be in the range of approximately 5 · X to approximately 12 · X.

[0034] In a particularly advantageous casting resin, the filler particles comprise first filler particles from a first size class with first mean particle sizes PG1 in the range of 3 µm to 5 µm and a first volume fraction V1 in the range of 20% to 30% of the filler particles. Furthermore, second filler particles are provided, belonging to a second size class with second mean particle sizes PG2 in the range of 10 µm to 20 µm. These have a second volume fraction V2 in the range of 15% to 25% of the filler particles within the mixture. Finally, the filler particles comprise third filler particles from a third size class with third mean particle sizes PG3 in the range of 70 µm to 120 µm and a third volume fraction V3 in the range of 50% to 60% of the filler particles.

[0035] In some embodiments, the volume ratios can be subject to the conditions 22% ≤ V1 ≤ 28% and / or 17% ≤ V2 ≤ 22% and / or 53% ≤ V3 ≤ 58%. The sum of the volume fractions should in each case equal 100%.

[0036] Ideally, all filler particles should originate from one of these three size classes, so that the percentage volume fractions of the filler particles add up to 100%. In practical terms, it is usually sufficient if almost exclusively only these three size classes are present, such that at least 95% of all filler particles originate from one of the three size classes. Therefore, according to the invention, the condition 0.95 ≤ (V1+V2+V3) ≤ 1 must apply.

[0037] One embodiment can be described as a filler mixture with a trimodal particle size distribution with (local) maxima at 4µm ± 0.5 µm, at 15 µm ± 1 µm and at 105 µm ± 5 µm.

[0038] This corresponds to a particle size distribution where the conditions PG1=X; PG2=X⋅4.5+ / −0.5; PG3=PG2⋅6.5+ / −0.5 These values ​​apply, provided that X ≈ 3.5 µm.

[0039] The particle size distribution within each size class or fraction should be relatively narrow. In other words, there should not be excessively large deviations from the mean value of the respective size class. This can be achieved, for example, by ensuring that, during the pretreatment and selection of the filler particles, the half-width of the particle sizes within each size class is no more than 30% of the respective mean value of the size class.

[0040] Preferably, the largest filler particles (those of the third size class) are on average at least four, five, or six times larger than those of the medium-sized filler particles of the second size class. In other words, a characteristic of such mixtures is that practically no or only negligible filler particles should originate from the intermediate size range between the medium-sized and large particles, i.e., with intermediate sizes ranging from approximately 30 µm to approximately 70 µm.

[0041] By adhering to these mixing ratios and the corresponding particle size classes, highly thermally conductive casting resins or casting resin compositions are possible, in which the volume fraction of the filler particles in the casting resin is 70% or more, and the volume fraction of the solid particles in the casting resin is preferably between 75% and 85%. According to the inventor's findings, this allows the majority of the heat conduction within the cast resin to occur via filler particles of high thermal conductivity that are in direct contact with one another, so that the poor thermal conductivity of the epoxy resin material does not have a particularly strong effect on the thermal conductivity of the casting resin.

[0042] Preferably, the casting resin is essentially free of nanoparticles, i.e., free of filler particles whose average particle size in the nanometer range is significantly below 1 µm. A larger proportion of nanoparticles can increase viscosity, which is undesirable here with regard to its use in casting components. Although nanoparticles may enter the casting resin in small quantities as a byproduct of the mechanical pre-processing of the intended filler particles, care should be taken to ensure that their volume fraction remains negligible.

[0043] If these mixing ratios are adhered to sufficiently well, thermal conductivities of well over 3.0 W / m·K can be achieved for the finished thermally conductive casting resin with a sufficiently high filler particle content. Values ​​of 3.5 W / m·K or more, or even 4.0 W / m·K or more, are also achievable. It is even possible to produce highly flowable casting resins that wet thermally critical components and, with a thermal conductivity of approximately 8 W / m·K or more, dissipate this heat into cooling surfaces.

[0044] The thermal conductivity of casting resins gradually decreases as the ambient temperature increases. In advantageous embodiments, this reduction in thermal conductivity with increasing temperature is very small compared to the prior art. Commercially available "high thermal conductivity" casting resins lose up to 50% of their thermal conductivity in the temperature range between 20°C and 180°C. In contrast, some of the casting resins described here experience a loss of only about 15% in the same temperature range. Thus, in addition to the already very high thermal conductivity in the operating range of various applications above 100°C, sufficient heat dissipation remains.

[0045] Despite the high filler content, which is beneficial for heat conduction, the flowability remains sufficient for processing during casting; the casting resin does not become too pasty to be processed by casting, even with a high filler particle content.

[0046] The casting compound is preferably pasty at room temperature (20 °C). This is advantageous because it largely prevents sedimentation during storage. Upon heating the mixture, the viscosity improves noticeably until a free-flowing, self-leveling casting compound is formed. As is well known, increasing the processing temperature reduces the base viscosity of the resin, and thermal expansion increases the volume, so that the free resin volume, with rising temperature, provides the necessary thinning.

[0047] In some embodiments, the casting resin has a viscosity at room temperature (20 °C) that can be in the range of at least 500,000 mPa·s, preferably from several hundred thousand mPa·s up to one million mPa·s or more. In contrast, at typical processing or casting temperatures in the range of approximately 60 °C to approximately 120 °C, the viscosity values ​​are frequently below 50,000 mPa·s, with the viscosity in the upper end of this temperature range (e.g., in the range of 100 °C to 120 °C) preferably being in the range of less than 10,000 mPa·s, e.g., in the range of 2,000 to 8,000 mPa·s.

[0048] The viscosity of the casting resin can therefore, when poured, be in the range of thin to viscous honey or certain types of syrup, and thus significantly lower than that of pasty materials used as modeling clay or molding compounds, which can only be pressed into cavities under external pressure. The viscosities can be in a range where the flowable material self-levels to form a smooth surface and can, for example, effectively saturate or impregnate coils and windings.

[0049] It is currently assumed that the surprisingly low viscosity, considering the high proportion of filler particles, results, among other things, from the fact that, when the specified size and volume ratios are maintained, the contact between the filler particles is such that the internal friction of the casting resin remains comparatively low. It is therefore assumed that the filler particles in contact with each other do not become strongly interlocked, but rather remain relatively mobile, thus providing sufficient flowability for casting.

[0050] The highest packing densities can be achieved with a high proportion of spherical fillers. Spherical filler particles can also be combined with non-spherical ones. In this case, the packing density decreases to maintain a fluid viscosity, and consequently, so does the thermal conductivity. However, the material becomes less expensive because spherical fillers are significantly more expensive to manufacture than angular ones. One advantage of the solution proposed here is that, for example, at a thermal conductivity of 5 W / mK, one has the choice between a slightly higher viscosity and lower cost or a lower viscosity and higher cost.

[0051] In some embodiments, the choice of material may result in only relatively weak attractive forces between the particles in contact with each other, i.e., relatively low cohesion.

[0052] According to a further development, at least 50% of all filler particles, preferably 70% or more, or 80% or more, or 90% or more, or 95% or more of the filler particles belong to the same type of filler particle, which here means that they consist of the same base material, e.g., aluminum oxide.

[0053] According to a further development, at least 50% of all filler particles, preferably 70% or more, or 80% or more, or 90% or more, or 95% or more of the filler particles are spherical filler particles.

[0054] According to a further development, at least 80% of all filler particles, preferably 90% or more of the filler particles, are spherical and / or granular aluminium oxide particles.

[0055] Preferably, nitrides are not used. Although nitrides have high thermal conductivity, they are expensive and have disadvantages in processing. For example, aluminum nitride powder exhibits high hydrolysis sensitivity. In water, incomplete decomposition of aluminum nitride into aluminum hydroxide and ammonia can be observed. Due to its hydrolysis sensitivity, powdered aluminum nitrides are difficult to store and, even after being incorporated into the resin, can absorb water, potentially leading to decomposition within the resin or hardener. Contact with atmospheric moisture is sufficient to initiate this decomposition process.

[0056] Although boron nitride has excellent thermal conductivity, its flat shape results in a large surface area relative to its volume. This means that a significant amount of liquid is bound across the surface, leading to a rapid increase in viscosity even with small additions.

[0057] These substances should therefore be avoided, but may well be present.

[0058] During the development process, it was further discovered that it might be possible to utilize the underlying concepts of the invention in another way. In a series of tests, several thermally conductive casting resins were produced with filler particles of different trimodal size distributions. The three different types of filler particles (with average particle sizes PG1, PG2, and PG3) in each of the casting resins had the size ratios (in particular 3 ≤ PG2 / PG1 ≤ 5 and 4 ≤ PG3 / PG2 ≤ 8) and volume ratios (in particular 20% ≤ V1 ≤ 30%, 15% ≤ V2 ≤ 25%, and 50% ≤ V3 ≤ 60%) already described. However, the individual casting resins, also referred to here as base casting resins, differed significantly with respect to the absolute sizes of the first (smallest), second (average), and third (largest) particles.

[0059] Each of these base casting resins was very liquid at processing temperature and therefore suitable for casting, and after hardening exhibited the characteristic, relatively high thermal conductivities.

[0060] These casting resins (base resins) with varying particle sizes served as intermediates for the production of a final product in the form of another thermally conductive casting resin, also referred to here as a combination casting resin. This combination casting resin comprised a mixture of a first thermally conductive (base) casting resin and at least one different second thermally conductive (base) casting resin, each of which, when its size and volume ratios were carefully controlled, was readily pourable and, after curing, exhibited high thermal conductivity. These two-component mixtures of two different base casting resins again yielded a thermally conductive casting resin (combination casting resin), which can be considered a combination product of the base casting resins that served as intermediates. The aim was to create mixtures that were as homogeneous as possible.Such a combination casting resin made from two different base casting resins can be described as containing two different, each trimodal, size distributions of filler particles.

[0061] It has been shown that, under certain conditions, the resulting combination casting resin also had a sufficiently low viscosity for good processing and, after curing, could exhibit a thermal conductivity that was also, in some cases, considerably higher than the state of the art, even if the thermal conductivities of the combination casting resins generally did not quite reach the higher values ​​of the intermediate products (base casting resin with exactly three filler particles of different size classes).

[0062] If the first mean particle size PG1 of the first base casting resin is designated PG1-1 and that of the second base casting resin PG1-2, and accordingly the particles of mean particle sizes of the middle and largest particles of the respective trimodal mixtures PG2-1, PG2-2 and PG3-1 and PG3-2 respectively, excellent values ​​for viscosity and thermal conductivity were found when the following conditions were met: 1.5 <PG1−2 / PG1−1<3;1,5<PG2−2 / PG2−1<3 und1,5<PG3−2 / PG3−1<3, The abbreviation PGi-j denotes the i-th mean particle size of the j-th casting resin, where i=1, 2, 3 and j =1, 2. In other words: PG1-1 represents the mean particle size of the smallest filler particles of the first base casting resin, PG2-1 represents the medium filler particles of the first casting resin, etc., and PG1-2, PG2-2 and PG3-2 represent the smallest, medium and largest particle sizes of the second casting resin, which contained comparatively larger filler particles overall.

[0063] The experiments indicate that the smallest, medium, and largest particle sizes in the casting resin with the coarser grains (second base casting resin) should be at least 1.5 times and at most 3 times larger than their corresponding counterparts in the second base casting resin with the coarser particles. The same should apply to the respective medium and largest particle sizes.

[0064] It is currently assumed that if the base casting resins each exhibited the size and volume ratios of their particles that have been found to be favorable, then certain mixtures of two casting resins with trimodal size distributions would again exhibit exceptionally good values ​​for viscosity and thermal conductivity.

[0065] It is currently assumed that, provided the conditions for the size ratios between the corresponding particles of the coarser and finer mixtures are met, the finer particles can settle into the spaces remaining in the framework of the trimodal mixture containing the coarser filler particles. Accordingly, the space filling can be further increased, potentially resulting in a slight increase in thermal conductivity. For some size ratios, the tendency to sedimentation was also lower, without a significant subsequent reduction in thermal conductivity.

[0066] For many intended applications, for example in the context of potting electric motors or electrically operated devices, mixtures of two different casting resins have proven effective, for which the following conditions apply: First base casting resin: 1.5μm <PG1−1<2,5μm;3,0μm<PG2−1<6,0μm;60μm<PG3−1<80μm

[0067] Second base casting resin: 4.0 µm < PG1-2 < 6.0 µm; 15 µm < PG2-2 < 25 µm; 110 µm < PG3-2 < 140 µm

[0068] Even with these mixtures of two individually highly thermally conductive casting resins, it has been shown that particularly good properties can be achieved if the filler particles of the casting resin contain only a negligible number of filler particles from an intermediate size range with intermediate sizes from approximately 30 µm to approximately 60 µm.

[0069] The invention also relates to a method for manufacturing an electrotechnical assembly. The method comprises the step of providing an electrically operable functional unit with electrically conductive components that generate heat when an electric current is passed through them during operation of the assembly, as well as a cooling unit associated with the functional unit, the cooling unit having a heat-absorbing surface facing the functional unit, wherein a gap is formed between the functional unit and the heat-absorbing surface. In the method, a castable potting material containing a thermally conductive casting resin is filled into the gap such that the potting material substantially fills the gap in a predetermined filling area. This potting step is followed by a curing step in which the potting material hardens.The material is cured and thereby forms a thermally conductive solid that at least partially encapsulates the functional unit and mechanically and thermally connects it to the cooling unit. The method is characterized in that a potting material with a casting resin according to the claimed invention is used.

[0070] The electrotechnical assembly could, for example, be a stator unit of an electric machine. Such a stator unit comprises a stator housing with an inner surface that encloses a circumferential interior space, and a stator with electrically conductive windings arranged within the housing in such a way that a gap remains between the stator and the inner surface of the housing. The stator with its windings forms the electrical functional unit, while the stator housing acts as a heat sink or cooling unit, absorbing and dissipating heat through its inner surface facing the stator. The electric machine could, for example, be an electric motor or a generator. The stator housing can be actively cooled, for instance, by incorporating cooling channels within the housing through which a coolant flows.Heat can be transferred from the stator windings to the coolant, thus removing heat from the stator and preventing overheating of the stator windings.

[0071] During potting, the surfaces of heat-conducting parts should be wetted as thoroughly as possible, and the surrounding air or other gases should ideally be completely displaced. Displacement can be problematic. In narrow capillaries and gaps, such as inside copper coils and other confined spaces between components, the gases are often prevented from being displaced by the flowing casting compound. These voids reduce direct, all-around wetting and the filling of bores and gaps. To promote penetration and homogeneous distribution of the casting compound, processing under vacuum is recommended. For this purpose, the potting compound should first be prepared under vacuum, e.g., at approximately 5 mbar, since the addition of fillers introduces a significant amount of air into the resin. If the compound is not sufficiently prepared under vacuum, subsequent degassing within the component can lead to foaming during the potting process.The air contained in the resin then counteracts the effects of the added fillers and significantly reduces the thermal conductivity. The actual vacuum casting should then be carried out at a higher pressure or a lower vacuum (e.g., at approximately 10 mbar) to prevent subsequent degassing. Ideally, no foam bubbles should be visible on the casting surface, and the surface should have a mirror-like shine and be perfectly level. The resin, homogeneously cast within the component, then cures at temperatures between 100 and 160°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Further advantages and aspects of the invention will become apparent from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures. Fig. Figure 1 shows a schematic section through a stator of an electric motor with a space to be filled with casting resin; Fig. 2 is a diagram that schematically shows the volume fractions of the three different particle sizes as a function of the particle size for an exemplary embodiment; Fig. 3A, Fig. Figure 3B schematically illustrates the size ratios of the filler particles within the casting resin; Fig. Figure 4 shows a diagram illustrating the dependence of the viscosity of the casting resin on the temperature; Fig. Figure 5 shows a diagram illustrating the dependence of the thermal conductivity of the casting resin on the temperature; Fig. Figure 6 shows a diagram of the thermal expansion of various materials during a temperature change from room temperature to 200 °C; Fig. Figure 7 shows, for comparison, the temporal development of the stator temperature of an electric motor during continuous operation on the test bench using a conventional casting resin (SdT) and when using a casting resin of an exemplary embodiment; Fig.Figure 8 shows the temperature increase at the stator during the tests of Fig. 7. The abbreviation OFF indicates that the conventionally impregnated electric motor shuts down after exceeding the specified limit temperature of 160 °C. In an electric motor with potting material according to the exemplary embodiment (curve HTC), the stator temperature remained a minimum of ΔT below the limit temperature. DETAILED DESCRIPTION OF THE EXECUTION EXAMPLES

[0073] The following describes embodiments of casting resins or casting resin compositions of the type first described in this application and their use in the manufacture of electric motors, which can be used, for example, as drives in the automotive sector.

[0074] These casting resins are specifically designed for targeted and efficient heat dissipation in electrotechnical components and can contribute to significantly more efficient designs for encapsulated assemblies such as chokes, transformers, and electric motors. This is because their high heat dissipation capacity allows for a significant reduction in the operating temperatures of critical areas. This creates power reserves that can be used either to deliver more power before reaching critical operating temperatures or to increase the power density relative to the installation volume. A crucial contribution to this is made by the highly flowable casting resins of the type described here, which wet thermally critical components and, with their high thermal conductivity, can transfer this heat to dedicated cooling surfaces.

[0075] Based on Fig.Section 1 describes, by way of example, a method for manufacturing a stator unit SE of an electric machine, for example, an electric motor for a pump or for another electric drive. A stator housing SG, for example, essentially cylindrical, is provided, which has an inner surface IS that encloses a housing interior in the circumferential direction. A stator ST is arranged in the stator housing, comprising a stator core SK and electrically conductive stator windings SW. These can be, for example, wound coils or constructed using plug-in coils (hairpins). The stator is arranged in the stator housing such that a space ZW remains between the stator and the inner surface of the stator housing.To optimize heat conduction between the stator and stator housing, while simultaneously mechanically stabilizing the overall assembly and electrically insulating the gaps, a flowable potting compound consisting of a thermally conductive casting resin is poured into the gap in such a way that the potting compound essentially fills the gap within a predetermined fill area. The potting compound is then cured, forming a thermally conductive solid that encapsulates the stator and mechanically and thermally connects it to the stator housing. This is particularly relevant for the [specific applications / requirements - context needed]. Fig. 1. The winding heads WK, which are still shown exposed, are thermally well conductive and mechanically stable when connected to the stator housing SG. Production

[0076] The following describes exemplary applications of casting resins and their processing, in which free-flowing casting resins are used that wet the thermally critical components and, with a thermal conductivity in the range of 3 W / m·K or more, dissipate this heat to associated cooling surfaces. In some exemplary applications, thermal conductivities in the range of more than 3.5 W / m·K up to approximately 8 W / m·K were achieved.

[0077] The starting point of the production process is a basic formulation with components that can be combined in such a way that, by combining the most efficient and usually also most expensive fillers and additives, high thermal conductivities of currently up to approximately 8 W / m·K can be achieved, whereby by adding less efficient fillers and additives, the more cost-effective variants of the casting resin composition or the casting resin with lower thermal conductivity can also be formulated successively.

[0078] The following table shows an example of a basic formulation that can serve as a basis for designing casting resins for specific applications. The abbreviations FS1, FS2, etc., represent specific filler particle types, PG1, PG2, and PG2 represent specific (average) particle sizes, "sp" denotes spherical particle types, and V1, V2, etc., represent the volume fractions of filler particles to be used relative to the total volume of filler particles. The volume fractions of epoxy resin and hardener refer to the total volume of the casting resin with filler. The additives refer to the volume of the resin mixture without filler. position Description specification Quantities 1 epoxy resin thin liquid Resin-hardener mixture total approx. 15 - 25% by volume 2 Harder thin liquid 3 FS1 PG3 V3 4 FS2 PG3 V3 5 FS3 PG1 V1 6 FS4 PG3 - sp V3 7 FS5 PG3 - sp V3 8 FS6 PG2 - sp V2 9 FS7 PG2 - sp V2 10 FS8 PG1 - sp V1 11 FS9 PG1 - sp V1 12 AD1 Disp X% 13 AD2 Reactive diluents Y% 14 AD3 Thix Z% 15 AD4 accelerator var 16 pigment A%

[0079] According to the invention, at least 95% of all filler particles belong to one of the three particle types: small, medium, or large, such that the condition 0.95 ≤ (V1 + V2 + V3) ≤ 1 is fulfilled. For the size ratios, the conditions 3 ≤ PG2 / PG1 ≤ 5 and 4 ≤ PG3 / PG2 ≤ 8 must be met according to the invention. In particular, the conditions PG1 = X; PG2 = X·4.5 ± 0.5; PG3 = PG2·6.5 ± 0.5 should apply as far as possible. For volume ratios, the conditions 20% ≤ V1 ≤ 30%, 15% ≤ V2 ≤ 25%; 50% ≤ V3 ≤ 60% must be met according to the invention.

[0080] In this example, all filler particles were made of aluminum oxide (Al₂O₃). Provided the particle sizes are adhered to, cross-mixtures with other fillers such as silicon dioxide, quartz, glass, nitrides, or other granules of electrically insulating materials are also possible. This allows for relatively high packing densities, but depending on the amount added, characteristic and mechanical properties are lost.

[0081] The resin as the raw material can be selected according to the intended application, taking into account the temperature range and the type of components to be encapsulated. For stator units of electric drives, resins of thermal class "H" (180 °C) should be used because operating temperatures of up to 150 °C or more can easily be reached. In corresponding applications, a commercially available, low-viscosity epoxy resin is used. Some of these have a very high glass transition temperature (TG) (the temperature at which the polymer transforms from a hard, glassy material into a soft, rubbery one) and high chemical and UV resistance. A further advantage is the lowest possible base viscosity of the resin.

[0082] In preferred embodiments, a hot-curing hardener is used. These hardeners can also be very low in viscosity (50 mPa·s), thus significantly diluting the base resin.

[0083] An example of a casting resin can be composed according to the following table. Position Description specification Crowd 1 Resin hardener mixture thin liquid 15-25% by volume 2 Rheology additive Additive 0.5-1% on resin 3 Reactive diluents Additive 10% off resin 4 FS 1 PG 1 cant. V 1 5 FS 2 PG 2 kant. V 2 6 FS 3 PG 3 kant. V 3 7 FS 4 PG 1 sph. V1 8 FS 5 PG 2 sph. V 2 9 FS 6 PG 3 sph. V 3

[0084] In the production of the casting resin, the resin and hardener are first mixed separately according to the formula. The fillers are preferably premixed dry in the desired particle size distribution. The mixing of the filler particles with the resin and hardener preferably takes place with the addition of heat and under a vacuum of, for example, approximately 20 mbar, preferably within a temperature range of approximately 80 °C to approximately 100 °C.

[0085] The resin matrix is ​​already very thin at room temperature, with viscosities below 1000 mPa·s, which is advantageous for achieving extremely high filler levels. Applying heat can further reduce the viscosity to a level similar to that of water. To keep the mixture thin even at lower temperatures, some variants include the addition of a reactive diluent, for example, up to 10% in the resin-hardener mixture. This can alter the stoichiometric mixing ratio. The reactive diluent may reduce the thermal conductivity (TG), resulting in a higher coefficient of thermal expansion at high temperatures.

[0086] Such a low-viscosity casting resin can accommodate very high filler concentrations; however, sedimentation of the filler particles can occur, resulting in an uneven distribution of the fillers within the resin matrix. Commercially available sedimentation-inhibiting additives can be used to reduce this tendency. The casting resin can be prevented from sedimenting by stirring or circulation. Preferably, a filler concentration close to the percolation point is used, so that the fillers support each other and prevent settling.

[0087] The result is a hot-curing casting resin system that can be thermally accelerated. This means that the reactivity can be influenced by the temperatures in the individual process steps. The reactivity can also be further adjusted with catalysts, which allows for parameter adjustments to optimize the process over time. Furthermore, energy can be saved when using catalysts because the process temperatures can be reduced somewhat due to the higher exothermicity of the reaction, which releases additional energy in a relatively short time.

[0088] Series trials have shown that maintaining minimum casting temperatures of approximately 80°C to 90°C can be advantageous. Pre-hardening temperatures can range from 110°C to 130°C, particularly around 120°C. During tempering, even higher temperatures, such as approximately 160°C or more, can be reached temporarily. Size distributions

[0089] Experiments have led to the development of casting resin compositions and resins that offer comparatively good flow and casting properties while maintaining a very high solids content. Since particle friction increases with increasing filler content, and consequently viscosity rises, measures must be taken to minimize particle friction. The maximum filler factor is therefore limited by the maximum viscosity that can still be tolerated for casting a component, such as a stator.

[0090] According to the inventor's findings, the type and distribution of filler particles are of particular importance. In one embodiment, a trimodal size distribution of filler particles was used. In this example, this means that at least 95% of all filler particles belong to one of only three selected particle types. The diagram in Fig.Figure 2 shows the volume fractions of the three different particle sizes PG as a function of particle size for an exemplary embodiment. First particles P1 have sizes from a first size range around a first mean particle size PG1 and are present with a first volume fraction V1. Second particles P2, with sizes from a second size range around a second mean particle size PG2, are present with a second volume fraction V2. The second mean particle size is larger than the first mean particle size. Third particles P3, with sizes from a third size range around a third mean particle size PG3 > PG2, are present with a third volume fraction V3. In the exemplary embodiment, the first mean particle sizes are in the range of 3 µm to 5 µm, particularly around 4 µm, and represent a first volume fraction V1 in the range of 20% to 35% of the filler particles (in the example, approximately 23% to 27%).The comparatively larger second filler particles have a second mean particle size PG2 in the range of 10 µm to 20 µm and are present in a second volume fraction V2, which is in the range of approximately 15% to approximately 25% of the filler particles, specifically in the range of 17% to 22%. The third filler particles P3 have a third mean particle size PG3 in the range of approximately 70 µm to approximately 120 µm, with values ​​between approximately 100 µm and 110 µm in this example. These relatively largest filler particles are present in a third volume fraction V3, which in this example is in the range of 45% to 60% of the filler particles, specifically between 50% and 58%.

[0091] Favorable size ratios can also be determined starting from the smallest particle size PG1 = X ≈ 3.5 µm via the conditions PG2= X·4.5 + / -0.5 and PG3=PG2·6.5+ / -0.5.

[0092] In this distribution of filler particle quantities and sizes, it is particularly noticeable that practically no filler particles with medium particle sizes of ≥ 20 µm to 100 µm are present. Furthermore, the selection and pre-processing of the filler particles ensured a relatively narrow particle size variation within each size class, so that the half-width of the particle sizes is at most 30% of the respective mean value of the size class. It has been shown that, using this size distribution, the volume fraction of solid particles in the casting resin composition can be increased to 75% or more, and possibly even up to approximately 85%, without simultaneously increasing the viscosity to such an extent that processing by casting is no longer possible.

[0093] The Fig.Figures 3A and 3B schematically illustrate the conditions within the casting resin. For illustrative purposes, the conditions for spherical particles are shown in each case. The filler particles are dimensioned relative to each other such that the largest filler particles P3 are in contact with as many neighbors as possible in three-dimensional space. More or less tetragonal spaces TZ are formed between the largest filler particles (see Fig.3B), in which the smaller particles of the first and second size classes are located. The average particle sizes of the medium-sized particles are dimensioned such that they come into contact with as many adjacent large particles as possible, without being so large as to prevent direct contact between the largest particles. The remaining spaces are then filled with the relatively smallest first particles, which are also sized to come into contact with as many adjacent medium-sized particles as possible, without being so large as to prevent direct contact between the medium-sized particles.

[0094] It is currently assumed that, if the mixing ratios specified in the claimed invention are observed, a particularly large number of effective heat conduction paths exist within the casting resin or the casting resin composition. The filler particles of the third size class, i.e., the largest filler particles, apparently form a basic framework in which the large particles are in direct contact with each other in the area of ​​contact points or thermal bridges. A large proportion of the heat energy to be transported from a warmer side to a cooler side then passes through the highly thermally conductive large filler particles, primarily via these thermal bridges created by direct contact, to the cold side. Three-dimensionally extended spaces then remain between the largest filler particles in direct contact with each other.These are predominantly filled by filler particles of the medium and smallest fractions, whereby a relatively large number of thermal bridges are still formed through direct contact between adjacent filler particles. It is assumed that the formation of this highly thermally conductive framework from the largest filler particles is possible, among other reasons, because the second largest size class is significantly smaller than the largest size class, so that the formation of direct contact points between the largest filler particles is not prevented by excessively large particles of other size classes.Rather, the mean particle size of the medium size class is in such a favorable ratio to the mean particle size of the largest size class that the spaces remaining between the largest particles can be well filled with the second largest filler particles without significantly blocking direct contact between the largest filler particles. Properties of the casting resin

[0095] Casting resins according to embodiments of the invention are very easy to process due to their high flowability. To live up to its name, the casting resin is pourable, i.e., liquid. For the purposes of the invention, "liquid" refers to a material that self-levels, even in thin layers, and is capable of wetting and flowing around surfaces, even in narrow gaps. The casting resin has a pasty consistency at room temperature, which prevents excessive sedimentation. A viscosity sufficient for processing is reached at approximately 60 to 70°C. Since the casting resin requires high temperatures to polymerize, temperatures above 70 to 90°C are not problematic. For example, it can be cast well at temperatures up to 120°C and becomes less fluid at higher temperatures. However, since components of the resin can evaporate at excessively high temperatures and negatively affect the final properties, temperatures above approximately 150°C to 160°C should not be maintained for extended periods.In this range, a dilution effect usually only occurs through the thermal expansion of the liquid and the associated increase in its volume. Viscosity is primarily influenced by the liquid freely available in the matrix, in which the particles float. The liquid bound by surface wetting of the particles contributes only minimally to flowability. For the filler, the particle size distribution, particle shape, and the ratio of the particle volumes of individual fractions are crucial. A key to flowability therefore lies in the correct, precise particle size distribution. When using spherical fillers, the entire mass acts like a multiple ball bearing.

[0096] In Fig.Figure 4 shows an exemplary diagram of the temperature behavior of viscosity V in a specific embodiment. The stated viscosity values ​​for the dynamic viscosity V (unit: mPa·s) were obtained from measurements using a viscometer according to EN ISO 3219. A rotational viscometer was used for the measurements. In this method, a motor rotates a body within the liquid. The required torque is measured during rotation. From this, as well as from the exact geometry of the rotating body and the rotational speed, the dynamic viscosity of the liquid can then be determined. Properties after potting

[0097] The following describes some properties of the casting resin composition in its processed state, as well as properties of the components manufactured with it.

[0098] Thermogravimetric analysis (TGA) was used to determine the solids content of organic substances (resin) and inorganic substances (mineral fillers). The test is a pyrolytic test in which a casting resin sample is heated to 1000°C in a laboratory furnace, causing all organic components to pyrolytically vaporize. What remains is the non-combustible filler. The filler content of the sample, expressed as a percentage, is calculated from the difference between the sample weight and the remaining mass. Measurements revealed filler contents of over 90% (e.g., 91.5%) with correspondingly low resin contents (less than 10%, e.g., 8.5%).

[0099] The diagram in Fig.Figure 5 shows measured values ​​for the thermal conductivity of a cast and subsequently cured resin composition at different temperatures according to an exemplary embodiment. The measurements were taken using the hot disk method. The hot disk method allows the measurement of the thermal properties thermal conductivity, thermal diffusivity, and heat capacity. The measuring device operates according to the so-called "transient plane source" method and complies with ISO standard 22007-2. The method operates dynamically and is therefore very fast. A flat sensor serves as both a heat source and a temperature sensor. In standard measurements, it is placed between two samples of identical material.

[0100] The measurement curve shows a thermal conductivity of approximately 8.4 W / m·K at room temperature, measured using the hot-disk method. For preferred applications, thermal conductivity at high temperatures is also crucial. Most datasheets only specify thermal conductivity at room temperature. However, good thermal conductivity in electrotechnical components is only necessary at higher temperatures, usually above 50°C. Therefore, it is important to ensure sufficient thermal conductivity even at elevated temperatures. This is because thermal conductivity decreases with increasing temperature. This is particularly important when running simulation programs. Errors can occur in the thermal design of components, which can become costly if the calculated performance is not achieved due to thermal issues.

[0101] In Fig.Figure 5 illustrates how the thermal conductivity of a casting resin gradually decreases as the ambient temperature increases, according to one embodiment. The material tested here is intended for potting electric motor stators.

[0102] High-performance motors often reach peak temperatures of over 180°C. In the temperature range between room temperature and 180°C, the potting material loses 15-20% of its thermal conductivity. However, the thermal conductivity of the casting resins presented here is still quite high at over 5 W / m·K at 180°C.

[0103] For the temperature dependence of the thermal conductivity in the exemplary embodiment, a value of approximately -0.0056 W / m·K / K is obtained in the temperature range from 20°C to 180°C. In other words, the thermal conductivity decreases by only about 15% when the temperature is increased from 20°C to 180°C, i.e., by less than 20%.

[0104] The cured potting compound continues to exhibit excellent insulating properties, even at higher temperatures. Stable insulation resistance values ​​of around 2.3 GΩ (gigaohms) were measured at approximately 150°C, and even at the limit of approximately 180°C, the measured values ​​were around 1.2 GΩ.

[0105] A major problem in many applications, for example in the automotive sector, is the formation of cracks in the casting resin during significant temperature changes, such as during thermal shock testing. In contrast, casting resin compositions of the type proposed here show virtually no tendency to crack. This is partly due to the success in approximating the coefficient of thermal expansion of the casting compound to that of the surrounding metals. This is attributed, among other things, to the high filler content achievable in the casting resin matrix.

[0106] The thermal expansion and glass transition temperature (TG) were determined using a Hitachi TMA 7100 instrument via thermomechanical analysis (TMA). TMA is a thermal analysis method primarily used to measure the coefficient of thermal expansion (CTE). Thermomechanical analysis measures expansion under controlled load. The displacement of a pushrod in contact with the sample is measured as a function of temperature.

[0107] The diagram in Fig.Figure 6 describes the thermal expansion during a temperature change from room temperature to 200 °C. It is evident that the coefficient of thermal expansion of the casting resins described here (abbreviation HTC, derived from: High Thermal Conductivity) is on the order of magnitude of the values ​​for aluminum (Al) and iron (Fe), while conventional casting resin compositions with a high glass transition temperature TG (HTG) or LTG with a low TG show significantly higher values.

[0108] Casting resins typically have a coefficient of thermal expansion (CTE) of 100–150 ppm below the thermal conductivity (TG). Mineral fillers have a CTE of approximately 5 ppm, depending on the type. In a resin-filler mixture, the CTE of the cured casting resin decreases considerably with increasing solids content. An extremely high packing density of the filler particles contributes to a coefficient of thermal expansion of approximately 10–16 ppm / K, below the TG. This allows the casting resin composition to expand or contract in parallel with the surrounding metals during temperature changes, thus preventing cracking. This is confirmed by thermal shock tests.

[0109] The embodiments measured for Fig. 6 achieved approximately 11 ppm in the range of 80°C–175°C. The value for TG, which can also be determined via this measurement, was above 180°C. Properties of the cast components

[0110] The positive effects of using highly thermally conductive casting resin compositions on the properties of components cast with them can be illustrated using the example of a stator for a pump drive. The available continuous power increased from approximately 130 kW to over 200 kW. At a power input of 180 kW, the temperature was still about 27 °C below the threshold temperature that would lead to derating.

[0111] To test the crack resistance, temperature shock tests were performed between a maximum temperature of 180 °C and a minimum temperature of -40 °C with temperature cycle times of 40 minutes. In each temperature chamber, the process time was 240 minutes, and 100 temperature cycles were performed. The potting compound showed no cracks upon visual inspection or examination with magnification. The insulation resistance of the potting compound should be as high as possible because excessively low electrical resistance can allow current to flow, causing the inverter of an electric motor to shut down. In the temperature range between approximately 20 °C and 140 °C, constant values ​​on the order of 2.2 GΩ were measured. Above 140 °C up to approximately 180 °C, the insulation resistance decreased to still good values ​​of approximately 1.3 GΩ at 180 °C.

[0112] In the endurance test on the test bench during the simulation of a circular track, the stator temperature Ts was systematically below the stator temperature of an identical stator potted with a conventional standard impregnation when using casting resin in an exemplary embodiment. Fig. Figure 7 shows the measured values.

[0113] In other test bench measurements, the electric motor was operated constantly at maximum power. The diagram in Fig. Figure 8 shows the corresponding temperature increase at the stator. In a conventionally impregnated electric motor, the motor was switched off after approximately 200 to 300 seconds due to excessive stator temperature, because the specified limit temperature of 180 °C was exceeded. In an electric motor with potting material according to the exemplary embodiment, the stator temperature remained permanently below the limit temperature; a shutdown only occurred due to an increased gearbox oil temperature.

[0114] The highest thermal conductivity values ​​currently achieved, around 8 W / m·K, were attained with fillers containing a high proportion of spherical particles. For economic reasons, it may be necessary to use less expensive fillers, for example, those that are granular or angular due to pre-processing and therefore do not roll against each other as well. To maintain sufficient viscosity, it is generally necessary to accept a reduction in thermal conductivity. However, even with correspondingly less expensive mixtures, thermal conductivities between approximately 3 W / m·K and 6 W / m·K can be achieved with acceptable flow properties.

[0115] Recent investigations by the inventors using thermally conductive casting resins of the type described here show that under certain circumstances it is also possible to produce combination casting resins with excellent castability and thermal conductivity properties by creating two different base casting resins, each containing filler particles with a trimodal size distribution of the type described here, wherein the absolute sizes of the largest particles, the medium particles and the smallest particles of the respective trimodal size distributions differ significantly in a certain way.

[0116] In this example, the first casting resin is the base resin with the overall finer filler particles, and the second casting resin is the base resin with the comparatively coarser filler particles. The following notation applies: PG1-1 denotes the first average particle size in the first casting resin, i.e., the resin with the overall finer filler particles. Similarly, PG3-2 denotes the third average particle size (i.e., the relatively largest filler particles) in the second casting resin, i.e., the one with the coarser filler particles. Accordingly, the ratio PG1-2 / PG1-1 represents the size ratio of the smallest particles of the coarser fraction to the finer fraction, and so on. Two casting resins (first casting resin, second casting resin) were produced. In the first casting resin, the following particle sizes were observed: PG1-1 = 2 µm; PG2-1 = 10 µm; and PG3-1 = 70 µm.The following filler particles were used for the second casting resin: PG1-2 = 5 µm; PG2-2 = 20 µm; PG3-2 = 120 µm. For the volume fractions of the first, second, and third particles, the following conditions applied in each of the base casting resins: 20% ≤ V1 ≤ 30%; 15% ≤ V2 ≤ 25%; and 50% ≤ V3 ≤ 60%.

[0117] Both base casting resins were present in the same quantity (same total volume) and were mixed until the solid particles of the different size classes (a total of six different average sizes) were distributed as homogeneously as possible in the material.

[0118] Compared to a base casting resin with only a trimodal particle size distribution, a lower tendency to sedimentation was occasionally observed. The combination casting resin, which was mixed from two "simple" casting resins (the base casting resins), was somewhat more viscous than the comparison resins with only a trimodal particle size distribution. The achievable thermal conductivities were systematically lower than those of the "pure" base casting resins, but at approximately 5.5 W / m·K, significantly higher than the values ​​previously known for conventional thermally conductive casting resins (maximum approximately 3.3 W / m·K at room temperature).

Claims

[1] Comprising a heat-conducting casting resin comprising: at least one resin and filler particles of different size classes; characterized by , that The filler particles have a trimodal size distribution with a first volume fraction V1 of first particles with sizes from a first size range around a first mean particle size PG1, a second volume fraction V2 of second particles with sizes from a second size range around a second mean particle size PG2 > PG1 and a third volume fraction V3 of third particles with sizes from a third size range around a third mean particle size PG3 > PG2, wherein (i) at least 95% of all filler particles belong to one of the three particle types, such that the condition 95% ≤ (V1 + V2 + V3) ≤ 100% applies; (ii) for size ratios the conditions 3≤PG2 / PG1≤5 and 4≤PG3 / PG2≤8 apply and (iii) for volume ratios the conditions 20%≤V1≤30%, 15%≤V2≤25%; 50%≤V3≤60% apply. [2] Casting resin according to claim 1, characterized by that the size ratios meet the following conditions: PG1=X;PG2=X⋅4.5+ / −0.5; PG3=PG2⋅6.5+ / −0.

5. [3] Casting resin according to claim 1 or 2, characterized by , that the first size range includes particle sizes from 2 µm to 5 µm and / or that the second size range includes particle sizes from 10 µm to 20 µm and / or that the third diameter range includes particle sizes from 70 µm to 120 µm. [4] Casting resin according to any one of the preceding claims, characterized by , that the filler particles include: first filler particles from a first size class with first mean particle sizes PG1 in the range of 3 µm to 5 µm and a first volume fraction V1 in the range of 20% to 30% of the filler particles; second filler particles from a second size class with second mean particle sizes PG2 in the range of 10 µm to 20 µm and a second volume fraction V2 in the range of 15% to 25% of the filler particles; and third filler particles from a third size class with third mean particle sizes PG3 in the range of 70 µm to 120 µm and a third volume fraction V3 in the range of 50% to 60% of the filler particles. [5] Casting resin according to any one of the preceding claims, characterized by , that the filler particles form a filler mixture with a tri-modal particle size distribution with local maxima at 4µm ± 0.5 µm, at 15 µm ± 1 µm and at 105 µm ± 5 µm. [6] Casting resin according to any one of the preceding claims, characterized by , that within the size classes the half-width of the particle sizes is at most 30% of the respective mean value of the size class. [7] Casting resin according to any one of the preceding claims, characterized by that the filler particles of the third size class are on average at least four times, five times, or six times larger than those of the filler particles of the second size class and / or that the filler particles contain a negligible number of filler particles from an intermediate size range between medium and large particles, in particular with intermediate sizes from the range of approximately 30 µm to approximately 60 µm. [8] Casting resin according to any one of the preceding claims, characterized by that a volume fraction of the filler particles in the casting resin is 70% or more, wherein preferably the volume fraction of the solid particles in the casting resin is between 75% and 85%. [9] Casting resin according to any one of the preceding claims, characterized bythat the casting resin has a thermal conductivity of 3.5 W / m·K or more and / or 4.0 W / m·K or more, preferably the thermal conductivity being in a range of 4.0 W / m·K to approximately 8 W / m·K. [10] Casting resin according to any one of the preceding claims, characterized by that the casting resin has a pasty consistency at room temperature (20 °C) with a viscosity of at least 500,000 mPa·s and / or that at processing temperatures in the range of 60°C to 120°C the viscosity is at most 50,000 mPa·s, wherein preferably the viscosity in the upper end range of this temperature window is in the range of less than 10,000 mPa·s, in particular in the range of 2,000 to 8,000 mPa·s. [11] Casting resin according to any one of the preceding claims, characterized by that at least 50% of all filler particles, preferably 70% or more or 80% or more or 90% or more or 95% or more of the filler particles are spherical filler particles. [12] Casting resin according to any one of the preceding claims, characterized by that at least 80% of all filler particles, preferably 90% or more of the filler particles, are spherical and / or granular aluminium oxide particles. [13] Containing a heat-conducting casting resin: a mixture with a first thermally conductive casting resin and at least a second thermally conductive casting resin, each of these casting resins contains at least one resin and filler particles of different size classes, wherein the filler particles of each of the casting resins each have a trimodal size distribution with a first volume fraction V1 of first particles with sizes from a first size range around a first mean particle size PG1, a second volume fraction V2 of second particles with sizes from a second size range around a second mean particle size PG2 > PG1 and a third volume fraction V3 of third particles with sizes from a third size range around a third mean particle size PG3 > PG2, wherein (i) at least 95% of all filler particles belong to one of the three particle types, such that the condition 95% ≤ (V1 + V2 + V3) ≤ 100% applies; (ii) for size ratios the conditions 3≤PG2 / PG1≤5 and 4≤PG3 / PG2≤8 apply and (iii) for volume ratios the conditions 20%≤V1≤30%, 15%≤V2≤25%; 50%≤V3≤60% apply, The following additional conditions apply: 1.5 <PG1−2 / PG1−1<3 1.5 <PG2−2 / PG2−1<3 und 1.5 <PG3−2 / PG3−1<3, where PGi-j is the i. mean particle size of the j. casting resin, where i=1, 2, 3 and j =1, 2. [14] Casting resin according to claim 13, characterized by that the following conditions are met: 1.5 μm <PG1−1<2,5 μm 3.0 μm <PG2−1<6,0 μm 60 μm <PG3−1<80 μm 4.0 μm <PG1−2<6,0 μm 15 μm <PG2−2<25 μm 110 μm <PG3−2<140 μm [15] Casting resin according to claim 13 or 14, characterized by , that the filler particles of the casting resin contain virtually no filler particles from an intermediate size range with intermediate sizes from approximately 30 µm to approximately 60 µm [16] Method for manufacturing an electrotechnical assembly comprising: Providing an electrically operated functional unit with electrically conductive components that generate heat when an electric current is passed through the assembly during operation, and with a cooling unit associated with the functional unit and a heat absorption surface facing the functional unit, wherein a space is formed between the functional unit and the heat absorption surface; Pouring a flowable potting material made of a thermally conductive casting resin into the gap such that the potting material substantially fills the gap in a predetermined filling area; Curing of the potting material to form a thermally conductive solid that at least partially encapsulates the functional unit and is mechanically connected to the cooling unit. thermally connects, characterized by , that a potting material is used in conjunction with a casting resin according to one of claims 1 to 15. [17] Method according to claim 16, characterized by , that the potting compound is prepared under vacuum at a first vacuum, in particular at approximately 5 mbar, and that the filling of the flowable potting compound takes place under vacuum at a second vacuum, in particular at approximately 10 mbar, wherein the second vacuum is weaker than the first vacuum prevailing during the preparation of the potting compound, and / or that the curing of the potting compound is carried out in a temperature range of 100°C to 160°C. [18] Method according to claim 16 or 17, characterized by that the electrotechnical assembly is a stator unit of an electric machine and that the procedure comprises the following steps: Providing a stator housing with an inner surface that encloses a housing interior in the circumferential direction, and a stator that has a stator core with electrically conductive stator windings and is arranged in the stator housing in such a manner, that a gap remains between the stator and the inside of the stator housing; Pouring a flowable potting compound containing the thermally conductive casting resin into the gap in such a way that the potting compound substantially fills the gap in a predetermined filling area; Curing of the potting material to form a thermally conductive solid that encapsulates the stator and connects it mechanically and thermally to the stator housing.

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