HIGH-TEMPERATURE SUPERIOR INSULATION

Description

[0001] The present disclosure relates to a high-temperature thermal insulation material in the form of a powder mixture comprising expanded perlite in powder form and an opacifier in powder form. Furthermore, the disclosure relates to high-temperature thermal insulation elements comprising this high-temperature thermal insulation material, which are suitable for temperatures between 100°C and 800°C.

[0002] Thermal insulation material made of powdered expanded perlite or fumed silica with opacifiers is known, for example, from DE102008003400A1, DE102008003421A1, DE102005045726A1, DE102010029513A1, DE202007013074U1, DE4432896C2, WO2017 / 097768A1, US3147877A, and WO2016 / 108794 A1. These known thermal insulation materials and vacuum superinsulation elements made from them are suitable for temperatures in the range between -100°C and +80°C, but are hardly suitable for high-temperature applications in the range between 100°C and 800°C.

[0003] The effective thermal conductivity k_eff of a thermal insulation material or thermal insulation element consists of three components: k_eff = k_Gas + k_FK + k_Strahlung k_Gas is caused by convection and gas heat conduction. This fraction can be reduced by evacuation. k_FK results from solid-state heat conduction. This fraction can be reduced by the smallest contact points between the individual particles, e.g., powder with a granular structure. The following applies to thermal conductivity due to thermal radiation: k_Strahlung = 16 * SBK * T 3 / 3 * E with Stefan-Boltzmann constant SBK, the extinction coefficient E and the mean radiation temperature T. Due to the T 3< dependence of the thermal conductivity on thermal radiation, this component of the thermal conductivity is dominant in high-temperature applications. k_radiation can be reduced by increasing the extinction coefficient E. In this context, an opacifier is generally understood to be a material which, in combination with another material, called the base material, has a higher extinction coefficient for thermal radiation than the base material, so that the extinction coefficient of the combination of base material and opacifier is higher than for the pure base material. From a lecture by Dr. Thomas Beikircher on 29 March 2017 in Karlsruhe on vacuum superinsulation (https: / / www.hs-karlsruhe.de / fileadmin / hska / EIT / Aktuelles / seminar_erneurbare_energien / Sommer_2017 / Fo lien / 29032017VSIBeikircher.pdf) it is known that with fumed silica as the base material, effective thermal conductivities k_eff in the range between 0.02 and 0.04 W / mK can be achieved by adding opacifiers in the temperature range between 400°C and 800°C. The disadvantage of fumed silica is its high price. For the considerably cheaper expanded perlite as the base material, no experimentally determined values ​​for thermal conductivity at higher temperatures are yet available. However, model calculations suggest (see Beikircher) that comparable values ​​for effective thermal conductivity can be achieved with expanded perlite containing opacifiers.

[0004] It is therefore an object of the present invention to provide a high-temperature thermal insulation material based on expanded perlite with opacifier and high-temperature thermal insulation elements with this thermal insulation material.

[0005] According to a first aspect of the invention, the above object is achieved by a high-temperature thermal insulation material according to claim 1.

[0006] Experiments by the inventor have shown that in the range between 400°C and 800°C for a powder mixture consisting of expanded pearlite and an opacifier made of boron carbide, silicon carbide, rutile, ilmenite or carbon black or a mixture of these substances, thermal conductivities k_eff in the range between 0.013 and 0.05 W / mK can be achieved, provided the total density of the powder mixture is in the range between 160 and 500 kg / m 3< , and provided the powder mixture contains between 20 and 80% by weight of silicon carbide as an opacifier. The total density of the powder mixture can be determined experimentally by determining the mass and volume of a powder bed and relating them to one another. The proportion of the opacifier in weight %, based on the total powder mixture, can be determined by weighing.

[0007] Examples of opacifiers are boron carbide, silicon carbide, rutile, ilmenite, carbon black, or a mixture of these substances. By adding opacifiers, k_eff can be reduced at higher temperatures. According to the invention, the opacifier is silicon carbide (SiC); it is inexpensive and leads to low values ​​for k_eff. Boron carbide (B4C) enables even lower values ​​for k_eff, but is very expensive and therefore only of interest for special applications. Rutile (TiO2) and ilmenite (FeTiO3) are similarly inexpensive to silicon carbide, but yield somewhat higher values ​​for k_eff in the powder mixture. Carbon black yields similarly good values ​​for k_eff as boron carbide, but is somewhat more difficult to handle.

[0008] According to a preferred embodiment, the total density of the powder mixture is in the range between 180 and 340 kg / m 3 . This allows particularly low thermal conductivities k_eff in the range between 0.013 and 0.04 W / mK to be achieved.

[0009] According to a further preferred embodiment, the expanded perlite has an average grain size of 2 to 100 µm, and in particular of 10 to 70 µm. Expanded perlite of this type has proven particularly suitable. The average grain size is the mean value of the individual grain sizes.

[0010] According to a particularly preferred embodiment, the expanded perlite has an average grain size of 35 µm and a grain size distribution of 5 to 140 µm. Such perlite is offered as Perlite Eurocell®< 140. The average grain size and distribution are determined by sieve analysis according to DIN 66165.

[0011] According to a further preferred embodiment, the powdered opacifier has an average particle size of 1 to 10 µm, and preferably 1.5 to 4 µm. Tests have shown that the best results are achieved with an average particle size within this range.

[0012] According to a particularly preferred embodiment, the opacifier is silicon carbide with an average grain size of 2 µm and a grain size distribution between 1 and 6 µm. Tests have shown that the best results are achieved with silicon carbide, and the very best with silicon carbide with an average grain size of 2 µm and a grain size distribution between 1 and 6 µm. A silicon carbide with such properties is available, for example, under the name Silcar®< P8. The average grain size and distribution are determined by laser diffraction according to ISO 13320.

[0013] According to the invention, the proportion of silicon carbide in the powder mixture is 20 to 80% by weight, preferably 20 to 60% by weight, and particularly preferably 30 to 50% by weight. This allows particularly low values ​​for the effective thermal conductivity k_eff to be achieved with silicon carbide as an opacifier.

[0014] According to a second aspect of the invention, the above-mentioned object is achieved with respect to a high-temperature thermal insulation element in that at least one molded part is pressed from high-temperature thermal insulation material according to the first aspect of the invention.

[0015] A molded part can, for example, be a flat insulation panel for insulating a flat surface, a hollow cylinder or a cylindrical half-shell for insulating a cylindrical pipe, or another shaped molded part. By pressing a molded part from high-temperature thermal insulation material according to the first aspect of the invention, components of various geometries can be efficiently insulated in various insulation applications.

[0016] According to a preferred embodiment, the molded part contains fiber elements in addition to the high-temperature thermal insulation material. For example, the molded part contains glass fibers. The addition of fiber elements can improve the dimensional stability of the molded parts.

[0017] According to a further preferred embodiment, the molded part is enclosed in a vacuum-tight shell and evacuated. Vacuum-tight means that the shell is sufficiently tight so that the interior of the vacuum-tight shell can be evacuated. Evacuated means that the gas pressure inside the vacuum-tight shell is lower than the ambient gas pressure or lower than 300 mbar (cf. DIN 28400 Part 1). For example, the gas pressure inside the vacuum-tight shell is less than 10 mbar, for example 0.05 mbar or 0.001 mbar. By evacuating, particularly low thermal conductivities k_eff can be achieved. The molded part is, for example, a flat insulation panel, a hollow cylinder, a cylindrical half-shell, or another shaped molded part that is surrounded by a vacuum-tight shell and is evacuated.In particular, the molded part can also be created by evacuation, in which the high-temperature thermal insulation material is filled into a vacuum-tight shell, the shell is sealed, and the high-temperature thermal insulation material is compressed during evacuation by the gas pressure of the environment acting from the outside.

[0018] According to a further preferred embodiment, the vacuum-tight envelope consists of a welded metal foil, and in particular of a stainless steel metal foil. Such metal foils are characterized by particularly good vacuum tightness at high temperatures. Thus, the gas pressure inside the vacuum-tight envelope can be kept low over a long period of time, for example, several months. Accordingly, the thermal conductivity k_eff can also be kept low over this period. The thickness of the metal foil is preferably between 50 and 100 µm. This ensures sufficient vacuum tightness; at the same time, thermal bridges that result from the metal foil at the edge of the high-temperature thermal insulation element are kept small.

[0019] According to the second aspect of the invention, a high-temperature thermal insulation element with a dimensionally stable, vacuum-tight metal shell is also described, into which high-temperature thermal insulation material according to the first aspect of the invention is filled, and which is evacuated. In this case, the high-temperature thermal insulation material is not pressed into a molded part, but rather the powder mixture is filled into a dimensionally stable, vacuum-tight metal shell. Vacuum-tight means that the metal shell is sufficiently tight so that the interior of the dimensionally stable, vacuum-tight metal shell can be evacuated. Dimensionally stable means that the metal shell is sufficiently stable to retain its shape when the interior of the dimensionally stable, vacuum-tight metal shell is evacuated, and the ambient gas pressure acts on the dimensionally stable, vacuum-tight metal shell.For example, the gas pressure inside the dimensionally stable, vacuum-tight metal shell is less than 10 mbar, for example, 0.05 mbar or 0.001 mbar. Evacuation allows for particularly low thermal conductivities (k_eff) to be achieved.

[0020] According to a preferred embodiment, the dimensionally stable, vacuum-tight metal shell encloses a cavity, resulting in a double-walled container. For example, this results in a cylindrical, double-walled container; however, a spherical, double-walled container or a double-walled container of another shape can also be produced. For example, the high-temperature thermal insulation material is filled into the annular gap of the, for example, cylindrical, double-walled container. For example, a heat storage medium is introduced into the volume enclosed by the inner wall of the, for example, cylindrical, double-walled container. Such an arrangement is suitable, for example, as a heat storage device, which is particularly well thermally insulated due to the low thermal conductivity k_eff of the high-temperature thermal insulation material filled into the annular gap and evacuated.

[0021] According to the second aspect of the invention, a high-temperature thermal insulation element in the form of a powder bed of the high-temperature thermal insulation material according to the first aspect of the invention is also described, wherein the powder bed is located in a gas atmosphere that is not evacuated. "Not evacuated" means that the gas pressure of the gas atmosphere is substantially equal to or higher than the ambient gas pressure and at the same time is at least 300 mbar (cf. DIN 28400 Part 1). For example, the gas pressure of the gas atmosphere is between 500 and 1500 mbar, for example in the range between 900 and 1100 mbar.

[0022] For example, the powder bed is located in the ambient air as a gas atmosphere. However, other gases, particularly heat-insulating gases such as argon or krypton, are also conceivable. Because the powder bed is not evacuated, costs can be saved, especially for expensive vacuum systems. A vacuum-tight enclosure is also not required for a high-temperature thermal insulation element in the form of a non-evacuated powder bed. However, the powder bed can still be surrounded by a enclosure, for example, to hold it in place. However, the enclosure does not need to be vacuum-tight or dimensionally stable.

[0023] Preferred embodiments of the invention are described below with reference to Tables 1 to 5, which show the k_eff values ​​achieved for various powder mixtures, and with reference to the drawings. The drawings show Fig. 1 is a schematic sectional view of a high-temperature thermal insulation element according to the second aspect of the invention, Fig. 2 is a schematic sectional view (longitudinal section) of a further high-temperature thermal insulation element according to the second aspect, Fig. 3 is a further schematic sectional view (cross section) of the high-temperature thermal insulation element from Fig. 3 , and in Fig. 4 a schematic sectional view of another high-temperature thermal insulation element according to the second aspect.

[0024] Tables 1 to 3 below show various measurement series. In all cases, Eurocell ®< 140 perlite with an average grain size of 35 µm and a distribution of 5 to 140 µm was used. The thermal conductivity k_eff was measured using the guarded hot plate method described in DIN 52612, DIN EN 12667, or ASTM C177. In all cases, the k_eff values ​​were determined in an evacuated state, approximately 0.05 mbar. The average grain size of the silicon carbide powder Silcar ®< P8 used as an opacifier is 2 µm with a distribution of approximately 1 to 6 µm.

[0025] In the first series of measurements, Eurocell 140 perlite was used without the addition of opacifiers. The measurement results presented in Table 1 show that, even without the addition of opacifiers, k_eff values ​​were achieved that were lower than those of conventional insulation materials such as mineral wool, silicate fiber mats, and calcium silicate boards. Regarding the k_eff values ​​for conventional insulation materials, reference is made to the aforementioned presentation by Beikircher. The density of the perlite powder is 180 kg / m³. Table 1: First series of measurements with pure perlite Eurocell 140 (density: 180 kg / m 3< ) at different temperatures in evacuated state Temperature [°C] Thermal conductivity [W / mK] 100 0,005 200 0,009 400 0,022 600 0,040

[0026] The second series of measurements (Table 2) shows various powder mixtures of expanded perlite (Perlite Eurocell 140) with varying amounts of silicon carbide as an opacifier at 400°C. With the exception of the first value – no opacifier – the k_eff values ​​for all concentrations mentioned are below 0.017 W / mK, thus in the range of fumed silica with opacifier. The density of the powder mixture is between 180 and 342 kg / m³. Table 2: Second series of measurements at 400°C in evacuated condition with powder mixtures of perlite Eurocell 140 and opacifier SiC under variation of the mixture proportion and thus also the density: SiC content [wt.%] Density [kg / m 3 ] Thermal conductivity [W / mK] 0 180 0,022 20 214 0,016 30 236 0,014 40 263 0,013 50 297 0,014 60 342 0,016

[0027] The third series of measurements (Table 3) shows a powder mixture of 60 wt.% Eurocell 140 perlite and 40 wt.% silicon carbide powder as an opacifier. The total density of the powder mixture is 263 kg / m3. The measured k_eff values ​​for temperatures between 100°C and 600°C are between 0.005 and 0.020 W / mK. Table 3: Third series of measurements with powder mixture of Perlite Eurocell 140 and opacifier SiC (SiC content: 40 wt.%, density: 263 kg / m 3< ) at different temperatures in evacuated state: Temperature [°C] Thermal conductivity [W / mK] 100 0,005 200 0,008 400 0,014 600 0,020

[0028] The following Tables 4 and 5 show further series of measurements in which the materials under investigation were not evacuated. The measurements were carried out in a nitrogen atmosphere at approximately 980 mbar. The fourth series of measurements (Table 4) shows Perlite Eurocell 140 without the addition of opacifiers. The density of the perlite powder is 180 kg / m³. The fifth series of measurements (Table 5) shows a powder mixture consisting of 60 wt.% Perlite Eurocell 140 and 40 wt.% silicon carbide powder as opacifier. The total density of the powder mixture is 263 kg / m³. The measurement results presented in Tables 4 and 5 show that in the non-evacuated state, k_eff values ​​were achieved that are in the range of conventional insulation materials such as mineral wool, silicate fiber mats, and calcium silicate boards. Regarding the k_eff values ​​for conventional insulation materials, reference is again made to the aforementioned presentation by Beikircher. By avoiding evacuation, costs can be saved. Table 4: Fourth series of measurements with pure perlite Eurocell 140 (density: 180 kg / m 3< ) at different temperatures in the non-evacuated state Temperature [°C] Thermal conductivity [W / mK] 100 0,059 200 0,070 400 0,095 600 0,119 Table 5: Fifth series of measurements with powder mixture of Perlite Eurocell 140 and opacifier SiC (SiC content: 40 wt.%, density: 263 kg / m 3< ) at different temperatures in the non-evacuated state: Temperature [°C] Thermal conductivity [W / mK] 100 0,059 200 0,068 400 0,085 600 0,099

[0029] In Fig. 1A schematic sectional view of a high-temperature thermal insulation element according to the second aspect of the invention is shown. The high-temperature thermal insulation element (11) comprises a pressed molded part (12), here in the form of a flat panel consisting of high-temperature thermal insulation material according to the first aspect of the invention, and a vacuum-tight sleeve (13). The molded part (12) does not necessarily contain fiber elements; however, it can also contain fiber elements, such as glass fibers, to improve dimensional stability. The vacuum-tight sleeve (13) consists of a welded metal foil, here, for example, a stainless steel foil with a thickness of, for example, 100 µm. The space enclosed by the vacuum-tight sleeve (13), in which the molded part (12) is located, is evacuated, for example to a gas pressure of 0.05 mbar.Due to the panel shape of the molded part (12), the high-temperature thermal insulation element (11) is particularly suitable for thermal insulation of flat surfaces.

[0030] Fig. 2 shows a schematic longitudinal section of another high-temperature thermal insulation element according to the second aspect of the invention; Fig. 3shows a corresponding schematic cross-section. The high-temperature thermal insulation element (21) here is a cylindrical, double-walled container with a dimensionally stable, vacuum-tight metal shell (23) into which high-temperature thermal insulation material (22) according to the first aspect of the invention is filled. The space formed by the two walls of the dimensionally stable, vacuum-tight metal shell (23), into which the high-temperature thermal insulation material (22) is filled, is evacuated, for example, to 0.05 mbar. Since the dimensionally stable, vacuum-tight metal shell (23) is dimensionally stable, the high-temperature thermal insulation element (21) retains its shape, even if the ambient gas pressure acts on the high-temperature thermal insulation element (21) as a result of the evacuation. The dimensionally stable, vacuum-tight metal shell (23) also encloses a cavity (24), resulting in a double-walled container, here a cylindrical, double-walled container.The space formed by the two walls of the dimensionally stable, vacuum-tight metal shell (23), into which the high-temperature thermal insulation material (22) is filled, is also referred to as the annular gap. A heat storage medium, for example, can be introduced into the cavity (24) enclosed by the dimensionally stable, vacuum-tight metal shell (23). The high-temperature thermal insulation element (21) then insulates the heat storage medium in the cavity (24), making the entire assembly suitable as a heat storage device.

[0031] Fig. 4Finally, a schematic sectional view of another high-temperature thermal insulation element according to the second aspect of the invention is shown. The high-temperature thermal insulation element (31) is in the form of a powder bed (32) made of high-temperature thermal insulation material according to the first aspect of the invention. The powder bed (32) is located in a non-evacuated gas atmosphere. Here, the powder bed (32) is located, for example, in the ambient air as a gas atmosphere, which has a gas pressure of, for example, 1000 mbar. The powder bed is in contact with a surface (33) to be insulated. Here, the surface (33) has a flat shape; however, it can also have a different, in particular an irregular, shape. In this case, the powder bed (32) adapts to the irregular shape of the surface (33).The powder bed (32) does not necessarily have to be enclosed by a casing; however, it may nevertheless be enclosed by a casing (not shown here), for example, to hold the powder bed in place. However, the casing does not need to be vacuum-tight or dimensionally stable.

Claims

1. High-temperature thermal insulation material in the form of a powder mixture with expanded perlite in powder form and opacifier in powder form, characterized in that the powder mixture consists of expanded perlite and an opacifier, wherein the powder mixture has a bulk density in the range between 160 and 500 kg / m3, and wherein 20 to 80 % by weight of silicon carbide is contained in the powder mixture as an opacifier.

2. High-temperature thermal insulation material according to claim 1, characterized in that the powder mixture has a bulk density in the range between 180 and 340 kg / m3.

3. High-temperature thermal insulation material according to claim 1 or 2, characterized in that the expanded perlite has a mean grain size of 2 to 100 µm and preferably of 10 to 70 µm, the mean grain size being determined by sieve analysis according to DIN 66165.

4. High-temperature thermal insulation material according to claim 3, characterized in that the expanded perlite has a mean grain size of 35 µm and a grain size distribution of 5 to 140 µm, the mean grain size and the grain size distribution being determined by sieve analysis according to DIN 66165.

5. High-temperature thermal insulation material according to one of claims 1 to 4, characterized in that the opacifier has a mean grain size of 1 to 10 µm and preferably of 1.5 to 4 µm, the mean grain size being determined by laser diffraction according to ISO 13320.

6. High-temperature thermal insulation material according to claim 5, characterized in that the opacifier is silicon carbide with a mean grain size of 2 µm and a grain size distribution of 1 to 6 µm, the mean grain size and the grain size distribution being determined by laser diffraction according to ISO 13320.

7. High-temperature thermal insulation material according to one of claims 1 to 6, characterized in that 20 to 60 % by weight, and preferably 30 to 50 % by weight, of silicon carbide is contained in the powder mixture as an opacifier.

8. High-temperature thermal insulation element, with at least one pressed molded part made of high-temperature thermal insulation material according to one of claims 1 to 7.

9. High-temperature thermal insulation element according to claim 8, characterized in that the moulded part contains fibre elements.

10. High-temperature thermal insulation element according to claim 8 or 9, characterized in that the moulded part is enclosed in a vacuum-tight envelope and evacuated.

11. High-temperature thermal insulation element according to claim 10, characterized in that the vacuum-tight envelope consists of a welded metal foil and in particular of a stainless-steel metal foil.

12. High-temperature thermal insulation element, with a dimensionally stable, vacuum-tight metal shell, into which high-temperature thermal insulation material according to one of claims 1 to 7 is filled, and which is evacuated.

13. High-temperature thermal insulation element according to claim 12, characterized in that the dimensionally stable, vacuum-tight metal shell encloses a cavity, so that a double-walled container results.

14. High-temperature thermal insulation element in the form of a powder bulk of the high-temperature thermal insulation material according to one of claims 1 to 7, wherein the powder bulk is located in a gas atmosphere which is not evacuated.