JP7792863B2 - Heat shielding structure - Google Patents

Description

The present invention relates to a heat shielding structure, particularly a heat shielding structure made of ceramics that provides excellent heat shielding effects in high temperature ranges.

Heat-blocking structures, or heat-shielding structures, can be made of a single material or made of a component made of one material with another material formed on its surface.

However, when constructed from a single material, it is difficult for that single material to satisfy all of the properties required for a structural material, such as heat resistance, heat insulation, and impact resistance, so it is usually used in a form where other materials are combined with the surface of a component made of one material.

Known examples of components made of one material with another heat-insulating material formed on the surface include a structure in which the surface (high-temperature side) is made of a highly absorptive layer such as SiC or C, which converts radiant light into heat, and the back (low-temperature side) is made of a heat-insulating layer made of a material with low thermal conductivity, or a structure in which a single material is made of a metal with a high melting point and low emissivity, which forms a layer that reflects linearly polarized radiant light.

One form of heat-shielding structure with particularly high heat resistance is a technology that forms a coating made of a heat-resistant material on the surface of a component made of an aggregate of various heat-resistant fibers.

On the other hand, the mainstream heat-shielding structure for building materials uses a low-emissivity material with low radiation absorption rate on the surface and an insulating layer on the back. In this way, materials with different properties are combined depending on the application.

A material made of porous magnesia spinel with a characteristic pore size distribution is known to have excellent heat resistance and heat insulation properties, particularly in temperatures above 1000°C.

For example, Patent Document 1 describes a composite refractory insulating material comprising a heat insulating material made of porous ceramics and a refractory material having a compressive strength greater than that of the porous ceramics, the porous ceramics having a porosity of 65 vol% to 90 vol% and being a spinel material expressed by the chemical formula _MgAl2O4 , in which coarse pores having a pore size greater than 1000 _μm account for 25 vol% or less of the total pore volume, and micropores having a pore size of 0.45 μm or less account for 5 vol% to 40 vol% of the volume of pores having a pore size of 1000 μm or less, and having at least one pore size distribution peak within the pore size range of 0.14 μm to 10 μm, and comprising ceramic particles having an arithmetic mean particle size of 0.04 μm to 1 μm.

Patent No. 5877821

In recent years, there has been a need for heat-shielding structures that provide excellent heat-shielding effects at high temperatures, particularly in environments above 1000°C, and that are lightweight, small in volume, and highly durable.

When attempting to create a heat-shielding structure that meets these needs, for example, conventional heat-resistant heat-shielding structures convert radiant light into heat, resulting in high surface temperatures, and the insulating layer on the backside must be sufficiently thick and insulating. This is disadvantageous for applications requiring a lightweight, low-volume structure.

In addition, in one embodiment of a thermal barrier structure in which a high-melting-point metal is used for the thermal barrier layer, an oxide film forms on the metal surface in an oxygen-containing atmosphere, and the formation of this oxide film increases emissivity. Therefore, the structure can only be used in a high vacuum, limiting its applications. In particular, it is difficult to apply the structure to environments exposed to the atmosphere or combustion gases.

Alternatively, some low-emissivity materials used in heat-shielding structures in building materials combine a heat-shielding layer with a heat-insulating layer, but the low-emissivity materials used in heat-shielding layers have low heat resistance, and none can be used at high temperatures, particularly above 1000°C.

Here, magnesia-spinel porous ceramics, such as those described in Patent Document 1, have excellent heat insulating properties in the temperature range above 1000°C, so it is easy to imagine their application to heat shield structures.

However, while the porous ceramics described in Patent Document 1 suppress the increase in thermal conductivity in the temperature range above 1000°C, there are many unknowns about their heat-shielding properties, and simply applying them to obtain a heat-shielding structure that is lightweight, low-volume, and highly durable will not easily achieve the expected properties.

The present invention was made in light of the above, and aims to provide a heat shielding structure that, compared to conventional technology, provides superior radiant insulation performance, particularly in environments exceeding 1000°C, and is lightweight, small in volume, yet highly durable.

A thermal barrier structure according to one aspect of the present invention includes a substrate and a thermal barrier layer formed on one main surface of the substrate, wherein the substrate is made of porous ceramics, the thermal barrier layer contains MgAlO4 and has a bulk density of ₁ g/cm3 ^or greater and 1.7 g/cm3 ^or less , the proportion of pores having a pore diameter of 0.5 μm or greater and less than 10 μm in the total pore volume of the thermal barrier layer is 50 vol% or greater and 77 vol% or less , the proportion of pores having a pore diameter of 1 μm or greater and less than 5 μm in the pore volume of pores having a pore diameter of 0.5 μm or greater and less than 10 μm is 61 vol% or greater and 84 vol% or less, and the proportion of pores having a pore diameter of more than 5.5 μm in the total pore volume of the thermal barrier layer is 3 vol% or greater and 21 vol% or less .

This configuration allows for a heat-shielding structure that provides superior heat-shielding performance, particularly in environments exceeding 1000°C, compared to conventional technology.

A thermal barrier structure according to another aspect of the present invention comprises a substrate and a thermal barrier layer formed on one main surface of the substrate, wherein the substrate is made of a ceramic material having an emissivity of 0.75 or more, the thermal barrier layer contains MgAlO4 and has a bulk density of ₁ g/cm3 ^or more and 1.7 g/cm3 or ^less , the proportion of pores having a pore diameter of 0.5 μm or more and less than 10 μm in the total pore volume of the thermal barrier layer is 50 vol% or more and 77 vol% or less , the proportion of pores having a pore diameter of 1 μm or more and less than 5 μm in the pore volume of pores having a diameter of 0.5 μm or more and less than 10 μm is 61 vol% or more and 84 vol% or less, and the proportion of pores having a pore diameter larger than 5.5 μm in the total pore volume of the thermal barrier layer is 3 vol% or more and 21 vol% or less . A specific aspect is one in which the substrate is made of ceramic fiber cloth.

In this case, the total thickness of the substrate and the heat shield layer can be 2 mm or less, making it possible to use it as a lightweight, low-volume heat shield structure.

According to the present invention, by coating conventional materials that can be used at high temperatures with a highly reflective heat-shielding layer optimized to provide an appropriate heat-shielding effect, it is possible to obtain a heat-shielding structure that can be used at high temperatures and has excellent heat-shielding effects. Furthermore, by coating such a highly reflective heat-shielding layer on a heat-shielding layer made of a highly absorptive material in order to suppress the transmission of radiant light, it is possible to obtain a heat-shielding effect equal to or better than that of conventional products with a thickness that is about one-third of the conventional thickness, and it is also possible to obtain a heat-shielding structure that is lightweight and has a low volume.

FIG. 1 is a schematic diagram showing a method for evaluating a heat flux ratio and a temperature ratio used in verifying the present invention.

The present invention will be described in detail below. One aspect of the present invention is a thermal barrier structure comprising a substrate and a thermal barrier layer formed on one main surface of the substrate, wherein the substrate is made of porous ceramics, the thermal barrier layer has a bulk density of 2 g/ ^cm3 or less, and the proportion of pores having a pore diameter of 0.5 μm or more and less than 10 μm in the total pore volume of the thermal barrier layer is 40 vol% or more, the proportion of pores having a pore diameter of 1 μm or more and less than 5 μm in the pore volume of pores having a diameter of 0.5 μm or more and less than 10 μm is 50 vol% or more, and the proportion of pores having a pore diameter of more than 5.5 μm in the total pore volume of the thermal barrier layer is 55 vol% or less.

In one aspect of the present invention, the substrate is a porous material. A wide range of known porous materials can be used depending on the intended use of the heat shield structure, as long as they function as a thermal insulating layer. Examples include heat-resistant ceramic materials such as alumina and silicon carbide, and foam ceramics. Ceramics with added fibers or the like may also be used.

The heat-shielding layer according to one aspect of the present invention is, in a broad sense, composed of diffuse reflection components with particles of 1 to 5 μm diameter and pores of 1 to 5 μm diameter as scatterers. It can be said that the heat-shielding layer achieves its heat-shielding effect through diffuse reflection based on the Mie theory, i.e., Mie scattering. Here, Mie scattering is scattering that occurs when the wavelength of radiated light and the size of the scatterers are similar.

However, since other materials within the scope of conventional technology can provide sufficient heat-shielding effects in the temperature range below 1000°C, in order to differentiate this invention from conventional technology, it is assumed that it will be used in temperatures above 1000°C.

In other words, in this invention, since the dominant wavelength of radiant light at temperatures between 1000°C and 2000°C is 1 to 5 μm, Mie scattering is caused by making the pores and particles approximately this size. Note that Mie scattering does not occur at exactly the same size as the wavelength, but at a size close to it (approximately 1/10 to 10 times larger).

However, in order for particles to act as scatterers, it is important that the layer has a certain degree of refractive index difference, and therefore the layer needs to contain not only particles but also many pores. Thus, in the present invention, the present inventors further investigated these conditions and found that the optimal conditions are that the bulk density of the thermal barrier layer is 2 g/ ^cm3 or less, the proportion of pores having a pore diameter of 0.5 μm or more and less than 10 μm to the total pore volume of the thermal barrier layer is 40 vol% or more, the proportion of pores having a pore diameter of 1 μm or more and less than 5 μm to the pore volume of pores having a pore diameter of 0.5 μm or more and less than 10 μm is 50 vol% or more, and the proportion of pores having a pore diameter of more than 5.5 μm to the total pore volume of the thermal barrier layer is 55 vol% or less.

First, by setting the bulk density of the thermal barrier layer according to the present invention to 2 g/ ^cm3 or less, the porosity can be increased and solid-state heat transfer can also be suppressed. Here, suppression of solid-state heat transfer includes not only the heat transfer within the thermal barrier layer but also the effect of increasing the contact resistance at the interface between the thermal barrier layer and the substrate, which is the heat insulating layer. However, if the bulk density is too low, the strength decreases and the layer becomes brittle, it becomes difficult to obtain the pore distribution of the present invention described below, sufficient diffuse reflection does not occur, and there is a concern that the production costs will increase. Therefore, in the present invention, it is preferable that the lower limit of the bulk density be 0.3 g/ ^cm3 .

Next, in the heat shield layer according to the present invention, the proportion of pores with a pore diameter of 0.5 μm or more but less than 10 μm relative to the total pore volume of the heat shield layer is 40 vol% or more. If this proportion is less than 40 vol%, it falls outside the wavelength range of radiant light assumed in the present invention, which is undesirable as it reduces the Mie scattering effect. Furthermore, the closer this proportion is to 100 vol%, the more scatterers there are, so it can be said that it is theoretically more effective.

The thermal barrier layer according to the present invention has a pore volume with a pore diameter of 0.5 μm or more but less than 10 μm, in which the proportion of pores with a pore diameter of 1 μm or more but less than 5 μm is 50 vol % or more. If this proportion is 50 vol % or more, Mie scattering can occur efficiently.

In the heat shield layer of the present invention, the proportion of pores with a pore diameter greater than 5.5 μm relative to the total pore volume of the heat shield layer is 55 vol% or less. By keeping this proportion at 55 vol% or less, it is possible to suppress the transmission of radiant light.

Compared to structures consisting only of a thermal insulation layer on a substrate, the thermal barrier structure of the present invention, equipped with the above-described configuration, can easily and significantly improve radiant thermal insulation performance simply by coating the surface layer of the substrate. Coating can be performed using conventional methods, such as spray coating, screen printing, brush coating, electrodeposition coating, application with a bar coater, application with a doctor blade, and spin coating.

The heat shield structure can be manufactured, for example, by applying a coating material made from a mixture of spinel porous powder, ceramic adhesive, and water to a substrate using the method described above, and then firing the coating at approximately 1,000°C.

Here, considering that the material for the thermal barrier layer is used at 1000°C or higher and that the above-mentioned pore size distribution can be obtained relatively easily, a porous body made of a high-melting-point composition, i.e., ceramics, is preferred. Furthermore, oxide ceramics have oxidation resistance. Using high-melting-point ceramics can provide heat resistance and, since _they can contain grain boundaries, can also provide the effect _of suppressing solid-state heat transfer. Specific examples of high-melting-point ceramics include _MgAl2O4 and _LaAl11O18 .

In addition, the heat shield layer of the present invention is preferably not made of a high-melting-point ceramic alone, but rather contains reactive components such as Si and Ca as additives in a range that ensures the pore size distribution of the present invention. This is because the inclusion of reactive components promotes sintering between particles, preventing the heat shield layer from detaching from the base material, the thermal insulation layer or high-absorption heat shield layer. Specifically, the content of the additive is preferably 0.05 to 20 wt%.

The heat shield layer of the present invention may also contain aggregate components such as particles or hollow particles made of ceramic materials (alumina, silicon carbide, etc.), or fibers, to the extent that the above-mentioned pore size distribution can be maintained. Note that the inclusion of fibers is more preferable, as it has the effect of suppressing the amount of shrinkage during the production of the heat shield layer.

To summarize what we have discussed so far, the heat shield layer effectively reflects radiant light onto the surface of the heat shield structure, the base material has the effect of suppressing solid and gas heat transfer, and interfacial thermal resistance occurs at the interface between the heat shield layer and the base material. Interfacial thermal resistance suppresses solid heat transfer because the heat shield layer is porous, and if the base material is made of fiber, the contact area between the fiber and the heat shield layer is reduced, improving interfacial thermal resistance. Furthermore, because the heat shield layer has low emissivity, radiant light is suppressed not only from the surface but also from the heat shield layer to the insulation layer.

As described above, the heat shielding structure according to one aspect of the present invention can easily impart excellent heat shielding properties to conventional heat insulating materials by providing a heat shielding layer with oxidation resistance and heat resistance on its surface. Conventional heat insulating materials have an issue with radiant light transmission due to their high porosity, but this issue can be resolved by combining them with this heat shielding material.

Next, another aspect of the present invention will be described. Another aspect of the present invention is a thermal barrier structure comprising a substrate and a thermal barrier layer formed on one main surface of the substrate, wherein the substrate is made of a ceramic material having an emissivity of 0.75 or more, the thermal barrier layer has a bulk density of 2 g/ ^cm3 or less, and the proportion of pores having a pore diameter of 0.5 μm or more and less than 10 μm in the total pore volume of the thermal barrier layer is 40 vol% or more, the proportion of pores having a pore diameter of 1 μm or more and less than 5 μm in the pore volume of pores having a diameter of 0.5 μm or more and less than 10 μm is 50 vol% or more, and the proportion of pores having a pore diameter of more than 5.5 μm in the total pore volume of the thermal barrier layer is 55 vol% or less.

In other words, the substrate in one embodiment of the present invention is porous ceramic, while in another embodiment the substrate is made of a ceramic material with an emissivity of 0.75 or higher. However, the structure of the heat shield layer in both embodiments is the same.

In another aspect of the present invention, by using a ceramic material with an emissivity of 0.75 or more for the base material, the thickness of the entire heat shielding structure can be made even thinner while still achieving equivalent heat shielding properties.

Specific examples of the substrate include heat-resistant _, highly absorptive materials such as SiC, C, and _Si3N4 . Furthermore, by using a fiber cloth structure for the substrate, the thermal interface resistance between the heat shield layer and the substrate can be increased, improving the radiation insulation performance and providing the advantage of greater freedom in shape, making it advantageous for on-site construction and heat shielding of special shapes.

Furthermore, when a combination of SiC fiber cloth for the substrate _{and MgAl2O4} for the heat shield layer is used, the total thickness of the heat shield structure can be kept to 2 mm or less, even when used in an oxidizing atmosphere at 1000°C or higher. In conventional structures, to achieve the same level of heat shielding performance, the substrate and heat shield layer had to be made thick. In contrast, in a more preferred embodiment of the present invention, a sufficient heat shielding effect can be obtained even with a thickness of 1 mm.

In another aspect of the present invention, a heat shield structure can be manufactured by applying a coating material made by mixing spinel porous powder, ceramic adhesive, and water onto a substrate, and then firing the coating material, similar to the heat shield structure described above.

In particular, in the above-mentioned embodiment, the heat-shielding layer effectively reflects radiant light toward the surface of the heat-shielding structure, and because gaps exist between the fibers at the interface between the heat-shielding layer and the substrate, the contact area with the heat-shielding layer is small, suppressing heat transfer between solids and improving heat-shielding properties at the interface. Furthermore, because the substrate is made of SiC fiber cloth, the temperature distribution within the substrate is small, thereby reducing heat re-distribution. The heat-shielding structure of the present invention, which combines all of these factors, can achieve approximately the same heat-shielding effect in a lighter form compared to conventional thermal insulation material alone or thermal insulation material with a heat-shielding layer formed on it. The present invention does not place any particular restrictions on the form of the SiC fiber cloth (fiber bundle diameter, density, folded shape, short fiber to long fiber content, etc.), and it can be widely applied.

In another aspect of the present invention, a conductive material with low radiant light transmission can also be used for the substrate. Here, a conductive material with low radiant light transmission is a metal material (e.g., an alloy such as platinum or stainless steel).

Conductive materials that transmit little radiant light are opaque bodies similar to heat-resistant, highly absorptive materials such as SiC, Si ₃ N ₄ , and C, and can be used as the substrate of the present invention as long as heat resistance is ensured, thereby achieving a heat-shielding effect similar to that of SiC or C. However, they are somewhat disadvantageous compared to ceramic materials in terms of weight reduction and corrosion resistance.

In addition, Mie scattering is caused by diffuse reflection with little directionality. For this reason, applying this coating to the innermost layer of a heat treatment furnace is expected to not only provide a heat-shielding effect, but also reduce temperature distribution within the furnace.

The present invention will be described in detail below based on examples, but the present invention is not limited to the examples shown below.

[Examples 1 to 8]
11 mol of hydraulic alumina powder (BK-112: manufactured by Sumitomo Chemical Co., Ltd.) and 9 mol of magnesium oxide powder (MGO11PB: manufactured by Kojundo Chemical Laboratory Co., Ltd.) were mixed, and pure water was added to prepare a slurry. The mixture was then hydraulically molded to obtain a molded body measuring 60 mm x 70 mm x 20 mm. This molded body was fired in an oxygen atmosphere at 1500°C for 3 hours to obtain a spinel porous body. This spinel porous body was pulverized using an alumina mortar and pestle and sieved with 1 mm mesh to obtain a spinel porous powder. The obtained spinel porous powder was mixed with a commercially available ceramic adhesive (FF adhesive: manufactured by Nichias Corporation) in a weight ratio of 3:2 or 4: ₁ , and alumina ( _Al2O3 ) fiber (Denka Arsen B100: manufactured by Denka Co., Ltd.) was added at an outer percentage of 0 wt% or 10 wt%, and pure water was added in an amount to obtain fluidity to prepare a coating material.
The obtained coating material was applied to a thickness of 1 to 2 mm on the surface of a substrate (diameter 50 mm, thickness 0.3 to 4 mm) that had been processed into a disk shape, and the coating material was then fired at 1000°C for 3 hours to produce the heat shield structures of Examples 1 to 7, and fired at 1500°C for 3 hours to produce the heat shield structure of Example 8.
In Examples 1 to 6 and 8, a porous ceramic fiber-based heat insulating material (Fibermax 1600P board, manufactured by Isolite Kogyo Co., Ltd.) was used as the substrate, and in Example 7, a silicon carbide continuous fiber cloth (Hi-Nicalon Type S, manufactured by Nippon Carbon Co., Ltd.) made of a silicon carbide ceramic material with an emissivity of 0.86 was used.
The mixing ratio of the spinel porous powder to the ceramic adhesive, the heat shield layer and substrate layer that form the resulting heat shield structure, and the overall thickness are shown in Table 1. In Table 1, the substrates of Examples 1 to 6 and Example 8 are described as heat insulating layers, and the substrate of Example 7 is described as a high absorptivity material.

[Example 9]
11 mol of hydraulic alumina and 11 mol of lanthanum oxide were mixed and added with pure water to form a slurry. This slurry was cast to obtain a molded body measuring 60 mm x 70 mm x 20 mm. This molded body was fired at 1700 °C for 3 hours to obtain lanthanum hexaaluminate ceramics. This lanthanum hexaaluminate was crushed using an alumina mortar and pestle and sieved with a 1 mm mesh size to obtain lanthanum hexaaluminate powder. The resulting lanthanum hexaaluminate powder was mixed with a commercially available ceramic adhesive (FF adhesive: manufactured by Nichias Corporation) in a weight ratio of 4:1, and a sufficient amount of pure water was added to obtain a coating material.
The obtained coating material was applied to a fiber-based thermal insulation material in a thickness of 1 mm, and then fired at 1500°C for 3 hours in a 100 _% oxygen atmosphere to obtain a heat shield structure having a coating layer made of lanthanum hexaaluminate consisting of _LaAl11O18 .

[Comparative Examples 1 to 4]
Comparative Example 1 uses only the substrate (heat insulating layer) without using a thermal barrier layer. In Comparative Examples 2 to 4, the type and amount of ceramic adhesive were appropriately changed relative to the manufacturing conditions of each example so as to produce structures outside the scope of the present invention.

Table 1 shows the production conditions for Examples 1 to 9, and Table 2 shows the structures and heat-shielding effects (temperature ratio and heat flux ratio) for Examples 1 to 9 and Comparative Examples 1 to 4. Conditions 1 to 4 shown in Table 2 are as follows: Condition 1 is the proportion of pore volume with a pore diameter of 0.5 μm or greater but less than 10 μm to the total pore volume. Condition 2 is the proportion of pore volume with a pore diameter of 1 μm or greater but less than 5 μm to the total pore volume of pores with a diameter of 0.5 μm or greater but less than 10 μm. Condition 3 is the proportion of pores larger than 5.5 μm to the total pore volume. Condition 4 is bulk density.

[evaluation]
For the structure of the thermal barrier layer in Table 2, the pore size distribution in the thermal barrier layer was measured according to JIS R1655:2003 "Test method for pore size distribution of molded fine ceramics by mercury intrusion porosimetry," which is a method for measuring pore size distribution by mercury intrusion porosimetry.
The heat-shielding effect was measured using a tungsten lamp surface-irradiation type infrared vacuum furnace (IVF29VS manufactured by Thermo Riko Co., Ltd.). Specifically, a disk-shaped sample with a diameter of 50 mm and a thickness of 1.6 to 5 mm was prepared. Next, the sample was placed on a 50 mm diameter, 1 mm thick SiC disk. The sample surface was irradiated with a tungsten lamp. The heat flux at a position 2 mm from the rear surface of the SiC disk was measured, and the temperature was measured using a K-type thermocouple sandwiched between the sample and the sample. The heat flux ratio and temperature ratio were determined with and without the sample. A small heat flux ratio and temperature ratio indicate a high heat-shielding effect. The tungsten lamp output was adjusted to perform measurements at two levels: 50% (equivalent to a wavelength temperature of 1965 K) and 80% (equivalent to a wavelength temperature of 2318 K). Figure 1 is a schematic diagram illustrating the evaluation method for the heat flux ratio and temperature ratio.

From Tables 1 and 2, it can be seen that by combining a heat shield layer with a substrate (thermal insulation layer), Examples 1 to 9 have lower temperatures and heat fluxes than Comparative Example 1, an insulation material without a heat shield layer, and therefore achieve a heat shielding effect.

Here, the combination of Example 1 and Example 2, _and the combination of Example 3 and Example 4 are both combinations of materials containing 10 wt% _Al2O3 fiber and not containing it, but Table 2 shows that there is no significant difference in the heat-shielding effect as long as the pore diameter structure is within the range.

Furthermore, Example 7 is composed of a heat-shielding layer and a substrate (high-absorption material), which is a structure according to another aspect of the present invention, and even though its thickness is about one-third of the overall thickness of Examples 1 to 6, it still achieves a heat-shielding effect that is not significantly inferior to Examples 1 to 6.

The waveform of the radiated light differs between lamp outputs of 50% and 80%. The higher the output, the more light there is on the short wavelength side, and the higher the energy. Therefore, the heat flux and temperature are higher at 80% lamp output than at 50%. The reason why the temperature ratio does not change significantly between 50% and 80% in Examples 8 and 9 is thought to be because the proportion of pore volume with pore diameters of 0.5 μm or more and less than 10 μm was 77% or more, which is the highest among the examples.

In Comparative Examples 2 to 4, the heat shield layers were prepared by adjusting the pore size structure and bulk density to include those outside the range. Furthermore, all of these examples include those with a temperature ratio exceeding 0.50 or a heat flux ratio exceeding 0.16. Compared to Examples 1 to 9, the temperature ratio and heat flux ratio are larger, indicating inferior heat shielding properties.

Claims (5)

A thermal barrier structure comprising a substrate and a thermal barrier layer formed on one main surface of the substrate,
the substrate is made of porous ceramics,
the thermal barrier layer contains MgAlO4 and has a bulk density of 1 g/cm3 or ^more and 1.7 _g /cm3 ^or less ,
a ratio of pores having a pore diameter of 0.5 μm or more and less than 10 μm to a total pore volume of the thermal barrier layer of 50 vol % to 77 vol % ; a ratio of pores having a pore diameter of 1 μm or more and less than 5 μm to a total pore volume of pores having a diameter of 0.5 μm or more and less than 10 μm of 61 vol % to 84 vol % ; and a ratio of pores having a pore diameter of more than 5.5 μm to a total pore volume of the thermal barrier layer of 3 vol % to 21 vol % . A thermal barrier structure comprising a substrate and a thermal barrier layer formed on one main surface of the substrate,
the substrate is made of a ceramic material having an emissivity of 0.75 or more,
the thermal barrier layer contains MgAlO4 and has a bulk density of 1 g/cm3 or ^more and 1.7 _g /cm3 ^or less ,
a ratio of pores having a pore diameter of 0.5 μm or more and less than 10 μm to a total pore volume of the thermal barrier layer of 50 vol % to 77 vol % ; a ratio of pores having a pore diameter of 1 μm or more and less than 5 μm to a total pore volume of pores having a diameter of 0.5 μm or more and less than 10 μm of 61 vol % to 84 vol % ; and a ratio of pores having a pore diameter of more than 5.5 μm to a total pore volume of the thermal barrier layer of 3 vol % to 21 vol % . The heat shield structure according to claim 2, characterized in that the substrate is made of ceramic fiber cloth. The thermal barrier structure described in claim 2, characterized in that the total thickness of the substrate and the thermal barrier layer is 2 mm or less. The thermal barrier structure described in claim 3, characterized in that the total thickness of the substrate and the thermal barrier layer is 2 mm or less.