Thermal insulating material

Abstract

To provide a thermal insulating material that exhibits excellent thermal insulation performance under high temperatures, and also possesses high strength and hardness.SOLUTION: A thermal insulating material is obtained by molding a mixture containing 62 to 86 mass% of fumed silica and 10 to 30 mass% of an infrared-shielding material. The mode diameter in the pore size distribution within the range of 70 nm or less is 20 nm or less, the total number of pores is 3.5×1016 pores / g or more, the thermal conductivity at 800°C is 0.06 W / (m K) or less, the compressive strength is 0.4 MPa or more, and the hardness is 75 or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a heat insulating material.

Background Art

[0002] As efforts towards carbon neutrality, improvement in the heat insulation performance of heat insulating materials, particularly the heat insulation performance at high temperatures, is required for reducing heat loss in steelmaking furnaces and the like and improving the power generation efficiency of fuel cells. On the other hand, for the handling property during the construction and assembly of the heat insulating material, the pressure resistance against the thermal expansion of the content during use at high temperatures, and the vibration resistance during transportation, etc., it is also required to be high strength and high hardness. Generally, the heat insulation performance is in an inverse relationship with strength and hardness, and it is difficult to achieve both.

[0003] Conventionally, as a raw material for heat insulating materials, ultrafine powders of silica such as fumed silica have been widely used. For example, in Patent Document 1, in order to mold ultrafine powders of silica alone into a porous body, techniques for obtaining a silica molded body having good heat insulation properties and moldability by devising powder characteristics, the microstructure of the molded body, molding methods, etc. are disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the technology of Patent Document 1, since the ultrafine powder of silica is molded alone, there is a concern that the heat insulation performance will rapidly deteriorate at high temperatures (400 °C or higher) where the influence of radiation is large. That is, in the silica molded body (heat insulating material) obtained by the technology of Patent Document 1, there was a problem particularly in the heat insulation performance at high temperatures. In addition, since the strength and hardness of the silica molded body (heat insulating material) obtained by the technology of Patent Document 1 are premised on the molding of silica alone, it may not be sufficient when it is a molded body of a mixture with other raw materials. In the technology of Patent Document 1, it is not originally assumed to mix additives (fibers, infrared impermeable materials, etc.) with the ultrafine powder of silica and perform compression molding, and it is imagined that the moldability will deteriorate if additives are mixed.

[0006] In view of the above, the problem to be solved by the present invention is to provide a heat insulating material that is excellent in heat insulation performance at high temperatures and has high strength and hardness.

Means for Solving the Problems

[0007] According to one aspect of the present invention, the following heat insulating material is provided. A heat insulating material obtained by molding a mixture containing 62 to 86% by mass of fumed silica and 10 to 30% by mass of an infrared impermeable material, The mode diameter in the range of 70 nm or less of the pore distribution is 20 nm or less, and the total number of pores is 3.5 × 10 16 pieces / g or more, and the heat conductivity at 800 °C is 0.06 W / (m·K) or less, the compressive strength is 0.4 MPa or more, and the hardness is 75 or more.

Effects of the Invention

[0008] According to the present invention, a heat insulating material that is excellent in heat insulation performance at high temperatures and has high strength and hardness can be obtained.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention will be described. The average free path of molecules in air at room temperature is about 70 nm. Therefore, in a porous body having voids with a diameter of 70 nm or less, heat transfer by convection and conduction of air is suppressed, and such a porous body is known to exhibit excellent heat insulation properties. In the present invention, in order to make the heat insulating material have significantly lower thermal conductivity, higher strength, and higher hardness than conventional ones, attention was paid to the pore distribution in the range of 70 nm or less in the porous body and the number of pores inside the heat insulating material, and intensive studies were carried out.

[0011] In the case of a heat insulating material composed of ultrafine powder, the pore size distribution is generally classified into the following three categories. (1) 100 nm or less Pores surrounded by chain-like primary particles in primary aggregates (mode diameter A shown in FIGS. 1 and 2) (2) 100 nm to 10 μm Pores formed in the gaps between primary aggregates in secondary aggregates or in the gaps between fibers (mode diameter B shown in FIGS. 1 and 2) (3) 10 μm or more Relatively large spaces due to gaps between secondary aggregates, defects in the molded body, cracks, etc. (not present in the case of the samples in FIGS. 1 and 2)

[0012] The fumed silica used as the main raw material of the heat insulating material in the present invention has high purity and high heat resistance of 800 °C or higher. As schematically shown in Fig. 3, it is obtained by generating ultrafine particles of silica by burning silicon tetrachloride in oxygen and hydrogen in a flame (1000 °C or higher) (flame hydrolysis method). The primary particles (particle size of about 5 to 30 nm) combine with other particles in the flame to form primary aggregates (particle size of about 100 to 400 nm) having a complex chain structure. It is difficult to decompose the primary aggregates into primary particles, and the primary aggregates become the substantial minimum unit as a powder. The inventors of the present invention have minimized the pore diameter (mode diameter) in the range of 70 nm or less of the primary aggregates as much as possible, and further increased the number of pores formed by the entanglement of the chain structure of the aggregates and the gaps between the aggregates, etc., so that the heat insulating material can be made to have lower thermal conductivity, higher strength, and higher hardness than before.

[0013] In order to minimize the pore diameter of the primary aggregates, for example, it can be achieved by increasing the specific surface area of the fumed silica to make the primary particles smaller. In order to make the primary particles smaller, for example, the production conditions such as the supply amount of silicon tetrachloride and the combustion time are adjusted so that they are combined before the primary particles grow and form primary aggregates. Since the pore diameter in the primary aggregates with a smaller primary particle diameter becomes smaller, the mode diameter in the pore size distribution becomes smaller. Also, the complexity of the chain structure of the primary aggregates (the number of branches, the length of the chain (structure), the surface properties (the number of surface silanol groups, etc.)) is considered to affect the pore diameter and the number of pores, and it is considered that these factors are combined to determine the heat insulating performance, strength, and hardness of the heat insulating material.

[0014] As a reason why minimizing the mode diameter in the range of 70 nm or less of the pore distribution contributes to reducing the thermal conductivity of the heat insulating material, it is speculated that even though the pores are the gentle spaces of chain-like primary particles in the primary aggregates, due to the movement of molecules such as vibration, rotation, translation, contraction, and elongation, the size of the pores changes partially, so that heat transfer by gas molecules cannot be completely suppressed. Therefore, the smaller the pore diameter (mode diameter) is (especially when it is 20 nm or less), the smaller it becomes than the mean free path of molecules in the air even if the size of the pores changes somewhat, which is considered to contribute to suppressing heat transfer by gas molecules. In addition, secondary aggregates (particle size of about 10 to 50 μm) are formed when the primary aggregates are physically bonded to other aggregates by van der Waals forces. The bonding force is not relatively strong and can be dispersed into primary aggregates by applying a strong shearing force or the like to crush it, but it is likely to re-aggregate. Therefore, it is difficult to completely eliminate pores of 100 nm or more formed mainly within the secondary aggregates. Therefore, it is effective to increase the number of pores in the primary aggregates or secondary aggregates by making the chain structure in the primary aggregates larger and more complex. As a result, the number of partitions of the space by the chain-like primary particles increases, so that heat transfer by convection and conduction of air is more suppressed and the heat insulation property is improved. Furthermore, if the number of partitions increases, the pores become denser and the point contact between particles increases, so that the hardness and strength are improved structurally.

[0015] In the present invention, based on the above technical considerations, the following specific configuration is adopted to achieve low thermal conductivity, high strength, and high hardness of the heat insulating material. That is, the heat insulating material of the present invention is obtained by molding a mixture containing 62 to 86% by mass of fumed silica for creating fine pores and 10 to 30% by mass of an infrared impermeable material for reflecting radiant heat in the high temperature range, and has a mode diameter in the range of 70 nm or less of the pore distribution of 20 nm or less and a total number of pores of 3.5×10 16 It is an ultrafine porous material of more than particles / g. Thereby, a heat insulating material having a thermal conductivity of 0.06 W / (m·K) or less, a compressive strength of 0.4 MPa or more, and a hardness of 75 or more at 800°C can be obtained.

[0016] For the content ratios of the respective raw materials in the mixture, it is more preferable that the fumed silica is 70 to 86% by mass and the infrared ray impermeable material is 10 to 25% by mass for the purpose of reducing the thermal conductivity, increasing the strength, and increasing the hardness of the heat insulating material. Here, in the present invention, the mixture refers to a material obtained by mixing each raw material of the heat insulating material and a binder described later, and the content ratio of each raw material in the mixture refers to the ratio occupied in 100% by mass of the total amount of each raw material of the heat insulating material and the solid content of the binder.

[0017] Furthermore, in the present invention, by setting the mode diameter in the range of 70 nm or less of the pore distribution to 16 nm or less and the total number of pores to 4.5×10 16 pieces / g or more, a heat insulating material having a thermal conductivity of 0.055 W / (m·K) or less at 800°C can be obtained. In the present invention, the thermal conductivity is measured in accordance with the hot wire method, specifically JIS R 2251-1.

[0018] The fumed silica used as the main raw material of the heat insulating material in the present invention preferably has a DBP absorption amount of 370 mL / 100 g or more, and more preferably 390 mL / 100 g or more. The magnitude of the DBP absorption amount corresponds to the magnitude of the structure in the fumed silica. The structure is a chain-like or branched structure formed by the connection of a plurality of primary particles, and is related to the size of the primary aggregate and the spread of the structure. When the structure is large (long), as shown in FIG. 4, it is considered that the entanglement of the chain-like structure in the primary aggregate increases, so that the average pore diameter becomes smaller and the number of pores increases, and thus, as described above, it contributes to the improvement of the heat insulating property, strength, and hardness of the heat insulating material. Therefore, it is considered that the control of the structure is effective for adjusting such an aggregated structure. In addition, the specific surface area of the fumed silica (BET method, N2 adsorption: BET specific surface area) is preferably 300 to 500 m 2 / g, and more preferably 400 to 450 m 2It is more preferable that it is / g. As described above, the larger the specific surface area, the lower the thermal conductivity and the higher the hardness and strength. That is, when the specific surface area is small, the pore diameter becomes large and the thermal conductivity becomes high. However, if the specific surface area is too large, there is a concern that the cohesive force is strong, making it difficult to disperse the aggregates, and since the primary particles are small, sintering progresses easily, resulting in a decrease in heat resistance. In addition, for improving heat resistance by suppressing sintering, the mixture may contain fumed alumina, precipitated alumina, etc.

[0019] In heat insulation at high temperatures (400 °C or higher), heat transfer by radiation becomes much larger compared to heat conduction and convection. Therefore, for suppressing heat transfer at high temperatures, reducing radiation is effective. Thus, in the present invention, an infrared impermeable material is contained in the mixture as a material that reflects, absorbs, etc. infrared rays. As the infrared impermeable material, materials with a high refractive index such as silicon carbide, iron oxide, zirconium silicate, titanium oxide, etc. can be used. In order to efficiently reflect radiant heat, considering the infrared wavelength, it is preferable that the average particle diameter (D50) of the infrared impermeable material is 1 to 5 μm, and more preferably 2 to 4 μm. Also, for shielding infrared rays (radiation), the content rate and dispersibility in the mixture are important. The content rate of the infrared impermeable material is set to 10 to 30 mass%, and it is preferably pulverized as much as possible by mixing with fumed silica using a mixer or the like and causing them to collide with each other, and dispersion and mixing are performed. Note that when the content rate of the infrared impermeable material is less than 10 mass%, heat transfer by radiation increases and the thermal conductivity becomes high. Also, when the content rate of the infrared impermeable material exceeds 30 mass%, the content rate of fumed silica decreases, resulting in coarsening of the pores, not only a decrease in strength and hardness, but also an increase in thermal conductivity.

[0020] In addition, in order to improve the shape retention of the heat insulating material which is a molded body, inorganic fibers may be included in the mixture. Inorganic fibers are fibers made of inorganic substances, and silica fibers, glass fibers, alumina fibers, etc. can be used. Considering heat resistance and cost, it is preferable to use silica fibers (SiO2 content of 95% by mass or more). The content of the inorganic fibers in the mixture is preferably 1 to 10% by mass, more preferably 2 to 5% by mass, from the viewpoints of moldability and workability. Also, as the shape of the inorganic fibers, it is preferable that the average fiber diameter is 4 to 12 μm and the average fiber length is 3 to 10 mm. A more preferable range is that the average fiber diameter of the inorganic fibers is 5 to 9 μm and the average fiber length is 5 to 8 mm.

[0021] Although a higher bulk density of the heat insulating material can provide higher hardness and strength, if it is too high, there is a risk of deterioration of the heat insulating property, and if it is too low, there is a risk of reduction in strength. Therefore, it is preferably 240 to 350 kg / m 3 and more preferably 260 to 310 kg / m 3 .

[0022] The heat insulating material of the present invention can be obtained by a manufacturing method including the following steps. (1) Mixing step: The raw materials of the heat insulating material, that is, fumed silica, infrared ray impermeable material, inorganic fibers, etc. are dry-mixed in a mixer, and further dry-mixed in a mixer while spraying an inorganic binder (such as a colloidal silica solution) to produce a powdery mixture. (2) Forming step: Using the mixture obtained in the mixing step, dry uniaxial compression molding is performed with a press molding machine having upper and lower press parts provided with a large number of holes.

[0023] In the above-mentioned mixing step, fumed silica having a large specific surface area and structure is sufficiently crushed, dispersed, and mixed. As a result, the pore diameter can be made smaller and the number of pores can be increased. As a result, the thermal conductivity is reduced, and furthermore, the strength and hardness are improved. Also, since the infrared ray impermeable material having a high infrared ray shielding effect is also sufficiently crushed, dispersed, and mixed, high heat insulating property can be obtained even at high temperatures. The mixer used in the mixing process is not particularly limited as long as it has appropriate shearing force, impact force, etc. for crushing powders (especially aggregates). A Henschel mixer, a super mixer, a Spartan mixer, etc. can be used. Regarding the shape of the blades used in the mixer, any shape that can exhibit high shearing force may be selected, and an appropriate one is chosen.

[0024] In addition, the heat insulating material of the present invention can be manufactured without including a heating process for heating the molded body obtained in the above-described molding process. Therefore, since it can be manufactured with a short tact time and heating equipment is not required, both the initial cost and the running cost can be suppressed, and thus the total manufacturing cost can be reduced. Furthermore, since the amount of CO2 emissions during manufacturing is small, it also contributes to carbon neutrality. As described above, in the manufacturing method of the heat insulating material of the present invention, post-treatment such as heat treatment is not required. Therefore, not only is the cost low, but also the manufacturing is simple, the tact time is short, and a heat insulating material with high heat resistance, low thermal conductivity, and high strength can be obtained.

[0025] The use of the heat insulating material of the present invention is not particularly limited. For example, it can be used for heat insulation of steelmaking furnaces such as ladles, mixer cars, and tundishes, rolling furnaces such as soaking pits and heating furnaces, heat treatment furnaces such as carburizing furnaces, metal sintering furnaces, and induction treatment furnaces, non-ferrous metal furnaces such as melting furnaces and fuel heating furnaces, ceramic furnaces such as glass melting furnaces, cement firing furnaces, and refractory firing furnaces, or for heat insulation of the spaces between cells or modules of secondary batteries and high-temperature parts such as reformers and cell stacks of fuel cells.

Examples

[0026] Table 1 shows the raw material formulations and physical property values of the heat insulating materials obtained for the examples and comparative examples of the present invention.

[0027]

Table 1

[0028] The measuring methods for each physical property value shown in Table 1 are as follows. Unless otherwise specified, the measurements are taken at room temperature (about 20 - 25°C). [DBP Absorption Amount (Oil Absorption Amount) of Fumed Silica] The DBP (dibutyl phthalate) absorption amount (oil absorption amount) of fumed silica, which is a raw material of the heat insulating material, was measured using an absorption amount measuring device (S410E manufactured by Asahi R & D Co., Ltd., compliant with JIS K 6217 - 4). The dropping rate of the oil was set at 4 mL / min, and the DBP dropping amount (mL) at 70% of the maximum torque was calculated and converted to the value per 100 g of the sample, which was defined as the DBP absorption amount (mL / 100 g). [Pore Distribution] The pore distribution of the heat insulating material was measured using a mercury porosimeter (Autopore IV 9520 manufactured by Micromeritics) based on JIS R1655. The sample was processed into a size that could be inserted into the holder (length, width, and thickness all 10 mm or less), and the measurement conditions were a surface tension of 0.480 N / m and a contact angle of 140°. The measurement range was 0.004 - 500 μm. In the Log differential pore volume distribution (dV / d(logD)), the pore diameter at the peak within the range of 70 nm or less was defined as the "mode diameter in the range of 70 nm or less of the pore distribution" (hereinafter referred to as "mode diameter (≤70 nm)"). For the total number of pores (pieces / g), assuming the shape of the pores is spherical, the volume per pore (mL / piece) was obtained using the formula for the volume of a sphere (4 / 3×π×(radius) 3 ) and then the total pore volume (mL / g) was obtained by dividing the total pore volume by the volume per pore. [Thermal Conductivity] The thermal conductivity (W / (m·K)) of the heat insulating material was measured using a thermal conductivity measuring device (HWM - 15 manufactured by Spain Lab Co., Ltd., compliant with JIS R 2251 - 1) by the heat ray method (orthogonal method). The measurement atmosphere was air, and the temperature was set at room temperature (25°C) or 800°C. The sample size was 230 mm in length, 114 mm in width, and 20 mm in thickness, and two pieces were used as a set. [Compressive Strength] The compressive strength of the heat insulating material was measured using an autograph manufactured by Shimadzu Corporation. The sample size was 50 mm in length, 50 mm in width, and 20 mm in thickness, and the compression speed was 0.5 mm / min. At this time, the test force (N) at the point of 10% compression from the initial thickness was divided by the area of the compression surface (mm 2 ) to obtain the compressive strength (MPa). [Hardness] The hardness of the heat insulating material was measured using a durometer (type C, conforming to JIS K 7312 / JIS S 6050). The sample size was 50 mm in length, 50 mm in width, and 20 mm in thickness. Five arbitrary points were measured and the average value was calculated. [Bulk density] Regarding the bulk density (kg / m 3 ) of the heat insulating material, the dimensions and mass of the sample for measuring the thermal conductivity were measured, and the mass was divided by the volume obtained from the dimensions to calculate it.

[0029] In Example 1, a heat insulating material was prototyped as follows. As raw materials for the heat insulating material, 2.88 kg (72 mass%) of fumed silica (BET specific surface area 400 m 2 / g, DBP absorption 437 mL / 100 g), 0.16 kg (4 mass%) of fumed alumina (BET specific surface area 100 m 2 / g), 0.8 kg (20 mass%) of an infrared ray impermeable material (silicon carbide (average particle diameter (D50) of about 3 μm)), and 0.12 kg (3 mass%) of inorganic fiber (silica fiber (average fiber diameter of about 7 μm, average fiber length of about 6 mm)) were prepared. Further, as a binder, 0.133 kg (1 mass% (solid content)) of a colloidal silica (silica sol) solution (solid content 30 mass%) was prepared (the total of each raw material and the binder solid content was 4 kg). First, the total amounts of fumed silica, fumed alumina, infrared ray impermeable material, and inorganic fiber were dry-mixed in a Henschel mixer (rotation speed of about 3000 rpm) for 14 minutes, and then the total amount of the binder was dry-mixed in the mixer (same conditions as above) for 6 minutes while adding it by spraying with a spray nozzle to obtain a powdery mixture (total mixing time of 20 minutes). After that, a net was installed on the bottom side of the pressing surface of the metal frame in the press machine, about 0.92 kg of the mixture was charged, a net was also installed on the upper surface side, and the mixture was dry uniaxially pressed for 1 minute while degassing at a forming pressure of about 1 MPa by the upper and lower pressing parts provided with a large number of holes to obtain a raw plate of the heat insulating material sample (length 400 mm, width 300 mm, thickness 26 mm). Next, the surface of the raw plate was smoothed by milling with a machining center, the thickness was reduced to 20 mm, and then cut into predetermined dimensions to obtain each test piece.

[0030] For Examples 2 to 7 and Comparative Examples 1 to 3, heat insulating materials were also trial-produced in the same manner as in Example 1 to obtain each test piece. The differences between Examples 2 to 7 and Comparative Examples 1 to 3 and Example 1 are as follows. In Example 2, as the fumed silica, one with a BET specific surface area of 300 m 2 / g and a DBP absorption amount of 394 mL / 100 g was used. In Example 3, as the fumed silica, one with a BET specific surface area of 450 m 2 / g and a DBP absorption amount of 449 mL / 100 g was used. In Example 4, the content rates of fumed silica and the infrared ray impermeable material were 62% by mass and 30% by mass, respectively. In Example 5, the content rates of fumed silica and the infrared ray impermeable material were 82% by mass and 10% by mass, respectively. In Example 6, the content rates of fumed silica, fumed alumina, and the infrared ray impermeable material were 86% by mass, 0% by mass, and 10% by mass, respectively. In Example 7, as the fumed silica, one with a BET specific surface area of 380 m 2 / g and a DBP absorption amount of 378 mL / 100 g was used. In Comparative Example 1, as the fumed silica, one with a BET specific surface area of 200 m 2 / g and a DBP absorption amount of 354 mL / 100 g was used. In Comparative Example 2, the content rates of fumed silica and the infrared ray impermeable material were 92% by mass and 0% by mass, respectively. In Comparative Example 3, the content ratios of fumed silica and infrared ray impermeable material were 52% by mass and 40% by mass, respectively.

[0031] On the other hand, in Comparative Example 4, the original plate of Nitias Losslim Board GH (Losslim is a registered trademark), which is a commercially available silica-based heat insulating material (length: 500 mm, width: 500 mm, thickness: 25 mm), was smoothed by milling with a machining center, the thickness was reduced to 20 mm, and then cut into predetermined dimensions to obtain each test piece.

[0032] Table 1 shows the measurement results of each physical property value in Examples 1 to 7 and Comparative Examples 1 to 4. Table 2 shows the reduction rate of thermal conductivity with respect to Comparative Examples 1 to 4 in Examples 1 to 7 (the ratio of the value obtained by dividing the thermal conductivity of the example by the thermal conductivity of the comparative example and subtracting 1).

[0033]

Table 2

[0034] As shown in Table 1, in Example 1, the mode diameter (≦70 nm) was 11.2 nm, and the total pore number was 6.0×10 16An insulating material with a specific surface area of 10.5 m² / g, a thermal conductivity (800 °C) of 0.047 W / (m·K), a compressive strength of 0.58 MPa, and a hardness of 81 was obtained. Also, as shown in Table 2, when compared with Comparative Examples 1 to 4, a significant reduction in thermal conductivity (800 °C) reaching 23 to 83% was observed. Furthermore, despite the high heat insulation performance, both the compressive strength and hardness were high, and the handling property was better than that of the insulating materials of Comparative Examples 1 and 3. In the insulating materials of Comparative Examples 1 and 3, crumbling and chipping were observed during molding and handling of the insulating material samples. In Example 1, it is presumed that due to the high specific surface area and large structure of fumed silica, the pore diameter in the primary aggregates became smaller and the number of pores increased. As a result, the mode diameter (≤70 nm) was further reduced compared to the prior art and commercial products. Moreover, although the total pore volume and the pore volume in the range of 70 nm or less of the pore distribution (pore volume (≤70 nm)) were comparable to those of Comparative Example 1, the heat insulation performance, compressive strength, and hardness were improved because the total number of pores became 7.5 times that of Comparative Example 1.

[0035] Regarding Examples 2 to 7 as well, insulating materials with a mode diameter (≤70 nm) of 8.9 to 15.8 nm, a total number of pores of 4.4 to 12.1×10 16 m² / g, a thermal conductivity (800 °C) of 0.046 to 0.057 W / (m·K), a compressive strength of 0.44 to 0.92 MPa, and a hardness of 77 to 85 were obtained. Similar to Example 1, an advantageous difference could be obtained compared to Comparative Examples 1 to 4, and it was possible to achieve both high heat insulation performance and high compressive strength and hardness.

Claims

1. A heat insulating material obtained by molding a mixture containing 62 to 86% by mass of fumed silica and 10 to 30% by mass of an infrared impermeable material, having a mode diameter of 20 nm or less in the range of 70 nm or less of the pore distribution, a total pore number of 3.5×10 １６ pieces / g or more, and having a thermal conductivity at 800°C of 0.06 W / (m·K) or less, a compressive strength of 0.4 MPa or more, and a hardness of 75 or more.

2. The heat insulating material according to Claim 1, wherein the DBP absorption amount of the fumed silica is 370 mL / 100 g or more.

3. The BET specific surface area of the fumed silica is 300 to 500 m ２ / g. The heat insulating material according to Claim 1 or 2.

4. having a mode diameter of 16 nm or less in the range of 70 nm or less of the pore distribution, a total pore number of 4.5×10 １６ pieces / g or more, the DBP absorption amount of the fumed silica is 390 mL / 100 g or more, and the thermal conductivity at 800°C is 0.055 W / (m·K) or less. The heat insulating material according to Claim 1.

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