JP2016026982A - Heat insulating material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat insulating material that suppresses an increase in thermal conductivity to maintain excellent thermal insulation properties even at a high temperature of 1,500°C or more, and is excellent in heat-resisting properties at 1,700°C or more.SOLUTION: In a heat insulating material made of a porous sintered body comprising MgAlOand having a porosity of not less than 60% and less than 73%, pores having a pore size of not less than 0.8 μm and less than 10 μm occupy not less than 70 vol% and less than 90 vol% of the total pore volume and pores having a pore size of not less than 0.01 μm and less than 0.8 μm occupy not less than 10 vol% and less than 20 vol% of the total pore volume, and the thermal conductivity at 1,000°C or more and 1500°C or less is made not to exceed 1.5 times the thermal conductivity at not less than 20°C and less than 1,000°C.SELECTED DRAWING: None

Description

The present invention relates to a heat insulating material comprising a porous sintered body of MgAl ₂ O ₄ and excellent in heat insulating properties at a high temperature of 1000 ° C. or higher.

  The heat insulating material is required to have low thermal conductivity, and fiber-based heat insulating materials such as glass fibers, ceramics having a low bulk density such as a ceramic porous body, and the like are generally used. The heat transfer factors that affect the thermal conductivity can be divided into solid heat transfer, gas heat transfer, and radiant heat transfer.

As the fiber-based heat insulating material, for example, in Patent Document 1, a heat insulating layer made of a fiber body filled with airgel and including an infrared reflecting agent is coated with a porous covering layer to suppress radiant heat transfer. Is described.
However, such a heat insulating material is mainly composed of silica airgel, has low heat resistance, and the thermal conductivity at a high temperature of 400 ° C. or higher is unknown.

On the other hand, in a ceramic porous body, by setting it as a high porosity, solid heat transfer is suppressed and thermal conductivity is reduced.
However, since the influence of radiant heat transfer becomes large at a high temperature of 400 ° C. or higher, conventionally, in a heat insulating material intended for use in such a high temperature range, metal oxides such as zirconia and titania and carbonization have been conventionally used. A material having a high emissivity such as silicon is added to suppress radiant heat transfer.

  Furthermore, the inventors of the present invention have a spinel ceramic porous body having a predetermined pore size distribution that can suppress solid heat transfer and radiant heat transfer, and has excellent heat resistance at a high temperature of 1000 ° C. or higher. It has been proposed that it can be used as a material (for example, see Patent Documents 2 and 3).

JP 2009-299893 A JP 2012-229139 A JP 2013-209278 A

  However, although the ceramic porous bodies described in Patent Documents 2 and 3 have heat resistance at 1000 ° C. or higher, which is higher than the conventional one, the heat resistant temperature is 1600 ° C. at most and the compressive strength is 0. It stopped at about 8 MPa.

  In recent years, there has been a need for higher performance heat insulating materials, and heat insulating materials having heat resistance, high strength, low thermal conductivity, and heat insulating properties even at a higher temperature of about 1800 ° C. It has been demanded.

  The present invention has been made in view of the above technical problem, and has improved heat insulation properties as a conventional material, has excellent heat resistance even at 1800 ° C., has high strength, and is 1000 ° C. An object of the present invention is to provide a heat insulating material in which an increase in thermal conductivity is suppressed even at the above high temperature and excellent heat insulating properties are maintained.

The heat insulating material according to the present invention is made of a porous sintered body made of MgAl ₂ O _{4 with} a porosity of 60% or more and less than 73%, and pores having a pore diameter of 0.8 μm or more and less than 10 μm are 70 vol% or more of the total pore volume. The pores having a pore diameter of less than 90 vol% and pore diameters of 0.01 μm or more and less than 0.8 μm occupy 10 vol% or more and less than 20 vol% of the total pore volume, and the thermal conductivity at 1000 ° C. or more and 1500 ° C. or less is 20 ° C. or more. It is characterized by not exceeding 1.5 times the thermal conductivity at less than 1000 ° C.
A porous sintered body having such a pore structure is suitable as a heat insulating material in which an increase in thermal conductivity is suppressed even at a high temperature of 1000 ° C. or higher and 1500 ° C. or lower, and heat resistance and compressive strength are maintained even at 1800 ° C. It is.

The heat insulating material preferably has a compressive strength of 1 MPa or more.
A heat insulating material having such a compressive strength is suitable for applications requiring high strength at high temperatures.

  The heat insulating material has an excellent heat insulating effect as the thermal conductivity at high temperature is smaller. Therefore, the thermal conductivity at 1000 ° C. or higher and 1500 ° C. or lower is 0.45 W / (m · K) or lower. Preferably, it is 0.40 W / (m · K) or less.

  Moreover, since the heat insulation effect which was excellent also in the high temperature range is acquired, so that the increase in the heat conductivity at high temperature is suppressed, the heat conductivity in 1000 to 1500 degreeC is 20 to 1000 degreeC. It is preferable not to exceed 1.2 times the thermal conductivity.

The heat insulating material according to the present invention has improved heat insulating material characteristics than before, has excellent heat resistance even at 1800 ° C, has improved compressive strength, and conducts heat even at a high temperature of 1000 ° C or higher. Since the increase in rate is suppressed and excellent heat insulating properties are maintained, it is suitable as a heat insulating material for use in a high temperature range.
Therefore, the heat insulating material according to the present invention is also suitable for various structural materials that are required to have high heat insulating properties in a high temperature environment of about 1800 ° C. and furnace materials such as refractory materials such as ceramics, glass, steel, and non-ferrous metals. Can be applied.

It is the graph which showed the pore diameter distribution by the mercury porosimeter of each porous sintered compact or heat insulation brick concerning an Example and a comparative example. It is the graph which showed the relationship between the temperature and thermal conductivity about each porous sintered compact or heat insulation brick which concerns on an Example and a comparative example.

Hereinafter, the present invention will be described in more detail.
The heat insulating material according to the present invention is a heat insulating material made of a porous sintered body made of MgAl ₂ O ₄ and having a porosity of 60% or more and less than 73%. The pores having a pore diameter of 0.8 μm or more and less than 10 μm occupy 70 vol% or more and less than 90 vol% of the total pore volume, and the pores having a pore diameter of 0.01 μm or more and less than 0.8 μm are 10 vol% or more of the total pore volume. It occupies less than 20 vol%, and further, the thermal conductivity at 1000 ° C. or higher and 1500 ° C. or lower does not exceed 1.5 times the thermal conductivity at 20 ° C. or higher and lower than 1000 ° C.
The present invention focuses on the pore structure of a porous sintered body, and is based on the finding that specific fine pores affect the heat resistance and heat insulation properties in a high temperature range. That is, in the heat insulating material according to the present invention, in the porous sintered body made of MgAl ₂ O ₄ , the heat resistance is maintained even at 1800 ° C. and the compressive strength is controlled by controlling the amount of specific fine pores. In addition, an increase in thermal conductivity is suppressed even at a high temperature of 1000 ° C. or higher, and excellent heat insulation can be maintained.

Therefore, the heat insulating material according to the present invention is improved in heat resistance and compressive strength as compared with conventional heat insulating materials, and higher heat insulating properties can be obtained even with the same thickness. And can contribute to energy saving.
For example, when the heat insulating material is applied to large equipment such as a furnace wall, sufficient strength and heat insulating properties can be obtained even if it is thin, and space saving of the equipment can be achieved. In addition, the amount of heat released from the surface of the furnace body can be reduced by reducing the surface area of the furnace body. Furthermore, since the said heat insulating material is a low heat capacity, the energy saving effect superior to the conventional heat insulation brick is acquired.

The material of the heat insulating material according to the present invention is spinel MgAl ₂ O ₄ .
Spinel porous sintered bodies have high heat resistance and excellent strength at high temperatures, which can reduce fluctuations in pore shape and size caused by grain growth and grain boundary bonding at high temperatures. And the effect of suppressing fluctuations in thermal conductivity can be maintained for a long time. In particular, MgAl ₂ O ₄ , that is, magnesia spinel, has a high structural stability in a high temperature region of 1000 ° C. or higher and has an isotropic crystal structure, so that unique grain growth and shrinkage even when exposed to high temperatures. Not shown. For this reason, it is a material suitable for the heat insulating material which can maintain the specific pore structure which is the feature of the present invention and is used at a high temperature.
The chemical composition and the spinel structure can be measured and identified by, for example, a powder X-ray diffraction method.

Moreover, the porosity of the porous sintered body made of MgAl ₂ O ₄ constituting the heat insulating material according to the present invention is 60% or more and less than 73%.
If the porosity is less than 60%, the proportion of MgAl ₂ O ₄ in the porous sintered body is high, solid heat transfer increases, and it may be difficult to reduce the thermal conductivity. On the other hand, in the case of 73% or more, since the proportion of MgAl ₂ O ₄ in the porous sintered body is relatively low, it becomes brittle and sufficient heat resistance may not be obtained.
The porosity is calculated according to JIS R 2614 “Method for measuring specific gravity and true porosity of refractory heat-insulating brick”.

The pore structure of the porous sintered body is such that pores having a pore diameter of 0.8 μm or more and less than 10 μm occupy 70 vol% or more and less than 90 vol% of the total pore volume, and pores having a pore diameter of 0.01 μm or more and less than 0.8 μm. It accounts for 10 vol% or more and less than 20 vol% of the total pore volume.
Thus, most of the pores of the porous sintered body are small pores having a pore diameter of less than 10 μm. When there are many pores having a pore diameter of 10 μm or more, the effect of scattering infrared rays is reduced, the influence of radiant heat transfer is increased, and a sufficient heat insulation effect at a high temperature cannot be obtained, and the strength of the heat insulating material may be reduced. There is.
Preferably, it has at least one pore size distribution peak in the range of 0.8 μm or more and less than 10 μm.

In the heat insulating material, among the pores of the porous sintered body, pores (micropores) having a pore diameter of 0.01 μm or more and less than 0.8 μm occupy 10 vol% or more and less than 20 vol% of the total pore volume. Shall.
The presence of such micropores in the above-described proportion can increase the number of pores per unit volume and enhance the infrared scattering effect. In particular, it is effective for suppressing radiant heat transfer that greatly affects the thermal conductivity in a high temperature region, and an increase in thermal conductivity at a high temperature is also suppressed, so that an excellent heat insulating effect can be obtained.

When the proportion of the micropores in the total pore volume is less than 10 vol%, the number of pores per unit volume is small, and the infrared scattering effect may not be sufficiently obtained. On the other hand, if the proportion of the micropores in the total pore volume is 20 vol% or more, the strength of the heat insulating material may be reduced.
The pore size distribution in the porous sintered body can be measured according to JIS R 1655 “Method for testing pore size distribution of molded ceramics by mercury porosimetry”.

Specifically, the thermal conductivity of the heat insulating material is that the thermal conductivity at 1000 ° C. or higher and 1500 ° C. or lower does not exceed 1.5 times the thermal conductivity at 20 ° C. or higher and lower than 1000 ° C., It shall not exceed 1.2 times.
Thus, the heat insulating material in which the increase in the thermal conductivity in the high temperature region is suppressed can maintain the same heat insulating effect as in the case of the lower temperature even in the high temperature region of 1000 ° C. or higher, and can be suitably applied even in the high temperature region. it can.

Furthermore, the heat insulating material preferably has a thermal conductivity of 0.45 W / (m · K) or less in a high temperature range of 1000 ° C. or more and 1500 ° C. or less, and is 0.40 W / (m · K) or less. Is more preferable.
Such a heat insulating material that is suppressed without increasing the thermal conductivity even at a high temperature of 1000 ° C. or higher has little variation in the heat insulating effect even at a high temperature and can be suitably used.

It should be noted that the pore diameter distribution peak may be within the range of the pore diameter of 10 μm or more. However, since coarse pores cause a decrease in heat insulation due to radiant heat transfer, the presence of pores having a pore diameter of more than 1000 μm is not preferable.
With such a pore size distribution, it is possible to obtain a heat insulating material with low thermal conductivity that contributes less to solid heat transfer while maintaining strength.

In the porous sintered body, it is preferable that primary particles having a particle size larger than 100 μm are not observed in an arbitrary cross section. More preferably, there are no primary particles having a particle size larger than 50 μm.
Thus, by suppressing the growth of crystal grains, the pore structure having the minute pores as described above can be maintained, and the heat insulation at high temperature is maintained.

  The method for producing the heat insulating material according to the present invention as described above is not particularly limited, and a known method for producing a porous sintered body can be applied. Formation and adjustment of the pore structure can be performed, for example, by adding a pore former or a foaming agent.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example, this invention is not restrict | limited by the following Example.

Example 1
11 mol of hydraulic alumina powder (BK-112; manufactured by Sumitomo Chemical Co., Ltd.) is mixed at a ratio of 9 mol of magnesium oxide powder (MGO11PB; manufactured by Kojundo Chemical Laboratory Co., Ltd.), and this is mixed with hydraulic alumina and magnesium oxide. A slurry was prepared by adding equal weight of pure water to the total weight of and uniformly dispersing pure water.
Then, 50 vol% of a granular acrylic resin having a diameter of 10 μm was added to the slurry as a pore former and mixed and molded to obtain a molded body of 60 mm × 70 mm × 20 mm.
This molded body was fired in the atmosphere at 1800 ° C. for 3 hours to produce a porous sintered body.

The porous sintered body obtained above was subjected to X-ray diffraction (X-ray diffractometer: RINT2500 manufactured by Rigaku Corporation, X-ray source: CuKα, voltage: 40 kV, current: 0.3 A, scanning speed: 0.06 ° When the crystal phase was identified at / s), a magnesia spinel phase was observed.
Moreover, the pore volume of this porous sintered body was measured using a mercury porosimeter (manufactured by Shimadzu Corporation, Autopore 49500). FIG. 1 shows the pore size distribution of this porous sintered body.

(Example 2 and Comparative Examples 1 and 2)
In Example 1, the magnesium oxide blending ratio and the pure water addition rate are not changed, the diameter and the amount of the pore former, the firing temperature and the firing time are appropriately changed, and the other methods are the same as in Example 1, A porous sintered body having a pore structure as shown in Example 2 and Comparative Examples 1 and 2 in Table 1 below was prepared.

(Comparative Example 3)
A commercially available alumina fireproof insulating brick (heat resistant temperature 1650 ° C.) was used as Comparative Example 3.

About each porous sintered compact or heat insulation brick of the said Example and comparative example, the pore volume was measured using the mercury porosimeter. FIG. 1 shows each pore size distribution.
Moreover, about each porous sintered compact or heat insulation brick of the said Example and comparative example, the heat conductivity was measured with reference to JISR2616. Further, the compressive strength was evaluated with reference to JIS R 2615 “Testing method for compressive strength of fireproof insulating brick”.
Various evaluation results are summarized in Table 1 below.

From the evaluation results shown in Table 1 and FIGS. 1 and 2, it is 0.4 W / (m · K) or less at 1000 ° C. or more and 1500 ° C. or less, and the increase in thermal conductivity is suppressed even in a high temperature range. confirmed.
On the other hand, when the micropores in the pore diameter range of 0.01 μm or more and less than 0.8 μm are 20 vol% or more of the total pore volume (Comparative Example 1), although the thermal conductivity is small, the compressive strength is low. . Moreover, when the micropores in the pore diameter range of 0.01 μm or more and less than 0.8 μm are less than 10 vol% of the total pore volume (Comparative Example 2), although the compressive strength is high, the thermal conductivity is 20 ° C. or more and 1000 It was very high compared with Examples 1 and 2 in any of the low temperature range below 1000C and the high temperature range of 1000 ° C to 1500 ° C.
Moreover, since a commercially available fireproof heat insulation brick (comparative example 3) does not have a micropore like Example 1, 2, the increase in radiant heat transfer is seen with a temperature rise, and thermal conductivity is large. Rose.

  By the way, in Patent Document 2, “molding is performed with hydraulics” (see (Experiment 1) in the Examples), but one embodiment of the present invention is simply “molding” (for example, See Example 1).

  More specifically, the “molding” in one embodiment of the present invention is a process in which a known defoaming process is performed at a stage where the slurry is solidified by hydraulic to remove coarse pores and then molded into a predetermined shape. It is. This is because, as described above, the present invention is because the presence of pores having a pore diameter of more than 1000 μm is not preferable because coarse pores cause a decrease in heat insulation due to radiant heat transfer.

  Here, pores having a pore diameter exceeding 1000 μm, that is, coarse pores, can be easily visually confirmed. Coarse pores are disclosed in, for example, Patent Document 2 [Table 4] Sample No. In 3-D, it can be visually confirmed, and in one embodiment of the present invention, it cannot be visually confirmed.

  Although one embodiment of the present invention is a slurry obtained by subjecting a slurry to a known defoaming treatment, the present invention is not limited to this, and any other wide variety can be used as long as coarse pores can be removed. A known method can be applied, and press molding is also applicable as an example.

  For reference, the ratio of pores having a pore diameter of 0.8 μm or more and less than 10 μm and pores having a pore diameter of 0.01 μm or more and less than 0.8 μm in the porous ceramics of Examples 3 and 4 described in Patent Document 3 [Table 2] Calculated, pores having a pore diameter of 0.8 μm or more and less than 10 μm account for 65% (Patent Document 3 [Table 2] Example 3), 62% (Patent Document 3 [Table 2] Example 4) of the total pore volume, In addition, pores having a pore diameter of 0.01 μm or more and less than 0.8 μm are 32% (Patent Document 3 [Table 2] Example 3) and 27% (Patent Document 3 [Table 2] Example 4) of the total pore volume. All are outside the scope of the present invention.

In Examples 3 and 4 of Patent Document 3, the firing temperature is 1300 ° C. or 1400 ° C., whereas in one embodiment of the present invention, the temperature is 1800 ° C. By increasing the firing temperature, sintering between the MgAl ₂ O ₄ particles proceeds and the particles are strongly bonded to each other, so that it can be said that the compressive strength of the entire porous sintered body is improved. Here, the compressive strengths of Examples 3 and 4 of Patent Document 3 were both 0.9 MPa. (Refer to [Table 2] for various evaluation results for the porous ceramics of Examples 3 and 4.)

  In the present invention, the firing temperature is 1800 ° C., but for the purpose of further improving the compressive strength, if the firing temperature is 1500 ° C. or higher, the porous sintered body having pore distribution as in the present invention is used. It can be said that sufficient compression strength can be improved as compared with a lower firing temperature.

  In the present invention, it is possible to improve the strength more appropriately by taking both the reduction of coarse pores and the improvement of the firing temperature together and appropriately, but the improvement in strength and the maintenance of low thermal conductivity are mutually contradictory. In the present invention, it is possible to obtain a heat insulating material having desired characteristics by optimizing these three conditions together with the adjustment of the pore volume ratio.

Claims (5)

A porous sintered body composed of MgAl ₂ O _{4 with} a porosity of 60% or more and less than 73%,
The pores having a pore diameter of 0.8 μm or more and less than 10 μm occupy 70 vol% or more and less than 90 vol% of the total pore volume, and the pores having a pore diameter of 0.01 μm or more and less than 0.8 μm are 10 vol% or more and 20 vol% of the total pore volume. Less than,
A heat insulating material characterized in that the thermal conductivity at 1000 ° C. or higher and 1500 ° C. or lower does not exceed 1.5 times the thermal conductivity at 20 ° C. or higher and lower than 1000 ° C.   The heat insulating material according to claim 1, wherein the compressive strength is 1 MPa or more.   The heat insulating material according to claim 1 or 2, wherein the heat conductivity at 1000 ° C or more and 1500 ° C or less is 0.45 W / (m · K) or less.   The heat conductivity in 1000 degreeC or more and 1500 degrees C or less is 0.40 W / (m * K) or less, The heat insulating material as described in any one of Claims 1-3 characterized by the above-mentioned.   The heat conductivity in 1000 degreeC or more and 1500 degrees C or less does not exceed 1.2 times the heat conductivity in 20 degreeC or more and less than 1000 degreeC, The heat insulating material as described in any one of Claims 1-4 characterized by the above-mentioned. .

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