JP4395864B2 - Heating element for microwave firing furnace - Google Patents

Description

  The present invention relates to a heating element that generates heat by irradiating microwaves, for example, a heating element for baking ceramic materials in a furnace by irradiating microwaves in the furnace, particularly baking an alumina material. The present invention relates to a heating element suitable for firing at a high temperature of 1600 ° C. or higher.

  Conventionally, an electric furnace, a gas furnace, or the like is generally used for firing ceramic materials or the like.

  However, from the standpoints of energy saving and environmental load reduction, the microwave baking method has been attracting attention as an effective baking method.

  For example, Patent Document 1 and Patent Document 2 disclose a heating element (refractory heat insulating material) having a microwave absorption characteristic substantially equivalent to a material to be baked when the material to be baked is self-heated by microwave irradiation. ) Discloses a microwave firing method in which the temperature difference between the fired product and the heating element (fireproof heat insulating material) is eliminated by surrounding the fired product so that the fired product can be uniformly fired. Has been.

Further, Patent Document 3 discloses a heating element and a refractory heat insulating material that are suitable for firing an alumina material that is fired at a high temperature of 1600 ° C. or higher, by further developing the above technique.
JP 2002-130960 A JP 2003-240451 A Japanese Patent Application No. 2004-070464

  A typical example of the fine ceramic material is alumina ceramics. This alumina ceramic is generally fired at a high temperature of 1600 ° C. or higher. When this alumina ceramic material is fired by self-heating by microwave irradiation, there are the following problems.

  Alumina has low dielectric loss at low temperatures, so even when it is irradiated with microwaves, it does not generate heat due to its own microwave absorption. Therefore, by applying the technology disclosed in Patent Document 1 and Patent Document 2, a heating element having microwave absorption characteristics equivalent to alumina that is a fired object, and surrounding the alumina that is the fired object, Even with irradiation, it is difficult to raise the temperature to the target firing temperature.

  Further, when the temperature of alumina rises to 1000 ° C. or higher by some external means, the microwave absorption (dielectric loss) of alumina itself increases. Once the alumina is heated to a high temperature, self-heating due to absorption of microwaves becomes remarkable. Thereafter, the temperature is raised by self-heating and finally reaches the firing temperature.

  Therefore, in the case where microwave firing is performed by a method in which alumina that is a material to be fired is surrounded by a heating element having an equivalent microwave absorption characteristic (for example, the same alumina material as the material to be fired), The unit could not be self-heated by microwave absorption unless a unit composed of an alumina and a heating element surrounding it was heated to 1000 ° C. or higher by another heating means.

  However, in order to raise the temperature of the unit consisting of the alumina to be fired and the heating element surrounding it to a temperature at which it self-heats by microwave absorption, it is complicated to design the firing furnace. In addition, there is a disadvantage that the temperature control during the operation is complicated.

  Therefore, even if it is a material with low microwave absorption at a low temperature such as alumina, it is one kind of heating element that generates heat substantially equivalently to the object to be fired without providing another heating means. A technique for uniformly firing a certain alumina has been demanded.

  In order to fire the alumina material by self-heating by microwave absorption with only one type of heating element that generates heat substantially equivalently to alumina as the object to be fired without providing another heating means, the heating element However, it is required to have both of the following characteristics: 1) self-heating from room temperature to 1000 ° C. by microwave irradiation, and 2) heat generation equivalent to that of alumina as a fired object at 1000 ° C. or higher.

As a result of repeated studies, the inventor of the present application is that if the heating element is formed of an aggregate and an inorganic binder, the aggregate is alumina particles, and the inorganic binder is an inorganic binder containing a Na ₂ O component, the above 2 The inventors have found that a heating element having both characteristics can be obtained, and filed a patent document 3. The heating element disclosed in Patent Document 3 mentions sodium silicate as a suitable inorganic binder containing a Na ₂ O component. The presence of the Na ₂ O component in this inorganic binder causes the heating element to exhibit the above-mentioned characteristic 1), that is, the self-heating characteristic from room temperature to 1000 ° C.

Furthermore, through repeated studies, the present inventors have found that when the Na ₂ O component, which is an alkali, is exposed to a high temperature, it reacts with the refractory heat insulating material adjacent to the heating element and may deteriorate the refractory heat insulating material. I found it to get.

  In view of the above, an object of the present invention is to provide a heating element having both the above characteristics 1) and 2), and 3) the deterioration of the adjacent refractory heat insulating material.

  Examples of the solving means of the present invention are as follows.

(1) A heating element that generates heat when irradiated with microwaves, and is formed of an aggregate and an inorganic binder, the aggregate is alumina particles, and the inorganic binder includes Na ₂ O and SiO ₂ components, and a range of inorganic binder _{Na 2} O / SiO 2 ratio (weight% ratio) is from 0.0075 to 0.025, solid blending amount of the inorganic binding material with respect to the aggregate 100 parts by weight A heating element characterized by being 8 to 20 parts by weight per minute.

  (2) The heating element described above, wherein the heating element further contains inorganic fibers as reinforcing fibers.

  (3) The heating element described above, wherein the inorganic fiber is at least one selected from alumina fiber and mullite fiber.

  (4) A coating material comprising water and a thickener in addition to aggregates, inorganic binders, and inorganic fibers as a material for forming the above-described heating element.

  (5) A refractory heat insulating material for a microwave firing furnace, wherein a heat generating layer is provided on one side of the base material, the base material is mainly composed of an inorganic fibrous material, and the heat generating layer is composed of the above-described heat generating element. Refractory insulation characterized by that.

  (6) A refractory heat insulating material for a microwave firing furnace, wherein a heat generating layer is provided on one side of the base material, the base material is mainly composed of an inorganic fibrous material, and the heat generating layer is made of the coating material described above. Refractory insulation characterized by that.

  According to the present invention, even if the material to be baked is made of a material that has low microwave absorption at low temperatures and is difficult to be baked by self-heating, there is no need to provide a separate heating means. It becomes possible to fire the object to be fired uniformly. Therefore, the configuration of the microwave baking furnace is not complicated, and temperature control during the operation can be easily performed. Furthermore, the heating element does not deteriorate the refractory heat insulating material adjacent thereto.

The inventor of the present application has studied in view of the above problems, and as a result, the Na ₂ O component is indispensable in order for the heating element to exhibit the characteristics of 1) above, that is, self-heating characteristics from room temperature to 1000 ° C. The conclusion was reached. Then, the heating element is formed from an aggregate and an inorganic binder, the aggregate is alumina powder, the inorganic binder is an inorganic binder containing a Na ₂ O component and a SiO ₂ component, and the inorganic binder is An inorganic binder having a Na ₂ O / SiO ₂ ratio (weight% ratio) in the range of 0.0075 to 0.025 is used, and the inorganic binder is used in a solid content of 8 to 100 parts by weight with respect to 100 parts by weight of the aggregate. It has been found that when a heating element is prepared by blending 20 parts by weight, a heating element for a microwave baking furnace having the above three characteristics can be obtained. The present invention has been made based on such knowledge.

  In the present invention, the heating element is constituted by the aggregate and the inorganic binder.

In the heating element of the present invention, alumina particles made of the same material as the material to be fired are used as the aggregate, and an inorganic binder containing a Na ₂ O component and a SiO ₂ component is used as the inorganic binder. ) The Na ₂ O / SiO ₂ ratio (weight ratio) of the inorganic binder used is in the range of 0.0075 to 0.025, and 2) the blending amount of the inorganic binder is 100 parts by weight of aggregate. The solid content is 8 to 20 parts by weight.

  Considering that the aggregate of the heating element needs to generate heat equivalent to the alumina that is the object to be fired at a high temperature, the aggregate of the heating element is composed of alumina particles that are the same material as the object to be fired. preferable.

  The average particle size of the alumina particles used as the aggregate (JIS R 1692 (1997) “average particle size measured by laser diffraction / scattering method of fine ceramic raw material”) is the same. A range of ˜100 μm is more preferable. When the average particle diameter of the aggregate is within the above range, as described later, when the heating element is formed as an indeterminate coating material, the coating property and workability of the coating material can be kept good.

As the inorganic binder for binding the aggregate, an inorganic binder containing a Na ₂ O component and a SiO ₂ component is used. In particular, in the present invention, the Na ₂ O / SiO ₂ ratio (weight ratio) is 0.0075. An inorganic binder that is in the range of ~ 0.025 is used.

  As described above, the aggregate of the heating element is alumina particles that are the same material as the material to be fired. When alumina reaches a high temperature of 1000 ° C. or higher, the dielectric loss rapidly increases (microwave absorption increases) and self-heats. However, at a temperature lower than this, since the microwave absorption of alumina itself is small, it is difficult to raise the temperature by self-heating of alumina itself.

  Therefore, in the low temperature range of 1000 ° C. or lower, the aggregate of the heating element itself does not generate heat due to self-heating. Therefore, it is necessary to cause the heating element to generate heat by self-heating even in a low temperature range where the microwave absorption of alumina, which is an aggregate of the heating element, is small.

Therefore, in the present invention, an inorganic binder containing an Na ₂ O component is used as an inorganic binder for bonding the alumina particles that are the aggregates of the heating element, and the resulting heating element does not self-heat alumina. Even in the low-temperature range, excellent self-heating characteristics are expressed by microwave absorption.

As a result of further research, it has been found that the Na ₂ O component contained in the heating element may react with the refractory insulation adjacent to the heating element and deteriorate the refractory insulation at a high temperature. It was. Therefore, in order to reduce the deterioration of the refractory heat insulating material adjacent to the heating element, further intensive studies were made on the chemical composition of the inorganic binder to be blended in the heating element. As a result, the Na ₂ O component is indispensable for the self-heating characteristics in the low temperature range. However, 1) The Na ₂ O / SiO ₂ ratio (weight ratio) of the inorganic binder used is 0.0075-0. .2) and 2) the amount of the inorganic binder is 8 to 20 parts by weight in solid content with respect to 100 parts by weight of the aggregate, the self-heating of the heating element in the low temperature range It has been found that a heating element that does not deteriorate the adjacent refractory heat insulating material while maintaining good characteristics can be obtained.

The inventors of the present application firstly made alumina particles of the same material as the material to be fired as the aggregate, and Na ₂ O / SiO ₂ wt% ratio of the inorganic binder containing the Na ₂ O component and the SiO ₂ component as the inorganic binder. In order to evaluate the relationship between the heat generation characteristics of the heating element due to microwave absorption, the following experiment was conducted.

100 parts by weight of alumina powder (average particle size 5 μm), 9 parts by weight of mullite fiber, 1 part by weight of inorganic thickener, 0.8 part by weight of organic thickener, 57 parts by weight of water, and 0.4 parts of Na ₂ O component Silica sol (colloidal silica solution) containing 40% by weight of a solid content of 40% by weight and SiO ₂ component (Na ₂ O / SiO ₂ weight ratio is 0.01) in a range of 4 to 25 parts by weight in solid content. The mixture was agitated and kneaded with a mixer, and an irregular coating material for constituting a heating element was produced. The composition is shown in Table 1.

  Next, the function of the prepared coating material as a heating element was evaluated. The evaluation method is described below.

  The coating material produced by the above-mentioned composition in Table 1 was applied to one side of a 25 mm thick ceramic fiber board (FMX-17R manufactured by Saint-Gobain TM Co., Ltd.) with a thickness of 2 mm. Then, it was dried at 100 ° C. for 3 hours and then calcined at 1000 ° C. for 1 hour to obtain a refractory heat insulating material provided with the heating element of the present invention.

  Next, using this refractory heat insulating material, a closed space of 40 × 40 × 25 mm was created with the surface provided with the heating element inside. A pellet of a temperature history sensor (JFCC reference thermometer) for measuring temperature is installed in this closed space, and a microwave of 2.45 GHz is irradiated for 150 minutes at a power of 500 W using a small microwave oven. The temperature in the closed space was measured by a temperature history sensor. This method is for simply evaluating the function as a heating element.

  If the temperature in the closed space reaches 1550 ° C. or higher according to the conditions of this evaluation method (frequency: 2.45 GHz, 500 W, 150 minutes), as described later, for firing an alumina material in an actual furnace. It has a sufficient heat generation function as a heating element.

  Table 1 shows the temperature reached in the closed space surrounded by the heating element obtained by each composition.

  FIG. 1 shows the relationship between the blending amount of the inorganic binder of the heating element (parts by weight of the solid content of the inorganic binder when the aggregate is 100 parts by weight) and the temperature reached in the closed space.

  As can be seen from FIG. 1, when the added amount (solid content) of the binder is 8 parts by weight, the ultimate temperature in the closed space is rapidly increased, and the heat generation characteristics of the obtained heating element are improved. Can do. Furthermore, when the addition amount (solid content) of the binder is in the range of 8 to 20 parts by weight, it can be found that the temperature in the closed space is almost constant and stable heat generation characteristics can be obtained. Moreover, when the addition amount (solid content) of a binder exceeds 20 weight part, it turns out that the temperature in closed space rises further.

  However, as will be described later, when the addition amount (solid content) of the binder exceeds 20 parts by weight, the resulting heat generating element and the adjacent refractory heat insulating material tend to be deteriorated by reaction. In general, it is known that the microwave absorption characteristics of a substance increase when it is in a molten state. Therefore, when the added amount (solid content) of the binder exceeds 20 parts by weight, the temperature in the closed space further increases because the heating element and the refractory heat insulating material adjacent to it react with each other. It can be considered that this is because the temperature in the closed space further rises because a portion that forms a molten state is formed.

  Next, an experiment for firing alumina ceramics in an actual microwave firing furnace was performed.

  The coating material produced by the above-mentioned formulation in Table 1 was applied to one side of a 25 mm thick ceramic fiber board (FMX-17R manufactured by Saint-Gobain TM) at a thickness of 2 mm. Thereafter, it was dried at 100 ° C. for 3 hours and calcined at 1000 ° C. for 1 hour to obtain a refractory heat insulating material provided with the heating element of the present invention.

  Next, using this refractory heat insulating material, a furnace internal space of 200 × 200 × 200 mm was created with the surface provided with the heating element inside. In order to further insulate this furnace space, two layers of 25 mm thick ceramic fiber boards (FMX-16CV and 14R manufactured by Saint-Gobain TM) were disposed on the outside thereof to provide a heat insulating layer.

  Next, as a material to be fired, a green body having a size of 100 × 50 × 35 mm was prepared by adding appropriate amounts of carboxymethyl cellulose and water to alumina powder. Two substrates of the alumina molded product were irradiated with microwaves having a frequency of 2.45 GHz in the aforementioned closed space.

  Table 1 shows the results of the firing test of the alumina porcelain when the heating element obtained by each composition was used.

  In Table 1, “O” indicates that firing was possible and “x” indicates that firing was not possible in the column “Baking of alumina porcelain”. In Table 1, in the column of “State of refractory insulation”, “O” indicates that the refractory insulation was in a good state without deterioration even after the temperature increase, and “×” indicates that after the temperature increase. The deterioration of the refractory heat insulation material was remarkable, indicating that it was partially melted.

  As a result, the heating element (# 2 to # 5) obtained when the binder was added in an amount of 8 to 20 parts by weight in solids increased in temperature to 1600 ° C. in about 200 minutes, and in a short time The temperature could be raised. It was also confirmed that all two alumina substrates were uniformly fired. However, when the heating element (# 1) having a binder addition amount (solid content) of less than 8% by weight was used, it was impossible to raise the temperature in the furnace to 1600 ° C. When the heating element (# 6) in which the amount of added binder (solid content) exceeds 20 parts by weight was used, the temperature could be raised to 1600 ° C, but the ceramic fiber board adjacent to the heating element Was deteriorated and partially melted, and a state of peeling with the heating element was confirmed.

  The heating element of the present invention is not a dense body, but is formed by bonding aggregated alumina particles with an inorganic binder, and the density of the heating element is reduced.

  The internal structure of the heating element of the present invention is not a state in which the alumina particles that are aggregates are bonded together by sintering, but a state in which an inorganic binder that is continuous in a film shape bonds the aggregate particles together. And it is rich in voids. Such a structure is suitable for improving the thermal shock resistance of the heating element. The characteristics of microwave baking include high-speed temperature rise and high-speed cooling. Therefore, excellent thermal shock resistance has a practically important effect on the heating element of the microwave baking furnace.

The density of the heating element of the present invention is preferably 1000 to 1500 kg / m ³ .

Also from the viewpoint of thermal shock resistance of the resulting heating element, the blending amount of the inorganic binder with respect to 100 parts by weight of the aggregate is preferably 20 parts by weight or less. When the blending amount of the inorganic binder with respect to 100 parts by weight of the aggregate exceeds 20 parts by weight, voids in the heating element are excessively filled in the inorganic binder, and the thermal shock resistance may be lowered.

  Furthermore, when the heating element of the present invention contains inorganic fibers as a reinforcing material, the thermal shock resistance of the resulting heating element is improved, which is more preferable. As the inorganic fiber serving as a reinforcing material, alumina silica fiber, alumina fiber, and mullite fiber are preferable. Since the firing temperature of alumina is as high as 1600 ° C. or higher, among these, alumina fibers and mullite fibers excellent in heat resistance at higher temperatures can be used more preferably.

  As described in the above description regarding the experiment, it is more preferable that the heating element of the present invention is formed of an irregular coating material such as slurry or cement. When the heating element is formed by the coating material, a thickener and water can be appropriately used in addition to the aggregate, the inorganic binder, and the inorganic fiber.

  A structure in which an inorganic fibrous material is used as a base material and the heating element is provided on one surface of the base material is suitable as a refractory heat insulating material for a microwave firing furnace.

  The base material on which the heating element is provided is preferably a material that can transmit microwaves and has excellent heat insulating properties. When the microwave is absorbed by the base material and the consumption of the microwave by the base material is increased, the amount of energy required for firing the object to be fired increases as a result.

  Moreover, in order to suppress the temperature effect of the heating element due to radiation cooling, the base material preferably has high heat insulation. In particular, the firing temperature of alumina is as high as 1600 ° C. or higher, and the heat insulating property of the base material is important in order to suppress the radiative cooling of the exothermic heating element.

 Furthermore, in the microwave firing, since the high-speed temperature rise and the high-speed cooling are performed, the substrate is preferably excellent in thermal shock resistance.

  Examples of the substrate satisfying such characteristics include inorganic fiber materials such as ceramic fiber boards mainly composed of alumina fibers, mullite fibers, and alumina silica fibers. The ceramic fiber board can transmit microwaves and has excellent thermal shock resistance in addition to excellent heat insulation and heat resistance, and can be preferably used.

Next, the present invention will be specifically described with reference to examples.

  Based on the blending composition shown in Table 2, a predetermined amount of raw materials were blended, and this was stirred and kneaded with a mixer to obtain an amorphous coating material for forming a heating element. The average particle size of the aggregates shown in Table 2 was measured by JIS R 1692 (1997) “Method for measuring particle size distribution of fine ceramic raw material by laser diffraction / scattering method”.

  Next, the function of the prepared coating material as a heating element was evaluated. The evaluation method is described below.

  The coating material produced with the composition shown in Table 2 was applied to one side of a 25 mm thick ceramic fiber board (FMX-17R manufactured by Saint-Gobain TM Co., Ltd.) with a thickness of 2 mm. Then, it was dried at 100 ° C. for 3 hours and then calcined at 1000 ° C. to obtain a refractory heat insulating material provided with the heating element of the present invention.

  Next, using this refractory heat insulating material, a closed space of 40 × 40 × 25 mm was created with the surface provided with the heating element inside. A pellet of a temperature history sensor (JFCC reference thermometer) for measuring temperature is installed in this closed space, and a microwave of 2.45 GHz is irradiated for 150 minutes at a power of 500 W using a small microwave oven. The temperature in the closed space was measured by a temperature history sensor. This method is for simply evaluating the function as a heating element.

  If the temperature in the closed space reaches 1550 ° C. or higher due to the conditions of this evaluation method (frequency: 2.45 GHz, 500 W, 150 minutes), it is sufficient as a heating element for firing an alumina material in an actual furnace. Has a heat generation function.

  Table 2 shows the ultimate temperature in the closed space surrounded by the heating element obtained by each composition.

  Next, an experiment for firing alumina ceramics in an actual microwave firing furnace was performed.

  The coating material produced by the above blending was applied at a thickness of 2 mm to one side of a 25 mm thick ceramic fiber board (FMX-17R manufactured by Saint-Gobain TM). Thereafter, it was dried at 100 ° C. for 3 hours and calcined at 1000 ° C. for 1 hour to obtain a refractory heat insulating material provided with the heating element of the present invention.

  Next, using this refractory heat insulating material, a furnace internal space of 200 × 200 × 200 mm was created with the surface provided with the heating element inside. In order to further insulate this furnace space, two layers of 25 mm thick ceramic fiber boards (FMX-16CV and 14R manufactured by Saint-Gobain TM) were disposed on the outside thereof to provide a heat insulating layer.

  Next, as a material to be fired, a green body having dimensions of 100 × 50 × 35 mm was prepared by adding appropriate amounts of carboxymethyl cellulose and water to alumina powder. Two substrates of the alumina molded product were irradiated with microwaves having a frequency of 2.45 GHz in the aforementioned closed space.

  Table 2 shows the results of the firing test of the alumina porcelain when the heating element obtained by each blending composition was used. In Table 2, in the column of “Alumina porcelain firing”, “O” indicates that firing was possible, and “x” indicates that firing was not possible. Further, in Table 1, in the column of “State of fireproof insulation”, “O” indicates that the fireproof insulation was in a good state without deterioration even after the temperature rise, and “x” indicates that after the temperature rise. The deterioration of the “refractory heat insulating material” was remarkable, indicating that it was partially melted.

Examples 1 to 4 consist of 100 parts by weight of alumina powder (average particle size 5 μm), 9 parts by weight of mullite fiber, and an inorganic binder (Na ₂ O component 0) having a Na ₂ O / SiO ₂ weight ratio of 0.01. A colloidal silica solution containing a solid content of 4 wt% and SiO ₂ component 40 wt% is a heating element containing 4 to 20 parts by weight of the solid content. As shown in Table 2, the heat generating function of the heat generating element was good, and firing of the alumina ceramic was possible. Moreover, the state of the fireproof heat insulating material after temperature rising was also favorable.

  In Example 5, the average particle diameter of the alumina powder as the aggregate is 14 μm, and in Example 6, the average particle diameter of the alumina powder as the aggregate is 57 μm, and the average particle diameter of the alumina particles as the aggregate is Except for the difference, is the same as Examples 1-4. As shown in Table 2, the heat generating function of the heat generating element was good, and firing of the alumina ceramic was possible. Moreover, the state of the fireproof heat insulating material after temperature rising was also favorable.

  In Comparative Example 1, the amount of the inorganic binder when the aggregate is 100 parts by weight is 4 parts by weight (the amount of the inorganic binder when the aggregate is 100 parts by weight is 8 to 20 parts by weight) This is the same as the first to fourth embodiments except for the above. As shown in Table 2, the heat generating function of the heating element was insufficient, and the alumina-based ceramic could not be fired.

  In Comparative Example 2, the amount of the inorganic binder when the aggregate is 100 parts by weight is 24 parts by weight (the amount of the inorganic binder when the aggregate is 100 parts by weight is 8 to 20 parts by weight) This is the same as the first to fourth embodiments except for the above. As shown in Table 2, a part of the refractory heat insulating material after the temperature rise was melted and deteriorated.

In Comparative Example 3, the inorganic binder has a Na ₂ O / SiO ₂ weight ratio of 0.26 (the inorganic binder has a Na ₂ O / SiO ₂ weight ratio outside the range of 0.0075 to 0.025). Other than the above, the second embodiment is the same as the first embodiment. As shown in Table 2, a part of the refractory heat insulating material after the temperature rise was melted and deteriorated.

In Comparative Example 4, the inorganic binder has a Na ₂ O / SiO ₂ weight ratio of 0.0024 (the inorganic binder has a Na ₂ O / SiO ₂ weight ratio outside the range of 0.0075 to 0.025). Except for this, this is the same as Example 4. As shown in Table 2, the heat generating function of the heating element was insufficient, and the alumina-based porcelain could not be fired.

Na ₂ O / SiO ₂ weight ratio of 0.01 was obtained by blending the inorganic binder solid content (weight part of the inorganic binder solid content when the aggregate was 100 parts by weight) and the blending thereof. It is a graph which shows the relationship with the ultimate temperature in the closed space enclosed with the heat generating body.

Claims (6)

A heating element that generates heat when irradiated with microwaves, and is formed of an aggregate and an inorganic binder, the aggregate is alumina particles, the inorganic binder includes Na ₂ O and SiO ₂ components, and The Na ₂ O / SiO ₂ ratio (weight% ratio) of the inorganic binder is in the range of 0.0075 to 0.025, and the blending amount of the inorganic binder with respect to 100 parts by weight of the aggregate is 8 in solid content. A heating element characterized by being 20 parts by weight.   The heating element according to claim 1, wherein the heating element further contains inorganic fibers as reinforcing fibers.   The heating element according to claim 2, wherein the inorganic fibers are at least one selected from alumina fibers and mullite fibers.   As a material for forming the heating element according to any one of claims 1 to 3, water and a thickener are contained in addition to the aggregate, the inorganic binder, and the inorganic fiber. Coat material.   It is a refractory heat insulating material for a microwave firing furnace, and a heating layer is provided on one side of the substrate, the substrate is mainly composed of an inorganic fibrous material, and the heating layer is any one of claims 1 to 3. A fireproof heat insulating material comprising the heating element according to the item. It is a refractory heat insulating material for a microwave firing furnace, and a heat generating layer is provided on one side of the base material, the base material is mainly composed of an inorganic fibrous material, and the heat generating layer is from the coating material according to claim 4. A fireproof heat insulating material characterized by

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