JP2008143731A - Heat resistant expanded graphite sheet and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an expanded graphite sheet having heat resistance and oxidation consumption resistance required as the ones sufficiently satisfying durability even in the temperature range of >700°C, and to provide its production method. <P>SOLUTION: The heat resistant expanded graphite sheet essentially composed of expanded graphite, and comprises phosphorous pentoxide and phosphate, and at least either a boron compound or a yttrium compound in a dispersed manner. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a heat-resistant expanded graphite sheet excellent in heat resistance and oxidation wear resistance and a method for producing the same.

Japanese Patent Publication No.54-30678 Patent No. 3430243

  Expanded graphite sheet is made of natural graphite, quiche graphite, pyrolytic graphite, etc., concentrated sulfuric acid, concentrated nitric acid, concentrated sulfuric acid and potassium chlorate, concentrated sulfuric acid and potassium nitrate or hydrogen peroxide, strong oxidizing agents such as bromine or aluminum chloride. An interlayer compound is formed by treatment with a halide such as, and the graphite particles (acid-treated graphite) on which the interlayer compound is formed are subjected to a rapid heat treatment, for example, at a high temperature of 950 ° C. or higher for 1 to 10 seconds. A cracked gas is generated, and the graphite layer is expanded by the gas pressure to form expanded graphite particles. The expanded graphite particles are produced by compression molding or roll molding in the presence or absence of a binder. The expanded graphite sheet produced in this way is used in a wide field such as a gasket, a heat insulating material, and a cushion material.

  Of the expanded graphite particles used in the expanded graphite sheet, those having a low expansion ratio, for example, expanded graphite particles of about 20 to 70 times, are difficult to form in the absence of a binder, and it is necessary to use a binder. There is a problem that the use of this binder reduces the purity and various physical properties.

  On the other hand, those having a high expansion ratio, for example, expanded graphite particles of about 200 to 300 times can be formed into a sheet in the absence of a binder, so that the purity is high and various physical properties are excellent. . Therefore, at present, expanded graphite particles having a high expansion ratio are usually used for manufacturing expanded graphite sheets.

  However, the conventional expanded graphite sheet has a problem in heat resistance in air, particularly in air at a high temperature of 700 ° C. or higher, and as a result, the so-called oxidation consumption rate is high, which causes oxidation consumption of graphite. There are fatal drawbacks.

  In order to eliminate this defect, an expanded graphite sheet obtained by subjecting expanded graphite particles having a low expansion ratio of about 20 to 70 times to oxidation inhibition treatment of phosphoric acid or phosphate (described in Patent Document 1) or about 200 to 300 times An expanded graphite sheet (described in Patent Document 2) in which expanded graphite particles having a high expansion ratio are subjected to oxidation inhibition treatment of phosphoric acid and phosphate has been proposed.

  According to the description of Patent Document 1, it is said that by using phosphoric acid or a phosphate, it is possible to form a sheet using expanded graphite particles having a low expansion coefficient without a binder. Since no binder is used, various physical properties as an expanded graphite sheet, in particular, mechanical properties, sheet uniformity, and the like are not satisfactory. Moreover, although it is said that the oxidation resistance is improved, this is also insufficient, and especially when exposed for a long time, the oxidation consumption is intense and it is never satisfactory. On the other hand, the expanded graphite sheet described in Patent Document 2 has a low oxidation consumption rate in air up to about 700 ° C. and sufficient heat resistance, but has sufficient durability in a high temperature region exceeding 700 ° C. There is a problem in that it does not yet have an oxidation consumption rate required to satisfy the above.

  The present invention has been made in view of the above-mentioned points, and the object of the present invention is that it can sufficiently satisfy the durability even in a high temperature region exceeding 700 ° C. as well as about 700 ° C. An object of the present invention is to provide an expanded graphite sheet having required heat resistance and oxidation resistance and its manufacturing method.

  The heat-resistant expanded graphite sheet of the present invention comprises expanded graphite as a main component, and phosphorus pentoxide and phosphate, and at least one of a boron compound and an yttrium compound are dispersed therein.

  According to such a heat-resistant expanded graphite sheet, heat resistance is imparted by phosphorus pentoxide and phosphate and at least one of a boron compound and an yttrium compound, and at a high temperature region exceeding 700 ° C. due to an oxidation inhibiting action. Even during use, oxidation consumption is sufficiently reduced.

  In the heat-resistant expanded graphite sheet of the present invention, phosphorus pentoxide and phosphate impart an oxidation inhibiting action on expanded graphite. The phosphorus pentoxide is preferably dispersed and contained at a ratio of 0.5 to 8% by weight, and the phosphate is preferably dispersed and contained at a ratio of 1 to 10% by weight.

  The phosphate includes a dispersion selected from any one of primary lithium phosphate, secondary lithium phosphate, primary calcium phosphate, secondary calcium phosphate, primary aluminum phosphate, and secondary aluminum phosphate. It is good to have.

  The boron compound or the yttrium compound has an effect of further promoting the oxidation inhibitory action of phosphorus pentoxide and phosphate, and each of them may be dispersed and contained alone, or both may be dispersed and contained simultaneously. The boron compound is preferably composed of boron oxide and boron phosphate and dispersed and contained at a ratio of 0.1 to 1.5% by weight. The yttrium compound is composed of yttrium oxide and yttrium phosphate. It is good to be dispersed and contained at a ratio of 0.5 to 5.0% by weight.

  The method for producing a heat-resistant expanded graphite sheet of the present invention comprises spraying a solution obtained by mixing a phosphoric acid aqueous solution, a phosphoric acid aqueous solution, boric acid and at least one of yttrium chloride with acid-treated graphite powder obtained by treating graphite powder with a strong acid. Then, these are stirred and blended, and the acid-treated graphite powder blended with the solution is expanded 200 to 300 times at a temperature of 950 to 1200 ° C. and then formed into a sheet. The expanded graphite sheet includes 0.5 to 8 wt% phosphorus pentoxide and 1 to 10 wt% phosphate, 0.1 to 1.5 wt% boron compound and 0.5 wt%. At least one of yttrium compounds in a proportion of ˜5.0% by weight is dispersed and contained.

According to such a method for producing a heat-resistant expanded graphite sheet, phosphoric acid generates phosphorus pentoxide (P ₂ O ₅ ) by a dehydration reaction, phosphate is desorbed from water in the structural formula, and boric acid is boron oxide ( Boron compounds such as B ₂ O ₃ ) and boron phosphate (BPO ₄ ) are produced, and yttrium chloride produces yttrium compounds such as yttrium oxide (Y ₂ O ₃ ) and yttrium phosphate (YPO ₄ ).

  Then, the expanded graphite sheet thus prepared is mixed with 0.5 to 8% by weight of phosphorus pentoxide, 1 to 10% by weight of phosphate, and 0.1 to 1.5% by weight of boron compound. And at least one of 0.5% to 5.0% by weight of the yttrium compound is dispersedly contained.

  According to the present invention, heat resistance is imparted by phosphorus pentoxide and phosphate dispersed in the expanded graphite sheet and at least one of a boron compound and yttrium compound, and phosphorus pentoxide, phosphate and boron are added. Since the oxidation inhibiting action with at least one of the compound and the yttrium compound is imparted, even when exposed to a high temperature region exceeding 700 ° C. for a long time, the oxidation consumption rate in air is low, It is possible to provide a heat-resistant expanded graphite sheet excellent in durability applicable to various uses and a method for producing the same.

  Next, it demonstrates still in detail based on the example of preferable embodiment of this invention. The present invention is not limited to these examples.

  The heat-resistant expanded graphite sheet of the present invention comprises expanded graphite as a main component, in which phosphorus pentoxide and phosphate, and at least one of a boron compound and an yttrium compound are dispersed and contained.

  The content of the heat-resistant expanded graphite sheet is 0.5 to 8% by weight of phosphorus pentoxide, 1 to 10% by weight of phosphate, and 0.1 to 1.5% of boron compound. 0.5% to 5.0% by weight of the yttrium compound instead of 0.1 to 1.5% by weight of the boron compound or in addition to 0.1 to 1.5% by weight of the boron compound. It is.

  In the heat-resistant expanded graphite sheet, if the content of phosphorus pentoxide is less than 0.5% by weight, the oxidation consumption rate of the heat-resistant expanded graphite sheet cannot be reduced, and if it exceeds 8% by weight, No effect is observed in reducing the oxidation consumption rate. Further, when the phosphate content is less than 1% by weight, no effect is observed in reducing the oxidation consumption rate of the heat-resistant expanded graphite sheet. When the phosphate content exceeds 10% by weight, the expanded graphite sheet tends to be hardened. Appears and hinders the flexibility of the expanded graphite sheet.

  In the heat-resistant expanded graphite sheet, if the content of boron oxide and boron phosphate as the boron compound is less than 0.1% by weight, the effect of the oxidation inhibiting action imparted by phosphorus pentoxide and phosphate cannot be promoted, On the other hand, if the content exceeds 1.5% by weight, the expanded graphite sheet tends to be hardened as in the case of the phosphate, and the flexibility of the expanded graphite sheet is impaired.

  In the heat-resistant expanded graphite sheet, when the content of yttrium oxide and yttrium phosphate as the yttrium compound is less than 0.5% by weight, the effect of the oxidation inhibiting action imparted by phosphorus pentoxide and phosphate is the same as that of the boron compound. When the content exceeds 5.0% by weight, the expanded graphite sheet has a tendency to become hard as in the case of the phosphate and boron compounds, and the expanded graphite sheet has flexibility. Will be inhibited.

  In a heat-resistant expanded graphite sheet, heat resistance is imparted to expanded graphite by phosphorus pentoxide and phosphate and at least one of a boron compound and an yttrium compound, and a high-temperature region exceeding 700 ° C. due to an oxidation inhibiting action, for example, 750 Oxidation consumption is reduced even in the use in a high temperature region of ° C.

  Next, the manufacturing method of a heat resistant expanded graphite sheet is demonstrated.

Preparation of acid-treated graphite particles Acid-treated graphite particles are obtained by treating graphite with sulfuric acid and an oxidizing agent according to a conventional method, and drying the graphite according to a conventional method. This will be described in detail as follows. While stirring concentrated sulfuric acid having a concentration of 98%, a 60% aqueous solution of hydrogen peroxide is added as an oxidizing agent to make a reaction solution. The reaction solution is cooled and maintained at a temperature of 10 ° C., and scaly natural graphite powder having a particle size of 30 to 80 mesh is added to the reaction solution and reacted for 30 minutes. After the reaction, the acid-treated graphite is separated by suction filtration, and the washing process of stirring the acid-treated graphite with 300 parts by weight of water for 10 minutes and suction filtration is repeated twice to sufficiently remove the sulfuric acid content from the acid-treated graphite. To do. Next, acid-treated graphite particles are produced by drying the acid-treated graphite from which sulfuric acid has been sufficiently removed in a drying furnace maintained at a temperature of 110 ° C. for 3 hours.

  Examples of graphite subjected to the acid treatment include quiche graphite, pyrolytic graphite and the like in addition to scaly natural graphite.

Examples of phosphoric acid uniformly mixed in the acid-treated graphite particles include orthophosphoric acid (H ₃ PO ₄ ), metaphosphoric acid (HPO ₃ ), polyphosphoric acid, specifically pyrophosphoric acid (H ₄ P ₂ O ₇ ), and tripolyphosphoric acid. It is selected from chain condensed phosphoric acid such as (H ₅ P ₈ O ₁₀ ) and polymetaphosphoric acid, specifically cyclic condensed phosphoric acid such as trimetaphosphoric acid and tetrametaphosphoric acid, and is usually used in the form of an aqueous solution.

As the phosphate uniformly mixed with the acid-treated graphite particles, a primary phosphate and a secondary phosphate are used, and among them, alkali metal salts and alkaline earth metal salts are preferable, and lithium and calcium are particularly preferable. Moreover, an aluminum salt can also be used as a metal salt. Specifically, primary lithium phosphate (LiH ₂ PO ₄ ), secondary lithium phosphate (Li ₂ HPO ₄ ), primary calcium phosphate [Ca (H ₂ PO ₄ ) ₂ ], dicalcium phosphate (CaHPO ₄ ) Primary aluminum phosphate [Al (H ₂ PO ₄ ) ₃ ] and secondary aluminum phosphate [Al ₂ (HPO ₄ ) ₃ ], and these phosphates are used in the form of an aqueous solution.

Boron uniformly mixed in the acid-treated graphite particles is used in the form of an aqueous solution of boric acid (H ₃ BO ₃ ) or in the form of scaly crystal powder. Specifically, it is used as a solution obtained by mixing a saturated boric acid aqueous solution with one aqueous solution of phosphoric acid and phosphate, or as a solution obtained by dispersing and mixing boric acid powder in one aqueous solution of phosphoric acid and phosphate.

As yttrium uniformly mixed with the acid-treated graphite particles, yttrium chloride (YCl ₃ · 6H ₂ O) is used, and is used as a solution mixed with an aqueous solution of phosphoric acid and phosphate.

  While preparing acid-treated graphite particles (powder) and stirring the acid-treated graphite powder, the acid-treated graphite powder is mixed with a phosphoric acid aqueous solution with a concentration of 84%, a phosphoric acid aqueous solution with a concentration of 50%, a boric acid saturated aqueous solution, and an yttrium chloride aqueous solution. A solution containing at least one of them is sprayed, and these are agitated and mixed uniformly to produce a wettable mixture.

  After the wettable mixture is dried in a drying furnace, the mixture is subjected to an expansion treatment at a temperature of 950 to 1200 ° C. for several seconds to several minutes to generate cracked gas. The expanded graphite powder expanded to 300 times is manufactured. In this expansion treatment, phosphoric acid in the component undergoes a dehydration reaction to produce phosphorus pentoxide, and the phosphate is contained together with phosphorus pentoxide as water in the structural formula is eliminated. Boric acid is partially oxidized to produce boron oxide, and part of it produces boron phosphate. As a boron compound, it coexists with phosphorus pentoxide and phosphate, or phosphorus pentoxide, phosphate and yttrium compound. Contained together. Yttrium chloride partially produces yttrium oxide, partially produces yttrium phosphate, and coexists with phosphorus pentoxide and phosphate as an yttrium compound, or coexists with phosphorus pentoxide, phosphate and boron compound. Contained.

  The expanded graphite powder thus prepared is supplied between rolls adjusted to a desired gap and roll-molded to prepare a heat-resistant expanded graphite sheet having a desired thickness.

  The heat-resistant expanded graphite sheet thus prepared contains a predetermined amount of phosphorus pentoxide and phosphate and at least one of a boron compound and an yttrium compound in a dispersed manner.

  EXAMPLES Hereinafter, although an Example demonstrates this invention still in detail, this invention is not limited to a following example, unless the summary is exceeded.

Examples 1-4
While stirring 300 parts by weight of concentrated sulfuric acid having a concentration of 98%, 5 parts by weight of a 60% aqueous solution of hydrogen peroxide was added as an oxidizing agent, and this was used as a reaction solution. The reaction solution was cooled and maintained at a temperature of 10 ° C., 100 parts by weight of natural flake graphite powder having a particle size of 30 to 80 mesh was added to the reaction solution, and the reaction was performed for 30 minutes. After the reaction, the acid-treated graphite is separated by suction filtration, and the washing process of stirring the acid-treated graphite with 300 parts by weight of water for 10 minutes and suction filtration is repeated twice to sufficiently remove the sulfuric acid content from the acid-treated graphite. did.

  Next, the acid-treated graphite from which the sulfuric acid content was sufficiently removed was dried for 3 hours in a drying furnace maintained at a temperature of 110 ° C. to obtain an acid-treated graphite powder.

  An orthophosphoric acid aqueous solution having a concentration of 84% as phosphoric acid, an aqueous solution of primary aluminum phosphate having a concentration of 50% as a phosphate, and a saturated aqueous boric acid solution were prepared. 1.74 to 7.94 parts by weight of orthophosphoric acid, 4.21 parts by weight of primary aluminum phosphate, 0.15 to 1.45 parts by weight of boric acid, and 0.84 to 8 parts of water based on 100 parts by weight of the acid-treated graphite powder. A solution composed of 34 parts by weight was mixed and stirred uniformly to prepare a wettable mixture.

The wettable mixture was dried for 2 hours in a drying oven maintained at a temperature of 120 ° C. Next, the dried mixture was subjected to an expansion treatment at a temperature of 1000 ° C. for 5 minutes to generate decomposition gas, and a graphite powder (expansion magnification: 240 times) was expanded by expanding the graphite layer by the gas pressure. In this expansion treatment step, orthophosphoric acid in the component generates a dehydration reaction to generate phosphorus pentoxide, and the primary aluminum phosphate is desorbed from water in the structural formula [Al (H ₂ PO ₄ ) ₃ ] to form Al ( PO ₃ ) ₃ and coexisting with phosphorus pentoxide, boric acid partly generates boron oxide, part of it generates boron phosphate and coexists with phosphorus pentoxide and primary aluminum phosphate as a boron compound. It was confirmed that it was contained. This expanded graphite powder was supplied between rollers adjusted to a roll gap of 0.33 mm and roll-formed to produce a heat-resistant expanded graphite sheet having a thickness of 0.36 mm. This heat-resistant expanded graphite sheet was cut into a square shape with a side of 25 mm and used as a test piece.

Table 1 shows the results of testing the component composition of the heat-resistant expanded graphite sheet thus obtained and the oxidation consumption rate of the heat-resistant expanded graphite sheet. The evaluation of the oxidation consumption rate of the heat-resistant expanded graphite sheet is based on the weight reduction rate (%) of the heat-resistant expanded graphite sheet after being left in the air maintained at 700 ° C. and 750 ° C. for 3 hours. ).

Examples 5-8
An acid-treated graphite powder was produced in the same manner as in the above example.

  An orthophosphoric acid aqueous solution having a concentration of 84% as phosphoric acid, a primary aluminum phosphate aqueous solution having a concentration of 50% as a phosphate, and an aqueous yttrium chloride solution were prepared. 1.74 to 7.94 parts by weight of orthophosphoric acid, 4.21 parts by weight of primary aluminum phosphate, 0.56 to 5.65 parts by weight of yttrium chloride, and 0.69 to 6 parts of water based on 100 parts by weight of the acid-treated graphite powder. A solution consisting of .89 parts by weight was sprayed and stirred to prepare a mixture having wettability.

The wettable mixture was dried for 2 hours in a drying oven maintained at a temperature of 120 ° C. Next, the dried mixture was subjected to an expansion treatment at a temperature of 1000 ° C. for 5 minutes to generate decomposition gas, and a graphite powder (expansion magnification: 240 times) was expanded by expanding the graphite layer by the gas pressure. In this expansion treatment step, orthophosphoric acid in the component generates a dehydration reaction to generate phosphorus pentoxide, and the primary aluminum phosphate is desorbed from water in the structural formula [Al (H ₂ PO ₄ ) ₃ ] to form Al ( PO ₃ ) ₃ and coexisting with phosphorus pentoxide, part of yttrium chloride forms yttrium oxide, part of it produces yttrium phosphate and coexists with phosphorus pentoxide and primary aluminum phosphate as yttrium compounds. It was confirmed that it was contained. This expanded graphite powder was supplied between rollers adjusted to a roll gap of 0.33 mm and roll-formed to produce a heat-resistant expanded graphite sheet having a thickness of 0.36 mm. This heat-resistant expanded graphite sheet was cut into a square shape with a side of 25 mm and used as a test piece. Table 2 shows the results of testing the component composition of the heat-resistant expanded graphite sheet thus obtained and the oxidation consumption rate of the heat-resistant expanded graphite sheet.

Examples 9-10
An acid-treated graphite powder was produced in the same manner as in the above example. An orthophosphoric acid aqueous solution having a concentration of 84% as phosphoric acid, a primary aluminum phosphate aqueous solution having a concentration of 50% as a phosphate, a saturated boric acid aqueous solution, and an aqueous yttrium chloride solution were prepared. 1.74 to 4.50 parts by weight of orthophosphoric acid, 4.21 parts by weight of primary aluminum phosphate, 0.15 to 0.72 of boric acid, and 0.56 to 2 of yttrium chloride with respect to 100 parts by weight of the acid-treated graphite powder. A solution composed of 82 parts by weight and 0.84 to 4.17 parts by weight of water was sprayed and stirred to prepare a mixture having wettability.

The wettable mixture was dried for 2 hours in a drying oven maintained at a temperature of 120 ° C. Next, the dried mixture was subjected to an expansion treatment at a temperature of 1000 ° C. for 5 minutes to generate decomposition gas, and a graphite powder (expansion magnification: 240 times) was expanded by expanding the graphite layer by the gas pressure. In this expansion treatment step, orthophosphoric acid in the component generates a dehydration reaction to generate phosphorus pentoxide, and the primary aluminum phosphate is desorbed from water in the structural formula [Al (H ₂ PO ₄ ) ₃ ] to form Al ( PO ₃ ) ₃ and coexisting with phosphorous pentoxide, boric acid partially producing boron oxide, partially producing boron phosphate and coexisting with phosphorus pentoxide and primary aluminum phosphate as a boron compound, It was confirmed that yttrium chloride partly produced yttrium oxide and partly produced yttrium phosphate to coexist with phosphorus pentoxide, primary aluminum phosphate and boron compound as an yttrium compound. This expanded graphite powder was supplied between rollers adjusted to a roll gap of 0.33 mm and roll-formed to produce a heat-resistant expanded graphite sheet having a thickness of 0.36 mm. This heat-resistant expanded graphite sheet was cut into a square shape with a side of 25 mm and used as a test piece. Table 3 shows the results of testing the component composition of the heat-resistant expanded graphite sheet thus obtained and the oxidation consumption rate of the heat-resistant expanded graphite sheet. The evaluation of the oxidation consumption rate of the heat-resistant expanded graphite sheet and the expanded graphite sheet was performed in the same manner as in the above examples.

Comparative Examples 1-2
An acid-treated graphite powder was produced in the same manner as in the above example.

  An orthophosphoric acid aqueous solution having a concentration of 84% was prepared as phosphoric acid, and a primary aluminum phosphate aqueous solution having a concentration of 50% was prepared as a phosphate. A solution composed of 0.5 to 1.3 parts by weight of orthophosphoric acid and 4.3 to 8.5 parts by weight of primary aluminum phosphate is added to 100 parts by weight of the acid-treated graphite powder, and the wettability is improved by stirring uniformly. A mixture having was prepared.

  The wettable mixture was dried for 2 hours in a drying oven maintained at a temperature of 120 ° C. Next, the dried mixture was subjected to an expansion treatment at a temperature of 1000 ° C. for 5 minutes to generate decomposition gas, and a graphite powder (expansion magnification: 240 times) was expanded by expanding the graphite layer by the gas pressure. In this expansion treatment step, it was confirmed that the orthophosphoric acid in the component produced a dehydration reaction to produce phosphorus pentoxide, and the primary aluminum phosphate remained almost unchanged and contained together with phosphorus pentoxide. This expanded graphite powder was supplied between rollers adjusted to a roll gap of 0.33 mm and roll-formed to produce a heat-resistant expanded graphite sheet having a thickness of 0.36 mm. This heat-resistant expanded graphite sheet was cut into a square shape with a side of 25 mm and used as a test piece.

Table 3 shows the test results of the component composition of the expanded graphite sheet thus obtained and the oxidation consumption rate of the heat-resistant expanded graphite sheet. The evaluation of the oxidation consumption rate of the heat-resistant expanded graphite sheet and the expanded graphite sheet was performed in the same manner as in the above examples.

  From the above test results, at a temperature of 700 ° C., there is no significant difference in oxidation consumption rate between the heat-resistant expanded graphite sheets of Examples 1 to 9 and the expanded graphite sheets of Comparative Examples 1 and 2, but 750 ° C. It can be seen that there is a difference in the oxidation consumption rate at the temperature of. This difference is due to the improvement of the oxidation inhibiting action due to the synergistic effect with at least one of boron compound and yttrium compound dispersed in addition to phosphorus pentoxide and primary aluminum phosphate dispersed in the expanded graphite sheet. It is due.

Claims (8)

  A heat-resistant expanded graphite sheet comprising expanded graphite as a main component, in which phosphorus pentoxide and phosphate, and at least one of a boron compound and an yttrium compound are dispersedly contained.   The heat-resistant expanded graphite sheet according to claim 1, wherein phosphorus pentoxide is dispersed and contained at a ratio of 0.5 to 8% by weight.   The phosphate is selected from at least one of primary lithium phosphate, secondary lithium phosphate, primary calcium phosphate, secondary calcium phosphate, primary aluminum phosphate and secondary aluminum phosphate, The heat-resistant expanded graphite sheet according to claim 1 or 2, which is dispersed and contained at a ratio of 10% by weight.   The heat-resistant expanded graphite sheet according to any one of claims 1 to 3, wherein the boron compound comprises boron oxide and boron phosphate and is dispersed and contained at a ratio of 0.1 to 1.5% by weight.   The heat-resistant expanded graphite sheet according to any one of claims 1 to 4, wherein the yttrium compound comprises yttrium oxide and yttrium phosphate, and is dispersed and contained at a ratio of 0.5 to 5.0% by weight.   A solution prepared by mixing a phosphoric acid aqueous solution and a phosphate aqueous solution with at least one of boric acid and yttrium chloride is sprayed on the acid-treated graphite powder obtained by treating graphite powder with a strong acid, and these are mixed by stirring. A method of producing a heat-resistant expanded graphite sheet by expanding the blended acid-treated graphite powder 200 to 300 times at a temperature of 950 to 1200 ° C. and producing a heat-resistant expanded graphite sheet. 5-8% by weight phosphorus pentoxide and 1-10% by weight phosphate, 0.1-1.5% by weight boron compound and 0.5-5.0% by weight A method for producing a heat-resistant expanded graphite sheet in which at least one of the yttrium compounds is dispersedly contained.   The method for producing a heat-resistant expanded graphite sheet according to claim 6, wherein the phosphoric acid is selected from at least one of orthophosphoric acid, metaphosphoric acid, polyphosphoric acid, and polymetaphosphoric acid.   The phosphate is selected from at least one of primary lithium phosphate, secondary lithium phosphate, primary calcium phosphate, secondary calcium phosphate, primary aluminum phosphate, and secondary aluminum phosphate. Of producing a heat-resistant expanded graphite sheet.