JP2006280084A - Stator for motor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnetic member that is excellent in heat resistance having coils comprising conductors covered by a glass-system insulating material, that hardly causes a fracture including a break or a crack of the insulation cover, and that makes it easy to repair such a fracture even if it occurs. <P>SOLUTION: This stator for a motor comprises a magnetic material core, coils of conductors provided around the magnetic material core and covered by a glass layer made of an inorganic oxide and whose softening temperature is 400°C or larger and less than 800°C, a protective sheet layer inserted between the magnetic material core and the glass layer and that has higher softening temperature than the glass layer, and a molded layer provided on the outside circumference of the coil. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a motor stator including a coil for generating a magnetic field, and more particularly to a motor stator in a coil / core integrated motor.

  There is a demand for a small, high-output motor for powering electric vehicles and hybrid vehicles. In order to obtain a small and high output motor, it is desirable that the current density to the coil provided in the stator of the motor is high. However, since the heat generation of the coil increases due to the high current density, a material having excellent heat resistance is required for the insulating material covering the coil.

  A polyimide resin is known as an insulating material having excellent heat resistance, and a polyimide wire that is insulation-coated with a polyimide resin achieves a heat resistant life of about 250 ° C. Recently, however, power motors and the like have been required to be smaller and have higher output, and coils coated with an insulating material having higher heat resistance than conventional polyimide resins have been required for motors. .

  An inorganic glass-based insulating material is expected as an insulating material having higher heat resistance than a polyimide resin. Many glass-based insulating materials have excellent insulating properties, and usually have a glass transition point and softening point that are much higher than the temperature (about 250 ° C.) of the heat resistance life of polyimide wires, and may have excellent heat resistance. Many.

  However, glass-based insulating materials are fragile, and breakage such as cracks or cracks is likely to occur due to vibration or bending. Therefore, when a glass-based insulating material is used for the insulation coating of the stator coil of the motor, the coil and the magnetic core are rubbed when the coil is mounted on a magnetic core (iron core, etc.), and the magnetic vibration shock when the motor is used It has been thought that breakage due to the above is likely to occur, and a decrease in insulation becomes a problem.

  The present invention is a stator for a motor including a coil for generating a magnetic field, and the coil is coated with a glass-based insulating material having excellent heat resistance, and thus can be used in a high-temperature environment. It is an object of the present invention to provide a stator for a motor having a feature that breakage such as cracks or cracks in an insulating coating hardly occurs, or that the breakage is easy to repair.

  As a result of intensive studies, the present inventor used a glass-based insulating material having a softening temperature in a specific range, and protected a coil covered with the insulating material having a softening temperature higher than that of the insulating material. Covering with a sheet layer reduces the occurrence of breakage of the insulation coating and makes it possible to repair the breakage. Also, by providing a mold layer for fixing the coil on the outer periphery of the coil, It has been found that the occurrence of breakage can be further reduced. The present invention has been completed based on such findings.

  That is, the present invention provides, as claim 1, a magnetic core, a conductor provided around the magnetic core and covered with a glass layer made of an inorganic oxide and having a softening temperature of 400 ° C. or higher and lower than 800 ° C. A protective sheet layer made of an inorganic oxide and having a softening temperature higher than that of the glass layer, and a mold layer provided on the outer periphery of the coil, provided between the coil, the magnetic core and the glass layer. A motor stator is provided.

  The material of the conductor forming the coil included in the motor stator of the present invention is not particularly limited, and is made of copper, silver or an alloy containing these as a main component, or copper or silver is plated with nickel or the like. In general, copper or an alloy containing copper as a main component is used. The shape of the conductor is not particularly limited, but a conductor having a rectangular cross section is preferable in order to increase the space factor of the conductor in the stator and prevent electric field concentration.

  The surface of this conductor is insulated with a glass layer made of an inorganic oxide. Since the coating has a dense molecular structure vitrified, excellent insulating properties can be obtained. Furthermore, the effect of contributing to the oxidation prevention and corrosion prevention of the conductor is also exhibited.

  The glass layer has a softening temperature in the range of 400 ° C. or higher and lower than 800 ° C. In order to obtain high heat resistance far exceeding that of the polyimide resin, the softening temperature needs to be 400 ° C. or higher. Here, the softening temperature refers to the temperature at the inflection point when measured by the TMA method under conditions of a load of 10 g and a temperature increase of 10 ° C./min.

Further, since the glass layer has a softening temperature of 400 ° C. or more and less than 800 ° C., the viscosity of the glass layer can be made 10 ⁵ poises or less in this temperature range, and the width is 50 μm by annealing in this temperature range. Fine cracks to the extent can be easily eliminated. Therefore, fine cracks caused by rubbing, impact, coil deformation, etc. during the manufacturing process of the stator can be eliminated by this annealing, and the insulating coating in which the cracks have occurred can be self-repaired. When the softening temperature of the glass layer is 800 ° C. or higher, it becomes difficult to repair a crack or the like of the glass layer by annealing, and it is difficult to select a material for the protective sheet layer. In a glass containing P oxide and B oxide, a softening temperature within this range can be easily obtained.

As a glass layer which is an insulating material which coat | covers the surface of this conductor, what has a thermal expansion coefficient in the range of 8-23 * 10 < ^-6 > / degreeC in a glass transition point is preferable. Claim 2 corresponds to this preferable mode.

As a result of having a coefficient of thermal expansion within the above range, at a temperature ranging from room temperature to the glass transition point, the glass layer is composed of copper (thermal expansion coefficient: 17 × 10 ⁻⁶ / ° C.), silver (thermal expansion coefficient: 19 ×). 10 ⁻⁶ / ° C.), gold (thermal expansion coefficient: 14 × 10 ⁻⁶ / ° C.), etc., and thermal expansion close to nickel (thermal expansion coefficient: 13 × 10 ⁻⁶ / ° C.) used as conductor plating Therefore, it is possible to prevent the occurrence of breakage such as a crack caused by a change in environmental temperature accompanying energization or the like.

  The coefficient of thermal expansion within the above range can be obtained by using P oxide, B oxide and two or more kinds of alkali metal oxides as the inorganic oxide constituting the glass layer. The coefficient of thermal expansion varies depending on the ratio of P oxide and B oxide and the content of alkali metal oxide. Therefore, the thermal expansion coefficient in the above range is obtained by adjusting these within the range described later.

  The glass layer composed of P oxide, B oxide, and two or more kinds of alkali metal oxides, in addition to the feature that this coefficient of thermal expansion can be obtained, is resistant to crystallization, and a stable glass state is obtained. In addition, it has excellent characteristics such as good discharge resistance and good high-temperature insulation properties, and does not contain lead-based substances harmful to the human body, so it is preferable as a glass layer for coil insulation coating. Moreover, the range of 400-800 degreeC of softening temperature can be easily obtained with the glass which consists of P oxide, B oxide, and 2 or more types of alkali metal oxides. Claim 3 corresponds to this preferred embodiment, wherein the glass stator has (1) P oxide and B oxide and (2) two or more kinds of alkali metal oxides as a main component. The present invention provides a stator for a motor characterized by being contained.

  Note that P oxide, B oxide, and alkali metal oxide do not exist as independent molecules in the glass. Therefore, “containing (1) and (2)” is the same as the case where it is assumed that P oxide, B oxide and two or more kinds of alkali metal oxides exist as independent molecules. It means that the glass has a composition.

The ratio of (1) P oxide and B oxide to (2) two or more alkali metal oxides is preferably in the range of 1: 0.1 to 1: 1.2 in terms of molar ratio. When the content of (2) is less than 0.1 mol with respect to 1 mol of (1), the coefficient of thermal expansion decreases and it becomes difficult to set the content in the range of 8 to 23 × 10 ⁻⁶ / ° C. On the other hand, when it exceeds 1.2 mol, it becomes difficult to vitrify. In order to obtain higher heat resistance and insulation resistance, the ratio of (1) and (2) is more preferably in the range of 1: 0.1 to 1: 0.6 in terms of molar ratio. (1) Examples of P oxide and B oxide include P ₂ O ₅ and B ₂ O ₃ , respectively.

  Alkali metals have high ionic conductivity, and thus reduce the insulating properties of the glass. However, by containing two or more kinds of alkali metals in the glass, it is possible to prevent a decrease in the insulating properties of the glass.

As the two or more types of alkali metal oxides, Li ₂ O, Na ₂ O, or K ₂ O is preferable because of its availability. Further, the closer the ratio between two or more types of alkali metal oxides is to an equimolar ratio, the greater the effect of preventing a decrease in insulation. Therefore, in these compositions, the content of other alkali metal oxides is preferably 0.8 to 1.2 mol with respect to 1 mol of one type of alkali metal oxide. Among the combinations selected from Li ₂ O, Na ₂ O and K ₂ O, the combination of Li ₂ O and K ₂ O is particularly preferable from the viewpoint of preventing a decrease in insulating properties.

  The inorganic oxide constituting the glass layer more preferably contains (3) an oxide having a melting point higher than that of the P oxide and the B oxide in addition to the above (1) and (2). Claim 4 corresponds to this preferred embodiment. By containing (3), the water resistance of the glass layer can be improved. In particular, when the ratio of P oxide to B oxide is large, the water resistance tends to decrease, so the content of (3) is preferable. (3) is a so-called inorganic modified oxide, which does not form glass, but has an effect of changing the physical properties of the glass by entering into gaps between atoms forming the glass.

(3) As an oxide having a higher melting point than P oxide and B oxide, Al ₂ O ₃ and SiO ₂ are preferably exemplified. As in the cases of (1) and (2) above, including (3) does not necessarily include (3) as an independent molecule, but assumes that (3) is included as a molecule. It means that the glass has the same composition as the case.

  The glass layer is, for example, a method in which a conductor to be insulated is immersed in a melt of an inorganic oxide constituting the glass layer, pulled up, cooled and solidified, or after the conductor is immersed in a dispersion of glass powder and then heated. Then, the glass powder can be formed by melting, cooling and solidifying the glass powder. If the shape of the conductor is processed after the formation of the glass layer, the glass layer is likely to break. Therefore, after forming the glass layer in advance, the glass layer is formed. It is preferable to keep it to a minimum.

  When a glass powder dispersion is used, the solvent for dispersing the glass powder is preferably a solvent having a high boiling point of 150 ° C. or higher, and examples thereof include polyethylene glycol and α-terpineol. Further, 1-methyl-2-pyrrolidone or the like is preferably added to the solvent in order to lower the viscosity.

  The protective sheet layer constituting the motor stator of the present invention is provided between the magnetic core and the glass layer. That is, the outer periphery of the glass layer is covered and the glass layer is protected. The protective sheet layer can mitigate shocks applied from the magnetic core, such as when a coil is inserted into the magnetic core or due to magnetic vibration when a motor is used. Can be reduced.

The protective sheet layer has a softening temperature higher than the softening temperature of the glass layer. As a result, annealing at a temperature higher than the softening temperature of the glass layer becomes possible, and cracks and the like generated in the glass layer by this annealing can be repaired. By annealing at a temperature higher than the softening temperature of the glass layer, the viscosity of the glass layer becomes about 10 ⁵ poises or less, but the glass layer is covered with a protective sheet layer, so that the glass layer is damaged. Absent.

  In order to sufficiently repair cracks and the like, the annealing temperature is preferably several tens of degrees Celsius or higher than the softening temperature of the glass layer, and in order to sufficiently prevent softening of the protective sheet layer, The softening temperature is preferably higher by several tens of degrees C. than the annealing temperature. Therefore, preferably, the protective sheet layer has a softening temperature that is 100 ° C. or more higher than the softening temperature of the glass layer.

  The protective sheet layer is made of an inorganic oxide. As this inorganic oxide, a material which is not easily cracked by vibration or impact, for example, a cushioning material is preferably used. From this viewpoint, a porous material having a porosity of 10% by volume or more and less than 90% by volume is preferably used as the material for the protective sheet layer. Claim 5 corresponds to this preferable mode. Here, the porosity is the ratio of the volume of the pores to the total volume of the porous material. The value after assembling on the glass layer, for example, when the protective sheet layer is compressed during the assembling, is compressed. It is a later value.

  By using a porous material having a porosity of 10% by volume or more, the protective sheet layer has a cushioning property, and the glass layer is more unlikely to be broken by vibration or impact from the magnetic core. On the other hand, when the porosity is 90% by volume or more, the strength of the protective sheet layer is low, and it is easy to break during processing. In addition, when the motor is used, the insulation performance may be deteriorated due to partial discharge occurring in the space in the pores or entry of conductive foreign matter. The porosity is more preferably in the range of 40% by volume or more and less than 60% by volume.

Examples of the inorganic oxide constituting the protective sheet layer include Al ₂ O ₃ and SiO ₂ . Since the melting points of Al ₂ O ₃ and SiO ₂ far exceed 1000 ° C., a softening temperature higher than the softening temperature of the glass layer by at least 100 ° C. can be easily achieved by using these as the main components.

Examples of the porous material having a porosity of 10% by volume or more and less than 90% by volume include materials made of insulating inorganic fibers. More specifically, examples thereof include alumina fibers and SiO ₂ glass fibers, woven fabrics and nonwoven fabrics thereof.

  When an organic binder is used for forming the protective sheet layer, the organic resin is thermally deteriorated, so that it is difficult to use the motor stator at a high temperature of 300 ° C. or higher. Therefore, the protective sheet layer is preferably formed without using an organic binder. By using a woven fabric or a non-woven fabric, a protective sheet layer can be easily formed without using an organic material binder. Therefore, the porous material constituting the protective sheet layer is preferably a woven fabric or a non-woven fabric. Claim 6 corresponds to this preferred embodiment.

  Specifically, the assembling of the protective sheet layer onto the glass layer is performed, for example, by a method in which an inorganic fiber or a woven or non-woven fabric is wound around a coil covered with the glass layer after the glass layer is formed. Or it can carry out by the method of affixing a woven fabric and a nonwoven fabric on the surface at the side of the magnetic core of the coil covered with the glass layer. In order to more reliably adhere to the glass layer and to obtain a predetermined porosity when the protective sheet layer is a porous material, the protective sheet is applied during or after attaching or after attaching or winding. Layer compression may be performed.

  The magnetic core constituting the motor stator of the present invention is a core (core) made of a ferromagnetic material such as iron, and is manufactured by using a magnetic powder such as iron powder and molding it by a powder mold. can do. It is also possible to manufacture by laminating silicon steel plates or the like having a predetermined shape.

  The stator for a motor of the present invention has a magnetic core, an insulating coating coil provided around the magnetic core, and a protective sheet layer between the magnetic core and the coil. A layer is provided. The mold layer is provided to fix the coil in the stator. By fixing the coil, collision between the coil and the magnetic core can be prevented even when magnetic vibration occurs, and the breakage of the insulating coating due to impact can be reduced.

  The mold layer also functions as an insulating layer. That is, if a voltage difference generated between adjacent coils, for example, a three-phase AC motor has a large interphase voltage, the withstand voltage is insufficient with only the insulating coating of glass. In this case, the mold layer serves to supplement the withstand voltage. The thickness of the mold layer is not less than the thickness necessary for fixing the coil and obtaining the required withstand voltage.

  The outer periphery of the coil is a part where the temperature is relatively difficult to rise when the motor is used. If the coil has a heat generation exceeding the heat resistant life of 250 ° C of the polyimide resin, this part is also close to 250 ° C. There is. Therefore, the mold layer is preferably formed of a material having a softening temperature of 250 ° C. or higher. Claim 7 corresponds to this preferable mode.

  Specific examples of the material for forming the mold layer include thermoplastic polyimide, fluorine resin, thermosetting polyimide, polyamideimide, liquid crystal polymer containing inorganic short fibers such as glass, polyether ketone, polyphenylene sulfide. And glass-based inorganic solutions such as polysilazane.

  In the stator for motors of the present invention, for example, a coil that is insulated and coated with a glass layer and further formed with a protective sheet layer is inserted into the magnetic core that constitutes the stator, and then the magnetic material provided with the coil in this way. It can be obtained by combining a plurality of body cores and forming a mold layer on the outer periphery of the coil. In this case, in order to easily insert the magnetic core, a lubricating component may be applied on the protective sheet layer. Also, a powder material that forms a magnetic core, for example, an iron powder coated with an insulating material, is inserted with a coil that is coated with insulation and further formed with a protective sheet layer, and powder molding is performed integrally with the coil. Thereafter, a method of forming a mold layer on the outer periphery of the coil is also conceivable. Usually, the combination of magnetic cores is performed after the mold layer is formed.

  The mold layer can be formed by injection molding when the material is a thermoplastic material, and by a method such as impregnation when the material is other than thermoplastic. It is also possible to employ a method in which a molded sheet that has been molded in the shape of the mold layer is prepared in advance and this molded sheet is inserted into the mold layer.

  The motor stator of the present invention includes a coil coated with a glass-based insulating material excellent in heat resistance, and has a higher temperature environment than a conventional motor stator using a coil coated with a polyimide-based resin. Can be used below. Therefore, a high current density to the coil can be achieved, and a small and high output motor can be manufactured. Furthermore, since breakage such as cracks and cracks of the glass-based insulating coating is unlikely to occur, or even if the breakage occurs, it can be easily repaired by annealing, and therefore, it is suitably used for a small and high-power motor or the like.

  The stator for a motor of the present invention is particularly suitably used for a coil / core integrated motor. The coil / core integrated motor is more desired to have heat resistance, and the insulating coating is liable to break, so that the excellent effect of the present invention is particularly exerted.

  Next, the best mode for carrying out the invention will be described by way of examples. The examples are not intended to limit the scope of the invention.

  FIG. 1 is a partial cross-sectional view showing a portion of a motor stator of this embodiment. As shown in FIG. 1, the coil 1 is composed of a rectangular conductor 2 (copper) that is wider toward the tip of the stator (in the direction of arrow Y in the figure), and this conductor is covered with a glass layer 3. Has been. The coil 1 is provided around the iron core 4, and the iron core 4 is coupled to the iron core 4 'to form a core. A protective sheet layer 5 is provided between the iron cores 4, 4 ′ and the glass layer 3, that is, on the end face side of the coil and the inner peripheral side of the coil (in the direction of arrow X in the figure). The glass layer 3 is protected from impact and the like by the sheet layer 5. A mold layer 6 is provided on the outer peripheral side of the coil, and the coil 1 is fixed so as not to vibrate.

Next, manufacture of the motor stator shown in FIG. 1 will be described.
[Manufacture of glass]
As raw materials, B ₂ O ₃ (manufactured by Soekawa Riken, purity 99.9%), Li ₂ PO ₃ (manufactured by Hakuho Chemical Laboratory, purity 3N), K ₂ PO ₃ (manufactured by Hakuho Chemical Laboratory, purity) 3N) and Al ₂ O ₃ (manufactured by Katayama Chemical Co., Ltd., purity 98% or more) were used. The blending amount of each raw material is as follows.
B ₂ O _3: 12.7 parts by weight Li ₂ PO _3: 15.7 parts by weight K ₂ PO _3: 21.6 parts by weight Al ₂ O _3: 0.771 parts by weight

Each raw material was sufficiently mixed and melted in an electric furnace (1100 to 1300 ° C., in the air) for 0.5 to 1 hour. Thereafter, the glass was taken out from the furnace and cast into a stainless steel mold or a graphite plate to obtain glass. As a result, 50 parts by weight of glass containing P ₂ O ₅ : B ₂ O ₃ : Li ₂ O: K ₂ O: Al ₂ O ₃ in a composition of 33: 33: 16.5: 16.5: 2 It was made. The melting point (softening point) of this glass was 471 ° C. The coefficient of thermal expansion was 13.7 × 10 ⁻⁶ / ° C.
[Create dispersion]
After pulverizing the glass thus obtained, a glass powder having a specific particle size was obtained using a sieve. A dispersion of glass powder (insulating paint) was prepared by adding 50 parts by weight of α-terpineol as a dispersion solvent to 50 parts by weight of the obtained glass powder.

[Immersion process]
A flat rectangular conductor 2 made of copper was processed into a coil having a diameter of 3 cm, and then immersed in the dispersion for 1 minute and then pulled up.

[Heat treatment process]
Thereafter, this coil was kept in an oven at 570 ° C. for 10 minutes, heat-treated, and then cooled to obtain a coil having a glass layer 3 having a thickness of about 100 μm on the outer periphery of the conductor. Thereafter, the coil was compressed in the length direction (Y direction in FIG. 1) so that the glass layers 3 covering the conductor 2 of the coil were in contact with each other, and the coil 1 was obtained.

[Protective sheet]
On the inner peripheral side and the end face of the coil 1 obtained as described above, the material composition is Al ₂ O ₃ —SiO ₂ based, and is composed of short fibers having a diameter of 1 to 10 μm and a length of 0.5 to 5 mm. A woven cloth (nonwoven insulating sheet: MC paper manufactured by Nippon Inorganic Co., Ltd., softening temperature 1500 ° C.) is pasted so that the thickness is about 0.5 mm, and the porosity is 70 in the non-woven form after pasting. A protective sheet layer 5 having a volume% was formed.

[Iron core (magnetic core)]
An iron core 4 was produced by a powder mold of iron powder, and this iron core 4 was inserted into the coil 1 on which the protective sheet layer 5 was formed as described above. Thereafter, the iron core 4 ′ was inserted into the coil 1 from the side opposite to the iron core 4, and the iron core 4 and the iron core 4 ′ were joined.

[Mold layer]
After joining the iron core 4 and the iron core 4 ′, a mold layer 6 was formed on the outer periphery of the coil. The mold layer 6 is formed using a liquid crystal polymer (Sumikasuper LCP E5008, manufactured by Sumitomo Chemical), polyetheretherketone (150GL30, manufactured by Victorex MC) or polysilazane (NN310, manufactured by Clariant Japan), respectively. Three types of structures, that is, a core made of iron cores 4 and 4 ', a protective sheet layer 5, a coil 1 insulated with a glass layer 3 and a mold layer 6 were obtained.

  In the case of a liquid crystal polymer (Sumika Super LCP E5008, manufactured by Sumitomo Chemical Co., Ltd.), an injection molding machine manufactured by Nissei Plastic Industry is used and injection is performed under molding conditions of a temperature of 390 ° C., a pressure of 51 MPa, a speed of 98 mm / second, and a mold temperature of 80 ° C. A mold layer having a film thickness of 300 μm was formed by molding.

  In the case of polyether ether ketone (150GL30, manufactured by Victorex MC), injection was performed using an injection molding machine manufactured by Nissei Plastic Industry under the molding conditions of a temperature of 400 ° C., a pressure of 115 MPa, a speed of 175 mm / sec, and a mold temperature of 190 ° C. A mold layer having a thickness of 500 μm was formed by molding.

  In the case of polysilazane (NN310, manufactured by Clariant Japan), the solvent is dried at 200 ° C. × 1 hour in a pulling constant temperature bath after being fully immersed in the polysilazane solution, and this total immersion and drying process is repeated 10 times in total, and then 400 ° C. × By curing for 1 hour, a mold layer having a thickness of about 9 μm was formed.

  After the formation of the mold layer 6, the obtained structure, that is, the core composed of the iron cores 4, 4 ′, the protective sheet layer 5, the coil 1 insulated with the glass layer 3, and the structure composed of the mold layer 6, has a plurality of circular shapes. A stator for a motor was formed by combining them in a circumferential shape.

It is a fragmentary sectional view showing the portion of the stator for motors of an example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Coil 2 Conductor 3 Glass layer 4, 4 'Iron core 5 Protection sheet layer 6 Mold layer

Claims (7)

  A magnetic core, a coil of a conductor provided around the magnetic core and made of an inorganic oxide and covered with a glass layer having a softening temperature of 400 ° C. or higher and lower than 800 ° C., between the magnetic core and the glass layer A motor stator comprising a protective sheet layer made of an inorganic oxide and having a softening temperature higher than that of the glass layer, and a mold layer provided on the outer periphery of the coil. 2. The motor stator according to claim 1, wherein the glass layer has a thermal expansion coefficient of 8 to 23 × 10 ⁻⁶ / ° C. at a glass transition point.   3. The motor stator according to claim 2, wherein the glass layer contains (1) P oxide and B oxide, and (2) two or more kinds of alkali metal oxides as main components.   The motor stator according to claim 3, wherein the glass layer further contains (3) an oxide having a melting point higher than that of the P oxide and the B oxide.   The motor stator according to any one of claims 1 to 4, wherein the protective sheet layer is made of a porous material having a porosity of 10 to 90% by volume.   The motor stator according to claim 5, wherein the protective sheet layer is made of a woven fabric or a non-woven fabric. The stator for a motor according to any one of claims 1 to 6, wherein the mold layer is made of a material having a softening temperature of 250 ° C or higher.

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