JPH08245273A - Production of carbon fiber reinforced carbon composite material - Google Patents

Abstract

PURPOSE: To efficiently produce a high density and high strength carbon fiber reinforced carbon composite material. CONSTITUTION: A matrix binder is impregnated into carbon fibers, combined, molded and carbonized by firing in a nonoxidizing atmosphere to obtain a C/C composite as a base material. This C/C base material is densified through 1st and 2nd densifying processes. In the 1st process, pitch is impregnated into the base material and carbonized by firing at 800-1,200 deg.C in a nonoxidizing atmosphere and the impregnation-carbonization is repeated plural times so as to increase the bulk density of the resultant material to 1.1-1.5g/cc. In the 2nd process, a liq. thermosetting resin is impregnated, cured and carbonized by firing at 800-1,200 deg.C in a nonoxidizing atmosphere and the impregnation- curing-carbonization is repeated plural times so as to increase the bulk density of the resultant material to >=1.6g/cc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a carbon fiber reinforced carbon composite material (hereinafter referred to as "C / C" material) having a high density and a high strength, particularly a high interlaminar shear strength.

[0002]

2. Description of the Related Art C / C materials have excellent specific strength and specific elastic modulus due to the composite of carbon fibers, and also have the light weight characteristic of carbon materials and excellent heat resistance and chemical stability. , Structural materials for aviation and spacecraft, dies for hot pressing, members for high temperature furnaces, etc. Typical techniques for producing this C / C material are: (1) Lamination of carbon fiber woven fabric impregnated with a matrix binder, compression molding to a predetermined shape by pressing, etc., and then non-oxidizing the prepreg molded body. A method of calcining and carbonizing in an atmosphere, (2) a method of forming a tow of carbon fibers soaked in a matrix binder into a predetermined shape by a filament winding method, and calcining and carbonizing this prepreg molded body in the same manner, (3) carbon A method of depositing carbon in a preform structure of a fiber by using CVD (Chemical Vapor Deposition) is known.

Of these, the method (3) for compounding using CVD requires a large-scale vapor phase reaction apparatus, which makes it difficult to manufacture large materials, and is not practical as an industrial means. . On the other hand, the methods (1) and (2) by the composite carbonization of the matrix binder are considered to be industrially advantageous manufacturing means because there are few restrictions on the equipment. However, when adopting these methods, a considerable amount of matrix binder is extruded to the outside during compression molding, or volatile components contained in the matrix binder are volatilized at the stage of firing and carbonizing the prepreg molded body. Occurs, the material structure of the obtained C / C composite has extremely low density and low strength. In order to compensate for such a phenomenon, secondary densification is carried out by forcibly impregnating the porous structure of the C / C composite with a binder resin such as a carbonizing phenol resin or furan resin, or a coal-based or petroleum-based pitch, and then firing. Processing is generally done.

For example, in JP-A-2-283666, a pitch-based carbon fiber oriented two-dimensionally or three-dimensionally is impregnated with coal tar pitch and / or petroleum pitch, and then carbonized in the impregnated state. And then subjecting this treated material to a graphitization treatment at 2000 to 3000 ° C.,
Then, as a densification treatment, the graphitized material is impregnated with coal tar pitch and / or petroleum-based pitch having a softening point of 150 to 250 ° C. and substantially containing no quinoline insoluble matter, followed by carbonization-graphitization treatment. A method for producing a C / C material, in which the steps of applying are repeated until a desired density is obtained, is also disclosed in Japanese Patent Application Laid-Open No. Hei 5
No. 139832 discloses that carbon fiber is impregnated with a mixture of a thermosetting resin containing 10 to 60% by weight of an optically isotropic coal tar pitch having a softening point of 200 to 300 ° C. to form a prepreg, which is molded. Next, the primary fired body obtained by carrying out the carbonization treatment is impregnated with a high softening point pitch having a softening point of 150 to 250 ° C substantially containing no quinoline insoluble matter, and subsequently infusible at 200 to 350 ° C in air. After that, a step of performing carbonization-graphitization treatment under an inert atmosphere,
A method for producing a carbon material has been proposed which is repeated until the bulk density becomes 1.6 g / cc or more.

In addition, the applicant of the present invention uses carbon fiber with a residual carbon rate of 45%.
After composite molding with the matrix binder composed of the above thermosetting resin liquid, 1400 to 1 in a non-oxidizing atmosphere.
Carbonization is performed in a temperature range of 700 ° C. to form a primary fired body having a porosity of 1% or less, and the primary fired body is impregnated with a thermosetting resin liquid having a residual carbon rate of 45% or more, and then cured in a non-oxidizing atmosphere. A method for producing a C / C material, which is heat-treated under a temperature range of 2000 ° C. or higher (JP-A-5-229868), has been developed and proposed.

[0006]

[Problems to be Solved by the Invention] However, C / C
The composite performance of the material is greatly affected not only by the bonding force between the carbon fiber that is the filler and the matrix binder that forms the matrix, but also by the bonding force between the base material structure and the binder that is impregnated by the densification treatment, impregnating the pitch. Then, the densification treatment increases the adhesive force between the carbon fibers and improves the interlaminar shear strength, but develops anisotropy, and thus cracks easily occur along the oriented surface of the carbon hexagonal mesh plane. On the other hand, when the thermosetting resin liquid is impregnated and subjected to a densification treatment, the adhesive strength between the carbon fibers is reduced as compared with the pitch impregnation, so that the interlaminar shear strength is reduced and the structure is isotropic. There is a drawback that brittle fracture is likely to occur.

In the method of mixing pitch and thermosetting resin liquid and impregnating the primary fired body to densify it, not only is it difficult to adjust the viscosity because a uniform mixture is difficult to prepare, but also firing carbonization. There is a problem in that the pitch and the behavior of the thermosetting resin are different at the time and an effective densification treatment cannot be performed.

The present inventor has found that the primary fired body, that is, C / C
As a result of repeated studies on the method of densifying the composite, the pitch was first impregnated for densification, and then the two-step treatment of impregnating and densifying with a thermosetting resin liquid was performed, resulting in high density and high strength of the material. It has been found that a high C / C material can be obtained.

The present invention has been completed based on the above findings, and its object is to improve the efficiency of a C / C material having a high density and a high material strength such as interlaminar shear strength by improving a densification treatment method. , To provide a manufacturing method that can be industrially produced.

[0010]

In order to achieve the above object, a method for producing C / C according to the present invention is such that carbon fiber is impregnated with a matrix binder to carry out composite molding, followed by firing carbonization in a non-oxidizing atmosphere. The C / C composite obtained as described above is used as a base material, and the C / C base material is impregnated with pitch under a non-oxidizing atmosphere.
The first densification step to make the bulk density of the material 1.1 to 1.5 g / cc by repeating the process of firing and carbonizing at 0 to 1200 ° C. a plurality of times, and then impregnating and hardening the thermosetting resin liquid to non-oxidize The constitutional feature is that a second densification step for sequentially increasing the bulk density of the material to 1.6 g / cc or more by sequentially repeating the process of firing and carbonizing at 800 to 1200 ° C. under a strong atmosphere is performed.

In the present invention, as the carbon fiber to be the reinforcing material, plain weave, satin weave, twill weave and the like woven fabrics produced from various raw materials such as polyacrylonitrile type, rayon type and pitch type can be used in one-dimensional or multi-dimensional directions. Oriented fibrous bodies, felts and tows are used. Further, as the matrix binder, a thermosetting resin liquid having a residual carbon content of 50% by weight or more, such as phenol or furan, or coal or petroleum pitch is used.

The carbon fiber is impregnated with the matrix binder by dipping or coating it in the matrix binder, and the surface of the carbon fiber is sufficiently wet with the binder and then semi-cured to form a prepreg. Pressure molding is performed to produce a composite molded body. In addition, in this compound stage,
Fiber volume content when carbon fiber amount is C / C composite
It is desirable to set (Vf) in advance so as to be 50 to 65% from the viewpoint of ensuring strength.

The formed composite compact is then fired and carbonized in a carbonization furnace maintained in a non-oxidizing atmosphere to obtain a C / C composite. As the carbonization furnace, there are used a lead hammer furnace of a type in which firing and carbonization are performed while being covered with a carbonaceous packing material such as coke powder, or an electric furnace in which the system is held by a non-oxidizing gas such as nitrogen or argon. The burning carbonization is usually performed at a temperature of 800 ° C. or higher.

The present invention provides the C / C thus obtained.
The composite is used as a base material, and the C / C base material is sequentially subjected to a two-step densification treatment of a first densification step and a second densification step. The flow chart of the densification process of the invention is shown in the system diagram.

As shown in FIG. 1, the first densification step is C /
Pitch impregnate the base material of the C composite in a non-oxidizing atmosphere for 8
This is a step of repeatedly performing a process of heating and carbonizing by heating at a temperature of 00 to 1200 ° C. a plurality of times until the bulk density of the base material becomes 1.1 to 1.5 g / cc. Petroleum pitch is used. Impregnation of the pitch is, for example, C
The / C composite is preferably immersed in a pitch and forcedly impregnated under pressure and heating at a pressure of 5 to 15 kg / cm ² and a temperature of 200 to 350 ° C. The C / C base material impregnated with pitch is heat-treated at a temperature of 800 to 1200 ° C. in a carbonization furnace held in a non-oxidizing atmosphere such as nitrogen or argon to be calcined and carbonized, whereby the impregnated pitch is converted into a carbide. To do. In the first densification step, the bulk density of the C / C base material is 1.1 to 1.
It is necessary to repeat it several times until it reaches the range of 5 g / cc. When the bulk density exceeds 1.5 g / cc, it becomes difficult to smoothly impregnate the thermosetting resin liquid in the second densification step,
On the other hand, when the bulk density is less than 1.1 g / cc, the effect of densification by pitch impregnation becomes insufficient, the adhesive force between the carbon fibers decreases, and it becomes difficult to improve the interlaminar shear strength.

C / which has been subjected to the first densification step treatment
The C base material is then subjected to the treatment of the second densification step shown in FIG. The second densification step is a treatment in which a thermosetting resin liquid is impregnated and cured, and the mixture is heated to a temperature of 800 to 1200 ° C. in a non-oxidizing atmosphere and calcined and carbonized so that the bulk density of the base material is 1.6 g / cc.
This is a process of repeating the process a plurality of times until the above. An initial condensate such as a phenol resin, a furan resin, or an epoxy resin is used as the thermosetting resin liquid, and the thermosetting resin liquid is added to the C / C base material treated in the first densification step at room temperature. 15
After pressure impregnation at a pressure of Kg / cm ² , the resin is cured by heating at a temperature of 200 to 300 ° C., then 800 to 1 in a carbonization furnace kept in a non-oxidizing atmosphere such as nitrogen or argon.
The resin is calcined and carbonized by heating to a temperature of 200 ° C. This second densification step needs to be repeated a plurality of times until the bulk density of the material reaches 1.6 g / cc or more. This is because if the bulk density is less than 1.6 g / cc, the material strength cannot be sufficiently improved.

It is preferable that the C / C material subjected to the first densification step and the second densification step is further heat-treated at a temperature of 2000 ° C. or higher in a non-oxidizing atmosphere such as nitrogen or argon. By this heat treatment, the material structure of the C / C material can be further strengthened. The surface of the C / C material manufactured in this manner may be coated with an oxidation resistant coating, if necessary.

[0018]

According to the present invention, the C / C material is densified by the first densification step of impregnating pitch and firing carbonization, and the second densification step of impregnating thermosetting resin liquid and firing carbonization. It is technically characterized in that the densification process is carried out in two stages and is repeated a plurality of times until each densification process reaches a specific bulk density. The reason why the pitch is impregnated as the first densification step is to fill the porous structure of the primary fired body and enhance the adhesive force between the carbon fibers,
As a result, the interlaminar shear strength can be increased.

Further, the impregnated pitch is 1 in the firing carbonization process.
Although pores of about μm are generated, the thermosetting resin liquid impregnated in the second densification step effectively fills the pores and densifies them, and improves the material strength.

As described above, by using the pitch as the impregnating material in the first densification step and the thermosetting resin liquid in the second densification step, the respective functions act synergistically to achieve high density. It is possible to efficiently manufacture a C / C material having excellent strength characteristics.

[0021]

Hereinafter, examples of the present invention will be described in comparison with comparative examples.

Example 1 (1) Preparation of C / C composite: polyacrylonitrile-based plain-woven carbon fiber cloth [W6101 manufactured by Toho Rayon Co., Ltd.] on the surface of a phenol resin initial condensate as a matrix binder [Dainippon Ink Chemical Industry Co., Ltd.] was applied and sufficiently impregnated, and air-dried for 48 hours to prepare a prepreg sheet. 200 x 1 by stacking 16 sheets of this prepreg sheet
The mixture was put into a mold of 00 mm and heated at a temperature of 110 ° C. and pressurized at a pressure of 20 kg / cm ^{2 to} perform composite molding. After heating the composite molded body to a temperature of 250 ° C. to completely cure it, it is transferred to an electric furnace kept in a nitrogen gas atmosphere, heated to 1000 ° C. at a temperature rising rate of 5 ° C./hr, and held for 5 hrs, and baked. A carbon / carbon composite was produced by carbonization. The obtained C / C composite has a fiber volume content (Vf) of 60% and a bulk density of 1.09.
It was g / cc.

(2) First densification step: This C / C composite was dipped in a coal-based pitch, and the pitch was impregnated by heating to 240 ° C. while applying a pressure of 7 kg / cm ² to the system. After that, it was placed in an electric furnace maintained in a nitrogen gas atmosphere, heated to a temperature of 1000 ° C. at a temperature rising rate of 10 ° C./hr, and held for 1 hour for calcination and carbonization. By repeating this treatment three times, a C / C composite having a bulk density of 1.30 g / cc was obtained.

(3) Second densification step: The C / C composite subjected to the treatment of the first densification step described above is dipped in a phenol resin initial condensate [manufactured by Dainippon Ink and Chemicals, Inc.]. At room temperature, the system was pressurized to 7 kg / cm ² to impregnate the phenol resin initial condensate, and then heated to 250 ° C. to cure the resin. Then put in an electric furnace maintained in a nitrogen gas atmosphere,
It was heated to a temperature of 1000 ° C. at a temperature rising rate of 5 ° C./hr, held for 1 hour, and calcined and carbonized. This treatment was repeated 3 times to obtain a C / C material having a bulk density of 1.76 g / cc.

The obtained C / C material was further heated to 2 ° C. in an electric furnace kept in a nitrogen gas atmosphere at a temperature rising rate of 10 ° C./hr.
Heat treatment was performed at a temperature of 000 ° C. The bending strength and the interlaminar shear strength of the C / C material thus manufactured were measured, and the results are shown in Table 1 together with the densification treatment conditions.
The flexural strength and the interlaminar shear strength were measured as follows.

Bending strength: length 150 mm, width 12.5 mm,
A test piece having a thickness of 4 mm was subjected to a three-point bending test under conditions of a fulcrum distance of 64 mm and a crosshead speed of 6 mm / min. In addition,
Other conditions were in accordance with ASTM D790. Interlaminar shear strength: length 40mm, width 6mm, thickness 4mm test piece distance between fulcrums 20mm, crosshead speed 1.25mm
It was measured under the condition of / min. Other conditions are ASTM
D2344-67 short beam method was applied.

Example 2 A C / C composite produced by the same method as in Example 1 was impregnated with coal-based pitch by the same method as in Example 1 as the first densification step, followed by treatment by calcining and carbonizing. Repeated times to obtain a C / C composite with a bulk density of 1.46 g / cc. Then, in the second densification step, a furan resin initial condensate is used instead of the phenol resin initial condensate [Sumitomo Durres Co., Ltd.].
In the same manner as in Example 1 except that was used, the furan resin initial condensate was impregnated and cured, and carbonized by firing. This process 3
Repeated times to obtain a C / C material having a bulk density of 1.78 g / cc. This C / C material was heated at 2000 ° C. by the same method as in Example 1 to produce a C / C material, and the bending strength and the interlaminar shear strength were measured by the same method as in Example 1, and the measurement was performed. The results are also shown in Table 1 together with the densification treatment conditions.

Example 3 A C / C composite was prepared in the same manner as in Example 1 except that the composite molding pressure was changed to 15 kg / cm ² when the C / C composite was prepared. The bulk density of the obtained C / C composite is 1.08 g / cc
Met. The C / C composite was impregnated with coal-based pitch by the same method as in Example 1 as the first densification step and calcined and carbonized, and the bulk density was 1.14 g /
After making cc, the second densification step was carried out by impregnating and curing the phenol resin initial condensate by the same method as in Example 1 and repeating the firing carbonization three times to obtain C having a bulk density of 1.76 g / cc.
/ C material was obtained. Then, using the same method as in Example 1, 20
C / C material was manufactured by heat treatment at 00 ° C., bending strength and interlaminar shear strength were measured by the same method as in Example 1, and the measurement results are also shown in Table 1 together with the densification treatment conditions.

Comparative Example 1 A C / C composite was prepared in the same manner as in Example 1 except that the composite molding pressure was changed to 10 kg / cm ² when the C / C composite was prepared. The bulk density of the obtained C / C composite is 1.02 g / cc
Met. The C / C composite was impregnated with coal-based pitch by the same method as in Example 1 as the first densification step and calcined and carbonized once to obtain a bulk density of 1.08 g /
After making cc, the process of impregnating and curing the initial condensation product of the phenol resin by the same method as in Example 1 as the second densification step and firing carbonization were repeated 5 times to obtain C having a bulk density of 1.69 g / cc.
/ C material was obtained. Then, using the same method as in Example 1, 20
C / C material was manufactured by heat treatment at 00 ° C., bending strength and interlaminar shear strength were measured by the same method as in Example 1, and the measurement results are also shown in Table 1 together with the densification treatment conditions.

Comparative Example 2 A C / C composite produced by the same method as in Example 1 was impregnated with coal-based pitch by the same method as in Example 1 as the first densification step, and treated by firing and carbonization. Repeated times to obtain a C / C composite having a bulk density of 1.63 g / cc. Then, as a second densification step, a process of impregnating and curing a phenol resin initial condensate and calcining and carbonizing is repeated twice in the same manner as in Example 1 to produce a C / C material having a bulk density of 1.70 g / cc. did. This C / C material was made into 2 by the same method as in Example 1.
After heat treatment at 000 ° C., bending strength and interlaminar shear strength were measured by the same method as in Example 1, and the measurement results are also shown in Table 1 together with the densification treatment conditions.

Comparative Example 3 With respect to the C / C composite prepared by the same method as in Example 1, the C / C composite was immersed in the phenol resin initial condensate in the first densification step, and the system was allowed to stand at room temperature. After pressurizing to 7 kg / cm ² and impregnating the phenolic resin initial condensate, 25
The resin was cured by heating to 0 ° C. Then, it was placed in an electric furnace maintained in a nitrogen gas atmosphere and heated at a heating rate of 5 ° C./hr for 10
It was heated to a temperature of 00 ° C. and held for 1 hour for calcination and carbonization.
This treatment was repeated 3 times to obtain a bulk density of 1.42 g / cc, and then, as a second densification step, immersing the coal-based pitch in the system and applying a pressure of 7 Kg / cm ^{2 to} the temperature of 240 ° C. After being heated to 1 to impregnate the pitch, it was placed in an electric furnace maintained in a nitrogen gas atmosphere, heated to a temperature of 1000 ° C. at a temperature rising rate of 10 ° C./hr, and held for 1 hour to perform a calcination carbonization treatment. By repeating this treatment three times, C having a bulk density of 1.65 g / cc was obtained.
/ C material was obtained. This C / C material was subjected to the same method as in Example 1 at 10 ° C./hr in an electric furnace maintained in a nitrogen gas atmosphere.
The heat treatment was carried out at a temperature of 2000 ° C. at a heating rate of. The bending strength and the interlaminar shear strength of the C / C material thus manufactured were measured by the same method as in Example 1, and the measurement results are also shown in Table 1 together with the densification treatment conditions.

Comparative Example 4 A C / C composite prepared by the same method as in Example 1 was immersed in a phenol resin initial condensate, and the inside of the system was heated to 7 kg at room temperature.
After the pressure was increased to / cm ² to impregnate the phenol resin initial condensate, it was heated to 250 ° C. to cure the resin. Next, the C / C material with a bulk density of 1.70 g / cc was placed 6 times by placing it in an electric furnace maintained in a nitrogen gas atmosphere and heating it to 2000 ° C. at a temperature rising rate of 5 ° C./hr to calcinate and carbonize. Was manufactured. The bending strength and the interlaminar shear strength of the C / C material thus manufactured were measured by the same method as in Example 1,
The measurement results are shown in Table 2 together with the densification treatment conditions.

Comparative Example 5 The C / C composite was impregnated and cured with the furan resin precondensate in the same manner as in Comparative Example 4 except that the furan resin precondensate was used in place of the phenol resin precondensate. The carbonization treatment by firing was repeated 6 times to obtain C having a bulk density of 1.73 g / cc.
/ C material was manufactured. The bending strength and the interlaminar shear strength of the obtained C / C material were measured by the same method as in Example 1, and the measurement results are shown in Table 2 together with the densification treatment conditions.
It was also published in.

Comparative Example 6 A C / C composite prepared by the same method as in Example 1 was immersed in petroleum oil pitch and heated to 240 ° C. while pressurizing the inside of the system to a pressure of 7 kg / cm ² to obtain a pitch. Was impregnated, then placed in an electric furnace maintained in a nitrogen gas atmosphere, heated to a temperature of 2000 ° C. at a temperature rising rate of 10 ° C./hr, and held for 1 hour to perform a calcination carbonization treatment. By repeating this treatment 6 times, a C / C material having a bulk density of 1.65 g / cc was obtained. The bending strength and the interlaminar shear strength of this C / C material were measured by the same method as in Example 1, and the measurement results are also shown in Table 2 together with the densification treatment conditions.

Comparative Example 7 A C / C composite was impregnated with coal-based pitch in the same manner as in Comparative Example 6 except that coal-based pitch was used instead of petroleum-based pitch, and then calcined and carbonized 6 times. Repeatedly, a C / C material having a bulk density of 1.68 g / cc was obtained. Bending strength and interlayer shear strength of the obtained C / C material were measured by the same method as in Example 1, and the measurement results are also shown in Table 2 together with the densification treatment conditions.

[0036]

[Table 1]

[0037]

[Table 2]

From the results shown in Tables 1 and 2, it is understood that the C / C material produced by the manufacturing method of the present invention has improved bulk density and strength characteristics as compared with the C / C material of the comparative example. That is, in Comparative Example 1 in which the bulk density by the pitch impregnation and firing carbonization in the first densification step is small, the interlaminar shear strength is significantly reduced because the densification by the pitch is small, while the pitch impregnation and firing in the first densification step are performed. It is recognized that in Comparative Example 2 in which the bulk density due to the carbonization treatment is high, the bending strength is largely reduced because the amount of the thermosetting resin liquid impregnated is small. Furthermore, in Comparative Example 3 in which the thermosetting resin liquid was impregnated in the first densification step and the pitch was impregnated in the second densification step, it is clear that the bending strength and the interlaminar shear strength are low. Further, in Comparative Examples 4 and 5 which are densification treatments using only the thermosetting resin liquid, the interlaminar shear strength is remarkably low, while Comparative Example 6 in which the densification treatment is performed using only the pitch.
It was found that No. 7 had a significantly low bending strength.

[0039]

As described above, according to the present invention, the bending strength and the interlaminar shear strength can be increased at a high density by performing the two-step densification treatment of the first densification step and the second densification step under specific conditions. It is possible to efficiently manufacture the excellent C / C material. Therefore, it is extremely useful as a method for industrially producing a high performance C / C material.

[Brief description of drawings]

FIG. 1 is a system diagram showing a flow sheet of a first densification step and a second densification step of the present invention.

Claims (2)

[Claims] 1. A C / C composite obtained by impregnating a carbon fiber with a matrix binder to form a composite and then firing and carbonizing it in a non-oxidizing atmosphere is used as a base material, and the C / C base material is provided with a pitch. The treatment of impregnating and calcining at 800 to 1200 ° C. in a non-oxidizing atmosphere is repeated a plurality of times so that the bulk density of the material is 1.1 to
First densification step to 1.5g / cc, then impregnation and hardening with thermosetting resin liquid, 800-1200 ° C under non-oxidizing atmosphere
A method for producing a carbon fiber reinforced carbon composite material, which comprises sequentially performing a second densification step of making the bulk density of the material 1.6 g / cc or more by repeating the treatment for firing and carbonizing a plurality of times. 2. The first densification step according to claim 1,
Next, a method for producing a carbon fiber reinforced carbon composite material, in which a second densification step is sequentially performed and then heat treatment is performed at a temperature of 2000 ° C. or higher in a non-oxidizing atmosphere.