WO2010013109A1

WO2010013109A1 - Method for forming thin sio2 film on magnetic material

Info

Publication number: WO2010013109A1
Application number: PCT/IB2009/006320
Authority: WO
Inventors: Yusuke Oishi; Toshiya Yamaguchi; Eisuke Hoshina; Kazuhiro Kawashima; Takeshi Hattori
Original assignee: Fine Sinter Co Ltd; Toyota Motor Corp
Current assignee: Fine Sinter Co Ltd; Toyota Motor Corp
Priority date: 2008-08-01
Filing date: 2009-07-23
Publication date: 2010-02-04
Anticipated expiration: 2011-02-01
Also published as: JP2010040666A

Abstract

A method for forming a thin SiO2 film on a magnetic material by which a thin SiO2 film is formed on a surface of a magnetic material including iron and silicon as main components, the method including: an Fe oxide removal process of removing Fe oxide present on the surface of the magnetic material by performing a reduction treatment with respect to the magnetic material; and an oxidation treatment process of forming a thin SiO2 film on the surface of the magnetic material by performing an oxidation treatment with respect to the magnetic material from which the Fe oxide has been removed in the Fe oxide removal process. An electric resistance value of the magnetic material can be increased and eddy current loss can be decreased.

Description

METHOD FOR FORMING THIN SiO₂ FILM ON MAGNETIC MATERIAL

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a method of forming a thin SiO₂ film in which a magnetic material is subjected to an oxidation treatment to form a thin SiO₂ film thereon. 2. Description of the Related Art

[0002] Components obtained by processing and molding magnetic materials such as electromagnetic steel sheets and magnetic powders have been used in products that employ electromagnetism, and a technology of forming a thin SiO₂ film by performing an oxidation treatment in a weakly oxidizing atmosphere is available for improving the performance of such magnetic materials, in particular increasing electric resistivity (for example, see Japanese Patent No. 2698003, Japanese Patent Application Publication No. 2005-146315 (JP-A-2005-146315), and Japanese Patent No. 4010296).

[0003] Japanese Patent No. 2698003 describes a method for forming a thin insulating coating film on an electromagnetic steel sheet by which the steel is subjected to an oxidation treatment, SiO₂ is formed, and adhesion of the coating film is improved.

[0004] JP-A-2005-146315 and Japanese Patent No. 4010296 describe a method for subjecting a magnetic powder to an oxidation treatment, oxidizing mainly a second element with a high oxidation reactivity, and forming an oxide coating film.

[0005] An Fe-Si system is described as a representative example in the methods of the above-described related art, and the performance of the magnetic materials is improved, for example, electric resistivity is increased, by forming silicon oxide SiO₂ in the vicinity of the surface layer of the magnetic material by subjecting the magnetic material to an oxidation treatment.

[0006] The process flows disclosed in the aforementioned documents representing the related art will be compared below to get better understanding of the related art. A method 1 of the related art that is illustrated by FIG. 6 is described in Japanese Patent No. 2698003 and includes conducting an oxidation treatment of a steel sheet as a base material and then performing insulating coating by using a coating liquid or the like. A method 2 of the related art that is illustrated by FIG. 6 is a method for producing a magnetic powder and the like that is described in JP-A-2005-146315 and Japanese Patent No. 4010296, and this method involves only a process of subjecting a base material to an oxidation treatment. A method 3 of the related art that is illustrated by FIG 6 is a method for producing a magnetic powder and the like that is described in Japanese Patent No. 4010296, and this method involves repeating a processing cycle including a process of subjecting a base material to an oxidation treatment and a process of performing a treatment for preventing the diffusion of oxygen component after the oxidation treatment process.

[0007] However, the following problem is associated with the related art. Thus, a uniform SiO₂ coating film cannot be formed due to the effect of oxides located on the magnetic material surface that were originally present on the magnetic material surface or were produced by oxidation with the passage of time and, therefore, a high electric resistivity that can satisfy a high level of requirements for properties of the magnetic material cannot be obtained.

[0008] Furthermore, the object of the invention disclosed in the aforementioned Japanese Patent No. 2698003 is to produce a SiO₂ film by performing an oxidation treatment prior to coating an insulating film in order to increase the adhesion of the insulating film coated on an electromagnetic steel sheet and an effect of increasing the electric resistance by oxidation treatment (effect of increasing electric resistivity by forming SiO₂; referred to hereinbelow as "resistance increase effect") is not realized.

[0009] In the invention disclosed in Japanese Patent No. 4010296, a steel sheet of the invention disclosed in Japanese Patent No. 2698003 is replaced with a powder, the oxidation treatment and the oxidation component diffusion preventing treatment are performed alternately, and the resistance is also described. However, the resistance increase effect is not realized.

[0010] The problem associated with all the inventions of related art is that a production method by which SiO₂ is formed by conducting only an oxidation treatment cannot be expected to demonstrate the resistance increase effect, and eddy current loss increases in a case where the magnetic material coated with the thin SiO₂ film, disclosed in the related art, is used as a soft magnetic material.

SUMMARY OF THE INVENTION

[0011] The invention provides a method for forming a thin SiO₂ film on a magnetic material by which an electric resistance value is increased to reduce eddy current loss in the magnetic material. [0012] The results of a surface analysis of magnetic materials obtained in the comprehensive research conducted by the inventors demonstrated that SiO₂ can be formed by conducting an oxidation treatment in a weakly oxidizing atmosphere according to any of the above-described inventions of the related art, but a sufficient resistance increase effect is not demonstrated because uniform SiO₂ is not formed. More specifically, SiO₂ itself has a high electric resistivity, but where SiO₂ is formed unevenly on a magnetic material, the electric resistivity decreases significantly and when zones where SiO₂ has not been formed are present on the magnetic material surface, the desired electric resistivity cannot be obtained. This is because an Fe oxide is mainly present in the zones where SiO₂ has not been formed, SiO₂ does not form a uniform film because of the Fe oxide presence, and the electric resistivity decreases. The inventors have discovered that an Fe oxide that has already been present as a natural oxide or the like prior to the oxidation treatment of the magnetic material is the main factor (= zones where SiO₂ is not formed = presence of Fe oxide and the like) inhibiting the formation of a uniform SiO₂ film. Methods for eliminating this factor have been investigated and the results obtained led to the creation of the invention.

[0013] An aspect of the invention relates to a method for forming a thin SiO₂ film of a magnetic material by which a thin SiO₂ film is formed on a surface of a magnetic material including iron and silicon as main components, characterized by including: performing an Fe oxide removal process of removing Fe oxide present on the surface of the magnetic material by performing a reduction treatment with respect to the magnetic material; and performing an oxidation treatment process of forming a thin SiO₂ film on the surface of the magnetic material by performing an oxidation treatment with respect to the magnetic material from which the Fe oxide has been removed in the Fe oxide removal process.

[0014] The Fe oxide present on the surface of the magnetic material may be removed in the Fe oxide removal process by performing the reduction treatment with respect to the magnetic material by using either or both C and H₂.

[0015] Conditions of the reduction treatment in a case where the reduction treatment is performed with respect to the magnetic material by using C in the Fe oxide removal process may include a heating temperature of 700 to 1300⁰C and a CO partial pressure (Pco) of equal to or less than 1 Pa.

[0016] Conditions of the reduction treatment in a case where the reduction treatment is performed with respect to the magnetic material by using H₂ in the Fe oxide removal process may include a heating temperature of 700 to 1300⁰C and a dew point of equal to or less than -40°C.

[0017] In this aspect, in the oxidation treatment process, only Si present on a magnetic material surface may be preferentially oxidized and a thin SiO₂ film may be formed on the surface of the magnetic material by performing the oxidation treatment. [0018] The oxidation treatment process may be performed within a heating temperature range of 700 to 1300⁰C in a hydrogen gas flow with a partial pressure ratio (P_H2_O/P_H2) of a partial pressure of water vapor (P_H2O) to a partial pressure of hydrogen (PH₂) of 1 x 10^'5 to 1 x 10^'1.

[0019] The oxidation treatment process may be performed within a heating temperature range of 700 to 1300⁰C in a carbon monoxide gas flow with a partial pressure ratio (Pco₂/Pco) of a partial pressure of carbon dioxide (Pco2) to a partial pressure of carbon monoxide (Pco) of 1 x 10^"6 to 1 x 10^" .

[0020] In this aspect, Fe oxide may be not present on the magnetic material surface after the oxidation treatment performed in the oxidation treatment process. [0021] In this aspect, the Fe oxide may be FeO_x.

[0022] In accordance with the invention, a more uniform thin SiO₂ film can be formed on the magnetic material surface and, therefore, a desirable high electric resistivity is obtained for the magnetic material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements, and wherein:

FIG 1 is a process flow diagram of an embodiment of the invention; FIG 2 shows Si element state analysis results obtained by XPS analysis for the outermost surface of the magnetic material;

FIG 3 shows Fe element state analysis results obtained by XPS analysis for the outermost surface of the magnetic material;

FIGS. 4A and 4B are formation image diagrams of thin SiO₂ films; FIG 4A is a formation image diagram of a thin SiO₂ film formed by the method for forming a thin SiO₂ film in accordance with the invention; and FIG 4B is a formation image diagram of a thin SiO₂ film formed by the method according to the related art; FIG 5 shows a resistant value after the oxidation treatment; and

FIG 6 shows a process flow diagram of the method according to the related art.

DETAILED DESCRIPTION OF EMBODIMENTS

[0024] An embodiment of the invention will be described below. FIG. 1 is a process flow diagram of an embodiment of the invention. FIG 2 shows Si element state analysis results obtained by XPS analysis for the outermost surface of the magnetic material. FIG. 3 shows Fe element state analysis results obtained by XPS analysis for the outermost surface of the magnetic material. FIGS. 4A and 4B are formation image diagrams of thin SiO₂ films. FIG. 4A is a formation image diagram of a thin SiO₂ film formed by the method for forming a thin SiO₂ film in accordance with the invention. FIG. 4B is a formation image diagram of a thin SiO₂ film formed by the method according to the related art. FIG. 5 shows a resistant value after the oxidation treatment. FIG. 6 shows a process flow diagram of the method according to the related art. A method for forming a thin SiO₂ film by which a thin SiO₂ film is formed on the surface of an Fe-Si magnetic powder, which is a magnetic material, will be described below, bμt the invention is not limited to the magnetic powder and can be generally applied to a wide range of magnetic materials of any shape, such as a powder and a steel sheet, provided that the magnetic material includes Fe and Si as the main components. [0025] A method for forming a thin SiO₂ film by which a thin SiO₂ film is formed on the surface of a base material that is a magnetic material will be explained below with reference to FIG. 1. As shown in FIG 1, the main processing flow of the method for forming a thin SiO₂ film involves performing in the order of description an Fe oxide removal process of removing an Fe oxide present on the surface of the magnetic material by performing a reduction treatment with C (carbon) or H₂ with respect to a base material that is a treatment object, and an oxidation treatment process of forming a thin SiO₂ on the surface of the magnetic material by performing an oxidation treatment with respect to the magnetic material from which the Fe oxide has been removed by the Fe oxide removal process. These processes will be described below in greater detail. [0026] First, a magnetic material (base material) that is the treatment object to be subjected to the treatment performed in the aforementioned processes will be described. The base material that is a treatment object is a magnetic material including Fe and Si as the main components and can be in the form of a steel sheet or a powder. In the embodiment, an example will be explained in which a magnetic powder of an Fe-Si system that includes Fe and Si as the main components is used, this magnetic material being an example of a soft magnetic material.

[0027] Magnetic powders of an Fe-Si system are widely used as soft magnetic material powders because of a comparatively high electric resistivity and a comparatively low cost. The amount of Si in the magnetic powder of an Fe-Si system is determined by a balance of specific resistance value and magnetic flux density. For example, the amount of Si may be 1 to 10 wt.%, or 1 to 7 wt.%, or 2 to 5 wt.%. Where the amount of Si is too low, the electric resistivity is small and the eddy current loss cannot be reduced. Where the amount of Si is too high, magnetic properties are degraded. [0028] The magnetic powder of an Fe-Si system may be an alloy powder including a predetermined amount of Si and Fe and unavoidable impurities. This alloy is not necessarily a two-component alloy and may include appropriate amounts of carbon (denoted hereinbelow by C), aluminum (Al), tin (Sn), nickel (Ni), and cobalt (Co).

[0029] The magnetic powder may be an atomized powder obtained by gas atomization or water atomization or a ground powder obtained by grinding an alloy ingot in a ball mill or the like.

[0030] In a case where reduction is performed using C (carbon) in the below-described Fe oxide removal process, the original base material may include C, or C may be added to the base material by a carburization treatment. [0031] The Fe oxide removal process will be described below.

[0032] Because Fe oxide is originally present on the surface of the base material powder, the Fe oxide removal process is performed to remove this Fe oxide. In the Fe oxide removal process, a reduction treatment is performed, while conducting heating with a heating means such as an electric furnace, and in this process, the Fe oxide is removed from the surface of the base material powder by heating the base material at a predetermined temperature and a predetermined dew point for a predetermined reduction treatment time in a reducing atmosphere satisfying the below-described reduction treatment conditions and reducing the Fe oxide present on the base material powder surface. After the Fe oxide removal process, the below-described oxidation treatment process is performed continuously therewith. The Fe oxide as referred to herein is an Fe oxide represented by FeO_x, such as FeO and Fe₂O₃.

[0033] The reduction treatment performed in the Fe oxide removal process can be performed using either of C and H₂, or the reduction treatment of the base material powder is performed by using both the C and the H₂, thereby removing the Fe oxide present on the surface of the base material powder.

[0034] A dew point adjustment device that can adjust a dew point of gases is connected to the electric furnace in the intermediate section of each gas piping serving to introduce a gas for the reduction treatment and a gas for the oxidation treatment into the electric furnace. A configuration may be used in which a dew point on the inlet port side and a dew point on the outlet port side of the electric furnace that are located downstream of the dew point adjustment devices be measured with dew point meters of an electrostatic capacity system.

[0035] In the Fe oxide removal process, a heating temperature of 700 to 1300⁰C may be selected as a condition of the reduction treatment in a case where C is used for the reduction treatment. Furthermore, with consideration for a CO reaction (C + O → CO), a partial pressure of CO (Pco) may be equal to or less than 1 Pa. A heating temperature of 700 to 1300⁰C may be selected as a condition of the reduction treatment in a case where H₂ is used instead of C for the reduction treatment. The dew point may be equal to or less than -40⁰C. Where the dew point is higher than -40⁰C, SiO₂ is generated before the Fe oxide is removed by the reduction treatment. Furthermore, the reduction treatment in the case both the C and the H₂ are used for the reduction treatment may be performed under conditions such that the conditions relating to the above-described case in which C is used and the case in which H₂ is used are satisfied. As for the reduction treatment time, the sufficient treatment can be performed for about 1 to 4 h, but the reduction treatment time is not particularly limited and may be appropriately set correspondingly to various conditions such as the base material shape, amount to be treated, and specifications of the apparatus used. The reducing atmosphere is not particularly limited, and for example ammonia gas may be used. [0036] Described below is the oxidation treatment process in which the oxidation treatment is performed continuously after the above-described Fe oxide removal process with respect to the base material from which the Fe oxide has been removed.

[0037] In the oxidation treatment process, a thin silicon oxide film SiO₂ is formed uniformly on the base material surface by conducting heat treatment under oxidation treatment conditions (in the embodiment, the temperature condition may be 700 to 1300⁰C and the dew point conditions may be equal to or less than 50⁰C ) in an oxidizing atmosphere having a predetermined temperature and a predetermined dew point with respect to the surface of the base material (magnetic powder of an Fe-Si system) that has been subjected to the reduction treatment in the Fe oxide removal process, the oxidation treatment process being conducted, for example, by using an electric furnace similar to that used in the above-described Fe oxide removal process in an oxidizing atmosphere (for example, in a hydrogen gas flow that is an oxidizing atmosphere maintained at a predetermined oxygen concentration (oxygen partial pressure)). The thickness of the silicon oxide film formed on the base material surface can be appropriately adjusted according to the temperature conditions of heating, heating time, and Si content in the base material. Furthermore, by performing the oxidation treatment under a dew point condition of equal to or lower than 50⁰C, it is possible to oxidize preferentially only Si on the base material surface and produce a thin SiO₂ film with good efficiency. [0038] Where heating is performed in the above-described temperature condition range of the oxidation treatment, Si, which has an oxidation rate higher than that of Fe, diffuses into the surface layer of the base material powder and is oxidized. As a result, the surface of the base material powder is covered uniformly with Si oxide.

[0039] The oxidizing atmosphere is usually most often formed in a hydrogen gas flow or a carbon monoxide gas flow. In a case where the oxidizing atmosphere is formed in a hydrogen gas flow, a partial pressure ratio (PmofPrn) of a partial pressure of water vapor (P_H2O) to a partial pressure of hydrogen (P_H2) may be 1 x 10^"5 to 1 x 10^"1.

[0040] In a case where the oxidizing atmosphere is formed in a carbon monoxide gas flow, a partial pressure ratio (Pco2/Pco) of a partial pressure of carbon dioxide (Pco2) to a partial pressure of carbon monoxide (Pco) rnay be 1 x 10^"6 to 1 x 10^"1.

[0041] In a case where the oxidizing atmosphere is formed in a hydrogen gas flow, the oxidizing atmosphere can be also attained, for example, by controlling the dew point (temperature) in the hydrogen gas flow. The dew point can be easily observed with a dew point thermometer or the like. In the embodiment, as described hereinabove, the dew point of water vapor in the hydrogen gas flow may be equal to or less than +50⁰C. Incidentally, the dew point (temperature) is a temperature at which the water vapor contained in a gas reaches the saturation and condensates and, for example, is an ambient temperature at a relative humidity of 100%. Where the amount of moisture in an oxidizing atmosphere is small, the dew point temperature is low. Conversely, where the amount of moisture in the oxidizing atmosphere is large, the dew point temperature thereof rises. Essentially, the dew point is an indicator of the amount of moisture contained in the oxidizing atmosphere, and the dew point temperature has no correlation with the temperature of the oxidizing atmosphere itself. However, in the atmosphere gas inlet and outlet ports of the electric furnace where the heat treatment is carried out, the treatment is performed under a condition of a gas pressure of 1 atm, and the dew point referred thereto means a value under 1 atm (0.1 MPa). In the oxidation treatment process, a thin SiO₂ film may be also formed on the base material surface by using a heating means other than the electric furnace. [0042] Example embodiments relating to the invention will be described below. A magnetic powder including Fe and Si as the main components and serving as a base material (starting material) (in the example embodiment, the magnetic powder has a composition of Fe - 6% Si - 0.5% C; units: wt.%) is placed into the above-described electric furnace, the inside of the electric furnace is evacuated, and then a reducing gas serving as a reducing atmosphere is introduced into the electric furnace (in the example embodiment, hydrogen gas is used: the reduction treatment conditions are CO partial pressure 10^"2, dew point -70⁰C). Dew point adjustment devices that can adjust the dew point of the reducing gas and the below-described oxidizing gas are connected to the intermediate section of gas piping for introducing the reducing gas and the below-described oxidizing gas into the electric furnace. The dew point at the inlet port side and the dew point at the outlet port side of the electric furnace that were located downstream of the dew point adjustment devices were measured with dew point meters of an electrostatic capacity type. The dew point inside the electric furnace was stabilized at a constant value. In the example embodiment, the dew point inside the electric furnace was considered and handled as being almost equal to the dew point at the inlet port side (inlet port dew point) (same hereinbelow). Furthermore, the dew point specified the reducing gas and the below-described oxidizing gas after dew point adjustment in a state under 1 aim. [0043] After the dew point has been confirmed to be stable, the temperature inside the electric furnace was raised at a predetermined temperature increase rate and held for 1 h at 1100⁰C, followed by cooling. The reduction treatment (Fe oxide removal process) was thus conducted for a predetermined time under the conditions of a heating temperature of 1100⁰C, a dew point of -70⁰C, and a CO partial pressure of 10^"2 in the electric furnace with a reducing atmosphere created by a hydrogen gas flow having reducing ability. Because of the reduction treatment in the Fe oxide removal process, C contained in the magnetic powder that was the base material and O (oxygen) contained in FeO and Fe₂O₃ present on the base material surface reacted with each other, CO was generated (CO reaction), FeO and Fe₂O₃, which are iron (Fe) oxides, were converted (reduced) into Fe, and FeO and Fe₂O₃ were removed from the base material surface (see the results of the below-described XPS analysis). Furthermore, after the Fe oxide removal process, the oxidation treatment process was performed for a predetermined time (in the example embodiment, 1 h) under the conditions of a heating temperature of 1100⁰C and a dew point of 0⁰C in an electric furnace with an oxidizing atmosphere constituted by a hydrogen gas flow (oxidizing gas) with a partial pressure ratio (Pmo/Pm) of a partial pressure of water vapor (P_H2O) to a partial pressure of hydrogen (P_H2) of about 0.2. As a result of the oxidation treatment process, a thin silicon dioxide (SiO₂) film, which is an insulating film, was formed on the surface of the magnetic powder. As described above, the CO partial pressure, which is one of the reduction treatment condition, was set to a value equal to or less than 1 Pa (in the example embodiment, 10^"2) for the sake of advancing the equilibrium of the above-described CO reaction to the CO generation side.

[0044] The Si elemental state on the outermost surface of the magnetic powder was analyzed by an XPS analysis (XPS: X-ray Photoelectron Spectroscopy) with respect to the magnetic powder having SiO₂ formed on the surface thereof by subjecting the base material to the above-described Fe oxide removal process and oxidation treatment process. The SiO₂ peaks were compared for the magnetic powder ((I) in FIG. 2) obtained by forming SiO₂ on the base material surface by the method for forming a thin SiO₂ film in accordance with the invention, as in the example embodiment, the comparative magnetic powder ((II) in FIG 2; method disclosed in Japanese Patent No. 4010296) obtained by forming SiO₂ on the surface by performing only the oxidation treatment process, without performing the Fe oxide removal process, as a method according to the related art, and the untreated base material ((III) in FIG. 2), which is the base material. As shown in FIG. 2, the comparison clearly demonstrate that by continuously performing the above-described Fe oxide removal process and oxidation treatment process, it was possible to form a sufficient thin SiO₂ film on the magnetic powder produced in the example embodiment ((I) in FIG 2).

[0045] The Fe elemental state on the outermost surface was also analyzed by the XPS analysis with respect to the magnetic powder formed by implementing the above-described Fe oxide removal process and oxidation treatment process and the magnetic powder (comparative sample) that was subjected only to the oxidation treatment as in the method according to the related art. The results demonstrated, as shown in FIG 3, that in the magnetic powder that is a comparative sample treated by the method according to the related art, a FeO-Fe₂O₃ peak was demonstrated and FeO and Fe₂O₃ (Fe oxides represented by FeO_x), which are Fe oxides, remained on the surface of the magnetic powder under the treatment ((II) in FIG 3), whereas in the magnetic powder produced in the example embodiment, the FeO Fe₂O₃ peak was not demonstrated and the residues of FeO and Fe₂O₃ (Fe oxides represented by FeO_x), which are Fe oxides, were not found on the surface ((I) in FIG. 3). This result indicates that a uniform thin SiO₂ film was formed by the method for forming a thin SiO₂ film in accordance with the invention.

[0046] The analysis results suggest that the formation states of thin SiO₂ films after the oxidation treatment differ between the method in accordance with the invention and the method according to the related art. FIG. 4B shows an image of a thin SiO₂ film formed by the method according to the related art (a method in which only the oxidation treatment is performed). In this case, a uniform thin SiO₂ film cannot be produced on the surface of magnetic powder because of the presence of FeO and Fe₂O₃, which are Fe oxides and locally present on the original surface. FIG. 4A shows an image of a thin SiO₂ film formed by the method for forming a thin SiO₂ film in accordance with the invention. Because FeO and Fe₂O₃, which are Fe oxides, are removed in advance by the Fe oxide removal process, FeO and Fe₂O₃ (Fe oxides represented by FeO_x), which are Fe oxides, are not present on the base material surface. As a result, a uniform thin SiO₂ film can be produced on the surface of magnetic powder.

[0047] Comparison of the resistance values after the oxidation treatment that were obtained with the method according to the related art and the method in accordance with the invention demonstrates, as shown in FIG 5, that the resistance value ((I) in FIG. 5) of the magnetic powder formed by the method for forming a thin SiO₂ film in accordance with the invention is greatly increased with respect to that ((II) of FIG. 5) of the magnetic powder formed by the method according to the related art. Thus, by applying the method for forming a thin SiO₂ film in accordance with the invention, it is possible to obtain a resistance increase effect. As shown in FIGS. 4 A and 4B, in a case where Fe oxide is present on the surface of the base material powder prior to the treatment, the resistance increase effect cannot be obtained. Therefore, the resistance of a magnetic material can be increased by introducing the Fe oxide removal process performed in accordance with the invention in the preliminary stage of the method according to the related art. In the example embodiment, a magnetic powder is described as the magnetic material, but a thin SiO₂ film can be also formed under the treatment conditions similar to those described above on a steel material.

[0048] Thus, by applying a method for forming a thin SiO₂ film on a magnetic material that is a method for forming a thin SiO₂ film by which a thin SiO₂ film is formed on a surface of a magnetic material including iron and silicon as the main components, this method including an Fe oxide removal process of removing an Fe oxide present on the surface of the magnetic material by performing a reduction treatment with respect to the magnetic material; and an oxidation treatment process of forming a thin SiO₂ on the surface of the magnetic material by performing an oxidation treatment with respect to the magnetic material from which the Fe oxide has been removed by the Fe oxide removal process, it is possible to form a more uniform thin SiO₂ film on the magnetic material surface and, therefore, the desirable high electric resistivity can be obtained for the magnetic material.

[0049] A method for forming a thin SiO₂ film on a magnetic material can be applied in which the Fe oxide present on the surface of the magnetic material is removed in the Fe oxide removal process by performing the reduction treatment with respect to the magnetic material by using either or both C and H₂, and a reduction treatment method can be appropriately selected with consideration for the starting material composition, starting material form, and the treatment volume.

[0050]' The method for forming a thin SiO₂ film in accordance with the invention has been discovered by specifying FeO and Fe₂O₃ (inhibiting factors that decrease the resistance value) that could not be specified as problems in the methods according to the related art, and a technique for forming a more uniform thin SiO₂ film has been established by which an Fe oxide removal process of reducing Fe oxide is provided as a pretreatment of an oxidation treatment when a thin SiO₂ film is formed by the oxidation treatment on the base material surface, and the reduction treatment and oxidation treatment are performed continuously with respect to the base material. The invention can be introduced when required in the process of forming a thin SiO₂ film by the oxidation treatment alone that has been performed in the related art.

[0051] While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the invention.

Claims

1. A method for forming a thin SiO₂ film of a magnetic material by which a thin SiO₂ film is formed on a surface of a magnetic material including iron and silicon as main components, characterized by comprising: performing an Fe oxide removal process of removing Fe oxide present on the surface of the magnetic material by performing a reduction treatment with respect to the magnetic material; and performing an oxidation treatment process of forming a thin SiO₂ film on the surface of the magnetic material by performing an oxidation treatment with respect to the magnetic material from which the Fe oxide has been removed in the Fe oxide removal process.

2. The method for forming a thin SiO₂ film on a magnetic material according to claim 1, wherein the Fe oxide present on the surface of the magnetic material is removed in the Fe oxide removal process by performing the reduction treatment with respect to the magnetic material by using either or both C and H₂.

3. The method for forming a thin SiO₂ film on a magnetic material according to claim 1, wherein conditions of the reduction treatment in a case where the reduction treatment is performed with respect to the magnetic material by using C in the Fe oxide removal process include a heating temperature of 700 to 1300⁰C and a CO partial pressure (Pco) of equal to or less than 1 Pa.

4. The method for forming a thin SiO₂ film on a magnetic material according to claim 1, wherein conditions of the reduction treatment in a case where the reduction treatment is performed with respect to the magnetic material by using H₂ in the Fe oxide removal process include a heating temperature of 700 to 1300⁰C and a dew point of equal to or less than -40⁰C.

5. The method for forming a thin SiO₂ film on a magnetic material according to any one of claims 1 to 4, wherein in the oxidation treatment process, only Si present on the magnetic material surface is preferentially oxidized and a thin SiO₂ film is formed on the surface of the magnetic material by performing the oxidation treatment.

6. The method for forming a thin SiO₂ film on a magnetic material according to claim 5, wherein the oxidation treatment process is performed within a heating temperature range of 700to 1300⁰C in a hydrogen gas flow with a partial pressure ratio (P_H2O/P_H2) of a partial pressure of water vapor (P_H2O) to a partial pressure of hydrogen (P_H2) of 1 X lO^"5 to 1 x 10^"1.

7. The method for forming a thin SiO₂ film on a magnetic material according to claim 5, wherein the oxidation treatment process is performed within a heating temperature range of 700 to 1300⁰C in a carbon monoxide gas flow with a partial pressure ratio (Pco₂/Pco) of a partial pressure of carbon dioxide (Pco2) to a partial pressure of carbon monoxide (P_co) of 1 x 10^"6 to 1 x 10^"1.

8. The method for forming a thin SiO₂ film on a magnetic material according to any one of claims 1 to 7, wherein Fe oxide is not present on the magnetic material surface after the oxidation treatment performed in the oxidation treatment process.

9. The method for forming a thin SiO₂ film on a magnetic material according to any one of claims 1 to 8, wherein the Fe oxide is FeO_x.