JP5892421B2 - Metal powder, manufacturing method thereof, and dust core - Google Patents
Metal powder, manufacturing method thereof, and dust core Download PDFInfo
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本発明は、モータコアやリアクトル、インダクタ用途で高効率の圧粉磁心とすることが可能な金属粉末、その製造方法、及び圧粉磁心に関する。 The present invention relates to a metal powder that can be used as a highly efficient dust core for motor cores, reactors, and inductors, a manufacturing method thereof, and a dust core.
モータコアやリアクトル、インダクタ等の各用途では高効率な圧粉磁心が要求される。圧粉磁心は軟磁性の金属粉末をバインダと共に所定形状に圧縮成形したものであり、金属粉末として一般的に水アトマイズ法等で製造された純鉄または鉄基の金属粉末が用いられている。圧粉磁心の磁心損失を小さくすることで各用途での高効率化が可能であり、省エネルギー化に貢献できる。磁心損失はヒステリシス損失と渦電流損失に大別できる。 A high-efficiency dust core is required for each application such as a motor core, a reactor, and an inductor. The dust core is formed by compressing a soft magnetic metal powder into a predetermined shape together with a binder, and pure metal or iron-based metal powder produced by a water atomization method or the like is generally used as the metal powder. By reducing the core loss of the dust core, it is possible to increase the efficiency in each application and contribute to energy saving. Core loss can be broadly divided into hysteresis loss and eddy current loss.
圧縮成形すると圧粉磁心内に歪みが残留して保磁力が増大してしまうので、ヒステリシス損失が大きくなってしまう。そのため、圧縮成形後に歪取り熱処理（以下、焼鈍処理）を施す工程が設けられる。 When compression molding is performed, strain remains in the dust core and the coercive force increases, resulting in an increase in hysteresis loss. Therefore, a step of applying a strain relief heat treatment (hereinafter, annealing treatment) after compression molding is provided.
ヒステリシス損失が使用周波数に比例して大きくなるのに対し、渦電流損失は使用周波数の２乗に比例して大きくなる。金属粉末の周囲に絶縁膜を被覆して金属粉末同士を導通させないようにすることで渦電流損失を小さくすることができる。この絶縁膜は焼鈍処理の温度でも破壊されない高耐熱性とする必要がある。 Hysteresis loss increases in proportion to the operating frequency, whereas eddy current loss increases in proportion to the square of the operating frequency. The eddy current loss can be reduced by covering the metal powder with an insulating film so that the metal powder is not electrically connected. This insulating film needs to have high heat resistance that is not destroyed even at the annealing temperature.
絶縁膜として例えばリン酸塩ガラスが用いられる。しかしリン酸塩ガラスによる絶縁膜は焼鈍処理の際に結晶化が進行し、それに伴い被覆が破壊されて金属粉末同士が通電し、その結果として絶縁性が低下することが知られている。よってリン酸塩ガラスを用いる場合には焼鈍処理を400℃程度に留めているのが実情である。 For example, phosphate glass is used as the insulating film. However, it is known that the insulating film made of phosphate glass is crystallized during the annealing process, and the coating is destroyed accordingly, and the metal powders are energized, and as a result, the insulating property is lowered. Therefore, when using phosphate glass, it is the actual situation that the annealing treatment is kept at about 400 ° C.
特許文献１や特許文献２には、酸化鉄粉末を炭素で固相還元して得られたＦｅ基の金属粒子に炭素皮膜を形成した金属粉末が開示されている。炭素皮膜はリン酸塩ガラスの絶縁膜よりも耐熱性が高く、高温での焼鈍処理が可能である。 Patent Documents 1 and 2 disclose metal powders in which a carbon film is formed on Fe-based metal particles obtained by solid-phase reduction of iron oxide powder with carbon. The carbon film has higher heat resistance than the insulating film of phosphate glass, and can be annealed at a high temperature.
特許文献１や特許文献２に記載されるようにＦｅ基の金属粒子と絶縁膜との間に炭素皮膜を設けた金属粉末とすることで金属粒子同士が焼結することを抑制して渦電流損失の低下を防止することができるが、酸化鉄粉末の組成により炭素皮膜の形成が十分にできない場合があることがわかった。 As described in Patent Document 1 and Patent Document 2, by using a metal powder in which a carbon film is provided between an Fe-based metal particle and an insulating film, sintering of the metal particles is suppressed and eddy currents are suppressed. Although it was possible to prevent the loss from being reduced, it was found that the formation of the carbon film may not be sufficiently performed depending on the composition of the iron oxide powder.
よって本発明は、焼鈍処理を施しても渦電流損失の増大を抑制できる圧粉磁心に好適な金属粉末、その金属粉末を用いた圧粉磁心を提供することを目的とする。またこの金属粉末を得ることが容易な製造方法を提供することを目的とする。 Accordingly, an object of the present invention is to provide a metal powder suitable for a powder magnetic core that can suppress an increase in eddy current loss even after annealing, and a powder magnetic core using the metal powder. It is another object of the present invention to provide a production method that makes it easy to obtain this metal powder.
第１の本発明は、Ｆｅ基の金属粒子の表面に炭素皮膜を形成した金属粉末であって、前記金属粉末は平均粒径が５μｍ以上５０μｍ以下であり、焼鈍前の前記金属粉末の表面をXPS法により分析した分析値が、Cの割合が原子比で８７ａｔ％以上、Ｆｅの割合が２．０ａｔ％以下、Ｍｎの割合が１．０ａｔ％以下であることを特徴とする。 1st this invention is the metal powder which formed the carbon membrane | film | coat on the surface of the metal particle of Fe group, Comprising: The said metal powder is an average particle diameter of 5 micrometers or more and 50 micrometers or less, The surface of the said metal powder before annealing is used. The analysis value analyzed by the XPS method is characterized in that the C ratio is 87 atomic% or more, the Fe ratio is 2.0 atomic% or less, and the Mn ratio is 1.0 atomic% or less.
第２の本発明は、第１の本発明に記載の金属粉末であって、焼鈍前の前記金属粉末の表面をXPS法により分析した分析値が、Cの割合が原子比で８８ａｔ％以上、Ｆｅの割合が２．０ａｔ％以下、Ｍｎの割合が０．１ａｔ％以下であることを特徴とする。 2nd this invention is the metal powder as described in 1st this invention, Comprising: The analysis value which analyzed the surface of the said metal powder before annealing by XPS method, the ratio of C is 88 atomic% or more by atomic ratio, The ratio of Fe is 2.0 at% or less, and the ratio of Mn is 0.1 at% or less.
第３の本発明は、第１又は第２の本発明に記載の金属粉末を用いた圧粉磁心である。 3rd this invention is a powder magnetic core using the metal powder as described in 1st or 2nd this invention.
第４の本発明は、第１又は第２の本発明に記載の金属粉末の製造方法であって、Mnが１．０ｍａｓｓ％以下の酸化鉄粉末と、炭素粉末を混合し、混合した粉末を非酸化性雰囲気中において熱処理することで酸化鉄粉末を還元して金属粒子とするとともに、前記金属粒子に炭素皮膜を形成させた金属粉末とすることを特徴とする。 4th this invention is a manufacturing method of the metal powder as described in 1st or 2nd this invention, Comprising: Iron oxide powder whose Mn is 1.0 mass% or less, and carbon powder are mixed, The mixed powder is used. The iron oxide powder is reduced to metal particles by heat treatment in a non-oxidizing atmosphere, and the metal particles are formed by forming a carbon film on the metal particles.
第５の本発明は、第４の本発明に記載の金属粉末の製造方法であって、前記酸化鉄粉末と炭素粉末はモル比率ｍ[炭素粉末/酸化鉄粉末]が３．５以上４．５以下の範囲で混合されることを特徴とする。 The fifth aspect of the present invention is the method for producing a metal powder according to the fourth aspect of the present invention, wherein the iron oxide powder and the carbon powder have a molar ratio m [carbon powder / iron oxide powder] of 3.5 or more. It is characterized by being mixed within a range of 5 or less.
本発明により、高温の焼鈍にも耐えられる炭素皮膜を形成した金属粉末を提供することが容易になり、焼鈍しても金属粒子同士が焼結して電気的に導通することを抑制できるので、高温の焼鈍での渦電流損失の増大を抑制した圧粉磁心を提供することができる。 According to the present invention, it becomes easy to provide a metal powder formed with a carbon film that can withstand high-temperature annealing, and even when annealed, metal particles can be suppressed from being sintered and electrically connected, It is possible to provide a dust core in which an increase in eddy current loss due to high-temperature annealing is suppressed.
以下に本発明を具体的に説明する。
Ｍｎは熱処理によってＦｅ基の金属粒子の表面に析出する性質があり、Ｆｅ基の金属粒子に炭素皮膜を形成する際、金属粉末の表面におけるＭｎ濃度が高いと焼鈍時に炭素皮膜を破壊する原因となることを知見した。
The present invention will be specifically described below.
Mn has the property of precipitating on the surface of Fe-based metal particles by heat treatment. When a carbon film is formed on Fe-based metal particles, if the Mn concentration on the surface of the metal powder is high, the carbon film may be destroyed during annealing. I found out that
本発明の金属粉末は、Ｆｅ基の金属粒子の表面に炭素皮膜を形成した金属粉末であって、平均粒径が５μｍ以上５０μｍ以下であり、焼鈍前の前記金属粉末の表面をXPS（X-ray
Photoelectron Spectroscopy：X線光電子分光）法により分析した分析値が、Cの割合が原子比で８７ａｔ％以上、Ｆｅの割合が２．０ａｔ％以下、Ｍｎの割合が１．０ａｔ％以下であることを特徴とする。
The metal powder of the present invention is a metal powder in which a carbon film is formed on the surface of Fe-based metal particles, the average particle size is 5 μm or more and 50 μm or less, and the surface of the metal powder before annealing is treated with XPS (X− ray
The analysis value analyzed by the Photoelectron Spectroscopy (X-ray photoelectron spectroscopy) method shows that the C ratio is 87 atomic% or more, the Fe ratio is 2.0 atomic% or less, and the Mn ratio is 1.0 atomic% or less. Features.
金属粉末の表面におけるＭｎ濃度が１．０ａｔ％以下であるため、圧粉磁心にして焼鈍する際に炭素皮膜が破壊され難く、粒子同士の焼結を抑制して圧粉磁心の渦電流損失の増大を小さくすることができる。
また、Ｃの割合が８７ａｔ％未満、もしくはＦｅの割合が２．０ａｔ％を超えてしまうと金属粒子の露出部が多くなり、圧粉磁心にして焼鈍する際に金属粒子同士が焼結しやすくなり、渦電流損失が増大する。なお、XPS法ではC、Fe、Mn以外にも不可避不純物としてO、N、S等が検出され、本願発明のC、Fe、Mn量はこれらの総和を１００％とした時の値である。なお、金属粉末の表面におけるＭｎ濃度が０．１ａｔ％以下であれば、さらに圧粉磁心での焼鈍時における渦電流損失の増大を小さくできるので好ましい。
Since the Mn concentration on the surface of the metal powder is 1.0 at% or less, the carbon film is not easily destroyed when annealing to a powder magnetic core, and the eddy current loss of the powder magnetic core is suppressed by suppressing the sintering of particles. The increase can be reduced.
In addition, when the proportion of C is less than 87 at% or the proportion of Fe exceeds 2.0 at%, the exposed portion of the metal particles increases, and the metal particles are easily sintered when annealed to a dust core. As a result, eddy current loss increases. In the XPS method, in addition to C, Fe, and Mn, O, N, S, and the like are detected as inevitable impurities, and the amounts of C, Fe, and Mn in the present invention are values when the sum of these is 100%. In addition, it is preferable if the Mn concentration on the surface of the metal powder is 0.1 at% or less because an increase in eddy current loss during annealing in the dust core can be further reduced.
本発明の金属粉末の平均粒径は５μｍ以上５０μｍ以下である。平均粒径が５μｍ未満であると圧粉磁心とした際に成形体の密度が低下してしまう。平均粒径が５０μｍを超えると炭素皮膜が不完全となり、Ｃ、Ｆｅ、Ｍｎの表面濃度が好適範囲を維持できなくなる。特にＭｎ濃度が１．０ａｔ％を超えてしまうため好ましくない。好ましい平均粒径の範囲は１０μｍ以上３０μｍ以下とする。 The average particle size of the metal powder of the present invention is 5 μm or more and 50 μm or less. When the average particle size is less than 5 μm, the density of the molded body is reduced when a dust core is formed. When the average particle diameter exceeds 50 μm, the carbon film becomes incomplete, and the surface concentration of C, Fe, and Mn cannot be maintained within a preferable range. In particular, the Mn concentration exceeds 1.0 at%, which is not preferable. A preferable range of the average particle diameter is 10 μm or more and 30 μm or less.
本発明の金属粉末を得るのに好適な製造方法について説明する。
例えば製造方法として、Mnが１．０ｍａｓｓ％以下の酸化鉄粉末と、炭素粉末を混合し、混合した粉末を非酸化性雰囲気中において還元処理することで酸化鉄粉末を還元して金属粒子とし、かつ、前記金属粒子に炭素皮膜を形成させた金属粉末とすることができる。酸化鉄粉末中のＭｎ含有量が１．０ｍａｓｓ％以下であれば、還元処理後（焼鈍前）の金属粉末の表面のＭｎ濃度を１．０ａｔ％以下とすることができる。酸化鉄粉末のＭｎ量は好ましくは０．５ｍａｓｓ％以下、さらに好ましくは０．１ｍａｓｓ％以下とする。
A production method suitable for obtaining the metal powder of the present invention will be described.
For example, as a manufacturing method, iron oxide powder with Mn of 1.0 mass% or less and carbon powder are mixed, and the mixed powder is reduced in a non-oxidizing atmosphere to reduce the iron oxide powder to metal particles, And it can be set as the metal powder which formed the carbon film in the said metal particle. If the Mn content in the iron oxide powder is 1.0 mass% or less, the Mn concentration on the surface of the metal powder after the reduction treatment (before annealing) can be 1.0 at% or less. The amount of Mn in the iron oxide powder is preferably 0.5 mass% or less, more preferably 0.1 mass% or less.
酸化鉄粉末はFe2O3、Fe3O4、FeOなどの酸化鉄を用いることができる。特にFe2O3は安価に入手が可能であり好ましい。磁性粒子の平均粒径は0.01μｍ以上、3.0μm以下が好ましい。平均粒径が0.01μm未満であると最終的に得られる金属粉末の平均粒径が５μｍ未満となってしまい好ましくない。一方、3.0μmを超える酸化鉄粉末の製造は困難であり、工業材料としての入手や製造が難しく実用的ではない。 As the iron oxide powder, iron oxides such as Fe 2 O 3 , Fe 3 O 4 , and FeO can be used. In particular, Fe 2 O 3 is preferable because it can be obtained at low cost. The average particle size of the magnetic particles is preferably 0.01 μm or more and 3.0 μm or less. If the average particle size is less than 0.01 μm, the average particle size of the finally obtained metal powder becomes less than 5 μm, which is not preferable. On the other hand, it is difficult to produce iron oxide powder exceeding 3.0 μm, and it is difficult and practical to obtain and manufacture as an industrial material.
本発明において炭素皮膜とは金属粒子の表面が炭素で被覆された膜を指す。X線光電子分光（X-ray
Photoelectron Spectroscopy：XPS）分析によって得られる粉末表面の炭素濃度は８７ａｔ％以上、さらには８８ａｔ％以上が好ましい。金属粒子中のＸ元素量が１質量％以上の金属粉末であれば炭素濃度を９０ａｔ％以上とすることもできる。
In the present invention, the carbon film refers to a film in which the surface of metal particles is coated with carbon. X-ray photoelectron spectroscopy (X-ray
The carbon concentration of the powder surface obtained by Photoelectron Spectroscopy (XPS) analysis is preferably 87 at% or more, more preferably 88 at% or more. If the amount of X element in the metal particles is a metal powder of 1% by mass or more, the carbon concentration can be 90 at% or more.
炭素粉末はグラファイトやカーボンブラック、天然黒鉛の炭素粉末が適している。炭素を含む化合物であってもよい。すなわち石炭や活性炭、コークスや脂肪酸、ポリビニルアルコールなどの高分子であってもよい。上記炭素粉末の平均粒径は0.01〜100μmが好ましく、より好ましくは0.01〜50μmである。0.01μm未満の炭素粉末は高価で実用的ではない。また100μmを超えると酸化鉄との混合に偏りが生じ、酸化鉄粉末に対する炭素粉末の固相還元が不十分となって好ましくない。炭素はグラファイト状の炭素を用いているが、アモルファス状であっても構わない。Fe基の金属粒子が上記炭素で被覆されていることにより、成形体において焼鈍時にFe基金属粒子同士の焼結を抑制することができる。 As the carbon powder, graphite, carbon black, or natural graphite carbon powder is suitable. A compound containing carbon may also be used. That is, it may be a polymer such as coal, activated carbon, coke, fatty acid, or polyvinyl alcohol. The average particle size of the carbon powder is preferably 0.01 to 100 μm, more preferably 0.01 to 50 μm. Carbon powder less than 0.01 μm is expensive and impractical. On the other hand, if it exceeds 100 μm, the mixing with iron oxide is biased, and the solid phase reduction of the carbon powder with respect to the iron oxide powder is not preferable. As the carbon, graphite-like carbon is used, but it may be amorphous. By covering the Fe-based metal particles with the carbon, sintering of the Fe-based metal particles can be suppressed during annealing in the molded body.
平均粒径とはレーザー散乱式粒度分布計から得られるd50の値とする。 The average particle diameter is a value of d50 obtained from a laser scattering particle size distribution analyzer.
酸化鉄粉末と炭素粉末の混合粉を非酸化性雰囲気で熱処理すると、酸化鉄粉末は炭素粉末側に酸素を奪われ還元される事で鉄基の金属粒子となり、その後、金属粒子の表面に炭素皮膜が形成される。熱処理時の雰囲気は非酸化性雰囲気であることが好ましく、Ar、Heなどの不活性ガスや水素、窒素、炭酸ガスなどが選択される。特に安全かつ安価な点では窒素雰囲気がより好ましい。 When heat treatment is performed on a mixed powder of iron oxide powder and carbon powder in a non-oxidizing atmosphere, the iron oxide powder becomes iron-based metal particles by depriving the carbon powder side of oxygen and reducing it, and then carbon on the surface of the metal particles. A film is formed. The atmosphere during the heat treatment is preferably a non-oxidizing atmosphere, and an inert gas such as Ar or He, hydrogen, nitrogen, carbon dioxide gas or the like is selected. In particular, a nitrogen atmosphere is more preferable in terms of safety and low cost.
還元処理後は、本発明の金属粉末と余剰炭素粉末が混在した状態であるため、磁石等で金属粉末を磁気捕集して余剰炭素粉末を除去する。 After the reduction treatment, since the metal powder of the present invention and the surplus carbon powder are mixed, the surplus carbon powder is removed by magnetically collecting the metal powder with a magnet or the like.
酸化鉄粉末と炭素粉末はモル比率ｍ[炭素粉末/酸化鉄粉末]で３．５以上４．５以下の範囲で混合することが好ましい。
モル比率ｍが３．５未満であると、固相還元時にＦｅ粒子同士の焼結粒成長が促進され、粒径が粗大化して炭素の被覆が不完全となるため好ましくない。またモル比率ｍが４．５を超えると金属粉末の収率が低下する上、磁気捕集における余剰炭素粉末の除去が困難となり生産効率が低下するので好ましくない。
The iron oxide powder and the carbon powder are preferably mixed in a molar ratio m [carbon powder / iron oxide powder] in the range of 3.5 to 4.5.
When the molar ratio m is less than 3.5, the growth of sintered grains between Fe particles is promoted during solid-phase reduction, and the particle size becomes coarse and the coating of carbon becomes incomplete. On the other hand, when the molar ratio m exceeds 4.5, the yield of the metal powder is lowered, and it is difficult to remove the surplus carbon powder in the magnetic collection and the production efficiency is lowered.
上記のモル比率にした場合、還元処理の温度Ｔ（℃）は、前記モル比率ｍ、酸化鉄粉末のMn量ｎに対して、Ｔ≧−１００ｍ＋１６１０＋１００ｎで表される式を満たす温度であることが好ましい。温度Ｔがこの式を満たす数値ではない場合、炭素皮膜が十分に厚くならず炭素濃度が８７ａｔ％未満になり、焼鈍の際にFe 基の金属粒子同士が焼結してしまう。
但し還元処理の温度Tの上限は１６００℃以下の範囲が好ましい。温度Ｔが１６００℃を超えると熱処理炉に高い耐熱性が要求され製造コストが嵩んでしまう。さらに１５００℃以下であれば炭素皮膜の厚さを過剰にすることなく耐熱性を維持できる。
還元処理の温度Ｔの下限は、少なくとも１１５０℃以上であることが好ましい。１１５０℃以上とすることで還元後の金属粉末の形状を球形とすることができ、圧粉成形で金型内に給粉する時の粉末流動性を高めることができる。
When the molar ratio is set to the above, the temperature T (° C.) of the reduction treatment is a temperature satisfying the expression represented by T ≧ −100 m + 1610 + 100 n with respect to the molar ratio m and the Mn amount n of the iron oxide powder. preferable. When the temperature T is not a numerical value satisfying this equation, the carbon film is not sufficiently thick, the carbon concentration is less than 87 at%, and the Fe-based metal particles are sintered during annealing.
However, the upper limit of the temperature T of the reduction treatment is preferably in the range of 1600 ° C. or less. When temperature T exceeds 1600 degreeC, high heat resistance is requested | required of the heat processing furnace, and manufacturing cost will increase. Furthermore, if it is 1500 degrees C or less, heat resistance can be maintained, without making the thickness of a carbon film excessive.
The lower limit of the temperature T of the reduction treatment is preferably at least 1150 ° C. By setting the temperature to 1150 ° C. or higher, the shape of the metal powder after reduction can be made spherical, and the powder fluidity when powdered into the mold by compacting can be improved.
還元処理の好ましい熱処理時間は０．１時間（ｈ）以上１０ｈ以下である。０．１ｈ未満であると粒成長の効果が十分に得られず、１０ｈを超えると第１熱処理での工程時間が長くなり工業的に好ましくない。さらに好ましい熱処理時間は０．２ｈ以上５ｈ以下である。 A preferable heat treatment time for the reduction treatment is 0.1 hour (h) or more and 10 h or less. If it is less than 0.1 h, the effect of grain growth cannot be sufficiently obtained, and if it exceeds 10 h, the process time in the first heat treatment becomes long, which is not industrially preferable. A more preferable heat treatment time is 0.2 h or more and 5 h or less.
金属粒子は鉄を主成分とする。Al、Co、Ni、Siから選ばれる少なくとも1種の元素Xを添加してFe-X合金粒子とすることもできる特にSiは前記熱処理における金属粒子の表面での炭素析出効果が大きいので好適である。Ｘ元素は０．５質量％以上７．０質量％以下で含む組成となることが好ましい。金属粒子中のＸ元素量が０．５％未満であると、炭素皮膜による耐熱性の効果が十分に得にくい。また、鉄中に存在する炭化鉄が残留しやすく、その結果として金属粉末の硬度が高くなり圧縮性が低下して圧縮成形が難しくなる。Ｘ元素量が７．０質量％を超えると金属粉末の透磁率が下がり、圧粉磁心として用いる際に磁気飽和して軟磁気特性が低下しやすくなる。上記範囲のＸ元素を含む本発明の金属粉末は、X線光電子分光法で分析した表面炭素濃度が90at％以上である。 The metal particles are mainly composed of iron. At least one element X selected from Al, Co, Ni, and Si can be added to form an Fe-X alloy particle. Particularly, Si is suitable because it has a large carbon precipitation effect on the surface of the metal particle in the heat treatment. is there. It is preferable that the element X has a composition containing 0.5% by mass or more and 7.0% by mass or less. When the amount of X element in the metal particles is less than 0.5%, it is difficult to sufficiently obtain the heat resistance effect by the carbon film. In addition, iron carbide existing in iron tends to remain, and as a result, the hardness of the metal powder is increased, the compressibility is lowered, and compression molding becomes difficult. When the amount of X element exceeds 7.0% by mass, the magnetic permeability of the metal powder is lowered, and when used as a dust core, magnetic saturation occurs and soft magnetic properties tend to be lowered. The metal powder of the present invention containing the X element in the above range has a surface carbon concentration of 90 at% or more analyzed by X-ray photoelectron spectroscopy.
金属粒子の組成に元素Xを含めるには、元素Xは元素X単体、酸化物、炭化物、窒化物の化合物粉末、具体的な例としてAl4C3、AlN、CoO、Co2O3、Co3O4、Ni2O3、NiO、Si3N4、SiC等を用い、酸化鉄粉末、炭素粉末と共に混合して還元熱処理する。元素Xとしてアルミナやシリカといった酸化物を用いると熱力学的に安定であるので熱還元された鉄と反応することが困難となり相応しくない。固相反応性を考慮すると、元素Xの化合物粉末の粒径は0.001〜5μmの範囲内であるのが好ましい。粒径が０．００１μm未満では比表面積が大きすぎて容易に酸化し、取り扱いが困難である。また5μm超では比表面積が小さすぎるため、鉄との反応性が低く添加効果が期待できない。より好ましくは０．００１〜1μmが好ましい。 In order to include element X in the composition of metal particles, element X is element X alone, oxide, carbide, nitride compound powder, specific examples are Al 4 C 3 , AlN, CoO, Co 2 O 3 , Co 3 O 4 , Ni 2 O 3 , NiO, Si 3 N 4 , SiC, etc., are mixed with iron oxide powder and carbon powder and subjected to reduction heat treatment. When an oxide such as alumina or silica is used as the element X, it is difficult to react with the thermally reduced iron because it is thermodynamically stable, which is not suitable. Considering the solid phase reactivity, the particle size of the element X compound powder is preferably in the range of 0.001 to 5 μm. If the particle size is less than 0.001 μm, the specific surface area is too large and it is easily oxidized and difficult to handle. If it exceeds 5 μm, the specific surface area is too small, so the reactivity with iron is low and the effect of addition cannot be expected. More preferably, 0.001 to 1 μm is preferable.
本発明の金属粉末を圧縮成形すると、１０ｋＨｚ~１０MHｚの周波数帯域で優れた磁気特性を示す圧粉磁心を得ることができる。この圧粉磁心は、窒素中で700℃以上で焼鈍しても炭素皮膜の剥離や破壊を極力抑えられるので渦電流損失の増大を抑制できる。 When the metal powder of the present invention is compression molded, a powder magnetic core exhibiting excellent magnetic properties in a frequency band of 10 kHz to 10 MHz can be obtained. This powder magnetic core can suppress an increase in eddy current loss because the carbon film can be prevented from peeling and breaking as much as possible even when annealed at 700 ° C. or higher in nitrogen.
以下、さらに詳細に実施例を述べる。 Hereinafter, examples will be described in more detail.
（実施例１〜４）
酸化鉄粉末中のＭｎ量ｎを0.095〜0.47 mass%に変え、炭素皮膜が形成された金属粉末の表面の組成に与える影響を調べた。
Mn含有量が0.095mass%の酸化鉄粉とカーボンブラック粉を、モル比率でC/Fe2O3＝4.4となるように配合し、更に還元後の組成がFe-1mass%SiとなるようにSiC粉末を上記配合粉に対して0.8mass%添加した後、イソプロピルアルコール（IPA）中で17時間ボールミル混合した。
また、Ｍｎ含有量が0.26〜0.47mass%の酸化鉄粉とカーボンブラック粉を、モル比率でC/Fe2O3＝3.9となるように配合し、更に還元後の組成がFe-1mass%SiとなるようにSiC粉末を上記配合粉に対して0.8mass%添加した後、イソプロピルアルコール（IPA）中で17時間ボールミル混合した。混合後のスラリーをドラフト内で乾燥し、得られた原料混合粉を窒素雰囲気中において1400℃で2時間保持して還元処理することで酸化鉄粉末を還元して金属粒子とし、かつ、前記金属粒子に炭素皮膜を形成させた。熱処理後の生成物をIPA中で超音波洗浄した後、磁石で金属粉末のみを磁気捕集した。
得られた金属粉末の平均粒径、および金属粉末のC、Mn、Feの表面組成を表１に示す。平均粒径は粒度分布測定装置（日機装（株）、マイクロトラックMT3300）で測定し、表面組成はXPS分光分析装置（アルバック・ファイ（株）、PHI Quantera II）により分析した。
本実施形態の金属粉末は、その表面のＸＰＳ法によって測定されたＭｎ量を1.0at％以下である。この金属粉末は700℃以上で焼鈍しても炭素皮膜の剥離や破壊が少なく、この金属粉末を用いた圧粉磁心の渦電流損失の増大を抑制できる。
(Examples 1-4)
The amount n of Mn in the iron oxide powder was changed to 0.095 to 0.47 mass%, and the influence on the surface composition of the metal powder on which the carbon film was formed was examined.
Mn content of 0.095 mass% iron oxide powder and carbon black powder are blended so that the molar ratio is C / Fe 2 O 3 = 4.4, and the composition after reduction is Fe-1 mass% Si After adding 0.8 mass% of SiC powder to the above blended powder, ball mill mixing was performed in isopropyl alcohol (IPA) for 17 hours.
In addition, iron oxide powder having a Mn content of 0.26 to 0.47 mass% and carbon black powder are blended so that the molar ratio is C / Fe 2 O 3 = 3.9, and the composition after reduction is Fe-1 mass% Si After adding 0.8 mass% of the SiC powder to the above blended powder, ball mill mixing was performed in isopropyl alcohol (IPA) for 17 hours. The mixed slurry is dried in a fume hood, and the obtained raw material mixed powder is reduced in a nitrogen atmosphere at 1400 ° C. for 2 hours to reduce the iron oxide powder into metal particles, and the metal A carbon film was formed on the particles. The heat-treated product was ultrasonically cleaned in IPA, and then only the metal powder was magnetically collected with a magnet.
Table 1 shows the average particle diameter of the obtained metal powder and the surface composition of C, Mn, and Fe of the metal powder. The average particle size was measured with a particle size distribution analyzer (Nikkiso Co., Ltd., Microtrac MT3300), and the surface composition was analyzed with an XPS spectrometer (ULVAC-PHI Co., Ltd., PHI Quantera II).
In the metal powder of this embodiment, the amount of Mn measured by the XPS method on the surface is 1.0 at% or less. Even if this metal powder is annealed at 700 ° C. or higher, the carbon film is hardly peeled off or broken, and an increase in eddy current loss of a dust core using this metal powder can be suppressed.
（比較例１）
Ｍｎ含有量が1.20mass%の酸化鉄粉とカーボンブラック粉を、モル比率でC/Fe2O3＝3.9となるように配合し、更に還元後の組成がFe-1mass%SiとなるようにSiC粉末を上記配合粉に対して0.8mass%添加した後、イソプロピルアルコール（IPA）中で17時間ボールミル混合した。以降は実施例１と同様にして金属粉末を作製した。金属粉末の表面のＭｎ量は1.0at％を超えている。この金属粉末を用いた圧粉磁心の渦電流損失は焼鈍前後で大きく増大していた。
(Comparative Example 1)
The iron oxide powder and carbon black powder with Mn content of 1.20 mass% are blended so that the molar ratio is C / Fe 2 O 3 = 3.9, and the composition after reduction is Fe-1 mass% Si. After adding 0.8 mass% of SiC powder to the above blended powder, ball mill mixing was performed in isopropyl alcohol (IPA) for 17 hours. Thereafter, a metal powder was produced in the same manner as in Example 1. The amount of Mn on the surface of the metal powder exceeds 1.0 at%. The eddy current loss of the dust core using this metal powder increased greatly before and after annealing.
（実施例５〜１０）
酸化鉄粉末のMn量ｎおよびモル比率[C/Fe2O3]ｍに対して、好ましい還元温度の範囲を調べた。
Mn含有量が0.47mass%の酸化鉄粉とカーボンブラック粉を、モル比率でC/Fe2O3＝3.5、3.7、3.9、4.4となるように配合し、更に還元後の組成がFe-1mass%SiとなるようにSiC粉末を上記配合粉に対して0.8mass%添加した後、イソプロピルアルコール（IPA）中で17時間ボールミル混合した。混合後のスラリーをドラフト内で乾燥し、得られた原料混合粉を窒素雰囲気中において1200〜1400℃で2時間保持することで、酸化鉄から金属鉄へと固相還元反応を行なった。熱処理後の生成物をIPA中で超音波洗浄した後、磁石で金属粉末のみを磁気捕集した。
得られた金属粉末の平均粒径、および金属粉末のC、Mn、Feの表面組成を表２に示す。平均粒径は粒度分布測定装置（日機装（株）、マイクロトラックMT3300）で測定し、表面組成はXPS分光分析装置（アルバック・ファイ（株）、PHI Quantera II）により分析した。
(Examples 5 to 10)
A preferable reduction temperature range was examined with respect to the Mn amount n and the molar ratio [C / Fe 2 O 3 ] m of the iron oxide powder.
An iron oxide powder having a Mn content of 0.47 mass% and a carbon black powder are blended so that the molar ratio is C / Fe 2 O 3 = 3.5, 3.7, 3.9, 4.4, and the composition after reduction is Fe-1 mass After adding 0.8 mass% of SiC powder to the above blended powder so as to be% Si, ball mill mixing was performed in isopropyl alcohol (IPA) for 17 hours. The slurry after mixing was dried in a fume hood, and the obtained raw material mixed powder was held at 1200 to 1400 ° C. for 2 hours in a nitrogen atmosphere to perform a solid-phase reduction reaction from iron oxide to metallic iron. The heat-treated product was ultrasonically cleaned in IPA, and then only the metal powder was magnetically collected with a magnet.
Table 2 shows the average particle diameter of the obtained metal powder and the surface composition of C, Mn, and Fe of the metal powder. The average particle size was measured with a particle size distribution analyzer (Nikkiso Co., Ltd., Microtrac MT3300), and the surface composition was analyzed with an XPS spectrometer (ULVAC-PHI Co., Ltd., PHI Quantera II).
（比較例２〜６）
モル比率C/Fe2O3を3.5、3.7、3.9、4.4とし、それぞれの原料混合粉を表２に示す所定の温度で熱処理した以外は実施例１と同様にして金属粉末を得た。得られた金属粉末の平均粒径、およびC、Mn、Feの表面組成を表２に示す。比較例は表面の炭素濃度が70at％未満と低く、Ｍｎ濃度が１．０ａｔ％を超えている。
(Comparative Examples 2-6)
A metal powder was obtained in the same manner as in Example 1 except that the molar ratio C / Fe 2 O 3 was 3.5, 3.7, 3.9, and 4.4, and each raw material mixed powder was heat-treated at a predetermined temperature shown in Table 2. Table 2 shows the average particle diameter of the obtained metal powder and the surface composition of C, Mn, and Fe. In the comparative example, the surface carbon concentration is as low as less than 70 at%, and the Mn concentration exceeds 1.0 at%.
（実施例１１〜１３）
Mn含有量が0.095mass%の酸化鉄粉を用い、モル比率でC/Fe2O3＝4.4とした以外は実施例１と同様に金属粉末を得た。
得られた金属粉末の平均粒径、および表面におけるC、Mn、Feの組成を表２に示す。
(Examples 11 to 13)
A metal powder was obtained in the same manner as in Example 1 except that iron oxide powder having an Mn content of 0.095 mass% was used and the molar ratio was C / Fe 2 O 3 = 4.4.
Table 2 shows the average particle diameter of the obtained metal powder and the composition of C, Mn, and Fe on the surface.
酸化鉄のMn含有量ｎが0.47mass%の場合、モル比率ｍが３．５および３．７の場合は還元処理の温度Ｔ（℃）を１４００℃とした実施例５，６の試料、モル比率が３．９および４．４の場合は還元処理の温度Ｔ（℃）を１３００℃と１４００℃とした実施例７〜１０の試料で、金属粉末の表面の組成においてCの割合が８７ａｔ％以上、Ｆｅの割合が２．０ａｔ％以下、Ｍｎの割合が１．０ａｔ％以下である。
Mn含有量が0.095mass%、モル比率ｍが４．４の場合、1200℃〜1400℃のいずれの実施例でも、金属粉末の表面の組成においてCの割合が８７ａｔ％以上、Ｆｅの割合が２．０ａｔ％以下、Ｍｎの割合が１．０ａｔ％以下である。
実施例における還元温度Ｔと、モル比率ｍ、酸化鉄粉末のMn量ｎは、Ｔ≧−１００ｍ＋１６１０＋１００ｎで表される式を満たしている。対して、この式の関係を満たさない比較例２〜６の試料では、還元温度が低いためにCの割合が８７ａｔ％に満たず、またＭｎの割合が１．０ａｔ％超である。比較例の金属粉末は炭素皮膜が十分な厚さにならずに磁性粒子が露出していると予想され、後述するように焼鈍による磁心損失の増加が大きい。
When the Mn content n of iron oxide is 0.47 mass%, when the molar ratio m is 3.5 and 3.7, the samples and moles of Examples 5 and 6 in which the temperature T (° C) of the reduction treatment is 1400 ° C When the ratio is 3.9 and 4.4, the samples of Examples 7 to 10 in which the temperature T (° C.) of the reduction treatment is 1300 ° C. and 1400 ° C., the ratio of C is 87 at% in the composition of the surface of the metal powder. As mentioned above, the ratio of Fe is 2.0 at% or less, and the ratio of Mn is 1.0 at% or less.
When the Mn content is 0.095 mass% and the molar ratio m is 4.4, the ratio of C in the composition of the surface of the metal powder is 87 at% or more and the ratio of Fe is 2 in any of the examples of 1200 ° C. to 1400 ° C. 0.0 at% or less, and the ratio of Mn is 1.0 at% or less.
In the examples, the reduction temperature T, the molar ratio m, and the Mn amount n of the iron oxide powder satisfy the expression represented by T ≧ −100 m + 1610 + 100 n. On the other hand, in the samples of Comparative Examples 2 to 6 that do not satisfy the relationship of this formula, the ratio of C is less than 87 at% because the reduction temperature is low, and the ratio of Mn is more than 1.0 at%. In the metal powder of the comparative example, the carbon film is not sufficiently thick, and the magnetic particles are expected to be exposed. As described later, the increase in the core loss due to annealing is large.
表３に実施例５と実施例１３の金属粉末のICP分析によるＭｎ量を示す。酸化鉄粉末のMn含有量よりも若干減少した値であり、いずれも１．０mass%以下である。 Table 3 shows the amount of Mn by ICP analysis of the metal powders of Example 5 and Example 13. It is a value slightly reduced from the Mn content of the iron oxide powder, and both are 1.0 mass% or less.
図１は比較例６の金属粉末の粒子断面のＳＥＭ観察写真、図２は図１の金属粉末の表面の析出物（図中a）を拡大したＳＥＭ観察写真である。図２中ｂは鉄基の金属粒子である。表４は析出物と金属粒子の組成をオージェ分光分析で分析した結果である。析出物の主組成はMnSと同定された。この析出物ａは金属粒子と隣接しており炭素皮膜の形成を妨害している要因と推察される。 FIG. 1 is a SEM observation photograph of a particle cross section of the metal powder of Comparative Example 6, and FIG. 2 is an SEM observation photograph in which the precipitate (a in the figure) on the surface of the metal powder of FIG. In FIG. 2, b is iron-based metal particles. Table 4 shows the results of analyzing the composition of the precipitate and the metal particles by Auger spectroscopic analysis. The main composition of the precipitate was identified as MnS. This precipitate a is adjacent to the metal particles, and is presumed to be a factor hindering the formation of the carbon film.
比較例５と実施例７，８の金属粉末を窒素雰囲気下800℃で2時間焼鈍した後に表面の組成をXPS法で分析した。表５は焼鈍前後のC、Mn、Feの表面組成を示す。いずれの試料においても焼鈍によってMn量が増加しており、比較例５については焼鈍後にMnが６．６at%も検出されている。前記のようにMnが金属粉末の表面に多量に存在すると炭素による被覆が不十分となる。この傾向は、XPS法で得た金属粉末の表面のMn濃度とC濃度の関係にも現れている（図３）。金属粉末の表面のC濃度が低いとMn濃度が高くなっている。焼鈍後は更にMn濃度が増加するが、焼鈍前の金属粉末の表面のＭｎ濃度が１at%以下であれば、焼鈍後においても表面のＭｎの増加を極力抑えることができ、炭素皮膜が破壊されることを防ぐことができる。 After the metal powders of Comparative Example 5 and Examples 7 and 8 were annealed at 800 ° C. for 2 hours in a nitrogen atmosphere, the surface composition was analyzed by XPS. Table 5 shows the surface composition of C, Mn, and Fe before and after annealing. In any sample, the amount of Mn was increased by annealing, and in Comparative Example 5, Mn of 6.6 at% was detected after annealing. As described above, when a large amount of Mn is present on the surface of the metal powder, the coating with carbon becomes insufficient. This tendency also appears in the relationship between the Mn concentration and C concentration on the surface of the metal powder obtained by the XPS method (FIG. 3). When the C concentration on the surface of the metal powder is low, the Mn concentration is high. Although the Mn concentration further increases after annealing, if the Mn concentration on the surface of the metal powder before annealing is 1 at% or less, the increase in surface Mn can be suppressed as much as possible even after annealing, and the carbon film is destroyed. Can be prevented.
（実施例１４）
実施例１１の金属粉末に対してゾルゲル法を用いてシリカ被覆を施し、外径１３．４ｍｍ、内径７．７ｍｍの金型を用いてリング形状の圧粉体を作製した。成形圧力は１５００ＭＰa（15ton/cm2）とした。得られたリング状圧粉体の磁心損失をB-Hアナライザ（IWATSU、SY8232）を用いて測定した。更に圧縮時の歪みを除去するために上記リング状圧粉体を窒素雰囲気下で８００℃で２時間焼鈍し、焼鈍前と同様に磁心損失を測定した。測定周波数５０ｋHz、励磁磁束密度５０ｍTとした時のそれぞれの磁心損失値を表６に示す。
(Example 14)
The metal powder of Example 11 was coated with silica using the sol-gel method, and a ring-shaped green compact was produced using a mold having an outer diameter of 13.4 mm and an inner diameter of 7.7 mm. The molding pressure was 1500 MPa (15 ton / cm 2). The core loss of the obtained ring compact was measured using a BH analyzer (IWATSU, SY8232). Furthermore, in order to remove the distortion at the time of compression, the ring-shaped green compact was annealed at 800 ° C. for 2 hours in a nitrogen atmosphere, and the core loss was measured in the same manner as before annealing. Table 6 shows the respective core loss values when the measurement frequency is 50 kHz and the excitation magnetic flux density is 50 mT.
（実施例１５）
還元処理時間を０．２時間とした以外は実施例１１と同様にして金属粉末を作製し、その表面組成をＸＰＳ分析したところ、Cは８８at%、Feは２．０at%、Mnは１．０at%であった。当該金属粉末を用いて実施例１４と同様にしてリング形状圧粉体を作製し、磁心損失を測定した。結果を表６に示す。
(Example 15)
A metal powder was prepared in the same manner as in Example 11 except that the reduction treatment time was 0.2 hours, and the surface composition was analyzed by XPS. As a result, C was 88 at%, Fe was 2.0 at%, and Mn was 1. It was 0 at%. A ring-shaped green compact was produced using the metal powder in the same manner as in Example 14, and the magnetic core loss was measured. The results are shown in Table 6.
（比較例７）
比較例６の金属粉末を用いた以外は実施例１４と同様にして圧粉体の磁心損失を測定した。測定周波数５０ｋHz、励磁磁束密度５０ｍTとした時のそれぞれの磁心損失値を表６に示す。
(Comparative Example 7)
The core loss of the green compact was measured in the same manner as in Example 14 except that the metal powder of Comparative Example 6 was used. Table 6 shows the respective core loss values when the measurement frequency is 50 kHz and the excitation magnetic flux density is 50 mT.
（比較例８）
還元処理温度を１２００℃とした以外は実施例２と同様にして金属粉末を作製し、その表面組成をＸＰＳ分析したところ、Cは８３at%、Feは２．４at%、Mnは１．２at%であった。当該金属粉末を用いて実施例１４と同様にしてリング形状圧粉体を作製し、磁心損失を測定した。結果を表６に示す。
(Comparative Example 8)
A metal powder was prepared in the same manner as in Example 2 except that the reduction treatment temperature was 1200 ° C., and its surface composition was analyzed by XPS. C was 83 at%, Fe was 2.4 at%, and Mn was 1.2 at%. Met. A ring-shaped green compact was produced using the metal powder in the same manner as in Example 14, and the magnetic core loss was measured. The results are shown in Table 6.
（比較例９）
炭素皮膜の無い鉄粉で圧粉体を作製した。
ガスアトマイズ鉄粉（平均粒径２４μｍ）を用いた以外は実施例１４と同様にしてリング形状圧粉体を作製し、磁心損失を測定した。歪み除去のための焼鈍は実施例１４と同様の８００℃に加え、５００℃と６００℃を行った。焼鈍前後の磁心損失を表６に示す。
(Comparative Example 9)
A green compact was made of iron powder without a carbon film.
A ring-shaped green compact was prepared in the same manner as in Example 14 except that gas atomized iron powder (average particle size: 24 μm) was used, and the core loss was measured. In addition to 800 degreeC similar to Example 14, annealing for distortion removal performed 500 degreeC and 600 degreeC. Table 6 shows the core loss before and after annealing.
比較例７においては焼鈍後に渦電流損失が顕著に増大しており、全損失が焼鈍前よりも大きくなっている。これに対して実施例１４の磁心損失は、焼鈍による渦電流損失の増大が極めて少なく、結果として全損失を低減できている。また比較例９の磁心損失は５００℃で焼鈍した場合は渦電流損失の増大は殆ど見られないものの、ヒステリシス損失の値が大きく歪み除去が不十分である。十分歪み除去すべく焼鈍温度を６００℃に上げると渦電流損失が増大して全損失も焼鈍前より大きくなってしまう。８００℃で焼鈍した場合は磁心損失の増大が一層顕著となる。 In Comparative Example 7, the eddy current loss significantly increases after annealing, and the total loss is larger than that before annealing. On the other hand, in the magnetic core loss of Example 14, the increase in eddy current loss due to annealing is extremely small, and as a result, the total loss can be reduced. Further, when the magnetic core loss of Comparative Example 9 is annealed at 500 ° C., the eddy current loss hardly increases, but the hysteresis loss is large and the distortion removal is insufficient. When the annealing temperature is raised to 600 ° C. to sufficiently remove the strain, the eddy current loss increases and the total loss becomes larger than before the annealing. When annealed at 800 ° C., the increase in magnetic core loss becomes more remarkable.
したがって、本発明の磁性金属粉末は金属粒子が炭素で被覆されているため焼鈍に伴う粒子同士の焼結が抑制されており、渦電流損失の増大を抑制できる。このため歪みを十分に除去できる７００〜８００℃での焼鈍が可能となり、渦電流損失を低減することができる。
Therefore, in the magnetic metal powder of the present invention, since the metal particles are coated with carbon, sintering between particles accompanying annealing is suppressed, and an increase in eddy current loss can be suppressed. For this reason, annealing at 700 to 800 ° C. that can sufficiently remove the strain is possible, and eddy current loss can be reduced.
Claims (5)
前記金属粉末は平均粒径が５μｍ以上５０μｍ以下であり、
前記金属粉末の表面をXPS法により分析した分析値が、Cの割合が原子比で８７ａｔ％以上、Ｆｅの割合が２．０ａｔ％以下、Ｍｎの割合が１．０ａｔ％以下であることを特徴とする金属粉末。 A metal powder having a carbon film formed on the surface of Fe-based metal particles,
The metal powder has an average particle size of 5 μm or more and 50 μm or less,
Analytical values obtained by analyzing the surface of the metal powder by the XPS method are characterized in that the C ratio is 87 atomic% or more, the Fe ratio is 2.0 atomic% or less, and the Mn ratio is 1.0 atomic% or less. Metal powder.
前記金属粉末の表面をXPS法により分析した分析値が、Cの割合が原子比で８８ａｔ％以上、Ｆｅの割合が２．０ａｔ％以下、Ｍｎの割合が０．１ａｔ％以下であることを特徴とする金属粉末。 The metal powder according to claim 1,
Analyzed values obtained by analyzing the surface of the metal powder by the XPS method are characterized in that the C ratio is 88 atomic% or more, the Fe ratio is 2.0 atomic% or less, and the Mn ratio is 0.1 atomic% or less. Metal powder.
Mnが１．０ｍａｓｓ％以下の酸化鉄粉末と、炭素粉末を混合し、混合した粉末を非酸化性雰囲気中において熱処理することで酸化鉄粉末を還元して金属粒子とするとともに、前記金属粒子に炭素皮膜を形成させた金属粉末とすることを特徴とする金属粉末の製造方法。 It is a manufacturing method of the metal powder according to claim 1 or 2,
Iron oxide powder with Mn of 1.0 mass% or less and carbon powder are mixed, and the mixed powder is heat-treated in a non-oxidizing atmosphere to reduce the iron oxide powder to metal particles, A method for producing a metal powder, characterized in that the metal powder has a carbon film formed thereon.
前記酸化鉄粉末と炭素粉末はモル比率ｍ[炭素粉末/酸化鉄粉末]が３．５以上４．５以下の範囲で混合されることを特徴とする金属粉末の製造方法。 It is a manufacturing method of the metal powder according to claim 4,
The method for producing metal powder, wherein the iron oxide powder and the carbon powder are mixed in a molar ratio m [carbon powder / iron oxide powder] in the range of 3.5 to 4.5.
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