JP3629390B2

JP3629390B2 - High frequency powder magnetic core and method for manufacturing the same

Info

Publication number: JP3629390B2
Application number: JP33489899A
Authority: JP
Inventors: 民夫高田
Original assignee: Hitachi Powdered Metals Co Ltd
Current assignee: Hitachi Powdered Metals Co Ltd
Priority date: 1999-11-25
Filing date: 1999-11-25
Publication date: 2005-03-16
Anticipated expiration: 2019-11-25
Also published as: JP2001155914A

Description

【０００１】
【発明の属する技術分野】
この発明は各種の電気・電子機器に使用される圧粉磁心に係るもので、とくに１ｋＨｚ〜１ＭＨｚ程度までの高周波領域で用いられる例えばチョークコイル，ノイズフィルター，リアクトルなどのコアに好適な、磁束密度が高く周波数特性に優れ、機器の小型化に対応し得る圧粉磁心に関するものである。
【０００２】
【従来の技術】
交流磁場内で使用される軟磁性材料の鉄心（磁心）では、磁束密度および透磁率が大きいことと、鉄損が小さいことが特に要求される。珪素鋼板はこの条件に適合するが、薄板の打ち抜き品を積層して作る関係で形状に制約を受けるため、形状が複雑な場合は、任意の形状に成形可能な粉末冶金による磁心（粉末磁心）によることが多い。粉末磁心には圧粉体を焼結した所謂焼結鉄心と、焼結せずに純鉄，Ｆｅ−Ｓｉ合金，センダスト，パーマロイなど強磁性金属の粉末を熱硬化性樹脂，水ガラスその他適宜の結合材で固化させた圧粉磁心とがあるが、鉄損の主要部分を占める渦電流損は鉄心の厚さの自乗に比例するため、一体成形の焼結鉄心では鉄損が大きくなるという問題がある。
【０００３】
この点では、圧粉磁心の場合は鉄粉粒子の間に非磁性の樹脂が介在するために渦電流損が小さいという本質的特徴がある。あとは、圧粉磁心の場合磁束密度は磁心の密度比によって一義的に定まるので、高密度に成形して磁束密度を高めれば要求特性を充足することができる訳である。圧粉体の密度を高めるためには、粉末を圧縮成形する際の成形圧力を高くすることと、それに伴って粉末相互間や粉末と金型との間に生じる摩擦抵抗を減じる必要があり、その手段として、一般的には粉末潤滑剤を原料粉に混合している。しかしその量によっては、焼結工程のない圧粉磁心の場合には摩擦抵抗は減じるものの却って圧粉密度の低下を招いたり、樹脂硬化の過程で溶融した粉末潤滑剤が鉄粉と樹脂の接合および樹脂層の良好な形成を妨げ、その結果圧粉体の強度（抗折力）の低下を招くことがある。従って圧粉磁心の場合は粉末潤滑剤の添加は出来るだけ少量に留め、押型潤滑を併用するのが好ましい。
【０００４】
【発明が解決しようとする課題】
以前、本件出願人は特公昭４９−１５６８４号において、通常の溶解法による鉄系磁性合金の場合と同じく粉末磁心の場合も鉄にＳｉ，Ａｌ，Ｎｉなど各成分単味の固有抵抗，または鉄との固溶体の固有抵抗が鉄単味の固有抵抗よりも高い成分の添加が磁気特性の向上に有効であること；特に、これらの成分またはその拡散部の薄層が鉄粒子を被覆して存在する組織構造にすると交流磁気特性（磁束密度，鉄損）が著しく改善されること；そしてこの様な組織構造は、Ｓｉを例にとると例えば珪素樹脂の水溶液に鉄粉を浸漬，乾燥すれば表面が珪素樹脂で被覆された鉄粉が得られるので、その鉄粉の使用により容易に実現されることを開示した。
【０００５】
しかし近年、各種電気・電子機器の小型化が進むにつれてこれらに使用される磁心も小型化する必要が生じた結果、小型化しても従来の機能を損わない、即ち高磁束密度・高透磁率・低鉄損などの磁気特性を具え、且つ高強度の磁心が求められるに至った。ところが表面に固有抵抗の高い層を形成した粉末を用いても、これを高密度に圧粉するため粉末潤滑剤の添加量を減らして成形圧力を高くすると粉末相互の摩擦摩耗が増大する結果、従来の単なる造粒処理や前述の浸漬処理程度では折角の被覆層が剥離してしまい、所期の特性には達しなかった。そこでこの発明の課題は、被覆層のより一層の材質的改良と，その被覆層を鉄粉などの強磁性金属粉末（以下この明細書では鉄粉で強磁性金属粉末を代表させる。）の表面に強固に結合させる手段を見出すことにある。
【０００６】
【課題を解決するための手段】
発明者は種々研究の結果、先ず被覆層の材質については、被覆層が無機絶縁物と有機絶縁物の双方を含有し、この両者が微細に分散した状態で鉄粉粒子の表層部と融合した組織状態の被覆層が形成されれば所望の磁気特性が得られること；その際、鉄粉と被覆材（無機絶縁物および有機絶縁物）の配合割合については、体積比で無機絶縁物と有機絶縁物の和が１〜６％（その内無機絶縁物が０．５〜５．５％）および残り鉄粉の組成範囲が特に好ましいこと；この様な組織状態の被覆層は前述の通り従来の慣用手段では得られないが、鉄粉と絶縁物粉末を所定割合に配合した混合粉に、強力な圧縮・剪断作用を機械的に反復負荷する処理を施せば容易に得られることを見出した。
【０００７】
この処理に適する装置としては圧縮剪断型の機械式粒子複合化装置と呼ばれる装置があり、被覆型複合粒子の作製，粒子の表面改質，形状制御，固体粒子間の融合の促進，精密混合などに応用できるとされている。市販品にはホソカワミクロン社のメカノフュージョン（表面融合）システム，奈良機械製作所のハイブリダイゼーションシステム，徳寿工作所のシータコンポーザ（何れも商品名）その他があり、原理は何れも類似している。筆頭のものを例にとると、装置は、回転する容器とその中に装着された円弧状ヘッドを持つ腕部材からなり、投入された粉体は遠心力によって容器内面に押し付けられて容器とともに回転してヘッドと容器内面の間で強力な圧縮・剪断作用を受け、容器内面に付着しスクレーパーで掻き取られる。これらが高速で繰り返されて粒子複合化などの効果が出る。容器内面とヘッドとの隙間は被処理粉末の種類や処理目的に合わせて調整されるが、概ね５０〜５００μｍ程度である。
【０００８】
この処理を鉄粉と絶縁物粉末との混合粉に施すと、鉄粉を核としてその表面に絶縁物を主体とする被覆層が形成されるが、この被覆層はＸ線回折その他の試験結果によれば核の金属相と微細化した絶縁物粒子が交互に分散し、一部非晶質化した組織となって極めて高抵抗の絶縁性を示している。そして核の金属相と絶縁物粒子との界面近傍では、両者の成分の濃度分布が一方の成分は正の勾配，他の一方の成分は負の勾配をもって連続的に変化していることから、界面においては両者が融合していることが判る。この様に、この処理を施した複合粉末の場合は被覆層と核とが強固に一体化しているためこれを高圧力で圧縮成形しても造粒，浸漬その他の従来の処理法の場合と異なり、被覆層の破壊や剥離を生じて特性の劣化を招くことはない。ちなみに、この処理に相応しい名称として、この明細書では“融合処理”と呼ぶこととする。その趣旨は、同じく被覆を意図する処理でも単なる造粒や従来の複合とは被覆の効果が全く異なるので、これらとの区別を図ることにある。
【０００９】
この融合処理を鉄粉と絶縁物粉末との混合粉に施すことにより前述の組織構造で核と強固に一体化した被覆層が形成される機序については、次のように考えられる。即ち、混合粉が処理装置の容器内面とヘッドとの間を通過する際に受ける強力な圧縮・剪断作用によって、鉄粉（核粒子）に挟まれた絶縁物粉末が微細に粉砕，分断され、核粒子の表面に付着する。核粒子の表面上では付着した絶縁物粉末が圧縮力によって埋め込まれ、核の金属相と微細化した絶縁物粒子との混合相（被覆層）が徐々に形成される。そして核の金属相と絶縁物粒子との界面では核粒子相互間に生じる摩擦熱によって両者が部分的に反応し固着（融合）する。この繰り返しの結果、所望の特徴を具えた被覆層が得られる訳である。
【００１０】
強磁性金属粉末としては軟磁気特性に優れ、飽和磁束密度の高い鉄系金属粉末が望ましい。中でも価格的に純鉄粉は好適であり、アトマイズ法，還元法その他の各種製造法による鉄粉が用いられるが、圧縮成形性や純度の点から還元鉄粉が望ましい。ちなみに還元鉄粉の純度は通常９９．９質量％以上である。純鉄以外の強磁性金属粉末としてはＦｅ−Ｓｉ系，Ｆｅ−Ｓｉ−Ａｌ系，Ｆｅ−Ｎｉ系，Ｆｅ−Ｃｏ系，Ｆｅ−Ｍｏ−Ｎｉ系なども挙げられるが、所望の磁束密度、透磁率およびコストに応じて適宜に選択される。強磁性金属粉末の粒径については、粒径が小さいほど渦電流損は小さくなり高周波特性が向上するが、小さ過ぎると粉末の流動性，圧縮成形性が悪くなり、高密度の圧粉磁心が得られない。従って粒径は１５０μｍ以下，好ましくは１０〜１００μｍ程度が適当である。
【００１１】
無機絶縁物としてはＡｌ_２Ｏ_３，ＳｉＯ_２，ＴｉＯ_２，ＣａＣＯ_３などの酸化物粉末、およびカオリン（白陶土），珪藻土，タルク（滑石）などの鉱産物粉末が用いられる。無機絶縁物の粒径は微細混合相を得るため出来るだけ小さい方が望ましく、５０μｍ以下が好ましい。ただし微細に過ぎると取り扱いやコストの問題があるので、０．５μｍ以上とするのがよい。有機絶縁物にはフェノール，エポキシ，ポリイミドなどの熱硬化性樹脂粉末や、ポリアミド，ポリエチレン，ポリフェニレンサルファイドなどの熱可塑性樹脂粉末が使用される。有機絶縁物の粒径については、無機絶縁物について述べたのと同様の理由で１〜５０μｍが好ましい。
【００１２】
有機絶縁物としての合成樹脂は、圧粉磁心の結合剤を兼ねる必須の成分であるが、これ単独では充分な絶縁性は得難く、特に高周波領域での絶縁性を確保するためには、無機絶縁物と併用する必要がある。そしてその場合、強磁性金属粉末（鉄粉）への配合量は、体積比で無機絶縁物と有機絶縁物の和が１〜６％（その内無機絶縁物が０．５〜５．５％）および残り鉄粉の組成範囲が特に好ましい。その理由は後述する実施例のデータが示すように、無機絶縁物が０．５％未満，無機絶縁物と有機絶縁物の和が１％未満では高周波領域での所望の絶縁性および圧粉磁心の強度が確保できず、一方、無機絶縁物が５．５％を越え、無機絶縁物と有機絶縁物の和が６％を越えて過剰になると、磁束密度および透磁率が著しく低下し、同時に磁心の強度も低下することによる。なおこの明細書における配合割合，添加量などは、特に断わらない限り全て体積比で示してある。
【００１３】
【発明の実施の形態】
先ず原料粉については、強磁性金属粉末の代表としての純鉄粉には粒径１００μｍ以下（以下、−１００μｍのように記す。）の還元鉄粉，およびこの鉄粉にリン酸被膜処理を施したものの二種類を、絶縁物粉末は、無機絶縁物にはタルク粉末（−３μｍ），アルミナ粉末（−５μｍ），シリカ粉末（−３μｍ），炭酸カルシウム粉末（−３０μｍ）の四種類を、有機絶縁物にはフェノール樹脂粉末（−５０μｍ）およびポリフェニレンサルファイド樹脂（ＰＰＳ；−３０μｍ）粉末の二種類を用意した。有機樹脂は、圧粉磁心における結合材を兼ねている。次いでこれらの原料粉を表１に示した各所定の割合に配合して、それぞれ組成の異なる試料１〜試料２３用の混合粉を調製した。表中、鉄粉の欄の☆印はリン酸被膜処理を施した鉄粉であることを、（有機）樹脂の欄の＊印はポリフェニレンサルファイド樹脂（無印はフェノール樹脂）であることを示している。
【００１４】
次にこの発明の特徴とする融合処理の作用効果を見るため、試料２０〜２３を除く他の試料はそれぞれの混合粉にシータコンポーザ（徳寿工作所の商品名）を使用して融合処理を施した後、粉末潤滑剤としてエチレンビスステアロアミドの粉末０．０５％（各試料一律）を添加・混合する。一方、試料２０〜２３については無処理の比較例として、融合処理を施さずにエチレンビスステアロアミドの粉末０．０５％（各試料一律）を添加・混合する。次いで各試料ごとに磁気特性測定用の磁心，および強度測定用の試験片を作製する訳であるが、圧粉時の成形圧力や圧粉体を固化する加熱温度の影響を見るため、これらの因子を各試料それぞれ、表１に示すように割り付けてある。
【００１５】
磁気特性測定用の磁心は内径２０ｍｍ，外径３０ｍｍ，厚さ５ｍｍのリング状である。また、強度測定用の試験片は長さ３１．８ｍｍ，幅１２．７ｍｍ，厚さ５ｍｍの平板状であり、その抗折力をもって強度を評価する。これらを成形する金型には成形の都度、予め押型潤滑剤としてエチレンビスステアロアミドのアルコール懸濁液を塗布・乾燥しておく。成形後の固化処理のための加熱時間は加熱温度が１８０℃の場合は１時間，３５０℃では３０分間，５００℃では２０分間とする。
【００１６】
（実施例）先ず表１の試料９の欄に示す条件に従い、リン酸被膜処理を施した還元鉄粉に無機絶縁物として滑石（タルク）粉末４．５％と，結合材を兼ねる有機絶縁物としてフェノール樹脂の粉末０．５％を配合してシータコンポーザにより融合処理を施し、粉末潤滑剤を０．０５％添加して成形圧力９８０ＭＰａで所定の形状に成形後、温度１８０℃で１時間の加熱を行なって圧粉磁心と抗折力試験片を作製した。以下同様にして、表１に示すそれぞれの配合組成および処理条件に従って試料１〜試料８および試料１０〜試料２３に係る圧粉磁心と抗折力試験片を作製し、特性試験に供した。なお、シータコンポーザの運転条件は容器内面とヘッドとの隙間５００μｍ，両者の回転速度差２．５ｍ／秒，処理時間は３０分間である。
【００１７】
次に磁気特性の試験については、交流磁気特性は１次コイル２０回，２次コイル２０回の捲線を磁心に施して周波数１ｋＨｚ，４００ｋＨｚおよび１ＭＨｚにおける実効透磁率μａを測り、１ＭＨｚにおける実効透磁率の１ｋＨｚにおける実効透磁率に対する比（表には１Ｍ／１ｋと略記）を求めた。この比は高周波特性を評価する物差の一つであり、この値が１に近いほど、所望の周波数範囲内における磁心の実効透磁率が安定していることを、即ち高周波特性が優れていることを示す訳である。直流磁気特性は１次コイル２００回，２次コイル２０回の捲線を磁心に施して磁束密度Ｂ_１００を測定した。また、抗折力は試験片を材料試験機に支点間距離２５．４ｍｍで載置し、その中心に負荷して破壊強度を求めた。
【００１８】
この様にして得られた試料１〜試料２３の磁気特性，抗折力のデータを表２に示す。表１とこの表２は、本来は１枚の表を紙面の都合で分割したものなので、見易くするために表１には表２の備考欄を，表２には表１の配合欄の概要と融合処理の有無を、それぞれ重複して掲載してある。表における試料の配列は、試料１〜１９は全てこの発明の骨子とする融合処理を施したもので、絶縁物配合量の少ない順に並べてある。表２のデータが示すように、試料１〜試料３は１ｋＨｚから１ＭＨｚまで周波数が高くなるにつれて透磁率が激減してしまうので、この高周波領域での磁心には使えない。これは、絶縁物含有量が１％未満で足りないために、所要の絶縁性を持つ被覆層が形成されないことによると考えられる。
【００１９】
一方、試料１７〜試料１９は絶縁物含有量が６％超と多い（過剰）ため絶縁性被覆層の形成が充分で高周波特性は優れるものの、その反面磁心中の鉄粉の割合が不足するために実効透磁率，磁束密度および抗折力が劣り、所望の品質に達しない。これに対して、特に好ましいのは絶縁物が５％（鉄粉９５％）であるが、絶縁物が１〜６％の範囲内にある試料４〜試料１６は磁気特性，強度とも所望の品質を充分満たしている。従って試料４〜試料１６がこの発明の実施例であり、試料１〜試料３は絶縁物（無機絶縁物と有機絶縁物の和）の配合量が１％未満で高周波特性が劣ること；試料１７〜試料１９は絶縁物が６％超と過剰で透磁率，磁束密度，強度ともに劣ることにより、何れもこの発明の範囲外とされる。次の試料２０〜試料２３はいわゆる従来型の圧粉磁心で、絶縁物の配合量は実施例中最適な５％を採択し、原料粉を混合したまま融合処理を省いて圧縮成形した比較例である。
【００２０】
ここで、この発明の特徴とする融合処理についての検討に先立ち、他の要因に関する検討結果を説明する。先ず無機絶縁物の種類については、配合量が等しく４．５％の試料６〜試料１５を見ると、各特性とも炭酸カルシウムの数値が幾分高いものの全体として有意差は認められない。これは絶縁物配合量が過剰の試料１８，１９からも同様である。
【００２１】
次に鉄粉へのリン酸被膜処理の効果については、融合処理をしない比較例では一応有効と思われるが、融合処理を施した場合は試料７〜試料１２を見ると相反するデータがあり、明らかに有効とは言い難い。その理由は、融合処理の過程で受ける強力な摩擦および圧縮・剪断作用によってリン酸被膜の効果が減殺されるためと考えられる。なおリン酸被膜処理を施した鉄粉は流動性などの粉末特性が改善されるためか、融合処理の有無に拘らず抗折力が高めになっている。
【００２２】
成形時の成形圧力については、その影響を受けるのは主に磁束密度と抗折力の筈であり、事実低圧で成形した試料６〜試料８および試料１３〜試料１５と高圧で成形した試料９〜試料１２とを比べると磁束密度，抗折力ともに有意である。また抗折力については、成形後の加熱温度との相関傾向も見受けられる。
【００２３】
成形後の加熱（固化）処理温度の影響については、この加熱は一般に圧粉体を強化させ、また成形圧力によって圧粉体に残留する歪みを除去する。圧粉磁心に残留する歪みは透磁率，保磁力，鉄損（ヒステリシス損失）などの構造敏感性の磁気特性を阻害するので、加熱によって歪みが消失すれば、これらの特性が向上する筈である。しかしその一方で、鉄粉の表面に形成された被覆層の絶縁成分が加熱によって鉄粉内部に拡散すると、本来の絶縁効果が損われ、結果として磁気特性を劣化させる。加熱温度１８０℃の試料７，９；３５０℃の試料６，１０；５００℃の試料１１，１２が示すように、高温になるほど抗折力と透磁率は向上するものの高周波特性は劣化の傾向にあるのは、歪みの消失と拡散の進行という効果が相反する作用のためと考えられる。
【００２４】
【表１】

【００２５】
【表２】

【００２６】
最後に、原料粉に対する融合処理の効果は、この処理を施した試料７〜試料９および試料１１を見ると、その全てが実効透磁率，高周波特性，磁束密度などの磁気特性、抗折力とも高周波用圧粉磁心として充分な品質を具え、中でも高周波特性は高圧で成形した場合でも、僅かな低下に留まっている。その理由は、融合処理を経た被覆層は内部の鉄粉と強固に接合しているために、圧粉成形時の高い圧力や摩擦摩耗に充分耐え得ることにある。これに対して、融合処理を施さない試料２０〜試料２３では、周波数が４００ｋＨｚを超えると低圧で成形した場合でも実効透磁率が急激に減少し、高周波特性が著しく劣化している。これは被覆層と内部の鉄粉との接合が弱いため、成形時に受ける圧力や摩擦により被覆層の剥離，損傷が生じたことを意味する。そしてこの事実は、この発明の特徴とする融合処理の有効性を如実に示している。なお抗折力が成形圧力や加熱温度の上昇につれて向上しているのは、常識的に首肯されるところである。
【００２７】
【発明の効果】
従来、１ＭＨｚを超えるような高周波領域では、絶対値は低い（０．４前後）ものの磁束密度が安定しているフェライトコアが専ら用いられ、一方数十ｋＨｚ程度までの領域では珪素鋼板積層品が用いられているが、その中間の領域用には適切な磁心材が無かった。例えばセンダストは、周波数１ＭＨｚでも磁束密度が安定してはいるがそのレベルはフェライトコアより若干高い程度のため、当初に述べた機器の小型化に対応することは出来ない。しかるに、この発明に係る圧粉磁心は周波数１ＭＨｚまで実効透磁率が殆ど低下しないため高周波特性が優れ、且つ高い磁束密度（１．１前後）を示している。従ってこの発明は圧粉磁心の用途範囲を拡大するとともに、電気・電子機器の小型化への対応を可能にしたものである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a dust core used in various electric and electronic devices, and particularly suitable for cores such as choke coils, noise filters, reactors, etc. used in a high frequency range of about 1 kHz to 1 MHz. The present invention relates to a dust core that is high in frequency and excellent in frequency characteristics and can be used for downsizing of devices.
[0002]
[Prior art]
An iron core (magnetic core) of a soft magnetic material used in an alternating magnetic field is particularly required to have a high magnetic flux density and magnetic permeability and a small iron loss. Silicon steel plates meet these conditions, but because the shape is limited by the relationship of making thin punched products, if the shape is complex, a magnetic core made of powder metallurgy that can be formed into any shape (powder magnetic core) Often due to For the powder magnetic core, a so-called sintered iron core in which a green compact is sintered, and a powder of ferromagnetic metal such as pure iron, Fe-Si alloy, Sendust, Permalloy, etc., without being sintered, is used as thermosetting resin, water glass, or other suitable materials. There is a powder magnetic core solidified with a binder, but the eddy current loss that occupies the main part of the iron loss is proportional to the square of the thickness of the iron core, so the iron loss increases in the integrally formed sintered core There is.
[0003]
In this respect, in the case of a dust core, there is an essential feature that eddy current loss is small because nonmagnetic resin is interposed between iron powder particles. After that, in the case of a dust core, the magnetic flux density is uniquely determined by the density ratio of the magnetic core. Therefore, if the magnetic flux density is increased by forming the magnetic core at a high density, the required characteristics can be satisfied. In order to increase the density of the green compact, it is necessary to increase the molding pressure when compressing the powder and to reduce the frictional resistance generated between the powders and between the powder and the mold, As a means for this, a powder lubricant is generally mixed with the raw material powder. However, depending on the amount, in the case of a powder magnetic core without a sintering process, the frictional resistance decreases, but on the contrary, the powder density decreases, or the powder lubricant melted in the resin curing process causes the iron powder and the resin to join. Further, good formation of the resin layer is hindered, and as a result, the strength (bending strength) of the green compact may be reduced. Therefore, in the case of a powder magnetic core, it is preferable to add a powder lubricant as little as possible and to use push-type lubrication together.
[0004]
[Problems to be solved by the invention]
Previously, in Japanese Patent Publication No. 49-15684, the applicant of the present invention disclosed a specific resistance of each component such as Si, Al, Ni, etc. in the case of a powder magnetic core as in the case of an iron-based magnetic alloy by a normal melting method, or iron The addition of a component whose specific resistance of the solid solution is higher than that of iron alone is effective in improving the magnetic properties; in particular, these components or a thin layer of the diffusion part thereof is coated with iron particles. AC structure (magnetic flux density, iron loss) is remarkably improved when the structure is made; and such structure can be obtained by immersing and drying iron powder in an aqueous solution of silicon resin, for example. It has been disclosed that iron powder whose surface is coated with silicon resin can be obtained, and can be easily realized by using the iron powder.
[0005]
However, in recent years, as various electric and electronic devices have been miniaturized, the magnetic cores used for these devices have to be miniaturized. As a result, even if they are miniaturized, conventional functions are not impaired, that is, high magnetic flux density and high magnetic permeability. -Magnetic properties such as low iron loss and high strength magnetic cores have been demanded. However, even if a powder having a layer with a high specific resistance is used on the surface, if the molding pressure is increased by reducing the addition amount of the powder lubricant in order to compact this, the frictional wear between the powders increases. In the conventional mere granulation treatment or the above-described dipping treatment, the folded coating layer is peeled off, and the desired characteristics are not achieved. Therefore, the object of the present invention is to further improve the material properties of the coating layer and to form the coating layer on the surface of a ferromagnetic metal powder such as iron powder (hereinafter, the ferromagnetic metal powder is represented by iron powder in this specification). It is to find a means to firmly bond to each other.
[0006]
[Means for Solving the Problems]
As a result of various studies, the inventor first fused the material of the coating layer with the surface layer portion of the iron powder particles in a state in which the coating layer contains both an inorganic insulator and an organic insulator and both are finely dispersed. The desired magnetic properties can be obtained if a coating layer in a textured state is formed; at that time, the mixing ratio of the iron powder and the coating material (inorganic insulator and organic insulator) is an inorganic insulator and organic by volume ratio. The sum of the insulating materials is 1 to 6% (of which the inorganic insulating material is 0.5 to 5.5%) and the composition range of the remaining iron powder is particularly preferred; However, it was found that it can be easily obtained by subjecting the mixed powder containing iron powder and insulating powder to a predetermined ratio to a mechanical compression and shearing process. .
[0007]
Equipment suitable for this treatment is a so-called compression-shear type mechanical particle compositing device, such as production of coated composite particles, surface modification of particles, shape control, promotion of fusion between solid particles, precision mixing, etc. It can be applied to. Commercially available products include Hosokawa Micron's mechano-fusion (surface fusion) system, Nara Machinery's hybridization system, Deoksugaku Factory's Theta Composer (both trade names), and the principle is similar. Taking the first one as an example, the device consists of a rotating container and an arm member with an arc-shaped head mounted in it, and the charged powder is pressed against the inner surface of the container by centrifugal force and rotates with the container. Then, it is subjected to a strong compression / shearing action between the head and the inner surface of the container, adheres to the inner surface of the container, and is scraped off by a scraper. These are repeated at a high speed to produce effects such as particle compositing. The gap between the inner surface of the container and the head is adjusted according to the type of powder to be processed and the purpose of processing, but is generally about 50 to 500 μm.
[0008]
When this treatment is applied to a mixed powder of iron powder and insulator powder, a coating layer mainly composed of an insulator is formed on the surface of the iron powder as a core. This coating layer is the result of X-ray diffraction and other tests. According to the above, the metal phase of the nucleus and the finely divided insulating particles are alternately dispersed to form a partially amorphous structure, which shows an extremely high insulating property. And in the vicinity of the interface between the core metal phase and the insulator particles, the concentration distribution of both components changes continuously with one component having a positive gradient and the other component having a negative gradient. It can be seen that both are fused at the interface. In this way, in the case of the composite powder subjected to this treatment, the coating layer and the core are firmly integrated, so even if this is compression molded at high pressure, granulation, immersion and other conventional treatment methods Unlikely, the coating layer is not broken or peeled off, and the characteristics are not deteriorated. Incidentally, the name suitable for this processing is called “fusion processing” in this specification. The purpose of this is to distinguish between these processes, which are also intended for coating, because the coating effect is completely different from mere granulation and conventional composites.
[0009]
The mechanism for forming the coating layer firmly integrated with the core in the above-described structure by applying this fusion treatment to the mixed powder of iron powder and insulator powder is considered as follows. That is, the insulating powder sandwiched between the iron powder (nuclear particles) is finely pulverized and divided by the powerful compression / shearing action received when the mixed powder passes between the inner surface of the container of the processing apparatus and the head, It adheres to the surface of the core particle. On the surface of the core particle, the adhered insulator powder is embedded by compressive force, and a mixed phase (coating layer) of the core metal phase and the miniaturized insulator particle is gradually formed. Then, at the interface between the core metal phase and the insulator particles, both of them partially react and adhere (fuse) due to frictional heat generated between the core particles. As a result of this repetition, a coating layer having desired characteristics is obtained.
[0010]
As the ferromagnetic metal powder, iron-based metal powder having excellent soft magnetic characteristics and high saturation magnetic flux density is desirable. Among these, pure iron powder is preferable in terms of price, and iron powder produced by various methods such as an atomizing method, a reduction method, or the like is used, but reduced iron powder is preferable in terms of compression moldability and purity. Incidentally, the purity of the reduced iron powder is usually 99.9% by mass or more. Examples of ferromagnetic metal powders other than pure iron include Fe—Si, Fe—Si—Al, Fe—Ni, Fe—Co, and Fe—Mo—Ni. It is appropriately selected according to the magnetic susceptibility and cost. With regard to the particle size of the ferromagnetic metal powder, the smaller the particle size, the smaller the eddy current loss and the higher the high frequency characteristics. However, if the particle size is too small, the powder fluidity and compression moldability deteriorate, resulting in a high-density dust core. I can't get it. Accordingly, the particle size is suitably 150 μm or less, preferably about 10 to 100 μm.
[0011]
As the inorganic insulator, oxide powders such as Al ₂ O ₃ , SiO ₂ , TiO ₂ , and CaCO ₃ , and mineral product powders such as kaolin (white ceramic clay), diatomaceous earth, and talc (talc) are used. The particle size of the inorganic insulator is desirably as small as possible to obtain a fine mixed phase, and is preferably 50 μm or less. However, if it is too fine, there are problems of handling and cost, so 0.5 μm or more is preferable. Thermosetting resin powders such as phenol, epoxy, and polyimide, and thermoplastic resin powders such as polyamide, polyethylene, and polyphenylene sulfide are used for the organic insulator. The particle size of the organic insulator is preferably 1 to 50 μm for the same reason as described for the inorganic insulator.
[0012]
Synthetic resin as an organic insulator is an indispensable component that also serves as a binder for the dust core, but it is difficult to obtain sufficient insulation by itself, especially in order to ensure insulation in the high-frequency region. It is necessary to use it together with an insulator. And in that case, the blending amount into the ferromagnetic metal powder (iron powder) is 1 to 6% of the sum of the inorganic insulator and the organic insulator by volume ratio (of which the inorganic insulator is 0.5 to 5.5%) ) And the remaining iron powder composition range is particularly preferred. The reason for this is that, as shown in the data of the examples described later, if the inorganic insulator is less than 0.5% and the sum of the inorganic insulator and the organic insulator is less than 1%, the desired insulation and dust core in the high frequency region On the other hand, if the inorganic insulating material exceeds 5.5% and the sum of the inorganic insulating material and the organic insulating material exceeds 6%, the magnetic flux density and the magnetic permeability are remarkably lowered. This is because the strength of the magnetic core also decreases. In addition, unless otherwise indicated, the blending ratio, addition amount, and the like in this specification are all shown in volume ratios.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
First, as for the raw material powder, the reduced iron powder having a particle size of 100 μm or less (hereinafter referred to as −100 μm) is applied to pure iron powder as a representative of ferromagnetic metal powder, and the iron powder is subjected to a phosphoric acid coating treatment. Insulator powder, inorganic insulator is talc powder (-3μm), alumina powder (-5μm), silica powder (-3μm), calcium carbonate powder (-30μm), organic Two types of insulation were prepared: phenol resin powder (-50 μm) and polyphenylene sulfide resin (PPS; −30 μm). The organic resin also serves as a binder in the dust core. Subsequently, these raw material powders were blended in the predetermined ratios shown in Table 1 to prepare mixed powders for Samples 1 to 23 having different compositions. In the table, the asterisk in the iron powder column indicates that the iron powder has been subjected to a phosphoric acid coating, and the asterisk in the (organic) resin column indicates that it is a polyphenylene sulfide resin (no mark is a phenol resin). Yes.
[0014]
Next, in order to see the operational effects of the fusion process, which is a feature of the present invention, the other samples except Samples 20 to 23 were subjected to fusion treatment using a theta composer (trade name of Tokuju Factory) for each mixed powder. After that, 0.05% ethylene bisstearamide powder (uniform for each sample) is added and mixed as a powder lubricant. On the other hand, for samples 20 to 23, as an untreated comparative example, 0.05% ethylene bisstearamide powder (each sample is uniform) is added and mixed without being subjected to the fusion treatment. Next, a magnetic core for measuring magnetic properties and a test piece for measuring strength are prepared for each sample. In order to see the influence of the molding pressure during compaction and the heating temperature at which the compact is solidified, Factors are assigned to each sample as shown in Table 1.
[0015]
A magnetic core for measuring magnetic properties has a ring shape with an inner diameter of 20 mm, an outer diameter of 30 mm, and a thickness of 5 mm. Moreover, the test piece for strength measurement is a flat plate shape having a length of 31.8 mm, a width of 12.7 mm, and a thickness of 5 mm, and the strength is evaluated by its bending strength. The mold for molding these is preliminarily coated with an alcohol suspension of ethylenebisstearamide as a pressing lubricant and dried every time the molding is performed. The heating time for the solidification treatment after molding is 1 hour when the heating temperature is 180 ° C., 30 minutes at 350 ° C., and 20 minutes at 500 ° C.
[0016]
(Example) First, in accordance with the conditions shown in the column of Sample 9 in Table 1, 4.5% talc powder as an inorganic insulator and reduced iron powder subjected to a phosphate coating treatment, and an organic insulator that also serves as a binder As a blender, 0.5% of a phenol resin powder is blended and subjected to a fusion treatment by a theta composer. After adding 0.05% of a powder lubricant and molding into a predetermined shape at a molding pressure of 980 MPa, the temperature is 180 ° C. for 1 hour. Heating was performed to prepare a dust core and a bending strength test piece. In the same manner, dust cores and bending strength test pieces according to Samples 1 to 8 and Samples 10 to 23 were prepared according to the respective composition and processing conditions shown in Table 1, and subjected to a characteristic test. The operating conditions of the theta composer are a gap of 500 μm between the inner surface of the container and the head, a rotational speed difference of 2.5 m / second, and a processing time of 30 minutes.
[0017]
Next, regarding the magnetic characteristic test, the AC magnetic characteristic is measured by measuring the effective permeability μa at frequencies of 1 kHz, 400 kHz and 1 MHz by applying a winding of 20 times of the primary coil and 20 times of the secondary coil to the magnetic core, and measuring the effective permeability at 1 MHz. The ratio to the effective permeability at 1 kHz (abbreviated as 1M / 1k in the table) was determined. This ratio is one of the physical differences for evaluating the high frequency characteristics. The closer this value is to 1, the more stable the effective permeability of the magnetic core in the desired frequency range, that is, the high frequency characteristics are excellent. That is the reason. DC magnetic properties primary coil 200 times, the magnetic flux density was measured B ₁₀₀ secondary coil 20 times windings subjected to the magnetic core. The bending strength was determined by placing the test piece on a material testing machine at a distance between supporting points of 25.4 mm and applying it to the center to determine the breaking strength.
[0018]
Table 2 shows the magnetic characteristics and bending strength data of Samples 1 to 23 thus obtained. Since Table 1 and Table 2 are originally a single table divided due to space limitations, Table 1 contains the remarks column in Table 1 and Table 2 shows the formulation column in Table 1 for ease of viewing. And the presence or absence of fusion processing are listed in duplicate. In the table, the samples 1 to 19 are all subjected to the fusion process as the gist of the present invention, and are arranged in ascending order of the amount of the insulator. As shown in the data in Table 2, since the magnetic permeability of Samples 1 to 3 decreases drastically as the frequency increases from 1 kHz to 1 MHz, it cannot be used for the magnetic core in this high frequency region. This is presumably due to the fact that the insulating layer content is less than 1%, so that the coating layer having the required insulating properties is not formed.
[0019]
On the other hand, Samples 17 to 19 have an insulating content of more than 6% (excessive), so that the insulating coating layer is sufficiently formed and the high frequency characteristics are excellent, but the ratio of iron powder in the magnetic core is insufficient. However, the effective permeability, magnetic flux density and bending strength are inferior, and the desired quality is not achieved. On the other hand, it is particularly preferable that the insulator is 5% (iron powder 95%), but samples 4 to 16 in which the insulator is in the range of 1 to 6% have the desired quality in terms of both magnetic properties and strength. Is fully satisfied. Therefore, Sample 4 to Sample 16 are examples of the present invention, and Sample 1 to Sample 3 are inferior in high-frequency characteristics when the blending amount of the insulator (sum of inorganic insulator and organic insulator) is less than 1%; Sample 17 -Sample 19 is in excess of 6% of the insulator and is inferior in permeability, magnetic flux density, and strength. The following samples 20 to 23 are so-called conventional powder magnetic cores, and the optimum blending amount of the insulator is 5% in the examples, and a comparative example in which the molding process is omitted while mixing the raw material powder and compression molding is performed. It is.
[0020]
Here, prior to the examination of the fusion process, which is a feature of the present invention, the examination results regarding other factors will be described. First, regarding the types of inorganic insulators, when Samples 6 to 15 having the same blending amount of 4.5% are viewed, no significant difference is recognized as a whole although the numerical values of calcium carbonate are somewhat high for each characteristic. The same applies to the samples 18 and 19 in which the amount of the insulator is excessive.
[0021]
Next, as for the effect of the phosphate coating treatment on the iron powder, it seems to be effective in the comparative example in which the fusion treatment is not performed, but when the fusion treatment is performed, there is a conflicting data when the samples 7 to 12 are viewed, Obviously not effective. The reason is considered to be that the effect of the phosphate coating is diminished by the strong friction and compression / shearing action received during the fusion process. The iron powder that has been subjected to the phosphoric acid coating treatment has a higher bending strength regardless of the presence or absence of the fusion treatment, probably because the powder properties such as fluidity are improved.
[0022]
The molding pressure at the time of molding is mainly influenced by the magnetic flux density and bending strength. In fact, Sample 6 to Sample 8 and Sample 13 to Sample 15 molded at low pressure and Sample 9 molded at high pressure. When compared with the sample 12, both the magnetic flux density and the bending strength are significant. In addition, regarding the bending strength, there is also a correlation tendency with the heating temperature after molding.
[0023]
Regarding the influence of the heating (solidification) processing temperature after molding, this heating generally strengthens the green compact and removes the strain remaining in the green compact by the molding pressure. The strain remaining in the powder magnetic core interferes with the magnetic properties of structural sensitivity such as permeability, coercive force, and iron loss (hysteresis loss). If the strain disappears due to heating, these properties should be improved. . However, on the other hand, when the insulating component of the coating layer formed on the surface of the iron powder diffuses into the iron powder by heating, the original insulating effect is lost, resulting in deterioration of the magnetic properties. As shown by Samples 7 and 9 at a heating temperature of 180 ° C., Samples 6 and 10 at 350 ° C. and Samples 11 and 12 at 500 ° C., the higher the temperature, the higher the bending strength and permeability, but the high frequency characteristics tend to deteriorate. It is thought that this is because the effects of the disappearance of strain and the progress of diffusion are contradictory.
[0024]
[Table 1]

[0025]
[Table 2]

[0026]
Finally, the effects of the fusion treatment on the raw material powder are as follows. Samples 7 to 9 and 11 subjected to this treatment all have effective magnetic permeability, high frequency characteristics, magnetic characteristics such as magnetic flux density, and bending strength. It has sufficient quality as a high-frequency powder magnetic core, and the high-frequency characteristics remain low even when molded at high pressure. The reason is that the coating layer that has undergone the fusion treatment is firmly bonded to the internal iron powder, and therefore can sufficiently withstand the high pressure and frictional wear during compacting. On the other hand, in Samples 20 to 23 which are not subjected to the fusion treatment, when the frequency exceeds 400 kHz, the effective magnetic permeability rapidly decreases even when molded at a low pressure, and the high frequency characteristics are remarkably deteriorated. This means that the bond between the coating layer and the internal iron powder is weak, and the coating layer is peeled off and damaged by the pressure and friction received during molding. This fact clearly shows the effectiveness of the fusion process that is a feature of the present invention. The fact that the bending strength is improved as the molding pressure and the heating temperature are increased is commonplace.
[0027]
【The invention's effect】
Conventionally, in a high frequency region exceeding 1 MHz, a ferrite core having a low absolute value (around 0.4) but having a stable magnetic flux density is exclusively used, whereas in a region up to about several tens of kHz, a silicon steel sheet laminate is used. Although used, there was no suitable core material for the middle region. For example, although Sendust has a stable magnetic flux density even at a frequency of 1 MHz, its level is slightly higher than that of a ferrite core, so it cannot cope with the downsizing of the equipment described at the beginning. However, the dust core according to the present invention has excellent high frequency characteristics because the effective magnetic permeability hardly decreases up to a frequency of 1 MHz, and exhibits a high magnetic flux density (around 1.1). Therefore, this invention expands the application range of the powder magnetic core and makes it possible to cope with downsizing of electric / electronic devices.

Claims

In a dust core made of a ferromagnetic metal powder having a strong insulating coating layer formed on the surface, the coating layer contains both an inorganic insulator and an organic insulator, and the ratio of the ferromagnetic metal powder to the insulator. However, a total of 1 to 6% of organic insulators that also serve as an inorganic insulator and a binder by volume ratio (of which 0.5 to 5.5% of inorganic insulators) and ferromagnetic metal powder are the balance, and this The structure near the interface of the coating layer has a ferromagnetic metal powder as the nucleus, and the metal phase of this nucleus and the insulator particles are continuous with one component having a positive gradient and the other component having a negative gradient. A powder magnetic core for high frequency, characterized by exhibiting a textured state fused with a surface layer portion of a ferromagnetic metal powder in a finely dispersed state with a concentration distribution that varies with time.

Powerful compression with mixed powder containing 1-6% total volume of inorganic insulator and organic insulator that also serves as a binder in ferromagnetic metal powder (including 0.5-5.5% inorganic insulator)・ After applying a fusion process that mechanically repeatedly applies a shearing action to form a strong insulating coating layer on the surface of the ferromagnetic metal powder, the powder is compression-molded into a required shape and heated and solidified. The method for producing a high-frequency powder magnetic core according to claim 1, wherein the method is characterized in that: