JP2004026621A

JP2004026621A - Sheet-like iron oxyhydroxide, iron oxide, magnetite, and its manufacturing method

Info

Publication number: JP2004026621A
Application number: JP2002189726A
Authority: JP
Inventors: Kyoko Suganuma; 菅沼　恭子
Original assignee: CHEMI LIGHT KOGYO KK
Current assignee: CHEMI LIGHT KOGYO KK
Priority date: 2002-06-28
Filing date: 2002-06-28
Publication date: 2004-01-29
Anticipated expiration: 2022-06-28
Also published as: JP3942963B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sheet-like iron oxide useful for raw materials for sheet-like ferrite magnetic materials using form anisotropy, sheet-like pigments for anticorrosive coatings having a high ultraviolet ray shielding effect, and pigments having glossiness and blackness. <P>SOLUTION: When iron oxyhydroxide is precipitated from an iron salt solution at normal pressure, it is precipitated as a polycrystal film having almost uniform crystalline directions, and stripped to obtain sheet-like particles of iron oxyhydroxide (FeOOH). Sheet-like iron oxide (Fe<SB>2</SB>O<SB>3</SB>) and magnetite (Fe<SB>3</SB>O<SB>4</SB>) are obtained by heating, dehydration, and reduction and oxidation as needed under conditions for holding the shape. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、不要高周波輻射の吸収に有効とされる板状フェライト粒子の原料並びに高透磁率複合磁性成型物の構成物である板状フェライト磁性体の原料としての板状の鉄酸化物、優れた防錆効果を示すとされる天然ＭＩＯ（雲母状酸化鉄）顔料に替わりうる板状酸化鉄、さらに生体に安全な光沢顔料、黒色顔料としての板状酸化鉄、板状マグネタイトおよびその製造方法に関するものである。
【０００２】
【従来の技術】
近年の目覚ましい電子部品、電子機器の小型化、高速化に伴って不要輻射ノイズの低減は大きな課題となっている。このための研究が種々なされた結果、高周波帯域での不要輻射は、吸収素子を構成するフェライト磁性体の形状が扁平（板状と同義）であるとき、優れた吸収特性を示すことがわかってきた。例えば、特開２００１−２１０９２４号公報では、扁平状フェライト粒子を有機絶縁樹脂内に同一方向に配向させることによって、１ＧＨｚ以上の高周波帯域において優れたノイズ吸収特性を示す回路基板および電子部品について開示している。
【０００３】
しかし、ここで用いた扁平状フェライト粒子は、特開２００１−２８４１１８号公報で開示されているように、粒状酸化鉄を原料として製造した通常のフェライト粒子をシート状に成型したのち焼成、粉砕して見かけ上扁平にしたものである。この方法では製造工程が複雑であるばかりでなく、扁平粒子の厚みを薄くすることに限界があり扁平形状に由来する反磁界係数の低下が十分でないため、所望の磁気特性が得られないとされる。また、等方性粒子を扁平状に焼結させたものであるから、扁平面内の強度が特に大きいわけではなく、樹脂との混練などの後工程で扁平性の維持が困難である。
【０００４】
また、特開２０００−２５２１１３号公報では、オートクレーブを用いて水熱合成した板状α−Ｆｅ_２Ｏ_３を原料として、組成や焼成温度を工夫して板状フェライト粒子を製造し、これを用いた低周波帯での比透磁率が高く、高周波帯での電磁波吸収特性に優れたフェライト粒子複合体が開示されている。しかし、ここでは水熱合成反応を用いるため、まず製造コストが高い。次に水熱合成品は粒子が緻密なのでフェライト化には高温が必要で、高温では板状が壊れ易いという難点があり、実用化には至っていない。
【０００５】
さらに、高密度焼結フェライトの原料として板状フェライトを利用する提案が、特開昭６２−３０２１号公報、特許第３２４２７４４号にあるが、前者はオートクレーブを用いて盤状ゲーサイトを製造し盤状酸化物を得る方法、後者は粒状微粉末フェライトをシート状に乾燥して見掛け上の板状フェライトを製造するもので、前述と同じ難点を有している。また、フラックス法を用いて板状フェライト粒子粉末を製造する方法が、特開平５−４５５２７号公報に示されているが、フラックス法利用は高温での煩雑な作業でコスト高になるうえに、板状粒子としては小さいものしか得られず、あまり実用的ではない。
【０００６】
次に、板状酸化鉄（α−Ｆｅ_２Ｏ_３）は塗膜中で層状に配向するため、塗膜表面からの水分、ガスなどの侵入を防ぎ、仮に表面のビヒクル（展色材：塗料の中の顔料以外の成分）が劣化しても板状多層面が風化と腐食に耐え下層の塗膜を保護するという仕組みで耐候性に優れていることが知られている。しかしながら、これまで実用に供された板状酸化鉄は天然ＭＩＯ（雲母状酸化鉄）のみで、これも不純物含有量の少ない良質品の埋蔵量が少なく、現在では殆ど流通していないと言われる。
【０００７】
一方、天然ＭＩＯの特性を持つ板状酸化鉄の合成が試みられ、特公昭４９−４４８７８号公報、特公昭５５−１６９７８号公報にその製法が開示されているが、これらも水熱合成反応を利用しているため製造コストが嵩む上に大きい粒子を得難いという欠点があり、実用化には至っていない。
さらに、無機顔料は有機顔料に比べ、生体や環境への安全性が高いことが知られているが、着色性や発色性に劣ることから、化粧品や身の回り品の着色材としての利用はそれほど多くはない。光沢性・装飾性の高い安価で安全な無機顔料が望まれている。
【０００８】
【発明が解決しようとする課題】
以上のように、フェライト磁性体、防錆塗料および顔料等の分野においては、酸化鉄の形状を板状にすれば多くの利点があることは分かっているが、これまでは粒子そのものが板状である酸化鉄を安価で安定的に製造することが出来なかった。さらに、最近の高周波用フェライト磁性体の分野では、板状のアスペクト比（ｄ／ｔ）を大きくすることが求められるようになってきたが、従来の技術ではやはり安価に安定的に製造できないばかりでなく十分な磁気特性が得られていない。本発明は、アスペクト比（ｄ／ｔ）の大きさを用途に応じて自由に調整した粒子そのものが板状である酸化鉄を安価に安定的に提供することを目的とする。また、板状の酸化鉄が得られればその形状を維持したマグネタイト、マグヘマイトは通常の方法で製造することが出来る。本発明はそのような板状のオキシ水酸化鉄、酸化鉄の製造方法を提供することも目的とする。
【０００９】
【課題を解決するための手段】
上述したように、水熱合成反応やフラックス法によって製造される板状酸化鉄は、粒子そのものが板状の単結晶体であるがいくつかの欠点がある。上記の課題を解決するには、粒子そのものが板状で、かつアスペクト比（ｄ／ｔ）を大きくできれば、酸化鉄粒子は必ずしも単結晶体である必要はなく、多結晶体でも良い。本発明はこの点に着目して見出したもので、常圧で室温〜７０℃の比較的低い温度の溶液から、結晶性のオキシ水酸化鉄膜を析出させることによって、板状の酸化鉄多結晶体を得るものである。
【００１０】
その発明の要旨とするところは、
（１）第１および第２鉄イオンを含む水溶液から常圧で析出させるオキシ水酸化鉄であって、結晶性の膜として析出し、剥離して板状粒子となる多結晶オキシ水酸化鉄。
（２）前記（１）に記載の多結晶オキシ水酸化鉄を加熱・脱水してなる板状α型酸化鉄。
（３）前記（２）に記載の板状α型酸化鉄を還元してなる板状マグネタイト。
（４）前記（３）に記載の板状マグネタイトを酸化してなる板状γ型酸化鉄。
【００１１】
（５）前記（１）または（２）に記載の板状オキシ水酸化鉄または板状α型酸化鉄の剥離面が緻密で光沢性を示すこと、および／またはＸ線回折で特定結晶面が発達した多結晶体であることを特徴とする板状のオキシ水酸化鉄または板状α型酸化鉄。
（６）前記（１）〜（４）に記載の板状鉄酸化物であって、最大長径ｄが５μｍ以上数ｍｍ以下の範囲にあり、最大長径ｄと厚みｔとの比（ｄ／ｔ）が、２以上である板状鉄酸化物。
【００１２】
（７）前記（１）に記載の板状オキシ水酸化鉄を製造するに際し、ｐＨ＝−１．０〜２．０、Ｆｅ^２＋／Ｆｅ^３＋＝０．２〜２０に設定することによって、結晶性のオキシ水酸化鉄膜を析出させ、剥離して板状粒子となすことを特徴とする板状のオキシ水酸化鉄の製造方法。
（８）前記（２）に記載の板状α型酸化鉄の製造方法であって、請求項１に記載の板状オキシ水酸化鉄を加熱・脱水するに際し、脱離したＨ_２Ｏ等加熱時発生成分を直ちに排出させることによってオキシ水酸化鉄の板状性を保持することを特徴とする板状α型酸化鉄の製造方法である。なお、加熱時発生成分とは、Ｈ_２Ｏは勿論、Ｃｌ分、ＳＯ_２、ＮＯ_２等を意味する。
【００１３】
以下、本発明について詳細に説明する。
鉄塩溶液から沈澱・析出する化合物はその種類が非常に多く、反応系は複雑である。Ｆｅイオンの種類と濃度、反応系の温度、ｐＨ、共存する陰イオンの種類、酸化剤の有無によって、多様な沈澱が生成する。その代表的なものは、オキシ水酸化鉄（ＦｅＯＯＨ、α、β、γ、δの構造異性体がある）で、このうちα型オキシ水酸化鉄は、磁気記録材料の中心をなすγ−Ｆｅ_２Ｏ_３の原料として知られている。これらを含め、これまで知られている鉄塩溶液からの沈澱・析出物は無定形あるいは結晶性に拘わらず細かい粒子として母液から分離され、乾燥すると粉末状態になる。
【００１４】
ところが、本発明では、系の条件を選ぶことによって、溶液内の固体表面にオキシ水酸化鉄の粒子が方位を揃えて析出、結晶化し、２次元の広がりを持った結晶性の膜を形成することを見出した。この膜は時間と共に厚くなり、一定の厚み（約３μｍ）以上になると剥離して板状の粒子になる。このように成長した板状粒子は、剥離面が緻密で光沢を示し、緻密度が増すとＸ線回折で特異成長した結晶面が認められる単結晶性を示すが多結晶体であって、板状性は極めて安定である。なお、固体表面への結晶性膜の析出と同時に液表面に結晶性膜が少量析出することもある。この膜は固体表面に析出したものより緻密度が高いが、板状性、結晶性は同じである。
【００１５】
図１は粒子のＳＥＭ像を示す顕微鏡写真である。図１（ａ）はＦｅＯＯＨのＳＥＭによる５０倍の顕微鏡写真であり、図１（ｂ）はα−Ｆｅ_２Ｏ_３のＳＥＭによる５０倍の顕微鏡写真であり、また、図１（ｃ）はα−Ｆｅ_２Ｏ_３のＳＥＭによる５００倍の顕微鏡写真である。この図に示すように、板状断面のＳＥＭ観察によると、針状または柱状の１次粒子が固体表面に対して直角方向に平行または放射状に整列して成長し、これが結晶化した２次粒子が基本単位となっていることが分かる。常圧でかつ室温〜中温度の溶液からの析出物としては、従来知られていない結晶性の２次および３次構造体を形成していると考えられ、その結果、単なる１次粒子の集合体にはない機械的強度を発現し、板状形状が安定になると考えられる。
【００１６】
板状面の形は不定形であるが、軽く粉砕することによって任意の板面径に調節することができ、５μｍから最大数ｍｍの大きさまでが可能である。大きさの調整にはメッシュ篩を使用することができる。厚みは反応時間に比例し、剥離が容易になる３μｍ以上であれば任意に調整できるので、アスペクト比を自由に調整できる。最大長径ｄと厚みｔとのアスペクト比（ｄ／ｔ）は２以上であれば形状異方性の効果が現われ、この値が大きい程その効果も大きくなる。析出させる固体の材質は特に選ばない。ガラス、磁器、プラスティック、金属など何でもよい。
【００１７】
この板状オキシ水酸化鉄を、板状性を損なわない条件で加熱・脱水すると板状のα型酸化鉄が得られる。加熱することによって結晶性が向上し、より強固な板状晶になる。これを、前述のフェライト用原料や塗料用顔料に利用する。なお、フェライト用には、脱水前のオキシ水酸化鉄のままでも十分利用できる。オキシ水酸化鉄からの方が低温でフェライト化し易いという利点があるが、組成合わせなどの作業性はα−Ｆｅ_２Ｏ_３にしてからの方が良い。
【００１８】
本発明による板状酸化鉄が多結晶体であることは、フェライト磁性体の原料として有利である。天然ＭＩＯ並びにオートクレーブを用いた水熱合成反応で得た単結晶体の板状酸化鉄は、粒子の緻密度が高いためにフェライト化には高温反応を必要とする上に、フェライト化反応と共に起こる結晶構造の組替えが形状にまで反映して板状が壊れ易い。本発明による板状酸化鉄は緻密度がほどほどであるから、フェライト化のような他成分との固相反応は低温で進み、かつ結晶構造の組替えが起こっても多結晶体なので形状の変化までは影響が及ばないので、コストおよび形状の維持の両面で優位である。板状マグネタイト（Ｆｅ_３Ｏ_４）およびマグヘマイト（γ−Ｆｅ_２Ｏ_３）は、上記の板状酸化鉄を還元・酸化して得られる。マグネタイトおよびマグヘマイトはヘマタイトとともに磁性材料としての用途の他に、光沢性を利用した装飾顔料として、さらにマグネタイトはカーボンに替わり得る生体安全性の高い黒色顔料としての利用が考えられる。
【００１９】
【発明の実施の形態】
本発明に係る結晶性オキシ水酸化鉄膜の析出は以下のように実施できる。
水溶液中でのＦｅ^２＋、Ｆｅ^３＋イオンは、通常正八面体の６つの頂点に水分子を配位した中心にあって、水和イオンになっているといわれる。このうちＦｅ^３＋イオンが他の水分子と反応してＨ^＋（Ｈ_３Ｏ^＋）を放出する反応が加水分解反応で、プロトリシス（ｐｒｏｔｏｌｙｓｉｓ）と呼ばれ下記にように進み、溶液の酸性度を増す。
Ｆｅ（Ｈ_２Ｏ）_６ ^３＋＋Ｈ_２Ｏ　←→　Ｆｅ（ＯＨ）（Ｈ_２Ｏ）_５ ^２＋＋Ｈ_３Ｏ^＋
…　（１）
Ｆｅ（ＯＨ）（Ｈ_２Ｏ）_５ ^２＋＋Ｈ_２Ｏ　←→　Ｆｅ（ＯＨ）_２（Ｈ_２Ｏ）_４　^＋
＋Ｈ_３Ｏ^＋　…　（２）
【００２０】
なお、Ｆｅ^３＋イオンの加水分解は、配位したＨ_２Ｏ分子６ケが一つずつＯＨ基に置き換わっていく。上記にその内の初めの２つを、下記に最後の式を示し、中間の式３つは省略した。
Ｆｅ（ＯＨ）_５（Ｈ_２Ｏ）^２−＋Ｈ_２Ｏ　←→　Ｆｅ（ＯＨ）_６ ^３−＋Ｈ_３Ｏ^＋
…　（３）
プロトリシスで生成した錯イオンは互いに重合反応を起こし、最終的にはオキシ水酸化鉄などとして沈澱・析出する。
【００２１】
２Ｆｅ（Ｈ_２Ｏ）_５（ＯＨ）^２＋　←→　［（Ｈ_２Ｏ）_５Ｆｅ−Ｏ−Ｆｅ（Ｈ_２
Ｏ）_５］^４＋＋Ｈ_２Ｏ　…　（４）
［Ｆｅ（Ｈ_２Ｏ）_５−Ｏ−Ｆｅ（Ｈ_２Ｏ）_５］^４＋＋Ｈ_３Ｏ^＋　←→　［（Ｈ_２
Ｏ）_５Ｆｅ−ＯＨ−Ｆｅ（Ｈ_２Ｏ）_５］^５＋＋Ｈ_２Ｏ　　…　（５）
重合反応は、−Ｏ−または−ＯＨ−による架橋反応で、（４）式の反応はｏｘｏｌａｔｉｏｎ、（５）式の反応はｏｌａｔｉｏｎと呼ばれる。通常はまず正八面体が２個つながった２核錯体ができ、それが基本となって重合が進むが、重合ポリマーが無定形になるか、結晶性の粒子に成長するかは、Ｆｅイオンの種類と濃度、反応系の温度、ｐＨ、共存する陰イオンの種類と濃度、溶存酸素量または酸化剤の有無等によって異なり大変複雑であることが知られている。
【００２２】
重合の模型を以下に示す。
【００２３】
【化１】

【００２４】
鉄塩溶液からオキシ水酸化鉄の多結晶体を膜状に析出させるためには、加水分解反応と重合反応の速度を適度に抑制してやれば良い。加水分解が急速に進んで価数の低い錯イオンが増えると、重合も急速に進み生成ポリマーは不規則になりやすく、析出するオキシ水酸化鉄は無定形になり易い。本発明は上記の知見に基づいているものである。つまり、加水分解速度をできるだけ遅くするために、系のｐＨを加水分解が始まるとされる下限域（−１．０〜２．０）に設定し、全Ｆｅ濃度を１．５〜５．０ｍｏｌ／ｌとやや高めにして、アルカリや酸化剤は用いずに溶液に接する気相から溶解してくる酸素のみを酸化に用い、温度は室温〜７０℃の比較的低温で行う。すなわち、ｐＨ：−１．０〜２．０とした理由は、ｐＨ−１．０未満では加水分解に先立つＦｅ^２＋イオンのＦｅ^３＋イオンへの酸化が殆ど進まず、また、ｐＨ２．０を超えるとＦｅ^３＋イオンの加水分解速度が大きすぎて生成物は膜を形成しない。従って、その範囲を−１．０〜２．０とした。
【００２５】
Ｆｅ^３＋イオンを含む溶液に、Ｆｅ^２＋イオンを適当量（Ｆｅ^２＋／Ｆｅ^３＋＝０．２〜２０）添加して静置すると、５０℃の場合は３日〜４日で剥離可能な厚さに成長する。Ｆｅ^２＋／Ｆｅ^３＋が０．２未満では膜の成長は極めて遅く、２０を超えると加水分解速度が大きすぎて微細粒子となり膜は成長しない。膜の析出に伴い系のｐＨは上昇してくる。これは良く知られているＦｅイオンの加水分解並びに酸化反応とは異なる。通常のＦｅイオンの加水分解、酸化反応ではｐＨは必ず低下する。本発明の系でも条件を所定の範囲からずらすと、つまり加水分解、酸化反応速度を大きくし過ぎるとｐＨは低下し、一般的に知られている細かい粒子のオキシ水酸化鉄沈澱が得られ膜は生成しない。
【００２６】
また、系に接する気相は、空気または空気と酸素の混合ガスで常圧である。酸素分圧が大きいほど析出速度は大きくなる。反応温度が高いほど、Ｆｅ^２＋イオン濃度が大きいほど、析出速度は大きくなるが、速度を大きくし過ぎると生成した膜内の１次粒子の配向性が低下し、板状粒子の緻密度が低下する。
【００２７】
α−Ｆｅ_２Ｏ_３はＦｅＯＯＨを加熱・脱水して得るが、板状形態を維持するには、脱離するＨ_２ＯやＣｌ成分等を速やかに排出させる条件で行うことが必要である。Ｈ_２ＯやＣｌ成分等の分圧が上がると、板状は壊れて普通の粉末になる。加熱は、４００〜８００℃で３０分〜１時間で十分で、加熱炉内に空気を送るか、または十分に内容積の大きい密閉ではない加熱炉を用いれば良い。加熱温度が高いと結晶性がより良好なα−Ｆｅ_２Ｏ_３を得るが、その後フェライト化などの固相反応を行う場合には、あまり結晶性を上げて緻密にしない方が良い。
【００２８】
図２はα−Ｆｅ_２Ｏ_３のＸ線回折パターンを示す図である。図２（ａ）は本発明による板状酸化鉄のＸ線回折パターンを示す図であり、図２（ｂ）はフェライト原料として通常利用されている高温熱分解で生成した酸化鉄のＸ線回折パターンを示す図である。板状酸化鉄では通常酸化鉄で２番目の強度を示す（１１０）回折線が（１０４）回折線強度より大きくなり、（１１０）面が特異成長していることが分かる。１次または２次粒子の整列度合いは反応速度によって左右され、反応速度を小さくすると配向性は向上し、板状粒子としての機械的強度も向上する。配向性の高い試料では、膜の厚みが薄くても容易に剥離して板状形状の安定な粒子となる。
【００２９】
一方、配向度が低くても剥離するものは結晶性粒子であることに変わりはなく板状面の強度は十分にある。むしろ、フェライト化のように他成分との固相反応をする場合は、配向性はほどほどの方が有利である。剥離させずに反応を続行すれば厚みは任意の厚さまで成長する。本発明の方法によれば、板状粒子の緻密さ（配向性）と厚さは目的に応じて任意に調整できる。従って、不要高周波輻射の吸収や高透磁率用の板状フェライトの原料として必要な高アスペクト比（ｄ／ｔ）をもつ板状粒子を任意に製造できる。
【００３０】
【実施例】
以下、本発明について実施例および比較例によって具体的にせつめいする。
Ｆｅ^２＋／Ｆｅ^３＋＝１．３、全Ｆｅ濃度＝２．５ｍｏｌ／ｌのＦｅＣｌ_２＋ＦｅＣｌ_３水溶液の初期ｐＨを０．３に調整し、液の厚みが５ｍｍ〜１ｃｍになるようにポリエチレン製のバットに注ぐ。水分の蒸発によるＦｅ濃度の変化を防ぐため、ラップ類で覆ったのち、５０℃に設定されている恒温槽に入れて４日間保定した。ここで接する気相は空気であった。液量に対して接する気相の量を十分にするためバットの深さは液相厚みの１０倍以上になるようにした。保定後の上澄み溶液のｐＨは約１．０であった。上澄み溶液を分離した後バットを水洗すると、底から赤褐色の薄いフィルムが剥離してきた。これをＸ線回折で調べたところβ型オキシ水酸化鉄のパターンを示し、ＳＥＭ観察で図１のような板状粒子を示した。
【００３１】
上述した板状粒子を平たい蒸発皿に厚さ約５ｍｍ以下に載せて空気流通下で６００℃、１時間加熱したところ、黒褐色の光沢面と深紅色の裏面を持った板状粒子が得られた。これをＸ線回折で調べたところ図２（ａ）のように（１１０）面が特異成長したα−Ｆｅ_２Ｏ_３のパターンを示した。また、ＳＥＭ観察で図１（ｂ）（ｃ）のような板状粒子を示し、図１（ａ）の形状がそのまま継承されていることが分かった。なお、管球はＦｅである。この板状α型酸化鉄を平底蒸発皿に載せて、ＣＯ気流中、６００℃、１時間加熱したら板状マグネタイトになった。さらに、この板状マグネタイトを平底蒸発皿に載せて空気中、４００℃、１時間加熱したら板状マグヘマイトになった。
【００３２】
（比較例）
Ｆｅ^２＋／Ｆｅ^３＋＝１．３、全Ｆｅ濃度＝２．５ｍｏｌ／ｌのＦｅＣｌ_２＋ＦｅＣｌ_３水溶液の初期ｐＨを２．３に調整し、液の厚みが５ｍｍ〜１ｃｍになるようにポリエチレン製のバットに注ぐ。水分の蒸発によるＦｅ濃度の変化を防ぐため、ラップ類で覆ったのち、７０℃に設定されている恒温槽に入れて４日間保定した。ここで接する気相は空気であった。液量に対して接する気相の量を十分にするためバットの深さは液相厚みの１０倍以上になるようにした。保定後の上澄み溶液のｐＨは約０．３であった。上澄み溶液を分離した後バットを水洗すると、底に黄土色の粒子が付着していた。これを薬さじで剥がして乾燥したら微細な粉末になった。Ｘ線回折で調べたところβ型オキシ水酸化鉄のパターンを示し、ＳＥＭ観察で短冊状粒子であることが分かった。これを平たい蒸発皿に厚さ約５ｍｍ以下に載せて空気流通下で６００℃、１時間加熱したところ、赤色の粉末が得られ、Ｘ線回折で調べたところ図２（ｂ）と同じ通常のα−Ｆｅ_２Ｏ_３のパターンを示した。また、ＳＥＭ観察では短冊状粒子であることが確認された。
【００３３】
【発明の効果】
以上述べたように、本発明によれば、結晶性板状のオキシ水酸化鉄が常圧の水溶液から簡単に得られ、これを原料として結晶性板状の酸化鉄が容易に得られる。同様に板状のマグネタイト、マグヘマイトも容易に得られ、フェライト原料のような電子材料としてだけでなく、塗料、顔料の分野でも特性および経済性の両面で大いに役立つ工業的に極めて優れた材料を提供できる。
【図面の簡単な説明】
【図１】
粒子断面のＳＥＭ像を示す顕微鏡写真である。
【図２】
α−Ｆｅ_２Ｏ_３のＸ線回折パターンを示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention provides a plate-like ferrite particle as a raw material of a plate-like ferrite particle which is effective for absorbing unnecessary high-frequency radiation and a plate-like ferrite magnetic material as a raw material of a plate-like ferrite magnetic material which is a component of a high-permeability composite magnetic molding. Iron oxide which can replace natural MIO (mica-like iron oxide) pigment, which is said to exhibit anti-rust effect, furthermore, gloss pigment safe for living body, plate-like iron oxide as black pigment, plate-like magnetite and method for producing the same It is about.
[0002]
[Prior art]
With recent remarkable miniaturization and high-speed of electronic components and electronic devices, reduction of unnecessary radiation noise has become a major issue. As a result of various studies for this purpose, it has been found that unnecessary radiation in the high frequency band exhibits excellent absorption characteristics when the shape of the ferrite magnetic material constituting the absorbing element is flat (synonymous with the plate shape). Was. For example, Japanese Patent Application Laid-Open No. 2001-210924 discloses a circuit board and an electronic component that exhibit excellent noise absorption characteristics in a high frequency band of 1 GHz or more by orienting flat ferrite particles in an organic insulating resin in the same direction. ing.
[0003]
However, as disclosed in JP-A-2001-284118, the flat ferrite particles used here are formed by molding ordinary ferrite particles manufactured using granular iron oxide as a raw material into a sheet shape, followed by firing and pulverization. It is apparently flattened. In this method, not only the manufacturing process is complicated, but also there is a limit in reducing the thickness of the flat particles and the reduction of the demagnetizing field coefficient due to the flat shape is not sufficient, so that desired magnetic properties cannot be obtained. You. Further, since the isotropic particles are sintered in a flat shape, the strength in the flat surface is not particularly large, and it is difficult to maintain the flatness in a post-process such as kneading with a resin.
[0004]
Further, in Japanese Patent Application Laid-Open No. 2000-252113, plate-like ferrite particles are produced by using a plate-like α-Fe ₂ O ₃ hydrothermally synthesized using an autoclave as a raw material, devising the composition and the firing temperature, and using this. A ferrite particle composite having a high relative magnetic permeability in a low frequency band and excellent electromagnetic wave absorption characteristics in a high frequency band has been disclosed. However, here, the production cost is high because a hydrothermal synthesis reaction is used. Next, since the hydrothermally synthesized product has a dense particle, a high temperature is required for ferrite formation, and the plate shape is easily broken at a high temperature, so that it has not been put to practical use.
[0005]
Further, proposals using plate-like ferrite as a raw material for high-density sintered ferrite are disclosed in Japanese Patent Application Laid-Open No. Sho 62-3021 and Patent No. 3242744. In the latter method, the powdery ferrite is dried into a sheet to produce an apparent plate-like ferrite, which has the same disadvantages as described above. Further, a method for producing plate-like ferrite particles using a flux method is disclosed in Japanese Patent Application Laid-Open No. 5-45527. However, the use of the flux method requires a complicated operation at a high temperature and increases the cost. Only small particles are obtained as plate-like particles, which is not very practical.
[0006]
Next, since the plate-like iron oxide (α-Fe ₂ O ₃ ) is oriented in a layered manner in the coating film, the penetration of moisture, gas, etc. from the coating film surface is prevented, and the surface of the vehicle (coloring material: paint) It is known that even if the component (other than the pigment in the above) deteriorates, the plate-like multilayer surface is resistant to weathering and corrosion and protects the lower coating film, and thus has excellent weather resistance. However, the only plate-like iron oxide that has been practically used so far is only natural MIO (mica-like iron oxide), which is also said to be hardly distributed at present because the reserves of high-quality products with low impurity content are small. .
[0007]
On the other hand, synthesis of plate-like iron oxide having the characteristics of natural MIO has been attempted, and its production method is disclosed in Japanese Patent Publication No. 49-44878 and Japanese Patent Publication No. 55-16978. Since it is used, the production cost is increased and large particles are hardly obtained, so that it has not been put to practical use.
In addition, inorganic pigments are known to be more safe for living organisms and the environment than organic pigments.However, because they are inferior in coloring and coloring properties, they are not widely used as coloring materials for cosmetics and personal items. There is no. Inexpensive and safe inorganic pigments having high gloss and decorativeness are desired.
[0008]
[Problems to be solved by the invention]
As described above, in the fields of ferrite magnetic materials, rust preventive paints, pigments, etc., it is known that there are many advantages to making the shape of iron oxide into a plate shape. Cannot be stably manufactured at low cost. Furthermore, in the field of ferrite magnetic material for high frequency recently, it has been required to increase the plate-like aspect ratio (d / t), but the conventional technology still cannot be manufactured stably at low cost. However, sufficient magnetic characteristics have not been obtained. An object of the present invention is to provide inexpensively and stably an iron oxide in which the size of the aspect ratio (d / t) is freely adjusted in accordance with the application and the particles themselves are plate-like. In addition, if plate-like iron oxide is obtained, magnetite and maghemite maintaining the shape can be produced by a usual method. Another object of the present invention is to provide a method for producing such plate-like iron oxyhydroxide and iron oxide.
[0009]
[Means for Solving the Problems]
As described above, plate-like iron oxide produced by a hydrothermal synthesis reaction or a flux method is a plate-like single crystal body in itself, but has some drawbacks. In order to solve the above-mentioned problem, as long as the particles themselves are plate-shaped and the aspect ratio (d / t) can be increased, the iron oxide particles need not necessarily be a single crystal, but may be a polycrystal. The present invention has been found by paying attention to this point. By depositing a crystalline iron oxyhydroxide film from a solution having a relatively low temperature of room temperature to 70 ° C. under normal pressure, a plate-like iron oxide oxyhydroxide is obtained. A crystal is obtained.
[0010]
The gist of the invention is that
(1) An iron oxyhydroxide that is precipitated from an aqueous solution containing first and second ferric ions at normal pressure, and is a polycrystalline iron oxyhydroxide that is deposited as a crystalline film and exfoliates into plate-like particles.
(2) A plate-like α-type iron oxide obtained by heating and dehydrating the polycrystalline iron oxyhydroxide according to (1).
(3) A plate magnetite obtained by reducing the plate α-type iron oxide according to the above (2).
(4) A plate-like γ-type iron oxide obtained by oxidizing the plate-like magnetite according to (3).
[0011]
(5) The exfoliated surface of the plate-like iron oxyhydroxide or the plate-like α-type iron oxide according to the above (1) or (2) is dense and shows gloss, and / or a specific crystal surface is determined by X-ray diffraction. Plate-like iron oxyhydroxide or plate-like α-type iron oxide, which is a developed polycrystal.
(6) The plate-like iron oxide according to (1) to (4), wherein the maximum major axis d is in a range of 5 μm or more and several mm or less, and a ratio (d / t) between the maximum major axis d and the thickness t. ) Is 2 or more.
[0012]
(7) When producing the plate-like iron oxyhydroxide according to the above (1), by setting the pH to -1.0 to 2.0 and Fe ²⁺ / Fe ³⁺ = 0.2 to 20, the crystal is produced. A method for producing a plate-like iron oxyhydroxide, comprising depositing an exfoliated iron oxyhydroxide film and separating the film into plate-like particles.
(8) The method for producing plate-like α-type iron oxide according to (2), wherein the plate-like iron oxyhydroxide according to claim 1 is heated and dehydrated by heating such as desorbed H ₂ O. A method for producing a plate-like α-type iron oxide, wherein the plate-like property of iron oxyhydroxide is maintained by immediately discharging a time-generated component. The components generated at the time of heating mean not only H ₂ O but also Cl components, SO ₂ , NO _{2 and the} like.
[0013]
Hereinafter, the present invention will be described in detail.
There are many kinds of compounds precipitated and precipitated from the iron salt solution, and the reaction system is complicated. Various precipitates are formed depending on the type and concentration of Fe ions, the temperature and pH of the reaction system, the type of coexisting anions, and the presence or absence of an oxidizing agent. A typical example is iron oxyhydroxide (having structural isomers of FeOOH, α, β, γ, and δ), of which α-type iron oxyhydroxide is γ-Fe which forms the center of a magnetic recording material. It is known as a raw material for ₂ O ₃ . These and other known precipitates and precipitates from iron salt solutions are separated from the mother liquor as fine particles irrespective of their amorphousness or crystallinity and become powder when dried.
[0014]
However, in the present invention, by selecting the conditions of the system, the iron oxyhydroxide particles are deposited and crystallized on the solid surface in the solution in a uniform orientation to form a crystalline film having a two-dimensional spread. I found that. This film becomes thicker with time, and when it exceeds a certain thickness (about 3 μm), it peels off and becomes plate-like particles. The plate-like particles grown in this way have a dense and exfoliated surface, show a gloss, and when the density increases, show a single crystal characteristic in which a crystal surface that has grown specifically by X-ray diffraction is observed. The state is extremely stable. Note that a small amount of the crystalline film may be deposited on the liquid surface simultaneously with the deposition of the crystalline film on the solid surface. This film has a higher density than that deposited on the solid surface, but has the same plate shape and crystallinity.
[0015]
FIG. 1 is a micrograph showing an SEM image of the particles. FIG. 1A is a 50 × microscopic photograph of FeOOH by SEM, FIG. 1B is a 50 × microscopic photograph of α-Fe ₂ O ₃ by SEM, and FIG. 500 photomicrograph of by SEM of -Fe ₂ O _3. As shown in this figure, according to the SEM observation of the plate-shaped cross section, needle-like or columnar primary particles grow in parallel or radial alignment in a direction perpendicular to the solid surface, and are crystallized secondary particles. Is a basic unit. It is considered that the precipitates from the solution at normal pressure and room temperature to medium temperature form crystalline secondary and tertiary structures which have not been known so far. It is thought that it develops mechanical strength not found in the body, and the plate shape becomes stable.
[0016]
Although the shape of the plate surface is indefinite, it can be adjusted to an arbitrary plate surface diameter by lightly pulverizing, and a size of 5 μm to a maximum of several mm is possible. A mesh sieve can be used for adjusting the size. The thickness is proportional to the reaction time, and can be arbitrarily adjusted if the thickness is 3 μm or more that facilitates peeling, so that the aspect ratio can be freely adjusted. If the aspect ratio (d / t) between the maximum major axis d and the thickness t is 2 or more, the effect of shape anisotropy appears, and the effect increases as this value increases. The material of the solid to be deposited is not particularly limited. Any material such as glass, porcelain, plastic, and metal can be used.
[0017]
When the plate-like iron oxyhydroxide is heated and dehydrated under conditions that do not impair the plate-like property, a plate-like α-type iron oxide is obtained. By heating, the crystallinity is improved and a stronger plate-like crystal is obtained. This is used for the aforementioned ferrite raw material and paint pigment. For ferrite, iron oxyhydroxide before dehydration can be sufficiently used. Iron oxyhydroxide has the advantage that ferrite is easily formed at a low temperature, but workability such as composition adjustment is better to use α-Fe ₂ O ₃ .
[0018]
The fact that the plate-like iron oxide according to the present invention is polycrystalline is advantageous as a raw material for a ferrite magnetic material. Single crystal plate-like iron oxide obtained by hydrothermal synthesis reaction using natural MIO and autoclave requires high-temperature reaction for ferrite formation due to high density of particles, and occurs together with ferrite formation reaction. The reshaping of the crystal structure reflects the shape, and the plate is easily broken. Since the plate-like iron oxide according to the present invention has a moderate density, the solid-phase reaction with other components such as ferrite formation proceeds at a low temperature, and even if the crystal structure is rearranged, it is polycrystalline, so that the shape changes. Has no effect, and is advantageous in both cost and shape maintenance. Plate-like magnetite (Fe ₃ O ₄ ) and maghemite (γ-Fe ₂ O ₃ ) are obtained by reducing and oxidizing the above-mentioned plate-like iron oxide. Magnetite and maghemite may be used not only as magnetic materials together with hematite, but also as decorative pigments utilizing glossiness, and magnetite may be used as a highly biosafety black pigment which can replace carbon.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
The deposition of the crystalline iron oxyhydroxide film according to the present invention can be carried out as follows.
It is said that Fe ²⁺ and Fe ³⁺ ions in an aqueous solution are usually hydrated ions at the center where water molecules are coordinated at six vertices of an octahedron. Of these, the reaction in which Fe ³⁺ ions react with other water molecules to release H ⁺ (H ₃ O ⁺ ) is a hydrolysis reaction, which is called protolysis, and proceeds as described below to reduce the acidity of the solution. Increase.
_{_{^{Fe (H 2 O) 6 3+}}} + H 2 O ← → Fe (OH) (H 2 O) 5 2+ + H 3 O +
… (1)
Fe (OH) (H ₂ O) ₅ ²⁺ + H ₂ O ← → Fe (OH) ₂ (H ₂ O) ₄ ⁺
+ H ₃ O ⁺ ... (2)
[0020]
In the hydrolysis of Fe ³⁺ ions, six coordinated H ₂ O molecules are replaced by OH groups one by one. The first two are shown above, the last one below, and the middle three are omitted.
_{_{^{Fe (OH) 5 (H 2}}} O) 2- + H 2 O ← → Fe (OH) 6 3- + H 3 O +
… (3)
The complex ions generated by the protolysis cause a polymerization reaction with each other, and eventually precipitate and precipitate as iron oxyhydroxide or the like.
[0021]
2Fe (H ₂ O) ₅ (OH) ²⁺ ← → [(H ₂ O) ₅ Fe—O—Fe (H ₂
O) ₅ ] ⁴⁺ + H ₂ O (4)
[Fe (H ₂ O) ₅ —O—Fe (H ₂ O) ₅ ] ⁴ ++ H ₃ O ⁺ ← → [(H ₂
O) ₅ Fe—OH—Fe (H ₂ O) ₅ ] ⁵⁺ + H ₂ O (5)
The polymerization reaction is a cross-linking reaction by -O- or -OH-, and the reaction of the formula (4) is called oxolation, and the reaction of the formula (5) is called olation. Usually, first, a dinuclear complex consisting of two octahedrons is formed, and the polymerization proceeds on the basis of this. The type of Fe ion depends on whether the polymer becomes amorphous or grows into crystalline particles. It is known that the temperature varies depending on the temperature and pH of the reaction system, the type and concentration of coexisting anions, the amount of dissolved oxygen, the presence or absence of an oxidizing agent, and the like, and is very complicated.
[0022]
A model of the polymerization is shown below.
[0023]
Embedded image

[0024]
In order to deposit the polycrystalline iron oxyhydroxide in the form of a film from the iron salt solution, the rates of the hydrolysis reaction and the polymerization reaction may be appropriately suppressed. When hydrolysis proceeds rapidly and the number of complex ions having a low valence increases, polymerization proceeds rapidly and the resulting polymer tends to be irregular, and the precipitated iron oxyhydroxide tends to be amorphous. The present invention is based on the above findings. That is, in order to make the hydrolysis rate as slow as possible, the pH of the system is set to the lower limit range (-1.0 to 2.0) where hydrolysis is started, and the total Fe concentration is 1.5 to 5.0 mol. / L, and use only oxygen dissolved from the gas phase in contact with the solution without using an alkali or an oxidizing agent for oxidation, and the temperature is relatively low from room temperature to 70 ° C. That is, the reason for setting the pH at -1.0 to 2.0 is that if the pH is less than -1.0, the oxidation of Fe ²⁺ ions to Fe ³⁺ ions prior to hydrolysis hardly progresses, and the pH exceeds 2.0. And the rate of hydrolysis of Fe ³⁺ ions is too high and the product does not form a film. Therefore, the range was -1.0 to 2.0.
[0025]
When an appropriate amount of Fe ²⁺ ions (Fe ²⁺ / Fe ³⁺ = 0.2 to 20) is added to a solution containing Fe ³⁺ ions and the solution is allowed to stand, a thickness that can be peeled in 3 to 4 days at 50 ° C. To grow. If the ratio of Fe ²⁺ / Fe ³⁺ is less than 0.2, the growth of the film is extremely slow, and if it exceeds 20, the hydrolysis rate is too high to form fine particles and the film does not grow. The pH of the system increases with the deposition of the film. This differs from the well known hydrolysis and oxidation of Fe ions. In ordinary hydrolysis and oxidation reactions of Fe ions, the pH always drops. In the system of the present invention, when the conditions are deviated from the predetermined range, that is, when the hydrolysis and oxidation reaction rates are excessively increased, the pH decreases, and a generally known fine particle iron oxyhydroxide precipitate is obtained. Is not generated.
[0026]
The gas phase in contact with the system is air or a mixed gas of air and oxygen at normal pressure. The higher the oxygen partial pressure, the higher the deposition rate. The higher the reaction temperature and the higher the Fe ²⁺ ion concentration, the higher the deposition rate. However, if the reaction rate is too high, the orientation of the primary particles in the formed film decreases, and the compactness of the plate-like particles decreases. I do.
[0027]
α-Fe ₂ O ₃ is obtained by heating and dehydrating FeOOH, but in order to maintain a plate-like form, it is necessary to carry out the conditions under which H ₂ O and Cl components to be desorbed are quickly discharged. When the partial pressure of the H ₂ O or Cl component increases, the plate becomes broken and becomes a normal powder. Heating at 400 to 800 ° C. for 30 minutes to 1 hour is sufficient, and air may be sent into the heating furnace or a non-closed heating furnace having a sufficiently large internal volume may be used. When the heating temperature is high, α-Fe ₂ O ₃ having better crystallinity is obtained. However, when a solid phase reaction such as ferrite formation is performed thereafter, it is better to increase the crystallinity and not to make the crystal dense.
[0028]
FIG. 2 is a diagram showing an X-ray diffraction pattern of α-Fe ₂ O ₃ . FIG. 2 (a) is a diagram showing an X-ray diffraction pattern of the plate-like iron oxide according to the present invention, and FIG. 2 (b) is an X-ray diffraction of iron oxide produced by high-temperature pyrolysis generally used as a ferrite raw material. It is a figure showing a pattern. In the plate-like iron oxide, the (110) diffraction line, which usually shows the second intensity with iron oxide, is larger than the (104) diffraction line intensity, and it can be seen that the (110) plane is growing singularly. The degree of alignment of the primary or secondary particles depends on the reaction rate. When the reaction rate is reduced, the orientation improves, and the mechanical strength of the plate-like particles also increases. In a sample having a high orientation, even if the film thickness is small, it is easily peeled off to obtain stable particles having a plate-like shape.
[0029]
On the other hand, what peels off even if the degree of orientation is low is still crystalline particles, and the strength of the plate-like surface is sufficient. Rather, when a solid phase reaction with another component is performed, such as ferrite formation, moderate orientation is advantageous. If the reaction is continued without separation, the thickness grows to an arbitrary thickness. According to the method of the present invention, the density (orientation) and thickness of the plate-like particles can be arbitrarily adjusted according to the purpose. Therefore, it is possible to arbitrarily produce plate-like particles having a high aspect ratio (d / t) required as a raw material of plate-like ferrite for absorbing unnecessary high-frequency radiation and high magnetic permeability.
[0030]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
The initial pH of an aqueous solution of FeCl ₂ + FeCl _{3 having} a concentration of Fe ²⁺ / Fe ³⁺ = 1.3 and a total Fe concentration of 2.5 mol / l was adjusted to 0.3, and a polyethylene solution was prepared so that the thickness of the solution was 5 mm to 1 cm. Pour into the bat. In order to prevent a change in Fe concentration due to evaporation of water, the sample was covered with wraps and then placed in a thermostat set at 50 ° C. and kept for 4 days. The gas phase in contact here was air. In order to make the amount of the gas phase in contact with the liquid amount sufficient, the depth of the vat was set to be 10 times or more the liquid phase thickness. The pH of the supernatant solution after the retention was about 1.0. When the vat was washed with water after separating the supernatant solution, a thin reddish-brown film came off from the bottom. When this was examined by X-ray diffraction, it showed a pattern of β-type iron oxyhydroxide, and SEM observation showed plate-like particles as shown in FIG.
[0031]
When the above-mentioned plate-like particles were placed on a flat evaporating dish to a thickness of about 5 mm or less and heated at 600 ° C. for 1 hour under an air flow, plate-like particles having a black-brown glossy surface and a crimson back surface were obtained. . When this was examined by X-ray diffraction, as shown in FIG. 2A, the (110) plane showed a pattern of α-Fe ₂ O ₃ in which growth was specific. Further, SEM observation showed plate-like particles as shown in FIGS. 1B and 1C, and it was found that the shape of FIG. 1A was inherited as it was. The tube is Fe. The plate-shaped α-type iron oxide was placed on a flat-bottomed evaporating dish and heated at 600 ° C. for 1 hour in a CO gas stream to form plate-shaped magnetite. Further, the plate-like magnetite was placed on a flat-bottomed evaporating dish and heated in air at 400 ° C. for 1 hour to form plate-like maghemite.
[0032]
(Comparative example)
The initial pH of an aqueous solution of FeCl ₂ + FeCl _{3 having} a concentration of Fe ²⁺ / Fe ³⁺ = 1.3 and a total Fe concentration of 2.5 mol / l was adjusted to 2.3, and polyethylene was adjusted so that the thickness of the solution was 5 mm to 1 cm. Pour into the bat. In order to prevent a change in Fe concentration due to evaporation of water, the sample was covered with wraps and placed in a thermostat set at 70 ° C. and kept for 4 days. The gas phase in contact here was air. In order to make the amount of the gas phase in contact with the liquid amount sufficient, the depth of the vat was set to be 10 times or more the liquid phase thickness. The pH of the supernatant solution after the retention was about 0.3. When the vat was washed with water after separating the supernatant solution, ocher particles adhered to the bottom. This was peeled off with a spoonful and dried to a fine powder. Examination by X-ray diffraction showed a pattern of β-type iron oxyhydroxide, and it was found by SEM observation that the particles were strip-shaped particles. This was placed on a flat evaporating dish with a thickness of about 5 mm or less, and heated at 600 ° C. for 1 hour under an air stream, to obtain a red powder. The powder was examined by X-ray diffraction. The pattern of α-Fe ₂ O ₃ was shown. SEM observation confirmed that the particles were strip-shaped particles.
[0033]
【The invention's effect】
As described above, according to the present invention, crystalline plate-like iron oxyhydroxide can be easily obtained from an aqueous solution at normal pressure, and from this, crystalline plate-like iron oxide can be easily obtained. Similarly, plate-like magnetite and maghemite are easily obtained, providing industrially superior materials that are extremely useful not only as electronic materials such as ferrite raw materials but also in the fields of paints and pigments in terms of both properties and economy. it can.
[Brief description of the drawings]
FIG.
It is a micrograph which shows the SEM image of a particle cross section.
FIG. 2
It is a diagram showing an X-ray diffraction pattern of the _α-Fe 2 _{O 3.}

Claims

A polycrystalline iron oxyhydroxide that is precipitated from an aqueous solution containing ferrous and ferric ions at normal pressure, and is deposited as a crystalline film and exfoliated into plate-like particles.

A plate-like α-type iron oxide obtained by heating and dehydrating the polycrystalline iron oxyhydroxide according to claim 1.

A plate-like magnetite obtained by reducing the plate-like α-type iron oxide according to claim 2.

A plate-like γ-type iron oxide obtained by oxidizing the plate-like magnetite according to claim 3.

3. A polycrystalline body in which the exfoliated surface of the plate-like iron oxyhydroxide or the plate-like α-type iron oxide according to claim 1 or 2 is dense and shows a gloss, and / or a specific crystal surface is developed by X-ray diffraction. A plate-like iron oxyhydroxide or a plate-like α-type iron oxide.

The plate-shaped iron oxide according to any one of claims 1 to 4, wherein a maximum major axis d is in a range of 5 µm or more and several mm or less, and a ratio (d / t) between the maximum major axis d and the thickness t is 2 or more. Some plate-like iron oxides.

In preparing the plate-like iron oxyhydroxide according to claim 1, by setting pH = -1.0 to 2.0 and Fe ²⁺ / Fe ³⁺ = 0.2 to 20, crystalline oxy water is set. A method for producing a plate-like iron oxyhydroxide, wherein an iron oxide film is deposited and peeled to form plate-like particles.

3. The method for producing a plate-like α-type iron oxide according to claim 2, wherein, when heating and dehydrating the plate-like iron oxyhydroxide according to claim 1, components generated during heating, such as H ₂ O, desorbed. A method for producing plate-like α-type iron oxide, wherein the plate-like property of iron oxyhydroxide is maintained by immediately discharging.