JP2004182545A - Method for producing porous ferrite and radio wave absorber using the same - Google Patents

Abstract

[PROBLEMS] A method for producing a porous ferrite capable of performing sintering in which cracks and cracks are prevented and capable of appropriately forming pores in a sintered body, and increasing the radio wave absorption characteristics to a higher frequency and a wider area by making the porous ferrite. To provide a radio wave absorber excellent in mechanical strength and weather resistance. [Solution] In manufacturing, a ferrite powder as a base material is weighed and calcined at a predetermined temperature, and then the calcined body is pulverized. Granulate. The granulated ferrite powder is mixed with fine organic particles together with a thermoplastic powder having a low melting point and a binder function. The additive is burned off to obtain a porous sintered body. Organic particles (starch) and thermoplastic powder (paraffin fine particles) are added in a total amount of 0 to 30 wt%, and the mixing ratio of the organic particles is increased in a range of 0 to 5 wt% in accordance with the amount of the thermoplastic powder. . When the thickness of the sample d is 0.8 cm, the return loss is about 20 dB or more at about 100 to 750 MHz, which corresponds to almost the entire VHF to UHF band.
[Selection diagram] FIG.

Description

【００２７】
図１は、有機物粒子，熱可塑性粉体の添加量をパラメータにした各試料の密度を示すグラフ図である。各試料の添加量と密度の関係は同図に示すように、
添加量： ０ｗｔ％の試料ａは密度：５．１ｇ／ｃｍ^３、
添加量： ５ｗｔ％の試料ｂは密度：４．６ｇ／ｃｍ^３、
添加量：１０ｗｔ％の試料ｃは密度：４．０ｇ／ｃｍ^３、
添加量：１５ｗｔ％の試料ｄは密度：３．７ｇ／ｃｍ^３、
添加量：２０ｗｔ％の試料ｅは密度：３．２ｇ／ｃｍ^３、
添加量：３０ｗｔ％の試料ｆは密度：２．７ｇ／ｃｍ^３
という結果が得られた。
【００２８】
したがって同図から明らかなように、製造した焼結体の密度は、有機物粒子（デンプン）と熱可塑性粉体（パラフィン）の添加量と相関があり、添加量を増すに連れて密度が低減することを確認した。なお、添加量０ｗｔ％の試料ａは、有機物粒子，熱可塑性粉体をまったく添加しないで磁性粉体のみを焼成した比較例である。
【００２９】
また、製造した各試料を顕微鏡で観察した。その結果、有機物粒子，熱可塑性粉体を添加したもの（試料ｂ〜ｆ）において、大きさのそろった微細な空孔または連続した空孔が全体的に均一に分布していることを確認した。
【００３０】
すなわち、有機物粒子，熱可塑性粉体の混合を調製することでは、焼結体中における空孔部分を直接的に設定することになる。したがって、それら添加剤の混合割合や粒子径等を調製することにより多孔質化の程度つまり空孔の分布，比率等を制御でき、焼結体の密度を適宜に変えることができる。このことは後述するように、空孔率を大きく設定した多孔質化により電波吸収特性を高周波化，広域化することができることにつながる。
【００３１】
次に、試料の製造条件について説明する。ＮｉＺｎ系フェライト紛体は、基本成分であるＦｅ_２Ｏ_３，ＮｉＯ，ＺｎＯからなり、その他、微量成分が含まれている。なお、母剤の磁性粉体はＮｉＺｎ系フェライトに限らず、ＭｇＺｎ系，ＭｎＺｎ系などのフェライト粉体としてもよい。
【００３２】
つまり、フェライト粉体の組成は、例えば、Ｆｅ_２Ｏ_３が５０〜５８モル％，ＭｎＯが１２〜４７モル％，ＺｎＯが３〜３０モル％とすることができる。また、Ｆｅ_２Ｏ_３が４６〜４９モル％，ＭｇＯが２４〜２７モル％，ＺｎＯが１８〜２１モル％，ＭｎＯが４〜７モル％，ＣｕＯが１〜４モル％の組成とすることもできる。さらには、Ｆｅ_２Ｏ_３が４３〜５０モル％，ＺｎＯが１０〜３５モル％，ＣｕＯが３〜１５モル％，残部がＮｉＯの組成としてもよい。
【００３３】
熱可塑性粉体としては、本発明ではパラフィンを用いるが，ポリエステルなどの樹脂材料とすることも可能である。有機物粒子には、デンプン，グルテン，小麦粉，コーンスターチ，米粉，片栗粉，こんにゃく粉，ショ糖，ポリビニルアルコール（ＰＶＡ），木粉，竹粉，米糠などがよい。これらの有機物は入手が容易で安価であるため量産化面で有利性がある。
【００３４】
また、熱可塑性粉体の配合量を、１〜４０ｗｔ％とし、あるいは有機物粒子と熱可塑性粉体とを併せた配合量を、１〜４０ｗｔ％とし、有機物粒子と熱可塑性粉体の粒子サイズを、約１０〜１０００μｍに設定し、当該粒子サイズは混合割合に応じて適宜に調整することもよい。また、粒子サイズおよび混合割合を調整することにより、空孔率を０〜５０％にすることもよい。
【００３５】
また、磁性粉体のみを温度５００〜１３００℃で仮焼成し、次に当該仮焼成体を粉砕して有機物粒子および熱可塑性粉体を混合して、成形後に焼成する。そして、仮焼成体の粉砕において、粒子サイズを１０ｍｍ以下にすることもよい。
【００３６】
また、磁性粉体のみを所定温度で仮焼成し、次に当該仮焼成体を粉砕して１〜２ｗｔ％のポリビニルアルコールなどのバインダとともに造粒し、この後に有機物粒子および熱可塑性粉体を混合して、成形後に焼成する。そして、仮焼成体の粉砕において、粒子サイズを１ｍｍ以下にすることも好ましい。
【００３７】
なお、熱可塑性粉体の添加量をむやみに増すと、圧縮成形の際に金型への粘着が増加するため成形性が悪化する。これは熱可塑性粉体の添加を、共に添加する有機物粒子の添加量に見合う適量に調製することで防止でき、また有機物粒子の粒径が熱可塑性粉体と同等あるいはそれ以下であることが好ましい。
【００３８】
（測定結果）
各試料ａ〜ｆの反射減衰量は、図２〜図７に示すような結果が得られた。各図には、各試料の厚みをパラメータとして変化させたときの反射減衰量を求めてあり、反射減衰量が２０ｄＢ以上つまり電波吸収率９９％以上となる特性領域に注目すると、電波吸収の帯域は密度の低下に伴って高周波側に移行し、帯域幅も広がることが分かる。
【００３９】
また、図５に示すように、試料ｄの厚み０．８ｃｍでは、約１００〜７５０ＭＨｚの周波数帯域で反射減衰量が２０ｄＢ以上となり、これはＶＨＦ〜ＵＨＦ帯域のほぼ全域に対応する。
【００４０】
このように、焼結体の密度（空孔率）を変更することにより、反射減衰（電波吸収）の帯域を容易に変化させることができ、所望する周波数帯域について電波吸収特性を適宜に発現させることができる。つまり、電波吸収体としては、多孔質化によって軽量化ができるとともに、目的の周波数帯域において透磁率はあまり低下させずに誘電率を大きく低下させることができるので、電波吸収特性の広帯域化，高周波化ができる。
したがって本発明によれば、高い周波数帯域で良好な電波吸収特性を得ることができ、しかもその電波吸収の帯域設定を広帯域で適宜に設定可能である。
【００４１】
（電波吸収体への適用）
また、以上のように試作を行った結果、本発明に係る多孔質フェライトの製造方法によれば焼成体をタイル形状に形成することが容易であり、一辺が１０ｃｍ以上の方形の焼結体を製作できることを確認した。これは面積サイズが比較的に大きく、電波吸収体としての適用が好ましい。つまり、こうしたタイル形状の焼結体（フェライトタイル）は、多数個を敷き並べて任意の面積サイズに施工できるメリットがある。
【００４２】
電波吸収体をＴＶゴースト障害の対策に用いる場合は、多孔質化のもので有効な電波吸収特性を得るために、空孔率が上がるほど厚みを増さなければならない。しかし、厚みの増加を考慮しても多孔質化のものではすべての条件において約１０％以上の軽量化が可能である。
【００４３】
さらに、試料ｄの厚み０．８ｃｍでは、約１００〜７５０ＭＨｚで反射減衰量が２０ｄＢ以上となりＶＨＦ〜ＵＨＦ帯域のほぼ全域に対応する実用上問題がないきわめて広帯域な電波吸収特性を有することになる。
【００４４】
（結論）
以上から明らかなように、本発明に係る製造方法では、母剤の磁性粉体には、微細な有機物粒子とともに、融点が低くバインダ機能がある熱可塑性粉体を併せて混合し、この混合物を圧縮成形する。このため、熱可塑性粉体がバインダ機能し、圧縮成形の際に混練状態が不均一になることを防げる。
【００４５】
その結果、成形体の強度を上げることができ、混合粉体を混練する際に水分調整がいらず、割れや亀裂を防止した成形体を容易に得ることができる。そして、融点が低い熱可塑性粉体は焼成工程の初期の段階でスムーズに揮発し、割れや亀裂を防止した焼成を行うことができ、焼結体中には空孔が均一に分布することから、良好な品質の多孔質フェライトを得ることができる。さらにこの場合、製造工程は従来と同様で増加する工程がなく、製造を容易に行えて歩留まりもよくなり生産性が高い。
【００４６】
なお、試料の製造では、母剤となるＮｉＺｎ系フェライト紛体に有機物粒子をまったく混合しないで熱可塑性粉体のみを混合した試料も製造してみた。そして顕微鏡で観察した結果、この試料であっても微細な空孔が全体に均一に分布していることを確認でき、多孔質フェライトが得られることを確認した。
【００４７】
ところで上記した第１の実施の形態では、母剤のフェライト粉体は仮焼成した後に粉砕して造粒を行ったが、粉砕，造粒を行わない工程手順にしてもよい。すなわち、第２の実施の形態では、仮焼成した後のフェライト粉体に、微細な有機物粒子とともに、融点が低くバインダ機能がある熱可塑性粉体を併せて混合して、成形後に焼成する。これにより、当該両成分を焼成時に焼失させて多孔質化した焼結体を得る。
【００４８】
ここでも本発明に係る製造方法の効果を実証するため、製造条件を替えて各種の試料を製造し、各試料について高周波特性を測定した。この特性測定の手順は、第１の実施の形態と同様でありその説明を省略する。
【００４９】
（測定結果）
各試料の反射減衰量は、図８から図１１に示すように得られた。ここで各試料にあっては、
添加量： ５ｗｔ％の試料ｂは密度：４．７ｇ／ｃｍ^３、
添加量：１５ｗｔ％の試料ｄは密度：３．５ｇ／ｃｍ^３、
添加量：２０ｗｔ％の試料ｅは密度：３．２ｇ／ｃｍ^３、
添加量：３０ｗｔ％の試料ｆは密度：２．８ｇ／ｃｍ^３
という結果が得られた。
【００５０】
そして、各図には、それぞれ各試料の厚みをパラメータとして変化させたときの反射減衰量を求めてあり、反射減衰量が２０ｄＢ以上つまり電波吸収率９９％以上となる特性領域に注目すると、電波吸収の帯域は密度の低下に伴って高周波側に移行し、帯域幅も広がることが分かり、粉砕，造粒を行わない単に仮焼成した後の粉体においても同様な効果を確認した。
【００５１】
またこの場合、空孔率が大きくなると、造粒後の粉体を用いたときに比べて電波吸収性能が劣ることを確認した。しかし、焼成に関して仮焼粉体は粒子径が大きいために、その分解，燃焼が速やかに容易に行われることになる。さらに、粉砕，造粒を行わないことから工程数が減り、生産性，コスト面に有利性がある。なお、その他の構成並びに作用効果は、第１の実施の形態と同様であるので、その詳細な説明を省略する。
【００５２】
図１２は、本発明の第３の実施の形態を示している。本実施の形態では、上記した本発明に係る多孔質フェライトの利用形態の一例であり、多層構造の電波吸収体に構成するようにしている。
【００５３】
具体的には、図１２に示すように、多孔質フェライト層２の表裏を非磁性体層１，３で覆う３層構造に形成し、非磁性体層１，３は河川や湖の汚泥あるいは産業廃棄汚泥を母剤にする。
【００５４】
ここではフェライト層２の原料には、ＮｉＺｎ系のフェライト粉体を配合して約９００℃の温度で仮焼成し、その仮焼成したフェライト粉体に、熱可塑性粉体としてパラフィン微粒子を５〜３０ｗｔ％の割合で添加した混合粉体を用いた。このとき、パラフィンの混合比は、フェライト層２の収縮率に大きく影響するため非磁性体層１，３の収縮率から予測することが好ましい。つまり、非磁性体層１，３の収縮率に合うように設定される。
【００５５】
非磁性体層１，３の原料には湖から採取した汚泥を用いた。汚泥は乾燥させた後に粉砕工程を経て粉体にする。本実施の形態で用いた汚泥は、河川の汚泥であり、その組成は下記表の通りであった。
【００５６】
【表１】

【００５７】
次に、調製した各原料粉体を金型内に順に充填して、非磁性体層１，フェライト層２，非磁性体層３の順に３層に分離する状態に圧縮成形した。本実施の形態では金型は一辺が１２ｃｍの方形のものを用い、そして電気炉に入れて温度１０５０℃で２時間の焼成を行った。この焼成した焼結体を顕微鏡で観察したところ、焼結体には割れ，膨れ，反りはなく、多孔質フェライト層２と非磁性体層１，３との界面もヒビ割れなどがなく、しっかりと結合していることを確認した。そして、３層の収縮率がほぼ等しく設定されるため、界面での剥離もない。
【００５８】
ここで本発明に係る製造方法の効果を実証するため、製造条件を替えて各種の試料を製造し、各試料について高周波特性を測定した。この特性測定の手順は、第１の実施の形態と同様でありその説明を省略する。
【００５９】
（測定結果）
各試料の反射減衰量は、図１３（資料ｇ），図１４（資料ｈ）に示すように得られた。ここで各試料は各層の厚みを適宜に設定したものであって、
試料ｇは非磁性体層１が１ｃｍ，
フェライト層２が１ｃｍ，
非磁性体層３が０〜０．９ｃｍであり、
試料ｈは非磁性体層１が０〜２．４ｃｍ，
フェライト層２が１ｃｍ，
非磁性体層３が０．３ｃｍである。
【００６０】
なお、当該多層焼結体は図１２に示すように、電波吸収のための対象物４に対して非磁性体層１側を覆い被せる設定とし、このため図１３，図１４には非磁性体層１側あるいは非磁性体層３側の厚みをパラメータとして変化させたときの反射減衰量を求めてある。反射減衰量が２０ｄＢ以上つまり電波吸収率９９％以上となる特性領域に注目すると、電波吸収特性は、表面層になる非磁性体層３の厚みを増すほど劣化してしまうが（図１３）、内側層になる非磁性体層１の厚みを適切に設定することにより、高周波側に広げるとともに電波吸収率も増大できることを確認した（図１４）。
【００６１】
したがって、多孔質フェライトを用いた多層（３層）焼結体にあっては、電波吸収体への適用では、表面層は腐食耐性が良好に得られる程度に薄くし、対象物４に接する内側層は電波吸収特性が最適になる厚みに設定すればよい。これにより、機械的な強度および耐候性が要求される屋外等に施工が好ましい電波吸収体を得ることができる。
【００６２】
非磁性体層１，３の原料に汚泥を用いることでは、汚泥をリサイクル利用することができ、省力化が行えて好ましい。しかし、これに限定されるものではなく、誘電率が低い材料を非磁性体層１，３の原料に用いることで電波吸収特性がより向上することは明らかである。また、産業リサイクルの目的から汚泥を用いたが、他の非磁性体材料でも適用できることは明らかであり、各非磁性体層１，３は相違した材料から形成してもよい。
【００６３】
ところで、多層焼結体の製造にあっては、フェライト層２は多孔質化のための添加剤を調製することにより収縮率を制御でき、このため非磁性体層１，３の側と収縮率を一致させる設定により、性状の異なる多層を同一工程で圧縮成形して焼成することができる。このように、多層を同一工程によって成形，焼成できることから製造が容易になり生産性が高く、歩留まりもよくなるためコスト低減が行える。
【００６４】
多層焼結体では、フェライト層，非磁性体層の層数や厚さを適宜に変更できることから、電波吸収帯域を選択的に設定することができ、電波吸収体としての特性を制御でき、様々な特性要求に対応可能で適用が広がるメリットがある。
【００６５】
（付記）
１．磁性粉体に、微細な有機物粒子を混合して混合粉体を製造し、成形後に焼成して前記有機物粒子を焼失させる多孔質フェライトの製造方法において、前記有機物粒子とともに、融点が低くバインダ機能がある熱可塑性粉体を併せて混合して当該両成分を前記焼成時に焼失させることにより多孔質フェライトを製造することができる。
【００６６】
２．また、前記熱可塑性粉体は、パラフィン，ポリエステルなどの樹脂材料とすることができる。
【００６７】
３．上記した１，２において、前記有機物粒子は、デンプン，グルテン，小麦粉，コーンスターチ，米粉，片栗粉，こんにゃく粉，ショ糖，ポリビニルアルコール，木粉，竹粉，米糠などから構成することができる。
【００６８】
４．上記した１から３において、前記磁性粉体は、ＮｉＺｎ系，ＭｇＺｎ系，ＭｎＺｎ系などのフェライト粉体とすることができる。この場合に、そのフェライト粉体は、Ｆｅ_２Ｏ_３が５０〜５８モル％，ＭｎＯが１２〜４７モル％，ＺｎＯが３〜３０モル％の組成としたり、あるいはＦｅ_２Ｏ_３が４６〜４９モル％，ＭｇＯが２４〜２７モル％，ＺｎＯが１８〜２１モル％，ＭｎＯが４〜７モル％，ＣｕＯが１〜４モル％の組成としたり、さらにはＦｅ_２Ｏ_３が４３〜５０モル％，ＺｎＯが１０〜３５モル％，ＣｕＯが３〜１５モル％，残部がＮｉＯの組成とすることができる。
【００６９】
５．上記した１から４において、前記熱可塑性粉体の配合量を、１〜４０ｗｔ％とすることができる。
【００７０】
６．上記した１から５において、前記有機物粒子と前記熱可塑性粉体とを併せた配合量を、１〜４０ｗｔ％とすることができる。
【００７１】
７．上記した１から６において、前記有機物粒子と前記熱可塑性粉体の粒子サイズを、約１０〜１０００μｍに設定し、当該粒子サイズは混合割合に応じて適宜に調整することができる。この場合に、前記粒子サイズおよび前記混合割合を調整することにより、空孔率を０〜５０％にするとよい。
【００７２】
８．上記した１から７において、前記磁性粉体のみを温度５００〜１３００℃で仮焼成し、次に当該仮焼成体を粉砕して前記有機物粒子および前記熱可塑性粉体を混合して、成形後に焼成することができる。この場合に、前記仮焼成体の粉砕において、粒子サイズを１０ｍｍ以下にするとよい。
【００７３】
９．上記した１から８において、前記磁性粉体のみを所定温度で仮焼成し、次に当該仮焼成体を粉砕して１〜２ｗｔ％のポリビニルアルコールなどのバインダとともに造粒し、この後に前記有機物粒子および前記熱可塑性粉体を混合して、成形後に焼成することができる。この場合に、前記仮焼成体の粉砕において、粒子サイズを１ｍｍ以下にするとよい。
【００７４】
１０．上記した１から１５に記載の製造方法により焼成体をタイル形状に形成することにより形成された多孔質フェライトを用いることにより、電波吸収特性が、高周波化，広帯域化したタイル状の電波吸収体を構成することができる。そして、好ましくは、係る多孔質フェライトの膜層と非磁性体からなる膜層とを多層に一体化して形成することである。この場合に、非磁性体層は、河川や湖の汚泥あるいは産業廃棄汚泥を母剤にすることができる。
【００７５】
上記した付記に示した各製造方法では、母剤の磁性粉体には、微細な有機物粒子とともに、融点が低くバインダ機能がある熱可塑性粉体を併せて混合し、この混合物を圧縮成形する。このため、熱可塑性粉体がバインダ機能し、圧縮成形の際に混練状態が不均一になることを防げる。
【００７６】
その結果、成形体の強度を上げることができ、混合粉体を混練する際に水分調整を特にすることなく、割れや亀裂を防止した成形体を容易に得ることができる。熱可塑性粉体として用いたパラフィンは融点が低く１００℃以下の低温で熱分解するため焼成工程の初期の段階でスムーズに揮発し、割れや亀裂を防止した焼成を行うことができる。
【００７７】
また、有機物粒子，熱可塑性粉体の混合を調製することでは、焼結体中における空孔部分を直接的に設定することになる。したがって、それら添加剤の混合割合や粒子径等を調製することにより多孔質化の程度つまり空孔の分布，比率等を制御でき、焼結体の密度を適宜に変えることができる。そして、空孔率を大きく設定した多孔質化により電波吸収特性を高周波化，広域化することができる。
多孔質フェライトを用いた多層焼結体は、フェライト層を覆う非磁性体層が保護膜層になり、このため強度を向上でき、腐食耐性も良好にできる。
【００７８】
また、多層焼結体の製造にあっては、フェライト層は多孔質化のための添加剤を調製することにより収縮率を制御でき、このため非磁性体層側と収縮率を一致させる設定により、性状の異なる多層を同時に成形，焼成することができる。
【００７９】
【発明の効果】
以上のように、本発明に係る多孔質フェライトの製造方法では、造孔材として１００μｍ以下のパラフィン粉末を用いたため、焼成工程の初期の段階でスムーズに揮発し、割れや亀裂を防止した焼成を行うことができ、焼結体中には空孔が均一に分布することから、良好な品質の多孔質フェライトを得ることができる。さらにこの場合、製造工程は従来と同様で増加する工程がなく、製造を容易に行えて歩留まりもよくなり生産性が高い。
【００８０】
焼失性有機物粒子とパラフィン粉末の混合を調製するようにした場合には、焼結体中における空孔部分を直接的に設定することになり、多孔質化の程度つまり空孔の分布，比率等を制御でき、焼結体の密度を適宜に変えることができる。電波吸収体としては、空孔率を大きく設定した多孔質化によって軽量化ができるとともに、目的の周波数帯域において透磁率はあまり低下させずに誘電率を大きく低下させることができるので、電波吸収特性の広帯域化，高周波化ができる。
【００８１】
多孔質フェライトの少なくとも一面に非磁性体層を一体的に設けた複数層からなる焼結体は、フェライト層を覆う非磁性体層が保護膜層になるため強度を向上でき、腐食耐性も良好にできる。したがって、電波吸収体への利用において、機械的な強度，耐候性が要求される屋外等への施工に好ましく適用できる。
【００８２】
複数層からなる焼結体では、フェライト層，非磁性体層の層数や厚さを適宜に変更できることから、電波吸収帯域を選択的に設定することができ、電波吸収体としての特性を制御でき、様々な特性要求に対応可能で適用が広がるメリットがある。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態を説明するための有機物粒子，熱可塑性粉体の添加量をパラメータにした各試料の密度を示すグラフ図である。
【図２】本発明の第１の実施の形態の効果を実証するための周波数をパラメータにした試料ａの反射減衰量を示すグラフ図である。
【図３】本発明の第１の実施の形態の効果を実証するための周波数をパラメータにした試料ｂの反射減衰量を示すグラフ図である。
【図４】本発明の第１の実施の形態の効果を実証するための周波数をパラメータにした試料ｃの反射減衰量を示すグラフ図である。
【図５】本発明の第１の実施の形態の効果を実証するための周波数をパラメータにした試料ｄの反射減衰量を示すグラフ図である。
【図６】本発明の第１の実施の形態の効果を実証するための周波数をパラメータにした試料ｅの反射減衰量を示すグラフ図である。
【図７】本発明の第１の実施の形態の効果を実証するための周波数をパラメータにした試料ｆの反射減衰量を示すグラフ図である。
【図８】本発明の第２の実施の形態の効果を実証するための周波数をパラメータにした試料ｂの反射減衰量を示すグラフ図である。
【図９】本発明の第２の実施の形態の効果を実証するための周波数をパラメータにした試料ｄの反射減衰量を示すグラフ図である。
【図１０】本発明の第２の実施の形態の効果を実証するための周波数をパラメータにした試料ｅの反射減衰量を示すグラフ図である。
【図１１】本発明の第２の実施の形態の効果を実証するための周波数をパラメータにした試料ｆの反射減衰量を示すグラフ図である。
【図１２】本発明の第３の実施の形態である多孔質フェライトを用いた多層焼結体（電波吸収体）の断面図である。
【図１３】周波数をパラメータにした試料ｇの反射減衰量を示すグラフ図である。
【図１４】周波数をパラメータにした試料ｈの反射減衰量を示すグラフ図である。
【符号の説明】
１，３ 非磁性体層
２ 多孔質フェライト層
４ 対象物[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a porous ferrite and a radio wave absorber using the same. More specifically, the present invention relates to a porous ferrite in which a burnable powder material is mixed with a magnetic powder, and the mixture is molded and sintered. Improvement of properties of porous ferrite.
[0002]
[Prior art]
As a manufacturing technique for firing and sintering a ceramic material such as ferrite, a technique for making a sintered body porous is already well known, and is disclosed in, for example, Patent Literature 1 and Patent Literature 2. In other words, a well-known manufacturing method is to add particles of a substance which is decomposed, burned and scattered during firing (referred to as “porous material” in this specification) to ceramic powder in advance and mix them. There is a method of molding and firing. During the firing, the particles are decomposed, burned, gasified, and scattered to form pores in the sintered body. The decomposable and combustible substance particles include various organic substances and synthetic resins.
[0003]
Making the sintered body porous is important, for example, by using a ferrite sintered body as a radio wave absorber. This is because ferrite sintered bodies of normal density have problems such as heavy weight and the radio wave absorption band is limited to a relatively low frequency band side. Attention has been paid to a technique for appropriately forming pores in a sintered body.
[0004]
[Patent Document 1]
JP-A-5-117058
[Patent Document 2]
JP-A-5-294753
[0005]
[Problems to be solved by the invention]
However, such a conventional manufacturing technique for making porous has the following problems. That is, since a plurality of different kinds of powders are kneaded, the state of kneading tends to be non-uniform at the time of compression molding due to their density difference. This causes problems such as cracks and cracks in the compact and the sintered body, and uneven distribution of pores in the sintered body.
[0006]
Further, in order to prevent cracking during molding, it is necessary to appropriately adjust the water content of the mixture, but it is difficult to maintain the optimum water content. Further, there is a problem that cracks and cracks are generated in the molded body (sintered body) due to heat and gas generated by decomposition and burning of the additional component for making the porous body.
[0007]
On the other hand, since the ferrite sintered body that has been made porous is porous, its strength is reduced and its corrosion resistance is also reduced. In addition, water absorption increases due to the porous nature, and when applied to radio wave absorbers, the dielectric constant tends to increase due to wetting and radio wave absorption characteristics tend to deteriorate, requiring mechanical strength and weather resistance. There is a problem that it is difficult to perform construction outdoors.
[0008]
Further, regarding the radio wave absorber, there is a demand that such a radio wave absorber be installed on the surface of a high-rise building or the like in order to prevent TV ghosts, for example. For this reason, reduction in weight and thickness is required. Furthermore, it is necessary to provide a structure having a certain flat area or more, such as a tile (including a panel), for ease of construction. In addition, there is a need to have a broadband characteristic in which the radio wave absorption band can cover the high frequency side in response to the shift of the frequency band to the high frequency side (UHF band) with the digitization of TV broadcasting. Then, only those exhibiting the radio wave absorption performance only in a narrow frequency band such as only the VHF band or only the UHF band.
[0009]
The present invention has been made in view of the above-described background, and aims at solving the above-described problems, performing firing with preventing cracks and cracks, and forming pores in a sintered body. An object of the present invention is to provide a method for producing a porous ferrite that can be appropriately formed. By increasing the porosity of the porous ferrite, it is possible to increase the frequency of the radio wave absorption and widen the range, and it is excellent in mechanical strength and weather resistance, and is suitable for outdoor applications. An object of the present invention is to provide a used radio wave absorber.
[0010]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, in the method for producing a porous ferrite according to the present invention, a magnetic powder is mixed with a pore-forming material, and the formed pore-forming material is fired to burn out the pore-forming material. In the method for producing a porous ferrite that forms pores, the pore former is configured to include a fine paraffin powder of 100 μm or less.
[0011]
The predetermined hole is, for example, a hole for increasing the radio wave absorption characteristic to a higher frequency and a wider band. The pore-forming material is capable of forming pores by being burned out (pyrolyzed at a low melting point) at least by heat during firing.
[0012]
Preferably, together with the paraffin powder, burnable organic particles having a particle size equal to or smaller than the particle size of the paraffin powder are mixed with the magnetic powder, and the burnable organic particles are made of starch, gluten , Flour, corn starch, rice flour, potato starch, konjac flour, sucrose, polyvinyl alcohol, wood flour, bamboo flour, rice bran.
[0013]
The magnetic powder may be a ferrite powder of NiZn type, MgZn type, MnZn type or the like. In this case, for example, the ferrite powder is Fe₂O₃Is 50 to 58 mol%, MnO is 12 to 47 mol%, and ZnO is 3 to 30 mol%. Further, the ferrite powder is made of Fe₂O₃46 to 49 mol%, MgO 24 to 27 mol%, ZnO 18 to 21 mol%, MnO 4 to 7 mol%, and CuO 1 to 4 mol%. Further, the ferrite powder is made of Fe₂O₃43-50 mol%, ZnO 10-35 mol%, CuO 3-15 mol%, and the balance NiO.
[0014]
Further, the magnetic powder is pre-fired at a temperature of 500 to 1300 ° C., and then the pre-fired body is pulverized, and the pore-forming material or the pore-forming material and the burnable organic particles are mixed and fired after molding. Steps can be added.
[0015]
Furthermore, only the magnetic powder is pre-fired at a predetermined temperature, the pre-fired body is pulverized, and then granulated with a binder such as 1 to 2 wt% of polyvinyl alcohol. The pore material and the burnable organic particles may be mixed, and fired after molding.
[0016]
On the other hand, the radio wave absorption band according to the present invention is a radio wave absorber made of sintered ferrite, is configured in a porous tile shape, and has a high frequency and wide band radio wave absorption characteristic. In addition, it is also possible to form a fired body formed into a porous tile shape by each of the manufacturing methods described above.
[0017]
Further, it is preferable to include the above-described radio wave absorber (tile-shaped porous ferrite) and a non-magnetic layer integrally formed on at least one surface of the radio wave absorber. In this case, the nonmagnetic layer is preferably made of sludge from a river or lake or industrial waste sludge as a base material. Further, it is more preferable to form the porous ferrite into a three-layer structure in which the front and back surfaces of the ferrite are covered with a nonmagnetic layer.
[0018]
Since the paraffin used as the pore former has a low melting point and is thermally decomposed at a low temperature of 100 ° C. or lower, it can be volatilized smoothly in the initial stage of the firing step, and can be fired while preventing cracks and cracks. In addition, by using paraffin, continuous pores (pores) can be easily formed due to large plasticity at the time of molding.
[0019]
Further, by adjusting the mixing ratio between the burnable organic particles and the paraffin, it is possible to directly set the void portion in the sintered body. Therefore, the degree of porosity, that is, the distribution and ratio of pores, can be controlled by adjusting the mixing ratio and the particle size of these additives, and the density of the sintered body can be appropriately changed. Further, the radio wave absorption characteristics can be increased in frequency and broadened by making the porosity porous. In addition, the burnable organic substance also has a function of preventing the paraffin powder from sticking to a mold generated during molding.
[0020]
In the multilayer sintered body using the porous ferrite, the nonmagnetic layer covering the ferrite layer serves as a protective film layer, so that the strength can be improved and the corrosion resistance can be improved. In the production of a sintered body consisting of multiple layers, the shrinkage rate of the ferrite layer can be controlled by preparing an additive for making it porous, so that the shrinkage rate matches that of the non-magnetic layer. By setting so that a plurality of layers having different properties can be formed and fired at the same time. Therefore, each layer does not peel off at the interface.
[0021]
Further, when sludge is used as a raw material of the non-magnetic layer, the sludge can be recycled, which is preferable because labor can be saved. Of course, the non-magnetic layer is not limited to one formed from sludge. The sludge component is preferably a silica component SiO.₂Is 10 wt% or more. This is because, as the properties of the nonmagnetic layer (sludge) used in the present invention, it is necessary to keep the dielectric constant low, and the sintering temperature is suppressed to a ferrite sintering temperature (for example, 1300 ° C.) or lower. This is because it is necessary to keep Therefore, special sludge containing a large amount of Al or Cu is not preferable because the firing temperature becomes high.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
A first embodiment of the present invention will be described. The present embodiment is an example of a method for producing a porous ferrite (sintered body) using a NiZn-based ferrite material. In the production, a predetermined amount of a NiZn-based ferrite powder serving as a base material is weighed and calcined at a prescribed temperature, and then the calcined body is pulverized and granulated. The granulated ferrite powder is mixed with fine organic particles together with a thermoplastic powder having a low melting point and a binder function, followed by firing after molding. As a result, a porous sintered body is obtained by burning out both components during firing. The calcining temperature is, for example, 500 to 1300 ° C., and the particle size is granulated to, for example, 10 mm or less in pulverizing the calcined body.
[0023]
In the present embodiment, starch is used as the organic particles, and paraffin fine particles are used as the thermoplastic powder, and these are combined and added at a ratio of 0 to 30 wt%. The mixing ratio of the organic particles (starch) is increased in the range of 0 to 5 wt% in accordance with the increase in the amount of the thermoplastic powder (paraffin). As the paraffin, a fine powder having a particle size of 100 μm or less was used.
[0024]
Then, the mixed powder was filled in a mold, molded into a predetermined shape by applying pressure, and placed in an electric furnace and fired at 1080 ° C. for 2 hours. The sample (porous ferrite) was manufactured by processing the obtained sintered body into a toroidal shape (outer diameter: 7 mm, inner diameter: 3 mm, thickness: 3 mm).
[0025]
In order to demonstrate the effect of the manufacturing method according to the present invention, various samples were manufactured under different manufacturing conditions, and the high-frequency characteristics of each sample were measured. Specifically, a coaxial tube (outer diameter: 7 mm, inner diameter: 3 mm) serving as an S-parameter measurement jig and a vector network analyzer are used. Then, while the sample is set in the coaxial tube, S11 (complex reflectance) and S21 (complex transmittance) parameters are measured in a frequency band of 30 MHz to 60 GHz, and from these values, the material constant of the sample, that is, the complex relative magnetic permeability. (Μr) and the complex relative permittivity (εr) are obtained. Next, the obtained material constants are substituted into the following equations (1) and (2) to determine the return loss (Return Loss), and the radio wave absorption characteristics are evaluated from the return loss (RL). did.
[0026]
(Equation 1)

[0027]
FIG. 1 is a graph showing the density of each sample with the amounts of organic particles and thermoplastic powder added as parameters. The relationship between the amount of addition and the density of each sample is shown in the figure,
Addition amount: 0% by weight of sample a has a density of 5.1 g / cm³,
Addition amount: 5 wt% sample b has a density of 4.6 g / cm³,
Sample c having an addition amount of 10 wt% has a density of 4.0 g / cm.³,
Sample d having an addition amount of 15 wt% has a density of 3.7 g / cm.³,
Sample e having an addition amount of 20 wt% has a density of 3.2 g / cm.³,
Sample f having an addition amount of 30 wt% has a density of 2.7 g / cm.³
The result was obtained.
[0028]
Therefore, as is clear from the figure, the density of the manufactured sintered body has a correlation with the addition amount of the organic particles (starch) and the thermoplastic powder (paraffin), and the density decreases as the addition amount increases. It was confirmed. The sample a with the added amount of 0 wt% is a comparative example in which only the magnetic powder was fired without adding any organic particles and thermoplastic powder.
[0029]
In addition, each manufactured sample was observed with a microscope. As a result, it was confirmed that fine holes or continuous holes having a uniform size were uniformly distributed as a whole in the sample to which the organic particles and the thermoplastic powder were added (samples b to f). .
[0030]
That is, by preparing the mixture of the organic particles and the thermoplastic powder, the pores in the sintered body are directly set. Therefore, the degree of porosity, that is, the distribution and ratio of pores, can be controlled by adjusting the mixing ratio and the particle size of these additives, and the density of the sintered body can be appropriately changed. This leads to a higher frequency and a wider range of radio wave absorption characteristics by making the porosity large, as described later.
[0031]
Next, the manufacturing conditions of the sample will be described. The NiZn-based ferrite powder has a basic component of Fe₂O₃, NiO, ZnO, and other minor components. The magnetic powder of the base material is not limited to NiZn-based ferrite, but may be MgZn-based or MnZn-based ferrite powder.
[0032]
That is, the composition of the ferrite powder is, for example, Fe₂O₃Can be 50 to 58 mol%, MnO can be 12 to 47 mol%, and ZnO can be 3 to 30 mol%. Also, Fe₂O₃46 to 49 mol%, MgO 24 to 27 mol%, ZnO 18 to 21 mol%, MnO 4 to 7 mol%, and CuO 1 to 4 mol%. Furthermore, Fe₂O₃May be 43 to 50 mol%, ZnO is 10 to 35 mol%, CuO is 3 to 15 mol%, and the balance is NiO.
[0033]
As the thermoplastic powder, paraffin is used in the present invention, but a resin material such as polyester can also be used. Organic particles include starch, gluten, flour, corn starch, rice flour, starch, konjac flour, sucrose, polyvinyl alcohol (PVA), wood flour, bamboo flour, rice bran and the like. Since these organic substances are easily available and inexpensive, they are advantageous in terms of mass production.
[0034]
Further, the blending amount of the thermoplastic powder is set to 1 to 40 wt%, or the combined amount of the organic particles and the thermoplastic powder is set to 1 to 40 wt%, and the particle size of the organic particles and the thermoplastic powder is reduced. The particle size may be appropriately adjusted according to the mixing ratio. The porosity may be set to 0 to 50% by adjusting the particle size and the mixing ratio.
[0035]
Further, only the magnetic powder is pre-fired at a temperature of 500 to 1300 ° C., and then the pre-fired body is pulverized, mixed with organic particles and thermoplastic powder, and fired after molding. In the pulverization of the calcined body, the particle size may be reduced to 10 mm or less.
[0036]
In addition, only the magnetic powder is pre-fired at a predetermined temperature, and then the pre-fired body is pulverized and granulated with a binder such as 1 to 2 wt% of polyvinyl alcohol, and thereafter, the organic particles and the thermoplastic powder are mixed. Then, it is fired after molding. In the pulverization of the calcined body, the particle size is preferably set to 1 mm or less.
[0037]
In addition, if the amount of the thermoplastic powder is excessively increased, the adhesiveness to a mold during compression molding increases, so that the moldability deteriorates. This can be prevented by adjusting the addition of the thermoplastic powder to an appropriate amount corresponding to the amount of the organic particles added together, and the particle diameter of the organic particles is preferably equal to or less than the thermoplastic powder. .
[0038]
(Measurement result)
As for the return loss of each of the samples a to f, results as shown in FIGS. 2 to 7 were obtained. In each figure, the return loss when the thickness of each sample is changed as a parameter is obtained. Focusing on the characteristic region where the return loss is 20 dB or more, that is, the radio wave absorption rate is 99% or more, the band of the radio wave absorption is considered. It can be seen that the frequency shifts to the high frequency side as the density decreases, and the bandwidth also increases.
[0039]
As shown in FIG. 5, when the thickness of the sample d is 0.8 cm, the return loss is 20 dB or more in the frequency band of about 100 to 750 MHz, which corresponds to almost the entire range of the VHF to UHF band.
[0040]
As described above, by changing the density (porosity) of the sintered body, the band of the reflection attenuation (radio wave absorption) can be easily changed, and the radio wave absorption characteristics can be appropriately expressed in a desired frequency band. be able to. In other words, the radio wave absorber can be made lighter by making it porous, and the dielectric constant can be greatly reduced without significantly lowering the magnetic permeability in the target frequency band. Can be
Therefore, according to the present invention, good radio wave absorption characteristics can be obtained in a high frequency band, and the radio wave absorption band can be appropriately set in a wide band.
[0041]
(Application to radio wave absorber)
Further, as a result of the trial production as described above, according to the method for producing a porous ferrite according to the present invention, it is easy to form a fired body in a tile shape, and a square sintered body having a side of 10 cm or more can be obtained. I confirmed that it can be manufactured. This has a relatively large area size and is preferably applied as a radio wave absorber. That is, there is an advantage that such tile-shaped sintered bodies (ferrite tiles) can be laid in a large number and arranged to have an arbitrary area size.
[0042]
When the radio wave absorber is used for countermeasures against TV ghost failure, the thickness must be increased as the porosity increases in order to obtain effective radio wave absorption characteristics with a porous material. However, even if the increase in thickness is taken into consideration, a porous material can reduce the weight by about 10% or more under all conditions.
[0043]
Further, when the thickness of the sample d is 0.8 cm, the return loss is about 20 dB or more at about 100 to 750 MHz, and has an extremely wide band radio wave absorption characteristic corresponding to almost the entire range of the VHF to UHF band and having no practical problem.
[0044]
(Conclusion)
As is clear from the above, in the production method according to the present invention, the magnetic powder of the base material is mixed with fine organic particles together with a thermoplastic powder having a low melting point and a binder function. Compression molding. For this reason, the thermoplastic powder functions as a binder, and the kneading state during compression molding can be prevented from becoming uneven.
[0045]
As a result, the strength of the molded article can be increased, and the molded article in which cracks and cracks are prevented can be easily obtained without adjusting the water content when kneading the mixed powder. In addition, the thermoplastic powder having a low melting point volatilizes smoothly in the initial stage of the firing process, and can be fired with cracks and cracks prevented, and the pores are uniformly distributed in the sintered body. And porous ferrite of good quality can be obtained. Further, in this case, the number of manufacturing steps is the same as in the conventional case, and there is no additional step.
[0046]
In the production of the sample, a sample in which only the thermoplastic powder was mixed without mixing the organic particles at all with the NiZn-based ferrite powder as the base material was also produced. As a result of observation with a microscope, it was confirmed that even in this sample, fine pores were uniformly distributed throughout, and that porous ferrite was obtained.
[0047]
By the way, in the first embodiment described above, the base ferrite powder is preliminarily calcined and then pulverized and granulated. However, a process procedure in which pulverization and granulation are not performed may be adopted. That is, in the second embodiment, the pre-baked ferrite powder is mixed with fine organic particles together with a thermoplastic powder having a low melting point and a binder function, followed by firing after molding. As a result, a porous sintered body is obtained by burning out both components during firing.
[0048]
Again, in order to demonstrate the effects of the manufacturing method according to the present invention, various samples were manufactured under different manufacturing conditions, and the high-frequency characteristics of each sample were measured. The procedure of this characteristic measurement is the same as that of the first embodiment, and the description is omitted.
[0049]
(Measurement result)
The return loss of each sample was obtained as shown in FIGS. Here, for each sample,
Addition amount: 5 wt% sample b has a density of 4.7 g / cm³,
Sample d having an addition amount of 15 wt% has a density of 3.5 g / cm.³,
Sample e having an addition amount of 20 wt% has a density of 3.2 g / cm.³,
Sample f having an addition amount of 30 wt% has a density of 2.8 g / cm.³
The result was obtained.
[0050]
In each figure, the return loss when the thickness of each sample is changed as a parameter is obtained. When attention is paid to the characteristic region where the return loss is 20 dB or more, that is, the radio wave absorption rate is 99% or more, It was found that the absorption band shifted to the high frequency side as the density decreased, and the band width was also widened. The same effect was confirmed in the powder that had been simply calcined without performing pulverization and granulation.
[0051]
Also, in this case, it was confirmed that when the porosity was large, the radio wave absorption performance was inferior to that when the granulated powder was used. However, regarding the calcination, the calcined powder has a large particle size, so that its decomposition and combustion are quickly and easily performed. Further, since no pulverization or granulation is performed, the number of steps is reduced, which is advantageous in productivity and cost. Note that the other configuration, operation, and effect are the same as those of the first embodiment, and a detailed description thereof will be omitted.
[0052]
FIG. 12 shows a third embodiment of the present invention. The present embodiment is an example of an application of the above-described porous ferrite according to the present invention, and is configured as a radio wave absorber having a multilayer structure.
[0053]
Specifically, as shown in FIG. 12, the porous ferrite layer 2 is formed in a three-layer structure in which the front and back surfaces of the porous ferrite layer 2 are covered with non-magnetic layers 1 and 3, and the non-magnetic layers 1 and 3 are formed of sludge from rivers or lakes. Use industrial waste sludge as a base.
[0054]
Here, a NiZn-based ferrite powder is blended as a raw material of the ferrite layer 2 and pre-fired at a temperature of about 900 ° C., and the pre-fired ferrite powder is mixed with 5 to 30 wt. % Mixed powder was used. At this time, since the mixing ratio of paraffin greatly affects the shrinkage of the ferrite layer 2, it is preferable to predict the mixing ratio from the shrinkage of the nonmagnetic layers 1 and 3. That is, it is set so as to match the contraction rate of the nonmagnetic layers 1 and 3.
[0055]
Sludge collected from a lake was used as a raw material for the nonmagnetic layers 1 and 3. The sludge is dried and turned into a powder through a pulverizing process. The sludge used in the present embodiment was river sludge, and the composition was as shown in the following table.
[0056]
[Table 1]

[0057]
Next, each of the prepared raw material powders was sequentially filled in a mold and compression-molded so as to be separated into three layers of a nonmagnetic layer 1, a ferrite layer 2, and a nonmagnetic layer 3 in this order. In the present embodiment, a mold having a square shape with a side of 12 cm was used, and was fired at a temperature of 1050 ° C. for 2 hours in an electric furnace. Observation of the fired sintered body with a microscope revealed that the sintered body did not crack, swell, or warp, and that the interface between the porous ferrite layer 2 and the nonmagnetic layers 1 and 3 did not have any cracks. It was confirmed that it was bonded. Since the contraction rates of the three layers are set to be substantially equal, there is no separation at the interface.
[0058]
Here, in order to demonstrate the effect of the manufacturing method according to the present invention, various samples were manufactured under different manufacturing conditions, and the high-frequency characteristics of each sample were measured. The procedure of this characteristic measurement is the same as that of the first embodiment, and the description is omitted.
[0059]
(Measurement result)
The return loss of each sample was obtained as shown in FIG. 13 (document g) and FIG. 14 (document h). Here, each sample is one in which the thickness of each layer is appropriately set,
In sample g, the nonmagnetic layer 1 was 1 cm,
Ferrite layer 2 is 1 cm,
The nonmagnetic layer 3 is 0 to 0.9 cm,
In the sample h, the nonmagnetic layer 1 is 0 to 2.4 cm,
Ferrite layer 2 is 1 cm,
The nonmagnetic layer 3 is 0.3 cm.
[0060]
As shown in FIG. 12, the multi-layer sintered body is set so as to cover the non-magnetic material layer 1 side with respect to the object 4 for radio wave absorption. The return loss is obtained when the thickness of the layer 1 or the nonmagnetic layer 3 is changed as a parameter. Focusing on the characteristic region where the return loss is 20 dB or more, that is, the radio wave absorption rate is 99% or more, the radio wave absorption characteristics deteriorate as the thickness of the nonmagnetic layer 3 serving as the surface layer increases (FIG. 13). It was confirmed that by appropriately setting the thickness of the nonmagnetic layer 1 serving as the inner layer, it is possible to increase the radio wave absorption and increase the radio wave absorptance (FIG. 14).
[0061]
Therefore, in the case of a multilayer (three-layer) sintered body using a porous ferrite, when applied to a radio wave absorber, the surface layer is made thin enough to obtain good corrosion resistance, and the inner side is in contact with the object 4. The layer may be set to a thickness at which the radio wave absorption characteristics are optimized. This makes it possible to obtain a radio wave absorber that is preferably installed outdoors or the like where mechanical strength and weather resistance are required.
[0062]
It is preferable to use sludge as a raw material of the nonmagnetic layers 1 and 3 because the sludge can be recycled and labor can be saved. However, the present invention is not limited to this, and it is clear that the use of a material having a low dielectric constant as the raw material of the nonmagnetic layers 1 and 3 further improves the radio wave absorption characteristics. Although sludge is used for the purpose of industrial recycling, it is clear that other non-magnetic materials can be applied, and each of the non-magnetic layers 1 and 3 may be formed from a different material.
[0063]
By the way, in the production of the multilayer sintered body, the shrinkage rate of the ferrite layer 2 can be controlled by preparing an additive for making the ferrite layer porous. Can be compression-molded and fired in the same step. As described above, since the multilayer can be molded and fired in the same process, the production becomes easy, the productivity is high, and the yield is improved, so that the cost can be reduced.
[0064]
In a multilayer sintered body, the number and thickness of the ferrite layer and the non-magnetic layer can be changed as appropriate, so that the radio wave absorption band can be selectively set, and the characteristics as the radio wave absorber can be controlled. It has the merit of being able to meet various characteristic requirements and expanding its application.
[0065]
(Note)
1. In a method for producing a porous ferrite in which magnetic powder is mixed with fine organic particles to produce a mixed powder, and baked after molding to burn off the organic particles, the organic particles have a low melting point and a low binder function. A porous ferrite can be produced by mixing and mixing certain thermoplastic powders and burning off both components during the firing.
[0066]
2. Further, the thermoplastic powder may be a resin material such as paraffin or polyester.
[0067]
3. In the above-mentioned 1 and 2, the organic particles can be composed of starch, gluten, flour, corn starch, rice flour, potato starch, konjac flour, sucrose, polyvinyl alcohol, wood flour, bamboo flour, rice bran and the like.
[0068]
4. In the above items 1 to 3, the magnetic powder may be a ferrite powder of NiZn, MgZn, MnZn or the like. In this case, the ferrite powder is Fe₂O₃Is 50 to 58 mol%, MnO is 12 to 47 mol%, and ZnO is 3 to 30 mol%.₂O₃Is 46 to 49 mol%, MgO is 24 to 27 mol%, ZnO is 18 to 21 mol%, MnO is 4 to 7 mol%, and CuO is 1 to 4 mol%.₂O₃43-50 mol%, ZnO 10-35 mol%, CuO 3-15 mol%, and the balance NiO.
[0069]
5. In the above items 1 to 4, the blending amount of the thermoplastic powder can be 1 to 40 wt%.
[0070]
6. In 1 to 5 described above, the combined amount of the organic particles and the thermoplastic powder may be 1 to 40 wt%.
[0071]
7. In the above items 1 to 6, the particle size of the organic particles and the thermoplastic powder is set to about 10 to 1000 μm, and the particle size can be appropriately adjusted according to the mixing ratio. In this case, the porosity may be set to 0 to 50% by adjusting the particle size and the mixing ratio.
[0072]
8. In the above items 1 to 7, only the magnetic powder is calcined at a temperature of 500 to 1300 ° C., and then the calcined body is pulverized to mix the organic particles and the thermoplastic powder. can do. In this case, in the pulverization of the calcined body, the particle size may be set to 10 mm or less.
[0073]
9. In the above items 1 to 8, only the magnetic powder is calcined at a predetermined temperature, and then the calcined body is pulverized and granulated with a binder such as polyvinyl alcohol of 1 to 2% by weight. And the thermoplastic powder can be mixed and fired after molding. In this case, in the pulverization of the calcined body, the particle size may be set to 1 mm or less.
[0074]
10. By using the porous ferrite formed by forming the fired body into a tile shape by the manufacturing method described in 1 to 15 above, a tile-shaped radio wave absorber having a radio wave absorption characteristic of a higher frequency and a wider band can be obtained. Can be configured. Preferably, the film layer of the porous ferrite and the film layer made of a non-magnetic material are integrally formed in a multilayer. In this case, the nonmagnetic layer can use sludge from rivers and lakes or industrial waste sludge as a base material.
[0075]
In each of the production methods described in the above supplementary notes, a thermoplastic powder having a low melting point and a binder function is mixed with the magnetic powder of the base material together with fine organic particles, and the mixture is compression-molded. For this reason, the thermoplastic powder functions as a binder, and the kneading state during compression molding can be prevented from becoming uneven.
[0076]
As a result, the strength of the molded product can be increased, and a molded product in which cracks and cracks are prevented can be easily obtained without particularly adjusting the water content when kneading the mixed powder. Paraffin used as a thermoplastic powder has a low melting point and is thermally decomposed at a low temperature of 100 ° C. or less, so that it can be volatilized smoothly in the initial stage of the firing step, and can be fired with cracks and cracks prevented.
[0077]
In addition, by preparing the mixture of the organic particles and the thermoplastic powder, the pore portion in the sintered body is directly set. Therefore, the degree of porosity, that is, the distribution and ratio of pores, can be controlled by adjusting the mixing ratio and the particle size of these additives, and the density of the sintered body can be appropriately changed. Further, the radio wave absorption characteristics can be increased in frequency and broadened by making the porosity porous.
In the multilayer sintered body using the porous ferrite, the nonmagnetic layer covering the ferrite layer serves as a protective film layer, so that the strength can be improved and the corrosion resistance can be improved.
[0078]
In the production of a multilayer sintered body, the shrinkage rate of the ferrite layer can be controlled by preparing an additive for making the ferrite layer porous. In addition, multiple layers having different properties can be simultaneously molded and fired.
[0079]
【The invention's effect】
As described above, in the method for producing a porous ferrite according to the present invention, since a paraffin powder having a diameter of 100 μm or less was used as a pore-forming material, it was volatilized smoothly in the initial stage of the firing step, and firing was performed with cracks and cracks prevented. Since the pores are uniformly distributed in the sintered body, good quality porous ferrite can be obtained. Further, in this case, the number of manufacturing steps is the same as in the conventional case, and there is no additional step.
[0080]
When the mixture of the burnable organic particles and the paraffin powder is prepared, the porosity in the sintered body is directly set, and the degree of porosity, that is, the distribution and ratio of vacancies, and the like are determined. Can be controlled, and the density of the sintered body can be appropriately changed. As a radio wave absorber, it is possible to reduce the weight by making it porous with a high porosity, and it is possible to greatly reduce the dielectric constant without significantly lowering the magnetic permeability in the target frequency band. The bandwidth and frequency can be increased.
[0081]
A sintered body composed of a plurality of layers in which a nonmagnetic layer is integrally provided on at least one surface of a porous ferrite can improve strength because the nonmagnetic layer covering the ferrite layer becomes a protective film layer, and has good corrosion resistance. Can be. Therefore, in application to a radio wave absorber, it can be preferably applied to construction outdoors where mechanical strength and weather resistance are required.
[0082]
In a sintered body composed of multiple layers, the number and thickness of the ferrite layer and the nonmagnetic layer can be changed as appropriate, so that the radio wave absorption band can be selectively set and the characteristics of the radio wave absorber can be controlled. It has the merit that it can meet various characteristic requirements and its application can be expanded.
[Brief description of the drawings]
FIG. 1 is a graph showing the density of each sample with the addition amount of organic particles and thermoplastic powder as a parameter for explaining the first embodiment of the present invention.
FIG. 2 is a graph showing the return loss of a sample a with frequency as a parameter for demonstrating the effect of the first embodiment of the present invention.
FIG. 3 is a graph showing the return loss of a sample b with frequency as a parameter for demonstrating the effect of the first embodiment of the present invention.
FIG. 4 is a graph showing the return loss of a sample c using frequency as a parameter to demonstrate the effect of the first embodiment of the present invention.
FIG. 5 is a graph showing the return loss of a sample d with frequency as a parameter for demonstrating the effect of the first embodiment of the present invention.
FIG. 6 is a graph showing the return loss of a sample e with frequency as a parameter for demonstrating the effect of the first embodiment of the present invention.
FIG. 7 is a graph showing the return loss of a sample f with frequency as a parameter for demonstrating the effect of the first embodiment of the present invention.
FIG. 8 is a graph showing the return loss of a sample b using frequency as a parameter to demonstrate the effect of the second embodiment of the present invention.
FIG. 9 is a graph showing the return loss of a sample d using frequency as a parameter for demonstrating the effect of the second embodiment of the present invention.
FIG. 10 is a graph showing the return loss of a sample e with frequency as a parameter for demonstrating the effect of the second embodiment of the present invention.
FIG. 11 is a graph showing the return loss of a sample f with frequency as a parameter for demonstrating the effect of the second embodiment of the present invention.
FIG. 12 is a cross-sectional view of a multilayer sintered body (wave absorber) using a porous ferrite according to a third embodiment of the present invention.
FIG. 13 is a graph showing the return loss of sample g using frequency as a parameter.
FIG. 14 is a graph showing the return loss of a sample h using frequency as a parameter.
[Explanation of symbols]
1,3 Non-magnetic layer
2 Porous ferrite layer
4 Object

Claims (9)

In the method for producing a porous ferrite, which forms predetermined holes by burning out the pore-forming material by mixing the pore-forming material with the magnetic powder and firing after molding,
The method for producing a porous ferrite, wherein the pore former contains fine paraffin powder of 100 μm or less. Along with the paraffin powder, burnable organic particles having a particle size equal to or less than the particle size of the paraffin powder to be mixed with the magnetic powder,
2. The method according to claim 1, wherein the burnable organic particles are at least one of starch, gluten, flour, corn starch, rice flour, potato starch, konjac flour, sucrose, polyvinyl alcohol, wood flour, bamboo flour, and rice bran. A method for producing the porous ferrite according to the above. The method according to claim 1, wherein the magnetic powder is a ferrite powder of NiZn, MgZn, MnZn, or the like. Temporarily baking the magnetic powder at a temperature of 500 to 1300 ° C.,
Next, the calcined body is crushed,
Mixing the pore former or the pore former and the burnable organic particles,
The method for producing a porous ferrite according to any one of claims 1 to 3, wherein the firing is performed after molding. Preliminarily firing only the magnetic powder at a predetermined temperature,
Crushing the calcined body,
Next, granulate with a binder such as polyvinyl alcohol of 1 to 2 wt%,
Thereafter, the pore former or the pore former and the burnable organic particles are mixed,
The method for producing a porous ferrite according to any one of claims 1 to 4, wherein the method is performed after forming. A sintered ferrite radio wave absorber,
A radio wave absorber characterized in that it is formed in a porous tile shape and has a radio wave absorption characteristic of a higher frequency and a wider band. A radio wave absorber comprising a fired body formed into a porous tile shape by the manufacturing method according to claim 1. A radio wave absorber comprising: the radio wave absorber according to claim 6; and a nonmagnetic layer integrally formed on at least one surface of the radio wave absorber. 9. The radio wave absorber according to claim 8, wherein the nonmagnetic layer uses sludge from a river or lake or industrial waste sludge as a base material. 10.

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