JP3715582B2 - Magnetic material - Google Patents

Description

【００６２】
図４に、供試体１について、温度対エントロピーの変化量の関係を示す。図４に見るように、外部磁場を０から０．５テスラまで、または０から１テスラまでの間で変化させた場合、温度約１９５Ｋから２１０Ｋの間に、エントロピーの変化量にピークが現われている。
【００６３】
表１に、各供試体について、エントロピーの変化量にピークが現われる温度（Ｔpeak）における、磁場変化ΔＨに対するエントロピーの変化量（ΔＳmax）の計算結果を示す。なお、表１中には、比較のため、磁気冷凍作業用の磁性材料のプロトタイプであるＧｄの電子磁気スピン系のエントロピーの変化量も、併せて示してある。
【００６４】
【表１】

【００６５】
表１から分るように、供試体１〜４では、１テスラ以下の低磁場において大きなエントロピーの変化が観察されており、Ｇｄに対して遥かに優位であることが分る。これに対して、供試体５〜７では、Ｇｄと同程度またはそれ以下のエントロピー変化が測定されており、供試体１〜４と比べて大幅に劣っていることが分る。
【００６６】
なお、先に記載したように、磁性材料を用いて実際に磁気冷凍を行う場合には、磁場の変化（ΔＨ）に伴うエントロピー変化量ΔＳ（Ｔ，ΔＨ）の大きさのみではなく、エントロピー変化量にピークが現れる温度幅（“実効的な温度幅”）も重要な要素である。以下に、目安として、各供試体について、ΔＳ（Ｔ，ΔＨ＝１テスラ）及びΔＳ（Ｔ，ΔＨ＝０．５テスラ）についての温度幅を示す。
【００６７】
外部磁場変化ΔＨが１テスラの場合、供試体１〜４のエントロピー変化量のピークの温度幅は、下記の通りである：
供試体１：約１９０〜２１０Ｋ
供試体２：約１９５〜２２０Ｋ
供試体３：約２１５〜２４５Ｋ
供試体４：約１９５〜２１７Ｋ
また、外部磁場変化ΔＨが０．５テスラの場合、供試体１〜４のエントロピー変化量のピークの温度幅は、下記の通りである：
供試体１：約１９４〜２０７Ｋ
供試体２：約１９７〜２１８Ｋ
供試体３：約２２０〜２３７Ｋ
供試体４：約１９６〜２１８Ｋ
このように、供試体１〜４は、外部磁場変化ΔＨが０．５テスラの場合であっても、１０Ｋ以上の温度幅を有しており、実用上問題がないと言える。
【００６８】
以上のように、供試体１〜４では、特性温度（Ｔｃｒｉ）よりも高温側において、１テスラ以下の比較的低い磁場で電子磁気スピンの配列の状態に大きな変化が生じ、磁化曲線に変曲点が現れる。更に、これらの供試体では、磁化曲線に変曲点が現れる温度の近傍において、電子磁気スピン系に極めて大きなエントロピー変化が観察されることが確認された。
【００６９】
なお、供試体１〜４については、Ｘ線回折により、主相は立方晶の構造であり、第二層としてαＦｅ相が僅かに析出していることが判明した。
【００７０】
（例２）
下記の組成を備えた３種類の供試体を作製し、その磁化曲線及び磁場変化に伴うエントロピー変化について調べた。なお、下記の供試体の内、供試体１１及び１２は本発明に基づく磁性材料に該当し、供試体１３は比較例である。
【００７１】
供試体１１：Ｆｅ₆₇Ｈｆ₂₈Ｔａ₅
供試体１２：Ｆｅ₆₇Ｈｆ₂₇Ｔａ₆
供試体１３：Ｆｅ₆₇Ｈｆ₂₉Ｔａ₄
アーク溶解によって上記各組成の材料を調整した後、真空中で９５０℃から１０００℃の温度で約１００時間の均一化熱処理を施し、母合金を作製した。次いで、この母合金から、プラズマスプレー法を用いて粒子状の供試体を作製した。その結果、０．１ｍｍ〜０．３ｍｍ程度の長径を有する粒子が多く得られた。このようにして得られた各供試体について、その磁化曲線を測定した。
【００７２】
図５に、供試体１１の、温度２３７．５Ｋから３０７．５Ｋまでの範囲の磁化曲線を示す。図６に、供試体１３の、温度２７７．５Ｋから３３２．５Ｋまでの範囲の磁化曲線を示す。
【００７３】
供試体１１の磁化曲線（図５）では、Ｔ＝２７７．５Ｋより低温の領域においては、非常に低い外部磁場によって磁化が急激に増大し、１テスラ以下の磁場の範囲で、磁化の磁場に対する二回微分係数が負（上に凸）であることが分る（図５中の符号ａ〜ｅ）。
【００７４】
このような磁化曲線の形状は、Ｔ＝２８０Ｋ近傍で大きく変化している。Ｔ＝２８０．５Ｋ（符号ｆ）及び２８２．５Ｋ（符号ｇ）においては、０．０１テスラ程度の非常に低い磁場では小さな磁化の値を示しているが、磁場の増大に伴い、それぞれ、磁場Ｈ＝０．２７テスラ近傍とＨ＝０．５テスラ近傍で磁化が急激に増大し、磁化曲線に変曲点が現れている。磁化の増大は、変曲点を通過した後、次第に緩やかになる。
【００７５】
例１の供試体１の場合と同様に、更に温度を上げてゆくと、Ｈｃ（磁化曲線に変曲点が現れる磁場）の値が増大するとともに、Ｈｃ近傍での磁化の変化量が減少する。Ｔ＝２９２．５Ｋ（符号ｊ）より高温になると、Ｈｃ近傍での転移は次第に消失する（符号ｊ〜ｌ）。
【００７６】
なお、供試体１２についても、同様な磁化曲線が観測され、Ｔ＝２４５Ｋ近傍で、１テスラ以下の磁場において磁化曲線に変曲点が現れた。
【００７７】
これに対して、供試体１３の磁化曲線（図６）では、先の例における供試体６（比較例）の場合と同様に、磁気相転移温度の近傍で磁化の値が大きく変化するものの、１テスラ以下の磁場の範囲で、磁化の磁場に対する二回微分係数は常に負であり、磁化曲線の形状（上に凸）には大きな変化が見られないことが分る。
【００７８】
次に、上記の３種の各供試体について、外部磁場を変化させたときの電子磁気スピン系のエントロピーの変化量ΔＳ（Ｔ，ΔＨ）を、例１の場合と同様な方法で評価した。表２に、各供試体の、エントロピーの変化量ΔＳがピークを示す温度（Ｔpeak）における磁場変化ΔＨに対するエントロピーの変化量（ΔＳmax）の計算結果を示す。
【００７９】
【表２】

【００８０】
表２から分るように、供試体１１及び１２では、１テスラ以下の低磁場において大きなエントロピーの変化が観察されており、供試体１３と比べて遥かに優位であることが分る。
【００８１】
さらに、Ｆｅを６７原子％程度含み、Ｔｉを２５〜３０原子％程度含むＦｅ−Ｔｉ−Ｓｃ系の磁気物質においても、上記の供試体１１及び１２と同様な結果が得られた。
【００８２】
（例３）
下記の組成を備えた２種類の供試体を作製し、その磁化曲線及び磁場変化に伴うエントロピー変化について調べた。
【００８３】
供試体２１：Ｍｎ_63.4Ｃｒ_3.3Ｓｂ_33.3
供試体２２：Ｍｎ₅₀Ａｓ₃₅Ｓｂ₁₅
粉末状の原料を調合し、坩堝に封入して、８００℃〜９５０℃の温度で長時間保持して（供試体２１では約２日間、供試体２２では約１週間）、ゆっくり反応させた後、真空中で５５０℃から７００℃の温度で約１００時間の均一化熱処理を施した。このようにして得られた各供試体について、その磁化曲線を測定した。
【００８４】
図７に、供試体２１の、温度３００Ｋから３１５Ｋまでの範囲の磁化曲線を示す。供試体２１の温度３１５Ｋの磁化曲線では、１テスラ以下の磁場の範囲で、磁化の磁場に対する二回微分係数が常に負（上に凸）であることが分る（図７中の符号ｇ）。
【００８５】
供試体２１では、供試体１や１１の場合とは逆に、温度を下げて行くと、磁化曲線に下に凸な形状の部分が現れるようになる。
【００８６】
温度３１０Ｋ（符号ｃ）では、磁場０．４テスラ以下の範囲で、磁化曲線は上に凸な形状を示す。磁場０．４テスラ近傍で変曲点が現われ、磁場約０．４テスラから０．８テスラまでの範囲では、磁化曲線は下に凸な形状となる。更に磁場が増加すると、磁場０．８テスラ近傍で再び変曲点が現われ、磁場０．９テスラ以上の範囲では、磁化曲線は上に凸な形状に戻る。
【００８７】
温度を下げて行くと、磁化曲線の形状が下に凸から上に凸な形状に変化する変曲点が高磁場側へシフトし、３０７．５Ｋ、３０５Ｋ、３０２．５Ｋ、３００Ｋ（符号ｄ〜ｇ）の各温度において磁化曲線の変曲点の位置は、それぞれ、磁場１テスラ、２テスラ、２．６テスラ、３．５テスラ程度となった。
【００８８】
また、供試体２２では、Ｔ＝２３２．５Ｋにおいて、磁場０．８テスラ近傍で、磁化曲線に変曲点が現れ、磁場０．８テスラ以下の範囲では磁化曲線は下に凸な形状であり、磁場０．８テスラ以上の範囲で上に凸な形状になることが観測された。
【００８９】
供試体２２では、温度２００Ｋから２３０Ｋの範囲では、磁場１テスラ以下の範囲で、磁化曲線は常に上に凸な形状である。温度を上げて行くと、温度２３２．５Ｋでは、上記のように、磁化曲線の形状が下に凸から上に凸な形状に変化する。更に温度を上げて行くと、磁化曲線の変曲点は高磁場側にシフトし、２４０Ｋでの磁化曲線の変曲点の位置は、磁場３．８テスラとなった。
【００９０】
次に、上記の供試体２１及び２２について、外部磁場を変化させたときの電子磁気スピン系のエントロピーの変化量ΔＳ（Ｔ，ΔＨ）を、例１の場合と同様な方法で評価した。供試体２１及び２２では、磁場を０テスラか１テスラの範囲で変化させた場合、それぞれ、３１１Ｋ及び２３１Ｋ近傍で、エントロピー変化にピークが観察された。
【００９１】
表３に、各供試体の、エントロピーの変化量ΔＳがピークを示す温度（Ｔpeak）における磁場変化ΔＨに対するエントロピーの変化量（ΔＳmax）を、単位磁化当たりに値に換算した値を示す。
【００９２】
【表３】

【００９３】
表３から分るように、供試体２１及び２２では、１テスラ以下の低磁場において、単位磁化当たりのエントロピーの変化量は、Ｇｄと比べて遥かに優位であることが分る。
【００９４】
なお、先に記載したように、磁性材料を用いて実際に磁気冷凍を行う場合には、エントロピー変化量にピークが現れる温度幅（“実効的な温度幅”）も重要な要素である。以下に、目安として、各供試体について、ΔＳ（Ｔ，ΔＨ＝１テスラ）及びΔＳ（Ｔ，ΔＨ＝０．５テスラ）についての温度幅を示す。
【００９５】
外部磁場変化ΔＨが１テスラの場合、供試体２１及び２２のエントロピー変化量のピークの温度幅は、下記の通りである：
供試体２１：約３０４〜３１５Ｋ
供試体２２：約２１４〜２３６Ｋ
また、外部磁場変化ΔＨが０．５テスラの場合、供試体２１及び２２のエントロピー変化量のピークの温度幅は、下記の通りである：
供試体２１：約３０５〜３１６Ｋ
供試体２２：約２１５〜２３５Ｋ
このように、供試体２１及び２２は、外部磁場変化ΔＨが０．５テスラの場合であっても、１０Ｋ以上の温度幅を有している。
【００９６】
（磁気冷凍システムの構成について）
本発明に基づく磁性材料を用いる磁気冷凍システムは、主要な構成要素として、磁気冷凍作業室、導入配管、排出配管及び永久磁石を備える。磁性材料は磁気冷凍作業室の内部に充填される。熱交換媒体は、導入配管を介して磁気冷凍作業室の中に導入され、排出配管を介して排出される。永久磁石は、磁気冷凍作業室の近傍に配置される。磁気冷凍作業室に対する永久磁石の相対位置を変化させることによって、磁性材料に対する磁場の印加及び除去を行う。磁性材料は、磁場を除去した時に冷却される。熱交換媒体は、このようにして冷却された磁性材料との熱交換によって冷却される。
【００９７】
好ましくは、上記の排出配管は二つの系統に分けられる。第一の排出配管は、磁気冷凍作業室から内部の予冷に使用された熱交換媒体を取り出す際に使用される。第二の排出配管は、磁気冷凍作業室から内部で冷却された熱交換媒体を取り出す際に使用される。磁気冷凍作業室に対する永久磁石の相対位置を変化させるため、駆動装置が設けられ、この駆動装置に永久磁石が取り付けられる。永久磁石の相対位置の変化に同期させて、磁気冷凍作業室からの熱交換媒体の排出経路を第一排出配管と第二排出配管の間で切替えることによって、磁気冷凍サイクルが構成される。
【００９８】
好ましくは、磁性材料は、前記磁気冷凍作業室の内部に５０％以上７５％以下の体積充填率で充填される。
【００９９】
上記の磁気冷凍システムにおいて、磁性材料は、磁気冷凍作業室の中に、熱交換媒体の流路となる空間が確保されるような状態で充填される。ここで、磁気冷凍作業室内での磁性材料の充填率が低い場合には、熱交換媒体との間での熱交換の際、熱交換媒体の流れによって磁性材料が攪拌され、互いに衝突する。このような衝突は、磁性材料にクラックを生じさせ、更にその破壊を招く。磁性材料の破壊により生じた微細粉は、熱交換媒体の圧力損失を高め、冷凍能力を低下させる要因となる。悪い場合には微粉が配管の一部に堆積して詰まり、熱交換媒体の流れを阻害する。従って、そのような事態を回避するために、磁性材料は、磁性冷凍作業室の内部に５０％以上７５％以下の体積充填率で収容されていることが好ましい。さらには、６０％以上７０％以下の体積充填率で収容されていることがより好ましい。
【０１００】
好ましくは、上記の磁性材料として、粒径（長径）が０．１ｍｍ以上１．５ｍｍ以下であり、その８７ｗｔ％以上が、アスベクト比が２以下である粒子を使用する。
【０１０１】
高い冷却能力を実現するためには、磁気冷凍作業室の内部に充填された磁性材料と熱交換媒体との間で熱交換が充分に行われることが重要である。熱交換を充分に行わせるためには、磁性材料の比表面積を大きくする必要がある。本発明の磁性材料の場合、比表面積を大きくするために、粒径を小さく設定することが効果的である。但し、粒径が小さ過ぎる場合には、熱交換媒体の圧力損失が増大するので、これを勘案して、最適な粒径を選択する必要がある。ここで、上記の磁性材料の粒子径は、好ましくは、０．１〜１．５ｍｍ程度であり、更に好ましくは、０．２〜０．８ｍｍ程度である。
【０１０２】
また、上記の磁性材料の粒子形状は、表面に突起がない滑らかな形状であることが好ましく、例えば、球形または回転楕円体形であることが好ましい。このような形状にすることによって、粒子の破壊に伴う微細粉の発生を防止するとともに、熱交換媒体の圧力損失の増大を抑えることができる。
【０１０３】
例えば、磁気冷凍作業室に充填された粒子の内、８７％ｗｔ以上の粒子がアスペクト比が２以下の形状を有することが好ましい。これは、ほぼ球形状の粒子に、アスペクト比２以上の異形粒子を混在させて実験を行ったところ、異形粒子の混在量が１３％以上の場合には、熱交換媒体の流れに長期間曝される結果、微細粉が発生し、流体の圧力損失が増大してしまったからである。
【０１０４】
なお、熱交換媒体としては、熱サイクルの運転温度域に合わせて、鉱物油、溶剤、水やこれらの混合液などを選択することができる。上記の磁性材料の粒子径も、使用される熱交換媒体の粘性（表面張力）やポンプの能力に応じて、上記の範囲内で最適な粒子径を選ぶことが望ましい。
【０１０５】
図８に、本発明に基づく磁性材料が使用される磁気冷凍システムの概略構成を示す。図９に、この磁気冷凍システムにおける熱交換用媒体の循環系統の概略構成を示す。図中、１は磁性材料、２は磁気冷凍作業室、３は導入配管、４は排出配管、５ａ及び５ｂは永久磁石、６ａ及び６ｂは回転盤、２５は低温消費施設、２６は放熱器を表す。
【０１０６】
図８に示すように、磁気冷凍作業室２は、矩形断面の筒型の形状を備えている。磁気冷凍作業室２の両端部の近傍には、それぞれ、メッシュグリッド１１、１２が取り付けられ、それらの間に、本発明に基づく磁性材料１が充填されている。磁性材料１は、例えば、平均径０．４ｍｍの球状であり、磁気冷凍作業室２内に６２％の容積充填率で充填されている。また、メッシュグリッド１１、１２のメッシュサイズは＃８０、Ｃｕ線径０．１４ｍｍである。磁気冷凍作業室２の一方の端には、熱交換用媒体の導入配管３が接続され、もう一方の端には、熱交換用媒体の排出配管４が接続されている。なお、この例では、同一形状の二つの磁気冷凍作業室２が設けられ、互いに平行に並べられて配置されている。
【０１０７】
二つの磁気冷凍作業室２を間に挟むように、一対の回転盤６ａ、６ｂが設けられている。回転盤６ａ、６ｂは共通の軸７で支持されている。この軸７は二つの磁気冷凍作業室２の中央に位置している。回転盤６ａ、６ｂの周縁近傍の内側には、それぞれ永久磁石５ａ、５ｂが保持されている。永久磁石５ａ、５ｂは、互いに対向するとともに、ヨーク（図示せず）を介して互いに結合されている。これによって、互いに対を成す永久磁石５ａ、５ｂの間隙部分に、強い磁場空間が形成される。なお、この例では、二つの磁気冷凍作業室２にそれぞれ対応するように、二対の永久磁石５ａ、５ｂが設けられ、軸７を中央に挟んで配置されている。
【０１０８】
回転盤６ａ、６ｂを９０度回転させる毎に、永久磁石５ａ、５ｂが磁気冷凍作業室２に対して接近及び離反を繰り返す。各一対の永久磁石５ａ、５ｂが各磁気冷凍作業室２の側壁に最も接近した状態では、永久磁石５ａ、５ｂの間に形成された磁場空間の中に磁気冷凍作業室２が入り、その中に収容されている磁性材料１に磁場が印加される。
【０１０９】
磁性材料１に対して磁場が印加された状態から、除去された状態に切り替わる際、電子磁気スピン系のエントロピーが増加し、格子系と電子磁気スピン系の間でエントロピーの移動が起こる。それによって、磁性材料１の温度が低下し、それが熱交換用媒体に伝達され、熱交換用媒体の温度が低下する。このようにして温度が低下した熱交換用媒体は、磁気冷凍作業室２から排出配管４を通って排出され、外部の低温消費施設（２５：図９）に冷媒として供給される。
【０１１０】
図９に示すように、導入配管３の上流側には、熱交換用媒体が貯えられるタンク２１が設けられ、導入配管３の途中にはポンプ２２が設けられている。排出配管４は、磁気冷凍作業室２から出た後に二つの系統に分けられ、二つの循環ラインが構成されている。一方の循環ライン（冷却ライン２３）の途中には、バルブＶ１、低温消費施設２５及びバルブＶ３が設けられ、冷却ライン２３の末端はタンク２１に接続されている。もう一方の循環ライン（予冷ライン２４）の途中には、バルブＶ２、放熱器２６及びバルブＶ４が設けられ、予冷ライン２４の末端はタンク２１に接続されている。
【０１１１】
次に、この磁気冷凍システムの運転について説明する。この磁気冷凍システムは、予冷工程及び冷却工程を交互に繰り返すことによって運転される。
【０１１２】
先ず、予冷工程では、バルブＶ１及びバルブＶ３を閉じた状態で、バルブＶ２及びＶ４を開き、熱交換用媒体を予冷ライン２４内で循環させる。この状態で、磁気冷凍作業室２に永久磁石（５ａ、５ｂ：図８）を近付ける。磁性材料１に磁場が印加されると、磁性材料１の温度が上昇し、それが熱交換用媒体に伝達され、熱交換用媒体の温度が上昇する。このようにして暖められた熱交換用媒体は、磁気冷凍作業室２から排出配管４を通って排出され、バルブＶ２を通って放熱室２６に導入され、そこで冷却される。冷却された熱交換用媒体は、バルブＶ４を通ってタンク２１内へ戻る。
【０１１３】
磁気冷凍作業室２内の磁性材料１の温度が、導入配管３を通って磁気冷凍作業室２に供給される熱媒体の温度の近傍まで低下したところで、バルブＶ２及びＶ４を閉じ、予冷工程を終了させて冷却工程に移る。
【０１１４】
冷却工程では、先ず、磁気冷凍作業室２から永久磁石（５ａ、５ｂ：図８）を遠ざける。次いで、バルブＶ１及びバルブＶ３を開き、熱交換用媒体を冷却ライン２３内で循環させる。磁性材料１から磁場が除去されると、磁性材料１の温度が低下し、それが熱交換用媒体に伝達され、熱交換用媒体の温度が低下する。このようにして冷却された熱交換用媒体は、磁気冷凍作業室２から排出配管４を通って排出され、バルブＶ１を通って低温消費施設２５に導入される。熱交換用媒体は、低温消費施設２５内で使用されて温度が上昇した後、バルブＶ３を通ってタンク２１内へ戻る。
【０１１５】
磁気冷凍作業室２内の磁性材料１の温度が、導入配管３を通って磁気冷凍作業室２に供給される熱媒体の温度の近傍まで上昇したところで、バルブＶ１及びＶ３を閉じ、冷却工程を終了させて、再び予冷却工程に移る。
【０１１６】
この磁気冷凍システムの制御装置（図示せず）は、永久磁石５ａ、５ｂの動きに同期させてバルブＶ１〜Ｖ４を制御し、上記の予冷工程及び冷却工程を交互に繰り返す。
【０１１７】
【発明の効果】
本発明の磁性材料では、常温域において、比較的低い磁場で磁化曲線に変曲点が現れるとともに、磁化曲線に変曲点が現れる温度の近傍において、電子磁気スピン系に大きなエントロピー変化が観察される。従って、本発明の磁性材料を使用すれば、上記の温度の近傍において電子磁気スピン系と格子系との間でエントロピーの授受を行わせることによって、比較的低い磁場を用いて磁気冷凍を実現することが可能になる。
【０１１８】
また、このような磁性材料と永久磁石とを組み合わせることによって、超電導磁石を用いることなく、小型、簡便、低価格の磁気冷凍システムを構成することができる。
【図面の簡単な説明】
【図１】本発明に基づく磁性材料（供試体１）の磁化曲線の例を示す図。
【図２】本発明に基づく磁性材料（供試体４）の磁化曲線の例を示す図。
【図３】比較例として用いた磁性材料（供試体６）の磁化曲線の例を示す図。
【図４】本発明に基づく磁性材料（供試体１）のエントロピー変化の温度依存性を示す図。
【図５】本発明に基づく磁性材料（供試体１１）の磁化曲線の例を示す図。
【図６】比較例として用いた磁性材料（供試体１３）の磁化曲線の例を示す図。
【図７】本発明に基づく磁性材料（供試体２１）の磁化曲線の例を示す図。
【図８】本発明に基づく磁性材料が使用される磁気冷凍システムの概略構成図を示す図。
【図９】本発明に基づく磁性材料が使用される磁気冷凍システムにおける熱交換用媒体の循環系統の概略構成図を示す。
【符号の説明】
１・・・磁性材料、
２・・・磁気冷凍作業室、
３・・・導入配管（第一の流路）、
４・・・排出配管（第二の流路）、
５ａ、ｂ・・・永久磁石、
６ａ、ｂ・・・回転盤（駆動装置）、
７・・・軸、
１１、１２・・・メッシュグリッド、
２１・・・タンク、
２２・・・ポンプ、
２３・・・冷却ライン、
２４・・・予冷ライン、
２５・・・低温消費施設、
２６・・・放熱器。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic material, and more particularly to a magnetic material capable of realizing magnetic refrigeration using a relatively low magnetic field in a normal temperature range.
[0002]
[Prior art]
Currently, a gas compression / expansion cycle is mainly used in a refrigeration system in a normal temperature range, which is closely related to human daily life, such as a refrigerator, a freezer, and an air conditioner. However, with regard to the gas compression / expansion cycle, environmental destruction associated with the discharge of the specific chlorofluorocarbon gas becomes a serious problem, and there is a concern about the influence of the alternative chlorofluorocarbon gas on the environment. Against this background, there is a demand for practical use of a clean and highly efficient refrigeration technique that does not have the problem of environmental destruction associated with the disposal of working gas.
[0003]
In recent years, as one of such environmentally friendly and highly efficient refrigeration technologies, expectations for magnetic refrigeration have increased, and research and development of magnetic refrigeration technologies for normal temperature regions have been activated. In magnetic refrigeration, a low temperature is generated as follows by applying the magnetocaloric effect (a phenomenon in which the temperature changes when an external magnetic field is changed in an adiabatic state with respect to a magnetic substance).
[0004]
In a magnetic substance, entropy changes due to the difference in the degree of freedom of the electron magnetic spin system between a state when a magnetic field is applied and a state when a magnetic field is removed. With such entropy change, entropy shift occurs between the electron magnetic spin system and the lattice system. In magnetic refrigeration, a magnetic material with a large electron magnetic spin is used, and entropy is transferred between the electron magnetic spin system and the lattice system by using a large change in entropy between applying and removing the magnetic field. Let it be done, thereby producing a low temperature.
[0005]
In the temperature region of 1K or higher, the “magnetic substance” usually refers to a substance that exhibits magnetism due to electron spin. On the other hand, in the temperature region of several mK or less, magnetism due to the nuclear spin becomes relatively obvious. For example, PrNi₅It has been reported that an ultra-low temperature of 27 μK was generated using In the following, unless otherwise specified, the “magnetic substance” refers to a substance exhibiting magnetism due to electron spin, and the target temperature range is 1K or more.
[0006]
In the early 1900s, as a magnetic substance having a magnetocaloric effect, Gd₂(SO₄)₃8H₂Paramagnetic salts such as O, Gd₃Ga₅O₁₂A magnetic refrigeration system using a paramagnetic compound represented by (Gadolinium Gallium Garnet; “GGG”) has been developed. However, in most cases, the magnetic refrigeration system using a paramagnetic substance has been applied to a cryogenic region of 20K or less. The reason is that as the lattice vibration increases as the temperature rises, the magnitude of the external magnetic field required to change the degree of freedom of the electron magnetic spin system also increases, so it can be obtained using a superconducting magnet. This is because, assuming a magnetic field of about 10 Tesla, the temperature at which magnetic refrigeration can be realized using a paramagnetic substance is limited to a cryogenic region.
[0007]
On the other hand, in order to realize magnetic refrigeration in a higher temperature range, research on magnetic refrigeration using a magnetic phase transition between a paramagnetic state and a ferromagnetic state in ferromagnetic materials has been actively conducted since the 1970s. It was. As a result, lanthanum rare earth elements such as Pr, Nd, Dy, Er, Tm, and Gd alone, or two or more rare earth alloy materials such as Gd-Y and Gd-Dy, RAl₂(R represents a rare earth element, the same applies hereinafter), RNi₂Many magnetic materials containing rare earths with large electron magnetic spin per unit volume, such as rare earth intermetallic compounds such as GdPd, have been proposed.
[0008]
In these ferromagnetic materials, by applying an external magnetic field in the vicinity of the ferromagnetic phase transition temperature (Curie temperature; Tc), the electron magnetic spin is changed from the paramagnetic state to the ferromagnetic state, and the entropy at that time is obtained. Magnetic refrigeration is realized using changes. Accordingly, the application temperature range is limited to the vicinity of the ferromagnetic phase transition temperature (Tc) of each magnetic substance, but the magnitude of the external magnetic field may be as much as necessary to cause the magnetic phase transition, and 20K. It falls within the range that can be produced even in a temperature range much higher than that.
[0009]
In 1974, Brown, USA, realized magnetic refrigeration for the first time at room temperature using a plate of ferromagnetic material Gd having a ferromagnetic phase transition temperature (Tc) of about 294K. However, in Brown's experiment, since the integrated Gd plate was used from the high temperature side to the low temperature side, the refrigeration cycle was operated continuously, but there were problems with the stability of heat exchange in the refrigeration cycle. .
[0010]
Here, there is an essential problem in the magnetic refrigeration in the medium temperature to room temperature range far higher than 20K. That is, as the temperature rises, the lattice vibration becomes active, and the lattice entropy becomes relatively larger than the magnetic entropy of the electron magnetic spin system in the temperature range of 100 to 150K or higher. For this reason, even if entropy is exchanged between the electron magnetic spin system and the lattice system by changing the external magnetic field, the magnetocaloric effect, that is, the temperature drop (ΔTad) of the substance remains very small.
[0011]
In 1982, Barclay in the United States was an obstacle to magnetic refrigeration in the middle to room temperature range (or higher temperature range where the lattice entropy is relatively higher than the magnetic entropy). In addition to the magnetic refrigeration work due to the magnetocaloric effect, the magnetic entropy, which was positioned as a magnetic energetic effect, was also considered to be actively used. We proposed a freezing method (US-Pat.4,332,135). This magnetic refrigeration system is called an AMR system (“Active Magnetic Refrigeration”).
[0012]
In 1997, Zimm, Gschneidner, Pecharsky et al. Of the United States made a prototype of an AMR type magnetic refrigerator using a filled cylinder filled with fine spherical Gd, and succeeded in continuous steady operation of the magnetic refrigeration cycle at room temperature. (Advances in Cryogenic Engineering, Vol. 43 (1998) 1759). According to this, by using a superconducting magnet in the room temperature range, changing the external magnetic field from 0 Tesla to 5 Tesla succeeded in freezing at about 30 ° C., and the freezing temperature difference between the high temperature end and the low temperature end It has been reported that when (ΔT) is 13 ° C., a very high refrigeration efficiency (COP = 15; except for the input power to the magnetic field generating means) is obtained. Incidentally, the refrigeration efficiency (COP) in a compression / expansion cycle (conventional refrigerator, etc.) using conventional chlorofluorocarbon is about 1 to 3. Here, the value of COP changes depending on the refrigeration temperature range, and when comparing between different refrigeration systems, it is necessary to handle the definition strictly. It can be seen that there is a great expectation as a refrigeration system that can realize high refrigeration efficiency.
[0013]
In addition to the above-mentioned technical demonstration of the AMR cycle magnetic refrigeration system using Gd, Pecharsky and Gschneidner et al. In 1997 announced that Gd is a magnetic substance that can produce a very large entropy change in the room temperature region.₅(Ge, Si)₄A system was developed (US-Pat. 5,743,095). For example, Gd₅(Ge_0.5Si_0.5)₄Then, when the external magnetic field is changed from 0 to 5 Tesla at about 277 K, an entropy change (ΔS) of about 20 J / (kg · K) is shown, and when the external magnetic field is changed from 0 to 2 Tesla, about 15 J / The entropy change (ΔS) of (kg · K) is shown. That is, a large entropy change more than twice Gd is observed near room temperature.
[0014]
However, in the above-mentioned example of Zimm, Gschneidner, Pecharsky et al., A superconducting magnet is used to apply a large external magnetic field of about 2 to 5 Tesla to Gd, which is a magnetic material for magnetic refrigeration work. At present, a cryogenic environment of about 10K is required to operate the superconducting magnet, which causes a problem that the system becomes large. Furthermore, when using a superconducting magnet, a cryogen such as liquid helium or a cryogenic generator for cryogenic generation is required. Therefore, such a system is used for daily use such as refrigeration and air conditioning. Applying to is not realistic.
[0015]
In addition to the superconducting magnet, there is a large-capacity electromagnet as means for generating a large magnetic field. However, when an electromagnet is used, it is necessary to supply a large current or to cool the water against Joule heat generation, which increases the size of the system and increases the operating cost. Therefore, as in the case of a superconducting magnet, it is not practical to apply a system using an electromagnet to daily use.
[0016]
There is a permanent magnet as a small and simple magnetic field generating means. However, it is difficult to generate a large magnetic field of about 2 to 5 Tesla using a permanent magnet. Although an experimental result using an NdFeB-based permanent magnet and Gd as a magnetic material for magnetic refrigeration work has been reported, the cooling temperature in the room temperature region is as extremely low as 1.6 ° C. because the magnetic field is small, There is a large difference between the refrigeration capacity of the conventional gas compression and expansion cycle.
[0017]
[Problems to be solved by the invention]
The present invention has been made in view of the problems of the magnetic refrigeration technology in the normal temperature range as described above. An object of the present invention is to provide a magnetic material for magnetic refrigeration work that can realize magnetic refrigeration using a relatively low magnetic field in a normal temperature range.
[0018]
[Means for Solving the Problems]
The magnetic material of the present invention has an inflection point at which the second derivative with respect to the magnetic field of the magnetization curve changes from positive to negative within the strength range of the magnetic field formed using a permanent magnet only in a part of the temperature range. It is characterized by that.
[0019]
Preferably, the magnetic material of the present invention is characterized by having the above-mentioned inflection point in the magnetization curve within a strength range of a magnetic field of 1 Tesla or less only in a part of a temperature range of 200 K or more and 350 K or less.
[0020]
According to the present invention, an external magnetic field is applied to a magnetic material having a magnetization curve that meets the above conditions using a permanent magnet in the vicinity of the temperature indicating the inflection point, and the value of the external magnetic field is changed. By doing so, entropy is exchanged between the electron magnetic spin system and the lattice system, and magnetic refrigeration can be realized.
[0021]
The inventors of the present application are within a range of a normal temperature of 200 K to 350 K (that is, a temperature closely related to daily life from dry ice to hot water) and within a relatively low magnetic field strength range of 1 Tesla or less, As one means for causing the above inflection point to appear in the magnetization curve, it has been found that it is effective to compete for ferromagnetic magnetic interaction and antiferromagnetic magnetic interaction. The reason why the above inflection point appears in the magnetization curve is considered as follows. By competing the ferromagnetic magnetic interaction and the antiferromagnetic magnetic interaction, a plurality of electronic states that are magnetically close in energy are formed. Such mutual relationship between the electronic states is changed by applying a magnetic field. For this reason, the magnetic arrangement changes partially or entirely inside the material system by applying a magnetic field, and as a result, an inflection point appears in the magnetization curve.
[0022]
The important point here is that the relationship of energy between electronic states forms a delicate state in a normal temperature range of about 200K to 350K. By forming such a special state, it becomes possible for the first time to obtain a large entropy change by applying a relatively low magnetic field, that is, to realize a highly efficient magnetic refrigeration.
[0023]
Further, when magnetic refrigeration is actually performed using a magnetic material, not only the magnitude of the entropy change amount ΔS (T, ΔH) accompanying the change in magnetic field (ΔH) but also the temperature range in which the peak of the entropy change amount appears. Is also an important factor. That is, even if a large entropy change is obtained, the magnetic refrigeration cycle cannot be stably operated when it can be obtained only in a very narrow temperature range (for example, about 1 to 2 K). That is, the steady state of the refrigeration cycle is not established, or even if it is established, the capacity as a refrigerator becomes extremely low.
[0024]
For example, in the case of AMR type magnetic refrigeration, since the magnetic material plays the heat storage function simultaneously with the magnetic refrigeration function, a temperature gradient is generated inside the magnetic refrigeration chamber during steady operation of the refrigeration cycle. That is, even when the temperature of the magnetic material is substantially uniform in the magnetic refrigeration chamber at the start of operation, a temperature gradient is gradually generated in the magnetic refrigeration chamber by repeating the refrigeration cycle. The both ends of each become a high temperature end and a low temperature end. For this reason, the magnetic materials in the magnetic refrigeration chamber work in different temperature cycles depending on their positions, and each temperature cycle is also in a steady state during steady operation. Here, when the magnetic material has a peak in the entropy change amount only in a very narrow temperature range (for example, about 1 to 2K), the refrigeration cycle is established only in this narrow temperature range. . Therefore, it is difficult to stably operate a large refrigeration temperature difference (for example, 10K to 20K or more).
[0025]
It is also conceivable to sequentially arrange magnetic materials exhibiting a peak of entropy variation in each temperature range from the high temperature end to the low temperature end of the magnetic refrigeration chamber in accordance with the temperature gradient during the steady operation. However, since it gradually approaches the steady state through different temperature cycles as the refrigeration cycle repeats from the start of operation, it is required that the substance has an entropy change in a temperature range wider than the temperature amplitude during steady operation. The
[0026]
For the reasons described above, a magnetic material for magnetic refrigeration is required to have a large entropy change amount and a wide temperature range in which a peak appears in the entropy change amount. In addition, the temperature width of the peak of the entropy change amount indicates not the half value width but the peak tail width. This is because the effective temperature width of the peak is effective in the actual temperature cycle. Here, the effective temperature width of the peak is the peak width when the error level is removed at the bottom portion.
[0027]
When the amount of entropy change (temperature dependence) at temperature T with respect to a specific external magnetic field change ΔH is ΔS (T, ΔH) and the peak value is ΔSmax, the effective temperature width of the peak is as follows: Defined: Range of temperature T where ΔS (T, ΔH)> 0.1 * ΔSmax or 0.1 * ΔSmax> 1 [J / (1) with a value of one tenth of ΔSmax as a base level reference kg, K)] is satisfied, a temperature that satisfies ΔS (T, ΔH)> 1 [J / (kg, K)] with 1 [J / (kg, K)] as the skirt level reference. T range.
[0028]
In order to realize a magnetic refrigeration cycle using a single magnetic material, the effective temperature width of the peak of the entropy change amount ΔS (T, ΔH) needs to be at least 3K or more. Preferably, it is necessary to secure an effective temperature range of about 5K or more, more preferably 8K or more.
[0029]
Furthermore, regarding the peak of the entropy change amount, it is preferable that there is no temperature hysteresis, and even if it exists, it is 8K or less, preferably 3K or less, more preferably 1K or less.
[0030]
However, in past studies, there is often a trade-off between the amount of entropy change and the temperature range of its peak, so it is important to adjust the balance between the two.
[0031]
When the peak width of the entropy change is as narrow as about 1 to 2K, it is effective to have a slight composition fluctuation as a means for widening the peak width so as to be practical. By slightly altering the physical properties of the magnetic substance itself by giving a slight compositional fluctuation, the electronic state balance in the micro portion is slightly changed, and the temperature at which the inflection point appears is distributed in the micro region. Can do. As a result, the peak width of the entropy change of the magnetic substance can be widened.
[0032]
In addition, when viewed in the metal structure, the physical properties of the magnetic substance itself are greatly changed in the same manner as described above by precipitating a small amount of a second phase having a different crystal structure from the main phase. It is also possible to widen the peak width of the entropy change. If such a second phase is, for example, 30% by volume or less, there is no practical problem, and it is possible to widen the peak width of ΔS.
[0033]
In addition, as a specific method, it is possible to mention a preparation composition slightly shifted from a predetermined composition, a slight addition of an additive element, or a rapid cooling from a molten metal state during synthesis. .
[0034]
Here, in order to obtain a large entropy change, it is required that the internal degree of freedom of the magnetic material is large. In order to increase the magnetic internal degree of freedom of magnetic substances, transition metal elements such as Fe, Ni, Co, Mn, and Cr, and rare earth elements such as Pr, Nd, Gd, Tb, Dy, Er, Ho, and Tm are used. It is preferable to use the element as the main constituent element.
[0035]
Furthermore, in order to make the above inflection point appear in the magnetization curve in a low magnetic field range of 1 Tesla or less in a normal temperature range of about 200K to 350K, it is necessary to use Fe, Ni, Co, Mn, or Cr. It is effective to include any of these alone or in total at least 50 atomic%. This is because when the ratio of the transition metal element such as Fe is small, it becomes difficult to make the inflection point appear in a high temperature range of 200 K or more with a magnetic field of 1 Tesla or less.
[0036]
In the case of Gd, Sm, and Tb elements that have a relatively strong magnetic interaction among the rare earth elements, the above inflection point appears at a temperature of 200 K or higher, so that Fe, Co, Ni, The total amount with transition metal elements such as Mn and Cr is preferably 60 atomic% or more.
[0037]
As a magnetic material that meets the above conditions, for example,
Including one or more elements selected from the group consisting of Fe, Co, Ni, Mn, and Cr in a total of 50 atomic% to 96 atomic%,
Including one or more elements selected from the group consisting of Si, C, Ge, Al, B, Ga, and In, in a total of 4 atomic% to 43 atomic%,
4 atomic% or more in total of one or more elements selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb The magnetic material containing 20 atomic% or less can be mentioned.
[0038]
In the second group, Si or Ge is preferable, and Si is particularly preferable to be 4 atomic% or more and 25 atomic% or less.
[0039]
As a representative example of such a magnetic material, R (T, M)₁₃, R (T, M)₁₂, R₂(T, M)₁₇, R₃(T, M)₂₉Where R is a rare earth element, T is a transition metal, and M is the above-described element of Group 3B or 4B. Particularly preferably, (La, Pr, Ce, Nd) (Fe, T, Si)₁₃, (La, Pr, Ce, Nd) (Fe, T, Si, M)₁₃It is.
[0040]
Furthermore, as another magnetic material that meets the above conditions, for example,
Including 60 or more atoms and 96 or less atoms in total of one or more elements selected from the group consisting of Fe, Co, Ni, Mn, and Cr,
A magnetic material containing one or more elements selected from the group consisting of Sc, Ti, Y, Zr, Nb, Mo, Hf, Ta, and W in a total of 4 atomic% to 40 atomic%. Can do. It is particularly preferable that two or more elements selected from these second groups are used.
[0041]
In the second group, Ti, Zr, Nb, and Hf are particularly preferable, and it is preferable that the total amount is 25 atomic% or more.
[0042]
As a representative example of such a magnetic material, (Hf, Ta) Fe₂, (Ti, Sc) Fe₂, (Nb, Mo) Fe₂There is.
[0043]
Such a magnetic material for magnetic refrigeration preferably has a large magnetic entropy change per unit weight or unit volume so that the weight and capacity can be reduced from a practical viewpoint. Furthermore, from another practical viewpoint, it is also preferable that the change in magnetic entropy per unit magnetization is large. The reason is that the magnetic material receives an external force (magnetic force) proportional to the magnitude of the magnetization of the material in a magnetic field gradient, so in a practical situation, when controlling the relative position of the permanent magnet and the magnetic material, This is because a strong magnetic force becomes an obstacle.
[0044]
Other magnetic materials that meet the above conditions include, for example:
Including one or more elements selected from the group consisting of Fe, Co, Ni, Mn, and Cr in a total of 50 atomic% to 80 atomic%,
A magnetic material containing a total of 20 atomic percent or more and 50 atomic percent or less of one or more elements selected from the group consisting of Sb, Bi, P, and As can be given.
[0045]
As a representative example of such a magnetic material, (Mn, Cr)₂(Sb, As, P), (Mn, Cr) (Sb, As, P, Bi), (Co, Mn, Fe, Ni)₂(P, As), (Fe, Co, Mn)₃P and so on. Among these, (Mn, Cr)₂Sb, (Mn, Cr) Sb, (Co, Mn)₂P, (Fe, T)₂(P, As) and the like are more preferable.
[0046]
In addition, in order to finely control the electronic state, a part (about 10% or less) of a 3b transition metal element such as Fe, Co, Ni, Mn, or Cr may be replaced with a 4b metal element such as Rh or Pd. It is valid. Furthermore, the electronic state can be finely controlled by substituting a part (about 20% or less) of 5b elements such as Sb, Bi, P, and As with light elements such as B and C.
[0047]
When the content of oxygen is large, when the magnetic material is manufactured, in the melting step (the step of melting and mixing the raw materials), oxygen and the metal element combine to form a high melting point oxide. This is formed and floats as a high melting point impurity in the molten metal layer, which becomes a factor that hinders the production of high-quality materials in the melting process and the resolidification process. Therefore, in order to suppress the formation of such an oxide as much as possible, it is preferable to suppress the oxygen content to 1 atomic% or less.
[0048]
Since the inflection point as described above appears within a relatively low magnetic field, the magnetic material of the present invention is small without using a superconducting magnet or a large current capacity electromagnet near the temperature at which the inflection point appears. A magnetic refrigeration system can be realized using a permanent magnet.
[0049]
DETAILED DESCRIPTION OF THE INVENTION
Next, some examples of the magnetic material according to the present invention will be described.
[0050]
(Example 1)
Seven types of specimens having the following compositions were prepared, and their magnetization curves and entropy changes accompanying magnetic field changes were examined. Of the following specimens, specimens 1 to 4 correspond to the magnetic material according to the present invention, and specimens 5, 6 and 7 are comparative examples.
[0051]
Specimen 1: Fe_81.7Si_11.1La_7.2
Specimen 2: Fe_80.8Si_12.1La_7.1
Specimen 3: Fe_82.6Co_0.9Si_9.3La_7.2
Specimen 4: Fe_81.7Si_10.2Ga_0.9La_7.2
Specimen 5: Fe_69.7Al_23.2La_7.1
Specimen 6: Fe_75.8Si_17.1La_7.1
Specimen 7: Gd₉₅Y_Five
After adjusting the material of each said composition by arc melting, the homogenization heat processing for 2 weeks was performed in the temperature of 900 to 1100 degreeC in the vacuum. The magnetization curve of each specimen prepared in this way was measured.
[0052]
FIG. 1 shows the magnetization curve of the specimen 1 in the temperature range from 157.5K to 232.5K. FIG. 2 shows a magnetization curve of the specimen 4 in the temperature range from 192.5K to 217.5K. FIG. 3 shows a magnetization curve of the specimen 6 in the range from the temperature 200K to 270K.
[0053]
In the magnetization curve of the specimen 1 (FIG. 1), in the temperature region lower than T = 196.5K, the magnetization rapidly increases due to a very low external magnetic field, and the magnetic field of magnetization in the magnetic field range of 1 Tesla or less. 2 is negative (convex upward) (signs a to d in FIG. 1).
[0054]
The shape of such a magnetization curve changes greatly in the vicinity of T = 200K. At T = 200.5K (symbol e) and 202.5K (symbol f), a very low magnetic field of about 0.01 Tesla shows a small magnetization value, but as the magnetic field increases, Magnetization rapidly increases near H = 0.35 Tesla and H = 0.6 Tesla, and an inflection point appears in the magnetization curve. The increase in magnetization becomes gradually slower after passing through the inflection point. The value of the magnetic field at which such an inflection point appears in the magnetization curve will be expressed as Hc below.
[0055]
As the temperature is further increased, Hc increases and the amount of change in magnetization near Hc decreases. When the temperature becomes higher than T = 207.5K (symbol g), the range of the magnetic field in which the magnetization increases in the vicinity of Hc is broadened and gradually disappears (symbols h to k in FIG. 1).
[0056]
In the magnetization curve of the specimen 4 (FIG. 2), in the temperature range lower than T = 202.5K, the magnetization rapidly increases due to a very low external magnetic field, and the magnetization magnetic field is within a range of 1 Tesla or less. 2 is negative (convex upward) (signs a to c in FIG. 2).
[0057]
The shape of such a magnetization curve changes greatly at T = 207.5K. At T = 207.5K (symbol d), the shape of the magnetization curve protrudes downward in the vicinity of the magnetic field H = 0.3 Tesla to H0.4 Tesla, and the magnetization curve of the magnetic field H = 0.5 Tesla or higher. The shape changes upward. That is, an inflection point appears in the magnetization curve near the magnetic field H = 0.45 Tesla.
[0058]
Note that a similar magnetization curve was observed for the specimen 2 and the specimen 3. In Specimen 2, an inflection point appeared in the magnetic field range of 1 Tesla or less near T = 207K and in Specimen 3 near T = 230K.
[0059]
On the other hand, in the magnetization curve of the specimen 6 (FIG. 3), although the magnetization value changes greatly in the vicinity of the magnetic phase transition temperature, the second derivative of the magnetization with respect to the magnetic field in the magnetic field range of 1 Tesla or less. Is always negative, and it can be seen that there is no significant change in the shape (convex upward) of the magnetization curve. Note that a similar magnetization curve was observed for the specimen 5 and the specimen 7, and the second derivative of the magnetization relative to the magnetic field was always negative in the magnetic field range of 1 Tesla or less.
[0060]
Next, for each of the above seven types of specimens, the amount of change ΔS (T, ΔH) in the entropy of the electron magnetic spin system when the external magnetic field was changed was determined from the magnetization measurement data using the following equation. . In any case, a peak appeared in the entropy change amount ΔS at a specific temperature (Tpeak) with respect to an arbitrary magnetic field change amount ΔH.
[0061]
[Expression 1]

[0062]
FIG. 4 shows the relationship between the amount of change in temperature and entropy for the specimen 1. As shown in FIG. 4, when the external magnetic field is changed from 0 to 0.5 Tesla or from 0 to 1 Tesla, a peak appears in the amount of change in entropy at a temperature of about 195 K to 210 K. Yes.
[0063]
Table 1 shows the calculation result of the entropy change amount (ΔSmax) with respect to the magnetic field change ΔH at the temperature (Tpeak) at which the peak appears in the entropy change amount for each specimen. In Table 1, the amount of entropy change in the Gd electron magnetic spin system, which is a prototype of a magnetic material for magnetic refrigeration work, is also shown for comparison.
[0064]
[Table 1]

[0065]
As can be seen from Table 1, in specimens 1 to 4, a large entropy change was observed in a low magnetic field of 1 Tesla or less, indicating that it is far superior to Gd. On the other hand, in the specimens 5 to 7, the entropy change comparable to or less than that of Gd is measured, and it can be seen that the specimens are significantly inferior to the specimens 1 to 4.
[0066]
As described above, when magnetic refrigeration is actually performed using a magnetic material, not only the magnitude of the entropy change amount ΔS (T, ΔH) accompanying the change in magnetic field (ΔH) but also the entropy change The temperature range in which the peak appears in the quantity (“effective temperature range”) is also an important factor. Below, the temperature width about (DELTA) S (T, (DELTA) H = 1 Tesla) and (DELTA) S (T, (DELTA) H = 0.5 Tesla) is shown as a standard about each specimen.
[0067]
When the external magnetic field change ΔH is 1 Tesla, the temperature width of the peak of the entropy change amount of the specimens 1 to 4 is as follows:
Specimen 1: about 190 to 210K
Specimen 2: about 195 to 220K
Specimen 3: about 215 to 245K
Specimen 4: about 195 to 217K
When the external magnetic field change ΔH is 0.5 Tesla, the temperature width of the peak of the entropy change amount of the specimens 1 to 4 is as follows:
Specimen 1: about 194 to 207K
Specimen 2: Approximately 197 to 218K
Specimen 3: about 220 to 237K
Specimen 4: 196 to 218K
Thus, the specimens 1 to 4 have a temperature range of 10K or more even when the external magnetic field change ΔH is 0.5 Tesla, and it can be said that there is no practical problem.
[0068]
As described above, in the specimens 1 to 4, a large change occurs in the arrangement state of the electron magnetic spins at a relatively low magnetic field of 1 Tesla or higher on the higher temperature side than the characteristic temperature (Tcri), and the magnetization curve is inflected. A point appears. Furthermore, in these specimens, it was confirmed that an extremely large entropy change was observed in the electron magnetic spin system in the vicinity of the temperature at which the inflection point appears in the magnetization curve.
[0069]
For specimens 1 to 4, it was found by X-ray diffraction that the main phase had a cubic structure and the αFe phase was slightly precipitated as the second layer.
[0070]
(Example 2)
Three types of specimens having the following compositions were prepared, and their magnetization curves and entropy changes accompanying magnetic field changes were examined. Of the following specimens, specimens 11 and 12 correspond to the magnetic material according to the present invention, and specimen 13 is a comparative example.
[0071]
Specimen 11: Fe₆₇Hf₂₈Ta_Five
Specimen 12: Fe₆₇Hf₂₇Ta₆
Specimen 13: Fe₆₇Hf₂₉Ta_Four
After adjusting the materials of the above-mentioned compositions by arc melting, a uniform heat treatment was performed in a vacuum at a temperature of 950 ° C. to 1000 ° C. for about 100 hours to produce a master alloy. Next, a particulate specimen was produced from this mother alloy using a plasma spray method. As a result, many particles having a major axis of about 0.1 mm to 0.3 mm were obtained. The magnetization curve of each specimen thus obtained was measured.
[0072]
FIG. 5 shows the magnetization curve of the specimen 11 in the temperature range from 237.5K to 307.5K. FIG. 6 shows a magnetization curve of the specimen 13 in the temperature range from 277.5K to 332.5K.
[0073]
In the magnetization curve of the specimen 11 (FIG. 5), in the region lower than T = 277.5K, the magnetization rapidly increases due to a very low external magnetic field, and in the magnetic field range of 1 Tesla or less, It can be seen that the second derivative is negative (convex upward) (signs a to e in FIG. 5).
[0074]
The shape of such a magnetization curve changes greatly in the vicinity of T = 280K. At T = 280.5K (symbol f) and 282.5K (symbol g), a very low magnetic field of about 0.01 Tesla shows a small magnetization value. Magnetization rapidly increases near H = 0.27 Tesla and H = 0.5 Tesla, and an inflection point appears in the magnetization curve. The increase in magnetization becomes gradually slower after passing through the inflection point.
[0075]
As in the case of the specimen 1 of Example 1, when the temperature is further increased, the value of Hc (a magnetic field in which an inflection point appears in the magnetization curve) increases and the amount of change in magnetization near Hc decreases. . When the temperature is higher than T = 292.5 K (symbol j), the transition near Hc gradually disappears (symbols j to l).
[0076]
A similar magnetization curve was also observed for the specimen 12, and an inflection point appeared in the magnetization curve in a magnetic field of 1 Tesla or less near T = 245K.
[0077]
On the other hand, in the magnetization curve of the specimen 13 (FIG. 6), as in the case of the specimen 6 in the previous example (comparative example), although the magnetization value changes greatly in the vicinity of the magnetic phase transition temperature, It can be seen that in the range of a magnetic field of 1 Tesla or less, the twice differential coefficient of the magnetization with respect to the magnetic field is always negative, and there is no significant change in the shape of the magnetization curve (convex upward).
[0078]
Next, for each of the three types of specimens described above, the amount of entropy change ΔS (T, ΔH) of the electron magnetic spin system when the external magnetic field was changed was evaluated by the same method as in Example 1. Table 2 shows the calculation result of the entropy change amount (ΔSmax) with respect to the magnetic field change ΔH at the temperature (Tpeak) at which the entropy change amount ΔS shows a peak for each specimen.
[0079]
[Table 2]

[0080]
As can be seen from Table 2, in the specimens 11 and 12, a large entropy change is observed in a low magnetic field of 1 Tesla or less, and it can be seen that it is far superior to the specimen 13.
[0081]
Furthermore, the same results as those of the specimens 11 and 12 were also obtained in an Fe—Ti—Sc magnetic material containing about 67 atomic% Fe and about 25 to 30 atomic% Ti.
[0082]
(Example 3)
Two types of specimens having the following compositions were prepared, and their magnetization curves and entropy changes accompanying magnetic field changes were examined.
[0083]
Specimen 21: Mn_63.4Cr_3.3Sb_33.3
Specimen 22: Mn₅₀As₃₅Sb₁₅
After preparing powdery raw materials, enclosing them in a crucible and holding them at a temperature of 800 ° C. to 950 ° C. for a long time (about 2 days for Specimen 21 and about 1 week for Specimen 22), after slowly reacting Then, a uniform heat treatment was performed in a vacuum at a temperature of 550 ° C. to 700 ° C. for about 100 hours. The magnetization curve of each specimen thus obtained was measured.
[0084]
FIG. 7 shows a magnetization curve of the specimen 21 in the temperature range from 300K to 315K. In the magnetization curve of the specimen 21 at a temperature of 315K, it can be seen that the second derivative of the magnetization with respect to the magnetic field is always negative (convex upward) in the magnetic field range of 1 Tesla or less (sign g in FIG. 7). .
[0085]
In the specimen 21, contrary to the specimens 1 and 11, when the temperature is lowered, a downward convex portion appears in the magnetization curve.
[0086]
At a temperature of 310 K (symbol c), the magnetization curve has a convex shape in a magnetic field of 0.4 Tesla or less. An inflection point appears in the vicinity of a magnetic field of 0.4 Tesla, and the magnetization curve has a downwardly convex shape in the range of about 0.4 Tesla to 0.8 Tesla. When the magnetic field further increases, an inflection point appears again in the vicinity of the magnetic field of 0.8 Tesla, and the magnetization curve returns to an upwardly convex shape in the range of the magnetic field of 0.9 Tesla or more.
[0087]
When the temperature is lowered, the inflection point at which the shape of the magnetization curve changes from a downward convex shape to an upward convex shape shifts to the high magnetic field side, and is 307.5K, 305K, 302.5K, 300K (reference symbols d˜). At each temperature of g), the position of the inflection point of the magnetization curve was about 1 Tesla, 2 Tesla, 2.6 Tesla, and 3.5 Tesla, respectively.
[0088]
In the specimen 22, at T = 232.5K, an inflection point appears in the magnetization curve near the magnetic field of 0.8 Tesla, and the magnetization curve has a downward convex shape in the range of the magnetic field of 0.8 Tesla or less. A convex shape was observed in the magnetic field range of 0.8 Tesla or higher.
[0089]
In the specimen 22, in the temperature range of 200K to 230K, the magnetization curve always has an upwardly convex shape in the range of a magnetic field of 1 Tesla or less. When the temperature is raised, at the temperature of 232.5 K, as described above, the shape of the magnetization curve changes from a downward convex to an upward convex shape. As the temperature was further increased, the inflection point of the magnetization curve shifted to the high magnetic field side, and the position of the inflection point of the magnetization curve at 240 K became a magnetic field of 3.8 Tesla.
[0090]
Next, with respect to the specimens 21 and 22 described above, the amount of entropy change ΔS (T, ΔH) of the electron magnetic spin system when the external magnetic field was changed was evaluated by the same method as in Example 1. In specimens 21 and 22, when the magnetic field was changed in the range of 0 Tesla or 1 Tesla, peaks in entropy change were observed in the vicinity of 311 K and 231 K, respectively.
[0091]
Table 3 shows values obtained by converting the entropy change amount (ΔSmax) with respect to the magnetic field change ΔH at the temperature (Tpeak) at which the entropy change amount ΔS shows a peak value into a value per unit magnetization.
[0092]
[Table 3]

[0093]
As can be seen from Table 3, in the specimens 21 and 22, the amount of change in entropy per unit magnetization is far superior to Gd in a low magnetic field of 1 Tesla or less.
[0094]
As described above, when magnetic refrigeration is actually performed using a magnetic material, the temperature width (“effective temperature width”) at which a peak appears in the entropy change amount is also an important factor. Below, the temperature width about (DELTA) S (T, (DELTA) H = 1 Tesla) and (DELTA) S (T, (DELTA) H = 0.5 Tesla) is shown as a standard about each specimen.
[0095]
When the external magnetic field change ΔH is 1 Tesla, the temperature width of the peak of the entropy change amount of the specimens 21 and 22 is as follows:
Specimen 21: about 304 to 315K
Specimen 22: about 214 to 236K
When the external magnetic field change ΔH is 0.5 Tesla, the temperature width of the peak of the entropy change amount of the specimens 21 and 22 is as follows:
Specimen 21: about 305 to 316K
Specimen 22: about 215 to 235K
Thus, the specimens 21 and 22 have a temperature range of 10K or more even when the external magnetic field change ΔH is 0.5 Tesla.
[0096]
(Configuration of magnetic refrigeration system)
A magnetic refrigeration system using a magnetic material according to the present invention includes a magnetic refrigeration chamber, an introduction pipe, a discharge pipe, and a permanent magnet as main components. The magnetic material is filled into the magnetic refrigeration chamber. The heat exchange medium is introduced into the magnetic refrigeration chamber through the introduction pipe and discharged through the discharge pipe. The permanent magnet is disposed in the vicinity of the magnetic refrigeration chamber. A magnetic field is applied to and removed from the magnetic material by changing the relative position of the permanent magnet with respect to the magnetic refrigeration chamber. The magnetic material is cooled when the magnetic field is removed. The heat exchange medium is cooled by heat exchange with the magnetic material thus cooled.
[0097]
Preferably, the discharge pipe is divided into two systems. The first discharge pipe is used when taking out the heat exchange medium used for the internal precooling from the magnetic refrigeration chamber. The second discharge pipe is used when taking out the heat exchange medium cooled inside from the magnetic refrigeration chamber. In order to change the relative position of the permanent magnet with respect to the magnetic refrigeration chamber, a driving device is provided, and the permanent magnet is attached to the driving device. A magnetic refrigeration cycle is configured by switching the discharge path of the heat exchange medium from the magnetic refrigeration chamber between the first discharge pipe and the second discharge pipe in synchronization with the change in the relative position of the permanent magnet.
[0098]
Preferably, the magnetic material is filled in the magnetic refrigeration chamber with a volume filling rate of 50% to 75%.
[0099]
In the above magnetic refrigeration system, the magnetic material is filled into the magnetic refrigeration chamber in such a manner that a space serving as a flow path for the heat exchange medium is secured. Here, when the filling rate of the magnetic material in the magnetic refrigeration chamber is low, the magnetic materials are stirred by the flow of the heat exchange medium and collide with each other during the heat exchange with the heat exchange medium. Such a collision causes a crack in the magnetic material and further breaks it. The fine powder produced by the destruction of the magnetic material increases the pressure loss of the heat exchange medium and decreases the refrigeration capacity. When it is bad, fine powder accumulates in a part of the pipe and becomes clogged, thereby obstructing the flow of the heat exchange medium. Therefore, in order to avoid such a situation, it is preferable that the magnetic material is accommodated in the magnetic refrigeration chamber with a volume filling rate of 50% to 75%. Furthermore, it is more preferable that it is accommodated at a volume filling rate of 60% or more and 70% or less.
[0100]
Preferably, particles having a particle diameter (major axis) of 0.1 mm or more and 1.5 mm or less and 87 wt% or more of which the aspect ratio is 2 or less are used as the magnetic material.
[0101]
In order to realize a high cooling capacity, it is important that heat exchange is sufficiently performed between the magnetic material filled in the magnetic refrigeration chamber and the heat exchange medium. In order to sufficiently perform heat exchange, it is necessary to increase the specific surface area of the magnetic material. In the case of the magnetic material of the present invention, it is effective to set the particle size small in order to increase the specific surface area. However, when the particle size is too small, the pressure loss of the heat exchange medium increases, so that it is necessary to select an optimum particle size in consideration of this. Here, the particle diameter of the magnetic material is preferably about 0.1 to 1.5 mm, and more preferably about 0.2 to 0.8 mm.
[0102]
Further, the particle shape of the magnetic material is preferably a smooth shape having no protrusions on the surface, for example, a spherical shape or a spheroid shape. By adopting such a shape, it is possible to prevent generation of fine powder due to particle destruction and to suppress an increase in pressure loss of the heat exchange medium.
[0103]
For example, it is preferable that 87% wt or more of particles filled in the magnetic refrigeration chamber have an aspect ratio of 2 or less. When an experiment was performed by mixing irregularly shaped particles having an aspect ratio of 2 or more with substantially spherical particles, when the amount of irregularly shaped particles mixed was 13% or more, it was exposed to the flow of the heat exchange medium for a long time. As a result, fine powder is generated and the pressure loss of the fluid increases.
[0104]
In addition, as a heat exchange medium, mineral oil, a solvent, water, these liquid mixture, etc. can be selected according to the operating temperature range of a heat cycle. As for the particle size of the magnetic material, it is desirable to select an optimum particle size within the above range according to the viscosity (surface tension) of the heat exchange medium used and the capacity of the pump.
[0105]
FIG. 8 shows a schematic configuration of a magnetic refrigeration system in which the magnetic material according to the present invention is used. FIG. 9 shows a schematic configuration of a circulation system for a heat exchange medium in the magnetic refrigeration system. In the figure, 1 is a magnetic material, 2 is a magnetic refrigeration chamber, 3 is an introduction pipe, 4 is a discharge pipe, 5a and 5b are permanent magnets, 6a and 6b are rotating disks, 25 is a low temperature consumption facility, and 26 is a radiator. Represent.
[0106]
As shown in FIG. 8, the magnetic refrigeration chamber 2 has a cylindrical shape with a rectangular cross section. Mesh grids 11 and 12 are attached in the vicinity of both end portions of the magnetic refrigeration work chamber 2, and the magnetic material 1 according to the present invention is filled between them. The magnetic material 1 has, for example, a spherical shape with an average diameter of 0.4 mm, and is filled in the magnetic refrigeration work chamber 2 with a volume filling rate of 62%. The mesh size of the mesh grids 11 and 12 is # 80 and the Cu wire diameter is 0.14 mm. A heat exchange medium introduction pipe 3 is connected to one end of the magnetic refrigeration chamber 2, and a heat exchange medium discharge pipe 4 is connected to the other end. In this example, two magnetic refrigeration chambers 2 having the same shape are provided and arranged in parallel with each other.
[0107]
A pair of turntables 6a and 6b are provided so as to sandwich the two magnetic refrigeration chambers 2 therebetween. The turntables 6a and 6b are supported by a common shaft 7. The shaft 7 is located at the center of the two magnetic refrigeration chambers 2. Permanent magnets 5a and 5b are held on the inner sides in the vicinity of the peripheral edges of the rotary disks 6a and 6b, respectively. The permanent magnets 5a and 5b face each other and are coupled to each other via a yoke (not shown). As a result, a strong magnetic field space is formed in the gap portion between the permanent magnets 5a and 5b paired with each other. In this example, two pairs of permanent magnets 5a and 5b are provided so as to correspond to the two magnetic refrigeration working chambers 2, respectively, and are arranged with the shaft 7 in the center.
[0108]
Each time the turntables 6a and 6b are rotated 90 degrees, the permanent magnets 5a and 5b repeatedly approach and leave the magnetic refrigeration chamber 2. In a state where each pair of permanent magnets 5a and 5b is closest to the side wall of each magnetic refrigeration working chamber 2, the magnetic refrigeration working chamber 2 enters the magnetic field space formed between the permanent magnets 5a and 5b. A magnetic field is applied to the magnetic material 1 accommodated in.
[0109]
When switching from a state in which a magnetic field is applied to the magnetic material 1 to a state in which the magnetic material 1 is removed, the entropy of the electron magnetic spin system increases and entropy movement occurs between the lattice system and the electron magnetic spin system. As a result, the temperature of the magnetic material 1 is lowered, which is transmitted to the heat exchange medium, and the temperature of the heat exchange medium is lowered. The heat exchange medium whose temperature has been lowered in this manner is discharged from the magnetic refrigeration work chamber 2 through the discharge pipe 4 and supplied as a refrigerant to the external low-temperature consumption facility (25: FIG. 9).
[0110]
As shown in FIG. 9, a tank 21 for storing a heat exchange medium is provided upstream of the introduction pipe 3, and a pump 22 is provided in the middle of the introduction pipe 3. The discharge pipe 4 is divided into two systems after leaving the magnetic refrigeration work chamber 2, and two circulation lines are configured. In the middle of one circulation line (cooling line 23), a valve V 1, a low temperature consumption facility 25 and a valve V 3 are provided, and the end of the cooling line 23 is connected to the tank 21. A valve V 2, a radiator 26 and a valve V 4 are provided in the middle of the other circulation line (pre-cooling line 24), and the end of the pre-cooling line 24 is connected to the tank 21.
[0111]
Next, the operation of this magnetic refrigeration system will be described. This magnetic refrigeration system is operated by alternately repeating a pre-cooling process and a cooling process.
[0112]
First, in the precooling step, the valves V2 and V4 are opened with the valves V1 and V3 closed, and the heat exchange medium is circulated in the precooling line 24. In this state, the permanent magnet (5a, 5b: FIG. 8) is brought close to the magnetic refrigeration chamber 2. When a magnetic field is applied to the magnetic material 1, the temperature of the magnetic material 1 rises and is transmitted to the heat exchange medium, and the temperature of the heat exchange medium rises. The heat exchange medium warmed in this way is discharged from the magnetic refrigeration working chamber 2 through the discharge pipe 4, introduced into the heat radiation chamber 26 through the valve V2, and cooled there. The cooled heat exchange medium returns to the tank 21 through the valve V4.
[0113]
When the temperature of the magnetic material 1 in the magnetic refrigeration chamber 2 has dropped to near the temperature of the heat medium supplied to the magnetic refrigeration chamber 2 through the introduction pipe 3, the valves V2 and V4 are closed, and the precooling step is performed. Finish and go to the cooling process.
[0114]
In the cooling process, first, the permanent magnets (5a, 5b: FIG. 8) are moved away from the magnetic refrigeration chamber 2. Next, the valve V <b> 1 and the valve V <b> 3 are opened, and the heat exchange medium is circulated in the cooling line 23. When the magnetic field is removed from the magnetic material 1, the temperature of the magnetic material 1 is lowered, which is transmitted to the heat exchange medium, and the temperature of the heat exchange medium is lowered. The heat exchange medium thus cooled is discharged from the magnetic refrigeration chamber 2 through the discharge pipe 4 and introduced into the low temperature consumption facility 25 through the valve V1. The heat exchange medium is used in the low temperature consumption facility 25 and the temperature rises, and then returns to the tank 21 through the valve V3.
[0115]
When the temperature of the magnetic material 1 in the magnetic refrigeration chamber 2 rises to the vicinity of the temperature of the heat medium supplied to the magnetic refrigeration chamber 2 through the introduction pipe 3, the valves V1 and V3 are closed and the cooling process is performed. End and go to the pre-cooling process again.
[0116]
A control device (not shown) of the magnetic refrigeration system controls the valves V1 to V4 in synchronization with the movement of the permanent magnets 5a and 5b, and alternately repeats the precooling process and the cooling process.
[0117]
【The invention's effect】
In the magnetic material of the present invention, an inflection point appears in the magnetization curve at a relatively low magnetic field in the normal temperature region, and a large entropy change is observed in the electron magnetic spin system near the temperature at which the inflection point appears in the magnetization curve. The Therefore, if the magnetic material of the present invention is used, magnetic refrigeration is realized using a relatively low magnetic field by transferring entropy between the electron magnetic spin system and the lattice system in the vicinity of the above temperature. It becomes possible.
[0118]
Further, by combining such a magnetic material and a permanent magnet, a small-sized, simple, and low-cost magnetic refrigeration system can be configured without using a superconducting magnet.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a magnetization curve of a magnetic material (specimen 1) according to the present invention.
FIG. 2 is a diagram showing an example of a magnetization curve of a magnetic material (specimen 4) according to the present invention.
FIG. 3 is a diagram showing an example of a magnetization curve of a magnetic material (specimen 6) used as a comparative example.
FIG. 4 is a graph showing the temperature dependence of the entropy change of the magnetic material (specimen 1) according to the present invention.
FIG. 5 is a diagram showing an example of a magnetization curve of a magnetic material (specimen 11) according to the present invention.
FIG. 6 is a diagram showing an example of a magnetization curve of a magnetic material (specimen 13) used as a comparative example.
FIG. 7 is a diagram showing an example of a magnetization curve of a magnetic material (specimen 21) according to the present invention.
FIG. 8 is a schematic configuration diagram of a magnetic refrigeration system in which a magnetic material according to the present invention is used.
FIG. 9 shows a schematic configuration diagram of a circulation system of a heat exchange medium in a magnetic refrigeration system in which a magnetic material according to the present invention is used.
[Explanation of symbols]
1 ... Magnetic material,
2 ... Magnetic refrigeration room,
3 ... introduction piping (first flow path),
4 ... discharge piping (second flow path),
5a, b ... permanent magnets,
6a, b ... turntable (drive device),
7 ... axis,
11, 12 ... mesh grid,
21 ... Tank,
22 ... pump,
23 ... Cooling line,
24 ... Pre-cooling line,
25 ... Low temperature consumption facility,
26 ... radiator.

Claims (2)

Magnetism characterized by having an inflection point at which the second derivative with respect to the magnetic field of the magnetization curve changes from positive to negative within the strength range of the magnetic field of 1 Tesla or less only in a part of the temperature range of 200 K to 350 K. Magnetic material for freezing. In the magnetic refrigeration method that creates a refrigeration cycle by changing the external magnetic field acting on the magnetic material and using the entropy change accompanying the magnetic phase transition,
As a magnetic material, a magnetic material having an inflection point at which the second derivative with respect to the magnetic field of the magnetization curve changes from positive to negative within a strength range of a magnetic field of 1 Tesla or less only in a part of a temperature range of 200 K to 350 K. A magnetic refrigeration method using a material.

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