JP4070110B2

JP4070110B2 - Thermoelectric conversion material comprising Ln2S3 sintered body and method for producing the same

Info

Publication number: JP4070110B2
Application number: JP2002353348A
Authority: JP
Inventors: 伸治平井; 聡之西村; 揚一郎上村; 成紀森田; 道広太田
Original assignee: Japan Science and Technology Agency; Nitto Denko Corp; National Institute for Materials Science; National Institute of Japan Science and Technology Agency
Current assignee: Japan Science and Technology Agency; Nitto Denko Corp; National Institute for Materials Science; National Institute of Japan Science and Technology Agency
Priority date: 2002-12-05
Filing date: 2002-12-05
Publication date: 2008-04-02
Anticipated expiration: 2022-12-05
Also published as: JP2004186529A

Description

【０００１】
【発明の属する技術分野】
本発明は、特に、室温付近から高温域まで熱電材料として有用な、比抵抗が小さく、大きなパワーファクターを有する熱電変換材料用ランタノイド三二硫化物焼結体およびその製造方法に関する。
【０００２】
【従来の技術】
熱電変換材料の応用は、多岐に亘っている。熱エネルギーを電気エネルギーに変換するクリーンエネルギー源としての利用が最も期待されるところであるが、ペルチェ効果を利用するものとして小型冷凍器、放熱板、恒温槽、電熱用等が考えられ、また実現されている。
【０００３】
熱起電力は、２種の電気伝導体を接合したときに２接点間の温度差ΔＴにより発生する電圧Ｖであり、それらの間にはＶ＝αΔＴの関係がある。このαのことをゼーベック係数という。この熱起電力を利用して、熱エネルギーを電気エネルギーに変換する際に、熱電材料の有効性を示す指標として、電気伝導度σ、熱伝導度κを使って、式、Ｚ＝α²σ／κ、で示される性能指数Ｚが用いられている。このＺの値の大きい材料ほど優れた熱電材料となる。α²σは、一般にパワーファクターと呼ばれ、熱電変換材料から取り出せる電力の大きさの指標になる。
【０００４】
既に報告され、又は利用されている熱電材料は多く、現在、最も大きい性能指数が得られているのはＢｉ−Ｔｅ系の物質で、約３×１０^-3（／Ｋ）の値を示しているが、それらの物質のゼーベック係数の値は、約２００（μＶ／Ｋ）程度である。
【０００５】
希土類元素の硫化物は、大きなゼーベック係数を持ち、ランタノイド三二硫化物の中でもＬａからＮｄまでの硫化物は、低温安定相である斜方晶のα相から正方晶のβ相、さらに、高温安定相である立方晶Ｔｈ₃Ｐ₄型のγ相へと不可逆変態し、特に、Ｌａ₂Ｓ₃は、３７３Ｋで＋３５４μＶ・ｄｅｇ^-1、Ｃｅ₂Ｓ₃は、３７３Ｋで＋５７４μＶ・ｄｅｇ^-1のゼーべック係数を有する熱電材料であることが報告されている（例えば、非特許文献１、２）。
【０００６】
また、硫化ランタンＬａ_3-xＳ₄およびＬａ−Ａ−Ｓ系（ＡはＣａ又はＢａ）において、最大２．９×１０^-4（／Ｋ）の性能指数が得られたことが報告されている（非特許文献３）。しかし、そこで報告されているゼーベック係数は最大値で約１００（μＶ／Ｋ）である。
【０００７】
本発明者らは、ランタン硫化物系において従来報告されていたゼーベック係数よりも約一桁大きいゼーベック係数を有する物質を見出し、特許出願した（特許文献１、非特許文献４）。この物質は、組成がＬａ₂ Ｓ₃で示され、結晶構造がβとγの混合相からなり、ゼーベック係数がγ単相のときより大きい値を有することを特徴とする硫化ランタン焼結体であり、ゼーベック係数が６０℃で１０００（μＶ／Ｋ）以上の値を有する。
【０００８】
高融点材料には難焼結性というイメージがあるが、本発明者らは、ランタノイド三二硫化物を真空中でホットプレスし、焼結温度をγ変態温度以上に設定しγ単相の焼結体を製作すると、Ｌａ〜Ｓｍまでのいずれの場合も、相対密度が９７〜１００％のものが得られることや、低酸素含有量の合成粉末を用いるとγ単相の焼結体が得られ、高酸素含有量の合成粉末ではγとβの混合相あるいはβ単相の焼結体が得られること、焼結体の残留酸素含有量は、焼結条件が高温、長時間ほど減少する傾向を示すこと、さらには、ランタノイド三二硫化物は１０００Ｋを超す高温で大きなゼーベック係数を有するため、今後キャリア濃度を綿密に調整しながら電気抵抗を下げることができれば、熱電性能指数も増大し次世代の高温域熱電材料として期待できることを報告した（非特許文献５）。
【０００９】
さらに、Ｌａ₂Ｓ₃を予め一定の圧力を加えることにより冷間成形した圧粉体を焼結した常圧焼結体も、同じ温度でホットプレス法により作製した焼結体とゼーベック係数は殆ど遜色がないことを報告した（非特許文献６）。
【００１０】
本発明者らは、その後、この焼結体にＰｄ粒子を添加することにより比抵抗が減少し性能指数が向上することを見出した（非特許文献７、特願２００２−５６５５９）。
【００１１】
【非特許文献１】
ゲ・ヴェ・サムソノフ他著,「硫化物便覧」,日・ソ通信社,1974年,p108
【非特許文献２】
C.Wood et al.,「Thermoelectric properties of lanthanum sulfide」, J.Appl.Phys.,Vol.58,No.4,1985年8月15日,pp1542-1547
【非特許文献３】
勝山茂他,「３元系希土類カルコゲナイドLa-A-S(A=Ca,Ba)の熱電特性」,熱電変換シンポジウム´９９論文集,熱電変換研究会,1999年,pp56-57
【非特許文献４】
平井伸治他,「α-La₂S₃の合成と熱電特性」,日本金属学会秋期（第125回）大会講演概要,1999年11月,p317
【非特許文献５】
平井伸治他,「ランタノイド系二元系硫化物の合成と焼結」,金属,Vo.70,No.8,2000年,pp629-635
【非特許文献６】
平井伸治他,「耐火材料や熱電材料として期待されるランタノイド二元系硫化物」,金属,Vo.70,No.11,2000年,pp960-965
【非特許文献７】
上村揚一郎他,「Pdを添加したLa₂S₃常圧焼結体の熱電特性」,日本物理学会2001年秋期大会講演概要集,第56巻，第2号，第4分冊,2001年,p530
【特許文献１】
特開2001-335367号公報
【００１２】
【発明が解決しようとする課題】
熱電変換材料は、放熱板として電子機器の冷却、センサー等、その応用範囲は広く、地球上に限らず、人工衛星においても存在する温度差を電気エネルギーとして利用することは、究極のクリーンエネルギー源であり、地球環境の悪化を考えるとき、その開発は急務である。
【００１３】
熱電変換素子を利用したクリーンエネルギーの実用化が実現すれば、その効果は非常に大きい。また、放熱板等への応用はＩＣの集積度緻密化による発熱問題を解決し、小型化、高集積化等この方面での一層の進展が期待される。
【００１４】
ランタノイド三二硫化物（Ｌｎ₂Ｓ₃）は、低温で安定な斜方晶のα相から温度の上昇に連れて正方晶のβ相へ、さらに高温ではβ相から立方晶のγ相へといずれも不可逆的な相変態を示す。
【００１５】
本発明者らが先に発明した硫化ランタン又は硫化セリウムなどのランタノイド三二硫化物焼結体は、大きなゼーベック係数αを持ち、高い性能指数Ｚを持つ材料であるが、電気抵抗が大きく、性能指数の値も小さかった。
【００１６】
熱電変換材料の上述の性能指数Ｚを求める式において、３種の物理的性質がその値を決定しているが、ゼーベック係数αの値は２乗でＺの値を大きくすることから、このα値の大きな物質がより優れた熱電材料となり得る。
【００１７】
そこで、本発明は、ランタノイド三二硫化物において、大きなゼーベック係数を持ち、電気伝導性を付加することで、比抵抗が小さく大きなパワーファクターを有する熱電変換素子用ランタノイド三二硫化物焼結体を提供することを目的とする。
【００１８】
【課題を解決するための手段】
本発明者らは、高温でβ相から立方晶のγ相へと不可逆的な相変態をするランタノイド三二硫化物焼結体について、種々の元素の添加により電気伝導度の改善を検討する過程で、原料にＧｅ粉末，Ｎｂ粉末の少なくとも１種類を混合して焼結すると、上記の課題を解決できることを見出した。また、炭素粉末の添加によっても同様な結果が得られることを見出した。
【００１９】
すなわち、本発明は、（１）組成式Ｌｎ₂Ｓ₃（Ｌｎは、Ｌａ，Ｃｅ，Ｐｒ，Ｎｄ，Ｓｍ，Ｅｕ，Ｇｄ，Ｔｂ，Ｄｙ，Ｈｏ，Ｅｒ，Ｔｍ，Ｙｂ，Ｌｕの群から選ばれる少なくとも１種）で表されるβ型ランタノイド三二硫化物粉末にＧｅ粉末又はＮｂ粉末の少なくとも１種を混合して焼結した、結晶構造がβ型を主成分とするランタノイド三二硫化物焼結体中にＧｅ又はＮｂの少なくとも１種類が金属粒子として分散し、比抵抗が１０Ωｃｍ以下であることを特徴とするＬｎ₂Ｓ₃焼結体からなる熱電変換材料、である。
また、本発明は、（２）焼結体中に分散するＧｅ又はＮｂの少なくとも１種類の含有量が０．５〜１０質量％であることを特徴とする上記（１）のＬｎ₂Ｓ₃焼結体からなる熱電変換材料、である。
【００２０】
また、本発明は、（３）組成式Ｌｎ₂Ｓ₃（Ｌｎは、Ｌａ，Ｃｅ，Ｐｒ，Ｎｄ，Ｓｍ，Ｅｕ，Ｇｄ，Ｔｂ，Ｄｙ，Ｈｏ，Ｅｒ，Ｔｍ，Ｙｂ，Ｌｕの群から選ばれる少なくとも１種）で表されるβ型ランタノイド三二硫化物粉末に炭素粉末を混合して焼結した結晶構造がβ型を主成分とするランタノイド三二硫化物焼結体中に炭素が粒子状に分散し、比抵抗が１０Ωｃｍ以下であることを特徴とするＬｎ₂Ｓ₃焼結体からなる熱電変換材料、である。
また、本発明は、（４）焼結体中に分散する炭素の含有量が０．１〜５質量％であることを特徴とする上記（３）のＬｎ₂Ｓ₃焼結体からなる熱電変換材料、である。
【００２１】
また、本発明は、（５）組成式Ｌｎ₂Ｓ₃（Ｌｎは、Ｌａ，Ｃｅ，Ｐｒ，Ｎｄ，Ｓｍ，Ｅｕ，Ｇｄ，Ｔｂ，Ｄｙ，Ｈｏ，Ｅｒ，Ｔｍ，Ｙｂ，Ｌｕの群から選ばれる少なくとも１種）で表されるβ型ランタノイド三二硫化物粉末に平均粒径が１００μｍ以下のＧｅ粉末，Ｎｂ粉末の少なくとも１種類を混合し、成型後又は成型と同時に１３００℃から１７００℃の温度範囲で焼結することを特徴とする上記（１）又は（２）のＬｎ₂Ｓ₃焼結体からなる熱電変換材料の製造方法、である。
【００２２】
また、本発明は、（６）組成式Ｌｎ₂Ｓ₃（Ｌｎは、Ｌａ，Ｃｅ，Ｐｒ，Ｎｄ，Ｓｍ，Ｅｕ，Ｇｄ，Ｔｂ，Ｄｙ，Ｈｏ，Ｅｒ，Ｔｍ，Ｙｂ，Ｌｕの群から選ばれる少なくとも１種）で表されるβ型ランタノイド三二硫化物粉末に平均粒径が１μｍ以下の炭素粉末を混合し、成型後又は成型と同時に１３００℃から１７００℃の温度範囲で焼結することを特徴とする上記（３）又は（４）のＬｎ₂Ｓ₃焼結体からなる熱電変換材料の製造方法、である。
また、本発明は、（７）β型ランタノイド三二硫化物粉末は不純物としての酸素含有量が０．９重量％以上であることを特徴とする上記（５）又は（６）のＬｎ₂Ｓ₃焼結体からなる熱電変換材料の製造方法、である。
【００２３】
このように、原料にＧｅ粉末、Ｎｂ粉末の少なくとも１種や炭素粉末を混合して焼結することにより、なぜ大きなパワーファクターが得られるか、その機構は明確ではないが、炭素は導電性を高める物質であり、Ｇｅ又はＮｂを添加して焼結を行うと一部がランタノイド三二硫化物中の硫黄と反応して炭素と同様に導電性のある硫化物を形成するために抵抗が下がるのではないかと推測される。
【００２４】
原料に混合したＧｅ粉末、Ｎｂ粉末はランタノイド三二硫化物焼結体中の結晶粒界や粒内に分散して存在するが、このＧｅ、Ｎｂの少なくとも１種の含有量は０．５〜１０質量％とする。これらの含有量が０．５質量％より少ないと電気抵抗が大きく、これらの含有量が１０質量％より多いと良質な焼結体を得るのが困難になりパワーファクターが低下する。ランタノイド三二硫化物焼結体中の炭素の含有量は０．１〜５質量％とする。この含有量が０．１質量％より少ないと電気抵抗が大きく、５質量％より多いと電気抵抗は低くなるが、ゼーベック係数が小さくなり、パワーファクターも減少する。
【００２５】
本発明のランタノイド三二硫化物焼結体は、例えば、温度差を利用したクリーンエネルギーの発電材料として、宇宙船等での補助電源、熱電対温度計、ペルチェ効果を利用した、電熱器、小型冷凍機、吸熱板、放熱板、恒温槽などに利用される。
【００２６】
【発明の実施の形態】
本発明の熱電変換材料の製造方法において、β型ランタノイド三二硫化物粉末は、ランタノイドの酸化物をＣＳ₂ガスを使用して硫化したものを使用でき、これらの粉末は市販品として入手できる。粒径範囲は約１００〜０．１μｍで、酸素が約０．９質量％〜１．２質量％含有されており、純度は約９９．９％である。ランタノイド三二硫化物の酸素濃度が１．２重量％を上回ると、γ相が生成せずに電気的に絶縁体となってしまう。
更に、本発明者らが報告してきた（非特許文献５）ランタノイドの酸化物をＣＳ₂ガスを使用して高温で硫化して得られた高純度β型ランタノイド三二硫化物粉末も使用できる。例えば、時間２８．８Ks、温度１２７３Kで硫化して得られたβ型硫化ランタン粉末では酸素濃度０．１８質量％であり、粒径範囲は約３００〜０．１μｍである。なお、粒径範囲はSEM観察から見積った。
【００２７】
β型ランタノイド三二硫化物粉末に混合するＧｅ粉末、Ｎｂ粉末、炭素粉末の少なくとも１種は平均粒径１００μｍ以下で、混合量はＧｅ粉末、Ｎｂ粉末は０．５〜１０質量％、炭素粉末は０．１〜５質量％とする。
Ｇｅ粉末、Ｎｂ粉末の平均粒径は、より好ましくは５０μｍ以下、特に２０μｍ〜０．１μ程度が好ましい。これらの粉末の平均粒径が１００μｍを超えると、焼結体中に含まれるＧｅ又はＮｂが熱電変換材料を長時間使用した場合に偏析して、電気抵抗が１×１０¹Ωｃｍよりも大きくなり、好ましくない。
Ｇｅ粉末、Ｎｂ粉末の混合量が１０質量％を超えるとＧｅ又はＮｂが偏析して焼結を阻害するため、好ましくない。より好ましくは１〜５質量％である。
炭素粉末の平均粒径は、より好ましくは１μｍ以下、特に０．１μｍ〜０．０１μ程度が好ましい。炭素粉末の平均粒径が１００μｍを超える場合、焼結を阻害する為、好ましくない。炭素粉末の混合量が５質量％を超えるとゼーベック係数が減少しパワーファクターが低下するため、より好ましくは０．５〜３質量％である。
【００２８】
焼結方法は真空中又は不活性ガス中でのホットプレス焼結法、通電パルス焼結法、常圧焼結法などのいずれの焼結法でもよい。焼結温度が１３００℃未満では、γ相が生成せず、焼結体は電気的に絶縁体のままであり、１７００℃を超えると、β相が減少し、ゼーベック係数が小さくなり不適当である。焼結保持時間は焼結方法や焼結温度に関係し、比較的低い温度では長時間の焼成が必要になるが、長くても２時間程度まででよい。焼結温度が比較的高い場合は保持時間は０分でも良い。
【００２９】
常圧焼結法の場合は、常温において、２５ＭＰａ以上の圧力で圧縮成型する。圧縮成型の圧力が２５ＭＰａ未満では、焼結体はポーラスで電気抵抗が１×１０¹⁵Ω・ｃｍ以上に大きくなり、不適当である。圧力の上限は特に限定されないが、必要以上に大きくする必要はなく、好ましくは１５０ＭＰａ程度以下とする。
【００３０】
このように、圧縮成型と常圧焼結法を採用することにより結晶構造がβ型を主成分とし、微量のγ型成分を有する硫化ランタン焼結体が得られる。この方法により、焼結体の脆さや電気的性質が改善され、比抵抗が１０Ωｃｍ以下を実現することができる。
【００３１】
【実施例】
実施例１
Ｌａ₂Ｓ₃粉末(酸素含有量約１質量％：高純度化学（株）)にＮｂ粉末（平均粒径２０μｍ：田中貴金属（株））をそれぞれ１．０質量％、３．３質量％、６．２質量％混ぜた混合粉末を六方晶窒化ホウ素で内部を被覆した黒鉛ジグに入れ、２０MPａの圧力を加えながら真空中で１５００℃、４５分間保持することで、ホットプレス焼結を行った。得られた焼結体はＸ線回折測定でβ相のみが確認できた。焼結体の特性を１００℃で測定した結果を表１に示す。
【００３２】
実施例２
添加金属としてＧｅ粉末（平均粒径１０μｍ：田中貴金属（株））３．１質量％を用いる以外は、実施例１と同じ方法で焼結体を作製した。得られた焼結体はＸ線回折測定でβ相のみが確認できた。焼結体の特性を１００℃で測定した結果を表１に示す。
【００３３】
実施例３
添加金属として炭素粉末（平均粒径０．０２２μｍ：東海カーボン（株））をそれぞれ０．６質量％、１．０質量％、４．７質量％混用いる以外は、実施例１と同じ方法で焼結体を作製した。得られた焼結体の特性を表１に示す。
【００３４】
比較例１
添加物を加えない以外は、実施例１と同じ方法で焼結体を作製した。得られた添加元素無添加の焼結体の特性を表１に示す。
【００３５】
【表１】

【００３６】
比較例２
実施例１のＮｂ粉末に代えてＰｄ粒（平均粒径０．４μｍ：田中貴金属（株））を用いた以外は、実施例１と同じ方法で焼結体を作製した。得られた焼結体は、比抵抗＞１０¹⁰Ωｃｍ、ゼーベック係数の値は０μV/Kであった。
【００３７】
実施例３
Ｌａ₂Ｓ₃粉末(酸素含有量約１質量％：高純度化学（株）)にＮｂ粉末（平均粒径２０μｍ：田中貴金属（株））をそれぞれ１．０質量％、６．０質量％、１０．０質量％混ぜた混合粉末をグラファイトのシートで内部を被覆した黒鉛ジグに入れ、３０MPａの圧力を加えながら真空中で１５００℃、３０分間保持することで、通電パルス焼結を行った。得られた焼結体はＸ線回折測定でβ相のみが確認できた。焼結体の特性を測定した結果を表２に示す。抵抗率は室温、ゼーベック係数は６０℃で測定を行った。パワーファクターは室温での抵抗率と６０℃でのゼーベック係数から算出した。
【００３８】
【表２】

【００３９】
【発明の効果】
本発明のランタノイド三二硫化物焼結体からなる熱電変換材料は、大きなゼーベック係数を持ち、比抵抗が小さく、公知のパラジウムを添加したもの（６×１０^-9）よりも大きなパワーファクター（１×１０^-7以上）を有し、熱電変換材料から取り出せる電力が大きいので、室温から高温域まで幅広く使用できる。特に、材料の安定性から高温域熱電材料として、火力発電所の廃熱や地熱を利用した発電システムに利用が期待できる。[0001]
BACKGROUND OF THE INVENTION
The present invention particularly relates to a lanthanoid tridisulfide sintered body for a thermoelectric conversion material having a small specific resistance and a large power factor, which is useful as a thermoelectric material from around room temperature to a high temperature range, and a method for producing the same.
[0002]
[Prior art]
Applications of thermoelectric conversion materials are diverse. Although it is most expected to be used as a clean energy source that converts thermal energy into electrical energy, small refrigerators, heat sinks, thermostats, electric heaters, etc. can be considered and realized as those using the Peltier effect. ing.
[0003]
The thermoelectromotive force is a voltage V generated by a temperature difference ΔT between two contact points when two kinds of electrical conductors are joined, and there is a relationship of V = αΔT between them. This α is called Seebeck coefficient. When this thermal electromotive force is used to convert thermal energy into electrical energy, the electrical conductivity σ and thermal conductivity κ are used as indices indicating the effectiveness of the thermoelectric material, and the equation Z = α ² σ A figure of merit Z indicated by / κ is used. A material having a larger value of Z is a superior thermoelectric material. α ² σ is generally called a power factor and is an index of the amount of power that can be extracted from the thermoelectric conversion material.
[0004]
There are many thermoelectric materials that have already been reported or used. Currently, the largest figure of merit has been obtained for Bi-Te based materials, indicating a value of about 3 × 10 ⁻³ (/ K). However, the Seebeck coefficient of these substances is about 200 (μV / K).
[0005]
Rare earth element sulfides have a large Seebeck coefficient, and among lanthanoid tridisulfides, sulfides from La to Nd are low-temperature stable phases orthorhombic α phase to tetragonal β phase, and high temperature It is irreversibly transformed into a cubic Th ₃ P ₄ type γ phase, which is a stable phase. In particular, La ₂ S ₃ is +354 μV · deg ⁻¹ at 373 K, and Ce ₂ S ₃ is +574 μV · deg ⁻¹ at 373 K. It has been reported that it is a thermoelectric material having a Seebeck coefficient (for example, Non-Patent Documents 1 and 2).
[0006]
In addition, it has been reported that a figure of merit of 2.9 × 10 ⁻⁴ (/ K) at maximum was obtained in the lanthanum sulfide La _3-x S ₄ and La-AS systems (A is Ca or Ba). (Non-patent Document 3). However, the Seebeck coefficient reported there is a maximum value of about 100 (μV / K).
[0007]
The present inventors have found a substance having a Seebeck coefficient that is about an order of magnitude larger than the conventionally reported Seebeck coefficient in lanthanum sulfide systems, and have applied for a patent (Patent Document 1, Non-Patent Document 4). This material is a lanthanum sulfide sintered body characterized in that the composition is La ₂ S ₃ , the crystal structure is a mixed phase of β and γ, and the Seebeck coefficient is larger than that of a γ single phase. Yes, the Seebeck coefficient has a value of 1000 (μV / K) or more at 60 ° C.
[0008]
Although the high melting point material has an image that it is difficult to sinter, the present inventors hot-pressed the lanthanoid tridisulfide in a vacuum, set the sintering temperature to be equal to or higher than the γ transformation temperature, and sintered the γ single phase. When a bonded body is manufactured, in any case from La to Sm, a material having a relative density of 97 to 100% can be obtained, or a synthetic powder having a low oxygen content can be used to obtain a γ single-phase sintered body. In the case of synthetic powders with high oxygen content, it is possible to obtain a γ and β mixed phase or β single phase sintered body, and the residual oxygen content of the sintered body decreases as the sintering conditions increase at higher temperatures. Since lanthanoid tridisulfide has a large Seebeck coefficient at a high temperature exceeding 1000 K, if the electric resistance can be lowered while carefully adjusting the carrier concentration in the future, the thermoelectric figure of merit will increase. Expected as a high-temperature thermoelectric material of the next generation It reported Rukoto (Non-Patent Document 5).
[0009]
Furthermore, an atmospheric pressure sintered body obtained by sintering a green compact formed by cold forming La ₂ S ₃ in advance at a constant pressure has almost the same Seebeck coefficient as a sintered body produced by hot pressing at the same temperature. It was reported that there was no fading (Non-patent document 6).
[0010]
Subsequently, the inventors have found that by adding Pd particles to this sintered body, the specific resistance is reduced and the figure of merit is improved (Non-patent Document 7, Japanese Patent Application No. 2002-56559).
[0011]
[Non-Patent Document 1]
Ge Ve Samsonov et al., “Sulphide Handbook”, Japan-So-Soshinsha, 1974, p. 108
[Non-Patent Document 2]
C. Wood et al., `` Thermoelectric properties of lanthanum sulfide '', J. Appl. Phys., Vol. 58, No. 4, August 15, 1985, pp1542-1547
[Non-Patent Document 3]
Shigeru Katsuyama et al., “Thermoelectric Properties of Ternary Rare Earth Chalcogenide La-AS (A = Ca, Ba)”, Thermoelectric Conversion Symposium '99 Proceedings, Thermoelectric Conversion Study Group, 1999, pp56-57
[Non-Patent Document 4]
Shinji Hirai et al., “Synthesis and Thermoelectric Properties of α-La ₂ S ₃ ”, Abstracts of Annual Meeting of the Japan Institute of Metals (125th), November 1999, p317
[Non-Patent Document 5]
Shinji Hirai et al., "Synthesis and Sintering of Lanthanoid Binary Sulfides", Metals, Vo.70, No.8, 2000, pp629-635
[Non-Patent Document 6]
Shinji Hirai et al., “Lantanoid binary sulfides expected as refractory and thermoelectric materials”, Metals, Vo.70, No.11, 2000, pp960-965
[Non-Patent Document 7]
Uemura, Yoichiro et al., “Thermoelectric properties of La ₂ S ₃ normal-pressure sintered body with Pd added”, Japanese Physical Society of Japan 2001 Fall Conference, Vol.56, No.2, Volume 4, 2001, p530
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-335367
[Problems to be solved by the invention]
Thermoelectric conversion materials have a wide range of applications, such as cooling of electronic equipment and sensors as heat sinks, and the use of temperature differences that exist not only on the earth but also on satellites as electrical energy is the ultimate clean energy source. The development is urgent when considering the deterioration of the global environment.
[0013]
If practical use of clean energy using a thermoelectric conversion element is realized, the effect is very large. Further, application to a heat sink or the like solves the problem of heat generation due to the densification of IC integration, and further progress in this direction such as downsizing and higher integration is expected.
[0014]
Lanthanoid tridisulfide (Ln ₂ S ₃ ) is transformed from an orthorhombic α phase, which is stable at low temperatures, to a tetragonal β phase as the temperature increases, and from a β phase to a cubic γ phase at higher temperatures. Both show irreversible phase transformations.
[0015]
A lanthanoid tridisulfide sintered body such as lanthanum sulfide or cerium sulfide previously invented by the present inventors is a material having a large Seebeck coefficient α and a high figure of merit Z. The index value was also small.
[0016]
In the above formula for obtaining the above-mentioned figure of merit Z of the thermoelectric conversion material, three kinds of physical properties determine the value. Since the value of the Seebeck coefficient α is squared, the value of Z is increased. Substances with large values can be better thermoelectric materials.
[0017]
Therefore, the present invention provides a lanthanoid tridisulfide sintered body for a thermoelectric conversion element having a large Seebeck coefficient and adding electrical conductivity to have a small specific resistance and a large power factor. The purpose is to provide.
[0018]
[Means for Solving the Problems]
The present inventors have investigated the improvement of electrical conductivity by adding various elements to lanthanoid tridisulfide sintered bodies that undergo irreversible phase transformation from β phase to cubic γ phase at high temperature. Thus, it has been found that the above-mentioned problems can be solved by mixing and sintering at least one of Ge powder and Nb powder in the raw material. Moreover, it discovered that the same result was obtained also by addition of carbon powder.
[0019]
That is, the present invention provides (1) composition formula Ln ₂ S ₃ (Ln is selected from the group of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. Lanthanoid tridisulfide having a crystal structure of β-type as a main component, which is sintered by mixing at least one kind of Ge powder or Nb powder with β-type lanthanoid tridisulfide powder represented by A thermoelectric conversion material comprising a Ln ₂ S ₃ sintered body, wherein at least one kind of Ge or Nb is dispersed as metal particles in the sintered body and the specific resistance is 10 Ωcm or less.
Further, the present invention is, (2) Ln ₂ S ₃ of (1) the content of at least one of Ge or Nb dispersed in the sintered body, characterized in that from 0.5 to 10 wt% A thermoelectric conversion material made of a sintered body.
[0020]
In the present invention, (3) composition formula Ln ₂ S ₃ (Ln is selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In the lanthanoid tridisulfide sintered body mainly composed of β type, carbon particles are obtained by mixing and sintering carbon powder with β type lanthanoid tridisulfide powder represented by A thermoelectric conversion material comprising a Ln ₂ S ₃ sintered body, characterized in that the specific resistance is 10 Ωcm or less.
The present invention also provides (4) a thermoelectric comprising the Ln ₂ S ₃ sintered body of (3) above, wherein the content of carbon dispersed in the sintered body is 0.1 to 5% by mass. Conversion material.
[0021]
In the present invention, the composition formula (5) is selected from the group consisting of Ln ₂ S ₃ (Ln is La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). At least one kind of Ge powder and Nb powder with an average particle size of 100 μm or less, and after molding or simultaneously with molding, 1300 ° C. to 1700 ° C. A method for producing a thermoelectric conversion material comprising the Ln ₂ S ₃ sintered body according to (1) or (2), wherein sintering is performed in a temperature range.
[0022]
In the present invention, (6) composition formula Ln ₂ S ₃ (Ln is selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. A carbon powder having an average particle size of 1 μm or less is mixed with β-type lanthanoid tridisulfide powder represented by the formula (1) and sintered at a temperature range of 1300 ° C. to 1700 ° C. after molding or simultaneously with molding. A method for producing a thermoelectric conversion material comprising the Ln ₂ S ₃ sintered body according to (3) or (4) above.
In the present invention, (7) the β-type lanthanoid tridisulfide powder has an oxygen content as an impurity of 0.9% by weight or more, and the Ln ₂ S of (5) or (6) above ₃ is a method for producing a thermoelectric conversion material comprising a sintered body.
[0023]
As described above, the mechanism of why a large power factor can be obtained by mixing and sintering at least one of Ge powder and Nb powder and carbon powder as raw materials is not clear, but carbon has conductivity. It is a substance to increase, and when Ge or Nb is added and sintered, a part of it reacts with sulfur in the lanthanide tridisulfide to form a conductive sulfide similar to carbon, resulting in a decrease in resistance. It is guessed that.
[0024]
The Ge powder and Nb powder mixed in the raw material are dispersed in the crystal grain boundaries and grains in the lanthanoid tridisulfide sintered body, but the content of at least one kind of Ge and Nb is 0.5 to 10% by mass. If the content is less than 0.5% by mass, the electric resistance is large, and if the content is more than 10% by mass, it becomes difficult to obtain a high-quality sintered body and the power factor is lowered. The carbon content in the lanthanoid tridisulfide sintered body is 0.1 to 5% by mass. When the content is less than 0.1% by mass, the electrical resistance is high. When the content is more than 5% by mass, the electrical resistance is low, but the Seebeck coefficient is reduced and the power factor is also reduced.
[0025]
The lanthanoid tridisulfide sintered body of the present invention is, for example, an auxiliary power source in a spacecraft or the like, a thermocouple thermometer, an electric heater using a Peltier effect, a small size as a clean energy power generation material using a temperature difference Used in refrigerators, heat absorbing plates, heat sinks, thermostats, etc.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
In the method for producing a thermoelectric conversion material of the present invention, β-type lanthanoid tridisulfide powder obtained by sulfurizing a lanthanoid oxide using CS ₂ gas can be used, and these powders are available as commercial products. The particle size range is about 100 to 0.1 μm, oxygen is contained in an amount of about 0.9% to 1.2% by mass, and the purity is about 99.9%. If the oxygen concentration of the lanthanoid tridisulfide exceeds 1.2% by weight, a γ phase is not generated and an electrical insulator is formed.
Furthermore, high purity β-type lanthanoid tridisulfide powder obtained by sulfiding a lanthanoid oxide at a high temperature using CS ₂ gas, which has been reported by the present inventors (Non-patent Document 5), can also be used. For example, β-type lanthanum sulfide powder obtained by sulfiding at a time of 28.8 Ks and a temperature of 1273 K has an oxygen concentration of 0.18% by mass and a particle size range of about 300 to 0.1 μm. The particle size range was estimated from SEM observation.
[0027]
At least one of Ge powder, Nb powder, and carbon powder to be mixed with β-type lanthanoid tridisulfide powder has an average particle size of 100 μm or less, the mixing amount is Ge powder, Nb powder is 0.5 to 10% by mass, carbon powder Is 0.1 to 5% by mass.
The average particle size of the Ge powder and Nb powder is more preferably 50 μm or less, and particularly preferably about 20 μm to 0.1 μm. When the average particle diameter of these powders exceeds 100 μm, Ge or Nb contained in the sintered body segregates when the thermoelectric conversion material is used for a long time, and the electric resistance becomes larger than 1 × 10 ¹ Ωcm. It is not preferable.
If the mixing amount of Ge powder and Nb powder exceeds 10% by mass, Ge or Nb is segregated to inhibit sintering, which is not preferable. More preferably, it is 1-5 mass%.
The average particle size of the carbon powder is more preferably 1 μm or less, and particularly preferably about 0.1 μm to 0.01 μm. When the average particle diameter of the carbon powder exceeds 100 μm, sintering is hindered, which is not preferable. When the mixing amount of the carbon powder exceeds 5% by mass, the Seebeck coefficient decreases and the power factor decreases. Therefore, the amount is more preferably 0.5 to 3% by mass.
[0028]
The sintering method may be any sintering method such as a hot press sintering method in vacuum or in an inert gas, a current pulse sintering method, a normal pressure sintering method, or the like. If the sintering temperature is less than 1300 ° C., the γ phase is not generated, and the sintered body remains an electrically insulating material. If the sintering temperature exceeds 1700 ° C., the β phase decreases, and the Seebeck coefficient decreases, which is inappropriate. is there. The sintering holding time is related to the sintering method and the sintering temperature, and long time firing is required at a relatively low temperature, but it may be up to about 2 hours at the longest. If the sintering temperature is relatively high, the holding time may be 0 minutes.
[0029]
In the case of the normal pressure sintering method, compression molding is performed at room temperature at a pressure of 25 MPa or more. If the compression molding pressure is less than 25 MPa, the sintered body is porous and has an electric resistance of 1 × 10 ¹⁵ Ω · cm or more, which is inappropriate. The upper limit of the pressure is not particularly limited, but need not be increased more than necessary, and is preferably about 150 MPa or less.
[0030]
In this way, by employing compression molding and atmospheric pressure sintering, a lanthanum sulfide sintered body having a crystal structure mainly composed of β type and a small amount of γ type component can be obtained. By this method, the brittleness and electrical properties of the sintered body are improved, and a specific resistance of 10 Ωcm or less can be realized.
[0031]
【Example】
Example 1
La ₂ S ₃ powder (oxygen content about 1% by mass: High Purity Chemical Co., Ltd.) and Nb powder (average particle size 20 μm: Tanaka Kikinzoku Co., Ltd.) 1.0% by mass, 3.3% by mass, The mixed powder mixed with 6.2% by mass was placed in a graphite jig coated with hexagonal boron nitride, and held at 1500 ° C. for 45 minutes in a vacuum while applying a pressure of 20 MPa to perform hot press sintering. . Only the β phase was confirmed in the obtained sintered body by X-ray diffraction measurement. Table 1 shows the results of measuring the characteristics of the sintered body at 100 ° C.
[0032]
Example 2
A sintered body was produced in the same manner as in Example 1 except that 3.1% by mass of Ge powder (average particle size: 10 μm: Tanaka Kikinzoku Co., Ltd.) was used as the additive metal. Only the β phase was confirmed in the obtained sintered body by X-ray diffraction measurement. Table 1 shows the results of measuring the characteristics of the sintered body at 100 ° C.
[0033]
Example 3
The same method as in Example 1 except that carbon powder (average particle size 0.022 μm: Tokai Carbon Co., Ltd.) was used as an additive metal in a mixture of 0.6 mass%, 1.0 mass%, and 4.7 mass%, respectively. A sintered body was produced. Table 1 shows the characteristics of the obtained sintered body.
[0034]
Comparative Example 1
A sintered body was produced in the same manner as in Example 1 except that no additive was added. Table 1 shows the characteristics of the obtained sintered body without additive elements.
[0035]
[Table 1]

[0036]
Comparative Example 2
A sintered body was produced in the same manner as in Example 1 except that Pd grains (average particle size 0.4 μm: Tanaka Kikinzoku Co., Ltd.) were used instead of the Nb powder in Example 1. The obtained sintered body had a specific resistance> 10 ¹⁰ Ωcm and a Seebeck coefficient of 0 μV / K.
[0037]
Example 3
La ₂ S ₃ powder (oxygen content about 1% by mass: High Purity Chemical Co., Ltd.) and Nb powder (average particle size 20 μm: Tanaka Kikinzoku Co., Ltd.) are respectively 1.0% by mass, 6.0% by mass, The mixed powder mixed with 10.0% by mass was placed in a graphite jig whose interior was covered with a graphite sheet, and kept at 1500 ° C. for 30 minutes in a vacuum while applying a pressure of 30 MPa, thereby conducting energization pulse sintering. Only the β phase was confirmed in the obtained sintered body by X-ray diffraction measurement. Table 2 shows the results of measuring the characteristics of the sintered body. The resistivity was measured at room temperature, and the Seebeck coefficient was measured at 60 ° C. The power factor was calculated from the resistivity at room temperature and the Seebeck coefficient at 60 ° C.
[0038]
[Table 2]

[0039]
【The invention's effect】
The thermoelectric conversion material comprising the lanthanoid tridisulfide sintered body of the present invention has a large Seebeck coefficient, a small specific resistance, and a power factor (1 × 1) larger than that of a known palladium added (6 × 10 ⁻⁹ ). × 10 ⁻⁷ or more) and a large amount of electric power that can be extracted from the thermoelectric conversion material, it can be used widely from room temperature to high temperature range. In particular, it can be expected to be used in power generation systems that utilize waste heat and geothermal heat from thermal power plants as high-temperature thermoelectric materials due to the stability of the materials.

Claims

It is represented by a composition formula Ln ₂ S ₃ (Ln is at least one selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). At least one kind of Ge powder or Nb powder is mixed with β-type lanthanoid tridisulfide powder and sintered, and a lanthanoid tridisulfide sintered body having a crystal structure of β-type as a main component contains Ge or Nb. A thermoelectric conversion material comprising a Ln ₂ S ₃ sintered body, wherein at least one kind is dispersed as metal particles and the specific resistance is 10 Ωcm or less.

The thermoelectric conversion material comprising a Ln ₂ S ₃ sintered body according to claim 1, wherein the content of at least one of Ge or Nb dispersed in the sintered body is 0.5 to 10% by mass.

It is represented by a composition formula Ln ₂ S ₃ (Ln is at least one selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). Carbon powder is mixed with β-type lanthanoid tridisulfide powder and sintered, and the crystal structure is dispersed as particles in a lanthanoid tridisulfide sintered body whose main component is β-type, and the specific resistance is 10 Ωcm. A thermoelectric conversion material comprising a Ln ₂ S ₃ sintered body, characterized in that:

The thermoelectric conversion material comprising a Ln ₂ S ₃ sintered body according to claim 3, wherein the content of carbon dispersed in the sintered body is 0.1 to 5% by mass.

It is represented by a composition formula Ln ₂ S ₃ (Ln is at least one selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). It is characterized by mixing at least one kind of Ge powder or Nb powder having an average particle diameter of 100 μm or less with β-type lanthanoid tridisulfide powder and sintering it at a temperature range of 1300 ° C. to 1700 ° C. after molding or simultaneously with molding. A method for producing a thermoelectric conversion material comprising the Ln ₂ S ₃ sintered body according to claim 1 or 2.

It is represented by a composition formula Ln ₂ S ₃ (Ln is at least one selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). 5. A carbon powder having an average particle size of 1 μm or less is mixed with β-type lanthanoid tridisulfide powder, and sintered in a temperature range of 1300 ° C. to 1700 ° C. after molding or simultaneously with molding. A method for producing a thermoelectric conversion material comprising the described Ln ₂ S ₃ sintered body.

The process for producing a thermoelectric conversion material comprising an Ln ₂ S ₃ sintered body according to claim 5 or 6, wherein the β-type lanthanoid tridisulfide powder has an oxygen content as an impurity of 0.9 wt% or more. Method.