JP3580778B2

JP3580778B2 - Thermoelectric conversion element and method of manufacturing the same

Info

Publication number: JP3580778B2
Application number: JP2001020616A
Authority: JP
Inventors: 啓久瀬知; 広一田中; 健一田島; 和博西薗
Original assignee: Kyocera Corp
Current assignee: Kyocera Corp
Priority date: 2001-01-29
Filing date: 2001-01-29
Publication date: 2004-10-27
Anticipated expiration: 2021-01-29
Also published as: JP2002223013A

Description

【０００１】
【発明の属する技術分野】
本発明は、熱電変換素子及びその製造方法に関し、特に、熱電冷却素子、熱電発電素子として好適に使用される熱電変換素子の製造方法に関する。
【０００２】
【従来技術】
Ｂｉ、Ｔｅ、Ｓｂ、Ｓｅ等の化合物は、熱電変換材料として知られているが、それ自体は、融点が低く、化学的安定性に欠け、焼結性も悪い。そのため、多結晶材料を採用し、熱電変換性能を改善する方法が提案され、研究されている。
【０００３】
この熱電変換材料の性能は、αをゼーベック係数、ρを比抵抗、κを熱伝導率としたとき、Ｚ＝α^２／ρ・κで定義される性能指数Ｚで表され、Ｚが大きいほど変換性能が優れた材料であることが知られている。従って、ゼーベック係数が大きく、比抵抗及び熱伝導率が小さいほど性能指数Ｚは大きくなる。
【０００４】
熱電変換材料が多結晶材料の場合、焼結性と特性とを両立することが難しかった。即ち、結晶粒子径の小さな原料粉末は焼結性が良好であるものの、比抵抗の増大を招き、逆に、結晶粒子径の大きい原料粉末は、焼結性が悪く、得られた焼結体の比抵抗は低いものの熱伝導率が大きくなるため、性能指数Ｚの高い焼結体が得られていなかった。
【０００５】
そこで、同一組成で粒子径の異なる２種類の原料粉末を組み合わせることで、焼結性の改善と比抵抗増大抑制を両立する方法が、特開平９−５５５４２号公報で提案されている。
【０００６】
また、ｐ型熱電半導体の焼結性を改善するために酸素濃度３０ｐｐｍ以下の非酸化性ガス雰囲気で粉砕、成形し、成形体体積が炉容積との体積比率１％以上において常圧焼成を行ったのち、一軸プレスやＣＩＰによって加圧し、熱処理することによって緻密体を得ることが、特開平１１−２８４２３７号公報で提案されている。
【０００７】
【発明が解決しようとする課題】
しかしながら、特開平９−５５５４２号公報の方法は、粒度配合を用いることによって緻密化を促進させるものであり、その結果、熱伝導率を低減させる効果があるものの、電気伝導率も低下するため、性能指数Ｚの改善は十分ではないという問題点があった。
【０００８】
また、特開平１１−２８４２３７号公報の方法では、焼成後に一軸プレスやＣＩＰによる加圧ならびに熱処理を必要とするため、工程が複雑となり、コストが高くなるという問題点があった。
【０００９】
従って、本発明は、熱伝導率及び比抵抗が小さく、性能指数の高い熱電変換素子とそれを低コストで製造する方法を提供することを目的とする。
【００１０】
【課題を解決するための手段】
本発明は、熱電変換材料からなる原料粉末の粒度配合に加えて、気孔径及び気孔分布の制御を行うことによって熱伝導率及び比抵抗を低減し、相対密度を向上させることができるという知見に基づき、その結果、高い性能指数を実現するものである。
【００１１】
即ち、熱電変換材料からなる結晶を主体とし、相対密度が９０〜９８％の焼結体であり、該焼結体内に平均径１〜５μｍの気孔が分布していることを特徴とするものである。このような微細な気孔を均一に分布させ、相対密度を制御することにより、熱伝導率のさらなる低減を図ることが出来る。ならびに原料粉末の粒度配合による粗大粒の存在により比抵抗の改善が可能となる。
【００１２】
特に、比抵抗が１．０×１０^−５Ω・ｍ以下であること、又は熱伝導率が１．５Ｗ／ｍ・Ｋ以下であることが好ましい。これにより、さらに性能指数Ｚを高くすることができる。
【００１３】
また、前記熱電変換材料がビスマス、テルル、アンチモン及びセレンのうち少なくとも２種からなることが好ましい。これにより、室温付近（２０〜１００℃）でのゼーベック係数が高くなる結果、性能指数Ｚが向上するため、特に室温付近で使用する熱電変換素子として優れた特性を示すことができる。
【００１４】
また、本発明の熱電変換素子の製造方法は、平均粒子径１０〜１００μｍと０．１〜５μｍとの熱電変換材料からなる２種類の原料粉末を混合し、混合粉末を成形た後に５〜１００気圧の非酸化性ガス雰囲気中で、相対密度が９０〜９８％となるように焼成することを特徴とするもので、これにより、焼結体の相対密度を向上させて電気伝導性を向上させると共に、雰囲気圧力、温度プロファイルを制御することで気孔分布ならびに気孔径を制御し、低熱伝導化を実現することができる。
【００１５】
特に、前記混合粉末より作成した成形体を、前記焼成の前に、あらかじめ４気圧以下の圧力で予備焼成することが好ましい。これにより、相対密度向上を図ると共に焼結体中に微細な気孔を分布させることで熱伝導率の更なる低減を図ることができる。
【００１６】
また、前記焼成が４００℃〜５００℃で行われることが好ましい。これにより、構成成分の揮発を防ぎつつ焼結体の緻密化を図ることが可能となる。
【００１７】
さらに、前記原料粉末がビスマス、テルル、アンチモン及びセレンのうち少なくとも２種からなることが好ましい。これにより、室温付近で優れた特性を示す熱電変換素子を作製することができる。
【００１８】
【発明の実施の形態】
本発明の熱電変換素子は、相対密度が９０〜９８％の焼結体であることが重要であり、特に９３〜９８％、さらには９５〜９８％が好ましい。相対密度が９０％未満では性能指数が小さく、９８％を越えると気孔率が低下し熱伝導率が増加し、性能指数が低下するため、上記の範囲にすることが必要である。
【００１９】
また、本発明によれば、焼結体内に存在する気孔は、熱伝導率を低減させる作用を有するため、焼結体中に残存する気孔を微細化すること重要であり、これによって電気抵抗率を増加させることなく、熱伝導率の低減を図ることが可能となる。
【００２０】
その結果、焼結体内に平均径１〜５μｍの気孔が分布していることが重要であり、特に気孔の平均径は１〜３μｍ、さらには１〜２μｍであることが好ましい。気孔の平均径が１μｍ未満では、熱伝導率低減効果が低下し、５μｍを越えると比抵抗の増大を招く。
【００２１】
特に、上記の気孔は、均一に焼結体中に分布していることが好ましく、これにより熱電変換特性と機械特性の均一化と熱電変換素子の信頼性及び安定性を向上することが可能となる。
【００２２】
さらに、本発明の熱電変換素子は、比抵抗が１．０×１０^−５Ω・ｍ以下、特に８．５×１０^−６Ω・ｍ以下、さらには５×１０^−６Ω・ｍ以下であることが好ましい。これによって、性能指数Ｚを更に高くすることができる。
【００２３】
また、熱伝導率が１．５Ｗ／ｍ・Ｋ以下、特に１．０Ｗ／ｍ・Ｋ以下、さらには０．５Ｗ／ｍ・Ｋ以下であることが好ましい。これによって、性能指数Ｚを更に高くすることができる。
【００２４】
さらに、前記熱電変換材料がビスマス（Ｂｉ）、テルル（Ｔｅ）、アンチモン（Ｓｂ）及びセレン（Ｓｅ）のうち少なくとも２種からなることが好ましい。上記の元素を２種選択するのは室温付近でのゼーベック係数が高いためであり、例えば、Ｂｉ−Ｔｅ、Ｂｉ−Ｓｂ等の組合せを用いることができる。これらの材料は室温付近でのゼーベック係数が高いため、性能指数Ｚを向上させることが出来る。
【００２５】
以上のように構成される本発明の熱電変換素子は、高い性能指数Ｚを有するため、特に熱電冷却素子や熱電発電素子として好適に使用することができる。
【００２６】
次に、本発明の熱電変換素子を製造する方法を説明する。
【００２７】
本発明の熱電変換材料の製造方法において、熱電変換材料の原料組成は半導体組成を適用する。このような組成としては、特に、室温付近で性能指数の高いＢｉ−Ｔｅ、Ｂｉ−Ｓｂ、Ｔｅ−Ｓｂ等を用いることが好ましく、以下に、Ｂｉ−Ｔｅ系熱電半導体を製造する場合を例として取り上げて説明する。
【００２８】
本発明によれば、平均粒子径１０〜１００μｍの粗大粒子と０．１〜５μｍの微細粒子との２種類のＢｉ−Ｔｅ粉末を用いても良いが、平均粒子径０．５〜２００μｍのＢｉ及びＴｅ粉末からＢｉ−Ｔｅの粗大粉末及び微細粉末を作製してもよい。以下は、ＢｉとＴｅ粉末を用いる場合について説明する。
【００２９】
粗大粒子と微細粒子はそれぞれ単独で焼成すると得られる特性が異なる。即ち、粗大原料を用いた焼結体は、熱伝導率が高いものの、比抵抗とゼーベック係数が低い。一方、微細原料を用いた焼結体は、熱伝導率が低いものの比抵抗とゼーベック係数が高い。そして、性能指数を算出すると粗大原料を用いても、粒子原料を用いても大きな差はない。
【００３０】
しかし、同一の化学組成で粗大粒子と微細粒子とからなる粉末材料を用いて焼結体を作製し、熱電変換材料として利用する場合、材料の熱電変換特性を決定する個々の基本的物性である熱伝導率、電気抵抗率、ゼーベック係数は２種類の熱電変換材料粉末の混合比率（複合比率）に対してそれぞれ互いに異なる変化挙動を示す。即ち、熱伝導率は複合比率に対して非線形の変化を示し、ある値を閾値として急峻な変化を示す傾向があり、混合比率を制御することによって高くすることができる。そのため、粗大粒子と微細粒子の比が９：１〜５：５、特に８：２〜６：４であることが好ましい。
【００３１】
粗大粒子の平均粒子径は１０〜１００μｍとすることが比抵抗を低減させるために重要であり、特に３０〜９０μｍ、さらには５０〜８０μｍが好ましい。１０μｍ未満では微細粒子との粒子径差が小さくなって粒度配合の効果が小さく、特に比抵抗低減の効果が薄れる。また、１００μｍを越えると焼結性が悪化し、相対密度が９０％以下となり性能指数Ｚが低下するためである。
【００３２】
また、微細粒子の平均粒子径は０．１〜５μｍとすることが重要であり、特に０．２〜３μｍ、さらには０．３〜２μｍが好ましい。これにより焼結性を向上させると共に焼結体全体の熱伝導率を低減させることが出来るが、平均粒子径が０．１μｍ未満の場合、長時間の粉砕が必要となり原料作製コストが高騰し現実的でない。また、粒子径が５μｍを越えると焼結性が悪化し、相対密度が９０％以下となり性能指数Ｚが低下する。
【００３３】
準備したＢｉ粉末、Ｔｅ粉末を仮焼し、ライカイ機等により粉砕して粗大粉末と微細粉末を作製する。例えば、Ｂｉ粉末及びＴｅ粉末をＢｉ_２Ｔｅ_３組成となるように秤量し、非酸化性雰囲気中で２０〜９６時間の熱処理を行い、得られた仮焼体を乳鉢にて粗粉砕した後、回転ボールミル、振動ミル、スタンプミル、遊星ボールミルを用いて乾式粉砕を行い、粉砕時間に応じて平均粒子径が１０〜１００μｍの原料粉末Ａと０．１〜５μｍまで微粉砕した原料粉末Ｂを作製することができる。
【００３４】
得られた２種類の粒子径を有する原料粉末Ａ及びＢを所定の割合に混合した混合粉末を用いて各種の公知の成形方法、例えば一軸プレス法、ＣＩＰ法、鋳込み法、射出成形法等により成形体を作製する。プレス法による成形方法を用いる場合、成形圧１００ＭＰａで成形して所望の形状を得ることができる。
【００３５】
このようにして作製した成形体を、５〜１００気圧の非酸化性ガス雰囲気中で、相対密度が９０〜９８％となるように焼成することが重要であり、圧力は特に５０〜１００気圧、さらには、８０〜１００気圧であることが好ましい。この圧力が５気圧未満であると気孔径の制御が難しくなり、１００気圧を超えると焼成炉の構造が複雑となり焼成の費用が高くなる。
【００３６】
また、非酸化性ガスを雰囲気に用いるのは、酸化による熱電性能低下防止のためであり、具体的には、窒素、水素、アンモニア、不活性ガス及びこれらの混合ガスを例示できる。
【００３７】
焼成温度は、十分な焼結性を確保し、かつ原料成分の揮発を防ぐために４００〜５００℃の範囲であることが好ましい。
【００３８】
また、本発明によれば、上記の成形体を、前記焼成の前に、あらかじめ４気圧以下の圧力で予備焼成することが好ましい。つまり、一次焼成として成形体を４気圧以下の圧力で焼成し、次いで、二次焼成として、５〜１００気圧の非酸化性ガス雰囲気中で焼成するものであり、これによって気孔径制御が容易となる。なお、一次焼成は１〜８時間、特に３〜６時間、二次焼成は１〜１５時間、特に３〜１０時間行うことが、相対密度９０〜９８％、気孔の平均径１〜５μｍの焼結体を得るために好ましい。
【００３９】
このように、粒度配合とともに焼成方法及びその条件を選択することによって気孔の平均径及び分布状態を制御することができ、その結果、本発明の性能指数Ｚの高い熱電変換素子を低コストで実現することができる。
【００４０】
【実施例】
まず、出発原料として準備した純度９９．９９％以上のビスマス、テルルをＢｉ_２Ｔｅ_３となるように秤量し、これらの混合粉末をそれぞれパイレックスガラス管に真空封入しロッキング炉にて溶融・攪拌後冷却することにより熱電半導体材料インゴットを作製した。その後スタンプミルを用いて粗粉砕した後、エタノール溶媒中で２時間および４８時間回転ミルを施し、表１に示す粒径の粗大原料粉末Ａと微細原料粉末Ｂを作製した。
【００４１】
次に、原料粉末ＡとＢを所定の割合に混合した混合粉末を一軸プレスにて成形圧１００ＭＰａで成形し、直径１０ｍｍ、厚さ１ｍｍ及び縦４ｍｍ、横５ｍｍ、長さ２０ｍｍの２種類の成形体を作製した。
【００４２】
これらの成形体を、表１に示す条件で焼成を行った。なお、一次焼成は常圧焼成法、二次焼成はＧＰＳ（ガス圧）焼成法を用いた。得られた焼結体は、比抵抗ρは、ＪＩＳＣ２１４１に準拠した３端子法により、２５℃で測定した。熱伝導率κは、レーザフラッシュ法によって測定した。
【００４３】
また、ゼーベック係数αは市販の熱起電力測定装置（真空理工製ＺＥＭ−１）にて測定し、性能指数Ｚ（×１０^−３／Ｋ）をＺ＝α^２／ρ・κの計算式で算出した。さらに、相対密度は、焼結体の比重をアルキメデス法で測定するとともに、ＪＩＳＣ２１４１−１９９２に規定される方法に準拠して測定した粉末試料の真密度から相対密度と気孔率を算出した。
【００４４】
さらにまた、平均気孔径は、走査型電子顕微鏡（ＳＥＭ）による４００倍の写真から気孔の長径及び短径とを測定し、その平均値を気孔径とするとともに、１００個の気孔について気孔径を測定し、平均値を算出した。結果を表１に示す。
【００４５】
【表１】

【００４６】
本発明の試料Ｎｏ．２〜７、９、１１〜１３、１５〜１７、２０〜２２は、相対密度が９１〜９８％、平均気孔径が１〜５μｍで、性能指数Ｚが全て２．８×１０^−３／Ｋ以上と熱電性能に優れた素子であった。
【００４７】
一方、常圧焼成のみで、本発明の範囲外の試料Ｎｏ．１は、相対密度が８５％と低く、平均気孔径が５０μｍと大きく、性能指数Ｚが１．６４×１０^−３／Ｋと低い値であった。
【００４８】
また、焼成時のガス加圧保持時間が０．５時間と短く、本発明の範囲外の試料Ｎｏ．８は相対密度が８８％と低く、性能指数Ｚが１．５２×１０^−３／Ｋと低い値であった。
【００４９】
さらに、粗大原料Ａの平均粒子径が２μｍと小さく、本発明の範囲外の試料Ｎｏ．１０は、相対密度が８８％と低く、性能指数Ｚが２．１×１０^−３／Ｋと低い値であった。
【００５０】
さらにまた、粗大原料Ａの平均粒子径が１５０μｍと大きく、本発明の範囲外の試料Ｎｏ．１４は、相対密度が８８％と低く、平均気孔径が１０μｍと大きく、性能指数Ｚが１．９７×１０^−３／Ｋと低い値であった。
【００５１】
また、微細原料Ｂの粒径が８μｍと大きく、本発明の範囲外の試料Ｎｏ．１８は、相対密度が８８％と低く、平均気孔径が７．２μｍと大きく、性能指数Ｚが１．９８×１０^−３／Ｋと低い値であった。
【００５２】
さらに、微細原料Ｂを含まず、粗大原料Ａのみで、本発明の範囲外の試料Ｎｏ．１９は、相対密度が８０％と低く、平均気孔径が１０μｍと大きく、性能指数Ｚが２．０７×１０^−３／Ｋと低い値であった。
【００５３】
さらにまた、粗大原料Ａを含まず、微細原料Ｂのみで、相対密度が９９％と大きい本発明の範囲外の試料Ｎｏ．２３は、平均気孔径が０．０５μｍと小さく、性能指数Ｚが１．６０×１０^−３／Ｋと低い値であった。
【００５４】
また、二次焼成が酸化性雰囲気で、本発明の範囲外の試料Ｎｏ．２４は、試料が酸化したため、相対密度が８３％と低く、平均気孔径が１２μｍと大きく、性能指数Ｚがほぼ０であった。
【００５５】
【発明の効果】
本発明は、Ｔｅ系熱電変換材料において焼結性の向上と熱伝導率の低減及び電気抵抗率の増大抑制を同時に達成して性能指数が優れた熱電変換材料を得られることが認められた。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a thermoelectric conversion element and a method for manufacturing the same, and more particularly, to a method for manufacturing a thermoelectric conversion element suitably used as a thermoelectric cooling element and a thermoelectric power generation element.
[0002]
[Prior art]
Compounds such as Bi, Te, Sb, and Se are known as thermoelectric conversion materials, but themselves have low melting points, lack chemical stability, and have poor sinterability. Therefore, a method of improving the thermoelectric conversion performance by using a polycrystalline material has been proposed and studied.
[0003]
The performance of this thermoelectric conversion material is represented by a performance index Z defined by Z = α ² / ρ · κ, where α is the Seebeck coefficient, ρ is the specific resistance, and κ is the thermal conductivity. It is known that the material has excellent conversion performance. Therefore, the larger the Seebeck coefficient and the smaller the specific resistance and the thermal conductivity, the larger the figure of merit Z.
[0004]
When the thermoelectric conversion material is a polycrystalline material, it has been difficult to achieve both sinterability and characteristics. That is, the raw material powder having a small crystal particle diameter has good sinterability, but causes an increase in specific resistance. Conversely, the raw material powder having a large crystal particle diameter has poor sinterability, and the obtained sintered body is poor. Although the specific resistance was low, the thermal conductivity increased, and a sintered body with a high figure of merit Z was not obtained.
[0005]
Japanese Patent Application Laid-Open No. H9-55542 proposes a method of combining two types of raw material powders having the same composition and different particle diameters to achieve both improvement in sinterability and suppression of increase in specific resistance.
[0006]
Further, in order to improve the sinterability of the p-type thermoelectric semiconductor, the p-type thermoelectric semiconductor is pulverized and molded in a non-oxidizing gas atmosphere having an oxygen concentration of 30 ppm or less, and normal pressure firing is performed at a volume ratio of 1% or more with respect to the furnace volume. After that, it has been proposed in Japanese Patent Application Laid-Open No. 11-284237 to obtain a dense body by applying pressure by a uniaxial press or CIP and performing heat treatment.
[0007]
[Problems to be solved by the invention]
However, the method disclosed in Japanese Patent Application Laid-Open No. 9-55542 promotes densification by using a mixture of particle sizes. As a result, although the effect of reducing thermal conductivity is obtained, the electrical conductivity is also reduced. There is a problem that the improvement of the figure of merit Z is not sufficient.
[0008]
In addition, the method disclosed in Japanese Patent Application Laid-Open No. H11-284237 requires pressurization by a uniaxial press or CIP and heat treatment after firing, so that the process is complicated and the cost is increased.
[0009]
Therefore, an object of the present invention is to provide a thermoelectric conversion element having a small thermal conductivity and a low specific resistance and a high figure of merit, and a method of manufacturing the same at low cost.
[0010]
[Means for Solving the Problems]
The present invention is based on the finding that, in addition to the particle size blending of the raw material powder composed of a thermoelectric conversion material, by controlling the pore diameter and the pore distribution, the thermal conductivity and the specific resistance can be reduced and the relative density can be improved. As a result, a high figure of merit is realized.
[0011]
That is, the sintered body is mainly composed of a crystal made of a thermoelectric conversion material and has a relative density of 90 to 98%, and pores having an average diameter of 1 to 5 μm are distributed in the sintered body. is there. By uniformly distributing such fine pores and controlling the relative density, the thermal conductivity can be further reduced. The specific resistance can be improved by the presence of coarse particles due to the particle size combination of the raw material powder.
[0012]
In particular, the specific resistance is preferably 1.0 × 10 ⁻⁵ Ω · m or less, or the thermal conductivity is preferably 1.5 W / m · K or less. Thereby, the figure of merit Z can be further increased.
[0013]
Further, it is preferable that the thermoelectric conversion material is made of at least two of bismuth, tellurium, antimony, and selenium. As a result, the Seebeck coefficient near room temperature (20 to 100 ° C.) increases, and the figure of merit Z improves, so that excellent characteristics can be exhibited particularly as a thermoelectric conversion element used near room temperature.
[0014]
Further, the method for producing a thermoelectric conversion element of the present invention comprises mixing two types of raw material powders each composed of a thermoelectric conversion material having an average particle size of 10 to 100 μm and 0.1 to 5 μm, forming a mixed powder, It is characterized by firing in a non-oxidizing gas atmosphere at atmospheric pressure so that the relative density becomes 90 to 98%, thereby improving the relative density of the sintered body and improving the electrical conductivity. At the same time, by controlling the atmospheric pressure and the temperature profile, the pore distribution and the pore diameter can be controlled, and low thermal conductivity can be realized.
[0015]
In particular, it is preferable to pre-fire a formed body made from the mixed powder at a pressure of 4 atm or less before the firing. Thereby, the relative density can be improved, and fine pores are distributed in the sintered body, so that the thermal conductivity can be further reduced.
[0016]
Further, it is preferable that the calcination is performed at 400 ° C. to 500 ° C. As a result, it is possible to densify the sintered body while preventing volatilization of the constituent components.
[0017]
Further, it is preferable that the raw material powder comprises at least two of bismuth, tellurium, antimony and selenium. Thereby, a thermoelectric conversion element exhibiting excellent characteristics near room temperature can be manufactured.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
It is important that the thermoelectric conversion element of the present invention is a sintered body having a relative density of 90 to 98%, particularly preferably 93 to 98%, more preferably 95 to 98%. If the relative density is less than 90%, the figure of merit is small, and if it exceeds 98%, the porosity decreases, the thermal conductivity increases, and the figure of merit decreases.
[0019]
Further, according to the present invention, the pores present in the sintered body have a function of reducing the thermal conductivity, so that it is important to make the pores remaining in the sintered body fine, Without increasing the thermal conductivity.
[0020]
As a result, it is important that pores having an average diameter of 1 to 5 μm are distributed in the sintered body. In particular, the average diameter of the pores is preferably 1 to 3 μm, and more preferably 1 to 2 μm. When the average diameter of the pores is less than 1 μm, the effect of reducing the thermal conductivity decreases, and when it exceeds 5 μm, the specific resistance increases.
[0021]
In particular, the pores are preferably uniformly distributed in the sintered body, which makes it possible to make the thermoelectric conversion characteristics and mechanical characteristics uniform and improve the reliability and stability of the thermoelectric conversion element. Become.
[0022]
Furthermore, the thermoelectric conversion element of the present invention has a specific resistance of 1.0 × 10 ⁻⁵ Ω · m or less, particularly 8.5 × 10 ⁻⁶ Ω · m or less, and further preferably 5 × 10 ⁻⁶ Ω · m or less. Preferably, there is. Thereby, the figure of merit Z can be further increased.
[0023]
Further, the thermal conductivity is preferably 1.5 W / m · K or less, particularly preferably 1.0 W / m · K or less, more preferably 0.5 W / m · K or less. Thereby, the figure of merit Z can be further increased.
[0024]
Further, it is preferable that the thermoelectric conversion material is made of at least two of bismuth (Bi), tellurium (Te), antimony (Sb), and selenium (Se). The reason for selecting two types of the above elements is that the Seebeck coefficient near room temperature is high, and for example, a combination of Bi-Te, Bi-Sb, or the like can be used. Since these materials have a high Seebeck coefficient near room temperature, the figure of merit Z can be improved.
[0025]
Since the thermoelectric conversion element of the present invention configured as described above has a high figure of merit Z, it can be suitably used particularly as a thermoelectric cooling element or a thermoelectric power generation element.
[0026]
Next, a method for manufacturing the thermoelectric conversion element of the present invention will be described.
[0027]
In the method for producing a thermoelectric conversion material of the present invention, a semiconductor composition is used as a raw material composition of the thermoelectric conversion material. As such a composition, in particular, it is preferable to use Bi-Te, Bi-Sb, Te-Sb, or the like having a high figure of merit near room temperature. In the following, a case where a Bi-Te-based thermoelectric semiconductor is manufactured will be described as an example. Take it up and explain.
[0028]
According to the present invention, two types of Bi-Te powder of coarse particles having an average particle diameter of 10 to 100 μm and fine particles having a particle diameter of 0.1 to 5 μm may be used, but Bi having an average particle diameter of 0.5 to 200 μm may be used. Alternatively, a coarse powder and a fine powder of Bi-Te may be produced from the Te powder and the Te powder. Hereinafter, the case where Bi and Te powders are used will be described.
[0029]
Coarse particles and fine particles have different properties when fired independently. That is, the sintered body using the coarse raw material has high thermal conductivity, but low specific resistance and Seebeck coefficient. On the other hand, a sintered body using a fine raw material has a low thermal conductivity but a high specific resistance and a high Seebeck coefficient. When the figure of merit is calculated, there is no significant difference between using a coarse raw material and using a particle raw material.
[0030]
However, when a sintered body is produced using a powder material composed of coarse particles and fine particles with the same chemical composition and used as a thermoelectric conversion material, it is an individual basic physical property that determines the thermoelectric conversion characteristics of the material. The thermal conductivity, the electrical resistivity, and the Seebeck coefficient change differently from each other with respect to the mixing ratio (composite ratio) of the two types of thermoelectric conversion material powders. That is, the thermal conductivity tends to show a non-linear change with respect to the composite ratio, and show a steep change with a certain value as a threshold, and can be increased by controlling the mixture ratio. Therefore, the ratio of coarse particles to fine particles is preferably from 9: 1 to 5: 5, particularly preferably from 8: 2 to 6: 4.
[0031]
It is important that the average particle diameter of the coarse particles is 10 to 100 μm in order to reduce the specific resistance, and it is particularly preferably 30 to 90 μm, further preferably 50 to 80 μm. If it is less than 10 μm, the difference in particle size from the fine particles is small, and the effect of the particle size blending is small, and the effect of reducing the specific resistance is particularly diminished. On the other hand, if it exceeds 100 μm, the sinterability deteriorates, the relative density becomes 90% or less, and the figure of merit Z decreases.
[0032]
It is important that the average particle size of the fine particles is 0.1 to 5 μm, particularly 0.2 to 3 μm, and more preferably 0.3 to 2 μm. As a result, the sinterability can be improved and the thermal conductivity of the entire sintered body can be reduced. However, when the average particle diameter is less than 0.1 μm, pulverization for a long time is required, so that the raw material production cost rises and the actual cost increases. Not a target. If the particle diameter exceeds 5 μm, the sinterability deteriorates, the relative density becomes 90% or less, and the figure of merit Z decreases.
[0033]
The prepared Bi powder and Te powder are calcined and pulverized by a raikai machine or the like to produce a coarse powder and a fine powder. For example, Bi powder and Te powder are weighed so as to have a Bi ₂ Te ₃ composition, heat-treated for 20 to 96 hours in a non-oxidizing atmosphere, and the obtained calcined body is roughly pulverized in a mortar, Dry pulverization is performed using a rotary ball mill, vibration mill, stamp mill, or planetary ball mill, and a raw material powder A having an average particle diameter of 10 to 100 μm and a raw material powder B finely pulverized to 0.1 to 5 μm are produced according to the pulverization time. can do.
[0034]
Various known molding methods, for example, a uniaxial pressing method, a CIP method, a casting method, an injection molding method, and the like, using a mixed powder obtained by mixing the obtained raw material powders A and B having two kinds of particle diameters in a predetermined ratio, are used. A molded body is produced. When a molding method using a pressing method is used, a desired shape can be obtained by molding at a molding pressure of 100 MPa.
[0035]
It is important that the formed body thus formed is fired in a non-oxidizing gas atmosphere of 5 to 100 atm so that the relative density becomes 90 to 98%. Further, the pressure is preferably 80 to 100 atm. If the pressure is less than 5 atm, it is difficult to control the pore diameter, and if it exceeds 100 atm, the structure of the firing furnace becomes complicated and the cost of firing increases.
[0036]
The use of a non-oxidizing gas in the atmosphere is for preventing a decrease in thermoelectric performance due to oxidation, and specific examples thereof include nitrogen, hydrogen, ammonia, an inert gas, and a mixed gas thereof.
[0037]
The firing temperature is preferably in the range of 400 to 500 ° C. in order to ensure sufficient sinterability and prevent volatilization of the raw material components.
[0038]
Further, according to the present invention, it is preferable that the above-mentioned molded body is preliminarily fired at a pressure of 4 atm or less before the firing. In other words, the compact is fired at a pressure of 4 atm or less as the primary firing, and then fired in a non-oxidizing gas atmosphere at 5 to 100 atm as the secondary firing. Become. The primary firing is performed for 1 to 8 hours, particularly 3 to 6 hours, and the secondary firing is performed for 1 to 15 hours, particularly 3 to 10 hours. The firing is performed at a relative density of 90 to 98% and an average pore diameter of 1 to 5 μm. It is preferable to obtain a union.
[0039]
As described above, the average diameter and distribution of pores can be controlled by selecting the firing method and the conditions together with the particle size blending. As a result, a thermoelectric conversion element having a high figure of merit Z of the present invention can be realized at low cost. can do.
[0040]
【Example】
First, bismuth and tellurium having a purity of 99.99% or more prepared as starting materials are weighed so as to be Bi ₂ Te _3, and these mixed powders are each vacuum-sealed in a Pyrex glass tube and melted and stirred in a rocking furnace. By cooling, a thermoelectric semiconductor material ingot was produced. Then, after coarsely pulverizing using a stamp mill, a rotary mill was performed in an ethanol solvent for 2 hours and 48 hours to produce coarse raw material powder A and fine raw material powder B having the particle diameters shown in Table 1.
[0041]
Next, a mixed powder obtained by mixing the raw material powders A and B at a predetermined ratio is molded by a uniaxial press with a molding pressure of 100 MPa, and two types of moldings having a diameter of 10 mm, a thickness of 1 mm, a length of 4 mm, a width of 5 mm, and a length of 20 mm are formed. The body was made.
[0042]
These compacts were fired under the conditions shown in Table 1. Note that the primary firing was performed under the normal pressure firing method, and the secondary firing was performed using the GPS (gas pressure) firing method. The specific resistance ρ of the obtained sintered body was measured at 25 ° C. by a three-terminal method based on JIS C2141. The thermal conductivity κ was measured by a laser flash method.
[0043]
The Seebeck coefficient α is measured with a commercially available thermoelectromotive force measuring device (ZEM-1 manufactured by Vacuum Riko), and the figure of merit Z (× 10 ⁻³ / K) is calculated by the formula of Z = α ² / ρ · κ. Calculated. Further, the relative density was determined by measuring the specific gravity of the sintered body by the Archimedes method and calculating the relative density and the porosity from the true density of the powder sample measured according to the method specified in JIS C2141-1992.
[0044]
Furthermore, the average pore diameter is obtained by measuring the long diameter and the short diameter of the pores from a 400 × photograph by a scanning electron microscope (SEM), and taking the average value as the pore diameter. It measured and calculated the average value. Table 1 shows the results.
[0045]
[Table 1]

[0046]
Sample No. of the present invention 2 to 7, 9, 11 to 13, 15 to 17, and 20 to 22 have a relative density of 91 to 98%, an average pore diameter of 1 to 5 μm, and a performance index Z of 2.8 × 10 ⁻³ / K. Thus, the element was excellent in thermoelectric performance.
[0047]
On the other hand, the sample No. which was out of the scope of the present invention only under normal pressure firing. In No. 1, the relative density was as low as 85%, the average pore diameter was as large as 50 μm, and the figure of merit Z was as low as 1.64 × 10 ⁻³ / K.
[0048]
Further, the gas pressure holding time during firing was as short as 0.5 hour, and Sample No. out of the range of the present invention was used. 8, the relative density was as low as 88%, and the figure of merit Z was as low as 1.52 × 10 ⁻³ / K.
[0049]
Further, the average particle diameter of the coarse raw material A was as small as 2 μm, and the sample No. 2 which was out of the range of the present invention. In No. 10, the relative density was as low as 88%, and the figure of merit Z was as low as 2.1 × 10 ⁻³ / K.
[0050]
Furthermore, the average particle size of the coarse raw material A was as large as 150 μm, and the sample Nos. In No. 14, the relative density was as low as 88%, the average pore diameter was as large as 10 μm, and the figure of merit Z was as low as 1.97 × 10 ⁻³ / K.
[0051]
In addition, the particle size of the fine raw material B was as large as 8 μm, and the sample No. which was out of the range of the present invention. In No. 18, the relative density was as low as 88%, the average pore diameter was as large as 7.2 μm, and the figure of merit Z was as low as 1.98 × 10 ⁻³ / K.
[0052]
Furthermore, the sample No. which does not contain the fine raw material B but contains only the coarse raw material A and which is outside the scope of the present invention In No. 19, the relative density was as low as 80%, the average pore diameter was as large as 10 μm, and the figure of merit Z was as low as 2.07 × 10 ⁻³ / K.
[0053]
Furthermore, the sample No., which does not include the coarse raw material A but contains only the fine raw material B and has a relative density as large as 99%, which is out of the range of the present invention. In No. 23, the average pore diameter was as small as 0.05 μm, and the figure of merit Z was as low as 1.60 × 10 ⁻³ / K.
[0054]
In addition, the secondary firing was performed in an oxidizing atmosphere, and the sample Nos. In Sample No. 24, the sample was oxidized, so the relative density was as low as 83%, the average pore diameter was as large as 12 μm, and the figure of merit Z was almost 0.
[0055]
【The invention's effect】
According to the present invention, it has been found that a thermoelectric conversion material having an excellent figure of merit can be obtained by simultaneously achieving improvement in sinterability, reduction in thermal conductivity, and suppression of increase in electrical resistivity in a Te-based thermoelectric conversion material.

Claims

A thermoelectric conversion element mainly comprising a crystal made of a thermoelectric conversion material and having a relative density of 90 to 98%, wherein pores having an average diameter of 1 to 5 μm are distributed in the sintered body.

2. The thermoelectric conversion element according to claim 1, wherein the specific resistance is 1.0 × 10 ⁻⁵ Ω · m or less.

The thermoelectric conversion element according to claim 1, wherein the thermal conductivity is 1.5 W / m · K or less.

The thermoelectric conversion element according to any one of claims 1 to 3, wherein the thermoelectric conversion material comprises at least two of bismuth, tellurium, antimony, and selenium.

Two kinds of raw material powders composed of thermoelectric conversion materials having an average particle diameter of 10 to 100 μm and 0.1 to 5 μm are mixed, and the mixed powder is molded. A method for manufacturing a thermoelectric conversion element, wherein the firing is performed so as to be 90 to 98%.

6. The method for manufacturing a thermoelectric conversion element according to claim 5, wherein the green body made of the mixed powder is preliminarily fired at a pressure of 4 atm or less before the firing.

The method according to claim 5, wherein the firing is performed at 400 ° C. to 500 ° C. 8.

The method according to any one of claims 5 to 7, wherein the thermoelectric material comprises at least two of bismuth, tellurium, antimony, and selenium.