JP2005019910A

JP2005019910A - Thermoelectric transduction segment element and manufacturing method of the same

Info

Publication number: JP2005019910A
Application number: JP2003186047A
Authority: JP
Inventors: Teruo Noguchi; 照夫野口; Atsushi Yamamoto; 淳山本
Original assignee: National Institute of Advanced Industrial Science and Technology AIST
Current assignee: National Institute of Advanced Industrial Science and Technology AIST
Priority date: 2003-06-30
Filing date: 2003-06-30
Publication date: 2005-01-20
Anticipated expiration: 2023-06-30
Also published as: JP4918672B2

Abstract

<P>PROBLEM TO BE SOLVED: To ease a thermal stress between respective thermoelectric materials and between the thermoelectric materials and electrode-forming metal materials in a thermoelectric transduction segment element. <P>SOLUTION: Between different kinds of thermoelectric materials having mutually different thermal expansion coefficient, one or two kinds of other material layers, which are different in kind from both thermoelectric materials and have a thermal expansion coefficient of a medium between that of both thermoelectric materials, are inserted to ease a thermal stress between the thermoelectric materials, thus enable the employment of a sintering method. Further, other metal materials, which are different in kind from the thermoelectric materials and the electrode-forming metal materials and have a thermal expansion coefficient of a medium between that of both thermoelectric materials and electrode-forming metal materials, are inserted between the thermoelectric materials and the electrode-forming metal materials. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、熱膨張率の異なる熱電変換材料を接合してなるセグメント構造の熱電変換セグメント素子及びその製造方法に関する。
【０００２】
【従来の技術】
熱電発電素子は一般的に、棒状、柱状の構造をもち、その片端を高温に、他端を低温に保って、その温度差に比例した電力を得るものである。しかし、熱電素子に用いる材料は性能を最適にする使用温度範囲があり、使用温度において発電出力又は発電効率を最大にするために、複数の材料を温度差に沿うように接合して用いることがある。この際、それらの材料を機械構造的にも電気的にも、図５のように直列に接合してなる素子をセグメント素子とよぶ。
【０００３】
コバルト（Ｃｏ）、アンチモン（Ｓｂ），テルル（Ｔｅ）からなる化合物、Ｃｏ（Ｓｂ_１−ｘＴｅ_ｘ）_３（以下構成元素の頭字を取ったＣＳＴと略す）は、約２００〜数１００℃程度の温度域で優れた特性を有するＮ型熱電変換材料として、またビスマス（Ｂｉ）、テルル（Ｔｅ）、アンチモン（Ｓｂ）からなる化合物Ｂｉ_３（Ｔｅ _１−ｘＳｂ _ｘ）_３（以下構成元素の頭字を取ったＢＴＳ）は約２００℃以下で優れたＮ型材料として知られる。
【０００４】
もし、この２つの材料を機械的電気的に接合して、図５のようにＣＳＴ（材料２）を温度の高い側にＢＴＳ（材料１）を低い側に使用できるならば、数百℃以下で特性の優れたＮ型熱電素子（この様な構造をセグメント式熱電素子と称する）を形成することができると考えられる。
【０００５】
しかし、ＣＳＴとＢＴＳという二種類の材料は熱膨張率が、前者は８ｘ１０^−６程度、後者は１８ｘ１０^−６程度であって、２倍以上の相違がある。従って、機械的強度が十分な接合を得ることは容易ではない。例えば、この二種材を用いて、一体構造のセグメント素子をホットプレスによる粉末焼結法で製造することを考える。この際ＣＳＴとＢＴＳとは融点が異なるため、焼結温度を変えざるを得ない。実際、前者は６００℃程度、後者は４５０℃前後がホットプレス時の最適温度であるから、同時に二つを焼結することはできず、先ず前者（ＣＳＴ）を焼結した後、その上にＢＴＳの粉末を乗せてホットプレスを行う事が必要である。しかし、この際、以下の問題が生じるので、健全な素子を形成せしめることは困難である。
【０００６】
１）ＣＳＴ材の上にそのままＢＴＳ材を乗せて焼結しても、前述の熱膨張率の相違による応力が材料の強度を超え、接合部付近で割れが生じる。
２）また、仮に接合して発電素子を製作できたとしても、室温と数１００℃の間を頻繁に往復する発電運転をする間に、使用中の熱応力によって短寿命となる危険がある。
３）熱電素子はその高温度端と低温度端とに、電流を出入させるための金属電極を形成して実際の使用に供する。この金属電極は、実際に使用する際には銅や鉄のリード線と接合することを考慮してその材質が選択される。このため、熱電材料とＦｅ、Ｎｉ、Ｃｕ等電極用金属材料とでは熱膨張率が著しく異なるのが通常である。実際、ＢＴＳなどではＦｅが一般に使用されるが、幸いＢＴＳとＦｅとでは熱膨張率の差が比較的小さいので、問題は生じない。しかし、ＣＳＴはＦｅ、Ｎｉ等に比して１／２以下程度の熱膨張率であるから、接合することは困難である。
【０００７】
【発明が解決しようとする課題】
異種材料の熱膨張率が著しく異なる場合、接合法の如何を問わず、接合時と冷却後の温度差による熱膨張の相違が、大きな応力を材料にもたらし、材料を破壊する。特にロウ材を使わない焼結法などを採用して直接接合する場合に、この熱応力破壊を避けることは極めて困難である。
本発明は、上記の問題を克服するため、ＣＳＴとＢＴＳとの間の熱応力、及び各熱電材料と電極用金属材料の間の熱応力を緩和することを目的としている。
【０００８】
【課題を解決するための手段】
本発明では、熱膨張率の異なる異種の熱電材料の間に、何れとも異なる種類で、熱膨張率が両材料の中間の値をもつ別の１〜２種の材料層を挿入することにより熱応力を緩和して、前述した焼結法の採用を可能にするものである。
さらに、熱電材料と電極用金属材料との間に、何れとも異なる種類で、熱膨張率が両材料の中間の値をもつ別の金属材料層を挿入するものである。
【０００９】
【発明の実施の形態】
ＣＳＴへの金属材料の接合
まず、焼結温度が高い方の材料（ＣＳＴ）の両端に何らかの金属材料を接合することが必要となる（図２）。これまで、ＣＳＴと金属との接合を、ホットプレスによる一体同時焼結によって試みた。その際に用いた材料と、それらの結果を表１にまとめる。
【表１】

【００１０】
表１で見る限り、Ｔｉは有望であるが、ＣＳＴとは熱膨張率が近いために巧く接合するものの、ＢＳＴとは接合できない。ＢＴＳと接合できる金属は、Ｆｅ、Ｎｉ等熱膨張率が１３ｘ１０^−６程度以上の材料である。また、Ｔｉは活性で酸化性が極めて強いため、大気中では半田付けや銀ロー付けなどが不可能であり、このまま電極材にはなり得ない。そこで図２の構造の両端に更にもう一つの金属を接合することを試みた（図３）。
【００１１】
金属２はＢＴＳと熱膨張率が近く、かつ半田づけ等の容易に行えるものが望ましい。金属１はＴｉとして、まず金属２としてＦｅを試みた。この場合、ＣＳＴ、金属１、金属２の厚さも応力の大きさに関係し、特に金属１と金属２の厚さの比は考慮する必要があった。実際、ＣＳＴ：Ｔｉ：Ｆｅ＝３ｍｍ：０．２ｍｍ：０．２ｍｍでは、ＣＳＴとＴｉとの境界近傍で割れが発生た。Ｔｉの厚さを０．４ｍｍにすると、ＣＳＴの割れは無くなるが、金属２（Ｆｅ）と金属１（Ｔｉ）の界面近傍、又は金属２の内部で割れが発生し、剥離が生じた。
【００１２】
次に、金属２として、Ｆｅ−１０ｗｔ％Ｔｉ合金を用いたところ、割れ、剥離などは何処にも生じないことを見いだした。この際、Ｆｅ−１０ｗｔ％Ｔｉ合金は、ＦｅとＴｉの粉末を遊星ボールミルによって５時間粉砕・混合をして得た混合粉末を用いた。
以上を見やすくするため、表２にまとめた。
【表２】

【００１３】
何故、Ｆｅ−１０ｗｔ％Ｔｉ合金がよいのかについては、必ずしも明確でないが、Ｆｅ−１０ｗｔ％Ｔｉの熱膨張率とヤング率の値が、結果として良好な接合を齎したものと考えられる。測定したヤング率は、〜１．１ｘ１０^６ｋｇｆ／ｃｍ^２で、この値はＴｉにかなり近く、Ｆｅの約６０％であった。
【００１４】
ＢＴＳへの金属材料の接合
上述のＦｅ−１０Ｔｉ／Ｔｉ／ＣＳＴ／Ｔｉ／Ｆｅ−１０ＴｉにＢＳＴを接合してセグメント構造の素子とする。ここで、問題はＣＳＴの端部にある固体のＦｅ−１０ｗｔ％ＴｉとＢＳＴとの接合である。この下準備のため、Ｆｅ−１０ｗｔ％Ｔｉの予め焼結した板の上にＢＳＴの粉末とＦｅの粉末を充填して一体焼結成型することを試みた（図４）。
この場合、焼結温度は４５０℃程度とした。Ｆｅ−１０ｗｔ％ＴｉとＢＳＴ、及びＦｅとＢＳＴともに接合性はよく、割れや剥離は生じない。
【００１５】
ＣＳＴとＢＳＴを接合して成るセグメント式ｎ型素子
以上の準備による知見に基づき、図１に示すような最終的なＣＳＴ−ＢＴＳの一体焼結Ｎ型セグメント素子を製造した。製造方法は以下の如くである。
ＣＳＴ、Ｔｉ、Ｆｅ−１０ｗｔ％Ｔｉの粉末を原料として、図３の構造をした棒状焼結体をＣＳＴの焼結条件によって製作する。この焼結体の片側端上にＢＴＳとＦｅの粉末をこの順序で充填し、ＢＴＳの焼結条件で焼結する。
Ｔｉが大気中では酸化が著しくリード線との接合が困難であるのに対して、例示の構成は、実際に使用する際には素子最外側に位置するＦｅ−１０ｗｔ％Ｔｉの鉄基合金電極が、銅や鉄のリード線と良好に接合されることになる。
【００１６】
【発明の効果】
本発明では、熱膨張率の異なる異種の熱電材料の間に、何れとも異なる種類で、熱膨張率が両材料の中間の値をもつ別の１〜２種の材料層を挿入することにより熱応力を緩和して、前述した焼結法の採用を可能にするものである。
【図面の簡単な説明】
【図１】ＣＳＴ−ＢＴＳの一体焼結Ｎ型セグメント素子を示す図である。
【図２】焼結温度が高い方の材料（ＣＳＴ）の両端に金属材料を接合することを説明する図である。
【図３】図５の構造の両端に更にもう一つの金属を接合することを説明する図である。
【図４】Ｆｅ−１０ｗｔ％Ｔｉの予め焼結した板の上にＢＳＴの粉末とＦｅの粉末を充填して一体焼結成型することを説明する図である。
【図５】一般的構成を有するセグメント素子を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a segmented thermoelectric conversion segment element formed by joining thermoelectric conversion materials having different coefficients of thermal expansion, and a method for manufacturing the same.
[0002]
[Prior art]
A thermoelectric power generation element generally has a rod-like or columnar structure, and one end is kept at a high temperature and the other end is kept at a low temperature to obtain electric power proportional to the temperature difference. However, the materials used for thermoelectric elements have a working temperature range that optimizes performance, and in order to maximize the power generation output or power generation efficiency at the working temperature, it is necessary to use a plurality of materials that are joined along the temperature difference. is there. At this time, an element formed by joining these materials in series, as shown in FIG. 5, both mechanically and electrically is called a segment element.
[0003]
Cobalt (Co), antimony (Sb), tellurium (Te) compound, Co (Sb _1-x Te _x ) ₃ (hereinafter abbreviated as CST with constituent elements abbreviations) is about 200 to several hundred degrees Celsius. As an N-type thermoelectric conversion material having excellent characteristics in the temperature range of the above, a compound Bi ₃ (Te _1-x Sb _x ) ₃ (hereinafter referred to as a constituent element) composed of bismuth (Bi), tellurium (Te), and antimony (Sb) BTS) is known as an excellent N-type material at about 200 ° C. or less.
[0004]
If these two materials are mechanically and electrically joined and CST (material 2) can be used on the higher temperature side and BTS (material 1) on the lower side as shown in FIG. It is considered that an N-type thermoelectric element (such a structure is referred to as a segmented thermoelectric element) having excellent characteristics can be formed.
[0005]
However, two types of material thermal expansion rate of CST and the BTS, the former 8 ^{x10 -6} mm, the latter is of the order of 18 × 10 ^-6, there is a difference more than twice. Therefore, it is not easy to obtain a bond having sufficient mechanical strength. For example, it is considered that a segment element having an integral structure is manufactured by a powder sintering method using hot pressing using these two kinds of materials. At this time, since CST and BTS have different melting points, the sintering temperature must be changed. Actually, the optimum temperature for hot pressing is about 600 ° C. for the former and about 450 ° C. for the latter, so it is impossible to sinter the two at the same time. It is necessary to hot-press with BTS powder. However, at this time, since the following problems occur, it is difficult to form a sound element.
[0006]
1) Even if the BTS material is directly placed on the CST material and sintered, the stress due to the difference in the thermal expansion coefficient exceeds the strength of the material, and cracks occur near the joint.
2) Even if the power generation element can be manufactured by joining, there is a risk that the lifespan may be shortened due to thermal stress during use during power generation operation that frequently reciprocates between room temperature and several hundred degrees Celsius.
3) The thermoelectric element is used for actual use by forming metal electrodes for allowing current to flow in and out at the high temperature end and the low temperature end. The material of the metal electrode is selected in consideration of joining with a copper or iron lead wire when actually used. For this reason, the coefficient of thermal expansion is usually significantly different between thermoelectric materials and metal materials for electrodes such as Fe, Ni and Cu. Actually, Fe is generally used in BTS and the like, but fortunately there is no problem because the difference in coefficient of thermal expansion between BTS and Fe is relatively small. However, since CST has a thermal expansion coefficient of about ½ or less than that of Fe, Ni, etc., it is difficult to join.
[0007]
[Problems to be solved by the invention]
When the thermal expansion coefficients of different types of materials are significantly different, the difference in thermal expansion due to the temperature difference at the time of bonding and after cooling causes a large stress to the material and breaks the material regardless of the bonding method. In particular, it is extremely difficult to avoid this thermal stress failure when directly joining by employing a sintering method that does not use a brazing material.
The present invention aims to alleviate the thermal stress between the CST and the BTS and the thermal stress between each thermoelectric material and the electrode metal material in order to overcome the above problems.
[0008]
[Means for Solving the Problems]
In the present invention, heat is inserted by inserting another one or two kinds of material layers having different thermal expansion coefficients between the two different types of thermoelectric materials having different thermal expansion coefficients. The stress is relieved and the above-described sintering method can be adopted.
Further, another metal material layer is inserted between the thermoelectric material and the electrode metal material, which is different from each other and has a thermal expansion coefficient intermediate between both materials.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Joining of metal material to CST First, it is necessary to join some metal material to both ends of the material having higher sintering temperature (CST) (FIG. 2). Until now, joining of CST and metal has been attempted by integral simultaneous sintering by hot pressing. Table 1 summarizes the materials used at that time and the results.
[Table 1]

[0010]
As can be seen from Table 1, Ti is promising, but although it is skillfully joined to CST because of its close thermal expansion coefficient, it cannot be joined to BST. The metal that can be bonded to the BTS is a material having a thermal expansion coefficient of about 13 × 10 ⁻⁶ or more, such as Fe and Ni. Further, since Ti is active and extremely oxidizable, it cannot be soldered or brazed in the atmosphere, and cannot be used as an electrode material. Therefore, another metal was tried to be bonded to both ends of the structure of FIG. 2 (FIG. 3).
[0011]
It is desirable that the metal 2 has a thermal expansion coefficient close to that of the BTS and can be easily soldered. Metal 1 was tried as Ti, and first as metal 2 Fe was tried. In this case, the thicknesses of CST, metal 1 and metal 2 are also related to the magnitude of stress, and in particular, the ratio of the thicknesses of metal 1 and metal 2 must be taken into consideration. In fact, when CST: Ti: Fe = 3 mm: 0.2 mm: 0.2 mm, cracks occurred near the boundary between CST and Ti. When the thickness of Ti was 0.4 mm, CST cracking disappeared, but cracking occurred in the vicinity of the interface between metal 2 (Fe) and metal 1 (Ti) or inside metal 2 and peeling occurred.
[0012]
Next, when an Fe-10 wt% Ti alloy was used as the metal 2, it was found that no cracks, peeling, etc. occurred anywhere. At this time, as the Fe-10 wt% Ti alloy, a mixed powder obtained by pulverizing and mixing Fe and Ti powder with a planetary ball mill for 5 hours was used.
To make it easier to see, the results are summarized in Table 2.
[Table 2]

[0013]
The reason why the Fe-10 wt% Ti alloy is good is not necessarily clear, but the values of the thermal expansion coefficient and Young's modulus of Fe-10 wt% Ti are considered to result in good bonding. The measured Young's modulus was ˜1.1 × 10 ⁶ kgf / cm ^2, which was quite close to Ti and about 60% of Fe.
[0014]
Bonding of metal material to BTS A segment structure element is formed by bonding BST to the above-described Fe-10Ti / Ti / CST / Ti / Fe-10Ti. Here, the problem is the joining of solid Fe-10 wt% Ti at the end of CST and BST. For this preparation, an attempt was made to integrally sinter-mold by filling BST powder and Fe powder on a pre-sintered plate of Fe-10 wt% Ti (FIG. 4).
In this case, the sintering temperature was about 450 ° C. Both Fe-10 wt% Ti and BST and Fe and BST have good bondability, and no cracking or peeling occurs.
[0015]
A final CST-BTS integrally sintered N-type segment element as shown in FIG. 1 was manufactured based on the knowledge obtained by preparing more than the segment type n-type element formed by joining CST and BST. The manufacturing method is as follows.
A rod-like sintered body having the structure shown in FIG. 3 is manufactured under the CST sintering conditions using CST, Ti, and Fe-10 wt% Ti powder as raw materials. BTS and Fe powders are filled in this order on one end of the sintered body and sintered under the BTS sintering conditions.
Whereas Ti is easily oxidized in the atmosphere and joining to the lead wire is difficult, the example configuration is an Fe-10 wt% Ti iron-based alloy electrode positioned on the outermost side of the device when actually used. However, it will be satisfactorily bonded to copper or iron lead wires.
[0016]
【The invention's effect】
In the present invention, heat is inserted by inserting another one or two kinds of material layers having different thermal expansion coefficients between the two different types of thermoelectric materials having different thermal expansion coefficients. The stress is relieved and the above-described sintering method can be adopted.
[Brief description of the drawings]
FIG. 1 is a view showing an integrally sintered N-type segment element of CST-BTS.
FIG. 2 is a diagram for explaining that a metal material is bonded to both ends of a material (CST) having a higher sintering temperature.
FIG. 3 is a diagram illustrating that another metal is bonded to both ends of the structure of FIG. 5;
FIG. 4 is a view for explaining integral sintering molding by filling BST powder and Fe powder on a pre-sintered plate of Fe-10 wt% Ti.
FIG. 5 is a diagram showing a segment element having a general configuration.

Claims

In a thermoelectric conversion segment element having a segment structure in which thermoelectric conversion materials having different coefficients of thermal expansion are joined and metal materials for electrodes are provided on both sides,
Thermal stress can be alleviated by inserting one or two other layers with different thermal expansion coefficients between different types of thermoelectric materials with different thermal expansion coefficients. Thus, a thermoelectric conversion segment element that can be integrally joined.

The thermoelectric conversion segment element according to claim 1, wherein another metal material layer having a different thermal expansion coefficient between the two materials is inserted between the thermoelectric material and the electrode metal material.

In a method for manufacturing a segmented thermoelectric conversion segment element in which thermoelectric conversion materials having different coefficients of thermal expansion are joined and a metal material for an electrode is provided on both sides,
Thermal stress can be alleviated by inserting one or two other layers with different thermal expansion coefficients between different types of thermoelectric materials with different thermal expansion coefficients. And the manufacturing method of the thermoelectric conversion segment element which enabled integral joining.

4. The thermoelectric conversion segment element according to claim 3, wherein another metal material layer having a different thermal expansion coefficient between the two materials is inserted between the thermoelectric material and the electrode metal material. Method.