新型熱電材料綜述

王 超，張 蕊，杜 欣，張晨貴，欒春紅，姜 晶，胡 強(qiáng)，王軍喜，杜志游，李天笑，馬鐵中，嚴(yán) 冬，尹志堯

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新型熱電材料綜述

王 超1,2，張 蕊1,2，杜 欣1,2，張晨貴1,2，欒春紅1,2，姜 晶1,2，胡 強(qiáng)3，王軍喜4，杜志游5，李天笑5，馬鐵中6，嚴(yán) 冬6，尹志堯5

(1. 電子科技大學(xué)微電子與固體電子學(xué)院 成都 611731；2. 電子科技大學(xué)電子薄膜與集成器件國(guó)家重點(diǎn)實(shí)驗(yàn)室 成都 611731；3. 廣東省中科宏微半導(dǎo)體設(shè)備有限公司 廣州 510530；4. 中國(guó)科學(xué)院半導(dǎo)體研究所 北京海淀區(qū) 100083； 5. 中微半導(dǎo)體設(shè)備(上海)有限公司 上海浦東區(qū) 201201；6. 北京智朗芯光科技有限公司 北京昌平區(qū) 102206)

熱電材料能夠?qū)崿F(xiàn)熱能和電能之間的相互轉(zhuǎn)化，利用溫度差進(jìn)行發(fā)電是一種潛在的能源利用的方法，另外利用電學(xué)對(duì)熱量的轉(zhuǎn)化，可以進(jìn)行溫度的精確控制，在傳感器和集成電路中有著廣闊的應(yīng)用前景。該文綜述了近年來(lái)幾類熱電材料的種類、發(fā)展歷程和研究現(xiàn)狀，包括碲化物、硫族層狀化合物、氧化物、籠合物，Half-Heusler材料，方鈷礦材料、Zintl相熱電材料以及銅硫族類材料。另外，對(duì)熱電材料的應(yīng)用做了一些歸納總結(jié)，希望能擴(kuò)展熱電器件的應(yīng)用，實(shí)現(xiàn)未來(lái)的規(guī)模產(chǎn)業(yè)化。

新型熱電材料; 功率因子; seebeck系數(shù); 熱電器件; 熱電性能

1 研究背景介紹

隨著半導(dǎo)體光刻技術(shù)和密集封裝技術(shù)的發(fā)展，半導(dǎo)體芯片上的熱能產(chǎn)生已經(jīng)達(dá)到將近100 W/cm2[1]。文獻(xiàn)[2]提出溫度每升高2 K，硅芯片的穩(wěn)定性將會(huì)降低10%。文獻(xiàn)[3]的研究也表明了超過(guò)一半的電路實(shí)驗(yàn)失敗與溫度有關(guān)。熱問(wèn)題制約著互補(bǔ)金屬氧化物半導(dǎo)體(complementary metal oxide semiconductor, CMOS)的發(fā)展，解決熱問(wèn)題對(duì)于片上系統(tǒng)的設(shè)計(jì)也變得至關(guān)重要。一直以來(lái)，熱電材料都以其體積小、重量輕、堅(jiān)固、無(wú)噪音、無(wú)污染、壽命長(zhǎng)、易于控制等優(yōu)點(diǎn)而備受關(guān)注[4]。不僅可運(yùn)用于熱能回收發(fā)電、制冷，也可將其運(yùn)用于集成電路，解決芯片熱問(wèn)題。

熱電現(xiàn)象最早是在1823年由德國(guó)人Seebeck發(fā)現(xiàn)的。當(dāng)兩種不同導(dǎo)體構(gòu)成閉合回路時(shí)，如果兩個(gè)接點(diǎn)的溫度不同，則兩接點(diǎn)間有電動(dòng)勢(shì)產(chǎn)生，且在回路中有電流通過(guò)，即溫差電現(xiàn)象或Seebeck效應(yīng)。Seebeck系數(shù)可表示為：

式中，表示電勢(shì)；表示溫度，的大小和符號(hào)取決于兩種材料和兩個(gè)結(jié)點(diǎn)的溫度。原則上講，當(dāng)載流子是電子時(shí)，冷端為負(fù)，是負(fù)值；如果空穴是主要載流子類型，那么熱端是負(fù)，是正值。

1834年，法國(guó)鐘表匠Pletier發(fā)現(xiàn)了Seebeck效應(yīng)的逆效應(yīng)，即電流通過(guò)兩個(gè)不同導(dǎo)體形成的接點(diǎn)時(shí)，接點(diǎn)處會(huì)發(fā)生放熱或吸熱現(xiàn)象，稱為Peltier效應(yīng)。Peltier系數(shù)可表示為：

式中，表示單位時(shí)間接頭處所吸收(釋放)的帕爾貼熱；表示外加電源所提供的電流強(qiáng)度。

1854年，Thomson發(fā)現(xiàn)當(dāng)電流通過(guò)一個(gè)單一導(dǎo)體，且該導(dǎo)體中存在溫度梯度時(shí)，就會(huì)產(chǎn)生可逆的熱效應(yīng)，稱為T(mén)homson效應(yīng)。Peltier效應(yīng)和Thomson效應(yīng)都是電制冷(或電制熱)效應(yīng)，但是由于Thomson效應(yīng)是一種二級(jí)效應(yīng)，實(shí)際應(yīng)用價(jià)值不大。

在實(shí)際應(yīng)用過(guò)程中，以無(wú)量綱的ZT值來(lái)衡量材料的熱電性能：

常見(jiàn)的改善ZT值的策略有兩種，一種是通過(guò)摻雜和能帶工程，調(diào)控遷移率和載流子濃度，改變態(tài)密度有效質(zhì)量，進(jìn)而最大化功率因子[4-6]。另一種策略是通過(guò)納米結(jié)構(gòu)或聲子工程降低晶格熱導(dǎo)率K[7-8]。調(diào)控載流子濃度，常見(jiàn)的方法是通過(guò)摻雜，但摻雜過(guò)程不可避免地引入了晶格缺陷和畸變，這極大地影響了材料的物理性質(zhì)。除了通過(guò)摻雜的手段，還可以通過(guò)使用電場(chǎng)[9]、磁場(chǎng)[10]、光輻射[11]來(lái)激發(fā)和調(diào)控載流子濃度。傳統(tǒng)的熱電材料包括低溫(300～500 K)的Bi2Te3，中溫(500～900 K)PbTe和高溫(900～1200 K)的SiGe合金。

隨著科技的進(jìn)步以及材料合成技術(shù)的發(fā)展，人們除了對(duì)傳統(tǒng)材料進(jìn)行進(jìn)一步研究以及改善其性能外，大量的新型熱電材料也備受人們的關(guān)注。例如，金屬氧化物熱電材料克服了傳統(tǒng)材料制備困難、成本高、易氧化、強(qiáng)度低等缺點(diǎn)；填充式方鈷礦作為一類新型熱電材料具有低熱導(dǎo)率的優(yōu)點(diǎn)；金屬硅化物型熱電材料相比于其他傳統(tǒng)的熱電材料具有熔點(diǎn)高的特點(diǎn)，并且其原料來(lái)源豐富，高溫條件下具有較好的抗氧化性。近年來(lái)，納米技術(shù)在改善材料熱電性能方面起到了舉足輕重的作用，形成了一系列新型熱電材料，例如：“聲子玻璃-電子晶體”(PGEC)熱電材料[12]、納米線和納米管熱電材料、納米超晶格熱電材料等。圖1為近年來(lái)主要塊體熱電材料的發(fā)展趨勢(shì)圖。

圖1 近年來(lái)主要熱電材料的發(fā)展趨勢(shì)

2 新型熱電材料

2.1 碲化物

碲化物材料由于自身具有較低的熱導(dǎo)率，因此是被視為一種比較有發(fā)展?jié)摿Φ臒犭姴牧?。基于Bi2Te3和PbTe的體材料均展現(xiàn)出比較優(yōu)異的熱電性能。Bi2Te3體系[13]適用于低溫，在室溫附近熱電優(yōu)值達(dá)到1(相應(yīng)的熱電轉(zhuǎn)換效率約為7%～8%)，被公認(rèn)為是最好的熱電材料，目前大多數(shù)制冷元器件都是使用這類材料。PbTe體系[14]適用于500～900 K的中溫，熱電優(yōu)值最大可達(dá)0.8，主要用于溫差發(fā)電。這兩類熱電材料體系目前已經(jīng)得到了廣泛的應(yīng)用并且研究較為成熟，本文就不再具體討論。

2.1.1 Ag2Te

Ag2Te熱電材料是一種具有在一定溫度下相轉(zhuǎn)變現(xiàn)象的材料：418 K溫度下，單斜相α-Ag2Te是比較穩(wěn)定的；418～1 075 K溫度范圍內(nèi)，面心立方相β-Ag2Te是比較穩(wěn)定的；1 075～1 233 K溫度范圍內(nèi)，γ-Ag2Te是比較穩(wěn)定的[15]。370 K時(shí)，Ag2Te熱電材料的優(yōu)值達(dá)到最大值0.27[16]。

Sb元素替換部分Ag元素形成的AgSbTe2熱電材料具有較好的熱電性能。AgSbTe2的熱電優(yōu)值隨著燒結(jié)技術(shù)的不同而展現(xiàn)出巨大差異。等離子輔助燒結(jié)技術(shù)(plasma assisted sintering, PAS)制備的AgSbTe2材料，溫度為300 K時(shí)，熱電優(yōu)值的最大值為0.29[17]。高溫高壓技術(shù)制備的AgSbTe2材料，溫度為513 K時(shí)，材料熱電優(yōu)值的最大值為1.07[18-19]。通過(guò)熔融紡絲和SPS工藝制備的AgSbTe2材料的熱電性能比用傳統(tǒng)的熔融和自然冷卻方法制備的熱電性能好，溫度為570 K時(shí)，熱電優(yōu)值的最大值可達(dá)1.65[20]。通過(guò)摻雜或替代的方法可以有效改善AgSbTe2材料的熱電性能。例如，在Ag0.99Na0.01SbTe2.02材料中，Na原子取代部分Ag原子占據(jù)晶格點(diǎn)，溫度為570 K時(shí)熱電優(yōu)值達(dá)到1.50，比未摻雜的AgSbTe2的熱電性能提升了很多[21]。除此以外，在AgSbTe2中摻雜NaSe、TlTe等也可以提高材料的熱電性能[22-24]。

2.1.2 TAGS[(AgSbTe2)1?x(GeTe)x]

TAGS是由碲、銻、鍺、銀組成的，是碲化銀在碲化鍺中的固溶體，其組分為(AgSbTe2)1?x(GeTe)，是P型材料。研究發(fā)現(xiàn)當(dāng)在0.8～0.85范圍內(nèi)時(shí)，P型TAGS材料的熱電性能最好，因?yàn)榇藭r(shí)材料的熱導(dǎo)率最小[16](=0.8, 稱之為T(mén)AGS80；=0.85, 稱之為T(mén)AGS85)。溫度為773 K時(shí)，熱壓法制備的TAGS80的熱電優(yōu)值可達(dá)1.75，TAGS85的熱電優(yōu)值也可以達(dá)到1.4[25]。目前，TAGS85已經(jīng)被應(yīng)用于空間放射性同位素?zé)犭姲l(fā)電機(jī)。

TAGS熱電材料中的Sb2Te3替換部分Ag2Te可以形成一種新的P型熱電材料 (GeTe)0.8[(Ag2Te)0.4(Sb2Te3)0.6]0.2，700 K時(shí)，熱電優(yōu)值約為1.47[26]。同時(shí)，制備樣品時(shí)的環(huán)境條件會(huì)對(duì)TAGS材料的熱電性能產(chǎn)生一定的影響，實(shí)驗(yàn)表明氬氣氛圍下制備出的TAGS材料具有最大的熱電優(yōu)值[27]。摻雜也可以改善TAGS材料的熱電性能，通過(guò)向TAGS85材料中加入磁性稀土元素可以形成稀磁半導(dǎo)體，從而提高材料的塞貝克系數(shù)，改善材料的熱電性能(730 K時(shí)，加入磁性稀土元素的TAGS85的熱電優(yōu)值大于1.5)[28-29]。

2.1.3 Sb2Te3

Sb2Te3材料的塞貝克系數(shù)比較小，因此熱電性能不是特別的突出，但是通過(guò)合金化或摻雜的手段可以有效提高材料的塞貝克系數(shù)，從而改善材料的熱電性能。

冷壓技術(shù)制備的CuBi0.5Sb1.5?xTe3(= 0?0.4)合金的熱電性能強(qiáng)烈地依賴于Cu離子的含量，溫度為442 K，在0.05～0.1范圍時(shí)，合金CuBi0.5Sb1.5?xTe3的最大熱電優(yōu)值可達(dá)0.74[30]。通過(guò)SPS(spark plasma sintering)技術(shù)制備的(Cu4Te3)-(Bi0.5Sb1.5Te3)1?x材料具有較大的電導(dǎo)率、低的晶格熱導(dǎo)率，同時(shí)，材料的塞貝克系數(shù)隨著溫度的升高而線性增加，當(dāng)=0.025，溫度為474 K時(shí)材料展現(xiàn)出較好的熱電性能，此時(shí)熱電優(yōu)值達(dá)到1.26[31]。Sb2Te3材料與Ag元素形成的合金具有比較優(yōu)異的熱電性能，478 K時(shí)，材料(Ag0.365Sb0.558Te)0.025-(Bi0.5Sb1.5Te3)0.975的熱電優(yōu)值為1.1[32]。Sb2Te3材料中摻雜S元素也可以改善它的熱電性能，423 K時(shí)，Sb2Te3材料中摻雜0.1%～0.5%的S元素時(shí)，熱電優(yōu)值為0.95[33]。

2.2 硫族層狀化合物

2.2.1 SnSe

SnSe晶體在室溫環(huán)境下具有層狀斜方晶系結(jié)構(gòu)。這種晶體材料的原料來(lái)源比較豐富，且所含元素對(duì)環(huán)境無(wú)毒無(wú)污染，是一種環(huán)境友好型熱電材料，同時(shí)具有非常低的熱導(dǎo)率和比較理想的熱電性能[34]。實(shí)驗(yàn)發(fā)現(xiàn)SnSe熱電材料的晶體結(jié)構(gòu)還具有各向異性，如圖2所示，3幅圖分別是SnSe晶體沿、、軸方向看過(guò)去的晶體結(jié)構(gòu)，由于沿各個(gè)軸的晶體結(jié)構(gòu)是不相同的，因此SnSe的熱電性能也具有各向異性，923 K時(shí)，沿軸方向的熱電優(yōu)值為0.8，沿軸方向的熱電優(yōu)值為2.3，沿軸方向的熱電優(yōu)值高達(dá)2.62；室溫條件下，SnSe晶體材料沿軸方向的熱電優(yōu)值也可以達(dá)到0.12[34]。

SnSe熱電材料在高溫條件下會(huì)發(fā)生相變[35-36]。室溫環(huán)境下，SnSe具有pbmn空間群?,?,?)結(jié)構(gòu)，800 K時(shí)，晶體結(jié)構(gòu)發(fā)生變化，變?yōu)镃mcm空間群?,?,?)結(jié)構(gòu)。接近相變溫度或者高于相變溫度時(shí)，軸可以強(qiáng)烈的散射聲子獲得很低的熱導(dǎo)率而保持比較高的電導(dǎo)率，因此SnSe晶體材料成為近年來(lái)研究的熱點(diǎn)[37]。

摻雜可以改善單晶SnSe的熱電性能，并且還可以拓寬它的溫度應(yīng)用范圍。單晶SnSe材料中摻雜Na元素可以改變材料的費(fèi)米能級(jí)EF，如圖3所示[38]，材料的費(fèi)米能級(jí)EF(圖中虛線所示)隨著Na元素含量的增加而降低，但是材料的能帶結(jié)構(gòu)不隨摻雜而發(fā)生改變。因此，Na元素的引入可以有效改善SnSe材料的熱電性能(沿軸方向的熱電優(yōu)值)，特別是重?fù)诫s的Sn0.97Na0.03Se熱電材料，800 K時(shí)，Sn0.97Na0.03Se材料的熱電優(yōu)值甚至超過(guò)2；300～800 K的溫度范圍內(nèi)，材料Sn0.97Na0.03Se的平均熱電優(yōu)值為1.17[38]，這個(gè)值比目前正在探索研究的大部分熱電材料在300～800 K溫度范圍內(nèi)的平均熱電優(yōu)值要高[14,29,34,39-44]。單晶SnSe材料中摻雜Ag元素也可以改善其熱電性能，但是效果沒(méi)有摻雜Na元素的改善效果好。

2.2.2 SnS

SnS具有和SnSe結(jié)構(gòu)類似的層狀結(jié)構(gòu)，如圖4所示[45]，每個(gè)原胞內(nèi)含有8個(gè)原子，這8個(gè)原子構(gòu)成雙層結(jié)構(gòu)[46-47]，每層中的Sn原子和S原子由共價(jià)鍵連接在一起，層與層之間的結(jié)合力主要來(lái)源于Sn和S原子之間長(zhǎng)鍵的相互作用力，鍵能比較小，因此層與層之間的聯(lián)系比較弱。SnS材料由于其原料來(lái)源豐富、價(jià)格低廉、無(wú)毒無(wú)污染而備受業(yè)內(nèi)人士青睞，過(guò)去幾十年有關(guān)于SnS材料的研究主要集中在它的光學(xué)性能上，包括光電導(dǎo)性和折射率[48-50]。

室溫條件下，SnS是一種具有pbmn空間群結(jié)構(gòu)的斜方晶系[51]，835 K時(shí)，SnS材料會(huì)發(fā)生相變，由斜方晶系變?yōu)檎骄礫51-52]。文獻(xiàn)[51]通過(guò)第一性原理計(jì)算了SnS材料的能帶結(jié)構(gòu)，指出SnS材料是一種具有較大塞貝克系數(shù)和較小熱導(dǎo)率的間接半導(dǎo)體。最近，文獻(xiàn)[53]也通過(guò)計(jì)算指出SnS材料是一種很有發(fā)展?jié)摿Φ臒犭姴牧?。由于SnS材料的電導(dǎo)率比較小，溫度為873 K時(shí)，材料SnS的最大熱電優(yōu)值僅僅接近0.16[45]。

摻雜可以明顯改善SnS材料的熱電性能。文獻(xiàn)[45]的實(shí)驗(yàn)表明：SnS摻雜Ag離子，雖然材料的晶格常數(shù)不會(huì)發(fā)生變化，這可能與Ag離子和Sn離子的半徑長(zhǎng)度大致相同有關(guān)，但是材料的電導(dǎo)率會(huì)顯著增加。SnS材料中摻雜0.5%的Ag元素時(shí)，材料的熱電性能得到大幅度提升，溫度為923 K時(shí)，熱電優(yōu)值達(dá)到0.6。SnS-SnSe形成的固溶體也可以改善材料的熱電性能。固溶體SnS1-xSe中，隨著Se元素的增加，固溶體的熱電優(yōu)值也隨之增加，823 K時(shí)，固溶體SnS0.2Se0.8的熱電優(yōu)值可達(dá)0.82[54]。

2.3 氧化物

氧化物自身的電導(dǎo)率比較小，但是堿金屬或堿土金屬的層狀鈷化物由于具有特殊的晶體結(jié)構(gòu)(由A層和CoO2層沿軸方向交替排列而成。A離子在兩層CoO2層之間呈50%～70%的無(wú)規(guī)則分布。CoO2層負(fù)責(zé)導(dǎo)電，A離子層起到散射聲子、降低氧化物熱導(dǎo)率的作用)從而改善了氧化物自身的電導(dǎo)率較小的缺點(diǎn)，如圖5所示[55]，使得層狀鈷化物成為一種典型的氧化物熱電材料。

實(shí)驗(yàn)發(fā)現(xiàn)[41,43,56-69]：相對(duì)于合金體系的熱電材料，氧化物的熱電性能相對(duì)較低，如圖6所示。該圖展示了在一個(gè)較寬的溫度范圍內(nèi)，氧化物及合金體系熱電性能隨溫度的變化趨勢(shì)，對(duì)比兩圖可以發(fā)現(xiàn)在整個(gè)溫度范圍內(nèi)氧化物的熱電優(yōu)值都沒(méi)有合金化合物的熱電優(yōu)值大。但是氧化物在高溫領(lǐng)域表現(xiàn)出來(lái)的化學(xué)穩(wěn)定性及熱穩(wěn)定性促使人們更多的關(guān)注其熱電性能的優(yōu)化。

圖5 層狀鈷化物的晶體結(jié)構(gòu)

a. 合金體系熱電材料的熱電優(yōu)值

b. 氧化物體系熱電材料的熱電優(yōu)值

圖6 兩種體系熱電材料的熱電優(yōu)值

2.3.1 NaxCo2O4

NaCo2O4化合物是典型的氧化物熱電材料，根據(jù)化合物中Na元素含量的不同，NaCo2O4化合物會(huì)有4種不同的結(jié)構(gòu)[70]：在1.8～2.0范圍時(shí)，具有-NaCo2O4結(jié)構(gòu)；等于1.5時(shí)，具有-NaCo2O4結(jié)構(gòu)；在1.1～1.2范圍時(shí)，具有-NaCo2O4結(jié)構(gòu)；在1.0～1.4范圍時(shí)，具有-NaCo2O4結(jié)構(gòu)。其中，-NaCo2O4結(jié)構(gòu)的化合物具有最好的熱電性能[71]。

NaCo2O4是一種層狀結(jié)構(gòu)的過(guò)渡金屬氧化物，由Na層和CoO2層沿軸方向交替排列而成。Na離子在兩層CoO2層之間呈50%～70%的無(wú)規(guī)則分布[72]。CoO2層負(fù)責(zé)導(dǎo)電，Na離子層起到散射聲子、降低氧化物熱導(dǎo)率的作用，這實(shí)際上相當(dāng)于形成了一種新的“聲子玻璃-電子晶體”。因此，NaCo2O4化合物具有較好的熱電性能，300 K時(shí)，該氧化物的塞貝克系數(shù)為100 V/K，電阻率為200W·cm，載流子濃度在1021～1022cm-3范圍內(nèi)。

摻雜可以改善NaCo2O4化合物的熱電性能。Rh元素的加入會(huì)使NaCo2O4的熱導(dǎo)率減??；Pd元素的加入可以增加NaCo2O4的電導(dǎo)率，723 K時(shí)，化合物NaCo1.9Pd0.1O4的熱電優(yōu)值可達(dá)到0.045[73]。摻雜Ni元素可以減小材料的晶粒尺寸，從而降低熱導(dǎo)率，另外，Ni元素的添加還可以優(yōu)化塞貝克系數(shù)，因此起到提高NaCo2O4化合物熱電性能的作用，673 K時(shí)，NaCo0.9Ni0.1O2的熱電優(yōu)值為0.176[74]。Zn元素替換部分Co元素可以增加材料的電導(dǎo)率和塞貝克系數(shù)，從而改善材料的熱電性能[75]。

NaCo2O4化合物的化學(xué)性質(zhì)不太穩(wěn)定，在空氣中容易潮解，而且高溫時(shí)該氧化物的性質(zhì)也不是很穩(wěn)定，超過(guò)800℃時(shí)，Na離子容易揮發(fā)，因此NaCo2O4氧化物的應(yīng)用受到了很大的限制。于是，人們開(kāi)始研究另外一種新的氧化物熱電材料。

2.3.2 Ca3Co4O9

文獻(xiàn)[76]首先發(fā)現(xiàn)了Ca3Co4O9材料具有較好的熱電性能，直至今日，層狀鈷化物Ca3Co4O9仍然一直受到人們的廣泛關(guān)注[77-79]。層狀鈷化物Ca3Co4O9具有和NaCo2O4化合物類似的晶體結(jié)構(gòu)，如圖7所示，Ca3Co4O9比NaCo2O4的結(jié)構(gòu)穩(wěn)定性要好，但其熱電性能不如NaCo2O4化合物。單晶Ca3Co4O9在973 K時(shí)，熱電優(yōu)值為0.87[80]。

制備方法可以影響Ca3Co4O9的熱電性能。用傳統(tǒng)燒結(jié)方法制備的Ca3Co4O9晶體，700 ℃時(shí)，材料的熱電優(yōu)值為0.052，但用放電等離子燒結(jié)(SPS)技術(shù)制備的Ca3Co4O9晶體，其電導(dǎo)率和功率因子顯著提高，從而達(dá)到優(yōu)化晶體熱電性能的作用，700 ℃時(shí)，材料Ca3Co4O9的熱電優(yōu)值為0.16[81]。采用放電等離子燒結(jié)技術(shù)制備Ca3Co4O9晶體時(shí)，加工過(guò)程中施加的壓力和保溫溫度會(huì)對(duì)材料的熱電性能產(chǎn)生影響，但冷卻速率卻對(duì)材料的熱電性能沒(méi)有顯著的影響[82]。在一定溫度及壓力范圍內(nèi)，材料的熱電性能隨溫度和壓力的增加而增加，溫度為900 ℃、壓力為50 MPa時(shí)，材料的熱電性能達(dá)到最優(yōu)值。

圖7 Ca3Co4O9的晶體結(jié)構(gòu)

摻雜或是替換某些元素也可以提高Ca3Co4O9晶體材料的熱電性能。少量Cr元素替換Ca3Co4O9晶體中的Co元素可以顯著提高材料的熱電性能[83]，材料Ca3Co4-xCrO9(≤0.05)的熱電性能比不摻雜的Ca3Co4O9晶體的熱電性能提升了25%；Bi元素的摻雜也可以提升材料的熱電優(yōu)值[84]，Bi離子替代部分Ca離子可以增大Ca3Co4O9晶體的電導(dǎo)率和塞貝克系數(shù)，同時(shí)降低材料的熱導(dǎo)率，700℃時(shí)，材料(Ca0.95Bi0.05)3Co4O9的熱電優(yōu)值高達(dá)0.25。添加Ag元素對(duì)Ca3Co4O9晶體材料熱電性能的改善作用是最明顯的，少量Ag元素的摻雜可以增加材料的電導(dǎo)率和功率因子，從而起到改善熱電性能的效果，1000 K時(shí)，材料Ca2.7Ag0.3Co4O9的熱電優(yōu)值達(dá)到0.5[63]。

2.4 籠合物

籠合物是一類具有典型“電子晶體-聲子玻璃”特性的熱電材料[12]?；\合物熱電材料可以分為4大類：Ⅰ型籠合物、Ⅱ型籠合物、Ⅲ型籠合物、Ⅷ型籠合物，其中，Ⅷ型籠合物的熱電性能相比于其他3類要好一些。因此，本文僅就Ⅷ型籠合物來(lái)討論它的熱電性能。Ⅷ型籠合物包含4種化合物：Ba8Ga16Sn30，a-Eu8Ga16Ge30，Sr8Ga16-xAlGe30(6≤≤10)，Sr8Ga16-xAlSi30(8≤≤10)，其中，Ⅷ型籠合物Ba8Ga16Sn30的熱電性能最好[85-88]。

研究發(fā)現(xiàn)，Ga和Sn元素在Ⅷ型籠合物Ba8Ga16Sn30中的化學(xué)計(jì)量比可以決定材料的載流子類型[89-90]，當(dāng)Ga元素過(guò)量時(shí)，Ⅷ型籠合物Ba8Ga16Sn30表現(xiàn)為P型半導(dǎo)體的性能；當(dāng)Sn元素過(guò)量時(shí)，Ⅷ型籠合物Ba8Ga16Sn30表現(xiàn)為N型半導(dǎo)體的性能。450～500 K的溫度范圍內(nèi)，P型單晶籠合物Ba8Ga16Sn30的最大熱電優(yōu)值為1.0，N型單晶籠合物Ba8Ga16Sn30的最大熱電優(yōu)值為0.9[89]。

通過(guò)摻雜或是替換某些元素可以改善Ⅷ型籠合物Ba8Ga16Sn30的熱電性能。Sb元素替換單晶籠合物Ba8Ga16Sn30中的部分Sn元素會(huì)使Ga含量增加，從而改變材料的熱電性能，本文將這種材料記為Ba8Ga16+xSn30?x?ySb(< 0.9,< 0.9)，當(dāng)==0.7，溫度為480 K時(shí)，材料的熱電優(yōu)值為1.0[89]。通過(guò)Ge元素取代Sn元素可以制備出P型單晶籠合物Ba8Ga15.9Sn30.1?xGe(0≤≤4.73)，其塞貝克系數(shù)和電阻率顯著增加，相比不摻雜的籠合物，Ge元素的引入會(huì)使材料的塞貝克增加1.3倍，電阻率增加2倍，當(dāng)=0.07，溫度為540 K時(shí)，材料的最大熱電優(yōu)值達(dá)到0.87[91]。摻雜Cu元素也可以改善P型單晶籠合物Ba8Ga16Sn30的熱電性能[92]，材料的電導(dǎo)率及載流子遷移率均隨著Cu元素含量的增加而增加，實(shí)驗(yàn)顯示：300 K時(shí)，當(dāng)增加到0.033，籠合物Ba8Ga16-xCuSn30(0≤≤0.033)的電阻率相比于沒(méi)有摻雜的Ba8Ga16Sn30材料的電阻率減小39%，載流子遷移率增加兩倍，塞貝克系數(shù)僅下降10%；540 K時(shí)，P型單晶籠合物Ba8Ga16-xCuSn30(=0.033)的熱電優(yōu)值可達(dá)1.35。P型多晶籠合物Ba8Ga16Sn30中引入Ge元素，可以提高材料的載流子遷移率，從而改善籠合物的熱電性能。550 K時(shí)，多晶籠合物Ba8Ga16.4Sn25.0Ge4.6的最大熱電優(yōu)值為0.62；600 K時(shí)，多晶籠合物Ba8Ga16.9Sn19.8Ge9.3的最大熱電優(yōu)值為0.63[93]。

2.5 Half-Heusler材料

Half-Heusler體系是一種典型的窄帶隙半金屬材料，適用于中高溫范圍，由于其高溫穩(wěn)定性和良好的機(jī)械性能而備受關(guān)注。Half-Heusler的通式為ABX，其中A是元素周期表中左邊的過(guò)渡元素(鈦或釩族)，B為元素周期表中右邊的過(guò)渡元素(鐵，鈷，鎳族)，X為主族元素(鎵、錫、或銻)，其為立方MgAgAs型結(jié)構(gòu)，晶體結(jié)構(gòu)如圖8所示[94]。

圖中大、小2種實(shí)心圓圈分別代表A和B原子，空心圓圈代表X原子，B原子占據(jù)AX亞結(jié)構(gòu)立方間隙的一半[94]。A原子格子和B原子格子一起構(gòu)成NaCl型結(jié)構(gòu)，形成4個(gè)小立方體，若4個(gè)小立方體的所有空隙中心均被B原子填滿，則材料的結(jié)構(gòu)為ABZX，即所謂的Heusler結(jié)構(gòu)化合物，但圖中小立方體的空隙中心只有一半被B原子占據(jù)，另一半是空的，因此稱之為Half-Heusler合金。A、B和X晶格位置都具有高的可替代性，由于質(zhì)量起伏和應(yīng)力起伏效應(yīng)，對(duì)A、B元素的取代可降低晶格熱導(dǎo)率，同時(shí)X的取代也可調(diào)整載流子濃度，進(jìn)一步調(diào)控seebeck和電導(dǎo)率。N型Half-Heusler合金MNiSn和MCoSb(M=Ti, Zr, Hf)是被研究的最多的[95-96]，P型材料也有如MCoSb[97-98]，LnPdSb[99]，ErNiSn[100-101]，HfPtSn[102-103]，ZrPtSn[104]等。

工業(yè)和汽車排放的尾氣平均溫度為500～600 ℃[105]，這使得中高溫?zé)犭姴牧细哂泄I(yè)應(yīng)用價(jià)值，中高溫?zé)犭姴牧系拇頌镻bTe、方鈷礦和Half-Heusler體系。Half-Heusler材料的應(yīng)用溫度貼近于絕大部分的工業(yè)熱源，PbTe材料具有毒性，且機(jī)械效率差，方鈷礦的熱穩(wěn)定性差，Half-Heusler的優(yōu)勢(shì)在于沒(méi)有這類缺點(diǎn)，但高熱導(dǎo)率一直制約著它的發(fā)展。人們通過(guò)以下4種方法來(lái)降低其熱導(dǎo)率。

1) 最常見(jiàn)的是通過(guò)化學(xué)替位摻雜來(lái)降低其晶格熱導(dǎo)率。如ZrNiSn，其晶格熱導(dǎo)率為10 W/Mk，人們通過(guò)形成固溶體Zr0.5Hf0.5NiSn有效的減少了其晶格熱導(dǎo)率[95]。

2) 另外還可以通過(guò)形成納米結(jié)構(gòu)來(lái)降低晶格熱導(dǎo)率。文獻(xiàn)[96]通過(guò)高能球磨加直流熱壓法制成納米復(fù)合相Half-Heusler材料增加了聲子散射，降低了晶格熱導(dǎo)率，使P型Zr0.5Hf0.5CoSb0.8Sn0.2的ZT值從0.5上升到0.8，N型的Hf0.75Zr0.25NiSn0.99Sb0.01的ZT值從0.8增加到1.0[106]。值得注意的是，納米復(fù)合材料中載流子遷移率并無(wú)顯著降低，但熱壓后的晶粒尺寸會(huì)變大，這制約了熱導(dǎo)率的進(jìn)一步降低。

3) 隨后人們通過(guò)增大原子之間質(zhì)量和尺寸的差異來(lái)進(jìn)一步降低熱導(dǎo)率。文獻(xiàn)[107]利用原子質(zhì)量和尺寸相差更大的Hf-Ti組合代替Hf-Zr組合進(jìn)一步降低熱導(dǎo)率，在800 ℃時(shí)P型HH材料的ZT值達(dá)1.0。之后，文獻(xiàn)[108]用價(jià)格更低廉的Ti取代Hf得到的N型Hf0.75-xTiZr0.25NiSn0.99Sb0.01實(shí)現(xiàn)了500 ℃下ZT值達(dá)到1，有利于中溫區(qū)域(汽車尾氣回收)的應(yīng)用。

4) 此后人們又著眼于Half-Heusler的三元合金材料的研究，文獻(xiàn)[109]在之前的研究基礎(chǔ)上著手于三元合金(Ti,Zr,Hf)CoSb0.8Sn0.2的研究，得到其800℃下的ZT值大于1。此后文獻(xiàn)[110]減少了Hf的用量同樣使ZT值在700 ℃達(dá)到了1.0。文獻(xiàn)[111]對(duì)N型Half-Heusler體系進(jìn)行研究，減少Hf的用量為之前的三分之一，同樣獲得了1.0的ZT值，且其成本僅為之前的一半。

目前，對(duì)于Half-Heusler體系的熱電性能的研究仍在不斷進(jìn)行。例如，文獻(xiàn)[112]探索了P型NbFeSb基的HH得到的ZT值在700 ℃達(dá)到1.0，隨后文獻(xiàn)[97]發(fā)現(xiàn)P型FeNb0.88Hf0.12Sb和FeNb0.86Hf0.14Sb合金的ZT值在1 200 K時(shí)可接近1.5，原型器件的功率密度達(dá)2.2 W/cm2。文獻(xiàn)[113]通過(guò)將接近熔點(diǎn)附近的材料快速退火這種方式減小結(jié)構(gòu)的無(wú)序性，提升了功率因子，制得了非納米結(jié)構(gòu)但ZT值達(dá)1.2的(Hf, Zr)NiSn合金。但ZT值距離3.0的目標(biāo)依舊很遠(yuǎn)。文獻(xiàn)[114]系統(tǒng)地調(diào)查了大量Half-Heusler化合物的熱導(dǎo)率，發(fā)現(xiàn)LaPtSb的熱導(dǎo)率低至1.72 W/mk。文獻(xiàn)[115]通過(guò)理論計(jì)算預(yù)測(cè)了LaPtSb這種N型Half-Heusler合金的ZT值可達(dá)2.2。文獻(xiàn)[116]發(fā)現(xiàn)HH拓?fù)浣^緣體LaPtBi在單軸拉伸應(yīng)變下具有不錯(cuò)的熱電性質(zhì)，預(yù)測(cè)了Half- Heusler拓?fù)浣^緣體作為熱電材料的潛質(zhì)。

通過(guò)納米復(fù)合技術(shù)、增強(qiáng)合金散射、三元合金等措施，Half-Heusler的最高ZT值大幅增加了。

2.6 Skutterudite方鈷礦材料

具有Skutterudite晶體結(jié)構(gòu)的熱電材料，又稱為方鈷礦材料，最初在挪威小鎮(zhèn)Skutterud以礦物形式被發(fā)現(xiàn)的。是一類通式為MX3的化合物(其中M是金屬元素，如Ir、Co、Rh、Fe等；X是V族元素，如P、As、Sb等)[117]，適用于中溫區(qū)(400～600 K)。是PGEC(phonon glass electron crystal)設(shè)計(jì)理念的典型體現(xiàn)。方鈷礦為立方晶格結(jié)構(gòu)，最初來(lái)源于CoAs3礦物，而后擴(kuò)展到相同族的其他化合物中。一個(gè)單位晶胞包含了8個(gè)AB3分子，共32個(gè)原子，每個(gè)晶胞內(nèi)還有兩個(gè)較大的空隙[1]，通過(guò)往空隙內(nèi)填充原子形成填充式方鈷礦，進(jìn)而減少晶格熱導(dǎo)率，而電子輸運(yùn)情況基本不受影響，以下幾種方式可以提升其熱電效率。

1) 元素置換，形成固溶體合金。在CoSb3化合物中，Co的位置可被Fe、Ni、Ir等取代，Sb的位置可被Te、Se、Sn等替代。文獻(xiàn)[118]通過(guò)用Ge、Te來(lái)部分替代Sb，實(shí)現(xiàn)了在750 K下ZT值約為1.1，文獻(xiàn)[119]用Sn、Te替代Sb，實(shí)現(xiàn)了約550℃下ZT值為1.1。對(duì)Co位用Fe部分或全部替代也同樣是研究的熱點(diǎn)[120-121]。

2) 通過(guò)形成填充式方鈷礦材料來(lái)降低熱導(dǎo)率。這種填充式方鈷礦材料通式為RM4X12，其中X為磷、砷或銻；M為鐵、釕、鋨；而R為填充元素。CoSb3基方鈷礦材料RxCo4Sb12是一種優(yōu)秀的n型熱電材料，其具有高電子遷移率、高有效質(zhì)量、低熱導(dǎo)率。R可以為堿金屬[122-123]、堿土金屬[124-125]、稀有金屬[126]和一些其他元素如鉈[127]、錫[128]等。在850 K溫度下，通過(guò)摻Na可實(shí)現(xiàn)ZT值為1.25[123]，摻Ba可實(shí)現(xiàn)ZT值為1.1[125]。0 K下，摻In可實(shí)現(xiàn)ZT值達(dá)1.2[129]。文獻(xiàn)[130]通過(guò)摻Y(jié)b實(shí)現(xiàn)了Yb0.3Co4Sb12在850 K下的ZT值為1.3，隨后又用更加便宜的Ce代替Yb，實(shí)現(xiàn)了Ce0.14Co4Sb12在相同溫度下同樣的ZT值[131]。

3) 除了單填以外，還可以進(jìn)行多填。文獻(xiàn)[132]通過(guò)往CoSb3中填充Ba、La、Yb元素實(shí)現(xiàn)了850 K下ZT值達(dá)1.7。文獻(xiàn)[133]通過(guò)填充Sr,Ba,Yb實(shí)現(xiàn)了823 K溫度下ZT值達(dá)到1.4。除了可對(duì)CoSb3進(jìn)行填充外，最近文獻(xiàn)[134]開(kāi)始對(duì)與CoSb3具有相似性質(zhì)的IrSb3進(jìn)行單原子填充得到N型半導(dǎo)體，得到685 K時(shí)最大ZT值為0.44，這個(gè)值能夠通過(guò)多原子填充進(jìn)一步提高。

4) 對(duì)材料低維化處理：將熱電材料制成多晶材料，并減小晶粒尺寸，增大晶界密度，形成納米化材料。納米材料的高密度晶界可以強(qiáng)烈地散射聲子，減少熱導(dǎo)率。文獻(xiàn)[135]利用放電等離子體燒結(jié)(SPS)法制成微納米級(jí)CoSb3，發(fā)現(xiàn)隨著晶粒尺寸的減小，熱電性能得到極大改善。

5) 另外，合成具有微氣孔的方鈷礦材料也能有效的降低熱導(dǎo)率，文獻(xiàn)[136]通過(guò)對(duì)CoSb3基的熱電材料引入氣孔極大地提升了ZT值。

2.7 Zintl相熱電材料

Zintl相是一類符合PGEC概念的熱電材料，由電負(fù)性差別較大的陰陽(yáng)離子組成。其中陽(yáng)離子為堿金屬、堿土金屬、稀有金屬。陽(yáng)離子轉(zhuǎn)移電子給陰離子，而轉(zhuǎn)移過(guò)去的電子不能填滿陰離子的最外層，陰離子間形成共價(jià)鍵，由此共價(jià)鍵形成穩(wěn)定的框架結(jié)構(gòu)，起到“電子晶體”的作用。結(jié)構(gòu)內(nèi)部嵌有的結(jié)合較弱的離子區(qū)域，可以有效散射聲子，起到“聲子玻璃”的作用，這種離子鍵和共價(jià)鍵共存結(jié)構(gòu)就為Zintl相結(jié)構(gòu)[137]。這種化合物通常都是窄帶隙半導(dǎo)體，由于其復(fù)雜的結(jié)構(gòu)，晶格熱導(dǎo)率通常都很低[138]。Zintl相熱電材料形式多樣，絕大多數(shù)為三元化合物，如A14MPn11(A = Ca, Sr, Ba, Eu, Yb; M = Mg, Nb, Mn, Zn, Cd; Pn = P, As, Sb, Bi)這種形式，通常簡(jiǎn)稱為14-1-11。根據(jù)Zintl相熱電材料不同的陰離子結(jié)構(gòu)，可以分為孤立基團(tuán)(14-1-11，Zn4Sb3等)[139-141]，一維四面體鏈(5-2-6[142-145], 3-1-3[146-147], 9-4-9[148-149]等)，二維層狀(1-2-2[150], Mg3Sb2[151-152]等)，三維框架結(jié)構(gòu)(包括金屬間籠形物[153-154]，方鈷礦[155]，Mo3Sb7[156]等)。

其中研究較多的為Sb基，如Yb14MnSb11[157]，Zn4Sb3[140], Mg3Sb2[158], BaGa2Sb2[159], Eu5In2Sb6[160], CaYb1-xZn2Sb2[161], EuZn2Sb2[162], YbCd2-xZnSb2[163]等。CaAl2Si2結(jié)構(gòu)的三元銻化物AX2Sb2(X=Cd, Zn; A =Sr, Ca, Yb, Eu)在中溫區(qū)具有較好的熱電性質(zhì)。YbCd1.6Zb0.4Sb2在700 K下的ZT值可達(dá)1.2[163]。高溫區(qū)的代表是Yb14MnSb11，相對(duì)分子質(zhì)量為為典型熱電材料PbTe的10倍。大分子質(zhì)量使其擁有的室溫?zé)釋?dǎo)率非常低。低熱導(dǎo)率使其在1 200 K下的ZT值約為1.0[164]，可用于深空探測(cè)等高溫環(huán)境工作。這種材料打破了高溫?zé)犭姴牧习l(fā)展的停滯不前，比起SiGe可實(shí)現(xiàn)4倍的效率和幾乎兩倍的ZT值[157]，這種材料已經(jīng)被美國(guó)國(guó)家航空航天局噴氣推進(jìn)實(shí)驗(yàn)室發(fā)展作為其下一代的放射性同位素?zé)犭姲l(fā)生器(RTG)[165]。

除了Sb基的Zintl相化合物外，Te基的Ag9TlTe5也是Zintl相化合物的典型代表，其彈性模量低，化學(xué)鍵結(jié)合相對(duì)弱，不利于聲子傳輸。熱導(dǎo)率從室溫到650 K一直保持相對(duì)恒定的低值。其ZT值在673 K可達(dá)到1.25[166]。另外，對(duì)于Bi基的Zintl相化合物也開(kāi)始被研究。文獻(xiàn)[167]第一次報(bào)道了CaMg2Bi2和YbMg2Bi2的熱電性質(zhì)，隨后，文獻(xiàn)[168]通過(guò)球磨熱壓法合成了ZT值約為1的Bi基化合物。

2.8 Cu-S族類熱電材料

Cu2-xS(Se，Te)是一類新型的P型熱電材料。與傳統(tǒng)的熱電材料相比，其不含昂貴稀少的重金屬元素Pb,Te,Bi,Ge,Co,Sb等，組成的硫族元素(S，Se，Te)和Cu元素均在地殼中含量豐富且無(wú)毒。銅硫?qū)倩衔镫m然有簡(jiǎn)單的化學(xué)式，但其原子排布十分復(fù)雜。在高溫下，這種化合物主要由占據(jù)固定位置的主體晶格結(jié)構(gòu)S(Se, Te)和占據(jù)亞晶格結(jié)構(gòu)的可自由移動(dòng)的銅離子組成。當(dāng)溫度升高時(shí)Cu離子可以像液體一樣在亞晶格中自由遷移。Cu離子的這種液相行為可以強(qiáng)烈地散射聲子，降低聲子的平均自由程，使熱導(dǎo)率降低，但不會(huì)降低載流子的遷移率。文獻(xiàn)[169]拓展了原有的“聲子玻璃，電子晶體”的概念，提出了“聲子液體，電子晶體”來(lái)解釋這種結(jié)構(gòu)的液相行為。這種非同尋常的結(jié)構(gòu)使得其成為了理想的熱電材料。

Cu2-xSe具有高溫固液立方相β和低溫穩(wěn)定相α。以Cu2Se為例，低溫相的Cu2Se具有單斜晶體結(jié)構(gòu)，薄層狀，對(duì)稱性差，銅離子是固定的，無(wú)液相行為[170-172]。當(dāng)溫度上升到400 K左右時(shí)，α相的Cu2Se轉(zhuǎn)變?yōu)楦邷卅孪?空間群為)。在相變過(guò)程中，Cu離子沿著<111>方向有序堆疊，形成簡(jiǎn)單的反螢石結(jié)構(gòu)[173]。這種相變通過(guò)降溫可逆。文獻(xiàn)[169]通過(guò)熱壓和等離子燒結(jié)制成的Cu2Se，伴隨著升溫，Cu離子的高遷移率和液相流動(dòng)行為使得晶格熱導(dǎo)率降低到0.4～0.6 W/mK。ZT值在1 000 K時(shí)可以達(dá)到1.5。之后，文獻(xiàn)[174]發(fā)現(xiàn)相變對(duì)熱電性能的影響很突出，在連續(xù)的相變過(guò)程中，強(qiáng)烈的結(jié)構(gòu)波動(dòng)引起臨界電子和聲子的散射增加，熱導(dǎo)率降低，在摻I的Cu2Se中，ZT值增加到400 K附近的2.3左右。文獻(xiàn)[25]通過(guò)水熱法合成了高質(zhì)量的β相納米片狀Cu2Se，經(jīng)過(guò)放電等離子燒結(jié)(SPS)處理后，由于納米材料小角度高密度的晶界和晶界內(nèi)部的位移極大增加了聲子散射，使得晶格熱導(dǎo)率低至約0.2 W/mK，ZT值在850 K左右達(dá)到了1.82[175]，摻雜Al時(shí)達(dá)到了2.62[176]。

Cu2-xS不僅是一種有光電材料[177]，也是有潛力的熱電材料，在自然界中以多種形式存在。在Cu元素豐富的環(huán)境下，已經(jīng)確定的Cu2-xS化合物就有輝銅礦(Cu2S)[178]、久輝銅礦(Cu1.94S)[179]、藍(lán)輝銅礦(Cu1.8S)[180]、斜方藍(lán)輝銅礦(Cu1.75S)[181]。在S的晶格中，Cu原子的位置是不確定的，隨著的變化而變化。對(duì)于Cu2S，在不同溫度下，存在3種輝銅礦相位：低于370 K時(shí)為單斜γ相(L銅輝礦)，370～700 K為六方形固液雜交[182]β相(H銅輝礦)，高于700 K轉(zhuǎn)為帶有自由移動(dòng)Cu離子的立方α相[183]。文獻(xiàn)[184]通過(guò)熱壓法和等離子燒結(jié)(SPS)制得了液相Cu2-xS，其熱導(dǎo)率在300～1 000 K范圍內(nèi)低于0.6 W/mK)，在1 000 K時(shí)，Cu1.97S的ZT值可達(dá)到1.7。該實(shí)驗(yàn)除了證明在高溫下，Cu2-xS可以達(dá)到高ZT值外，在低溫下(298 K)，Cu2-xS的載流子濃度和塞貝克系數(shù)可以低至4.8×1 018 cm-3和0.1 μV/K，相比于其他典型的熱電材料，這是相當(dāng)?shù)偷闹?。之后，文獻(xiàn)[185]通過(guò)金屬固態(tài)技術(shù)，成功合成了970 K的ZT值約為1.9的高密度a-相Cu2S和Cu1.97S多晶塊體材料，其中Cu1.97S不僅展現(xiàn)了很好的熱電性能和可重復(fù)性，也展現(xiàn)了非常優(yōu)秀的機(jī)械性質(zhì)，硬度達(dá)到約1 Gpa，比起之前的熱壓法，不但減小了成本，而且在一定程度上避免了Cu離子的遷移。其后，又發(fā)現(xiàn)當(dāng)X位為S和Te兩種元素同時(shí)固溶時(shí)，ZT值可以達(dá)到2.1[186]。

3 應(yīng) 用

熱電轉(zhuǎn)換技術(shù)是一種利用半導(dǎo)體材料直接將熱能與電能相互轉(zhuǎn)換的技術(shù)。隨著環(huán)境保護(hù)形勢(shì)的日益嚴(yán)峻，研究和開(kāi)發(fā)清潔能源已成為全球科學(xué)研究的重點(diǎn)領(lǐng)域。其中，熱電轉(zhuǎn)換技術(shù)憑借系統(tǒng)體積小、可靠性高、不排放污染物質(zhì)、使用溫度范圍廣等特點(diǎn)，被重點(diǎn)關(guān)注[187]。

熱電器件主要由P型熱電材料、N型熱電材料、Cu和Al2O3陶瓷基板組成，如圖9所示。目前，熱電器件已經(jīng)實(shí)現(xiàn)產(chǎn)業(yè)化，國(guó)內(nèi)外有不少?gòu)S商在生產(chǎn)和銷售熱電器件。國(guó)外的廠商主要有Marlow、TE Technology、TEC Microsystems等。國(guó)內(nèi)的廠商有廣東富信科技股份有限公司、香河?xùn)|方電子有限公司、上海申和熱磁電子有限公司、杭州大和熱磁電子有限公司、河南鴻昌電子有限公司、江西納米克熱電電子股份有限公司等。

熱電器件的應(yīng)用主要用于溫差發(fā)電和制冷兩個(gè)方面，如圖10所示。其中溫差發(fā)電主要基于Seebeck效應(yīng)，熱電制冷主要基于Peltier效應(yīng)。

溫差發(fā)電技術(shù)最早開(kāi)始于20世紀(jì)40年代，由于其具有結(jié)構(gòu)簡(jiǎn)單、堅(jiān)固耐用、無(wú)運(yùn)動(dòng)部件、無(wú)噪聲、使用壽命長(zhǎng)等優(yōu)點(diǎn)，使其在航天、航空、軍事等領(lǐng)域得到廣泛應(yīng)用。在深空中，熱電器件主要用在放射性同位素?zé)犭姍C(jī)(radioisotope thermoelectric generator, RTG)，如圖11所示。隨著化石能源的日趨枯竭，美國(guó)、日本、歐盟等發(fā)達(dá)國(guó)家更加重視溫差發(fā)電技術(shù)在民用領(lǐng)域的研究，并取得了長(zhǎng)足的進(jìn)展。目前，溫差發(fā)電技術(shù)可以合理利用太陽(yáng)能、地?zé)崮堋⒐I(yè)廢熱、汽車尾氣廢熱、人體熱等低品位能源轉(zhuǎn)換成電能[188-194]，如圖12和圖13所示。

熱電制冷技術(shù)又稱為半導(dǎo)體制冷或溫差電制冷。熱電制冷器是固體電子元件，具有體積小、加熱/制冷迅速、加熱制冷切換方便等特點(diǎn)，已經(jīng)在民用、電子、醫(yī)療、光學(xué)、計(jì)量等幾個(gè)領(lǐng)域得到廣泛的應(yīng)用[195]。而熱電制冷技術(shù)的應(yīng)用又分為熱電制冷和精確控溫兩個(gè)方面。

熱電制冷主要應(yīng)用有民用領(lǐng)域的車載冰箱、除濕器、小型飲料機(jī)、車用冷杯、冷帽、汽車座椅、化妝品存儲(chǔ)箱等[196-197]，以及電子領(lǐng)域的主板北橋制冷芯片、CPU測(cè)試平臺(tái)、冷風(fēng)裝置、冷卻板、大功率LED散熱器、投影儀制冷等[198-207]，如圖14和圖15所示。

熱電精確控溫主要應(yīng)用在一些對(duì)溫度要求高的精密設(shè)備或組件中。在光學(xué)領(lǐng)域中，熱電器件主要用在對(duì)紅外線探測(cè)器、高靈敏度CCD、激光發(fā)射器、分光光度計(jì)、色譜儀等儀器和部件的溫度控制上[208-212]。文獻(xiàn)[213]采用內(nèi)置TEC的激光發(fā)射器設(shè)計(jì)立方星通信系統(tǒng)，在功耗小于10 W的條件下使衛(wèi)星對(duì)地傳輸高達(dá)50 Mbps。文獻(xiàn)[214]采用四級(jí)熱電制冷片研制了星載CCD相機(jī)快速制冷系統(tǒng)，能在1 min內(nèi)將CCD冷卻到-73 ℃，精度±0.1 ℃。在醫(yī)療領(lǐng)域，熱電器件主要用于DNA擴(kuò)增儀、生物試劑檢測(cè)裝置、低溫藥劑保存箱以及各種高精度醫(yī)療儀器[215-222]，如圖16所示。

4 結(jié)束語(yǔ)

綜上所述，近年來(lái)熱電材料取得了一系列的進(jìn)展，新的高性能熱電材料不斷涌現(xiàn)，發(fā)現(xiàn)了一些結(jié)構(gòu)上可以大幅降低熱導(dǎo)率又同時(shí)保持高的電導(dǎo)率的SnSe、SnS類的材料，使ZT值有了進(jìn)一步的增加。另一方面，熱電器件也在擴(kuò)展到新的領(lǐng)域里面，在對(duì)溫度控制要求較高的傳感器，集成電路中有了新的應(yīng)用，同時(shí)也出現(xiàn)在可穿戴的電子設(shè)備上。熱電材料的存在已經(jīng)超過(guò)一百年了，其大規(guī)模的應(yīng)用至今尚未形成，其主要原因就是成本過(guò)高，這兩者相互影響。本文通過(guò)對(duì)近年來(lái)一些新型熱電材料和器件的回顧，希望能夠促使找到新的應(yīng)用，從而形成熱電材料和器件的規(guī)?；a(chǎn)業(yè)。

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編 輯 葉 芳

Research Progress of New Thermoelectric Materials

WANG Chao1,2, ZHANG Rui1,2, DU Xin1,2, ZHANG Chen-gui1,2, LUAN Chun-hong1,2, JIANG Jing1,2, HU Qiang3, WANG Jun-xi4, DU Zhi-you5, LI Tian-xiao5, MA Tie-zhong6, YAN Dong6, and YIN Zhi-yao5

(1. School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China Chengdu 611731; 2. State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China Chengdu 611731； 3. MTM Semiconductor Equipment Co., Ltd. Guangzhou 510530; 4. Institute of Semiconductors, Chinese Academy of Sciences Haidian Beijing 100083; 5. Advanced Micro-Fabrication Equipment Inc. Pudong Shanghai 201201; 6. BEI OPTICS Technology Co.,Ltd. Changping Beijing 102206)

Thermoelectric materials can realize transfer between electricity and heat. On one hand, harvesting electricity from temperature difference can be a future way to achieve energy; on the other hand, the temperature of thermoelectric materials can be precisely controlled. After decades of development, thermoelectric materials have been grown rapidly, but the overall efficiency is still low. In order to further enhance the device efficiency, the novel thermoelectric materials need to be discovered. This review summarize the recent development of some new kinds of thermoelectric materials with their applications in sensors and integrated circuits. It aims to prosper the applications of the thermoelectric materials.

new thermoelectric materials; power factor; seebeck coefficient; thermoelectric devices; thermoelectric properties

TN415

A

10.3969/j.issn.1001-0548.2017.01.019

2016-06-21；

2016-10-10

國(guó)家自然科學(xué)基金(51672037,61604031); 四川省科技計(jì)劃(2014GZ0151,2016JQ0022);中央高校基本科研業(yè)務(wù)費(fèi)(ZYGX2013J115,ZYGX2014J087, ZYGX2015J029)

王超(1978-)，男，博士，教授，主要從事能源材料方面的研究.