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1研究背景

近年來(lai)全(quan)球(qiu)的(de)能源需(xu)求(qiu)量(liang)呈現加速增長(chang)的(de)態勢,多種(zhong)燃料(liao)的(de)消耗增速及(ji)全(quan)球(qiu)碳排放增長(chang)量(liang)都達到近十年來(lai)的(de)最大值(zhi)。各國當(dang)前(qian)面臨既要滿(man)足全(quan)球(qiu)電(dian)(dian)氣(qi)化背(bei)景下不(bu)斷增長(chang)的(de)電(dian)(dian)力需(xu)求(qiu),又要促進(jin)(jin)能源轉(zhuan)型和(he)控(kong)制環境污染的(de)雙(shuang)重矛盾。同時,推(tui)進(jin)(jin)高效(xiao)的(de)能源開發(fa)利用(yong)設備(bei)和(he)技術(shu)的(de)研究以及(ji)開發(fa)清潔的(de)新能源,是我國當(dang)前(qian)能源建設的(de)發(fa)展(zhan)方向(xiang)。積極開發(fa)基于核能、太陽(yang)能等新能源的(de)高效(xiao)高溫發(fa)電(dian)(dian)系統(tong),具(ju)有重要的(de)工程價值(zhi)和(he)社會意義(yi)。

熱電(dian)子發(fa)(fa)(fa)射(she)能(neng)量轉(zhuan)(zhuan)換(TEC)發(fa)(fa)(fa)電(dian)系統(tong)是(shi)值得關注的高(gao)效能(neng)高(gao)溫發(fa)(fa)(fa)電(dian)系統(tong)。熱電(dian)子發(fa)(fa)(fa)射(she)能(neng)量轉(zhuan)(zhuan)換器是(shi)一種基于熱電(dian)子發(fa)(fa)(fa)射(she)效應,在1500K至2000K的運行溫度(du)下直接(jie)(jie)將熱能(neng)轉(zhuan)(zhuan)化為電(dian)能(neng)的發(fa)(fa)(fa)電(dian)裝置,其(qi)(qi)特點為無需(xu)化學反應、流(liu)(liu)體介質或移動部件,結構簡(jian)單,可靠性高(gao);在發(fa)(fa)(fa)電(dian)過程中無噪音、無磨損、無介質泄露,使用壽命長;同時其(qi)(qi)又(you)具備(bei)可擴展性高(gao),單位面積輸出電(dian)流(liu)(liu)密度(du)及輸出功(gong)率大等(deng)優點,是(shi)目前理論發(fa)(fa)(fa)電(dian)效率最(zui)高(gao)的熱電(dian)能(neng)量直接(jie)(jie)轉(zhuan)(zhuan)換裝置。

在(zai)(zai)(zai)核(he)(he)能(neng)利(li)(li)用領(ling)域(yu),熱(re)(re)電(dian)(dian)(dian)子發射(she)能(neng)量(liang)(liang)轉(zhuan)換器(qi)能(neng)夠在(zai)(zai)(zai)高(gao)溫(wen)環(huan)境下(xia)(xia)直接(jie)將(jiang)核(he)(he)裂變(bian)產(chan)生的(de)熱(re)(re)能(neng)轉(zhuan)化(hua)為電(dian)(dian)(dian)能(neng)。常(chang)(chang)規的(de)核(he)(he)電(dian)(dian)(dian)站的(de)蒸(zheng)汽循環(huan)溫(wen)度通常(chang)(chang)在(zai)(zai)(zai)800K以下(xia)(xia),核(he)(he)裂變(bian)反應的(de)高(gao)溫(wen)端存在(zai)(zai)(zai)大(da)量(liang)(liang)未(wei)被有效利(li)(li)用的(de)熱(re)(re)能(neng),采用運行(xing)溫(wen)度更高(gao)的(de)熱(re)(re)電(dian)(dian)(dian)子發射(she)能(neng)量(liang)(liang)轉(zhuan)換器(qi)作為頂循環(huan),可(ke)(ke)大(da)幅提高(gao)熱(re)(re)能(neng)的(de)利(li)(li)用率。在(zai)(zai)(zai)空(kong)間核(he)(he)動力(li)系(xi)(xi)統(tong)的(de)研(yan)究中,相較于蒸(zheng)汽渦輪發電(dian)(dian)(dian)方案,熱(re)(re)電(dian)(dian)(dian)子發射(she)能(neng)量(liang)(liang)轉(zhuan)換器(qi)在(zai)(zai)(zai)可(ke)(ke)靠性、系(xi)(xi)統(tong)發射(she)重(zhong)量(liang)(liang)、使用壽命(ming)等(deng)(deng)方面(mian)具有較為明顯的(de)優勢。目前美、俄在(zai)(zai)(zai)銫蒸(zheng)汽型(xing)熱(re)(re)電(dian)(dian)(dian)子能(neng)量(liang)(liang)轉(zhuan)換器(qi)結合空(kong)間核(he)(he)電(dian)(dian)(dian)系(xi)(xi)統(tong)的(de)應用研(yan)究上已經取得了一(yi)定(ding)的(de)進展(zhan)。在(zai)(zai)(zai)太(tai)(tai)(tai)陽(yang)能(neng)光熱(re)(re)轉(zhuan)換領(ling)域(yu),通過(guo)菲涅爾聚(ju)光鏡等(deng)(deng)輔助器(qi)件聚(ju)焦太(tai)(tai)(tai)陽(yang)輻射(she)加熱(re)(re)發射(she)電(dian)(dian)(dian)極,可(ke)(ke)有效利(li)(li)用太(tai)(tai)(tai)陽(yang)能(neng)進行(xing)熱(re)(re)電(dian)(dian)(dian)轉(zhuan)化(hua)。該裝置在(zai)(zai)(zai)空(kong)間太(tai)(tai)(tai)陽(yang)能(neng)電(dian)(dian)(dian)站、獨(du)立軍事設備供電(dian)(dian)(dian)、偏遠地區小型(xing)分布式(shi)能(neng)源供應等(deng)(deng)方面(mian)有巨大(da)應用潛力(li)。

綜上,TEC高(gao)溫(wen)發電裝(zhuang)置有(you)望應用于軍事、航(hang)天動力轉換等(deng)對系統(tong)穩定性(xing)和無(wu)噪聲性(xing)有(you)要求的領域(yu),也(ye)可以(yi)在民用發電領域(yu)減少(shao)運營成本和提高(gao)發電效率(lv)。

2基本原理

熱(re)(re)電(dian)(dian)子發(fa)(fa)(fa)射現(xian)象(xiang)于十九(jiu)世紀八(ba)十年(nian)代(dai)被(bei)發(fa)(fa)(fa)現(xian),早期稱之(zhi)為“愛迪生效應”。其(qi)現(xian)象(xiang)為,將(jiang)兩(liang)電(dian)(dian)極(ji)(ji)置于真(zhen)空(kong)中(zhong)(zhong),加熱(re)(re)其(qi)中(zhong)(zhong)一個電(dian)(dian)極(ji)(ji)時可(ke)測得兩(liang)個電(dian)(dian)極(ji)(ji)間存(cun)在(zai)電(dian)(dian)流。熱(re)(re)電(dian)(dian)子發(fa)(fa)(fa)射電(dian)(dian)流可(ke)使用如圖1所示實(shi)驗電(dian)(dian)路進行測量,其(qi)中(zhong)(zhong)加熱(re)(re)裝(zhuang)置可(ke)控制發(fa)(fa)(fa)射電(dian)(dian)極(ji)(ji)的(de)溫度(du)。在(zai)兩(liang)電(dian)(dian)極(ji)(ji)之(zhi)間施加額外的(de)偏(pian)壓,并(bing)從零(ling)逐漸增加,熱(re)(re)電(dian)(dian)子發(fa)(fa)(fa)射電(dian)(dian)流先線性增加,然后逐漸達到飽和。

形(xing)成(cheng)熱電(dian)(dian)(dian)(dian)(dian)子(zi)發射電(dian)(dian)(dian)(dian)(dian)流(liu)的(de)(de)(de)(de)(de)原(yuan)因在(zai)于,金屬材(cai)料溫度升(sheng)高后材(cai)料內部電(dian)(dian)(dian)(dian)(dian)子(zi)的(de)(de)(de)(de)(de)能(neng)量(liang)增加(jia),進(jin)入(ru)能(neng)級較(jiao)高的(de)(de)(de)(de)(de)能(neng)態,當(dang)其(qi)能(neng)量(liang)大于材(cai)料表(biao)面(mian)的(de)(de)(de)(de)(de)逸出(chu)功(功函數)時,電(dian)(dian)(dian)(dian)(dian)子(zi)就會越過表(biao)面(mian)勢壘進(jin)入(ru)真空。若進(jin)入(ru)真空的(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)子(zi)能(neng)量(liang)并未耗盡,能(neng)夠繼續越過電(dian)(dian)(dian)(dian)(dian)極之間的(de)(de)(de)(de)(de)附加(jia)勢壘達到(dao)另(ling)一側電(dian)(dian)(dian)(dian)(dian)極則形(xing)成(cheng)電(dian)(dian)(dian)(dian)(dian)流(liu)。Richardson于1902年推導出(chu)金屬表(biao)面(mian)熱電(dian)(dian)(dian)(dian)(dian)子(zi)發射電(dian)(dian)(dian)(dian)(dian)流(liu)密度的(de)(de)(de)(de)(de)大小(xiao)與溫度T和金屬的(de)(de)(de)(de)(de)功函數φ有如下(xia)關系[1]:

其中,A為(wei)Richardson常數A≈120A/cm2·K2,kB為(wei)玻爾茲(zi)曼常數kB≈8.6×105eV/K。這就是Richardson方(fang)程(cheng),表明溫(wen)度越高、功函數越小,則熱(re)電子發射的電流密度就越大。

熱(re)(re)(re)電子(zi)發(fa)射(she)(she)(she)(she)現象的(de)(de)應用(yong)之一是(shi)熱(re)(re)(re)電子(zi)發(fa)射(she)(she)(she)(she)能量(liang)(liang)轉(zhuan)換器。熱(re)(re)(re)電子(zi)發(fa)射(she)(she)(she)(she)能量(liang)(liang)轉(zhuan)換器仍然(ran)是(shi)一種熱(re)(re)(re)機,其直接(jie)以熱(re)(re)(re)量(liang)(liang)作為(wei)(wei)能量(liang)(liang)來源(yuan),且可視(shi)為(wei)(wei)以電子(zi)作為(wei)(wei)工質(zhi)進行(xing)發(fa)電。真空(kong)型熱(re)(re)(re)電子(zi)轉(zhuan)換裝置(zhi)的(de)(de)基(ji)本形式如(ru)圖2所示,它的(de)(de)主要(yao)構(gou)件包括真空(kong)罩、發(fa)射(she)(she)(she)(she)電極(ji)(ji)(ji)、收(shou)集(ji)電極(ji)(ji)(ji)、熱(re)(re)(re)源(yuan)和(he)熱(re)(re)(re)沉(chen)等五(wu)部分。兩電極(ji)(ji)(ji)平行(xing)放置(zhi),中間(jian)(jian)留有(you)一定空(kong)隙,兩電極(ji)(ji)(ji)中間(jian)(jian)的(de)(de)空(kong)隙為(wei)(wei)超高真空(kong)環境。當在(zai)發(fa)射(she)(she)(she)(she)極(ji)(ji)(ji)和(he)收(shou)集(ji)極(ji)(ji)(ji)連接(jie)一負(fu)(fu)載(zai)時,電子(zi)從熱(re)(re)(re)源(yuan)吸收(shou)能量(liang)(liang),克服發(fa)射(she)(she)(she)(she)極(ji)(ji)(ji)的(de)(de)表面(mian)功函數逸出,穿過兩極(ji)(ji)(ji)間(jian)(jian)空(kong)隙到達收(shou)集(ji)極(ji)(ji)(ji),隨后由(you)于接(jie)觸勢和(he)外加偏(pian)壓作用(yong),到達收(shou)集(ji)極(ji)(ji)(ji)電子(zi)經由(you)外電路(lu)及負(fu)(fu)載(zai)輸(shu)出功率并返回發(fa)射(she)(she)(she)(she)極(ji)(ji)(ji),構(gou)成(cheng)完整的(de)(de)電流(liu)回路(lu)。

3研究進展

熱電(dian)(dian)子(zi)(zi)(zi)(zi)發(fa)(fa)(fa)(fa)射(she)(she)現(xian)象(xiang)于(yu)1885年(nian)由Edison發(fa)(fa)(fa)(fa)現(xian),隨后(hou)Thomson于(yu)1897年(nian)發(fa)(fa)(fa)(fa)現(xian)電(dian)(dian)子(zi)(zi)(zi)(zi)。1902年(nian)Richardson對熱電(dian)(dian)子(zi)(zi)(zi)(zi)發(fa)(fa)(fa)(fa)射(she)(she)進行了定量(liang)的(de)物理描述并推(tui)導了熱電(dian)(dian)子(zi)(zi)(zi)(zi)發(fa)(fa)(fa)(fa)射(she)(she)電(dian)(dian)流(liu)密度(du)(du)方程,即Richardson-Dushman方程。1923年(nian)Langmuir指出熱電(dian)(dian)子(zi)(zi)(zi)(zi)發(fa)(fa)(fa)(fa)射(she)(she)的(de)電(dian)(dian)極(ji)板(ban)間存在空間電(dian)(dian)荷(he)效(xiao)應[2]。空間電(dian)(dian)荷(he)效(xiao)應即發(fa)(fa)(fa)(fa)射(she)(she)極(ji)不(bu)斷逸出的(de)低速電(dian)(dian)子(zi)(zi)(zi)(zi)會在發(fa)(fa)(fa)(fa)射(she)(she)極(ji)與(yu)收集(ji)極(ji)之間形成(cheng)電(dian)(dian)子(zi)(zi)(zi)(zi)云(yun),電(dian)(dian)子(zi)(zi)(zi)(zi)云(yun)產生的(de)電(dian)(dian)場阻礙后(hou)續逸出的(de)電(dian)(dian)子(zi)(zi)(zi)(zi)到達收集(ji)極(ji),從而削弱熱電(dian)(dian)子(zi)(zi)(zi)(zi)能(neng)量(liang)轉換(huan)器的(de)實(shi)際輸(shu)出電(dian)(dian)流(liu)密度(du)(du)。

1950年代(dai)中期(qi),耐高溫材料技術(shu)、原子(zi)(zi)(zi)能(neng)發(fa)電(dian)技術(shu)的(de)(de)(de)(de)(de)發(fa)展(zhan)以(yi)及航(hang)(hang)天(tian)領域的(de)(de)(de)(de)(de)高效緊湊型(xing)電(dian)源需求促使各國研(yan)究(jiu)人員開(kai)展(zhan)對(dui)熱(re)(re)(re)(re)(re)電(dian)子(zi)(zi)(zi)發(fa)射能(neng)量(liang)(liang)轉(zhuan)(zhuan)(zhuan)換(huan)(huan)器(qi)(qi)的(de)(de)(de)(de)(de)實質(zhi)研(yan)究(jiu),蘇聯(lian)的(de)(de)(de)(de)(de)Marchuk、美國的(de)(de)(de)(de)(de)Wilson、Grover等進行了(le)相關研(yan)究(jiu)[1]。早期(qi)熱(re)(re)(re)(re)(re)電(dian)子(zi)(zi)(zi)發(fa)射能(neng)量(liang)(liang)轉(zhuan)(zhuan)(zhuan)換(huan)(huan)器(qi)(qi)被考慮應用(yong)(yong)于(yu)(yu)太(tai)陽(yang)能(neng)和放射性同位素空(kong)間動力系(xi)統(tong),但是至1965年該技術(shu)無法取代(dai)相對(dui)成(cheng)熟的(de)(de)(de)(de)(de)光伏及半導體熱(re)(re)(re)(re)(re)電(dian)技術(shu)作為(wei)航(hang)(hang)天(tian)動力技術(shu)方(fang)案。首(shou)個太(tai)陽(yang)能(neng)熱(re)(re)(re)(re)(re)電(dian)子(zi)(zi)(zi)轉(zhuan)(zhuan)(zhuan)換(huan)(huan)器(qi)(qi)在(zai)太(tai)空(kong)任務中的(de)(de)(de)(de)(de)能(neng)量(liang)(liang)轉(zhuan)(zhuan)(zhuan)化效率為(wei)4～7%,遠低(di)于(yu)(yu)其理論效率[1]。1965年后美國、蘇聯(lian)、西(xi)德、法國等將(jiang)該技術(shu)的(de)(de)(de)(de)(de)應用(yong)(yong)研(yan)究(jiu)重點調整(zheng)為(wei)熱(re)(re)(re)(re)(re)電(dian)子(zi)(zi)(zi)能(neng)量(liang)(liang)轉(zhuan)(zhuan)(zhuan)化與核反(fan)應堆結合的(de)(de)(de)(de)(de)工(gong)程開(kai)發(fa),至1990年美、俄先后開(kai)發(fa)了(le)熱(re)(re)(re)(re)(re)電(dian)子(zi)(zi)(zi)燃料元件(TEF)、基于(yu)(yu)熱(re)(re)(re)(re)(re)電(dian)子(zi)(zi)(zi)能(neng)量(liang)(liang)轉(zhuan)(zhuan)(zhuan)換(huan)(huan)器(qi)(qi)的(de)(de)(de)(de)(de)TOPAZ核電(dian)系(xi)統(tong)。這一(yi)階段(duan)各國的(de)(de)(de)(de)(de)對(dui)熱(re)(re)(re)(re)(re)電(dian)子(zi)(zi)(zi)發(fa)射能(neng)量(liang)(liang)轉(zhuan)(zhuan)(zhuan)換(huan)(huan)器(qi)(qi)的(de)(de)(de)(de)(de)研(yan)究(jiu)關注點主要(yao)(yao)在(zai)整(zheng)體系(xi)統(tong)壽命、航(hang)(hang)天(tian)發(fa)射重量(liang)(liang)、輸出功率等方(fang)面,且發(fa)電(dian)系(xi)統(tong)主要(yao)(yao)采用(yong)(yong)銫蒸汽型(xing)熱(re)(re)(re)(re)(re)電(dian)子(zi)(zi)(zi)能(neng)量(liang)(liang)轉(zhuan)(zhuan)(zhuan)換(huan)(huan)器(qi)(qi)。

1996年(nian)Naito等(deng)報道了一(yi)種(zhong)半(ban)導體熱電系統和熱電子(zi)能量(liang)轉化(hua)串(chuan)聯(lian)的(de)太(tai)陽能發電系統,其聯(lian)合轉換效率(lv)接近40%。2019年(nian)廖天軍等(deng)基于石墨(mo)烯發射極[3],以發射極功函(han)數、費米能級、熱源溫度為(wei)變(bian)量(liang)進(jin)行熱電子(zi)功率(lv)器件的(de)參數優化(hua),理論模(mo)型的(de)最高效率(lv)為(wei)60%[4]。

在(zai)近(jin)年對熱電子(zi)轉換器(qi)的研(yan)究中(zhong),多假(jia)設發射(she)(she)極與收(shou)集極之間(jian)為無(wu)結構(gou)支撐(cheng)件的高(gao)真空狀態,并且在(zai)理論效率計算中(zhong)忽略熱損失,或并未著重關注熱損失造成轉化效率下降的問題。然(ran)而,在(zai)熱電子(zi)器(qi)件的實際工作過(guo)程中(zhong),兩(liang)電極間(jian)存在(zai)熱輻射(she)(she),且由(you)于各類(lei)結構(gou)件的影響,電極間(jian)的熱傳導無(wu)法(fa)忽略。

4關鍵問題及解決途徑

熱(re)電(dian)(dian)子(zi)發射(she)能量(liang)(liang)轉(zhuan)(zhuan)換器的(de)(de)(de)(de)(de)性能和大(da)規模應用主要受到三方(fang)面影響:如(ru)何(he)降低(di)電(dian)(dian)極材(cai)料功(gong)函數(shu)的(de)(de)(de)(de)(de)大(da)小。具有低(di)功(gong)函數(shu)的(de)(de)(de)(de)(de)材(cai)料內部的(de)(de)(de)(de)(de)電(dian)(dian)子(zi)逸出所需能量(liang)(liang)更少,能夠使熱(re)電(dian)(dian)子(zi)高溫發電(dian)(dian)系統(tong)(tong)在處(chu)于相(xiang)對(dui)較低(di)的(de)(de)(de)(de)(de)運(yun)行(xing)溫度(du)時獲(huo)得較大(da)的(de)(de)(de)(de)(de)輸出電(dian)(dian)流,從而(er)拓寬(kuan)其應用范圍(wei);如(ru)何(he)減(jian)小空(kong)(kong)間(jian)電(dian)(dian)荷(he)效(xiao)應對(dui)電(dian)(dian)流的(de)(de)(de)(de)(de)影響。空(kong)(kong)間(jian)電(dian)(dian)荷(he)效(xiao)應會造(zao)成電(dian)(dian)子(zi)在極間(jian)空(kong)(kong)間(jian)散射(she),并(bing)對(dui)逸出的(de)(de)(de)(de)(de)電(dian)(dian)子(zi)施加額外的(de)(de)(de)(de)(de)勢壘阻(zu)礙,進而(er)削(xue)弱單位時間(jian)到達收集(ji)極的(de)(de)(de)(de)(de)電(dian)(dian)子(zi)數(shu),降低(di)電(dian)(dian)流密(mi)度(du)。有效(xiao)克服空(kong)(kong)間(jian)電(dian)(dian)荷(he)效(xiao)應能夠提(ti)高輸出電(dian)(dian)流密(mi)度(du),進而(er)提(ti)高系統(tong)(tong)實際輸出功(gong)率;如(ru)何(he)減(jian)少裝置(zhi)熱(re)損失并(bing)提(ti)高能量(liang)(liang)轉(zhuan)(zhuan)化效(xiao)率。

4.1低功函數的電極(ji)材料(liao)

功函(han)數(shu)通(tong)常由真(zhen)(zhen)空能(neng)級(ji)與材(cai)料(liao)費米能(neng)級(ji)之差定義,即電子(zi)(zi)(zi)(zi)從材(cai)料(liao)內部發(fa)射到緊靠(kao)固體表(biao)面的(de)(de)(de)真(zhen)(zhen)空中的(de)(de)(de)一(yi)點(dian)所(suo)需(xu)的(de)(de)(de)最(zui)小(xiao)能(neng)量。在熱電子(zi)(zi)(zi)(zi)能(neng)量轉換器(qi)中,發(fa)射極(ji)(ji)與收集(ji)極(ji)(ji)的(de)(de)(de)功函(han)數(shu)影響著極(ji)(ji)板(ban)間勢(shi)(shi)壘的(de)(de)(de)變化,較高的(de)(de)(de)勢(shi)(shi)壘阻礙了(le)發(fa)射極(ji)(ji)電子(zi)(zi)(zi)(zi)向收集(ji)極(ji)(ji)運(yun)動。為(wei)了(le)使(shi)發(fa)射極(ji)(ji)上更多的(de)(de)(de)電子(zi)(zi)(zi)(zi)克服勢(shi)(shi)壘到達收集(ji)極(ji)(ji),一(yi)般的(de)(de)(de)熱電子(zi)(zi)(zi)(zi)能(neng)量轉換器(qi)理想(xiang)運(yun)行溫度在1500K以(yi)(yi)上。采用低功函(han)數(shu)的(de)(de)(de)材(cai)料(liao)作為(wei)電極(ji)(ji)可使(shi)電子(zi)(zi)(zi)(zi)逸出并飛(fei)越極(ji)(ji)板(ban)空間所(suo)需(xu)能(neng)量更少,以(yi)(yi)獲得更大輸出電流或降(jiang)低發(fa)射極(ji)(ji)運(yun)行的(de)(de)(de)溫度條件限制。

早(zao)期熱電(dian)子發(fa)(fa)射(she)陰極材(cai)料(liao)(liao)以鎢材(cai)料(liao)(liao)為主,其后以硼(peng)(peng)化鑭(lan)系列(lie)材(cai)料(liao)(liao)研究(jiu)發(fa)(fa)展起來。其中六硼(peng)(peng)化鑭(lan)是(shi)一種高(gao)熔點(dian)、高(gao)化學(xue)穩定(ding)性(xing)、高(gao)導電(dian)率(lv)、低功(gong)函(han)數(shu)的電(dian)極材(cai)料(liao)(liao),功(gong)函(han)數(shu)范(fan)圍(wei)在(zai)2.41eV～3.0eV,是(shi)一種常用的熱電(dian)子發(fa)(fa)射(she)電(dian)極材(cai)料(liao)(liao)。2012年Lee等研究(jiu)表(biao)明(ming)Ba或BaO涂層添加在(zai)聚SiC發(fa)(fa)射(she)極表(biao)面(mian),可以使其功(gong)函(han)數(shu)降低至2.1eV,并使熱電(dian)子電(dian)流擴大5～6個(ge)數(shu)量級(ji)[5]。

除了低功函(han)數(shu)的(de)(de)要求(qiu)外,熱電(dian)子發射(she)(she)能(neng)量轉換器因其結構(gou)特點和(he)實際工(gong)作狀(zhuang)況,對電(dian)極的(de)(de)其他性能(neng)也(ye)有(you)一定要求(qiu)。發射(she)(she)極材(cai)料應(ying)(ying)具有(you)極高的(de)(de)熔點,高溫下機械(xie)強度高,熱導和(he)電(dian)導性能(neng)良好(hao),發射(she)(she)面的(de)(de)電(dian)子發射(she)(she)性能(neng)穩定。收集極的(de)(de)基本要求(qiu)與發射(she)(she)極一致,其功函(han)數(shu)應(ying)(ying)低于發射(she)(she)極約(yue)1eV,以(yi)便獲(huo)得較大的(de)(de)輸出(chu)電(dian)壓(ya)。

4.2空間電荷效應

熱(re)電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)(zi)發射和接(jie)收(shou)的(de)(de)兩(liang)(liang)個電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)板間(jian)(jian)存在(zai)(zai)空(kong)間(jian)(jian)電(dian)(dian)(dian)(dian)(dian)荷效應,于1923年由Langmuir提出(chu)。大量電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)(zi)連續(xu)逸出(chu)的(de)(de)過程中不(bu)斷有電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)(zi)處于上述趨(qu)勢,繼而在(zai)(zai)靠(kao)近發射極(ji)(ji)的(de)(de)某一區域形(xing)成負(fu)電(dian)(dian)(dian)(dian)(dian)荷團,負(fu)電(dian)(dian)(dian)(dian)(dian)荷相(xiang)互的(de)(de)斥力導致后續(xu)發射的(de)(de)部分電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)(zi)向其他方向散射,無法到達收(shou)集極(ji)(ji),削弱熱(re)離(li)子(zi)(zi)(zi)能量轉(zhuan)換器的(de)(de)實際(ji)輸(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)(dian)流(liu)密度(du),繼而降低輸(shu)(shu)出(chu)功率(lv)和轉(zhuan)化效率(lv)。該效應在(zai)(zai)理論分析時可(ke)等(deng)效為極(ji)(ji)板之間(jian)(jian)的(de)(de)額(e)外勢壘高度(du),隨著極(ji)(ji)板間(jian)(jian)距(ju)的(de)(de)減(jian)小(xiao)而減(jian)小(xiao)。目前存在(zai)(zai)三種(zhong)主流(liu)方案(an)降低空(kong)間(jian)(jian)電(dian)(dian)(dian)(dian)(dian)荷效應對(dui)輸(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)(dian)流(liu)密度(du)的(de)(de)影(ying)響:直接(jie)調控并減(jian)小(xiao)發射電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)與收(shou)集電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)間(jian)(jian)的(de)(de)距(ju)離(li)至亞微米級(ji);兩(liang)(liang)電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)間(jian)(jian)通入Cs蒸汽,中和低速(su)電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)(zi);兩(liang)(liang)電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)間(jian)(jian)增加(jia)電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)(zi)加(jia)速(su)柵格等(deng)額(e)外結構(gou),實現對(dui)低速(su)電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)(zi)的(de)(de)加(jia)速(su)和偏轉(zhuan)。

縮(suo)小(xiao)兩電(dian)極(ji)(ji)的(de)間(jian)(jian)(jian)(jian)距是(shi)一(yi)種削弱空間(jian)(jian)(jian)(jian)電(dian)荷效應的(de)有效手段。隨著發射極(ji)(ji)和集電(dian)極(ji)(ji)之(zhi)間(jian)(jian)(jian)(jian)的(de)距離變得足(zu)夠(gou)小(xiao),沒(mei)有足(zu)夠(gou)的(de)空間(jian)(jian)(jian)(jian)和時間(jian)(jian)(jian)(jian)使行進的(de)電(dian)子相(xiang)互碰撞,從而在更短的(de)時間(jian)(jian)(jian)(jian)內到達收集極(ji)(ji),但(dan)是(shi)上世紀五六十年代起開展的(de)研究(jiu)中,由(you)于技術所限,精確控制電(dian)極(ji)(ji)保(bao)持亞微米級的(de)間(jian)(jian)(jian)(jian)距極(ji)(ji)為困難。

在縮小電(dian)極間(jian)(jian)距(ju)以減小空(kong)(kong)間(jian)(jian)電(dian)荷效應的同時,保持(chi)發射極和收集極的溫(wen)度(du)差成為(wei)了(le)另(ling)一個問題。較大(da)的極間(jian)(jian)距(ju)會導致(zhi)空(kong)(kong)間(jian)(jian)電(dian)荷效應,從而限(xian)制電(dian)流(liu)傳輸,而極間(jian)(jian)距(ju)減小到(dao)一定程度(du)則會導致(zhi)發射極與收集極之(zhi)間(jian)(jian)的過(guo)度(du)傳熱(re),稱為(wei)近場輻射傳熱(re)現(xian)象(xiang),若間(jian)(jian)距(ju)過(guo)小則會使傳熱(re)提(ti)高多個數量級(ji)。

另(ling)一種空(kong)間(jian)(jian)電(dian)(dian)荷效應的(de)削(xue)弱方法是將(jiang)帶正電(dian)(dian)的(de)離(li)子(zi)(zi)注(zhu)入(ru)兩電(dian)(dian)極(ji)間(jian)(jian)的(de)空(kong)間(jian)(jian),用(yong)于(yu)中(zhong)和負(fu)電(dian)(dian)荷團,由于(yu)銫的(de)電(dian)(dian)離(li)勢較(jiao)低(di),常將(jiang)其作(zuo)為中(zhong)和材料。當(dang)銫注(zhu)入(ru)電(dian)(dian)極(ji)間(jian)(jian)的(de)間(jian)(jian)隙后,銫原(yuan)子(zi)(zi)首(shou)先會(hui)(hui)吸附在電(dian)(dian)極(ji)金屬(shu)表面(mian),使得電(dian)(dian)極(ji)功函數降低(di),隨后由于(yu)電(dian)(dian)極(ji)的(de)升(sheng)溫,其表面(mian)的(de)銫原(yuan)子(zi)(zi)熱(re)離(li)化成為分布(bu)在電(dian)(dian)極(ji)間(jian)(jian)隙之間(jian)(jian)的(de)銫離(li)子(zi)(zi),對一部分低(di)速電(dian)(dian)子(zi)(zi)進行中(zhong)和,削(xue)弱空(kong)間(jian)(jian)電(dian)(dian)荷效應。然而部分電(dian)(dian)子(zi)(zi)會(hui)(hui)與銫離(li)子(zi)(zi)發生碰撞散(san)射,因此達到收集極(ji)的(de)電(dian)(dian)子(zi)(zi)相較(jiao)于(yu)理想情(qing)況仍然有所(suo)減少(shao)。

填充Cs蒸汽和增加(jia)電子加(jia)速柵格兩類方(fang)案不僅(jin)使設備的結(jie)構(gou)復(fu)雜度、系統復(fu)雜度提高,降(jiang)低(di)可靠性,還增加(jia)運(yun)行的額外(wai)功耗和設備重量(liang),在一定(ding)程度上削弱(ruo)了該(gai)裝(zhuang)置單位(wei)面積功率高、結(jie)構(gou)簡單、運(yun)行穩定(ding)的優勢(shi)。