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1研究背景

近(jin)年來全球的(de)(de)(de)(de)能(neng)(neng)源(yuan)需求量呈(cheng)現加速增(zeng)長(chang)的(de)(de)(de)(de)態(tai)勢,多(duo)種燃料的(de)(de)(de)(de)消耗增(zeng)速及全球碳排(pai)放增(zeng)長(chang)量都(dou)達到(dao)近(jin)十年來的(de)(de)(de)(de)最大(da)值(zhi)(zhi)。各國(guo)當(dang)(dang)前面臨(lin)既(ji)要(yao)(yao)滿(man)足全球電(dian)氣(qi)化背景下(xia)不斷增(zeng)長(chang)的(de)(de)(de)(de)電(dian)力(li)需求,又(you)要(yao)(yao)促進能(neng)(neng)源(yuan)轉型和控制環境(jing)污染(ran)的(de)(de)(de)(de)雙重(zhong)矛盾。同時,推進高(gao)效(xiao)的(de)(de)(de)(de)能(neng)(neng)源(yuan)開發(fa)利用(yong)設備和技(ji)術的(de)(de)(de)(de)研究以及開發(fa)清潔的(de)(de)(de)(de)新(xin)能(neng)(neng)源(yuan),是我國(guo)當(dang)(dang)前能(neng)(neng)源(yuan)建設的(de)(de)(de)(de)發(fa)展方(fang)向。積(ji)極開發(fa)基(ji)于核(he)能(neng)(neng)、太(tai)陽能(neng)(neng)等新(xin)能(neng)(neng)源(yuan)的(de)(de)(de)(de)高(gao)效(xiao)高(gao)溫(wen)發(fa)電(dian)系統,具有重(zhong)要(yao)(yao)的(de)(de)(de)(de)工程(cheng)價值(zhi)(zhi)和社會意(yi)義。

熱(re)電(dian)(dian)子(zi)發(fa)射(she)(she)(she)能(neng)量(liang)轉(zhuan)(zhuan)換(huan)(TEC)發(fa)電(dian)(dian)系統是值得關(guan)注的(de)高(gao)效能(neng)高(gao)溫發(fa)電(dian)(dian)系統。熱(re)電(dian)(dian)子(zi)發(fa)射(she)(she)(she)能(neng)量(liang)轉(zhuan)(zhuan)換(huan)器是一種基于熱(re)電(dian)(dian)子(zi)發(fa)射(she)(she)(she)效應,在1500K至(zhi)2000K的(de)運行溫度(du)下直接(jie)將熱(re)能(neng)轉(zhuan)(zhuan)化(hua)為電(dian)(dian)能(neng)的(de)發(fa)電(dian)(dian)裝置,其(qi)特點為無需(xu)化(hua)學(xue)反應、流體介質(zhi)或移(yi)動部(bu)件,結(jie)構(gou)簡單(dan),可(ke)靠性高(gao);在發(fa)電(dian)(dian)過程中無噪音、無磨損、無介質(zhi)泄(xie)露,使用壽命長;同時其(qi)又具備可(ke)擴展性高(gao),單(dan)位面積輸出電(dian)(dian)流密度(du)及輸出功率大等優點,是目前理論發(fa)電(dian)(dian)效率最高(gao)的(de)熱(re)電(dian)(dian)能(neng)量(liang)直接(jie)轉(zhuan)(zhuan)換(huan)裝置。

在(zai)(zai)核能(neng)(neng)(neng)利用領(ling)域,熱(re)(re)(re)電(dian)(dian)(dian)子(zi)發(fa)射(she)能(neng)(neng)(neng)量(liang)(liang)轉(zhuan)換器能(neng)(neng)(neng)夠在(zai)(zai)高溫環(huan)境下直接將核裂變產生的(de)(de)熱(re)(re)(re)能(neng)(neng)(neng)轉(zhuan)化為電(dian)(dian)(dian)能(neng)(neng)(neng)。常規的(de)(de)核電(dian)(dian)(dian)站的(de)(de)蒸(zheng)汽循環(huan)溫度通常在(zai)(zai)800K以下,核裂變反應(ying)(ying)的(de)(de)高溫端存(cun)在(zai)(zai)大量(liang)(liang)未被有(you)效(xiao)利用的(de)(de)熱(re)(re)(re)能(neng)(neng)(neng),采用運行(xing)溫度更高的(de)(de)熱(re)(re)(re)電(dian)(dian)(dian)子(zi)發(fa)射(she)能(neng)(neng)(neng)量(liang)(liang)轉(zhuan)換器作(zuo)為頂循環(huan),可大幅提高熱(re)(re)(re)能(neng)(neng)(neng)的(de)(de)利用率(lv)。在(zai)(zai)空間核動力系統(tong)的(de)(de)研究中,相較于蒸(zheng)汽渦輪(lun)發(fa)電(dian)(dian)(dian)方案,熱(re)(re)(re)電(dian)(dian)(dian)子(zi)發(fa)射(she)能(neng)(neng)(neng)量(liang)(liang)轉(zhuan)換器在(zai)(zai)可靠性、系統(tong)發(fa)射(she)重量(liang)(liang)、使用壽命等(deng)方面具(ju)有(you)較為明顯的(de)(de)優勢。目前美、俄在(zai)(zai)銫蒸(zheng)汽型(xing)熱(re)(re)(re)電(dian)(dian)(dian)子(zi)能(neng)(neng)(neng)量(liang)(liang)轉(zhuan)換器結合空間核電(dian)(dian)(dian)系統(tong)的(de)(de)應(ying)(ying)用研究上(shang)已經取得(de)了一定的(de)(de)進(jin)(jin)展(zhan)。在(zai)(zai)太陽(yang)能(neng)(neng)(neng)光(guang)熱(re)(re)(re)轉(zhuan)換領(ling)域,通過菲涅爾聚(ju)光(guang)鏡等(deng)輔助器件聚(ju)焦太陽(yang)輻射(she)加熱(re)(re)(re)發(fa)射(she)電(dian)(dian)(dian)極,可有(you)效(xiao)利用太陽(yang)能(neng)(neng)(neng)進(jin)(jin)行(xing)熱(re)(re)(re)電(dian)(dian)(dian)轉(zhuan)化。該裝置在(zai)(zai)空間太陽(yang)能(neng)(neng)(neng)電(dian)(dian)(dian)站、獨(du)立軍事設備供(gong)電(dian)(dian)(dian)、偏遠地區(qu)小型(xing)分布(bu)式(shi)能(neng)(neng)(neng)源供(gong)應(ying)(ying)等(deng)方面有(you)巨大應(ying)(ying)用潛力。

綜(zong)上,TEC高(gao)溫(wen)發(fa)電(dian)裝置有(you)望應(ying)用于軍事、航天動力轉(zhuan)換等對(dui)系統穩(wen)定性和無噪聲性有(you)要求(qiu)的領域,也可以在民用發(fa)電(dian)領域減少運營(ying)成本和提高(gao)發(fa)電(dian)效率。

2基本原理

熱(re)(re)電(dian)子(zi)發(fa)(fa)射(she)(she)現(xian)象(xiang)于(yu)十(shi)九(jiu)世紀八(ba)十(shi)年代被發(fa)(fa)現(xian),早(zao)期稱之(zhi)為(wei)“愛迪生效應(ying)”。其(qi)現(xian)象(xiang)為(wei),將(jiang)兩電(dian)極(ji)(ji)置于(yu)真空(kong)中(zhong),加(jia)(jia)熱(re)(re)其(qi)中(zhong)一個(ge)電(dian)極(ji)(ji)時(shi)可測得兩個(ge)電(dian)極(ji)(ji)間(jian)(jian)存在電(dian)流(liu)。熱(re)(re)電(dian)子(zi)發(fa)(fa)射(she)(she)電(dian)流(liu)可使用如圖(tu)1所(suo)示實驗電(dian)路進行測量(liang),其(qi)中(zhong)加(jia)(jia)熱(re)(re)裝置可控制發(fa)(fa)射(she)(she)電(dian)極(ji)(ji)的溫度。在兩電(dian)極(ji)(ji)之(zhi)間(jian)(jian)施加(jia)(jia)額(e)外(wai)的偏壓,并從(cong)零逐漸增(zeng)加(jia)(jia),熱(re)(re)電(dian)子(zi)發(fa)(fa)射(she)(she)電(dian)流(liu)先(xian)線性增(zeng)加(jia)(jia),然后逐漸達到(dao)飽(bao)和。

形(xing)成熱(re)電(dian)(dian)(dian)子(zi)發(fa)射(she)電(dian)(dian)(dian)流的(de)(de)(de)原因(yin)在(zai)于,金(jin)屬(shu)材(cai)料(liao)(liao)溫度升高后材(cai)料(liao)(liao)內部(bu)電(dian)(dian)(dian)子(zi)的(de)(de)(de)能(neng)量增加,進入能(neng)級較高的(de)(de)(de)能(neng)態,當其能(neng)量大于材(cai)料(liao)(liao)表面(mian)的(de)(de)(de)逸出(chu)功(功函數)時(shi),電(dian)(dian)(dian)子(zi)就會越過表面(mian)勢壘進入真(zhen)空。若(ruo)進入真(zhen)空的(de)(de)(de)電(dian)(dian)(dian)子(zi)能(neng)量并未耗盡,能(neng)夠繼續越過電(dian)(dian)(dian)極之間(jian)的(de)(de)(de)附加勢壘達到另一(yi)側電(dian)(dian)(dian)極則形(xing)成電(dian)(dian)(dian)流。Richardson于1902年推導出(chu)金(jin)屬(shu)表面(mian)熱(re)電(dian)(dian)(dian)子(zi)發(fa)射(she)電(dian)(dian)(dian)流密度的(de)(de)(de)大小與(yu)溫度T和金(jin)屬(shu)的(de)(de)(de)功函數φ有如下關系(xi)[1]:

其中,A為(wei)Richardson常數A≈120A/cm2·K2,kB為(wei)玻爾茲曼常數kB≈8.6×105eV/K。這就是Richardson方程(cheng),表明溫(wen)度越(yue)(yue)高、功函數越(yue)(yue)小(xiao),則熱電子(zi)發(fa)射(she)的(de)電流密度就越(yue)(yue)大。

熱(re)電(dian)(dian)(dian)子發(fa)(fa)射現象的(de)(de)應(ying)用之一(yi)是(shi)熱(re)電(dian)(dian)(dian)子發(fa)(fa)射能(neng)量轉(zhuan)(zhuan)換器(qi)。熱(re)電(dian)(dian)(dian)子發(fa)(fa)射能(neng)量轉(zhuan)(zhuan)換器(qi)仍(reng)然是(shi)一(yi)種熱(re)機(ji),其直接(jie)以熱(re)量作為(wei)(wei)能(neng)量來(lai)源,且(qie)可視(shi)為(wei)(wei)以電(dian)(dian)(dian)子作為(wei)(wei)工質進行發(fa)(fa)電(dian)(dian)(dian)。真空(kong)型熱(re)電(dian)(dian)(dian)子轉(zhuan)(zhuan)換裝置的(de)(de)基本(ben)形(xing)式如(ru)圖2所示,它的(de)(de)主要構件(jian)包括真空(kong)罩(zhao)、發(fa)(fa)射電(dian)(dian)(dian)極(ji)(ji)、收集(ji)電(dian)(dian)(dian)極(ji)(ji)、熱(re)源和熱(re)沉等(deng)五部分。兩(liang)電(dian)(dian)(dian)極(ji)(ji)平(ping)行放(fang)置,中(zhong)間留有(you)一(yi)定空(kong)隙(xi),兩(liang)電(dian)(dian)(dian)極(ji)(ji)中(zhong)間的(de)(de)空(kong)隙(xi)為(wei)(wei)超高真空(kong)環境。當在發(fa)(fa)射極(ji)(ji)和收集(ji)極(ji)(ji)連接(jie)一(yi)負載(zai)時,電(dian)(dian)(dian)子從熱(re)源吸收能(neng)量,克服發(fa)(fa)射極(ji)(ji)的(de)(de)表(biao)面功函數(shu)逸出(chu),穿過兩(liang)極(ji)(ji)間空(kong)隙(xi)到達(da)收集(ji)極(ji)(ji),隨后由于(yu)接(jie)觸勢和外加偏壓作用,到達(da)收集(ji)極(ji)(ji)電(dian)(dian)(dian)子經由外電(dian)(dian)(dian)路及(ji)負載(zai)輸出(chu)功率并返(fan)回發(fa)(fa)射極(ji)(ji),構成完整的(de)(de)電(dian)(dian)(dian)流回路。

3研究進展

熱(re)電(dian)(dian)(dian)子發(fa)射(she)(she)現(xian)(xian)象于(yu)1885年(nian)由(you)Edison發(fa)現(xian)(xian),隨(sui)后Thomson于(yu)1897年(nian)發(fa)現(xian)(xian)電(dian)(dian)(dian)子。1902年(nian)Richardson對(dui)熱(re)電(dian)(dian)(dian)子發(fa)射(she)(she)進行了定量的物理描述并推導了熱(re)電(dian)(dian)(dian)子發(fa)射(she)(she)電(dian)(dian)(dian)流密度方(fang)程,即Richardson-Dushman方(fang)程。1923年(nian)Langmuir指出(chu)熱(re)電(dian)(dian)(dian)子發(fa)射(she)(she)的電(dian)(dian)(dian)極板(ban)間存在(zai)空間電(dian)(dian)(dian)荷效(xiao)應[2]。空間電(dian)(dian)(dian)荷效(xiao)應即發(fa)射(she)(she)極不斷逸出(chu)的低(di)速(su)電(dian)(dian)(dian)子會在(zai)發(fa)射(she)(she)極與收(shou)集極之間形成電(dian)(dian)(dian)子云(yun),電(dian)(dian)(dian)子云(yun)產生的電(dian)(dian)(dian)場阻礙后續逸出(chu)的電(dian)(dian)(dian)子到(dao)達(da)收(shou)集極,從(cong)而(er)削(xue)弱熱(re)電(dian)(dian)(dian)子能量轉換器的實際輸(shu)出(chu)電(dian)(dian)(dian)流密度。

1950年代中(zhong)期(qi),耐高溫材(cai)料(liao)技(ji)術、原子(zi)(zi)能(neng)發(fa)電(dian)(dian)(dian)(dian)技(ji)術的(de)發(fa)展以及(ji)航(hang)天(tian)領域(yu)的(de)高效(xiao)緊湊型(xing)電(dian)(dian)(dian)(dian)源(yuan)需(xu)求促使各國(guo)研究人員開展對(dui)熱(re)電(dian)(dian)(dian)(dian)子(zi)(zi)發(fa)射能(neng)量轉(zhuan)換(huan)器(qi)的(de)實質研究,蘇(su)聯的(de)Marchuk、美(mei)(mei)國(guo)的(de)Wilson、Grover等(deng)進(jin)行(xing)了(le)相關研究[1]。早(zao)期(qi)熱(re)電(dian)(dian)(dian)(dian)子(zi)(zi)發(fa)射能(neng)量轉(zhuan)換(huan)器(qi)被(bei)考(kao)慮應用(yong)于太陽能(neng)和(he)放射性同位(wei)素空間(jian)動力(li)系(xi)統(tong)(tong),但是(shi)至1965年該(gai)技(ji)術無法(fa)取代相對(dui)成(cheng)熟(shu)的(de)光伏及(ji)半(ban)導(dao)體熱(re)電(dian)(dian)(dian)(dian)技(ji)術作(zuo)為航(hang)天(tian)動力(li)技(ji)術方(fang)案。首個太陽能(neng)熱(re)電(dian)(dian)(dian)(dian)子(zi)(zi)轉(zhuan)換(huan)器(qi)在太空任務中(zhong)的(de)能(neng)量轉(zhuan)化(hua)效(xiao)率為4～7%,遠(yuan)低于其(qi)理論效(xiao)率[1]。1965年后(hou)美(mei)(mei)國(guo)、蘇(su)聯、西德、法(fa)國(guo)等(deng)將(jiang)該(gai)技(ji)術的(de)應用(yong)研究重點(dian)調整為熱(re)電(dian)(dian)(dian)(dian)子(zi)(zi)能(neng)量轉(zhuan)化(hua)與核反應堆結合(he)的(de)工(gong)程(cheng)開發(fa),至1990年美(mei)(mei)、俄(e)先(xian)后(hou)開發(fa)了(le)熱(re)電(dian)(dian)(dian)(dian)子(zi)(zi)燃(ran)料(liao)元(yuan)件(TEF)、基于熱(re)電(dian)(dian)(dian)(dian)子(zi)(zi)能(neng)量轉(zhuan)換(huan)器(qi)的(de)TOPAZ核電(dian)(dian)(dian)(dian)系(xi)統(tong)(tong)。這一階段(duan)各國(guo)的(de)對(dui)熱(re)電(dian)(dian)(dian)(dian)子(zi)(zi)發(fa)射能(neng)量轉(zhuan)換(huan)器(qi)的(de)研究關注點(dian)主(zhu)要在整體系(xi)統(tong)(tong)壽命、航(hang)天(tian)發(fa)射重量、輸出功率等(deng)方(fang)面,且(qie)發(fa)電(dian)(dian)(dian)(dian)系(xi)統(tong)(tong)主(zhu)要采用(yong)銫(se)蒸汽(qi)型(xing)熱(re)電(dian)(dian)(dian)(dian)子(zi)(zi)能(neng)量轉(zhuan)換(huan)器(qi)。

1996年Naito等(deng)報道了(le)一(yi)種半導(dao)體熱(re)(re)電系統和熱(re)(re)電子(zi)能量轉化(hua)串聯的太陽能發(fa)(fa)電系統,其聯合轉換效(xiao)率(lv)接(jie)近40%。2019年廖(liao)天軍等(deng)基于石墨(mo)烯(xi)發(fa)(fa)射極[3],以(yi)發(fa)(fa)射極功函(han)數、費(fei)米能級、熱(re)(re)源溫度為(wei)變量進行熱(re)(re)電子(zi)功率(lv)器(qi)件(jian)的參數優化(hua),理論模型(xing)的最(zui)高(gao)效(xiao)率(lv)為(wei)60%[4]。

在近(jin)年(nian)對熱(re)電(dian)子轉換(huan)器的(de)研究中(zhong),多(duo)假設(she)發(fa)射極與(yu)收集(ji)極之(zhi)間為無結(jie)構支撐件的(de)高真空狀(zhuang)態,并且(qie)在理論效率(lv)計算中(zhong)忽(hu)略熱(re)損(sun)失,或并未著(zhu)重關(guan)注熱(re)損(sun)失造成轉化效率(lv)下(xia)降的(de)問題。然而,在熱(re)電(dian)子器件的(de)實際工(gong)作過程中(zhong),兩(liang)電(dian)極間存在熱(re)輻射,且(qie)由于(yu)各類結(jie)構件的(de)影(ying)響,電(dian)極間的(de)熱(re)傳導無法(fa)忽(hu)略。

4關鍵問題及解決途徑

熱(re)電(dian)(dian)子(zi)發射(she)能(neng)量轉換器的(de)(de)(de)(de)(de)性能(neng)和大(da)(da)(da)規模(mo)應(ying)(ying)用(yong)主要(yao)受到(dao)三方面影響:如(ru)何降低電(dian)(dian)極(ji)材料功(gong)函數(shu)的(de)(de)(de)(de)(de)大(da)(da)(da)小(xiao)。具有(you)低功(gong)函數(shu)的(de)(de)(de)(de)(de)材料內(nei)部的(de)(de)(de)(de)(de)電(dian)(dian)子(zi)逸出(chu)(chu)所需(xu)能(neng)量更少,能(neng)夠使熱(re)電(dian)(dian)子(zi)高(gao)溫(wen)(wen)發電(dian)(dian)系(xi)統在處于(yu)相對較(jiao)低的(de)(de)(de)(de)(de)運行(xing)溫(wen)(wen)度時(shi)獲得較(jiao)大(da)(da)(da)的(de)(de)(de)(de)(de)輸(shu)出(chu)(chu)電(dian)(dian)流,從(cong)而(er)拓寬其應(ying)(ying)用(yong)范圍;如(ru)何減(jian)小(xiao)空(kong)(kong)間(jian)電(dian)(dian)荷效應(ying)(ying)對電(dian)(dian)流的(de)(de)(de)(de)(de)影響。空(kong)(kong)間(jian)電(dian)(dian)荷效應(ying)(ying)會造成(cheng)電(dian)(dian)子(zi)在極(ji)間(jian)空(kong)(kong)間(jian)散射(she),并對逸出(chu)(chu)的(de)(de)(de)(de)(de)電(dian)(dian)子(zi)施加額(e)外(wai)的(de)(de)(de)(de)(de)勢壘阻礙,進(jin)而(er)削弱單位時(shi)間(jian)到(dao)達收(shou)集極(ji)的(de)(de)(de)(de)(de)電(dian)(dian)子(zi)數(shu),降低電(dian)(dian)流密(mi)(mi)度。有(you)效克服空(kong)(kong)間(jian)電(dian)(dian)荷效應(ying)(ying)能(neng)夠提(ti)高(gao)輸(shu)出(chu)(chu)電(dian)(dian)流密(mi)(mi)度,進(jin)而(er)提(ti)高(gao)系(xi)統實際輸(shu)出(chu)(chu)功(gong)率(lv);如(ru)何減(jian)少裝置(zhi)熱(re)損(sun)失并提(ti)高(gao)能(neng)量轉化效率(lv)。

4.1低功函數的(de)電(dian)極材料

功(gong)函數(shu)通常由真(zhen)(zhen)空能(neng)級與材料費米能(neng)級之差定義,即電(dian)(dian)(dian)(dian)(dian)(dian)子從(cong)材料內部發射(she)(she)到緊靠固體表面的(de)(de)(de)真(zhen)(zhen)空中的(de)(de)(de)一點(dian)所需的(de)(de)(de)最小(xiao)能(neng)量(liang)(liang)。在(zai)熱電(dian)(dian)(dian)(dian)(dian)(dian)子能(neng)量(liang)(liang)轉換器(qi)中,發射(she)(she)極(ji)(ji)(ji)與收集(ji)極(ji)(ji)(ji)的(de)(de)(de)功(gong)函數(shu)影響著極(ji)(ji)(ji)板間(jian)勢壘(lei)的(de)(de)(de)變化,較(jiao)高的(de)(de)(de)勢壘(lei)阻(zu)礙了發射(she)(she)極(ji)(ji)(ji)電(dian)(dian)(dian)(dian)(dian)(dian)子向收集(ji)極(ji)(ji)(ji)運(yun)動。為了使發射(she)(she)極(ji)(ji)(ji)上更(geng)多的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)子克服勢壘(lei)到達收集(ji)極(ji)(ji)(ji),一般的(de)(de)(de)熱電(dian)(dian)(dian)(dian)(dian)(dian)子能(neng)量(liang)(liang)轉換器(qi)理想運(yun)行溫(wen)度在(zai)1500K以上。采用(yong)低(di)功(gong)函數(shu)的(de)(de)(de)材料作為電(dian)(dian)(dian)(dian)(dian)(dian)極(ji)(ji)(ji)可使電(dian)(dian)(dian)(dian)(dian)(dian)子逸出(chu)并(bing)飛(fei)越極(ji)(ji)(ji)板空間(jian)所需能(neng)量(liang)(liang)更(geng)少,以獲得(de)更(geng)大(da)輸出(chu)電(dian)(dian)(dian)(dian)(dian)(dian)流或降(jiang)低(di)發射(she)(she)極(ji)(ji)(ji)運(yun)行的(de)(de)(de)溫(wen)度條(tiao)件限(xian)制。

早期熱(re)(re)電(dian)(dian)(dian)(dian)子(zi)發射陰極材(cai)料以(yi)鎢材(cai)料為主,其(qi)后以(yi)硼化(hua)鑭(lan)系(xi)列材(cai)料研究發展起來。其(qi)中六硼化(hua)鑭(lan)是一種高熔點、高化(hua)學(xue)穩定性、高導電(dian)(dian)(dian)(dian)率、低功函(han)數(shu)(shu)的(de)電(dian)(dian)(dian)(dian)極材(cai)料,功函(han)數(shu)(shu)范圍在2.41eV～3.0eV,是一種常用的(de)熱(re)(re)電(dian)(dian)(dian)(dian)子(zi)發射電(dian)(dian)(dian)(dian)極材(cai)料。2012年(nian)Lee等(deng)研究表明Ba或BaO涂層添加(jia)在聚SiC發射極表面,可(ke)以(yi)使其(qi)功函(han)數(shu)(shu)降(jiang)低至2.1eV,并使熱(re)(re)電(dian)(dian)(dian)(dian)子(zi)電(dian)(dian)(dian)(dian)流擴大5～6個數(shu)(shu)量(liang)級[5]。

除了低功(gong)(gong)函數的(de)要求外,熱(re)電(dian)子發(fa)(fa)(fa)射能(neng)量轉換器(qi)因其結(jie)構特點(dian)(dian)和(he)實際工作狀況(kuang),對電(dian)極的(de)其他(ta)性能(neng)也有一定要求。發(fa)(fa)(fa)射極材料應具(ju)有極高的(de)熔點(dian)(dian),高溫(wen)下機械強度高,熱(re)導(dao)和(he)電(dian)導(dao)性能(neng)良好,發(fa)(fa)(fa)射面的(de)電(dian)子發(fa)(fa)(fa)射性能(neng)穩(wen)定。收(shou)集極的(de)基本要求與發(fa)(fa)(fa)射極一致,其功(gong)(gong)函數應低于發(fa)(fa)(fa)射極約1eV,以(yi)便獲(huo)得較(jiao)大的(de)輸出電(dian)壓。

4.2空間電荷效應

熱電(dian)(dian)(dian)子(zi)發(fa)射和(he)(he)接收(shou)的(de)(de)兩(liang)個電(dian)(dian)(dian)極(ji)板間(jian)(jian)(jian)(jian)存在空(kong)間(jian)(jian)(jian)(jian)電(dian)(dian)(dian)荷(he)效(xiao)(xiao)應(ying)(ying),于(yu)1923年(nian)由Langmuir提出。大量電(dian)(dian)(dian)子(zi)連續(xu)逸(yi)出的(de)(de)過(guo)程(cheng)中不(bu)斷有(you)電(dian)(dian)(dian)子(zi)處(chu)于(yu)上述趨勢,繼而(er)在靠(kao)近發(fa)射極(ji)的(de)(de)某(mou)一區域形(xing)成負(fu)電(dian)(dian)(dian)荷(he)團(tuan),負(fu)電(dian)(dian)(dian)荷(he)相互的(de)(de)斥力導致后續(xu)發(fa)射的(de)(de)部分電(dian)(dian)(dian)子(zi)向(xiang)其(qi)他方向(xiang)散射,無法到(dao)達收(shou)集極(ji),削弱熱離(li)子(zi)能量轉換器的(de)(de)實際輸出電(dian)(dian)(dian)流密(mi)度(du),繼而(er)降(jiang)低(di)(di)輸出功率(lv)和(he)(he)轉化效(xiao)(xiao)率(lv)。該效(xiao)(xiao)應(ying)(ying)在理(li)論分析時可等效(xiao)(xiao)為極(ji)板之間(jian)(jian)(jian)(jian)的(de)(de)額外(wai)勢壘高(gao)度(du),隨著極(ji)板間(jian)(jian)(jian)(jian)距的(de)(de)減小而(er)減小。目前存在三種主流方案降(jiang)低(di)(di)空(kong)間(jian)(jian)(jian)(jian)電(dian)(dian)(dian)荷(he)效(xiao)(xiao)應(ying)(ying)對輸出電(dian)(dian)(dian)流密(mi)度(du)的(de)(de)影響(xiang):直接調控(kong)并減小發(fa)射電(dian)(dian)(dian)極(ji)與收(shou)集電(dian)(dian)(dian)極(ji)間(jian)(jian)(jian)(jian)的(de)(de)距離(li)至亞(ya)微米級(ji);兩(liang)電(dian)(dian)(dian)極(ji)間(jian)(jian)(jian)(jian)通入Cs蒸汽,中和(he)(he)低(di)(di)速(su)(su)電(dian)(dian)(dian)子(zi);兩(liang)電(dian)(dian)(dian)極(ji)間(jian)(jian)(jian)(jian)增加電(dian)(dian)(dian)子(zi)加速(su)(su)柵格等額外(wai)結構,實現對低(di)(di)速(su)(su)電(dian)(dian)(dian)子(zi)的(de)(de)加速(su)(su)和(he)(he)偏轉。

縮(suo)小兩電(dian)(dian)極(ji)(ji)的(de)間(jian)(jian)距是一(yi)種(zhong)削(xue)弱空(kong)間(jian)(jian)電(dian)(dian)荷(he)效應的(de)有效手段。隨著發射極(ji)(ji)和(he)集電(dian)(dian)極(ji)(ji)之(zhi)間(jian)(jian)的(de)距離變得(de)足夠小,沒有足夠的(de)空(kong)間(jian)(jian)和(he)時(shi)間(jian)(jian)使(shi)行進的(de)電(dian)(dian)子相互碰撞,從而在(zai)更(geng)短的(de)時(shi)間(jian)(jian)內到達收集極(ji)(ji),但是上世紀五(wu)六十年代起開展的(de)研究中(zhong),由于技術(shu)所(suo)限,精確控制電(dian)(dian)極(ji)(ji)保(bao)持亞(ya)微米級(ji)的(de)間(jian)(jian)距極(ji)(ji)為困難。

在縮(suo)小電(dian)極(ji)(ji)間(jian)(jian)距(ju)以減小空間(jian)(jian)電(dian)荷效應的(de)同時,保(bao)持發射(she)極(ji)(ji)和(he)收集極(ji)(ji)的(de)溫度差成為(wei)了另一個(ge)問題。較大的(de)極(ji)(ji)間(jian)(jian)距(ju)會(hui)導致空間(jian)(jian)電(dian)荷效應,從而限(xian)制電(dian)流傳(chuan)輸,而極(ji)(ji)間(jian)(jian)距(ju)減小到(dao)一定程度則會(hui)導致發射(she)極(ji)(ji)與收集極(ji)(ji)之(zhi)間(jian)(jian)的(de)過度傳(chuan)熱(re)(re)(re),稱為(wei)近場輻射(she)傳(chuan)熱(re)(re)(re)現象(xiang),若間(jian)(jian)距(ju)過小則會(hui)使傳(chuan)熱(re)(re)(re)提高多個(ge)數(shu)量級(ji)。

另(ling)一種空間(jian)電(dian)(dian)(dian)(dian)(dian)荷(he)效應的(de)(de)削(xue)弱(ruo)(ruo)方法是(shi)將帶正(zheng)電(dian)(dian)(dian)(dian)(dian)的(de)(de)離(li)(li)子(zi)(zi)注入兩電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)間(jian)的(de)(de)空間(jian),用于中和(he)負電(dian)(dian)(dian)(dian)(dian)荷(he)團,由(you)于銫的(de)(de)電(dian)(dian)(dian)(dian)(dian)離(li)(li)勢較低(di)(di),常(chang)將其(qi)作為中和(he)材料。當銫注入電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)間(jian)的(de)(de)間(jian)隙后,銫原(yuan)子(zi)(zi)首先(xian)會(hui)吸附(fu)在電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)金(jin)屬表(biao)面,使(shi)得電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)功函數降低(di)(di),隨后由(you)于電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)的(de)(de)升(sheng)溫(wen),其(qi)表(biao)面的(de)(de)銫原(yuan)子(zi)(zi)熱(re)離(li)(li)化成為分(fen)布在電(dian)(dian)(dian)(dian)(dian)極(ji)(ji)間(jian)隙之(zhi)間(jian)的(de)(de)銫離(li)(li)子(zi)(zi),對(dui)一部(bu)分(fen)低(di)(di)速電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)進行中和(he),削(xue)弱(ruo)(ruo)空間(jian)電(dian)(dian)(dian)(dian)(dian)荷(he)效應。然而部(bu)分(fen)電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)會(hui)與銫離(li)(li)子(zi)(zi)發生碰撞散(san)射,因此達到收集極(ji)(ji)的(de)(de)電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)相較于理想情況仍然有所(suo)減少。

填充(chong)Cs蒸汽和增加(jia)(jia)電子(zi)加(jia)(jia)速柵格兩類方案不僅使設備的(de)結構復雜度、系統復雜度提高,降(jiang)低可靠(kao)性,還(huan)增加(jia)(jia)運(yun)行的(de)額外(wai)功耗和設備重(zhong)量(liang),在一定程度上削(xue)弱了該裝置單位面積功率高、結構簡單、運(yun)行穩定的(de)優(you)勢。