裴先茹,陳秀麗

(鄭州師范學(xué)院化學(xué)系,河南鄭州 450044）

鈦酸鍶鋇基陶瓷鐵電可調(diào)微波介質(zhì)材料的研究進(jìn)展

裴先茹＊,陳秀麗

(鄭州師范學(xué)院化學(xué)系,河南鄭州 450044）

綜述了鈦酸鍶鋇基陶瓷鐵電可調(diào)微波介質(zhì)材料的研究進(jìn)展,著重從離子摻雜和介質(zhì)材料復(fù)合兩個(gè)方面概括分析了當(dāng)前Ba1-xSrxTiO3(BST)基鐵電陶瓷微波介質(zhì)可調(diào)材料的發(fā)展?fàn)顩r,同時(shí)對(duì)比分析了兩種方法的優(yōu)缺點(diǎn).

鈦酸鍶鋇基陶瓷;微波介質(zhì)材料;研究進(jìn)展

Abstract:A review is provided of the recent advance in the study on ferroelectric tunable microwave materials made of barium strontium titanate-based ceramics.The development of ferroelectric tunable microwave materials is highlighted in relation to iron-doping and material hybridization.In the meantime,the advantages and disadvantages of iron-doping and material hybridization are compared.

Keywords:barium strontium titanate-based ceramics;microwave material;research progress

BaxSr1-xTiO3(BST)是BaTiO3和SrTiO3的無(wú)限固熔體,具有優(yōu)異的介電性能,如介電常數(shù)和居里溫度(TC)調(diào)節(jié)方便(如圖1所示),低介電損耗(tgδ),高可調(diào)率等,從而使其在調(diào)諧微波器件、鐵電移相器、諧振器和濾波器等方面有著廣泛的應(yīng)用前景[1-4],因此鈦酸鍶鋇材料被普遍認(rèn)為是最有前途的鐵電移相材料.與傳統(tǒng)的鐵氧體移相器相比,鈦酸鍶鋇材料在微波可調(diào)器件方面(尤其在微波移相器方面)具有自身特有的優(yōu)勢(shì),例如:小尺寸、響應(yīng)快、低損耗等[5].根據(jù)理論分析和實(shí)際應(yīng)用中的認(rèn)識(shí),高性能的鈦酸鍶鋇鐵電可調(diào)微波陶瓷材料需要具備以下幾個(gè)方面的特點(diǎn):合適的介電常數(shù) (εr=50～1 000),在微波頻段,介質(zhì)損耗要小(tgδ<0.005),高可調(diào)率(T>10%)[6].然而具有高介電常數(shù)的BST陶瓷材料很難滿足其與激勵(lì)源內(nèi)部的阻抗匹配和大功率的要求,且其在微波頻段下具有較大的介電損耗,這在很大程度上限制了它在微波可調(diào)器件領(lǐng)域的應(yīng)用.因此,如何制備出既具有適中介電常數(shù)、低的介電損耗,又具有寬的介電可調(diào)性的材料是一個(gè)技術(shù)難點(diǎn)[7-9].本文作者主要從離子摻雜和微波介質(zhì)材料復(fù)合兩個(gè)方面來(lái)分析當(dāng)前微波介質(zhì)可調(diào)材料的研究進(jìn)展.

1 BST基微波介質(zhì)材料的研究狀況

當(dāng)前,國(guó)內(nèi)外為了提高BST基微波介質(zhì)的綜合性能,主要從兩方面入手:(1)采用離子摻雜,即通過(guò)不同離子的化學(xué)計(jì)量比或非化學(xué)計(jì)量比摻雜;(2)與微波介質(zhì)材料復(fù)合,實(shí)現(xiàn)性能上互相取長(zhǎng)補(bǔ)短,產(chǎn)生協(xié)同效應(yīng),使復(fù)合材料綜合性能優(yōu)于原組成材料,從而得到能滿足介電可調(diào)微波器件(電調(diào)諧諧振器、LC濾波器、耦合器、相控陣天線移相器等)的要求.

圖1 居里溫度與Ba含量的變化關(guān)系(a)及介電常數(shù)與溫度的變化關(guān)系(b)圖Fig.1 The relation between Curie temperature and barium concentration(a)and the relation between permittivity and temperature(b)

1.1 離子摻雜

1.1.1 稀土金屬氧化物摻雜BST基微波介質(zhì)材料

常用的稀土氧化物改性劑主要有 :Yb2O3、Dy2O3、Sm2O3、Nd2O3、CeO2、La2O3[10-33]等 ,這些氧化物加入到BST基體中,由于稀土元素特有的活性,摻雜的稀土離子以不同的方式進(jìn)入晶體材料的晶格中,引起材料微觀結(jié)構(gòu)的改變,從而有效地調(diào)整和改進(jìn)BST材料體系的相關(guān)性能.研究發(fā)現(xiàn)在BST基體中摻雜少量稀土元素氧化物,其介電常數(shù)就可以產(chǎn)生十分明顯的變化,而且其他性能如居里溫度、燒結(jié)性能和介電常數(shù)的溫度穩(wěn)定性等都能夠得到很大的改善,通常摻雜摩爾分?jǐn)?shù)0.5%左右的稀土氧化物[34],并且通過(guò)控制Ba2+(或者 Sr2+)和 Ti4+之間的空位補(bǔ)償,能很好地改善BST介電可調(diào)材料的介電特性,如 Q值(1/tgδ),居里溫度 TC等[35].如少量的La2O3加入到Ba0.55Sr0.45TiO3-MgO復(fù)合陶瓷中,與未摻雜的樣品相比,介電常數(shù)降低,最重要的是提高了材料在微波頻段下?lián)p耗因子(Q×f)的值[36].如果摻雜稀土元素過(guò)多,往往會(huì)引起材料中缺陷的增加,使得材料的絕緣性能大大降低,并使得基體的居里溫度迅速向低溫方向移動(dòng),其介電性能就會(huì)大大惡化.

1.1.2 其他金屬氧化物(MgO、Al2O3、MnO2、ZnO等)摻雜BST基微波介質(zhì)材料

迄今為止,利用單一的金屬氧化物MgO作為改性劑[37-39],對(duì)降低Ba1-xSrxTiO3陶瓷材料的介電損耗和介電常數(shù)也有顯著的效果.Sengupta等人對(duì)Ba1-xSrxTiO3(x=0.5～0.6)-MgO進(jìn)行了詳細(xì)的研究,結(jié)果表明:隨著MgO含量的增加,其介電常數(shù)逐漸減小,在微波頻段下具有低的介電損耗,同時(shí)保持一定的可調(diào)率[6].其他單一的金屬氧化物如:SiO2、Fe2O3、Nb2O5、ZrO2、Al2O3、MnO2等也可以作為改性劑來(lái)改善Ba1-xSrxTiO3材料的介電性能.對(duì)BST介電材料的特性也起到了較好的調(diào)控作用,有效地降低了材料的介電損耗,然而由于離子的取代導(dǎo)致居里溫度向低溫移動(dòng),引起摻雜后的可調(diào)性降低.例如Mn置換A/B位離子Ba0.4Sr0.6TiO3陶瓷改性,低濃度受主摻雜能有效改善BST鐵電材料的介電性能.Mn2+離子置換Ti4+離子將產(chǎn)生一個(gè)氧空位,但由于Mn2+離子易于向Mn3+、Mn4+離子轉(zhuǎn)化,從而導(dǎo)致氧空位濃度降低.Mn2+置換Sr2+離子雖使其電負(fù)性差值減小,但結(jié)構(gòu)容許因子更接近1,有利于基體中鈣鈦礦結(jié)構(gòu)穩(wěn)定.隨著Mn2+置換量的增加,大部分Mn2+占據(jù)B位,且被氧化成Mn3+,同時(shí)有效地抑制了 Ti4+向 Ti3+離子的轉(zhuǎn)化.隨著Mn2+置換量的增加,其介電常數(shù)減小,而介電可調(diào)率變化不大(T=10.0%,30 kV/cm),但Q值從601逐漸減小到187,是一個(gè)很好的介電材料體系[40].

1.2 低介電常數(shù)高Q值的微波介質(zhì)材料和BST材料復(fù)合

由于復(fù)合材料可通過(guò)設(shè)計(jì),即通過(guò)對(duì)原材料的選擇、各組分分布和工藝條件的保證等,來(lái)實(shí)現(xiàn)性能上互相取長(zhǎng)補(bǔ)短,產(chǎn)生協(xié)同效應(yīng),使復(fù)合材料的綜合性能優(yōu)于原組成材料而滿足不同的要求,因而選用低介電常數(shù),低損耗的微波介質(zhì)材料和BST固溶體復(fù)合成為優(yōu)化BST鐵電材料可調(diào)介電性能的一種有效的手段,在某些方面優(yōu)于離子摻雜BST的介電特性.目前,復(fù)合較多的主要為含Mg的一些鹽類,如MgTiO3、Mg2TiO4、BaO-TiO2微波介質(zhì)材料體系、Mg2SiO4、MgAl2O4等低介電常數(shù),高 Q值的微波介質(zhì)材料,并從中取得了一些有意義的結(jié)論.如Ba0.5Sr0.5TiO3-Mg2TiO4,介電偏壓和微波性能測(cè)試結(jié)果表明,BST-MT復(fù)合材料仍能保持較高的可調(diào)性,且微波性能得到了明顯改善.樣品50wt%Ba0.5Sr0.5TiO3-50wt%Mg2TiO4在10 kHz下的介電常數(shù)為335,介電損耗為0.000 3,在外加30 kV/cm偏置電場(chǎng)作用下,可調(diào)性達(dá)到18.4%,在2.065 GHz頻率下的介電損耗為0.004 2[41].可見(jiàn)通過(guò)微波介質(zhì)材料復(fù)合可有效地調(diào)節(jié)材料的介電性能,從而滿足介電可調(diào)微波器件的要求.然而由于單純的BST和微波介質(zhì)材料兩相復(fù)合,不一定能滿足器件所需求的介電性能,往往采取兩種微波介質(zhì)材料和鐵電材料形成三相復(fù)合,其復(fù)合后的介電性能得到明顯的改善.例如當(dāng)Ba0.6Sr0.4TiO3/35wt%Mg2SiO4/15wt%MgO三相復(fù)合時(shí),獲得了低的介電常數(shù)(εr=118),高的可調(diào)率(T=13%,E=20 kV/cm)[42].介電可調(diào)材料與高Q值微波介質(zhì)材料復(fù)合,由于其保持了介電可調(diào)材料的高調(diào)制特性,還能擁有微波材料的高Q特性,從而成為優(yōu)化介電可調(diào)材料微波性能的另一途徑.

1.3 兩種方法的優(yōu)缺點(diǎn)

總之,不論通過(guò)離子摻雜還是與微波介質(zhì)材料復(fù)合,都能在一定程度上改善材料的介電特性.對(duì)于離子摻雜,少量的摻雜對(duì)于改變材料的介電特性較為明顯,但是隨著摻雜量的增加,往往會(huì)導(dǎo)致材料中缺陷的增加,同時(shí)介電損耗也往往隨之增加.對(duì)于用微波介質(zhì)材料復(fù)合來(lái)改變材料的介電特性,隨著復(fù)合量的增加,大大降低了基體材料的介電常數(shù),同時(shí)具有較低的損耗,并保持了一定的可調(diào)率,這種特性往往可通過(guò)合理的優(yōu)化來(lái)達(dá)到一種平衡.

2 微波介質(zhì)材料的發(fā)展趨勢(shì)

隨著介電可調(diào)材料的不斷發(fā)展,現(xiàn)代移動(dòng)通信、無(wú)線局域網(wǎng)、軍事雷達(dá)等設(shè)備正向小型、輕量、高頻、多功能及低成本化方向發(fā)展,對(duì)以微波介質(zhì)陶瓷為基礎(chǔ)的微波元器件提出了更高要求.為滿足此要求,利用低溫共燒陶瓷(L TCC)技術(shù)設(shè)計(jì)制造片式多層微波器件已成為當(dāng)今的研究熱點(diǎn).通過(guò)摻雜低熔點(diǎn)氧化物或玻璃相:Li2O、B2O3-Li2O玻璃等燒結(jié)助劑進(jìn)行液相燒結(jié)[43],同樣也可以獲得具有介電可調(diào)特性且可以低溫?zé)Y(jié)的新型微波介質(zhì)陶瓷材料.

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Progress on barium strontium titanate-based ceramics as ferroelectric tunable microwave materials

PEI Xian-ru＊,CHEN Xiu-li

(Department of Chemistry,Zhengzhou Normal College,Zhengzhou450044,Henan,China)

O 629.9

A

1008-1011(2011)02-0079-05

2010-11-11.

河南省基礎(chǔ)與前沿技術(shù)研究計(jì)劃項(xiàng)目(092300410119).

裴先茹(1975-),女,碩士,主要從事分子基功能材料研究,E-mail:xianrupei@163.com.