钛酸锶钡基陶瓷铁电可调微波介质材料的研究进展

裴先茹,陈秀丽

(郑州师范学院化学系,河南郑州 450044）

钛酸锶钡基陶瓷铁电可调微波介质材料的研究进展

裴先茹＊,陈秀丽

(郑州师范学院化学系,河南郑州 450044）

综述了钛酸锶钡基陶瓷铁电可调微波介质材料的研究进展,着重从离子掺杂和介质材料复合两个方面概括分析了当前Ba1-xSrxTiO3(BST)基铁电陶瓷微波介质可调材料的发展状况,同时对比分析了两种方法的优缺点.

钛酸锶钡基陶瓷;微波介质材料;研究进展

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的无限固熔体,具有优异的介电性能,如介电常数和居里温度(TC)调节方便(如图1所示),低介电损耗(tgδ),高可调率等,从而使其在调谐微波器件、铁电移相器、谐振器和滤波器等方面有着广泛的应用前景[1-4],因此钛酸锶钡材料被普遍认为是最有前途的铁电移相材料.与传统的铁氧体移相器相比,钛酸锶钡材料在微波可调器件方面(尤其在微波移相器方面)具有自身特有的优势,例如:小尺寸、响应快、低损耗等[5].根据理论分析和实际应用中的认识,高性能的钛酸锶钡铁电可调微波陶瓷材料需要具备以下几个方面的特点:合适的介电常数 (εr=50～1 000),在微波频段,介质损耗要小(tgδ<0.005),高可调率(T>10%)[6].然而具有高介电常数的BST陶瓷材料很难满足其与激励源内部的阻抗匹配和大功率的要求,且其在微波频段下具有较大的介电损耗,这在很大程度上限制了它在微波可调器件领域的应用.因此,如何制备出既具有适中介电常数、低的介电损耗,又具有宽的介电可调性的材料是一个技术难点[7-9].本文作者主要从离子掺杂和微波介质材料复合两个方面来分析当前微波介质可调材料的研究进展.

1 BST基微波介质材料的研究状况

当前,国内外为了提高BST基微波介质的综合性能,主要从两方面入手:(1)采用离子掺杂,即通过不同离子的化学计量比或非化学计量比掺杂;(2)与微波介质材料复合,实现性能上互相取长补短,产生协同效应,使复合材料综合性能优于原组成材料,从而得到能满足介电可调微波器件(电调谐谐振器、LC滤波器、耦合器、相控阵天线移相器等)的要求.

图1 居里温度与Ba含量的变化关系(a)及介电常数与温度的变化关系(b)图Fig.1 The relation between Curie temperature and barium concentration(a)and the relation between permittivity and temperature(b)

1.1 离子掺杂

1.1.1 稀土金属氧化物掺杂BST基微波介质材料

常用的稀土氧化物改性剂主要有 :Yb2O3、Dy2O3、Sm2O3、Nd2O3、CeO2、La2O3[10-33]等 ,这些氧化物加入到BST基体中,由于稀土元素特有的活性,掺杂的稀土离子以不同的方式进入晶体材料的晶格中,引起材料微观结构的改变,从而有效地调整和改进BST材料体系的相关性能.研究发现在BST基体中掺杂少量稀土元素氧化物,其介电常数就可以产生十分明显的变化,而且其他性能如居里温度、烧结性能和介电常数的温度稳定性等都能够得到很大的改善,通常掺杂摩尔分数0.5%左右的稀土氧化物[34],并且通过控制Ba2+(或者 Sr2+)和 Ti4+之间的空位补偿,能很好地改善BST介电可调材料的介电特性,如 Q值(1/tgδ),居里温度 TC等[35].如少量的La2O3加入到Ba0.55Sr0.45TiO3-MgO复合陶瓷中,与未掺杂的样品相比,介电常数降低,最重要的是提高了材料在微波频段下损耗因子(Q×f)的值[36].如果掺杂稀土元素过多,往往会引起材料中缺陷的增加,使得材料的绝缘性能大大降低,并使得基体的居里温度迅速向低温方向移动,其介电性能就会大大恶化.

1.1.2 其他金属氧化物(MgO、Al2O3、MnO2、ZnO等)掺杂BST基微波介质材料

迄今为止,利用单一的金属氧化物MgO作为改性剂[37-39],对降低Ba1-xSrxTiO3陶瓷材料的介电损耗和介电常数也有显著的效果.Sengupta等人对Ba1-xSrxTiO3(x=0.5～0.6)-MgO进行了详细的研究,结果表明:随着MgO含量的增加,其介电常数逐渐减小,在微波频段下具有低的介电损耗,同时保持一定的可调率[6].其他单一的金属氧化物如:SiO2、Fe2O3、Nb2O5、ZrO2、Al2O3、MnO2等也可以作为改性剂来改善Ba1-xSrxTiO3材料的介电性能.对BST介电材料的特性也起到了较好的调控作用,有效地降低了材料的介电损耗,然而由于离子的取代导致居里温度向低温移动,引起掺杂后的可调性降低.例如Mn置换A/B位离子Ba0.4Sr0.6TiO3陶瓷改性,低浓度受主掺杂能有效改善BST铁电材料的介电性能.Mn2+离子置换Ti4+离子将产生一个氧空位,但由于Mn2+离子易于向Mn3+、Mn4+离子转化,从而导致氧空位浓度降低.Mn2+置换Sr2+离子虽使其电负性差值减小,但结构容许因子更接近1,有利于基体中钙钛矿结构稳定.随着Mn2+置换量的增加,大部分Mn2+占据B位,且被氧化成Mn3+,同时有效地抑制了 Ti4+向 Ti3+离子的转化.随着Mn2+置换量的增加,其介电常数减小,而介电可调率变化不大(T=10.0%,30 kV/cm),但Q值从601逐渐减小到187,是一个很好的介电材料体系[40].

1.2 低介电常数高Q值的微波介质材料和BST材料复合

由于复合材料可通过设计,即通过对原材料的选择、各组分分布和工艺条件的保证等,来实现性能上互相取长补短,产生协同效应,使复合材料的综合性能优于原组成材料而满足不同的要求,因而选用低介电常数,低损耗的微波介质材料和BST固溶体复合成为优化BST铁电材料可调介电性能的一种有效的手段,在某些方面优于离子掺杂BST的介电特性.目前,复合较多的主要为含Mg的一些盐类,如MgTiO3、Mg2TiO4、BaO-TiO2微波介质材料体系、Mg2SiO4、MgAl2O4等低介电常数,高 Q值的微波介质材料,并从中取得了一些有意义的结论.如Ba0.5Sr0.5TiO3-Mg2TiO4,介电偏压和微波性能测试结果表明,BST-MT复合材料仍能保持较高的可调性,且微波性能得到了明显改善.样品50wt%Ba0.5Sr0.5TiO3-50wt%Mg2TiO4在10 kHz下的介电常数为335,介电损耗为0.000 3,在外加30 kV/cm偏置电场作用下,可调性达到18.4%,在2.065 GHz频率下的介电损耗为0.004 2[41].可见通过微波介质材料复合可有效地调节材料的介电性能,从而满足介电可调微波器件的要求.然而由于单纯的BST和微波介质材料两相复合,不一定能满足器件所需求的介电性能,往往采取两种微波介质材料和铁电材料形成三相复合,其复合后的介电性能得到明显的改善.例如当Ba0.6Sr0.4TiO3/35wt%Mg2SiO4/15wt%MgO三相复合时,获得了低的介电常数(εr=118),高的可调率(T=13%,E=20 kV/cm)[42].介电可调材料与高Q值微波介质材料复合,由于其保持了介电可调材料的高调制特性,还能拥有微波材料的高Q特性,从而成为优化介电可调材料微波性能的另一途径.

1.3 两种方法的优缺点

总之,不论通过离子掺杂还是与微波介质材料复合,都能在一定程度上改善材料的介电特性.对于离子掺杂,少量的掺杂对于改变材料的介电特性较为明显,但是随着掺杂量的增加,往往会导致材料中缺陷的增加,同时介电损耗也往往随之增加.对于用微波介质材料复合来改变材料的介电特性,随着复合量的增加,大大降低了基体材料的介电常数,同时具有较低的损耗,并保持了一定的可调率,这种特性往往可通过合理的优化来达到一种平衡.

2 微波介质材料的发展趋势

随着介电可调材料的不断发展,现代移动通信、无线局域网、军事雷达等设备正向小型、轻量、高频、多功能及低成本化方向发展,对以微波介质陶瓷为基础的微波元器件提出了更高要求.为满足此要求,利用低温共烧陶瓷(L TCC)技术设计制造片式多层微波器件已成为当今的研究热点.通过掺杂低熔点氧化物或玻璃相:Li2O、B2O3-Li2O玻璃等烧结助剂进行液相烧结[43],同样也可以获得具有介电可调特性且可以低温烧结的新型微波介质陶瓷材料.

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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.

河南省基础与前沿技术研究计划项目(092300410119).

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