【作者】 张光祖；

【导师】 姜胜林；

【作者基本信息】 华中科技大学 ， 微电子学与固体电子学， 2010， 博士

【摘要】 作为一种典型的铁电材料,钛酸锶钡(BST)具有极强的非线性介电特性,其介电性能可随外加直流偏场变化(场致效应),被广泛用于压控滤波器、振荡器、微波移相器和场致热释电红外探测器中。为此,本文对BST材料的场致效应及其在非制冷红外探测器方面的应用进行了系统研究,为高性能非制冷红外焦平面阵列的研制奠定了基础。对BST场致效应的掺杂理论进行了论述,分析了掺杂物化合价、离子半径等对BST晶格常数、电畴结构等微观特性和介电可调率、场致热释电系数等宏观电学性能的影响。由于陶瓷晶粒尺寸和气孔等微结构与材料场致效应关系密切,本文在Johnson理论中引入了修正因子α,用以描述微结构对有效电场分布的调控作用。理论研究指出：随着晶粒尺寸的减小,晶界比例上升,晶界对电场的散射作用使陶瓷晶粒上的有效电场下降,材料抗值减小,介电可调率降低；当陶瓷中存在气孔时,圆形气孔上分配的有效电场要大于不规则形状气孔,气孔为不规则形状的多孔陶瓷的α值较大,晶粒上有效电场较高,与圆形气孔多孔陶瓷相比,其介电可调率较大。为了研究掺杂对材料场致效应的影响,进行了Mn受主掺杂实验。随着掺杂量的增大,BST的介电峰被压低、展宽,介电可调率减小,但介电损耗显著降低。当掺杂量为0.3 mol%时,室温下BST在400 V/mm直流电场下的介电可调率为20%,损耗降至0.2%,可调率优值FoM>100,场致热释电系数为84x 102μC/m2℃,探测率优值为17.3×10-5Pa-0.5,随着Mn掺杂量的继续增大,材料介电损耗上升,可调率和探测率优值急剧下降。为进一步提高BST的场致热释电性能,中和过量受主掺杂对介电损耗造成的负面影响,在Mn掺杂基础上进行了Y、Mn施、受主共掺杂实验。随着Y、Mn掺杂量的增大,BST介电常数减小,介电峰展宽,当Y、Mn掺杂量分别为1.2 mol%和0.6 mol%时,BST的介电损耗低于0.2%,场致热释电系为85×102μC/m2℃,探测率优值较Mn单元素掺杂提高到19.0×10-5Pa-0.5。为了研究晶粒尺寸对BST场致效应的影响,系统实验了二次烧结工艺,制备了晶粒尺寸在0.3μm至4μm的BST致密陶瓷,探讨了晶粒尺寸对BST掺杂元素分布、晶格常数、电畴结构等微观参数和介电可调率、场致热释电系数等宏观电学性能的影响。随着晶粒尺寸的减小,BST介电峰展宽,可调率下降,但介电损耗变化不大。晶粒的适当减小有助于BST场致热释电系数的提高,但若晶粒过小,晶粒间巨大的内应力使BST的晶格四方率急剧减小,铁电性下降,热释电性变差。实验显示BST的最佳晶粒尺寸为1μm,此时材料在500 V/mm直流偏场下的场致热释电系数为105×102μC/m2℃,探测率优值为22.0×10-4 Pa-0.5。根据理论研究设计了多孔陶瓷的微结构,研究了多孔陶瓷制备工艺。用聚甲基丙烯酸甲酯(PMMA)作为造孔剂制备了不同气孔率、气孔形状和孔径的多孔陶瓷,对理论进行了验证。实验证明晶粒尺寸大于1μm的致密陶瓷和气孔率小于14%、孔径大于10μm的陶瓷,其微结构因子α接近于1；对于气孔为非圆形的陶瓷,其α值大于1；而当气孔率大于22.3%时,BST的α值为0.9,说明改进后的理论使原有模型和实验结果之间的偏差缩小了约10%。气孔可削弱晶粒间的内应力和电场对介电峰的压峰效应,减小材料的体积热容,这都有利于热释电性能的提高,因此本文提出并制备了BST多孔场致热释电陶瓷。当气孔率为9.6%,外加直流偏场为400 V/mm时,材料的场致热释电系数为80×102μC/m2℃,探测率优值为27.0×10-5 Pa-05,综合热释电性能优于致密陶瓷。为进一步加大气孔对陶瓷内应力和有效电场分布的调控作用,本文采用多壁碳纳米管作为造孔剂制备了微气孔BST多孔场致热释电陶瓷。气孔率为9.5%的微气孔陶瓷的场致热释电系数和探测率优值分别可达到95×102μc/m2℃和32.0×10-5Pa-0.5,较PMMA制备的多孔陶瓷具有更理想的场致热释电性能。最后,通过陶瓷微加工工艺对BST多孔热释电陶瓷进行剪薄和切割,通过负胶光刻、金属蒸发和剥离工艺在陶瓷表面制备了银和镍铬合金电极,制作了红外信号探测元；针对BST的场致热释电效应,结合微电容检测原理,设计并搭建了红外探测器信号读出电路,使探测元实现了对黑体红外辐射信号的探测,这同时证明了本文制备的BST陶瓷和探测元性能良好。更多还原

【Abstract】 As a kind of typical ferroelectric material, barium strontium titanate (BST) is of very strong nonlinear dielectric properties, which may vary due to DC field-induced effect. It is being broadly used in voltage-controlled filter, oscillator, microwave phase shifter and DC field-induced pyroelectric infrared detector. Therefore, this paper conducted the systematic research on the DC field-induced effect of BST material and its application on the uncooled infrared detector and thus laying a basis for the development of high-performance uncooled infrared focal plane array.The doping theory of BST material’s DC field-induced effect was described, and the influences of the valence of dopant and ionic radius on the microscopic properties such as lattice constant and domain structure and the macroscopic properties like dielectric tunability, DC field-induced pyroelectric coefficient etc. were analyzed. Since the microstructure of ceramic grain, e.g. dimension and pore, had a close relationship with the DC field-induced effect of the material, this paper introduced the modifying factor a into Johnson theory for describing the microstructure’s control over the electric field distribution. As pointed out by the theory, the grain size decreased, the proportion of the grain boundary rose, the scattering effect of the grain boundary made the effective electric field on the ceramic grain decrease, the a value of the material became smaller, and the dielectric tunability was weakened; there were pores in the ceramics, the effective electric field distributed on round pores was larger than that on pores of irregular shapes, the a value of porous ceramics with irregular-shape pores were larger and the effective electric field on their grains were higher, so compared with porous ceramics with round pores, their dielectric tunability was better.In order to research the doping effect on the DC field-induced effect of BST material, the Mn-doping experiment was made. As more Mn was doped, the permittivity peak was depressed and broadened, the dielectric tunability was reduced, but the dielectric loss significantly declined. When 0.3 mol% Mn was doped, the dielectric tunability of BST at room temperature and under 400 V/mm DC bias field was 20%, the dielectric loss declined to 0.2%, the FoM (figure of merit) of tunability>100, the DC field-induced pyroelectric coefficient was 84×102μC/m2℃and the FoM of detectivity was 17.3×10-5 Pa-0.5. As Mn-dopant was increased, the dielectric loss became larger, but the FoMs of tunability and detectivity fell drastically. For further enhancing the DC field-induced pyroelectric properties of BST and neutralizing the negative effects of excessive doping on the dielectric loss, the Y, Mn acceptor-donor co-doping was conducted on the basis of Mn-doping. With more Y and Mn doped, the dielectric constant decreased, and the permittivity peak broadened. When the Y and Mn doping were respectively 1.2mol% and 0.6mol%, the dielectric loss was lower than 0.2%, DC field-induced pyroelectric coefficient of BST was 85×102μC/m2℃and the detectivity FoM increased to 19.0×10-5 Pa-0.5 as compared with Mn-doping.For exploring the influence of grain size on the DC field-induced effect of BST, the two-step sintering process was adopted to prepare compact BST ceramics with the grain size ranging between 0.3μm to 4μm and discussed the influences of grain size on the microscopic parameters, e.g. BST doping element, lattice constant and domain structure, and on the macroscopic parameters such as dielectric tunability and field-induced pyroelectric coefficient etc. As the grain size became smaller, the permittivity peak was broadened and the tunability decreased, but the dielectric loss changed slightly. The proper decrease of grain size was helpful for the increase of field-induced effect of BST. If the grain size was too small, the big internal stress between grains made the tetragonal lattices reduced radically, the ferroelectric properties declined and the pyroelectric properties became worse. The experiment indicated that the best grain size of BST was 1μm and now the field-induced pyroelectric coefficient of the material under 500 V/mm DC bias field was 105×102μC/m2℃and the FoM of the detectivity was 22.0×10-5 Pa-0.5.According to theory, the microstructure of porous ceramics was designed and its preparation process was explored, with polymethyl methacrylate (PMMA) as the pore former to prepare the porous ceramics of different porosities, pore shapes and apertures for verifying the theory. The experiment indicated that, for dense ceramics with grain size larger than 1μm and ceramics with the porosity smaller than 14% and aperture larger than 10μm, the microstructure factor a was close to 1; for ceramics with non-round pores, the a value was larger than.1; when the pority was larger than 22.3%, theαvalue was 0.9. This indicated that the theory after improvement reduced about 10% of the deviation of original model from the experimental results.The pores could weaken the internal stress between grain size and the peak-depressing effect of electric field on the dielectric peak and reduce the heat capacity per volume of materials, and thus being beneficial for enhancing the pyroelectric properties. Therefore, this paper proposed and prepared field-induced porous pyroelectric BST ceramics. When the porosity was 9.6% and the DC bias field was 400V/mm, the field-induced pyroelectric coefficient was 80×102μC/m2℃, the FoM of the detectivity was 27.0×10-5 Pa-0.5, and the comprehensive pyroelectric properties were better than those of dense samples. In order to further strengthen the tuning of pores on the internal stress of ceramics and the effective electric field distribution. This paper adopted the multiwall carbon nanotubes as the pore former to prepare the microporous field-induced pyroelectric BST ceramics. The field-induced pyroelectric coefficient and the FoM of detectivity of the microporous ceramics with the porosity of 9.5% could reach 95×102μc/m2℃and 32.0×10-5Pa-0.5, thus it had more ideal DC field-induced pyroelectric properties than PMMA-prepared porous ceramics.Finally, through shear thinning and cutting of porous pyroelectric BST ceramics by the micro-processing technology, the silver and nickel chromium alloy electrode were prepared on the ceramics surface by the photolithography, metal evaporation and negative photoresist stripping processes, and the detecting element of infrared detector was fabricated; aiming at the field-induced pyroelectric effect of BST, according to the micro capacitance measurement technology, the readout circuit of infrared detector unit was designed and built, enabling the detecting element to detect the black-body infrared radiation signal, and demonstrating that the BST ceramics and detecting element prepared in this paper had good performance.更多还原