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钛酸铋钠钾压电厚膜的制备与性能研究

Preparation and Properties of Sodium-Potassium Bismuth Titanate Piezoelectric Thick Films

【作者】 张海波

【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, 2008, 博士

【摘要】 厚度为10-100μm的压电厚膜兼具陶瓷与薄膜的优点,随着电子元器件向小型、高灵敏、集成、多功能方向发展,压电厚膜及器件成为国内外研究热点。本文采用丝网印刷法制备出高致密钛酸铋钠钾无铅压电厚膜,并研究了厚膜微结构和性能。采用双层印刷法在Al2O3衬底上制备了厚度约为8μm的Pt底电极,研究表明第一层和第二层分别在600℃和1200℃下烧结,Pt电极具有最大表面覆盖率和最小表面电阻,其方阻为0.044Ω/ ,平均粗糙度为0.6μm;采用传统固相法制备(Na0.82K0.18)0.5Bi0.5TiO3(NKBT)陶瓷粉体,以5wt.%的乙基纤维素溶入到松油醇中配制粘合剂溶液,加入2wt.%的二乙二醇丁醚醋酸酯作分散剂,1wt.%的邻苯二甲酸二丁酯和聚乙二醇作增塑剂,将NKBT粉体与粘合剂溶液按3:1的质量比混合碾磨,得到30℃时18-1s剪切速率下粘度为20-80Pa.s的厚膜浆料。通过在NKBT中掺入La2O3和MnO2进行材料的A位和B位掺杂改性,在厚膜浆料中加入低熔点氧化物0.6Bi2O3-0.4Li2O后实现NKBT厚膜的低温烧结。结果表明适量的MnO2掺杂NKBT厚膜晶粒明显长大、尺寸更均匀,致密度提高,四方相含量增加,掺MnO2量为1.0 mol%厚度约为40μm的NKBT厚膜介电常数为735,损耗为2.2%(10kHz),纵向压电系数d33为88pC/N,剩余极化强度为28.5μC/cm2,矫顽场强为54kV/cm,室温热释电系数为3.8×10-4 C/m2℃;La2O3的掺入使NKBT厚膜晶粒细化,四方相含量增加,反铁电相含量增加,最佳烧结温度升高。掺La2O3后NKBT厚膜的室温介电常数增大,最大介电常数εmax随掺杂量的增加而降低,介电峰呈现更加宽化的趋势。掺La2O3量为1.0mol%时,NKBT厚膜具有最小漏电流密度为1.56×10-9A/cm(2外加电场为100kV/cm),压电系数d33达到最大值87pC/N,掺La2O3量为0.5mol%的NKBT厚膜具有最大剩余极化强度为19.3μC/cm2;掺入3wt.%的0.6Bi2O3-0.4Li2O的NKBT厚膜在1000℃下烧结后相对密度可达到95.54%,剩余极化强度为19.6μC/cm2。研究了NKBT厚膜的烧结特性、微结构和性能,包括溶胶渗透填充对NKBT厚膜的微结构与性能的影响,NKBT厚膜中的残余应力及其对电性能的影响,结果表明内偏场以及低介电层的出现使NKBT厚膜的性能与陶瓷有很大不同。采用复合溶胶渗透填充,在纯溶胶中添加了相同成份的NKBT功能纳米粉体,增加了填充到气孔中的有效成分。当溶胶中粉体含量为1.5g/ml时,填充6次后的NKBT厚膜具有较好的介电性能和铁电性能,其介电常数为768,损耗为2.3%,剩余极化强度为21.3μC/cm2,矫顽场强为59kV/cm。添加3wt.%的烧结助剂后的NKBT厚膜,再采用复合溶胶填充12次后,其纵向压电系数d33达到102pC/N。在1100℃下烧结的厚度约80μm的NKBT厚膜具有最大残余张应力约136MPa,随膜厚增加,厚膜残余张应力逐渐较小,当残余应力减少到84MPa时,剩余极化强度增大到25.3μC/cm2、矫顽场强减小至48kV/cm。研究了NKBT厚膜的疲劳、老化特性,结果表明NKBT厚膜疲劳、老化的主要原因是缺陷浓度增加,畴壁运动被遏制,以及用来稳定90°畴结构的缺陷偶极子的缓慢转向。温度升高使氧空位浓度增加,钉扎作用增强,导致疲劳加剧,适当的Bi过量减少了NKBT厚膜中因Bi挥发而产生的A空位,使其疲劳、老化特性变好。

【Abstract】 Piezoelectric thick films (with thickness in the range of 10 to 100μm) which have the merits of both the bulk material and thin film are of great interest due to the drive for miniaturisation, high power/sensitivity, multi-function, and system integration with the electric circuits. The purpose of the present work is to fabricate and evaluate sodium-potassium bismuth titanate (NKBT) lead free piezoelectric thick film on electroded alumina substrate by the screen printing.Double layers Pt bottom electrodes for NKBT thick films have been screen printed on alumina substrates. As the first layer sintered at 600℃, the double Pt layers with sequential heat treatments at 1200℃exhibt improved densification and surface coverage. The double Pt layers with the minimum average roughness of 0.6μm and the smallest sheet resistivity of 0.044Ω/(?) have been produced. NKBT lead free piezoelectric ceramic powders were prepared by using solid phase synthesis, The organic vehicles consisted 5wt.% ethyl cellulose as binder, 2wt.% 2-(2-n-Butoxyethoxy) ethyl as dispersing agent, 1wt.% dibutyl phthalate and DBP as a plasticizer, and 92wt.%α-terpineol as solvent (α-terpineol). The screen printing pastes were produced by mixing the ceramic powders and the organic vehicles together with the mol ratio of 3:1. The viscosity of the prepared paste was adjusted by viscosimeter in the range 20-80 Pa.s for shear rate 18-1 s.NKBT thick films with A and B site substitution have been produced by La2O3 and MnO2 doping, respectively. In order to reduce the annealing temperature, the NKBT thick films doped with 0.6Bi2O3-0.4Li2O as sintering aids were manufactured, and their microstructural, dielectric and piezoelectric properties were investigated. The results show that MnO2 doping increases the grain size, grain uniformity, density, and content of tetragonal phase of the NKBT thick film. The resulting 40μm thick films have maximum relative permittivity of 735, (at 10 kHz), longitudinal piezoelectric coefficient d33 of 88 pC/N, remanent polarization of 28.5μC/cm2, and pyroelectric coefficient of 3.8×10-4 C/m2℃(25℃), minimum dissipation of 2.2%, and coercive field of 54 kV/cm. The La2O3 doped NKBT thick films have smaller grain, higher conten of tetragonal phase and antiferroelectric phase, and higher optimal annealing temperature than undoped NKBT thick films. The dielectric constant of the NKBT thick film increases with increasing La2O3 content, and the maximum dielectric,εmax of the NKBT thick film decreases with increasing La2O3 content. The NKBT thick films have the minimum leakage current density of 1.56×10-9A/cm2 (100kV/cm applied bias ) and maximum longitudinal piezoelectric coefficient d33 of 87 pC/N when doped 1.0 mol% La2O3. The NKBT thick films doped 0.5 mol% La2O3 have the higher remanent polarization of 19.3μC/cm2. When 3 wt.% 0.6Bi2O3-0.4Li2O were added, the NKBT thick film can be sintered at 1000℃, the apparent density of 95.54% and remanent polarization of 19.6μC/cm2.NKBT thick films have been produced using a combination of screen printing and subsequent infiltration of corresponding composite sol. The densification mechanism and the effects of residual stress on electric characteristics of NKBT thick films have been investigated. The results show that the differences in dielectric and piezoelectric properties between the thick film and corresponding ceramic result from internal bias and low dielectric layer in thick film.Their structure, dielectric, ferroelectric and piezoelectric properties were investigated with variation in the number of composite sol infiltrations and the nanopowder loading in composite sol. The NKBT thick films which were infiltrated by the composite precursor solution with higher solids loading (1.5 g/ml) exhibited superior dielectric properties compared with the NKBT thick film infiltrated with pure sol. The best performance of the NKBT thick films with six infiltrations were dielectric constant of 768, dielectric loss of 2.3% at 10 kHz, remnant polarization of 21.3μC/cm2, and coercive field of 59 kV/cm respectively. When 3wt.% sintering aids added, the NKBT thick films with twelve infiltrations have the maximum longitudinal piezoelectric coefficient d33 of 102 pC/N. The effects of the residual stress on the electrical properties of NKBT thick films were investigated. The resulting 80μm thick films sintered at 1100℃have the higher tensile stress of 136MPa, and the residual stress decreases with increasing thickness of the thick films.The dielectric and piezoelectric aging were attributed to the decrease of the domain wall contribution to the permittivity which corresponds to a gradual change of the 90? domain wall with time, and the ferroelectric fatigue was resulted from pinning of domain wall due to increase of defect concentrations. Moreover, the increase of the test temperature enhanced the pinning effect which related to oxygen vacancy concentration and resulted in quicher aging and fatigue. The aging and fatigue characteristics of NKBT thick films were improved by adding excessive Bi2O3 due to decreases of A site vacancies.

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