【作者】 蔡苇；

【导师】 高家诚；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2011， 博士

【摘要】 钛酸钡具有优良的压电、热释电和铁电性,可制作多层陶瓷电容器、铁电随机存储器、热释电探测器、倍频器、介质移相器、压控滤波器等众多的功能元器件。为了进一步提高其介电非线性和减小低频下的介质损耗,通过A位掺Sr2+形成钛酸锶钡(Ba_1-xSr_xTiO₃,简称BST)材料。但钛酸锶钡材料当外加直流电场超过几百kV/cm后,其漏电流密度以数量级上升,并在2MV/cm左右发生击穿。人们通过改进制备工艺、掺杂等多种措施仍未能解决这一重要问题,因而不得不考虑用其它材料代替钛酸锶钡。锆钛酸钡(BaZr_xTi_1-xO₃,简称BZT)漏电流低、耐电强度高,是一种可能代替钛酸锶钡的材料,而锡钛酸钡(BaSn_xTi_1-xO₃,简称BTS)作为一种典型的钛酸钡基弛豫铁电材料也受到大家的重视,对这些钛酸钡基材料进行掺杂改性是需要进一步研究的重要问题。本文系统研究了掺杂种类及浓度与钛酸钡基陶瓷微结构、介电性能和铁电性能的关系,对掺杂机理进行了探讨,并采用第一性原理讨论了钛酸钡基材料的电子结构。取得的成果如下:①锆钛酸钡陶瓷的晶粒尺寸直接影响其电畴类型,较大晶粒尺寸的BZT陶瓷中没有90°畴存在;在BZT陶瓷中存在鱼骨状、薄片状和水波纹等形状的电畴;随晶粒尺寸增大,BZT陶瓷的矫顽场强逐渐减小,剩余极化强度逐渐增大。②掺杂锆钛酸钡陶瓷中,Mn⁴⁺、Al3+、Hf⁴⁺、Ce⁴⁺、Ni³⁺（Ni₂O₃量<1 at.%）均进入BZT陶瓷的晶胞取代B位的Ti⁴⁺和Zr⁴⁺,但Ni₂O₃量≥2 at.%时,出现第二相Ni₂O₃。MnO₂、Al₂O₃、CeO₂、Ni₂O₃和HfO₂均具有细晶作用。MnO₂、CeO₂、HfO₂、Ni₂O₃可降低BZT陶瓷的介质损耗,但Al₂O₃使其介质损耗升高。BZT陶瓷相变弥散程度随MnO₂、Al₂O₃量增加而减弱,随Ni₂O₃、HfO₂量增加先减弱后增强,掺MnO₂、Al₂O₃、Ni₂O₃、HfO₂时无频率色散现象;掺1 at.%CeO₂的BZT陶瓷介电峰宽化且存在明显频率色散。BZT陶瓷剩余极化强度和矫顽场强随MnO₂、CeO₂量增加而减小;随Ni₂O₃量增加,BZT陶瓷剩余极化强度逐渐减小,矫顽场强先减小后增大;BZT陶瓷矫顽场强随Al₂O₃量增加而增大;随HfO₂量增加,BZT陶瓷的剩余极化强度先增加后减小,矫顽场强先减小后增大。综合考虑,B位可变价的MnO₂的引入可使锆钛酸钡陶瓷在室温下具有较大的介电常数和较低的介质损耗,是一种较为理想的掺杂改性物。③当x为0⁰.30时,BaSn_xTi_1-xO₃陶瓷随锡量（x）增加由四方相向立方相过渡。Mn⁴⁺、Zn²⁺取代锡钛酸钡B位的Ti⁴⁺和Sn⁴⁺。Sn⁴⁺、Mn⁴⁺和Zn²⁺均具有细晶作用。BaSn_xTi_1-xO₃陶瓷室温介质损耗低于钛酸钡陶瓷。BaSn_xTi_1-xO₃陶瓷的相变弥散程度随锡量增加逐渐增强。BaTi0.9Sn0.1O3陶瓷相变弥散程度随MnO₂量增加逐渐减弱,而ZnO可使其相变弥散程度增强。BaSn_xTi_1-xO₃陶瓷剩余极化强度随锡量（x）增加而减小;当x为0.10⁰.20,锡钛酸钡陶瓷矫顽场强随锡量增加而增大。④掺杂钛酸钡陶瓷中,V⁵⁺和La³⁺分别取代钛酸钡B位的Ti⁴⁺和A位的Ba2+;MgO量<1.5at.%时, Mg²⁺取代B位的Ti⁴⁺,当MgO量≥1.5 at.%时,出现第二相MgO。MgO和La₂O₃具有细晶作用,但钛酸钡陶瓷的晶粒尺寸随V₂O₅量增加而增大。V₂O₅可降低钛酸钡陶瓷介质损耗,而MgO使其介质损耗增加;La₂O₃量达一定程度可使钛酸钡陶瓷介质损耗降低。钛酸钡陶瓷中掺MgO和La₂O₃会使介电峰有一定程度宽化。随V₂O₅量增加,钛酸钡陶瓷剩余极化强度先增加到最大值随后减小;掺V₂O₅后,钛酸钡陶瓷矫顽场强明显降低。综合考虑,B位可变价的V₂O₅的引入可使钛酸钡陶瓷具有较低的介质损耗和较大的剩余极化强度,是一种较为理想的掺杂改性物。⑤电子结构的第一性原理计算表明:钛酸钡中Ba-O、Ti-O分别以离子键、共价键形式结合,其相变是Ti原子相对位置发生改变引起的。Ti和O的杂化主要是由O 2p态和Ti 3d态所提供。铁电相钛酸钡的Ti-O杂化程度低于顺电相钛酸钡。掺锆钛酸钡禁带宽度随锆量增加而增大。掺Zr⁴⁺后,总态密度和分态密度变化明显,这是Zr-4d电子与O-2p电子间发生杂化所致。掺铪钛酸钡禁带宽度随铪量增加先增大后减小,其能带结构简并度随铪量增加而降低,其铁电性强于纯钛酸钡。掺Hf后,O-Ti键布居数增加,共价键结合增强,而O-Ba键布居数绝对值减小,离子键结合减弱。O-Ti、O-Hf之间存在强烈杂化,这是掺铪钛酸钡铁电性形成的原因。更多还原

【Abstract】 Barium titanate is one of the important ferroelectric materials with perovskite structure, which has been used as multi-layer ceramic capacitor （MLCC）, ferroelectric random access memories （FRAM）, pyroelectric detector, frequency multiplier, dielectric phase shifter and voltage-controlled filter, and so forth because of its excellent piezoelectric, pyroelectric and ferroelectric properties. There are still some disadvantages in barium titanate materials. In order to further improve the dielectric nonlinearity and reduce dielectric loss at the low frequency, Sr2 + are doped into barium titanate to substitute for Ba2+ on A-site to form barium strontium titanate (Ba_1-xSr_xTiO₃, short for BST) material. However, when the applied electric field exceeds a few hundred kV/cm, the leakage current density of barium strontium titanate increases by an order of magnitude, and the breakdown occurs at about the 2MV/cm. The important problem has not yet been solved by improving the preparation process and doping. Therefore, other materials have been developed to replace barium strontium titanate. Barium zirconate titanate (BaZr_xTi_1-xO₃, short for BZT) is a possible good substitute because of low leakage current density and high breakdown strength. Barium stannate titanate (BaSn_xTi_1-xO₃, short for BTS) is a typical barium titanate-based relaxor ferroelectrics, which have been attracted much attention. Further research on doping and modification of these barium titanate-based materials is needed.In this paper, the influence of the doping type and concentration on microstructures, dielectric and ferroelectric properties of barium titanate-based ceramics has been investigated systematically and the doping mechanism has been discussed. The electronic structure of barium titanate-based materials has been studied by first-principle calculation. The results are as follows:①Grain size of barium zirconate titanate ceramics can affect the type of its domain structure. There is no 90°domain in the BZT ceramics with the larger grain size. Domains with herribone, lamellar and water-mark characters are observed in BZT ceramics. As the grain size increases, the coercive field of BZT ceramics decreases, while the remnant polarization increases.②Mn⁴⁺, Al3+, Hf⁴⁺, Ce⁴⁺ and Ni³⁺ （Ni₂O₃ content<1 at.%） enter into the cell of BZT ceramics to substitute for Ti⁴⁺ and Zr⁴⁺ on the B sites. When content of Ni₂O₃ is more than 2 at.%, there is the second phase Ni₂O₃. MnO₂, Al₂O₃, CeO₂, Ni₂O₃ and HfO₂ have grain refinement.MnO₂, CeO₂, HfO₂ and Ni₂O₃ can reduce dielectric loss, but Al₂O₃ increases the dielectric loss. The diffuseness of phase transition of BZT ceramics weakens with the increasing of MnO₂ and Al₂O₃ content, and the diffuseness of phase transition initially increases and then decreases with the increasing of Ni₂O₃ and HfO₂ content. There is no obvious frequency dispersion phenomenon in barium zirconnate titante doped with MnO₂, Al₂O₃, Ni₂O₃ and HfO₂. The dielectric peak of BZT ceramics doped with 1 at.% CeO₂ broadens and there is apparent frequency dispersion phenomenon. The remnant polarization and coercive field of BZT ceramics decrease with increasing of MnO₂ and CeO₂ content. As Ni₂O₃ content increases, the remnant polarization decreases and the coercive field initially decreases and then increases. The coercive field of Al-doped BZT ceramics increases with the increase of Al₂O₃ content. As HfO₂ content increases, the remnant polarization initially increases and then decreases, while the coercive field initially decreases and then increases. In conclusion, variable-valent MnO₂ on B sites is an ideal modifier, because that it can lead to the larger dielectric constant and lower dielectric loss of barium zirconate titanate ceramics at room temperature.③When x is 0⁰.30, BaSn_xTi_1-xO₃ ceramics transit from the tetragonal phase to cubic phase as x increases. Mn⁴⁺ and Zn²⁺ enter into the cell to substitute for Ti⁴⁺ and Sn⁴⁺ on B sites. Sn⁴⁺, Mn⁴⁺ and Zn²⁺ have grain refinement. The dielectric loss of BaSn_xTi_1-xO₃ ceramics at room temperature is lower than that of barium titanate ceramics. The diffuseness of phase transition of BaSn_xTi_1-xO₃ ceramics enhances gradually with the increase of tin content. The diffuseness of phase transition of BaTi0.9Sn0.1O3 ceramics weakens with the increasing of MnO₂ content and addition of ZnO can enhance the diffuseness of phase transition. The remnant polarization of BaSn_xTi_1-xO₃ ceramics decreases with the increasing of tin content. When x is 0.10⁰.20, the coercive field increases with the increase of tin content.④In doped barium titanate ceramics, V⁵⁺ and La³⁺ enter into the cell to substitute for Ti⁴⁺ on B sites and Ba2+ on sites, respectively. When MgO content is less than 1.5 at.%, Mg²⁺ enters into the cell to replace Ti⁴⁺ on B sites, and there is second phase （MgO） when MgO content is above 1.5 at.%. MgO and La₂O₃ have grain refinement. Grain size of barium titanate ceramics increases with the increasing of V₂O₅ content.V₂O₅ can reduce dielectric loss, and MgO increases the dielectric loss. When La₂O₃ content is above the critical value, it can reduce the dielectric loss of barium titanate ceramics. The dielectric peak of MgO-doped and La₂O₃-doped BZT ceramics broadens. As V₂O₅ content increases, the remnant polarization of barium titanate ceramics initially increases the maximum and then decreases. The coercive field of V₂O₅-doped barium titanate ceramics is much lower than that of pure barium titanate ceramics. In conclusion, variable-valent V₂O₅ on B sites is an ideal modifier, because that it can lead to the lower dielectric loss and larger remnant polarization of barium titanate ceramics.⑤Electronic structure by first-principles calculations shows that Ba-O and Ti-O of barium titanate is ionic bonds and covalent bonds, respectively. The phase transition is due to the change in the relative position of Ti atoms. Hybridization of Ti and O is mainly from the O 2p states and Ti 3d states. Hybridization of ferroelectric phase of barium titanate is lower than that of paraelectric phase.Band gap of Zr-doped barium titanate increases with the increasing of zirconium content. When Zr⁴⁺ is doped, the total density of states and partial density of states changes significantly, which is due to hybridization between Zr-4d electron and the O-2p. As hafnium content increases, band gap of Hf-doped barium titanate initially increases and then decreases, and the degeneracy of band structure increases. Ferroelectricity of Hf-doped barium titanate is stronger than pure barium titanate. When hafnium is doped, the populations of O-Ti bond increase and covalent bond enhances, while the absolute value of populations of O-Ba bond decreases and ionic bond weakens. There is strong hybridization between O-Ti and O-Hf, which is the cause of formation of ferroelectricity.更多还原