Sandwiched BaNd2Ti4O12/Bi4Ti3O12/BaNd2Ti4O12 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) combined with heat treatment was investigated to prepare laminated ceramics from compositions having quite different sintering temperatures. Although the optimal sintering temperature of BaNd₂Ti₄O₁₂ (BNT) ceramics was much higher than that of Bi₄Ti₃O₁₂ (BIT) ceramics, sandwiched BaNd₂Ti₄O₁₂/Bi₄Ti₃O₁₂/BaNd₂Ti₄O₁₂ (BNT/BIT/BNT) composite ceramics were successfully prepared with this new method. The results of scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) showed that the BNT layers and the BIT layer were well bonded and no significant diffusion between them was observed. The temperature coefficient of dielectric constant of the laminated ceramic was found to be much smaller than that of BNT ceramic.     

0D/2D Z-scheme heterojunctions of g-C3N4 quantum dots/ZnO nanosheets as a highly efficient visible-light photocatalyst

Constructing Z-scheme heterojunctions comprising of constituents with different dimensionality is an effective strategy to spatially separate electron and hole. To fully utilize the synergistic coupling effect of dimensionality, herein, we first immobilize g-C₃N₄ quantum dots (CNQDs) onto ZnO nanosheets with oxygen vacancies (OV-ZnO) to create a 0D/2D hybrid via a facile and cost-effective approach. The CNQDs/OV-ZnO heterojunctions display CNQDs content-dependent performance in visible-light photocatalytic activity. The optimal CNQDs/OV-ZnO heterojunction exhibits high photocatalytic activity for degradation of methyl blue and bisphenol A, where the kinetic constant is 11.4 and 32.5 fold of pure OV-ZnO, respectively. Photoluminescence, electrochemical impedance spectroscopy and photocurrent verify that the photogenerated electron-hole pairs in this 0D/2D Z-scheme heterojunction have been effectively separated. The enhanced photocatalytic activity could be attributed to the synergistic effect of efficient Z-scheme charge separation, highly dispersed 0D CNQDs, coordinating sites of 2D OV-ZnO nanosheets and the strong coupling between them. In addition, the 3D flower-like structure constructed by 2D nanosheets greatly inhibits the leaching and loss of the photocatalyst in the recycling process, and ensures the high recycling ability of CNQDs/OV-ZnO. This work paves the way toward designing novel visible-light 0D/2D photocatalysts in the application of solar energy.     

非化学计量Bi2O3对制备Bi4Ti3O12粉体影响的研究

制备晶粒形貌生长各向异性的中间化合物Bi4Ti3O12粉体是采用流延成型和模板晶粒生长(TGG)等工艺方法制备(Na0.5Bi0.5)TiO3基无铅压电织构陶瓷的关键技术.以NaCl-KCl熔盐法制备了生长各向异性的Bi4Ti3O12粉体,利用X射线衍射和扫描电子显微镜研究了非化学计量Bi2O3对粉体相结构和微观形貌的影响,优化了制备Bi4Ti3O12粉体的工艺参数,并探讨了Bi4Ti3O12粉体在熔盐中的生长机理.研究表明,随着Bi2O3过量程度的增加,所得Bi4Ti3O12粉体颗粒的平均尺寸和均匀程度均增加,当Bi2O3过量7.5%时达到最佳值,其平均粒径为8～10 μm.     

硅衬底Bi_4Ti_3O_(12)和Bi_(3.25)La_(0.75)Ti_3O_(12)铁电薄膜的慢正电子束研究

对硅衬底生长的Bi4Ti3O12和Bi3.25La0.75Ti3O12薄膜样品测量了慢正电子多普勒展宽谱,得到了S参数随正电子注入能量的变化。通过对S参数和W参数的分析,讨论了这类材料中的捕获态特征和结构特点,结果表明,薄膜与硅衬底界面的缺陷为空位-氧复合体,La的掺杂有助于阻止空位-氧复合体向界面的扩散。     

Sintering and characterization of Bi4Ti3O12 ceramics

Polycrystalline ferroelectric Bi₄Ti₃O₁₂ ceramics have been prepared by the method of reactive liquid phase sintering. The sintering behaviour of the Bi₂O₃-TiO₂ composite was examined by plotting the isothermal densification curves. The results indicate that the starting oxides are involved in the reaction even at temperatures lower than or equal to 800°C, but the reaction advances at a very slow rate. Above solidus, the liquid phase promotes an extended reaction. Saturation observed in two densification curves, at 875 and 1100°C demonstrate that the reaction proceeds by two steps. A completion of the Bi₄Ti₃O₁₂ formation occurs after 60 min of sintering at 1100°C. Optical micrographs of sintered bismuth titanate ceramics show randomly oriented ferroelectric grains separated by a paraelectric intergranular layer. The Bi₄Ti₃O₁₂ crystallites exhibit a platelike morphology, similar in the appearance to mica, as evidenced by scanning electron micrographs. Isothermal annealing (750 to 950°C) does not affect the microstructure and electric properties of sintered bismuth titanate. The considerable value of dielectric permittivity and the appearance of hysteresis have been correlated to the presence of oxygen vacancies within the pseudotetragonal structure of Bi₄Ti₃O₁₂. The oxygen vacancies are preferentially sited in the vicinity of bismuth ions as evidenced by X-ray photoemission data. XPS and AES measurements confirm that the surface concentration of cations comprising the Bi₄Ti₃O₁₂ ceramics does not deviate from the nominal bulk composition.     

花瓣状Fe_3O_4/Bi_2O_3复合粒子的制备及光催化活性

用共沉淀法制备的Fe3O4纳米粒子作为种子,通过水热法获得了微米尺寸的Fe3O4/Bi2O3复合粒子。X射线衍射和X光电子能谱表征结果说明复合粒子是由Fe3O4和Bi2O3组成。扫描电子显微镜照片表明复合粒子形貌基本呈规则球形,并且具有花瓣状的三维多级结构。以罗丹明B的催化降解实验为模型考察了不同反应组成、不同反应介质、不同反应温度条件下制备的复合粒子的催化活性。结果表明,当反应条件中m(Bi(NO3)3·5H2O)/m(Fe3O4)为1.9 g∶0.2 g,水作反应介质在160℃时,所制备的复合粒子催化活性最高,对罗丹明B的降解率达95.4%。降解完成后,用磁铁吸附,Fe3O4/Bi2O3很快从体系中分离,可以重新催化降解罗丹明B,实现磁场控制的循环催化。实验发现,Fe3O4/Bi2O3经6次循环利用后,对罗丹明B的降解率仍达88.5%。     

Structural and electrical studies on Bi2VO5.5/Bi4Ti3O12 multilayer thin films

The textured multilayer (ML) thin films of bismuth layered ferroelectric (FE) compounds, Bi₂VO_5.5 (BVO) and Bi₄Ti₃O₁₂ (BTO) with different individual layer thicknesses were fabricated via pulsed laser deposition technique on Pt(111)/TiO₂/SiO₂/Si substrates. X-ray diffraction studies confirmed that BVO and BTO retained their respective crystal structures in these multilayer (ML) thin films. The atomic force microscopy and scanning electron microscopy studies showed smooth and dense microstructures. The polarization hysteresis (P?CE) studies on a representative (BVBT30) ML thin film at 300 K confirmed the remnant polarization (2P _r ) and coercive field (E _c ) to be ~20 ??C/cm² and 250 kV/cm, respectively. The value of P _r obtained was greater than that of the single layer thin film of BVO (P _r  ~ 5.6 ??C/cm²). The room temperature dielectric constant (??_r??) and the loss (D) for BVBT30 ML measured at 100 kHz were 170 and 0.01, respectively. The frequency and temperature dependent dielectric constant, impedance, modulus and ac conductivity of these ML thin films were studied as a function of frequency (100 Hz?C1 MHz) in the 25?C300 °C temperature range. Two distinct electrical responses were observed in these films, which were attributed to the grain effects at low temperatures and grain boundary effects at higher temperatures. The frequency dependent electrical conductivity was fitted well with the double power law which evidenced two different types of contributions to the conductivity; the low frequency conductivity being due to the short range translational hopping and the high frequency conductivity was due to the localized or reorientational hopping.     

Nb掺杂Bi4Ti3O12层状结构铁电陶瓷的电行为特性研究

采用固相烧结工艺制备了Nb⁵⁺掺杂的Bi₄Ti₃O₁₂层状结构铁电陶瓷.运用XRD 和AFM对Bi₄Ti_3-xNb_xO_12+x/2材料的微观结构进行表征,发现所制备的陶瓷均具有单一的正交相结构,抛光热腐蚀表面晶粒的显微形貌表现为随机排列的棒状结构.通过对材料直流电导率与温度关系的Arrhenius拟合,分析丁Bi₄Ti_3-xNb_xO_12+x/2的导电机理. Nb⁵⁺掺杂提高了材料的介电常数,但居里温度随掺杂含量的增加呈线性下降趋势.DSC结果显示Bi₄Ti_3-xNb_xO_12+x/2材料在居里温度处经历了一级铁电相变.样品的铁电性能测试结果表明, Nb⁵⁺掺杂Bi₄Ti₃O₁₂提高了材料的剩余极化Pr,这主要是由于^_Nb5+取代Ti⁴⁺大大降低了材料中氧空位的浓度,使得氧空位对畴的钉扎作用减弱的缘故.     

Preparation and sintering behaviour of submicronic Bi4Ti3O12 powders

Submicronic powders of Bi₄Ti₃O₁₂ with different morphologies were prepared by both the oxalate coprecipitation and the conventional mixing oxides methods. Compacts of the two calcined powders were sintered at 850–1100 °C in air, and the densification process was studied by non-isothermal and dilatometric experiments. A rapid densification (> 97% theoretical density) below 875 °C took place in the Bi₄Ti₃O₁₂ oxalate powder which was attributed to an extremely uniform pore-size distribution in the green compact. The possible formation of a transient liquid which promotes densification also was taken into account. The development of plate-like morphology in the conventional Bi₄Ti₃O₁₂ powder, broad pore-size distribution, and the plate-like colony formation, hindered rapid densification of the green compacts at low temperature. Microstructural development was studied; preliminary dielectric and electrical results are also reported.     

熔盐法合成板状钛酸铋微晶

采用熔盐法制备了板状Bi4Ti3O12微晶.用示差扫描量热-热重分析(DSC-TG)、X射线衍射(XRD)及扫描电镜(SEM)研究了不同温度对合成Bi4Ti3O12相结构与形貌的影响.结果表明,板状Bi4Ti3O12微晶的最佳合成温度为800～1000℃,低于800℃合成产物中容易出现杂相;1030℃时Bi4Ti3O12出现结构变化;1165℃出现杂相Na0.5Bi0.5Ti7O27;温度继续升高到1250℃时Bi4Ti3O12出现分解.实现板状Bi4Ti3O12微晶形貌可控的最佳温度范围为850～950℃.     

Pressure dependence of the Raman-active modes in Bi4Ti3O12

Bismuth titanate (Bi₄Ti₃O₁₂), a layer-type ferroelectric material crystallizing in the monoclinic system, has been investigated by high-pressure Raman technique up to 17 GPa in a diamond anvil cell. A pressure-induced phase transition occurs near 3 GPa, and the softening of the lowest Raman mode near 31 cm⁻¹ seems to be associated with this transition. Furthermore, the Raman data as well as absorption measurements indicate that a second phase transition occurs near 11 GPa. This transition may be due to a change from the ferroelectric to the paraelectric phase under the influence of pressure.     

Dielectric enhancement of the trilayered Bi3.15Eu0.85Ti3O12/Bi3.15Nd0.85Ti3O12/Bi3.15Eu0.85Ti3O12 thin film deposited on Pt/Ti/SiO2/Si substrate

The trilayered Bi_3.15Eu_0.85Ti₃O₁₂/Bi_3.15Nd_0.85Ti₃O₁₂/Bi_3.15Eu_0.85Ti₃O₁₂ (BET/BNT/BET) thin film was deposited on Pt/Ti/SiO₂/Si(100) substrates by metal organic decomposition at annealing temperature of 650 °C, and the microstructure, chemical composition, leakage current, dielectric and ferroelectric properties were investigated by field emission scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, semiconductor characterization system, impedance analyzer and ferroelectric tester. The trilayered thin film is of crack-free and dense surface with some discrete cluster distribution, and typical Bi-layered perovskite polycrystalline phase. The dielectric constant ε _r and dissipation factor tanδ are 1,233 and 0.0215 at 100 kHz for the trilayered thin film. Comparing with the pure BET and BNT thin films, the dielectric constant of trilayered thin film is enhanced, which is due to the space charge and the intermediate superlattice. The trilayered thin film shows excellent dielectric properties and can be promisingly used for the high dielectric layer of silicon-based embedded capacitors in package substrate.     

Properties of aurivillius phases in the Bi4Ti3O12-BiFeO3 system

A number of new Bi _m+1Fe _m?3TiO_3m+3 compounds with integer and fractional m values are identified in the Bi₄Ti₃O₁₂-BiFeO₃ system. All of the compounds are shown to have Aurivillius-type structures. The phase-transition and decomposition temperatures of the new and earlier known compounds of the Bi₄Ti₃O₁₂-BiFeO₃ system are determined. The linear thermal expansion coefficients of the synthesized compounds are evaluated using dilatometry, and their sintering onset temperatures are determined.     

Fabrication of Bi2WO6/In2O3 photocatalysts with efficient photocatalytic performance for the degradation of organic pollutants: Insight into the role of oxygen vacancy and heterojunction

Photocatalysis is an attractive and green strategy for organic pollutant removal. The development of alternative and effective photocatalysts has attracted great attention. Herein, we rationally engineer an alternative rich-oxygen vacancies (OVs) Bi₂WO₆/In₂O₃ composite photocatalyst via integrating the calcination and hydrothermal method for removing organic dyes (rhodamine B). Thanks to the synergistic effect of OVs and heterojunction structure, the 80 wt% Bi₂WO₆/In₂O₃ (BiIn80) displays enhanced photocatalytic degradation effect. The degradation rate of BiIn80 is up to 97.3% under light irradiation within 120 min and the reaction rate constant k value (0.03221 min⁻¹) is about 15-fold and 4.17-fold as high as those of In₂O₃ (0.00203 min⁻¹) and Bi₂WO₆ (0.00772 min⁻¹), respectively. The heterostructure of Bi₂WO₆/In₂O₃ can extend the lifespan of the photogenerated charge carriers. Moreover, the density functional theory (DFT) calculations reveal that the OVs in Bi₂WO₆/In₂O₃ can boost visible light absorbability by decreasing band gap value and serve as the extra electron transfer channels to enhance the separation efficiency of photogenerated electron-hole pairs. This study not only provides an alternative route for fabricating highly efficient heterojunction photocatalysts, but also obtains better understanding of the synergistic effect of OVs and heterojunction on enhancing the photocatalytic performance.     

Effects of a Bi2Ti2O7 seeding layer on properties of Bi3.5Nd0.5Ti3O12 thin film

A thin-film bilayer structure consisting of polycrystalline Bi_0.5Nd_0.5Ti₃O₁₂ (BNdT) and preferential (111)-orientated Bi₂Ti₂O₇ (BTO) has been prepared on n-type Si (100) substrate by a chemical solution deposition and spin-coating technique. The crystallization of BNdT film can be enhanced and the surface consists of larger grains because of the intervention of BTO. We fabricated the Au/BNdT/BTO/Si and Au/BNdT/Si structures, respectively. Compared with BNdT, the BNdT/BTO film shows better insulating and memory properties. The room temperature resistivity is 10¹¹ Ω cm at a dc bias of 5 V. The C–V hysteresis curve is referred to as polarization-type switching and the memory window is about 3 V. All these results show that the BTO is an effective diffusion-barrier and plays a brilliant role as a seeding layer.     

A位掺杂Bi4Ti3O12陶瓷的铁电性能研究

用传统固相烧结工艺,制备了纯相的Bi4Ti3O12 (BTO)陶瓷,A位掺杂的Bi3.25La0.75Ti3O12(BLT),Bi3.15Nd0.85Ti3O12(BNdT)铁电陶瓷.X射线衍射结果表明,所有样品均为单一的层状钙钛矿结构,La、Nd掺杂未改变BTO的晶体结构.铁电测试结果表明,BTO、BLT和BNdT陶瓷的剩余极化2Pr值分别为12.4、23.8和39.4μC/cm2.A位掺杂后,BLT、BNdT的2Pr值比未掺杂的BTO分别提高了1.92和3.18倍.漏电流测试表明,BLT、BNdT陶瓷的漏电流密度比BTO明显降低.A位掺杂显著提高了BTO陶瓷的铁电性能.     

La掺杂的Bi4Ti3O12陶瓷的动态变温X射线衍射研究

利用动态变温X射线衍射技术,研究了La掺杂Bi4Ti3O12（BLT）陶瓷相的形成过程、微结构的变化及其与温度的关系。动态跟踪比较了不同物相的反应速率与消长规律。比较了大气与真空不同的气氛环境对BLT相结构形成的影响。初步给出了固态成相的温度范围和所需时间。     

熔盐法制备片状钛酸铋粉体的研究

采用NaCl-KCl熔盐法制备了生长各向异性的片状Bi4Ti3O12粉体.研究了预烧温度及熔盐含量对粉体形貌及显微结构的影响.结果表明,750℃预烧所得Bi4Ti3O12粉体为单一的钙钛矿型结构,随熔盐含量的增加,Bi4Ti3O12粉粒的尺寸及生长的各向异性程度均有所增大,预烧温度的增加也促进了粉粒的生长,获得明显生长各向异性的钛酸铋粉体的最佳预烧温度为950～1000℃.