Effects of excess Bi2O3 on grain orientation and electrical properties of CaBi4Ti4O15 ceramics

A CaBi₄Ti₄O₁₅ (CBTO) ceramic in which the Bi₂O₃ concentration was controlled from 0 to 10 wt% was fabricated using a solid-state reaction method. Structural analysis by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) indicated differences in the preferred grain orientation and size of the plate-like grains according to the Bi₂O₃ concentration. The orientation of plate-like grains was also found to vary with the Bi₂O₃ concentration. There was no noticeable change trend of dielectric properties with different Bi₂O₃ concentrations. Relatively low dielectric constants (about 135) were exhibited by the CBTO ceramic with 1 wt% Bi₂O₃ and CBTO ceramic with 10 wt% Bi₂O₃ only, and similar values (about 150) were exhibited by the other ceramics. The dielectric loss exhibited a low value in the range of 0.01–0.09 for all samples (frequency range of 1–100 kHz). Regarding the ratio changes of the piezoelectric coefficient (d₃₃) and the ratio of a-axis orientation of plate-like grains, the trends of these two values were shown to be similar. These results suggest that the addition of Bi₂O₃ greatly influences the microstructure of CBTO ceramics, including the grain size and orientation of plate-like grains. In particular, the change in the preferred grain orientation is closely related to the change in the piezoelectric properties.     

Effect of amorphous TiO2 buffer layer on the phase formation of CaBi4Ti4O15 ferroelectric thin films

Amorphous buffer layers of TiO₂ were introduced between CaBi₄Ti₄O₁₅ thin films and Pt bottom electrodes through chemical solution deposition techniques. Several tens of nanometers thick alkoxy-derived TiO₂ layers were found to be critical for acceleration of the phase transition in the thin films. Unlike thin film crystallized directly onto a highly (111) oriented Pt bottom electrode, the thin film on an amorphous TiO₂ layer was almost single phase perovskite and showed random orientation. The crystallinity of the CaBi₄Ti₄O₁₅ thin film was much higher than that crystallized directly onto the Pt bottom electrode. The dielectric and ferroelectric properties of the CaBi₄Ti₄O₁₅/TiO₂ stacked thin films on Pt coated Si substrates are evaluated, leading to the potential of the amorphous buffer layer for the integrated devices being clarified. PACS 77.84.-s; 68.37.-d; 81.15.-z     

Microstructure and dielectric properties of pulsed-laser-deposited CaCu 3 Ti 4 O 12 thin films on LaNiO 3 buffered Pt/Ti/SiO 2 /Si substrates

CaCu₃Ti₄O₁₂ (CCTO) thin films have been prepared by a pulsed-laser-deposition method on LaNiO₃ buffered Pt/Ti/SiO₂/Si substrates, and their microstructure and dielectric properties have been compared with those of the films deposited directly on Pt/Ti/SiO₂/Si substrates. The crystalline structure and the surface morphology of the CCTO thin films were markedly affected by the bottom electrodes. Both the films show temperature-independent dielectric properties in a wide temperature range, which is similar to those properties obtained in single-crystal or epitaxial thin films, while the room-temperature dielectric constant of the 350-nm-thick CCTO films on LaNiO₃/Pt/Ti/SiO₂/Si substrates at 100 kHz was found to be 2300, which was increased significantly compared with that obtained in the films on Pt/Ti/SiO₂/Si substrates. Using the impedance spectroscopy technique, it has been suggested that the high dielectric constant response of the CCTO thin films originates from the grain boundary layer mechanism as found in internal barrier layer capacitors. PACS 77.55.+f; 81.15.Fg; 68.55.-a     

Improved electrical properties in La- and V-co-doped Na0.5Bi4.5Ti4O15 thin films

Ferroelectric La- and V-co-doped Na_0.5Bi_4.5Ti₄O₁₅ (NLBTV) thin film was prepared on Pt(111)/Ti/SiO₂/Si substrates by using a chemical solution deposition method and annealed at 750?°C under oxygen atmosphere. Crystal structure of the thin film was investigated by X-ray diffraction and Raman scattering. Surface morphology of the thin film was investigated by scanning electron microscopy. The NLBTV thin film capacitor exhibited better ferroelectric properties such as larger remnant polarization and smaller coercive electric field than Na_0.5Bi_4.5Ti₄O₁₅ (NBT) thin film capacitor. Reduced leakage current was observed in the NLBTV thin film capacitor compared to the NBT thin film capacitor. Almost no polarization fatigue was observed up to 1.44×10¹⁰ switching cycles.     

Mo掺杂SrBi4Ti4O15陶瓷的铁电介电性能

用传统的固相烧结工艺,制备了钼掺杂铁电陶瓷样品SrBi4Ti4O15(SBTi)铁电陶瓷SrBi4-2x/3Ti4-xMoxO15(x=0.00,0.003,0.012,0.03,0.06,0.09).X射线衍射的结果表明,样品均为单一的层状钙钛矿结构相,Mo掺杂未改变SBTi的晶体结构.通过扫描电子显微镜观测发现,样品晶粒为片状,随掺杂量的增加,晶粒逐渐变小.铁电测量表明,Mo掺杂使SBTi的铁电性能得到较大改善.随掺杂量x的增加,样品的剩余极化(2Pr)呈现出先增大,后减小的规律.在x=0.06时,2Pr达到最大值26.5 μC/cm2,与SrBi4TiO15(2Pr=12.2 μC/cm2)相比,提高117%.材料的矫顽场Ec在掺杂后增加仅为20%左右.SBTi的居里温度受掺杂的影响甚微,说明Mo对SrBi4Ti4O15的掺杂基本未影响材料原有的良好的热稳定性.     

Structure and magnetic properties of annealed metastable FeAg/Pt films

FeAg and FeAg/Pt films were prepared by dc magnetron sputtering at room temperature. The effects of Ag volume fraction in FeAg films and postannealing temperature and time on structure and magnetic properties of FeAg and FeAg/Pt films have been investigated. The results show that the as-deposited FeAg films are metastable. After annealing at 300°C, the phase separation of metastable FeAg films happened and the highest coercivity is obtained in Fe₅₀Ag₅₀/Pt film. With increasing annealing temperature, the ordering and the magnetic properties of the Fe₅₀Ag₅₀/Pt films were improved. When the Fe₅₀Ag₅₀/Pt films are annealed at 600°C for different annealing times, a long annealing time enhances the ordering of the metastable Fe₅₀Ag₅₀/Pt films and affects the orientation development. When the films are annealed for a long time, the grain size and the magnetic domain size also increase, which lead to an increase of correlation length due to the growth of FePt grains.     

Studies of structural and electrical properties on four-layers Aurivillius phase BaBi4Ti4O15

BaBi₄Ti₄O₁₅ (BBT) ceramic was synthesized using mixed oxide route and the structural and electrical properties were investigated systematically. The structural studies confirmed it to be an n=4 member of the Aurivillius oxide. A broad dielectric peak with frequency dependent dielectric maximum temperature was observed. The dielectric relaxation obeyed the Vogel–Fulcher relation wherein f₀=8.37E+14 Hz, E_a=0.13 eV, and T_f=608.18 K. The diffuseness parameter γ established the relaxor nature and it was attributed to the A-site cationic disorder. The specimen exhibited the excellent reproducibility in the measurements of displacement current, a remnant polarization of 5.4 μC/cm², and a coercive field of 4.03 MV/m. The room temperature piezoelectric coefficient d₃₃ was found to be 23 pC/N and the field-induced strain S was about 0.018% at the 8 MV/m electric field.     

La掺杂SrBi4Ti4O15铁电材料性能研究

按x=0 0 0 ,0 10 ,0 2 5 ,0 5 0 ,0 75和 1 0 0 ,采用固相烧结工艺 ,制备了不同La掺杂量的SrBi4-xLaxTi4O1 5的陶瓷样品 .用x射线衍射对其微结构进行了分析 ,并测量了铁电、介电性能 .结果发现 ,La掺杂未改变SrBi4Ti4O1 5的晶体结构 .随掺杂量的增加 ,样品的矫顽场 (Ec)下降 ,剩余极化 ( 2Pr)先增大 ,后减小 .在x =0 2 5时 ,2Pr 达到极大值 ,为2 4 2 μC·cm- 2 ,这时Ec=60 8kV·cm- 1 ,与SrBi4Ti4O1 5相比 ,2Pr 增加了近 5 0 % ,而Ec 下降了近 2 5 % ,材料铁电性能显著提高 .SrBi4-xLaxTi4O1 5的相变温度Tc 随x的增加逐渐降低 ,x =0 2 5时 ,Tc=45 1℃ .在x =0 75 ,1 0 0时 ,样品出现弛豫铁电体的典型特征     

Structure and piezoelectric properties of microporous polyvinylidene fluoride films

A method to produce microporous polyvinylidene fluoride films characterized by a high content of a piezoactive crystalline phase and a developed relief surface is elaborated. The porous films are obtained in the process based on melt extrusion with subsequent isometric annealing and uniaxial extension. It is found that the samples have an oriented lamellar structure containing both α- and β-modifications of the crystallites. The effect of temperature and degree of uniaxial extention of the annealed films on the characteristics of the porous samples, that is, the content of the piezoactive crystalline β-phase, overall porosity, breakdown voltage and the piezoelectric modulus is studied.

     

Dielectric and piezoelectric properties of the Ba0.92Ca0.08Ti0.95Zr0.05O3 thin films grown on different substrate

Lead free Ba_0.92Ca_0.08Ti_0.95Zr_0.05O₃ (BCZT) thin films were deposited on Pt/Ti/SiO₂/Si and LaNiO₃(LNO)/Pt/Ti/SiO₂/Si substrates by a sol–gel processing technique, respectively. The effects of substrate on structure, dielectric and piezoelectric properties were investigated in detail. The BCZT thin films deposited on LNO/Pt/Ti/SiO₂/Si substrates exhibit (100) orientation, larger grain size and higher dielectric tunability (64%). The BCZT thin films deposited on Pt/Ti/SiO₂/Si exhibit (110) orientation, higher Curie temperature (75 °C), better piezoelectric property (d₃₃ of 50 pm/V) and lower dielectric loss (0.02). The differences in dielectric and piezoelectric properties in the two kinds of oriented BCZT films should be attributed to the difference of structure, in-plane stress and polarization rotation in orientation engineered BCZT films.     

Quartz crystal microbalance measurements of O/Ti and CO/Ti atom ratios on very thin Ti films

The adsorption of O₂ and CO on Ti films with thicknesses in the range 0–200 Å was investigated by quartz crystal microbalance (QCM) measurements. The O/Ti and CO/Ti ratios in the limit of zero film thickness was determined to be 1.5 and 0.5 respectively using the film thickness as a parameter. This result suggests that the oxide formed is Ti₂O₃. The Ti + O₂ results are discussed in a model where the thinnest metal films are assumed to consist of metal islands. From the model an “oxidation depth” of ≈10 Å is estimated.     

(Bi, La)4Ti3O12-Sr(Bi, La)4Ti4O15共生结构铁电材料性能研究

采用固相烧结工艺,制备了不同La掺杂量(x=0.00,0.25,0.50,0.75,1.00,1.25和1.50)的(Bi, La)4Ti3O12-Sr(Bi, La)4Ti4O15 (SrBi8-xLaxTi7O27)共生结构铁电陶瓷样品.用x射线衍射对其进行微结构分析,并测量铁电、介电性能.结果发现,La掺杂未改变Bi4Ti3O12-SrBi4Ti4O15共生结构铁电材料的晶体结构.随掺杂量的增加,样品的矫顽场(Ec)略有增加,剩余极化(2Pr)先增大,后减小.在x＝0.50时,2Pr达到极大值,为25.6 μC*cm-2,与Bi4Ti3O12-SrBi4Ti4O15相比,2Pr增加了近60%,而Ec仅增加约10%.随La掺杂量的增加,样品的居里温度TC逐渐降低,x＝0.50时,TC=556 ℃.在x=1.50时,样品出现弛豫铁电体的典型特征.     

Structure and properties of ultrafast-quenched Sn-Bi foils

The effect of bismuth on the microstructure of ultrafast-quenched foils of Sn-Bi alloys has been investigated. Bismuth phase precipitation is observed in alloys containing more than 2 at % Bi. With an increase in the bismuth concentration to 8 at %, most precipitates acquire a needle-like shape. Cross-sectional inhomogeneity of the initial foils is revealed. It is found that addition of bismuth leads to an increase in the foil microhardness by a factor of 2.5.     

Multiferroic and resistive switching behaviors in BiFe0.95Cr0.05O3 thin films deposited on Pt/Ti/SiO2/Si substrates

Multiferroic and resistive switching properties of single-phase polycrystalline perovskite BiFe_0.95Cr_0.05O₃ (BFCO) thin films grown on Pt/Ti/SiO₂/Si substrates by radio-frequency magnetron sputtering were investigated. The BFCO film shows ferroelectric and magnetic properties simultaneously at room temperature, and also exhibits a good piezoelectric property with remanent effective piezoelectric coefficient d _33,f ～55±4 pm/V. An obviously resistive switching behavior was observed in the BFCO thin film at room temperature, which was discussed by the filamentary conduction mechanism associated with the redistribution of oxygen vacancies.     

Effects of repetitive polarization switching on the coercive voltage of Pt/Pb(Zr0.52Ti0.48)O3/Pt thin films analyzed using impedance spectroscopy

We investigated the effect of repetitive switching of polarization on the ferroelectric Pt/Pb(Zr_0.52Ti_0.48)O₃/Pt thin film capacitor by using impedance spectroscopy. From the Cole-Cole plot, the equivalent circuit is described as a combination of the bulk part (a capacitor), the interface part (the constant phase element (CPE), and a parallelly-connected resistor). The circuit parameters were analyzed at various stages of switching. An early increase and a subsequent decrease of the bulk capacitance may represent the wake-up and fatigue phenomena, respectively. The change in the interface part was characterized by an increase in resistance and the growth of n, the exponent of CPE, which may have come from a reduction of defects and the diminished inhomogeneity in the interfacial layer, respectively. The change in the resistance and the coefficient of the CPE in the interface part collectively resulted in an increase in the interfacial impedance. The coercive voltage, which may have intrinsically increased due to the repetitive switching, was even larger as a result of the increased interfacial impedance.     

Structure and magnetic properties of magnetron-sputtered [(Fe/Pt/Fe)/Au]n multilayer films

Using dc magnetron sputtering, Fe/Pt/Au multilayer films were prepared, and the effects of Au layer thickness and annealing temperature on structure and magnetic properties of the Fe/Pt/Au multilayer films were investigated. The as-deposited Fe/Pt/Au multilayer films have good periodic structure with composition modulation along the growth direction. The stress stored in the as-deposited films promoted the ordering of the films annealed at 400 °C. When the films were annealed at 500 °C, the thicker Au layer could restrain the order-disorder transformation region volume and lead to the decrease of the ordered volume fraction with Au layer thickness increasing.     

Structural and dielectric properties of Bi4Ti3O12 thin films prepared by metalorganic solution deposition

Ferroelectric Bi₄Ti₃O₁₂ thin films with single phase and nanosized microstructure were prepared on Pt/Ti/SiO₂/Si(111) substrate by metalorganic solution deposition using titanium butoxide and bismuth nitrate at relatively low annealing temperatures. The internal strain in Bi₄Ti₃O₁₂ thin films was calculated from the peak shifts and broadening of XRD patterns. With increase in annealing temperature, the uniform strain decreased from positive to zero and then to negative, and the non-uniform strain decreased and was negative. The total strain was negative and in the range of -0.2%–-1.0%, from which the stress of the films was calculated to be about -1.4×10⁹ N/m². The mode values of strain decreased with increase in annealing temperature and increased with increase in film thickness. The dielectric constant increased with increase in annealing temperature and film thickness. The dielectric properties were interpreted by considering the influence of strain, grain size, and grain boundaries. The strain lowered the polarization and increased the dielectric constant. The larger the grain size and the thinner the grain boundary, the greater the dielectric constant. The influence of grain size and grain boundary was stronger than that of the strain. Received: 23 September 1998 / Accepted: 6 January 1999 / Published online: 24 March 1999     

溶胶-凝胶法制备Sr2Bi4Ti5O18薄膜及其铁电性能研究

采用溶胶-凝胶法,在氧气氛中和层层晶化的工艺条件下,成功地制备了沉积在Pt/Ti/SiO2/Si(100)衬底上的铁电性能优良的Sr2Bi4Ti5O18(SBTi)薄膜,并研究了SBTi薄膜的微结构、表面形貌、铁电性能和疲劳特性.研究表明:薄膜具有单一的层状钙钛矿结构,且为随机取向;薄膜表面光滑,无裂纹,厚度约为725nm;铁电性能测试显示较饱和、方形的电滞回线,当外电场强度为275kV/cm时,其剩余极化2Pr和矫顽场2Ec分别为24.0μC/cm2和137.8kV/cm;疲劳测试发现薄膜经过4.4×1010次极化反转后,基本没有显示疲劳.     

Electrical performance of Ti/Pt thin films on glass-ceramic substrates after high temperature loading

In this study, the influence of post-deposition annealings (PDA) up to temperatures of T _PDA=700°C on the room-temperature resistivity of e-beam evaporated titanium/platinum (Ti/Pt) bi-layers on low temperature co-fired (LTCC) substrates covered with a glass encapsulate is investigated. The thickness of the platinum top layer is varied between 24 and 95 nm (titanium film thickness: 5 nm) and between 23 and 90 nm (titanium film thickness: 15 nm), respectively. In the “as-deposited” state and up to post-deposition annealing temperatures of T _PDA=450°C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. When applying, however, solely the Fuchs-Sondheimer model for evaluation, the effective mean free path for electrons is substantially above the value reported for crystalline platinum at room temperature. Compared to similar investigations on smooth Si/SiO₂ substrates yielding interpretable results within this theoretical approach, this is due to the increase of the thickness-dependent fraction in film resistivity which is strongly affected by the enhanced LTCC/glass surface roughness. At T _PDA>600°C, diffusion of titanium into the platinum top layer and the roughening of the LTCC/glass substrate dominate the electrical behavior, both causing an increase in film resistivity above average. In contrast to Si/SiO₂ substrates, thermal induced grooving effects in the Pt top layer play a minor role as the temperature coefficients of expansion of metallization and glass-ceramic substrate match better and the effective temperature difference for stress generation is lower due a glass softening temperature of about 450°C.     

Structure-related infrared optical properties of BaTiO3 thin films grown on Pt/Ti/SiO2/Si substrates

BaTiO₃ thin films with different thickness have been grown on Pt/Ti/SiO₂/Si substrates by a modified sol-gel method. X-ray diffraction analyses show that the BaTiO₃ thin films are polycrystalline. The crystalline quality of the films is improved with increasing thickness. The infrared optical properties of the BaTiO₃ thin films have been investigated using an infrared spectroscopic ellipsometry in the wave number range of 800-4000 cm⁻¹ (2.5-12.5 μm). By fitting the measured pseudodielectric functions with a three-phase model (Air/BaTiO₃/Pt), and a derived classical dispersion relation for the thin films, the optical constants and thicknesses of the thin films have been simultaneously obtained. The refractive index of the BaTiO₃ thin films increases and on the other hand, the extinction coefficient does not change with increasing thickness in the entirely measured wave number range. The dependence of the refractive index on the film thickness has been discussed in detail and was mainly due to both the crystalline quality of the films and packing density. Finally, the absorption coefficient was calculated in the infrared region for applications in the pyroelectric IR detectors.