Ab Initio Study of Doping Mechanisms in Ba TiO_3-BiYO_3

A density functional plane-wave pseudopotential method is used to study the doping mechanisms of impurity defects(Bi_(Ba),Y_(Ti)) in BaTi O_3-BiYO_3. Single Bi_(Ba) and Y_(Ti) impurities have little structure distortion. Bi forms ionic bond with nearby O atom in single Bi impurity, Y formed [YO_6] octahedral in single Y impurity. However, in the co-doped Bi_(Ba) and Y_(Ti) structure, Bi formed three valence bonds with nearby O atom, which causes the large structure distortion. The doped ion makes the mobile of Ti~(4+) difficult and loss local ferroelectricity, which will broaden the dielectric constant temperature curve and increase the temperature stability of BaTiO_3 ceramic matrix.     

Structure and Electric Properties of （1-x）（Bi1/2Na1/2） TiO3-xBaTiO3 Systems

The structural, dielectric and piezoelectric properties of （1-x）（Bi1/2Na1/2） TiO3-xBaTiO3 ceramics were investigated for the compositional range, x=0.02, 0.04, 0.06, 0.08, 0.10. The samples were synthesized by a conventional solid-state reaction technique. All compositions show a single perovskite structure, and X-ray powder diffraction patterns can be indexed using a rhombohedral structure. Lattice constants and lattice distortion increase while the amount of BaTiO3 increases. The X-ray diffraction results show the morphotropic phase boundary （MPB） of （1-x）（Bi1/2Na12） TiO3-xBaTiO3 exists in near x=0.06-0.08. Temperature dependence of dielectric constant eT33/ε0 measurement reveals that all compositions experience one structural phase and two ferroelectric phases transition below 400℃： rhombohedral （or rhombohedral plus tetragonal） ferroelectric phase ←→ tetragonal antiferroelectric phase ←→ tetragonal paraelectric phase. Relaxor behaviors exist in the course of ferroelectric to antiferroelectric phase transition. Dielectric and piezoelectric properties are enhanced in the MPB range for （ 1-x）（Bi1/2Na1/2）TiO3-xBaTiO3.     

Effect of La doping on microstructure and ferroelectric properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films prepared by sol-gel method

The Bi4Ti3Oi2 and Bi3.25La0.75Ti3O12 thin films were prepared on the Pt/Ti/SiO2/Si substrate using the sol-gel method. The effect of La doping on the microstructure and ferroelectric properties of Bi4Ti3O12 films were investigated. Both the Bi4Ti3O12 and Bi3.25La0.75Ti3O12 thin films exhibited typical bismuth layered perovskite structure. The 2Pr （remanent polarization） value of Bi3.25La0.75Ti3O12 thin films is 18.6 μC/cm^2, which is much larger than that of Bi4Ti3O12 thin films. And the Bi3.2eLa0.75Ti3O12 films show fatigue-free behavior, while the Bi4Ti3O12 thin films exhibit the fatigue problem. The mechanism of improvement of La doping was discussed.     

Preparation and ferroelectric properties of Bi<Subscript>3.4</Subscript>Ce<Subscript>0.6</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films grown by sol-gel method

We have investigated the preparation and properties of Bi_3.4Ce_0.6Ti₃O₁₂ thin films. The Bi_3.4Ce_0.6Ti₃O₁₂ thin films were fabricated on the Pt/Ti/SiO₂/Si substrates using sol-gel method. The structure and morphology of the films were characterized using X-ray diffraction and atomic force microscopy. The thin films showed a perovskite phase and dense microstructure. The dielectric constant and the dissipation factor of the Bi_3.4Ce_0.6Ti₃O₁₂ thin films were about 172 and 0.031 at 1 kHz, respectively. The 2P _r and 2E _c of the Bi_3.4Ce_0.6Ti₃O₁₂ thin films were 67.1 μC/cm² and 299.7 kV/cm, respectively, under an applied field of 600 kV/cm. The Bi_3.4Ce_0.6Ti₃O₁₂ film did not show fatigue up to 4.46×10⁹ switching cycles at a frequency of 1 MHz, and showed good insulating behavior according to the test of leakage current. Supported by the Natural Science Foundation of Hubei Province (Grants No. 2004ABA082)     

Dielectric properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics by impedance spectroscopic method

Various lead-free ceramics have been investigated in search for new high-temperature dielectrics. In particular, Bi₄Ti₃O₁₂ is a type of ferroelectric ceramics, which is supposed to replace leadcontaining ceramics for its outstanding dielectric properties in the near future. Ferroelectric ceramics of Bi₄Ti₃O₁₂ made by conventional mixed oxide route have been studied by impedance spectroscopy in a wide range of temperature. X-ray diffraction patterns show that Bi₄Ti₃O₁₂ ceramics are a single-phase of ferroelectric Bi-layered perovskite structure whether it is calcined at 800 °C or after sintering production. This study focused on the effect of the grain size on the electric properties of BIT ceramics. The BIT ceramics with different grain sizes were prepared at different sintering temperatures. Grain becomes coarser with the sintering temperature increasing by 50 °C, relative permittivity and dielectric loss also change a lot. When sintered at 1 100 °C, r values peak can reach 205.40 at a frequency of 100 kHz, the minimum dielectric losses of four different frequencies make no difference, all close to 0.027. The values of E _a range from 0.52 to 0.68 eV. The dielectric properties of the sample sintered at 1 100 °C are relatively better than those of the other samples by analyzing the relationship of the grain, the internal stresses, the homogeneity and the dielectric properties. SEM can better explain the results of the dielectric spectrum at different sintering temperatures. The results show that Bi₄Ti₃O₁₂ ceramics are a kind of dielectrics. Thus, Bi₄Ti₃O₁₂ can be used in high-temperature capacitors and microwave ceramics.     

Enhanced Ferroelectric Polarization in Laser-ablated Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Thin Films by Controlling Preferred Orientation

Polycrystalline Bi₄Ti₃O₁₂ thin films with various fractions of a-axis, c-axis and random orientations have been grown on Pt(111)/Ti/SiO₂/Si substrates by laser-ablation under different kinetic growth conditions. The relationship between the structure and ferroelectric property of the films was investigated, so as to explore the possibility of enhancing ferroelectric polarization by controlling the preferred orientation. The structural characterization indicated that the large growth rate and high oxygen background pressure were both favorable for the growth of non-c-axis oriented grains in the Bi₄Ti₃O₁₂ thin films. The films with high fractions of a-axis and random orientations, i e, f (a-sxis) = 28.3% and f (random) = 69.6%, could be obtained at the deposition temperature of 973 K, oxygen partial pressure of 15 Pa and laser fluence of 4.6 J/cm², respectively. It was also noted that the variation of ferroelectric polarization was in accordance with the evolution non-c-axis orientation. A large value of remanent polarization (2Pr = 35.5 μC/cm²) was obtained for the Bi₄Ti₃O₁₂ thin films with significant non-c-axis orientation, even higher than that of rare-earth-doped Bi₄Ti₃O₁₂ films.     

Effect of Constituent Core-sizes on Microstructure and Dielectric Properties of BaTiO<Subscript>3</Subscript>@(0.6Ba-TiO<Subscript>3</Subscript>-0.4BiAlO<Subscript>3</Subscript>) Core-Shell Material

The fundamental characteristics of varied initial core-sizes of BaTiO₃(BT) and its influential role on the morphology and dielectric properties of BaTiO₃@0.6BaTiO₃-0.4BiAlO₃(BT@0.6BT-0.4BA) ceramic samples were studied. Alkoxide sol-precipitation method was adopted as revised chemical route to synthesize the constituent “core” BT powders in a dispersed phase, whereas the distinctive initial nano-sized particles were affected by the pre-calcination temperatures (600-900 °C).The microstructure of the uncoated BT ceramics revealed an exaggerated grain growth with an optimized dielectric constant (ε_max >9 000) whilst the coated ceramics behaved otherwise (grain growth inhibited) when sintered at an elevated temperature. Regardless of the previously studied solubility limit (about 0.1%) of BT-BA samples, BT@0.6BT-0.4BA maintained a maximum dielectric constant (ε_max) ranging from 1 592 to 1 708 and tan δ less than 2% under a unit mole ratio at room temperature. In view of all these analyses, the initial nanometer sizes of the as-prepared BT-core powders combined with the increase effect of cation substitutions of Bi³⁺ and Al³⁺ in the shell content, induced the diffuse transition phase of BT@0.6BT-0.4BA composition.     

Phase Structure and Dielectric Properties of Doped BaTiO3 Ceramics

The effects of two rare earth oxides such as CeO2 and Sm2O3 on the phase structure and dielectric properties of BaTiO3 ceramic were investigated. Results indicate that the dielectric constant of this system will increase greatly with the increasing content of these two oxides, and Ce^4＋ substitutes for Ba^2＋ located at A-site in ABO3 structure. Quantitative XRD analysis shows that c/a ratio in the sample with addition of CeO2 will increase, which implies the increase of tetragonality in system, causing the augment of dielectric constant, and the decrease of the crystal＇s geometrical symmetry results in curie-temperature moving towards low temperature; Sm^3＋（0.096 nm） substitutes for Ba^2＋（0.135 nm） possessing larger radius in A-site and the electrovalency in A-site increases, the mutual effect is strengthened, so the polarization is enhanced, and the dielectric constant increases notablely.     

Low-temperature sintering and microwave dielectric properties of Ba<Subscript>2</Subscript>Ti<Subscript>3</Subscript>Nb<Subscript>4</Subscript>O<Subscript>18</Subscript> ceramics

The effects of CuO and H3BO3 additions on the low-temperature sintering,microstructure,and microwave dielectric properties of Ba2Ti3Nb4O18 ceramics were investigated.The addition of less amount of CuO (< 1 wt%) considerably facilitated the densification of Ba2Ti3Nb4O18 ceramics.Appropriate addition of H3BO3 (< 3.5 wt%) remarkably improved the microwave dielectric properties of ceramics.The addition of H3BO3 and CuO successfully reduced the sintering temperature of Ba2Ti3Nb4O18 ceramics from 1300 to 1050 ℃.B...     

Improved Breakdown Strength in (Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>)<Subscript>0.85</Subscript>Bi<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> Ceramics with Addition of CaZrO<Subscript>3</Subscript> for Energy Storage Application

(Ba_0.6Sr_0.4)_0.85Bi_0.1TiO₃ ceramics doped with x wt%CaZrO₃ (x= 0-10) were synthesized by solid-state reaction method. The effects of CaZrO₃ amount on the dielectric properties and structure of (Ba_0.6Sr_0.4)_0.85Bi_0.1TiO₃ ceramics were investigated. X-ray diffraction results indicated a pure cubic perovskite structure for all samples and that the lattice parameter increased till x=5 and then slightly decreased. A homogenous microstructure was observed with the addition of CaZrO₃. Dielectric measurements revealed a relaxor-like characteristic for all samples and that the diffusivity γ reached the maximum value of 1.78 at x=5. With the addition of CaZrO₃, the dielectric constant dependence on electric field was weakened, insulation resistivity enhanced and dielectric breakdown strength improved obviously and reached 19.9 kV/mm at x=7.5. In virtue of low dielectric loss (tan δ<0.001 5), moderate dielectric constant (ε_r >1 500) and high breakdown strength (E_b >17.5 kV/mm), the CaZrO₃ doped (Ba_0.6Sr_0.4)_0.85Bi_0.1TiO₃ ceramic is a potential candidate material for high power electric applications.     

Microstructures and fatigue-free properties of the La<Superscript>3+</Superscript> and Nd<Superscript>3+</Superscript> doped Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films prepared by modified sol-gel technique

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) and Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin films were fabricated on Pt/TiO₂/SiO₂/Si (100) substrates by a modified sol-gel technique. X-ray diffraction indicated that these films were of single phase with random polycrystalline orientations. The surface morphologies of the films were observed by scanning electron microscope, showing uniform, dense films with grain size of 50–100 nm. Well-saturated hysteresis loops of the films were obtained in metal-ferroelectric-metal type capacitors with Cu top electrodes at an applied voltage of 400 kV/cm, giving the remanent polarization (2P _r) and coercive field (2E _c) values of the films of 25.1 μC/cm² and 203 kV/cm for BLT, and 44.2 μC/cm² and 296 kV/cm for BNT, respectively. Moreover, these capacitors did not show fatigue behaviors after up to 1.75×10¹⁰ switching cycles at the test frequency of 1 MHz, suggesting a fatigue-free character. The influences of the La³⁺ and Nd³⁺ doping on the properties of the films were comparatively discussed. Supported by the National Key Basic Research and Development Program of China (Grant No. 2006CB932305) and the Natural Science Foundation of Hubei Province, China (Grant No. 2004ABA082)     

Structure and Electrical Properties of （Na0.5Bi0.5）0.94Ba0.06TiO3 Ceramicwith0.5wt% of MnO

The structure and electrical properties of （Na0.5Bi0.5）0.94Ba0.06TiO3 ceramic doped with 0.5 wt% of MnO were investigated in comparison with those of （Na0.5Bi0.5）0.94Ba0.06TiO3 ceramic. It was ascertained that the MnO addition did not cause remarkable change in crystal structure and microstructure. The MnO addition mainly displayed a hard effect on the electrical properties, an increase of coercive field （E） and mechanical quality factor （Qm） together with a decrease of dielectric constant （εr） and piezoelectric constant （d33）. An enhancement of electromechanical coupling factor （kp） with the MnO addition was obtained too. An essential relation between the piezoelectric properties and ferroelectric nature of the ceramics was detected. It was found that the piezoelectric properties of the ceramics highly depended on the corporative contribution of remanent polarization （Pt） and coercive field.     

Structure and Electric Conductivity of Ce-doped Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> Electrolyte Synthesized by Reverse Titration Chemical Coprecipitation

Ce-doped Bi₂O₃ nanopowders were prepared by reverse titration chemical coprecipitation from Bi³⁺ and Ce⁴⁺ containing aqueous solution. Techniques of X-ray diffraction (XRD), transmission electron microscopic (TEM) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the as-synthesized materials. The XRD patterns indicated that the peaks can be easily indexed to β-Bi₂O₃ and no diffraction peaks of Ce or other impurity phases were detected in the prepared samples. The calculated average crystalline size decreased from 31.72 to 11.96 nm when the Ce content increased from 1 wt% to 10 wt%. The morphology changed from flake-like into the spherical-like with increase in Ce content. The electric conductivity of Ce-doped Bi₂O₃ electrolyte was also investigated by two probe DC method. Conductivity analysis exhibited that the rate of conductivity increased with increasing Ce²⁺ ratio, when the Ce concentration was up to 5 wt%, the as-synthesized Ce-doped Bi₂O₃ electrolyte showed the maximum value of conductivity(0.295 S·cm^–1).     

Domain structure and defects of highly ordered Bi<Subscript>4</Subscript>Si<Subscript>3</Subscript>O<Subscript>12</Subscript> micro-crystals

Highly ordered Bi₄Si₃O₁₂ micro-crystals were prepared at normal atmosphere. Phase identification of the prepared crystals was accomplished by X-ray diffractometer (XRD). Domain structure and defects were characterized by environmental scanning electron microscopy (ESEM). XRD shows that the obtained micro-crystals are of eulytite structure with chemical formulation of Bi₄Si₃O₁₂. A highly ordered growth pattern is confirmed due to the faster growth of the {124} faces than that of the {204} faces by ESEM. The growing process of the domain structure is of pollen parent and filial generation pattern. The filial generations of Bi₄Si₃O₁₂ crystals are generated from the pollen parent. Cracks generate from the defect areas and propagate along the {124} faces due to their lower binding energy under a proper temperature gradient, contributing to the total transcrystalline fracture. It is confirmed that the generation and development of the voids in the crystal grains can be developed when unmatched dimensions of the two opposite faces are formed. And the development of the voids is dependent on the dimensions and orientations of the two opposite faces. Supported by the Innovation Research Team Funds of Shaanxi University of Science & Technology (Grant No. SUST-A04)     

Ferroelectric properties of Bi<Subscript>4</Subscript>Zr<Subscript>0.5</Subscript>Ti<Subscript>2.5</Subscript>O<Subscript>12</Subscript> thin films prepared on LaNiO<Subscript>3</Subscript> bottom electrode by sol-gel method

The Bi4Zr0.5Ti2.5O12 (BZT) thin films were fabricated on the LaNiO3 bottom electrode using sol-gel method. The structure and morphology of the films were characterized using X-ray diffraction, AFM and SEM. The results show that the films have a perovskite phase and dense microstructure. The 2Pr and 2Vc of the Pt/BZT/LaNiO3 capacitor are 28.2 μC/cm2 and 14.7 V respectively at an applied voltage of 25 V. After the switching of 1×1010 cycles, the Pr value decreases to 87% of its pre-fatigue values. The dielectric constant (ε) and the dissipation factor (tanδ) of the BZT thin films are about 204 and 0.029 at 1 kHz, respectively. The films show good insulating behavior according to the test of leakage current. The clockwise C-V hysteresis curve observed shows that the Pt/BZT/LaNiO3 structure has a memory effect because of the BZT film's ferroelectric polarization.     

Dielectric tunable properties of BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript>-doped Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> microwave composite ceramics

BaTi₄O₉-doped Ba_0.6Sr_0.4TiO₃ (BST) composite ceramics were prepared by the conventional solid-state reaction and their structure, dielectric nonlinear characteristics and microwave dielectric properties were investigated. The secondary phase of the orthorhombic structure Ba₄Ti₁₃O₃₀ is formed among BST composite ceramics with the increase of BaTi₄O₉. At the same time, a duplex or bimodal grains size distribution shows fine grains in a coarse grain matrix. The degree of frequency dispersion of dielectric permittivity below T _m is increased initially and then decreased with respect to BaTi₄O₉. As the BaTi₄O₉ content increases, the tunability of composite ceramics decreases, while the Q value increases. Interestingly, 70 wt% BaTi₄O₉-doped BST has a tunability ∼4.0% (under 30 kV/cm biasing) versus a permittivity ∼68 and quality factor ∼134.1 (at ∼3.2 GHz). Supported by the Ministry of Science and Technology of China through 973-project (Grant No. 2009CB623302), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (Grant No.707024), Shanghai Committee of Science and Technology (Grant No. 07DZ22302), and Shanghai Foundation Project under 06JC14070     

Effect of Pb substitution on the microstructure and ferroelectric properties of Ba<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>TiO<Subscript>3</Subscript> ceramic

Ferroelectric Ba0.7Sr0.3TiO3（BST） and partially Pb^2＋ substituted for Ba^2＋ ceramics （Ba0.7-xPbx）Sr0.3TiO3 （x=0.1-0.4, BPST） were prepared by using conventional solid-reaction method. XRD analysis shows that the samples microstructure changes from cubic phase to tetragonal one with the Pb^2＋ content increasing. ESEM analysis shows that the Pb^2＋ substituted samples have a denser and more uniform surface morphology than that of pure BST. Measured electrical properties suggest that the Pb^2＋ substitution for Ba^2＋ in the BST system enhances the ferroelectric performance obviously when x=0.2. In addition, the substitution increases the samples Curie temperature （To） r （Ba0.5Pb0.2）Sr0.3TiO3 ceramic has good ferroelectric properties measured at a maximal electric field of 30 kV/cm under the condition of room temperature. The corresponding saturated polarization （Ps）, remnant polarization （Pr） and coercive field （Ec） is respectively 15.687 μC/cm^2, 8.100 μ C/cm^2 and 6.611 kV/cm. The measured Tc of （Ba0.5Pb0.2）Sr0.3TiO3 is 117 ℃.     

Electrochemical preparation of V<Subscript>2</Subscript>O<Subscript>3</Subscript> from NaVO<Subscript>3</Subscript> and its reduction mechanism

Vanadium trioxide (V₂O₃) was directly prepared by NaVO₃ electrolysis in NaCl molten salts. Electrolysis products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The existing state and electrochemical behavior of NaVO₃ were also studied. The results indicated that V₂O₃ can be obtained from NaVO₃. VC and C were also formed at high cell voltage, high temperature, and long electrolysis time. During electrolysis, NaVO₃ was dissociated to Na⁺ and VO₃ ^? in NaCl molten salt. NaVO₃ was initially electro- reduced to V₂O₃ on cathode and Na₂O was released simultaneously. Na₂CO₃ was formed due to the reaction between Na₂O and CO₂. The production of C was ascribed to the electro-reduction of CO₃ ^2?. VC was produced due to the reaction between C and V₂O₃.     

Optimization of<Emphasis Type="Italic">p</Emphasis>-type segmented Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>/CoSb<Subscript>3</Subscript> thermoelectric material prepared by spark plasma sintering

1 IntroductionIt is well known that the performance of monolithicthermoelectric materials can be characterized as figure-of-meritZ,Z=α2σ/κ, where ,α,σandκare the See-beck coefficients ,the electrical conductivity andthe ther-mal conductivity,respectively.In the past years , greatefforts have been made inlookingforthermoelectric mate-rials with a higher Seebeck coefficient ,electrical conduc-tivity and alowerthermal conductivity[1-9].Butfor monol-ithic thermoelectric materials , their f…     

Study on BaTiO<Subscript>3</Subscript> films prepared by AC power microarc oxidation

BaTiO₃ films were prepared by microarc oxidation (MAO) with an in-house built alternating current (AC) power supply in Ba(OH)₂ solution in this study. The surface morphology, combinability with the substrate and phase composition of the films were investigated by XRD, SEM and TEM. The BaTiO₃ films were annealed at a temperature range of 900 to 1200°C and phase compositions were tested thereafter. The results showed that the BaTiO₃ films were mainly composed of the primitive hexagonal phase with relatively small amount of tetragonal and amorphous phases. Moreover, the amount of amorphous phase decreased with the time of annealing. The transformation of BaTiO₃ from hexagonal structure to tetragonal stucture became obvious until the annealing temperature reached 1200°C. The film consisted mainly of BaTiO₃ tetragonal structure with ferroelectric property. The influence of annealing on surface morphology and mechanism of phase transformation of the films were also discussed. Supported by Guangdong Natural Science Foundation (Grant No. 0500649)