溶胶-凝胶法合成的Fe-Al氧化物纳米复合磁性颗粒

利用溶胶-凝胶法合成了Fe-Al氧化物纳米复合磁性颗粒.X射线衍射(XRD)分析表明,合成的产物为多相Fe-Al氧化物纳米复合磁性颗粒.利用振动样品磁强计(VSM)测量了所制得样品的磁化曲线,并由此得出了样品的饱和磁化强度和矫顽力.实验结果表明,在同一热处理温度下,当样品中Fe元素与Al2O3的质量比越大,所制得的Fe...     

Structural,dielectric and magnetic properties of particulate composites of relaxor (BaTi<Subscript>0.85</Subscript>Sn<Subscript>0.15</Subscript>O<Subscript>3</Subscript>) and ferrite (NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>) synthesized by gel-combustion method

Particulate composites of (1-x) BaTi_0.85Sn_0.15O₃ – x NiFe₂O₄ (with x?=?5, 10,15 and 20 wt%) were synthesized using the solid-state reaction method by sintering at 1350 °C for 4 h. Formation of the diphase composites was confirmed by X-ray diffraction (XRD) and Fourier Transform Infra-red (FTIR) techniques. Temperature (RT-200 °C) and frequency (20 Hz- 1 MHz) dependent of AC conductivity, dielectric constant and dissipation have been studied. The dielectric constant exhibits strong frequency dispersion in the range 20 Hz-1 kHz which is attributed to Maxwell-Wagner interfacial polarization occurring at grain-grain-boundaries interface/interface of grains of BTS-NF. The M-H curve of all the composites exhibited a hysteresis loops typical charcateistic of a ferromagnetic material. The ferromagnetic ordering in the composites on account of NiFe₂O₄ as a constituent is explained using bound magnetic polarons (BMPs) model. The experimental magnetic data have been fitted to BMP model. Value of M_s is smaller, whereas of H_c and M_r are higher of the composites compared to value for NiFe₂O₄. The temperature at which divergence in the M vs. T plot in ZFC and FC starts is higher for the composites than for NiFe₂O₄.     

Dielectric and Piezoelectric Properties of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-K<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-NaNbO<Subscript>3</Subscript>Lead-Free Ceramics

The ternary lead-free piezoelectric ceramics system of (1 – x) [0.88Na_0.5Bi_0.5TiO₃-0.12K_0.5Bi_0.5TiO₃] – xNaNbO₃(x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) were synthesized by conventional solid state reaction method. The crystal structure, dielectric, piezoelectric properties and P-E hysteresis loops were investigated. The crystalline structure of all compositions is mono-perovskite phase ascertained by XRD, and the lattice constant was calculated from the XRD data. Temperature dependence of dielectric constant _r and dissipation factor tan measurement revealed that all compositions experienced two phase transitions: from ferroelectric to anti-ferroelectric and from anti-ferroelectric to paraelectric, and these two phase transitions have relaxor characteristics. Both transition temperatures T_d and T_m are lowered due to introduction of NaNbO₃. P-E hysteresis loops show that 0.88Na_0.5Bi_0.5TiO₃-0.12K_0.5Bi_0.5TiO₃ ceramics has the maximum P_r and E_c corresponding to the maximum values of electromechanical coupling factor K_p and piezoelectric constant d_33. The piezoelectric constant d₃₃ and electromechanical coupling factor K_p decrease a little, while the dielectric constant ₃₃^T/₀ improves much more when the concentration of NaNbO₃ is 8 mol%.     

Electrical conductivity of the Al-stabilized La<Subscript>2/3</Subscript>TiO<Subscript>3</Subscript>

A-site deficient lanthanum titanate (La_2/3TiO₃) materials with perovskite structure are attractive due to their electrical applications such as ion conductors and dielectrics. However, its stability at room temperature in air is obtained only if Na or Li etc. is incorporated into La site or Al into Ti site. In this study, the electrical conductivities of La_0.683(Ti_0.95Al_0.05)O₃ have been measured in oxygen partial pressure (Po₂) between 1 and 10⁻¹⁸ atm at 1000~1400°C. The electrical conductivity exhibited −1/4, −1/6 and −1/5 dependence (log σ ∝ log , n = −1/4, −1/6, −1/5) depending upon temperature and Po₂. The defect model explaining the observation was proposed and discussed. The chemical diffusion coefficient was estimated from the electrical conductivity relaxation.     

The effect of TiO<Subscript>2</Subscript> and P<Subscript>2</Subscript>O<Subscript>5</Subscript> on densification behavior and properties of Anortite-Diopside glass-ceramic substrates

In this work the synthesis of anorthite-diopside glass-ceramics (GCs) was carried out via sintering and crystallization of glass powder compacts in the temperature interval 800°C and 950°C. Glass powder compacts with mean particle size of 2 μm were prepared. The effects of adding TiO₂ and P₂O₅ on the sintering behavior of glass powder compacts and on the properties of resultant glass-ceramics were studied. Mechanical, thermal, chemical and dielectric properties of sintered GCs were investigated with the aim to evaluate the potential of the GCs as substrate materials for microelectronics applications.     

Dielectric Properties and Sintering Characteristics of CaTiO<Subscript>3</Subscript>-(Li<Subscript>1/2</Subscript>Nd<Subscript>1/2</Subscript>)TiO<Subscript>3</Subscript> Ceramics

The dielectric properties and the sintering effect upon microstructure of (1–x) CaTiO₃-x(Li_1/2Nd_1/2)-TiO₃ Ceramics are investigated in this paper. Nd³⁺ and Mg^{2 +} ions co-substitution for Ca^{2 +} on A site improves the sintering characteristic of CaTiO₃ ceramics with forming orthorhombic perovskite structure. The structure of (1 – x) CaTiO₃-x(Li_1/2Nd_1/2)TiO₃ changes from orthorhombic to tetragonal as (Li_1/2Nd_1/2)TiO₃ addition increasing. Limited solubility of (Li_1/2Nd_1/2)TiO₃ in CaTiO₃ forming a part solid solution compound achieves the adjustment of for CaTiO₃ at low sintering temperature. The proper dielectric properties with = 78, tan = 0.0006, = +7 ppm/C are obtained for 0.8Ca_0.67(Nd,Mg)_0.22TiO₃-0.2(Li_1/2Nd_1/2)TiO₃ ceramics.     

Enhancement of thermoelectric performance in rare earth-doped Sr<Subscript>3</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> by symmetry restoration of TiO<Subscript>6</Subscript> octahedra

Aiming at the realization of the enhancement of thermoelectric performance through structural modification, the present work has clarified the significant effects of rare earth (RE=Gd, Sm, Nd, and La) doping at Sr-sites in Sr₃Ti₂O₇, both on the structural restoration of distorted TiO₆ octahedra and on the Seebeck coefficient, especially at high temperatures. The preferential substitution of RE ³⁺ at the nine-coordinate Sr-sites can facilitate the degeneration of the conduction band (Ti 3d-t _2g ) orbital, owing to its special capability in restoring TiO₆ octahedra to a higher state of symmetry and thus enhance the density of states (DOS) effective mass of the carriers, which gives rise to a rather large increase in the Seebeck coefficient. The present findings have affirmed the effectiveness of structural restoration in enhancing the Seebeck coefficient by Sr-site-doping, which will help establish a useful solution for Ti-based thermoelectric oxides with inherently distorted TiO₆ octahedra to achieve high thermoelectric performance.     

Effects of MnO<Subscript>2</Subscript> addition on microstructure and electrical properties of (Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>) <Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>TiO<Subscript>3</Subscript> ceramics

In this letter, MnO₂-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (BNBT-6) lead-free piezoelectric ceramics were synthesized by solid state reaction, and the microstructure and electrical properties of the ceramics were investigated. X-ray diffraction (XRD) reveals that all specimens take on single perovskite type structure, and the diffraction peaks shift to a large angle as the MnO₂ addition increases. Scanning electron microscopy shows that the grain sizes increases, and then decreases with increasing the MnO₂ content. The experiment results indicate that the electrical properties of ceramics are significantly influenced by the MnO₂ content, and the ceramics with homogeneous microstructure and excellent electrical properties are obtained with addition of 0.3 wt% MnO₂ and sintered at 1160°C. The piezoelectric constant (d₃₃), the electromechanical coupling factor (k _p ), the dissipation factor (tan δ) and the dielectric constant (ɛ _r ) reach 160 pC/N, 0.29, 0.026 and 879, respectively. These excellent properties indicate that the MnO₂-doped BNBT-6 ceramics can be used for actuators.     

Raman and infrared spectroscopy of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> - BaTiO<Subscript>3</Subscript> ceramics

Lead-free Na_0.5Bi_0.5TiO₃ -BaTiO₃ ceramics have been prepared in the whole range of concentrations and studied at room-temperature by means of X-ray, Raman scattering and infrared techniques. X-ray measurements revealed rhombohedral, rhombohedral-tetragonal boundaries and tetragonal modifacations depending on the contents of BaTiO₃. The distinct changes of the Raman and infrared spectra with increasing of BaTiO₃ content, which were correlated with X-ray results, were observed. The broad phonon spectra indicated the disorder in the A site of Na_0.5Bi_0.5TiO₃ -BaTiO₃ system.     

Improvement in microwave dielectric properties of Sr<Subscript>2</Subscript>TiO<Subscript>4</Subscript> ceramics through post-annealing treatment

Sr₂TiO₄ ceramics were synthesized via the conventional solid-state reaction process, and the effects of post-annealing treatment in air on the microwave dielectric properties and defect behavior of title compound were investigated systematically. The Q?×?f values could be effectively improved from 107,000 GHz to 120,300 GHz for the specimens treated at 1450 °C for 16 h. The thermally stimulated depolarization currents (TSDC) revealed two kinds of defect dipoles [\( \left({\mathrm{Ti}}_{\mathrm{Ti}}^{\hbox{'}}-{V}_{\mathrm{O}}^{\bullet \bullet}\right) \) and \( \left({V}_{\mathrm{Sr}}^{"}-{V}_{\mathrm{O}}^{\bullet \bullet}\right) \)] and oxygen vacancies \( \left({V}_{\mathrm{O}}^{\bullet \bullet}\right) \) were considered the main defects in Sr₂TiO₄. Under a post-annealing treatment in air, the concentrations of such defects in the ceramics decreased. Meanwhile, the impedance spectrum revealed the activation energy of the grain boundaries increased. These evidences could account for the improvement of Q?×?f values. Accompanied with a high ε_r of 40.4 and a large τ_f of 126 ppm/°C, the enhanced high-Q Sr₂TiO₄ ceramics can be good candidates for applications in wireless passive temperature sensing.     

N-doping effects on the oxygen sensing of TiO<Subscript>2</Subscript> films

Gas sensors based on Titanium dioxide films have been a subject of interest due to its high sensitivity and low cost. The sensing mechanism of this type of sensors is based on the conduction mechanism, which is governed by the potential barriers formed at the inter-grains of the polycrystalline structure. The shape of these potential barriers strongly depends on the characteristics of the material, and then it is expected that a doping aggregation will affect significantly the conductivity and thus the sensitivity of the sensor. In this work, we study the effect in the oxygen sensitivity of titanium dioxide films due to N-doping. We developed a model in order to explain our experimental results based on the fact that, for the particle size of our titanium dioxide samples, grains are completely depleted of carriers.     

Formation of a KNbO<Subscript>3</Subscript> single crystal using solvothermally synthesized K<Subscript>2-m</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6-m/2</Subscript> pyrochlore phase

A K_2-mNb₂O_6-m/2 single crystal with a pyrochlore phase formed when the Nb₂O₅?+?x mol% KOH specimens with 0.6?≤?x?≤?1.2 were solvothermally heated at 230 °C for 24 h. They have an octahedral shape with a size of 100 μm, and the composition of this single crystal is close to K_1.3Nb₂O_5.65. The single-crystal KNbO₃ formed when the single-crystal K_2-mNb₂O_6-m/2 was annealed at a temperature between 600 °C and 800 °C with K₂CO₃ powders. When annealing was conducted at 600 °C (or with a small amount of K₂CO₃), the KNbO₃ single crystal has a rhombohedral structure that is stable at low temperatures (< ? 10 °C). The formation of the rhombohedral KNbO₃ structure can be explained by the presence of the K⁺ vacancies in the specimen. The KNbO₃ single crystal with an orthorhombic structure formed when the K_2-mNb₂O_6-m/2 single crystal was annealed at 800 °C with 20 wt% of K₂CO₃.     

Multiferroic,magnetoelectric and magneto-impedance properties of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/(Pb,Sr) TiO<Subscript>3</Subscript> bilayer films

We prepared NiFe₂O₄/(Pb, Sr)TiO₃ (NFO/PST) bilayer films by the chemical solution method and investigated their multiferroic, magnetoelectric and magneto-impedance properties. Multiferroic properties have been observed at room temperature. The bilayer films exhibit saturation polarization P _s ? 26.6 μC/cm² and saturation magnetization M _s ? 134 emu/cm³. With increasing Sr content, M _s. and P _s values of the NFO/PST bilayer films decrease. The variation may be ascribed to the influence of interfacial strain and decrease in tetragonality. High magnetoelectric coupling effect has been observed in the NFO/PST bilayer films with maximum value of α _E  = 6.35 Vcm^?1 Oe^?1 measured at H _DC  ~ 1 kOe and f ~ 10 kHz for Sr ~ 10% (NFO/PST10). The magneto-impedance measurements establish a strong dependence on magnetic field, further confirming magnetoelectric response in NFO/PST bilayer films. An alternative approach for impedance analysis of NFO/PST10 bilayer film provides direct evidence of strain mediated magnetoelectric coupling at room temperature. The results indicate that NFO/PST bilayer films can be considered as a potential multiferroic magnetoelectric material.     

The Similar Defect Chemistry of Highly-Doped SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> and SrBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>

The equilibrium electrical conductivities of undoped SrBi₂Ta₂O₉ (SBT) and SrBi₂Nb₂O₉ (SBN) have been shown to behave quite differently. SBT has the behavior expected for a 1% acceptor-doped oxide, while SBN behaves like a 1% donor-doped oxide. This difference has been related to the substantial cation place exchange that occurs between the Bi^{+ 3} and Sr^{+ 2} ions in the alternating layers of the structure. It was proposed that this place exchange is not entirely self-compensating, as would be expected for a simple, isotropic oxide, but that there is some local compensation within each layer by lattice and/or electronic defects. It is now shown that the equilibrium conductivity of 3% donor-doped SBT is similar to that of undoped SBN, while the equilibrium conductivity of 3% acceptor-doped SBN resembles that of undoped SBT. Thus the defect chemistrys of the two compounds are quite similar, but the equilibrium conductivities are displaced along a doping axis.     

Improvement of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Co<Subscript>2</Subscript>O<Subscript>y</Subscript> thermoelectric performances by Na doping

Bi₂Sr_2-xNa_xCo₂O_y thermoelectric materials with x = 0, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15 have been prepared by the classical solid state reaction. Microstructure has shown an important grain growth when Na is added, leading to very high bulk densities confirmed through density measurements. These modifications have produced a drastic decrease of electrical resistivity without significant modification of Seebeck coefficient. As a consequence, Power Factor has been increased in all Na-doped samples, reaching the maximum value (0.21 mW/K².m at 650 °C) for 0.075 Na samples, which is fairly close to the reported for single crystals.     

Phase transition,microstructure and properties of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)<Subscript>1-x</Subscript>Ba<Subscript>x</Subscript>TiO<Subscript>3</Subscript> lead-free piezoelectric ceramics

In this article, (Na_0.5Bi_0.5)_1-xBa_xTiO₃ lead-free piezoelectric ceramics were prepared by solid-state reaction. The influence of Ba contents on phase structures, compositional distribution and electrical properties of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ ceramics were systematically investigated to further understand the nature of phase transition. It was found that the phase structure of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ transforms from rhombohedral to tetragonal symmetry at x = 0.06 ~ 0.07 and Ba²⁺ segregation forms the coexistence of Ba-rich tetragonal and Ba-deficient rhombohedral phases close to MPB. The electrical properties of prepared samples regularly changed with Ba content, which is closely related to the distribution of rhombohedral and tetragonal phases. The prepared sample near MPB exhibited the largest dielectric constant and the excellent piezoelectric properties (the maximal measuring field reached 78 kV/cm and the piezoelectric constant d ₃₃ = 151pC/N).     

Properties and crystallization kinetics of low temperature co-fired Li<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> electroceramics

The glass-ceramic in the Li₂O-Al₂O₃-SiO₂ system has been prepared by melt quenching route. The crystallization kinetics was studied by differential scanning calorimetry. The effects of sintering temperature on the phase transformation, sintering behavior, bulk density, microstructure, thermal expansion, bending strength and dielectric properties were also investigated by X-ray diffractometry and scanning electron microscopy. (Li, Mg, Zn)_1.7Al₂O₄Si₆O₁₂ is the first crystalline phase forming in the glass-ceramic and transforms to LiAlSi₃O₈ phase at 800 °C. The other two crystalline phases of ZrO₂ and CaMgSi₂O₆ precipitate at 700 and 750 °C, respectively. The densification of this LAS glass-ceramic starts at around 730 °C and stops at about 805 °C. The coefficient of thermal expansion increases with the increasing sintering temperature. The sample sintered at 800 °C for 30 min exhibited excellent properties. The nonisothermal activation energy of crystallization is 149 kJ/mol and the values of Avrami constant (n) are in the range of 3.2 to 3.9. The LAS glass-ceramic sintered at 800 °C for 30 min showed excellent properties. This makes that this material suitable for a number of LTCC applications.     

Microwave Dielectric Properties of CuO-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Doped ZnNb<Subscript>2</Subscript>O<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

Microwave dielectric properties of low temperature sintering ZnNb₂O₆ ceramics doped with CuO-V₂O₅-Bi₂O₃ additions were investigated systematically. The co-doping of CuO, V₂O₅ and Bi₂O₃ can significantly lower the sintering temperature of ZnNb₂O₆ ceramics from 1150 to 870C. The secondary phase containing Cu, V, Bi and Zn was observed at grain boundary junctions, and the amount of secondary phase increased with increasing CuO-V₂O₅-Bi₂O₃ content. The dielectric properties at microwave frequencies (7–9 GHz) in this system exhibited a significant dependence on the relative density, content of additives and microstructure of the ceramics. The dielectric constant ( _r) of ZnNb₂O₆ ceramics increased from 21.95 to 24.18 with increasing CuO-V₂O₅-Bi₂O₃ additions from 1.5 to 4.0 wt%. The quality factors (Q× f) of this system decreased with increasing CuO-V₂O₅-Bi₂O₃ content and ranged from 36118 to 67100 GHz for sintered ceramics, furthermore, all Q× f values of samples with CuO-V₂O₅-Bi₂O₃ additions are lower than that of un-doped ZnNb₂O₆ ceramics sintered at 1150C for 2 h. The temperature coefficient of resonant frequency ( _f) changed from –33.16 to –25.96 ppm/C with increasing CuO-V₂O₅-Bi₂O₃ from 1.5 to 4.0 wt%     

Influence of substitution on dielectric diffuseness in Ba<Subscript>(1-x)</Subscript>Bi<Subscript>(2+2x/3)</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics prepared by chemical precursor decomposition method

Barium bismuth niobate, Ba_(1-x)Bi_(2+2x/3)Nb₂O₉ (BBN with x = 0.0, 0.1, 0.2, 0.3, 0.4) ceramic powders in the nanometer range were prepared by chemical precursor decomposition method (CPD). The single phase layered perovskite was prepared throughout the composition range studied. No intermediate phase was found during heat treatment at and above 600°C. The crystallite size and the particle size, obtained from XRD and TEM respectively, were in the range of 15–30 nm. The addition of Bi₂O₃ substantially improved the sinterability associated with high density (96%) which was otherwise difficult in the case of pure BaBi₂Nb₂O₉ (BBN x = 0.0). The sintering was done at 900°C for 4 h. The relative permittivity of BBN ceramics at both room temperature and in the vicinity of the temperature of maximum permittivity (T_m) has increased significantly with increase in bismuth content and loss is also decreased to a certain level of bismuth doping. T_m increased with increase in Bi₂O₃. The diffuseness (γ) in the phase transition was found to increase from 1.54 to 1.98 with the increase in Ba²⁺ substitution level from x = 0.0 to x = 0.3.     

Theoretical analysis of band alignment and charge carriers migration in mixed-phase TiO<Subscript>2</Subscript> systems

Photocatalysts based on mixtures of rutile and anatase forms of titania usually show a better catalytic performance than each individual component. In order to understand this behavior, several experimental and theoretical approaches have been proposed in the past, looking for an adequate reference frame for aligning energy bands, and arriving sometimes to opposite results. In this work, the theoretical results obtained for the band alignment applying a modified common anion rule for different possibilities of mixed-phase (anatase–rutile) interaction are presented. According to our results, mixed-phase systems involve the transfer of electrons from rutile to anatase and holes from anatase to rutile. This analysis would be applicable to real samples of mixed phase of titania with large particle size. However, for heterogeneous size particulate systems, it is not only necessary to consider the alignment of bands of the bulk system, but also those of the corresponding surfaces. In keeping with the analysis performed, the best mixed systems are those composed by large particles of both polymorphs or by small particles of anatase dissolved in rutile. Our results could explain the disagreement found in the literature regarding the experimental works.