钽掺杂对钛酸钡基陶瓷介电性能影响的研究

研究了微量钽掺杂对钛酸钡基陶瓷介电常数、介电损耗以及温度稳定性等介电性能的影响.利用XRD、SEM等现代分析手段分析了材料的显微结构,分析了显微结构与材料介电性能的关系,得到了性能较为优良的电容器基料配方.     

Lattice Constants and Dielectric Properties of Barium Titanate—Barium Stannate—Strontium Titanate Bodies

Lattice constants and temperatures corresponding to maximum dielectric permittivity were investigated for compositions in the barium titanate-rich portion of the system barium titanate–barium stannate–strontium titanate. Substitution of tin for titanium increases the lattice constant at room temperature, and substitution of strontium for barium decreases this parameter. Both types of substitution decrease the Curie temperature; tin is about twice as effective as strontium in this regard. Properties of bodies containing both tin and strontium are intermediate between those of bodies containing either alone.     

Effect of Direct-Current Biasing on the Dielectric Properties of Barium Strontium Titanate

Ba_{1− x} Sr _x TiO₃ (BST, x = 0 to x = 1) ferroelectric ceramics doped with 1.0 mol% MgO and 0.05 mol% MnO₂ were prepared with a rate-controlled sintering profile. As the strontium molar fraction x increased, the average grain size decreased from 14 μm for x = 0 to 2 μm for x = 1. Temperature dependence of the dielectric properties was measured as a function of dc biasing. At 10°C above the Curie temperature, the changes of dielectric constant at 5000 V/cm were 28%, 17%, 26%, and 36% for x = 0, 0.25, 0.5, and 0.75, respectively. The reduction of dielectric constant by the applied dc biasing was fitted by a phenomenological equation that was based on Devonshire's theory. From this phenomenological equation, an anharmonic coefficient, which was an order parameter of the anharmonic interactions, was calculated for each composition. As the strontium molar fraction increased, these coefficients increased from 2.45 × 10⁻¹⁹ cm²V⁻² for x = 0 to 5.90 × 10⁻¹⁹ cm²V⁻² for x = 0.75. A similar trend was observed on the dc field dependence of the dielectric loss, except at high field for x = 0, of which a high loss was obtained. A two-element circuit model was proposed to explain the effect of high dc biasing on the dielectric loss.     

Crystallographic Facetting in Sintered Barium Titanate

A commercial TiO₂-excess BaTiO₃ powder has been sintered and its microstructure analyzed for crystallographic facetting via both scanning and transmission electron microscopy (SEM and TEM). Facetted grain surfaces are developed initially from {111} at a low temperature of 1215°C, which are then altered to {111} and {100} at 1290°C in the presence of a grain-boundary liquid phase. The grain shape is also modified correspondingly from platelike to polygonal. Facetting of the intragranularly located residual pores in BaTiO₃ along the {141} planes further develops on the (quasi-)equilibrium shape after annealing at 1400°C for 100 h from the initially well-characterized {111}, {110}, and {100} in as-sintered samples sintered at the same temperature for 10 h. The Wulff plots derived from the residual pores in as-sintered and annealed samples are constructed for the 〈011〉 zone. Microstructural analysis also suggests that the shape of grains and intragranular residual pores is modified progressively upon annealing. The initial solid–vapor surface energy has become less anisotropic crystallographically. Abnormal grain growth in relation to the surface energy anisotropy is discussed.     

Microwave Dielectric Properties of Gallium-Doped Hexagonal Barium Titanate Ceramics

High-permittivity and low-loss ceramics with composition BaTi_0.92Ga_0.08O_2.96 have been prepared in the BaO–Ga₂O₃–TiO₂ system using the mixed-oxide route. This compound forms as the hexagonal polymorph (6 H ) of BaTiO₃ with the space group P 6₃/ mmc . The dielectric properties of dense ceramics have been studied, at microwave frequencies, with the ceramics fired at 1450°C under flowing oxygen gas; the results are a relative permittivity, ɛ_r, of ∼74 and a quality factor, Q · f _r, of ∼7815 at 5.5 GHz. The quality factor is strongly influenced by the sintering conditions (temperature and atmosphere), whereas the relative permittivity is not influenced significantly by ceramic processing for pellets ≥93% of the theoretical X-ray density. To our knowledge, this is the first report of microwave dielectric resonance in a perovskite-type BaTiO₃-based ceramic.     

Effect of Two-Dimensional Pressure on the Permittivity of Fine- and Coarse-Grained Barium Titanate

The effect of two-dimensional pressure on the permittivity of fine-and coarse-grained ceramic BaTiO₃ was investigated. It was shown that, unlike coarse-grained (10μ or above) barium titanate, the permittivity of the fine-grained (1μ mean) material increased with the increase in two-dimensional compression perpendicular to the electrode, whereas for an intermediate grain size, the permittivity was almost independent of pressure. The apparent anomalous effect of the fine grain size on the pressure coefficient of permittivity was explained satisfactorily on the basis of an internal stress model using a modified Devonshire thermodynamic treatment.     

Effect of Crystal-Axis Orientation on Dielectric Properties of Ceramics Prepared from Fibrous Barium Titanate

Barium titanate (BaTiO₃) ceramics with high degrees of a- and c -axis orientation were fabricated by combining hydrothermal synthesis of fibrous particles, formation of green compacts with aligned extruded rods in the mold, and normal sintering. The orientation of crystal axes in such ceramics gives rise to high dielectric permittivity. Ceramics with an orientation degree f =76% showed values of ε/ε₀ which are 1.5 times (ε/ε₀= 2700) as large as those of a randomly oriented (commercial) ceramic (ε/ε₀= 1800).     

Effect of Two-Dimensional Pressure on the Permittivity of Fine- and Coarse-Grained Barium Titanate

The effect two-dimensional pressure on the permittivity of fine- and coarse-grained ceramic BaTiO₃ was investigated. It was shown that, unlike coarse-grained (10° or above) barium titanate, the permittivity of the fine-grained (1μ mean) material increased with the increase in two-dimensional compression perpendicular to the electrode, whereas for an intermediate grain size, the permittivity was almost independent of pressure. The apparent anomalous effect of the fine grain size on the pressure coefficient of permittivity was explained satisfactorily on the basis of an internal stress model using a modified Devon-shire thermodynamic treatment.     

球磨石用量对钛酸钡介电性能的影响

通过改变料球水的比例以及采用沉降测试、水煮损耗测试、温度特性、电导率和化学分析等测试手段，研究了在制备钛酸钡的球磨工艺中球磨石的用量对钛酸钡粉体均匀性及钛酸钡介电性能的影响。研究结果表明，采用不同的料球水比例制得的电容器性能差别很大，当球：料：水=0．5：1：1时制得的钛酸钡性能最好。     

Effects of Zirconia on Microstructure and Dielectric Properties of Barium Titanate Ceramics

Dielectric properties and structural characteristics of BaTiO₃ ceramics are significantly influenced by small addition (2 wt%) of ZrO₂. SEM and TEM observations revealed enhanced microstructural uniformity and retarded grain growth depending on sintering temperature. Above 1320°C, Zr diffusion into the BaTiO₃ lattice resulted in a chemical modification of the tetragonal structure and the development of core–shell grains. Below 1320°C, TEM analysis showed ZrO₂ at the grain boundaries as discrete particles (∼0.03μm). X-ray diffraction analysis revealed a decrease in the axial (c/a) ratio with decreasing grain size. A corresponding decrease in the spontaneous polarization, and twinned domain structures, were also observed in the fine-grained ceramics. These samples also showed a flattened permittivity response with temperature and significantly lower losses.     

Dopant Effect on Local Dielectric Properties in Barium Titanate Based Electroceramics Determined by Transmission EELS

Local electronic excitations of Nb-doped BaTiO₃ electroceramic were investigated using low-loss electron energy loss (EEL) spectroscopy with a transmission electron microscope. The variations in electronic structure of the BaTiO₃ were monitored as a function of Nb content by using Kramers-Krönig analysis of the low-loss EEL spectra. We found that the valence state of Nb (+5) as compared with that of the Ti (+4) introduces changes in the valence and conduction band states of BaTi_{1− x} Nb _x O₃ samples. The real part of the dielectric function, Re(1/ɛ), displays an increasingly less negative character with higher amounts of dopant and the valence plasmon exhibits "quasi-plasmon" characteristics with the addition of Nb (at 0.0–1.8 at.%). Further, the plasmon energy shifts (by about 0.5 eV) to higher values with Nb additions. Significant changes take place in oscillator strengths of excitations in local (nanometer-scale) regions of the perovskite samples. This investigation demonstrates a method to quantitatively assess electronic properties, at the submicrometer scale, of doped ceramics used in electronic and electrooptical applications.     

Effect of Firing Cycle on Structure and Some Dielectric and Piezoelectric Properties of Barium Titanate Ceramics

In the preparation of barium titanate ceramics for dielectric and piezoelectric applications, careful attention must be given to many factors which determine the ultimate performance of the ceramic. In this study structural differences were observed with the aid of electron micrographs as the time and temperature of firing were varied. At lower temperatures and shorter firing intervals crystals remained small, grain-boundary areas were relatively large, and there were numerous voids where several crystals joined. As the time and/or temperature were increased, crystals became larger with an improved grain-boundary condition that resulted in better structure. An optimum condition was soon reached beyond which rounding of edges at grain boundaries where several crystals joined became more pronounced. This caused an increase in the number of voids. Accompanying these changes, the 1-kc. dielectric constant, the piezoelectric constants (thickness mode), and coupling coefficients first improved, reached optimum values, and then declined slightly, but the resistance to depoling continually decreased as the crystal size increased. An experimental technique of rapid heating and cooling was used to prepare dense specimens of chemically pure barium titanate with relatively small average crystal sizes. Dielectric constants as high as 3000 with Q values of 80 were obtained for these high-density ceramics, accompanied by low effective piezoelectric constants and coupling coefficients.     

Effect of Acceptor and Donor Dopants on the Dielectric and Tunable Properties of Barium Strontium Titanate

The effects of different concentrations of Mn²⁺, Mg²⁺, Al³⁺, Fe³⁺, La³⁺, and Nb⁵⁺ on the dielectric and tunable properties of Ba_0.6Sr_0.4TiO₃ ceramics were investigated. It was found that doping in small amounts with acceptor ions such as Mg²⁺, Fe³⁺, and Al³⁺ could meliorate the dielectric properties clearly. Decrease of dielectric loss was attributed to the formation of compensating defects originating from acceptor substitution. It was concluded that the tunability was linked to both the dielectric constant and the grain size. A higher figure of merit was obtained by doping the ceramics with smaller ions of Al and Fe, compared to Ti.     

Phenomenological Theory of High Permittivity in Fine-Grained Barium Titanate

The anomalously high permittivity in fine-grained ceramic barium titanate is explained by the absence of 90° twinning within the grains, giving rise to internal stresses as the ceramic cools below the Curie temperature. Using the Devonshire phenomenological thermodynamic method and a simplified model, the stresses involved are calculated. At these stress levels, the reduced strain calculated on this model agrees well with X-ray data for the fine-grained material. The limitations and possible developments of the proposed model are discussed.     

Phenomenological Theory of High Permittivity in Fine-Grained Barium Titanate

The anomalously high permittivity in fine-grained ceramic barium titanate is explained by the absence of 90° twinning within the grains, giving rise to internal stresses as the ceramic cools below the Curie temperature. Using the Devonshire phenomenological thermodynamic method and a simplified model, the stresses involved are calculated. At these stresses levels, the reduced strain calculated on this model agrees well with X-ray data for the fine-grained material. The limitations and possible developments of the proposed model are discussed.     

Dielectric Properties of Fluxed Barium Titanate Ceramics with Zirconia Additions

BaTiO₃ compacts, when fluxed with <2 vol% of a complex borate glass phase, were sintered to near theoretical density at temperatures <1175°C in 2 h. Microstructural analysis showed a uniform grain size <1.0 μm with 0.75 wt% ZrO₂ added to the flux phase as a grain growth inhibitor. TEM analysis revealed a microcrystalline grain-boundary phase with the ZrO₂ resident along the grain boundaries. These samples displayed an essentially flat dielectric profile, low dissipation factors (<2%) over the range 25° to 125°C, a near linear dependence (≅±15%) between 25° and −55°C, and significantly increased voltage stability. X-ray diffraction analyss of these small-grained materials indicated a suppression of the tetragonal structure toward a more cubic modification.     

Direct-Current Field Dependence of Dielectric Properties in Alumina-Doped Barium Strontium Titanate

The dielectric properties of (Ba_0.6Sr_0.4)TiO₃ and Al₂O₃-doped (Ba_0.6Sr_0.4)TiO₃ have been characterized. The grain size of the specimen is maximum for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 1 wt% Al₂O₃. The density and the real part of the relative dielectric constant each decrease as the Al₂O₃ content increases. The loss factor is minimum for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 2 wt% Al₂O₃. The dielectric constant of the specimens decreases as the applied dc field increases. The influence of the dc field on the loss factor is much less than that on the dielectric constant. The tunability is ∼24% for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 1 wt% Al₂O₃.     

Effect of Precursor Pyrolysis on the Dielectric Properties of Hydrothermally Derived Barium Titanate Thin Films

BaTiO₃ thin films were processed hydrothermally on Ag-coated quartz substrates at 90°C by reacting films of titanium dimethoxy dineodecanoate (TDD) in aqueous solutions of Ba(OH)₂. Two reaction sequences were used: either the TDD was reacted directly in aqueous Ba(OH)₂, or the TDD was first pyrolyzed in air at temperatures ranging from 200° to 500°C before hydrothermal reaction. Depending on the processing conditions, the dielectric constant of the thin films ranged from 5 to 170, the dielectric constant increasing with increasing pyrolysis temperature. Thin film porosimetry data suggest that the improvement in film dielectric performance is related to decreases in thin film residual porosity after hydrothermal reaction.     

Depletion-layer Dielectric Properties of Positive Temperature Coefficient of Resistance Barium Titanate

For pure and impurity-added positive temperature coefficient of resistance (PTCR) barium titanate ceramic samples, a −11°C shift of the Curie point at the grain-boundary/depletion-layer region was observed. This result is obtained by fitting the PTCR grain-boundary resistance and capacitance data to a theory which combines a double-depletion-layer model with the Devonshire thermodynamic theory of barium titanate. The parameters used in the fitting are obtained from independent experiments. The shift of the Curie point is believed to result from the grain-boundary clamp ing effect near the cubic-tetragonal phase transition point.