过渡金属掺杂BaTiO3的晶格结构和相变特性

采用溶胶一凝胶法制备纯净BaTiO3,将1．0mol％的Cr、Fe、Mn、Co掺杂BaTiO2和浓度分别为1．25mol％、1．5mol％、1．75mol％、2．0mol％的Fe掺杂BaTiO2前驱粉体,经热压烧结,并在1350℃下烧结成多晶陶瓷样品。对样品进行XRD和DSC表征的研究。分析了掺杂离子种类和浓度对样品的晶格四方性、相变温度（居里点）和相变潜热的影响。分析结果表明,掺杂BaTiO2的四方性、居里点和相变潜热低于纯净BaTiO。且随Cr、Fe、Mn、Co的掺杂顺序以及Fe掺杂浓度的增加而减少。过渡元素掺杂BaTiO2的居里点和相变潜热髓晶格四方性的降低逐渐减小。     

过渡金属掺杂BaTiO3的晶格结构和相变特性

采用溶胶-凝胶法制备纯净BaTiO3,将1.0mol%的Cr、Fe、Mn、Co掺杂BaTiO3和浓度分别为1.25mol%、1.5mol%、1.75mol%、2.0mol%的Fe掺杂BaTiO3前驱粉体,经热压烧结,并在1350℃下烧结成多晶陶瓷样品。对样品进行XRD和DSC表征的研究。分析了掺杂离子种类和浓度对样品的晶格四方性、相变温度(居里点)和相变潜热的影响。分析结果表明,掺杂BaTiO3的四方性、居里点和相变潜热低于纯净BaTiO3且随Cr、Fe、Mn、Co的掺杂顺序以及Fe掺杂浓度的增加而减少。过渡元素掺杂BaTiO3的居里点和相变潜热随晶格四方性的降低逐渐减小。     

钛酸锶钡结构与制备工艺研究进展

钛酸锶钡[(Ba,Sr)TiO3,SBT]是1种重要的电子陶瓷材料,具有高介电常数、低漏电流、高热释电系数,高的介电可调等特性,广泛用于动态随机存储器、热释电探测器、介质移相器等电子元件的制备。综述了钛酸锶钡[(Ba,Sr)TiO3,SBT]的结构和制备工艺研究进展,评述了制备工艺和掺杂等对钛酸锶钡的结构和性能的影响,并指出了钛酸锶钡尚待解决的问题。     

Effect of Twin-Plane Reentrant Edge on the Coarsening Behavior of Barium Titanate Grains

When BaTiO₃ ceramics were sintered at relatively low temperatures (≤1250°C), the grains with reentrant edges caused by a (111) double twin grew exclusively. As a result, a microstructure with a bimodal grain-size distribution composed of platelike large grains and fine matrix grains was obtained. In contrast, at the usual sintering temperature between 1250° and 1350°C, grains containing a (111) double twin did not exhibit any growth advantage. In this case, a coarse and uniform microstructure was obtained. When this coarse-grained specimen was further heat-treated at 1365°C, the grains possessing a double twin were observed to grow exclusively again. The results were explained in terms of a coarsening process controlled by two-dimensional nucleation.     

Temperature Dependence of the Coarsening Behavior of Barium Titanate Grains

The coarsening behavior of large seed particles during the sintering of BaTiO₃ ceramics has been investigated. At 1350°C, the grains are faceted, and the seed particles grow extensively. At 1380°C, however, the grains are spherical, and coarsening of the seed particles is limited. The observed difference is discussed in terms of the growth mechanism and the atomic structure of the interfaces.     

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.     

Effects of Excess Barium Ions on Aqueous Barium Titanate Tape Properties

The effects of excess free barium ions in aqueous barium titanate slip on the resulting BaTiO₃ tape properties were investigated in terms of the slip behavior, green/sintered tape density and morphology, and dielectric properties. The excess free barium ions expressed by means of the Ba/Ti ratio adversely affected most tape properties. Increase in the slip viscosity, green porosity, and agglomeration along with a decrease in mechanical properties and green/sintered density were found with the increase in the Ba/Ti ratio. However, dielectric permittivity was increased with increase in the Ba/Ti ratio. An effort was made to correlate these phenomena with Ba²⁺ leaching in water for realistic multilayer ceramic capacitor applications.     

Effect of Particle Size on the Room-Temperature Crystal Structure of Barium Titanate

The room-temperature tetragonal-to-cubic transformation in BaTiO₃ powders with decreasing particle size has been carefully studied, using materials prepared mainly by hydrothermal methods. Hydrothermal BaTiO₃ powders exhibited a more uniform particle size distribution than oxalate-route powders, with X-ray diffraction and electron microscopy indicating that powders 0.19 μm in size were fully cubic while powders 0.27 μ were completely tetragonal (within a 5% detection limit for cubic material) at room temperature. The tetragonal-to-cubic transformation temperature was also found to lie in the range of 121°± 3°C for BaTiO₃ powders with room-temperature ( c/a ) values > 1.008. No transformation could be detected using differential scanning calorimetry for BaTiO₃ particles with a ( c/a ) > 1.008 at room temperature. BaTiO₃ powder with a particle size just too small (0.19 μm) to be tetragonal at room temperature remained cubic down to 80 K. Different models for the cubic-to-tetragonal room-temperature transformation are discussed. Hydroxyl ions do not appear to greatly affect the cubic-to-tetragonal transformation, which appears to be essentially dependent on particle size. It is concluded that a model based on surface free energy, as previously discussed for the monoclinic-to-tetragonal transformation at room temperature of fine ZrO₂ particles, is consistent with the experimental data.     

Core-Shell Structure of Acceptor-Rich, Coarse Barium Titanate Grains

Heavily doped barium titanate (BaTiO₃) powders, one with a donor-rich and the other with an acceptor-rich composition, were prepared. After sintering, the donor-rich specimen exhibited a fine-grained microstructure but significant grain growth occurred in the acceptor-rich specimen. A stable dielectric behavior was observed only in the donor-rich fine-grained specimen over the temperature range studied. However, in both specimens, an undoped core region several hundred nanometers in size was detected. The core-shell structure appeared to be maintained in BaTiO₃ under the conventional sintering conditions_.     

Fabrication of High-Curie-Point Barium-Lead Titanate PTCR Ceramics

High-Curie-point semiconducting barium-lead titanate positive temperature coefficient of resistivity (PTCR) ceramics of composition Ba_0.897Pb_0.1La_0.003TiO₃ and Ba_0.5Pb_0.5La_0.003TiO₃ were prepared. The starting powders were synthesized by reacting commercial BaTiO₃, PbO, and TiO₂. To avoid the nonstoichiometry due to the volatilization of Pb during the sintering process, a lead atmosphere sintering approach with PbTiO₃ as packing powder was used. The samples being fabricated by this method show a PTCR effect of 3 to 4.5 orders of magnitude above the Curie point. The curie points were about 180°C for Ba_0.897Pb_0.1La_0.003TiO₃ and about 360°C for Ba_0.497Pb_0.5La_0.003TiO₃.     

Electrical Properties of PTCR Barium Titanate

When properly doped, barium titanate ceramics display positive temperature coefficient resistance (PTCR) behavior. This has been proved to be a Schottky barrier type of grain-boundary effect. However there has not yet been a complete point-to-point comparison between the experimental data and theory for the entire set of the material nonlinear dielectric properties. In this study, a methodology has been developed which allows the study of the depletion layer dielectric properties while the PTCR effect is being investigated. An equivalent dielectric constant, the value of which is to be determined from this experiment, is treated as an average of the dielectric properties of the depletion layer and is used to analyze the grain-boundary resistance and capacitance data based on a simple double-depletion-layer model. The theoretical relationship between this equivalent dielectric constant and the material dielectric properties is also explored in this study.     

Anomalous Grain Growth in Donor-Doped Barium Titanate with Excess Barium Oxide

Grain growth and semiconductivity of donor-doped BaTiO₃ceramics with an excess of BaO and additions of SiO₂or B₂O₃were studied. The microstructures and electrical measurements on sintered samples revealed that their electrical properties are related to the microstructure development of the sintered samples. Samples heated with an excess of BaO developed a normal microstructure during sintering, as a consequence of normal grain growth (NGG), and were yellow and insulating. In contrast, samples with an excess of BaO and an addition of SiO₂or B₂O₃exhibited anomalous grain growth (AGG) and were dark blue and semiconducting after sintering. When some BaTiO₃seed grains were embedded in a sample of donor-doped BaTiO₃with an excess of BaO (without SiO₂or B₂O₃), AGG was observed, i.e., some seed grains grew into large grains and were blue and semiconducting. An explanation is given for why AGG is responsible for the oxygen release and the formation of semiconducting grains in donor-doped BaTiO₃and not NGG.     

XPS Analysis of Submicrometer Barium Titanate Powder

A commercial submicrometer BaTiO₃ powder was analyzed using X-ray photoelectron spectroscopy. The analysis revealed the powder surfaces to be covered with a layer of physisorbed H₂O and chemisorbed –OH ions. A BaCO₃ residual not detected with XPS was shown to be present in the powder using X-ray diffraction, suggesting that the carbonate takes the form of discrete particles rather than of a continuous surface layer. A relaxed surface phase detected in previous XPS analyses of bulk BaTiO₃ was also shown to be present. Depth profiling revealed the powder surfaces to be Ti-rich, confirming the presence of a phase, or phases, to stoichiometrically balance the barium carbonate.     

Point Defect Concentrations in Barium Titanate Revisited

An analytic link between the oxygen partial pressure and the concentrations of the point defects for given temperatures and A/B conditions is presented for perovskites. A clear distinction between three different conditions is made. These are the sintering conditions, an intermediate metastable state, and a low-temperature metastable state. The analytical solution for a metastable state resulting from nonequilibrated metal vacancies permits a more accurate and self-consistent approach to calculating the equilibrium constants from conductivity– P (O₂) data. One of the reasons for the higher accuracy is that there is no need to divide the existence regime into subregimes with different approximations to the electroneutrality equation (Brouwer approximation). An excellent fit of the experimental conductivity data to a single function with only two adjustable parameters over all conductivity– P (O₂) space is obtained. The relative importance of frozen-in metal vacancies and foreign acceptors is discussed for BaTiO₃.     

Necessary Conditions for the Formation of {111} Twins in Barium Titanate

The experimental conditions for {111} twin formation in BaTiO₃ were investigated. When BaTiO₃ compacts without excess TiO₂ were sintered either in an oxidizing atmosphere (air) or in a reducing atmosphere (95N₂–5H₂), no {111} twins formed within the BaTiO₃ grains and no abnormal grain growth occurred. In contrast, many {111} twins were present within the abnormally grown grains in the excess-TiO₂-containing BaTiO₃ samples sintered in air, while no twins were observed in the excess-TiO₂-containing samples sintered in 95N₂–5H₂. X-ray diffraction analysis showed that excess TiO₂ forms a Ba₆Ti₁₇O₄₀ phase during sintering with the space group A 2/ a in air and a Ba₆Ti₁₇O_{40− x} phase with the space group C in 95N₂–5H₂. It appears therefore that excess TiO₂ and an oxidizing atmosphere are necessary for {111} twin formation in BaTiO₃. These results may also indicate that the interface structure between BaTiO₃ and Ba₆Ti₁₇O₄₀ influences the twin formation.     

钛酸钡纳米粉体表面包覆方法的研究

钛酸钡纳米粉体的表面包覆不仅解决了粉体的团聚和分散问题，而且赋予粉体诸多的优异性能，因此粉体的表面包覆技术受到诸多关注并且已取得了一定的研充进展。文章主要综述了近年来钛酸钡纳米粉体表面包覆方法，以及包覆后对钛酸钡纳米粉体的表面性质乃至粉体的相结构和性质的改善作用。     

Experimental Evaluation of the Acceptor-States Compensation in Positive-Temperature-Coefficient-Type Barium Titanate

Experimental evidence shows that the acceptor-state levels in Sb-doped positive-temperature-coefficient-type BaTiO₃ are compensated up to a critical acceptor-state density. Using the slope of the natural logarithm of the resistivity with respect to 1/ T , instead of maximum resistivity as a measure for the acceptor-state density, it is possible to estimate this critical value. The value obtained (4.2 × 10¹⁷ m⁻²) is believed to be the first reported estimate based on experimental data. It is in good agreement with the estimate of 6 × 10¹⁷ m⁻² (first reported by Jonker) obtained from the spontaneous polarization of BaTiO₃. This shows that the ferroelectric behavior of BaTiO₃ is indeed a feasible explanation for the low resistivity below the Curie point, as proposed by Jonker.     

Influence of Solids Concentration on the Isoelectric Point of Aqueous Barium Titanate

We report on findings that the particle surface charge is influenced by solids concentration in aqueous suspensions of BaTiO₃. Three decades in solids concentration were analyzed by combining results from two different electrokinetic methods. Combined results demonstrate a systematic acidic shift in the isoelectric pH with decreasing solids concentration. The shift is attributed to the development of a Ba-depleted, TiO₂-rich surface layer. Using kinetic arguments, it is shown that the thickness of this layer will be proportional to the surface-to-volume ratio.     

Intermediate Phase Development in Phosphorus-Doped Barium Titanate

In the present work, the phase formation and thermal evolution in phosphorus-doped BaTiO₃ have been studied using differential thermal analysis, X-ray diffractometry, scanning electron microscopy coupled with energy-dispersive spectroscopy, transmission electron microscopy, and high-temperature nuclear magnetic resonance. Phosphorus cations that are incorporated from ester phosphate form a surface layer that covers the BaTiO₃ particles. This layer acts as a reactive coating during sintering. Phosphorus-doped BaTiO₃ samples that have been treated at temperatures of 650°–900°C show the presence of crystalline Ba₂TiP₂O₉ and/or Ba₃(PO₄)₂ phases. The appearance of secondary phases is dependent on the cooling rate. Higher temperatures (900°–1200°C) result in the presence of a phosphorus–BaO-rich phase that covers the BaTiO₃ particles. As a consequence, the remaining titanium-rich BaTiO₃ drives the formation of a liquid phase at temperatures >1200°C. In regard to the reported sintering behavior of P⁵⁺-doped BaTiO₃, the formation of a phosphorus–BaO-rich phase that covers the BaTiO₃ particles could be the origin of the improved porosity coalescence and removal that is observed at the earlier stages of sintering.     

Grain-Boundary Chemistry of Barium Titanate and Strontium Titanate: II, Origin of Electrical Barriers in Positive-Temperature-Coefficient Thermistors

Scanning transmission electron microscopy (STEM) of positive-temperature-coefficient (PTC) BaTiO₃ thermistors shows that the grain-boundary oxygen content in as-received (oxidatively cooled) materials is slightly enriched compared to quenched samples, and the acceptor-rich space-charge present at high temperatures is retained upon cooling. The defect density of the space charge is approximately equal to the acceptor state density at PTC boundaries determined by electrical measurements. Accordingly, it is proposed that the electrical barrier forms when acceptor defects already segregated in the ionic space charge at high temperature become active interface states when compensating donor defects in the grain-boundary core are oxidized. These acceptor defects appear to be primarily barium vacancies, but need not form upon cooling in the manner proposed by Daniels and Wernicke. Acceptor solutes when present can also contribute to barrier formation through space-charge segregation; the increase in interface state density upon addition of Mn is consistent with the magnitude of the expected segregation.