钛酸铅微晶玻璃介电性能研究

主要对钛酸铅系铁电性微晶玻璃的介电性能做了研究,研制的微晶玻璃初始组成由形成主晶相的氧化物和形成玻璃相的氧化物构成,经过熔融、热处理可以得到以钛酸铅为主晶相的微晶玻璃,分析了不同组成情况下微晶玻璃的介电性能.     

Bubble-Induced Nucleation and Crystal Growth in Glass

Evidence that bubbles can cause annular nucleation and subsequent crystal growth on the surface of Bao-Al₂O₃-TiO₂-SiO₂, glasses is presented. The crystalline rings were identified as hexacelsian, BaAl₂Si₂O₈ (the hexagonal or high-temperature polymorph of celsian). Size, character, and suggestions as to the genesis of these rings are given.     

Nucleation and Crystallization of a Lead Halide Phosphate Glass by Differential Thermal Analysis

The nucleation and crystallization mechanisms of a lead halide phosphate glass [40P₂O₅·30PbBr₂·30PbF₂ (mol%)] were investigated by differential thermal analysis (DTA) and X-ray diffraction analysis. There were two crystalline phases in the crystallized samples: the major phase was PbP₂O₄, and the minor phase was PbP₂O₆. The average activation energy for crystallization, E , for two different particle sizes of this glass was determined to be 119 ± 4 kJ/mol by the Kissinger method and 124 ± 4 kJ/mol by the Augis–Bennett method. The Avrami constants were determined to be 1.6 and 2.5 for particle sizes of 203 and 1040 μm, respectively, by the Ozawa equation, and 1.7 and 2.4 for particle sizes of 203 and 1040 μm, respectively, by the Augis–Bennett equation. The decrease in the crystallization peak height in the DTA curve with increasing particle size suggested that the particles crystallize primarily by surface crystallization. A nucleation-rate type curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T _p, or the height of the crystallization peak, (δ T )_p, as a function of nucleation temperature, T _n. The temperature where nucleation can occur for this glass ranges from 360°–450°C and the maximum nucleation rate is at 420°± 10°C.     

Subcritical Crack Growth in Lead Zirconate Titanate

Subcritical crack growth in terms of velocity–stress intensity factor ( v – K ) curves in lead zirconate titanate (PZT) were experimentally characterized on poled and unpoled compact tension specimens. The poled specimens were tested under open- and short-circuit electrical boundary conditions, which resulted in an increase in fracture toughness by 0.2 MPa·m^1/2 for the accessible velocity range ( v = 10⁻⁹ to 10⁻⁴ m/s) in the open-circuit case. Subcritical crack growth of unpoled specimens was obtained under ambient (relative humidity = 35%) and dry (relative humidity ∼ 0.02%) conditions over a regime in stress intensity factor of 0.5 MPa·m^1/2.     

Nucleation and Growth of Sodium Disilicate Crystals in Sodium Disilicate Glass

A high-temperature microscope for use with transmitted or reflected light up to × 400 magnification and up to 1400°C. was constructed. Nucleation and crystal growth in sodium disilicate glass was studied with this microscope. Heterogeneous nucleation occurred at the interface between the glass and the platinum heating element from 450° to 870° C. with a maximum at 600° C. Heterogeneous nucleation also occurred at the glass-atmosphere interface from 25° to 375° C. as a result of surface reactions with water vapor. No homogeneous nucleation was observed. Linear growth rates of α and β sodium disilicate crystals were measured from 600° to 870° C. in two sodium disilicate glasses. Metastable growth of both α and β was observed, and growth rates of α and β were the same at a given temperature for a given glass. An equation relating fluidity and undercooling to growth rate was applied.     

Topotatic-Like Phase Transformation of Amorphous Lead Titanate to Cubic Lead Titanate

The crystallization process of lead titanate (PT) prepared using the polymeric precursor method was investigated using X-ray diffractometry, Raman spectroscopy, electron microscopy, and X-ray absorption spectroscopy techniques. The results showed that amorphous PT was formed by an O–Ti–O structure composed of fivefold and sixfold oxygen-coordinated titanium. The local structure of the amorphous PT phase was similar to that of the cubic PT phase, i.e., similar coordination number and similar bond lengths, leading to a topotactic-like transformation during the phase transformation from amorphous to cubic perovskite PT. Because of the low crystallization temperature, every transformation observed during the crystallization process was associated with a short-range rearrangement process.     

Nucleation and Inhibition Control during the Hydrothermal–Electrochemical Growth of Barium Titanate Films

The control of the microstructure of BaTiO₃ films grown on titanium by the hydrothermal–electrochemical method was investigated. Experiments were conducted in a three-electrode high-pressure electrochemical cell in a 0.1 M Ba(OH)₂ electrolyte at 150°C. Results showed that the spontaneous initial nucleation linked to pure hydrothermal BaTiO₃ formation can be inhibited by cathodically protecting the titanium electrode from the moment it is immersed in the electrolyte. The application of initial nucleation pulses of varying cathodic potentials affected the grain size of the deposit. It is suggested that the formation of a titanium oxide layer is a necessary step previous to the nucleation of BaTiO₃.     

Grain Growth and Densification of Hot-Pressed Lead Zirconate-Lead Titanate Ceramics Containing Bismuth

Grain growth and densification were studied in a hot-pressed ferroelectric composition of 65/35 (Zr/Ti molar ratio) lead zirconate-lead titanate containing 2 at.% bismuth. The grain growth and densification rate processes were measurable from 1050° to 1300°C and from 700° to 1100°C, respectively. Grain growth as a function of time followed a ⅓ power law ( D (grain size) = kt ^⅓). An activation energy of 95 kcal/mole was calculated. Densification was a two-stage process as a function of time: a rapid initial stage and a slower final stage. The initial stage of densification behaved viscously with an activation energy of 36.7 kcal/mole. Grain boundary sliding and the Nabarro-Herring mechanism of stress-directed movement of vacancies are suggested as the densification mechanisms. Results indicated that densification is sensitive to stoichiometry and to additives.     

Nucleation and Crystallization of a Lithium Aluminosilicate Glass

An aluminosilicate glass of composition 61SiO₂6Al₂O₃10MgO6ZnO·12Li₂O·5TiO₂ (mol%) has been prepared by a melting process and investigated as far as crystallization is concerned. Glass-ceramic is easily obtained because glass shows a high tendency to crystallize starting from 700°C. The crystalline phases evolve with temperature, showing the aluminosilicates to be the main phase up to 1050°C, followed by metasilicates and silicates, some of which have lower melting points. The titanates of Mg and Zn develop from the phase-separated glass, soon after T _g, and grow to form nucleation centers for the other crystalline phases. The evolution from phase-separated glass to glass-ceramic has been followed by many thermal, diffractometric, spectroscopic, and microscopic techniques.     

Nanocrystalline Lead Titanate and Lead Titanate/Vinylidene Fluoride-Trifluoroethylene 0-3 Nanocomposites

Lead titanate (PT) powders of nanometer particle sizes were prepared using a sol-gel process. The PT gel was annealed at various temperatures to produce powders with different particle sizes. X-ray diffractometry (XRD), thermogravimetric analysis (TGA), and differential thermal analysis (DTA) revealed that the PT gel initially crystallized at an annealing temperature of 500°C. The crystallite diameter measured by XRD increased from 27 to 54 nm, while the size of PT particles increased from 30 to 270 nm as the annealing temperature increased from 500° to 900°C. Lead titanate/vinylidene fluoride-trifluoroethylene (PT/P(VDF-TrFE)) 0-3 nanocomposites with PT volume fractions of 0.18 and 0.41 were prepared using PT powders annealed at various temperatures. Because the pyroelectric coefficients of the ceramic and polymer phases had like signs and the piezoelectric coefficients had opposite signs, the poled composite had high pyroelectric but very low piezoelectric activity, thereby reducing vibration-induced electrical noise in pyroelectric sensor applications.     

Low-Temperature Preparation of Nanocrystalline Lead Zirconate Titanate and Lead Lanthanum Zirconate Titanate Powders Using Triethanolamine

Nanocrystalline lead zirconate titanate (PZT) powders, with a Zr:Ti ratio of 60:40, have been prepared from a solution of triethanolamine (TEA) and Ti⁴⁺, Zr⁴⁺, and Pb²⁺ ions. The metal ions were in solution through complex formation with TEA. The soluble metal-ion–TEA complex formed the precursor material when it was completely dehydrated. Heat treatment of the precursor at 450°C resulted in single-phase PZT powders. The precursor and the heat-treated powders have been characterized by using thermal analysis and X-ray diffractometry (XRD) studies. The average particle size, as measured from X-ray line broadening and transmission electron microscopy studies, was ∼20 nm. PZT powders modified with 3 mol% of lanthanum (PLZT) also were prepared through this route and were investigated via XRD studies. The dielectric constants of the PZT and PLZT powders were 12475 and 11262, and their corresponding Curie temperatures were 362° and 315°C, respectively.     

Mechanochemical Synthesis of Lead Zirconate Titanate from Mixed Oxides

High-density lead zirconate titanate (PZT) ceramics have been successfully prepared by using a novel mechanochemical fabrication technique, which skips the phase-forming calcination at an intermediate temperature that is always required in the industrial processes currently in use. The fabrication technique starts with mixing of the low-cost industrial oxide powders, and the designed PZT perovskite phase is formed by reacting the oxide constituents in a mechanochemical chamber that consists of a cylindrical alumina vial and one stainless-steel ball inside it. The solid-state reaction among constituent oxides is activated via mechanical energy instead of high temperature. When mechanochemically activated for 20 h, an ultrafine PZT powder of perovskite structure with a minimized degree of particle agglomeration is obtained. The resulting PZT powder sinters to 99.0% of theoretical density at 1100°C for 1 h. The sintered PZT ceramic exhibits a dielectric constant of 1340 and a dielectric loss of 0.6% at a frequency of 1 kHz at room temperature.     

Sintering of Lead Titanate Using a Spark-Plasma-Sintering Technique

Lead titanate (PbTiO₃) is difficult to sinter without additives, even when a spark-plasma-sintering technique is applied. The high tetragonality, c / a , of PbTiO₃ causes destructive strain after sintering. We found that a sintered body of PbTiO₃ could be obtained, when the PbO/TiO₂ value was <0.9. However, decreasing the ratio did not increase the bulk density; rather, it caused a decrease in the grain size. The mechanical strain that resulted from the phase change from cubic, at the sintering temperature, to tetragonal, at room temperature after the sintering, was released by the decrease in the grain size.     

Ferroelectric Properties of Lead Titanate

The eutectic and solidus temperatures of the PbTiO₃-KF binary system were investigated by DTA for four compositions. Optical studies of crystal habit as a function of crystallization conditions were used to determine favorable conditions for growing PbTiO₃ crystals from a KF flux. Results of X-ray studies of these crystals agree closely with those of earlier studies. Interferometric measurements of thermal expansion confirmed the ferroelectric transition at 490° C. Specific heat studies indicated a transition energy of about 1550 cal/mole with an entropy change of 2.055 cal/mole °C. Spontaneous polarization was calculated as 66 μcoulombs/cm².     

Densification and Crystallization of Lead Titanate Aerogels

Crack-free monolithic lead titanate (PbTiO₃, or PT) aerogels were prepared via a sol-gel process and drying in supercritical carbon dioxide. The material was characterized by thermogravimetric analysis, dilatometry, helium pycnometry, and X-ray diffraction. Skeletal densification was the main reason for the observed shrinkage of the heat-treated porous material at temperatures <450°C. The transition (at temperatures of 500°-600°C) from an amorphous state to the crystalline (tetragonal) PT was accompanied by significant particle coarsening, as revealed by transmission electron microscopy. Nitrogen adsorption measurements confirmed that crystallization was preceded by the formation of interparticle necks.     

Lead Zirconate Titanate Hollow-Sphere Transducers

Millimeter-sized, hollow spheres of lead zirconate titanate were fabricated by blowing gas through a fine-grained slurry of PZT-5. After they were sintered, the spheres were poled in two ways: radially between inside and outside electrodes, and tangentially between two outside electrodes. The capacitance and vibration modes were modeled and measured for these two poling configurations. The two principal modes of vibration were a breathing mode near 700 kHz and a wall thickness mode near 10 MHz. These spheres have potential uses in medical ultrasound, nondestructive testing, and low-density transducer arrays.