Characterization and photoluminescence studies on hydrothermally synthesized Mn-doped barium titanate nano powders

Manganese doped cubic barium titanate nanocrystals with [Mn/Ti] mole percent varying from 0.1 to 3 were prepared through hydrothermal route at 150 °C. The mean crystallite diameters obtained for different Mn concentrations varied within 26 to 30 nm. TEM of a typical sample showed most of the particles as single crystallites (particle size 15-40 nm) with some weakly agglomerated particles. The photo luminescent (PL) spectra of each sample showed a sharp peak in the blue band centered approximately at 464 nm and a weaker peak in the green band centered nearly at 494 nm. The luminescent efficiency passed through maxima for the sample with 2% [Mn/Ti] molar ratio. Effect of calcination temperature in the range of 200 to 600 °C on PL spectra showed that the intensity of peak corresponding to blue region decreased with the increase in calcination temperature from 200 to 600 °C and for the sample calcined at 600 °C, only a broad peak corresponding to green luminescence region was observed.     

Novel methodology to investigate superconducting dielectric resonators

An analytical method is presented for investigating the resonant behavior of a c-axis oriented YBa₂Cu₂O_7–(0) thin film on a resonator composed of LaAlO₃ (001). The concept of the negative dielectric medium for a superconductor is introduced within the framework of the two-fluid model, which permits us to treat a superconductor as any other penetrable materials so that only its electromagnetics are concerned. A conformai transformation is further suggested to map the original open boundary-value problem to a closed one. This not only makes the original problem readily solvable by using the variational technique, but is also a powerful tool for analyzing some kinds of problems such as the propagation characteristics of the superconducting microstripe and coplanar waveguide structures.     

High-Q sapphire-rutile frequency-temperature compensated microwave dielectric resonators

A sapphiro-rutile composite resonator was constructed from a cylindrical sapphire monocrystal with two thin disks of monocrystal rutile held tightly against the ends. Because rutile exhibits low loss and an opposite temperature coefficient of permittivity to sapphire, it is an ideal material for compensating the frequency-temperature dependence of a sapphire resonator. Most of the electromagnetic modes in the composite structure exhibited turning points (or compensation points) in the frequency-temperature characteristic. The temperatures of compensation for the WG quasi TM modes were measured to be below 90 K with Q-factors of the order of a few million depending on the mode. For WG quasi TE modes, the temperatures of compensation were measured to be between 100 to 160 K with Q-factors of the order of a few hundreds of thousands, depending on the mode. The second derivatives of the compensation points were measured to be of the order 0.1 ppm/K(2 ), which agreed well with the predicted values.     

Studies on the dielectric and relaxor behavior of sol-gel derived barium strontium zirconate titanate thin films

The dielectric behavior of sol-gel derived Ba_0.80Sr_0.20(Zr_xTi_1−x)O₃ (0.0 ≤ x ≤ 0.50) thin films is studied. A relaxor behavior is observed for x ≥ 0.35. The degree of relaxation increases with Zr content. The frequency dependence of the polar regions follows Vogel-Fulcher relation with a characteristic cooperative freezing at freezing temperature (T_f). Below T_f, a long range polarization ordering is likely to take place. The plausible mechanism of the relaxor behavior of BSZT thin films with Zr contents ≥ 0.35 has been proposed based on the measured temperature as well as frequency dependent dielectric data. The solid solution system is visualized as a mixture of Ti^+ 4 rich polar regions and Zr^+ 4 rich regions; with the increase in Zr content the volume fraction of the polar regions is progressively reduced. At and above 35.0 at.% Zr substitution the polar regions exhibit typical relaxor behavior.     

Cryogenic sapphire dielectric resonators: performance and potential

Sapphire dielectric resonators (SDR) at cryogenic temperatures have been used to produce oscillators of exceptional short- and medium-term stability. We discuss the demonstrated performance achieved at the National Physical Laboratory (NPL) as well as potential long-term capabilities of this type of oscillator both as a “fly-wheel” standard for atomic clocks and in a possible determination of the fundamental physical constants     

Studies of structural,dielectric and electrical characteristics of nickel-modified barium titanate for device applications

Journal of Materials Science: Materials in Electronics - In the present report, an attempt has been made to develop (via synthesis and characterization) a lead-free multifunctional material by...     

钛酸锶钡(BST)薄膜的介电性能研究

采用射频磁控溅射法制备了BaxSr1-xTiO2（简称BST）薄膜材料,研究了不同膜厚,晶粒尺寸的BST薄膜的介电系数温度特性（ε1-T),频率特性（ε-r-f),电压特性（εr-U)及损耗的温度特性（tgδ-T),频率特性（tgδ-f),找出了BST薄膜的非线性,损耗随尺度变化的规律。     

Aging effects on dielectric properties of barium neodymium titanium tantalate ceramics

The aging effects on crystal structures and dielectric properties were investigated in Ba_pNd_6−pTi_8−pTa_2+pO₃₀ (p = 3, 4 and 5) ceramics with tungsten bronze structure. From XRD analyses on fresh and aged specimens, the slight change of crystal constant was concluded in Ba_pNd_6−pTi_8−pTa_2+pO₃₀ ceramics. Strong aging effects on the dielectric properties were observed in the present ceramics. After aging, a slightly decreased dielectric constant was observed combined with a significantly reduced dielectric loss in all the three compositions. These results had been discussed in term of the disorder-order state of ions at A- and B-sites. From tolerance factor and electronegativity difference, the stability was discussed for the filled type tungsten bronze phase.     

铌酸锶钡陶瓷材料的制备及介电性能研究

以BaCO3，SrCO3和Nb205作为原料，采用高能球磨工艺制备SBN50陶瓷粉体。球磨后的粉体不经煅烧，直接压片成型，在1250～1350℃下保温1．5～12h可制备出SBN50陶瓷材料，并对此进行了X射线衍射分析、扫描电镜观察和性能测试。结果表明：球磨30h的粉体在1100℃时合成SBN50单相；随着烧结温度的升高和保温时间的延长，SBN50陶瓷的介电常数先增大后减小，晶粒大小呈有规律的变化。1300℃下保温3h制得的陶瓷样品介电常数最高（εmax=1447），居里温度（L）为130℃。     

A new red-emitting Ce, Mn-doped barium lithium silicate phosphor for NUV LED application

A series of new red-emitting Ce³⁺, Mn²⁺-doped barium lithium silicate phosphors, with general formula Ba₂Li₂Si₂O₇: Ce³⁺, Mn²⁺, were synthesized by solid-state reactions. The Mn²⁺ in this system can be effectively excited in a wide UV region especially in the NUV range, and has broad red emission after NUV excitation through Ce³⁺-Mn²⁺ energy transfer, as well as excellent thermal stability. The blue-shift behavior of Mn²⁺ emission with increasing temperature can be described in terms of the phonon-electron interaction. The promising luminescence properties make it a red candidate for application in NUV chip pumped LED.     

Antennas for wireless communication: recent advances using dielectric resonators

Over the last few years, developments in wireless communications have presented many challenges to the antenna and microwave communities in terms of special requirements for antenna design, fabrication and integration. These requirements include new characterisation in terms of antenna performance, miniaturised size and shape and new suitable fabrication and implementation techniques to fit with the devices. These aspects are addressed with a focus on recent developments in dielectric resonator antenna research to meet some of the new challenges. A few recent representative designs, which outline new innovations and improved antenna features, as well as directions for future research, are briefly described.     

Microwave synthesis of barium iron niobate and dielectric properties

A microwave synthesis was applied to prepare barium iron niobate (BFN) powders for the first time. Pure perovskite phase of BFN with a cubic symmetry was obtained when precursors were calcined at 850 °C for 14 h. Average particle size of pure BFN powder was estimated to be in submicron range. The examination of dielectric spectra indicated that BFN ceramics prepared by the present method exhibited rather high dielectric constant (∼ 30,000 at 300 °C for 1 kHz) with low dielectric loss (less than 0.6).     

Influence of core-shell structured grain on dielectric properties of cerium-modified barium titanate

The dielectric properties and chemical inhomogenity of BaTiO₃ ceramics sintered with additions of CeO₂:1.5TiO₂ were investigated using TEM XRD and EDS. Grains had three regions: grain shell severely doped with cerium, grain core BaTiO₃ of greater purity, and a concentration gradient region where Curie temperatures were distributed. With decreasing temperatures, the volume fraction of ferroelectric domains in the concentration gradient region increased. Addition of CeO₂:1.5TiO₂ to BaTiO₃ decreased the tetragonality (c/a ratio) and the grain size, and lowered the Curie temperature by approximately 30 C per mol%. Even a small addition of 0.005 mol CeO₂:1.5TiO₂ to BaTiO₃ reduced the average grain size from 25 m to less than 1.25 m and grains of an average grain size were completely doped with cerium, which explained why the 130 C ferroelectric transition peak did not exist. The inhomogenity of the cerium concentration had a large influence on the dielectric temperature characteristic.     

Simple design rules for optimal design of dielectric temperature-compensated sapphire resonators

It has been shown that the use of two dielectric crystals with opposite temperature coefficient of permittivity allows the realization of a resonator with a zero temperature coefficient of frequency. By using sapphire and rutile materials, which have low-loss tangents, some compensated resonators with very high Q-factors have been realized. In this work we develop rules that greatly simplify the design of a dielectric-compensated resonator. We show that the optimum design for compensation at a specific temperature may be determined by simply selecting the aspect ratio of the sapphire resonator.     

Tuning the properties of microwave dielectric resonators in a stack configuration

The microwave properties of ceramic dielectric resonators in a stack configuration are investigated using two different dielectric materials with opposite temperature coefficients of resonant frequency in the temperature range 50–300 K. The two dielectrics, which have been studied, are Ba₅Nb₄O₁₅ and 5ZnO–Nb₂O₅, synthesized by the conventional solid-state ceramic route. The temperature coefficient of resonant frequency of the stack resonator is tuned by varying the volume fraction of 5ZnO–Nb₂O₅. Temperature coefficient of resonant frequency slowly changed from a non-linear to almost a linear behavior with increase in the volume fraction of 5ZnO–Nb₂O_5.     

Electrostatic transducers for micromechanical resonators: free space and solid dielectric

Three electrostatic transduction methods are analyzed for a micromechanical, longitudinal mode, beam resonator. The conventional parallel plate transducer placed at the location of maximum displacement is compared to two solid, dielectric transducers internal to the resonator. Although the solid dielectric offers higher permittivity than the free-space-filled transducers, the unfavorable locations of the internal transducers reduce or even remove the performance advantage of the higher permittivity.