Fabrication and electrical properties of textured Na1/2Bi1/2TiO3-BaTiO3 ceramics by reactive-templated grain growth

The <001> fiber-textured Na_1/2 Bi_1/2TiO₃-BaTiO₃ (6 mol% BaTiO₃) ceramics were fabricated by reactive-templated grain growth (RTGG), using plate-like Bi₄Ti₃O₁₄ (BiT) particles prepared by a molten salt method as templates. The effects of sintering conditions on texture development and microstructure evolvement were both studied, and the mechanisms of grain orientation and densification were discussed. High Lotgering factor (≥96%) and high density (≥96% theoretic density) textured Na_1/2 Bi_1/2TiO₃-6BaTiO₃ ceramics were prepared by using the max templates concentration supplying 100% Bi in the final product, and sintering at 1200 °C for 10 h. The NBT-6BT obtained exhibited good piezoelectric performance with piezoelectric coefficient d ₃₃ ?=?241pC/N, and electromechanical coupling factor k _p ?=?41.2%, k _t ?=?66.5% at room temperature.     

Nanocomposite ceramics based on La-doped BaTi2O5 and BaTiO3 with high temperature-independent permittivity and low dielectric loss

Nanocomposite ceramics containing a mixture of two ferroelectric phases, La-doped BaTi₂O₅ and BaTiO₃, with carefully-controlled phase amounts and ceramic microstructure have temperature-independent permittivity and low dielectric loss over very wide temperature ranges: ɛ = 95 ± 10 from 25 to 600 °C; tan δ = 0.02(2) from 25 to 400 °C, measured at 100 kHz. Further optimisation of properties should be possible.     

Novel materials in magnetic resonance imaging: high permittivity ceramics,metamaterials, metasurfaces and artificial dielectrics

This article reviews recent developments in designing and testing new types of materials which can be: (i) placed around the body for in vivo imaging, (ii) be integrated into a conventional RF coil, or (iii) form the resonator itself. These materials can improve the quality of MRI scans for both in vivo and magnetic resonance microscopy applications. The methodological section covers the basic operation and design of two different types of materials, namely high permittivity materials constructed from ceramics and artificial dielectrics/metasurfaces formed by coupled conductive subunits, either in air or surrounded by dielectric material. Applications of high permittivity materials and metasurfaces placed next to the body to neuroimaging and extremity imaging at 7 T, body and neuroimaging at 3 T, and extremity imaging at 1.5 T are shown. Results using ceramic resonators for both high field in vivo imaging and magnetic resonance microscopy are also shown. The development of new materials to improve MR image quality remains an active area of research, but has not yet found significant use in clinical applications. This is mainly due to practical issues such as specific absorption rate modelling, accurate and reproducible placement, and acceptable size/weight of such materials. The most successful area has been simple “dielectric pads” for neuroimaging at 7 T which were initially developed somewhat as a stop-gap while parallel transmit technology was being developed, but have continued to be used at many sites. Some of these issues can potentially be overcome using much lighter metasurfaces and artificial dielectrics, which are just beginning to be assessed.

     

Fabrication of three-dimensional electromagnetic band-gap structure with high-K dielectric ceramics by rapid-prototyping

Three-Dimensional diamond structure electromagnetic band-gap (EBG) structures containing high-K dielectric ceramic Bi(Nb_0.992 V_0.008)O₄ (BVN) fabricated by rapid-prototyping (RP) technique were investigated. The simulations based on finite element method (FEM) were employed to model the band diagram. The influences of structure dimensions, aspect ratio and permittivity contrast on the band gap width were studied. The optimal band gap width EBGs were fabricated and investigated experimentally. EBG structures composed of epoxy resin diamond lattice and inverse-diamond lattice made of high-K BVN ceramic with silica gel were fabricated by RP method. The transmission characteristics of the EBG structures were measured by transmission/reflection (T/R) methods with a vector network analyzer. Obvious wide band-gaps of two EBGs with different lattice parameters were observed in the curve of transmission characteristics, which agreed well with the simulation results.     

Ferroelectric and piezoelectric properties of (Bi1/2K1/2)TiO3 ceramics fabricated by hot-pressing method

Bismuth potassium titanate, (Bi_1/2K_1/2)TiO₃ (BKT), ceramics were prepared by the hot-pressing (HP) method without dopant and with dopants of Bi₂O₃, La₂O₃ and MnCO₃. The relative density of BKT ceramics hot-pressed at 1,060 and 1,080 °C (hereafter abbreviated to BKT-HP1060°C and BKT-HP1080°C) and x mass% Bi₂O₃, La₂O₃ and MnCO₃ doped BKT ceramics hot-pressed at 1,060 °C (hereafter abbreviated as BKTBix; x?=?0.1–0.6, BKTLax; x?=?0.1–0.6 and BKTMnx; x?=?0.1–0.3) were all higher than 97%. In this study, the ferroelectric properties of BKT ceramics were successfully obtained, and the remanent polarization P _r and coercive field E _c of BKT-HP1080°C were 22.2 μC/cm² and 52.5 kV/cm, respectively. A small amount of La tends to increase P _r, and the P _r of BKTLa0.1 was 19.2 μC/cm². The piezoelectricities were improved to optimize poling conditions, and the electromechanical coupling factor k ₃₃ and piezoelectric constant d ₃₃ of BKT-HP1080°C were 0.34 and 82.8 pC/N, respectively.     

Colossal dielectric permittivity of BaTiO3-based nanocrystalline ceramics sintered by spark plasma sintering

In pursuit of high permittivity materials for electronic application, there has been a considerable interest recently in the dielectric properties of various perovskite oxides like calcium copper titanate or lanthanum doped barium titanate. When processed in a particular way, this later material present at ambient temperature and at f?=?1 kHz unusual interesting dielectric properties, a so called “colossal” permittivity value up to several 10⁶ with relatively low dielectric losses. Moreover and contrary to what is classically expected and evidenced for this type of materials, no temperature dependence is observed. This behavior is observed in nanopowders based ceramics. An assumption to explain the observed properties is proposed. These results have important technological applications, since these nanoceramics open a new route to the fabrication of very thin dielectric films.     

The optical properties of microbulb alumina ceramics at high temperature

The normal spectral emissivity of two types of high-temperature ceramics made of alumina microbulbs as a function of the wavelength and temperature was investigated experimentally for the first time. Data for samples of various thicknesses (2–8 mm) at wavelengths of 1.0–5.0 μm and temperatures of 1073–1873 K were obtained. The ceramics optical properties in the forms of an effective absorption factor and a radiation diffusion factor were calculated in the wavelength band from 3.2 to 4.6 μm using the dependence of emissivity on thickness. The effect the porosity of ceramics has on its optical properties was analyzed.     

Sintering and dielectric properties of Al2O3 ceramics doped by TiO2 and CuO

The effects of CuO and TiO₂ additives on the microstructure and microwave dielectric properties of Al₂O₃ ceramics were investigated. Al₂O₃ ceramics with CuO and TiO₂ additions can be well sintered to achieve 93∼98% theoretical densities below 1,360 °C due to Ti₄Cu₂O liquid phase sintering effect. The Qf values decreased with increasing CuO and TiO₂ content, due to the formation of the second phase Ti₄Cu₂O. However, the varying behaviors of the dielectric constant (ɛ _r ) and temperature coefficients (τ _f ) were associated with phase constitutions, as a result of the change of CuO and TiO₂content. The τ _f can be shifted close to 0 ppm/°C by controlling the content of CuO and TiO₂. The specimens with 0.5 wt.% CuO and 7 wt.% TiO₂ sintered at 1,360 °C for 4 h showed ɛ _r of 11.8, Qf value of 30,000 GHz, and τ _f of −7 ppm/°C.     

Fabrication and characterization of micropatterned barium titanate ceramics

A new patterning method combining electron beam (EB) lithography and electrophoretic deposition (EPD) for fabricating micropatterned barium titanate (BaTiO₃) thin films was investigated. At first, resist molds with high resolution were prepared using EB lithography on Pt/Ti/Si substrates. Then BaTiO₃ nanoparticles were deposited on the substrates by EPD from a transparent suspension of monodispersed BaTiO₃ nanoparticles; a mixed solvent of 2-methoxyethonal and acetylacetone with a 9:1 volumetric ratio was used as a dispersion medium. The nanoparticles with an average size of about 10 nm were synthesized at a low temperature of 90 °C by a high concentration sol-gel process. EPD layers superfluously deposited on the resist molds were mechanically polished away, followed by chemically removing the molds in a resist remover to leave micropatterns of BaTiO₃ nanoparticles on the substrates, which were finally sintered to yield micropatterned BaTiO₃ ceramic thin films. The method developed may be used to fabricate other micropatterned electroceramic thin films.     

Fabrication of amorphous bulk and multi-phase ceramics by melting method in the HfO2-Al2O3-Gd2O3-Eu2O3system

Ceramics have generally been fabricated from powders by shape forming & sintering methods except for glasses and glass ceramics. Glasses and glass ceramics can be fabricated by melting methods. The melting method has not only higher productivity but also higher shape forming ability than powder processes via forming & sintering methods. Thus we have reinvestigated melting methods in binary and ternary oxides systems to fabricate amorphous bulk ceramics and bulk nano composites. We have successfully fabricated amorphous phases by simple melt solidification methods in ternary eutectic melts in the HfO₂-Al₂O₃-Gd₂O₃system. The present study demonstrates the formation of the amorphous phases in quaternary systems HfO₂-Al₂O₃-Gd₂O₃-Eu₂O₃. Furthermore, we have also succeeded to fabricate nano-structured bulk ceramics, which consisted of constituent oxide grains with 20–100 nm in size, by post annealing of the amorphous phase.     

Microwave dielectric properties of low-firing BiNbO4 ceramics with V2O5 substitution

The effect of V₂O₅ substitution on the sintering behavior and the microwave dielectric properties of BiNbO₄ ceramics were studied. The sintering temperatures of Bi(V _x Nb_1?x )O₄ ceramics decrease from 990 to 810°C with x value increasing from 0.002 to 0.064. The size of grains increased with the sintering temperature increasing and decreased with the substitution amount increasing. The dielectric properties are affected by the microstructures very much. The quality factor Q value is from 2500 to 4000 at about frequency?=?5 GHz and reach to the maximum when x?=?0.032. With the different x value, the Q _f values change between 15000 to 20000 GHz; the τ _f values changes between 0 and +20 ppm/°C between temperature range 25～85°C and decreased with the increasing of x value.     

Acoustic properties of PLZT ceramics studied by Brillouin scattering

Brillouin spectra of lanthanum lead zirconate titanate (Pb_1−x La_x)(Zr_yTi_1−y )O₃ (PLZT-100(x/y/1−y)) relaxor ferroelectric ceramics with the famous composition of PLZT-10/65/35 have been investigated using a high-resolution Brillouin spectroscopy. The central peak of PLZT-10/65/35 appeared below ∼600 K, which is similar to the Burns temperature. The integrated intensity of the central peak grew on cooling and reached its maximum value at around 370 K, which is higher than the dielectric maximum temperatures. The overall shape and width of central peaks did not change in the electric field range of 0–7 kV/cm at room temperature. These observations were discussed in relation with the dynamics of nano-sized polar clusters, which have been suggested as the origin of the complex dynamical behaviors of ferroelectric relaxors.     

Dielectric and piezoelectric properties of PZT ceramics with anisotropic porosity

Pb(Zr,Ti)O₃ ceramics with homogenous, anisotropic porosity were manufactured and the influence of the anisotropic porosity on the dielectric and piezoelectric properties was studied. Image analysis allowed the quantification of the preferential orientation and the calculation of the distribution of the aspect ratio of the pores. The different dependence of the permittivities e₃₃^Tvarepsilon _{33}^T and e₁₁^Tvarepsilon _{11}^T on the porosity content demonstrates that the population of pores is constituted of a high volume of pores with low aspect ratio and a low volume of pores with high aspect ratio. In view of the quantitative results obtained on the microstructure, an explanation of the frequency dependence of the dielectric and piezoelectric behaviour is suggested. Furthermore, the higher d₃₁ piezoelectric coefficient in the material with anisotropic pores than in material with isotropic pores is related to higher effective fields which are a consequence of the enhanced longitudinal mobility of the material under transverse stress.     

Microwave dielectric properties of Mg4Nb2O9 ceramics produced by hydrothermal synthesis

Mg₄Nb₂O₉ ceramics have been prepared by a hydrothermal synthesis in order to reduce the sintering temperature. The sintering and microwave dielectric properties of the hydrothermally processed Mg₄Nb₂O₉ were studied under various sintering temperatures ranging from 900 to 1300°C. The highest Q×f _o value of 26,069 GHz was obtained at the sintering temperature of 1300°C and is attributed to the increased density and appropriate grain growth. τ _f value of ?17.1 ppm/°C was improved by the addition of TiO₂ and τ _f value of 6.7 ppm/°C was obtained at 20 wt% TiO₂. Chemical compatibility of Mg₄Nb₂O₉ with Ag was tested to identity the possibility of using Mg₄Nb₂O₉ for an LTCC application. Since any secondary phase was not observed in the XRD pattern of the mixtures of Mg₄Nb₂O₉ and Ag powder heat treated at 900°C, it was considered that the Mg₄Nb₂O₉ system is applicable to the multilayer microwave devices using Ag as an electrode.     

Self-constrained sintering of Al2O3/glass/Al2O3 ceramics by glass infiltration

Though need for precise alignment of interlayer patterning in LTCC application, there have been few reports about zero-shrinkage sintering techniques. In this study, ceramic substrate with minimal x–y shrinkage was prepared by glass infiltration method with ‘Al₂O₃/glass/Al₂O₃’ structure. Glass infiltration into alumina particle layer was observed with variation of both sintering temperature (700?≤?T _sint.?≤?900 °C) and alumina particle size distribution (0.5?≤?D ₅₀?≤?1.8 μm). Since glass had low viscosity enough to infiltrate at 700 °C, infiltration started at that temperature and infiltrated up to 20 μm or so with temperature increase, but infiltration depth did not increase noticeably above 750 °C. Based on these results, when sintered at 900 °C with controlled sheet thickness of both glass and alumina, the shrinkage in x–y direction was calculated as less than 0.2%, with 40% in z direction. Dielectric constant (? _r) measured 6.19 with quality factor (Q) of 552 at 1 GHz of frequency. From these results, it is thought that zero-shrinkage ceramic substrates would be obtained without de-lamination.     

高倍率LiMn_2O_4锂离子电池的制作与性能

采用商品化的LiMn2O4和石墨作为正极材料制作锰酸锂动力电池,并利用XRD、SEM等分析手段表征了LiMn2O4原料。研究了不同面密度和导电剂含量对锰酸锂电池倍率性能的影响。研究发现,锰酸锂电池的倍率性能随着面密度的减小而改善,随着导电剂含量的增加先改善后变差。当正极面密度未2.5 g/dm2,导电剂含量为3%时电池的倍率性能最好。20 C放电容量为1 C的94.1%,1C充电5 C放电,100次循环后容量保持率为92%。     

Fabrication of yttrium aluminum garnet transparent ceramics from yttria nanopowders synthesized by carbonate precipitation

Nanocrystalline yttria powders have been successfully synthesized by using yttrium nitrate as starting salt and ammonium hydrogen carbonate as precipitant. It was found that a small amount of ammonia sulfate in the yttrium nitrate solution can effectively reduce the agglomeration and the resultant powders display good dispersion. Pure cubic phase yttria powders were prepared by calcining the precipitate at 1100 °C for 4 h. The size of the as-prepared yttria powders was about 60 nm in diameter and showed excellent sintering activity. The as-prepared yttria nanopowders and commercial ultrafine alumina powders were used to fabricate YAG ceramics by solid-state reaction method and transparent YAG ceramics have been obtained by vacuum sintering at 1760 °C for 6 h.     

Band-gap properties of 2D photonic crystal made by silica matrix doped with magnetic nanoparticles

We present a study of the band gaps of a variety of two-dimensional photonic crystal made by \(\text {SiO}_{2}/\text {ZrO}_{2}\) and \(\text {SiO}_{2}/\text {TiO}_{2}\) matrix doped with magnetic nanoparticles as function of the filling factor parameter for different refractive index contrasts. The results obtained are useful for better designs of magneto photonic crystal devices.     

Multiferroic ceramics Pb(Fe1/2Nb1/2)O3 doped by Li

We present the results of obtaining and investigating multiferroic ceramics Pb(Fe_1/2Nb_1/2)O₃ (PFN) and PFN doped by various amounts of Li (PFN:Li). Ceramics have been obtained from oxides by two-step synthesis. For obtained samples the X-ray diffraction patterns, microstructure, DC electric conductivity, the temperature dependencies of dielectric permittivity and dielectric losses, electromechanical properties and hysteresis loops have been investigated. Obtained results have shown that the introduction of Li decreases electric conductivity and improves dielectric and electromechanical parameters important for applications.     

Fabrication and properties of 1-3-2 multi-element piezoelectric composite

A design method is presented for 1-3-2 type multi-element piezoelectric composite so as to provide basis for the development and study of transducer and transducer array. The composite samples have been fabricated by a twice dice-filling process from lead zirconate titanate (PZT-4), epoxy resin and silicone, the properties of the composites have been measured and investigated physically. The results show that the piezoelectric, dielectric and electromechanical coupling properties of the composite exhibit a good consistency, while it has a high thickness electromechanical coupling factor K _t and a wide bandwidth of pulse-echo signal. The impedance plot of every element indicates good behavior that all elements show excellent frequency consistency, and on the other hand the chance of coupling of lateral resonance and thickness resonance is very small, and with a clear thickness resonance mode at around 100?kHz. Silicon has extremely decoupling properties which can efficiently block the transmission of the acoustic vibrations and eliminate coupling effect among elements. And the farther the distance between the incentive element and receiving element is, the smaller the response signal become, while the response signal decay rate exhibits the trend of increase, the largest up to 95%.