Year 2015, Volume 35

Abstract

Ba(Zr_xTi_1-x)O₃ (BZT) films with different Zr contents were deposited on (100)MgO substrates by RF-magnetron reactive sputtering using metal targets. The BZT films epitaxially grew on MgO substrates with only (001)/(100) orientation and had a single perovskite phase. In all cases, the ratio of Ba/Ti was stoichiometry and BZT films possess a dense microstructure. The effect of Zr content on the crystal structure was studied. The grain size was decreased with increasing Zr content. The dielectric properties of the BZT films were also measured at high frequencies region. The reliability of the high-frequency dielectric properties extracted from the measured S11 reflection coefficients. The capacitance for BZT film showed little dispersion and low dielectric loss at 1-18 GHz. These results indicated that we succeeded in depositing high-quality and potential tunable ferroelectrics for high frequency applications.

Abstract

MgNb₂O₆, a well-known ceramic material with columbite crystal structure, has attracted much attention in microwave integrated circuits and optical applications due to its superior properties, such as its high dielectric constant and an optical band gap. In this work, transparent amorphous-MgNb₂O₆ thin films were fabricated on ITO/glass substrates using the sol-gel method. The average optical transmission percentage in the visible range (λ=400–800 nm) is over 80% for all MgNb₂O₆/ITO/glass samples, while the optical band gap is estimated at ~4 eV. On the other hand, the dielectric properties of the MgNb₂O₆ thin films were very sensitive to the annealing conditions. In this study, the dielectric constant of the films was calculated to be higher than 30 under a 100 kHz AC electric field. The effects of the annealing temperature and atmosphere on the dielectric properties of the MgNb₂O₆ thin films were investigated.

Abstract

Submicron-structured (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (BNBT6) dense ceramics, from nanometric powder synthesized by sol gel auto-combustion at 500ºC and obtained by hot-pressing (800ºC-2h) and subsequent recrystallization at moderate temperature (1000-1050ºC-1h), have been studied. In-situ measurements at the shear mode of electromechanical resonance of non-standard thickness-poled shear plates as a function of the temperature show higher depolarization temperature than measurements at the radial mode of thin disks. Shear mode related material coefficients are measurable up to 160ºC, being k₁₅≈30% and d₁₅≈250 pC.N^-1 at 130ºC. Depolarization is a complex phenomena caused by a ferroelectric (FE) macrodomains thermal randomization and a phase transition from the field-induced FE phase to a relaxor phase. The early stage of such a transition involves a non-negligible piezoelectricity arising most probably by the percolative coexistence of ferroelectric macrodomains in the resonator under the given stress field for each resonance mode.

Abstract

A sol-gel method has been utilized to prepare lead-free 2 mol% Er-doped K_0.5Na_0.5NbO_3- -xLiNbO₃ (Er-KNN_-xLN, x = 0, 0.08 and 0.10) ceramics. The X-ray diffraction results show that the ceramics have the perovskite structure and Er³⁺ ions have diffused into the host lattice. Compared to the solid-state reaction method, the sol-gel method is favor to reduce the sintering temperatures (~ 50 C) of ceramics. Better compositional homogeneity, as well as finer and uniform grains is resulted from the sol-gel process, which have been confirmed by the scanning electron microscope images. The photoluminescence properties of ceramic bulks have been systemically studied. Under the excitation of 980 nm, the ceramics exhibit visible up-conversion green (510-570 nm) and red (640-685 nm) emissions. The emission colors of them are located in the yellowish green region. The down-conversion emissions in near-infrared (1450-1650 nm) and mid-infrared (2600-2870 nm) regions have been exhibited. The ferroelectric, dielectric and piezoelectric properties of ceramics have also been investigated. Because of the good electrical and excellent photoluminescence performances, our sol-gel-derived Er-doped KNN-LN ceramics should have great potential for multifunctional optoelectronic applications, such as optical-electro integrated materials and devices.

Abstract

K0.5Na0.5NbO3 (KNN) was manufactured by spark plasma sintering (SPS), which is a fast sintering method allowing to control the grain growth. Different samples of KNN are sintered with SPS at 920°C under 50 MPa for 5 minutes. High densities over than 97% are achieved. In order to make domain engineering, KNN crystals are grown by floating zone method. Stable molten zone is reached when oxygen or nitrogen gas flux is used, leading up to 50 mm length of crystals. High electromechanical coupling factor
kt about 46%, kp around 45% and εs33/ε0 of 253 are achieved for KNN ceramics poled at optimum electric field about 3 kV / mm. KNN crystal boule exhibits kt
 about 40% against 34% for KNN ceramic, both poled at 1 kV / mm. These results are promising to replace PZT for transducers applications.

Abstract

Material research and development on piezoelectric ceramics, especially lead-free ceramics, was proposed from a viewpoint of relationships between piezoelectricity and elastic constants such as Young’s modulus and Poisson’s ratio. We developed a method to be convenient to measure acoustic wave velocities by an ultrasonic thickness gauge with high-frequency. From the change in longitudinal and transvers wave velocities before and after DC poling, it was found that the ceramic bulk density was important to improve the piezoelectricity in lead-free ceramics. As a result, the candidates of lead-free ceramic compositions with higher piezoelectricity were proposed. Furthermore, the ratio of transvers wave velocity to longitudinal wave velocity was clarified to estimate compositions with higher piezoelectricity. The measurement of sound velocities was an effective method for researching and developing piezoelectric materials, and it was possible to design the material compositions of lead-free piezoelectric ceramics as well as lead-containing ceramics by the novel measuring method.

Abstract

Samples exhibiting substitution of Titanium in Bismuth-Sodium-Titanate (BNT) by isovalent Zirconium, Tin and Germanium (Bi_0.5Na_0.5Ti(_1-x)D_xO₃ with D = Zr, Ge and Sn and x = 0.0025; 0.0050; 0.01; 0.02; 0.04; 0.08) were investigated with respect to microstructure, relative permittivity, loss factor and polarization. The substituents differ in mass, ionic radius and d-electron configuration. The shift of transitions observed in plots of permittivity and loss factor vs. temperature indicate the influence of the ionic radius. The occurrence of a pinched hysteresis loop at ambient temperature in the Sn-doped samples and the absence of any pinching in the hysteresis loops of Zr- and Ge-doped samples give rise to the assumption that this feature of polarization is connected to the ionic radius combined with the d¹⁰ electron configuration of Sn⁴⁺.

Abstract

Recently, wireless energy transfer techniques are being developed for hybrid (HEV) or electric (EV) vehicles. However, without taking any measures, magnetic field strength around energy transfer systems that employ solenoid type antennas is known to exceed regulated values in most countries. In this study, the application of magnetic sheets, which are widely used as EMC components, is proposed to suppress the magnetic field leakage from an automobile energy transfer system and the ability of magnetic leakage suppression with the magnetic sheet has been evaluated by means of electromagnetic simulation. Through these investigations, desirable system arrangements are discussed. As a result, if combined with a metal plate, non-conductive magnetic materials having large permeability has been found to be preferable.

Abstract

BiFeO₃ films undoped and doped with Ba and/or Ti have been fabricated through Metal-Organic Chemical Vapor Deposition (MOCVD) on SrTiO₃ (100), SrTiO₃:Nb (100) and YSZ (100) substrates. Films have been deposited using a multi-metal source, consisting of the Bi(phenyl)₃, Fe(tmhd)₃, Ba(hfa)₂•tetraglyme and Ti(tmhd)₂(O-iPr)₂ (phenyl= -C₆H₅, H-tmhd=2,2,6,6- tetramethyl-3,5-heptandione; O-iPr= iso-propoxide; H-hfa=1,1,1,5,5,5-hexafluoro-2,4- pentanedione; tetraglyme = CH₃O(CH₂CH₂O)₄CH₃) precursor mixture. The structural and morphological characterization of films has been carried out using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Chemical compositional studies have been performed by energy dispersive X-ray (EDX) analysis. Structural and morphological characterizations point to the formation of crystalline phases and homogeneous surfaces for both undoped and doped BiFeO₃ films. Piezoresponse force microscopy (PFM) and piezoresponce force spectroscopy (PFS) have been applied to study the piezoelectric and ferroelectric properties of the films.

Abstract

Cu wires were heated by electric currents to fabricate Cu₂O thin films. The films were successfully deposited on glass substrates placed above the wires at air pressures in the range of 100 - 40 Pa. Then the films were annealed in a controlled atmosphere and investigated the crystallinity, morphology and the time response to illumination of the films. After annealing at 800 oC at atmospheric pressure with oxygen partial pressure of 12 Pa, the crystallinity was increased and the time response of photoconduction was successfully improved.

Abstract

The transient stage is critical due to the stress induced by the chemical and thermal strain. In order to predict this strain, the oxygen activity field through the membrane needs to be known. Usually, the membrane is divided into three zones: the bulk where diffusion takes place and the two surfaces where exchanges between atmosphere and membrane take place. Oxygen bulk diffusion is well described by the Wagner theory. A consensus has not yet emerged regarding the surface exchange models proposed in the literature. Moreover, these models describe the permanent state, and cannot be extended to the transient stage. A new macroscopic surface exchange model which allows computing transient stage is proposed. This model assumed that the oxygen flux is governed by the association/dissociation of adsorbed oxygen and by the high energetic cost of oxygen reduction/oxidation. Then, the balance of transient specie only present on the surface is introduced to account for these two phenomena. The oxygen activity fields predicted by the proposed model are in agreement with the measures of chemical potential drop between the membrane and the atmosphere in permanent state. Transient stage measured during isothermal expansion test is partially reproduced.

Abstract

Ceramics based on Y-doped BaCeO₃ were prepared by citrate process. The X- ray fluorescence
shows that barium stoichiometry deviation is established during calcination step. Through the SEM
analysis of fracture surface of sintered samples it was possible to infer that powder highly deficient
of barium shows higher diffusion coefficient during sintering than stoichiometric powder promoting
grain growth and, consequently, trapping pores into the grains.