X. Yao

Dielectric nonlinear characteristics of Ba(Zr0.35Ti0.65)O3 thin films grown by a sol-gel process

Ba(Zr0.35Ti0.65)O3 (BZT) thin films were deposited via a sol-gel process on Pt-coated silicon substrates. The BZT films were in the perovskite phase and had polycrystalline structure. Temperature-dependent dielectric measurements revealed that the thin films have relaxor behavior and diffuse phase transition characteristics. The tunability K of the dielectric constant (at 600 kV/cm) is about 40% in the temperature range of 179–293 K. The improved temperature stability from this BZT thin film is beneficial to applications requiring a wide range of operating temperatures, thereby eliminating the need for environmental controls. Although the K value is not extraordinarily large compared to (Ba,Sr)TiO3, the low dielectric constant of BZT is attractive for microwave frequency applications. This provides an additional possibility in balancing K and dielectric constant through materials engineering for optimum device performance.

Microwave characterization of ferrite particles

Several different mixture equations have previously been used to characterize the microwave properties of the ferrite particle-insulating medium mixtures. For very low particle concentrations, these equations give nearly the same results. But when the particle concentration is higher than a few per cent, these equations generally give different results. Based on microwave measurements and numerical calculations, the validity of five well-known mixture equations has been examined in this paper. The experimental results show that both the Bruggeman and QCA-CP (`quasi-crystalline approximation with coherent potential) equations can accurately describe the microwave properties of the ferrite-medium mixtures, and that the Lichtenecker, Logarithm and Maxwell-Garnett equations are not suitable for characterizing the ferrite-medium mixtures over a wide particle concentration range at microwave frequencies. The microwave intrinsic permeability and permittivity spectra of some BaZn2-xCoxFe16O27, Ba4Zn2-xCoxFe36O60 and Ba2Zn2-xCoxFe12O22 ferrite particles have been presented, which were calculated from the measurement data of the ferrite-wax mixtures using the Bruggeman equation. The microwave properties of these ferrite particles have also been discussed. In the 1-6?GHz range, both the real and imaginary parts of permeability of BaZn2-xCoxFe16O27 and Ba4Zn2-xCoxFe36O60 ferrite particles evidently increase with the increasing Co concentration. Both the real and imaginary parts of permittivity of Ba2Zn2-xCoxFe12O22 ferrite particles apparently decrease as the Co concentration increases.

Amorphous Pb(Zr, Ti)O3 thin film hydrogen gas sensor

Abstract The capacitive Pd/lead zirconate titanate (PZT)/Pt devices have been fabricated with amorphous Pb(Zrx, Ti1−x)O3 (x=0, 30, 53, 65, 90) thin films deposited using the sol–gel spin-coating technology. The PZT films have been characterized by TGA, DTA, X-ray diffraction (XRD), dielectric and electrical properties, and gas sensitivity measurement. It has been shown that the amorphous PZT film can be operative as a hydrogen gas sensor material. The gas sensitivity of Pd/PZT/Pt devices has been systematically studied, and it has been observed that the sensitivity of the turn-on voltage shift in dc I–V curves is as large as 2.3xa0V at 1000xa0ppm hydrogen gas diluted in air. Among the PZT films with different Zr/Ti ratios, it has been investigated that the PZT film with Zr/Ti=30/70 has the lowest dielectric loss, lowest leakage current, and highest gas sensitivity to hydrogen.

Electromagnetic anisotropy of magnetic iron fibres at microwave frequencies

The theoretical formulae of electromagnetic parameters of magnetic metal fibres were derived. Based on these formulae, the microwave electromagnetic parameters of magnetic iron fibres were calculated. The calculated results show that the magnetic iron fibres exhibit strong electromagnetic anisotropy at microwave frequencies; the permeability µ∥ parallel to the fibre is higher than the permeability µ⊥ perpendicular to the fibre; the permittivity e∥ parallel to the fibre is significantly higher than the permittivity e⊥ perpendicular to the fibre. Magnetic iron fibres of diameter about 2-5 µm were fabricated by the bundle-drawing method and anisotropic samples were prepared by dispersing iron fibres randomly in paraffin. During the preparation of the samples, a magnetic field was applied to orient the iron fibres. A method for determining the microwave electromagnetic parameters of anisotropic samples was described in great detail. Using this method, the electromagnetic parameters of the anisotropic samples containing oriented iron fibres were measured in the S band. The measurement results validated the electromagnetic anisotropy of the magnetic iron fibres.

The Synthesis, Characterization and Microwave Properties of ZnCo-Substituted W-Type Barium Hexaferrite, from a Sol-Gel Precursor

The preparation of BaZn2 − xCoxFe16O27 W-type hexaferrites powders by a citrate sol-gel method has been investigated. The samples were characterized by TG-DSC, X-ray diffraction (XRD), scanning electron microscopy (SEM). The complex dielectric constant and complex permeability of hexaferrite-paraffin wax composites were measured by the transmission/reflection coaxial line method in the range from 50 MHz to 3 GHz. The dependence of complex dielectric constant and permeability on annealing temperature, composition and measuring frequency was presented.

Enhanced electric field tunable dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films

Highly (100)-oriented barium stannate titanate Ba(Sn0.15Ti0.85)O3 (BTS) thin films were deposited on SrTiO3 (STO) and Nb doped SrTiO3 (NSTO) single-crystal substrates through sol-gel process, respectively. Coplanar capacitance and parallel plate capacitance structure configurations were formed by preparing interdigital electrodes (IDEs) and parallel plate electrodes (PPEs) on BTS thin film. The tunability of films with IDE was 45.54%, while the tunability of films with PPE was only 11.54% at the frequency of 1 MHz with an applied electric field of 80 kV/cm. This result showed that the tunability in the a-b plane contributes markedly to the measurements obtained with the IDE.

Improvement of dielectric properties of graded Co-doped (Ba0.7Sr0.3) TiO3 thin films fabricated by sol-gel method

The undoped, homogeneous and graded Co-doped (Ba0.7Sr0.3) TiO3 (BST) thin films with compositionally gradient from 0 mol% to 5 mol% were prepared on Pt /Ti /SiO2/Si (100) substrates by sol–gel technique. All of the thin films crystallized into a pure perovskite structure after post-deposition annealing. Dielectric properties of undoped, homogeneous and graded Co-doped BST thin films were investigated as a function of frequency and direct current bias field. These results showed that the graded acceptor doping was a promising technique to reduce the loss tangent and leakage current while maintaining weak change in tunability of BST thin film.

Preparation and Characteristics of Porous Silica Films by a Modified Base-Catalyzed Sol-Gel Process Containing PVA: II. Film Preparation

Porous SiO2 films were successfully deposited on silicon substrates by a modified base-catalyzed Sol-Gel process (MBCP) containing polyvinyl alcohol (PVA). The process conditions, such as the gelation time, the synthesis temperature, the stabilizing agent of the precursor solution and the spin coating speed, the heat-treatment, the annealing temperature of the film on the microstructure and porosity of porous SiO2 films were systematically investigated by SEM, XRD and ellipsometry techniques. This study provides a novel preparation technique for the porous SiO2 film. Using this process, the resultant film can reach a thickness of 3.6 μm for one layer, a porosity of 25–50%, a low thermal conductivity of 0.11 W/m·K. This film will be used as a low dielectric layer, an thermal-insulating layer and a low refractive index layer.

Photoluminescent and Dielectric Characterizations of Pr Doped CaBi2Nb2O9 Multifunctional Ferroelectrics

We examine the Pr3+ doped CaBi2Nb2O9 (CBNO) bismuth layered ferroelectrics and demonstrate that a certain amount of Pr3+ doped ceramic sample shows a red photoluminescence with blue and ultraviolet excitation at room temperature. The blue excitation attributes to the f-f transitions from the 3H4 ground state to the 3PJ (J = 0, 1, 2) excited states of Pr3+ and the ultraviolet excitation is associated with the electronic transition within the host solid respectively. The red emission is ascribed to 1D2→3H4 transition. The excitation of the blue wavelength region covers the emission wavelength of the commercial blue LED chips; meaning that Pr doped CBNO could be used for white LEDs. At the same time, CBNO is an intrinsic ferroelectric and piezoelectric material. Pr doped CBNO would be useful in multifunctional sensor, future electrical–optical integration and coupling devices.

Effect of Compositional Variations on Field-Induced Phase Transition of (Pb,La)(Zr,Sn,Ti)O3 Antiferroelectric Ceramics

(Pb,La)(Zr,Sn,Ti)O 3 ceramics near AFE-FE phase boundary were studied as a function of composition, electric field and temperature. Below the critical temperature T FE − AFE of FE to AFE phase transition, the electric field-induced FE state can maintain metastable state when electric field was removed. As temperature increased over T FE − AFE , the metastable FE recovered to AFE. As Ti content increased, the structure of unpoled PZST ceramics changed from tetragonal AFE phase to rhombohedral FE phase, the temperature T FE − AFE increased, while the switching field E AFE − FE and E FE − AFE decreased. The forward switching field E AFE − FE and hysteresis Δ E were observed to be dependent on T FE − AFE and Tc, respectively.