I. M. Kotelyanskii

Elastic properties of thin-film palladium for surface acoustic wave (SAW) sensors

Abstract The elastic constants and density of supported palladium film have been determined, together with their changes produced by exposure to a gas concentration of 1% H2 in N2 at room temperature and atmospheric pressure. The results have been obtained by comparing the experimental group and phase velocities of surface acoustic waves (SAWs) propagating along different substrates covered with the Pd film under test, with the theoretical values calculated using the perturbative approach. Measurements of the phase velocity have been made on the films both before and after exposure to H2. The data obtained have been compared with those relative to polycrystalline and single-crystal palladium and palladium hydride (PdH0.66).

Analysis of the different contributions to the response of SAW gas sensors

Numerical estimations of the mass and elastic loadings are performed for different possible material combinations. In order to do this, the density and elastic constants of the film are changed consequently and one by one for each of the combination selected in accordance with Farnell-Adlers theory. In addition, the perturbation approach is applied to analyse the SAW gas response in analytical form. On considering, as an example, Pd coated SAW delay lines exposed to hydrogen, the relative changes in Pd parameters are measured, the mass and elastic loadings are estimated and the SAW hydrogen responses are theoretically predicted and experimentally verified. An increase of a factor 10 in the SAW gas response is demonstrated for Pd film deposited on different substrate materials and/or crystallographic orientations.

Deposition of CeO2 films including areas with the different orientation and sharp border between them

Epitaxial films from one material, with sharp borders between contacting regions having different film orientation are grown on one surface of the substrate for the first time. The main reason for the deposition of thin ceria layers with mixed (001) and (111) orientations on a (1102) sapphire substrate is determined. We suggest that this is related to the availability of surface defects which, in thin near-surface layers, deviate from stoichiometric composition. This in turn is connected with the loss of oxygen. A technique for influencing CeO2 film orientation is demonstrated. This involves specific preliminary processing of the substrate, and the selection of oxygen partial pressure during the deposition process. High quality thin (30–50 nm) “protective” (001) CeO2 epitaxial layers are prepared on (1102) Al2O3. Structures comprising two epitaxial protective CeO2 layers, orientations (001) and (111), are made on the base of (0001) and (1102) sapphire substrates. The interface between the epitaxial layers is <1 000 nm. Preliminary results using this method are described, and the possibility of creating a “bi-epitaxial” transition in thin YBa2Cu3O7−x layers is explored.

Magnetic field tunable acoustic resonator with ferromagnetic-ferroelectric layered structure

High overtone acoustic resonator with yttrium iron garnet/zinc oxide layered structure was theoretically considered, fabricated, and experimentally investigated. The theory of the resonator, containing an arbitrary number of magnetic and nonmagnetic dielectric/ferroelectric layers, placed in a transverse magnetic field is presented. The simulation shows the possibility to tune the resonant frequency in the range of ±1 MHz by magnetic field. This tuning is due to the resonance magnetoelastic interaction in the saturated ferrite film and the total phase shift of acoustic wave in the structure. The experiment proves the magnetic field influence on resonance frequencies and attenuation of transverse wave with polarization vector quasicollinear with the field direction. The tuning about 0.25 MHz near the acoustic resonant frequency 2 GHz was obtained in the field 260 Oe. This frequency is close to the ferromagnetic resonance frequency in ferrite film, corresponding to the field applied.

Analysis of acoustic waves in multilayers using compound matrices

A modification of the matrix formalism for studying acoustic wave propagation in anisotropic piezoelectric multilayers is developed. The modification is based upon the use of the compound matrices, well known from the theory of the matrices. Using the modified method, analytical expressions for the relationship between the real and imaginary parts of the determinant of the boundary conditions are derived. One of the advantages of the method is demonstrated for thick plates and thick film structures. On considering the dispersion curves for thick multilayers, the slow dispersion regions inherent for anisotropic materials are discovered. The regions are related with the cutoff velocities of the bulk waves in the film materials.

Terahertz spectroscopy based on high-Tc Josephson junctions

A review of the authors’ studies on the spectroscopy employing the ac Josephson effect in high-temperature superconducting junctions is presented. It is demonstrated that Hilbert spectroscopy and admittance spectroscopy can be successfully implemented in the terahertz frequency range with the use of high-temperature superconductor bicrystal Josephson junctions.

Real time characterization of elastic variations in palladium films produced by hydrogen adsorption

The magnitude and time variations of the SAW propagation velocity /spl Delta/v/v, of the mass and elastic loading, and of the fractional changes in the density /spl Delta//spl rho///spl rho/ and in the elastic modulii /spl Delta/c/sub ij//c/sub ij/ of Pd films are experimentally evaluated at room temperature and atmospheric pressure for 0.1, 0.5, 1% H/sub 2/, 1% CO, 1% N/sub 2/O and 1% NO gas mixtures in pure nitrogen. Two different thermally evaporated Pd films have been tested: one as evaporated, the other one after annealing at 423/spl deg/K and 10/sup -5/ Pa for 15 hours. The annealing is shown to change all set of the parameters. For not annealed films, hydrogen gives rise to large variations in the elastic constants (/spl sime/5-40%) and to an increase in the phase velocity; while for annealed films, it produces small variations in the elastic constants (/spl sime/1-2%) and a decrease in the phase velocity. The difference is attributed to weakly and strongly coupled oxygen preadsorbed in the film. The results obtained can allow an improvement of Pd-based sensors.

New buffer sublayers for heteroepitaxial III–V nitride films on sapphire substrates

AbstractIt is suggested to deposit III–V nitride films onto sapphire substrates upon preliminary deposition of a buffer sublayer of a crystalline material with a cubic structure. It is shown experimentally that the deposition of a heteroepitaxial niobium sublayer onto a (0001)-oriented sapphire substrate or a niobium nitride sublayer onto a (

The growth and domain structure of YBa2Cu3Ox films on neodymium gallate substrates with a deviation of the normal to the surface from the [110] direction in NdGaO3

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