V.I. Anisimkin

Monitoring of NH3 gas by LB polypyrrole-based SAW sensor

Abstract Surface acoustic wave (SAW) devices have been fabricated and tested as sensors of NH3 in gaseous phase. Polypyrrole films, prepared by Langmuir-Blodgett (LB) technique, have been deposited onto the surface of SAW devices as gas absorbent layers. Simultaneous measurements of SAW phase velocity and attenuation have been carried out in order to investigate the sensing mechanisms. The sensor response shows high sensitivity towards NH3 gas and excellent selectivity with respect to main interfering gases (CO, CH4, H2, O2), at room temperature. The low cross-sensitivity permits the recognition of NH3 gas as one component of a mixture of interfering gases.

Gas sensing properties of Langmuir-Blodgett polypyrrole film investigated by surface acoustic waves

The gas sensing properties of organic polypyrrole (PPS) film, deposited onto LiNbO/sub 3/ substrate by Langmuir-Blodgett (LB) technique, have been monitored by surface acoustic wave (SAW) delay lines and studied with respect to sensitivity, selectivity, response time, stability, repeatability, and aging. The SAW PPy elements demonstrate high sensitivity toward NH/sub 3/ gas with high selectivity against CH/sub 4/, CO, H/sub 2/, and O/sub 2/. The detectable threshold concentration has been estimated as 20 ppm NH/sub 3/ in air; the response time is in the 10s range, and the recovery time is about 15 min; the repeatability of the SAW response toward eight sequential NH/sub 3/ gas exposures is within 6%; the aging of the PPy film is within 4% over a month; and the effect of humidity on SAW NH/sub 3/ gas response is negligible for the typical conditions at room ambient air. Partially reversible SAW response recognizing NH/sub 3/ gas as one component of an interfering gases-mixture has been observed. Simultaneous chemoresponses of SAW phase and insertion loss have been performed in order to investigate the sensing mechanisms. By merging with electrical conductivity gas response, the dominant SAW sensing effects for NH/sub 3/ gas detection are defined as elastic loading.

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).

Surface acoustic wave humidity sensors: a comparison between different types of sensitive membrane

Surface acoustic wave (SAW) sensors have been successfully exploited for the detection of both chemical and physical quantities. In this paper we report more recent results on SAW sensors for the detection of relative humidity (R.H.). Different chemical interactive materials (CIMs), of both organic and inorganic compounds, are tested and their performances in the R.H. range between 10 and 80% are reported and compared. All the devices analysed have been shown to operate in a reversible way, with a fairly good repeatibility and fast response times. The CIMs have been shown to produce a variety of different behaviours versus R.H. percentage, with respect to both the amplitude and linearity of the response. The response to R.H. has also been investigated for different orientations of both quartz and lithium niobate bare substrates. These data can be useful in order to investigate the effect of the R.H. of the surroundings on the performances of SAW based devices. In addition this test is of great importance in order to know how humidity influences the response of both chemical and physical sensors. The response of organic CIMs has also been tested toward hydroxylated molecules such as acetone and ethanol, and the relative calibration curves are reported.

Attenuation of acoustic normal modes in piezoelectric plates loaded by viscous liquids

Attenuation alphan versus viscosity eta of adjacent liquid is measured for each normal mode n generated in 30 different plates of commercially available, piezoelectric crystals with thickness-to-wavelength ratio in the range h/lambda = 0.6-2.5. Two modes with an optimal combination of sensitivity (0.1 dB/mmmiddotcP), insertion loss (<35 dB), and stop-band rejection (>15 dB) are found in liquid-loaded 128degY, X + 90deg-LiNbO3 with h/lambda = 1.67. Both modes are suited for viscosity measurements and other sensing tasks in viscous liquids. They have predominantly longitudinal displacement and large propagation velocity vn about 15,000 m/s

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.

General properties of the Anisimkin jr. plate modes

Acoustic plate modes of different orders n, having equal velocities vn close to that of the longitudinal BAW vL, are numerically studied in crystals of different symmetries. Three families of the modes with v_n ~ vL, each at relevant plate thickness h/λ = (h/λ)_n, are found (h is the thickness, λ is the wavelength): the generalized Lamb mode with comparable longitudinal u₁, shear-horizontal u₂, and shear-vertical u₃ displacements, the Anisimkin Jr. (AN) mode with u₁ ≫ u₂ and u₃, and u₁ ~ constant ≠ 0 at any depth, and the quasilongitudinal (QL) mode with u₁ > u₂, and u₃, but u₁ ≠ constant over the plate thickness. Existence of the families does not depend on anisotropy or piezoelectric properties of the plate, but on the closeness of the mode velocity v_n to the BAW velocity v_L, the value of the dispersion slope dv_n/d(h/λ) at v_n = v_L and h/λ = (h/λ)_n, and the proximity of the plate thickness (h/λ)_n supporting the mode, to the thickness (h/λ)_R providing transverse BAW resonance between plate faces. The Lamb modes approach v_n = v_L at irregular (h/λ)_n far from resonance (h/λ)_R and at large dv_n/d(h/_n) ~10³ m/s. The two other modes are characterized by lower dispersion dv_n/d(h/_n) <; 10³ m/s and regular (h/λ)_n close to the resonance (h/λ)_R. Because both modes have small vertical displacement on plate faces and propagate almost entirely within the crystals, they are attractive for liquid sensing.

Gas thermal conductivity sensor based on SAW

An uncoated surface acoustic wave (SAW) delay line integrated with a suitable film heater is used as a gas sensitive element based on thermal conductivity change. It detects concentrations from 0.1-100% of H/sub 2/, O/sub 2/, He, and Ar in N/sub 2/ in the temperature range from 20-150/spl deg/C. The value and sign of the SAW gas response are controlled by substrate material and reference gas.

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.

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.