Gas-sensitive Films Research Articles

Partial Casting Method on Water Surface and Preparation of Multi-Channel Saw Chemical Sensor

Surface acoustic wave (SAW) devices can be used as vapor phase chemical sensors. If we make use of SAW devices with high frequency properties and multi-channel structures, highly sensitive and multi gas sensors can be realized. However, it is generally difficult to deposit different gas sensitive films on the propagation paths of monolithic multi-channel SAW devices. This paper describes the new coating method by which several types of gas-sensitive molecular films can be coated on the paths. The method is called partial casting method on water surface. It makes use of the water surface segmented by a trapezoidal frame, and the molecular film spread in the frame is compressed by the vertical movement of the frame and deposited on the substrate placed on the frame top. Several bilayers of dialkyl polyion complex and dimyristoyl phosphatidyl ethanolamine were coated on 36°Y-X LiTaO 3 SAW delay line oscillators, and odor sensor characteristics were successfully measured. It was confirmed from the present experiments that the method can be applicable to fabrication of multi-channel SAW chemical sensors with various kinds of molecular films.

NO x response of tin dioxide based gas sensors

Three types of SnO 2 based sensors developed at Swansea have been tested in NO x containing environments. Polycrystalline Bi 2O 3 doped devices exhibit a reverse sensitivity to NO x both in air and in nitrogen at temperatures of 300 °C or less. In this case sensor resistance falls markedly upon exposure to the oxidising gas. In contrast sensors prepared from undoped SnO 2 which had been precalcined at 1500 °C behave in a conventional manner, where marked resistance increases are observed in the presence of NO x . Gas sensitive thin films fabricated via the oxidation of a metallic Sn layer display high sensitivity to NO x while remaining unresponsive to common interfering gases such as carbon monoxide or methane.

A new microstructured silicon substrate for ultrathin gas-sensitive films

Microstructured substrates featuring interdigitated electrodes with a high length-to-width ratio allow the evaluation of thin films with very high sheet resistivity even in environments with large electromagnetic interference. However, thin phthalocyanine (Pc) films vacuum evaporated on substrates with conventional interdigitated (ID) electrodes deposited on top of a planar surface, develop cracks at the edges of the contact stripes. For samples with a film thickness of 200 monolayers (ML) corresponding to 80 nm this causes a resistance two orders of magnitude higher than expected for the chosen length-to-width ratio. A new microstructured Si substrate with in-plane interdigitated (IPID) electrodes is designed to achieve minimum height differences on the surface. This substrate features interdigitated electrodes with a length-to-width ratio of 52 000:1 and, additionally, an integrated heating element and a temperature-sensitive resistor. Thin Pc films with various thicknesses between 50 ML ( = 20 nm) and 500 ML ( = 200 nm) exhibit sheet resistivities as were expected for the chosen length-to-width ratio. These sensing elements were successfully employed in the detection of diesel exhaust gases in ambient air and they have proven their stability and sensitivity in polluted air during several months.

The biomimetic property of gas-sensitive films for odorants constructed by the Langmuir-Blodgett technique

The method of the quantitative structure-activity relationship is widely used in drug design. Many kinds of activities related to the cytoplasmic membrane can be analyzed by this method. It was examined whether this method could be applied to the gas-sensitivity of an LB film. The result demonstrates that an LB film has biomimetic properties and that a gas-sensitive device, possessing properties similar to an olfactory cell, could be realized.

Organophosphorus compound detection with a supported copper + cuprous oxide island film. 1. Gas-sensitive film physical characteristics and direct current studies

The electronic properties modified by the exposure of a dielectric supported island film of copper + cuprous oxide to small concentrations (part-per-million levels) of diisopropyl methylphosphonate (DIMP) are reported. Electron microscopy and electron diffraction measurements confirmed the film's structure and composition. Auger spectroscopy verified the adsorption of DIMP. Isothermal direct current measurements suggest a space-charge conduction mechanism whereby carriers are thermally excited, field induced, and transported via shallow trapping and impurity centers.

Organophosphorus compound detection with a supported copper + cuprous oxide island film. 2. Alternating current studies and sensor performance

The alternating current properties modified by the exposure of a dielectric supported discontinuous film of copper + cuprous oxide to parts-per-million concentration levels of diisopropyl methylphosphate (DIMP) are reported. In addition, the merit of integrating these films with the distributed RC notch network sensor is investigated. The frequency-dependent resistance (R/sub p/(f)) is interpreted in terms of a simple equivalent circuit composed of frequency-independent parameters whose values were determined by using a nonlinear least-squares analysis. The parameter changes caused by DIMP exposure can be differentiated between those associated with the bulk conductivity of the islands compared to their gaps. A detection scheme based upon the distributed RC notch network is evaluated. In operation, changes made in the lumped notch tuning resistance to reestablish a preexposed notch depth directly correlate with the independently measured resistance change of the gas-sensitive film.

Preparation of gas sensitive film by deposition of ultrafine tin dioxide particles

An evaporation-reaction type aerosol generator for ultrafine tin dioxide particles has been developed, and the effect of its evaporating temperature on particle size distributions and number concentrations of the generated ultrafine tin dioxide particles has been studied. The particles are drawn with high velocity into a deposition chamber with a pressure of about 400 Pa, and are deposited by inertia on a glass substrate, where they form a thin film. The electrical characteristics of this film are evaluated. The conductance of the film increases with the decrease of film thickness and with the increase of the substrate temperature. The current-voltage relation of the film is ohmic. It is shown that the ultrafine tin oxide particle film can selectively detect the gases N 2 and O 2 and is very sensitive to H 2.

Preparation of Gas Sensitive Film by Deposition of Ultrafine Tin Oxide Particles

Preparation of Gas Sensitive Film by Deposition of Ultrafine Tin Oxide Particles