Vehicle Exhaust Gas Chemical Sensors Using Acoustic Wave Resonators

Under Sandia's Laboratory Directed Research and Development (LDRD) program, novel acoustic wave-based sensors were explored for detecting gaseous chemical species in vehicle exhaust streams. The need exists for on-line, real-time monitors to continuously analyze the toxic exhaust gases --- nitrogen oxides (NO x ), carbon monoxide (CO), and hydrocarbons (HC) --- for determining catalytic converter efficiency, documenting compliance to emission regulations, and optimizing engine performance through feedback control. In this project, we adapted existing acoustic wave chemical sensor technology to the high-temperature environment and investigated new robust sensor materials for improving gas detection sensitivity and selectivity. This report describes one new sensor that has potential use as an exhaust stream residual hydrocarbon monitor. The sensor consists of a thickness shear mode (TSM) quartz resonator coated with a thin mesoporous silica layer ion-exchanged with palladium ions. When ..

Comparison of lumped-element and transmission-line models for thickness-shear-mode quartz resonator sensors

Both a transmission-line model and its simpler variant, a lumped-element model, can be used to predict the responses of a thickness-shear-mode quartz resonator sensor. Relative deviations in the parameters computed by the two models (shifts in resonant frequency and motional resistance) do not exceed 3% for most practical sensor configurations operating at the fundamental resonance. If the ratio of the load surface mechanical impedance to the quartz shear characteristic impedance does not exceed 0.1, the lumped-element model always predicts responses within 1% of those for the transmission-line model

Sensing liquid properties with shear-mode resonator sensors

Liquid properties are measured from the changes they induce in the resonant frequency and damping of thicknessshear mode quartz resonators. A smooth-surfaced resonator viscously entrains the contacting fluid and responds to the density-viscosity product. Separation of density and viscosity is accomplished using two devices: one with a smooth surface and one with a corrugated surface that traps fluid. By observing the difference in stored and dissipated energies in the contacting fluid, its non-Newtonian characteristics can also be determined

Error analysis of quartz crystal resonator applications

Quartz crystal resonators in chemical sensing applications are usually configured as the frequency determining element of an electrical oscillator. By contrast, the shear modulus determination of a polymer coating needs a complete impedance analysis. The first part of this contribution reports the error made if common approximations are used to relate the frequency shift to the sorbed mass. In the second part the authors discuss different error sources in the procedure to determine shear parameters

Modeling the Responses of TSM Resonators under Various Loading Conditions

We develop a general model that describes the electrical responses of thickness shear mode resonators subject to a variety of surface loadkgs. The model incorporates a physically diverse set of single component loadings, including rigid solids, viscoelastic media and fluids (Newtonian or Maxwellian). The model allows any number of these components to be combined in any configuration. Such multiple loadings are representative of a variety of physical situations encountered in electrochemical and other liquid phase applications, as well as gas phase applications. In the general case, the response of the composite is not a linear combination of the individual component responses. We discuss application of the model in a qualitative diagnostic fashion, to gain insight into the nature of the interracial structure, and in a quantitative fashion, to extract appropriate physical parameters, such as liquid viscosity and density and polymer shear moduli

Equivalent-Circuit Model for the TSM Resonator with a Viscoelastic Layer

This paper describes a new equivalent-circuit model for the thickness shear mode resonator with a surface viscoelastic layer operating near film resonance. The electrical impedance of the film is represented by a simple three-element parallel circuit containing a resistor, a capacitor, and an inductor. These elements describe the film's viscous power dissipation, elastic energy storage, and kinetic energy storage, respectively. Resonator response comparisons between this lumped-element model and the general transmission-line model show good agreement over a range of film phase conditions and not just near film resonance

An Examination of System Architectures for Distributing Sensor Data Via Ethernet Networks

Application of the World Wide Web (WWW) for the transfer of sensor data from remote locations to laboratories and offices is a largely ignored application of the WWW. We have investigated several architectures for this application including simple web server/client architectures and variations of this approach. In addition, we have evaluated several commercial approaches and other techniques that have been investigated and are in the literature. Finally, we have provided conclusions based on the results of our study offering suggestions about the advantages and disadvantages of each of the approaches studied