Coupled microstructural and transport effects in n-type sensor response modeling for thin layers

Abstract

The chemical gas sensor system of CO detection in a SnO 2 matrix was considered. A model was formulated which incorporated the coupled processes of gases diffusing into a porous ceramic and then participating in surface chemical reactions of adsorption, ionization and desorption. Microstructural properties of the sensor matrix were coupled with the diffusion and surface chemistry processes. The consequent surface chemical state served to partition bulk and grain boundary contributions to the n-type material conductance. Conductivity levels determined both with and without the presence of the target gas, CO, allowed sensor response to be determined as a function of film thickness.This simulation represents a modeling advance as it is the first to couple spatial variation of microstructural properties with diffusing gas species and the attendant surface chemistry and electroceramic properties, to predict sensor response as a function of film thickness. This will serve to be a useful design tool for ensuing materials research work towards improved sensor device development.Crown

Thin film SnO2-based gas sensors: Film thickness influence

“…According to the second approach, the gas sensing properties of metal oxide devices are determined by the competitive influence of gas diffusion and surface reactions taking place in pores. Unfortunately, further work in this direction had been suspended until a diffusion-reacting approach was explored once again in recent years in order to interpret the obtained results [32,[69][70][71][72]. Moreover, Yamazoe and co-workers in his research tried to design a phenomenological model of metal oxide gas sensors based on the indicated approach [14,73,74].…”

Nanostructured metal oxide gas sensors prepared by electrospinning

“…It is rather difficult to mutually satisfy both of these requirements using chemical solution deposition techniques or conventional deposition and sintering methods of ceramic pastes that typically yield mesoporous layers in which the pore size scales with the particle size (as demonstrated, for example, Fig. 3) and the gas penetration depth into the layer is rather small 66, 74. Unlike these methods, electrospinning enables to obtain highly porous layer with large and small pores and nanosized particles that optimally satisfy both of these requirements 9, 10.…”

Coupled Gas Flow, Diffusion and Reaction in a ppb-level SnO2-NiO Formaldehyde Sensor System

“…The source terms R in Eqn 5 account for the chemical reactions which occur among the diffusing gases and surface species, arising from the catalytic oxidation mechanism for HCHO [7,16] on SnO 2 -NiO, outlined below:…”

“…The total number of surface sites (S 0 ) was calculated in [16]. The mass action calculation for the equilibrium concentrations of the species specified by the four constants (k 1 Finally, the conductivity in the SnO 2 -NiO was determined based on a model developed by Windischmann and Mark [17], where σ is the local conductivity, and σ 0 is the conductivity in the absence of HCHO.…”

“…Estimating responses for data collected at equilibrium ( Figure 5), along with the sensor response to feed gasses requiring a chamber volume exchange (Figure 6), allowed the determination of k 2 and k Ϫ2 . Given that the reactions took place on SnO 2 , activation energies were taken from previous work, [16] while the catalytic effect of the nickel was taken as the basis for an increased pre-exponential constant. The sensor model (Eqns 5 through 8) was applied to experimental data from [14], shown in Figure 6a.…”