Modeling the sensing characteristics of chemi-resistive thin film semi-conducting gas sensors.

For chemi-resistive thin film gas sensors a generic theoretical model is proposed to predict the variation of sensor response with the operating temperature and thickness of the sensing film. A diffusion equation is formulated assuming that inflammable target gases move through the sensing film by Knudsen diffusion and react with the adsorbed oxygen following first-order kinetics. We have assumed a realistic non-linear variation between the conductance and test gas concentration and derived a general expression relating the sensor response to the operating temperature and thickness of the film. Assuming Langmuir adsorption kinetics, we have theoretically predicted the response and recovery transients during gas sensing using thin film sensing elements. It is predicted that for irreversible type sensing, the response time is reduced with an increase in test gas concentration, whereas for reversible sensing, the response time is independent of test gas concentration. For zinc oxide thin film sensors, an excellent match is obtained between the model prediction and experimental data for their thickness (122 nm to 380 nm) and temperature variation (200 °C to 325 °C) in 500 ppm carbon monoxide (CO) sensing. The maximum CO response% (∼53%) was achieved in 320 nm thick ZnO films. The conductance transients for response and recovery for CO sensing closely follow Langmuir adsorption kinetics and as predicted theoretically, indeed for irreversible sensing, the response time reduces from 350 s to 220 s with an increase in test gas concentration from 20 to 550 ppm. In the case of reversible sensing we found that the response time is ∼55 s irrespective of the CO gas concentration in the range of 5-500 ppm. The models developed in the present work are quite generic in nature and we have discussed their applicability to a wide variety of sensing materials with various types of surface morphologies.

Qualitative and quantitative differentiation of gases using ZnO thin film gas sensors and pattern recognition analysis.

In the present work we have grown highly textured, ultra-thin, nano-crystalline zinc oxide thin films using a metal organic chemical vapor deposition technique and addressed their selectivity towards hydrogen, carbon dioxide and methane gas sensing. Structural and microstructural characteristics of the synthesized films were investigated utilizing X-ray diffraction and electron microscopy techniques respectively. Using a dynamic flow gas sensing measurement set up, the sensing characteristics of these films were investigated as a function of gas concentration (10-1660 ppm) and operating temperature (250-380 °C). ZnO thin film sensing elements were found to be sensitive to all of these gases. Thus at a sensor operating temperature of ~300 °C, the response% of the ZnO thin films were ~68, 59, and 52% for hydrogen, carbon monoxide and methane gases respectively. The data matrices extracted from first Fourier transform analyses (FFT) of the conductance transients were used as input parameters in a linear unsupervised principal component analysis (PCA) pattern recognition technique. We have demonstrated that FFT combined with PCA is an excellent tool for the differentiation of these reducing gases.

Nanomagnets La0.8Pb0.2(Fe0.8Co0.2)O3 assembled with a bonded surface graphene oxide: sensitive for sensing small gas molecules.

Nanocrystallites La0.8Pb0.2(Fe0.8Co0.2)O3 (LPFC) when bonded through a surface layer (carbon) in small ensembles display surface sensitive magnetism useful for biological probes, electrodes, and toxic gas sensors. A simple dispersion and hydrolysis of the salts in ethylene glycol (EG) in water is explored to form ensembles of the nanocrystallites (NCs) by combustion of a liquid precursor gel slowly in microwave at 70-80 dgrees C (apparent) in a closed container in air. In a dilute sample, the EG molecules mediate hydrolyzed species to configure in small groups in process to form a gel. Proposed models describe how a residual carbon bridges a stable bonded layer of a graphene-oxide-like hybrid structure on the LPFC-NCs in attenuating the magnetic structure. SEM images, measured from a pelletized sample which was used to study the gas sensing features in terms of the electrical resistance, describe plate shaped NCs, typically 30-60 nm widths, 60-180 nm lengths and -50 m2/g surface area (after heating at -750 degrees C). These NCs are arranged in ensembles (200-900 nm size). As per the X-ray diffraction, the plates (a Pnma orthorhombic structure) bear only small strain -0.0023 N/m2 and oxygen vacancies. The phonon and electronic bands from a bonded surface layer disappear when it is etched out slowly by heating above 550 degrees C in air. The surface layer actively promotes selective H2 gas sensor properties.

Hydrogen sensing characteristics of wet chemical synthesized tailored Mg0.5Zn0.5Fe2O4 nanostructures.

Solution based synthesis routes are attractive for making tailor-made nanostructures for electroceramics in a simple and cost-effective way. The gas sensing characteristics of semiconducting oxide gas sensors strongly depend on the adsorption and desorption of gases over the sensing surface. The morphology of the sensing element is known to influence the adsorption and desorption of gases and thereby the sensing performance of the material. In the present work a Pechini based solution synthesis route is adopted in order to synthesize magnesium zinc ferrite gas sensors in nanoparticle, nanotube and thin film forms. The hydrogen gas sensing characteristics of these sensing elements are compared as a function of test gas concentration and operating temperature. The influences of the morphology of the magnesium zinc ferrite sensing elements on the hydrogen sensing characteristics are discussed.

Reducing gas sensing behavior of nano-crystalline magnesium-zinc ferrite powders.

As an effective alternative of simple binary oxides, cubic spinel oxides are considered to be attractive to make sensitive and stable gas sensor, selective to a specific gas. We have focused the present work on the investigation of the gas sensing characteristics of cubic spinel based nano-crystalline magnesium zinc ferrite powders. A wet chemical synthesis route is adopted to synthesize nano-crystalline magnesium zinc ferrite powders. The phase formation behavior and microstructure evolution of the synthesized powder has been investigated using infrared spectroscopy in conjunction with X-ray diffraction analyses and electron microscopy. The n-type semiconducting magnesium-zinc ferrite ceramic exhibits reasonably good sensitivity towards a variety of gases including carbon monoxide, hydrogen, methane and nitrous oxide. It is demonstrated that these sensors can be made selective to hydrogen gas sensing by modulating the operating temperature. The conductance transients during response and recovery processes have been modeled using Langmuir adsorption isotherm and activation energies for gas adsorption and desorption processes have been estimated from the respective thermally activated kinetic processes.