K.I. Gnanasekar

Preparation and characterisation of Cr2−xTixO3+δ and its sensor properties

Abstract Compounds with nominal compositions Cr 2− x Ti x O 3+ δ (where x =0.1–0.4) were prepared by a solid state chemical reaction at 1273 K. Characterisation of the products by XRD showed the presence of a solid solution of TiO 2 in Cr 2 O 3 as the major phase along with CrTiO 3 as the minor phase. Conductivity studies indicated the absence of a significant influence of titanium concentration on the activation energy ( E a ) for conduction in the temperature range 300–700 K in ambient air. Sensor studies showed that the compound with nominal composition Cr 1.8 Ti 0.2 O 3+ δ is a suitable material for sensing NH 3 , H 2 and LPG in ambient around 573 K. The enhanced sensitivity of this compound when compared with it’s parent compounds, is proposed to be due to the presence of a solid solution of TiO 2 in Cr 2 O 3 .

Electrical and sensor properties of FeNbO4: a new sensor material

Abstract We report here the preparation, electrical, magnetic and gas sensor characterisation of the monoclinic phase (wolframite structure) of FeNbO 4 . Crystallographic and surface morphological studies were carried out by the powder X-ray diffraction (XRD) and scanning electron microscopic (SEM) techniques, respectively. The high electrical conductivity of FeNbO 4 is due to the mixed valence nature (Fe 2+ /Fe 3+ ) of Fe-ions in the FeOFe framework as confirmed by the temperature dependent magnetic susceptibility studies. Impedance studies on electrical conductivity as a function of temperature in the range of 300–673 K provided E a as 0.210 eV. Sensor studies on FeNbO 4 at various operating temperatures show that the material is highly sensitive to H 2 and H 2 S at 573–623 K. A moderate sensitivity for LPG is obtained at around 673 K.

A LOW TEMPERATURE H2 SENSOR BASED ON INTERMEDIATE HYDROXY TIN OXIDE

Abstract Nanosize powders of SnO2 were prepared by two precipitation methods, namely tin chloride hydrolysis and tin ethoxide hydrolysis. The precipitation was followed by a sequential washing procedure using solvents of decreasing polarity. This process helped to enhance the surface area of the resulting powders and to make them suitable for sensor applications by preventing their agglomeration. The sensors fabricated from such powders were found to exhibit a sensitivity of ∼90% for 100 ppm H2 at 443 K and ∼50% for 500 ppm LPG at an operating temperature of 523 K. Their ability to sense H2 at such low temperatures is attributed to the large surface area which offers a large number of active sites for reaction with H2 and the possible participation of hydroxyl groups of the oxy-hydroxy phase in the sensing action.

Ag6Mo10O33 — a new silver ion conducting ammonia sensor material

Electrical and gas sensing properties of Ag6Mo10O33 are investigated for the first time. Wagner’s polarization experiment carried out on this material shows that it is a good silver ion conductor in the temperature range of 533 to 657 K. Studies show that it possesses good selectivity and sensitivity towards ammonia with fast response and retrac times indicating its potential as a sensor material for this gas below its TLV value. Morphological changes associated with this compound when exposed to ammonia are studied by using SEM and the corresponding compositional changes associated with each grain is studied by using EDAX. The results indicate that the compound breaks down when exposed to ammonia. When equilibrated with air the reaction products recombine fast to form the original compound.

Ammonia sensing properties of thick and thin films of Ag6Mo10O33 and Cr1·8Ti0·2O3+δ

Abstract Thick film of silver decamolybdate (Ag6Mo10O33) was screen printed and thin film of chromium titanate Cr1·8Ti0·2O3+δ was laser deposited on alumina substrates. Morphological features of the films were characterised and ammonia sensing properties were studied. Resistance of Ag6Mo10O33 thick films decrease on introduction of NH3 gas and these films were capable of sensing 2 ppm of ammonia in air at 578 K. Cr1·8Ti0·2O3+δ thin films could sense 20 ppm of ammonia in air at 673 K, which exhibited increase in resistance towards NH3 gas.

Application of principal component analysis to gas sensing characteristics of Cr0.8Fe0.2NbO4 thick film array.

The transient changes in resistances of Cr0.8Fe0.2NbO4 thick film sensors towards specified concentrations of H2, NH3, acetonitrile, acetone, alcohol, cyclohexane and petroleum gas at different operating temperatures were recorded. The analyte-specific characteristics such as slopes of the response and retrace curves, area under the curve and sensitivity deduced from the transient curve of the respective analyte gas have been used to construct a data matrix. Principal component analysis (PCA) was applied to this data and the score plot was obtained. Distinguishing one reducing gas from the other is demonstrated based on this approach, which otherwise is not possible by measuring relative changes in conductivity. This methodology is extended for three Cr0.8Fe0.2NbO4 thick film sensor array operated at different temperatures.

Analysis of Nano-Structured

Nano-structured In2O3 thin film was made using a pulsed laser deposition technique. The surface topography and structural properties of the thin film were characterized by atomic force microscopy and X-ray diffraction, respectively. Complex impedance spectroscopy of In2O3 thin film gas sensor was investigated from 275 °C to 425 °C when exposed into clean air and air containing a trace level of NOx. Significant NOx sensing characteristics of thin film were observed at 325 °C by ac impedance spectroscopic analysis. The resistance and capacitance of indium oxide film increased when exposed into the trace level of NOx. A mechanism for this increase of resistance and capacitance is proposed.

{\rm In}_{2}{\rm O}_{3}

Highly a-axis-oriented nanostructured thin films of 0.5 atom % Ba-doped In 2 O 3 exhibited high sensitivity toward 3 ppm (threshold limit values of NO 2 ) of NO x , consistently and reproducibly. Thin films prepared by pulsed laser (KrF,λ = 248 nm) deposition are highly granular, exhibiting wirelike geometry with an average diameter of ~80 to 100 nm. Sensing mechanism as investigated by X-ray photoemission studies reveals that adsorption of NO x on the surface of the film acts as a trap for conduction electrons due to its high electron affinity, which provides a different chemical environment for the surface indium ions.

Thin Film

Nanostructured thin films of Ba-doped indium oxide have been examined for monitoring ppm levels of NOx in ambient air, and their response patterns have been recorded at different operating conditions. These films are found to be capable of sensing down to 0.5 ppm NOx concentration in air. The optimum operating temperature for sensing is found to be 300°C. Response and retrace times are typically 60 s and 300 s respectively. They are highly selective to NOx and do not show significant response to hydrogen and petroleum gas up to 5000 ppm.

{\rm NO}_{\rm x}

Use of AgI_1-xCl_x (x= 0 and 0.025) towards sensing of Cl₂ gas has been explored. AgI_1-xCl_x has been demonstrated to sense ~ 10 vppb of Cl₂ in air.