G. Periaswami

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 .

Effects of silica additions on H2S sensing properties of CuO–SnO2 sensors

Abstract CuO–SnO2 sensors with and without silica additive were prepared by different methods. These were then studied for their response to H2S gas in ambient. The influence of the different procedures adopted for preparation on their properties such as resistivity and sensitivity is brought out in the light of microstructural aspects obtained from SEM and surface area measurements. The response characteristics of the sensors have been accounted for in terms of p–n junctions formed between CuO and SnO2 particles and modification of the effective surface area due to the presence of porous silica additive.

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.

Low-temperature synthesis of β-aluminas by a sol-gel technique

New intermediates observed in the process of preparing sodium β-aluminas by a sol-gel method using metal alkoxides as precursors are described. Results of studies to optimise the conditions for the preparation by changing parameters such as calcination temperatures and initial concentration of isopropoxides are reported. Based on IR, TG/DTA and XRD studies, different steps in the process leading to the formation of β-aluminas are discussed. These studies show that the formation of β-alumina starts at as low a temperature as 1273 K possibly through a modified cubic-close-packed γ-alumina structure stuffed with sodium ions and completes at 1473 K.

Evaluation of BaSnO3 and Ba(Zr,Sn)O3 solid solutions as semiconductor sensor materials

Abstract Pure BaSnO 3 and BaSn 1 − x Zr x O 3 compositions ( x = 0.05, 0.10 and 0.20) were studied as sensor materials for H 2 S in ambient conditions. Solid solution formation in the BaSnO 3 -BaZrO 3 system was studied by analysing the phases present. Improved sensing characteristics of these materials are explained in the light of solid solution formation and their electrical conductivity behaviour.

Low-temperature zirconia oxygen gauges based on RuO2 electrode

Abstract A low temperature oxygen gauge based on zirconia electrolyte has been developed. It makes use of RuO 2 as electrode material in place of platinum in conventional gauges. The low interfacial impedance of the RuO 2 electrode makes it possible to keep the cell resistance below 10 6 ω even at low temperatures. Nernsts law tests indicate that this cell can give theoretical outputs down to 498 K campared to 923 K for gauges with platinum electrodes. Faradays law tests confirm its good performance over a wide range of oxygen concentrations. High electronic conductivity, single oxide phase, slight non-stoichiometry and good adherence are responsible for the good performance of RuO 2 as electrode. An activation energy of 90.95 kJ/mole observed for the interface shows that the vacancy movement in the electrolyte is the rate controlling step. The evaporation of RuO 2 as RuO 4 gives rise to flow dependent output. This can be overcame by operating the cells at low temperatures though at the cost of speedy response. The low operating tempratures lead to a compact gauge with good stability.

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.

Preparation of proton–β/β″-aluminas by the ion-exchange under hydrothermal conditions and their characterisation

Abstract Preparation of proton–β/β″-aluminas is realised by an ion-exchange process under hydrothermal conditions using dilute acetic acid or sulphuric acid as the medium, and the corresponding alkali β/β″-aluminas as the starting materials. The optimum conditions involved in this process is around 425–450 K under the autogenerated pressure below 50 MPa, and a duration of 2–10 h. At higher temperatures and pressures or extended duration, proton–β-alumina converts to boehmite, γ-AlOOH, in the same medium. This method yields products with a variable range of composition with [H 2 O]:[Al 2 O 3 ]=1:5 to 1:17. The resulting products convert to α-alumina when heated to temperatures above 1273 K under ambient pressures. High resolution transmission electron microscopy (HRTEM) indicates the variable contents of microsyntactic defects and stacking faults with the changing composition. This study also shows the retention of the β-alumina structure during the displacement of alkali by hydroxonium ions under optimum hydrothermal conditions.

Extended stability ranges of β-aluminas with increasing ionic radii of metal alkali ions

Abstract Alkali (M)-β-aluminas were prepared by the soft chemistry method of gel-to-crystalline conversion (G–C) technique. The β-aluminas showed an extended compositional field of stability with respect to the alumina-content with increasing ionic radius of the alkali metal ions. Thus, the compositional stability of β-alumina is extended from M 2 O/Al 2 O 3 of 1:11 for Na + to 1:17 for K + and 1:22 for Rb + . High resolution electron microscopy (HREM) reveals that the decreasing concentration of alkali ions in the conducting planes is balanced by the changing widths of spinel blocks arising from the shift of tetrahedral Al 3+ to octahedral sites and the accompanying increase in stacking defects.