A P Chatterjee

ZnO thin film sensor

Abstract The electrical and gas sensing properties of chemically deposited zinc oxide (ZnO) films were investigated. Two activation energy values, 0.3 eV and 0.8–0.9 eV, were determined in the temperature range 300–400 K which are attributed to oxygen vacancy (VO) donor and heat of chemisorption of the O2− species. The adsorption of oxygen on ZnO leads to a stable and highly resistive surface suitable for sensor operation in the resistive-mode. A high sensitivity (S>99%) is observed for 3 vol% hydrogen (H2) in air along with a strong surface reduction. For liquid petroleum gas (LPG), a high sensitivity (S=50–75%) is observed in the 0.4–1.6 vol% concentration range in air which is an important commercial range for LPG alarm development.

Chemically deposited zinc oxide thin film gas sensor

Zinc oxide (ZnO) thin films were prepared by a low cost chemical deposition technique using sodium zincate bath. Structural characterizations by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM) indicate the formation of ZnO films, containing 0.05–0.50 μm size crystallites, with preferred c-axis orientation. The electrical conductance of the ZnO films became stable and reproducible in the 300–450 K temperature range after repeated thermal cyclings in air. Palladium sensitised ZnO films were exposed to toxic and combustible gases e.g., hydrogen (H2), liquid petroleum gas (LPG), methane (CH4) and hydrogen sulphide (H2S) at a minimum operating temperature of 150 °C; which was well below the normal operating temperature range of 200–400 °C, typically reported in literature for ceramic gas sensors. The response of the ZnO thin film sensors at 150 °C, was found to be significant, even for parts per million level concentrations of CH4 (50 ppm) and H2S (15 ppm).

Chemical deposition of ZnO films for gas sensors

Zinc oxide (ZnO) thin films were prepared following a chemical, deposition technique using a sodium zincate bath. Structural characterizations by scanning electron microscopy (SEM) and X-ray diffraction (XRD) indicate the formation of ZnO film with a preferred c-axis orientation. The electrical conductance of the ZnO films became stable and reproducible in the 300–500 K temperature range with two activation energy barrier values of 0.3 eV and 0.8 eV in the low temperature (300–420 K) and high temperature (430–500 K) ranges, respectively. The ZnO films prepared by this method are highly resistive, indicating the presence of a large density of oxygen adsorbed acceptor-like trap states (O2-, O-, etc.). Palladium sensitized ZnO films were exposed to hydrogen (H2) with air as a carrier gas at different operating temperatures ranging between 150–375°C and the response is evaluated.

Thermal barrier coatings from sol gel-derived spray-grade Y2O3-ZrO2 microspheres

For the development of ceramic thermal barrier coatings, spray-grade yttria-stabilized zirconia microspheres were prepared by the sol-gel technique. Oxide microspheres were obtained by calcination of the corresponding gel spheres at 1000 °C. Scanning electron microscopic and optical microscopic observations revealed the material thus obtained to have a predominantly spherical morphology and the requisite size distribution (5–50 μm). The dense, calcined microspheres showed good flowability. X-ray diffraction studies indicated the presence of the tetragonal polymorph of ZrO2 as the major phase, in addition to about 14% monoclinic ZrO2. The plasma-sprayed YSZ coatings made from the sol-gel-derived microspheres showed a further decrease in the monoclinic ZrO2 content (6%). The coatings survived 40–50 thermal cycles (30 min at 1200 °C followed by a water quench), indicating good thermal shock resistance.

Tin dioxide thin film gas sensor

Abstract Tin dioxide (SnO 2 ) is the singular, most important material utilised in commercially manufactured sensors for toxic and combustible gases. In the present work, tin dioxide thin film sensors with porous microstructure and thickness of 0.22 μ m have been fabricated on commercially available glass slides by a novel, low-cost, modified chemical deposition technique. Here we report, for the first time, the details of structural and deposition characteristics of the same films as a function of experimental parameters such as the number of dippings, bath temperature and bath concentration. In addition, the electrical properties were studied for both as deposited and palladium sensitised films as a function of temperature (300–500 K) in a closed quartz tube furnace. Further, the gas sensitivity of the palladium sensitised tin dioxide thin film sensors was evaluated in air inside the same closed quartz tube furnace as a function of the operating temperature (150–300°C) for a fixed concentration (3 vol%) of hydrogen gas with nitrogen as the carrier gas. The sensor response could be recorded at an operating temperature of as low as 150°C. Maximum sensitivity of 90% was found to occur at a low temperature of only 200°C. Above this cut-off temperarure, sensitivity of the present thin film sensors was found to suffer only moderate degradation.

Electrodeposition and characterisation of cuprous oxide films

Abstract Semiconducting cuprous oxide films were prepared by electrodeposition onto copper substrates from an alkaline CuSO4 bath at temperatures between 40 and 60°C. The Cu2O films, which were deposited using a potentiostatic method, were found to exhibit exponential growth kinetics. X-ray diffraction studies revealed the formation of only Cu2O films with (200) preferred orientation. The observed current-voltage characteristics of the Cu 2 O Cu device structures were found to be similar to that of a metal-insulator-semiconductor (MIS) tunnel diode, indicating the presence of a thin unidentified interfacial insulating layer between the copper substrate and the cuprous oxide film.

Preparation of ZrO2CaO and ZrO2MgO fibres by alkoxide Sol-Gel processing

Abstract Extensive studies were made on the spinnability of Zr(O n C 3 H 7 ) 4 -derived sols for the preparation of ZrO 2 CaO and ZrO 2 MgO fibres. An improved preparative route was proposed in which acetylacetone controlled the hydrolyzability of the alkoxide and 2-methoxyethanol served as a slow-drying solvent. The advantages of the method of sol preparation over the known methods were highlighted. Crystallization behaviour of the gel fibres was studied mostly up to 1300°C for ZrO 2 CaO fibres (up to 20% mol CaO) and up to 1000°C for ZrO 2 MgO fibres (up to 12% mol MgO). ZrO 2 CaO fibres (7–11% mol CaO) were found to yield only the tetragonal and cubic polymorphs of ZrO 2 after heating at 1300°C/4h and furnace cooling. ZrO 2 MgO fibres, on the other hand, showed exsolution of MgO above 800°C and development of monoclinic ZrO 2 .

High purity zirconia powders via wet chemical processing: A comparative study

Abstract Comparison has been made of the achievable purity levels and agglomeration characteristics of zirconia powders obtained via six different wet chemical routes where the starting material was an impure Zr-oxychloride solution obtained from Indian zircon sand. For the highest purity (2–15 ppm of other oxides except Hf), two routes were found to be effective: crystallization of the oxychloride via the basic sulphate, or double crystallization of the oxychloride, followed by dissolution and hydroxide precipitation. The average agglomerate size was the largest (10–15 μm) for the powders obtained by direct decomposition of the basic sulphate at 1000°C, though the size distribution was narrow and distinctly monomodal. For smaller agglomerates (1·5–2·5 μm), on the other hand, the size distribution was very wide. Calcination at 800°C of powders obtained via hydroxide precipitation from a solution always yielded only tetragonal zirconia as the crystalline phase.

Characterization of basic zirconium sulphate, a precursor for zirconia

Conditions de preparation a partir de ZrOCl 2 et de H 2 SO 4 en solution aqueuse. Analyse thermique et examen des cristaux par microscopie electronique