Showing papers by "Somnath C. Roy published in 2007"

Highly sensitive SnO2 thin film NO2 gas sensor operating at low temperature

Abstract: Thin films of pure nano-crystalline SnO 2 and WO 3 -doped SnO 2 in different concentrations (3 wt.% and 5 wt.%) are deposited using sol–gel spin coating technique on glass substrates. The structural and morphological properties of these films are investigated using XRD, TEM and AFM. The sensitivity and selectivity of these films are tested to different reducing and oxidizing gases such as SO 2 , NH 3 , NO 2 and ethanol. A SnO 2 thin film with 5 wt.% WO 3 shows very high responses to NO 2 (four orders of magnitude in the value of ( R g − R a )/ R a ; R g and R a are the resistance in NO 2 and in air, respectively) and excellent selectivity for NO 2 gas at a low operating temperature of 150 °C. The effect of WO 3 on the sensing characteristics of these films towards NO 2 is discussed.

Effect of Fe doping on the gas sensing properties of nano-crystalline SnO2 thin films

Abstract: In this paper, we report the gas sensing properties of sol–gel derived Fe-doped nano-crystalline SnO2 thin films. Films were doped with Fe in two different concentrations (2 and 5 wt.%). The crystallite size was found to decrease with increase in stacking fault density resulting from increasing Fe content in the SnO2 films. Gas sensing characteristics were investigated for different target gases such as carbon monoxide (CO), ammonia (NH3) and ethanol (C2H5OH). The films with 2% Fe content showed high response and excellent selectivity for CO compared to other gases at 200 °C. But, at 5% Fe content, the response factor to CO decreased while it increased for NH3 and ethanol. However, faster response and recovery times were observed for CO at higher Fe content in the films. These results have been correlated with the defect chemistry and crystallite size effect resulting from Fe incorporation in the SnO2 thin films.

An orthogonal ferromagnetically coupled tetracopper(II) 2 × 2 homoleptic grid supported by µ-O4 bridges and its DFT study

Abstract: A pyrazole based ditopic ligand (PzOAP), prepared by the reaction between 5-methylpyrazole-3-carbohydrazide and methyl ester of imino picolinic acid, reacts with Cu(NO3)2·6H2O to form a self-assembled, ferromagnetically coupled, alkoxide bridged tetranuclear homoleptic Cu(II) square grid-complex [Cu4(PzOAP)4(NO3)2] (NO3)2·4H2O (1) with a central Cu4[µ-O4] core, involving four ligand molecules. In the Cu4[µ-O4] core, out of four copper centers, two copper centers are penta-coordinated and the remaining two are hexa-coordinated. In each case of hexa-coordination, the sixth position is occupied by the nitrate ion. The complex 1 has been characterized structurally and magnetically. Although Cu–O–Cu bridge angles are too large (138–141°) and Cu–Cu distances are short (4.043–4.131 A), suitable for propagation of expected antiferromagnetic exchange interactions within the grid, yet intramolecular ferromagnetic exchange (J = 5.38 cm−1) is present with S = 4/2 magnetic ground state. This ferromagnetic interaction is quite obvious from the bridging connections (dx2−y2) lying almost orthogonally between the metal centers. The exchange pathways parameters have been evaluated from density functional calculations.

Study on Flow and Turbulence Inside a Stirred Tank and Investigation on the Effects of Macroinstability on Trailing Vortex Structures

Abstract: Chemical process industries use stirred tank reactors (STR) for a variety of mixing and blending operations. Turbulent flow inside a baffled stirred tank reactor with a 45° pitched blade impeller is numerically studied using a large-eddy simulation (LES) technique with a body-fitted curvilinear mesh. The moving impeller geometries are modeled using an immersed boundary method (IBM). The instantaneous as well as time-averaged flow field suggests formation of trailing vortex due to the interaction of the fluid streams from the side and top edges of the blade. An enhanced distribution of turbulent kinetic energy has been observed in the vicinity of the trailing vortices. Instabilities occurring at a frequency lower than the frequency of impeller rotation are noticed from the time signal of the velocity components. The growth and break-up of the trailing vortices with this macro-instability frequency is observed. Turbulence is shown to be strongly anisotropic.Copyright © 2007 by ASME