On-line Magazine 'Sensors & Transducers' (S&T e-Digest)
(ISSN 1726- 5479)
Vol. 70, Issue 8, August 2006, pp. 671-678
Novel Non-Stoichiometric Manganese – Cobalt – Nickel – Oxide Composite as Humidity Sensor Through Solid-State Electrical Conductivity Measurements
R. Sundaram, K. Maniraj*
Department of Chemistry, *Department of Physics, Science and Humanities,
Maamallan Institute of Technology, Sriperumbudur 602 105, Tamil Nadu, India
Tel.: +91-44-2710 7071, 2716 3667, fax:+91-44-2716 3682
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Received: 24 July 2006 /Accepted: 22 August 2006 /Published: 27 August 2006
Abstract. Equimolar amounts of manganese(II) chloride, cobalt(III) nitrate and nickel(II) chloride in aqueous solution were reacted with ammonia and the resulting precipitate of hydroxides was heated to 7500 C in 6h to yield a non stoichiometric oxides having a composition of Mn0.06Co0.6Ni0.6O2.5 as analyzed by atomic absorption spectroscopy to a pellet and sintered at 6000 C. Characterization of the material has been made with AAS, Far-IR, TG-DTA, XRD, SEM, VSM and electrical conductance measurement. The far-IR spectra indicated the presence of metal-oxygen bonds and the discrete nature of the oxide was established from power X-ray diffraction pattern recorded at room temperature. The thermogravimetric data indicated the successive loss and gain of fraction of oxygen atoms, a specific feature of non-stoichiometric metal oxides. It was subjected to solid-state DC electrical conductivity measurements at room temperature. The current increases linearly with applied field and exponentially with increase in temperature showing conformance to ohmic law and semiconducting nature. The scanning electron microscopy (SEM) studies were carried out to study the surface and pores structure of the sensor materials. The Brunauer-Emmett-Teller (BET) surface adsorption studies showed that the radiuses of the pore sizes were found to be distributed from 10-45A with the pore specific volume being 0.01 cm3 g-1. As the composites having micropores are preferred for humidity sensing properties, the material was subjected to water vapour of different humidity achieved by various water buffers at room temperature and the electrical conductivity was measured as a function of relative humidity (RH). The electrical resistivity drastically decreases with increase in humidity, proving the material to be a good water vapour sensor. The sensitivity factor (Sf) was 55000 in the range 5–98% RH, meaning the resistivity falls by a factor of 5.5 x 104 when the atmospheric RH increases from 5-98%. Such a high value of sensitivity for water vapour has seldom been reported in literature, making the composite possibly the most sensitive water vapour sensor. The response and recovery time for this humidity-sensing composite was also studied.
Keywords: electrical conductivity, humidity sensors, manganese oxide, nickel oxide, cobalt oxide, non-stoichiometric tri-metallic oxides
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