Sensors and Actuators B 115 (2006) 589–595 Structure and humidity sensitive electrical properties of the Sn 4+ and/or Mo 6+ substituted Mg ferrite N. Rezlescu ∗ , C. Doroftei, E. Rezlescu, P.D. Popa Institute of Technical Physics, Bld. D. Mangeron 47, 700050 Ia¸ si, Romania Received 8 July 2005; received in revised form 12 October 2005; accepted 18 October 2005 Available online 28 November 2005 Abstract Pure and Sn 4+ and/or Mo 6+ substituted Mg ferrites with compositions Mg 1-x Sn x Fe 2-y Mo y O 4 (x = 0.00, 0.10 and y = 0.00, 0.02) have been prepared by sol–gel autocombustion method. The ceramic samples were sintered at 1100 ◦ C for 4 h. The phase composition and lattice parameters were determined by XRD, while effect of Sn and Mo substitutions on the granular structure was studied by SEM. Experimental results revealed that some physical properties are in close relation with the structural changes induced by the presence of Sn and Mo ions. We have found that the humidity sensitivity of the electrical resistivity largely depends on composition, crystallite size, surface area and porosity. The best results concerning humidity sensitivity were obtained for Mg 0.9 Sn 0.1 Fe 2 O 4 ferrite. The Sn ions enhance the humidity sensitivity of the Mg ferrite and assure a nanocrystalline structure of this ferrite. © 2005 Elsevier B.V. All rights reserved. Keywords: Ferrites; Humidity sensor; Magnesium ferrite; Substitutions; Sol–gel; Selfcombustion 1. Introduction During the last years, many kinds of ceramic oxides have been investigated actively as humidity sensing materials [1–11]. Humidity sensors based on semiconducting oxides have cer- tain advantages compared to other types of humidity sensors, such as low cost, simple construction, small size and ease of placing the sensor in the operating environment. Basically, a ceramic sensor can detect humidity on the principle of mea- suring a change in the resistance by water vapor adsorption. The change in the resistance of the sensor can be explained by the fact that water molecules act as an electron-donating gas [12]. Seiyama et al. [13] have investigated the sensing mechanism of semiconductive humidity sensors. When water molecules are adsorbed on semiconductive oxide, the conduc- tivity increases or decreases according to whether the oxides are of n-type or p-type. This means that electrons are appar- ently transferred from water molecules to oxides. The ability of a metal oxide to sense the presence of water molecules depends on the interaction between water molecules and the ∗ Corresponding author. Tel.: +40 232 430680; fax: +40 232 231132. E-mail address: reznic@phys-iasi.ro (N. Rezlescu). surface of the metal oxides, i.e. the reactivity of its surface. The reactivity depends on composition and electronic and morphologic structure [14]. These surface characteristics are heavily affected by the preparation procedure and composi- tion. Seki et al. [15] have reported the utilization of ferrites for water vapors detection as humidity sensitive active elements. Because the ferrites behave as a n-type semiconductor, conduc- tivity will be increased in the presence of water vapor. Vain- gankar et al. [16] have investigated the humidity sensitivity of CuZn ferrite. The use of Mg ferrite as humidity sensor has been reported by Shimizu et al. [17]. Much interest was focused on Mg ferrite owing to its high electrical resistivity and high poros- ity [1,14,18]. Taking into consideration that the sensing effect mainly takes place on material surface, the control of particle size will be one of the first requirements for enhancing the sensor’s humidity sen- sitivity. The nanograined materials offer a high surface area for water vapor detection, in comparison with micrograined mate- rial. In this work we study the influence of the composition on morphology and electrical properties of Mg ferrite. This allowed us to correlate these results with sensitivity towards water vapors. As substituents we selected two ions, Sn 4+ 0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.10.028