Production, electrical conductivity, and gas-sensing properties of thin nickel ferrite films

Abstract

83 Oxide materials based on nickel ferrite are well known owing to their magnetic properties. However, in recent years, their electrical properties have also been intensely studied. In particular, NiFe2O4�based ferrites are promising as sensing materials for gas sen� sors (1). The mechanism of the sensing of oxide mate� rials is based on the change in the electrical conductiv� ity of a thin film of a material in the course of the adsorption of molecules of a gas being sensed (meth� ane, propane-butane mixtures, etc.). At the same time, there are a number of difficulties in creating a gassensing module that would include a ferritebased sensing element. First of all, this is because of the instability and ambiguity of the physical properties while producing thin nanostructured layers of oxide materials. Ferrites are of the spinel structure type, and their physical properties are significantly dependent on the lattice parameters and also on the type and nature of the ion distribution in the lattice (2, 3). By the present time, certain results have been achieved and methods have been developed for obtain� ing ferrite films by laser deposition, aerosol pyrolysis, sol-gel processes, and ion beam sputtering (4). Investi� gation of the properties of thin ferrite films and the improvement of their production methods will allow one to relate the structural and physical characteristics with the parameters determining the gas sensing of a given material. In the context of solving this problem, in this work, we studied the possibility of producing a NiFe2O4�based material with predictable properties and stable composition by ion beam sputtering of a fer� rite powder of a given composition. This method allows one to apply thin films of oxide materials to substrates made of either conductive or nonconductive materials. The main advantage of the method is the possibility of forming layers ~1 μm thick within a single stage. In gas sensors, oxide layers consisting of nanosized particles are mainly used. A most important factor determining the gas sensing of a layer is its specific sur� face area because the main mechanism of the sensing of the sensing layer is the change in the electrical con� ductivity during the adsorption of molecules of a gas being sensed (5). Therefore, since the adsorption occurs in the nearsurface zone and the sensing layer is usually applied to a nonconductive substrate with recording electrodes, it is necessary to produce the thinnest possible layer with a highly developed ferrite surface for using as a sensing material. In view of the above, in this work, we studied the electrical conduction characteristics of films applied to glass or silicon substrates and the effect of the gas concentration on the behavior of the electrical resis� tance of a thin ferrite film.