The production of selective tin dioxide based semiconducting gas sensors

• Main Author: Bond, S. E.
• Published: Swansea University 1993
• Subjects: 546
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636118

Previous work has produced a tin dioxide based semiconducting gas sensor which is selective to CO in the presence of the lower hydrocarbons. The sensing material comprises a mixture of bismuth oxide and tin dioxide. When this mixture is heated at 800<SUP>o</SUP>C all of the bismuth oxide is converted to bismuth stannate (Bi<SUB>2</SUB>Sn<SUB>2</SUB>O<SUB>7</SUB>). This reaction commences at ≃ 650°C. This is one of a group of mixed oxides having the formula M_2Sn_2O_7 which possess a pyrochlore structure. The thesis describes the characterisation of novel sensors. These were produced with the aim of reproducing the behaviour of the original device. Sensor materials comprised mixtures of metal oxides (M_2O_3 where M = lanthanum, neodymium, samarium, gadolinium, ytterbium, dysprosium, thulium, holmium and lutetium, and also yttrium) and tin dioxide which formed pyrochlore stannates at 1500^oC. Furthermore, scandium oxide (Sc_2O_3) and antimony oxide (Sb_2O_3) mixed with tin dioxide and heated at appropriate temperatures produced stannate materials which do not possess the pyrochlore structure. Other sensor materials comprised titanium dioxide and lanthanum oxide or neodymium oxide which formed pyrochlore titanates at suitable temperatures. Tin dioxide sintered at 1500°C exhibits resistance increases or decreases upon exposure to identical gases at different operating temperatures. In a further attempt to understand the gas-sensing behaviour of these materials their response upon exposure to reducing gases, for a range of sintering temperatures ≤1500°C, was compared to the behaviour of a tin dioxide sensor identically treated. The physical properties of unheated and heated sensor materials were investigated by x-ray diffraction (room temperature and temperature-programmed), fourier transform infra-red (FTIR) spectroscopy, thermogravimetry and differential scanning calorimetry (TG/DSC) and B.E.T. surface area equipment. Hence the temperature of production of stannate and titanate compounds and the chemical and physical processes which occurred during heating were determined.