Tham khảo tài liệu 'microsensors, mems and smart devices - gardner varadhan and awadelkarim part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | BIO CHEMICAL SENSORS 283 Figure a Basic structure of a Taguchi-type tin oxide gas sensors and b photograph of a series-8 commercial gas sensor Courtesy of Figaro Engineering Japan Tin oxide devices are operated at various high temperatures and doped with different materials to enhance theữ specificity. The response of a tin oxide sensor in terms of its relative conductance Gs Go where Gs is the conductance of a gas of fixed concentration and Gq is the conductance in air is shown in Figure Yamazoe el al. 1983 . The devices are operated at high temperatures typically between 300 and 400 C for several reasons. First and most important the chemical reaction is more specific at higher temperatures and second the reaction kinetics are much faster that is the device responds in just a few seconds. Finally operating the device well above a temperature of 100 C ameliorates the effect of humidity upon its response - a critical factor for many chemical sensors. The basic reactions that occur within the porous sintered film can be represented by the following reactions. First vacant sites within the nonstoichiometric tin oxide lattice react with atmospheric oxygen to abstract electrons out of the conduction band of the tin oxide creating chemisorbed oxygen sites such as 0 07 and so on. _ 1 __ mO2 vacancy e . Om Figure Variation of the response of three doped tin oxide gas sensors with temperature for four different gases. Adapted from Yamazoe et al. 1983 284 MICROSENSORS Next this reversible reaction is disturbed when the analyte molecule X reacts with the chemisorbed oxygen species to release electrons and promulgate further reactions X Om XOm e - In a simple physical description the tin oxide behaves like an n-type semiconductor and therefore there is an increase in the electron carrier density n and hence in the electrical conductivity Ơ of the material with increased gas concentration where Act iineA n
