The Study Of Pd Doped CO Gas Sensors Based On Ceramic Micro-processing

A novel gas sensor based on micro-machined ceramic hotplate and a wire-free bonding is presented, which is aim at the shortcomings of high power consumption and difficult to package existing in the current commercial ceramic gas sensors. The finite element method (FEM) is utilized to simulate the thermal characteristics of micro-hotplate and provide a theoretical guidance for optimizing the device's structure. The steady-state heat transfer model of ceramic hotplate is established, and the results from model calculation are compared with those obtained by FEM simulation. On the basis of the exploration of the wet etching technology in micro-machining ceramic materials, the excimer laser micro-machining technique is determined, by which batches of ceramic hotplates with different structural parameters are produced successfully. The test results show that the minimum heating power required for an average temperature of 300℃is about 200 mW and the heating power of 425 mW will enable the average temperature of the hotplate to reach 600℃. Working at 600℃, Pt micro-heater has good thermal stability and thermal shock resistance. Ceramic micro hotplate can be used for low-power, high temperature gas sensors.Low-power gas sensor based on ceramic hotplate is developed to detect CO gas on constant-voltage heating mode. It is suggested that gas-sensing film can be prepared with the heater on the same side of the substrate when hotplate is used as a gas sensor. The equivalent circuit model is built on and the feasibility of the above mentioned method is analyzed theoretically. According to the results of the analysis, it is found that the heating power can be simultaneously used as the power of the test circuit.SnO₂ nano-powders are prepared by direct precipitation method, sol-gel method by citric acid dissolving and sol-gel method by ammonia dissolving, respectively. In the view of particle size, sol-gel method by ammonia dissolving is the best preparation method. On this basis, three Pd doped powders with atom doping concentration of 0.2%, 2% and 10% are prepared. Ceramic thick-film CO gas sensors based on hot plate are produced with above prepared Pd-doped materials and the morpHology of the sensitive film is characterized.The tests on gas-sensing characteristics of four sensors with different sensitive materials are carried out in the self-made test platform. The impact of the concentrations of Pd-doping on CO gas sensitivity are studied from the sensitivity, selectivity, stability, response time, environmental impact, such as temperature and humidity. From the comprehensive evaluation of the test results, it is found that 0.2% Pd-doping for CO detection is the best doping concentration. At the best working temperature of 200℃, the device with 0.2% Pd-doping has a low heating power of 130 mW and good response to CO, which is expected to be a practical constant-voltage heated CO gas sensor.Combined with the experimental results, density functional theory (DFT) method is used to calculate the sensitive mechanism of the SnO₂-based CO gas sensor. The models of stoichiometric and reduced SnO₂ (110) surface are established respectively and their surface structure and electrical characteristics are calculated. Several metals of Pt, Pd, Cu and Ag which are commonly added in the CO gas sensors are selected to study their impact on the catalytic reaction of CO adsorption. The calculation provides a theoretical foundation for the selection of Pd as a dopant in the CO gas sensors. The models of CO molecule adsorption on SnO₂ (110) surface are established. By comparing the difference of adsorption properties of CO adsorbing on the undoped and Pd-doped SnO₂ (110) surface, the mechanism that Pd doping can change the sensing properties of CO gas sensors is investigatetd.