Gas Detection Using Breakdown Voltage of a Cold Pressed Thick-Layer ZnO Varistor

Effects of target gas (TG) on I-V characteristics of a Zinc Oxide varistor was investigated. The sensitive layer was fabricated by partial sintering of a mm-thick layer formed by low-pressure cold pressing of ZnO powder. SEM and Arrhenius method, respectively characterized the microstructure and electrical properties of the layers. The sensor was fabricated by providing Ohmic contacts, and installation of a custom-made micro-heater beneath the layer. The I-V characteristics of the device at the presence of Ethanol and Oxygen were recorded using an initiated circuit. It was found that the breakdown voltage shifts to lower and higher voltages upon exposure to reducing and oxidizing gases, respectively. On the other hand, the I-V curve of the device was analyzed based on a mathematical model. The model-based analysis of I-V curve was fitted to the experimental data, and the semi-empirical model parameters were resulted as the fitting parameters. By application of model parameters, the steady state gas sensitivity (S = J(gas)/J(Bair)) of the device was estimated. By comparison of gas detection sensitivity of the device in resistive and varistor operation, it was found that the gas sensitivity of a ZnO varistor is one order of magnitude higher than that of ZnO resistive gas sensor. The I-V characteristics of the device at different operating temperature and gas concentration were investigated. The result revealed that the device breakdown voltage and gas sensitivity strongly depends to device temperature as well as gas concentration.