Carbon monoxide (CO) gas sensitivity of CuO-infiltrated ZnO ceramics

Abstract

Distinction of carbon monoxide (CO) gas from other combustible gases (especially from H 2 gas) is difficult using conventional SnO2-based ceramic sensors. Special methods, such as measurements under periodic temperature cycling between 100 and 300 °C, are required for CO gas sensing by SnO2based ceramic sensors [1]. A heterocontact of pand n-type semiconducting ceramics is one type of sensor using an interface [2-5]. For making heterocontacts, sintered ceramics of p-type and n-type semiconductors are physically contacted by pressing. In these sensors, the contact points are open to atmosphere gases due to the surface roughness, and the gas phases can react at the interface. This reaction changes the electrical properties of the interface, so that the devices can be used as gas sensors. In previous experiments, a CuO/ZnO sensor has shown excellent CO gas selectivity and a tunable sensing property by control of the applied bias voltages [5, 6]. The preparation of the ceramic contacts is simple, but these contacts do not provide the quantitative reproducibilities important for industrial gas sensors. To make stable p-n contacts, CuO/ZnO thin films were prepared by the sputtering method. This showed sensing properties for humidity and CO gas similar to those of the ceramic heterocontact, but CO gas sensitivity is not high due to the small open-interface area [7, 8]. In the present study, in order to increase the contact area and improve the reproducibility of the CO gas sensing properties, a ZnO-CuO composite was prepared by infiltration of aqueous solution of copper into porous ZnO, and firing. Microstructure and gas sensitivities for CO and H2 gases were examined for the CuO-infiltrated ZnO ceramics. Powder of ZnO (99.99% purity) was pressed into rectangular bars of 5 x 3 x 15 mm at 98 MPa, and sintered at 700 °C for 3 h. The relative density of the ZnO ceramics was 70-80% of the theoretical value. An aqueous solution of copper was obtained from 99.9% cupric nitrate (Cu(NO3)23H20; Kojundo Chemical). A part of the ZnO ceramics was dipped into 0.7 N cupric solution and fired at 600 °C for 1 h. The firing temperature was determined from the results of differential thermal analysis (DTA) which shows three endothermic peaks at 109 °C, 223 °C and 435 °C. Phases in the sample were confirmed