Gyeong Man Choi

ELECTRICAL AND CO GAS SENSING PROPERTIES OF ZNO-SNO2 COMPOSITES

Abstract ZnO–SnO 2 composites of 0–100 mol% composition range were fabricated in the form of pellet by sintering at 900°C and their electrical conductivity and CO gas sensitivity were measured between 70 and 500°C. SnO 2 -rich composites showed higher sensitivity values than pure SnO 2 . ZnO-rich composites have more porous microstructure and thus are more sensitive to CO gas at all temperatures than pure ZnO. The effects of microstructure and composition on the gas sensitivity were discussed. SnO 2 –ZnO grain boundary was also proposed to be responsible for the gas sensitivity.

Electrical and CO gas sensing properties of layered ZnO–CuO sensor

The electrical conductivity and the CO gas-sensing characteristics of ZnO-CuO-layered samples were studied and compared with those of ZnO samples modified with various dopants. Layered ZnO-CuO samples were fabricated by sequentially pressing ZnO and CuO powders in a die followed by co-firing at 8008C for 3 h. The control of porosity of both ZnO and CuO enabled a stable contact between ZnO and CuO. The addition of SnO into ZnO prevented the densification of ZnO and further addition of Al O and CuO, respectively, 2 23 increased and decreased the electrical conductivity of porous ZnO without further changing the microstructure. All layered samples showed the electrical conductivity mostly determined by the heterocontact interface. The layered sample with Al-doped ZnO showed the highest CO gas sensitivity among all samples. q 2000 Elsevier Science S.A. All rights reserved.

Electrical and reducing gas sensing properties of ZnO and ZnO–CuO thin films fabricated by spin coating method

Abstract ZnO and ZnO–CuO composite thin films were fabricated by sol-gel spin coating method and their electrical conductivities and the reducing gas sensitivities were measured between 100 and 500°C. As the thickness of ZnO film linearly increased with the repeated coating, the grain size increased and the film became denser. As a result, the electrical conductivity increased. CuO addition is shown to increase the grain size of ZnO–CuO composite film. As the number of coating increased, the thickness of ZnO–CuO film also increased together with the grain size. The electrical conductivity increase was also observed for ZnO–CuO film with repeated coatings. The 10 mol.% CuO addition eliminated the CO gas sensitivity of the film and severely reduced H 2 gas sensitivity. The co-addition of CuO and Al 2 O 3 gave effects of contribution of p-type conduction on the reducing gas sensitivity of ZnO film.

Nonstoichiometry and electrical conduction of CuO

Abstract Electrical conductivity, thermoelectric power and weight changes were measured between room temperature and 800 or 1000 °C on CuO with varying sintering temperatures to determine nonstoichiometric and defect properties of CuO. CuO was confirmed to be a metal deficient p-type semiconductor with copper vacancies at low temperature. Copper vacancy formation was favored with decreasing sintering temperature indicating the negative enthalpy of the defect generation reaction. Nonstoichiometry in Cu 1−y O was a few percent at 700–900 °C. Conduction in Cu 1−y O was judged to be due to hopping of charge carriers with an activation energy of 0.1 eV. The band gap energy of CuO, estimated from high temperature intrinsic conductivity was ~ 1.2 eV.

CO gas sensing properties of ZnO–CuO composite

Abstract In order to correct the reproducibility problem inherent in the hetero-contact gas sensor and to see the possibility of using the electrical composite material as a gas sensor, porous ZnO(n)–CuO(p) composite system was selected for the investigation. The electrical conductivity and CO gas sensing properties were examined between 70 and 530°C at a fixed relative humidity of 23%. ZnO–CuO samples containing pure CuO showed small gas sensitivity to 200 ppm CO, however those containing Al-doped CuO showed high sensitivity to the same gas near 200°C. The shape of the electrical conductivity curve was related to the gas sensitivity. The temperature showing maximum CO gas sensitivity decreased with the addition of CuO. These observations were explained by both the microstructure and the electrical properties of p/n contact of the composite.

Microstructure and CO gas sensing properties of porous ZnO produced by starch addition

Abstract Effects of corn starch addition on the microstructure and CO gas response were studied by sintering ZnO at 600∼900°C for 3 h in air. The addition of 5 wt% corn starch as the fugitive phase decreased both the grain size and the sintered density of ZnO at all sintering temperatures and thus increased the sensitivity to 200 ppm CO. The increasing relative density and grain size with sintering temperature was accompanied by a decreasing CO gas sensitivity after maximum at 700°C. The temperature showing the maximum CO gas sensitivity decreased with decreasing grain size. Humidity decreased both the hysteresis of electrical conductivity and CO gas sensitivity.

Current–voltage characteristics and selective CO detection of Zn2SnO4 and ZnO/Zn2SnO4, SnO2/Zn2SnO4 layered-type sensors

Abstract The current–voltage characteristics and the CO gas-sensing properties of porous Zn 2 SnO 4 were examined. ZnO/Zn 2 SnO 4 and SnO 2 /Zn 2 SnO 4 layered-type samples were also fabricated for the investigation of the interfacial effect between ZnO (or SnO 2 ) and Zn 2 SnO 4 by sequentially die-pressing respective powders. To form a stable contact between ZnO (or SnO 2 ) and Zn 2 SnO 4 , the composition and thus the sinterability of ZnO (or SnO 2 ) was controlled. The electrical and gas-sensing properties of the layered samples were compared with those of individual ZnO, SnO 2 and Zn 2 SnO 4 . Zn 2 SnO 4 showed the higher sensitivity values to CO gas than to H 2 gas at relatively low temperature. The sensitivity of Zn 2 SnO 4 decreased with the increasing magnitude of applied voltage at high temperature. Both SnO 2 /Zn 2 SnO 4 and ZnO/Zn 2 SnO 4 layered-type samples showed the sensing characteristics varying with the direction of applied bias. The interface between Pt and Zn 2 SnO 4 was responsible for the observed non-ohmic current–voltage characteristics and the resultant gas sensitivity.

Computer simulation of the electrical conductivity of composites: the effect of geometrical arrangement

Abstract A computational methodology is presented to simulate d.c. and a.c. transport properties of two-dimensional composites. Using this method, average electrical properties, or, more specifically, d.c. conductivity and a.c. impedance spectra, of macroscopic mixtures of hard spheres which have random or regular arrangements of the components are studied. Numerical results are compared with the calculated results of analytic equations that have generally been used to describe the d.c. and a.c. electrical properties of composites. It is shown in this study that they are sensitive functions of the filling fraction, sample size and the geometrical arrangement of the components. The geometrical arrangement effects, which hardly can be considered in analytical formulae, become more important as the sample size decreases and the filling fraction approaches the effective percolation threshold.

Percolation Behavior of Conductor-Insulator Composites with Varying Aspect Ratio of Conductive Fiber

The electrical properties of composite electroceramics are determined by the concentration, shape and distribution of filler phase in matrix. The carbon fiber-filled polymer was chosen as a model system and the electrical conductivity was measured as a function of carbon fiber content and the aspect ratio (AR) of the fibers to understand the percolation behavior of the composites. The composites of carbon fiber (1∼9 vol.%) and thermoplastic polymer were fabricated in a mold press with the aspect ratio of carbon fiber varying between 4 and 10. The percolation threshold volume concentrations (Vc) of transition from the insulator to the conductor decreased as the fiber aspect ratio increased. With the fibers segregated at the polymer-polymer interfaces in the present study, Vc values were much smaller than those with the fibers randomly distributed in the matrix shown in other studies. The inverse relation between Vc and AR was found as expected. From the comparison with other experimental and simulated data, we concluded that the slope in 1/Vc versus AR plot is a strong function of fiber segregation.

Electrical and CO gas-sensing properties of ZnO/SnO2 hetero-contact

Abstract Stable and reproducible hetero-contact samples of SnO2:3Zn and ZnO:5Sn were successfully fabricated by co-firing two oxide layers at 800°C for 3 h. Hetero-contact sample showed higher resistivity and sensitivity to 200 ppm CO gas than individual SnO2:3Zn or ZnO:5Sn. The electrical and CO gas-sensing properties of hetero-contact samples were strongly dependent on the sample thickness; the thinner hetero-contact sample showed the higher resistivity and the higher sensitivity to 200 ppm CO gas than the thicker one. In other words, the interfacial element has more distinctive influence on the gas sensitivity. From these results, it is proposed that SnO2(n)/ZnO(n) hetero-contact interface works as a good sensing element.