Zhang Tianshu

Ionic conductivity in the CeO2–Gd2O3 system (0.05≤Gd/Ce≤0.4) prepared by oxalate coprecipitation

Abstract High purity cerium and gadolinium salts were used to form ceria-based solid solution (Ce1−xGdxO2−δ, 0.05≤x≤0.4) through the oxalate coprecipitation. Crystal structure and microstructure were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The ionic conductivity (i.e., lattice, grain boundary (GB) and total conductivities) in this system were systematically studied as a function of dopant content over the temperature range from 200 to 850 °C in air using an impedance spectroscopy. The grain boundary conductivity increases rapidly at x 0.2. The Gd content has a slighter influence on the lattice conductivity in the Gd content ranging from 0.05 to 0.2, while a rapid decrease in the lattice conductivity is observed at x>0.2. A maximum total conductivity is observed for the composition x=0.2.

Selective detection of ethanol vapor and hydrogen using Cd-doped SnO2-based sensors

Abstract The effect of CdO doping on microstructure, conductance and gas-sensing properties of SnO 2 -based sensors has been presented in this study. Precursor powders with Cd/Sn molar ratios ranging from 0 to 0.5 were prepared by chemical coprecipitation. X-ray diffraction (XRD) analysis indicates that the solid-state reaction in the CdO–SnO 2 system occurs and α-CdSnO 3 with pervoskite structure is formed between 600 and 650°C. CdO doping suppresses SnO 2 crystallite growth effectively which has been confirmed by means of XRD, transmission electron microscopy (TEM) and BET method. The 10 mol% Cd-doped SnO 2 -based sensor shows an excellent ethanol-sensing performance, such as high sensitivity (275 for 100 ppm C 2 H 5 OH), rapid response rate (12 s for 90% response time) and high selectivity over CO, H 2 and i-C 4 H 10 . On the other hand, this sensor has good H 2 -sensing properties in the absence of ethanol vapor. The sensor operates at 300°C, the sensitivity to 1000 ppm H 2 is up to 98, but only 16 and 7 for 1000 ppm CO and i-C 4 H 10 , respectively.

Early-stage sintering mechanisms of Fe-doped CeO2

Cerium oxide is a very useful base material used as catalyst supports, ion conductors and gas sensors. It is also well known that ceria-based materials are difficult to densify below 1500°C. However, a small amount of Fe doping obviously promotes the densification rate and reduces the sintering temperatures. In this study, the early stage sintering mechanisms of undoped and Fe-doped CeO2 were investigated, based on two sintering models. We confirm that pure CeO2 exhibits volume-diffusion controlled sintering, while a viscous flow mechanism dominates the early stage sintering of Fe-doped CeO2.

Sintering and densification behavior of Mn-doped CeO2

Abstract The effect of Mn doping on sintering and densification behavior of commercial CeO 2 powder has been investigated in this study. The results from both X-ray diffraction (XRD) and thermal gravimetric-differential thermal analysis methods indicate no binary compounds are formed between CeO 2 and MnO 2 , and Mn element exists in the state of Mn 2+ (i.e. MnO) in the samples sintered above 1200°C. Dilatometer measurement and scanning electron microscopy observation confirm that small amount of Mn doping reduces sintering temperature dramatically and promotes grain boundary mobility. About 1% Mn-doped CeO 2 sintered at 1300°C for 2 h reaches almost full densification (>99.0% of relative density), compared with ∼96% of relative density for pure CeO 2 sintered at 1525°C for 2 h, and both samples have almost the same grain size.

Improvements in α-Fe2O3 ceramic sensors for reducing gases by addition of Sb2O3

The microstructure, electrical properties and gas-sensing characteristics of Sb-doped α-Fe2O3 were investigated. Powder precursors with Sb/Fe = 0–0.1 were prepared by chemical coprecipitation method. Sb-doped α-Fe2O3 powders were characterized by means of thermal gravimetric-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), BET surface area and scanning electron microscope (SEM). It was found that the raw powders underwent crystallization into the corundum structure of α-Fe2O3 at a temperature which increased somewhat with increasing Sb content; a proper amount of Sb doping suppressed both crystallite growth and the formation of hard agglomerates. The doping of Sb2O3 decreased the sensor resistance by one order of magnitude and increased the sensitivities to some hydrocarbon gases markedly. The former can be attributed to the substitution of Sb5+ for Fe3+ sites in α-Fe2O3 generating more free electrons; the latter is closely related to Sb-doped samples accommodating a higher density of chemisorbed oxygen.

FeSbO4 semiconductor ceramics: a new material for sensing liquid-petroleum gas

FeSbO4-based semiconducting ceramics used as a promising candidate for sensing liquid-petroleum gas (LPG) are presented here for the first time. Precursor powders of FeSbO4 were prepared by two different methods (i.e., ball-milling and chemical coprecipitation). The solid-state reaction in the Fe2O3-Sb2O3 system was investigated by means of thermal gravimetric-differential thermal analysis (TG-DTA) and X-ray diffraction (XRD). Based on our experimental results and previous work, the diffusion of antimony oxide onto α-Fe2O3 is assumed to be a controlling step of the solid-state reaction. Scanning electron microscopy (SEM) and the BET method were used to characterize the samples calcined at 550 to 1000 °C. It was found that a sudden change in specific surface area, crystallite size and particle size takes place between 550 °C calcining and 600 °C calcining, which has an obvious influence on gas-sensing properties. FeSbO4-based sensors operating at 370 °C show a high sensitivity and selectivity to liquid-petroleum gas (LPG) over H2 CO or i-C4H10. Addition of Pd shows a fair increase in sensitivity to LPG, but a remarkable improvement in response and recovery times which is advantageous for its practical application.

Effect of A-Site Substitution of Calcium on Zr-Rich Lead Zirconate Titanate

Effect of calcium substitution on the A-site of Zr-rich lead zirconate titanate Pb(Zr 0.94 Ti 0.06 )O 3 and Pb(Zr 0.92 Ti 0.08 )O 3 is studied. It is found that the rhombohedral structure of Zr-rich lead zirconate titanate changes to tetragonal and finally to cubic with increasing amount of calcium substitution. Both the dielectric constant and the Curie temperature decrease with the substitution of calcium on the A-site of the ABO 3 perovskite structure. A pyroelectric current peak is observed which is resulted from the phase transition from the low temperature rhombohedral ferroelectric phase F R ( LT ) to the high temperature F R ( HT ) one. Small amount of calcium substitution increases the peak value and also shifts the peak position to higher temperatures. However, further increase in the calcium content lowers the peak value and finally smears out the peak.

Stress Effect on the Pyroelectric Properties of Lead Titanate Thin Films

In the application of thin ferroelectric films to IR sensors, problems may arise to affect the sensitivities of the devices. The effects to influence the pyroelectric properties can be mainly categorized into two groups. One is the intrinsic effect due to the reduction in grain size. The other is the effect caused by residual stress. In this paper the stress effect on the pyroelectric properties of PbTiO 3 thin films will be studied by using the Landau-Ginsburg-Devonshire (LGD) phenomenological theory. It is found that a compressive stress increases the figure of merit while a tensile stress increases the pyroelectric coefficient for the films with the polarization vectors normal to the surface.

Stress- and strain-relaxation in lead zirconate titanate based ceramics

The stress- and strain-relaxation behavior of lead zirconate titanate based ceramics is studied by using the three-point bending test. The relaxation behavior has been explained based on the domain reorientation mechanism. It is found that the stress relaxation behavior obeys a logarithmic time law. It has also been experimentally confirmed that above the Curie temperature where all the domains disappear, there is no relaxation in the ferroelectric ceramics.