Dongxiang Zhou

Physically Flexible, Rapid‐Response Gas Sensor Based on Colloidal Quantum Dot Solids

A gas sensor based on PbS colloidal quantum dots (CQDs) is constructed on a paper substrate, yielding flexible, rapid-response NO₂ gas sensors, fabricated from the solution phase. The devices are highly sensitive and fully recoverable at room temperature, which is attributed to the excellent access of gas molecules to the CQD surface, realized by surface ligand removal, combined with the desirable binding energy of NO₂ with the PbS CQDs.

Preparation and characteristic of the thermistor materials in the thick-film integrated temperature–humidity sensor

Abstract The designing, preparation, and characteristic of the thermistor materials in a thick-film integrated temperature–humidity sensor were investigated in detail. This article presented the relationship between resistance–temperature characteristics and the composition of the negative temperature coefficient thermistor materials, which matched with humidity sensitive material in the integrated sensor. The effects of the fabrication processing on the R–T characteristic and the material granularity, granularity uniformity were presented. Finally this article provided some experimental data of the thermistor materials and the characteristic curves of the thick-film integrated temperature–humidity sensor prepared from the material.

Influence of TiO2/Sb2O3 ratio on ZnO varistor ceramics

Abstract The influence of TiO2/Sb2O3 (Ti/Sb) ratio on the microstructure development and electrical properties of varistor ceramics in ZnO–(Bi2O3–TiO2–Sb2O3)–CoO–MnO2 system was investigated. Characterization of varistors was performed by XRD, SEM, d.c. degradation behavior, and electrical measurements. It is found that the Ti/Sb ratio greatly influences the microstructure and electrical properties of the ZnO varistor. The specimen with Ti/Sb ratio of 5 has the lowest breakdown field (20V/mm) and improved d.c. degradation behavior.

Study of piezoelectric ceramic materials for high-temperature and high-frequency applications

Abstract In this paper, some methods to get piezoelectric ceramic materials for high-temperature and high-frequency applications based on modified PbTiO3 piezoelectric ceramics have been introduced. A solid solution system (1 − X)PbTiO3 + XPb(Cd1/3Nb2/3)O3 has been studied for its piezoelectric properties as the B-position complex substitution of ABO3 perovskite structure. The system with X=14 mol% has good piezoelectric properties by adding some suitably modified additives, such as WO3, MnO2 and NiO. The Curie temperature is over 450 °C and the dielectric constant, es, is about 200 at room temperature. The piezoelectric coupling factor Kt is 0.45 and the mechanical quality factor Qmcan be changed from 500 to 2400 according to different situations. The high stability of these materials makes it possible for them to be used in high-temperature and high-frequency situations.

Degradation phenomena due to dc bias in low-voltage ZnO varistors

Abstract Relationship between the electrical characteristics of the low-voltage ZnO varistors and dc degradation is investigated in this study. The degradation phenomena of the low-voltage ZnO varistors due to long-time bias are studied by current–voltage ( I – V ) and ac measurements. After degradation, the changes observed in the properties of deep bulk trap are attributed to the alteration of defect structure.

Magnetron sputtering electrode on the BaTiO3-based PTCR ceramics and the effect of heat treatment on their properties

Electrode materials, including aluminum-, nickel- and nickel-based alloys, are deposited on the BaTiO3-based PTCR thermistors. Experimental results indicated that there could form good ohmic contacts between BaTiO3-based PTCR thermistors and those electrode materials. We compared the properties of PTCR sputtering electrode with others, and found that sputtering method was a good technique to make electrodes on the BaTiO3-based PTCR thermistors. We also discussed the effect of heat treatment on the properties of PTCR sputtering electrodes.

Analysis on the aging characteristics of PTCR of donor-doped barium titanate

The resistance versus time curves of room temperature unload ageing, high temperature unload ageing and high temperature continuous load test ageing of PTC thermistors are measured. At the initial aging stage under both high temperature unload ageing and high temperature continuous load test ageing, the resistance decreases first, and then increases, which are different from that of the room temperature aging. Such abnormal phenomena exist in the initial ageing stage and are attributed to buffering of the unstable mechanical stress and desorption of the absorbed oxygen molecules under the thermal action. R � /T characteristic curves of aged sample are different from that of oxidized and reduced samples. A new mechanism is proposed to interpret the aging characteristics of the donor-doped BaTiO3 PTC thermistor. # 2002 Elsevier Science B.V. All rights reserved.

Preparation and electrical properties of ZnO-glass ceramic films

ZnO-glass ceramic films doped with Sb2O3, MnO, Co2O3 and Cr2O3 were fabricated by a novel sol-gel process. The precursors were prepared by dispersing the ZnO nano-powders throughout into the sols, which were prepared by dissolving zinc acetate dihydrate and the dopants into 2-methoxyethanol and stabilized by diethanolamine and glacial acetate. The films were prepared on Au/Si substrates by spin coating method and were heat-treated and annealed in air at 450 and 750 °C, respectively. The films with the thickness of about 2 µm were obtained after 10-20-layers deposition. The crystal phases, the residual stress in the crystals and the microstructure of films were studied via XRD analysis, Raman spectroscopy and SEM, respectively. The films were used as film varistors after the upper Au electrodes, the area of which is 3 mm2, were made on the surface of the films by vacuum evaporation. The electrical properties were studied by varistor characteristics testing instrument, and the remarkable nonlinear V-I response was observed. The nonlinear voltage and the nonlinear coefficient (α), which were 3-19 V and 5-15, respectively, were affected obviously by glass content. The highest nonlinearity coefficient with nonlinear voltage of 6.5 V could be achieved when the films contain 8 wt% glass.

Theoretical Analysis of Wireless Passive Impedance-Loaded SAW Sensors

This paper concentrates on the theoretical analysis of wireless passive surface acoustic wave (SAW) impedance-loaded sensors. A general method is proposed for simulating the impedance-loaded SAW sensors. It is based on the combined finite-element method and boundary element method (FEM/BEM). A FEM is used to account for the mass loading effect of electrodes and a Greens function is used to model the piezoelectric substrate. Comparison between the simulations and measurements on SAW devices shows a good agreement. The calculated amplitude variation of the impulse response in time domain shows a resonant characteristic with the change of the loaded impedance. It is found that the return loss reaches the maximum value when the resonant frequency of the loaded circuits matches the center frequency of the short-circuited SAW transponder. This phenomenon is successfully explained by using the proposed model. Some high-performance sensors with greater amplitude modulation and larger sensitive range could be designed using this method.

A new method for microwave dielectric measurement of low loss ceramics

Abstract In this paper a novel resonant method was proposed for the determination of dielectric constant and loss tangent of low loss ceramic materials used for dielectric resonators at microwave frequencies. The details of the design and fabrication of the cavity, the input and output coupling transmission lines and the substrate were discussed. The radial mode matching (RMM) technique was used to determine the exact frequencies of this resonant configuration as a function of the permittivity of the sample. And the relation between the loss tangent of the sample and the unloaded quality factor ( Q l ) of this structure was obtained on the basis of the electromagnetic field distribution. Some numerical results for the direct and inverse problems were presented to validate these relationships. The absolute uncertainties of the measurement were presented. Lastly the procedure of measurement was discussed.