Zhengkuan Jiao

Thermoanalytical characteristic of nanocrystalline brookite-based titanium dioxide

Abstract Using differential-thermoanalytical (TG-DTA) measurement and X-ray diffraction (XRD) analysis, the characteristics of nanocrystalline brookite-based TiO 2 with a certain amount of anatase and rutile have been studied. There are two irreversible endothermic peaks on the DTA curve. The first peak (with weight loss on thermal-gravity TG and differential thermalgravity DTG curves) corresponds to the desorption of physical or chemical species, while the second one (without weight loss on TG and DTG curves) is related to the structural phase transition from brookite to anatase then to rutile, and the structural phase transitions are beneficial to the grain growth. In this paper, the grain size and the content of each individual phase in specimens at different temperatures are also reported.

The thermal stability of nanocrystalline maghemite

Nanocrystalline maghemite O was prepared by a wet chemical method. The thermal stability of this material has been investigated by using differential thermal analysis (DTA) measurements and x-ray diffraction analyses. One exothermic peak without weight loss appears irreversibly on the DTA curve. It was confirmed for the first time that this exothermic peak corresponds completely to the maghemite-to-hematite (-to-) structural phase transition. The nanocrystalline maghemite O particles grow very slowly when the temperature is lower than the range of the exothermic peak; whereas, during and after the transition, the nanocrystalline hematite O particles grow rapidly with increasing temperature. Moreover, the transition temperature of nanocrystalline maghemite O is higher than that of the coarse-grained counterpart. In this paper, the authors have also suggested some preliminary physical interpretations of the experimental results.

High dielectric constant in CaCu3Ti4O12 thin film prepared by pulsed laser deposition

Abstract We investigate the dielectric properties of c -axis-oriented epitaxial CaCu 3 Ti 4 O 12 thin film prepared by pulsed laser deposition. The dielectric constants are found to be in the order of 10 3 –10 4 , depending on the mean grain size for the as-deposited thin films. Furthermore, ac conductivity is measured in the frequencies of 0.1–100 kHz and temperatures of 77–350 K. We find that the dissipation is due to hopping polarization of charge carried at high frequencies and is influenced by interfacial effect at low frequencies. Our results of high dielectric constant and its variation with temperature, the low dielectric loss and the stability of the film show that CaCu 3 Ti 4 O 12 thin film may become a good candidate for certain technological applications.

Growth behavior and surface morphology of Ag rough thin films deposited on silicone oil surfaces

The growth behavior and surface morphology of a rough film system, deposited on silicone oil drop surfaces by a r.f. magnetron sputtering method, have been studied. An anomalous film growth relaxation is observed during the deposition process. The relaxation rate is extremely sensitive to the substrate temperature. The surface morphology at the micrometer length scale is very susceptible to the substrate temperature, the incident r.f. capacity, and the nature of the solid substrate on which the oil drop is dripped. A discussion on the physical origins of these phenomena is also presented.

Quench characteristics and normal zone propagation of an MgB2 superconducting coil

We have studied experimentally the quench characteristics and normal zone propagation of Cu–Ni sheathed MgB2 superconducting monofilament tape. The quench dynamical process and the stabilities, i.e. minimum quench energy (MQE) and normal zone propagation velocity vp, are investigated through the double-solenoid experiment at 4.2 K. By measuring the MQE and vp of MgB2 tape, we derive the dependence of MQE and vp on transport current and magnetic field.

Structural and electrical properties of an Au film system deposited on silicone oil surfaces

An Au thin film system, deposited on silicone oil surfaces by the thermal deposition method, has been fabricated and its structure as well as electrical properties has been studied. A web-shaped characteristic surface morphology of the films is observed. The dc sheet resistance R of the metal films on the liquid surfaces is measured during and after deposition in situ by the four-probe method. The time dependence of the sheet resistance can be explained in terms of the film growth mechanism on the oil surface. The anomalous I–V characteristics of the film system can be interpreted as a competition among the local Joule heating, hopping and tunnelling effects. It is found that the dc third-harmonic coefficient B0 and the zero-power resistance R0 satisfy the power-law relation B0 R02+w and the exponent w is close to zero. This result indicates that the hopping and tunnelling effects in the samples are much stronger than those of the other film systems. We also find Im R0−β with β = 0.79 ± 0.08 and Im B0−γ with γ = 0.49 ± 0.07, where Im is the critical current. The physical origins of the phenomena are discussed.

The grain size dependence of the resistance behaviors in doped lanthanum manganite polycrystalline films

Using the sol–gel spin-coating method, La0.67Sr0.33MnO3 and La0.7Ca0.3MnO3 polycrystalline films have been prepared. The polycrystalline films show much lower resistivity peak temperature Tp than that of the corresponding epitaxial film, and Tp shifts to higher temperature when the annealing temperature increases. The magnetization measurement reveals that all of the polycrystalline films have the same magnetic transition temperature as the corresponding epitaxial films. These results are different from the previous works. We ascribe the difference to the grain size of our polycrystalline samples being much smaller, so the conduction through the grain boundaries predominates over the intragrain conduction. In this case, spin disorder and magnetocrystalline anisotropy should be taken into account in explaining the magnetotransport behavior.

Dielectrical behavior of nanostructured α-Fe2O3 with different annealed temperatures

The spectra of the dielectrical constant and dielectrical loss in three nanostructured α-Fin23 samples at different annealed temperatures are measured by using ac LRC method. The relaxation times of polarization and the conductance are calculated by means of the Debye mechanism and the interface effects in two kinds of polarization processes with two different relaxation times are proposed.

Size effect on structure and infrared behavior in nanocrystalline magnesium oxide

Abstract From the results of Fourier Transform Infrared (FTIR) spectra and X-ray Diffraction (XRD), we found that there exists size effect on the structure and infrared behavior in nanocrystalline MgO. When the grain size decreases, the crystal lattice dilates anisotropically, and the red shift of stretching vibrational mode coexists with the blue shift of the bending vibrational mode. We give a preliminary interpretation to the above abnormal phenomena.

Polaronic effect on the binding energy of an impurity with varying position in parabolic quantum dots

Abstract The Feynman-Haken variational path integral theory is generalized to calculate the binding energy E b of an electron coupled simultaneously to an impurity with varing position and to a longitudinal-optical (LO) phonon field in parabolic quantum dots. Our calculations are applied to some semiconductor materials and the results for the binding energy are obtained for different confinement length R of the dot and arbitrary position of the impurity. It is shown that the polaronic correction to E b decreases with the displacement of the impurity and increases with the confinement length of the dot. More interestingly, it is not so strong as the polaronic correction to the ground-state energy of the system, and the behaviours of their variation with R are totally different.