S.T Li

Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles

It is shown that a “nanofluid” consisting of copper nanometer-sized particles dispersed in ethylene glycol has a much higher effective thermal conductivity than either pure ethylene glycol or ethylene glycol containing the same volume fraction of dispersed oxide nanoparticles. The effective thermal conductivity of ethylene glycol is shown to be increased by up to 40% for a nanofluid consisting of ethylene glycol containing approximately 0.3 vol % Cu nanoparticles of mean diameter <10 nm. The results are anomalous based on previous theoretical calculations that had predicted a strong effect of particle shape on effective nanofluid thermal conductivity, but no effect of either particle size or particle thermal conductivity.

Measuring Thermal Conductivity of Fluids Containing Oxide Nanoparticles

Oxide nanofluids were produced and their thermal conductivities were measured by a transient hot-wire method. The experimental results show that these nanofluids, containing a small amount of nanoparticles, have substantially higher thermal conductivities than the same liquids without nanoparticles. Comparisons between experiments and the Hamilton and Crosser model show that the model can predict the thermal conductivity of nanofluids containing large agglomerated Al{sub 2}O{sub 3} particles. However, the model appears to be inadequate for nanofluids containing CuO particles. This suggests that not only particle shape but size is considered to be dominant in enhancing the thermal conductivity of nanofluids.

Enhanced thermal conductivity through the development of nanofluids

Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids required in many industrial applications. To overcome this limitation, a new class of heat transfer fluids is being developed by suspending nanocrystalline particles in liquids such as water or oil. The resulting nanofluids possess extremely high thermal conductivities compared to the liquids without dispersed nanocrystalline particles. For example, 5 volume % of nanocrystalline copper oxide particles suspended in water results in an improvement in thermal conductivity of almost 60% compared to water without nanoparticles. Excellent suspension properties are also observed, with no significant settling of nanocrystalline oxide particles occurring in stationary fluids over time periods longer than several days. Direct evaporation of Cu nanoparticles into pump oil results in similar improvements in thermal conductivity compared to oxide-in-water systems, but importantly, requires far smaller concentrations of dispersed nanocrystalline powder.

Formation and characteristics of Pb(Zr,Ti)O3 field-effect transistor with a SiO2 buffer layer

Ferroelectric heterostructures of Au/Pb(Zr0.52Ti0.48)O3/SiO2/Si and Au/Pb(Zr0.52Ti0.48)O3/Si have been fabricated by using laser ablation technique. Electrical properties of these ferroelectric field-effect transistors have been characterized through both the current vs voltage and capacitance vs voltage (C–V) measurements. The C–V characteristics of Au/Pb(Zr0.52Ti0.48)O3/SiO2/Si heterostructures demonstrate a polarization switching behavior, showing a memory window as much as 1 V at 1 kHz. In addition, the experimental results reveal that a SiO2 buffer layer is essential for memory properties in the Au/Pb(Zr0.52Ti0.48)O3/SiO2/Si gate structure.

Nonlinear optical dynamics and Eu3+ spectral holeburning in strontium barium niobate thin film grown by pulsed laser deposition

Optical quality SrxBa1−xNb2O6 (SBN) thin films, both undoped and Eu3+-doped, of thickness less than 0.5 μm have been successfully grown on fused quartz substrates using a pulsed laser deposition technique. Optical properties of these films were characterized in high-resolution spectroscopic experiments in time and frequency domains. For undoped SBN thin films, broadband emission in the UV region extending to the visible was observed following excitation at 355 nm. This emission is attributed to exciton luminescence of the SBN film. Nonlinear optical response in the picosecond regime and the third-order nonlinear susceptibility, χ(3), were studied using degenerate four-wave-mixing methods. In transverse alignment, χ(3) is enhanced by two orders of magnitude in comparison with its bulk counterpart. A thermal annealing process, monitored via changes in spectral properties of Eu3+, was employed to convert the as-grown amorphous film into a polycrystalline film. High-resolution spectroscopic measurements in th...

Optical properties of undoped and Eu3+-doped SBN thin film grown by pulsed laser deposition

SBN (Sr x Ba 1-x Nb 2 O 6 ) thin films were successfully grown by pulsed laser deposition (PLD) and characterized as to their microstructure, and both linear and nonlinear optical properties. High-resolution spectroscopic measurements in both time domain and frequency domain were conducted for the first time in this material. For pure SBN thin film without doping, a broad-band UV emission extending to the visible was observed. It is attributed to the exciton luminescence of the SBN film host. High-resolution nonlinear optical response in picosecond region as well as the third-order susceptibility were characterized by degenerative four-wave-mixing (DFWM) measurements. A considerable enhancement of the third-order nonlinear susceptibility X (3) was recorded. For SBN thin films doped with Eu 3+ , high-resolution spectroscopic measurements in frequency domain were conducted at low temperature. The fine structure of the 5 D 0 - 7 F multiplet emission was obtained. It is suggested that Eu 3+ ions may substitute Nb, occupying 6-fold sites. The line width measurement shows that the 5 D 0 - 7 F 0 transition is inhomogeneously broadened, and can be drastically suppressed by annealing treatment of the thin films obtained.

Optical property of Eu in strontium barium niobate optical thin film grown by pulsed laser deposition

Abstract Thin films of strontium barium niobate Sr x Ba 1− x Nb 2 O 6 (SBN) were successfully fabricated on fused quartz substrates using pulsed laser deposition (PLD) technique. The optical and non-linear optical properties were characterized. High-resolution spectroscopic measurements in either time domain or frequency domain were conducted. For undoped SBN thin films, a broad-band emission at UV and extending to the visible was observed by excitation at 355 nm. It is attributed to the exciton luminescence of the SBN host in the film. In degenerate four-wave-mixing (DFWM) measurements, a considerable enhancement of the third order susceptibility χ (3) by 2 orders of magnitude, in transverse alignment was obtained. High-resolution spectroscopic measurements in frequency domain were conducted at the temperatures varying from 270 K down to 2.3 K. The very well resolved fine structure of 5 D 0 – 7 F multiplet emission was observed in the annealed sample. The hole-burning experiment shows a hole of width 100 MHz with depth as high as 30% with the laser pumping at 5774 A. It is suggested that Eu 3+ ions may substitute Nb, occupying 6-fold sites.

Effects of self-radiation damage on electronic properties of 244Cm3+ in an orthophosphate crystal of YPO4

The electronic energy level structure of the α-emitting isotope 244 Cm 3+ (t 1/2 =18.1 years) doped into single crystals of YPO 4 has been studied using site-selected laser spectroscopic methods. Electronic transitions between the nominal 8 S 7/2 ground state and the 6 D 7/2 state of Cm 3+ were utilized to characterize the effects of α-decay-induced structural damage. The total splitting of the four doublets in the ground multiplet is 12.5 cm -1 , and that of the excited multiplet is 611 cm -1 . Due to radiation damage accumulated in 17 years since the crystals were grown, the inhomogeneous line width of the 8 S 7/2 ↔ 6 D 7/2 transitions is broader than 50 cm -1 as measured without site selection. The line width of resonant fluorescence line narrowing (RFLN) is less than 1 cm -1 at 4 K. A total of 12 satellite lines were observed symmetrically spacing about the RFLN line. The position, width, and intensity of these satellite lines have been analyzed to gain information on the electronic and structural properties of the actinide ions in the disordered lattice of Cm 3+ :YPO 4 .

Crystal field analysis and Monte Carlo simulation of lattice disordering for Cm3+ in YPO4 and LuPO4

A method of analyzing inhomogeneous line broadening with crystal-field theory is presented. For actinide ions with unfilled f-shells in solids, the most important contribution to line broadening of f-electron transitions is from short-range interactions with distorted lattice. Optically active curium ions are used to measure the degree of radiation damage in the vicinity of their lattice site. The radiation damage was produced by decay of distant 244Cm3+ ions. Fluorescence line narrowing (FLN) spectra show that inhomogeneous line broadening induced by α-decay of the actinide ion 244Cm3+ in single crystals of LuPO4 and YPO4 has a microscopic nature. Namely, correlation is absent between energy levels of the Cm3+ ions at different local environments. Monte Carlo simulation of lattice distortion and crystal-field calculation of Cm3+ energy levels have been conducted to provide a quantitative interpretation of the experimental results. This method should be applicable to 4f- and 3d-ions in crystalline materia...

GAS-CONDENSATION SYNTHESIS OF NANOCRYSTALLINE BATIO3

Nanocrystalline BaTiO3 has been prepared by a gas-condensation process. BaTiO3 and Ti sources are vaporized simultaneously in either a helium or an oxygen environment using an electron beam evaporation system. The stoichiometry of nanocrystalline BaTiO3 powders can be controlled precisely and reproducibly. Nanocrystalline BaTiO3 powders, with an average particle size of less than 20 nm, can be obtained by postannealing the as-evaporated powders at a temperature of 700 °C. These powders show good sintering behavior with a high density at a sintering temperature as low as 1250 °C. Differential thermal analysis indicated that nanocrystalline BaTiO3was formed through the reaction of Ba/Ti oxidized clusters. Dielectric properties of ceramics from nanocrystalline BaTiO3 are also reported.