Di Yin

High performance of polyimide/CaCu3Ti4O12@Ag hybrid films with enhanced dielectric permittivity and low dielectric loss

This work reports the excellent dielectric properties of polyimide (PI) embedded with CaCu3Ti4O12(CCTO)/Ag nanoparticles (CCTO@Ag). By functionalizing the surface of CCTO nanoparticles with Ag coating, the dielectric permittivity of PI/CCTO@Ag composites is significantly increased to 103 (100 Hz) at 3 vol% filler loading. The enhancement of dielectric permittivity is attributed to the increment of conductivity of the interlayer between CCTO and PI by Ag, which enhances the space charge polarization and Maxwell–Wagner–Sillars (MWS) effect. The experimental results fit well with percolation theory. Moreover, the low loss (0.018 at 100 Hz) achieved is attributed to the blockage of charge transfer by insulating polyimide chains. It is shown that the electrical field distortion is significantly improved by decorating the surface of CCTO nanoparticles with Ag using Comsol Multiphysics. This plays an important role in the enhancement of the dielectric properties.

The temperature dependence of magnetic properties for cobalt ferrite nanoparticles by the hydrothermal method

Cobalt ferrite nanoparticles were synthesized via the hydrothermal route with the addition of trisodium citrate dihydrate (Na(3)CA center dot 2H(2)O). The characterizations including x-ray diffraction (XRD) and transmission electron microscope showed that the products of this hydrothermal reaction are composed of cobalt ferrite nanocrystallite and a small amount of FeOOH, and the average crystallite size of the nanoparticles is 7.6 +/- 0.3 nm by XRD. The magnetic measurements revealed the temperature-dependent magnetic properties: The superparamagnetism occurs above 380 K due to the overcoming of energy barrier for the flip of spins, which arises from the magnetocrystalline anisotropy energy and the interparticle interactions due to the aggregation of the nanoparticles; a frozen spin-glasslike state was observed below 20 K, which is accompanied with the decrease in coercivity and high-field paramagnetic susceptibility, as well as the enhancement of saturated magnetization and the effective magnetic anisotropy constant

Polyimide/nanosized CaCu3Ti4O12 functional hybrid films with high dielectric permittivity

This work reports the high dielectric permittivity of polyimide (PI) embedded with CaCu3Ti4O12 (CCTO) nanoparticles. The dielectric behavior has been investigated over a frequency of 100 Hz-1 MHz. High dielectric permittivity (e = 171) and low dielectric loss (tan δ = 0.45) at 100 Hz have been observed near the percolation threshold. The experimental results fit well with the Percolation theory. We suggest that the high dielectric permittivity originates from the large interface area and the remarkable Maxwell-Wagner-Sillars effect at percolation in which nomadic charge carriers are blocked at internal interfaces between CCTO nanoparticles and the polyimide matrix.

Far infrared optical properties of the pyrochlore spin ice compound Dy2Ti2O7

Near normal incidence far infrared reflectivity spectra of [111] dysprosium titanate (Dy2Ti2O7) single crystal have been measured at different temperatures. Seven phonon modes (eight at low temperature) are identified at frequencies below 1000 cm−1. Optical conductivity spectra are obtained by fitting all the reflectivity spectra with the factorized form of the dielectric function. Both the Born effective charges and the static optical permittivity are found to increase with decreasing temperature. Moreover, phonon linewidth narrowing and a phonon mode shift with decreasing temperature are also observed, which may result from enhanced charge localization. The redshift of several low frequency modes is attributed to the spin–phonon coupling. All observed optical properties can be explained within the framework of the nearest neighbour ferromagnetic (FM) spin ice model.

Improving thermostability of CrO2 thin films by doping with Sn

Chromium dioxide (CrO2) is an ideal material for spin electronic devices since it has almost 100% spin polarization near Fermi level. However, it is thermally unstable and easily decomposes to Cr2O3 even at room temperature. In this study, we try to improve the thermal stability of CrO2 thin films by doping with Sn whose oxide has the same structure as CrO2. High quality epitaxial CrO2 and Sn-doped CrO2 films were grown on single crystalline TiO2 (100) substrates by chemical vapor deposition. Sn4+ ions were believed to be doped into CrO2 lattice and take the lattice positions of Cr4+. The magnetic measurements show that Sn-doping leads to a decrease of magnetocrystalline anisotropy. The thermal stabilities of the films were evaluated by annealing the films at different temperatures. Sn-doped films can withstand a temperature up to 510 °C, significantly higher than what undoped films can do (lower than 435 °C), which suggests that Sn-doping indeed enhances the thermal stability of CrO2 films. Our study als...

Ftir and dielectric studies of electrical aging in polyimide under AC voltage

The Kapton 100HN polyimide films were aged under 3 kV ac voltage (50 Hz), and the aging evolution were studied by the Fourier transform infrared spectroscopy (FTIR) and the dielectric measurement. The FTIR spectra show that the ether linkage, and some of C-H bonds in aromatic rings and C-N-C bonds transverse stretching vibration in imide are firstly broken by corona discharge. Then, with the increase of aging time, the cleavage of the imide group including C-N-C and C=O vibrations occurs. At the same time, the oxidative degradation produces compounds such as carboxylic acid, ketones, and aldehydes. Accordingly, the variations of dielectric constant ε1 and the dissipation factor tan δ are dependent on the formation of the polar groups and the breakage of some polar bonds in polyimide.

Degradation of Polyimide Film by Corona Aging under Bipolar Pulse Voltage

Corona aging of 100CR polyimide (PI) film under bipolar pulse voltage was studied by using atomic force microscopy (AFM) and Confocal Raman microspectroscopy. AFM results show that the 100CR film near the breakdown hole is much rougher than the outside region. In the region away from the breakdown hole, no Raman shift is detected, but the intensity of all Raman peaks shows clearly decrease with the measuring region approaching more closely to the aging center. It is suggested that the damage of the PI film initiates from the surface in the presence of voids due to the partial discharge, and then gradually extends to interior, and these partial discharge cause progressive deterioration, and ultimately lead to the electrical breakdown of PI film with continuous aging.

Structure and Magnetic Properties of Fe-Doped TiO2 Nano-Crystals by Nonaqueous Synthesis

Ti1-xFexO2 nanocrystals with x = 0.01, 0.02, 0.03 were prepared via a nonaqueous synthesis route. X-ray diffraction, transmission electron microscopy and high resolution transmission electron microscopy characterization confirmed the formation of anatase-phase nanocrystals with the average crystallite sizes of around 10 nm. The lattice constants alternate with the increase of the Fe content and no iron clusters were generated. X-ray photoelectron spectroscopy measurements showed that the substitutional Fe ions present mainly the valence of +3. The magnetic hysteresis loops measured at room temperature (RT) 300K showed that all the doped samples are atypically ferromagnetic, and the coercivity (Hc) of all the Fe-doping samples is around 0.1 T. An interpretation for the intrinsic RT ferromagnetism is put up based on the free carriers and defects induced interaction between Fe3+ ions.