Preparation and characterization of a poled nanocrystal and polymer composite PbTiO3/PEKc film for electro-optic applications

Composite thin films of PbTiO₃ nanocrystals and high-transparency PEKc polymer for applications in electro-optical devices were prepared using the spin coating technique. The size of the PbTiO₃ nanocrystals was estimated to be 30–40 nm using a transmission electron microscope. The transmission technique, a simple method for measuring the electro-optic coefficients of poled composite polymer films was developed. The electro-optic coefficient γ₃₃ of poled PbTiO₃/PEKc composite polymer films was measured to be 18.34 pm  V^-1 at 633 nm under room temperature. The index at 633 nm and the dielectric constant at 100 kHz under room temperature were determined to be 1.65248 and 7.32, respectively. The figure of merit F₂=n⁷γ²/ε was estimated to be 1546, showing very good electro-optical properties. Received: 5 February 2002 / Accepted: 12 March 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: +852/2788-7791, E-mail: eeytc@cityu.edu.hk     

Observation of nonlinear acoustic effects at isotropic solid-solid interfaces

The second harmonic generation of SV shear waves at isotropic solid-solid interfaces is experimentally studied. The amplitude of shear waves is measured for the interfaces of glass-air, glass-iron, glass-copper, and glass-aluminum. The measured angular relation of amplitude of the second harmonic wave is compared with theory and the agreement is reasonably good. The influence of the physical state of the interface on second harmonic generation is also observed. It is found that the second harmonic generation is sensitive to the interface state.     

Quantitative analysis of bending efficiency in photonic-crystal waveguide bends at lambda = 1.55 mum wavelengths

Based on a photonic-crystal slab structure, a 60 degrees photonic-crystal waveguide bend is successfully fabricated. Its bending efficiency within the photonic bandgap is measured, and near 100% efficiency is observed at certain frequencies near the valence band edge. The bending radius is ~1mum at a wavelength of lambda~1.55mum . The measured eta spectrum also agrees well with a finite-difference time-domain simulation.     

Novel behavior of bond-centered muonium in heavily doped n-type silicon: Curie-like spin susceptibility and charge screening

Bond-centered muonium ( Mu(0)(BC)) has been observed in very heavily doped n-type Si:P. It exhibits a Curie-like electronic spin susceptibility which leads to a giant negative shift in the muon spin precession frequency. At high dopant levels, the Mu(0)(BC) hyperfine parameters, deduced from a model involving spin exchange with free carriers, are significantly reduced from those in intrinsic Si. This indicates that the spin density distribution for Mu(0)(BC) in metallic Si:P is altered significantly by charge screening effects, likely a general phenomenon for deep impurities in materials with high carrier concentrations.     

Origin of antimony segregation in GaInSb/InAs strained-layer superlattices

We show how cross-sectional scanning tunneling microscopy may be used to reconstruct the Sb segregation profiles in GaInSb /InAs strained-layer superlattices. These profiles are accurately described by a one-dimensional model parametrizing the spatial evolution of an Sb seed at the InAs-on-GaInSb interface in terms of two-anion-layer exchange. We argue that the segregation seed, which decreases from 2 / 3 to 1 / 2 monolayer when growth conditions are made less anion rich, has its origin in the Sb-bilayer reconstruction maintained during GaInSb epitaxy.     

Charge ordering in the TMTTF family of molecular conductors

Using one- and two-dimensional NMR spectroscopy applied to 13C spin-labeled (TMTTF)2AsF6 and (TMTTF)2PF6, we demonstrate the existence of an intermediate charge-ordered phase in the TMTTF family of charge-transfer salts. At ambient temperature, the spectra are characteristic of nuclei in equivalent molecules. Below a continuous charge-ordering transition temperature T(co), there is evidence for two inequivalent molecules with unequal electron densities. The absence of an associated magnetic anomaly indicates only the charge degrees of freedom are involved and the lack of evidence for a structural anomaly suggests that charge-lattice coupling is too weak to drive the transition.     

在玻璃转变温度及其以上温区聚苯乙烯/聚氧化乙烯共混物的动力学弛豫行为

用熔融共混法制备了不同组分的聚苯乙烯(PS)与聚氧化乙烯(PEO)共混物(PS/PEO).在玻璃转变温度T_g及其以上温区,利用相对能量耗散技术研究了该共混物的动力学行为.结果发现,在能量耗散-温度曲线上出现了两个弛豫型的耗散峰(α峰和α′峰).分析表明,α峰是与PS玻璃转变有关的特征耗散峰; α′峰则对应于一种“液-液转变”.两者的弛豫时间τ都不满足Arrhenius关系.此外,还研究了组分对这两个弛豫型耗散峰的影响,并给予了定性的解释. 关键词： 相对能量耗散 玻璃转变 力学弛豫     

Fundamental relationship between the nonequilibrium glassy state and yield stress of amorphous polymers

This paper reports the theoretical prediction and experimental verification of the connection between the yield stress of amorphous polymers and the physical aging phenomenon. The analysis reveals the existence of a fundamental relationship between the nonequilibrium glassy state and the thermally activated process controlling viscoelastic and plastic deformation. The results show that the volume relaxation and deformation kinetics share the same relaxation times, and that the activation energy for deformation below T_g is much smaller than previously mentioned in the literature. This indicates that the phenomenon of physical aging plays a very important role in the deformation and processing of polymers at low temperatures. The effect of quenching and annealing on the yield stress is described in terms of the mean energy of hole formation, the departure of volume from its equilibrium state, the distribution of hole energies, and lattice volume. The same set of molecular parameters obtained from the molecular kinetic theory of the glass transition and volume relaxation predicts the yield stress as a function of cooling rate, annealing time, temperature, and strain rate.     

Diastereoselective Aldol Addition Using Boron Trichloride or Alkoxydichloroborane

Under carefully controlled conditions, boron trichloride or alkoxydichloroborane/ethyldiisopropylamine in CH₂Cl₂ can be used to effect diastereoselective aldol additions of ethyl ketones to saturated, α, β-unsaturated, or aromatic aldehydes. The C? C bond formation takes place with relative topicity ul (‘syn,’ configuration of the aldols), in selectivities ranging from 90 to 99% ds (Tables 1–3). Mechanistic aspects of the reaction are discussed.     

A Synthesis of Thalidomide

A synthesis of racemic thalidomide ( 1 ) was described and the important formal [3+3] cycloaddition strategy was a keystep. The total yield of thalidomide ( 1 ) was 18% in five steps from known 3 .