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161.
162.
Dielectric measurements between ?50 and 60°C have been made on isotropic and oriented samples of poly(chlorotrifluoroethylene) with draw ratios λ of 1 to 3.5 at frequencies ranging from 30 Hz to 1 MHz. For the oriented samples, the dielectric loss has been measured with the electric field normal (ε) and parallel (ε) to the draw direction. At low frequency (say 60 Hz) the loss data for the oriented samples reveal two peaks at 25 and ?5°C, which are associated with the amorphous (γa) and the crystalline (γc) relaxations, respectively. Analysis of these data using a two-phase model yields values for the amorphous orientation function fa which are only about 25 to 60% of those for the crystalline orientation function fc. Upon annealing, the anisotropy ε/ε at the γa peak decreases significantly while that at the γc peak remains largely unchanged. This implies a roughly unaltered fc and a large decrease in fa, which is consistent with the results of wide-angle x-ray diffraction and birefringence measurements. 相似文献
163.
164.
Heat capacities of poly(vinylidene fluoride) (PVF2) and polytetrafluoroethylene (PTFE) have been measured between 5 and 100°K with an accuracy of (1–5)% by adiabatic calorimetry. Calculations based on contributions from known optical lines and the Tarasov continuum model are in good agreement with experimental results down to 30°K for PVF2 and 10°K for PTFE, and yield characteristic temperatures θ1 and θ3 which are consistent with previous values determined from high-temperature (100—350°K) data. At lower temperature the measured heat capacity is significantly higher [(30–100)%] than the model prediction, and can be satisfactorily accounted for by the introduction of localized vibrators at a concentration of about 1% as compared to acoustical oscillators and at a characteristic temperature of about 20°K. Using established data on polyethylene for comparison, the principle of additivity for heat capacities is found to be valid down to at least 20°K, convering the region (<60°K) where interchain vibrations contribute significantly to the heat capacity. Possible reasons for this unexpected behavior are discussed. 相似文献
165.
Antifreeze protein type III aggregates once the concentration exceeds a critical value, the so-called critical aggregation concentration (CAC). It was found for the first time that the aggregation of antifreeze protein exerts a direct impact on the antifreeze efficiency. It follows from our measurements that the AFP III above CAC will enhance the antifreeze activity because of the increase of the kink kinetics barrier of surface integration. This is attributed to the optimal packing of AFP III molecules on the surface of the ice nucleus as well as ice crystals above CAC. This study will extend our understanding of the antifreeze mechanism of antifreeze protein monomers as well as antifreeze aggregates on ice nucleation and shed light on the selection of antifreeze agents. 相似文献
166.
Dongliang Tao Wallace C.H. Choy 《Applied Physics A: Materials Science & Processing》2008,91(1):157-160
The growth mechanism of single-crystal ZnO nanorods synthesized by the method of polymer complexing with zinc salts is investigated.
The annealing temperature is controlled at about the decomposition temperature of dihydrate zinc acetate (Zn(O2CCH3)2·2H2O) of 573 K. By changing the annealing time, the ZnO nanostructures can be modified from nanoparticles to nanorods. As a result,
the formation of single-crystal ZnO nanorods can be observed. Through investigating the Fourier transform infrared spectra
of (a) polyvinyl pyrrolidone (PVP), (b) Zn(O2CCH3)2·2H2O and (c) the mixture of PVP and Zn(O2CCH3)2(H2O)2, the interaction between PVP and Zn(O2CCH3)2·2H2O can be observed. PVP plays an important role in the growth of the single-crystal ZnO nanorods. We analyze the growth process
of ZnO nanorods by observing their TEM images at different moments. Consequently, our results indicate that the single-crystal
ZnO nanorods were formed by self-assembling the ZnO nanoparticles.
PACS 61.46.Hk; 61.46.Df; 78.30.-j; 81.07.-b; 81.16.Be 相似文献
167.
Detecting a weak signal from chaotic time series is of general interest in science and engineering. In this work we introduce and investigate a signal detection algorithm for which chaos theory, nonlinear dynamical reconstruction techniques, neural networks, and time-frequency analysis are put together in a synergistic manner. By applying the scheme to numerical simulation and different experimental measurement data sets (Henon map, chaotic circuit, and NH(3) laser data sets), we demonstrate that weak signals hidden beneath the noise floor can be detected by using a model-based detector. Particularly, the signal frequencies can be extracted accurately in the time-frequency space. By comparing the model-based method with the standard denoising wavelet technique as well as supervised principal components analysis detector, we further show that the nonlinear dynamics and neural network-based approach performs better in extracting frequencies of weak signals hidden in chaotic time series. 相似文献
168.
S.H. Choy X.X. Wang H.L.W. Chan C.L. Choy 《Applied Physics A: Materials Science & Processing》2007,89(3):775-781
Lead-free (0.90-x)(Bi1/2Na1/2)TiO3-0.05(Bi1/2K1/2)TiO3-x(Bi1/2Li1/2)TiO3-0.05BaTiO3 piezoelectric ceramics (abbreviated as BNKLBT-100x, with x ranged from 0 to 2.5 mol %) were prepared by a conventional mixed
oxide method. Effects of the amount of (Bi1/2Li1/2)TiO3 (BLT) on the electrical properties and crystal structure of the BNKLBT ceramics were examined. BNKLBT-1.5 ceramics have good
properties with piezoelectric constant d33=163 pC/N, electromechanical coupling factor kp=0.33, kt=0.53, relative permittivity εr=785 and dissipation factor cosδ=2.2% at 1 kHz. The sample has larger remnant polarization than BNKLBT-0 ceramics and the
same coercive field as BNKLBT-0 ceramics. X-ray diffraction analysis shows that the incorporated BLT diffuses into the BNT–BKT–BT
lattice to form a solid solution during sintering, but changes the crystal structure from rhombohedral to tetragonal symmetry
at higher BLT amounts. Depolarization temperature (Td) of the BNKLBT-100x ceramics increases from 102 °C to ∼136 °C for BNKLBT-0 to BNKLBT-2.5. BNKLBT-1.5 is used as the transduction
element in compressive type accelerometer and its sensitivity is calibrated by the back-to back method. Within the ±2.5% tolerance,
the lead-free accelerometer has a mean value of 2.97 pC/ms-2 within 50 Hz–12.45 kHz and the lead-based accelerometer has a mean value of 4.34 pC/ms-2 within 50 Hz to 8.24 kHz.
PACS 77.22.Ej; 77.84.-s; 85.50.-n 相似文献
169.
In this paper we consider a special kind of binary trees where each right edge is associated with a positive number and each left edge with a positive number( ). Given, and the number of nodesn, an optimal tree is one which minimizes the total weighted path length. An algorithm for constructing an optimal tree for given, , n is presented, based on which bounds for balances and total weighted path lengths of optimal trees are derived. 相似文献
170.
J.Z.?Liu K.W.?KwokEmail author H.L.W.?Chan C.L.?Choy 《Applied Physics A: Materials Science & Processing》2004,78(7):1033-1036
The microstrip-ring-resonance technique has been applied to determine the dielectric properties of high-permittivity barium strontium titanate (BaxSr1-xTiO3) ceramics at microwave frequencies. A microstrip ring resonator of diameter 5 mm has been fabricated on the ceramics by the standard photolithography process. The transmission coefficient S21 spectra for the resonators have been measured using a vector network analyzer and simulated using a commercial electromagnetic simulation package, IE3D. By fitting the observed spectra to the simulated spectra, and tan of the BaxSr1-xTiO3 ceramics have been determined. The results obtained by this technique are in agreement with those determined by other conventional methods. This technique is relatively simple, especially for high-permittivity materials, since there are less rigorous requirements on the sample and the test conditions. PACS 77.22.-d; 77.84.Dy; 84.40.Az 相似文献