Coupling model for calculating prestress loss caused by relaxation loss,shrinkage,and creep of concrete

The calculation model for the relaxation loss of concrete mentioned in the Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts(JTG D62—2004) was modified according to experimental data. Time-varying relaxation loss was considered in the new model. Moreover, prestressed reinforcement with varying lengths(caused by the shrinkage and creep of concrete) might influence the final values and the time-varying function of the forecast relaxation loss. Hence, the effects of concrete shrinkage and creep were considered when calculating prestress loss, which reflected the coupling relation between these effects and relaxation loss in concrete. Hence, the forecast relaxation loss of prestressed reinforcement under the effects of different initial stress levels at any time point can be calculated using the modified model. To simplify the calculation, the integral expression of the model can be changed into an algebraic equation. The accuracy of the result is related to the division of the periods within the ending time of deriving the final value of the relaxation loss of prestressed reinforcement. When the time division is reasonable, result accuracy is high. The modified model works excellently according to the comparison of the test results. The calculation result of the modified model mainly reflects the prestress loss values of prestressed reinforcement at each time point, which confirms that adopting the finding in practical applications is reasonable.

Electrical and dielectric properties of Na_(1/2)La_(1/2)Cu_3Ti_4O_(12) ceramics prepared by high energy ball-milling and conventional sintering

We report on the measurements of the electrical and dielectric properties of Na1/2La1/2Cu3Ti4O12 （NLCTO） ce- ramics prepared by high energy ball-milling and conventional sintering without any calcination steps. The x-ray powder diffraction analysis shows that pure perovskite-like CCTO phase is obtained after sintering at 1025 ℃-1075 ℃. Higher sintering temperatures result in multi-phase ceramics due to thermal decomposition. Scanning electron microscope observations reveal that the grain size is in a range of - 3 μm-5 μm for these ceramics. Impedance spectroscopy measurements performed in a wide frequency range （1 Hz-10 MHz） and at various temperatures （120 K-470 K） are used to study the dielectric and electrical properties of NLCTO ceramics. A good compromise between high ε′ （5.7 ×10^3 and 4.1 × 10^3 at 1.1 kHz and 96 kHz, respectively） and low tan δ （0.161 and 0.126 at 1.1 kHz and 96 kHz, respectively） is obtained for the ceramic sintered at 1050 ℃. The observed high dielectric constant behavior is explained in terms of the internal barrier layer capacitance effect.

Dielectric relaxation in pure and Co-doped Bi_(12)GeO_(20) single crystals

We report the results of investigation of dielectric spectroscopy study of single crystals of Bi_(12)GeO_(20) and Bi_(12)GeO_(20) doped with Co nanoparticles.The complex dielectric constant was measured in the temperature interval from 5 to 450K and frequencies from 1Hz to 1MHz.The electrical conductivity of both samples was thermally activated with potential barriers of 0.55eV and 0.59eV,respectively.Doped samples had bigger complex dielectric constants and σ′ exhibited slightly steeper temperature dependence than in the pure sample.The dielectric relaxation was observed in pure and doped single crystals and relaxation frequencies showed similar activation energies as electrical conductivities.

Polaronic behavior of La_(0.8)Te_(0.2)MnO_(3)

We have analyzed recently published data of the ac electrical conductivity and dielectric constant in La_(0.8)Te_(0.2)MnO_(3)(LTMO),and at temperatures below 500 K both quantities follow the universal dielectric response(UDR)typical of hopping or tunneling of localized charge carriers.A detailed analysis of the temperature dependence of the UDR parameter s in terms of the theoretical model for tunneling of small polarons revealed that below 500 K,this mechanism governs the charge transport in LTMO system.

Isothermal depolarization current in metal-oxide varistor ceramics under various polarizing fields

A method of analysis of experimental kinetic dependences of isothermal depolarization currents in metal-oxide varistor ceramics composed of ZnO–Bi_(2)O_(3)–CoO–Sb_(2)O_(3)–SnO_(2)–MnO–B_(2)O_(3) has been proposed and tested.The method is based on reconstruction of dependences with the use of the Kohlrausch–Williams–Watt function.Expansion of the distribution and reduction of the average time of dielectric relaxation of the depolarization were found as the magnitude of the polarizing electric field increased.These findings have been interpreted in the framework of the notion of participation in the recharge of localized states more distant from the Fermi level in the crystallites of zinc-oxide ceramics under increased polarizing fields.