We report the results of magnetic, magnetocaloric properties, and critical behavior investigation of the double-layered perovskite manganite La1.4(Sr0.95Ca0.05)1.6Mn2O7. The compounds exhibits a paramagnetic (PM) to ferromagnetic (FM) transition at the Curie temperature TC = 248 K, a Neel transition at TN = 180 K, and a spin glass behavior below 150 K. To probe the magnetic interactions responsible for the magnetic transitions, we performed a critical exponent analysis in the vicinity of the FM–PM transition range. Magnetic entropy change (??SM) was estimated from isothermal magnetization data. The critical exponents β and γ, determined by analyzing the Arrott plots, are found to be TC = 248 K, β = 0.594, γ = 1.048, and δ = 2.764. These values for the critical exponents are close to the mean-field values. In order to estimate the spontaneous magnetization MS(T) at a given temperature, we use a process based on the analysis, in the mean-field theory, of the magnetic entropy change (??SM) versus the magnetization data. An excellent agreement is found between the spontaneous magnetization determined from the entropy change [(??SM) vs. M2] and the classical extrapolation from the Arrott curves (µ0H/M vs. M2), thus confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in this system and in other compounds as well. 相似文献
The principal objective of cognitive radio (CR) networks is to configure and share dynamically the spectrum resources in order to avoid user interference and congestion. This goal is limited by the effect of errors synchronization between primary and secondary users. In this paper, we study the impact of the asynchronism on the cyclic prefix-based orthogonal frequency division multiplexing modulation (CP-OFDM) including nonlinear HPA model. The considered system includes a reference primary user perfectly synchronized with its reference base station and Nsu interfering secondary users. We provide a new theoretical aspect of interference analysis in the context of the OFDM based CR network. Furthermore, on the basis of this analysis, we derive the accurate expression of bit error rate in the presence of a Rayleigh flat fading channel. Finally, to solve the problems of asynchronism and nonlinearity, a hybrid iterative method of compensation and parallel interference cancellation have been developed based on these two conditions.
This study, which deals with the coating and granulation of solid particles by aqueous solutions of polymers or inorganic salts, aims to understand the effect of:
process-related variables such as the excess gas velocity, atomizer location, liquid flow rate and concentration, and atomizing air flow rate,
physicochemical-related variables such as the viscosity of solutions, wettability of the granulating liquid on solid particle surfaces, initial particle mean size, and porosity of the particles on the agglomeration kinetics of solid particles in a fluidized bed.
The results showed that for a given particle size, the fluidizing air velocity was the most important factor affecting the growth kinetics and the stability of the operation. An increase of the relative humidity, depending on the liquid flow rate as well as the air flow rate, favor agglomeration mechanism especially for values greater than 0.4. An increase in the particle initial size leads to an enhancement of the layering mechanism, especially for values greater than 300 μm.
The effect of the interfacial tension is investigated by adding different concentrations of a non-ionic surfactant to the binding solution. The effect of the contact angle is then studied using non-hydrophobic, partly hydrophobic, or totally hydrophobic particles. The growth of agglomerates appears to be favoured when the interfacial tension increases and the contact angle decreases. The viscosity of the solution has less effect than the interfacial parameters. The results show that the dominant forces in the granulation process are the capillary forces. 相似文献
The recent increase in machining productivity is closely related to longer tool life and good surface quality. In the present study, an experimental technique is proposed to evaluate the performance of a cemented carbide inset during the machining of AISI D3 steel. The aim of this technique is to find a relationship between the vibratory state of the cutting tool and the corresponding wear during machining in order to detect the beginning of the transition period to excessive wear. A spectral indicator named spectral center of gravity, SCG, is proposed to highlight the three phases of tool wear using the spectra of the accelerations measured. Very promising results are obtained which can be used to underpin an industrial monitoring system capable of detecting the onset of transition to excessive wear and alerting the user of the end of the tool’s life. The purpose of this study is to review the vibration analysis techniques and to explore their contributions, advantages and drawbacks in monitoring of tool wear. 相似文献
This paper presents a numerical investigation of turbulent forced convective flow in a horizontal channel. An exchanger isothermal test plate is embedded in the lower wall, in the fully developed region of the flow close to the exit of the channel. Above this isothermal plate, on the upper surface, a black coated isothermally heating resistance facing downwards is installed. This absorbing surface provides a controlled radiative heat flux on the lower test plate. In this study, custom-built tangential gradient fluxmeters (TGFM) are used to provide local measurements of convective heat transfer so as to validate the numerical predictions. Then, parametric studies are carried out. The profiles for the heat flux are presented for different Reynolds numbers in the flow direction along the cold isothermal lower plate. Then, the influence of the presence of an obstacle, located on the lower surface, on the heat flux is also investigated. All numerical predictions are carried out with Fluent, previously calibrated against benchmark problems and experimental measurements. In the paper, special emphasis is given in the systematic comparison between experimental and numerical results. 相似文献
Bulk Co samples having a mean grain size of ~300 nm were processed by hot isostatic pressing of a high purity Co nanopowder synthesized by chimie douce. The grain interior exhibited a highly faulted nanoscale lamellar microstructure comprising an intricate mixture of face-centered cubic, hexagonal close-packed phases and nanotwins. Room temperature compression tests carried out at a strain rate of ~2 × 10?4 s?1 revealed a yield stress of ~1 GPa, a strain to rupture of ~5%. During straining it was found that the hexagonal close-packed phase content increased from 55% to 65% suggesting a deformation mechanism based on stress-assisted face-centered cubic to hexagonal close-packed phase transformation. In addition, an apparent activation volume of ~3b3 was computed which indicates that the deformation mechanism was controlled by dislocation nucleation from the numerous boundaries. Nonetheless, in such an intricate microstructure, the overall mechanical properties are discussed in term of a complex interplay between lattice dislocation plasticity, transformation-induced plasticity and possibly twin-induced plasticity. 相似文献
Evapotranspiration is a process driven by weather, vegetation, and soil conditions. The complex interrelations among these parameters have been modelled by numerous remote-sensing energy balance algorithms. When estimating evapotranspiration on a regional scale, the spatial variability of the weather parameters is important and thus closer attention to the meteorological input data is required. The aim of this work is to improve the accuracy of estimating actual evapotranspiration by integrating outputs from a meteorological model into a remotely sensed energy balance model. In order to achieve this, a time series of Terra Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images were processed to retrieve daily evapotranspiration values using raster meteorological data. The ITA-MyWater tool implementing the ReSET-Raster algorithm was used in the Tâmega trans-boundary watershed shared by Portugal and Spain. The results were compared to the global MODIS evapotranspiration products for validation, achieving a coefficient of correlation of 0.61 and a root mean square error of 0.92 mm day–1. Compared with an actual evapotranspiration map that was generated using weather station data, there were improvements in the spatial distribution, especially in dry areas where differences between evapotranspiration estimations of up to 1.88 mm day–1 were noticed. The proposed methodology contributes to the improved estimation of water use, an important parameter of water cycles, using satellite remote-sensing data. 相似文献
Treated wastewater (TWW) reuse has increasingly been integrated in the planning and development of water resources in Tunisia.
The present study aimed the evaluation of the environmental and health impact that would have the reuse of TWW for crops direct
irrigation or for the recharge of the local aquifer in Korba (Tunisia). For this purpose water analyses were carried on the
TWW intended for the aquifer recharge and on underground water of this area. As for underground water before recharge, no
contamination by organic matter or heavy metals is shown but high salinity, nitrate, potassium and chloride concentrations
are detected. The bacteriological analyses show the occurrence of faecal streptococcus, thermo-tolerant coliforms, total coliforms
and E coli, but absence of salmonella. These results indicate that this water is not suitable for irrigation worse still for
drinking purpose. The monitoring of TWW pollutants has demonstrated that oxygen demands (COD and BOD) do not exceed the Tunisian
standards for TWW used in agriculture (NT 106.03) except for August when samples reach high values (COD = 139 mg O2 L − 1, BOD = 34). It is also the case for temperature, electrical conductivity (EC), salinity and pH. Heavy metal concentrations
are under the detection limit. The determination of nutrients shows relatively low concentrations of nitrates, nitrites and
orthophosphate (the maxima in mg L − 1 are respectively 6.6, 5.6 and 0.92) whereas the potassium levels are high (up to 48.8 mg L − 1) and the ammonia levels very high, reaching 60.6 mg L − 1. As for bacteriological pollution, while no salmonella and intestinal nematods are detected, high concentrations of total
coliforms, thermo-tolerant coliforms, faecal streptococci and E. coli are analysed. Consequently, the better use of TWW in
this region would be the use of infiltration basins for the recharge of the deteriorated aquifer by TWW. It would give the
opportunity to better the quality of the TWW reaching the groundwater by an additional treatment for bacteriological and suspended
solid (TSS) contaminants while being an alternative water for the aquifer recharge and a coastal barrier against seawater
intrusion. 相似文献