Microwave absorption properties of SiC@SiO2@Fe3O4 hybrids in the 2–18 GHz range

To enhance the microwave absorption performance of silicon carbide nanowires（SiCNWs）, SiO_2 nanoshells with a thickness of approximately 2 nm and Fe_3O_4 nanoparticles were grown on the surface of SiCNWs to form SiC@SiO_2@Fe_3O_4 hybrids. The microwave absorption performance of the SiC@SiO_2@Fe_3O_4 hybrids with different thicknesses was investigated in the frequency range from 2 to 18 GHz using a free-space antenna-based system. The results indicate that SiC@SiO_2@Fe_3O_4 hybrids exhibit improved microwave absorption. In particular, in the case of an SiC@SiO_2 to iron（III） acetylacetonate mass ratio of 1：3, the microwave absorption with an absorber of 2-mm thickness exhibited a minimum reflection loss of-39.58 d B at 12.24 GHz. With respect to the enhanced microwave absorption mechanism, the Fe_3O_4 nanoparticles coated on SiC@SiO_2 nanowires are proposed to balance the permeability and permittivity of the materials, contributing to the microwave attenuation.

Sound Absorption Enhancement by Thin Multi-Slit Hybrid Structures

We report an extraordinary sound absorption enhancement in low and intermediate frequencies achieved by a thin multi-slit hybrid structure formed by incorporating micrometer scale micro-slits into a sub-millimeter scale meso-slit matrix. Theoretical and numerical results reveal that this exotic phenomenon is attributed to the noticeable velocity and temperature gradients induced at the junctures of the micro- and meso-slits, which cause significant loss of sound energy as a result of viscous and thermal effects. It is demonstrated that the proposed thin multi-slit hybrid structure with micro-scale configuration is capable of controling low frequency noise with large wavelength, which is attractive for applications where the size and weight of a sound absorber are restricted.

Mathematical model of absorption and hybrid heat pump

Recovering waste heat from industrial processes is bene ficial in order to reduce the primary energy demands and heat pumps can be used to this purpose.Absorption heat pumps are energy-saving and environment-friendly because use working fluids that do not cause ozone depletion and can reduce the global warming emissions.The hybrid heat pump processes combine the conventional vapor-compression and the absorption heat pump cycles.Studies about the simulations and modeling of hybrid heat pumps are few in literature.In this research a mathematical model for single effect absorption and hybrid heat pump is carried out with Chem Cad? 6.0.1.LiBr–H_2O is used as working fluid while electrolytic NRTL and electrolytes latent heat are used as thermodynamic model due to the better results.Binary parameters of activity coef ficients are regressed from experimental vapor pressure data while default constants are used for the solubility expressions.A design of heat pumps is developed and a new modeling of generator is analyzed.The coef ficient of performance of absorption heat pump and hybrid heat pump is equal to 0.7 and 0.83 respectively.For absorption heat pump a sensitivity analysis is carried out to evaluate the effect of temperature and pressure generator,the concentration of Li–Br solution on coef ficient of performance,cooling capacity and working fluid temperature.For hybrid heat pump,the different coef ficients of performance,the primary energy ratio,the generator heat,and the compressor power are analyzed for different values of compressor proportion.Results show that comparing the two systems the hybrid pump allows to save more primary energy,costs and carbon dioxide emissions with respect to absorption heat pump with the increasing of compressor proportion parameter.Future researches should focus on the construction of this heat pumps integrated in chemical processes as a biogas plant or trigeneration systems.

Low-temperature compression-assisted absorption thermal energy storage using ionic liquids

Thermal energy storage technologies play a significant role in building energy efficiency by balancing the mis-match between renewable energy supply and building energy demand.The absorption thermal energy storage(ATES)stands out due to its high energy storage density(ESD),high coefficient of performance(COP),low charg-ing temperature and wider application flexibility.A hybrid compression-assisted ATES(CATES)using ionic liquid(IL)-based working fluids is investigated to address the problems of the existing ATES cycle.Models for mixture property and cycle performance are established with verified accuracies.Four ILs([DMIM][DMP],[EMIM][Ac],[EMIM][DEP],and[EMIM][EtSO_(4)])are compared with H_(2)O/LiBr.Results show that the CATES effectively avoid the crystallization,decreases the circulation ratio,lowers the charging temperature,and improves the COP/ESD.H_(2)O/[DMIM][DMP]has the highest COP and performs better than H_(2)O/LiBr with generation temperatures above 86℃,while H_(2)O/[EMIM][EtSO_(4)]shows the highest COP with generation temperatures below 75℃.Among the H_(2)O/IL mixtures,H_(2)O/[EMIM][Ac]shows the highest ESD with generation temperatures above 86℃,otherwise H_(2)O/[EMIM][EtSO_(4)]shows the highest.The optimal compression ratio is 1.6-2.8 for H_(2)O/[DMIM][DMP]under the generation temperatures of 90-70℃with the maximum COP of 0.758-0.727.The ESD increases significantly with the compression ratio.