Water adsorption performance of UiO-66 modified by MgCl_(2) for heat transformation applications

UiO-66 is a potential material for adsorption heat transformation(AHT)with high specific surface area,and excellent thermal and chemical stability.However,the low water adsorption capacity of UiO-66 in the low relative pressure range(0<P/P_(0)<0.3)limits its application in AHT.We prepare the UiO-66 modified by MgCl_(2)through using the solvothermal method and impregnation method,and study their water vapor adsorption performances and heat storage capacities.Attributed to the extremely high saturated water uptake and excellent hydrophilicity of MgCl_(2),the water adsorption performance of UiO-66 is improved,although the introduction of MgCl_(2)reduces its specific surface area and pore volume.The water adsorption capacity at P/P_(0)=0.3 and the saturated water adsorption capacity of the UiO-66(with MgCl_(2)content of 0.57 wt%)modified by the solvothermal method are 0.27 g/g and 0.57 g/g at 298 K,respectively,which are 68.8%and 32.6%higher than the counterparts of pure UiO-66,respectively.Comparing with pure UiO-66,the water adsorption capacity of the UiO-66(with MgCl_(2)content of 1.02 wt%)modified by the impregnation method is increased by 56.3%and 14.0%at the same pressure,respectively.During 20 water adsorption/desorption cycles,the above two materials show high heat storage densities(~1293 J/g and 1378 J/g).Therein,the UiO-66 modified by the solvothermal method exhibits the excellent cyclic stability.These results suggest that the introduction of an appropriate amount of MgCl_(2)makes UiO-66more suitable for AHT applications.

Effect of carbon nanotubes addition on thermoelectric properties of Ca_(3)Co_(4)O_(9)ceramics

Increasing the phonon scattering center by adding nanoparticles to thermoelectric materials is an effective method of regulating the thermal conductivity.In this study,a series of Ca3 Co_(4)O_(9)/x wt.%CNTs(x=0,3,5,7,10)polycrystalline ceramic thermoelectric materials by adding carbon nanotubes(CNTs)were prepared with sol-gel method and cold-pressing sintering technology.The results of x-ray diffraction and field emission scanning electron microscopy show that the materials have a single-phase structure with high orientation and sheet like microstructure.The effect of adding carbon nanotubes to the thermoelectric properties of Ca_(3)Co_(4)O_(9)was systematically measured.The test results of thermoelectric properties show that the addition of carbon nanotubes reduces the electrical conductivity and Seebeck coefficient of the material.Nevertheless,the thermal conductivity of the samples with carbon nanotubes addition is lower than that of the samples without carbon nanotubes.At 625 K,the thermal conductivity of Ca3 Co_(4)O_(9)/10 wt.%CNTs sample is reduced to0.408 W·m^(-1)·K^(-1),which is about 73%lower than that of the original sample.When the three parameters are coupled,the figure of merit of Ca_(3)Co_(4)O_(9)/3 wt.%CNTs sample reaches 0.052,which is 29%higher than that of the original sample.This shows that an appropriate amount of carbon nanotubes addition can reduce the thermal conductivity of Ca_(3)Co_(4)O_(9)ceramic samples and improve their thermoelectric properties.

Thermoelectric properties of lower concentration K-doped Ca_(3)Co_(4)O_(9) ceramics

The tuning of electron and phonon by ion doping is an effective method of improving the performances of thermoelectric materials.A series of lower concentration K-doped Ca_(3-x)K_(x)Co_(4)O_(9)(x=0,0.05,0.10,0.15)polycrystalline ceramic samples are prepared by combining citrate acid sol-gel method with cold-pressing sintering method.The single-phase compositions and plate-like grain morphologies of all samples are confirmed by x-ray diffraction and field emission scanning electron microscope.The effects of lower concentration K doping on the thermoelectric properties of the material are evaluated systematically at high temperatures(300–1026 K).Low concentration K doping causes electrical conductivity to increase up to 23%with little effect on the Seebeck coefficient.Simultaneously,the thermal conductivity of K-doped sample is lower than that of the undoped sample,and the total thermal conductivity reaches a minimum value of approximately1.30 W·m^(-1)·K^(-1),which may be suppressed mainly by the phonon thermal conduction confinement.The dimensionless figure-of-merit ZT of Ca_(2.95)K_(0.05)Co_(4)O_(9)is close to 0.22 at 1026 K,representing an improvement of about 36%compared with that of Ca_(3)Co_(4)O_(9),suggesting that lower concentration K-doped Ca_(3)Co_(4)O_(9)series materials are promising thermoelectric oxides for high-temperature applications.