High thermoelectric performance at room temperature of n-type Mg_(3)Bi_(2)-based materials by Se doping

Bi_(2)Te_(3) based alloys have been the most widely used thermoelectric material at low temperature for many decades.Here we report Se doped n-type Mg_(3)Bi_(2) based materials with a thermoelectric figure-of-merit ZT of 0.82 at 300 K and a peak ZT of 1.24 at 498 K,which is comparable to the n-type Bi_(2)Te_(3) and Te doped Mg_(3)Bi_(1.4)Sb_(0.6).The improved thermoelectric performance is benefited from the high carrier concentration and mobility as well as the thermal conductivity reduction.The reduced resistivity increased the power factor at all measured temperatures,leading to a higher engineering ZT(ZTeng)and engineering power factor(PFeng)for n-type Mg_(3)Bi_(2).The n-type Mg_(3)Bi_(1.4)Sb_(0.6) materials are promising for thermoelectric power generation and cooling applications near room temperature.

Bismuth-free Mg_(3)Sb_(2) with enhanced room-temperature thermoelectric and mechanical properties

Thermoelectric materials are competitive candidates for special cooling applications.Mg_(3)Sb_(2)-based materials consisting of inexpensive ingredients have profound thermoelectric properties.At present,alloying with Mg_(3)Bi_(2) is the most effective approach to optimize the thermoelectric properties of Mg_(3)Sb_(2)-based materials.However,the extremely low abundance of bismuth in the crust contradicts its economic expectation.In this work,the ZrO_(2) micro-particles were separated into the Mg_(3.2)Sb_(1.99)Te_(0.01).The doping effect of Zr atoms at Mg sites increased the electrical conductivity,and the combined secondary phase lowered the lattice thermal conductivity.With acceptable degradation in the Seebeck coefficient,the sample combined with 5%(in mass)ZrO_(2) exhibited a dimensionless figure of merit(zT)of 0.49 and a power factor of 2.7 mW·m^(−1)·K^(−2) near room temperature.The average zT in the range from 300 K to 500 K reached 0.8,on par with the Mg_(3)Sb_(2) single bond Mg_(3)Bi_(2) alloys.Besides,the compressive and bending strengths reach 669 MPa and 269 MPa,respectively,far superior to the common room-temperature thermoelectrics.This secondary phase showed a surprising and uncostly promotion of the Mg_(3)Sb_(2)-based thermoelectric materials,impelling the realization of its commercial application.

Electrical property enhancement and lattice thermal conductivity reduction of n-type Mg_(3)Sb_(1.5)Bi_(0.5)-based Zintl compound by In&Se co-doping

Mg_(3)Sb_(1.5)Bi_(0.5)-based Zintl compounds have attracted extensive attention as potential thermoelectric materials due to their earth-abundant elements.However,pure and intrinsic Mg_(3)Sb_(1.5)Bi_(0.5)manifests a poor thermoelectric performance because of its low electrical conductivity of about 3×10^(2)S/m at room temperature.In this work,In and Se co-doping was carried out to optimize the thermoelectric perfor-mance of n-type Mg_(3)Sb_(1.5)Bi_(0.5)-based material.The experimental results revealed that the carrier con-centration and mobility of Mg_(3)Sb_(1.5)Bi_(0.5)significantly increased after In and Se co-doping,leading to an improvement of power factor.Simultaneously,lattice thermal conductivity was significantly reduced due to the large mass difference between In and Mg.A maximum zT of 1.64 at 723 K was obtained for the Mg_(3.17)In_(0.03)Sb_(1.5)Bi_(0.49)Se_(0.01)sample.And an average zT value of about 1.1 between 300 and 723 K was achieved,which insures its possible application at medium temperature range as a non-toxic and low-cost TE material.

A straightforward spectral emissivity estimating method based on constructing random rough surfaces

Spectral emissivity is an essential and sensitive parameter to characterize the radiative capacity of the solid surface in scientific and engineering applications,which would be non-negligibly affected by surface morphology.However,there is a lack of assessment of the effect of roughness on emissivity and a straightforward method for estimating the emissivity of rough surfaces.This paper established an estimating method based on constructing random rough surfaces to predict rough surface(Geometric region)emissivity for metal solids.Based on this method,the emissivity of ideal gray and non-gray body surfaces was calculated and analyzed.The calculated and measured spectral emissivities of GH3044,K465,DD6,and TC4 alloys with different roughness were compared.The results show that the emissivity increases with the roughness degree,and the enhancement effect weakens with the increase of roughness or emissivity due to the existing limit(emissivityε=1.0).At the same time,the roughness would not change the overall spectral distribution characteristics but may attenuate the local features of the spectral emissivity.The estimated results are in good agreement with the experimental data for the above alloys'rough surfaces.This study provides a new reliable approach to obtaining the spectral emissivity of rough surfaces.This approach is especially beneficial for measuring objects in extreme environments where emissivity is difficult to obtain.Meanwhile,this study promotes an understanding of surface morphology's effect mechanism on emissivity.

In-situ growth of flexible 3D hollow tubular Cu_(2)S nanorods on Cu foam for high electrochemical performance supercapacitor

For pursing high-performance supercapacitors,both of the design strategy and structural characteristic of electrode materials are crucial.Herein,we report the in-situ growth of flexible self-assembled 3D hollow tubular Cu_(2)S nanorods on Cu foam substrate(Cu_(2)S@Cu).The Cu substrate is simultaneously acted as a copper source and a collector,which reduces the contact resistance.Moreover,the highly ordered 3D unique structure increases the redox reactive sites and enhances the ion transmission effectively,resulting in greatly improved electrochemical performance.Based on the Cu_(2)S@Cu electrode,the supercapacitor exhibits high areal capacitance of 1000 mF cm^(-2) at a current density of 2 mA cm^(-2),and great cycle stability,maintaining 96.9% capacitance after 10,000 cycles.Furthermore,the supercapacitor also shows an excellent flexibility with no significant decrease in the twisting or bending state.The capacity retention rates are 99.8% and 86.1%,respectively,and finally recover to 99.3%,confirming its great potential in practical application for portable electronic devices.

Effect of electrolyte concentration o n the tribological performance of MAO coatings on aluminum alloys

Micro-arc oxidation(MAO)is an efficient approach to improve the hardness,wea r resistance,and other properties of aluminum alloys.In order to investigate the effect of the electrolyte concentration on the properties of MAO coatings for LY12 alloy,the voltage variation during the MAO process was recorded.The surface morphologies and phase compositions of the coatings produced with different electrolytes were investigated using scanning electron microscopy and X-ray diffraction,respectively.The roughness and thickness of the coatings were measured using a pocket roughness meter a nd an eddy-current thickness meter,respectively.The tribological performances of the coatings wer e investigated against GCr15 bearing steel on aball-on-disc wear tester in open air.The results showed that with an increase in the Na2SiO3 content,the working voltage of the MAO process decreased,the roughness a nd thickness of the coatings increased significantly,a nd the relative content of the α-Al2O3 phase decreased.With an increase in the KOH content,the working voltage decreased slightly,the roughness and thickness of the coatings increased slightly,and the α-and γ-Al2O3 phase contents remained unchanged.The friction coefficient and wear rate of the coatings increased with an increase in the Na2Sio3 and KoH concentrations.A decrease in the porosity and roughness and an increase in the α-Al2O3 content of the coatings reduced their wear mass loss.

Enhanced thermoelectric performance in Ga_(x)Co_(4)Sb_(12.3-y)Te_(y) skutterudites via suppressing bipolar effects for commercial application

The n-type filled and doped skutterudites Ga_(x)Co_(4)Sb_(12.3) and Ga_(0.2)Co_(4)Sb_(11.3)Te composites with the welldistributed GaSb nanoinclusions are synthesized through the manipulating of metastable Ga fillers and enrichment of Sb by an in-situ method with a proper annealing procedure.Ga atoms can fill the icosahedron cages of skutterudite at high temperature,but at low temperature,they are driven out from the lattice voids and form the second phase of GaSb at grain boundaries.The presence of GaSb second phases reduces the thermal conductivity effectively.Te doping leads to a significant increase in carrier concentration of Ga_(0.2)Co_(4)Sb_(11.3)Te,thus largely suppresses the bipolar effect of Ga_(x)Co_(4)Sb_(12.3),resulting in a great enhancement in power factor.Moreover,Te doping induces mass and strain fluctuation,which decreases the lattice thermal conductivity further.Consequently,the maximum ZT is increased from 0.56 for Ga_(0.2)Co_(4)Sb_(12.3) at 573 K to 1.48 for Ga_(0.2)Co_(4)Sb_(11.3)Te at 873 K,which is advantageous to improve the thermoelectric conversion efficiency for commercial application.