稀有金属(英文版) 2019,38(04),350-358

Microstructures and thermal properties of Sn-Bi-Pb-Zn alloys as heat storage and transfer materials

Qing-Meng Wang Xiao-Min Cheng Yuan-Yuan Li Guo-Ming Yu Zhi Liu

School of Materials Science and Engineering, Wuhan University of Technology

School of Mechanical and Electrical Engineering, Huanggang Normal University

作者简介：*Xiao-Min Cheng,e-mail:Chengxm@whut.edu.cn;

收稿日期：8 January 2018

基金：financially supported by the National Key Technology Research & Development Program of China (No. 2012BAA05B05);the Key Technology Research & Development Program of Hubei (No. 2015BAA111);the Fundamental Research Funds for the Central Universities (No.WUT: 2017II23GX);

Microstructures and thermal properties of Sn-Bi-Pb-Zn alloys as heat storage and transfer materials

Qing-Meng Wang Xiao-Min Cheng Yuan-Yuan Li Guo-Ming Yu Zhi Liu

School of Materials Science and Engineering, Wuhan University of Technology

School of Mechanical and Electrical Engineering, Huanggang Normal University

Abstract：

Low-melting-point alloy has the characteristics of high thermal conductivity, low solidus temperature and the wide range of use temperature, which is a potential heat transfer medium. The microstructure and thermal properties of Sn-Bi-Pb-Zn alloys as heat transfer and storage material were investigated in this paper. The phase compositions, microstructure and thermal properties were investigated by X-ray diffusion(XRD), electron probe microanalysis(EPMA) and differential scanning calorimeter(DSC) analysis, respectively. The results show that the phases of Sn-Bi-Zn and Sn-Pb-Zn alloys are mainly eutectic formed by solid solution, while the formation of Pb₇Bi₃ intermetallic compounds decreases the melting point of Sn-Bi-Pb and Bi-Pb-Zn. The thermal properties of the zinc-containing alloys are better than that of Sn-Bi-Pb, but the weight of the zinc-containing alloys significantly reduces above 900 ℃. As the density, specific heat capacity and thermal diffusivity change with temperature and physical state, the thermal conductivity of the alloys first decreases and then increases. These results demonstrate the feasibility of using low-melting alloys as the heat transfer and storage material.

Keyword：

Heat transfer; Microstructure; Metal alloy; Thermal properties; Solar power;

Received： 8 January 2018

1 Introduction

Solar energy is the most ideal renewable energy because of its rich resources,widely distributed and almost no pollution to the environment [ 1] .The choice of heat transfer and storage materials is very important to improve the efficiency of the concentrating solar power (CSP) system [ 2] .From the present point of view,heat-conducting oil and molten salt has been widely used as heat transfer and storage material in the CSP system.Owing to the high phase change temperature of the molten salts,it is necessary to add auxiliary heating equipment in the CSP system to prevent the freezing and clogging of the heat transfer pipes,and the use temperature of heat-conducting oil and molten salt can only reach about 600℃ [ 3, 4, 5, 6] ;it is difficult to meet the requirements of low-freezing-point and hightemperature efficiency in solar thermal power generation.Therefore,the development of a heat transfer and storage material used in wide temperature range needs to be considered.

Metal and its alloys have the advantages of high heat storage density,high thermal conductivity and low vapor pressure.Scholars have proposed liquid metals as heat storage and transfer material [ 7, 8, 9, 10] :Wood's metal (PbSn-Bi-Cd,melting point 65-80℃) [ 11, 12] ,Rose metal(Bi-Sn-Pb,melting point 93℃) and Pb-Bi eutectic alloy(melting point 125℃) [ 13] are also candidates for this application.It has attracted many experts and scholars in the field of energy storage and transfer in the recent years [ 14, 15, 16, 17] .For instance,Frazer et al. [ 18] have identified that the alloying materials can increase the operating temperature of the solar power plant to 900℃.Mg-Bi alloy as a latent heat energy storage material for CSP applications is reported by Fang et al. [ 19] .They found that the higher the proportion of Mg+Mg3Bi₂ eutectic phases is in alloy,the higher the value of melting enthalpy is.Kang et al. [ 20] determined that the heat capacity and thermal diffusivity gradually decrease with the increase in the volume fraction of the Mg2Sn phase in the Mg-Sn binary alloys.Kaya et al. [ 21] investigated that the melting temperature of Bi-Sn-Ag ternary eutectic alloy was measured to be 416.63 K.The values of the enthalpy of fusion and the specific heat for alloy were found to be 53.432 J·g^-1 and 0.128 J·g-1·K^-1,respectively.At present,in electrical energy storage field,storage devices such as battery [ 22, 23, 24] and supercapacitors [ 25, 26, 27] have been widely used.However,in heat transfer and storage field,the research on the application of materials mainly focuses on the molten salts,relatively few studies on the thermal properties of alloys in different states.Alloys composed of Sn,Bi,Pb and Zn have low melting point,high boiling point and good thermal conductivity.Hence,they can be the potential materials used in the wide temperature range for energy transfer and storage in solar thermal power generation system.However,compared with high-temperature heat storage materials,the latent heat storage is also not good because of the low phase change heat.

This paper aims to develop a new type of low-melting heat transfer and storage medium based on the Sn-Bi-PbZn alloys.The relationship between the microstructures and the thermal properties of the test alloys and the thermal properties of different state will also be discussed.

2 Experimental

2.1 Materials and preparation

Bi-43Pb-2Zn,Sn-56Bi-34.5Pb,Sn-22Pb-4.5Zn and Sn-50Bi-2Zn eutectic alloys were prepared from pure Sn(99.9%),Bi (99.9%),Pb (99.9%),and Zn (99.99%) metals by melting in a graphite crucible under the protection of high-purity nitrogen atmosphere in order to strictly prevent the specimens from oxidizing during the preparation process.The chemical compositions of the four designed alloys were measured by X-ray fluorescence (XRF,Axios advanced),as shown in Table 1.After heat insulation at300℃for 30 min,the melts were poured into an iron mold to form rectangular ingots with dimensions ofΦ30 mm×100 mm.

2.2 Analysis methods

The microstructures and phase compositions of the alloys were examined using electron probe microanalysis(EPMA,JXA-8230) equipped with an energy-dispersive X-ray (EDX,INCAX-ACT) analysis,and an AXS D8 for X-ray diffractometer (XRD).The melting temperatures of the alloys were analyzed by a differential scanning calorimetry (DSC,STA449C/3/G) under argon atmosphere protection at a heating rate of 5℃·min^-1 in the temperature range of 30-250℃.

The linear thermal expansion coefficient of the samples was measured by pushrod-type expansion meter (DIL402C) at the heating rate of 2℃·min^-1 in the solid state.The thermogravimetric analysis of alloy samples was tested at a heating rate of 2℃·min^-1 by thermogravimetry/differential thermal analyzer (TG/DTA,Perkin Elmer Diamond TG/DTA) under the protection of high-purity nitrogen atmosphere.The alloy samples for thermal expansion coefficient were processed to 5 mm×5mm×20 mm by wire cutting.

The thermal diffusivity of the alloy with the size of 2.5 mm × 10 mm × 10 mm at 50-250℃ was measured by laser flash method (LFA 457).The temperature conditions were set to 50,150 and 250℃,respectively.The data were averaged after multiple measurements.The thermal conductivity was calculated from relation [ 28] :

where α is the thermal diffusivity,ρ the density,and cp the specific heat capacity at constant pressure.The density of the alloy at high temperature was calculated from Eq.(2) [ 19] :

where ΔL/L0 is the relative elongation and ρ0 is the density of the sample at room temperature measured by the Archimedes method.The relationship between the saturated vapor pressure and the temperature of the liquid metal can be expressed as [ 29] :

  下载原图

Table 1 Components of Sn-Bi-Pb-Zn alloys measured by XRF

3 Results and discussion

3.1 Microstructure analysis

XRD patterns of as-cast Sn-Bi-Pb-Zn alloys are shown in Fig.1.It can be seen from Fig.1a that the Bi-43Pb-2Zn alloy is mainly composed of Bi phase,Pb phase,Zn phase and Pb₇Bi₃.The diffraction intensity of Zn is not obvious because the content of Zn is only 2 wt%.As seen in Fig.1b,the phase structure changes in the Sn-56Bi-34.5Pb alloy.It mainly consists of Sn phase,Bi phase,Pb phase and Pb₇Bi₃.Figure 1c shows that the phases in Sn-22Pb-4.5Zn alloys are mainly Sn phase,Pb phase and Zn phase,and the diffraction intensity of Zn is higher than that of other alloys.Figure 1d indicates that the Sn-50Bi-2Zn alloys are mainly composed of Sn phase,Bi phase and Zn phase.There is no intermetallic compound formed in both Sn-22Pb-4.5Zn and Sn-50Bi-2Zn alloys.

The micros truc tures images of Sn-Bi-Pb-Zn alloys are shown in Fig.2.Table 2 shows the compositions of the phases exhibited in Fig.2,obtained from EDX analysis.As shown in Fig.2a,b,the micro structure of Bi-43Pb-2Zn eutectic alloy consists of white Bi-rich phase (A),dark BiPb phase (B) and black Bi+Zn phase (C).The composition of white phase approaches to pure Bi dissolved with 4.41at%Pb.From EDX analysis of the gray phase,it reveals that the phase is composed of Pb₇Bi₃.According to the Sn-56Bi-34.5Pb microstructure diagram (Fig.2c,d),the micro structure and phase compositions are different with those of Bi-43Pb-2Zn alloy,which mainly consist of white Bi-rich phase (A),dark Bi-Pb phase (B) and gray Sn-rich phase (D).The composition of the gray phase analyzed with EDX approaches to pure Sn dissolved with 4.14 at%Bi.Because of the lower mass content of Pb in the alloys,there is still some Bi remaining after forming Pb₇Bi₃ the alloy.When the alloy composition changes to Sn-22Pb-4.5Zn (Fig.2e,f),the microstructure of the ternary alloy can be distinguished into three phases,the gray Sn-rich phase (D),the white Pb-rich phase (E) and the black Znrich phase (F).It was found that the black phase is Zn-rich phase dissolved with large amount of Sn (73.87 at%) and Pb (3.86 at%).As can be seen from the micros truc ture diagram of Sn-50Bi-2Zn in Fig.2g,h,it mainly consists of white Bi-rich phase,gray Sn-rich phase and black Znrich phase.The gray phase is composed of Bi and Sn,while seldom Zn is found in the white and gray phase.Large amount of Sn and Bi are found to be distributed in the Znrich phases.From the microstructure diagrams of Sn-BiPb-Zn alloys,the micro-morphology of Bi-43Pb-2Zn,Sn-22Pb-4.5Zn and Sn-50Bi-2Zn alloys is denser and smaller than that of Sn-56Bi-34.5Pb alloy,which indicates that the microstructure of the Zn-containing alloys has more interface exists.

Fig.1 XRD patterns of test alloys:a Bi-43Pb-2Zn,b Sn-56Bi-34.5Pb,c Sn-22Pb-4.5Zn,and d Sn-50Bi-2Zn

Fig.2 EPMA images of Sn-Bi-Pb-Zn alloys:a,b Bi-43Pb-2Zn alloy;c,d Sn-56Bi-34.5Pb alloy;e,f Sn-22Pb-4.5Zn alloy;g,h Sn-50Bi-2Zn alloy

  下载原图

Table 2 Chemical composition of intermetallic phases in Fig.2 (at%)

3.2 Phase change temperatures and specific heat capacity

Figure 3 shows the melting point and melting enthalpy curves of Sn-Bi-Pb-Zn alloys.The data in Table 3 show that the melting enthalpies of the test alloys are 20.44,19.59,59.80 and 47.62 Jg-1 with the melting points of126.79,90.78,171.75 and 135.49℃,respectively.Results show that the values of melting point and the enthalpies of the zinc-containing alloys are higher than those of Sn-56Bi-34.5Pb alloy.This is because the principle of alloy melting is the bonds between the metal atoms destroyed by the energy.The more the interfaces there are,the more the metal bonds are between the atoms,so more energy was needed when the alloy melts.On the other hand,the content of the high-melting-point Bi elemental reduces by the formation of the Pb₇Bi₃ intermetallic compound,resulting in a decrease in the melting point of the alloy matrix.

Fig.3 DSC curves of Sn-Bi-Pb-Zn alloys:a Bi-43Pb-2Zn,b Sn-56Bi-34.5Pb,c Sn-22Pb-4.5Zn,and d Sn-50Bi-2Zn

  下载原图

Table 3 Thermophysical properties of Sn-Bi-Pb-Zn alloys

The temperature dependence of the specific heat capacity of the samples is shown in Fig.4.Result indicates that the specific heat of the test alloys increases with Zn content increasing.According to the microstructure analysis of the alloys,with the number of micro-interfaces formed between the structures and phases increasing,the alloy will absorb more energy when heated,resulting in a significant increase in the specific heat of the alloy.As can be seen from Table 4,the specific heat capacity of the liquid alloy is slightly higher than that of the solid state,but not very different,indicating that the alloy can absorb more heat in the liquid state.

3.3 Saturated vapor pressure and thermogravimetric analysis

The higher the vapor pressure is,the easier it is to evaporate the metal element.Under the same conditions,the element with high vapor pressure is more likely to evaporate than the element with low vapor pressure.The relationship between the saturated vapor pressure and the temperature of pure Sn,Bi,Pb and Zn can be calculated from Eq.(3) and the evaporation constant of the alloy component in Table 5.As can be seen from Fig.5,the saturated vapor pressure of the alloying element increases with temperature increasing,but the rate of increase in Zn is much larger than that of other elements.The evaporation order obtained by saturation vapor pressure standard is Zn>Bi>Pb>Sn.Therefore,when the temperature increases,Zn will take precedence over other elements to evaporate into the gas phase.

The thermogravimetric analysis is displayed in Fig.6,and the weight change of the alloys is recorded at25-1000℃.As can be seen from Fig.6,the weight does not change significantly before 900℃,indicating that the Sn-Bi-Pb-Zn alloy has good thermal stability.It also can be seen that the weight of the zinc-containing alloys significantly reduces above 900℃.The reason may be that the saturated vapor pressure of Zn is much larger than that of other elements,especially at high temperatures,so part of Zn evaporates into gas phase,resulting in a reduction in the quality of the alloy.

Fig.4 Temperature dependence of specific heat capacity of samples:a Bi-43Pb-2Zn,b Sn-56Bi-34.5Pb,c Sn-22Pb-4.5Zn,and d Sn-50Bi-2Zn

  下载原图

Table 4 Average specific heat capacity in different states of samples(J·g^-1·K^-1)

3.4 Thermal expansion and density

Figure 7 shows the results of thermal expansion of test alloys.As seen in Fig.7a,the elongation of all of the samples increases with temperature increasing.The values of the elongation of the Sn-Bi-Pb-Zn alloys are18.14×10^-6,17.82×10^-6,24.11×10^-6 and14.29×10^-6℃^-1,respectively.The values display a slight trend to increase with Zn content increasing.The linear expansion coefficients of pure Sn,Bi,Pb and Zn are19.5×10^-6,14.0×10^-6,29.3×10^-6 and36×10^-6℃C^-1,respectively.Therefore,the thermal expansion coefficient of the Sn-22Pb-4.5Zn alloy with higher content of Pb and Zn is relatively high.Since Zn in the sample alloys leads to the increase in thermal expansion coefficient and elongation of the alloy,the higher the Zn content is,the greater the thermal expansion coefficient is.

  下载原图

Table 5 Evaporation constant of alloy component

Fig.5 Relationship between saturated vapor pressure and tempera-ture of alloy component

Fig.6 Weight gain of Sn-Bi-Pb-Zn alloys

Density is one of the parameters for thermal conductivity calculation.The density at different temperatures was measured by the Archimedes method.The densities of pure Sn,Bi,Pb and Zn are 7.31,9.25,11.35 and 7.13 g·cm^-3,respectively.The density of the Bi-43Pb-2Zn alloy is the largest because of the high proportion of Pb,and the density of the Sn-22Pb^-4.5Zn alloy is smaller than that of the other three eutectic alloys because of the high proportion of Sn and Zn.Based on Eq.(1) and data above,the curves of densities with temperature are also obtained in Fig.8.The density of all the alloys decreases with temperature increasing because of the heat expansion.Owing to the special nature of Bi,the density of Sn-50Bi-2Zn alloy increases when it changes to a liquid state,but this phenomenon disappears in Bi^-43Pb-2Zn and Sn-56Bi-34.5 Pb alloys because the Pb₇Bi₃ intermetallic compounds were formed.

Fig.8 Temperature dependence of density of test alloys

3.5 Thermal diffusivity and thermal conductivity

Thermal diffusivity measured by laser flash method is another parameter for thermal conductivity calculation.The thermal diffusivity values of Sn-Bi-Pb-Zn alloys at different temperatures are shown in Table 6.It can be seen that the alloys with high Zn content have higher thermal diffusivity,while the alloy with high content of Bi has lower thermal diffusivity.The thermal diffusivity of the alloy decreases with temperature increasing.It can be seen from Fig.9 that the thermal conductivity of zinc-containing alloys is higher than that of Sn-56Bi-34.5Pb.Although the increase in Zn content leads to the decrease in the density,the thermal conductivity of the alloys still obviously increases due to the increase in thermal diffusivity and specific heat capacity.Results in Fig.9 also indicate that due to the change in density and thermal properties of the alloy from solid state to liquid state,the thermal conductivity tends to decrease firstly and then increase,but the thermal conductivity of Sn-22Pb-4.5Zn alloy at 250℃decreases significantly because of the high melting point.It is foreseen that the thermal conductivity of the alloys will rise with temperature in liquid state increasing.

Fig.7 Temperature dependence of linear thermal expansion coefficient of test alloys:a relative elongation (ΔL/L₀) and b linear thermal expansion coefficient (α)

  下载原图

Table 6 Thermal diffusivity of test alloys at different temperatures(mm²·s^-1)

Fig.9 Temperature dependence of thermal conductivity of samples

3.6 Case study:a comparison with other materials as heat transfer materials

The Sn-Bi-Pb-Zn alloys have higher boiling point,lower melting point and higher energy density than the traditional heat transfer materials,so it is selected to conduct the comparison.Table 7 shows the most important thermophysical properties of water/steam,heat-conducting oil and molten salt as well as those of the Sn-Bi-Pb-Zn alloy.As shown in Table 7,the melting temperatures of these materials are in the range of 100-250℃.The thermal conductivity of Sn-Bi-Pb-Zn alloys is 5-30 times as large as that of any other heat transfer materials.The melting enthalpies of water/steam,heat-conducting oil and molten salt are higher than those of Sn-Bi-Pb-Zn alloys.The heat capacity is between 40%and 88%smaller and the density is between 4.3 and 7.5 times higher than those of Sn-BiPb-Zn alloys than other materials.It can be seen that the main advantage of the Sn-Bi-Pb-Zn alloys over the other materials is the high thermal conductivity and high density.

4 Conclusion

The results indicate that the melting point of Sn-Bi-Pb-Zn alloys is between 90.78 and 171.75℃,and phase change enthalpy is between 19.59 and 59.8 J·g^-1.The specific heat capacity of the liquid alloys is slightly higher than that of the solid alloys.The microstructure of the zinc-containing alloys forms more micro-interfaces than that of Sn-56Bi-34.5Pb alloy,thus improving the thermal properties of the solid-state alloys.The thermal conductivity of the alloys increases with temperature increasing in liquid state and thermogravimetric analysis,showing that the zinc-containing alloys exhibit significant weight loss above 900℃,consistent with the calculated vapor pressure results.

  下载原图

Table 7 Comparison of thermophysical properties of Sn-Bi-Pb-Zn alloys with other materials

a Values from Refs. [2-6] ,b Liquid, c Solid

参考文献

[1] Reilly J, Paltsev S, Felzer B, Wang X, Kicklighter D, Melillo J,Prinn R, Sarofim M, Sokolov A, Wang C. Global economic effects of changes in crops, pasture, and forests due to changing climate, carbon dioxide, and ozone. Energy Policy. 2007;35(11):5370.

[2] Vignarooban K, Xu XH, Arvay A, Hsu K, Kannan AM. Heat transfer fluids for concentrating solar power systems—a review.Appl Energy. 2015;146(1):383.

[3] Lorenzin N, Abanades A. A review on the application of liquid metals as heat transfer fluid in concentrated solar power technologies. Int J Hydrogen Energy. 2016;41(17):6990.

[4] Chen YY, Zhao CY. Thermophysical properties of Ca(NO_3)_2-NaNO_3-KNO_3 mixtures for heat transfer and thermal storage. Sol Energy. 2017;146(1):172.

[5] Cheng XM, Li G, Yu GM, Li YY, Han JQ. Effect of expanded graphite and carbon nanotubes on the thermal performance of stearic acid phase change materials. J Mater Sci. 2017;52(20):12370.

[6] Zhu C, Cheng XM, Li YY, Tao BM. Influence of heat treatment on solidus temperature of NaN03-KN03 molten salt. Sol Energy. 2015;118(1):303.

[7] Pei YZ, Zhou XY, Zhu TJ. Editorial for rare metals, special issue on advanced thermoelectric materials. Rare Met. 2018;37(4):257.

[8] Porteiro J, Miguez JL, Crespo B, Gonzalez LML, De Lara J.Experimental investigation of the thermal response of a thermal storage tank partially filled with different PCMs(phase change materials)to a steep demand. Energy. 2015;91(1):202.

[9] Ben X, Li P, Chan C. Application of phase change materials for thermal energy storage in concentrated solar thermal power plants:a review to recent developments. Appl Energy. 2015;160(1):286.

[10] Ge HS, Li HY, Mei SF, Liu J. Low melting point liquid metal as a new class of phase change material:an emerging frontier in energy area. Renew Sustain Energy Rev. 2013;21(5):331.

[11] Zhang Y, Li PW. Minimum system entropy production as the FOM of high temperature heat transfer fluids for CSP systems.Sol Energy. 2017;152(1):80.

[12] Aksoz S, Ocak Y, Marasli N, Keslioglu K. Determination of thermal conductivity and interfacial energy of solid Zn solution in the Zn-Al-Bi eutectic system. Exp Thermal Fluid Sci. 2011;35(2):395.

[13] Khatib Hisham. IEA World Energy Outlook 2010—a comment.Energy Policy. 2011;39(5):2507.

[14] Fukahori R, Nomura T, Zhu CY, Sheng N, Okinaka N, Akiyama T. Thermal analysis of Al-Si alloys as high-temperaturephase-change material and their corrosion properties with ceramic materials. Appl Energy. 2016;163(1):1.

[15] Blanco-Rodriguez P, Rodriguez-Aseguinolaza J, Risueno E,Tello M. Thermophysical characterization of Mg-51%Zn eutectic metal alloy:a phase change material for thermal energy storage in direct steam generation applications. Energy. 2014;75(7):414.

[16] Wang ZY, Wang H, Li XB, Wang DZ, Zhang QY, Chen G, Ren ZF. Aluminum and silicon based phase change materials for high capacity thermal energy storage. Appl Therm Eng. 2015;89(1):204.

[17] Risueno E, Faik A, Rodriguez-Aseguinolaza J, Blanco-Rodriguez P, Gil A, Tello M, D'Aguanno B. Mg-Zn-Al eutectic alloys as phase change material for latent heat thermal energy storage. Energy Procedia. 2015;69(1):1006.

[18] Frazer D, Stergar E, Cionea C, Hosemann P. Liquid metal as a heat transport fluid for thermal solar power applications. Energy Procedia. 2014;49(1):627.

[19] Fang D, Sun Z, Li YY, Cheng XM. Preparation, microstructure and thermal properties of Mg, Bi alloys as phase change materials for thermal energy storage. Appl Therm Eng. 2016;92(1):187.

[20] Kang H, Suh JY, Kang SW, Bae DH. Effect of Sn on thermal conductivity of Mg-5Zn based alloys. Mater Trans. 2015;56(7):1144.

[21] Kaya H, Engin S, Aker A, Buyuk U, Cadirli E. Microstructural,mechanical, electrical, and thermal properties of the Bi-Sn-Ag ternary eutectic alloy. J Wuhan Univ Technol. 2017;32(1):147.

[22] Wang JG, Liu HY, Liu HZ, Fu ZH, Nan D. Facile synthesis of microsized MnO/C composites with high tap density as high performance anodes for Li-ion batteries. Chem Eng J. 2017;328(1):591.

[23] Momeni MM. Dye-sensitized solar cells based on Cr-doped TiO2 nanotube photoanodes. Rare Met. 2017;36(11):865.

[24] Wang JG, Sun HH, Liu HY, Jin DD, Zhou R, Wei BQ.Edge-oriented SnS2 nanosheet arrays on carbon paper as advanced binder-free anodes for Li-ion and Na-ion batteries.J Mater Chem A. 2017;5(1):23115.

[25] Wang JG, Zhang ZY, Zhang XY, Yin XM, Li X, Liu XR, Kang FY, Wei BQ. Cation exchange formation of prussian blue analogue submicroboxes for high-performance Na-ion hybrid supercapacitors. Nano Energy. 2017;39(1):647.

[26] Guo P, Shen Y, Song Y, Ma J, Lin YH, Nan CW. Self-etching Ni-Co hydroxides@Ni-Cu nanowire arrays with enhancing ultrahigh areal capacitance for flexible thin-film supercapacitors.Rare Met. 2017;36(9):691.

[27] Wang JG, Li HZ, Sun HH, Hua W, Wang HW, Liu XR, Wei BQ. One-pot synthesis of nitrogen-doped ordered mesoporous carbon spheres for high-rate and long-cycle life supercapacitors.Carbon. 2018;127(1):85.

[28] Yuan JW, Zhang K, Zhang XH, Li XG, Li T, Li YJ, Ma ML, Shi GL. Thermal characteristics of Mg-Zn-Mn alloys with high specific strength and high thermal conductivity. J Alloy Compd.2013;578(6):32.

[29] Lan Z, Ma XH, Hao ZL, Jiang R. Experiments on saturated vapor pressure of aqueous lithium bromide solution at high temperatures. Int J Refrig. 2017;76(1):73.