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Study on thermophysical performance of Mg–Bi–Sn phase-change alloys for high temperature thermal energy storage
Journal Pre-proof Study on thermophysical performance of Mg-Bi-Sn phase-change alloys for high temperature thermal energy storage Dong Fang, Xiaomin Cheng, Xicong Ye, Haihua Wu, Congyang Zhang, Kai Xu, Ning Wang PII:
S0042-207X(20)30001-4
DOI:
https://doi.org/10.1016/j.vacuum.2020.109164
Reference:
VAC 109164
To appear in:
Vacuum
Received Date: 17 October 2019 Revised Date:
27 December 2019
Accepted Date: 31 December 2019
Please cite this article as: Fang D, Cheng X, Ye X, Wu H, Zhang C, Xu K, Wang N, Study on thermophysical performance of Mg-Bi-Sn phase-change alloys for high temperature thermal energy storage, Vacuum, https://doi.org/10.1016/j.vacuum.2020.109164. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Study on thermophysical performance of Mg-Bi-Sn phase-change alloys for high temperature thermal energy storage Dong Fang a,b, Xiaomin Chengb,*,Xicong Ye a,*,Haihua Wu a,Congyang Zhang a,Kai Xub, Ning Wangc a
Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance, China Three Gorges University, Yichang 443002, China
b
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
c
National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan 430070, China
Abstract: Mg-based alloys are potential high temperature phase change materials for thermal energy storage due to their high melting enthalpy, high thermal conductivity and excellent compatibility with Fe-based containment materials. This work mainly focuses on studying the
microstructure and thermophysical characterization of Mg-Bi-Sn alloys phase change materials with 4 different components and their thermal
energy storage performance. The experimental results show that the Mg-33Bi-17Sn, Mg-39Bi-17Sn and Mg-45Bi-17Sn alloys mainly consist of
primary α-Mg phase and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure, and the Mg-33Bi-23Sn alloy mainly consist of primary α-Mg phase, primary Mg2Sn+Mg3Bi2 phases and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. Their melting enthalpies are respectively 18.5 J/g, 168.8 J/g, 106.3 J/g and 140.3 J/g, while their melting temperatures are about 515°C ~ 525°C. As from the results, the Mg-39Bi-17Sn alloy obtains the highest melting
enthalpy, which attributes to its higher proportion of eutectic α-Mg+Mg2Sn+Mg3Bi2 structure. However, the Mg-45Bi-17Sn alloy shows the highest thermal conductivity. In addition, the melting point of Mg-39Bi-17Sn alloys increase by about 1.8℃, and the melting enthalpy decreases
by about 4.2% after 300 thermal cycling. Based on all results, the Mg-39Bi-17Sn alloy shows with great thermalphsycial performance and is
expected to be used as thermal energy storage material.
Keywords: Latent heat storage; Phase change material; Metallic materials; Mg-Bi-Sn alloy
1. Introduction
Solar energy promotes and promises sustainable development due to its proverbial advantages (e.g. widespread, non-toxic, sustainable)
[1, 2]
. However, the distribution of solar energy is intermittent in time and space, therefore, thermal energy
storage (TES) becomes a key link in solar thermal utilization [3-5]. As heat carrier, phase change materials (PCMs) based on latent heat storage (LHS) used in TES systems are being widely studied around the world because of its high heat storage density and the characteristics of releasing heat at nearly constant temperature [6].
The operating temperature of PCMs are normally classified as three temperature regions, low temperature (<90℃), medium temperature (90-400℃) and high temperature (>400℃). For solar thermal power generation, their effective working temperature are often between 120 -1000℃[7]. Based on the working temperature range above, medium and high temperature PCMs are considered for TES systems, such as molten salts, metals and alloys. Kenisarin [7] et al and Zalba [8] et al summarized inorganic substances and organic substances, with melting point from ambient temperature to 1000℃. The most well-studied binary inorganic salt is the NaNO3-KNO3 and the most representative of them is the solar salt, which has operation temperature range from to 300℃ to 550℃, can be used as PCMs for tower solar power station. However, several defects of inorganic salt limit its application, such as low thermal conductivity, strong corrosive effect on packaging materials, and large volume changes before and after phase change. In order to address these issues of inorganic salts, metals and alloys are considered as the high temperature PCMs for TES. The advantages of metallic PCMs are naturally higher thermal conductivity and lower thermal expansion coefficient than inorganic salts. The reports on metallic PCMs have been published for long time, for instance, many binary and ternary metallic eutectic alloys with melting temperatures above 300℃ were reported [9,10]. In recent years, the research on metallic PCMs focused on aluminum alloys. He and Zhang [11] studied the heat storage properties of two components Al-Si alloy. It was found that the phase transition temperatures of AlSi12 and AlSi20 were 576 °C and 585 °C, respectively, and latent heat was 560 J/g and 460 J/g. For the long time cycling [12], the thermal reliability results showed that the melting temperature and the latent heat of fusion of Al-34.02Mg-6.62Zn (wt. %) decreased 3.1-5.3 K and 10.98% after 1000 times thermal cycles, respectively. Takahiro Nomura[13] studied the thermal properties of Al-Si alloys for high-temperature (above 500℃) PCMs and the corrosion behavior of the alloys on ceramic materials. Based on their research, it is feasibility to use Al-Si alloys as the PCMs and ceramics as container materials. Nevertheless, the serious corrosiveness of aluminum alloys PCMs to iron-based container material limits its application in LHS system [14]. According to Mg-Fe phase diagram, liquid aluminum alloy has corrosive effect on iron-based containers from 400℃ to 600℃. Mg and Al alloys have similar melting temperature, high latent heat of fusion and thermal conductivity, and thus Mg alloys can also be considered as PCMs. Magnesium alloys are generally used as structural materials [15,16], there are few researches on Mg alloys PCMs. The
thermophysical characterizations of Mg-51Zn and Mg-24.9Zn-5.1Al alloys were investigated by Rodríguez-Aseguinolaza et al [17-19]. Bi and Sn have low melting point, high heat storage density and good heat stability. Hence, they can be used as PCMs. The heat storage properties of Mg-Bi and Mg-Sn alloys were investigated by our previous research [20, 21], and the melting enthalpy of Mg-54Bi, Mg-37Sn were also proved to be higher than others. According to the requirement of operating temperature range of the heat storage material in the solar thermal power generation system, the Mg alloy materials with a wide temperature range should be prepared and studied. The melting temperature of Mg-Bi-Sn is lower than that of Mg-Bi (550-560 °C), Mg-Sn (555-560 °C) alloy, it has potential to be used as heat storage material to satisfy with the demand of different heat storage systems. In the present work, the novel Mg-Bi-Sn phase-change alloys with different proportions were prepared by gravity casting method, and their microstructures were characterized by XRD, SEM, TEM and the effect on phase change enthalpy of alloys was also investigated to explore the phase change heat storage mechanism. Finally, their thermophysical performance as PCMs for TES were measured and the optimal components of the phase change alloys was confirmed. Finally, the microstructure and heat storage properties of Mg-39Bi-17Sn alloy after thermal cycling were studied to ensure thermal stability of the PCMs during long-term service.
2. Experimental setup
Four types of Mg-Bi-Sn alloys were smelted from pure magnesium ingot (99.98 wt. %), bismuth ingot (99.99% wt. %) and tin ingot (99.99 wt. %) in furnace resistance under protected by Ar atmosphere at 700°C. The furnace was vacuum pumping to 0.1 torr before the Ar argon was added. The melt was refined by RJ-2 flux. The nominal compositions of Mg-Bi-Sn alloys are given in Table 1. The compositions of RJ-2 flux and coating agent are given in our previous investigation [18, 19]. It was then static melting at 580 °C for about 20 min, and casted into steel mould. The preheating temperature of steel mould was 200 °C. The size of casting mould cavity is φ30 mm×100 mm. Finally, the samples were polished and etched in nitric acid alcohol (4ml nitric acid+96ml ethyl alcohol) for the next observation.
Table 1 The nominal compositions of Mg-Bi-Sn alloys Samples
Alloy
a b c d
Mg-33Bi-17Sn Mg-39Bi-17Sn Mg-45Bi-17Sn Mg-33Bi-23Sn
Composition (wt. %) Mg
Bi
Sn
50 44 38 44
33 39 45 33
17 17 17 23
The microstructure and composition were examined by X-ray diffraction (XRD, D8 Adwance, BRUKER AXS, Germany), electron probe micro-analysis (EPMA, JXA-8230, JEOL, Japan) with an energy-dispersive X-ray spectrometer (EDS,NCAX-ACT, JEOL, Japan) and transmission electron microscope(TEM, Talos F200S, Thermo Fisher, USA) with an EDS. The melting temperature, latent heat and specific heat capacity of the composites were tested by Differential scanning calorimeter (DSC, STA449C/3/G, Netzsch, Germany) in an Ar atmosphere. The heating rate was 5 K/min, and testing temperature range was from room temperature to 600 °C. The thermal diffusivity measurements were measured by Nanoflash device(LFA457, Netzsch ,Germany) over the temperature range of 50-400°C basing on Laser-flash method. The size of block sample was 10mm×10mm×2.5mm. The testing temperature interval was set to 50°C and repeated least three measurements at each temperature points. The relative elongation of samples were measured in an Ar atmosphere using pushrod type dilatometer (DIL 402C, Netzsch ,Germany) in the range of 30 °C to 450 °C. The density at different temperatures is calculated by the following equation [20]: ρ = ρ0 (1 +△L/L0)-3
(1)
where ρ0 is the density of the alloy at 20 °C obtained by the Archimedes method, and ∆L/L0 is the relative elongation. The thermal conductivity can be calculated from the equation: λ = a·ρ·cp
(2)
where a is the thermal diffusivity, ρ is the density and cp is the specific heat at constant pressure. The Mg-39Bi-17Sn alloy was chosen for thermal cycling experiments due to its best thermalphsycial performance among the four alloys. Thermal cycling experiment is as follows: the alloy was placed in a stainless-steel tube with a
diameter of 40 mm and closed at both ends. The stainless-steel tube was placed in resistance furnace at 600°C for 20 minutes. Then they were placed outside for 20minutes air cooling. That is one thermal cycling. A stainless-steel tube was taken out after 300 cycles, and the upper of steel tube was cut off. After that the steel tube was heated to melt the magnesium alloy. Finally, the magnesium alloy was cast into the metal mold. The XRD, EMPA, melting enthalpy, and thermal conductivity of samples were tested after 300 cycles.
3. Results and discussion
Fig. 1 depicts the XRD pattern of Mg-Bi-Sn alloys. From Fig. 1, it is known that the four alloys have α-Mg, Mg3Bi2 and Mg2Sn phases. When the diffraction angle(2θ) is 26°, compared with the Mg-33Bi-17Sn alloy, the diffraction peak height in Mg-39Bi-17Sn alloy is significantly enhanced, and the diffraction peak height in Mg-45Bi-17Sn alloy is slightly decreased. The reason for this phenomenon may be the increase of Bi content leads to the increase of Mg3Bi2 phase. The diffraction peak height (2θ=23°,37°,44°) of Mg2Sn phase increases from a alloy to d alloy. It is possible that the proportion of Mg2Sn phase has changed due to the difference of the ratio of Mg to Sn.
Fig. 1. XRD pattern of the Mg-Bi-Sn alloys Fig. 2 shows the EPMA secondary electron images of the four alloys. Table 2 shows the elemental compositions of the intermetallic obtained from EDS. The point(C,F point) and region(A,B,D,E,G,H,J,K,L) were chosen in the process of EDS. The EPMA image of the Mg-33Bi-17Sn, Mg-39Bi-17Sn and Mg-45Bi-17Sn in Fig. 2(a,b,c) reveals that the microstructure of these three alloys consisting of a mixture of bright and gray contrast areas. The volume fraction of the bright areas is estimated to be 16.5%, 57.4% and 43.5%, respectively. Combined with the high magnification images, XRD patterns, and
the results of EDS, the black dendrite shape area is composed of primary α-Mg phase (A, D, G), and the grey punctate shape area is composed of α-Mg+Mg2Sn+Mg3Bi2 eutectic structure (B,E,H). The composition of point I reveal that there are flaky Mg3Bi2 in Mg-45Bi-17Sn alloy. The Fig(d) displays that there is an another bright phase(L) in Mg-33Bi-23Sn alloy except the two phases described above. According to the XRD and FTIR results, the bright phase is primary Mg2Sn+Mg3Bi2 phase, which are polygons with 50-80µm on a side. The volume fraction of the α-Mg+Mg2Sn+Mg3Bi2 eutectic structure areas is estimated to be 60.7%.
Fig. 2. EPMA micrographs of Mg-Bi-Sn alloys Table 2 Composition of intermetallic phases in EPMA images. Chemical composition（at.%）
Selected area
Mg
Bi
Sn
A B C D E F G H I J K L
98.52 84.75 77.94 97.16 89.04 65.76 97 82.56 59.92 95.89 79.12 64.73
2.57 3.09 0.76 11.48 16.95 0.81 8.08 40.08 1.03 10.77 25.41
1.48 12.68 18.97 2.08 11.90 17.29 2.19 9.36 3.08 10.11 9.86
Closest phase α-Mg α-Mg+Mg2Sn+Mg3Bi2 α-Mg+Mg2Sn+Mg3Bi2 α-Mg α-Mg+Mg2Sn+Mg3Bi2 α-Mg+Mg2Sn+Mg3Bi2 α-Mg α-Mg+Mg2Sn+Mg3Bi2 Mg3Bi2 α-Mg α-Mg+Mg2Sn+Mg3Bi2 Mg2Sn+Mg3Bi2
Fig. 3 shows the TEM images of Mg-39Bi-17Sn alloy. Table 3 shows the compositions of the phases obtained from EDS. According to Fig. 3 and Table 3, it can be deduced that the black area (point a) and grey area (point b) are eutectic Mg2Sn+Mg3Bi2 and eutectic α-Mg phases, respectively. These two phases are alternately distributed. The spacing between two adjacent black phases is about 0.5-5µm.
Fig. 3. TEM images of Mg-39Bi-17Sn alloy Table 3 Composition of phases in TEM images. Composition（.at%） Selected area a b
Closest phase Mg
Bi
Sn
41.8 97.6
42.3 0.3
15.8 2.1
Mg2Sn+Mg3Bi2 α-Mg
Fig. 4 shows the high resolution TEM morphology and selective electron diffraction photographs of Mg-39Bi-17Sn alloy. Selected-area diffraction patterns in alloy reveal that the black areas mainly consist of Mg3Bi2. The significant diffraction spot of Mg2Sn are not found. It is possible to consider that the content of Mg2Sn is too low lead to the diffraction spot is too weak. Combined with EDS above, the SADPs in Fig. 3(B) and (C) obtained from the bright and dark phases in Fig. 3(A) correspond to the [-3 6 -3 0] and [0 0 0 1] zone axes of the Mg3Bi2 and α-Mg phases, respectively.
Fig. 4. High resolution TEM morphology and selective electron diffraction photographs of Mg-39Bi-17Sn alloy: (A) high resolution TEM；(B) selective electron diffraction photograph of Mg3Bi2; (C) selective electron diffraction of Mg3Bi2 Fig. 5 displays the Mg-Bi-Sn ternary system phase diagram and the positions (a-d points) that represent the composition of the four alloys. The a, b, c and d points show the simple a, b, c and d, respectively. As can be seen from the figure, the point b is located at the eutectic composition point E3, where eutectic transformation occurs: L→α-Mg+Mg2Sn+Mg3Bi2. However, in this study, there will be primary phases due to the solidification in non-equilibrium, which changes the growth mode of the eutectic structure, so that the dendritic phase (primary α-Mg) is precipitated as the primary phase, and then the eutectic structure is formed. Hence, the Mg-39Bi-17Sn alloy mainly consists of primary α-Mg phases and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. The three other alloys locate around the point E3 of composition, the microstructure of them mainly also contain primary α-Mg phases and eutectic structure. The Mg-33Bi-23Sn alloy contain primary Mg2Sn+Mg3Bi2 phase berceuse of high Sn content.
Fig. 5. The phase diagram of the Mg-Bi-Sn alloy [23]
Fig. 6. DSC thermograms of Mg-Bi-Sn alloys Fig. 6 shows the DSC thermograms of the Mg-Bi-Sn alloys. As can be seen, the solidus temperature of Mg-33Bi-17Sn, Mg-39Bi-17Sn, Mg-45Bi-17Sn and Mg-33Bi-23Sn alloys are 524.1℃, 515.8℃, 515.2℃ and 515.0℃, respectively. The solidus temperature of Mg-33Bi-17Sn alloy is higher than three others because the alloy melting is inconsistent. The solidus temperature of Mg-Sn, Mg-Bi alloys are 545℃-560℃. Compared with Mg-Sn and Mg-Bi alloys, the solidus temperature of ternary alloys decreased by 30℃-45℃. Mg-39Bi-17Sn and Mg-33Bi-17Sn have the largest and the lowest latent heat of 168.8J/g, 18.5J/g respectively. The composition of b sample close to the eutectic composition of the Mg-Bi-Sn alloy (E3 point). This result suggests that the latent heat of eutectic composition is higher than others in the same alloy system. The similar conclusion was concluded in our previous research and Takahiro Nomura’s study
[13]
. To further explore the heat
storage mechanism of the alloy, the relationship between phase change enthalpies and proportion of eutectic structure were investigated shown as Fig.7. From the Fig.7, it can be seen that the changing trend of enthalpies and proportion of eutectic structure were generally positively correlated. So it is expected that the Mg-39Bi-17Sn has higher latent heat because it
contains more proportion of α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. The reasons may be as follows: these four alloys are all prepared under almost identical conditions. The main difference between a, b and c samples is the proportion of eutectic structure to primary α-Mg phases. The essence of alloy melting is the breaking of metal bonds when the energy is provided outside. The primary α-Mg phase contains Mg-Mg bond, the α-Mg+Mg2Sn+Mg3Bi2 eutectic structure contain Mg-Mg bond, Mg-Sn bond and Mg-Bi bond. The greater the bond energy, the greater the heat required to break the bond energy. The closer the atomic spacing in the unit cell, the higher bond energy between atoms is. The atomic spacing of Mg-Sn, Mg-Bi are less than Mg-Mg, so the bond energies of Mg-Sn bond and Mg-Bi bond are higher than which of Mg-Mg. The bond energies of eutectic structure are higher than primary α-Mg phases. Mg-39Bi-17Sn contains more eutectic structure, so it has higher melting enthalpy. Mg-39Bi-17Sn contains more eutectic structure, so it has higher melting enthalpy. Mg-39Bi-17Sn contains more eutectic structure, so it has higher melting enthalpy. Comparing Mg-39Bi-17Sn with Mg-33Bi-23Sn, the proportion of eutectic structure of Mg-33Bi-23Sn and Mg-39Bi-17Sn are almost the same, however, the melting enthalpy of Mg-33Bi-23Sn is 17% lower than Mg-39Bi-17Sn. That may because the polygon primary Mg2Sn+Mg3Bi2 phase affected the structure of the alloy, which leads to the decrease of the melting enthalpy.
Fig. 7. Phase change enthalpies and proportion of eutectic structure of Mg-Bi-Sn alloys
Fig. 8. The relative elongation of Mg-Bi-Sn alloys 4.4 4.0 (c)
Density[g⋅ cm-3]
3.6 3.2
(b)
2.8
(d)
2.4 2.0
(a)
1.6 0
100
200
300
400
500
Temperature[ ]
Fig. 9. Density of Mg-Bi-Sn alloys at different temperatures The relative elongations of Mg-Bi-Sn alloys are given in Fig. 8. The experimental uncertainty is estimated to be at most 4.2%. The result reveals that the relative elongation of all alloys increases with temperature. The increase rate of relative elongation of four alloys slow down after 350℃. Compared to the other three alloys, the relative elongation increasing rate of Mg-33Bi-23Sn reduces significantly. The relative elongation of Mg-33Bi-17Sn is similar to these of Mg-39Bi-17Sn and Mg-45Bi-17Sn, and Mg-33Bi-23Sn has less elongation than these of three alloys above at the same temperature. The reason may be as follows: the Mg-Bi-Sn alloys are composed of multiple phases, which are α-Mg, Mg2Sn and Mg3Bi2 phases. The relative elongation depends on the properties of these phases. The Mg-33Bi-23Sn alloy contain more primary Mg2Sn+Mg3Bi2 phases than other alloys, so it is inferred that the relative elongation of Mg2Sn and Mg3Bi2 phases is less than α-Mg phase. Tian Xizhi[24] considered that the greater the concentration coefficient, the larger the relative elongation of crystal. The concentration coefficient of α-Mg is higher than Mg2Sn and Mg3Bi2 phases, so the relative elongation of Mg2Sn and Mg3Bi2 phases is less than α-Mg phase. The densities of Mg-Bi-Sn alloys at different temperatures are shown in Fig. 9. The experimental uncertainty is estimated to be at most 2.0 %. The densities at room temperature are measured by Archimedean principle. According to equation (1), the densities of Mg-Bi-Sn alloys at different temperature are calculated. It is obvious that the density of all alloys decreases as temperature increasing. The density of bulk Mg, Sn and Bi under room temperature are 1.738 g/cm3, 7.265 g/cm3 and 9.780 g/cm3, respectively [25]. The density of these three pure metals is also one of the factors affecting the density of the alloys. The higher the content of Mg in alloys, the lower the density, while the higher the content of Bi in alloys, the larger the density. Therefore, the Mg-33Bi-17Sn and Mg-45Bi-17Sn have least and largest density
among all alloys under the same temperature, respectively.
Fig. 10. Thermal diffusivity of Mg-Bi-Sn alloys as a function of temperature Fig. 10 shows thermal diffusivity of Mg-Bi-Sn alloys as a function of temperature. The experimental uncertainty is estimated to be at most 1.0%. As clearly seen from this figure, thermal diffusivity of all alloys raise with increase of temperature from 50℃ to 350℃, then it has a decrease after 350℃. This is mainly due to Mg2Sn and Mg3Bi2 precipitates dissolve while the temperature of phase boundary reaches a certain value [22]. A decrease of thermal diffusivity is induced by the reversible process as increasing temperature. It can be seen that the thermal diffusivity of Mg-33Bi-23Sn is less than these of three other alloys at the same temperature, which can be attributed to it contained more proportion of α-Mg phases.
Fig. 11. Specific heat capacity of Mg-Bi-Sn alloys as a function of temperature The specific heat capacities of Mg-Bi-Sn alloys at various temperatures were measured and the specific heat capacity data are shown in Fig. 11, in which the error bars are also shown. The experimental uncertainties are estimated to be at most 3.0%. As we can see in Fig. 11, the specific heat capacities of all alloys raise with increasing temperature from 50℃ to 400℃. The reason is as follows: the calculated values of heat capacity can be obtained by the Neumann-Kopp rule, which consider that the heat capacity of alloy depends on the elements it contains. The specific heat of Mg, Bi and Sn displays an increasing trend from 20 ºC to 400 ºC. The four alloys all contain Mg, Bi and Sn. So it can be concluded that the specific
heat capacities of all alloys increase when the temperature rises. Moreover, the Mg-33Bi-17Sn has largest specific heat capacity, and Mg-45Bi-17Sn has least specific heat capacity among the four alloys, which can be attributed to difference of Mg content.
Fig. 12. Thermal conductivity of Mg-Bi-Sn alloys as a function of temperature According to equation (2), the thermal conductivity can be calculated by the thermal diffusivity, density and heat capacity of alloys. The calculation results are shown as Fig. 12. The experimental uncertainties are estimated to be at most 6%. As seen in Fig. 12, the thermal conductivity increases as temperature increases from 50℃ to 350℃, and then it has a decline up to 400°C. The trend of thermal conductivity with temperature is similar to thermal diffusivity. That is because the density of all alloys decreases with increasing temperature, while the specific heat capacity increases when temperature rises. Combine two opposite factors, the product of them did not change much when temperature changes. In other words, the thermal diffusivity is a leading factor in the calculation of thermal conductivity. As seen on Fig.12, the thermal conductivity of Mg-33Bi-17Sn is close to that of Mg-33Bi-23Sn alloy, and is less than that of Mg-39Bi-17Sn alloy at the same temperature. The thermal conductivity of Mg-45Bi-17Sn alloy is highest among four alloys. The reasons for this phenomenon are as follows: the volume fraction of α-Mg is the greatest influence on thermal conductivity in alloys. Based on previous estimates, the volume fraction of α-Mg phase of Mg-33Bi-17Sn, Mg-39Bi-17Sn, Mg-45Bi-17Sn and Mg-33Bi-23Sn is about 83.5%, 42.6%, 56.5% and 39.3%.In addition to the Mg-33Bi-17Sn alloy, the thermal conductivity of other three alloys increases as the volume fraction of α-Mg increasing. The reason for the lower thermal conductivity of the Mg-33Bi-17Sn alloy may be it has lowest density in all alloys. Kinetic molecular theory proposed that the thermal conductivity of materials increases when the mean free path of electrons and
phonons raise. For Mg-Bi-Sn alloy, Bi, Sn in solid solution may act as scattering centers for electrons and phonons, which limit the mean free path of electrons and phonons
[26, 27]
. In addition, the increase of volume fraction of Mg2Sn+Mg3Bi2
phases form more interfaces that hinder the transmission of phonons and electrons, resulting in a decrease in thermal conductivity. Therefore, the thermal conductivity of Mg-39Bi-17Sn, Mg-45Bi-17Sn and Mg-33Bi-23Sn are 49, 56, 33 W/mK at 400℃, respectively. 3.2 The microstructures and thermophysical performance of Mg-39Bi-17Sn alloy after thermal cycling Fig. 13 shows the XRD pattern of the Mg-39Bi-17Sn alloys before and after 300 thermal cycling. It can be seen that the phase composition of the alloy before and after the thermal cycle has not changed. However, the diffraction peak height of the phase in alloy after the cycle becomes smaller.
Fig. 13. XRD pattern of the Mg-39Bi-17Sn alloys before and after 300 thermal cycling Fig. 14 reveals the EPMA secondary electron image of the Mg-39Bi-17Sn alloy after 300 thermal cycling. Table 4 shows the elemental compositions of the intermetallic obtained from EDS. The region (A, B, C) were chosen in the process of EDS. It can be seen that there are bright and gray contrast areas in image. According to EDS results, the gray contrast areas are primary α-Mg phase (A), the coarse acicular areas are Mg2Sn+Mg3Bi2 structure(B), and the bright contrast areas are α-Mg+Mg2Sn+Mg3Bi2 eutectic structure (C). Compared to Mg-39Bi-17Sn alloys without cycle, the volume fraction of primary α-Mg phase in alloy decrease, while the volume fraction of primary Mg2Sn+Mg3Bi2 structures increase. The α-Mg+Mg2Sn+Mg3Bi2 eutectic structure of Mg-39Bi-17Sn alloy are coarser after 300 thermal cycling.
Fig. 14. EPMA micrographs of Mg-39Bi-17Sn alloys after 300 thermal cycling Table 4 Composition of intermetallic phases in EPMA images after 300 thermal cycling Chemical composition（at.%）
Selected area
Mg
Bi
Sn
A B C
96.87 57.85 68.25
0.51 `16.74 16.32
2.62 25.41 15.43
Closest phase α-Mg Mg2Sn+Mg3Bi2 α-Mg+Mg2Sn+Mg3Bi2
Fig. 15 illustrates the DSC thermograms of the Mg-39Bi-17Sn alloys after 300 thermal cycles. It can be seen that the solidus temperature increase from 515.8℃ to 517.6℃, while the melting enthalpy decrease from 168.8J/g to 161.5 J/g after 300 thermal cycles. The melting enthalpy decreases by about 4.3%. The reason may be as follows: the melting of the alloy generally begins at the grain boundary or phase interface. The grains of the alloy becomes coarser after repeated melting-solidification, the grains become coarser, the phase interface in alloys decrease, resulting in less energy required to melt the alloy. Therefore, the melting enthalpy and solidus temperature of the alloy decreases. It can be considered that the Mg-39Bi-17Sn alloy has a good thermal stability.
Fig. 15. DSC thermograms of the Mg-39Bi-17Sn alloys after 300 thermal cycling Fig. 16 displays the relative elongation and density of Mg-39Bi-17Sn alloys at different temperatures before and after 300 thermal cycling. The experimental uncertainties are estimated to be at most 4.2% and 5.3%, respectively. As seen
on Fig.13(a), the relative elongation of two alloys is close when the temperature is lower than 100℃。The relative elongation of Mg-39Bi-17Sn alloy after 300 thermal cycling is less than the alloy before thermal cycling. As seen on Fig.13(b), the density of alloys before and after thermal cycling is relatively close, which decrease with increasing temperature.
Fig. 16. The relative elongation(a) and density(b) of Mg-39Bi-17Sn alloys at different temperatures before and after 300 thermal cycling. Fig.17 shows the specific heat capacity and thermal diffusivity of Mg-39Bi-17Sn alloys as a function of temperature before and after 300 thermal cycling. The experimental uncertainties are estimated to be at most 2.0% and 0.7%, respectively. It can be seen that the specific heat capacity and thermal diffusivity of alloys all decrease after 300 thermal cycles.
Fig.17. Specific heat capacity(a) and thermal diffusivity(b) of Mg-39Bi-17Sn alloys as a function of temperature before and after 300 thermal cycling. Fig. 18 depicts the thermal conductivity of Mg-39Bi-17Sn alloys as a function of temperature before and after 300 thermal cycling. It can be seen that the thermal conductivity of alloy changes with temperature similar to that before the
cycle. Thermal conductivity of Mg-39Bi-17Sn alloys decrease by about 5%-8% after 300 thermal cycles. The reason may be as follows: oxidation and impurity precipitation may occur during the thermal cycling, lead to an increase of the second phase and defects in the alloy, resulting in a decrease in thermal conductivity.
Fig. 18. Thermal conductivity of Mg-Bi-Sn alloys as a function of temperature before and after 300 thermal cycling.
4. Conclusions
In summary, several types of Mg-Bi-Sn alloys were fabricated and considered as PCMs for TES. The microstructure and thermal properties were measured and the relationship between them was also discussed in the paper. The results are as follows: (1) The Mg-33Bi-17Sn, Mg-39Bi-17Sn and Mg-45Bi-17Sn alloys mainly consist of primary α-Mg phase and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. The Mg-33Bi-23Sn alloy mainly consist of primary α-Mg phase, primary Mg2Sn+Mg3Bi2 phases and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. (2) The melting enthalpies of Mg-33Bi-17Sn, Mg-39Bi-17Sn, Mg-45Bi-17Sn and Mg-33Bi-23Sn alloys are 18.5, 168.8, 106.3 and 140.3J/g, with the melting temperatures in temperature range 515°C to 525°C, respectively. The Mg-39Bi-17Sn alloy has the highest melting enthalpy that may due to the higher proportion of eutectic α-Mg+Mg2Sn+Mg3Bi2 structure. (3) The thermal conductivity of alloys studied increase as temperature increasing from 50℃ to 350℃, and then it has a decline up to 400°C. The thermal conductivity of Mg-33Bi-17Sn is similar to that of Mg-33Bi-23Sn alloy, and is less than that of Mg-39Bi-17Sn alloy at the same temperature. The thermal conductivity of Mg-45Bi-17Sn alloy is the highest among four alloys. (4) The solidus temperature of Mg-39Bi-17Sn alloys increase by about 1.8℃, and the melting enthalpy decreases by
about 4.2% after 300 thermal cycling. Thermal conductivity of Mg-39Bi-17Sn alloys decrease by about 5%-8%. It can be noted that the Mg-39Bi-17Sn alloy has good thermal stability as a PCM. (5) Base on the thermophysical characterization of four Mg-Bi-Sn alloys, the Mg-39Bi-17Sn alloy is expected to be used as thermal energy storage material because of its high melting enthalpy and thermal conductivity. Moreover, the compatibility of Mg-Bi-Sn alloy with constructional materials at high temperature has been researched and the paper about this will be published in the future.
Acknowledgement
This work was supported by the National Key Technology Research & Development Program of China (Grant No. 2012BAA05B05), the National Natural Science Foundation of China (No. 51604161 and No. 61605148).
References
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The microstructure and thermophysical performance of Mg-Bi-Sn alloys are determined. The relationship between melting enthalpies and phase composition are studied. The thermal stability of Mg-39Bi-17Sn alloy after thermal cycling is investigated. Mg-39Bi-17Sn alloy is proposed as a phase change material at high temperature.
Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
S0042-207X(20)30001-4
DOI:
https://doi.org/10.1016/j.vacuum.2020.109164
Reference:
VAC 109164
To appear in:
Vacuum
Received Date: 17 October 2019 Revised Date:
27 December 2019
Accepted Date: 31 December 2019
Please cite this article as: Fang D, Cheng X, Ye X, Wu H, Zhang C, Xu K, Wang N, Study on thermophysical performance of Mg-Bi-Sn phase-change alloys for high temperature thermal energy storage, Vacuum, https://doi.org/10.1016/j.vacuum.2020.109164. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Study on thermophysical performance of Mg-Bi-Sn phase-change alloys for high temperature thermal energy storage Dong Fang a,b, Xiaomin Chengb,*,Xicong Ye a,*,Haihua Wu a,Congyang Zhang a,Kai Xub, Ning Wangc a
Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance, China Three Gorges University, Yichang 443002, China
b
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
c
National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan 430070, China
Abstract: Mg-based alloys are potential high temperature phase change materials for thermal energy storage due to their high melting enthalpy, high thermal conductivity and excellent compatibility with Fe-based containment materials. This work mainly focuses on studying the
microstructure and thermophysical characterization of Mg-Bi-Sn alloys phase change materials with 4 different components and their thermal
energy storage performance. The experimental results show that the Mg-33Bi-17Sn, Mg-39Bi-17Sn and Mg-45Bi-17Sn alloys mainly consist of
primary α-Mg phase and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure, and the Mg-33Bi-23Sn alloy mainly consist of primary α-Mg phase, primary Mg2Sn+Mg3Bi2 phases and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. Their melting enthalpies are respectively 18.5 J/g, 168.8 J/g, 106.3 J/g and 140.3 J/g, while their melting temperatures are about 515°C ~ 525°C. As from the results, the Mg-39Bi-17Sn alloy obtains the highest melting
enthalpy, which attributes to its higher proportion of eutectic α-Mg+Mg2Sn+Mg3Bi2 structure. However, the Mg-45Bi-17Sn alloy shows the highest thermal conductivity. In addition, the melting point of Mg-39Bi-17Sn alloys increase by about 1.8℃, and the melting enthalpy decreases
by about 4.2% after 300 thermal cycling. Based on all results, the Mg-39Bi-17Sn alloy shows with great thermalphsycial performance and is
expected to be used as thermal energy storage material.
Keywords: Latent heat storage; Phase change material; Metallic materials; Mg-Bi-Sn alloy
1. Introduction
Solar energy promotes and promises sustainable development due to its proverbial advantages (e.g. widespread, non-toxic, sustainable)
[1, 2]
. However, the distribution of solar energy is intermittent in time and space, therefore, thermal energy
storage (TES) becomes a key link in solar thermal utilization [3-5]. As heat carrier, phase change materials (PCMs) based on latent heat storage (LHS) used in TES systems are being widely studied around the world because of its high heat storage density and the characteristics of releasing heat at nearly constant temperature [6].
The operating temperature of PCMs are normally classified as three temperature regions, low temperature (<90℃), medium temperature (90-400℃) and high temperature (>400℃). For solar thermal power generation, their effective working temperature are often between 120 -1000℃[7]. Based on the working temperature range above, medium and high temperature PCMs are considered for TES systems, such as molten salts, metals and alloys. Kenisarin [7] et al and Zalba [8] et al summarized inorganic substances and organic substances, with melting point from ambient temperature to 1000℃. The most well-studied binary inorganic salt is the NaNO3-KNO3 and the most representative of them is the solar salt, which has operation temperature range from to 300℃ to 550℃, can be used as PCMs for tower solar power station. However, several defects of inorganic salt limit its application, such as low thermal conductivity, strong corrosive effect on packaging materials, and large volume changes before and after phase change. In order to address these issues of inorganic salts, metals and alloys are considered as the high temperature PCMs for TES. The advantages of metallic PCMs are naturally higher thermal conductivity and lower thermal expansion coefficient than inorganic salts. The reports on metallic PCMs have been published for long time, for instance, many binary and ternary metallic eutectic alloys with melting temperatures above 300℃ were reported [9,10]. In recent years, the research on metallic PCMs focused on aluminum alloys. He and Zhang [11] studied the heat storage properties of two components Al-Si alloy. It was found that the phase transition temperatures of AlSi12 and AlSi20 were 576 °C and 585 °C, respectively, and latent heat was 560 J/g and 460 J/g. For the long time cycling [12], the thermal reliability results showed that the melting temperature and the latent heat of fusion of Al-34.02Mg-6.62Zn (wt. %) decreased 3.1-5.3 K and 10.98% after 1000 times thermal cycles, respectively. Takahiro Nomura[13] studied the thermal properties of Al-Si alloys for high-temperature (above 500℃) PCMs and the corrosion behavior of the alloys on ceramic materials. Based on their research, it is feasibility to use Al-Si alloys as the PCMs and ceramics as container materials. Nevertheless, the serious corrosiveness of aluminum alloys PCMs to iron-based container material limits its application in LHS system [14]. According to Mg-Fe phase diagram, liquid aluminum alloy has corrosive effect on iron-based containers from 400℃ to 600℃. Mg and Al alloys have similar melting temperature, high latent heat of fusion and thermal conductivity, and thus Mg alloys can also be considered as PCMs. Magnesium alloys are generally used as structural materials [15,16], there are few researches on Mg alloys PCMs. The
thermophysical characterizations of Mg-51Zn and Mg-24.9Zn-5.1Al alloys were investigated by Rodríguez-Aseguinolaza et al [17-19]. Bi and Sn have low melting point, high heat storage density and good heat stability. Hence, they can be used as PCMs. The heat storage properties of Mg-Bi and Mg-Sn alloys were investigated by our previous research [20, 21], and the melting enthalpy of Mg-54Bi, Mg-37Sn were also proved to be higher than others. According to the requirement of operating temperature range of the heat storage material in the solar thermal power generation system, the Mg alloy materials with a wide temperature range should be prepared and studied. The melting temperature of Mg-Bi-Sn is lower than that of Mg-Bi (550-560 °C), Mg-Sn (555-560 °C) alloy, it has potential to be used as heat storage material to satisfy with the demand of different heat storage systems. In the present work, the novel Mg-Bi-Sn phase-change alloys with different proportions were prepared by gravity casting method, and their microstructures were characterized by XRD, SEM, TEM and the effect on phase change enthalpy of alloys was also investigated to explore the phase change heat storage mechanism. Finally, their thermophysical performance as PCMs for TES were measured and the optimal components of the phase change alloys was confirmed. Finally, the microstructure and heat storage properties of Mg-39Bi-17Sn alloy after thermal cycling were studied to ensure thermal stability of the PCMs during long-term service.
2. Experimental setup
Four types of Mg-Bi-Sn alloys were smelted from pure magnesium ingot (99.98 wt. %), bismuth ingot (99.99% wt. %) and tin ingot (99.99 wt. %) in furnace resistance under protected by Ar atmosphere at 700°C. The furnace was vacuum pumping to 0.1 torr before the Ar argon was added. The melt was refined by RJ-2 flux. The nominal compositions of Mg-Bi-Sn alloys are given in Table 1. The compositions of RJ-2 flux and coating agent are given in our previous investigation [18, 19]. It was then static melting at 580 °C for about 20 min, and casted into steel mould. The preheating temperature of steel mould was 200 °C. The size of casting mould cavity is φ30 mm×100 mm. Finally, the samples were polished and etched in nitric acid alcohol (4ml nitric acid+96ml ethyl alcohol) for the next observation.
Table 1 The nominal compositions of Mg-Bi-Sn alloys Samples
Alloy
a b c d
Mg-33Bi-17Sn Mg-39Bi-17Sn Mg-45Bi-17Sn Mg-33Bi-23Sn
Composition (wt. %) Mg
Bi
Sn
50 44 38 44
33 39 45 33
17 17 17 23
The microstructure and composition were examined by X-ray diffraction (XRD, D8 Adwance, BRUKER AXS, Germany), electron probe micro-analysis (EPMA, JXA-8230, JEOL, Japan) with an energy-dispersive X-ray spectrometer (EDS,NCAX-ACT, JEOL, Japan) and transmission electron microscope(TEM, Talos F200S, Thermo Fisher, USA) with an EDS. The melting temperature, latent heat and specific heat capacity of the composites were tested by Differential scanning calorimeter (DSC, STA449C/3/G, Netzsch, Germany) in an Ar atmosphere. The heating rate was 5 K/min, and testing temperature range was from room temperature to 600 °C. The thermal diffusivity measurements were measured by Nanoflash device(LFA457, Netzsch ,Germany) over the temperature range of 50-400°C basing on Laser-flash method. The size of block sample was 10mm×10mm×2.5mm. The testing temperature interval was set to 50°C and repeated least three measurements at each temperature points. The relative elongation of samples were measured in an Ar atmosphere using pushrod type dilatometer (DIL 402C, Netzsch ,Germany) in the range of 30 °C to 450 °C. The density at different temperatures is calculated by the following equation [20]: ρ = ρ0 (1 +△L/L0)-3
(1)
where ρ0 is the density of the alloy at 20 °C obtained by the Archimedes method, and ∆L/L0 is the relative elongation. The thermal conductivity can be calculated from the equation: λ = a·ρ·cp
(2)
where a is the thermal diffusivity, ρ is the density and cp is the specific heat at constant pressure. The Mg-39Bi-17Sn alloy was chosen for thermal cycling experiments due to its best thermalphsycial performance among the four alloys. Thermal cycling experiment is as follows: the alloy was placed in a stainless-steel tube with a
diameter of 40 mm and closed at both ends. The stainless-steel tube was placed in resistance furnace at 600°C for 20 minutes. Then they were placed outside for 20minutes air cooling. That is one thermal cycling. A stainless-steel tube was taken out after 300 cycles, and the upper of steel tube was cut off. After that the steel tube was heated to melt the magnesium alloy. Finally, the magnesium alloy was cast into the metal mold. The XRD, EMPA, melting enthalpy, and thermal conductivity of samples were tested after 300 cycles.
3. Results and discussion
Fig. 1 depicts the XRD pattern of Mg-Bi-Sn alloys. From Fig. 1, it is known that the four alloys have α-Mg, Mg3Bi2 and Mg2Sn phases. When the diffraction angle(2θ) is 26°, compared with the Mg-33Bi-17Sn alloy, the diffraction peak height in Mg-39Bi-17Sn alloy is significantly enhanced, and the diffraction peak height in Mg-45Bi-17Sn alloy is slightly decreased. The reason for this phenomenon may be the increase of Bi content leads to the increase of Mg3Bi2 phase. The diffraction peak height (2θ=23°,37°,44°) of Mg2Sn phase increases from a alloy to d alloy. It is possible that the proportion of Mg2Sn phase has changed due to the difference of the ratio of Mg to Sn.
Fig. 1. XRD pattern of the Mg-Bi-Sn alloys Fig. 2 shows the EPMA secondary electron images of the four alloys. Table 2 shows the elemental compositions of the intermetallic obtained from EDS. The point(C,F point) and region(A,B,D,E,G,H,J,K,L) were chosen in the process of EDS. The EPMA image of the Mg-33Bi-17Sn, Mg-39Bi-17Sn and Mg-45Bi-17Sn in Fig. 2(a,b,c) reveals that the microstructure of these three alloys consisting of a mixture of bright and gray contrast areas. The volume fraction of the bright areas is estimated to be 16.5%, 57.4% and 43.5%, respectively. Combined with the high magnification images, XRD patterns, and
the results of EDS, the black dendrite shape area is composed of primary α-Mg phase (A, D, G), and the grey punctate shape area is composed of α-Mg+Mg2Sn+Mg3Bi2 eutectic structure (B,E,H). The composition of point I reveal that there are flaky Mg3Bi2 in Mg-45Bi-17Sn alloy. The Fig(d) displays that there is an another bright phase(L) in Mg-33Bi-23Sn alloy except the two phases described above. According to the XRD and FTIR results, the bright phase is primary Mg2Sn+Mg3Bi2 phase, which are polygons with 50-80µm on a side. The volume fraction of the α-Mg+Mg2Sn+Mg3Bi2 eutectic structure areas is estimated to be 60.7%.
Fig. 2. EPMA micrographs of Mg-Bi-Sn alloys Table 2 Composition of intermetallic phases in EPMA images. Chemical composition（at.%）
Selected area
Mg
Bi
Sn
A B C D E F G H I J K L
98.52 84.75 77.94 97.16 89.04 65.76 97 82.56 59.92 95.89 79.12 64.73
2.57 3.09 0.76 11.48 16.95 0.81 8.08 40.08 1.03 10.77 25.41
1.48 12.68 18.97 2.08 11.90 17.29 2.19 9.36 3.08 10.11 9.86
Closest phase α-Mg α-Mg+Mg2Sn+Mg3Bi2 α-Mg+Mg2Sn+Mg3Bi2 α-Mg α-Mg+Mg2Sn+Mg3Bi2 α-Mg+Mg2Sn+Mg3Bi2 α-Mg α-Mg+Mg2Sn+Mg3Bi2 Mg3Bi2 α-Mg α-Mg+Mg2Sn+Mg3Bi2 Mg2Sn+Mg3Bi2
Fig. 3 shows the TEM images of Mg-39Bi-17Sn alloy. Table 3 shows the compositions of the phases obtained from EDS. According to Fig. 3 and Table 3, it can be deduced that the black area (point a) and grey area (point b) are eutectic Mg2Sn+Mg3Bi2 and eutectic α-Mg phases, respectively. These two phases are alternately distributed. The spacing between two adjacent black phases is about 0.5-5µm.
Fig. 3. TEM images of Mg-39Bi-17Sn alloy Table 3 Composition of phases in TEM images. Composition（.at%） Selected area a b
Closest phase Mg
Bi
Sn
41.8 97.6
42.3 0.3
15.8 2.1
Mg2Sn+Mg3Bi2 α-Mg
Fig. 4 shows the high resolution TEM morphology and selective electron diffraction photographs of Mg-39Bi-17Sn alloy. Selected-area diffraction patterns in alloy reveal that the black areas mainly consist of Mg3Bi2. The significant diffraction spot of Mg2Sn are not found. It is possible to consider that the content of Mg2Sn is too low lead to the diffraction spot is too weak. Combined with EDS above, the SADPs in Fig. 3(B) and (C) obtained from the bright and dark phases in Fig. 3(A) correspond to the [-3 6 -3 0] and [0 0 0 1] zone axes of the Mg3Bi2 and α-Mg phases, respectively.
Fig. 4. High resolution TEM morphology and selective electron diffraction photographs of Mg-39Bi-17Sn alloy: (A) high resolution TEM；(B) selective electron diffraction photograph of Mg3Bi2; (C) selective electron diffraction of Mg3Bi2 Fig. 5 displays the Mg-Bi-Sn ternary system phase diagram and the positions (a-d points) that represent the composition of the four alloys. The a, b, c and d points show the simple a, b, c and d, respectively. As can be seen from the figure, the point b is located at the eutectic composition point E3, where eutectic transformation occurs: L→α-Mg+Mg2Sn+Mg3Bi2. However, in this study, there will be primary phases due to the solidification in non-equilibrium, which changes the growth mode of the eutectic structure, so that the dendritic phase (primary α-Mg) is precipitated as the primary phase, and then the eutectic structure is formed. Hence, the Mg-39Bi-17Sn alloy mainly consists of primary α-Mg phases and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. The three other alloys locate around the point E3 of composition, the microstructure of them mainly also contain primary α-Mg phases and eutectic structure. The Mg-33Bi-23Sn alloy contain primary Mg2Sn+Mg3Bi2 phase berceuse of high Sn content.
Fig. 5. The phase diagram of the Mg-Bi-Sn alloy [23]
Fig. 6. DSC thermograms of Mg-Bi-Sn alloys Fig. 6 shows the DSC thermograms of the Mg-Bi-Sn alloys. As can be seen, the solidus temperature of Mg-33Bi-17Sn, Mg-39Bi-17Sn, Mg-45Bi-17Sn and Mg-33Bi-23Sn alloys are 524.1℃, 515.8℃, 515.2℃ and 515.0℃, respectively. The solidus temperature of Mg-33Bi-17Sn alloy is higher than three others because the alloy melting is inconsistent. The solidus temperature of Mg-Sn, Mg-Bi alloys are 545℃-560℃. Compared with Mg-Sn and Mg-Bi alloys, the solidus temperature of ternary alloys decreased by 30℃-45℃. Mg-39Bi-17Sn and Mg-33Bi-17Sn have the largest and the lowest latent heat of 168.8J/g, 18.5J/g respectively. The composition of b sample close to the eutectic composition of the Mg-Bi-Sn alloy (E3 point). This result suggests that the latent heat of eutectic composition is higher than others in the same alloy system. The similar conclusion was concluded in our previous research and Takahiro Nomura’s study
[13]
. To further explore the heat
storage mechanism of the alloy, the relationship between phase change enthalpies and proportion of eutectic structure were investigated shown as Fig.7. From the Fig.7, it can be seen that the changing trend of enthalpies and proportion of eutectic structure were generally positively correlated. So it is expected that the Mg-39Bi-17Sn has higher latent heat because it
contains more proportion of α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. The reasons may be as follows: these four alloys are all prepared under almost identical conditions. The main difference between a, b and c samples is the proportion of eutectic structure to primary α-Mg phases. The essence of alloy melting is the breaking of metal bonds when the energy is provided outside. The primary α-Mg phase contains Mg-Mg bond, the α-Mg+Mg2Sn+Mg3Bi2 eutectic structure contain Mg-Mg bond, Mg-Sn bond and Mg-Bi bond. The greater the bond energy, the greater the heat required to break the bond energy. The closer the atomic spacing in the unit cell, the higher bond energy between atoms is. The atomic spacing of Mg-Sn, Mg-Bi are less than Mg-Mg, so the bond energies of Mg-Sn bond and Mg-Bi bond are higher than which of Mg-Mg. The bond energies of eutectic structure are higher than primary α-Mg phases. Mg-39Bi-17Sn contains more eutectic structure, so it has higher melting enthalpy. Mg-39Bi-17Sn contains more eutectic structure, so it has higher melting enthalpy. Mg-39Bi-17Sn contains more eutectic structure, so it has higher melting enthalpy. Comparing Mg-39Bi-17Sn with Mg-33Bi-23Sn, the proportion of eutectic structure of Mg-33Bi-23Sn and Mg-39Bi-17Sn are almost the same, however, the melting enthalpy of Mg-33Bi-23Sn is 17% lower than Mg-39Bi-17Sn. That may because the polygon primary Mg2Sn+Mg3Bi2 phase affected the structure of the alloy, which leads to the decrease of the melting enthalpy.
Fig. 7. Phase change enthalpies and proportion of eutectic structure of Mg-Bi-Sn alloys
Fig. 8. The relative elongation of Mg-Bi-Sn alloys 4.4 4.0 (c)
Density[g⋅ cm-3]
3.6 3.2
(b)
2.8
(d)
2.4 2.0
(a)
1.6 0
100
200
300
400
500
Temperature[ ]
Fig. 9. Density of Mg-Bi-Sn alloys at different temperatures The relative elongations of Mg-Bi-Sn alloys are given in Fig. 8. The experimental uncertainty is estimated to be at most 4.2%. The result reveals that the relative elongation of all alloys increases with temperature. The increase rate of relative elongation of four alloys slow down after 350℃. Compared to the other three alloys, the relative elongation increasing rate of Mg-33Bi-23Sn reduces significantly. The relative elongation of Mg-33Bi-17Sn is similar to these of Mg-39Bi-17Sn and Mg-45Bi-17Sn, and Mg-33Bi-23Sn has less elongation than these of three alloys above at the same temperature. The reason may be as follows: the Mg-Bi-Sn alloys are composed of multiple phases, which are α-Mg, Mg2Sn and Mg3Bi2 phases. The relative elongation depends on the properties of these phases. The Mg-33Bi-23Sn alloy contain more primary Mg2Sn+Mg3Bi2 phases than other alloys, so it is inferred that the relative elongation of Mg2Sn and Mg3Bi2 phases is less than α-Mg phase. Tian Xizhi[24] considered that the greater the concentration coefficient, the larger the relative elongation of crystal. The concentration coefficient of α-Mg is higher than Mg2Sn and Mg3Bi2 phases, so the relative elongation of Mg2Sn and Mg3Bi2 phases is less than α-Mg phase. The densities of Mg-Bi-Sn alloys at different temperatures are shown in Fig. 9. The experimental uncertainty is estimated to be at most 2.0 %. The densities at room temperature are measured by Archimedean principle. According to equation (1), the densities of Mg-Bi-Sn alloys at different temperature are calculated. It is obvious that the density of all alloys decreases as temperature increasing. The density of bulk Mg, Sn and Bi under room temperature are 1.738 g/cm3, 7.265 g/cm3 and 9.780 g/cm3, respectively [25]. The density of these three pure metals is also one of the factors affecting the density of the alloys. The higher the content of Mg in alloys, the lower the density, while the higher the content of Bi in alloys, the larger the density. Therefore, the Mg-33Bi-17Sn and Mg-45Bi-17Sn have least and largest density
among all alloys under the same temperature, respectively.
Fig. 10. Thermal diffusivity of Mg-Bi-Sn alloys as a function of temperature Fig. 10 shows thermal diffusivity of Mg-Bi-Sn alloys as a function of temperature. The experimental uncertainty is estimated to be at most 1.0%. As clearly seen from this figure, thermal diffusivity of all alloys raise with increase of temperature from 50℃ to 350℃, then it has a decrease after 350℃. This is mainly due to Mg2Sn and Mg3Bi2 precipitates dissolve while the temperature of phase boundary reaches a certain value [22]. A decrease of thermal diffusivity is induced by the reversible process as increasing temperature. It can be seen that the thermal diffusivity of Mg-33Bi-23Sn is less than these of three other alloys at the same temperature, which can be attributed to it contained more proportion of α-Mg phases.
Fig. 11. Specific heat capacity of Mg-Bi-Sn alloys as a function of temperature The specific heat capacities of Mg-Bi-Sn alloys at various temperatures were measured and the specific heat capacity data are shown in Fig. 11, in which the error bars are also shown. The experimental uncertainties are estimated to be at most 3.0%. As we can see in Fig. 11, the specific heat capacities of all alloys raise with increasing temperature from 50℃ to 400℃. The reason is as follows: the calculated values of heat capacity can be obtained by the Neumann-Kopp rule, which consider that the heat capacity of alloy depends on the elements it contains. The specific heat of Mg, Bi and Sn displays an increasing trend from 20 ºC to 400 ºC. The four alloys all contain Mg, Bi and Sn. So it can be concluded that the specific
heat capacities of all alloys increase when the temperature rises. Moreover, the Mg-33Bi-17Sn has largest specific heat capacity, and Mg-45Bi-17Sn has least specific heat capacity among the four alloys, which can be attributed to difference of Mg content.
Fig. 12. Thermal conductivity of Mg-Bi-Sn alloys as a function of temperature According to equation (2), the thermal conductivity can be calculated by the thermal diffusivity, density and heat capacity of alloys. The calculation results are shown as Fig. 12. The experimental uncertainties are estimated to be at most 6%. As seen in Fig. 12, the thermal conductivity increases as temperature increases from 50℃ to 350℃, and then it has a decline up to 400°C. The trend of thermal conductivity with temperature is similar to thermal diffusivity. That is because the density of all alloys decreases with increasing temperature, while the specific heat capacity increases when temperature rises. Combine two opposite factors, the product of them did not change much when temperature changes. In other words, the thermal diffusivity is a leading factor in the calculation of thermal conductivity. As seen on Fig.12, the thermal conductivity of Mg-33Bi-17Sn is close to that of Mg-33Bi-23Sn alloy, and is less than that of Mg-39Bi-17Sn alloy at the same temperature. The thermal conductivity of Mg-45Bi-17Sn alloy is highest among four alloys. The reasons for this phenomenon are as follows: the volume fraction of α-Mg is the greatest influence on thermal conductivity in alloys. Based on previous estimates, the volume fraction of α-Mg phase of Mg-33Bi-17Sn, Mg-39Bi-17Sn, Mg-45Bi-17Sn and Mg-33Bi-23Sn is about 83.5%, 42.6%, 56.5% and 39.3%.In addition to the Mg-33Bi-17Sn alloy, the thermal conductivity of other three alloys increases as the volume fraction of α-Mg increasing. The reason for the lower thermal conductivity of the Mg-33Bi-17Sn alloy may be it has lowest density in all alloys. Kinetic molecular theory proposed that the thermal conductivity of materials increases when the mean free path of electrons and
phonons raise. For Mg-Bi-Sn alloy, Bi, Sn in solid solution may act as scattering centers for electrons and phonons, which limit the mean free path of electrons and phonons
[26, 27]
. In addition, the increase of volume fraction of Mg2Sn+Mg3Bi2
phases form more interfaces that hinder the transmission of phonons and electrons, resulting in a decrease in thermal conductivity. Therefore, the thermal conductivity of Mg-39Bi-17Sn, Mg-45Bi-17Sn and Mg-33Bi-23Sn are 49, 56, 33 W/mK at 400℃, respectively. 3.2 The microstructures and thermophysical performance of Mg-39Bi-17Sn alloy after thermal cycling Fig. 13 shows the XRD pattern of the Mg-39Bi-17Sn alloys before and after 300 thermal cycling. It can be seen that the phase composition of the alloy before and after the thermal cycle has not changed. However, the diffraction peak height of the phase in alloy after the cycle becomes smaller.
Fig. 13. XRD pattern of the Mg-39Bi-17Sn alloys before and after 300 thermal cycling Fig. 14 reveals the EPMA secondary electron image of the Mg-39Bi-17Sn alloy after 300 thermal cycling. Table 4 shows the elemental compositions of the intermetallic obtained from EDS. The region (A, B, C) were chosen in the process of EDS. It can be seen that there are bright and gray contrast areas in image. According to EDS results, the gray contrast areas are primary α-Mg phase (A), the coarse acicular areas are Mg2Sn+Mg3Bi2 structure(B), and the bright contrast areas are α-Mg+Mg2Sn+Mg3Bi2 eutectic structure (C). Compared to Mg-39Bi-17Sn alloys without cycle, the volume fraction of primary α-Mg phase in alloy decrease, while the volume fraction of primary Mg2Sn+Mg3Bi2 structures increase. The α-Mg+Mg2Sn+Mg3Bi2 eutectic structure of Mg-39Bi-17Sn alloy are coarser after 300 thermal cycling.
Fig. 14. EPMA micrographs of Mg-39Bi-17Sn alloys after 300 thermal cycling Table 4 Composition of intermetallic phases in EPMA images after 300 thermal cycling Chemical composition（at.%）
Selected area
Mg
Bi
Sn
A B C
96.87 57.85 68.25
0.51 `16.74 16.32
2.62 25.41 15.43
Closest phase α-Mg Mg2Sn+Mg3Bi2 α-Mg+Mg2Sn+Mg3Bi2
Fig. 15 illustrates the DSC thermograms of the Mg-39Bi-17Sn alloys after 300 thermal cycles. It can be seen that the solidus temperature increase from 515.8℃ to 517.6℃, while the melting enthalpy decrease from 168.8J/g to 161.5 J/g after 300 thermal cycles. The melting enthalpy decreases by about 4.3%. The reason may be as follows: the melting of the alloy generally begins at the grain boundary or phase interface. The grains of the alloy becomes coarser after repeated melting-solidification, the grains become coarser, the phase interface in alloys decrease, resulting in less energy required to melt the alloy. Therefore, the melting enthalpy and solidus temperature of the alloy decreases. It can be considered that the Mg-39Bi-17Sn alloy has a good thermal stability.
Fig. 15. DSC thermograms of the Mg-39Bi-17Sn alloys after 300 thermal cycling Fig. 16 displays the relative elongation and density of Mg-39Bi-17Sn alloys at different temperatures before and after 300 thermal cycling. The experimental uncertainties are estimated to be at most 4.2% and 5.3%, respectively. As seen
on Fig.13(a), the relative elongation of two alloys is close when the temperature is lower than 100℃。The relative elongation of Mg-39Bi-17Sn alloy after 300 thermal cycling is less than the alloy before thermal cycling. As seen on Fig.13(b), the density of alloys before and after thermal cycling is relatively close, which decrease with increasing temperature.
Fig. 16. The relative elongation(a) and density(b) of Mg-39Bi-17Sn alloys at different temperatures before and after 300 thermal cycling. Fig.17 shows the specific heat capacity and thermal diffusivity of Mg-39Bi-17Sn alloys as a function of temperature before and after 300 thermal cycling. The experimental uncertainties are estimated to be at most 2.0% and 0.7%, respectively. It can be seen that the specific heat capacity and thermal diffusivity of alloys all decrease after 300 thermal cycles.
Fig.17. Specific heat capacity(a) and thermal diffusivity(b) of Mg-39Bi-17Sn alloys as a function of temperature before and after 300 thermal cycling. Fig. 18 depicts the thermal conductivity of Mg-39Bi-17Sn alloys as a function of temperature before and after 300 thermal cycling. It can be seen that the thermal conductivity of alloy changes with temperature similar to that before the
cycle. Thermal conductivity of Mg-39Bi-17Sn alloys decrease by about 5%-8% after 300 thermal cycles. The reason may be as follows: oxidation and impurity precipitation may occur during the thermal cycling, lead to an increase of the second phase and defects in the alloy, resulting in a decrease in thermal conductivity.
Fig. 18. Thermal conductivity of Mg-Bi-Sn alloys as a function of temperature before and after 300 thermal cycling.
4. Conclusions
In summary, several types of Mg-Bi-Sn alloys were fabricated and considered as PCMs for TES. The microstructure and thermal properties were measured and the relationship between them was also discussed in the paper. The results are as follows: (1) The Mg-33Bi-17Sn, Mg-39Bi-17Sn and Mg-45Bi-17Sn alloys mainly consist of primary α-Mg phase and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. The Mg-33Bi-23Sn alloy mainly consist of primary α-Mg phase, primary Mg2Sn+Mg3Bi2 phases and α-Mg+Mg2Sn+Mg3Bi2 eutectic structure. (2) The melting enthalpies of Mg-33Bi-17Sn, Mg-39Bi-17Sn, Mg-45Bi-17Sn and Mg-33Bi-23Sn alloys are 18.5, 168.8, 106.3 and 140.3J/g, with the melting temperatures in temperature range 515°C to 525°C, respectively. The Mg-39Bi-17Sn alloy has the highest melting enthalpy that may due to the higher proportion of eutectic α-Mg+Mg2Sn+Mg3Bi2 structure. (3) The thermal conductivity of alloys studied increase as temperature increasing from 50℃ to 350℃, and then it has a decline up to 400°C. The thermal conductivity of Mg-33Bi-17Sn is similar to that of Mg-33Bi-23Sn alloy, and is less than that of Mg-39Bi-17Sn alloy at the same temperature. The thermal conductivity of Mg-45Bi-17Sn alloy is the highest among four alloys. (4) The solidus temperature of Mg-39Bi-17Sn alloys increase by about 1.8℃, and the melting enthalpy decreases by
about 4.2% after 300 thermal cycling. Thermal conductivity of Mg-39Bi-17Sn alloys decrease by about 5%-8%. It can be noted that the Mg-39Bi-17Sn alloy has good thermal stability as a PCM. (5) Base on the thermophysical characterization of four Mg-Bi-Sn alloys, the Mg-39Bi-17Sn alloy is expected to be used as thermal energy storage material because of its high melting enthalpy and thermal conductivity. Moreover, the compatibility of Mg-Bi-Sn alloy with constructional materials at high temperature has been researched and the paper about this will be published in the future.
Acknowledgement
This work was supported by the National Key Technology Research & Development Program of China (Grant No. 2012BAA05B05), the National Natural Science Foundation of China (No. 51604161 and No. 61605148).
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The microstructure and thermophysical performance of Mg-Bi-Sn alloys are determined. The relationship between melting enthalpies and phase composition are studied. The thermal stability of Mg-39Bi-17Sn alloy after thermal cycling is investigated. Mg-39Bi-17Sn alloy is proposed as a phase change material at high temperature.
Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.