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Preparation, microstructure and thermal properties of MgBi alloys as phase change materials for thermal energy storage
Applied Thermal Engineering 92 (2016) 187–193
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Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Preparation, microstructure and thermal properties of Mg—Bi alloys as phase change materials for thermal energy storage Dong Fang, Zheng Sun, Yuanyuan Li, Xiaomin Cheng * School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
H I G H L I G H T S
• • • •
The microstructure and thermal properties of Mg—Bi alloys are determined. The relationship between melting enthalpies and phase composition are studied. The activation energy of Mg—54%Bi alloy is calculated by multiple DSC technology. Mg—54%Bi alloy is proposed as a phase change material at high (>420 °C) temperature.
A R T I C L E
I N F O
Article history: Received 25 June 2015 Accepted 21 September 2015 Available online 1 October 2015 Keywords: Metallic materials Mg—Bi alloy Latent heat storage Phase change material
A B S T R A C T
Comparing with Al-based phase change material, Mg-based phase change material is getting more and more attention due to its high corrosion resistance with encapsulation materials based on iron. This study focuses on the characterization of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi (wt. %) alloys as phase change materials for thermal energy storage at high temperature. The phase compositions, microstructure and phase change temperatures were investigated by X-ray diffusion (XRD), electron probe micro-analysis (EPMA) and differential scanning calorimeter (DSC) analysis, respectively. The results indicates that the microstructure of Mg—36%Bi and Mg—54%Bi alloys are mainly composed of α-Mg matrix and α-Mg + Mg3Bi2 eutectic phases, Mg—60%Bi alloy are mainly composed of the Mg3Bi2 phase and α-MgMg3Bi2 eutectic phases. The melting enthalpies of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys are 138.2, 180.5 and 48.7 J/g, with the phase change temperatures of 547.6, 546.3 and 548.1 °C, respectively. The Mg—54%Bi alloy has the highest melting enthalpy in three alloys. The main reason may be that it has more proportion of α-Mg + Mg3Bi2 eutectic phases. The thermal expansion of three alloys increases with increasing temperature. The values of the thermal conductivity decrease with increasing Bi content. Besides, the activation energy of Mg—54%Bi was calculated by multiple DSC technology. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Phase change materials (PCMs) are drawing worldwide increasing attention in thermal energy storage (TES) systems due to their high performance in energy storage density, energy conversion efficiency, storing and releasing thermal energy at nearly constant temperature [1,2]. Selection of PCMs for TES applications depends on thermal properties such as the operating temperature, heat capacity, thermal conductivity and thermal reliability. Hoshi et al. [3] classified the PCMs in high temperature range over 420 °C associated with a concentrating solar power system. Main emphasis on high temperature PCMs is molten salts such as alkali metal nitrates, alkali metal nitrites and their mixtures [4,5].
* Corresponding author. Tel.: +86 13507117513; Fax:+86 02787651779. E-mail address: [email protected] (X. Cheng). http://dx.doi.org/10.1016/j.applthermaleng.2015.09.090 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
However, one of the main drawbacks of inorganic molten salts is their low thermal conductivity, resulting in the need of a more sophisticated heat exchanger for charging and discharging processes of the TES system. An alternative to inorganic salts can be metal alloys [6]. Metal alloy PCMs were proposed as TES materials more than three decades ago [7,8], and have been experimentally studied to some extent for TES applications [9–11]. Recently, scientists have studied thermal physical properties of Al-based heat storage material. For instance, Birchenall et al. [8,12] analyzed thermal physical properties of binary and multiple alloys containing A1, Cu, Mg, Si, Zn and other elements, and considered that metal materials with high phase change latent heat value usually include high melting point elements, and found that the alloys that are rich in elements of Al and Si are ideal phase change heat storage materials. Huang et al. [13] determined the specific heat of liquid and solid forms band the latent heat of fusion of Al—Si, Al—Si—Mg and Al—Si—Cu alloys. Wang et al. [11] developed a novel high temperature space heater using Al—12%Si (wt.%) alloy as heat storage
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Table 1 The components of Mg—Bi alloys. Samples
a b c
Compounds
Chemical composition (wt.%)
Mg—36%Bi Mg—54%Bi Mg—60%Bi
Mg
Bi
O
63.51 45.26 39.95
36.03 54.15 59.72
0.46 0.59 0.33
medium. In addition, the research of Achard [14] indicated that Al—Mg alloy would be suitable for heat storage material at about 450 °C. The thermal reliability and corrosion behavior of Al—Mg—Zn alloy with respect to the number of thermal cycles for TES systems were studied by Sun et al. [9]. They found that the melting temperature changed by 3–5 K and the latent heat of fusion decreased after 1000 thermal cycles. One major drawback of Al-based alloys is that aluminum melt is highly corrosive to encapsulation materials based on iron [15], which limits its long-term and large-scale applications. Mg—Fe system is thermodynamically stable and immiscible in the concerned temperature range of 400–600 °C according to the phase diagram [16]. Most studies of domestic and overseas scholars focus on Al-based alloys as PCMs [9,11,14], and studies about phase change thermal storage materials based on magnesium are relatively lacking so far. Metal Mg has appropriate melting point, high melting enthalpy and good thermal conductivity, and metal Bi also has low melting point, high heat storage density and good heat stability. Hence, they can be used as PCMs. In this work we focused on thermal properties of three components of Mg—Bi alloys as PCM for TES applications from 400 °C to 600 °C. The phase composition, microstructure, and the significant thermal properties were investigated in order to provide reference for the design and application of magnesium alloy phase change thermal energy storage materials. 2. Experimental 2.1. Materials and preparation Pure magnesium ingot (99.98% purity) and bismuth ingot (99.99% purity) were used to prepare Mg—Bi alloys. The chemical compositions of Mg—Bi alloys measured by XRF are shown in Table 1. The synthesis of the alloy was melted (600 g melt) in a graphite crucible in a pit type resistance furnace. The RJ-2 flux refining agent was used in the melting process. The components of RJ-2 flux and coating agent are shown in Table 2. A high purity argon gas (99.999%) atmosphere was also offered in order to strictly prevent the specimens from oxidizing during the preparation process. The specimens with a diameter of 20 mm were cast in the iron mold (φ30 × 100 mm) preheated at 200 °C.
Differential scanning calorimetry (DSC, STA449C/3/G) analyses were performed at a constant heating rate of 5 K/min in the range of 25–600 °C under argon atmosphere. The linear thermal expansion coefficients of samples were measured by pushrod type dilatometer (DIL 402C) in the temperature range 30–450 °C at the heating rate of 5 K/min. The sample for dilatometry test was machined to dimensions of 5 × 5 × 20 mm. Laser-flash method (LFA 457) in the temperature range of 30– 400 °C was adopted for the thermal diffusivity measurements of the block sample with dimensions of 10 × 10 × 2.5 mm. The step of the testing temperatures was set to 50 °C, and at least three measurements were performed at each testing point. The density at high temperature was calculated using the relation [17]:
ρ = ρ0 (1 + ΔL L0 )
−3
(1)
where ρ0 is the density of the alloy at 20 °C, which can be measured by the Archimedes method, and ΔL/L0 is the relative elongation. The specific heat capacity was calculated using the Neumann– Kopp rule and published data [18]. The thermal conductivity can be obtained using the following Equation:
λ = a ⋅ ρ ⋅cp
(2)
where a is the thermal diffusivity, ρ the density and cp the specific heat capacity at constant pressure. 3. Results and discussion 3.1. Microstructure analysis Fig. 1 shows X-ray diffraction patterns of as-cast Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys. According to XRD patterns, it is confirmed that the phases of the alloys only consist of α-Mg and Mg3Bi2. As seen in Fig. 1, the phase structure is not changed in these Mg—Bi alloys. Fig. 1 displays that diffraction intensity of α-Mg gradually decreases as increasing Bi content. It may be the reason that the proportion of α-Mg phase decreases with increasing Bi content. Fig. 2 shows electron probe micro-analysis (EPMA) images of the three alloys. Table 3 shows compositions of the intermetallic phases exhibited in Fig. 2, obtained from EDS analysis. In the low magnification images of Mg—36%Bi and Mg—54%Bi alloys, it is seen that
2.2. Analysis methods The phases of the samples were analyzed by X-ray diffraction (XRD, D8 ADVANCE). Microstructural analysis was carried out using electron probe micro-analysis (EPMA, JXA-8230) equipped with energy-dispersive X-ray spectrometer (EDS) (INCAX-ACT).
Table 2 The components of RJ-2 flux. Composition (mass%)
RJ-2 flux Coating agent
MgCl2
KCl
NaCl
CaCl2
CaF2
BaCl2
43–55 65–74
20–30 10–20
20–30 15
3–5 3–5
10–15 4–7
3–5 –
Fig. 1. XRD patterns of Mg—Bi alloys.
D. Fang et al./Applied Thermal Engineering 92 (2016) 187–193
189
Fig. 2. EPMA images of Mg—Bi alloys.
the microstructure is mainly composed of two distinct phases, which are distinguished by black and gray. The black phase takes the dendrite shape and the gray phase takes punctate shape which can be clearly seen in high magnification images. Combined with the results of analysis of XRD (see Fig. 1) and EDS (see Table 3), it is believed that the black phase is primary α-Mg phase, and the gray phase is α-Mg + Mg3Bi2 eutectic phase. It can be seen that the phase compositions are the same in these two alloys, and the only difference is the proportion of the two phases in alloys. Combined with the results of analysis of XRD and EDS, Fig. 1(c) shows that the microstructure mainly composes of three phases. The gray punctate shape is α-Mg + Mg3Bi2 eutectic phase (I). The black phase around the gray punctate phase is α-Mg phase (H), the gray acicular and flake structure is primary Mg3Bi2 phase (E). According to Mg—Bi binary phase diagram seen Fig. 3, Bi has a relatively high solubility in the magnesium alloy. The solubility of Bi in Mg is about 8.85% at 551 °C, and when the temperature decreases to 200 °C, its solubility only reduces to below 1%. The eutectic point in Mg—Bi binary alloy is at 44.2% Mg and 55.8% Bi, at which eutectic reaction L→α-Mg + Mg3Bi2 takes place. The positions of Mg—36%Bi (point a) and Mg—54%Bi (point b) alloys are seen in phase diagram (Fig. 3). So a reaction: L→α-Mg takes place before eutectic
reaction occurred during alloy solidification. Then, the eutectic reaction L→α-Mg + Mg3Bi2 takes place in the rest of liquid-phase. Therefore, the primary α-Mg is formed in these two alloys. However, at point c (Mg—60%Bi) in the phase diagram, a reaction: L→Mg3Bi2 takes place before eutectic reaction occurred during alloy solidification. Therefore, the primary Mg3Bi2 phase is formed in the alloy. 3.2. Phase change temperatures and enthalpies Fig. 4 reveals the measured phase change temperatures and melting enthalpies of three compositions of Mg—Bi alloys. As shown in Fig. 3, only one endothermic peak is observed in the solid– liquid transition in agreement with the phase diagram. The data in Table 4 show that the melting enthalpies of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys are 138.2, 180.5 and 48.7 J/g, with the phase change temperatures of 547.6, 546.3 and 548.1 °C, respectively. The melting enthalpies of Mg—54%Bi alloy are higher than those of Mg—36%Bi and Mg—60%Bi. The reasons may be as follows: the preparation conditions of the Mg—Bi alloys are almost the same. The species and morphology of phases are the same in Mg—36%Bi and Mg—54%Bi alloys (Fig. 2a and b). The main difference is the proportion of the two phases. The proportion of α-Mg + Mg3Bi2 eutectic
Table 3 Chemical composition of intermetallic phases in Fig. 2 (in at.%). Phase
A
B
C
D
E
F
G
Mg Bi Closest phase
98.97 1.03 α-Mg
82.41 17.59 α-Mg + Mg3Bi2
98.94 1.06 α-Mg
80.74 19.26 α-Mg + Mg3Bi2
61.66 38.34 Mg3Bi2
95.06 4.94 α-Mg
80.82 19.18 α-Mg + Mg3Bi2
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Fig. 3. Phase diagram of Mg—Bi system [19]. Fig. 5. DSC curves of Mg—54%Bi alloy.
phases in EPMA images is calculated, which accounts for 72%– 75%, 90%–93% and 45%–48% in Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys, respectively. The melting enthalpies increase by 30.6% and the proportion of α-Mg + Mg3Bi2 eutectic phases increases by about 24%–25% when Bi content increases from about 36% to 54%. So it can be concluded that the proportion of α-Mg + Mg3Bi2 eutectic phases may be a main reason for the increase of melting enthalpies in these two alloys. Because the essence of the molten of alloy is that the bonds between the metal atoms are destroyed when the energy outside is provided, the regular arrangement of atoms is broken. The more proportion of α-Mg + Mg3Bi2 eutectic phases has, the more bonds the metal atoms have. It needs more energy to
destroy these bonds, thus the melting enthalpy is higher. Comparing Mg—54%Bi with Mg—60%Bi, the phase compositions change when Bi content increases from about 54% to 60%. Mg—54%Bi alloy is mainly composed of α-Mg matrix and α-Mg + Mg3Bi2 eutectic phases, and Mg—60%Bi alloy is mainly composed of the Mg3Bi2 phases and α-Mg + Mg3Bi2 eutectic phases. On one hand, the proportion of α-Mg + Mg3Bi2 eutectic phases decreases to 45%–48% in Mg—60%Bi. On the other hand, the massive acicular and flake primary Mg3Bi2 phases in Mg—60%Bi may lead to dramatic decrease of the melting enthalpy. Non-isothermal melting process dynamics of Mg—54%Bi alloy is measured by multiple DSC technology. Kissinger [20] method is one of the more commonly used methods in data processing for nonisothermal transformation kinetics. Kissinger approximate equation is:
⎛ β ⎞ E⎛ 1 ⎞ ln ⎜ 2 ⎟ = − ⎜ ⎟ + C R ⎝ TP ⎠ ⎝ Tp ⎠
Fig. 4. DSC curves of Mg—Bi alloys.
Table 4 Thermophysical properties of Mg—Bi alloys. Compounds
Mg—36%Bi Mg—54%Bi Mg—60%Bi
Melting temperature/(°C)
Melting enthalpy/(J/g)
Onset
Peak
End
ΔHm
547.6 546.3 548.1
550.5 551.6 551.4
555.3 559.8 558.7
138.2 180.5 48.7
(3)
where β is a heating rate in DSC analyses, Tp the peak temperature at various heating rates in analyses, R the universal gas constant and E the apparent activation energy of the melting process. The relationship between ln(β/Tp2) and 1/Tp is linear, thus the corresponding slope, intercept and correlation coefficient can be calculated by using linear regression at different heating rates of the peak temperature obtained by DSC curves. According to Equation (3) and Fig. 6, the slope of the line is −E/R, and R is the universal gas constant. So the activation energy can be calculated by slope. Fig. 5 shows the DSC curves at various heating rates. It can be seen that the peak temperature of DSC curves moves to higher temperature with increasing heating rates. The reason is that the heating rate is higher, and the heat flow is higher, which lead to larger thermal effect produced in unit time. Fig. 6 shows the curve according to ln(β/T p 2 ) to 1/T p and it is a linear fit by using the experimental data in Table 5. It can be obtained apparent activation energy E in melting process according to the slope of the line in Fig. 5. The activation energy E is 1322.8 kJ/mol. A similar study about shape-stabilized PCMs composed of polyethylene glycol (PEG) and mesoporous active carbon (AC) was also observed by Feng et al. [21]. The phase change activation energies for PEG in the PEG/ AC PCMs (50, 60 and 70 wt %) were determined to be 1030.6, 696.0 and 624.6 kJ/mol, respectively. It can be seen that the activation energy of Mg—54%Bi is higher than those of three PEG/AC PCMs.
D. Fang et al./Applied Thermal Engineering 92 (2016) 187–193
Fig. 6. Plot ln(β/Tp2) versus 1/Tp.
3.3. Thermal expansion Fig. 7 shows the results of thermal expansion of test alloys. As seen in Fig. 7(a), the elongation of all of the samples increases with increasing temperature. It exhibits that the elongation of Mg—60%Bi alloy is highest above 300 °C when compared with those of Mg—36%Bi and Mg—54%Bi alloys. As seen in Fig. 7(b), the thermal expansions of three alloys also increase with increasing temperature. It shows that the thermal expansion of Mg—60%Bi alloy is the highest above 285 °C when compared with those of Mg—36%Bi and
Table 5 The peak temperature of DSC curves for sample under various heating rates. β/(K·min−1)
Peak temperature/K
5 10 15 20
824.7 827.9 828.3 830.7
(a)
191
Fig. 8. Temperature dependence of density of test alloys.
Mg—54%Bi alloys. The reason may be as follows: a spot of simple substance of Bi may exists in alloy due to the high Bi content in Mg—60%Bi, whose melting point is 271.3 °C. The simple substance of Bi melts when the temperature increases to over 285 °C. The phase change due to melting can lead to the increase of thermal expansion coefficient. However, the thermal expansion coefficient cannot increase dramatically because the simple substance of Bi content is small in alloy. Comparing the microstructure of three alloys, it can be found that there are massive acicular and flake primary Mg3Bi2 phases in Mg—60%Bi, so it is inferred that the thermal expansion coefficient of primary Mg3Bi2 phases may increase dramatically when the temperature increases to over 285 °C. This may be the main reason that cause the thermal expansion coefficient (elongation) increasing dramatically. Density is one of the parameter for thermal conductivity calculation. The density at 20 °C has been measured by the Archimedes method. The densities of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi are 2.947, 3.094 and 3.221 g/cm3, respectively. Base on Equation (1) and data above, the curves of densities are also obtained in Fig. 8. The densities of all of the alloys increase with increasing Bi content and
(b)
Fig. 7. Temperature dependence of the linear thermal expansion coefficient of test alloys: (a) relative elongation, (b) linear thermal expansion coefficient.
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(a)
(b)
Fig. 9. Temperature dependence of the specific heat capacity thermal conductivity: (a) specific heat capacity, (b) thermal conductivity.
decrease with increasing temperature. The reason is that the densities of pure Mg and Bi are 1.74 g/cm3 and 9.78 g/cm3, respectively. So the densities of the Mg—Bi alloys increase with increasing Bi content. 3.4. Thermal conductivity
the increasing addition of Bi atoms cause large distortion of Mg matrix and destroy the periodicity of lattice, thus the mean free path of electrons is reduced, and the thermal conductivity decreases dramatically [23]. 4. Case study: a comparison with other materials as PCM
Fig. 9 shows the temperature dependence of specific heat capacity and thermal conductivity of Mg—Bi alloys. A linear growth specific heat curve is shown below phase change temperature in Fig. 9(a). It should be noted that the rule may be used only in a temperature range where no phase transition occurs. As mentioned above, the specific heat of alloys increases with increasing temperature and decreases with increasing Bi content. Based on the values of thermal diffusivity, density and specific heat capacity obtained, the thermal conductivity can be calculated using Equation (2). The temperature dependences of the thermal conductivity for the three Mg—Bi alloys are shown in Fig. 9(b). From Fig. 9(b), the thermal conductivity of the Mg—Bi alloys shows a slight increase from 40 °C up to 300 °C. Then, it exhibits a very light decrease up to 400 °C. The reason may be as follows: when the phase boundary reached a certain temperature, dissolution of Mg3Bi2 precipitates occurs, and a reversible process leads to a decrease in thermal conductivity with increasing temperature. For instance, the thermal conductivity of the Mg—Bi alloys exhibits a light decrease above 300 °C [17]. It can be noticed that the thermal conductivity of alloys was weakly temperature dependent, with small positive temperature coefficients [22]. As shown in Fig. 9(b), it is seen that the values of the thermal conductivity decrease remarkably with increasing Bi content. Comparing to Mg—36%Bi alloy, the values of thermal conductivity of Mg—54%Bi and Mg—60%Bi alloys roughly drop by 10 and 60 W/mK, respectively. The reason is that
The Mg—54%Bi alloy has higher melting enthalpy and energy density than those of other Mg—Bi alloys, so it is selected to conduct the comparison. Table 6 shows the most important thermophysical properties of binary and ternary inorganic salts as well as those of the Mg—54%Bi alloy. As shown in Table 6: the melting temperatures of these three materials are in the range of 500–550 °C. The thermal conductivity of Mg—54%Bi alloy is 40–70 times as great as that of any salt. The melting enthalpies of binary and ternary inorganic are higher than that of Mg—54%Bi alloy. The heat capacity of Mg—54%Bi alloy is between 37% and 66% smaller than molten salts. The energy density of the alloy is between 8% and 17% smaller than molten salts. It can be seen that the main advantage of Mg—54Bi% over the inorganic salts is the high thermal conductivity. 5. Adaptability to solar thermal power technologies Latent heat thermal energy storage systems (LHTES) that utilize PCMs have received great attention in solar thermal applications because of their large heat storage capacity and their isothermal behavior during charging and discharging processes [25]. The present review analyzes the state of studies and developments of PCMs, which can be used to store thermal and solar energy in the range of 120–1000 °C [24].The Mg—54%Bi alloy discussed here has the capability to be used as the PCMs in LHTES in solar thermal applications
Table 6 Comparison of Mg—54%Bi thermophysical properties with other materials. Material
Density (g·cm−3)
Melting enthalpy (J g−1)
Vol. heat storage (MJ m−3)
Heat capacity (kJ kg−1 K−1)
Thermal conductivity (W m−1 K−1)
Melting temperature (°C)
NaCl(33)—67CaCl2 Li2CO3(20)—60Na2CO3—20K2CO3 Mg—54%Bi
2.16a 2.38a 3.09
281a 283a 181
607a 673a 559
0.84a,c 1.59a,c 0.53c
1.02a,b 1.83a,b 73c
500a 550a 546
a b c
Values from Ref. 24. Liquid. Solid.
D. Fang et al./Applied Thermal Engineering 92 (2016) 187–193
due to its large heat storage capacity, high thermal conductivity and suitable working temperature. 6. Conclusions Comparing with Al-based phase change material, Mg-based phase change material is getting more and more attention due to its high corrosion resistance with encapsulation materials based on iron. In this paper, most significant thermal properties of three components of Mg—Bi alloy as a latent heat energy storage material for CSP applications are reported. Firstly, the results indicate that the microstructures of Mg—36%Bi and Mg—54%Bi alloys are mainly composed of primary crystal α-Mg solid solution matrix and α-Mg + Mg3Bi2 eutectic phases, and the microstructure of Mg—60%Bi alloy is mainly composed of the primary Mg 3 Bi 2 phase and α-Mg + Mg3Bi2 eutectic phases. The melting enthalpies of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys are 138.2, 180.5 and 48.7 J/g, respectively, and the phase change temperatures of three compositions are in the range of 546–548 °C. The activation energy of Mg—54%Bi is 1322.8 kJ/mol. The Mg—54%Bi alloy has a higher value of melting enthalpy. The main reason may be that it has higher proportion of Mg + Mg3Bi2 eutectic phases in alloy. Furthermore, the values of the specific heat capacity and thermal conductivity decrease with increasing Bi content. Comparing to Mg—36%Bi alloy, the values of thermal conductivity of Mg—54%Bi and Mg—60%Bi alloys roughly drop by 10 and 60 W/mK, respectively. The elongation and linear thermal expansion coefficient also increase with the temperature from 40 °C to 450 °C. At last, it can be concluded that the investigated Mg—54%Bi alloy has potential to improve the energy storage material for CSP applications. Besides, a further study of the compatibility of Mg-based alloys with encapsulating materials will be reported in the future.
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The authors would like to thank National Key Technology Research & Development Program of China (Grant No. 2012BAA05B05) and National Natural Science Foundation of China (Grant No. 51206125). References
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Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Preparation, microstructure and thermal properties of Mg—Bi alloys as phase change materials for thermal energy storage Dong Fang, Zheng Sun, Yuanyuan Li, Xiaomin Cheng * School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
H I G H L I G H T S
• • • •
The microstructure and thermal properties of Mg—Bi alloys are determined. The relationship between melting enthalpies and phase composition are studied. The activation energy of Mg—54%Bi alloy is calculated by multiple DSC technology. Mg—54%Bi alloy is proposed as a phase change material at high (>420 °C) temperature.
A R T I C L E
I N F O
Article history: Received 25 June 2015 Accepted 21 September 2015 Available online 1 October 2015 Keywords: Metallic materials Mg—Bi alloy Latent heat storage Phase change material
A B S T R A C T
Comparing with Al-based phase change material, Mg-based phase change material is getting more and more attention due to its high corrosion resistance with encapsulation materials based on iron. This study focuses on the characterization of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi (wt. %) alloys as phase change materials for thermal energy storage at high temperature. The phase compositions, microstructure and phase change temperatures were investigated by X-ray diffusion (XRD), electron probe micro-analysis (EPMA) and differential scanning calorimeter (DSC) analysis, respectively. The results indicates that the microstructure of Mg—36%Bi and Mg—54%Bi alloys are mainly composed of α-Mg matrix and α-Mg + Mg3Bi2 eutectic phases, Mg—60%Bi alloy are mainly composed of the Mg3Bi2 phase and α-MgMg3Bi2 eutectic phases. The melting enthalpies of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys are 138.2, 180.5 and 48.7 J/g, with the phase change temperatures of 547.6, 546.3 and 548.1 °C, respectively. The Mg—54%Bi alloy has the highest melting enthalpy in three alloys. The main reason may be that it has more proportion of α-Mg + Mg3Bi2 eutectic phases. The thermal expansion of three alloys increases with increasing temperature. The values of the thermal conductivity decrease with increasing Bi content. Besides, the activation energy of Mg—54%Bi was calculated by multiple DSC technology. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Phase change materials (PCMs) are drawing worldwide increasing attention in thermal energy storage (TES) systems due to their high performance in energy storage density, energy conversion efficiency, storing and releasing thermal energy at nearly constant temperature [1,2]. Selection of PCMs for TES applications depends on thermal properties such as the operating temperature, heat capacity, thermal conductivity and thermal reliability. Hoshi et al. [3] classified the PCMs in high temperature range over 420 °C associated with a concentrating solar power system. Main emphasis on high temperature PCMs is molten salts such as alkali metal nitrates, alkali metal nitrites and their mixtures [4,5].
* Corresponding author. Tel.: +86 13507117513; Fax:+86 02787651779. E-mail address: [email protected] (X. Cheng). http://dx.doi.org/10.1016/j.applthermaleng.2015.09.090 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
However, one of the main drawbacks of inorganic molten salts is their low thermal conductivity, resulting in the need of a more sophisticated heat exchanger for charging and discharging processes of the TES system. An alternative to inorganic salts can be metal alloys [6]. Metal alloy PCMs were proposed as TES materials more than three decades ago [7,8], and have been experimentally studied to some extent for TES applications [9–11]. Recently, scientists have studied thermal physical properties of Al-based heat storage material. For instance, Birchenall et al. [8,12] analyzed thermal physical properties of binary and multiple alloys containing A1, Cu, Mg, Si, Zn and other elements, and considered that metal materials with high phase change latent heat value usually include high melting point elements, and found that the alloys that are rich in elements of Al and Si are ideal phase change heat storage materials. Huang et al. [13] determined the specific heat of liquid and solid forms band the latent heat of fusion of Al—Si, Al—Si—Mg and Al—Si—Cu alloys. Wang et al. [11] developed a novel high temperature space heater using Al—12%Si (wt.%) alloy as heat storage
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Table 1 The components of Mg—Bi alloys. Samples
a b c
Compounds
Chemical composition (wt.%)
Mg—36%Bi Mg—54%Bi Mg—60%Bi
Mg
Bi
O
63.51 45.26 39.95
36.03 54.15 59.72
0.46 0.59 0.33
medium. In addition, the research of Achard [14] indicated that Al—Mg alloy would be suitable for heat storage material at about 450 °C. The thermal reliability and corrosion behavior of Al—Mg—Zn alloy with respect to the number of thermal cycles for TES systems were studied by Sun et al. [9]. They found that the melting temperature changed by 3–5 K and the latent heat of fusion decreased after 1000 thermal cycles. One major drawback of Al-based alloys is that aluminum melt is highly corrosive to encapsulation materials based on iron [15], which limits its long-term and large-scale applications. Mg—Fe system is thermodynamically stable and immiscible in the concerned temperature range of 400–600 °C according to the phase diagram [16]. Most studies of domestic and overseas scholars focus on Al-based alloys as PCMs [9,11,14], and studies about phase change thermal storage materials based on magnesium are relatively lacking so far. Metal Mg has appropriate melting point, high melting enthalpy and good thermal conductivity, and metal Bi also has low melting point, high heat storage density and good heat stability. Hence, they can be used as PCMs. In this work we focused on thermal properties of three components of Mg—Bi alloys as PCM for TES applications from 400 °C to 600 °C. The phase composition, microstructure, and the significant thermal properties were investigated in order to provide reference for the design and application of magnesium alloy phase change thermal energy storage materials. 2. Experimental 2.1. Materials and preparation Pure magnesium ingot (99.98% purity) and bismuth ingot (99.99% purity) were used to prepare Mg—Bi alloys. The chemical compositions of Mg—Bi alloys measured by XRF are shown in Table 1. The synthesis of the alloy was melted (600 g melt) in a graphite crucible in a pit type resistance furnace. The RJ-2 flux refining agent was used in the melting process. The components of RJ-2 flux and coating agent are shown in Table 2. A high purity argon gas (99.999%) atmosphere was also offered in order to strictly prevent the specimens from oxidizing during the preparation process. The specimens with a diameter of 20 mm were cast in the iron mold (φ30 × 100 mm) preheated at 200 °C.
Differential scanning calorimetry (DSC, STA449C/3/G) analyses were performed at a constant heating rate of 5 K/min in the range of 25–600 °C under argon atmosphere. The linear thermal expansion coefficients of samples were measured by pushrod type dilatometer (DIL 402C) in the temperature range 30–450 °C at the heating rate of 5 K/min. The sample for dilatometry test was machined to dimensions of 5 × 5 × 20 mm. Laser-flash method (LFA 457) in the temperature range of 30– 400 °C was adopted for the thermal diffusivity measurements of the block sample with dimensions of 10 × 10 × 2.5 mm. The step of the testing temperatures was set to 50 °C, and at least three measurements were performed at each testing point. The density at high temperature was calculated using the relation [17]:
ρ = ρ0 (1 + ΔL L0 )
−3
(1)
where ρ0 is the density of the alloy at 20 °C, which can be measured by the Archimedes method, and ΔL/L0 is the relative elongation. The specific heat capacity was calculated using the Neumann– Kopp rule and published data [18]. The thermal conductivity can be obtained using the following Equation:
λ = a ⋅ ρ ⋅cp
(2)
where a is the thermal diffusivity, ρ the density and cp the specific heat capacity at constant pressure. 3. Results and discussion 3.1. Microstructure analysis Fig. 1 shows X-ray diffraction patterns of as-cast Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys. According to XRD patterns, it is confirmed that the phases of the alloys only consist of α-Mg and Mg3Bi2. As seen in Fig. 1, the phase structure is not changed in these Mg—Bi alloys. Fig. 1 displays that diffraction intensity of α-Mg gradually decreases as increasing Bi content. It may be the reason that the proportion of α-Mg phase decreases with increasing Bi content. Fig. 2 shows electron probe micro-analysis (EPMA) images of the three alloys. Table 3 shows compositions of the intermetallic phases exhibited in Fig. 2, obtained from EDS analysis. In the low magnification images of Mg—36%Bi and Mg—54%Bi alloys, it is seen that
2.2. Analysis methods The phases of the samples were analyzed by X-ray diffraction (XRD, D8 ADVANCE). Microstructural analysis was carried out using electron probe micro-analysis (EPMA, JXA-8230) equipped with energy-dispersive X-ray spectrometer (EDS) (INCAX-ACT).
Table 2 The components of RJ-2 flux. Composition (mass%)
RJ-2 flux Coating agent
MgCl2
KCl
NaCl
CaCl2
CaF2
BaCl2
43–55 65–74
20–30 10–20
20–30 15
3–5 3–5
10–15 4–7
3–5 –
Fig. 1. XRD patterns of Mg—Bi alloys.
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Fig. 2. EPMA images of Mg—Bi alloys.
the microstructure is mainly composed of two distinct phases, which are distinguished by black and gray. The black phase takes the dendrite shape and the gray phase takes punctate shape which can be clearly seen in high magnification images. Combined with the results of analysis of XRD (see Fig. 1) and EDS (see Table 3), it is believed that the black phase is primary α-Mg phase, and the gray phase is α-Mg + Mg3Bi2 eutectic phase. It can be seen that the phase compositions are the same in these two alloys, and the only difference is the proportion of the two phases in alloys. Combined with the results of analysis of XRD and EDS, Fig. 1(c) shows that the microstructure mainly composes of three phases. The gray punctate shape is α-Mg + Mg3Bi2 eutectic phase (I). The black phase around the gray punctate phase is α-Mg phase (H), the gray acicular and flake structure is primary Mg3Bi2 phase (E). According to Mg—Bi binary phase diagram seen Fig. 3, Bi has a relatively high solubility in the magnesium alloy. The solubility of Bi in Mg is about 8.85% at 551 °C, and when the temperature decreases to 200 °C, its solubility only reduces to below 1%. The eutectic point in Mg—Bi binary alloy is at 44.2% Mg and 55.8% Bi, at which eutectic reaction L→α-Mg + Mg3Bi2 takes place. The positions of Mg—36%Bi (point a) and Mg—54%Bi (point b) alloys are seen in phase diagram (Fig. 3). So a reaction: L→α-Mg takes place before eutectic
reaction occurred during alloy solidification. Then, the eutectic reaction L→α-Mg + Mg3Bi2 takes place in the rest of liquid-phase. Therefore, the primary α-Mg is formed in these two alloys. However, at point c (Mg—60%Bi) in the phase diagram, a reaction: L→Mg3Bi2 takes place before eutectic reaction occurred during alloy solidification. Therefore, the primary Mg3Bi2 phase is formed in the alloy. 3.2. Phase change temperatures and enthalpies Fig. 4 reveals the measured phase change temperatures and melting enthalpies of three compositions of Mg—Bi alloys. As shown in Fig. 3, only one endothermic peak is observed in the solid– liquid transition in agreement with the phase diagram. The data in Table 4 show that the melting enthalpies of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys are 138.2, 180.5 and 48.7 J/g, with the phase change temperatures of 547.6, 546.3 and 548.1 °C, respectively. The melting enthalpies of Mg—54%Bi alloy are higher than those of Mg—36%Bi and Mg—60%Bi. The reasons may be as follows: the preparation conditions of the Mg—Bi alloys are almost the same. The species and morphology of phases are the same in Mg—36%Bi and Mg—54%Bi alloys (Fig. 2a and b). The main difference is the proportion of the two phases. The proportion of α-Mg + Mg3Bi2 eutectic
Table 3 Chemical composition of intermetallic phases in Fig. 2 (in at.%). Phase
A
B
C
D
E
F
G
Mg Bi Closest phase
98.97 1.03 α-Mg
82.41 17.59 α-Mg + Mg3Bi2
98.94 1.06 α-Mg
80.74 19.26 α-Mg + Mg3Bi2
61.66 38.34 Mg3Bi2
95.06 4.94 α-Mg
80.82 19.18 α-Mg + Mg3Bi2
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Fig. 3. Phase diagram of Mg—Bi system [19]. Fig. 5. DSC curves of Mg—54%Bi alloy.
phases in EPMA images is calculated, which accounts for 72%– 75%, 90%–93% and 45%–48% in Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys, respectively. The melting enthalpies increase by 30.6% and the proportion of α-Mg + Mg3Bi2 eutectic phases increases by about 24%–25% when Bi content increases from about 36% to 54%. So it can be concluded that the proportion of α-Mg + Mg3Bi2 eutectic phases may be a main reason for the increase of melting enthalpies in these two alloys. Because the essence of the molten of alloy is that the bonds between the metal atoms are destroyed when the energy outside is provided, the regular arrangement of atoms is broken. The more proportion of α-Mg + Mg3Bi2 eutectic phases has, the more bonds the metal atoms have. It needs more energy to
destroy these bonds, thus the melting enthalpy is higher. Comparing Mg—54%Bi with Mg—60%Bi, the phase compositions change when Bi content increases from about 54% to 60%. Mg—54%Bi alloy is mainly composed of α-Mg matrix and α-Mg + Mg3Bi2 eutectic phases, and Mg—60%Bi alloy is mainly composed of the Mg3Bi2 phases and α-Mg + Mg3Bi2 eutectic phases. On one hand, the proportion of α-Mg + Mg3Bi2 eutectic phases decreases to 45%–48% in Mg—60%Bi. On the other hand, the massive acicular and flake primary Mg3Bi2 phases in Mg—60%Bi may lead to dramatic decrease of the melting enthalpy. Non-isothermal melting process dynamics of Mg—54%Bi alloy is measured by multiple DSC technology. Kissinger [20] method is one of the more commonly used methods in data processing for nonisothermal transformation kinetics. Kissinger approximate equation is:
⎛ β ⎞ E⎛ 1 ⎞ ln ⎜ 2 ⎟ = − ⎜ ⎟ + C R ⎝ TP ⎠ ⎝ Tp ⎠
Fig. 4. DSC curves of Mg—Bi alloys.
Table 4 Thermophysical properties of Mg—Bi alloys. Compounds
Mg—36%Bi Mg—54%Bi Mg—60%Bi
Melting temperature/(°C)
Melting enthalpy/(J/g)
Onset
Peak
End
ΔHm
547.6 546.3 548.1
550.5 551.6 551.4
555.3 559.8 558.7
138.2 180.5 48.7
(3)
where β is a heating rate in DSC analyses, Tp the peak temperature at various heating rates in analyses, R the universal gas constant and E the apparent activation energy of the melting process. The relationship between ln(β/Tp2) and 1/Tp is linear, thus the corresponding slope, intercept and correlation coefficient can be calculated by using linear regression at different heating rates of the peak temperature obtained by DSC curves. According to Equation (3) and Fig. 6, the slope of the line is −E/R, and R is the universal gas constant. So the activation energy can be calculated by slope. Fig. 5 shows the DSC curves at various heating rates. It can be seen that the peak temperature of DSC curves moves to higher temperature with increasing heating rates. The reason is that the heating rate is higher, and the heat flow is higher, which lead to larger thermal effect produced in unit time. Fig. 6 shows the curve according to ln(β/T p 2 ) to 1/T p and it is a linear fit by using the experimental data in Table 5. It can be obtained apparent activation energy E in melting process according to the slope of the line in Fig. 5. The activation energy E is 1322.8 kJ/mol. A similar study about shape-stabilized PCMs composed of polyethylene glycol (PEG) and mesoporous active carbon (AC) was also observed by Feng et al. [21]. The phase change activation energies for PEG in the PEG/ AC PCMs (50, 60 and 70 wt %) were determined to be 1030.6, 696.0 and 624.6 kJ/mol, respectively. It can be seen that the activation energy of Mg—54%Bi is higher than those of three PEG/AC PCMs.
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Fig. 6. Plot ln(β/Tp2) versus 1/Tp.
3.3. Thermal expansion Fig. 7 shows the results of thermal expansion of test alloys. As seen in Fig. 7(a), the elongation of all of the samples increases with increasing temperature. It exhibits that the elongation of Mg—60%Bi alloy is highest above 300 °C when compared with those of Mg—36%Bi and Mg—54%Bi alloys. As seen in Fig. 7(b), the thermal expansions of three alloys also increase with increasing temperature. It shows that the thermal expansion of Mg—60%Bi alloy is the highest above 285 °C when compared with those of Mg—36%Bi and
Table 5 The peak temperature of DSC curves for sample under various heating rates. β/(K·min−1)
Peak temperature/K
5 10 15 20
824.7 827.9 828.3 830.7
(a)
191
Fig. 8. Temperature dependence of density of test alloys.
Mg—54%Bi alloys. The reason may be as follows: a spot of simple substance of Bi may exists in alloy due to the high Bi content in Mg—60%Bi, whose melting point is 271.3 °C. The simple substance of Bi melts when the temperature increases to over 285 °C. The phase change due to melting can lead to the increase of thermal expansion coefficient. However, the thermal expansion coefficient cannot increase dramatically because the simple substance of Bi content is small in alloy. Comparing the microstructure of three alloys, it can be found that there are massive acicular and flake primary Mg3Bi2 phases in Mg—60%Bi, so it is inferred that the thermal expansion coefficient of primary Mg3Bi2 phases may increase dramatically when the temperature increases to over 285 °C. This may be the main reason that cause the thermal expansion coefficient (elongation) increasing dramatically. Density is one of the parameter for thermal conductivity calculation. The density at 20 °C has been measured by the Archimedes method. The densities of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi are 2.947, 3.094 and 3.221 g/cm3, respectively. Base on Equation (1) and data above, the curves of densities are also obtained in Fig. 8. The densities of all of the alloys increase with increasing Bi content and
(b)
Fig. 7. Temperature dependence of the linear thermal expansion coefficient of test alloys: (a) relative elongation, (b) linear thermal expansion coefficient.
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(a)
(b)
Fig. 9. Temperature dependence of the specific heat capacity thermal conductivity: (a) specific heat capacity, (b) thermal conductivity.
decrease with increasing temperature. The reason is that the densities of pure Mg and Bi are 1.74 g/cm3 and 9.78 g/cm3, respectively. So the densities of the Mg—Bi alloys increase with increasing Bi content. 3.4. Thermal conductivity
the increasing addition of Bi atoms cause large distortion of Mg matrix and destroy the periodicity of lattice, thus the mean free path of electrons is reduced, and the thermal conductivity decreases dramatically [23]. 4. Case study: a comparison with other materials as PCM
Fig. 9 shows the temperature dependence of specific heat capacity and thermal conductivity of Mg—Bi alloys. A linear growth specific heat curve is shown below phase change temperature in Fig. 9(a). It should be noted that the rule may be used only in a temperature range where no phase transition occurs. As mentioned above, the specific heat of alloys increases with increasing temperature and decreases with increasing Bi content. Based on the values of thermal diffusivity, density and specific heat capacity obtained, the thermal conductivity can be calculated using Equation (2). The temperature dependences of the thermal conductivity for the three Mg—Bi alloys are shown in Fig. 9(b). From Fig. 9(b), the thermal conductivity of the Mg—Bi alloys shows a slight increase from 40 °C up to 300 °C. Then, it exhibits a very light decrease up to 400 °C. The reason may be as follows: when the phase boundary reached a certain temperature, dissolution of Mg3Bi2 precipitates occurs, and a reversible process leads to a decrease in thermal conductivity with increasing temperature. For instance, the thermal conductivity of the Mg—Bi alloys exhibits a light decrease above 300 °C [17]. It can be noticed that the thermal conductivity of alloys was weakly temperature dependent, with small positive temperature coefficients [22]. As shown in Fig. 9(b), it is seen that the values of the thermal conductivity decrease remarkably with increasing Bi content. Comparing to Mg—36%Bi alloy, the values of thermal conductivity of Mg—54%Bi and Mg—60%Bi alloys roughly drop by 10 and 60 W/mK, respectively. The reason is that
The Mg—54%Bi alloy has higher melting enthalpy and energy density than those of other Mg—Bi alloys, so it is selected to conduct the comparison. Table 6 shows the most important thermophysical properties of binary and ternary inorganic salts as well as those of the Mg—54%Bi alloy. As shown in Table 6: the melting temperatures of these three materials are in the range of 500–550 °C. The thermal conductivity of Mg—54%Bi alloy is 40–70 times as great as that of any salt. The melting enthalpies of binary and ternary inorganic are higher than that of Mg—54%Bi alloy. The heat capacity of Mg—54%Bi alloy is between 37% and 66% smaller than molten salts. The energy density of the alloy is between 8% and 17% smaller than molten salts. It can be seen that the main advantage of Mg—54Bi% over the inorganic salts is the high thermal conductivity. 5. Adaptability to solar thermal power technologies Latent heat thermal energy storage systems (LHTES) that utilize PCMs have received great attention in solar thermal applications because of their large heat storage capacity and their isothermal behavior during charging and discharging processes [25]. The present review analyzes the state of studies and developments of PCMs, which can be used to store thermal and solar energy in the range of 120–1000 °C [24].The Mg—54%Bi alloy discussed here has the capability to be used as the PCMs in LHTES in solar thermal applications
Table 6 Comparison of Mg—54%Bi thermophysical properties with other materials. Material
Density (g·cm−3)
Melting enthalpy (J g−1)
Vol. heat storage (MJ m−3)
Heat capacity (kJ kg−1 K−1)
Thermal conductivity (W m−1 K−1)
Melting temperature (°C)
NaCl(33)—67CaCl2 Li2CO3(20)—60Na2CO3—20K2CO3 Mg—54%Bi
2.16a 2.38a 3.09
281a 283a 181
607a 673a 559
0.84a,c 1.59a,c 0.53c
1.02a,b 1.83a,b 73c
500a 550a 546
a b c
Values from Ref. 24. Liquid. Solid.
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due to its large heat storage capacity, high thermal conductivity and suitable working temperature. 6. Conclusions Comparing with Al-based phase change material, Mg-based phase change material is getting more and more attention due to its high corrosion resistance with encapsulation materials based on iron. In this paper, most significant thermal properties of three components of Mg—Bi alloy as a latent heat energy storage material for CSP applications are reported. Firstly, the results indicate that the microstructures of Mg—36%Bi and Mg—54%Bi alloys are mainly composed of primary crystal α-Mg solid solution matrix and α-Mg + Mg3Bi2 eutectic phases, and the microstructure of Mg—60%Bi alloy is mainly composed of the primary Mg 3 Bi 2 phase and α-Mg + Mg3Bi2 eutectic phases. The melting enthalpies of Mg—36%Bi, Mg—54%Bi and Mg—60%Bi alloys are 138.2, 180.5 and 48.7 J/g, respectively, and the phase change temperatures of three compositions are in the range of 546–548 °C. The activation energy of Mg—54%Bi is 1322.8 kJ/mol. The Mg—54%Bi alloy has a higher value of melting enthalpy. The main reason may be that it has higher proportion of Mg + Mg3Bi2 eutectic phases in alloy. Furthermore, the values of the specific heat capacity and thermal conductivity decrease with increasing Bi content. Comparing to Mg—36%Bi alloy, the values of thermal conductivity of Mg—54%Bi and Mg—60%Bi alloys roughly drop by 10 and 60 W/mK, respectively. The elongation and linear thermal expansion coefficient also increase with the temperature from 40 °C to 450 °C. At last, it can be concluded that the investigated Mg—54%Bi alloy has potential to improve the energy storage material for CSP applications. Besides, a further study of the compatibility of Mg-based alloys with encapsulating materials will be reported in the future.
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Acknowledgement [20]
The authors would like to thank National Key Technology Research & Development Program of China (Grant No. 2012BAA05B05) and National Natural Science Foundation of China (Grant No. 51206125). References
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