FAN Xiaoming, YE Peiyi, ZHAO Tong, XU Lianyou, CHENG Xiaomin

(School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China)

Abstract: The microstructure and thermal characteristics of Mg-36%Ga, Mg-43%Ga, and Mg-45%Ga(wt%) alloys were investigated. The experimental results show that the microstructure of Mg-36%Ga alloy is mainly composed of primary α-Mg phase and α-Mg+Mg5Ga2 eutectic phase, the microstructure of Mg-43%Ga alloy is mainly composed of α-Mg+Mg5Ga2 eutectic phase, and the microstructure of Mg-45%Ga alloy is mainly composed of primary Mg5Ga2 phase and α-Mg+Mg5Ga2 eutectic phase. The melting enthalpies of Mg-36%Ga, Mg-43%Ga, and Mg-45%Ga are 146.41, 171.90, and 113.90 J/g, with the phase change temperature of 422.57, 422.70, and 422.90 ℃, respectively. Mg-43%Ga alloy contains the highest melting enthalpy because of the highest content of α-Mg+Mg5Ga2 eutectic phase. In addition, the thermal expansion of the three alloys increases with increasing temperature, while the thermal diffusivity and thermal conductivity decreases with increasing content of Ga.

Key words: Mg-Ga alloy; phase change material; thermal energy storage; latent heat storage

1 Introduction

Thermal energy storage(TES) system is a set of energy conversion device that stores energy in the form of thermal energy and converts it into required energy when needed. Obviously, this is competitive to solve the mismatch of supply and demand of energy[1].In practice, TES system can be not only used in concentrated solar power(CSP), but applied in the other fields such as “peak load shifting” of grid, industrial waste heat and waste heat recovery and utilization,thermal insulation system and so on.

Thermal storage material is one of the key part of TES system and its selection is also very important,which will directly affect the thermal storage efficiency and life of the system. Phase change materials(PCMs)uses the heat absorption and exothermic during the two-phase transformation to complete the storage and release of heat energy. It has the advantages of high heat storage density, small volume and simple principle, and the temperature of the material is roughly constant in the use of process, with less heat loss. Therefore, it has occupied current research on heat storage materials. Metals, especially some alloy phase change heat storage materials, have high thermal conductivity, high energy storage density, good thermal cycling stability and low subcooling degree during phase change, which have great potential in high temperature phase change heat storage. As early as the end of the 20th century, some scholars have studied the thermophysical properties of aluminum alloy and tried to apply it to the field of thermal energy storage. For example, Birchenall[2]found that the intermetallic eutectic phase with ordered structure in Al-Si alloy and Al-Mg alloy produced a large amount of heat exchange when forming and melting. The results indicate that the phase change latent heat of Al-37.5%Mg (at%) alloy and A1-13%Si alloy are 310 and 515 J/g, with phase transition temperature of 724℃ and 854 ℃, respectively, and their performance of thermal conduction are also outstanding, so they have a great potential applications in the field of high temperature thermal storage. There are also scholars who have studied Fe-based and Cu-based heat storage alloys[3,4]. But comparatively speaking, the researchers at home and abroad have a relatively deep research of various heat storage properties of Al-based alloys for the moment[5]. However, long-term research and application have revealed that the melted Al-based heat storage materials have strong corrosion effect on the iron container, which will undoubtedly greatly limit its long time and large-scale application in TES system.With high temperature stability with iron, appropriate melting point, good thermal stability and higher phase change latent heat, magnesium has the potential to be used as PCMs for TES system. For instance, Blanco-Rodríguez et al have studied the thermophysical properties of Mg-51%Zn eutectic alloy, and analyzed the possibility of using Mg-based alloys as PCMs for TES in CSP applications[6].

There is no doubt that phase change latent heat,thermal conductivity, thermal expansion, thermal diffusivity and specific heat capacity and other thermal physical parameters are important basis for measuring and selecting heat storage materials. As well as according to the different application environment for TES systems, the selection of the heat storage material with appropriate working temperature is also one of the important principles of confirming PCMs. Therefore, it is of great significance to research PCMs with different operating temperatures to meet the demand[7]. Fang Donget al[8-10]have studied the structure and related thermophysical properties of Mg-Sn, Mg-Bi and Mg-Bi-Sn alloys by adding metals with low melting temperature such as tin and bismuth to magnesium to reduce the melting temperature of the alloys. The results show that the melting temperature of Mg-37%Sn, Mg-54%Bi and Mg-39%Bi-17%Sn are 554.4,546.3, and 515.8 ℃, respectively, and the addition of new components, especially low-melting components,can effectively decrease the phase change temperature of magnesium alloys.

With low melting temperature(29 ℃), nonvolatileh, high boiling temperature(2 403 ℃) and thermal stability, gallium is an excellent heat transfer medium in nuclear power plants as well as bismuth.Compared with Mg-Bi alloys and Mg-Sn alloys, the phase change temperature of Mg-Ga alloys is lower,so as to the working temperature of Mg-based heat storage materials will be further reduced to meet the potential use of more fields. Moreover, with high density, the heat storage density of Mg-Ga alloys can be guaranteed, so there are the promising use of Mg-Ga alloys as PCMs for TES system. So far, the number of studies on Mg-Ga alloys is rather limited.Some scholars have studied the hydrogen storage properties[11]and characterization and corrosion behavior[12]of Mg-Ga alloys, but different fields claim different properties of materials. There are also some researchers who have simulated the thermophysical properties of Mg-Ga alloys[13], but the actual tests and systematic studies on Mg-Ga alloys as phase change heat storage materials have not been reported.Therefore, the study of heat storage properties of Mg-Ga alloys is of great significance to the development of Mg-based heat storage alloys. In consequence, we are aimed at preparing three components of Mg-Ga alloys,and investigate their microstructure and thermophysical performance as PCMs for TES system, so as to provide reference for research and application in phase change heat storage.

2 Experimental

2.1 Materials and preparation

Pure magnesium ingot(99.98%) and 4N grade pure gallium ingot(99.99%) were used to prepare three type of Mg-Ga alloys. The synthesis of alloys were melted in a graphite crucible in a well type resistance furnace under the protection of argon gas(99.999%).The chemical compositions of Mg-Ga alloys were measured by X-ray fluorescence(XRF, Zetium)spectrometer and are shown in Table 1. The smelting temperature was 600 ℃. RJ-2 flux refining agent was used to facilitate slag removal and reduce the oxidation of magnesium in the smelting process. The composition of RJ-2 flux refining agent and coating agent are shown in Table 2. The steel mold with the cavity dimension ϕ30 mm×100 mm was preheated up to about 200 ℃ for casting alloy ingot. Finally, the samples were sectioned from the bottom center of casting, which were etched by an etchant of 4vol% nitric acid + ethylalcohol.

Table 1 The composition of Mg-Ga alloys

Table 2 The composition of RJ-2 flux

2.2 Analysis methods

The microstructure and phase composition of the samples were analyzed by X-ray diffraction(XRD, D8 Advance) and electron probe microanalysis(EPMA, JAXA-8230) with an energy-dispersive X-ray spectrometer (EDS, NCAX-ACT). Differential scanning calorimeter (DSC, DSC8500) analyses were adopted for the measurements of phase change temperature,latent heat and specific heat capacity in the range of 25-500 ℃ at the heating rate of 10K/min. The thermal expansion of Mg-Ga alloy samples with dimensions of 5 mm×5 mm×20 mm were measured by pushrod type dilatometer (DIL 402C) in the range of 25-350℃ at a constant heating rate of 5 K/min. The thermal diffusivity measurements of the block sample with dimensions of 10 mm×10 mm×2.5 mm were performed by laser-flash method (LFA457) in the range of 25-350℃. Each sample was measured at least three times at temperatures of 50, 100, 150, 200, 250, 300, and 350℃, respectively. The density at high temperature was calculated using the equation below[14]:

where,ρ0is the density of the alloy at 20 ℃, which can be measured by the Archimedes method, and ΔL/L0is the relative elongation, which can be calculated with the help of the values of measured thermal expansion.

Thermal conductivity was calculated using the relation[9]:

where,ais the thermal diffusivity,ρis the density, andcpis the specific heat capacity at constant pressure.

3 Results and discussion

3.1 Microstructure analysis

The XRD patterns of Mg-36%Ga, Mg-43%Ga,and Mg-45%Ga alloys are shown in Fig.1. As shown in Fig.1,the phases of three type of alloys are all composed of α-Mg and Mg5Ga2, indicating that the chemical composition changes do not change the phase composition of the alloys, but only resulting in the difference of proportions and characteristic peak intensity of the two phases. It can be found that the diffraction peak intensity of α-Mg decreases gradually with increasing the content of Ga in the alloys, which indicates that the proportion of α-Mg phase decreases with increasing Ga content, thus it can be inferred that the proportion of Mg5Ga2phase will increase gradually.

Fig.1 XRD patterns of Mg-Ga alloys

Fig.2 shows the electron probe analysis (EPMA)micrographs of the three Mg-Ga alloys, and Table 3 shows the chemical compositions of representative regions (A,E) and points (B, C, D, F, G) chosen in the process of EDS. As can be seen from Fig.2, the microstructure of Mg-Ga alloys are significantly different. The EPMA image of the Mg-36%Ga, Mg-43%Ga, and Mg-45%Ga in Fig.2 reveals that the microstructure of these three alloys consisting ofa mixture of black, gray and white contrast areas.The microstructure of Mg-36%Ga alloy consists of gray matrix and black primary phase (Fig.2(a)). It is obvious that the gray matrix is composed of black and white areas interlaced in the high magnification images. Combined with the results of analysis of XRD(Fig.1) and EDS (Table 3), it is believed that the black primary phase is α-Mg phase and the gray matrix is α-Mg+Mg5Ga2eutectic phase.

Fig.2 EPMA micrographs of Mg-Ga alloys

Table 3 Chemical composition of intermetallic phases in Fig.2/at%

Fig.3 is the phase diagram of Mg-Ga binary alloy.As seen in Fig.3, Ga element has a relatively high solubility in the magnesium alloy, which can nearly reach 4% (at%) at 420 ℃, and when the temperature decreases to 200 ℃, its solubility only reduces to about 1%(at%). The positions of Mg-36%Ga (point A) alloy is 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 + Mg5Ga2takes place in the rest of liquid-phase.Hence, the primary α-Mg is formed in Mg-36%Ga alloy, and it can be inferred that the primary α-Mg phase dissolved some of Ga in the process of precipitation,and the solubility of Ga element in α-Mg phase reaches the maximum when the temperature drops to eutectic temperature. After the eutectic reaction is completed,temperature continues to decline, and then the solubility of Ga element in α-Mg solid solution gradually decreases, and the excess gallium is precipitated gradually with the decrease of temperature. As the sample solidified quickly in the steel mold, gallium in α-Mg solid solution could not be precipitated normally in accordance with the equilibrium phase diagram(Fig.3). As a result, the α-Mg in region A (Fig.2(a))contained 3.53at%Ga. As for Mg-43%Ga alloy, at point b in the phase diagram, its microstructure is clearly shown in Fig.2(b). There is no obvious characteristic on the micrograph but uniform gray, and obvious black and white layered structure can be observed in the high magnification images. In Table 3, the chemical composition of point C is basically the same that of point D. Combined with the component analysis, it can be determined that both point C and D are composed of α-Mg+Mg5Ga2eutectic phase. It can be inferred that microstructure of Mg-43%Ga alloy is only consist of α-Mg+Mg5Ga2eutectic phase. However, as can be seen from the binary phase diagram of Mg and Ga shown in Fig.3, Mg-43%Ga is not the eutectic composition,but almost complete eutectic phase is obtained. The reason is also be that the solidification temperature of the alloy is lower than the eutectic temperature caused by the rapid solidification of the sample in the die,thus the complete non-equilibrium eutectic structure is obtained, that is, so called pseudoeutectic structure.With respect to Mg-45%Ga alloy, at point C (Mg-45%Ga) in the phase diagram, a reaction: L→Mg5Ga2takes place before eutectic reaction occurred during alloy solidification. Therefore, the primary Mg5Ga2phase is formed in the alloy, and combined with the chemical composition analysis of region E in Table 3,white raindrop shaped area should be primary Mg5Ga2phase. It can be clearly observed in EPMA images from Fig.2(c) that microstructure is composed of white raindrop primary Mg5Ga2phase and gray α-Mg+Mg5Ga2matrix. It can also be seen in the high magnification images that the gray matrix is still composed of black and white layered structure, and its composition is roughly the same as point B, C, and D in Mg-36%Ga and Mg-43% Ga. As expected, its structure should also be eutectic structure α-Mg+Mg5Ga2.

Fig.3 Binary phase diagrams of Mg and Ga[15]

3.2 Phase change temperature and enthalpy

Fig.4 shows the DSC curves of the three alloys measured, and the specific analysis data are shown in Table 4. As can be seen from Table 4, the melting enthalpies of Mg-36%Ga, Mg-43%Ga, and Mg-45%Ga are 146.41, 171.90, and 113.90 J/g, with the melting temperatures of 422.57, 422.70, and 422.90℃, respectively. The results show that the melting temperature of the three alloys do not differ much,but the melting enthalpy of Mg-43%Ga alloy is significantly higher than that of Mg-36%Ga and Mg-45%Ga, rise by 17.4% and 50.9%, respectively. The reason should mainly be the different contents of α-Mg+Mg5Ga2eutectic phase distributed in three alloys.It can be clearly observed from the EPMA image in Fig.2 that the α-Mg+Mg5Ga2eutectic phase content of Mg-36%Ga, Mg-43%Ga and Mg-45%Ga account for 70%-75%, 95%-99%, and 50%-55%, respectively.On the basis of Fig.3, combined with DSC curves in Fig.4, it is clear that α-Mg+Mg5Ga2eutectic phase will start to melt when heating to above 422 ℃, and is only heated up to 425 ℃ or so enough to finish the melting of the eutectic phase in the range of 410-450 ℃, and the melting temperature of α-Mg and Mg5Ga2are 650℃ and 460 ℃, respectively, so the process of melting α-Mg+Mg5Ga2eutectic phase will absorb more phase change latent heat, which is generally represented by melting enthalpy. Therefore, it can be doped out that the higher the content of α-Mg+Mg5Ga2eutectic phase, the higher the heat absorbed during melting and the higher the melting enthalpy. It is confirmed that the Mg-43% Ga alloy with the highest content of α-Mg+Mg5Ga2eutectic phase has the highest melting enthalpy. In addition, Fig.4 also shows that a long melting process and a tendency to appear a second heat absorption peak in Mg-45%Ga alloy, indicating its poor uniform fusibility.

Table 4 Phase change properties of Mg-Ga alloys

Fig.4 DSC curves of Mg-Ga alloys

3.3 Thermal expansion

The temperature dependence of the relative elongation of test alloys are shown in Fig.5.

As seen in Fig.5, the thermal relative elongation of three alloys enhances with increasing temperature,showing an obvious linear relationship. When the temperature exceeds 150 ℃, the elongation of Mg-43%Ga alloy is higher when compared with those of the other two alloys, and the elongation curves of the other two alloys gradually coincide. Density is one of the parameter for thermal conductivity calculation. The density at 20 ℃ has been measured by the Archimedes method. The densities of Mg-36%Ga, Mg-43%Ga,and Mg-45%Ga alloys are 2.384, 2.526, and 2.646 g/cm3, respectively. Based on elongation of alloys and equation (1), the curves of temperature dependence of densities of alloys are obtained, as shown in Fig. 6. The results indicate that the densities of the alloys increase gradually with increasing Ga content. The reason is that the densities of Ga and Mg are 5.90 g/cm3and 1.74 g/cm3, respectively. Therefore, the increase of Ga will lead to a significant increase in densities of alloys.In addition, the densities of the three alloys decreased gradually with increasing temperature, and the decrease amplitude was basically the same, which was consistent with little difference in elongation of alloys as shown in Fig.5.

Fig.5 Temperature dependence of the relative elongation of Mg-Ga alloys

Fig.6 Temperature dependence of density of Mg-Ga alloys

3.4 Thermal conductivity

The thermal conductivity plays an important role in whether the alloy can be used as PCMs for TES system[16]. Based on the values of thermal density,diffusivity and specific heat capacity obtained, the thermal conductivity can be calculated using equation(2), and the results are shown in Fig.9. When the temperature increased from 50 ℃ to 250 ℃, the thermal conductivity of the three alloys increased slightly with the increase of temperature, then the temperature increases up to 300 ℃, the thermal conductivity of the three alloys increases slightly or is basically flat with increasing temperature. Finally,when the temperature increased from 300 ℃ to 350℃, the thermal conductivity of Mg-36%Ga and Mg-43%Ga alloys decreased mildly with the increasing temperature.

Fig.7 Temperature dependence of thermal diffusivity of Mg-Ga alloys

Fig.8 Temperature dependence of specific heat capacity of Mg-Ga alloys

Fig.9 Temperature dependence of thermal conductivity of Mg-Ga alloys

The reason is possibly that when the phase boundary reaches a certain temperature, precipitates is dissolved, and a reversible process leads to a decrease in thermal conductivity with increasing temperature.While the thermal conductivity of Mg-45%Ga alloy increased slightly with increasing temperature. It can be noticed that the thermal conductivity of alloys was weakly temperature dependent, with small positive temperature coefficients.

By the comprehensive analysis of Figs.7-9, it can be found that alloy composition has a significant effect on thermal diffusivity, specific heat capacity and thermal conductivity, of which all the alloys decrease with increasing Ga content. The reason may be that the solubility of Ga atoms into α-Mg phase, resulting in greatly lattice distortion and destroying the periodicity of lattice, thus reducing the free path of electrons. And with the increase of Ga content, the solutes in the Mg-Ga alloys increased, acting as electron contributors and scattering centers of phonons to thermal transfer.According to the kinetic molecular theory, the thermal conductivity of materials increases when the mean free path of electrons and phonons raise. Therefore, the thermal conductivity, thermal diffusivity and specific heat capacity of the alloys decrease with increasing Ga content.

4 Conclusions

Three types of Mg-Ga alloys intended to be used as PCMs for TES system were prepared, and their thermophysical properties were investigated. The main conclusions are shown as follows:

a) The microstructure of Mg-36%Ga alloy is mainly composed of primary α-Mg phase and α-Mg+Mg5Ga2eutectic phase, the microstructure of Mg-43%Ga alloy is mainly composed of α-Mg+Mg5Ga2eutectic phase and the microstructure of Mg-45%Ga alloy is mainly composed of Mg5Ga2primary phase and α-Mg+Mg5Ga2eutectic phase.

b) The melting enthalpies of Mg-36%Ga, Mg-43%Ga, and Mg-45%Ga are 146.41, 171.90, and 113.90 J/g, respectively. And the phase change temperatures of three alloys are in the range of 422-423 ℃. The melting enthalpy of Mg-43%Ga alloy is significantly higher than those of the other two alloys,possibly because it contains a higher proportion of α-Mg+Mg5Ga2eutectic phase than the other two alloys.

c) The thermal diffusivity, specific heat capacity and thermal conductivity of the three Mg-Ga alloys decrease with increasing Ga content at 50-350 ℃.The reasons may be that solubility of Ga in α-Mg phase cause a large lattice distortion, destroys the periodicity of the lattice, and thus reduces the free path of electrons.

d) Mg-43%Ga alloy is a potential PCM used for TES. Besides, a further study of the compatibility of Mg-Ga alloys with encapsulating materials will be reported in the future.