Mg-Bi-Zn ternary phase change energy storage alloy and preparation method and application thereof

Abstract

The invention relates to the technical field of preparation of phase-change energy storage materials, in particular to an Mg-Bi-Zn ternary phase-change energy storage alloy and a preparation method and application thereof.The Mg-Bi-Zn ternary phase-change energy storage alloy prepared through the method has the chemical formula of Mg-0. 2Bi-xZn, x ranges from 24.9 to 34.9, x ranges from 0 to 2, y ranges from 0 to 2, y ranges from 0 to 2, y ranges from 0 to 2, y ranges from 0 to 2, y ranges from 0 to 2, y ranges from 0 to 2, and y ranges from 0 to 2. The Mg-Bi-Zn ternary phase change energy storage alloy is mainly composed of an alpha-Mg matrix, a petal-shaped MgZn phase and a granular and short-rod-shaped Mg3Bi2 phase, the phase change area is about 350 DEG C, and the Mg-Bi-Zn ternary phase change energy storage alloy is suitable for heat storage of most industrial waste heat.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of phase change energy storage materials, and particularly relates to a Mg-Bi-Zn ternary phase change energy storage alloy, a preparation method thereof, and an application thereof. Background Art

[0002] Thermal energy is an important form of energy in human life, and more than 90% of the energy needs to be converted and utilized through the form of thermal energy. Thermal energy storage technology is an effective method to improve energy utilization efficiency, and has broad application prospects in the fields such as solar energy utilization, "peak shaving and valley filling" of electric power, and recovery and utilization of waste heat and residual heat. At present, there are three thermal energy storage methods: sensible heat storage, phase change heat storage, and chemical reaction heat storage. Compared with sensible heat storage and chemical heat storage, phase change heat storage has many advantages such as small volume change during the phase change process, large amount of thermal energy that can be stored at a constant temperature, and large energy storage capacity. It is the most ideal thermal energy storage method considering various factors at present.

[0003] In phase change heat storage, the phase change material plays a key role. As the core of the heat storage system, phase change heat storage materials can be roughly divided into organic phase change heat storage materials, inorganic phase change heat storage materials, metal phase change heat storage materials, and composite phase change heat storage materials. Metals and their alloys have the advantages of relatively high phase change temperature, large phase change latent heat, high thermal conductivity, large energy storage density, small phase change volume change, almost no supercooling degree, and suitable price when used as phase change heat storage materials. Common metal phase change heat storage materials are all solid-liquid phase change materials, including aluminum-based alloys, magnesium-based alloys, zinc-based alloys, etc. Mg has thermal physical properties equivalent to those of Al, and because Mg has good compatibility with iron-based container materials, it has received attention as a phase change heat storage material. Researchers have found that Mg-based alloys such as Mg-Zn, Mg-Cu, Mg-Ca, Mg-Sn, and Mg-Bi have good energy storage performance and are good phase change materials. However, at present, the phase change heat storage temperature of most Mg-based alloys is above 400 °C, which is a high-temperature phase change heat storage material, and to a certain extent, limits the application range of metal-based phase change heat storage materials. Summary of the Invention

[0004] Aiming at the deficiencies of the above-mentioned prior art, the purpose of the present invention is to provide a Mg-Bi-Zn ternary phase change energy storage alloy, a preparation method thereof, and an application thereof. The phase change energy storage alloy prepared by the present invention is mainly composed of an α-Mg matrix, petal-shaped MgZn phases, and granular and short rod-shaped Mg3Bi2 phases. The phase change region of the phase change energy storage alloy is about 350 °C, which is suitable for the heat storage of most industrial waste heat.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A Mg-Bi-Zn ternary phase change energy storage alloy, and the chemical formula of each element in the composition of the Mg-Bi-Zn ternary phase change energy storage alloy is Mg-0.2Bi-xZn by mass percentage, where 0.2 and x respectively represent the mass percentages of Bi and Zn, x is 24.9 to 34.9, and the balance is Mg.

[0007] For the Mg-Bi-Zn hypoeutectic alloy prepared by the present invention, increasing the zinc content is beneficial to increasing the latent heat of the material. Combining with the alloy microstructure, it can be seen that when the zinc content is increased, the alloy structure is refined. After refinement, the grain boundaries in the alloy increase. The increase in grain boundaries means that more energy is required to overcome the grain boundary obstacles during the phase change process, so that the solid-liquid phase change needs to absorb more latent heat, and thus the phase change temperature is reduced.

[0008] In a preferred embodiment of the present invention, the phase change temperature of the Mg-Bi-Zn ternary phase change energy storage alloy is 342.96 °C to 343.09 °C.

[0009] In a preferred embodiment of the present invention, the room temperature structure of the Mg-Bi-Zn ternary phase change energy storage alloy is mainly composed of an α-Mg matrix, MgZn phase, and granular and short rod-shaped Mg3Bi2 phases.

[0010] In a preferred embodiment of the present invention, the Mg-Bi-Zn ternary phase change energy storage alloy is a hypoeutectic alloy.

[0011] Another object of the present invention is to provide a preparation method of the Mg-Bi-Zn ternary phase change energy storage alloy described in any one of the above, including the following steps:

[0012] Weigh raw materials pure magnesium, pure zinc and pure bismuth according to the mass percentage of Mg-0.2Bi-xZn, and mix them, where x is 24.9 to 34.9.

[0013] Repeatedly melt the mixed raw materials to obtain a Mg-Bi-Zn ternary phase change energy storage alloy with uniform composition.

[0014] In a preferred embodiment of the present invention, the melting temperature is 675 °C to 685 °C.

[0015] In a preferred embodiment of the present invention, a refining agent and a covering agent are added during the melting process to reduce the oxidation of magnesium during the melting process.

[0016] In a preferred embodiment of the present invention, the purities of pure magnesium, pure zinc and pure bismuth are all ≥99.9%.

[0017] The third object of the present invention is to provide an application of the Mg-Bi-Zn ternary phase change energy storage alloy described in any one of the above in a phase change heat storage material.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] 1. For the Mg-Bi-Zn ternary phase change energy storage alloy prepared by the present invention, the elements of each component are calculated by mass percentage, and the chemical formula is Mg-0.2Bi-xZn, where x is 24.9-34.9. The Mg-Bi-Zn ternary phase change energy storage alloy is mainly composed of an α-Mg matrix, petal-shaped MgZn phases, and granular and short rod-shaped Mg3Bi2 phases. The phase change region is around 350°C, which is suitable for heat storage of most industrial waste heat.

[0020] 2. For the hypoeutectic alloy of Mg-Bi-Zn prepared by the present invention, increasing the content of zinc is beneficial to increasing the latent heat of the material. Combining with the alloy microstructure, it can be seen that when the zinc content is increased, the alloy structure is refined. After refinement, the grain boundaries in the alloy increase. The increase in grain boundaries means that more energy is required to overcome the obstacles of grain boundaries during the phase change process, so that more latent heat needs to be absorbed during the solid-liquid phase change, and thus the phase change temperature is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is the XRD pattern of alloys with different compositions of the present invention.

[0022] Figure 2 It is the microstructure of alloys with different compositions of the present invention, (a) Mg-0.2Bi-24.9Zn; (b) Mg-0.2Bi-29.9Zn; (c) Mg-0.2Bi-34.9Zn.

[0023] Figure 3 It is the EDS element distribution map of the Mg-0.2Bi-29.9Zn magnesium alloy prepared in Example 1.

[0024] Figure 4 It is the EDS energy spectrum of different morphology tissues, where spectrum Figure 1 is the granular tissue, spectrum Figure 2 is the short rod-shaped tissue, spectrum Figure 3 is the gray tissue, spectrum Figure 4 is the petal-shaped tissue.

[0025] Figure 5 It is the (a) TEM photo, (b) selected atomic arrangement, and (c) electron diffraction pattern of the Mg-0.2Bi-29.9Zn magnesium alloy prepared in Example 1.

[0026] Figure 6 It is the TEM photo of the Mg-0.2Bi-29.9Zn magnesium alloy prepared in Example 1 and the electron diffraction patterns of different tissues.

[0027] Figure 7 It is the DSC curve of alloys with different compositions.

[0028] Figure 8 Specific heat capacity curves of alloys with different compositions. Specific implementation manners

[0029] The following combines the embodiments of the present invention, and uses preferred embodiments and accompanying drawings for detailed description. The technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0030] It should be noted that all the professional terms used in the present invention are only for the purpose of describing specific embodiments, and are not intended to limit the protection scope of the present invention. Unless otherwise specifically stated, all kinds of raw materials, reagents, instruments and equipment used in the following embodiments of the present invention can be obtained through the market or prepared by existing methods.

[0031] Example 1

[0032] A preparation method of a Mg-0.2Bi-24.9Zn ternary phase change energy storage alloy, comprising the following steps:

[0033] (1) Weigh raw materials pure magnesium, pure zinc and pure bismuth according to the mass percentages of Mg-0.2Bi-24.9Zn.

[0034] (2) Use a pit resistance furnace to melt the mixed raw materials in a graphite crucible, and melt repeatedly 3 times to ensure uniform alloy composition. The melting temperature is 680 °C. In order to facilitate slag removal and reduce the oxidation of magnesium during melting, 0.5% of RJ-2 refining agent based on the mass of the metal melt and 3% of covering agent sodium chloride based on the mass of the furnace charge are selected to be added.

[0035] (3) Use a metal mold to pour the sample. Preheat it to 220 °C before pouring, and protect the pouring port with inert gas. The mold size is φ30mm×100mm.

[0036] Example 2

[0037] A preparation method of a Mg-0.2Bi-29.9Zn ternary phase change energy storage alloy, comprising the following steps:

[0038] (1) Weigh raw materials pure magnesium, pure zinc and pure bismuth according to the mass percentages of Mg-0.2Bi-24.9Zn.

[0039] (2) The mixed raw materials are melted in a pit resistance furnace in a graphite crucible, and melted repeatedly three times to ensure uniform alloy composition. The melting temperature is 680 °C. To facilitate slag removal and reduce the oxidation of magnesium during melting, 0.5% of RJ-2 refining agent based on the mass of the molten metal and 3% of covering agent sodium chloride based on the mass of the charge are selected to be added.

[0040] (3) The specimens are cast using a metal mold. Before casting, it is preheated to 220 °C, and an inert gas is used to protect the pouring gate. The mold size is φ30mm×100mm.

[0041] Example 3

[0042] A preparation method of a Mg-0.2Bi-34.9Zn ternary phase change energy storage alloy, comprising the following steps:

[0043] (1) The raw materials pure magnesium, pure zinc and pure bismuth are weighed according to the mass percentages of Mg-0.2Bi-24.9Zn.

[0044] (2) The mixed raw materials are melted in a pit resistance furnace in a graphite crucible, and melted repeatedly three times to ensure uniform alloy composition. The melting temperature is 680 °C. To facilitate slag removal and reduce the oxidation of magnesium during melting, 0.5% of RJ-2 refining agent based on the mass of the molten metal and 3% of covering agent sodium chloride based on the mass of the charge are selected to be added.

[0045] (3) The specimens are cast using a metal mold. Before casting, it is preheated to 220 °C, and an inert gas is used to protect the pouring gate. The mold size is φ30mm×100mm.

[0046] Result analysis

[0047] ICP test results

[0048] The ICP test was completed at Xi'an Hantang Analysis and Testing Co., Ltd. The test results are shown in Table 1. It can be seen that there are slight deviations between the contents of the specimens and the designed contents, but the deviation of the Bi element content does not exceed 0.03%, the deviation of the Zn element content does not exceed 1%, and the deviation of the Mg element content does not exceed 1%, all within the error range. Therefore, the three specimens obtained are reliable.

[0049] Table 1 ICP test results of three specimens prepared in Example 1 to Example 3

[0050] Mg (wt.%) Bi (wt.%) Zn (wt.%) Mg-0.2Bi-24.9Zn 74.68 0.23 25.09 Mg-0.2Bi-29.9Zn 70.05 0.21 29.29 Mg-0.2Bi-34.9Zn 65.84 0.20 33.96

[0051] XRD analysis

[0052] Figure 1XRD patterns of the three alloys prepared in Examples 1 to 3. Combining the Mg-Zn phase diagram and the Mg-Bi phase diagram, it can be seen that the three alloys are all hypoeutectic compositions. During the equilibrium solidification process of the alloys, primary α-Mg is first precipitated from the liquid phase. When cooled to the eutectic point temperature of 340 °C, the remaining liquid phase undergoes eutectic transformation to form a eutectic structure of Mg + Mg7Zn3. When further cooled to 325 °C, Mg7Zn3 undergoes eutectoid transformation to form an α-Mg + MgZn eutectoid cluster. During the process of cooling to room temperature, the Mg3Bi2 phase is precipitated from α-Mg. The actual solidification process is non-equilibrium and segregation will inevitably occur. The first-crystallized part contains more components with high melting points, and the later-crystallized part contains more components with low melting points, making the composition inside the grains uneven. Therefore, the room temperature structure after non-equilibrium solidification is mainly composed of primary α-Mg, (α-Mg + Mg7Zn3) eutectic structure, (α-Mg + MgZn) eutectoid structure, and the precipitated Mg3Bi2 structure.

[0053] It can be seen from the XRD analysis results that the three alloys are mainly composed of α-Mg, Mg7Zn3, and MgZn2 phases. The Mg3Bi2 phase may not be marked because the Bi content is only 0.2%, which is relatively low. Both the Mg7Zn3 phase and the MgZn phase are unstable compounds, while MgZn2 is a stable compound. Therefore, the MgZn2 phase finally exists in the alloy.

[0054] Microstructure analysis

[0055] Figure 2 Microstructures of three alloys with different compositions, (a) Mg-0.2Bi-24.9Zn; (b) Mg-0.2Bi-29.9Zn; (c) Mg-0.2Bi-34.9Zn. Comparison Figure 2 In (a) - (c), it can be seen that the primary phase of the alloy is petal-shaped. As the Zn content increases, the color of the petal-shaped structure becomes darker, indicating that the amount of magnesium-zinc phase in the eutectic phase and the eutectoid phase increases. Secondly, as the Zn content increases, the grains of the petal phase significantly decrease. This is because as the Zn content increases, the liquidus temperature of the alloy decreases, the temperature at which the primary phase precipitates decreases, and the growth time of the primary phase during the cooling process is shortened, so it is finer. In addition, there are also granular and short rod-shaped second phases in the alloy. Combining the phase diagram analysis, it may be the Bi-containing phase.

[0056] The area scanning EDS element distribution map of the Mg-0.2Bi-29.9Zn alloy is as Figure 3 shown, where green is the Mg element, red is the Zn element, and yellow is the Bi element. From Figure 4It can be seen that the region corresponding to the dark gray petal-shaped tissue mainly contains Mg element and trace amounts of Zn element. Therefore, the dark gray petal-shaped tissue should be primary α-Mg and a small amount of eutectic and eutectoid tissues. The region corresponding to the light gray reticular tissue mainly contains Zn element. Therefore, the light gray reticular tissue should be eutectic and eutectoid tissues. Most of the Bi element is distributed in granular form, and a very small amount is distributed in short rod form. Therefore, the granular and short rod-shaped secondary phases distributed on the dark gray and light gray tissues should be Mg3Bi2 phase.

[0057] The SEM image and EDS spectrum of Mg-0.2Bi-29.9Zn alloy are as Figure 4 . Combining Figure 4 with the spectra Figure 1 and Figure 2 and the above analysis, it can be seen that the granular and short rod-shaped secondary phases distributed on the α-Mg matrix and magnesium-zinc phase are Mg3Bi2 phase. Combining spectrum Figure 3 , it can be seen that the dark gray petal-shaped phase is mainly α-Mg phase, and there are a small amount of Mg7Zn3, MgZn2 phases and Mg3Bi2 phase. Combining spectrum Figure 3 , it can be seen that the light gray reticular phase is mainly Mg7Zn3, MgZn2 phases, and there is a small amount of Mg3Bi2 phase.

[0058] Figure 5 are the TEM photo, selected area atomic arrangement and electron diffraction pattern of Mg-0.2Bi-29.9Zn magnesium alloy. It can be seen that the selected area atomic arrangement rule is

[010] Mg. After analysis, the interplanar spacing of the crystal plane is 0.2475nm, the interplanar spacing of the crystal plane is 0.2342nm, the interplanar spacing of the crystal plane is 0.2474nm.

[0059] Figure 6 are the TEM bright field image of the precipitation phase morphology of Mg-0.2Bi-29.9Zn magnesium alloy and the calibration result of the diffraction pattern. From Figure 6 , it can be seen that the dark gray tissue is

[010] Mg, the light gray tissue is

[101] MgZn2, and the granular tissue is Mg3Bi2, which further illustrates the phase composition in the alloy.

[0060] DSC and density analysis

[0061] The DSC test results of alloys with different compositions are as Figure 7 shown. From Figure 7It can be seen that the temperatures at which phase transitions start in Mg-0.2Bi-xZn (x = 24.9, 29.9, 34.9) are 342.96 °C, 342.86 °C, and 343.09 °C respectively. The latent heat of phase transition of the specimens was obtained by calculating the area of the peak of the DSC curve. The latent heats of phase transition of the three alloys are 53.16 J / g, 66.68 J / g, and 81.36 J / g respectively. It can be seen that for hypoeutectic Mg-Bi-Zn alloys, increasing the zinc content is beneficial to increasing the latent heat of the material. Combining with the microstructure of the alloy, it can be known that when the zinc content is increased, the alloy structure is refined. After refinement, the number of grain boundaries in the alloy increases. The increase in grain boundaries means that more energy is required to overcome the obstacles of grain boundaries during the phase transition process, so that more latent heat needs to be absorbed during the above solid-liquid phase transition.

[0062] Thermal capacity analysis

[0063] Figure 8 Fig. is the specific heat capacity curves of alloys with different compositions. From the above DSC data, it can be seen that the phase temperatures of the three alloys are all around 340 °C, that is, 627.15 K. Therefore, the heat capacity was measured in the temperature range of 540 K to 658 K. Comparing the specific heat capacity curves of the three alloy compositions, it can be seen that as the Zn content increases, the peak value of the heat capacity increases, which further indicates that the heat storage capacity is enhanced. Small phase transition regions began to appear in the specific heat capacity curves of the three alloys at about 330 °C. Research shows that the melting point of the MgZn phase is about 367 °C, and the melting point of the Mg3Bi2 phase is 821 °C. Therefore, this peak may be the heat capacity peak of the melting of the MgZn phase with a small amount of impurities. Large and sharp phase transition peaks began to appear in the three alloys at about 340 °C. This region is the phase transition interval for the heat storage and release of the obtained phase change material. Therefore, the following heat capacity data analysis is carried out around this phase transition interval.

[0064] The least squares analysis is a mathematical optimization technique that finds the best function match for a set of data by minimizing the sum of the squares of the errors. To ensure the accuracy of the fitting results, the least squares method was used to fit the temperature range before the phase transition.

[0065] The formula for calculating the reduced transition temperature is as follows:

[0066]

[0067] Where X is the reduced transition temperature, T is the experimental temperature, and T1 to T2 is the temperature range where no phase transition occurs.

[0068] The polynomial calculation formulas and fitting coefficients of the specific heat capacity (Cp) in the temperature range before the phase transition of the three different alloys and the corresponding reduced transition temperature X are as follows.

[0069] Table 2 Fitting data in the temperature range before the phase transition (542 K - 608 K) of three different alloys

[0070]

[0071] From the fitting coefficients, it can be seen that the fitting accuracies of the first two alloys are both higher than 99%, and the fitting accuracy of the third alloy is higher than 95%.

[0072] The experimental heat capacities of the three phase change energy storage alloys were calculated by the least squares method, and the polynomial equation of the smoothed heat capacity (Cp') varying with the reduced temperature (X) and the smoothed heat capacity values every 1 K are shown in Table 3. In the non-phase change temperature range, with the increase of the Zn content in the alloy, the smoothed heat capacity of the energy storage material shows an upward trend.

[0073] Table 3 Smoothed heat capacities of three different alloys

[0074]

[0075]

[0076]

[0077] In summary, the three alloys of Mg-0.2Bi-xZn (x = 24.9, 29.9, 34.9) prepared by the present invention are mainly composed of an α-Mg matrix, petal-shaped MgZn phases, and granular and short rod-shaped Mg3Bi2 phases. With the increase of the Zn content, the MgZn phases increase, and the petal morphology has a tendency to shrink inward. The phase change initiation temperatures of the three alloys are 342.96 °C, 342.86 °C, and 343.09 °C respectively, and the latent heats of phase change are 53.16 J / g, 66.68 J / g, and 81.36 J / g respectively. It can be seen that the phase change regions of the three alloys are around 350 °C, which are suitable for the heat storage of most industrial waste heat. The experimental heat capacities of the three alloys were fitted with data using Origin software, and the polynomial calculation formula of the heat capacity (Cp) before phase change and the corresponding reduced transformation temperature X was calculated. The fitting function has high accuracy. In the non-phase change temperature range, with the increase of the Zn content in the alloy, the smoothed heat capacity of the energy storage material shows an upward trend.

[0078] It should be noted that when the present invention involves numerical ranges, it should be understood that both endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods adopted are the same as those in the embodiments, in order to prevent repetition, the present invention describes the preferred embodiments. Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the appended claims are intended to be construed as including the preferred embodiments and all changes and modifications falling within the scope of the present invention.

[0079] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and variations.

Claims

1. A Mg-Bi-Zn ternary phase change energy storage alloy, characterized in that, The Mg-Bi-Zn ternary phase change energy storage alloy consists of the following components by mass percentage: 0.2% Bi, 24.9% - 34.9% Zn, and the balance is Mg, with a total of 100%. Its chemical formula is Mg-0.2Bi-xZn, where x is 24.9 - 34.

9.

2. The Mg-Bi-Zn ternary phase change energy storage alloy according to claim 1, wherein The phase change temperature of the Mg-Bi-Zn ternary phase change energy storage alloy is 342.96°C - 343.09°C.

3. The Mg-Bi-Zn ternary phase change energy storage alloy according to claim 1, wherein The room temperature structure of the Mg-Bi-Zn ternary phase change energy storage alloy mainly consists of an α-Mg matrix, MgZn phase, and granular and short rod-shaped Mg3Bi2 phases.

4. The Mg-Bi-Zn ternary phase change energy storage alloy according to claim 1, characterized in that, The Mg-Bi-Zn ternary phase change energy storage alloy is a hypoeutectic alloy.

5. A method for preparing the Mg-Bi-Zn ternary phase change energy storage alloy according to any one of claims 1 to 4, characterized in that, It includes the following steps: Weigh the raw materials pure magnesium, pure zinc, and pure bismuth according to the mass percentage of Mg-0.2Bi-xZn, and mix them, where x is 24.9 - 34.9; Repeatedly melt the mixed raw materials to obtain a Mg-Bi-Zn ternary phase change energy storage alloy with uniform composition.

6. The preparation method of the Mg-Bi-Zn ternary phase change energy storage alloy according to claim 5, wherein The melting temperature is 675°C - 685°C.

7. The preparation method of the Mg-Bi-Zn ternary phase change energy storage alloy according to claim 5, characterized in that, Add a refining agent and a covering agent during the melting process to reduce the oxidation of magnesium during melting.

8. The preparation method of the Mg-Bi-Zn ternary phase change energy storage alloy according to claim 5, characterized in that, The purities of pure magnesium, pure zinc, and pure bismuth are all ≥99.9%.

9. Application of the Mg-Bi-Zn ternary phase change energy storage alloy according to any one of claims 1 - 4 in a phase change heat storage material.

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