Study on formation and properties of Al–Li–Sm alloy containing whiskers in molten salts

Abstract

The electrochemical behaviors of samarium ions on a liquid aluminum electrode in LiCl–KCl melts at 973 K were investigated by cyclic voltammetry, square wave voltammetry and a cathodic polarization test. The results show that only one kind of Al–Sm intermetallic compound can be formed under our experimental conditions. The Al–Li–Sm alloy containing whiskers was obtained by galvanostatic electrolysis in LiCl–KCl melts after the addition of 10 wt% SmCl₃ on a liquid aluminum electrode at 973 K. The alloy containing whiskers was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS). Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to examine the percentage composition of each element. Transmission electron microscopy (TEM) with electron diffraction was employed to characterize the crystal structure of the needle-like precipitates. The mechanical properties of the micro hardness and Young’s modulus were characterized by the Leitz micro hardness tester and the tool for Young’s modulus, respectively.

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1. Introduction

Light metals, often referred to as having a density which is less than 3.5 g cm⁻³, include aluminum, magnesium, beryllium and alkali metals. The most representative light metals are aluminum and magnesium, corresponding to aluminum and magnesium alloys. Lithium is the lightest alloying element. The density of an aluminum alloy will be reduced by three percent after each one weight percent of lithium is added.^1–3 Thus, the Al–Li alloys which have a low density, high specific strength and high specific stiffness can not only satisfy the requirements of the aerospace industry, but also show wide application prospects.^3–5 However, its large-scale application in the aerospace industry does not materialize because Al–Li alloys have the disadvantages of a low Young’s modulus, high toughness, high fracture toughness, low strength, and high anisotropy of mechanical properties.⁵ To solve the above problems, rare earth elements were added into Al–Li alloys. Rare earth elements can improve the properties of Al based alloys by forming an Al–RE intermetallic compound, removing the impurities and changing the form of a precipitate during melting processes.^5,6 Whereas, the Young’s modulus and stiffness of the alloy increase a little.

A whisker is a perfect crystal. Researchers have adopted the whiskers and fibers of SiC, B₄N and Saffil Al₂O₃ (5% SiO₂ δ-Al₂O₃) to reinforce the properties of alloys and make considerable headway.^7–10 However, since the whiskers and fibers are adscititious, the binding force with the alloy matrix interface is confined. The range of strength has not been enhanced much. Therefore, the prospect that if the whiskers can grow in the alloy interior and strengthen the mechanical property of the alloy obtained by galvanostatic electrolysis/potentiostatic electrolysis in molten salts attracted our attention. In the electroextraction process of europium in LiCl–KCl–AlCl₃–Eu₂O₃, lots of the needle-like precipitates were obtained by Yan et al.¹¹ The results from XRD and SEM show that the needle-like precipitates are Al–Eu intermetallic components. However, further investigation has not been conducted to research whether the needle-like precipitates are whiskers. In addition, to the best of our knowledge, the whiskers prepared by galvanostatic electrolysis/potentiostatic electrolysis in molten salts on a liquid cathode have never been reported up to the present moment.

The method of the liquid cathode is a process to form an alloy with the assistance of a liquid metal as the cathode. This method can reduce one or more metallic ions from a molten salt system to a metallic atom by an electrode reaction at a relatively positive potential. Adopting this method to manufacture alloys can reduce the activity of the alloy, pollution and the loss of dissolution, and it can electrodeposit high-melting metals to a low melting point alloy at a lower temperature. In recent years, the preparation of an alloy and the extraction of lanthanides and actinides on a liquid cathode have been carried out by many researchers. Castrillejo et al.¹² investigated the electrochemical behavior of praseodymium ions in LiCl–KCl melts on liquid cadmium and bismuth electrodes and successfully prepared various kinds of Pr–Cd and Pr–Bi intermetallic compounds, respectively. Kim et al.¹³ presented the electrochemical behavior of cerium on solid W and liquid cadmium electrodes and calculated the Gibbs free energies for the formation of Ce–Cd intermetallic compounds. Kim et al.¹⁴ and Koyama et al.¹⁵ investigated the electrochemical behavior of uranium in LiCl–KCl molten salts on a liquid cadmium electrode. Their results show that the reduction of uranium is a one step reaction with a transfer of three electrons on an inert electrode, and the difference in the reduction potential between the inert and liquid cadmium electrode is caused by the activity decrease on the liquid electrode. Under the condition of the differences of reduction potentials, the electrorefining of uranium on the liquid cadmium electrode was successfully carried out. Our group exhibited the electrochemical behavior of neodymium¹⁶ in NaCl–KCl molten salts on a liquid aluminum electrode, and successfully prepared Al–Nd alloys. Based on the literature mentioned above, all electrochemical behavior and electrolysis experiments were carried out on a liquid electrode. However, information about the preparation of Al–Li–RE alloys containing whiskers on a liquid aluminum electrode in LiCl–KCl melts has never been reported up to now. Therefore, it is of great significance to explore the electrochemical behavior of REs and prepare Al–RE intermetallic compound whiskers on a liquid aluminum electrode.

Generally speaking, it is difficult to obtain bulk alloys and a high reaction rate on a solid cathode at a low operation temperature in chloride melts. Moreover, the reaction rate is controlled by the diffusion rate of Ln reduction and the diffusion rate of Ln atoms into the aluminum substrate. Therefore, liquid aluminum at a high temperature is selected as the working electrode to prepare the Al–Li–Sm alloy containing whiskers. At the same time, the micro hardness and Young’s modulus of the Al–Li–Sm alloy with and without whiskers were characterized by the Leitz micro hardness tester and the tool for Young’s modulus, respectively.

2. Experimental

2.1. Preparation and purification of the melts

The mixture of eutectic LiCl–KCl was first dried under vacuum for more than 48 h at 573 K to remove excess water, and then put into an alumina crucible which was placed in an electric furnace. The mass ratio of LiCl–KCl was 1 : 1. The samarium ions were introduced into the alumina crucible in the form of anhydrous SmCl₃ (99.0%) powder. The whole process of the electrochemical measurements and the electrolyte were protected by a dry argon atmosphere to avoid electrolyte contact with oxygen and water. The temperature of the molten salts was monitored by the nickel–chromium thermocouple sheathed with an alumina tube.

2.2. Electrochemical apparatus and electrodes

All electrochemical measurements were measured using an Autolab potentiostat/galvanostat, which was controlled by the Nova 1.6 software package. For the cyclic voltammetry, square wave voltammetry and cathodic polarization curve, the counter electrode was a spectral pure graphite rod of 6 mm diameter. A silver wire (d = 1 mm, 99.99% purity) dipped into a solution of AgCl (1 wt%) in the LiCl–KCl eutectic contained in a Pyrex tube was used as the reference electrode. All potentials were referred to this Ag⁺/Ag couple. The working electrode was a liquid aluminum electrode placed in an alumina crucible. A Mo wire (d = 1 mm), which was polished thoroughly using SiC paper and then cleaned ultrasonically with ethanol prior to use, placed in an alumina sleeve was immersed in the liquid aluminum electrode and used as the electric lead. The active electrode surface area was 1.76 cm², which was determined by the cross sectional areas of the alumina crucible and alumina sleeve.

2.3. Preparation and characterization of Al–Li–Sm alloys

The Al–Li–Sm alloy containing whiskers was prepared by the galvanostatic electrolysis at −0.17 A cm⁻² for 5 hours on the liquid aluminum electrode. After the electrolysis, the samples were abraded and polished with 300#, 1500# and 2000# SiC paper followed by ultrasonic cleaning in ethylene glycol and ethanol (99.8% purity) in an ultrasonic bath for 15 min and stored in the glove box until their analysis. The samples were analyzed by XRD (Rigaku D/max-TTR-III diffractometer) using Cu-Kα radiation at 40 kV and 150 mA. Scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) (JSM-6480A; JEOL Co., Ltd) was used to analyze the microstructure and micro-zone chemical of bulk the Al–Li–Sm alloys. To determine the content of Al, Sm and Li in the sample, the sample was dissolved in aqua regia (HNO₃ : HCl : H₂O = 1 : 3 : 8, v/v). The solution was diluted and analyzed by ICP-AES (Thermo Elemental, IRIS Intrepid II XSP). The crystal structure of the Al–Li–Sm alloy was analyzed by transmission electron microscopy (JEOL JEM-2010) with electron diffraction. The TEM sample preparation steps were as follows: the samples with a thickness of about 0.4 mm were made by line cutting, and then were abraded and polished to a thickness of about 50 μm with 200#, 400# and 1500# SiC paper. After that, the sample was made into a circular sheet with a diameter of 3 mm, and was thinned by a Precision Ion Polishing System (Gatan691). The voltage was 4 kV. The current was 1.5 mA and the angle was −15–15 degrees. The whole process of thinning took 2 h. The Leitz micro hardness tester with a loading force of 200 N and a holding pressure time of 15 s was used to measure the micro hardness of the alloy. The Young’s modulus of the alloy was measured by a 5077PR square ware pulser Receiver and Tektronix (DPO 3034) oscillograph.

3. Results and discussions

3.1. The electrochemical behavior of Sm(III) on a liquid aluminum electrode in LiCl–KCl melts

Cyclic voltammetry. Firstly, typical cyclic voltammetry was employed to investigate the electrochemical behavior of Sm(III) in chloride melts. Fig. 1 elucidates the cyclic voltammograms in LiCl–KCl melts before (curve 1) and after (curves 2 and 3) the addition of 2 wt% SmCl₃ on the liquid aluminum electrode at 973 K. Two pairs of redox signals are observed before the addition of SmCl₃ (curve 1). At the beginning of curve 1, an anodic signal A′ is observed, which is ascribed to the dissolution of the liquid aluminum electrode. As the potential turns negative, the reduction signal A at about −0.95 V is related to the reduction of Al(III) ions on the aluminum substrate. The huge cathodic signal B at about −2.0 V at the end of curve 1 is related to the formation of an Al–Li alloy, which is caused by the underpotential deposition of lithium ions on the liquid aluminum electrode. In the direction of the anode, an anodic signal B′ at around −1.35 V is attributed to the dissolution of an Al–Li alloy. After the addition of 2 wt% SmCl₃, a new pair of redox signals between A and B appears. Curve 2 shows the typical cyclic voltammogram obtained in LiCl–KCl–SmCl₃ (2 wt%) melts on the liquid aluminum electrode at 973 K. The reduction signal C at around −1.40 V is attributed to the formation of an Al–Sm intermetallic compound. And the anodic signal C′ between the signals A′ and B′ at around −1.15 V is related to the dissolution of the Al–Sm intermetallic compound. In order to investigate the corresponding relationship of the cathodic/anodic signals, curve 3 was plotted from −0.50 to −1.60 V. It is obvious that the cathodic/anodic signals B/B′ disappear and there are no other cathodic/anodic signals in the potential range. That is to say, only one kind of Al–Sm intermetallic compound can be formed under our experimental conditions. The formation potential of the Al–Sm intermetallic compound is more positive than the reduction potential of Sm(II) on an inert electrode. The reason for the potential difference is due to the activity of the deposited metal decreasing in the liquid aluminum electrode.¹⁷

Fig. 1 Typical cyclic voltammograms obtained in LiCl–KCl melts before (curve 1) and after (curves 2 and 3) the addition of SmCl₃ (2 wt%) on the liquid aluminum electrode (S = 1.76 cm²) at 973 K. Scan rate: 0.1 V s⁻¹.

Square wave voltammetry. Square wave voltammetry is a more sensitive method than cyclic voltammetry. Therefore, square wave voltammetry was employed to further investigate the electrochemical behavior of Sm(III) on the liquid aluminum electrode. Fig. 2 illustrates the square wave voltammograms plotted from −0.90 to −2.0 V in LiCl–KCl melts before (curve 1) and after (curve 2) the addition of 2 wt% SmCl₃ on the liquid aluminum electrode at 973 K with a step potential of 1 mV, frequency of 10 Hz and pulse height of 25 mV. Two reduction signals can be observed (curve 1) before the addition of SmCl₃. The first signal A at beginning of curve 1 is related to the reduction of aluminum ions on an aluminum substrate. The second signal B at the end of the curve is attributed to the formation of an Al–Li alloy, which is formed by the underpotential deposition of lithium on the liquid aluminum electrode. After the addition of 2 wt% SmCl₃ (curve 2) to the LiCl–KCl melts on the liquid aluminum electrode at 973 K, a new reduction signal C which begins at −1.07 V and terminates at −1.55 V appears. The reduction signal C is attributed to the formation of an Al–Sm intermetallic compound. The results from the square wave voltammetry are in accordance with the results from the cyclic voltammetry.

Fig. 2 Square wave voltammograms of LiCl–KCl melts before (curve 1) and after (curve 2) the addition of SmCl₃ (2 wt%) on the liquid aluminum electrode (S = 1.76 cm²) at 973 K. Pulse height: 25 mV; potential step: 1 mV; frequency: 10 Hz.

Cathodic polarization test. The cathodic polarization curve with a low scan rate is a steady-state process, and is an excellent method to investigate the formation of alloys. Thus, the cathodic polarization curve was used to verify the results from the cyclic voltammetry and square wave voltammetry. Fig. 3 illustrates the cathodic polarization curve obtained in LiCl–KCl melts after the addition of 2 wt% SmCl₃ on the liquid aluminum electrode at 973 K. The scan rate is 5 mV s⁻¹. At the beginning of the curve, the signal A′ is related to the oxidation process of the aluminum metal. When the potential reaches about −0.76 V, the signal A is ascribed to the reduction of Al(III). When the potential reaches −1.23 V, the first turning point C can be observed. The turning point C corresponds to the formation of an Al–Sm intermetallic compound. The last turning point B is ascribed to the formation of an Al–Li alloy, which is formed by the underpotential deposition of lithium ions on the liquid aluminum electrode. The results from the polarization test are consistent with the results from the cyclic voltammetry and square wave voltammetry.

Fig. 3 Cathodic polarization curve of the LiCl–KCl melts before and after the addition of SmCl₃ (2 wt%) on the liquid aluminum electrode (S = 1.76 cm²) at 973 K (scan rate: 5 mV s⁻¹).

3.2. Preparation and characterization of Al–Li–Sm alloys containing whiskers in LiCl–KCl–SmCl₃ melts at 973 K on the liquid aluminum electrode

Based on the above electrochemical results, galvanostatic electrolysis was carried out to prepare an Al–Li–Sm alloy containing whiskers on a liquid aluminum electrode. In this method, galvanostatic electrolysis was applied for a given time, and then it was interrupted. After electrolysis, the alloy was kept in the melt and gradually cooled to room temperature (cooling rate is 2 °C min⁻¹). To avoid the electrolyte contacting oxygen and water, the whole process was protected under an argon atmosphere. Then, the alloy was separated from the melts and stored inside the glove box until analysis.

Fig. 4 formulates the typical evolution of cathodic potentials during galvanostatic electrolysis in LiCl–KCl melts after the addition of 10 wt% SmCl₃ on the liquid aluminum electrode at 973 K, the current density is −0.17 A cm⁻². In the electrolysis process, the electrolytic potential is no more than −1.825 V. In other words, there is a small amount of lithium deposition on the liquid aluminum electrode. Fig. 5 illustrates the XRD pattern of the Al–Li–Sm alloy obtained on the liquid aluminum electrode in LiCl–KCl–SmCl₃ (10 wt%) melts at 973 K after galvanostatic electrolysis for 5 hours. It is easy to see that the Al–Li–Sm alloy mainly contains Al₄Sm and Al phases. Twelve XRD diffraction peaks corresponding to the Al₄Sm intermetallic compound can be seen in Fig. 5. By comparing them with the JCPDS of Al₄Sm in the XRD databases, the values are obtained and listed in Table 1. The theta values corresponding to the diffraction peaks are nearly the same. It is worthwhile to note that there are no diffraction peaks for the Al–Li intermetallic compounds. This phenomenon may be caused by two reasons. One is that the chemical affinity of Al–Sm is higher than that of Al–Li. Another probability is that the lithium content of the alloy is low. Thus, ICP was used to examine the percentage composition of each element. The result from the ICP shows that the composition of the alloy includes 1.5 wt% Li, 7.9 wt% Sm and 90.6 wt% Al. SEM with EDS map analysis was employed to investigate the surface morphology of the alloy. Fig. 6 illustrates the SEM with EDS map analysis of the Al–Li–Sm alloy obtained on a liquid aluminum electrode (S = 1.76 cm²) in LiCl–KCl (10 wt%) SmCl₃ melts after galvanostatic electrolysis for 5 hours at 973 K. From the SEM image, it can clearly be seen that a lot of needle-like precipitates form in the Al–Li–Sm alloy, and the ratio of length to diameter of the needle-like precipitates is greater than 10 : 1. EDS map analysis was employed to investigate the distribution of the elements. The results from the EDS map analysis indicate that the samarium element mainly distributes in needle-like precipitates and the aluminum element mainly distributes on the grey area. In order to further investigate the situation of the element distribution, EDS quantitative analysis of the Al–Li–Sm alloy was employed. Fig. 7 shows the SEM with EDS quantitative analysis of the Al–Li–Sm alloy. Points 1 and 2 locate on the needle-like precipitates and point 3 locates on the grey area. The EDS results of the points labeled 1 and 2 taken from needle-like precipitates indicate that the deposit is composed of elements Al and Sm.

Fig. 4 Typical evolution of cathodic potentials during galvanostatic electrolysis in LiCl–KCl melts after the addition of 10 wt% SmCl₃ on the liquid aluminum electrode at 973 K, current density is −0.17 A cm⁻².

Fig. 5 XRD pattern of Al–Li–Sm alloy obtained on the liquid aluminum electrode (S = 1.76 cm²) in LiCl–KCl melts after the addition of 10 wt% SmCl₃ at 973 K.

Table 1 Comparison of JCPDS of Al₄Sm in the XRD databases and the present work values

	Standard	Present work
2Theta	17.96	17.71
	22.68	22.47
	29.59	29.51
	34.68	34.58
	36.31	36.02
	42.13	42.17
	48.30	48.30
	50.69	50.50
	55.57	55.45
	64.29	64.03
	68.11	68.00
	72.27	72.21

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Fig. 6 SEM with EDS map analysis of Al–Li–Sm alloy obtained on the liquid aluminum electrode (S = 1.76 cm²) in LiCl–KCl melts after the addition of 10 wt% SmCl₃ at 973 K.

Fig. 7 SEM with EDS quantitative analysis of Al–Li–Sm alloy obtained on the liquid aluminum electrode (S = 1.76 cm²) in LiCl–KCl melts after the addition of 10 wt% SmCl₃ at 973 K.

As long as the single crystal structure is met and the ratio of length to diameter is greater than 10 to 1, it can be concluded that it is whisker. From the SEM images, it is obvious that the ratio of length to diameter of the needle-like precipitates is greater than 10 : 1. Therefore, it is important to verify whether the needle-like precipitate is a single crystal structure. Thus, transmission electron microscopy with electron diffraction was adopted to research the crystal structure of the needle-like precipitate. Fig. 8 shows the TEM image (a), EDS analysis (b) and electron diffraction pattern (c) of the Al–Li–Sm alloy containing whiskers obtained by galvanostatic electrolysis for 5 hours at 973 K on a liquid aluminum electrode in LiCl–KCl–SmCl₃ (10 wt%) melts. From the TEM image (Fig. 8a), the needle-like precipitate can be observed in the alloy. Fig. 8b illustrates the EDS/TEM analysis of the needle-like precipitate. It is easy to see that the needle-like precipitate is composed of elements Al and Sm. Then, point A was taken from a random area of the needle-like precipitate, and the electron diffraction image (Fig. 8c) was obtained. The result from the electron diffraction image shows that the diffraction spots are arranged in an orderly manner. Thus, the crystal structure of the needle-like precipitate is a single-crystal structure. Combining the above results of the SEM & EDS and TEM & EDS, the needle-like precipitates can be regarded as Al–Sm intermetallic compound whiskers.

Fig. 8 TEM image (a), EDS analysis (b) and electron diffraction pattern (c) of the Al–Li–Sm alloy containing whiskers obtained by galvanostatic electrolysis for 5 hours at 973 K on a liquid aluminum electrode in LiCl–KCl–SmCl₃ (10 wt%) melts.

3.3. The test of mechanical properties

Micro hardness. The purpose of the synthetic Al–Li–Sm alloy containing whiskers is mainly to verify whether the whiskers can improve the mechanical properties of the alloy. Thus, the Leitz micro hardness tester was used to measure the micro hardness of the alloy. First, two kinds of alloys with nearly the same composition of Al, Li and Sm were prepared under the same experimental conditions. However, the cooling rate for preparing these two kinds of Al–Li–Sm alloy was different. The cooling rate for the Al–Li–Sm alloy containing whiskers was 2 °C min⁻¹, but the Al–Li–Sm alloy without whiskers was separated from the melts and directly cooled to room temperature after galvanostatic electrolysis for 5 h with the protection of an argon atmosphere. Then, a series of hardness tests were completed by the micro hardness tester. Fig. 9 illustrates the comparison diagram of the micro hardness of the Al–Li–Sm alloy with (column 1) and without whiskers (column 2). Column 1 illustrates the micro hardness of the Al–Li–Sm alloy containing whiskers obtained by galvanostatic electrolysis for 5 hours at 973 K on the liquid aluminum electrode in LiCl–KCl–SmCl₃ (10 wt%) melts. From column 1, the micro hardness of the Al–Li–Sm alloy containing whiskers can be identified as 63.6 ± 0.8 HV. Column 2 shows the micro hardness of the Al–Li–Sm alloy without whiskers. The micro hardness from column 2 is 54.3 ± 0.6 HV. It is easy to see that the micro hardness of the Al–Li–Sm alloy containing whiskers improves by 17.1% in comparison with the Al–Li–Sm alloy without whiskers.

Fig. 9 The comparison diagram of the micro hardness of the Al–Li–Sm alloys with (column 1) and without whiskers (column 2).

Young’s modulus. In order to further investigate the mechanical property of the Al–Li–Sm alloy containing whiskers, the elasticity modulus of the alloy was calculated using the Young’s modulus. The Young’s modulus was calculated using the following formula:¹⁸
where E is Young’s modulus, ν is Poisson’s ratio, ρ is the density of the Al–Li–Sm alloy and V_L is the propagation speed of longitudinal waves. The density of the alloy was measured by drainage. The Poisson’s ratio was calculated by the following equation:¹⁸
where V_T is the velocity of the transverse waves. The propagation speed of the longitudinal waves and the velocity of the transverse waves can be calculated by the wave length of the longitudinal and transverse waves and the propagation time. Fig. 10 exhibits the comparison diagram of the Young’s modulus of the Al–Li–Sm alloy with (column 1) and without whiskers (column 2). The Young’s modulus values of the Al–Li–Sm alloys with and without whiskers are 80.54 ± 0.54 GPa and 77.44 ± 0.38 GPa, respectively. It is obvious that the Young’s modulus of the Al–Li–Sm alloy containing whiskers improves by 4% in comparison with the Al–Li–Sm alloy without whiskers.

Fig. 10 Comparison diagram of the Young’s modulus of the Al–Li–Sm alloy with (column 1) and without whiskers (column 2).

4. Conclusions

The electrochemical behavior of samarium ions on a liquid aluminum electrode in LiCl–KCl melts was investigated by different electrochemical measurements. Only one kind of Al–Sm intermetallic compound (Al₄Sm) can be formed under the Al-rich experimental condition. The XRD analysis of the Al–Li–Sm alloy containing whiskers obtained by galvanostatic electrolysis in LiCl–KCl–SmCl₃ (10 wt%) melts on a liquid aluminum electrode illustrate that an Al₄Sm phase is formed. The SEM shows that many needle-like precipitates form on the surface of the aluminum substrate, and these precipitates are finally identified as Al–Sm intermetallic compound whiskers by TEM with an electron diffraction test. Both the elastic modulus and micro hardness of the Al–Li–Sm alloy containing whiskers are further improved. The micro hardness and Young’s modulus of the Al–Li–Sm alloy containing whiskers improve by 17.1% and 4% in comparison with the Al–Li–Sm alloy without whiskers, respectively.

Acknowledgements

The work was financially supported by the China Scholarship Council, the National Natural Science Foundation of China (21103033, 21101040, 91226201 and 51574097), the Fundamental Research funds for the Central Universities (HEUCF20151007), the Foundation for University Key Teacher of Heilongjiang Province of China and Harbin Engineering University (1253G016 and HEUCFQ1415), and the Special Foundation Heilongjiang Postdoctoral Science Foundation (LBH-TZ0411).

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