Journal of Alloys and Compounds Effect of urea on

Journal of Alloys and Compounds 530 (2012) 144–151

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Effect of urea on the size and morphology of AlN nanoparticles synthesized from combustion synthesis precursors Aimin Chu a,b , Mingli Qin a,∗ , Rafi-ud-din a,c , Baorui Jia a , Huifeng Lu a , Xuanhui Qu a a b c

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China School of Electro-mechanism Engineering, Hunan University of Science and Technology, Xiangtan 411201, China Chemistry Division, PINSTECH, Post Office Nilore, Islamabad, Pakistan

a r t i c l e

i n f o

Article history: Received 10 May 2011 Received in revised form 7 December 2011 Accepted 25 December 2011 Keywords: Aluminum nitride nanoparticles Carbothermal reduction Combustion synthesis Precursor Urea

a b s t r a c t AlN nanoparticles were synthesized by carbothermal reduction method using a combustion synthesis precursor derived from aluminum nitrate, glucose, and urea mixed solution. Effects of urea on the combustion temperature of solutions, the particle size and morphology of precursors, the intermediate formed ␥-alumina, and the synthesized AlN were studied in detail. The results indicated that the homogeneous mixture of amorphous (Al2 O3 + C) precursor might be prepared by selecting an optimum molar ratio of urea to aluminum nitrate (U/Al) in solution by combustion synthesis method. Furthermore, a regular variation in the particle size and morphology of precursors had been observed with increasing (U/Al). The nitridation products, synthesized at 1500 ◦ C, retained the characteristics of ␥-alumina in the precursors. The nitridation products, prepared with (U/Al = 0.5–2), comprised of well-distributed spherical particles of AlN with the average size ranging from 30 to 80 nm. Moreover, the nitridation reactivity of products with (U/Al = 0.5–2) had been found at 99%, which was significantly higher than that of the nitridation products prepared with (U/Al = 0.3, 2.5, 3) and without urea. © 2012 Published by Elsevier B.V.

1. Introduction Recently, aluminum nitride (AlN) has attracted a considerable interest for many applications such as advanced electronic substrate and packaging materials owing to its high thermal conductivity (theoretical value is 320 W m−1 K−1 ), a high electrical resistivity (>1016 m), a low dielectric constant (8.8 at 1 MHz) as well as the low dielectric loss (3–10 at 1 MHz), and a low thermal expansion coefficient (20–500 ◦ C, 4.6 × 10−6 K−1 ) matching closely to that of silicon [1,2]. Generally, AlN powders have been synthesized commercially by two methods [3,4]. One is the direct nitridation of Al with N2 and the other is the carbothermal reduction and nitridation (CRN) of Al2 O3 with carbon black in the presence of N2 . Although the CRN method can yield the AlN powders exhibiting high purity, facile sinterability, and stability against humidity, however, this method has also displayed some limitations such as difficulty in mixing the starting materials homogeneously, the high cost based on the high calcination temperature, and the need for high purity Al2 O3 and carbon black for the fabrication of high purity AlN powders. During the past years, in order to

∗ Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, 100083 Beijing, PR China. Tel.: +86 10 6233 2700; fax: +86 10 6233 4321. E-mail address: [email protected] (M. Qin). 0925-8388/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.jallcom.2011.12.133

ameliorate the CRN method and to reduce the fabrication cost, a lot of efforts have been employed. For example, Silverman [5] has synthesized the AlN powders by using the colloidal aluminum oxide precursors trapped in a polymer matrix. Jung and Ahn [6] have utilized the (hydroxo) (succinato) aluminum (III) complex to prepare the AlN powders. These reports have surmised that the particle size and the mixing homogeneity of the staring materials have a great influence on the reaction conditions as well as on the properties of the synthesized AlN powders. Therefore, the proper selection of raw materials and the preparation of a homogeneous mixture of (Al2 O3 + C) precursors are the two essential tasks needed to accomplish the successful synthesis of AlN powders by the CRN method [5–13]. It is well-known that the low temperature combustion synthesis (LCS) method is considered to be one of the most appropriate methods to prepare the oxide-based materials [14]. The LCS method has presented many advantages. Firstly, it saves energy rendering the combustion reaction simple and instantaneous. On the other hand, the synthesized ceramic powders have exhibited the welldefined chemical compositions with homogeneous distribution of the elements. During the LCS process, the heat generated by the chemical reaction between fuel and metal nitrates is utilized to convert the nitrates to the target materials. Therefore, the heat generated during the reaction usually governs the characteristics of the final reaction products. Many reports [15–18] have demonstrated that the constitution of a fuel-oxidizer mixture is one of

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the most important factors influencing the amount of heat generated during the combustion reaction. Although many reports [19,20] have been documented about the synthesis of AlN powders by the carbothermal reduction of the LCS precursor, however, no significant systematic studies have been conducted to investigate the effects of fuel/oxidizer molar ratio (U/Al) on the combustion temperature of solutions, the particle size and the morphology of precursors, ␥-alumina, and the synthesized AlN. In this paper, the fuel/oxidizer molar ratio (U/Al) has been optimized for the synthesis of nano-sized AlN powders via a modified CRN route. Moreover, the effects of the fuel/oxidizer molar ratio (U/Al) on the combustion temperature of solutions, the particle size and the morphology of precursors, ␥-alumina, and the synthesized AlN have been investigated. The correlation among the various investigated parameters has also been discussed systematically. 2. Experimental details 2.1. Starting materials Aluminum nitrate (Al(NO3 )3 ·9H2 O, 99% in purity), urea (CO(NH2 )2 , 99% in purity) and glucose (C6 H12 O6 ·H2 O, 99% in purity) were used as raw materials to prepare aluminum nitride nanopowder. The molar ratio of urea to aluminum nitrate (U/Al) in solution was in the range of 0–3, and the mixing molar ratio of glucose to aluminum nitrate (C/Al) was constant at 8. 2.2. Synthesis procedure Each sort of solutions was filled into a 2000 ml glass and then was heated in air in an electrical temperature-controlled furnace to prepare the precursor. The electrical furnace could be operated upto 350 ◦ C with a heating rate of 20 K min−1 . As the heating is continued, the solution with urea swelled accompanying with the release of a lot of gases. The whole process only took several minutes resulting in finally a porous and foamy mixture of (Al2 O3 + C) precursor. The same reaction phenomena were not observed in the solution without urea. On the contrary, a brown mixture of (Al2 O3 + C) precursor was obtained following the complete reaction. The combustion temperature of the solution system was measured through the thermocouple of K model and display instrument of XMT-101 model. The measuring site for the combustion temperature of each solution system was in 3 cm distance from the bottom of glass. The prepared precursors were ground to a fine powder before starting the nitridation reaction. The nitridation reaction of precursors was performed in a vertical graphite furnace. A strict temperature program was followed in all runs with the heating at a constant rate of 10 K min−1 up to the plateau temperature of 1200 or 1500 ◦ C, which was controlled within ±1 K. A constant nitrogen pressure at 1 atm was maintained in the reactor during all the runs. The precursors were heated in a flowing nitrogen gas at various temperatures for 120 min, and then the nitridation products were cooled naturally to room temperature. The flow rate of nitrogen was 2 l/min. The residual carbon in each nitridation product was removed by firing at 700 ◦ C in air for 60 min. 2.3. Characterization X-ray diffraction study of the precursor powders and calcined powders were carried out in an X-ray diffractometer using CuKa radiation (XRD, Rigaku, D/maxRB12). The morphology and particle size of the precursors and calcined powders particles were observed by scanning electron microscopy (SEM, JSM-6301F) and field emission scanning electron microscopy (FE-SEM, JSM-6701F), respectively. The samples for FE-SEM observations were dispersed in ethanol by ultrasonic vibration for 30 min. The specific surface area (SSA) of nitridation products was determined by the BET method. The content of nitrogen in the nitridation products was determined by a LECO TN-114 nitrogen–oxygen analyzer.

3. Results and discussion 3.1. Effects of urea on the preparation of the precursors Fig. 1 depicts the variation of combustion temperature of the solutions with various (U/Al). It is evident that the combustion temperature of solutions increases with increasing (U/Al) and reaches the maximum (577 ◦ C) at 2.5. Subsequently, the temperature declines slightly with a further increment in the (U/Al). It is well known that the combustion temperature is significantly influenced by the heat generated during the combustion reaction (determined

Fig. 1. The variation in the combustion temperature of the solutions with various (U/Al).

by the constitution of the fuel (urea in our study) relative to the oxidizer (aluminum nitrate in our study)). A higher amount of heat generated during the combustion reaction leads to a significant enhancement in combustion temperature of the reaction system. It implies that the (U/Al = 2.5) results in the highest amount of heat generated during the combustion reaction system [Fig. 1]. According to the theory of propellant chemistry [21], the “stoichiometric” composition involving a fuel-to-nitrate ratio, exhibiting the complete reaction of fuel with metal nitrate, usually corresponds to the release of the maximum energy for the reaction. The calculations of the “stoichiometric” composition are usually based on a simple assumption that the valence of nitrogen in the product species is 0 (as in form of N2 ) implying that the combustion reaction proceeds as follows: 2Al(NO3 )3 · 9H2 O + 5CO(NH2 )2 → Al2 O3 + 5CO2 + 28H2 O + 8N2 (1) Eq. (1) clearly indicates that the theoretical value of (U/Al), at which the maximum heat is generated during reaction, is 2.5 in accordance with the above assumption. The results of the combustion temperature, measured in our work, are in complete agreement with the theory of propellant chemistry [21]. Fig. 2 shows the effects of (U/Al) on the morphology and particle size of the precursors. It is obvious that the morphology and particle size of the precursors display a regular variation with an increase in the (U/Al). The precursor prepared without urea consists of big blocky-shaped particles of upto ∼15 ␮m extent [Fig. 2a]. The precursor prepared with (U/Al = 0.3) is comprised of the mixture of blocky-shaped as well as flake-like particles [Fig. 2b]. The present appearance of this precursor may be due to the slight dispersant effect of the spot of gases generated by the reaction between urea and aluminum nitrate resulting in the diminution of precursor particle size compared to that of the precursor prepared without urea. A further increase in (U/Al) renders the flake-like particles more attenuated with a decrease in their size [Fig. 2a–d]. Moreover, the flake-like particles [Fig. 2d] exhibit some holes at the surface, which may be ascribed to the strong dispersant effect of gases generated during the reaction. As (U/Al) is further increased, the rapid evolution of large volume of gases, generated during the reaction, reinforces the dispersant effect on the particles of precursors culminating in the further breaking of the particles into the smaller pieces [Fig. 2e–f]. Moreover, an increase in the (U/Al) also transforms the particle morphology of the precursor into favaginous mass [Fig. 2g] as well as the hard aggregates [Fig. 2h]. Table 1 displays the SSA of the precursors prepared with various (U/Al). It is vividly discernible that at first the SSA increases with

146


Fig. 2. SEM micrographs of precursors prepared with various (U/Al): (a) 0; (b) 0.3; (c) 0.5; (d) 1.0; (e) 1.5; (f) 2; (g) 2.5; and (h) 3.

the increasing (U/Al) and reaches the maximum (14.043 m2 /g) at the (U/Al = 2.0). This behavior may be ascribed to the rapid growth in the amount of gases generated during the reaction reinforcing the dispersant effect on the particles of precursors. As the (U/Al) surpasses 2.0, the SSA of the precursors starts decreasing with an

Table 1 Specific surface area of the precursors prepared with various (U/Al). (U/Al)

0

0.3

0.5

1.0

1.5

2.0

2.5

3.0

SSA (m2 /g)

2.327

7.339

8.060

10.387

11.787

14.043

10.648

5.060

increase in (U/Al) owing to the enhancement in the combustion temperature and time during the reaction rendering the precursors to display the similar behavior to that of sintering. Furthermore, it is evident that the SSA of the precursors prepared with urea is much higher than that of without urea (2.327 m2 /g). For the precursors prepared with urea, a combustion reaction takes place between urea and aluminum nitrate in the solutions resulting in the rapid evolution of large volume of gases in the reaction that effectively inhibits the agglomeration among particles and moreover transforms the precursor with the appearance of a porous and foamy mass [20]. However, no such combustion reaction has been observed during the heating for the precursor without urea. The


147

Fig. 3. SEM elemental distribution photographs of Al and C elements in precursors prepared with (U/Al = 1.5) (a) and without urea (b).

dispersant effect on the particles of the precursor, prepared without urea, has been observed to be insignificant. This insignificant dispersant effect may be ascribed to the insignificant and intermittent amount of gases generated by the decomposition of aluminum nitrate during the reaction. Consequently, the precursor prepared without urea becomes aggregated resulting in big particles with a smaller SSA. However, it is obvious that the results depicted by Table 1 are in contradiction with that depicted in Fig. 1. As the (U/Al) increases beyond 2.5, the combustion temperature of reaction system decreases [Fig. 1]. Moreover, the amount of gases generated during the reaction also becomes gradually bigger because of the fact that the more fuel (urea) can yield more gases. This phenomenon may be ascribed to the reason that with (U/Al) above 2.5, the particles of precursors get smaller and smaller. On the contrary, it is found that the average particle size of precursors prepared with the (U/Al) above 2.5 is larger than that of precursors with the (U/Al) of 2.5. In fact, the phenomenon of continuous combustion of carbon has been observed during the reaction. As the (U/Al) becomes equal to or beyond 2.5, the reaction has been stopped by inhibiting the supply of heat. However, it has been observed that all the precursors with (U/Al) surpassing 2.5 still have exhibited the continuous combustion lasting for about 1–3 min and exhibiting the similar behavior of sintering to that observed under the higher temperature effect for more time. Especially, as the (U/Al) reaches up to 3, the particle morphology of precursors transforms into the bigger and harder aggregates [Fig. 2h]. The continuous combustion in the precursors may be attributed to the generation of the more amounts of gases generated with the increasing (U/Al). The higher

concentration of gases, carrying off the more amount of residual water in the precursors, renders the precursors more drier leading to the occurrence of continuous combustion. Fig. 3a and b show the SEM elemental distribution micrographs of Al and C elements in the precursors prepared with (U/Al = 1.5) and without urea, respectively. It can be clearly seen in Fig. 3a that the particles, consisting of amounts of Al and C elements, exhibit

Fig. 4. XRD patterns of ␥-alumina prepared with various (U/Al): (a) 0; (b) 0.3; (c) 0.5; (d) 1.0; (e) 1.5; (f) 2; (g) 2.5; and (h) 3.

148


Fig. 5. FESEM micrographs of ␥-alumina prepared with various (U/Al): (a) 0; (b) 0.3; (c) 0.5; (d) 1.0; (e) 1.5; (f) 2; (g) 2.5; and (h) 3.

better anastomotic shapes. It is obvious that the particles of the precursor prepared with (U/Al = 1.5) exhibit the homogeneous elemental distribution of Al and C. However, it is also evident in Fig. 3b that the shapes of particle consisting of various amounts of Al and C elements are poorly anastomotic. It implies that the precursor particles prepared without urea exhibit an inhomogeneous elemental distribution of Al and C. Furthermore, the EDS spectrum and XRD pattern reveal that the main composition of all of the precursors is comprised of amorphous carbon as well as amorphous alumina. 3.2. Effects of urea on the size and morphology of -alumina particles Hashimoto et al. [22] have reported that the particle size of synthesized AlN is governed by the particle size of an intermediate formed ␥-alumina. In order to investigate the effects of urea on

the size and morphology of ␥-alumina prior to nitridation, the precursors prepared with various (U/Al) are calcined at 1200 ◦ C in the nitrogen flow for 120 min. Subsequently, the residual carbon, in the calcined samples, is removed by firing at 700 ◦ C for 60 min in air. Fig. 4 demonstrates the X-ray diffraction (XRD) patterns of samples prepared with various (U/Al) calcined at 1200 ◦ C. Fig. 4 reveals that only ␥-alumina phase can be detected in these samples, which indicates that the nitridation reaction of amorphous alumina has not yet taken place in the precursors and moreover, the amorphous alumina has only transformed into ␥-alumina during the calcination process. Table 2 presents the SSA of ␥-alumina, prepared from precursors, as a function of (U/Al). It is obvious that the SSA of ␥-alumina, prepared with various (U/Al), increases dramatically compared to that of their corresponding precursors consisting of (Al2 O3 + C) mixture. The SSA of ␥-alumina prepared without urea is 277.209 m2 /g,


Fig. 6. XRD patterns of nitridation products prepared with various (U/Al): (a) 0; (b) 0.3; (c) 0.5; (d) 1.0; (e) 1.5; (f) 2; (g) 2.5; and (h) 3.

which is higher than that of other samples prepared with urea. These results indicate that the SSA of ␥-alumina decreases with an increase in (U/Al). Fig. 5 presents the effects of (U/Al) on the size and morphology of ␥-alumina particles. It can be observed that the average size of ␥-alumina particles, prepared without urea [Fig. 5a], is much smaller than that of other samples. Moreover, the distribution distance among ␥-alumina particles is rather large, and the particle-size distribution is extremely wide. With an increase in the (U/Al), the particle size of ␥-alumina, prepared with various (U/Al), increases slightly owing to the improvement in combustion temperature of reaction system. These results are in good agreement with that obtained from Table 2. Furthermore, the distribution distance among various ␥-alumina particles becomes narrow gradually, and the agglomeration becomes visible with the (U/Al) reaching beyond 2 [Fig. 5f]. With the continuous increase in (U/Al), the amount of agglomeration gets severe and the boundary division among ␥-alumina particles becomes inconspicuous with the (U/Al) surpassing 3 [Fig. 5h]. This phenomenon implies that the distribution distance among ␥-alumina particles in these samples is also affected by some other factors. Since this space has not been observed among particles in the samples prior to decarbonization, therefore, it is presumed that the space width among ␥-alumina particles, seen at the surface of samples, is caused by the evolution of CO2 during the decarbonization process.

Fig. 7. Reactivity of nitridation products prepared with various (U/Al) and calcined at 1500 ◦ C.

149

Fig. 8. XRD patterns of nitridation products calcined at 1500 ◦ C in argon for 2 h: (a) (U/Al = 0); (b) (U/Al = 0.3); (c) (U/Al = 2.5); and (d) (U/Al = 3.0).

3.3. Effects of urea on the size and morphology of nitridation product particles For X-ray diffraction analysis, the preparation route of nitridation products involves the calcination of the precursors at 1500 ◦ C in the nitrogen flow for 120 min. On the other hand, for the specific surface area, FE-SEM, and pore size distribution analyses, the preparation route of nitridation products involves the calcination of the precursors at 1500 ◦ C in the nitrogen flow for 120 min, and then firing at 700 ◦ C for 60 min in air. X-ray diffraction (XRD) patterns of nitridation products prepared with various (U/Al) are presented in Fig. 6. Apparently, all the nitridation products exhibit only peaks assigned to AlN, as shown in Fig. 6a–h, respectively. Therefore, it can be assumed that all of the nitridation products, synthesized at 1500 ◦ C, have accomplished the nitridation reaction. Fig. 7 displays the extent of reactivity of nitridation products prepared with various (U/Al). Since AlN reoxidizes easily during the decarbonization and its oxidation level depends on its surface area, therefore, the reactivity of nitridation has been investigated for the calcined samples prior to decarbonization. The reactivity of nitridation is calculated by dividing the weight percent of nitrogen, contained in the samples, by 34.19%, which correspond to the theoretical nitrogen content in AlN following the subtraction of the residual carbon contents. It is evident in Fig. 7 that the reactivity of the nitridation products, prepared with (U/Al = 0.5–2), reaches at 99% which is higher than that of both the samples with (U/Al = 0.3, 2.5, 3) and without urea. It indicates that some unreacted amorphous alumina in the nitridation products may remain leading to the lower reactivity of the nitridation products prepared with (U/Al = 0.3, 2.5, 3) and without urea. In order to validate the above conclusion, the nitridation products, prepared with (U/Al = 0.3, 2.5, 3) and without urea, are calcined again at 1500 ◦ C in the argon flow for 2 h. Subsequently, the calcined products are subjected to phase analysis. The XRD patterns of calcination products, calcined at 1500 ◦ C in the argon flow for 2 h, are shown in Fig. 8. It is visible that the four calcination products exhibit the mixture of AlN, ␣-alumina and ␥-alumina indicating the existence of some unreacted amorphous alumina in the four nitridation products. The unreacted amorphous alumina transforms into ␣-alumina and ␥alumina followed by the calcination at 1500 ◦ C in the argon flow for 120 min. The effects of (U/Al) on the size and morphology of the nitridation products particles are depicted in Fig. 9. With increasing

150


Table 2 Specific surface area of ␥-alumina prepared with various (U/Al). (U/Al)

0

0.3

0.5

1.0

1.5

2.0

2.5

3.0

SSA (m2 /g)

277.209

248.015

205.357

188.2247

169.236

139.834

86.377

74.218

(U/Al), the average particle size of the nitridation products becomes larger first reaching the maximum at (U/Al = 1.5), and then becomes smaller. Moreover, the nitridation product, prepared without urea, consists of irregular multi-phase particles with particle size

ranging from 20 to 100 nm [Fig. 9a]. Due to the weaker combustion reaction, the nitridation product prepared with (U/Al = 0.3) consists of fine, coarse, irregular, and spherical shaped particles [Fig. 9b]. AlN particles, prepared with appropriate (U/Al = 0.5–2), have

Fig. 9. FESEM micrographs of nitridation products prepared with various (U/Al): (a) 0; (b) 0.3; (c) 0.5; (d) 1.0; (e) 1.5; (f) 2; (g) 2.5; and (h) 3.


151

Table 3 Specific surface area of the nitridation products prepared with various (U/Al). (U/Al)

0

0.3

0.5

1.0

1.5

2.0

2.5

3.0

SSA (m2 /g)

55.334

47.119

38.672

26.181

18.063

21.163

25.227

30.352

exhibited the homogenously diapered spherical particles morphology with the average size varying from 30 to 80 nm, respectively [Fig. 9c–f]. It has already been reported [23,24] that the particle size and morphology of AlN, synthesized by carbothermal reduction method, are closely related to the size and morphology of aluminum sources in the precursors. It is evident that the particle-size distribution of ␥-alumina, prepared without urea, is wide in our system [Fig. 5a]. During the subsequent nitridation process, the wide particle-size distribution of ␥-alumina does not further improve, and moreover, the nitridation product still exhibits the wide particle-size distribution [Fig. 9a]. Table 3 demonstrates the SSA of the nitridation products prepared with various (U/Al). Apparently, an increase in (U/Al) first decreases the SSA of nitridation products. It reaches the minimum (18.063 m2 /g) at (U/Al = 1.5) and then increases. It is also evident that the SSA of the nitridation product, prepared without urea, is the highest. Moreover, it is also clear that the SSA of the nitridation products prepared with (U/Al = 2.5, 3) are higher than that of the nitridation products prepared with (U/Al = 1.5, 2). This phenomenon may be attributed to the presence of the large amount of unreacted amorphous alumina, with the higher SSA, in the nitridation products prepared with (U/Al = 2.5, 3) as evidenced by the analysis of XRD shown in Fig. 8. 4. Conclusions Effects of urea on the combustion temperature of solutions, the particle size and morphology of precursors, ␥-alumina, and the synthesized AlN have been investigated. The following conclusions can be drawn: (1) A homogeneous mixture of amorphous (Al2 O3 + C) precursors can be prepared with optimum (U/Al) by using the LCS method. The particle size and morphology of precursors have exhibited a regular variation with increasing (U/Al) owing to the various amounts of gases generated from the reaction at the combustion temperature. (2) The molar ratio of urea to aluminum nitrate (U/Al) governs the ␥-alumina particle size in the precursors by influencing the combustion temperature and time. The average size of ␥-alumina particles increases with increasing (U/Al). The particle-size distribution of ␥-alumina prepared without urea is found to be extremely wide.

(3) The nitridation products, synthesized at 1500 ◦ C, retain the characteristics of ␥-alumina. The nitridation products, prepared with (U/Al = 0.5–2), consist of well-distributed spherical particles of AlN. Furthermore, with an increase in (U/Al), the nitridation products exhibit a narrow particle-size distribution with the average particle size ranging from 30 to 80 nm. The nitridation reactivity of the products, prepared with (U/Al = 0.5–2), reaches upto 99%, which is much higher than that of the nitridation products prepared with (U/Al = 0.3, 2.5, 3) and without urea. Acknowledgments This work is financially supported by National Natural Science Foundation Program of China (50802006) and (51172017), Natural Science Foundation Program of Beijing (2102028), and the Fundamental Research Funds for the Central Universities. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

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