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Vol. 8. Issue 5.
Pages 4687-4698 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4687-4698 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.08.014
Open Access
Production of different morphologies and size of metallic W particles through hydrogen reduction
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Yijie Wua, Zepeng Lva, Haibo Sunb, Jie Danga,b,
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jiedang@cqu.edu.cn

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a College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
b School of Materials Science and Energy Engineering, Foshan University, Foshan, 528000, Guangdong, China
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Table 1. Phases in samples with different fraction of Li2CO3 reduced at 923 K (PDF#4-806).
Abstract

In this investigation, the Li2CO3-assisted hydrogen reduction method was proposed to produce different morphologies and size of metallic tungsten particles. The effects of temperature and content of Li2CO3 additive were both considered. The experimental results showed that both the temperature and additive had great effects on the morphology and size of produced tungsten particles. As the increases of temperature and fraction of Li2CO3, the morphology of tungsten particles was changed from spherical to polyhedral; while the size of these particles was increased; and the obtained products became more and more dispersed. The addition of Li2CO3 also affected the reduction rate dramatically. It was found that the Li2CO3 additive retarded the initial reduction reaction, but it could obviously improve the later reduction (WO2 to W) at a high temperature, furthermore, the higher fraction of Li2CO3 made this promotion more remarkable. Based on above results, the reduction mechanism was proposed: without additive, the reduction entirely obeys pseudomorphic transformation mechanism at low temperature, and obeys CVT mechanism at high temperature; with additive, the reduction follows a mechanism of mixture of nucleation, pseudomorphic transformation and CVT at low temperature, while at high temperature, the sublimation of WO3, nucleation and CVT mechanisms all play important roles.

Keywords:
Li2CO3
Nucleation
Reduction
Morphology and size
Tungsten
Full Text
1Introduction

Tungsten, one of the refractory metals, has lots of excellent properties including: the highest melting point among all metallic elements, high electrical conductivity, high elastic modulus, high density, good corrosion resistance, and excellent physical and chemical properties at elevated temperatures [1,2]. These excellent properties make tungsten become a choice of the raw material for many applications, such as filaments, electrical contacts, heating elements, cutting tools, catalysts and kinetic energy penetrators [2–6]. And metallic tungsten cannot be substituted in many technical fields. Tungsten particles with desired morphologies and size can be used to prepare high-end products. For example, ultrafine tungsten powders with a grain size below 0.5 μm are key raw materials for fabricating ultrafine cemented carbides [7], and spherical and dense tungsten particles can be used to produce porous tungsten matrix with homogeneous pore distribution and open pore channel [8].

A number of methods of producing metallic tungsten have been proposed, such as the electrochemical reduction of tungsten compounds [9], the self-propagating high-temperature synthesis [10], wire explosion process [11], chemical vapor synthesis [12] and the thermal decomposition of tungsten hexacarbonyl [13]. However, the main well-established industrial process for production of tungsten is a one step hydrogen reduction process or two-step hydrogen reduction process (the first step is the reduction of WO3 to WO2, and the second step is to reduce WO2 to W). The reduction mechanism of tungsten oxides using H2 has been investigated by many researchers [14–18]. The reduction process is not only a pseudomorphic transformation from tungsten oxide to its metal, but also combined with a chemical vapor transport which is responsible for the final powder characteristics [19–21]. With the generation, migration, reduction, and deposition of a gaseous transport species (WO2(OH)2), the morphologies of the products become greatly different from those of the raw material. Thus, it was considered that the morphology and size of final tungsten powders are mainly affected by the reaction mechanism [22,23]. However, the controllable preparation of desired morphologies and size of tungsten has remained a difficult problem, especially in the industrial production.

Zhang et al. [24] reported that the addition of a small amount of additives into MoO2 has a great influence on the shape and size of prepared metallic Mo powders. Panatarani et al. [25] investigated the synthesis of ZnO nanoparticles by a salt-assisted spray pyrolysis method. Mann et al. [26] reported that single-crystalline Fe2O3 nanometer and micron particles with controlled shapes and phases could be prepared by using aerosol assisted molten salt syntheses. Other investigations also reported employing the salt-assisted method to produce nanoparticles and micron particles with controlled shapes and sizes [27–29].

Therefore, the present study aims to produce tungsten particles with controllable morphologies and size by hydrogen reduction of tungsten oxides with employing salt-assisted method. The effects of temperature and amount of additive on the particle’s morphologies and size are illustrated. The reduction and affecting mechanisms after an addition of Li2CO3 were investigated as well.

2Experimental section2.1Materials

Commercially available WO3 powders (99.9% metals basis, Shanghai Aladdin Biochemical Technology Co., Ltd) were used as the raw material. Fig. 1 presents the X-ray diffraction patterns, SEM image and size distribution of WO3 powders, which clearly shows that large WO3 powders are presented as parallelepiped-shaped and irregular blocky-shaped. The mean particle size of WO3 powders is around 63 μm. Laser particle size analysis (MASTERSIZER 2000, Malvern instruments) were employed to determine particle size of WO3. The assisted additive in the present study was reagent-grade Li2CO3 (99%, Shanghai Titan Scientific Co., Ltd). The additive was completely dissolved in deionized water and then sprayed into the WO3 powders. Mixed WO3 powders with different amounts of Li2CO3 (0.1 mass %, 0.5 mass % and 1 mass %) were prepared, respectively. Then, the mixtures were dried at 373 K for 4 h before the reduction experiments.

Fig. 1.

(a) XRD patterns, the SEM image and (b) size distribution of raw WO3 powders.

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2.2Experimental

In order to monitor the weight change of samples during the reduction continuously, a thermo-gravimetric analyzer (TGA) (HCT-3, Beijing Hengjiu, Instrument Ltd., China) was used, and the corresponding schematic diagram of the experimental apparatus is shown in Fig. 2. A sample of around 100 mg was used in each experimental run and after an alumina crucible (8 × 8 mm) with samples was put into the system, high-purity argon was introduced to flush air out of the furnace. In an isothermal reduction, the furnace was first heated from room temperature up to the different desired reduction temperatures with a ramping rate of 10 K/min. After an equilibration time of 20 min to stabilize the sample’s temperature, hydrogen was introduced into the furnace. After reacting for a certain time, the reaction gas was switched to pure argon again, and samples were cooled down to room temperature quickly.

Fig. 2.

Schematic diagram of the experimental apparatus for the thermogravimetry analysis. 1) Ar gas flow controller; 2) H2 gas flow controller; 3) calibrated alumina crucible; 4) experimental alumina crucible; 5) thermogravimetry analyzer; 6) data collector; 7) beaker flask A; 8) beaker flask B.

(0.19MB).

In all experimental runs, a constant gas flow rate of 60 mL/min (about 0.318 × 10−2 m/s at 298 K) was maintained. The flow rate of gas was controlled by gas flow controllers (Alicant, Model MC-500SCCM-D). X-Ray Diffraction (XRD) (PANalytical X’Pert Powder, Panalytical B.V.) measurements were conducted for samples. Morphologies of these samples were observed using SEM (TESCAN VEGA 3 LMH, Czech Republic) technique.

3Results and discussion3.1Isothermal reduction

Fig. 3 shows the TG (Thermogravimetry) and DTG (Differential thermogravimetry) curves of reduction of pure WO3 and mixed powders with H2 at three different temperatures from 923 K to 1323 K. Due to the small fraction of addition, the effect of Li2CO3 decomposition on weight change can be ignored. According to the weight loss, all the final products are confirmed as metallic tungsten. The Fig. 3 also demonstrates that the temperature has a significant influence on the reduction rate. At a low temperature of 923 K, the time required for complete reduction is around 200 min; at 1123 K, it takes roughly 15 min; while only less than 6 min is needed to complete the reduction at a high temperature of 1323 K. DTG curves show that the initial reaction rate of samples with an additive is smaller than that of samples without additives, and at lower temperatures (923 K and 1123 K), the initial reaction rate for samples with 1 mass % additive is the lowest. However, as the reaction proceeds, the reaction rate for samples with additives increases and with increase of the amount of additive accelerates as well. At 1123 K and 1323 K, the reaction rate for samples with additives is apparently higher than that of samples without an additive in the later stage of reaction (WO2 to W) according to the DTG curves. The effect on the change of reduction rate will be discussed in later section.

Fig. 3.

Reduction curves of pure WO3 and mixed powders with H2 at different temperatures: (A1), (A2) TG and DTG at 923 K; (B1), (B2) TG and DTG at 1123 K; (C1), (C2) TG and DTG at 1323 K.

(0.87MB).

To further confirm the reduction products, samples reduced at 923 K were subjected to XRD analysis. Table 1 shows XRD results for the reduction products of pure WO3 as well as mixtures of WO3 and Li2CO3 by H2 gas. It can be clearly seen that both pure WO3 and mixed powders are reduced to metallic tungsten powders completely (the diffraction peaks of additive are not detected, which is resulted from their small amounts).

Table 1.

Phases in samples with different fraction of Li2CO3 reduced at 923 K (PDF#4-806).

T (K)  Fraction of Li2CO3 (%)  Phase 
923
0.1 
0.5 
3.2The morphological analysis

Pure WO3 without any Li2CO3 was used as a blank reference. SEM micrographs of metallic tungsten produced at different temperatures are shown in Figs. 4–6, which present that the additive and their fraction have significant effects on both morphologies and size of prepared tungsten. Estimation from SEM micrographs was employed to determine particle size of samples. As a whole, the addition of Li2CO3 affects the morphology of products; and the amount of additive changes both the morphology and size of products.

Fig. 4.

SEM images and size distribution of W products obtained by reducing WO3 at 923 K: (a1), (a2) without additive; (b1), (b2), (b3) with 0.1 mass % Li2CO3; (c1), (c2), (c3) with 0.5 mass % Li2CO3; (d1), (d2), (d3) with 1 mass % Li2CO3.

(0.87MB).
Fig. 6.

SEM images and size distribution of W products obtained by reducing WO3 at 1323 K: (a1), (a2), (a3) without additive; (b1), (b2) with 0.1 mass % Li2CO3; (c1), (c2) with 0.5 mass % Li2CO3; (d1), (d2) with 1 mass % Li2CO3.

(0.88MB).

Fig. 4(a) illustrates at 923 K, the tungsten product obtained by reducing WO3 (without additive) with H2 almost keeps the same parallelepiped-shaped and irregular blocky-shaped morphology and the same size as raw WO3 powders, except many cracks and pores are formed during the reduction resulting from the loss of oxygen. With increasing temperature from 923 K to 1123 K (Fig. 5(a)), although some of the large powders also keep the parallelepiped-shaped and irregular blocky-shaped morphology, these large powders are composed of many small spherical, ellipsoidal and other shapes’ particles, and the mean particle size is 0.648 μm. While reacting at 1323 K (Fig. 6(a)), the large parallelepiped and irregular blocky-shaped particles are degraded and a large amount of small rod-shaped particles are formed. But apparently, these rod-shaped particles still aggregate together.

Fig. 5.

SEM images and size distribution of W products obtained by reducing WO3 at 1123 K: (a1), (a2), (a3) without additive; (b1), (b2), (b3) with 0.1 mass % Li2CO3; (c1), (c2), (c3) with 0.5 mass % Li2CO3; (d1), (d2), (d3) with 1 mass % Li2CO3.

(0.81MB).

Figs. 4(b), 5(b) and 6(b) show the SEM of tungsten obtained by reducing WO3 (with 0.1 mass % Li2CO3) at different temperatures. It is worth noting that all the experimental conditions were kept the same as in the reduction experiments for samples without Li2CO3 addition. It can be obtained from Fig. 4(b) that although large parallelepiped and irregular blocky-shaped morphologies are maintained to a certain extent, the products are all composed of a large amount of small, uniform and spherical particles (0.405 μm). With further increasing the reaction temperature to 1123 K (Fig. 5(b)), the particles become larger (1.434 μm) but the overall irregular blocky-shaped morphology is still not changed for some powders. As the temperature reaches to 1323 K (Fig. 6(b)), particles continue to grow up (8.131 μm) and the irregular blocky-shaped morphology disappears. Under this condition, the obtained tungsten particles are highly dispersed. Further increasing the amount of added Li2CO3 to 0.5 mass % and 1 mass %, the particle’s shape is changed from spherical to polyhedral, and the size of tungsten particles increases as well from nanoscale to micrometer scale. For the sample with 1 mass % additive reacted at 1323 K, the size even rises to as much as 22.146 microns. By comparison, the addition of 0.1 mass % Li2CO3 has the most significant effect on the morphology and size of the obtained products.

Therefore, based on above analysis, the effects of temperature and additive on the morphology and size of produced tungsten can be summarized. Fig. 7 shows these influences in detail. The evolution tendency of obtained tungsten can be drawn: as the increases of temperature and amount of additive of Li2CO3, the morphology of particles is changed from spherical to polyhedral; the size of these particles is increasing; and the obtained products become more and more dispersed. As a result, the tungsten particles produced at 923 K by reducing WO3 with 0.1 mass % Li2CO3 present a smallest size (0.405 μm).

Fig. 7.

Effects of the temperature and additive on the morphologies and size of produced metallic tungsten.

(0.52MB).
3.3Reduction mechanism

Based on the results of isothermal reduction and SEM analysis sections, the following facts can be drawn:

  • 1)

    After adding Li2CO3, the large parallelepiped-shaped powders disappear and large irregular blocky-shaped powders are degraded, and the morphology of small tungsten particles changes dramatically.

  • 2)

    As a whole, the size of tungsten particles increases with the increase of amount of Li2CO3. Furthermore, with increasing the addition of Li2CO3, the obtained products become much more dispersed.

  • 3)

    At all studied temperatures, the addition of Li2CO3 retards the initial reduction reaction. However, the additive can obviously increase the reduction rate of WO2 to W at high temperature, furthermore, the higher content of Li2CO3 makes this promotion more remarkable.

3.3.1The influence of additive on the morphologies of products

The final products for samples with or without additives present different morphologies, which are resulted from different reaction processes and mechanisms. In our previous study [30] for the reduction of WO3 with CH3OH vapor, it was found that there existed competition between two reaction mechanisms (pseudomorphic transformation mechanism and chemical vapor transport (CVT) mechanism): at low temperature, the saturation vapor pressure of gaseous transport species (WO2(OH)2) was low, and thus the pseudomorphic transformation mechanism played a dominant role which caused the unchanged shape of the large powders; while at higher temperature, the amount of gaseous transport phase WO2(OH)2 increased, and the reaction obeyed the chemical vapor transport mechanism.

When pure WO3 is reduced by hydrogen at a low temperature of 923 K, Tungsten products still maintain the original parallelepiped-shaped and irregular blocky-shaped morphology. This is because at lower temperature, the reduction reaction occurs very slow and the amount of water vapor formed (it can react with tungsten or its oxides to form WO2(OH)2) during the reaction is relatively small; furthermore, the formed water can be quickly removed by the H2 flow, which finally causes low partial pressure of water vapor existing in the system. And thus the amount of formed gaseous transport phase WO2(OH)2 is very small, and pseudomorphic transformation mechanism plays a dominant role resulting in the unchanged shape of the tungsten powders. However, with increasing the reaction temperature to 1123 K, the reaction rate increases dramatically (as shown in Fig. 3) and a large amount of water vapor are formed in a short time, and in this case the amount of local gaseous transport phase WO2(OH)2 increases significantly. Therefore, the CVT mechanism can play an important role during the reduction. Further increasing the temperature to 1323 K, more gaseous transport phase is formed and, herein, the CVT mechanism plays a dominant role. Therefore, the large powders are degraded.

With adding Li2CO3, the large parallelepiped-shaped powders disappear and irregular blocky-shaped powders are degraded even at a low temperature of 923 K. This phenomenon strongly suggests that the chemical vapor transport mechanism also plays an important role to some extent during the reduction of samples with additive. It has been reported [31] that the catalytic action of liquid alkali compounds upon the reaction with water vapor can form the volatile oxide hydrate. Therefore, the CVT mechanism will be enhanced. In addition, based on the similarity of chemical properties of Li and H, the first main group elements, Li and their compounds can form volatile substances with tungsten oxide as well (WO3·nLi2O) [32]. Therefore, samples with Li2CO3 can also form volatile substances enhancing the CVT mechanism at lower temperature and make particles degenerate even with a lower vapor partial pressure of water.

To further investigate other reasons for the caused change of the morphologies after adding additive, samples with or without additive were only heated to a desired temperature in Ar gas but without reduction and quickly cooled to room temperature, and then were subjected to the SEM analysis. Fig. 8 shows the SEM images of these samples. Fig. 8a and b illustrate that the parallelepiped-shaped and irregular blocky-shaped morphologies are maintained at 923 K. The morphology for the sample without additive is the same as that of sample with 1 mass % Li2CO3 at a lower temperature. As further increasing temperature to 1123 K, the sample without adding Li2CO3 almost does not change (Fig. 8c), while the surface of sample with adding 1 mass % Li2CO3 becomes smooth (Fig. 8d). It is well know that the melting temperature of Li2CO3 is 993 K [33] and the decomposition of Li2CO3 terminates at 1349 K [34]. Thus, Li2CO3 exists as liquid phases at most experimental temperatures except at 923 K. During the experimental process, the partial liquid Li2CO3 could be decomposed to Li2O and CO2 according to reaction (1) [34], and then Li2O reacted with WO3 to form Li2WO4 (reaction 2). What’s more, the decomposition rate of Li2CO3 increased with increasing temperature and terminated at 1349 K. [34] On the other hand, Li2WO4 could be formed between Li2CO3 and WO3 directly (reaction 3) [35,36].

Li2CO3 = Li2O + CO2 (1)
Li2O + WO3 = Li2WO4 (2)
Li2CO3 + WO3 = Li2WO4 + CO2 (3)

Fig. 8.

SEM images of WO3 powders obtained by heating to a desired temperature and then cooling to room temperature at Ar atmosphere: (a), (c) and (e) without additive at 923 K, 1123 K and 1323 K; (b), (d) and (f) 1 mass % Li2CO3 at 923 K, 1123 K and 1323 K.

(1.09MB).

Because the melting point of lithium tungstate is 1015 K [37], accordingly, the smooth surface is caused by liquid-phase lithium tungsten and Li2CO3 condensation. It should be noted that small spherical particles are also found on the smooth surface of large powders at 1123 K, and these small spherical particles are deposited tungsten trioxide. This suggests that the addition of Li2CO3 considerably promotes the sublimation of tungsten trioxide. As the temperature reaches to 1323 K, for pure WO3 sample shown in Fig. 8e, although the parallelepiped-shaped and irregular blocky-shaped morphologies are maintained, these powders are all composed of a large number of small, rod-shaped particles; for sample with 1 mass % Li2CO3, the rod-shaped particles are also formed, while compared with Fig. 8e, these rod-shaped particles become larger; another important phenomena for sample with additive is the original parallelepiped-shaped and irregular blocky-shaped morphologies are changed. These rod-shaped particles should be WO3, which is resulted from the sublimation and recrystallization of WO3. The cause of the difference of rod’s size can be explained by the homogenous nucleation and nonhomogeneous nucleation theory. For sample with additive, it obeys nonhomogeneous nucleation during the recrystallization. It is favorable for WO3 steam recrystallization and growth through nonhomogeneous nucleation, and thus the size is larger. However, for pure WO3 sample, it belongs to the homogenous nucleation, and under certain supersaturation, more nuclei are formed, therefore, more particles are recrystallized and the size is smaller. The sublimation and recrystallization of WO3 also affect the reduction process and the final products, especially the size and morphologies. The sublimation helps to degrade the larger parallelepiped-shaped and irregular blocky-shaped powders and to form small particles.

Another possible reason for additive influencing the morphologies of tungsten products is that the decomposed Li2CO3, Li2O or formed Li2WO4 and LiOH (formed during the reduction according to reaction 4 [34]) act as the nuclei.

Li2O + H2O = 2LiOH (4)

The initial nucleus generated from gaseous tungsten compounds may play an important role in the morphology of the tungsten particle. The additive can create an ideal place for the nucleation of tungsten particles, and sufficient molten subjects can help tungsten crystals to reach the stable shape. This mechanism has been proposed by Sun et al. [22], who studied the effect of chlorine salts on the reduction of MoO2 to produce Mo.

3.3.2The influence of additive on the size of products

First, the reaction temperature has a marked effect on the size of the products and there exist two sides of the effect: (1) the increase of temperature is beneficial for the CVT mechanism, and it is helpful to form more particles and thus to decrease the particle’s size; (2) the increase of temperature can accelerate the migration of tungsten atoms and thus increase the size of particles.

From the aspect of nucleation, the addition of Li2CO3 also causes two opposite sides of the effect on the size of the products. On one hand, the additive acts as nuclei for the formation and growth of W particles. Increasing the content of Li2CO3 can provide more nuclei, and thus decrease the size of tungsten products. On the other hand, a higher content of Li2CO3 results in more Li2WO4 and LiOH formation, and these liquid phases can promote the migration of W atoms, which increases the size of products. From the aspect of influencing the reaction mechanism, the addition of Li2CO3 affects the CVT mechanism. The addition and increase of the content of Li2CO3 are beneficial for the CVT mechanism by forming volatile substances, and thus it can decrease the particle’s size. Furthermore, the addition of Li2CO3 can promote the sublimation and recrystallization of WO3 at high temperature. If the size of recrystallized WO3 particles is small, it is reasonable to obtain small particles of final tungsten products.

Regarding the high dispersion of the obtained tungsten products after adding the additive of Li2CO3, the main reason is that the additive covers or is attached to the surface of particles and thus inherits the aggregation of small particles. Therefore, the as-prepared tungsten particles are highly dispersed after adding the additive.

Based on above discussion, the affecting and reaction mechanism can be described as Fig. 9 Without additive, the reduction entirely obeys pseudomorphic transformation mechanism at low temperature, and at high temperature, CVT mechanism plays a dominant role. With additive, the reduction follows a mechanism of mixture of nucleation, pseudomorphic transformation and CVT at low temperature. While at high temperature, the sublimation of WO3, nucleation and CVT mechanisms all play important roles.

Fig. 9.

Schematic diagram of the proposed reduction mechanisms: (a) without Li2CO3, (b) with adding Li2CO3.

(0.55MB).
3.3.3The influence of additive on the reduction rate

As discussed above, the added Li2CO3 can be partial decomposed to Li2O and CO2, and then Li2O reacts with WO3 to form Li2WO4. First, the additive covers the WO3 powders decreasing the reaction surface area, and the formed liquid phase Li2CO3, Li2WO4 and LiOH at high temperature fills with the pores and inhibits gases diffusion; Second, the formation of Li2WO4 reduces the reaction activity. And thus, the addition of Li2CO3 retards the initial reduction of WO3, and this retardation is more remarkable at a higher content of additive.

For the later reduction of WO2 to W, the reduction rate is accelerated by the addition of Li2CO3 at 1123 K and 1323 K. The main reason is that at higher temperatures, the addition facilitates the sublimation and recrystallization of WO3 and makes the reactant more dispersed during the later reduction, therefore, the kinetic condition is improved. The increase of the portion of CVT process is another factor which can increase the reduction rate.

4Conclusions

In the current study, the hydrogen reduction of WO3 was investigated with employing a Li2CO3-assisted method to produce different morphologies and size of metallic tungsten particles. The following conclusions can be drawn.

  • i

    Both the temperature and addition had noticeable effects on the morphologies and size of produced tungsten particles.

  • ii

    Without additive: the obtained tungsten products kept the original WO3 morphology at low temperature; although fine tungsten particles were produced at high temperature, those fine particles still aggregated together.

  • iii

    After adding Li2CO3, the large parallelepiped-shaped powders disappeared and irregular blocky-shaped powders were degraded, and the morphologies of small tungsten particles changed dramatically.

  • iv

    As the increases of temperature and amount of Li2CO3, the morphology of small particles was changed from spherical to polyhedral; while the size of these particles was increased; and the obtained products became more and more dispersed.

  • v

    The reduction and affecting mechanisms after an addition of Li2CO3 were proposed as well.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

Thanks are given to the financial supports from the National Natural Science Foundation of China (51604046, 51674053), Fundamental and Frontier Research Project of Chongqing (cstc2017jcyjAX0322), Young Elite Scientists Sponsorship Program by CAST (2018QNRC001) the Venture & Innovation Support Program for Chongqing Overseas Returnees (cx2018055)and the Fundamental Research Funds for the Central Universities (2018CDPTCG0001/32).

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