Variation of voids and inter-layer shear strength of advanced polymer-matrix composites at different pressures with high-pressure microwave

Abstract

Due to its rapid and volumetric heating capabilities, microwave heating has been conceived as an alternative cost-effective route for curing advanced polymer-matrix composites. Different curing pressures were applied to the pre-impregnated laminates of advanced polymer-matrix composites in a given high-pressure microwave curing process. The variation of voids’ shapes in the advanced polymer-matrix composites was revealed, and the shear failure mechanism of advanced polymer-matrix composites with different porosity was analyzed. During the shear failure process of the advanced polymer-matrix composites, sounds ceaselessly occur. The experiment results of the optical digital microscopy, the scanning electron microscope, and the inter-laminar shear properties test showed that in high-pressure microwave curing process, pressure was not only the main factor that affects the porosity and the mechanical properties but also influences the bonding strength of the resin and the carbon fiber interface significantly. There is a threshold value for the effect of curing pressure on porosity and inter-laminar shear strength of advanced polymer-matrix composites. The internal relationship among porosity, inter-laminar shear strength, and curing pressure of advanced polymer-matrix composites could be summarized as a piecewise power function.

Keywords

Advanced polymer-matrix composites high-pressure microwave pressure voids inter-laminar shear strength

Introduction

Advanced polymer-matrix composites (APMCs) are widely used in the aerospace industry due to its advantages of light weight, high strength, large rigidity, and excellent elasticity.¹ Autoclave pressure hinders the growth of voids or even leads to the collapse of the bubbles in the pre-impregnated material, thereby producing APMC part with excellent properties and very low porosity to meet the aerospace requirements.²

At present, autoclave molding process (AMP) is the main method to produce high-performance composite parts, because the products made by this way have uniform resin content and stable mechanical performance. Despite the ascendency above, AMP is time- and energy-consuming. Because in this thermal curing process, the carbon fiber–reinforced epoxy resin pre-impregnated material is heated by heat transfer and convection of high temperature compressed gas which consumes considerable energies.

In today’s competitive global market, the demands for APMCs are rapidly increasing, then how to produce good quality parts with lower cost and time has been a priority for manufacturers.³ Unlike the conventional thermal heating, microwave heating has the volumetric and selective heating ability that allows for more quick and uniform curing of the APMCs.^3–5 The microwave energy can transfer electromagnetically and relatively evenly throughout the APMC part and generate more even temperature distribution.^3,6–10 The molecules of matter are composed of atoms (atoms or ions). Each atom has an equal amount of positive and negative charges. The center of gravity of the positive and negative charges of the molecules in the polar dielectric does not coincide (the molecule has a dipole moment), and each molecule has the property of an electric dipole. When there is no external electric field, due to the influence of molecular thermal motion, the electric dipole moment of each molecule in the medium is randomly arranged and the synthesized electric dipole moment is zero, and the external macroscopic electric effects cancel each other without presenting charging phenomenon, as shown in Figure 1(a). When a polar molecule is subjected to an external electric field E₀, the electric dipole is rotated by the action of the electric field. The electric dipole turns from the disordered arrangement to the direction of the external electric field, and the polar dielectric presents a charged phenomenon. When the external electric field changes constantly, the electric dipole will continue to rotate and follow the electric field changes, as shown in Figure 1(b). Thereby, the molecules of the polar medium are constantly rubbed together, thereby generating heat, so that the temperature of the polar medium is increased as a whole, as shown in Figure 1.¹¹ In conventional autoclave heating matter, the heat is transferred from the surface to the inside of the matter by heat conduction, as shown in Figure 1(c) and (d).

Figure 1.

The principle of microwave heating: (a) no microwave, (b) microwave application, (c) heat conduction heating, and (d) microwave heating.

Microwave curing is such a promising technology to manufacture the APMC parts than conventional thermal curing.^7,10,12 In order to take advantage of the microwave heating to reduce production costs and improve efficiency, previous researchers have done a lot of research on microwave-cured APMCs under pressure.¹³ However, as far as we know, there is no literature about curing the high-performance APMCs by microwave under high-pressure environment, which has been detailed in our previous article.¹³

The high-pressure microwave curing equipment was developed by our team on the basis of the autoclave,¹³ as shown in Figures 2 –4.¹³ The equipment used a power control closed-loop system to control the temperature of the specimen. The equipment’s working principle is shown in Figure 4.¹³ Through the small holes of the microwave cavity, the high-pressure gas of autoclave got into and put on the vacuum bag after being vacuumed.¹³ The basic principles and control methods of the high-pressure microwave curing equipment have been elaborated in the previous paper.¹³ The high-pressure microwave-cured APMC specimen is shown in Figure 5. The experiment showed that microwaves can easily penetrate 12-cm-thick APMC specimen and cure it.

Figure 2.

Schematic diagram of the structure of the high-pressure microwave curing equipment.

Figure 3.

High-pressure microwave curing equipment.

Figure 4.

Schematic diagram of temperature closed-loop control.

Figure 5.

High-pressure microwave-cured APMC specimen: (a) cured and cut APMC specimen; (b) APMC specimen thickness.

During the processing of APMC pre-impregnated material (cutting, paving, packaging, and curing), the manufacturing defects are apt to occur due to the complexity of manufacturing environment, such as artificial errors, structural design, process conditions, and slight leakage of the vacuum bag,¹² as shown in Figure 6. It should be noted that, in AMP, the excellent performance of product could not always be achieved due to the delamination, voids, and pores. These defects may have a great impact on the mechanical properties and fatigue resistance of the APMC parts, and even cause part scrap, which leads to serious economic losses.^2,14–16 In previous reports, the effects of voids and pores on mechanical properties of the APMC parts have been investigated in various ways.^{2,14,15,17–21} However, none of them have considered the situation with the high-pressure microwave curing, which is a new processing way, after all.¹³

Figure 6.

Large APMC aircraft bearing part produced by the AMP: (a) after vacuuming, the part before the AMP curing; (b) cured part by the AMP.

Previous research of our team showed that microwave curing process improved the curing rate of epoxy resin composite materials effectively and reduced the activation energy of the curing reaction, while the introduction of curing pressure to the curing reaction of epoxy resin composite materials has certain role in promoting, compared with the traditional thermal curing process.²² At the same time, the APMC laminates cured by the high-pressure microwave curing equipment not only increased efficiency but also significantly reduced the energy cost.¹³ The tensile strength of the APMC laminates increased by 9.2% in a direction perpendicular to the fiber, and the inter-laminar shear strength (ILSS) increased by 4.2%.¹³ During the APMC molding process with high-pressure microwave, the curing pressure field is one of the main physical fields. It not only allows the APMCs to adhere to the mold surface, ensuring the molding accuracy of the part, but also promotes resin flow and fiber densification and directly affects the void content in the APMCs. When the curing pressure ranged between 0.1 and 0.4 MPa, the porosity and the ILSS were relatively sensitive to the curing pressure.¹² The porosity sharply deceased, while the curing pressure increased, and the ILSS increased by 35%.¹² When the curing pressure reached 0.4 MPa, there were basically no voids inside the parts, and the ILSS was no longer sensitive to the curing pressure, which indicated there was a threshold value (0.4 MPa) of mechanical property for forming the APMCs by the high-pressure microwave curing.¹²

In this article, under an identical temperature profile and different curing pressures, a series of carbon fiber–reinforced epoxy resin pre-impregnated laminates were cured by the high-pressure microwave, which revealed the evolution of voids in parts, the effect of pressure on the fiber–resin interface, and the internal relationship between pressure and pores and the mechanical properties of APMC parts.

When high-pressure microwaves are used to produce APMC parts, it can provide reference for process engineers to eliminate defects and to evaluate internal defects in the APMC parts. At the same time, it also provides a theoretical basis for airworthiness certification.

Experiment

T800 aeronautical APMC material

The experimental material was the T800 aeronautical APMC pre-impregnated material, with carbon fiber reinforcement. Pre-impregnated sheets were supplied by Commercial Aircraft Co., Ltd. It was a unidirectional continuous fiber composite, and the resin system was based on bismaleimide. The material was stored in a sealed package at −12°C. The physical properties of the pre-impregnated material are shown in Table 1.¹³

Table 1.

Physical properties of pre-impregnated material.

Name	Ply thickness (mm)	Laminate density (g/cm³)	Fiber volume (%)	Glass transition, dry (°C)
Nominal value	0.21	1.570	57.55	180

Preparation of unidirectional APMC laminates

Each of the unidirectional APMC laminates was designed with 10 plies [0]_10s and paved by manual. The dimension of the laminates was 30 cm (length) × 40 cm (width) × 1.91 mm (thickness). The components were edged with aluminum foil to prevent the arcing between the carbon fibers in microwave field.^12,13 The quartz glass plate was used as a mold for curing these laminates, because the microwave would penetrate the quartz glass without heating it.

Six unidirectional APMC laminates were cured under different curing pressures (set as 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 MPa) in the high-pressure microwave curing process. The cure cycle of these laminates using the high-pressure microwave curing is shown specifically in Figure 7.¹² To exclude the air and bubbles inside the pre-impregnated material for improving the quality of the APMC laminates, the curing pressure was used in the whole curing process for each laminate.

Figure 7.

Vacuum bagging arrangement and the plot of temperature versus time and pressure: (a) schematic diagram of a vacuum bag system and (b) plot of temperature versus time and pressure.

Optical digital microscopy

In order to study the microscopic structure of the APMC specimens, the optical digital microscope (ODM; OLYMPUS DS ×500) was employed. The specimens were 10 × 10 mm² in size, taken from each piece of specimen cured by different curing pressures. According to the national standard JC/T 773-2010, the samples were mosaic, polished, and cleaned.

Porosity test

The porosity was mainly calculated according to the following equation

P_{P} % = \frac{S_{P}}{S_{W}} %

(1)

where $P_{P} %$ is the percentage of area occupied by pores in the ODM micrograph, $S_{P}$ is the area occupied by all pores on the ODM micrograph, and $S_{W}$ is the total area of the ODM micrograph.

After each sample was photographed by ODM, the ODM photos were divided into square grid of the same size and uniformity. Thus, the area occupied by the pores in each sample and the proportion of the area occupied by the pores in the cross section of the sample are known. The porosity of the six samples was averaged to know the area factor of the pores in the sample at a particular pressure.

Inter-laminar shear properties test

The mechanical properties of the APMC specimens were evaluated by the inter-laminar shear properties test (JC/T 773-2010). These tests were carried out with the CMT5105 tensile testing apparatus (Sansi Taijie Co., Ltd, China) at room temperature. The performance test was performed with the same geometry of the specimen. The specimen edges were polished before testing, in order to avoid pre-mature rupture due to the stress concentration caused by machining scratches.

The inter-laminar shear specimen size is shown in Table 2. The fillet radius of the loaded head was 5 mm, the fillet radius of the bearing was 2 mm, and the pivot span was 12.5 mm. The schematic diagram of the inter-laminar shear properties test is shown in Figure 8.

Table 2.

Size of inter-laminar shear specimen (unit: mm).

Thickness, h	Length, l	Width, b
2 ± 0.2	20 ± 1	10 ± 0.2

Figure 8.

Equipment photo and schematic diagram of the inter-laminar shear properties test.

The ILSS, $τ_{m}$ (MPa), of the composite specimens was determined according to equation (2)

τ_{m} = \frac{3}{4} \times \frac{F}{b h}

(2)

where $F$ wasis the maximum load or failure load (N), b is the specimen width (mm), and h is the specimen thickness (mm).

Microscopic morphology study

In order to study the bonding condition between the resin and the carbon fibers of the APMC specimens after the inter-laminar shear properties test, the scanning electron microscope (SEM; TESCAN MIRA3 LUM) was employed to observe the surface microstructure of the APMC specimens.

Results and discussions

Microstructure characterization of the APMC specimens

Figures 8 –11 show how the curing pressure affects the internal porosity of the APMC parts in the high-pressure microwave curing process. First, the pores generally appeared only in the resin layers. With the increasing curing pressure, the pores were more inclined and appeared in the center of the parts. When the curing pressure reached 0.4 MPa, the pores almost disappeared. Second, with the increasing curing pressure, the rules of void variation in the APMCs are as follows: the void shape gradually changes from the big, flat, disk-shaped pores to the small, flat, disk-shaped pores, and then into an ellipsoidal void, and then into a small round void. Third, during the high-pressure microwave curing process, the viscosity of the resin decreased with the increasing temperature. The edges of the pores were arc-shaped (due to the liquid surface tension), which indicated that the pores may generally be formed when the resin was in liquid state. Fourth, the location of the pores indicated that the pores came from the air entrapped during the laying-up of the pre-impregnated materials, and the volatiles generated when the resin system was cured or from moisture absorption. The growth of pores can be accomplished by the diffusion of these gases or by the consolidation of the surrounding pores. Finally, the curing pressure is still the decisive factor affecting the porosity of the APMC parts in the high-pressure microwave curing process.

Figure 9.

ODM morphologies of longitudinal section of the APMCs under different pressures with high-pressure microwave curing: (a) 0.1 MPa, (b) 0.2 MPa, (c) 0.3 MPa, (d) 0.4 MPa, (e) 0.5 MPa, and (f) 0.6 MPa.

Figure 10.

ODM morphologies of transversal surface of the APMCs under different pressures with high-pressure microwave curing:¹² (a) 0.1 MPa, (b) 0.2 MPa, (c) 0.3 MPa,(d) 0.4 MPa, (e) 0.5 MPa, and (f) 0.6 MPa.

Figure 11.

Pore size change trend diagram under different pressures with microwave curing.

Mechanical performance analysis

The ILSS between the resin layer and the carbon fiber layer was not only an indicator of adhesion strength on the interface but also an important way to characterize the mechanical properties of the APMC parts.

Through the inter-laminar shear properties test, it could be concluded that the APMC parts’ fracture form is different from that of metal. As shown in Figure 12(a), the APMC parts’ shear failure process mainly had two stages.

Figure 12.

The load–displacement curve of the APMC parts with microwave curing: (a) typical load–displacement curve; (b) the load–displacement curve of 0.1–0.6 MPa.

First, the APMC parts deformed elastically under loading, and when the specimens reached the extreme of elastic deformation (the elastic limit point C), the specimens would issue a “bang” sound. The occurrence of this sound indicates that the specimens’ matrix shear damage occurred between the layers. During the elastic deformation stage, elastic deformation only occurred in the specimens, while the resin and fibers of the APMC specimen could bear the loading together.

Second, when the elastic deformation of the APMC specimens ended, the specimens began to enter the “toughness fracture phase” as the loading increased. The specimens continued to issue “bang” and “creak” sound. At the same time, phenomena such as the adhesive failure, delamination, matrix cracking, fiber breakage, and debonding inside the APMC specimens appeared. During this stage, the carrying capacity of the specimens did not decrease with the increase in internal defects in the specimens, but continued to increase. The reason for this phenomenon is that as the deformation of the specimens increases and the matrix cracking occurs, more and more carbon fibers begin to share the loading together, and some carbon fibers still withstand some of the loading when pulled out from the matrix even though they have been broken. As shown in Figures 12 and 13, the resin did not deformed plastically, so the entire specimens presented a brittle fracture. Finally, when the displacement reached the point B, the carrying capacity of the specimens reached the maximum value (the breaking point F). Under the action of loading, the delamination stopped and most of the carbon fibers were broken or pulled out or unbonded. With the continuous issue of “creak” sound, the carrying capacity of the workpiece dropped sharply. It is totally different from the metal parts which do not sound when it is deformed; when the APMC parts’ inter-laminar shear damage occurs, it will make a big sound. The sound can be provided as an alarm signal to the service and maintenance personnel. The appearance of the fracture after the destruction of the specimens is shown in Figure 13.

Figure 13.

ODM morphology of the specimen after the inter-laminar shear test: (a) fracture local morphology; (b) fracture macroscopic appearance.

With the increase in curing pressure, the extreme point and the maximum loading of shear failure of the specimens gradually became larger too, as shown in Figure 12(b). In order to analyze the changes of the inter-laminar shear properties of the APMC parts, which were cured under different curing pressures in the high-pressure microwave curing process, specimens were prepared for each curing pressure condition. The average value of the ILSS was obtained after the three-point bending test, as shown in Table 3.

Table 3.

Specimens’ ILSS under different curing pressures by the microwave curing.

Curing pressure (MPa)	0.1	0.2	0.3	0.4	0.5	0.6
Average ILSS value (MPa)	78.65	92.60	101.00	106.30	107.84	108.64

ILSS: inter-laminar shear strength.

As shown in Figures 12(b) and 14, when the curing pressure changed from 0.1 to 0.4 MPa, the ILSS of the APMC specimens increased rapidly. It also showed that in the microwave curing process, the curing pressure did affect the void content, ILSS, and modulus of the APMCs. When the curing pressure exceeded 0.4 MPa, the increasing rate of the ILSS of the APMC specimens was slowed down obviously, though the curing pressure kept increasing. The result showed that the ILSS was less sensitive to the curing pressure after the curing pressure exceeds 0.4 MPa. It indicated that 0.4 MPa was the threshold value of the APMC specimens, which was discussed in our previous study.¹²

Figure 14.

Relationship between different pressures and laminates’ ILSS by the high-pressure microwave curing.¹²

Fracture morphology microscopic structural analysis

After the inter-laminar shear test, the fracture surfaces of the APMC specimens were examined using SEM (Figure 15). The carbon fibers did not need the surface spray treatment due to their electrical conductivity properties. To analyse the failure mechanism of carbon fiber and resin bonding interface in specimens, the microstructure of the specimen section was amplified 2000 times, respectively.

Figure 15.

SEM morphology of composites under different cure pressures by microwave curing: (a) 0.1 MPa, (b) 0.2 MPa, (c) 0.3 MPa, (d) 0.4 MPa, (e) 0.5 MPa, and (f) 0.6 MPa.

From Figures 9, 10, and 15, the effect of different curing pressures on the bonding interface between carbon fibers and resin under microwave curing could be observed clearly. When the curing pressure was 0.1 MPa, the APMCs were loose and porous inside. The interfacial strength between the resin and the carbon fibers was weak, and there was scarcely any resin adhered to the carbon fibers’ surface after the inter-laminar shear properties test, the resin crumbed as block mass after shear failure, and the hackles at the failure plane were not obvious.

When the curing pressure was 0.2 MPa, there were resin fragments stuck to the fibers’ surface. The resin no longer crumbed as block mass after shear failure but wrapped around the carbon fibers, which indicated that the interfacial strength between the resin and the carbon fibers was enhanced. The hackles at the failure plane were still not much, indicating that the load-carrying capacity in the parts was still not strong.

When the curing pressure was 0.3 MPa, the fiber surface was surrounded by a layer of resin fragments. The hackles at the failure plane after shearing were increased obviously, indicating that the parts had been compacted to an extent from curing, and the interfacial strength between the resin and the carbon fibers had been enhanced further.

When the curing pressure reached 0.4 MPa or above, as shown in Figure 15(d)–(f), the carbon fibers at the failure plane were surrounded by a thick layer of resin after the inter-laminar shear test, the hackles were closely arranged as strips, and the carbon fibers were closely arranged either, indicating that, under these curing pressures, the APMCs were tight inside after curing, the resin and carbon fibers were infiltrated sufficiently in curing, and the interfacial strength between the resin and the carbon fibers was relatively strong. Damage often occurred inside the resin matrix, and debonding was not easy at the interface. These conclusions were consistent with the previous theory, that the interface is a synthesis of mechanics and chemistry.

It could be concluded that, first, when the curing pressure was less than 0.4 MPa, the curing pressure played an important role in decreasing the porosity of the parts and enhancing the interfacial properties between the resin and the carbon fibers. Second, the curing pressure was a benefit for the resin flowing and the sufficient infiltration of resin and carbon fibers. Third, when the curing pressure reached 0.4 MPa or above, the effect of curing pressure on the porosity and the interfacial properties of the APMCs decreased accordingly.

Quantitative relationship characterization

The functional relationship between the ILSS and the curing pressure was measured using a non-linear fitting method. As shown in Figure 16, when the threshold value was 0.4 MPa, the section function could describe the relationship between ILSS and curing pressure more accurately. And the experimental data were consistent with the fitted curve. The functional relationship could be expressed by equation (3)

{\begin{cases} I_{1} = 56.7 + 267.66667 X - 522.5 X^{2} + 408.33333 X^{3}, \\ X \in [0 \leq p \leq 0.4] \\ I_{2} = 92.74 + 48.7 X - 37 X^{2}, X \in [0.4 \leq p \leq 0.6] \end{cases}

(3)

where $I$ is the ILSS value (MPa) and p is the curing pressure (MPa). Figure 17 shows the plot of porosity factor versus curing pressure of microwave curing.

Figure 16.

Plot of ILSS value versus curing pressure of microwave curing.

Figure 17.

Plot of porosity factor versus curing pressure of microwave curing.

The functional relationship between the porosity factor and the curing pressure is shown in Figure 17. Because there is a threshold point of 0.4 MPa, the section function could describe the relationship between the porosity factor and the curing pressure more accurately. As shown in Figure 17, the experimental data were consistent with the fitted curve. The functional relationship could be expressed by equation (4)

{\begin{cases} F_{1} = 13.8282 - 106.707 X + 293.17 X^{2} - 279.2 X^{3}, \\ X \in [0 \leq p \leq 0.4] \\ F_{2} = 2.757 - 10.109 X + 9.19 X^{2}, \\ X \in [0.4 \leq p \leq 0.6] \end{cases}

(4)

It is interesting to know that the curing pressure affects not only the porosity factor but also the ILSS of the APMCs directly, as shown in Figures 11, 16, and 17. Some scholars believe that the level of porosity is a major factor affecting the mechanical properties of the APMCs.^15,17,18,23 From our experimental results, the curing pressure is still the main factor affecting the internal voids of the APMCs under the high-pressure microwave curing, which is consistent with the previous point of view.

When the curing pressure increased from 0.1 to 0.4 MPa, the voids became smaller and the ILSS of the APMCs gradually increased. After the curing pressure reached the threshold point (0.4 MPa), the voids in the APMCs gradually disappeared, while the pressure continued to increase. This was mainly because the air bubbles dissolved in the resin gradually with the increase in curing pressure. However, as curing pressure continued to increase beyond the threshold point, the ILSS of APMCs was still slowly increasing. It is a good illustration of the fact that curing pressure is also one of the factors that affects the ILSS of APMCs, but not a main one. The SEM morphology (Figure 15) also illustrated that the curing pressure can affect the bonding condition between the resin and the carbon fibers of the APMCs. All these showed that the curing pressure should be higher than 0.4 MPa in the high-pressure microwave curing.

Equations (3) and (4) can be provided as a reference for the process engineers to develop a curing process in production practice. At the same time, it can provide the theoretical basis for the quality inspection of the airworthiness certification.

Conclusion

Under a given high-pressure microwave curing process, with the gradual increase in curing pressure, the rules of void variation in the APMCs are as follows: the void shape gradually changes from the big, flat, disk-shaped pores to the small, flat, disk-shaped pores, and then into an ellipsoidal void, and then into a small round void as the curing pressure increases. When the curing pressure exceeds 0.4 MPa, the void basically disappears.

The shear failure process of the APMCs can be divided into two stages: the elastic deformation and the ductile fracture. During the shear failure process of the APMCs, sounds ceaselessly occur. At the same time, the failure mechanism of the ductile fracture of APMCs was described.

The results of ODM, SEM, and the inter-laminar shear properties test show that the pressure is the major factor affecting the porosity and ILSS of APMCs in high-pressure microwave curing process. At the same time, the pressure also has a significant impact on the bonding strength of the resin and the carbon fiber interface. There is a threshold for the effect of pressure on porosity and ILSS of APMCs. When the pressure is higher than 0.4 MPa, the voids in APMCs basically disappeared, and the growth of ILSS becomes slow. The functional relationship between the porosity and the ILSS of APMCs can be expressed by the following piecewise functional equations.

ILSS value versus curing pressure

{\begin{cases} I_{1} = 56.7 + 267.66667 X - 522.5 X^{2} + 408.33333 X^{3}, \\ X \in [0 \leq p \leq 0.4] \\ I_{2} = 92.74 + 48.7 X - 37 X^{2}, X \in [0.4 \leq p \leq 0.6] \end{cases}

Porosity factor versus curing pressure

{\begin{cases} F_{1} = 13.8282 - 106.707 X + 293.17 X^{2} - 279.2 X^{3}, \\ X \in [0 \leq p \leq 0.4] \\ F_{2} = 2.757 - 10.109 X + 9.19 X^{2}, \\ X \in [0.4 \leq p \leq 0.6] \end{cases}

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the National Program on Key Basic Research Project of China (973 Program, grant no. 2014CB046502) and sponsored by the Innovation Driven Project of Central South University (grant no. 2015CX002) and the National Natural Science Foundation of China (no. 51675538). The authors would like to gratefully acknowledge the support of composite research team of the Central South University.

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