Effect of Different Catalysts on Properties of Coal Tar Pitch Modified by Cinnamaldehyde

Zhang, Wenjuan; Tian, Wenhong; Song, Shihua; Zeng, Xianren; Gao, Peng; Mao, Lili; Li, Tiehu

doi:https://doi.org/10.1155/2019/6040173

Journal of Spectroscopy

On this page

Abstract Introduction Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2019 | Article ID 6040173 | https://doi.org/10.1155/2019/6040173

Effect of Different Catalysts on Properties of Coal Tar Pitch Modified by Cinnamaldehyde

Wenjuan Zhang,¹Wenhong Tian,²Shihua Song,¹Xianren Zeng,¹Peng Gao,¹Lili Mao,³and Tiehu Li⁴

Academic Editor: Jose S. Camara

Received19 Sept 2018

Accepted03 Feb 2019

Published03 Mar 2019

Abstract

Cinnamaldehyde- (CMA-) modified coal tar pitches (CTPs) are prepared in the presence of acids. In this paper, the effect of boric acid and p-toluene sulfonic acid on the pyrolysis and graphitization process of CMA-modified CTP was studied. The pyrolysis process was studied by Fourier transform infrared spectroscopy, thermogravimetric analysis and derivative thermogravimetry, and polarized-light microscopy. In addition, the graphitization process was studied by X-ray diffraction and Raman spectroscopy. The results indicate the carbon yield of CMA-modified CTP with boric acid as catalyst (B₇C₁₀) is higher than that of CMA-modified CTP with p-toluene sulfonic acid as a catalyst (P₇C₁₀). In addition, under the same experimental condition (heated at 400°C and held for 1 h), the mesophase spheres of B₇C₁₀ are more regular than those of P₇C₁₀ and the largest diameter of the mesophase spheres can reach to 40 um. Further, after the graphitization process, the graphitization degree of B₇C₁₀ is higher than that of P₇C₁₀. So, it is more effective to modify CTP with boric acid as a catalyst.

1. Introduction

Coal tar pitch (CTP) is a composite material composed of aromatic compounds with wide molecular weight distribution [1]. As a low cost precursor and because of the ability to produce graphitized carbon, CTP is a promising candidate for the production of carbon materials and carbon-carbon composite materials. [2]. However, it has low carbon yield, low density, and large coking pore volume, which require a series of impregnation/carbonization steps for subsequent densification under high pressure. Therefore, it is time-consuming and expensive to treat carbon materials or carbon-carbon composites with CTP as a matrix precursor [3]. Therefore, the key to simplify and reduce the cost of the preparation process is to improve the carbon yield of CTP by specific treatment methods. [4].

It is well known that the carbonization yield of CTP can be improved by physical separation and chemical modification. Physical separation can be used to separate components with different average molecular weights in CTP [5]. On the other hand, chemical modifications involve chemical reactions between CTP and polystyrene, rosin, lignin/silica hybrid, and so on [6–12]. Compared with physical separation, chemical modification has advantages in saving resources, reducing waste treatment, and simplifying the preparation process. Therefore, it has been the typical way to improve the carbonization yield of CTP.

Cinnamaldehyde (CMA) is a kind of alpha, beta-unsaturated aldehyde, in which the benzene ring is conjugated with C=O and C=C groups. Therefore, it is a potential natural crosslinker [13]. Hydrogenation reactions can occur simultaneously in C=O and C=C groups [14]. In our previous study, CMA was used to modify CTP with boric acid and p-toluene sulfonic acid as catalysts, respectively [15, 16]. Higher carbonization yield and better properties of modified CTP can be obtained because of the cross-linking reaction between CMA and CTP. However, the catalysts also play important roles in the modification of CTP with CMA. In this paper, we investigate the effect of the two catalysts on the pyrolysis and graphitization process of CMA-modified CTP.

2. Experiment

2.1. Materials

CTP was purchased from the Steel Co. Ltd. (Wuhan, China). CMA, toluene, quinoline, boric acid, and PTS were of analytical grade. Some properties of the CTP are included in Table 1.

2.2. Modification of CTP

In our previous research, the optimal experimental condition for CMA-modified CTP is 100 g CTP and 7 g PTS mixed with 10 ml of CMA [16]. For comparison, CMA-modified CTPs with boric acid and p-toluene sulfonic acid as catalysts were prepared under the same experimental conditions. The detailed preparation method was described in references [15, 16]. The pitches studied in this paper are the parent CTP (B₀C₀), the CTP modified with 10 ml of CMA with 7 g of boric acid as a catalyst (B₇C₁₀), and the CTP modified with 10 ml of CMA with 7 g of PTS as a catalyst (P₇C₁₀), respectively.

2.3. Preparation of Mesocarbon Microbeads

The mesophase spheres of the parent CTP and modified CTPs were prepared in a high-pressure reaction kettle. About 5 g of the sample was heated to 400°C at a heating rate of 1°C·min⁻¹ and kept for 1 h at this temperature. In the whole process, nitrogen gas was introduced into the kettle.

2.4. Graphitization of CTPs

The graphitization experiment was carried out in a graphitizing furnace. First, the samples were carbonized at 900°C. Then, the carbonized samples were placed in a graphite crucible and heated to 2400°C at a heating rate of 10°C·min⁻¹ for 2 h. The resultant graphitized products of B₀C₀, B₇C₁₀, and P₇C₁₀ were denoted as B₀C₀–2400, B₇C₁₀–2400, and P₇C₁₀–2400, respectively.

2.5. Characteristics of CTP and CMA-Modified CTPs

The compositions of CTP and CMA-modified CTPs were analyzed by standard methods: softening point (SP), ring and ball method, ASTM D36-66; coking value (CV), ISO 6998; toluene insolubles (TI), ISO 6376-96; quinoline insolubles (QI), ISO 6791-81.

2.6. Measurements

Elemental analysis of C, H, and N was performed on a Vario EL-III analyzer. Fourier transform infrared (FT-IR) spectroscopy was acquired on a Bruker Tenser-27 FT-IR spectrometer with thin films of KBr in the range of 4000–400 cm⁻¹. Thermogravimetric analysis (TG-DTG) was performed on a Mettler-Toledo 851^e thermal analyzer under N₂ atmosphere with a heating rate of 10°C·min⁻¹.

The optical textures of the semicokes were observed using an OLYMPUS-B061 polarized-light microscope. X-ray diffraction (XRD) was measured on a PANalytical X’Pert PRO X-ray diffractometer with CuKα (λ = 1.5406 Å) radiation at 40 kV and 35 mA. Raman spectra were performed on the Raman microscope (inVia, Renishaw, London, England) at 900–2000 cm⁻¹. The spectral excitation was provided by an Ar ion laser, using the 514.5 nm line and with proper power density on the sample surface.

3. Results and Discussion

3.1. Characteristics of the Parent CTP and Two CMA-Modified CTPs

The main characteristics of the parent CTP (B₀C₀) and two CMA-modified CTPs (B₇C₁₀ and P₇C₁₀) are listed in Table 1. It can be observed that B₇C₁₀ and P₇C₁₀ have lower softening points, TI content, and C/H ratio than those of B₀C₀. While the CV and QI content of B₇C₁₀ and P₇C₁₀ are higher than that of B₀C₀. The highest CV can be obtained for B₇C_10. So, after the modification of CTP with CMA in the presence of acid, the CV of CTP is indeed improved.

3.2. FT-IR Analysis

FT-IR spectra of B₀C₀, B₇C₁₀, and P₇C₁₀ are shown in Figure 1. The attribution of the peaks of B₀C₀ is described in references [15–17]. For the FT-IR spectra of B₇C₁₀ and P₇C₁₀, the disappearance of the peak at 3420 cm⁻¹ indicates the reaction between CMA and O–H. In addition, the peaks at 1670 cm⁻¹, 1120 cm⁻¹, and 700 cm⁻¹ are the characteristic peaks of CMA [18]. In the FT-IR spectrum of B₇C₁₀, the peak attributed to the O–H stretching vibration of boric acid is at 3200 cm⁻¹, while the peak at 1192 cm⁻¹ is due to the asymmetric stretching vibration of the tetrahedral BO₄ [19]. In the spectrum of P₇C₁₀, the characteristic peaks of PTS are at 1226 cm⁻¹, 1180 cm⁻¹, 1035 cm⁻¹, and 567 cm⁻¹ [20]. The appearance of the peaks indicates the presence of the matter.

3.3. TG-DTG Analysis

The TG curves of B₀C₀, B₇C₁₀ and P₇C₁₀ are shown in Figure 2. The results show that both parental CTP and modified CTP decompose at a mass loss stage in the temperature range of 25–800°C. Weight loss is mainly due to the removal of light compounds and gases produced by thermal polymerization and aromatic ring side chain cracking [1]. The carbonization yield of B₀C₀, B₇C₁₀, and P₇C₁₀ at 800°C are 41.04%, 46.68%, and 46.12%, respectively, indicating that the carbonization yield of CTP increases after CTP is modified by CMA.

The physical and chemical changes during the CTP pyrolysis can be well understood by the combination analysis of TG and DTG results. The DTG curves of the parent CTP and two CMA-modified CTPs are shown in Figure 3. The DTG curve of B₀C₀ is characterized by a single peak centered at 362°C, indicating that the mass loss rate at this temperature reaches a maximum. However, in the DTG curves of B₇C₁₀ and P₇C₁₀, two small peaks appeared at a temperature of about 370°C, which may be caused by complex changes in this temperature. In addition, in the DTG curve of B₇C₁₀, the peak at about 150°C is caused by dehydration of boric acid [21]. Otherwise, at 500°C, the peak corresponding to the thermal polymerization at this temperature is more pronounced in the DTG curves of B₇C₁₀ and P₇C₁₀ than in B₀C₀.

3.4. Optical Texture of Resultant Semicokes

The optical texture observed by a polarizing microscope is closely related to the conductivity, thermal expansion, mechanical strength, and graphitization properties of carbon materials and is one of the most relevant characteristics of carbon materials [22]. When CTP is heated above 350°C, mesophase spheres will appear in its optical structure, which is a good precursor of high-performance carbon materials [23].

The optical structure of the product prepared from B₀C₀, B₇C₁₀, and P₇C₁₀ after heating at 400°C for 1 hour is shown in Figure 4. The optical micrograph of the parent CTP (Figure 4(a)) shows that some small mesophase spheres are produced. These spheres have a maximum diameter of about 5 um and are very dispersed. Figure 4(b) shows the mesophase spheres obtained from B₇C₁₀ after heating at 400°C for 1 h. Compared to B₀C₀ (Figure 4(a)), the number of mesophase spheres increases and the shape is more regular. These spheres have a maximum diameter of 40 um. The optical structure of P₇C₁₀ treated under the same conditions is shown in Figure 4(c). Compared to B₀C₀ (Figure 4(a)), the number of mesophase spheres increases, but the shapes of these balls are less regular and the size is about 15 um. From this, it can be seen that the number of mesophase spheres increases after the modification of CTP by CMA. But the B₇C₁₀ can get larger sizes and more regular spheres.

3.5. XRD Analysis

To study the effect of boric acid and PTS on the graphitization process of CMA-modified coal tar pitch, B₀C₀–2400, B₇C₁₀–2400, and P₇C₁₀–2400, which are the graphitized products of B₀C₀, B₇C₁₀, and P₇C₁₀ heated at 2400°C and held for 2 h, respectively, were prepared and characterized by XRD and Raman spectra.

Figures 5(a)–5(c) show the XRD patterns of the three CTPs and their corresponding graphitized products. Figure 5(d) is the magnified XRD patterns of B₀C₀–2400, B₇C₁₀–2400, and P₇C₁₀–2400. By comparing the XRD patterns of CTP with its corresponding graphitized products (Figures 5(a)–5(c)), it can be observed that the diffraction peak centered at 25°, which is generally indexed to (002) diffraction of graphite [24], becomes sharp after graphitization while the other diffraction peaks of graphite loomed. Otherwise, in the XRD patterns of B₇C₁₀ (Figure 5(b)), the peaks centered at 14.67° and 28.1° are the diffraction peaks of boric acid [25]. To see clearly the small peaks of the graphitized products of B₀C₀–2400, B₇C₁₀–2400, and P₇C₁₀–2400, the XRD patterns are magnified (Figure 5(d)). It is worth noting that (201) and (114) crystal planes of graphite can be obverted in the XRD patterns of B₇C₁₀–2400 but cannot be found in the XRD patterns of B₀C₀–2400 and P₇C₁₀–2400, which indicates B₇C₁₀–2400 possesses a higher degree of graphitization than B₀C₀–2400 and P₇C₁₀–2400.

(a)

(b)

(c)

(d)

According to the XRD patterns, the crystal structure parameters (d₀₀₂, the interlayer spacing; L_c, the crystallite sizes along the c-axis; G, the graphitization degree) of B₀C₀–2400, B₇C₁₀–2400, and P₇C₁₀–2400 are calculated by the Bragg formula and the Debye-Scherrer equation, respectively [26], and the results are listed in Table 2. It can be observed that d₀₀₂ and L_c values of B₇C₁₀–2400 are smaller than those of B₀C₀–2400, and the graphitization degree of B₇C₁₀–2400 is larger than that of B₀C₀–2400. This indicates the graphitization degree of CMA-modified CTP can be increased obviously with boric acid as a catalyst. For P₇C₁₀–2400, the value of d₀₀₂ is larger than that of B₀C₀–2400, but the values of L_c and G are smaller than those of B₀C₀–2400. This shows that the graphitization degree of CMA-modified CTP with PTS as a catalyst cannot be increased although the carbon yield is increased.

3.6. Raman Analysis

The Raman spectrum of a single graphite crystal usually shows a very strong peak corresponding to the E2G mode at 1575 cm⁻¹ (G band). Polycrystalline graphite and disordered carbon exhibit additional peaks at 1355 cm⁻¹ (D band). The I_G/I_D intensity ratio is considered to be an indicator of the degree of sample disorder. The larger the I_G/I_D value, the lower the degree of disorder [27].

Figure 6 shows the Raman spectra of B₀C₀–2400, B₇C₁₀–2400, and P₇C₁₀–2400. The two peaks of P₇C₁₀–2400 is little different from that of B₀C₀–2400. But for B₇C₁₀–2400, the two peaks shift to higher frequency and the intensity increases strongly.

Table 3 lists the I_G/I_D value of Raman spectra of B₀C₀–2400, B₇C₁₀–2400, and P₇C₁₀–2400. It is obvious that B₇C₁₀–2400 has higher I_G/I_D value than B₀C₀–2400 and P₇C₁₀–2400, which indicates B₇C₁₀–2400 has a higher degree of graphitization. So, the result of the Raman spectroscopy is consistent with the XRD.

From the XRD and Raman spectroscopy analysis, it can be concluded that B₇C₁₀ has a higher graphitization degree than P₇C₁₀. The reason may be due to the good compatibility between boron atoms and carbon atoms. The covalent radius of boron atoms and carbon atoms is 0.088 nm and 0.077 nm, respectively. In addition, the diffusion coefficient of boron atoms in the direction of the graphite crystal is as high as 6320 cm²/S⁻¹ [28]. Therefore, it is possible that boron atoms may occupy the disordered carbon structure by the diffusion of solid solution, and the defects of the disordered structure are eliminated [29]. So, the modified CTP with boric acid as a catalyst has a higher graphitization degree than that with p-toluene sulfonic acid as a catalyst.

4. Conclusions

The effect of boric acid and p-toluene sulfonic acid on the pyrolysis and graphitization process of CMA-modified CTP was compared. The results show that larger size and more regular mesophase spheres can be obtained from B₇C₁₀ compared with P₇C₁₀ under the same experimental condition. Furthermore, the product of B₇C₁₀ graphitized at 2400°C possesses a higher graphitization degree than that of P₇C₁₀. Therefore, boric acid is better than p-toluene sulfonic acid as a catalyst in the modification of CTP.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was supported by the Science & Technology Project of Department of Education of Jiangxi Province (GJJ170948).

References

M. Pérez, M. Granda, R. Santamarı́a, T. Morgan, and R. Menéndez, “A thermoanalytical study of the co-pyrolysis of coal-tar pitch and petroleum pitch,” Fuel, vol. 83, no. 9, pp. 1257–1265, 2004.
View at: Publisher Site | Google Scholar
Y.-X. Wang, S.-L. Chou, J. H. Kim, H.-K. Liu, and S.-X. Dou, “Nanocomposites of silicon and carbon derived from coal tar pitch: cheap anode materials for lithium-ion batteries with long cycle life and enhanced capacity,” Electrochimica Acta, vol. 93, pp. 213–221, 2013.
View at: Publisher Site | Google Scholar
L. M. Manocha, M. Patel, S. M. Manocha, C. Vix-Guterl, and P. Ehrburger, “Carbon/carbon composites with heat-treated pitches: I. Effect of treatment in air on the physical characteristics of coal tar pitches and the carbon matrix derived therefrom,” Carbon, vol. 39, no. 5, pp. 663–671, 2001.
View at: Publisher Site | Google Scholar
V. Prada, M. Granda, J. Bermejo, and R. Menéndez, “Preparation of novel pitches by tar air-blowing,” Carbon, vol. 37, no. 1, pp. 97–106, 1999.
View at: Publisher Site | Google Scholar
J. J. Fernández, A. Figueiras, M. Granda, J. Bermejo, and R. Menéndez, “Modification of coal-tar pitch by air-blowing - I. Variation of pitch composition and properties,” Carbon, vol. 33, no. 3, pp. 295–307, 1995.
View at: Publisher Site | Google Scholar
W. Ciesinska, J. Zielinski, and T. Brzozowska, “Thermal treatment of pitch-polymer blends,” Journal of Thermal Analysis and Calorimetry, vol. 95, no. 1, pp. 193–196, 2009.
View at: Google Scholar
Q. Lin, W. Su, and Y. Xie, “Effect of rosin to coal-tar pitch on carbonization behavior and optical texture of resultant semi-cokes,” Journal of Analytical and Applied Pyrolysis, vol. 86, no. 1, pp. 8–13, 2009.
View at: Publisher Site | Google Scholar
T. Li, X. Liu, C. Wang, and H. Wang, “Structural characteristics of mesophase spheres prepared from coal tar pitch modified by phenolic resin,” Chinese Journal of Chemical Engineering, vol. 14, no. 5, pp. 660–664, 2006.
View at: Publisher Site | Google Scholar
Q. Lin, H. Tang, C. Li, and L. Wu, “Carbonization behavior of coal-tar pitch modified with lignin/silica hybrid and optical texture of resultant semi-cokes,” Journal of Analytical and Applied Pyrolysis, vol. 90, no. 1, pp. 1–6, 2011.
View at: Publisher Site | Google Scholar
X. Wang, Y. Ma, R. Niu, Q. Wang, and X. Wang, “Pyrolysis behaviour and kinetic of coal tar pitch modified with paraformaldehyde,” Waste and Biomass Valorization, vol. 8, no. 1, pp. 209–216, 2016.
View at: Publisher Site | Google Scholar
W. C. Shi, L. M. Dong, Q. Li, C. Wang, and T. X. Liang, “Study on the variety of rheological properties and thermal analysis of modified coal tar pitch with 1,4-benzenedimethanol,” Key Engineering Materials, vol. 602-603, pp. 304–307, 2014.
View at: Publisher Site | Google Scholar
Q. Lin, T. Li, Y. Ji, W. Wang, and X. Wang, “Study of the modification of coal-tar pitch with p-methyl benzaldehyde,” Fuel, vol. 84, no. 2-3, pp. 177–182, 2005.
View at: Publisher Site | Google Scholar
M. P. Balaguer, J. Gómez-Estaca, R. Gavara, and P. Hernandez-Munoz, “Functional properties of bioplastics made from wheat gliadins modified with cinnamaldehyde,” Journal of Agricultural and Food Chemistry, vol. 59, no. 12, pp. 6689–6695, 2011.
View at: Publisher Site | Google Scholar
T. Vergunst, F. Kapteijn, and J. A. Moulijn, “Kinetics of cinnamaldehyde hydrogenation-concentration dependent selectivity,” Catalysis Today, vol. 66, no. 2–4, pp. 381–387, 2001.
View at: Publisher Site | Google Scholar
W. Zhang, T. Li, M. Lu, and C. Hou, “Comparative study of the modification of coal tar pitch for higher carbonization yield and better properties,” Chinese Journal of Chemical Engineering, vol. 21, no. 12, pp. 1391–1396, 2013.
View at: Publisher Site | Google Scholar
W. Zhang, T. Li, H. Liu et al., “Preparation and carbonization behavior of cinnamaldehyde modified coal tar pitch,” Journal of Analytical and Applied Pyrolysis, vol. 94, pp. 63–67, 2012.
View at: Publisher Site | Google Scholar
J. Alcañiz-Monge, D. Cazorla-Amorós, and A. Linares-Solano, “Characterisation of coal tar pitches by thermal analysis, infrared spectroscopy and solvent fractionation,” Fuel, vol. 80, no. 1, pp. 41–48, 2001.
View at: Publisher Site | Google Scholar
M. E. Carlotti, S. Sapino, R. Cavalli, M. Trotta, F. Trotta, and K. Martina, “Inclusion of cinnamaldehyde in modified γ-cyclodextrins,” Journal of Inclusion Phenomena and Macrocyclic Chemistry, vol. 57, no. 1, pp. 445–450, 2007.
View at: Publisher Site | Google Scholar
Z. Liu, S. Gao, and S. Xia, “FT-IR spectroscopic study of phase transformation of chloropinnoite in boric acid solution at 303 K,” Spectrochimica Acta Part A, vol. 59, no. 2, pp. 265–270, 2003.
View at: Google Scholar
F. Selampinar, U. Akbulut, E. Yildiz, A. Güngör, and L. Toppare, “Synthesis of a novel poly(arylene ether ketone) and its conducting composites with polypyrrole,” Synthetic Metals, vol. 89, no. 2, pp. 111–117, 1997.
View at: Publisher Site | Google Scholar
Q. Wang, J. Li, and J. E. Winandy, “Chemical mechanism of fire retardance of boric acid on wood,” Wood Science and Technology, vol. 38, no. 5, pp. 375–389, 2004.
View at: Publisher Site | Google Scholar
B. Grzyb, J. Machnikowski, J. V. Weber, A. Koch, and O. Heintz, “Mechanism of co-pyrolysis of coal-tar pitch with polyacrylonitrile,” Journal of Analytical and Applied Pyrolysis, vol. 67, no. 1, pp. 77–93, 2003.
View at: Publisher Site | Google Scholar
Y.-G. Wang, Y.-C. Chang, S. Ishida, Y. Korai, and I. Mochida, “Stabilization and carbonization properties of mesocarbon microbeads (MCMB) prepared from a synthetic naphthalene isotropic pitch,” Carbon, vol. 37, no. 6, pp. 969–976, 1999.
View at: Publisher Site | Google Scholar
A.-H. Lu, W.-C. Li, E.-L. Salabas, B. Spliethoff, and F. Schüth, “Low temperature catalytic pyrolysis for the synthesis of high surface area, nanostructured graphitic carbon,” Chemistry of Materials, vol. 18, no. 8, pp. 2086–2094, 2006.
View at: Publisher Site | Google Scholar
C. Zha, G. R. Atkins, and A. F. Masters, “A spectroscopic study of an anhydrous,” Journal of Sol-Gel Science and Technology, vol. 13, no. 1, pp. 103–107, 1998.
View at: Publisher Site | Google Scholar
G. Wei, W. Su, Z. Wei et al., “Effect of the graphitization degree for electrospun carbon nanofibers on their electrochemical activity towards VO 2+/VO 2 + redox couple,” Electrochimica Acta, vol. 199, pp. 147–153, 2016.
View at: Publisher Site | Google Scholar
D. Mattia, M. P. Rossi, B. M. Kim, G. Korneva, H. H. Bau, and Y. Gogotsi, “Effect of graphitization on the wettability and electrical conductivity of CVD-carbon nanotubes and films,” Journal of Physical Chemistry B, vol. 110, no. 20, pp. 9850–9855, 2006.
View at: Publisher Site | Google Scholar
L. E. Jones and P. A. Thrower, “Influence of boron on carbon fiber microstructure, physical properties, and oxidation behavior,” Carbon, vol. 29, no. 2, pp. 251–269, 1991.
View at: Publisher Site | Google Scholar
H. Wang, Q. Guo, J. Yang et al., “Microstructure and thermophysical properties of B₄C/graphite composites containing substitutional boron,” Carbon, vol. 52, pp. 10–16, 2013.
View at: Publisher Site | Google Scholar

Copyright

Copyright © 2019 Wenjuan Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

PDF Download Citation

Download other formats

Order printed copies