Effect of Carbon Nanotubes on the Microstructure and Thermal Property of Phenolic/Graphite Composite

Nguyen, The Huu; Vu, Minh Thanh; Le, Van Thu; Nguyen, Tuan Anh

doi:https://doi.org/10.1155/2018/6329651

International Journal of Chemical Engineering

On this page

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

Research Article | Open Access

Volume 2018 | Article ID 6329651 | https://doi.org/10.1155/2018/6329651

Effect of Carbon Nanotubes on the Microstructure and Thermal Property of Phenolic/Graphite Composite

The Huu Nguyen,¹Minh Thanh Vu,²Van Thu Le,³and Tuan Anh Nguyen⁴

Academic Editor: Maurizio Volpe

Received10 May 2018

Revised19 Jul 2018

Accepted26 Jul 2018

Published02 Sept 2018

Abstract

In this work, composites based on phenolic resin (P), graphite powder (G), carbon fibers (CFs), and carbon nanotubes (CNTs) were prepared by using hot compression molding. The effect of CNTs on the microstructure and thermal property of these composites was investigated. FESEM analysis indicated that the surface structure of phenolic/graphite composites became more dense and homogeneous with the presence of CNTs. The carbonization behavior of composites was characterized by using thermal gravimetric analyses (TGAs). TGA data showed that the presence of CNTs in phenolic/graphite composites enhanced their thermal stability, by increasing the temperature of maximum weight loss rate (T_m) during pyrolysis and decreasing the weight loss after pyrolysis. In addition, incorporation of CNTs into phenolic/graphite composites reduced significantly their open porosity.

1. Introduction

Phenolic resins are the oldest synthetic plastics commercially applied in a broad range of applications such as paints, adhesives, and composites. There are two types of phenolic resin, the resole type (self-curing, one-step resin) and the novolac type (two-stage resin). It was reported that phenolic resin can provide the intermolecular hydrogen bonding, which acts as the domain driving force that interacts with hydroxyl, carbonyl, amide, ester, and other hydrogen-bonding functional groups [1]. Therefore, phenolic resins have been widely used as a matrix for composites.

Recently, carbon nanotubes (CNTs) have been used as reinforcements in polymer matrices to improve their mechanical and electrical properties [2–4]. Although some experimental data indicated the strength enhancement by the presence of CNTs in the polymer matrix [1, 5, 6], no strength enhancement or limited enhancement was observed in other works [7–9]. There are two important factors that might limit the applications of this reinforcement. The first one is the homogeneous dispersion of CNTs in the polymer matrix. The second one is the effective translation of CNT strengths (interaction between the CNTs and polymer matrix). For the effective translation of strength, the strong CNT-matrix bonds are required; thus, surface properties of CNTs became very important. Various methods have been used to enhance the dispersion of CNTs, such as the covalent modification, using surfactants, and polymers wrapping [10].

It was reported that carbon-carbon (C-C) composites combined both high strength and high stiffness with high thermal resistance (∼2000°C) and lightweight. The microconstituents of these C-C composites include carbon fibers, pyrolytic carbon matrix, and pores [11]. Adding the CNT component into C-C composites is the new approach for improving the property of C-C composites. CNTs could have played important roles in C-C composites, such as an interlock with higher heat friction [12], an antioxidizing agent and a thermal protective barrier [13], and a heat dissipator throughout the phenolic matrix [14]. Saeed et al. [15] successfully fabricated the carbon fiber-reinforced CNT-phenolic nanocomposites by melt mixing. They reported that the addition of CNTs improved thermal stability and ablation properties of the carbon fiber-reinforced composites. Adding 0.05% CNTs into their composites increased the char yield from 47% to 49% (at 900°C). The authors signaled that the CNT-phenolic interface could have an important role in thermal resistance of the composite. Saghar et al. [16] recently also reported that addition of 0.1 wt.% CNTs into carbon fiber-phenolic matrix composites reduced their ablation from 0.042 mm/s to 0.038 mm/s.

On the contrary, Newling and Walker [17] reported that addition of graphite powder decreased the shrinkage tendency of the phenolic matrix during carbonization and also enhanced the carbon yield. Fitzer et al. [18] reported that the strength translation of the carbon fiber-reinforced composite increased from 40% to 60% by adding 50% graphite powders into the resin matrix prior to its carbonization. Besides, during the pyrolysis (carbonization), the resin matrix was converted to carbon, thus increasing the porosity and decreasing the density. The typical densification process was thereafter carried out by using either resin impregnation or chemical vapor infiltration (CVI) and then by the high-temperature heat treatment (graphitization). It was reported in the literature that the main factors influencing the physical properties of the C-C composite are (i) its components (composition, property, and interaction), (ii) carbonization, (iii) deposition of pyrocarbon, and (iv) graphitization, with the composition of constituent parts being the most important factor [19–22].

In this work, CNTs were added into the phenolic resin/graphite composite fabricated by the hot compression molding. The effects of CNTs on the morphology and properties of phenolic/graphite composites were investigated.

2. Experimental

2.1. Materials

The phenolic resin powder was synthesized by the reaction of phenol and formalin. The specific gravity of the as-synthesized powder was 0.337 and its melting point was 84°C. To provide the mechanical strength of composites, the carbon fibers (T-650/35 PAN-based plain weave) of diameter ∼7 µm were used in this study. The density of carbon fibers (CF) is 1.77 g/cm³, and their thermal conductivity is ∼14 W/mK. Before using for fabrication of composites, these fibers were thermally treated in air (at 400°C for 2 hours).

The custom-made carbon nanotubes (CNTs) were produced by the CVD method as supplied by the Institute for Materials Science (Vietnam). The as-prepared CNTs have the outer diameter of 10–30 nm, inner diameter of 5–15 nm, and length of 5–30 µm. Thermal conductivity and density of these CNTs are ∼1950 W/mK and ∼1.6 cm³, respectively. Before using for fabrication of composites, carboxylic groups have been grafted on the surface of these CNTs under chemical treatment using the H₂SO₄/HNO₃ mixture [23, 24].

Flake-shaped graphite powder (particle size of 30–50 µm and density of ∼2.2 g/cm³) was purchased from Sigma-Aldrich.

2.2. Fabrication of Composite

To fabricate the phenolic sample and its composites, a mixture (P solution) with the phenolic resin : hexamethylenetetramine : ethanol ratio by weight of 50 : 6 : 44 was prepared by using a mechanical mixer.

Graphite powder (G), chopped carbon fiber (CF), and CNTs were then added into the P solution to form the composites. For each composite sample, the ratio of P : G : CF : CNT by weight was 1 : 3 : 0.2 : 0.05.

These as-prepared 6 samples, such as P, G/P, G-CF/P, CNT/P, CNT-CF/P, and CNT-CF-G/P, were then heated up to 80°C for 3 hours (for removing the residual solvents) and subsequently hot-pressed. The hot-pressed process took place at 120°C (for 30 minutes) and then at 165°C (for 30 minutes) under a pressure of 150 kg/cm². After cooling to room temperature, all samples have been machined in the dimensions of 10 mm × 10 mm × 10 mm.

Table 1 shows the mix design for the 6 types of samples: P, G/P, G-CF/P, CNT/P, CNT-CF/P, and CNT-CF-G/P composites.

2.3. Composite Characterization

The morphological investigation of the prepared composites was carried out by using the field-emission scanning electron microscope (FESEM S4800, Hitachi, Japan). The carbonization behavior of composites was characterized by using thermal gravimetric analyses (TGAs). TGA was conducted by using the STA 409PC Netzsch equipment. These composites were heated from room temperature to 1000°C at a rate of 20°C·min⁻¹ under nitrogen atmosphere with a flow rate of 50 cm³·min⁻¹.

The density of composites was determined by dividing their mass by the volume of cubic sample. The open porosity of composites was measured by using the Archimedes method with water as the immersing agent (water impregnation method [11]).

3. Results and Discussion

3.1. Microstructure Study

For the comparative studies, we divide these 6 samples into two groups, such as P, G/P, and G-CF/P samples and CNT/P, CNT-CF/P, and CNT-CF-G/P samples.

Figure 1 presents the SEM images of P, G/P, and G-CF/P samples before and after carbonization. As can be seen in Figure 1(a), before pyrolysis, the SEM images indicate the relatively homogeneous microstructure without the appearance of large pores on the surface of resin P. However, the surface of these pyrolyzed composites was porous and heterogeneous (Figure 1(b)). These results indicated that P resin was strongly decomposed by pyrolysis, especially in its external surface. It should be noted that the deep pores appearing inside the microstructure indicated poor and fast pyrolysis behavior of G/P and G-CF/P composites (Figures 1(d) and 1(f)). In addition, after pyrolysis, the presence of G in the P matrix increased its porosity (Figures 1(b) and 1(d)). Higher porosity was also observed for G-CF/P composites (Figure 1(e)). This high porosity could be explained by the low P resin content in these composites that made the graphite powder incompletely wet, leading to the weak binding and increasing porosity. In carbonization, carbon from phenolic resins contained high content of closed pores [25]. However, to obtain the high-strength C-C composites, resin content should be limited below 15 wt.%, due to the pyrolysis shrinkage of phenolic resins [6, 7]. Therefore, it was not recommended to increase the resin content for reducing the porosity of composites. Incorporation of nanofillers (CNTs) into the P matrix is expected to reduce its porosity.

(a)

(b)

(c)

(d)

(e)

(f)

Figures 2 and 3 present the SEM images at high magnifications of CNTs (before and after acid treatment) and CNT/P nanocomposites (before and after pyrolysis), respectively. Regarding CNT component, before acid treatment, CNT bundles and aggregates have been observed in Figures 2(a) and 2(c). To fabricate the CNT/resin composite, these bundles and aggregates could reduce highly the mechanical property of nanocomposites. As can be seen in Figures 2(b) and 2(d), by surface modification, these aggregations of CNTs were significantly reduced, leading to the homogeneous dispersion of CNTs in the resin matrix. The SEM images of CNT/P nanocomposite samples (Figure 3) indicated that the surface of CNTs was coated by thick coatings. The outer diameter of coated CNTs (after pyrolysis) was ∼50 nm (Figure 3(b)), which was much larger than that of untreated CNTs (e.g., ∼30 nm). These coatings were still strongly adherent on the surface of CNTs after pyrolysis, indicating the strong interaction between modified CNTs and resin matrices (via the carboxylic groups on the surface of CNTs [24]). Thus, acid modification of CNTs might enhance the load transfer at the CNT-resin interface.

(a)

(b)

(c)

(d)

(a)

(b)

Regarding 3 other composites of CNT/P, CNT-CF/P, and CNT-CF-G/P (Figure 4), the effect of CNT on the microstructure of these composites was very impressive, especially for the CNT-CF-G/P sample. As shown in Figures 4(a) and 4(b), for both before and after pyrolysis, the incorporation of CNT into the P matrix reduced significantly its porosity and made its surface more compact. However, the small pores were still observed substantially after pyrolysis, at high magnification for CNT/P composites (Figure 3(b)). To improve the microstructure of CNT/P composites, other fillers, such as G and CF, were used simultaneously with CNTs for the P matrix. As can be seen in Figures 4(c)–4(f), the addition of both G and CF into CNT/P improved significantly its surface morphology, not only before pyrolysis but also after pyrolysis. This finding indicated the slow pyrolysis behavior of G-CF-CNT/P, which did not decompose the composite structure.

(a)

(b)

(c)

(d)

(e)

(f)

3.2. Thermal Property

The effect of CNTs on the thermal stability of phenolic resin was studied using thermogravimetric analysis (TGA). The composite samples were heated from room temperature to 1000°C. This heating process converts gradually the phenolic resin matrix to a carbon matrix.

The TGA curves for phenolic resin (P), G/P, and G-CF/P composites are presented in Figure 5. Figure 6 presents the TGA and DTG curves of CNT/P, CNT-CF/P, and CNT-CF-G/P composites.

(a)

(b)

(a)

(b)

As can be seen in Figure 5(a), the pyrolysis (thermal degradation) of phenolic resin exhibits three thermolytic regions. The first region below 400°C was the thermal decomposition of free cross-linked molecules, such as water, unreacted phenol, and formaldehyde [15]. Thus, in this region, the weight loss should be caused by the evolution of unreacted monomers. In this region, the mass loss rate maximum (DTG diagram) was obtained at 107.2°C (T_m1). The T_m1 represented the temperature at which the maximum weight loss rate occurred in region 1.

The second and third regions were located at 400–600°C and 600–1000°C, respectively. These two regions could be attributed to the decomposition of the cross-linked molecules with the evolution of carbon monoxide, carbon dioxide, methane, phenols, and cresols [5]. In region 2, the mass loss rate maximum was obtained at 512.1°C (T_m2).

For better comparison of TGA data, we assume the temperature at 5% loss in mass (T_initial) as the initial thermal degradation temperature. It was found that the T_initial of the neat phenolic resin was 109°C. Table 2 presents the values of T_m, T_initial, and % weight remaining at 600°C and 1000°C for phenolic resin and its composites, respectively.

Regarding the contribution of graphite reinforcement, as seen in Table 2, it was found that the presence of graphite in phenolic resin (Figure 6) increased the T_m1, T_m2, and T_initial values. Adding the graphite into phenolic resin also significantly reduced the mass loss of resin during the pyrolysis. At 1000°C, the char yields of the neat phenolic resin and G/P composite were found to be 47% and 91%, respectively. Compared to CNT (sample CNT/P), adding the graphite into phenolic resin has a superior enhancement in thermal stability.

For CF reinforcement, adding CF into the G/P composite (G-CF/P) increased its char yield, from 92.7% to 95.4% (at 600°C) and from 91% to 93.7% (at 1000°C). Similar results are observed when adding CF into the CNT/P composite (CNT-CF/P), but its enhancement in thermal stability was lower than that of the G/P composite.

An interesting finding is that adding CNTs into the G-CF/P composite has maximum enhancement in thermal stability, among these 6 samples. As can be seen in Table 2, the presence of CNTs in the G-CF/P composite enhanced its thermal stability, by increasing T_m2 (from 511°C to 522°C), T_initial (from 635°C to 1000°C), and char yield (from to 93.7% to 95%, at 1000°C). This suggests that CNTs increase the bonding between CF and G with the phenolic matrix, resulting in dense carbon-carbon composites.

3.3. Density and Open Porosity of Composites

By converting the phenolic resin matrix to a carbon matrix, the pyrolysis process could enhance the shear modulus due to the high stiffness of the carbon matrix. However, the shrinkage and mass loss during pyrolysis created the porous microstructure that reduced the strength of the composite [26]. It was reported that carbon from phenolic resins contained a high content of closed pores [25], whereas the open porosity could affect the thermal expansion and oxidative stability of the C-C composite.

In this study, before and after the pyrolysis process, the apparent density and open porosity of all composite samples were determined and compared. Table 3 summarizes the values of their apparent density and open porosity.

As can be seen in Table 3, the presence of CNTs (as nanofillers) in the composite reduced significantly its open porosity after pyrolysis. In addition, the apparent density of unpyrolyzed and pyrolyzed composites has the lowest value for G-CF-CNT/P samples, followed by G-CF/P and G/P composites. This small reduction could be attributed to the reduction of graphite powder content in the composites (graphite has a higher value of density, as compared to both CF and CNTs). These values of density and open porosity are strongly coherent with their morphological investigation represented in Figures 1 and 2.

High-performance carbon-carbon composites required a high value of density (e.g., >1.8 g/cm³). However, there are significant differences between the coefficient of thermal expansion of the carbon fibers and resin matrix; thus, during the cooling process, they could generate the internal stresses and stress cracks inside the composite [27]. In addition, the shrinkage during pyrolysis created pores and cracks, thus reducing the density (Table 3). To obtain the desired value of density (>1.9 g/cm³), this carbonized CNT-CF-G/P composite should be then reinfiltrated with carbon (via its open pores) by using CVI (chemical vapor infiltration).

4. Conclusions

The main findings of this research were as follows:(i)The homogeneous and dense G/P, G-CF/P, and G-CF-CNT/P composites were successfully fabricated by using hot compression molding.(ii)The TGA data indicated that the presence of CNTs in the phenolic/graphite composite enhanced its thermal stability, by increasing T_m2 (from 511°C to 522°C), T_initial (from 635°C to 1000°C), and char yield (from 93.7% to 95%, at 1000°C). This suggests that CNTs increase the bonding between CF and G with the phenolic matrix, resulting in dense carbon-carbon composites.(iii)The incorporation of CNTs into the phenolic/graphite composite reduced significantly its open porosity (from 15% to 10%).

The CVI process for densification of this carbonized CNT-CF-G/P composite is now under investigation. After the CVI process (CH₄-Ar gases, at 1100°C during 4 hours), the final CNT-CF-G/P composite could have a higher value of density (∼1.9 g/cm³) and lower open porosity (<3%), as compared to its values before CVI.

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 there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work was financially supported by the Vietnam Academy of Science and Technology (Project no. VAST.TĐ.AN-QP.01/17-19).

References

C. C. Ma, S. C. Sung, F. Y. Wang, L. Y. Chiang, L. Y. Wang, and C. L. Chiang, “Thermal, mechanical, and morphological properties of novolac-type phenolic resin blended with fullerenol polyurethane and linear polyurethane,” Journal of Polymer Science Part B: Polymer Physics, vol. 39, no. 20, pp. 2436–2443, 2001.
View at: Publisher Site | Google Scholar
T. W. Kim, H. R. Lee, S. S. Kim, and Y. S. Lim, “Electromagnetic interference shielding properties of carbon nanotubes reinforced composites,” Advanced Materials Research, vol. 26–28, pp. 305–308, 2007.
View at: Publisher Site | Google Scholar
R. Andrews and M. C. Weisenberger, “Carbon nanotube polymer composites,” Current Opinion in Solid State and Materials Science, vol. 8, no. 1, pp. 31–37, 2004.
View at: Publisher Site | Google Scholar
P. Garg, B. P. Singh, G. Kumar et al., “Effect of dispersion conditions on the mechanical properties of multi-walled carbon nanotubes based epoxy resin composites,” Journal of Polymer Research, vol. 18, no. 6, pp. 1397–1407, 2011.
View at: Publisher Site | Google Scholar
J. N. Coleman, M. Cadek, R. Blake et al., “High performance nanotube-reinforced plastics: understanding the mechanism of strength increase,” Advanced Functional Materials, vol. 14, no. 8, pp. 791–798, 2004.
View at: Publisher Site | Google Scholar
N. H. Tai, M. K. Yeh, and J. H. Liu, “Enhancement of the mechanical properties of carbon nanotube/phenolic composites using a carbon nanotube network as the reinforcement,” Carbon, vol. 42, no. 12-13, pp. 2774–2777, 2004.
View at: Publisher Site | Google Scholar
J. N. Coleman, U. Khan, W. J. Blau, and Y. K. Gunko, “Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites,” Carbon, vol. 44, no. 9, pp. 1624–1652, 2006.
View at: Publisher Site | Google Scholar
O. Breuer and U. Sundararaj, “Big returns from small fibers: a review of polymer/carbon nanotube composites,” Polymer Composites, vol. 25, no. 6, pp. 630–645, 2004.
View at: Publisher Site | Google Scholar
T. H. Ko, W. S. Kuo, and Y. H. Chang, “Influence of carbon-fiber felts on the development of carbon–carbon composites,” Composites Part A: Applied Science and Manufacturing, vol. 34, no. 5, pp. 393–401, 2003.
View at: Publisher Site | Google Scholar
M. M. Chehimi, J. Pinson, and Z. Salmi, “Carbon nanotubes: surface modification and applications,” in Applied Surface Chemistry of Nanomaterials, J. Pinson and M. M. Chehimi, Eds., pp. 95–143, Nova Science Publishers, Hauppauge, NY, USA, 2013, ISBN: 978-1628083514.
View at: Google Scholar
S. Dietrich, J.-M. Gebert, G. Stasiuk et al., “Microstructure characterization of CVI-densified carbon/carbon composites with various fiber distributions,” Composites Science and Technology, vol. 72, no. 15, pp. 1892–1900, 2012.
View at: Publisher Site | Google Scholar
M. Ahmad, U. Farooq, and T. Subhani, “Effect of multiwall carbon nanotubes on the ablative properties of carbon fiber-reinforced epoxy matrix composites,” Arabian Journal for Science and Engineering, vol. 40, no. 5, pp. 1529–1538, 2015.
View at: Publisher Site | Google Scholar
J. S. Tate, S. Gaikwad, N. Theodoropoulou, E. Trevino, and J. H. Koo, “Carbon/phenolic nanocomposites as advanced thermal protection material in aerospace applications,” Journal of Composites, vol. 2013, Article ID 403656, 9 pages, 2013.
View at: Publisher Site | Google Scholar
J. M. Park, D. J. Kwon, Z. Ji et al., “Effects of carbon nanotubes and carbon fiber reinforcements on thermal conductivity and ablation properties of carbon/phenolic composites,” Composites Part B: Engineering, vol. 67, pp. 22–29, 2014.
View at: Publisher Site | Google Scholar
S. Saeed, S. Hakeem, M. Faheem et al., “Effect of doping of multi-walled carbon nanotubes on phenolic based carbon fiber reinforced nanocomposites,” Journal of Physics: Conference Series, vol. 439, Article ID 012017, 2013.
View at: Publisher Site | Google Scholar
A. Saghar, M. Khan, I. Sadiq, and T. Subhani, “Effect of carbon nanotubes and silicon carbide particles on ablative properties of carbon fiber phenolic matrix composites,” Vacuum, vol. 148, pp. 124–126, 2018.
View at: Publisher Site | Google Scholar
D. L. Newling and E. J. Walker, “High-performance ‘graphitized’ carbon-carbon composites,” in Proceedings of International Conference on Carbon Fibres, Their Composites and Application, London, UK, 1971.
View at: Google Scholar
E. Fitzer, K. H. Geigl, and W. Huttner, “Studies on matrix precursor materials for carbon carbon composites,” in Proceedings of 5th International Conference on Carbon and Graphite, vol. 1, pp. 493–506, London, UK, September 1978.
View at: Google Scholar
H. Weisshaus, S. Kening, and A. Siegmann, “Effect of materials and processing on the mechanical properties of C/C composites,” Carbon, vol. 29, no. 8, pp. 1203–1220, 1991.
View at: Publisher Site | Google Scholar
T. H. Ko, W. S. Kuo, and Y. R. Lu, “The influence of post-cure on properties of carbon/phenolic resin cured composites and their final carbon/carbon composite,” Polymer Composites, vol. 21, no. 1, pp. 96–103, 2000.
View at: Publisher Site | Google Scholar
E. T. Thostenson, Z. Ren, and T. W. Chou, “Advances in the science and technology of carbon nanotubes and their composites: a review,” Composites Science and Technology, vol. 61, no. 13, pp. 1899–1912, 2001.
View at: Publisher Site | Google Scholar
Q. M. Gong, Z. Li, X. W. Zhou, J. J. Wu, Y. Wang, and J. Liang, “Synthesis and characterization of in situ grown carbon nanofiber/nanotube reinforced carbon/carbon composites,” Carbon, vol. 43, no. 11, pp. 2426–2429, 2005.
View at: Publisher Site | Google Scholar
T. H. Nguyen, M. T. Nguyen, and E. G. Rakov, “Acid functionalization of carbon nanofibers,” Inorganic Materials, vol. 46, no. 10, pp. 1077–1083, 2010.
View at: Publisher Site | Google Scholar
V. T. Le, C. L. Ngo, Q. T. Le, T. T. Ngo, D. N. Nguyen, and M. T. Vu, “Surface modification and functionalization of carbon nanotube with some organic compounds,” Advances in Natural Sciences: Nanoscience and Nanotechnology, vol. 4, no. 3, Article ID 035017, 2013.
View at: Publisher Site | Google Scholar
M. C. Huang and H. Teng, “Urea impregnation to enhance porosity development of carbons prepared from phenol-formaldehyde resins,” Carbon, vol. 40, no. 6, pp. 955–958, 2002.
View at: Publisher Site | Google Scholar
H. P. Silva, L. C. Pardini, and E. Bittencourt, “Shear properties of carbon fiber/phenolic resin composites heat treated at high temperatures,” Journal of Aerospace Technology and Management, vol. 8, no. 3, pp. 363–372, 2016.
View at: Publisher Site | Google Scholar
J. Jortner, “Macroporosity and interface cracking in multi-directional carbon-carbons,” Carbon, vol. 24, no. 5, pp. 603–613, 1986.
View at: Publisher Site | Google Scholar

Copyright

Copyright © 2018 The Huu Nguyen 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

Views

1696

Downloads

1139

Citations