Journal Home > Volume 12 , Issue 7

Copper (Cu)-containing single-source precursors (SSPs) for the preparation of SiCuCN-based ceramic nanocomposites were successfully synthesized for the first time using polysilazane (PSZ), copper(II) acetate monohydrate (CuAc), and 2-aminoethanol via nucleophilic substitution reactions at silicon (Si) centers of PSZ. The synthesis process, polymer-to-ceramic transformation, and high-temperature microstructural evolution of the prepared ceramics were characterized. Dielectric properties and electromagnetic wave (EMW) absorbing performance of the ceramics were investigated as well. The results show that the polymer-to-ceramic transformation finishes at ca. 900 ℃, and Cu nanoparticles are homogeneously distributed in a SiCN matrix, forming a SiCN/Cu nanocomposite. After annealing at 1200 ℃, the Cu nanoparticles completely transform into copper silicide (Cu3Si). Interestingly, the thermal stability of the Cu nanoparticles can be strongly improved by increasing the free carbon content, so that a part of metallic Cu nanoparticles can be detected in the ceramics annealed even at 1300 ℃, forming a SiCN/Cu/Cu3Si/C nanocomposite. Compared with SiCN, the SiCuCN-based nanocomposites exhibit strongly enhanced dielectric properties, which results in outstanding EMW absorbing performance. The minimum reflection coefficient (RCmin) of the SiCN/Cu/Cu3Si/C nanocomposites annealed at 1300 ℃ achieves −59.85 dB with a sample thickness of 1.55 mm, and the effective absorption bandwidth (EAB) broadens to 5.55 GHz at 1.45 mm. The enhanced EMW absorbing performance can be attributed to an in situ formed unique network, which was constructed with Cu and Cu3Si nanoparticles connected by ring-like carbon ribbons within the SiCN matrix.


menu
Abstract
Full text
Outline
About this article

Microstructural evolution and electromagnetic wave absorbing performance of single-source-precursor-synthesized SiCuCN-based ceramic nanocomposites

Show Author's information Jincan YangaQingbo Wena ( )Bo FengbYalei WangaXiang Xionga( )
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
Fundamentals Department, Air Force Engineering University, Xi’an 710051, China

Abstract

Copper (Cu)-containing single-source precursors (SSPs) for the preparation of SiCuCN-based ceramic nanocomposites were successfully synthesized for the first time using polysilazane (PSZ), copper(II) acetate monohydrate (CuAc), and 2-aminoethanol via nucleophilic substitution reactions at silicon (Si) centers of PSZ. The synthesis process, polymer-to-ceramic transformation, and high-temperature microstructural evolution of the prepared ceramics were characterized. Dielectric properties and electromagnetic wave (EMW) absorbing performance of the ceramics were investigated as well. The results show that the polymer-to-ceramic transformation finishes at ca. 900 ℃, and Cu nanoparticles are homogeneously distributed in a SiCN matrix, forming a SiCN/Cu nanocomposite. After annealing at 1200 ℃, the Cu nanoparticles completely transform into copper silicide (Cu3Si). Interestingly, the thermal stability of the Cu nanoparticles can be strongly improved by increasing the free carbon content, so that a part of metallic Cu nanoparticles can be detected in the ceramics annealed even at 1300 ℃, forming a SiCN/Cu/Cu3Si/C nanocomposite. Compared with SiCN, the SiCuCN-based nanocomposites exhibit strongly enhanced dielectric properties, which results in outstanding EMW absorbing performance. The minimum reflection coefficient (RCmin) of the SiCN/Cu/Cu3Si/C nanocomposites annealed at 1300 ℃ achieves −59.85 dB with a sample thickness of 1.55 mm, and the effective absorption bandwidth (EAB) broadens to 5.55 GHz at 1.45 mm. The enhanced EMW absorbing performance can be attributed to an in situ formed unique network, which was constructed with Cu and Cu3Si nanoparticles connected by ring-like carbon ribbons within the SiCN matrix.

Keywords: microstructure, dielectric property, polymer-derived ceramics (PDCs), electromagnetic wave (EMW) absorbing, copper (Cu)

References(94)

[1]
Xue JM, Yin XW, Cheng LF. Induced crystallization behavior and EMW absorption properties of CVI SiCN ceramics modified with carbon nanowires. Chem Eng J 2019, 378: 122213.
[2]
Zeng XJ, Cheng XY, Yu RH, et al. Electromagnetic microwave absorption theory and recent achievements in microwave absorbers. Carbon 2020, 168: 606–623.
[3]
Song CK, Liu YS, Ye F, et al. Enhanced mechanical property and tunable dielectric property of SiCf/SiC–SiBCN composites by CVI combined with PIP. J Adv Ceram 2021, 10: 758–767.
[4]
Zeng XJ, Li E, Xia GH, et al. Silica-based ceramics toward electromagnetic microwave absorption. J Eur Ceram Soc 2021, 41: 7381–7403.
[5]
Yuan KK, Han DY, Liang JF, et al. Microwave induced in-situ formation of SiC nanowires on SiCNO ceramic aerogels with excellent electromagnetic wave absorption performance. J Adv Ceram 2021, 10: 1140–1151.
[6]
Wang S, Gong HY, Zhang YJ, et al. Microwave absorption properties of polymer-derived SiCN(CNTs) composite ceramics. Ceram Int 2021, 47: 1294–1302.
[7]
Li W, Yu ZJ, Wen QB, et al. Ceramic-based electromagnetic wave absorbing materials and concepts towards lightweight, flexibility and thermal resistance. Int Mater Rev 2022, .
[8]
Shao GF, Shen XD, Huang XG. Multilevel structural design and heterointerface engineering of a host–guest binary aerogel toward multifunctional broadband microwave absorption. ACS Mater Lett 2022, 4: 1787–1797.
[9]
Li DX, Jia DC, Yang ZH, et al. Principles, design, structure and properties of ceramics for microwave absorption or transmission at high-temperatures. Int Mater Rev 2022, 67: 266–297.
[10]
Duan WY, Yin XW, Li Q, et al. A review of absorption properties in silicon-based polymer derived ceramics. J Eur Ceram Soc 2016, 36: 3681–3689.
[11]
Li Q, Yin XW, Duan WY, et al. Dielectric and microwave absorption properties of polymer derived SiCN ceramics annealed in N2 atmosphere. J Eur Ceram Soc 2014, 34: 589–598.
[12]
Shah SR, Raj R. Nanoscale densification creep in polymer-derived silicon carbonitrides at 1350 ℃. J Am Ceram Soc 2001, 84: 2208–2212.
[13]
Seo D, Jung S, Lombardo SJ, et al. Fabrication and electrical properties of polymer-derived ceramic (PDC) thin films for high-temperature heat flux sensors. Sensor Actuat-A Phys 2011, 165: 250–255.
[14]
Sun ZL, Zhou Y, Jia DC, et al. Mechanical and thermal physical properties of amorphous SiCN(O) ceramic bulks prepared by hot-press sintering. Mater Lett 2012, 72: 57–59.
[15]
Song Y, He LH, Zhang XF, et al. Highly efficient electromagnetic wave absorbing metal-free and carbon-rich ceramics derived from hyperbranched polycarbosilazanes. J Phys Chem C 2017, 121: 24774–24785.
[16]
Shao GF, Ding CX, Yu GY, et al. Bridged polysilsesquioxane-derived SiOCN ceramic aerogels for microwave absorption. J Am Ceram Soc 2023, 106: 2407–2419.
[17]
Haluschka C, Engel C, Riedel R. Silicon carbonitride ceramics derived from polysilazanes Part II. Investigation of electrical properties. J Eur Ceram Soc 2000, 20: 1365–1374.
[18]
Yin XW, Kong L, Zhang LT, et al. Electromagnetic properties of Si–C–N based ceramics and composites. Int Mater Rev 2014, 59: 326–355.
[19]
Guo X, Feng YR, Lin X, et al. The dielectric and microwave absorption properties of polymer-derived SiCN ceramics. J Eur Ceram Soc 2018, 38: 1327–1333.
[20]
Xiao FF, Sun HB, Li J, et al. Electrospinning preparation and electromagnetic wave absorption properties of SiCN fibers. Ceram Int 2020, 46: 12773–12781.
[21]
Yuan KK, Han DY, Zhao WY, et al. Structure regulation and microwave absorption property of SiCN ceramic aerogels produced by catalytic pyrolysis. Ceram Int 2021, 47: 31561–31566.
[22]
Ye F, Zhang LT, Yin XW, et al. SiCN-based composite ceramics fabricated by chemical vapor infiltration with excellent mechanical and electromagnetic properties. Mater Lett 2013, 111: 169–172.
[23]
Ren FY, Xue JM, Liu XL, et al. In situ construction of CNWs/SiC-NWs hybrid network reinforced SiCN with excellent electromagnetic wave absorption properties in X band. Carbon 2020, 168: 278–289.
[24]
Xue JM, Hu S, Li XP, et al. Enhanced microwave absorbing properties of Y2O3 modified PDC SiCN ceramics with heterogeneous amorphous interface. J Alloys Compd 2023, 931: 167499.
[25]
Lu JB, Feng YR, Liu J, et al. Improved electromagnetic wave absorbing performance of PDCs–SiCN(Ni) fibers with different nickel content. Ceram Int 2022, 48: 23578–23589.
[26]
Guo X, Xiao FF, Li J, et al. Fe-doped SiCN composite fibers for electromagnetic waves absorption. Ceram Int 2021, 47: 1184–1190.
[27]
Liu Y, Feng YR, Gong HY, et al. Electromagnetic wave absorption properties of nickel-containing polymer-derived SiCN ceramics. Ceram Int 2018, 44: 10945–10950.
[28]
Feng YR, Guo X, Gong HY, et al. The influence of carbon materials on the absorption performance of polymer-derived SiCN ceramics in X-band. Ceram Int 2018, 44: 15686–15689.
[29]
Liu Y, Feng YR, Gong HY, et al. Microwave absorbing performance of polymer-derived SiCN (Ni) ceramics prepared from different nickel sources. J Alloys Compd 2018, 749: 620–627.
[30]
Liu XM, Yu ZJ, Chen LQ, et al. Role of single-source-precursor structure on microstructure and electromagnetic properties of CNTs–SiCN nanocomposites. J Am Ceram Soc 2017, 100: 4649–4660.
[31]
Liu XM, Yu ZJ, Ishikawa R, et al. Single-source-precursor synthesis and electromagnetic properties of novel RGO–SiCN ceramic nanocomposites. J Mater Chem C 2017, 5: 7950–7960.
[32]
Glatz G, Schmalz T, Kraus T, et al. Copper-containing SiCN precursor ceramics (Cu@SiCN) as selective hydrocarbon oxidation catalysts using air as an oxidant. Chemistry 2010, 16: 4231–4238.
[33]
Qin F, Brosseau C. A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles. J Appl Phys 2012, 111: 061301.
[34]
Wen QB, Feng Y, Yu ZJ, et al. Microwave absorption of SiC/HfCxN1−x/C ceramic nanocomposites with HfCxN1−x–carbon core–shell particles. J Am Ceram Soc 2016, 99: 2655–2663.
[35]
Seifollahi Bazarjani M, Kleebe HJ, Müller MM, et al. Nanoporous silicon oxycarbonitride ceramics derived from polysilazanes in situ modified with nickel nanoparticles. Chem Mater 2011, 23: 4112–4123.
[36]
Liu GW, Kaspar J, Reinold LM, et al. Electrochemical performance of DVB-modified SiOC and SiCN polymer-derived negative electrodes for lithium-ion batteries. Electrochim Acta 2013, 106: 101–108.
[37]
Guo X, Feng YR, Liu Y, et al. Cross-linking behavior and dielectric properties of SiCN precursor. Ceram Int 2017, 43: 16866–16871.
[38]
Li HB, Zhang LT, Cheng LF, et al. Effect of the polycarbosilane structure on its final ceramic yield. J Eur Ceram Soc 2008, 28: 887–891.
[39]
Liu Y, Lin X, Gong HY, et al. Electromagnetic properties and microwave absorption performances of nickel-doped SiCN ceramics pyrolyzed at different temperatures. J Alloys Compd 2019, 771: 356–363.
[40]
Günthner M, Wang KS, Bordia RK, et al. Conversion behaviour and resulting mechanical properties of polysilazane-based coatings. J Eur Ceram Soc 2012, 32: 1883–1892.
[41]
Musumeci AW, Frost RL, Waclawik ER. A spectroscopic study of the mineral paceite (calcium acetate). Spectrochim Acta A 2007, 67: 649–661.
[42]
Kuz’mina NE, Palkina KK, Polyakova NV, et al. Synthesis, crystal structure, and IR absorption spectra of the adduct of copper(II) acetate monohydrate with 1-amino-4-aza-9-fluorenone Cu2(CH3COO)4(H2O)2·C12H8N2O. Russ J Coord Chem+ 2001, 27: 711–716.
[43]
Sun J, Wen QB, Li T, et al. Phase evolution of SiOC-based ceramic nanocomposites derived from a polymethylsiloxane modified by Hf- and Ti-alkoxides. J Am Ceram Soc 2020, 103: 1436–1445.
[44]
Ionescu E, Kleebe HJ, Riedel R. Silicon-containing polymer-derived ceramic nanocomposites (PDC-NCs): Preparative approaches and properties. Chem Soc Rev 2012, 41: 5032–5052.
[45]
Seifollahi Bazarjani M, Kleebe HJ, Müller MM, et al. Nanoporous silicon oxycarbonitride ceramics derived from polysilazanes in situ modified with nickel nanoparticles. Chem Mater 2011, 23: 4112–4123.
[46]
Gutierrez O, Tellis JC, Primer DN, et al. Nickel-catalyzed cross-coupling of photoredox-generated radicals: Uncovering a general manifold for stereoconvergence in nickel-catalyzed cross-couplings. J Am Chem Soc 2015, 137: 4896–4899.
[47]
Jones GD, McFarland C, Anderson TJ, et al. Analysis of key steps in the catalytic cross-coupling of alkyl electrophiles under Negishi-like conditions. Chem Commun 2005: 4211–4213.
[48]
Su D, Li YL, Hou F, et al. Synthesis and characterization of ethylene-bridged copolycarbosilazane as precursors for silicon carbonitride ceramics. J Am Ceram Soc 2014, 97: 1311–1316.
[49]
Seitz J, Bill J, Egger N, et al. Structural investigations of Si/C/N-ceramics from polysilazane precursors by nuclear magnetic resonance. J Eur Ceram Soc 1996, 16: 885–891.
[50]
Wu YH, Huang JL, Hou SC, et al. Cu3Si enhanced crystallinity and dopamine derived nitrogen doping into carbon coated micron-sized Si/Cu3Si as anode material in lithium-ion batteries. Electrochim Acta 2021, 387: 138495.
[51]
Guo JF, Pei SE, He ZS, et al. Novel porous Si–Cu3Si–Cu microsphere composites with excellent electrochemical lithium storage. Electrochim Acta 2020, 348: 136334.
[52]
Francis A, Ionescu E, Fasel C, et al. Crystallization behavior and controlling mechanism of iron-containing Si–C–N ceramics. Inorg Chem 2009, 48: 10078–10083.
[53]
Hojamberdiev M, Prasad RM, Fasel C, et al. Single-source-precursor synthesis of soft magnetic Fe3Si- and Fe5Si3-containing SiOC ceramic nanocomposites. J Eur Ceram Soc 2013, 33: 2465–2472.
[54]
Okamoto H. Cu–Si (copper–silicon). J Phase Equilib Diff 2012, 33: 415–416.
[55]
Hurwitz FI, Heimann P, Farmer SC, et al. Characterization of the pyrolytic conversion of polysilsesquioxanes to silicon oxycarbides. J Mater Sci 1993, 28: 6622–6630.
[56]
Colombo P, Paulson TE, Pantano CG. Synthesis of silicon carbide thin films with polycarbosilane (PCS). J Am Ceram Soc 1997, 80: 2333–2340.
[57]
Kurtenbach D, Martin HP, Müller E, et al. Crystallization of polymer derived silicon carbide materials. J Eur Ceram Soc 1998, 18: 1885–1891.
[58]
Danko GA, Silberglitt R, Colombo P, et al. Comparison of microwave hybrid and conventional heating of preceramic polymers to form silicon carbide and silicon oxycarbide ceramics. J Am Ceram Soc 2000, 83: 1617–1625.
[59]
Modena S, Sorarù GD, Blum Y, et al. Passive oxidation of an effluent system: The case of polymer–derived SiCO. J Am Ceram Soc 2005, 88: 339–345.
[60]
Mera G, Riedel R, Poli F, et al. Carbon-rich SiCN ceramics derived from phenyl-containing poly(silylcarbodiimides). J Eur Ceram Soc 2009, 29: 2873–2883.
[61]
Iwamoto Y, Völger W, Kroke E, et al. Crystallization behavior of amorphous silicon carbonitride ceramics derived from organometallic precursors. J Am Ceram Soc 2001, 84: 2170–2178.
[62]
Laine RM, Babonneau F, Blowhowiak KY, et al. The evolutionary process during pyrolytic transformation of poly(N-methylsilazane) from a preceramic polymer into an amorphous silicon nitride/carbon composite. J Am Ceram Soc 1995, 78: 137–145.
[63]
Long X, Zhang S, Shao CW, et al. Effects of heat-treatment on the microstructure, electromagnetic wave absorbing properties, and mechanical properties of SiCN fibers. Front Mater 2020, 7: 563891.
[64]
Luo CJ, Duan WY, Yin XW, et al. Microwave-absorbing polymer-derived ceramics from cobalt-coordinated poly(dimethylsilylene)diacetylenes. J Phys Chem C 2016, 120: 18721–18732.
[65]
Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000, 61: 14095–14107.
[66]
Janakiraman N, Aldinger F. Fabrication and characterization of fully dense Si–C–N ceramics from a poly (ureamethylvinyl) silazane precursor. J Eur Ceram Soc 2009, 29: 163–173.
[67]
Ji ZH, Zhang LL, Tang DM, et al. A review on the controlled growth of single-wall carbon nanotubes from metal catalysts. Acta Metall Sin 2018, 54: 1665–1682. (in Chinese)
[68]
Zhou WW, Han ZY, Wang JY, et al. Copper catalyzing growth of single-walled carbon nanotubes on substrates. Nano Lett 2006, 6: 2987–2990.
[69]
Geng DC, Wu B, Guo YL, et al. Uniform hexagonal graphene flakes and films grown on liquid copper surface. PNAS 2012, 109: 7992–7996.
[70]
Zhao WB, Hu BS, Yang Q, et al. Synergetic interaction between copper and carbon impurity induces low temperature growth of highly-defective graphene for enhanced electrochemical performance. Carbon 2019, 150: 371–377.
[71]
Logesh G, Sabu U, Srishilan C, et al. Tunable microwave absorption performance of carbon fiber-reinforced reaction bonded silicon nitride composites. Ceram Int 2021, 47: 22540–22549.
[72]
Wang S, Ashfaq MZ, Qi DS, et al. Electromagnetic wave absorption properties of polymer-derived magnetic carbon-rich SiCN-based composite ceramics. Ceram Int 2022, 48: 4986–4998.
[73]
Li Q, Yin XW, Feng LY. Dielectric properties of Si3N4–SiCN composite ceramics in X-band. Ceram Int 2012, 38: 6015–6020.
[74]
Zhang XF, Li YX, Liu RG, et al. High-magnetization FeCo nanochains with ultrathin interfacial gaps for broadband electromagnetic wave absorption at gigahertz. ACS Appl Mater Interfaces 2016, 8: 3494–3498.
[75]
Wang C, Han XJ, Xu P, et al. The electromagnetic property of chemically reduced graphene oxide and its application as microwave absorbing material. Appl Phys Lett 2011, 98: 072906.
[76]
Cao MS, Song WL, Hou ZL, et al. The effects of temperature and frequency on the dielectric properties, electromagnetic interference shielding and microwave-absorption of short carbon fiber/silica composites. Carbon 2010, 48: 788–796.
[77]
Micheli D, Apollo C, Pastore R, et al. X-band microwave characterization of carbon-based nanocomposite material, absorption capability comparison and RAS design simulation. Compos Sci Technol 2010, 70: 400–409.
[78]
Suh S, Yoon H, Park H, et al. Enhancing the electrochemical performance of silicon anodes for lithium-ion batteries: One-pot solid-state synthesis of Si/Cu/Cu3Si/C electrode. Appl Surf Sci 2021, 567: 150868.
[79]
Zhang YG, Du N, Jiang JW, et al. Enhanced electrochemical properties of Cu3Si-embedded three-dimensional porous Si synthesized by one-pot synthesis. J Alloys Compd 2019, 792: 341–347.
[80]
Lee SS, Nam KH, Jung H, et al. Si-based composite interconnected by multiple matrices for high-performance Li-ion battery anodes. Chem Eng J 2020, 381: 122619.
[81]
Ma JB, Zhao B, Xiang HM, et al. High-entropy spinel ferrites MFe2O4 (M = Mg, Mn, Fe, Co, Ni, Cu, Zn) with tunable electromagnetic properties and strong microwave absorption. J Adv Ceram 2022, 11: 754–768.
[82]
Guo X, Lu JB, Liu J, et al. Enhanced electromagnetic wave absorption properties of PDCs–SiCN(Ni) fibers by in situ formed CNTs and Ni2Si. Ceram Int 2022, 48: 20495–20505.
[83]
Liu MJ, Liu YH, Guo HC, et al. A facile way to enhance microwave absorption properties of rGO and Fe3O4 based composites by multi-layered structure. Compos Part A-Appl S 2021, 146: 106411.
[84]
Feng YM, Xia L, Ding CH, et al. Boosted multi-polarization from silicate-glass@rGO doped with modifier cations for superior microwave absorption. J Colloid Interface Sci 2021, 593: 96–104.
[85]
Liu XG, Ou ZQ, Geng DY, et al. Influence of a graphite shell on the thermal and electromagnetic characteristics of FeNi nanoparticles. Carbon 2010, 48: 891–897.
[86]
Li MH, Zhu WJ, Li X, et al. Ti3C2Tx/MoS2 self-rolling rod-based foam boosts interfacial polarization for electromagnetic wave absorption. Adv Sci 2022, 9: 2201118.
[87]
Zhang WD, Zhang X, Zhu Q, et al. High-efficiency and wide-bandwidth microwave absorbers based on MoS2-coated carbon fiber. J Colloid Interface Sci 2021, 586: 457–468.
[88]
Zhang WM, Dai FZ, Xiang HM, et al. Enabling highly efficient and broadband electromagnetic wave absorption by tuning impedance match in high-entropy transition metal diborides (HE TMB2). J Adv Ceram 2021, 10: 1299–1316.
[89]
Feng YR, Guo X, Gong HY, et al. Enhanced electromagnetic microwave absorption of Fe/C/SiCN composite ceramics targeting in integrated structure and function. Ceram Int 2021, 47: 3842–3852.
[90]
Wang S, Gong HY, Ashfaq MZ, et al. Introducing MWCNTs conductive network in polymer-derived SiCN ceramics for broadband electromagnetic wave absorption. Ceram Int 2022, 48: 23989–24002.
[91]
Wang S, Lin X, Ashfaq MZ, et al. Microwave absorption properties of SiCN ceramics doped with cobalt nanoparticles. J Mater Sci: Mater Electron 2020, 31: 3803–3816.
[92]
Liu J, Liu CM, Tong YC, et al. Enhanced EMW absorption properties of SiCN/Fe/Ni ceramics modified with Fe/Ni bimetal. Ceram Int 2022, 48: 30206–30217.
[93]
Feng YR, Guo X, Lu JB, et al. Enhanced electromagnetic wave absorption performance of SiCN(Fe) fibers by in situ generated Fe3Si and CNTs. Ceram Int 2021, 47: 19582–19594.
[94]
Liu XL, Tang ZM, Xue JM, et al. Enhanced microwave absorption properties of polymer-derived SiC/SiCN composite ceramics modified by TiC. J Mater Sci: Mater Electron 2021, 32: 25895–25907.
Publication history
Copyright
Acknowledgements
Rights and permissions

Publication history

Received: 02 January 2023
Revised: 03 March 2023
Accepted: 22 March 2023
Published: 08 May 2023
Issue date: July 2023

Copyright

© The Author(s) 2023.

Acknowledgements

Qingbo Wen thanks the National Natural Science Foundation of China (No. 52102085), the National Science Fund for Excellent Young Scholars (Overseas), and State Key Laboratory of Powder Metallurgy, Central South University (No. 621022222) for financial support. Xiang Xiong thanks the National Natural Science Foundation of China (No. 52072410) for financial support.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Return