Materials Letters 82 (2012) 188–190

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Materials Letters

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Electromagnetic performances of composites with promising carbons derived frombacterial cellulose

Bo Dai ⁎, Xiaoping Shao, Yong Ren, Gaihua Wang, Chonghua Pei, Yongjun MaState Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, School of Materials Science and Engineering, Southwest University of Science and Technology,Mianyang 621010, PR China

⁎ Corresponding author. Fax: +86 816 2419492.E-mail address: bodai31@vip.sina.com (B. Dai).

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.05.094

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 April 2012Accepted 17 May 2012Available online 29 May 2012

Keywords:Carbon materialsBacterial celluloseNanocompositesMicrostructureElectromagnetic properties

Carbon-matrix composites with interconnected carbon nanoribbon filler were fabricated using bacterial cel-lulose as the carbon precursor, and their use for electromagnetic interference shielding and microwave ab-sorption was investigated. After carbonization at 800 °C, bacterial cellulose (BC) was converted into acarbon (CBC) with a novel three-dimensional web built of entangled and interconnected cellulose ribbons.Composites of CBC impregnated with paraffin wax exhibit high complex permittivity over a frequencyrange of 2–18 GHz, depending on the CBC content. The shielding effectiveness reaches 20 dB in the measuredfrequency range for the 40 wt.% CBC/paraffin wax composite with only a 1 mm thickness. Rhombic shapeFe3O4 nanoparticles were prepared by oxidation precipitation and mixed with paraffin wax in a mass ratioof 2:1. Incorporating 2.5 wt.% CBC into the Fe3O4/paraffin wax composite decreases the reflection loss atleast 5 times. CBC dramatically improves the dielectric loss and attenuation constant through the increaseof imaginary permittivity, leading to improved microwave-absorption properties.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

With the fast growth in the application of gigahertz electronic de-vices, such as local area networks, radar systems, and personal commu-nications, electromagnetic interference (EMI) has been recognized aserious problem with electronic and digital equipment. To provide aneffective solution for the problem, considerable investigations havebeen focused on EMI shielding. Microwave absorbing materials(MAM) are also in high demand in the defense and aerospace indus-tries. To achieve significant absorption properties, the MAM musthave suitable values of complex permeability (μr=μ′− jμ″) and per-mittivity (εr=ε′− jε″). Recently, studies on shielding and absorbingmaterials using nanocarbon materials such as nanofibers, nanotubes,and nanofilaments, have become more intensive because of the struc-tural and electrical advantages of these nanoscale fillers over conven-tional carbon fillers [1–5]. Their excellent electrical properties as wellas large aspect ratios can be used to impart electrical conductivity to di-electric hosts, thereby improving electrostatic charge dissipation andelectromagnetic EMI shielding or microwave absorbing efficiency [6].Chemically reduced graphene oxide (r-GO) was also explored formicrowave absorption [7]. However, homogenous dispersion of nano-carbons into the matrix is quite difficult. The cost and limited supplyalso block the application of nanocarbons as fillers for EMI shieldingand microwave absorption. Our aim is to use natural materials as

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carbonaceous sources, which have self-assembled interconnectednanoribbon networks, to fabricate carbon-matrix composites. Hereinwe report the first example of microwave composites with bacterialcellulose nanoribbons. These composites exhibit high permittivity inthe frequency range of 2–18 GHz, and thus can be excellent high-lossmaterials, for example as an EMI material or high-performance micro-wave absorbing material. The interesting electromagnetic characteris-tics are due to the novel three-dimensional web-like networks whichestablish additional electrical conduction pathways throughout thewhole system.

2. Experimental section

Carbonized bacterial cellulose (CBC) was obtained by heat treatingbacterial cellulose (BC), which was pyrolyzed at 800 °C for 4 h undernitrogen atmosphere. 1 g BC would yield about 0.15 g CBC afterthe pyrolysis. Dried CBC fibers were then mechanically milled intopowder for measurement of electromagnetic parameters. The knownoxidation precipitation method [8] was used to prepare Fe3O4.The CBC, Fe3O4 and CBC-Fe3O4/paraffin wax samples were preparedby uniformly mixing the powders in paraffin wax matrix. A seriesof CBC/paraffin wax composites was prepared with CBC loading upto 40 wt.%. The Fe3O4-to-paraffin wax ratio was 2:1 in weight. TheCBC concentration in the CBC-Fe3O4/paraffin wax composite was2.5 wt.%. A drop of diluted supernatant containing dispersed CBCwas put onto a carbon-coated Cu grid and then examined with trans-mission electronmicroscopy (TEM, Tecnai F20). Complex permittivityand permeability measurements were performed on an Agilent

Fig. 1. TEM images of CBC. Inset is the highmagnification of the areamarked by the square.

6

8a b

ε'

ivity 1.2

1.6

2.0

μ'bilit

y

189B. Dai et al. / Materials Letters 82 (2012) 188–190

E8363B vector network analyzer in the frequency range of 2–18 GHz.Three samples were tested for each electromagnetic parameter mea-surement, and the reported result is the average.

3. Results and discussion

BC fiber is an extracellular product excreted in the form of pelli-cles. It is structured in a web-like network by self-assembly of contin-uous nanofibers about 10 nm thick and 50 nmwide. Each nanofiber isa bundle of cellulose microfibrils, each of which is about 4 nm thickand 4 nm wide. The web-like network leads BC to be homogenouslydispersed into the matrices [9], and its composites have significantlyhigh mechanical strength and extremely low thermal-expansion co-efficients [10,11]. Although BC has been used for many years in vari-ous areas, most previous work focused on its preparation andmechanical properties; its electromagnetic wave characteristicshave not been reported.

We use BC as the starting material. Upon carbonizing at tempera-tures up to 800 °C under nitrogen atmosphere, BC was converted intoa kind of carbon nanoribbons which reserved the pristine structure ofBC. TEM image shown in Fig. 1c clarifies that the carbonized bacterialcellulose networks can be described as a three-dimensional web builtof entangled and interconnected cellulose ribbons. The width and

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0

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30

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b5 wt.%

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Fig. 2. Frequency dependencies of complex permittivity (a) real and (b) imaginaryparts of CBC/paraffin wax composites with various CBC loadings.

thickness of the nanoribbons are on the order of tens of nanometersand a few nanometers, respectively. A higher magnification showseach ribbon assembly is composed of a number of extended chainsof bacterial fibrils (the inset figure). These fibrils are seen to be inclose contact with one another and to twist as a whole.

Fig. 2 shows the real and the imaginary parts of the complex per-mittivity dependencies on the CBC concentrations. Both ε′ and ε″ in-crease explicitly with increasing mass fractions of CBC. The realpermittivity of the composite with 10 wt.% CBC loading can reacharound 8.5, which is almost four times that of pure paraffin wax. Forthe composites with 20 wt.% and 40 wt.% CBC loading, this value canbe more than 22 and 30, respectively. It should be noted that theimaginary permittivity increases much more dramatically. Althoughthe ε″ value of pure paraffin wax is almost zero, that of 10 wt.% CBCloading composite is up to 2.1 and of 40 wt.% is no less than 76. Forhigh concentrations (20 wt.% and 40 wt.%), both real and imaginaryparts decrease rapidly with frequency in the lower frequency region,level off in the midfrequency range, and maintain a high value of per-mittivity thereafter. The rapid increase in the permittivity with con-centration is attributed to the addition of CBC and the subsequentonset of percolation, similar to that of carbon nanotubes (CNTs)[12]. In addition, it is noteworthy that the loss tangent (tan δe=ε″/ε′) of the composites with 10 wt.% CBC enhances nearly three ordersover pure paraffin wax. Especially, when the concentrations of CBCincrease to 20 wt.% and 40 wt.%, the magnitudes of loss tangent ap-proach 1, even exceed 1. This can be understood according to the fol-lowing. First, there are vast mobile charge carriers (electrons orholes) with great mobility in CBC to interact with electromagneticfields by oscillating in radiation, just like those in CNTs. Second, wepropose that the web-like networks in CBC also establish bridges formobile charge carriers and they can move freely along these bridges.These additional channels interact with the electromagnetic fieldover a short range, resulting in higher dielectric loss.

Whatever the EMI shielding or microwave absorption, the attenu-ation constant α, which determines the attenuation properties of ma-terials, is one of the key factors and can be calculated [13]. In our case,α has a starting value of about 620 Np/m at 2 GHz and increases al-most linearly with the frequency until up to 2000 Np/m at 18 GHz.

2 4 6 8 10 12 14 16 18 2 4 6 8 10 12 14 16 180

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mea

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0.0

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μ"

μ'

Fig. 3. Frequency dependence of complex permittivity (a) and complex permeability(b) of Fe3O4/paraffin wax composite. Frequency dependence of complex permittivity(c) and complex permeability (d) of CBC-Fe3O4/paraffin wax.

-3

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Fig. 4. Reflections loss of (a) Fe3O4/paraffin wax and (b) of CBC-Fe3O4/paraffin waxcomposite at different sample thicknesses. The t1, t2, t3, and t4 correspond to the thick-ness of 1.2 mm, 1.4 mm, 1.6 mm and 1.8 mm, respectively.

190 B. Dai et al. / Materials Letters 82 (2012) 188–190

For EMI shielding, the total shielding effectiveness SET is alwaysexpressed by SET=SEA+SER+SEI. The SEA and SER are the absorptionand reflection shielding efficiencies, and can be described asSEA=8.686αl and SER=20 lg|1+n|2/4|n| [14], respectively. Byusing the measured electromagnetic parameters, we calculated theSEA (with the thickness l assumed to be 1 mm), SER, and SEA+SER. Itwas found that the contribution to the total EMI SE is mainly fromthe reflection in the low frequency range while from the absorptionin the high range. In the whole measured frequency range, SET is larg-er than 20 dB and meet the commercial application SE demands.

Although CBC has large dielectric loss, it cannot be used as a mi-crowave absorber due to the serious imbalance of the electromagnet-ic match and, hence, the drastic reflection as discussed above. Onemay expect there is a better match between the dielectric loss andthe magnetic loss for magnetic-nanoparticle/CBC composites. There-fore, we synthesized Fe3O4 and CBC composites in the aim of micro-wave absorption. Fig. 3a and b shows the complex permittivity andcomplex permeability of the Fe3O4/paraffin wax composite. The εr′value gradually declines from 7.3 to 6.3 towards high frequencieswhile εr″ slightly increase from 0.6 to 0.9, thus, tan δe~0.1. Fig. 3cand d presents the electromagnetic parameters of the compositewhich consists of 2.5 wt.% CBC. One can clearly see a two-fold growthin εr′ and a sharp increase of at least 6 times in εr″. Therefore, tan δecan be as much as ~0.4. Fig. 4 shows the frequency dependence ofthe reflection loss (RL) at various sample thickness (t=1.2, 1.4, 1.6,and 1.8 mm), where RL was calculated according to Natio et al. [15].RL of the Fe3O4 sheet is only a few decibels even for thicknesses upto 1.8 mm. As a matter of fact, one cannot expect a value less than−20 dB (99% power absorption) until the thickness is up to7.0 mm. Considerable improvement of microwave absorption is ob-served for CBC-Fe3O4/paraffin wax composite; RL can be as little as−21.3 dB for the thickness of only 1.2 mm. The minimum RL obvious-ly shifts to a lower frequency with increasing thickness. The signifi-cant enhancement of microwave absorption is greatly related to thedielectric loss, tan δe~0.1 for Fe3O4 and tan δe~0.4 for CBC-Fe3O4

composite discussed previously. Also, this enhancement can be un-derstood since the attenuation constant α is greater than 150 Np/m

wherever the frequency is beyond 10 GHz for CBC-Fe3O4 while nomore than 60 Np/m for Fe3O4.

4. Conclusions

CBC has been found to retain the unique interconnected web-likenetwork of bacterial nanoribbons, and used to fabricate carbon-matrix composites. These composites have remarkable imaginarypermittivity and lead to huge dielectric loss. The electromagneticproperties may be further optimized by manipulating the bacterialnanoribbons through adjusting experimental parameters such as car-bonizing temperature and transition metal impregnation. Togetherwith other advantages, such as light weight, easy processability,high mechanical strength, and good dispersion in the matrices, suchCBC can be a highly competitive candidate to produce new type ofEMI shielding and microwave absorbing composites.

Acknowledgments

This work was supported by National Basic Research Program ofChina (Grant No. 2011CB612212) and Outstanding Young ResearcherFund from Sichuan Province of China (Grant No. 09ZQ026-002).

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.matlet.2012.05.094.

References

[1] Watts PCP, Hsu WK, Barnes A, Chambers B. High permittivity from defectivemultiwalled carbon nanotubes in the X-band. Adv Mater 2003;15:600–3.

[2] Tang N, Zhong W, Au C, Yang Y, Han M, Lin K, et al. Synthesis, microwave electro-magnetic and microwave absorption properties of twin carbon nanocoils. J PhysChem C 2008;112:19316–23.

[3] Liu XG, Ou ZQ, Geng DY, Han Z, Jiang JJ, Liu W, et al. Influence of a graphite shell onthe thermal and electromagnetic characteristics of FeNi nanoparticles. Carbon2010;48:891–7.

[4] Wu ZP, Li MM, Hu YY, Li YS, Wang ZX, Yin YH, et al. Electromagnetic interferenceshielding of carbon nanotube macrofilms. Scr Mater 2011:809–12.

[5] Su QM, Zhong G, Li J, Du GH, Xu BS. Fabrication of Fe/Fe3C-functionalized carbonnanotubes and their electromagnetic and microwave absorbing properties. ApplPhys A Mater 2012;106:56–7.

[6] Sen R, Zhao B, Perea D, Itkis ME, Hu H, Love J, et al. Preparation of single-walledcarbon nanotube reinforced polystyrene and polyurethane nanofibers and mem-branes by electrospinning. Nano Lett 2004;4:459–64.

[7] Wang C, Han XJ, Xu P, Zhang XL, Du YC, Hu SR, et al. The electromagnetic propertyof chemically reduced graphene oxide and its application as microwave absorbingmaterial. Appl Phys Lett 2011;98(7) 072906–3.

[8] Vereda F, Vicente J, Morales MP, Rull F, Alvarez RH. Synthesis and characterizationof single-domain monocrystalline magnetite particles by oxidative aging ofFe(OH)2. J Phys Chem C 2008;112:5843–9.

[9] Fernandes SCM, Freire CSR, Silvestre AJD, Neto CP, Gandini A, Burglund LA, et al.Transparent chitosan films reinforced with a high content of nanofibrillated cellu-lose. Carbohydr Polym 2010;81:394–401.

[10] Nishno T, Takano K, Nakamae K. Elastic modulus of the crystalline regions of cel-lulose polymorphs. J Polym Sci, Part B: Polym Phys 1995;33:1647–51.

[11] Yano H, Sugiyama J, Nakagaito AN, Nogi M, Matsuura T, Hikita M, et al. Opticallytransparent composites reinforced with networks of bacterial nanofibers. AdvMater 2005;1:153–5.

[12] Grimes CA, Mungle C, Kouzoudis D, Fang S, Eklund PC. The 500 MHz to 5.50 GHzcomplex permittivity spectra of single-wall carbon nanotube-loaded polymercomposites. Chem Phys Lett 2000;319:460–4.

[13] Lu B, Huang H, Dong XL, Zhang XF, Lei JP, Sun JP, et al. Influence of alloy compo-nents on electromagnetic characteristics of core/shell-type Fe–Ni nanoparticles.J Appl Phys 2008;104:114313–6.

[14] Joo J, Epstein AJ. Electromagnetic radiation shielding by intrinsically conductingpolymers. Appl Phys Lett 1994;65:2278–80.

[15] Natio Y, Suetake K. Application of ferrite to electromagnetic wave and its charac-teristics. IEEE Trans Microw Theory Tech 1971;19:65–73.