New Electromagnetic Shielding Materials Based on Viscose-Carbon Nanotubes Composites.
The hybrid materials based on natural resources, that is, cellulose have gain a lot of interest, because these materials can be successfully introduced in sustainable, biodegradable, and eco-friendly products. The properties of these composites depend on the properties of the constituents, their composition ratio, as well as on the preparation method. In the recent years, several studies have focused on composites made of polysaccharides loaded with carbon nanotubes (CNT) [1,2]. Among polysaccharides, cellulose plays a privilege role due to its largest availability, renewability, and good mechanical properties. Viscose fibers, obtained from regenerated cellulose, are attractive, readily available, nontoxic, biodegradable, biocompatible, and an exceptional candidate for a wide range of applications [3, 4], In many cases, the functionalization of hydroxyl groups of cellulose chain using different chemical protocols such as esterification, etherification, or oxidation is considered, in order to increase the versatility of the cellulose to achieving desired properties. The conversion of the hydroxyl to carboxyl groups is often employed to gain new functionalities in the cellulose chain, especially by using the water-soluble stable nitroxyl radical 2,2,6,6-tetramethylpyperidine-l-oxyl (TEMPO) [5-8], but also involving the nonpersistent nitroxyl radicals, as we have recently reported [6, 9, 10]. Semiconductive viscose fibers can be prepared by layer by layer assembly, chemical plating, electroless plating, in situ polymerization, atomic layer deposition, and the dipping and drying method, and so forth. [11-14]. However, these methods use rather complicated procedures, causing severe pollution issues due to utilization of heavy metals.
CNT, discovered in 1991 , attracted extensive attention to researchers, due to their thermal, electrical, and mechanical properties, their unique structure, and narrow size distribution, in nanometer range, highly accessible surface area, low resistivity, and high stability [16, 17]. These exceptional properties of CNT have been investigated for devices such as field-emission displays, scanning probe microscopy tips, and microelectronic devices [18-21]. Viscose fibers--CNT composites combine the advantages of the high conductivity performance of CNT with relatively low-cost and easily processed organic polymers. The composites of cellulose and multiwalled CNT have a good electrical conductivity, as has been recently reported . These composites are useful in fabrication devices acting as protective tools at electromagnetic interferences. Conducting polymers composites with their unique combination of electrical, dielectric, magnetic, thermal, and mechanical properties are beneficial for suppression of electromagnetic noises (electromagnetic shielding). Electromagnetic shielding refers to the reflection and/or adsorption of electromagnetic radiation by a material, which acts as a shield against the penetration of the radiation through the shield.
When an electromagnetic wave encounters an obstacle, radiation can be reflected and/or refracted and/or absorbed (Fig. 1). E and H represents the power (P) of electromagnetic field (electric and magnetic field intensity), [sigma], [mu], [epsilon] represents constants of the shield material (electric conductivity, electric permittivity, and magnetic permeability) .
Three different mechanisms, namely reflection loss (R), absorbance loss (A), and internal reflections loss (or multiple internal reflection-MR), may contribute to overall attenuation of the electromagnetic field. Mathematically shielding effectiveness (SE) can be expressed in logarithmic scale (Eq. (1)) [24, 25]:
SE [dB] = [SE.sub.R] + [SE.sub.A] + [SE.sub.MR] = 10[1og.sub.10] ([P.sub.t]/[P.sub.i]) = 20[log.sub.10] ([E.sub.t]/[E.sub.i]) = 20[log.sub.10] ([H.sub.t]/[H.sub.i]) (1)
where [SE.sub.R] is the shielding effectiveness for reflection, [SE.sub.A] is the shielding effectiveness for absorption, [SE.sub.MR] is the shielding effectiveness for multiple reflection, [P.sub.t] [P.sub.i] is the power of wave (i = incident wave, t = transmitted wave), [E.sub.t] or i, [H.sub.t] or i is the intensity of electric, magnetic field.
If the shield is thin or the material is made by nanofibers, nanotubes, nanometric pyramidal structures, the reflected wave from the second boundary is re-reflected from the first boundary and returns to the second boundary to be reflected again and again. This means internal reflections or multiple internal reflections and the attenuation ([SE.sub.MR]) can be mathematically expressed as in Eq. (2) :
[SE.sub.MR] = 20 [log.sub.10]) 1 - [e.sup.2t]/[delta]] (2)
For dielectric material [24, 25]:
[delta] =[(2/[omega][[mu].sub.r][sigma]).sup.1/2] (3)
Where [omega] is angular frequency ([omega] = 2[pi][upsilon]), [upsilon] is electric field frequency, [mu] is the magnetic relative permeability of the shield material, [sigma] is the conductivity of shield material (measured in S/m), and t is the shield thickness.
For shields made of dielectric materials absorption loss occurs because currents induced in the medium produce ohmic losses and heating of the material so in this case [E.sub.t] and [H.sub.t] can be expressed as below :
[E.sub.t] = [E.sub.i][e.sup.-t/[delta]] (4)
[H.sub.t] = [H.sub.i][e.sup.-t/[delta]] (5)
With Eqs. (1), (4) and (5), we can write the magnitude of absorption term [SE.sub.A]:
[mathematical expression not reproducible] (6)
[sigma] = [omega][[epsilon].sub.0][epsilon]" (7)
Where [[epsilon].sub.0] is free space (vacuum) permittivity: 8,854-[10.sup.-12] F/m, [epsilon]" is absorption permittivity.
These parameters of shield material (t, [sigma], [[mu].sub.r]) directly affect the obtained value of electromagnetic shielding ([SE.sub.A] component of total SE).
The reflection loss ([SE.sub.R]) is related to the relative impedance mismatch between the shield's surface and propagating wave and is specific for conductive materials with approximately 1 [OMEGA]/cm conductivity. The magnitude of this reflection loss under plane wave can be expressed as below (Eq. 8) [24--26]:
[SE.sub.R] [dB] = -10 [log.sub.10] ([sigma]/16[omega][[epsilon].sub.0][[mu].sub.r]) (8)
A literature survey indicated a large variety of composites used for electromagnetic shielding. Thus Melvin et al. obtained a hybrid nanocomposites barium titanate/carbon nanotube with calculated reflexion loss of -29.6 dB (for a shield thickness t = 1 mm) , Abbas et al. prepared polyaniline/barium titanate/carbon-based composites with maximum loss of -25 dB (for a 2.5-mm thick sample) at 11.2 GHz and band width of 2.7 GHz , Rotaru et al. prepared in various compositions through an ultrasound/microwave-assisted procedure viscose/barium titanate composites with maximum shielding obtained for frequencies between [10.sup.4] and [10.sup.5] Hz from -18 dB to -22 dB ,
Herein, we purpose to produce new flexible and biobased composites by incorporation of multiwalled CNT in viscose fibers (V) or its [C.sub.6]-oxidized analogue (VO), as electromagnetic shielding materials, in particular for electric field within industrial frequency (50-55 Hz).
Viscose fibers (V), (Lenzing AG, Austria; linear density, 1.3 dtex; fiber length, 39 mm), multi-walled CNT (Timesnano, China) with diameters of less than 9 nm, lengths of 5 [micro]m, 95% carbon purity. The other used chemicals: 2,6,6-tetramethylpiperidinel-yl)-oxyl radical (TEMPO), sodium periodate, sodium bromide, 9% (wt) sodium hypochlorite, were purchased from SigmaAldrich and used without further purification.
Synthesis of the Oxidized Viscose
Viscose fibers (5 g) was suspended in 700 mL of distilled water under vigorous stirring, following by the addition of TEMPO (0.08 g, 0.5 mmol) and NaBr (0.5 g, 5 mmol). 9% NaCIO solution (1.89 g, 25 mmol) was added to the viscose fibers slurry under continuous stirring for 1 h at room temperature. During this time, the pH value was carefully maintained at about 10 by adding 2 m NaOH solution. The oxidation reaction was stopped by adding a 5 mL of ethanol. The oxidized viscose fibers separated by filtration were vacuum drying at 40[degrees]C for 24 h.
Preparation of the Viscose/Oxidized Viscose-Embedded CNT
The device involved in the processes of preparation was ultrasound generator Sonics Vibracell (750 W nominal electric power, 20 kHz ultrasound frequencies, display for information of the energy delivered to the end of the probe, sensor for temperature). The CNT powder (0.2519 g) was dispersed in 200 mL Milli-Q ultrapure water by sonication for 60 min. The CNT dispersion liquid and the viscose/oxidized viscose (3 g) were mixed for 60 min at room temperature. Then, the viscose/oxidized viscose was put into the coagulation bath containing 5% [Na.sub.2]S[O.sub.4] and 5% [H.sub.2]S[O.sub.4] (100 mL) for 10 min. The viscose/oxidized viscose was washed with distilled water several times, divided into two fractions, and finally one fraction is dried in air at room temperature for 24 h (V-CNT1, VO-CNT1), and the second fraction was dried in vacuum at 120[degrees]C for 20 min (V-CNT2 and VO-CNT2).
X-Ray Photoelectron Spectroscopy (XPS). The compositional analysis of the viscose (unoxidized and oxidized) samples was carried out by X-ray photoelectron spectroscopy (XPS) using a PHI-5000 VersaProbe photoelectron spectrometer ([PSI] ULVACPHI, INC.) with a hemispherical energy analyzer (0.85 eV binding energy resolution for organic materials). A monochromatic Al K[alpha] X-ray radiation (hv = 1,486.7 eV) was used as excitation source. The standard take-off angle used for analysis was 45[degrees], producing a maximum analysis depth in the range of 3-5 nm. Spectra were recorded from at least three different locations on each sample, with a 1 mm x 1 mm area of analysis. Low-resolution survey spectra were recorded in 0.5 eV steps with 117.4 eV analyzer pass energy. In addition, high-resolution carbon (1 s) spectra were recorded in 0.1 eV steps with 58.7 eV analyzer pass energy.
FT-IR Analysis. Infrared absorption spectra of original viscose, oxidized viscose, CNT, and oxidized viscose treated with CNT were recorded using a Bruker Vertex 70 spectrometer at a scan range from 4,000 to 650 [cm.sup.-1], at a resolution of 2 [cm.sup.-1] and 32 scans. Samples were measured as a KBr pellet.
Raman Spectroscopy. Raman spectra of the vibrational were recorded at a temperature of 22 [degrees]C, using a Renishaw InVia Reflex spectrometer with a radiation source formed by a GaAlAs diode laser, having wavelength 785 nm and energy 10 mW on the sample. The analyses were done at 180[degrees] backscattering geometry with respect to the incident beam.
Scanning Electron Spectroscopy (SEM)
Scanning electron spectroscopy (SEM) was performed by means of colloidal silver on copper supports. All samples were covered with a thin layer of gold by sputtering (EMITECH K 550x). The coated surface was further studied using an Environmental Scanning 200 instrument, working at 5 kV with secondary electrons in high vacuum mode.
The thermal analysis was performed using TGA-SDTA 851e Mettler Toledo system. The analyses were done by using of 3-5 mg of sample. The samples were heated in temperature range from 25 to 700[degrees]C. The heating rate was 10[degrees]C/min. Analyses were made in inert atmosphere, acquired by a continuous nitrogen flow of 20 mL/min.
Dielectrically Properties and Shielding Calculation
The dielectric constants of the samples were measured on a Concept 40 Novocontrol Dielectric Spectrometer in a frequency range of [10.sup.6] - [10.sup.0] Hz, at room temperature, with silver electrodes. The samples were prepared as pellets with the thickness of 1-1.5 mm. We used in the calculation of SE formulae (2) and (3) for multiple reflection loss ([SE.sub.MR]) and formula (6) and (7) for the magnitude of absorption term ([SE.sub.A]). We did not take into account reflection term ([SE.sub.R]), because the precursors and the composites are insulated materials and not conductive. In Eq. (3), the magnetic relative permeability is equal to one (material without magnetic properties). Graphs were processed with the Origin Pro 8 program based on data obtained from analysis of dielectric spectroscopy (dielectric constant, [epsilon]' and dielectric loss, [epsilon]").
RESULTS AND DISCUSSION
The introduction of carboxyl groups at the fibers' surface could be evidenced by using XPS technique. XPS technique allows the study of elemental composition study of a large variety of polysaccharides materials. The first XPS experiment was done to perform a low-resolution scan of the original and TEMPO-oxidized fibers in order to determine the percentages of carbon and oxygen (Fig. 2 and Table 1). It can be seen that the O/C ratio increases from 0.66 in the original sample to 0.74 in the oxidized sample, as a result of introduction of one more oxygen atom originating from carboxylic groups. Supplemental information can be acquired from a high-resolution scan, performed on the C Is region. The high-resolution C Is peak provide details about (a) types and (b) amounts of carbon-oxygen bond which are present, see Fig. 2 and Table 1. According with the experimental results, the chemical shift of carbon (C 1s) in XPS could be divided as C1, unoxidized carbon (C--C); C2, carbon with one oxygen bond (C--O); C3, carbon with two oxygen bonds (O--C--O or C=0); and C4, carbon with three oxygen bonds (O--C=0). For the four categories, we have found the peaks at 284.6, 286.4, 287.7, and 288.3 eV, respectively.
After the characterization of the oxidized fibers, the preparation of the composites has been performed. Figure 3 shows schematically the typical process for fabrication of the fibers-CNT composites. Due to the incorporation of the CNT at the fibers surface, the original color of the fibers is changed, becoming darker. The inhomogeneous appearance of the prepared samples is explainable due to the lack of uniformity of the fiber's surface in the case of unoxidized viscose and also the randomly introduction of COOH groups at the superficial level in the case of oxidized viscose fibers.
Characterization of the Composites
FTIR Analysis. The FTIR technique can be used as a straightforward method to evaluate the structural changes, which have occurred in the viscose after oxidation and embedding of CNT. In these conditions, the carboxylic group formed from TEMPO oxidation appears in the FTIR spectrum, between 1,700 and 1,750 [cm.sup.-1]. Figure 4 shows the FTIR spectra of the viscose (V), oxidized viscose (VO), CNT, and the composites V-CNT1, V-CNT2, V-O-CNT1, and VO-CNT2. In the FTIR spectrum of V/VO, the broadband vibration of OH groups are found in 3445 [cm.sup.-1], and the stretching for aliphatic C-H bonds in 2892 [cm.sup.-1]. The symmetric bending of C[H.sub.2] and C-O groups of the pyranose ring of V/VO are found, respectively, at 1441 and 1,377 [cm.sup.-1]. In the range of 1,200-1,000 [cm.sup.-1], the symmetric and asymmetric stretching of ether bond (C--O--C) are assigned. The absorption peak at 1016 [cm.sup.-1] corresponds to the C--O ether groups. After oxidation, for the sample VO can be observed, the presence of a new peak at 1735 [cm.sup.-1], absorption attributed to the C=O stretching frequency of carboxyl groups in their acidic form. The band is absent in the initial samples (V) and V-CNT composites (V-CNT1 and V-CNT2) but can be detected in an oxidized sample (VO) and VO-CNT composites (VO-CNT1 and VOCNT2). Thus, as we can see the CNT composite curves does not generate new groups, the infrared activity of the CNT being too low. Nevertheless, at 2892 and 1,643 [cm.sup.-1], the peaks of the CNT composites (V-CNT1 and VO-CNT1) is a very slow increase compared with that the V and VO. This is due to the increasing water bound in the micro-dissolution process (the sample VCNT1 and VO-CNT1 were dried in air at room temperatures for 24 h compared with the sample V-CNT2 and VO-CNT2 that were dried in vacuum at 120[degrees]C, for 20 min). Also, the change in intensity of the peak at 3445 [cm.sup.-1] may be due to the formation of intermolecular hydrogen bond between CNT and V/VO.
Raman Spectroscopy. Raman spectroscopy is a successfully, nondestructive technique, largely used in the last years to characterize carbon-based materials, including CNT. For CNT-based composites, Raman spectroscopy is a helpful analysis performed to assess the level of CNT dispersion, as well as the degree of interaction between the carbonaceous material and organic matrix, based on peaks shifting or width changes [30, 31]. The Raman spectrum of the CNT presents two main bands: one located around 1,593 [cm.sup.-1] (G band) coresponding to the C--C band in-plane vibration, and another one at 1301 [cm.sup.-1] (D band) proieminent in disordered carbon systems (see Fig. 5 and Supporting Information section). There is also preset a weak band at 2594 [cm.sup.-1] (called G' band), which is the overtone of the D band.
The Raman spectrum of viscose fibers (see Supporting Information) is composed by two main regions: 1500-900 [cm.sup.-1] and 600-300 [cm.sup.-1]. The first region is located with bands of skeletal, symmetric and asymmetric glycosidic ring (1,092, and 1,121 [cm.sup.-1]), methylene bending, rocking, and wagging (bands at 1469, 1374, 1,267, and 900 [cm.sup.-1]). In the second region, mainly CCC and COC ring deformation bands could be detected, at 457, 421, 371, 346, and 303 [cm.sup.-1] . The Raman spectra of the CNT composite samples (V-CNT2 and VOCNT2) are barely dominated by the characteristic bands of the CNT, Fig. 5 and Supporting Information, but these bands are slightly shifted to the higher wavenumbers, from 1,301, and 1,593 [cm.sup.-1] (in CNT sample) to 1,311, and ~1,600 [cm.sup.-1], respectively, (in V-CNT2, and VO-CNT2 samples), which indicates a lower intertube interaction.
Termogravimetric Analysis. The thermostability of the CNT-composites was evaluated by means of thermogravimetric experiments carried out at a heating rate of 10[degrees]C/min under inert atmosphere, acquired by a continuous nitrogen flow of 20 mL/min. The values of initial weight loss, the maximum degradation temperatures, and total weight loss at 700[degrees]C are listed in Table 2. Figure 6 shows the weight loss rate curves obtained from TGA experiments.
The initial weight loss of all samples around 100[degrees]C was between 6 and 10%, which relates to residual moisture present in all samples. For V, pyrolysis started at 281[degrees]C and for V-CNT composites pyrolysis started at 207[degrees]C and for all continued until around 330[degrees]C leading to depolymerization of viscose fibers to various anhydro-monosaccharides, dehydrated species, carbon oxide, and char. Regarding of the VO and VO-CNT composites, pyrolysis started around 210[degrees]C and continued until around 350[degrees]C.
In Fig. 5, the thermal degradation of VO show two peaks around 241[degrees]C (correspond to the degradation of anhydro-glucuronate units) and 319[degrees]C (correspond to the degradation of viscose chains containing more unstable anhydroglucuronate units in the crystal surface), the two peaks being below the degradation point of the V (331[degrees]C) thus confirming the formation of carboxylic groups at the C6 primary hydroxyls groups (the carboxylic groups leads to a decrease in the thermal degradation point).
Scanning Electron Spectroscopy
Microphotographs of the V and the VO are shown in Fig. 7. After oxidation for 1 h, the morphologically of VO was identical to those of the V, only some fine particles formed by deoxidation have been observed. The oxidized sample VO shows insignificant regions of deterioration only at the surface level. Likewise, as we can see in SEM imagines, the powder of CNT consists of agglomerated particles characterized by a quite uniform shape. For the CNT composites, in all cases, the surface became rough and brittle, and the CNT were lowly adsorbed only on the surface of the V/VO fibers without aggregations. The dark regions in the SEM images were attributed to the CNT.
Dielectric and Shielding Properties
The real part of dielectric permittivity (dielectric constant, [epsilon]') and the imaginary part (dielectric loss, [epsilon]") has been obtained from dielectric measurements performed in the frequency range of [10.sup.6]-[10.sup.0] Hz. The dielectric constant and dielectric losses of insulating materials are attributed to the polarizations of the electrons and molecules, which includes five types of polarization: electronic, vibrational (atomic), orientation (dipolar), ionic, and interfacial polarizations. As one may see from Fig. 8, for all samples, as the frequency increases, the real ([epsilon]') and imaginary ([epsilon]") permittivity decrease and remain constant at higher frequencies, indicating the occurrence of dielectric dispersion (ionic and orientation polarizabilities) [29, 33]. This may be attributed to the dipoles resulting from changes in valence states of cations and space-charge polarization [27, 34, 35]. Also the interfacial polarizabilities (common to composites with precursors who have different dielectric properties) can contribute to this decrease in low-frequency region . The chemical oxidation of the viscose increase the polarization (high value of [epsilon]' and [epsilon]" for VO comparing with pristine V). Thermal treatment (dried in vacuum procedure at 120[degrees]C) increased electronic polarization (e' for CNT2 composites, shows higher value than [epsilon]' for CNT1 composites), which can be attributed to an increased charge carriers, due to increased [pi]-clectrons in the nanocomposites. Values for industrial frequency (50-55 Hz) are given in Table 3.
In terms of shielding effectiveness (Fig. 9), V and VO have no shielding properties (SE V > 0 dB). The composites present a maximum shielding for low frequency (-100 to -75 dB at 0-10 Hz) and a minimum for high frequency (approximately -25 dB for [10.sup.5]-[10.sup.6] Hz). The thermal treatment at 120[degrees]C (for VCNT2 and VO-CNT2 composites) lead to a decrease of the shielding properties. Multiple reflection shielding is the high component of the total shielding effectiveness while absorption shielding component is almost negligible at low frequencies (approximately 0 dB for 1-[10.sup.4] Hz and-1.5-3 dB for [10.sup.4]-[10.sup.6] Hz). For industrial frequency (50-55 Hz) shielding effectiveness is -75-100 dB, provided a good potential for blocking electromagnetic interferences in electric devices. For comparison, in Table 4, we present some values for shielding effectiveness obtained for composites with cellulose fibers and/or graphite or CNT, cited in the literature for [10.sup.0] -[10.sup.8] Hz frequency or even for GHz values ([10.sup.9] Hz).
It is generally assumed that three mechanisms are involved in electromagnetic interference shielding: reflection, absorption, and multiple-reflection . When CNT are incorporated into polymer matrix, absorption plays a primordial role, followed by shielding by reflection. Some theoretical analysis revealed that multiple reflection within the CNT internal surfaces might have an obstructive impact on the overall electromagnetic interference shielding, because the diameter of the CNT is orders of magnitude smaller than the skin depth. Multiple reflection between external surfaces of CNT also diminish the overall shielding, but this influence is lower than that between the internal surfaces (according to the theoretical calculations) .
In this work, we have used both viscose and its TEMPO-oxidized analogue as organic matrices for the incorporation of CNT using ultrasonication process. The shielding values (dB) for both viscose and oxidized viscose--CNT composites, have larger values (60-110 and 70-80 dB. respectively) than those reported in the literature at 50-55 Hz. Therefore, the as prepared bio-based composites, containing multiwalled CNT and viscose fibers or [C.sub.6]-oxidized analogue, can be easily adapted to various requirements due to their flexibility, being efficient alternatives to the existing electromagnetic shielding materials, in particular for electric field within industrial frequency (50-55 Hz).
This work was supported by a grant of Ministry of Research and Innovation, CNCS--UEFISCDI, project number PN-III-P4-ID-PCE-2016-0349, within PNCDI in.
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Madalina Elena Culica, (1) Gabriela Biliuta, (1) Razvan Rotaru, (1) Gabriela Lisa, (2) Raluca loana Baron, (1) Sergiu Coseri [iD] (1)
(1) Petru Poni Institute of Macromolecular Chemistry of Romanian Academy, 41 A, Grigore Ghica Voda Alley, 700487 lasi, Romania
(2) Faculty of Chemical Engineering and Environmental Protection, Gheorghe Asachi Technical University, 73 Prof, dr. docent Dimitrie Mangeron Street, 700050, lasi, Romania
Additional Supporting Information may be found in the online version of this article.
Correspondence to: S. Coseri; e-mail: email@example.com
Caption: FIG. 1. Schematic representation of shielding mechanism (i: Incident, r: Reflected, t: Transmitted).
Caption: FIG. 2. XPS survey spectra and scan of C Is region of the original and TEMPO-oxidized fibers. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 3. Schematization of the V/VO-CNT composites fabrication, emphasizing the color changes of the fibers. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 4. The FTIR curves of CNT, V, VO and the composites V-CNT1, VCNT2, VO-CNT 1, VO-CNT2. The dashed rectangle area delineate the specific absorption band of the >C=0 moiety in carboxyl group. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 5. Raman spectra of CNT, V-CNT2, and V0-CNT2 samples. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 6. DTG curves of CNT, V, VO, VO-CNT2, VO-CNT1, V-CNT2, and V-CNT1 samples. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 7. SEM microphotographs of V, V-CNT1, V-CNT2, VO, VO-CNT1, VO-CNT2, and CNT.
Caption: FIG. 8. Dispersion ([epsilon]' = [epsilon]'(v)) and absorption spectra ([epsilon]" = [epsilon]" (v)) for V, VO, and their composites (V-CNT1, V-CNT2, VOCNT1, and VO-CNT2) with frequency in logarithmic scale (base ten). [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 9. Multiple reflection shielding (SEMR), absorption shielding (SEA), and total shielding effectiveness (SE) with frequency in logarithmic scale (base ten) for V, VO, and their composites (V-CNT1, V-CNT2, VO-CNT1, and VOCNT2). [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. XPS analysis of the original and TEMPO-oxidized fibers. Binding energy (eV) 284.6 286.4 287.7 288.3 Fiber O/C C1 (at. %) C2 (at. %) C3 (at. %) C4 (at. %) Original 0.66 24.92 52.04 20.30 2.77 Oxidized 0.74 30.66 50.12 12.33 6.91 TABLE 2. The initial weight loss, maximum degradation, and total weight loss of V, VO, and composites samples. Samples Initial weight Maximum Total weight loss around degradation loss at 100[degrees]C (%) temperature 700[degrees]C (%) ([degrees]C) V 10 331 80 V-CNT1 8 339 76 V-CNT2 9 320 70 VO 9 319 91 VO-CNT1 6 335 86 VO-CNT2 7 319 74 CNT 1 567 57 TABLE 3. Value of [epsilon]' and [epsilon]" for V, VO, and their composites, for industrial frequency. Sample V VO V-CNT1 V-CNT2 VO-CNT1 VO-CNT2 [epsilon]' 18.9 46.5 18.8 80.8 23.2 69.7 [epsilon]" 43.1 249.3 15.7 541.2 21 12.7 TABLE 4. Shielding effectiveness of various cellulose fibers--CNT composites. Composite Thickness Shielding type [mm] Viscose/barium titanate 3 Absorption Viscose/barium titanate 3 Absorption Polyaniline coated nickel - Refraction spheres/carbon black/co poly(ethylene-propylene) Polyaniline/silver, graphite, - Refraction and carbon black Barium titanate/carbon nanotube 1 Reflection Barium titanate/carbon nanotube 1.1 Reflection Cellulose/carbon nanotube 0.2 Reflection, multiple reflection Viscose/carbon nanotube 1-1.5 Multiple reflection Viscose/carbon nanotube 1-1.5 Multiple reflection Oxidized viscose/carbon nanotube 1-1.5 Multiple reflection Oxidized viscose/carbon nanotube 1-1.5 Multiple reflection Composite Shielding value [dB] Viscose/barium titanate 18-22 Viscose/barium titanate 4-9 Polyaniline coated nickel [greater than or spheres/carbon black/co equal to]20 poly(ethylene-propylene) Polyaniline/silver, graphite, [greater than or and carbon black equal to]20 Barium titanate/carbon nanotube 29.6 Barium titanate/carbon nanotube 56.5 Cellulose/carbon nanotube 20 Viscose/carbon nanotube 60-110 Viscose/carbon nanotube 25 Oxidized viscose/carbon nanotube 70-80 Oxidized viscose/carbon nanotube 20 Composite Frequency [Hz] Reference Viscose/barium titanate [10.sup.4]-[10.sup.5]  Viscose/barium titanate 50-55  Polyaniline coated nickel [10.sp.4]-[10.sup.8] [371 spheres/carbon black/co poly(ethylene-propylene) Polyaniline/silver, graphite, [10.sup.8]-[10.sup.9]  and carbon black Barium titanate/carbon nanotube 13.6 x [10.sup.9]  Barium titanate/carbon nanotube 13.2 x [10.sup.9]  Cellulose/carbon nanotube (15-40)-109 [351 Viscose/carbon nanotube 50-55 This work Viscose/carbon nanotube [10.sup.6] This work Oxidized viscose/carbon nanotube 50-55 This work Oxidized viscose/carbon nanotube [10.sup.6] This work
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|Author:||Culica, Madalina Elena; Biliuta, Gabriela; Rotaru, Razvan; Lisa, Gabriela; Baron, Raluca Ioana; Cose|
|Publication:||Polymer Engineering and Science|
|Date:||Jul 1, 2019|
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