Improved Electromagnetic Interference Shielding Response of Poly

May 14, 2012 - Improved Electromagnetic Interference Shielding Response of Poly(aniline)-Coated Fabrics Containing Dielectric and Magnetic Nanoparticl...
24 downloads 12 Views 656KB Size
Download PDF
Article pubs.acs.org/JPCC

Improved Electromagnetic Interference Shielding Response of Poly(aniline)-Coated Fabrics Containing Dielectric and Magnetic Nanoparticles Parveen Saini,*,† Veena Choudhary,‡ N. Vijayan,§ and R. K. Kotnala¶ †

Polymeric and Soft Materials Section, National Physical Laboratory, CSIR, New Delhi 110 012, India Centre for Polymer Science and Engineering, Indian Institute of Technology, New Delhi 110 016, India § Crystal Growth Section, National Physical Laboratory, CSIR, New Delhi 110 012, India ¶ Multiferroics and Magnetic Lab, National Physical Laboratory, CSIR, New Delhi 110 012, India ‡

S Supporting Information *

ABSTRACT: Composite absorbers based on conducting fabrics possessing moderate conductivity and dielectric/magnetic properties were prepared by in situ incorporation of nanoparticles of BaTiO3 (15−25 nm) or Fe3O4 (25−40 nm) within coated poly(aniline) (PANI) matrix. The X-ray diffraction patterns and transmission electron microscopy images confirmed the formation of PANI coating and incorporation of BaTiO3 or Fe3O4 nanoparticles. Scanning electron microscopy images show formation of thick and uniform coating of PANI over individual fibers and in interweave regions. The dielectric studies show that incorporation of BaTiO3 lead to enhancement of dielectric properties of PANI whereas magnetization measurements revealed that incorporation of Fe3O4 resulted in noticeable improvement in magnetic properties with saturation magnetization of 17.9 emu/g. The Ku-band (12.4−18.0 GHz) shielding studies revealed that pure PANI-coated fabric show total shielding effectiveness (SET) of −15.3 dB which enhanced to −16.8 and −19.4 dB after incorporation of BaTiO3 and Fe3O4 nanoparticles respectively. Such an improvement can be attributed to the better matching of input impedance, reduction of skin depth, and additional dielectric/magnetic losses. The high value of absorption-dominated SET (i.e., 97−99% attenuation) and specific shielding effectiveness value of 17−20 dB cm3/g demonstrate the potential of these fabrics as promising microwave-shielding material. In addition, these fabrics also display good antistatic response with static charge decay time of only 0.11 s.

1. INTRODUCTION The proliferation of electronics and widespread instrumentation has generated electromagnetic interference (EMI) as an offshoot.1 In a true sense, EMI is a novel kind of pollution which tries to interrupt the usual functioning of electronic appliances or may also lead to complete disruption of a gadget’s performance. The consequent hazards may be loss of revenue, energy, time, or even precious human life. Therefore, some blocking mechanism must be provided to isolate the internals of an appliance from the surroundings, i.e., to limit the amount of EMI radiation from the external environment that can penetrate the circuit or conversely to regulate the electromagnetic (EM) energy generated by the circuit that can escape into the external environment.2 Therefore, considerable efforts have been made in the past to develop lightweight and broadband absorbing shielding materials.3−8 The primary mechanism of EMI shielding is usually reflection for which the shield must have mobile charge carriers (electrons or holes) that can interact with the electromagnetic fields in the radiation. Consequently, metals have been widely applied for shielding of electromagnetic (EM) radiations9−11 due to the high value of © 2012 American Chemical Society

electrical conductivity (σ). However, their use is limited due to heavy weight, high processing cost, and corrosion susceptibility.2,12 As an alternative, carbon-based materials such as carbon black/graphite, single or multiwall carbon nanotubes (SWNTs or MWNTs), graphene, carbon fibers, or mesoporous carbon have been suggested.13−17 However, micrometer-size fillers like carbon black or graphite possess poor dispersibility and high percolation thresholds.18,20 In contrast, although carbon-based nanofillers such as CNTs and graphene display extremely low percolation thresholds,20−24 they are costly, difficult to produce at bulk scale, and often need complicated purification/functionalization steps.3,19,22,23 A secondary mechanism of EMI shielding is absorption for which the shield should possess electric and/or magnetic dipoles that can interact with electric (E) and magnetic (H) vectors of incident EM radiation. The electric/magnetic dipoles may be provided by materials having high value of dielectric Received: March 5, 2012 Revised: May 11, 2012 Published: May 14, 2012 13403

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412

The Journal of Physical Chemistry C

Article

2. EXPERIMENTAL SECTION 2.1. Materials Used. Aniline (Loba Chemie, India) was freshly double distilled before use. 4-Dodecylbenzenesulfonic acid (DBSA, Merck, India), ammonium persulfate (APS, Merck, India), and isopropyl alcohol (Merck, India) were all of analytical grade and used as received. Tetragonal BaTiO3 nanoparticles were prepared by the sol−gel technique46 whereas Fe3O4 nanoparticles were procured from Sigma Aldrich, India. Millipore water (resistivity >106 Ω cm) was used for synthesis and washings. 2.2. Synthesis of BaTiO3 Nanoparticles. Tetragonal BaTiO3 nanoparticles were prepared by typical sol−gel procedure46 using suitable Ba and Ti sources. In a typical reaction, calculated amounts of Ba(OH)2·8H2O and titanium isopropoxide were separately dissolved in methanol to prepare 0.2 M solutions. These solutions were then mixed slowly under continuous stirring conditions and maintaining temperature at 80 °C. After 2 h, the temperature was raised to 100 °C and stirring was continued until the solution turned into a viscous gel. Continued heating resulted in complete evaporation of solvent leaving behind an opaque white powder of the precursor. This precursor was then heated at 1200 °C (under air) for 1 h to obtain the creamy white powder of BaTiO3. The BaTiO3 particles so obtained were subjected to further size reduction (4 h) using planetary ball mill (Retsch PM-400) and tungsten carbide jars. 2.3. Preparation of PANI-DBSA-Fe3O4/BaTiO3-Coated Fabrics. The coating of PANI-DBSA (PDB) was prepared by in situ emulsion polymerization of aniline (AN).33 In a typical reaction, 0.3 mol of dopant DBSA was homogenized with 1.0 L of water to form an emulsion. Subsequently, 0.1 mol of AN monomer was added and homogenized further to form ANDBSA micelles. The cotton fabric (15 cm × 15 cm) was placed in a glass reactor containing the above miceller solution and the polymerization was initiated by the dropwise addition of aqueous ammonium peroxydisulfate (0.1 mol) with continuous shaking of the reaction mixture throughout the course of reaction (6 h). A thick coating of PANI was formed over the surface of the fabric during polymerization marked by the appearance of dark green color. Finally, the fabric was removed from the reaction system, dipped in 2-propanol, and dried at 60 °C under vacuum. The coating process was repeated one more time and the resultant double-coated fabric was designated as CPDB. In the similar fashion, BaTiO3 and Fe3O4 nanoparticles containing coated fabrics, designated as CPBT11 and CPFF12, respectively, were also prepared by adding predetermined amounts of BaTiO3/Fe3O4 after addition of DBSA and keeping all other ingredients and reaction conditions same.

constant (e.g., ZnO, SiO2, TiO2, or BaTiO3, etc.) or magnetic permeability (e.g., carbonyl iron, Ni, Co, or Fe metals, γ-Fe2O3, Fe3O4, etc.).18,19,25−27 However, these materials or their composites possess problems like low permittivity or permeability at gigahertz frequencies, weight penalties, narrow-band action, and processing difficulties.19,25−30 Besides, they often suffer from problems like irreproducible results and inferior electrical/mechanical properties primarily due to poor dispersion and agglomeration effects associated with the abovementioned fillers.31,32 Furthermore, in poorly dispersed compositions, the leakage of incident radiation from the patches of microwave transparent host matrix (empty spaces devoid of filler phase) results in deterioration of the radiation blocking efficiency. In this regard, conducting polymers with finite conductivity and nontransparency to microwaves are found to offer an attractive solution.33−35 Particularly, the synthetic metals like poly(aniline) (PANI)-based compositions have received special attention due to tunable conductivity, adjustable permittivity or permeability, low density, noncorrosiveness, nominal cost, and good thermal and environmental stability36 resulting in a wealth of technocommercial applications.37,38 However, their commercial utility is limited by difficult processing and poor mechanical properties.36 The direct coating of conducting polymers on the surface of textiles (via in situ polymerization route) is a powerful technique to bypass the need of otherwise necessary thermal or solution processability.39 These fabrics combine the mechanical properties, stitchability, and flexibility of fabrics with the conductivity of doped π-conjugated polymers.40,41 Therefore, several attempts have been made in the recent past to design the conducting-polymer-coated fabrics as flexible and large area EM shield.39,42−44 However, the achieved shielding efficiencies remained rather low due to low conductivity, leakage of radiation from the microwave-transparent interweave spacings, and absence of any secondary shielding mechanism like strong dielectric or magnetic losses leading to absorption.42−44 Any attempt to enhance absorption by incorporation of inorganic fillers (within coated conducting polymer matrix) using conventional inorganic dopants like HCl or H2SO4 resulted in microscopic heterogeneity and inferior electromagnetic properties.45 Most importantly, the corrosive nature of the above inorganic dopants can severely affect the dielectric/magnetic properties of added inorganic fillers, rendering them useless. Herein we tried to use surfactant dopant, i.e., dodecylbenzenesulfonate (DBSA), as a soft and noncorrosive acid to achieve uniform dispersion of inorganic fillers such as BaTiO3 or Fe3O4 without affecting their intrinsic properties, so that superior shielding performance can be realized. Therefore, the present work is an attempt to design mechanically flexile microwave shields based on conductingpolymer-coated cotton fabrics which inherited dielectric or magnetic attributes from in situ incorporated BaTiO3 or Fe3O4 nanoparticles respectively. These fabrics were characterized for their structural, morphological, and magnetic attributes using specific probes such as XRD, TEM/HRTEM/SEM, and VSM. Their electrical conductivity was measured by four-probe technique, and antistatic response was measured using a static decay meter. Furthermore, the electromagnetic shielding response and electromagnetic attributes like complex permittivity and permeability of these fabrics were also measured in the 12.4−18.0 GHz frequency range (i.e., Ku-band) using a vector network analyzer.

3. CHARACTERIZATION The phase identification was performed using X-ray diffractometer (XRD, Brooker Advanced D8 system) in the diffraction (2θ) range of 10−70°. The measurements were taken at a scan rate of 0.02°/sec and slit width of 0.1 mm and using Cu Kα (λ = 1.540 598 Å) radiation source. The surface morphologies were gathered from scanning electron microscope (SEM, VP-EVO, MA-10, Carl-Zeiss, UK) operating at an accelerating potential of (10.0 kV) and attached with an energydispersive X-ray (EDX) analysis unit. The bulk morphological information was gathered from transmission electron microscope (TEM, TEM/HRTEM, Tecnai G2 F30 S-Twin) operating at an accelerating potential of (300 kV) and attached 13404

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412

The Journal of Physical Chemistry C

Article

Scheme 1. Schematic Representation of in-Situ Coating of Poly(aniline)-BaTiO3/Fe3O4 over Fabric Substrate by Polymerization of Aniline Using DBSA as Surfactant Dopant and APS as Oxidant

Figure 1. XRD patterns of (a) PANI, PANI-BaTiO3, and PANI-Fe3O4 and (b) BaTiO3/Fe3O4 nanoparticles and coated fabrics, viz., CPDB, CPBT11, and CPFF12.

above fillers. DBSA molecules consist of a polar sulfonic acid (i.e., −SO3H) group as head and a long nonpolar dodecyl (−C12H25) chain as tail, which are responsible for its surfactant characteristics. Furthermore, being a proton donor (due to −SO3H group), DBSA can also work as an effective dopant for poly(aniline).47 Therefore, as shown in Scheme 1, when polymerization of an aniline monomer takes place in the presence of fabric substrate and DBSA functionalized BaTiO3/ Fe3O4 nanoparticles, the filler particles get uniformly dispersed within the coated polymer matrix. Most importantly, DBSA, being an organic sulfonic acid, is a soft and noncorrosive material so that filler properties remain unaffected and superior shielding performance can be realized. 4.2. XRD Studies. Figure 1a shows the XRD patterns of pure poly(aniline) (PANI), PANI-BaTiO3, and PANI-Fe3O4, whereas Figure 1b displays the diffractogram of pure BaTiO3/ Fe3O4 nanoparticles and coated fabrics, viz., CPDB, CPBT11, and CPFF12. The main diffraction peaks of BaTiO3 nanoparticles are located at 2θ values of 22.2° (d = 3.99 Å), 31.6° (d = 2.83 Å), 38.9° (d = 2.31 Å), 45.2° (d = 2.01 Å), 50.9° (d = 1.79 Å), 56.3° (d = 1.63 Å), and 66.1° (d = 1.41 Å) corresponding to (100), (101), (111), (200), (201), (211), and (202) planes. These peaks match the standard XRD pattern of BaTiO348,49 (JCPDS No. 89-1428) with lattice parameters value of a = b = 3.9994 Å and c = 4.0058 Å, showing tetragonal structure. The tetragonal nature of BaTiO3 particles was also explicitly confirmed by observation of DSC transition around 120 °C (see Supporting Information, Figure I). The tetragonality (i.e., c/a ratio) was found to be 1.002 and good dielectric properties are expected. Similarly, the characteristic

with an energy-dispersive X-ray (EDX) analysis unit. The magnetic measurements of the samples were performed using the vibrating sample magnetometer (VSM) model 7304 Lakeshore Cryotronics Inc., USA, with a maximum magnetic field of 1.2 T using a Perspex holder vibrating horizontally at frequency of 76 Hz. The room temperature conductivity of the coated fabric was measured by a two-probe technique using programmable current source (Keithley-220) and a nanovoltmeter (Keithley-181). The measurement of static decay time of the conducting fabric was done using a charge decay test unit (John Chubb Instrument, UK, Model-JCI-155v5). Both shielding effectiveness (SE) and electromagnetic attributes (complex permittivity and permeability) were measured using a vector network analyzer (VNA E8263BAgilent Technologies) by keeping the samples in a holder placed between flanges of Ku-band (12.4−18.0 GHz) waveguide.

4. RESULTS AND DISCUSSION 4.1. Polymerization. The proper polymerization mechanism plays a crucial role in the development of conducting polymer composites and in the realization of uniform dispersion of nanoparticles within a polymer matrix. The relatively high density of ferrites/titanate fillers (∼5−7 g/cm3) compared to available monomers/dopants/solvents (∼1.0 g/ cm3) often results in settling tendency and poor dispersion of filler within the conducting polymer matrix. Such phase separation and microscopic heterogeneity result in poor electromagnetic properties. However, aqueous solutions of amphiphilic molecules like DBSA possess enough viscosity to counteract the agglomeration and settling tendencies of the 13405

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412

The Journal of Physical Chemistry C

Article

peaks of Fe3O4 nanoparticles are located at 2θ values of 18.5° (d = 4.80 Å), 30.3° (d = 2.95 Å), 35.6° (d = 2.52 Å), 43.3° (d = 2.09 Å), 53.8° (d = 1.70 Å), 57.3° (d = 1.61 Å), and 63.9° (d = 1.46 Å) corresponding to (111), (220), (311), (400), (422), (511), and (440) reflections of cubic Fe3O450,51 (JCPDS No. 88-0315) with lattice parameter values of a = b = c = 8.375 Å. It is observed that when DBSA-doped PANI (PDB) was coated over cotton fabric, the formed CPDB gives superimposed XRD patterns of PDB and cotton, where the peaks at 2θ values of 19.9°, 25.0°, and 26.9° were contributed by coating of PDB whereas the 16.6° and 22.8° peaks represent characteristic signatures of cellulosic structure (JCPDS No. 50-2241). In particular, the main characteristic peaks located at 2θ values of ∼20° and ∼25° represent periodicity parallel and perpendicular to the chain axis and are characteristic of the doped form of poly(aniline).33,37 Therefore, relative intensity of these two peaks (i.e., I25°/I20° ratio) can be used as a qualitative measure of achieved doping levels. When BaTiO3 (dielectric) or Fe3O4 (magnetic) nanoparticles were incorporated within the coated PANI matrix, the additional peaks corresponding to the elemental presence of BaTiO3 or Fe3O4 phases were observed (for CPBT11 and CPFF12) besides characteristic peaks of CPDB. Notably, peaks at 2θ values of 31.6° (for CPBT11) and 35.6° (for CPFF12) can be attributed to the (101) planes of tetragonal BaTiO3 and (311) planes of cubic Fe3O4, respectively, and confirm their presence in the coated PANI matrix. Further, the characteristic peaks of BaTiO3 or Fe3O4 exhibit only a slight shifting and broadening in the composites; however, no extra peaks (except that of PANI) were observed. This ruled out the formation of any chemical linkage between PANI chains and BaTiO3/Fe3O4 nanoparticles and revealed that interactions are purely physical in nature. Such interactions originate possibly due to capping of nanoparticles by PANI chains or DBSA molecules. The above results confirm that in situ polymerization in the presence of noncorrosive dopants like DBSA did not disturb the phase and preserve the electromagnetic properties of above nanofillers. Therefore, coated fabrics are expected to exhibit good dielectric or magnetic properties contributed by incorporated BaTiO3 or Fe3O4 nanoparticles. Furthermore, the careful analysis of XRD data revealed that the I25°/I20° ratio for CPBT11 and CPFF12 was slightly lower than that for CPDB, indicating the decrease in doping level upon incorporation of BaTiO3 or Fe3O4 nanoparticles. These results are in accord with the decreased electrical conductivity value; therefore, reduced reflectivity and better input impedance matching for incident electromagnetic radiations are expected, which in turn is expected to enhance shielding performance. 4.3. Morphological Investigations. Figure 2 shows the representative TEM images of pure (uncoated) and PANIcoated BaTiO3 and Fe3O4 nanoparticles. It is observed that pure nanoparticles are highly agglomerated with size in the range of 15−25 and 25−40 nm for BaTiO3 (Figure 2a) and Fe3O4 (Figure 2b) nanoparticles, respectively. The agglomeration tendency is a direct consequence of the small particle size and consequent enhancement of surface energy. At nanoscale level, the number of surface atoms, surface area, and dangling bonds/unsatisfied valences get enormously increased.52 Therefore, nanoparticles often reduce their surface energies by forming clusters/agglomerates or by adsorbing other molecules, e.g., moisture. It has been observed that incorporation of the above particles within the PANI matrix greatly reduces their agglomeration tendency due to the coating of poly(aniline).

Figure 2. TEM images of (a) BaTiO3 and (b) Fe3O4 nanoparticles and scratched coatings from (c) CPBT11 and (d) CPFF12. Insets of images (c) and (d) show SAED patterns. Images (e) and (f) show high-resolution TEM lattice fringes of particles from CPBT11 and CPFF12, respectively, whereas the corresponding insets show related interplanar spacings.

Therefore, the TEM images of scratched particles from CPBT11 (Figure 2c) and CPFF12 (Figure 2d) clearly show the presence of entrapped cube-shaped tetragonal BaTiO3 (red arrows) and distorted icosahedron-shaped cubic Fe3O4 (pink arrows) nanoparticles (dark areas), respectively, within the PANI matrix (light areas marked by green arrows). The insets of Figure 2, c and d, present selected area electron diffraction (SAED) patterns of PANI-BaTiO3 and PANI-Fe3O4 particles, respectively, that display diffused halo with dim dots. This can be attributed to the presence of amorphous coating of poly(aniline) over nanoparticles. Nevertheless, the above patterns matched with the standard SAED patterns of BaTiO3 and Fe3O4 with (101) and (311), respectively, as dominant planes. Moreover, the radial distance of dots (with inverse correlation) exactly matched with the XRD d-values of BaTiO3 (d = 2.83 Å) and Fe3O4 (d = 2.52 Å), further confirming their incorporation within PANI matrix. HR-TEM image of CPBT11 (Figure 2e) and CPFF12 (Figure 2f) shows the characteristic lattice fringes of BaTiO 3 and Fe 3 O 4 respectively, which reflect the nanocrystalline nature of these particles. The enlarged view of these lattices fringes gives the average interplanar spacing of ∼2.85 Å (inset Figure 2e) and 2.54 Å (inset Figure 2f) for BaTiO3 and Fe3O4, respectively, 13406

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412

The Journal of Physical Chemistry C

Article

which is in excellent agreement with the d-spacing found by XRD technique. Furthermore, the absence of lattice fringes in the poly(aniline) containing regions (bright regions) reconfirms the amorphous nature of poly(aniline) coating. Figure 3 shows the SEM images of coated fabrics (i.e., CPDB, CPBT11, and CPFF12) and PANI-BaTiO3/PANI-

Information, Figure IV). These embedded nanoparticles provide additional dielectric or magnetic properties to the PANI coating which is desirable for introducing secondary shielding mechanism, i.e., absorption within the PANI-based shield. 4.4. Magnetic Properties. Figure 4 shows the room temperature magnetization plots of pure Fe3O4 nanoparticles

Figure 4. VSM plots of Fe3O4 nanoparticles (upper inset), pristine PANI (lower inset), and scratched particles from CPBT11 and CPFF12, showing saturation magnetization (Ms), coercivity (Hc), and retentivity (Mr) values.

and pristine PANI as well as scratched particles from CPBT11 and CPFF12. The results revealed pure Fe3O4 failed to saturate even at 0.5 T magnetic field and exhibit narrow hysteresis loop (upper inset) with saturation magnetization (Ms) value of 75.4 emu/g (at 5.0 kG). Furthermore, extremely small retentivity (Mr ∼ 2.3 emu/g) and coercivity (Hc ∼ 17.2 G) of Fe3O4 indicate the fast relaxing and nearly superparamagnetic (SPM) nature of these particles. According to Stoner−Wohlfarth theory, magnetic anisotropy (EA) energy for single-domain magnetic nanoparticles can be expressed as53

Figure 3. SEM pictures of CPDB at (a) low (50.0×) and (b) high (500.0×) magnification, and of (c) CPBT11, (d) CPFF12, (e) PANIFe3O4, and (f) PANI-BaTiO3.

Fe3O4 particles from coating. The low-magnification SEM image (Figure 3a) CPDB revealed that thick coating of poly(aniline) (PDB) forms conducting domains that were extended over several fiber including interfiber regions. The high-resolution image (Figure 3b) also revealed that PDB produces thick and uniform coating over individual fibers along with formation of thick conducting film covering the interweave spacings. This reduces the number of microwave transparent empty spaces which can be helpful in achieving the blocking of incident electromagnetic radiation in a more efficient manner. The SEM images of CPBT11 (Figure 3c) and CPFF12 (Figure 3d) revealed the presence of incorporated BaTiO3 and Fe3O4 nanoparticles, respectively, appearing only as bright dots due to their nano dimensions. Similarly, SEM images of coated PANIFe3O4 (Figure 3e) or PANI-BaTiO3 (Figure 3f) particles show Fe3O4/BaTiO3 nanoparticles (bright regions) dispersed within PANI matrix. However, the actual presence of these nanoparticles was explicitly confirmed by recording the energydispersive X-ray spectroscopic (EDS) patterns of coated particles (see Supporting Information, Figures II, III, and IV) which gives the characteristic peaks of BaTiO3 (Ba, Ti, and O, Supporting Information, Figure II) or Fe3O4 (Fe and O, Supporting Information, Figure III) along with superimposed peaks due to the elemental presence of C and N of PANI (matrix) and C, O, and S of DBSA (dopant) (Supporting

EA = KV sin 2 θ

(1) 3

where K is the magnetic anisotropy constant, V (4/3πr ) is the volume of the nanoparticles of radius (r), and θ is the angle between magnetization direction and the easy axis of the nanoparticle. For a particle with uniaxial anisotropy EA = KV and the condition for SPM becomes KV = 25kBT; K is the magnetocrystalline anisotropy.54 The anisotropy represents the energy barrier to prevent the change of the magnetization direction. When the size of nanoparticles is reduced to a threshold value, EA becomes less than the thermal activation energy and the magnetization direction of nanoparticles can be easily moved away from the easy axis. Therefore, the registered SPM character of Fe3O4 particles arises due to their small size, i.e., approaching toward the single domain limit55 so that room temperature thermal energy (kBT ≈ 25 meV) becomes sufficient to rotate the entire domain, resulting in randomization. As the PANI itself possesses weak ferromagnetic behavior (lower inset) with Ms value of 0.015 emu/g, incorporation of diamagnetic BaTiO3 nanoparticles (Ms ∼ −0.0013 emu/g at 5.0 kG, see Supporting Information, Figure 13407

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412

The Journal of Physical Chemistry C

Article

charging voltages of ±5.0 kV. The results show that accepted charge decays in an exponential manner marked by a sharp initial decay followed by a plateau region. The blank cotton (Figure 6a) gives a peak at 150.09 V which indicates that cotton accepted only ∼3.0% of the applied voltage (5.0 kV). Further, the accepted surface voltage decays to 10% cutoff limit in 3.54 s which was higher than the limiting time58 of 2.0 s. However, on coating cotton with conducting PANI (PDB), charge acceptability and retention capability drastically decreases due to rapid charge dissipation. Therefore, when a positive potential of 5.0 kV was applied to CPDB (Figure 6b), it accepted only 41.4 V. Moreover, the received voltage was rapidly dissipated and reached the 10% cutoff limit within 0.11 s, demonstrating that the sample clearly passes the antistatic criteria of 2.0 s. Similarly, other fabrics, i.e., CPBT11 and CPFF12, also display rapid charge drainage with decay time of 0.13 s (Figure 6c) and 0.16 s (Figure 6d) respectively. The good antistatic response of these textiles arises due to coated fiber that forms conducting domains having high aspect ratio accruing from the long lengths of individual fibers. The aforementioned acceptable conductivity as well as inherent magnetic properties suggests that these coated fabrics are also expected to exhibit good microwave attenuation efficiency for which the recommended conductivity range is 10−3−0.1 S/cm.58 4.6. Microwave Shielding Response. The EMI shielding is a direct consequence of reflection, absorption, and multiple internal reflection losses. Therefore, as shown in Figure 7, some part of the incident wave gets reflected, other gets absorbed, and the rest comes out as transmitted wave. The total shielding effectiveness (SET) that includes contributions due to reflection and absorption can be expressed as3,8,38

V) within the PANI matrix resulted in poor magnetic property (e.g., Ms ∼ 2.23 × 10 −5 emu/g for the PANI-BaTiO 3 composite). However, the incorporation of Fe3O4 resulted in noticeable enhancement in magnetic properties (Ms ∼17.9 emu/g for PANI-Fe3O4) as compared to pristine PANI. It is important to note that coating of PANI over Fe3O4 caused enhancement of both retentivity (Mr ∼ 2.3 emu/g) and coercivity (Hc ∼ 17.2 G) compared to pure Fe3O4. Such enhancement is marked by appearance of hysteresis loop which reflects transition from superparamgnetic to ferromagnetic behavior. The increased interaction between PANI and Fe3O4 nanoparticles is responsible for reduction of particle−particle exchange interactions allowing easy alignment of PANI-coated nanoparticles with the applied magnetic field resulting in ferromagnetic character. Therefore, PANI matrix allows each nanoparticle to behave independently. These results are of particular interest as good magnetic properties are expected to improve the microwave attenuation efficiency. In order to enhance the microwave absorption, the initial permeability (μi) of the absorber should be as high as possible. The μi of ferromagnetic materials can be mathematically expressed as56 ⎞ ⎛ Ms 2 ⎟ μi = ⎜ ⎝ akHcMs + bλξ ⎠

(2)

where a and b are two constants determined by the material composition, λ is the magnetostriction constant, ξ is the elastic strain parameter of the crystal, and k is a proportionality coefficient. The above equation shows that permeability can be enhanced either by enhancing Ms or by reducing Hc. In the present system, the incorporation of Fe3O4 within the PANI (Ms ∼0.015 emu/g, Hc ∼121 G) matrix resulted in enhancement of Ms (∼17.9 emu/g) with simultaneous reduction of Hc (∼73 G). This is favorable for the improvement of μi value (according to eq 2) which in turn is expected to enhance the microwave absorption capability. 4.5. Electrical Conductivity and Antistatic Performance. The room temperature electrical conductivity of Fe3O4/ BaTiO3 nanoparticles, their composites with PANI (i.e., PANIFe3O4 or PANI-BaTiO3), and coated fabrics (i.e., CPDB, CPBT11, and CPFF12) is shown in Figure 5. It can be seen that coated fabrics (with conductivity in the range 1.7 × 10−3 to 9.4 × 10−3 S/cm) conform to the conductivity range (10−9− 10−5 S/cm) specified in the Electronics Industries Association (EIA) standard for ensuring efficient dissipation of electrostatic charges.57 To probe further, the static charge decay profiles (Figure 6) of the above fabrics were also recorded for corona

SE T(dB) = 10 log10(PT/PI) = 20 log10(E T /E I) = log10(HT/20HI)

(3)

where PI (EI or HI) and PT (ET or HT) are the power (electric or magnetic field intensity) of incident and transmitted EM waves, respectively. The scattering parameters S11 (S22) and S12 (S21) of VNA are related to reflectance (R) and transmittance (T) respectively, i.e., T = |ET/EI|2 = |S12|2 (=|S12|2), R = |ER/EI|2 = |S11|2 (=|S22|2). Once, the values of R and T are known, absorbance (A) can be calculated as A = (1 − R − T). Further, the relative intensity of the EM wave inside the shield (after primary reflection from the face of incidence) is based on the quantity (1 − R), and effective absorbance (Aeff) can be described as Aeff = (1 − R − T)/(1 − R). Therefore, attenuations due to reflection (SER) and absorption (SEA) can be conveniently expressed as SE R = 10 log10(1 − R )

(4)

SEA = 10 log(1 − Aeff ) = 10 log10[T /(1 − R )]

(5)

According to classical EM wave theory, far-field (kr ≫ 1) losses for good conductor8,38 (σT/ωεo > 0) and electrically thick samples (t > δ), angular frequency (ω) dependence of reflection and absorption losses can be expressed in the terms of total conductivity (σT) real permeability (μ′), skin depth (δ), and thickness (t) of the shield material as

Figure 5. Electrical conductivity of coated fabrics, BaTiO3/Fe3O4 nanoparticles, and their PANI-based nanocomposites. 13408

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412

The Journal of Physical Chemistry C

Article

Figure 6. Static decay profiles of (a) pure cotton fabric and (b) CPDB, (c) CPBT11, and (d) CPFF12 showing accepted surface voltage, decay profile, and charge decay time.

⎛ σ ωμ′ ⎞1/2 ⎟ SEA (dB) = −8.68t ⎜ T ⎝ 2 ⎠

The above equations revealed that both conductivity (closely related with dielectric properties, i.e., loss factor or imaginary permittivity) and magnetic properties are important; therefore, use of PANI-BaTiO3 or PANI-Fe3O4 based fabrics are expected to display superior shielding performance compared to CPDB. Figure 8 shows the frequency dependence of shielding effectiveness and electromagnetic attributes of the coated fabrics, i.e., CPDB, CPBT11, and CPFF12. The results (Figure 8a) revealed that for CPDB the losses due to reflection (SER) and absorption (SEA) were lying in the range of −4.4 to −3.7 dB and −10.9 to −10.3 dB, respectively. However, inclusion of dielectric (BaTiO3) or magnetic (Fe3O4) nanoparticles leads to enhancement of absorptive attenuation with concomitant reduction of reflection loss. Therefore, the SER values of CPBT11 and CPFF12 were found to be in the range −3.7 to −1.8 dB and −3.1 to −2.8 dB, respectively. In contrast, the corresponding SEA values were −13.1 to −13.2 dB (for CPBT11) and −15.8 to −16.6 dB (and CPFF12). These figures represent maximum total shielding effectiveness (SET = SER + SEA), value of −15.3 dB (CPDB), −16.8 dB (CPBT11) and −19.4 dB (CPFF12) corresponding to attenuation levels of more than 97%, 97.9%, and 98.9%, respectively. The decrease of SER values with loading of dielectric/magnetic filler can be attributed to the better matching of input impedance, whereas the SEA enhancement can be attributed to the reduction of skin depth/moderate microwave conductivity (see Supporting Information, Figure VI) as well as additional dielectric/ magnetic losses. Furthermore, the specific EMI shielding efficiency (i.e., attenuation per unit density) is a more appropriate scale for comparing the shielding performance of

Figure 7. Schematic representation of electromagnetic interference shielding mechanism displaying interaction of the shield with incident plane-wave radiation.

⎛ σT ⎞ SE R (dB) = −10 log10⎜ ⎟ ⎝ 16ωε0μ′ ⎠

(6)

⎛t ⎞ t SEA (dB) = −20 log10 e = − 8.68⎜ ⎟ ⎝δ⎠ δ

(7)

σT = (σac + σdc) = ωε0ε″

(8)

(9)

where σ ac and σdc are frequency-dependent (ac) and -independent (dc) components of σT whereas ε″ is imaginary permittivity, k is wavenumber, and r is the distance between radiation source and detector. The skin depth (δ) is defined as the depth of penetration at which the incident EM radiation is reduced to 33% of its original strength and can be expressed in terms of real permeability (μ′), σT, and ω as δ = [2/(σTωμ′)]1/2 which gives absorption loss as 13409

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412

The Journal of Physical Chemistry C

Article

Figure 8. Frequency dependence of (a) losses due to reflection (SER) and absorption (SEA), (b) complex permittivity (ε*), (c) complex permeability (μ*), and (d) total loss (tan δe + tan δm) for CPDB, CPBT11, and CPFF12.

BaTiO3 can be attributed to its strong dielectric properties compared to Fe3O4. The complex permeability (μ*) spectra (Figure 8c) of the samples were also recorded to investigate magnetic properties and associated losses. When the frequency of the applied field increases, magnetocrystalline anisotropy comes into picture62 and the induced magnetization (B) vector starts to lag behind the applied field (H) resulting in magnetic losses.63 Anisotropy effects become much stronger at the nanoscale level, resulting in large difference between B and H vectors leading to enhanced magnetic losses.64 The real permeability values of CPBT11 and CPFF12 were found to be higher than that of CPDB. This was due to the improvement of the magnetic properties along with parallel reduction of eddy current losses. Similarly, the sample CPFF12 shows the higher magnetic losses than either of CPDB or CPBT11 which in turn leads to enhanced absorption of the electromagnetic microwaves. Moreover, the introduced magnetic properties also lead to better matching of input impedance along with reduction of skin depth, further contributing toward improvement of absorption loss. In order to make a better comparison of microwave absorption ability of these materials, the loss (imaginary) parts of the permittivity or permeability were normalized with corresponding polarization (real) parts so that dielectric or magnetic loss tangent values (i.e., tan δe = ε′/ε′ and tan δm = μ″/μ′) can be deduced.3,8 The consolidated loss tangent values, i.e., tan δe + tan δm (Figure 8d), revealed that the total losses were highest for CPFF12, thus accounting for highest absorption loss for CPFF12-based shield.

various materials especially for high-tech areas like space and defense where weight of shield in an important design parameter. Thus, the specific EMI shielding efficiency of our coated fabrics was calculated to be 17−20 dB cm3/g which is much higher than that of typical metals (e.g., ∼10 dB cm3/g for solid copper59). The amenability of these fabrics toward chemical coating process for producing large area and mechanically flexible shields is considered as an added advantage from the technocommercial and industrial viewpoint. In order to establish correlation between observed shielding response and electromagnetic attributes, complex permittivity (ε*) and permeability (μ*) values of samples were also calculated from experimental scattering parameters using the Nicolson−Ross−Weir algorithm.60,61 The complex permittivity (ε*) spectra of the samples (Figure 8b) show that both real (ε′) and imaginary (ε″) permittivity values exhibit decreasing trend with frequency. This can be attributed to the decreasing ability of the dipoles (present in the system) to maintain the in-phase movement with rapidly pulsating electric vector of the incident EM wave. The results also revealed that the dielectric constant (ε′) as well as dielectric loss (ε″) values of the CPDB exhibit noticeable enhancement after addition of dielectric or magnetic nanoparticles. This can be attributed to the enhancement of the polarization due to high permittivity values of the embedded particles. In conjugated polymers such as poly(aniline), strong polarization occurs due to the presence of localized defects known as polaron/bipolaron.3,37 Further, increased interfacial polarization due to significant electrical conductivity difference between PANI (∼2.1 S/cm) and above nanofiller (i.e., BaTiO3 ∼ 10−7 S/cm and Fe3O4 ∼ 10−5 S/cm) also contribute toward polarization. Similarly, the associated relaxation effects lead to enhancement of ε″ values. The higher values in the case of 13410

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412

The Journal of Physical Chemistry C

Article

(12) Oh, K. W.; Kim, D. J.; Kim, S. H. Polymer (Korea) 2001, 25, 302. (13) Kim, H. M.; Kim, K.; Lee, C. Y.; Joo, J.; Cho, S. J.; Yoon, H. S.; Pejaković, D. A.; Yoo, J. W.; Epstein, A. J. Appl. Phys. Lett. 2004, 84, 589. (14) Saini, P.; Choudhary, V.; Dhawan, S. K. Polym. Adv. Technol. 2009, 20, 355. (15) Kim, S. H.; Jang, S. H.; Byun, S. W.; Lee, J. Y.; Joo, J. S.; Jeong, S. H.; Park, M.-J. J. Appl. Polym. Sci. 2003, 87, 1969. (16) Zhou, J.; He, J.; Li, G.; Wang, T.; Sun, D.; Ding, X.; Zhao, J.; Wu, S. J. Phys. Chem. C 2010, 114, 7611. (17) Yang, Y.; Gupta, M. C.; Dudley, K. L.; Lawrence, R. W. Adv. Mater. 2005, 17, 1999. (18) Olmedo, L.; Hourquebie, P.; Jousse, F. Handbook of Organic Conductive Molecules and Polymers; John Wiley & Sons Ltd.: Chichester, UK, 1997; Vol. 2. (19) Eswai, A. M. K.; Farag, M. M. Mater. Des. 2007, 28, 2394. (20) Singh, B. P.; Prabha; Saini, P.; Gupta, T.; Garg, P.; Kumar, G.; Pandey, I.; Pandey, S.; Seth, R. K.; Dhawan, S. K.; Mathur, R. B. J. Nanopart. Res. 2011, 13, 7065. (21) Pandey, S.; Singh, B. P.; Mathur, R. B.; Dhami, T. L.; Saini, P.; Dhawan, S. K. Nanoscale Res. Lett. 2009, 4, 327. (22) Moniruzzaman, M; Winey, K. I. Macromolecules 2006, 39, 5194. (23) Liang, J.; Wang, Y.; Huang, Y.; Ma, Y.; Liu, Z.; Cai, J.; Zhang, C.; Gao, H.; Chen, Y. Carbon 2009, 47, 922. (24) Varrla, E.; Venkataraman, S.; Sundara, R. Macromol. Mater. Eng. 2011, 296, 894. (25) Cho, H. S.; Kim, S. S. IEEE Trans. Magn. 1999, 35, 3151. (26) Haijun, Z.; Zhichao, L.; Chenliang, M.; Xi, Y.; Liangying, Z. Mater. Sci. Eng., B 2002, 96, 289. (27) Singh, P.; Babbar, V. K.; Razdan, A.; Srivastava, S. L.; Goel, T. C. Mater. Sci. Eng., B 2000, 78, 70. (28) Abbas, S. M.; Chattterjee, R.; Dixit, A. K.; Kumar, A. V. R.; Goel, T. C. J. Appl. Phys. 2007, 101, 074105. (29) Singh, P.; Babbar, V. K.; Razdan, A.; Srivastava, S. L.; Goel, T. C. Mater. Sci. Eng., B 1999, 67, 132. (30) Shin, J. Y.; Oh, J. H. IEEE Trans. Magn. 1993, 29, 3437. (31) Saini, D. R.; Shenoy, A. V.; Nadkarni, V. M. J. Appl. Polym. Sci. 1984, 29, 4123. (32) Dang, Z.-M.; Fan, L.-Z.; Shen, Y.; Nan, C.-W. Chem. Phys. Lett. 2003, 369, 95. (33) Saini, P.; Choudhary, V.; Sood, K. N.; Dhawan, S. K. J. Appl. Polym. Sci. 2009, 113, 3146. (34) Joo, J.; Epstein, A. J. Appl. Phys. Lett. 1994, 65, 2278. (35) Saini, P.; Choudhary, V.; Dhawan, S. K. Polym. Adv. Technol. 2010, 21, 1. (36) Trivedi, D. C. Handbook of Organic Conductive Molecules and Polymers; John Wiley & Sons Ltd.: Chichester, UK, 1997; Vol. 2. (37) Saini, P.; Jalan, R.; Dhawan, S. K. J. Appl. Polym. Sci. 2008, 108, 1437. (38) Colaneri, N. F.; Shacklette, L. W. IEEE Trans. Instrum. Meas. 1992, 41, 291. (39) Saini, P.; Choudhary, V.; Dhawan, S. K. J. Appl. Polym. Sci. 2012, 23, 343. (40) Seshadri, D. T.; Bhat, N. V. Sen’i Gakkaishi 2005, 61, 103. (41) dall’Acqua, L.; Tonin, C.; Peila, R.; Ferrero, F.; Catellani, M. Synth. Met. 2004, 146, 213. (42) MacDiarmid, A. G.; Jones, W. E., Jr.; Norris, I. D.; Gao, J.; Johnson, A. T., Jr.; Pinto, N. J.; Hone, J.; Han, B.; Ko, F. K.; Okuzaki, H.; Llaguno, M. Synth. Met. 2001, 119, 27. (43) Dhawan, S. K.; Singh, N.; Venkatachalam, S. Synth. Met. 2002, 129, 261. (44) Hakansson, E.; Amiet, A.; Kaynak, A. Synth. Met. 2007, 157, 1054. (45) Aksit, A. C.; Onar, N.; Ebeoglugi, M. F.; Birlik, I.; Celik, E.; Ozdemir, I. J. Appl. Polym. Sci. 2009, 113, 358. (46) Gomi, K.; Tanaka, K.; Kamiya, H. J. Ceram. Soc. Jpn. 2003, 111, 67. (47) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992, 48, 91.

5. CONCLUSIONS This work provides a promising methodology to fabricate flexible and lightweight microwave shields based on poly(aniline)-coated fabric with incorporated dielectric/magnetic nanoparticles. These coated fabrics inherited dielectric or magnetic properties from the in situ incorporated BaTiO3 or Fe3O4 nanoparticles. The incorporation of BaTiO3 or Fe3O4 resulted in enhancement of total shielding (SET) to −16.8 or −19.4 dB, respectively, compared to PANI-coated fabric (SET ∼ −15.3 dB). The high value of specific attenuation of these fabrics, i.e., 17−20 dB cm3/g which is much higher than that of copper (∼10 dB cm3/g) support their potential as futuristic microwave absorbers. The enhanced absorption response can be attributed to the better matching of input impedance, reduction of skin depth as well as additional dielectric or magnetic losses. In addition, these fabrics also displayed good antistatic response with static charge decay time of 0.11 s, which clearly passes the antistatic criteria of 2.0 s.

■

ASSOCIATED CONTENT

S Supporting Information *

DSC scanning of BaTiO3 nanoparticles; EDS patterns of PANIBaTiO3, PANI-Fe3O4, and PANI particles; VSM plot of BaTiO3 nanoparticles; and skin depth (δ) and microwave conductivity (σT) values of particles from different conducting fabrics. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 91-011-45609505. Fax: 91-011-45609310. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the Director, NPL, for his keen interest in the work. Authors are also thankful to Mr. K. N Sood for SEM images and Dr. V. N. Singh and Dr. Renu Pasricha for HRTEM analysis.

■

REFERENCES

(1) Yang, Y.; Gupta, M. C.; Dudley, K. L.; Lawrence, R. W. Nanoletters 2005, 5, 2131. (2) Ott, H. W. Noise Reduction Techniques in Electronic Systems, 2nd ed.; John Wiley & Sons: New York, 1988. (3) Saini, P.; Choudhary, V.; Singh, B. P.; Mathur, R. B.; Dhawan, S. K. Mater. Chem. Phys. 2009, 113, 919. (4) Che, R. C.; Peng, L. M.; Duan, X. F.; Chen, Q.; Liang, X. L. Adv. Mater. 2004, 16, 401. (5) Thomassin, J. M.; Lou, X. D.; Pagnoulle, C.; Saib, A.; Bednarz, L.; Huynen, I.; Jérôme, R.; Detrembleur, C. J. Phys. Chem. C 2007, 111, 11186. (6) An, Z. G.; Pan, S. L.; Zhang, J. J. J. Phys. Chem. C 2009, 113, 2715. (7) Xu, P.; Han, X. J.; Jiang, J. J.; Wang, X. H.; Li, X. D.; Wen, A. H. J. Phys. Chem. C 2007, 111, 12603. (8) Saini, P.; Choudhary, V.; Singh, B. P.; Mathur, R. B.; Dhawan, S. K. Synth. Met. 2011, 161, 1522. (9) Joo, J.; Song, H. G.; Jang, K. S.; Oh, E. J. Synth. Met. 1999, 102, 1349. (10) Shui, X.; Chung, D. D. L. J. Electron. Mater. 1995, 24, 107. (11) Li, L.; Chung, D. D. L. Composites 1994, 25, 215. 13411

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412

The Journal of Physical Chemistry C

Article

(48) Habib, A.; Stelzer, N.; Angerer, P.; Haubner, R. Bull. Mater. Sci. 2011, 34, 19. (49) Vinothini, V.; Singh, P.; Balasubramanian, M. Ceram. Int. 2006, 32, 99. (50) Sun, J.; Zhou, S.; Hou, P.; Yang, Y.; Weng, J.; Li, X.; Li, M. J. Biomed. Mater. Res. Part A 2007, 80, 333. (51) Ma, M.; Zhang, Y.; Yu, W.; Shen, H.; Zhang, H.; Gu, N. Colloids Surf. A: Physicochem. Eng. Aspects 2003, 212, 219. (52) Kittle, C. Introduction to Solid State Physics, 8th ed.; John Wiley & Sons Inc.: New York, 2005. (53) Stoner, E. C.; Wohlfarth, E. P. Philos. Trans. R. Soc., London A 1948, 240, 599. (54) Neel, L. Rev. Mod. Phys. 1953, 25, 293. (55) Qiao, R.; Yang, C.; Gao, M. J. Mater. Chem. 2009, 19, 6274. (56) Tian, M. Magnetic Materials; Tsinghua University Press: Beijing, 2001. (57) Electronics Industries Association (EIA), Standard 541, Section 2. (58) Ono, H.; Ohsawa, A.; Tabata, Y. J. Electrostat. 2003, 57, 355. (59) Shui, X.; Chung, D. D. L. J. Electron. Mater. 1997, 26, 928. (60) Nicolson, A. M.; Ross, G. F. IEEE Trans. Instrum. Meas. 1970, 19, 377. (61) Weir, W. B. Proc. IEEE 1974, 62, 33. (62) Dimitrov, D. A.; Wysin, G. M. Phys. Rev. B 1995, 51, 11947. (63) Shilov, V. P.; Bacri, J. C.; Gazeau, F.; Gendron, F.; Perzynski, R.; Raikher, Y. L. J. Appl. Phys. 1999, 85, 6642. (64) Dar, M. A.; Kotnala, R. K.; Verma, V.; Shah, J.; Siddiqui, W. A.; Alam, M. J. Phys. Chem. C 2012, 116, 5277.

13412

dx.doi.org/10.1021/jp302131w | J. Phys. Chem. C 2012, 116, 13403−13412