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novolac phenolic resin composite for microwave absorber applications
Composites: Part B 42 (2011) 1291–1297
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Synthesis and microwave characterization of expanded graphite/novolac phenolic resin composite for microwave absorber applications Jyoti Prasad Gogoi a, Nidhi S. Bhattacharyya a,⇑, K.C. James Raju b a b
Microwave Engineering Laboratory, Department of Physics, Tezpur University, Assam 784 028, India School of Physics, University of Hyderabad, Hyderabad 500 046, Andhra Pradesh, India
a r t i c l e
i n f o
Article history: Received 3 November 2010 Received in revised form 13 January 2011 Accepted 23 January 2011 Available online 1 February 2011 Keywords: A. Particle-reinforcement B. Electrical properties B. Thermal properties E. Compression molding Microwave characterization
a b s t r a c t Chemical oxidation and thermal treatment method is used to synthesize expanded graphite from natural graphite flakes of size 2 lm. Novolac phenolic resin (NPR) is used as base matrix and mechanically mixed with expanded graphite (EG) in different weight ratios (30 wt.%, 40 wt.% & 50 wt.% of EG). The mixture is molded at 100–150 °C using compression molding technique to pellets of EG/NPR composite for electrical and microwave characterization. Surface morphology and structural characterization are investigated using scanning electron microscope and X-ray diffraction respectively. 50 wt.% EG composite shows a maximum value of electrical conductivity 147 S/cm and thermal stability 350 °C. The composite is highly dissipating with (e00r /e0r > 1) over the entire X band. Reflection loss >13 dB is observed throughout out the frequency range. The composite has a potential for broadband microwave absorber application. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The compatibility of electronic devices with various electromagnetic environments has become an important issue in recent years with increasing use of gigahertz electromagnetic waves in wireless communications, satellite communications, radar systems, and military applications. This in turn increases the demands on electromagnetic interference shielding and electromagnetic wave absorbing materials in the GHz range. Radar absorbing materials (RAM) have been widely used to prevent or minimize electromagnetic reflections from large structures such as aircraft, ships and tanks and to cover the walls of anechoic chambers [1–5]. In general, RAMs are fabricated in the form of sheets that consist of insulating polymer, like rubber, and magnetic or dielectric loss materials such as ferrite, permalloy, carbon black, and short carbon fiber [6,7]. An electromagnetic wave absorption characteristic of material depends on its dielectric properties (complex permittivity, er = e0r je00r ), magnetic properties (complex permeability, lr = l0r jl00r ), thickness and frequency range. Dielectric composite absorption at microwave frequencies depends on the ohmic loss of energy, generally achieved by adding conductive fillers like carbon black, graphite or metal particles. On the other hand, magnetic composite absorption depends on magnetic hysteresis effect of magnetic materials, like ferrite, incorporated into the matrix ⇑ Corresponding author. Tel.: +91 3712 267008/5555; fax: +91 3712 267005/6. E-mail address: [email protected] (N.S. Bhattacharyya). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.01.026
[8–10]. Of the two techniques, the magnetic composite absorber has two main shortcomings; firstly, density of the magnetic materials is too high to use them in large quantity as filler of absorbers. Secondly, the resonance frequency range showing effective characteristics exist in the MHz range and hence the efficiency of absorbers decreases rapidly in the GHz and beyond this range. Thus, the technical requirement for the absorber limits the number of ferromagnetic materials that can be used in the microwave range [11]. Additional issues that have to be met for free space applications of RAM are broadband absorption, light weight, good mechanical and thermal stability [12–14]. The inclusions properties influence the absorption properties of the composites. Polymer nanocomposites with carbon nano tubes as reinforcers have got widest attention in RAM application, due to their good mechanical, electrical, and thermal properties [15,16]. However, cost effectiveness and complexity of processing are the major issues for their commercial applications. Less cost effective alternative is using graphite flakes in composites as referred in [17], where the flake thickness was of the order of few microns and good absorption is observed above 12 GHz. Another promising composite reinforcement can be expanded graphite flakes, with the characteristics of very low density, good electrical, thermal and mechanical properties [18–20,21]. Though the cost of making expanded graphite is to some extent higher as compared to graphite flakes inclusions, the density is almost 200 times less than natural graphite flakes [22]. Another aspect to be considered while fabricating microwave absorbing material is the influence of
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base matrix. Ref. [23] reports, that the use of phenolic resin matrix in carbon black composite instead of epoxy resin matrix enhances the electrical properties of the composite. In the present investigation, novolac phenolic resin based composite with EG as inclusions is developed with a view to develop high dielectric loss, light weight, cost effective broadband microwave absorber. The developed composite are characterized for electrical conductivity, thermal behavior and complex permittivity in X band. Reflection loss of the composite is additionally addressed, using transmission line (TL) technique for conductor backed single layer structure. 2. Experimental 2.1. Preparation of expanded graphite Expanded graphite is synthesized by method of chemical oxidation and thermal treatment method. Natural graphite flakes of size less than 2 lm (from Mass Graphite and Carbon products, Mehsana, India) are dried at 75 °C in vacuum oven for 8 h to remove the moisture content and then mixed with saturated acid consisting of sulfuric acid and concentrated nitric acid in a volume ratio 3:1 for 12 h to form graphite intercalated compound (GIC). Nitric acid serves as an oxidizer and sulfuric acid as an intercalant. The mixture is stirred from time to time to obtain uniform intercalation of each flake. The chemically treated flakes are then thoroughly rinsed with water until the pH level of the solution reaches seven and dried at 60 °C, in vacuum oven, for 5 h. GIC are then suddenly exposed to high temperature in a muffle furnace maintained at temperature 800–900 °C to form EG. 2.2. Preparation of composites Novolac type phenolic resin with 10% hexamethylene tetramine as harderner (supplied by Pheno Organic Limited, New Dehli) is used as matrix for the composite in the present investigation. The hydroxyl and methylene linkages present in NPR chemical structure, facilitates bonding for composite formation [33]. NPR has good heat resistance, electrical insulation, dimensional stability, flame and chemical resistance and is also cost effective [25,26]. The EG flakes and NPR powder are mechanically blended to obtain uniform mixture with weight ratios of EG to NPR varying as 30:70, 40:60 and 50:50. The mixture is placed in a specially designed three-piece die-mould and initially heated up to 95–100 °C. A pressure up to 1.5–2 tons is slowly applied and the fixture with the sample is isothermally heated at 150 °C for 2 h and then allowed to cool at room temperature. Pellets of dimensions, 10.38 mm 22.94 mm 3.7 mm are obtained for X band microwave and other characterization.
q ¼ ðV=IÞ2pS where q is the resistivity, V is the applied voltage, I is the measured current through the sample, and S is the distance between probes. Through-plane electrical conductivity is measured by two probe method using Keithley 2400C – source meter. A silver paste is applied on two opposite faces of the rectangular pellets to assure proper ohmic contact. The resistivity is calculated using the measured value of resistance and physical dimension (area and thickness) of the samples. The reciprocal of resistivity gives the conductivity of the samples. Thermal Gravimetric Analysis (TGA) of the materials is performed on Thermal Analyzer, (Model STA 6000, Perkin Elmer). Microwave characterization: Transmission/reflection method, using Agilent WR-90 X11644A rectangular waveguide line compatible with Agilent 8722ES vector network analyzer, is used to measure scattering parameter of the composites. Complex permittivity is computed from S11 and S21values using Nicolson–Ross method [27]. 3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 shows the XRD patterns of natural graphite (NG) and expanded graphite (EG). NG exhibits peaks at 2h = 26.56° and 2h = 54.85° corresponds to (0 0 2) and (0 0 4) diffraction respectively, whereas EG exhibits peaks at 2h = 26.52° and 2h = 54.85° corresponding to (0 0 2) and (0 0 4) diffraction, respectively. It is seen that diffraction peak of expanded graphite is broad and shifted to lower diffraction angle 26.52°. Graphite is made up of layered planes of hexagonal arrays or networks of carbon atoms lying over the midpoint of the carbon hexagon. The separation of layer is 3.361 Å (c-axis) and this distance is too large for the
2.3. Analysis and characterization techniques Microstructural characterization: X-ray diffraction patterns of the prepared materials are obtained using Cu Ka radiation from Rigaku, Miniflex 200 X-ray diffractometer at room temperature. Surface morphology and growth structure of the materials are studied by scanning electron microscope (SEM), model JEOL JSM-6390LV. Electrical and thermal characterization: In-plane and throughplane dc electrical conductivity of the EG/NPR composite is measured as shown in the Fig. 3c. In-plane electrical conductivity of the composites is measured using four-probe technique at room temperature using constant current source (model CCS-01) and digital micro voltmeter (model DMV-001). The conductivity is calculated with following equation: Fig. 1. XRD of natural graphite (NG) and expanded graphite (EG).
J.P. Gogoi et al. / Composites: Part B 42 (2011) 1291–1297
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Fig. 2. SEM micrograph of (a) NG, (b) and (c) EG and (d) fractured EG/NPR composite.
formation of strong chemical bond. Weak van der Waals’ forces exist between the layers of graphite. On treating the graphite flakes with concentrated sulfuric acid and concentrated nitric acid, in a volume ratio 3:1 and sudden exposure of the graphite intercalated compound to high temperature causes chemical oxidation and leads to expansion of the graphite basal planes and the expansion is almost 300 times [21]. The XRD pattern shows that the interlayer spacing for expanded graphite is 3.395 Å.The value of intensity (in a.u) along y-axis in the XRD plot shows a very large variation (70,000 for natural graphite and 2500 for expanded graphite) showing that for same volume sample the graphite crystallite is less in expanded graphite. 3.2. SEM analyses Fig. 2a shows the SEM micrograph of natural graphite (NG). Fig. 2b is of expanded graphite flake with 200 magnification, vermicular or worm shaped with loose and porous structure that is due to opening of planar carbon networks wedged at the edge surface of crystallite by surface groups. Fig. 2c shows EG at higher magnification (4300), where it can be seen that the structure of EG basically consists of numerous graphite sheets of nanometers thickness and micrometer diameter. This structure endows EG with high surface area. Fig. 2d shows the uniform distribution of EG inside the bulk of novolac phenolic resin composite. 3.3. DC conductivity study Fig. 3a shows the variation of in-plane electrical conductivity with increasing expanded graphite percentage in the composite. The conductivity increases with values for 30 wt.%, 40 wt.% and
50 wt.% EG in composite and are around 10 S/cm, 100 S/cm and 147 S/cm respectively. Fig. 3b shows through-plane electrical conductivity variation with increasing EG content in the composites. The through-plane conductivity value for 30 wt.%, 40 wt.% and 50 wt.% are 0.10 S/cm, 0.125 S/cm and 0.325 S/cm respectively. EG/NPR composite consists of insulating phenolic resin phase (1011 S/cm) and conducting EG flakes (104 S/cm [18]). The percentage of insulating phase is higher than conducting EG flakes in 30 wt.% EG composite and hinders the electrical conduction. With increasing EG content, the conducting phases increases which facilitate the electrical conduction. In this investigation, EG is synthesized from NG using 3:1 volume ratio of concentrated sulfuric acid and nitric acid, this assists in increasing the surface area [28], removing contaminants from EG surface, reducing the pores and unbonded interfaces between EG and phenolic resin, thus establishing a conductive network of EG in the composite system and hence enhanced conductivity. 3.4. Thermal Gravimetric Analysis (TGA) Thermal stability of NPR, EG and 50 wt.% EG/ NPR composite measured by TGA in air atmosphere is shown in Fig. 4. TGA curve of NPR shows that there is small weight loss up to 150 °C. The major weight loss occurs due to evolution of volatiles in between the temperature 350–450 °C. Overall there is 75–80% weight loss up to 800 °C in air atmosphere. From the figure it is seen that TGA curve of EG shows thermal stability up to 600 °C. From 600 °C to 800 °C moderate weight loss and above 800 °C continuous weight loss up to 850 °C is observed, this is due to surface complexes formed during oxidation. The total weight loss up to 850 °C is 10–15%. The 50 wt.% EG/NPR composite shows there is very small weight
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Fig. 4. Thermo gravimetric analysis (TGA) curves of EG, NPR and 50 wt.% EG/NPR composite.
Fig. 3. Measured dc conductivity of EG/NPR composites (a) in-plane conductivity, (b) through-plane conductivity.
loss up to 300 °C. From 300 to 400 °C, a moderate weight loss and above 400 °C continuous weight loss is observed up to 850 °C. Overall, there is 45–50% weight loss up to 800 °C in air atmosphere. It can be seen from the curves that, addition of EG flakes in the matrix enhances the thermal stability in comparison to pure matrix. Thus, TGA curve shows that the developed composite is thermally stable up to 350 °C. 3.5. Complex permittivity and dielectric loss studies Fig. 5a and b show the frequency dependence of the real part (e0r ) and imaginary part (e00r ) of relative complex permittivity of
Fig. 3c. Schematic diagram of plane of dc conductivity measurement.
EG/NPR composites in the frequency range 8–12 GHz. Composite with 50 wt.% of EG loading shows decline in the values of e0r and e00r from 5 to 3.5 and from 6.2 to 3.9, respectively, over the frequency range 8–12 GHz. The other two composites of 30 wt.% and 40 wt.% EG loading also show similar trends. Although e0r values do not show much variation, a noticeable decrease in e00r values with decrease in EG content is observed. Fig. 5b shows a high resolution of imaginary permittivity curves up to 9.5 GHz with varying EG content. The EG/NPR composites contains NPR as insulating section and EG as conducting section, with EG encapsulated within the polymer. The insulating sections acts like a high resistive path for flow of free electrons, however some electrons may still conduct by tunneling effect [29]. Within the composite system, there could be two paths of electromagnetic propagation as shown in Fig. 5c. In one path, line of electric flux can pass from EG flakes-polymer-EG flakes and in the other path through direct contact between the EG flakes. This is in analogy to reference [30] where Matsumoto and Miyata have discussed a model for metal–polymer composite, with filler conductivity of 104 S/cm. The second path of conduction, however, will be more effective for higher filler concentration which increases the interaction between the fillers. As EG conductivity is quite high as compared to polymer, increasing EG content increases the effective conductivity which in turn increases the loss factor e00r . In the same reference, the modeling done for higher concentration metal particles embedded in polymer matrix showed a decreasing trend for real and imaginary part of permittivity with frequency, as is observed in EG/ NPR composite system. The use of NPR as matrix material shows an increase in imaginary permittivity values over that with epoxy resin matrix referred in [24], because of increase in interaction between EG and NPR [23]. Fig. 6(a) shows the dielectric loss tangent of the composites. A high resolution curve is shown in the graph Fig. 6(b) which shows that increasing EG wt.% in the composite increases (e00r /e0r ) factor. A high value (e00r /e0r ) >1 has been observed for the three compositions in the entire X band. The composition with 50 wt.% EG has a dielectric loss of 1.21 in the lower frequency range and slightly declined to 1.20 in the upper frequency range. 40 wt.% EG and 30 wt.% EG composite shows dielectric loss of about 1.19 and 1.17 respectively, over the entire X band. Dielectric loss greater than unity suggest that the material has high dissipation factor rather than storage capacity. Thus, the EG/NPR composite has the potential characteristics for broadband microwave absorption.
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Fig. 6a. Dielectric loss tangent of EG/NPR composites.
Fig. 5. Complex permittivity of EG/NPR composite at X band, (a) real permittivity and (b) imaginary permittivity.
Fig. 6b. Dielectric loss tangent of EG/NPR composites in higher resolution.
3.6. Reflection loss study Reflection loss is determined from the measured values of e0r and e00r of the composites, by transmission line theory for conductor backed single-layered structure [31]. The reflection loss (RL) of the incident electromagnetic wave normal to the planar single-layered structure is expressed as
Z in Z 0 RL ¼ 20 log Z in þ Z 0 rffiffiffiffiffi Z in ¼ Z 0
ð1Þ
lr 2pfd pffiffiffiffiffiffiffiffiffi er lr tanh j c er
ð2Þ
The composites are nonmagnetic, hence the values of
l0r ¼ 1 andl00r ¼ 0. Fig. 7 shows the reflection loss curves of EG/
Fig. 5c. Schematic diagram of electromagnetic propagation within the composite system.
NPR composites of different filler to polymer weight ratios of thickness, d = 3.7 mm. It is observed that the reflection loss enhances from 13 dB at lower frequencies to 16.5 dB at higher frequencies in the X band. In fact, the reflection loss of EG/NPR composites determined so far, is found to be quite high in comparison to unexpanded graphite flakes stated in reference [17] in the frequency ranges 8–12 GHz. Although dielectric loss tangent is higher for higher EG content, the reflection loss is less, as summarized in Table 1. This could possibly be, due to impedance mismatch at the air-absorber interface [32]. The condition for good microwave
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cost effective. Absorption can be further enhanced by structural modifications like graded structure or modifying radar absorbing structure.
Acknowledgement The authors would like to sincerely thank Mr. Rama Obelesu, University of Hyderabad for extending help in talking microwave measurements.
References
Fig. 7. Reflection loss curve of EG/NPR composites with variation of EG concentration.
Table 1 Reported microwave properties of EG/NPR composites of thickness 3.7 mm at 10 GHz. EG/NPR composites (wt.%)
e0r
e00r
e00r /e0r
Reflection loss(dB)
30 40 50
4.02 4.02 4.03
4.67 4.74 4.86
1.13 1.14 1.17
14.93 14.81 14.67
absorption is that (1) an incident wave should completely enter into the absorbing layer and (2) subsequently get absorbs due to lossy nature of the material. For condition (1) to be satisfied, there should impedance matching at the air-absorber interface as seen by the incident electromagnetic wave. The impedance of free space, Z0, is 377 ohm. Input impedance of the absorber is elucidated by equation
Z in ¼ Z 0
sffiffiffiffi 1
2pfd pffiffiffiffi tanh j er c er
ð3Þ
As seen from the above equation, with increase in e00r , er increases and consequently impedance mismatch occurs. High impedance mismatch leads to low reflection loss as described by Eq. (1). However, by using graded layered structure and optimizing thickness of each layer the impedance matching condition can be obtained. 4. Conclusions The EG flakes synthesized by chemical oxidation and thermal treatment method is mixed with NPR in different EG wt.% and complex permittivity are investigated in the frequency ranges 8–12 GHz. The e0r permittivity of the composite shows no significant variation with EG wt.% loading in the composite, e00r also increases with increase in EG wt.% in the composite. The dc electrical conductivity values show an increase with increase in filler loading. The composite shows thermal stability up to 350 °C and can be used for high temperature applications. High performance microwave absorption with potential for applications in broadband frequency can be achieved using the EG/NPR composites which have high dielectric loss behavior over the entire X-band. Further, the developed composites are light weight and relatively
[1] Pinhoa MS, Gregoria ML, Nunes RCR, Soares BG. Aging effect on the reflectivity measurements of polychloroprene matrices containing carbon black and carbonyl- iron powder. J Polym Degrad Stab 2001;73:1–5. [2] Knott EF, Shaeffer JF, Tuley MT. Radar cross section. New York: Artech House; 1993. p. 9. [3] Carvalho A, Gregori ML, Chambers B. Determination of microwave intrinsic properties of hexaferrite samples. Micro Opt Tech Lett 1997;16:393–7. [4] Smith FC, Chambers B, Bennett JC. Methodology for accurate free-space characterization of radar absorbing materials. IEE Proc Sci Meas Tech 1994;141:538–46. [5] Petrov VM, Gagulin VV. Microwave absorbing materials. J Inorg Mater 2001;37:93–8. [6] Elimat ZM. AC electrical conductivity of poly (methyl methacrylate)/carbon black composite. J Phys D 2006;39:2824–8. [7] Chung DDL. Electromagnetic interference shielding effectiveness of carbon materials. Carbon 2001;39:279–85. [8] Du JH, Sun C, Bai S, Su G, Ying Z, Cheng HM. Microwave electromagnetic characteristics of a microcoiled carbon fibers/paraffin wax composite in Ku band. J Mater Res Soc 2002;17:1232–6. [9] Luo X, Chung DDL. Electromagnetic interference shielding using continuos carbon-fiber carbon–matrix and polymer–matrix composite. Compos: Part B 1999;30:227–31. [10] Das NC, Khastgir D, Chaki TK, Chakraborty A. Electromagnetic interference shielding effectiveness of carbon black and carbon fibre filled EVA and NR Based composite. Compos: Part A 2000;31:1069–81. [11] Wallace JL. Broadband magnetic microwave absorbers: fundamental limitations. IEEE Trans Magn 1993;29:4209–14. [12] Qi X, Yang Y, Zhong W, Deng Y, Au C, Du Y. Large-scale synthesis, characterization and microwave absorption properties of carbon nanotubes of different helicities. J Solid State Chem 2009;182:2691–7. [13] Michielssen E, Sajer JM, Ranjithan S, Mittra R. Design of lightweight, broadband microwave absorbers using genetic algorithms. IEEE Trans Microwave Theory Tech 1993;41:1024–30. [14] Yusoff AN, Abdullah MH, Ahmad SH, Jusoh SF, Mansor AA, Hamid SAA. Electromagnetic and absorption properties of some microwave absorbers. J Appl Phys 2002;92:876–82. [15] Grimes CA, Mungle C, Kouzoudis D, Fang S, Eklund PC. The 500 MHz–5.50 GHz complex permittivity spectra of single-wall carbon nanotube-loaded polymer composites. Chem Phys Lett 2000;319:460–4. [16] Grimes CA, Dickey EC, Mungle C, Ong KG, Qian D. Effect of purification of the electrical conductivity and complex permittivity of multiwall carbon nanotubes. J Appl Phys 2001;90:4134–7. [17] Fan Y, Yang H, Li M, Zou G. Evaluation of the microwave absorption property of flake graphite. J Mater Chem Phys 2009;115:696–8. [18] Celzard A, Marêché J F, Furdin G, Puricelli S. Electrical conductivity of anisotropic expanded graphite-based monoliths. J Phys D: Appl Phys 2000;33:3094–101. [19] Chen Guo-Hua, Wu Da-Jun, Wen-Gui Weng, Bin He, Wen-li Yan. Preparation of polystyrene–graphite conducting nanocomposites via intercalation polymerization. J Polym Int 2001;50:980–5. [20] Chen Guo-hua, Cuiling Wu, Wengui Weng, Dajun Wu, Wenli Yan. Preparation of polystyrene/graphite nanosheet composite. J Polym Commun 2003;44:1781–4. [21] Debelak B, Lafdi K. Use of expanded graphite filler to enhance polymer physical properties. Carbon 2007;45:1727–34. [22] Yasmin A, Jyi-Jiin Luo, Daniel IM. Processing of expanded graphite reinforced polymer nanocomposites. Compos Sci Tech 2006;66:1179–86. [23] Kakati BK, Deka D. Effect of resin matrix precursor on the properties of graphite composite bipolar plate for PEM fuel cell. Energy Fuel 2007;21:1681–7. [24] Lee Sang-Eui, Choi Oyoung, Hahn TH. Microwave properties of graphite nanoplatelet/epoxy composites. J Appl Phys 2008;104:033705-1–5-7. [25] Bishop GR, Sheard PA. Fire-resistant composites for structural sections. Compos Struct 1992;21:85–9. [26] Knop A, Pilato LA. Phenolic resins/chemistry, applications and performance. Future directions. Berlin: Springer; 1985. [27] Nicolson AM, Ross GF. Measurement of the intrinsic properties of materials by time-domain techniques. IEEE Trans Instrum Meas 1970;IM-19: 377–82.
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Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Synthesis and microwave characterization of expanded graphite/novolac phenolic resin composite for microwave absorber applications Jyoti Prasad Gogoi a, Nidhi S. Bhattacharyya a,⇑, K.C. James Raju b a b
Microwave Engineering Laboratory, Department of Physics, Tezpur University, Assam 784 028, India School of Physics, University of Hyderabad, Hyderabad 500 046, Andhra Pradesh, India
a r t i c l e
i n f o
Article history: Received 3 November 2010 Received in revised form 13 January 2011 Accepted 23 January 2011 Available online 1 February 2011 Keywords: A. Particle-reinforcement B. Electrical properties B. Thermal properties E. Compression molding Microwave characterization
a b s t r a c t Chemical oxidation and thermal treatment method is used to synthesize expanded graphite from natural graphite flakes of size 2 lm. Novolac phenolic resin (NPR) is used as base matrix and mechanically mixed with expanded graphite (EG) in different weight ratios (30 wt.%, 40 wt.% & 50 wt.% of EG). The mixture is molded at 100–150 °C using compression molding technique to pellets of EG/NPR composite for electrical and microwave characterization. Surface morphology and structural characterization are investigated using scanning electron microscope and X-ray diffraction respectively. 50 wt.% EG composite shows a maximum value of electrical conductivity 147 S/cm and thermal stability 350 °C. The composite is highly dissipating with (e00r /e0r > 1) over the entire X band. Reflection loss >13 dB is observed throughout out the frequency range. The composite has a potential for broadband microwave absorber application. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The compatibility of electronic devices with various electromagnetic environments has become an important issue in recent years with increasing use of gigahertz electromagnetic waves in wireless communications, satellite communications, radar systems, and military applications. This in turn increases the demands on electromagnetic interference shielding and electromagnetic wave absorbing materials in the GHz range. Radar absorbing materials (RAM) have been widely used to prevent or minimize electromagnetic reflections from large structures such as aircraft, ships and tanks and to cover the walls of anechoic chambers [1–5]. In general, RAMs are fabricated in the form of sheets that consist of insulating polymer, like rubber, and magnetic or dielectric loss materials such as ferrite, permalloy, carbon black, and short carbon fiber [6,7]. An electromagnetic wave absorption characteristic of material depends on its dielectric properties (complex permittivity, er = e0r je00r ), magnetic properties (complex permeability, lr = l0r jl00r ), thickness and frequency range. Dielectric composite absorption at microwave frequencies depends on the ohmic loss of energy, generally achieved by adding conductive fillers like carbon black, graphite or metal particles. On the other hand, magnetic composite absorption depends on magnetic hysteresis effect of magnetic materials, like ferrite, incorporated into the matrix ⇑ Corresponding author. Tel.: +91 3712 267008/5555; fax: +91 3712 267005/6. E-mail address: [email protected] (N.S. Bhattacharyya). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.01.026
[8–10]. Of the two techniques, the magnetic composite absorber has two main shortcomings; firstly, density of the magnetic materials is too high to use them in large quantity as filler of absorbers. Secondly, the resonance frequency range showing effective characteristics exist in the MHz range and hence the efficiency of absorbers decreases rapidly in the GHz and beyond this range. Thus, the technical requirement for the absorber limits the number of ferromagnetic materials that can be used in the microwave range [11]. Additional issues that have to be met for free space applications of RAM are broadband absorption, light weight, good mechanical and thermal stability [12–14]. The inclusions properties influence the absorption properties of the composites. Polymer nanocomposites with carbon nano tubes as reinforcers have got widest attention in RAM application, due to their good mechanical, electrical, and thermal properties [15,16]. However, cost effectiveness and complexity of processing are the major issues for their commercial applications. Less cost effective alternative is using graphite flakes in composites as referred in [17], where the flake thickness was of the order of few microns and good absorption is observed above 12 GHz. Another promising composite reinforcement can be expanded graphite flakes, with the characteristics of very low density, good electrical, thermal and mechanical properties [18–20,21]. Though the cost of making expanded graphite is to some extent higher as compared to graphite flakes inclusions, the density is almost 200 times less than natural graphite flakes [22]. Another aspect to be considered while fabricating microwave absorbing material is the influence of
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base matrix. Ref. [23] reports, that the use of phenolic resin matrix in carbon black composite instead of epoxy resin matrix enhances the electrical properties of the composite. In the present investigation, novolac phenolic resin based composite with EG as inclusions is developed with a view to develop high dielectric loss, light weight, cost effective broadband microwave absorber. The developed composite are characterized for electrical conductivity, thermal behavior and complex permittivity in X band. Reflection loss of the composite is additionally addressed, using transmission line (TL) technique for conductor backed single layer structure. 2. Experimental 2.1. Preparation of expanded graphite Expanded graphite is synthesized by method of chemical oxidation and thermal treatment method. Natural graphite flakes of size less than 2 lm (from Mass Graphite and Carbon products, Mehsana, India) are dried at 75 °C in vacuum oven for 8 h to remove the moisture content and then mixed with saturated acid consisting of sulfuric acid and concentrated nitric acid in a volume ratio 3:1 for 12 h to form graphite intercalated compound (GIC). Nitric acid serves as an oxidizer and sulfuric acid as an intercalant. The mixture is stirred from time to time to obtain uniform intercalation of each flake. The chemically treated flakes are then thoroughly rinsed with water until the pH level of the solution reaches seven and dried at 60 °C, in vacuum oven, for 5 h. GIC are then suddenly exposed to high temperature in a muffle furnace maintained at temperature 800–900 °C to form EG. 2.2. Preparation of composites Novolac type phenolic resin with 10% hexamethylene tetramine as harderner (supplied by Pheno Organic Limited, New Dehli) is used as matrix for the composite in the present investigation. The hydroxyl and methylene linkages present in NPR chemical structure, facilitates bonding for composite formation [33]. NPR has good heat resistance, electrical insulation, dimensional stability, flame and chemical resistance and is also cost effective [25,26]. The EG flakes and NPR powder are mechanically blended to obtain uniform mixture with weight ratios of EG to NPR varying as 30:70, 40:60 and 50:50. The mixture is placed in a specially designed three-piece die-mould and initially heated up to 95–100 °C. A pressure up to 1.5–2 tons is slowly applied and the fixture with the sample is isothermally heated at 150 °C for 2 h and then allowed to cool at room temperature. Pellets of dimensions, 10.38 mm 22.94 mm 3.7 mm are obtained for X band microwave and other characterization.
q ¼ ðV=IÞ2pS where q is the resistivity, V is the applied voltage, I is the measured current through the sample, and S is the distance between probes. Through-plane electrical conductivity is measured by two probe method using Keithley 2400C – source meter. A silver paste is applied on two opposite faces of the rectangular pellets to assure proper ohmic contact. The resistivity is calculated using the measured value of resistance and physical dimension (area and thickness) of the samples. The reciprocal of resistivity gives the conductivity of the samples. Thermal Gravimetric Analysis (TGA) of the materials is performed on Thermal Analyzer, (Model STA 6000, Perkin Elmer). Microwave characterization: Transmission/reflection method, using Agilent WR-90 X11644A rectangular waveguide line compatible with Agilent 8722ES vector network analyzer, is used to measure scattering parameter of the composites. Complex permittivity is computed from S11 and S21values using Nicolson–Ross method [27]. 3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 shows the XRD patterns of natural graphite (NG) and expanded graphite (EG). NG exhibits peaks at 2h = 26.56° and 2h = 54.85° corresponds to (0 0 2) and (0 0 4) diffraction respectively, whereas EG exhibits peaks at 2h = 26.52° and 2h = 54.85° corresponding to (0 0 2) and (0 0 4) diffraction, respectively. It is seen that diffraction peak of expanded graphite is broad and shifted to lower diffraction angle 26.52°. Graphite is made up of layered planes of hexagonal arrays or networks of carbon atoms lying over the midpoint of the carbon hexagon. The separation of layer is 3.361 Å (c-axis) and this distance is too large for the
2.3. Analysis and characterization techniques Microstructural characterization: X-ray diffraction patterns of the prepared materials are obtained using Cu Ka radiation from Rigaku, Miniflex 200 X-ray diffractometer at room temperature. Surface morphology and growth structure of the materials are studied by scanning electron microscope (SEM), model JEOL JSM-6390LV. Electrical and thermal characterization: In-plane and throughplane dc electrical conductivity of the EG/NPR composite is measured as shown in the Fig. 3c. In-plane electrical conductivity of the composites is measured using four-probe technique at room temperature using constant current source (model CCS-01) and digital micro voltmeter (model DMV-001). The conductivity is calculated with following equation: Fig. 1. XRD of natural graphite (NG) and expanded graphite (EG).
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Fig. 2. SEM micrograph of (a) NG, (b) and (c) EG and (d) fractured EG/NPR composite.
formation of strong chemical bond. Weak van der Waals’ forces exist between the layers of graphite. On treating the graphite flakes with concentrated sulfuric acid and concentrated nitric acid, in a volume ratio 3:1 and sudden exposure of the graphite intercalated compound to high temperature causes chemical oxidation and leads to expansion of the graphite basal planes and the expansion is almost 300 times [21]. The XRD pattern shows that the interlayer spacing for expanded graphite is 3.395 Å.The value of intensity (in a.u) along y-axis in the XRD plot shows a very large variation (70,000 for natural graphite and 2500 for expanded graphite) showing that for same volume sample the graphite crystallite is less in expanded graphite. 3.2. SEM analyses Fig. 2a shows the SEM micrograph of natural graphite (NG). Fig. 2b is of expanded graphite flake with 200 magnification, vermicular or worm shaped with loose and porous structure that is due to opening of planar carbon networks wedged at the edge surface of crystallite by surface groups. Fig. 2c shows EG at higher magnification (4300), where it can be seen that the structure of EG basically consists of numerous graphite sheets of nanometers thickness and micrometer diameter. This structure endows EG with high surface area. Fig. 2d shows the uniform distribution of EG inside the bulk of novolac phenolic resin composite. 3.3. DC conductivity study Fig. 3a shows the variation of in-plane electrical conductivity with increasing expanded graphite percentage in the composite. The conductivity increases with values for 30 wt.%, 40 wt.% and
50 wt.% EG in composite and are around 10 S/cm, 100 S/cm and 147 S/cm respectively. Fig. 3b shows through-plane electrical conductivity variation with increasing EG content in the composites. The through-plane conductivity value for 30 wt.%, 40 wt.% and 50 wt.% are 0.10 S/cm, 0.125 S/cm and 0.325 S/cm respectively. EG/NPR composite consists of insulating phenolic resin phase (1011 S/cm) and conducting EG flakes (104 S/cm [18]). The percentage of insulating phase is higher than conducting EG flakes in 30 wt.% EG composite and hinders the electrical conduction. With increasing EG content, the conducting phases increases which facilitate the electrical conduction. In this investigation, EG is synthesized from NG using 3:1 volume ratio of concentrated sulfuric acid and nitric acid, this assists in increasing the surface area [28], removing contaminants from EG surface, reducing the pores and unbonded interfaces between EG and phenolic resin, thus establishing a conductive network of EG in the composite system and hence enhanced conductivity. 3.4. Thermal Gravimetric Analysis (TGA) Thermal stability of NPR, EG and 50 wt.% EG/ NPR composite measured by TGA in air atmosphere is shown in Fig. 4. TGA curve of NPR shows that there is small weight loss up to 150 °C. The major weight loss occurs due to evolution of volatiles in between the temperature 350–450 °C. Overall there is 75–80% weight loss up to 800 °C in air atmosphere. From the figure it is seen that TGA curve of EG shows thermal stability up to 600 °C. From 600 °C to 800 °C moderate weight loss and above 800 °C continuous weight loss up to 850 °C is observed, this is due to surface complexes formed during oxidation. The total weight loss up to 850 °C is 10–15%. The 50 wt.% EG/NPR composite shows there is very small weight
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Fig. 4. Thermo gravimetric analysis (TGA) curves of EG, NPR and 50 wt.% EG/NPR composite.
Fig. 3. Measured dc conductivity of EG/NPR composites (a) in-plane conductivity, (b) through-plane conductivity.
loss up to 300 °C. From 300 to 400 °C, a moderate weight loss and above 400 °C continuous weight loss is observed up to 850 °C. Overall, there is 45–50% weight loss up to 800 °C in air atmosphere. It can be seen from the curves that, addition of EG flakes in the matrix enhances the thermal stability in comparison to pure matrix. Thus, TGA curve shows that the developed composite is thermally stable up to 350 °C. 3.5. Complex permittivity and dielectric loss studies Fig. 5a and b show the frequency dependence of the real part (e0r ) and imaginary part (e00r ) of relative complex permittivity of
Fig. 3c. Schematic diagram of plane of dc conductivity measurement.
EG/NPR composites in the frequency range 8–12 GHz. Composite with 50 wt.% of EG loading shows decline in the values of e0r and e00r from 5 to 3.5 and from 6.2 to 3.9, respectively, over the frequency range 8–12 GHz. The other two composites of 30 wt.% and 40 wt.% EG loading also show similar trends. Although e0r values do not show much variation, a noticeable decrease in e00r values with decrease in EG content is observed. Fig. 5b shows a high resolution of imaginary permittivity curves up to 9.5 GHz with varying EG content. The EG/NPR composites contains NPR as insulating section and EG as conducting section, with EG encapsulated within the polymer. The insulating sections acts like a high resistive path for flow of free electrons, however some electrons may still conduct by tunneling effect [29]. Within the composite system, there could be two paths of electromagnetic propagation as shown in Fig. 5c. In one path, line of electric flux can pass from EG flakes-polymer-EG flakes and in the other path through direct contact between the EG flakes. This is in analogy to reference [30] where Matsumoto and Miyata have discussed a model for metal–polymer composite, with filler conductivity of 104 S/cm. The second path of conduction, however, will be more effective for higher filler concentration which increases the interaction between the fillers. As EG conductivity is quite high as compared to polymer, increasing EG content increases the effective conductivity which in turn increases the loss factor e00r . In the same reference, the modeling done for higher concentration metal particles embedded in polymer matrix showed a decreasing trend for real and imaginary part of permittivity with frequency, as is observed in EG/ NPR composite system. The use of NPR as matrix material shows an increase in imaginary permittivity values over that with epoxy resin matrix referred in [24], because of increase in interaction between EG and NPR [23]. Fig. 6(a) shows the dielectric loss tangent of the composites. A high resolution curve is shown in the graph Fig. 6(b) which shows that increasing EG wt.% in the composite increases (e00r /e0r ) factor. A high value (e00r /e0r ) >1 has been observed for the three compositions in the entire X band. The composition with 50 wt.% EG has a dielectric loss of 1.21 in the lower frequency range and slightly declined to 1.20 in the upper frequency range. 40 wt.% EG and 30 wt.% EG composite shows dielectric loss of about 1.19 and 1.17 respectively, over the entire X band. Dielectric loss greater than unity suggest that the material has high dissipation factor rather than storage capacity. Thus, the EG/NPR composite has the potential characteristics for broadband microwave absorption.
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Fig. 6a. Dielectric loss tangent of EG/NPR composites.
Fig. 5. Complex permittivity of EG/NPR composite at X band, (a) real permittivity and (b) imaginary permittivity.
Fig. 6b. Dielectric loss tangent of EG/NPR composites in higher resolution.
3.6. Reflection loss study Reflection loss is determined from the measured values of e0r and e00r of the composites, by transmission line theory for conductor backed single-layered structure [31]. The reflection loss (RL) of the incident electromagnetic wave normal to the planar single-layered structure is expressed as
Z in Z 0 RL ¼ 20 log Z in þ Z 0 rffiffiffiffiffi Z in ¼ Z 0
ð1Þ
lr 2pfd pffiffiffiffiffiffiffiffiffi er lr tanh j c er
ð2Þ
The composites are nonmagnetic, hence the values of
l0r ¼ 1 andl00r ¼ 0. Fig. 7 shows the reflection loss curves of EG/
Fig. 5c. Schematic diagram of electromagnetic propagation within the composite system.
NPR composites of different filler to polymer weight ratios of thickness, d = 3.7 mm. It is observed that the reflection loss enhances from 13 dB at lower frequencies to 16.5 dB at higher frequencies in the X band. In fact, the reflection loss of EG/NPR composites determined so far, is found to be quite high in comparison to unexpanded graphite flakes stated in reference [17] in the frequency ranges 8–12 GHz. Although dielectric loss tangent is higher for higher EG content, the reflection loss is less, as summarized in Table 1. This could possibly be, due to impedance mismatch at the air-absorber interface [32]. The condition for good microwave
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cost effective. Absorption can be further enhanced by structural modifications like graded structure or modifying radar absorbing structure.
Acknowledgement The authors would like to sincerely thank Mr. Rama Obelesu, University of Hyderabad for extending help in talking microwave measurements.
References
Fig. 7. Reflection loss curve of EG/NPR composites with variation of EG concentration.
Table 1 Reported microwave properties of EG/NPR composites of thickness 3.7 mm at 10 GHz. EG/NPR composites (wt.%)
e0r
e00r
e00r /e0r
Reflection loss(dB)
30 40 50
4.02 4.02 4.03
4.67 4.74 4.86
1.13 1.14 1.17
14.93 14.81 14.67
absorption is that (1) an incident wave should completely enter into the absorbing layer and (2) subsequently get absorbs due to lossy nature of the material. For condition (1) to be satisfied, there should impedance matching at the air-absorber interface as seen by the incident electromagnetic wave. The impedance of free space, Z0, is 377 ohm. Input impedance of the absorber is elucidated by equation
Z in ¼ Z 0
sffiffiffiffi 1
2pfd pffiffiffiffi tanh j er c er
ð3Þ
As seen from the above equation, with increase in e00r , er increases and consequently impedance mismatch occurs. High impedance mismatch leads to low reflection loss as described by Eq. (1). However, by using graded layered structure and optimizing thickness of each layer the impedance matching condition can be obtained. 4. Conclusions The EG flakes synthesized by chemical oxidation and thermal treatment method is mixed with NPR in different EG wt.% and complex permittivity are investigated in the frequency ranges 8–12 GHz. The e0r permittivity of the composite shows no significant variation with EG wt.% loading in the composite, e00r also increases with increase in EG wt.% in the composite. The dc electrical conductivity values show an increase with increase in filler loading. The composite shows thermal stability up to 350 °C and can be used for high temperature applications. High performance microwave absorption with potential for applications in broadband frequency can be achieved using the EG/NPR composites which have high dielectric loss behavior over the entire X-band. Further, the developed composites are light weight and relatively
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