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Effect of Graphite filler on dielectric properties of polymer composite
Bhabani Sankar Rana
Department of Mechanical Engineering, MEMS, Balasore Email: [email protected]
Abstract: In recent years the composites have attached substantial importance as potential structure materials.
The attractive features of composites are their low cost, light weights, high specific modulus, and renewability and biodegradability. Increasingly enabled by the introduction of newer polymer resin matrix materials and high performance reinforcement fibers of glass, carbon and amide, the penetration of these advance materials has witnessed a steady expansion in uses and volume. Keeping this in view the present work has been undertaken to develop a polymer matrix composite (epoxy resin) using graphite fiber as reinforcement and epoxy as matrix to study its dielectric properties. The composites are prepared with different volume fraction of graphite. The composites are prepared with different volume fraction of graphite ranging from 5, 7 and 10 vol%. The composites have been prepared by hand lay-out process. The dielectric constant and dielectric loss values were evaluated with both frequency and temperature for different volume fraction of graphite by using computer interfaced LCR meter.
I. INTRODUCTION
Composites are engineering materials made from two or more constituents that remain separate on a macroscopic level while forming a single component. There are two types of constituent materials : matrix and reinforcement. The matrix materials surround and support the reinforcement materials by maintaining their relative positions. The primary functions of the matrix are to transfer stresses between the reinforcing fiber/
particles and to protect them from mechanical and environmental damage where as the presence of fiber / particles in a composite improves its mechanical properties such as strength stiffness etc.
1.1 Why a Composite?
Over the last thirty years composite materials, plastics and ceramics have been the dominant emerging materials. The volume and number of applications of composite materials have grown steadily, penetrating and conquering new markets relentlessly. Modern composite materials constitute a significant proportion of the engineered materials market ranging from everyday products to sophisticated niche applications.
While composites have already proven their worth as weight-saving materials, the current challenge is to make them cost effective. The efforts to produce economically attractive composite components have resulted in several innovative manufacturing techniques currently being used in the composites industry. It is obvious, especially for composites, that the
improvement in manufacturing technology alone is not enough to overcome the cost hurdle. It is essential that there be an integrated effort in design, material, process, tooling, quality assurance, manufacturing, and even program management for composites to become competitive with metals.
1.2 Classification of Composites
Basically, composites can be classified into the following three groups on the basic of matrix
1. Metal Matrix Composites(MMCs) 2. Ceramic Matrix Composites(CMCs) 3. Polymer Matrix Composites(PMCs)
II. LITERATURE SURVEY
The purpose of this literature survey is to provide background information on the issues to be dissertation and to emphasize the relevance of the present study.
This treatise some related aspects of EPOXY-Graphite composite with special reference to their mechanical and dielectric characteristics.
J. Cho et al [6] have worked on the mechanical characterization of graphite/epoxy nano composites by using the multi-scale analysis. Mechanical properties of nano composites consisting of epoxy matrix reinforced with randomly oriented graphite platelets were studied by the Mori–Tanaka approach in conjunction with molecular mechanics. Elastic constants of graphite nano platelets, which are the inclusion phase in the micromechanical model, were calculated based on their molecular force field. The calculated elastic constants compared well with both experimental data and other published theoretical predictions. The calculations confirm that the modulus of the nano composites studied here is strongly dependent on the aspect ratio of the reinforcing particles, but not on their size. The predicted module compares favourably with experimental results of several nano composites with graphite particles of various aspect ratios and sizes.
Smrutisikha Bal [7] had studied on effect of carbon nanofiber upon the epoxy composites. Addition of very low (up to 1 wt. %) amount of CNFs brought improvement in mechanical and electrical properties of epoxy composite. The curing of nano composites at refrigerated temperature facilitated better dispersion by optimizing adhesion between epoxy and CNF. These
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samples cured at low temperature showed significant enhancement in flexural modulus and hardness that is attributed to flexibility of CNFs inside a stretched matrix. Raman spectra of these refrigerated samples compared to room temperature samples clearly indicated shifting and increasing of G-band that confirmed better reinforcement and stress transfer from matrix to fiber.
Insulator epoxy behaved like a semiconductor with very low infusion of CNFs that was confirmed from electrical measurement. Electrical conductivity was found to be best at higher content of CNF in room temperature sample and in the direction of fiber alignment. However, in case of refrigerated sample electrical conductivity was better for low content of CNFs.
III. RAW MATERIALS
Raw materials used in the experimental work are listed below
1. Graphite Powder 2. Epoxy resin 3. Hardener 3.1 Graphite
Graphite occurs in metamorphic rocks as a result of the reduction of sedimentary carbon compounds during metamorphism. It also occurs in igneous rocks and in meteorites. Minerals associated with graphite include quartz, calcite, micas and tourmaline. In meteorites it occurs with toilette and silicate minerals. Small graphitic crystals in meteoritic iron are called cliftoni.
3.1.1Properties
The acoustic thermal properties of graphite are highly anisotropic, since photons propagate very quickly along the tightly-bound planes, but are slower to travel from one plane to another. Graphite can electricity due to the vast electron delocalization within the carbon layers, the phenomenon is called aromaticity. These valence electrons are free to move, so are able to conduct electricity. However, the electricity is primarily conducted within the plane of the layers. The conductive properties of powdered graphite allowed its use as a semiconductor substitute in early carbon microphones.
Graphite and graphite powder are valued in industrial applications for their self-lubricating and dry lubricating properties. There is a common belief that graphite's lubricating properties are solely due to the loose inter lamellar coupling between sheets in the structure.
3.1.2 Uses
Natural graphite is mostly consumed for refractory, batteries, steelmaking, expanded graphite, brake linings, foundry facings and lubricants. Graphene which occurs naturally in graphite has unique physical properties and might be one of the strongest substances known;
however, the process of separating it from graphite will require some technological development before it is economically feasible to use it in industrial processes.
The materials are strong, stiff and lightweight. Polymer graphite fiber composite is the material of choice for application where lightweight & superior performance is paramount, such as component for spacecrafts, fighter aircrafts and race cars.
3.2 Epoxy resin
Epoxy resins are polymeric or semi-polymeric materials, and as such rarely exist as pure substances, since variable chain length results from the polymerisation reaction used to produce them. High purity grades can be produced for certain applications, e.g. using a distillation purification process. One disadvantage of high purity liquid grades is their tendency to form crystalline solids due to their highly regular structure, which require melting to enable processing. Epoxy resin and graphite was heated separately inside a vacuum over for 1 hr at 600C. Mixing of resin with graphite was carried out by H.S.M (high intensity mechanical liquid stirrer) at approximately 500 rpm speed and 600C for 1 hr. Curing agent was added to the mixture at predetermined parts per hundred ratio. The solution is gently mixed for 15-20 minutes to avoid introduction of any air bubbles due to mixing action. The final slurry free from air bubbles was poured into the preheated mould (120*120*4 mm) and cured in the vacuum .Post curing was carried out into two phase: first at 1200C for 2hrs inside vacuum over and room temperature for 48hrs.The above procedure will be repeated for different weight of graphite to get the sheet of composites and details is given table.
Table No.3.1 Different composition of fiber and matrix
Composition of fiber (%)
Weight of graphite (gm)
Weight of Epoxy (gm)
Weight of Hardener (gm)
5% 9.1 96.85 9.685
7% 12 94.81 9.481
10% 18.1 91.76 9.176
Table No. 3.2 Detailed composition and processing condition of test samples Sl.
No
Epoxy Hardener Clay Preheating Epoxy/clay
Stirring
Epoxy+ graphite+
hardener
Curing Postcure
1. GY-257 A-140 5 wt%
graphite
1hrat600C /4hr at 800C
1hr & 600 rpm /15 minutes
1hr at 800C 48 hrs at Room temp.
2. GY-257 A-140 7 wt%
graphite
1hrat600C /4hr at 800C
1hr & 600 rpm/15minutes
1hr at 800C 48 hrs at Room temp.
3. Gy-257 A-140 10 wt%
graphite
1hrat600C /4hr at 800C
1hr & 600 rpm /15 minutes
1hr at 800C 48 hrs at Room temp.
Table No. 3.3 Characteristics of hardener
Product name Characteristics Viscosity 250C[mpa.s] Amine value[KOH/g] H+active eqiv[g/Eq]
Aradur 140 Polyminoimidazoline 300-600 at 750C 370-410 ~95
IV. RESULT AND DISCUSSION
4.1 Frequency Dependence of Dielectric Constant (r) and Dielectric Loss (tanδ)
Frequency dependence of dielectric constant (εr) and dielectric loss (tanδ) at room temperature (RT) at 1 kHz frequency of Graphite-(1-)Epoxy composites with
= 0.05, 0.07 and 0.10 volume fractions are shown in Fig 4.1 (a-b). When a dielectric material is subjected to an ac electric field, the displacement of charges and ions and orientation of the dipole moments tries to follow the direction of the electric field. But the switching of the displacement and orientations with the fast reversal of the electric field becomes more difficult at higher frequencies. As the frequency increases, ionic and orientation sources of polarizability decreases and finally disappear due to the inertia of the molecules and ions. Therefore among the three types of polarizability, the electronic polarizability which involves electrons only exists up to very high frequencies [14,15].
Fig 4.1 (a) shows the decrease in r value with the increase in frequency for all the volume fractions of Graphite-Epoxy composites. The reason for decrease in dielectric constant is due to the damping out of the successive polarization mechanisms. Again the r values for vol% 5 and 7 remain almost constant in the frequency range 103 to 106 Hz. While for vol% 10 of Graphite-Epoxy composites, the dielectric value first decreases rapidly and then decreases slowly with the increase in frequency. This decrease in dielectric value with the increase in frequency can be explained on the basis of net polarization. As it is commonly known as polarization of a dielectric material is the sum of the contributions of dipolar, electronic, ionic and interfacial polarizations [16]. At low frequencies, all the polarizations respond easily to the time varying electric field but as the frequency of the electric field increases different polarization contributions filters out, as a result, the net polarization of the material decreases which leads to the decrease in the value of εr.
Again Fig 4.1 (b) shows the same trend as follows by the dielectric constant. The loss values decrease with the increase in frequency for all the volume fractions of Graphite-Epoxy composites. For the vol% of 5 and 7 of Graphite-Epoxy composites, the dielectric loss curves show slow decrease in the values with the increase in the frequency. While for vol% 10 of Graphite-Epoxy composites, the tan first decreases rapidly from 102 to
104 Hz and then decreases slowly with the increase in the frequency from 104 to 106 Hz. The reason for decrease in tan values of the Graphite-Epoxy composites can also be related to the mechanism of polarizations [17,18]
Fig. 4.1 Frequency dependence of (a) r and (b) tan of Graphite-Epoxy composites at 1 kHz frequency.
4.2 Temperature Dependence of Dielectric Constant (r) and Dielectric Loss (tanδ)
Fig 4.2 (a-b) shows the temperature dependence of dielectric constant of Graphite-(1-)Epoxy composites with = 0.05, 0.07 and 0.10 volume fractions at four different frequencies, 1 kHz, 10 kHz, 100 kHz and 1 MHz, respectively. It is observed that the
r values increase first with the increase in temperature and shows anomalous behavior near the phase transition temperatures (Tc). The increase in the value of εr with temperature can be explained on the basis of increase in domain wall mobility. Generally the phase transition in perovskite type materials generally occurs due to instability of temperature dependent low frequency optical soft mode frequency [19,20]. Again a shift of phase transition is observed with the increase in the vol% of graphite in the composite samples.
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Fig. 4.2 Temperature dependence of r of Graphite- Epoxy composites for (a) vol% 5, (b) vol% 7, and (c)
vol% 10, respectively at four different frequencies.
Table 4.1 (Details of the dielectric properties of Graphite-Epoxy composites at four different frequencies)
Graphite- (1-)Epoxy composites
Dielectric properties
r at RT at 1 kHz
r at RT at 10 kHz
r at RT at 100 kHz
r at RT at 1 MHz
tan at RT at 1 kHz
tan at RT at 10 kHz
tan at RT at 100 kHz
tan at RT at 1 MHz
= 0.05 12.8958 10.2235 7.6535 5.6391 0.1509 0.1113 0.0519 0.0403
= 0.07 33.1270 28.1512 24.1779 22.5737 0.258 0.1815 0.2078 0.1574
= 0.10 50.0175 34.2293 28.0208 20.1201 2.5618 1.4076 0.4606 0.1794
V. CONCLUSION & RECOMMENDATION FOR FUTURE RESEARCH
The following conclusions are drawn from the present study:
The dielectric properties of graphite-epoxy composites have been studied as a function of both frequency and temperature.
The RT dielectric constant values increase with the increase in the vol % of graphite with the maximum
r ~ 12.89 obtained for vol% 10 of the composites.
Again the RT dielectric loss shows a increase in the value with increase of volume fraction of graphite powder in the graphite-epoxy composites.
Temperature dependence of dielectric constant value shows first an increase in r and then a decrease with the increase in temperature.
The dielectric loss curves for all the compositions of the composites show an increasing trend with the increase in temperature.
Graphite can be used with conducting polymer to fabricate the composites with conducting as par with metallic conductors to find its application suitable in electronics field. Other tests like surface resistivity, volume resistivity, thermo gravimetric analysis, impendence etc. can also be performed using graphite as filler and epoxy as polymer material.
REFERENCES
[1] I.M. Afanasova,, V.A. Morozov, A.V. Kepmana, S.G. Ionov, A.N. Seleznev, G. Van Tendeloo, V.V. Avdeev Preparation, electrical and thermal properties of new exfoliated graphite-based composites.( 2 0 0 9 ) 2 6 3 –2 7 0.
[2] Spain IL. Electronic transport properties of graphite, carbons, and related materials. In:
Walker Jr PL, Thrower PA, editors. Chemistry and physics of carbon, vol 16. New York and Basel: Dekker; 1981. p. 119–304.
[3] Saunders D, Galea S, Deirmendjian G. The development of fatigue damage around fastener holes on thick graphite/epoxy composite laminates. Composites 1993;24(4):309–21.
[4] Ezquerra Y, Kulescza M, Balta-Calleja F.
Electrical transport in polyethylene–graphite composite materials. Synth Met 1991;41(3):915–
20.
[5] Archana Nigrawal and Navin Chand Electrical and Thermal Investigations on Exfoliated Graphite filled Epoxy Gradient Composites.
Malaysian Polymer Journal, Vol. 5, No. 2, p 130- 139, 2010.
[6] J. Cho, J.J. Luo, I.M. Daniel of Composites Science and Technology 67 (2007) 2399–2407 [7] Smrutisikha Bal Department of Metallurgical and
Materials Engineering, N.I.T., Rourkela 769 008, India Materials and Design 31 (2010) 2406–2413
[8] Zhengtao Yang a, Ken McElrath b, Jeff Bahr c, Nandika Anne D’Souza a, COMPOSITE PART B 43 (2012) 2079-2086.
[9] Yong Zhao et al Composites Materials [44] 01 September 2012
[10] S.N. Wosu a, David Hui , L. Daniel Department of Mechanical Engineering and Materials Science, Part B 43 (2012) 841–855
[11] S. Chatterjee a, J.W. Wangb, W.S. Kuo c, N.H.
Tai d, C. Salzmann e, W.L. Li b, R. Hollertz F.A.
Nüesch a, B.T.T. Chu Chemical Physics Letters 531 (2012) 6–10.
[12] F. Aymerich, P. Priolo, C.T. Sun Composites Science and Technology 63 (2003) 907–917 [13] M. Szczepanik*, J. Stabik, M. £azarczyk, A.
Dybowska Archives of Materials Science and Engineering Volume 37 Issue 1 May 2009 Pages 37-44.
[14] Shaikh P.A.; Kambale R.C.; Rao A.V.; Kolekar Y.D.J. of Alloys and Compounds.2009,482, 276.
