MICROWAVE CHARACTERIZATION OF ALUMINA FILLED POLYSTYRENE COMPOSITE

Juti R. Deka and Nidhi S. Bhattacharyya Dept. of Physics, Tezpur University, Tezpur, Assam-784028 E-mail:[email protected]

Abstract

The recent strides in polymer technology have spawned enormous application possibilities of polymers as microwave substrate materials. In the present investigation, variations in microwave permittivity and loss tangent with increasing quantities of the inclusions (alumina) in the polystyrene (PS) matrix have been conducted. It is observed from the microwave measurement that real part of complex permittivity of the PS-alumina composite system at 9.68 GHz increases from 2.5 to 2.785 as the filler (inclusion) percentage increases from 0-7%Volume Fraction (VF). Dispersive behavior of this composite system with improved dielectric property is also studied. A frequency sweep from 8.4 to 12.0 GHz shows an increase in the real part permittivity of composite by 0.31- 0.39% as the volume fraction of filler in the composite increases from 0 to 7%VF and the loss factor varies in the range 0.0035-0.005.

I.INTRODUCTION

Major new applications in aeronautics, space and telecommunication technology using composite materials are springing at an ever increasing pace. High permittivity and low dielectric loss is the primary characteristic of the substrate for design applications as microstrip circuits at microwave frequencies. Polymer suffers from poor microwave dielectric properties, feeble dimensional stability, large thermal expansion coefficients and poor thermal conductivity when compared with other microwave material choices viz. ceramics and ferrites [1-3]. A great technological interest have developed in particle-filled polymer composites as improvements in properties of polymers can be done by reinforcing it with additional materials so that overall properties of the composites are superior to those of the individual components as properties of the composite depends on the intrinsic properties of the reinforce material [4- 7]. Moreover, mass production of these composites is efficient and economical. In the present investigation a polymer composite is developed where alumina particles are in reinforcing phase and polystyrene (PS) is in matrix phase. Microwave property of the composite material with the change in percentage volume fraction (%VF) is examined. A significant improvement in the microwave property of polystyrene (PS) is observed when reinforced with alumina fillers.

II. EXPERIMENTAL

II.A Sample Preparation Technique

A 2 gm of crosslink polystyrene (Aldrich) is dissolved in 50ml cyclohexane (E Merck). In order to reduce the viscosity of the solution so as the quantity of alumina can be increased 2ml tetraethyl orthotitanate (Aldrich) or titanium ethoxide [Ti(OC2H5)4] is used as surfactant to the PS solution [8-9]. The surfactant mixed solution is stirred for half an hour. Alumina (E Merck) is grounded and sieved to get particle size quite less than the probing EM wavelength (which is ~36×103 µm at the lower 8-12.4 GHz range) with different volume fraction is mixed with the solution in a homogenizer at 8000 rpm rotor speed for about 15 minutes. This homogenous mixture is casted in rectangular shape and allowed to dry at room temperature. The preparation and environmental conditions are kept same for one batch of samples.

II.B Microwave Measurement

Measurements of microwave properties of the composite at 9.68GHz are carried out using cavity resonator technique [1], waveguide technique [10] and resonance method [11-12]. The frequency dependence of the wave velocity in a transmission line is called dispersion property and it is an important parameter when the line is used in a wide frequency range. The frequency dependence of dielectric constant gives rise to ‘material dispersion’ in which the wave velocity is frequency dependent. For time domain measurement of complex permittivity in the 8.4 to 12.0 GHz range sweep frequency measurement set up is used.

III. RESULTS AND DISCUSSION

III.A Complex Permittivity of PS-Alumina Composite at 9.68 GHz

Fig. 1 shows an increase of permittivity of PS-Alumina composite system at 9.68GHz from 2.5 to 2.785 when the filler percentage varies from 1 to 7%VF. Dielectric response of a particulate filled polymer composites relies on characteristic of each phase in the composites [13-14]. When an electric field Ē is applied to material - cations and anions in the dielectric are displaced in response to the field [15]. This displacement produces an induced dipole moments and the aggregate of induced or permanent dipoles produces a net polarization in the material. Permittivity also depends on the volume fraction of filler and eventually on the spatial arrangement in the mixture. With the increase in filler percentage the local potential increases in response to the same external applied electric field. This increase in potential increases the capacitance of the material. Higher value of capacitance results in higher effective permittivity of the composite.

0.006 2. 8 BG CR CR Res 2. 75 WG Res 0.005

2.7 0.004

r o ty i 2.65 ct v

a tti

i 0.003 s f s m

r o e 2.6 L P 0.002 2. 55 0.001 2. 5

2.45 0 0246802468 % Volume fraction of filler % Volume fraction of filler

Fig 1: Permittivity of PS- alumina composite versus %VF of Fig 2: Loss factor of PS-alumina composite versus filler at 9.68GHz. %VF of filler at 9.68GHz (CR-cavity resonator, WG- wave guide, Res- resonance method (CR-cavity resonator, Res- resonance method) BG- Bruggmans theory)

In fig. 1 theoretical result determined with the Bruggemans effective medium theory (BG) is also given for comparison of the experimental results. The experimental results (CR-cavity resonator, WG - wave guide, Res – Resonance method) show a deviation of effective permittivity from the Bruggman’s effective medium approach. The surfactant reduces the interphase region between the low dielectric polymer and high dielectric filler thus changing the effective dielectric constant of the composite [16]. In PS-alumina composite, the loss factor varies in the range 0.0035-0.005 for different volume fraction, as shown in fig. 2. The additional dielectric loss is from the interfacial polarization between two different dielectrics in the composite and under the influence of electric field [17].

III. B Dispersive Complex Permittivity of PS-Alumina Composite at X-Band Effective permittivity value of composites shows dispersive behavior. A frequency sweep from 8.4 to 12 GHz increases the real part permittivity of PS-alumina composite by 0.31- 0.39% as the volume fraction of filler in the composite increases from 1 to 7%. The dielectric spectra of 1% and 3%VF of PS-alumina measured using resonance and waveguide method is shown in fig 3. The loss ranges from 0.004 to 0.016 in PS-alumina composite.

3 3 Res WG Cr Res WG CR 2.8 2.8

y ty t 2.6 2.6 i i v v i t tti i it

m m r r e 2.4 P Pe 2.4

2.2 2.2

2 2 8.4 9 9.6 10.2 10.8 11.4 12 8.4 9 9.6 10.2 10.8 11.4 12 Frequency (GHz) Frequency (GHz) Fig 3a: Dispersive permittivity of PS -1%VF alumina filler Fig 3b: Dispersive permittivity of PS - 3%VF alumina filler

At microwave frequencies according to classical dispersion theory of dielectric [13], the dielectric constant is unchanged and the dielectric loss increases with frequency. Therefore the product of quality factor and frequency describes loss property of the dielectric. The complex permittivity depends on the frequency but it stays almost constant over a small range of frequency spectrum [18-20]. The complex permittivity (ε) of / // / a material is a function of angular frequency i.e. ε(ω) = εo[εr (ω)-jεr (ω)] where εr is the time dependent // relative permittivity, εr is the corresponding loss factor and ω is the radian frequency. Resonance dispersion law widely applied for the qualitative characterization of the dielectric properties of the composites [21] and is given by A ε(f) = ε + d 2 ⎛ f ⎞ ⎛ f ⎞ 1− i⎜ ⎟ − ⎜ ⎟ ⎝ frel ⎠ ⎝ fres ⎠ where ‘A’ is the scale factor depends on the percentage of filler and frel and fres are the relaxation and resonance frequencies defining the shape of dispersion curve.

IV. CONCLUSION Permittivity of the system increases with the increase in filler percentage. From the loss factor measurement it can be conclude that the composite is a low loss material. The composite system can be used as substrate for MIC’s because of the improvement in the value of permittivity. As circuits on high dielectric material get congested and hence reduce the power handling capability, these composites can replace quartz at higher frequency. The composite studied in the present investigation finds a more economic alternative to costly microwave substrate materials.

References: [1] V. R. K. Murthy, S. Sunderam and B. Viswanathan, “Materials and Technology for Microwave Integrated Circuits”, Microwave Materials, pp. 30-36, Narosa Publishing House, New Delhi, 1993. [2] I. Bahl and K Ely, “Modern Microwave Substrate Materials”, Microwave Journal, vol 33, pp 131-138, 1990. [3] G R Traut, “Clad Laminates of PTFE Composites for Microwave Antennas”, Microwave Journal, vol 23, pp 47-54, 1980. [4] Christian Brosseu, Patrick Queffelec and Phillippe Talbot, “Microwave Characterization of Filled Polymer”, J.App. Phys, vol 89, pp 4532-4540, April, 2001. [5] H. M. Musal Jr., H. T. Hahn and G. G. Bush, “Validation of Mixture Equation for Dielectric-Magnetic Composite”, J.App. Phys, vol.63, pp.3768-3770, April, 1988. [6] T. Nakamura, T. Tsutaoka and K. Hatakeyama, “Frequency Dispersion of Permeability in Ferrite Composite Materials”, J.Magnetism and Magnetic Materials, vol.138, pp.319-323, July,1994. [7] J.P. Ganne, M. Pate, b. Grammaticos, A.Ramani and T. Robin, “ Microwave Permeability of Ferrite Loaded Composites: Comparison Between Model and Experiment” , ICF 7, Bordeaux,1996. [8] E.K Sichel, “ Carbon Black- Polymer Composites”, Marcel Dekker, New York, 1982. [9] K.L Mittal, “ Silanes and Other Coupling Agents”, Adhesion Society, vol.2, 2000 [10] S. Max and F. Jerome, “Handbook of Microwave Measurements,” John Willey and Sons, New York, vol. 2, 1963. [11] E. Yamashita, K. Atsuki and T. Hirahata, “Microstrip Dispersion in a Wide Frequency Range,” IEEE Trans Microwave Theory and Techniques, vol. 29, pp. 610-612, 1981. [12] E. Yamashita, K. Atsuki and T. Ueda, “An Approximate Dispersion Formula of Microstrip Lines for Computer Aided Design of Microwave Integrated Circuits,” IEEE Trans Microwave Theory and Techniques, vol. 27, pp. 1036-1039, 1979. [13] W. G. Spitzer, R. C. Miller, D. A. Kleinman and L. E. Howarth, “Far Infrared Dielectric Dispersion in BaTiO3, SrTiO3 and TiO2,” Physical Review, vol. 126, pp. 1710-1715,1962. [14] M. G. Todd and F. G. Shi, “Molecular Basis of the Interphase Dielectric Properties of Microelectronic and Opto-electronic Packaging Materials,” IEEE Trans on Components and Packaging Technologies, vol. 26, pp. 667-670, 2003. [15] J. B. Jarvis, R. G. Geyer, J. H. Grosvenor, Jr., M. D. Genezic, C. A. Jones, B. Riddle and C. M. Weil, “Dielectric Characterization of Low Loss Materials Measurement Techniques,” IEEE Trans on Dielectrics and Electrical Insulation, vol. 5, pp. 571-574, 1998. [16] M. G. Todd and F. G. Shi, “Characterization of the Interphase Dielectric Constant of Polymer Composite Materials: Effect of Chemical Coupling Agents,” J. of Appl. Phys, vol. 94, pp. 4551-4556, 2003. [17] Z. Yutao, C. Cheng, J. Weifang, X. Hengkun and L. Yaonan, “Relationship between Dielectric Loss and Interphase Structure of Filled-types Polymer Composite,” State Key Lab of Electrical Insulation for Power Equipments, pp- 77. [18] W. J. Chudobiak, O. P. Jain and V. Makios, “Dispersion in Microstrip”, IEEE Trans Microwave Theory and Techniques, vol. 19, pp. 783-788, 1971. [19] B. Riddle and J. B. Jarvis, “Complex Permittivity Measurement of Common Plastic Over Variable Temperature,” IEEE Trans Microwave Theory and Techniques, vol. 51, pp. 727-732, 2003. [20] A. N. Lagarkov, S. M. Matytsin, K. N. Rozanov and A. K. Sarychev, “Dielectric Permittivity of Fiber-filled Composites: Comparison of Theory and Experiment,” Physica A, vol. 241, pp. 58-61, 1997. [21] W. M. Merrill, R. E. Diaz and N. G. Alexopoulos, “A Recasting of the Effective Parameters of Composite Mixtures into the Language of Artificial Dielectrics,” Physica A, vol. 241, pp. 334-337, 1997.