Polymer ceramic composites for microwave substrate

Polymer ceramic composites for microwave substrate and antenna applications ) ) 2 2 S. George , S. Raman , P. Mohanan and M. T. Sebastian *

lMaterials and Minerals Division, National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram-695019, India. 2 Department of Electronics, Cochin University of Science and Technology, Cochin-682022, India. Abstract The preparation and microwave dielectric properties Cal(Lil/�bz/3)o.8nI.lI03-' ceramic filled polymer composites for microwave substrate applications is discussed in this paper. The prediction of the dielectric properties of polymer-ceramic composites using different theoretical models is also discussed. A Coplanar Waveguide Monopole Antenna is designed and fabricated using epoxy-CLNT polymer-ceramic composite and compared its response with standard FR-4 epoxy.

1. Introduction The lower regions of microwave spectrum are increasingly getting overcrowded in the communication sector with the advent of modem communication systems. This has necessitated the need for microwave dielectric materials with low dielectric loss, optimum relative permittivity, low coefficient of thermal expansion, moisture absorption resistance and good mechanical stability [1,2]. In the context of low permittivity materials, several ceramics such as silicates and aluminates with excellent microwave dielectric, thermal and mechanical properties have been developed for substrate and packaging applications [3,4]. However, they are brittle and have high processing temperature. Recently, Button et at. proposed a composite strategy of combining the advantages of ceramic and polymer to achieve a superior property balance [5]. Polymer-ceramic composites offer an attractive combination of processibility and properties, which cannot be attained from individual components. In the present investigation, we have used polyethylene and epoxy [6] as the polymer bases and Ca[(LiI/3Nb2/3)l-xTix]03-o (CLNT) [7] ceramics as the filler for polymer-ceramic composites.

prepared by sigma blending technique [8]. The epoxy based polymer-ceramic composites were prepared by mechanical mixing and subsequent curing in vacuum. The surface morphology of the composites was studied by scanning electron microscope (JEOL-JSM 5600 LV, Tokyo, Japan). The microwave dielectric properties (x-band) of the sample were measured by the cavity perturbation technique using HP 8510 C Network Analyzer (Agilent Technologies). The 40 mm x 40 mm x 2 mm epoxy-CLNT composites were prepared and laminated with copper. The coplanar antenna was designed and etched the pattern and compared its return loss characteristics with standard FR-4 Epoxy. 3. Results and discussion Typical microstructure (fractured surface) of polyethylene-CLNT (PE - 0.4 Vr CLNT) and epoxy- 0.3 Vr CLNT polymer­ ceramic composites is shown in Fig. 1.

2. Experimental (CLNT) Ca[(Li1/3Nb2/3)osTiozj03_o ceramics were prepared by conventional solid­ state ceramic route. The sintered CLNT ceramics were ground well to form fine powders. Polyethylene-CLNT composites were

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Fig. 1. The microstructure (fractured surface) of (a) polyethylene - 0.4 Vr CLNT and (b) epoxy0.3 VrCLNT polymer-ceramic composites.

by Subodh et al. [8]. For 0.3 volume fraction of filler loading, the epoxy-CLNT composite shows an Er of 7.1 and tan 0 of 0.026.

It can be observed that the CLNT ceramic particles are randomly distributed throughout the polymer. The fractured surface shows a dense microstructure and the polymer surrounds each ceramic particles. It is clear from the micrograph that the polymers act as good adhesives for combining ceramic particles.

.,.-:--TO.006 lS..----....--------:Polyethylene

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Figure 2 shows the microwave dielectric properties of both PE-CLNT and Epoxy-CLNT polymer-ceramic composite as a function of ceramic loading. It is found that, at 9 GHz both the relative permittivity and dielectric loss increase with increase in the ceramic loading in polymer-ceramic composite. The increase in the relative permittivity of polymer-ceramic composite is expected since; the particulate filler has much higher relative permittivity (=38.6) compared with that of the polymer matrix «3). The variation of relative permittivity of polymer-ceramic composite with respect to the filler concentration can be attributed to an increase in the total polarizability of the composite material. The composite may be considered to be a homogeneous phase of composite particles. Each composite particle consists of a ceramic particle surrounded by a layer of polymer, although there can be distributions in the layer thickness. As the ceramic content increases with a corresponding decrease in the polymer ratio, the homogeneity of the composites decreases because the composite now consists of some ceramic particles which are not covered by the polymer layer (due to a reduction in the polymer content), thus leading to a heterogeneous mixture. As the ceramic content increases, the heterogeneity of composite increases and this lead to an increase in relative permittivity and dielectric loss of polymer ceramic composites. As the filler content increased from 0 to 0.50 volume fractions, the relative permittivity and dielectric loss increased from 2.3 to 9 and 0.0006 to 0.005 respectively for polyethylene-CLNT composite. For 0.4 volume fraction of filler loading, the Polyethylene-CLNT composite shows Er = 7.7 and tan 0 0.004 at 9 GHz. As the filler content increases from 0 to 0.40 volume fractions, the relative permittivity of epoxy-CLNT increases from 3.02 to 9.60. The loss tangent initially shows an increasing trend up to a filler concentration of 0.10 volume fraction and with further filler addition, the loss tangent starts decreasing. This could be due to the low dielectric loss of ceramics (tan 0 =10.4) 2 compared to that of epoxy (tan 0 =10. ). Similar observations in loss tangent have been reported

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