ELECTROMAGNETIC SHIELDING PROPERTIES OF CARBON BASED COMPOSITES

Transcription

1 ELECTROMAGNETIC SHIELDING PROPERTIES OF CARBON BASED COMPOSITES ADELA BARA, ANA MARIA BONDAR, IULIAN IORDACHE, RADU VASILESCU-MIREA, CRISTINA BANCIU, DELIA PATROI, SILVIA HODOROGEA Key wrds: Carbn-ceramic cmpsites, Shielding prperties, Cmpsite characterisatin. Carbn-ceramic cmpsites are prmising materials fr shielding electrmagnetic radiatin and reducing r eliminatin f electrmagnetic interference (EMI) because f their cnductivity and dielectric cnstant and ease f cntrl f these prperties thrugh chemical prcessing. This paper presents ur results regarding t the shielding prperties f this type f cmpsites. The btained carbn-ceramic cmpsites were structural and micr-structural investigated by ptical micrscpy, X Ray Diffractin (XRD) and functinal characterized frm electrical and electrmagnetic attenuatin pint f view. 1. INTRODUCTION With the rapid advances and brad implementatin f cmputer and telecmmunicatin technlgies there is increased interest in shielding f electrmagnetic radiatin, especially in the radi and micrwave frequency ranges. The usual shielding techniques fcus n the use f standard metals and their cmpsites, which have disadvantage due t limited mechanical fleibility, heavy weight, crrsin, and difficulty f tuning the shielding efficiency. Carbn-ceramic cmpsites are prmising materials fr shielding electrmagnetic radiatin and reducing r eliminatin f electrmagnetic interference (EMI) because f their cnductivity and dielectric cnstant and ease f cntrl f these prperties thrugh chemical prcessing [1, 2]. The carbn-ceramic cmpsites develped are ceramic-matri cmpsites having dielectric cmpnents (calin and ) with inclusin f a cnducting phase (natural graphite) [3]. These cmpsites were designed t display a cmbinatin f characteristics f cmpnents as micr-electrical netwrks, micrresistrs (carbn) and micr-capacitrs (insulatrs), randmly dispsed in space. INCDIE ICPE-CA Natinal Institute fr Research and Develpment in Electrical Engineering, 313 Splaiul Unirii, Bucharest-3, Rmania, tel.: , fa: , abara@icpe-ca.r Rev. Rum. Sci. Techn. Électrtechn. et Énerg., 3, Suppl., p. 13, Bucarest, 08

2 14 Adela Bara et al. 2 Electrical cnductivity f such systems is strngly dependent n the cncentratin f cnductive phase [4]. When weight rati f cnductive phase achieve a specific critical value, named perclatin threshld, appears a dielectriccnductive micrstructural phase transitin. 2. EXPERIMENTAL The raw materials used in this stage are presented in table belw (Table 1). Table 1 The raw materials used Graphite Alumina Calin Sdium silicate < 63 µm < 80 µm < 0 µm Water slutin with a density 2g/cm3 The filler materials (graphite, alumina, calin) were hmgenised in a duble-cn mier fr 30 min with small speed. The mied pwder are hmgenised with sdium silicate as binder and then dry in free air, ne day. After that the cmpsite granules are crushed and srted t a granulatin f 63 µm. The carbn/ceramic cmpsites samples are btained by frming the pwder in a metal die t a pressure f abut MPa. The green samples are dried 24 h t a cnstant temperature f abut ± C and then cmpsites are thermal treated at abut 900 C. The cmpsitinal map fr designed C-ceramic cmpsites, ceramic-matri fr electrmagnetic shielding applicatins is presented in Table 2. Table 2 Cmpsitinal map fr designed C-ceramic cmpsites Cmpunds Cmpsitins in weight (wt) [%] P4 P P6 P

3 3 Electrmagnetic shielding prperties f carbn based cmpsites RESULTS AND DISCUSSION 3.1. STRUCTURAL INVESTIGATION This electrical behavir is cnfirmed by ptical micrscpy and XRD investigatins Fig. 1. Intensity (a.u.) P-41 - C - Al2O3 - SiO2 0 P θ Intensity (a.u.) P-4 - C - Al2O3 - SiO2 P θ

4 16 Adela Bara et al. 4 Intensity (a.u.) P-7 - C - Al2O3 - SiO θ Fig. 1 Structure investigatin f the sample P41, P4 and P7: XRD; ptical micrscpy. P ELECTRICAL CHARACTERIZATION Electrical characterizatin f the btained materials were dne by V-A methd. It was established the electrical resistivity behaviur functin f the calin/alumina rati (wt %) between : and 2:. In Fig. 2 is presented the variatin f electrical resistivity functin f the alumina cntent (wt %) fr all series f designed cmpsites: P4, P, P6, P7. Resistivity (µωm) P 4 P 7 P 6 P % Al2O3 Fig. 2 Electrical resistivity vs. alumina cntent.

5 Electrmagnetic shielding prperties f carbn based cmpsites 17 It was bserved an accentuated variatin f resistivity in the case f P4 and P samples. The electrical resistivity is decreasing in these case with the increasing f alumina cntent. In the case f samples having 60 % r 70 % wt graphite (P6 and P7), the alumina cntent variatin des nt influence the electrical resistivity ELECTROMAGNETIC SHIELDING EVALUATION There is investigated EMI shielding by epsing cmpsite samples t different frequencies electrmagnetic fields (100 khz 12 GHz). The principle f measuring methd and the equipment arrangement in general testing cnfiguratin are presented belw (Fig. 3). Fig. 3 Schematic cnfiguratin f testing methd and equipment. It is knwn that in general a material t be cnsidered as EM shielding shuld be placed ver the limit f db. The btained results are pltted as absrptin in db vs frequencies (Figs. 4 7). Fig. 4 Attenuatin vs. frequency fr P4 samples.

6 18 Adela Bara et al. 6 Fig. Attenuatin vs. frequency fr P samples. Fig. 6 Attenuatin vs. frequency fr P6 samples.

7 7 Electrmagnetic shielding prperties f carbn based cmpsites 19 Fig. 7 Attenuatin vs. frequency fr P7 samples. By testing and evaluatin f sample behaviur in the frequancy range between 100 khz MHz, have been btained the fllw cnclusins: The all samples shwn EM shielding prperties in the frequency range between 1-10 MHz, with the highest registered values fr shielding effectivenes f 60-70dB. There was a slight difference f 6 db between tested samples In the range f 300 MHz culd be seen sharp mdificatins f shielding effectiveness, between and 60 db. This mdificatin was epected due t resnant fenmena. Hwever, we chse t perfrm these measurements nly t have an ideea f the shielding effectiveness limits ic case f cmpsite materials. In the range f MHz the samples shwn the same behaviur regarding the shielding prperties fr the all tested samples, with a large difference f 18 t 42 db between samples. The tests between 1 12 GHz shwn large differencess beteween the values f shielding effectiveness. 4. CONCLUSIONS The carbn-ceramic cmpsites develped in this stage are ceramic-matri cmpsites having dielectric cmpnents (calin and ) with inclusin f a cnducting phase (natural graphite).

8 Adela Bara et al. 8 The dielectric phases shw strng respnses where they became active and hence giving a crrespnding specific range f frequencies. This representatin crrespnd t a teture f dielectric netwrks with cnductive inclusins, which is cnfirmed by ptical micrscpy and XRD investigatins.cnsidering the btained results, we recmand the utilizatin f carbn/ceramic cmpsites in applicatins regarding EMI shielding in the frequency range f 1 t 10 GHz. There is pssibily fr realizatin f a shielding system with etended utilizatin between 100 khz 12 GHz, fr attenuatin in large areas. Received n 1 September, 07 REFERENCES 1. V. I. Trefilv, Ceramic- and Carbn-matri Cmpsites, Chapman & Hall, pp. 4 7, N. C. Lubara, B. Rand, N. H. Brett, Br.Ceramic Trans. J., 88, pp (1989). 3. Ana. Maria Bndar, I. Irdache, Carbn/Ceramic cmpsites designed fr electrical applicatin, Jurnal f Optelectrnics and Advanced Materials, 8, 2, pp (06). 4. A. Celzard, J. F. Mareche, F. Payt, D. Begin, G. Furdin, Electrical cnductivity f antharacite as a functin f heat treatment temperature, Carbn, 38, pp (00).