EMI shielding

Electromagnetic interference (EMI) testing has become more prevalent for materials that either emit or are affected by EMI. Shielding efficiency (SE) of materials is deterrnined by measuring electric field strength between a transmitter and receiver with or without the presence of the material under test. Several researchers have suggested a correlation between volume resistivity and SE values (300,301). 

The metal fillers act as a reinforcing material that results in added strength and stiffness (126). They color the plastic gray for nickel, 2inc, stainless steel, and aluminum, and brown for copper. Metal additives are more expensive than carbon black or surface-active agents, but they get extensive use in EMI shielding appHcations. 

Pitch-based fibers generally have higher moduh but lower strengths than theh PAN-based counterparts. The specific properties of the various types of carbon fibers are compared in Figure 4. Pitch-based fibers also have higher electrical conductivity, which can be an important consideration in certain circumstances, for example, for use in electromagnetic inductance (EMI) shielding. 

The philosophy of any EMI shield is to encourage eddy currents to flow within the surfaces, thus dissipating the noise energy. Also, the assembled enclosure should act as a gaussian enclosure where there is good electrical conduction totally around the enclosure. So removable hatches and enclosure members need very good electrical connections around their peripheries. RF gasketing is sometimes used in particularly troublesome cases. 

Liu, N.-I. and van der Meer, R Synergistic effect of metal flake and metal or metal coated fiber on EMI shielding effectiveness of thermoplastics, US Patent 4 566 990, 1986, and references therein. 

Table 3.17 shows some examples using aluminium powder for EMI shielding. 

EMI shielding for RF shielding and antenna dishes, a metal wire mat structure can be located in the empty mould and then impregnated by the flowing plastic. The metallizing... 

Fig. 30 a Shielding effectiveness of various composite systems (at 12 GHz), b EMI shielding effectiveness as a function of conductivity at 16 wt% filler loading... 

Figure 30a depicts the comparative plots of the EMI shielding effectiveness (SE) of all the three composite systems at a particular frequency of 12 GHz [196]. 

The process consists of pre-etching, etching, etch neutralization, catalyst application, catalyst activation, and plating. Most commercial applications, except RFI/EMI shielding, use the initial copper or nickel deposit as a base for subsequent electrolytic plating of electrolytic copper, nickel, or chromium. The exact types and thicknesses of metal used are determined by part usage, eg, automotive exterior, decorative, plumbing, and others (24). 

This list can be divided into three main classes based mainly on function and redox state. First, applications that utilize the conjugated polymer in its neutral state are often based around their semi-conducting properties, as in electronic devices such as field effect transistors or as the active materials in electroluminescent devices. Secondly, the conducting forms of the polymers can be used for electron transport, electrostatic charge dissipation, and as EMI-shielding mate-... 

Fig. 9.17 Range of soft ferrite components (i) TV scanning yoke (components kindly supplied by Philips Components Ltd.) (ii) UR core and TV line output transformer (iii) E core for switched mode power supply (iv) wide band transformer core (v) core giving good magnetic shielding (vi) high Q (adjustable) filter core (cf. Fig. 9.48) (vii) precision ferrite antenna for transponder (viii) multilayer EMI suppressors (ix) toroids for laser and radar pulse applications (x) typical EMI shields for cables, ((ii)—(x) Courtesy of Ferroxcube UK .)...

Microwave-absorbing materials are currently in high demand for many expanded EMI shielding and radar cross section reduction applications with both commercial and defense purposes. The nanostructured materials have attraction for microwave radiation absorbing and shielding materials in the high-frequency range due... 

Keywords carbon nanotubes, PMMA, dispersion, interface, composites, strength, electrical conductivity, EMI shielding. 

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