Quantitative Interpretation of Electromagnetic Interference Shielding Efficiency: Is It Really a Wave Absorber or a Reflector?

As electromagnetic (EM) pollution continues to increase, electromagnetic interference (EMI) shielding materials have been intensively evaluated in terms of two main shielding mechanisms of reflection and absorption. Since the shielding effectiveness (SE) is represented in the logarithmic scale and in a coupled way of transmission (SET), absorption (SEA), and reflection (SER), often there is a misinterpretation that the EM wave reflectors are regarded as EM wave-absorbing materials. Surprisingly, we found that many materials reported as an EM wave absorber in the literature provide, in fact, less than 50% of EM wave-absorbing capability, i.e., over 50% of EM wave-reflecting feature. According to the theory and definition of EMI SE, the absorption-dominant EMI shielding materials should have the ratio of absorption to incident energy (A) as A > 0.5, which corresponds to a necessary condition that SER < 3.01 dB. The SER subsequently gives SEA in relation to SET. Using this criterion, we classified the EMI shielding materials with their shielding mechanism. The proposed methodology provides significant insight into the evaluation and development of EMI shielding materials.

Introduction

Electromagnetic (EM) waves generated by electronic devices have detrimental impacts on the performance of nearby devices and can also be harmful to human health.[1−3] With the exponential increase in the number of electronic devices, highly efficient materials for electromagnetic interference (EMI) shielding are desperately needed, particularly specified for specific shielding mechanisms of reflection or absorption. Among the EM wave-reflecting and -absorbing characteristics, EM wave-reflecting materials, say metals, do not eliminate incident EM waves but just redirect them in random directions, which could lead to secondary interference issues around the neighboring devices or source detectors.[4] On the other hand, the EM wave-absorbing materials could eliminate the incident or self-induced EM waves in an exclusive way, which is the most desirable characteristic of EMI shielding materials.

In material characterization and evaluation of EMI shielding capability, we have found that often there is a misinterpretation that the EM wave-reflecting materials are regarded as the absorbing materials. This misinterpretation stems from a simple comparison of the shielding effectiveness (SE) of materials that shield by reflection and absorption, viz:[5,6]where T, R, and A are the transmission, reflection, and absorption coefficients of the EMI shielding materials and SET, SER, and SEA are the total, reflection, and absorption SE, respectively. Notably, it can be found that the SEA is simply calculated by subtracting the SER from the SET.

The EMI shielding materials possessing an SET of over 50 dB corresponding to 99.999% of shielding of incident EM waves may be classified as a good EMI shielding material composed of both absorption and reflection capabilities.[6−9] For example, the layered structure of pure Ti3C2T (MXene) with a thickness of 8 μm is reported to have the SET, SEA, and SER as around 58, 35, and 23 dB, respectively, which correspond to their EMI shielding coefficients of T (0.0000016), A (0.005), and R (0.995).[6] Since SEA is larger than SER, the material could be interpreted as a good EM wave-absorbing material, which is, in fact, not true because over 99% of EM waves is not absorbed but reflected, i.e., absorption efficiency = 0.5% and reflection efficiency = 99.5%, based on the absorption and reflection coefficients. This confusing data results from the formulas that are used to calculate the SEs in eqs –4.[10] Consequently, a simple comparison of SEA and SER values may cause significant misinterpretation of material capabilities of EM wave absorption and reflection.

In this study, we investigated EMI shielding properties of four layers of aluminum foil and various EMI shielding models. It was thoroughly demonstrated that the EM wave absorption performance of shielding materials can be dramatically exaggerated with high SEA values and the EMI shielding coefficients are more relevant parameters than SEs for evaluating the shielding mechanism of materials. In addition, the previously reported EMI shielding materials were classified with their dominant shielding mechanisms.

Results and Discussion

EMI shielding properties of four layers of aluminum foil with a thickness of 16.3 μm each are shown in Figure , which is a typical metallic reflector that may have been used in EMI shielding purposes. The average SET, SEA, and SER of this aluminum foil are 104.7, 89.3, and 15.4 dB, respectively, in the X-band frequency range (Figure A). The aluminum foil exhibits a significantly higher SEA (89.3 dB) than SER (15.4 dB). Note that the four layers of aluminum foil can serve the interfaces for multiple internal reflections, in which the EM waves are reflected back and forth between the layers, as schematically shown in Figure S1. Since the internally reflected EM waves are finally absorbed within the material, the multiple internal reflection effect can be included in absorption and subsequently may increase the contribution from absorption to total EMI SE.[6] Nevertheless, the reflection, absorption, and transmission fractions of the foil are 95.93, 4.07, and 0.000000007%, respectively (Figure B). The transmission fraction of the aluminum foil is nearly zero because most incident EM waves are blocked by the metallic barrier. The reflection fraction is over 88% in the whole X-band frequency range, verifying that this material is a very good reflector. On the other hand, it absorbs under 12% of the incident EM waves over the measured frequency range despite its notably high SEA. Absorption is not the dominant shielding mechanism even though the SEA is much higher than the SER. Conclusively, SER and SEA values may be misrepresented in evaluating the reflection and absorption capabilities of materials.

Figure 1

(A) Comparison of EMI SET, SEA, and SER, and (B) reflection, absorption, and transmission fractions of four layers of aluminum foil as a function of frequency in the X-band range.

The materials that satisfy the common commercial EMI shielding requirement (>20 dB) can be classified with their dominant shielding mechanisms.[9] Specifically, the materials providing the absorption fraction of over 50% for incident EM waves can be classified to absorption-dominant shielding materials and vice versa. To be an absorption-dominant shielding material, its SER value should be lower than 3.01 dB, which is calculated by eq .[10] In experiments, the directly measured one is the amount of reflected energy, and thus the SER is determined by the experimental value of the reflection coefficient, i.e., the fraction to reflected energies of incident EM waves, using eq , which subsequently gives SEA by eq . Therefore, SEA depends on SER or the reflection coefficient associated with SET, which often results in misleading of SEA. For example, if two types of shielding materials have an SET of 40 and 60 dB with the same SER of 3.01 dB, the materials may exhibit transmission fractions of 0.01 and 0.0001%, respectively, and identical reflection fractions of 50%. Subsequently, although they also have almost the same absorption fractions of 50%, the corresponding SEA values are 36.99 and 56.99 dB, respectively, which are quite different values.

To clarify the correlation between the SEs (i.e., SER and SEA) and shielding mechanisms (i.e., reflection and absorption), the SER ranges are divided into four domains to classify various EMI shielding materials with shielding mechanisms, as shown in Table . Absorption-dominant shielding materials should have an SER of less than 3.01 dB (reflect less than 50% of EM waves); these materials belong to domain 1. The other materials, which possess an SER greater than 3.01 dB, could be classified as EM wave-reflecting materials and belong to domains 2, 3, and 4 based on their reflection fractions (50–90%, 90–99%, and 99–100%, respectively). Along with these domains, we also provide EMI shielding models to promote a comprehensive understanding of the SEs. The SEA graphs are plotted with various fixed SET of 20, 40, 60, 80, and 100 dB as a function of SER and reflection fraction, as shown in Figure A,B, respectively. Figure A shows linear graphs of SEA with a slope of −1. Each graph can be used as a boundary line that determines the SE of the materials. For example, the data points that fall above the SEA line with an SET of 40 dB exhibit SET greater than 40 dB, whereas those below the line exhibit an SET below 40 dB. The data for the aluminum foil (104.7 dB) is located on the upper side of the SEA line with an SET of 100 dB. On the other hand, the curves of SEA vs reflection fraction, which are calculated using eq , are logarithmic (Figure B). With a reflection fraction of 50%, the SER is fixed at 3.01 dB, whereas the SEA differs greatly with various SET values. In particular, with a fixed SET of 20 dB, when the SER and SEA are both 10 dB, the reflection fraction is 90%, although it seems that half of incident EM waves are reflected and the other half are absorbed. Thus, it should be noted that the equal values of SER and SEA never mean that the contributions of reflection and absorption are the same.

Table 1

Four Domains of SER to Classify the Materials with Their Shielding Mechanisma

domains	SER ranges (dB)	reflection ranges (%)
domain 1	0 < SER < 3.01	0 < R < 50
domain 2	3.01 < SER < 10	50 < R < 90
domain 3	10 < SER < 20	90 < R < 99
domain 4	20 < SER	99 < R < 100

SER is shown, with the corresponding ranges of reflection fraction.

Figure 2

EMI shielding models with SEA as a function of (A) SER and (B) reflection fraction.

Interestingly, the differences between the SEs and real shielding mechanism are dramatically exaggerated at high SET. As shown in Figure , when the absorption fraction curves with different SET are plotted as a function of SER/SET, if the proportion of SER implies the contribution of reflection, then the absorption fraction should be 50% at SER/SET = 0.5 with a linear graph. However, the absorption fractions with SET of 10, 20, 40, 60, and 100 dB at SER/SET = 0.5 are 21.6, 9, 1, 0.1, and 0.001%, respectively. In other words, if the SEs are thought to represent the contributions of reflection and absorption, then EMI shielding materials with a higher SET could look like more efficient EM wave-absorbing materials. Thus, the absolute amount of reflected/absorbed EM waves should be considered according to the SER, as shown in the models. An absorption-dominant mechanism can be only exerted when the SER is lower than 3.01 dB (domain 1), regardless of the SEA.

Figure 3

Absorption fraction curves with different SET as a function of SER/SET.

We identified the shielding mechanism of various typical EMI shielding materials (Figure and Table S1). Using the criterion SER value of 3.01 dB (50% reflection), all materials can be classified by their dominant shielding mechanisms, as shown in Figure A. Almost all researched materials with an SER greater than 3.01 dB accomplish EMI shielding mainly by reflection despite their high SEA. This notion is verified, as shown in Figure B. The data points on the left side of the boundary line (reflection = 50%) exhibit absorption-dominant EMI shielding, whereas those on the right side exhibit reflection-dominant shielding. Specifically, the MXene, metals, and their composites show relatively high shielding performances with reflection-dominant shielding. This feature mainly originates from their high electrical conductivities, in which abundant free electrons directly interact with the incident EM waves, thus dissipating the radiation power by reflection.[11−13] By contrast, the carbon-based materials such as graphene and carbon nanotube (CNT) and their composites absorb some EM waves and exhibit absorption-dominant shielding with their electric and/or magnetic dipoles, which can transform the EM energy to thermal energy.[14−16] Note that, among the materials that give an SET of greater than 40 dB, hardly any are absorption-dominant shielding materials, emphasizing the need of high-performance EM wave-absorbing materials to address secondary or repeated EM pollution issues.

Figure 4

(A) Comparison of SEA and SET of typical EMI shielding materials as a function of (A) SER and (B) reflection fraction, respectively. A detailed description of each data point is presented in Table S1.

Conclusions

We have developed various EMI shielding models to give an insight in shielding mechanism, in which the shielding materials can be thoroughly classified with their dominant shielding mechanisms. Although it seems that a higher SEA than SER means absorption-dominant EMI shielding, we verified that the absorption-dominant shielding can be only exerted for materials with a smaller SER than 3.01 dB. We have also emphasized that the difference between SEs and real shielding mechanism is exaggerated in a higher SET for materials. Accordingly, it is desirable that the EMI shielding mechanism of materials is interpreted using reflection and absorption coefficients, which show reflection and absorption fractions for the incident EM waves, respectively, rather than SER and SEA. It is believed that this study would correct many researchers’ misinterpretations who work on EMI shielding materials that require controlled shielding mechanisms.

Experimental Section

The aluminum foil samples used in this study were commercially available products. The thickness of the samples was determined using a thickness gauge (ID-C112B, Mitutoyo Corp.). The aluminum foil samples were prepared with a rectangular dimension of 40 mm × 40 mm × 65 μm to fit the specimen holder. The EMI SE of the samples was measured in the frequency range 8.2–12.4 GHz (X-band) by WR-90 rectangular waveguide using a two-port vector network analyzer (E5071C, Keysight Technologies, USA). The scattering parameters (S33, S34, S43, and S44) of each sample were recorded and used to calculate the R and T coefficients and EMI SE as