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70 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 54, NO. 1, FEBRUARY 2012

Microwave Frequency Characteristics ofMagnetically Functionalized Carbon

Nanotube ArraysVladimir A. Labunov, Vadim A. Bogush, Alena L. Prudnikava, Boris G. Shulitski, Ivan V. Komissarov,

Alexander S. Basaev, Beng Kang Tay, Member, IEEE, and Maziar Shakersadeh

Abstract—This paper reports the results of a comprehensivestudy of the interaction of electromagnetic radiation (EMR) ofthe wide frequency range (8–12, 26–37, and 78–118 GHz) with ar-rays of vertically aligned and disordered carbon nanotubes (CNTs)which have been obtained by the floating catalyst chemical va-por deposition method. The obtained nanotubes represent a com-posite of multiwall CNTs with encapsulated magnetic nanopar-ticles of iron phases, i.e., magnetically functionalized nanotubes(MFCNTs). MFCNTs were formed on silicon substrates, and dis-ordered arrays in the form of powder were obtained by separatingthe MFCNT arrays mechanically from the walls of the quartz reac-tor. The frequency dependences of the reflection and transmissioncoefficients of EMR of MFCNTs of two types were investigated. Thehigh electromagnetic shielding efficiency (40 dB) of MFCNTs as-sociated with the reflection of electromagnetic waves was detected.Possible mechanisms of attenuation of electromagnetic signals byaligned and disordered MFCNTs were discussed.

Index Terms—Carbon nanotubes (CNTs), electromagnetic radi-ation (EMR), microwave frequency, shielding.

I. INTRODUCTION

THE increasing use of active electromagnetic resource, in-crement of power and the number of emitters of electro-

magnetic waves, and expanding the range of electromagneticsignals with the transition to the high-frequency region requirethe establishment and improvement of the high-performancebroadband electromagnetic radiation (EMR) screens and radioabsorbing coatings [1], [2]. EMR screens are designed to protectradio equipment and wildlife [3], to shield radiation sources andhigh-sensitivity sensors, to protect various objects from detec-tion [4].

Manuscript received December 20, 2011; accepted December 26, 2011. Dateof current version February 17, 2012. This work was supported in part by theScientific Technical Program of the Union State “Nanotechnologiya-SG” underGrant 1.3.1/09-1034.

V. A. Labunov, V. A. Bogush, A. L. Prudnikava, B. G. Shulitski, andI. V. Komissarov are with the Department of Micro- and Nanoelectronics,Laboratory of Integrated Micro- and Nanosystems, Belarusian State Univer-sity of Informatics and Radioelectronics, Minsk 220013, Belarus (e-mail:[email protected]; [email protected]; [email protected]; [email protected]; [email protected]).

A. S. Basaev is with Technological Center, Scientific and ManufacturingComplex, Moscow Institute of Electronic Technology, Zelenograd 124498,Moscow, Russia (e-mail: [email protected]).

B. K. Tay and M. Shakersadeh are with the School of Electrical andElectronic Engineering, Nanyang Technological University, 639798 Singapore(e-mail: [email protected]; [email protected]).

Digital Object Identifier 10.1109/TEMC.2012.2182772

The EMR shielding efficiency (SE) in a given frequency rangeis defined by the specific properties of materials, i.e., the com-plex magnetic and dielectric permeabilities, specific conductiv-ity, dimensions, and mutual influence of structural elements incomposite media [5], [6].

Development of technologies for the creation and study ofmaterials and composites for the microwave frequency rangeapplications is of current importance because they could po-tentially be used to create devices for the microwave signalsprocessing, such as filters, moderating systems, phase changers,and directional couplers [7]. In addition, they can be used toform the elements of hybrid systems, such as heteromagneticmicro- and nanosystems [8], [9].

II. ELECTROMAGNETIC SHIELDING AND

SHIELDING MATERIALS

Electromagnetic shielding phenomenon is associated with adecrease of the electromagnetic field strength in a shielded re-gion of space, which is achieved by the reflection of electromag-netic waves from the surface of the screen and/or absorption ofEMR energy by the screen material. At the same time SE of thescreen, defined as the ratio of the field strength in a protectedarea before mounting the screen to the field strength at the samepoint in space after installing it, is a dimensionless quantity(≥1), and can be characterized by the attenuation of the EMRscreen [1], [10]. Thus, the attenuation is generally determinedby the physical processes of reflection and absorption of EMR.

In order to obtain a high reflection coefficient of the screen,most often metal thin films, meshes, particles, etc., are used asthe screen coatings. The drawbacks of such screens are low cor-rosion resistance and thermal stability, high weight and cost,proneness to abrasion, and the appearance of scratches andcracks, which can be secondary sources of radiation and, thus,reduce the SE as the frequency of EMR increases. In addition,a high value of the reflection coefficient of the screen for manypractical applications is undesirable because the reflected EMRmay have negative effects on the objects in the shielded area.

Another mechanism of electromagnetic shielding is the ab-sorption of EMR which is observed in the materials havingelectric and/or magnetic dipoles interacting with an alternatingelectromagnetic field. Electric dipoles can be found in the mate-rials with high dielectric permittivity, such as BaTiO3 . Magneticdipoles are contained in materials having a high value of mag-netic permeability, such as Fe3O4 , etc. [11], [12]. Absorbing

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LABUNOV et al.: MICROWAVE FREQUENCY CHARACTERISTICS OF MAGNETICALLY FUNCTIONALIZED CARBON NANOTUBE ARRAYS 71

properties of a material determine the attenuation of electromag-netic waves in a medium which is proportional to the distancepropagated by the wave [10], [11].

In addition to the mechanisms of reflection and absorptionof EMR, there is a mechanism of multiple internal reflectionswhich is associated with the reflection of an electromagneticwave from various surfaces and interfaces present in the screenmaterial [13]. This mechanism takes place when the shieldingmaterial has a high specific surface area (for example, porousmaterials) or a high specific area of boundaries (composite ma-terials containing high specific surface area fillers). The smallerthe dimensions of the pores or filler, the greater the specific sur-face area and the specific area of boundaries, respectively. Thelimiting case is the nanostructured materials [14].

Thus, the electromagnetic SE is determined by the total lossesdue to reflection, absorption, and multiple internal reflections ofEMR. The losses caused by multiple reflections can be neglectedwhen the distances between the reflecting surfaces are largerthan the skin layer depth.

The composites based on a polymer matrix which containsa conductive filler have been increasingly used as the materialof the screens. They have the advantage of the combination ofthe required mechanical properties of the binder (matrix) andthe high EMR attenuation value of the filler in a single de-vice. The dielectric polymer matrices, which do not affect theradiation shielding, but provide both the contact between theconductive particles of the filler and the high SE, are most com-monly used [15], [16]. In the composite materials with conduc-tive fillers due to the skin effect, SE increases with decreasingthe particle size, which must be comparable to or less than theskin layer depth. Conductive polymer matrices are getting morepopular nowadays. The presence of the conductive filler pro-vides the electrical contact between the filler particles, therebyincreasing the conductivity of the matrix. However, numeroustechnological problems and their higher cost prevent them froma widespread use [17]. Along with polymers, the conductivecements, carbon materials, and their composites can be used asconducting matrices. Carbon materials are widely used in elec-tronics because of their electrical and mechanical properties,availability, and ease of processing [18]. The peculiarities oftheir interaction with EMR make them attractive as shieldingmaterials [19], [20].

In order to ensure a high SE of the screen, the matrix shouldcontain a high quantity of the conductive filler. However, thehigh concentration of the filler would deteriorate the strengthand elasticity of the composite. Therefore, the use of the fillersthat are effective at low concentrations is preferable, since italso reduces the cost and weight of the screen. Material ofthe matrix and geometrical parameters of the filler are veryimportant for the creation of the efficient broadband composite-based EMR shields. The conductive fillers such as metallic orcarbon fibers are able to ensure a high specific surface areasince they have small (submicron) diameters and long length.Theoretically, metals can provide better electromagnetic SE thancarbon, because of their high conductivity. However, carbon hasa higher thermal stability and corrosion resistance, as well astechnological advances in the production of nanofibers [8], [21].

The performed analysis suggests that carbon nanotubes arethe promising material for the EMR screens. They almost com-pletely satisfy the requirements mentioned earlier.

Carbon nanotubes (CNTs) are carbon molecules with a uniqueset of specific mechanical, electrical, emission, optical, andchemical properties. Regular studies of their properties resultedin the discovery of new effects and fields of application withenormous commercial potential [22], [23]. CNTs are advanta-geous for creation of efficient broadband screens of EMR andother microwave devices since both multi-wall and single-wallCNTs possess high conductivity (higher than metals), high ther-mal conductivity and thermal stability, and increased chemicaland corrosion resistance. Moreover, they have a high aspect ratiogeometry and can be formed by standard low-cost methods.

CNT arrays with encapsulated magnetic nanoparticles of ironphases, i.e., magnetically functionalized CNTs (MFCNTs), areof particular interest for EMR screens’ application [24]. In thiscase, conductive CNTs, serving as a matrix, should ensure highreflection of EMR, and the encapsulated in CNTs magneticnanoparticles, as a filler of the matrix, should ensure effectiveabsorption. It is important that the magnetic filler being encap-sulated in the CNTs is not subjected to corrosion.

There are several methods of introducing a substance intoCNTs [25]. One of them is common sputtering of carbon inarc discharge using the anode containing the material that isto be encapsulated into CNTs. Such technology is applicablemainly to the materials that can “survive” under extreme con-ditions of an electric arc. The second method is the opening ofCNT tips chemically, followed by the introduction of the fillermaterial inside the tubes by the capillary effect. This methodhas proved itself better than the first one and can be used forthe introduction of the filler out of the melt (liquid phase). Thethird method involves the synthesis of CNTs by chemical vapordeposition (CVD). In this case, CNTs are synthesized using ananostructured catalyst that is formed either on the surface ofthe substrate before starting the pyrolysis process in the reac-tion chamber (localized catalyst CVD) or during the pyrolysisprocess, when the catalyst is formed by the decomposition of avolatile source, such as metallocene usually mixed with liquidhydrocarbon (floating catalyst CVD). Both CVD methods arecompatible with the microelectronics technology.

The floating catalyst CVD method is usually more preferable,primarily due to its simplicity. During the growth process ofCNTs by this method, the catalyst particles are embedding inCNTs, sometimes fully filling their channels. The catalyst can bealso found in-between CNT walls, as well as outside of CNTsin the form of nanoparticles incapsulated in graphene shells.An important fact is that the catalysts for CNTs growth areferromagnetic materials, such as Fe, Ni, and Co. Therefore, thismethod is most suitable for the synthesis of MFCNTs.

MFCNT arrays represent a “porous” material having a highspecific surface area of both the tubes and filler, and a highspecific area of the interfaces between them. This means thatthe use of such arrays may implement a mechanism of multiplereflection of EMR.

EMR screens can be created using MFCNT arrays formed onvarious substrates. MFCNTs, being a composite material itself,


can be additionally put into the matrix made of the aforemen-tioned materials, in order to give the shield additional mechani-cal, chemical, etc., properties.

Up-to-date methods allow obtaining aligned or disorderedMFCNT arrays. Such arrays can be synthesized over the largesurface areas what is sufficient for creating commercial EMRscreens. Moreover, the mass production of MFCNT arrays isavailable.

This study is devoted to the investigation of the interaction ofmicrowave EMR of the wide frequency range (8–12, 26–37, and78-118 GHz) with the magnetically functionalized CNT arraysobtained by the floating catalyst CVD method, discussion of themechanisms of EMR attenuation in such systems, and possibleapplications of the obtained results.

III. SYNTHESIS OF MFCNT ARRAYS, THEIR STRUCTURE, AND

MAGNETIC AND ELECTRIC PROPERTIES

MFCNTs were synthesized by the floating catalyst CVDmethod realized by a high temperature pyrolysis of fer-rocene/xylene [C8H10] solution (i.e., feeding solution) at at-mospheric pressure using Ar as a carrier gas. The boron-dopedsilicon wafers (ρ = 12 Ω·cm) with crystal orientation (1 1 1),dimensions 8 mm × 25 mm, were used as substrates, whichwere placed in the middle of a tubular quartz reactor having aninternal diameter of 8 mm. The aerosol of the feeding solutionwas delivered into the synthesis zone by a dosed injection. Inthe synthesis process, the ferrocene concentration in xylene was10%, the injection rate of the solution in the reaction zone 1 mLmin−1 , temperature 1170 K, the Ar flow rate 100 cm3 min−1 ,and the duration of the process 4 min. As shown in [24], undersuch conditions both on the surface of silicon wafers and onthe walls of the quartz reactor, the arrays of vertically aligned,dense CNTs filled with nanoparticles of catalyst were formed,i.e., MFCNTs.

The formation of MFCNTs on the surface of silicon wafersand on the walls of the quartz reactor made it possible to createtwo types of shielding material. One of them represents arraysof vertically aligned MFCNTs formed on silicon substrates (seeFig. 1). The second type is a powder-like material obtained bymechanical separation of MFCNTs from the reactor walls (seeFig. 2).

Fig. 1(a) and (b) shows the SEM images of the MFCNT arrayon the silicon substrate at different magnifications (scanningelectron microscope Hitachi S-4800). As can be seen, underthe realized synthesis conditions the vertically aligned arraysof dense nanotubes were formed. The height of the array is∼320 μm [see Fig. 1(a)], and the diameters of MFCNTs arewithin 10–50 nm [see Fig. 1(c)].

Fig. 2 shows the SEM images of the powder-like MFCNTs atdifferent magnifications. Fig. 2(a) and (b) shows general viewsof the powder-like material. It is seen that the obtained materialrepresents disordered blocks of MFCNTs arrays. In turn, eachblock consists of an array of aligned MFCNTs [see Fig. 2(c) and(d)]. It is clear that the orientation of the nanotubes on the reactorwalls was, as in the case of a silicon substrate, perpendicular tothe wall surface. Composition of the MFCNT arrays synthesized

Fig. 1. SEM images of the MFCNT array synthesized on the silicon substrateat different magnifications: (a) ×130; (b) ×20 000 (a) ×100 000.

on Si substrates as well as on the walls of the quartz reactor wasdetermined by X-ray analysis at room temperature in Cu Kα-radiation. As shown in [24], in both cases the MFCNTs arrayscontain a small amount of carbon in the amorphous state, butmostly graphite with a rhombohedral structure, characteristicof CNTs. Also, α-Fe, cementite Fe3C, and carbide Fe5C2 weredetected. It was calculated that more than 90% represents Fe3Crelative to Fe and Fe5C2 content [24].


Fig. 2. SEM images of the powder-like MFCNTs separated mechanically from the walls of the quartz reactor, at different magnifications. (a), and (b) Generalviews of the powdered material. (c) Blocks of MFCNT arrays. (d) High-magnification image of MFCNTs.

Fig. 3. Structure of MFCNTs encapsulating the nanoparticles of iron-containing phases: (1) Filler of cylindrical shape in the channel of MFCNTs.(2, inset) Filler of the ellipsoidal shape in a channel of MFCNTs (TEM).(3) Filler in the walls of CNTs (TEM).

The structure of the nanotubes in the arrays was examinedby transmission electron microscopy (JEM-100 CX, Jeol). Ascan be seen from Fig. 3, MFCNTs are multiwall and have largediameter distribution (10–50 nm) and various number of tubewalls. It is seen that MFCNTs are filled with nanoparticles,which are, according to X-ray diffraction analysis, primarilyFe3C. Filler particles are located in the channels (see Fig. 3,markers 1 and 2 in the inset) and in the walls (marker 3) ofMFCNTs. The filler in the channels has elongated cylindrical(marker 1) or ellipsoidal shape (marker 2, inset).

Fig. 4. Temperature dependence of the specific magnetization of the powder-like MFCNTs (upon heating).

The presence of Fe3C mainly and a small amount of Fe5C2in the MFCNT arrays suggests that in the process of nanotubegrowth by the floating catalyst CVD, iron particles are not justembedding into the nanotubes, but enter into a chemical reactionwith them.

In order to estimate the skin layer depth of the MFCNT array,the data on the specific resistivity and magnetic permeability ofthe array in the investigated frequency range will be necessary.

The specific magnetization of the powder-like MFCNTs wasmeasured by the static ponderomotive method upon heating inthe temperature range 78 ≤ T < 600 K [13] (see Fig. 4). Theanomalous point in the dependence corresponds to the Curie


point TC ∼ 480 K for Fe3C. This fact testifies that the obtainednanotubes represent a composite ferromagnetic material and canbe regarded as MFCNTs indeed.

Investigation of the specific magnetization behavior uponheating and cooling showed that the synthesized MFCNTs pos-sess reversible magnetic properties in a wide temperature range(78 ≤ T≤ 720 K), which testifies to the high temperature stabil-ity of ferromagnetic nanoparticles inside CNT channels what, inturn, can lead to creation of reliable devices based on MFCNTs,such as EMR shields.

Taking the value of the relative magnetic permeability of thearray μ= 1 and the value of the specific resistivity of∼10−3 [26]in the frequency range of 26–37 GHz, we obtained the valuesof the skin layer depth d in the range of 10−4–10−5m. Oneshould consider that this estimate does not take into accountthe frequency dependences of the mentioned parameters andrequires additional experimental data.

IV. METHODS OF STUDYING THE INTERACTION OF MFCNTS

WITH THE EMR OF THE MICROWAVE FREQUENCY RANGE

In order to determine the interaction of EMR of the microwaverange with MFCNT arrays as a material for the EMR screens’applications, it is necessary to obtain information about SE andmechanism of attenuation of EMR.

The existing measurement systems provide the informationabout the level of reflected and transmitted through the screensignals, and allow measuring, respectively, the reflection co-efficient and attenuation of EMR. The latter is defined by theratio of levels of the incident and transmitted signals. Thus, theattenuation is determined by the level of EMR passed throughthe screen, which, in turn, depends on the reflection of elec-tromagnetic waves from the “free space–screen” interface andabsorption of EMR in the material of the screen. Some devicesallow measuring the transmission coefficient, which is inverselyproportional to attenuation.

In the microwave range, the parameters of EMR screens andother devices are measured using the vector and scalar net-work analyzers. Besides, the characteristics of electromagneticshields can also be measured by the panoramic meters of atten-uation and voltage standing-wave ratio (VSWR).

Vector network analyzers represent automated devices thatallow the measurement of complex values of elements of thewave scattering matrix [S] which describes a linear four-terminalnetwork (FTN), i.e., an electromagnetic screen [27]:

[S] =[

S11 S12

S21 S22

](1)

where S11 , S22 are the complex reflection coefficients of EMRof the input and output of FTN, correspondingly, and S21 , S12are the complex transmission coefficients of FTN in the forwardand reverse directions, respectively [28].

The matrix of S-parameters measured using the vector an-alyzer allows determining unambiguously the parameters ofEMR propagation by solving a differential equation system [29].However, this method of analysis is very complex, and the mea-suring systems are rather expensive. For many practical appli-

cations, it is sufficient to measure only the absolute value ofthe reflection coefficient or VSWR to characterize the reflec-tion, and the direct transmission coefficient to characterize theattenuation of the EMR shield.

The measurements of the frequency dependence of these char-acteristics (absolute values of complex elements of the wavescattering matrix) are performed using the scalar network an-alyzers (SNA). For the devices with a symmetric scatteringmatrix, such as uniform EMR screens, the analysis can beconducted also using the panoramic meters of the VSWR andattenuation.

The VSWR is related to the absolute value of the reflectioncoefficient by the following expression [30]:

VSWR =1 + Γ1 − Γ

(2)

where Γ = |S11 | is the absolute value of the reflection coefficientat the input of FTN.

Determination of SE of the screen on the basis of the measuredabsolute value of the direct transmission coefficient of FTN isperformed taking into account the attenuation of EMR in theenvironment of its propagation (air) according to the expression

SE =

∣∣∣∣∣E1

E2eγ ′′x

∣∣∣∣∣ = |S21 |−1e−γ ′′x (3)

where |S21 | = |E2 |/|E1 | is the absolute value of the direct trans-mission coefficient of FTN; E1 and E2 are the field strengthsat the input and output of FTN, or in front and behind of it,correspondingly.

Considering that the absorption coefficient in free space γ′′ isclose to zero, and the distance between the transmitter and thereceiver also approaches to zero, the value of the absorption infree space is very small. This allowed us to equate the factore−γ ′′x to 1. Using (3), the calculated SE in this case is equal to thereciprocal value of the absolute value of the direct transmissioncoefficient of passive FTN [27].

Investigation of the frequency dependences of the reflec-tion and transmission coefficients (i.e., attenuation of EMR)of MFCNT arrays was performed in the ranges of 8–12, 26–37,and 78–118 GHz. The choice of the frequency intervals was de-termined by the peculiarities of EMR propagation, the availablemeasuring equipment, and the possible profit of the used materi-als in applications such as remote sensing, radio-wave imaging,etc.

Depending on the frequency range, the panoramic meters ofthe VSWR and attenuation or the SNA were used (see Table I).

In the range of 8–12 GHz, the panoramic meters of the VSWR(or attenuation) operating on the principle of selection and directdetection of the incident and reflected (or transmitted) wavelevels were used. Signal proportional to the power supplied tothe load is selected by the directional detector of the incidentwave (D1). The signal reflected from the sample (or passedthrough the screen) is selected by the directional detector of thereflected wave (D2), what implies the presence of two schemesfor the detectors coupling for measuring the attenuation (seeFig. 5) and reflection (see Fig. 6) coefficients, correspondingly.


TABLE IEQUIPMENT USED FOR MEASURING THE MICROWAVE CHARACTERISTICS OF MFCNTS

Fig. 5. Measurement scheme of EMR attenuation (the absolute value of thedirect transmission coefficient).

Fig. 6. Measurement scheme of the reflection coefficient of EMR (VSWR):(a) with a reflector and (b) with a matched load.

During the study of the reflection coefficient, an extra mea-surement using a matched load was carried out [see Fig. 7(b)].This allows eliminating the error which might occur duringthe estimation of the reflection coefficient for materials hav-ing low attenuation, and also gives additional information aboutthe mechanism of EMR propagation in the sample. This er-ror usually arises if the thickness of the screen is divisible bya quarter of wavelength (the self-quenching effect is used inthe quarter-wave resonant absorbers of EMR). The proposedadditional measurement of the absolute value of the reflectioncoefficient using the matched load minimizes the probability

Fig. 7. Measurement scheme of the absolute values of (a) direct transmissionand (b) reflection coefficients of the samples using SNA.

of reflection of the electromagnetic wave from the interface atthe output of the screen, and allows characterizing directly theproperties of the “free space–screen” interface. For samples withhigh attenuation, such additional measurement is not necessary.

In frequency ranges of 26–37 and 78–118 GHz, the absolutevalues of the reflection |S11 | and transmission |S21 | coefficientswere measured directly using automated SNA having improvedmetrological characteristics (see Fig. 7). The measurements us-ing SNA were carried out in a panoramic mode, and the relativeerror for the reflection coefficient did not exceed 20%, and forthe transmission coefficient 5%.

MFCNT orientation relative to the EMR wavefront is an im-portant factor that determines the SE of the screen [24]. There-fore, we investigated two types of samples, that is, the arrayof MFCNTs aligned perpendicularly to the Si substrate (seeFig. 1) and the powder-like MFCNTs consisting of disorderedblocks of aligned MFCNT arrays (see Fig. 2) placed betweentwo radio transparent polymer films, what provided their spatialfixation, mechanical contact with each other, thus forming a sin-gle conductive system (see Fig. 8). Considering the thickness ofthe whole sample, ∼1 mm, and the thickness of each polymerfilm, ∼300 μm, the thickness of the active layer of powder-like MFCNTs was 1 mm–2 × 300 μm = ∼400 μm, which is


Fig. 8. Scheme of the second-type sample of the powder-like MFCNTs con-sisting of disordered blocks of aligned MFCNT arrays placed between two radiotransparent polymer films.

approximately equal to the height of the MFCNT array formedon the Si substrate [∼320 μm, see Fig. 1(a)]. Hence, the twosamples had the same thickness, an important parameter thatdetermines the shielding properties of a material.

For measurement of attenuation (or transmission coefficient),the samples were fixed in the measuring section between theflanges of the waveguide system or horn antennas perpendic-ularly to the waveguide axis (see Figs. 5–7). The electromag-netic power leakage was minimized by using a specially de-signed measuring cell with coaxially fixed wave guides (orhorns). This junction was taken into account during calibra-tion. It was revealed that the microwave power loss did notexceed 15% and it was compensated by the increasing of thegenerator power. The position of the sample was kept the sameduring the measurement of the VSWR (or reflection coefficient),which corresponds to the most “severe conditions” (maximumreflection coefficient) in the case when plane-parallel samplesand the plane wavefront are used. The position of the samplesin the measuring cell was fixed perpendicularly to the EMRpropagation.

Analysis of the mechanisms of EMR attenuation by the ar-rays of MFCNTs was carried out on the assumption that theincident electromagnetic wave is partially reflected from the“free space–screen” interface, partially attenuated in the vol-ume of the material due to absorption and multiple reflections,and partly transmitted through the sample.

V. RESULTS AND DISCUSSION

Since the equipment used in different frequency ranges (8–12, 26–37, and 78–118 GHz) has some distinctions in the waysof obtaining the data, initially all measured values were reducedto a single representation. The measured VSWR was convertedinto the absolute values of the reflection coefficient |S11 |, and theattenuation values into the absolute values of the transmissioncoefficient |S21 |. The obtained results were grouped in accor-dance with the frequency range, and are shown in Figs. 9–11.

Fig. 9. Frequency dependence of the absolute values of (a) reflection and(b) transmission coefficients of the Si substrate (1), the arrays of alignedMFCNTs on Si substrate (2), and powder-like MFCNTs (3) in the range of 8–12 GHz. The results of measurements using the reflector are labeled as primednumbers [see Fig. 7(b)].

In the frequency range of 8–12 GHz for the samples of thefirst type, the microwave characteristics of Si substrates werealso measured. The frequency dependence of the absolute valueof the reflection coefficient was measured using both schemesshown in Fig. 6 (with the reflector and the matched load). Asseen in Fig. 9(a), the use of the reflector or the matched loadaffects the measured reflection coefficient of a silicon substrate(1). It is characterized by a high value of |S11 | (about −1 dB)if the measurement is performed using the reflector [see Fig.6(a)], and a low value (around −10 dB) using the matchedload [see Fig. 6(b)]. At the same time, the absolute value ofthe transmission coefficient of the Si substrate (1) is quite high,∼–2 dB, [see Fig. 9(b)] suggesting the low EMR attenuationand, therefore, its negligible influence on the EMR propagationin the 8–12 GHz frequency range. Fig. 9(a) also shows thatat 8–12 GHz the values of |S11 | measured using the reflectorand the matched load both for the arrays of aligned MFCNTson the Si substrate (2) and powder-like MFCNTs (3) differinsignificantly (for about 1.5 dB). Therefore, MFCNT arrayscontribute mainly to the measurements of EMR interaction,rather than the substrate on which they are formed.

In [8] and [17], similar results were obtained for materialshaving high EMR reflection (like MFCNTs) in the frequencyranges of 26–37 and 78–118 GHz. This means that at these


Fig. 10. Frequency dependence of the absolute values of (a) reflection and(b) transmission coefficients of the Si substrate (1), the arrays of alignedMFCNTs on the Si substrate (2), and powder-like MFCNTs (3) in the range of26–37 GHz.

frequencies both samples will not demonstrate any substantialdifference in the values of the reflection coefficients for varioustypes of loads. In this regard, the reflection coefficients of Sisubstrates in the frequency range of 26–37 GHz and of MFCNTsamples in the ranges of 26–3 and 78–118 GHz were measuredusing the “classical” scheme with the reflector [see Fig. 7(b)].In the range of 78–118 GHz, the measurement of Si substrateswas not performed.

The frequency dependences of |S11 | and |S21 | of Si substratesin the range of 26–37 GHz are characterized by their high valuesas well. |S11 | is −2 dB when measured with the reflector [seeFig. 10(a)]. |S21 | is also quite high (−5 dB to −8 dB) [seeFig. 10(b)].

As seen in Figs. 9–11, the sample of the first type (2) demon-strates high values of |S11 | (−5 to 1 dB in all frequency ranges),which suggests the high reflectivity of these samples, and thusthe basic mechanism of EMR shielding is the reflection of theelectromagnetic waves. It should be noted that in all the fre-quency ranges, the reflection coefficient of MFCNT arrays onthe Si substrate does not show a monotonic dependence on thefrequency. In the range of 78–118 GHz, the features of resonantinteractions (reflections) are observed.

For the sample of the second type (3), |S11 | decreases withincreasing frequency: from −3 to −5 dB (8–12 GHz), from−9 to −13 dB (26–37 GHz), and from −12 to −25 dB (78–118 GHz). Besides, in the frequency range of 8–12 GHz |S11 |decreases slightly, in the range of 26–37 GHz the slope of thecurve increases, and in the range of 78–118 GHz it shows a pro-

Fig. 11. Frequency dependence of the absolute values of (a) reflection and(b) transmission coefficients of the arrays of aligned MFCNTs on the Si substrate(1) and powder-like MFCNTs (2) in the range of 78–118 GHz.

nounced resonant interaction, reducing the reflection coefficientto −25 dB due to the resonance absorption at the frequencies of96–97 GHz [see Figs. 9(a), 10(a), and 11(a)]. The observed dropof the reflection coefficient with increasing frequency may be as-sociated with changing the conditions of electromagnetic wavespropagation at the interface “free space–screen”; commensura-bility of the EMR wavelength with its penetration depth intothe material; dimensions of the conducting heterogeneities, i.e.,blocks of MFCNT arrays of ∼320 μm in length.

The absolute values of the transmission coefficient of EMRfor both types of samples decrease with increasing frequencyif three frequency ranges are compared (attenuation of EMRby the sample increases). The attenuation of EMR by the first-type sample is 8–10 dB (8–12 GHz), 27 dB (26–37 GHz), 17–27 dB (78–118 GHz); and by the second-type sample is 18–21 dB (8–12 GHz), 38–40 dB (26–37 GHz), and 35–40 dB (78–118 GHz). At the frequencies above 26 GHz, the |S21 | valuesreach the measurement limit (40 dB). The observed decrease ofthe absolute value of the transmission coefficient is obviouslycaused by the decrease of the skin layer depth with increasingfrequency what leads to the increase of attenuation of EMR bythe material.

It should be noted that in all frequency ranges, the |S21 |value of MFCNT arrays on the Si substrate is much higherthan that of the powder-like MFCNTs. It differs for more than


TABLE IIABSOLUTE VALUES OF THE REFLECTION AND TRANSMISSION COEFFICIENTS OF MFCNTS ARRAYS

10 dB [see Figs. 9(b), 10(b), and 11(a)] what is most likely dueto the different values of the effective electrical conductivityof the nanotube arrays having different spatial orientation inthe investigated samples. Multiple internal reflections of theEMR in the powder-like disordered MFCNTs samples can alsocontribute to the observed high attenuation.

In all frequency ranges, the absolute value of the transmis-sion coefficient has almost no dependence on the frequency[see Figs. 9(b), 10(b), and 11(b)]. However, in the range of 78–118 GHz there is a pronounced resonance interaction in bothsamples, which can be related to the peculiarities of the dis-tribution of the electromagnetic signals, measurement featuresin this range, as well as the special properties of CNTs, whichdimensions are comparable with the wavelength of EMR inthe material. The presence of the resonant characteristics andthe absolute values of the reflection and transmission coeffi-cients of −25 dB and −38 dB at 96–97 GHz, respectively, forthe powder-like disordered MFCNTs, suggest the high SE ofelectromagnetic signals and the resonant absorption of EMR.The resonant interaction of microwave EMR with the arrays ofmagnetically functionalized CNTs in the frequency range of 78–118 GHz has not been observed previously. This phenomenonrequires special investigation.

Microwave characteristics of the aligned and disorderedMFCNT arrays in the investigated frequency ranges using thedata of Figs. 9–11 are presented in Table II.

The analysis of the present parameters suggests that the mainmechanism of EMR attenuation in the range of 8–12 GHz ofboth samples is the reflection of electromagnetic waves due tothe difference of the wave impedances of free space and thescreen material [31]. As the frequency is increasing, the reflec-tion coefficient of the powdered MFCNTs decreases, while thereflection by the aligned MFCNTs remains almost unchanged.Probably, in the comparable short-wave frequency range differ-ent orientation of conductive elements, which dimensions arecomparable with the EMR wavelength in the sample, promotesthe reduction of the reflection. At the same time, at higher wave-lengths the scale of structure inhomogeneities has no effect, andthe MFCNTs act like a solid material [32].

The higher absolute values of EMR attenuation by thepowder-like MFCNTs as compared to the aligned MFCNTson the Si substrate (for about 10–15 dB) are explained by themore developed network of electrical contacts between the struc-tural elements (blocks of MFCNT arrays) and the lower waveimpedance of disordered MFCNT arrays in general.

The total resistance of the array of nanotubes is determinedby the resistance of the junctions between different CNTs andcan be estimated using the percolation theory. As shown in [33],the electrical conductivity of the percolation network of CNTs

remains practically unchanged up to 100 MHz and then dra-matically increases at the frequencies above 10 GHz. In thiscase, the CNT arrays cannot be considered as a homogeneousmaterial, and the discrete element analysis must be employed.In this case, the system can be reckoned using the theory ofelementary dipoles (antennas). Interaction of the antenna withEMR is most effective when the length of the antenna is of thesame order of magnitude as the wavelength of the incident ra-diation. At the same time, it is necessary to consider the signalsuppression, if the electric field of the incident radiation is po-larized perpendicularly to the axis of the antenna dipole, and theresonant response, if the antenna’s length is divisible by a halfwavelength of the radiation.

The presence of the magnetic nanostructured filler inMFCNTs can lead to the decrease of the penetration depth ofEMR in the material dozens of times, and to the possibility ofradiation channeling in the MFCNT array at higher frequencies.Moreover, one should also take into account the anisotropy ofthe electrical properties of aligned MFCNTs, which can influ-ence the formation of the reflected signals and the overall SE.Most likely, the mechanism of EMR attenuation in these materi-als is based on the multiple reflections of the wave by structuralelements, transformation of the incident plane electromagneticwave with its subsequent attenuation [32], [34].

The obtained and investigated materials may be used forapplications such as lightweight composite materials for highperformance electromagnetic screens and devices for the pro-cessing and channeling of electromagnetic signals [35]. Fur-ther research will be aimed at the identification of the mech-anisms of interaction of EMR of the microwave range withMFCNT arrays of different orientations, containing nanostruc-tured magnetic filler, nature of the observed resonant behaviorin order to find their optimal characteristics for future practicalapplications.

VI. CONCLUSION

We have reported the results of measurements of the fre-quency dependences of the reflection and transmission coeffi-cients of magnetically functionalized CNTs of two types, i.e.,the array of vertically aligned CNTs on a silicon substrate andthe powder-like material representing disordered blocks of nan-otube arrays mounted between dielectric radio transparent poly-mer film. It has been revealed that both samples, especially thesecond, have high SE of EMR. The attenuation in the frequencyrange of 8–12 GHz reached 22 dB (for the second-type sample)what was the result of high reflection of electromagnetic waves.In the frequency ranges of 26–37 and 78–118 GHz, the value ofattenuation ran up to 40 dB (and was limited by the measuring


system capability), which was associated with peculiar conduc-tive and magnetic properties of MFCNT arrays, that provided ahigh degree of reflection of EMR from their structural elements.

As the frequency increased, the absolute value of the transfercoefficient decreased to−40 dB (i.e., the attenuation increased),and the reflection coefficient decreased as well (to −25 dB),what can be associated with the electromagnetic wave scatter-ing on the structural elements (blocks of MFCNT arrays) havingthe length (∼300 μm) commensurable with the length of theelectromagnetic waves in the investigated material. In the fre-quency range of 78–118 GHz, the frequency dependence of thetransmission and reflection coefficients demonstrated resonancebehavior, which also confirms the presence of size effects.

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Vladimir A. Labunov received the Ph.D. and doc-toral degrees in physics of solids from the BelarusianState University of Informatics and Radioelectronics,Minsk, Belarus.

He is currently a Professor, Lecturer, and ChiefScientific Researcher in the Department of Micro-and Nanoelectronics, Belarusian State University ofInformatics and Radioelectronics, and a member ofBelarusian National Academy of Sciences. His scien-tific carrier started from microelectronics, where themain focus of the scientific research was “interaction

of plasma, ion, electron beams, and optical radiation with solids for applicationin production of integrated circuits.” In this area, he has more than 400 scientificpapers and 500 patents. During the last ten years, the group that carried outunder his leadership the research focused on the development of nanostructuredinorganic and organic functional materials based on carbon, porous silicon, andoxides of valve metals (such as nanoparticles, nanotubes, and nanowires) forcreation of nanocomponents for information and communication systems ofnew generation, such as transistors, magnetic field sensors, magnetic storageand information processing devices, microwave elements, solar cells, displays,organic LED panels, microfuel cells, etc. In this field, he has more than 100scientific publications and 5 patents. He is a Scientific Supervisor of the nationalprograms such as “Nanoelectronics and Microsistems,” “Electronics,” and “In-formation Technologies.”


Vadim A. Bogush received the M.S. and Ph.D. de-grees in radio engineering and material science andthe degree of Doctor of Sciences in microelectronicstechnologies from the Belarusian State University ofInformatics and Radioelectronics, Minsk, Belarus, in1997, 2000, and 2007, respectively.

He is the Deputy Chairman of the National Statis-tical Committee of the Republic of Belarus. Prior tothis, he was an Associate Professor and is the Head ofthe Department of the Belarusian State University ofInformatics and Radioelectronics, and the Vicerector

of the Academy of Public Administration under the aegis of the President of theRepublic of Belarus. He has also been a Postdoctoral Fellow at the Departmentof Physical electronics, Tel Aviv University, Tel Aviv, Israel, since 2001. Hehas published more than 120 scientific papers in microelectronics technology,computer science, and information security. His main research interests includeelectroless processing for nanostructure fabrication, development of compositematerials for electromagnetic compatibility and information security applica-tions, and statistical data processing and information systems.

Dr. Bogush was a member of the Electrochemical Society in 2002. He alsotook part in the organization of several international conferences on informationsecurity and telecommunication systems during 1997–2007.

Alena L. Prudnikava received the M.S. degreein electronic engineering and the Ph.D. degree inphysics from the Belarusian State University ofInformatics and Radioelectronics (BSUIR), Minsk,Belarus, in 2006 and 2011, respectively.

She is a Researcher with the Department ofMicro- and Nanoelectronics, Laboratory of Inte-grated Micro- and Nanosystems, BSUIR. She hasdone research on CNTs synthesis by CVD, CNTfield emitters, CNT graphite nanolayer structures, andmagnetic and shielding properties of CNTs.

Boris G. Shulitski received the Graduate degree fromthe Faculty of Physics, Department of Physical Op-tics, Belarusian State University, Minsk, Belarus, in1989.

From 1989 to 1990, he worked as an Engineerat the Institute of Solid State and SemiconductorPhysics of the National Academy of Sciences ofBelarus, and from 1990 to 2001 as a Researcher at theResearch Institute of Applied Physical Problems. Heis currently a Senior Researcher with the Laboratoryof Integrated Micro- and Nanosystems, Department

of Micro- and Nanoelectronics, BSUIR. His main scientific activities are relatedto technology and nanomaterials in renewable energy sources. He is a Scien-tific advisor of five projects devoted to this subject, such as development andinvestigations of carbon nanotubes arrays and molecular crystallites for com-mercialization in the sphere of nanotransistors, field effect displays, solar cells,gas-, chemi-, biosensors and fuel cells. He is the author of more than 50 publi-cations, some of which are devoted to the philosophical aspects of nanoscience.

Ivan V. Komissarov received the M.S. degree fromthe Donetsk State University, Donetsk, Ukraine, andthe Ph.D. degree from the Institute of Physics, Pol-ish Academy of Science, Warsaw, Poland, both inphysics, in 1997 and 2004, respectively.

From 2005 to 2006, he was a Postdoctoral Re-searcher in the Laboratory for Laser Energetics, Uni-versity of Rochester, NY. From 2006 to 2008, he wasa Postdoctoral Researcher at Leiden University, Lei-den, The Netherlands. He is currently a Research Fel-low at the Belarusian State University of Informatics

and Radioelectronics, Minsk, Belarus. His current research interest includes thedevelopment of CNT synthesis technology for microwave and field emissionapplications.

Alexander S. Basaev received the Ph.D. degreein physics and mathematics from the Faculty ofPhysics and Technology, Moscow Institute of Elec-tronic Technology (MIET), Moscow, Russia, in 1979.

From 1979 to 1990, he was a Research Assistant,Senior Engineer, and Head of the CAD Laboratoryin MIET. In 1990, he joined the State Research Cen-tre of Russia “SMC Technological Centre of MIET”(SMC TC) as a Head of the CAD Department. Since1994, he has been a Deputy Director of SMC TCand the Head of the research Laboratory “Integrated

Technologies” in MIET. He is currently responsible for the Strategic Relationsof SMC TC and is a member of the MIET management board. He is a ScientificSupervisor of several projects funded by the Russian Federal Agency on Scienceand Innovation. His main scientific research interests include computer-aideddesign, nanomaterials, and nanodimensional elements for IC and MEMS.

Beng Kang Tay (M’10) received the B.Eng. (Hons.)and M.Sc. degrees from the National University ofSingapore, Singapore, in 1985 and 1989, respectively.He received the Ph.D. degree in electrical and elec-tronic engineering from the Nanyang TechnologicalUniversity (NTU), Singapore, in 1999.

He is currently a Professor and Associate Dean(Research) at the College of Engineering, NTU. Hisresearch works in plasma processing of materialsspan over 15 years and have resulted in more than300 publications. He is the holder of nine U.S. patents

based on filtered cathodic vacuum arc technology.Dr. Tay and coworkers are the recipients of the ASEAN Outstanding En-

gineering Award and the National Technology Award, Singapore, in 1997 and2000, respectively, for outstanding and pioneering R&D contributions on a newfiltered cathodic vacuum arc technology. His team also received the 2007 IES(the Institution of Engineers Singapore) Prestigious Engineering AchievementAwards for their work in nanoengineered carbon hybrid systems.

Maziar Shakerzadeh received the Bachelor degreein materials science and engineering from Shiraz Uni-versity, Shiraz, Iran, in 2006, and is currently workingtoward the Ph.D. degree from Nanyang Technologi-cal University (NTU), Singapore.

He is currently pursuing his research in Data Stor-age Institute (DSI), a research institute of Agencyfor Science, Technology and Research (A∗STAR),Singapore, on mechanical properties and thermal sta-bility of nanostructures and ultrathin films. Prior tothat, he was with NTU as a Project Officer where he

studied the graphitization of amorphous carbon thin films. His research interestsinclude fabrication, structural characterization, and properties of carbon-basednanostructures and thin films.

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