Thermal Conductivity and Electromagnetic Interference (EMI)

polymers

Article Thermal Conductivity and Electromagnetic Interference (EMI) Absorbing Properties of Composite Sheets Composed of Dry Processed Core–Shell Structured Fillers and Silicone Polymers

Hyun-Seok Choi 1,2,*, Ji-Won Park 3 , Kyung-Sub Lee 4, Sang-Woo Kim 5 and Su-Jeong Suh 1,*

1 Advanced Materials Science and Engineering, Sung Kyun Kwan University, Suwon 16419, Korea 2 R&D Center, SMT Co., Ltd., Suwon 16643, Korea 3 R&D Center, JB Lab. Coporation, Seoul 08826, Korea; [email protected] 4 R&D Center, Nopion Corporation, Suwon 16336, Korea; [email protected] 5 Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Korea; [email protected] * Correspondence: [email protected] (H.-S.C.); [email protected] (S.-J.S.); Tel.: +82-10-2425-7551 (H.-S.C.) Received: 15 September 2020; Accepted: 6 October 2020; Published: 10 October 2020

Abstract: This paper proposes dual-functional sheets (DFSs) that simultaneously have high thermal conductivity (TC) and electromagnetic interference (EMI) absorbing properties, making them suitable for use in mobile electronics. By adopting a simple but highly efficient dry process for manufacturing core–shell structured fillers (CSSFs) and formulating a close-packed filler composition, the DFSs show 1 1 high performance, TC of 5.1 W m− K− , and a 4 dB inter-decoupling ratio (IDR) at a 1 GHz frequency. − 1 Especially, the DFSs show a high dielectric breakdown voltage (BDV) of 3 kV mm− , which is beneficial for application in most electronic devices. The DFSs consist of two kinds of CSSFs that are blended in accordance with the close-packing rule, Horsfield’s packing model, and with polydimethylsiloxane (PDMS) polymers. The core materials are soft magnetic Fe-12.5%Cr and Fe-6.5%Si alloy powders of different sizes, and Al2O3 ceramic powders of a 1-µm diameter are used as the shell material. The high performance of the DFS is supposed to originate from the thick and stable shell layer and the maximized filler loading capability owing to the close-packed structure.

Keywords: dual-functional sheet; thermal conductivity; inter-decoupling ratio; core–shell structured fillers; dielectric breakdown voltage

1. Introduction Digital mobile electronics have dramatically evolved since their appearance in the late 1990s. With the evolution of mobile electronics, plenty of subsidiary technical challenges have been solved. Especially, thermal and electromagnetic interference (EMI) issues have become critical hurdles that decide the ﬁnal product availability for market launches. In many cases, thermal- and EMI-related problems are generated in an electronic device simultaneously [1–4]. To solve these issues, thermal interface materials (TIMs), EMI shielding materials, and EMI absorbers have been used separately to date. EMI absorbers should be used to reduce the secondary reﬂection electromagnetic waves that are generated from EMI shielding materials like EMI shield cans or EMI shielding coatings or gaskets. Unlike EMI shielding materials, TIMs and EMI absorbers are generally found in the form of sheets or pads. Nowadays, because there is not enough space to apply TIMs and EMI absorbers separately in recent slim mobile electronic devices, such as smartphones, tablet PCs, or laptop PCs,

Polymers 2020, 12, 2318; doi:10.3390/polym12102318 www.mdpi.com/journal/polymers PolymersPolymers2020 2020, 12, 12, 2318, x FOR PEER REVIEW 2 ofof 1112

recent slim mobile electronic devices, such as smartphones, tablet PCs, or laptop PCs, many manyresearchers researchers have havebeen been working working on ondeveloping developing soluti solutionsons to tomeet meet the the existing existing industrial industrial needsneeds viavia usingusing a a combined combined TIM TIM and and EMI EMI absorbing absorbing material material as as a a single single solution solution [ 1[1,5].,5]. SomeSome researchers researchers have have attempted attempted to to achieve achieve both both thermal thermal conductivity conductivity (TC) (TC) and and EMI EMI absorbing absorbing propertiesproperties atat the the same same time time by by applying applying a a blended blended filler filler system system composed composed of of thermally thermally conductive conductive ceramicceramic powderspowders andand magneticmagnetic powderspowders [[6];6]; however,however, itit is is di difficultfficult to to maximize maximize each each property property with with aa blendedblended fillerfiller system because because there there is is a alimit limit to tothe the filler filler loading loading capability. capability. Some Some researchers researchers have havetried triedto develop to develop core–shell core–shell structured structured fillers fillers(CSSFs) (CSSFs) to enhance to enhance the TC theby TCreducing by reducing the thermal the thermalcontact contactresistance, resistance, and these and attempts these attempts have been have carried been out carried via conventional out via conventional wet processes wet processes with high withtemperature high temperature reduction reduction reactions reactions [6–8]; [howe6–8];ver, however, wet wetprocesses processes are are not not practical practical forfor commercializationcommercialization becausebecause of of the the inherent inherent low low productivity, productivity, as aswell well as the as theenvironmental environmental issues issues from froman industrial an industrial perspective. perspective. On Onthe thecontrary, contrary, dry dry processes processes for for manufacturing manufacturing CSSFs CSSFs are relativelyrelatively simplesimple and and stable stable thick thick shell shell layers layers can can be be formed formed in in a a short short period period of of time time [ 9[9,10].,10]. DryDry processes processes for for manufacturing manufacturing CSSFs CSSFs have have rarely rarely been researched been researched via comparison via comparison with wet processes, with wet typicallyprocesses, because typically of a because lack of concepts of a lack or of highly concepts efficient or highly equipment. efficient Henschel equipment. mixers Henschel and super mixers mixers and havesuper been mixers widely have used been as dry widely processes used for as surfacedry proc coatingesses orfor the surface modification coating of or various the modification types of fillers; of however,various types in order of fillers; to manufacture however, in stable order and to manu uniformfacture shell stable layers and of uniform CSSFs, anothershell layers effective of CSSFs, and sophisticatedanother effective dry processand sophisticated needs to be dry applied. process This needs is because to be softapplied. magnetic This materials,is because as soft core magnetic bodies, arematerials, usually heavyas core ferrous bodies, alloy are metals usually in general,heavy ferrous and aluminum alloy metals oxide in as general, a shell layer andalso aluminum has a high oxide density. as a Ashell usual layer Henschel also has mixer a high or super density. mixer A is usual not powerful Henschel enough mixer to or circulate super mixer heavy is metals not powerful or ceramic enough powders to fromcirculate the bottom heavy to metals the top or of aceramic vessel. Therefore,powders afrom specially the bottom designed to impeller the top structure of a vessel. should Therefore, be applied, a andspecially the revolving designed speed impeller should structure also be increasedshould be to applied, an extremely and highthe revolving level. Recently, speed some should traditional also be mixerincreased makers to an have extremely developed high state-of-the-art level. Recently, technologies some traditional and equipment mixer make for manufacturingrs have developed metal- state- and ceramic-basedof-the-art technologies CSSFs. and equipment for manufacturing metal- and ceramic-based CSSFs. InIn thisthis work,work, twotwo sizes of CSSFs CSSFs are are manufactured manufactured by by an an effective effective dry dry process process and and blended blended in inaccordance accordance with with Horsfield’s Horsfield’s close-packing close-packing model model in in order order to to achieve TC andand EMIEMI absorbingabsorbing propertiesproperties atat high high levels. levels. SomeSome studiesstudies ofof dual-functionaldual-functional sheetssheets (DFSs)(DFSs) havehave beenbeen conductedconducted withwith unimodalunimodal CSSFs;CSSFs; however,however, inin orderorder toto achieveachieve thethe close-packingclose-packing structure,structure, itit isis necessarynecessary toto useuse aa bimodalbimodal filler filler system system [11 [11,12].,12]. There There have have been been few reportsfew reports of using of bimodalusing bimodal or trimodal or trimodal CSSF systems. CSSF Itsystems. is especially It is especially rare to find rare studies to find about studies DFSs about that featureDFSs that simultaneous feature simultaneous TC and EMI TC absorbing and EMI propertiesabsorbing whenproperties using when bimodal using CSSFs. bimodal Figure CSSFs.1 shows Figure concepts 1 shows of concepts DFSs made of DFSs of three made types of three of fillertypes systems, of filler namelysystems, simple namely blended simple filler blended systems, filler unimodalsystems, unimod fillers, andal fillers, bimodal and fillers. bimodal fillers.

FigureFigure 1. 1.Concepts Concepts of of three three kinds kinds of dual-functional of dual-functional sheets sheets (DFSs) (DFSs) made ofmade simple of blendedsimple fillerblended systems, filler unimodal core–shell structured fillers (CSSFs), and bimodal CSSFs. systems, unimodal core–shell structured fillers (CSSFs), and bimodal CSSFs.

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2. Materials and Methods 2. Materials and Methods Spherical Fe-12.5%Cr alloy powders (D50 = 50 μm) (410L, Höganäs, Belgium) were used as the Spherical Fe-12.5%Cr alloy powders (D50 = 50 µm) (410L, Höganäs, Belgium) were used as the core material for large size fillers and semi-spherical Fe-6.5%Si powders (D50 = 15 μm) (Mega Flux®, core material for large size fillers and semi-spherical Fe-6.5%Si powders (D = 15 µm) (Mega Flux®, Changsung Corp., Incheon, Korea) were used as the core material of small50 size fillers. The shell Changsung Corp., Incheon, Korea) were used as the core material of small size fillers. The shell material material used was aluminum oxide powder (D50 = 1 μm) (DAW-01, Denka Company Limited, Omuta, used was aluminum oxide powder (D = 1 µm) (DAW-01, Denka Company Limited, Omuta, Japan). Japan). The core–shell structured fillers50 (CSSFs) were manufactured by the dry process equipment The core–shell structured fillers (CSSFs) were manufactured by the dry process equipment brand-named brand-named COMPOSI (MP5 model, Nippon Coke & Engineering Co. Ltd., Tochigi, Japan). Surface COMPOSI (MP5 model, Nippon Coke & Engineering Co. Ltd., Tochigi, Japan). Surface pre-treatment of pre-treatment of the Fe-12.5%Cr and Fe-6.5%Si core materials was not needed for forming strongly the Fe-12.5%Cr and Fe-6.5%Si core materials was not needed for forming strongly bonded shell layers due bonded shell layers due to the high shear stress between the rotor, stator, or chamber wall, where the toso-called the high mechanochemical shear stress between effect the rotor,at the stator, surface or chamberof the core wall, powders where the and so-called the surface mechanochemical was highly effectactivated at the [13,14]. surface The of thestructure core powders of a rotor and and the surfacestator in was the highly dry process activated equipment [13,14]. The and structure the particle of a rotormoving and paths stator are in the shown dry process in Figure equipment 2. The andcore theand particle shell powders moving paths were are poured shown into in Figure the chamber2. The core of andthe mixer shell powders at the same were time. poured The into chamber the chamber volume of the was mixer 1.6 L at and the samethe quantities time. The of chamber the core volume and shell was 1.6powders L and thewere quantities 800 and of42 the g, corerespectively. and shell Next, powders the werecore and 800 andshell 42 powders g, respectively. were milled Next, theat the core shear and shellrate of powders 8000 rpm were for milled approximately at the shear 8–60 rate ofmin. 8000 Th rpme ambient for approximately gas was air 8–60 and min. the Thetemperature ambient gas inside was airthe andchamber the temperature rose to 80 °C inside during the chamberprocessing. rose to 80 ◦C during processing.

Figure 2. Schematic ofof thethe impellorimpellorstructure structure of of the the dry dry process process equipment equipment (COMPOSI, (COMPOSI, brand brand name name of Nipponof Nippon Coke Coke & Engineering’s& Engineering’s super super mixer) mixe andr) and the the moving moving paths paths of particles.of particles.

The particle particle size size distribution distribution (PSD) (PSD) was was analyzed analyzed by a particle by a particle size analyzer size analyzer(Cilas920, (Cilas920, Malvern MalvernInstruments, Instruments, Malvern, Malvern, UK). The UK). microstructure The microstructure and atomic and atomic compositions compositions of CSSFs of CSSFs were were analyzed analyzed by bya planar a planar and and cross-sectional cross-sectional scanningscanning electron electron microscope microscope (SEM) (SEM) with awith focused a focused ion beam ion (FIB) beam technique (FIB) (XL-30technique FEG, (XL-30 Philips FEG, FEI andPhilips 3D FEGFEI and FEI Quanta,3D FEG Hillsboro,FEI Quanta, OR, Hillsboro, USA), and OR, additionally USA), and a highadditionally resolution a transmissionhigh resolution electron transmission microscope electron (TEM, Tecnai micros G2-F20,cope FEI)(TEM, and Tecnai an energy G2-F20, dispersive FEI) spectrometer and an energy (EDS) (EDAXdispersive PV9761, spectrometer AMETEK, (EDS) Berwyn, (EDAX PA, USA).PV9761, AMETEK, Berwyn, PA, USA). Two manufacturedmanufactured CSSFsCSSFs werewere blendedblended inin accordanceaccordance with Horsfield’sHorsfield’s mixingmixing model and were dispersed inin addition-curableaddition-curable polydimethylsiloxane polydimethylsiloxane (PDMS) (PDMS) polymers polymers (HR-G500A (HR-G500A/B,/B, HRS HRS Silicone Silicone Co., Ltd.,Co., Ltd., Pyeongtaek, Pyeongtaek, Korea). Korea). The compositionsThe compositions were designedwere designed with bimodal with bimodal CSSFs, CSSFs, unimodal unimodal CSSFs, andCSSFs, a simply and a simply blended blended filler system, filler system, as shown as sh inown Table in1 Table. The 1. compounds The compounds were dispersedwere dispersed well bywell a Thinkyby a Thinky mixer mixer (ARV-310P, (ARV-310P, Thinky Thinky Corpoaration, Corpoaration, Tokyo, Tokyo, Japan) andJapan) were and then were processed then processed to produce to sheetsproduce of sheets a 3-mm of thickness a 3-mm thickness by a typical by hot-pressa typical hot-press method. Themethod. curing The condition curing condition of the PDMS of the polymers PDMS waspolymers 150 ◦ Cwas for 150 30 °C min. for The30 min. thermal The thermal conductivity conduc (TC)tivity of (TC) the curedof the cured specimen specimen was measured was measured by a modifiedby a modified transient transient plane sourceplane source method method (TCi thermal (TCi thermal analyzer, analyzer, C-Therm, C-Therm, Fredericton, Fredericton, NB, Canada) NB, andCanada) the inter-decouplingand the inter-decoupling ratio (IDR), ratio as (IDR), the EMI as absorbingthe EMI absorbing property, property, was evaluated was evaluated by a network by a analyzernetwork (PNAanalyzer 8364A, (PNA Keysight, 8364A, Keysight, Santa Rosa, Santa CA, Rosa, USA). CA, The USA). breakdown The breakdown voltage (BDV) voltage was (BDV) measured was bymeasured a Hipot by tester a Hipot (TOS5101, tester (TOS5101, Kikusui, Kobayashi, Kikusui, Ko Japan)bayashi, at cut-o Japan)ff current at cut-off of 10current mA. of 10 mA.

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Table 1. Compositions of three kinds of dual-functional sheets (DFSs) and their porosity values. PDMS: Polydimethylsiloxane.

Composition (wt %) 1 Sample Filler Filler Porosity Fe-12.5%Cr Fe-6.5%Si Al2O3 PDMS Total wt % vol % (%) BM1 56.81 30.59 4.60 8 100 92 63.27 0.08 BM2 57.43 30.92 4.65 7 100 93 66.56 0.10 BM3 58.05 31.25 4.70 6 100 94 70.12 0.09 Bimodal CSSFs BM4 58.66 31.59 4.75 5 100 95 73.99 0.12 BM5 59.28 31.92 4.80 4 100 96 78.24 0.15 BM6 59.90 32.25 4.85 3 100 97 82.89 0.28 UM1 87.40 - 4.60 8 100 92 62.01 0.13 UM2 88.35 - 4.65 7 100 93 65.34 0.15 UM3 89.30 - 4.70 6 100 94 68.92 0.16 Unimodal CSSFs UM4 90.25 - 4.75 5 100 95 72.95 0.24 UM5 91.20 - 4.80 4 100 96 77.30 0.28 UM6 92.15 - 4.85 3 100 97 82.11 0.35 SB1 87.40 - 4.60 8 100 92 54.97 0.14 Simple blended SB2 88.35 - 4.65 7 100 93 58.11 0.16 filler system SB3 89.30 - 4.70 6 100 94 61.56 0.23 1 Porosity is defined as the difference between theoretically calculated and real measured densities.

3. Results and Discussion

3.1. Characterization of Core–Shell Structured Fillers (CSSFs) Figure3 shows the particle size distribution (PSD) analysis results of the Fe-12.5%Cr CSSFs with the milling time. As the milling time increased, the proportion of ﬁne aluminum oxide powders with an average diameter of about 1 µm gradually disappeared. This indicates that most of the aluminum oxide powders are attached onto the Fe-12.5%Cr core surfaces. The small quantity of aluminum oxide residue, shown as a tail in the PSD graphs, can act as a close-packing intermediate and help increase the breakdown voltage (BDV) of the ﬁnal dual-functional sheets (DFSs). As can be seen in Figure3, the average diameter value of the CSSFs is not greatly changed because most of the aluminum oxide particles are embedded into the core surfaces and intermixed layers of core and shell materials are formed. These intermixed layers can be observed by a cross-sectional transmission Polymerselectron 2020, 12 microscope, x FOR PEER (TEM).REVIEW 5 of 12

Figure 3. Particle size distribution (PSD) analysis of CSSFs with milling time. Figure 3. Particle size distribution (PSD) analysis of CSSFs with milling time.

Figure 4 shows planar images of the Fe-12.5%Cr and Al2O3 raw materials and cross-sectional images of Fe-12.5%Cr CSSFs with milling time, where the results were found via a scanning electron microscope (SEM) with a focused ion beam (FIB). The thickness of the shell layer increases up until 30 min, and after 30 min the thickness of the shell layer does not change much. It can be considered that once stable shell layers form, additional aluminum oxide powders cannot attach onto the shell surface because the aluminum oxide itself is a stable material and it is not likely that it will bond together via simple mechanical stress alone [15]. There are only hydroxyl (-OH) functional groups on the surfaces of the aluminum oxide particles, and these are not sufficient to act as bonding intermediates. The thickness of the shell layer is approximately ~1–3 μm at the milling time of 30 min, and this is considered to be enough to act as an electrical insulation layer for the BDV property of DFSs. Based on Figures 3 and 4, a milling time of 30 min can be decided as the optimum milling time.

Figure 4. Planar scanning electron microscope (SEM) images of (a) Fe-12.5%Cr core powders, (b) aluminum oxide shell powders, and (c–f) cross-sectional SEM back-scattered images of Fe-12.5%Cr CSSFs with milling time.

The final shapes of Fe-12.5%Cr CSSFs and Fe-6.5%Si CSSFs milled for 30 min by the dry process are shown in Figure 5, displaying planar and cross-sectional SEM images. The surfaces of the CSSFs

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Polymers 2020, 12, 2318 5 of 11 Figure 3. Particle size distribution (PSD) analysis of CSSFs with milling time.

FigureFigure4 4shows shows planar planar images images of of the the Fe-12.5%Cr Fe-12.5%Cr and and Al Al2O2O33raw raw materialsmaterials andandcross-sectional cross-sectional imagesimages of of Fe-12.5%Cr Fe-12.5%Cr CSSFs CSSFs with with milling milling time,time, wherewhere the the results results were were found found via via a a scanning scanning electron electron microscopemicroscope (SEM)(SEM) with a a focused focused ion ion beam beam (FIB). (FIB). The The thickness thickness of the of theshell shell layer layer increases increases up until up until30 min, 30 and min, after and 30 after min 30 the min thickness the thickness of the shell of the layer shell does layer notdoes change not much. change It can much. be considered It can be consideredthat once stable that once shell stable layers shell form, layers additional form, additional aluminum aluminum oxide powders oxide powderscannot attach cannot onto attach the ontoshell thesurface shell surfacebecause because the aluminum the aluminum oxide oxideitself itselfis a stable is a stable material material and and it is it isnot not likely likely that that it it will bondbond togethertogether via via simple simple mechanical mechanical stress stress alone alone [ 15[15].]. ThereThere areare onlyonly hydroxylhydroxyl (-OH)(-OH) functionalfunctional groupsgroups onon thethe surfaces surfaces of theof the aluminum aluminum oxide oxide particles, particles, and these and are these not su areﬃcient not tosufficient act as bonding to act intermediates. as bonding Theintermediates. thickness of The the thickness shell layer of the is approximately shell layer is approximately ~1–3 µm at the ~1–3 milling μm at time the milling of 30 min, time and of 30 this min, is consideredand this is toconsidered be enough to to be act enough as an electrical to act as insulation an electrical layer insulation for the BDV layer property for the of BDV DFSs. property Based on of FiguresDFSs. Based3 and4 on, a Figures milling 3 time and of 4, 30a milling min can time be decidedof 30 min as can the be optimum decided milling as the optimum time. milling time.

FigureFigure 4.4. PlanarPlanar scanning scanning electron electron microscope microscope (SEM) (SEM) images images of of(a) ( aFe-12.5%Cr) Fe-12.5%Cr core core powders, powders, (b) (baluminum) aluminum oxide oxide shell shell powders, powders, and and (c (–cf–)f )cross-sectional cross-sectional SEM SEM back-scattered back-scattered images ofof Fe-12.5%CrFe-12.5%Cr CSSFsCSSFs with with milling milling time. time. The final shapes of Fe-12.5%Cr CSSFs and Fe-6.5%Si CSSFs milled for 30 min by the dry process The final shapes of Fe-12.5%Cr CSSFs and Fe-6.5%Si CSSFs milled for 30 min by the dry process are shown in Figure5, displaying planar and cross-sectional SEM images. The surfaces of the CSSFs are are shown in Figure 5, displaying planar and cross-sectional SEM images. The surfaces of the CSSFs reasonably smooth in terms of compounding in PDMS polymers, and the shell layer thicknesses of the Fe-12.5%Cr CSSFs and Fe-6.5%Si CSSFs are 2.5 and 0.5 µm, respectively. The difference of thicknesses between two CSSFs is originated from the difference of average momentum (=mass velocity) between × Fe-12.5%Cr and Fe-6.5%Si particles when they collide with aluminum oxide particles, because their density is 7.78 and 7.48 g/cm3 and, accordingly, their average mass (=volume density) is 5.09 10 8 × × − and 1.32 10 9 g, respectively. Each shell layer of the two types of CSSFs has a dense cross-sectional × − microstructure and can function as an electrical barrier to withstand high voltages. As shown in Figure6, the cross-sectional TEM and EDS analysis of Fe-12.5%Cr CSSFs shows that there are areas where atoms from both the core and shell layers coexist. By EDS dark field spot analysis (Figure6a,d–f) it can be seen that there is an intermixed layer (area 2) and that the core materials are embedded in the shell layers (area 3). A matrix of Al2O3 shell particles surrounds the core materials in the shell layers. Because of the intermixed layers, a kind of interlocking effect can sustain bonding between the core and shell layers. The thickness of intermixed layers can be analyzed by the EDS line profile method (Figure6b,c). The thickness of intermixed layers here is about 20–30 nm, which is naturally affected by the original particle size of the shell layers. In the shell layers, small amounts of the core materials (Fe, Cr) were detected by cross-sectional selected area electron diffraction (SAED) analysis (Figure7). The dim di ffraction pattern of Fe-12.5%Cr elements and the bright pattern of Al2O3 means that there are intermediate Fe-12.5%Cr core materials with the Al2O3 shell particles. Polymers 2020, 12, x FOR PEER REVIEW 6 of 12

are reasonably smooth in terms of compounding in PDMS polymers, and the shell layer thicknesses of the Fe-12.5%Cr CSSFs and Fe-6.5%Si CSSFs are 2.5 and 0.5 μm, respectively. The difference of thicknesses between two CSSFs is originated from the difference of average momentum (= mass × Polymers 2020, 12, 2318 6 of 11 velocity) between Fe-12.5%Cr and Fe-6.5%Si particles when they collide with aluminum oxide particles, because their density is 7.78 and 7.48 g/cm3 and, accordingly, their average mass (= volume −8 −9 This metallic× density)component is 5.09 × 10 and could 1.32 be × 10 act g, as respectively. an inorganic Each binder shell layer for interlocking of the two types core of CSSFs and shell has particles, a dense cross-sectional microstructure and can function as an electrical barrier to withstand high as conﬁrmedvoltages. by the high resolution TEM images here.

FigurePolymersFigure 5. Shapes2020 5. ,Shapes 12, x of FOR (ofa) PEER( Fe-12.5%Cra) Fe-12.5%Cr REVIEW CSSFs CSSFs and and (b(b)) Fe-6.5%SiFe-6.5%Si CSSFs CSSFs milled milled for for 30 30min min (found (found via viaa 7 a of planar 12 Polymers 2020, 12, x FOR PEER REVIEW 7 of 12 SEM)planar and theSEM) thickness and the thickness of shell of layers shell layers (found (found by a by cross-sectional a cross-sectional SEM). SEM).

As shown in Figure 6, the cross-sectional TEM and EDS analysis of Fe-12.5%Cr CSSFs shows that there are areas where atoms from both the core and shell layers coexist. By EDS dark field spot analysis (Figure 6a,d,e,f) it can be seen that there is an intermixed layer (area 2) and that the core materials are embedded in the shell layers (area 3). A matrix of Al2O3 shell particles surrounds the core materials in the shell layers. Because of the intermixed layers, a kind of interlocking effect can sustain bonding between the core and shell layers. The thickness of intermixed layers can be analyzed by the EDS line profile method (Figure 6b,c). The thickness of intermixed layers here is about 20–30 nm, which is naturally affected by the original particle size of the shell layers. In the shell layers, small amounts of the core materials (Fe, Cr) were detected by cross-sectional selected area electron diffraction (SAED) analysis (Figure 7). The dim diffraction pattern of Fe-12.5%Cr elements and the bright pattern of Al2O3 means that there are intermediate Fe-12.5%Cr core materials with the Al2O3 shell particles. This metallic component could be act as an inorganic binder for interlocking core and shell particles, as confirmed by the high resolution TEM images here.

Figure 6.Figure(a) Cross-sectional 6. (a) Cross-sectional TEM TEM image, image, ( b(b,c,c)) EDSEDS line line profile proﬁle of ofareas areas 1, 2,1, 3, 2,and 3, ( andd–f) EDS (d–f atomic) EDS atomic Figure 6. (a) Cross-sectional TEM image, (b,c) EDS line profile of areas 1, 2, 3, and (d–f) EDS atomic compositioncomposition analysis analysis of a Fe-12.5%Crof a Fe-12.5%Cr CSSF CSSF at at areas areas 1, 2, 2, 3. 3. composition analysis of a Fe-12.5%Cr CSSF at areas 1, 2, 3.

FigureFigure 7. Cross-sectional 7. Cross-sectional selected selected area area electron electron diffrac diﬀractiontion (SAED) (SAED) analysis analysis of a Fe-12.5%Cr of a Fe-12.5%Cr CSSF. CSSF. Figure 7. Cross-sectional selected area electron diffraction (SAED) analysis of a Fe-12.5%Cr CSSF. 3.2. Characterization of Dual-Functional Sheets (DFSs) 3.2. Characterization of Dual-Functional Sheets (DFSs) 3.2.1. Thermal Conductivity (TC) 3.2.1. Thermal Conductivity (TC) Figure 8 shows the TC values of DFSs made from bimodal CSSFs, unimodal CSSFs, and a simple blendedFigure filler 8 shows system. the The TC TCvalues values of DFSs of the made Fe-12.5% fromCr bimodal and Fe-6.5%Si CSSFs, unimodal core materials CSSFs, were and 16 a simpleand 35 blendedW m−1K− filler1 here, system. respectively. The TC The values TC of of the the pure Fe-12.5% aluminumCr and oxide Fe-6.5%Si was 32 core W m materials−1 K−1 here. were The 16TC and values 35 Wof mDFSs−1K− 1made here, respectively.from bimodal The CSSFs TC ofwere the higherpure aluminum than those oxide of DFSs was made32 W m from−1 K− 1unimodal here. The CSSFsTC values or a ofsimple DFSs blendedmade from filler bimodal system. CSSFs The reason were higherfor the th highan those TC of of the DFSs bimodal made CSSFsfrom unimodal is the increase CSSFs in or the a simplevolume blended fraction filler of Fe-6.5%Si system. (aThe high reason TC material), for the high and TC the of total the fillerbimodal loading CSSFs capability is the increase also increases in the volumedue to the fraction close-packing of Fe-6.5%Si structure (a high of TC the material), two sizes and of CSSFs,the total which filler loadingis known capability as Horsfield’s also increases packing duemodel to the[16,17]. close-packing The optimal structure ratio of of a largerthe two sized sizes Fe-12.5%Cr of CSSFs, whichCSSF tois aknown smaller as sized Horsfield’s Fe-6.5%Si packing CSSF modelis 6.5 to [16,17]. 3.5, as The can optimalbe seen ratioin Figure of a larger8a. At sized this point, Fe-12.5%Cr the maximum CSSF to aTC smaller of 5.1 sizedW m− 1Fe-6.5%Si K−1 is achieved CSSF isand 6.5 this to 3.5, value as canis over be seen two in times Figure higher 8a. At than this that point, of the maximumsimple blended TC of filler 5.1 W system, m−1 K− 1as is shownachieved in andFigure this 8b. value In the is caseover of two a simple times blendedhigher than filler that syst ofem, the the simple limit of blended capable filler filler system, loading asis aroundshown in94 Figure 8b. In the case of a simple blended filler system, the limit of capable filler loading is around 94

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3.2. Characterization of Dual-Functional Sheets (DFSs)

3.2.1. Thermal Conductivity (TC) Figure8 shows the TC values of DFSs made from bimodal CSSFs, unimodal CSSFs, and a simple blended filler system. The TC values of the Fe-12.5%Cr and Fe-6.5%Si core materials were 16 and 1 1 1 1 35 W m − K− here, respectively. The TC of the pure aluminum oxide was 32 W m− K− here. The TC values of DFSs made from bimodal CSSFs were higher than those of DFSs made from unimodal CSSFs or a simple blended filler system. The reason for the high TC of the bimodal CSSFs is the increase in the volume fraction of Fe-6.5%Si (a high TC material), and the total filler loading capability also increases due to the close-packing structure of the two sizes of CSSFs, which is known as Horsfield’s packing model [16,17]. The optimal ratio of a larger sized Fe-12.5%Cr CSSF to a smaller sized Fe-6.5%Si CSSF is 6.5 to 3.5, as can be 1 1 seen in Figure8a. At this point, the maximum TC of 5.1 W m − K− is achieved and this value is over two times higher than that of the simple blended filler system, as shown in Figure8b. In the case of a simple blended filler system, the limit of capable filler loading is around 94 wt %; however, in the case of CSSF systems, the filler loading capability increases up to 97 wt %. There are also saturation points for the filler loading capability and TC for the two types of CSSFs. After the filler loading exceeds 96 wt %, the TC values of DFSs made of the two types of CSSFs become saturated, and even the TC of a DFS made of unimodal CSSFs decreases slightly. This phenomenon is mainly related to the dispersion qualities. At an extremely high filler loading content of 97 wt % (82 vol %), compound dispersibility is greatly affected by compounding process parameters, such as batch size, temperature, dispersion time, shear rate, and so forth, and a high-degree of techniques are required to achieve complete dispersion [18]. The dispersion property is evaluated by the porosity of cured DFSs, which is expressed as the difference between the theoretical density (designed density) and the real measured density of the DFSs, as shown in Table1. In case of unimodal CSSFs, the porosity of a DFS filled with a 97 wt % CSSF is larger than that of a 96 wt % CSSF. Therefore, it is reasonable to assume that the TC decreases in spite of the increased filler loading from 96 to 97 wt % in DFSs made of unimodal CSSFs. In the case of bimodal CSSFs, the TC increases slightly when the filler loading content increases from 96 to 97 wt %. A slight discrepancy in behaviors of varying TC can be detected between the two CSSFs systems. In the case of the unimodal CSSFs system, the saturation point of filler loading for the increase in TC is considered to be at 95 wt %, but in the case of the bimodal CSSFs system, the saturation point is thought to be at 96 wt %, as can be seen in Figure8b. The actual values of TC in the bimodal CSSFs system at 96 and 97 wt % are 5.08 and 5.10, respectively. Despite the porosity increase, the filler loading increase affects the increase in TC more strongly. Moreover, dispersion capability due to the close-packing structure of the bimodal CSSFs system also has a positive effect on the increase in TC at 97 wt %. The effect of porosity on TC values will be discussed specifically in future works. When calculated via the equation suggested by Pal et al. [19,20], the theoretical TC values of DFSs made of bimodal CSSFs are much higher than the measured values. It is considered that the reason for the large difference of these TC values is largely because the suggested theoretical models of TC estimation are usually limited to within a low filler loading area of under 60 vol %; however, the volume fraction of the maximum filler loading in this work is over 80 vol % and the deviation of the measured TCs from the theoretical values increases as the filler loading content increases. Therefore, new practical models that contain the effect of the naturally contained porosity during the compounding and sheet-making processes are needed to estimate more accurate TC values with high filler loading contents. This is out of the bounds of this paper and will be discussed in future works. Polymers 2020, 12, x FOR PEER REVIEW 8 of 12 wt %; however, in the case of CSSF systems, the filler loading capability increases up to 97 wt %. There are also saturation points for the filler loading capability and TC for the two types of CSSFs. After the filler loading exceeds 96 wt %, the TC values of DFSs made of the two types of CSSFs become saturated, and even the TC of a DFS made of unimodal CSSFs decreases slightly. This phenomenon is mainly related to the dispersion qualities. At an extremely high filler loading content of 97 wt % (82 vol %), compound dispersibility is greatly affected by compounding process parameters, such as batch size, temperature, dispersion time, shear rate, and so forth, and a high-degree of techniques are required to achieve complete dispersion [18]. The dispersion property is evaluated by the porosity of cured DFSs, which is expressed as the difference between the theoretical density (designed density) and the real measured density of the DFSs, as shown in Table 1. In case of unimodal CSSFs, the porosity of a DFS filled with a 97 wt % CSSF is larger than that of a 96 wt % CSSF. Therefore, it is reasonable to assume that the TC decreases in spite of the increased filler loading from 96 to 97 wt % in DFSs made of unimodal CSSFs. In the case of bimodal CSSFs, the TC increases slightly when the filler loading content increases from 96 to 97 wt %. A slight discrepancy in behaviors of varying TC can be detected between the two CSSFs systems. In the case of the unimodal CSSFs system, the saturation point of filler loading for the increase in TC is considered to be at 95 wt %, but in the case of the bimodal CSSFs system, the saturation point is thought to be at 96 wt %, as can be seen in Figure 8b. The actual values of TC in the bimodal CSSFs system at 96 and 97 wt % are 5.08 and 5.10, respectively. Despite the porosity increase, the filler loading increase affects the increase in TC more strongly. Moreover, dispersion capability due to the close-packing structure of the bimodal CSSFs system also has a positive effect on the increase in TC at 97 wt %. The effect of porosity on TC values Polymers 2020, 12, 2318 8 of 11 will be discussed specifically in future works.

Figure 8. (a) Thermal conductivities (TCs) of dual-functional sheets with a blending ratio of larger to smallerFigure sized8. (a) CSSFsThermal and conductivities the (b) comparison (TCs) ofof the dual-functional TCs of DFSs made sheets of with bimodal a blending CSSFs, unimodalratio of larger CSSFs, to andsmaller simple sized blended CSSFs filler and systems.the (b) comparison of the TCs of DFSs made of bimodal CSSFs, unimodal CSSFs, and simple blended filler systems. 3.2.2. Electromagnetic Interference (EMI) Absorbing Property When calculated via the equation suggested by Pal et al. [19,20], the theoretical TC values of In this work, the EMI absorbing property is represented by the inter-decoupling ratio (IDR, Rde), DFSs made of bimodal CSSFs are much higher than the measured values. It is considered that the which is measured by a micro-loop antenna method [21]. The IDR is defined as the difference between the reason for the large difference of these TC values is largely because the suggested theoretical models transmission S-parameter (S21) with and without an EMI absorber. The related equation is given as follows: of TC estimation are usually limited to within a low filler loading area of under 60 vol %; however, the volume fraction of the maximum filler loading in this work is over 80 vol % and the deviation of S21 (or Rde) = S21 Air S21 Sample [dB] (1) the measured TCs from the theoretical values increases− as the filler loading content increases. whereTherefore,S21 Air newand practicalS21 Sample modelsare that the contain S-parameters the effe ofct of a the transmitted naturally contained wave with porosity and without during the an EMIcompounding absorber, and respectively. sheet-making The processes EMI absorbing are needed property to estimate is also more improved accurate in TC the values case with of DFSs high madefiller loading of bimodal contents. CSSFs This when is out compared of the bounds with DFSs of this made paper of and unimodal will be CSSFs discussed or a in simple future blended works. filler system. In Figure9, at a frequency of 1 GHz, the DFS featuring bimodal CSSFs shows an IDR value of 4 dB, and this value is nearly twice those found with unimodal CSSFs or a simple − blended filler system. It is known that the IDR of a material is mainly related to the material’s relative permeability [21,22]. The aluminum oxides in the shell layers have only dielectric permittivity, so the permeability of the CSSFs relies on the magnetic property of the Fe-based core materials. In the case of a Fe-12.5%Cr core material, the initial permeability at a frequency of 1 kHz is 150 and that of Fe-6.5%Si is 100 [23,24]. By a simple average calculation, in spite of decreasing average permeability, the IDR of DFSs made of bimodal CSSFs is higher than that of unimodal CSSFs. It is considered that the close-packed structure of DFSs consisting of bimodal CSSFs shows high density and therefore low transmission. In the process of evaluating a single performance, it is possible to confirm the results that are superior to each evaluation item, but in the case of [25,26] a core-shell structure, to show a maximum of 2.2 W m 1 K 1 of TC and 23~ 40 dB of shielding effectiveness [27]. − − − − 3.2.3. Dielectric Breakdown Voltage (BDV) Most metal-cored CSSFs have a weak point of a low BDV for electronic applications because of an incompact shell structure and shallow thickness [28]; however, the densely intermixed shell structure and thick shell thickness of CSSFs manufactured by the dry process in this work enables the DFSs to show high BDV values. In the case of the simple blended filler systems here, the BDV values were measured with an alternating current (AC) at 250 V with a cut-off current of 10 mA. In the case of the unimodal CSSFs and bimodal CSSFs, the BDV values increase up to 2400 and 3200 V (AC) at the same cut-off current for each, respectively. The reasons for the difference of BDVs in the two CSSFs systems are mainly attributed to the porosity effect. The dielectric breakdown voltage of air is known to be 0.4~3.0 kV/mm depending on the conditions, and that of cured polydimethylsiloxane (PDMS) Polymers 2020, 12, x FOR PEER REVIEW 9 of 12

3.2.2. Electromagnetic Interference (EMI) Absorbing Property

In this work, the EMI absorbing property is represented by the inter-decoupling ratio (IDR, Rde), which is measured by a micro-loop antenna method [21]. The IDR is defined as the difference between the transmission S-parameter (S21) with and without an EMI absorber. The related equation is given as follows:

S21 (or Rde) = S21 Air – S21 Sample [dB] (1) where S21 Air and S21 Sample are the S-parameters of a transmitted wave with and without an EMI absorber, respectively. The EMI absorbing property is also improved in the case of DFSs made of Polymersbimodal2020 CSSFs, 12, 2318 when compared with DFSs made of unimodal CSSFs or a simple blended9 filler of 11 system. In Figure 9, at a frequency of 1 GHz, the DFS featuring bimodal CSSFs shows an IDR value of −4 dB, and this value is nearly twice those found with unimodal CSSFs or a simple blended filler polymer and aluminum oxide is 15 and 14 kV/mm, respectively. The densities of air, PDMS polymer, system. It is known that the IDR of a material is mainly related to the material’s relative permeability Fe-12.5%Cr CSSFs and Fe-6.5%Si CSSFs are 0.001, 0.98, 6.91 and 6.00 g/cm3, respectively. The densities [21,22]. The aluminum oxides in the shell layers have only dielectric permittivity, so the permeability of the two CSSFs are obtained from geometric calculation. The porosity of the unimodal CSSFs system of the CSSFs relies on the magnetic property of the Fe-based core materials. In the case of a Fe-12.5%Cr is 25% larger than that of the bimodal CSSFs system. It means that volume ratio of air in the unimodal core material, the initial permeability at a frequency of 1 kHz is 150 and that of Fe-6.5%Si is 100 [23,24]. CSSFs system is higher than in the bimodal CSSFs system. The density of air is thousands of times By a simple average calculation, in spite of decreasing average permeability, the IDR of DFSs made smaller than that of PDMS and core-shell fillers, and the effect of air on the BDV is much greater in the of bimodal CSSFs is higher than that of unimodal CSSFs. It is considered that the close-packed unimodal CSSFs system than in the bimodal CSSFs system because the BDV of air is lower compared structure of DFSs consisting of bimodal CSSFs shows high density and therefore low transmission. with other materials. These BDV values of over 1500 V (AC) mean that the DFSs made of CSSFs can be In the process of evaluating a single performance, it is possible to confirm the results that are superior utilized as conventional thermal interface materials (TIMs) and EMI noise suppressors for general to each evaluation item, but in the case of [25,26] a core-shell structure, to show a maximum of 2.2 W electronic applications. Figure 10 shows a comparison of the BDV values between the simple blended m−1 K−1 of TC and −23~−40 dB of shielding effectiveness [27]. filler systems and unimodal and bimodal CSSFs.

Polymers 2020, 12, x FOR PEER REVIEW 10 of 12 g/cm3, respectively. The densities of the two CSSFs are obtained from geometric calculation. The porosity of the unimodal CSSFs system is 25% larger than that of the bimodal CSSFs system. It means that volume ratio of air in the unimodal CSSFs system is higher than in the bimodal CSSFs system. The density of air is thousands of times smaller than that of PDMS and core-shell fillers, and the effect of air on the BDV is much greater in the unimodal CSSFs system than in the bimodal CSSFs system because the BDV of air is lower compared with other materials. These BDV values of over 1500 V (AC) mean that the DFSs made of CSSFs can be utilized as conventional thermal interface materials (TIMs) and EMI noise suppressors for general electronic applications. Figure 10 shows a comparison of theFigure BDV 9. valuesComparison between of the the inter-decoupling simple blended ratio fi (IDR)ller systems of DFSs made andof unimodal bimodal CSSFs, and bimodal unimodal CSSFs. CSSFs, andFigure simple 9. Comparison blended filler of systems. the inter-decoupling ratio (IDR) of DFSs made of bimodal CSSFs, unimodal CSSFs, and simple blended filler systems.

3.2.3. Dielectric Breakdown Voltage (BDV)

Most metal-cored CSSFs have a weak point of a low BDV for electronic applications because of an incompact shell structure and shallow thickness [28]; however, the densely intermixed shell structure and thick shell thickness of CSSFs manufactured by the dry process in this work enables the DFSs to show high BDV values. In the case of the simple blended filler systems here, the BDV values were measured with an alternating current (AC) at 250 V with a cut-off current of 10 mA. In the case of the unimodal CSSFs and bimodal CSSFs, the BDV values increase up to 2400 and 3200 V (AC) at the same cut-off current for each, respectively. The reasons for the difference of BDVs in the two CSSFs systems are mainly attributed to the porosity effect. The dielectric breakdown voltage of air is known to be 0.4~3.0 kV/mm depending on the conditions, and that of cured polydimethylsiloxane (PDMS) polymer and aluminum oxide is 15 and 14 kV/mm, respectively. The densitiesFigure of 10. air,Breakdown PDMS polymer, voltage (BDV)Fe-12.5%Cr of DFSs CSSFs made and of bimodal Fe-6.5%Si CSSFs, CSSFs unimodal are 0.001, CSSFs, 0.98, and 6.91 simple and 6.00 blended ﬁller systems. Figure 10. Breakdown voltage (BDV) of DFSs made of bimodal CSSFs, unimodal CSSFs, and simple 4. Conclusionsblended filler systems.

4. ConclusionsFor fabricating core–shell structured fillers, as raw materials for thermally conductive and EMI absorbing dual-functional sheets, a simple but highly efficient dry process that has economic and environmentalFor fabricating advantages core–shell compared structured to the fillers, conventional as raw materials wet processes for thermally was adopted conductive here. A and thermal EMI absorbing dual-functional1 sheets,1 a simple but highly efficient dry process that has economic and conductivity of 5.1 W m− K− was achieved in the case of the dual-functional sheets made of bimodal core–shellenvironmental structured advantages fillers, featuringcompared larger to the sized conventional Fe-12.5%Cr wet and processes smaller was sized adopted Fe-6.5%Si here. blended A thermal at a −1 −1 ratioconductivity of 6.5 to 3.5.of 5.1 The W inter-decoupling m K was achieved ratio, in as the an indexcase of of the the dual-functional EMI absorbing property,sheets made was of measured bimodal tocore–shell be 4 dB structured at a frequency fillers, of 1featuring GHz here. larger Especially, sized Fe-12.5%Cr by forming aand thick smaller and stable sized shell Fe-6.5%Si layer structure blended at a ratio− of 6.5 to 3.5. The inter-decoupling ratio, as an index of the EMI absorbing property, was1 for the core–shell structured fillers, the dielectric breakdown voltage was measured to be 3.2 kV mm− measured to be −4 dB at a frequency of 1 GHz here. Especially, by forming a thick and stable shell layer structure for the core–shell structured fillers, the dielectric breakdown voltage was measured to be 3.2 kV mm−1 and was much higher than that of the simple blended filler systems here. The three critical properties of high thermal conductivity, EMI absorption, and a high breakdown voltage allow beneficial application of the dual-functional sheets developed here in use with real electronic applications.

Funding: This research was funded by Ministry of Trade, Industry, and Energy (MOTIE) of the South Korean government (grant no. 10031944).

Conflicts of Interest: The authors declare no conflict of interest.

Polymers 2020, 12, 2318 10 of 11 and was much higher than that of the simple blended ﬁller systems here. The three critical properties of high thermal conductivity, EMI absorption, and a high breakdown voltage allow beneﬁcial application of the dual-functional sheets developed here in use with real electronic applications.

Author Contributions: Conceptualization, S.-J.S.; Formal analysis, S.-W.K. and K.-S.L.; Writing—original draft, H.-S.C.; Writing—review and editing, J.-W.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ministry of Trade, Industry, and Energy (MOTIE) of the South Korean government (grant no. 10031944). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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