Thermal Conductivity and Electromagnetic Interference (EMI)
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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