materials Article Structure and Thermal Expansion of Cu-90 vol. % Graphite Composites Andrej Opálek 1, * , Štefan Emmer 2 , Roman ˇ Ciˇ cka 3 , Nad’a Beronská 1 , Peter Oslanec, Jr. 1 and Jaroslav Ková ˇ cik 1   Citation: Opálek, A.; Emmer, Š.; ˇ Ciˇ cka, R.; Beronská, N.; Oslanec, P.J.; Kovᡠcik, J. Structure and Thermal Expansion of Cu−90 vol. % Graphite Composites. Materials 2021, 14, 7089. https://doi.org/10.3390/ma 14227089 Academic Editor: Dinara Sobola Received: 28 October 2021 Accepted: 17 November 2021 Published: 22 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Dúbravská Cesta 9, 845 13 Bratislava, Slovakia; nada.beronska@savba.sk (N.B.); peter.oslanec@savba.sk (P.O.J.); jaroslav.kovacik@savba.sk (J.K.) 2 IVMA STU, Vazovova 5, 812 43 Bratislava, Slovakia; stefan.emmer11@gmail.com 3 Institute of Materials Science, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology, Jána Bottu 25, 917 24 Trnava, Slovakia; roman.cicka@stuba.sk * Correspondence: andrej.opalek@savba.sk Abstract: Copper–graphite composites are promising functional materials exhibiting application potential in electrical equipment and heat exchangers, due to their lower expansion coefficient and high electrical and thermal conductivities. Here, copper–graphite composites with 10–90 vol. % graphite were prepared by hot isostatic pressing, and their microstructure and coefficient of thermal expansion (CTE) were experimentally examined. The CTE decreased with increasing graphite volume fraction, from 17.8 × 10 −6 K −1 for HIPed pure copper to 4.9 × 10 −6 K −1 for 90 vol. % graphite. In the HIPed pure copper, the presence of cuprous oxide was detected by SEM-EDS. In contrast, Cu–graphite composites contained only a very small amount of oxygen (OHN analysis). There was only one exception, the composite with 90 vol. % graphite contained around 1.8 wt. % water absorbed inside the structure. The internal stresses in the composites were released during the first heating cycle of the CTE measurement. The permanent prolongation and shape of CTE curves were strongly affected by composition. After the release of internal stresses, the CTE curves of composites did not change any further. Finally, the modified Schapery model, including anisotropy and the clustering of graphite, was used to model the dependence of CTE on graphite volume fraction. Modeling suggested that the clustering of graphite via van der Waals bonds (out of hexagonal plane) is the most critical parameter and significantly affects the microstructure and CTE of the Cu–graphite composites when more than 30 vol. % graphite is present. Keywords: metal matrix composites; hot isostatic pressing; thermal expansion; Schapery model 1. Introduction Electronic components in applications such as central processing units of comput- ers, phones, broadcast radio and television receivers, and other daily used appliances in households and workplaces suffer from severe overheating. Silicon (Si) chips represent the typical electronic component that is usually produced. This chip, in commercial devices, needs to be maintained in a stress-free condition. [1–3]. For heat dissipation from the chip, the installation of a heat sink must be considered. The material of a heat sink should have high thermal conductivity. However, on the other hand, this material has to avoid any stress. Materials in contact with the chip should have a coefficient of thermal expansion (CTE) close to 4 × 10 −6 K −1 , that of Si [4,5]. Developing a high-performance heat sink for the purpose mentioned above has led to increasing industry requests for highly efficient materials. These are heat-dissipating materials with faster heat removal and minimization of thermal stresses [4,5]. More rapid heat removal can be achieved by higher thermal conductivity (TC), and thermal stresses can be reduced by modifying the coefficient of thermal expansion (CTE). Compatibility Materials 2021, 14, 7089. https://doi.org/10.3390/ma14227089 https://www.mdpi.com/journal/materials