# The Influence of Copper Oxide Particle Size on the Properties of Epoxy Resin
## 3 Results and Discussion
### Thermal Conductivity  
Analyzing Table 4 and Figure 9, it can be seen that the addition of copper in the amount of 10% by weight has a positive effect on thermal conductivity. The greatest increase is noticeable for particles with a size of 40-60 nm. Taking into account the given graphs, it can be concluded that the increase is caused by the natural conductive properties of copper which, after reaching a given value, creates percolation paths, enabling thermal flow. The smaller the particles, the higher the thermal conductivity value until it reaches a critical value; for Cu particles, this is in the range of 10-30 nm. The decrease in thermal conductivity may be caused by an insufficient distribution of copper particles and, consequently, the creation of significant spaces, which prevent the creation of uninterrupted conductive paths.  
Figure 8: Macroscopic photos of sample surfaces after the accelerated thermal aging process.  
Figure 7: Comparison of strain at break values before and after the accelerated thermal aging process.  
Composites reinforced with copper particles are good conductors of electricity and heat; this is possible thanks to a phenomenon called percolation. In order for a composite to become a conductor throughout the entire material, it is necessary to form continuous conductive paths. Thanks to the formation of a conductive network, it is possible to observe the free flow of electrons through the connected paths, which guarantees an increase in thermal and electrical conductivity. A percolation barrier describes the minimum concentration of reinforcement particles needed to form conductive beams. If a given value is not achieved, the particles are randomly distributed in the material, which causes disturbances in the network, reducing the flow of electrons. This phenomenon is crucial when forming composites reinforced with copper particles because, thanks to the appropriate selection of concentration, we can achieve the desired conductivity. Moreover, the phenomenon of network formation can be improved using surface treatment techniques to improve dispersion and reduce particle agglomeration in the composite matrix [39]. [PERSON] and colleagues investigated the mechanical properties as well as the electrical and thermal conductivity of epoxy composites modified with copper and nickel. It was found that the electrical conductivity of the composites exhibited percolation behavior at specific threshold values of 9.9% and 4.0% vol% for EP-Cu and EP-Ni composites, respectively. Using the Lichtenecker model, the theoretical thermal conductivity of the dispersed metallic phase in the composites was estimated to be 35 W/mK for Cu powder and 13 W/mK for Ni powder. The findings of the theoretical and experimental studies overlapped. Unlike electrical conductivity, thermal conductivity did not exhibit percolation properties. These values are nearly an order of magnitude lower than the thermal conductivity of solid metals. This phenomenon can be attributed to the presence of high thermal resistances at the particle-to-particle and particle-polymer interfaces which impede heat transfer in the metal-filled polymer matrix [46].  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Parameter** & **EP** & **10\% MikroCu** & **10\% 40–60 nmCu** & **10\% 10–30 nmCu** \\ \hline Thermal conductivity, W/(m\(\cdot\)K) & 0.21717 \(\pm\) 0.007 & 0.24741 \(\pm\) 0.007 & 0.25824 \(\pm\) 0.008 & 0.25238 \(\pm\) 0.008 \\ \hline Average temperature, K & 10.5 \(\pm\) 0.03 & 10.6 \(\pm\) 0.03 & 10.7 \(\pm\) 0.03 & 10.7 \(\pm\) 0.03 \\ \hline Percentage change, \% & 13.92 \(\pm\) 0.42 & 18.91 \(\pm\) 0.57 & 16.21 \(\pm\) 0.49 \\ \hline \end{tabular}
\end{table}
Table 4: Thermal conductivity results.  
Figure 9: Percentage increase in thermal conductivity compared to the initial value of thermal conductivity of the epoxy resin.