# Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review
## 2 Literature Review
### Nanomaterials
#### 2.2.2 Experimental Work
In the work [61], an experimental technique using carbon nanotubes (CNTs) reinforced with the specific volumetric fraction of CNT to improve mechanical and tribological properties, especially to improve the stiffness, wear resistance and rigidity of the HDPE composite material using injection molding, is reported. The results showed that the melting point and oxidation temperature of the composites are not affected by the addition of CNTs. However, its crystallinity seems to increase. By increasing the volume fraction of CNTs, the properties of the material improve considerably. In the work [62], the researchers reported that some mechanical properties of HDPE, like all polymers, are very sensitive to the service temperature. In general, all polymers at temperatures significantly below their glass transition temperatures (Tg) undergo brittle fracture. Thus, an increase in mechanical properties results in variations in the material properties. These are tested using different materials such as HDPE, nanodiamonds, multi-walled carbon nanotubes and graphite nanoplatelets. A quasi-static nanoindentation test was used to evaluate the mechanical properties of hardness, plasticity, the tensile modulus of the pure HDPE and different HDPEs with nanocomposites. The authors conclude that the filled hybrid of nanodiamonds and Graphite Nanoplatelets had higher surface properties. For this reason, HDPE, nanodiamonds and graphite nanoplatelets could improve the quasi-static and dynamic properties of materials. In [63], two different configurations of graphite were used. One had a different distribution of particle sizes and the other had a different specific surface to improve the diffusivity and thermal and electrical conductivity of HDPE. The authors reported the graphite characterization, thermal conductivity and diffusivity, Young's modulus of elasticity, elongation, the stress at break, and the electrical conductivity of HDPE/graphite composites. The latter showed that the different types of graphite have a different influence on the percolation concentration of the composites. A summary of the previous results of experimental works related to nanomaterials is given in Table 5. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Authors** & **Nanomaterials** & **Applications** & **Results and Remarks** \\ \hline [PERSON] et al. [48] & GNPs and EG & Analysis of the thermal, mechanical, and electrical properties of nanocomposites & The best result was gained with 40 wt\% of GNPs with HDPE; the \(\kappa\) of the composite increases at 1.32 W/mK while pure HDPE was 0.36 W/mK given in Table 5.  
\begin{tabular}{c} \hline \hline  & & & A vertical pipe TC of 0.41 W/m/\({}^{\circ}\)C (HDPE) vs. \\  & & 0.71 W/m/\({}^{\circ}\)C (GEOverform). Reduction in \\  & & borehole lengths (10\% less). The heat dissipated \\  & & the thermal performance of boreholes \\  & & & 25\% higher (2300 W). The borehole thermal \\  & & & resistance of the well was 17\% lower \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of experimental studies on heat exchanger pipes using nanomaterials.