## 5 Conclusions  
The results obtained in this work confirmed the Bluesil low volatility and good chemical stability up to 450 \({}^{\circ}\)C under a nitrogen atmosphere. This behavior is comparable to the one reported for DPMS studied under similar conditions. Hydrogen is the main degradation product, and it is possible to assume a partial segmentation of the silicone chains, indicated by an increase in the fluid viscosity, due to the formation of cross-linked compounds. However, the extent of these variations is limited, even at 450 \({}^{\circ}\)C, as demonstrated by calorimetric, chromatographic and spectroscopic analyses, which show that only small changes are produced regarding specific heat and oil composition. A kinetic equation was defined regarding the hydrogen production rate, which can be used to estimate a realistic lifetime for the fluid at different temperatures.  
A comparison with commercial thermal oils shows the expected higher chemical resilience of the siloxanes structure with respect to the aromatic one, resulting in an increase in the maximum temperature limit of about 100 \({}^{\circ}\)C for Bluesil. This feature can make silicone oils quite interesting for CSP applications, considering their low flammability. On the other hand, their high price still represents a disadvantage for large utilization in solar power plants.  
Therminol 66 and Therminol SP present different degradation processes, that is, polymerization for the former and molecular cracking for the latter.  
Conceptualization, E.M., G.T. and S.S.; Methodology, E.M., I.B., G.T., A.C.T. and A.C.; Software, I.B., G.C., F.F., N.R. and A.S. ([PERSON]); Validation, S.S.; Formal analysis, N.C., N.R., A.S. ([PERSON]) and A.C.T.; Investigation, E.M., G.C., F.P. and A.S. ([PERSON]); Data curation, E.M., N.C., F.P., S.S., A.S. ([PERSON]) and A.C.T.; Writing--original draft, E.M.; Writing--review & editing, I.D.S., S.S., A.C.T. and A.C.; Visualization, I.D.S. and F.F.;  
Figure 10: Infrared spectra analysis of fresh and aged Bluesil. The black line stands for the fresh sample and the red line stands for the aged sample.  
Supervision, A.C. and M.C.; Project administration, A.C. and M.C.; Funding acquisition, M.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by French Alternative Energies and Atomic Energy Commission.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
This work was developed in the framework of the ARDECO project (ASTRID Research and Development Cooperation) CEA-ENEA agreement; the authors are grateful for the revision and suggestions provided by their CEA colleagues. The presented work is also part of a doctoral thesis coordinated by [PERSON], whom the authors would like to thank.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} HTF & Heat transfer fluid \\ HT & Heat transfer \\ GC & Gas chromatograph \\ TCD & Thermal conductivity detector \\ GC-MS & Gas chromatography coupled to mass spectrometry \\ TGA & Thermo gravimetric analysis \\ DSC & Differential scanning calorimetry \\ \end{tabular}