## 1 Introduction  
Heat transfer fluids (HTFs) play a key role in several application areas, from human bodies to engineering systems. Heat transfer processes are deeply important in heat exchangers and, consequently, the chemical and thermophysical characteristics of the working fluids can affect the overall heat to power efficiency. Therefore, enhancement of thermophysical properties like specific heat, thermal conductivity, stability, materials compatibility and viscosity have gained much attention from researchers in the past decade [1]. From nuclear reactors, where thermal energy is released by fission, up to thermodynamic power systems, HTFs are involved in different operating steps, and several studies were performed over the years to select the highest-performing thermal fluids [2]. HTFselection is a crucial parameter also for renewable energy technologies such as thermal solar energy plants, as it strongly affects the economic effectiveness and dispatchability of these systems [3]. An ideal heat transfer fluid should possess favorable thermo-physical properties and should be chemically stable, non-corrosive, non-toxic, non-flammable and able to operate over a large temperature range without undergoing any adverse changes. Though water is the most obvious choice as a primary working fluid, it has limitations in terms of operating temperatures and pressures, hence the necessity to develop more suitable thermal exchange fluids that can efficiently operate at high temperatures. The modern HTFs are mainly synthetic, and, among them, thermal oils can operate, pressurized with an inert gas, at temperatures of up to 400 \({}^{\circ}\)C [4], whereas mineral and hot oils are generally thermally stable up to a maximum temperature of 300 \({}^{\circ}\)C [4]. For this reason, aromatic species such as Therminol 66 and Dowtherm Q are typically employed in nuclear and solar plants [5; 6]. Unlike other oil-based materials [7; 8], very little data are currently available about thermal oil's chemical stability and possible composition changes as a function of the operating temperatures. Actually, the instability near and above the maximum operating temperature represents one of the most critical limitations for these materials due to the occurrence of modifications in their chemical structure along with production of flammable gasses like, for instance, hydrogen. In addition, the degradation reactions could lead to a worsening of the fluids in thermo-physical features and to fouling processes, resulting in circuitry pollution [9; 10; 11]. Since the former and more traditional application of oils was as coolants in the nuclear field, these materials have been generally characterized and tested under simultaneous heat and irradiation stress [12; 13], and only a small amount of material is available in the scientific literature regarding their thermal stability [14]. Silicone oils, unlike hydrocarbon analogs, are scarcely flammable and, hence, they are more popular for applications in which the fire safety and ecologic requirements are high, additionaaly, in recent years, they have gained great interest also as cooling media for Lithium-Ion Battery (LIB) modules [15] and, in general, for electronics systems [16]. These organosilcone fluids possess a unique combination of properties and are virtually chemically and corrosion inert, non-explosive and low in terms of toxicity. According to manufacturer recommendations [17; 18; 19; 20] silicone oils are suitable for operation at temperatures of up to 400 \({}^{\circ}\)C in a closed-loop system and 250 \({}^{\circ}\)C in open systems. However, the low volatility and the characteristics of these compounds suggest that they can present greater thermal stability.