Thermionic emission and heat transfer between nanoparticles and gas at high temperatures

• Role of thermionic emission in conductive heat transfer is emphasized. • Consequences of neglecting effect under consideration are discussed. • Experimental overestimation of energy accommodation coefficient is quantified. An accurate description of the heat transfer between nanoparticles and the bath gas is essential to describe the temporal evolution of the particulate. In particular, modeling the processes of nanoparticle formation in flame synthesis heavily relies on the temperature of the generated particles and, therefore, requires knowledge of this temperature. In its turn, the temperature depends on conductive heat transfer from the particle surface to the surrounding gas. Traditionally, the latter process is described using the so-called energy accommodation coefficient (EAC). In this communication, we analyze the current state of experimental research regarding the value of EAC at high temperatures. We further conclude that depending on the temperature range, such as moderately (e.g., metal combustion) and extremely high (e.g., laser-induced incandescence), the EAC magnitudes required to quantify the experiment vary substantially. We discuss that the main distinction of the extremely high temperature range, where the large EAC values are reported, is the possibility of the significant thermionic emission of electrons. By considering these electrons as carriers for conductive heat transfer, we show that at the extremely high temperatures the corresponding channel becomes comparatively efficient with the usual conductive heat transfer of gas molecules. This leads to the need to rethink the generally accepted approaches to the description of heat transfer in two-phase high-temperature systems and, in particular to the estimates of the EAC values, which is also done in the current work.