Coulomb collisions are believed to be a fundamental process governing the thermodynamic equilibrium of solar wind plasmas, yet their role within the Alfvénic slow solar wind remains poorly understood. In this study, we carry out a statistical analysis of proton core parameters measured by the Helios spacecraft to examine how its temperature anisotropy (T⊥/T∥) varies with collisional age for three types of solar wind, namely, fast wind, Alfvénic slow wind, and non-Alfvénic slow wind. Here, T⊥ and T∥ denote the proton temperatures perpendicular and parallel to the ambient magnetic field. Consistent with previous findings, we confirm that Coulomb collisions play a negligible role in the fast wind but strongly regulate the thermodynamics of non-Alfvénic slow wind. Remarkably, however, the Alfvénic slow wind displays two distinct regimes: a weakly collisional regime characterized by significant temperature anisotropy, T⊥/T∥ > 1, and a collisionally regulated regime characterized by T⊥/T∥ ≈ 1. In addition, a close examination of the radial evolution of several well-defined Alfvénic slow wind streams indicates that their thermodynamic behavior is strongly governed by several key plasma parameters, notably the solar wind speed. This in turn implies that the origin of the streams—either the cores of coronal holes or their over-expanded edges—determines the dominant processes governing the dynamics and thermodynamics of the Alfvénic slow wind.

Abstract In the present study, the effect of nickel nitrate addition as a catalytic precursor for the in situ formation of Ni nanoparticles during the heating process has been investigated on the modification of microstructure and graphitization of amorphous carbon resulting from pyrolysis of phenolic resin. For this purpose, the prepared resin samples were cured in carbon substrate with and without additives at temperatures of 800, 1000, and 1250 °C. XRD, FESEM, and TEM studies were performed to investigate the phase and microstructural changes in the samples during the heating process. In addition to phase and microstructural studies, thermodynamic calculations of the reactions performed for the in situ formation of nickel nanoparticles and their effective factors during the curing process were performed. The results indicated that nickel nitrate is transformed to nickel nanoparticles of different sizes during the reduction process in a reduced atmosphere. The in situ formation of nickel nanoparticles and its catalytic effect led to the graphitization of carbon resulting from the pyrolysis of phenolic resin at a temperature of 800 °C and above. By increasing temperature, the morphology of the formed graphite changed and hollow carbon nanotubes, carbon cells, and onion skin carbon were formed in the microstructure. It was also observed that by increasing the temperature and the amount of additive, carbon nanotubes and their size are increased. A noteworthy point from thermodynamic calculations during the formation of nickel nanoparticles was that the nickel nanoparticles themselves acted as accelerators of nickel oxide reduction reactions and the formation of nickel nanoparticles. This increases the amount of amorphous carbon graphitization resulting from the pyrolysis of phenolic resin which leads to the formation of more carbon nanotubes at higher temperatures.