## 1 Introduction
A comparison of the thermophysical properties and cost of some solid particulates is shown in Table 1. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Material**} & **Specific Heat** & **Melting Point** & **Sintering Temperature** & **Energy Density** & **Density** & **Thermal** & **Cost** \\  & **(kJ/kgK)** & **(\({}^{\circ}\)C)** & **(\({}^{\circ}\)C)** & **(kJ/m\({}^{3}\))** & **(kg/m\({}^{3}\))** & **Conductivity (W/mK)** & **(\%/ton)** \\ \hline Quartz; SiO\({}_{2}\) & 0.95 & 1670 & 1670–1714 & 2480 & 2600 & 0.4 & 150 \\ \hline Sand; SiO\({}_{2}\) & 0.95 & 1700 & 1760 & 2490 & 2550 & 1.2 & 75 \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of some thermophysical properties and the cost of solid particulates. Adapted from [15; 16; 17; 18; 19; 20; 21; 22; 23]. Numerous investigations have been carried out to understand how particles affect the receiver's performance. The first to employ calcined coke gemstone solid particles as the HTM in solar receivers was the National Renewable Energy Laboratory [5]. The study conducted by our collaborators at the Georgia Institute of Technology, and it investigated how temperature and pressure affected the particles' sintering characteristics [24]. However, the sintering study was limited to temperatures up to 800 \({}^{\circ}\)C.  
Numerous experts have studied the specific heat of solid particles utilized in next-generation CSP reactors [25; 26; 27; 28; 29]. They either measured or calculated the specific heat at high temperatures up to a maximum of 800 \({}^{\circ}\)C. [PERSON] et al. [21] calculated the thermal conductivity of various particulates up to 1000 \({}^{\circ}\)C. Nonetheless, their calculation was based on a predictive model. [PERSON] et al. [29] measured the thermal conductivity of only desert sand. Thermal conductivity was measured at temperatures close to ambient conditions only. An obstructed flow particle heating receiver (PHR) using sand was designed and developed by King Saud University (KSU) and Georgia Institute of Technology (GIT) [30; 31]. Several on-sun tests were conducted in the 300 kW\({}_{\text{th}}\) particle-based CSP plant at KSU [32; 33]. Riyadh red sand (RRS) and Carbobead (CB) were used as HTMs. The system demonstrated that the particle temperature at the outlet of the PHR reached 720 \({}^{\circ}\)C. Our team is conducting ongoing de-risking activities to ensure the reliability, efficiency, and economic viability of particle-based CSP systems before their widespread commercial deployment [34; 35]. Based on this initiative, a 1.3 MWe pre-commercial plant will be built by our team in Waad-Al-Shamal, Saudi Arabia [36]. Many studies have been carried out by researchers worldwide, as seen in the literature reviewed herein. Nonetheless, those studies are somewhat limited. In particular, no study has been found that experimentally investigated thermal conductivity at high temperatures and sintering at both high temperatures and pressures for many particulate materials of interest. Since the authors are the developers of an innovative CSP system [37], investigation of these properties is needed. Consequently, this paper provides the experimental measurements of the sintering behavior at high pressure and temperature, thermal conductivity at high temperature, and other pertinent properties of the candidate particulate materials. Four naturally occurring particulate materials, namely, Riyadh white sand (RWS), Riyadh red sand (RRS), Saudi olivine sand (SOS), and US olivine sand (USOS), and one engineered particulate material called Carbobead (CB) have been investigated. RWS, RRS, and SOS are naturally occurring sands readily available in Saudi Arabia, making them cost-effective and practical for large-scale CSP systems. CB is a commercially engineered ceramic particle, widely used in energy applications due to its high solar absorptance, thermal stability, and durability. CB allows for benchmarking the performance of natural particulates against a well-established reference material.