Fabrication and Characterization of a Novel Carbon/Carbon Composite with Micro-channels for Concentrated Solar Power Gas Receivers

Concentrated solar power (CSP) gas receivers have recently been proposed due to their potentially higher efficiencies when combined with sCO₂ (super-critical CO₂) Brayton cycles. The state-of-the-art of modular micro-channel receivers are made with nickel-alloys. However, these receivers face thermal fatigue issues due to daily start-up operations, limiting their operating temperatures to around 560 °C. To overcome these challenges, a novel modular carbon/carbon (C/C) composite with micro-channels for CSP sCO₂ gas receivers is proposed. These composites offer comparable thermal and mechanical properties while having a significantly lower coefficient of thermal expansion. This mitigates thermal fatigue problems, allowing for much higher operating temperatures to be used (&gt; 800 °C), which in turn can greatly increase efficiency. In addition, their lightweight nature can reduce installation costs.<br/><br/>In this work, the fabrication of the C/C composite modules starts by combining carbon fibers with a resorcinol-modified phenolic resin. Between the layers of carbon fibers, 3D printed PLA (polylactic acid) channels are placed, serving as a template for the desired micro-channel network. The resin is then cured in a vacuum bag and in-house designed and built autoclave setup (up to 150 °C, 160 psi) to produce a void-free prepreg. The novel approach of adding resorcinol allows the resin to be first B-stage cured at 50 °C, which is below the glass transition temperature of PLA (52.4 °C), preventing channel squishing due to the high pressure provided by the autoclave. Next, PLA is depolymerized under vacuum at 290 °C, leaving a cleared channel network in its place. The cured phenolic resin in the prepreg is then carbonized at 1000 °C under a nitrogen atmosphere where gas evolution causes a decrease in density. Due to matrix shrinkage, residual stresses build up between the carbon matrix and the fibers, which are alleviated through graphitization at 2200 °C. To obtain composites with the desired properties, a densification procedure is done by using a novel CVI (chemical vapor infiltration) method with methane above 900 °C and vacuum pressures below 250 mbar with processing times between 20-100 hours. During this process, methane is broken down to deposit carbon in the pores of the C/C composite, increasing the density of the material. The densified composite is then graphitized a second time at 2200 °C under argon, greatly improving its thermal conductivity.<br/><br/>Carbon materials are known to oxidize in air and CO₂ at temperatures above 500 °C. Thus, an anti-oxidative coating is applied to the C/C composite modules to prevent mass loss due to both outside air and the heat transfer fluid (sCO₂). A multilayered coating approach using pure SiC and a mixture of ZrB₂-SiC is shown to be effective due to their high temperature stability. Furthermore, the formation of oxides when exposed to air (SiO₂, ZrO₂, and ZrSiO₄) aids in protecting the substrate as well as providing self-healing abilities. The first layer consists of SiC, produced through pack cementation using a powder mixture (Si, C, Al₂O₃, and SiC) in a closed graphite crucible heat treated at 1800 °C. This SiC layer has excellent adhesion and acts as a buffer to alleviate thermal stresses between the C/C composite and the outer ZrB₂-SiC layer, which is in turn applied through a slurry/dip coating process. The slurry, in which SiC coated samples are dipped, consists of a mixture of ZrB₂ powders with a polymeric ceramic precursor. Once heat treated above 1000 °C, the precursor acts as a binder by forming SiC. Isothermal tests at 800 °C for 3 days were done and show that after an initial weight increase of around 3.4 wt% due to oxide formation, the specimens remain stable with no weight loss. A combination of SEM (scanning electron microscopy), SEM/EDS (energy-dispersive spectroscopy), XRD (X-ray diffraction) and TGA (thermogravimetric analysis) are used to characterize the produced C/C composites.