CARBIDE AND CARBONITRIDE NANOCOMPOSITE BASED PHOTOTHERMAL SOLAR ABSORBERS – CARAPASS

Due to its intrinsic properties (thermal, chemical and mechanical resistance, etc.), silicon carbide (SiC) (and its carbonitride derivatives) is considered to be one of the materials of choice for the design of “high temperature solar absorbers«. A technological breakthrough is targeted through the development of MX / SiC type nanocomposites (M = Ti, Zr, Hf; X = CxN1-x, 0 = x = 1) to improve the optical selectivity of SiC while avoiding the oxidation of metal carbides / nitrides at the target temperatures with a controlled design, structure and open porosity, by combining two ceramic synthesis approaches with forming processes and a modeling approach. Understanding (and mastering) the synthesis of nanocomposites and their shaping as well as the phenomena governing the development of their optical selectivity are key steps in this process.

Major results emerge from this project. They are listed below according to the experimental (synthesis) and modeling approaches implemented:
? Tailored synthesis (fine control of the Ti content) of precursors of TiCxN1-x / Si-B-C (N) nanocomposites by the PDCs route (Polymer-Derived Ceramics)
? Flexibility in terms of shaping of nanocomposites
? Integration of sintering agents at the molecular level
? Fine control of the crystallinity of the phases constituting the nanocomposites
? Sol-gel synthesis of precursors of SiC-TiC nanocomposites allowing the control of the TiC content in the final material
? Control of the final density of SiC-TiC nanocomposite pellets through SPS (T) sintering parameters
? The increase in relative density and the TiC content improve the optical selectivity at room temperature of the SiC-TiC material
? Development of a modeling method allowing:
i) calculate the optical properties of nanoparticles distributed in a matrix
ii) identify an optimal chemical composition for material intended for CSP

The optical properties and the performances predicted for a TiC 30% - SiC 70% composite with a relative density of 83% are close to those estimated for a conventional receiver with non-selective absorbent paint.

The future prospect of the project is on several levels and demonstrate that the work is still vast and open. For example, in terms of the forming process, additive manufacturing would make it possible to manufacture lattice structures ideal as volumetric absorbers while tape-casting would make it possible to produce relatively thick, self-supporting films with a selective surface that would be particularly suited to the intended application as a surface absorber. The advantage of the synthesis routes implemented in the CARAPASS project is that they are relatively flexible and can be coupled with different shaping processes. This opens up many perspectives that could be exploited through a large-scale project at European level.

Five papers in the form of publications in high-IF journals have been accepted so far. More specifically, they relate to the development of materials and the optical properties that are developed, as well as modeling. 8 more articles are expected to be published by the end of 2022.

A common industrial challenge to improve the efficiency of the solar-to-electricity conversion for concentrating solar power (CSP) is to operate at high temperatures (900-1000°C). Research and development efforts on over recent years have therefore focused on the materials that compose the solar absorber which plays the key role in the overall CSP system performance. Silicon carbide (SiC) exhibits a chemical inertness, a high temperature oxidation resistance and a robustness compatible with the operating conditions of further CSP systems. However, despite a good sunlight absorption, SiC has a high thermal emittance, leading to a poor optical selectivity. Promising properties for absorber materials can be found in transition metal carbides and nitrides of column IV according to their refractivity, their inherent spectral selectivity and a lower thermal emittance compared to SiC. However, their major limitation is their tendency to oxidize in the targeted temperature range. By entering the scope of the Challenge 3 “Stimuler le renouveau industriel” (theme “Matériaux et procédés” and more particularly the priority 14), the CARAPASS project proposes to prepare nanocomposites of the type MX/SiC (M = Ti, Zr, Hf; X = CxN1-x, 0 = x = 1) by combining SiC and transition metal carbide and/or nitride in the same materials with the goal to combine optical selectivity, thermomechanical properties and oxidation resistance to fit the requirements of the next generation of high temperature absorber materials. These materials are prepared as dense monoliths to maintain their mechanical strength and robustness at high temperature. The four year CARAPASS collaborative research project brings together specialists in materials synthesis, materials characterization, and computational approaches. It is built from five French research institutes, IEM, ICSM, SPCTS, PROMES and CRM2, with complementary expertises in chemistry, in processing, in characterization of materials - especially for CSP - and in modeling which have already collaborated in the past. To reach our objectives, the project is based on the promising results obtained by IEM and ICSM with TiC/SiC nanocomposites. CARAPASS is subdivided into five interconnected scientific tasks. The first task is focused on the preparation of nanocomposite powders using two chemical routes investigated by IEM and ICSM. The second task consists to prepare dense materials following three strategies based on pressing, casting and Spark Plasma Sintering processes to be characterized in tasks 3 and 4. Physical and chemical characterization of nanocomposites is the topic of the task 3. In addition to standard material science techniques available in each institute, the thermostructural, mechanical and thermal properties of the nanocomposite monoliths will be evaluated before and after thermal ageing. The task 4 studies the optical characterization of the nanocomposites to demonstrate the selective behavior of nanocomposites The optical properties will also be measured after accelerated ageing. A theoretical work will be done in task 5 using density functional theory and the GW approximation. The present project is built to elaborate materials that are expected to lead to benefits for the advancement of science, industry and society and should allow France to be in place on this growing thematic at international scale.