Scientific Papers & Presentations

Presentation - Chromium-Silicon Alloys with Iron and Nickel for Structural High Temperature Applications

Cr-Si based-alloys are promising candidates due to their high melting point and good oxidation resistance. They are designed for working temperatures beyond Ni-base superalloys. The major drawbacks of Cr- based alloys are the embrittlement by Cr2N formation and high ductile to brittle transition temperatures (DBTT). This work investigates the effect of alloying Cr-Si-alloys with the elements Fe and Ni and targets the microstructure and precipitation of A15 phase. Varied compositions are manufactured by arc-melting and heat treatment at 1200°C, which enables controlled precipitation hardening by the Cr3Si-A15 phase.Ni can cause the effect of solution softening in Cr and increases the low-temperature ductility as a result. For Fe it is shown, that it stabilizes the two-phase structure consisting of nitration resistant A15 phase and Crss. Oxidation exposures at 1200°C in synthetic air indicate that Fe additions up to 5 at.% increase also the nitridation resistance of Crss.

Michael Kerbstadt, Emma White, Mathias Galetz

TMS 2023
San Diego, California, USA, March 19-23, 2023

Project Results at ASTEP Workshop

Project Results Presented at ASTEP Workshop 

Daniel Benitez
ASTEP Workshop
October 20th, 2022

Poster - Optimization of Spinel Absorber Coatings for CSP Particle Receivers

In order to improve the current commercial plants of concentrating solar power system (CSP), new system designs are being explored within the framework of COMPASsCO2 project to reach high temperatures. Solid particles are potential candidates to be used as receiver in CSP plants to increase their operating temperature (in order of 1000 °C) and efficiency, while can be also used as thermal storage medium. So, the integration of CSP particle systems into a highly efficiency-CO2 Brayton power cycle for electricity production is pursued. Different kinds of particles developed by Saint-Gobain have been coated with new developed coatings in order to ensure good optical properties and study their high temperatures and abrasion resistance. Solar absorptance values as high as 0.98 are obtained by applying 4 layers of porous black copper, manganese and iron spinel, heated at 1000ºC. A system to study abrasion processes by particle-particle collisions was also presented.

Farchado, R. Tordesillas, G. San Vicente, N. Germán, A. Morales

SolarPACES 2022
Albuquerque, NM, USA, September 27-30, 2022

COMPASsCO2 Results at ACHEMA 2022 Conference

COMPASsCO2 Results at ACHEMA 2022 Conference

Michael Kerbstadt
ACHEMA 2022 Conference
Frankfurt am Main, August 23rd, 2022

Presentation - Estimation of the mechanical behavior of Ni-base alloys at high temperatures using Small punch tests

The COMPASsCO2 project will investigate materials for the extreme environments of heat exchanger (temperature, pressure, etc.) from commercial alloys such as advanced Ni-Superalloys as well as novel materials. Small punch (SP) testing is a very successful screening method to estimate mechanical properties of metallic. Yield strength and Ultimate tensile strength values have been evaluated successfully from the SP tests for the commercial materials for the design of the heat exchanger. The obtained results correlated very well with the results of tensile validation tests. The testing method can be used for supporting the development of the novel materials to obtain the requested tensile properties, when the available material is not enough for conventional tensile specimen production.

R. Hernández, J. San, Andrés, M. Serrano, D. Plaza, A. Muñoz, E., Muñoz-Cortés
CNMAT 2022 Conference
Ciudad Real , 28 June – 1 July, 2022

COMPASsCO2 Results at Processes4Planet Forum

COMPASsCO2 Results at Processes4Planet Forum

Maxime Rouzès
Processes4Planet Forum
Brussels, June 9th, 2022

Presentation at SolarConcentra
General project presentation 

Daniel Benitez
November 11th, 2021

Saint-Gobain's New Material Solutions for Thermal Energy Storage
The poster presentation developed and presented by Saint-Gobin CREE,one of the COMPASsCO2 project participants, at the 27th SolarPACES conference edition, which takes place online from September 27th to October 1st. The presentation gives an overview of all the thermal energy storage media manufactured by Saint-Gobain for CSP, ACAES and sensible heat applications, which includes preliminary results on development of granulated and fused particles in comparison to current state-of-the-art sintered bauxite. 

Samuel Marlin and Idris Amirouche
September 27th-October 1st, 2021

Presentation at sCO2-Flex Final Event

General project presentation 

Daniel Benitez
sCO2-Flex Final Event
June 16th, 2021

Novel Cr/Si-Slurry Diffusion Coatings for High Temperatures

Surface enrichment in Al, Si, and Cr can greatly improve high temperature oxidation resistance of many alloys. Al, Si, and Cr coatings are commonly applied via simple slurries or more
complex pack cementation processes. Due to the high melting point of Cr, the deposition of Cr-based diffusion coatings by the slurry technique has proved challenging, and to date, Cr has mostly been applied by pack cementation. Here, a novel Cr-Si coating process via the slurry technique is described which has been developed and then demonstrated on two Ni-based superalloys, Rene 80 and Inconel 740H. The addition of Si to the slurry lowers the melting point via a Cr-Si eutectic and enables the formation of a liquid phase during heat treatment. Through this Cr-Si slurry coating process diffusion layers enriched by Cr and Si of about 150 m were achieved. Oxidation behavior was studied through isothermal exposures at 900 C for 1000 h in lab air. Uncoated Rene 80 and IN740H both showed formation of a Ti-containing Cr2O3 scale below a thin TiO2 top layer. Underneath the external scale a zone of internally oxidized Al grew over the exposure time and reduced the load-bearing cross-section progressively. In comparison, the Cr/Si-coated samples did not show internal Al oxidation, but a slow-growing Si-rich oxide film underneath the external Cr2O3 scale. This subscale represents an additional oxygen diffusion barrier. Thus, the weight gain during exposure for the coated samples was significantly lower than for the uncoated materials.

Michael Kerbstadt, Emma Marie Hamilton White and Mathias Christian Galetz

Materials Journal
December 2, 2023

Using an Al-Incorporated Deep Black Pigment Coating to Enhance the Solar Absorptance of Iron Oxide-Rich Particles

The use of solid particles as direct heat absorbance and storage media promises enhanced storage densities in concentrated solar power (CSP) technologies. The long-term optical performance of those particles, which aim to be operational over years, is crucial. Dry powder coating with a deep black Cu-Mn-oxide pigment in a resonant acoustic mixer and subsequent sintering was employed to improve the long-term optical performance of hematite-rich spherical particles, which aimed to replace the state-of-the-art bauxite proppants. Due to the specific reactivity of the hematite particles, a new strategy using an Al-modified composition of the initial deep black pigment was required. The Al modification diminishes cation diffusion into hematite, allowing the formation of spinel-type Fe-Mn-Cu-Al-oxide coatings with favorable long-term temperature and optical stability. The effect of chemical composition of the coating layer on the coating process mechanism was discussed and the need for an elongated sintering time was noticed to ensure the termination of stable spinel phase formation. The structural and optical measurements revealed the enhancement of the properties of hematite absorber particles through this new modified coating process.

Gözde Alkan, Peter Mechnich  and Johannes Pernpeintner

Coatings Journal
November 10, 2023

Chromium-based bcc-superalloys strengthened by iron supplements

Chromium alloys are being considered for next-generation concentrated solar power applications operating > 800 °C. Cr offers advantages in melting point, cost, and oxidation resistance. However, improvements in mechanical performance are needed. Here, Cr-based body-centred-cubic (bcc) alloys of the type Cr(Fe)-NiAl are investigated, leading to ‘bcc-superalloys’ comprising a bcc-Cr(Fe) matrix (β) strengthened by ordered-bcc NiAl intermetallic precipitates (β’), with iron additions to tailor the precipitate volume fraction and mechanical properties at high temperatures. Computational design using CALculation of PHAse Diagram (CALPHAD) predicts that Fe increases the solubility of Ni and Al, increasing precipitate volume fraction, which is validated experimentally. Nano-scale, highly-coherent B2-NiAl precipitates with lattice misfit ∼ 0.1% are formed in the Cr(Fe) matrix. The Cr(Fe)-NiAl A2-B2 alloys show remarkably low coarsening rate (∼102 nm3/h at 1000 °C), outperforming ferritic-superalloys, cobalt- and nickel-based superalloys. Low interfacial energies of ∼ 40/20 mJ/m2 at 1000/1200 °C are determined based on the coarsening kinetics. The low coarsening rates are principally attributed to the low solubility of Ni and Al in the Cr matrix. The alloys show high compressive yield strength of ∼320 MPa at 1000 °C. The Fe-modified alloy exhibits resistance to age softening, related to the low coarsening rate as well as the relatively stable Orowan strengthening as a function of precipitate radius. Microstructure tailoring with Fe additions offers a new design route to improve the balance of properties in “Cr-superalloys”, accelerating their development as a new class of high-temperature materials.

Kan Ma, Thomas Blackburn, Johan P. Magnussen, Michael Kerbstadt, Pedro A. Ferreir´os, Tatu Pinomaa, Christina Hofer, David G. Hopkinson, Sarah J. Day, Paul A.J. Bagot, Michael P. Moody, Mathias C. Galetz, Alexander J. Knowles.

Acta Materialia Journal
July 26, 2023

Variability and associated uncertainty in image analysis for soiling characterization in solar energy systems

The accumulation of soiling on photovoltaic modules, and on the mirrors of concentrating solar power systems cause non-negligible energy loss with economic consequences. These can be mitigated, or even prevented, through appropriate actions if the magnitude of soiling is known. Particle counting analysis is a common procedure to characterize soiling as it can be easily performed on micrographs of glass coupons or solar devices that have been exposed to the environment. Soiling studies have typically assumed that particle counting produces invariant and inter-institution consistent results. However, particle size distribution analysis is affected by the operator and the utilized methodology. The results of a round-robin study are presented in this work to explore and elucidate the uncertainty related to particle counting and its effect on the characterization of the soiling of glass surfaces used in solar energy conversion systems. An international group of soiling experts analysed the same 8 micrographs using the same open-source ImageJ software package. The variation in the particle analyses results were investigated to identify specimen characteristics with the lowest coefficient of variation and the least uncertainty among the various operators. The mean particle diameter showed the lowest variance among the investigated characteristics , whereas the number of particles exhibited the largest variation. Additional parameters, such as the fractional area coverage and the distribution’s skewness, yielded an intermediate variance. These results can provide useful information on optical and microscope-based soiling monitoring and characterization.

Greg P. Smestad, Cody Anderson, Michael E. Cholette, Pavan Fuke, Ahmed Amine Hachicha, Anil Kottantharayil, Klemens Ilse, Mounia Karim, Muhammad Zahid Khan, Herbert Merkle, David C. Miller, Jimmy M. Newkirk, Giovanni Picotti, Florian Wiesinger, Guido Willers, Leonardo Micheli.

Solar Energy Materials and Solar Cells Journal
July 4, 2023

Accurate identification and measurement of the precipitate area by two-stage deep neural networks in novel chromium-based alloys

The performance of advanced materials for extreme environments is underpinned by their microstructure, such as the size and distribution of nano- to micro-sized reinforcing phase(s). Chromium-based superalloys are a recently proposed alternative to conventional face-centred-cubic superalloys for high-temperature applications, e.g., Concentrated Solar Power. Their development requires the determination of precipitate volume fraction and size distribution using Electron Microscopy (EM), as these properties are crucial for the thermal stability and mechanical properties of chromium superalloys. Traditional approaches to EM image processing utilise filtering with a fixed contrast threshold, leads to weak robustness to background noise and poor generalisability to different materials. It also requires an enormous amount of time for manual object measurements on large datasets. Efficient and accurate object detection and segmentation are therefore highly desired to accelerate the development of novel materials like chromium-based superalloys. To address these bottlenecks, based on YOLOv5 and SegFormer structures, this study proposes an end-to-end, two-stage deep learning scheme, DT-SegNet, to perform object detection and segmentation for EM images. The proposed approach can thus benefit from the training efficiency of CNNs at the detection stage (i.e., a small number of training images required) and the accuracy of the ViT at the segmentation stage. Extensive numerical experiments demonstrate that the proposed DT-SegNet significantly outperforms the state-of-the-art segmentation tools offered by Weka and ilastik regarding a large number of metrics, including accuracy, precision, recall and F1-score. This model forms a useful tool to aid alloy development microstructure examinations, and offers significant advantages to address the large datasets associated with highthroughput alloy development approaches.

Zeyu Xia, Kan Ma, Sibo Cheng, Thomas Blackburn, Ziling Peng, Kewei Zhu, Weihang Zhang, Dunhui Xiao, Alexander J Knowles and
Rossella Arcucci

PCCP Journal
May 26, 2023

Improved Performance of Ceramic Solar Absorber Particles Coated with Black Oxide Pigment Deposited by Resonant Acoustic Mixing and Reaction Sintering

A deep-black Cu, Mn, Fe- pigment with a spinel structure was employed to coat standard proppants in order to improve long term solar absorptance. The coating process was performed by high-energy, high-speed mixing of proppants and small quantities of spinel powders in a resonant acoustic mixer. A continuous powder coating is achieved by electrostatic attraction between the proppant surface and the coating particles. Consolidation and strong attachment of the coating is achieved by the subsequent sintering beyond the spinel melting temperature. Chemical reaction and bonding between spinel coating and proppant lead to the incorporation of Al, Mg and Ti into the spinel structure. Coated bauxite proppants exhibit a significantly improved, long-term stable solar absorption accompanied by a promising abrasion resistance. The presented coating methodology is considered to be scalable to industrial production.

Gözde Alkan, Peter Mechnich and Johannes Pernpeintner
Coatings Journal
May 31th, 2022

Informative & Outreach Papers

COMPASsCO2 Poster – an overview of the project

The COMPASsCO2 poster gives a general overview of the COMPASsCO2 project, including addressed challenges, research focus (new particles and new metals alloys for the particle/sCO2 heat exchanger and their validation) as well as stakeholders engagement/dissemination and the composition of the consortium.


This article is a first of a series of informative papers aiming at disseminating the main achievements of COMPASsCO2 project to a wide audience, in order to increase awareness on sustainable energy technologies and highlight the efforts of both research and industry for the transition to a carbon neutral energy mix in Europe. It summarises the activities conducted in the first 18 months of implementation (November 2020 – April 2022), and the main accomplishments, with a specific focus on the development and testing of novel materials.