Showing papers by "J. W. Coenen published in 2021"

Advanced Self-Passivating Alloys for an Application under Extreme Conditions

Abstract: Self-passivating Metal Alloys with Reduced Thermo-oxidation (SMART) are under development for the primary application as plasma-facing materials for the first wall in a fusion DEMOnstration power plant (DEMO). SMART materials must combine suppressed oxidation in case of an accident and an acceptable plasma performance during the regular operation of the future power plant. Modern SMART materials contain chromium as a passivating element, yttrium as an active element and a tungsten base matrix. An overview of the research and development program on SMART materials is presented and all major areas of the structured R&D are explained. Attaining desired performance under accident and regular plasma conditions are vital elements of an R&D program addressing the viability of the entire concept. An impressive more than 104-fold suppression of oxidation, accompanied with more than 40-fold suppression of sublimation of tungsten oxide, was attained during an experimentally reproduced accident event with a duration of 10 days. The sputtering resistance under DEMO-relevant plasma conditions of SMART materials and pure tungsten was identical for conditions corresponding to nearly 20 days of continuous DEMO operation. Fundamental understanding of physics processes undergone in the SMART material is gained via fundamental studies comprising dedicated modeling and experiments. The important role of yttrium, stabilizing the SMART alloy microstructure and improving self-passivating behavior, is under investigation. Activities toward industrial up-scale have begun, comprising the first mechanical alloying with an industrial partner and the sintering of a bulk SMART alloy sample with dimensions of 100 mm × 100 mm × 7 mm using an industrial facility. These achievements open the way to further expansion of the SMART technology toward its application in fusion and potentially in other renewable energy sources such as concentrated solar power stations.

Improving the W Coating Uniformity by a COMSOL Model-Based CVD Parameter Study for Denser Wf/W Composites

Abstract: Tungsten (W) has the unique combination of excellent thermal properties, low sputter yield, low hydrogen retention, and acceptable activation. Therefore, W is presently the main candidate for the first wall and armor material for future fusion devices. However, its intrinsic brittleness and its embrittlement during operation bears the risk of a sudden and catastrophic component failure. As a countermeasure, tungsten fiber-reinforced tungsten (Wf/W) composites exhibiting extrinsic toughening are being developed. A possible Wf/W production route is chemical vapor deposition (CVD) by reducing WF6 with H2 on heated W fabrics. The challenge here is that the growing CVD-W can seal gaseous domains leading to strength reducing pores. In previous work, CVD models for Wf/W synthesis were developed with COMSOL Multiphysics and validated experimentally. In the present article, these models were applied to conduct a parameter study to optimize the coating uniformity, the relative density, the WF6 demand, and the process time. A low temperature and a low total pressure increase the process time, but in return lead to very uniform W layers at the micro and macro scales and thus to an optimized relative density of the Wf/W composite. High H2 and low WF6 gas flow rates lead to a slightly shorter process time and an improved coating uniformity as long as WF6 is not depleted, which can be avoided by applying the presented reactor model.

Yttria-Coated Tungsten Fibers for Use in Tungsten Fiber-Reinforced Composites: A Comparative Study on PVD vs. CVD Routes

Abstract: Tungsten fiber-reinforced tungsten (Wf/W) composites are being developed to improve the intrinsic brittleness of tungsten. In these composites, engineered fiber/matrix interfaces are crucial in order to realize toughening mechanisms. For such a purpose, yttria (Y2O3), being one of the suitable interface materials, could be realized through different coating techniques. In this study, the deposition of thin films of yttria on a 150 µm tungsten wire by physical and chemical vapor deposition (PVD and CVD) techniques is comparatively investigated. Although fabrication of yttria is feasible through both CVD and PVD routes, certain coating conditions such as temperature, growth rate, oxidation of Wf, etc., decide the qualitative nature of a coating to a particular extent. In the case of PVD, the oxidation of Wf is highly reduced compared to the WO3 formation in high-temperature CVD coating processes. Yttria-coated tungsten fibers are examined comprehensively to characterize their microstructure, phase, and chemical composition using SEM, XRD, and Raman spectroscopy techniques, respectively.

Characteristics of Microstructure Evolution during FAST Joining of the Tungsten Foil Laminate

Abstract: The tungsten (W) foil laminate is an advanced material concept developed as a solution for the low temperature brittleness of W. However, the deformed W foils inevitably undergo microstructure deterioration (crystallization) during the joining process at a high temperature. In this work, joining of the W foil laminate was carried out in a field-assisted sintering technology (FAST) apparatus. The joining temperature was optimized by varying the temperature from 600 to 1400 °C. The critical current for mitigating the microstructure deterioration of the deformed W foil was evaluated by changing the sample size. It is found that the optimal joining temperature is 1200 °C and the critical current density is below 418 A/cm2. According to an optimized FAST joining process, the W foil laminate with a low microstructure deterioration and good interfacial bonding can be obtained. After analyzing these current profiles, it was evident that the high current density (sharp peak current) is the reason for the significant microstructure deterioration. An effective approach of using an artificial operation mode was proposed to avoid the sharp peak current. This study provides the fundamental knowledge of FAST principal parameters for producing advanced materials.