Low-Temperature H2/D2 Plasma–W Material Interaction and W Dust Production for Fusion-Related Studies

In this paper, results concerning hydrogen and deuterium plasma (RF, 13.56 MHz) interactions with tungsten surfaces, were reported. We used the Hollow-Cathode (HC) configuration for plasma–tungsten surface interaction experiments, along with the collection of tungsten dust, at different distances. Further on, the plasma-exposed tungsten surfaces and the collected dust were morphologically analyzed by contact profilometry, scanning electron microscopy, and energy dispersive spectroscopy measurements, along with chemical investigations by the X-ray photoelectron spectroscopy technique. The results showed that exposing the tungsten surfaces to the hydrogen plasma induces surface erosion phenomena along with the formation of dust and interconnected W structures. Herein, the mean ejected material volume was ~1.1 × 105 µm3. Deuterium plasma facilitated the formation of blisters at the surface level. For this case, the mean ejected material volume was ~3.3 × 104 µm3. For both plasma types, tungsten dust within nano- and micrometer sizes could be collected. The current study offers a perspective of lab-scaled plasma systems, which are capable of producing tungsten fusion-like surfaces and dust

Influence of Laser-Designed Microstructure Density on Interface Characteristics and on Preliminary Responses of Epithelial Cells

International audienceCurrent trends in designing medical and tissue engineering systems rely on the incorporation of micro- and nano-topographies for inducing a specific cellular response within the context of an aimed application. As such, dedicated studies have recently focused on understanding the possible effects of high and low density packed topographies on the behavior of epithelial cells, especially when considering their long-term viability and functionality. We proposed to use stair-like designed topographies with three different degrees of distribution, all created in polydimethylsiloxane (PDMS) as active means to monitor cell behavior. Our model cellular system was human bronchial epithelial cells (BEAS-2B), a reference line in the quality control of mesenchymal stem cells (MSCs). PDMS microtextured substrates of 4 µm square unit topographies were created using a mold design implemented by a KrF Excimer laser. Varying the spacing between surface features and their multiscale level distribution led to irregular stairs/lines in low, medium and high densities, respectively. Profilometry, scanning electron and atomic force microscopy, contact angle and surface energy measurements were performed to evaluate the topographical and interface characteristics of the designed surfaces, while density-induced cellular effects were investigated using traditional cell-based assays. Our analysis showed that microstructure topographical distribution influences the adhesion profiles of epithelial cells. Our analysis hint that epithelial organoid formation might be initiated by the restriction of cell spreading and migration when using user-designed, controlled micro-topographies on engineered surfaces

Shellac Thin Films Obtained by Matrix-Assisted Pulsed Laser Evaporation (MAPLE)

We report on the fabrication of shellac thin films on silicon substrates by matrix-assisted pulsed laser evaporation (MAPLE) using methanol as matrix. Very adherent, dense, and smooth films were obtained by MAPLE with optimized deposition parameters, such as laser wavelength and laser fluence. Films with a root mean square (RMS) roughness of 11 nm measured on 40 × 40 µm2 were obtained for a 2000-nm-thick shellac film deposited with 0.6 J/cm2 fluence at a laser wavelength of 266 nm. The MAPLE films were tested in simulated gastric fluid in order to assess their capabilities as an enteric coating. The chemical, morphological, and optical properties of shellac samples were investigated by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM)

Material Erosion and Dust Formation during Tungsten Exposure to Hollow-Cathode and Microjet Discharges

International audienceTungsten erosion and dust occurrence are phenomena of great interest for fusion technology. Herein, we report results concerning the material damage and dust formation in the presence of high temperature and large area or concentrated discharges in helium and argon. In order to generate adequate plasmas, we used tungsten electrodes in two experimental discharge systems, namely a hollow discharge and a microjet discharge. In both exposure cases, we noticed surface modification, which was assigned to sputtering, melting, and vaporization processes, and a significant dust presence. We report the formation on electrode surfaces of tungsten fuzz, nano-cones, nanofibers, and cauliflower- and faced-like particles, depending on the discharge and gas type. Dust with various morphologies and sizes was collected and analyzed with respect to the morphology, size distribution, and chemical composition. We noticed, with respect to erosion and particle formation, common behaviors of W in both laboratory and fusion facilities experiments

Thin Films of Metal-Organic Framework Interfaces Obtained by Laser Evaporation

Properties such as large surface area, high pore volume, high chemical and thermal stability, and structural flexibility render zeolitic imidazolate frameworks (ZIFs) well-suited materials for gas separation, chemical sensors, and optical and electrical devices. For such applications, film processing is a prerequisite. Herein, matrix-assisted pulsed laser evaporation (MAPLE) was successfully used as a single-step deposition process to fabricate ZIF-8 films. By correlating laser fluency and controlling the specific transfer of lab-synthesized ZIF-8, films with user-controlled physical and chemical properties were obtained. Films’ characteristics were evaluated by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The analysis showed that frameworks of ZIF-8 can be deposited successfully and controllably to yield polycrystalline films. The deposited films maintained the integrity of the individual ZIF-8 framework, while undergoing minor crystalline and surface chemistry changes. No significant changes in particle size were observed. Our study demonstrated control over both the MAPLE deposition conditions and the outcome, as well as the suitability of the listed deposition method to create composite architectures that could potentially be used in applications ranging from selective membranes to gas sensors

Induced Hydrophilicity and In Vitro Preliminary Osteoblast Response of Polyvinylidene Fluoride (PVDF) Coatings Obtained via MAPLE Deposition and Subsequent Thermal Treatment

Recent advancements in biomedicine have focused on designing novel and stable interfaces that can drive a specific cellular response toward the requirements of medical devices or implants. Among these, in recent years, electroactive polymers (i.e., polyvinylidene fluoride or PVDF) have caught the attention within the biomedical applications sector, due to their insolubility, stability in biological media, in vitro and in vivo non-toxicity, or even piezoelectric properties. However, the main disadvantage of PVDF-based bio-interfaces is related to the absence of the functional groups on the fluoropolymer and their hydrophobic character leading to a deficiency of cell adhesion and proliferation. This work was aimed at obtaining hydrophilic functional PVDF polymer coatings by using, for the first time, the one-step, matrix-assisted pulsed evaporation (MAPLE) method, testing the need of a post-deposition thermal treatment and analyzing their preliminary capacity to support MC3T3-E1 pre-osteoblast cell survival. As osteoblast cells are known to prefer rough surfaces, MAPLE deposition parameters were studied for obtaining coatings with roughness of tens to hundreds of nm, while maintaining the chemical properties similar to those of the pristine material. The in vitro studies indicated that all surfaces supported the survival of viable osteoblasts with active metabolisms, similar to the “control” sample, with no major differences regarding the thermally treated materials; this eliminates the need to use a secondary step for obtaining hydrophilic PVDF coatings. The physical-chemical characteristics of the thin films, along with the in vitro analyses, suggest that MAPLE is an adequate technique for fabricating PVDF thin films for further bio-applications

Second harmonic generation (SHG) in pentacene thin films grown by matrix assisted pulsed laser evaporation (MAPLE)

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Stearic Acid/Layered Double Hydroxides Composite Thin Films Deposited by Combined Laser Techniques

We report on the investigation of stearic acid-layered double hydroxide (LDH) composite films, with controlled wettability capabilities, deposited by a combined pulsed laser deposition (PLD)-matrix-assisted pulsed laser evaporation (MAPLE) system. Two pulsed lasers working in IR or UV were used for experiments, allowing the use of proper deposition parameters (wavelength, laser fluence, repetition rate) for each organic and inorganic component material. We have studied the time stability and wettability properties of the films and we have seen that the morphology of the surface has a low effect on the wettability of the surfaces. The obtained composite films consist in stearic acid aggregates in LDH structure, exhibiting a shift to hydrophobicity after 36 months of storage

Impact of microwave plasma treatment on tritium retention in submicronic tungsten dust

International audienceIn the current paper, we have studied the impact of microwave (2.45 GHz) plasma treatment of submicronic (250-600 nm) tungsten dust, upon tritium gas retention. Herein, the con-ducted experiments have emphasized the role of dust treatments in pure hydrogen gas versus hydrogen plasma, before the tritiation process at different pressures. The obtained tritiated dust was analyzed via room temperature desorption and dissolutions. Additionally, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy analyses were per-formed to observe the changes induced by the plasma discharge. The results have shown that the Specific Surface Area of dust is enhanced by using microwave hydrogen plasma treatments, resulting in a high tritium gas retention inside the submicronic tungsten dust. & COP

Modelling tritium adsorption and desorption from tungsten dust with a surface kinetic model

International audienceDuring tokamak operation, nano-to-micrometer-sized dust will be created due to plasma-wall interactions. The loading of such dust with tritium can be problematic as it may significantly rise the in-vessel tritium inventory causing a safety issue. This work contributes to the assessment of the monitoring of tritium inventory in dust through modelling adsorption and desorption of tritium gas on the tungsten (W) dust surfaces.We based our approach on a classical kinetic model of sticking and desorption of tritium molecules. The desorption energies are parametrized with density functional theory (DFT) calculations which have shown that it depends on the hydrogen coverage of the surface (1.4 eV for bare surface and <0.8 eV for saturated surface).Using this model, we simulate and successfully reproduce tritium loading and desorption from 10μm-sized dust typical of what has been collected in WEST. For such dust, the calculated and measured tritium activity after loading under 1 bar of T2 at 743 K is about 500 MBq/g. The tritium desorption from these types of dust is also well reproduced with the DFT desorption energies: in 200 h, the tritium dust lose 200 MBq/g. The model proposed, focused on surface only, will be compared to a more classic rate- diffusion approach in order to evaluate the maximum tritium retained in the dust volume over time.Then, we show that untritiated dust placed in conditions relevant with the floor of the ITER machine are loaded with a high amount of tritium even at very low in-vessel tritium pressure (10-5 Pa). We investigate possible scenarios for tritium recovery from the W dust using isotopic exchange. The simulations show that exposures of tritiated dust at 1×104 Pa of H2 at 300 K (room temperature for an outside-the-vessel scenario) or 1 Pa of D2 at 373 K (for an in-vessel scenario) are efficient ways to recover the total quantity of tritium as soon as H2 or D2 remain the main isotopes in the gas phase. It is a possible way to validate this surface model (by designing isotopic exchange experiments) and offers opportunity to handle the tritium inventory in W dust, and by extension to any W surfaces, during ITER operation