National Institute Of Materials Physics - Romania

The present study aims to describe the role of the grain size on the properties of submicron- and nano-structured Ba0.8Sr0.2TiO3 (BST) ceramics. Dense (1 - 2% porosity) ceramics with average grain sizes in the range of (77 234) nm were consolidated under different spark plasma sintering conditions starting from nanopowders with a mean particle size of 70 nm, synthesized via the acetate variant of the sol-gel method. The structural analysis based on XRD data revealed a mixture of cubic and tetragonal modifications at room temperature for the precursor powders and for all the investigated ceramics. The structural heterogeneity of the individual ceramic grains with coexistence of cubic and tetragonal polymorphs was confirmed by HR-TEM investigations. Accordingly, a "brick-wall" model with cubic grain boundary regions and tetragonal grain cores is proposed. By increasing the grain size, from 77 to 234 nm, a decrease of the phase transitions diffuseness accompanied by an increase of the permittivity maxima (from 650 to 4500) and dielectric losses (from 5 to 7.5%, at 100 Hz), was detected by broadband dielectric spectroscopy. No variation of the Curie temperature in the investigated Ba0.8Sr0.2TiO3 ceramics was detected, unlike typically reported for BaTiO3 ceramics with similar grain sizes. The Curie-Weiss temperature and the Curie constant decrease when grain size is diminished, indicating an overall reduction of the ferroelectric active volume, as a scaling effect. The ferroelectric switching was demonstrated for all the selected fine-grained BST ceramics, either at nanoscale or macroscopically, with an increased ferroelectric character for the coarser submicron-structured ceramics, with respect to the nanocrystalline one. The observed properties of the fine-grained Ba0.8Sr0.2TiO3 ceramics are explained in the frame of multiphase coexistence and ferroelectricity "dilution" due to the increasing role of non-ferroelectric grain boundaries when reducing grain size and complete the knowledge on the scale-dependent properties of dense fine-grained BaTiO3-based ceramics.

The European Fusion Reactor (DEMO, Demonstration Power Plant) relies significantly on joining technologies in its design. Current research within the EUROfusion framework focuses on developing materials for the first wall and divertor applications, emphasizing the need for suitable joining processes, particularly for tungsten. The electric field-assisted sintering technique (FAST) emerges as a promising alternative due to its high current density, enabling rapid heating and cooling rates for fast sintering or joining. In this study, FAST was employed to join tungsten and EUROFERE97 steel, the chosen materials for the first wall, using 50-mu m-thick Cu foils as interlayers. Three distinct joining conditions were tested at 980 degrees C for 2, 5, and 9 min at 41.97 MPa to optimize joint properties and assess FAST parameters influence. Hardness measurements revealed values around 450 HV0.1 for tungsten, 100 HV0.1 for copper, and 390 HV0.1 for EUROFER97 under all joining conditions. Increasing bonding time improved joint continuity along the EUROFER97/Cu and W/Cu interfaces. Notably, the 5 min bonding time resulted in the highest shear strength, while the 9 min sample exhibited reduced strength, possibly due to Kirkendall porosity accumulation at the EUROFER97/Cu interface. This porosity facilitated crack initiation and propagation, diminishing interfacial adhesion properties.

Herein, as a novel idea, a microwave (MW) post-treatment strategy is proposed to modify the structure and surface characteristics of already prepared activated carbons (ACs) for application in non-aqueous Electric Double-Layer Capacitors. Pistachio nutshell-derived carbon is first KOH-activated and then subjected to MW irradiation for 0, 2, 5, and 10 minutes. X-ray Diffraction and Raman analyses show that MW post-treatment leads to structural modifications, and FTIR and XPS analyses reveal relative elimination of surface functional groups which results in subsequent enhancement in water contact angle and renders more favorable surface wetting of carbon by non-aqueous organic electrolyte. The performance characteristics of symmetrical non-aqueous supercapacitors incorporated with the prepared ACs show a significant positive effect of MW irradiation in such a way that 0 and 10-minute-irradiated ACs demonstrate 152 and 392 F g-1 capacities, respectively, at 1.75 A g-1, with the corresponding specific energies and powers of 340 Wh kg-1 and 11 kW kg-1 for AC-10, respectively. This remarkable enhancement in the electrochemical performance is attributed to the effective role of the MW post-treatment in modifying the AC structure as well as providing AC surfaces that have better wettability with less polar non-aqueous electrolyte. Moreover, this strategy is additionally applicable to make hydrophobic activated carbons for other applications as the absorption of less polar contaminants from liquid or gaseous environments.

We investigate the transport and energy spectrum properties of a three-dimensional high-order topological structure formed by stacking two-dimensional square diatomic Chern insulator lattices. Electron-hole symmetry and the energy spectrum degeneracy at individual points in the semimetallic phase are proven to be due to chiral and antiunitary symmetries in the periodic system. Additionally, we explore the influence of boundary conditions in a slab system with varying surface atom connectivity, and we demonstrate analytically the presence of zero-energy surface states in specific configurations. Moreover, we describe the emergence of two chiral hinge states driven by a perpendicular phase in the nanowire geometry. Next, the quantum Hall resistance is computed in the cross-configuration of a four-lead device. In this paper, we demonstrate that the trajectories of hinge states, determined by the number of layers in parallelepiped finite structure, give rise to fractional Hall plateaus.

Magneto-optical measurements enable the identification of Fe2+ and Fe3+ in the R and S blocks of BaFe12O19 Mhexaferrite powder obtained by sol -gel processing, followed by thermal annealing at 900 C for 3 hours. The ferromagnetic phase is evidenced by the spin-majority configuration (Fe3+) due to their unpaired electrons between d -d orbitals, while the Fe2+ in the low spin possesses diamagnetic behavior strongly dependent on the surrounded crystal field. Additionally, the irradiation with gamma rays changes the ratio between Fe2+/Fe3+, mainly on the surface of BaFe12O19 nanocrystals. These changes were confirmed by X-ray Photoelectron Spectroscopy measurements, in which the concentration of Fe2+ increased from 69% to 82%, while the one of Fe3+ decreased from 31% to 18%. he thermoluminescent measurements reveal the same changes of Fe3+ in Fe2+ by electron capturing during irradiation, which is released as a red emission after recombination processes. The changes are explained by the increasing of some Fe-O bonds along the c-axis, mainly due to breaking a part of these bonds. The X-ray analysis confirms the changing of the parameters for the BaFe12O19 hexagonal structure.

The precipitation-hardenable nickel-based superalloy Rene 41 exhibits remarkable mechanical characteristics and high corrosion resistance at high temperatures, properties that allow it to be used in high-end applications. This research paper presents findings on the influence of thermal shocks on its microstructure, hardness, and thermal diffusivity at temperatures between 700 and 1000 degrees C. Solar energy was used for cyclic thermal shock tests. The samples were characterized using microhardness measurements, optical microscopic analysis, scanning electron microscopy coupled with EDS elemental chemical analysis, X-ray diffraction, and flash thermal diffusivity measurements. Structural transformations and the variation of properties were observed with an increase in the number of shocks applied at the same temperature and with temperature variation for the same number of thermal shocks.

Considering the increasing need for sustainable and economical energy storage solutions, the integration of layered materials such as MoS2 into these systems represents an important step toward enhancing energy sustainability and efficiency. Exploring environmentally responsible fabrication techniques, this study assesses wrinkled MoS2 thin films synthesized from distinct Mo and MoS2 targets, followed by sulfurization conducted in a graphite box. We utilized magnetron sputtering to deposit precursor Mo and MoS2 films on Si substrates, achieving thicknesses below 20 nm. This novel approach decreases sulfur by up to tenfold during sulfurization due to the confined space technique, contributing also to avoiding the formation of toxic gases such as SO2 or the necessity of using H2S, aligning with sustainable materials development. Thinner MoS2 layers were obtained post-sulfurization from the MoS2 precursors, as shown by X-ray reflectometry. Raman spectroscopy and grazing X-ray diffraction analyses confirmed the amorphous nature of the as-deposited films. Post-sulfurization, both types of films exhibited crystalline hexagonal MoS2 phases, with the sulfurized Mo showing a polycrystalline nature with a (100) orientation and sulfurized MoS2 displaying a (00L) preferred orientation. The X-ray photoelectron spectroscopy results supported a Mo:S ratio of 1:2 on the surface of the films obtained using the MoS2 precursor films, confirming the stoichiometry obtained by means of energy dispersive X-ray spectroscopy. Scanning electron microscopy and atomic force microscopy images revealed micrometer-sized clusters potentially formed during rapid cooling post-sulfurization, with an increased average roughness. These results open the way for the further exploration of wrinkled MoS2 thin films in advanced energy storage technologies.

To advance gas sensor technologies, it is essential to identify materials that exhibit both high selectivity and sensitivity. Here, we unravel the gas-sensing capabilities of indium sulfide (InS) nanosheets, particularly in relation to nitrogen dioxide (NO2) detection. Utilizing a synergistic approach that combines in situ and operando experimental methodologies with density functional theory calculations, we demonstrate that these nanosheets offer outstanding sensitivity toward NO2, characterized by a remarkably low detection threshold of 180 ppb at an operational temperature of 350 degrees C. This remarkable sensitivity is ascribed to the electronic charge redistribution around the Fermi level, facilitated by an oxygen-deficient In2O3-x surface layer that forms naturally when the InS surface is exposed to ambient conditions. A pivotal aspect of our investigation was the exploration of the influence of sulfur abstraction on these surface modifications. We demonstrate that sulfur abstraction plays a critical role in the formation and operational efficacy of the In2O3-x layer, thereby acting as a key element in the sensor mechanism. This unique surface chemistry not only amplifies the sensitivity to NO2 but also confers unparalleled selectivity over other gases and volatile organic compounds. Notably, this level of performance exceeds that of other 2D semiconductors and metal oxides, thus establishing InS nanosheets as an ideal platform for high-performance gas sensors suitable for demanding environments. Moreover, unlike many state-of-the-art sensor materials, InS-based sensors can withstand a wider variety of environmental conditions due to their superior water adsorption resistance.

In this study, we synthesized thin films of semiconductor Cu 2 CoSnS 4 (CCTS). We investigated the mechanism of CCTS electrodeposition precursor onto fluorine -doped tin oxide (FTO) surface. This investigation utilized various mixed additives (Trisodium citrate, Glycine, and Boric acid) through voltammetric and chronoamperometric techniques. The polarization cathodic indicated that the additives narrowing the potential range for electrodeposition of the four elements. The reduction of S 2 O 3 2- was mainly induced by the effect of metal ions. The current transient was analyzed using the Astley and Scharifker-Hills models. Trisodium citrate electrolyte showed an instantaneous model followed by 3D diffusion -limited growth. Both trisodium citrate mixed with glycine and trisodium citrate mixed with boric acid shifted towards the progressive nucleation model. Trisodium citrate with tartaric acid showed a strong agreement with progressive nucleation. In -situ electrochemical impedance spectroscopy (EIS) evaluated a low charge transfer resistance for CCTS precursor electrodeposition in trisodium citrate electrolyte. The X-ray diffraction and Raman analysis study revealed the stannite structure of the obtained Cu 2 CoSnS 4 thin film. The morphological properties and thickness of the films were investigated using a scanning electron microscope (SEM). The compositions were determined using energy dispersive spectroscopy which indicated different atomic ratios of Cu-Co-Sn-S. The maximum absorption was observed within the 1.5 eV range for the film deposited in the Trisodium citrate bath, as determined by spectroscopic ellipsometry.