National Institute Of Materials Physics - Romania

VO2-based thermochromic thin films are promising for energy-efficient smart window applications due to their reversible metal-insulator transition near 68 degrees C. However, practical limitations such as high transition temperature, limited visible transmittance and substrate compatibility remain challenges. In this study, VO2/TiO2 bilayer thin films were deposited by PLD on glass, fused silica, and quartz substrates to investigate the effect of buffer layer, film thickness, and strain on their structural, optical, and electrical properties. Comprehensive characterization using in situ grazing incidence X-ray diffraction, synchrotron GIXRD, and high-resolution Transmission Electron Microscopy revealed that TiO2 buffer layers, in the anatase or brookite phase, significantly influences the growth orientation and strain distribution in VO2 films. Residual strain mapping indicated up to 5 % compressive strain in VO2 and >10 % tensile strain in the TiO2, promoting partial stabilization of the intermediate M2 phase. Thickness-dependent studies showed a strong trade-off between optical transmittance and electrical switching amplitude: thinner films exhibited higher luminous transmittance (up to 30 %) and sharper IR switching, while thicker films yielded larger resistivity switching contrast. These results demonstrate that interfacial strain, film thickness, and substrate choice are critical parameters for tailoring VO2-based coatings and optimizing thermochromic performance for scalable smart window technologies.

This study explores strategies to enhance the ionic conductivity of LiTi2(PO4)(3) (LTP), a promising NASICON-type solid electrolyte for next-generation solid-state batteries. Although LTP exhibits an open-framework structure favorable for Li+ diffusion, its intrinsic low ionic conductivity limits practical application. To address this, this research investigates the impact of co-doping titanium (Ti4+) with iron (Fe3+) and yttrium (Y3+) in Li1+xFeyYx-yTi2-x(PO4)(3) (where x = 0.3 and y = 0.1, 0.15, 0.2), aiming to increase charge carrier concentration and induce structural distortion beneficial for ion transport. Samples were synthesized via solid-state reaction and characterized by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The local environment of Fe3+ was further analyzed using electron paramagnetic resonance and Mossbauer spectroscopy. Spark Plasma Sintering (SPS) was employed to obtain dense ceramics and suppress secondary phase formation. The electrical properties were investigated by impedance spectroscopy over the 150K-400K temperature range, and the relaxation dynamics and conduction mechanisms were analyzed using the modulus formalism and a.c. methods conductivity. The Li1.3Fe0.15Y0.15Ti1.7(PO4)(3) composition exhibited the highest room-temperature conductivity of 8.21 x 10(-4) S/cm, confirming the combined effect of Fe3+/Y3+ co-doping and SPS densification in enhancing the ionic transport properties of LTP-based solid electrolytes.

In 2025, the Year of Quantum Science and Technology (https://quantum2025.org/), we celebrate a century of quantum mechanics, witnessing a surge in activities that illuminate its inherent strangeness and drive technological innovation. Superconductivity, discovered 114 years ago, stands as a prime example, offering direct and compelling evidence of macroscopic quantum phenomena. Beyond its ability to conduct immense currents without loss, superconductivity reveals the quantum realm operating on a scale we can directly observe and manipulate. The macroscopic quantum coherence, where an ensemble of particles is described by a single wave function, leads to remarkable consequences: dissipation-less current and flux quantization-the basic properties exploited in superconducting quantum circuit fabrication. This Roadmap has been inspired by intensive discussions and collaborations emerging from the European Cooperation in Science & Technology COST-Action CA21144 (SuperQuMap-Superconducting Nanodevices and Quantum Materials for Coherent Manipulation). The aim of the COST Action SuperQuMap is to establish a strong European network centered on macroscopic quantum behavior in superconductors, bringing together groups of different backgrounds and more than 30 countries. The roadmap outlines the network's concrete activities, driving advancements in superconductor-based quantum technologies and charting future directions. Spanning fundamental research to practical applications, the roadmap incorporates insights from industry partners developing quantum computation. It begins by exploring quantum materials, highlighting how topology and electronic correlations could catalyze a quantum leap in technology. We then delve into manipulating the superconducting phase, leveraging advancements in magnetism, 3D fabrication, and tunable correlations. Further, we showcase the advanced microscopy techniques-such as angle-resolved photoemission spectroscopy and scanning probes-used to visualize quantum behavior. Finally, and crucially, we detail the quantum devices developed within the network, and their transformative impact on modern quantum computing approaches.

The paper presents a comparative experimental study of heat-transfer behavior in three alloys considered candidate materials for nuclear reactors: the austenitic stainless steel 316L, Zircaloy-4 (currently used in CANDU reactors), and an ODS alloy with a ferritic matrix. The investigation was conducted across five temperature intervals, each sample being subjected to a thermal shock through short-term overheating to the upper limit of its respective interval. The variation of thermal diffusivity in the three alloys was determined as a function of both measurement temperature and applied thermal shock, and trends in heat-transfer behavior were compared across the five temperature ranges. The experimental results show that up to 400 degrees C, Zircaloy-4 exhibits the highest thermal diffusivity, followed by the ODS alloy, with the lowest values measured for 316L steel. At approximately 450 degrees C, the ratio between 316L and the ODS alloy reverses. Beyond this point, increasing the temperature up to 900 degrees C is accompanied by a continuous rise in thermal diffusivity for both 316L stainless steel and Zircaloy-4. In contrast, for the ODS steel, increasing temperature leads to a continuous decrease in thermal diffusivity, reaching a minimum near the Curie point. The novelty of the study lies in the comparative assessment of the influence of temperature on the heat-transfer process in three alloys relevant to nuclear energy, covering the operating temperature ranges of CANDU and ALFRED reactors, as well as potential accidental overheating up to 900 degrees C. A particular feature of the work is the prior application of a short-duration overheating step produced using solar energy. The results are relevant not only for nuclear reactors but also for other high-temperature applications in corrosive environments.

This review paper delves into the concept of critical current density (Jc) in high-temperature superconductors (HTS) across macroscopic, mesoscopic, and microscopic perspectives. Through this exploration, a comprehensive range of connections is unveiled aiming to foster advancements in the physics, materials science, and the engineering of applied superconductors. Beginning with the macroscopic interpretation of Jc as a central material law, the review traces its development from C.P. Bean's foundational work to modern extensions. Mesoscopic challenges in understanding vortex dynamics and their coherence with thermodynamic anisotropy regimes are addressed, underscoring the importance of understanding the limitations and corrections implicit in the macroscopic measurement of J0, linked with mesoscopic phenomena such as irradiation effects, defect manipulation, and vortex interactions. The transition to supercritical current densities is also discussed, detailing the superconductor behavior beyond critical thresholds with a focus on flux-flow instability regimes relevant to fault current limiters and fusion energy magnets. Enhancing J0 through tailored material microstructures, engineered pinning centers, grain boundary manipulation, and controlled doping is explored, along with radiation techniques and their impact on large-scale energy systems. Emphasizing the critical role of J0, this review focuses on its physical optimization and engineering manipulation, highlighting its significance across diverse sectors.

Cu2ZnSnS4 (CZTS) is an emerging material with significant potential as an absorber layer for solar cells. Precise control over the film preparation process is crucial for attaining optimal transport, electrical, and optical properties. This study investigates the effect of sulfurization duration on the properties of CZTS films deposited onto soda lime glass substrates via spray pyrolysis, followed by annealing at 550 degrees C in a sulfur-rich environment under argon flow. X-ray diffraction and Raman spectroscopy confirmed the formation of monophasic CZTS, with the highest phase purity observed for films sulfurized for 5 min. Scanning electron microscopy demonstrated notable morphological and microstructural enhancements due to the sulfurization process, while energydispersive spectroscopy confirmed near-ideal stoichiometric composition (Cu:Zn:Sn:S approximate to 2:1:1:4). Optical spectroscopy determined the band gap of the films to be between 1.40 and 1.50 eV. The electrical transport properties were investigated up to 130 degrees C, revealing p-type conductivity, with Seebeck coefficients ranging from 30 to 70 mu V K-2 and low electrical resistivity, displaying semiconductor-like behavior. The maximum power factor achieved was 0.36 mu W mK-2 at 130 degrees C for the sample sulfurized for 5 min. These findings suggest that a 5-min sulfurization time is optimal for producing single-phase CZTS films characterized by uniform morphology, accurate stoichiometric composition, and an ideal direct band gap. Given its favorable thermoelectric properties, CZTS shows significant promise as a material for thermoelectric applications, particularly in waste heat recovery systems. The results indicate that CZTS films could be further optimized for use in thermoelectric devices, and future studies could focus on enhancing their thermoelectric performance by adjusting sulfurization conditions and exploring material modifications.

Some alloys exhibit not only shape memory but also thermal memory, retaining information about the highest temperature reached during heating. This phenomenon occurs when the alloy starts in the martensite phase and is then heated up to an "arrest temperature" that lies strictly within the martensite-austenite transformation range, without the transformation being completed. The "reading" of this memory is performed by cooling the alloy back into martensite and then reheating it to full austenite in a calorimeter, where the phase transition heat flow displays a dip near the arrest temperature. This unique behavior naturally qualifies such materials as temperature sensors, more precisely as maximum thermometers, which by definition indicate the maximum temperature reached during a given time interval. In this paper, we extend existing thermal memory studies to various polycrystalline shape memory alloys with Heusler structure, prepared by rapid solidification and based on NiFeGa (with Co, Al, Gd, Nd additions) and NiMnGa compositions. We analyze the possibility of shifting the transformation temperatures - and implicitly the thermal memory sensitivity range - through composition variations and thermal treatments. The thermal memory effect was consistently observed, and in fact quite readily, across all samples at various temperatures within the sensitivity interval. In contrast to classical maximum thermometers, these materials are capable of also memorizing multiple temperatures, as long as they are recorded in a strictly decreasing order. The use of sample groups and calibration aspects are discussed. Finally, we emphasize that shape memory alloys with these compositions and preparation methods show potential for recording temperatures across a wide range - from 0 degrees C to above 100 degrees C. A statistical geometry model, based on the redistribution of the martensite plates sizes, qualitatively reproduces the observed thermal memory features.

The precise control of the magnetic compensation temperature (theta c) in ferrimagnetic garnets is essential for the development of cutting-edge ultrafast customizable spintronic devices. In this work we demonstrate how fine variation in stoichiometry and cation distribution in iron gadolinium garnets significanty influences theta c. Two samples of Gd3Fe5O112 garnets synthesized via a new hydrothermal method and a conventional solid-state reaction, respectively, were considered. The complex study was carried out using a complex approach combining X-ray diffraction, magnetometry, and M & ouml;ssbauer spectroscopy. Atomic-scale analysis revealed with unprecedent accuracy a cationic inversion between Fe3+ ang Gd3+ at octahedral and dodecahedral sites in both samples, and their chemical compositions were determined as Gd2.70Fe4.76O11.9 and Gd2.96Fe4.68O11.5, respectively. These local rearrangements have been shown to have a consistent influence on theta c (290 K and 317 K, respectively) around room temperature, emphasizing the high sensitivity of exchange interactions to internal atomic order. Results clearly illustrate the strong correlation between the processing, atomic configuration and macroscopic magnetic behavior, establishing a new paradigm for the design of garnet-based materials with tunable theta c. The strategy for the accurate determination of cation inversion illustrated in this work exhibits great potential in guiding material innovations for next-generation spintronics.

In this study, thermal analysis (including differential thermal analysis and thermogravimetric analysis coupled with mass spectrometry) was used to design the sintering process of alumina as well as Al2O3-Mo and Al2O3-Ni green bodies obtained by digital light processing (DLP) 3D printing. The measurements were performed for selected organic additives, which are commonly used in the DLP technique, such as photoinitiators, dispersing agents, and organic monomers. Additionally, metallic powders (Ni, Mo), as well as ceramic and composite green bodies, have been subjected to thermal analysis. The obtained results allowed us to determine proper sintering conditions for a two-step sintering program. Firstly, the organic phase was burnt out at 400 degrees C in the air. At this temperature, metallic powders have not yet started to oxidize, and most of the organic additives have already been eliminated from the sample. The second step was performed using spark plasma sintering at 1150 degrees C with a pressure equaling 60 MPa in an argon atmosphere to prevent the oxidation of metals. The samples were gradually cooled down to 800 degrees C at a cooling rate of 35 degrees C min-1 and then furnace-cooled to room temperature, preventing the formation of intrinsic defects (microcracks) in a multicomponent ceramic-metal composite. The XRD and SEM-EDS analysis allowed us to conclude that the obtained composites are well densified, no other phases apart from alumina and metals are present in the samples, and that the alumina grain growth is smaller than for conventional sintering. An increase in fracture toughness for the composite samples was observed compared to pure alumina.

Mg2Si1-xSnx solid solutions are promising thermoelectric materials due to the inexpensive and abundant nature of their components, their environmental friendliness and their flexibility in terms of optimising their thermoelectric properties. In this study, we report on the thermoelectric properties enhancement of n-type Mg2Si0.3Sn0.7 solid solutions doped with aliovalent elements, donor (Bi) and acceptor (Ag), respectively. Samples were synthesized via conventional melting followed by spark plasma sintering. Co-doping led to carrier concentration and mobility tuning, resulting in a significantly increased Seebeck coefficient while maintaining high electrical conductivity. Consequently, the power factor reached 44.67 x 10(-3) Wm(-)(1) K-2 at 650 K for Mg1.98Ag0.02Si0.29Sn0.69Bi0.02. Furthermore, lattice thermal conductivity was reduced via enhanced phonon scattering caused by point defects and suppressed bipolar conductivity attributed to a widened band gap and increased carrier density. The combined effects yielded improved ZT values, peaking at 1.12 at 674 K and 1.13 at 721 K for Mg1.99Ag0.01Si0.29Sn0.69Bi0.02 and Mg1.98Ag0.02Si0.29Sn0.69Bi0.02, respectively. These results highlight the potential of Ag-Bi co-doping as a robust strategy to enhance the thermoelectric efficiency of Mg2Si1-xSnx based materials.