National Institute Of Materials Physics - Romania

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.

Bulk MgB2 disks (20 mm in diameter and 3.8 mm in thickness) added with 0, 1.5, 3.0, 4.5, 6.0, and 9.0 wt% silver are obtained by in situ spark plasma sintering. Samples are characterized by structural, microstructural, magnetic, and magnetic levitation zero-field-cooling and field-cooling measurements. Superconducting transitions are sharp with critical temperatures being in the range of 36.5-37.9 K. The vertical (F z,ZFC) and lateral (F x) levitation force show maximum values for samples added with 1.5 or 3 wt Ag%. At 20 K they are +17.78 N and 6.37 N, respectively. A maximum for zero field critical current density and pinning force is found for the sample added with 3 wt%. Roughly, as expected, levitation force correlates with zero-field critical current density, but other details influence this dependance. Results indicate that silver is an effective addition to MgB2 for enhancement of levitation performance.

Terahertz (THz) spintronic emitters represent a novel class of heterostructures composed of ferromagnetic (FM) and non-magnetic (NM) metallic layers that strongly emit terahertz (THz) radiation upon femtosecond laser pulse excitation. The optimal geometric configuration to maximize the strength of the emission is currently considered a trilayer structure, NM1/FM/NM2, where the FM layer is confined between two NM layers with opposite spin Hall angles. To investigate this, ultrathin Ta/Fe/Pt trilayers are fabricated and their THz emission profiles are analyzed. These results show that the highest THz emission is achieved for the sample of Ta (1.5 nm)/Fe (2 nm)/Pt (2 nm), demonstrating a significant enhancement compared to standard FM/NM bilayers. Furthermore, the thickness dependence of the THz emission is modeled in Ta (t1 nm)/Fe (2 nm)/Pt (t2 nm), varying t1 and t2 from 1 nm to 3 nm. From this analysis, spin diffusion lengths of lambda Pt = 1.2 nm and lambda Ta = 0.85 nm are extracted. The structure-property relationship is assessed via transmission electron microscopy, revealing that an epitaxial single-crystalline Ta layer covers the MgO surface with Ta adopting a high-resistivity fcc allotropic phase with a lattice parameter of a = 0.436 nm. This phase, together with the prerequisite for low Ta+Pt thickness, emerges as a key factor in achieving high THz emission from trilayer structures.

Although the DPPH (2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl) radical is known for its indefinite stability, both in solid and in solution, and therefore no dimerization reaction occurs, the DPPH-dimer has been obtained by an alternative synthesis. Oxidation of the DPPH-dimer led to the corresponding DPPH-diradical, practically exhibiting all of the known properties of the simple DPPH radical. The structures were confirmed using 1H and 13C NMR, IR, UV-vis, HR-MS, and electron spin resonance (for the diradical) analyses. Additionally, cyclic voltammetry and superconducting quantum interference device (SQUID) measurements were performed to investigate the electrochemical and magnetic properties of the DPPH-diradical. DFT calculations revealed that the ground state was an open-shell singlet. The diradical character y of the ground state and vertical S-T gap were 0.279 and -5.81 kcal mol-1, respectively.

In this study we present the effect of the irradiation of sintered MgB2 with protons of intermediate energies, from 8.6 to 15.07 MeV, at a constant fluence of 2.86 x 10(16) p/cm(2). We found clear evidence that the disorder generated by irradiation leads to a weak suppression of the critical temperature T-c and an increase of the critical current density J(c) with increasing proton energy, as expected. However, it was found that thermomagnetic instabilities and vortex creep phenomena strongly depend on the ratio between proton range and the sample thickness. The number of the macroscopic flux jumps (MFJ) is highest and manifest up to 28 K in the sample with the proton range shorter than sample thickness (P-1) and decrease as the proton energy increases. Similarly, the relaxation rate of the irreversible magnetization is the lowest in the sample P-1 and increases with proton energy. We tentatively attribute this effect to the protons that stop within MgB2 and interact with the local structure/atomic composition.

Recent achievements in ultrafast spin physics have enabled the use of heterostructures composed of ferromagnetic (FM)/non-magnetic (NM) thin layers for terahertz (THz) generation. The mechanism of THz emission from FM/NM multilayers has been typically ascribed to the inverse spin Hall effect (ISHE). In this work, we probe the mechanism of the ISHE by inserting a second ferromagnetic layer in the form of an alloy between the FM/NM system. In particular, by utilizing the co-sputtering technique, we fabricate Fe/L1(0)-FePt/Pt ultra-thin heterostructures. We successfully grow the tetragonal phase of FePt (L1(0)-phase) as revealed by X-ray diffraction and reflection techniques. We show the strong magnetic coupling between Fe and L1(0)-FePt using magneto-optical and Superconducting Quantum Interference Device (SQUID) magnetometry. Subsequently, by utilizing THz time domain spectroscopy technique, we record the THz emission and thus we the reveal the efficiency of spin-to-charge conversion in Fe/L1(0)-FePt/Pt. We establish that Fe/L1(0)-FePt/Pt configuration is significantly superior to the Fe/Pt bilayer structure, regarding THz emission amplitude. The unique trilayer structure opens new perspectives in terms of material choices for the future spintronic THz sources.

Superconducting bulk disks, of 20 mm in diameter and similar to 3.5-mm thickness of MgB2 were prepared by spark plasma sintering. Samples are co-added with 10 wt. % hexagonal BN (h-BN) or graphene (G) and other additives (B4C, Te, cubic BN, fullerene C-60, or Repa-C6H10O7Ge2 (GEP)), where h-BN and G are introduced in the composite to provide full machinability by chipping of the composite and the other additives to modify microstructure and superconducting characteristics. Measurements of trapped magnetic field B-tr for a fixed rate of the applied magnetic field decrease (0.00015 T/s) indicate that samples with G show less flux jumps, but a higher thermomagnetic stability is accompanied by lower values of B-tr than for samples with h-BN. The highest maximum B-tr at 12 K for samples added with h-BN or graphene was recorded for MgB2(Te)(0.01) + 10wt.% h-BN (3.48 T) and MgB2(B4C)(0.01) + 10wt.% G (2.73 T), respectively. These values of maximum trapped field were determined for an applied field of - 2.5 and - 1.8 T. Results suggest that machinable MgB2-based composites show potential for bulk superconducting magnet applications.

Exchange biasing of oxide-DMSs (dilute magnetic semiconductors) in exchange bias nanostructures, which are essential for use in spin-based electronic devices, presents a challenge because, in the available literature, exchange coupling typically occurs between secondary phases, with oxide-DMS not being involved. The experimental results presented in this study are consistent with the participation of Fe-doped ZnO (oxide-DMS) as a ferromagnetic (FM) component in the exchange bias (EB) of the ZnFeO system. The study was conducted on ZnFeO nanostructured systems with a 2 at% iron concentration, which were processed by the decomposition of Zn-Fe-propionates and annealing in an Ar-5%H-2 atmosphere at temperatures between 480 degrees C and 510 degrees C and times between 60 min and 100 min. The magnetization investigations support the coexistence of an FM phase with a Curie temperature above 300 K, an antiferromagnetic phase with a Neel transition, T-N, at about 200 K and a disordered spin-glass-like phase with a transition, T-SG, at about 50 K. All samples show the EB effect, characterised by a high EB blocking temperature, T-B approximate to 200 K, and/or a low EB blocking temperature, T-B approximate to 50 K. The analysis of experimental data provides evidence for the exchange biasing of Fe-ZnO (oxide-DMS) by the FeO (AFM) phase and/or SG phase. The exchange-biasing of Fe-ZnO oxide-DMS is promising for developing new materials (e.g., FM electrodes) to improve the efficiency of injecting highly spin-polarised currents in spintronic devices and to increase the device operating temperature.

Design of products involves functional and sensory aspects, where surfaces play an important role. This study uses (i) sensory attributes to show that tactile sensation is highly dependent on surface roughness, and (ii) variation in pupil diameter to suggest that roughness close to fingerprint geometry causes less arousal. A panel of over 30 participants explored six plexiglass surfaces with different roughness generated by variations in milling speed and depth. The pattern obtained on the samples is periodic in one direction, with an average wavelength between 113 mu m to 600 mu m and an average height between 13 mu m and 123 mu m. During a blind touch, the sensory attributes of smoothness, grip and quality of each sample were evaluated by the panellists, as well as the emotional attributes of valence and arousal. The evolution of pupil diameter over time was also recorded, and its average value during the first two seconds of touch was considered as a new emotional attribute. These attributes made it possible to calculate six centred indicators, ranging between -1 and 1, for each panellist and each sample. Statistical analysis of these indicators showed that the declared valence is correlated with smoothness, grip, and quality, all gradually decreasing as roughness increases. These results will allow product designers to improve the hedonic experience of future users. To more precisely analyse arousal, valence, and the evolution of pupil diameter, three of the six samples, manufactured with the same cutting tool, were considered. Valence and arousal appeared relatively difficult to verbalised, but the pupil diameter allowed the samples to be discriminated. The sample with a roughness close to the geometry of the fingerprint appeared to be the least emotional.