Search Results (233)

Most astrophysical systems (except for very compact objects such as, e.g., black holes and neutron stars) in our Universe are characterized by shallow gravitational potentials, with dimensionless compactness $|\Phi |\equiv r_s/R\ll 1$, where $r_s$ and R are their Schwarzschild radius and typical size, respectively. While the existence and characteristic scales of such virialized systems depend on gravity, we demonstrate that the value of $|\Phi |$—and thus the non-relativistic nature of most astrophysical objects—arises from microphysical parameters, specifically the fine structure constant and the electron-to-proton mass ratio, and is fundamentally independent of the gravitational constant, G. In fact, the (generally extensive) gravitational potential becomes ‘locally’ intensive at the system boundary; the compactness parameter corresponds to the binding energy (or degeneracy energy, in the case of quantum degeneracy pressure-supported systems) per proton, representing the amount of work that needs to be done in order to allow proton extraction from the system. More generally, extensive properties of gravitating systems depend on G, whereas intensive properties do not. It then follows that peak rms values of large-scale astrophysical velocities and escape velocities associated with naturally formed astrophysical systems are determined by electromagnetic and atomic physics, not by gravitation, and that the compactness, $|\Phi |$, is always set by microphysical scales—even for the most compact objects, such as neutron stars, where $|\Phi |$ is determined by quantities like the pion-to-proton mass ratio. This observation, largely overlooked in the literature, explains why the Universe is not dominated by relativistic, compact objects and connects the relatively low entropy of the observable Universe to underlying basic microphysics. Our results emphasize the central but underappreciated role played by dimensionless microphysical constants in shaping the macroscopic gravitational landscape of the Universe. In particular, we clarify that this independence of the compactness, $|\Phi |$, from G applies specifically to entire, virialized, or degeneracy pressure-supported systems, naturally formed astrophysical systems—such as stars, galaxies, and planets—that have reached equilibrium between self-gravity and microphysical processes. In contrast, arbitrary subsystems (e.g., a piece cut from a planet) do not exhibit this property; well within/outside the gravitating object, the rms velocity is suppressed and G reappears. Finally, we point out that a clear distinction between intensive and extensive astrophysical/cosmological properties could potentially shed new light on the mass hierarchy and the cosmological constant problems; both may be related to the large complexity of our Universe. Full article

Mg-Sc body-centered cubic (BCC) phase-structured alloys not only exhibit superior room-temperature ductility and quasi-isotropic deformation behaviors compared to conventional hexagonal close-packed (HCP) Mg alloys in mechanical applications, but they also demonstrate a shape-memory effect that is applicable to intelligent devices. Due to the introduction of a dual-phase microstructure feature, the unveiled strengthening/toughening mechanism, and the potential benefit of Sc alloying in BCC creep deformation, it is necessary to investigate the composition and time-dependent creep behaviors of BCC Mg-Sc alloys, such as creep resistance and strain rate sensitivity at room temperature, through nano-indentation on the Mg-Sc diffusion couple. A critical finding is that as the Sc content increases from 23.01 at.% to 33.56 at.%, the BCC Mg-Sc alloy exhibits a progressive enhancement in creep resistance at room temperature, evidenced by the creep stress exponent (n) rising from 49.02 to 66.22. Furthermore, the strain rate sensitivity (m) increases from 0.02 at 26.94 at.% Sc to 0.11 at 32.63 at.% Sc, along with the Sc composition gradient. These phenomena can be attributed to the formation of ordered structures with the increasing Sc concentration, which introduce short-range local barriers to dislocation motion, as confirmed through atomic-scale microstructural analysis. Full article

To our knowledge, no magnetic B2 phase in the Al–Mn system of near-equiatomic compositions has been reported so far. Here, we investigate the structural and magnetic characteristics of Al₄₅Mn_41.8X_13.2 (X = Fe, Co or Ni) alloys. We demonstrate that adding 13.2 atomic percent magnetic 3d metal to AlMn stabilizes a ferromagnetic B2 structure, where Al and X occupy different sublattices. We employ density functional theory calculations and experimental characterizations to underscore the role of the late 3d metals for the phase stability of the quasi-ordered ternary systems. We show that these alloys possess large local magnetic moments primarily due to Mn atoms partitioned to the Al-free sublattice. The revealed magneto-chemical effect opens alternative routes for tailoring the magnetic properties of B2 intermetallic compounds for various magnetic applications. Full article

Stoichiometric single crystals of Mn₄Al₁₁ were synthesized from the elements using Sn as a flux. The crystal structure of Mn₄Al₁₁ was investigated using single crystal X-ray diffraction and showed a complex triclinic structure with a relatively small unit cell and interpenetrating networks of Mn and Al atoms. While our results generally agree with the previously reported data in the basic structure features such as triclinic symmetry and structure type, the atomic parameters differ significantly, likely due to different synthetic techniques producing off-stoichiometry or doped crystals used in the previous works. Our structural analysis showed that the view of the Mn substructure as isolated zigzag chains is incomplete. Instead, the Mn chains are coupled in corrugated layers by long Mn-Mn bonds. The high quality of the crystals with the stoichiometric composition also enabled us to study magnetic behavior in great detail and reveal previously unobserved magnetic ordering. Our magnetization measurements showed that Mn₄Al₁₁ is an antiferromagnet with T_N of 65 K. The presence of the maximum above T_N also suggests strong local interactions indicative of low-dimensional magnetic behavior, which likely stems from lowered dimensionality of the Mn substructure. Full article

Nitriding of austenitic stainless steels at low temperatures hinders the precipitation of chromium nitrides and causes the formation of a supersaturated solid solution of nitrogen atoms in the austenite lattice, known as expanded austenite. In this study, the corrosion behavior of low-temperature nitrided AISI 316L is investigated in a NaCl solution using different electrochemical techniques, electrochemical impedance spectroscopy, cyclic potentiodynamic polarization and galvanostatic tests, in order to assess the effect of test conditions. The nitrided layer has an enhanced resistance to localized corrosion, but its ability to repassivate depends on the damage extent caused by the different tests. Full article

Protein association and aggregation are fundamental processes that play critical roles in a variety of biological phenomena from cell signaling to the development of incurable diseases, including amyloidoses. Understanding the basic biophysical principles governing protein aggregation processes is of crucial importance for developing treatment strategies for diseases associated with protein aggregation, including sarcopenia, as well as for the treatment of pathological processes associated with the disruption of functional protein complexes. This work, using a set of methods such as atomic force microscopy (AFM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction, as well as bioinformatics analysis, investigated the structures of complexes formed by titin and myosin-binding protein C (MyBP-C). TEM revealed the formation of morphologically ordered aggregates in the form of beads during co-incubation of titin and MyBP-C under close-to-physiological conditions (175 mM KCl, pH 7.0). AFM showed the formation of a relatively homogeneous film with local areas of relief change. Fluorimetry with thioflavin T, as well as FTIR spectroscopy, revealed signs of an amyloid-like structure, including a signal in the cross-β region. X-ray diffraction showed the presence of a cross-β structure characteristic of amyloid aggregates. Such structural features were not observed in the control samples of the investigated proteins separately. In sarcomeres, these proteins are associated with each other, and this interaction plays a partial role in the formation of a strong sarcomeric cytoskeleton. We found that under physiological ionic-strength conditions titin and MyBP-C form complexes in which an amyloid-like structure is present. The possible functional significance of amyloid-like aggregation of these proteins in muscle cells in vivo is discussed. Full article

The study of the impact of anthropogenic and natural pollution on living organisms has become a major social issue. In this context, the objective of this work is to assess the use of the polychaete annelid Hediste diversicolor as a bioindicator organism for the quality of the marine environment. The concentration of four heavy metals (lead, copper, zinc, and cadmium) was determined in natural populations of H. diversicolor captured from four locations along the Tunisian coast using atomic absorption spectroscopy. Concentration ranges (µg/g dry weight) across all sites were as follows: Cd (0.12–0.43), Cu (3.80–6.45), Zn (18.35–42.78), and Pb (22.64–63.91). Statistical analysis confirmed significant spatial variation (Pb: F = 12.15, p < 0.001; Zn: F = 3.32, p = 0.04; Cd: F = 48.66, p < 0.001; Cu: F = 9.08, p < 0.001), with peak Pb in Bizerte and Cu in Sfax. These results highlight the influence of local environmental factors, such as industrial and urban pollution on metal accumulation in Hediste diversicolor. In this study, the accumulation of the analyzed elements in the tissues of H. diversicolor follows an increasing order as follows: Cd < Cu < Zn < Pb. Additionally, lead metal concentrations were higher than those of cadmium, zinc, and copper for all four studied locations. To our knowledge, this is the first study in Tunisia to assess heavy metal accumulation in H. diversicolor. The recorded levels were similar to, or lower than, those reported in other studies worldwide. These findings underscore the potential of H. diversicolor as a sensitive and effective bioindicator for monitoring coastal contamination and guiding environmental management strategies in Tunisia. Full article

The nanostructural features of a mechanically alloyed Sb-doped (Ti_0.4Zr_0.6)_0.7Hf_0.3NiSn thermoelectric (TE) Half-Heusler (HH) compound were addressed using Transmission Electron Microscopy (TEM) coupled with Energy Dispersive Spectroscopy measurements and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The EXAFS measurements at the Ni-K, Sn-K, Zr-K, and Hf-L₃-edge were implemented in an effort to reveal the influence of Hf and Zr incorporation into the crystal with respect to their previously measured TE properties. The substitution of Ti by Hf and Zr is expected to yield local lattice distortions due to the different atomic sizes of the dopants or/and electronic charge redistribution amongst the cations. However, the material is characterised by a high degree of crystallinity in both the short and long-range order, on average, and the nominal stoichiometry is identified as (Zr_0.42Hf_0.30Ti_0.28)NiSn_0.98Sb_0.02. The synergistic effect of minimization of extended structural defects or lattice distortions and considerable alloying-induced point defect population contributes to the improved TE properties and leads to the previously reported enhancement of the figure of merit of the mixed HHs. Full article

The present work aimed to develop a simple simulation tool to support studies of slip and other non-traditional boundary conditions in solid–fluid interactions. A mesoscale particle model (movable automata) was chosen to enable performant simulation of all relevant aspects of the system, including phase changes, plastic deformation and flow, interface phenomena, turbulence, etc. The physical system under study comprised two atomically flat surfaces composed of particles of different sizes and separated by a model fluid formed by moving particles with repulsing cores of different sizes and long-range attraction. The resulting simulation method was tested under a variety of particle densities and conditions. It was shown that the particles can enter different (solid, liquid, and gaseous) states, depending on the effective temperature (kinetic energy caused by surface motion and random noise generated by spatially distributed Langevin sources). The local order parameter and formation of solid domains was studied for systems with varying density. Heating of the region close to one of the plates could change the density of the liquid in its proximity and resulted in chaotization (turbulence); it also dramatically changed the system configuration, the direction of the average flow, and reduced the effective friction force. Full article

Calcite (CaCO₃), a widely used mineral in materials science and environmental engineering, exhibits excellent stability but has limited mechanical strength and a wide electronic band gap, restricting its broader functional applications. To address these limitations, we systematically investigated the effects of Fe(II) doping on the electronic and mechanical properties of calcite using density functional theory calculations. The results reveal that Fe atoms preferentially form a layered distribution within the lattice and significantly alter the electronic structure, notably reducing the band gap through the introduction of Fe 3d-derived states near the Fermi level. Concurrently, the incorporation of Fe strengthens the elastic constants and enhances the shear resistance, especially in directions aligned with the dopant layering. These improvements are attributed to the strong Fe-O bonding and localized lattice distortions. Furthermore, the interplay between the dopant distribution and magnetic ordering suggests that spin polarization could serve as a potential handle for property tuning. This study highlights Fe-doped calcite as a promising candidate for functional mineral-based materials and provides theoretical insights into the magnetic and structural design of carbonate systems. Full article

Quantum field theory (QFT) and general relativity (GR) are pillars of modern physics, each supported by extensive experimental evidence. QFT operates within Lorentzian spacetime, while GR ensures local Lorentzian geometry. Despite their successes, these frameworks diverge significantly in their estimations of vacuum energy density, leading to the cosmological constant problem—a discrepancy where QFT estimates exceed observed values by 123 orders of magnitude. This paper addresses this inconsistency by tracing the cooling evolution of the universe’s gauge symmetries—from $SU\left(3\right)\times SU\left(2\right)\times U\left(1\right)$ at high temperatures to $SU\left(3\right)$ alone near absolute zero—motivated by the experimental Meissner effect. This symmetry reduction posits that $SU\left(3\right)$ forms the fundamental “atoms” of vacuum energy. Our analysis demonstrates that the calculated number of $SU\left(3\right)$ vacuum atoms reconciles QFT’s predictions with empirical observations, effectively resolving the cosmological constant problem. The third law of thermodynamics, by preventing the attainment of absolute zero, ensures the stability of $SU\left(3\right)$ vacuum atoms, providing a thermodynamic foundation for quark confinement. This stability guarantees a strictly positive mass gap defined by the vacuum energy density and implies a Lorentzian quantum structure of spacetime. Moreover, it offers insights into the origins of both gravity/gauge duality and gravity/superconductor duality. Full article

During the thermal treatment of kaolinite, the main mineral phase in kaolin rock, dehydroxylation occurs, forming metakaolin through a process that has significant industrial applications. This study experimentally analyzed dehydroxylation in two kaolinite samples: a well-crystallized reference sample from the Clay Mineral Society and a locally sourced, industrial kaolin sample. The mechanism and kinetic parameters were evaluated from a series of thermogravimetric measurements. Non-isothermal kinetic analysis revealed that dehydroxylation followed a third-order (F3) reaction mechanism, with activation energies ($E_a$) ranging from 35 to 60 kcal/mol. Additionally, theoretical calculations based on Density Functional Theory were performed on four systems in which a water molecule was removed by combining OH group and H atom vacancies in the kaolinite unit cell. These models represented the onset of dehydroxylation and provided values for the reaction energy Q from first-principles calculations, which served as reference values for $E_a$. The results confirm that water molecule formation involving both OH at the kaolinite outer surface and inner surface are energetically competitive and highlight the crucial role of structural relaxations following water removal to determine Q values in the range of 30–50 kcal/mol, in very good agreement with the experiments. Full article

In the present work, the adsorption of vitamin B12 (VB12) on sugarcane-derived activated carbon (AC) was investigated with the purpose of developing a hybrid material able to degrade highly toxic and recalcitrant chlordecone (CLD) for remediating the severe environmental issue of pesticide pollution of water and soil. The AC used is made from locally accessible sugarcane bagasse. The experimental kinetic and isothermic data of VB12 adsorption on AC were modeled using multiple models, including Pseudo-Order 1 (Lagergren), Pseudo-Order 2, Elovich, and Brouers–Sotolongo models for the kinetics. The isotherms models used were Langmuir, Freundlich, Hill–Sips, Brouers–Sotolongo (BS), Brouers-Gaspard (BG), General Brouers–Sotolongo (GBS), and Redlich–Peterson (RP) models. The results showed that the BG model is the most suitable to satisfactorily describe the adsorption of VB12 on the studied AC, involving a heterogeneous adsorption mechanism onto a heterogeneous surface and providing the maximum adsorption capacity, a convenient tool to estimate the saturation level of adsorbate (i.e., chlordecone (CLD)) onto the adsorbent (AC). Voltammetric studies confirm the interaction between VB12 and the AC. Finally, molecular modeling is used to provide atomic insights showing the entrapment of VB12 inside the porous system to form a new hybrid material. The calculations provide the conformations with the best binding energy in the GaudiMM environment. Full article

Multiband and nodal-like superconductivity (SC) with s- + d-wave pairing symmetry have implied that tetragonal iron sulphide (FeS) is a distinctive testbed for exploring unexpected electronic correlations. In particular, the low-moment disordered static magnetism originating from the Fe moment leads to the possibility of the coexistence of magnetic orders (MOs) in the superconducting ground state via the tuning of electronic configurations. Here, guided by density functional theory (DFT) calculations, we found that slightly substitutionally doped chromium (Cr) atoms in tetragonal FeS single crystals can induce both considerable d-orbital reconstruction around the Fermi surface and a local magnetic moment of 2.4 µ_B at each doping site, which could highly modulate the SC ground states of the host. On this basis, a clear magnetic transition and reduced anisotropy of SC were experimentally observed. In particular, SC can survive with a doping content below 0.05. This coexistence of SC and MOs suggests strong spin correlations between Cr dopants and the host through exchange coupling. Further, an electronic temperature-related phase diagram of FeS with Cr doping contents from 0 to 0.07 is also provided. These results demonstrate that the continuous injection of local moments can be a controllable method to use to tune collective orders in unconventional iron-based superconductors. Full article

CoCrNi medium-entropy alloy (MEA) coatings prepared using laser cladding (LC) with unique properties have aroused great interest in recent years and have been widely studied. However, limited studies have been conducted on the effect of adding Al/Fe on the wear properties of CoCrNi MEA coatings prepared on the surface of stainless steel. In this study, AlCoCrFeNi, CoCrFeNi, and CoCrNi MEA LC coatings were prepared on a stainless steel substrate. The grain structures and microscopic morphologies of coatings were characterized, and the wear mechanisms were analyzed using the nano-indentation and wear tests. The hardness-strengthening mechanism was theoretically investigated using phase diagrams and molecular dynamics (MD). The phase diagram results show that the addition of Al lowered the nucleation initiation temperature, thereby increasing the nucleation rate and forming more grains. Moreover, according to the Voronoi volumes and mean–square atomic displacements (MASD) results using MD, the addition of Al makes the appearance of severe localized lattice distortions, while the addition of Fe tends to form short-range ordered structures. In summary, fine-grain strengthening and the hardness strengthening caused by local lattice distortion were the main strengthening mechanisms of AlCoCrFeNi. These findings are highly significant for expanding the application potential and provide profound insights into the wear properties of the CoCrNi MEA coatings. Full article