Materials Science (cond-mat.mtrl-sci)

Abstract: The influence of microstructure on thermoelectricity is increasingly recognized. Approaches for microstructural engineering can hence be exploited to enhance thermoelectric performance, particularly through manipulating crystalline defects, their structure, and composition. Here, we focus on a full-Heusler Fe2VAl-based compound that is one of the most promising thermoelectric materials containing only Earth-abundant, non-toxic elements. A Fe2VTa0.05Al0.95 cast alloy was atomized under a nitrogen-rich atmosphere to induce nitride precipitation. Nanometer- to micrometer-scale microstructural investigations by advanced scanning electron microscopy and atom probe tomography (APT) are performed on the powder first and then on the material consolidated by spark-plasma sintering for an increasing time. APT reveals an unexpected pick-up of additional impurities from atomization, namely W and Mo. The microstructure is then correlated with local and global measurements of the thermoelectric properties. At grain boundaries, segregation and precipitation locally reduce the electrical resistivity, as evidenced by in-situ four-point probe measurements. The final microstructure contains a hierarchy of structural defects, including individual point defects, dislocations, grain boundaries, and precipitates, that allow for a strong decrease in thermal conductivity. In combination, these effects provide an appreciable increase in thermoelectric performance.

Abstract: The bilayer of CrI3 is a prototypical van der Waals 2D antiferromagnetic material with magnetoelectric effect. It is not generally known, however, that for symmetry reasons the flexomagnetic effect, i.e., the strain gradient-induced magnetization, is also possible in this material. In the present paper, based on the first principle calculations, we estimate the flexomagnetic effect to be 200 {\mu}B{\AA} that is two orders of magnitude higher than it was predicted for the referent antiperovskite flexomagnetic material Mn3GaN. The two major factors of flexomagnetic effect enhancement related to the peculiarities of antiferromagnetic structure of van der Waals magnets is revealed: the strain-dependent ferromagnetic coupling in each layer and large interlayer distance separating antiferromagnetically coupled ions. Since 2D systems are naturally prone to mechanical deformation, the emerging field of flexomagnetism is of special interest for application in spintronics of van der Waals materials and straintronics in particular.

Abstract: We present a group theoretical and ab-initio analysis of lattice point defects in fluorographene, with a focus on neutral and negative $\text{V}_{\text{CF}}$ vacancies. By using a combination of density functional theory calculations and group theory analysis, we investigate the many-body configurations of the defects and calculate the vertical absorption and zero-phonon line energies of the excited states and their dependence with strain. The description of the defects is extended by computing their formation energy, as well as further relevant parameters as the Jahn-Teller energy for neutral $\text{V}_{\text{CF}}$ and the zero field splitting for negative $\text{V}_{\text{CF}}$ vacancies. Based on our results, we discuss possible quantum applications of these color centers when coupled to mechanical oscillation modes of the hosting two-dimensional material. The symmetry and active orbitals of the defects exhibit a parallelism with those of the extensively studied NV centers in diamond. In this context, the studied defects emerge as interesting candidates for the development of two-dimensional quantum devices based on fluorographene.

Abstract: Additive manufacturing (AM) of multi-principal element alloys (MPEAs) has gained significant attention in recent years. However, the intricate nature of phenomena such as rapid solidification, heat gradients, and residual stresses presents challenges in controlling the properties of printed components. To overcome these challenges, this study utilized machine learning (ML) approach to investigate the correlations between composition, processing parameters, testing conditions, and yield strength of single-phase MPEAs within the CoCrFeMnNi system, produced via laser-melt deposition and laser powder-bed fusion. Multiple algorithms, including Random Forest, Gradient Boosting, and Extreme Gradient Boosting, were trained, and tested. SHapley Additive exPlanations algorithm was employed to analyze the contributions of input features. All models exhibited reasonable accuracy, with Random Forest performing the best. The impact of data sparsity was examined, and minimal sensitivity to data splitting was observed. Notably, the research yielded valuable insights into the key features influencing the yield strength of MPEAs, showcasing the potential of ML in accurately modeling the material properties of additively manufactured components.

Abstract: We have looked into the structural, mechanical, optoelectronic, superconducting state and thermophysical aspects of intermetallic compound Zr$_3$Ir using the density functional theory (DFT). Many of the physical properties, including direction dependent mechanical properties, Vickers hardness, optical properties, chemical bonding nature, and charge density distributions, are being investigated for the first time. According to this study, Zr$_3$Ir exhibits ductile features, high machinability, significant metallic bonding, a low Vickers hardness with low Debye temperature, and a modest level of elastic anisotropy. The mechanical and dynamical stabilities of Zr$_3$Ir have been confirmed. The metallic nature of Zr$_3$Ir is seen in the electronic band structures with a high electronic energy density of states at the Fermi level. The bonding nature has been explored by the charge density mapping and bond population analysis. The tetragonal Zr$_3$Ir shows a remarkable electronic stability, as confirmed by the presence of a pseudogap in the electronic energy density of states at the Fermi level between the bonding and antibonding states. Optical parameters show very good agreement with the electronic properties. The reflectivity spectra reveal that Zr$_3$Ir is a good reflector in the infrared and near-visible regions. Zr$_3$Ir is an excellent ultra-violet (UV) radiation absorber. High refractive index at visible photon energies indicates that Zr$_3$Ir could be used to improve the visual aspects of electronic displays. All the optical constants exhibit a moderate degree of anisotropy. Zr$_3$Ir has a moderate melting point, high damage tolerance, and very low minimum thermal conductivity. The thermomechanical characteristics of Zr$_3$Ir reveal that it is a potential thermal barrier coating material. The superconducting state parameters of Zr$_3$Ir are also explored.

Abstract: The expanded phthalocyanine (EPc) single-layer sheets with double transition metals (labeled as TM2EPc, TM = Sc-Zn) are predicted to be a new class of two-dimensional (2D) metal-organic materials with a series of favorable functional properties by means of systematic first-principle calculations and molecular dynamics simulations. The strong coordination between metal and EPc substrate accounts for the excellent structural stability. Chemical bonding analysis has demonstrated the absence of TM-TM bonding. Each metal center is isolated, but connected to the organic framework by four 2c-2e TM-N {\sigma}-bonds to form an extended 2D network. Unexpectedly, it is found that the V2EPc is an antiferromagnetic metal with Dirac cone, while Cr2EPc exhibits ferromagnetic Dirac half-metallicity, which is not common in 2D materials. Excitingly, the ferromagnetic Cr2EPc and antiferromagnetic Mn2- and Fe2-EPc have high magnetic transition temperatures of 223, 217, and 325 K, respectively, which are crucial for the practical applications of spintronics. Cr2EPc can maintain the Dirac half-metallicity under -6 % ~ 2 % biaxial strains, and Fe2EPc can transform from semiconductor to half-metal by applying -6 % ~ -10 % compressive strains. Additionally, the TM2EPc monolayers exhibit a full response to visible light and some materials have strong absorption in the ultraviolet and infrared regions in addition to visible light, showing extraordinary solar light-harvesting ability. Notably, the designed type-II heterojunctions Fe2EPc/SnC, Co2EPc/GeS, and Ni2EPc/2H-WSe2 have high power conversion efficiency (PCE > 15%), especially the PCE of Ni2EPc/2H-WSe2 reaches 25.19%, which has great potential in solar cell applications. All these desired properties render 2D TM2EPc monolayers promising candidates for future applications in nanoelectronics, spintronics,optoelectronics, and photovoltaic devices.

Abstract: We describe the formation of swift heavy ion tracks in polyethylene (PE) by combining the Monte Carlo code TREKIS, which models electronic excitation in nanometric proximity of the ion trajectory, with the molecular dynamics simulating a response of the atomic system to the perturbation. The model predicts circular tracks in amorphous PE but elliptical ones in crystalline PE caused by preferential propagation of excitation along polymer chains during the cooling stage. The obtained track sizes and shapes agree well with the high-resolution microscopy of tracks in PE. The velocity effect in PE is shown: the track parameters differ for ions with the same energy losses but different velocities.

Abstract: Slip intermittency and stress oscillations in titanium alloy Ti-7Al-O that were observed using in-situ far-field high energy X-ray diffraction microscopy (ff-HEDM) are investigated using a discrete dislocation plasticity (DDP) model. The mechanistic foundation of slip intermittency and stress oscillations are shown to be dislocation escape from obstacles during stress holds, governed by a thermal activation constitutive law. The stress drop events due to <a>-basal slip are larger in magnitude than those along <a>-prism, which is a consequence of their differing rate sensitivities, previously found from micropillar testing. It is suggested that interstitial oxygen suppresses stress oscillations by inhibiting the thermal activation process. Understanding of these mechanisms is of benefit to the design and safety assessment of jet engine titanium alloys subjected to dwell fatigue.

Abstract: The binary fluorite oxide Hf0.5Zr0.5O2 tends to grab a significant amount of notice due to the distinct and superior ferroelectricity found in its metastable phase. Stabilizing the metastable ferroelectric phase and delineating the underlying growth mechanism, however, are still challenging. Recent discoveries of metastable ferroelectric Hf0.5Zr0.5O2 epitaxially grown on structurally dissimilar perovskite oxides have triggered intensive investigations on the ferroelectricity in materials that are nonpolar in bulk form. Nonetheless, the growth mechanism for the unique fluorite/perovskite heterostructures has yet to be fully explored. Here we show that the metastable ferroelectric Hf0.5Zr0.5O2 films can be stabilized even on a one-unit-cell-thick perovskite La0.67Sr0.33MnO3 buffer layer. In collaboration with scanning transmittance electron microscopy (STEM) based characterizations, we show that monolayer reconstruction driven by interfacial redox reactions plays a vital role in the formation of a unique heterointerface between the two structurally dissimilar oxides, providing the template monolayer that facilitates the epitaxial growth of the metastable HZO films. Our findings offer significant insights into the stabilization mechanism of the ferroelectric Hf0.5Zr0.5O2, and this mechanism could be extended for exploring functional metastable phases of various metal oxides.

Abstract: Vector magnetometry provides more information than scalar measurements for magnetic surveys utilized in space, defense, medical, geological and industrial applications. These areas would benefit from a mobile vector magnetometer that can operate in extreme conditions. Here we present a scanning fiber-coupled nitrogen vacancy (NV) center vector magnetometer. Feedback control of the microwave excitation frequency is employed to improve dynamic range and maintain sensitivity during movement of the sensor head. Tracking of the excitation frequency shifts for all four orientations of the NV center allow us to image the vector magnetic field of a damaged steel plate. We calculate the magnetic tensor gradiometry images in real time, and they allow us to detect smaller damage than is possible with vector or scalar imaging.