Effects of Low-Temperature Heat Treatment on Mechanical and Thermophysical Properties of Cu-10Sn Alloys Fabricated by Laser Powder Bed Fusion

This study investigated the impact of low-temperature heat treatments on the mechanical and thermophysical properties of Cu-10Sn alloys fabricated by a laser powder bed fusion (LPBF) additive manufacturing (AM) process. The microstructure, phase structure, and mechanical and thermal properties of the LPBF Cu-10Sn samples were comparatively investigated under both the as-fabricated (AF) condition and after low-temperature heat treatments at 140, 180, 220, 260, and 300 °C. The results showed that the low-temperature heat treatments did not significantly affect the phase and grain structures of the Cu-10Sn alloys. Both pre- and post-treatment samples displayed consistent grain sizes, with no obvious X-ray diffraction angle shift for the α phase, indicating that atom diffusion of the Sn element is beyond the detection resolution of X-ray diffractometers (XRD). However, the 180 °C heat-treated sample exhibited the highest hardness, while the AF samples had the lowest hardness, which was most likely due to the generation of precipitates according to thermodynamics modeling. Heat-treated samples also displayed higher thermal diffusivity values than their AF counterpart. The AF sample had the longest lifetime of ~0.19 nanoseconds (ns) in the positron annihilation lifetime spectroscopy (PALS) test, indicating the presence of the most atomic-level defects.

Investigating the Free Volumes as Nanospaces in Human Stratum Corneum Lipid Bilayers Using Positron Annihilation Lifetime Spectroscopy (PALS)

This work is the first one that provides not only evidence for the existence of free volumes in the human stratum corneum but also focuses on comparing these experimental data, obtained through the unique positron annihilation lifetime spectroscopy (PALS) method, with theoretical values published in earlier works. The mean free volume of 0.269 nm was slightly lower than the theoretical value of 0.4 nm. The lifetime τ3 (1.83 ns with a coefficient of variation CV of 3.21%) is dependent on the size of open sites in the skin. This information was used to calculate the free volume radius R (0.269 nm with CV 2.14%), free volume size Vf (0.081 nm3 with CV 4.69%), and the intensity I3 (9.01% with CV 10.94%) to estimate the relative fractional free volume fv (1.32 a.u. with CV 13.68%) in human skin ex vivo. The relation between the lifetime of o-Ps (τ3) and the radius of free volume (R) was formulated using the Tao-Eldrup model, which assumes spherical voids and applies to sites with radii smaller than 1 nm. The results indicate that PALS is a powerful tool for confirming the existence of free volumes and determining their size. The studies also focused on describing the probable locations of these nanospaces in SC lipid bilayers. According to the theory, these play an essential role in dynamic processes in biological systems, including the diffusion of low-molecular-weight hydrophobic and moderately hydrophilic molecules. The mechanism of their formation has been determined by the molecular dynamics of the lipid chains.

Developing a novel positronium biomarker for cardiac myxoma imaging

PURPOSE

Cardiac myxoma (CM), the most common cardiac tumor in adults, accounts for 50-75% of benign cardiac tumors. The diagnosis of CM is often elusive, especially in young stroke survivors and transthoracic echocardiography (TTE) is the initial technique for the differential diagnostics of CM. Less invasive cardiac computed tomography (CT) and magnetic resonance imaging (MRI) are not available for the majority of cardiac patients. Here, a robust imaging approach, ortho-Positronium (o-Ps) imaging, is presented to determine cardiac myxoma extracted from patients undergoing urgent cardiac surgery due to unexpected atrial masses. We aimed to assess if the o-Ps atom, produced copiously in intramolecular voids during the PET imaging, serves as a biomarker for CM diagnosing.

METHODS

Six perioperative CM and normal (adipose) tissue samples from patients, with primary diagnosis confirmed by the histopathology examination, were examined using positron annihilation lifetime spectroscopy (PALS) and micro-CT. Additionally, cell cultures and confocal microscopy techniques were used to picture cell morphology and origin.

RESULTS

We observed significant shortening in the mean o-Ps lifetime in tumor with compare to normal tissues: an average value of 1.92(02) ns and 2.72(05) ns for CM and the adipose tissue, respectively. Microscopic differences between tumor samples, confirmed in histopathology examination and micro-CT, did not influenced the major positronium imaging results.

CONCLUSIONS

Our findings, combined with o-Ps lifetime analysis, revealed the novel emerging positronium imaging marker (o-PS) for cardiovascular imaging. This method opens the new perspective to facilitate the quantitative in vivo assessment of intracardiac masses on a molecular (nanoscale) level.

Multicomponent Quantum Mechanics/Molecular Mechanics Study of Hydrated Positronium

We propose a model for solvated positronium (Ps) atoms in water, based on the sequential quantum mechanics/molecular mechanics (s-QM/MM) protocol. We developed a Lennard-Jones force field to account for Ps-water interactions in the MM step. The repulsive term was obtained from a previously reported model for the solvated electron, while the dispersion constant was derived from the Slater-Kirkwood formula. The force field was employed in classical Monte Carlo (MC) simulations to generate Ps-solvent configurations in the NpT ensemble, while the quantum properties were computed with the any-particle molecular orbital method in the subsequent QM step. Our approach is general, as it can be applied to other liquids and materials. One basically needs to describe the solvated electron in the environment of interest to obtain the Ps solvation model. The thermodynamical properties computed from the MC simulations point out similarities between the solvation of Ps and noble gas atoms, hydrophobic solutes that form clathrate structures. We performed convergence tests for the QM step, with particular attention to the choice of basis set and expansion centers for the positronic and electronic subsystems. Our largest model was composed of the Ps atom and 22 water molecules in the QM region, corresponding to the first solvation shell, surrounded by 128 molecules described as point charges. The mean electronic and positronic vertical detachment energies were (4.73 ± 0.04) eV and (5.33 ± 0.04) eV, respectively. The latter estimates were computed with Koopmans' theorem corrected by second-order self-energies, for a set of statistically uncorrelated MC configurations. While the Hartree-Fock wave functions do not properly account for the annihilation rates, they were useful for numerical tests, pointing out that annihilation is more sensitive to the choice of basis sets and expansion centers than the detachment energies. We further explored a model with reduced solute cavity size by changing the Ps-solvent force field. Although the pick-off annihilation lifetimes were affected by the cavity size, essentially the same conclusions were drawn from both models.

Defect engineering in thermoelectric materials: what have we learned?

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Adjusting the Valence State of Vanadium in VO2 (B) by Extracting Oxygen Anions for High-Performance Aqueous Zinc-Ion Batteries

VO₂ generally has a higher theoretical capacity and layered structure suitable for the intercalation/extraction of zinc ions. However, Zn²⁺ ions with high charge density interact with the crystal lattice and limit further improvement in electrochemical performance. Defect engineering is a potential modification method with very promising application prospects, but the established procedures for preparing defects are complicated. In this study, VO_2-x (B) with oxygen deficiency is prepared by a simple solution reaction with NaBH₄ . The presence of oxygen deficiencies is confirmed by positron annihilation lifetime spectroscopy, UV/Vis absorbance spectroscopy and others. Owing to the presence of oxygen defects, the aqueous Zn/VO_2-x (B) battery exhibits improved specific capacity, excellent reversibility, and structural stability. Ex situ characterization techniques are employed to demonstrate the reversible insertion-extraction mechanism of Zn²⁺ ions from and into the host material. In addition, the Zn/VO_2-x (B) batteries still exhibit considerable electrochemical performance, even with high-loading electrodes (about 4 mg cm^-2 ).

High-Barrier Polyimide Containing Carbazole Moiety: Synthesis, Gas Barrier Properties, and Molecular Simulations

A high-barrier polyimide (2,7-CPI) was synthesized through the polymerization of pyromellitic dianhydride (PMDA) and a novel diamine (2,7-CDA) containing carbazole moiety. The synthesized diamine and polyimide were fully characterized by elemental analyses, FTIR and NMR. The 2,7-CPI displays very attractive barrier performances, with oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) low to 0.14 cm³·m⁻²·day⁻¹ and 0.05 g·m⁻²·day⁻¹, respectively. Meanwhile, 2,7-CPI also exhibits exceptional thermal stability with a glass transition temperature (T_g) of 467 °C, 5% weight-loss temperature (T_d5%) of 550 °C under N₂ and coefficient of thermal expansion (CTE) of 3.4 ppm/K. The barrier performances of 2,7-CPI are compared with those of a structural analogue (2,7-CPPI) and a typical polyimide (Kapton). Their barrier performances with respect to microstructure were investigated by molecular simulations, wide angle X-ray diffraction (WAXD), and positron annihilation lifetime spectroscopy (PALS). The results show that 2,7-CPI possesses better coplanar structure and more number of intermolecular hydrogen bonds among the three PIs, which result in tight chain packing and thereby high crystallinity, low free volume, and decreased chains mobility. That is, the high crystallinity and low free volume of 2,7-CPI reduce the diffusion and solubility of gases. Meanwhile, the poor chains mobility further decreases the gases diffusion. The reduced diffusion and solubility of gases consequently promote the improvement of barrier properties for 2,7-CPI. The polyimide has a wide application prospect in the field of flexible electronic packaging industries.

Probing the optical properties and luminescence mechanism of a UV-emitting SrBPO5 :Ce3+ phosphor

Ce-doped (1 × 10^-5 to 3.0 mol%) SrBPO₅ phosphors were synthesized using a conventional solid-state reaction route at 1273 K in an air atmosphere. Phase and morphology of the samples were studied from powder X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) micrographs, respectively. The band gap energies of the pure and Ce-doped SrBPO₅ phosphors were calculated from the recorded diffuse reflectance spectra. Photoluminescence (PL) and Ce³⁺ lifetime were recorded at 300 and 77 K. Photoluminescence lifetime measurements revealed two-lifetime values for Ce³⁺ at both 300 K (17 and 36 nsec) and 77 K (12 and 30 nsec), suggesting the presence of two different environments around Ce³⁺ . Time-resolved emission spectroscopy (TRES) studies confirmed the presence of Ce³⁺ in two different environments. In addition, SrBPO₅ :Ce exhibited intense UV emission, signifying its possible use as an efficient sensitizer for solid-state lighting applications. The effect of γ-irradiation on PL was also determined. Thermally stimulated luminescence (TSL) glow curves of the γ-irradiated phosphor, along with trap parameters, dose-response, and the possible TSL mechanism were also investigated. Positron annihilation lifetime spectroscopy was carried out to probe defects present in undoped and Ce-doped SrBPO₅ .

Magnetic performance and defect characterization studies of core-shell architectured MgFe2O4@BaTiO3 multiferroic nanostructures

Multiferroics that permit manipulation of the magnetization vector exclusively by electric fields have spawned extensive interest for memory and logic device applications. In line with this understanding, we herein report the encapsulation of non-ferroelectric magnesium ferrite (MgFe2O4) nanoparticles in a ferroelectric shell of BaTiO3 to produce a system with engineered dielectric, magnetic, magneto-electric and ferroelectric properties. The interface effect on the strain transfer was observed to strongly influence the magneto-electric coupling and the electric and magnetic properties of the system. The model polyhedral image of MgFe2O4@BaTiO3 has helped to get an insight into the core-shell structure. The multiferroicity induced by the excellent coupling between the ferroelectric and magnetostrictive phases at the core-shell interface unlocks wide prospects for device downscaling and information storage applications. The influence of magnetostrictive stress on the magneto-electric coupling effects and domain dynamics was further studied using transmission electron microscopy (TEM) and atomic force microscopy images. Interestingly, the realization of a superparamagnetic multiferroic system has been a breakthrough and facilitates ultra high density magnetic data storage technologies. Evidence for spontaneous polarization and the ferroelectric trait exhibited by the multiferroic samples was revealed from the P-E hysteresis loop. The investigation of defect evolution in the system was carried out using positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS) of annihilation radiation and the studies revealed thermal diffusion of positrons into the interfacial regions within the core-shell structure and the "formation and pick-off annihilation of orthopositronium atoms". It is concluded that interface engineering is a strong means for manipulation of the magnetic, dielectric and magneto-electric properties in multiferroic heterostructures for high density electrical energy and magnetic data storage.

Selective Coassembly of Aromatic Amino Acids to Fabricate Hydrogels with Light Irradiation-Induced Emission for Fluorescent Imprint

Controlling the structural parameters in coassembly is crucial for the fabrication of multicomponent functional materials. Here a proof-of-concept study is presented to reveal the α-substituent effect of aromatic amino acids on their selective coassembly with bipyridine binders. With the assistance of X-ray scattering technique, it is found that individual packing in the solid state as well as bulky effect brought by α-substitution determines the occurrence of coassembly. A well-performed hydrogels based on the complexation between certain aromatic amino acids and bipyridine units are successfully constructed, providing unprecedented smart materials with light irradiation-triggered luminescence. Such hydrogels without the phase separation and photobleaching during light irradiation are able to behave fluorescent imprint materials. This study provides a suitable protocol in rationally designing amino acid residues of short peptides for fabricating self-assembled multicomponent materials. In addition, this protocol is useful in screening potential functional materials on account of diverse self-assembly behavior.

Linking the structures, free volumes, and properties of ionic liquid mixtures

The formation of ionic liquid (IL) mixtures has been proposed as an approach to rationally fine-tune the physicochemical properties of ILs for a variety of applications. However, the effects of forming such mixtures on the resultant properties of the liquids are only beginning to be understood. Towards a more complete understanding of both the thermodynamics of mixing ILs and the effect of mixing these liquids on their structures and physicochemical properties, the spatial arrangement and free volume of IL mixtures containing the common [C₄C₁im]⁺ cation and different anions have been systematically explored using small angle X-ray scattering (SAXS), positron annihilation lifetime spectroscopy (PALS) and ¹²⁹Xe NMR techniques. Anion size has the greatest effect on the spatial arrangement of the ILs and their mixtures in terms of the size of the non-polar domains and inter-ion distances. It was found that differences in coulombic attraction between oppositely charged ions arising from the distribution of charge density amongst the atoms of the anion also significantly influences these inter-ion distances. PALS and ¹²⁹Xe NMR results pertaining to the free volume of these mixtures were found to strongly correlate with each other despite the vastly different timescales of these techniques. Furthermore, the excess free volumes calculated from each of these measurements were in excellent agreement with the excess volumes of mixing measured for the IL mixtures investigated. The correspondence of these techniques indicates that the static and dynamic free volume of these liquid mixtures are strongly linked. Consequently, fluxional processes such as hydrogen bonding do not significantly contribute to the free volumes of these liquids compared to the spatial arrangement of ions arising from their size, shape and coulombic attraction. Given the relationship between free volume and transport properties such as viscosity and conductivity, these results provide a link between the structures of IL mixtures, the thermodynamics of mixing and their physicochemical properties.