An Investigation on Substitution of Ag Atoms for Al or Ti Atoms in the Ti2AlC MAX Phase Ceramic Based on First-Principles Calculations

The present work introduced first-principles calculation to explore the substitution behavior of Ag atoms for Al or Ti atoms in the Ti2AlC MAX phase ceramic. The effect of Ag substitution on supercell parameter, bonding characteristic, and stability of the Ti2AlC was investigated. The results show that for the substitution of Ag for Al, the Al-Ti bond was replaced by a weaker Ti-Ag bond, decreasing the stability of the Ti2AlC. However, the electrical conductivity of the Ti2AlC was enhanced after the substitution because of the contribution of Ag 4d orbital electrons toward the density of states (DOS) at the Fermi level coupled with the filling of Ti d orbital electrons. For the substitution of Ag for Ti, new bonds, such as Ag-Al bond, Ag-C bond, Al-Al bond, Ti-Ti anti-bond, and C-C anti-bond were generated in the Ti2AlC. The Ti-Ti anti-bond was strengthened as well as the number of C-C anti-bond was increased with increasing the substitution ratio of Ag for Ti. Similar to the substitution of Ag for Al, the stability of the Ti2AlC also decreased because the original Al-Ti bond became weaker as well as the Ti-Ti and C-C anti-bonds were generated during the substitution of Ag for Ti. Comparing with the loss of Ti d orbital electrons, Ag 4d orbits contributed more electrons to the DOS at the Fermi level, improving the electrical conductivity of the Ti2AlC after substitution. Based on the calculation, the substitution limit of Ag for Al or Ti was determined. At last, the substitution behavior of Ag for Al or Ti was compared to discriminate that Ag atoms would tend to preferentially substitute for Ti atoms in Ti2AlC. The current work provides a new perspective to understand intrinsic structural characteristic and lattice stability of the Ti2AlC MAX phase ceramic.

Structures, and Thermophysical Properties Characterizations of (La1-xHox)3NbO7 Solid Solutions as Thermal Barrier Coatings

A sequence of (La1-xHox)3NbO7 solid solutions were fabricated in this work, which were studied as candidate for thermal insulation materials. The lattices were identified via XRD, when SEM and EDS were used to characterize the microstructures and element distributions. The results showed that the highest modulus, hardness, and toughness of (La1-xHox)3NbO7 were 196 GPa, 9.2 GPa, and 1.6 MPa m1/2, respectively, and they accorded with the mechanical property requirements. Also, a low thermal conductivity (1.06 W m−1 K−1) and high thermal expansion coefficients (TECs: 11.3 × 10−6 K−1) were simultaneously realized in (La3/6Ho3/6)3NbO7, at high temperatures. No phase transition was detected up to 1,200°C, which proved their good high-temperature lattice stability. The intense anharmonic lattice vibrations might contribute to the outstanding thermal properties of (La1-xHox)3NbO7 ceramics. The suitable modulus, high hardness, low thermal conductivity, and high TECs of (La1-xHox)3NbO7 solid solutions proclaimed that they were exceptional thermal insulation ceramics.

Revisiting polyanionic LiFePO4 battery material for electric vehicles

Although oxide cathodes have been widely used in these Li-ion batteries, these cathodes suffer from instability of the oxygen close-packed structure. In contrast, polyanionic phosphates such as LiFePO4 have incredible lattice stability and safety features owing to the strong covalent bond of P-O, which constrains the oxygen atoms and minimizes the defects of the oxygen site, resulting in stable frameworks. In addition, the presence of the strong P-O covalent bond stabilizes the anti-bonding transitional metal redox couple through an M-O-P inductive effect to generate a relatively high potential. Hence, polyanionic LiFePO4 has been an ideal choice of cathode materials for batteries deployed in electric vehicles. In this review, we revisit the basic features and development of LiFePO4, as an attempt to speeding its future deployment in massive electric vehicles.

PHASE TRANSITIONS IN SOLID SOLUTIONS BASED ON CDAS2 AT PRESSURES UP TO 50 GPA

Research has been performed into baric dependences between electrical resistance and thermo-EMF of eutectic solid solutions based on cadmium diarsenide of various compositions (Cd0.97Zn0.03As2 and Cd0.95Zn0.05As2) at pressures from 16 to 50 GPa and room temperature. Pressures were created in a chamber with conducting diamond anvils that served as contacts with the sample. Structural changes were recorded by changing electrical resistance and thermo-EMF. When replacing cadmium atom, zinc forms closer bonds with arsenic, which must result in the crystal lattice strengthening and higher pressures of phase transitions. Solid solutions preserve phase transitions observed in pure cadmium diarsenide. An increase in the crystal lattice stability of the solutions is confirmed compared to initial cadmium diarsenide. It is shown that with an increase in the zinc concentration pressures of phase transitions are shifted into the zone of higher pressures. The compounds preserve electronic conductivity in the range of pressures under consideration. At pressures higher than 30 GPa the thermo-EMF values become close to zero. This may be due to both an increase on concentrations of minority charge carriers and the impact of additional donating levels in the forbidden zone of solid solutions.

A stable immature lattice packages IP6 for HIV capsid maturation

HIV virion assembly begins with the construction of an immature lattice consisting of Gag hexamers. Upon virion release, protease-mediated Gag cleavage leads to a maturation event in which the immature lattice disassembles and the mature capsid assembles. The cellular metabolite inositiol hexakisphosphate (IP6) and maturation inhibitors (MIs) both bind and stabilize immature Gag hexamers, but whereas IP6 promotes virus maturation, MIs inhibit it. Here we show that HIV is evolutionarily constrained to maintain an immature lattice stability that ensures IP6 packaging without preventing maturation. Replication-deficient mutant viruses with reduced IP6 recruitment display increased infectivity upon treatment with the MI PF46396 (PF96) or the acquisition of second-site compensatory mutations. Both PF96 and second-site mutations stabilise the immature lattice and restore IP6 incorporation, suggesting that immature lattice stability and IP6 binding are interdependent. This IP6 dependence suggests that modifying MIs to compete with IP6 for Gag hexamer binding could substantially improve MI antiviral potency.