Elucidating Stress-Strain Relations of ZrB12 from First-Principles Studies.

Transition-metal boron-rich compounds exhibit favorable synthesis conditions and mechanical properties that hold great promise for wide-ranging applications. However, the complex bonding networks of these compounds produce diverse structural and mechanical behaviors that require in-depth studies. A notable case is ZrB12, which has been reported to possess high Vickers hardness comparable to those of ReB2 and WB4. Surprisingly, first-principles calculations of stress-strain relations reveal unexpected low indentation strengths of ZrB12 well below those of ReB2 and WB4. Such contrasting results are reconciled by noting that the additional presence of a boron-rich phase of ZrB50 in the experimental synthesis process likely plays a key role in the extrinsic strengthening. These findings uncover mechanisms for the higher measured strength of ZrB12 and offer insights for elucidating extrinsic hardening phenomena that may exist in other transition-metal compounds.

Flame Retardancy and Mechanism of Novel Phosphorus-Silicon Flame Retardant Based on Polysilsesquioxane.

A novel phosphorus-silicon flame retardant (P5PSQ) was prepared by bonding phosphate to silicon-based polysilsesquioxane (PSQ) and used as flame retardant of poly (lactic acid) (PLA). The results show that PLA with 10 wt % P5PSQ has a limiting oxygen index (LOI) 24.1%, the peak heat release rate (PHRR) and total heat release (THR) of PLA decrease 21.8% and 25.2% compared to neat PLA in cone calorimetric test, indicating that P5PSQ shows better flame retardancy in comparison to PSQ. Furthermore, the study for the morphology and composition of carbon residue after the combustion of PLA and the gas release of PLA during combustion illustrate that P5PSQ has flame retardancy in condensed phase and gas phase simultaneously. In condensed phase, phosphorus from phosphate promotes the formation of more stable and better carbon layer containing Si and P, which inhibits the transfer of heat and oxygen in the combustion. In gas phase, the phosphate in P5PSQ emits phosphorus-containing compound that can restrain the release of C-O containing products, which may have effective flame retardancy for PLA in gas phase to a certain extent. In one word, P5PSQ is denoted as a good phosphorus-silicon synergistic flame-retardant.

Improving the stability and ductility of polylactic acid via phosphite functional polysilsesquioxane.

To improve the stability and ductility of polylactic acid (PLA), chain extender or crosslinking agent of phosphite functional polysilsesquioxane (PPSQ) was synthesized by the reaction of phosphite group with the amino group of poly(amino-epoxy)silsesquioxane (PSQ). First, the reaction of PPSQ with PLA was characterized by molecular weight (M w) and melt mass flow rate (MFR) of PLA after melting reaction. The results showed a 6.6% increase in the M w of PLA and a 24.5% decrease in MFR value at the PPSQ loading content of 2 wt% in PLA, indicating that PPSQ takes chain extension or crosslinking in PLA. Then, this result was further supported by the thermal stability improvement of PLA, which was testified by the increase of oxidative activation energy and the oxygen onset temperature (OOT) value. PPSQ improved the water resistance and mechanical properties of PLA. The hydrolysis rate decreased by 46.8%, and the tensile strength and impact strength increased by 17.2% and 89.4%. Taken together, these results indicate that the addition of PPSQ can produce the PLA with excellent thermal stability, hydrolytic stability and mechanical properties.

Strain-induced modulations of electronic structure and electron-phonon coupling in dense H3S.

A ∼200 K superconducting phase in dense hydrogen sulfide is an important milestone for the development of novel superconductors with high critical temperature. Here, we systematically studied the effect of uniaxial strain on the electronic and superconducting properties in dense H3S using density functional calculations. Our theoretical results show that inducing strain is an effective tool to control the electronic Fermi surface topology, logarithmic average frequency, and electron-phonon coupling parameter of dense H3S. Thus, uniaxial strain induces sensitive and considerable changes in superconducting critical temperature, which stem from the energy-level shifts and softening lattice vibrations.

Effect of pressure on the structural, electronic and mechanical properties of ultraincompressible W2B.

The crystal structures of W2B have been extensively investigated by the swarm structure searching method at ambient and high-pressure conditions. Our calculated thermodynamic enthalpy data suggests that the tetragonal phase with I4/m symmetry is the most stable at 0-50 GPa. The theoretical elastic properties and phonon spectroscopy confirmed that I4/m W2B is both mechanically and dynamically stable. The calculated band structure and density of states show that I4/m W2B is metallic and the electronic properties are sensitive to changes in external pressure with the occurrence of an electronic topological transition. The simulated high elastic modulus, hardness and strain-stress relationships reveal that W2B exhibits excellent ultraincompressible properties and high strength. The combination of superior conductivity and mechanical properties reveals that W2B can be used for hard coatings and electrical measurements.