Fanzhi Meng

First-principles study of La2CoMnO6: a promising cathode material for intermediate-temperature solid oxide fuel cells due to intrinsic Co-Mn cation disorder

AbstractLa2CoMnO6 has attracted intensive research interest because of its prospect for novel technological applications and rich fundamental physics. And, due to the similar radii size as well as small covalent difference (as large as 2) between Co and Mn, cation disorder should be intrinsic within this perovskite. We performed comprehensive first-principles calculations on both La2CoMnO6 (LCMO) and LCMO with CoMn-MnCo antisite defects (AD:LCMO), focusing on the formation of bulk oxygen vacancies, which plays a key role in oxygen ion diffusion process in solid oxide fuel cell (SOFC) electrodes. First, it is found that the covalent states are 2 and +4 for Co and Mn at their regular sites while they are both prone to be +3 in the antisites. The formation energies for oxygen vacancies are predicted to follow the trend Co2+-O-Mn4+ > Co2+-O-Co3+ > Mn3+-O-Mn4+, and the underlying microscopic mechanism is attributed to the more electron delocalization between mixed-covalent transition metals (Co2+-O-Co3+ and Mn3+-O-Mn4+), which is beneficial to diminish the electronic repulsion and help to stabilize the vacancy. Therefore, we could conclude that oxygen ionic conductivity should be enhanced in the compounds with higher degree of cation disorder. Our results indicate that AD:LCMO should be a promising intermediate-temperature solid oxide fuel cell cathode material. Graphical AbstractBased on first-principles calculations, we studied ordered LCMO as well as LCMO with CoMn-MnCo antisite defects (AD:LCMO). First, it was found that the covalent states are +2 and +4 for Co and Mn at regular sites while they are both prone to be +3 in the antisites. The formation energies for oxygen vacancies are predicted to follow the trend Co2+-O-Mn4+ > Co2+-O-Co3+ > Mn3+-O-Mn4+, and the underlying microscopic mechanism for this trend is attributed to the more delocalization of the electrons within AD:LCMO due to the mixed-covalent transition-metal pairs.

Broadband down-conversion for silicon solar cell by ZnSe/phosphor heterostructure

Down-conversion is a feasible way to improve conversion efficiency of silicon solar cell. However, the width of excitation band for down-converter based on trivalent lanthanide ions is still not satisfying. Here, we designed and fabricated a heterostructural down-converter composed of Y₂O₃: [(Tb³⁺-Yb³⁺), Li⁺] quantum cutting phosphor and ZnSe. The ZnSe phase was used to absorb the incident light with energy larger than its bandgap, and transfer the energy to Tb³⁺-Yb³⁺ quantum cutting couple. Short-wavelength incident light was finally converted into a strong Yb³⁺ emission at about 1000 nm, locating at the maximal spectral response of silicon solar cell. The excitation band of the down-conversion covers a wide region of 250-550 nm. Benefiting from the energy match between ZnSe bandgap and ⁷F₆→⁵D₄ absorption of Tb³⁺ ions, the bandwidth of down-conversion is almost maximized.

High power 4.65 μm single-wavelength laser by second-harmonic generation of pulsed TEA CO2 laser in AgGaSe2 and ZnGeP2

A high power 4.65 μm single-wavelength laser by second-harmonic generation (SHG) of TEA CO2 laser pulses in silver gallium selenide (AgGaSe2) and zinc germanium phosphide (ZnGeP2) crystals is reported. Experimental results show that the average output power of SHG laser is not only restricted by the damage threshold of the nonlinear crystals, but also limited by the irradiated power of fundamental-wave laser depending on the operating repetition-rate. It is found that ZnGeP2 can withstand higher 9.3 μm laser irradiation intensity than AgGaSe2. As a result, using a parallel array stacked by seven ZnGeP2 crystals, an average power of 20.3 W 4.65 μm laser is obtained at 250 Hz. To the best of our knowledge, it is the highest output power for SHG of CO2 laser by far.

Synergistic effects of intrinsic cation disorder and electron-deficient substitution on ion and electron conductivity in La1-xSrxCo0.5Mn0.5O3-δ (x = 0, 0.5, and 0.75).

The effects of intrinsic cation disorder and electron-deficient substitution for La1-xSrxCo0.5Mn0.5O3-δ (LSCM, x = 0, 0.5, and 0.75) on oxygen vacancy formation, and their influence on the electrochemical properties, were revealed through a combination of computer simulation and experimental study. First-principles calculations were first performed and found that the tendency of the oxygen vacancy formation energy was Mn(3+)-O*-Mn(4+) < Co(2+)-O*-Co(3+) < Co(2+)-O*-Mn(4+), meaning that antisite defects not only facilitate the formation of oxygen vacancy but introduce the mixed-valent transition-metal pairs for high electrical conductivity. Detailed partial density of states (PDOS) analysis for Mn on Co sites (MnCo) and Co on Mn sites (CoMn) indicate that Co(2+) is prone to being Co(3+) while Mn(4+) is prone to being Mn(3+) when they are on antisites, respectively. Also it was found that the holes introduced by Sr tend to enter the Co sublattice for x = 0.5 and then the O sublattice when x = 0.75, which further promotes oxygen vacancy formation, and these results are confirmed by both the calculated PDOS results and charge-density difference. On the basis of microscopic predictions, we intentionally synthesized a series of pure LSCM compounds and carried out comprehensive characterization. The crystal structures and their stability were characterized via powder X-ray Rietveld refinements and in situ high-temperature X-ray diffraction. X-ray photoelectron spectroscopy testified to the mixed oxidation states of Co(2+)/Co(3+) and Mn(3+)/Mn(4+). The thermal expansion coefficients were found to match the Ce0.8Sm0.2O2-δ electrolyte well. The electrical conductivities were about 41.4, 140.5, and 204.2 S cm(-1) at doping levels of x = 0, 0.5, and 0.75, and the corresponding impedances were 0.041, 0.027, and 0.022 Ω cm(2) at 850 °C, respectively. All of the measured results testify that Sr-doped LaCo0.5Mn0.5O3 compounds are promising cathode materials for intermediate-temperature solid oxide fuel cells.

Computer modeling and experimental study of non-chain pulsed electric-discharge DF laser

Computer simulation and experimental study of a pulsed electrical-discharge DF laser pumped by the SF(6)-D(2) non-chain reaction are presented. The computer model encompassing 28 reactions is based on laser rate equations theory, and applied to approximately describe the chemical processes of non-chain DF laser. A comprehensive study of the dependence of number density on time for all particles in the gain area is conducted by numerical calculation adopting Runge-Kutta method. The output performance of non-chain pulsed DF laser as a function of the output mirror reflectivity and the mixture ratio are analyzed. The calculation results are compared with experimental data, showing good agreement with each other. Both the theoretical analysis and experimental results present that the laser output performance can be improved by optimizing the mixture ratio and output mirror reflectivity. The optimum values of mixture ratio and output mirror reflectivity are respectively 10:1 and 30%. The single pulse energy of 4.95J, pulse duration of 148.8ns and peak power of 33.27 MW are achieved under the optimum conditions.

Detailed Structures and Formation Mechanisms of Well-Known Al10RE2Mn7 Phase in Die-Cast Mg–4Al–4RE–0.3Mn Alloy

The detailed structures and the corresponding formation mechanisms of the well-known Al10RE2Mn7 phase in the conventional die-cast Mg–4Al–4RE–0.3Mn alloy were thoroughly investigated using transmission electron microscopy (TEM). The results indicate that the Al10RE2Mn7 phase ordinarily contains both normal

Fabrication of a high strength Mg–11Gd–4.5Y–1Nd–1.5Zn–0.5Zr (wt%) alloy by thermomechanical treatments

(11\overline{2} 1)

Assessment of LaM0.25Mn0.75O3-δ (M = Fe, Co, Ni, Cu) as promising cathode materials for intermediate-temperature solid oxide fuel cells

(112¯1) twins and orientation twins. Both detailed TEM observations and density functional theory calculations indicate that the Al10RE2Mn7 phase is transferred from the Al8REMn4 phase following an orientation relationship as