Investigation of interfacial reactions between metallic substrates and n-type bulk bismuth telluride thermoelectric material

Journal of Materials Science - In practical applications of bismuth telluride thermoelectric materials, the materials need to be connected with a metallic electrode before they can be used;...     

Capturing the non‐spherical shape of granular media and its trickle flow characteristics using fully‐Lagrangian method

We performed a numerical analysis for simulating granular media structures containing non‐spherical elements and the liquid trickle flow characteristics of such structures. Fully‐Lagrangian numerical simulation methods can track all motion information for solid or liquid elements at each point in time. We introduced suitable compressibility to moving particle semi‐implicit (MPS) and performed individual packing behavior calculations for non‐spherical elements, based on discrete element method (DEM) with expanded functions. Rigid bodies‐DEM is a method using a DEM contact force model that is expanded to handle the motion of freely shaped solids. It expresses complex shapes to enable low calculation costs and intuitive mounting. We used the boundary for the granular media configured with non‐spherical elements to implement a trickle flow simulation based on weakly compressible‐MPS. Even for elements of equal volume, different shapes changed the liquid passage velocity and hold‐up amount. The mean downflow velocity of the liquid phase was not always dependent on the void fraction. For the plane of projection, we obtained a good correlation with the mean downflow velocity in each packed structure, and successfully performed arrangements according to the new liquid‐passage shape coefficient. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2257–2271, 2017     

Experimental determination of solidified lithium disilicate crystal bandgap energy using EELS and XPS

Lithium disilicate (LS2) has been a crucial parent composition for glass-ceramics since the 1950s because of its excellent chemical and physical durability. In addition, a wide range of electrical properties can be obtained by changing the composition and crystallinity. Bandgap energy is one of the critical electrical properties for designing new lithium silicate-based materials. In this study, the bandgap energy of a synthesized LS2 crystal is evaluated using electron energy-loss spectroscopy and X-ray photoelectron spectroscopy. These two techniques unambiguously establish that the bandgap energy of LS2 is 7.7-7.8 eV, which is in the vacuum ultraviolet region. This confirms the insulating nature of the LS2 crystal.     

Effect of Mg2+ and fluorine on the network and highly efficient photoluminescence of Eu3+ ion in MgF2–BaO–B2O3 glasses

The glass structure and photoluminescence of new oxyfluoride glasses with the composition of xMgF₂–(66.7−2x/3)BaO–(33.3−x/3)B₂O₃ (x = 10-50 mol%) were investigated in this work. The structure of the glasses was investigated by magic-angle spinning NMR, XAS, and Raman scattering spectroscopies. It was revealed that the glasses are mainly composed of BO₃ units with a disconnected borate network consisting mainly of ortho- and pyro-borate units, and ortho-borate increases with the addition of MgF₂. The fluorine atoms are surrounded by Mg²⁺ and Ba²⁺ ions. The photoluminescence of Eu³⁺-doped samples were investigated. It was indicated that asymmetry of the Eu³⁺ site increased with the addition of MgF₂. The photoluminescence quantum yield (η) of the glasses are very high and increased with MgF₂ addition; red photoluminescence is observed with η = 82% for 10MgF₂ and η = 98% for 50MgF₂ for excitation at 393 nm.     

Thermal conductivity of Na2O–B2O3–SiO2 melts

Borosilicate glasses are candidate materials for the immobilization of high-level radioactive waste. The values of thermal conductivity of different borosilicate melts are thus indispensable information when optimizing the temperature distribution in a glass melting furnace. In this study, the thermal effusivity of Na₂O–B₂O₃–SiO₂ melts was measured using a front heating–front detection laser-flash method. The thermal conductivity, which can be obtained by combining the measured thermal effusivity with the specific heat capacity and density, was calculated using the least-squares method; the values for the Na₂O–B₂O₃–SiO₂ melts either slightly decreased linearly with increasing temperature or remained almost constant over the investigated temperature range. The values of thermal conductivity of the Na₂O–B₂O₃–SiO₂ melts were higher than those of B₂O₃–SiO₂ melts and lower than those of CaO–B₂O₃–SiO₂ melts. Furthermore, the thermal conductivity of the Na₂O–B₂O₃–SiO₂ melts was compared with those of the B₂O₃–SiO₂ and CaO–B₂O₃–SiO₂ samples.     

Incorporation limit of MoO3 in sodium borosilicate glasses

Optimizing the concentration of molybdenum incorporated in a borosilicate glass matrix is essential in the vitrification of high-level radioactive waste. However, the incorporation limit of MoO₃ in fundamental borosilicate systems has been rarely correlated with the local structure of the molybdenum cations. This study investigates the variations in the incorporation limit of MoO₃ in ternary sodium borosilicate glass upon varying the B₂O₃/(SiO₂ + B₂O₃) ratio (i.e., B)_. The incorporation limit of MoO₃ was less than 3 mol% in the low-B region (B < 0.7), where molybdenum cations mainly existed as [MoO₄]²⁻. However, when B was higher than 0.85, the incorporation limit was higher than 6 mol%, and the Raman spectra indicated the presence of octahedrally coordinated molybdenum cations, essential to stabilize the Mo–O–Mo linkage. The variation in the local structure of molybdenum cations can be explained by the available amount of non-framework cations compensating for the negative charge near [MoO₄]²⁻. These results allow the development of glass compositions with a high incorporation limit of MoO₃ simply by controlling the local structure near the molybdenum cations.