Clean Grain Boundary Found in C14/Body-Center-Cubic Multi-Phase Metal Hydride Alloys
The grain boundaries of three Laves phase-related body-center-cubic (bcc) solid-solution, metal hydride (MH) alloys with different phase abundances were closely examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and more importantly, electron backscatter diffraction (EBSD) techniques. By using EBSD, we were able to identify the alignment of the crystallographic orientations of the three major phases in the alloys (C14, bcc, and B2 structures). This finding confirms the presence of crystallographically sharp interfaces between neighboring phases, which is a basic assumption for synergetic effects in a multi-phase MH system.
Studies in molybdenum/manganese content in the dual body-centered-cubic phases metal hydride alloys
Abstract A series of TiVCrMn-based body-centered-cubic (BCC) alloys with design compositions of Ti40V30Cr15Mn15−xMox, where x = 0, 2, 4, 6, 8, 10, and 12, were prepared by an arc melter. Their microstructures were characterized and compared to the hydrogen storage properties in both gaseous phase and electrochemistry. X-ray diffraction results indicate the alloys are dominated by either one (x = 0 and 2) or two BCC phases (x = 4 to 12). As the Mo-content in the alloy increases, much of the first BCC phase is replaced by the second BCC phase with a stronger metal–hydrogen bond, lattice constant of the first BCC phase increases while that of the second BCC phase remains about the same, and both gaseous and electrochemical hydrogen storage capacities and plateau pressure increases and then decreases. The boundary between the two BCC phases is found to be crucial for the hydrogen storage properties. Alloy with a composition of Ti40V30Cr15Mn11Mo4 shows a good balance among various electrochemical properties, and alloys with higher Mo-contents (8 at% for example) achieved discharge capacity up to 639 mAh g−1 at 4 mA g−1 but are less stable in 30% KOH due to cracking and surface oxidation. Using higher concentration of KOH at 50% as the electrolyte is able to improve 8 at% Mo-containing alloy by nearly 60% by reducing the adverse effect from passivation.
Structure and electrochemical properties of Ti0.25V0.35Cr0.40−xNix (x=0.05–0.40) solid solution alloys
Abstract The structure and electrochemical properties of Ti 0.25 V 0.35 Cr 0.40−x Ni x (x=0.05 –0.40) solid solution alloys have been studied in the paper. It is found that these alloys consist of a main solid solution phase with body centered cubic (bcc) structure and a little TiNi-based secondary phase. The solid solution alloys show easier activation behavior at 313 K as a negative electrode material for MH–Ni battery. With changing x from 0.05 to 0.30, the discharge capacity and the high-rate dischargeability increase. The discharge capacity increases with increasing temperature. The maximum discharge capacity is 240 mA h / g at 313 K for x=0.30 and 280 mA h / g at 343 K for x=0.15.