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Silicon electrodes with the columnar macroporous structure were investigated to determine the effect of variations in the columnar pore morphology on lithiation and energy storage capacity in Li-ion cells. Several variants of macroporous Si columnar electrodes were electrochemically cycled against the Li reference electrode. The changes in macro-pore size and Si wall thickness of the columnar architecture greatly affected the cyclic Li storage and discharge capacities. A strong correlation of the Li-storage capacity with the ratio of Si wall thickness to pore diameter is found to exist. Specifically, one columnar Si electrode with an optimum macroporous structure exhibited a very high reversible specific capacity of ~1250 mAh/g (total capacity 1.2 mAh/cm2) for over 200 cycles. Electron microscopy revealed that the high reversible Li-storage capacity is due to the macropores accommodating the change in volume of lithiation and providing nearly complete reconstruction of Si walls upon delithiation. The present observations can lead to practical, high-capacity, and damage-resistant Si electrodes for Li-ion batteries.

A rigorous analysis of yield strength of pure iron over a wide grain size scale, using an extensive compilation of experimental data, indicates that the common Hall–Petch relationship is not obeyed with large deviations at the extremes of grain size. The author proposes here a phenomenological exponential function to represent the grain size effect on strength over multiple length scales. It is shown that the exponential function describes the grain size dependence of strength remarkably well, on the basis of a large set of experimental data for pure Fe. A nonlinear regression analysis indicated that the function provided a very high degree of correlation of data. The validity of the function is also supported by its conformation to physical boundary conditions at the extremes of grain size, that is, by asymptotically reaching the limiting stress for dislocation nucleation at infinitesimal grain size, and, the strength of single crystal at infinite grain size. The exponential form is a significant improvement over the Hall–Petch relationship and may be used as a guide to develop a reliable theory of grain size strengthening of iron.

The design and the processing of a new class of titanium boride (TiB)-based bulk cermets containing a metallic phase (β-Ti phase) for toughening is presented. The general approach is rapid reaction and densification, using starting powders of Ti, TiB2, Fe, and Mo, by electric-field-activated sintering. The cermets consist of two-phase microstructures in which the boride phase formed as a networked structure of TiB whiskers that were created in situ upon the reaction between the powders. Hardness, flexural strength, and fracture toughness measurements of these materials revealed that they possess an interesting set of properties up to: hardness values of 1090 kg/mm2, flexural strength values of 953 MPa, and fracture toughness values of 18 MPa m1/2. A remarkable finding is that although the metallic phase fractured by microscopic cleavage, the cermets showed good fracture toughness values. The present study not only illustrates the process details and microstructure leading to these properties but also provides a broad powder metallurgical approach to design and synthesize cermets that may yield further improved properties.