Type I Clathrates as Novel Silicon Anodes: An Electrochemical and Structural Investigation.

Silicon clathrates contain cage-like structures that can encapsulate various guest atoms or molecules. An electrochemical evaluation of type I silicon clathrates based on Ba8Al y Si46-y as the anode material for lithium-ion batteries is presented here. Postcycling characterization with nuclear magnetic resonance and X-ray diffraction shows no discernible structural or volume changes even after electrochemical insertion of 44 Li (≈1 Li/Si) into the clathrate structure. The observed properties are in stark contrast with lithiation of other silicon anodes, which become amorphous and suffer from large volume changes. The electrochemical reactions are proposed to occur as single phase reactions at approximately 0.2 and 0.4 V versus Li/Li+ during lithiation and delithiation, respectively, distinct from diamond cubic or amorphous silicon anodes. Reversible capacities as high as 499 mAh g-1 at a 5 mA g-1 rate were observed for silicon clathrate with composition Ba8Al8.54Si37.46, corresponding to ≈1.18 Li/Si. These results show that silicon clathrates could be promising durable anodes for lithium-ion batteries.

Micromechanical modeling of R-curve behaviors in human cortical bone.

The risk of bone fracture increases with age because of a variety of factors that include, among others, decreasing bone quantity and quality due to increasing porosity and crack density with age. Experimental evidence has indicated that changes in bone microstructure and trace mineralization with age can result in different crack-tip strain field and fracture response, leading to different fracture mechanisms and R-curve behaviors. In this paper, a micromechanical modeling approach is developed to predict the R-curve response of bone tissue by delineating fracture mechanisms that lead to microdamage and ligament bridging by incorporating the influence of increasing porosity and crack density with age. The effects of age on fracture of human femur cortical bone due to porosity (bone quantity) and bone quality (crack density) with age are then examined via the micromechanical model.

Non-destructive characterization of microdamage in cortical bone using low field pulsed NMR.

The microcracking and damage accumulation process in human cortical bone was characterized by performing cyclic loading under four-point bending at ambient temperature. A non-destructive nuclear magnetic resonance (NMR) spin-spin (T(2)) relaxation technique was applied to quantify the apparent changes in bone porosity as a function of cyclic loading and prior damage accumulation, first to unloaded cortical bone to quantify the initial porosity and then to fatigued cortical bone that was subjected to cyclic loading to various levels of modulus degradation and microdamage in the form of microcracks. The NMR T(2) relaxation time and amplitude data of the fatigued bone were compared against the undamaged state. The difference in the T(2) relaxation time data was taken as a measure of the increase in pore size, bone porosity or microcrack density due to microdamage induced by cyclic loading. A procedure was developed to deduce the number and size distributions of microcracks formed in cortical bone. Serial sectioning of the fatigued bone showed the formation of microcracks along the cement lines or within the interstitial tissue. The results on the evolution of microdamage derived from NMR measurements were verified by independent experimental measurements of microcrack density using histological characterization techniques. The size distribution and population of the microcracks were then utilized in conjunction with an analytical model to predict the degradation of the elastic modulus of cortical bone as a function of damage accumulation.

Grindability of alpha-case formed on cast titanium.

The hardened alpha-case (alpha-case) layer inevitably forms on the surface of titanium castings when prepared by investment casting. Because the hardness of the alpha-case is incomparable to that of the interior structure, the perception exists that the alpha-case is difficult to remove during cutting, grinding and polishing. Grindability (ease of grinding) of cast cpTi and cast Ti-6Al-4V was evaluated by grinding cast specimens incrementally using a SiC abrasive wheel. The present study revealed that the presence of the brittle alpha-case with lower fracture toughness is beneficial in grinding titanium. The alpha-case on the ductile cpTi can be ground much easier than its bulk interior structure. In less ductile Ti-6Al-4V, the grinding rate is much higher than that of cpTi, and the alpha-case and its interior structure are at similar levels since the fracture toughness of its alpha-case and the bulk material is not large enough.

Relating crack-tip deformation to mineralization and fracture resistance in human femur cortical bone.

The risk of bone fracture increases with age because of a variety of factors that include, among others, decreasing bone quantity and quality. Despite recent advances, the roles of bone microstructure and trace mineralization in the fracture process are not well understood. In this study, we utilize a combination of in-situ fracture toughness testing, digital strain mapping, and X-ray photoelectron spectroscopy techniques to characterize the near-tip strain field, fracture toughness, and chemical elements on the fracture surface of bone specimens from donors of two ages (48-year-old and 78-year-old females). We show that age-related embrittlement of bone fracture is associated with higher near-tip strains by lamellar shear and crack deflection at lamellar interfaces in the young bone and their absence in the old bone. The different near-tip deformation behaviors may be associated with the presence of Si and Zn in the young bone but more Ca and P and the lack of Si and Zn in the old bone.

Modeling wear of cast Ti alloys.

The wear behavior of Ti-based alloys was analyzed by considering the elastic-plastic fracture of individual alloys in response to the relevant contact stress field. Using the contact stresses as the process driving force, wear was computed as the wear rate or volume loss as a function of hardness and tensile ductility for Ti-based cast alloys containing an alpha, alpha+beta or beta microstructure with or without the intermetallic precipitates. Model predictions indicated that wear of Ti alloys increases with increasing hardness but with decreasing fracture toughness or tensile ductility. The theoretical results are compared with experimental data to elucidate the roles of microstructure in wear and contrasted against those in grindability.