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Effect of microstructure on mixed‐mode (mode I + II), high‐cycle fatigue thresholds in a Ti‐6Al‐4V alloy is reported over a range of crack sizes from tens of micrometers to in excess of several millimeters. Specifically, two microstructural conditions were examined—a fine‐grained equiaxed bimodal structure (grain size ∼20 µm) and a coarser lamellar structure (colony size ∼500 µm). Studies were conducted over a range of mode‐mixities, from pure mode I (Δ K II /Δ K I = 0) to nearly pure mode II (Δ K II /Δ K I ∼ 7.1), at load ratios (minimum load/maximum load) between 0.1 and 0.8, with thresholds characterized in terms of the strain‐energy release rate (Δ G ) incorporating both tensile and shear‐loading components. In the presence of through‐thickness cracks—large (> 4 mm) compared to microstructural dimensions—significant effects of mode‐mixity and load ratio were observed for both microstructures, with the lamellar alloy generally displaying the better resistance. However, these effects were substantially reduced if allowance was made for crack‐tip shielding. Additionally, when thresholds were measured in the presence of cracks comparable to microstructural dimensions, specifically short (∼200 µm) through‐thickness cracks and microstructurally small (< 50 µm) surface cracks, where the influence of crack‐tip shielding would be minimal, such effects were similarly markedly reduced. Moreover, small‐crack Δ G TH thresholds were some 50–90 times smaller than corresponding large crack values. Such effects are discussed in terms of the dominant role of mode I behaviour and the effects of microstructure (in relation to crack size) in promoting crack‐tip shielding that arises from significant changes in the crack path in the two structures.
The epitaxial liftoff approach has been attracting increasing interest as an alternative to lattice-mismatched heteroepitaxy. A thin-film GaAs double heterostructure injection diode laser fabricated on a glass substrate by the epitaxial liftoff technique is reported. This presages the integration of the two major optical communication materials, III-V semiconductor crystals with SiO/sub 2/ glass.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
The accuracy of the nonlinear static procedure (NSP) using the lateral force distributions specified in the FEMA-356 document (ASCE 2000), now standard in engineering practice, and the recently developed dynamics-based modal pushover analysis (MPA) procedure (Chopra and Goel 2002) was evaluated in the companion paper by Goel and Chopra (2003). The median seismic demands were computed by these procedures for six SAC buildings, each analyzed for 20 ground motions, and compared with ‘‘exact’’ results obtained from nonlinear response history analysis (RHA).
Alkali-ion intercalation compounds are widely used as cathode materials for rechargeable batteries, including Li-, Na-, and K-ion batteries. For many years, researchers have developed layered oxide cathode materials for Na- and K-ion batteries given that the layered oxides show high reversible capacity and high working voltage in Li-ion technology. However, our works have demonstrated that the layered oxide compounds may not be good cathode candidates for Na and K systems. There are two main issues: [1-4] (i) Na and K transition metal oxide compounds form non-stoichiometric composition ( x <1.0 in A x MO 2 , A= Na and K). It leads practical difficulty of realizing Na- and K-ion batteries because all the Na and K ions should come from the cathode in rocking-chair batteries. (ii) As the insertion ion size increases, the voltage curve becomes much sloped. The sloped voltage curves result in low specific capacity and average voltage. Both the problems are attributable to much stronger Na + -Na + and K + -K + interaction than Li + -Li + in the layered oxide structure. In this respect, polyanion compounds that have 3 dimensional arrangements of Na and K ions, resulting in longer Na + -Na + and K + -K + distance, will be better candidates. We proposed KVPO 4 F [5] as an example of polyanion compounds and it has stoichiometric composition and provides high average voltage of ~4.3 V ( vs. K/K + ) with a reversible capacity of ~105 mAh/g. We further investigated the effect of intercalation ion species in K x VPO 4 F ( x ~0). [6] Our work demonstrates the voltage for Na intercalation is even higher than that for Li insertion unlike the common belief that Li insertion voltage is always higher than Na insertion. The lower Li intercalation voltage is likely attributed to unstable Li site in large cavity, making less stable discharged product upon Li insertion vs. Na insertion. In addition, we found that Li intercalation is more sluggish than Na and K intercalation in K x VPO 4 F ( x ~0). Since Li ion is too small compared to cavity in the cathode, Li ions are undercoordinated in the transition state, in which a Li ion is coordinated by two anions only. Therefore, Li ion migration barrier is much higher than Na and K migration. This finding teaches us that large cavity size (or channel size) is not always good for fast alkali ion migration and we need to finely tune the cavity size suitable for each intercalating ion species. References [1] H. Kim, J. C. Kim, S.-H. Bo, T. Shi, D. –H. Kwon, G. Ceder. K‐Ion Batteries Based on a P2‐Type K 0.6 CoO 2 Cathode. Adv. Energy Mater. 7, (2017) 1700098 [2] H. Kim, D. –H. Seo, A. Urban, J. Lee, D. –H. Kwon, S. –H. Bo, T. Shi, J. Papp, B. McCloskey, G. Ceder. Stoichiometric Layered Potassium Transition Metal Oxide for Rechargeable Potassium Batteries. Chem. Mater. 30 (2018) 6532-6539. [3] H. Kim, D. –H. Seo, J. C. Kim, S. –H. Bo, L. Liu, T. Shi, G. Ceder. Investigation of Potassium Storage in Layered P3‐Type K 0.5 MnO 2 Cathode. Adv. Mater. 29 (2017) 1702480. [4] H. Kim, H. Ji, J. Wang, and G. Ceder. Trands in Chem. 1 (2019) 682. [5] H. Kim, D. –H. Seo, M. Bianchini, R. Clement, H. Kim, J. C. Kim, Y. Tian, T. Shi, W. –S. Yoon, G. Ceder. A New Strategy for High‐Voltage Cathodes for K‐Ion Batteries: Stoichiometric KVPO 4 F. Adv. Energy Mater. 8, (2018) 1801591. [6] H. Kim, Y. Ishado, Y. Tian, G. Ceder. Investigation of Alkali‐Ion (Li, Na, and K) Intercalation in K x VPO 4 F ( x ∼ 0) Cathode. Adv. Funct. Mater. 29 (2019) 1902392.
Abstract not Available.
Neutral tetraazacyclophanes were prepared in a one-step palladium-catalyzed amination reaction. Simple oxidation of these materials creates dication diradicals that are stable at room temperature and that are geometrically well defined. The electronic and magnetic properties of the dications were investigated by CV, UV−vis, and EPR spectroscopy. These spectral data and solution phase magnetic susceptibility measurements indicate high spin ground states in certain media. EPR zero field splitting parameters for the diradical suggest that the distance between the two radical sites can be approximated simply by the distance between alternating nitrogens in the macrocycle.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
