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Microtubes of near-equiatomic nickel-titanium (NiTi) alloys can be created via the Kirkendall effect during Ni Ti interdiffusion, when nickel wires are surface-coated with titanium via pack cementation and subsequently homogenized. This study explores the effect of diffusion distance upon Kirkendall microtube formation in NiTi by considering a range of Ni wire diameters. For Ni wire diameters of 25, 50 and 100 μm, titanized at 925 °C for 0.5, 2, and 8 h to achieve average NiTi composition, partial interdiffusion occurs concurrently with Ti surface deposition, resulting in concentric shells of NiTi2, NiTi and Ni3Ti around a Ni core, with some Kirkendall porosity created within the wires. Upon subsequent homogenization at 925 °C, near-single-phase NiTi wires are created and the Kirkendall porosity increases, leading to a variety of pore/channel structures: (i) for 25 μm Ni wires where diffusion distances and times are short, a high volume fraction of micropores is created near the final NiTi wire surface, with 1–2 larger pores near its core; (ii) for 50 μm Ni wires, a single, ∼20 μm diameter pore is created near the NiTi wire center, transforming the wires into microtubes, and; (iii) for 100 μm Ni wires, a ∼50 μm diameter irregular pore is formed near the NiTi wire center, along with an eccentric crescent-shaped pore of similar cross-section, resulting from interruption of a single diffusion path, due to the longer diffusion distances and times.

The use of burnable absorbers can be beneficial for nuclear reactors by extending the fuel's operational cycle, providing additional criticality control, and flattening the power profile. In this work, three burnable absorber materials (boron carbide, aluminum boride, and zirconium boride) embedded in aluminum have been fabricated into foils and clad in AA-6061 for potential use in high performance research reactors. The as-fabricated boron-containing phases were determined using transmission electron microscopy to be AlB2, B4C, and ZrB2. TEM also revealed incomplete bonding at the B4C-matrix interface. SEM showed a relatively uniform spatial distribution of boron-containing phases for all the candidate materials. Higher porosity was observed in the foil containing ZrB2 in its as-rolled condition. The porosity in the ZrB2 foil was reduced by hot isostatic pressing. The size and shape distributions of the boron-containing phases were analyzed on the criteria of cross-sectional area, perimeter, roundness, circularity, and aspect ratio. A method of converting the 2D burnable absorber dispersoids seen in cross-sectional microscopy images into 3D volumes was derived using both spherical and ellipsoidal geometry models. The difference in calculated burnable absorber dispersoid average volume between the two models ranges from 20% to 100%, which could impact burnable absorber burnout rates due to differences in neutron self-shielding.

In-situ and ex-situ X-ray 3D-tomography is used to characterize the microstructure of Ni microwires, with wire diameters spanning 25–100 μm, (i) after vapor-phase deposition of Ti onto their surface and (ii) after subsequent homogenization to achieve the near-equiatomic NiTi composition desired for shape-memory or superelastic behavior. After Ti deposition at 925 °C, wires are partially homogenized, exhibiting a pure Ni core surrounded by concentric shells of Ni3Ti, NiTi and NiTi2 intermetallic phases. Because of the imbalanced Ni and Ti diffusive fluxes, Kirkendall porosity is formed near the center of the wire, which often merges into a single pore in cross-sections, due to spatial confinement of the wire geometry. During subsequent homogenization at 925 °C, these Kirkendall pores grow due to further Ni-Ti interdiffusion, and they coalesce into a single, hollow channel near the central axis of the wire, thus forming a NiTi microtube. In some cases, off-center pores form in addition to the central pore, but these off-center pores do not form continuous channels.

The volume/issue for reference 25 of this article1 was published incorrectly; the citation should appear as follows: 25. P. Lloreda-Jurado, S. Wilke, K. Scotti, A. Paul-Escolano, D. Dunand and R. Sepulveda: Structure–processing relationships of freeze-cast iron foams fabricated with various solidification rates and post-casting heat treatment J. Mater. Res., 35(19), 2587–2596 (2020).

In order for lithium ion batteries to be successfully deployed into many emerging applications, such as transportation and advanced portable electronics, these batteries must have higher volumetric and gravimetric energy densities, as well as the ability to quickly charge and store energy. Alloy-based anode materials, such as silicon and tin, are promising candidates for increasing capacity, energy and power density because they possess maximum gravimetric capacities up to ten times that of graphite, the current standard for commercial lithium-ion cells. However, these materials suffer from dramatic volume changes during (de)lithiation (up to 300%), which can severely limit their lifetime. One effective route towards reversibly accommodating these large volume changes, improving capacity retention and at the same time increasing power density is to take advantage of lower stress and strain values and gradients at smaller length scales using nanostructures such as the inverse opal structure. While many different nanostructures and morphologies have been explored, the rational design and optimization of these structures has been hindered by a dearth of experimentally measured, quantitative strain data for nanostructured NiSn and Si anodes. The amorphous nature of lithiation in nanostructured Si anodes and the unclear lithiation mechanism(s) in nanostructured NiSn anodes has greatly hindered in operando strain measurement to date, especially on microscale and larger format cells. Additionally, transient effects or mechanistic changes that may occur when cycling these anodes at higher rates have not been very well explored in the literature. Using synchrotron-based X-ray diffraction techniques at the Advanced Photon Source, lattice strains in Si and NiSn coated Ni nanostructured inverse opal scaffolds were measured in operando at a variety of rates in order to deduce mismatch stresses and strain evolution during (dis)charging in the active anode material thin film and the nickel scaffold. Since both the active anode materials form strong bonds with the inverse opal nickel scaffold, the elastic strains measured in the nickel are similar to those present in the anode material, allowing stress and strain states present in the Si or NiSn to be indirectly measured. These inverse opal anodes were cycled at rates between 1C and 20C, and 1C and 500C for Si and NiSn based anodes respectively, with charge and discharge current densities held constant and equivalent at each cycling rate. Additionally, asymmetric cycling parameters were utilized to explore, in operando, a fast charge, slow discharge behavior that may be more representative of current and emerging applications, where lithiation rates of up to 115C for NiSn and 60C for Si and fixed delithiation current densities corresponding to a symmetric 1C rate were utilized. Strains measured in the Ni scaffold were directly correlated with the electrochemical cycling of the anode. These strains are discussed in terms of elasto-plastic deformation mechanisms in the scaffold, cracking of the active materials and other potential stress relief mechanisms. As observed through in operando strain measurements, potential changes in lithiation mechanisms and possible mechanical failure modes at various (dis)charge rates are also discussed.