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We demonstrate an integrated non-destructive inspection methodology that employs the nonlinear ultrasonics-based sideband peak counting (SPC) technique in conjunction with topological acoustics (TA) sensing to comprehensively characterize the acoustic response of steel plates that contain differing levels of damage. By combining the SPC technique and TA, increased sensitivity to defect/damage detection as well as the ability to spatially resolve the presence of defects was successfully established. Towards this end, using a Rockwell hardness indenter, steel plates were subject to one, three and five centrally located indentations respectively. The acoustic response of the plate as a function of number of indentations was examined at a frequency range between 50 kHz and 800 kHz, from which the change in a global geometric phase was valuated. Here, geometric phase is a measure of the topological acoustic field response to the spatial locations of the indentations within the steel plates. The global geometric phase unambiguously showed an increase with increasing number of indentations. In addition, spatial variations in a ‘local’ geometric phase as well as spatial variations in the PC index (SPC-I) were also determined. Spatial variations in both the local geometric phase as well as the SPC-I were particularly significant across the indentations for frequencies below 300 kHz, and by combining the respective spatial variations in the SPC-I and geometric phase, the locations of the indentations were accurately identified. The developed SPC-TA nondestructive method represents a promising technique for detecting and evaluating defects in structural materials. 

evaluated by the sideband peak counting (SPC) nonlinear acoustics method and suitably validated by microfocus X-ray computed tomography (XCT). A wide-band chirp acoustic wave was used to inspect the microstructures of IN718 samples with five distinct process parameters, and the results reveal that the number of sidebands, which result from the non-linearity induced by porosity, is significantly influenced by the distribution and size of pores, in addition to the volume fraction. There was a clear correlation between extent of porosity and the corresponding value of the SPC index. XCT analysis corroborated these findings, providing quantitative insights into the porosity characteristics that affect the ensuing acoustic responses. The findings demonstrated that the porosity with varying sizes and distributions generate different SPC profiles, which were correlated to XCT results to quantitatively assess the size and spatial distributions of the porosity. Fusion of SPC and XCT characterization techniques provides a new strategic approach for non-destructive testing, where the SPC method offers rapid, qualitative evaluation, while XCT provides detailed spatial resolution for defect quantification. The integration of SPC could lead to the development of more cost-effective and advanced quality control protocols, ensuring the reliability of AM-manufactured components regardless of their geometry and composition. 

We used the transfer matrix method to investigate the conditions supporting the existence of directional bulk waves in a two-dimensional (2D) phononic crystal. The 2D crystal was a square lattice of unit cells composed of rectangular subunits constituted of two different isotropic continuous media. We established the conditions on the geometry of the phononic crystal and its constitutive media for the emergence of waves, which, for the same handedness, exhibited a non-zero amplitude in one direction within the crystal’s 2D Brillouin zone and zero amplitude in the opposite direction. Due to time-reversal symmetry, the crystal supported propagation in the reverse direction for the opposite handedness. These features may enable robust directional propagation of bulk acoustic waves and topological acoustic technology. 

Bone mineralization is critical to maintaining tissue mechanical function. The application of mechanical stress via exercise promotes bone mineralization via cellular mechanotransduction and increased fluid transport through the collagen matrix. However, due to its complex composition and ability to exchange ions with the surrounding body fluids, bone mineral composition and crystallization is also expected to respond to stress. Here, a combination of data from materials simulations, namely density functional theory and molecular dynamics, and experimental studies were input into an equilibrium thermodynamic model of bone apatite under stress in an aqueous solution based on the theory of thermochemical equilibrium of stressed solids. The model indicated that increasing uniaxial stress induced mineral crystallization. This was accompanied by a decrease in calcium and carbonate integration into the apatite solid. These results suggest that weight-bearing exercises can increase tissue mineralization via interactions between bone mineral and body fluid independent of cell and matrix behaviours, thus providing another mechanism by which exercise can improve bone health. This article is part of a discussion meeting issue ‘Supercomputing simulations of advanced materials’. 

Additive engineering is a common strategy to improve the performance and stability of metal halide perovskite through the modulation of crystallization kinetics and passivation of surface defects. However, much of this work has lacked a systematic approach necessary to understand how the functionality and molecular structure of the additives influence perovskite performance and stability. This paper describes the inclusion of low concentrations of 5-aminovaleric acid (5-AVA) and its ammonium acid derivatives, 5-ammoniumvaleric acid iodide (5-AVAI) and 5-ammoniumvaleric acid chloride (5-AVACl), into the precursor inks for methylammonium lead triiodide (MAPbI3) perovskite and highlights the important role of halides in affecting the interactions of additives with perovskite and film properties. The film quality, as determined by X-ray diffraction (XRD) and photoluminescence (PL) spectrophotometry, is shown to improve with the inclusion of all additives, but an increase in annealing time from 5 to 30 min is necessary. We observe an increase in grain size and a decrease in film roughness with the incorporation of 5-AVAI and 5-AVACl with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Critically, X-ray photoelectron spectroscopy (XPS) measurements and density functional theory (DFT) calculations show that 5-AVAI and 5-AVACl preferentially interact with MAPbI3 surfaces via the ammonium functional group, while 5-AVA will interact with either amino or carboxylic acid functional groups. Charge localization analysis shows the surprising result that HCl dissociates from 5-AVACl in vacuum, resulting in the decomposition of the ammonium acid to 5-AVA. We show that device repeatability is improved with the inclusion of all additives and that 5-AVACl increases the power conversion efficiency of devices from 17.61 ± 1.07 to 18.07 ± 0.42%. Finally, we show stability improvements for unencapsulated devices exposed to 50% relative humidity, with devices incorporating 5-AVAI and 5-AVACl exhibiting the greatest improvements. 

Understanding the fundamental mechanisms that underlie the synthesis of fullerene molecules in the interstellar medium (ISM) and in the environments of astrophysical objects is an open question. In this regard, using classical molecular dynamics, we demonstrate the possibility of in situ formation of fullerene molecules, such as C 60 from graphite, which is known to occur in the ISM, in particular, circumstellar environments. Specifically, when graphite is subjected to thermal and mechanical stimuli that are typical of circumstellar shells, we find that the graphite sheet edges undergo significant restructuring and curling, leading to edge-induced interlayer-interactions and formation of mechanically strained five-membered-ring structural units. These units serve as precursors for the formation of fullerene structures, such as pristine and metastable C 60 molecules. The pathways leading to molecular C 60 formation consist of a series of steps that involve bond-breakage and subsequent local rearrangement of atoms, with the activation energy barriers of the rate-limiting step(s) being comparable to the energetics of Stone–Wales rearrangement reactions. The identified chemical pathways provide fundamental insights into the mechanisms that underlie C 60 formation. Moreover, they clearly demonstrate that top-down synthesis of C 60 from graphitic sources is a viable synthesis route at conditions pertaining to circumstellar matter.