Periodic Lattice Research Articles

Strain regulation retards natural operation decay of perovskite solar cells.

Perovskite solar cells (pero-SCs) have undergone a rapid development in the last decade. However, there is still a lack of systematic studies to investigate whether the empirical rules of working lifetime assessment used in silicon solar cells can be applied to pero-SCs. It is commonly believed that pero-SCs show enhanced stability under day/night cycling due to the reported self-healing effect in the dark.1,2 While we discovered that the degradation of highly efficient FAPbI3 pero-SCs is in fact much faster under natural day/night cycling mode, questioning the widely accepted approach to estimate the operational lifetime of pero-SCs based on continuous mode testing. We reveal the key factor to be the lattice strain caused by thermal expansion/shrinking of the perovskite during the operation, an effect that gradually relaxes under the continuous-illumination mode but cycles synchronously under the cycling mode.3,4 The periodic lattice strain under the cycling mode results in deep trap accumulation and chemical degradation during operation, decreasing the ion migration potential and hence the device lifetime.5 We introduce phenylselenenyl chloride (Ph-Se-Cl) to regulate the perovskite lattice strain during day/night cycling, which achieved the certified efficiency of 26.3% and a 10-time improved T80 lifetime under the cycling mode after the modification.

Sub-THz and THz Cherenkov radiation source with two-dimensional periodic surface lattice and multistage depressed collector

We present the theory, concept and design of an efficient, megawatt coherent Cherenkov radiation source based on a two-dimensional periodic surface lattice (2D-PSL) cavity combined with a novel energy recovery system for the generation of highly efficient (> 50%) single-frequency radiation. We demonstrate the scalability of the transverse dimension of the 2D-PSL cavity of the Cherenkov source and thus the potential for efficient, continuous-wave, high-power (> 1 MW) operation; fundamental to the eventual realization of clean, fusion energy. These new sources, with the capacity to operate in the 0.1-10THz range, hold strong promise to address the long-standing “Terahertz gap”. By combining a Cherenkov oscillator driven by a non-gyrating beam with an innovative four-stage depressed collector energy recovery system, the overall device efficiency can be increased to be competitive with gyrotrons in the requirements for heating and current drive in fusion plasma. In these Cherenkov devices, the frequency independence of the magnetic guide field enables advantageous frequency scaling without deployment constraints, making them especially attractive for high-impact applications in fusion science, turbulence diagnostics, non-destructive testing and biochemical spectroscopy. The novel energy recovery techniques presented in this paper have broad applicability to many electron-beam driven devices, bringing revolutionary potential to future THz source technologies.

Numerical study of dispersion characteristics of cylindrical spiral lines and periodic gratings based on them

Numerical studies of the dispersion characteristics of cylindrical spiral lines have been carried out. One-dimensional and two-dimensional periodic lattices formed by such lines are also considered. The eigenvalue method was used in combination with periodic boundary conditions. The dispersion characteristics are calculated in the range of parameters for which the conditions of applicability of the known approximate numerical-analytical methods for analyzing spiral lines are not met. The dispersion characteristics of the first two eigen-waves in one-dimensional and two-dimensional periodic lattices of spirals are obtained for different values of phase shifts between lattice periods. It is shown that the main type of wave in them is a wave slowed down relative to the axes of the lines with the direction of rotation of the field vector determined by the direction of winding of the spiral.

Extraordinary viscous absortion by phononic supercrystals

An effective medium theory has been developed to study the viscous absorption of a doubly periodic phononic crystal, here denominated as supercrystal (SC). In practice, the SC is made of hard cylindrical scatterers embedded in a viscous fluid and consists of a combination of two phononic crystals with lattice periods, $a_1$ and $a_2$. The decay coefficient of sound due to viscosity is calculated analytically at low frequencies when both lattices homogenize. The viscous absorbance of finite SC slabs calculated with the analytical model is supported by exact calculations based on the finite element method (COMSOL simulations). In addition, results for the complex sound speed show a good agreement with the experimentally extracted sound speed.

Bjerrum defects in s-II gas hydrate

The energy and structure of Bjerrum defects in structure II gas hydrates were investigated by using first-principle calculations for finite-size clusters and periodic 3D lattice systems. The formation energies of these defects were calculated for the first time when the cages of the structure II structure were completely empty and the large cage was filled with a THF molecule. Analogous to findings in ice structures, one of the hydrogen atoms forming the D defect was noted to orient toward the cage. If the excess proton resides in the large cage, it acts as an attraction center for the polar guest molecule, i.e., THF. Therefore, the large cage guest THF molecule stabilizes the D/L defect pair and isolated D/L defect formation energies by forming hydrogen bonds with the D defect. In such cases, the defect structure representing a D/L defect pair containing a THF molecule interacting with one of the hydrogen atoms of the D defect mirrors the guest-induced ones. Notably, the classical Bjerrum defect and the guest-induced Bjerrum defect exhibit a similar phenomenon in defective structures. Contrary to existing literature, it is evident that guest-induced Bjerrum defects involve both the L and D components. The insights gained from this study could potentially offer an alternative perspective to understand various experimental observations, such as those related to dielectric and NMR properties.

Topologically protected bound modes in non-neighboring hexagonal lattices

In the last decade, the study of wave localization and guiding has gained a renewed interest thanks to the introduction of the concept of topological protection. Originally developed in the field of quantum mechanics, topological protection has successively inspired the search for its analogue in other physical domains, including elasticity. In this context, periodic hexagonal lattices have been proposed as ideal candidates to realize these concepts. In this work, we explore the dynamic behavior of discrete hexagonal lattices with third nearest neighbor connections. First, the formation and transition of multiple Dirac cones above a critical value of the relative stiffness between the first and third nearest neighbor connections is derived. Then, the formation of extremely localized modes (i.e., bound modes) at the interface between regions formed by lattices which are inverted copies of each other is demonstrated. Our results show that this localization derives from a type of interface satisfying a point reflection symmetry for which the considered chain decouples into two identical copies at the high symmetry point of the Brillouin zone. Our findings open novel avenues for topological waveguiding and confinement with potential applications in surface acoustic wave devices.

Multi-objective topology optimization and numerical investigation of heat sinks based on triply periodic minimal surface lattices

In thermal management applications, such as heat sinks (HSs) for electronic devices, cellular materials have extensively been employed. In recent years, there has been a growing attention towards employing topology optimization for enhancing hydraulic and heat transfer performance of HSs by optimizing their topology. The utilization of triply periodic minimal surface (TPMS) based structures presents distinctive prospects for customizing the design and performance of HSs. However, their potential remains unexplored in the context of customizing additively manufactured porous optimized HSs. Consequently, there is a need for research aimed at examining their coupled hydraulic and thermal performance. Density mapping approaches are used to build a variable density TPMS-based HSs from the output of topology optimization by applying the TPMS level-set equations that relate relative density and the level-set constant. In this work, a relative density mapping methodology is applied to thermo-fluid optimization problem to design a TPMS-based convective cooling system. An in-house MATLAB code was developed to perform a multi-objective topology optimization. After that, uniform and variable density (mapped from topology optimization results) TPMS-based HSs are analyzed using Star-CCM + CFD software to investigate their hydraulic and heat transfer performance. An experimental setup was established, and the numerical results were validated using uniform TPMS-based heat sinks. Results showed that incorporating TPMS with topology optimization has a great potential in thermal management applications as pressure drop across the heat sink was reduced while maintaining the performance.

A review of structural diversity design and optimization for lattice metamaterials

Metamaterials are a type of groundbreaking engineered materials with unique properties not found in natural substances. Lattice metamaterials, which have a periodic lattice cell structure, possess exceptional attributes such as a negative Poisson’s ratio, high stiffness-to-weight ratios, and outstanding energy dissipation capabilities. This review provides a comprehensive examination of lattice metamaterials. It covers their various structures and fabrication methods. The review emphasizes the crucial role of homogenization methods and multi-scale modeling in assessing metamaterial properties. It also highlights the advancement of topology optimization through advanced computational techniques, such as finite element analysis simulations and machine learning algorithms.

Crashworthiness Investigations for 3D-Printed Multi-Layer Multi-Topology Engineering Resin Lattice Materials.

In comparison to monolithic materials, cellular solids have superior energy absorption capabilities. Of particular interest within this category are the periodic lattice materials, which offer repeatable and highly customizable behavior, particularly in combination with advances in additive manufacturing technologies. In this paper, the crashworthiness of engineering multi-layer, multi-topology (MLMT) resin lattices is experimentally examined. First, the response of a single- and three-layer single topology cubic and octet lattices, at a relative density of 30%, is investigated. Then, the response of MLMT lattices is characterized and compared to those single-topology lattices. Crashworthiness data were collected for all topology arrangements, finding that while the three-layer cubic and octet lattices were capable of absorbing 9.8 J and 7.8 J, respectively, up to their respective densification points, the unique MLMT lattices were capable of absorbing more: 19.0 J (octet-cube-octet) and 22.4 J (cube-octet-cube). These values are between 94% and 187% greater than the single-topology clusters of the same mass.

Electrified reformer for syngas production – Additive manufacturing of coated microchannel monolithic reactor

Here, an electrified reformer with a bespoke design of catalyst-coated and additively manufactured monoliths with 3-D pore architecture is presented. The 2D-pore architecture of the hexagonal honeycomb monolith was compared against the 3D-pore architecture of three broad classifications - triply periodic minimal surface (Gyroid), strut-based (Octet) and stochastic lattice (Voronoi). Gyroid structured reactor, coated with NiO/Ce0.8Gd0.2O2-δ catalyst and heated to 900 °C achieved >99 % conversion of CO2 and CH4 at WHSV = 11,000 L.h−1.kgcat−1, while remaining active for >42 h with no evidence of coke deposition. Whereas Octet and Voronoi reactors showed slightly lower conversions, with 6-fold and 4-fold higher pressure drops, respectively. This study combines Computational Fluid Dynamics and experimental evidence to reveal that the Gyroid lattice provides high catalyst deposition and catalytic activity at low pressure drops. This study demonstrates the feasibility of renewable energy-powered structured reactors to provide a pathway for low carbon footprint chemical production.

The deformation mechanism of low symmetric Ti–Pt intermetallic compounds containing high density of planar defects

Intermetallic compounds typically exhibit limited plastic deformation capacity due to challenges in activating dislocation slip and deformation twinning, coupled with a lack of alternative deformation mechanisms. Ti–Pt alloys are a prevalent type of intermetallic compound utilized in high-temperature shape memory alloys and as materials for energy applications in electric fields. However, they often exhibit poor deformation capability. Here, we prepared a low-symmetry intermetallic phase, Ti4Pt3, which demonstrates significant plastic deformation capability. This phase features a high density of parallel planar defects, resulting in an exceptionally large lattice periodicity perpendicular to these defects. Through in-situ scanning electron microscope compression tests, we observed substantial plastic deformation in this new phase. Analysis of the deformed Ti4Pt3 phase revealed that the dense planar defects create uniformly distributed sites of internal stress concentration, enabling a rapid increase in back stress within crystals. This phenomenon leads to notable lattice rotation and localized order-disorder transitions, both crucial mechanisms facilitating plastic deformation and enhancing deformation capacity. Our research underscores the potential of leveraging structural asymmetry to enable unconventional deformation mechanisms, thereby enhancing the plasticity of intermetallic materials.

Observation of nonlinear fractal higher order topological insulator

Higher-order topological insulators (HOTIs) are unique materials hosting topologically protected states, whose dimensionality is at least by 2 lower than that of the bulk. Topological states in such insulators may be strongly confined in their corners which leads to considerable enhancement of nonlinear processes involving such states. However, all nonlinear HOTIs demonstrated so far were built on periodic bulk lattice materials. Here, we demonstrate the first nonlinear photonic HOTI with the fractal origin. Despite their fractional effective dimensionality, the HOTIs constructed here on two different types of the Sierpiński gasket waveguide arrays, may support topological corner states for unexpectedly wide range of coupling strengths, even in parameter regions where conventional HOTIs become trivial. We demonstrate thresholdless spatial solitons bifurcating from corner states in nonlinear fractal HOTIs and show that their localization can be efficiently controlled by the input beam power. We observe sharp differences in nonlinear light localization on outer and multiple inner corners and edges representative for these fractal materials. Our findings not only represent a new paradigm for nonlinear topological insulators, but also open new avenues for potential applications of fractal materials to control the light flow.

Tailoring elastic bandgaps and moduli of triply periodic minimal surface structures by a hybrid technique

This work aimed to investigate elastic dispersion behaviors of triply periodic minimal surface (TPMS) lattice structures. The dispersion curves were established using the finite element (FE) Bloch reduction method, in which the Bloch periodic boundary condition was implied via the Bloch transform matrix. Four different types of both surface- and solid-based TPMS lattices with varying relative densities were considered. It was found that only the solid-based Neovius and Schwarz structures provided complete elastic bandgaps. Then, elastic stiffnesses of the structures obtained by representative volume element (RVE) models were evaluated with regard to their achieved band gaps. Reducing the volume fraction of lattices obviously enhanced the bandgap behaviors, while decreasing the specific moduli. Moreover, new hybrid phononic lattices were introduced for tailoring the elastic bandgap and mechanical properties. The results showed that incorporating the strut-based face-centered cubic (FCC) lattice in the TPMS structures could obviously improve the bandgap characteristics and simultaneously maintain reasonable specific elastic properties. Finally, wave suppression structure by using the dispersion curves was demonstrated by examining a simple beam infilled with the periodic lattices under excited loads. It was shown that the TPMS structures could be well applied as an effective wave suppression feature in load-bearing structures.

Janus Monolayer of 1T-TaSSe: A Computational Study.

Materials exhibiting charge density waves are attracting increasing attention owing to their complex physics and potential for applications. In this paper, we present a computational, first principles-based study of the Janus monolayer of 1T-TaSSe transition metal dichalcogenide. We extensively compare the results with those obtained for parent compounds, TaS2 and TaSe2 monolayers, with confirmed presence of 13×13 charge density waves. The structural and electronic properties of the normal (undistorted) phase and distorted phase with 13×13 periodic lattice distortion are discussed. In particular, for a normal phase, the emergence of dipolar moment due to symmetry breaking is demonstrated, and its sensitivity to an external electric field perpendicular to the monolayer is investigated. Moreover, the appearance of imaginary energy phonon modes suggesting structural instability is shown. For the distorted phase, we predict the presence of a flat, weakly dispersive band related to the appearance of charge density waves, similar to the one observed in parent compounds. The results suggest a novel platform for studying charge density waves in two-dimensional transition metal dichalcogenides.

Mott Transition for a Lieb-Liniger Gas in a Shallow Quasiperiodic Potential: Delocalization Induced by Disorder.

Disorder or quasidisorder is known to favor localization in many-body Bose systems. Here, in contrast, we demonstrate an anomalous delocalization effect induced by incommensurability in quasiperiodic lattices. Loading ultracold atoms in two shallow periodic lattices with equal amplitude and either equal or incommensurate spatial periods, we show the onset of a Mott transition not only in the periodic case but also in the quasiperiodic case. Switching from periodic to quasiperiodic potential with the same amplitude, we find that the Mott insulator turns into a delocalized superfluid. Our experimental results agree with quantum MonteCarlo calculations, showing this anomalous delocalization induced by the interplay between the disorder and interaction.

Evolution of the Fermi Surface of 1T-VSe2 across a Structural Phase Transition.

Periodic lattice distortion, known as the charge density wave, is generally attributed to electron-phonon coupling. This correlation is expected to induce a pseudogap at the Fermi level in order to gain the required energy for stable lattice distortion. The transition metal dichalcogenide 1T-VSe2 also undergoes such a transition at 110 K. Here, we present detailed angle-resolved photoemission spectroscopy experiments to investigate the electronic structure in 1T-VSe2 across the structural transition. Previously reported warping of the electronic structure and the energy shift of a secondary peak near the Fermi level as the origin of the charge density wave phase are shown to be temperature independent and hence cannot be attributed to the structural transition. Our work reveals new states that were not resolved in previous studies. Earlier results can be explained by the different dispersion natures of these states and temperature-induced broadening. Only the overall size of the Fermi surface is found to change across the structural transition. These observations, quite different from the charge density wave scenario commonly considered for 1T-VSe2 and other transition metal dichalcogenides, bring fresh perspectives toward correctly describing structural transitions. Therefore, these new results can be applied to material families in which the origin of the structural transition has not been resolved.

Structure and properties features of CA-PVD Ti-Al-Ni-N coatings deposited on carbide alloys and tool steel substrates

The results of studies of the properties of CA-PVD Ti-Al-Ni-N coatings deposited on widely used carbide alloys of the WC-Co, WC-TiC-Co, WC-TiC-TaC-Co groups and carbon steel are presented. It has been shown that the hardness and Young's modulus of the coatings on different substrates differ by up to 1.5 times, even though there is no significant difference in composition. The formation of globular aggregations on cell ridges, which is a characteristic feature of the morphology of coatings on steel substrates, and the presence of multilevel (stepped) structure were observed. The difference in CSR, microstrain and lattice period values for the nitride phase in the coatings on the substrates has been established. From the point of view of differences in thermal diffusivity of substrates, features of morphology and substructure of coatings are explained. The differences in the properties of coatings are also associated with the implementation of tensile macrostresses on steel in comparison with compressive stresses on carbide substrates. Tensile macrostresses and macrolayering are the reasons for the relatively low level of strength (2–3 times) of adhesion of the coating to tool steel compared to coatings on carbide substrates. At the same time, a decrease in the shear strength of the coating material on a steel substrate due to macro-layering may be the reason for the relatively low friction coefficient of this coating (~0.5) compared to coatings on a carbide base (0.69–0.53). The work provides recommendations for the most effective use of Ti-Ni-Al-N coating on carbide alloys and carbon steel.

Controlling the spontaneous emission of the telecom O-band centers in Silicon-on-Insulator with coherent dipole-quadrupole interactions on a silicon pillar lattice

The G-center in silicon-on-insulator (SOI) has emerged as a relevant single photon source due to the zero-phonon line at 1278 nm, matching the O-band of optical telecommunication wavelengths. Due to the weak emission of the G-center from planar SOI, it is necessary to enhance its emission, in terms of collection efficiency and fluorescence enhancement, to further study its optical properties for application in quantum technologies. Here we design a fabrication-ready SOI Mie-resonator made of a lattice of silicon nanopillars on silica to achieve the spontaneous emission enhancement of single G-centers. Using Si nanopillars lattice on silica with period and dimensions optimized, owing to the coherent superposition of the electric dipolar and magnetic quadrupolar electromagnetic Mie-scattering moments of the individual pillars to the periodic lattice, we show the G-center emission/decay rate enhancement averaged over it's possible dipole orientations is about 13 times compared to bulk. This corresponds to a fluorescence enhancement of about 125 times compared to bulk and about 5-6 times compared to an optimized single pillar with photon collection with a confocal objective. These results provide a fabrication pathway for out-of-plane single photon collection from the SOI G-center or other telecom O-band single-photon sources for their potential deployment in CMOS-compatible quantum optical technologies.

Superior damage tolerance observed in interpenetrating phase composites composed of aperiodic lattice structures

Interpenetrating Phase Composite (IPC) metamaterials, based on lattice topologies, have garnered significant attention as advanced materials for structural applications. However, conventional IPCs, which rely on periodic lattice unit cells, are prone to catastrophic failure due to their global deformation modes. To overcome this limitation, we present a novel IPC design utilizing aperiodic truss unit cells, inspired by the elusive “Einstein” monotile pattern. Our concept is demonstrated through IPC 3D printed via polymer jetting, using a hard polymer as the lattice filler and a soft polymer as the matrix. The distinctive mechanical properties of IPCs are characterized through single and cyclic quasi-static compression testing. Our findings demonstrate that aperiodic IPCs enable progressive deformation with gradual compression stress plateaus. Additionally, aperiodic IPCs exhibit remarkable damage tolerance, retaining 67.59 % of residual energy absorption and 73.83 % of ultimate strength after multiple cyclic compressions up to 30 % strain. These mechanisms are attributed to the synergistic deformation of interconnected unit cells, which lead to self-adjusting plastic collapse, progressive displacement evolution and delocalized deformation. This aperiodic concept paves the way for developing high-performance cushioning protection materials.

Pressure drop measurements over anisotropic porous substrates in channel flow

Previous theoretical and simulation results indicate that anisotropic porous materials have the potential to reduce turbulent skin friction in wall-bounded flows. This study experimentally investigates the influence of anisotropy on the drag response of porous substrates. A family of anisotropic periodic lattices was manufactured using 3D printing. Rod spacing in different directions was varied systematically to achieve different ratios of streamwise, wall-normal, and spanwise bulk permeabilities (κxx\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\kappa _{xx}$$\\end{document}, κyy\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\kappa _{yy}$$\\end{document}, and κzz\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\kappa _{zz}$$\\end{document}). The 3D printed materials were flush-mounted in a benchtop water channel. Pressure drop measurements were taken in the fully developed region of the flow to systematically characterize drag for materials with anisotropy ratios κxxκyy∈[0.035,28.6]\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{\\kappa _{xx}}{\\kappa _{yy}} \\in [0.035,28.6]$$\\end{document}. Results show that all materials lead to an increase in drag compared to the reference smooth wall case over the range of bulk Reynolds numbers tested (Reb∈[500,4000]\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Re}_b \\in [500,4000]$$\\end{document}). However, the relative increase in drag is lower for streamwise-preferential materials. We estimate that the wall-normal permeability for all tested cases exceeded the threshold identified in previous literature (κyy+>0.4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{\\kappa _{yy}}^+> 0.4$$\\end{document}) for the emergence of energetic spanwise rollers similar to Kelvin–Helmholtz vortices, which can increase drag. The results also indicate that porous walls exhibit a departure from laminar behavior at different values for bulk Reynolds numbers depending on the geometry.Graphical abstract