Honeycomb Materials Research Articles

Delineating the role of ripples on the thermal expansion of 2D honeycomb materials: graphene, 2D h-BN and monolayer (ML)-MoS2.

We delineated the role of thermally excited ripples on the thermal expansion properties of 2D honeycomb materials (free-standing graphene, 2D h-BN, and ML-MoS2), by explicitly carrying out three-dimensional (3D) and two-dimensional (2D) molecular dynamics simulations. In 3D simulations, the in-plane lattice parameter (a-lattice) of graphene and 2D h-BN shows thermal contraction over a wide range of temperatures and exhibits a strong system size dependence. The 2D simulations of the very same system show a reverse trend, where the a-lattice expands in the whole computed temperature range. In contrast to graphene and 2D h-BN, the a-lattice of ML-MoS2 shows thermal expansion in both 2D and 3D simulations and their system size dependence is marginal. By analyzing the phonon dispersion at 300 K, we found that the discrepancy between 2D and 3D simulations of graphene and 2D h-BN is due to the absence of out-of-plane bending modes (ZA) in 2D simulations, which is responsible for the thermal contraction of the a-lattice at low temperature. Meanwhile, all the phonon modes are present in the 2D phonon dispersion of ML-MoS2, which indicates that the origin of the ZA mode is not purely due to the out-of-plane movement of atoms and also its effect on thermal expansion is not significant as found in graphene and 2D h-BN.

Investigation of high-speed rubbing behavior of labyrinth-honeycomb seal for turbine engine application

The labyrinth-honeycomb seal has been widely used in gas turbine engines as an abradable gas path seal to protect the rotor from wear and damage in rubbing interaction. It usually works with a stepped labyrinth because the knife-edged tips could produce a special dynamic sealing system, and then the minimum clearance is possible between the rotor and stationary component. To investigate the high-speed rubbing behavior between a Hastelloy-X honeycomb material and a GH4169 double stepped labyrinth, nine rubbing tests were conducted using a high-speed abrasion test rig while the blade tip speed varied from 150 to 450 m/s, and the incursion rate from 120 to 360 µm/s. The abradability of honeycomb made from Hastelloy-X was fully verified by analyzing the visual rubbing observations, rubbing forces, and impact acceleration. It is shown that compression deformation happens to the honeycomb material during the rubbing process with the labyrinth blade except for a simple cutting mechanism, which is mainly affected by the parameter of incursion rate. Thermal ablation and oxidation were the main damage occurring on the labyrinth tip and appeared more obviously at a higher blade tip speed. Rubbing forces and impact acceleration were obtained from a piezoelectric dynamometer and acceleration sensor during the rubbing process. At a blade tip speed of 300 m/s and incursion rate of 360 µm/s, radial and tangential forces show their maximum values of 716 N and 871 N, respectively. The peak value of acceleration presents 341g with the highest blade tip speed of 450 m/s and the highest incursion rate of 360 µm/s. All testing results provide a great deal of effective information on high-speed rubbing behavior for the abradablility evaluation of a honeycomb.

Anisotropic failure and size effects in periodic honeycomb materials: A gradient-elasticity approach

This paper proposes a fracture mechanics model for the analysis of crack propagation in periodic honeycomb materials. The model is based on gradient-elasticity which enables us to account for the effect of the material structure at the macroscopic scale. For simulating the propagation of cracks along an arbitrary path, the numerical implementation is elaborated based on an extended finite element method with the required level of continuity. The two main features captured by the model are directionality and size effect. The numerical predictions are consistent with experimental results on honeycomb materials but also with results reported in the literature for microstructurally short cracks in metals.

Two Dimensional Honeycomb Materials: Random Fields, Dissipation and Fluctuations

In this paper, we propose a method to describe the many-body problem of electrons in honeycomb materials via the introduction of random fields which are coupled to the electrons and have a Gaussian distribution. From a one-body approach to the problem, after integrating exactly the contribution of the random fields, one builds a non-hermitian and dissipative effective Hamiltonian with two-body interactions. Our approach introduces besides the usual average over the electron field a second average over the random fields. The interplay of two averages enables the definition of various types of Green's functions which allow the investigation of fluctuation-dissipation characteristics of the interactions that are a manifestation of the many-body problem. In the current work we study only the dissipative term, through the perturbative analysis of the dynamics associated the effective Hamiltonian generated by two different kinds of couplings. For the cases analysed, the eigenstates of the effective Hamiltonian are complex and, therefore, some of the states have a finite life time. Moreover, we also investigate, in the mean field approximation, the most general parity conserving coupling to the random fields and compute the width of charge carriers $\Gamma$ as a function of the Fermi energy $E_F$. The theoretical prediction for $\Gamma (E_F)$ is compared to the available experimental data for graphene. The good agreement between $\Gamma_{theo}$ and $\Gamma_{exp}$ suggests that description of the many-body problem associated to the electrons in honeycomb materials can indeed be done via the introduction of random fields.

Kirigami honeycomb material exhibits a “Poisson’s switch”

Kirigami honeycomb material exhibits a “Poisson’s switch”

Mould design and manufacturing considerations of honeycomb biocomposites with transverse fibre direction for aerospace application

Sandwich structures with honeycomb core are known to significantly improve stiffness at lower weight and possess high flexural rigidity. They have found wide applications in aerospace as part of the primary structures, as well as the interior paneling and floors. High performance aluminum and aramid are the typical materials used for the purpose of honeycomb core whereas in other industries, materials such as fibre glass, carbon fibre, Nomex and also Kevlar reinforced with polymer are used. Recently, growing interest in developing composite structures with natural fibre reinforcement has also spurred research in natural fibre honeycomb material. The majority of the researches done, however, have generally emphasized on the usage of random chopped fibre and only a few are reported on development of honeycomb structure using unidirectional fibre as the reinforcement. This is mainly due to its processing difficulties, which often involve several stages to account for the arrangement of fibres and curing. Since the use of unidirectional fibre supports greater strength compared to random chopped fibre, a single-stage process in conjunction with vacuum infusion is suggested with a mould design that supports fibre arrangement in the direction of honeycomb loading.

Elastic wave dispersion in pre-strained negative stiffness honeycombs

A doubly periodic array of curved beams, known as a negative stiffness (NS) honeycomb, has recently been shown to exhibit tunable impact isolation by exploiting nonlinear mechanical behavior [Correa et al., Rapid Prototyping J., 21 (2), 2015]. The primary benefit of NS honeycombs over standard honeycomb materials lies in their ability to return to their original shape after experiencing an impact. The recoverable nature of their response is a result of the curved beam structure which has a single stable configuration but experiences one or more buckling events when loaded. The complex nonlinear elastic response of NS honeycombs lead to large variations in mechanical stiffness, which makes these materials compelling candidates for tunable control of elastic waves. The present work investigates linear elastic wave propagation of a representative NS honeycomb lattice at varying levels of pre-strain. Eigenfrequency finite element analysis is performed on the unit cell subjected to finite uniaxial deformation. The longitudinal and transverse phase speeds are shown to be anisotropic and highly dependent on uniaxial pre-strain. This is especially true near points of instability, where one observes very different functional dependence of longitudinal and transverse wave motion on pre-strain.

Topological edge states in correlated honeycomb materials with strong spin-orbit coupling

We study possible edge states in single layers of honeycomb materials such as $\alpha$-RuCl$_3$ and A$_2$IrO$_3$ (A=Li, Na) with strong spin-orbit coupling (SOC). These two dimensional systems exhibit linearly dispersing one-dimensional (1D) edge states when their 1D boundary forms a zig-zag shape. Using an effective tight-binding model based on first principles band structure calculations including Hubbard U and SOC, we find degenerate edge states at the zone center and zone boundary. The roles of chiral symmetry and time-reversal symmetry are presented. The implications to experimental signatures and the effects of disorder are also discussed.

Porous Materials with Tunable Structure and Mechanical Properties via Templated Layer-by-Layer Assembly.

The deposition of stiff and strong coatings onto porous templates offers a novel strategy for fabricating macroscale materials with controlled architectures at the micro- and nanoscale. Here, layer-by-layer assembly is utilized to fabricate nanocomposite-coated foams with highly customizable properties by depositing polymer-nanoclay coatings onto open-cell foam templates. The compressive mechanical behavior of these materials evolves in a predictable manner that is qualitatively captured by scaling laws for the mechanical properties of cellular materials. The observed and predicted properties span a remarkable range of density-stiffness space, extending from regions of very soft elastomer foams to very stiff, lightweight honeycomb and lattice materials.

Finite Element Calculation for the Tensile Elastic Modulus of Honeycomb Materials with Different Pore Shape

The square and hexagonal honeycomb materials were studied by finite element simulation of tensile test in order to calculate the elastic modulus in tension under different porosity and to analyze tensile properties affected by the pore shape of honeycomb materials. Research results show that the tensile elastic modulus of the square honeycomb is larger than the hexagonal honeycomb. The reason is that the square structure has a larger stiffness than hexagonal structure. Also, the equivalent elastic moduli of calculated values are higher than the theoretical values, because the finite element model is based on solid elements while the theoretical model is based on the Euler beam.

Ballistic Protection Evaluation of Sequencing the Composite Material Sandwich Panels for the Reliable Combination of Armor Layers

Impact problems are usually interesting for the military, either for defensive or offensive purposes to develop armor or ammunition. Recently, daily applications request safety of the products, therefore, it is essential to understand the material behavior under intense short duration or impact loadings. Metallic armor is extremely heavy and would not be popular for personal protection. However, reinforced fiber composites have been used for these purposes. In present study, carbon-fiber-reinforced aluminum honeycomb, aramid and plywood materials were used for armor matrix layers. For determining the capability of sequencing the composite layers, ballistic tests for all six combination of sequenced sandwich panels for three different composites were evaluated at a speed of 700 m/s using a 36 caliber one-cored projectile. To obtain cheaper and reliable solutions for further studies of various test conditions, computer aided ballistic simulations were analyzed. To make sufficient correlations, the test results and the computer simulations were compared to each other. Finally, plywood used between the aramid and the carbon-fiber-reinforced aluminum honeycomb sandwich panel has shown the most accurate and the reliable results of the tests and the computer simulations.

Magnetic Properties of Restacked 2D Spin 1/2 honeycomb RuCl3 Nanosheets.

Spin 1/2 honeycomb materials have gained substantial interest due to their exotic magnetism and possible application in quantum computing. However, in all current materials out-of-plane interactions are interfering with the in-plane order, hence a true 2D magnetic honeycomb system is still in demand. Here, we report the exfoliation of the magnetic semiconductor α-RuCl3 into the first halide monolayers and the magnetic characterization of the spin 1/2 honeycomb arrangement of turbostratically stacked RuCl3 monolayers. The exfoliation is based on a reductive lithiation/hydration approach, which gives rise to a loss of cooperative magnetism due to the disruption of the spin 1/2 state by electron injection into the layers. The restacked, macroscopic pellets of RuCl3 layers lack symmetry along the stacking direction. After an oxidative treatment, cooperative magnetism similar to the bulk is restored. The oxidized pellets of restacked single layers feature a magnetic transition at TN = 7 K if the field is aligned parallel to the ab-plane, while the magnetic properties differ from bulk α-RuCl3 if the field is aligned perpendicular to the ab-plane. The deliberate introduction of turbostratic disorder to manipulate the magnetic properties of RuCl3 is of interest for research in frustrated magnetism and complex magnetic order as predicted by the Kitaev-Heisenberg model.

Two iridates, two models, and two approaches: A comparative study on magnetism in three-dimensional honeycomb materials

Two recent theoretical works studied the role of Kitaev interactions in the newly observed incommensurate magnetic order in the hyper-honeycomb ($\beta$-Li$_2$IrO$_3$) and stripy-honeycomb ($\gamma$-Li$_2$IrO$_3$) iridates. Each of these works analyzed a different model ($JK\Gamma$ versus coupled zigzag chain model) using a contrasting method (classical versus soft-spin analysis). The lack of commonality between these works precludes meaningful comparisons and a proper understanding of these unusual orderings. In this study, we complete the unfinished picture initiated by these two works by solving both models with both approaches for both 3D honeycomb iridates. Through comparisons between all combinations of models, techniques, and materials, we find that the bond-isotropic $JK\Gamma$ model consistently predicts the experimental phase of $\beta$-Li$_2$IrO$_3$ regardless of the method used, while the experimental phase of $\gamma$-Li$_2$IrO$_3$ can be generated by the soft-spin approach with eigenmode mixing irrespective of the model used. To gain further insights, we solve a 1D quantum spin-chain model related to both 3D models using the density matrix renormalization group method to form a benchmark. We discover that in the 1D model, incommensurate correlations in the classical and soft-spin analysis survive in the quantum limit only in the presence of the symmetric-off-diagonal exchange $\Gamma$ found in the $JK\Gamma$ model. The relevance of these results to the real materials are also discussed.

Strain field of the monovacancy in silicene: First-principles study

The in-plane strain fields of single-vacancy silicene with different monovacancy (MV) concentrations, as well as the corresponding electronic band structures, are investigated by using the first-principle calculations. Firstly the self-healing MV is found to be the most stable ground structure in silicene, which is different from the other 2D hexagonal honeycomb materials, e.g. graphene, h-BN. In the isolated MV center, the bonds along the pentagons are compressed, creating a compress field, and those close to the distorted hexagons are stretched, creating a stretch field. As the MV concentration increasing, the interacted compress field tends to corrugate the defected silicene, while the interacted stretch field impacts little on the low-buckled structure. Especially, the corrugation presents in those supercells with small MV concentration, just as the (4, 5), (4, 6), (4, 7), (4, 8) supercells. The corrugations approach zero at both low and high MV concentrations, and the (4, 6) supercell with a MV concentration of about 0.021, has a peak value of 3.23Å. The electronic calculations show that the linear dispersion at Γ point in pristine silicene is broken by the lower lattice symmetry of the self-healing MV reconstruction, which translates it into metal as well.

Van der Waals heterostructures of germanene, stanene, and silicene with hexagonal boron nitride and their topological domain walls

We investigate van der Waals (vdW) heterostructures made of germanene, stanene, or silicene with hexagonal boron nitride (h-BN). The intriguing topological properties of these buckled honeycomb materials can be maintained and further engineered in the heterostructures, where the competition between the substrate effect and external electric fields can be used to control the tunable topological phase transitions. Using such heterostructures as building blocks, various vdW topological domain walls (DW) are designed, along which there exist valley polarized quantum spin Hall edge states or valley-contrasting edge states which are protected by valley(spin)- resolved topological charges and can be tailored by the patterning of the heterojunctions and by external fields.

A novel double-component MOAC honeycomb composite with pollen grains as a template for phosphoproteomics research

The enrichment and separation of phosphopeptides from mixed biological samples is a technologically very significance, but highly challenging work. Current designed materials are mainly based on the broad and effective adsorptive character of metal oxide affinity chromatography (MOAC). Though significant progress has been made in the enrichment of phosphopeptides with MOAC material, there are chances for further development. In this study, a novel pollen-based MOAC honeycomb material was firstly explored in which the suitable hydrophilic channels preferentially enrich much more endogenous phosphopeptides than nonphosphopeptides or proteins while doping binary metal oxides at the atomic level and the ultra-high specific surface area have further allowed it to possess more effective active sites. Based on these unique features, the pollen-based material exhibited high selectivity for β-casein (mass ratio of β-casein/BSA, 1:1500), ultra-low detection limit (0.1fmol), desirable reusability. Moreover, the bionics MOAC composites were investigated in the enrichment of phosphopeptides from nonfat milk, human serum (male and female at the same age) and mice liver, results of which indicate the great potential of the composite for the phosphoproteome analysis of complex biological samples through the cheap and environmentally friendly process.

Post-bifurcation and stability of a finitely strained hexagonal honeycomb subjected to equi-biaxial in-plane loading

The buckling and crushing mechanics of cellular honeycomb materials is an important engineering problem. Motivated by the pioneering experimental and numerical studies of Papka and Kyriakides (1994, 1999a,b), we review the literature on finitely strained honeycombs subjected to in-plane loading and identify two open questions: (i) How does the mechanical response of the honeycomb depend on the applied loading device? and (ii) What can the Bloch wave representation of all bounded perturbations contribute to our understanding of the stability of post-bifurcated equilibrium configurations? To address these issues we model the honeycomb as a two-dimensional infinite perfect periodic medium. We use analytical group theory methods (as opposed to the more common, but less robust, imperfection method) to study the honeycomb’s bifurcation behavior under three different far-field loadings that produce (initially) the same equi-biaxial contractive dilatation. Using an FEM discretization of the honeycomb walls (struts), we solve the equilibrium equations to find the principal and bifurcated equilibrium paths for each of the three loading cases. We evaluate the structure’s stability using two criteria: rank-one convexity of the homogenized continuum (long wavelength perturbations) and Bloch wave stability (bounded perturbations of arbitrary wavelength). We find that the post-bifurcation behavior is extremely sensitive to the applied loading device, in spite of a common principal solution. We confirm that the flower mode is always unstable, as previously reported. However, our (first ever) Bloch wave stability analysis of the post-bifurcated equilibrium paths shows that the flower mode is stable for all sufficiently short wavelength perturbations. This new result provides a realistic explanation for why this mode has been observed in the finite size specimen experiments of Papka and Kyriakides (1999a).

Two-dimensional transition metal dichalcogenides with a hexagonal lattice: Room-temperature quantum spin Hall insulators

So far, several transition metal dichalcogenides (TMDCs) based two-dimensional (2D) topological insulators (TIs) have been discovered, all of them based on a tetragonal lattice. However, in 2D crystals, the hexagonal rather than the tetragonal symmetry is the most common motif. Here, based on first-principles calculations, we propose a new class of stable 2D TMDCs of composition MX2 (M=Mo, W, X=S, Se, Te) with a hexagonal lattice. They are all in the same stability range as other 2D TMDC allotropes that have been demonstrated experimentally, and they are identified to be practical 2D TIs with large band gaps ranging from 41 to 198 meV, making them suitable for applications at room-temperature. Besides, in contrast to tetragonal 2D TMDs, their hexagonal lattice will greatly facilitate the integration of theses novel TI states van-der-Waals crystals with other hexagonal or honeycomb materials, and thus provide a route for 2D-material-based devices for wider nanoelectronic and spintronic applications. The nontrivial band gaps of both WSe2 and WTe2 2D crystals are 198 meV, which are larger than that in any previously reported TMDC-based TIs. These large band gaps entirely stem from the strong spin-orbit coupling strength within the d orbitals of Mo/W atoms near the Fermi level. Our findings will significantly broaden the scientific and technological impact of both 2D TIs and TMDCs.

Smart damping of geometrically nonlinear vibrations of functionally graded sandwich plates using 1–3 piezoelectric composites

ABSTRACTGeometrically nonlinear dynamic analysis of smart functionally graded (FG) sandwich plates integrated with the patches of active constrained layer damping (ACLD) treatment has been carried out by the finite element method. The constraining layer of the ACLD treatment is considered to be made of vertically/obliquely reinforced 1–3 piezoelectric composite while the constrained layer is made of a viscoelastic material, which is modeled using the Golla–Hughes–McTavish method in the time domain. The top and bottom faces of the substrate sandwich plate are composed of the FG isotropic material whose mechanical properties are assumed to vary according to a standard power-law distribution in terms of the volume fractions of the constituents while the core layer may be either a soft honeycomb material or a hard ceramic material. Several FG sandwich plates with different core configurations are studied to evaluate the numerical results. The numerical results indicate that the ACLD patches significantly improve the damping characteristics of the FG sandwich plates for suppressing their geometrically nonlinear vibrations. Effects of metal- or ceramic-rich top and bottom surfaces, the variation of power-law index on the control authority of the ACLD patches have been investigated. Emphasis has also been placed on investigating the effect of the variation of piezoelectric fiber orientation angle on the performance of the ACLD patches.

Development of an 1100C Crushable Nose Surrogate Vehicle for Low-Speed Impact Testing of Breakaway Hardware

In the development of breakaway roadside safety hardware, surrogate vehicles offer an efficient, cost-effective means of testing new breakaway design concepts and evaluating system performance in accordance with applicable standards. For the purposes of conducting low-speed impact testing, a crushable nose impactor, when combined with a gravity pendulum, can constitute a suitable surrogate alternative to a production vehicle. This paper describes the development of a crushable nose impactor that can serve as a surrogate version of the 1100C passenger car described in AASHTO’s Manual for Assessing Safety Hardware. The design of the new crushable nose was based on published force–deformation data collected from crash testing of a Kia Rio production passenger car. The new crushable nose differs from other designs in that it is intended specifically for use in low-speed impact testing of breakaway hardware; the nose uses tapered (i.e., trapezoidal) aluminum honeycomb cartridges, which permits improved approximation of the production vehicle force–deformation curve; and the cartridge design process incorporates insights gained through high-deformation compression testing of aluminum honeycomb materials. This paper documents ( a) the analysis techniques that were used iteratively to design the new crushable nose and ( b) the final dimensions of the configured cartridges. The results from rigid pole validation impact testing of the crushable nose impactor, conducted with the Florida Department of Transportation’s gravity pendulum, are presented and shown to be in good agreement with the production vehicle force–deformation data.