Albert C. To

Broadband wave filtering of bioinspired hierarchical phononic crystal

Broadband wave filtering effect is observed in a phononic crystal with hierarchical structure inspired by biocomposites. Through a multilayered model with hierarchical structure, it is demonstrated that the overall bandwidth covered by closely adjacent bandgaps for this structure is orders of magnitude broader in frequency than that of conventional periodic structure with single periodicity. This remarkable feature is attributed to the inherent multiscale periodicity in the microstructure, which roughly superimposes the bandgaps generated by individual periodic structures with periods corresponding to those at different hierarchies in the hierarchical structure.

Anomalous heat conduction behavior in thin finite-size silicon nanowires

Anomalous heat conduction behavior is observed for the first time using non-equilibrium molecular dynamics (NEMD) simulations to obtain the thermal conductivity of thin finite-size silicon nanowires (NWs) in the 001 lattice direction. In the series of simulations, the length dependence of thermal conductivity of thin silicon nanowires (NWs) ranging from 6 to 434 nm is analyzed. It is found that a transition occurs in the thermal conductivity versus length curve after the initial convergence trend appears near the mean free path of bulk silicon. Because no experimental measurements of thermal conductivity are available for sub-10 nm diameter silicon NWs, different NEMD methods are used to test and analyze this anomalous thermal behavior of thin Si NWs with different boundary conditions. The underlying mechanism of the observed behavior is inferred from MD simulations with different boundary conditions so that the anomalous behavior is mainly caused by border restriction and boundary scattering of the thin silicon NWs.

Heats of Vaporization of Room Temperature Ionic Liquids by Tunable Vacuum Ultraviolet Photoionization

The heats of vaporization of the room temperature ionic liquids (RTILs) N-butyl-N-methylpyrrolidinium bistrifluorosulfonylimide, N-butyl-N-methylpyrrolidinium dicyanamide, and 1-butyl-3-methylimidazolium dicyanamide are determined using a heated effusive vapor source in conjunction with single photon ionization by a tunable vacuum ultraviolet synchrotron source. The relative gas phase ionic liquid vapor densities in the effusive beam are monitored by clearly distinguished dissociative photoionization processes via a time-of-flight mass spectrometer at a tunable vacuum ultraviolet beamline 9.0.2.3 (Chemical Dynamics Beamline) at the Advanced Light Source synchrotron facility. Resulting in relatively few assumptions, through the analysis of both parent cations and fragment cations, the heat of vaporization of N-butyl-N-methylpyrrolidinium bistrifluorosulfonylimide is determined to be DeltaH(vap)(298.15 K) = 195 +/- 19 kJ mol(-1). The observed heats of vaporization of 1-butyl-3-methylimidazolium dicyanamide (DeltaH(vap)(298.15 K) = 174 +/- 12 kJ mol(-1)) and N-butyl-N-methylpyrrolidinium dicyanamide (DeltaH(vap)(298.15 K) = 171 +/- 12 kJ mol(-1)) are consistent with reported experimental values using electron impact ionization. The tunable vacuum ultraviolet source has enabled accurate measurement of photoion appearance energies. These appearance energies are in good agreement with MP2 calculations for dissociative photoionization of the ion pair. These experimental heats of vaporization, photoion appearance energies, and ab initio calculations corroborate vaporization of these RTILs as intact cation-anion pairs.

Ultra-low Thermal Conductivity in Si/Ge Hierarchical Superlattice Nanowire

Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced by introducing hierarchical structure to form Si/Ge hierarchical superlattice nanowire (H-SNW). The structural hierarchy introduces defects to disrupt the periodicity of regular SNW and scatters coherent phonons, which are the key contributors to thermal transport in regular SNW. Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW. By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved. Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%. It is concluded that the hierarchical structuring is an effective way of reducing thermal conductivity significantly in SNW, which can be a promising path for improving the efficiency of Si/Ge-based SNW thermoelectrics.

Highly Enhanced Damping Figure of Merit in Biomimetic Hierarchical Staggered Composites

Most composites exhibit a damping figure of merit, a crucial index of a materials dynamic behavior, lower than the value predicted by the Hashin–Shtrikman bound. This work found that the biomimetic hierarchical staggered composites inspired by bone structure can have a damping figure of merit tens of times higher than the Hashin–Shtrikman composite. The optimum state is achieved when the hard and soft phases contribute equally to the overall stiffness of the composite in the direction parallel to the platelets. At this optimal point, the model predicts that the overall stiffness is half the Voigt bound while the damping loss factor is half that of the soft phase. This behavior stems from a deformation mechanism transition from soft-phase-dominant to hard-phase-dominant as the platelets aspect ratio increases. The findings from this study may have important implications in the future design of composites to mitigate vibration and absorb shock in load-bearing structures.

Ligament and joint sizes govern softening in nanoporous aluminum

The present computational study demonstrates that softening of an open cell nanoporous aluminum structure subjected to tensile loading can be significantly reduced when the size of ligaments and the joints that connect them in the structure is designed to be sufficiently small. It is found using molecular dynamics simulations that the softening becomes slightly slower with increasing porosity for the structures with porosity less than or equal to 72%, and stress localization is observed during softening. In contrast, for structures with more than 75% porosity, softening is much slower, and stress delocalization occurs during softening. It is argued that at relatively high porosity, softening is governed by both the ligament size and the joint size because their compliance becomes high enough to allow the overloading stress due to ligament rupture to be redistributed more effectively throughout the structure.

Proportional Topology Optimization: A New Non-Sensitivity Method for Solving Stress Constrained and Minimum Compliance Problems and Its Implementation in MATLAB

A new topology optimization method called the Proportional Topology Optimization (PTO) is presented. As a non-sensitivity method, PTO is simple to understand, easy to implement, and is also efficient and accurate at the same time. It is implemented into two MATLAB programs to solve the stress constrained and minimum compliance problems. Descriptions of the algorithm and computer programs are provided in detail. The method is applied to solve three numerical examples for both types of problems. The method shows comparable efficiency and accuracy with an existing optimality criteria method which computes sensitivities. Also, the PTO stress constrained algorithm and minimum compliance algorithm are compared by feeding output from one algorithm to the other in an alternative manner, where the former yields lower maximum stress and volume fraction but higher compliance compared to the latter. Advantages and disadvantages of the proposed method and future works are discussed. The computer programs are self-contained and publicly shared in the website www.ptomethod.org.

Efficient design optimization of variable-density cellular structures for additive manufacturing: theory and experimental validation

Purpose The purpose of the paper is to propose a homogenization-based topology optimization method to optimize the design of variable-density cellular structure, in order to achieve lightweight design and overcome some of the manufacturability issues in additive manufacturing. Design/methodology/approach First, homogenization is performed to capture the effective mechanical properties of cellular structures through the scaling law as a function their relative density. Second, the scaling law is used directly in the topology optimization algorithm to compute the optimal density distribution for the part being optimized. Third, a new technique is presented to reconstruct the computer-aided design (CAD) model of the optimal variable-density cellular structure. The proposed method is validated by comparing the results obtained through homogenized model, full-scale simulation and experimentally testing the optimized parts after being additive manufactured. Findings The test examples demonstrate that the homogenization-based method is efficient, accurate and is able to produce manufacturable designs. Originality/value The optimized designs in our examples also show significant increase in stiffness and strength when compared to the original designs with identical overall weight.

Microseismic source deconvolution: Wiener filter versus minimax, Fourier versus wavelets, and linear versus nonlinear

Deconvolution is commonly performed on microseismic signals to determine the time history of a dislocation source, usually modeled as combinations of forces or couples. This paper presents a new deconvolution method that uses a nonlinear thresholding estimator, which is based on the minimax framework and operates in the wavelet domain. Experiments were performed on a steel plate using artificially generated microseismic signals, which were recorded by high-fidelity displacement sensors at various locations. The source functions were deconvolved from the recorded signals by Wiener filters and the new method. Results were compared and show that the new method outperforms the other methods in terms of reducing noise while keeping the sharp features of the source functions. Other advantages of the nonlinear thresholding estimator include (1) its performance is close to that of a minimax estimator, (2) it is nonlinear and takes advantage of sparse representations under wavelet bases, and (3) its computation is...

Mechanical properties of SWNT X-Junctions through molecular dynamics simulation

The mechanical behavior of seven different carbon nanotube (CNT) X-junctions with a varying number of bonds was investigated through molecular dynamics simulations. The X-junctions are composed of two (6,0) single-walled carbon nanotubes (SWNTs) created via vibration-assisted heat welding. The junctions, containing anywhere between one and seven bonds, are subject to uniaxial tensile, shear and torsional strain, and then the stiffness values are determined for each case. When subjected to tensile and shear strain, both the arrangement and orientation of bonds are found to affect the stiffness of junctions more substantially than the number of bonds, bond length or bond order. Surprisingly, anisotropic shear behavior is observed in the X-junctions, which can be attributed to the junctions bond orientation. Also, the stiffness of X-junctions tested under an applied torque (torsion) differs from the stiffness under tensile and shear strain, however, in that it is more substantially affected by the number of bonds present in the junction than by any other property.