Carbon nanotube-fullerene hybrid nanostructures by C60 bombardment: formation and mechanical behavior.

This study reports the investigation on C60 bombardment on the carbon nanotube (CNT) by molecular dynamic (MD) simulations. We found that carbon nanotube nanobuds or nanotube-fullerene hybrid nanostructures can be formed by C60 bombardment. Different from the nanobuds in earlier studies, more structural patterns are found in the bombardment formed nanobuds and nanotube-fullerene hybrid nanostructures. In addition, the attaching strengths of the carbon nanobuds are explored, and results indicate that the junction between C60 and CNTs in the nanobuds is very stable. Moreover, we also found that the bombardment formed nanobuds and nanotube-fullerene hybrid nanostructures generally decrease the maximum tensile strength and Young's modulus of carbon nanotubes.

Microstructures and Corrosion Properties of Wire Arc Additive Manufactured Copper-Nickel Alloys.

The 70/30 copper-nickel alloy is used mainly in critical parts with more demanding conditions in marine settings. There is a need for innovative methods that offer fast production and cost-effectiveness in order to supplement current copper-nickel alloy manufacturing processes. In this study, we employ wire arc additive manufacturing (WAAM) to fabricate the 70/30 copper-nickel alloy. The as-built microstructure is characterized by columnar grains with prominent dendrites and chemical segregation in the inter-dendritic area. The aspect ratio of the columnar grain increases with increasing travel speed (TS) at the same wire feed speed (WFS). This is in contrast with the equiaxed grain structure, with a more random orientation, of the conventional sample. The sample built with a WFS of 8 m/min, TS of 1000 mm/min, and a track distance of 3.85 mm exhibits superior corrosion properties in the 3.5 wt% NaCl solution when compared with the conventional sample, as evidenced by a higher film resistance and breakdown potential, along with a lower passive current density of the WAAM sample. The corrosion morphology reveals the critical roles played by the nickel element that is unevenly distributed between the dendrite core and inter-dendritic area.

Unusually high room and elevated-temperature tensile properties observed in direct aged wire-arc directed energy deposited Inconel 718.

Wire-arc directed energy deposition (DED) processed Inconel (IN) 718 is known to have coarse columnar grains, strong texture, and significant chemical and microstructural inhomogeneity in the as-fabricated condition. Homogenization treatment is commonly used prior to aging to eliminate the inhomogeneity and detrimental precipitation for better mechanical properties. In this study, however, direct aging (DA) at 700 °C without homogenization has resulted in room-temperature yield strength, ultimate tensile strength (UTS), and elongation that are comparable to wrought condition and among the highest reported properties for wire-arc DED IN718. The DA samples at between 650 and 750 °C aging also demonstrates remarkable ductility when deformed at elevated temperatures. In addition, when aged below 750 °C the DA IN718 possesses significantly higher UTS compared to those with homogenization treatment. These superior mechanical properties are highly likely due to the non-uniform and hierarchical precipitation consisting of disk-shaped γ″ in diameter from a few to tens of nm in the dendritic core area and micron-sized Laves phase and carbides in the inter-dendritic region.

Toward Real-Time Blood Pressure Monitoring via High-Fidelity Iontronic Tonometric Sensors with High Sensitivity and Large Dynamic Ranges.

Continuous, noninvasive blood pressure (CNIBP) monitoring provides valuable hemodynamic information that renders detection of the early onset of cardiovascular diseases. Wearable mechano-electric pressure sensors that mount on the skin are promising candidates for monitoring continuous blood pressure (BP) pulse waveforms due to their excellent conformability, simple sensing mechanisms, and convenient signal acquisition. However, it is challenging to acquire high-fidelity BP pulse waveforms since it requires highly sensitive sensors (sensitivity larger than 4 × 10-5 kPa-1 ) that respond linearly with pressure change over a large dynamic range, covering the typical BP range (5-25 kPa). Herein, this work introduces a high-fidelity, iontronic-based tonometric sensor (ITS) with high sensitivity (4.82 kPa-1 ), good linearity (R2 > 0.995), and a large dynamic range (up to 180% output change) over a broad working range (0 to 38 kPa). Additionally, the ITS demonstrates a low limit of detection at 40 Pa, a fast load response time (35 ms) and release time (35 ms), as well as a stable response over 5000 load per release cycles, paving ways for potential applications in human-interface interaction, electronic skins, and robotic haptics. This work further explores the application of the ITS in monitoring real-time, beat-to-beat BP by measuring the brachial and radial pulse waveforms. This work provides a rational design of a wearable pressure sensor with high sensitivity, good linearity, and a large dynamic range for real-time CNIBP monitoring.

Surface structure and properties of functionalized nanodiamonds: a first-principles study.

The goal of this work is to gain fundamental understanding of the surface and internal structure of functionalized detonation nanodiamonds (NDs) using quantum mechanics based density functional theory (DFT) calculations. The unique structure of ND assists in the binding of different functional groups to its surface which in turn facilitates binding with drug molecules. The ability to comprehensively model the surface properties, as well as drug-ND interactions during functionalization, is a challenge and is the problem of our interest. First, the structure of NDs of technologically relevant size (∼5 nm) was optimized using classical mechanics based molecular mechanics simulations. Quantum mechanics based density functional theory (DFT) was then employed to analyse the properties of smaller relevant parts of the optimized cluster further to address the effect of functionalization on the stability of the cluster and reactivity at its surface. It is found that functionalization is preferred over reconstruction at the (100) surface and promotes graphitization in the (111) surface for NDs functionalized with the carbonyl oxygen (C = O) group. It is also seen that the edges of ND are the preferred sites for functionalization with the carboxyl group (-COOH) vis-à-vis the corners of ND.

Integrating Geometric Data into Topology Optimization via Neural Style Transfer.

This research proposes a novel topology optimization method using neural style transfer to simultaneously optimize both structural performance for a given loading condition and geometric similarity for a reference design. For the neural style transfer, the convolutional layers of a pre-trained neural network extract and quantify characteristic features from the reference and input designs for optimization. The optimization analysis is evaluated as a single weighted objective function with the ability for the user to control the influence of the neural style transfer with the structural performance. As seen in architecture and consumer-facing products, the visual appeal of a design contributes to its overall value along with mechanical performance metrics. Using this method, a designer allows the tool to find the ideal compromise of these metrics. Three case studies are included to demonstrate the capabilities of this method with various loading conditions and reference designs. The structural performances of the novel designs are within 10% of the baseline without geometric reference, and the designs incorporate features in the given reference such as member size or meshed features. The performance of the proposed optimizer is compared against other optimizers without the geometric similarity constraint.

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.

Ultrahigh Thermal Rectification in Pillared Graphene Structure with Carbon Nanotube-Graphene Intramolecular Junctions.

In this letter, graded pillared graphene structures with carbon nanotube-graphene intramolecular junctions are demonstrated to exhibit ultrahigh thermal rectification. The designed graded two-stage pillared graphene structures are shown to have rectification values of 790.8 and 1173.0% at average temperatures 300 and 200 K, respectively. The ultrahigh thermal rectification is found to be a result of the obvious phonon spectra mismatch before and after reversing the applied thermal bias. This outcome is attributed to both the device shape asymmetry and the size asymmetric boundary thermal contacts. We also find that the significant and stable standing waves that exist in graded two-stage pillared graphene structures play an important role in this kind of thermal rectifier, and are responsible for the ultrahigh thermal rectification of the two-stage ones as well. Our work demonstrates that pillared graphene structure with SWCNT-graphene intramolecular junctions is an excellent and promising phononic device.

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.

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.