Modified Theoretical Model Predicts Radial Support Capacity of Polymer Braided Stents.

BACKGROUND AND OBJECTIVE: Self-expanding polymer braided stents are expected to replace metallic stents in the treatment of Peripheral Arterial Disease, which seriously endangers human health. To restore the patency of blocked peripheral arteries with different properties and functions, the radial supporting capacity of the stent should be considered corresponding to the vessel. A theoretical model can be established as an effective method to study the radial supporting capacity of the stent which can shorten the stent design cycle and realize the customization of the stent according to lesion site. However, the classical model developed by Jedwab and Clerc of radial force is only limited to metallic braided stents, and the predictions for polymer braided stents are deviated. METHODS: In this paper, based on the limitation of the J&C model for polymer braided stents, a modified radial force model for polymer braided stents was proposed, which considered the friction between monofilaments and the torsion of the monofilaments. And the modified model was verified by radial force tests of polymer braided stents with different structures and monofilaments. RESULTS: Compared with the J&C model, the proposed modified model has better predictability for the radial force of polymer braided stents that prepared with different braided structure and polymer monofilaments. The root mean squared error of modified model is 0.041±0.026, while that of the J&C model is 0.246±0.111. CONCLUSIONS: For polymer braided stents, the friction between the polymer monofilaments and the torsion of the monofilaments during the radial compression cannot be ignored. The radial force prediction accuracy of the modified model considering these factors was significantly improved. This work provides a research basis on the theoretical model of polymer braided stents, and improves the feasibility of rapid personalized customization of polymer braided stents.

Low voltage-driven high-performance thermal switching in antiferroelectric PbZrO3 thin films.

Effective control of heat transfer is vital for energy saving and carbon emission reduction. In contrast to achievements in electrical conduction, active control of heat transfer is much more challenging. Ferroelectrics are promising candidates for thermal switching as a result of their tunable domain structures. However, switching ratios in ferroelectrics are low (<1.2). We report that high-quality antiferroelectric PbZrO3 epitaxial thin films exhibit high-contrast (>2.2), fast-speed (<150 nanoseconds), and long-lifetime (>107) thermal switching under a small voltage (<10 V). In situ reciprocal space mapping and atomistic modelings reveal that the field-driven antiferroelectric-ferroelectric phase transition induces a substantial change of primitive cell size, which modulates phonon-phonon scattering phase space drastically and results in high switching ratio. These results advance the concept of thermal transport control in ferroic materials.

Unexpected doping effects on phonon transport in quasi-one-dimensional van der Waals crystal TiS3 nanoribbons.

Doping usually reduces lattice thermal conductivity because of enhanced phonon-impurity scattering. Here, we report unexpected doping effects on the lattice thermal conductivity of quasi-one-dimensional (quasi-1D) van der Waals (vdW) TiS3 nanoribbons. As the nanoribbon thickness reduces from ~80 to ~19 nm, the concentration of oxygen atoms has a monotonic increase along with a 7.4-fold enhancement in the thermal conductivity at room temperature. Through material characterizations and atomistic modellings, we find oxygen atoms diffuse more readily into thinner nanoribbons and more sulfur atoms are substituted. The doped oxygen atoms induce significant lattice contraction and coupling strength enhancement along the molecular chain direction while have little effect on vdW interactions, different from that doping atoms induce potential and structural distortions along all three-dimensional directions in 3D materials. With the enhancement of coupling strength, Young's modulus is enhanced while phonon-impurity scattering strength is suppressed, significantly improving the phonon thermal transport.

Regulating mechanical performance of poly (l-lactide acid) stent by the combined effects of heat and aqueous media.

Annealing process has been applied to the development of thermoforming polymer braided stent and treating its basic constitute monofilaments, especially for Poly (l-lactide acid) (PLLA) condensed by lactic acid monomer made from the plant starch. In this work, high performance monofilaments were produced by melting spun and solid-state drawing methods. Inspired by the effects of water plasticization on semi-crystal polymer, PLLA monofilaments were annealed with and without constraint in vacuum and aqueous media. Then, the co-effects of water infestation and heat on the micro-structure and mechanical properties of these filaments were characterized. Furtherly, mechanical performance of PLLA braided stents shaped by different annealing methods was also compared. Results showed that annealing in aqueous media generated more obvious structure change of PLLA filaments. Interestingly, the combined effects of aqueous phase and thermal effectively increased the crystallinity, and decreased the molecular weight and orientation of PLLA filaments. Therefore, higher modulus, smaller strength, and elongation at the break for filaments could be obtained, which could furtherly realize better radial compression resistance of the braided stent. This annealing strategy could provide new perspectives between anneal and material properties of PLLA monofilaments, and provide more suitable manufacturing technics for polymer braided stent.

Focus on the crucial deformation region to adjust the comprehensive performance of poly (L-lactic acid) stent.

The fully biodegradable polymer stent is considered as the fourth-generation vascular implant with good biocompatibility and long-term therapeutic potential. It has attracted much attention because it overcomes the disadvantage of the permanently implanted metal stent. However, compared with the metal stent, its mechanical properties are slightly inferior, which is an urgent problem. Based on previous studies, fully biodegradable polymer stents are prone to experience cracks and damage in large deformation region during the crimping and expansion process. The large deformation region is mainly located at the ring bend of the stent. We supposed that these damages are the leading causes of weakening the mechanical performance of polymer stents and are mainly affected by the crucial deformation region. For this purpose, this work studies the relationship between different crucial deformation regions and the mechanical performance of the polymer stent. Firstly, the volume of the crucial deformation region is improved by increasing the ring width. Although the radial strength of the stent is enhanced with the increase in ring width, the radial stiffness also increases, and correspondingly, the flexibility of the stent decreases. To obtain acceptable comprehensive mechanical performance, two types of slotting design in critical deformation region were proposed. The proposed slotted stent with a bulge has sufficient radial strength and low radial stiffness, having a good radial support capacity and flexibility. In other words, the proposed stent has improved the radial support without sacrificing flexibility. Overall, different crucial deformation regions cause different degrees of damage to the stent during crimping and expansion, which affects the mechanical properties of the stent. Reasonable structural design of the crucial deformation region is the key to adjust the comprehensive performance of the stent.

Modulating thermal conductance across the metal/graphene/SiO2 interface with ion irradiation.

Optimizing the efficiency of heat dissipation across an interface is a great challenge with the continuously increasing integration of microelectronic devices. In this work, an effective method in tuning the heat conduction across the Al/graphene/SiO2 interface is reported. It was found that the interfacial thermal conductance of Al/irradiated graphene/SiO2 can be increased by a factor of 3, as compared with that of Al/pristine graphene/SiO2. The X-ray photoelectron spectroscopy (XPS) analysis indicates that ion irradiation may promote the formation of CO bonds on the irradiated graphene surface, which is beneficial to the enhancement of interfacial thermal conductance. The density functional theory (DFT) calculations reveal that in addition to the formed bonds between O atoms and Al atoms, the adsorption strength between Al and irradiated graphene is intensified, which plays a dominant role in enhancing the interfacial thermal conductance of Al/graphene/SiO2.

Resonance in Atomic-Scale Sliding Friction.

Friction represents a major energy dissipation mode, yet the atomistic mechanism of how friction converts mechanical motion into heat remains elusive. It has been suggested that excess phonons are mainly excited at the washboard frequency, the fundamental frequency at which relative motion excites the interface atoms, and the subsequent thermalization of these nonequilibrium phonons completes the energy dissipation process. Through combined atomic force microscopy measurements and atomistic modeling, here we show that the nonlinear interactions between a sliding tip and the substrate can generate excess phonons at not only the washboard frequency but also its harmonics. These nonequilibrium phonons can induce resonant vibration of the tip and lead to multiple peaks in the friction force as the tip sliding velocity ramps up. These observations disclose previously unrecognized energy dissipation channels associated with tip vibration and provide insights into engineering friction force through adjusting the resonant frequency of the tip-substrate system.

Solid-State Thermal Memory of Temperature-Responsive Polymer Induced by Hydrogen Bonds.

Memory is an essential element for a computer to process information, which is integrated by logical circuits. Like electronic computing, thermal information can also be stored and read out by a thermal memory. Here, we show that a phase-changing polymer with hysteretic thermal transport properties can be experimentally processed into thermal memories at room temperature. We used a temperature-responsive and reversible polymer synthesized with melamine (M) and 6,7-dimethoxy-2,4[1H,3H]-quinazolinedione (Q) as a model system to demonstrate the manipulation of thermal transport at a molecular level. Fourier transform infrared spectroscopy and differential scanning calorimetry measurements indicate that this hysteretic behavior is based on the interaction of hydrogen bonds at high (317 K) and low (297 K) temperatures. This work demonstrates a controllable phonon transport process through the manipulation of hydrogen bonds, and thus it has potential applications in thermal memories.

The effects of contact atom distribution at the interface on the phonon transport.

At the nanometer scale, heat (phonon) transport is sensitive to the contact details at the interface due to the phonon wave property. However, the effects of contact atom distribution are ignored. In this work, the atomic Green's function (AGF) method and molecular dynamics (MD) simulation are applied to explore those effects. A parameter named as the average distance d[combining macron] is raised here to measure the distribution of contacted atoms at the interface. Based on the AGF method, phonon transmission profiles at different d[combining macron] (distribution) with the same number of contacted atoms have a coincident point, the reverse frequency fr. If the phonon frequency f is smaller (larger) than fr, smaller d[combining macron] has smaller (larger) phonon transmission. The overlap of the vibrational density of states from the MD simulation and the local density of states from the AGF method indicate that the reverse frequency is caused by the match degree of vibration modes across the interface. The existence of reverse frequency leads to the reverse temperature Tr. Increasing the contact area or the interfacial coupling strength can cause the blue shift of fr and the increase of Tr. The MD simulations observe a larger temperature jump at the interface for larger d[combining macron], similar to that from the AGF method at temperatures higher than Tr due to the high-temperature limit property in MD. The results are independent of the choice of cutoff distance in potential and interfacial coupling strength, indicating that the conclusion here is applicable for the general interface.

The enhancement of heat conduction across the metal/graphite interface treated with a focused ion beam.

In this work, we report a convenient and efficient approach to improve heat conduction across the metal/graphite interface. It is demonstrated that the interfacial thermal conductance between Al and graphite can be enhanced by a factor of ∼5 after milling the graphite with a focused ion beam. Such enhancement is attributed to the decreased Fermi level of the milled graphite compared with the pristine counterpart. Once graphite is milled with the focused ion beam, surface defects are formed that induce the redistribution of electrons at the interface between Al and graphite. The formation of enormous dipoles on the milled graphite/Al interface leads to the conversion of the interfacial interaction from physisorption to chemisorption, which is beneficial for phonon transmission across the interface. Based on the measured Fermi level difference, the non-equilibrium Green's function method predicts that the interfacial interaction strength in the Al/milled graphite is increased 4-fold compared with Al/pristine graphite, which causes the increase of the interfacial thermal conductance. Our theoretical model also predicts that the interfacial thermal conductance does not increase monotonically with the interaction strength. Once the interaction strength exceeds a critical value, the interface thermal conductance will decrease.

Measurement of intrinsic thermal conductivity of carbon fiber using direct electrical heating method.

It is usually very difficult to measure the intrinsic thermal conductivity of fibers using direct electrical heating method, due to the presence of lateral heat loss. In this study, we demonstrate that the intrinsic thermal conductivity and lateral heat transfer coefficient of fibers can be extracted simultaneously via multiple measurements on the same fiber. In our experiments, three samples of various lengths were prepared from an individual polyacrylonitrile-based carbon fiber of 5.6 µm in diameter and measured with the direct electrical heating method. From each sample, we can get a curve of thermal conductivity vs lateral heat transfer coefficient. We showed that the intrinsic thermal conductivity and lateral heat transfer coefficient can be extracted from the intersection of these curves. Our results also showed that ignoring the lateral heat loss can result in an overestimation in thermal conductivity of carbon fibers by more than 3 times.

Electrical and thermal conductivities of polycrystalline platinum nanowires.

Understanding the electrical and thermal transport properties of polycrystalline metallic nanostructures is of great interest for applications in microelectronics. In view of the diverse experimental results in polycrystalline metallic nanowires and nanofilms, it is a long-standing question whether their electrical and thermal properties can be well predicted by a practical model. By eliminating the effects of electrical and thermal contact resistances, we measure the electrical and thermal conductivities of three different polycrystalline Pt nanowires. The electron scattering at the surface is found to be diffusive, and the charge reflection coefficient at grain boundaries is proved to be a function of the melting point. The Lorenz number is observed to be suppressed from the free-electron value by about 30%, which can be explained by introducing a thermal reflection coefficient in calculating the thermal conductivity to account for the small angle scattering effect involving phonons at the grain boundaries. Using this model, both the electrical and thermal conductivities of the polycrystalline Pt nanowires are calculated at different diameters and temperatures.

Thermal transport through fishbone silicon nanoribbons: unraveling the role of Sharvin resistance.

Heat conduction has been shown to be greatly suppressed in Si nanomeshes, which has attracted extensive attention for potential thermoelectric applications, yet the precise suppression mechanism remains to be fully understood. Attempting to further disclose the underlying mechanisms, we report on the thermal conductivity of the building block for nanomeshes, i.e., Si nanoribbons with fins attached to the two opposite sides. By expanding only the fin width while keeping both the period length and the backbone size constant, we observed an unexpected non-monotonic trend of the effective thermal conductivity normalized with the backbone cross-section. Further analysis showed that the corrected thermal conductivity extracted with appropriate consideration of the geometrical effect on diffusion followed a monotonically decreasing trend, reaching a maximum thermal conductivity reduction of 18% at 300 K for a ribbon with the maximum explored fin width of 430 nm, as compared to that of the straight ribbon of 66 nm backbone width. We attribute the thermal conductivity reduction to the thermal constriction resistance induced by the cross-section reduction between the fin and backbone sections. For ribbons with a larger fin width, the effective phonon mean free path is longer for phonons arriving at the constriction, which boosts the ballistic constriction resistance, i.e., Sharvin resistance, and leads to a lower thermal conductivity.

Effect of Electrical Contact Resistance on Measurement of Thermal Conductivity and Wiedemann-Franz Law for Individual Metallic Nanowires.

The electrical and thermal properties of metallic nanostructures have attracted considerable fundamental and technological interests. Recent studies confirmed a dramatic decrease in the electrical and thermal conductivities when the dimension is comparable or even smaller than the electron mean free path. However, the verification of the Wiedemann-Franz law in these nanostructures remains hotly debated. The Lorenz number obtained from the two-probe measurement is found to be much larger than that from the four-probe measurement. Here, we reported the electrical and thermal properties of the individual silver nanowires measured by the two-probe and four-probe configurations. The measured electrical contact resistance is found to be nearly temperature-independent, indicating a ballistic-dominant electronic transport at the contacts. When the effect of thermal contact resistance is diminished, the Lorenz number measured by the four-probe configuration is comparable to the Sommerfeld value, verifying that the Wiedemann-Franz law holds in the monocrystalline-like silver nanowire. Comparatively, the derived electrical conductivity becomes smaller and the thermal conductivity becomes larger in the two-probe measurement, confirming that the electrical contact resistance will introduce a large error. The present study experimentally demonstrates a reasonable explanation to the discouragingly broad span in the Lorenz number obtained from different metallic nanostructures.

Significantly enhanced thermal conductivity of indium arsenide nanowires via sulfur passivation.

ABSTARCT: In this work, we experimentally investigated the effect of sulfur passivation on thermal transport in indium arsenide (InAs) nanowires. Our measurement results show that thermal conductivity can be enhanced by a ratio up to 159% by sulfur passivation. Current-voltage (I-V) measurements were performed on both unpassivated and S-passivated InAs nanowires to understand the mechanism of thermal conductivity enhancement. We observed a remarkable improvement in electrical conductivity upon sulfur passivation and a significant contribution of electrons to thermal conductivity, which account for the enhanced thermal conductivity of the S-passivated InAs nanowires.

Defect Facilitated Phonon Transport through Kinks in Boron Carbide Nanowires.

Nanowires of complex morphologies, such as kinked wires, have been recently synthesized and demonstrated for novel devices and applications. However, the effects of these morphologies on thermal transport have not been well studied. Through systematic experimental measurements, we show that single-crystalline, defect-free kinks in boron carbide nanowires can pose a thermal resistance up to ∼30 times larger than that of a straight wire segment of equivalent length. Analysis suggests that this pronounced resistance can be attributed to the combined effects of backscattering of highly focused phonons and required mode conversion at the kink. Interestingly, it is also found that instead of posing resistance, structural defects in the kink can actually assist phonon transport through the kink and reduce its resistance. Given the common kink-like wire morphology in nanoelectronic devices and required low thermal conductivity for thermoelectric devices, these findings have important implications in precise thermal management of electronic devices and thermoelectrics.

Thermal conductivity of individual silicon nanoribbons.

The thermal conductivities of two groups of silicon nanoribbons of ∼20 and ∼30 nm thickness and various widths have been measured and analyzed through combining the Callaway model and the Fuchs-Sondheimer (FS) reduction function. The results show that while the data for the ∼30 nm thick ribbons can be well-explained by the classical size effect, the measured thermal conductivities for the ∼20 nm thick ribbons deviate from the prediction remarkably, and size effects beyond phonon-boundary scattering must be considered. The measurements of the Young's modulus of the thin nanoribbons yield significantly lower values than the corresponding bulk value, which could lead to a reduced phonon group velocity and subsequently thermal conductivity. This study helps to build a regime map for thermal conductivity versus nanostructures' surface-area-to-volume ratio that clearly delineates two regions where size effects beyond the Casimir limit are important or not important.

Length-dependent thermal transport in one-dimensional self-assembly of planar π-conjugated molecules.

This work reports a thermal transport study in quasi-one-dimensional organic nanostructures self-assembled from conjugated planar molecules via π-π interactions. Thermal resistances of single crystalline copper phthalocyanine (CuPc) and perylenetetracarboxylic diimide (PTCDI) nanoribbons are measured via a suspended thermal bridge method. We experimentally observed the deviation from the linear length dependence for the thermal resistance of single crystalline ß-phase CuPc nanoribbons, indicating possible subdiffusion thermal transport. Interestingly, a gradual transition to the linear length dependence is observed with the increase of the lateral dimensions of CuPc nanoribbons. The measured thermal resistance of single crystalline CuPc nanoribbons shows an increasing trend with temperature. However, the trend of temperature dependence of thermal resistance is reversed after electron irradiation, i.e., decreasing with temperature, indicating that the single crystalline CuPc nanoribbons become 'amorphous'. Similar behavior is also observed for PTCDI nanoribbons after electron irradiation, proving that the electron beam can induce amorphization of single crystalline self-assembled nanostructures of planar π-conjugated molecules. The measured thermal resistance of the 'amorphous' CuPc nanoribbon demonstrates a roughly linear dependence on the nanoribbon length, suggesting that normal diffusion dominates thermal transport.

Thermal conductivity of zinc blende and wurtzite CdSe nanostructures.

Many binary octet compounds including CdSe can be grown in either the wurtzite (WZ) or zinc blende (ZB) phase, which has aroused great interest among the research community in understanding the phase dependence of the thermal transport properties of these compounds. So far, it has been debatable whether the ZB phase possesses higher thermal conductivity than the WZ phase. In this work, we report on thermal conductivity measurements of CdSe nanowires/nanoribbons with both WZ and ZB phases via a suspended device method. At room temperature, the thermal conductivity of all the ZB CdSe nanostructures measured in this work is higher than the bulk thermal conductivity of the WZ CdSe reported in the literature, suggesting that the bulk thermal conductivity of the ZB CdSe is higher than that of the WZ phase. Our result is different from previous experimental results in the literature for InAs nanowires which suggest similar thermal conductivity values for the bulk ZB and WZ InAs crystals. The higher thermal conductivity of the ZB CdSe can be explained by its lower anharmonicity and a smaller number of atoms per unit cell compared to the WZ phase.

Defect-Engineered Heat Transport in Graphene: A Route to High Efficient Thermal Rectification.

Low-dimensional materials such as graphene provide an ideal platform to probe the correlation between thermal transport and lattice defects, which could be engineered at the molecular level. In this work, we perform molecular dynamics simulations and non-contact optothermal Raman measurements to study this correlation. We find that oxygen plasma treatment could reduce the thermal conductivity of graphene significantly even at extremely low defect concentration (∼ 83% reduction for ∼ 0.1% defects), which could be attributed mainly to the creation of carbonyl pair defects. Other types of defects such as hydroxyl, epoxy groups and nano-holes demonstrate much weaker effects on the reduction where the sp(2) nature of graphene is better preserved. With the capability of selectively functionalizing graphene, we propose an asymmetric junction between graphene and defective graphene with a high thermal rectification ratio of ∼ 46%, as demonstrated by our molecular dynamics simulation results. Our findings provide fundamental insights into the physics of thermal transport in defective graphene, and two-dimensional materials in general, which could help on the future design of functional applications such as optothermal and electrothermal devices.