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In this study, the Liu–Murakami (LM) creep damage-coupled model was considered to evaluate the creep properties of martensitic stainless steel. The degree of creep damage was examined at two temperatures (565 ℃ and 593 ℃) to assess mechanically and thermally activated processes. A series of high applied stresses (applied stress/ultimate strength > 0.5) was considered for accelerated creep loadings. A full set of creep constants was determined by combining the Norton and LM models. Constitutive equations were used to quantitatively estimate experimental creep curves. The variation in creep constants was discussed based on stress sensitivity, such as stress triaxiality and applied stress, depending on the power of stress. The creep strain–time curves were successfully estimated. The comparison between the experimental and analytical results was in good agreement in the tertiary regime. In addition, the compensation of the two applied temperatures provides a supplementary explanation of the relationship between the ultimate strength and rupture time in terms of temperature sensitivity. The analytical results show that different applied stresses and temperatures could be compensated to characterize the creep behavior of the material. Thus, the creep strain–time and creep strain rate–certain rupture time curves were finally achieved. The analytical process in this study provides a laboratory-scale assessment of creep properties using the accelerated creep test and LM model.

Void growth, as a critical stage in ductile fracture, could play some important role in the ductility of high entropy alloys (HEAs). Under a high-strain-rate tension (strain rate: 0.1~0.4/ns), our atomistic simulations reveal a severely suppressed growth of void (initial diameter: 4.0~9.1 nm) in HEAs at a low temperature, compared with that in Ni metal or Ni-based binary alloys. The emission of unclosed glide loops is found critical to void growth. Void growth is suppressed, once dislocation emission is suppressed due to the solute drag effect. Regarding the alloying elements, we find the void suppression relevance as Fe>Co and Cr>Fe>Mn.

Water splitting is thermodynamically uphill reaction, hence it cannot occur easily, and also highly complicated and challenging reaction in chemistry. In electrocatalytic water splitting, the combination of oxygen and hydrogen evolution reactions produces highly clean and sustainable hydrogen energy and which attracts research communities. Also, fabrication of highly active and low cost materials for water splitting is a major challenge. Therefore, in the present study, γ-Fe2O3 nanowires were fabricated from highly available and cost-effective iron plate without any chemical modifications/doping onto the surface of the working electrode with high current density. The fabricated nanowires achieved the current density of 10 mA/cm2 at 1.88 V vs. RHE with the scan rate of 50 mV/sec. Stability measurements of the fabricated Fe2O3 nanowires were monitored up to 3275 sec with the current density of 9.6 mA/cm2 at a constant potential of 1.7 V vs. RHE and scan rate of 50 mV/sec.

A non-contact quantitative method for measuring the electrical conductivity of a SnO2 nanobelt field-effect transistor (FET) with nanometer-scale spatial resolution is reported. The topography and microwave images of the nanobelt FET were measured by microwave atomic force microscopy (M-AFM) under a constant source voltage and different back-gate voltages. The output characteristics of the nanobelt FET were measured using a two-probe measurement method. The local conductivity of the SnO2 nanobelt FET measured by M-AFM concurred with that obtained by the two-probe measurement. Therefore, M-AFM is a promising method for measuring the local conductivity of nanomaterial FETs.

Tumor-targeted core-shell nanoparticles were developed for a multifunctional drug delivery system (DDS) for cancer therapy. Fe3O4 nanoparticles with mesoporous silica (Fe3O4NPs@mSiO2), in which Fe3O4 and mSiO2 form the core and shell, respectively, were functionalized to deliver a hydrophobic anti-tumor drug and to heat targeted tumor cells with the energy of an alternating magnetic field. For targeting tumor cells, tLyP-1, a tumor cell homing and penetrating peptide, was engrafted on to the Fe3O4NPs@mSiO2 (Fe3O4NPs@mSiO2-tLyP-1). The fabricated Fe3O4NPs@mSiO2-tLyP-1 were loaded with camptothecin (CPT); they showed robust, selective targeting and penetrating efficacy for Hela cells, and induced cell death. The CPT-loaded Fe3O4NPs@mSiO2-tLyP-1 (Fe3O4NPs@mSiO2-tLyP-1(CPT)) reduced the cell viability of Hela cells to 28.8%. Furthermore, in combination with application of an alternating magnetic field to cause hyperthermia, the nanoparticles made it possible to decrease viability to 18.3%. The systemic toxicity of Fe3O4NPs@mSiO2-tLyP-1(CPT) to human mesenchymal stem cells (hMSCs) was minimal, because the nanoparticles selectively targeted and penetrated the tumor cells. This study indicates that the developed Fe3O4NPs@mSiO2-tLyP-1 have potential as a DDS in the chemo-hyperthermic treatment of cancer.

In this research, we proposed a highly sensitive hydrogen sensor based on a new suspended structure of cross-stacked multiwall carbon nanotube (MWCNT) sheet. MWCNT sheet is a kind of CNT film which has a super-high CNT alignment and can be easily prepared by drawing from the spinnable CNT array in large scales. By stacking the sheets onto an electrode with a 1 × 1 cm hole in mutually perpendicular directions, sensors with suspended cross-stacked structure were realized. Afterwards, a two-side Pd functionalization was introduced. The effects of suspended structure, cross-stacked structure and two-side Pd functionalization were investigated respectively. It was observed that the sample with 2 + 1 layers of cross-stacked MWCNT sheet and two-side 3 nm Pd deposition showed the best gas sensing performance with a relative resistance change of 35.30% at 4% H2. This result indicates that the proposed sensor is one of the best among all reported MWCNT based hydrogen sensors. The method demonstrated in this research gives a potential solution for the mass production of CNT-based sensors with high sensitivity and reliability.

Accidental chemotherapy extravasation exacerbates the side effects of anticancer drugs. Therefore, drug-delivery nanocarriers should be designed to avoid persistent drug release at off-target sites and promote burst drug release at on-target. Considering these requirements, poly(allylamine)-co-poly(allylurea) (PAU), a ureido-derivatized temperature responsive polymer with upper critical solution temperature (UCSTs), is an ideal material. This report describes the fabrication, characterization, and in vitro cellular toxicity of PAU polymer-grafted magnetic mesoporous silica nanoparticles as drug-delivery nanocarriers. A UCST of 43 °C and an ultranarrow transition temperature range of 39-43 °C was realized, ensuring that the nanocarriers suppressed undesirable leakage to below 10 % of the drug loading for 8 h in the absence of a thermal stimulus. A drug release burst of up to 75 % of the drug loading was achieved within 30 min after the stimulus, reducing the viability of the in vitro cancer cells to 12 %. Therefore, the ureido-derivatized polymer is one of the most suitable gatekeepers for drug-delivery nanocarriers.

This study evaluated the low-cycle fatigue (LCF) life of type 316 austenitic stainless steel to determine the effect of multiple high-density pulsed electric currents (HDPECs). Fatigue properties were analyzed with the fatigue crack growth (FCG) and LCF tests by considering different conditions of multiple HDPECs and its application. The HDPEC densities of 100, 150 and 200 A/mm2 with application numbers from 1 to 17 times were used. The LCF results were assessed by using fatigue models, and the effectiveness of the application methods was examined. Under the HDPEC density of 200 A/mm2, increasing the number of HDPECs during the period of crack propagation is the best way for delaying FCG. Multiple applications of HDPEC caused a decrease in the length and an increase in the depth of striations in the specimen’s fatigue fracture surface, and the degree of ductility was increased thereby leading to the delay in FCG. The proposed conditions pave the way toward improving the LCF life of the material.

We investigate the effect of pulsed electric current on fatigue-crack growth and the fatigue properties of aluminum alloy (A6061-T6) at current densities of 0–150 A/mm2. The fatigue life of most of the treated specimens increases substantially when compared with that of untreated specimens. SEM imaging of the treated specimens shows local melting on the fracture surfaces. To clarify the effect of local melting on fatigue-crack propagation, we examine crack propagation utilizing the plastic replication method. We find that the delay of crack propagation is attributable to the crack-shielding effect arising from current-induced local melting.

The power devices that are attracting attention in the recent development of electric vehicles, specifically, SiC and GaN power devices (power semiconductors) are expected as key technologies for the 21st century because of high current control and high-frequency operation. However, because of the operating temperature exceeds 300 degrees due to high power, thermal damage on the mounting part of the electronic package cannot be prevented by the present solder technique. We recently developed a metallic nanowire surface fastener (NSF) to resolve the abovementioned problems (Fig. 1). This fastener can be used to connect electronic components on a substrate at room temperature using the van der Waals force between each nanowire. The NSF would be expected to apply huge filed such as wearable devices, assembling of power devices in the inverter of electronic vehicle. This study demonstrates a 64-pin NSF that behaves like a ball grid array for application to actual electronic devices. Additionally, we demonstrate the fabrication of NSF on the flexible substrate. Through the comparison of above both, the adhesion mechanism of NSF is discussed in this present study. We also introduce how to control the adhesion morphology to improve the adhesion strength, electronic conductivity, and thermal conductivity of the NSF.

Water splitting is thermodynamically uphill reaction, hence it cannot occur easily, and also highly complicated and challenging reaction in chemistry. In electrocatalytic water splitting, the combination of oxygen and hydrogen evolution reactions produces highly clean and sustainable hydrogen energy and which attracts research communities. Also, fabrication of highly active and low cost materials for water splitting is a major challenge. Therefore, in the present study, γ-Fe2O3 nanowires were fabricated from highly available and cost-effective iron plate without any chemical modifications/doping onto the surface of the working electrode with high current density. The fabricated nanowires achieved the current density of 10 mA/cm2 at 1.88 V vs. RHE with the scan rate of 50 mV/sec. Stability measurements of the fabricated Fe2O3 nanowires were monitored up to 3275 sec with the current density of 9.6 mA/cm2 at a constant potential of 1.7 V vs. RHE and scan rate of 50 mV/sec.

We report a non-contact and quantitative method to measure the local permittivity of dielectric materials with a nanometer-scale spatial resolution. A theoretical model based on near-field approximation was developed to describe the effect of a microwave on the interaction between a probe and a sample. Under the non-contact mode, we successfully measured the force curves of Si, Al2O3, Ge, and ZrO2 using microwave atomic force microscopy and observed the variation in the force caused by the microwave. According to the established theoretical model, a quantitative non-contact evaluation of the local permittivity of dielectric materials was performed.We report a non-contact and quantitative method to measure the local permittivity of dielectric materials with a nanometer-scale spatial resolution. A theoretical model based on near-field approximation was developed to describe the effect of a microwave on the interaction between a probe and a sample. Under the non-contact mode, we successfully measured the force curves of Si, Al2O3, Ge, and ZrO2 using microwave atomic force microscopy and observed the variation in the force caused by the microwave. According to the established theoretical model, a quantitative non-contact evaluation of the local permittivity of dielectric materials was performed.

Since the fabrication technique for high-entropy alloy (HEA) nanowires/nanopillars is still in its infancy, neither experimental nor modeling analyses of their cold-welding performance have been reported. Based on insights accumulated in our previous experiments and simulations regarding cold-welded metallic nanowires, in this study, the cold-welding performance of HEA nanowires is probed by atomistic simulations. Among different materials, our simulations reveal that extensively twinned structures are formed in CoCrMnFeNi samples, but not in CoCrCuFeNi or Ni samples. The larger fracture strain in certain HEAs is due to the improved ductility around the fracturing area as well as multiple twinning. Unlike in Ni samples, the fracture strains in HEA samples, regardless of being cuboid or cylindrical, are improved by shrinking the sample size. Among different orientations, the [010]-direction monocrystalline nanowires fail at a strain over 0.6, which is almost double that of the [111] direction. The fracture strains in polycrystalline HEA samples are, on average, larger than those in polycrystalline Ni samples. Furthermore, fracture strains in randomly generated polycrystalline HEA samples are more predictable than those in polycrystalline Ni samples with identical grain configurations. As previously reported, dislocation emission is still a prerequisite to fracture in all cold-welded samples.

In this study, a non-contact and quantitative evaluation method was developed to measure the conductivity of metallic nanowires with a nanometer-scale spatial resolution. A coaxial probe was experimentally fabricated; using this probe, microwave images of the Al, Ag, and Cu nanowires and their topography images were simultaneously obtained via microwave atomic force microscopy (M-AFM) in the non-contact mode. A semi-near-field reflection model was established to describe the spatial distribution of a microwave between the tip of the probe and the sample. The local conductivities of metallic nanowires on the nanometer-scale can be quantitatively evaluated in a single scan, using a metal strip substrate to calibrate the reflection signal.

In this paper, we report a non-contact and quantitative method to evaluate the permittivity of semiconductor nanomaterials. A microwave atomic force microscopy (M-AFM) was used to obtain the topography and microwave images of nanomaterials in one scanning process. Morphology and microwave images of ZnO and CuO nanowires, and SnO2 nanobelts with high spatial resolution were obtained in the non-contact mode of M-AFM. The local relative permittivity of these one-dimensional metal oxide nanomaterials was quantitatively evaluated.

In this research, we propose a new simple method to fabricate high-density Fe2O3 nanowire arrays for solar water splitting, based on oxidation-assisted stress-induced atomic-diffusion. In the presence of water vapor, surface oxidation was promoted during the heating process. The driving force induced by the stress gradient was enhanced due to the expansion of the oxidation layer. Therefore, Fe2O3 nanowire arrays were fabricated at a relative low temperature (350 °C) with a high density (8.66 wire per μm2). Using the nanowire array as the photoanode, a photocurrent density of 0.65 mA cm−2 at 1.23 V vs. RHE was achieved in a three-electrode system.

Though dislocation emission was known to be critical in the deformation and fracture process of metallic nanowires, the corresponding mechanism in cold-welded metallic nanowires has never been clarified. In order to understand the dislocation-mediated deformation mechanism in cold-welded nanowires, large-scale atomistic simulations are employed in this work. During dislocation emission, the formation and motion of only unclosed shear loops are observed inside nanowires. We prove from a theoretical viewpoint that unclosed shear dislocation loops can induce material transport and hence plastically deform cold-welded nanowires. For a better clarification, through running simulation replicates, we propose a novel approach to single out the displacement field solely due to shear loop emission. Regarding the pattern of material transport, our simulated atom arrangement near the dislocation core is found consistent with that captured by high-resolution transmission electron microscopy (HRTEM) of a stressed Au nanocrystal. Our analysis clarifies that the emission of unclosed shear loop plays a crucial role in not only the tensile deformation, but also the fracture process of cold-welded nanowires. It should be regarded as a general mechanism that shear loop emission can induce surface deformation and redistribute mass in the process of plasticity.

In this paper, we have proposed a new crystal plane control technique of single crystal SnO2 nanobelts by thermal sublimation. The synthesis pressure was found to drastically influence the dimension of the cross-sectional aspect ratio (width–thickness) of the nanobelts. The pressure changed the surface free energy that contributes to the preferential growth of crystal planes of the nanobelt. The aspect ratio range of 1.2 to 7.39 for the nanobelts was achieved solely owing to the pressure variation. Also, as a catalyst style on the synthesis by thermal sublimation, the dispersed Au nanoparticles assisted the formation of uniform SnO2 nanobelts.

A drastic size effect in cold-welded metallic nanowire for surface fasteners was observed in our previous experiment. In this work, a large-scale molecular dynamics (MD) simulation and a finite element (FE) simulation coupled with the nonsingular dislocation theory are combined to probe the true origin of this drastic size effect. Very large diameter (up to 200 nm) is involved in the direct MD simulation, leaving no gap between the simulation and the experiment. It was once believed that the drastic size effect comes from the Van der Waals (vdW) force transmitted through the bonding interface, which does give a mathematically agreeable scaling law. However, based on present simulations, new understanding is established―the drastic size effect is linked to dislocation emission, rather than vdW force. Compared with a single nanowire, the externally induced stress surges near the bonding corner of cold-welded nanocrystals due to the enormous stress concentration. The dislocation emission is hence triggered. It is also revealed that, overwhelmingly, dislocation emission should be energetically favorable in cold-welded nanocrystals. Additionally, MD simulations with polycrystalline nanocrystals reveal that both the random orientation effect and the grain boundary effect are secondary to the enormous stress concentration effect. Under such less ideal situation, dislocation emission still determines the maximum stress.

Surface-mount techniques primarily depend on soldering. However, soldering techniques have encountered some challenges in recent years. These challenges include rare metal recycling, thermal problems, and Pb toxicity. We recently developed a metallic nanowire surface fastener (NSF) to resolve the abovementioned problems. This fastener can be used to connect electronic components on a substrate at room temperature using the van der Waals force between each nanowire. This study demonstrates a 64-pin NSF that behaves like a ball grid array (BGA) for application to actual electronic devices. The adhesion strength and electrical properties of the NSF were investigated by adjusting the nanowire parameters, such as diameter, length, density (number per area), preload, and shape. The shape control of the nanowires greatly contributed to the improvement of the properties. A maximum adhesion strength of 16.4 N/cm2 was achieved using a bent, hook-like NSF. This strength was 4–5 times the value of the straight NSF. The contact resistivity was 2.98 × 10−2 Ω∙cm2. The NSF fabricated through the simple template method showed the room temperature bonding ability and adaptability to a highly ordered electrode like the BGA.

In recent years, different types of techniques have been developed to observe the cells and extract a wealth of information about their physiological properties. The analysis of the electrical properties requires modeling the cells. However, the cells have complex structures and most of the analytical equations are limited. Therefore, in this laboratory, the microwave detection system is combined with the atomic force microscopy (AFM) to evaluate the local electrical properties of the materials. In this research, a new method to measure the dielectric constant of biomaterials based on microwave was developed on microwave atomic force microscopy (M-AFM). The theoretical model was established to analyze the mechanism of the force caused by microwave and was developed a method to quantitatively measure the dielectric constant. By using this measurement method, the dielectric constant of mesenchymal stem cells (MSCs) was measured.

The market for wearable devices has increased considerably in recent years. In response to this demand, flexible electronic circuit technology has become more important. The conventional bonding technology in electronic assembly depends on high-temperature processes such as reflow soldering, which result in undesired thermal damages and residual stress at a bonding interface. In addition, it exhibits poor compatibility with bendable or stretchable device applications. Therefore, there is an urgent requirement to attach electronic parts on printed circuit boards with good mechanical and electrical properties at room temperature. Nanowire surface fasteners (NSFs) are candidates for resolving these problems. This paper describes the fabrication of an NSF on a flexible substrate, which can be used for room temperature conductive bonding. The template method is used for preparing high-density nanowire arrays. A Cu thin film is layered on the template as the flexible substrate. After etching the template, a Cu NSF is obtained on the Cu film substrate. In addition, the electrical and mechanical properties of the Cu NSF are studied under various fabrication conditions. The Cu NSF exhibits high shear adhesion strength (~234 N cm^−2) and low contact resistivity (2.2 × 10^−4 Ω cm2)., ファイル公開：2019-07-27