The deteriorated degradation resistance of Mg alloy microtubes for vascular stent under the coupling effect of radial compressive stress and dynamic medium

The degradation of Mg alloys relates to the service performance of Mg alloy biodegradable implants.In order to investigate the degradation behavior of Mg alloys as vascular stent materials in the near service environment,the hot-extruded fine-grained Mg-Zn-Y-Nd alloy microtubes,which are employed to manufacture vascular stents,were tested under radial compressive stress in the dynamic Hanks'Balanced Salt Solution(HBSS).The results revealed that the high flow rate accelerates the degradation of Mg alloy microtubes and its degradation is sensitive to radial compressive stress.These results contribute to understanding the service performance of Mg alloys as vascular stent materials.

Synthesis of hexagonal monocrystal AlN microtubes and nanowires at low temperature

This paper reports that pure hexagonal aluminium nitride microtubes and nanowires growing along the [0001] direction have been successfully synthesized by directly reacting AlCl3 with NaN3 at low temperature （450℃） under condition of non-solvent system. The grey-white powder of reacting product was characterized by high-resolution transmission electron microscope （HRTEM）, which shows that the powder is long straight-wire morphology with outer diameter from 40nm to 300 nm and length up to several micrometres. The results of both electron diffraction （ED） and x-ray diffraction （XRD） indicate that the AlN microtubes have a pure hexagonal monocrystal tubular structure with the combination of the curled AlN nanobelts. Room-temperature photoluminescence spectrum of the synthesized sample showed an emission peak, which is closely related to the small size of the microtubes.

Science Letters: Equivalent thickness of materials of fused silica and stainless steel in the flow of microtubes

The deviation of flow characteristics from the predictions of the conventional theory for microtubes was attrib- uted to the change of fluid viscosity resulted from the interactions between the molecules on solid wall and in fluid. The degree of this departure is dependent on the microtubes materials. A concept of equivalent thickness with which conventional theory can be used to predict the flow in microtubes without modifying the fluid viscosity was put forward. The values of equivalent thickness for fused silica and stainless steel materials were determined as 1.8 μm and 1.5 μm, respectively, by repeated numeri- cal simulation.

In vitro corrosion properties of HTHEed Mg-Zn-Y-Nd alloy microtubes for stent applications:Influence of second phase particles and crystal orientation

Magnesium(Mg)alloys are promising materials for cardiovascular stent applications due to their good biocompatibility and biodegradability.However,in vitro and in vivo corrosion tests reveal that Mg alloy stents suffer from a rapid corrosion rate and severe localized corrosion,which is limiting their widespread application.To solve the problem of uneven degradation of stents,a HTHE(long-time and high-temperature heat treatment,large-reduction-ratio hot extrusion)process is used to manufacture Mg-Zn-Y-Nd alloy microtubes in this study.The heat treatment is to dissolve alloying elements and reduce the size of SPPs,and the hot extrusion is to acquire fine-grained and strongly textured microtubes.The microstructural characterization shows that coarse second phases in as-cast alloy are refined and uniformly distributed in matrix of microtubes.After hot extrusion,microtubes show strong texture with basal plain oriented parallel to the longitudinal section(LS).The corrosion testing indicates that severe localized corrosion occurs on the cross section(CS)while localized corrosion is alleviated on the LS.Based on the different corrosion properties of the LS and CS,HTHEed microtubes are promising for solving the problems of rapid corrosion rate and severe localized corrosion of Mg alloy stents.

Iron-doped Ag/Ni_(2)(CO_(3))(OH)_(2)hierarchical microtubes for highly efficient water oxidation

Doping of foreign atoms and construction of unique structures are considered as effective approaches to design high-activity and strongdurability electrocatalysts.Herein,we report Fe-doped nickel hydroxide carbonate hierarchical microtubes with Ag nanoparticles(denoted Ag/NiFeHC HMTs)through hydrolysis precipitation process.Experimental tests and density functional theory calculations reveal that Fe doping can tune the electron configuration to enhance the conductivity,markedly improve the electrochemical surface area to expose more active sites,and act as reactive centers to lower the free energy of the rate determination step.In addition,the unique hierarchical structure can also offer active sites and excellent cycling stability.Benefitting from these advantages,the as-obtained Ag/NiFeHC HMTs show excellent oxygen evolution reaction activity,with an overpotential of 208 mV at 10 mA cm^(−2)in 1.0M KOH.Also,it could achieve long-term stability at a current density of 20 mA cm^(−2)for 24 h.

Supramolecular electrostatic self-assembly of mesoporous thin-walled graphitic carbon nitride microtubes for highly efficient visible-light photocatalytic activities

For efficient solar energy conversion,the morphology engineering of hollow graphitic carbon nitride(gC3 N4)is one of the promising approachs benefiting from abundant exposed active sites and short photocarrier transport distances,but is difficult to control on account of easy structural collapse.Herein,a facile supramolecular electrostatic self-assembly strategy has been developed for the first time to fabricate mesoporous thin-walled g-C3N4 microtubes(mtw-CNT)with shell thickness of ca.13 nm.The morphological control of g-C3N4 enhances specific surface area by 12 times,induces stronger optical absorption,widens bandgap by 0.18 e V,improves photocurrent density by 2.5 times,and prolongs lifetimes of charge carriers from bulk to surface,compared with those of bulk g-C3N4.As a consequence,the transformed g-C3N4 exhibits the optimum photocatalytic H2-production rate of 3.99 mmol·h^-1·g^-1(λ>420 nm)with remarkable apparent quantum efficiency of 8.7%(λ=420±15 nm)and long-term stability.Moreover,mtw-CNT also achieves high photocatalytic CO2-to-CO selectivity of 96%(λ>420 nm),much better than those on the most previously reported porous g-C3N4 photocatalysts prepared by the conventional hard-templating and soft-templating methods.

Template-free synthesis of Co_(3)O_(4)microtubes for enhanced oxygen evolution reaction

To fully exploit the superiority of tubular structures,in this study,we systematically explore the optimal preparation conditions for Ni/Co_(3)O_(4),including cation species and content,additive species and content,and anion species.Our results reveal that the formation of an initial cobalt nickel acetate hydroxide prism is the key factor and directly affects the final microtubular structure.Moreover,P is subsequently doped into the Ni/Co_(3)O_(4)lattice to increase the M^(3+)/M^(2+)molar ratio(M=Co and Ni),promote reaction kinetics,and optimize electronic structure.Consequently,the oxygen evolution reaction performance of P-doped tubular Ni/Co_(3)O_(4)is significantly higher than that of undoped Ni/Co_(3)O_(4)and the state-of-the-art RuO_(2)electrocatalyst.

Flow characteristics of liquid with pressure-dependent viscosities in microtubes

It is obvious that the pressure gradient along the axial direction in a pipe flow keeps constant according to the Hagen-Poiseuille equation. However, recent experiments indicated that the distribution of the pressure seemed no longer linear for liquid flows in microtubes driven by high pressure （1-30MPa）. Based on H-P equation with slip boundary condition and Bridgman＇s relation of viscosity vs. static pressure, the nonlinear distribution of pressure along the axial direction is analyzed in this paper. The revised standard Poiseuille number with the effect of pressure-dependent viscosity taken into account agrees well with the experimental results. Therefore, the dependence of the viscosity on the pressure is one of the dominating factors under high driven pressure, and is represented by an important property coefficient α of the liquid.

Water slip flow in superhydrophobic microtubes within laminar flow region

The fabrication of superhydrophobic surfaces and the studies on water flow characteristics therein are of great significance to many industrial areas as well as to science and technology development. Experiments were car- ried out to investigate slip characteristics of water flowing in circular superhydrophobic microtubes within lam- inar flow region. The superhydrophobic microtubes of stainless steel were fabricated with chemical etching- fluorination treatment. An experimental setup was designed to measure the pressure drop as function of water flow rate. For comparison, superhydrophilic tubes were also tested. Poiseuille number Po was found to be smaller for the superhydrophobic microtubes than that for superhydrophilic ones. The pressure drop reduc- tion ranges from 8% to 31%. It decreases with increasing Reynolds number when Re 〈 900, owing to the transition from Cassie state to Wenze] state. However, it is almost unchanged with further increasing Re after Re 〉 900. The slip length in superhydrophobic microtubes also exhibits a Reynolds number dependence similarly to the pressure drop reduction. The relation between slip length and Darcy friction factor is theoretically analyzed with consideration of surface roughness effect, which was testified with the experimental results.

Plasmonic rhenium trioxide self-assembled microtubes for highly sensitive, stable and reproducible surface-enhanced Raman spectroscopy detection

Compared with noble metals, improving the sensitivity of semiconducting surface-enhanced Raman scattering(SERS) substrates is of great significance to their fundamental research and practical application of Raman spectroscopy. Herein, a simple chemical method is developed to synthesize a rhenium trioxide(ReO_(3)) microtubes assembled with highly crystalline nanoparticles. The ReO_(3) microtubes show a strong and well-defined surface plasmon resonance(SPR) behavior in visible region, which is rare for non-noble metals. As a low-cost SERS substrate, the plasmonic ReO_(3) microtubes exhibit a Raman enhancement factor of 8.9×10^(5) and a lowest detection limit of 1.0×10^(-9) mol/L for phenolic pollutants. Moreover, these ReO_(3) microtubule SERS substrates show excellent chemical stability and can resist the corrosion of strong acids and bases.

Template-free large-scale synthesis of g-C3N4 microtubes for enhanced visible light-driven photocatalytic H2 production

A template-free hydrothermal-assisted thermal polymerization method has been developed for the large-scale synthesis of one-dimensional （1D） graphitic carbon nitride （g-C3N4） microtubes. The g-C3N4 microtubes were obtained by simple thermal polymerization of melamine-cyanuric acid complex microrods under N2 atmosphere, which were synthesized by hydrothermal treatment of melamine solution at 180℃ for 24 h. The as-obtained g-C3N4 microtubes exhibited a large surface area and a unique one-dimensional tubular structure, which provided abundant active sites for proton reduction and also facilitated the electron transfer processes. As such, the g-C3N4 microtubes showed enhanced photocatalytic H2 production activity in lactic acid aqueous solutions under visible light irradiation （A 〉 420 nm）, which was - 3.1 times higher than that of bulk g-C3N4 prepared by direct thermal polymerization of the melamine precursor under the same calcination conditions.

General formation of Prussian blue analogue microtubes for high-performance Na-ion hybrid supercapacitors

One-dimensional(1D)hollow-structured nanomaterials with desirable compositions have aroused huge attention in the field of electrochemical energy storage.In the present study,1D hierarchical cobalt hexacyanoferrate(CoHCF)microtubes were initially fabricated using a facile self-templated method with the electrospun polyacrylonitrile(PAN)-cobalt acetate(Co(Ac)2)as templates.After the chemical reaction was performed between the templates and a potassium ferricyanide solution,the core-shell PANCo(Ac)2@CoHCF nanofibers were successfully fabricated.Subsequently,the CoHCF microtubes were finally obtained via the selective dissolution of the PAN-Co(Ac)2 cores.Benefiting from the unique structural characteristic,the CoHCF microtubes electrode exhibited prominent electrochemical characteristics in Na2SO4 aqueous electrolyte,including a high specific capacitance of 281.8 F g^-1(at 1 A g^-1),and good rate capability as well as long cycling stability(93%capacitance retention after 5000 cycles).The hybrid supercapacitor assembled with CoHCF microtubes and activated carbon(AC)as the positive and negative electrodes,respectively,exhibited a high energy density of 43.89 W h kg^-1,a power density of 27.78 k W kg^-1,as well as a long cycle life.Note that this versatile self-templated synthetic strategy could be extended to fabricate other 1D hollow Prussian blue(PB)or its analogues(PBA)with controllable composition,which have a potential application in a range of fields.

FACILE FABRICATION OF LARGE SCALE MICROTUBES WITH A NATURAL TEMPLATE—KAPOK FIBER

A newly natural fine template, kapok fiber, for microtube preparation was reported. Large scale microtubes with high length/diameter ratio and controllable wall thickness and morphology have been successfully fabricated with this template. It is a wildly available, low-cost, environmental friendly and fine structured natural template for microtubes. Its thin wall thickness is only about 1-2 um that means the whole template material is tiny and easy for removing. Even there is any residue the amount can be ignored. When the template is covered with a shell component, hollow structured microtube could be obtained by removing the thin inner template, and its shape could be the same as that of the original template （positive copy of the template＇s shape）. The products have high length/diameter ratio and uniform tubular structure. By further modifying the fabricating methods, facile fabrication not only exists for polypyrrole （PPy） in electrochemical deposition, but also for many other organic and inorganic materials. The surface morphology and wall thickness of the resultant microtubes can be easily modulated by controlling the processing conditions. This natural fiber is predicted to be a fine template for fabricating large scale microtubes with large cavity and high length/diameter ratio.

Friction resistance for gas flow in smooth microtubes

A new tube-cutting method was used to measure the pressure and Mach number distribution along a microtube of 108.3 μm. Experiments were also performed concerning the average Fanning friction factors of five kinds of microtubes whose diameters range from 80.0 to 166.6 μm. It is found that the pressure distribution in a microtube becomes nonlinear at a high Mach number and the product of measured average Fanning friction factors $\overline C _{_f } $ and Reynolds numberRe is higher than 16. Numerical results show that the gas compressibility leads to a variation of the velocity profile from parabolic, and results in a large velocity gradient at the tube inner wall surface. The transition from laminar to turbulence in microtubes also occurs atRe ≈ 2 300, and the phenomenon of early transition is not observed in the experiments.

Porous microtubes of nickel-cobalt double oxides as non-enzymatic hydrogen peroxide sensors

Non-enzymatic electrochemical sensors for the determination of hydrogen peroxide(H_(2)O_(2))have attracted more and more concerns.A series of nickel and cobalt double oxides(Ni_(x)Co_(y)-DO)with the different ratios of Ni/Co have been prepared by a polyol-mediated solvothermal method for H_(2)O_(2)detection.The obtained products exhibit honeycomb-like open porous microtubes constituted with the low-dimensional nanostructured Ni_(x)Co_(y)-DO blocks after the calcination treatment.Compared with nickel oxides,the introduced Co ions in Ni_(x)Co_(y)-DO can induce the production of surficial oxygen vacancies,and further enhance the electrode surface activity.In particular,the NiCo-DO sample(with an atomic ratio of Ni/Co=4:3)shows the richest surficial oxygen vacancies and presents the highest H_(2)O_(2)detection activity among all the as-prepared samples,demonstrating an excellent sensitivity of698.60μAL mmol^(-1)cm^(-2)(0~0.4 mmol/L),low detection limit(0.28μmol/L,S/N=3),as well as long stability,high selectivity and good reproducibility.This work lends a new impetus to the potential application of double metal oxides for the next generation of non-enzymatic sensors.

SnO_2-doped ɑ-Fe_2O_3 patulous microtubes for high performance formaldehyde sensing

SnO2-doped α-FezO3 patulous microtubes （SFPNs） are synthesized by an electrospinning method. The as-synthesized materials are characterized by scanning electron microscope, X-ray powder diffraction and energy dispersive spectroscopy. The gas sensing results show SFPNs possess an excellent sensing property to formalde- hyde. The response value of SFPNs gas sensor to 50 ppm formaldehyde is 25.4 at 220℃. The lowest detecting limit of 1 ppm formaldehyde is 3.2. Response and recovery characteristic curves of SFPNs gas sensors to 1, 2, 3, 5, 5, 3, 2 and 1 ppm formaldehyde are also tested. The results show a good reversibility and repeatability of SFPNs gas sensors. The sensor exhibits a high selectivity in the presence of acetone, ethanol, toluene, ammonia, hydrogen, carbon monoxide and butane. Moreover, the sensor has a good long-time stability.

Nitrogen-doped graphene microtubes with opened inner voids: Highly efficient metal-free electrocatalysts for alkaline hydrogen evolution reaction

A facile method was developed to fabricate nitrogen-doped graphene microtubes (N-GMT) with ultra-thin walls of 1–4 nm and large inner voids of 1–2 μm. The successful introduction of nitrogen dopants afforded N-GMT more active sites for significantly enhanced hydrogen evolution reaction (HER) activity, achieving a current density of 10 mA·cm–2at overpotentials of 0.464 and 0.426 V vs. RHE in 0.1 and 6 M KOH solution, respectively. This HER performance surpassed that of the best metal-free catalyst reported in basic solution, further illustrating the great potential of N-GMT as an efficient HER catalyst for real applications in water splitting and chlor-alkali processes. [Figure not available: see fulltext.] © 2015, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

Characteristics of pressure drop for singlephase and two-phase flow across sudden contraction in microtubes

Single-phase and gas-liquid two-phase pressure drops caused by a sudden con-traction in microtubes were experimentally investigated at room temperature and atmospheric pressure,using nitrogen and water. The experimental results on pressure drop with a novel measurement method,the tiny gaps on the tubes,were used to characterize the sudden contraction pressure drop for tube diameters from 850 to 330 μm. The ranges of the gas and liquid superficial velocity were 2.55―322.08 and 0.98―9.78 m/s in the smaller tube respectively. In single-phase flow experiments,the contraction loss coefficients were larger than the experimental results from conventional tubes in the laminar flow. While in the turbulent flow,the contraction loss coefficients were slightly smaller than those from conventional tubes and predicted well by Kc=0.5×(1-σ2)0.75. In two-phase flow experiments,the slip flow model with a velocity slip ratio S=(ρL/ρG)1/3 showed a good prediction that reveals the occurrence of velocity slip. An empirical correlation for two-phase flow pressure drops caused by the sudden contraction was developed based on the proposed contraction loss coefficients correlation for single-phase flow and Mar-tinelli factor.

Viscous Heating for Laminar Liquid Flow in Microtubes

Using de-ionized ultra-filtered water (DIUFW) as the working fluid, the effects of viscous dissipation in micro-tubes with inner diameters of 19.9μm and 44.2μm, respectively, have been studied by experiments, the theoretical analysis and the numerical simulation at laminar state. Based on thermal imaging technology of micro-area, the temperature rise resulted from the viscous dissipation in microtube is measured by employing IR camera with a specially magnifying lens at different Reynolds numbers. A 2-D model adapted to microtube is presented to simulate the viscous dissipation characteristic considering electric double layer effect (EDL). The investigation shows the calculating results are in rough agreement with the experimental data if removing the experimental uncertainties. Based on the experimental and the numerical simulation results, a viscous dissipation number which can describe the law of the viscous heating in microtube is summed up and it explains the abnormity of the flow resistance in microtubes.

On the Possible Cometary Nature of the Uchur Cosmic Body(Fall 3.08.1993)

An expeditionary study of the area of the alleged impact event that occurred on 3.08.1993 in the area of the Lower Konkuli River(southeast of the Aldan Highlands,Lurikan Range,Russia)was carried out.According to the materials of remote sensing,the places of collision with the earth of a cosmic body are determined.In the area of the impact of the shock wave on the Earth’s surface,peat samples were selected,the micro probe analysis of which showed the pres­ence of a cosmogenic substance in concentrations 6-8 times higher than the background.Silicate and magnetite micro spheres,native iron,moissanite,and carbon micro tubes coated with a film consisting of pure nickel were found.Of particular interest were the findings of specific Ni film micro structures that allow us to make an assumption about the cometary nature of the Uchur cosmic body.Most researchers associate the observed flights of fireballs with the sub­sequent fall of meteorites.Researchers are trying to find the massive body of the fallen space body.However,often,even after many years of searching,a massive cosmic body cannot be found.This happened when studying the site of the fall of the Tunguska cosmic body.In this case,it remains to be assumed that the cosmic body contained micro­scopic dust particles.The structure and composition of such particles can only be studied using microscopic research methods.When studying the Uchur cosmic body,the authors concluded that it could be of a cometary nature due to the findings of specific particles-thin films of pure nickel on the surface of plant remains of terrestrial origin.This hy­pothesis arose from the recent discovery of atomic nickel vapors in comets.