Porous Microtubes Research Articles

Synthesis of a hollow microtubular Ca/Al sorbent with high CO2 uptake by hard templating

A novel hollow microtubular Ca/Al CO2 sorbent was synthesized through a hard templating pathway using absorbent cotton as a template, limestone as a CaO precursor and aluminum nitrate as an Al2O3 precursor. The synthetic Ca/Al sorbent consists of hollow porous microtubes with diameters ranging from 0.5 to 5 μm and replicates the biomorph of the absorbent cotton template well. When the mass ratio of CaO/Al2O3 is 90:10, the hollow microtubular Ca/Al sorbent exhibits the highest CO2 capture capacities during the cycles. The CO2 capture capacities of the novel Ca/Al sorbent after 50 cycles under a mild calcination condition (850 °C in pure N2) and a severe calcination condition (920 °C in 70% CO2/30% N2) are respectively 0.55 and 0.38 g/g, which are 7.91 and 8.93 times greater than those of limestone. The main compositions of the walls of the microtubes in the novel Ca/Al sorbent are CaO and Ca12Al14O33. Ca12Al14O33, as a good supporter, maintains the structural stability of the microtubes during the CO2 capture cycles. CO2 diffuses into the porous wall of the microtube in both directions (simultaneously into the outer and inner surfaces), so the CO2 diffusion resistance in the microtubes is low. The calcium looping system using the novel Ca/Al sorbent shows much lower energy consumption of the calciner and lower cost, compared with using limestone. Thus, Ca-Al-T appears promising in the calcium looping.

Heat transfer of the MHD nanofluid in porous microtubes under the electrokinetic effects

The purpose of this paper is to focus on the heat transfer characteristics of thermally developed electrokinetic flow of the nanofluid (water-Al2O3) through a porous microtube in the presence of the imposed magnetic field. The wall of the microtube is subjected to the constant heat fluxes. Moreover, the local thermal equilibrium (LTE) model is considered. Combining with the electrokinetic effects of the wall, and utilizing the Brinkman model for the nanofluid flow in the porous medium, the conservative partial differential equations have been transformed into the ordinary equations. The analytical solution of velocity and numerical solution of temperature are derived based on relevant dimensionless parameters. Furthermore, the Nusselt number, local and total entropy generation analyses are further examined qualitatively.

Electrohydrodynamic transport of non-symmetric electrolyte through porous wall of a microtube.

Transport of salt through the wall of porous microtube is relevant in various physiological microcirculation systems. Transport phenomena based modeling of such system is undertaken in the present study considering a combined driving force consisting of pressure gradient and external electric field. Transport of salt is modeled in two domains, in the flow conduit and in the pores of porous wall of the microtube. The solute transport in the microtube is presented by convective-diffusive mass balance and it is solved using integral method under the framework of boundary layer analysis. The wall of the microtube is considered to be consisting of series of straight parallel cylindrical pores with charged inner surface. The solute transport through the pores is considered to be composed of diffusive, convective and electric potential gradient governed by Nernst-Planck equation. Transport in the microtube and pores is coupled through the osmotic pressure model for the solvent and Donnan equilibrium distribution for the solute. The simulated results agree remarkably well with the experimental data conducted by in-house experimental set up. The charge density of the porous wall is estimated through the minimization of errors involved between the experimental and simulated data for different operating conditions.

Nanoporous Microtubes via Oxidation and Reduction of Cu–Ni Commercial Wires

Metallic porous microtubes were obtained from commercial wires (200–250 µm diameter) of Cu-65Ni-2Fe, Cu-44Ni-1Mn and Cu-23Ni, alloys (wt. %) by surface oxidation at 1173 K in air, removal of the unoxidized core by chemical etching, and reduction in annealing in the hydrogen atmosphere. Transversal sections of the partially oxidized wires show a porous layered structure, with an external shell of CuO (about 10 μm thick) and an inner layer of NiO (70–80 μm thick). In partially oxidized Cu-44Ni-1Mn and Cu-23Ni, Cu2O is dispersed in NiO because the maximum solubility of Cu in NiO is exceeded, whereas in Cu-65Ni-2Fe, a Cu2O shell is present between CuO and NiO layers. Chemical etching removed the unoxidized metallic core and Cu2O with formation of porous oxide microtubes. Porosity increases with Cu content because of the larger amount of Cu2O in the partially oxidized wire. After reduction, the transversal sections of the metallic porous microtubes show a series of f.c.c.-(Cu,Ni) solid solutions with different compositions, due to the segregation of CuO and NiO during oxidation caused by the different diffusion coefficients of Ni and Cu in the respective oxides. Pore formation occurs at each step of the process because of the Kirkendall effect, selective phase removal and volume contraction.

Biodegradable Porous Silk Microtubes for Tissue Vascularization.

Cardiovascular diseases are the leading cause of mortality around the globe, and microvasculature replacements to help stem these diseases are not available. Additionally, some vascular surgeries needing small diameter vascular grafts present different performance requirements. In this work silk fibroin scaffolds based on silk/polyethylene oxide blends were developed as microtubes for vasculature needs and for different tissue regeneration times, mechanical properties and structural designs. Systems with 13, 14 and 15% silk alone or blended with 1 or 2% of polyethylene oxide (PEO) were used to generate porous microtubes using gel-spinning. Microtubes with inner diameters (ID) of 150-300 μm and 100 μm wall thickness were fabricated. The systems were assessed for porosity, mechanical properties, enzymatic degradability, and in vitro vascular endothelial cell attachment and metabolic activity. After 14 days all tubes supported the proliferation of cells and cell attachment increased with porosity. The silk tubes with PEO had similar crystallinity but higher elastic modulus compared with the systems without PEO. The silk (13%)/PEO (1%) system showed the highest porosity (20 μm pore diameters on average), highest cell attachment and fastest degradation profile. There was a good correlation between these parameters with silk concentration and the presence of PEO. The results demonstrate the ability to generate versatile and tunable tubular biomaterials based on silk-PEO-blends with potential for microvascular grafts.

Controllable synthesis of one-dimensional isolated Ni0.5Zn0.5Fe2O4 microtubes for application as catalyst support in RF heated reactors

One-dimensional isolated Ni0.5Zn0.5Fe2O4 microtubes have been prepared via a template assisted sol–gel method. Temperature dependence of the structural and magnetic properties was studied via XRD, N2 adsorption, SEM, TEM, and VSM. An increase in calcination temperature from 873 to 1273K caused a decrease in the specific surface area from 80.7 to 17.0m2/g due to an increase of the grain size from 25.3 to 112nm. All samples demonstrated anomalous coercivity behavior due to mechanical stresses acting on their domain walls. The porous microtubes calcined at 1073K have a mean external diameter of 3.7μm with a length-to-diameter ratio exceeding 12. The microtubes calcined at 973K have the highest coercivity of 88.1Oe and demonstrated the largest specific heating rate of 4.36W/g in a radiofrequency field at 295kHz.

Structural, magnetic and thermal properties of one-dimensional CoFe2O4 microtubes

One-dimensional CoFe2O4 microtubes have been prepared via a simple template-assembled sol–gel method. Influence of calcination temperature on structural and magnetic properties, heat capacity and specific heating rate under radiofrequency field 295 kHz was studied. A CoFe2O4 spinel was the main phase in all samples. As the calcination temperature increased, the average crystal size increased from 34.1 to 168 nm and the specific surface area decreased from 85.7 to 8.5 m2 g−1. When calcined at 1073 K, porous microtubes with a narrow size distribution in the range between 2.0 and 2.5 μm and a length to diameter ratio exceeding 20 were obtained. The heat capacity of the microtubes calcined at 973 K was 140.81 J mol−1 K−1 at 395 K, being close to the theoretic value. The sample calcined at 973 K showed highest rate of 0.293 K s−1 mg−1.

Reversibly pH-Responsive Nanoporous Layer-by-Layer Microtubes.

Nanoporous layer-by-layer (LbL) microtubes consisting of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) are prepared by LbL deposition in porous templates followed by postassembly acid treatment. The formation of the nanoporous structure is studied as a function of solution pH, treatment time, and number of layers. Pore formation is most effective at pH 1.8, requiring only 5 min to achieve a complete transition, and is shown to be reversible. Whereas the inner surface of the porous microtubes is rough, the outer surface is smoother and exhibited isolated pores, suggestive of an asymmetric porous structure.

Porous Ag3PO4 microtubes with improved photocatalytic properties

The Ag3PO4 porous microtubes are, for the first time, prepared by a one-pot synthesis using polyethylene glycol 200 (PEG200) as the reaction medium. This study establishes that PEG 200 plays a vital role in the formation of the unique structures. Under visible light irradiation (≥420nm), the porous sample exhibits a higher photocatalytic activity for the degradation of RhB than solid Ag3PO4 and Ag3PO4 tetrapods, which has been mainly ascribed to the novel hollow structure.

Effects of non-Newtonian power law rheology on mass transport of a neutral solute for electro-osmotic flow in a porous microtube.

Mass transport of a neutral solute for a power law fluid in a porous microtube under electro-osmotic flow regime is characterized in this study. Combined electro-osmotic and pressure driven flow is conducted herein. An analytical solution of concentration profile within mass transfer boundary layer is derived from the first principle. The solute transport through the porous wall is also coupled with the electro-osmotic flow to predict the solute concentration in the permeate stream. The effects of non-Newtonian rheology and the operating conditions on the permeation rate and permeate solute concentration are analyzed in detail. Both cases of assisting (electro-osmotic and poiseulle flow are in same direction) and opposing flow (the individual flows are in opposite direction) cases are taken care of. Enhancement of Sherwood due to electro-osmotic flow for a non-porous conduit is also quantified. Effects if non-Newtonian rheology on Sherwood number enhancement are observed.

Tunable properties induced by ion exchange in multilayer intertwined CuS microflowers with hierarchal structures

Novel hierarchical wool-ball-like copper sulfide (CuS) microflowers with a three-dimensional (3D) porous framework were successfully synthesized by the direct reaction of copper with sulfur powder using a one-pot in situ growth method at low temperature (60 °C). The CuS microflowers covered firmly the surface of the 3D porous framework. The formation mechanism was examined in detail by adjusting the amount of hydrochloric acid and reaction time. Most importantly, the chemical composition of the CuS microflowers was altered by the Se exchange without changing their morphology and structure. In this way, pure CuSe and Cu1.8Se crystalline materials were obtained on the surface of the porous microtube at different reaction times and the appropriate amount of Se powder. And interestingly, the core material remained as CuS. This behavior greatly affects the physical and chemical properties of the materials. The catalytic ability of the as-obtained CuSe@CuS and CuSe1.8@CuS composite materials to degrade methylene blue and rhodamine B is several times greater than that of the as-synthesized CuS microflowers.

Formation of α-TeO2 pearl-like microwires templated on porous microtubes through thermal oxidation of Te microtubes

The current work demonstrates the first reported successful synthesis of unique α-TeO2 hybrid microstructures through thermal oxidation of Te microtubes in a horizontal tube furnace at different temperatures in the range 220–470°C, under oxygen flow. The obtained microstructures were carefully characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. They exhibited characteristic morphologies with porous scaffold microtubes covered along their whole length with pearl-like microwires. The possible formation mechanism of the obtained microstructures was also discussed.

Hydrothermal Synthesis and Growth Mechanism of Mesoporous ZnO Microtubes

Porous ZnO microtubes have been synthesized via a facile hydrothermal method based on nanoparticle splicing growth and the preferential etching strategy in the hydrothermal reaction system with high supersaturation. The ZnO tubes have hexangular wurtzite structure with a dimension of about 6 to 12 m in length, 4 m in diameter and about 40 nm in wall thickness, which is made of numerous mesopores with diameter ranging from 40 nm to 200 nm.

Sherwood number in porous microtube due to combined pressure and electroosmotically driven flow

Mass transfer of a neutral solute in a porous microtube is quantified in this study. An analytical expression of the Sherwood number is developed from first principles for combined flow of pressure driven and electroosmotic flow. Similarity solution method is adopted for solution of convective-diffusive species balance equation with coupled velocity profile, within the mass transfer boundary layer. It is observed that the Sherwood number increases with decrease in the Debye length (as the electric double layer becomes more compact) and it becomes constant beyond scaled Debye length of 60. Effects of the Reynolds number, dimensionless suction velocity, ratio of driving force and scaled Debye length have been investigated in detail. The analysis is useful for efficient design of microfluidic devices and flow through porous media.

Nucleation−Dissolution−Recrystallization: A New Growth Mechanism for t-Selenium Nanotubes

We report a hydrothermal reaction to make t-selenium nanotubes, in the absence of a surfactant or polymer to direct nanoparticle growth, and without externally added forces (such as ultrasonic). A series of electron microscopy characterization results suggest that the growth of t-selenium nanotubes is governed by a nucleation-dissolution-recrystallization growth mechanism. In this mechanism, t-selenium nanoparticles were initially formed in the hydrothermal system, then the t-selenium nanoparticles started to dissolve into the solution and grow onto large nanoparticles of selenium, and spherelike microparticles were obtained. The spherelike microparticles then gradually dissolved to generate selenium atoms in the solution; these selenium atoms were renewedly transferred onto the surfaces of the spherelike microparticles and were recrystallized. Along with the dissolution-recrystallization process, the spherelike microparticles gradually evolved into novel groovelike nanostructures. The nanogrooves could grow along the circumferential direction and the tuber axis direction until all spherelike microparticles had been completely consumed, eventually growing into t-selenium nanotubes. Studies found that this growth mechanism is strongly affected by temperature and concentrations of NaOH. By adjusting temperature and concentrations of NaOH, t-selenium nanotubes, nanowires, microrods, porous microtubes, and polyhedrons can be synthesized, respectively.

Performance of a double catalyst fuel cell cathode with a tubular oxygen breathing and preliminary reduction zone

This paper discusses a mathematical model for a liquid phase reacting flow occurring at the cathode of a patent pending novel fuel cell geometry, where a non homogeneous catalysis carried by gold and Prussian Blue, with the first reducing air O2 and the second the resulting H2O2. The breathing zone is porous walls microtubes, with three different types of pores in its walls. Inside the microtubes there is water solution of sulfuric acid. The microtubes possess an external layer of extremely porous polymer hydrophobic agent. A Prussian Blue thin porous layer is over the selective membrane. Appropriate porous and tubular connecting elements close the fluid loop. The asymmetry induces proper current and electric potential profiles, which leads to a mainly electrocapillary electrokinetic flow, which enhances the oxygen transport and assures the H2O2 flow to its reduction layer.