Porous Top Layer Research Articles

Characteristics and composition of fouling caused by pig slurry in a tubular heat exchanger – Recommended cleaning systems

The structure and composition of the fouling deposits caused by pig slurry heated in a tubular heat exchanger were characterized to understand their formation and thus be able to minimize fouling and define effective routine cleaning methods. Two temperatures (55 °C and 80 °C) were investigated. Two types of fouling were identified: organic/mineral and biofilm. The first only formed at temperatures above 50 °C, often during the heating phase, and was the main problem encountered in treatments at 80 °C. Organic/mineral deposits formed a thin compact sub-layer and a thick porous top layer composed of 67–76% minerals, 9–15% proteins, 8–20% carbohydrates and 0–5% fats. Biofilms formed at temperatures between 25 °C and 70 °C in both the cooling and heating sections of the exchanger. This type of fouling predominated at temperatures below 55 °C. The biofilm covered a thin mineral base layer. Strongly acidic or alkaline washing cycle are recommended to clean Type I deposits, while in-line gas-rumbling is recommended for Type II fouling.

Defect minimization and morphology optimization in TiO2 nanotube thin films, grown on transparent conducting substrate, for dye synthesized solar cell application

TiO2 nanotube (TNT) arrays have proven to be a perspective material for dye- or semiconductor sensitized solar cells. Although their preparation by anodic oxidation method is well elaborated for Ti foil substrate, the synthesis of high quality, homogeneous TNT arrays on a transparent conductive oxide (TCO) is still associated with some experimental challenges. In this paper we present a way of preparation of defect-free, homogenous TNT film on a TCO substrate by a combination of high temperature Ti sputtering and controlled “interrupted” anodization in viscous electrolyte. High temperature of the substrate during Ti sputter coating was found crucial for good adhesion between the Ti thin film and TCO, which seems to be the most important condition for synthesis of TNTs from a Ti thin film. The reason of poor adhesion of the Ti layer sputtered at room temperature is discussed in terms of internal stress forces. An Ar-ion sputtering was proposed as a method for removal of the non-organized top porous layer to reveal a well organized tubular geometry of TNTs and control their final lengths. Such control of morphology of the top layer is important for preparation of solid state solar cells.

Corrosion behaviour of cold sprayed titanium coatings in simulated body fluid

A composite structure comprising high strength titanium (Ti) alloy core and a high surface area pure Ti coating can be ideal for implant applications. In this work, the corrosion behaviour of pure Ti coatings composed of porous top layer (∼100 μm) and dense bottom layer (∼500 μm) obtained by cold spray method has been investigated. The porous top layer satisfies the high surface area requirement for the implant and the dense bottom layer ensures a good corrosion protection of the substrate and also good bond strength. In order to further improve the corrosion and mechanical property of cold sprayed Ti coatings, heat treatment (850°C for 5 h) was applied. Electrochemical potentiodynamic and electrochemical impedance spectroscopy (EIS) measurements were used to characterise the electrochemical corrosion behaviour of the cold sprayed Ti coatings in Hanks’ solution. For comparison, the corrosion and mechanical properties of annealed and un-annealed wrought Ti substrates were also investigated. Our results showed that the as sprayed Ti coatings exhibited a relatively lower corrosion resistance than those for pure Ti substrate. However, post-spray heat treatment improved the corrosion resistance to a level close to that of the bulk material. Furthermore, EIS studies revealed that the newly formed dense and structurally stable oxide layer on the surface of the coating during potentiodynamic scanning was beneficial to the corrosion behaviour.

Ortho-Dichlorobenzene as a pore modifier for PEMFC catalyst electrodes and dense Nafion membranes with one porous surface

ortho-Dichlorobenzene (ODB) was investigated as a pore modifier for both Nafion membranes and proton exchange membrane fuel cell (PEMFC) catalyst layers. When ODB containing Nafion dispersions are cast into a film, evaporation leads to dense membranes with one highly porous surface layer occupying 20–30% of the overall thickness. Dense Nafion membranes with a porous top layer are interesting for use in membrane humidifiers, for increasing the electrode–membrane interface areas in electrochemical devices like fuel cells or electrolysers, and other applications. Fuel cell membrane electrode assemblies were obtained by spray coating commercial Nafion 212 membranes with ODB containing catalyst inks. ODB does not poison the catalyst, and 54–120% higher potentials in comparison to the 217 mV obtained with an ODB free ink were achieved at 1.5 stoic air flow, 100% rh and a current density of 1 A cm−2. This shows that the porous catalyst electrode structure is changed, resulting in a reduced contribution of mass transport limited overpotentials. Furthermore, MEAs prepared with ODB in the catalyst ink are less sensitive towards changes in the air flow rate and the cathode relative humidity (between 50% rh and 100% rh: 217–292 mV at 1 A cm−2 without ODB and 439–478 mV with ODB).

Investigations of PbSe layers after anodic electrochemical etching by scanning electron microscopy

AbstractInvestigations of the morphology of the surface of PbSe/CaF2/Si(111) epitaxial structures after anodic electrochemical etching in a Norr electrolyte with anodizing current density of 2‐8 mA/cm2 and duration of 10‐20 min were performed. Two possible morphology types of PbSe nanostructured surficial porous layers were distinguished. Agglomerated nanoislands with 80‐400 nm lateral sizes and 70 nm average separating gaps were observed for the anodized films with initial flat terraced surface. It is shown that in this case it is possible to fabricate PbSe agglomerated nanoisland structures directly on silicon substrate. A double‐layered porous structure was obtained for epitaxial PbSe films with initial granular surface. Top porous layer with a thickness of 75‐100 nm presents a developed hierarchical porous network. Bottom layer is characterized with a presence of mesopores with 7‐18 nm diameter propagating into the layer. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth mechanism of ZnO nanostructures in wet-oxidation process

Zinc (Zn) thin films were prepared by direct current magnetron sputtering as precursors with different deposition times. Zinc oxide (ZnO) nanostructures such as nanowires, nanobelts and nanoblades were then synthesized from the Zn precursors by wet-oxidation process. The microstructures of the Zn precursor and ZnO nanostructures have been studied by scanning electronic microscopy and X-ray diffractometry. The optoelectronic properties were analyzed by photoluminescence measurement. It was found that the Zn precursor film with a porous top layer consisting of well-crystallized Zn grains is an essential for formation of ZnO nanowires. Along with time dependence study and temperature dependence studies, the ZnO nanostructure growth mechanisms during the wet-oxidation process are proposed: water vapor has a major influence on the initial stage, and the final dimensions of the nanostructure are controlled by the vapor–solid process.

The effect of substrate composition on the electrochemical and mechanical properties of PEO coatings on Mg alloys

Abstract Plasma electrolytic oxidation (PEO) is a unique surface treatment technology which is based on anodic oxidation forming ceramic oxide coatings on the surface of light alloys such as Mg, Al and Ti. In the present study, PEO coatings prepared on AZ91D, AZ31B, AM60B and AM50B Mg alloys have been investigated. Surface morphology and elemental composition of coatings were determined using scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). SEM results showed that the coating exhibited a porous top surface layer and a subsequent dense layer with micro-pores and shrinkage cracks. Phase analysis of coatings was carried out by X-ray diffraction (XRD). XRD analyses indicated that PEO coatings on AZ alloys had higher amount of Periclase (MgO) followed by the presence of Spinel (MgAl 2 O 4 ) e.g. on the AZ91D alloy compared to that on AM series alloys. In order to examine the effect of substrate composition on adhesion strength of PEO coating scratch tests were carried out. Electrochemical corrosion tests were undertaken by means of potentiodynamic polarization technique in 3.5% NaCl solution at room temperature (20 ± 2 °C). Corrosion test results indicated that the corrosion rates of coated Mg alloys decreased by nearly two orders of magnitude as compared to bare Mg alloys. PEO coatings on AZ series alloys showed better corrosion resistance and higher adhesion properties than AM series alloys. In addition to the PEO processing parameters, such are mainly attributes of the compositional variations of the substrate alloys which are responsible for the formation, phase contents and structural properties of the PEO coatings.

The morphological evolution of solvent-containing PMMA membranes in various solvent removal processes

In the present work poly (methyl methacrylate) (PMMA) membranes were fabricated by immersing PMMA/1,4-dioxane cast films into the coagulation bath of methanol, ethanol, n-propanol, or n-butanol. Because the alcohols are not strong nonsolvent to PMMA, it was not easy to remove enough solvent from the cast film in the coagulation bath, to avoid the effect of the residual solvent contained in the gelled cast films. Due to the residual solvent, redissolution of the gelled films could occur when they were air dried, and redissolution followed by redemixing was observed when the gelled films were immersed in water bath. The redissolution and redemixing in the water bath was strongly related to the amount of the residual solvent contained in gelled cast film. Also, it was found that, with a fixed time period in the alcohol bath (30 min), the amount of residual solvent in the gelled film was strongly related to the carbon number of the alcohol coagulant. The alcohol with higher carbon number resulted in lower solvent removal rate in the coagulation bath and higher amount of residual solvent. With methanol as the coagulant, the residual dioxane contained in the gelled film was too little to induce any redissolution or morphology change of the gelled film in the water bath. With ethanol, only partial redissolution was observed. For the case that n-propanol was the coagulant of the first bath, complete redissolution of the gelled film, followed by immediate redemixing, was observed in the second coagulation bath (water bath). And with n-butanol as the coagulant, the redemixing was further prolonged to allow for delayed redemixing. When redemixing occurred immediately after redissolution, membranes with a three-layer structure were formed, consisting of a lacy-like top porous layer, a dense middle layer, and sponge-like bottom layer. Mechanisms are also proposed to account for the formation of the three-layer structure.

Pore-filling membrane for direct methanol fuel cells based on sulfonated poly(styrene-ran-ethylene) and porous polyimide matrix

We have synthesized a porous co-polyimide film by coagulating a polyimide precursor in the non-solvent and thermal imidization. Factors affecting the morphology, pore size, porosity, and mechanical strength of the film were discussed. The porous polyimide matrix consists of a porous top layer and a spongy sub-structure with micropores. It is used as a porous matrix to construct sulfonated poly(styrene-ran-ethylene) (SPSE) infiltrated composite membrane for direct methanol fuel cell (DMFC) application. Due to the complete inertness to methanol and the very high mechanical strength of the polyimide matrix, the swelling of the composite membrane is greatly suppressed and the methanol crossover is also significantly reduced, while high proton conductivity is still maintained. Because of its higher proton conductivity and less methanol permeability, single fuel cell performance test demonstrated that this composite membrane outperformed Nafion membrane. Based on these results, SPSE/polyimide composite membrane has great potential to be used in DMFCs.

Fabrication of the porous polyimide film as a matrix of the composite membrane of the direct methanol fuel cell

We successfully synthesized porous co-polyimide membranes from a polycondensation reaction of an aromatic dianhydride of 3,3′,4,4′-biphenyltetracarboxylicdianhydride (SBPDA) with diamines of p-phenylenediamine (PPDA) and 4,4′-oxydianiline (ODA) by a wet phase inversion process. Factors affecting the mechanical strength of the membrane and porosity were discussed. By tuning the parameters, a spongy type porous membrane with high mechanical strength and high porosity was obtained. In addition, the porous co-polyimide membrane consisted of a porous top layer and a spongy type sub-structure. This material is suitable as the matrix of a multifunctional composite membrane of a direct methanol fuel cell (DMFC) by filling protonic electrolytes in the micropores of the matrix.

Nanoscale architectural tuning of parylene patch devices to control therapeutic releaserates

The advent of therapeutic functionalized implant coatings has significantly impacted themedical device field by enabling prolonged device functionality for enhanced patienttreatment. Incorporation of drug release from a stable, biocompatible surface isinstrumental in decreasing systemic application of toxic therapeutics and increasing thelifespan of implants by the incorporation of antibiotics and anti-inflammatories. Inthis study, we have developed a parylene C-based device for controlled release ofDoxorubicin, an anti-cancer chemotherapy and definitive read-out for preserved drugfunctionality, and further characterized the parylene deposition condition-dependenttunability of drug release. Drug release is controlled by the deposition of a layer of20–200 nm thick parylene over the drug layer. This places a porous layer above theDoxorubicin, limiting drug elution based on drug accessibility to solvent andthe solvent used. An increase in the thickness of the porous top layer prolongsthe elution of active drug from the device from, in the conditions tested, theorder of 10 min to the order of 2 d in water and from the order of 10 min to noelution in PBS. Thus, the controlled release of an anti-cancer therapeutic has beenachieved via scalably fabricated, parylene C-encapsulated drug delivery devices.

Ferromagnetism in ion implanted amorphous and nanocrystallineMnxGe1−x

The structural, electronic, and magnetic properties of a ${\mathrm{Mn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloy prepared through room-temperature ion implantation ($100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, $2\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}\text{ions}∕{\mathrm{cm}}^{2}$) and subsequent $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ annealing have been investigated with several experimental techniques. The as-implanted sample shows a quasi-Gaussian Mn concentration depth profile with a projected range (peak Mn concentration $x\ensuremath{\simeq}12\phantom{\rule{0.3em}{0ex}}\mathrm{at.}∕%$) at $55\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and end of range at $140\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The structural investigation shows that the overall implanted Ge layer is amorphous. In particular, up to a depth of $60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the implanted layer is also porous and oxidized, whereas the deepest implanted region $(60\char21{}140\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ is purely composed of amorphous Ge with Mn atoms diluted in it. This sample manifests magnetic hysteresis up to $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and a strong nonlinear S-shaped magnetic response up to $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Upon annealing at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, the top porous layer remains essentially amorphous, whereas partial reconstruction into Ge nanocrystals ($\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter) occurs in the $60\char21{}140\text{\ensuremath{-}}\mathrm{nm}$-deep implanted region. Part of the Mn atoms, mainly belonging to the top porous layer, further diffuses toward the surface and forms chemical bonds with O contaminants, becoming magnetically inactive. The other Mn atoms, mainly in the region between 60 and $140\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ from the surface, remain trapped in the residual amorphous matrix or in the Ge nanocrystals, whereas formation of Mn-Ge extrinsic phases (like ${\mathrm{Mn}}_{11}{\mathrm{Ge}}_{8}$ and ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}$) is excluded. The magnetic response of the annealed sample originates from the existence of a soft and a harder magnetic component, assigned to the dilution of Mn atoms in residual amorphous Ge and Ge nanocrystals, respectively. The hard component, attributable to a ${\mathrm{Mn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ diluted magnetic semiconductor in nanocrystalline form, manifests magnetic hysteresis up to above $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Effects of anodization voltage onCaP/Al2O3–Ti nanometre biocomposites

Nanometre CaP/Al2O3–Ti biocomposites for implant applications were successfully fabricated usinga hybrid technique of anodization and hydrothermal treatment, in whichCaP/Al2O3 formed a double-layer coating on titanium with porous CaP as the top layer and anodicAl2O3 film as the intermediate layer. Techniques, such as x-ray diffraction (XRD),electron scanning microscopy and energy disperse x-ray analysis (SEM+ EDX), transmission electron microscopy (TEM) and atomic force microscopy (AFM), wereused to investigate the composition, microstructure and morphology of the fabricatedCaP/Al2O3 compositecoating and the CaP/Al2O3–Ti biocomposites. XRD results showed that the fabricated composite coating containedAl2O3 and various calcium phosphate phases. SEM and TEM micrographs confirmedthat CaP crystals were in nanometres, embedded in situ in the walls of thecylindrical structure of anodic alumina, and finally formed a thin and porous toplayer on the anodic alumina intermediate layer. The nanometre and T-shapeeffects of the CaP top layer, and the porous and cylindrical microstructure ofCaP/Al2O3 composite coating could produce an excellent combination of bioactivity and mechanical integrity ofthe CaP/Al2O3–Ti biocomposites.It was also found that the anodization voltage of the anodization process playedan important role on the composition and microstructure of the fabricatedCaP/Al2O3–Ti biocomposites. The contents of Ca and P incorporated in anodic aluminadepended strongly on the anodization voltage. Their variations could result indifferent CaP phases, CaP crystal shapes and sizes, and topography of theCaP/Al2O3–Ti biocomposites. The optimal anodization voltage in this study was found to be in the range40–60 V.

Fabrication of three‐dimensionally ordered microporous membrane by wet phase separation

AbstractWe report a novel porous fluorinated polyimide membrane with a cylinder structure fabricated by a wet phase inversion process, which is formed by a ternary system, polyimide/solvent/water. The porous polyimide membranes consisted of a thin top porous layer and three‐dimensionally ordered cylinder micropores. The porous membrane‐forming solvents were N‐methylpyrrolidone containing nonsolvent additives such as alcohol, and the height and width of the cylinder structure were controlled by the solvents. Water fluxes through the porous polyimide membranes were measured using a stirred dead‐end filtration cell, and the fluxes of the porous membrane with the cylinder‐type structure were approximately three times greater than those of the membrane with the finger‐type structure. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3016–3021, 2004

In situ spectroscopic ellipsometric study of porous alumina film dissolution

Porous alumina films have been synthesised by anodisation of high purity aluminium in 0.4 M phosphoric acid at constant current density. The dissolution process of those coatings in 2 M phosphoric acid has been investigated by in situ spectroscopic ellipsometry (SE) combined with electrochemical measurements. A sharp drop of the open current potential is observed due to film removal. The dissolution time depends on total barrier layer thickness. Those results are confirmed by SE measurements. A three layer optical model has been used to interpret SE spectra. During the dissolution process, the porous top layer has a quasi-constant thickness, whereas porosity increases significantly. The dissolution proceeds within the pores. Only the thickness of pure alumina barrier layer (upper part) decreases during the dissolution process whereas the interface barrier layer (lower part) does not change.

Thin film PV module

This paper proposes a model of PV module based on thin-film polycrystalline Si with porous silicon insertions. The application of porous silicon allows one to fabricate the PV module without assembling the separate silicon cells. The porous silicon is applied as a buffer layer between the supported substrate and deposited polycrystalline Si as vertical insulating bars between separate cells and as antireflection coating on the top of the cells. The contribution of antireflection effect of the top porous layer, light scattering, photoluminescence and absorption in porous insertions to the solar cell parameters are numerically assessed with the help of PC1D program. It is shown that the light trapping in the top porous layer, light scattering effects in the porous insulating bars and in porous buffer layer result in the increase of the individual cell output characteristics, whereas the effect of re-emission is negligible. Efficient solar cells and PV modules can be fabricated even for 5–10 μm silicon layers. One of the main factors restricting the efficiency of the cells with porous silicon is the optical absorption loss.

Ion beam studies of porphyrin adsorption on and leaching of soda lime glass

Porphyrin organic dye layers on oxidic surfaces may play a key role in the light absorption and charge separation of organic solar cells. We report a study of the adsorption characteristics of positively charged porphyrins from an aqueous solution onto soda lime glass. Advanced surface analytical techniques (RBS, ERD and XPS) have been employed to study the layer growth of the porphyrins, in order to supplement the optical measurements of the layers. A linear growth of the amount of absorbed material is found. Leaching of the material by the carbonated solution appears to result in an increasing surface area available for deposition of porphyrins in the porous top layer of the glass. The ERD measurements indicate a surplus of hydrogen in the layer, which suggest the presence of H 2O in the porous glass layer. It is shown that the MeV ion beam techniques, which are not routinely applied in studies of organic thin films, can give meaningful results if one is aware of possible ion beam damage. The coverages found with these techniques are in good agreement with coverages determined with optical measurements. They have the additional benefit of depth profiling of these layers and determining the leaching of the glass.

Characterization of membrane distillation membranes prepared by phase inversion

In this paper, flat membrane distillation membranes have been successfully manufactured from PVDF/DMAc and PVDF/DMF blends by using phase inversion induced by an immersion precipitation technique. The structure of the membranes is asymmetric with a porous top layer and macrovoids, as assessed by SEM. The existence of MD fluxes in these membranes is established by performing various pure water flux experiments. A maximum in the MD fluxes for a particular value of the polymer content in the casting solution from which the membrane is manufactured has been observed. The dependence of the magnitude of the fluxes on the membrane thickness is also discussed and the influence of temperature polarization evaluated.

Synthesis of hydrogen-permselective membranes by modified chemical vapor deposition. Microstructure and permselectivity of silica/carbon/Vycor membranes

A modified chemical vapor deposition technique was employed for the synthesis of inorganic membranes that are selective for hydrogen separation and can withstand temperatures up to 800 o C. In an opposing reactants geometry and at 750 o C triisopropylsilane (C 3 H 7 ) 3 SiH vapors reacted with oxygen within the mouth of the pores of Vycor glass substrates to deposit SiO 2 /C nanostructures. Subsequent reaction resulted in the growth of a porous top layer on the Vycor substrate

Electrochemical Preparation of Barium Titanate Films at Ultra-Low Temperatures

AbstractPreparation of BaTiO3 thin films on titanium substrates by hydrothermal-electrochemical methods at temperatures ranging from 80 to 200°C have recently been reported in literature. In the present work, polycrystalline, homogeneous, and electrically insulating films of BaTiO3 have been electrochemically synthesized at temperatures as low as 55°C. Effects of various electrochemical and thermodynamic parameters such as temperature, solution pH, electrolyte composition, current density, and atmosphere on phase stability and film characteristics have been investigated. Formation of BaTiO3 is favored only under highly alkaline conditions. This finding is consistent with the reported phase stability criteria for BaTiO3 in Ba-Ti-CO2-H2O system. Auger spectroscopic analyses with depth profiling indicate a titanium oxide layer, whose thickness depends on current density, acts as a precursor to BaTiO3. Use of highly oxidizing atmospheres promotes the formation of thick, well-crystallized and electrically insulating films of BaTiO3 at low temperatures. Films synthesized at 55'C exhibit a characteristic bilayer structure consisting of a large grained porous top layer above a fine grained and dense bottom layer. In contrast, the films prepared at higher temperatures are uniform, fine grained but porous. Preliminary experiments indicate the feasibility to electrochemically synthesize BaTiO3 at temperatures near 25°C.