Surface characteristics of titanium anodized in the four different types of electrolyte

This study evaluated the surface characteristics of titanium modified by anodic spark oxidation and a subsequent hydrothermal treatment. The electrolytic compositions of the experimental groups are as follows: GA: 0.015 M dl-α-glycerophosphate disodium salt hydrate (dl-α-GP) and 0.2 M calcium acetate (CA), GB: anodized in 0.015 M β-GP (glycerophosphate disodium salt) and 0.2 M CA, GC: anodized in 0.015 M GP (glycerophosphate disodium salt) and 0.2 M CA, and GD: anodized in 0.015 M GP-Ca (glycerophosphate calcium salt) and 0.2 M CA. Anodic spark oxidation was carried out at 30 mA/cm² to 290 V. In addition, the anodized samples were treated hydrothermally at 300 °C for 2 h in an autoclave system. Regardless of the electrolytic composition, the anodic oxide films on the titanium surface contained pores ∼5 μm in size and the diameter was larger at the protrusion parts than that at the lower parts. The phase of the anodic oxide layer consisted mainly of anatase with a small amount of rutile. HA crystals precipitated on the porous titanium oxide layer after a hydrothermal treatment. Moreover, the morphology of the HA crystals was a dense fine needle shape, which changed according to the electrolytic composition. The mean surface roughness (R_a) was highest in group GB at 0.437 μm. The R_a values of the hydrothermally treated group was approximately 0.14-0.2 μm higher than the anodized groups. Anodic spark oxidation and the hydrothermal treatment resulted in increasing corrosion potential and decreasing corrosion current density, which means an improvement in the corrosion resistance. The surface activity of the specimens in Hanks’ solution was GD > GA > GB > GC.     

Enhanced visible light photocurrent generation at surface-modified TiO2 nanotubes

Photoelectrodes consisting of TiO₂ nanotube layers with different thicknesses (0.5 μm, 1.7 μm, 3 μm, 6 μm, 9 μm, and 18 μm) were prepared by anodization of titanium substrates and subsequent surface modification by a heat treatment at 400 °C in the presence of urea pyrolysis products. In contrast to unmodified TiO₂ nanotubes, the modified photoelectrodes exhibit photocurrents under visible light irradiation down to 750 nm. Photocurrent transients indicate enhanced recombination unless a suitable hole-scavenger, like iodide, is present since the photogenerated holes do not oxidize water efficiently. In the visible light the photoconversion efficiency increases significantly with nanotube length. The maximum incident photon-to-current efficiency (IPCE) was observed for tubes with the length of 6-9 μm (IPCE ∼4.5% and 1.4% at 450 nm and 550 nm, respectively) and the photocurrent enhancement with increasing tube length is found to be stronger at longer irradiations wavelengths.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

Vertically aligned carbon nanotube arrays grown on a lamellar catalyst by fluidized bed catalytic chemical vapor deposition

Large amount of vertically aligned carbon nanotube (CNT) arrays were grown among the layers of vermiculite in a fluidized bed reactor. The vermiculite, which was 100-300 μm in diameter and merely 50-100 μm thick, served as catalyst carrier. The Fe/Mo active phase was randomly distributed among the layers of vermiculite. The catalyst shows good fluidization characteristics, and can easily be fluidized in the reactor within a large range of gas velocities. When ethylene is used as carbon source, CNT arrays with a relatively uniform length and CNT diameter can be synthesized. The CNTs in the arrays are with an inner diameter of 3-6 nm, an outer diameter of 7-12 nm, and a length of up to several tens of micrometers. The as-grown CNTs possess good alignment and exhibit a purity of ca. 84%. Unlike CNT arrays grown on a plane or spherical substrate, the CNT arrays grown in the fluidized bed remain their particle morphologies with a size of 50-300 μm and the good fluidization characteristics were preserved accordingly.     

Synthesis of functional gradient BCP/ZrO2 bone substitutes using ZrO2 and BCP nanopowders

BCP/BCP-ZrO₂/ZrO₂ scaffold with a functionally gradient layered structure (FG BCP/ZrO₂) was fabricated by the polymeric sponge replica method and subsequent dipping process. To enhance the compressive strength and bioactive properties of monolithic ZrO₂ scaffold, ZrO₂ and BCP phases were selected as a main frame and surface layer, respectively. The formation of microcracks was significantly decreased by incorporating an intermediate layer consisting of BCP-ZrO₂ phase. The thicknesses of the monolithic ZrO₂, BCP-ZrO₂, and BCP layer were around 10-30 μm, 3-5 μm, and 2-3 μm, respectively. The FG BCP/ZrO₂ scaffold showed highly interconnected pores as well as good material properties, which were 68% porosity and 7.2 MPa of compressive strength. Average pore size of FG BCP/ZrO₂ scaffold was about 220 μm in diameter. From MTT assay and SEM observation of osteoblast-like MG-63 cells, FG BCP/ZrO₂ scaffold showed good cell viability and faster proliferation behavior.     

Local field emission from individual vertical carbon nanofibers grown on tungsten filament

Individual high-aspect-ratio carbon nanofibers (CNFs) were grown on tungsten filament substrates by plasma-enhanced hot filament chemical vapor deposition. They are ∼100 nm in diameter and 6-30 μm in length with a density less than 10⁶/cm². The field emission property of single as-grown carbon nanofibers was measured in a scanning electron microscope equipped with a moveable nanoscale probe tip. The measurement results showed that the threshold field of single carbon nanofibers with different lengths was in the range of 4-5 V/μm with a corresponding emission current density of 20 μA/cm², but an evident difference in the enhancement of emitted current between nanofibers of different lengths could be found when the applied field was increased continuously. This indicates that the field emission property of single carbon nanofibers depends mainly upon their length, which is essentially attributed to the change of field enhancement factor of single carbon nanofibers. In addition, field emission of the different positions on the wall of a single carbon nanofiber was studied.     

Influence of surface defects on the biaxial strength of a silicon nitride ceramic - Increase of strength by crack healing

Failure of brittle materials starts in general from defects which exist in the volume or on the surface of the specimens. Surface flaws, which are more dangerous than volume flaws, can be introduced by machining. They decrease the strength of specimens and components.For this investigation silicon nitride specimens were produced using different machining conditions. About half of them were strength tested by use of the biaxial ball-on-three balls (B3B) test. It has been shown that better (more gentle) machining increases the strength but may also cause an increased scatter of strength data.The remaining specimens were heat treated (annealed) at 1000 °C in air and afterwards also strength tested using the B3B test. Compared to the non heat treated specimens a significant increase in strength could be proven, which was - depending on the machining conditions - between almost 300 MPa and more than 500 MPa. The scatter of strength data was largely decreased.The improvement was caused by the formation of a thin (0.5-2 μm) glassy layer which filled surface cracks and surface related pores during annealing.     

Influence of β-Si3N4 particle size and heat treatment on microstructural evolution of α:β-SiAlON ceramics

In the present study, we report the effects of starting β-Si₃N₄ particle sizes and post-sintering heat treatment on microstructure evolution and mechanical properties of prepared α-β SiAlON ceramics. Three different β-Si₃N₄ starting powders, with particle sizes of 2, 1 and 0.5 μm were used to prepare α-β SiAlON ceramics by gas-pressure sintering. Elongated β-SiAlON grain morphology was identified in the samples prepared using 0.5 μm particle size β-Si₃N₄ powder. Low-aspect ratio grain morphology was observed in samples prepared from starting powders with coarse particles (2 μm and 1 μm). The sintered samples were further heat treated to develop desired microstructure with elongated grains. The hardness and indentation fracture toughness values of sintered and heat treated samples were found to lie in the range of 12.4-14.2 GPa and 5.1-6.4 MPa m^1/2 respectively. It was revealed that fracture toughness increases with decrease in particle size of starting β-Si₃N₄ powder.     

Influence of catalytic particle size on the performance of fluidized-bed chemical vapor deposition synthesis of carbon nanotubes

In this paper, the impacts of catalytic particle size on the overall reactor performance for carbon nanotubes (CNTs) production using a fluidized-bed chemical vapor deposition (FBCVD) process have been studied. Six different particle size fractions (10-20 μm, 20-53 μm, 53-75 μm, 75-100 μm, 100-200 μm, and 200-300 μm) were selected. It was observed that the smaller the catalytic particle diameter, the greater the carbon deposition efficiency and the greater CNT synthesis selectivity. The 10-20 μm catalytic particles exhibited 30% higher carbon deposition efficiency than the 200-300 μm catalytic particles. The selectivity toward CNTs formation was also approximately 100%. These observations could be explained by the fact that when the diameter of the catalytic particle gets smaller, the breakthrough capacities during frontal diffusion will be bigger due to a shorter diffusion path length within the particle. Moreover, the fine particles ensured high interstitial velocity which subsequently enhances the heat and mass transfer, and consequently improves the CVD reaction.     

Bioactive and electrochemical characterization of TiO2 nanotubes on titanium via anodic oxidation

The selection of bioactive and electrochemically stable materials for implants having effective corrosion resistance during long-term use in the body is essential. In this study, the bioactive and electrochemical properties of titanium implant materials with a nanotube surface treatment and various types of post-treatments were examined. Two types of amorphous TiO₂ nanotubes were grown homogeneously on the surface: one with a larger diameter (approximately 85 nm) and one with a smaller diameter (approximately 50 nm). Amorphous TiO₂ nanotubes were partially crystallized to anatase and rutile by heat treatment at 500 °C for 2 h. The corrosion potential (E_corr) of the heat-treated sample (HT) had a novel value of 0.102 V due to the stable TiO₂ crystal phase compared to the −0.106 V observed in the anodic oxidation sample (AN). The corrosion current density (I_corr) ranged from 0.20 to 0.64 μA/cm² according to the post-treatment conditions. However, at 0.6 V, where a passive layer had formed, the corrosion resistance of the HT was approximately ten times that of the AN and untreated (UT) samples. After evaluating the hydroxyapatite (HA)-forming ability by immersion in a simulated body fluid (SBF) solution, the CP process induced the adsorption of Ca and P onto HT. A comparison of the time-dependent amount of Ca and P adsorption showed that Ca adsorption plays a role in determining the rate at which hydroxyapatite (HA) is formed. For the induction of HA formation, a level of Ca adsorption above a critical level is required.     

A green separation process for recovery of healthy oil from pumpkin seed

In this study, pumpkin seeds, called as “Ürgüp Sivrisi” and grown in Cappadocia region, were used as plant materials because of high aroma contents. In the supercritical fluid extraction of pumpkin seed oil, the effect of main process parameters as the particle size (250-2360 μm), the volumetric flow rate of supercritical solvent (0.06-0.30 L/h), the operating pressure (20-50 MPa), the operating temperature (40-70 °C), the type of entrainer (ethanol and n-hexane) and those concentrations (0-10 vol.%) on the extraction yield, the oil solubility and the initial extraction rate were investigated. A cross-over effect for the extraction of pumpkin seed oil using supercritical CO₂ was determined at the operating pressure of 20-30 MPa. The maximum extraction yield obtained with entrainer free was reached 0.50 g oil/g dry seed at 600-1180 μm, 0.12 L/h, 50 MPa and 70 °C for the operation time of 5 h. The maximum extraction yield obtained with ethanol as an entrainer in the experiments was reached 0.54 g oil/g dry seed at the conditions of 600-1180 μm, 0.12 L/h, 30 MPa, 40 °C and 8 vol.% for the operating time of 2 h. The oil compositions were determined by gas chromatography analysis and the results showed that the compositions of pumpkin seed oil which were obtained by means of organic solvent extraction and supercritical fluid extraction were similar. The average oil compositions determined as 9.3 (±0.43)% palmitic acid, 7.5 (±0.6)% stearic acid, 32.3 (±0.6)% oleic acid, 48.1 (±0.6)% linoleic acid and 0.7 (±0.3)% linolenic acid. The morphological changes in the seeds were determined by the scanning electron microscopy analysis.     

Effect of synthesis condition on morphology and yield of hydrothermally grown SnO2 nanorod clusters

In this present work, we report the synthesis of SnO₂ nanorod clusters by means of hydrothermal treatment of colloidal hydrous tin oxide at 200 °C. Effect of synthesis parameters including concentrations of Na₂SnO₃·3H₂O and NaOH, and hydrothermal time on morphology and yield of the products is investigated. At optimum synthesis condition, nanorod clusters consisting of single crystalline, tetragonal-shaped rutile SnO₂ nanorod with uniform shape and size of 190 ± 6 nm in diameter and 1.4 ± 0.2 μm in length were obtained. The influence of precursor concentration on yield and morphology development was discussed. Grown mechanism is described based on aggregation of nanocrystals and their subsequent growth homocentrically.     

Computational modelling of the impact of particle size to the heat transfer coefficient between biomass particles and a fluidised bed

The fluid-particle interaction and the impact of different heat transfer conditions on pyrolysis of biomass inside a 150 g/h fluidised bed reactor are modelled. Two different size biomass particles (350 μm and 550 μm in diameter) are injected into the fluidised bed. The different biomass particle sizes result in different heat transfer conditions. This is due to the fact that the 350 μm diameter particle is smaller than the sand particles of the reactor (440 μm), while the 550 μm one is larger. The bed-to-particle heat transfer for both cases is calculated according to the literature. Conductive heat transfer is assumed for the larger biomass particle (550 μm) inside the bed, while biomass-sand contacts for the smaller biomass particle (350 μm) were considered unimportant. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Biomass reaction kinetics is modelled according to the literature using a two-stage, semi-global model which takes into account secondary reactions. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of User Defined Function (UDF).     

Synthesis and characterization of new UV absorbing microspheres of narrow size distribution by dispersion polymerization of 2-(2′-hydroxy-5′-methacryloxyethylphenyl)-2H-benzotriazole

New UV absorbing microspheres of sizes ranging between 0.2 ± 0.03 and 3.0 ± 0.2 μm were formed by dispersion polymerization of the monomer 2-(2′-hydroxy-5′-methacryloxyethylphenyl)-2H-benzotriazole (trade name: NORBLOC) in methyl ethyl ketone as a continuous phase. The effect of various polymerization parameters, such as monomer concentration, initiator type and concentration, stabilizer concentration and crosslinker monomer concentration, on the size and size distribution, and on the polymerization yield of the produced PNORBLOC microspheres has been elucidated. Polyethylene/PNORBLOC resins and films of 150 ± 25 μm thickness were prepared by melt blending of low density polyethylene with 2% (w/w) PNORBLOC microspheres of 0.25 ± 0.03 μm diameters, followed by a tubular blown process at 170-190 °C. The UV irradiation (200-390 nm) cut-off efficiency of these films has been demonstrated.     

Direct anodic growth of thick WO3 mesosponge layers and characterization of their photoelectrochemical response

Thick mesoporous tungsten oxide (WO₃) layers can be formed by anodization of tungsten in a 10 wt% K₂HPO₄/glycerol electrolyte, if the electrolyte temperature is around 80-100 °C. At 90 °C, a regular mesoporous WO₃ layer was grown up to a thickness of approximately 9 μm. This WO₃ mesosponge layer consists of typical feature sizes of 20-30 nm and pore widths of 10-30 nm. The photoresponse of different layer thicknesses and different annealing treatments was characterized in a photoelectrochemical cell. The highest photocurrents were observed with a 2.5 μm thick WO₃ layer annealed at 550 °C consisting of a mixture of orthorhombic, triclinic and monoclinic phases. Incident photon to current efficiencies (IPCEs) of the samples were 73.4% in a 1 M HClO₄ electrolyte and 167.5% for methanol photo-oxidation in 0.1 M CH₃OH/1 M HClO₄ electrolyte, at 1 V vs. Ag/AgCl under illumination at a wavelength of 420 nm.     

Wastewater filtration performance of multilayer glassy microporous filters

The glassy composition (quartz, clinoptilolite and frit glass mixture) provides a filter having glassy pore wall microstructure and thus enables easily cleaning through the filter recovery by back flushing. The filter was obtained as multilayer compaction by one step slip cast-processing where a cylindrical filter, consisting of filtration layer on granular assemblies with specific interlayer was shaped by a fine particle migration phenomenon. The multilayer compaction has low resistance to liquid flow and thus the filter great potential to use for wastewater filtration. It is known that high capacity filtration also requires correct pore size/interval with respect to filtered particles. In this study, a wastewater overflow from marble factory (0.035 wt.% of solid with a size distribution of 0.58-1.46 μm) was filtered by different pore sizes of the glassy filters (pore size intervals: 0.4-10 μm, 0.2-4 μm, 0.1-1.5 μm and 0.04-2 μm) and significantly different filtering capacities was obtained; the irreversible fouling capacities were determined between 2.9 and 8.5 m³ of filtrate per m² of the filter area through the filtration produced 5 min intervals. The filtration pressure was 5 bar and backflushing was achieved at 1 bar. The high filterability (8.1 m³/m² in 5 min) with high filtrate clarity (∼0.5 nephelometric turbidity units) could be obtained using finer pore sized filters. The large size filter was seriously clogged during the filtration.     

Study of the high temperature oxidation performance of Thermal Barrier Coatings with HVOF sprayed bond coat and incorporating a PVD ceramic interlayer

The performance of an intermediate Cr₃C₂ ceramic layer applied by PVD between the bond coat and the ceramic top coat in a TBC system was evaluated. The thickness of the transitional layer was kept around 1–2 μm. Two substrate materials and two distinct bond coats were combined in the tests. High Velocity Oxygen Fuel (HVOF) and Atmospheric Plasma Spraying (APS) were used respectively for bond coat and top coat deposition. Isothermal oxidation tests were performed at 1000 °C in static air atmosphere. Thermal grown oxide (TGO) was measured and correlated to the exposition times. Results are discussed in terms of the TGO growth rate and changes in residual stresses. The results suggest an improvement in the oxidation resistance of the bond coat because of the presence of the intermediate layer.     

A comparison of the results obtained from grinding in a stirred media mill lignite coal samples treated with microwave and untreated samples

Various studies have been carried out on the effect of microwave-treatment on grinding different types of coal. However, the effect of microwave treatment on grinding coal samples −3.35 mm in size which can be considered to be fine is still under investigation. The purpose of this paper is to make contributions to these studies conducted. In the study, lignite coal samples with pyritic sulphur and 25% structural moisture were crushed below −3.35 mm particle size using jaw and cone crushers and then classified into three different mono size groups by Russel sieve. For a complete removal of the structural moisture from the lignite coal, a microwave application with 600 W needs approximately 35% more energy consumption than that with 850 W. The untreated coal samples and the ones treated with microwave at 850 W were ground for 5, 15, 30, 60, 120 s in a stirred media mill. The breakage rates of microwave-treated coal increased and accordingly the ground products of microwave-treated coal yielded finer particles than −106 μm as compared to untreated coals. The untreated and microwave-treated feed coals of −3350 μm and −1180 μm particle sizes were ground for 2 min in the stirred media mill. It was found that the increases in the rate of weight percentages for −106 μm particle size fraction after 2 min of grinding of untreated and microwave-treated feed coals of −3350 μm and −1180 μm were found to be 15.81% and 2.69%, respectively. Moreover, Hardgrove Index (HGI) test results of lignite coal showed that the HGI index value increased by approximately 23% after microwave treatment with 850 W.     

Synthesis of double-walled carbon nanotube films and their field emission properties

Continuous double-walled carbon nanotube (DWCNT) films were synthesized using an Fe-Mo catalyst by the arc discharge method. This new catalyst has dramatically improved the purity and selectivity of DWCNT product. High-resolution transmission electron microscopy indicates that the outer and inner diameter of DWCNT are 1.9-4.7 nm and 1.2-3.8 nm, respectively. The field emission properties of DWCNT films have been studied. The directly grown film was transferred onto quartz substrates and used as emission cathodes, and has demonstrated a quite good emission performance. Moreover, the emissions of DWCNT films have been further improved by heat treatment. The film after 400 °C oxidation shows excellent field emission property with a low turn-on (E_to = 0.6 V/μm) and threshold field (E_th = 0.9 V/μm) corresponding to the emission current density of 1 μA/cm² and 1 mA/cm², respectively.     

Numerical simulation of microparticles penetration and gas dynamics in an axi-symmetric supersonic nozzle for genetic vaccination

This paper describes two phase (solid particles/gas) flow in a supersonic nozzle that is part of a device for micromolecular vaccine/drug delivery. It accelerates micro solid particles to high speeds sufficient to penetrate the viable epidermis layer to achieve the pharmaceutical effect. Helium is used as the driving gas for the solid particles because of its high compressibility factor. A numerical parametric study was performed for gas pressures ranging between 3 and 6 MPa and gold particles of diameters 1.8 μm and 5 μm. The computed results show that uniform particle velocity was achieved at standoff distance of 2 exit diameters (D_e) downstream of the device exit with particles concentrated on the supersonic core jet. Increasing the helium pressure from 3 to 6 MPa caused an increase in the particle velocity of 24% for particles with a diameter of 1.8 μm and 7% for particles of diameter 5 μm at the standoff distance. Furthermore increased gas pressure has adverse effect on particles concentration. As the inlet pressure increases, the particles are concentrated more at the core of the nozzle. Semi-empirical particle penetration calculation confirms the numerical results that the 5 μm particles penetration distance is 45-135 μm and the 1.8 μm diameter penetration is 35-95 μm beneath the skin. Comparison of different geometries has been done in order to understand each section function and to gain optimum performance.