PVDF hollow fiber and nanofiber membranes for fresh water reclamation using membrane distillation

Polyvinylidene fluoride hollow fiber and nanofibrous membranes are engineered and successfully fabricated using dry-jet wet spinning and electrospinning techniques, respectively. Fabricated membranes are characterized for their morphology, average pore size, pore size distribution, nanofiber diameter distribution, thickness, and water contact angle. Direct contact membrane distillation (DCMD) performances of the fabricated membranes have been investigated using a locally designed and fabricated, fully automated MD bench scale unit and DCMD module. Electrospun nanofibrous membranes showed a water flux as high as 36 L m^?2 h^?1 whereas hollow fiber membranes showed a water flux of 31.6 L m^?2 h^?1, at a feed inlet temperature of 80 °C and at a permeate inlet temperature of 20 °C.     

Preparation of poly(vinyl alcohol)/tin glycolate composite fibers by combined sol–gel/electrospinning techniques and their conversion to ultrafine tin oxide fibers

Ultrafine composite fibers made from poly(vinyl alcohol) (PVA)/tin glycolate — a moisture-stable tin oxide containing compound — were prepared by a combined sol–gel processing and electrospinning technique. These fibers were subsequently converted to ultrafine tin oxide fibers by calcination treatment, with the aim of producing tin oxide fiber with a high surface area-to-mass ratio and a high specific conductivity value. The acidity of spinning solution plays an important role to the morphology and size of the obtained fibers. The average diameters of the obtained composite fibers were in the range of 87–166 nm. It was found that the ultrafine tin oxide fiber showed the high conductivity value of 1.59 × 10³ S cm^?1 at calcinations temperature of 600 °C, and the BET surface area was in a range of 71 and 275 m² g^?1. Moreover, the effect of calcinations temperature on the phase and the size of the tin oxide fibers were investigated in this study.     

Influence of growth parameters on the dopant configuration of nitrogen-doped graphene synthesized from phthalocyanine molecules

Synthesis of nitrogen-doped graphene (NDG) via chemical vapor deposition (CVD) using phthalocyanine, a solid precursor containing carbon and nitrogen, is reported. The effect of the growth parameters (temperature, time, and carrier gas) on the surface morphology, dopant configuration, and conductivity of the films was studied. The NDG films were synthesized at different substrate temperatures of 1050 °C, 950 °C, and 850 °C for different growth times of 5–15 min in the presence of an Ar?+?H₂ gas mixture. Significantly, pyrrolic-N type defects are observed predominantly after 5 min of growth time. At 1050 °C, pyrrolic N content is around 45.4% after 5 min of growth which decreased to 24.1% after 15 min of growth, while the graphitic-N content increased from 41.2 to 76% at the same time. It is demonstrated that the conversion of pyrrolic type of nitrogen to graphitic nitrogen defects can be arrested by changing the carrier gas from Ar?+?H₂ to Ar. The pyrrolic-N content increased to 64% by changing the gas from Ar?+?H₂ to Ar at 15 min. The electrolyte gated field-effect transistors were fabricated using the obtained films, and dopant-dependent mobility was observed. The mobility for pyrrolic-N-dominated film is 13.6 cm² V^?1 s^?1 increasing to 62.8 cm² V^?1 s^?1 for graphitic-N-dominated film.

     

Thermal expansion of hydroxyapatite between − 100 °C and 50 °C

Hydroxy apatite (HAp) ceramic was synthesized using traditional sintering. Dilatometric and lattice thermal expansion properties of a HAp ceramic were evaluated at temperatures of ? 100–50 °C. In that temperature range, the dilatometric thermal expansion coefficient and the lattice thermal expansion coefficient of the HAp ceramic were, respectively, 10.6 × 10^? 6/°C and 9.9 × 10^? 6/°C. Furthermore, thermal expansion properties of a human tooth were measured. The thermal expansion coefficient of the horizontal direction perpendicular to the growing direction of a tooth was 15.5 × 10^? 6/°C; that of the vertical direction along with the direction of tooth growth was 18.9 × 10^? 6/°C at the temperature range described above.     

Preparation and characterization of polyvinyl alcohol hydrogels crosslinked by biodegradable polyurethane for tissue engineering of cartilage

Polyurethane was prepared from hexamethylene diisocyanate (HMDI) and polycaprolactone diol (PCL) with stoichiometry ratio of two in a reactor to form prepolymer. Polyvinyl alcohol (PVA) at PVA/prepolymer ratios of 8, 4, 2 and 1 was crosslinked with the former degradable polyester polyurethane. Fourier transform infrared (FTIR) was employed to confirm polyurethane formation during the course of reactions. FTIR spectrum revealed bands at 1729–1733 cm^? 1 and 3347–3340 cm^? 1 which indicates carbonyl and NH of amine groups, respectively. Polyurethane formation was also confirmed by the absence of the isocyanate peaks (NCO) at 2270 cm^? 1. Dynamic mechanical thermal analysis (DMTA) showed that by increasing prepolymer concentration glass transition temperature decreases from 26 °C for PVA to 19 °C for sample with PVA/prepolymer ratio of 4 and then it rises up to 31 °C. Water uptake measurements illustrated about four fold reduction in swelling ratio of PVA after crosslinking and the sample with equal amounts of PVA and PPU had water uptake of 100%, close to that of a natural cartilage and much less than PVA (425%). All samples had compressive modulus in the range of the articular cartilage (1.9–14.4 MPa). The morphology of the isolated cells on the samples was evaluated by scanning electron microscopy (SEM) and revealed cell attachment and proliferation. The cell viability (3-4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, MTT) and GAG expression (dimethylmethylene blue, DMMB) assays with human chondrocytes on the sample with PVA/prepolymer ratio of one showed about 14 and 33% increase in cell viability and GAG expression after 14 days of culture compare to the PVA, respectively.     

Role of aluminum distribution on the growth of CuIn0.75Al0.25Se2 films and numerical simulation of p-CuIn0.75Al0.25Se2/n-CuAlSe2 heterojunction solar cell

Chalcopyrite thin films of CuIn_0.75Al_0.25Se₂ have been grown by a two-stage process containing e-beam evaporation of a threefold (In/Cu/Al/Se) precursor deposition onto glass substrates in a high vacuum followed by post selenization at various temperatures (300–550 °C) using a horizontal tubular furnace. The X-ray diffraction pattern of precursor layers selenized at?≤?525 °C shows the formation of co-existence of CuInSe₂ and Cu(In,Al)Se₂ phases, as well as a mixture of two Cu(In,Al)Se₂ phases with different Al content. The precursor layers selenized at 550 °C results in the formation of single-phase Cu(In,Al)Se₂ thin films. The presence of an intense A₁ mode at 174.7 cm^?1 in the Raman spectra of selenized films at 550 °C confirms the growth of Cu(In,Al)Se₂ phase. The energy-dispersive spectra of stacked layers selenized at 550 °C shows a Cu-poor and (In?+?Al)-rich composition with atomic ratios of Cu/(In?+?Al)?=?0.79, Al/(In?+?Al)?=?0.25, and Se/(Cu?+?In?+?Al)?=?0.98. The secondary ion mass spectra depth study reveals a shift in Al distribution from graded to uniform with an increase in selenization temperature to 550 °C. The stacked layers selenized at 550 °C reveal a uniform distribution of void-free dense grains (~?0.7 μm). Optical and electrical studies of selenized Cu(In,Al)Se₂ films at 550 °C show a direct band gap of 1.22 eV with a higher hole mobility of 14.0 cm²/V-s. The heterojunction solar cell of p-Cu(In,Al)Se₂/n-CuAlSe₂ was numerically simulated using SCAPS-1D software, yielding a high power conversion efficiency (η) of 21.01%.

     

Preparation and electrochemical characterization of lithium cobalt oxide nanoparticles by modified sol–gel method

Uniformly distributed nanoparticles of LiCoO₂ have been synthesized through the simple sol–gel method in presence of neutral surfactant (Tween-80). The powders were characterized by X-ray diffractometry, transmission electron microscopy and electrochemical method including charge–discharge cycling performance. The powder calcined at a temperature of 900 °C for 5 h shows pure phase layered LiCoO₂. The results show that the particle size is reduced in presence of surfactant as compared to normal sol–gel method. Also, the sample prepared in presence of surfactant and calcined at 900 °C for 5 h shows the highest initial discharge capacity (106 mAh g^?1) with good cycling stability as compared to the sample prepared without surfactant which shows the specific discharge capacity of 50 mAh g^?1.     

The effect of high temperature heat treatment on the structure and properties of anodic aluminum oxide

Nanoporous anodic aluminum oxide (AAO) membranes can be fabricated with highly controllable thickness and porosity, making them ideal for filtration applications. Use of these membranes is currently limited largely due to their size and overall fragility. The objective of this research was to improve mechanical properties of AAO membranes through use of high temperature heat treatment to induce phase transformations in the material. A repeatable two-step anodization process was developed for consistent sample fabrication and heat treatments were performed at 900 °C and 1200 °C in air. The pore morphology and phase composition of the as-anodized and heat-treated membranes were then observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microhardness testing was utilized to evaluate the mechanical behavior of the membranes before and after heat treatment. As-anodized AAO membranes were determined to be amorphous, and membranes heat-treated to 900 °C and 1200 °C were transformed to crystalline phases while retaining their original porous structure. Heat treatment to 900 °C resulted in formation of the γ-alumina transition phase in the skeleton regions of the membrane and nanocrystalline regions of α-alumina throughout the structure, while heat treatment to 1200 °C completely transformed the material to the stable α-alumina structure. The microhardness testing showed an increase in hardness from 2.5 ± 0.4 GPa to 4.7 ± 1.0 GPa in the transformation from amorphous to α-alumina.     

Carbon nanofibers produced from modified electrospun PAN/hydroxyapatite precursors as scaffolds for bone tissue engineering

In the current study we have proposed a method to obtain a carbon/HAp bioactive nanofibrous scaffold. The modified carbon nanofibrous nonwoven' fabrics were obtained by the use of electrospinning and subsequent stabilization and carbonization processes. The modified with HAp powder nanofibrous PAN nonwovens were thermally stabilized using a multi-stage process in the temperature ranging from 100 °C to 300 °C in an oxidative environment and then carbonized at 1000 °C in argon atmosphere. The changes of properties of composite precursor membranes taking place during stabilization and carbonization processes were investigated using the methods of: DSC, TGA, FTIR, SEM, EDX, WAXD and mechanical tests. Bioactivity was determined by assessing the formation of crystalline apatite on the surface of membranes upon immersion in Simulated Body Fluid (SBF). The FTIR, SEM and WAXD investigation clearly prove that hydroxyapatite added to the electrospinning solution was present also in composites nanofibrous nonwovens after stabilization and carbonization process. It was found that due to HAp addition: the significant decrease of fibers average diameter occurs and that the average pore size for modified membranes is smaller than for the unmodified one. On the other hand it was shown that the ceramic additive protects fibers from mass reduction during the stabilization treatment. Finally a drastic increase of mineralization activity of nCF/HAp scaffolds as compared to their nCF counterparts has been proved.     

Novel synthesis of high surface area MgAl2O4 spinel as catalyst support

A modified sol–gel route, by combining gelation and coprecipitation processes, was developed for the synthesis of high surface area MgAl₂O₄ spinel precursors. The obtained precursors were then calcined in flowing air at temperatures ranging from 500 to 900 °C. The formation of new phases upon calcinations was investigated using X-ray diffraction, thermal gravimetric analysis, and Fourier transform infrared spectroscopy (FTIR). Single-phase spinel powder with uniform pore size distribution was formed at temperatures as low as 600 °C. It was found that the thermal stability of the as-synthesized spinels is higher than that reported by other preparation methods. After calcinations at 800 and 950 °C for 8 h, the specific surface area reaches a level of 182 and 136 m²·g⁻¹, respectively. And the degree of crystallinity is higher than other preparation methods as illustrated by samples calcined at 800 °C. The amount of PVA added significantly affects the surface area of the samples. With increasing the ratio of M/PVA, the surface area of the resulting spinels increased accordingly.     

Synthesis and characterization of nanocrystalline Nd3+-doped gadolinium scandium aluminum garnet powders by a gel-combustion method

Nd³⁺-doped gadolinium scandium aluminum garnet (Nd:GSAG) precursor was synthesized by a gel combustion method using metal nitrates and citric acid as raw materials. The structure and morphology of the precursor and the sintered powders were studied by means of X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscopy (TEM). The results showed that the precursor transformed into pure GSAG polycrystalline phase at about 800 °C, and the powders sintered at 800–1000 °C were well-dispersed with average particle sizes in the range of 30–80 nm. Optical properties of Nd:GSAG nano-powders were characterized by using photoluminescence spectroscopy. The highest photoluminescence intensity was achieved for the powder sintered at 900 °C.     

Novel synthesis and electrochemical investigations of ZnO/C composites for lithium-ion batteries

For the first time, ZnO/C composites were synthesized using zinc glycerolate as a precursor through one-step calcination under a nitrogen atmosphere. The effect of the heat treatment conditions on the structure, composition, morphology as well as on the electrochemical properties regarding application in lithium-ion batteries are investigated. The products obtained by calcination of the precursor in nitrogen at 400—800 °C consist of zinc oxide nanoparticles and amorphous carbon that is in-situ generated from organic components of the glycerolate precursor. When used as anode material for lithium-ion batteries, the as-prepared ZnO/C composite synthesized at a calcination temperature of 700 °C delivers initial discharge and charge capacities of 1061 and 671 mAh g^?1 at a current rate of 100 mA g^?1 and hence 1.5 times more than bare ZnO, which reaches only 749/439 mAh g^?1. The native carbon improves the conductivity, allowing efficient electronic conductivity and Li-ion diffusion. By means of ex-situ XRD studies a two-step storage mechanism is proven.

     

Improved properties of PTFE composites filled with glass fiber modified by sol–gel method

PTFE/GF(glass fiber) composites are widely applied in high-frequency printed circuit board (PCB) substrate materials due to the excellent dielectric properties of PTFE and the low thermal expansion coefficient of GF. However, the poor interface compatibility between PTFE and GF affects the performance of the composite substrates. In this study, tetraethyl orthosilicate (TEOS) was used as the silicon source, and polydimethylsiloxane (PDMS) was the organic precursor to modify the surface of GF through the sol–gel method to promote the interface compatibility of GF and PTFE. The modified GF noted T-GF was filled in PTFE to prepare PTFE/T-GF composites. SEM, FTIR, XPS, and contact angle confirmed that organic–inorganic hybrids were successfully loaded on GF's surface. Moreover, compared with PTFE/GF and the conventional coupling agent modified GF filled PTFE composites, the PTFE/T-GF exhibited improved dielectric constant (2.305), decreased dielectric loss (9.08E^?4), higher bending strength (21.45 MPa) and bending modulus (522 MPa), better thermal conductivity (0.268 W/m*K) and lower CTE (70 ppm/°C), making it has promising application as the substrate materials for high frequency PCB.

     

Effect of pre‐processing annealing and ram speed on mechanical properties for low carbon steel processed by constrained groove pressing

Constrained groove pressing (CGP) has emerged for producing ultra‐fine‐grained materials with distinguished properties. Low carbon steel sheets were subjected to severe plastic deformation by constrained groove pressing process. The effect of pre‐processing annealing temperature, ram speed and number of passes on microstructure, mechanical properties and wear behaviour of the sheets were investigated. The 3 mm thick sheets were deformed by a constrained groove pressing die at ram speeds: 5 mm/min, 10 mm min^?1 and 20 mm min^?1. Furthermore, the as received sheets were annealed at 600 °C and 900 °C, then deformed at ram speed 20 mm min^?1. The annealing temperature 900 °C led to slightly coarser grains, lower strength and larger ductility compared to those obtained after annealing at 600 °C. With lowering the ram speed to 5 mm min^?1, the number of passes could be increased to 10 passes while increasing ram speed from 5 mm min^?1 to 20 mm min^?1 improved the mechanical properties; after 3 constrained groove pressing passes, the ultimate tensile strength increased from 420 MPa to 490 MPa, the hardness from 174 HV 1 to 190 HV 1 and the elongation from 7.6 % to 9.5 %. Finer grains were also obtained by increasing ram speed. Wear resistance was greatly enhanced by constrained groove pressing and by the increase in ram speed.     

Synthesis and characterization of smoke-like porous sol–gel indium tin oxide coatings on glass

A precursor for porous indium tin oxide (ITO) coatings with smoke-like surface feature was prepared from the hydrated metal [In(III)/Sn(IV)] salts and polyvinyl alcohol (PVA) solvated in a mixed aqueous-organic medium. Films were prepared by dip coating and cured at four temperatures (60 °C, 150 °C, 250 °C and 400 °C) where different surface features and morphological properties were obtained. The thickness of the films ranged from 0.5 to 1.2 μm. After the 60 °C cure, the surface showed a unique “smoke-like” feature of combustion products of PVA-ITO precursor. Increasing the cure temperature to 150 °C led to the development of In(III) and Sn(IV) moieties incorporated in crystalline PVA having the shape close to the hexagonal. Study of thermogravimetric analysis and differential thermal analysis of the material suggests that the gel to oxide transformation occurs by the removal of physisorbed water, PVA and nitrate ion followed by the condensation of hydroxide groups. Electrical parameters such as resistivity, conductivity, sheet resistance were evaluated by two- and four-point probe methods. Field emission scanning electron microscopy and transmission electron microscopy studies of the sample cured at 400 °C showed that the films were a porous network containing 5–40 nm clusters of ITO.     

Hybrid analogs for the production of porous calcium phosphate scaffolds

A polymer–inorganic sol mixture has been used to develop interconnected and highly porous calcium phosphate networks. The inorganic sol was developed by reacting triethyl phosphite and calcium nitrate. The sol was directly added to an aqueous solution of PVA with molecular weights between 40,500 and 155,000 g/mol. This mixture was electrospun at a voltage of 20 kV to produce fibers, whose diameter was less than 1 μm. This electrospun structure was calcined at 600 °C obtain to a highly interconnected sub-micron fibrous network (fiber size ∼ 200 nm) of calcium phosphate. The crystal size is on the order of 30 nm. Micropores could be introduced in each of the fibers by controlling the polymer molecular weight and the sol volume fraction. Such structures can have many potential uses in the repair and treatment of bone defects and in drug delivery.     

Synthesis and characterization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> nanocomposite powder by sucrose process

Nanocrystalline alumina–zirconia powders were prepared by a modified chemical route using sucrose, polyvinyl alcohol (PVA) and metal nitrates followed by a post calcination process. The process involved dehydration of Al³⁺–Zr⁴⁺ ions-sucrose–PVA solution to a highly viscous liquid which on decomposition process produced a black precursor material. The obtained precursor were then calcined at various temperatures: 1,050, 1,100, 1,150, 1,200 and 1,250 °C for different soaking times (1, 2, 4 h) in air. The formation of a nanocomposite composed of α-alumina (~20 nm) and tetragonal (t) zirconia (~19 nm) crystallites were confirmed for the sample calcined at 1,200 °C for 2 h, based on our XRD and TEM results. However, for the samples calcined below 1,150 °C the composite formed were composed of metastable alumina (γ, δ, θ) as well as t-zirconia phases. Interestingly, the zirconia phase retained its tetragonal structure for all the samples calcined above 1,050 °C. This is possibly related to the “size effect” and reduction of surface enthalpy of the zirconia crystallites surrounded by Al³⁺ cations.     

Effect of HNO<Subscript>3</Subscript> on crystalline phase evolution in lithium silicate powders prepared by sol–gel processes

An interesting observation is reported on the dramatic effect of HNO₃ on crystalline phase evolution in the 33.3 mol% Li₂O–SiO₂ glass–ceramic (stoichiometric composition of lithium disilicate Li₂Si₂O₅, LS₂) prepared by sol–gel processes from tetraethylorthosilicate (TEOS) and lithium ethoxide precursors. Nitric acid (65%), in molar ratio HNO₃/TEOS = 0.1, was added either to the precursor sol or to 95 °C dried gel. The product, which is amorphous at temperatures below 450 °C, transforms into crystalline lithium metasilicate (Li₂SiO₃, LS) at around 550 °C (starting temperature ∼450 °C), instead of forming crystalline LS₂. Phase separation in the glassy phase may be responsible for the formation of lithium metasilicate. XRD, ²⁹Si MAS, and ⁷Li static NMR were used to follow the crystallization evolution and network structures of the materials heat-treated at various temperatures.     

Preparation of porous poly(l-lactic acid)/tobermorite composite membranes via electrospinning and heat treatment

A novel poly(l-lactic acid) (PLLA)/tobermorite composite porous membrane for use as filter materials was synthesized by electrospinning a mixture of tobermorite modified by poly(dialyldimethylammoniumchloride) (PDDA) and PLLA. X-ray diffraction patterns of the sample indicated the existence of tobermorite in the sample. Scanning electron micrographs of the sample showed that the nonwoven membrane consists of a few microscopic fibers despite the existence of tobermorite particles. This was due to repulsive force and surface tension. However, no tobermorite particle was exposed at its fiber surface. Thermal analysis showed PLLA of the sample pyrolyzed with two steps of weight loss (44% at around 170 °C and 34% at around 270 °C). Heat treatment of the sample at 130 and 250 °C led to crystallization of a part of amorphous PLLA in the fiber and elimination of the remaining amorphous PLLA, respectively. After the treatment, the membrane maintained its porous structure and was coated with tobermorite particles at the fiber surface.     

Kinetics of thermal gradient chemical vapor infiltration of large-size carbon/carbon composites with vaporized kerosene

Large-size samples of carbon/carbon composites were prepared using thermal gradient chemical vapor infiltration with kerosene precursor at 950, 1020, 1100, 1180 and 1250 °C. The temperature gradient, kinetics and density distribution of these samples were studied and the microstructure of pyrolytic carbon was examined by polarized light microscopy. The results show that the initial infiltration rate increased from 5.81 to 21.32 g min^?1 by increasing deposition temperature from 950 to 1250 °C. The densification kinetics relied on deposition temperature and competition between reaction and diffusion, and the diffusion mechanism transformed from bulk to Knudsen diffusion regime. The calculated apparent activation energy is about 68.2 kJ mol^?1. The temperature range 1020–1100 °C is appropriate for fabricating composites with high final bulk density due to high degree of pore filling and the density of sample S3 infiltrated at 1100 °C is the highest among all investigated samples.