Добірка наукової літератури з теми "SiO2-tubes"

Double-shell tubular on-dimensional structure can be fabricated through a layer by layer method, in which the core template was removed to create the tubular shape. In this paper, we report, for the first time, the double nanoshell SiO2/TiO2 hollow tubes prepared through a layer-by-layer deposition method involving the sol-gel process for the SiO2 and TiO2 generation. During TEOS and TEOT hydrolysis/condensation for the SiO2 and TiO2 shell layer formation, cetyltrimethylammonium bromide (CTAB) is adopted both as the structure-directing template and as the mesopore-channel template distributing around the shell. The obtained double-nanoshell hollow tubes illustrate a large surface area and high pore volume. Also, mesoporous double-nanoshell SiO2/TiO2 hollow tubes have the inner and outer shell thickness of about 80 nm and 120 nm, respectively. Plus, the shell thickness of SiO2 and TiO2 is controllable depending on the used concentration of TEOS and TEOT during their sol-gel process. Therefore, the technique for the preparation of SiO2/TiO2 mesoporous double-nanoshell hollow tubes could provide new insights into the construction of mesoporous double-shell and hollow structure for other multicomponent and hierarchical hybrid systems.

The purpose of this study is to investigate the interfacial reactions between Mo foil and quartz in metal halide arc tubes. After welding to a tungsten cathode, the Mo foil was specially treated before Mo/SiO2 pinch seal operation. Mo foils in arc tubes show different performance in lamp depending on the type of the treatment. Mo/SiO2 interfacial reactions and amounts of Mo diffusion into the SiO2 were analyzed using TEM, SEM and EDS in order to understand the different properties of arc tubes.Arc tubes were treated with one of the following conditions : (A)untreated, (B)dry hydrogen fired, (C)process “B” plus humidity exposure. Cross-sectional TEM and SEM samples in the longitudinal direction of the arc tube through the center of Mo foil were prepared. Two Mo/SiO2 interfacial areas, one near the end of the W cathode and the other the longitudinal center of the foil were compared each other.

The application of porous carbon is versatile. It is used for high-performance catalyst support, electrode material in batteries, and gas storage. In each of these application fields nanostructuring improves the material properties. Supercapacitors store a high energy density. Exactly adapted carbon structures increase the life of lithium batteries and protect catalysts with increasing reaction rate and selectivity. Most of porous carbon materials have a spherical shape. To the best of our knowledge, there is no procedure to synthesize nanostructured cylindrical porous carbon systematically. Here, template glass fibres and SiO2-tubes were modified with nanostructured SiO2/phenolic resin and SiO2/poly(furfuryl alcohol) layers by surface twin polymerization (TP) of 2,2′-spirobi[4H-1,3,2-benzodioxasiline] and tetrafurfuryloxysilane. Afterwards the SiO2/polymer layer on the template is thermally transformed into a defect-free nanostructured SiO2/carbon layer. After completely removing the SiO2components microporous carbon tubes or rods are finally achieved. The diameters of the carbon rods and the inner as well as the outer diameter of the carbon tubes are adjustable according to the shape and size of the template. Thus, a huge variety of microporous carbon materials can be easily produced by template-assisted TP.

Al2O3-SiO2-ZrO2composite sol was prepared with inorganic precursors by sol-gel method. Using Al2O3ceramic tubes as supporter, well-surfaced Al2O3-SiO2-ZrO2composite microfiltration membrane was achieved by the film-forming technique of pretreatment of the ceramic tube supporter, 3 times recycled dipping-drying-sintering, being dried at 50°C thermostatic water bath and sintered at 900°C.

The flow field structure and cooling medium of tubes have major influence on the heat transfer performance of automotive radiator. In this study, two novel types of radiator tube (wasp-waisted tube 2# and wasp-waisted tube 3#) are developed, six types radiator tubes with different flow field structures and equal flow cross-sectional area are numerically simulated. In addition, four nanofluids with different concentrations (Al2O3/water, SiO2/water, TiO2/water and CuO/water) were studied in Reynolds number 2500-7500. The results show that the heat transfer capacity of the wasp-waisted tube 2# and the wasp-waisted tube 3# is significantly better than that of the other radiator tubes, followed by the wasp-waisted tube. Compared with the wasp-waisted tube, the heat transfer coefficient of the wasp-waisted tube 2# and the wasp-waisted tube 3# increased by 10.6% and 3.5% respectively. On the other hand, nanoparticles improve the heat transfer efficiency of base fluid. When Re reaches 7500 and the volume concentration is 3%, the Nunf/Nubf of SiO2/water is 5.52%, 5.22% and 8.70% higher than that of Al2O3 /water, TiO2/water and CuO/water, respectively. The comprehensive heat transfer capacity of SiO2/water-3% in the wasp-waisted tube is the best.

Nano-microscale fracture mechanisms, which affect fracture toughness, play an important role in improving the impact characterization of fiber reinforced polymer composites. Therefore, crack behaviors are tried to be controlled with fracture mechanisms by filling nanoparticles into polymer matrix for improving impact characteristics and fracture toughness in latest studies. In this study, it was aimed to investigate the effects of SiO2 nanoparticles addition into epoxy matrix on the low velocity impact characteristics and fracture toughness in basalt fiber reinforced filament wound composite tubes. SiO2 nanoparticle of 4% wt. filled and unfilled ± [55]6 filament wound basalt fiber reinforced/epoxy composite tubes were subjected to low velocity impact tests at 5 J, 10 J, and 15 J of energy levels. It was seen that while the addition of nanoparticles were increasing the maximum impact forces in the range of about 19%–32%, displacements and absorbed energies decreased because of the increase in the bending stiffness. Charpy impact tests were performed to three different notched arc shaped specimens for determining the impact fracture toughness. SiO2 nanoparticles increased the fracture toughness by 20%–23%. It was observed that SiO2 nanoparticles delayed the formation of failures such as debonding and delamination, and reduced the fiber breakage branching in low velocity impact tests. A liquid penetrant test was used to inspect the crack formations and progressions on the impacted surfaces of all composite tubes as practical inspection for industrial applications. It was seen that microscope and SEM analysis supported the liquid penetrant inspection, which is a non-destructive testing method.

An exo-templating synthesis process using polymeric fibers and inorganic sol particles deposited onto structured one-dimensional objects is presented. In particular, CeO2/ZrO2@SiO2 composite tubes were synthesized in a two-step procedure by using electrospun polystyrene fibers as fiber template. First, a sol–gel approach based on an exo-templating technique was employed to obtain polystyrene(PS)/SiO2 composite fibers. These composite fibers were subsequently covered by spray-coating with ceria and zirconia sol solutions. After drying and final calcination of the green body composites, the PS polymer template was removed, and composite tubes of the composition CeO2/ZrO2@SiO2 were obtained. The SiO2/ZrO2/CeO2 microtubes, which consist of interconnected silica particles, are held together by ceria and zirconia deposits formed during the thermal treatment process. These microtubes are mainly located in the pendentive connecting the individual spherical silica particles and glue them together. The composition and crystallinity of this material connecting the individual silica particles contains the elements Ce and Zr and O as mixed oxide solid solution identified by XRD, Raman and high-resolution TEM and EFTEM. High-resolution microscopy techniques allowed for an elemental mapping on the surface of the silica host structure and determination of the O, Zr and Ce elemental distribution with nm precision.

Polycarbonate etched ion-track membranes with about 30 µm long and 50 nm wide cylindrical channels were conformally coated with SiO2 by atomic layer deposition (ALD). The process was performed at 50 °C to avoid thermal damage to the polymer membrane. Analysis of the coated membranes by small angle X-ray scattering (SAXS) reveals a homogeneous, conformal layer of SiO2 in the channels at a deposition rate of 1.7–1.8 Å per ALD cycle. Characterization by infrared and X-ray photoelectron spectroscopy (XPS) confirms the stoichiometric composition of the SiO2 films. Detailed XPS analysis reveals that the mechanism of SiO2 formation is based on subsurface crystal growth. By dissolving the polymer, the silica nanotubes are released from the ion-track membrane. The thickness of the tube wall is well controlled by the ALD process. Because the track-etched channels exhibited diameters in the range of nanometres and lengths in the range of micrometres, cylindrical tubes with an aspect ratio as large as 3000 have been produced.

博士
國立臺灣大學
化學研究所
94
As the improvement in the nanoscience and nanotechnology, the synthesis and technology for the fabrication of nanosized materials have great development and become more complete than the past few years. In addition, it also has more structural changes in its dimensionality and size. In this thesis, four kinds of different one-dimensional semiconducting nanomaterials have been successfully fabricated using different physical or chemical synthetic methods and these nanomaterials are single-crystalline silicon nanowires, amorphous silicon dioxide nanotubes, single-crystalline tin dioxide nanobelts and single-crystalline copper indium diselenides nanorods. By mixing the pure silicon powders and the catalysts (including metal or silicon dioxide powders) and with assistance for the laser ablation technology, the single-crystalline silicon nanowires (SiNWs) can be fabricated with the diameters reach 5-40 nm and the lengths extend to tens of micrometers. While the metal powders (like Fe, Ru and Pr) are used as the catalysts for the syntheses of SiNWs, the most stable Si {111} facets are grown and the growth direction for the SiNWs is parallel to the facets growth direction, i.e., the wire growth direction is <111> for the metal-catalyzed SiNWs. Such SiNWs are grown via the typical vapor-liquid-solid (VLS) growth mechanism that existing the eutectic liquid droplet formation during the synthesis. On the other hand, as the silicon dioxide (SiO2) used as the catalysts for the fabrication of SiNWs, the Si {111} facets also grow; however, the wire growth direction, as the <112> direction, is perpendicular to the lattice plane growth direction. The SiNWs catalyzed by SiO2 follow an oxide-assisted (OA) growth mechanism during their growths. Furthermore, based on the different chamber pressure used during the experiments, it can be found that with the increasing of the pressure, the diameters for SiNWs enlarge and the lengths for SiNWs shorten. The Raman spectra for the different diameter SiNWs are measured and the most intense F2g phonon mode, which is located ~ 520 cm-1, can be found that with decreasing for the diameter of SiNWs, the red-shifted behavior of the F2g mode is clearly seen from the corresponding Raman spectra. By using the chemical vapor deposition method, the one-dimensional silicon dioxide nanotubes (SiO2NTs) are produced on the silicon substrate coated with Au nanoparticles which are preannealed at high temperature. The SiO2NTs can reach to 40-100 nm in diameters and extend to few micrometers in lengths. According to the electron diffraction (ED) pattern for the SiO2NTs, it can be confirmed that these nanotubes are amorphous. Besides this, the nanotubes can be separated into two groups, as thick- and thin-walled SiO2NTs, based on their different synthesis temperatures. Moreover, with the different reaction temperatures, the different shapes of Au nanoparticles are grown and this causes the different thicknesses of the SiO2NTs. With detailed analysis on the SiO2NTs, it can be figured out that the SiO2 species are diffused from the Au {111} facets and the walls of SiO2NTs are along the <022> direction of the thick-walled SiO2NTs while the <200> direction of the thin-walled SiO2NTs. Moreover, with the higher reaction temperature, the amorphous silicon dioxide nanowires (SiO2NWs) are synthesized on the silicon substrate. The Raman spectra of SiO2NTs and micro-crystallite SiO2 powders are taken and used for the characterization and both have the intense Raman peak (Si-O phonon mode) at ~ 467 cm-1. With the thermal evaporation-condensation method, the high purity single-crystalline tin dioxide nanobelts (SnO2NBs) are fabricated via the thermal heating of tin monoxide (SnO) powders in high temperature. The SnO2NBs have their belt width for 30-90 nm, the belt thickness for 20-30 nm and the belt length for tens of micrometers. Based on the X-ray diffraction (XRD) measurements of the SnO2NBs, it can be confirmed that the nanobelts are the pure rutile tetragonal structures. From the high-resolved transmission electron microscopic images, the {111} lattice planes are clearly seen and the SnO2 nanobelt grows along the <130> direction indexed from the corresponding ED pattern. The growth of the SnO2NBs can be attributed to the self-disproportion reaction of SnO bulk powders via the vapor-solid (VS) growth mechanism. In the Raman spectrum measurement, the rutile SnO2NBs have good signal to noise ratio and the peaks at 475.9, 635.5, and 777.2 cm-1 are resolved which are corresponding to the Eg, A1g, and B2g phonon modes, respectively. The last part in this thesis is the fabrication of the copper indium diselenide nanorods (CuInSe2NRs) which are commonly used in the solar cell technology. During the synthesis works, two ways are used for producing the CuInSe2 nanostructures, including the laser ablation/anodic aluminum oxide (AAO) membranes and the solvothermal methods. In the laser ablation/AAO membranes method, the AAO membranes have the highly uniform pore distribution and the CuInSe2 species can diffuse into the hollow channels to form the eutectic composites with the metal catalysts coated on the membranes and grow the one-dimensional CuInSe2 nanorods. The diameters of the CuInSe2NRs can reach 150-200 nm, however, the lengths of the can only extend ~2 micrometers. On the other hand, the CuInSe2NRs can also be synthesized by the solvothermal method, but the total reaction time is needed for at least 36 hours. Due to the long reaction time, the better aspect ratio and the product yield for the CuInSe2NRs can be acquired. The CuInSe2NRs fabricated by the solvothermal method have the diameter size of 50-100 nm and the lengths can extend to tens of micrometers. Moreover, from the high resolution image, the {112} lattice planes are found in the nanorods and can be indexed that the nanorods grow along the <331> direction. The Raman spectrum for CuInSe2NRs is taken and can be found that the most intense A1 phonon mode located at 175.1 cm-1 is clearly verified. This is another evidence tells us that the nanorods are purely with the CuInSe2 structures for the chemical composition.

The present thesis discusses the beneficial effects of confining biologically relevant molecules inside porous structures of varying morphology and dimensions. Confinement of a biomolecule such as protein, enzymes drugs leads alteration in structural features and to a significant improvement in its biophysical properties. These properties include electrochemical redox behavior (for electroactive biomolecules) and thermal stability (denaturation temperature) of the concerned biomolecule. Silica (SiO2) based materials were primarily used as substrates for confining proteins and drugs. The confinement effects were probed in depth using various electrochemical, spectroscopic, and scattering techniques. The outcomes of confinement were utilized for developing electrochemical biosensors for the protein detection. Confinement of drugs effects their structural properties which gets reflected in their release kinetics studies. Electrochemical sensing was carried out using porous structure modified electrodes. These were used not only for detection of biological analytes but also extended to environmental pollutants. In the thesis, Chapters 2-4 deal with discussions related to electrochemical, spectroscopic, and scattering studies of protein confined inside SiO2 as well as polymer capsules. In Chapter 5A and 5B, Titania (TiO2) based nanotubes were utilized for demonstration of realistic electrochemical biosensors for the detection of myoglobin and a penicillin binding protein. In Chapters 6 and 7, enzyme and inhibitors, drug release kinetics from mesoporous oxides and TiO2 tubes have been discussed. Using the same approach as in chapter 5, electrochemical sensing of model environmental pollutants using TiO2 microwires have been discussed in chapters 8A and B. The use of TiO2 microwires for photocatalytic applications have also been considered in detail. A brief discussion of the contents and highlights of the individual chapters are described below: Chapter 1 discusses in detail about the porous substrates extensively used for biomolecular (proteins and drugs) confinement and structural features of these substrates. These substrates include the mesoporous materials of varying pore dimensions and pore arrangement. The alteration in the biophysical properties of the molecules because of confinement within these mesoporous substrates and its effects on related applications such as bio catalysis, drug release rates, electrochemical biosensing have been considered. A brief discussion on the present state of art in the field of drug delivery, enzyme catalysis and electrochemical biosensing have been included. The principles related to electrochemical, spectroscopic and the scattering techniques used to characterize the properties have been discussed in detail in this chapter. Chapter 2 include discussions on the investigations on the structure and function of hemoglobin (Hb) confined inside sol-gel template synthesized silica tubes (SiO2-tubes) Immobilization of hemoglobin inside SiO2-tubes resulted in the facile electron transfer to electroactive heme center leading to an enhanced electrochemical response. The consequences of confinement on protein structures and activity were further probed via ligand binding and thermal stability studies. Reversible binding of n-donor liquid ligands such as pyridine and its derivatives and predictive variation in their redox potentials were obtained from detailed electrochemical investigations. The results suggested absence of adverse effect on structure and function of Hb confined inside the channels of SiO2-tubes. Additionally, the thermal stability of confined Hb was compared to that of free Hb in solution. The melting or denaturation temperature of Hb immobilized inside SiO2-tubes increased by approximately 4 oC compared to that of free Hb. In Chapter 3A, the configuration of hemoglobin (Hb) in solution and confined inside silica tubes (SiO2-tubes) have been studied using synchrotron small angle x-ray scattering (SAXS) and the consequences were correlated to its electrochemical activity. Confinement inside silica tubes aided in preventing protein aggregation compared to that observed for unconfined protein in solution. In case of confined Hb, the radius of gyration (Rg) and size polydispersity (p) was considerably lower than in solution. The difference in configuration between the confined and unconfined protein were reflected in their electrochemical response. Reversible electrochemical response (from cyclic voltammograms) were obtained in case of the confined hemoglobin in contrary to only cathodic response for the unconfined protein in solution. This led to the conclusion of difference in orientation of the electroactive heme center. The electron transfer coefficient () and electron transfer rate constant (ks) were also calculated to further support the structural differences between the unconfined and confined states of the hemoglobin. Thus, absence of any adverse effects on confinement of proteins inside the inorganic matrices such as silica nanotubes opens new prospects for utilizing inorganic matrices as protein “encapsulators” as well as sensors at varying temperatures. Chapter 3B discusses the implications of host dimensions on the protein structure. This is a very important parameter as it considerably influences the protein properties under confinement. This study probes the structure of same Hb molecules, confined inside silica tubes of pore diameters varying by one order in magnitude: ~ 20-200 nm. The confinement effect on structure was probed vis-à-vis the protein in solution. Small angle neutron scattering (SANS), which provides information on the protein tertiary and quaternary structures, was employed to study the influence of tube pore diameter on confined protein structure and configuration. Depending on the SiO2-tubes pore diameter, confinement significantly influenced the structural stability of Hb. High radius of gyration (Rg) and polydispersity (p) of Hb in case of the 20 nm diameter SiO2-tubes indicated that Hb undergoes significant amount of aggregation. However, for SiO2-tubes with pore diameters > 100 nm, Rg of Hb was found to be in very close proximity to that obtained from the protein data bank (PDB) reported structure. This strongly indicated that the protein has a preference for the more native like non-aggregated state when confined inside tubes of diameter ~ 100 nm. Further insight in to the Hb structure was obtained from distance distribution function, p(R) and ab-initio models calculated from the SANS patterns. These also suggest that the size of SiO2-tubes is a key parameter for the protein stability and structure. In Chapter 4 we have introduced an organic substrate to investigate the effect of confinement on structure of hemoglobin (Hb). Like as discussed in chapters 3(A and B), Hb transformed from an aggregated state in solution to non-aggregated state when confined inside the polymer capsules. Synchrotron small angle x-ray scattering (SAXS) studies directly confirmed this fact. The radius of gyration (Rg) and polydispersity (p) of the proteins in the confined state were smaller compared to that in solution. In fact, the Rg value was very similar to theoretical values obtained using protein structures generated from protein databank. The Rg value was almost constant in the temperature range (25-85 °C, Tm = 59 °C), for the confined Hb. This observation is in contrary to the increasing Rg values obtained for the free Hb in solution suggesting higher thermal stability of confined Hb inside the polymer capsule. Protein functions gets significantly altered as a result this. It resulted in an enhancement of the electroactivity of confined Hb. While Hb in solution showed dominance of the cathodic process (Fe3+→ Fe2+), efficient reversible Fe3+/Fe2+ redox response is observed in case of the confined Hb. This again gave an indication of the difference in orientation of electroactive heme group resulting it to reside in a chemically different environment compared to when it is in solution. This has important implications on protein functional properties and related applications. Thus, in this chapter we get a detailed overview of how confinement orients different groups’ viz., electroactive heme center to take up positions that makes it favorable to participate in biochemical activities such as sensing of analytes from small to macromolecules and controlled delivery of drugs. The conclusions derived from the studies in previous chapters have been utilized in chapters 5A and 5B for developing a realistic electrochemical biosensor. Since the sensing based on electrochemical response largely depends on the location of heme group, the location of the heme center was altered in a controlled manner using chemical treatment. Chapter 5A deals with an alternate antibody-free strategy for the rapid electrochemical detection of cardiac myoglobin (having heme center) using hydrothermally synthesized TiO2 nanotubes (TiO2-NT). In this strategy, myoglobin was unfolded using denaturants to expose deeply buried electroactive heme center into the solution very close to the electrode. This leads to an efficient reversible electron transfer from protein to electrode surface. The sensing performance of the TiO2-NT modified electrodes were compared vis á vis commercially available titania and GCE electrodes. The tubular morphology of the TiO2-NT led to facile transfer of electrons to the electrode surface which eventually provided linear current response (obtained from cyclic voltammetry) over a wide range of Mb concentration. The sensitivity of the TiO2-NT based sensor was remarkable and was equal to 18 A/ mg ml-1 (detection limit= 50 nM). This coupled with the rapid analysis time of few tens of minutes (compared to few days for ELISA) demonstrates its potential usefulness for the early detection of the acute myocardial infarction (AMI). Chapter 5B comprises of a discussion of employing a rapid electrochemical detection method of proteins without any electroactive center. The protein was transformed to an electrochemically active protein via metal tagging (Fe3+ in this case). This biosensor was also based on titania (TiO2-NT) nanotubes which was used to modify the working electrode. To reduce the detection volumes drastically, screen printed carbon electrodes (SPCE) was introduced in this detection. It was possible to detect as low as 1 ng l-1 of protein in very small sample volumes (as low as 30 l). The feasibility of this method for the detection of PBP2a, a marker for methicillin resistant Staphylococcus aureus (MRSA) was demonstrated here. This biosensor could effectively detect PBP2a in whole cell lysate samples. To mimic the practical detection conditions, the selectivity and efficiency was also validated using other non-selective proteins such as PTP10D, a protein tyrosine phosphatase, and bovine serum albumin (BSA). As already mentioned, this electrochemical detection strategy could reproducibly detect protein samples within minutes compared to standard ELISA methods (3-4 h) or a modified ELISA protocols (FAST-ELISA; 30 mins) excluding the time taken for sample preparation. These observations suggest the potential of the titania nanotube based electrochemical biosensor in both clinical and community settings for the detection of infectious pathogen In Chapter 6, the feasibility of utilizing mesoporous matrices of alumina and silica for inhibition of enzymatic activity have been presented. These studies were performed on a protein tyrosine phosphatase by the name chick retinal tyrosine phosphotase-2 (CRYP- 2), a protein that is identical in sequence to the human glomerular epithelial protein-1 and involved in hepatic carcinoma. The inhibition of CRYP-2 is of tremendous therapeutic importance. Inhibition of catalytic activity was examined using the sustained delivery of para nitrocatechol sulfate (pNCS) from bare and amine functionalized mesoporous silica (MCM-48) and mesoporous alumina (Al2O3). Amine functionalized MCM-48 was found to exhibit the best release of pNCS among the various mesoporous matrices studied and hence inhibition of CRYP-2 was maximum in this case. The maximum speed of reaction, vmax (= 160 ± 10 μmols min-1mg-1) and inhibition constant, Ki (= 85.0 ± 5.0 μmols) estimated using a competitive inhibition model were found to be very similar to inhibition activities of protein tyrosine phosphatases using other methods. In Chapter 7, we have demonstrated another very attractive application of the TiO2-NT which have been already used for protein sensing application. Due to the porous nature of the surface and its other attractive features, TiO2-NT has a great potential in drug delivery applications. The TiO2-NT mimicked the pore channels of the mesoporous substrates that have been used in the previous chapter. The drug release from these TiO2- NT exhibited a completely different sigmoidal release profile compared to our previous reports from the group. Additionally, the effect of surface functionalization and solution pH on drug release profile have also been considered during our studies. The release profiles were modelled with theoretical Hill equation to extract several physical parameters to explain the extent of drug substrate interactions. These results further supplemented the unique nature of the release profile. In Chapter 8A we have again demonstrated an electrochemical detection strategy but this time for chemical pollutants. Commercial textile industry effluents such as dyes were chosen for the model studies. Mesoporous anatase titania microwires synthesized via an optimized polyol method were used for sensing and photocatalysis of these dyes. Using spectroscopic investigations, we have showed that these titania microwires preferentially sense cationic (e.g. methylene blue, Rhodamine B) over anionic (e.g. Orange G, Remazol Brilliant Blue R) dyes. It was observed that variation in microwire dimensions and pH of dye solution, led to an increase in the concentration of the adsorbed dye. These findings were later corroborated with much faster electrochemical sensing. The effect of microwire length on electrochemical detection sensitivity have also been accounted in these studies. The photochemical performance of these titania microwires have been compared with the commercial P25-TiO2 nano powders. The photochemical performance was also studied as a function of exposure times and pH of dye solution. Excellent sensing ability and photocatalytic activity of the titania microwires was attributed to increased effective reaction area of the controlled nanostructured morphology. This makes them an attractive substrate for commercial sensing applications In Chapter 8B, anatase TiO2 microwires used in previous chapter were chemically modified to silver (Ag) decorated TiO2 microwires (Ag-TiO2). This was done with an aim to improve the detection sensitivity and photodegradation performance. The Ag-TiO2 microwires were synthesized via polyol synthesis route followed by a simple surface modification and chemical reduction approach for attachment of silver. The electrochemical sensing performance of Ag-TiO2 microwires have been subsequently compared with the base TiO2 microwires in the detection of cationic dye such as methylene blue. The superior performance of the Ag-TiO2 composite microwires was attributed to improved surface reactivity, mass transport and catalytic property because of decorating the TiO2 surface with Ag nanoparticles. Further studies were also carried out to compare its photocatalytic activity with TiO2 microwires at constant illumination protocols and observation times. As demonstrated the improved photocatalytic performance of Ag-TiO2 composite microwires was attributed to the formation of a Schottky barrier between TiO2 and Ag nanoparticles leading to a fast transport of photogenerated electrons to the Ag nanoparticles.

Abstract Protective tubes (Ø90x1500 mm) were manufactured by spraying with a water stabilized plasma gun. Mixtures of zircon (ZrSiO4) and alumina (Al2O3) were used as feedstock powders. Products made of these powders exhibit very good properties during thermal cycling. Previous results of the phase composition and phase changes were obtained from as-sprayed and annealed samples using X-ray diffractometry (XRD) and scanning electron microscopy. During plasma spraying zircon decomposed into ZrO2 and SiO2, which on impact and after rapid quenching formed a very fine eutectic mixture of tetragonal or monoclinic ZrO2 and amorphous SiO2. During this process alumina, in feedstock as α-phase (corundum), formed the spinel γ-phase. On annealing the y-phase transformed into the a-phase, whereas amorphous SiO2 crystallized and reacted with tetragonal ZrO2 to form ZrSiO4. Mullite (3Al2O3.2SiO2) was found at the highest annealing temperature of 1500°C when alumina reacted with SiO2. High temperature XRD was used for direct observation of phase changes during heating and cooling between room temperature and 1500°C in powdered as-sprayed deposits. This method confirmed the phase changes observed at room temperature in annealed samples, in particular the partial transformation of tetragonal to monoclinic ZrO2.

The interface thermal resistance (ITR) of the interface between vertically aligned multiwalled carbon nanotube (MWCNT) arrays and the SiO2/Si and Inconel substrates was investigated. Experimental results were compared with theoretical predictions for the ITR across nanoconstrictions. The model considers classical constriction effects and contributions due to diffuse mismatch thermal resistance. Experimental values of the ITR are much larger than the model predictions. The observed discrepancy may be due to the imperfect mechanical contact between the tubes and substrate or additional contributions to ITR due to the presence of a catalyst layer at the interface.

A study of the outside vapor deposition (OVD) process for the manufacture of fiber optic sleeve tubes is presented. High purity silicon dioxide (SiO2) is deposited on the outside of a rotating substrate via flame hydrolysis of silicon tetrachloride (SiCl4). Three double-flame burners hydrolyze the precursor forming streams of nominal 50 nm particulate, which are driven to the substrate surface via thermophoresis. The partial-premix burners utilize two concentric combustion chambers to provide fine control of the hydrolyzation process and heat flux to the preform surface. The bulk average deposition rate and efficiency to create a full-size sample are 4.93 gm/min/burner and 28%, respectively. The peak surface temperatures hover around 980 deg C at the bare quartz substrate surface, but then rapidly increase to 1200 deg C as the first four layers of SiO2 are deposited. These peak surface temperatures then monotonically decrease as the circumference and surface area of the porous preform increase. Similarly, the SiO2 layer density is 0.96 gm/cm3 at the substrate surface, but then decreases to 0.28 gm/cm3 as the porous preform grows to a diameter of 174 mm.

Highly-specular reflective surfaces that can withstand elevated-temperatures are desirable for many applications including reflective heat shielding in solar receivers and secondary reflectors, which can be used between primary concentrators and heat collectors. A high-efficiency, high-temperature solar receiver design based on arrays of cavities needs a highly-specular reflective surface on its front section to help sunlight penetrate into the absorber tubes for effective flux spreading. Since this application is for high-temperature solar receivers, this surface needs to be durable and to maintain its optical properties through the usable life. Degradation mechanisms associated with elevated temperatures and thermal cycling, which include cracking, delamination, corrosion/oxidation, and environmental effects, could cause the optical properties of surfaces to degrade rapidly in these conditions. Protected mirror surfaces for these applications have been tested by depositing a thin layer of SiO2 on top of electrodeposited silver by means of the sol-gel method. To obtain an effective thin film structure, this sol-gel procedure has been investigated extensively by varying process parameters that affect film porosity and thickness. Endurance tests have been performed in a furnace at 150°C for thousands of hours. This paper presents the sol-gel process for intermediate-temperature specular reflective coatings and provides the long-term reliability test results of sol-gel protected silver-coated surfaces.