ZnS/Me heat mirror systems

Selective spectral transmittance and heat conversion efficiencies of some heat-mirror systems consisting of ZnS/Me (Me=Al or Ag) under different configuration were studied. The 15 nm ZnS/20 nm Ag system has optimal selective spectral transmittance in the visible wavelength range and the highest reflection in the infrared wavelength range. For this system, Ag can keep its optical characteristics after heating at 200°C by the protection of the thick ZnS layer. The system consisting of 15 nm ZnS/10 nm Al is the most efficient heat mirror of the ZnS/Al systems.     

Improving hole mobility with the heterojunction of graphitic carbon nitride and titanium dioxide via soft template process in photoelectrocatalytic water splitting

In this current work, we have prepared graphitic carbon nitride@titanium dioxide (g-C₃N₄@TiO₂) nanocomposite material via simple one-step soft template synthesis to improve the efficacy of charge separation in photoelectrocatalytic water splitting. The g-C₃N_4, a photoactive component was incorporated in varying amounts into TiO₂, and the resulting composites were confirmed to have improved photoelectrocatalytic activity over the bare-TiO₂. Under the optimal experimental condition, the 20 wt % g-C₃N₄@TiO₂ composite exhibited the highest photoelectrocatalytic activity. The g-C₃N₄ has an ability to absorb the incident photons, resulting in excitation of electrons between the frontier orbitals. These excited electrons move to TiO₂ via the interfacial border, hindering the recombination of the photo-induced electrons and holes and thus, improves the overall performance of the photocatalyst. The improved photoelectrocatalytic performance by g-C₃N₄@TiO₂ was ascribed to the overall impact of g-C₃N₄ that increased its absorptive spectrum into the visible region. As such, the presence of heterojunction in the prepared composite not only aided the separation of the photogenerated charge carriers but also maintained its strong oxidation and reduction capability in the photoelectrocatalytic water splitting.     

Precipitated nanosized titanium dioxide for electrochemical applications

Two types of titanium dioxide samples precipitated from aqueous solutions of titanium tetrachloride are investigated. Crystalline materials are obtained by means of neutralization of TiCl₄ with the solution of an alkali metal hydroxide. The change of the order of mixing leads to amorphous materials. The evolution of the samples upon the thermal treatment is characterized using XRD, SEM, TEM and porosity studies. The application of crystalline TiO₂ as an electrode material in lithium-ion 2016 sample cells enable one to yield specific currents up to 3350 mA g⁻¹. On the other hand, the thermal treatment of initially amorphous materials does not lead to complete crystallization, and the presence of amorphous TiO₂ is well seen as the so-called capacity behavior of cyclic voltammetry curves.     

Synthesis and electrochemical evaluation of an amorphous titanium dioxide derived from a solid state precursor

Titanium oxides are an important class of lithium-ion battery electrodes owing to their good capacity and stability within the cell environment. Although most Ti(IV) oxides are poor electronic conductors, new methods developed to synthesize nanometer scale primary particles have achieved the higher rate capability needed for modern commercial applications. In this report, the anionic water stable titanium oxalate anion [TiO(C₂O₄)₂]²⁻ was isolated in high yield as the insoluble DABCO (1,4-diazabicyclo[2.2.2]octane) salt. Powder X-ray diffraction studies show that the titanium dioxide material isolated after annealing in air is initially amorphous, converts to N-doped anatase above 400 °C, then to rutile above 600 °C. Electrochemical studies indicate that the amorphous titanium dioxide phase within a carbon matrix has a stable cycling capacity of ∼350 mAh g⁻¹. On crystallizing at 400 °C to a carbon-coated anatase the capacity drops to 210 mAh g⁻¹, and finally upon carbon burn-off to 50 mAh g⁻¹. Mixtures of the amorphous titanium dioxide and Li₄Ti₅O₁₂ showed a similar electrochemical profile and capacity to Li₄Ti₅O₁₂ but with the addition of a sloping region to the end of the discharge curve that could be advantageous for determining state-of-charge in systems using Li₄Ti₅O₁₂.     

Metal-free hydrogen evolution over defect-rich anatase titanium dioxide

Commercial anatase phase titanium dioxide was annealed under various gases (hydrogen, nitrogen, argon, and air) to induce the formation of defects. While annealing in the absence of oxygen there was a notable increase in the concentration of paramagnetic defects as measured by Electron Paramagnetic Resonance (EPR) and X-ray Photoelectron Spectroscopy (XPS). The presence of these defects increased the metal-free photocatalytic activity of the samples towards hydrogen evolution from photocatalytic methane steam reforming (MSR) under UV illumination. Catalyst activity was stable for over 42 h while illuminated owing to the regeneration of Ti³⁺ defects by UV photoexcitation, but rapidly decayed in the dark. The high concentration of unique Ti³⁺ defect sites generated during annealing catalyze hydrogen evolution, avoiding the need for precious metal cocatalysts, while anatase lacking these defects is inactive. This work shows that the implementation of defect-rich anatase TiO₂ provides new catalytic pathways for hydrogen generation from photocatalytic methane steam reforming.     

Solar photocatalysts based on titanium dioxide nanotubes for hydrogen evolution from aqueous solutions of ethanol

Titanium foil was anodized to grow titanium dioxide nanotubes (TiO₂ NT) by using an original technique. A solution of ethylene glycol and ammonium fluoride with a concentration of 0.5 wt% was employed as electrolyte, corrosion-resistant steel was chosen as the cathode material. Anodic oxidation was carried out potentiostatically at a voltage of 60 V at ambient temperature for 90 min. Modification of as prepared TiO₂ NT were modified by calcination in air at 350°C for 4 h. Platinization of TiO₂ NT was carried out by chemical reduction of platinum from H₂PtCl₆ solution by NaBH₄ solution. Photocatalyst characterization was performed by using XRD, BET, SEM, XPS techniques. It was found that the calcination of as-prepared TiO₂ NT leads to 6-fold increase of the photocatalytic activity under UV-irradiation. The enhanced activity of TiO₂ NT is associated with the developed specific surface area and unique morphology of TiO₂ NT. This results in high rate of separation of photogenerated excited electrons and holes. Due to high separation rate the catalytic activity increases substantially.     

Solar photocatalytic degradation of real textile effluents by associated titanium dioxide and hydrogen peroxide

The present work investigated the photodegradation of real textile effluents by advanced oxidative process (AOP) using TiO₂/H₂O₂/sunlight system. The procedures were carried out at ambient conditions in March 2005. The results were evaluated by COD reduction concomitant to the increase in inorganic ion concentration (mineralization) and the analysis of the effluent characteristic spectral wavelengths: 228, 254, and 284 nm (simple aromatic compounds), 310 nm (conjugated aromatic compounds), and 390, 450, and 530 (color). As this study is not restricted to the decolorization process, it allows a more reliable evaluation of effluent mineralization. The results indicate that solar radiation is as efficient as or even more efficient than artificial radiation was in previous studies and that it also allows a reduction in effluent treatment operational costs.     

Optical property and cycling durability of polytetrafluoroethylene top-covered and metal buffer layer inserted Mg-Ni switchable mirror

Pd-capped Mg-rich Mg-Ni alloy thin film shows excellent reversible switching properties in optical transmittance by the exposure to hydrogen containing gas. However, it shows fast degradation due to the oxidization of magnesium and the switching durability is not good enough for practical applications. To resolve this problem we tried to improve its switching durability of Mg-Ni based switchable mirror by the combined use of metal buffer layer insertion between Pd and Mg-Ni layer and polytetrafluoroethylene (PTFE) protective coating. PTFE thin film has been prepared on the surface of Mg-Ni thin films by RF magnetron sputtering in the Ar and CF₄ mixed gas discharge plasma at room temperature and a power of 30 W. The sample of Pd/Ti/Mg₄Ni thin film with the protective coating of 900-nm-thick PTFE layer can be switched over 1600 switching cycles, which suppress the degradation by 15% of its initial transmission modulation level.     

Properties of spray deposited titanium dioxide thin films and their application in photoelectrocatalysis

Thin films of titanium dioxide were deposited onto optically transparent, electrically conducting substrates (fluorine doped tin oxide on glass). The two oxide layers, SnO₂ and TiO₂, were deposited sequentially by spray pyrolysis. TiO₂ films of up to 800 nm thickness were prepared by varying the quantity of sprayed solution (titanyl acetylacetonate in methanol), at a growth rate of 0.15 nm/s.The effect of film thickness on the structural, optical and photoelectrochemical properties of TiO₂ films was studied. Scanning electron microscopy showed that the polycrystalline anatase films were compact. The grain size increased up to 1100 nm with increase in film thickness, whereas the crystallite size remained constant (40 nm) as shown by X-ray diffraction. The films had a transmittance of more than 70% in the visible region.Junctions of the semiconducting films with aqueous electrolytes were rectifying and photoactive. Films of 330 or 600 nm were thick enough to exhibit maximum photoelectrochemical response for light of a wavelength of 313 or 365 nm, respectively. Under depletion conditions, an IPCE (incident photon to current conversion efficiency) of 0.8 for a 330 nm thick film at 313 nm was obtained.Oxalic acid degradation under UVA light and under sunlight, applying electrical bias, was demonstrated using these electrodes.     

Single-layer heat mirror films and an improved method for evaluation of its optical and radiative properties in infrared

A technique for preparing single heat mirror films with high quality is described. We also present an improved method for evaluating its optical and thermal radiative properties in the infrared region (2.5<λ<25 μm). The calculated results agree well with that obtained by experimental measurements. The results calculated by using the simple Hagen–Rubens relation are also given. It is shown that the Hagen–Rubens relation is not accurate to predict the thermal radiative properties for the transparent semiconducting oxide films, such as indium tin oxides, prepared in our studies.     

Ground heat storage with a double layer heat exchanger

A one-dimensional model of ground heat storage is used to simulate complex heat and moisture processes in the vicinity of double-and single-layer ground heat exchangers. Simulated results indicate that a ground heat pump with the double-layer heat exchanger has certain advantages over the single-layer system in terms of a higher seasonal performance factor, smaller ground area, and reduced ecological risk in the soil subsurface region. Solar energy injection into the ground improves the seasonal performance factor both in the single and in the double layer heat exchanger system. Latent heat released by the freezing of soil moisture is an important component in the total amount of heat extracted from the ground when a phase change takes place.     

High dispersion and electrocatalytic activity of Pd/titanium dioxide nanotubes catalysts for hydrazine oxidation

Pd/titanium dioxide nanotubes (Pd/TiO₂-NTs) catalysts were prepared by a simple reduction method using TiO₂-NTs as support. The structure and morphology of the resulting Pd/TiO₂-NTs were characterized by transmission electron microscopy and X-ray diffraction. The results showed that Pd nanoparticles with a size range from 6 to 13 nm were well-dispersed on the surface of TiO₂-NTs. The electrocatalytic properties of Pd/TiO₂-NTs catalysts for hydrazine oxidation were also investigated by cyclic voltammetry. Compared to that of pure Pd particles and Pd/TiO₂ particles, Pd/TiO₂-NTs catalyst showed much higher electrochemical activity. This may be attributed to the uniform dispersion of Pd nanoparticles on TiO₂-NTs, smaller particle size and unique properties of TiO₂-NTs support. In addition, the mechanism of hydrazine electrochemical oxidation catalyzed by Pd/TiO₂-NTs are also investigated. The oxidation of hydrazine was an irreversible process, which might be controlled by diffusion process of hydrazine.     

Investigation of sol–gel derived thin films of titanium dioxide doped with vanadium oxide

Thin films of titanium dioxide and titanium–vanadium oxide were obtained by a sol–gel method. The coatings are uniform, smooth with very good optical properties. The solutions of both kinds are stable for more than a year. Structure and vibrational properties were studied with the help of X-ray diffraction (XRD) analysis and infrared spectroscopy (IR). The refractive indices and film thicknesses were measured by an ellipsometer at a wavelength of 638.2 nm, as a function of annealing temperature. The optical properties were investigated by ultraviolet–visible (UV–VIS) spectroscopy.     

Experimental heat transfer of nanofluid through an annular duct

This paper is related to heat transfer performance of Al₂O₃/H₂O and TiO₂/H₂O nonofluids through an annular channel with constant wall temperature boundary condition under turbulent flow regime. The constant temperature is applied on the outer wall of the channel. Experimental investigation was done for a wide range of Al₂O₃ and TiO₂ nanoparticle concentrations and Reynolds number. Based on the experimental results, for specific Peclet number, Nusselt number of nanofluids is higher than that of the base fluid. The enhancement increases with increase of nanparticle concentration for both employed nanofluids. Based on the results of this investigation there is no significant difference on the heat transfer enhancement associated with two employed nanofluids.     

An experimental investigation of convection heat transfer to supercritical carbon dioxide in miniature tubes

Experimental results of convection heat transfer to supercritical carbon dioxide in heated horizontal and vertical miniature tubes are reported in this paper. Stainless steel circular tubes having diameters of 0.70, 1.40, and 2.16 mm were investigated for pressures ranging from 74 to 120 bar, temperatures from 20 to 110 °C, and mass flow rates from 0.02 to 0.2 kg/min. The corresponding Reynolds numbers and Prandtl numbers ranged from 10⁴ to 2×10⁵ and from 0.9 to 10, respectively. It is found that the buoyancy effects were significant for all the flow orientations, although Reynolds numbers were as high as 10⁵. The experimental results reveal that in downward flow, a significant impairment of heat transfer was discerned in the pseudocritical region, although heat transfer for both horizontal and upward flow was enhanced. The experimental results further indicate that in all the flow orientations, the Nusselt numbers decreased substantially as the tube diameter shrunk to <1.0 mm. Based on the experimental data, correlations were developed for the axially-averaged Nusselt number of convection heat transfer to supercritical carbon dioxide in both horizontal and vertical miniature heated tubes.     

Numerical prediction on the variation of temperature in the eroded blast furnace hearth with titanium dioxide in hot metal

This paper numerically predicts the decrease in temperature of the eroded hearth of the blast furnace with floating deadman (coke zone) case for Port Kembla no. 5 blast furnace (PKBF5) by solving the Navier–Stokes equation, the thermal-energy-balance equation with conjugated heat transfer, and the mass transfer equation with the effect of chemical reaction during tapping process at steady state. The chemical reaction is generated by titanium dioxide (TiO₂) reacting with saturated carbon in hot metal. The following calculations are done by using a commercial computational fluid dynamics (CFD) package, FLUENT (version 6.2), with a segregated model. As shown in the results, on the basis of the flow pattern of liquid iron, the inlet selection of adding TiO₂ may be far away from the taphole. The calculated temperature variation in the damaged hearth by reaction is significant, but that on the other side of the hearth without adding TiO₂ isn't influenced. Additionally, the numerical temperature data of different locations may be examined by the blast furnace operators to judge the effect of lowering temperature.     

Theoretical analysis of a thermodynamic cycle for power and heat production using supercritical carbon dioxide

A numerical study of a thermodynamic cycle is described: solar energy powered Rankine cycle using supercritical carbon dioxide as the working fluid for combined power and heat production. A model is developed to predict the cycle performance. Experimental data is used to verify the numerical formulation. Of interest in the present study is the thermodynamic cycle of 0.3–1.0 kW power generation and 1.0–3.0 kW heat output. The effects of the governing parameters on the performance are investigated numerically. The results show that the cycle has a power generation efficiency of somewhat above 20.0% and heat recovery efficiency of 68.0%, respectively. It is seen that the cycle performance is strongly dependent on the governing parameters and they can be optimized to provide maximum power, maximum heat recovery or a combination of both. The power generation and heat recovery are found to be increased with solar collector efficient area. The power generation is also increased with water temperature of the heat recovery system, but decreased with heat exchanging area. It is also seen that the effect of the water flow rate in the heat recovery system on the cycle performance is negligible.     

Conjugate-mixed convection heat transfer in a lid-driven enclosure with thick bottom wall

Conjugate heat transfer by mixed convection and conduction in lid-driven enclosures with thick bottom wall has been studied by a numerical method. The enclosure is heated from the bottom wall isothermally. Temperature of the top moving wall, which has constant flow speed, is lower than that of the outside of bottom wall. Vertical walls of the enclosure are adiabatic. Governing parameters are solved for a wide range of Richardson numbers (0.1 ≤ Ri ≤ 10), ratio of height of bottom wall to enclosure height (0.1 ≤ h/H ≤ 0.5) and thermal conductivity ratio (0.01 ≤ λ_f/λ_s ≤ 10). Obtained results showed that heat transfer decreases with increasing of λ_f/λ_s ratio, Richardson number and thickness ratio of the wall. Flow strength is affected for only higher values of λ_f/λ_s ratio.     

Titanium dioxide nano-heterostructure with nanoparticles decorating nanowires for high-performance photocatalysis

In this work, we have synthesized a nano-heterostructure of TiO₂ nanoparticles decorating TiO₂ nanowires (TiO₂ NPs/NWs). Both the TiO₂ NPs and NWs show an anatase crystal phase with {101} facet predominately exposed. It is found that the ratio of NPs to NWs affects the dispersion of TiO₂ NPs and their combination with TiO₂ NWs, as well as the separation of photo-generated charges and further the photocatalytic activity of the material. The heterostructure with a TiO₂ NPs to NWs ratio of 1:1 (NP_0.5/NW_0.5) shows high activity and excellent cycling stability in both Rhodamine B degradation and water splitting. A possible charge transfer mechanism on the TiO₂ NPs/NWs hybrid has been proposed to clarify the correlation between material structure and performance. This work not only demonstrates the advantage of 0D-1D nano-heterostructures, but also provides a facile approach of designing and fabricating excellent catalysts for energy and environmental applications.     

Electrospun nickel doped titanium dioxide nanofibers as an effective photocatalyst for the hydrolytic dehydrogenation of ammonia borane

In this study, Ni-doped titanium dioxide (TiO₂) electrospun nanofibers are introduced as novel material for dehydrogenation of ammonia borane (AB) complex. Hydrolysis experiments with introduced catalytic nanofibers are prevailed to rapidly release hydrogen from AB complex. Typically, Ni nanoparticles (NPs) behave as a catalyst, meanwhile the incorporation of nickel NPs lead to decrease in the electrons/holes recombination rate in TiO₂ which resulted in the increase of active ions in the solution to a rapid evolution of hydrogen gas at room temperature. The utilized physiochemical analyses indicate that the introduced Ni-doped TiO₂ nanofibers have a smooth surface and uniform diameters along their lengths. Under sunlight irradiation, the hydrogen production rate in case of utilizing Ni-doped TiO₂ nanofibers is rapidly increased compared to the pristine TiO₂ nanofibers, the maximum hydrogen equivalent in case of the doped nanofibers is 2.6 while the pristine one is 1.4. Both formulations exhibit almost equal low activity in daylight as the observed hydrogen equivalent is 0.4. Overall, this study proposes cheap, stable and effective material for AB dehydrogenation at room temperature.