2D reduced graphene oxide–titania nanocomposites synthesized under different hydrothermal conditions for treatment of hazardous organic pollutants

Two-dimensional reduced graphene oxide–titania (RGO–TiO₂) composites were prepared using a single-step hydrothermal method under various hydrothermal reaction conditions. The morphological and surface characteristics of the RGO–TiO₂ composites and reference materials were determined. The RGO–TiO₂ composites showed photocatalytic activity for the decomposition of two target pollutants that was superior to both pure TiO₂ and RGO under fluorescent daylight lamp illumination. The photocatalytic activity of the RGO–TiO₂ composite increased as the hydrothermal treatment time increased from 1 to 24 h, but then it decreased as the time increased to 36 h, which indicated the presence of an optimal treatment time. RGO–TiO₂ composites activated by violet light-emitting diodes (LEDs) displayed lower decomposition efficiency than those activated by a daylight lamp, likely because of the lower light intensity of violet LEDs (0.2 mW/cm²) when compared with that of the daylight lamp (1.4 mW/cm²). However, the photocatalytic decomposition of the target pollutants using the RGO–TiO₂ composite was more energy-efficient using the violet LEDs. The photocatalytic reaction rates increased as the residence time decreased, whereas the reverse was true for the decomposition efficiency.     

Green synthesis and enhanced photocatalytic activity of Ce-doped TiO2 nanoparticles supported on porous glass

A facile and green method to prepare Ce-doped TiO₂ nanoparticles supported on porous glass beads is reported. An ion exchange process and subsequent calcination yielded Ce-doped TiO₂ nanoparticles with a mean size of 4.8 ± 0.3 nm. The nanoparticles were dispersed on the surface of porous glass beads. The addition of Ce enhanced the visible light absorption of the TiO₂ nanoparticles in the 400–500 nm spectral window. The band gap of the as-prepared catalyst was 2.80 eV. The Ce-doped TiO₂ nanoparticles immobilized on porous glass beads exhibited excellent photocatalytic activity for the visible-light-degradation of methyl orange (MO) and rhodamine B (RhB); with rate constants of 0.095 and 0.230 min⁻¹; respectively. The effects of Ce dosage; reaction duration; and initial solution pH on the conversion of MO and RhB dyes were investigated. The green synthesis and favorable photocatalytic activity makes the Ce-doped TiO₂ nanoparticles immobilized on porous glass an attractive alternative for the efficient degradation of organic pollutants.     

High photocatalytic efficiency of spouting reactor compared with fluidized bed with top irradiation source

The removal of volatile organic compounds by photocatalytic degradation is one of the safest and most effective ways of removing pollutants from the air. This process is highly affected by the type of reactor, light exposure, and hydrodynamics. For scale up purposes, continuous reactors with high capacity are required for treating large amounts of feedstock. In this work, two types of reactors based on different hydrodynamics, fluidized and spouted reactors, were designed to work under light irradiation inside the reactor. The efficiency of the reactors for volatile organic compound removal from high flow rates of air under Hg lamp irradiation using N–F-TiO₂ photocatalyst was investigated. The performance of the fluidized bed and spouted bed were evaluated and compared at the same weight hourly space velocity of feed stream through the reactor. The results revealed that 80% of the initial acetaldehyde was removed in the fluidized bed after about 200 min, while in the spouted bed the acetaldehyde was totally removed after about 120 min.     

Electrochemical performance of polygonized carbon nanofibers as anode materials for lithium-ion batteries

Carbon nanofibers with a polygonal cross section (P-CNFs) synthesized using a catalytic chemical vapor deposition (CCVD) technology have been investigated for potential applications in lithium batteries as anode materials. P-CNFs exhibit excellent high-rate capabilities. At a current density as high as 3.7 and 7.4 A/g, P-CNFs can still deliver a reversible capacity of 198.4 and 158.2 mAh/g, respectively. To improve their first coulombic efficiency, carbon-coated P-CNFs were prepared through thermal vapor deposition (TVD) of benzene at 900 °C. The electrochemical results demonstrate that appropriate amount of carbon coating can improve the first coulombic efficiency, the cycling stability and the rate performance of P-CNFs. After carbon coating, P-CNFs gain a weight increase approximately by 103 wt%, with its first coulombic efficiency increasing from 63.1 to 78.4%, and deliver a reversible capacity of 197.4 mAh/g at a current density of 3.7 A/g. After dozens of cycles, there is no significant capacity degradation at both low and high current densities.     

Large-scale synthesis of hierarchical MnO2 for benzene catalytic oxidation

Hierarchical sea-urchin-shaped manganese oxide microspheres were synthesized via a facile method based on the reaction between KMnO₄ and MnSO₄ in HNO₃ solution at 50 °C. The average diameter of the microspheres is ∼850 nm. The microspheres consist of a core of diameter of ∼800 nm and nanorods of width ∼50 nm. The nanorods exist at the edge of the core. The Brunauer–Emmett–Teller surface area of the sea-urchin-shaped microspheres is 259.4 m²/g. A possible formation mechanism of the hierarchical sea-urchin-shaped microspheres is proposed. The temperature for 90% conversion of benzene (T_90%) on the hierarchical urchin-shaped MnO₂ microspheres is about 218 °C.     

Fly ash-based geopolymer as a novel photocatalyst for degradation of dye from wastewater

The geopolymer synthesized by alkali-activated fly ash was firstly used as a novel photocatalyst for degradation of methylene blue (MB) dye from wastewater. The geopolymer is composed of nanoparticulates with an average particle size of about 50 nm. More than 90% of pore volume in the fly ash-based geopolymer predominately centralized on the pore size in the range of 17?700 nm. The degradation efficiency of MB dye by fly ash-based geopolymer catalyst was up to 92.79% under UV irradiation due to the synergistic effect of adsorption and semiconductor photocatalysis. The pseudo-first-order and pseudo-second-order rate equations as well as intra-particle diffusion rate equation were employed to correlate analysis for the adsorption kinetics of MB dye. The experimental data agreed well with pseudo-second-order rate equation in both cases of with UV and without UV irradiations. The intra-particle diffusion process is not the rate determining step. The photocatalytic degradation of MB dye in solution obeys third-order reaction kinetics.     

In situ synthesis of CNTs/Fe–Ni/TiO2 nanocomposite by fluidized bed chemical vapor deposition and the synergistic effect in photocatalysis

Carbon nanotube (CNTs)/Fe–Ni/TiO₂ nanocomposite photocatalysts have been synthesized by an in situ fluidized bed chemical vapor deposition (FBCVD) method. The composite photocatalysts were characterized by XRD, Raman spectroscopy, BET, FESEM, TEM, UV–vis spectroscopy, and XPS. The results showed that the CNTs were grown in situ on the surface of TiO₂. Fe(III) in TiO₂ showed no chemical changes in the growth of CNTs. Ni(II) was partly reduced to metal Ni in the FBCVD process, and the metal Ni acted as a catalyst for the growth of CNTs. The photocatalytic activities of CNTs/Fe–Ni/TiO₂ decreased with the rise of the FBCVD reaction temperature. For the sample synthesized at low FBCVD temperature (500 °C), more than 90% and nearly 50% of methylene blue were removed under UV irradiation in 180 min and under visible light irradiation in 300 min, respectively. The probable mechanism of synergistic enhancement of photocatalysis on the CNTs/Fe–Ni/TiO₂ nanocomposite is proposed.     

Coupled annealing temperature and layer thickness effect on strengthening mechanisms of Ti/Ni multilayer thin films

A systematic study was performed on mechanical and microstructural properties of Ti/Ni multilayers with layer thickness from 200 nm to 6 nm and annealing temperature from room temperature to 500 °C. Based on the observed hardness evolution, a coupled layer-thickness and annealing-temperature dependent strengthening mechanism map is proposed. For as-deposited films, the deformation behavior follows the traditional trend of dislocation mediated strengthening to grain boundary mediated softening with decreasing layer thickness. For annealed films, grain boundary relaxation is considered to be the initial strengthening mechanism with higher activation temperature required for thicker layers. Under further annealing, solid solution hardening, intermetallic precipitation hardening, and fully intermixed alloy structure continue to strengthen the thin layered films, while recrystallization and grain-growth lead to the eventual softening of thick layered films. For the films with intermediate layer thickness, a strong orientation dependent hardness behavior is exhibited under high temperature annealing due to mechanism switch from grain growth softening to intermetallic precipitation hardening when changing the loading orientation from perpendicular to parallel to the layer interfaces.     

Evolution of planetary boundary layer under different weather conditions,and its impact on aerosol concentrations

A field experiment was conducted in Tianjin, China from September 9–30, 2010, focused on the evolution of Planetary Boundary Layer (PBL) and its impact on surface air pollutants. The experiment used three remote sensing instruments, wind profile radar (WPR), microwave radiometer (MWR) and micro-pulse lidar (MPL), to detect the vertical profiles of winds, temperature, and aerosol backscattering coefficient and to measure the vertical profiles of surface pollutants (aerosol, CO, SO₂, NO_x), and also collected sonic anemometers data from a 255-m meteorological tower. Based on these measurements, the evolution of the PBL was estimated. The averaged PBL height was about 1000–1300 m during noon/afternoon-time, and 200–300 m during night-time. The PBL height and the aerosol concentrations were anti-correlated during clear and haze conditions. The averaged maximum PBL heights were 1.08 and 1.70 km while the averaged aerosol concentrations were 52 and 17 μg/m³ under haze and clear sky conditions, respectively. The influence of aerosols and clouds on solar radiation was observed based on sonic anemometers data collected from the 255-m meteorological tower. The heat flux was found significantly decreased by haze (heavy pollution) or cloud, which tended to depress the development of PBL, while the repressed structure of PBL further weakened the diffusion of pollutants, leading to heavy pollution. This possible positive feedback cycle (more aerosols → lower PBL height → more aerosols) would induce an acceleration process for heavy ground pollution in megacities.     

Improvement in the thermal conductivity of aluminum substrate for the desktop PC Central Processing Unit (CPU) by the Taguchi method

This paper examines the thermal conductivity of thin films (Cu or Ag) deposited on 1050 aluminum alloy substrates (99.57% purity) by various sputtering. The Taguchi method was used to clarify the influence of various deposition conditions (target, sputtering method, power, deposition time and annealing temperature). This paper employs the signal-to-noise (S/N) ratio and analysis of variance (ANOVA) to study the coating operation performance. The experimental results point out the optimum conditions of highly thermal conduction were the Ag target, sputtering method of RF, power of 300 W, deposition time of 15 min, and no annealing temperature. The sputtering method and power are the most significant factors among the five controllable factors affecting the thermal conductivity of aluminum substrate in the sputtering process.     

Immobilization of Ti02 nanoparticles on activated carbon fiber and its photodegradation performance for organic pollutants

The immobilization of titanium dioxide （TiO2） on activated carbon fiber （ACF）, （TiO2/ACF）, was accomplished by sol-gel-adsorption method followed by calcination at temperatures varying from 300 to 600℃ in an argon atmosphere. The material properties were determined by scanning electron microscope （SEM）, X-ray diffraction （XRD） and nitrogen adsorption. The photodegradation behavior of TiO2 /ACF was investigated in aqueous solutions using phenol and methyl orange （MO） as target pollutants. The effects of calcination temperature, photocatalyst dosage, initial solution pH and radiation time on the degradation of organic pollutants were studied. It was found that organic pollutants could be removed rapidly from water by the TiO2/ACF photocatalyst and the sample calcined at 500℃ exhibited the highest removal efficiency. Kinetics analysis showed that the photocatalytic degradation reaction can be described by a first-order rate equation. In addition, the possibility of cyclic usage of the photocatalyst was also confirmed. Moreover, TiO2 is tightly bound to ACF and can be easily handled and recovered from water. It can therefore be potentially applied for the treatment of water contaminated by organic pollutants.     

Effects of cerium incorporation on the catalytic oxidation of benzene over flame-made perovskite La1−xCexMnO3 catalysts

Perovskite-type La_1−xCe_xMnO₃ (x = 0–10%) catalysts were prepared by flame spray pyrolysis and their activities during the catalytic oxidation of benzene were examined over the temperature range of 100–450 °C. The structural properties and reducibility of these materials were also characterized by X-ray diffraction (XRD), N₂ adsorption/desorption, H₂ temperature-programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS). The incorporation of Ce was found to improve the benzene oxidation activity, and the perovskite in which x was 0.1 exhibited the highest activity. Phase composition and surface elemental analyses indicated that non-stoichiometric compounds were present. The incorporation of Ce had a negligible effect on the specific surface area of the perovskites and hence this factor has little impact on the catalytic activity. Introduction of Ce⁴⁺ resulted in modification of the chemical states of both B-site ions and oxygen species and facilitated the reducibility of the perovskite. The surface Mn⁴⁺/Mn³⁺ ratio was increased as a result of Ce⁴⁺ substitution, while a decrease in the surface-adsorbed O/lattice O (O_ads/O_latt) ratio was observed. The relationship between the surface elemental ratios and catalytic activity was established to allow a better understanding of the process by which benzene is oxidized over perovskites.     

A novel method for fast and continuous preparation of superfine titanium dioxide nanoparticles in microfluidic system

Previously we had developed a microfluidic system that can be easily fabricated by bending a stainless-steel tube into large circular loops. In this study, a fast and continuous preparation method for superfine TiO₂ nanoparticles (TiO₂-NPs) was developed for the aforementioned microfluidic system. The proposed method can yield anatase TiO₂ in 3.5 min, in contrast to the traditional hydrothermal reaction method, which requires hours or even days. Different reaction conditions, such as reaction temperature (120–200 °C), urea concentration (20–100 g/L), and tube length (5–20 m) were investigated. X-ray diffraction and Brunauer–Emmett–Teller analysis indicate that the as-prepared TiO₂-NPs have crystalline sizes of 4.1–5.8 nm and specific surface areas of 250.7–330.7 m²/g. Transmission electron microscopy images show that these TiO₂-NPs have an even diameter of approximately 5 nm. Moreover, because of their small crystalline sizes and large specific surface areas, most of these as-prepared TiO₂-NPs exhibit considerably better absorption and photocatalytic performance with methylene blue than commercial P5 TiO₂ does.     

Experimental pool boiling investigations of FC-72 on silicon with artificial cavities and integrated temperature microsensors

In this experimental study, fluorinert FC-72 is boiled on a silicon chip with artificial cavities and integrated microsensors. The horizontal silicon chip with dimensions of 39.5 × 19 × 0.38 mm is completely immersed in FC-72. The integrated nickel–titanium temperature microsensors on the back of the chip are calibrated individually and exhibit a near-linear increase of electrical resistance with temperature. The applied heat fluxes and the resulting wall superheat at the boiling surface are varied by means of an integral thin-film resistance heater (95% Al, 4% Cu and 1% Si), also on the back of the silicon chip. Artificial cylindrical cavities with a mouth diameter of 10 μm and depths of 40, 80 or 100 μm situated above the microthermometers serve as artificial nucleation sites, due to trapped vapour. Bubble growth rates, frequencies, departure diameters of bubbles and waiting times between bubbles from an isolated cavity for different wall superheats and pressures were obtained by analysing high-speed video images and the simultaneously measured temperature below the artificial cavity.     

Effect of temperature on the effective thermal conductivity of n-tetradecane-based nanofluids containing copper nanoparticles

Nanofluids were prepared by dispersing Cu nanoparticles (∼20 nm) in n-tetradecane by a two-step method. The effective thermal conductivity was measured for various nanoparticle volume fractions (0.0001–0.02) and temperatures (306.22–452.66 K). The experimental data compares well with the Jang and Choi model. The thermal conductivity enhancement was lower above 391.06 K than for that between 306.22 and 360.77 K. The interfacial thermal resistance increased with increasing temperature. The effective thermal conductivity enhancement was greater than that obtained with a more viscous fluid as the base media at 452.66 K because of nanoconvection induced by nanoparticle Brownian motion at high temperature.     

Spatially resolved wall temperature measurements during flow boiling in microchannels

Spatial and temporal variations of channel wall temperature during flow boiling microchannel flows using infrared thermography are presented and analyzed. In particular, the top channel wall temperature in a branching microchannel silicon heat sink is measured non-intrusively. Using this technique, time-averaged temperature measurements, with a spatial resolution of 10 μm, are presented over an 18 mm × 18 mm area of the heat sink. Also presented, within a specific sub-region of the heat sink, are intensity maps that are recorded at a rate of 120 frames per second. Time series data at selected point locations in this sub-region are analyzed for their frequency content, and dominant temperature fluctuations are extracted using proper orthogonal decomposition.Results at low-vapor-quality boiling condition indicate that temperatures can be determined from recorded radiation intensities with a temperature uncertainty varying from 0.9 °C at 25 °C to 1.0 °C at 125 °C. The time series data indicate periodic wall temperature fluctuations of approximately 2 °C that are attributed to the passage of vapor slugs. A dominant band of frequencies around 2–4 Hz is suggested by the frequency analysis. Proper orthogonal decomposition results indicate that first six orthogonal modes account for approximately 90% of the variance in temperature. The first mode reconstruction accounts for temporal variations in the dataset in the sub-region analyzed; however the magnitude of fluctuations and spatial variations in temperature are not accurately captured. A reconstruction using the first 25 modes is considered sufficient to capture both the temporal and spatial variations in the data.     

Catalytic oxidation of benzene over Ce–Mn oxides synthesized by flame spray pyrolysis

Flame spray pyrolysis (FSP) was utilized to synthesize Ce–Mn oxides in one step for catalytic oxidation of benzene. Cerium acetate and manganese acetate were used as precursors. The materials synthesized were characterized using X-ray diffraction (XRD), N₂ adsorption, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectroscopy, and H₂-temperature programmed reduction (H₂-TPR) and their benzene catalytic oxidation behavior was evaluated. Mn ions were evidenced in multiple chemical states. Crystalline Ce–Mn oxides consist of particles with size <40 nm and specific surface areas (SSA) of 20–50 m²/g. Raman spectrums and H₂-TPR results indicated the interaction between cerium and manganese oxides. Flame-made 12.5%-Ce–Mn oxide exhibited excellent catalytic activity at relatively low temperatures (T₉₅ about 260 °C) compared to other Ce–Mn oxides with different cerium-to-manganese ratios. Redox mechanism and strong interaction conform to structure analysis that Ce–Mn strong interaction formed during the high temperature flame process and the results were used to explain catalytic oxidation of benzene.     

Spatiotemporal variations of PM2.5 and PM10 concentrations between 31 Chinese cities and their relationships with SO2, NO2, CO and O3

The variations of mass concentrations of PM_2.5, PM₁₀, SO₂, NO₂, CO, and O₃ in 31 Chinese provincial capital cities were analyzed based on data from 286 monitoring sites obtained between March 22, 2013 and March 31, 2014. By comparing the pollutant concentrations over this length of time, the characteristics of the monthly variations of mass concentrations of air pollutants were determined. We used the Pearson correlation coefficient to establish the relationship between PM_2.5, PM₁₀, and the gas pollutants. The results revealed significant differences in the concentration levels of air pollutants and in the variations between the different cities. The Pearson correlation coefficients between PMs and NO₂ and SO₂ were either high or moderate (PM_2.5 with NO₂: r = 0.256–0.688, mean r = 0.498; PM₁₀ with NO₂: r = 0.169–0.713, mean r = 0.493; PM_2.5 with SO₂: r = 0.232–0.693, mean r = 0.449; PM₁₀ with SO₂: r = 0.131–0.669, mean r = 0.403). The correlation between PMs and CO was diverse (PM_2.5: r = 0.156–0.721, mean r = 0.437; PM₁₀: r = 0.06–0.67, mean r = 0.380). The correlation between PMs and O₃ was either weak or uncorrelated (PM_2.5: r = −0.35 to 0.089, mean r = −0.164; PM₁₀: r = −0.279 to 0.078, mean r = −0.127), except in Haikou (PM_2.5: r = 0.500; PM₁₀: r = 0.509).     

Microstructure and mechanical properties of ZrO2 (Y2O3)–Al2O3 nanocomposites prepared by spark plasma sintering

Zirconia (yttria)–alumina ceramic nanocomposites were fabricated from different powders by spark plasma sintering (SPS). One powder was a commercially available nanocomposite powder TZP-3Y20A, consisting of 3 mol% yttria-stabilized zirconia (3-YSZ) reinforced with 20 wt% alumina, and the other, used as a comparison, was a conventional mechanically mixed powder 3YSZ-20A, a blend made of 3 mol% yttria-stabilized zirconia powder ZrO₂ (3Y) and 20 wt% α-alumina powder. The effect of the sintering temperature on the densification, the sintering behavior, the mechanical properties and the microstructure of the composites was investigated. The results showed that the density increased with increasing sintering temperature, and thus, the mechanical properties were strengthened because of the increased densification. The nanocomposite powder TZP-3Y20A was easily sintered, and good mechanical properties were achieved as compared with the powder from the conventional mechanically mixed method, the maximum flexural strength and fracture toughness of which were 967 MPa and 5.27 MPa m^1/2, respectively.     

Experimental and numerical analyses on the effect of increasing inflow temperatures on the flow mixing behavior in a T-junction

Thermal degradation of piping induced by high cycle thermal fatigue (HCTF) is of significant importance as operating Nuclear Power Plants (NPP) become older and lifetime extension activities are initiated. In particular, HCTF incidents related to turbulent thermal mixing of fluids in a T-junction piping system are not well understood and could not be adequately monitored using common thermocouple instrumentation. To investigate this phenomenon, an experimental T-junction test facility was commissioned at the University of Stuttgart, known as the Fluid Structure Interaction (FSI) test facility. The paper presents the experimental investigation and the corresponding numerical validation using the large eddy simulation (LES) method to study T-junction flow mixing. Three experimental test cases are investigated with temperature differences (∆T) of 51.5 K (Case 1), 76 K (Case 2) and 97 K (Case 3) between the mixing fluids. A constant mass flow rate ratio (main/branch) of 4:1 is maintained in all the investigated cases. Flow mixing is observed to be incomplete in all the cases, resulting in a thermally stratified flow with an oscillating stratification layer downstream of the T-junction. Mean temperature and root mean square (RMS) temperature fluctuations predicted by LES in the mixing region are found to be in good agreement with measurement data, with the exception of few positions. Amplitudes of temperature fluctuations are observed to be higher near the stratification layer, ranging from 6.3–9.9% of ∆T. Power spectral density (PSD) analyses of temperature fluctuations indicate no dominant frequency (spectral peak) under prevailing flow conditions, an important factor in thermal fatigue analysis, and the energy of these fluctuations are mainly contained in the frequency range of 0.1–2 Hz for all the investigated cases. LES is performed using the CFD software ANSYS CFX 14.0.