Magnetic and electrochemical behavior of rhombohedral α-Fe2O3 nanoparticles with (1 0 4) dominant facets

Uniform rhombohedral α-Fe₂O₃ nanoparticles, ～60 nm in size, were synthesized via a triphenylphosphine-assisted hydrothermal method. Scanning electron micrograph (SEM) and transmission electron micrograph (TEM) analyses showed that the as-synthesized rhombohedral nanoparticles were enclosed by six (1 0 4) planes. The concentration of triphenylphosphine played an important role in morphological evolution of the α-Fe₂O₃ nanoparticles. The as-prepared rhombohedral nanoparticles possessed remanent magnetization M_r of 2.6 × 10^?3 emu/g and coercivity H_C of 2.05 Oe, both lower than those of other α-Fe₂O₃ particles with similar size, indicating their potential applications as superparamagnetic precursor materials. Furthermore, these rhombohedral α-Fe₂O₃ nanoparticles exhibited good sensor capability toward H₂O₂ with a linear response in the concentration range of 2–20 mM.     

Fabrication of nano-sized Ca(OH)2 with excellent adsorption ability for N2O4

Uniform nano-sized calcium hydroxide (Ca(OH)₂) monocrystal powder was synthesized from calcium oxide in a surfactant solution via a digestion method by decreasing the surface tension of the reaction system to control the growth of crystalline Ca(OH)₂. The Ca(OH)₂ monocrystal powder samples were characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), and Fourier transform-infrared spectroscopy (FT-IR). The NO_x adsorption ability of the samples was evaluated, and the influence of various types and concentrations of surfactants on powder agglomeration and then the specific surface area in the precipitation process were studied. The specific surface area of the samples was found as high as 58 m²/g and 92 m²/g and the particle size, 300–400 nm and 200–300 nm in the presence of 10 wt% PEG600 and 0.086 mL/L SDS at a reaction time of 5 h, respectively. The product has an exceptionally strong adsorption ability for NO_x, which makes it a highly promising adsorbent for emission control and air purification.     

Mixed-solvent thermal synthesis and magnetic properties of flower-like microstructured nickel

Flower-like microstructured nickel was synthesized by a facile mixed-solvent thermal process. The structure, morphology, and magnetic properties of the reaction products were investigated, respectively, by X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). The results showed that the products consisted of a face-centered cubic (fcc) structure with lattice constant of α = 3.524 Å. The average diameter of flower-like microstructured nickel was about 5 μm and the thickness of a single flake was about 100 nm. Magnetic measurement showed that these powders exhibited ferromagnetic characteristics.     

Preparation and gas sensing performances of palladium surface-modified flower-like SnO2 nanopowders

Flower-like SnO2 nanopowders prepared by a hydrothermal method were surface modified with palla- dium via impregnation. The crystal structure, morphology, and surface chemistry states of the samples were characterized by means of X-ray diffraction （XRD）, scanning electron microscopy （SEM）, and X-ray photoelectron spectroscopy （XPS）, respectively. The gas sensing performances were also investigated. For a hydrothermal temperature of 220 ℃, flower-like SnO2 nanoparticles consist of nanorods with diameters of 40 nm and lengths of 100 nm. The XPS and XRD results reveal that palladium exists in the Pd0 chemical state but the crystal is too small to be detected. The 0.3 wt% Pd modified SnO2 sensor shows better sensi- tivity, up to 21, for 70 μL/L ethanol gas at an optimal working temperature of 250 ℃. The quick response time （3 s） and fast recovery time （-20 s） are the main characteristics of this sensor.     

Synthesis of potassium sodium niobate powders using an EDTA/citrate complexing sol–gel method

Potassium sodium niobate (KNN) powders were synthesized by a modified sol–gel method, using as starting chemicals potassium carbonate, sodium carbonate, and niobium hydroxide, and, as esterification and chelating agents, respectively, ethylene glycol (EG) and ethylene diamine tetraacetic acid (EDTA)/citrate. The effects of citric acid (CA), EG, and EDTA on the stability of the precursor sol were systemically investigated. The powders and gels were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis-differential scanning calorimetry (TGA-DSC). The results indicated that a stable precursor sol was formed when n(CA):n(Mⁿ⁺) = 3:1, n(EDTA):n(NH₄OH) = 1:3.5, and n(CA):n(EG) = 1:2. The xerogel was calcined at 500–950 °C to prepare the KNN powder. Pure KNN perovskite phase with a cube-like structure was synthesized at 850 °C from the precursor sol for a K/Na molar ratio of 1.2. The formation mechanism of the KNN perovskite phase was also discussed.     

Emission of organic carbon,elemental carbon and water-soluble ions from crop straw burning under flaming and smoldering conditions

Emissions from major agricultural residues were measured using a self-designed combustion system. Emission factors (EFs) of organic carbon (OC), elemental carbon (EC), and water-soluble ions (WSIs) (K⁺, NH₄⁺, Na⁺, Mg²⁺, Ca²⁺, Cl⁻, NO₃⁻, SO₄^2–) in smoke from wheat and rice straw were measured under flaming and smoldering conditions. The OC₁/TC (total carbon) was highest (45.8% flaming, 57.7% smoldering) among carbon fractions. The mean EFs for OC (EF_OC) and EC (EF_EC) were 9.2 ± 3.9 and 2.2 ± 0.7 g/kg for wheat straw and 6.4 ± 1.9 and 1.1 ± 0.3 g/kg for rice straw under flaming conditions, while they were 40.8 ± 5.6 and 5.8 ± 1.0 g/kg and 37.6 ± 6.3 and 5.0 ± 1.4 g/kg under smoldering conditions, respectively. Higher EC ratios were observed in particulate matter (PM) mass under flaming conditions. The OC and EC for the two combustion patterns were significantly correlated (p < 0.01, R = 0.95 for wheat straw; p < 0.01, R = 0.97 for rice straw), and a higher positive correlation between OC₃ and EC was observed under both combustion conditions. WSIs emitted from flaming smoke were dominated by Cl⁻ and K⁺, which contributed 3.4% and 2.4% of the PM mass for rice straw and 2.2% and 1.0% for wheat straw, respectively. The EFs of Cl⁻ and K⁺ were 0.73 ± 0.16 and 0.51 ± 0.14 g/kg for wheat straw and 0.25 ± 0.15 and 0.12 ± 0.05 g/kg for rice straw under flaming conditions, while they were 0.42 ± 0.28 and 0.12 ± 0.06 g/kg and 0.30 ± 0.27 and 0.05 ± 0.03 g/kg under smoldering conditions, respectively. Na⁺, Mg²⁺, and NH₄⁺ were vital components in PM, comprising from 0.8% (smoldering) to 3.1% (flaming) of the mass. Strong correlations of Cl⁻ with K⁺, NH₄⁺, and Na⁺ ions were observed in rice straw and the calculated diagnostic ratios of OC/EC, K⁺/Na⁺ and Cl⁻/Na⁺ could be useful to distinguishing crop straw burning from other sources of atmospheric pollution.     

Experimental investigation on flow asymmetry in solid entrance region of a square circulating fluidized bed

To study the influence of back feeding particles on gas-solid flow in the riser, this paper investigated the flow asymmetry in the solid entrance region of a fluidized bed by particle concentration/velocity measurements in a cold square circulating fluidized beds （CFB）. The pressure drop distribution along the riser and the saturation carrying capacity of gas for Geldart-B type particles were first analyzed. Under the condition of u0 = 4 m/s and Gs = 21 kg/（m^2 s）, the back feeding particles were found to penetrate the lean gas-solid flow near the entrance （rear） wall before reaching the opposite （front） wall, thus leading to a relatively denser region near the front wall in the bottom bed. Higher solid circulation rate （u0 =4 m/s, Gs = 33 kg/（m^2 s）） resulted in a higher particle concentration in the riser. However the back feeding particles with higher momentum increased the asymmetry of the particle concentration/velocity profile in the solid entrance region. Lower air velocity （u0 =3.2 m/s） and Gs =21 kg/（m2 s）, beyond the saturation carrying capacity of gas, induced an S-shaped axial solid distribution with a denser bottom zone. This limited the penetration of the back feeding particles and forced the flnidizing air to flow in the central region, thus leading to a higher solid holdup near the rear wall. Under the conditions of uo = 4 m/s and Gs = 21 kg/（m^2 s）, addition of coarse particles （dp= 1145 μm） into the bed made the radial distribution of solids more symmetrical.     

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.     

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.     

Real-time measurements of PM2.5, PM10–2.5, and BC in an urban street canyon

A continuous dichotomous beta gauge monitor was used to characterize the hourly content of PM_2.5, PM_10–2.5, and Black Carbon (BC) over a 12-month period in an urban street canyon of Hong Kong. Hourly vehicle counts for nine vehicle classes and meteorological data were also recorded. The average weekly cycles of PM_2.5, PM_10–2.5, and BC suggested that all species are related to traffic, with high concentrations on workdays and low concentrations over the weekends. PM_2.5 exhibited two comparable concentrations at 10:00–11:00 (63.4 μg/m³) and 17:00–18:00 (65.0 μg/m³) local time (LT) during workdays, corresponding to the hours when the numbers of diesel-fueled and gasoline-fueled vehicles were at their maximum levels: 3179 and 2907 h⁻¹, respectively. BC is emitted mainly by diesel-fueled vehicles and this showed the highest concentration (31.2 μg/m³) during the midday period (10:00–11:00 LT) on workdays. A poor correlation was found between PM_2.5 concentration and wind speed (R = 0.51, P-value > 0.001). In contrast, the concentration of PM_10–2.5 was found to depend upon wind speed and it increased with obvious statistical significance as wind speed increased (R = 0.98, P-value < 0.0001).     

Synthesis of layered cathode materials Li[CoxNiyMn1−x−y]O2 from layered double hydroxide precursors

Cathode materials Li[Co_xNi_yMn_1?x?y]O₂ for lithium secondary batteries have been prepared by a new route using layered double hydroxides (LDHs) as a precursor. The resulting layered phase with the α-NaFeO₂ structure crystallizes in the rhombohedral system, with space group R-3m having an interlayer spacing close to 0.47 nm. X-ray photoelectron spectroscopy (XPS) was used to measure the oxidation states of Co, Ni and Mn. The effects of varying the Co/Ni/Mn ratio on both the structure and electrochemical properties of Li[Co_xNi_yMn_1?x?y]O₂ have been investigated by X-ray diffraction and electrochemical tests. The products demonstrated a rather stable cycling behavior, with a reversible capacity of 118 mAh/g for the layered material with Co/Ni/Mn = 1/1/1.     

Hydrothermal synthesis of Mn-Zn ferrites from spent alkaline Zn-Mn batteries

Nanocrystalline Mn-Zn ferrites （Mno.GZno.4Fe204） with particle size of 12 nm were synthesized hydrotherreally using spent alkaline Zn-Mn batteries, and accompanied by a study of the influencing factors. The nanocrystals were examined by powder X-ray diffraction （XRD） for crystalline phase identification, and scanning electron microscopy （SEM） for grain morphology. The relationship between concentration of Fe（II）, Mn（II）, and Zn（II） and pH value was obtained through thermodynamic analysis of the Fe（II）-Mn（II）-Zn（II）-NaOH-H2O system. The results showed that all ions were precipitated completely at a pH value of 10-11. The optimal preparation conditions are： co-precipitation pH of 10.5, temperature of 200 ℃ and time of 9 h.     

Hydrodynamic forces on a rotating sphere

The wake dynamics of a rotating sphere with prescribed rotation axis angles are quantitatively analysed by carrying out numerical simulations at Reynolds numbers of Re = 100, 250 and 300, non-dimensional rotational rates Ω¹ = 0–1 and rotation axis angles α = 0, π/6, π/3 and π/2 measured from the free stream axis. These parameters are the same as those in an earlier study (Poon et al., 2010, Int. J. Heat Fluid Flow) where the instantaneous flow structures were discussed qualitatively. This study extends the findings of the earlier study by employing phase diagrams (C_Lx, C_Ly) and (C_D, C_L) to provide a quantitative analysis of the time-dependent behaviour of the flow structures. At Re = 300 and Ω¹ = 0.05, the phase diagrams (C_Lx, C_Ly) show ‘saw tooth’ patterns for both α = 0 and π/6. The ‘saw tooth’ pattern indicates that the flow structures comprise a higher frequency oscillation component at a Reynolds number of 300 which is not observed until Re ≈ 800 for a stationary sphere. This ‘saw tooth’ pattern disappears as Ω¹ increases. The employment of the phase diagrams also reveals that different flow structures induce different oscillation amplitudes on both lateral force coefficients. With the exception of the vortices formed from a shear layer instability, all other flow regimes show larger fluctuations in C_L than C_D.     

Hydrothermal synthesis of ZnO whiskers from ɛ-Zn(OH)2 in NaOH/Na2SO4 solution

The influence of Na₂SO₄ on the formation of ZnO whiskers was investigated in this paper. ZnO whiskers with aspect ratios of up to 50 were synthesized by dissolving ɛ-Zn(OH)₂ precursor in NaOH/Na₂SO₄ solution at room temperature, followed by aging of the resulting solution at 140 °C for 6 h. Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy analyses revealed that SO₄²⁻ ions were primarily adsorbed on the (1 0 0) plane of the ZnO whiskers via an outer-sphere complex configuration (OH···O), thereby promoting the one-dimensional growth of ZnO whiskers along the c-axis.     

Flow boiling heat transfer of R134a,R236fa and R245fa in a horizontal 1.030 mm circular channel

This research focuses on acquiring accurate flow boiling heat transfer data and flow pattern visualization for three refrigerants, R134a, R236fa and R245fa in a 1.030 mm channel. We investigate trends in the data, and their possible mechanisms, for mass fluxes from 200 to 1600 kg/m²s, heat fluxes from 2.3 kW/m² to 250 kW/m² at T_sat = 31 °C and ΔT_sub from 2 to 9 K. The local saturated flow boiling heat transfer coefficients display a heat flux and a mass flux dependency but no residual subcooling influence. The changes in heat transfer trends correspond well with flow regime transitions. These were segregated into the isolated bubble (IB) regime, the coalescing bubble (CB) regime, and the annular (A) regime for the three fluids. The importance of nucleate boiling and forced convection in these small channels is still relatively unclear and requires further research.     

Atomistic simulation of crystallization of a polyethylene melt in steady uniaxial extension

We present simulation results of flow-induced crystallization of a dense polymeric liquid subjected to a strong uniaxial elongational flow using a rigorous nonequilibrium Monte Carlo method. A distinct transition between the liquid and the crystalline phases occurred at critical values of flow strength, with an abrupt, discontinuous transition of the overall chain conformation. The flow-induced crystalline phase matched quantitatively the experimental X-ray diffraction data of the real crystals remarkably well, including the sharp Bragg peaks at small wavenumbers, k < 1.5 Å^?1, indicating the existence of a global long-range ordering. We also found that the enthalpy change (ΔH = 225 J/g) during the phase transition was quantitatively very similar to the experimental heat of fusion (276 J/g) of polyethylene crystals under quiescent conditions. Furthermore, a detailed analysis of the configuration-based temperature provided a sound microscopic physical origin for the effective enhancement of the crystallization (or melting) temperature that has been observed in experiments. Simulation results also allow for the deduction of potential nonequilibrium expressions for thermodynamic quantities, such as temperature and heat capacity.     

Molecular distribution and seasonal variation of hydrocarbons in PM2.5 from Beijing during 2006

Normal (n)-alkanes and polycyclic aromatic hydrocarbons (PAHs) in PM_2.5 were collected from Beijing in 2006 and analyzed using a thermal desorption-GC/MS technique. Annual average concentrations of n-alkanes and PAHs were 282 ± 96 and 125 ± 150 ng/m³, respectively: both were highest in winter and lowest in summer. C₁₉–C₂₅ compounds dominated the n-alkanes while benzo[b]fluoranthene, benzo[e]pyrene, and phenanthrene were the most abundant PAHs. The n-alkanes exhibited moderate correlations with organic carbon (OC) and elemental carbon (EC) throughout the year, but the relationships between the PAHs, OC and EC differed between the heating and non-heating seasons. The health risks associated with PAHs in winter were more than 40 times those in spring and summer even though the PM_2.5 loadings were comparable. Carbon preference index values (<1.5) indicated that the n-alkanes were mostly from fossil fuel combustion. The ratios of indeno[123-cd]pyrene to benzo[ghi]pyrelene in summer and spring were 0.58 ± 0.12 and 0.63 ± 0.09, respectively, suggesting that the PAHs mainly originated from motor vehicles, but higher ratios in winter reflected an increased influence from coal, which is extensively burned for domestic heating. A comprehensive comparison showed that PAH pollution in Beijing has decreased in the past 10 years.     

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.     

Drag reduction for liquid flow through micro-apertures

Pressure drops in the flow through micro-orifices and capillaries were measured for silicone oils, aqueous solutions of polyethylene glycol (PEG), and surfactant aqueous solutions. The diameter of micro-orifices ranged from 5 μm to 400 μm. The corresponding length/diameter ratio was from 4 to 0.05 and capillary diameters were 105 μm and 450 μm. The following results were obtained: silicone oils of 10^?6 m²/s and 10^?5 m²/s in kinematic viscosity generated a reduction of pressure drop (RPD), that is, drag reduction, similar to the RPD of water and a glycerol/water mixture reported in the previous paper by the present authors. When RPD occurred, the pressure drop (PD) of silicone oils of 10^?6 m²/s and 10^?5 m²/s had nearly the same magnitude. Namely, the difference in viscosity did not influence RPD. A 10³ ppm aqueous solution of PEG20000 provided almost the same PD as that of PEG8000 for the 400 μm to 15 μm orifices, but a greater PD than that of PEG8000 for the 10 μm to 5 μm orifices. A non-ionic surfactant and a cationic surfactant were highly effective in RPD compared with anionic surfactants: the non-ionic and cationic surfactant solutions had PD one order of magnitude lower than that of water under some flow conditions in the concentration range from 1 ppm to 10⁴ ppm, but the anionic surfactant solutions did not generate RPD except in the case of the smallest orifice of 5 μm in diameter. The PD of the non-ionic surfactant solution showed a steep rise at a Reynolds number (Re_t) for 400 μm to 15 μm orifices. The Re_t provides the relationship Re_t = K/D, where D is the orifice diameter, and K is a constant of 2 × 10^?2 m for the 100–20 μm orifices irrespective of liquid concentration. Capillary flow experiment revealed that the PEG, non-ionic and cationic surfactant solutions generated RPD also in a laminar flow through the capillary of 105 μm in diameter, but not in the flow through the capillary of 450 μm in diameter. In order to clarify the cause of RPD, an additional experiment was carried out by changing the orifice material from metal to acrylic resin. The result gave a different appearance of RPD, suggesting that RPD is related to an interfacial phenomenon between the liquid and wall. The large RPDs found in the present experiment are very interesting from both academic and practical viewpoints.     

Mass and isotopic concentrations of water-insoluble refractory carbon in total suspended particulates at Mt. Waliguan Observatory (China)

Mass concentration and isotopic values δ¹³C and ¹⁴C are presented for the water-insoluble refractory carbon (WIRC) component of total suspended particulates (TSP), collected weekly during 2003, as well as from October 2005 to May 2006 at the WMO-GAW Mt. Waliguan (WLG) site. The overall average WIRC mass concentration was (1183 ± 120) ng/m³ (n = 79), while seasonal averages were 2081 ± 1707 (spring), 454 ± 205 (summer), 650 ± 411 (autumn), and 1019 ± 703 (winter) ng/m³. Seasonal variations in WIRC mass concentrations were consistent with black carbon measurements from an aethalometer, although WIRC concentrations were typically higher, especially in winter and spring. The δ¹³C PDB value (−25.3 ± 0.8)‰ determined for WIRC suggests that its sources are C₃ biomass or fossil fuel combustion. No seasonal change in δ¹³C PDB was evident. The average percent Modern Carbon (pMC) for ¹⁴C in WIRC for winter and spring was (67.2 ± 7.7)% (n = 29). Lower pMC values were associated with air masses transported from the area east of WLG, while higher pMC values were associated with air masses from the Tibetan Plateau, southwest of WLG. Elevated pMC values with abnormally high mass concentrations of TSP and WIRC were measured during a dust storm event.