High-frequency dielectric properties of BSTO ceramic prepared with hydrothermal synthesized SrTiO3 and BaTiO3 powders

BSTO dielectric ceramic was prepared from SrTi03 and BaTi03 powders synthesized by hydrothermal method, as well as from Bao.sSro.4TiO3 powder synthesized by conventional solid-state reaction. The former can be sintered at a relatively low temperature of 1120 ℃. Characterization by SEM showed that the grain shapes of both ceramics are cubical, though the grain size of the former is much smaller. Dielectric constants measured at 20℃ were shown to vary with frequency in the range from I kHz to 2 MHz and dc bias field, and further that the dielectric loss of the former to be less than 2 × 10^- 3 in the frequency range of 20 kHz to 1 MHz, much smaller than that of the latter sample. For the former, temperature dependence of dielectric constant is much flatter and there exists an extended phase transition diffusion covering a wide temperature range of Curie temperature To. The smaller grain size of the former depresses the dc bias electrical field dependence of dielectric constant. The tunability is 7% under a bias field of 0.6 kV/mm dc.     

Preparation of SiO2/TiO2 Janus particles by electrostatic assembly,hydrolysis and calcination

This paper describes a novel chemical method for preparing SiO₂/TiO₂ Janus particles. First, polystyrene (PSt)/SiO₂ particles with a raspberry-like structure are prepared by electrostatic assembly. The influences of the reaction time of sulfonation and the treatment times of polyelectrolyte solutions (PDADMAC, PSS) on PSt and SiO₂ are investigated with respect to the surface charge density of the particles. SiO₂/TiO₂ Janus particles are then obtained by hydrolysis of butyl titanate on the surface of PSt/SiO₂ particles followed by a calcination process. Particle size analyzer, Zeta potential instrument, FTIR, TEM and SEM are used to characterize the particle size, the amount of charge on the surface of PSt and SiO₂ particles and the compositions and morphologies of PSt/SiO₂, SiO₂/TiO₂ and PSt/SiO₂/TiO₂. The diameters of the PSt, SiO₂, PSt/SiO₂ and SiO₂/TiO₂ particles are 2.0 μm, 303 nm, 2.7 μm and 330 nm, respectively.     

Highly dispersed Mn2O3 microspheres: Facile solvothermal synthesis and their application as Li-ion battery anodes

Nanostructured transition metal oxides are promising alternative anodes for lithium ion batteries. Li-ion storage performance is expected to improve if high packing density energy particles are available. Herein, Mn₂O₃ microspheres with a ca. 18 μm diameter and a tapped density of 1.33 g/cm³ were synthesized by a facile solvothermal–thermal coversion route. Spherical MnCO₃ precursors were obtained through solvothermal treatment and they decomposed and converted into Mn₂O₃ microspheres at an annealing temperature of 700 °C. The Mn₂O₃ microspheres consisted of Mn₂O₃ nanoparticles with an average 40 nm diameter. These porous Mn₂O₃ microspheres allow good electrolyte penetration and provide an ion buffer reservoir to ensure a constant electrolyte supply. The Mn₂O₃ microspheres have reversible capacities of 590 and 320 mAh/g at 50 and 400 mA/g, respectively. We thus report an efficient route for the fabrication of energy particles for advanced energy storage.     

Solid-state synthesis of Li[Li0.2Mn0.56Ni0.16Co0.08]O2 cathode materials for lithium-ion batteries

Layered Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode materials were synthesized via a solid-state reaction for Li-ion batteries, in which lithium hydroxide monohydrate, manganese dioxide, nickel monoxide, and cobalt monoxide were employed as metal precursors. To uncover the relationship between the structure and electrochemical properties of the materials, synthesis conditions such as calcination temperature and time as well as quenching methods were investigated. For the synthesized Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ materials, the metal components were found to be in the form of Mn⁴⁺, Ni²⁺, and Co³⁺, and their molar ratio was in good agreement with stoichiometric ratio of 0.56:0.16:0.08. Among them, the one synthesized at 800 °C for 12 h and subsequently quenched in air showed the best electrochemical performances, which had an initial discharge specific capacity and coulombic efficiency of 265.6 mAh/g and 84.0%, respectively, and when cycled at 0.5, 1, and 2 C, the corresponding discharge specific capacities were 237.3, 212.6, and 178.6 mAh/g, respectively. After recovered to 0.1 C rate, the discharge specific capacity became 259.5 mAh/g and the capacity loss was only 2.3% of the initial value at 0.1 C. This work suggests that the solid-state synthesis route is easy for preparing high performance Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode materials for Li-ion batteries.     

Rheology of SiO2/(acrylic polymer/epoxy) suspensions. I. Linear viscoelasticity

Linear viscoelastic properties of SiO₂/(AP/EP) suspension with various SiO₂ volume fractions (ϕ) in a blend of acrylic polymer (AP) and epoxy (EP) were investigated at various temperatures (T). The AP/EP contained 70 vol.% of EP. The SiO₂ particles were treated with epoxy silane coupling agent. The effects of the SiO₂ particles are more pronounced in the terminal zone: a transition from viscoelastic liquid (ϕ ≤ 30 vol.%) to viscoelastic solid (ϕ ≥ 40 vol.%) was observed which can be interpreted as a critical gelation occurring at a critical particle content and critical gel temperature. The SiO₂/(AP/EP) systems exhibited a critical gel behavior at ϕ ≅ 35 vol.% and T ≅ 100°C characterized with a power–law relationship between the storage and loss moduli (G ^′ and G ^″) and frequency (ω); G ^′ = G ^″/tan(nπ/2) ∝ ω ⁿ . The critical gel exponent (n) was estimated to be about 0.45. The gelation occurred with increasing T.     

Effects of variable viscosity and thermal conductivity of Al2O3–water nanofluid on heat transfer enhancement in natural convection

Heat transfer enhancement in horizontal annuli using variable properties of Al₂O₃–water nanofluid is investigated. Different viscosity and thermal conductivity models are used to evaluate heat transfer enhancement in the annulus. The base case uses the Chon et al. expression for conductivity and the Nguyen et al. experimental data for viscosity which take into account the dependence of these properties on temperature and nanoparticle volume fraction. It was observed that for Ra  10⁴, the average Nusselt number was reduced by increasing the volume fraction of nanoparticles. However, for Ra = 10³, the average Nusselt number increased by increasing the volume fraction of nanoparticles. For Ra  10⁴, the Nusselt number was deteriorated every where around the cylinder surface especially at high expansion ratio. However, this reduction is only restricted to certain regions around the cylinder surface at Ra = 10³. For Ra  10⁴, the difference in Nusselt number between the Maxwell Garnett and Chon et al. model prediction is small. But, there was a deviation in prediction at Ra = 10³ and this deviation becomes more significant at high volume fraction of nanoparticles. The Nguyen et al. data and Brinkman model gives completely different predictions for Ra  10⁴ where the difference in prediction of Nusselt number reached 30%. However, this difference was less than 10% at Ra = 10³.     

不同配副下ZrB2-SiC复相陶瓷的摩擦学性能

以微米级ZrB_2和SiC粉末为原料,采用热压烧结制备ZrB_2-SiC复相陶瓷,考察了SiC含量,摩擦对偶,速度和载荷对ZrB_2-SiC复相陶瓷摩擦磨损特性的影响.结果表明:ZrB_2-SiC复相陶瓷的摩擦系数和磨损率对SiC含量和摩擦对偶的变化较为敏感,速度和载荷变化,摩擦系数和磨损率的波动较大;以WC为对偶,速度0.1 m/s,载荷5 N时的ZrB_2-SiC复相陶瓷的平均摩擦系数和磨损率分别仅为0.4和2.41×10–4 mm3/(N·m).ZrB_2-SiC复相陶瓷的磨损机制以机械磨损为主,伴有轻微摩擦氧化,摩擦层的形成有利于摩擦系数的减小.     

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.     

Experimental X-ray characterization of Gekko-XII laser propagation through very low-density aerogels (2–5 mg/cc) creating multi-keV photons from a titanium solid foil

This work describes measurements of laser propagation through very low-density aerogels and subsequent multi-keV photon production from titanium foils. For efficient foil heating, SiO₂ aerogel with densities of 2 and 5 mg/cm³ have been cast into a plastic cylinder, which are then mounted to Ti foils that are 3–20 μm thick. Experiments have been performed on the GEKKO-XII laser facility to characterize laser propagation through the aerogel and X-ray production from the Ti foil. Multi-keV emission is diagnosed with a full set of diagnostics giving laser-to-X-ray conversion efficiencies, time-dependent X-ray power and two-dimensional X-ray imaging.     

Microfluidic platform serves as controllable fabrication of binary Mo/Ir nanodots/carbon hetero-material for efficient electrocatalytic nitrogen reduction

Electrocatalytic nitrogen reduction reaction (NRR) is regarded as a potential routine to achieve environment-friendly ammonia production, because of its abundant nitrogen resources, clean energy utilization and flexible operation. However, it is hindered by low activity and selectivity, in which condition well-designed catalysts are urgently in need. In this work, a binary Mo/Ir nanodots/carbon (Mo/Ir/C) hetero-material is efficiently constructed via microfluidic strategy, of which the nanodots are homogeneously distributed on the carbon skeleton and the average size is approximately 1 nm. Excellent performance for NRR is obtained in 1 mol L⁻¹ KOH, of which the optimized ammonia yield and faradic efficiency are 7.27 μg h⁻¹ cm⁻² and 2.31% respectively. Moreover, the optimized ammonia yield of 6.20 μg h⁻¹ cm⁻² and faradic efficiency of 10.59% are also obtained in 0.005 mol L⁻¹ H₂SO₄. This work achieves the continuous-flow synthesis and controllable adjustment of hetero-materials for favorable morphologies, which provides an innovative pathway for catalyst design and further promotes the development of ammonia production field.     

SiO2-coated pure anatase TiO2 catalysts for enhanced photo-oxidation of naphthalene and anthracene

Our current efforts reveal the preparation of SiO₂@TiO₂ nanocomposites having different thicknesses of silica shell and the relationship to photocatalytic activity (PCA) for the photo-oxidation of naphthalene and anthracene. The presence of SiO₂ coating over TiO₂ surface was demonstrated by FT-IR analysis, with peaks corresponding to SiOSi (1081 cm⁻¹) and SiOTi (950 cm⁻¹) bonds observed. High-resolution transmission electron microscopy analysis confirmed the presence of SiO₂ in the as-prepared nanocomposites and the amount of Si, Ti, and O was determined by energy dispersive X-ray spectroscopy analysis. Increasing the SiO₂ shell thickness increases the surface area of the nanocomposites (69–235 m²/g), which enhances naphthalene/anthracene adsorption. However, the observed PCA trend presents an inverse correlation to the adsorption studies, where the as-prepared samples possessing the highest surface areas exhibited the least PCA, while catalysts having lower surface areas (among silica coated samples) displayed the highest PCA in the degradation of naphthalene and anthracene to CO₂. Despite complete degradation of naphthalene and anthracene, incomplete mineralization occurred, ascribed to the formation of various intermediates, identified by GC–MS analysis.     

Effect of high stirring speed on the agglomerate behaviors for cohesive SiO2 powders in gas fluidization

The effects of superficial gas velocity and mechanical stirring speed on the precise regulation of flow regimes for cohesive SiO₂ powders (mean diameter is 16 μm) were experimentally investigated in a stirring-assisted fluidized bed. The results showed that compared with the agglomerates formed in the non-assisted fluidization of cohesive SiO₂ powders, the introduction of mechanical stirring could effectively reduce the size of agglomerates and well disperse the agglomerates during fluidization. The best regulation range of agglomerate particulate fluidization can be achieved at 600 rpm when agglomerate sizes were reduced to below 200 μm. Further investigation based on the operational phase diagram revealed that transformations of flow regimes were dominated by both stirring speed and gas velocity. The stirring applied enlarges the operational range of agglomerate particulate fluidization (APF) with a delayed onset of bubbling for cohesive particles. However, the exorbitant speed increases the collision velocity and contact area between small agglomerates, which results in the formation of unstable agglomerates and the whirlpool of powder.     

Chemical characteristics of PM2.5 during dust storms and air pollution events in Chengdu,China

Daily fine particulate (PM_2.5) samples were collected in Chengdu from April 2009 to February 2010 to investigate their chemical profiles during dust storms (DSs) and several types of pollution events, including haze (HDs), biomass burning (BBs), and fireworks displays (FDs). The highest PM_2.5 mass concentrations were found during DSs (283.3 μg/m³), followed by FDs (212.7 μg/m³), HDs (187.3 μg/m³), and BBs (130.1 μg/m³). The concentrations of most elements were elevated during DSs and pollution events, except for BBs. Secondary inorganic ions (NO₃^?, SO₄^2?, and NH₄⁺) were enriched during HDs, while PM_2.5 from BBs showed high K⁺ but low SO₄^2?. FDs caused increases in K⁺ and enrichment in SO₄^2?. Ca²⁺ was abundant in DS samples. Ion-balance calculations indicated that PM_2.5 from HDs and FDs was more acidic than on normal days, but DS and BB particles were alkaline. The highest organic carbon (OC) concentration was 26.1 μg/m³ during FDs, followed by BBs (23.6 μg/m³), HDs (19.6 μg/m³), and DSs (18.8 μg/m³). In contrast, elemental carbon (EC) concentration was more abundant during HDs (10.6 μg/m³) and FDs (9.5 μg/m³) than during BBs (6.2 μg/m³) and DSs (6.0 μg/m³). The highest OC/EC ratios were obtained during BBs, with the lowest during HDs. SO₄^2?/K⁺ and TCA/SO₄^2? ratios proved to be effective indicators for differentiating pollution events. Mass balance showed that organic matter, SO₄^2?, and NO₃^? were the dominant chemical components during pollution events, while soil dust was dominant during DSs.     

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.     

Preparation of modified MFI (ZSM-5 and silicalite-1) zeolites for potassium extraction from seawater

Modified nanosized MFI (ZSM-5 and silicalite-1) zeolites were successfully synthesized by a hydrothermal method using aluminum isopropoxide and tetraethylorthosilicate as the raw materials. The synthetic zeolites were characterized by X-ray diffraction, energy dispersion spectroscopy, and scanning electron microscopy. The ZSM-5 and silicalite-1 zeolites exhibited ellipse-like and cubic columns, respectively. The K⁺ ion-exchange equilibrium and ion-exchange capacity of the synthetic zeolites in seawater were investigated. The K⁺ ion-exchange of synthetic zeolites was rapid and reached an ion-exchange balance in approximately 20 min. The K⁺ ion-exchange capacity of ZSM-5 and silicalite-1 in seawater was 56.7 and 48.7 mg/g, respectively. The synthetic zeolites have high selectivity toward K⁺, and therefore, they can be used to selectively extract potassium from seawater.     

Evaporation heat transfer coefficient in single circular small tubes for flow natural refrigerants of C3H8, NH3, and CO2

An experimental study of evaporation heat transfer coefficients for single circular small tubes was conducted for the flow of C₃H₈, NH₃, and CO₂ under various flow conditions. The test matrix encompasses the entire quality range from 0.0 to 1.0, mass fluxes from 50 to 600 kg m⁻² s⁻¹, heat fluxes from 5 to 70 kW m⁻², and saturation temperatures from 0 to 10 °C. The test section was made of circular stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 2000 mm in a horizontal orientation. The test section was uniformly heated by applying electric power directly to the tubes. The effects of mass flux, heat flux, saturation temperature, and inner tube diameter on the heat transfer coefficient are reported. Among the working refrigerants considered in this study, CO₂ has the highest heat transfer coefficient. Laminar flow was observed in the evaporative small tubes, and was considered in the modification of boiling heat transfer coefficients and pressure drop correlations.     

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.     

Preparation of nano-sized Al2O3–2SiO2 powder by sol–gel plus azeotropic distillation method

Nano-sized amorphous Al₂O₃–2SiO₂ powder was prepared by a sol–gel method coupled with azeotropic distillation. The structure of the powder was investigated by DTS, BET, TEM, FT-IR, TG-DTA and XRD, showing that n-butanol azeotropic distillation could effectively remove water from the aluminosilicate gels and prevent the formation of hard agglomerates in the drying process. The average particle diameter of the powder was about 70 nm. The largest BET specific surface area of the powder was 669 m²/g. To examine the alkali-activation reactivity of the powder, alkali-activation tests were performed with the powder reacting with sodium silicate solution. The synthetic powder was found to be highly reactive.     

Observation and analysis of near-surface atmospheric aerosol optical properties in urban Beijing

Year-round measurements of the mass concentration and optical properties of fine aerosols (PM_2.5) from June 2009 to May 2010 at an urban site in Beijing were analyzed. The annual mean values of the PM_2.5 mass concentration, absorption coefficient (Ab), scattering coefficient (Sc) and single scattering albedo (SSA) at 525 nm were 67 ± 66 μg/m³, 64 ± 62 Mm⁻¹, 360 ± 405 Mm⁻¹ and 0.82 ± 0.09, respectively. The bulk mass absorption efficiency and scattering efficiency of the PM_2.5 at 525 nm were 0.78 m²/g and 5.55 m²/g, respectively. The Ab and Sc showed a similar diurnal variation with a maximum at night and a minimum in the afternoon, whereas SSA displayed an opposite diurnal pattern. Significant increases in the Ab and Sc were observed in pollution episodes caused by the accumulation of pollutants from both local and regional sources under unfavorable weather conditions. Aerosol loadings in dust events increased by several times in the spring, which had limited effects on the Ab and Sc due to the low absorption and scattering efficiency of dust particles. The frequency of haze days was the highest in autumn because of the high aerosol absorption and scattering under unfavorable weather conditions. The daily PM_2.5 concentration should be controlled to a level lower than 64 μg/m³ to prevent the occurrence of haze days according to its exponentially decreased relationship with visibility.     

Light attenuation cross-section of black carbon in an urban atmosphere in northern China

Fine particulate matter (PM_2.5) samples were collected over two years in Xi’an, China to investigate the relationships between the aerosol composition and the light absorption efficiency of black carbon (BC). Real-time light attenuation of BC at 880 nm was measured with an aethalometer. The mass concentrations and elemental carbon (EC) contents of PM_2.5 were obtained, and light attenuation cross-sections (σ_ATN) of PM_2.5 BC were derived. The mass of EC contributed ∼5% to PM_2.5 on average. BC σ_ATN exhibited pronounced seasonal variability with values averaging 18.6, 24.2, 16.4, and 26.0 m²/g for the spring, summer, autumn, and winter, respectively, while averaging 23.0 m²/g overall. σ_ATN varied inversely with the ratios of EC/PM_2.5, EC/[SO₄²⁻], and EC/[NO₃⁻]. This study of the variability in σ_ATN illustrates the complexity of the interactions among the aerosol constituents in northern China and documents certain effects of the high EC, dust, sulfate and nitrate loadings on light attenuation.