Diffusion in nanocrystalline material

The paper reports on first investigations of the diffusion in nanocrystalline materials. The self-diffusion of the radioisotope ⁶⁷Cu in nanocrystalline copper has been measured by serial sectioning with the aid of ion-beam sputtering. The values of the diffusion coefficients which were found at 353 K and 393 K are 2×10⁻¹⁸ m²/s and 1.7×10⁻¹⁷ m²/s, respectively, i.e., they are about 16 or 14 orders of magnitude larger than the bulk self-diffusion and about 3 orders of magnitude larger than the grain-boundary self-diffusion in copper. In comparison to the bulk, small values for the activation enthalpy, 0.64 eV, and the pre-exponential factor of the self-diffusion coefficient, 3×10⁻⁹ m²/s, have been observed.     

Density Modulation Experiments on HT-7 Tokamak

Density modulation experiments are successfully conducted on HT-7 ohmic discharge to investigate particle transport coefficients: diffusion coefficients D and convection velocity V. The particle transport is studied at low (1.5×10¹⁹m^-3) and high (3×10¹⁹ m^-3) density regimes. The clear differences are observed that D is 0.42m²/s and 0.17m²/s, V is 4.7m/s (outward) and 1.6 m/s (inward) for low and high density plasmas respectively, where spatially constant D and V(r) = (r/a)V₀ were assumed for the analysis.     

Synergetic effects of ultrasound and sodium alginate coating on mass transfer and qualities of osmotic dehydrated pumpkin

This research studied the effects of combined ultrasound and 3% sodium alginate (SA) coating pretreatment (US + Coat) on mass transfer kinetics, quality aspects, and cell structure of osmotic dehydrated (OD) pumpkin. The results of the pretreatment were compared with the results of control (non-pretreated osmotic dehydration) and other three pretreatment methods, which were 1) ultrasound in distilled water for 10 min (USC), 2) ultrasound in 70% (w/w) sucrose solution (US) for 10, 20 and 30 min, and 3) coating with 1%, 2%, 3% (w/w) SA. The coating pretreatments with SA resulted in a higher water loss (WL) but lower water activity and solid gain (SG) than other treatments. US pretreatments resulted in the highest effective diffusion coefficients of water (D_w) and solid (D_s) but the cell structure of the product was deformed. The 3% SA coating treatment had the highest WL/SG (5.28) but with the longest OD time (12 h). Using the US + Coat pretreatment gave satisfactory high WL/SG (5.18), D_w (1.09 × 10⁻¹⁰ m²s⁻¹) and D_s (5.15 × 10⁻¹¹ m²s⁻¹), reduced the OD time to 9 h, and preserved the cell structure of the product. This research suggests that US + Coat pretreatment can be an effective processing step in the production of OD pumpkin.     

Diffusion of sodium,potassium, calcium,manganese, and radon in tuff and clinoptilolite under leaching

Nuclear physics methods are used to determine the diffusion coefficients of Na, Ca, Mn, K, and ²²²Rn in clinoptilolite (Sokirnitsa occurrence, Ukraine) and in natural tuff (Yucca Mountain, Nevada, United States) and in tuff irradiated by γ-quanta (E _max = 23 MeV) to a dose of 10⁷ Gy at a leaching temperature of 37°C. The diffusion coefficients of sodium and potassium in clinoptilolite are found to differ considerably: 4 × 10⁻¹⁷ and 2 × 10⁻²⁰ m²/s, respectively. This indicates the influence of aquacomplexes on the cation transfer. The diffusion coefficient of radon in these materials is determined: in clinoptilolite it equals 2.5 × 10⁻¹² m²/s.     

Flow boiling heat transfer of refrigerant R21 in microchannel heat sink

Boiling heat transfer in a refrigerant R 21 flow in a microchannel heat sink is studied. A stainless steel heat sink with a length of 120 mm contains ten microchannels with a size of 640×2050 μm at cross-section with a wall roughness of 10 μm. The local heat-transfer coefficient distribution along the heat sink length is obtained. The ranges of parameters are: mass flow from 68 to 172 kg/m²s, heat fluxes from 16 to 152 kW/m², and vapor quality from 0 to 1. The maximum values of the heat transfer coefficient are observed at the inlet of microchannels. The heat transfer coefficients decrease substantially along the length of channels under high heat flux conditions and, on the contrary, change insignificantly under low heat flux condition. A comparison with the well-known models of flow boiling heat transfer is performed and the range of applicability is defined.     

Diffusivity of a two-dimensional lattice fluid: CH4 adsorbed on MgO(100)

The diffusive motions of a 0.8 layer of CH₄ adsorbed on MgO(100) are measured at 72, 88 and 97 K by quasi-elastic neutron scattering. It is shown that at 72 K the methane film is solid and its molecules perform an isotropic rotational motion. At 88 and 97 K, the adsorbed layer is in a two-dimensional fluid state in which the molecules jump between equidistant (4.21 Å) lattice sites of the MgO surface. The mean residence time has been determined ( ∼ 1 × 10 ⁻¹⁰ s at 88 K and ∼ 4 × 10⁻¹¹ s at 97 K). The corresponding translational diffusion coefficients are ∼ 5 × 10⁻⁶ cm² s⁻¹ at 88 K and 12 × 10⁻⁶ cm² s⁻¹ at 97 K. The diffusivity of this lattice fluid is compared to that of the same molecules adsorbed on graphite (0001) previously reported. The reduced mobility observed in the case of CH₄/MgO(100) is related to the important depth of the potential wells on the MgO(100) surface.     

Silicon diffusion in an Fe-based amorphous alloy

The segregation rate of Si in amorphous as well as precrystallized Fe₈₁B_13.5Si_3.5C₂ was determined as a function of time at different temperatures below the crystallization temperature. Analysis of the segregation kinetics yielded diffusion parameters of E = 72±9 kJ/mol, D₀ = 1.7 × 10⁻¹⁵m²/s for the amorphous specimen and E = 154±5 kJ/mol, D₀ = 3.6 × 10⁻¹⁰m²/s for the crystalline specimen. The difference is explained by the initial presence of an oversaturated concentration of structural defects. The correct interpretation of segregation kinetics results for amorphous alloys is discussed.     

Potential-induced sonoelectrochemical graphene nanosheets with vacancies as hydrogen peroxide reduction catalysts and sensors

Defective graphene nanosheets (dGN_4V) with 5-9, 5-8-5, and point defects were synthesised by a sonoelectrochemical method, where a potential of 4 V (vs. Ag/AgCl) was applied to drive the rapid intercalation of phosphate ions between the layers of the graphite foil as a working electrode. In addition to these vacancies, double vacancy defects were also created when the applied potential was increased to 8 V (dGN_8V). The defect density of dGN_8V (2406 μm⁻²) was higher than that of dGN_4V (1786 μm⁻²). Additionally, dGN_8V and dGN_4V were applied as catalysts for the hydrogen peroxide reduction reaction (HPRR). The mass activity of dGN_8V (1.31 × 10⁻² mA·μg⁻¹) was greater than that of dGN_4V (1.17 × 10⁻² mA·μg⁻¹) because of its high electrochemical surface area (ECSA, 1250.89 m²·g⁻¹) and defect density (N_D, 2406 μm⁻²), leading to low charge transfer resistance on the electrocatalytic interface. The ECSA and N_D of dGN_4V were 502.7 m²·g⁻¹ and 1786 μm⁻², respectively. Apart from its remarkable HPRR activity, the cost-effective dGN_8V catalyst also showed potential as an amperometric sensor for the determination of H₂O₂.     

Numerical simulation of ultrasonic enhancement on mass transfer in liquid–solid reaction by a new computational model

Mass transfer coefficient is an important parameter in the process of mass transfer. It can reflect the degree of enhancement of mass transfer process in liquid–solid reaction and in non-reactive systems like dissolution and leaching, and further verify the issues by experiments in the reaction process. In the present paper, a new computational model quantitatively solving ultrasonic enhancement on mass transfer coefficient in liquid–solid reaction is established, and the mass transfer coefficient on silicon surface with a transducer at frequencies of 40 kHz, 60 kHz, 80 kHz and 100 kHz has been numerically simulated. The simulation results indicate that mass transfer coefficient increases with the increasing of ultrasound power, and the maximum value of mass transfer coefficient is 1.467 × 10⁻⁴ m/s at 60 kHz and the minimum is 1.310 × 10⁻⁴ m/s at 80 kHz in the condition when ultrasound power is 50 W (the mass transfer coefficient is 2.384 × 10⁻⁵ m/s without ultrasound). The extrinsic factors such as temperature and transducer diameter and distance between reactor and ultrasound source also influence the mass transfer coefficient on silicon surface. Mass transfer coefficient increases with the increasing temperature, with the decreasing distance between silicon and central position, with the decreasing of transducer diameter, and with the decreasing of distance between reactor and ultrasound source at the same ultrasonic power and frequency. The simulation results indicate that the computational model can quantitatively solve the ultrasonic enhancement on mass transfer coefficient.     

Comportement thermique d'un générateur à sorption solide

The thermal behaviour of a solid sorption generator of active carbon/alcohol machine, is studied during heating and cooling phases with a preheated air flow. A bicylindrical walls generator that contains 0.9 kg of a granular adsorbent in the presence of residual gas is tested during a cycle of an average duration of 3 hours; the grains of active carbon are rod-shaped of 0.003 m diameter and 0.008 m average length. The thermal contact conductance of adsorbent to the wall has an important influence on the rate of heat transfer between the generator and the external source of heat. In the absence of alcohol, heat transfer occuring without mass transfer in active carbon is essentially due to the conduction. A numerical bidimensional model allows one to justify experimentally the observed evolution and proposes thermal contact conductance between active carbon pellets and the generator wall. A parametric study of the thermal contact conductance gives 6.5 W·m⁻²·K⁻¹ as the best value. A simulation of heating and cooling phases with average conductance values between 5 and 30 W·m⁻²·K⁻¹ gives model estimated heating and cooling phases duration.     

HEAT AND MASS TRANSFER AND WALL SHEAR STRESS IN VERTICAL GAS-LIQUID FLOW

Results of an experimental investigation of heat and mass transfer and wall shear stress at gas-liquid flow in a vertical tube are presented. Local wall shear stress and mass transfer coefficients were measured by an electrochemical method. Experiments were performed in the range of Reynolds number variation with respect to liquid Rc_i, = 8.5 × 10³-5.4 × 10⁴, gas Re_g = 3 × 10³-1.4 × 10⁵, pressure 0.1-1 MPa. The relationship between heat and mass transfer and wall shear at gas-liquid flows is shown to exist. The results of measuring heat and mass transfer coefficients are generalized by formulas applied to calculate heat and mass transfer in single-phase turbulent flow.     

Can scalar neutrinos or massive Dirac neutrinos be the missing mass?

Scalar neutrinos and massive Dirac neutrinos in the mass range 2–20 GeV have been proposed as candidates to provide the dark matter in the halo of our galaxy. If so, the particles are captured inthe Earth with an efficiency of 10⁻¹⁰ − 10⁻⁷. For Dirac neutrinos more massive than about 9 GeV and scalar neutrinos more massive than abour 12 GeV, enough are captured to produce an observable neutrino flux at the surface of the Earth (∼ 10⁻² cm⁻² s⁻¹ for sneutrinos and ∼ 1.4 × 10⁻³ cm⁻² s⁻¹ for Dirac neutrinos), several orders of magnitude above atmospheric background and above what is observed. Hence stable scalar neutrinos of mass 12–20 GeV or Dirac neutrinos of mass 9–20 GeV cannot be the dominant component of the halo.     

Infrared plasma reflectivity minimum in heavily doped n-Si

This paper presents a numerical analysis of infrared (IR) plasma reflectivity minimum in ultra heavily doped (UHD) n-Si (impurity concentration N up to 6 × 10²¹cm⁻³) by using a self-consistent method (SCM) and a complex physical model. The necessity of taking into account the dependence of effective mass on impurity concentration is shown. The scattering on defects (N_def = 5 × 10¹⁷ cm⁻³) and dislocation (N_dis = 5 × 10¹¹ cm⁻²) is included. The approximate relation for the wavelength λ_m(N) of the reflectivity minimum is given. The results obtained are compared with the experimental results for n-Si and satisfactory agreement is found.     

Kinetics of lithium intercalation into carbon anodes: in situ impedance investigation of thickness and potential dependence

Electrochemical impedance spectroscopy (EIS) in three-electrode sandwich configuration was applied to investigate the kinetics of Li intercalation into mesocarbon-microbeads (MCMB) based anodes. A new frequency domain model, considering porous macroscopic structure of the active material and spherical diffusion inside the particles, has been applied to analyze the potential and thickness dependence of impedance spectra. Values of several kinetically relevant parameters like specific conductivity of the layer of intercalation material (2.6×10⁻⁴ S cm⁻¹), chemical diffusion coefficient of Li-ions in MCMB (2.2×10⁻⁹ cm² s), charge transfer resistance (184 Ω cm²), are obtained from the analysis. Applicability of the proposed model to prediction of time-domain charge curves has been tested using numerical inversion of Laplace transformation (NILT). The time domain modeling led to the conclusion that phase nucleation at the boundaries between Li-rich and Li-poor phases coexistent during intercalation is the rate-limiting step at the initial stage of the impulse-discharge.     

Effect of ultrasound on the kinetics of anti-solvent crystallization of sucrose

The effect of ultrasound on the kinetics of anti-solvent crystallization of sucrose was studied. The influence of temperature, stirring rate, supersaturation and ultrasonic power on the anti-solvent crystallization of sucrose was investigated. The relationship between infrared spectral characteristic band of sucrose and supersaturation was determined with an online reaction analyzer. The crystal size distribution of sucrose was detected by a laser particle-size analyzer. Ultrasound accelerated the crystallization process, and had no impact on the crystal shape. Abegg, Stevens and Larson model was fitted to the experimental data, and the results were the following: At 298.15 K, the average size of crystals was 133.8 μm and nucleation rate was 4.87 × 10⁹ m⁻³·s⁻¹ without ultrasound. In an ultrasonic field, the average size was 80.5 μm, and nucleation rate was 1.18 × 10¹¹ m⁻³·s⁻¹. Ultrasound significantly reduced the average size of crystals and improved the nucleation rate. It was observed that the crystal size decreased with the increase of stirring rate in silent environment. When the stirring rate increased from 250 to 400 rpm, the average size decreased from 173.0 to 132.9 μm. However, the stirring rate had no significant impact on the crystal size in the ultrasonic field. In addition, the activation energy of anti-solvent crystallization of sucrose was decreased, and the kinetic constant of nucleation rate was increased due to the effect of ultrasound. In the ultrasonic field, the activation energy was reduced from 20422.5 to 790.5 J·mol⁻¹, and the kinetic constant was increased from 9.76 × 10² to 8.38 × 10⁸.     

Flow boiling heat transfer of water in microchannel heat sink

The flow boiling heat transfer of water in a microchannel heat sink with variable initial vapor quality at the inlet is investigated. The stainless steel microchannel heat sink contains ten 640 × 2050 μm channels with a length of 120 mm; the wall roughness is 10 μm. The data on the local heat-transfer coefficient distribution in heat sink length are obtained in the range of mass fluxes from 30 to 90 kg/m²s, heat fluxes from 40 to 170 kW/m², and vapor qualities from 0 to 1. The heat transfer instability associated with dry spots resulting from insufficient wetting of channel walls introduces substantial contribution to the heat transfer mechanism and leads to decreasing heat transfer in heat sink length downward the flow. The developed method for calculating the flow boiling heat transfer of water in a microchannel heat sink allows more accurate prediction of heat transfer drop than available methods.     

Analysis of ultrasound-assisted convective heating/cooling process: Development and application of a Nusselt equation

In this study, the convective heating/cooling process assisted by US irradiation is analyzed with the aims of developing a new convective heat transfer correlation. Heat transfer experiments were conducted with different copper machined geometries (cube, sphere and cylinder), fluid velocities (0.93–5.00 × 10⁻³ m/s), temperatures (5–60 °C), and US intensities (0–6913 W/m²) using water as heat transfer fluid. The Nusselt (Nu) equation was obtained by assuming an apparent Nu number in the US-assisted process, expressed as the sum of contributions of the forced convection and cavitation-acoustic streaming effects. The Nu equation was validated with two sets of experiments conducted with a mixture of ethylene glycol and water (1:1 V/V) or a CaCl₂ aqueous solution (30 g/L) as immersion media, achieving a satisfactory reproduction of experimental data, with mean relative deviations of 17.6 and 17.8%, respectively. In addition, a conduction model with source term and the proposed correlation were applied to the analysis of US-accelerated heating kinetics of dry-cured ham reported in literature. Results demonstrated that US improves heating of ham slices because of the increased heat transfer coefficients and the direct absorption of US power by the foodstuff.     

The formation and decomposition kinetics of alkylidynes on Pt(111)

The formation and decomposition kinetics of ethylidyne and propylidyne on Pt(111), were studied using static secondary ion mass spectrometry and temperature programmed desorption. For the maximum amounts of dissociatively adsorbed ethylene and propylene formed during adsorption at 200 K and subsequent temperature programmed desorption, the following activation energies (E) and pre-exponential factors (A) are determined: (a) for ethylidyne formation: E = 17±1 kcal mol⁻¹ and A = 1×10^12±1 s⁻¹; (b) ethylidyne decomposition: E = 27 ±2 kcal mole⁻¹ and A = 6×10^11±1 s⁻¹; (c) propylidyne formation: E = 17.5 ± 2 kcal mol⁻¹ and A = 7×10^12±1 s⁻¹; and (d) propylidyne decomposition: E = 22.5±2 kcal mol⁻¹ and A = 4×10^{11 ± 1} s⁻¹.     

Surface modification and adhesion of wood-plastic composite (WPC) treated with UV/ozone

Because of the surfaces of wood-plastic composite (WPC) materials are enriched in polymers of low surface energy, they exhibit low adhesion properties. UV/ozone is proposed as surface treatment for increasing the surface energy and adhesion of WPC materials made with different polymers (polyethylene, polypropylene and polyvinyl chloride). UV lamp-WPC surface distance and time of UV exposure were varied for optimizing UV/ozone treatment of WPC, and UV dose used ranged between 2.02 × 10^?14 and 5.05 × 10^?12 J·s/m². UV/ozone treatment created new carbon-oxygen polar groups in WPC surfaces and increased their surface energy, mainly their polar component. Furthermore, ablation of the outermost WPC surface was produced, more noticeably by reducing the distance between WPC surface and UV lamp and by increasing the duration of the treatment. Noticeable increase in 180° peel adhesion was obtained in the joints made with UV/ozone treated WPC at 10–30 mm distance during 1–5 min (i.e., UV dose between 5.61 × 10^?14 and 2.53 × 10^?12 J·s/m²). Although 180° peel strength of joints made with acrylic adhesive tape and UV/ozone treated WPC for 10 min and 10 mm distance (UV dose: 5.05 × 10^?12 J·s/m²) was not increased because of dominant effect of ablation over creation of polar groups, the cross-hatch adhesion to different coatings was highly improved, irrespective of the polymer used and the wood content of WPC; however, the surface modifications and adhesion of UV/ozone treated WPC were more marked when its wood content was higher and by using UV dose between 0.10 × 10^?12 and 2.53 × 10^?12 J·s/m².     

Coverage of foreign atoms on surfaces as a function of adsorption,sputtering, and diffusion rates

During sputtering of a surface the coverage of foreign atoms may be changed by diffusion from the bulk to the surface and by adsorption from the gas phase. Assuming sufficiently high negative surface segregation enthalpy i.e. high ratios of surface to bulk concentration in equilibrium, simple expressions of the coverage time function for finite and semi-infinite samples are derived. The equations were applied to experimental results of oxygen coverage on a niobium surface where the three processes mentioned above were active. From the comparison with these results the diffusion coefficients of O in Nb D = 2 × 10^?13 m²/s at 520°C and D = 9 × 10^?13 m²/s at 600°C were evaluated.