A dimensionless criterion of dispersion for crystalline solids

A dimensionless criterion of dispersion for crystalline solids is proposed based on a comparison of the excess free energy in the compact and disperse state: D_L=12σ/Gb²pL, where σ is the surface energy, G is the shear modulus, b is the Burgers vector, p is the dislocation density, and L is the particle size. According to this, the critical size of particles corresponding to D_L=1 is approximately the same (L₁=500 nm) for all metals. Classification of disperse crystalline solids with respect to the parameter D_L is more justified thermodynamically than characterization by the particle size.     

Effect of supercritical drying parameters on the electrochemical properties of vanadium oxide-based aerogels

This paper examines the effect of supercritical drying parameters (including the nature of the solvent) on the electrochemical properties of vanadium oxide-based aerogels. It is shown that, among vanadium oxide-based aerogels prepared using different solvents (supercritical CO₂, n-hexane, and n-octane), the material obtained in n-hexane is superior in discharge capacity characteristics: its capacity is 350 to 250 mAh/g in the first seven cycles. The aerogel obtained in supercritical CO₂ has a far lower capacity: 80–105 mAh/g.     

Study of effect of soft inclusion on the mechanical behavior of soilcrete mixtures: experimental and modeling approaches

The influence of the volume fraction and size of soft soil inclusions on the mechanical behavior of soilcretes was experimentally and numerically investigated. Spherical kaolinite chunks with a specific water content and volume were added to the soilcrete samples during the filling phase of the molds. Different mechanical properties such as the effective static elastic modulus (E) and the unconfined compressive strength (UCS) of soilcrete specimens with different volume fraction of inclusions were determined. E and UCS of soilcrete specimens with different volume fraction of inclusions were also determined using 2D (unit cell) and 3D (digital specimens) finite element models. UCS and E decrease considerably as the volume fraction of soft inclusions increases. Experimental results show that these UCS and E drops are, respectively, about 47% and 20% for a volume fraction of inclusions f_v = 9%. A good agreement between the experimental and simulated data was observed, especially for elastic modulus.     

Comparison of the Physicochemical and Electrochemical Properties of Vanadium Oxide-Based Nanomaterials Prepared by Cryochemical Synthesis and Supercritical Drying Technique

Vanadium oxide-based nanomaterials have been prepared by cryochemical synthesis (CCS) and supercritical drying (SCD) in n-hexane and acetone. We have performed the first comparative analysis which demonstrates differences in the physicochemical and electrochemical properties of the products, related to the key features of the effect of the CCS and SCD approaches. The nanomaterials prepared from the same precursor using CCS and SCD (in acetone and n-hexane) have been shown to differ in phase composition and morphology. The oxidizing annealing of the resultant aerogels and cryogel at 500°C in air leads to the formation of only one phase: α-V₂O₅. In all cases except the aerogel prepared using SCD in n-hexane, the crystalline α-V₂O₅ has a higher discharge capacity in comparison with the unannealed aerogels and cryogel. The highest discharge capacity among the annealed aerogels is offered by the sample prepared using SCD in acetone (255 mAh/g), and the highest discharge capacity among the unannealed materials is offered by the sample prepared using SCD in n-hexane (280 mAh/g). The samples range in energy density from 110 to 640 Wh/kg. The highest energy density is also offered by the aerogel prepared using SCD in n-hexane.     

Preparation and characterization of monodisperse silica nanoparticles via miniemulsion sol–gel reaction of tetraethyl orthosilicate

Monodisperse silica nanoparticles were prepared via miniemulsion sol–gel reaction of tetraethyl orthosilicate (TEOS). Hexadecane (HD) or hexadecyltrimethoxysilane was used as costabilizer to effectively retard the Ostwald ripening process involved in TEOS miniemulsion. The Ostwald ripening behavior was characterized by dynamic light scattering (DLS), and it was adequately described by the modified Kabal’nov equation. The miniemulsion sol–gel reaction of TEOS/HD with a volume fraction (φ _c) of 0.024 at 80 °C is stable in the pH range 6–10. By contrast, gelation of reacting miniemulsions occurs at 70 and 100 min at pH 4 and 5, respectively. The weight-average silica particle size (d _w) of colloidal products prepared at 80 °C and pH 7 decreases from 59 to 36 nm with low polydispersity index (PDI, in the range 1.02–1.03), determined by transmission electron microscopy, when the φ _c of HD increases from 0.024 to 0.23. At constant φ _c (0.024), the resultant silica nanoparticles show larger d _w (83 nm) and PDI (1.35) for the TEOS/HD system at pH 10 as compared to the counterpart of pH 7. Furthermore, for the TEOS/HD system at pH 7 and low φ _c (0.024), d _w increases significantly with temperature being increased from 25 to 80 °C. By contrast, the effect of temperature on silica nanoparticle size becomes insignificant when a high level of HD (φ _c = 0.23) is used. Zeta potential measurements and field emission scanning electron microscopy were used to characterize the surface charge density and morphology of resultant silica nanoparticles.     

Nanomagnetics with lasers

Both liquid and vapour phase pulsed laser deposition (PLD) techniques have been used to synthesize nanophase magnetic alloys of CoPt. While the liquid route results in soft phase (disordered fcc) nanoparticles of CoPt near equiatomic composition dispersed in a surfactant-polymer matrix, the conventional vapour phase PLD allows growth of high coercivity nanoscale structures of CoPt on (001) SrTiO₃. The magnetization, M(T), dynamics of the colloidal particles is examined. Two distinct particle distributions are established from analysis of M(T) data, in conformity with results of electron microscopy. In vapour deposited films at low growth rate (～0.4 Å/s), morphology changes from a self-similar fractal to nanodots as the deposition temperature is raised from 700–800°C. The large lattice mismatch between (001) SrTiO₃ and the ac/bc plane of L1₀ ordered phase imparts tensile strain to the films whose morphological manifestations can be suppressed at high growth rates.     

Synthesis and photocatalytic activity of anatase-based aerogels

V_xTi_1–xO₂ (x = 5 and 10 mol %) solid solutions have been synthesized through supercritical drying in isopropanol at t = 250°C and p = 10 MPa. Their physicochemical properties and photocatalytic performance have been compared to those of an earlier synthesized Zn_xTi_1–xO₂ aerogel containing 10 mol % Zn [1]. It has been shown that increasing the vanadium content of V_xTi_1–xO₂ from 5 to 10 mol % leads to a decrease in hydrogen evolution rate in methanol/water splitting reaction under UV irradiation from 190 to 32 µL/(min g_catal), whereas in the case of the anatase-based aerogel solid solution containing 10 mol % Zn an opposite picture is observed: the hydrogen evolution rate in methanol/water splitting reaction under UV irradiation increases sharply to 700 µL/(min g_catal).     

Oscillating cylinder in viscous fluid: calculation of flow patterns and forces

Laminar and large-eddy-simulation (LES) calculations with the dynamic Smagorinsky model evaluate the flow and force on an oscillating cylinder of diameter D = 2R in otherwise calm fluid, for β = D ²/νT in the range 197–61400 and Keulegan–Carpenter number K = U _m T/D in the range 0.5–8 (ν kinematic viscosity, T oscillation period, U _m maximal velocity). Calculations resolving the streakline patterns of the Honji instability exemplify the local flow structures in the cylinder boundary layer (β ~ 197–300, K ~ 2) but show that the drag and inertia force are not affected by the instability. The present force calculations conform with the classical Stokes–Wang solution for all cases below flow separation corresponding to K < 2 (with β < 61400). The LES calculations of flow separation and vortical flow resolve the flow physics containing a large range of motion scales; it is shown that the energy in the temporal turbulent fluctuations (in fixed points) are resolved. Accurate calculation of the flow separation occurring for K > 2 has strong implication for the force on the cylinder. Present calculations of the force coefficients for K up to 4 and β = 11240 are in agreement with experiments by Otter (Appl Ocean Res 12:153–155, 1990). Drag coeffients when flow separation occurs are smaller than found in U-tube experiments. Inertia coefficients show strong decline for large K (up to 8) and moderate β = 1035 but is close to unity for K = 4 and β = 11240. The finest grid has 2.2 × 10⁶ cells, finest radial Δr/R = 0.0002, number of points along the cylinder circumference of 180, Δz/R = 0.044 and a time step of 0.0005T.     

Calculation of sublimation with allowance for impurity diffusion

We have calculated the impurity distribution in a spherical material during sublimation. The solutions found depend on two dimensionless parameters: separation factor β and diffusional Peclet number Pe = wR ₀/(ρ₁ D), where w is the mass vaporization rate, R ₀ is the initial sphere radius, ρ₁ is the density of the material, and D is the impurity diffusion coefficient. For the limiting cases Pe ? 1 and Pe < 1, we derived analytical expressions for the impurity distribution in the subliming material and for the average impurity concentrations in the residue and condensate as functions of time t or the degree of sublimation g(t). The present results can be used to optimize the dimensions of subliming materials and sublimation conditions.     

Methyl <Emphasis Type="Italic">tert</Emphasis>-butyl ether as a new solvent for the preparation of SiO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> binary aerogels

SiO₂–TiO₂ binary aerogels have been synthesized for the first time using methyl tert-butyl ether as supercritical fluid. It has been shown that the aerogels prepared in methyl tert-butyl ether and isopropanol contain nanocrystalline anatase and that amorphous SiO₂–TiO₂ aerogels with a homogeneous distribution of their components can be obtained in CO₂. A considerable contribution to the large specific surface area of the aerogels is made by micropores, especially when supercritical drying is carried out in CO₂ and isopropanol.     

Development of Σ3<Superscript><Emphasis Type="Italic">n</Emphasis></Superscript> CSL boundaries in austenitic stainless steels subjected to large strain deformation and annealing

The development of annealing twins was studied in chromium–nickel austenitic stainless steels subjected to cold or warm working. The annealing behavior can be characterized by an austenite reversal, recrystallization, and grain growth, depending on the deformation microstructures. The grain coarsening during recrystallization followed by a grain growth was accompanied by the development of twin-related Σ3 ⁿ CSL boundaries. The fraction of Σ3 ⁿ CSL boundaries and their density are defined by a unique parameter that is a relative change in the grain size, i.e., a ratio of the annealed grain size to that one evolved by preceding plastic working (D/D ₀). The fraction of Σ3 ⁿ CSL boundaries rapidly increased at early stage of recrystallization and grain growth while the ratio of D/D ₀ attained 5. Then, the rate of increase in the fraction of Σ3 ⁿ CSL boundaries slowed down significantly during further grain coarsening. On the other hand, the density of Σ3 ⁿ CSL boundaries increased to its maximum at a ratio of D/D ₀ about 2.5 followed by a gradual decrease during subsequent grain growth.     

Photoelastic strain measurement in GaP (100) wafers under external stresses

The sign of the absolute value of residual strain measured in LEC-grown GaP wafers with scanning infrared polariscope (SIRP) has been determined by measuring the residual strain in the round-shape wafer with and without compressive load to the diametric edges. Since the external stress induced by the compressive load is additive to the residual strain in wafer, the residual strain measured in the wafer with the compressive load is increased if the sign is negative while it is decreased if the sign is positive. The measurement of residual strain was successively performed in various LEC-grown GaP (100) wafers clamped their diametric edges in a specially-made vise, in which a movable jaw is pulled by a coil spring to slide parallel toward a fixed jaw with a compressive load. It is found from their results that the residual strain component of (S _r ? S _t), where S _r is the radial strain component and S _t is the tangential strain component defined in the cylindrical coordinate system matching to the round-shape wafer, is negative; that is, radially compressive in most wafers.     

Experimental study on a solar air heater with various perforated covers

In this study, the thermal performances of single- and counter-flow solar air heaters with a normal cover and with quarter- and half-perforated covers were investigated experimentally. In this work, on two of the perforated covers, the holes were made in the first quarter at the top side of the covers. As for the other two covers, half of the cover area on the top side was perforated. The hole diameter, D, was 0.3 cm. The holes in the covers had a centre-to-centre distance of 20D (6 cm) or 10D (3 cm). It was found that the efficiency of the air heater with the quarter-perforated cover was slightly higher than that of the one with the half-perforated cover for both single- and counter-flow collectors. The average efficiencies of the double-pass solar collector with 20D and 10D quarter-perforated covers were 51.38% and 54.76%, respectively, and the ones for the collector with 20D and 10D half-perforated covers were 48.21% and 51.17%, respectively, at mass flow rate of 0.032 kg/s. At the same mass flow rate, the average efficiency of the double-pass air heater with normal cover was 50.92%.     

Testing of concrete by rebound method: Leeb versus Schmidt hammers

Hardness is considered as an important property of concrete; it can be used to estimate compressive strength of concrete in situ. The classic Schmidt rebound hammer is the most popular nondestructive method to measure concrete surface hardness, while the Leeb rebound hammer has been extensively studied in geological and metallographic fields over decades, and its use for testing concrete is almost not known. The national and international standards for the measurement of hardness are reviewed. Concrete made different w/c ratios (0.33, 0.4 and 0.5) were tested by both methods. The simple linear correlation between rebound numbers (both Schmidt and Leeb) and concrete compressive strength are proposed. Schmidt rebound number was differently correlated with compressive strength for concretes with different w/c ratios, while the Leeb rebound numbers were more consistent and could be applied in predicting concrete compressive strength within 10% error for all w/c ratios. It was also concluded that Schmidt test can be considered as a semi-destructive method, because of significant strength reduction (in average by 10.5 MPa) that was observed after application of Schmidt hammer impact on specimens, while the Leeb rebound test procedure did not result in any damage of concrete. This difference can be explained by the dramatic difference in impact energy of the two hammers (2207 and 11 N·mm - for Schmidt rebound hammers of N-type and Leeb hammers of D-type, respectively). Moreover, the classic Schmidt rebound hammer is not recommended to be used on the concrete specimens, which are aimed for compressive tests at early age (less than 3 days) or when expected compressive strength is less than 7 MPa. These constraints do not apply to lower impact Leeb rebound devices, which can be considered as perfectly invasive (non-destructive). At the same time, as expected, Leeb rebound test is sensitive to the surface conditions, such as carbonation and surface moisture.     

Effect of inter-electrode spacing on structural and electrical properties of RF sputtered AlN films

An attempt has been made to correlate the morphological and electrical properties of RF sputtered aluminum nitride (AlN), with target to substrate distance (D _ts) in sputter chamber. AlN films, having thickness around 3,000 Å, were deposited on silicon substrates with different D _ts values varying from 5 to 8 cm. XRD results indicated that the crystallinity of c-axis oriented films increase significantly with decrease in D _ts and the FTIR absorption band of the films became prominent at shorter D _ts. The surface roughness increased from 1.85 to 2.45 nm with that in D _ts. A smooth surface with smaller grains was found at shorter D _ts. The capacitance–voltage (C–V) measurements revealed that the insulator charge density (Q _in) increased from 3.3 × 10¹¹  to 7.3 × 10¹¹ cm^?2 and the interface state density (D _it) from 1.5 × 10¹¹  to 7.3 × 10¹¹ eV^?1cm^?2 with the increase in D _ts.     

Influence of moisture condition on chloride diffusion in partially saturated ordinary Portland cement mortar

Experiments have been carried out to study the influence of moisture condition, including moisture content and its distribution, on the chloride diffusion in partially saturated ordinary Portland cement mortar. The mortar samples with water-to-cement (w/c) ratios of 0.4, 0.5 and 0.6, cured for 1 year, were preconditioned to uniform water saturations ranging from 18 to 100%. The interior relative humidities of these partially saturated cement mortars, i.e. water vapour desorption isotherm (WVDI), were measured. The WVDI results in relation to the pore structures obtained from the mercury intrusion porosimetry tests of paste samples with the same w/c ratios were analyzed, which provided a basic insight into the moisture distribution in the non-saturated cement mortars. The relative chloride diffusion coefficients of cement mortars at various water saturations were determined based on the Nernst-Einstein equation and conductivity technique. It is found that the relative chloride diffusion coefficient D_rc depends on the degree of water saturation S_w and WVDI. At a given S_w level, the D_rc is larger for a higher w/c ratio. The role of the w/c ratio in the D_rc–S_w relation, however, becomes less pronounced with increasing w/c ratio. There exists a critical saturation, below which the water-filled capillary pores are discontinuous and the D_rc-value tends towards infinitely small. An increase of the w/c ratio results in a decrease of the critical saturation level.     

The influence of gate length on the electron injection of velocity in an AlGaN/AlN/GaN НЕМТ channel

Field-effect high-electron-mobility transistors (HEMTs) based on AlGaN/AlN/GaN heterostructures with various gate lengths L _g have been studied. The maximum values of current and power gaincutoff frequencies (f _T and f _max, respectively) amounted to 88 and 155 GHz for HEMTs with L _g = 125 nm, while those for the transistors with L _g = 360 nm were 26 and 82 GHz, respectively. Based on the measured S-parameters, the values of elements in small-signal equivalent schemes of AlGaN/AlN/GaN HEMTs were extracted and the dependence of electron-injection velocity v_inj on the gate–drain voltage was determined. The influence of L _g and the drain–source voltage on v_inj has been studied.     

Vortex ring propagation and interactions studies

This paper experimentally investigates the vortex ring propagation and interactions with thin cylindrical and flat surfaces. Dye-based visualization technique is adopted for the interaction studies. Vortex rings are generated from a circular nozzle of 19 mm diameter with the stroke length ratios (length of the fluid slug to nozzle diameter, L_N/D_N) of 1 to 5, and ejection velocities in the range of 0.05 to 0.2 m/s. Vortex interaction studies are carried out with two different bodies; firstly, with the circular cylinders having the diameters of 0.2, 0.6, 1.5 and 2.5 mm, and secondly with a flat solid surface. Results indicate that the trails in the vortex ring start following at L_N/D_N = 4. The influence of the initial velocity is found to be insignificant on the vortex ring diameter, however, found to depend on stroke length ratio. Vortex-cylinder interaction studies indicated that vortex velocity decreases with increase in cylinder diameter after the interaction. Reconnections of vortex rings are observed in lower cylinder diameter cases. In case of vortex ring interaction with the flat surface, stretching of the vortex core is observed leading to a considerable increase in the vortex ring diameter.     

Preparation and properties of Pr<Superscript>3+</Superscript>/Ce<Superscript>3+</Superscript>:YAG phosphors using triethanolamine as dispersant and pH regulator

Pr³⁺/Ce³⁺:YAG precursors were co-precipitated using triethanolamine as dispersant and pH regulator. The different dosages of triethanolamine (D) vs. the properties of Pr³⁺/Ce³⁺:YAG phosphors were discussed. When D = 0.5 vol%, the pH of titration process was controlled in the range of ～7.94–8.16 to guarantee the uniform distribution of Al, Y, Ce and Pr in the precursors. The relatively higher pH could decrease the loss of Ce and Pr in the precursors and increase the particle size of the obtained powders, which was beneficial to the enhancement of luminescent intensity. Therefore, the precursors directly converted to pure-phase YAG at 900°C, and the phosphors calcined at 1000°C showed the best dispersity due to the dispersion effect of triethanolamine and the most excellent luminescent property. When D ≥ 2 vol%, although pure-phase YAG was detected, the emission intensity of the phosphors decreased due to the decrease of dispersity and purity. Moreover, the co-doped Pr³⁺ enhanced the red emission of Pr³⁺/Ce³⁺:YAG phosphors.     

Correlation between the electrical and luminescent properties of high-purity gallium arsenide

Epitaxial n-GaAs layers with a background impurity concentration of N_D-N_A<10¹⁵ cm^?3, grown by chloride vapor phase epitaxy in an open system, exhibit correlation between the electrical properties and the long persistence of the edge photoluminescence lines D⁰x and D⁰h related to the hole trapping centers. An increase in the concentration of such trapping centers in n-GaAs leads to a decrease in the mobility of free charge carriers.