Quantification of surface area and intrinsic mass transfer coefficient for ultrasound-assisted dissolution process of a sparingly soluble solid dispersed in aqueous solutions

The efficacy of power ultrasound of 20 kHz in enhancing the volumetric mass transfer coefficient was investigated in this study. Breakage and dissolution of sparingly soluble benzoic acid dispersed in either water or 24% aqueous glycerol was monitored as a function of time and ultrasound power input. Particle size measurements were carried out at intermediate times during the experiment to estimate the mean particle size and surface area. Linear combination of lognormal distributions was found to fit the experimental particle size distribution data. The De Brouckere mean diameters (d₄₃) obtained from the particle size distributions decreased with increase in the ultrasonic power level. Empirical correlations were developed for the evolution of surface area as a function of ultrasonic energy input per unit mass. The effect of ultrasound on the intrinsic mass transfer coefficient (k_c) could be decoupled from the volumetric mass transfer coefficient (k_ca) as the surface area was also estimated. Different approaches involving either constant or variable intrinsic mass transfer coefficients were employed when carrying out the delineation. Mass transfer rates were enhanced due to both higher ultrasound induced intrinsic convective mass transfer coefficient and additional surface area created from particle breakage. To delineate the effects of particle breakage from solid dissolution, experiments were also carried out under non-mass transfer conditions by pre-saturating the solvents with benzoic acid. Both the solid-liquid systems examined in the present study attained saturation concentration when the ultrasonic energy input per unit mass was approximately 60 kJ/kg, irrespective of the ultrasonic power level setting.     

Probability distribution functions for a single-particle vibrating in one dimension: experimental study and theoretical analysis

We consider the form of the rebound velocity, ν₀, particle velocity, ν, and height, h, probability density functions (PDFs) for the one-dimensional motion of a single particle on a sinusoidally oscillating base. The motion is considered in the limit of high excitation (vibration frequency ⪢ collision rate). Experimentally, we find that these PDFs are well-approximated by P_ν0(ν₀) ∞ ν₀ exp(− αν₀²), a Gaussian P_ν(ν) ∞ exp(− αν²) and a Boltzmann-type function P_h(h) ∞ exp(− 2αgh), where α is a constant and g is the acceleration due to gravity. We develop an analytical model which accurately predicts the general form for the rebound velocity PDF; the other two PDFs are then analytically shown to follow as a consequence. Scaling laws for the particle granular temperature with peak base velocity and particle-base restitution coefficient, determined from previous work, can also be predicted from the PDF. A fine scale “spiky” structure in the rebound velocity PDF is found, using numerical simulations, to be a consequence of resonance phenomena between the particle and vibrating base. Good agreement between scaling laws from the theory and simulation is found but insufficient data is obtainable to derive accuracy exponents experimentally.     

Analysis of pulsed discharge characteristics of solid-state switch (IGBT) based 16?kHz repetition rate, 100?W average power copper–HBr lasers

We present for the first time a comprehensive analysis (both time resolved and time averaged) of the gas-discharge characteristics of a solid-state switch (IGBT) based on high average power (100?W class), high pulse repetition rate (16?kHz) copper?CHBr laser under various excitation conditions. We evaluate various discharge-plasma parameters such as the electrical inductance, electrical resistance, active laser-head voltage, active electrical power, pre-pulse electron density, and axial gas temperature by numerical processing of the measured laser head voltage?Ccurrent waveforms. For the first time, we evaluate fractional losses at various intermediate stages of the circuit elements as well as effective coupling for the laser excitation process during transfer of energy from the wall plug to the laser-discharge plasma. We conclude that irrespective of the capacitive storage input power (P _in), a constant fraction of ~40% of P _in is coupled into the laser-discharge plasma. With a low to moderate specific input power of 0.4?C0.7?kW/?, the tube produced 70?C110?W average output power at an efficiency of????3.2?C2.8%, respectively. The average laser performances at various P _in are correlated to its time-resolved and average gas-discharge parameters such as the inter-pulse electron density and axial gas temperature.     

Discharged generator of singlet oxygen for oxygen-iodine laser. Transport kinetics of O2(a 1δg) and O2(b 1σ g + ) molecules and O(3 P) atoms in Ar:O2 and He:O2 flows excited by a 13.56-MHz discharge

To understand and reveal the basic physical factors providing the possibility of scaling of a discharged singlet oxygen generator (DSOG) in an oxygen-iodine laser, the production, and transport kinetics of metastable O₂(a ¹δ_g) and O₂(b ¹σ _g ⁺ ) molecules, as well as O(³ P) atoms, were investigated in Ar:O₂ and He:O₂ gas flows excited by a 13.56-MHz discharge in a wide range of pressures (4–40 Torr) and oxygen percentages. It is shown that the densities and transport kinetics of O₂(a ¹δ_g), O₂(b ¹σ _g ⁺ ), and O(³ P) appear similar for oxygen mixtures with argon and helium in the same conditions independent of discharge mode. Compared to pure O₂, the dilution of oxygen with an inert gas allows higher energy inputs per an oxygen molecule to achieved, especially under conditions of the homogeneous discharge mode (α-mode), which gives a higher efficiency of O₂(a ¹δ_g) excitation in Ar:O₂ and He:O₂ mixtures. But the maximum attainable yield of singlet oxygen in Ar:O₂ and He:O₂ at fixed partial O₂ pressure is found to be comparable with the O₂(a ¹δ_g) yield in pure oxygen at the same pressure. The reason for this is the increased three-body deactivation of O₂(a ¹δ_g) by atomic oxygen in the mixtures because of the greater total pressure. The estimation of the rate constant of O₂(a ¹δ_g) three-body quenching by O(³ P) in Ar:O₂ and He:O₂ mixtures as (1.5 ± 0.5) × 10^?32 cm⁶/s was carried out from the analysis of transport kinetics of singlet and atomic oxygen in the discharge afterglow at high pressures exceeding ～10 Torr. A similar analysis for the lower pressures has revealed that losses both of metastable O₂(a ¹δ_g) and O₂(b ¹σ _g ⁺ ) molecules, and of O(³ P) atoms on the surface of the discharge tube, are determined by the density of each of the components. The obtained loss probabilities of O₂(a ¹δ_g), O₂(b ¹σ _g ⁺ ), and O(³ P) on the silica surface show that the surface loss probabilities of all the species can increase noticeably under the discharge exposure. Thus, the key parameters determining the maximal O₂(a ¹δ_g) yield in the DSOG are a homogeneous volumetric mode of the discharge, energy input per oxygen molecule in this mode, and a low rate of O₂(a ¹δ_g) quenching. Just three-body quenching of O₂(a ¹δ_g) by O(³ P) limits the singlet oxygen yield with increasing pressure. The fast removal of atomic oxygen both in discharge and in the earlier afterglow could provide DSOG scaling with pressure.     

Two independent magnetic sublattices of Er in the Er5 − x LaxGe3 system

Magnetic measurements of Er_{5 ? x} La_xGe₃ with 1 ? x ? 5 have been done between 4·2 and room temperature. The compounds crystallize in the hexagonal (D8₈) structure (Mn₅Si₃ type) with space group P6₃/mcm. Two independent, ferromagnetic and antiferromagnetic, sublattices of Er exist. The magnetic interaction between the Er atoms in the 6(g) sites is ferromagnetic, while that between the Er atoms in the 4(d) sites is antiferromagnetic. The antiferromagnetic exchange interaction is weak in this system. The first and second La atom substitutes into the 6(g) site, while the third La in Er₂La₃Ge₃ is located in the 4(d) position. The fourth La is probably substituted in such a manner that all the Er atoms are left in the 6(g) sites.     

Optical-Optical Double Resonance Spectroscopy of the 1g(P1)-AΠ (1u)-XΣg Transition of I2

Optical-optical double resonance spectroscopy was used to study the 1_g(³P₁) ion-pair state of I₂ correlating to I⁻(¹S)+I⁺³P₁) at the dissociation limit. We gained access to the 1_g(³P₁) state though the A³Π (1_u) state in the (1+1) photon-excitation scheme. The pump laser excited the A³Π (1_u)-X¹Σ_g⁺ transition at a fixed frequency for state selection. The probe laser was scanned to detect the 1_g(³P₁)-A³Π (1_u) resonance by monitoring the ultraviolet emission from the 1_g(³P₁) state at 278 nm. The 1_g(³P₁) state was observed in a vibrational progression consisting of P and R doublets. An energy level analysis was carried out for the 1_g(³P₁) state in the 0≤ v ≤ 14 and 12≤J≤135 range, which led to a set of molecular parameters including the Ω-doubling constant. The Ω-doubling of the 1_g(³P₁) state was discussed by the pure precession model and interpreted to occur through the heterogeneous coupling with the 0_g⁻(³P₁) state correlating to the same ionic asymptote.     

Gravity modulation study on opposed flame spread over thin solid fuels

In this work a numerical investigation has been carried out to study the effect of g-jitter on zero-gravity (0g_e) opposed flow spreading flame over thin solid fuels. For comparison simulations have also been carried out for normal gravity (1g_e) downward spreading flames. G-jitter is emulated by gravity modulation of sinusoidal (A_ge sin(2πt/T_ge)) gravity perturbation (g-perturbation) of a particular time-period (T_ge) and amplitude (A_ge) over a selected base gravity level (0g_e or 1g_e). The response of flames at 0g_e base gravity and at 1g_e base gravity was different to the imposed g-perturbation. While at 0g_e the mean and the amplitude of the oscillatory flame spread rate (FSR) magnified with increase in the time period of g-perturbation, interestingly for the 1g_e flame a maximum mean FSR and oscillation amplitude occurs at certain perturbation time period. Further, at very small perturbation time-periods (T_ge) the FSR at 1g_e was lower than the steady state FSR. The amplitude of oscillatory FSR increased with increase in perturbation amplitude (A_ge). However, the 0g_e flame which is little affected (compared to 1g_e flame) at small perturbation amplitude (A_ge) is affected severely at large perturbation amplitude (A_ge). Both the gas phase and fuel pyrolysis (or fuel response) follow perturbation signal with a lag but fuel pyrolysis is more sluggish and lags behind gas phase. The phase lag between fuel pyrolysis and gas increases at smaller time-periods (T_ge) and tends to enhance the effect of external perturbation whereas at larger time-periods (T_ge) this lag inhibits the effect of external perturbation.     

On the energy transport velocity in a dissipative medium

An exact definition of the group velocity v _g is proposed for a wave process with arbitrary dispersion relation ω = ω′(k) + iω″(k). For the monochromatic approximation, a limit expression v _g (k) is obtained. A condition under which v _g (k) takes the form of the Kuzelev–Rukhadze expression [1] dω′(k)/dk is found. In the general case, it appears that v _g (k) is defined not only by the dispersion relation ω(k), but also by other elements of the initial problem. As applied to the dissipative medium, it is shown that v _g (k) defines the field energy transfer velocity, and this velocity does not exceed thee light speed in vacuum. An expression for the energy transfer velocity is also obtained for the case where the dispersion relation is given in the form k = k′(ω) + ik″(ω) which corresponds to the boundary problem.     

Investigation of mass transfer intensification under power ultrasound irradiation using 3D computational simulation: A comparative analysis

This paper aims at investigating the influence of acoustic streaming induced by low-frequency (24 kHz) ultrasound irradiation on mass transfer in a two-phase system. The main objective is to discuss the possible mass transfer improvements under ultrasound irradiation. Three analyses were conducted: i) experimental analysis of mass transfer under ultrasound irradiation; ii) comparative analysis between the results of the ultrasound assisted mass transfer with that obtained from mechanically stirring; and iii) computational analysis of the systems using 3D CFD simulation. In the experimental part, the interactive effects of liquid rheological properties, ultrasound power and superficial gas velocity on mass transfer were investigated in two different sonicators. The results were then compared with that of mechanical stirring. In the computational part, the results were illustrated as a function of acoustic streaming behaviour, fluid flow pattern, gas/liquid volume fraction and turbulence in the two-phase system and finally the mass transfer coefficient was specified. It was found that additional turbulence created by ultrasound played the most important role on intensifying the mass transfer phenomena compared to that in stirred vessel. Furthermore, long residence time which depends on geometrical parameters is another key for mass transfer. The results obtained in the present study would help researchers understand the role of ultrasound as an energy source and acoustic streaming as one of the most important of ultrasound waves on intensifying gas-liquid mass transfer in a two-phase system and can be a breakthrough in the design procedure as no similar studies were found in the existing literature.     

Collisional relaxation of ionic2 P 3/2 excited states in rare gases

Optical pumping with theD ₂ line is used to study the depolarization of the first excited² P _3/2 states of alkaline earth ions and Yb⁺ ions in collisions with rare gas atoms. The deorientation cross sections for the even isotopes of Ba⁺ and Yb⁺ ions are obtained (in units of 10^?16 cm²): rare gas He Ne Ar Kr Xeσ ₁ (Ba⁺ 6p ² P _3/2) 79(11) 89(13) 123(18) 152(22) 204(30)σ ₁ (Yb⁺ 6p ² P _3/2) 60(10) 62(9) 107(18) 133(17) 167(37) The cross sections are discussed in comparison with theoretical calculations and those of isoelectronic atoms. The comparison of the² P _3/2 relaxation of even and odd (I=3/2) isotopes of Ba⁺ allows to draw conclusions on the nature of the depolarization interaction.     

Measurements of theg J -factors of12C in the3 P 1 and3 P 2 states

Using the atomic beam magnetic resonance method the Zeeman interactions of¹²C in the³ P ₁ and³ P ₂ states at magnetic fields of about 3.4 kOe have been measured. The measured quantities areg _J (³ P ₁)?g_J(³ P ₂)=15.4(1.0)·10^?6 g _J (³ P ₂)=1.5010616 (50), from which the following value for g_J(³P₁) can be calculated:g _J (³ P ₁)=1.5010770 (50). The experimental results are in moderate agreement with theoretical calculations.     

A finite element-spherical harmonics radiation transport model for photon migration in turbid media

In this paper, we solve the steady-state form of the Boltzmann transport equation in homogeneous and heterogeneous tissue-like media with a finite element-spherical harmonics (FE-P_N) radiation transport method. We compare FE-transport and diffusion solutions in terms of the ratio of absorption to reduced scattering coefficient, (μ_a/μ_s′) and the anisotropy factor g. Two different scattering phase function formulas are employed to model anisotropic scattering in the slab media with high g-value. Influence of void-like heterogeneities, and of their boundaries with the surrounding medium on the transport of photons are also examined.     

Compositional effects on the optical and thermal properties of sodium borophosphate glasses

A series of sodium borophosphate glasses of the composition (1−x)NaPO₃−xB₂O₃ have been synthesised from Na₂CO₃, B₂O₃ and P₂O₅ and their optical and thermal properties investigated. The results show that refractive index (n) and glass transition temperature (T_g) show a maximum at about B/(B+P)=0.6 while thermal expansion coefficient (α) and thermo-optic coefficient (dn/dT) change monotonically with the B/(B+P) ratio. These observations can be interpreted based on the incorporation of BO₃ and BO₄ units into the glass structural network.     

A note on the induced pseudo-scalar coupling constant in μ− + 12C(g.s)→ 12B(1+,g.s+ ν μ

The sum of the induce pseudo-scalar g_P and the second-class induced tensor g_T couplings in muon capture is found to be (g_P + g_T)g_A= (13.3 + 1.8) by comparing with recent data on the recoil polarization of ¹²B(1⁺) in the μ^? + ¹²C→¹²B+ ν_μ process, even aaking into account the corrections due to the gamma decay of the excited states of ¹²B to ¹²B(1⁺;g.s.). With the canonical value for g_P and in the absence of meson exchange effects, our results indicate a large positive value for g_T which is in strong contradiction with the conclusion of the Louvain-Saclay—ETH experiment.     

Structural and photo-luminescence properties of nanocrystalline silicon films deposited at low temperature by plasma-enhanced chemical vapor deposition

Nanocrystalline silicon (nc-Si) films were prepared by a plasma-enhanced chemical vapor deposition method at a deposition temperature below 220 °C with different dynamic pressures (P_g), hydrogen flow rates ([H₂]), and RF powers, using SiH₄/H₂/SiF₄ mixtures. We examined the photo-luminescence (PL) spectra and the structural properties. We observed two stronger and weaker PL spectra with a peak energies around E_PL = 1.8 and 2.2-2.3 eV, respectively, suggesting that the first band was related to nanostructure in the films, and another band was associated with SiO-related bonds. The nc-Si films with rather large PL intensity was obtained for high [H₂] and/or low pressure values, However, effects of [H₂] are likely to be different from those of P_g. The average grain size (δ) and the crystalline volume fraction (ρ) at first rapidly increase, and then slowly increase, with increasing P_g. Other parameters exhibited opposite behaviors from those of δ or ρ. These results were discussed in connection with the changes in the PL properties with varying the deposition conditions.     

Investigation of photochromic cluster systems based on molybdenum oxides by ESR spectroscopy

The electron-spin resonance (ESR) spectra of cluster polyoxometalate systems—a finely dispersed powder of the (NH₄)₆[Mo₇O₂₄]-citrie acid complex, molybdic acid, and molybdenum(VI) oxide—are investigated. The initial samples are colored under exposure to ultraviolet (UV) irradiation (photochromic effect) and thermal annealing. The ESR signal (g _⊥ = 1.94, g _‖ = 1.92) which is observed for the (NH₄)₆[Mo₇O₂₄]-citric acid photocolored samples corresponds to an electron of the molybdenum atom. This is in agreement with the data derived from the electronic spectrum. In addition, the (NH₄)₆[Mo₇O₂₄]-citric acid colored system exhibits an ESR signal (g=2.02) which corresponds to a hole at the organic ligand. This confirms the previously advanced model of intramolecular electron transfer under UV irradiation. The thermally colored molybdic acid has a similar ESR spectrum (g=1.88, 1.92, 1.93, and 1.98). For the other samples, the ESR signal is not observed. It is demonstrated that an unpaired electron of molybdenum atoms is substantially delocalized over all metal atoms in the cluster.     

High-field/high-frequency EPR/ENDOR — A powerful new tool in photosynthesis research

In photosynthesis research the elucidation of the spatial and electronic structures of the electron donor and acceptor ion radicals is very important for an understanding of the light-induced electron transfer process. Recent 3 mm (W-band, 95 GHz) high-field EPR and ENDOR studies on the primary donor cation radicals P^+. (bacteriochlorophyll dimer), the acceptor anion radicals Q^+. (quinones), and the charge-separated radical pair (P^+.Q^?.) in photosynthetic bacteria and biomimetic model systems are presented. From single crystals of, for instance,Rb. sphaeroides R-26 reaction centers both the hyperfine tensors of various protons and theg-tensor of P₈₆₅ ^+. have been determined and compared with calculated tensor values based on recent X-ray structure data. The results consistently reveal a breaking of the local C₂ symmetry of the electronic structure at the primary donor side of the reaction center. This is of particular interest since it might be relevant for the vectorial electron transfer along the protein complex. Among the quinone radical anions studied are frozen solutions of the electron acceptors of bacterial and plant reaction centers (ubiquinone and plastoquinone, respectively). The increased electron Zeeman interaction in high-field EPR leads to almost completely resolvedg-tensor components even in disordered samples. Theg-tensors and component linewidths are sensitive probes for specific anisotropic interactions with the environment. In the case of the transient correlated coupled radical pair P₈₆₅ ^+.-Q_A ^?. ofRb. sphaeroides (Fe replaced by Zn) the spin-polarized high-field EPR spectra allow an unambiguous determination of the relative orientation of theg-tensors of the donor and acceptor parts. Thereby high-precision structure information is obtained on the electron transfer pigments after light-induced charge separation.     

Simulation numérique de l'écoulement et du transfer de chaleur autour d'un cylindre

This paper deals with the numerical study of heat transfer and air flow around a heated cylinder in crossflow. A finite-volume method with a third-order convective scheme (Quick) has been used along with the Trio software to solve the heat and momentum balance equations. Local velocity and temperature fields have been determined. Besides, transient (Strouhal number) and time-averaged (wall shear stress, pressure coefficient and Nusselt number) results have been obtained and compared with the literature data. A good agreement has been obtained for both the local and averaged values.     

Spectral dependence of photoinduced optical absorption in doped yttrium iron garnet single crystals

The photoinduced optical absorption α of doped yttrium iron garnets (YIG) is investigated. It is found that the optical absorption α at a wavelength of 1.1 μm depends on the wavelength of irradiating light in the range 0.6–1.9 μm. It is demonstrated that, in the Y₃Fe₅O₁₂ crystal with an acceptor Ba impurity, the photoinduced increase in α is due to the formation of Fe⁴⁺ ions in octahedral sites. The charge transfer occurs through photoexcitation of the ⁶ A _1g (⁶ S) → ⁴ T _1g (⁴ G) and ⁶ A _1g (⁶ S) → ⁴ T _2g (⁴ G) transitions of octahedral Fe³⁺ ions. In the crystal with a donor Si impurity, the increase in α is caused by the formation of Fe²⁺ ions upon photoionization of silicon. __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 7, 2001, pp. 1233–1235. Original Russian Text Copyright ? 2001 by Doroshenko, Nadezhdin.     

Structure and properties of rare-earth-doped glassy systems

Glasses of composition 90Na₂B₄O₇-(10−x) V₂O₅-xPr₆O₁₁ [x=0, 0.5, 1, 2, 4, 6, 8 and 10 mol%] were prepared by the melt-quenching technique. The density (ρ) and molar volume (V_M) were obtained and were found to increase with increase in Pr₆O₁₁ content. Such behaviour was accounted for by the difference between atomic weight and the atomic radii of the V and Pr ions. The rare-earth ion concentration (N) and ionic radius (r_p) were calculated. The differential thermal analysis (DTA) thermographs were used to determine the glass transition temperature (T_g) and crystallization temperature (T_c), for the examined compositions. Both T_g and T_c values were observed to increase with increased Pr₆O₁₁ content. The glass-forming ability (GFA) was estimated and it was found to increase with increase in the Pr₆O₁₁ content.