温度和颗粒浓度对纳米流体粘度的影响

采用"两步法"制备了含有不同质量分数的TiO2-水纳米流体,并观察了其稳定性。测量了不同质量分数的纳米流体在15～40℃时的粘度,结果表明,纳米流体的粘度随颗粒浓度的增加而增大,随温度的升高而以指数形式降低,并且各种纳米流体的粘度随温度的变化趋势相似。结合实验数据,对已有粘度计算公式进行修正,提出了涉及温度和颗粒浓度的纳米流体粘度计算公式。     

Conductivity and voltammetry in liquid and supercritical halogenated solvents

A previous study of the voltammetry of ferrocene in liquid and supercritical chlorodifluoromethane revealed electrochemically reversible behavior. However, shifts in the half-wave potential as a function of fluid conditions were observed which were tentatively attributed to ohmic distortion due to changes in fluid resistance. To more completely understand the voltammetry in this fluid, conductivity measurements have been made for a range of fluid conditions. Additionally, a second reference couple, cobaltocenium hexafluorophosphate, has been introduced, and the difference in half-wave potentials between the two redox couples has been examined as a function of fluid conditions. In the liquid, the difference in the half-wave potentials of the two couples corrected for ohmic distortion decreases as the fluid temperature increases (25-85 °C) at constant pressure (5.2 MPa). In the supercritical fluid at constant temperature (115 °C), the difference in the half-wave potentials corrected for ohmic distortion is constant at 1.276 ± 0.005 V over a considerable range of fluid pressure (10-30 MPa). Ion aggregation in the supercritical fluid is indicated both by the conductivity measurements and by the rather large hydrodynamic radius of cobaltocenium computed from the voltammograms. Preliminary voltammetry in supercritical trifluoromethane is also presented.     

Effect of crystal orientation on freeze concentration of solutions

Freeze concentration is one of the methods to concentrate solution by making pure ice in the solution. In general, the ratio of condensation is focused on, and so the research on the concentration captured in ice is very limited. However, quality improvement of eliminating impurities from wastewater is a very important task. In this study, the difference of initial morphology of ice was focused on and the influence on the freeze concentration was studied. It was found that the concentration of solute captured in ice was in the following order, from bigger value, multi-crystal ice, a single crystal ice with growth direction in a-axis and a single crystal ice with growth direction in c-axis. It was clarified that the ratio of concentration between the solute captured in ice and the solute in the mother solution varied from 1/10 to 1/250, depending on the crystal orientation of the ice.     

Generalized theory of steady-state voltammetry without a supporting electrolyte. Effect of product and substrate diffusion coefficient diversity

A generalized theory of the steady-state voltammetric response of a microelectrode in the absence of supporting electrolyte and for any values of diffusion coefficients of the substrate and the product of an electrode process is presented. The treatment applies to any reasonable combination of the charge numbers of the substrate, its counterion, and the product. A way to incorporate the activation polarization into the model is also demonstrated. It has been shown that the height, position, and shape of the migrational voltammogram are affected by the ratio of the product to substrate diffusivity (theta). In particular, for the electrode processes with sign retention, unequal diffusivities of electroactive species influence both characteristic points of the voltammogram (the limiting current and the half-wave potential). For charge neutralization processes (uncharged product), the changes in theta parameter are accompanied only by a shift in the half-wave potential. The most dramatic changes in the I-E relation can be observed for the charge reversal processes. In this case, a consecutive increase in theta results in the transition of the voltammogram shape from rapid exponential growth (theta < 1), through ramp shape (theta = 1), to common wave shape (theta > 1). On the basis of the expressions derived for the limiting current (exact and linearized), a possibility of the determination of the diffusion coefficient of the electrode reaction product is demonstrated. In addition, the ranges of theta where the assumption of equal diffusivities of the substrate and the product is obeyed within an insignificant error have been determined quantitatively. The theory has been experimentally verified using voltammetric oxidation of hexacyanoferrate(II).     

Effect of electrolyte concentration on microstructure and properties of micro arc oxidized hydroxyapatite/titania nanostructured composite

Micro arc oxidation was employed to grow TiO₂/hydroxyapatite composite layer on titanium substrate. The correlation between electrolyte concentration, diameter and density of the pores in fabricated layers was investigated. Therefore, the effect of electrolyte concentration on composition and morphology of grown layers was studied using SEM, EDX, XRD and XPS techniques. Samples were coated in electrolytes containing 5, 10 and 15 g/l calcium acetate and 1, 3 and 5 g/l β-glycerophosphate, at optimized voltage for 3 min. Pore size variations obey a similar pattern by the addition of both calcium acetate and β-glycerophosphatein various concentrations based on SEM observations. However, completely different volume fraction of porosities in as-obtained coatings was obtained as a result of these two components addition. Indeed, size and volume fraction of porosities in fabricated layers are directly affected by electrolyte concentration. The average crystallite size of TiO₂ and hydroxyapatite was determined as 45 nm and 48.5 nm, respectively. Moreover, the biocompatibility of selected specimens has been proved by MTT test.     

Interaction-site representation of polar mixtures and electrolyte solutions

In this work we present an interaction-site theory for mixture electrolyte solutions. The theory is semi theoretically adjusted for dielectrical properties. The theory preserves the correct asymptotic limits of the dielectric constant when the solution approaches the limit of any of its pure component. In the intermediate concentration range it interpolates the dielectric constants according to a quadratic mixing rule in relative dipole densities. For the simplified interaction-site models applied here, the excess energies of water/methanol at 293.15 K and experimentally measured densities are in fair agreement with experimental values. The estimated average number of hydrogen bonds for a mole fraction of water equal to 0.75 also compares well with published simulations. Estimated energies with and without the dielectric correction indicate that the dielectric constant may have a significant impact on the energy for this molecule. Excess energies for mixtures of water and ethanol are estimated to be too low.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Production of powders and films by the rapid expansion of supercritical solutions

A new process utilizing the rapid expansion of supercritical fluid solutions (RESS) is described for the production of fine powders and thin films by the rapid nonequilibrium precipitation of nonvolatile compounds from dense gas solutions upon expansion. A variety of the fluid solution expansion parameters, including solute and solvent identity, solute concentration, expansion temperature, and expansion nozzle configuration, has been shown to affect the product characteristics of materials formed during the RESS process. Conditions favouring thin film formation include very dilute solutions and short nozzles minimizing residence time during expansion. Particle formation is favoured by more concentrated solutions. The process has been demonstrated to produce products of widely varying morphology by the adjustment of RESS parameters, and examples of SiO₂ GeO₂ and various polymeric materials are presented. Unique features of the RESS process relevant to other powder and film production methods are described and potential applications are discussed.     

An experimental investigation and modelling of the viscosity refrigerant/oil solutions

This paper presents experimental data for the viscosity of solutions of refrigerant R600a (isobutane) with mineral compressor oils Azmol, Reniso WF 15A, and R245fa (1,1,1,3,3-pentafluoropropane) with polyolester compressor oil Planetelf ACD 100 FY on the saturation line. The experimental data were obtained for solution of R600a with mineral compressor oil Azmol in the temperature range from 294.7 to 338.1 K and the concentration range 0.04399 ≤ w_R ≤ 0.3651, the solution of R600a with mineral compressor oil Reniso WF 15A at the temperatures from 285.8 to 348.4 K and the concentration range 0.03364 ≤ w_R ≤ 0.2911, the solution of R245fa with polyolester compressor oil Planetelf ACD 100 FY at the temperatures from 309 to 348.2 and the concentration range 0.06390 ≤ w_R ≤ 0.3845. The viscosity was measured using a rolling ball method. The method for prediction of the dynamic viscosity for refrigerant/oil solutions is reported.     

Effects of electrolyte concentration and current density on the properties of electro-deposited NiFeW alloy coatings

NiWP alloy coatings were prepared by electrodeposition, and the effects of ferrous chloride (\(\hbox {FeCl}_{2})\), sodium tungstate (\(\hbox {Na}_{2}\hbox {WO}_{4})\) and current density (\(D_{\mathrm{K}}\)) on the properties of the coatings were studied. The results show that upon increasing the concentration of \(\hbox {FeCl}_{2}\), initially the Fe content of the coating increased and then tended to be stable; the deposition rate and microhardness of coating decreased when the cathodic current efficiency (\(\eta \)) initially increased and then decreased; and for a \(\hbox {FeCl}_{2}\) concentration of \(3.6\, \hbox {g\,l}^{-1}\), the cathodic current efficiency reached its maximum of 74.23%. Upon increasing the concentration of \(\hbox {Na}_{2}\hbox {WO}_{4}\), the W content and microhardness of the coatings increased; the deposition rate and the cathode current efficiency initially increased and then decreased. The cathodic current efficiency reached the maximum value of 70.33% with a \(\hbox {Na}_{2}\hbox {WO}_{4}\) concentration of 50 g \(\hbox {l}^{-1}\), whereas the deposition rate is maximum at 8.67 \(\upmu \hbox {m}\,\hbox {h}^{-1}\) with a \(\hbox {Na}_{2}\hbox {WO}_{4}\) concentration of \(40\, \hbox {g\,l}^{-1}\). Upon increasing the \(D_{\mathrm{K}}\), the deposition rate, microhardness, Fe and W content of the coatings increased, the cathodic current efficiency increases first increased and then decreased. When \(D_{\mathrm{K}}\) was 4 A dm\(^{-2}\), the current efficiency reached the maximum of 73.64%.     

Micronization of dihydroartemisinin by rapid expansion of supercritical solutions

The purpose of this study was to prepare fine particles of antimalarial drug dihydroartemisinin (DHA) by rapid expansion of supercritical solutions (RESS) using carbon dioxide as supercritical fluid. The mechanical grinding by jet mill and additional vibration rod mill also was performed as a comparative method. In the RESS process, drug particles were prepared by varying processing conditions, including extraction condition, pre-expansion condition, nozzle diameter, nozzle temperature, and collecting distance. Particle size and morphology and physicochemical characteristics of the drug particles were investigated. The RESS process could produce the smaller drug particles (about 1-2 microm) when compared to mechanical grinding method (about 7 microm). All RESS processing parameters had an effect on size and morphology of drug particles. The particle size of drug was related to the solubility of drug in supercritical CO(2) at each processing condition. The fine particles of DHA (about 1 microm) with narrow size distribution could be obtained at extraction pressure of 18 MPa and extraction temperature of 32 degrees C, which was closed to the critical temperature of supercritical CO(2) whereas broad size distribution was obtained at extraction temperature of 60 degrees C. Powder X-ray diffraction study indicated that the RESS-processed particles were in crystalline form. The results revealed that RESS process is applicable for micronization of DHA.     

Metal/polypyrrole quasi-reference electrode for voltammetry in nonaqueous and aqueous solutions

As an alternative to the usual commercial reference and quasi-reference electrodes (QREs), we propose metal (Pt, stainless steel) coated with partially oxidized polypyrrole as a QRE. The electrode is easily fabricated by cyclic voltammetry (CV) with the metal electrode in an acetonitrile or CH2Cl2 solution of 10 mM pyrrole containing 0.1 M Bu4NPF6. These QREs were more stable than the widely used metal (e.g., Ag, Pt) wire QRE as demonstrated by testing in several aqueous and organic media, with the stability checked by CV with ferrocenemethanol as the redox species. The results obtained demonstrate good stability of these QREs over a period of 1 day and relatively reproducible potential in a given solvent and supporting electrolyte. This electrode has the advantage that it does not contaminate the test solution with ions or solvent from a usual liquid electrolyte (e.g., the KCl in an Ag/AgCl). It is also simple to fabricate very small electrodes for use in nanocells, e.g., in scanning tunneling and electrochemical microscopy.     

Steady-state voltammetry of hydroxide ion oxidation in aqueous solutions containing ammonia

An oxidation process observed in dilute aqueous solutions of ammonia was investigated under steady-state conditions with gold microelectrodes with radii in the range 2.5-30 microm. Over the ammonia concentration range 0.1-10 mM, a well-defined voltammetric wave was observed at approximately 1.4 V versus Ag/AgCl. It was attributed to the oxidation of hydroxide ions that arise from the dissociation of the weak base. The steady-state limiting current was found to depend on the concentration of supporting electrolyte, and in solution with low electrolyte, it was enhanced by migration contribution, as expected for a negatively charged species that oxidizes on a positively charged electrode. In addition, the steady-state limiting current was proportional to both the ammonia concentration and the electrode radius. The overall electrode process was analyzed in terms of a CE mechanism (homogeneous chemical reaction preceding the heterogeneous electron transfer) with a fast chemical reaction when measurements were carried out in solutions containing NH3 at < or = 5 mM and with electrodes having a radius of > or = 5 microm. This was ascertained by comparing experimental and theoretical data obtained by simulation. The formation of the soluble complex species Au(NH3)2+ was also considered as a possible alternative to explain the presence of the oxidation wave. This process however was ruled out, as the experimental data did not fit theoretical predictions in any of the conditions employed in the investigation. Instead, the direct oxidation of NH3, probably to N2O, was invoked to explain the anomalous currents found when the CE process was strongly kinetically hindered. Throughout this study, a parallel was made between the CE mechanism investigated here and that known to occur during the hydrogen evolution reaction from weak acids.     

Automatic express-control of the concentration of aqueous solutions

We classify methods for the determination of the total concentration of salts in water. We developed variants of an acoustic method for express-control of the concentration of salts in water when results did not depend on a variation in the temperature. Measurement and functional conversion of multiply reflected signals along the measuring base in reference liquids and liquids under investigation are the foundations of there variants. We proposed principles of construction and the structure of a three-channel microprocessor system, which allows us to automatize processes of measurement and calibration, improve sensitivity, and extend the measurement range. Karpenko Physicomechanical Institute. Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 5, pp. 122–128, September–October, 1997.     

A polymer NMR cell for the study of high-pressure and supercritical fluid solutions

Nuclear magnetic resonance (NMR) offers researchers unique, highly localized molecular information. The importance of this technique is well established in studies using chemical shift, spin coupling, and relaxation times providing detailed structural information, determining chemical equilibria and kinetics, and understanding molecular dynamic processes. However, the widespread application of NMR spectroscopy to high-pressure liquids and supercritical fluids has been limited due to the complexity of the necessary instrumentation. One approach to these studies is to build a dedicated high-pressure probe. Another involves the utilization of a high-pressure cell designed to fit in commercially available probes. Here we present the design and implementation of a simple, three-piece, high-pressure NMR cell constructed of high-performance polymers. The present cell has pressure capabilities of up to 400 bar; however, the ultimate temperature and pressure limits will be determined by the specific polymer chosen. High-resolution NMR spectra of methanol modified and tributyl phosphate (IBP) modified supercritical CO2 are presented. An example of supercritical fluid phase behavior monitored with NMR is demonstrated for the TBP system in which the chemical shift changes in the 31P nucleus as a function of density are indicative of solution phase separation. The multinuclear NMR data demonstrate the utility of this cell for studying supercritical fluid solution systems relevant to analytical separations and extractions.     

Evaporation rate of concentrated electrolyte solutions from fine capillaries

Solution concentration at the meniscus is elevated above the initial value, but does not vary during evaporation from capillaries. The reduction in evaporation rate as compared to water determined experimentally is in agreement with previously developed theory.Notation mass of a mole of water, g/mole - D diffusion coefficient of vapor through air, cm²/sec - relative vapor pressure in chamber (relative air humidity) - M relative vapor pressure above meniscus - density of water, g/cm³ - R gas constant, erg/mole·deg - T temperature, °K - D_* diffusion coefficient for molecules in solution, cm²/sec Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 5, pp. 849–853, November, 1979.     

Investigation of precipitation selectivity and particle size concentration dependences in supercritical antisolvent method via online supercritical fluid chromatography

Supercritical antisolvent (SAS) precipitation technique, although being versatile and ecologically friendly, suffers from the lack of convenient methods for necessary thermodynamic parameters measurement. Recently we have proposed a method for solubility measurement in binary fluids based on an online hyphenation of supercritical antisolvent method and supercritical fluid chromatography (SAS-SFC). In this paper, we demonstrate the applicability of this method to the investigation of both selective precipitation from solution and particle size tuning in SAS using lower dicarboxylic acids as model objects. Measured solubility values adequately reflect selective crystallization from solution. SAS precipitation was observed only for those components, which concentration was above solubility in CO₂-solvent mixture as predicted by SAS-SFC method. Also, concentration dependences of particle size plotted in supersaturation coordinates instead of direct concentration in initial solution give additional insight into crystallization behaviour in SAS.     

Evaluation of the Hall parameter of electrolyte solutions in thermosyphonic MHD flow

The objective of this work is to determine experimentally the Hall parameter of electrolyte solutions using a closed loop thermosyphonic magnetohydrodynamic flow. The upper and lower parts of the loop, which represent the heat sink and heat source of the system respectively, are constructed from copper pipe coated with varnish on the inside surface. The middle region, connecting the upper and lower parts of the loop, is made from plastic vertical pipes, with segmented copper electrodes placed vertically opposite to each other on each side of the loop plastic walls and connected as a Hall generator to measure the open circuit voltage. A transverse magnetic field is imposed in the middle non-conducting plastic-wall region by a set of permanent magnets. The magnets provide a magnetic field strength of up to 0.225 T, whereas the driving temperature difference between the hot and cold portion of the loop ranges from 10 to 80 °C. Measurements of the induced flow rate and induced open circuit voltage are reported as a function of driving temperature difference and magnetic field strength.The analytical one-dimensional model of Ghaddar [Int. J. Heat Mass Transfer 41 (8-9) (1998) 1075] is extended to account for the electrode design and the Hall effect pertinent to electrolyte solutions. The open circuit voltage is related to the driving temperature difference, flow characteristic, magnetic field strength, electrolyte electric properties and electrode design. The developed 1-D model and the measured open circuit voltage are used to evaluate the Hall parameter (ωτ), which is a property of the fluid of the electrolyte liquid. It is found that ωτ can be as large as 100 for electrolytes and causes a significant loss in power output at the electrodes due to electron drift in the fluid leading to generation of current in an axial direction at the expense of the current flowing in the transverse direction between the electrodes.