G. Wiegand

Bio: G. Wiegand is an academic researcher from Karlsruhe Institute of Technology. The author has contributed to research in topics: Supercritical fluid & Supercritical carbon dioxide. The author has an hindex of 11, co-authored 26 publications receiving 358 citations. Previous affiliations of G. Wiegand include ETH Zurich.

Interfacial tension between water and non-polar fluids up to 473 K and 2800 bar

Abstract: The method of the Pendant Drop or Standing Bubble is applied to measure interfacial tensions between water and nonpolar fluids to high temperatures and pressures. The high pressure cell with two sapphire windows and the auxiliary equipment with several feed autoclaves is described. The shapes and sizes (about 2 mm) of drops and bubbles are recorded with microscope and video camera. A digital image processing procedure was developed which permits fast, objective and precise determination of the contour parameters. The six gases helium, neon, argon, nitrogen, methane, and propane have been investigated to 473 K (with nitrogen to 573 K) and (in part) to 2800 bar. Gas densities came close to liquid density values. For comparison, water plus liquid n-hexane, n-decane, and toluene was investigated to 473 K and 3000 bar. For these liquid hydrocarbons, the interfacial tension γ always increases with pressure. At 373 K for water-n-hexane γ is 41.8 mN/m at 100 bar and 47.3 mN/m at 2600 bar, respectively. In the water-gas systems γ decreases with pressure and passes through a flat minimum around 1000 bar. For water-nitrogen at 373 K γ = 52.5, 46.5 and 48.3 mN/m at 200, 1400 and 2800 bar. Only with water-helium γ increases continuously with pressure.

Bio-inspired, large scale, highly-scattering films for nanoparticle-alternative white surfaces.

Abstract: Inspired by the white beetle of the genus Cyphochilus, we fabricate ultra-thin, porous PMMA films by foaming with CO2 saturation. Optimising pore diameter and fraction in terms of broad-band reflectance results in very thin films with exceptional whiteness. Already films with 60 µm-thick scattering layer feature a whiteness with a reflectance of 90%. Even 9 µm thin scattering layers appear white with a reflectance above 57%. The transport mean free path in the artificial films is between 3.5 µm and 4 µm being close to the evolutionary optimised natural prototype. The bio-inspired white films do not lose their whiteness during further shaping, allowing for various applications.

Enhanced Photoluminescence in Quantum Dots–Porous Polymer Hybrid Films Fabricated by Microcellular Foaming

Abstract: DOI: 10.1002/adom.201900223 emission linewidth, tunable peak emission wavelength, and high photoluminescence quantum yield (PLQY).[1–4] In such applications, QDs are dispersed into a polymeric matrix to form hybrid films, which usually serve as free-standing, remote-type configurations excited by an external UV/blue light source.[5–7] In this layout, the light conversion process is notably hindered by the limited absorption of the excitation light by the QDs. Indeed, the QD nanoscale size does not result in efficient light scattering that could increase the UV/blue light absorption probability.[8–10] Besides, the reabsorption of the converted light by the QDs as well as their current costs prevent the use of too high QD concentrations.[11,12] Lastly, the converted light propagating in the hybrid film can be trapped and guided to the film’s edges upon consecutive total internal reflection events.[13] In order to improve the absorption of the excitation light and the extraction of the converted photons, QDs can be included within photonic crystal slabs or deposited on their surface.[14–16] By engineering the coupling to leaky modes, a high energy density can be obtained at resonance in the direct vicinity of the QDs, and the emission directionality together with its polarization The color conversion efficiency of thin polymeric layers embedding quantum dots (QDs) is limited by their negligible light scattering ability and by the insufficient absorption of the excitation photons. In this study, a route is presented to tackle these optical shortcomings by introducing a tailored network of micropores inside these hybrid films. This is achieved by exploiting the microcellular foaming approach which is rapid, cost effective and only makes use of a green solvent (supercritical carbon dioxide). With an appropriate combination of the applied pressure and temperature during foaming, and by using a proper film thickness, the photoluminescence (PL) intensity is enhanced by a factor of up to 6.6 compared to an equivalent but unfoamed hybrid film made of CdSe/ZnS QDs in a polymethyl methacrylate matrix. Spectroscopic measurements and ray tracing simulations reveal how the porous network assists UV/blue light absorption by the QDs and the subsequent outcoupling of the converted light. The approach improves the PL for various QD concentrations and can be easily scaled up and extended to other polymeric matrices as well as light converting materials.

The viscosity of n-decane to high temperatures of 573 K and high pressures of 300 MPa

Abstract: The dynamic viscosity of liquid n-decane has been measured with the "Oscillating Disk Method" from 293 K to 573 K and from 0.1 MPa to 300 MPa. A high pressure autoclave of 50 mm internal diameter contained between two rigid disks an oscillating stainless steel disk of 44 mm diameter suspended on a platinum-tungsten wire. Pressure was generated with a spindle press filled with a suitable hydraulic fluid separated from the n-decane. Thus heating and pressure variation could be performed at selected constant volumes. The density dependence of the viscosity could be determined. The viscosity increases at 373 K from 375 to 1602 μPas in the pressure range from 0.1 to 200 MPa, at 473 K from 200 to 1114 μPas in the pressure range from 10 to 300 MPa, and at 573 K from 223 to 675 μPas in the pressure range from 60 to 300 MPa. The mean accuracy of the viscosity measurements is about 1.3%. A polynomial function describing the viscosity variation with pressure and temperature is given. The maximum average deviation is 0.46%, the mean average deviation between measured and calculated data is 0.11%. The viscosity as a function of density is also shown. With Arrhenius plots, the energies of activation are obtained.

Organic Chemical Reactions in Supercritical Water.

Abstract: Water near or above its critical point (374 C, 218 atm) is attracting increased attention as a medium for organic chemistry. Most of this new attention is driven by the search for more green or environmentally benign chemical processes. Using near-critical or supercritical water (SCW) instead of organic solvents in chemical processes offers environmental advantages and may lead to pollution prevention. Interest in doing chemistry in SCW is not entirely new, however. There has been much previous research in this area with applications in synthetic fuels production, biomass processing, waste treatment, materials synthesis, and geochemistry. Water near its critical point possesses properties very different from those of ambient liquid water. The dielectric constant is much lower, and the number and persistence of hydrogen bonds are both diminished. As a result, high-temperature water behaves like many organic solvents in that organic compounds enjoy high solubilities in near-critical water and complete miscibility with SCW. Moreover, gases are also miscible in SCW so employing a SCW reaction environment provides an opportunity to conduct chemistry in a single fluid phase that would otherwise occur in a multiphase system under more conventional conditions. The paper discusses the use of supercritical water in the following reactions:more » hydrogenation/dehydrogenation; C-C bond formation; rearrangements; hydration/dehydration; elimination; hydrolysis; partial oxidation; H-D exchange; decomposition; and oxidation.« less

Short Fundamental Equations of State for 20 Industrial Fluids

Abstract: In a preceding project, functional forms for “short” Helmholtz energy equations of state for typical nonpolar and weakly polar fluids and for typical polar fluids were developed using simultaneous optimization. In this work, the coefficients of these short forms for the equations of state have been fitted for the fluids acetone, carbon monoxide, carbonyl sulfide, decane, hydrogen sulfide, 2-methylbutane (isopentane), 2,2-dimethylpropane (neopentane), 2-methylpentane (isohexane), krypton, nitrous oxide, nonane, sulfur dioxide, toluene, xenon, hexafluoroethane (R-116), 1,1-dichloro-1-fluoroethane (R-141b), 1-chloro-1,1-difluoroethane (R-142b), octafluoropropane (R-218), 1,1,1,3,3-pentafluoropropane (R-245fa), and fluoromethane (R-41). The 12 coefficients of the equations of state were fitted to substance specific data sets. The results show that simultaneously optimized functional forms can be applied to other fluids out of the same class of fluids for which they were optimized without significant loss of a...

Corrosion in high-temperature and supercritical water and aqueous solutions: a review

Abstract: The aim of the present article is to review some of the common corrosion phenomena and describe the predominant corrosion mechanisms in high-temperature and supercritical water. Corrosion in aqueous systems up to supercritical temperatures is determined by several solution-dependent and material-dependent factors. Solution-depending factors are the density, the temperature, the pH value, and the electrochemical potential of the solution, and the aggressiveness of the attacking anions. Material-dependent parameters include alloy composition, surface condition, material purity, and heat treatment. Corrosion phenomena that are observed include intergranular corrosion, pitting, general corrosion, and stress corrosion cracking. The solubility and dissociation of both attacking species and corrosion products play the most important role for corrosion in high-temperature water. Both solubility and dissociation processes are strongly influenced by the density, or the ionic product, respectively, of the solvent. High values of both parameters favor ionic reactions, and thus, accelerate electrochemical forms of corrosion. At low densities, water behaves like a non-polar solvent, and thus, ions associate. In these cases, the concentation of e.g. aggressive H + drops down and thus, solutions containing species such as HCl become neutral and thus less aggressive. Further, corrosion products plug the surface and material loss stops. Materials parameters have influence especially on the initiation of corrosion. In the present article, these factors are linked with the physical and chemical properties of high-temperature and supercritical water. An outlook is also given for future research needs.

Supercritical water as a solvent.

Abstract: Water is not restricted to moderate temperatures and low pressures, but can exist up to very high temperatures, far above its critical point at 647 K. In this supercritical regime, water can be gradually compressed from gas-like to liquid-like densities. The resulting dense supercritical states have extraordinary properties which can be tuned by temperature and pressure, and form the basis for innovative technologies. This Review covers the current knowledge of the major properties of supercritical water and its solutions with nonpolar, polar, and ionic compounds, and of the underlying molecular processes.