Kinetic Analysis of the Thermal Processing of Silica and Organosilica

The incorporation of an organic group into sol–gel-derived silica causes significant changes in the structure and properties of these materials. Therefore, the thermal treatment of organosilica materials may require a different approach. In the present paper, kinetic parameters (activation energy, pre-exponential constant, and reaction models) have been determined from mass loss data for the dehydration, dehydroxylation, and decomposition reactions that take place upon heating silica and organosilica. Parameters were obtained by employing model-free isoconversional methods to data obtained under multiple heating rates as well as by multivariate analysis of the kinetics using a multistep reaction model with distributed activation energy. For silica, it can be concluded that the reaction atmosphere (i.e., inert or thermo-oxidative) has no influence on the reaction rate of the dehydration and dehydroxylation reactions that are responsible for the densification of the material. Under inert atmosphere, full dehydration can be reached without affecting the organic moiety. Achieving complete dehydroxylation of the organosilica is practically impossible as decomposition does manifest itself under commonly employed calcination temperatures. This indicates that prudence is required in designing a heat treatment program for these hybrid materials. To aid in optimizing the thermal treatment, a predictive model was developed, which can be used to forecast the extent of dehydration, dehydroxylation, and decomposition reactions under a multitude of temperature program

Highly permeable and mechanically robust silicon carbide hollow fiber membranes

Silicon carbide (SiC) membranes have shown large potential for applications in water treatment. Being able to make these membranes in a hollow fiber geometry allows for higher surface-to-volume ratios. In this study, we present a thermal treatment procedure that is tuned to produce porous silicon carbide hollow fiber membranes with sufficient mechanical strength. Thermal treatments up to 1500 °C in either nitrogen or argon resulted in relatively strong fibers, that were still contaminated with residual carbon from the polymer binder. After treatment at a higher temperature of 1790 °C, the mechanical strength had decreased as a result of carbon removal, but after treatments at even higher temperature of 2075 °C the SiC-particles sinter together, resulting in fibers with mechanical strengths of 30–40 MPa and exceptionally high water permeabilities of 50,000 L m−2 h−1 bar−1. Combined with the unique chemical and thermal resistance of silicon carbide, these properties make the fibers suitable microfiltration membranes or as a membrane support for application under demanding condition

Temperature calibration procedure for thin film substrates for thermo-ellipsometric analysis using melting point standards

Precise and accurate temperature control is pertinent to studying thermally activated processes in thin films. Here, we present a calibration method for the substrate–film interface temperature using spectroscopic ellipsometry. The method is adapted from temperature calibration methods that are well developed for thermogravimetric analysis and differential scanning calorimetry instruments, and is based on probing a transition temperature. Indium, lead, and zinc could be spread on a substrate, and the phase transition of these metals could be detected by a change in the C signal of the ellipsometer. For water, the phase transition could be detected by a loss of signal intensity as a result of light scattering by the ice crystals. The combined approach allowed for construction of a linear calibration curve with an accuracy of 1.3 C or lower over the full temperature range

Synthesis of Porous Inorganic Hollow Fibers without Harmful Solvents

A route for the fabrication of porous inorganic hollow fibers with high surface-area-to-volume ratio that avoids harmful solvents is presented. The approach is based on bio-ionic gelation of an aqueous mixture of inorganic particles and sodium alginate during wet spinning. In a subsequent thermal treatment, the bio-organic material is removed and the inorganic particles are sintered. The method is applicable to the fabrication of various inorganic fibers, including metals and ceramics. The route completely avoids the use of organic solvents, such as N-methyl-2-pyrrolidone, and additives associated with the currently used fiber fabrication methods. In addition, it inherently avoids the manifestation of so-called macro voids and allows the facile incorporation of additional metal oxides in the inorganic hollow fibers

Swellipsometry in Twente: Measuring Polymer Swelling by In-Situ Ellipsometry

Thin and ultra-thin (<100 nm) polymer films are frequently used in important technological areas, including coatings, barrier and membrane applications. In these areas the films are frequently exposed to interacting penetrants. The interactions may significantly change equilibrium and dynamic properties of the thin film systems, thereby influencing their performance. In addition, it is known that the reduction of polymer film thickness below about 100 nm may result in the manifestation of, so called, nano-confinement effects. This term relates to the pronounced departure of the ultra-thin polymer properties from those of the bulk. In-situ spectroscopic ellipsometry is a powerful technique for monitoring dynamic changes in properties of thin films in contact with penetrants. This non-intrusive technique allows for very high precision, accuracy, and temporal resolution. In this contribution we show several examples in which the potential of the technique is utilized. The examples include studies on temperature-induced transitions of penetrant diffusion mechanisms in the vicinity of glass transition, probing surface diffusion in ultra-thin glassy films, and thin film composite membrane behavior under non-equilibrium permeation conditions

Tellipsometry in Twente: Dynamics of Thin Film Membranes Under Applied Temperature Profiles

We use in-situ ellipsometry to study the structural and chemical evolution of thin films as function of the temperature (‘Tellipsometry’). Particular focus is on organic, inorganic, and hybrid materials that are relevant to artificial membrane fabrication and operation. Our poster shows some illustrative examples

Swelling of 9 polymers commonly employed for solvent-resistant nanofiltration membranes: A comprehensive dataset

The presence of a solvent in a dense polymeric nanofiltration membrane layer can affect the macromolecular dynamics of the polymer material and the mobility of the solvent penetrant molecules. In addition, even the affinity of the swollen material for the solvent molecules can be distinct from that of the pure polymer material. These effects can substantially affect the membrane's performance. This paper provides sorption and swelling data of 9 thin polymer films that are commonly used for organic solvent nanofiltration (P84, Matrimid, PEI, PAN, PES, PSf, PEBAX, PTMSP, PDMS) swollen by 10 common solvents (hexane, toluene, dichloromethane, ethyl acetate, methyl ethyl ketone, acetone, isopropanol, ethanol, methanol, water). The paper describes the swelling dynamics during short-term solvent exposure (0–8 h), assesses the stability upon long-term solvent exposure (up to 2 months), and provides quantitative data on the solvent volume fractions inside the swollen layers. Among the surprising findings are the glubbery behavior of PTMSP and the completely different response of PES and PSf to toluene exposure. The results of this work demonstrate three crucial findings relevant to organic solvent nanofiltration membranes and other applications: 1. For many polymers, the swelling changes over long timescales of up to 2 months and longer. Results obtained on short timescales do however not always allow for direct extrapolation to longer time scales.2. Structural similarity of polymers does not guarantee similar swelling behavior.3. Swelling behavior of solvents cannot be solely explained by classifying solvents into aprotic, polar aprotic and polar protic solvents.The results of this work can aid in constructing transport models and can help predicting polymer-solvent compatibility and membrane performance in OSN applications