Erich Robens

Application of coupled thermal analysis techniques to thermodynamic studies of water interactions with a compressible ionic polymer matrix

Abstract The swelling of a compressible matrix (perfluorosulfonated membrane “Nafion”) was studied using different thermal analysis techniques coupled with thermogravimetry. Free swelling isotherms, molar heats of interaction, isochore swelling isotherms, swelling pressure, changes of external volume, D.S.C. peaks gave informations on the thermodynamic state of water in the membrane which were confirmed by spectroscopic measurements like NMR, ESR, Mossbauer. From all data it appears clearly that water molecules are distributed in small droplets around the sulfonate dipoles. The interaction energies are small and show at high water contents the contribution of endothermic terms due to compression of the matrix. The swelling pressures are important (300 to 600 bars). The water content is lower in isochore experiments and it diminishes at low temperatures. So, during cooling of a 20°C saturated sample there is a demixion process with phase separation of excess water. The DSC peaks must be interpreted in terms of desorption-sorption in thermal cycling and not in terms of porometry. Such a behaviour cannot be explained with classical sorption model. Recently new thermodynamic models of elastic matrices have been developped; the simplest has only two parameters describing the matrix: its dielectric constant and its shear modulus. With the aid of all experimental data, this model will be discussed for a complex system like Nafion.

Volumetric and gravimetric methods of determining monolayer capacities

Abstract A comparison is made between the volumetric and the gravimetric methods for measuring adsorption isotherms. The discussion includes equipment, handling, price, accuracy and sensitivity. Dead space and buoyancy errors are calculated and compared. The aim of the work is to initiate a detailed discussion of these methods of determining specific surface area and pore size distribution.

Disturbances in weighing - Part I A survey of work presented at the preceding VMT conferences

Abstract A survey is given on disturbances which determine the accuracy of weighing in vacuum. The nature of the disturbances covers the range from fundamental to purely practical. Apart from the physical description of the disturbing phenomena, numerical estimates for beam-type balances are presented. Part I covers the following phenomena: • Brownian motion • Knudsen forces • Cavity forces • Unequal thermal expansion of balance arms • Magnetostatic effects • Radiation pressure • Vibrations of the building • Other effects Additional effects will be discussed in Part II during the 21st Conference, 1985 at Dijon.

Measurement of sorption, vaporization and decomposition of materials used in vacuum technology by means of an electromagnetic microbalance

Abstract An apparatus is described by means of which the weight change of solid bodies can be measured at different temperatures in different atmospheres and under vacuum; the sensitivity is 10−6g. The apparatus can be used for testing materials and for studying sorption processes in vacuum technology. For a number of substances, such as sealing materials (Neoprene and Viton ∗ ), a polytetrafluoroethylene (Gaflon ∗ ) fabric, several vacuum greases and an hydrocarbon-resistant grease, the sorption of water vapour and nitrogen was measured and their thermal stability investigated. From a study of the observed decrease in weight with time, it is possible to determine whether this loss is caused by emission of sorbed gas, vaporization or decomposition of the material.

A new arrangement for the production of thermal gas flows and the measurement of radiometer forces

Abstract This paper was first presented (in German) at the first Annual Meeting of the Deutsche Gesellschaft fur Vakuumtechnik e.V. (German Society for Vacuum Technique) September 1962. Because of its interest it has been selected, with the collaboration of the Society, for translation and wider dissemination by publication in VACUUM in advance of the publication of the complete Proceedings. Using an electromagnetic microbalance, radiometer forces were measured on specimens exposed to a temperature gradient at low pressures. These forces, which were large compared with the sensitivity of the balance, cannot be adequately accounted for by available theories. The forces were measured within a pressure range from 10−5 to 100 torr as a function of the type of gas, the temperature conditions, the dimensions of the arrangement and the materials and geometry used for the specimens. Equations are given which provide a sufficiently accurate description of the results for the two limiting cases “mean free path large or small compared with the inside dimensions of the apparatus“. Since the forces are dependent on pressure, they may be used in a pressure gauge which, combined with a buoyancy measurement, covers the range from 10−6 torr to atmospheric pressure.

Computerised reduction of the weighing time required for the determination of adsorption isotherms on activated carbons

Abstract The method of extrapolation of equilibrium values of adsorbed mass to determine the nitrogen isotherm as presented in Berlin involves application of an exponential equation. This method has been found suitable for determinations in the meso- and macropore region. For the micropore range transformation using a quadratic equation and subsequent application of linear regression is preferable in several cases. The resultant values are evaluated by means of the Dubinin-Radushkevich method. The results of the determination by these two extrapolation methods are in good agreement. Linear regression is also suitable for the evaluation of adsorption kinetics measurements.

Some remarks on the balance as a symbol

Abstract Today just as in very early illustrations the balance is regarded a a symbol. It is an attribute of merchants and various trade guilds. In the hands of the Gods of Greece, it indicates victory or death of the hero. In the Egyptian religion, it first appears as an instrument in the death tribunal. In Judaism and for the Romans, it was the Symbol of justice.

Methods for the Elimination of Weighing Troubles Due to Convection in a Microbalance

The paper deals with an investigation into the fluctuations encountered in thermogravimetric measurements using a microbalance with suspended pans, in a temperature range up to 1200 C. The convection resulting at gas pressures above 200 torr, which is responsible for these fluctuations, depends on the pressure, the type of gas used, the temperature gradient, and the geometry of the arrangement. The difficulties have been overcome by using long, thin suspension wires; shielding samples and balance beam with horizontally suspended foils attached to the suspension wire or to the balance tube; using heaters with a homogeneous temperature zone and a helium gas stream protecting the measuring system.

The ancient Egyptian balance

Abstract The oldest known balances are equal-armed instruments that have been found in Egypt and are represented on Egyptian drawings and reliefs. In its simplest form the beam was made of wood or stone and provided with a central boring for the balance suspension and two borings at the ends of the beam for the pan suspensions. The existence of a balance standard in the Fifth Dynasty is testified. A plummet line was hung parallel to the balance suspension so that the horizontal position of the beam could be checked by comparing the right angles formed between beam and plummet line. From the time of the Middle Kingdom the pans were suspended by four cords. In the New Kingdom tubular beams were introduced whose ends were shaped in lotus flower or papyrus form. The strings of the pans came out together from inside the beam and diverged to the sides, the lower beam ends acting as knife edges. In most cases a pointer is either missing or so short that it is hardly helpful. This type of elaborate standard balance which was also distributed in Syria, Greece and Persia, is often depicted in the Books of Death of the New Kingdom in the scene showing the “Death Tribunal” with “Weighing of the Heart”. Unclear drawings led to misinterpretations as to the design and mode of operation of these balances. By style analysis and comparison with finds exhibited in the Cairo Egyptian Museum, it was possible, however, to clarify details.

The corrosion induced change of pore size distribution and surface area of a fuel element graphite

Abstract The transport behaviour of fission products in HTGR-fuel element graphite may be latered by changes of the pore structure of the graphite due to corrosion. The corrosion rate was measured at temperature between 850 and 900 °C in a CO2 atmosphere by a thermogravimeric method. Surface area and pore size distribution were determined before and after corrosion by measuring nitrogen adsorption isotherms at 77 K. The corrosion rate remained constant regardless of the pronounced increase in the specific surface area.