E. Robens

Physics lecturing with audience paced feedback

For some time we have experimented at the Eindhoven University of Technology with a system which provides students with an electronic feedback path to the lecturer, audience paced feedback (APF). In this paper we describe this APF system, and give indications of its effectiveness.

High-resolution sorption studies of argon and nitrogen on large crystals of microporous zeolite ZSM-5

SummaryHigh resolution adsorption (HRADS) with argon and nitrogen at 77 K were performed on large crystals of zeolite ZSM-5 using a novel volumetric device. Multi-step isotherms for both adsorptives could be observed for the first time. The micropore filling was followed by low temperature microcalorimetry. Exothermic heats of adsorption were found to be correlated with steps in the adsorption isotherms. Based on results from atom-atom potential energy calculations (AAP) as well as from independent model building it is shown that 24 kinetic adsorbate molecules can be situated in a ZSM-5 unit cell. Localized adsorption is presented as possible filling mechanisms. Experimental results are reasonably interpreted assuming primary filling of narrow channels and secondary adsorption in the wider channel intersections. At least for nitrogen there is evidence for a transition of fluid-like to a solid-like adsorbate phase.Compared to argon, Henrys constants and the initial isosteric heat of adsorption indicate a stronger adsorption of nitrogen which is thought to be due to additional interactions of the nitrogen quadrupole moment with the zeolite framework.

Investigation of characterizing methods for the microstructure of cement

Volumetric, gravimetric, calorimetric, flow methods, mercury porosimetry and laser granulometry were used to investigate the surface structure and the sorption behavior of industrial cements and hydrated cement paste. The suitability of the measuring methods is assessed.

Sources of Error in Sorption and Density Measurements

In sorption measurements, volumetric or gravimetric procedures are commonly used to determine the amount adsorbed. At low pressures, thermomolecular flow and pressure differences according to Knudsens law disturb measurements. In volumetry, calibration of the dead space is required; in gravimetry, the influence of buoyancy has to be taken into account. In both cases, adsorption of the calibrating gas, usually helium, may disturb the measurements [1]. From the calibration measurements, the density of the sample can in principle be calculated. However, it has been observed in many experiments that its value depends on the calibrating gas.

A note on sorption measuring instruments

Gas sorption phenomena can be used to characterise porous solids and dispersed materials. Usually isotherms of nitrogen and noble gases like He, Ar, Kr are measured at low temperatures (77–90 K). Other gases and vapours like water, CO2 and benzene are used at near ambient conditions of pressure and temperature. From the amount of gas adsorbed on the (external or internal) surface characteristic quantities like the specific surface area, specific pore volume and pore spectrum of the material are derived by standardised methods. Experimental techniques most often used are the carrier gas, the volumetric/manometric and the gravimetric method. A comprehensible overview of todays available instruments, their advantages and drawbacks is given.

Thermogravimetric and Sorption Measurement Techniques/Instruments

The survey covers commercially offered instruments for the determination of the surface structure of porous solids based on isothermal measurement of physisorption of inert gases and of thermogravimetric instruments. Basic assumptions, features and experimental uncertainties are compared.

Berechnung der Fraktaldimension einiger poröser Feststoffe aus der Stickstoff-Adsorptionsisotherme

The surface fractal dimension, dfS, is a parameter for the characterization of the surface roughness which is independent of the size of the scale applied on the sample surface. By means of a thermodynamic method dfs as well as the limits of fractality can be obtained from the nitrogen adsorption isotherm at 77 in the region of capillary condensation. The applicability of this method is demonstrated on several disperse materials ofdifferent origin.

GRAVIMETRIC MEASUREMENT OF WATER VAPOUR SORPTION, MOISTURE AND DRY MASS

The gravimetric measurement of the moisture content of solids is discussed in comparison to other usual methods. Results of measurements of silica gel are reported. Furthermore, we inform on standardisation work.

A Jäntti Approach for Quick Calculations of Sorption Equilibria

Jäntti introduced a method to calculate the adsorption equilibrium by measuring the actual adsorbed amount three times after a change to the gas pressure. He successfully applied the method to gas/solid systems in which simple adsorption processes occur. In the present paper we discuss a more general approach for different modes of measurement (adsorption and desorption), including sorption processes obeying second order differential equations.

Criticism on Jäntti's Three Point Method on curtailing gas adsorption measurements

Jäntti introduced a method to reduce the time required for the stepwise measurement of adsorption isotherms (Jäntti et al., Progress in Vacuum Microbalance Techniques, Vol. 1, Heyden, London, pp. 345–353, 1972). After a pressure change he measured the adsorbed mass three times and calculated its equilibrium value for the new pressure. In the present paper we discuss the applicability of this method in a broader scope (without starting from a single combination of gas and solid adsorbent) and also discuss the influence of measurement inaccuracies. It is shown that the method can serve as an early warning system, which decides whether an improvement of the adsorption model is necessary.