Benno Wessely

Particle Sizes of Fumed Silica

Fumed silica is a synthetic amorphous silicon dioxide produced by burning silicon tetrachloride in an oxygen-hydrogen flame. Surface areas range from 50-400 m 2 /g. Using particle sizing techniques, fumed silica shows micron sized particles leading to surface areas markedly lower than expected. Fumed silica appears as a fluffy solid with bulk densities down to 0.03 g/cm 3 , being invariant over the wide range of surface areas. Attempts to relate the variation of the surface area directly to the performance of fumed silica in technical applications, such as its thickening efficiency in fluids, mainly fail and remain ambiguous.

Particle characterisation in highly concentrated dispersions using ultrasonic backscattering method

Determining particle size and concentration in highly concentrated suspensions and emulsions is challenging, especially under process conditions. In general, ultrasound therefore can be used for particle characterisation due to the ability of sound waves to pass opaque dispersions, whereas optical detection principles mostly are limited to low particulate contents. An established acoustic method, the ultrasonic attenuation spectroscopy, uses a transmission setup for measuring the attenuation of a dispersion. A major drawback of this measurement method is caused by the fact, that the measuring gap tends to plug, which again limits the inline capability. To overcome this limitation, an ultrasonic reflection setup is used for gathering the sound waves, which are reflected, respectively backscattered by the dispersion. Statistically analysing the corresponding backscattering signal yields the sound attenuation as well as a scattering intensity equivalent. Both measurement parameters can be shown to be sensitive against particle size and concentration. Based on a single scattering theory, a semi-empirical approach is presented for interpretation of measurement results with respect to particle size and concentration. Measurements, performed on a glass beads in water dispersion, show good agreement with theory for dimensionless wave number 0.1

CONSTRUCCIÓN Y CALIBRACIÓN DE UNA CELDA DE SEDIMENTACIÓN GRAVIMÉTRICA PARA LA MEDICIÓN FOTOMÉTRICA DE LA DISTRIBUCIÓN DE TAMAÑO DE MICRO PARTÍCULAS

This paper present the theoretical basis, development, implementation and results of from a gravimetric sedimentation cell for determination of size distribution and median diameter of a sample of micro particles. The measurement is based on the transmittance of light through a suspension particles in water. The cell was built in modular form and was calibrated with suspensions of amorphous silicion dioxid with a median diameter of 1.5 µm. The results indicate that the sedimentation cell is very simple to use and that the size distribution of micro particles can be determined at low cost. This procedure may be conveniently applied in the many industrials processes.

Evaluation of preparation methods for suspended nano-objects on substrates for dimensional measurements by atomic force microscopy

Dimensional measurements on nano-objects by atomic force microscopy (AFM) require samples of safely fixed and well individualized particles with a suitable surface-specific particle number on flat and clean substrates. Several known and proven particle preparation methods, i.e., membrane filtration, drying, rinsing, dip coating as well as electrostatic and thermal precipitation, were performed by means of scanning electron microscopy to examine their suitability for preparing samples for dimensional AFM measurements. Different suspensions of nano-objects (with varying material, size and shape) stabilized in aqueous solutions were prepared therefore on different flat substrates. The drop-drying method was found to be the most suitable one for the analysed suspensions, because it does not require expensive dedicated equipment and led to a uniform local distribution of individualized nano-objects. Traceable AFM measurements based on Si and SiO2 coated substrates confirmed the suitability of this technique.

Statistical signal processing for ultrasonic particle characterization

An ultrasonic minimal intrusive reflection method for characterization of randomly moving particles in high concentrated suspensions is described. The statistical features of multiple consecutive scattered sound waves are used that represent the scattering amplitude and its decrease over time. Those parameters correlate with the particle size and concentration and deliver equivalent information like the attenuation calculated by conventional extinction measurements for particle characterization. For empirical validation the exponential decrease of the standard deviation was compared with the attenuation measured with a transmission-mode ultrasonic spectrometer. Especially the dependency for high particle concentration is shown.

Monitoring of highly concentrated suspensions by ultrasonic scattering

A new method for inline characterization of particles in high concentrated suspensions by ultrasonic scattering is described by extracting the statistical features of scattered sound waves that correlate with the particle size and concentration. To provide a high spatial resolution and range - in case of media with high attenuation - ultrasonic sensors with a high bandwidth and intensity are used in combination with signal coding techniques. The effects of particle dependent attenuation and multiple reflections are investigated by processing the signals in the frequency and time domain. Further, a spatial dependent attenuation coefficient is calculated that is equivalent to the values gained by conventional transmission methods.

Inline-Partikelmessung an hochkonzentrierten Suspensionen

Zusammenfassung Ein methodisch neuer Ansatz zur Charakterisierung der Feststoffphase in Suspensionen mit Ultraschall wird vorgestellt. Gegenüber bekannten Transmissionsverfahren ermöglicht die Messung in Reflexion auch die Analyse höher konzentrierter Stoffsysteme. Mit der inline aus dem Schallsignal abgeleiteten Information zur Partikelkonzentration bietet sich eine verfahrenstechnische Alternative zu Partikelmesstechniken, die auf aufwändiger Offlineanalytik basieren. Abstract A new method is presented for the characterization of the solid phase in suspensions using ultrasound. In contrast to the established transmission techniques, the analysis of higherly concentrated media is enabled through the reflection-based operation mode. With the information of particle concentration, deduced from the ultrasonic signal, an inline alternative is available to measurement techniques for offline analytics.

Construção e calibração de uma cela de sedimentação gravimétrica para a medição fotométrica da distribuição do tamanho de micropartículas

This paper present the theoretical basis, development, implementation and results of from a gravimetric sedimentation cell for determination of size distribution and median diameter of microparticles.

Dynamische Extinktionsspektroskopie – Eine Chance zur Erweiterung des Messbereiches photometrischer Partikelmesstechniken

Dynamic extinction spectroscopy is based on the simultaneous application of dynamic extinction measurement and optical extinction spectroscopy for online particle size measurement. It covers a range of particle sizes and size distributions from approx. 80 nm to 250 pm. Different applications will underline the potential of this measuring method for online process control.