Inorganic Particle Size Measurement Below 100 nm - Where Life Gets Difficult
Several different methods, photon correlation spectroscopy (PCS), an X-ray disc centrifuge (XDC), X-ray powder diffraction (XRD) and high resolution scanning and transmission electron microscopy combined with image analysis have been used to characterise the particle size distributions of several inorganic powders ( Al2O3, FexOy, SiO2). The different powders cover the size range from 5 nm to around 100nm and have different shapes and degrees of agglomeration. The gamma alumina is highly agglomerated, the silicas are roughly spherical and the iron oxide slightly elongated but well dispersed (low agglomeration). The state of agglomeration and how different treatments such as milling and surface modification affect the state of agglomeration is often poorly described in most nanoparticle studies. We shall illustrate how agglomeration can described for an “as received” and attrition milled gamma alumina, both as a function of milling time and bead size. Results are presented after consideration of the hydrodynamic density in the sedimentation methods, and light scattering for the optical based methods. The results with model spherical silica powders (Dv50 of 12, 35 and 80 nm) show how the accuracy of the sedimentation methods depends strongly on a knowledge of the particle hydrodynamic density. This can be affected greatly by particle or agglomerate porosity, the thickness of the electrical double layer or strongly bound hydration layer often found for nanoparticles in the aqueous dispersions investigated. Precipitated iron oxide particles with sizes around 10 nm start pushing the limits of several of the methods. A careful protocol had to be developed using XRD and TEM to allow quick and reliable measurements to be made using PCS. This allowed the investigation of the effect of PVA adsorption on the size of the iron oxides important for biomedical applications.
Role of Electrostatic Repulsion on the Viscosity of Bidisperse Silica Suspensions.
The flow behavior of bidisperse aqueous silica suspensions has been studied at different electrolyte concentrations as a function of shear rate, total volume fraction of the particles, and volume ratio of small to large particles. It is shown that the range of the electrostatic repulsion plays an important role in determining the viscosity of the suspension. Binary mixtures of particles of longer range repulsive forces showed higher viscosities than the suspensions of shorter range electrostatic interactions. Bimodal suspensions of long-range interactions showed non-Newtonian behavior over wider ranges of shear due to the deformation of the ionic cloud around the particles, which is larger in these systems. The viscosity of bimodal suspensions used in this study was scaled with respect to the viscosity of the related monosized systems and the viscosity of one bimodal suspension at a fixed total volume fraction of the particles, employing our earlier scaling method. The model normalizes the effect of colloidal forces by introducing a scaling factor that collapses the data into a single curve for bimodal suspensions of a particular size ratio, and it is shown that the model is valid for systems with both short-range and long-range repulsive forces. Copyright 1999 Academic Press.
Evolution of carbide precipitates in Hastelloy N joints during welding and post weld heat treatment
Abstract The effects of welding and post weld heat treatment (PWHT) on the morphology, composition and lattice parameter of the precipitates in the welding joints of Hastelloy N alloy have been investigated. Joint samples were prepared using a gas tungsten arc welding process with ERNiMo-2 fillers, and then solid-solution treated at 1100 °C for 20 min. After welding, eutectic M6C carbides can be observed in the heat affected zones (HAZ) and weld zones (WZ). These eutectic M6C carbides become spheroidized during the following PWHT. The concentration of Si decreases during the transformation from the primary ones to eutectic ones, and then increases during the spheroidizing process. The total concentration of Si + Cr in M6C carbides remains stable during welding and PWHT. At the same time, the heat leads to the outward diffusion of Mo from the M6C carbides, which results in a decrease of the lattice parameter.