Agglomerate formation during drying
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Andrew Allen Ceramics Division, National Institute of Standards and Technology, Building 223/Room A163, 100 Bureau Drive, Gaithersburg, Maryland 20899
Merrilea J. Mayoa) Department of Materials Science & Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 1 July 2002; accepted 21 November 2002)
The evolution of agglomerate structure during drying of particles from suspension has been studied for a nanocrystalline Y2O3 (8% mol fraction)-stabilized ZrO2 powder. Agglomerates in drying and dried suspensions were examined at the smallest size scales (1 nm to 1 m) using ultra-small angle x-ray scattering (USAXS) and at the largest size scales (100 nm to 10 m) using scanning electron microscopy. The results were correlated with the degree of particle dissolution in each suspension (measured by flame absorption spectroscopy of the suspension filtrate) and the zeta potential of the particles in suspension prior to drying. Results show that large agglomerates readily form across a pH range from 2 to 9. The fact that Y+3 ion dissolution varies by over four orders of magnitude in this range leads to the conclusion that there is little direct correlation between the degree of Y dissolution and agglomeration in this system (Zr ion dissolution was below the detection limit at all pH values studied). The observation of large agglomerates well before the introduction of air-water interfaces into the drying mass likewise leads to the conclusion that capillary forces are not essential to agglomerate formation. Instead, agglomerates appear to form as a direct consequence of increasing suspension concentration. Zeta potential also plays a role. Specifically, there was a notable change in agglomerate morphology as the isoelectric point was approached, at approximately pH 8. Here USAXS shows the particles in suspension to have a layered interior structure, with small primary particles aggregated in sheets to form each blocky particle. This is in contrast to the more rounded agglomerates formed away from the isoelectric point, which appear to be composed of the same primary particles arranged in chainlike structures. USAXS of powders from the dried suspensions confirms that the structures seen after drying are the same as those present in suspension. The two structural morphologies are attributed to diffusion-limited (sheets) versus reaction-limited (chains) aggregation, respectively.
I. INTRODUCTION
The formation of agglomerates during ceramic powder processing is a pervasive problem that, in most cases, prohibits the fabrication of fully dense ceramics. Although it is well known that drying of liquid-borne powders will create agglomerates, it is not known exactly how or why those agglomerates evolve in certain size and shape ranges. The problem of agglomeration is particularly acute in the nanometer-size range, where it a)
Presently adjunct to the Department of Materials and Nuclear Engineering, University of Maryland, Building 090, College Park, Maryland 20742-2115 J. Mater. Res., Vol. 1
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