Structure of Milled Galena (PbS) Particles as a Result of Grinding: Observations by Electron Microscopy
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P. Penhoud and P. Veyssie`re Laboratoire d’e´tude des microstructures, CNRS UMR 104, ONERA, BP 72, 92322 Chatillon Cedex, France (Received 14 January 2002; accepted 1 April 2002)
We have examined galena powders with the aim of providing information about the preparation mode of such powders from ancient Egyptian burial objects. Two extreme conditions of milling have been used to prepare galena powders in the laboratory, and the resulting products have been examined using scanning electron microscopy and transmission electron microscopy (TEM). The microstructure of hand-crushed coarse particles consists mainly of dislocation tangles. Annealing at 300 °C promotes a substantial recovery of the dislocation structure with the formation of subboundaries. Energetic ball milling produces a large variety of particle sizes, from 10 nm to several micrometers, with grains containing very high dislocation densities. Although PbS is a soft plastic compound, its fragmentation occurs down to very small sizes along various fracture regimes like in many brittle materials. Comparisons are made between TEM observations and the data obtained from x-ray diffraction peak profile analysis.
I. INTRODUCTION
Granular materials play an important role in a large number of industrial sectors such as construction, ceramic, food, drug, cosmetic, etc., and this extends back in time to the earliest human technological activities. Yet, many aspects of the behavior of particles are still not well understood. In particular, techniques for particle size reduction (crushing, milling, etc.) are widely used while their impact on materials has received limited attention. In the process of milling, fracture is expected to determine the final size and morphology of the particles. However, the fragmentation mechanisms involved at small sizes are unclear; fracture mechanics no longer applies below approximately 5 m. In addition to fracture, the stress can generate lattice defects (dislocations, etc.), induce phase transformation, or promote the formation of interfaces (grain boundaries, etc.). The resulting microstructures are known to strongly influence the fracture properties of materials and the milling process, as well as their physical or chemical properties. In this paper, we examine properties of galena (PbS) particles ground in the laboratory. The aim of the work is to reproduce, and therefore identify, the process used by the Egyptians to manufacture cosmetics about 4000 years ago. We have analyzed the mechanisms of size reduction and the microstructures induced in the particles. Galena is easily cleaved along {100} planes, and its favored slip J. Mater. Res., Vol. 17, No. 7, Jul 2002
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system for plastic deformation (i.e., 〈110〉{001}) operates at rather low stresses.1 Hence galena is a brittle and soft crystal and this is at the origin of characteristic features of ground PbS powders which will be compared to those observed in ancient cosmetics. In particular, dislocation structures, which can
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