Crystallite growth of nanocrystalline transition metals studied in situ by high temperature synchrotron X-ray diffraction

The thermal stability of nanocrystalline iron; Palladium and oxygen-doped nickel was investigated in situ by synchrotron X-ray diffraction. Synchrotron experiments allow the determination of crystallites up to the mum-range and the high flux of the synchrotron source enables in situ experiments with very high time resolution. For the normal crystallite growth of nanocrystalline iron we found two different growth regimes. At low temperatures (663 < T < 739) the activation energy of the growth rate constant is about 100 kJ/mol whereas 173 kJ/mol can be determined for higher temperatures (T > 739 K). Our advanced X-ray technique allows also the detection of a bimodal crystallite size distribution. The crystallite growth of nanocrystalline palladium shows above 628 K an anomalous behaviour. Nanocrystalline nickel can be stabilized by the pulse-reverse electrodeposition technique due to co-deposition of oxygen in the grain boundaries. The effect of nickel oxide on the thermal stability of n-Nickel was studied using samples with oxygen contents between 956 and 6039 wt.-ppm. At 873 K samples with a low oxygen content grow immediately up to the mum-range; Samples with 2000 ppm oxygen can be stabilized at final crystallite sizes of about 100 nm whereas n-nickel with the highest oxygen concentration even after several hours exhibits a constant crystallite size of only 40 nm. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.