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

The thermal stability of nanocrystalline iron; Palladium and oxygen-doped n ickel was investigated in situ by synchrotron X-ray diffraction. Synchrotro n experiments allow the determination of crystallites up to the mum-range a nd the high flux of the synchrotron source enables in situ experiments with very high time resolution. For the normal crystallite growth of nanocrysta lline iron we found two different growth regimes. At low temperatures (663 < T < 739) the activation energy of the growth rate constant is about 100 k J/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 cr ystallite size distribution. The crystallite growth of nanocrystalline pall adium shows above 628 K an anomalous behaviour. Nanocrystalline nickel can be stabilized by the pulse-reverse electrodeposition technique due to co-de position of oxygen in the grain boundaries. The effect of nickel oxide on t he thermal stability of n-Nickel was studied using samples with oxygen cont ents between 956 and 6039 wt.-ppm. At 873 K samples with a low oxygen conte nt grow immediately up to the mum-range; Samples with 2000 ppm oxygen can b e stabilized at final crystallite sizes of about 100 nm whereas n-nickel wi th the highest oxygen concentration even after several hours exhibits a con stant crystallite size of only 40 nm. (C) 2001 Acta Materialia Inc. Publish ed by Elsevier Science Ltd. All rights reserved.