The Application Of Positron Annihilation Technique In The Study Of Aluminum Nanopowders

The combustibility of Aluminum (Al) Nanopowders is much better than microsizedAl, and it is an irresistable trend for Al nanopowders replacing microsized Al powders insolid propellant. Some studies have found that the same sized Al nanopowders made bydifferent methods showed different combustibility. The reason was thought to be thatdifferent defects were introduced during the preparation process. The main content of thispaper is to investigate whether these defects exist, and their links to the preparationmethods. Positron annihilation technique (PAT) has an unique advantage in analysis ofthe size and concentration of atomic-level defects in the solid materials, so thistechnology was used to research the defects of Al nanopowders. At the meanwhile, inorder to make a comprehensive characterization of surface and defects of Alnanopowders, XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy) andnitrogen adsorption isotherm analysis were supplemented to analyze the instruction.Through understanding of the principles of positron annihilation, most of theannihilation events happened on the nano grain surface and interface. In this paper,positron annihilation experiments were used to compare the positron lifetime between thecoarse-grained Al and nanocrystals Al. The results confirmed that the positions ofpositrons annihilated in nanocrystals were different from coarse grain. The Alnanopowders with the same size but prepared by different methods, which were electricalexplosion wires method (E-nAl), laser-induction complex heating method (L-nAl) andplasma arc method (P-nAl), were found that the concentration of surface defects of E-nAlwas the maximum, the concentration of L-nAl and P-nAl were second and thirdrespectively, the size of these defects were about as large as5monovacancies. Then theisochronal annealing were carried out in two of the samples, the positron lifetimespectroscopy (PLS) were also carried out in the two samples after the annealing. The results of PLS showed that as the annealing temperature increased, the concentration ofsurface defects was becoming lower and the size shrinked. The final states of differentsamples after annealing at high temperature500℃tended to be the same. These generalinstructed the difference of the surface microstate were brought in by the differentpreparation methods.The XRD test in subsequent found lattice constants of the three kinds of Alnanopowders were different, and the number of defects were deduced as Cd(E-nAl)>Cd(L-nAl)> Cd(P-nAl), where the Cdis the concentration of defects. Furthermore, thesurface vacancies of Al nanopowders were found to be the absence of Al atoms in theXPS test, and also the concentration of vacancies, got from the XPS results of differentsamples, arranged the same with the results of PAT. Although the nitrogen adsorptionisotherm can not reflect the atomic-scale roughness in the surface, but, in the macro, theBET results of the three kinds of Al nanopowders which got from the nitrogen adsorptionisotherm also showed the arrangement as SE-nAl> SL-nAl> SP-nAl, where S was the specificsurface.