Defect density measurements in shocked single crystal ammonium perchlorate by x-ray photoelectron spectroscopy
- PDF / 1,713,776 Bytes
- 9 Pages / 576 x 792 pts Page_size
- 25 Downloads / 187 Views
W. L. Elban Department of Electrical Engineering and Engineering Science, Loyola College, Baltimore, Maryland 21210 (Received 3 June 1992; accepted 6 August 1992)
The linewidths of x-ray photoelectron spectra have been correlated with dislocation densities in a shock-damaged crystal of ammonium perchlorate (AP). A centimeter-size AP crystal was loaded at several sites with a diamond pyramid (Vickers) indenter, creating localized strain centers. The crystal was nonuniformly damaged by a rapidly decaying shock (peak pressure of 24.4 kbar at the entry surface), recovered intact, and cleaved through the indentations. The cleaved planes permitted interior analysis of the crystal by x-ray photoelectron spectroscopy (XPS) over a pattern of 1 mm by 1 mm areas. The linewidth of the Cl(2p3/2) spectra ranged from 1.70 eV for the region of greatest visible damage to 1.22 eV for the region of no visible damage, the same linewidth as that obtained for unshocked AP (control). The observed damage was compared to photographs in the literature of gamma-ray irradiated AP crystals, for which dislocation densities were reported. This provided an approximate correlation of dislocation density versus XPS linewidth. The correlation was refined by chemically etching and determining densities on another cleaved plane in the recovered crystal. By this technique, a —100X increase in dislocation density was determined for the region of greatest shock damage relative to an unshocked crystal. The strain fields associated with the impressions were found to enhance perchlorate decomposition when driven by shock. Distortion of the molecular lattice in the vicinity of a dislocation is the physical mechanism responsible for the broadening of the photoelectron lines. Ab initio calculations of the Cl(2p) energy level in the perchlorate anion predicted variations of 0.1 to 0.46 eV. Variations of this magnitude are sufficient to produce the observed linewidth broadening. I. INTRODUCTION X-ray photoelectron spectroscopy (XPS) has a rich history in its application to the assessment of a material's surface elemental, chemical, and structural nature. In the current work, XPS was used to examine an AP crystal that had been shock loaded near its reaction threshold. Of special interest in this study was the relative effect of crystal defect concentrations on the initiation of shock-induced chemistry. Those defect concentrations existed within strain fields surrounding diamond pyramid (Vickers) hardness impression placed in the crystal prior to shock loading. The shocked crystal was recovered intact and cleaved through the impressions so that the XPS analysis could be performed in a systematic pattern of small area analyses on what was the interior of the crystal. In addition to the observation of additional XPS peaks indicating localized chemical decomposition,1 there was a systematic variation in the linewidth of the XPS peaks. The pattern of the XPS Cl(2p3/2) linewidth essentially duplicated the variation of visible shock damage in the recovered crystal. Ass
Data Loading...
