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A thickness variation of only one Ångström makes a significant difference in the current through a tunnel junction due to the exponential thickness dependence of the current. It is thus important to achieve a uniform thickness along the barrier to enhance, for example, the sensitivity and speed of single electron transistors based on the tunnel junctions. Here, we have observed that grooves at Al grain boundaries are associated with a local increase of tunnel barrier thickness. The uniformity of the barrier thickness along the tunnel junction thus increases with increasing Al grain size. We have studied the effect of oxidation time, partial oxygen pressure and also temperature during film growth on the grain size. The implications are that the uniformity improves with higher temperature during film growth. [ABSTRACT FROM AUTHOR]

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Abstract: A binder phase grain boundary was located in a WC–Co–Cr cemented carbide using electron backscatter diffraction. A transmission electron microscopy specimen was produced including the grain boundary of interest. An energy dispersive X-ray analysis showed segregation of W and Cr to the grain boundary. However, no impurity atoms were found. [Copyright &y& Elsevier]

Abstract: The aging of a model 1wt.% Pt/Al2O3 catalyst was performed stepwise under different reactive atmosphere to study the evolution of metal dispersion and NO oxidation activity. After each aging step the dispersion was evaluated by CO chemisorptions and the activity of the catalyst for NO oxidation was measured using 500ppm NO and 8% O2 diluted in Ar. After a degreening step at 500°C, aging was performed at 600, 700, 800 and 900°C. Five wash-coated cordierite monoliths were aged in Ar, 10% O2, 1% H2, 30ppm SO2 and 30ppm SO2 +10% O2, respectively. The general trend showed a linear decrease in dispersion when increasing the aging temperature for the lower aging temperatures and for the highest ones the dispersion levels off. When the platinum dispersion decreased the NO oxidation activity increased, due to that the reaction is structure sensitive. H2 seemed to hinder sintering at low aging temperature. Interestingly, after aging in 10% oxygen at 600°C the NO oxidation activity was significantly higher compared to the Ar aged sample, although the dispersions were similar. Aging in oxygen at higher temperatures resulted in a decrease of dispersion and a slightly decreasing NO oxidation activity. Moreover lower dispersion limit was reached with oxygen aging. Aging in SO2 provoked a severe dispersion drop at low aging temperature meanwhile the activity increased only moderately. However, activity kept increasing with further treatments at higher temperature. The combination of O2 and SO2 enabled to decrease rapidly the dispersion and to greatly enhance the catalytic NO oxidation activity after the first aging step at only 600°C. The best overall conversion was obtained for the catalyst treated with this mixture after aging at 800°C. A 22-h aging at 250°C in a mixture containing 500ppm NO, 10% O2 and 30ppm SO2 led to a significant decrease of Pt dispersion, which shows the ability of SO2 to promote platinum sintering already 250°C. The low temperature sintering was confirmed with STEM measurements. Several larger particles were observed, but also many small particles remained. Thus the SO2 +O2 induced low temperature sintering results in a large variation of particle sizes. This treatment resulted in an increase of the maximum NO conversion (after reduction of the sample) from 45% to 76%. The different aging experiments show that it is beneficial to add SO2 during aging and the reason is the increased particle size, but also a clear chemical effect was observed. [Copyright &y& Elsevier]

Abstract: In this investigation different experimental methods to determine the platinum dispersion of Pt/BaO/Al2O3 NO x storage catalysts are compared. The dispersion of platinum is determined independently using temperature programmed desorption of CO, dissociation of N2O, static volumetric CO and H2 chemisorption, dynamic CO chemisorption and transmission electron microscopy. For Pt/BaO/Al2O3, reproducible results are obtained when the platinum dispersion is determined from N2O dissociation, CO and H2 chemisorption. However, repeated CO-TPD experiments result in decreased amount of desorbed CO, which limits this method to determine platinum dispersion for Pt/BaO/Al2O3 samples. For Pt/Al2O3, similar values for platinum dispersion are obtained for all methods used in the study. In addition to the platinum dispersion investigations, the surface morphology of Pt/BaO/Al2O3 samples and the distribution of different elements are studied using SEM-EDS. The SEM-EDS analyses show that the platinum is evenly distributed over the entire sample surface, while the distribution of barium is more uneven. [Copyright &y& Elsevier]