310 Daniel Haley et al.


Table 4. The most significant contaminant species are oxy- gen, presumably from sample transport, and Hg, which is assumed to be a volatile contaminant in the industrial-grade purging gas. Some unidentified contaminants, possibly organic components from pneumatic lubricants within the charging device were detected, but are not examined for the purposes of this work. As indicated in Table 4, the decom- posed deuterium content was variable, between 0.9 and 3 at%, with slightly higher D contents in the 500 kPa (5 bar) charging. However, this is radically lower than the thermodynamically predicted amounts of D in the bulk phase (40–45 at%) for pure Pd. There is some change in D after holding for 6 days, however as a single data point this result is not statistically robust. Nevertheless, it does demonstrate the stability of the formed hydride within the time and length scales of the APT experiment. As the systemunder investigation is within a two-phase


Table 3 shows the relative quantity of D within each peak from the overlapped Pd/Pd-H/Pd-D signal for three sets of charging experiments and subsequent analyses. The applied peak decomposition uses a flat-time-of-flight-based algorithm on a nearby peak-free region (90–96Da) to estimate back- ground noise, and a least-squares nonnegative solver for fitting. Statistical testing for nonnormality (Anderson-Darling test; National Institute of Standards and Technology, 2001) was performed to ensure normality of the background histogram, as used in a previous work (Haley et al., 2015). Concentration data for each analysis is reported in


Table 4. Relative Composition of Charged Samples (at%, back- ground corrected, decomposed).


Pressure (kPa, abs.) 500


Species H


D C


Unidentified O


Rh Pd


0.1


3.07 1.46 1.05


500


1.91 1.57 2.13 8.59


11.79 10.38 7.23


74.70 69.31 6.12 500 (6-day delay)


Composition (at%) 3.61


1.92 0.43 5.73 4.61 7.10


76.57 200


2.15 0.92 0.93 3.59


12.61 6.98


72.51


Unidentified species have been treated as a single component for decom- position.


by the isoconcentration surface, there is a clear relative increase in D concentration up to 10 at% D content in the core of the highlighted region. This may be a remnant of some β within the material, as it is clear that there is no preference for increased evaporation of H, which would be an artifact of the APT analysis, and only D which is real signal from the charging process.


region, one could reasonably expect to observe some spatial inhomogeneity within the distribution of D within the atom probe reconstruction due to remaining β phase. Indeed a small localized region increased in D is identifiable, within which there is no corresponding increase in H. Figure 3 shows a reconstruction of a postcharged sample, within which two isoconcentration surface analyses have been applied. The first (red) identifies regions within which the concentration of D is at least 5 at%, whereas the second (gray) highlights regions in the reconstruction which the concentration of H is at least 5 at%. Note that there is no apparent relationship between the two regions. As shown in the proximity histogram (proxigram), which measures the chemistry as a function of distance from the interface defined


Table 3. Relative Fraction of D from Each Identified Pd-H/D Species Within the Data Set (1+ Charge State), After Resolving Overlapped Peaks.


5 Pd


Pd-H Pd-D R2


0.96 0.01 0.04


>0.999


D2 Charging Pressure (kPa, abs.) 500


0 0.04 >0.999 200


Relative Species Fraction (At. Fr.) 0.96


0.96 0


0.04 >0.999


Surface ions were omitted from the analysis using a Pd >50% isosurface to exclude contamination.


Figure 3. 5% (ionic) D and H isoconcentration surfaces, showing enhanced D zone within the data set and proxigram of that zone. D concentration increases to up to 10% D within the marked zone, H is present primarily as a surface artifact. Data set is not calibrated, so dimensional data should be considered approximate. Note composition scale is logarithmic.


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