Atom Probe Analysis of Ex Situ Gas-Charged Stable Hydrides 309
After charging, D2 was vented from the system and then a further purging cycle was conducted to safely remove deuterium gas. Charging times were 1 h for Pd–Rh, in order to ensure sufficient time for diffusion of D into the sample. For V, the exposure time was 30 min, as the diffusion coefficient of D within V at room temperature is somewhat higher, ~1×10− 2m2/s. Three charging conditions for Pd–Rh were examined,
200, 500, and 500 kPa with an additional in-vacuum holding delay of 6 days, to examine the stability of the formed Pd-D over extended periods, and at different pressures. For V, charging was performed at a single pressure of 500 kPa.
RESULTS
Palladium Alloy Table 1 presents the uncharged composition for the three
experiments for the Pd–Rh alloy. It shows that the primary fluctuation in the measured composition is in the oxygen and Pd content. This is likely due to specimen preparation. After the charging process some surface contamination
was found to be present, presumably from the industrial- grade nitrogen source. However, this contamination is restricted to the approximately first 1×105 ions of surface material. Comparative pre and postcharging mass spectra, focusing on the low-mass region and around the Pd+ isotope peaks are presented in Figure 2. Note that the comparison spectra were obtained from the exact same atom probe needle.
Figure 2 shows that peaks due to D containing species
are visible in the postcharging mass spectrum at m/n 2, 3, and 4. In Table 2, the measured H:{H2,H3,H4} ratios obtained in the uncharged data set are presented. These
ratios will depend strongly on the intensity of the electric field applied during the APT
analysis.Noting that the sample should not appreciably change shape due to H exposure as Pd–Rh is a relatively noble alloy, then the field at which the experiment is conducted should be similar to pre and post- charging with D. However, in the case of the charged samples some of these peaks will also contain contributions from D. Thus using the H:{H2,H3,H4} ratios from the uncharged samples, the H2 component of the D and the H3 component
Table 1. Composition for Uncharged State, as Measured Using Voltage Run.
Species Pd
Rh O H
Unidentified N C
Mean Composition (at%) (n = 3) 85.94
8.81 2.49 2.31 0.35 0.09 0.02
2×SE 3.66
0.11 2.15 1.27 0.25 0.04 0.01
Data are average composition over three runs (n = 3), error bounds are ±2 SE.
Figure 2. Comparison of charged (500kPa abs.) and uncharged Pd–6.25%Rh alloy (same needle), showing peaks at 2 and 4 in the charged state. Normalization is by mass-to-charge spectrum total area.
of theDHpeaks, at 2 and 3 Da, respectively, in the analysis of the charged specimen can be estimated and is presented in Table 2. The primary contaminants detected in the analyses were: C, some unidentified peaks in the range of 13–46 Da, and trace amounts of Hg. Deuterium is detected not only in the form of D and D2
at low-mass peaks, but also as a molecular ion Pd-D+ in the APT experiment. Pd-H2 molecular ions, which would be indistinguishable from Pd-D, are not considered to have formed as these were not detected within the analysis of the uncharged data set. Examining Figure 2, Pd-D formation can be readily seen via 110Pd2-H, which has no overlap and thus
can be uniquely identified. As a slightly more complex peak, 105Pd2-H only overlaps with 106Pd1-H, and is thus separable from Pd at mass 107. 108Pd1-H and 110Pd1-H can be unambiguously identified at mass 109 and 111. As there exists, in general, overlap between Pd, Pd-H and Pd-D peaks, numerical separation of the relative contributions (Miller et al., 1996) of these components to each peak must be performed based on natural isotopic abundances of the individual elements that constitute the ion.
Table 2. Estimated Counts (Background Corrected) for Uncharged and Charged Pd–Rh Alloy, (500 kPa, 1 h Charging) in the Low-Mass Region of the Data Set.
Uncharged State
Species H
H2 H3 H4
Background- Corrected Counts Species
2,177.4 H
123.0 D+H2 58.4 DH+H3 0.0 D2
Charged State
Background- Corrected Counts
2,428.5 2,477.6 166.2 40.9
Estimated H Counts
–
137.2 65.1 0.0
2,340.3 101.0 40.9
Estimated D Counts
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