Atom Probe Analysis of Ex Situ Gas-Charged Stable Hydrides 311
Table 5. Composition of Uncharged Vanadium Samples as Measured by Voltage Atom Probe Tomography.
Concentration (at%)
V H O Si N C
Unidentified Vanadium
The initial acquisition of vanadium APT data was performed using laser-pulsing mode to remove surface oxide from the specimen. After removal of the oxide cap, the experiment was then switched to voltage-pulsing mode. The results for the voltage-pulsing are shown in Table 5 for two uncharged runs. Hydrogen and oxygen are the two major contaminant species. Hydrogen was detected as H and H2 within the sample.
DISCUSSION
charge state is well explained by fitting the peaks to a mixture of contributions from Pd, Pd-H, and Pd-D ions. The results in Table 3 show that Pd-H ions do not seem to occur in the APT experiment in large quantities, and that primarily Pd-D is observed. Significant Rh-H and Rh-D contributions seem unlikely, owing to the very good fit to the observed peak amplitudes that can be obtained using only the Pd-based isotopes. The inability to detect Pd-H ions cannot be directly attributed to a lack of Pd-H surface interaction, but is more likely simply due to the excess D that has been introduced as compared with the backgroundHlevel. From the presence of D, D2, and Pd-D, it is clear that during the charging process D has been taken up successfully in each case. The relative concentrations of D, as shown in Table 4, are significantly below the thermodynamically expected
Pd–Rh Alloy From Table 2, a snapshot of the D:H ratios that occur during APT field evaporation can be examined. We can further separate the relative contributions within these total signals, that is D and D2 and HandH2. The estimated ratios of these relative contributions are 17.7 for H:H2 and 57.2 for D:D2. The values of the two measurements are within the same order of magnitude, but the significant difference is sur- prising if the ionization process is assumed to occur at a fixed conversion rate. The presence of H is clearly an artifact with respect to the charging process, and can only originate from either within the high-vacuum chamber or as an adsorbed species during specimen transfer from the rig to the atom probe. The difference in themeasured H:H2 and D:D2 ratios may simply be a reflection of this. The Pd data in mass spectra obtained from the post-
87.16 10.37 2.13 0.16
0.074 0.067 0.044
Oxide region has been previously removed by laser atom probe.
94.34 2.10 3.46 –
0.060 –
0.031
values, even if the H component, an artifact of the experi- ment, is included in the total concentration. Several possibilities to explain this observation exist:
1. The D concentration has been underestimated due to unidentified species with significantly changed D content in the form of highly deuterated complex ions in the small quantity of unidentified peaks. This seems very unlikely, but is possible.
2. The level of deuteration that has been achieved is less than equilibrium.
3. Postcharging diffusion-loss of D through the surface of the specimen has occurred.
4. There is nonuniformity above the scale of the atom probe experiment
5. An unidentified APT-specific loss mechanism is in effect. Of note, these concentrations seem consistent with other works (Kesten et al. 2002), where D/metal ratios in a Fe/Pd/V multilayer reported D:V ratios of at most 0.18 (15 at%), despite the pressure difference.
To examine the possibility of postcharging diffusion-
after significant time, indicating that diffusion does not appear to be amajor barrier to the study ofDintroduced into the Pd– Rh system. This is consistent with electrochemical results that exhibit a decreased effective diffusion rate for thin films, which is attributed to surface barrier effects (Li & Cheng, 1996), and bulk diffusion is reported elsewhere as not being the limiting factor in Pd-D diffusion (Schwarz et al., 2005). At room temperature, the sample should equilibrate
loss, a sample was stored for 6 days within the APT “buffer” chamber at high-vacuum conditions. Interestingly, the D content reported for this analysis, D:Pd = 0.025, was slightly higher than the D content reported for the analysis more immediately transferred, D:Pd = 0.022, despite both analyses being generated from same specimen. Consequently, thiswork demonstrates thatDis detectable
purely within the β phase. Upon cooling, or a reduction in D content consistent with observations as shown in Figure 3, the sample should transition into an α+β region. These two phases are both FCC, but differ in lattice constant (Schwarz et al., 2005). Thus, we anticipate that before charging the sample is α, and then during charging it transitions to β,either partially or completely. Possibly there is some reversal during the experiment after the charging has been undertaken. It is possible that the enriched zone observed byAPTis this β phase.
Vanadium
Similarly to the Pd–Rh alloy, V also forms a hydride phase with H or D, and therefore this study also investigated the capability for APT to examine D using an ex situ method. Previous work by the author has been reported on some preliminary in situ low-pressure charging elsewhere, and low- pressure (hPa range) charging ofVcontainingmultilayers has been performed (Kesten et al., 2002; Gemma et al., 2012). The experiments in this study show the underlying metal as having no detectable deuterium uptake due to the
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