Pulsed field desorption mass spectrometric study of the oxidation of hydrogen-doped Ni3Zr

The oxidation of a Ni3Zr field emitter, electrochemically doped with hydrogen, has been studied over a temperature range from 100°C to 250°C in an oxygen atmosphere with pressures ranging from 10−4 Torr to 3 × 10−1 Torr using PFDMS. Water was observed immediately after the first oxidation at 100°C and 10−4 Torr O2 and was observed throughout the series of oxidations. Pulse field desorption produced a wide range of ions: Ni+, Ni2+, H+, O+, O2+, O2+, H2O+, Ni(OH)2+, NiOH+, NiOH2+, NiOH3+, Ni2O2+, Ni(CO)+, Ni(CO)2+, Ni(CO)3+, Zr+, ZrO23+, ZrO22+, ZrO2+, Zr2O+, ZrO3+, ZrO2+ and ZrO+; which differ substantially from an atom probe investigation of the oxidation (dry, no Habs) of the same alloy. The data show, for all oxidation temperatures and pressures, an initial strong enrichment of Ni at the surface. At lower temperatures and oxygen pressures this layer appears to be very thin. Previous low temperature/pressure thermal oxidation experiments performed on Ni3Zr (no Habs) resulted in the preferential oxidation and segregation of Zr to the surface. Only at higher temperatures and/or oxygen pressures has preferential Ni oxidation and segregation been previously observed. This behavior is indeed observed during these experiments after the 250°C and 3 × 10−1 Torr O2 oxidation. The unusual H-doped Ni3Zr behavior at low temperatures/pressures appears to involve the formation of a thin layer of Ni(OH)x which is more stable than NiO. The results from the study of H-doped Ni3Zr yield some important new findings regarding the effect of hydrogen absorption on the alloy oxidation. In addition, the reaction of absorbed hydrogen with oxygen provides a unique way of introducing water on a sample in a UHV environment.