Sublimation

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Based on the APID Volume 7 [1]

Contents

Introduction

Radiation-enhanced sublimation is an erosion mechanism peculiar to carbon-based materials, and affects only carbon atoms within those materials. It is similar to physical sputtering in that it does not involve chemical reactions, and that it does require incident particles to have sufficient energy to dislodge carbon atoms from their lattice sites. But, the process does not require that carbon atoms be ejected from the surface via momentum transfer alone, as in the case of physical sputtering. Once in interstitial spaces, the carbon atoms may diffuse to a surface, where they are weakly bound. The atoms may leave the surface by a thermal mechanism, such that there is an exponential increase in their release with increasing temperature. The activation energy for this release, that is, the atom binding energy to the surface, is generally in the range 0.5-1 eV significantly less than the sublimation energy for carbon 7.4 eV. This basic understanding of the process involved in RES has been established for many years. Models of the RES process have been reasonably successful, however, some questions remain with regard to the flux density dependence.


Features of Radiation-Enhanced Sublimation

Figure 1.  Time-of flight spectrum of emitted carbon atoms under 5 keV Ar+ bombardment of carbon at 2000 K.  Physically sputtered carbon atoms appear around 20 µs, and RES-emitted carbon atoms appear at times in agreement with a Maxwellian velocity distribution corresponding to the target temperature

Several fundamental aspects of RES have been investigated experimentally the most obvious of these is the temperature dependence, and thus the activation energy for the carbon atom release. The fact that the activation energy is about 10% of that for thermal sublimation clearly separates the two mechanisms. The released carbon atoms do, however, have a near thermal energy distribution which is similar to that seen for thermal sublimation. The time-of-flight spectra of carbon atoms released due to 5 keV Ar+ bombardment of carbon at 2000 K is presented in Fig. 1. There are two clear groups of atoms leaving the surface; a fast group, attributed to physical sputtering, and a thermal group attributed to RES; the two groups are clearly distinguishable. This provides solid evidence that carbon atoms released by RES involve a thermal process. In addition, measurements have shown that the C atoms are released with nearly a cosine distribution, as would be expected for a thermal release process.


A further feature which separates RES from physical sputtering and thermal sublimation is the fraction of carbon released as C2 and C3 molecules. In RES, the carbon is almost entirely released as single atoms (See Fig. 2), while for physical sputtering only about 80% is released as a C1, the rest being released as C2 and C3. In thermal sublimation, the fraction released as C2 and C3 increases with temperature, with single carbon atoms accounting for less than 20% at 2500 K. The predominant release of single carbon atoms in RES is a natural consequence of the mechanism which involves the transport of individual carbon atoms through interstitial spaces to the surface.


There are also differences in the angular dependence of the RES erosion yield as compared to physical sputtering. For example (Fig 3), the total erosion yield due to 1 keV C+ shows a smaller increase with angle than the physical sputtering yield. In fact, most of the increase shown for the total yield at 1500 K is attributable to the physical sputtering component. The weak angular dependence is a natural consequence of the RES mechanism. At large angles of incidence, carbon interstitials would simply be created closer to the surface, provided the mean diffusional range of the interstitials is less than the ion range, the number of C atoms emitted from the surface will not be affected.


Figure 2. Temperature dependence of the ratio of emitted C2/C1 and C3/C1 under 5 keV Ar+ bombardment of graphite. Data for T> 2200 K (in the insert) are obtained during thermal sublimation alone, i.e., with the ion beam off.
Figure 3. Angular dependence of physical sputtering and radiation-enhanced sublimation.



A large number of experiments have been performed investigating the temperature dependence of RES erosion yields for various pure and doped graphites. There is general agreement on the exponential nature of the temperature dependence, of the form Y<math>\propto</math> exp-E/T. It is noted that erosion yields for C+ self-sputtering can exceed unity, thus leading to the possibility of runaway erosion.


The dependent of RES erosion yields on incident ion energy and on the incident flux density have been much less studied than the temperature dependence. For fusion materials selection, however, these are critical issues. In Fig. 4, RES erosion yields are presented as a function of ion energy. It is important to note the clear indication of threshold energy, similar to that observed for physical sputtering. In Fig. 5 the flux dependence of RES erosion is compiled.


Figure 4. Energy dependence of radiation-enhanced sublimation, from several sources.
Figure 5. Flux dependence of RES. Unless otherwise indicated, the specimens were pyrolytic graphite.



Available Data Sets

Data Sets compiled in the APID Volume 7[1] are available at IAEA ALADDIN Database.

References

  1. 1.0 1.1 W. Eckstein, J. A. Stephens, R. E. H. Clark, J. W. Davis, A. A. Haasz, E. Vietzke and Y. Hirooka Physical Sputtering and Radiation-Enhanced Sublimation , Atomic and Plasma-Material Interaction Data for Fusion, Vol. 7 Part B (2000)
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