Christopher J. Wetteland

A comparison between the irradiation damage response of spinel and zirconia due to Xe ion bombardment

The mechanical properties of Xe-implanted spinel and cubic zirconia surfaces, as determined by nano-indentation measurements, are distinct and the differences can be related to their microstructures. Upon Xe(2+) ion irradiation at cryogenic temperature (120K), the Young`s modulus of irradiated spinel increases slightly (a few percent) then falls dramatically until the modulus is only about 3/4 the unirradiated value. The maximum modulus occurs concurrent with the formation of a metastable crystalline phase of spinel. The subsequent elastic softening at higher Xe(2+) doses is an indication of the onset of amorphization of the spinel. Xe-implanted zirconia surfaces behaves differently, in all cases showing almost no change in elastic modulus with increasing Xe(2+) ion dose. This is consistent with microstructural observations of Xe-implanted zirconia crystals which, unlike spinel, show no change in crystal structure with increasing ion dose. The defected layer in zirconia due to ion damage simply thickens with increasing Xe(2+) dose. This thickening may be a consequence of compressive stresses that form in the ion- implanted surface region. The hardness of both spinel and zirconia increases slightly for low Xe(2+) ion doses. At higher doses, zirconia shows little change in hardness, while the hardness of the implanted spinel falls by more than a factor of two. The initial increase in hardness of both spinel and zirconia is probably due to point defect accumulation and the precipitation of small interstitial clusters, while the drop in hardness of spinel at high Xe(2+) ion doses is due to the formation of an amorphous phase.

Ultraviolet-Ozone Oxidation of Metal Films

Metal oxides such as zirconia and hafnia are being investigated as new materials for application as gate dielectrics in future complementary metal-oxide-semiconductor devices. In this paper, we present results on oxidation of metal films such as Zr, Hf, and Al by the ultraviolet (UV) ozone oxidation method. A nuclear reaction analysis technique, the 16 O(d,α) 14 N nuclear reaction, was used to quantify the oxygen concentration in the dielectric stacks. The method was found to be sensitive to monolayer levels of oxygen. It was found that the oxidation kinetics of the metals increased significantly due to the presence of UV light. The oxidation rate was also found to depend on the oxygen partial pressure. The oxidation rate of Zr was greater than that of Hf, while Al oxidized more slowly than Hf for the UV-ozone oxidation conditions investigated. Possible reasons for the observed oxidation behavior are discussed in detail.

Helium behaviour and defect evolution in amorphous spinel during thermal annealing

Abstract MgAl 2 O 4 -spinel has been widely investigated as inert matrix for actinide-transmutation. Under impact of fission fragments, it becomes amorphous. During reactor irradiation, crystalline and amorphous spinel are expected in the fuel, containing, among others xenon and helium. Gas-release measurements and positron beam analysis (PBA) were performed on amorphized specimens and compared to results on crystalline spinel. Helium was released in two stages between 575 and 800 K in first-order desorption processes with activation enthalpies of 1.9 and 2.7 eV and attempt frequencies of about 10 13 s −1 . Xenon was released between 1050 and 1450 K. PBA experiments indicated that defect clustering occurred near the surface at low temperatures and in deeper regions at about 950–1200 K. Recrystallization was observed between 1000 and 1350 K and defect annealing was completed at 1600 K. PBA indicates two different damage zones, corresponding to displacement zones with and without xenon.

Radiation effects in corundum structure derivatives

Abstract The radiation response of α-Al2O3 (R3c), FeTiO3 ( R 3 ), MgTiO3 ( R 3 ), and LiTaO3 (R3c) was investigated using 200 keV Ar2+ and 1 MeV Kr+ and a combination of Rutherford backscattering spectrometry and transmission electron microscopy. All of these materials have the corundum or a corundum-derivative crystal structure. This family of oxides is of interest due to the simple but significant differences in the structure of the cation sublattices in the different space groups. These materials are also of interest because of their range of melting temperatures, the range of melting temperatures of their oxide components, differences in bonding characteristics, and the complete solid solution between FeTiO3 and MgTiO3. Our results show that α-Al2O3 and MgTiO3 are consistently more radiation tolerant than FeTiO3 and that LiTaO3 amorphizes substantially easier than the other three oxides. The greater stability of MgTiO3 than FeTiO3 appears to be analogous to the response of Mg-Fe silicates to ion irradiation and high pressure. A characteristic consistent with the radiation tolerance trend of the four oxides studied is a decreasing melting temperature of the component oxides. Thus, the lower stability of Fe–O octahedra in FeTiO3 and Li–O octahedra in LiTaO3 may adversely affect the radiation tolerance of these oxides.

Effects of Polishing on the Photoluminescence of Single Crystal ZnO

Abstract The photoluminescence (PL) response of single crystal ZnO subjected to mechanical and chemo-mechanical polishing has been investigated. Zn-terminated and O-terminated surfaces of (0001)-oriented ZnO crystals were prepared by mechanical polishing with 1/4 μm and 1 μm diamond abrasives and by chemomechanical polishing. The spectrally resolved room temperature PL of the polished surfaces was measured, and changes in both the spectral content and PL intensity introduced by polishing were assessed. The PL results were compared to a direct measure of subsurface damage obtained by ion channeling. Room temperature PL spectroscopy is shown to exhibit potential as a sensitive tool for the characterization of subsurface damage in polished ZnO.

Role of low-level impurities and intercalated molecular gases in the α particle radiolysis of polytetrafluoroethylene examined by static time-of-flight secondary-ion-mass spectrometery

The structural degradation of polytetrafluoroethylene (PTFE) upon irradiation with MeV alpha (α) particles is accompanied by the proliferation of hydrogenated and oxygen-functionalized fluorocarbon species. In this article, we explore the origin of monoxide- and dioxide-functionalized fluorocarbon species that emerge upon α particle irradiation of PTFE. Samples of neat PTFE were irradiated to α doses in the range of 107–5×1010rad using 5.5MeV He2+4 ions produced in a tandem accelerator. Static time-of-flight secondary-ion-mass spectrometry (TOF-SIMS), using a 20keV C60+ source, was employed to probe chemical changes as a function of α particle irradiation. Chemical images and high-resolution mass spectra were collected in both the positive and negative polarities. Residual gas analysis, utilized to monitor the liberation of molecular gases during α particle irradiation of the PTFE in vacuum, is discussed in relationship to the TOF-SIMS data.

Volatile Evolution from Polymer Materials Induced by Irradiation with Accelerated He ++ Ions

Experimentally investigating ageing caused by irradiation with energetic particles is very difficult. Radioactive sources can be employed but these are difficult to handle and contaminate the material being irradiated precluding subsequent chemical and physical characterisation. The penetration of energetic particles also tends to be small so any change is localised in the near surface region so only a small amount of material is irradiated. Analysing changes in such thin layers causes a number of problems. To simulate ageing induced by particle radiation polymer samples have been exposed to fast He ++ ions in an accelerated ion beam. The ions pass through a 10μm thick window of Havar foil before impacting upon the sample. Volatile species evolved from the materials upon bombardment are contained within the irradiation chamber by the foil window. Analysis of such species is shown to be a highly sensitive probe for investigating chemical changes in the exposed materials. A number of important chemical changes induced in polymer materials have been identified. Trends in the relative rates of volatile evolution have been identified which correlate with chemical changes identified in other radiation experiments. As these experiments are performed at far slower irradiation rates the large acceleration factors used in ion beam irradiation are discussed along with the implications for using ion beams to simulate alpha particle irradiation.

Vanadium-spinel composites for structural applications in hostile environments

Vanadium-spinel composites are promising materials for structural applications in radiation environments. Powders of two Vanadium-spinel composites, 20/80 vol. %, were prepared by (a) ball milling mixtures of vanadium and spinel powders (alloy VSLP) and (b) through a self-sustained reaction synthesis of vanadium, MgO, and Al powders (alloy VSHP). These powders were consolidated by hot isostatic pressing. Most of the V and spinel domains in the the compacts are sub-micron in size. The compacts have K{sub c} toughness values of 3.9, about three times the toughness obtained by hipping mixtures of commercial powders.