Barbara Okray Hall

Mechanical response of single crystal Si to very high fluence H+ implantation☆

Abstract Single crystals of Si are doped at room temperature with hydrogen from 2 × 1016 H/cm2 to 1.6 × 1018 H/cm2 in a range of energy from 20 keV to 1 MeV. Experiments using RBS/channeling, profilometry and cross-section TEM are reported.

Implanted hydrogen effects at high concentrations in model low Z shielding materials

Abstract Shielding of near-plasma structural components has long been recognized as an important consideration in Tokamak operation. Various low Z materials have been proposed and tested to determine sputtering characteristics, hydrogen retention and isotope exchange kinetics. The mechanical response of these materials to high fluence atomic displacement damage and accumulation of hydrogen and helium is receiving extensive attention at the present time. We report on detailed mechanistic studies of a model shielding material. More extensive treatments are given elsewhere. 1,2

Highly conductive poly(phenylene sulfide) prepared by high‐energy ion irradiation

Ion irradiation of poly(phenylene sulfide) (PPS) films with various ions produces highly conductive films. Using 5.6‐MeV fluorine ions, the conductivity of irradiated films at a dose of 1×1016 ions/cm2 is comparable to chemically doped films (0.77 S/cm), and higher conductivities should be possible with higher doses. Iodine ions of 50 MeV produced films with similar conductivities at a dose of 1×1014 ions/cm2: the highest conductivity reported for an organic material irradiated by such a low dose. Films irradiated with 0.32‐MeV lithium ions did not change their conductivity up to 1×1015 ions/cm2. We attribute the higher conductivities obtained in our iodine irradiated films to the substantially higher electronic energy deposition associated with 50‐MeV I ions. The Li‐implanted films showed no substantial change in conductivity probably because their energy deposition rates are too low.

Sintering and characterization of alkaline-earth-doped and zirconium-defficient Na3Zr2Si2PO12

Abstract The sintering behavior of Na3Zr2Si2PO12, Na3.08Zr1.96X0.04Si2PO12 (X = Mg, Ca, Sr, Ba), Na3Zr1.5 Si2PO12, and Na3Zr1.3Si2PO12 was studied between 900 and 1300°C. Microstructural examination indicated that liquid-phase sintering had occurred and that free ZrO2 was present in all of the samples. X-ray diffraction indicated that the NASICON and ZrO2 were monoclinic and that doping the Na3Zr2Si2PO12 with 0.02 mole% alkaline-earth ions (compared to the ZrO2 content) did not significantly increase the unit-cell volume.

Damage accumulation in hydrogen-implanted silicon

Abstract A phenomenological model has been developed to describe the evolution of the damage distribution in silicon implanted with hydrogen ions. The model includes annealing, stabilization of damage by hydrogen trapping, hydrogen detrapping and diffusion. Calculations for a variety of model parameters show that the model reproduces the behavior of profiles measured by the RBS/channeling technique.

Calculation of the lattice distortion due to carbon inα-iron

A method for calculating the static displacements of lattice atoms near a defect atom is presented and applied to carbon occupying the octahedral site in α-iron. The interaction between lattice atoms and the defect is assumed to extend to second neighbors, but a defect-lattice atom interatomic potential is neither assumed nor constructed. Instead the displacements of the first and second neighbors of the defect atom are treated as adjustable parameters, which can be determined experimentally, and the displacement field is calculated as a function of these parameters. A central, two-body Morse potential is used to describe the interaction of lattice atoms, and the lattice atom displacements are calculated for both harmonic and anharmonic (quartic) approximations of the potential. The latter calculation requires an iterative procedure that obtains self-consistent anharmonic displacements without sacrificing the convenience of the matrix inversion technique that is used to solve the harmonic problem.

Trap theory of helium partitioning at low doses

Abstract A time-dependent interstitial clustering model, based on the chemical reaction-rate formalism, has been developed for calculating concentrations of point defects, interstitial clusters, interstitial (free) and trapped helium concentrations during the early stages of irradiation. The various structural defects are assumed to provide saturable and reversible trap sites for helium, and capture and emission by vacancies, interstitials, interstitial clusters, and dislocations are included. When calculations are performed with parameters appropriate for nickel and several helium-dislocation detrapping energies E T D , the model predicts (1) that the helium concentration at vacancy traps increases monotonically with time, while that at interstitial clusters and dislocations attains a maximum and then decreases; (2) that the interstitial cluster number density does not depend on E T D for the range of values considered and is therefore insensitive to direct helium-interstitial and helium-dislocation interactions; and (3) that helium partitions primarily to vacancies rather than to interstitial clusters or dislocations, and therefore a majority of helium will be in cavities after long irradiation times.

Evolution of cavity size distributions in dual-ion irradiated austenitic stainless steel and Fe-Cr-Ni ternary alloys☆

Abstract The first four moments of experimentally measured cavity size distributions in dual-ion irradiated 304SS, Fe-12Ni-15Cr and Fe-30Ni-15Cr alloys have been calculated for a range of fluences, helium injection rates, and irradiation temperatures. The moments are shown to correctly describe the effects of alloy composition, fluence, helium and temperature on the evolution of the cavity size distributions. Experimentally determined moments are compared with those calculated from cavity nucleation and growth theories. The moments reflect the competition between nucleation and growth processes and provide insight into the details of the transient low swelling regime.

Helium partitioning between voids and dislocations

Abstract A model that describes the interaction of mobile helium with void and interstitial-loop distributions in irradiated metals has been developed. Both evolving loops and voids are assumed to provide saturable and reversible trap sites for interstitial helium and detrapping by thermal emission and by irradiation displacement is included. To avoid the computational limitations of standard rate-theory methods, differential equations have been derived for the moments of the cavity and loop distributions in the two-dimensional size spaces that include both point defect and helium number. Equations for the first and second moments are included in the partitioning model, along with the necessary mobile defect equations. The time dependence of the mean cavity size and gas content and the mean loop size and trap occupation number have been obtained in closed form for simplified conditions as an illustration.

Structural study of Nasicon and related compounds

Abstract A rigid-ion model has been developed for NaZr 2 P 3 O 12 and structurally related compounds. The materials are treated as perfectly ionic, and the crystal cohesive energy is approximated by electrostatic and repulsive energy terms. Repulsive potential parameters λ Zr-O and λ P-O have been evaluated from the equilibrium equations. Lattice parameters have been calculated for crystals with homovalent replacements for Na + and Zr 4+ , and for the system Na 1+x Zr 2 Si x P 3−x O 12 (0 ≤ x ≤ v3). The model correctly predicts increase or decrease in unit-cell volume with respect to that of the reference crystal but not the maximum in cell volume at x ∼ 2 in the sodium zirconium phosphosilicate system, which appears to arise from polarization effects.