Titanium Tritides Research Articles

Helium atom diffusion in titanium tritides (TiT2-x, −0.02 ≤ x ≤ 0.25): A first-principles and kinetic Monte Carlo study

The tritium (T, or 3H) and helium (3He) diffusion properties are critical to the helium bubble nucleation and growth behaviors in tritium storage materials such as titanium tritide (TiT2-x). Hereby, the possible atom migration paths of T and He in TiT2-x as well as their barriers and prefactors are calculated by density functional theory calculations, and are further applied in kinetic Monte Carlo simulations to determine the overall diffusion rates. The correlations of diffusion coefficients of the He atom with temperature in different TiT2-x (0 ≤ x ≤ 0.25) are shown as Arrhenius plots. The results show that two diffusional regimes, migration more via tetrahedral vacancies or octahedral interstitials, are presented depending on temperature and composition for both stoichiometric and non-stoichiometric tritides. Comparing with T, He exhibits similar diffusion mechanisms but faster diffusion rates. Our results suggest that the tritium (or vacancy) concentration, temperature, and axial strain are effective for tuning the diffusion mechanism and rate of a helium atom in titanium tritides.

Regulating the helium bubble nucleation in the titanium tritides by environment temperature during the early aging period

The nucleation of helium bubbles in the early aging stage of tritium storage materials will have an important influence on the bubble growth behavior in the whole of aging. Herein, the nucleation of helium bubbles was adjusted by environment temperature at the first 15 days immediately after preparation of polycrystalline titanium tritide films. The influence of the environment temperature on the nucleation of helium bubbles were studied by X-ray diffraction (XRD), positron annihilation spectroscopy (PAS), thermal helium desorption spectroscopy (THDS) and scanning transmission electron microscopy (STEM). Results demonstrate that the helium bubble nucleation is significantly affected by the environment temperature of −60 °C, 20 °C and 120 °C. The higher the environment temperature, the lower number density of helium bubble in the titanium tritide films. The helium bubble nucleation mechanism is strongly correlated to the helium atom diffusion coefficient that is controlled by the environment temperature. During the following aging, the rate of helium atoms captured by a single helium bubble increases in titanium tritides with low number density bubbles, which accelerates the growth of helium bubbles in the matrix. However, the low number density bubbles also decrease the capture probability of free helium atoms by helium bubbles, resulting in the increase of helium atoms diffusing to infinite size defects such as grain boundaries macroscopically, and accelerating the evolution of helium in titanium tritides.

Thermal desorption of tritium and helium in aged titanium tritide films

Titanium films were deposited onto molybdenum substrates and then converted into titanium tritides (TiT1.5–1.8) films inside a tritiding apparatus loaded with pure tritium gas. Evolution of tritium and helium in the titanium tritide films over a period of four years was investigated using a thermal desorption technique, together with X-ray diffraction analysis. Results showed that desorption profiles of the tritium varied significantly with the evolution of He contents. Apart from the primary peak from tritium desorption located at a temperature between 610 and 840 K, another higher temperature tritium desorption peak (at ∼950 K) was observed, attributed to damages in the lattice structures induced by generation of 3He bubbles. Release of helium in the tritide film became significant after a long term aging process (i.e., after a few years). Depending on the amount of the 3He bubbles generated due to the decay of tritium, spectra of the thermal helium desorption showed five peaks in the range from room temperature to ∼1750 K, corresponding to different states of helium evolution during aging of the titanium tritide films. The amounts of helium desorption in different stages were estimated, and the dissociation energy of helium from different trap states as a function of the aging duration was obtained.

Ab initio study of helium behavior in titanium tritides

Ab initio calculations based on density functional theory have been performed to investigate the relative stability of titanium tritides and the helium behavior in stable titanium tritides. The results show that the β-phase TiT1.5 without two tritium along the [100] direction (TiT1.5[100]) is more stable than other possible structures. The stability of titanium tritides decrease with the increased generation of helium in TiT1.5[100]. In addition, helium generated by tritium decay prefers locating at a tetrahedral site, and favorably migrates between two neighbor vacant tetrahedral sites through an intermediate octahedral site in titanium tritides, with a migration energy of 0.23eV. Furthermore, helium is easily accumulated on a (100) plane in β-phase TiT1.5[100].

Deuterium, tritium, and helium desorption from aged titanium tritides

Six samples of tritium-aged bulk titanium have been examined by thermal desorption and isotope exchange chemistry. The previously reported discovery of a lower temperature hydrogen desorption state in these materials has been confirmed in one new sample. The samples’ helium release shows the more severe effects obtained from longer aging periods, i.e. higher initial He/M ratios. Several of the more aged samples were spontaneously releasing helium. The hydrogen isotope and helium desorption of the samples will be discussed.

Helium release behavior of aged titanium tritides

One sample of bulk Ti has been loaded with a 50%/50% deuterium/tritium (D/T) mixture and statically aged for 6.5 years. Thermal desorption of the sample shows an initial release of hydrogen isotopes followed by He 3 release. Subsequent D 2 loading/desorption was used to quantify the trapped tritium heel. The sample shows an excess hydrogen capacity as a second thermal desorption peak that partially disappears and shifts with annealing at 923–973 K. The main hydrogen desorption peak also shifts to higher temperature, indicating a partial reversal of the tritium-decay induced damage by annealing.

Introduction of helium into metals by magnetron sputtering deposition method

High concentration helium, up to 16 at.%, was introduced into Ti films through magnetron sputtering method in a He/Ar complex atmosphere. The introduced helium distributes homogeneously in the films and mainly forms small helium bubbles. Helium thermodesorption experiments were carried out, from which it was found that the thermodesorption properties of the introduced 4He are similar to those of radiogenic 3He in titanium tritides. Titanium alloy films containing helium were also prepared through this way and a comparison of thermodesorption properties was made between them and Ti–He films.

Self-absorption of tritium betas in metal tritide particles.

Inhaling metal tritide particles is a potential occupational hazard. The radiation dose to tissue from tritide particles depends on their solubility and retention in the body. In each tritide particle, a portion of the beta particles from decay of tritium is absorbed by the metal matrix and therefore cannot contribute to absorbed radiation dose to tissue. A theoretical model for estimating the self-absorption of tritium betas in spherical metal tritide particles is presented. Numerical calculations are made with this method for titanium, zirconium, and erbium particles from 0.5 to 50 microm in diameter. The tritium spectrum is divided into energy groups to facilitate estimation of the energy that escapes the particle for dose calculations. Our results show considerable absorption of beta particles and their energy, even for respirable particles smaller than 5 microm. Limited experimental data of self-absorption for titanium and zirconium tritides supported the theoretical calculation. It is concluded that the self-absorption factors should be required for counting tritide particle samples as well as for estimating absorbed radiation dose to tissue.

3He bubble formation in titanium tritides at elevated temperatures A TEM study

Abstract Ti-tritides containing about 10% (H/M) tritium were aged at 300 and 550[ddot]C for several weeks and then inspected for 3He bubble formation by transmission electron microscopy. Micrographs of α-phase regions aged at 300[ddot]C displayed platelike He precipitates on (0001) habit planes accompanied by very strong contrast. Plates of the δ-tritide which were also present during ageing showed the usual bubble structure, somewhat coarser than at ambient temperature, α-phase regions aged at 550[ddot]C were found to contain a significantly reduced number density of rather large faceted bubbles. The present and previous results are interpreted in the light of the current understanding of He bubble formation in metals. The difference in the bubble morphology at different temperatures is attributed to differences in the availability of thermal vacancies during He clustering. Evidence is provided that up to about 600 K, He atoms diffuse interstitially and form interstitial clusters whereas at higher temper...