Tritide Films Research Articles

Optimization design of 4H–SiC-based betavoltaic battery using 3H source

This paper describes the theoretical calculation and optimization design of the PN junction betavoltaic batteries with 4H–SiC-based energy converter and titanium tritide source. The self-absorption of radioactive isotope sources and the energy deposition distribution in the semiconductor converter are simulated using the Monte Carlo method. The relationship between doping concentrations and basic factors such as minority carrier diffusion lengths and the width of the depletion region are analyzed via the calculation formulas. Then the maximum output power density and energy conversion efficiency are calculated. The optimal thickness of the titanium tritide film is about 0.7 μm, the doping concentrations are 2.5 × 1016 cm−3, and the junction depth of PN junction is 0.1 μm. The surface recombination velocities of electron and hole are 1 × 106 cm/s, respectively. The maximum output power density and energy conversion efficiency are 0.22 μW/cm2 and 2.37%, respectively.

Influencing factors of helium bubble growth in erbium tritides: Grain size and impurity element

The influencing factors of helium bubble growth in the erbium tritides were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The Er tritide films with different grain size and Hf impurity content were fabricated by electron beam vapor deposition and magnetron sputtering evaporation. The helium bubble grows along the {111} preferential orientation in the Er deuteride with a columnar grain structure, while the helium bubble grows as spherical shape in the Er tritide with a nano-sized grain structure. The residual stress is proposed as the major reason affecting the helium bubble growth behavior in the Er tritide, and the helium bubbles tend to grow as spherical shape under compressive stress. The helium bubble size increases slowly with the aging time once the nucleation in the Er tritide. With addition of Hf impurity in the Er tritide, the helium bubble size changes to be smaller and the instantaneous helium release fraction is lower at least by an order of magnitude than pure Er tritide. The above results demonstrate that the grain size and impurity element are two key factors affecting the helium bubble growth in the Er tritide.

Optimization of target lifetime for production of 14 MeV neutrons

Two methods are examined for extending the life of tritium targets for production of 14 MeV neutrons by the 3H(2H,n)4He nuclear reaction. With thick film targets the neutron production rate decreases with time due to isotope exchange of tritium in the film with implanted deuterium. In this case, the target life is maximized by operating the target at elevated temperature where the implanted deuterium mixes by thermal diffusion throughout the entire thickness of the film. The number of neutrons obtained from a target is then proportional to the initial tritium content of the film. A novel thin-film target design was also developed and tested. With these thin-film targets, the incident deuterium is implanted through the tritide into the underlying substrate material. A thin permeation barrier layer between the tritide film and substrate, reduces the rate of tritium loss from the tritide film. Good thin-film target performance was achieved using W and Fe for the barrier and substrate materials respectively. Thin-film targets were fabricated and tested and shown to produce similar number of neutrons as thick-film targets while using only a small fraction of the amount of tritium.

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.

Optimization design of betavoltaic battery based on titanium tritide and silicon using Monte Carlo code

This article presents the optimization design and simulation of a betavoltaic battery composed of a silicon p-n junction converter and a titanium tritide film as an isotope source. The self-absorption of β particles emitted from the tritium radioisotope in the titanium tritide film and the energy deposition of β particles in the silicon converter are investigated by the Monte Carlo simulation with the Geant4 radiation transport toolkit. The relationships between doping concentrations and basic parameters such as depletion region width, minority carrier diffusion length and leakage current of the PN junction are discussed through the calculation formulas. By optimizing the doping concentrations in the P-type and N-type regions, the optimized betavoltaic battery can maximize the output power and the conversion efficiency based on the energy deposition in the silicon. The results show that the optimal thickness of the titanium tritide film is about 0.7 µm and the optimal doping concentrations of the battery with a PN junction depth of 50 nm are Na=5.75×1019cm−3,Nd=2.95×1018cm−3. Under these parameters, the size 1mm×1mm proposed battery with 2.9 mCi/mm2 3H can achieve the output power 0.902 nW and the conversion efficiency 0.91%. The open circuit voltage, short circuit current and fill factor of the battery are 0.389 V, 3.03 nA and 0.766, respectively.

Evolution of 3He bubble microstructure in TiT2 films during aging

TEM observation was performed on titanium tritide films at He:Ti ratios of 0.036–0.356. Most helium atoms remained in the TiT2 lattice until at a He/Ti ratio of 0.06, almost all helium atoms entered the bubbles. The density of helium bubbles in Ti tritide increased with the increase in helium content, while their size distribution remained almost unchanged until the bubbles in the samples begin to interconnect at a He/Ti ratio of 0.247. Helium bubbles preferred to distribute in Ti {111} planes for all the samples. The morphology of helium bubble distribution changes not only with the density of bubbles but also with the strain in the films. Helium tended to migrate to the grain boundaries, and plenty of cracks were developed along the grain boundaries. The extension of cracks to the film surface is the cause for the large bursts of 3He into the atmosphere. The observed microstructural evolution and its dependence on helium concentration, density of bubble, and stress distribution are discussed in terms of diffusion of interstitial He atoms and their clustering.

Thermal desorption behavior of helium in aged titanium tritide films

The desorption behavior of helium in TiT(1.5∼1.8)−x3Hex film samples (x = 0.0022–0.22) was investigated by thermal desorption technique in vacuum condition in this paper. The thermal helium desorption spectrometry (THDS) of aging titanium tritide films prepared by electron beam evaporation revealed that, depending on the decayed 3He concentration in the samples, there are more than four states of helium existing in the films. The divided four zones in THDS based on helium states represent respectively: (1) the mobile single helium atoms with low activation energy in all aging samples resulted from the interstitial sites or dissociated from interstitial clusters, loops and dislocations, (2) helium bubbles inside the grain lattices, (3) helium bubbles in the grain boundaries and interconnected networks of dislocations in the helium concentration of 3Hegen/Ti > 0.0094, and (4) helium bubbles near or linked to the film surface by interconnected channel for later aging stage with 3Hegen/Ti > 0.18. The proportion of helium desorption in each zone was estimated, and dissociated energies of helium for different trapping states were given.

3He retention and structural evolution in erbium tritides: Phase and aging effects

Effects of phase changes on 3He release/retention and crystal lattice evolution during aging of erbium (Er) tritide films were investigated using X-ray diffraction. The contents of α phase and γ phase in the Er tritide films showed significant different effects on 3He release/retention. The initial tritium stoichiometry or excess tritium atoms accommodated in the octahedral sites and the microstructure (i.e., the texture and Er2O3 oxide inclusions) played an important role for the 3He release and the evolution of 3He bubbles in the β phase Er tritide films. In the β+γ region, evolution of 3He in the β phase was apparently influenced by the γ phase, which could result in a strongly anisotropic lattice dilation and an earlier inflection point of the expansion rate of (111) lattice parameter. A preferred occupation of 3He in basal plane of the hexagonal γ phase and the lattice expansion along the hexagonal direction were identified.

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.

The radiation damage of crystalline silicon PN diode in tritium beta-voltaic battery

A tritium beta-voltaic battery using a crystalline silicon convertor composed of (100)Si/SiO2/Si3N4 film degrades remarkably with radiation from a high intensity titanium tritide film. Simulation and experiments were carried out to investigate the main factor causing the degradation. The radiation damages mainly comes from the x-ray emitted from the titanium tritide film and beta particle can relieve the damages. The x-ray radiation induced positive charges in the SiO2 film destroying the output property of the PN diode with the induction of an electric field.

Simulations about self-absorption of tritium in titanium tritide and the energy deposition in a silicon Schottky barrier diode

Simulations on the self-absorption of tritium electrons in titanium tritide films and the energy deposition in a silicon Schottky barrier diode are carried out using the Geant4 radiation transport toolkit. Energy consumed in each part of the Schottky radiovoltaic battery is simulated to give a clue about how to make the battery work better. The power and energy-conversion efficiency of the tritium silicon Schottky radiovoltaic battery in an optimized design are simulated. Good consistency with experiments is obtained.

Progress of Helium Evolution in Aging Titanium Tritide Film

Helium release from titanium tritide films at room temperature have been studied. The evolution of lattice defects in long-aged titanium tritide films is also investigated by X-ray diffraction (XRD) over a period of about 1600 days (>4 years). And the thermal desorption (TD) has been used to investigate the 3He release from titanium tritide film with 3He/Ti atom ratio from 0.006 to 0.325. Results of XRD, TD and helium release were synthesized. A continuum-scale evolutionary model of helium for aging titanium tritide film is described which accounts for major features of the tritide experiment data. The combined stress-assisted-block loop punching growth for random bubble arrays and an average ligament stress criterion predicts an onset of inter-bubble fracture in good agreement with the He/Ti ratio observed for rapid He release.

Development of bubble microstructure in ErT2 films during aging

Helium bubbles form in metal tritide films as tritium decays into H3e, influencing mechanical properties and long-term film stability. The bubble nucleation and growth mechanisms comprise an active research area, but there has been only one previous systematic experimental study of helium bubble growth in metal tritides, on zirconium tritides. There have been no such studies on tritides such as ErT2 that form platelike bubbles and lack a secondary bubble population on a network of line dislocations, and yet such a study is needed to inform the modeling of helium bubble microstructure development in a broader range of metal tritides. Transmission electron microscopy has been used to study the growth and evolution of helium bubbles in ErT2 films over a four-year period. The results have been used to test the present models of helium bubble nucleation and growth in metal tritides, particularly those forming platelike bubbles. The results support the models of Trinkaus and Cowgill. The observations of nonuniform bubble thicknesses and the pattern of grain-boundary bubble formation, however, indicate that these models could be strengthened by closer attention to details of interfacial energy. It is strongly recommended that efforts be made (either experimentally or by calculation) to determine anisotropy of tritide/helium interfacial energy, both for clean, stoichiometric interfaces, and also allowing for such factors as nonstoichiometry and segregation.

Aging of ErT2 thin films: ERD analysis and mechanical property changes

Abstract Rare earth tritide films evolve as tritium decays into 3He, which forms bubbles that influence long-term film stability in applications such as neutron generators. We followed the properties of model ErT2 films as they aged using ERD analysis to monitor T and 3He profiles, nanoindentation testing for mechanical properties and transmission electron microscopy to characterize bubble growth. The profiles of T and 3He are separately measured in ERD using a ΔE − E detector, taking advantage of the differences in energy loss within the ΔE detector. The composition measured by ERD followed the expected build-up of 3He up to near critical release, where 3He begins to escape from the film as bubbles overlap. These measurements complement observations of the changing mechanical properties of these films, where the observed behavior divided into two regimes: a substantial increase in layer hardness but elasticity little changed over ∼18 months, followed by a decrease in elastic stiffness and a modest decrease in hardness over the final 24 months. The evolution of properties has been explained by a combination of dislocation pinning by the bubbles, elastic softening as the bubbles occupy an increasing fraction of the material, and details of bubble growth modes. The ERD measurements confirm that the changes in properties are due to changes in bubble morphology and not to changes in 3He or T content.

X-ray diffraction analysis of titanium tritide film during 1600 days

The generation and accumulation of 3He by tritium decay modified the physical and chemical properties of tritides. Here the evolution of lattice defects in long-aged titanium tritide films is investigated by X-ray diffraction and changes in the positions, intensities and line shapes of diffraction peaks have been determined over a period of about 1600days (>4years). Texture effects are also observed by biased intensities in standard θ–2θ scans. The results show that the TiT1.5 film keeps an fcc structure during 1600days and reveals an hkl-dependent unit-cell expansion and line width broadening which are interpreted in terms of isolated tetrahedral interstitial 3He atoms and isolated bubble growth models by dislocation loop-punching or dislocation dipole expansion combined with Krivoglaz theory. In the first 12days of aging, isolated tetrahedral interstitial 3He atoms or 3He clusters are formed, then interstitial 3He atoms diffuse into (111) planes and precipitate into clusters. The spontaneous formation of Frenkel pairs, the self-interstitial atoms produced are built into dislocations resulting in formation platelet bubbles and dislocation dipoles between 12 and 27days. Above 27days, multiple stages of 3He bubbles growth appear: (1) between 27 and 85days platelet helium bubbles growth by dislocation dipoles expansion, (2) between 85 and 231days the transition from platelet bubbles to sphere bubbles by loop emission, (3) after 231days sphere bubbles growth by dislocation loop-punching and probably formation of sub-grain boundaries by dislocation rearrangement.

Evolution of mechanical properties in ErT2 thin films

The mechanical properties of rare earth tritide films evolve as tritium decays into H3e, which forms bubbles that influence long-term film stability in applications such as neutron generators. Ultralow load nanoindentation, combined with finite-element modeling to separate the mechanical properties of the thin films from their substrates, has been used to follow the mechanical properties of model ErT2 films as they aged. The size of the growing H3e bubbles was followed with transmission electron microscopy, while ion beam analysis was used to monitor total T and H3e content. The observed behavior is divided into two regimes: a substantial increase in layer hardness but elasticity changed little over ∼18 months, followed by a decrease in elastic stiffness and a modest decease in hardness over the final 24 months. We show that the evolution of properties is explained by a combination of dislocation pinning by the bubbles, elastic softening as the bubbles occupy an increasing fraction of the material, and details of bubble growth modes.

Microstructural Features in Aged Erbium Tritide Films

Abstract Erbium is used as a storage medium for tritium. Microstructural study of helium bubble generation from tritium decay in erbium tritide can provide an unusual example of bubble development with negligible radiation damage. Aged erbium tritide film specimens were found to contain five distinctly different microstructural features. The general structure was of large columnar grains of ErT2. But on a fine scale, precipitates believed to be erbium oxy-tritides and helium bubbles could be identified. The precipitate size was in the range of ∼10 nm and the bubbles were of an unusual planar shape on {111} planes with an invariant thickness of ∼1 nm and a diameter on the order of 10 nm. Also, an outer layer containing no fine precipitate structure and only a few helium bubbles were present on the films. This layer is best described as a denuded zone which probably grew during aging in air. Finally, large embedded Er2O3 particles were found at low density and nonuniformly distributed, but sometimes extending through the thickness of the film. A failure mechanism allowing the helium to escape is suggested by observed cracking between bubbles closer to end of life.

Elastic recoil detection analysis of 3He

We give the results of a study using Monte Carlo ion interaction codes to simulate and optimize elastic recoil detection analysis for 3He buildup in tritide films. Two different codes were used. The primary tool was MCERD, written especially for simulating ion beam analysis using optimizations and enhancements for greatly increasing the probabilities for the creation and the detection of recoil atoms. MPTRIM, an implementation of the TRIMRC code for a massively parallel computer, was also used for comparison and for determination of absolute yield. This study was undertaken because of a need for high-resolution depth profiling of 3He and near-surface light impurities (e.g. oxygen) in metal hydride films containing tritium.

Electron emission at the surface of metal tritide films

Monte Carlo simulations of tritium decay electrons in three metal tritide films, LiT, MgT 2 and TiT 2, of effectively infinite thickness have been performed to determine distributions of the total electron energy and the normal energy E n at the surface of these materials. Monte Carlo calculations of electron emission at the surface of an effectively infinite `slab' of tritium gas (STP) have also been carried out. Using a planar retarding potential analyzer, measurement of the integrated electron flux and the distribution of normal energies E n from 50 eV to 7.4 keV have been carried out for lithium tritide films of effectively infinite thickness prepared with various tritium to lithium atomic ratios. A comparison of the measured and computed distributions shows that the measured flux is greater than the computed result for E n below approximately 1.5 keV, while for E n above 1.5 keV the two distributions are in good agreement. Calculations of high energy secondary and Auger electron contributions suggest that the observed discrepancy at lower energies can be largely accounted for by Auger electrons associated with impurities in the near-surface region of the lithium tritide films.

Electron Flux at the Surface of Titanium Tritide Films

Certain metal tritides have been investigated as reliable and quasi-constant sources of electrons for a number of practical purposes with particular attention to the dependence of the electron emis...