Electron Beam Induced Damage and Helium Accumulation in Lithium Metatitanate Ceramic Breeder Materials

Polycrystalline lithium metatitanate (Li2TiO3) is an attractive material for a range of applications including battery materials and microwave dielectrics, it has also attracted a great deal of attention as proposed tritium breeder material for use in nuclear fusion reactors. During the continued development of Li2TiO3, and similar Li-containing ceramics, for these advanced applications, transmission electron microscopy (TEM) is likely to be employed to study the nano-structure of these materials and, in the case of nuclear fusion applications, their response to radiation induced damage.
In this project, the effects of ceramic processing conditions on the microstructure of Li2TiO3 ceramics was first identified. The effects of ceramic microstructure on the behaviour of Li2TiO3 under electron irradiation was subsequently investigated. Results show that electron irradiation in a conventional TEM results in the formation of nanometre sized vacancy-type defects at room temperature, and that the size and number density of the vacancy-type defects are linked to ceramic microstructure and crystallographic disorder. Electron beam induced vacancy-type defect formation is discussed in terms of electron beam heating, electron beam induced radiolysis and electron beam induced displacement damage; the displacement of Li and O lattice atoms is proposed to be primarily responsible for the observed damage. Results suggest that the formation and growth of vacancy-type defects is initially driven by the supersaturation of vacancies due to the preferential loss of interstitials to the surfaces of thin film TEM specimens, followed by coalescence of larger vacancy-type defects, which leads to cavity growth. Results from X-ray diffraction suggest that growth at room temperature may be facilitated by the presence of CO2, produced by the decomposition of a Li2CO3 surface reaction layer under the electron beam. These results highlight that care must be taken when interpreting TEM data and undertaking the associated analysis, especially in Li-containing oxides.
Following the discovery of electron beam induced cavity formation in Li2TiO3, the thermal evolution of these cavities at temperatures relevant to fusion breeder blanket operating conditions was investigated using in-situ thermal annealing experiments, which were carried out under electron irradiation in a TEM. The effects of temperature, electron beam exposure time, and ceramic microstructure on cavity growth dynamics are discussed. The rate and extent of cavity growth was found to be significantly enhanced at elevated temperature, this is primarily attributed to the increased mobility of defects introduced as a result of electron beam induced displacement damage. Minimising exposure of thin-film TEM specimens to the electron beam was found to significantly
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reduce the extent of cavity growth, particularly at high temperatures, indicating that cavity growth is strongly dependent on electron fluence. The microstructural properties of low porosity and large grain size may increase the resistance of Li2TiO3 to electron beam damage. This is attributed to the longer diffusion path to defect sinks associated with large grain size and low porosity, resulting in a reduction of preferential interstitial loss and increased probability of defect recombination; thus reducing the excess vacancy concentration responsible for cavity growth in the bulk. A distinct correlation between the temperature at which significant quantities of CO2 were released from bulk Li2TiO3 samples (revealed by the results of thermal desorption spectroscopy (TDS) experiments) and accelerated cavity growth in thin film specimens was identified. Hence it is proposed that enhanced cavity growth under electron irradiation at elevated temperature in Li2TiO3 may be facilitated by CO2 gas produced as a result of the thermal decomposition of an Li2CO3 surface reaction layer.
The accumulation of helium in Li2TiO3 samples with different microstructural properties was investigated using in-situ helium ion implantation. The influence of the interactions of implanted helium with electron beam induced radiation defects on cavity / gas bubble growth at elevated temperatures was investigated using complimentary in-situ thermal annealing experiments. Helium was found to accumulate to a greater extent in samples of low porosity and large grain size. The extent of cavity / bubble growth was found to be significantly exacerbated at elevated temperatures in samples which exhibited high helium retention. Such cavities have the potential to trap tritium in a working breeder blanket, thereby impeding its release and increasing tritium inventory, both of which would be detrimental to material performance. In the absence of implanted helium, the microstructural properties of large grain size and low porosity appear to increase the radiation tolerance of Li2TiO3 to displacement damage induced by electron irradiation. However, since the presence of helium has been shown to severely exacerbate cavity growth in Li2TiO3, the increased helium retention by specimens of low porosity and large grain size in fact resulted in a greater extent of cavity growth in such specimens at temperatures relevant to breeder blanket operating conditions. As such, the properties of high porosity and small grain size are likely to be beneficial for the optimisation of the performance of Li2TiO3 ceramic breeder materials from the perspective of reducing cavity formation at breeder blanket operating temperatures when the effects of helium are taken into account.