Lithium Metatitanate Research Articles

Hyperfine interactions of paramagnetic radiation-induced defect centres in advanced ceramic breeder pebbles

Advanced ceramic breeder (ACB) pebbles consisting of 65 mol% lithium orthosilicate (Li4SiO4) and 35 mol% lithium metatitanate (Li2TiO3) are developed for tritium breeding in the European Union’s (EU) helium cooled pebble bed (HCPB) breeder blanket concept of the demonstration fusion power plant (DEMO). Electron-nuclear double resonance (ENDOR) spectroscopy is a powerful and widely used magnetic resonance technique that combines aspects of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). In the present work, ENDOR spectroscopy was applied to investigate hyperfine (HF) interactions of paramagnetic radiation-induced defect centres (containing unpaired electrons) in the biphasic ACB pebbles after irradiation with 10 MeV accelerated electrons. To separate individual EPR signals of the formed and accumulated paramagnetic centres, isochronal annealing of the irradiated ACB pebbles followed by EPR spectra measurements at X- and Q-microwave frequency bands at room and low temperature were performed. Afterwards, X-band ENDOR spectra at low temperature were recorded for selected EPR signals to reveal HF interactions between the unpaired electron spins with the spins of magnetic 7Li and 1H isotope nuclei. The g-factor and HF coupling values determined from EPR and ENDOR spectra simulations provide novel insights into the local structure of paramagnetic hole- and electron-type centres in the irradiated ACB pebbles.

Fabrication and characteristics of Li2TiO3 pebbles manufactured by using powder injection molding (PIM) process

The solid type of tritium breeder materials is usually used in a pebble form because of the high packing factor, efficiency of tritium extraction, and advantage of thermal conduction between the structural materials. The breeder pebbles are required a good sphericity and uniform microstructure from the viewpoint of a packing factor and a release of bred tritium. Therefore, in this study, the lithium metatitanate (Li2TiO3) pebbles have been fabricated by powder injection molding process in order to secure a high sphericity of pebbles. In addition, the physical, mechanical, and thermal properties of the pebbles have been investigated. The Li2TiO3 green pebbles have been fabricated by using the powder injection molding process (PIM process). The diameter and sphericity of green pebbles were about 1.000 mm and 1.001, respectively. Then, the green pebbles were sintered at 1000 °C for 3 hr in air atmosphere. The diameter of sintered pebbles is about 0.95 mm. The volume shrinkage ratio was about 16 % during the sintering process. The uniform microstructure with open pores on the inside and outside of the sintered pebbles was observed.

Molecular dynamics simulations of the effect of porosity on heat transfer in Li2TiO3

Heat transfer is a key consideration in the development of tritium breeder blankets for future fusion reactors. For solid tritium breeder materials there is a fine balance to be struck between high levels of porosity to encourage tritium release and minimising it to maintain the thermal and mechanical properties. Therefore, in this work we employ molecular dynamics simulations to understand how the introduction of porosity influences the thermal conductivity of lithium metatitanate ceramic breeder material. Our simulations predict that increasing the porosity leads to a decrease in the thermal conductivity which is in good agreement with previous experimental observations. By contrast, we do not observe the increase in the thermal conductivity at high temperatures, that is observed in some experiments. We argue that this increase is a consequence of sintering or some other modification of the experimental sample rather than a fundamental change in the heat conduction mechanism in the crystal matrix.

HICU PIE results of neutron-irradiated lithium metatitanate pebbles

Lithium metatitanate (Li2TiO3) pebbles were irradiated with neutrons within the HICU (High neutron fluence Irradiation of pebble staCks for fUsion) experiment to investigate their material properties and tritium release behaviour in a post-irradiation examination (PIE). The irradiation temperature is the most significant influence on the material. Besides a higher irradiation temperature, a higher initial Li–6 content tends to lead to an increased tritium generation resulting in the formation of the secondary lithium depleted phase Li4Ti5O12. A pre-compaction of the pebble bed does not have an apparent influence on the material properties. Tritium is released as semi-tritiated water and semi-tritiated hydrogen (HTO and HT) and its release is enhanced at elevated irradiation temperatures.

Comparative analysis of gas release from biphasic lithium ceramics pebble beds of various pebbles sizes and content under neutron irradiation conditions

This paper presents the results of 4 reactor campaigns on the irradiation of biphasic lithium ceramics containing different ratios of lithium orthosilicate (LOS) and lithium metatitanate (LMT) components (25 and 35 mol% LMT in LOS). The size distribution of pebbles in pebble beds was 250–1250 μm and 500–710 μm, respectively. The studies were carried out sequentially with each type of ceramics. In experiments carried out using the vacuum extraction method, the composition of gases released from lithium ceramic samples was registered in in-situ mode. The absence of purge gas during the experiments minimized the possibility of T2O and HTO formation, significantly simplifying processing of the results and providing more opportunities for results analysis. The main goal of the present paper was to identify the general patterns of the release of gases with mass numbers M2 (H2), M4 (He + HT), M6 (T2) and M18 (H2O) from ceramic samples throughout the entire irradiation experiment in 4 campaigns. Release trends of main gases with mass numbers M2, M4, M6 and M18 for all four campaigns are presented and their comparative analysis was performed. The average partial pressure of tritium release in the form of T2 and HT molecules for all campaigns was determined as 5.8 × 10-7 Torr. The dependences of formation rates of helium release peaks on the irradiation time were plotted. The nature of peak emissions does not have a monotonic relationship; upon irradiation, both an increase in the frequency of peaks and a decrease in it are observed. During irradiation, the process of peak helium release does not stop.The simulation was carried out by the finite element method, assuming that tritium release from the sample is determined by diffusion and desorption processes from the sample surface. The experimental curves are satisfactorily described by a number of sets of desorption and diffusion parameters. One way or another they lie in the range of specified values. The Arrhenius dependences of the effective diffusion coefficient and desorption coefficient obtained for lithium ceramics 35 LMT are equal to:D=5,2×10-11m2sexp-21kJmoleRT,K=1.21×10-4m2sexp-64kJmoleRT.The values of the effective diffusion coefficient and tritium desorption coefficient in 25 LMT ceramics were 15 and 20% lower than in 35 LMT ceramics.

Charge–discharge mechanism, lithium-ion diffusion in Al, Ca, and Cu doped lithium metatitanate based anodes for Li-ion batteries: first principles study

Understanding the charge–discharge mechanism at the atomic-scale and the evolution of electrochemical properties for lithiated/de-lithiated compounds is a key challenge in lithium ion batteries (LIBs).

Optimizing the synthesis process for Lithium-Ion sieve adsorbents: Effect of calcination temperature and heating rate on reaction efficiency and performance

Processing-structure–property relationships play a crucial role in tuning the performance of materials for a given application in order to attain a suitable set of optimal conditions, and it is therefore imperative to evaluate the parameters of either the synthesis, delithiation, adsorption, and desorption process. Through an orthogonal test design of the solid-state synthesis process, it was determined that a heating rate of 1 ℃/min had consistently higher reaction efficiencies of 68.1 %, 68.6 %, and 72.3 % at the calcination temperatures of 650 ℃, 700 ℃, and 750 ℃ respectively compared to heating rates of 4 ℃/min and 7 ℃/min. Moreover, a decrease in delithiation efficiency with increase in calcination temperature (99 % at 650 ℃, 96.1 % at 700 ℃, and 94.2 % at 750 ℃) at the 1 ℃/min heating rate was observed which was attributed to an increase in the average crystallite size. Adsorption performance of the synthesis-optimized zinc-doped lithium metatitanate showed consistently high adsorption efficiency (>90 %) despite the low adsorption capacity (4.9 mg/g) which was attributed to the low initial lithium-ion concentration of the lithium chloride solution and high adsorbent dosage.

Features of Helium and Tritium Release from Li2TiO3 Ceramic Pebbles under Neutron Irradiation.

The operation of fusion reactors is based on the reaction that occurs when two heavy hydrogen isotopes, deuterium and tritium, combine to form helium and a neutron with an energy of 14.1 MeV D + T → He + n. For this reaction to occur, it is necessary to produce tritium in the facility itself, as tritium is not common in nature. The generation of tritium in the facility is a key function of the breeder blanket. During the operation of a D-T fusion reactor, high-energy tritium is generated as a result of the 6Li(n,α)T reaction in a lithium-containing ceramic material in the breeder blanket. Lithium metatitanate Li2TiO3 is proposed as one of the promising materials for use in the solid breeder blanket of the DEMO reactor. Several concepts for test blanket modules based on lithium ceramics are being developed for testing at the ITER reactor. Lithium metatitanate Li2TiO3 has good tritium release parameters, as well as good thermal and thermomechanical characteristics. The most important property of lithium ceramics Li2TiO3 is its ability to withstand exposure to long-term high-energy radiation at high temperatures and across large temperature gradients. Its inherent thermal stability and chemical inertness are significant advantages in terms of safety concerns. This study was a continuation of research regarding tritium and helium release from lithium metatitanate Li2TiO3 with 96% 6Li during irradiation at the WWR-K research reactor using the vacuum extraction method. As a result of the analysis of experiments regarding the irradiation of lithium metatitanate in vacuum conditions, it has been established that, during irradiation, peak releases of helium from closed pores of the ceramics are observed, which open during the first 7 days of irradiation. The authors assumed that the reasons samples crack are temperature gradients over the ceramic sample, resulting from the internal heating of pebbles under the conditions of their vacuum evacuation, and contact with the bottom of the evacuated capsule. The temperature dependence of the effective diffusion coefficient of tritium in ceramics at the end of irradiation and the parameters of helium effusion were also determined.

Deuterium permeation behavior of zirconium oxide coating after exposure to solid breeder pebbles

Tritium permeation barrier has been developed using ceramic coatings to mitigate tritium leakage through structural materials in most of the fusion reactor blanket concepts. In a solid breeder blanket concept, corrosion of the coating is a serious issue in addition to tritium permeation; however, the effects of corrosion on tritium permeation have been rarely researched. Here, we report deuterium permeation behaviors of zirconium oxide coatings prepared by metal organic decomposition after exposure to two kinds of solid breeder pebbles with different compositions, followed by deuterium permeation measurements. The deuterium permeation flux varied with the pebble composition: the coated sample after exposure to the pebbles of 30 mol% lithium metatitanate and 70 mol% lithium orthosilicate was smaller than that after exposure to the pebbles of 20 mol% lithium metatitanate and 80 mol% lithium orthosilicate. Because the thickness of the corrosion layer formed by the pebbles with 30 mol% lithium metatitanate and 70 mol% lithium orthosilicate was larger, the composition of the pebbles affected the thickness of the corrosion layer and the permeation behaviors rather than coating integrity.

One‐step fabrication of Li2TiO3 ceramic pebbles using pulsed YAG laser

AbstractA large amount of Li‐containing ceramic breeder pebbles is packed in the solid breeding blanket of a nuclear fusion reactor. Several pebble fabrication technologies have been proposed in previous studies, including wet process, emulsion method, extrusion spheronization, additive manufacturing, and melt process. However, a simple, energy‐effective, and scalable fabrication technology remains to be developed for the automated mass production and reprocessing of used radioactive pebbles post‐operation. Selective laser melting potentially enables the quick and automated fabrication of breeder pebbles. Herein, we employ a high‐power density pulse laser to produce ceramic breeder pebbles. A pulsed YAG laser was irradiated over a lithium metatitanate (Li2TiO3) powder bed in air, and the corresponding temperature was monitored using fiber‐type infrared pyrometers. Spherical Li2TiO3 pebbles were successfully fabricated in a single step with an average diameter of 0.78 ± 0.13 μm and the sintering density of 87.4% ± 5.6% (input power: 7.9 J/pulse). The irradiated Li2TiO3 powder melted and turned spherical under surface tension and rapidly solidified, resulting in uniaxial fine grains and a decrease in the degree of long‐range cation ordering.

Simulations of complex electron paramagnetic resonance spectra for radiation-induced defect centres in advanced ceramic breeder pebbles

Advanced ceramic breeder (ACB) pebbles consisting of lithium orthosilicate (Li4SiO4) and the strengthening phase lithium metatitanate (Li2TiO3) are accepted as the European Union’s reference solid-state material for tritium breeding in thermonuclear fusion reactors. Previously, the influence of various radiation types on Li4SiO4-based ceramic breeder materials has already been investigated and described by several groups of researchers. Electron paramagnetic resonance (EPR) spectroscopy is one of the most frequently used analytical and non-destructive techniques that can be selectively applied for qualitative and quantitative research of radiation-induced defect centres with paramagnetic properties (electron spin S ≠ 0). In the present work, individual signals of paramagnetic radiation-induced defect centres in the ACB pebbles after irradiation with X-rays were separated by EPR spectra simulations for the first time. Multiple signals of spin S = ½ systems with distinctive symmetries and g-factor (g) values were identified and characterised. Individual signal decay at room temperature was monitored and isochronal annealing of the irradiated ACB pebbles was performed in order to evaluate the stability of accumulated radiation-induced defect centres. Based on the obtained results, it was determined that radiation-induced defect centres with EPR signals in the g > 2.00 region are similar to irradiated single-phase Li4SiO4 ceramic materials, while additions of Li2TiO3 as the second phase in the ACB pebbles also stimulates the formation of several titanium-related electron centres with signals in the g < 2.00 region and different thermal properties.

Modified Freeze-granulation method for fabricating Li2TiO3 ceramic tritium breeding pebbles

Lithium metatitanate (Li2TiO3) ceramic is an important candidate for tritium breeding material in fusion reactor blanket. Nevertheless, mass production has become an important factor restricting the application of Li2TiO3 pebbles. In this paper, freeze-granulation method is innovatively modified by drying frozen pebbles in acetone. The principle of modified freeze-granulation method is explained in detail. Influence of PVA content, sintering temperature and sintering time on mechanical performance of frozen pebbles dried in acetone are carefully discussed. The modified process could shorten the drying time in 30 min under the premise of good crushing load and thus conducive to improve the efficiency of mass production of tritium breeding ceramic pebbles potentially in the future. Finally, the Li2TiO3 pebbles present good sphericity, high average crushing load (38 N), abundant and uniform pores which are beneficial to tritium release.

EION-IMPLANTED DEUTERIUM RELEASE BEHAVIOR FROM Li-BASED ADVANCED CERAMICS

The behavior of ion-implanted deuterium in advanced lithium orthosilicate (Li4SiO4) pellets with addition of lithium metatitanate (Li2TiO3), and in reference Li4SiO4 pellets, has been studied. Thermal desorption spectroscopy (TDS) was used to examine the deuterium interaction with radiation defects in materials. Deuterium gas release from studied ceramics has revealed a clear dependence of deuterium trapping behavior on phase composition. The trap strength of radiation defects formed in lithium titanate was found to be higher than in lithium orthosilicate.

Reaction kinetics of lithium extraction and a recycling process for lithium meta-titanate pebbles

ABSTRACT Fusion reactors require large amounts of lithium meta-titanate (Li2TiO3) pebbles as tritium breeding materials for Japanese water-cooled solid breeder blanket. The development of a recycling process for tritium-breeding materials is essential for the early realization of fusion reactors. This study was proposed to understand the kinetics of Li extraction from Li2TiO3 by inducing chemical reactions and applying microwave heating and establish a highly efficient recycling process for spent Li2TiO3 pebbles. The results of Li extraction experiments performed under various conditions clarified that this extraction reaction could be applied via a surface chemical reaction with kct = 1 − (1 − f)1/3. The apparent activation energy associated with this extraction reaction was evaluated to be 52.78 kJ/mol, which is in agreement with that of the chemical reaction-controlled mechanism. Additionally, the reaction order for the dissolution of Li2TiO3 in the HCl solution was first-order. The highly efficient recycling process of Li2TiO3 pebbles was suggested.

Molecular dynamics modelling of amorphisation induced change in the mechanical properties of β-Li2TiO3

ABSTRACT The objective of this study is to understand the effect of irradiation on the mechanical properties of Lithium metatitanate (β-) at the molecular level. Computational samples with different levels of damage are subjected to tensile and compressive loads to study changes in elastic stiffness tensor and the crushing strength. Three levels of displacement ratios are chosen; stoichiometric, where atom types are chosen at random, secondly only Lithium atoms are displaced, and finally, atoms to be displaced are chosen in the ratio 60% Li 20% O 20% Ti. Further, local structural changes due to the interaction of stress and radiation are also analysed. It is observed that for stoichiometric and 60% Li, 20% Ti and 20% O displacement cases, the material becomes isotropic at around ≃0.5 dpa. However, for explicit Li displacement, the material continued to show an anisotropic response. This observation indicates that the collapse of the Ti/O sublattice was essential for amorphisation and deteriorating stiffness. However, it was found that very small disturbances in the O/Ti sub-lattice were sufficient to cause a drastic reduction in the crushing strength. Therefore, while small damages below 0.5 dpa may not drastically change the stiffness of the material, it could alter the crushing strength significantly.

EFFECT OF IRRADIATION AND DEUTERIUM BEHAVIOR IN LI-BASED TRITIUM BREEDING CERAMICS

The behavior of implanted deuterium in advanced lithium orthosilicate (Li4SiO4) pellets with addition of lithium metatitanate (Li2TiO3), and in reference Li4SiO4 pellets, has been investigated. Thermal desorption (TD) spectroscopy was used to study the deuterium interaction with radiation defects in materials. Computational evaluation of deuterium desorption within the framework of the diffusion-trapping model allowed to associate characteristics of experimental TD spectra with specific trapping sites in the material. It was found that deuterium desorption is limited mainly by intragranular diffusion of deuterium and its trapping by radiation defects associated with Li-vacancy traps. Deuterium gas release from both ceramics demonstrated similar trend indicating weak dependence of deuterium trapping behavior on phase composition. The change of the morphology and elemental composition of the pellets surface has been analyzed. SEM examination indicated that ion irradiation and subsequent thermal desorption annealing of two-phase ceramics leads to an increase in surface destruction processes.

Measurement of effective thermal conductivity of lithium metatitanate pebble beds by steady-state radial heat flow method

As a functional material of the tritium breeding blankets of the future fusion reactor, different candidates of the lithium-based ceramics have been considered to generate and release tritium. These tritium breeding materials in the form of packed pebble beds of nearly spherical-shaped particles have many advantages over the sintered pellets or blocks. For the design and analysis of the breeding blanket, the effective thermal conductivity (keff) of lithium-based ceramic pebble beds are required to be well characterized under the fusion-relevant conditions. In this study, an experimental setup has been fabricated and installed to estimate the keff of pebble beds as a function of bed temperature and filling gas pressure. The working principle of the experiment is based on the steady-state radial heat flow method. The various experiments have been performed on lithium metatitanate (Li2TiO3) pebbles having a diameter of 1 ± 0.15 mm to measure keff in the temperature range from 400 °C to 800 °C and in the environments of stagnant helium gas. The pressure of helium gas was adjusted from 0.105 MPa to 0.4 MPa (abs.) to account the effect of helium gas pressure. The increase in keff has been found with increasing the bed temperature and also increasing the helium gas pressure. The obtained results have been found comparable with the literature results with minimum difference.

Anisotropic thermal conductivity in Li2TiO3 ceramic breeder materials

The high anisotropy in the thermal conductivity of lithium metatitanate, Li2TiO3, is shown using the classical simulation method of Molecular Dynamics (MD). The thermal conductivity along the z-direction is markedly lower than that in x and y. This characteristic could be exploited in the fabrication of breeder blanket design to favourably adjust the thermal conductivity of Li2TiO3 by ensuring alignment along x or y. This work sets the foundation for investigating the effect of defects on the thermal conductivity of Li2TiO3, as anticipated to be formed during the lifetime of a breeder blanket.

Simultaneous measurement of effective thermal conductivity and effective thermal diffusivity of Li2TiO3 pebble bed using transient hot-wire technique

Lithium-based ceramics in the form of pebble beds have been considered for tritium breeder material in the fusion reactor's breeder blanket. It is essential to study the thermal characteristics of these ceramic pebble beds subjected to fusion relevant conditions. Effective thermal conductivity (k), effective thermal diffusivity (α) and effective specific heat (cp) of a packed bed are some of the crucial parameters for the design of breeder blanket module. In the present study, the transient hot-wire technique was used to measurek, α and cp of lithium metatitanate (Li2TiO3) pebble bed fabricated at Institute for Plasma Research, India. Thermal properties of Li2TiO3 pebble bed (1 ± 0.15 mm pebble diameter and ∼63% packing fraction) are measured within the temperature range of 45 °C to 800 °C in stagnant helium gas environment. The filling gas pressure variation effect on the Li2TiO3 pebble bed's thermal properties for the pressure range of 0.105 MPa to 0.4 MPa has also been studied. Equations are suggested for k and α of Li2TiO3 pebble bed as a function of temperature at different pressure in the helium gas environment.

Influence of Lithium Vacancy Defects on Tritium Diffusion in β-Li2TiO3.

Lithium metatitanate, Li2TiO3, is a leading candidate for application as a tritium breeding material in a future fusion reactor. Following transmutation of lithium, the tritium must escape the crystal in order to be extracted for use in the fusion plasma. The rate-limiting step to release tritium from the Li2TiO3 pebbles is diffusion through the crystal grains. In this work, the activation barriers for tritium diffusion have been calculated using density functional theory. The results show that tritium can diffuse as an interstitial with a barrier of 0.52 eV. However, when a tritium ion becomes bound to a lithium vacancy defect, the energy required to either detrap the tritium from the vacancy or for the cluster to diffuse increases to >1 eV. Overall, these results suggest that the introduction of lithium vacancies due to Li burn-up may lead to an increase in tritium retention in the pebbles.