Mass fabrication of Li2TiO3–Li4SiO4 ceramic pebbles with high strength by facile centrifugal granulation method

The bi-phase Li₂TiO₃–Li₄SiO₄ ceramic pebbles have been considered a promising breeder to realize the tritium self-sustainment in the blanket. However, up to now, the reported ceramic pebbles have the disadvantages of low yield, poor crushing load, and loose internal structure, which cannot meet the practical application requirements. In this work, the Li₂TiO₃–Li₄SiO₄ ceramic pebbles with excellent mechanical properties were fabricated successfully via the centrifugal granulation method with the assistance of introducing a spray-drying process, simulating particle trajectory by discrete element software and improving bonding interface between core and shell with ethylene glycol. The composition, microstructure, and inner structure of the Li₂TiO₃–Li₄SiO₄ ceramic pebbles were investigated, respectively, through X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray computed tomography (CT). It can be found that the employment of the ethylene glycol solution on the surface of Li₂TiO₃ can make the core and the shell combine well. Moreover, the effect of the rolling speed of the Li₂TiO₃–Li₄SiO₄ ceramic pebbles was investigated via discrete element method (EDEM) simulation and experiments. The experimental results displayed that the Li₂TiO₃–Li₄SiO₄ ceramic pebbles sintered at 1100°C for 2 h have a uniform diameter of 1 mm, a good sphericity of 0.97, and an excellent crushing load of 82.4 N, which are superior to those pebbles that obtained by using the traditional wet methods. Moreover, the CT results showed that the appropriate porosity of the core was 3.21% and of the shell was 10.73%. Therefore, the simple centrifugal granulation method can be applied to prepare the Li₂TiO₃–Li₄SiO₄ ceramic pebbles in a large scale and shed a light to investigate the relevant advanced biphasic tritium breeder materials in the future.     

Fabrication and characterization of Li4SiO4-Li2TiO3 composite ceramic pebbles using extrusion and spherodization technique

The effect of different amount of Li₂TiO₃(LT) (0–15?wt%) addition on the properties of composite Li₄SiO₄ (LS) ceramic pebbles were studied. The Li₄SiO₄-Li₂TiO₃ composite powder was prepared in-situ using solid state method at a calcination temperature as low as 800?°C. The composite pebbles were fabricated using a cost-effective and simple technique called extrusion-spherodization. The sintered pebbles were characterized for density, grain size, pore size distribution, crush load and moisture stability. The density of Li₄SiO₄ composite pebbles was improved for LS-5?wt% LT in comparison to LS pebbles when fired at 1000?°C. Moreover, the LS grain size in the composite pebbles was reduced (5.8?μm) in comparison to LS pebbles. We also found that the average crush load value of the LS-5?wt%LT composite pebbles had been improved by nearly 100% (33?N) to that of the pure LS pebbles (17?N). The LS-5?wt% LT pebbles showed improvement in stability to moisture.     

Study on the chemical compatibility between Li2TiO3 ceramic pebbles and diverse advanced structural materials

The tritium breeder and structural materials are necessary components in the blanket to realize tritium (T) self-sustainment of nuclear fusion. The long-term exposure between tritium breeders and structural materials will cause surface corrosion in irradiation environments and then further affect the tritium release behavior. In this study, chemical compatibility between Li₂TiO₃ ceramic pebbles and advanced structural materials was studied systematically at 700 °C for 300 h under He+0.1 % H₂ environment, respectively. The color of the Li₂TiO₃ ceramic pebbles changes from white to dark grey and black. Moreover, the grain size of Li₂TiO₃ ceramic pebbles increases to more than 5 μm, and the crushing load decreased slightly. For the structural materials, the Al-rich oxide layer with about 188.7 nm of 14Cr-5Al oxide dispersion strengthened (ODS) steel and Cr-Fe rich oxide layer with about 1.04 μm of 14Cr-ODS steel were observed on the cross-section. The effective diffusion coefficient of O element in Li₂TiO₃ ceramic moved into ODS steel at 700 °C was calculated to be 3.3 × 10⁻¹⁶cm²/s and 1.02 × 10⁻¹⁴ cm²/s. Unfortunately, when SiC ceramics were contacted with the pebbles, the crystal phase transformed into SiO₂, which severely limits its application. Therefore, these results will provide guidance for the selection of structural materials in the future.     

A novel mass production method for Li2TiO3 tritium breeder ceramic pebbles using polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) assisted granulation method

A novel mass production method of lithium titanite (Li₂TiO₃) tritium breeder ceramic pebbles using polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) assisted granulation method (APG) was proposed. A binder solution of polyvinyl alcohol (PVA) was used to modify the Li₂TiO₃ precursor powder. The powders with adhesive properties were prilled to form green pebbles (GPs) by spheronization at a low rotation speed and spraying with polyvinyl pyrrolidone (PVP), in several cycles. Then, the density and the crush load of the GPs were improved by high-speed rolling. Finally, the ceramic pebbles were produced by sintering. The phase, the microstructure, and the crush load of the ceramic pebbles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and with a universal tester, respectively.     

An innovative process for synthesis of superfine nanostructured Li2TiO3 tritium breeder ceramic pebbles via TBOT hydrolysis – solvothermal method

In this paper, a method combining hydrolysis of tetrabutyl orthotitanate (TBOT) and solvothermal reaction was first used to fabricate nanostructured Li₂TiO₃ tritium breeder ceramic pebbles. Initially, superfine nanostructured Li₂TiO₃ powders were synthesized with average particle size of about 10?nm, according to TEM. The surface area of precursor particles synthesized via this method was found to be 115.85?m²/g by BET analysis, which is much larger than that of the product obtained using traditional methods. The results showed that precursor particles had high sintering activity. XRD pattern revealed that the phase transition temperature for monoclinic phase Li₂TiO₃ prepared by this method was nearly 450?°C, which was the lowest phase transition temperature reported among all wet chemical methods to date. Subsequently, investigation of ceramic sintering demonstrated that Li₂TiO₃ ceramic pebbles with desired nano-crystalline sizes (27.98 ~ 55.03?nm) were obtained by sintering at 500 ~ 600?°C for 4?h. The possible mechanisms were proposed based on the reaction processes of TBOT hydrolysis, solvothermal reaction and sintering.     

Fabrication of attractive Li4SiO4 pebbles with modified powders synthesized via surfactant-assisted hydrothermal method

Li₄SiO₄ pebbles have been widely studied as attractive tritium breeding materials in the fusion reactor blanket of international thermonuclear experimental reactor (ITER). In this work, surfactant-assisted hydrothermal method was first employed to prepare ultrafine ceramic powders for fabricating attractive Li₄SiO₄ pebbles. SEM analysis revealed that the introduction of sodium dodecyl sulfate could eliminate the particle aggregation to prepare monodispersed precursor powders, and thus generated the green bodies of pebble with homogeneous microstructure, which was helpful to eventually obtain high-quality Li₄SiO₄ pebbles. Moreover, the effects of sintering temperature on the grain size, relative density, and crush load of Li₄SiO₄ pebbles were also investigated. Li₄SiO₄ pebbles sintered at 700 °C had a high crush load (average value 27.39 N), small grain size (average value 0.57 μm), satisfactory density (88.13%T.D.) and abundant pore structure, which were expected to show favorable tritium release behavior as a promising tritium breeding material for fusion reactor blanket.     

Comparison of the microwave and conventional sintering of Li2TiO3 ceramic pebbles

Microwave sintering was employed in the fabrication of Li₂TiO₃ ceramic pebbles using the powders synthesized via hydrothermal method. The as-prepared Li₂TiO₃ powders exhibited high reactivity with an average particle size as small as 40?nm. A comparative study between the microwave and conventional sintering behavior of Li₂TiO₃ pebbles was systematically investigated. The microstructure and density analyses showed that the presence of microwaves accelerated the densification and grain growth, thus decreasing the sintering temperature. Besides, an accelerated phase transformation from α-Li₂TiO₃ to β-Li₂TiO₃ was observed in microwave processing. The Li₂TiO₃ ceramic pebbles obtained by microwave sintering exhibited high density, good mechanical property and uniform microstructure, which might hold good potential as tritium breeding materials for blankets. The results showed that the microwave sintering was a promising process for the fabrication of Li₂TiO₃ pebbles.     

Efficient fabrication of high strength Li2TiO3 ceramic pebbles via improved rolling ball method assisted by sesbania gum binder

In this work, a method combining the spray-drying process and rolling ball method was first selected to mass fabricate Li₂TiO₃ tritium breeder ceramic pebbles. Herein, we overcome the issues namely complex production, high manufacturing cost, and lower mechanical strength in previous reports. The Li₂TiO₃ powder with high packing density after the spray drying process will self-agglomerate to form denser structured pebbles during the rolling ball process assisted by sesbania gum binder solution. The stability of slurry, different binder, binder concentrations, the formation mechanism, and the morphology of green pebbles were investigated by using viscometer, SEM. Moreover, the force on the Li₂TiO₃ green pebbles was also analyzed during the rolling ball process. After the debinding and densification process, the Li₂TiO₃ pebbles have a uniform diameter of 1.2 mm and good sphericity of 0.97. The Micro-CT instrument showed that the internal structure of the Li₂TiO₃ pebbles was dense. The experiment's confirmation shows that the Li₂TiO₃ ceramic pebbles sintered at 1000 °C have optimal mechanical properties such as a crushing load of 108 N and relative density of 92.4%TD, which is much larger than that of the pebbles obtained using traditional methods. This work not only overcomes the core-shell structure but also provides a new platform for better mechanical properties for studying the other materials systems in the future.     

A comparative study on the properties of Li2TiO3 powders and pebbles fabricated by different routes

Lithium titanate (Li₂TiO₃) is one of the promising candidate breeders for tritium self-sufficiency of deuterium(D)-tritium(T) fusion reaction. The differences in powder synthesis methods have a great impact on the properties of Li₂TiO₃ powders and the performance of Li₂TiO₃ ceramic pebbles. In this study, the Li₂TiO₃ powders were successfully synthesized by hydrothermal method and solid-state method, and then the pebbles were fabricated by the agar-based wet method. The mechanism of hydrothermal synthesis of Li₂TiO₃ powder was discussed. For the hydrothermal method, the Li₂TiO₃ powder with single phase can be obtained when the rate of Li/Ti = 2.4, and the powder presented two different morphology, which involved two reaction mechanisms, including in-situ phase transformation mechanism and dissolution-precipitation mechanism, the phase transformation from α-Li₂TiO₃ to β-Li₂TiO₃ accomplished at 400°C, which is lower than that of 750°C for solid-state method. Li₂TiO₃ pebbles prepared by the hydrothermal-wet method had a uniform pore distribution, an optimal grain size of 2.7 μm, a crushing load of 58.6 N, and relative density of 90.2%, respectively. In comparison, pebbles prepared by the solid-state-wet method also had better mechanical properties, which the crushing load and relative density were 53.9 N and 86.9% respectively under the optimal fabrication conditions.     

A novel process for fully automatic mass-production of Li2TiO3 ceramic pebbles with uniform structure and size

As an excellent promising tritium breeding material, Li₂TiO₃ ceramic pebbles will be required in large quantities in the future. For this reason, a fully automatic pneumatic eject device based on an improved wet process was employed in the mass-preparation of Li₂TiO₃ ceramic pebbles. The operating principle of the equipment, the preparation process parameters and the performance of the Li₂TiO₃ ceramic pebbles have been studied. The results showed that the spheroidization and solidification process of slurries droplets in the molding medium were critical to the sphericity of pebbles, and the size of the green pebbles was related to the droplet dropping speed、the control pressure and the nozzle inner diameter. It is revealed that more than 90% of the pebbles had an eccentricity of less than 1.1, and the diameter distribution was concentrated between 0.98 mm and 1.03 mm. The Li₂TiO₃ ceramic pebbles with the average grain size of 3.7 μm, the crushing load of 67 N, the relative density of 85.6%, and the porosity of 19.96% can be obtained after sintered at 1100 °C for 2 h. Also, the average pore size was 1.3 μm and the distribution was relatively concentrated. Therefore, this method is expected to meet the future demands of Li₂TiO₃ ceramic pebbles with excellent performance in bulk quantity.     

Preparation of Li4SiO4-xLi2O powders and pebbles for advanced tritium breeders

Li₄SiO₄ pebbles and Li₂O pebbles have been considered as the potential candidates for tritium breeders. Li₂O exhibits higher lithium density whereas worse lithium loss and hygroscopicity compared with Li₄SiO₄. It is anticipated to obtain an advanced breeder by combining Li₄SiO₄ with Li₂O. The coexistence of Li₄SiO₄ and Li₂O powders could not be obtained by solid state reaction using Li₂CO₃ as lithium source, while the biphasic Li₄SiO₄-xLi₂O (x=0.1, 0.2, 0.3, 0.4) powders were prepared by sol-gel method in this experiment. Meanwhile, the biphasic Li₄SiO₄-xLi₂O pebbles were fabricated by a wet method for the first time. The Li₂O aggregated at the grain boundaries and promoted the grain growth of the Li₄SiO₄. The grain size and the crush load of the Li₄SiO₄-0.3Li₂O pebbles reached 32.3 µm and 46.5 N, respectively.     

Accurate and uniform fabrication of Li2TiO3 pebbles with high properties based on Stereolithography technology

Li₂TiO₃ is a vital candidate breeder to solve tritium self-consistency of fusion reaction. In this study, Digital Light Processing (DLP) based on Stereolithography technology was used to fabricate Li₂TiO₃ pebbles for the first time. Ceramic suspensions with different solid loadings were prepared by mixing modified Li₂TiO₃ powder with UV-curable premixture. The size error of Li₂TiO₃ green pebbles was corrected by adjusting the deformation factor of equatorial radius, and the Li₂TiO₃ green pebbles were successfully fabricated with an accurate size. After sintering processing, the Li₂TiO₃ pebbles with a uniform diameter of 1.5 mm and better sphericity of 1.01 were yielded. The sintering behavior of Li₂TiO₃ pebbles were investigated. Results showed that the Li₂TiO₃ pebbles formed with 35 vol% solid loading, and sintered at 1050 ℃ had a uniform pore distribution, and also had optimal properties, such as high relative density of 93.6 %TD, and prominent crush load of 92.3 N.     

A preliminary study on the preparation of nanostructured Ti-doped Li4SiO4 pebbles by two-step sintering process

Li₄SiO₄ has been widely studied as attractive tritium breeding materials due to its innate merits. Considering the potential advantages of nanostructure in tritium breeding materials, a distinctive process was developed to obtain nanostructured Li₄SiO₄ pebbles. In brief, ultrafine precursor powders were synthesized by solvothermal method without using surfactants, and then indirect wet method was adopted to generate the green spheres with homogeneous microstructure. After that, the suitable sintering conditions were defined by studying the effects of sintering parameters on the grain size evolution, and nanostructured Ti-doped Li₄SiO₄ pebbles were first obtained by two-step sintering method. This study will be expected to provide references for fabricating other Li-based tritium breeding materials.     

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

A 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.     

Fabrication of nanostructured Li2TiO3 ceramic pebbles as tritium breeders using powder particles synthesised via a CTAB-assisted method

Nanostructured Li₂TiO₃ ceramics which may have effective thermal conductivity, excellent tritium release behaviour and good irradiation resistance are regarded as a promising solid tritium breeding material for the fusion reactor blanket of the International Thermonuclear Experimental Reactor (ITER). However, due to the limitations of the preparation technology, reports concerning Li₂TiO₃ nanoceramics have been rare. In this paper, uniform nano-Li₂TiO₃ powder particles which were essential to obtain nanostructured Li₂TiO₃ ceramics pebbles were synthesised via a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method, and then rare, homogeneous nanostructured Li₂TiO₃ ceramic pebbles were fabricated with the as-prepared powder particles. The mechanisms by which CTAB can reduce particle agglomeration and be of assistance in achieving a nanostructured Li₂TiO₃ ceramic were also investigated. In addition, systematic experiments on the relationship between the added amount of CTAB and the mechanical properties of the Li₂TiO₃ ceramic structure were also carried out. The results revealed that the desired Li₂TiO₃ nanoceramic could be fabricated when 3% CTAB was introduced, as the Li₂TiO₃ pebbles obtained had a small grain size (90 nm), high relative density (89.71%T.D.) and crush load (99.93 N), which were expected to show favourable potential as a promising tritium breeder material in the fusion reactor blanket.     

Preparation and characterization of Li2TiO3 ceramic pebbles by modification of the water-based sol–gel method

Li₂TiO₃ is considered as one of the best candidates for breeding materials. This article adopted a modification water-based sol–gel method to synthesize nano-Li₂TiO₃ powders, which overcomes the poor phase purity, coarse grain, and inferior crushing strength described in the previous literature. In this paper, the thermal effect of the precursor, the crystal phase, and the morphology of the powders were characterized by thermogravimetric analysis/differential thermal analysis (TG/DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. The nano-structured Li₂TiO₃ powders with good dispersion and an average particle size of 20–50 nm were successfully synthesized at 600°C by controlling PH and hydrolysis rate. Moreover, the phase transition temperature for the monoclinic phase β-Li₂TiO₃ was as low as 600°C, which is lower than 750°C using the traditional solid-state method. Meanwhile, the morphology, porosity, crushing load, and thermal conductivity of ceramic pebbles are characterized systematically by using scanning electron microscope (SEM), mercury injection meter, compression strength equipment, and laser scattering method, respectively. Experimental results showed that the Li₂TiO₃ ceramic pebbles with a sphericity of .98, crush load of 48.4 N, and relative density of 90.03 % were successfully prepared at 1050°C for 2 h. This method will provide new guidance for the preparation of tritium breeders.     

Long-term thermal stability of Li4TiO4–Li2TiO3 core–shell breeding pebbles under continuous heating in H2/Ar atmosphere

The long-term thermal stability of tritium breeding materials during service is a key factor to ensure efficient tritium release. In this study, the long-term thermal stability of advanced Li₄TiO₄–Li₂TiO₃ core–shell breeding pebbles under continuous heating in 1%H₂/Ar at 900°C was investigated for the first time. The results show that this core–shell material loses 3.4% Li mass after heating for 30 days, resulting in a reduction in Li density to .415 g/cm³, which is still significantly higher than other breeding materials. The moisture in the sample bed will determine the form of Li volatilization and thus affect the rate of Li mass loss. The core–shell pebbles maintain favorable phase stability during long-term heating, and the grain sizes of the Li₂TO₃ shell and Li₄TiO₄ core after 30 days of heating are 6.5 ± 1.5 and 6.9 ± 2.5 μm, respectively. Moreover, the samples did not crack or collapse during long-term heating and still had a satisfactory crushing strength of 37.61 ± 7.13 N after 30 days of heating. Overall, the high Li density and good thermal stability during long-term heating demonstrate that the Li₄TiO₄–Li₂TiO₃ core–shell breeding pebbles are a very reliable tritium breeding material for long-term service under harsh operating conditions.     

Low-cost fabrication of Li2TiO3 tritium breeding ceramic pebbles via low-temperature solid-state precursor method

Lithium metatitanate (Li₂TiO₃) ceramic pebbles were fabricated from the powder synthesised via low-temperature solid-state precursor method. Solid H₂TiO₃ and LiOH·H₂O react chemically during ball milling process to form a nano-sized precursor powder. Pure β-Li₂TiO₃ powder can be obtained by calcining the precursor powder at 500 °C, which is half the temperature of conventional solid-state method. The synthesis process is simple and low-cost, which would be more available to achieve batch production among all feasible techniques. The low-temperature calcination will effectively avoid hard particle aggregates and poor sinterability caused by high-temperature heat treatment, which is conducive to prepare ceramics with good properties. The results show that the powder exhibits high sinterability with small particle size of 19 nm. The Li₂TiO₃ ceramic pebbles sintered at 800 °C have small grain size (470 nm), high relative density (83%) and good crush load (45 N), which has great potential as tritium breeding materials for fusion reactors.     

Annihilation behavior of irradiation defects induced by γ-ray in biphasic tritium breeding materials xLi2TiO3-(1-x) Li4SiO4

For the operation of D-T fusion reactor, tritium breeding materials will be adopted to produce tritium. Li₂TiO₃–Li₄SiO₄ biphasic breeders are considered as promising breeding materials due to combining the advantages of Li₂TiO₃ and Li₄SiO₄. The annihilation kinetics of defects induced by γ-ray irradiation in xLi₂TiO₃-(1-x) Li₄SiO₄ (molar ratio: x = 0.25,0.5,0.75) have been investigated. ESR and Easyspin simulation were employed to elucidate the annihilation processes of the irradiation defects. The defects of E′-center and O^?-center were introduced by irradiation. The amount of defects decreased as the molar ratio of Li₂TiO₃/Li₄SiO₄ increased. It was indicated that the irradiation stability of Li₂TiO₃ was better than that of Li₄SiO₄. According to Easyspin simulation, the defect density of E′-center and O^?-center was obtained respectively and the amount of defects decreased as the annealing temperature increased. The activation energy and kinetics equation of E′-center and O^?-center in xLi₂TiO₃-(1-x) Li₄SiO₄ (x = 0.25,0.5,0.75) were obtained. Based on annihilation kinetics, the correlation between annihilation of irradiation defects and tritium release was given.     

Tritium and deuterium release behavior of Li2TiO3-0.5Li4SiO4–Pb ceramic

The neutron multiplier lead (Pb) containing Li₂TiO₃-0.5Li₄SiO₄–Pb was prepared, which aimed at the improvement of the tritium release performance of the lithium ceramic without an increase in lithium density. Li₂TiO₃-0.5Li₄SiO₄–Pb and that without Pb were irradiated at Kyoto University Research Reactor (KUR) with the thermal neutron flux of 5.5 × 10¹² n s⁻¹ cm⁻² and the fast neutron flux of 1.2 × 10¹² n s⁻¹ cm⁻². Fast neutrons can trigger the Pb (n, 2n) reaction. The tritium release experiments were conducted using the tritium thermal desorption spectroscopy (tritium-TDS) system. Compared with Li₂TiO₃-0.5Li₄SiO₄, the tritium release rate at the lower temperature range was higher than that for Li₂TiO₃-0.5Li₄SiO₄–Pb. The tritium-TDS spectrum was also shifted to the lower temperature side. The isothermal heating experimental results confirmed that tritium in the form of gas species (HT) was controlled by the diffusion process, while the trapping and de-trapping process contributed to the total tritium release. Further, 3 keV D₂⁺ implantation with the fluence of 2.0 × 10²² D m⁻² and subsequent TDS measurements were performed. It was shown that the D₂ release at the lower temperature side was increased for Li₂TiO₃-0.5Li₄SiO₄–Pb. Meanwhile, HD and HDO release at higher temperature side was significantly decreased.