Li2TiO3 Ceramic Pebbles Research Articles

Fabrication of fine‐grained Li2TiO3 ceramic with enhanced performance using high‐energy ball milling

AbstractNumerous efforts have been made to fabricate Li2TiO3 tritium breeding ceramics with satisfactory density and mechanical strength while maintaining a fine grain structure. In this study, high‐energy ball milling was performed to promote grain refinement and amorphization of TiO2 and Li2CO3 powders, thereby preparing Li2TiO3 ceramic at a reduced sintering temperature. In the presence of high‐energy ball milling, powder mixtures with particle sizes of 24–29 nm were obtained, and the formation of β‐Li2TiO3 occurred at 400°C. The relative density and crushing load of Li2TiO3 ceramic pebbles improved as the ball‐to‐powder ratio and milling time increased. The Li2TiO3 ceramic pebbles sintered at 900°C exhibited a tiny grain size, satisfactory relative density (89%), and excellent crushing load (121 ± 13 N). The results also demonstrated that Li2TiO3 ceramic possessed the maximum thermal conductivity and ionic conductivity under the conditions of a 20:1 ball‐to‐powder ratio and 20 h milling time, that is, 5.028 W/m·K and 1.87×10−2 S/m at room temperature. Overall, the high‐energy ball milling process has shown advantages in the fabrication of tritium breeding ceramics with fine‐grained structure and excellent comprehensive performance.

Enhancing the density and crush load of Li2TiO3 tritium breeding ceramic pebbles by adding LiNO3-Li2CO3

The fabrication of Li2TiO3 ceramic pebbles with satisfactory density and excellent crush load while maintaining a fine grain structure is one of the research objectives of solid tritium breeders. However, densification and grain refinement are often contradictory in lithium-containing ceramics during the sintering process. In this work, LiNO3-Li2CO3 was added during the preparation of Li2TiO3 green spheres by the wet process, and the formation of LiNO3-Li2CO3 molten salt resulted in the significant sintering improvement of Li2TiO3 ceramic pebbles. The addition of LiNO3-Li2CO3 increased the density of Li2TiO3 ceramic pebbles by 10.47 % (92.63 % vs. 82.16 %) and the crush load by more than four times (144.54 N vs. 28.2 N). ICP-OES analysis showed that the lithium content of the Li2TiO3 ceramic pebbles was slightly increased by the addition of LiNO3-Li2CO3. Overall, the difficulty of balancing grain refinement and densification in lithium-containing tritium breeders can be addressed by the proposed method.

Grain growth mechanism and thermal stability of Li2TiO3 pebbles fabricated by wet method

Li2TiO3 ceramic pebbles with uniform structure, high density and good mechanical property were successfully fabricated by wet process. The Li2TiO3 pebbles sintered at 1000 °C for 4 h had satisfactory density (91% T.D.) and crush load (a.v. 27.1 N). The apparent activation energy of Li2TiO3 grain growth was 201 ± 7 kJ/mol. Accelerated densification was observed in a reducing atmosphere, this might be attributed to the oxygen vacancies generated by the reduction of Ti4+ to Ti3+ in 0.1%H2+He atmosphere, which facilitate the diffusion of ions through the lattice. The grain growth was controlled by lattice diffusion under reducing atmosphere, and the apparent activation energy was 168 ± 6 kJ/mol. The mechanical results show that the crush load of Li2TiO3 pebbles decreased with the increase of temperature, and the average crush load at 500 °C was 13.9 N. In addition, the thermal cycling and long-time annealing experiments suggested that Li2TiO3 pebbles have good structural and performance stability. The Li2TiO3 ceramics prepared by wet process were satisfactory tritium breeder materials with ideal application potential.

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.

In-situ tritium release behavior and post-irradiation experiments of Li2TiO3 pebbles developed by freeze-drying method

To assess the overall performance of potential solid tritium breeders for China Fusion Engineering Test Reactor (CFETR), we conducted a series of In-situ irradiation experiments on the in-pile tritium production and extraction experimental platform at China Mianyang Research Reactor (CMRR). In 2019, we performed an in-pile tritium release experiment (IPTRE-2) on Li2TiO3 (∼90 T.D%, 6Li 7.5%, ∼30.17 g) ceramic pebbles, which are the latest developed breeders. The Li2TiO3 pebbles were produced at the Institute of Nuclear Physics and Chemistry (INPC), China Academy of Engineering Physics (CAEP), using a freeze-drying method. This paper reports on the In-situ tritium release behavior and the post-irradiation experiments of Li2TiO3 pebbles. We established the relationship between tritium residence time and temperature, which can be expressed as lgτ=-0.74+1622.4/T. Our findings indicate that the Li2TiO3 pebbles remained structurally stable without fragmentation, but their color darkened and the degradation of mechanical properties due to the reduction of Ti in Li2TiO3.

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

AbstractLi2TiO3 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.

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

AbstractThe 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 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.

Fabrication of fine-grained Li2TiO3 ceramic pebbles with enhanced crush load by rolling method: Optimization of Li/Ti ratio and sintering procedure

Li2TiO3 ceramic pebbles ought to have high crush strength and uniform microstructure for safety operation and efficient tritium release. Fabrication techniques should be optimized to achieve the desired properties. In our previous work, Li-rich Li2TiO3 ceramic pebbles with a satisfied crush load of 41 N were fabricated by a productive rolling method. Based on this work, effects of raw materials selection, Li/Ti ratio, and sintering atmosphere on Li-Ti-O phase formation were investigated. The pebbles with homogeneous microstructure and enhanced crush load of 92 N were obtained by optimizing the Li/Ti ratio and sintering procedure. The optimal Li/Ti ratio is determined to be 2.5, and the pebbles should be first sintered in air to achieve a high crush strength and then annealed in vacuum to remove the carbonate impurities.

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.

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.

Low-temperature synthesis of Li2TiO3 tritium breeder ceramic pebbles by water controlled-release solvothermal process

In this work, the water-controlled release solvothermal process (WCRSP) method was selected to synthesize nanostructured β-Li2TiO3 tritium breeder ceramic pebbles using anhydrous lithium acetate (LiOAc) as lithium source and tetrabutyl titanate (TBOT) as titanium source. The effects of the reaction system, temperature, and amount of acetic acid on the crystal structure and morphology of Li2TiO3 power were systematically investigated by using XRD, SEM, and TEM techniques, respectively. The urea or acetone in the reaction system affects the composition of Li2TiO3 in the water-controlled release solvothermal process at 550 °C. Initially, the nanostructured single-phase Li2TiO3 powders with good dispersion and an average particle size of 35 nm were successfully synthesized at 550 °C when the acetic acid content was 15 vol%. Moreover, the phase transition temperature for the monoclinic phase β-Li2TiO3 was as low as 450 °C. The morphology of Li2TiO3 powder presents two different shapes, one is micro-spherical, the other is spherical nanoparticles in different content acetic acid. The results indicate that using ethanol/acetic acid mixture as a reaction system is favorable for obtaining nanostructured Li2TiO3 powder. Finally, Li2TiO3 ceramic pebbles with grain size of 1.4 μm were successfully fabricated at 950 °C for 2 h at room temperature, and the crush load and relative density (theoretical density, T.D) of the pebbles reach 53 N and 85.2%, respectively.

Tritium release behavior of Li2TiO3 and 2Li2TiO3-Li4SiO4 biphasic ceramic pebbles fabricated by microwave sintering

Li2TiO3-Li4SiO4 biphasic ceramic is considered as an advanced tritium breeder material for its potential to combine the advantages of both Li2TiO3 and Li4SiO4. In this study, Li2TiO3 and 2Li2TiO3-Li4SiO4 biphasic pebbles with promising properties were fabricated by microwave sintering. The 2Li2TiO3-Li4SiO4 biphasic pebbles displayed porous structure with a grain size of 170 nm. Tritium release behavior for the microwave sintered ceramic breeder pebbles was investigated. It was found that the microwave sintered Li2TiO3 pebbles show the same release behavior with that of conventional sintered pebbles due to the similar microstructure and density properties. Besides, the tritium release results indicated that high closed porosity may lead to the increase of release temperature. The 2Li2TiO3-Li4SiO4 biphasic ceramic pebbles hold satisfactory tritium release behavior with a low release temperature around 450 °C. According to the tritium release results, it could be concluded that the interfaces between Li2TiO3 and Li4SiO4 can improve the tritium release behavior by promoting the surface reaction.

Characterization of Li-rich Li2TiO3 ceramic pebbles prepared by rolling method sintered in air and vacuum

Li2TiO3 with excess Li was developed as an advanced tritium breeder material for fusion reactors. To improve tritium release performance, the breeder pebbles should have a uniform microstructure with small grains. In this work, Li-rich Li2TiO3 ceramic pebbles were fabricated by an efficient spraying-rolling method and were sintered under air and vacuum atmospheres. Results show that the powder calcination temperature has a significant influence on crush load but negligible effect on grain growth. Excess Li2CO3 in pebbles was not just a lithium source to form the Li-rich phase, but also an effective sintering aid to improve sinterability. Small-grained pebbles with dense and homogeneous structure could be obtained by sintering at a low temperature of 1123 K in air. However, when sintered in vacuum, the pebbles exhibit a porous microstructure with very low crush strength. It is indicated that sintering in air is more suitable for the fabrication of Li-rich Li2TiO3 ceramic pebbles.

A process for fabrication of Li2TiO3 ceramic pebbles with high mechanical properties via non-hydrolytic sol-gel method

In this paper, the single-phase Li2TiO3 powder was successfully synthesized via non-hydrolytic sol-gel method (NHSG) using anhydrous lithium acetate (LiOAc) as lithium source and tetrabutyl titanate (TBOT) as titanium source. The effects of the temperature and the Li/Ti molar ratio on the synthesis of Li2TiO3 were investigated by XRD. FT-IR and XPS were used to characterize the synthesis mechanism and surface chemistry of Li2TiO3 powder heated at 650 °C for 2 h with 5 °C/min. Furthermore, the Li2TiO3 pebbles were successfully prepared using synthesized powder by indirect wet chemistry method. The results showed that the Li2TiO3 pebbles with diameter of 1.3mm–1.5 mm, crush load of 67 N, sphericity of 0.97, the relative density of 84.9%, the grain size of 2.85 μm and porosity of 19.84% were successfully fabricated when sintering temperature was 1050 °C for 2 h in air. Therefore，the NHSG method could provide a new efficient route for the production of Li2TiO3 ceramic pebbles.

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

Abstract 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.

Low-temperature preparation of nanostructured Li2TiO3 tritium breeder ceramic pebbles using CTAB-modified ultrafine powders by a mixed solvent-thermal method

Effects of cetyl trimethyl ammonium bromide (CTAB) on the formation of Li2TiO3 nanocrystals during ethanol-water solvothermal reactions were investigated. Addition of CTAB changed Li2TiO3 crystalline morphology from large to small aggregate particles due to the steric repulsion and electrostatic interaction of the CTAB ligands. Aggregate particle size of the Li2TiO3 precursor was reduced from 455.2 to 138.7 nm by preventing their agglomeration after CTAB addition. This was followed by sintering the green body prepared with the CTAB-modified precursor powders using an indirect wet method, then the nanostructured Li2TiO3 ceramic pebbles were obtained at relatively low temperatures from 750 to 800 °C for 4 h. Nanostructured Li2TiO3 ceramic pebbles had small grains and a large number of grain boundaries, which might potentially improve the radiation tolerance, tritium release rate, and mechanical properties of Li2TiO3 tritium breeder pebbles. In addition, grain growth mechanisms as function of different sintering temperatures, crush load as well as relative density of fabricated nanostructured Li2TiO3 ceramic pebbles were also investigated.

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.

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.

Fabrication of Li2TiO3 ceramic pebbles with fine microstructure by microwave sintering

Lithium metatitanate (Li2TiO3) ceramic pebbles were successfully fabricated by microwave sintering using the powders synthesized via solid state reaction. Nano-size Li2TiO3 powders with an average particle size of 45 nm were synthesized by solid state reaction, and wet process was employed to fabricate Li2TiO3 pebbles with good sphericity. The pebbles were sintered by microwave process with rapid heating rate. Due to the volumetric heating and enhanced diffusion of microwave sintering, the sintered Li2TiO3 ceramic pebbles show high density (>85%T.D.), high crush load and uniform microstructure with small grain size (100 nm–1 μm), which might hold good potential in tritium release and long-term stability. The results show that the microwave sintering is a promising process for the fabrication of Li2TiO3 pebbles with fine microstructure and good mechanical property.