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.     

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.     

Characterization of tritium breeding ceramic pebbles prepared by melt spraying

Li₄SiO₄ and Li₂TiO₃ have long been recognized as two excellent promising tritium breeding materials. In this paper, two kinds of ceramic pebbles, Li₄SiO₄ pure phase ceramic pebbles and Li₄SiO₄-xLi₂TiO₃ multiphase ceramic pebbles were prepared by a melt spraying method at a superheating temperature of 100 ℃ and then tested for their performance. The proportion of pebbles with a particle diameter of 0.8∼1.2 mm reached the maximum of 24.02 % when the spraying pressure is 0.04 MPa. The surface of the pebbles prepared by the spraying method was smooth, and the surface roughness was reported for the first time to reach 2.039 μm. The sphericity reached 1.027. When the Ti/Si molar ratio was 0.5, the crush load of the pebbles after heat treatment reached 71.6 N and the thermal conductivity of the materials reached its maximum of 3.098 W/(m·K) at 700 ℃.     

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.     

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.     

固态氚增值剂陶瓷微球制备工艺研究进展

根据聚变反应堆的结构设计,在基于核聚变反应的DEMO包层、ITER-DB包层和TBM包层都将需要约100吨的固态氚增殖陶瓷微球。在众多锂基陶瓷中,Li_4SiO_4和Li_2TiO_3陶瓷微球被认为是最有潜力的固态氚增殖剂。目前,国内外Li_4SiO_4和Li_2TiO_3微球的制备工艺已日趋成熟,如熔融法,湿法,机械滚动法等。但是,这些制备工艺绝大多数都还局限于实验室水平(公斤级),能同时满足微球综合性能好、易于扩大批量化且经济的制备工艺却较少。针对未来聚变堆包层对氚增殖陶瓷微球的巨大需求,本文从经济、环保、产量、生产效率、性能指标等方面全面评价了国内Li_4SiO_4和Li_2TiO_3固态氚增值剂陶瓷微球制备工艺的优缺点。同时,综述了目前国际上新型氚增殖陶瓷微球的研究现状。     

Influence of sintering atmosphere on phase,microstructure and mechanical properties of Li4Si0.7Ti0.3O4 tritium breeding ceramics

Li₄Si_1–xTi_xO₄ ceramic solid solution was considered as a promising tritium breeding material for fusion reactor blanket. In this work, the effects of the sintering atmosphere on the phase composition, microstructure, and mechanical properties of Li₄Si_0.7Ti_0.3O₄ ceramic pebbles were investigated for the first time, aiming to explore the optimal sintering atmosphere for this solid solution. The results show that compared with air and argon atmosphere, the negative pressure of vacuum atmosphere was more favorable for Ti to enter the Li₄SiO₄ crystal structure as a substitute for Si atom to form a solid solution. More importantly, sintering in argon-vacuum atmosphere can ensure the formation of solid solution while fully exerting the liquid phase sintering effect. The crushing load of the Li₄Si_0.7Ti_0.3O₄ ceramic pebbles sintered at 800 °C in argon-vacuum atmosphere reaches 30.3 ± 3.2 N. In general, the appropriate sintering atmosphere was of great significance for obtaining Li₄Si_0.7Ti_0.3O₄ solid solution to meet the performance requirements of tritium breeding blanket.     

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 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.     

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.     

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.     

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.     

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.     

Preparation of ZrO2 beads by an improved micro-droplet spray forming process

Monodisperse ZrO₂ ceramic beads with size larger than 1 mm have been prepared by an improved micro-droplet spray forming process, through which a compressor and a dispenser were employed to produce droplets continuously. Furthermore, the slurry recipe and drying temperature have been optimized to enhance the sphericity and smoothness of the beads. The sintered ZrO₂ ceramic beads present promising mechanical performance, including a relative density of 84.6%, a crush strength of 256.2 ± 36.6 N as well as a Vickers hardness of 1344.4 ± 58.3 HV. Such procedure reveals great potential in mass production of ceramic beads.     

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.     

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.     

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.     

Performance modulation and 3D printing parameters optimization of implantable medical tricalcium-silicate/polyetherimide composite

Tricalcium silicate (C3S)/polyetherimide (PEI) stents are manufactured through an additive manufacturing process using binder jetting. The key issues of C3S/PEI composite ceramic slurry and additive manufacturing process parameters are discussed in detail. Firstly, the low-temperature auxiliary sintering temperature of the sample was determined, and the influence of PEI content on the compressive strength and bending strength before and after sintering was studied. The sintering temperature and optimal PEI content are 340 °C and 10 wt%. Under this PEI content, the flow rate change during the printing process of the slurry was measured, and a C3S/PEI composite slurry suitable for binder jetting additive manufacturing was obtained, and it had excellent mechanical properties. The effect of the parameters of the binder jetting additive manufacturing process on the molding quality of the C3S/10PEI composite ceramic slurry was studied. The effect of the printed layer height on the deposition line width and height was explored, resulting in a selection rule for the printing layer height using nozzle diameters. The influence of the number of layers of the printed sample on the height and line width of the sample is studied. Under the condition that the height of the printing layer is 80% of the nozzle diameter and the hot air assisted drying, the maximum error of the forming size is only 3.13%. Finally, the biocompatibility and cell adsorption effect of the scaffold were studied, and it was found that the C3S/PEI scaffold, which was additively manufactured by binder jetting and sintered at low temperature, had good biological properties.     

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.     

Influence of hydrothermal carbon coating on the properties of CF/ZrB2/SiBCN prepared by slurry injection

With the advantages of short production cycle and facility, slurry injection technology will help explore new possibilities for the preparation of high-performance fiber toughened ultra-high-temperature ceramic (UHTC). Here, hydrothermal carbon (HTC) coating with abundant functional groups is developed on the surface of the carbon fiber (CF) to optimize the slurry injection process and finally enhance the performance of CF/ZrB₂/SiBCN. The enhanced surface energy of HTC coated CF could efficiency promote the injection process, ultimately contributing to the formation of the compact and uniform ceramic green body. The denser matrix of the finally product combined with the HTC coating establish an isolation between oxygen and fiber. Even in a more aggressive oxidation environment, the HTC coating could act as a sacrifice layer to prevent the fibers from damage. The results identify a unique attraction during the construction of CF/ZrB₂/SiBCN with optimal properties and could be extended to other CF/UHTCs.