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.     

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.     

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.     

Preparation of Li2TiO3 ceramic with nano-sized pores by ultrasonic-assisted solution combustion

Li₂TiO₃ is a candidate material for tritium breeding in the future nuclear fusion reactor. In this study, Li₂TiO₃ powder was synthesized by ultrasonic-assisted solution combustion synthesis (USCS) in a single step. The ultrasonic transducer with the power of 1000 W was introduced in the synthesis process. The crystallite size of Li₂TiO₃ powder prepared by utilization of ultrasonic power is significantly decreased to ∼5.0 nm, while the one obtained without ultrasonic power is 20.0 nm. Li₂TiO₃ ceramic sintered from USCS powder at 800 °C exhibits the small grain size of 330 nm and the open pores size of 140 nm. The crush load of the ceramic reaches 37.2 N although the structure is porous. Compared with the ceramic prepared by solid-state reaction and conventional solution combustion synthesis, USCS sample has a higher conductivity of 2.0 × 10⁻⁶ S m⁻¹ at room temperature, indicating the lower tritium diffusion barrier in the ceramic.     

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.     

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.     

Preparation of tritium breeding Li2TiO3 ceramic pebbles via newly developed piezoelectric micro-droplet jetting

Wet methods as an emerging technique for the preparation of millimeter-sized tritium breeding ceramic pebbles, but the imposed air pressure as the driving forces to extrude slurry droplets are fluctuating during the reciprocating extrusion process, which caused a slight inconsistency in pebble sizes. In this study, a piezoelectric micro-droplet jetting approach was proposed by introducing a piezo-driven valve and modifying the slurry barrel mechanism to realize the air pressure invariable. A self-developed piezoelectric micro-droplet jetting device was successfully utilized to prepare Li₂TiO₃ green pebbles with coefficients of variation being lower than 2.7%. The size of the green pebbles could be precisely controlled in the range of 0.88–1.37 mm by manipulating the nozzle diameter, the air pressure, and the jetting time. The pebbles sintered at 1000°C for 3 h possessed a small grain size of ∼5.9 μm, a satisfied relative density of ∼84.8% T.D., and a high crush load of ∼25.7 N, implying the prepared pebbles could be used as a promising solid tritium breeding material in fusion reactors. These findings are anticipated to provide new opportunities for the highly efficient preparation of size-controllable tritium breeding ceramic pebbles.     

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.     

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 ℃.     

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.     

Novel Li4SiO4-based sorbents from diatomite for high temperature CO2 capture

Using inexpensive porous diatomite as silicon source, novel Li₄SiO₄-based sorbents for high temperature CO₂ capture were prepared through the solid-state reaction method at lower temperature (700 °C). Effect of different raw material ratios on CO₂ absorption capacity was investigated. The results showed that CO₂ absorption capacity was dependent on the raw material ratio. When the raw material ratio was 2.6:1, the CO₂ absorption capacity reached 30.32 wt% (83% of the theoretical absorption capacity) in the atmosphere (50 mL/min N₂ and 50 mL/min CO₂). Meanwhile, it was found that the as-prepared Li₄SiO₄-based sorbents from diatomite exhibited good absorption–desorption performance.     

Fabrication of high strength Li-rich 2Li2TiO3–Li4SiO4 composite breeding ceramics at low-temperature by two-step sintering

In this study, Li-rich 2Li₂TiO₃–Li₄SiO₄ composite breeding ceramics were prepared for the first time through a two-step low-temperature sintering route to meet the increasingly stringent performance requirements of tritium breeding materials. The effects of excess Li addition and sintering atmosphere on the phase composition, mechanical properties, and microstructure of the composite breeding ceramics were systematically studied. A two-step low-temperature sintering method was proposed to integrate the advantages of different sintering environments. The results show that moderate Li addition can significantly enhance the crushing load of composite ceramics while maintaining the nanostructure. However, further increasing the amount of Li addition does not continue to increase the crushing load, and will even affect the structural uniformity of the composite breeding ceramics. Most importantly, the S2.5 (2Li_2.5TiO₃–Li₄SiO₄, molar ratio) composite breeding ceramics sintered at 750 °C in an air-vacuum environment exhibited a grain size of ～86 nm and a crushing load of ～66.9 N, which has not been achieved by previous studies.     

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.     

Melting salt assisted sol–gel synthesis of blue phosphor Y2SiO5:Ce

High brightness Y₂SiO₅:Ce phosphor powders with spherical shape and fine size were synthesized by melting salt assisted sol–gel method (MS&Sol–Gel). Commercial tetraethyl orthosilicate was used as the silica source and rare earth oxides were used as rare earth source. The prepared Y₂SiO₅:Ce powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), laser particle sizer, and fluorescentometer techniques. Y₂SiO₅:Ce powders were obtained at significantly lower temperature than that by conventional techniques. When sintered at 1200 °C for 2 h with 5 wt.% LiF and 2 wt.% KH₂PO₄ as fluxes, particles with spherical shape and narrow particle distribution could be obtained. Moreover, the grain size of the powders prepared through this process was in the range of 2–7 μm, strongly depending on the thermal treatments and the species of fluxes. PL intensity of the prepared Y₂SiO₅:Ce phosphor using 5 wt.% LiF and 2 wt.% KH₂PO₄ as fluxes was similar to that of commercial product.     

Diffusion of a tritium interstitial in Li4TiO4 from first-principles calculations

Li₄TiO₄ is an attractive tritium breeding material due to its high lithium density for the application in fusion reactors. Diffusion in the crystal lattice is a vital step of the tritium release process. In this paper, we report the diffusion of a tritium interstitial in the Li₄TiO₄ crystal from first-principles calculations. The tritium interstitial sites, diffusion paths and activation energies of diffusion are obtained. We find the most favorable tritium diffusion path is along <001> of the Li₄TiO₄ crystal with an activation energy of 0.410 eV. The activation energy is 0.366 eV for tritium at 450 K after the vibrational correction is applied. The calculated activation energy is in good agreement with the experimental value of 0.370 eV.     

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.     

Microwave dielectric properties of LBBS glass added (Zn0.95Co0.05)2SiO4 for LTCC technology

The effects of Li₂O–B₂O₃–Bi₂O₃–SiO₂ (LBBS) glass on the sintering characteristics and microwave dielectric properties of (Zn_0.95Co_0.05)₂SiO₄ were investigated in this study. (Zn_0.95Co_0.05)₂SiO₄ powders were fabricated by traditional solid-state preparation, and LBBS glass was synthesised by quenching method. The LBBS glass can effectively reduce the sintering temperature of (Zn_0.95Co_0.05)₂SiO₄ from 1300 °C to 900 °C and thus promote the densification and uniformity of the specimens. XRD patterns indicated that no other secondary phases existed in our doping range (0–2 wt%). To obtain the highest sintering density and a uniform microstructure when the samples were sintered at 900 °C, the optimal doping content was set to be 1.5 wt%. The sample also demonstrated the following excellent microwave dielectric properties: ɛ_r=6.16, Qf=33,000 GHz and τ_f=−59 ppm/°C.     

Effect of BaF2 as the source of Ba component and flux material in the preparation of Ba1.1Sr0.88SiO4:Eu0.02 phosphor by spray pyrolysis

The phosphor powders with the composition of Ba_1.1Sr_0.88SiO₄:Eu_0.02 were prepared by spray pyrolysis. Barium fluoride and barium nitrate were used as the source materials of Ba component. Ba_1.1Sr_0.88SiO₄:Eu_0.02 phosphor powders had broad excitation wavelength ranging from 220 to 460 nm irrespective of mole ratios of barium fluoride and barium nitrate. Ba_1.1Sr_0.88SiO₄:Eu_0.02 phosphor powders had broad emission spectrum between 470 and 600 nm and had the maximum peak intensity at 522 nm. Substitution of BaF₂ instead of some amount of barium nitrate improved the photoluminescence intensities of Ba_1.1Sr_0.88SiO₄:Eu_0.02 phosphor powders. The maximum photoluminescence intensity of Ba_1.1Sr_0.88SiO₄:Eu_0.02 phosphor powders prepared from spray solution with barium fluoride and barium nitrate was about 595% of the phosphor powders prepared from spray solution without barium fluoride. The sky blue light emitting LED with prepared phosphor powders showed (0.1958 and 0.2719) on the CIE chromaticity diagram and luminous intensity of 1.68 cd.     

Preparation of nanocrystalline ultra-high temperature Ta4ZrC5 ceramics by joint processes of solvothermal and carbothermal reaction

An attractive way to prepare nanocrystalline tantalum zirconium carbide ternary ceramics was proposed and confirmed experimentally. The experimental results showed the Ta₄ZrC₅ powders were successfully fabricated by joint processes of solvothermal and carbothermal reaction. The thermodynamic change process in the Ta₂O₅-ZrO₂-C system was studied. The reactions were substantially completed at relatively lower temperatures (∼1873 K/1 h) and the synthesized powders had a small average crystallite size (∼10 nm). The crystalline structure and the nitrogen sorption isotherms patterns of the product were studied. Besides, a monolithic Ta₄ZrC₅ ceramics was densified without sintering additives by pressureless sintering.