Li4SiO4 Pebbles Research Articles

A comprehensive study on the phase composition, mechanical properties and stability of Li4SiO4-Li2ZrO3 biphasic ceramics

Lithium orthosilicate (Li4SiO4) is regarded as a candidate for tritium breeding in fusion reactors. In this study, the Li4SiO4-Li2ZrO3 biphasic was developed to improve the sinterability, density, and mechanical properties of Li4SiO4. The Li4SiO4-xLi2ZrO3 (x = 0.5, 1) composite powders were prepared using solid-state reaction via in-situ method. Li6Zr2O7 existed in the ceramic powders at low calcination temperatures. When the calcination temperature was increased to 900 °C, Li6Zr2O7 was transformed into Li2ZrO3 due to the decomposition of Li6Zr2O7 at high temperatures. The TEM observation confirmed that the powders consisted of Li4SiO4 and Li2ZrO3. The Li4SiO4 and Li4SiO4-xLi2ZrO3 (x = 0.5, 1) pebbles were fabricated by the sol-gel method. The measurement results showed that the pebbles had a narrow size distribution and fine sphericity. The density of Li4SiO4-Li2ZrO3 pebbles reached 96.01% of the theoretical density when it was sintered at 1000 °C for 4 h. Compared with the Li4SiO4, the grain size of ceramic pebbles was significantly reduced. Owing to decreased grain size, 139.0 N crush load for the pebbles and 92.4 MPa bending strength for the sintered bodies were achieved. Besides, the ceramics with the Li4SiO4 to Li2ZrO3 ratio of 2: 1 exhibited preferable mechanical properties. Further, to investigate the chemical stability of biphasic ceramics, the structure and mechanical properties were examined under high temperatures and continuous inert gas purging, simulating the working condition of fusion reactors. It was shown that there was almost no change in phase composition and grain size after purging for 60 h at 650 °C. For the mechanical properties, the crush load was decreased initially due to the cracking in the surface region of the ceramic and then increased because the cracking was recovered.

Preparation of Li4SiO4 pebbles with high strength and inhibited lithium volatilization via lithium alginate

The tritium breeder's primary research focus is to inhibit lithium's volatilization during sintering and improve the mechanical properties of tritium breeder ceramic pebbles. In this study, ceramic pebbles were prepared by wet method using lithium alginate as a lithium supplement and polyvinyl alcohol. The effects of different contents of lithium alginate (LA) and polyvinyl alcohol (PVA) on ceramic pebbles were systematically studied. The results show that the lithium content in Li4SiO4 pebbles after sintering can be increased to 27.6wt% by adding 3wt% lithium alginate, which effectively inhibits the volatilization of lithium. In addition, when the sintering temperature is 1000℃ and the duration is 6 h, the crush load of ceramic pebbles can be increased to 50.9 N, and the density can reach 2.01 g/cm3. The above results indicate that Li4SiO4 ceramic pebbles rich in lithium, which can meet the demand for tritium breeding ceramics, can be prepared by adding LA.

Investigation of the size-dependent crushing behavior of Li4SiO4 pebbles via discrete element method

Li4SiO4 pebbles are commonly employed as breeder materials in solid breeder blankets. As a typical brittle material, the Li4SiO4 pebble sizes significantly affect their crushing behavior. In this study, the size-dependent crushing behavior of pebbles is investigated via discrete element method. Based on the Weibull theory, the probabilistic crushing strength of Li4SiO4 pebbles is analyzed. The numerical results demonstrate that both the characteristic crushing strength and Weibull modulus display a negative linear relationship with Li4SiO4 pebble size in a double-logarithmic plot. Furthermore, by considering both the axial force-displacement responses and fracture process of pebbles, three typical failure patterns of Li4SiO4 pebbles can be identified, namely, splitting failure (Pattern-I), progressive fracturing failure (Pattern-II) and explosive failure (Pattern-III). As the pebble size decreases, the failure pattern gradually shifts from Pattern-I to Pattern-II and to Pattern-III. Additionally, the fragment size distributions of crushed Li4SiO4 pebbles are analyzed based on the fractal theory. It is observed that the fragment size distributions of Li4SiO4 pebbles with varying sizes exhibit favorable fractal features. Smaller Li4SiO4 pebbles exhibit a more significant fragmentation, as evidenced by the increasing fractal dimension of the fragment size distribution with the decreasing pebble size. This work may provide valuable support for further investigations into the mechanical performance of a Li4SiO4 breeder blanket using DEM simulations.

Microfabrication of Li4SiO4 pebbles with complex pore structure via digital light processing: Accuracy control and properties optimizing

Li4SiO4 ceramic pebble is deemed as the most attractive tritium breeder for fusion reactor blanket. In this work, we fabricated a novel Li4SiO4 pebble with micrometer-scale pore structure by digital light processing to verify the feasibility of complex pore structure manufacturing for future breeders. The effects of processing parameters on printing accuracy, microstructure and mechanical properties of Li4SiO4 pebbles were investigated. For synthesizing the pure Li4SiO4 phase, the optimal stoichiometric ratio of Li2SiO3 to Li2CO3 was 1:1.09 in the premixed powder. The photosensitivity of dual-phase ceramic suspension was investigated according to an attenuation model, and the overgrowth was reduced to ∼50 μm by using the dimension formula. After optimizing the post-processing, the radial pore structure Li4SiO4 pebbles with excellent crush load (50.3 N, standard deviation = 4.4 N) and suitable open porosity (18.3 %) can be yielded. This work provides a fresh solution to control the pore structure of tritium breeding pebbles in an unprecedented micrometer scale.

Characterization of aged Li4SiO4 pebbles performance

Many studies and research have been carried out in the recent years to identify which breeder material and (in which) form, solid or liquid, are most suitable to be used in the Breeder Blanket (BB) modules. Nonetheless, the choice of a stable material, with high thermal conductivity and melting point remains unsolved and becomes even more complex when durability is considered. The breeder material must guarantee that the thermo-mechanical properties change as little as possible under environmental and operating conditions. The aim of this study is to investigate the ageing effects on the mechanical behaviour of lithium orthosilicate (Li4SiO4) in form of pebbles. The pebbles had been produced at the University of Pisa at room temperature by a hydrolytic sol-gel supported drip casting method and subsequently stored for 4 years in an environment simulating normal storage (i.e., standard condition). These pebbles of about 1.2–1.4 mm diameter have been subjected to uniaxial compression tests, without radial constraints, and SEM analysis. The results obtained were compared to those of non-aged pebbles to highlight the degradation caused by the ageing. SEM examination of aged pebbles showed increased porosity and a pebble surface that was not entirely homogeneous and uniform. This affected the crack shapes on the contact surface and the crushing load.

In-pile tritium release behavior and the post-irradiation experiments of Li4SiO4 fabricated by melting process

Understanding the tritium release and retention behavior of candidate tritium breeder materials is crucial for breeder blanket design. Recently, a melt spraying process was developed to prepare Li4SiO4 pebbles, which were subsequently subjected to the in-pile tritium production and extraction platform in China Mianyang Research Reactor (CMRR) to investigate their in-situ tritium release behavior and irradiation performance. The results demonstrate that HT is the main tritium release form, and adding hydrogen to the purge gas reduces tritium retention while increasing the HT percent in the purge gas. Post-irradiation experiments reveal that the irradiated pebbles darken in color and their grains swell, but the mechanical properties remain largely unchanged. It is concluded that the tritium residence time of Li4SiO4 made by melt spraying method at 467 °C is approximately 23.34 h. High-density Li4SiO4 pebbles exhibit tritium release at relatively low temperatures (＜600 °C) that is mainly controlled by bulk diffusion. The diffusion coefficient at 525 °C and 550 °C is 1.19 × 10−11 cm2/s and 5.34 × 10−11 cm2/s, respectively, with corresponding tritium residence times of 21.3 hours and 4.7 hours.

Li4SiO4 slurry conditions and sintering temperature for fabricating Li4SiO4 pebbles as tritium breeders for nuclear-fusion reactors

A tritium breeder is a lithium-based material capable of producing tritium. Many researchers designing nuclear-fusion energy are studying tritium production using pebbles, which are solid-type breeders. The sphericity and size of the pebbles are critical in obtaining pebbles with good tritium-breeding efficiency. Furthermore, tritium-release efficiency can be increased by using pebbles with appropriate porosities. Promising raw materials for tritium-breeding materials include Li4SiO4 and Li2TiO3. Li4SiO4 has a higher lithium density than Li2TiO3 and exhibits excellent tritium-breeding efficiency. However, it has the disadvantage of being easily decomposed during the Li4SiO4-green-pebble sintering process because of its low structural stability at high temperatures and high lithium density. In this study, we attempted to determine the optimal conditions for manufacturing Li4SiO4 pebbles using the droplet–freeze-drying method. The optimal Li4SiO4 slurry conditions and sintering temperatures were determined. The optimal Li4SiO4 slurry-fabrication conditions were 3 wt% polyvinyl alcohol and 75 wt% Li4SiO4 based on the deionized-water weight content. The sintering temperature at which Li4SiO4 did not decompose and exhibited the optimum porosity of 10.8% was 900 °C.

Effects of long-term annealing-induced microstructure evolution on mechanical performance and deuterium release behavior of Li4SiO4 pebbles

Lithium-based ceramic breeder Li4SiO4 will suffer long-term high temperature during operation. That may lead to changes to its microstructure, chemical phase, composition elements, and further impacts its mechanical performance and hydrogen isotope release behavior. In the present work, long-term annealing at 900 °C in He incl. 0.1 vol% H2 flow atmosphere for designated times was conducted. Li4SiO4 pebbles with poor or excellent mechanical performance (abbr. “poor pebbles” and “excellent pebbles”) have porous or dense microstructure, respectively. Coarsening process that grains and pores get larger as a function of annealing time was observed for Li4SiO4 poor pebbles. Li4SiO4 excellent pebbles can maintain its microstructure stability, and only slight grain growth was observed after 1000 h of annealing. Oxygen and lithium element loss was demonstrated over annealing time. Compression tests and Weibull analysis indicate that the distinct degradation of mechanical performance of Li4SiO4 pebbles mainly occur in the initial 100 h of annealing, and then the degradation process slows down. Thermal desorption spectroscopy reveals that 1000 h of annealing caused the disappearance of high temperature release peak (665 °C) and the shift of low temperature release peaks (around 300 °C) to lower temperature. The former is mainly attributed to decomposition of carbon impurities (Li2CO3) and the latter is attributed to carbon impurities and the transition from closed to open pores.

Carbon dioxide absorption in packed bed of lithium orthosilicate pebbles

ABSTRACT Lithium orthosilicate (Li4SiO4) has the highest carbon dioxide (CO2) absorption capacity among the other lithium-based ceramic materials and is also suitable for high-temperature application for CO2 absorption. In this work, lithium orthosilicate pebbles of 1.0 mm size with 20% excess lithium carbonate were synthesized and fabricated by using Solid State Reaction Process (SSRP) and experiments were carried out to study the effect of various process parameters on its adsorption capacity of CO2 from air-CO2 mixture at elevated temperatures. The various process parameters studied were: operating gas flow rate, bed wall temperature and CO2 concentration in air on which very limited works have been reported. Effects of operating temperature, flow rate and inlet concentration of CO2 on absorption capacity have been studied to develop break through curves. The cyclic tests have been carried out with 15% CO2 concentration in air at 400°C and 200 LPH flowrate using the same material and carried out 135 consecutive absorption-desorption experimental runs. It was found that the CO2-absorption capacity of Li4SiO4 pebbles at 400°C and 200 LPH flowrates is about 31% for 30% carbon dioxide concentration in air and 22% for 15% carbon dioxide concentration in air. Moreover, no reduction in CO2-absorption capacity of the material was observed even after 135 consecutive absorption-desorption cycles.

High efficiency CO2 sorption property of Li4SiO4 sorbent pebbles via a simple rolling ball method

At present, the Li4SiO4 sorbents have been getting widespread attention for mitigating the release of CO2 gas owing to its stable cyclic property, higher capturing ability, and lower regeneration temperatures. In this work, a facile rolling ball method is first proposed to fabricate Li4SiO4 sorbent pebbles, which overcomes the low production efficiency, poor adsorption stability, and high production cost adopted in the previous literature. In the rolling ball process, the spray granulation process was involved to decrease the specific surface area of ceramic powder, which makes it easier for powder self-agglomerate to form pebbles via the centrifugal force with the assistance of PVA polymer solution. Moreover, the phase composition, the specific surface area, the shaping principle, and the microstructure of Li4SiO4 sorbent pebbles were studied, respectively. The result shows that the Li4SiO4 pebbles sintered at 900 °C for 2 h present a superior comprehensive performance with a homogeneous diameter of 1.2 mm, good sphericity of 0.95, high crushing load of 65 N, and suitable porosity of 6.28 %. Compared with Li4SiO4 powder, the spherical pebbles performed superior CO2 adsorption ability (0.304 g CO2/g sorbent) and excellent cyclic stability in a 100% CO2 environment owing to its large number of uniform pores channels produced by binder solution at high temperature. Therefore, the rolling ball process is a potential method for the batch production of Li4SiO4 pebbles to capture CO2 stably in the future.

Thermal properties of paramagnetic radiation-induced defects in lithium orthosilicate containing breeder material

Lithium orthosilicate (Li4SiO4) containing ceramics are currently being developed as potential solid-state candidate materials for tritium breeding in future thermonuclear fusion reactors. Under the expected operational conditions, the tritium breeding material will be exposed to the simultaneous influence of several strong energetic factors, for example, radiation, temperature, magnetic field, etc. In the present work, thermal properties of the formed and accumulated paramagnetic radiation-induced defects (containing unpaired electrons) in the Li4SiO4 pebbles with a 2.5 wt.% surplus of silicon dioxide (SiO2) were investigated after irradiation with photons of different types and energies: X-rays with an energy up to 45 keV, gamma rays with an average energy of 1.25 MeV, and bremsstrahlung with an energy up to 6 MeV. The photon-irradiated pebbles were analysed using two complementary spectroscopic methods: electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL). Individual signals contributing to the acquired EPR spectra, TSL glow curves and spectra were distinguished, deconvoluted and simulated in order to obtain a more detailed understanding about the local structure, electron configuration, and thermal stability of the accumulated radiation-induced defects. The simulation and deconvolution data were also compared with the results, which have been acquired for the long-term neutron-irradiated pebbles from the HICU experiment (High neutron fluence Irradiation of pebble staCks for fUsion).

Lithium orthosilicate as nuclear fusion breeder material: Optimization of the drip casting production technology

The production of nuclear fusion energy, in the future DEMOnstration power plant (DEMO), is based on tritium self-sufficiency, achievable exploiting neutron-lithium reaction occurring in the breeding blanket modules. To fulfil the plant design conditions ceramic materials containing lithium are suggested. A lot of research efforts focused on the past years on the investigation and selection of the most suitable breeding material.In this study, the lithium orthosilicate, in pebble form, produced at room temperature at the University of Pisa was investigated and characterized from a thermo-mechanical point of view. The pebbles were produced by a drip casting forming technique, starting from an aqueous suspension of a Li4SiO4 precursor obtained by hydrolytic sol–gel method, and dropping it into a calcium chloride/lithium acetate solution. A high temperature thermal treatment was applied to green pebbles to synthesize Li4SiO4 and densify the structure. Thermal analysis and static uniaxial compression tests, without radial constraints, were carried out on different types of pebbles produced varying the composition of the precursor suspension by means of cellulose fiber additions. Results show the influence of the cellulose content on morphological and physical properties of the obtained pebbles. Li4SiO4 pebbles with a density of 88 % and a crush load of about 30 N were obtained when the suspension was modified by adding 15 wt% of cellulose fibers to the sol–gel precursor powders.

Li4SiO4 breeder pebbles fabrication by a sol-gel supported drip casting method

Lithium orthosilicate (Li4SiO4) is widely considered a potential candidate as tritium breeding material for blankets in thermonuclear fusion reactors. In this study, a low-temperature methodology based on the drip casting forming technique was proposed for Li4SiO4 pebbles fabrication. The hydrolytic sol-gel method (starting from tetraethyl orthosilicate and lithium hydroxide) was applied to synthesize Li4SiO4 precursor powders, which were used to prepare an aqueous suspension for dripping in calcium chloride/lithium acetate solution. Spherical Li4SiO4 pebbles were obtained after sintering (900-950°C) and exhibited regular roundness, high phase purity, and no macroscopic defects. A high density (> 70 % of the theoretical Li4SiO4 density) and a good crush load (35 N) were achieved when cellulose fibers were added to the precursor suspension and the sintering step was conducted up to 950°C. Results indicate that the produced Li4SiO4 pebbles can be suitable for the use in breeding blankets in future fusion reactors.

Investigation on effects of tritium release behavior in Li4SiO4 pebbles

For deuterium–tritium fusion reactor, the fuel of tritium is not available naturally. Tritium should be produced by the reaction of Li (n, ɑ) T. Li4SiO4 is one of promising candidates due to its high lithium density. In present work, effects of tritium release behavior from Li4SiO4 have been investigated and compared. The main release peak at lower temperature was attributed to tritium located on/near the grain surface and chemical adsorption on pore surface. The small peak at higher temperature was designated as tritium diffusion from grain including lattice and boundary. The tritium release behavior was simulated by diffusion model and surface reaction model. Tritium behavior is controlled by both diffusion and surface reaction for different stages. Based on the desorption theory, the kinetic parameters were obtained. The effects on tritium release behavior were studied. The porosity has significant effects on tritium release. Higher porosity has large specific surface providing more activation sites and fast channels for tritium release which results in lower temperature of tritium release. The main tritium release form was considered as tritium water. The release peak of deuterium moved towards lower temperature after adsorbing water vapor for 1800 h. It was demonstrated that adsorbing water was beneficial for tritium release due to isotope exchange. Compared the effects of porosity and water vapor, higher porosity has larger effects on tritium release in the present work. Based on the work, the tritium release is controlled by both diffusion and surface reaction at different stage and affected by comprehensive effects including porosity and adsorption water.

Preliminary investigation of Li4SiO4 pebbles structural performance

One of the main purposes of a breeder blanket is to remove the heat produced in the blanket by the fusion reaction neutrons, and to breed the tritium required to sustain it. To achieve these requirements, several breeder materials (solid or liquid lithium-bearing ones) have been investigated in the past decades.To date it has not yet been possible to identify a stable material, with high thermal conductivity and melting point.This paper deals with the mechanical characterization of the lithium orthosilicate (Li4SiO4) in form of pebbles, produced at the University of Pisa at room temperature by a drip casting forming technique, starting from an aqueous suspension of Li4SiO4 precursor prepared by a sol-gel synthesis method.To investigate also numerically, by means of FE code, the breeder blanket behaviour, it is of meaningful importance the mechanical characterization of such pebbles.To the purpose, either static or cyclic uniaxial compression tests, without radial constraints, have been performed on several produced pebbles of about 1.5 mm diameter in order to determine the collapse and crushing loads and the stiffness. Moreover, the carried-out post-test SEM examination allowed to evaluate the failure mode and the crack shapes on the contact surface.Results show the influence of the elastic properties and matrix flaw population on the crushing load.The pebbles produced by the sol-gel method showed also a high strength, the value of which is comparable to that of the pebbles obtained by melting process.

Preparation of coated Li2TiO3 and Li4SiO4 pebbles by fluidized bed chemical vapor deposition for advanced tritium breeders

• Coated breeders were proposed to address safety hazard. • Protective coatings were successfully fabricated by FBCVD for the first time. • Deposition and interface reaction behaviors were revealed. Lithium-based ceramic tritium breeders would attack the blanket structural materials and produce brittle corrosion layers, causing potential safety hazard. Herein, coated breeders were proposed as advanced tritium breeders to address the issue. Carbon (C), titanium nitride (TiN) and titanium carbide (TiC) were investigated as inert protective shells because they show excellent stability and chemical compatibility with Li 2 TiO 3 and Li 4 SiO 4 . Moreover, for the first time, the C and TiN coated Li 2 TiO 3 and Li 4 SiO 4 pebbles were successfully fabricated by fluidized bed chemical vapor deposition (FBCVD) process, which would significantly decrease the brittle corrosion layers and increase the safety. The deposition behavior and interface reactions were systemically investigated.

Feasibility of Li4SiO4 pebbles by drip casting and preliminary characterization

Abstract Open issues for fusion reactor technology are related to the breeder material to be used for the nuclear fusion reaction. Despite in the last decades several studies were carried out, the selection, characterization and reliability of breeder material are still open issues. In this study, the attention is focused on the production of lithium orthosilicate (Li4SiO4) by means of drip casting method. The feasibility of this method was verified using an experimental rig designed and constructed purposely at the Pisa University, Department of Civil and Industrial Engineering (DICI). The green pebbles were produced starting from a suspension of Li4SiO4 precursors (i.e. LiOH and Si(OCH2CH3)4) prepared by sol-gel method. These pebbles were dried at 40 °C for 4 h, and then sintered in air at 900 °C for 10 h. The morphological and physical properties of the obtained pebbles were analyzed by scanning electron microscope (SEM), X-ray and compression tests. The X-ray results showed the main characteristic peaks of Li4SiO4 crystalline phase and small peaks attributable to calcium metasilicate, while the SEM analysis showed a homogeneous and uniform structure of the pebbles. The obtained preliminary results showed the feasibility of dripping at room temperature and provided further useful information for the optimization of the process.

Preparation of Li4SiO4 pebbles with increased lithium content and crush load via molten salt assisted sintering

Improving the lithium density and performance of lithium-based ceramics is the main direction for the development of advanced tritium breeder. In this work, the previously used polymers or organics were replaced by eutectic LiNO3–LiCl in wet process for preparing spherical green body, and then Li4SiO4 pebbles were fabricated through molten salt assisted sintering. The eutectic salt used herein was eco-friendly and had little effect on the Li4SiO4 pebbles. Instead, the incorporation of eutectic salt improved the lithium density and sinterability of Li4SiO4. The prepared Li4SiO4 pebbles exhibited well-grained microstructure, and the grain size increased with the increasing contents of eutectic salt. Compressive tests showed that the crush load increased more than five times when 20 wt% eutectic salt was added. The proposed method may have great prospects in the preparation of tritium breeding materials.

Tritium release property of Li2TiO3-Li4SiO4 biphasic ceramics

The tritium release experiments of Li4SiO4 pebbles, Li2TiO3 pebbles and Li4SiO4-Li2TiO3 biphasic pebbles were conducted to investigated the tritium release behavior of Li4SiO4-Li2TiO3 biphasic pebbles. The results indicate that the biphasic ceramic pebbles with uniform structure and small grain size exhibit good tritium release performance. The tritium release temperature is around 450 °C, and most of tritium is released in the form of tritiated water. The tritium release behavior of Li4SiO4-Li2TiO3 biphasic pebbles is controlled by the release of both Li2TiO3 and Li4SiO4, and the interfaces between Li2TiO3 and Li4SiO4 can promote the tritium release. Besides, the surface reaction is the key process affected the tritium release of the Li4SiO4-Li2TiO3 biphasic pebbles, because the release behavior is significantly affected by the composition of purge gas. Under 0.1% H2+He purge gas, with the combined effect of the two-phase interface and the isotope exchange reaction, the tritium release properties of the biphasic pebbles are promoted.

Mechanical strength stability of tritium breeding materials under high temperature deuterium conditions

The mechanical strength of solid tritium breeding materials is one of the key factors for D-T fusion reactors. The crushing of ceramic breeder pebbles will block both tritium recovery circuit and thermal transmission. The mechanical strength changes of Li4SiO4 and Li2TiO3 pebbles exposed to high temperature and hydrogen isotope atmosphere were investigated. The crush load of Li4SiO4 decreased to 66 % of the original values when the breeders were exposed in deuterium gas at the temperature higher than 750 oC. Nevertheless, the crush load of Li2TiO3 remained stable at all the designated temperatures. Investigations involving phase analysis, defect analysis, grain size analysis, and lithium content analysis were performed to determine the causes of mechanical strength change. No new phase formation and no obvious lithium decrease were demonstrated by X-ray diffraction (XRD) and Inductively Coupled Plasma - Atomic Emission Spectrometer (ICP-AES). Defects arising from reduction reaction didn't play an important role in mechanical strength reduction. The main reason responsible for the decrease of mechanical strength of Li4SiO4 was attributed to grain growth. The grain growth behavior was understood by grain growth kinetics. The kinetic parameters of grain boundary migration for Li4SiO4 pebbles were obtained as n of 4 and activation energy Q of 140 kJ/mol. The mechanism of grain growth was given to demonstrate the mechanical strength change of ceramic breeder pebbles.