Lithium Metatitanate Research Articles

Characterization of the thermal conductivity for ceramic pebble beds

The evaluation of the thermal conductivity of breeder materials is one of the main goals to find the best candidate material for the fusion reactor technology.The aim of this paper is to evaluate experimentally the thermal conductivity of a ceramic material by applying the hot wire method at different temperatures, ranging from 50 to about 800°C.The updated experimental facility, available at the Department of Civil and Industrial Engineering (DICI) of the University of Pisa, used to determine the thermal conductivity of a ceramic material (alumina), will be described along with the measurement acquisition system. Moreover it will be also provided an overview of the current state of art of the ceramic pebble bed breeder thermos-mechanics R&D (e.g. Lithium Orthosilicate (Li4SiO4) and Lithium Metatitanate (Li2TiO3)) focusing on the up-to-date analysis.The methodological approach adopted is articulated in two phase: the first one aimed at the experimental evaluation of thermal conductivity of a ceramic material by means of hot wire method, to be subsequently used in the second phase that is based on the test rig method, through which is measured the thermal conductivity of pebble bed material. In this framework, the experimental procedure and the measured results obtained varying the temperature, are presented and discussed.

ChemInform Abstract: Recent Developments of Solid Breeder Fabrication

AbstractReview: 25 refs.

Hydrothermal Synthesis of Lithium Meta Titanate Nanocrystallites

Lithium meta titanate (Li2TiO3) is one of the most promising tritium breeding candidate materials. In this study, the nanocrystallites lithium-titanate with hexagonal and cubic crystal structures were synthesized at low temperature, 200°C for 12h by the hydrothermal method. The results showed that the monoclinic phase of Li2TiO3 nanostructure with high purity can be synthesized by further heat treatment of the hydrothermal synthesis powder above 700°C. In despite the heat treatment, electron microscope and X-ray diffraction analysis showed that the synthesized compounds had grain size smaller than 120nm. Li2TiO3 was also prepared by a solid-state reaction in order to study the influence of the synthesis rout on the morphology and particle size.

Solution based synthesis of mixed-phase materials in the Li2TiO3–Li4SiO4 system

As candidate tritium breeder materials for use in the ITER helium cooled pebble bed, ceramic multiphasic compounds lying in the region of the quasi-binary lithium metatitanate–lithium orthosilicate system may exhibit mechanical and physical advantages relative to single phase materials. Here we present an organometallic solution-based synthesis procedure for the low-temperature fabrication of compounds in the Li2TiO3–Li4SiO4 region and investigate phase stability and transformations through temperature varied X-ray diffraction and scanning calorimetry. Results demonstrate that the metatitanate and metasilicate phases Li2TiO3 and Li2SiO3 readily crystallise in nanocrystalline form at temperatures below 180°C. Lithium deficiency in the region of 5% results from Li sublimation from Li4SiO4 and/or from excess Li incorporation in the metatitanate phase and brings about a stoichiometry shift, with product compounds exhibiting mixed lithium orthosilicate/metasilicate content towards the Si rich region and predominantly Li2TiO3 content towards the Ti rich region. Above 1150°C the transformation of monoclinic to cubic γ-Li2TiO3 disordered solid-solution occurs while the melting of silicate phases indicates a likely monotectic type system with a solidus line in the region 1050–1100°C. Synthesis procedures involving a lithium chloride precursor are not likely to be a viable option for breeder pebble synthesis as this route was found to yield materials with a more significant Li-deficiency exhibiting the crystallisation of the Li2TiSiO5 phase at intermediate compositions.

Effects of helium and ambient water vapor on tritium release from Li2TiO3

The effects of hydrogen isotopes in tritium recovery gas and the presence of helium on the tritium release behaviors were investigated for lithium meta-titanate (Li2TiO3) to develop a tritium migration model. The tritium trapping sites as oxygen vacancies and oxygen atoms with dangling bonds were formed by energetic tritium ion implantation. Isotope exchange processes with water vapor in purge gas enhanced the recovery of tritium adsorbed on the surface of Li2TiO3 and trapped in oxygen vacancies. Replacement of tritium as hydroxyl groups with hydrogen isotopes from purge gas was hardly occurred due to the higher trapping energy of tritium as hydroxyl groups. Helium implantation would induce the formation of helium bubbles, which would occupy irradiation damages, contributing to the decrease in the densities of trapping site for tritium. Consequently, the addition of hydrogen isotopes in the tritium recovery gas and the presence of helium in tritium breeding materials can contribute to the efficient tritium recovery for tritium breeding materials.

Synthesis of Li2TiO3 by sol–gel combustion method and its gel-casting formation

Lithium metatitanate (Li2TiO3) has been recognized as a candidate of ceramic tritium breeder materials for its excellent performance. In this paper, Li2TiO3 powder was synthetized by a modified sol–gel combustion process, and the ratio of fuel to oxidizer was optimized. DTA–TG analysis indicated that an intense exothermic reaction occurred at 539 K, which led to the formation of a voluminous white powder. No additional annealing process was required, as pure crystalline β-Li2TiO3 was directly obtained from the combustion reaction. The powder had nano-scale structure and large specific surface area of 9.6m2/g. Li2TiO3 pebbles with desired size and shape were prepared by gel-casting technique using the as-prepared powder. The pebbles reached a high density (81–86% T.D) after sintered at a relatively low temperature (⩽1373K) for 2h. The sintering pebbles had a smooth surface, homogeneous microstructure and small grain size.

Fabrication of Li2TiO3 pebbles using PVA–boric acid reaction for solid breeding materials

Lithium metatitanate (Li2TiO3) is a candidate breeding material of the Helium Cooled Ceramic Reflector (HCCR) Test Blanket Module (TBM). The breeding material is used in pebble-bed form to reduce the uncertainty of the interface thermal conductance. In this study, Li2TiO3 pebbles were successfully fabricated by the slurry droplet wetting method using the cross-linking reaction between polyvinyl alcohol (PVA) and boric acid. The effects of fabrication parameters on the shaping of Li2TiO3 green body were investigated. In addition, the basic characteristics of the sintered pebble were also evaluated. The shape of Li2TiO3 green bodies was affected by slurry viscosity, PVA content and boric acid content. The grain size and average crush load of sintered Li2TiO3 pebble were controlled by the sintering time. The boron was completely removed during the final sintering process.

Preparation and Characterization of the Lithium Metatitanate Ceramics by Solution-Combustion Method for Indian LLCB TBM

In fusion DEMO reactors, the blanket requires lithium-containing ceramics as the tritium breeder material. Lithium metatitanate (Li2TiO3) is being considered as a promising tritium breeding material for thermonuclear fusion reactors because of its reasonable lithium atom density, prominent tritium release rate at low temperatures, low activation characteristics, low thermal expansion coefficient, high thermal conductivity, etc. Li2TiO3 will be used in the Indian Lead-Lithium–Cooled Ceramic Breeder concept to be tested in ITER. Li2TiO3 powder has been synthesized by the solution-combustion technique using a less expensive precursor of titanium, i.e., titanium dioxide (TiO2), at Institute for Plasma Research. Titanium oxynitrate [TiO(NO3)2] and lithium carbonate (Li2CO3) with citric acid fuel are used as the raw materials. The combustion reaction was carried out at citrate-to-metal ratios of 0.8 to 1.5, as well as for various pH values ranging from 1 to 5. Citric acid was used as a fuel material for the reaction. Calcination of the powder was carried out at 600°C. The powders were characterized for phase purity, grain size, and surface area using X-ray diffraction, scanning electron microscopy, and a Brunauer-Emmett-Teller surface area analyzer. Finally, Li2TiO3 pebbles were prepared by extrusion followed by spheronization with a diameter range from 1 to 1.5 mm. The details of the powder systemization, pebble formation, and their various characterizations are discussed in this paper.

Measurement of Thermal Diffusivity of Li2TiO3 Pellets by the Laser Flash Method and Comparison with Finite Element Simulation

Lithium metatitanate (Li2TiO3) is one of the candidate tritium-breeder materials for the ITER test blanket module (TBM) and the DEMO blanket. It will be used as the tritium-breeder material for the Indian lead-lithium–cooled ceramic breeder concept of the TBM to be tested in ITER. Thermal conductivity is one of the most important parameters in the design of TBMs using ceramic materials. The design of breeder blankets is strongly affected by the low values of the thermal conductivity and density of ceramic breeder pebble beds. A significant increase in both quantities would enhance thermal performance and lead to an increased tritium-breeding ratio. Thermal transport properties of Li2TiO3 were measured using the laser flash method, which determines the thermal diffusivity from the transient temperature rise measured at one side of a pellet with laser pulse heating at the other side. The thermal conductivity of Li2TiO3 pellets was then estimated from the experimentally obtained thermal diffusivity values. Finite element analysis using ANSYS software was performed to simulate the transient thermal measurements, and the results were compared with those obtained by the laser flash method. The experimental and simulation results were found to be in good agreement. The experimental details and ANSYS simulation are presented in this paper.

Thermo-mechanical test rig for experimental evaluation of thermal conductivity of ceramic pebble beds

The experimental determination of mechanical and thermal properties of ceramic pebble beds, such as the lithium orthosilicate or lithium metatitanate, is a key issue in the framework of fusion power technology, for the reason that they are possible candidates in the design of breeder blankets.The paper deals with an experimental method for the evaluation of the thermal conductivity of ceramic pebble beds versus the temperature and compressive strain, based on a steady state heat flux through a material (alumina) of known conductivity. The alumina thermal conductivity is determined by means of the hot wire method. To assess the experimental method, a thermo-mechanical characterization of alumina pebble beds (a material largely available), having different diameters, considering a wide range of temperatures and compression forces has been carried out.Moreover preliminary tests have been performed on lithium orthosilicate and lithium metatitanate pebble beds.

Preliminary results on the relationship between luminescence and crystalline structure of lithium metatitanate

Lithium metatitanate is a luminescent ceramic material of promising properties for numerous applications (breeder blanket in fusion reactors, ionic conductors). In some cases, the material will be submitted to aggressive environments (radiation, chemicals, high temperatures), which could induce changes in the crystalline structure. Small variations (presence of inclusions, impurities, or surface) in the lattice of crystalline solids are known to change the light emission behaviour, i.e. intensity and wavelength position of photoluminescence. Li2TiO3 ceramic samples were sintered in the range of 1050 and 1350°C to modify the high temperature crystalline structure, and then were characterized by means of their cathodoluminescence (CL) and thermoluminescence (TL) properties as a function of total radiation dose and time of storage.

Synthesis of Na‐Doped Lithium Metatitanate and Its Absorption for Carbon Dioxide

Na‐doped lithium metatitanate (Na‐doped Li2TiO3) absorbent was doped with Na2CO3 and lithium metatitanate (Li2TiO3) was prepared by a solid‐state reaction method from mixture of TiO2 and Li2CO3. The Na‐doped lithium metatitanate was characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM) and surface area. Carbon dioxide absorption on Na‐doped lithium metatitanate was investigated using TG‐DTA. The results reveal an increase of the CO2 absorption capacity of the Na‐doped materials with respect to pure Li2TiO3. XRD patterns of the doped samples suggest a limited substitution of Li by Na atoms within the Li2TiO3 structure. The results of experimental and modeling work were summarized to better understand the relationship between the sorbent microstructure and carbon dioxide absorption kinetics.

Enthalpy measurement of lithium meta-titanate by drop calorimetry and its derived heat capacity

Enthalpy of Li2TiO3, which was synthesized by a neutralizing method and its Li/Ti ratio was determined to be Li/Ti ratio (mol/mol)=1.97, was measured by a drop calorimeter, and its heat capacity was derived as a function of temperature. XRD (X-ray diffraction) analysis of the sample before and after the enthalpy measurement indicated no phase change during the measurement and a single phase of Li2TiO3 was observed. The enthalpy data were expressed as H(T)−H(323.17) (J/g)=2.2×10−5·T2+1.4·T+2.7×104/T−5.6×102 (373–1273K), where T is temperature in K. The heat capacity was calculated as Cp (J/gK)=2.2×2×10−5·T+1.4–2.7×104/T2 by differentiating the equation by temperature. These equations have accuracy of 3%.

Thermal transient and the temperature profile in a HELICA mock-up simulated by a new finite element homogenous model

This paper deals with a numerical approach for simulating the thermal and mechanical behaviour of pebble beds used as breeder and neutron multiplier in breeding blanket of nuclear fusion reactor. The model of the pebble beds is based on the results of a theoretical and experimental research activity performed by the Authors on ceramic pebble beds (lithium ortosilicate and lithium metatitanate). The results of this activity permitted to determine the effective thermal conductivity of the beds, versus the temperature and the axial pressure and to implement a homogenous model of pebble bed in a FEM code.This paper illustrates an application of the implemented model, considering pebble beds under several cycles of heating and cooling. The examined geometry corresponds to the HELICA mock-up tested by ENEA in the research centre Brasimone. The experimental tests performed on HELICA have been used as a benchmark problem in order to assess the different approaches for simulating pebble beds. In this paper, the simulations performed with two-dimensional models are illustrated. Moreover the numerical results are compared with the experimental ones. Finally, a discussion on results obtained by other authors involved in the benchmark is reported.

Vaporization property and crystal structure of lithium metatitanate with excess Li

The vaporization property and the crystal structure of lithium metatitanate with excess Li, which has been developed as an advanced tritium breeder, were studied. After synthesizing the lithium metatitanate specimens with Li/Ti=2.0–2.3 (at mixing of the starting materials), the vaporization properties were investigated by measuring the mass losses during heating at 1173K. The Li excessive specimens indicated higher rates of mass loss than the stoichiometric one during heating as long as excessive lithium atom exists. The crystal structures of stoichiometric and nonstoichiometric lithium metatitanates were discussed with the result of neutron diffraction and the refined structural parameters by Rietveld analysis. The refinement (Rwp=7.98, Rp=5.96 and Re=2.58) suggested that lithium metatitanate with excess Li has β-Li2TiO3 structure and excess Li atoms exist at unstable sites with the formation of reduced titanium or site vacancies.

Recent developments of solid breeder fabrication

Worldwide research and development has been performed on solid breeder materials as a tritium source for future fusion power reactors. These ceramic materials will be used in the International Thermonuclear Experimental Reactor (ITER) to check their performance and screen their applicability for use in fusion blankets. While all members of the ITER project are engaged in the development of solid breeder blanket concepts, five are actively involved in the fabrication of tritium breeding ceramics for the different test blanket modules to be tested in ITER.All solid breeder concepts proposed for ITER are based on the use of ceramics in the form of pebble beds with pebble diameters of about 1mm or smaller. Although alternative materials are still under investigation, the main candidates for tritium breeding ceramics are lithium metatitanate and lithium orthosilicate. Recent developments and preparatory work for ITER are presented.

Fabrication of modified lithium orthosilicate pebbles by addition of titania

Lithium orthosilicate pebbles are one of the ceramic tritium breeder materials destined for the European solid breeder test blanket modules of ITER, the large-scale scientific experiment intended to prove the viability of fusion as an energy source, presently under construction in Cadarache, France. While the current reference material is fabricated by melt-spraying with 2.5wt.% excess of silica, resulting in a two-phase material of lithium orthosilicate and metasilicate, a modified melt-based process was used to fabricate breeder pebbles with additions of titania in order to obtain pebbles with lithium metatitanate as a secondary phase. The fabricated two-phase pebbles exhibit a fine-grained microstructure and increased crush loads. The optimum titanate content has yet to be evaluated, nonetheless the pebbles may have the potential to combine the advantages of both lithium orthosilicate and metatitanate breeder ceramics.

Observations on microstructure and crystal structure of sintered lithium metatitanate with excess Li

Crystal grain growth and crystallization of lithium metatitanate with excess Li (Li2+xTiO3+y), which is expected as an advanced ceramic breeder for a future DEMO fusion reactor, were studied in this paper. By the observation of sintered pellets using scanning electron microscope, it was shown that the Li2.1TiO3+y specimens, which have small grains (1–2μm) and narrow size distribution, can be obtained by sintering in the temperature range from 1000 to 1100°C. The observation also showed the rapid grain growth in the Li2.1TiO3 specimens sintered above 1150°C. The changes in the crystal structure and the phase transformation with the increase of sintering temperature were also investigated by means of powder X-ray diffraction. Finally, the relationship between the grain growth and the phase state at high temperature was discussed.

Study of ceramic pebble beds in Post Irradiation Examination of the Pebble Bed Assemblies irradiation experiment

The irradiation experiment Pebble Bed Assemblies (PBA) consists of four mock-up representations (test elements) of the EU Helium Cooled Pebble Bed (HCPB) concept. The four test elements contain a ceramic breeder pebble bed sandwiched between two beryllium pebble beds and are regarded as one of the first DEMO representative HCPB blanket irradiation tests, with respect to temperatures and power densities. The design value of the PBA were to irradiate pebble beds at a power density of 20–26W/cc in the ceramic breeder, to a maximum temperature of 800°C.Two test elements contain lithium orthosilicate pebbles (Li4SiO4; FZK/KIT) and were irradiated with target temperatures of 600 and 800°C, respectively. The other test elements have lithium metatitanate (Li2TiO3; CEA) with different grain sizes and were both irradiated with a target temperature of 800°C. The PBA have been irradiated for 294 Full Power Days (12 cycles) in the High Flux Reactor (HFR) in Petten to a total neutron dose of 2–3dpa in Eurofer, and an estimated (total) lithium burnup of 2–3% in the ceramic pebbles.This work presents results of Post Irradiation Examinations (PIE) on the four HCPB test elements. Using e.g. SEM, the evolution of compressed pebble beds and pebble interactions like swelling, creep, sintering, etc., under irradiation and thermal loads are studied for the candidate pebble materials Li2TiO3 and Li4SiO4. (Chemical) interactions between ceramic pebbles and Eurofer (e.g. chrome diffusion) are observed. Looking at different sections of the pebble beds, correlations between temperatures and thermal–mechanical behaviour are clearly observed.

Atomistic Simulation of the Structural, Thermodynamic, and Elastic Properties of Li2TiO3

Lithium-based ceramics, such as lithium metatitanate, have been proposed for adoption in the breeder blanket region of a fusion reactor. In this article, we report a combination of empirical and density functional theory (DFT) simulations employing “on-the-fly” pseudopotentials for Li2TiO3. The smoothing parameters of the plane-wave pseudopotentials were optimized to ensure an appropriate level of precision for determination of structural, thermodynamic, and elastic properties. As the elastic properties of lithium metatitanate are not well-known, the efficacy of the DFT simulations employing the new pseudopotentials was explored using Li2O and TiO2 where experimental data are available. These pseudopotentials are then used to investigate the three intermediate temperature phases of Li2TiO3 (i.e., C2/c, C2/m, and P3112). Finally, we examine the elastic properties of Li2TiO3 using both DFT and an empirical potential model and find it to be, irrespective of space group, more resistant to deformation than other promising ceramic breeder materials.