Elsevier

Journal of Nuclear Materials

Volume 456, January 2015, Pages 151-161
Journal of Nuclear Materials

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

https://doi.org/10.1016/j.jnucmat.2014.09.028Get rights and content

Highlights

  • Investigation of phase stability in the quasi-binary Li2TiO3–Li4SiO4 system.

  • Sol-based syntheses of mixed phase materials from organometallic precursors.

  • LiCl based synthesis results in greater lithium deficiency than LiOH synthesis.

  • The Li2TiO3–Li4SiO4 quasi binary system appears to exhibit monotectic behaviour.

  • Mixed phase materials show liquid formation from melting of silicate material at 1100 °C.

Abstract

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.

Introduction

A key component of the ITER thermonuclear demonstrator reactor is the helium cooled pebble bed (HCPB) breeding blanket that utilises Li bearing ceramic pebbles for tritium breeding purposes [1], [2], [3]. Numerous candidate oxide compounds have been studied towards this goal. Material requirements include adequate thermo-mechanical stability, low activation levels, and sufficiently high lithium density. In recent years lithium metatitanate (Li2TiO3) and lithium orthosilicate (Li4SiO4), in some cases with excess Li [4], [5], [6], [7], have emerged as leading HCPB candidate materials, as these compounds are found to exhibit low activation relative to earlier studied compositions of Li2ZrO3 and LiAlO2 while maintaining favourable thermo-mechanical properties [1], [8], [9], [10]. The properties exhibited by candidate pebble bed materials are the subject of intensive investigation and review as these are critical to the viability and performance of the numerous solid breeder conceptual designs [11]. Consequently, increased versatility in the attainable properties of pebble material can be expected to yield meaningful benefits for breeder blanket design.

Research of breeder pebbles produced using melt-based processing methods involving the addition of TiO2 to Li4SiO4 melt has shown that materials comprised of biphasic mixtures of Li4SiO4 and Li2TiO3 exhibit improved mechanical properties relative to single phase materials, evident by increased crush stress [12], [13]. However, the stoichiometric and thermal limitations imposed by melt-processing restrict the range of compositions and microstructures attainable in the Li rich region of the ternary Li2O–SiO2–TiO2 system. For this reason the wet-chemical processes utilising precursor compounds in the form of salts and/or organometallic compounds to yield homogenous biphasic or triphasic oxide materials presents an attractive approach to breeder materials syntheses.

A number of recent studies have investigated methods for the fabrication of single phase Li2TiO3 and Li4SiO4 pebbles from powder-form precursor materials in dry and wet processes utilising binders and different spheroidisation techniques [14], [15], [16], [17], [18], [19], [20]. Such methods readily allow the fabrication of pebbles with well controlled geometry, density and consistency and reprocessing by melting and by wet chemical methods has further been the subject of various research efforts [21], [22]. In parallel, multiple solution based syntheses for lithium metatitanate and orthosilicate have been reported to date [23], [24], [25], [26], [27], [28]. Such processes generally utilise organometallic compounds and/or salts as Si, Ti and Li precursors for the synthesis of oxides of various stoichiometries. Relative to solid state processing, such syntheses have the advantage of better mixing and consequently shorter diffusion distances, imparting greater flexibility with respect to thermal processing, potentially allowing lower temperature synthesis of the desired oxide compounds, attainment of finer crystallite sizes and compositional versatility.

Li2TiO3 is a congruently melting phase exhibiting high temperature polymorphism while Li4SiO4 exists in a monoclinic structure up to temperatures of 1258 °C at which this phase melts congruently, although earlier studies reported a peritectic decomposition at 1255 °C [29], [30], [31], [32]. The metasilicate phase is also studied for potential applications in breeder pebbles and melts congruently at 1209 °C [30], [33]. As both endpoints are congruently melting phases, the Li2TiO3–Li4SiO4 composition range is considered to be a quasi-binary system. A summary of phase equilibria reported in the literature for the Li2O–TiO2–SiO2 (LTS) ternary system is shown in Fig. 1 [30], [31], [32], [34], [35], [36], [37]. In this figure the liquidus temperatures are taken from the most recent available references. Compositions 1–11 studied in the present work are further indicated in the diagram. Understanding of stability and equilibria in this system remains to date incomplete in the Li-rich ternary regime as tie-lines, liquidus surfaces and potential eutectic/eutectoid compositions have not been determined comprehensively in this region of the system. Reported results suggest a likely compatibility tie line between Li4SiO4 and Li2TiO3 as these two phases are found to coexist in melt-based products [12]. A further compatibility tie line or potentially mutual solubility is likely to exist between the ortho-phases (66.67 mol% Li2O) in the system owing to these compounds exhibiting similar cation-oxygen bond lengths and the isostructural relationship between high temperature β-Li4TiO4 (stable above 686 °C) and Li4SiO4 [31], [38], [39], [40].

Consequently, in an effort to investigate the synthesis of breeder materials of new compositions, we seek to explore the wet-chemical synthesis of compounds in what is likely to be a binary Li2TiO3–Li4SiO4 system.

Section snippets

Materials and methods

All sols fabricated in the presently reported work were ethanol based. Lithium hydroxide monohydrate and lithium chloride (both Alfa Aesar, 99%) were examined for use as Li precursor compounds following similar methods for solution processing of metal oxides [23], [27]. Prior to use, LiCl and LiOH·H2O were vacuum dried at 180 °C yielding anhydrous products as confirmed by mass balance. Organometallic Ti and Si precursor compounds used were respectively titanium tetra-isopropoxide (TTIP, 97%,

Experimental

The phase composition of fired pelletised samples described above was determined by powder X-ray diffraction, XRD, of crushed samples that was carried out over the range 15–70° 2θ using a Bruker D5000 diffractometer with Cu Kα emission in ambient air. In order to assess phenomena of crystallisation and phase stability in the synthesised materials fabricated using LiOH precursor, in situ Temperature varied XRD of dried powders was employed using a Bruker D8 Advance diffractometer equipped with

XRD

XRD spectra from samples fired at 900 °C fabricated from chloride and hydroxide precursors for three compositions corresponding to lithium metatitanate (LT), the intermediate composition (L3TS) and lithium orthosilicate (L2S) are shown in Fig. 2. It is evident that samples synthesised with the LiCl precursors showed a significant lithium deficiency relative to the target composition. This most likely is the result of the hygroscopic nature of the dried product and the consequent segregation of

Synthesis route and Li deficiency

The use of chloride precursors is frequently reported in sol–gel processing of metal oxides and allows better pH control in sols relative to hydroxide precursors. In the present work the use of this compound was found to be unfavourable owing to a resulting stoichiometric imbalance. Additionally the use of chloride is problematic for HCPB materials as residual chloride is undesirable in breeder blankets [73].

The greater extent of Li loss in the chloride based materials may result from the

Conclusions

Biphasic mixtures in the quasi-binary Li2TiO3–Li4SiO4 system offer the scope for improved properties, relative to single phase materials, for tritium breeding applications in fusion reactors. For this reason the solution based synthesis of materials in this system was studied using organometallic precursors with a focus on crystallisation and phase stability behaviour. Owing to deviation from target stoichiometries, the presently reported procedures did not result in purely biphasic Li4SiO4–Li2

Acknowledgements

This work was supported by the Group of Eight – DAAD Joint Research Cooperation Scheme.

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