Recent Progress in Research of Solid Tritium Breeder Materials Li₂TiO₃: A Review

Abstract

During the past decades, fusion reactor fuels such as deuterium and tritium have been extensively investigated due to increasing interest in nuclear fusion energy. Tritium, which is scarce in nature, needs to be fabricated by tritium breeder materials. Among the commonly investigated tritium breeder materials, lithium titanate (Li₂TiO₃) is recognized as one of the most promising solid tritium breeder materials because of its considerable lithium (Li) atomic density, low activation, excellent chemical stability, and low-temperature tritium release performance. This paper aims to provide a systematic review of the current progress in Li₂TiO₃ preparation methods as well as the high Li density, tritium release performance, irradiation behavior, and modification technologies of Li₂TiO₃ pebbles. Li₂TiO₃ can be synthesized by strategies such as solid-state, sol–gel, hydrothermal, solution combustion synthesis, and co-precipitation methods. Among them, the hydrothermal method is promising due to its simplicity and low cost. Many researchers have begun to focus on composite ceramic pebbles to further improve tritium breeder performance. This will provide a new direction for the future development of Li₂TiO₃ pebbles. The present review concludes with a summary of the preparation methods currently under development and offers an outlook of future opportunities, which will inspire more in-depth investigation and promote the practical application of Li₂TiO₃ in this field.

1. Introduction

With the rapid development of society, the consumption of non-renewable energy sources such as coal and fossil oil has led to an increasingly serious environmental crisis. Nuclear fusion energy using deuterium (D) and tritium (T) as fuels has become the preferred future energy source due to its safety, environmental friendliness, and abundant fuel resources [1]. Deuterium is abundant in seawater, while tritium is rare in nature. Therefore, tritium needs to be generated by the neutron irradiation of Li-containing tritium breeder blankets. Tritium breeder materials are considered prospective candidates on account of their high Li atomic density, low activation, good compatibility with structural materials, and excellent tritium release characteristics at low temperatures [2,3]. These materials include liquid tritium breeder and solid tritium breeder materials. Liquid breeder materials include pure Li, the eutectic Pb-17Li, low melting point ternary Li-Pb-X alloys, molten salts, and aqueous Li salt solutions [4]. Liquid breeder materials were first employed in breeder blankets due to their advantages of easy tritium extraction, resistance to irradiation damage, high thermal conductivity, and easily achieved manufacturing and specification requirements [5]. However, three major problems hinder the wider application of liquid breeder materials: (1) magnetohydrodynamic (MHD) effects increase fluid resistance and reduce liquid Li mobility [6]; (2) the corrosion candle of liquid Li on cladding structural materials [7]; (3) the inherent volatilization of liquid Li at operating temperatures leads to Li loss [8].

In this context, there has been a growing demand for new solid ceramic breeder materials since the late 1970s. Li-based ceramics have been proposed as promising tritium breeder materials for fusion reactor blankets [9]. Compared with liquid breeder materials, solid breeder materials show good promise for use in fusion power applications due to their lack of an MHD effect, high thermal stability, and chemical inertness [10]. These ceramics are usually fabricated into pebbles due to the ease of packing into equipment, the convenient replacement of ceramic materials, and excellent tritium diffusion in the grains [11].

A great deal of research has focused on the preparation–microstructure–property relationships of these processed pebbles [12]. Recent results have indicated that breeder ceramics with excellent tritium breeding performance should have the characteristics of high Li density, small grain size, and rich pore volume [13,14,15,16,17]. Generally, five types of tritium breeder pebbles have been proposed as promising materials: Li₂O, LiAlO₂, Li₂ZrO₃, Li₂TiO₃, and Li₄SiO₄ [11]. The main characteristics of commonly used solid breeder materials are listed in Table 1.

Li₂O is an attractive ceramic breeder material due to its high Li atom density, high thermal conductivity, and low activation. However, Li₂O also exhibits major drawbacks that limit its application, including poor thermal stability and the release characteristics of tritium from Li₂O structures [18]. LiAlO₂ is thermodynamically and chemically stable, but it has the lowest Li atom density among the solid breeder candidates (as shown in Table 1) [19]. Li₂ZrO₃ has good radiation resistance, a shorter tritium residence time, and lower tritium retention, but the preparation of a single monocline is difficult [20]. Li₄SiO₄ has high Li density, low activation performance, good thermophysical properties, and a high tritium release rate at low temperatures, but there are two Li₄SiO₄ phases and the phase transition is unstable [21]. Therefore, Li₂TiO₃ is considered to be one of the most promising solid tritium breeder materials due to its considerable Li atomic density, low activation, excellent chemical stability, and good low-temperature tritium release performance [22,23,24]. Furthermore, some biphasic materials such as Li₄SiO₄–Li₂TiO₃, Li₂ZrO₃–Li₂TiO₃, and Li₂ZrO₃–Li₄SiO₄ have also been successfully fabricated. In this article, an overview of the major progress in recent Li₂TiO₃ research is provided, with a focus on preparation methods and the corresponding tritium breeding characteristics. The structure of this review is shown in Figure 1. Normally, Li₂TiO₃ powders can be classified based on their preparation method: solid-state, sol–gel, hydrothermal, or solution combustion synthesis. As can be seen from the fabrication of Li₂TiO₃ powder, the crystal structure, morphology, stability, and other properties of Li₂TiO₃ are significantly affected by the synthesis method. Therefore, with the goal of preparing solid tritium breeder materials with good tritium release performance, exploring the preparation of Li₂TiO₃ powders with mass, low cost, and high performance is currently a hot research topic. At the same time, domestic and foreign researchers have also investigated the lithium density, tritium release properties, irradiation behavior, modification, and other aspects of Li₂TiO₃. Experimental physical, structural, and tritium performance data for Li₂TiO₃ lithium ceramic breeders have been accumulated to provide reliable performance data for the future selection of solid breeder materials.

Table 1. The comparison of characteristics of the solid tritium breeder candidates.

Ceramic Breeder	Li Density (g/cm3)	Crystalline Phase	Advantage	Disadvantage	Ref.
Li2O	0.91	Single phase	High-density Li High thermal conductivity Low activation	Reacting with water Radiation damage Li volatile	[24]
LiAlO2	0.28	Stable phase	Good chemical stability Excellent irradiation stability	Low-density Li Poor tritium release	[25]
Li2ZrO3	0.38 (Monoclinic) 0.33 (Tetragonal)	Stable blow 1100 °C Stable above 1100 °C	Low tritium release temperature	High activation	[26]
Li2TiO3	0.25(α) 0.43(β) 0.32(γ)	Instability Stable blow 1150 °C Stable above 1150 °C	Low tritium release temperature Water insensitive	~	[27]
Li4SiO4	0.55(α) 0.51(γ)	Stable blow 650 °C Stable above 650 °C	High-density Li Good chemical stability	Water absorption	[28]

Table 1. The comparison of characteristics of the solid tritium breeder candidates.

Ceramic Breeder	Li Density (g/cm3)	Crystalline Phase	Advantage	Disadvantage	Ref.
Li2O	0.91	Single phase	High-density Li High thermal conductivity Low activation	Reacting with water Radiation damage Li volatile	[24]
LiAlO2	0.28	Stable phase	Good chemical stability Excellent irradiation stability	Low-density Li Poor tritium release	[25]
Li2ZrO3	0.38 (Monoclinic) 0.33 (Tetragonal)	Stable blow 1100 °C Stable above 1100 °C	Low tritium release temperature	High activation	[26]
Li2TiO3	0.25(α) 0.43(β) 0.32(γ)	Instability Stable blow 1150 °C Stable above 1150 °C	Low tritium release temperature Water insensitive	~	[27]
Li4SiO4	0.55(α) 0.51(γ)	Stable blow 650 °C Stable above 650 °C	High-density Li Good chemical stability	Water absorption	[28]

2. Synthesis Methods of Li₂TiO₃ Powder

In general, Li₂TiO₃ ceramic pebbles should exhibit a spherical shape (0.5–1.0 mm diameter), a small grain size (less than 5 μm), a high relative density (>80% T.D. (theoretical density)), good sphericity, and high phase purity (>95%) [29]. The primary step for fabricating Li₂TiO₃ pebbles is the synthesis of Li₂TiO₃ powders. To date, several methods have been employed to prepare Li₂TiO₃ powders, including solid-state [30] and wet-chemistry methods. Wet-chemistry methods include sol–gel [31], hydrothermal [32], solution combustion synthesis [33], and co-precipitation methods.

2.1. Solid-State Methods

Solid-state methods are generally used for fabricating tritium breeder pebbles in mass production, in which the pebbles are prepared by the surface contact and diffusion of two or more solid phases with high input energy. These methods are based on the process sequence “mixing and milling, classification, sample preparation and synthesis/calcination, milling and recalcination, pebble fabrication, calcination and sintering” [34]. Solid-state methods are characterized by the chemical reaction between the solid raw materials, which consists of a two-step process: phase reaction and material migration. The initial temperature of the reaction is far lower than the melting point temperature of the reactants.

Li₂TiO₃ powders are mostly synthesized by sintering TiO₂ and Li₂CO₃ mixed powders at 700–1000 °C for 5–12 h [35,36,37]. The reaction during sintering is as follows:

$${\mathrm{Li}}_2{\mathrm{CO}}_3+{\mathrm{TiO}}_2={\mathrm{Li}}_2{\mathrm{TiO}}_3+{\mathrm{CO}}_2\uparrow$$

(1)

The particle size and relative density of these powders are both controlled by varying the reaction temperature and time. Ghosh et al. [34] fabricated Li₂TiO₃ pebbles by the “extrusion-spheroidization-sintering” of a Li₂TiO₃ powder based on Equation (1). Their results indicated that the pebble density increased with increasing sintering time. A total of 85%–90% T.D. was achieved, reaching the ideal density range for TBM materials in fusion reactors. Guo et al. [38] fabricated Li₂TiO₃ ceramic pebbles from a Li₂TiO₃ powder synthesized via a low-temperature solid-state precursor (LTSSP) method. The chemical reaction between solid H₂TiO₃ and LiOH·H₂O, shown in Equation (2), took place in a ball milling process. A nano-sized precursor powder was obtained. In order to obtain the pure Li₂TiO₃ powder, the precursor powder was calcined at 500 °C, which was half the temperature of traditional solid-state methods. The prepared pebbles sintered at 800 °C exhibited a high relative density (83%) and a good crush load (45 N) due to their uniform microstructure and small grain size (470 nm).

$${\mathrm{H}}_2{\mathrm{TiO}}_3+2\mathrm{LiOH}·{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Li}}_2{\mathrm{TiO}}_3+3{\mathrm{H}}_2\mathrm{O}$$

(2)

Recent developments in the fabrication of Li₂TiO₃ by the solid-state method are listed in

Table 2. Recent developments in the fabrication of Li₂TiO₃ by the solid-state method.

Organization	Pebble Size/mm	Density (% T.D.)	Grain Size	Crush Load (N)	Ref.
NFRI, Korea	1 mm	~	150 nm	7.5 N	[36]
SCU, China	~	85.1%	19.6 nm	42.8 N	[37]
PMD, India	~	98%	2–3 μm	~	[38]
France	0.8–1.2 mm	90%	1–2 μm	38–66 N	[39]
WUT, China	~	96.5%	2–3 μm	20–30 N	[40]

. Solid-state methods for the fabrication of Li₂TiO₃ ceramic powders have simple processes and can be easily applied at an industrial scale. However, the calcination temperatures required by these processes are high, and the resulting Li₂TiO₃ powders, therefore, have large particle sizes and uneven distribution.

2.2. Sol–Gel Methods

Sol–gel methods use the hydrolysis of metal alkoxides or inorganic salts to form a uniform sol of metal oxides or metal hydroxides. The solute is then polymerized into a transparent gel by evaporation and concentration, and finally, the gel is dried to remove organic components [41]. This method can be used to solidify compounds containing highly chemically active components by using the solution, sol, gel, and heat treatment steps to form oxides or other solid compounds. Sol–gel methods are characterized by the reaction in solution, which is easily carried out at low reaction temperatures [42,43,44].

A variety of Li sources, Ti sources, and chelating agents have been reported for synthesizing Li₂TiO₃ powders by sol–gel method. Li et al. [45] reported the fabrication of Li₂TiO₃ pebbles by a water-based sol–gel method utilizing Ti(C₄H₉O)₄ and LiNO₃ as the raw materials. The Li₂TiO₃ pebbles sintered at 1200 °C for 10 h achieved a density of ~68% T.D. The average diameter of these pebbles was about 1.4 mm. Li et al. also improved the density of their Li₂TiO₃ pebbles by choosing LiOH and CH₃COOLi as the Li source instead of LiNO₃ and Ti(C₄H₉O)₄) as the titanium source [46]. Their results demonstrated that Li₂TiO₃ pebbles with diameters of 1.18–1.3 mm and a density as high as 85% T.D. were obtained at a sintering temperature of 1100 °C for 4 h. Aggarwal et al. [47] investigated nano-sized Li₂TiO₃ by an internal gelation sol–gel process. TiOCl₂ and LiNO₃ were used as the Ti source and Li source, respectively, with a Li/Ti molar ratio of 2:1. They obtained Li₂TiO₃ pebbles with a diameter of 0.6–0.7 mm after sintering at 1100 °C for 2 h, and these pellets reached a density of ~90% T.D. M. Brykała et al. prepared spherical Li₂TiO₃ particles with diameters below 100 μm by a sol–gel process [48]. TiCl₄ in concentrated aqueous HCl and a sol emulsion in 2-ethylhexanol-1 containing SPAN-80 (EH) were mixed to extract water. LiOH was used as the Li source and the Li/Ti molar ratio was 2:1.

The synthesis of Li₂TiO₃ powder by the non-hydrolytic sol–gel (NHSG) method was first reported by Corriu in 1990 [49]. Zhang et al. [50] used this method to prepare Li₂TiO₃, and the sol was directly converted into a gel through the polycondensation of the reactants without undergoing the hydrolysis process. Thus, the uncontrollable hydrolysis process of the metal alcohol agent was avoided. The synthesis mechanism of these Li₂TiO₃ pebbles is shown in Figure 2. In addition, the NHSG process can achieve uniform precursor mixing at the atomic level, simplifying the preparation process. The Li₂TiO₃ powder sintered at 650 °C for 2 h displayed an average particle size of 24 nm with a diameter of 1.3–1.5 mm and an average grain size of 2.85 μm. The sol–gel parameters for preparing this Li₂TiO₃ material are summarized in

Table 3. Recent developments in the fabrication of Li₂TiO₃ by the sol–gel method.

Organization	Pebble Size/mm	Density (% T.D.)	Grain Size	Crush Load (N)	Ref.
SIC, China	1.4 mm	68%	10 μm	~	[44]
SIC, China	1.18–1.3 mm	85%	5 μm	~	[45]
FCD, India	0.6–0.7 mm	90%	100 μm	~	[46]
INCT, Poland	~	88%	250 nm	~	[47]
USTB, China	1.3–1.5 mm	84.9%	2.85 μm	67 N	[49]

. As can be seen, the sintered density is lower and the grain size is larger.

2.3. Hydrothermal Methods

Hydrothermal methods are used to synthesize powders under a high-temperature or high-vapor-pressure environment generated by heating the reaction medium in an autoclave. As the heating temperature increases, precursors such as oxides, hydroxides, or gels dissolve in the aqueous solution, eventually leading to supersaturation of the solution and the gradual formation of a new more stable phase. Hydrothermal methods exhibit the advantages of high purity, good dispersion, good crystal shape, and low production cost [51].

A significant number of works [52,53,54] have reported the synthesis of Li₂TiO₃ powders by the hydrothermal method, using various Ti and Li sources. Yanagisawa et al. [55] synthesized monoclinic β-Li₂TiO₃ nanoparticles using TiO₂ and LiOH·H₂O under hydrothermal conditions at 200 °C for 10 h. During the calcination process, the average particle size of the sample was about 114 nm, and there was no significant increase in particle size. A nanostructured Li₂TiO₃ powder was successfully fabricated by a water-controlled release solvothermal process method (WCRSP) [56]. This nanostructured Li₂TiO₃ powder, which had an average particle size of 35 nm, was successfully prepared at 550 °C. Lu et al. [57] prepared a Li₂TiO₃ power via a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method in which a small amount of CTAB was added into the LiOH·H₂O and TiO₂ solvents before transferring the solution into an autoclave for the hydrothermal reaction, which proceeded at 200 °C for 20 h. Their experimental results indicated that when 3% CTAB was added, the Li₂TiO₃ had an average particle size of about 90 nm and a relative density of 89.71% T.D.

The parameters used to prepare Li₂TiO₃ by the hydrothermal method are summarized in

Table 4. Recent developments in the fabrication of Li₂TiO₃ by hydrothermal method.

Organization	Pebble Size/mm	Density (% T.D.)	Grain Size	Crush Load (N)	Ref.
HUST, China	1.5 mm	81%	0.82 μm	34 N	[52]
HUST, China	1.0–1.2 mm	81%	100 nm	35 N	[53]
USTB, China	1.2 mm	85.2%	1.4 μm	53 N	[55]
SCU, China	1.7 mm	89.71%	90 nm	99.93 N	[56]
SCU, China	1.5 mm	89%	40 nm	90 N	[57]

. The products prepared by hydrothermal methods have controllable particle morphology, uniform particle size distribution, high phase purity, a simple process, low reagent cost, and good potential for easy industrial production.

2.4. Solution Combustion Synthesis

In solution combustion synthesis, a metal nitrate and aqueous fuel solution are heated and dehydrated, which causes an explosive combustion decomposition of the metal nitrate and fuel. This redox reaction releases a large amount of heat and promotes the smooth progress of the material synthesis reaction [58]. By using the combustion exotherm of the system, the synthesis can be performed at a relatively low temperature, and powders with large specific surface areas as well as fine crystal grains can be obtained [59]. Solution combustion methods use titanium alcohol as the raw material and glycine urea and alanine as the fuel, leading to high raw material costs and a complicated preparation process.

Jung et al. [33] synthesized Li₂TiO₃ by solution combustion using LiNO₃ and TiO(NO₃)₂ as a stock solution and stoichiometric glycine as the fuel. The synthesized Li₂TiO₃ had a high purity and excellent sinterability, and a high relative density of 85% T.D. was achieved at 1100 °C for 4 h. Xue et al. explored Li₂TiO₃ ceramics with nano-sized pores by ultrasonic-assisted solution combustion [60]. Ti(OC₄H₉)₄ and LiNO₃ were used as the Li and Ti sources and as oxidants in this solution combustion reaction, while C₆H₈O₇ was used as the fuel and reducing agent. The crystallite size of the Li₂TiO₃ powder prepared by ultrasonication was obviously refined to 5 nm. In contrast, the Li₂TiO₃ powder obtained without ultrasonication had a crystallite size of about 20 nm. The crush load of the ultrasonically prepared ceramic was maintained at 37.2 N even though the structure was porous.

Solution combustion synthesis parameters for preparing Li₂TiO₃ are summarized in

Table 5. Recent developments in the fabrication of Li₂TiO₃ by solution combustion synthesis.

Organization	Pebble Size/mm	Density (% T.D.)	Grain Size	Crush Load (N)	Ref.
HUST, China	~	85.8%	5 nm	37.2	[59]
HUST, China	~	92.6%	1 μm	~	[60]
NMTDD, Korea	2 mm	80%–87%	~	~	[61]
HUST, China	~	90%	800 nm	~	[62]
KAERI, China	~	~	0.3–0.5 μm	30 N	[63]

. Due to the addition of fuel and oxidant and the sublimation of Li during the solution combustion synthesis process, the final Li₂TiO₃ powder products obtained by using these methods have low purity. Moreover, solution combustion methods use TiO₂ or titanium alcohol as the raw materials, leading to high raw material costs and complicated preparation processes. The sharp rise in temperature during the combustion process means that the overall process is difficult to control, and the resulting powder morphology is irregular.

2.5. Co-Precipitation Methods

Co-precipitation methods involve the slow precipitation of dense and heavy amorphous or large grain crystalline particles via the controlled production of ions by an appropriate chemical reaction in a homogeneous solution [64]. Co-precipitation methods have two advantages: one, nano-powder materials with uniform chemical composition can be directly obtained by performing various chemical reactions in the solution, and two, these nano-powder materials can be obtained with small particle size and uniform distribution [65].

A Li₂TiO₃/Li₂MTi₃O₈ (M = Zn_1/3Co_2/3) nanocomposite was prepared by Hou et al. using a co-precipitation method [66]. ZnSO₄·7H₂O and CoSO₄·7H₂O (with a Zn/Co cationic molar ratio of 1:2) were mixed in deionized water in Tank 1 to obtain a 0.1 M solution under stirring to form the initial aqueous solution. Next, C₁₆H₃₆O₄Ti was dissolved in an ethanol solution in Tank 2. A mixed solution consisting of 1 M NaOH and 0.5 M NH₃·H₂O was prepared in Tank 3. The as-prepared precipitates were washed and dried to obtain the precursors. Then, the precursors were preheated at 500 °C for 4 h, followed by further mixing with a stoichiometric amount of Li₂CO₃. Finally, the mixture was calcined at 700 °C for 10 h in an air atmosphere to form the Li₂TiO₃/Li₂MTi₃O₈ nanocomposite.

Zhao et al. used a co-precipitation method to prepare Li₂TiO₃-coated LiNi_0.5Co_0.2Mn_0.3O₂ with enhanced electrochemical performance [67]. The [Ni_0.5Co_0.2Mn_0.3](OH)₂ powder was mixed with deionized water under constant stirring at 55 °C. Calculated amounts of Ti(SO₄)₂ and NaOH were separately dissolved in deionized water, and the obtained solutions were then subsequently injected dropwise into the [Ni_0.5Co_0.2Mn_0.3](OH)₂ suspension. After filtration, the precursor was washed with distilled water and alcohol several times. The dried precursor was mixed with an appropriate amount of LiOH, then ball-milled at 200 rpm for 1 h. The ball-milled mixture was first sintered in an air atmosphere at 850 °C for 12 h, then naturally cooled down to room temperature. The LiNi_0.5Co_0.2Mn_0.3O₂ samples coated with 1 wt.%, 3 wt.%, and 5 wt.% Li₂TiO₃ were denoted as 1% LTO@NCM, 3% LTO@NCM, and 5% LTO@NCM, respectively.

Co-precipitation methods require relatively higher reaction temperatures (usually above 700 °C). Thus, powders obtained by co-precipitation usually display lower sinterability. Furthermore, co-precipitation methods are complicated and require multi-step reaction routes with long-term reactions. Therefore, these are high-cost methods and are not used in many applications.

3. Fabrication of Li₂TiO₃ Pebbles with Enhanced Li Density

Recently, many reports have focused on the Li density [68,69,70,71], tritium release [72,73,74], irradiation [75,76,77], and modification [78,79,80] of Li₂TiO₃ pebbles.

The pebbles or pellets used in advanced tritium breeders are prepared after obtaining the Li₂TiO₃ powders. Current research has mainly focused on improving Li density. Li₂TiO₃ pebbles with higher Li density usually enable a higher tritium breeding ratio (TBR) and Li combustion at the end of life [13]. Due to their lower intrinsic Li density, the TBR of Li₂TiO₃ pebbles is considered to be lower than that of other breeders [76]. In theory, there are two methods for enhancing the Li density of Li₂TiO₃ pebbles: (a) increasing the pebble sintering density and (b) adding a Li₂O phase to the pebbles.

3.1. Improving Pellet Sintering Density

The sintering density of Li₂TiO₃ is affected by many factors. Related investigations have found that sintering density is significantly influenced by the sintering temperature, sintering method, additives, slurry content, and particle size. Zhang et al. [81] prepared Li₂TiO₃ pebbles using an improved wet industrial mass production process. By controlling the spheroidization and solidification process of the slurry droplets, a relative density of 85.6% was reached. Wen and coworkers [46] observed that the relative density of their pebbles dramatically increased with increasing temperature, reaching a relative density of 83% T.D. at a low temperature of 1000 °C (Figure 3a). Xue et al. [63] also reported a similar relationship between sintering density and temperature, with the density of their Li₂TiO₃ ceramic reaching 90.7% of theoretical density after treatment at 800 °C (Figure 3b). Lu’s team [58] demonstrated the influence of different preparation methods on sintering density. With a temperature of 750 °C, the relative density of microwave-sintered pebbles was 87% T.D., while the relative density of conventionally sintered pellets at the same temperature was only 70% T.D. (Figure 3c). Yu et al. [78] studied the influence of various additives on sintering density and found that adding 3% glycerin achieved the maximum relative density of 92.4% (Figure 3d). Chang et al. [12] reported the relationship between the solid content of the slurry and the particle size with relative density. As the solid content in the slurry was increased from 25% to 35%, the relative density of their Li₂TiO₃ pebbles slightly increased slightly, reaching the highest value when the solid phase content was 35% (Figure 3e). Figure 3f shows that as the particle size of the Li₂TiO₃ powder was decreased, the relative density of the pebbles increased, and the rate of increase was the highest at 200–300 mesh. When the mesh number reached 300, the density of Li₂TiO₃ pebbles reached the maximum value of 97.16%. However, increasing the sintering density of Li₂TiO₃ pellets only improved Li density to a limited degree, so many researchers have focused on increasing the Li content in the Li-Ti-O system.

3.2. Adding Li₂O Phase to Pebbles

Hoshino et al. [82] reported the influence of the Li/Ti ratio on the microstructure and thermal diffusivity of Li₂TiO₃. All specimens were sintered under the same sintering conditions, and grain growth was higher in samples with higher Li/Ti molar ratios. Therefore, their results demonstrated that increasing the Li/Ti ratio enhanced the sintering effect (Figure 4a). The relative density of the lithium-rich component samples increased. In the sintered Li₂TiO₃, the Li density increased due to the good sintering performance and the synergistic effect of the Li-rich composition (Figure 4b,c). However, Li₂TiO₃ pebbles with a higher Li/Ti ratio have not been reported yet. Based on this material’s phase diagram [83], when the Li/Ti molar ratio exceeds 2.25, Li₄TiO₄ is inevitably synthesized. To further increase the Li content, Zhang et al. [84] demonstrated a core–shell structure Li₂TiO₃ pebble with a Li₂TiO₃–Li₄TiO₄ complex phase core and a Li₂TiO₃ shell with tunable thickness. The direct synthesis strategy for obtaining these Li₂TiO₃ core–shell pebbles is displayed in Figure 4d. Figure 4e–g show the cross-section images and elemental distribution of the green pebbles at different washing times. The thickness of the shell was easily modified by precisely controlling the amount of water and the washing time. When the Li/Ti ratio was 2.7, a pebble was obtained with the theoretical Li density (0.434 g/cm³) of pure Li₂TiO₃.

Compared with pure Li₂TiO₃, Li-rich Li₂TiO₃ has a lower tritium release temperature [85]. Li₂TiO₃ with excessive Li is expected to be used as an advanced ceramic breeder material because its high lithium concentration can make up for the loss of lithium and prolong the pebble lifetime under operating conditions [86].

4. Tritium Release Property

Tritium release is one of the most critical properties of breeder materials. During fusion reactor operation, the tritium release rate in the proliferating cladding reaches a balance with the tritium generation rate [87]. A low tritium release rate, therefore, leads to serious tritium retention, which is unacceptable from economic and safety perspectives [88]. The tritium release processes in ceramic breeders include bulk diffusion, grain boundary diffusion, and surface desorption. Recently, the tritium release behavior of Li₂TiO₃ has been investigated [89]. Current research has mainly focused on the tritium release mechanism and tritium release properties.

4.1. Tritium Release Mechanism

Researchers initially believed that the overall release process of tritium was mainly controlled by the diffusion process of tritium within the grains of solid breeder materials. However, many recent studies have proposed that in addition to grain diffusion, the reaction on the grain surface is significant for understanding tritium release behavior [90].

Nishikawa et al. [91] reported the effect of surface water on the tritium release behavior of Li₂TiO₃. Their tritium release model representing the release behavior of the breeder tritium of Li₂TiO₃ is shown in Figure 5a. This model took into account the diffusion of tritium in solid breeder particles, the transfer of tritium to surface water at the particle interface layer, and the release of tritium from the surface water to the sweep gas during water adsorption/desorption. The reactions in this model were the isotope exchange reaction between hydrogen in the sweep gas and tritium in the surface water (isotope exchange reaction (1)), the isotope exchange reaction in the water vapor (isotope exchange reaction (2)), and the water production reaction of hydrogen in the sweep gas. The hydrogen in the sweep gas was exchanged with the tritium isotopes in the surface water (isotope exchange reaction (1)) and the tritium isotopes in the water vapor (isotope exchange reaction (2)), and the hydrogen in the purge gas produced water. By simulating the release behavior of water or tritium, it was confirmed that water production had a profound influence on the release behavior of tritium in blankets.

However, recent studies have shown that the tritium reaction with water vapor on sample surfaces is negligible. Kulsartov et al. [92] studied the tritium release mechanism of Li₂TiO₃ using a vacuum extraction method (Figure 5b). Tritium atoms were thermalized and diffused to the surface of the grain, interacting with defects/traps (absorbed/released by the defects/traps). Tritium atoms were then released on the surface of the grain and spread across the surface to local atoms or hydrogen atoms that were associated with the same tritium. Next, the tritium-bearing molecules were desorbed into the pore volume, upon which they followed a quick exit path away from the pebble. Because no increase in the vapor pressure of tritium water was detected in the experiment, these authors believed that the exchange reaction between tritium and water vapor on the surface of the sample was negligible.

According to the reported research mechanism, when water was involved in the reaction, tritium was mainly released in the form of HT, HTO + HT, and H₂O, while in the vacuum environment, tritium was mainly released as HT and T₂. The vacuum experiment eliminated the effect of water vapor on tritium, and these results significantly promoted the understanding of the mechanism of tritium release. In particular, when the effect of water vapor was ignored, the tritium release process was simpler.

4.2. Tritium Release Properties

Tritium release properties are mainly tested by blowing inert gas through Li₂TiO₃ and using an ionization chamber to analyze the released tritium content in the purge gas [92]. Research on the characteristics of tritium release has mainly focused on the form of tritium release, purge gas flow rate, temperature, content, and type of added gas.

As reported by Nishikawa et al., the forms of tritium released in pure He purge gas are mainly HT, HTO + HT, and H₂O [93]. Figure 6a shows the rapid release of H₂O in purge gas at the beginning of the operation. When the temperature was lower than 300 °C, tritium slowly diffused in the particles, delaying the tritium release process. HTO was formed in the presence of water at temperatures higher than 500 °C. Once neutron irradiation started, the tritium inventory in the bulk rapidly increased. Tsuchiya et al. studied the effects of scavenging gas flow rate, irradiation temperature, and other parameters on tritium release [94]. As the scavenging flow increased, the total tritium release temporarily and rapidly increased. With decreasing tritium concentration, the total amount of tritium released showed a rapid or slow downward trend. In addition, after several hours of changing the scavenging flow, the total amount of tritium released was stabilized (Figure 6b). Figure 6c shows that lowering the Li₂TiO₃ irradiation temperature reduced the total amount of tritium released. As the irradiation temperature was increased, the total amount of tritium released was increased.

In addition to pure He in the purge gas, a certain amount of H₂ or H₂O has been added in many tritium release experiments to increase tritium release. The effect of purge gas conditions on the tritium release properties of a Li₂TiO₃–Li₄SiO₄ biphasic material was reported by Lu et al. [95]. Under pure He purge gas, the main release peak of tritium was around 470 °C, and the release ratio of tritium mainly in the form of tritiated water was 72%. While under a He + 0.1% H₂ purge gas, the tritium release process was enhanced and the molecular fraction of tritium increased from 28% to 55% due to the isotope exchange reaction between H₂ and the tritium grain surface (Figure 6d,e). Kobayashi et al. reported the effect of He and water vapor as the purge gas on the release of tritium [96]. Figure 6f shows that the He and water vapor had a slight effect on the release of tritium in the low-temperature region. They also found that increasing the water vapor slightly reduced the retention of tritium in oxygen vacancies, indicating that tritium permeation from the surface promoted the detrapping reaction of tritium in oxygen vacancies.

The tritium release properties of Li₂TiO₃ are the main research focus of this tritium breeder material. Changing the parameters of the purge gas can effectively increase the tritium release rate and help enhance the ratio of tritium breeding in a real system.

5. Application of Deuterium in Spectroscopy

In the previous section, we discussed the tritium release behavior of Li₂TiO₃, but a comprehensive model of tritium release kinetics has still not been determined. Therefore, it is necessary to investigate the tritium migration process and understand the release mechanism of tritium in ceramic breeding materials. Due to the lack of 14 MeV neutron sources and the lower content of tritium in nature, the release behavior of deuterium in Li₂TiO₃ has been used to simulate the radiation effects of high-energy neutrons. The application of deuterium in spectroscopy has mainly focused on the deuterium release behavior and deuterium irradiation.

5.1. Deuterium Release Behavior

The damage caused by a D⁺ ion beam is not entirely comparable to that caused by a neutron beam. However, there are not sufficient 14 MeV neutron sources available for research, so energy ion beams have long been used to simulate the radiation effects of neutrons. Deuterium ion irradiation experiments have therefore been conducted with Li₂TiO₃ pebbles to simulate their tritium retention and desorption behavior.

Zhu et al. studied the release behavior of hydrogen isotopes from Li₂TiO₃ single crystal samples with different sizes by TDS experiments [97]. Figure 7a shows the TDS spectra of Li₂TiO₃ single crystal plate samples (thicknesses of 0.2–0.4 mm, 0.984 g). The deuterium thermally absorbed in the Li₂TiO₃ was mostly released as HDO, and three HDO peaks were observed. Peak (3) (around 1000 K) was caused by the recombination desorption of deuterium diffused from the bulk. The HDO peaks for the three Li₂TiO₃ single crystal samples are compared in Figure 7b. It should be noted that the amounts of these samples widely differed: 0.984 g for the plate sample, 0.031 g for the 50 μm powder sample, and 0.040 g for the 5 μm powder sample. No obvious HDO peak related to bulk diffusion was observed in the powder samples. This indicated that the rate-controlling step for the release of hydrogen isotopes shifted from bulk diffusion to surface processes as the size of the single crystal samples decreased to several μm.

T. Hino et al. investigated deuterium retention in Li₂TiO₃ pebbles [98]. Their results indicated that the deuterium retained in these pebbles desorbed in forms of HD, D₂, HDO, and D₂O. Meanwhile, H and O were retained before irradiation in lithium titanate as the impurities. The desorption rates of these gases decreased with increasing irradiation temperature. At an irradiation temperature of 773 K, the retained deuterium was completely desorbed during irradiation. In previous work, researchers reported that the main forms of deuterium released from Li₂TiO₃ were HDO and D₂O (Figure 7c).

Qi et al. reported the release behavior of hydrogen isotopes in deuterium-irradiated LiTiO₂₃, showing that the release was predominantly HD and D₂ [99]. The D₂ curve showed a single peak at 573 K, while the HD curve showed two peaks at 365 K and 590 K (Figure 7d). The D₂ release spectra obtained with heating rates of 5, 10, and 15 K/min are shown in Figure 7e. Only a single D₂ release peak was observed in the temperature region of 550–800 K for each spectrum. D₂ release peaks were located at 626 K, 665 K, and 690 K.

The main forms of released deuterium in lithium titanate are HD, D₂, HDO, and D₂O, but the release form of deuterium changes to HD and D₂ at high heating rates. The use of deuterium to simulate neutron irradiation is expected to provide some useful data for comprehensively investigating the behavior of tritium in ceramic breeding materials.

5.2. Deuterium Irradiation

The reaction of deuterium in Li₂TiO₃ includes the release behavior of deuterium as well as the influence of deuterium irradiation on the properties of Li₂TiO₃. Deuterium implantation is applied to simulate the irradiation damage in future fusion reactors.

Qi et al. investigated the influence of deuterium irradiation defects on the mechanical properties of the ceramic breeder material Li₂TiO₃ [100]. After deuterium irradiation, electron spin resonance (ESR) experiments were employed to investigate the irradiation defects. It was observed that the irradiation defects of F-centers increased as the irradiation dose was increased. This meant that more irradiation damage was produced in the Li₂TiO₃ sample exposed to a higher irradiation dose (Figure 8a). Figure 8b shows that the Vickers hardness of these samples was also affected by deuterium irradiation. Vickers hardness increases as the applied load decreases, which is consistent with Meyer’s theory. For these Li₂TiO₃ samples, the Vickers hardness decreased as the irradiation dose was increased.

Gu et al. reported the effects of deuterium irradiation and high temperature on the chemical states in Li₂TiO₃ [101]. The Li 1s, O 1s, Ti 2p, and C 1s XPS spectra of their samples are shown in Figure 8c. The O 1s peak shifted to a higher binding energy after both irradiation and deuterium exposure, indicating that O-D bonds were formed. The number of O-D bonds was enhanced with increasing irradiation and exposure temperature. The main deuterium atoms were trapped by defects in the irradiated samples. The annihilation of E-centers was thought to trigger the release of hydrogen isotopes. O-D bonds were the main deuterium trapping sites in the deuterium-exposed Li₂TiO₃. Deuterium recovered by detrapping these O-D bonds would require a higher temperature.

The influence of deuterium release from Li₂TiO₃ pebbles on the pebble surface conditions was investigated by Tsuchiya et al. [102]. Figure 8d shows the atomic composition depth profiles for their sample prior to irradiation/heating and the sample after seven irradiation/heating cycles. Clearly, lithium depletion in the surface region occurred due to these irradiation/heating cycles. Lithium depletion beyond the penetration range of the deuterium ions may be due to the synergetic effect of the induced damage and the heating process. Figure 8e shows the atomic compositions of the samples around the penetration depth of the deuterium ions as a function of the number of irradiation/heating cycles. The Li content decreased when increasing the number of irradiation/heating cycles. On the other hand, the Ti and O content increased when increasing the number of irradiation/heating cycles.

The use of deuterium irradiation to investigate Li₂TiO₃ has mainly focused on the properties, structures, and chemical valence states of Li₂TiO₃. The defects caused by deuterium irradiation greatly reduce the difficulty and cost of analysis, providing more experimental data for the tritium migration process in Li₂TiO₃.

6. Irradiation Performance

Li₂TiO₃ in operational fusion reactors is damaged by the radiation of fast neutrons, high-energy tritons, and helium ions produced in the ⁶Li (n,α)³H reaction [20,23]. Ionizing radiation damage can lead to changes in the microstructure of Li₂TiO₃, thereby affecting its tritium release behavior and thermodynamic properties [103]. Previous research has focused on the impact of radiation on the microstructure, irradiation decomposition, thermophysical parameters, changes to mechanical properties, and irradiation swelling [104]. Irradiation damage is classified by the irradiation source as neutron irradiation, energetic tritons, and ion irradiation damage. The irradiation performances of Li₂TiO₃ under both types of radiation are reviewed in this paper.

6.1. Neutron Irradiation

There are two main formation processes that lead to the creation of irradiation defects. The first process is the collision process in which recoil particles generated by atomic and nuclear reactions repel each other, and the second is the electronic excitation process. When excitons are formed, energy is transferred to the surrounding atoms to recoil particles, leading to the formation of irradiation defects [105]. Therefore, neutron irradiation and ion irradiation experiments have been carried out on Li₂TiO₃ to investigate the formation and annihilation of its irradiation defects.

Kobayashi et al. [106] researched the trapping effect of radiation damage on tritium and the effect of purge gas on the release behavior of tritium. The F⁺-center and O⁻-center were the main sites affected by the irradiation damage caused by neutrons in Li₂TiO₃, according to ESR measurements. The neutron flux of sample B was 3.3 × 10¹⁵ n·cm⁻²s⁻¹, while that of sample E was 2.2 × 10¹⁹ n·cm⁻²s⁻¹. The peak area of the absorption spectra shown in Figure 9a directly corresponded to the amount of irradiation damage. The irradiation damage density increased with increasing neutron flux. The ratio of tritium concentration to damage density also increased with increasing neutron flux. This research group also studied the effects of various γ-ray doses on the defect annihilation behavior of Li₂TiO₃ [105]. Figure 9b shows the ESR spectra of these Li₂TiO₃ samples before and after γ-ray irradiation. These spectra clearly indicated that the irradiation defects were generated by γ-ray irradiation, and three different irradiation defects were observed: E’-center, O⁻-center, and O₂-center defects. These studies mainly focused on the effects of γ-ray irradiation on Li₂TiO₃ induced by the electron excitation process. ESR was also used to track the annihilation of the irradiation defects in Li₂TiO₃ induced by γ-ray irradiation via annealing experiments [107]. The defect formation process differed under γ-ray and neutron irradiation and different defects such as O⁻-center and O₂-center defects were generated, resulting in varying irradiation defect annihilation behavior (Figure 9c).

Due to the uniform distribution of neutron-like irradiation in fusion reactors, γ-ray irradiation has been investigated in some of the literature [108]. It was observed that E-center and Ti³⁺ defects were the main defects introduced in Li₂TiO₃ irradiated by γ-rays. In addition, E-center defects negatively affected the thermal conductivity of Li₂TiO₃, while the existence of Ti³⁺ significantly enhanced thermal conductivity.

To investigate the effects of long-term heating on the release behavior of tritium, Li₂TiO₃ was irradiated by neutrons [109]. The authors stated that the tritium released at temperatures higher than 900 °C could not be recovered. With increasing heating time, the tritium recovery ratio decreased.

6.2. Ion Irradiation

The use of ion irradiation is an expedient method for evaluating the microstructural changes and irradiation defects of Li₂TiO₃ after irradiation. The ⁶Li (n, α) ³H reaction can be simulated by the irradiation of He ions and O ions to generate 2.1 MeV He ions. Both O ion and He ion irradiation can lead to less intense XRD and Raman peaks, indicating the production of long-range and short-range disorder after ion irradiation. In addition, both long-range and short-range destruction were strongly related to the ion species and energy. O ion irradiation more severely destroyed long-range order than He ion irradiation, as shown in Figure 10a [23]. ESR measurements demonstrated that E-center defects were produced in the damaged Li₂TiO₃ after He ion irradiation. In this irradiated Li₂TiO₃, the E-centers were a fundamental and dominant lattice defect.

Nakazawa’s team also reported the irradiation of Li₂TiO₃ with high-energy ions (Xe, O), and they used Raman spectroscopy to analyze their irradiated samples [110]. Analysis of the Li₂TiO₃ structure exposed to 160 MeV Xe ions showed that the Li₂TiO₃ structure was destroyed, while the Li₂TiO₃ structure exposed to 80 MeV O ions was not observably damaged. Therefore, the degree of disorder leading to the destruction of the Li₂TiO₃ structure was thought to be related to the electronic stopping power of incident ions (Figure 10b).

Zhou et al. [111] reported the preparation of Li₂TiO₃ and its structural evolution under high-energy Ar⁺ irradiation. As shown in Figure 10c, the displacement damage of their ceramic block reached a depth of about 200 nm, and the maximum value of the displacement damage was 49.93 at about 70 nm. The area affected by irradiation reached a depth of about 3 μm, but the structure of the ceramic block did not change. The high-energy ion irradiation affected the atomic order and structure on the surface of their small crystalline Li₂TiO₃ (Figure 10d). Furthermore, the Li₂TiO₃ microstructure changed due to irradiation. A biphasic Li₂TiO₃–Li₄SiO₄ mixture has also been synthesized and tested [112]. As shown in Figure 10e1,e2, some of the crystal grains in the Li₂TiO₃–Li₄SiO₄ sample were melted after irradiation. The cross-sectional SEM images in Figure 10e4,e5 show that the crystal grains in the outer layer were melted, while the grains in the inner layer remained granular. This research demonstrated the suitable irradiation performance of biphasic Li₂TiO₃–Li₄SiO₄.

In general, research investigating the irradiation of Li₂TiO₃ shows broad prospects for preparing tritium breeding materials in future fusion reactors and indicates that composite materials are a better choice for these tritium breeder materials.

7. Modification

Tritium breeder materials should exhibit the following characteristics: a high tritium multiplying ratio [113], easy extraction of tritium [114], physical and chemical stability at high temperatures [115], excellent compatibility [116], high thermal conductivity [117], and other characteristics. However, single-component materials are unable to meet all these requirements [118,119,120]. Therefore, to develop materials with improved tritium release performance, previous work has focused on the modification of material composition and structure. One modification strategy is the use of bulk doping to introduce oxides or related elements into solid tritium breeder materials [121,122]. Li₂TiO₃ is mainly modified by bulk doping. There are two main Li₂TiO₃ doping strategies: phase transformation and the formation of Li₂TiO₃ biphasic ceramics.

7.1. Phase Transformation

Wen’s team [123] reported that doping Li₂TiO₃ with Mg changed the density and pore size of Li₂TiO₃. With increasing Mg content, the density increased and the pore size decreased (Figure 11a). Their primitive Li₂TiO₃ and lightly doped Li_1.99Mg_0.01TiO_3.005 pellets showed pores as large as 3–10 μm. However, increasing the Mg content resulted in improved density and smaller pore sizes. When the Mg content was 0.1, a trans-granular fracture phenomenon was observed, indicating that the Li_1.90Mg_0.1TiO_3.05 pellets had a high density and favorable mechanical strength. It was also observed that Mg doping significantly improved the grain growth. The Mg-doped samples had a uniform microstructure, and the average grain size of Li_1.99Mg_0.01TiO_3.005, Li_1.97Mg_0.03TiO_3.015, and Li_1.95Mg_0.05TiO_3.025 was about 3 μm. Figure 11b shows the XRD patterns of the Li_2−2xMg_2xTiO_3+x powders with different doped Mg content. As can be seen, the XRD patterns of the Mg-doped samples and that of the primitive Li₂TiO₃ did not significantly differ. Li₂TiO₃ was the only phase detected in the XRD patterns of the samples in the 2x range of up to 0.1, which indicated that all the doped Mg was incorporated into the Li₂TiO₃ lattice.

Li et al. [124] reported the phase change of Li₂TiO₃ co-doped with magnesium and niobium. SEM images of their pure Li₂TiO₃ and doped ceramics are shown in Figure 11c. All the samples possessed a porous microstructure. The pure Li₂TiO₃ ceramics exhibited fuzzy grain boundaries, while the crystalline grains of the doped ceramics were well stacked and the grain boundaries were clear. The samples doped with either Mg²⁺ or Nb⁵⁺ showed clearly enhanced fuzzy grain boundaries compared with pure Li₂TiO₃. However, (Mg + Nb) co-doping eliminated the microcracks in the structure, in contrast to the use of Mg²⁺ as a single dopant. In addition, the (Mg + Nb) co-doped sample had a uniform microstructure.

Chen et al. [125] reported that the Q×f value of Li₂Ti_1−x(Zn_1/3Nb_2/3)_xO₃ was slightly improved by the small-level acceptor/donor complex substitution of Ti⁴⁺ with (Zn_1/3Nb_2/3). The dielectric properties of these composites were strongly dependent on their composition, densification, and microstructure. The Q×f value increased with increasing x value up to x = 0.2 and then decreased with a further increase in x value. Zuo et al. [126] investigated the microwave dielectric properties of a Li₂Ti_1−x(Al_0.5Nb_0.5)xO₃ system and reported a clear improvement in the value of Q×f. XRD patterns, which confirmed that a single-phase solid solution existed when 0 < x ≤ 0.2, while secondary phases started to appear when x > 0.2. The appearance of these secondary phases was accompanied by an order–disorder phase transition across the whole range. SEM observation indicated that the complex substitution of Al³⁺ and Nb⁵⁺ significantly affected the microstructural morphology. Both microcrack healing and grain growth contributed to the significantly enhanced Q × f values.

Figure 11. SEM images of the fracture surface of Li_2-2xMg_2xTiO_3+x pellets sintered at 1100 °C for 6 h. (a1) Li₂TiO₃, (a2) Li_1.99Mg_0.01TiO_3.005, (a3) Li_1.97Mg_0.03TiO_3.015, (a4) Li_1.95 Mg_0.05TiO_3.025, and (a5) Li_1.90 Mg_0.1TiO_3.05. (b) X-ray diffraction patterns of Mg-doped lithium titanate. (b1) 2x = 0; (b2) 2x = 0.01; (b3) 2x = 0.03; (b4) 2x = 0.05; (b5) 2x = 0.1. Reprinted with permission from ref. [122]. (c) Surface morphologies of (c1) pure Li₂TiO₃; (c2) LTM; (c3) LTN; (c4) LTMN. Reprinted with permission from ref. [123]. (d) Behavioral phase transformation diagram from a summary of temperature varied XRD and DSC data of the 11 compositions studied in heating mode. Reprinted with permission from ref. [126].

Figure 11. SEM images of the fracture surface of Li_2-2xMg_2xTiO_3+x pellets sintered at 1100 °C for 6 h. (a1) Li₂TiO₃, (a2) Li_1.99Mg_0.01TiO_3.005, (a3) Li_1.97Mg_0.03TiO_3.015, (a4) Li_1.95 Mg_0.05TiO_3.025, and (a5) Li_1.90 Mg_0.1TiO_3.05. (b) X-ray diffraction patterns of Mg-doped lithium titanate. (b1) 2x = 0; (b2) 2x = 0.01; (b3) 2x = 0.03; (b4) 2x = 0.05; (b5) 2x = 0.1. Reprinted with permission from ref. [122]. (c) Surface morphologies of (c1) pure Li₂TiO₃; (c2) LTM; (c3) LTN; (c4) LTMN. Reprinted with permission from ref. [123]. (d) Behavioral phase transformation diagram from a summary of temperature varied XRD and DSC data of the 11 compositions studied in heating mode. Reprinted with permission from ref. [126].

Hanaor et al. [127] reported the synthesis of solution-based mixed-phase materials in a Li₂TiO₃–Li₄SiO₄ system. The behavioral diagram shown in Figure 11d summarizes the observed phase transition behavior of the hydroxide sol precursor material during heating. This diagram was compiled based on XRD and DSC data. The currently reported method did not result in a pure two-phase Li₄SiO₄–Li₂TiO₃ mixture, but this two-phase mixture can be obtained by a solution-based method using appropriate process parameters, including the addition of excess Li.

7.2. Preparation of Li₂TiO₃ Biphasic Ceramic Pebbles

Li₄SiO₄ and Li₂TiO₃ are regarded as the most favorable ceramics due to their satisfactory thermo-mechanical properties [128]. Li₄SiO₄ has a better Li density, while Li₂TiO₃ shows excellent mechanical and tritium release properties [129]. Therefore, the biphasic Li₂TiO₃–Li₄SiO₄ breeding material has been researched and developed in recent years to combine the advantages of Li₄SiO₄ and Li₂TiO₃ [95,127,130]. This biphasic breeder material shows good prospects for obtaining enhanced mechanical properties and balanced tritium release properties [129]. In fact, both Li₄SiO₄ and Li₂TiO₃ phases can coexist in melt-based products [131].

The fabrication of biphasic Li₂TiO₃–Li₄SiO₄ breeder materials is another effective approach for improving the mechanical and release properties of tritium breeder pebbles. Lu et al. prepared 2Li₂TiO₃–Li₄SiO₄ pebbles with a grain size of 170 nm and a porous structure [132]. These 2Li₂TiO₃–Li₄SiO₄ biphasic ceramic pebbles exhibited a significantly reduced tritium release temperature and had good tritium release characteristics at 450 °C. The two-phase interface was rich and porous, leading to enhanced tritium release. This result indicated that the tritium release behavior of these biphasic ceramics can be enhanced by the surface reaction of the biphasic interface. A study on the microwave synthesis of Li₂TiO₃–Li₄SiO₄ biphasic ceramic pebbles was reported by Shi et al. [133]. The average grain size of the pebbles obtained by microwave sintering at 750 °C was about 148 nm. With increasing sintering temperature, the grains of the ceramic pebbles gradually grew and the microstructure was densified. When the temperature was 850 °C, the grain size distribution was uniform and homogeneously distributed open pores were observed. However, further increasing the sintering temperature to 900 °C resulted in the appearance of closed pores, which was potentially due to coarse grain growth and interfacial melting (Figure 12a). The XRD patterns shown in Figure 12b demonstrated that Li₂TiO₃ and Li₄SiO₄ had good compatibility. Moreover, the crush load of these biphasic pebbles was enhanced with increasing microwave sintering temperature (Figure 12c). Considering factors such as grain size/distribution, pore structure, and crush load, the optimum microwave sintering temperature was 850 °C. The grain size of the pebbles obtained under this temperature was 162 nm, and the crush load of these pebbles was 22.7 N. These results indicated that Li₂TiO₃–Li₄SiO₄ biphasic ceramic pebbles with distinguished mechanical properties are a promising tritium breeder material.

Some researchers have reported that the preparation of biphasic ceramic pebbles with core–shell structures may become the focus of research for developing new tritium breeder materials [134]. Core–shell structures are beneficial for increasing the crushing load and lithium density. Liao et al. prepared Li₄TiO₄–Li₂TiO₃ core–shell ceramic pebbles with thick shells and high strength through an improved granulation method [135]. The instability of Li₄TiO₄ in the presence of CO₂ and H₂O has been considered to be the main obstacle to the practical application of Li₄TiO₄ as a tritium breeding material. Therefore, using a Li₂TiO₃ ceramic coating as a physical barrier wrapped on the surface of Li₄TiO₄ pebbles is a potential solution to this problem. Figure 13a shows a schematic diagram of the PVP–assisted synthesis of Li₄TiO₄–Li₂TiO₃ core–shell green pebbles. As shown, the first step involved the synthesis of Li₄TiO₄ and Li₂TiO₃ powders through a solid-state reaction combined with pre-sintering heat treatment. Subsequently, the Li₄TiO₄ or Li₂TiO₃ powder was dispersed in a PVP solution. During the granulation process, PVP was effectively activated by the introduction of deionized water. Through the combined action of PVP and the liquid phase, the particles were grown layer by layer into Li₄TiO₄–Li₂TiO₃ pebbles with a core–shell structure.

Zhang et al. [136] prepared Li₂TiO₃-Li₄SiO₄ core–shell ceramic pebbles with improved crush load by a graphite bed process. Elemental mapping of a 50% Li₂TiO₃- 50% Li₄SiO₄ pebble indicated that Li₂TiO₃ and Li₄SiO₄ were physically compatible (Figure 13b). The average crush load of pure Li₄SiO₄ pebbles and Li₂TiO₃ pebbles fabricated by this graphite bed process were 40 N and 48 N, respectively. In contrast, the average crush load of the 50% Li₂TiO₃- 50% Li₄SiO₄ biphasic pebbles was 104.79 N. (Figure 13c,d). The average crush load of the ceramic pebbles increased with increasing sintering time. To the best of our knowledge, this was the highest crush load that has been reported so far. Li₂TiO₃-Li₄SiO₄ pebbles, therefore, show great development prospects and excellent promise as the next generation of progressive tritium breeder materials.

8. Summary and Outlook

In summary, this paper briefly reviewed the recent advances and current progress of Li₂TiO₃ in tritium breeder materials. The main issues of the major Li₂TiO₃ powder preparation methods, their advantages and disadvantages, and the particle size and density of the resulting powders were discussed. The representative studies summarized in this work and the discussion of Li₂TiO₃ development were divided into five parts: increasing the sintering density and adding a Li₂O phase to increase the Li density, the tritium release mechanism, and tritium release properties of Li₂TiO₃, the application of deuterium in spectroscopy, the effect of neutron and ion irradiation, and the preparation of biphasic ceramic materials and their phase transformation.

A significant amount of work has been reported with the goal of improving the performance of Li₂TiO₃. However, many problems still need to be addressed. First, most Li₂TiO₃ powers are currently synthesized by the hydrothermal method, and the yield of many hydrothermal Li₂TiO₃ powders is very low. Therefore, developing alternative preparation methods and enhancing the yield of Li₂TiO₃ are urgently required. This is crucial for the practical use of this material in fusion reactors, which require significant amounts of tritium breeder materials. Second, many researchers have focused on the development of composite ceramic materials to improve their tritium breeder performance. This will provide a new direction for the future development of Li₂TiO₃. Third, in terms of material preparation, researchers already possess semi-industrial production capabilities, and they have reported the recycling and reprocessing of tritium breeder ceramic microspheres and research on advanced tritium breeder materials.