Glycine-nitrate solution combustion synthesis of lithium zirconate: Effect of fuel-to oxidant ratio on phase, microstructure and sintering

Journal of the European Ceramic Society 40 (2020) 136–144

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Original Article

Glycine-nitrate solution combustion synthesis of lithium zirconate: Effect of fuel-to oxidant ratio on phase, microstructure and sintering

T

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Biranchi M. Tripathia,c, , Trupti Mohantya, Deep Prakasha,c, A.K. Tyagib,c, P.K. Sinhaa a

Powder Metallurgy Division, Bhabha Atomic Research Centre, Vashi Complex, Navi Mumbai, 400705, India Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India c Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094 b

A R T I C LE I N FO

A B S T R A C T

Keywords: D-T fusion Tritium breeder Ceramic Sintering Microstructure

Pure monoclinic lithium zirconate (Li2ZrO3) powder was synthesized by solution combustion route using glycine as fuel and nitrates as oxidants. Effect of fuel-to-oxidizer ratio (φ = 0.5–1.25) on synthesis condition, phase, powder morphology, sintering and thermodynamic aspects was systematically investigated. Thermodynamic analysis reveals; mode of combustion, adiabatic flame temperature, amount of gases and powder characteristics can be controlled by adjusting φ. The crystallite size of Li2ZrO3 powder was in the range of 18–40 nm. The powders consist of micrometric soft-agglomerates of irregular flake shape particles. The residual Li2O in Li2ZrO3 powder was found to be in the range 155–469 μg/g. The Li2ZrO3 powder synthesized by present method shows significantly enhanced sinter-ability as compared to conventional solid-state method. The Li2ZrO3 pellets were sintered close to 98% of T.D. at 1000 °C with grain size 1–2 μm. Increase of sintering temperature to 1050 °C results abnormal grain growth with large number of closed pores.

1. Introduction Lithium bearing ceramics viz., Li2TiO3, Li2ZrO3, Li4SiO4 and LiAlO2 due to their good thermal conductivity and satisfactory breeding performances have been widely studied for their application as tritium breeder in nuclear fusion [1–3]. Use of breeder materials have been reckoned in the form of sintered compacts e.g., pellet or pebble [4,5]. Density and microstructure are crucial parameters that control tritium breeding and release performance of sintered compacts. Generally fine grained microstructure and density close to 90% of theoretical (T.D.) are desired for good tritium breeding performance of sintered compacts [6–11]. However poor sinter-ability, susceptibility towards Li-evaporation and uncontrolled grain growth during sintering at high temperature are some of the serious drawbacks of tritium breeder materials [6,12–14]. Due to these limitations fabrication of advance tritium breeder materials by powder metallurgy route is quite challenging. One of the approaches to overcome these limitations is to synthesize enhanced sinterability breeder ceramic powders. The nanocrystalline ceramic powders often exhibit enhanced sinterability than their bulk counterpart. This is due to increased grain boundary volume fraction as the particle and crystallite sizes are reduced to nanometer regime leading to shorter diffusion pathways. Consequently, it might be possible to sinter nanocrystalline tritium breeder ceramic powders at

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relatively low temperature and in short duration [9,15–18]. This will eventually help in attaining higher densification as well as fine grained microstructure. Conventionally Li2ZrO3 powder is prepared via solid state reaction [19–25] which requires multiple annealing cycles at high temperature for extended period of time, which can last from several hours to few days leading to poor sinter-ability of the resulting powder. Meanwhile, the sublimation of Li2O can not be avoided due to high-temperature calcination. These aspects lead to poor tritium breeding properties of the Li2ZrO3 powder compacts prepared by the solid-state method. In order to improve sinterability, there has been concerted scientific effort during the last few years on synthesis of submicron to nanoscale powders. of tritium breeder ceramics by various wet chemical methods. Typical wet chemical methods include sol–gel, hydrothermal, solution combustion synthesis (SCS) and polymer solution method [9,17,26–33]. Lithium zirconate is also a promising candidate material for carbon dioxide (CO2) sequestration. Many research groups pointed out that CO2 capture properties can be enhanced significantly by fabricating Li2ZrO3 based materials using highly sinterable nanocrystalline Li2ZrO3 powders. Several wet chemical methods have been proposed for synthesis of nanocrystalline Li2ZrO3 powders [34–44]. SCS is fast and energy efficient alternative for preparation of

Corresponding author. E-mail address: [email protected] (B.M. Tripathi).

https://doi.org/10.1016/j.jeurceramsoc.2019.09.008 Received 26 May 2019; Received in revised form 4 September 2019; Accepted 6 September 2019 Available online 08 September 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.

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nanoscale tritium breeder powder which has recently drawn the attention of researchers due to the multiple advantages over the conventional methods. Typically, in solution combustion synthesis an exothermic redox reaction between metal nitrates and organic fuel is initiated by heating raw material solution at relatively low temperatures, and once combustion reaction is ignited, it evolves in self-propagated manner to yield the desired product [45,46]. For instance, Zhou et al. [9] reported that fine Li2TiO3 powder prepared by citric acid-nitrate SCS method exhibit very good sinter-ability. In this paper, we performed a systematic study on solution combustion synthesis of nanocrystalline Li2ZrO3 powders utilizing glycine as organic fuel. The outstanding benefits of SCS approach is discussed in relation to powder properties, sintering behavior, microstructure and thermodynamic aspects as well.

sintered pellets was determined by dissolving the pellet in aqua-regia and subsequently analyzing the sample by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Horiba, Germany). Relative density of sintered Li2ZrO3 pellets was measured at room temperature by Archimedes' method. Volumetric shrinkage was calculated by measuring change in dimensions of the pellets before and after sintering.

2. Experimental

= Li2ZrO3 (s) + 2.22 CO2 (g) + 2.555 N2 (g) + 2.775 H2O (g) + 2.5 O2 (g) (φ = 0.5) (R2)

3. Results and discussion 3.1. Thermodynamic aspects and mode of combustion According to reaction (R1), the combustion reaction for each fuel to oxidizer ratio can be expressed as: 2LiNO3 (aq.) + ZrO(NO3)2 (aq.) + 1.11 C2H5NO2 (aq.)

2.1. Synthesis of Li2ZrO3 powder

2LiNO3 (aq.) + ZrO(NO3)2 (aq.) + 1.665 C2H5NO2 (aq.)

The following raw materials were used for the preparation of Li2ZrO3 powder: LiNO3 (lithium nitrate), ZrO(NO3)2 (Zirconium oxynitrate), and C2H5NO2 (glycine). All reagents were of analytical grade and used as received without further purifications. Stoichiometric quantities of LiNO3 and ZrO(NO3)2 were added into deionized water to obtain a clear solution. Subsequently, aqueous solution of glycine was added to form precursor solution. Amount of glycine was varied so as to maintain different fuel-to-oxidizer ratios (φ). The transparent solution was transferred to a preheated hot plate maintained at 90 °C. A K-type thermocouple was appropriately placed to measure temperature of combustion reaction. When solution becomes viscous the temperature of hot plate was increased to ∼200 °C. Within a few minutes, combustion was ignited and self-propagated. A friable and voluminous powder resulted at the end of combustion reaction. The time elapsed from combustion reaction ignition until itsend was carefully noted. The combustion reaction is expressed as:

= Li2ZrO3 (s) + 3.33 CO2 (g) + 2.832 N2 (g) + 4.162 H2O (g) + 1.25 O2 (g) (φ = 0.75) (R3) 2LiNO3 (aq.) + ZrO(NO3)2 (aq.) + 2.22 C2H5NO2 (aq.) = Li2ZrO3 (s) + 4.44 CO2 (g) + 3.11 N2 (g) + 5.55 H2O (g) (φ = 1) (R4) 2LiNO3 (aq.) + ZrO(NO3)2 (aq.) + 2.775 C2H5NO2 (aq.) + 1.25 O2 (g) = Li2ZrO3 (s) + 5.55 CO2 (g) + 3.387 N2 (g) + 6.937 H2O (g) (φ = 1.25) (R5) In the above reactions, φ = 1 implies stoichiometric fuel-to-oxidizer ratio while φ > 1 and φ < 1 implies fuel-rich and fuel-lean conditions, respectively. In stoichiometric condition (R4), glycine is completely oxidized without atmospheric oxygen while in fuel-rich condition (R5) atmospheric oxygen is required for complete oxidation of excess glycine. Using the existing thermodynamic data available in the literature, the standard enthalpy of combustion reaction and adiabatic temperature were calculated for reactions (R2)–(R5). The amount of gases released was also calculated for reactions (R2)–(R5). The enthalpy of reaction and adiabatic temperature specific for each of the four reactions were calculated according to following relations:

2LiNO3 (aq.) + ZrO(NO3)2 (aq.) + 2.22φ C2H5NO2 (aq.) + 5(φ − 1) O2 (g) = Li2ZrO3 (s) + 4.44φ CO2 (g) + (2 + 1.11φ) N2 (g) + 5.55φ H2O (g) (R1) The stoichiometry of the redox reactions was calculated according to the basic concepts of propellant chemistry [47]. In our experiment, four fuel-to-oxidizer ratios, φ = 0.5, 0.75, 1.0 and 1.25 were chosen to delineate the effect of φ on powder properties and mode of SCS process. The as-synthesized powders were further annealed at 600–800 °C to understand effect of temperature on crystallinity of the powder.

0 Δr H298 =

0 0 ) product − ∑ n (Δf H298 ) reac tan t ∑ n (Δf H298

0 Δr H298 =

∫298 ∑ (n cp )products dt

Tad

0 Δr H298 ,CpTad

2.2. Sintering of Li2ZrO3 powders

(R6) (R7)

where, and n are the standard enthalpy of reaction, the heat capacity at constant pressure, the adiabatic temperature and the number of moles, respectively. For calculation of theoretical adiabatic temperature combustion reactions were assumed to be completed and excess fuel was oxidized by atmospheric oxygen. Also, it was assumed that heat is not lost either by radiation or conduction. The dependence of standard enthalpy of reaction, calculated adiabatic temperature, measured temperature and amount of gases released on fuel-to-oxidizer ratios are depicted in Figs.1–3 respectively. The negative values of the standard enthalpies of reaction suggest that, from the thermodynamic point of view, all four reactions are highly exothermic. As seen from Fig.1, the standard enthalpy of reaction increases from reaction (R2) to reaction (R5) with increasing φ suggesting that reaction (R5) is the most exothermic one. Similarly, the adiabatic temperature (Fig. 2) indicates that reaction R5 generates the highest adiabatic temperature (2554 °C), whilst reaction R2 generates the lowest adiabatic temperature (680 °C). On the other hand, gases generated in the combustion process affect

In order to investigate sinter-ability, pellets of 10 mm diameter and 3 mm thickness were fabricated by compacting as-synthesized Li2ZrO3 powders under 300 MPa pressure using hydraulic press without addition of binder. Subsequently, these pellets were sintered in muffle furnace at different temperatures ranging from 800 to 1000 °C for constant dwell time (2 h) in air. Rate of heating (10 °C/min) was kept constant. 2.3. Characterization Phase composition of Li2ZrO3 powder was identified by X-ray diffraction (XRD) (X-ray diffractometer Inel, France) where the X-ray source was Cu-Kα radiation (λ = 1.54056 Å). Particle size was measured by Laser diffraction particle size analyzer CILAS, France. The morphologies of as-synthesized powders and microstructure of sintered pellets were observed by scanning electron microscopy (SEM, Seron, Korea) operating at 20 kV accelerating voltage. Lithium content of 137

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Table 1 Mode of combustion reaction at different φ. φ

Mode of combustion

Duration of combustion reaction (s)

Colour of powder

0.5 0.75 1 1.25

VCS VCS VCS SHS-eruption

12 15 5 20

White White White White

and actual measured temperature is narrow and it gradually widens as φ increases. Also, unlike adiabatic temperature, the measured temperature decreases with increasing φ. This tendency cannot be solely assigned to the heat loss taking place during the reaction but also to the change in mode of combustion reaction in relation to φ. The change in combustion mode was carefully observed visually for all four reactions (R2–R5) and results are summarized in Table 1. It is to be noted that redox reactions corresponding to fuel lean (R2 and R3) and stoichiometric (R4) conditions can proceed to completion without use of atmospheric oxygen. In contrary, the redox reaction corresponding to fuel rich condition (R5) needs atmospheric oxygen to complete combustion reaction. However, supply of oxygen is impeded by sluggish diffusion of oxygen into the reaction mass resulting incomplete combustion of glycine in case of φ > 1. This violates our assumption made during calculation of adiabatic heat i.e. excess glycine was combusted by the atmospheric oxygen and released heat at the same time (R5). Thus in case of φ > 1 a more plausible explanation for large difference between adiabatic and measured maximum temperature is incomplete oxidation of glycine along with the heat loss due to release of gases during combustion process. As given in Table 1, the mode of combustion reaction depends on φ. For φ = 0.5, 0.75 and 1 very violent reactions are visually observed and combustion process belongs to VCS mode. For φ = 1.25, slightly moderate reaction is observed due to change in mode of combustion to SHS mode. Interestingly, the colour of as-synthesized powders was white even for φ > 1 which is different from grey appearance of powder observed by Zhou et al in case of citric acid as fuel. In fact, contrary to citric acid (Zhou et al.), the excess glycine in case of φ > 1 does not undergo endothermic decomposition generating carbon (C). Carbon contamination imparts grey appearance to the powder. Instead, excess glycine decomposes as follows:

Fig. 1. Effect of fuel-to-oxidizer ratio on standard enthalpy of reaction.

Fig. 2. Effect of fuel-to-oxidizer ratio on calculated and measured temperature.

2C2H5NO2 (s) + 4.5 O2(g) = 4CO2(g) + N2(g) + 5H2O(g)

(R8)

The calculated standard Gibbs free energy (-237 Kcal/mol) and enthalpy (−206 Kcal/mol) at 298 K shows that, from thermodynamic point of view, the above reaction is feasible and highly exothermic.

3.2. Phase composition and crystallite size The XRD pattern (Fig.4) reveals Li2ZrO3 exist as two polymorphs in the samples annealed at 700 and 750 °C for fuel-lean, stoichiometric as well as fuel-rich conditions. The assigned peaks are in good agreement with monoclinic and tetragonal phases of Li2ZrO3 as registered in the Joint Committee on the Powder Diffraction Strands Card (JCPDS Card No. 033-0843and 041-0324). The tetragonal phase is not indicated in XRD pattern of sample annealed at 600 °C. This is because at 600 °C tetragonal phase may be present in amorphous form. The existence of metastable tetragonal phase is consistent with eruption mode of combustion reaction (Table 1). In this mode combustion reaction completes within 5–20 sec (Table 1). Consequently, the time as well as energy required fell insufficient for reorganization of molecules to attain equilibrium monoclinic structure. Thus molecules are frozen into the metastable tetragonal phase. Very low intensity peaks corresponding to tetragonal phase indicates that tetragonal phase is minor in the samples. The relative intensities of monoclinic and tetragonal phases vary with fuel to oxidizer ratio as seen in XRD pattern. This is in accordance

Fig. 3. Effect of fuel-to-oxidizer ratio on amount of gases released.

the temperature to a large extent, because large quantity of heat would be carried away by the gas flow. As shown in Fig. 3, the amounts of gases increase with ϕ. In the fuel-rich system, the amount of gases generation is approximately two times more than that in the fuel-lean one. Because of these reasons, the actual measured maximum temperature during all combustion reactions (R2–R5) was much lower than the adiabatic temperature (Fig.2). Initially, the gap between adiabatic 138

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Fig. 4. XRD pattern of Li2ZrO3 powder synthesized at varying fuel-to-oxidizer ratios and temperatures (* = Monoclinic phase, # = Tetragonal phase) i) 600°C ii) 700°C iii) 750°C iv) 800°C.

(β2)hkl = [(β2)measured - (β2)instrumental

with thermodynamic analysis (Fig.1–3) which shows reaction temperature varies with fuel to oxidizer ratio. Consequently, the relative intensities of monoclinic and tetragonal phases also vary. Thus it is possible to tune fuel to oxidizer ratio to obtain pure monoclinic Li2ZrO3 phase. The well crystalline monoclinic polymorph of Li2ZrO3 was obtained in sample annealed at 800 °C as evident from sharp peaks in the XRD pattern (Fig.4) of the sample. It is to be noted that no impurity phases arising from unreacted reagents are observed in XRD pattern of the samples. This indicates that heat generated during combustion reactions (R2-R5) is adequate for conversion of precursor into Li2ZrO3 phase. This result is slightly different from the work of Zhou et al. in which unreacted LiNO3 and TiO2 were observed when citric acid was used as fuel. The crystallite size of Li2ZrO3 was calculated from the XRD peak broadening measured in terms of full width at half maximum (FWHM). It is necessary to subtract instrumental contribution and micro strain to broadening of a Bragg peak before the calculation of crystallite size. To subtract this contribution, XRD pattern of standard LaB6 sample was collected and instrumental broadening was determined in terms of FWHM of LaB6 Bragg peaks. The corrected FWHM (βhkl) corresponding to the diffraction peaks of Li2ZrO3 was estimated using the relation [24]:

(R9)

The average crystallite size was calculated invoking Scherrer’s equation. D = (0.9 λ/ βhkl cosθ)

(R10)

Where, D is crystallite size in nm, λ is wavelength of X-ray used, β is FWHM and θ is peak position. The calculated crystallite size of Li2ZrO3 annealed at 800°C for fuel lean, stoichiometric and fuel rich conditions are shown in Fig.5. As seen in Fig.5, the crystallite size decreases with increase in fuel-to-oxidizer ratio from φ = 0.5 (40 nm) to 0.75(32 nm) and then to 1(18 nm). Subsequently, at φ = 1.25 (fuel rich condition) crystallite size (25 nm) slightly increases. Decrease in crystallite size as φ increases from 0.5 to 0.75 and to 1 is obvious as the measured temperature of combustion reaction (Fig.2) also follows the similar trend. However, tenuous increase in crystallite size may be attributed to change in mode of combustion from VCS to SHS regime at φ = 1.25. In SHS regime, combustion reaction occurs for relatively longer duration as observed in Table 1, consequently the crystallite size increases. Thus, dependence of crystallite size on fuel-to-oxidizer ratio is consequence of many competing factors such as temperature, amount of gases released and mode of combustion. 139

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connectivity and presence of voids which contribute to morphological variations in Li2ZrO3 powder. The powder is highly porous and voluminous due to release of large quantity of gases during the combustion process. Large size gas channels are also evident especially in fuel rich regime (Fig.7d). The agglomerated Li2ZrO3 powder consists of large number of very fine Li2ZrO3 primary particles as evident from Fig.7(a–d). It is to be noted that in conventional solid-state-route, due to high temperature (> 1000°C), longer reaction time (several hours to days) and mild gaseous release, generally large and hard-agglomerated Li2ZrO3 powder is obtained which often results poor sinterability. On the other hand, in SCS using glycine as fuel, although adiabatic temperature is very high, however reaction time is extremely low and heat is dissipated very fast due release of large quantity of gases. Thus, high temperature witnessed by Li2ZrO3 particles is only transient which is in favor of soft agglomeration of Li2ZrO3 particles instead of formation of hard particle aggregates. These soft agglomerates due to very weak inter-particle bonding can be easily broken into primary particles with the help of pestle-mortar or mild ultrasonication before compaction and sintering and thus do not deteriorate sinterability of powder.

Fig. 5. Effect of fuel-to-oxidizer ratio on crystallite size of Li2ZrO3.

3.3. Powder morphology Mean particle size of Li2ZrO3 powder annealed at 800 °C and 1000 °C for fuel lean, stoichiometric and fuel rich conditions as measured by laser diffraction method is shown in Fig.6. It is evident from Fig.6 that irrespective of fuel-to-oxidizer ratio, the particle size is significantly larger than the crystallite size (Fig. 5) estimated by the XRD method. This indicates agglomeration of fine Li2ZrO3 particles due to heat produced by combustion reaction. As we have already discussed, the temperature of reaction depends on fuel-to-oxidizer ratio (Figs.1–3) and thus the degree of agglomeration too. Due to this the measured particle size also changes with fuel to oxidizer ratio (Fig.5). Decrease in particle size with increase in temperature from 800 to 1000 °C is obvious. The degree of agglomeration and its type (soft or hard) is an important parameter controlling sinterability of powder. Therefore, by tuning agglomeration characteristic with the help of optimizing fuel-tooxidizer ratio it is possible to enhance sinterability of Li2ZrO3 powders. The SEM images of Li2ZrO3 powder calcined at 800 °C at different fuel-to-oxidizer ratios are depicted in Fig.7(a–d). Powder displays morphology consisting of micrometric soft-agglomerates of irregular flake shape particles with large variation in size (5–30 μm) as shown in Fig.7(a–d). This is consistent with laser particle size data (Fig.6) which indicated agglomeration of Li2ZrO3 particles as discussed earlier. The variation in morphology is assigned to temperature difference arising due to change in fuel-to-oxidizer ratio. Additionally, there is possibility of temperature variation among different regions within the powder due to poor thermal conductivity of Li2ZrO3, less inter-particle

3.4. Chemical analysis (Li/Zr ratio) and residual Li2O content Mean Li/Zr mole ratio in Li2ZrO3 powder calcined at 1000°C for φ = 0.5, 0.75, 1 and 1.25 was estimated by ICP-AES using aqua-regia as dissolution medium. The estimated value (1.975 ± 0.006) of Li/Zr ratio is in close agreement with value implied by stoichiometry of Li2ZrO3 (i.e. 2). This result is very important as it confirms that no loss of lithium occurred during the synthesis of Li2ZrO3 by proposed SCS method. It is worth mentioning that as Li is a source of tritium, its evaporation at higher temperature is of serious concern in breeder materials. Despite high heat of combustion reaction, due to transient reaction and rapid dissipation of heat by large amount of evolving gases, the loss of lithium is restrained. The unreacted Li2O content in Li2ZrO3 powder calcined at 800°C was determined for different fuel-to-oxidizer ratios by a simple pH measurement method. In this method known quantity (50 mg) of Li2ZrO3 powder was suspended in distilled water for 15 min under mild ultrasonication at ambient temperature. During this period, it was assumed that Li2O impurity due to high solubility dissolves completely into water as following chemical reaction (R11) and thus alkalinity (pH) of water increases. Increase in pH of water was measured by pH meter and from this the Li2O impurity content in Li2ZrO3 powder was determined. Li2O (s) + H2O (l) = 2 LiOH (aq.)

(R11)

In order to confirm that Li2ZrO3 was not hydroxylated during Li2O dissolution, one of the sample was kept in water under the identical conditions for 24 h and XRD pattern of the sample before and after dissolution were recorded. The XRD pattern is shown in Fig.8. Any significant difference in the XRD pattern before and after dissolution has not been observed which confirms that Li2ZrO3 was not hydroxylated during Li2O dissolution. The quantity of Li2O in Li2ZrO3 powder is in the range 155–469 (μg/ g) as shown in Fig.9. It is often difficult to observe such a small quantity by XRD method. Therefore, presence of Li2O was not detected in XRD pattern (Fig.4). The Li2O originates from thermal decomposition of LiNO3 precursor during the combustion process and its quantity relies on several factors including temperature, release of gases, and mode of combustion. This is the reason for variation in Li2O content of Li2ZrO3 powder with fuel-to-oxidizer ratio (Fig.9). It is important to emphasize that the quantity of Li2O affects the tritium breeding performance of Li2ZrO3. During thermal desorption, tritiated water vapor is one of the chemical form in which tritium purges out of the breeder material. This tritiated water vapor chemically reacts with Li2O to form tritiated lithium hydroxide and thereby Li2O chemically traps tritium within

Fig. 6. Effect of fuel-to-oxidizer ratio on mean particle size of Li2ZrO3 powder (T1 = 800°C, T2 = 1000°C). 140

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Fig. 7. SEM images of Li2ZrO3 powder clacined at 800°C and fuel-to-oxidizer ratio a = 0.5, b = 0.75, c = 1, d = 1.25.

Fig. 8. XRD pattern of Li2ZrO3 powder before and after Li2O dissolution (* = monoclinic phase).

Fig. 9. Li2O content in Li2ZrO3 powder annealed at 800°C for different fuel-tooxidizer ratio.

breeder material [14,48–50]. In order to recover tritium from tritiated lithium hydroxide via thermal decomposition, a separate high temperature (> 1000°C) process is required which is disadvantageous in current TBM configuration. It is possible to optimize fuel-to-oxidizer ratio so as to ensure Li2O content as low as possible.

in pellet dimensions before and after sintering, the volumetric shrinkage was calculated at each sintering temperature. Density of the Li2ZrO3 pellets sintered at 900–11,000 °C was determined by Archimedes' principle using water as the immersion medium. The variation of relative density and volumetric shrinkage of Li2ZrO3 pellet as a function of sintering temperature is shown in Fig. 10. The density of the pellets increases with sintering temperature and close to 90% of T.D. is attained at temperature as low as 950 °C. This result is very encouraging in the sense that density close to 90% of T.D. is generally considered sufficient for tritium breeder application. Moreover, achieving this density (90% of T.D.) at temperature less that 1000°C is of paramount importance because in this temperature regime Li-evaporation may not be significant. On increasing sintering temperature, the density steeply rises to ∼98% of T.D. at 1000 °C. The density does not increase significantly on further increasing the sintering temperature to 1050 and 1100 °C (Fig.10). Jean-Daniel Lulewicz et., al., [51]

3.5. Sintering behavior of Li2ZrO3 To investigate sinterabilty, Li2ZrO3 powders synthesized under stoichiometric condition (φ = 1) and annealed at 800 °C were compacted in shape of pellets (10 mm diameter ×3 mm height). The as fabricated pellets were heated for sintering at different temperatures in the range 900–1100 °C for constant dwell time (2 h) and maintaining constant heating rate (10 °C/min). The geometrical dimension of pellets was measured after sintering at different temperatures. From difference 141

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Fig. 12. XRD patterns of Li2ZrO3 pellets after sintering (* = monoclinic phase).

which leads to grain enlargement without significant improvement in density. The volumetric shrinkage (%) of pellets follows similar pattern as that of relative density as a function of sintering temperature. The large volumetric shrinkage may be attributed to a lot of porosity initially present in green pellets. The SEM images of fracture surface of Li2ZrO3 pellets sintered at 900, 950, 1000 and 1050 °C temperature are depicted in Fig. 11(e–h). The SEM images demonstrate that all sintered Li2ZrO3 pellets have uniform grain morphology. This is consistent with the XRD result (Fig.4) which confirmed the formation of pure monoclinic phase of Li2ZrO3 at 800 °C annealing temperature. The pellet sintered at 900 and 950 °C exhibit characteristics of agglomerated sintering and has no fully developed microstructure (Fig.11(e–h)). However, well developed fine grained (grain size: 1–2 μm) microstructure is observed for Li2ZrO3 pellet sintered at 1000 °C as shown in Fig.11g. Further increase of sintering temperature to 1050 °C results abnormal grain growth with

Fig. 10. Effect of sintering temperature on relative density and volumetric shrinkage of Li2ZrO3 pellet.

reported density of sintered pebbles close to 70% of T.D. fabricated using Li2ZrO3 powder synthesized by solid state method. This clearly indicates that Li2ZrO3 powder synthesized by the present method has significantly high degree of sinterability compared to the conventional solid-state method. This densification behavior can be explained in relation to the kinetics of two competing thermally activated processes viz., densification and grain growth occurring during the sintering of Li2ZrO3 pellets at different temperatures. At temperature close to 1000 °C, densification is dominant process which is responsible for rapid densification of Li2ZrO3 pellet around this temperature. However, on further increasing sintering temperature, grain growth dominates

Fig. 11. SEM images of fractured surface of Li2ZrO3 sintered at e) 900°C f) 950°C g) 1000°C h) 1050°C. 142

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large number of closed pores (Fig.11h). The evolution of microstructure with increasing sintering temperature is in complete agreement with the densification curve of Li2ZrO3 pellet (Fig. 10). Because of dominance of densification process over grain growth at 1000 °C sintering temperature, high density and fine grained microstructure is evolved at this temperature (Fig. 11g). Thereafter (> 1000 °C) grain growth dominates which leads to abnormal coarse grained microstructure (Fig.11h) with no improvement in density (Fig10).

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3.6. Post sintering chemical analysis (Li/Zr ratio) and phase analysis of Li2ZrO3 The chemical analysis of Li2ZrO3 pellets after sintering was carried out by procedure already described in section 3.4. Mean Li/Zr mole ratio in Li2ZrO3 pellets sintered at 900–1100 °C for 2 h duration was estimated as value 1.9798 ± 000,602. The Li/Zr mole ratio is very close to the value implied by stoichiometry of Li2ZrO3 (i.e. 2). This result confirms that no significant loss of lithium occurred by sublimation during sintering of the Li2ZrO3 pellets. The XRD patterns of Li2ZrO3 pellets after sintering is presented in Fig. 12. Any significant modification in the XRD pattern has not been observed after sintering except minor variation in relative peak intensities. The peak intensity variations may be due to variation in crystallinity of sample with temperature. 4. Conclusion The paper investigated solution combustion synthesis of nancrystalline Li2ZrO3 powders using glycine as fuel. It is observed that fuel-tooxidizer ratio (φ) is crucial parameter which significantly influenced mode of combustion, phase composition and morphology of Li2ZrO3 powders. Thermodynamic analysis showed, the calculated adiabatic temperature and amount of gases increases with φ. However, due to their synergistic effect the actual measured maximum temperature of combustion reaction decreases with increasing φ and also the mode of combustion switches from VCS to SHS-eruption. Due to transient combustion reaction, minor metastable tetragonal phase is observed along with the equilibrium monoclinic phase of Li2ZrO3. However, post annealing at 800 °C temperature, well crystalline, phase pure monoclinic Li2ZrO3 powder was obtained. The crystallite size of Li2ZrO3 powder was calculated to be in the range of 18–40 nm depending on φ. The particle size considerably larger than crystallite size was observed due to agglomeration of Li2ZrO3 particles during combustion reaction. Powder morphology consists of micrometric soft-agglomerates of irregular flake shape particles. The residual Li2O in Li2ZrO3 powder was estimated to be in the range 155–469 μg/g. The Li2ZrO3 powders exhibit excellent sinterability and can be sintered to achieve density close to 90% of T.D at 950 °C and almost full densification at 1000 °C. Welldeveloped fine-grained microstructure with grain size: 1–2 μm was observed at 1000 °C sintering temperature. Further increase of sintering temperature to 1050 °C observed to be disadvantageous as it results abnormal grain growth with large number of closed pores. This solution combustion method provides an efficient alternative to produce Li2ZrO3 powder with excellent sinterability and thereby overcome critical issues such as Li-evaporation and grain growth often encountered during the processing of high-performance tritium breeder materials. References [1] T.K.E. Proust, L. Anzidei, M. Dalle Donne, U. Fischer, Solid breeder blanket design and tritium breeding, Fusion Eng. Des. 16 (1991) 73–84. [2] N. Roux, C. Johnson, K. Noda, Properties and performance of tritium breeding ceramics, J. Nucl. Mater. 191–194 (1992) 15–22. [3] A. Raffray, M. Akiba, V. Chuyanov, L. Giancarli, S. Malang, Breeding blanket concepts for fusion and materials requirements, J. Nucl. Mater. 307–311 (2002) 21–30. [4] J.G. Van Der Laan, H. Kawamura, N. Roux, D. Yamaki, Ceramic breeder research and development: progress and focus, J. Nucl. Mater. 283 (2000) 99–109.

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