- Email: [email protected]
Synthesis and characterization of lithium silicate powders
Fusion Engineering and Design 84 (2009) 2124–2130
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Synthesis and characterization of lithium silicate powders Tao Tang a,∗ , Zhi Zhang b , Jian-Bo Meng b , De-Li Luo a a b
National Key Laboratory for Surface Physics and Chemistry, P.O. Box 781-35, Mianyang, 621907, Sichuan, China China Academy of Engineering Physics, P. O. Box 919-71, Mianyang, 621900, Sichuan, China
a r t i c l e
i n f o
Article history: Received 13 July 2008 Received in revised form 2 October 2008 Accepted 10 February 2009 Available online 14 March 2009 Keywords: Test blanket module Tritium breeder Lithium silicates Solid-state reaction X-ray diffractometer Scanning electron microscope
a b s t r a c t Lithium-based ceramics, such as Li2 O, LiAlO2 , Li4 SiO4 , Li2 SiO3 , Li2 TiO3 and Li2 ZrO3 , have long been recognized as promising tritium breeding-materials for D-T fusion reactor blankets. Among these candidate materials, lithium orthosilicate (Li4 SiO4 ) and lithium metasilicate (Li2 SiO3 ) are recommended by many ITER research teams as the first selection for the solid tritium breeder. Li4 SiO4 has even been selected as the breeder material for the helium-cooled solid breeder test blanket module (HCSB TBM) in China and EU. In present study, the processes of solid-state reaction between amorphous silica and Li2 CO3 powders was studied by thermogravimetry analysis–differential scanning calorimetry (TGA/DSC); the lithium silicate powders were synthesized at 700, 800 and 900 ◦ C with Li:Si molar ratios of 0.5, 1, 2 and 4, respectively, using solid-state reaction method. The as-prepared lithium silicates were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results show that the phase composition and morphology of the as-prepared samples change with the different synthesis conditions. At low temperature of 700 ◦ C, all samples contain the amorphous silica, and the major crystalline phase is Li2 SiO3 with different microstructure for Li/Si ratio of 0.5, 1 and 2. As for Li/Si = 4, 98% purity of Li4 SiO4 can be obtained at 700 ◦ C. At high temperature of 900 ◦ C, the significant sinterization effect will occur in all samples and Li4 SiO4 will even decompose. The results also show that pure Li4 SiO4 can be synthesized by calcining at 800 ◦ C for 4 h, and its’ solid-state reaction synthesis may be divided into two steps: (1) 515–565 ◦ C: Li2 CO3 + SiO2 → Li2 SiO3 + CO2 ; (2) 565–754 ◦ C: Li2 CO3 + SiO2 → Li2 SiO3 + CO2 and then Li2 SiO3 + Li2 CO3 → Li4 SiO4 + CO2 . While Li/Si = 2, 99% purity of and pure Li2 SiO3 can be obtained at 800 and 900 ◦ C, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The tritium breeding blanket is a key component of the international thermonuclear experimental reactor (ITER) and DEMO reactor as it is directly involved in tritium breeding as well as energy extraction, both critical to the development of fusion energy. In recent years, there is a general agreement that lithium containing ceramics are the best option for tritium production and release through the 6 Li(n, ˛)3 H reaction [1]. These two properties determine the possible application of a tritium breeder materials into the fusion reactors. Different ceramics have been studied as attractive tritium breeder materials, such as lithium oxide (Li2 O), LiAlO2 , lithium titanate (Li2 TiO3 ), lithium zirconates (Li2 ZrO3 and Li8 ZrO6 ), lithium metasilicate (Li2 SiO3 ) and
∗ Corresponding author at: National Key Laboratory for Surface Physics and Chemistry, P.O. Box 718-35, Mianyang, 621907, China. Tel.: +86 816 3626457; fax: +86 816 3625900. E-mail address: [email protected] (T. Tang). 0920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2009.02.017
lithium orthosilicate (Li4 SiO4 ) among the others [2–4]. Several papers have been published about the syntheses, neutron irradiation performance, thermal stability and tritium release of these ceramics [5–13]. All these materials exhibit advantages in safety, lack of electromagnetic effects and tritium release. Among these candidates, lithium silicates (Li4 SiO4 and Li2 SiO3 ) have been demonstrated that these materials present good tritium solubility, they are compatible with other blanket and structural materials and they seem to have adequate thermo-physical, chemical and mechanical stability at high temperatures [8,14]. Actually, Li4 SiO4 has been selected as the breeder material for the heliumcooled solid breeder test blanket module (HCSB TBM) in China and EU. Nevertheless, the preparation of lithium silicate powders is the first step for the further investigations. In previous studies, lithium silicates have been synthesized by various methods [15–22]. For instance, solid mixtures of amorphous silica (SiO2 ) and a lithium compound, such as Li2 CO3 or LiOH [4], can be heated in air for long periods at temperatures between 370 and 1000 ◦ C to produce lithium silicate powders. The reaction between amorphous silica
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
(SiO2 ) suspended in water with aqueous solution of LiOH [7] has also been proposed. More recently, the sol–gel preparation method [17–22] has gained significant interest as a means of obtaining ceramic materials at low temperatures. Moreover, a modified combustion method has also been proposed to synthesize the Li2 SiO3 powder using LiOH, SiO2 · H2 O and urea as reaction precursors [23]. Control of the microstructure must be exercised when preparing the solid tritium breeding ceramics, as the microstructure may determine the rate of tritium release from the blanket [24]. Although numerous new methods have been developed, the conventional solid state reaction method was the dominating way to synthesize the ceramics according to literature because of its following advantages: (1) simple to operate, (2) no special needs for experiment conditions and (3) no external impurity introduced to the final product. In order to prepare the expected pure lithium silicate phase, it is important to investigate the solid-state reaction process in the synthesis of lithium silicates, however, which has seldom been studied even now. The objective of the present work is to investigate solid tritium breeder materials used for fusion reactor. Since this paper is part of a project to investigate the diffusion and release behavior of tritium and helium in the lithium silicate ceramics after neutron irradiation. The objective of the first phase is to prepare Li2 SiO3 and Li4 SiO4 . In present work, the conventional solid-state reaction method is utilized to synthesize the expectant lithium silicates using the mixtures of Li2 CO3 and amorphous silica as the reaction precursors. In order to optimize the synthesis conditions, the thermal analysis method is firstly employed to study the solid-state reaction process and mechanics. The composition and the microstructure of lithium silicate powders prepared at different reaction conditions will then be compared with each other. Based on these results, the optimized reaction conditions can be chosen.
(0.1054 nm) was selected with a diffracted beam monochromator. The resulting lithium silicates were identified by the corresponding Joint Committee on Powder Diffraction Standards (JCPDS) in virtue of MDI Jade 5.0 software. The relative percentages of the various compounds in the ceramics were estimated from the total area under the most intense diffraction peak for each phase identified by the corresponding JCPDS files [25,26]. An Autosorb-1-C surface area analyzer, Quantachrome Instruments, was used to determine the BET surface area of the silicates. All samples used for the surface area measurement were dried in vacuum at 200 ◦ C for 3 h prior to the analysis. The morphology of the crystals constituting the various samples was studied by scanning electron microscopy (SEM) (Philips XL-30). The samples were covered with gold to prevent the lack of conductivity. 3. Results and discussions 3.1. TGA/DSC Fig. 1 shows the result of TGA/DSC for the sample with Li:Si molar ratio of 4. For the sake of briefness, the analysis result behind 175 min is not illustrated in Fig. 1 because the mass loss of system is no more observed after 175 min. In order to identify whether solid state reaction occurs, simultaneously, the TGA/DSC result of Li2 CO3 obtained in same conditions is also shown in the right part of Fig. 1. According to the TGA analysis, the overall mass loss is about 45.78% deduced from the sample weight at the end of 4 h isothermal reaction at 900 ◦ C. However, the theoretical weight decrease is 42.54% for the Li4 SiO4 synthesis via the reaction of Li2 CO3 and SiO2 as 2Li2 CO3 + SiO2 = Li4 SiO4 + 2CO2 ↑
2. Experimental procedures 2.1. Preparation of lithium silicate powders Lithium silicates were prepared using solid-state reaction of mechanical mixtures of amorphous silica (SiO2 ) gel and lithium carbonate (Li2 CO3 ) at three temperature steps of 700, 800 and 900 ◦ C. All reactions were performed with the following Li:Si molar ratios: 0.5, 1, 2 and 4. The amorphous silica and Li2 CO3 were well mixed in a special blender which is designed for the dry powders mixing. In the synthesis of solid-state reaction, the reactants were calcined at 700, 800 and 900 ◦ C in air for 4 h, respectively. 2.2. Characterization techniques A STA449C, thermogravimetric analyzer appending differential scanning calorimetry (TGA/DSC), NETZSCH Instruments, was employed to measure the mass and heat variety of the system during whole the solid-state reaction process, where the integral heat deduced from the DSC is just a referential value and cannot be used in the quantitative analysis. TGA/DSC study was performed in flowing Ar (15 ml/min) from room temperature to 900 ◦ C with heating rate of 10 K/min and then holding at 900 ◦ C for 4 h. The well mixed reactants are pressed in a ∅5 × 3 mm pellet, and then placed at a Pt crucible. For eliminating the effect of instrumental error, a baseline was measured at the same experimental conditions prior to the analysis. TGA/DSC result shown here, consequently, is the real results of sample which has subtracted the baseline from the measured data. The as-prepared Lithium silicates compounds were identified by X-ray diffractometry (XRD). A diffractometer (Model D/max-RB) coupled to a copper-anode X-ray tube was used. The K˛ wavelength
2125
(1)
The reasons for the difference may be commonly thought as the release of water pre-adsorbed by and volatile contaminations in starting materials. From room temperature to 300 ◦ C, the mass loss of system is about 1.53% and it can be attributed to the water desorption. While from 300 to 515 ◦ C, sample losses weight as much as 2.70%, and it should not be the decomposition of Li2 CO3 because the decomposition temperature of Li2 CO3 is about 1230 ◦ C. Also, it cannot be attributed to the reaction between Li2 CO3 with SiO2 since a same weight decrease is measured in Li2 CO3 at this temperature range. As result, it must be the evaporation of volatile contaminations in starting materials. From 515 ◦ C to the end of 4 h isothermal calcination at 900 ◦ C, TGA result shows there exist three phases with mass loss of 4.74, 31.07 and 5.74%, respectively, which are in turn corresponding to two rapid phases of 515 → 565 ◦ C, and 565 → 754 ◦ C, and one slow phase of 754 → end of 4 h calcination at 900 ◦ C. Subtracting the effect of water and contaminates, the actual weight decrease is deduced as 43.38%, i.e. (0.0474 + 0.3107 + 0.0574)/(1 − 0.0153 − 0.027) × 100%, just 0.84% higher than the theoretical mass loss and it will be discussed later. (1) From 515 to 565 ◦ C, the sample weight decreases 4.74% accompanying with an obvious endothermic peak locating at 540 ◦ C. However, even no any weight loss is observed in Li2 CO3 at this temperature range. Therefore, one reaction resulting in release of the gaseous carbon dioxide must take place here. Considering the possible reaction between Li2 CO3 and SiO2 , this reaction may be seen as the formation of lithium metasilicate and release of CO2 (Li2 CO3 + SiO2 = Li2 SiO3 + CO2 ↑). (2) From 565 to 754 ◦ C, the DSC result shows more heat is absorbed by the system. While in the Li2 CO3 , one keen-edged DSC peak is observed at 724 ◦ C which consists with the melting point of Li2 CO3 (about 723 ◦ C). According to the TGA result of the sys-
2126
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
Fig. 1. TG/DSC results for the reaction between Li2 CO3 and SiO2 with Li/Si molar ratio of 4. Heating rate is 10 K/min. Both carrier gas and protection gas are Ar with flowing rate of 15 ml/min.
tem, the weight of system decreases steeply up to about 31.07%. As for Li2 CO3 , a significant mass loss is also measured after 730 ◦ C. However, the weight loss of Li2 CO3 should not result from the CO2 release because its’ decomposition temperature is about 1230 ◦ C, but comes from other reasons, such as the evaporation of Li2 CO3 and the reaction between Li and Pt. Due to no relative XRD analysis for the Li2 CO3 after TGA experiment, it is hard to affirm whether the reaction between Li and Pt occurs. However, when the TGA/DSC experiment for sample pellet with Li/Si molar ratio of 4 finished, it keeps the original shape accompanying with significant volume shrinkage. While for Li2 CO3 , we observed that it was coalescent with Pt crucible. It suggests in a certain extent that the mass loss in Li2 CO3 after 730 ◦ C might result from the molten Li2 CO3 reacting with Pt. Chang et al. [27] had studied the phase evolution of the heat treated powders which were prepared from two different precursors of LiOH ·H2 O and LiNO3 using wet chemistry approach. They observe that lithium orthosilicate is formed via the generation of Li2 SiO3 and some Li2 CO3 is detected at 500 ◦ C. Further heat treatment at 600 ◦ C and above leads to a reduction in the amount of Li2 CO3 due to itself decomposition. However, further decomposition of Li2 CO3 appears to favor the formation of Li2 SiO3 . Subsequent conversion to Li4 SiO4 of Li2 SiO3 is dependent on the decomposition of Li2 CO3 [27]. In present work, accordingly, the reaction in system during this temperature range may be described as Li2 CO3 + SiO2 = Li2 SiO3 + 2CO2 and then Li2 CO3 + Li2 SiO3 = Li4 SiO4 + CO2 . (3) From 754 ◦ C to the end of isothermal reaction at 900 ◦ C for 4 h, the sample mass decreases 5.74% which is not modified to actual value. As mentioned above, after eliminating the effects of water and other contaminates, the real weight losses 43.38% if the mass loss at this phase has been taken into account, only 0.84% higher than theoretical value. It suggests that the solidstate reaction should not finish after 754 ◦ C. That is to say the pure Li4 SiO4 should be obtained through isothermal calcination at the temperature higher than 754 ◦ C. However, TGA/DSC analysis result has given the mass loss difference of 0.84% between experiment and theoretical value, and a spreading endothermic peak is observed in this reaction step. It indicates that an endothermic reaction or phase transition process occurs at this temperature region. This will be discussed in detail whin the XRD analysis results. Accordingly, the synthesis process of Li4 SiO4 using solid-state reaction between Li2 CO3 and SiO2 may be described as the follow-
ing two steps: Li2 CO3 + SiO2 Li2 CO3 + SiO2
515–565 ◦ C
→
Li2 SiO3 + CO2
565–754 ◦ C
Li2 CO3 + Li2 SiO3
→
Li2 SiO3 + CO2
565–754 ◦ C
→
Li4 SiO4 + CO2
(2) (3a) (3b)
3.2. XRD Fig. 2 shows the XRD results of the two reactants: amorphous silica and Li2 CO3 . It reveals that there is no any diffraction peak for the amorphous SiO2 , but the highly crystalline Li2 CO3 is observed. All products obtained by solid-state reaction between different Li/Si molar ratio of Li2 CO3 and SiO2 at different temperature were analyzed by X-ray diffraction. Fig. 3 shows the XRD patterns of the samples obtained at different preparation conditions, in which (A), (B), (C) and (D) represents Li/Si molar ratio of 0.5, 1, 2 and 4 in raw reactant, respectively. Fig. 3(D) also shows the XRD pattern of the reactant mixture with Li/Si molar ratio of 4 before reaction. The results of XRD show that the amorphous silica always exists in all the the product samples prepared at 700 ◦ C. It reveals that
Fig. 2. The XRD patterns of amorphous silica and Li2 CO3 powders.
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
2127
Fig. 3. XRD patterns of as-prepared powders for different Li/Si molar ratio and reaction temperature. (A, B, C and D represents Li/Si molar ratio of 0.5, 1, 2 and 4, respectively; 1-Li2 SiO3 , 2-Li2 Si2 O5 , 3-Li4 SiO4 , 4-cristobalite, 5-quartz and 6- Li2 CO3 ).
amorphous silica can neither transform to the crystalline SiO2 nor completely react with Li2 CO3 at the low temperature as 700 ◦ C. The amount of amorphous silica decreases with the increasing Li/Si ratio in reactant mainly because the initial content of silica gel in reactant mixtures reduces. For the sample prepared at 700 ◦ C with Li/Si ratio of 0.5, see Fig. 3(A), a few of Li2 SiO3 and Li2 Si2 O5 can be detected and the former is the major crystalline phase. It is very difficult to identify whether the Li2 CO3 exists in the sample because both the degree of crystallization and the content of Li2 CO3 are so low that its’ X-ray diffraction characteristic line may be enshrouded by the amorphous silica. When the synthesization temperature increases, the content of Li2 Si2 O5 increases while Li2 SiO3 decreases and even disappears at 900 ◦ C. At the same time, crystalline SiO2 emerges as silicon oxide (quartz) at 800 ◦ C, and then transforms into cristobalite at 900 ◦ C. It means that the surplus amorphous silica will become into crystalline silica and furthermore take place phase transformation at higher temperature. From the Fig. 3(A), the main phase alters from Li2 SiO3 at 700 ◦ C to Li2 Si2 O5 at 800 ◦ C and crystalline SiO2 (cristobalite) at higher temperature of 900 ◦ C. For Li/Si = 1, see Fig. 3(B), the main crystalline phase Li2 SiO3 and considerable amorphous silica can be observed in the sample obtained at 700 ◦ C. Simultaneously, a small quantity of Li2 Si2 O5 is also detected. While at 800 ◦ C, almost equal contents of Li2 Si2 O5 and Li2 SiO3 are observed and a few quartz can be measured. At 900 ◦ C, the content of Li2 Si2 O5 increases to 86%, the Li2 SiO3 and cristobalite are still detected in the final sample. It agrees with the
result reported by Pfeiffer et al. [8] that Li2 SiO3 , Li2 Si2 O5 and quartz are the final products. However, they observed the main phase was Li2 SiO3 , and thought it should came from the smaller amount of Si presenting in the reactant [8]. Nevertheless, according to the phase diagram of Li2 O–SiO2 system [28], the Li2 Si2 O5 is stable at constant temperature when the 66.7 mol% SiO2 exists in this binary system. Consequently, pure Li2 Si2 O5 should be obtained in theory when the molar ratio of Li2 CO3 /SiO2 is 0.5 (i.e. Li/Si = 1) in reactants. Although the pure Li2 Si2 O5 is not obtained in present work, it is seen to be the major phase should be more reasonable comparing with the results given by Ref. [8]. When the Li/Si ratio increases to 2, see Fig. 3(C), a few Li2 CO3 , Li2 Si2 O5 and amorphous silica are detected in the samples obtained at 700 ◦ C. The main crystalline phase of Li2 SiO3 is identified to consist with the JCPDS file 74-2145. While at 800 and 900 ◦ C, the pure Li2 SiO3 is obtained and identified to accord with the JCPDS file 29-0829. The cell parameters are refined utilizing MDI Jade 5.0 software. Li2 SiO3 presents C-center Cmc21 space group (No. 36), and its’ lattice parameters are a = 9.392 Å, b = 5.398 Å, c = 4.672 Å for Li2 SiO3 obtained at 800 ◦ C and a = 9.416 Å, b = 5.409 Å, c = 4.671 Å for that synthesized at 900 ◦ C, respectively. These lattice constants are well consistent with the reported values [29]. However, it can be observed that the cell slightly expands when the sample prepared at 900 ◦ C. This should be so more serious sinterization effect accompanying with the growth of grains during long time higher temperature process that the stress increases and accordingly induces lattice expanding. Comparing their FWHM of
2128
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
Table 1 The phase composition (vol.%) of as-prepared sample. Temperature (◦ C)
Amorphous silica
0.5
700 800 900
84 42 0
0 0 0
0 13 43
16 11 0
0 34 57
0 0 0
1.0
700 800 900
60 34 0
0 0 0
0 6 4
33 26 11
7 34 86
0 0 0
2.0
700 800 900
38 0 0
17 0 0
0 0 0
43 99 100
2 1 0
0 0 0
4.0
700 800 900
0 0 0
<1 0 0
0 0 0
0 0 2
0 0 0
∼ 100 100 98
Li/Si
Li2 CO3
the strongest diffraction peak, it is broadened for those obtained at 800 ◦ C whose grain size is about 0.6 m, smaller than 0.8 m for 900 ◦ C. For the Li/Si molar ratio of 4, see Fig. 3(D), the major phase lithium orthosilicate containing Li2 CO3 and amorphous silica is detected for the sample obtained at 700 ◦ C. The content of expected lithium orthosilicate is 98%. According to the TGA result, the Li4 SiO4 starts formation from 565 ◦ C (Eqs. (3a) and (3b)), and this conversion will be accelerated with the increasing reaction temperature and will not finish till the temperature up to 754 ◦ C.
Quartz
Li2 SiO3
Li2 Si2 O5
Li4 SiO4
After that, there is, yet, about 5.74% mass loss. Therefore, both the XRD and TGA results indicate that pure Li4 SiO4 cannot be synthesized at 700 ◦ C even calcining for 4 h. At 800 ◦ C, only pure Li4 SiO4 phase is detected. The X-ray diffraction peaks are well consistent with the JCPDS file 37-1472. After structure refinement with the same method as above, its’ space group is P21 /m and lattice parameters are a = 5.303 Å, b = 6.113 Å, c = 5.154 Å, and ˇ = 90.33◦ . The grain size is deduced as 0.8 m. When the temperature is up to 900 ◦ C and isothermal calcination for 4 h, a very small amount of Li2 SiO3 is observed in the final product. However, Li2 O, LiOH or
Fig. 4. SEM micrographs of the as-prepared lithium silicate powders. Where A, B and C represents the synthesis reaction temperature of 700, 800 and 900 ◦ C, respectively; 1, 2, 3 and 4 denotes the Li:Si molar ratio of 0.5, 1, 2 and 4, respectively, in reactant.
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130 Table 2 BET specific surface area of as-prepared powders obtained at different condition (cm2 /g). Temperature (◦ C)
700 800 900
Li/Si molar ratios 0.5
1.0
2.0
4.0
76.60 22.16 3.54
48.52 12.11 1.08
18.12 2.42 1.24
9.14 6.30 1.31
LiOH ·H2 O can barely be found utilizing the XRD. Combining with the TGA/DSC result (see Fig. 1), a widened endothermic peak has been observed in this reaction phase. It indicates that an endothermal reaction process takes place during this temperature range and one of the products must be Li2 SiO3 . In fact, several authors have reported that lithium ceramics begin to lose lithium at temperature higher than 800 ◦ C [6,30,31]. However, at this temperature, there was only a small quantity of lithium that sublimated as Li2 O. Cruz et al. [32] had demonstrated that Li4 SiO4 began to decompose into small amounts of Li2 SiO3 (8%) after 30 min at 900 ◦ C. This behavior was constant up to 8 h of thermal treatment. Actually, only lithium present at the surface of the Li4 SiO4 particles must sublimate. In present work, the experiment mass loss is 0.84% higher than theoretical value, it should be attributed to the loss of Li2 O and this process takes place according to Li4 SiO4 →Li2 SiO3 + Li2 O(g)
(4)
Here, the sublimation of Li2 O at 900 ◦ C can well be used to explain why LiOH or LiOH ·H2 O cannot be measured in the sample. The contents of each phase and amorphous silica in the asprepared powders are shown in Table 1. 3.3. Specific surface area The BET specific surface areas of all samples were measured by nitrogen gas sorption method at liquid nitrogen temperature. Table 2 shows the BET specific surface area for all as-prepared samples. The results show that the surface area decreases with the increasing Li/Si molar ratio for the sample obtained at the same reaction temperature, and decreases with the increasing temperature for the same Li/Si molar ratio. This also can be demonstrated by the SEM morphology analysis described as following. 3.4. SEM All samples were studied by SEM. The micrograph of the lithium silicates demonstrate the crystal morphology differences due to the synthesis temperatures and the Li:Si molar ratio. Fig. 4 shows the representative SEM images of the samples obtained at different conditions, in which A, B and C represents the synthesis reaction temperature of 700, 800 and 900 ◦ C, respectively; 1, 2, 3 and 4 denotes the Li:Si molar ratio of 0.5, 1, 2 and 4, respectively, in reactant. For example, the SEM image labeled A-1 denotes the microstructure of the sample prepared at 700 ◦ C for the Li/Si ratio of 0.5. The rest may be deduced by analogy. For the sample with Li:Si ratio of 0.5 obtained at 700 ◦ C (see A-1 in Fig. 4), a lot of amorphous silica still exists and keeps their initial microstructure. Some homogeneous spherical crystals, having very smooth surface with a mean diameter of 2 m, are identified as crystalline phase of Li2 SiO3 by XRD. They are interspersed on the surface of amorphous silica. While calcination temperature increasing, the particles sphericity and the content of amorphous silica decreases (see B-1 and C-1 in Fig. 4). At 800 ◦ C, the spherical and polyhedral particles co-exist with the amorphous silica and the surface of spherical particles gets coarser. At 900 ◦ C, the sample
2129
shows non-homogeneous polyhedral agglomerates. The spherical particles and amorphous silica disappear. For Li/Si ratio of 1, the surface of spherical particles becomes corrugated and coalescent. It is hard to observe the separated spherical particle. While Li/Si = 2, the spherical particles is observed in the sample obtained at 700 ◦ C, however, the sphericity decreases. When the temperature is up to 800 ◦ C, see B-3 in Fig. 4, the homogeneous and coralloid crystal is observed. While at 900 ◦ C, see C-3 in Fig. 4, the particles show the obvious agglomeration. When the Li/Si molar ratio is up to 4, the particles show nonhomogeneous polyhedral crystals with the side length of about 30 m. 4. Conclusion As the first part of investigation for the tritium and helium diffusion and release behavior in lithium silicates, the lithium silicates powders have been synthesized using solid-state reaction between lithium carbonate with amorphous silica, and characterized with TGA/DSC, XRD, SEM and BET surface analysis technologies. For various Li:Si molar ratios in the mixtures of Li2 CO3 and amorphous silica, after a calcining treatment at 700, 800 and 900 ◦ C for 4 h, the solid-state reaction method produced a mixture of lithium silicates and quartz or single lithium silicate phase. The TGA study for the sample with Li/Si molar ratio of 4 shows that the solid-state reaction between lithium carbonate and amorphous silica powders consists of two steps as the formation of Li2 SiO3 , and the conversion of Li2 SiO3 to Li4 SiO4 . For the sample with Li:Si molar ratio of 0.5, 1 and 2 obtained at 700 ◦ C, the major crystalline phase is Li2 SiO3 , and its’ content increases with the increasing of Li/Si molar ratio. Pure Li2 Si2 O5 cannot be prepared when Li/Si = 1 in present work, but the pure Li2 SiO3 can be obtained at 900 ◦ C when Li/Si = 1. While for Li/Si = 4, the Li4 SiO4 with a purity of 98% can be obtained at 700 ◦ C. When the calcination temperature increases to 900 ◦ C, Li4 SiO4 will then decomposed into Li2 SiO3 and gaseous Li2 O. The ideal synthesis temperature for pure Li2 SiO3 and Li4 SiO4 is 900 and 800 ◦ C, respectively, in present experimental conditions. The SEM study shows that crystal morphology changes with the synthesis temperature, the initial Li/Si molar ratio in reactants and with the formed lithium silicates. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
A. Klix, Y. Verzilov, K. Ochiai, et al., Fusion Eng. Des. 72 (2005) 327. C.E. Johnson, K. Noda, N. Roux, J. Nucl. Mater. 263 (1998) 140. C.E. Johnson, J. Nucl. Mater. 270 (1999) 212. M. Taddia, P. Modesti, A. Albertazzi, J. Nucl. Mater. 336 (2005) 173. C. Alvani, S. Casadio, V. Contini, et al., J. Nucl. Mater. 307 (2002) 837. H. Pfeiffer, K.M. Knowles, J. Eur. Ceram. Soc. 24 (2004) 2433. N. Roux, C. Johnson, K. Noda, J. Nucl. Mater. 191C194 (1992) 15. H. Pfeiffer, P. Bosch, S. Bulbulian, J. Nucl. Mater. 257 (1998) 309. F. Li, K. Hu, J. Li, et al., J. Nucl. Mater. 300 (2002) 82. D. Cruz, S. Bulbulian, J. Nucl. Mater. 312 (2003) 262. J.J. Kingsley, L.R. Pederson, Mater. Lett. 18 (1993) 89. L.A. Chick, L.R. Pederson, G.D. Maupin, et al., Mater. Lett. 10 (1990) 6. J.G. van der Laan, H. Kawamura, N. Roux, et al., J. Nucl. Mater. 283 (2000) 99. A.R. Raffray, M.C. Billone, G. Federici, et al., Fusion Eng. Des. 28 (1995) 240. C.E. Johnson, K.R. Kummerer, E. Roth, J. Nucl. Mater. 155–157 (1988) 188. D. Vollath, H. Wedemeyer, E. Gunther, J. Nucl. Mater. 133&134 (1985) 221. L.C. Klein, Solid State Ionics 32&33 (1988) 639. P. Li, B.A. Ferguson, L.F. Francis, J. Mater. Sci. 30 (1995) 4076. M.A. Valenzuela, J.J. Becerril, P. Bosch, et al., J. Am. Ceram. Soc. 79 (1996) 455. L.C. Klein, C. Yu, R. Woodman, et al., Catal. Today 14 (1992) 165. T. Lopez, P. Bosch, M. Asomoza, et al., J. Mater. Synth. Process. 2 (1994) 99. ´ O. Rodrlguez, F. Gonzalez, P. Bosch, et al., Catal. Today 14 (1992) 243. D. Cruz, S. Bulbulian, J. Nucl. Mater. 312 (2003) 262. C.W. Turner, B.C. Clatworthy, A.Y.H. Gin, Symposium of Fabrication and Properties of Lithium Ceramics, Pittsburgh, PA, pp. 26–30, April 1987. J.J. Becerril, P. Bosch, S. Bulbulian, J. Nucl. Mater. 185 (1991) 304. H.P. Klug, L.E. Alexander, X-Ray Diffraction Procedures, John Wiley, New York, London, 1954.
2130
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
[27] C.-C. Chang, C.C. Wang, P.N. Kumta, Mater. Des. 22 (2001) 617. [28] A. Skokan, H. Wedemeyer, D. Vollath, et al., Proceedings of the 14th Symposium on Fusion Technology, Pergamon, Avignon, 1986, p. 1255. [29] K. Hesse, Acta Crystallogr. B. 33 (1977) 901.
[30] C.H. Lu, L. Wei-Cheng, J. Mater. Chem. 10 (2000) 1403. [31] K. Essaki, K. Nakagawa, M. Kato, H. Uemoto, J. Chem. Eng. Jpn. 37 (2004) 772. [32] D. Cruza, S. Bulbuliana, E. Limac, et al., J. Solid State Chem. 179 (2006) 909.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Synthesis and characterization of lithium silicate powders Tao Tang a,∗ , Zhi Zhang b , Jian-Bo Meng b , De-Li Luo a a b
National Key Laboratory for Surface Physics and Chemistry, P.O. Box 781-35, Mianyang, 621907, Sichuan, China China Academy of Engineering Physics, P. O. Box 919-71, Mianyang, 621900, Sichuan, China
a r t i c l e
i n f o
Article history: Received 13 July 2008 Received in revised form 2 October 2008 Accepted 10 February 2009 Available online 14 March 2009 Keywords: Test blanket module Tritium breeder Lithium silicates Solid-state reaction X-ray diffractometer Scanning electron microscope
a b s t r a c t Lithium-based ceramics, such as Li2 O, LiAlO2 , Li4 SiO4 , Li2 SiO3 , Li2 TiO3 and Li2 ZrO3 , have long been recognized as promising tritium breeding-materials for D-T fusion reactor blankets. Among these candidate materials, lithium orthosilicate (Li4 SiO4 ) and lithium metasilicate (Li2 SiO3 ) are recommended by many ITER research teams as the first selection for the solid tritium breeder. Li4 SiO4 has even been selected as the breeder material for the helium-cooled solid breeder test blanket module (HCSB TBM) in China and EU. In present study, the processes of solid-state reaction between amorphous silica and Li2 CO3 powders was studied by thermogravimetry analysis–differential scanning calorimetry (TGA/DSC); the lithium silicate powders were synthesized at 700, 800 and 900 ◦ C with Li:Si molar ratios of 0.5, 1, 2 and 4, respectively, using solid-state reaction method. The as-prepared lithium silicates were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results show that the phase composition and morphology of the as-prepared samples change with the different synthesis conditions. At low temperature of 700 ◦ C, all samples contain the amorphous silica, and the major crystalline phase is Li2 SiO3 with different microstructure for Li/Si ratio of 0.5, 1 and 2. As for Li/Si = 4, 98% purity of Li4 SiO4 can be obtained at 700 ◦ C. At high temperature of 900 ◦ C, the significant sinterization effect will occur in all samples and Li4 SiO4 will even decompose. The results also show that pure Li4 SiO4 can be synthesized by calcining at 800 ◦ C for 4 h, and its’ solid-state reaction synthesis may be divided into two steps: (1) 515–565 ◦ C: Li2 CO3 + SiO2 → Li2 SiO3 + CO2 ; (2) 565–754 ◦ C: Li2 CO3 + SiO2 → Li2 SiO3 + CO2 and then Li2 SiO3 + Li2 CO3 → Li4 SiO4 + CO2 . While Li/Si = 2, 99% purity of and pure Li2 SiO3 can be obtained at 800 and 900 ◦ C, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The tritium breeding blanket is a key component of the international thermonuclear experimental reactor (ITER) and DEMO reactor as it is directly involved in tritium breeding as well as energy extraction, both critical to the development of fusion energy. In recent years, there is a general agreement that lithium containing ceramics are the best option for tritium production and release through the 6 Li(n, ˛)3 H reaction [1]. These two properties determine the possible application of a tritium breeder materials into the fusion reactors. Different ceramics have been studied as attractive tritium breeder materials, such as lithium oxide (Li2 O), LiAlO2 , lithium titanate (Li2 TiO3 ), lithium zirconates (Li2 ZrO3 and Li8 ZrO6 ), lithium metasilicate (Li2 SiO3 ) and
∗ Corresponding author at: National Key Laboratory for Surface Physics and Chemistry, P.O. Box 718-35, Mianyang, 621907, China. Tel.: +86 816 3626457; fax: +86 816 3625900. E-mail address: [email protected] (T. Tang). 0920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2009.02.017
lithium orthosilicate (Li4 SiO4 ) among the others [2–4]. Several papers have been published about the syntheses, neutron irradiation performance, thermal stability and tritium release of these ceramics [5–13]. All these materials exhibit advantages in safety, lack of electromagnetic effects and tritium release. Among these candidates, lithium silicates (Li4 SiO4 and Li2 SiO3 ) have been demonstrated that these materials present good tritium solubility, they are compatible with other blanket and structural materials and they seem to have adequate thermo-physical, chemical and mechanical stability at high temperatures [8,14]. Actually, Li4 SiO4 has been selected as the breeder material for the heliumcooled solid breeder test blanket module (HCSB TBM) in China and EU. Nevertheless, the preparation of lithium silicate powders is the first step for the further investigations. In previous studies, lithium silicates have been synthesized by various methods [15–22]. For instance, solid mixtures of amorphous silica (SiO2 ) and a lithium compound, such as Li2 CO3 or LiOH [4], can be heated in air for long periods at temperatures between 370 and 1000 ◦ C to produce lithium silicate powders. The reaction between amorphous silica
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
(SiO2 ) suspended in water with aqueous solution of LiOH [7] has also been proposed. More recently, the sol–gel preparation method [17–22] has gained significant interest as a means of obtaining ceramic materials at low temperatures. Moreover, a modified combustion method has also been proposed to synthesize the Li2 SiO3 powder using LiOH, SiO2 · H2 O and urea as reaction precursors [23]. Control of the microstructure must be exercised when preparing the solid tritium breeding ceramics, as the microstructure may determine the rate of tritium release from the blanket [24]. Although numerous new methods have been developed, the conventional solid state reaction method was the dominating way to synthesize the ceramics according to literature because of its following advantages: (1) simple to operate, (2) no special needs for experiment conditions and (3) no external impurity introduced to the final product. In order to prepare the expected pure lithium silicate phase, it is important to investigate the solid-state reaction process in the synthesis of lithium silicates, however, which has seldom been studied even now. The objective of the present work is to investigate solid tritium breeder materials used for fusion reactor. Since this paper is part of a project to investigate the diffusion and release behavior of tritium and helium in the lithium silicate ceramics after neutron irradiation. The objective of the first phase is to prepare Li2 SiO3 and Li4 SiO4 . In present work, the conventional solid-state reaction method is utilized to synthesize the expectant lithium silicates using the mixtures of Li2 CO3 and amorphous silica as the reaction precursors. In order to optimize the synthesis conditions, the thermal analysis method is firstly employed to study the solid-state reaction process and mechanics. The composition and the microstructure of lithium silicate powders prepared at different reaction conditions will then be compared with each other. Based on these results, the optimized reaction conditions can be chosen.
(0.1054 nm) was selected with a diffracted beam monochromator. The resulting lithium silicates were identified by the corresponding Joint Committee on Powder Diffraction Standards (JCPDS) in virtue of MDI Jade 5.0 software. The relative percentages of the various compounds in the ceramics were estimated from the total area under the most intense diffraction peak for each phase identified by the corresponding JCPDS files [25,26]. An Autosorb-1-C surface area analyzer, Quantachrome Instruments, was used to determine the BET surface area of the silicates. All samples used for the surface area measurement were dried in vacuum at 200 ◦ C for 3 h prior to the analysis. The morphology of the crystals constituting the various samples was studied by scanning electron microscopy (SEM) (Philips XL-30). The samples were covered with gold to prevent the lack of conductivity. 3. Results and discussions 3.1. TGA/DSC Fig. 1 shows the result of TGA/DSC for the sample with Li:Si molar ratio of 4. For the sake of briefness, the analysis result behind 175 min is not illustrated in Fig. 1 because the mass loss of system is no more observed after 175 min. In order to identify whether solid state reaction occurs, simultaneously, the TGA/DSC result of Li2 CO3 obtained in same conditions is also shown in the right part of Fig. 1. According to the TGA analysis, the overall mass loss is about 45.78% deduced from the sample weight at the end of 4 h isothermal reaction at 900 ◦ C. However, the theoretical weight decrease is 42.54% for the Li4 SiO4 synthesis via the reaction of Li2 CO3 and SiO2 as 2Li2 CO3 + SiO2 = Li4 SiO4 + 2CO2 ↑
2. Experimental procedures 2.1. Preparation of lithium silicate powders Lithium silicates were prepared using solid-state reaction of mechanical mixtures of amorphous silica (SiO2 ) gel and lithium carbonate (Li2 CO3 ) at three temperature steps of 700, 800 and 900 ◦ C. All reactions were performed with the following Li:Si molar ratios: 0.5, 1, 2 and 4. The amorphous silica and Li2 CO3 were well mixed in a special blender which is designed for the dry powders mixing. In the synthesis of solid-state reaction, the reactants were calcined at 700, 800 and 900 ◦ C in air for 4 h, respectively. 2.2. Characterization techniques A STA449C, thermogravimetric analyzer appending differential scanning calorimetry (TGA/DSC), NETZSCH Instruments, was employed to measure the mass and heat variety of the system during whole the solid-state reaction process, where the integral heat deduced from the DSC is just a referential value and cannot be used in the quantitative analysis. TGA/DSC study was performed in flowing Ar (15 ml/min) from room temperature to 900 ◦ C with heating rate of 10 K/min and then holding at 900 ◦ C for 4 h. The well mixed reactants are pressed in a ∅5 × 3 mm pellet, and then placed at a Pt crucible. For eliminating the effect of instrumental error, a baseline was measured at the same experimental conditions prior to the analysis. TGA/DSC result shown here, consequently, is the real results of sample which has subtracted the baseline from the measured data. The as-prepared Lithium silicates compounds were identified by X-ray diffractometry (XRD). A diffractometer (Model D/max-RB) coupled to a copper-anode X-ray tube was used. The K˛ wavelength
2125
(1)
The reasons for the difference may be commonly thought as the release of water pre-adsorbed by and volatile contaminations in starting materials. From room temperature to 300 ◦ C, the mass loss of system is about 1.53% and it can be attributed to the water desorption. While from 300 to 515 ◦ C, sample losses weight as much as 2.70%, and it should not be the decomposition of Li2 CO3 because the decomposition temperature of Li2 CO3 is about 1230 ◦ C. Also, it cannot be attributed to the reaction between Li2 CO3 with SiO2 since a same weight decrease is measured in Li2 CO3 at this temperature range. As result, it must be the evaporation of volatile contaminations in starting materials. From 515 ◦ C to the end of 4 h isothermal calcination at 900 ◦ C, TGA result shows there exist three phases with mass loss of 4.74, 31.07 and 5.74%, respectively, which are in turn corresponding to two rapid phases of 515 → 565 ◦ C, and 565 → 754 ◦ C, and one slow phase of 754 → end of 4 h calcination at 900 ◦ C. Subtracting the effect of water and contaminates, the actual weight decrease is deduced as 43.38%, i.e. (0.0474 + 0.3107 + 0.0574)/(1 − 0.0153 − 0.027) × 100%, just 0.84% higher than the theoretical mass loss and it will be discussed later. (1) From 515 to 565 ◦ C, the sample weight decreases 4.74% accompanying with an obvious endothermic peak locating at 540 ◦ C. However, even no any weight loss is observed in Li2 CO3 at this temperature range. Therefore, one reaction resulting in release of the gaseous carbon dioxide must take place here. Considering the possible reaction between Li2 CO3 and SiO2 , this reaction may be seen as the formation of lithium metasilicate and release of CO2 (Li2 CO3 + SiO2 = Li2 SiO3 + CO2 ↑). (2) From 565 to 754 ◦ C, the DSC result shows more heat is absorbed by the system. While in the Li2 CO3 , one keen-edged DSC peak is observed at 724 ◦ C which consists with the melting point of Li2 CO3 (about 723 ◦ C). According to the TGA result of the sys-
2126
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
Fig. 1. TG/DSC results for the reaction between Li2 CO3 and SiO2 with Li/Si molar ratio of 4. Heating rate is 10 K/min. Both carrier gas and protection gas are Ar with flowing rate of 15 ml/min.
tem, the weight of system decreases steeply up to about 31.07%. As for Li2 CO3 , a significant mass loss is also measured after 730 ◦ C. However, the weight loss of Li2 CO3 should not result from the CO2 release because its’ decomposition temperature is about 1230 ◦ C, but comes from other reasons, such as the evaporation of Li2 CO3 and the reaction between Li and Pt. Due to no relative XRD analysis for the Li2 CO3 after TGA experiment, it is hard to affirm whether the reaction between Li and Pt occurs. However, when the TGA/DSC experiment for sample pellet with Li/Si molar ratio of 4 finished, it keeps the original shape accompanying with significant volume shrinkage. While for Li2 CO3 , we observed that it was coalescent with Pt crucible. It suggests in a certain extent that the mass loss in Li2 CO3 after 730 ◦ C might result from the molten Li2 CO3 reacting with Pt. Chang et al. [27] had studied the phase evolution of the heat treated powders which were prepared from two different precursors of LiOH ·H2 O and LiNO3 using wet chemistry approach. They observe that lithium orthosilicate is formed via the generation of Li2 SiO3 and some Li2 CO3 is detected at 500 ◦ C. Further heat treatment at 600 ◦ C and above leads to a reduction in the amount of Li2 CO3 due to itself decomposition. However, further decomposition of Li2 CO3 appears to favor the formation of Li2 SiO3 . Subsequent conversion to Li4 SiO4 of Li2 SiO3 is dependent on the decomposition of Li2 CO3 [27]. In present work, accordingly, the reaction in system during this temperature range may be described as Li2 CO3 + SiO2 = Li2 SiO3 + 2CO2 and then Li2 CO3 + Li2 SiO3 = Li4 SiO4 + CO2 . (3) From 754 ◦ C to the end of isothermal reaction at 900 ◦ C for 4 h, the sample mass decreases 5.74% which is not modified to actual value. As mentioned above, after eliminating the effects of water and other contaminates, the real weight losses 43.38% if the mass loss at this phase has been taken into account, only 0.84% higher than theoretical value. It suggests that the solidstate reaction should not finish after 754 ◦ C. That is to say the pure Li4 SiO4 should be obtained through isothermal calcination at the temperature higher than 754 ◦ C. However, TGA/DSC analysis result has given the mass loss difference of 0.84% between experiment and theoretical value, and a spreading endothermic peak is observed in this reaction step. It indicates that an endothermic reaction or phase transition process occurs at this temperature region. This will be discussed in detail whin the XRD analysis results. Accordingly, the synthesis process of Li4 SiO4 using solid-state reaction between Li2 CO3 and SiO2 may be described as the follow-
ing two steps: Li2 CO3 + SiO2 Li2 CO3 + SiO2
515–565 ◦ C
→
Li2 SiO3 + CO2
565–754 ◦ C
Li2 CO3 + Li2 SiO3
→
Li2 SiO3 + CO2
565–754 ◦ C
→
Li4 SiO4 + CO2
(2) (3a) (3b)
3.2. XRD Fig. 2 shows the XRD results of the two reactants: amorphous silica and Li2 CO3 . It reveals that there is no any diffraction peak for the amorphous SiO2 , but the highly crystalline Li2 CO3 is observed. All products obtained by solid-state reaction between different Li/Si molar ratio of Li2 CO3 and SiO2 at different temperature were analyzed by X-ray diffraction. Fig. 3 shows the XRD patterns of the samples obtained at different preparation conditions, in which (A), (B), (C) and (D) represents Li/Si molar ratio of 0.5, 1, 2 and 4 in raw reactant, respectively. Fig. 3(D) also shows the XRD pattern of the reactant mixture with Li/Si molar ratio of 4 before reaction. The results of XRD show that the amorphous silica always exists in all the the product samples prepared at 700 ◦ C. It reveals that
Fig. 2. The XRD patterns of amorphous silica and Li2 CO3 powders.
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
2127
Fig. 3. XRD patterns of as-prepared powders for different Li/Si molar ratio and reaction temperature. (A, B, C and D represents Li/Si molar ratio of 0.5, 1, 2 and 4, respectively; 1-Li2 SiO3 , 2-Li2 Si2 O5 , 3-Li4 SiO4 , 4-cristobalite, 5-quartz and 6- Li2 CO3 ).
amorphous silica can neither transform to the crystalline SiO2 nor completely react with Li2 CO3 at the low temperature as 700 ◦ C. The amount of amorphous silica decreases with the increasing Li/Si ratio in reactant mainly because the initial content of silica gel in reactant mixtures reduces. For the sample prepared at 700 ◦ C with Li/Si ratio of 0.5, see Fig. 3(A), a few of Li2 SiO3 and Li2 Si2 O5 can be detected and the former is the major crystalline phase. It is very difficult to identify whether the Li2 CO3 exists in the sample because both the degree of crystallization and the content of Li2 CO3 are so low that its’ X-ray diffraction characteristic line may be enshrouded by the amorphous silica. When the synthesization temperature increases, the content of Li2 Si2 O5 increases while Li2 SiO3 decreases and even disappears at 900 ◦ C. At the same time, crystalline SiO2 emerges as silicon oxide (quartz) at 800 ◦ C, and then transforms into cristobalite at 900 ◦ C. It means that the surplus amorphous silica will become into crystalline silica and furthermore take place phase transformation at higher temperature. From the Fig. 3(A), the main phase alters from Li2 SiO3 at 700 ◦ C to Li2 Si2 O5 at 800 ◦ C and crystalline SiO2 (cristobalite) at higher temperature of 900 ◦ C. For Li/Si = 1, see Fig. 3(B), the main crystalline phase Li2 SiO3 and considerable amorphous silica can be observed in the sample obtained at 700 ◦ C. Simultaneously, a small quantity of Li2 Si2 O5 is also detected. While at 800 ◦ C, almost equal contents of Li2 Si2 O5 and Li2 SiO3 are observed and a few quartz can be measured. At 900 ◦ C, the content of Li2 Si2 O5 increases to 86%, the Li2 SiO3 and cristobalite are still detected in the final sample. It agrees with the
result reported by Pfeiffer et al. [8] that Li2 SiO3 , Li2 Si2 O5 and quartz are the final products. However, they observed the main phase was Li2 SiO3 , and thought it should came from the smaller amount of Si presenting in the reactant [8]. Nevertheless, according to the phase diagram of Li2 O–SiO2 system [28], the Li2 Si2 O5 is stable at constant temperature when the 66.7 mol% SiO2 exists in this binary system. Consequently, pure Li2 Si2 O5 should be obtained in theory when the molar ratio of Li2 CO3 /SiO2 is 0.5 (i.e. Li/Si = 1) in reactants. Although the pure Li2 Si2 O5 is not obtained in present work, it is seen to be the major phase should be more reasonable comparing with the results given by Ref. [8]. When the Li/Si ratio increases to 2, see Fig. 3(C), a few Li2 CO3 , Li2 Si2 O5 and amorphous silica are detected in the samples obtained at 700 ◦ C. The main crystalline phase of Li2 SiO3 is identified to consist with the JCPDS file 74-2145. While at 800 and 900 ◦ C, the pure Li2 SiO3 is obtained and identified to accord with the JCPDS file 29-0829. The cell parameters are refined utilizing MDI Jade 5.0 software. Li2 SiO3 presents C-center Cmc21 space group (No. 36), and its’ lattice parameters are a = 9.392 Å, b = 5.398 Å, c = 4.672 Å for Li2 SiO3 obtained at 800 ◦ C and a = 9.416 Å, b = 5.409 Å, c = 4.671 Å for that synthesized at 900 ◦ C, respectively. These lattice constants are well consistent with the reported values [29]. However, it can be observed that the cell slightly expands when the sample prepared at 900 ◦ C. This should be so more serious sinterization effect accompanying with the growth of grains during long time higher temperature process that the stress increases and accordingly induces lattice expanding. Comparing their FWHM of
2128
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
Table 1 The phase composition (vol.%) of as-prepared sample. Temperature (◦ C)
Amorphous silica
0.5
700 800 900
84 42 0
0 0 0
0 13 43
16 11 0
0 34 57
0 0 0
1.0
700 800 900
60 34 0
0 0 0
0 6 4
33 26 11
7 34 86
0 0 0
2.0
700 800 900
38 0 0
17 0 0
0 0 0
43 99 100
2 1 0
0 0 0
4.0
700 800 900
0 0 0
<1 0 0
0 0 0
0 0 2
0 0 0
∼ 100 100 98
Li/Si
Li2 CO3
the strongest diffraction peak, it is broadened for those obtained at 800 ◦ C whose grain size is about 0.6 m, smaller than 0.8 m for 900 ◦ C. For the Li/Si molar ratio of 4, see Fig. 3(D), the major phase lithium orthosilicate containing Li2 CO3 and amorphous silica is detected for the sample obtained at 700 ◦ C. The content of expected lithium orthosilicate is 98%. According to the TGA result, the Li4 SiO4 starts formation from 565 ◦ C (Eqs. (3a) and (3b)), and this conversion will be accelerated with the increasing reaction temperature and will not finish till the temperature up to 754 ◦ C.
Quartz
Li2 SiO3
Li2 Si2 O5
Li4 SiO4
After that, there is, yet, about 5.74% mass loss. Therefore, both the XRD and TGA results indicate that pure Li4 SiO4 cannot be synthesized at 700 ◦ C even calcining for 4 h. At 800 ◦ C, only pure Li4 SiO4 phase is detected. The X-ray diffraction peaks are well consistent with the JCPDS file 37-1472. After structure refinement with the same method as above, its’ space group is P21 /m and lattice parameters are a = 5.303 Å, b = 6.113 Å, c = 5.154 Å, and ˇ = 90.33◦ . The grain size is deduced as 0.8 m. When the temperature is up to 900 ◦ C and isothermal calcination for 4 h, a very small amount of Li2 SiO3 is observed in the final product. However, Li2 O, LiOH or
Fig. 4. SEM micrographs of the as-prepared lithium silicate powders. Where A, B and C represents the synthesis reaction temperature of 700, 800 and 900 ◦ C, respectively; 1, 2, 3 and 4 denotes the Li:Si molar ratio of 0.5, 1, 2 and 4, respectively, in reactant.
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130 Table 2 BET specific surface area of as-prepared powders obtained at different condition (cm2 /g). Temperature (◦ C)
700 800 900
Li/Si molar ratios 0.5
1.0
2.0
4.0
76.60 22.16 3.54
48.52 12.11 1.08
18.12 2.42 1.24
9.14 6.30 1.31
LiOH ·H2 O can barely be found utilizing the XRD. Combining with the TGA/DSC result (see Fig. 1), a widened endothermic peak has been observed in this reaction phase. It indicates that an endothermal reaction process takes place during this temperature range and one of the products must be Li2 SiO3 . In fact, several authors have reported that lithium ceramics begin to lose lithium at temperature higher than 800 ◦ C [6,30,31]. However, at this temperature, there was only a small quantity of lithium that sublimated as Li2 O. Cruz et al. [32] had demonstrated that Li4 SiO4 began to decompose into small amounts of Li2 SiO3 (8%) after 30 min at 900 ◦ C. This behavior was constant up to 8 h of thermal treatment. Actually, only lithium present at the surface of the Li4 SiO4 particles must sublimate. In present work, the experiment mass loss is 0.84% higher than theoretical value, it should be attributed to the loss of Li2 O and this process takes place according to Li4 SiO4 →Li2 SiO3 + Li2 O(g)
(4)
Here, the sublimation of Li2 O at 900 ◦ C can well be used to explain why LiOH or LiOH ·H2 O cannot be measured in the sample. The contents of each phase and amorphous silica in the asprepared powders are shown in Table 1. 3.3. Specific surface area The BET specific surface areas of all samples were measured by nitrogen gas sorption method at liquid nitrogen temperature. Table 2 shows the BET specific surface area for all as-prepared samples. The results show that the surface area decreases with the increasing Li/Si molar ratio for the sample obtained at the same reaction temperature, and decreases with the increasing temperature for the same Li/Si molar ratio. This also can be demonstrated by the SEM morphology analysis described as following. 3.4. SEM All samples were studied by SEM. The micrograph of the lithium silicates demonstrate the crystal morphology differences due to the synthesis temperatures and the Li:Si molar ratio. Fig. 4 shows the representative SEM images of the samples obtained at different conditions, in which A, B and C represents the synthesis reaction temperature of 700, 800 and 900 ◦ C, respectively; 1, 2, 3 and 4 denotes the Li:Si molar ratio of 0.5, 1, 2 and 4, respectively, in reactant. For example, the SEM image labeled A-1 denotes the microstructure of the sample prepared at 700 ◦ C for the Li/Si ratio of 0.5. The rest may be deduced by analogy. For the sample with Li:Si ratio of 0.5 obtained at 700 ◦ C (see A-1 in Fig. 4), a lot of amorphous silica still exists and keeps their initial microstructure. Some homogeneous spherical crystals, having very smooth surface with a mean diameter of 2 m, are identified as crystalline phase of Li2 SiO3 by XRD. They are interspersed on the surface of amorphous silica. While calcination temperature increasing, the particles sphericity and the content of amorphous silica decreases (see B-1 and C-1 in Fig. 4). At 800 ◦ C, the spherical and polyhedral particles co-exist with the amorphous silica and the surface of spherical particles gets coarser. At 900 ◦ C, the sample
2129
shows non-homogeneous polyhedral agglomerates. The spherical particles and amorphous silica disappear. For Li/Si ratio of 1, the surface of spherical particles becomes corrugated and coalescent. It is hard to observe the separated spherical particle. While Li/Si = 2, the spherical particles is observed in the sample obtained at 700 ◦ C, however, the sphericity decreases. When the temperature is up to 800 ◦ C, see B-3 in Fig. 4, the homogeneous and coralloid crystal is observed. While at 900 ◦ C, see C-3 in Fig. 4, the particles show the obvious agglomeration. When the Li/Si molar ratio is up to 4, the particles show nonhomogeneous polyhedral crystals with the side length of about 30 m. 4. Conclusion As the first part of investigation for the tritium and helium diffusion and release behavior in lithium silicates, the lithium silicates powders have been synthesized using solid-state reaction between lithium carbonate with amorphous silica, and characterized with TGA/DSC, XRD, SEM and BET surface analysis technologies. For various Li:Si molar ratios in the mixtures of Li2 CO3 and amorphous silica, after a calcining treatment at 700, 800 and 900 ◦ C for 4 h, the solid-state reaction method produced a mixture of lithium silicates and quartz or single lithium silicate phase. The TGA study for the sample with Li/Si molar ratio of 4 shows that the solid-state reaction between lithium carbonate and amorphous silica powders consists of two steps as the formation of Li2 SiO3 , and the conversion of Li2 SiO3 to Li4 SiO4 . For the sample with Li:Si molar ratio of 0.5, 1 and 2 obtained at 700 ◦ C, the major crystalline phase is Li2 SiO3 , and its’ content increases with the increasing of Li/Si molar ratio. Pure Li2 Si2 O5 cannot be prepared when Li/Si = 1 in present work, but the pure Li2 SiO3 can be obtained at 900 ◦ C when Li/Si = 1. While for Li/Si = 4, the Li4 SiO4 with a purity of 98% can be obtained at 700 ◦ C. When the calcination temperature increases to 900 ◦ C, Li4 SiO4 will then decomposed into Li2 SiO3 and gaseous Li2 O. The ideal synthesis temperature for pure Li2 SiO3 and Li4 SiO4 is 900 and 800 ◦ C, respectively, in present experimental conditions. The SEM study shows that crystal morphology changes with the synthesis temperature, the initial Li/Si molar ratio in reactants and with the formed lithium silicates. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
A. Klix, Y. Verzilov, K. Ochiai, et al., Fusion Eng. Des. 72 (2005) 327. C.E. Johnson, K. Noda, N. Roux, J. Nucl. Mater. 263 (1998) 140. C.E. Johnson, J. Nucl. Mater. 270 (1999) 212. M. Taddia, P. Modesti, A. Albertazzi, J. Nucl. Mater. 336 (2005) 173. C. Alvani, S. Casadio, V. Contini, et al., J. Nucl. Mater. 307 (2002) 837. H. Pfeiffer, K.M. Knowles, J. Eur. Ceram. Soc. 24 (2004) 2433. N. Roux, C. Johnson, K. Noda, J. Nucl. Mater. 191C194 (1992) 15. H. Pfeiffer, P. Bosch, S. Bulbulian, J. Nucl. Mater. 257 (1998) 309. F. Li, K. Hu, J. Li, et al., J. Nucl. Mater. 300 (2002) 82. D. Cruz, S. Bulbulian, J. Nucl. Mater. 312 (2003) 262. J.J. Kingsley, L.R. Pederson, Mater. Lett. 18 (1993) 89. L.A. Chick, L.R. Pederson, G.D. Maupin, et al., Mater. Lett. 10 (1990) 6. J.G. van der Laan, H. Kawamura, N. Roux, et al., J. Nucl. Mater. 283 (2000) 99. A.R. Raffray, M.C. Billone, G. Federici, et al., Fusion Eng. Des. 28 (1995) 240. C.E. Johnson, K.R. Kummerer, E. Roth, J. Nucl. Mater. 155–157 (1988) 188. D. Vollath, H. Wedemeyer, E. Gunther, J. Nucl. Mater. 133&134 (1985) 221. L.C. Klein, Solid State Ionics 32&33 (1988) 639. P. Li, B.A. Ferguson, L.F. Francis, J. Mater. Sci. 30 (1995) 4076. M.A. Valenzuela, J.J. Becerril, P. Bosch, et al., J. Am. Ceram. Soc. 79 (1996) 455. L.C. Klein, C. Yu, R. Woodman, et al., Catal. Today 14 (1992) 165. T. Lopez, P. Bosch, M. Asomoza, et al., J. Mater. Synth. Process. 2 (1994) 99. ´ O. Rodrlguez, F. Gonzalez, P. Bosch, et al., Catal. Today 14 (1992) 243. D. Cruz, S. Bulbulian, J. Nucl. Mater. 312 (2003) 262. C.W. Turner, B.C. Clatworthy, A.Y.H. Gin, Symposium of Fabrication and Properties of Lithium Ceramics, Pittsburgh, PA, pp. 26–30, April 1987. J.J. Becerril, P. Bosch, S. Bulbulian, J. Nucl. Mater. 185 (1991) 304. H.P. Klug, L.E. Alexander, X-Ray Diffraction Procedures, John Wiley, New York, London, 1954.
2130
T. Tang et al. / Fusion Engineering and Design 84 (2009) 2124–2130
[27] C.-C. Chang, C.C. Wang, P.N. Kumta, Mater. Des. 22 (2001) 617. [28] A. Skokan, H. Wedemeyer, D. Vollath, et al., Proceedings of the 14th Symposium on Fusion Technology, Pergamon, Avignon, 1986, p. 1255. [29] K. Hesse, Acta Crystallogr. B. 33 (1977) 901.
[30] C.H. Lu, L. Wei-Cheng, J. Mater. Chem. 10 (2000) 1403. [31] K. Essaki, K. Nakagawa, M. Kato, H. Uemoto, J. Chem. Eng. Jpn. 37 (2004) 772. [32] D. Cruza, S. Bulbuliana, E. Limac, et al., J. Solid State Chem. 179 (2006) 909.