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Devitrification behavior and structure of Li2O–B2O3–Al2O3 composite gels from metal alkoxides
Journal of Non-Crystalline Solids 356 (2010) 2263–2267
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Devitrification behavior and structure of Li2O–B2O3–Al2O3 composite gels from metal alkoxides Shengchun Li a,b, B. Li b, X.B. Qi b,⁎, J.J. Wei a a b
Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, People's Republic of China Research Center of Laser Fusion, CAEP, Mianyang 621900, People's Republic of China
a r t i c l e
i n f o
Article history: Received 28 June 2009 Available online 9 September 2010 Keywords: Lithium aluminoborate; Sol–gel; Devitrification behavior; Structure
a b s t r a c t (30 − x/2)Li2O·(70 − x/2)B2O3·xAl2O3(x = 0, 5 and 10) composite gels have been fabricated by the sol–gel method. LiOCH3, B(OC4H9)3, and Al(OC4H9)3 were used as precursor for Li2O, B2O3, and Al2O3, respectively. B (OC4H9)3 and Al(OC4H9)3 were hydrolyzed separately and then mixed. The crystallization behavior and structure of the gels upon thermal treatment temperatures between 150 and 550 °C are characterized on the basis of SEM, XRD and IR analyses. Xerogel with x = 0 exhibits non-crystal features, whereas crystalline phases are found in the xerogels with x = 5 and 10. The crystalline phases are not found with increasing heat treatment temperatures from 150 to 450 °C, but crystalline phases appear present at 550 °C. The xerogel with x = 0, subject to thermal treatment below 450 °C, is found to be still amorphous, and a 550 °C heat treatment leads its structure changing from glassy to crystalline. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, borate-based glasses and ceramics have attracted much attention due to their latent applications as superionic conductors, nonlinear optical materials and radiation resistance stuff. Moreover, borate-based microspheres may have essential applications in many areas such as microelectronics, biotechnology, and pharmacy. On the other hand, the lower molecular weight of borate-based aerogels compared to silica-based aerogels indicates that they are potentially useful materials for some inertial confinement fusion (ICF) target designs. However, there were only a small number of reports studying the sol–gel route in the simple borate systems [1–11] such as alkali borate [1–8], lead borate [9] and aluminoborate [10,11]. The sol–gel route for making glasses has attained scientific and technological significance [12–14], and typically involved the hydrolysis and condensation of metal alkoxides. The major advantage of this approach is that it enables glasses to be gained with a high homogeneity which is difficult to prepare via conventional techniques of melting. Unlike silicon alkoxides, boron and aluminum alkoxides are coordinatively unsaturated and are able to adopt two or three stable coordination, respectively. Thus, the rates of hydrolysis are greater than for silicon alkoxides which are coordinatively saturated and display normally only one stable coordination number. Hence, oxide or hydroxide precipitation is easy to form. Consequently, the sol–gel method yields troubles for the formation of non-silicate glasses.
⁎ Corresponding author. Tel.: +86 816 2494893; fax: +86 816 2493148. E-mail address: [email protected] (X. Qi). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.08.017
In this paper, we report the preparation of lithium aluminoborate composite gels. As far as we know, there are no reports on the gels formation in the Li2O–B2O3–Al2O3 system. In succession, the crystalline behavior and structure changes in the gels upon thermal treatment temperatures are analyzed on the basis of X-ray diffraction, scanning electron microphotographs and infra-red spectra results. 2. Experimental methods 2.1. Fabrication of gels The compositions of the investigated gels are described by the general formula: (30 − x/2)Li2O·(70 − x/2)B2O3·xAl2O3 with x = 0, 5 and 10. The whole process for the preparation of lithium aluminoborate composite gels was shown in Fig. 1. Tri-n-butylborate and lithium methoxide were employed as starting materials, with anhydrous methanol as solvent. LiOCH3 (0.06 mol) was dissolved in methanol (30 ml) under stirring. The solution then was diluted with tetrahydrofuran (30 ml) in a closed flask at room temperature. Subsequently, doubly distilled water (3.2 ml) was added to this solution at constant stirring. Employing of larger amounts of water caused precipitates to produce while the B(OC4H9)3 was added. After 10 min, the B(OC4H9)3 was added slowly at room temperature because the B(OC4H9)3 is easy to hydrolyze. The solution was continuously stirred for 2 h. Separately, 0.01 mol of aluminum tri-sec-butoxide was hydrolyzed with 0.36 ml of distilled water in methanol (35 ml) at 60 °C. The molar ratio of water to aluminum tri-sec-butoxide is 2. Maintaining the molar ratio invariable, the amount of water was changed from 0.36 to 0.72 ml. Then, at this temperature, 2.0 ml of acetic acid was added in order to peptize the mono-hydroxide thus formed a transparent sol [15].
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Fig. 1. Schematic representation for preparing the lithium aluminoborate composite gel.
The two sols were mixed and stirred for 10 min. The final sol was placed in a desiccator to avoid attack from atmospheric moisture and held at 60 °C. Raw gels were gained about in 2 days. Afterwards, all specimens were dried for 96 h at 60 °C in air atmosphere, and they were dried exhaustively at 150 °C for 48 h. They become xerogel fragments now. 2.2. X-ray powder diffraction X-ray diffraction was utilized to detect the presence of any crystal phase in the samples. The sample diffractograms were attained making use of a X'Pert-PRO diffractometer. CuKα radiation was employed for analysis. The samples were scanned at 2θ angles from 5.02 to 89.98°. The crystalline phase was identified using the JCPDS indexes in the conventional way. 2.3. Scanning electron micrographs
Fig. 3. Scanning electron micrographs of the xerogels: (A) x = 0; (B) x = 5; and (C) x = 10.
SEM analysis employing a JSM-6700F Scanning Electron Micrograph (SEM) (JEOL) was performed on the xerogels fragments to observe the morphology of the samples.
2.4. Fourier transform infra-red characterization The structure of the studied samples was analyzed with the aid of PerkinElmer FT-IR Spectrometer using KBr pellets. FTIR transmittance
Fig. 2. XRD patterns (CuKα) of lithium aluminoborate xerogels; the mole ratio of B (OC4H9)3 to LiOCH3 is fixed at 7:3.
Fig. 4. XRD pattern of the xerogel with x = 0 after heat treatment at 550 °C for 1 h.
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Fig. 5. XRD patterns of lithium aluminoborate xerogel after heat treatment at different temperatures from 250 to 550 °C for 1 h. (○) Li2B4O7, (●) LiBO2, (◇) Li2AlB5O10, (◇) Li2CO3, (■) Li5AlO4, and (▲) unaware.
spectra were recorded in the 4000–450 cm− 1 region. A spectral resolution of 2 cm− 1 was selected. 3. Results and discussion 3.1. Crystallization behavior Primary information with regard to the devitrification behavior of the lithium aluminoborate xerogels was attained by employing SEM and X-ray diffraction analysis. As can be seen from Fig. 2, for x = 0 the xerogel exhibits a glass phase. This observation is consistent with other XRD analysis result covered in the literature [2]. For x = 5 and 10, however, crystallization is observed to take place in the xerogels. This is a strong indication that addition of
Fig. 6. FTIR spectra of the xerogels and xerogel with x = 10 after heat treatment at 550 °C for 1 h employing the KBr method.
alumina results in crystallization of these materials. The crystalline phase precipitated is not identified up to this point in our research. It is apparent that for x = 5 the xerogel has a stronger crystallization tendency than the xerogel do for x = 10. Typical SEM of the xerogels is shown in Fig. 3. As can be observed from these photographs, a particle develops when x is large. In Fig. 3 (B) and (C), particles were seen to precipitate randomly, and had different morphologies and sizes. Moreover, some agglomeration was highly obvious in Fig. 3(B). A comparison of the three types of the xerogels implies that lithium borate was much more uniform than lithium aluminoborate. These outcomes are in good agreement with the one affirmed by the aid of XRD. To further research the crystallization process, fragments of all the samples were heated for 1 h at 250, 350, 450 and 550 °C. Next, the specimens were analyzed with the aid of X-ray powder diffraction. Fig. 4 displays the change of X-ray diffraction patterns of lithium aluminoborate xerogel with temperatures of heat treatment. Figs. 4 and 5 show the X-ray diffraction patterns of the xerogels heated at 250, 350, 450 and 550 °C, separately, for 1 h. It is seen that the unknown crystalline phase nearly fully disappears at 450 °C in the xerogels for x = 5 and 10. As seen from Figs. 4 and 5, all the samples crystallize at 550 °C. The dominant crystalline phase precipitated is Li2B4O7 crystal for the xerogel containing no alumina. In the xerogel with x = 5 the main crystalline phase precipitated remains to be Li2B4O7 crystal, and there are large peaks attributed to LiBO2. This shows that the incorporation of up to 5 mol% Al2O3 into the Li2O–B2O3 binary borate gel does not change the major crystalline phase. The crystal structure of Li2B4O7 was determined and refined by Krogh-Moe [16]. However, in the xerogel with x = 10 the main crystalline phase precipitated is
Fig. 7. FTIR spectra of the xerogels for x = 0 and 10.
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Fig. 8. FTIR spectra of the xerogel with x = 10 after heat treatment at various temperatures from 250 to 550 °C for 1 h.
Li2AlB5O10. The crystal structure of this compound has been in detail described in [17]. The unique form of planar B–O rings in this configuration suggests that this substance may be an excellent birefringent material [18]. The aforesaid facts reveal that the xerogel with x = 0 displays a glassy phase, while xerogels with x = 5 and 10 exhibit, besides the amorphous phase, an additional phase which increases with increasing alumina content x values less than 5, and then decreases. 3.2. FTIR characterization The room temperature IR spectra of the studied xerogels in the 4000 to 1600 cm− 1 spectral region are shown in Fig. 6. These spectra are gained utilizing the KBr pellet technique. IR results show that all xerogels include B–OH groups, found at about 3430 cm− 1 [19]. Molecular water is also present in all xerogels, indicated by transmittance at or in the vicinity of 1640 cm− 1 [20]. Heat treat at 550 °C for 1 h virtually totally removed such water in all of the xerogels. Curve A in Fig. 6 shows this effect for the xerogel with x = 10. Fig. 7 exhibits the IR spectra, in the 2000 to 450 cm− 1 range, attained from the xerogels with x=0 and 10. IR spectra are almost identical for the xerogels containing 5 and 10 mol% alumina. Hence, here it is not shown. According to the literature [21], the strong transmittance bands in the 900–1500 cm− 1 region, centered at 1353 and 1035 cm− 1 are attributed to B–O symmetric stretching of BO3 groups and BO4 units contained in ditriborate and di-pentaborate groups. Moreover, in [22], the hydrated Na2O·2B2O3 crystalline spectra have a strongest transmittance band at 945 cm− 1 which is shifted in the deuterated sample. This fact suggests that hydroxyl motions might be responsible for the 1035 cm− 1 band. A shoulder around 890 cm− 1 can be indexed as B–O stretching of boroxol rings. The transmittance peaks observed between 600 and 800 cm− 1 are due to bending vibrations in B–O–B groups [20]. These transmittance peak assignments are in good agreement with those covered preceding infrared investigations with regard to alkali borate [23,24]. The band around 576 cm− 1 arises from B–O rocking [25]. Comparison of spectra for xerogel (x=0) and xerogels (x=5 and 10) points to two evident new peaks at
about 1426 cm− 1 and 1576 cm− 1. The former is attributed to the B–O asymmetric stretching vibrations in the BO3 units with one or two nonbridging oxygens. As it is known, the asymmetric triangle should display two transmittance bands in the 1100–1500 cm− 1 region, while the symmetric triangle only one. The latter is possibly indexed as Al–O stretching of AlO4 according to the literature [26]. Thus, one may conclude that the BO3 units in the xerogel with x=0 are symmetric, while they are asymmetric in the xerogels with x=5 and 10. The two peaks are very indicative of the presence of the crystalline phase. This is consistent with the above XRD analysis results. Moreover, there is an inconspicuous difference in the 1000– 1100 cm− 1 range. The xerogel containing 10 mol% alumina shows two transmittance humps appearance at the same frequency values in the region (1053 and 1025 cm− 1). Infra-red spectra provide some significant information on the structural and compositional variations of the xerogels upon heating. In Fig. 8, the IR spectra of the xerogel with x = 10 at different heat treatment temperatures are shown. The IR spectra of the xerogel with x = 0 at various temperatures have been reported in [2], and are not displayed herein. As can be observed from Fig. 8, the transmittance peak at 1576 cm− 1 decreased in intensity with increasing heat treatment temperature and faded away at 450 °C. Besides, the transmittance bands in the region from 1300 to 1500 cm− 1 were no longer seen. The IR spectrum of the xerogel with x = 10 after heat treatment at 450 °C displays broad transmittance bands containing no any sharp features characteristic of devitrification materials suggesting that the xerogel is amorphous after the heat treatment at 450 °C for 1 h as affirmed also by XRD. The above differences imply that, in the matrix of xerogel, subject to heat treatment at 450 °C for 1 h, there is a broader distribution of configuration units which consist of so-called ‘superstructure’ [21] units, such as boroxol and metaborate rings, metaborate chains, pentaborate, triborate, diborate and pyroborate with respect to the ones heat treatment below 450 °C for 1 h. However, the structural arrangement is not detected by XRD. The evidence of this behavior gained with the aid of the IR analysis is more sensitive than that of XRD to short range modification of anionic network. The IR spectra attained from the xerogels with x=0, 5 and 10 after heat treatment at 550 °C for 1 h are shown in Fig. 9. These IR spectra show very sharp transmittance peaks. The strong modes in the 800–1200 and 450–800 cm− 1 region indicate the presence of crystalline phases. A comparison of these spectra shows several main differences: 1. The peaks around 1140 and 900 cm− 1, according to the literature [2], associated with framework members, are absent in the spectrum of the xerogel with x = 10. However, the hump at about 1040 cm− 1 is absent in the spectrum of the xerogel containing no aluminum. 2. The strong transmittance bands in the 450–800 cm− 1, centered at 779, 654, 680, 547 and 510 cm− 1, present in the xerogel with x = 0,
Fig. 9. FTIR spectra of the xerogels with x=0, 5 and 10 after heat treatment at 550 °C for 1 h.
S. Li et al. / Journal of Non-Crystalline Solids 356 (2010) 2263–2267
are narrower than those of the xerogel with x = 5. Some of them are no longer observed. To clarify the attribute of these transmittance bands, additional investigation efforts are required with the aid of other experimental methods. 4. Conclusions In this work amorphous lithium aluminoborate raw gels have been fabricated successfully in a wide range of compositions investigated by the sol–gel technique. The xerogels were obtained by calcining the semi-dry gels at 150 °C for 48 h for further analysis. The xerogel with x=0 heated at 450 °C or lower was found to be always amorphous. However, after 1 h of heat treatment at 550 °C the Li2B4O7 phase yielded according to XRD analysis. Crystalline phase appeared in the xerogel with x=0, but were not identified in the xerogels with x=5 and 10. The crystalline phase was no longer found in the xerogels after heat treatment at 350 °C for 1 h. However, a new devitrification phase appeared in the xerogels after 1 h of heat treatment at 550 °C. Li2B4O7 and Li2AlB5O10 are major crystalline phase in the xerogels with x=5 and 10, respectively. References [1] N. Toghe, J.D. Mackenzie, J. Non-Cryst. Solids 68 (1984) 411. [2] M.C. Weinberg, G.F. Neilson, G.L. Smith, B. Dunn, G.S. Moore, J.D. Mackenzie, J. Mater. Sci. 20 (1985) 1501.
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[3] C.J. Brinker, K.J. Ward, K.D. Keefer, E. Holupka, P.J. Bray, R.K. Pearson, in: J. Fricke (Ed.), Aerogels, Springer, Berlin, 1986, p. 57. [4] T. Lopez, P. Bosch, M. Asomoza, E. Haro-Poniatowska, J. Mater. Synth. Process. 2 (1994) 99. [5] T. Lopez, E. Haro-Poniatowska, P. Bosch, M. Asomoza, R. Gomez, M. Massot, M. Balkanski, J. Sol–Gel Sci. Technol. 2 (1994) 891. [6] H. Yamashita, T. Yoko, S. Sakka, J. Mater. Sci. Lett. 9 (1990) 796. [7] H. Yamashita, T. Yoko, S. Sakka, J. Am. Ceram. Soc. 74 (1991) 1668. [8] M.P. Medda, A. Musinu, G. Piccaluga, G. Pinna, J. Mater. Sci. 29 (1994) 1330. [9] G.D. Chryssikos, A.P. Patsis, E.I. Kamitsos, J.A. Kapoutsis, M.A. Karakassides, C. Trapalis, E. Mylonas, G. Kordas, J. Mater. Sci. Lett. 14 (1995) 431. [10] F. Dumeignil, M. Guelton, M. Rigole, J.P. Amoureux, C. Fernandez, J. Grimblot, Colloids Surf A Physicochem. Eng. Aspects 158 (1999) 75. [11] O. Yamaguchi, M. Tada, K. Takeoka, K. Shimizu, Bull. Chem. Soc. Jpn 52 (1979) 2153. [12] R. Roy, J. Am. Ceram. Soc. 52 (1969) 244. [13] H. Dislisch, Angew. Chem. Int. Ed Engl. 10 (1971) 363. [14] M. Decottignies, J. Phalippou, J. Zarzycki, J. Mater. Sci. 13 (1978) 2605. [15] S. Grandi, L. Costa, J. Non-Cryst. Solids 225 (1998) 141. [16] J. Krogh-Moe, Acta Crystallogr. 15 (1962) 190. [17] M. He, H. Li, X.L. Chen, Y.P. Xu, T. Xu, Acta Crystallogr. 57 (2001) 1010. [18] M. He, X.L. Chen, B.Q. Hu, T. Zhou, Y.P. Xu, T. Xu, J. Solid State Chem. 165 (2002) 187. [19] M. Prassas, J. Phalippou, L.L. Hench, J. Zarzycki, J. Non-Cryst. Solids 48 (1982) 79. [20] M. Bengisu, E. Yilmaz, H. Farzad, S.T. Rei, J. Sol–Gel Sci. Technol. 45 (2008) 237. [21] P. Pernice, S. Esposito, A. Aronne, V.N. Sigaev, J. Non-Cryst. Solids 258 (1999) 1. [22] J. Wong, C.A. Angell, Glass Structure by Spectroscopy, Marcel Dekker Inc., New York, 1976, p. 434. [23] E.I. Kamitsos, A.P. Patsis, M.A. Karakassides, G.D. Chryssikos, J. Non-Cryst. Solids 126 (1990) 52. [24] J. Krogh-Moe, Phys. Chem. Glasses 6 (1965) 46. [25] A.F.L. Almeida, D. Thomazini, I.F. Vasconselos, M.A. Valente, A.S.B. Sombrac, Int. J. Inorg. Mater. 3 (2001) 829. [26] V.Z. Kreitsberga, V.G. Chekhovskii, A.P. Sizonenko, Y.U.Y.A. Keishs, S.E. Redala, P.G. Pauksh, Fiz. Khim. Stekla 17 (1991) 953.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Devitrification behavior and structure of Li2O–B2O3–Al2O3 composite gels from metal alkoxides Shengchun Li a,b, B. Li b, X.B. Qi b,⁎, J.J. Wei a a b
Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, People's Republic of China Research Center of Laser Fusion, CAEP, Mianyang 621900, People's Republic of China
a r t i c l e
i n f o
Article history: Received 28 June 2009 Available online 9 September 2010 Keywords: Lithium aluminoborate; Sol–gel; Devitrification behavior; Structure
a b s t r a c t (30 − x/2)Li2O·(70 − x/2)B2O3·xAl2O3(x = 0, 5 and 10) composite gels have been fabricated by the sol–gel method. LiOCH3, B(OC4H9)3, and Al(OC4H9)3 were used as precursor for Li2O, B2O3, and Al2O3, respectively. B (OC4H9)3 and Al(OC4H9)3 were hydrolyzed separately and then mixed. The crystallization behavior and structure of the gels upon thermal treatment temperatures between 150 and 550 °C are characterized on the basis of SEM, XRD and IR analyses. Xerogel with x = 0 exhibits non-crystal features, whereas crystalline phases are found in the xerogels with x = 5 and 10. The crystalline phases are not found with increasing heat treatment temperatures from 150 to 450 °C, but crystalline phases appear present at 550 °C. The xerogel with x = 0, subject to thermal treatment below 450 °C, is found to be still amorphous, and a 550 °C heat treatment leads its structure changing from glassy to crystalline. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, borate-based glasses and ceramics have attracted much attention due to their latent applications as superionic conductors, nonlinear optical materials and radiation resistance stuff. Moreover, borate-based microspheres may have essential applications in many areas such as microelectronics, biotechnology, and pharmacy. On the other hand, the lower molecular weight of borate-based aerogels compared to silica-based aerogels indicates that they are potentially useful materials for some inertial confinement fusion (ICF) target designs. However, there were only a small number of reports studying the sol–gel route in the simple borate systems [1–11] such as alkali borate [1–8], lead borate [9] and aluminoborate [10,11]. The sol–gel route for making glasses has attained scientific and technological significance [12–14], and typically involved the hydrolysis and condensation of metal alkoxides. The major advantage of this approach is that it enables glasses to be gained with a high homogeneity which is difficult to prepare via conventional techniques of melting. Unlike silicon alkoxides, boron and aluminum alkoxides are coordinatively unsaturated and are able to adopt two or three stable coordination, respectively. Thus, the rates of hydrolysis are greater than for silicon alkoxides which are coordinatively saturated and display normally only one stable coordination number. Hence, oxide or hydroxide precipitation is easy to form. Consequently, the sol–gel method yields troubles for the formation of non-silicate glasses.
⁎ Corresponding author. Tel.: +86 816 2494893; fax: +86 816 2493148. E-mail address: [email protected] (X. Qi). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.08.017
In this paper, we report the preparation of lithium aluminoborate composite gels. As far as we know, there are no reports on the gels formation in the Li2O–B2O3–Al2O3 system. In succession, the crystalline behavior and structure changes in the gels upon thermal treatment temperatures are analyzed on the basis of X-ray diffraction, scanning electron microphotographs and infra-red spectra results. 2. Experimental methods 2.1. Fabrication of gels The compositions of the investigated gels are described by the general formula: (30 − x/2)Li2O·(70 − x/2)B2O3·xAl2O3 with x = 0, 5 and 10. The whole process for the preparation of lithium aluminoborate composite gels was shown in Fig. 1. Tri-n-butylborate and lithium methoxide were employed as starting materials, with anhydrous methanol as solvent. LiOCH3 (0.06 mol) was dissolved in methanol (30 ml) under stirring. The solution then was diluted with tetrahydrofuran (30 ml) in a closed flask at room temperature. Subsequently, doubly distilled water (3.2 ml) was added to this solution at constant stirring. Employing of larger amounts of water caused precipitates to produce while the B(OC4H9)3 was added. After 10 min, the B(OC4H9)3 was added slowly at room temperature because the B(OC4H9)3 is easy to hydrolyze. The solution was continuously stirred for 2 h. Separately, 0.01 mol of aluminum tri-sec-butoxide was hydrolyzed with 0.36 ml of distilled water in methanol (35 ml) at 60 °C. The molar ratio of water to aluminum tri-sec-butoxide is 2. Maintaining the molar ratio invariable, the amount of water was changed from 0.36 to 0.72 ml. Then, at this temperature, 2.0 ml of acetic acid was added in order to peptize the mono-hydroxide thus formed a transparent sol [15].
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Fig. 1. Schematic representation for preparing the lithium aluminoborate composite gel.
The two sols were mixed and stirred for 10 min. The final sol was placed in a desiccator to avoid attack from atmospheric moisture and held at 60 °C. Raw gels were gained about in 2 days. Afterwards, all specimens were dried for 96 h at 60 °C in air atmosphere, and they were dried exhaustively at 150 °C for 48 h. They become xerogel fragments now. 2.2. X-ray powder diffraction X-ray diffraction was utilized to detect the presence of any crystal phase in the samples. The sample diffractograms were attained making use of a X'Pert-PRO diffractometer. CuKα radiation was employed for analysis. The samples were scanned at 2θ angles from 5.02 to 89.98°. The crystalline phase was identified using the JCPDS indexes in the conventional way. 2.3. Scanning electron micrographs
Fig. 3. Scanning electron micrographs of the xerogels: (A) x = 0; (B) x = 5; and (C) x = 10.
SEM analysis employing a JSM-6700F Scanning Electron Micrograph (SEM) (JEOL) was performed on the xerogels fragments to observe the morphology of the samples.
2.4. Fourier transform infra-red characterization The structure of the studied samples was analyzed with the aid of PerkinElmer FT-IR Spectrometer using KBr pellets. FTIR transmittance
Fig. 2. XRD patterns (CuKα) of lithium aluminoborate xerogels; the mole ratio of B (OC4H9)3 to LiOCH3 is fixed at 7:3.
Fig. 4. XRD pattern of the xerogel with x = 0 after heat treatment at 550 °C for 1 h.
S. Li et al. / Journal of Non-Crystalline Solids 356 (2010) 2263–2267
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Fig. 5. XRD patterns of lithium aluminoborate xerogel after heat treatment at different temperatures from 250 to 550 °C for 1 h. (○) Li2B4O7, (●) LiBO2, (◇) Li2AlB5O10, (◇) Li2CO3, (■) Li5AlO4, and (▲) unaware.
spectra were recorded in the 4000–450 cm− 1 region. A spectral resolution of 2 cm− 1 was selected. 3. Results and discussion 3.1. Crystallization behavior Primary information with regard to the devitrification behavior of the lithium aluminoborate xerogels was attained by employing SEM and X-ray diffraction analysis. As can be seen from Fig. 2, for x = 0 the xerogel exhibits a glass phase. This observation is consistent with other XRD analysis result covered in the literature [2]. For x = 5 and 10, however, crystallization is observed to take place in the xerogels. This is a strong indication that addition of
Fig. 6. FTIR spectra of the xerogels and xerogel with x = 10 after heat treatment at 550 °C for 1 h employing the KBr method.
alumina results in crystallization of these materials. The crystalline phase precipitated is not identified up to this point in our research. It is apparent that for x = 5 the xerogel has a stronger crystallization tendency than the xerogel do for x = 10. Typical SEM of the xerogels is shown in Fig. 3. As can be observed from these photographs, a particle develops when x is large. In Fig. 3 (B) and (C), particles were seen to precipitate randomly, and had different morphologies and sizes. Moreover, some agglomeration was highly obvious in Fig. 3(B). A comparison of the three types of the xerogels implies that lithium borate was much more uniform than lithium aluminoborate. These outcomes are in good agreement with the one affirmed by the aid of XRD. To further research the crystallization process, fragments of all the samples were heated for 1 h at 250, 350, 450 and 550 °C. Next, the specimens were analyzed with the aid of X-ray powder diffraction. Fig. 4 displays the change of X-ray diffraction patterns of lithium aluminoborate xerogel with temperatures of heat treatment. Figs. 4 and 5 show the X-ray diffraction patterns of the xerogels heated at 250, 350, 450 and 550 °C, separately, for 1 h. It is seen that the unknown crystalline phase nearly fully disappears at 450 °C in the xerogels for x = 5 and 10. As seen from Figs. 4 and 5, all the samples crystallize at 550 °C. The dominant crystalline phase precipitated is Li2B4O7 crystal for the xerogel containing no alumina. In the xerogel with x = 5 the main crystalline phase precipitated remains to be Li2B4O7 crystal, and there are large peaks attributed to LiBO2. This shows that the incorporation of up to 5 mol% Al2O3 into the Li2O–B2O3 binary borate gel does not change the major crystalline phase. The crystal structure of Li2B4O7 was determined and refined by Krogh-Moe [16]. However, in the xerogel with x = 10 the main crystalline phase precipitated is
Fig. 7. FTIR spectra of the xerogels for x = 0 and 10.
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Fig. 8. FTIR spectra of the xerogel with x = 10 after heat treatment at various temperatures from 250 to 550 °C for 1 h.
Li2AlB5O10. The crystal structure of this compound has been in detail described in [17]. The unique form of planar B–O rings in this configuration suggests that this substance may be an excellent birefringent material [18]. The aforesaid facts reveal that the xerogel with x = 0 displays a glassy phase, while xerogels with x = 5 and 10 exhibit, besides the amorphous phase, an additional phase which increases with increasing alumina content x values less than 5, and then decreases. 3.2. FTIR characterization The room temperature IR spectra of the studied xerogels in the 4000 to 1600 cm− 1 spectral region are shown in Fig. 6. These spectra are gained utilizing the KBr pellet technique. IR results show that all xerogels include B–OH groups, found at about 3430 cm− 1 [19]. Molecular water is also present in all xerogels, indicated by transmittance at or in the vicinity of 1640 cm− 1 [20]. Heat treat at 550 °C for 1 h virtually totally removed such water in all of the xerogels. Curve A in Fig. 6 shows this effect for the xerogel with x = 10. Fig. 7 exhibits the IR spectra, in the 2000 to 450 cm− 1 range, attained from the xerogels with x=0 and 10. IR spectra are almost identical for the xerogels containing 5 and 10 mol% alumina. Hence, here it is not shown. According to the literature [21], the strong transmittance bands in the 900–1500 cm− 1 region, centered at 1353 and 1035 cm− 1 are attributed to B–O symmetric stretching of BO3 groups and BO4 units contained in ditriborate and di-pentaborate groups. Moreover, in [22], the hydrated Na2O·2B2O3 crystalline spectra have a strongest transmittance band at 945 cm− 1 which is shifted in the deuterated sample. This fact suggests that hydroxyl motions might be responsible for the 1035 cm− 1 band. A shoulder around 890 cm− 1 can be indexed as B–O stretching of boroxol rings. The transmittance peaks observed between 600 and 800 cm− 1 are due to bending vibrations in B–O–B groups [20]. These transmittance peak assignments are in good agreement with those covered preceding infrared investigations with regard to alkali borate [23,24]. The band around 576 cm− 1 arises from B–O rocking [25]. Comparison of spectra for xerogel (x=0) and xerogels (x=5 and 10) points to two evident new peaks at
about 1426 cm− 1 and 1576 cm− 1. The former is attributed to the B–O asymmetric stretching vibrations in the BO3 units with one or two nonbridging oxygens. As it is known, the asymmetric triangle should display two transmittance bands in the 1100–1500 cm− 1 region, while the symmetric triangle only one. The latter is possibly indexed as Al–O stretching of AlO4 according to the literature [26]. Thus, one may conclude that the BO3 units in the xerogel with x=0 are symmetric, while they are asymmetric in the xerogels with x=5 and 10. The two peaks are very indicative of the presence of the crystalline phase. This is consistent with the above XRD analysis results. Moreover, there is an inconspicuous difference in the 1000– 1100 cm− 1 range. The xerogel containing 10 mol% alumina shows two transmittance humps appearance at the same frequency values in the region (1053 and 1025 cm− 1). Infra-red spectra provide some significant information on the structural and compositional variations of the xerogels upon heating. In Fig. 8, the IR spectra of the xerogel with x = 10 at different heat treatment temperatures are shown. The IR spectra of the xerogel with x = 0 at various temperatures have been reported in [2], and are not displayed herein. As can be observed from Fig. 8, the transmittance peak at 1576 cm− 1 decreased in intensity with increasing heat treatment temperature and faded away at 450 °C. Besides, the transmittance bands in the region from 1300 to 1500 cm− 1 were no longer seen. The IR spectrum of the xerogel with x = 10 after heat treatment at 450 °C displays broad transmittance bands containing no any sharp features characteristic of devitrification materials suggesting that the xerogel is amorphous after the heat treatment at 450 °C for 1 h as affirmed also by XRD. The above differences imply that, in the matrix of xerogel, subject to heat treatment at 450 °C for 1 h, there is a broader distribution of configuration units which consist of so-called ‘superstructure’ [21] units, such as boroxol and metaborate rings, metaborate chains, pentaborate, triborate, diborate and pyroborate with respect to the ones heat treatment below 450 °C for 1 h. However, the structural arrangement is not detected by XRD. The evidence of this behavior gained with the aid of the IR analysis is more sensitive than that of XRD to short range modification of anionic network. The IR spectra attained from the xerogels with x=0, 5 and 10 after heat treatment at 550 °C for 1 h are shown in Fig. 9. These IR spectra show very sharp transmittance peaks. The strong modes in the 800–1200 and 450–800 cm− 1 region indicate the presence of crystalline phases. A comparison of these spectra shows several main differences: 1. The peaks around 1140 and 900 cm− 1, according to the literature [2], associated with framework members, are absent in the spectrum of the xerogel with x = 10. However, the hump at about 1040 cm− 1 is absent in the spectrum of the xerogel containing no aluminum. 2. The strong transmittance bands in the 450–800 cm− 1, centered at 779, 654, 680, 547 and 510 cm− 1, present in the xerogel with x = 0,
Fig. 9. FTIR spectra of the xerogels with x=0, 5 and 10 after heat treatment at 550 °C for 1 h.
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are narrower than those of the xerogel with x = 5. Some of them are no longer observed. To clarify the attribute of these transmittance bands, additional investigation efforts are required with the aid of other experimental methods. 4. Conclusions In this work amorphous lithium aluminoborate raw gels have been fabricated successfully in a wide range of compositions investigated by the sol–gel technique. The xerogels were obtained by calcining the semi-dry gels at 150 °C for 48 h for further analysis. The xerogel with x=0 heated at 450 °C or lower was found to be always amorphous. However, after 1 h of heat treatment at 550 °C the Li2B4O7 phase yielded according to XRD analysis. Crystalline phase appeared in the xerogel with x=0, but were not identified in the xerogels with x=5 and 10. The crystalline phase was no longer found in the xerogels after heat treatment at 350 °C for 1 h. However, a new devitrification phase appeared in the xerogels after 1 h of heat treatment at 550 °C. Li2B4O7 and Li2AlB5O10 are major crystalline phase in the xerogels with x=5 and 10, respectively. References [1] N. Toghe, J.D. Mackenzie, J. Non-Cryst. Solids 68 (1984) 411. [2] M.C. Weinberg, G.F. Neilson, G.L. Smith, B. Dunn, G.S. Moore, J.D. Mackenzie, J. Mater. Sci. 20 (1985) 1501.
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