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Nuclear magnetic resonance studies of 0.139MO (or M′2O) · 0.673SiO2 · (0.188−x)Al2O3 · xB2O3 (M=Mg, Ca, Sr and Ba, M′=Na and K) glasses
Journal of Non-Crystalline Solids 331 (2003) 128–136 www.elsevier.com/locate/jnoncrysol
Nuclear magnetic resonance studies of 0.139MO (or M02O) Æ 0.673SiO2 Æ (0:188 x)Al2O3 Æ xB2O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ Na and K) glasses Hiroshi Yamashita, Kazuhiko Inoue, Takeshi Nakajin, Hyuma Inoue, Takashi Maekawa * Department of Applied Chemistry, Faculty of Engineering, Ehime University, Bunkyo-cho, Matsuyama 790-8577, Japan Received 12 February 2003; received in revised form 9 May 2003
Abstract The 11 B, 27 Al and 29 Si magic angle spinning (MAS) NMR spectra of 0.139MO (or M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 Æ (M ¼ Mg, Ca, Sr and Ba, M0 ¼ Na and K) glasses were examined. When three network forming oxides such as SiO2 , Al2 O3 and B2 O3 coexist, Al2 O3 reacts preferably with network modifier such as M02 O or MO. The reactivity of residual network modifier with SiO2 was high compared with B2 O3 in the order of K2 O < Na2 O < BaO < SrO < CaO < MgO. The apparent equilibrium constants, Kapp , of the reactions, Si(Q3 ) + B(Q3 ) ¼ Si(Q4 ) + B(Q4 ), were determined. Here, the Si(Qn ) or B(Q3 ) and B(Q4 ) represent the SiO2 structural units or three- and four-coordinated boron atoms, respectively. The subscript are the number of bridging oxygen atoms. The Kapp values indicate the reactivity of MO or M02 O with B2 O3 and SiO2 in the MO(M02 O) Æ SiO2 Æ Al2 O3 Æ B2 O3 glasses. The reaction proceeds to the left direction in the order of MgO > CaO > SrO > BaO > Na2 O > K2 O. The order of the reactivity is the same as that of the electronegativities among alkali and alkaline earth metals. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Aluminoborosilicate glasses have been used for liquid crystal display glasses. In particular alkali ion-free glasses have a bright prospect of the chemical stability and the mechanical strength. In order to see the physicochemical properties, it is
* Corresponding author. Tel.: +81-89 927 9926; fax: +81-89 927 9943. E-mail address: [email protected] (T. Maekawa).
important to see the relation between the structures and the composition of the glasses. It is also interesting that the glass-forming tendency of the aluminoborosilicate glasses is influenced by addition of alkaline earth metals into the glasses. However, it is not shed light on the tendency. Greenblatt et al. [1] reported the studies of xBaO Æ (1 x)B2 O3 glasses by 11 B NMR. It was shown that alkaline earth ions behave like alkali modifier ions and the fraction of four-coordinated boron atoms, N4 , follows the x=ð1 xÞ law as in alkali borate glasses. In fact the composition dependence of N4 in the barium borate system was found to be very similar to that
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.08.086
H. Yamashita et al. / Journal of Non-Crystalline Solids 331 (2003) 128–136
in the lithium and sodium borate glasses. Yiannopoulos et al. [2] allowed the quantification of network structure in terms of the fraction of N4 in the alkaline earth borate glasses by the infrared reflectance spectra. It was found that N4 increased with alkaline earth oxide content, MO, and attained maximum values at compositions depending on the M2þ ion type. N4 was found to decrease from Ba to Mg, i.e. upon increasing cation field strength, a trend opposite to that exhibited by alkali borate glasses. Kim et al. [3] determined on the N4 values in the MgO Æ Na2 O Æ B2 O3 glasses by 11 B NMR. The both Na2 O and MgO behaved as network modifiers in the glasses containing less than 0.15 molar fraction of MgO. As the MgO concentration increases further, MgO becomes progressively a network former, presumably in the form of MgO4 tetrahedra. Bishop et al. [4] measured the fraction of N4 in the calcium boroaluminate glasses by 11 B magic angle spinning (MAS)-NMR. For a given CaO concentration, the values of N4 in the aluminum-containing glasses were lower than those for the aluminum-free glasses. It was indicated that the aluminum atoms also assumed a tetrahedral bonding configuration. Jones et al. [5] reported for structural characterization of sodalime-silicate glasses by 29 Si and 23 Na MAS-NMR. The addition of CaO introduced a greater distortion into the structure. This result suggested that Ca preferentially arranged in the vicinity of Q2 species rather than Q3 . In particular the structure of alkali ion-free glasses have recently been studied by numerous workers [6–9]. In a previous paper, the 11 B, 27 Al and 29 Si MASNMR spectra of 0.188MO (or M02 O) Æ 0.673SiO2 Æ 0.077Al2 O3 Æ 0.062B2 O3 (M ¼ Ca, Sr and Ba, M0 ¼ Na) glasses were examined [10]. The compositions of the glasses of this series are based on the composition of alkali-free glass (code AN635) by Asahi glass Co. Ltd. [11]. The fraction of threecoordinated boron atoms decreased in the order of CaO > SrO > BaO > Na2 O. When three network forming oxides such as SiO2 , Al2 O3 and B2 O3 coexist in the 0.139Na2 O Æ 0.673SiO2 Æ (0:188 x) Al2 O3 Æ xB2 O3 glasses, Al2 O3 reacts preferably with Na2 O. The populations of four-coordinated boron atoms, N4 , are expressed well with r=ð1 rÞ, where r ¼ ([Na2 O] ) [Al2 O3 ])/([Na2 O] ) [Al2 O3 ] + [B2 O3 ]).
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In the present study, the difference of the reactivity among coexisting network forming oxides such as SiO2 , Al2 O3 and B2 O3 with alkali and alkaline earth oxides will be compared by the NMR measurements in the 0.139MO (or M02 O) Æ 0.673 SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ Na and K) glasses. This composition is chosen so that [MO] < ([Al2 O3 ] + [B2 O3 ]). The apparent equilibrium constants, Kapp , of the reactions, Si(Q3 ) + B(Q3 ) ¼ Si(Q4 ) + B(Q4 ), were determined.
2. Experimental Samples were prepared from reagent grade Na2 CO3 , K2 CO3 , H3 BO3 , SiO2 , Al2 O3 , MgO, CaCO3 , SrCO3 and BaO. Mixed starting materials were carefully melted in a platinum crucible at 1200–1600 °C using an electric furnace and poured into a cold metal plate. The obtained samples were optically clear. In all samples, a small amount of Gd2 O3 (0.05 mol%) was added to batches as a spin relaxation reagent for the 29 Si NMR measurements. The 11 B, 27 Al, and 29 Si MAS-NMR spectra were recorded on a JEOL JNM-CMX 300 spectrometer. Other experimental details were described in the previous papers [12,13]. The 29 Si NMR spectra were deconvoluted with Gaussian peaks. Setting values of the isotropic chemical shift, quadruple coupling constant and quadruple asymmetric parameters made the deconvolutions of 11 B spectra. X-ray powder patterns were recorded on a diffractometer and did not show crystalline phases in any glasses.
3. Results 3.1. 0.139M02 O0.673SiO2 (0:188 x)Al2 O3 xB2 O3 glasses Fig. 1(a) and (b) represent the 29 Si NMR spectra of 0.139M02 O Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. Here it is aimed to see the relative reactivity of Al2 O3 and B2 O3 with M02 O. With an increase in the B2 O3 content, the peak position moves to
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Fig. 1. 29 Si(a, b) and 11 B(c) MAS-NMR spectra of 0.139M02 O Æ 0.673SiO2 Æ (0:188 x)Al2 O2 Æ xB2 O2 glasses (a, c: M0 ¼ K, b: M0 ¼ Na). The solid and broken lines denote the experimental and deconvolution data, respectively.
higher magnetic field side and the peak width increases. The 11 B NMR spectra of 0.139K2 O Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses are shown in Fig. 1(c). The 11 B NMR spectra of the Na2 O glasses were reported in previous paper [10]. These spectra consist of two signals. One is a sharp signal around 0 ppm and the other is the broad ones from )20 to +20 ppm. The former is originated from the four-coordinated boron
and the latter from the three-coordinated boron atom. The fraction of three-coordinated boron atoms decreased slightly with an increase in B2 O3 content. Fig. 2 represents the 27 Al NMR spectrum of the glass (x ¼ 0:094). This spectrum consists of a peak around 50 ppm. As described previously [10], this peak is assigned to Al(Q4 ) [14]. The spectra of the other compositions were also similar to that shown in Fig. 2.
H. Yamashita et al. / Journal of Non-Crystalline Solids 331 (2003) 128–136
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SiO2 Æ Al2 O3 Æ B2 O3 glasses. A shoulder around )90 ppm in the K2 O Æ CaO glasses are large compared with the K2 O glasses (Fig. 1(a)). The 11 B NMR spectra of the glasses are shown in Fig. 5(b). The fraction of three-coordinated boron atoms in the former glasses is large compared with the latter ones (Fig. 1(c)).
4. Discussion 4.1. Reactivity of MO or M02 O with SiO2 , Al2 O3 and B2 O3
Fig. 2. 27 Al MAS-NMR spectra of 0.139MO(M02 O) Æ 0.673SiO2 Æ 0.094Al2 O3 Æ 0.094B2 O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ K) glasses.
3.2. 0.139MO 0.673SiO2 (0:188 x)Al2 O3 xB2 O3 glasses Fig. 3 represents the 29 Si NMR spectra of the 0.139MO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. In all the glasses, the apparent peak positions move to higher magnetic field with an increase in B2 O3 content. A shoulder around )90 ppm decreases in the order of MgO > CaO > SrO > BaO. The 11 B NMR spectra of the glasses are shown in Fig. 4. The fraction of three-coordinated boron atoms decreases in the order of MgO > CaO > SrO > BaO and with an increase in B2 O3 content. Fig. 2 represents the 27 Al NMR spectra of the 0.139MO Æ 0.673SiO2 Æ 0.094Al2 O3 Æ 0.094B2 O3 glasses. These spectra also consist of a peak around 50 ppm. As seen above this peak is assigned to Al(Q4 ). The peak positions do not differ among the 0.139MO Æ 0.673SiO2 Æ (0:188 x) Al2 O3 Æ xB2 O3 glasses. 3.3. K2 O CaO SiO2 Al2 O3 B2 O3 glasses Fig. 5(a) represents the 29 Si NMR spectra of 0.099K2 O Æ 0.040CaO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses as a representative of K2 O Æ CaO Æ
The change of the NMR spectra can be understood by a competition among the three network forming oxides (SiO2 , B2 O3 and Al2 O3 ) with basic oxides. Maekawa et al. investigated the structural study of sodium aluminosilicate glasses by 29 Si NMR [14]. These spectra were shifted to lower magnetic field with increasing aluminum oxide contents because Al2 O3 dissolves in silicate as NaAlO2 . In the previous paper [10], Na2 O reacted preferentially with Al2 O3 and only the residual Na2 O reacted with B2 O3 to form fourcoordinated boron atoms in the Na2 O Æ SiO2 Æ Al2 O3 Æ B2 O3 glasses. In the K2 O Æ SiO2 Æ Al2 O3 Æ B2 O3 glasses, it is also observed that the profiles of 27 Al NMR do not change with replacing Al2 O3 by B2 O3 . The 29 Si NMR spectra have a bulge on the low magnetic field side and the peak position moves toward high magnetic fields with an increase in B2 O3 content, which is due to the formation of Si(Q3 ). This means excision of silicate network proceeds. Table 1 represents the distribution of Si(Qn ) species determined by the deconvolution of the 29 Si NMR spectra, X obs . The proportion of Si(Q3 ) increases slightly with an increase in B2 O3 content. On the other hand, the proportion of B(Q4 ) increases largely with an increase in B2 O3 content. These results suggest that K2 O reacted preferentially with Al2 O3 and the residual K2 O reacted with B2 O3 rather than with SiO2 , i.e., K2 O was effectively employed the formation of B(Q4 ) species rather than the formation of Si(Q3 ) species. Fig. 6 represents a relation between populations of four-coordinated boron atoms and r ( ¼ ([K2 O] ) [Al2 O3 ])/([K2 O] ) [Al2 O3 + B2 O3 ])), as
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Fig. 3. 29 Si MAS-NMR spectra of the 0.139MO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses (a: Mg, b: Ca, c: Sr and d: Ba). These lines are the same as in Fig. 1.
described previously [10]. Here, the data of Fig. 1(c) is used. Generally the normalized standard errors of the deconvolution of the NMR spectra were within 9.9%. The fraction of fourcoordinated boron increases with an increase in r. A solid line is the theoretical one which is expected in xNa2 O Æ (1 x)B2 O3 binary glasses; the populations of four-coordinated boron atoms, N4 ¼ x=ð1 xÞ. Experimental values coincide well with the theoretical line. One also sees K2 O react preferentially with Al2 O3 and only the residual K2 O react with B2 O3 to form four-coordinated
boron atoms. However, in the MO glasses the difference from the r=ð1 rÞ increases in the order of BaO < SrO < CaO < MgO. This fact suggests that MO reacted preferentially with Al2 O3 and the reactivity of residual MO with SiO2 was high compared to the reactivity with B2 O3 in the order described above. 4.2. Stability constant Fig. 7 represents a relation between populations of four-coordinated boron atoms and of Si(Q3 ).
H. Yamashita et al. / Journal of Non-Crystalline Solids 331 (2003) 128–136
Fig. 4.
11
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B MAS-NMR spectra of the 0.139MO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses (a: Mg, b: Ca, c: Sr and d: Ba).
Here, the data of Figs. 1, 3 and 4, and previous data [10] are used. XBðQ4 Þ and XSiðQ3 Þ values increase with a decrease in Al2 O3 content. The slope increases in the order of MgO < CaO < SrO < BaO < Na2 O < K2 O. These results suggest that the reactivity of M02 O or MO with B2 O3 was high compared to the reactivity of M02 O or MO with SiO2 in the order described above. From these results, the apparent stability constant, Kapp , of Eq. (1) were determined. SiðQ3 Þ þ BðQ3 Þ ¼ SiðQ4 Þ þ BðQ4 Þ:
ð1Þ
The stability constants are expressed as Kapp ¼ [Si(Q4 )] [B(Q4 )]/[Si(Q3 )][B(Q3 )]. Table 2 represents
the Kapp values in the 0.139MO (or M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. The order of the reactivity is the same as that of the electronegativities among alkaline and alkaline earth metals. In binary glass melts containing Na2 O, the thermodynamic acid strength was determined by EMF measurements to be B2 O3 > SiO2 [15]. In sodium borosilicate glasses, the strong base such as Na2 O reacts mainly with the strong acid such as B2 O3 . Thus, the homogeneous glass separates into an alkali rich borate phase and SiO2 one by heat treatment. It is considered that the weak base such as MgO reacts mainly with the weak acid such as SiO2 . The
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Fig. 5. 29 Si(a) and 11 B(b) MAS-NMR spectra of 0.099K2 O Æ 0.040CaO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. These lines are the same as in Fig. 1. Table 1 Distribution of Si(Q3 ) determined by the deconvolution of the 11 B and 29 Si NMR spectra of 0.139MO (M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses MO or M02 O
x
B(Q4 )
XSiðQ3 Þobs
MgO
0.049 0.094 0.116
0.017 0.012 0.022
0.217 0.392 0.370
CaO
0.049 0.094 0.116 0.139
0.022 0.064 0.117 0.107
0.107 0.150 0.252 0.409
SrO
0.049 0.094 0.116 0.139
0.047 0.133 0.217 0.267
0.049 0.098 0.186 0.287
BaO
0.049 0.094 0.116 0.139
0.042 0.223 0.301 0.374
0.015 0.070 0.095 0.124
Na2 O
0.126 0.157 0.188
0.479 0.515 0.661
0.110 0.110 0.157
0.072 0.094 0.116 0.139
0.254 0.353 0.478 0.508
0.016 0.034 0.050 0.062
K2 O
Fig. 6. Correlation between glass composition and the population of threefold boron atoms from 11 B MAS-NMR spectra in 0.139MO (M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 systems. Here, r is ([MO or M02 O] ) [Al2 O3 ])/([MO or M02 O] ) [Al2 O3 ] + [B2 O3 ]).
strength of base coincides with the electronegativities. Fig. 8 represents a relation between populations of four-coordinated boron atoms and of Si(Q3 ) in the yK2 O Æ ð0:139 yÞCaO Æ 0.673SiO2 Æ (0:188 x)-
H. Yamashita et al. / Journal of Non-Crystalline Solids 331 (2003) 128–136
Fig. 7. Relation between XBðQ4 Þ and XSiðQ3 Þ in the 0.139MO (M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ K) glasses.
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Fig. 8. Relation between XBðQ4 Þ and XSiðQ3 Þ in the yK2 O Æ (0:139 y)CaO Æ 0.673SiO2 Æ (0:188 x)Al2 O2 Æ xB2 O3 glasses.
Table 2 Apparent equilibrium constants, Kapp , of the reactions, Si(Q3 ) + B(Q3 ) = Si(Q4 ) + B(Q4 ), in the 0.139 MO(M02 O) Æ 0.673 SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses Species
Kapp values
MgO CaO SrO BaO Na2 O K2 O
0.04 ± 0.04 0.3 ± 0.1 1.1 ± 0.3 4.0 ± 0.4 8±3 18 ± 5
Al2 O3 Æ xB2 O3 glasses. The slope increases with increase in the y value. These results suggest that the reactivity of K2 O with B2 O3 was high compared to the one of CaO. From their results, the apparent stability constant, Kapp , of the Eq. (1) were determined. Relation between log Kapp and [K2 O]/ ([K2 O] + [CaO])¼ z are shown in Fig. 9. This curve seems to have the inflection point at z ¼ 0:5. The dotted line at 0 5 z 5 0:5 was determined empirically as follows. Kapp values at z ¼ 0 and 1 are expressed KCa and KK , respectively. Kapp values were determined from Eq. (2): 1=Kapp ¼ ð1 2zÞ=KCa þ 2z=Kh :
ð2Þ
Here, log Kh equals ðlog KCa þ log KK Þ=2. The line at 0:5 5 z 5 1 was obtained by rotating 180° at
Fig. 9. Relation between log Kapp and [K2 O]/([K2 O] + [CaO]).
z ¼ 0:5 with the former line. This line showed agreement with the experimental data. From the Kapp values, the relation between glass composition and glass structural units can be completely elucidated.
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5. Conclusions The 11 B, 27 Al and 29 Si MAS-NMR spectra of 0.139MO(M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ Na and K) glasses were examined. In the M02 O systems, the populations of four-coordinated boron atoms, N4 , are expressed well with r=ð1 rÞ, where r ¼ ([M02 O])[Al2 O3 ])/([M02 O])[Al2 O3 ]+[B2 O3 ]). However, in the MO systems the difference from the r=ð1 rÞ increases in the order of BaO < SrO < CaO < MgO. In these glasses, the reactivity of MO with SiO2 was high compared to the reactivity of MO with B2 O3 in the order described above. The apparent equilibrium constants, Kapp , of the reactions, Si(Q3 ) + B(Q3 ) ¼ Si(Q4 ) + B(Q4 ), were determined. The Kapp values indicate the reactivity of MO or M02 O with B2 O3 and SiO2 in the 0.139MO (or M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. The reaction proceeds to the right direction in the order of MgO < CaO < SrO < BaO < Na2 O < K2 O. The order of the reactivity is the same as that of the electronegativities among alkaline and alkaline earth metals.
Acknowledgements The authors wish to thank the Advanced Instrumentation Center for Chemical Analysis and the Center for Cooperative Research and Devel-
opment, Ehime University, for the NMR measurements. This work was supported in part by a grant-in-aid for Scientific Research (no. 13450357) from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] S. Greenblatt, P.J. Bray, Phys. Chem. Glasses 8 (1967) 190. [2] Y.D. Yiannopoulos, G.D. Chryssikos, E.I. Kamitsos, Phys. Chem. Glasses 42 (2001) 164. [3] K.S. Kim, P.J. Bray, Phys. Chem. Glasses 15 (1974) 47. [4] S.G. Bishop, P.J. Bray, Phys. Chem. Glasses 7 (1966) 73. [5] A.R. Jones, R. Winter, G.N. Greaves, I.H. Smith, J. NonCryst. Solids 293–295 (2001) 87. [6] P. Zhao, S. Kroeker, J.F. Stebbins, J. Non-Cryst. Solids 276 (2000) 122. [7] V.P. Klyuev, B.Z. Pevzner, Phys. Chem. Glasses 41 (2000) 380. [8] G. Gruener, D. DeSousa Meneses, P. Odier, J.P. Loup, J. Non-Cryst. Solids 281 (2001) 117. [9] V.K. Shrikhande, V. Sudarsan, G.P. Kothiyal, S.K. Kulshreshtha, J. Non-Cryst. Solids 283 (2001) 18. [10] H. Yamashita, H. Yoshino, K. Nagata, H. Inoue, T. Nakajin, T. Maekawa, J. Non-Cryst. Solids 270 (2000) 48. [11] H. Amemiya, Am. Ceram. Soc. 29 (1993) 131. [12] H. Yamashita, H. Yoshino, K. Nagata, I. Yamaguchi, M. Ookawa, T. Maekawa, J. Ceram. Soc. Jpn. 106 (1998) 539. [13] H. Yamashita, K. Nagata, H. Yoshino, K. Ono, T. Maekawa, J. Non-Cryst. Solids 248 (1999) 115. [14] H. Maekawa, T. Maekawa, K. Kawamura, T. Yokokawa, J. Phys. Chem. 95 (1991) 6822. [15] T. Yokokawa, S. Kohsaka, J. Chem. Eng. Data 24 (1979) 167.
Nuclear magnetic resonance studies of 0.139MO (or M02O) Æ 0.673SiO2 Æ (0:188 x)Al2O3 Æ xB2O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ Na and K) glasses Hiroshi Yamashita, Kazuhiko Inoue, Takeshi Nakajin, Hyuma Inoue, Takashi Maekawa * Department of Applied Chemistry, Faculty of Engineering, Ehime University, Bunkyo-cho, Matsuyama 790-8577, Japan Received 12 February 2003; received in revised form 9 May 2003
Abstract The 11 B, 27 Al and 29 Si magic angle spinning (MAS) NMR spectra of 0.139MO (or M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 Æ (M ¼ Mg, Ca, Sr and Ba, M0 ¼ Na and K) glasses were examined. When three network forming oxides such as SiO2 , Al2 O3 and B2 O3 coexist, Al2 O3 reacts preferably with network modifier such as M02 O or MO. The reactivity of residual network modifier with SiO2 was high compared with B2 O3 in the order of K2 O < Na2 O < BaO < SrO < CaO < MgO. The apparent equilibrium constants, Kapp , of the reactions, Si(Q3 ) + B(Q3 ) ¼ Si(Q4 ) + B(Q4 ), were determined. Here, the Si(Qn ) or B(Q3 ) and B(Q4 ) represent the SiO2 structural units or three- and four-coordinated boron atoms, respectively. The subscript are the number of bridging oxygen atoms. The Kapp values indicate the reactivity of MO or M02 O with B2 O3 and SiO2 in the MO(M02 O) Æ SiO2 Æ Al2 O3 Æ B2 O3 glasses. The reaction proceeds to the left direction in the order of MgO > CaO > SrO > BaO > Na2 O > K2 O. The order of the reactivity is the same as that of the electronegativities among alkali and alkaline earth metals. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Aluminoborosilicate glasses have been used for liquid crystal display glasses. In particular alkali ion-free glasses have a bright prospect of the chemical stability and the mechanical strength. In order to see the physicochemical properties, it is
* Corresponding author. Tel.: +81-89 927 9926; fax: +81-89 927 9943. E-mail address: [email protected] (T. Maekawa).
important to see the relation between the structures and the composition of the glasses. It is also interesting that the glass-forming tendency of the aluminoborosilicate glasses is influenced by addition of alkaline earth metals into the glasses. However, it is not shed light on the tendency. Greenblatt et al. [1] reported the studies of xBaO Æ (1 x)B2 O3 glasses by 11 B NMR. It was shown that alkaline earth ions behave like alkali modifier ions and the fraction of four-coordinated boron atoms, N4 , follows the x=ð1 xÞ law as in alkali borate glasses. In fact the composition dependence of N4 in the barium borate system was found to be very similar to that
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.08.086
H. Yamashita et al. / Journal of Non-Crystalline Solids 331 (2003) 128–136
in the lithium and sodium borate glasses. Yiannopoulos et al. [2] allowed the quantification of network structure in terms of the fraction of N4 in the alkaline earth borate glasses by the infrared reflectance spectra. It was found that N4 increased with alkaline earth oxide content, MO, and attained maximum values at compositions depending on the M2þ ion type. N4 was found to decrease from Ba to Mg, i.e. upon increasing cation field strength, a trend opposite to that exhibited by alkali borate glasses. Kim et al. [3] determined on the N4 values in the MgO Æ Na2 O Æ B2 O3 glasses by 11 B NMR. The both Na2 O and MgO behaved as network modifiers in the glasses containing less than 0.15 molar fraction of MgO. As the MgO concentration increases further, MgO becomes progressively a network former, presumably in the form of MgO4 tetrahedra. Bishop et al. [4] measured the fraction of N4 in the calcium boroaluminate glasses by 11 B magic angle spinning (MAS)-NMR. For a given CaO concentration, the values of N4 in the aluminum-containing glasses were lower than those for the aluminum-free glasses. It was indicated that the aluminum atoms also assumed a tetrahedral bonding configuration. Jones et al. [5] reported for structural characterization of sodalime-silicate glasses by 29 Si and 23 Na MAS-NMR. The addition of CaO introduced a greater distortion into the structure. This result suggested that Ca preferentially arranged in the vicinity of Q2 species rather than Q3 . In particular the structure of alkali ion-free glasses have recently been studied by numerous workers [6–9]. In a previous paper, the 11 B, 27 Al and 29 Si MASNMR spectra of 0.188MO (or M02 O) Æ 0.673SiO2 Æ 0.077Al2 O3 Æ 0.062B2 O3 (M ¼ Ca, Sr and Ba, M0 ¼ Na) glasses were examined [10]. The compositions of the glasses of this series are based on the composition of alkali-free glass (code AN635) by Asahi glass Co. Ltd. [11]. The fraction of threecoordinated boron atoms decreased in the order of CaO > SrO > BaO > Na2 O. When three network forming oxides such as SiO2 , Al2 O3 and B2 O3 coexist in the 0.139Na2 O Æ 0.673SiO2 Æ (0:188 x) Al2 O3 Æ xB2 O3 glasses, Al2 O3 reacts preferably with Na2 O. The populations of four-coordinated boron atoms, N4 , are expressed well with r=ð1 rÞ, where r ¼ ([Na2 O] ) [Al2 O3 ])/([Na2 O] ) [Al2 O3 ] + [B2 O3 ]).
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In the present study, the difference of the reactivity among coexisting network forming oxides such as SiO2 , Al2 O3 and B2 O3 with alkali and alkaline earth oxides will be compared by the NMR measurements in the 0.139MO (or M02 O) Æ 0.673 SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ Na and K) glasses. This composition is chosen so that [MO] < ([Al2 O3 ] + [B2 O3 ]). The apparent equilibrium constants, Kapp , of the reactions, Si(Q3 ) + B(Q3 ) ¼ Si(Q4 ) + B(Q4 ), were determined.
2. Experimental Samples were prepared from reagent grade Na2 CO3 , K2 CO3 , H3 BO3 , SiO2 , Al2 O3 , MgO, CaCO3 , SrCO3 and BaO. Mixed starting materials were carefully melted in a platinum crucible at 1200–1600 °C using an electric furnace and poured into a cold metal plate. The obtained samples were optically clear. In all samples, a small amount of Gd2 O3 (0.05 mol%) was added to batches as a spin relaxation reagent for the 29 Si NMR measurements. The 11 B, 27 Al, and 29 Si MAS-NMR spectra were recorded on a JEOL JNM-CMX 300 spectrometer. Other experimental details were described in the previous papers [12,13]. The 29 Si NMR spectra were deconvoluted with Gaussian peaks. Setting values of the isotropic chemical shift, quadruple coupling constant and quadruple asymmetric parameters made the deconvolutions of 11 B spectra. X-ray powder patterns were recorded on a diffractometer and did not show crystalline phases in any glasses.
3. Results 3.1. 0.139M02 O0.673SiO2 (0:188 x)Al2 O3 xB2 O3 glasses Fig. 1(a) and (b) represent the 29 Si NMR spectra of 0.139M02 O Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. Here it is aimed to see the relative reactivity of Al2 O3 and B2 O3 with M02 O. With an increase in the B2 O3 content, the peak position moves to
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Fig. 1. 29 Si(a, b) and 11 B(c) MAS-NMR spectra of 0.139M02 O Æ 0.673SiO2 Æ (0:188 x)Al2 O2 Æ xB2 O2 glasses (a, c: M0 ¼ K, b: M0 ¼ Na). The solid and broken lines denote the experimental and deconvolution data, respectively.
higher magnetic field side and the peak width increases. The 11 B NMR spectra of 0.139K2 O Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses are shown in Fig. 1(c). The 11 B NMR spectra of the Na2 O glasses were reported in previous paper [10]. These spectra consist of two signals. One is a sharp signal around 0 ppm and the other is the broad ones from )20 to +20 ppm. The former is originated from the four-coordinated boron
and the latter from the three-coordinated boron atom. The fraction of three-coordinated boron atoms decreased slightly with an increase in B2 O3 content. Fig. 2 represents the 27 Al NMR spectrum of the glass (x ¼ 0:094). This spectrum consists of a peak around 50 ppm. As described previously [10], this peak is assigned to Al(Q4 ) [14]. The spectra of the other compositions were also similar to that shown in Fig. 2.
H. Yamashita et al. / Journal of Non-Crystalline Solids 331 (2003) 128–136
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SiO2 Æ Al2 O3 Æ B2 O3 glasses. A shoulder around )90 ppm in the K2 O Æ CaO glasses are large compared with the K2 O glasses (Fig. 1(a)). The 11 B NMR spectra of the glasses are shown in Fig. 5(b). The fraction of three-coordinated boron atoms in the former glasses is large compared with the latter ones (Fig. 1(c)).
4. Discussion 4.1. Reactivity of MO or M02 O with SiO2 , Al2 O3 and B2 O3
Fig. 2. 27 Al MAS-NMR spectra of 0.139MO(M02 O) Æ 0.673SiO2 Æ 0.094Al2 O3 Æ 0.094B2 O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ K) glasses.
3.2. 0.139MO 0.673SiO2 (0:188 x)Al2 O3 xB2 O3 glasses Fig. 3 represents the 29 Si NMR spectra of the 0.139MO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. In all the glasses, the apparent peak positions move to higher magnetic field with an increase in B2 O3 content. A shoulder around )90 ppm decreases in the order of MgO > CaO > SrO > BaO. The 11 B NMR spectra of the glasses are shown in Fig. 4. The fraction of three-coordinated boron atoms decreases in the order of MgO > CaO > SrO > BaO and with an increase in B2 O3 content. Fig. 2 represents the 27 Al NMR spectra of the 0.139MO Æ 0.673SiO2 Æ 0.094Al2 O3 Æ 0.094B2 O3 glasses. These spectra also consist of a peak around 50 ppm. As seen above this peak is assigned to Al(Q4 ). The peak positions do not differ among the 0.139MO Æ 0.673SiO2 Æ (0:188 x) Al2 O3 Æ xB2 O3 glasses. 3.3. K2 O CaO SiO2 Al2 O3 B2 O3 glasses Fig. 5(a) represents the 29 Si NMR spectra of 0.099K2 O Æ 0.040CaO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses as a representative of K2 O Æ CaO Æ
The change of the NMR spectra can be understood by a competition among the three network forming oxides (SiO2 , B2 O3 and Al2 O3 ) with basic oxides. Maekawa et al. investigated the structural study of sodium aluminosilicate glasses by 29 Si NMR [14]. These spectra were shifted to lower magnetic field with increasing aluminum oxide contents because Al2 O3 dissolves in silicate as NaAlO2 . In the previous paper [10], Na2 O reacted preferentially with Al2 O3 and only the residual Na2 O reacted with B2 O3 to form fourcoordinated boron atoms in the Na2 O Æ SiO2 Æ Al2 O3 Æ B2 O3 glasses. In the K2 O Æ SiO2 Æ Al2 O3 Æ B2 O3 glasses, it is also observed that the profiles of 27 Al NMR do not change with replacing Al2 O3 by B2 O3 . The 29 Si NMR spectra have a bulge on the low magnetic field side and the peak position moves toward high magnetic fields with an increase in B2 O3 content, which is due to the formation of Si(Q3 ). This means excision of silicate network proceeds. Table 1 represents the distribution of Si(Qn ) species determined by the deconvolution of the 29 Si NMR spectra, X obs . The proportion of Si(Q3 ) increases slightly with an increase in B2 O3 content. On the other hand, the proportion of B(Q4 ) increases largely with an increase in B2 O3 content. These results suggest that K2 O reacted preferentially with Al2 O3 and the residual K2 O reacted with B2 O3 rather than with SiO2 , i.e., K2 O was effectively employed the formation of B(Q4 ) species rather than the formation of Si(Q3 ) species. Fig. 6 represents a relation between populations of four-coordinated boron atoms and r ( ¼ ([K2 O] ) [Al2 O3 ])/([K2 O] ) [Al2 O3 + B2 O3 ])), as
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Fig. 3. 29 Si MAS-NMR spectra of the 0.139MO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses (a: Mg, b: Ca, c: Sr and d: Ba). These lines are the same as in Fig. 1.
described previously [10]. Here, the data of Fig. 1(c) is used. Generally the normalized standard errors of the deconvolution of the NMR spectra were within 9.9%. The fraction of fourcoordinated boron increases with an increase in r. A solid line is the theoretical one which is expected in xNa2 O Æ (1 x)B2 O3 binary glasses; the populations of four-coordinated boron atoms, N4 ¼ x=ð1 xÞ. Experimental values coincide well with the theoretical line. One also sees K2 O react preferentially with Al2 O3 and only the residual K2 O react with B2 O3 to form four-coordinated
boron atoms. However, in the MO glasses the difference from the r=ð1 rÞ increases in the order of BaO < SrO < CaO < MgO. This fact suggests that MO reacted preferentially with Al2 O3 and the reactivity of residual MO with SiO2 was high compared to the reactivity with B2 O3 in the order described above. 4.2. Stability constant Fig. 7 represents a relation between populations of four-coordinated boron atoms and of Si(Q3 ).
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Fig. 4.
11
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B MAS-NMR spectra of the 0.139MO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses (a: Mg, b: Ca, c: Sr and d: Ba).
Here, the data of Figs. 1, 3 and 4, and previous data [10] are used. XBðQ4 Þ and XSiðQ3 Þ values increase with a decrease in Al2 O3 content. The slope increases in the order of MgO < CaO < SrO < BaO < Na2 O < K2 O. These results suggest that the reactivity of M02 O or MO with B2 O3 was high compared to the reactivity of M02 O or MO with SiO2 in the order described above. From these results, the apparent stability constant, Kapp , of Eq. (1) were determined. SiðQ3 Þ þ BðQ3 Þ ¼ SiðQ4 Þ þ BðQ4 Þ:
ð1Þ
The stability constants are expressed as Kapp ¼ [Si(Q4 )] [B(Q4 )]/[Si(Q3 )][B(Q3 )]. Table 2 represents
the Kapp values in the 0.139MO (or M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. The order of the reactivity is the same as that of the electronegativities among alkaline and alkaline earth metals. In binary glass melts containing Na2 O, the thermodynamic acid strength was determined by EMF measurements to be B2 O3 > SiO2 [15]. In sodium borosilicate glasses, the strong base such as Na2 O reacts mainly with the strong acid such as B2 O3 . Thus, the homogeneous glass separates into an alkali rich borate phase and SiO2 one by heat treatment. It is considered that the weak base such as MgO reacts mainly with the weak acid such as SiO2 . The
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Fig. 5. 29 Si(a) and 11 B(b) MAS-NMR spectra of 0.099K2 O Æ 0.040CaO Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. These lines are the same as in Fig. 1. Table 1 Distribution of Si(Q3 ) determined by the deconvolution of the 11 B and 29 Si NMR spectra of 0.139MO (M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses MO or M02 O
x
B(Q4 )
XSiðQ3 Þobs
MgO
0.049 0.094 0.116
0.017 0.012 0.022
0.217 0.392 0.370
CaO
0.049 0.094 0.116 0.139
0.022 0.064 0.117 0.107
0.107 0.150 0.252 0.409
SrO
0.049 0.094 0.116 0.139
0.047 0.133 0.217 0.267
0.049 0.098 0.186 0.287
BaO
0.049 0.094 0.116 0.139
0.042 0.223 0.301 0.374
0.015 0.070 0.095 0.124
Na2 O
0.126 0.157 0.188
0.479 0.515 0.661
0.110 0.110 0.157
0.072 0.094 0.116 0.139
0.254 0.353 0.478 0.508
0.016 0.034 0.050 0.062
K2 O
Fig. 6. Correlation between glass composition and the population of threefold boron atoms from 11 B MAS-NMR spectra in 0.139MO (M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 systems. Here, r is ([MO or M02 O] ) [Al2 O3 ])/([MO or M02 O] ) [Al2 O3 ] + [B2 O3 ]).
strength of base coincides with the electronegativities. Fig. 8 represents a relation between populations of four-coordinated boron atoms and of Si(Q3 ) in the yK2 O Æ ð0:139 yÞCaO Æ 0.673SiO2 Æ (0:188 x)-
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Fig. 7. Relation between XBðQ4 Þ and XSiðQ3 Þ in the 0.139MO (M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ K) glasses.
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Fig. 8. Relation between XBðQ4 Þ and XSiðQ3 Þ in the yK2 O Æ (0:139 y)CaO Æ 0.673SiO2 Æ (0:188 x)Al2 O2 Æ xB2 O3 glasses.
Table 2 Apparent equilibrium constants, Kapp , of the reactions, Si(Q3 ) + B(Q3 ) = Si(Q4 ) + B(Q4 ), in the 0.139 MO(M02 O) Æ 0.673 SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses Species
Kapp values
MgO CaO SrO BaO Na2 O K2 O
0.04 ± 0.04 0.3 ± 0.1 1.1 ± 0.3 4.0 ± 0.4 8±3 18 ± 5
Al2 O3 Æ xB2 O3 glasses. The slope increases with increase in the y value. These results suggest that the reactivity of K2 O with B2 O3 was high compared to the one of CaO. From their results, the apparent stability constant, Kapp , of the Eq. (1) were determined. Relation between log Kapp and [K2 O]/ ([K2 O] + [CaO])¼ z are shown in Fig. 9. This curve seems to have the inflection point at z ¼ 0:5. The dotted line at 0 5 z 5 0:5 was determined empirically as follows. Kapp values at z ¼ 0 and 1 are expressed KCa and KK , respectively. Kapp values were determined from Eq. (2): 1=Kapp ¼ ð1 2zÞ=KCa þ 2z=Kh :
ð2Þ
Here, log Kh equals ðlog KCa þ log KK Þ=2. The line at 0:5 5 z 5 1 was obtained by rotating 180° at
Fig. 9. Relation between log Kapp and [K2 O]/([K2 O] + [CaO]).
z ¼ 0:5 with the former line. This line showed agreement with the experimental data. From the Kapp values, the relation between glass composition and glass structural units can be completely elucidated.
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5. Conclusions The 11 B, 27 Al and 29 Si MAS-NMR spectra of 0.139MO(M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 (M ¼ Mg, Ca, Sr and Ba, M0 ¼ Na and K) glasses were examined. In the M02 O systems, the populations of four-coordinated boron atoms, N4 , are expressed well with r=ð1 rÞ, where r ¼ ([M02 O])[Al2 O3 ])/([M02 O])[Al2 O3 ]+[B2 O3 ]). However, in the MO systems the difference from the r=ð1 rÞ increases in the order of BaO < SrO < CaO < MgO. In these glasses, the reactivity of MO with SiO2 was high compared to the reactivity of MO with B2 O3 in the order described above. The apparent equilibrium constants, Kapp , of the reactions, Si(Q3 ) + B(Q3 ) ¼ Si(Q4 ) + B(Q4 ), were determined. The Kapp values indicate the reactivity of MO or M02 O with B2 O3 and SiO2 in the 0.139MO (or M02 O) Æ 0.673SiO2 Æ (0:188 x)Al2 O3 Æ xB2 O3 glasses. The reaction proceeds to the right direction in the order of MgO < CaO < SrO < BaO < Na2 O < K2 O. The order of the reactivity is the same as that of the electronegativities among alkaline and alkaline earth metals.
Acknowledgements The authors wish to thank the Advanced Instrumentation Center for Chemical Analysis and the Center for Cooperative Research and Devel-
opment, Ehime University, for the NMR measurements. This work was supported in part by a grant-in-aid for Scientific Research (no. 13450357) from the Ministry of Education, Science, Sports and Culture, Japan.
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