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The influence of NiO on phase separation and crystallization of glasses of the MgO–Al2O3–SiO2–TiO2 system
Journal of Non-Crystalline Solids 345&346 (2004) 187–191 www.elsevier.com/locate/jnoncrysol
The influence of NiO on phase separation and crystallization of glasses of the MgO–Al2O3–SiO2–TiO2 system V.V. Golubkov a, T.I. Chuvaeva b, O.S. Dymshits b,*, A.A. Shashkin b, A.A. Zhilin b, W.-B. Byun c, K.-H. Lee c a
IV Grebenschikov Institute of Silicate Chemistry, Russian Academy of Science, Odoevskogo str., St. Petersburg 199155, Russian Federation b SI Vavilov State Optical Institute, ul. Babushkina 36/1, St. Petersburg 192171, Russian Federation c Korea Electrotechnology Research Institute, Kyonggi-Do 437-808, South Korea Available online 15 September 2004
Abstract The evolution of structure and phase composition of NiO-doped titania-containing magnesium aluminosilicate glasses has been studied using small angle X-ray scattering and X-ray diffraction analysis. After initial heat-treatments, three-phase immiscibility develops resulting in precipitation of two amorphous phases. One of them is enriched in MgO, Al2O3 and TiO2, the other in MgO and Al2O3. Nickel oxide enters both phases, which impedes to crystallization of the magnesium aluminotitanates phase; nickel-containing aluminomagnesium regions become centers of further crystallization of aluminomagnesium spinel. The spinel crystallization temperature, as well as mean sizes decreases with increasing the NiO content in the initial glass. Ó 2004 Published by Elsevier B.V. PACS: 61.10.E; 61.43.F; 81.05.P
1. Introduction Previously, we studied the influence of NiO on phase decomposition of titania-containing lithium aluminosilicate glasses [1]. In glasses doped by NiO at early stages of heat-treatment sizes of inhomogeneous regions and jDqj2, the square of the mean difference of electronic densities inside the inhomogeneous regions and the glass matrix, rapidly increase; the jDqj2 is proportional to NiO content. It implies that phase containing nickel oxide precipitates upon phase decomposition, the structure formed determines the structure of phase separated material and of resultant glass–ceramics. In [2] we studied the kinetics of structure and phase composition change upon heat-treatment of magnesium
*
Corresponding author. Tel.: +7 812 560 1911; fax: +7 812 560 1022. E-mail address: [email protected] (O.S. Dymshits). 0022-3093/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2004.08.020
aluminosilicate glasses nucleated with titania. Small angle X-ray scattering (SAXS) data showed that upon phase decomposition two kinds of amorphous phases are formed. After further heat-treatments magnesium aluminotitanates amorphous regions crystallize as MgO Æ 2TiO2–Al2O3 Æ TiO2 solid solutions, later spinel, MgAl2O4, crystallizes in aluminomagnesium regions. The objective of this paper was to study the influence of NiO on phase separation and crystallization of magnesium aluminosilicate glasses nucleated by TiO2. 2. Experimental procedure We present the results of SAXS and X-ray diffraction analysis (XRD) study of phase transformations in magnesium aluminosilicate glasses with equimolar contents of MgO and Al2O3 nucleated by TiO2 and doped by 0.5–5.0 mol% NiO. Glasses were melted in quartz crucibles at 1550 °C for 4 h with stirring, poured onto a metal
V.V. Golubkov et al. / Journal of Non-Crystalline Solids 345&346 (2004) 187–191
plate and annealed at 660 °C. XRD patterns of powdered samples were measured using Cu Ka radiation with a Ni filter. Relative fractions of crystallinity were estimated by measuring the square of the most intensive peak of a crystalline phase. The mean crystal sizes were estimated from broadening of X-ray peaks. We measured the intensity of X-ray scattering at angles of 8-550 using Cu Ka radiation. The samples were placed into a high-temperature furnace attached to a small angle X-ray setup. Heat-treatments under isothermal conditions in the temperature interval from 700 to 1000 °C were carried out in this furnace. The experimental details are described elsewhere [1–3].
16
7 14 12
6
10
Intensity / a.u.
188
5
8 6
4 3
4
3. Results
2
All initial glasses are inhomogeneous and X-ray amorphous. As is seen from Table 1, with increasing the NiO content, jDq2jof initial glasses increases. On heat-treatment, the intensity of scattering by inhomogeneous structure of glasses increases. All SAXS curves by heat-treated glasses demonstrate a maximum. Fig. 1 depicts scattering by samples doped by 3.0% NiO, a pronounced maximum appears on the curve after heating at 800 °C for 10 min, its location remains unchanged upon increasing time and temperature of heat-treatment (Fig. 1, curves 2–6). Its intensity constantly increases due to continued liquid-phase separation and crystalli-
0
2 1 0
10
20
30
40
50
60
70
80
90 100 110
Scattering angle, ϕ / ' Fig. 1. Intensity of SAXS by initial and heat-treated glasses doped with 3.0 mol% NiO: 1 – initial glass; 2 – 800 °C, 10 min; 3 – 800 °C, 2 h; 4 – 800 °C, 4 h; 5 – 800 °C, 4 h + 860 °C, 1 h; 6 – 800 °C, 4 h + 860 °C, 2 h; 7 – 800 °C, 4 h + 860 °C, 2 h + 960 °C, 2 h (scale 1:2).
zation in precipitated regions. For glasses doped by 0.5% and 1.0% NiO, during heating at 860 °C the position of the maximum at SAXS curves shifts to larger angles. For glass doped by 5.0 mol% NiO, the maximum
Table 1 Radii R of amorphous regions of inhomogeneity in initial and heat-treated glasses NiO content (mol%)
0
0.5
1.0
3.0
5.0
Heat-treatment schedule
SAXS data
T (°C)
˚) R (A
– 800 860 960 – 800 860 960 – 800 860 960 – 800 860 960 – 800 860 960
s (h)
– 21 6 6 – 3 3 2 – 3 3 2 – 4 3 2 – 3 3 2
60 53 53 55 55 34 41 30 25 27 40 22 26.5 28.5 40 38 25 27 39
XRD data ˚ 3 )2 jDqj2 Æ 102 (e/A
0.2 0.4 1.65 3.3 0.25 0.4 2 3.1 0.24 0.65 2 3.6 0.3 1.55 2.7 3.8 0.5 1.8 3 4
um ( 0 ) – 16 18 15 – 17 31 23 – 28 38 25 – 35 35 26 – – 32 26
2h = 37° ˚) R (A
S (cm2)
2h = 26° ˚) R (A
S (cm2)
– 25.0 20.0 27.0 – 20.0 22.0 24.0 – 11.5 20.0 25.0 – 16.0 18.5 22.0 – 15.0 16.5 21.0
– 2.9 8 19.5 – 4.7 9 19 – 5 6 19 – 3.2 5 18 – 3 8 18
– 60.0 70.0 70.0 – 40.0 57.5 65.0 – – 40.0 65.0 – – – 82.5 – – – –
– 3.8 6 7 – 3.5 5 7.5 – – 3 5.3 – – – 1.5 – – – –
jDqj2 magnitudes, positions of maxima on SAXS curves, um, integral intensities of reflections at 2h = 26 and 37° and radii of microcrystals derived from widths of these reflections.
V.V. Golubkov et al. / Journal of Non-Crystalline Solids 345&346 (2004) 187–191
o
4.0
960 C o
3.5
860 C
o
800 C I∆ρI2102 / (e/A3)2
189
3.0
4
2.5
3 2
2.0 1.5
1 1.0 0.5 0.0 0
2
4
6
8
time / h 60
o
o
o
800 C
55
860 C
960 C 1
Radius / angstrem
50 45 40
2
35 30 25 20 15 10 5 0 0
2
4
6
8
time / h
Fig. 2. Variation of jDqj2 (a) and of radii R of amorphous regions of inhomogeneity (b) with time of heat-treatment at 800 °C, with time of additional heat-treatment at 860 °C and with additional hold at 960 °C for 2 h. (a) 1 – without NiO; 2 – 1.0 mol% NiO; 3 – 3.0 mol% NiO; 4 – 5.0 mol% NiO. (b) 1 – without NiO; 2 – 3.0 mol% NiO.
is formed only during holding at 860 °C. For all glasses, after heat-treatment at 960 °C the maximum shifts to smaller angles. At every stage of heat-treatment, jDqj2 increases with time while its magnitude increases with increasing the NiO content (Fig. 2(a)). After certain holding time, the growth of jDqj2 is discontinued (Fig. 2(a)). The sizes of regions of inhomogeneity do not change pronouncedly: after short holding times they slightly decrease and then increase again. A significant growth of R is observed during heat-treatment at 960 °C. XRD pattern of glass without NiO heat-treated at 800 °C for 2 h and additionally at 860 °C for 3 h (Fig. 3, curve 1) shows the main peaks of magnesium aluminotitanates (the most intense one located at about 2h = 26°). This pattern also exhibits the peak at 2h = 37°, usually attributed to the most intense peak of spinel. However the absence of the other spinel reflections (marked by a dashed line in Fig. 3) implies that in this material the peak at 37° belongs to magnesium aluminotitanates. XRD patterns of glass with 3% NiO reveal that not only after heat-treatment at 800 °C for 4 h (Fig. 3, curve 2) but even after additional heating at 860 °C for 2 h (Fig. 3, curve 3) there is no any peak
Fig. 3. Parts of XRD pattern of: 1 – glass without NiO heat-treated at 800 °C, 2 h + 860 °C, 3 h; 2, 3 – glass with 3.0 mol% NiO heat-treated: 2 – 800 °C, 4 h; 3 – 800 °C, 4 h + 860 °C, 2 h.
of magnesium aluminotitanates crystals, spinel is the only crystalline phase. XRD patterns of glass–ceramics, prepared with the last hold at 960 °C, show that with increasing the NiO contents, the intensity of peaks of magnesium aluminotitanates gradually decreases to such extent that in glass– ceramics with 5.0% NiO those crystals do not precipitate at any crystallization temperature (Fig. 4, curve 1). The
Fig. 4. Parts of XRD patterns of glasses heat-treated at 800 + 860 + 960°. (1) – with 5.0 mol%; (2) – with 3.0 mol% NiO; (3) – with 1.0 mol% NiO; (4) – without NiO.
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intensity of XRD at 2h = 37° is independent of NiO content while that at 2h = 26° decreases with its increasing (Table 1). The peak at 2h = 37° is a superposition of reflections due to spinel and magnesium aluminotitanates. Assuming that intensities of peaks of magnesium aluminotitanates located at 26° and 37° are equal, one can see that the intensity of spinel peak increases with increasing the NiO content.
4. Discussion The kinetics of phase transformations in NiO-containing glasses and in glasses without NiO are similar. An increase of time of heat-treatment at 800 and 860 °C leads to a stabilization of jDqj2 which can be considered as a result of approaching to the steady state probably due to viscosity increasing upon enrichment of the residual glass by SiO2. Simultaneously with liquid-phase decomposition, crystallization takes place after heating for 3–4 h at 800 °C. After holding at 860 °C, jDqj2 increases. In magnesium aluminosilicate glasses doped by NiO like in NiO-free glass, the primary process resulting in a microinhomogeneous structure is a three-phase immiscibility with the formation of magnesium aluminotitanate and aluminomagnesium amorphous phases. In the first phase MgO Æ 2TiO2–Al2O3 Æ TiO2 solid solutions crystallize, in the second phase spinel precipitates. However due to addition of NiO, kinetics of phase decomposition, compositions and sizes of amorphous and crystalline regions of inhomogeneity change. The jDqj2 magnitude linearly depends on NiO content at any stage of heat-treatment (Fig. 5). This dependence is more pronounced at low than at high heat-treatment temperatures. It implies that NiO influences phase separation rather than crystallization probably due to increased NiO contents in liquid-phase separated regions, which
3
4.0
I∆ρI2102 / (e/A3)2
3.5
2
3.0 2.5
1
2.0 1.5 1.0 0.5 0.0 0
1
2
3
4
5
NiO / mol%
Fig. 5. Dependence of jDqj2 on NiO content after heat-treatments at different schedules: 1 – 800 °C, 3 h; 2 – 800 °C, 4 h + 860 °C, 3 h; 3 – 800 °C, 4 h + 860 °C, 3 h + 960 °C, 2 h.
is connected with higher mobility of NiO-containing structural elements. The addition of NiO pronouncedly influences the phase composition of heat-treated glasses and sequence of phase precipitation. In glasses containing 1% NiO and up, spinel precipitates prier to crystallization of magnesium aluminotitanates, after heat-treatment at 800 °C. Under additional heating at 860 °C, in the sample with 1% NiO appears the peak at 2h = 26 °, and the integral intensity of the peak at 37° increases by the magnitude of the integral intensity at 2h = 26°. So the increase of the integral intensity at 37° can be explained by crystallization of magnesium aluminotitanates and small increase in the volume of spinel can be considered. However according to Table 1, at this heat-treatment, jDqj2 increases by a factor of three, which is partially accounted for by crystallization, but mainly can be explained by the precipitation of amorphous phase. It means is this phase is aluminomagnesium, as after heat-treatment at 960 °C the volume of spinel pronouncedly increases. For glasses with 3.0% and 5.0% NiO crystallization of spinel is also observed at 800 °C. In these glasses the pronounced increase of integral intensity of the peak at 2h = 37° is also reached after heat-treatment at 960 °C. It implies that the gain in spinel precipitation is observed after that heat-treatment. We believe that appearance of spinel at 800 °C and cessation or slow-down of its crystallization at higher temperatures is due to the fact that at low heattreatment temperatures the aluminomagnesium inhomogeneous regions are enriched in NiO. Crystallization develops with high velocity, and at 800 °C NiAl2O4 spinel or solid solutions of NiAl2O4–MgAl2O4 precipitate. An appearance of the maximum on SAXS curves proves the spinodal mechanism of phase decomposition. Upon addition of NiO into initial glass, the maximum at the SAXS curve shifts to larger angles which reflects the decrease of the distance between centers, i.e., the increase of the number of regions, which leads to decrease of their sizes. At further heat-treatments these regions become the centers of precipitation of amorphous aluminomagnesium phase. As new regions do not appear, NiO exerts primary control over the final state of the material structure, distribution of aluminomagnesium regions and their sizes. Spinel crystallization is completed after heat-treatment at 960 °C, as sizes of microcrystals are determined by sizes of amorphous aluminomagnesium regions, they also depend on the NiO content in glass. The NiO enters into both precipitated phases. With increasing NiO content, the crystallization ability of magnesium aluminotitanate phase decreases, and the volume of aluminotitanate crystals decreases. Upon heat-treatment at 960 °C sizes of inhomogeneous regions increase and the position of the maximum shifts to smaller angles, which testifies the Ostwalds ripening processes. The regularity of microin-
V.V. Golubkov et al. / Journal of Non-Crystalline Solids 345&346 (2004) 187–191
homogeneous structure results in a pronounced decrease in SAXS scattering at close to zero angles and in decreasing the scattering of visible light. The decrease of crystal sizes upon addition of NiO leads to decreasing the light scattering as well.
5. Conclusions Addition of NiO to titania-containing magnesium aluminosilicate glasses leads to a change of phase separation kinetics, sizes and compositions of amorphous and crystalline phases. Upon heat-treatment of NiO-free glass three-phase immiscibility develops, and regions of magnesium aluminotitanate and aluminomagnesium amorphous phases precipitate in silicate matrix. In NiO-containing glasses nickel oxide enters both phases. Because of high mobility of NiO-containing structural elements, their precipitation begins at early stages of phase decomposition. Upon addition of NiO, tendency to crystallization of magnesium aluminotitanate phase decreases, and in the material with 5% NiO this phase remains amorphous after heat-treatment at 960 °C. Nickel-containing aluminomagnesium spinel crystallizes in Ni-enriched aluminomagnesium regions at
191
800 °C. At higher heat-treatment temperatures these regions become the centers of amorphous aluminomagnesium phase precipitation. Precipitation of NiOcontaining phase follows the mechanism of spinodal decomposition, which ensures high uniformity of phase distribution and determines the structure of the final glass–ceramics. The decrease in spinel crystal sizes provides additional decrease in intensity of light scattering and hence an increase of transparency of glass–ceramic materials.
Acknowledgments This work was supported by the Russian Foundation for Basic Research (projects nos. 01-03-32892 and 04-0332657) and by Grant NSH-1405.2003.3 of President of Russian Federation.
References [1] V.V. Golubkov, O.S. Dymshits, A.A. Zhilin, Glass Phys. Chem. 10 (1984) 155. [2] V.V. Golubkov, O.S. Dymshits, A.A. Zhilin, T.I. Chuvaeva, M.Ya. Tsenter, A.V. Chashkin, Glass Phys. Chem. 29 (3) (2003) 254 (in Russian). [3] V.N. Filipovich, J. Techn. Phys. 26 (1956) 398.
The influence of NiO on phase separation and crystallization of glasses of the MgO–Al2O3–SiO2–TiO2 system V.V. Golubkov a, T.I. Chuvaeva b, O.S. Dymshits b,*, A.A. Shashkin b, A.A. Zhilin b, W.-B. Byun c, K.-H. Lee c a
IV Grebenschikov Institute of Silicate Chemistry, Russian Academy of Science, Odoevskogo str., St. Petersburg 199155, Russian Federation b SI Vavilov State Optical Institute, ul. Babushkina 36/1, St. Petersburg 192171, Russian Federation c Korea Electrotechnology Research Institute, Kyonggi-Do 437-808, South Korea Available online 15 September 2004
Abstract The evolution of structure and phase composition of NiO-doped titania-containing magnesium aluminosilicate glasses has been studied using small angle X-ray scattering and X-ray diffraction analysis. After initial heat-treatments, three-phase immiscibility develops resulting in precipitation of two amorphous phases. One of them is enriched in MgO, Al2O3 and TiO2, the other in MgO and Al2O3. Nickel oxide enters both phases, which impedes to crystallization of the magnesium aluminotitanates phase; nickel-containing aluminomagnesium regions become centers of further crystallization of aluminomagnesium spinel. The spinel crystallization temperature, as well as mean sizes decreases with increasing the NiO content in the initial glass. Ó 2004 Published by Elsevier B.V. PACS: 61.10.E; 61.43.F; 81.05.P
1. Introduction Previously, we studied the influence of NiO on phase decomposition of titania-containing lithium aluminosilicate glasses [1]. In glasses doped by NiO at early stages of heat-treatment sizes of inhomogeneous regions and jDqj2, the square of the mean difference of electronic densities inside the inhomogeneous regions and the glass matrix, rapidly increase; the jDqj2 is proportional to NiO content. It implies that phase containing nickel oxide precipitates upon phase decomposition, the structure formed determines the structure of phase separated material and of resultant glass–ceramics. In [2] we studied the kinetics of structure and phase composition change upon heat-treatment of magnesium
*
Corresponding author. Tel.: +7 812 560 1911; fax: +7 812 560 1022. E-mail address: [email protected] (O.S. Dymshits). 0022-3093/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2004.08.020
aluminosilicate glasses nucleated with titania. Small angle X-ray scattering (SAXS) data showed that upon phase decomposition two kinds of amorphous phases are formed. After further heat-treatments magnesium aluminotitanates amorphous regions crystallize as MgO Æ 2TiO2–Al2O3 Æ TiO2 solid solutions, later spinel, MgAl2O4, crystallizes in aluminomagnesium regions. The objective of this paper was to study the influence of NiO on phase separation and crystallization of magnesium aluminosilicate glasses nucleated by TiO2. 2. Experimental procedure We present the results of SAXS and X-ray diffraction analysis (XRD) study of phase transformations in magnesium aluminosilicate glasses with equimolar contents of MgO and Al2O3 nucleated by TiO2 and doped by 0.5–5.0 mol% NiO. Glasses were melted in quartz crucibles at 1550 °C for 4 h with stirring, poured onto a metal
V.V. Golubkov et al. / Journal of Non-Crystalline Solids 345&346 (2004) 187–191
plate and annealed at 660 °C. XRD patterns of powdered samples were measured using Cu Ka radiation with a Ni filter. Relative fractions of crystallinity were estimated by measuring the square of the most intensive peak of a crystalline phase. The mean crystal sizes were estimated from broadening of X-ray peaks. We measured the intensity of X-ray scattering at angles of 8-550 using Cu Ka radiation. The samples were placed into a high-temperature furnace attached to a small angle X-ray setup. Heat-treatments under isothermal conditions in the temperature interval from 700 to 1000 °C were carried out in this furnace. The experimental details are described elsewhere [1–3].
16
7 14 12
6
10
Intensity / a.u.
188
5
8 6
4 3
4
3. Results
2
All initial glasses are inhomogeneous and X-ray amorphous. As is seen from Table 1, with increasing the NiO content, jDq2jof initial glasses increases. On heat-treatment, the intensity of scattering by inhomogeneous structure of glasses increases. All SAXS curves by heat-treated glasses demonstrate a maximum. Fig. 1 depicts scattering by samples doped by 3.0% NiO, a pronounced maximum appears on the curve after heating at 800 °C for 10 min, its location remains unchanged upon increasing time and temperature of heat-treatment (Fig. 1, curves 2–6). Its intensity constantly increases due to continued liquid-phase separation and crystalli-
0
2 1 0
10
20
30
40
50
60
70
80
90 100 110
Scattering angle, ϕ / ' Fig. 1. Intensity of SAXS by initial and heat-treated glasses doped with 3.0 mol% NiO: 1 – initial glass; 2 – 800 °C, 10 min; 3 – 800 °C, 2 h; 4 – 800 °C, 4 h; 5 – 800 °C, 4 h + 860 °C, 1 h; 6 – 800 °C, 4 h + 860 °C, 2 h; 7 – 800 °C, 4 h + 860 °C, 2 h + 960 °C, 2 h (scale 1:2).
zation in precipitated regions. For glasses doped by 0.5% and 1.0% NiO, during heating at 860 °C the position of the maximum at SAXS curves shifts to larger angles. For glass doped by 5.0 mol% NiO, the maximum
Table 1 Radii R of amorphous regions of inhomogeneity in initial and heat-treated glasses NiO content (mol%)
0
0.5
1.0
3.0
5.0
Heat-treatment schedule
SAXS data
T (°C)
˚) R (A
– 800 860 960 – 800 860 960 – 800 860 960 – 800 860 960 – 800 860 960
s (h)
– 21 6 6 – 3 3 2 – 3 3 2 – 4 3 2 – 3 3 2
60 53 53 55 55 34 41 30 25 27 40 22 26.5 28.5 40 38 25 27 39
XRD data ˚ 3 )2 jDqj2 Æ 102 (e/A
0.2 0.4 1.65 3.3 0.25 0.4 2 3.1 0.24 0.65 2 3.6 0.3 1.55 2.7 3.8 0.5 1.8 3 4
um ( 0 ) – 16 18 15 – 17 31 23 – 28 38 25 – 35 35 26 – – 32 26
2h = 37° ˚) R (A
S (cm2)
2h = 26° ˚) R (A
S (cm2)
– 25.0 20.0 27.0 – 20.0 22.0 24.0 – 11.5 20.0 25.0 – 16.0 18.5 22.0 – 15.0 16.5 21.0
– 2.9 8 19.5 – 4.7 9 19 – 5 6 19 – 3.2 5 18 – 3 8 18
– 60.0 70.0 70.0 – 40.0 57.5 65.0 – – 40.0 65.0 – – – 82.5 – – – –
– 3.8 6 7 – 3.5 5 7.5 – – 3 5.3 – – – 1.5 – – – –
jDqj2 magnitudes, positions of maxima on SAXS curves, um, integral intensities of reflections at 2h = 26 and 37° and radii of microcrystals derived from widths of these reflections.
V.V. Golubkov et al. / Journal of Non-Crystalline Solids 345&346 (2004) 187–191
o
4.0
960 C o
3.5
860 C
o
800 C I∆ρI2102 / (e/A3)2
189
3.0
4
2.5
3 2
2.0 1.5
1 1.0 0.5 0.0 0
2
4
6
8
time / h 60
o
o
o
800 C
55
860 C
960 C 1
Radius / angstrem
50 45 40
2
35 30 25 20 15 10 5 0 0
2
4
6
8
time / h
Fig. 2. Variation of jDqj2 (a) and of radii R of amorphous regions of inhomogeneity (b) with time of heat-treatment at 800 °C, with time of additional heat-treatment at 860 °C and with additional hold at 960 °C for 2 h. (a) 1 – without NiO; 2 – 1.0 mol% NiO; 3 – 3.0 mol% NiO; 4 – 5.0 mol% NiO. (b) 1 – without NiO; 2 – 3.0 mol% NiO.
is formed only during holding at 860 °C. For all glasses, after heat-treatment at 960 °C the maximum shifts to smaller angles. At every stage of heat-treatment, jDqj2 increases with time while its magnitude increases with increasing the NiO content (Fig. 2(a)). After certain holding time, the growth of jDqj2 is discontinued (Fig. 2(a)). The sizes of regions of inhomogeneity do not change pronouncedly: after short holding times they slightly decrease and then increase again. A significant growth of R is observed during heat-treatment at 960 °C. XRD pattern of glass without NiO heat-treated at 800 °C for 2 h and additionally at 860 °C for 3 h (Fig. 3, curve 1) shows the main peaks of magnesium aluminotitanates (the most intense one located at about 2h = 26°). This pattern also exhibits the peak at 2h = 37°, usually attributed to the most intense peak of spinel. However the absence of the other spinel reflections (marked by a dashed line in Fig. 3) implies that in this material the peak at 37° belongs to magnesium aluminotitanates. XRD patterns of glass with 3% NiO reveal that not only after heat-treatment at 800 °C for 4 h (Fig. 3, curve 2) but even after additional heating at 860 °C for 2 h (Fig. 3, curve 3) there is no any peak
Fig. 3. Parts of XRD pattern of: 1 – glass without NiO heat-treated at 800 °C, 2 h + 860 °C, 3 h; 2, 3 – glass with 3.0 mol% NiO heat-treated: 2 – 800 °C, 4 h; 3 – 800 °C, 4 h + 860 °C, 2 h.
of magnesium aluminotitanates crystals, spinel is the only crystalline phase. XRD patterns of glass–ceramics, prepared with the last hold at 960 °C, show that with increasing the NiO contents, the intensity of peaks of magnesium aluminotitanates gradually decreases to such extent that in glass– ceramics with 5.0% NiO those crystals do not precipitate at any crystallization temperature (Fig. 4, curve 1). The
Fig. 4. Parts of XRD patterns of glasses heat-treated at 800 + 860 + 960°. (1) – with 5.0 mol%; (2) – with 3.0 mol% NiO; (3) – with 1.0 mol% NiO; (4) – without NiO.
190
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intensity of XRD at 2h = 37° is independent of NiO content while that at 2h = 26° decreases with its increasing (Table 1). The peak at 2h = 37° is a superposition of reflections due to spinel and magnesium aluminotitanates. Assuming that intensities of peaks of magnesium aluminotitanates located at 26° and 37° are equal, one can see that the intensity of spinel peak increases with increasing the NiO content.
4. Discussion The kinetics of phase transformations in NiO-containing glasses and in glasses without NiO are similar. An increase of time of heat-treatment at 800 and 860 °C leads to a stabilization of jDqj2 which can be considered as a result of approaching to the steady state probably due to viscosity increasing upon enrichment of the residual glass by SiO2. Simultaneously with liquid-phase decomposition, crystallization takes place after heating for 3–4 h at 800 °C. After holding at 860 °C, jDqj2 increases. In magnesium aluminosilicate glasses doped by NiO like in NiO-free glass, the primary process resulting in a microinhomogeneous structure is a three-phase immiscibility with the formation of magnesium aluminotitanate and aluminomagnesium amorphous phases. In the first phase MgO Æ 2TiO2–Al2O3 Æ TiO2 solid solutions crystallize, in the second phase spinel precipitates. However due to addition of NiO, kinetics of phase decomposition, compositions and sizes of amorphous and crystalline regions of inhomogeneity change. The jDqj2 magnitude linearly depends on NiO content at any stage of heat-treatment (Fig. 5). This dependence is more pronounced at low than at high heat-treatment temperatures. It implies that NiO influences phase separation rather than crystallization probably due to increased NiO contents in liquid-phase separated regions, which
3
4.0
I∆ρI2102 / (e/A3)2
3.5
2
3.0 2.5
1
2.0 1.5 1.0 0.5 0.0 0
1
2
3
4
5
NiO / mol%
Fig. 5. Dependence of jDqj2 on NiO content after heat-treatments at different schedules: 1 – 800 °C, 3 h; 2 – 800 °C, 4 h + 860 °C, 3 h; 3 – 800 °C, 4 h + 860 °C, 3 h + 960 °C, 2 h.
is connected with higher mobility of NiO-containing structural elements. The addition of NiO pronouncedly influences the phase composition of heat-treated glasses and sequence of phase precipitation. In glasses containing 1% NiO and up, spinel precipitates prier to crystallization of magnesium aluminotitanates, after heat-treatment at 800 °C. Under additional heating at 860 °C, in the sample with 1% NiO appears the peak at 2h = 26 °, and the integral intensity of the peak at 37° increases by the magnitude of the integral intensity at 2h = 26°. So the increase of the integral intensity at 37° can be explained by crystallization of magnesium aluminotitanates and small increase in the volume of spinel can be considered. However according to Table 1, at this heat-treatment, jDqj2 increases by a factor of three, which is partially accounted for by crystallization, but mainly can be explained by the precipitation of amorphous phase. It means is this phase is aluminomagnesium, as after heat-treatment at 960 °C the volume of spinel pronouncedly increases. For glasses with 3.0% and 5.0% NiO crystallization of spinel is also observed at 800 °C. In these glasses the pronounced increase of integral intensity of the peak at 2h = 37° is also reached after heat-treatment at 960 °C. It implies that the gain in spinel precipitation is observed after that heat-treatment. We believe that appearance of spinel at 800 °C and cessation or slow-down of its crystallization at higher temperatures is due to the fact that at low heattreatment temperatures the aluminomagnesium inhomogeneous regions are enriched in NiO. Crystallization develops with high velocity, and at 800 °C NiAl2O4 spinel or solid solutions of NiAl2O4–MgAl2O4 precipitate. An appearance of the maximum on SAXS curves proves the spinodal mechanism of phase decomposition. Upon addition of NiO into initial glass, the maximum at the SAXS curve shifts to larger angles which reflects the decrease of the distance between centers, i.e., the increase of the number of regions, which leads to decrease of their sizes. At further heat-treatments these regions become the centers of precipitation of amorphous aluminomagnesium phase. As new regions do not appear, NiO exerts primary control over the final state of the material structure, distribution of aluminomagnesium regions and their sizes. Spinel crystallization is completed after heat-treatment at 960 °C, as sizes of microcrystals are determined by sizes of amorphous aluminomagnesium regions, they also depend on the NiO content in glass. The NiO enters into both precipitated phases. With increasing NiO content, the crystallization ability of magnesium aluminotitanate phase decreases, and the volume of aluminotitanate crystals decreases. Upon heat-treatment at 960 °C sizes of inhomogeneous regions increase and the position of the maximum shifts to smaller angles, which testifies the Ostwalds ripening processes. The regularity of microin-
V.V. Golubkov et al. / Journal of Non-Crystalline Solids 345&346 (2004) 187–191
homogeneous structure results in a pronounced decrease in SAXS scattering at close to zero angles and in decreasing the scattering of visible light. The decrease of crystal sizes upon addition of NiO leads to decreasing the light scattering as well.
5. Conclusions Addition of NiO to titania-containing magnesium aluminosilicate glasses leads to a change of phase separation kinetics, sizes and compositions of amorphous and crystalline phases. Upon heat-treatment of NiO-free glass three-phase immiscibility develops, and regions of magnesium aluminotitanate and aluminomagnesium amorphous phases precipitate in silicate matrix. In NiO-containing glasses nickel oxide enters both phases. Because of high mobility of NiO-containing structural elements, their precipitation begins at early stages of phase decomposition. Upon addition of NiO, tendency to crystallization of magnesium aluminotitanate phase decreases, and in the material with 5% NiO this phase remains amorphous after heat-treatment at 960 °C. Nickel-containing aluminomagnesium spinel crystallizes in Ni-enriched aluminomagnesium regions at
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800 °C. At higher heat-treatment temperatures these regions become the centers of amorphous aluminomagnesium phase precipitation. Precipitation of NiOcontaining phase follows the mechanism of spinodal decomposition, which ensures high uniformity of phase distribution and determines the structure of the final glass–ceramics. The decrease in spinel crystal sizes provides additional decrease in intensity of light scattering and hence an increase of transparency of glass–ceramic materials.
Acknowledgments This work was supported by the Russian Foundation for Basic Research (projects nos. 01-03-32892 and 04-0332657) and by Grant NSH-1405.2003.3 of President of Russian Federation.
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