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Effect of BaO addition on the structure and microstructure of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites
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Effect of BaO addition on the structure and microstructure of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites J. Partykan AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Ceramic and Refractory Materials, Cracow, Poland Received 29 April 2015; received in revised form 2 July 2015; accepted 3 July 2015
Abstract We examined the influence of barium oxide addition on the structure and properties of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramics. Glass– ceramics were prepared with constant SiO2/Al2O3 and Na2O/K2O ratios of 6.5 and 1.04, respectively, and the MgO content was maintained at 10.56 wt%. The barium oxide content was set at 4 wt%, 9 wt%, and 14 wt%. The weight ratios were recalculated from the BaCO3 added. To determine the microstructure of the glassy materials, scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS), X-ray diffraction (XRD), medium (MIR) and far infrared analysis (FIR), and Raman spectroscopy were used. Significant differences were observed in the phase composition and in the silica–alumina-oxide network of the glassy phase, which could be related to changes in the amount of barium oxide. The characteristic temperatures (beginning of sintering, sphere, half-sphere and melting), which were measured by using hot stage microscopy and dilatometry, also showed changes. & 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: B. Microstructure; B. Structure; D. Glass–ceramic; Phase; Crystallinity
1. Introduction Glass–ceramic composite materials are characterized by the presence fine crystalline structures within the fully dense glassy phase [1–3]. The amount of the crystalline phase may vary from 0.5 to 98 vol%; however, usually their content varies from 30 to 70 vol%. The initial oxide composition and the thermal treatment determine the type and quantity of the crystalline phase produced and the chemical composition of the glassy phase bonding the crystallite grains [2,3]. The final phase composition determines the functional properties of the obtained glass–ceramic composites. Here, we report the effect BaO addition on the microstructure and thermal properties of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites. The main components, i.e., silica and alumina, form a primary alumino-silico-oxide microstructural network, which is modified by the presence of the alkali metal oxides, Na2O and K2O and the alkaline earth oxide, MgO [4–6]. The SiO2/Al2O3 ratio n
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determines the characteristic temperatures such as temperatures of softening and of sphere and of half sphere formation [1,7] of the glass–ceramic materials and therefore, their applications. Characteristic temperatures may be changed by introducing alkaline metal oxides, such as Na2O and K2O (low- and medium-temperature fluxing agents) or alkali earth metal oxides MgO, BaO, which are medium and high-temperature fluxing agents. [5–7] Alkali metal oxides Na2O and K2O form eutectics with other oxides which enables the melting temperatures to be reduced below 1000 1C [1,2,4]. In the system SiO2–Al2O3–K2O can be found oxide compositions having a melting at 770, 975 or 1045 1C. In the system SiO2–Al2O3– Na2O, there are points having melting temperatures of 732 and 740 1C [23]. Similarly, sodium feldspar albite and potassium feldspar orthoclase have an eutectic point with melting temperature at 1063 1C. MgO is a refractory material with a high melting point of approximately 2800 1C. In the presence of SiO2, MgO participates in the formation of a liquid phase from 1170 1C. However, in the presence of alkali metal oxides with which MgO forms eutectics, the process can start below
http://dx.doi.org/10.1016/j.ceramint.2015.07.014 0272-8842/& 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: J. Partyka, Effect of BaO addition on the structure and microstructure of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.014
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1000 1C [5,6]. An increase in the MgO content results in the increased viscosity of the vitreous phase, which influences diffusion processes in the alloy and, in turn, the crystallization process [1,2,6,15]. MgO also reduces the thermal expansion coefficient, resulting in higher resistance to rapid temperature changes. A greater amount of MgO may induce crystallization of solid phases, such as forsterite, cordierite, diopside, and protoenstatite [1,2,6,15]. Barium oxide is the only fluxing agent in the system, which is active in small amounts only at temperatures exceeding 1250 1C. In the presence of alkali metal oxides, BaO begins to co-form the liquid phase below 1000 1C, which is similar to its role in the SiO2–BaO–K2O system at 907 1C [6–8] and in the SiO2–BaO–Na2O system at 785 1C [6,7]. Further, the addition of BaO effectively improves the mechanical performance and chemical resistance of the glass–ceramic composites [6,7,22]. 2. Experimental procedure In this work, the SiO2–Al2O3–Na2O–K2O–MgO þ BaO glass–ceramic composite system was used. The CaO/MgO molar ratio was fixed at 0.04; consequently, MgO was the major component (nearly 100 wt%). Barium oxide was then introduced into the system at 4 wt%, 9 wt%, and 14 wt%. These values were recalculated on the basis of the amount of barium carbonate added as the raw material. Other raw materials included were sodium feldspar (Na600) and potassium feldspar (K600) (supplied by SIBELCO), quartz (MK40, sourced from the SKSM Sobótka), kaolin (KOC, SURMIN), talcum A10H from Luzenac, and barium carbonate (BaCO3, supplied by Avantor Polska). Then, each batch was milled using a planetary mill to obtain a residue of approximately 1 wt% through a 56 mm sieve, which was then dried to obtain the raw powder. Each raw powder was examined by hot stage microscopy (HSM) to determine the various characteristic temperatures. Then, raw materials were fired in porcelain crucibles in a single cycle at a maximum temperature of 1230 1C for 14 h. Samples for scanning electron microscopy (SEM) and Raman spectroscopy were prepared by slicing out cubes from the fired material. The cubes were then polished to obtain the samples with dimensions 10 10 6 mm3. Samples for structural examination and chemical analysis were prepared by grounding the material in an agate mortar to obtain a powder with grain sizes lower than 63 mm. Chemical analysis was carried out using a wavelength dispersive X-ray fluorescence (WDXRF) spectrometer Axios mAX, Phillips-PANalytical (Table 1). All the characteristic temperatures were determined by HSM on a Misura 3 microscope, EES Expert System Solution S.r.l. (Table 2). The internal structure of the samples was analyzed by X-ray diffraction XRD, PANalytical X-ray diffractometer X'Pert Pro for phase composition determination (Table 3, Fig. 1), SEM Nova Nano SEM 200 with an energy dispersive spectrometer microanalyzer EDS-EDAX (Figs. 1–7), mid-infrared MIR, 4000–400 cm 1 and far-infrared FIR, 400–40 cm 1 analyses carried out on a Bruker Vertex 70v spectrometer (Figs. 8 and 9), Raman spectroscopy with excitation was carried out with
Table 1 Oxide compositions of the tested glazes (wt%). SiO2
Al2O3
CaO
MgO
Na2OþK2O
BaO
SiO2/Al2O3
70.92 67.65 64.58 59.03
11.04 10.55 10.11 9.15
0.42 0.43 0.46 0.38
10.85 10.64 10.27 10.48
6.78 6.88 6.70 6.75
0 3.85 7.88 14.20
6.42 6.41 6.39 6.45
Ar þ (514 nm) and He–Cd (325 nm) lasers, Jobin Yvon (Fig. 10). 3. Results and discussion The oxide composition of the glass–ceramic materials with and without barium oxide was analyzed to verify whether the introduction of various amounts of barium oxide would be the only factor influencing changes in the phase composition and the structure of this material. The presence of a very small amount of calcium oxide can be attributed to the addition of materials, such as feldspar and kaolin, which contain a small amount of CaO. Barium oxide was gradually introduced in amounts close to the assumed values of4 wt%, 9 wt%, and 14 wt%. Characteristic temperatures describe the behavior of the ceramic materials during heat treatment. The HSM measurement technique is particularly suitable for designing the firing curves. Using this technique, various characteristic temperatures, such as sintering, sphere, hemisphere, and full melt temperatures, which are related to the increase in the amount of the liquid silica–alumina phase, can be measured. Measurements using HSM are compatible with the ISO 540:1995 and DIN 51730-1998 standards. Using dilatometric measurements, two other characteristic temperatures, including the transformation temperature (Tg) and dilatometric softening temperature (Td), can be obtained. Tg characterizes the transition from an elastic brittle state to a viscous glassy state and corresponds to a dynamic viscosity of the order of 1013 Pa s. Td is the temperature at which the length of a sample in the dilatometer, with a considerable external force applied to the sample during heating at a constant rate, reaches a maximal value and begins to decrease with further increase in temperature. The appearance of a maximum on the dilatometric curve can be correlated with the parallel influence of sample dilatation with increasing temperature and sample deformation due to the viscous flow. The position of the maximum can correspond to a particular viscosity of the studied glass. Usually, this viscosity is assumed to be equal to the dynamic viscosity 1011 Pa s. All the characteristic temperatures are shown in Table 2. The inclusion of BaO to the SiO2–Al2O3–Na2O–K2O–MgO glass– ceramic system significantly increased the Td, sintering temperature, and sphere formation temperature, while the hemisphere and melting points only marginally increased. In contrast, BaO strongly reduces Tg, the transition point from solid to liquid state, which can be attributed to the reduction in the viscosity of the glassy phase formed during heating and to the increase in the BaO content in the presence
Please cite this article as: J. Partyka, Effect of BaO addition on the structure and microstructure of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.014
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Table 2 Characteristic temperatures of tested glass–ceramic materials form SiO2–Al2O3–Na2O–K2O–MgO with addition of BaO glass–ceramic materials. PORC 01-4 Mgþ BaO
Measurement technique HSM (hot stage microscope) Beginning of sintering
DL (dilatometer)
Sphere
Halfsphere
Melting
Transformation
Softening
1198 1251 (þ 53) 1248 (þ 50) 1247 (þ 48)
1269 1276 ( þ7) 1279 ( þ10) 1276 ( þ7)
1299 1316 (þ 17) 1320 (þ 21) 1325 (þ 26)
772.7 755.3 ( 17.4) 719.5 ( 53.2) 706.8 ( 66.7)
831.2 882.2 ( þ51.0) 986.5 ( þ155.3) 1138.3 ( þ307.1)
[1C] PORC PORC PORC PORC
01-4 01-4 01-4 01-4
Mgþ 0 wt% BaO Mgþ 4 wt% BaO Mgþ 9 wt% BaO Mgþ 14 wt% BaO
1120 1164 (þ 44) 1156 (þ 36) 1125 (þ 5)
Table 3 Phase composition of the SiO2–Al2O3–Na2O–K2O–MgOþ BaO glass–ceramic materials. Contents of barium oxide – BaO 0 wt%
4 wt%
9 wt%
14 wt%
Protoenstatite Quartz
Forsterite Hyalophane
Hyalophane Forsterite Celzjan
Hyalophane Forsterite Celzjan
of alkaline metal oxides. However, the increase observed in the other characteristic temperatures can be linked with the formation of new crystalline phases during heat treatment. Also, the phase composition of samples with BaO differed considerably in comparison to the input material, as shown in Table 3 and Fig. 1. The phase composition of the material without BaO shows the presence of protoenstatite Mg2(Si2O6) from the group of mono-ino-silicates, quartz, and the glassy phase. Protoenstatite was probably formed by crystallization from an aluminum-silicate melt. The composition point, which simplified to a three-component system of SiO2–Al2O3–MgO, was proximal to the protoenstatite primary crystallization field. In some areas, where the aluminum-silicate alloy was formed with progress in the heating, the oxide compositions differed. Hence, shifting the composition point to the crystallization field of this phase was possible, as confirmed by the SEM images (Fig. 2), which indicates areas with partly melted quartz crystals. In these areas, the SiO2 concentration was certainly higher than the average oxide content in the tested material. The presence of protoenstatite crystals can be observed in these areas as well. Barium oxide introduced in the lowest amount (4 wt%), in the presence alkali metal oxides, acted as a fluxing agent to a higher degree, which resulted in molten quartz grains. A higher SiO2 concentration in the alloy induced crystallization of forsterite (Mg2[SiO4]), instead of protoenstatite (Fig. 3). The presence of BaO also caused the occurrence of small clusters of crystals of barium feldspar, hyalophane, and tecto potassium–sodium–barium–aluminumsilicate ((Ba0.40K0.47Na0.13)(Si2.59Al1.41O8)) with structure similar to sanidine The clusters, which are not numerous, are evenly scattered within the volume of the material and are
separated by areas of vitreous phase and spots in which quartz and forsterite are present. With increase in the barium oxide amount (up to 9 wt%), further changes in the phase composition occurred. Apart from hyalophane and forsterite, barium feldspar–celsian–barium tectoaluminum-silicate: Ba0.8K0.2(Al1.77Si2.23)O8 (see Table 2 and Figs. 1 and 4) was also formed. The recalculated composition point resulting in a simplified three-component SiO2–Al2O3–BaO phase system showed that the point was located in the celsian crystallization field, indicating intensification of celsian crystallization. SEM images indicate a significantly higher concentration of the crystalline phases and a lower content of the vitreous phase. Forsterite crystals were still clustered in small areas of high SiO2 concentration in spots left by molten quartz (Fig. 4). In addition, the number and size of the hyalophane crystals increased (Figs. 4 and 5). Interestingly, the diversified hyalophane grain structure (Figs. 4 and 5) indicates the existence of two mechanisms i.e., dendrite and directional crystallization. The samples with the highest barium oxide content (14 wt%) showed a higher crystalline phase content (Figs. 6 and 7). The crystals have various sizes and structures, although disordered directional crystallization was predominant. The SEM images show a large number of hyalophane crystals K0.6Ba0.4(Al1.42Si2.58O8), lower amounts of celsian Ba0.8K0.2(Al1.77Si2.23)O8, while forsterite crystals were rare (Fig. 7). The type of bonds formed during the vitreous phase formation in the glass–crystalline materials exerts a major influence on the final properties of the materials. Silico-oxygen and/or silico-alumino-oxygen bonds are the basic elements of the vitreous phase structure. MIR and FIR analyses provide information on the silicon– aluminum–oxygen structure of the vitreous phase and can confirm the presence of crystalline phases [9,10]. In the MIR analysis, several characteristic (coordination number CK=4) silicon–oxygen bonding bands can be distinguished. The band at 1200 cm 1 is associated with the SiQO stretching vibrations, i.e., defects arising from the silico-oxygen tetrahedra connected with one another at the edges. The band at 1090 cm 1 is associated with the Si–O(Si) stretching vibrations and Si–O stretching vibrations at 980–1000 cm 1 [8–11]. These two bands represent the bridging oxygen atoms (BO, silico-oxygen bridges) and non-bridging oxygen atoms
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(NBO, broken silico-oxygen bridges). All the listed bands can be usually observed as one broad band within the wavenumber range 850–1400 cm 1, as shown in Fig. 8. From the shape of this band and the location of its maximum, the component bands (BO or NBO) predominant in a given material can be determined (Fig. 8). Moreover, the bands associated with bending vibrations of Si–O–Si at
790 cm 1 and bending vibrations of O–Si-O at 470 cm 1 can be identified in the MIR spectra. The bands within the 750–550 cm 1 range, which are associated with vibrations of the silico-oxygen rings and/or alumino-silico-oxygen rings with differing segmentations, are observed in the spectra of silicate materials [8–12]. Other bands may be related to bonds characteristic of some other crystalline phases and can be
Fig. 1. Phase composition of the SiO2–Al2O3–Na2O–K2O–MgO materials with a 4 wt%, 9 wt%, and 14 wt% BaO additive (Hy is Hyalophane, Cl is Celsian, Pr is Protoenstatite, and Q is quartz).
Fig. 2. SEM–EDS image of a SSiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composite sample without any BaO.
Fig. 3. SEM–EDS image of a SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composite with 4 wt% BaO.
Please cite this article as: J. Partyka, Effect of BaO addition on the structure and microstructure of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.014
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Fig. 4. SEM–EDS image of SiO2–Al2O3–Na2O–K2O–MgO samples with 9 wt% BaO.
Fig. 5. SEM image of a SiO2–Al2O3–Na2O–K2O–MgO sample with 9 wt% BaO.
Fig. 6. SEM image of a SiO2–Al2O3–Na2O–K2O–MgO sample with 14 wt% BaO.
interpreted by comparison with reference spectra of crystalline phases [12–14]. If MIR bands of materials without BaO are known, it is possible to define changes induced by BaO addition. MIR analysis (Fig. 8) was carried out within the 1600– 400 cm 1 range and bands with similar ranges of wavenumbers were observed for all the samples. The band at 608 cm 1 is an exception and occurs in samples with 4 wt% and 9 wt% BaO and can probably be attributed to forsterite [14–16]. There is also a band at 570 cm 1 for samples with 9 wt% and 14 wt% BaO. This band is could be attributed to sanidine, although the band could also originate from this phase because of its structural similarity to hyalophane (no hyalophane reference spectra were found in the MIR and FIR [20,21] structural databases). The disappearance of bands at 722– 724 cm 1 and 772–778 cm 1, which are associated with the bending vibrations of Si–O–Si [8,10–13,20,21], show the reduced order of the vitreous phase structure, i.e., a higher number of broken Si–O bonds (NBO). By observing the main bands within the 1400–840 cm 1 range, it can be noticed that there is a distinct shift in the maximum values of the bands towards lower wavenumbers, from 1059 cm 1 for the material without BaO to 1014 cm 1
for the sample with the highest (14 wt%) BaO content. This shift is clearly proportional to the barium oxide content. This band maximum shift indicates that the intensity of the component band within the 980–1000 cm 1 range increased. This band is associated with Si-O stretching vibrations, corresponding to broken silicon–oxygen bridges, i.e., the ones with an increased number of the NBOs. The presence of such bonds also reveals a higher degree of a disordered state of the silicon and oxygen structure in the vitreous phase in the sintered materials. Cations, which are located in the interstitial spaces, in this case Mg2 þ or Ba2 þ , may bond with unsaturated oxygen ions. Magnesium has a significantly lower ionic radius and therefore, preferentially occupies the interstitial positions in the silicon and oxygen structure and saturates the free oxygen bonds. The FIR spectra (Fig. 9) confirm the presence of crystalline phases with the sanidine structure (the bands at 97–111 cm 1) as well as protoenstatite and forsterite (bands at 291 cm 1 and 293–294 cm 1). To completely understand the structure and microstructure of the glass–ceramic materials dealt with in this study, it is essential to acquire information on the internal structure of the glassy phase, specifically by Raman spectroscopy. The Raman spectra
Please cite this article as: J. Partyka, Effect of BaO addition on the structure and microstructure of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.014
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Fig. 7. SEM–EDS image of a SiO2–Al2O3–Na2O–K2O–MgO sample with 14 wt% BaO.
Fig. 9. FIR spectra of SiO2–Al2O3–Na2O–K2O–MgO glass–crystalline composites with 4 wt%, 9 wt%, and 14 wt% BaO.
Fig. 8. MIR spectra of SiO2–Al2O3–Na2O–K2O–MgO glass–crystalline composites with 4 wt%, 9 wt%, and 14 wt% BaO.
of the glassy silico-oxygen or/and alumina–silico-oxygen materials shows a number of bands reflecting the multiple coordination of the Si–O bonds [18,19,22]. These bands can be linked to the three-dimensional structure of SiO2, indicated by Q0–Q4, the vibrational frequencies of which change with the polymerization degree of the silicate network, showing a distribution of SiO4 tetrahedra with 0, 1, 2, 3, or 4 BOs. Additionally, bands derived from the aluminum tetrahedra network with different configurations of NBO/BO ions can be observed. Fig. 10 shows the Raman spectra of the glassy phases of the tested samples. In all the spectra, the absence of the band at
710 cm 1 unambiguously indicates the absence of aluminooxygen octahedra [16–19,22] and thus, all the aluminum ions are arranged in a tetrahedral coordination, indicating that they take part in the formation of the glassy structure. In the sample without BaO, weak bands at 1135 cm 1 (corresponding to the Q4 group representing SiO2 and tectosilicate) and 821 cm 1 (linked to Q0 – monomeric SiO4) can be observed. The additional band at 770 cm 1 can be assigned to [AlO4] tetrahedra with three BOs and one NBO. The band at 583 cm 1 can be associated with the vibrations of the threemembered silico- and alumino-silico-oxygen rings. The addition of 4 wt% BaO led to the appearance of new bands in the 850–960 cm 1 range, which indicates the formation of the group Q1 – Si2O7 anion. The increase in the intensity of bands in the 800–850 cm 1 range shows an increase in the presence of monomeric SiO4 group Q0. The increase in the BaO content
Please cite this article as: J. Partyka, Effect of BaO addition on the structure and microstructure of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.014
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tectosilicate group, barium-potassium feldspar – hyalophane (K,Ba)[Al(Si,Al)Si2O8], barium feldspar–celsian (Ba0.8K0.2) (Al1.77Si2.23)O8, and forsterite-monosilicate Mg2(SiO4). 4) The presence of barium oxide promoted the formation of NBOs, resulting in the formation of a severely distorted silicon and oxygen structure with a vitreous phase in the tested materials. 5) The degree of the disordered state increased with increase in the amount of barium oxide added.
Acknowledgments This research has been carried out thanks to financing under the framework of NCBiR (Polish National Research and Development Committee) program Nos. N N508 477734 and PBS1/ B5/17/2012. References
Fig. 10. Raman spectrum of the glassy phase of the SiO2–Al2O3–Na2O–K2O– MgO samples with 4 wt%, 9 wt%, and 14 wt% BaO.
to 9 wt% did not cause significant changes in the Raman spectrum beyond the band shifts associated with the presence of the Si2O7 anion towards lower wavenumbers. Increase in the BaO content to 14 wt% significantly changed the structure of the glassy phase. The bands in the 800–850 cm 1 range completely disappeared, indicating the presence of SiO4 monomers, while the appearance of bands in the 1050– 1100 cm 1 and 1100–1200 cm 1 ranges may be linked to the presence of group Q2 with silicate chains and group Q4, comprising of SiO2 and tectosilicates. 4. Conclusion The key findings of our study are summarized below. 1) The addition of BaO to the SiO2–Al2O3–Na2O–K2O–MgO samples caused considerable changes with regard to the phase composition and an increase in the crystalline phase content. 2) With the lowest amount of barium oxide added, hyalophane, tecto potassium–sodium–barium–aluminum-silicate given by the formula (Ba0.40K0.47Na0.13)(Si2.59Al1.41O8), and small amounts of forsterite monosilicate were formed. 3) The crystalline phases formed with the addition of9 wt% and 14 wt% BaO were aluminum-silicate from the
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Effect of BaO addition on the structure and microstructure of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites J. Partykan AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Ceramic and Refractory Materials, Cracow, Poland Received 29 April 2015; received in revised form 2 July 2015; accepted 3 July 2015
Abstract We examined the influence of barium oxide addition on the structure and properties of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramics. Glass– ceramics were prepared with constant SiO2/Al2O3 and Na2O/K2O ratios of 6.5 and 1.04, respectively, and the MgO content was maintained at 10.56 wt%. The barium oxide content was set at 4 wt%, 9 wt%, and 14 wt%. The weight ratios were recalculated from the BaCO3 added. To determine the microstructure of the glassy materials, scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS), X-ray diffraction (XRD), medium (MIR) and far infrared analysis (FIR), and Raman spectroscopy were used. Significant differences were observed in the phase composition and in the silica–alumina-oxide network of the glassy phase, which could be related to changes in the amount of barium oxide. The characteristic temperatures (beginning of sintering, sphere, half-sphere and melting), which were measured by using hot stage microscopy and dilatometry, also showed changes. & 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: B. Microstructure; B. Structure; D. Glass–ceramic; Phase; Crystallinity
1. Introduction Glass–ceramic composite materials are characterized by the presence fine crystalline structures within the fully dense glassy phase [1–3]. The amount of the crystalline phase may vary from 0.5 to 98 vol%; however, usually their content varies from 30 to 70 vol%. The initial oxide composition and the thermal treatment determine the type and quantity of the crystalline phase produced and the chemical composition of the glassy phase bonding the crystallite grains [2,3]. The final phase composition determines the functional properties of the obtained glass–ceramic composites. Here, we report the effect BaO addition on the microstructure and thermal properties of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites. The main components, i.e., silica and alumina, form a primary alumino-silico-oxide microstructural network, which is modified by the presence of the alkali metal oxides, Na2O and K2O and the alkaline earth oxide, MgO [4–6]. The SiO2/Al2O3 ratio n
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determines the characteristic temperatures such as temperatures of softening and of sphere and of half sphere formation [1,7] of the glass–ceramic materials and therefore, their applications. Characteristic temperatures may be changed by introducing alkaline metal oxides, such as Na2O and K2O (low- and medium-temperature fluxing agents) or alkali earth metal oxides MgO, BaO, which are medium and high-temperature fluxing agents. [5–7] Alkali metal oxides Na2O and K2O form eutectics with other oxides which enables the melting temperatures to be reduced below 1000 1C [1,2,4]. In the system SiO2–Al2O3–K2O can be found oxide compositions having a melting at 770, 975 or 1045 1C. In the system SiO2–Al2O3– Na2O, there are points having melting temperatures of 732 and 740 1C [23]. Similarly, sodium feldspar albite and potassium feldspar orthoclase have an eutectic point with melting temperature at 1063 1C. MgO is a refractory material with a high melting point of approximately 2800 1C. In the presence of SiO2, MgO participates in the formation of a liquid phase from 1170 1C. However, in the presence of alkali metal oxides with which MgO forms eutectics, the process can start below
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1000 1C [5,6]. An increase in the MgO content results in the increased viscosity of the vitreous phase, which influences diffusion processes in the alloy and, in turn, the crystallization process [1,2,6,15]. MgO also reduces the thermal expansion coefficient, resulting in higher resistance to rapid temperature changes. A greater amount of MgO may induce crystallization of solid phases, such as forsterite, cordierite, diopside, and protoenstatite [1,2,6,15]. Barium oxide is the only fluxing agent in the system, which is active in small amounts only at temperatures exceeding 1250 1C. In the presence of alkali metal oxides, BaO begins to co-form the liquid phase below 1000 1C, which is similar to its role in the SiO2–BaO–K2O system at 907 1C [6–8] and in the SiO2–BaO–Na2O system at 785 1C [6,7]. Further, the addition of BaO effectively improves the mechanical performance and chemical resistance of the glass–ceramic composites [6,7,22]. 2. Experimental procedure In this work, the SiO2–Al2O3–Na2O–K2O–MgO þ BaO glass–ceramic composite system was used. The CaO/MgO molar ratio was fixed at 0.04; consequently, MgO was the major component (nearly 100 wt%). Barium oxide was then introduced into the system at 4 wt%, 9 wt%, and 14 wt%. These values were recalculated on the basis of the amount of barium carbonate added as the raw material. Other raw materials included were sodium feldspar (Na600) and potassium feldspar (K600) (supplied by SIBELCO), quartz (MK40, sourced from the SKSM Sobótka), kaolin (KOC, SURMIN), talcum A10H from Luzenac, and barium carbonate (BaCO3, supplied by Avantor Polska). Then, each batch was milled using a planetary mill to obtain a residue of approximately 1 wt% through a 56 mm sieve, which was then dried to obtain the raw powder. Each raw powder was examined by hot stage microscopy (HSM) to determine the various characteristic temperatures. Then, raw materials were fired in porcelain crucibles in a single cycle at a maximum temperature of 1230 1C for 14 h. Samples for scanning electron microscopy (SEM) and Raman spectroscopy were prepared by slicing out cubes from the fired material. The cubes were then polished to obtain the samples with dimensions 10 10 6 mm3. Samples for structural examination and chemical analysis were prepared by grounding the material in an agate mortar to obtain a powder with grain sizes lower than 63 mm. Chemical analysis was carried out using a wavelength dispersive X-ray fluorescence (WDXRF) spectrometer Axios mAX, Phillips-PANalytical (Table 1). All the characteristic temperatures were determined by HSM on a Misura 3 microscope, EES Expert System Solution S.r.l. (Table 2). The internal structure of the samples was analyzed by X-ray diffraction XRD, PANalytical X-ray diffractometer X'Pert Pro for phase composition determination (Table 3, Fig. 1), SEM Nova Nano SEM 200 with an energy dispersive spectrometer microanalyzer EDS-EDAX (Figs. 1–7), mid-infrared MIR, 4000–400 cm 1 and far-infrared FIR, 400–40 cm 1 analyses carried out on a Bruker Vertex 70v spectrometer (Figs. 8 and 9), Raman spectroscopy with excitation was carried out with
Table 1 Oxide compositions of the tested glazes (wt%). SiO2
Al2O3
CaO
MgO
Na2OþK2O
BaO
SiO2/Al2O3
70.92 67.65 64.58 59.03
11.04 10.55 10.11 9.15
0.42 0.43 0.46 0.38
10.85 10.64 10.27 10.48
6.78 6.88 6.70 6.75
0 3.85 7.88 14.20
6.42 6.41 6.39 6.45
Ar þ (514 nm) and He–Cd (325 nm) lasers, Jobin Yvon (Fig. 10). 3. Results and discussion The oxide composition of the glass–ceramic materials with and without barium oxide was analyzed to verify whether the introduction of various amounts of barium oxide would be the only factor influencing changes in the phase composition and the structure of this material. The presence of a very small amount of calcium oxide can be attributed to the addition of materials, such as feldspar and kaolin, which contain a small amount of CaO. Barium oxide was gradually introduced in amounts close to the assumed values of4 wt%, 9 wt%, and 14 wt%. Characteristic temperatures describe the behavior of the ceramic materials during heat treatment. The HSM measurement technique is particularly suitable for designing the firing curves. Using this technique, various characteristic temperatures, such as sintering, sphere, hemisphere, and full melt temperatures, which are related to the increase in the amount of the liquid silica–alumina phase, can be measured. Measurements using HSM are compatible with the ISO 540:1995 and DIN 51730-1998 standards. Using dilatometric measurements, two other characteristic temperatures, including the transformation temperature (Tg) and dilatometric softening temperature (Td), can be obtained. Tg characterizes the transition from an elastic brittle state to a viscous glassy state and corresponds to a dynamic viscosity of the order of 1013 Pa s. Td is the temperature at which the length of a sample in the dilatometer, with a considerable external force applied to the sample during heating at a constant rate, reaches a maximal value and begins to decrease with further increase in temperature. The appearance of a maximum on the dilatometric curve can be correlated with the parallel influence of sample dilatation with increasing temperature and sample deformation due to the viscous flow. The position of the maximum can correspond to a particular viscosity of the studied glass. Usually, this viscosity is assumed to be equal to the dynamic viscosity 1011 Pa s. All the characteristic temperatures are shown in Table 2. The inclusion of BaO to the SiO2–Al2O3–Na2O–K2O–MgO glass– ceramic system significantly increased the Td, sintering temperature, and sphere formation temperature, while the hemisphere and melting points only marginally increased. In contrast, BaO strongly reduces Tg, the transition point from solid to liquid state, which can be attributed to the reduction in the viscosity of the glassy phase formed during heating and to the increase in the BaO content in the presence
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Table 2 Characteristic temperatures of tested glass–ceramic materials form SiO2–Al2O3–Na2O–K2O–MgO with addition of BaO glass–ceramic materials. PORC 01-4 Mgþ BaO
Measurement technique HSM (hot stage microscope) Beginning of sintering
DL (dilatometer)
Sphere
Halfsphere
Melting
Transformation
Softening
1198 1251 (þ 53) 1248 (þ 50) 1247 (þ 48)
1269 1276 ( þ7) 1279 ( þ10) 1276 ( þ7)
1299 1316 (þ 17) 1320 (þ 21) 1325 (þ 26)
772.7 755.3 ( 17.4) 719.5 ( 53.2) 706.8 ( 66.7)
831.2 882.2 ( þ51.0) 986.5 ( þ155.3) 1138.3 ( þ307.1)
[1C] PORC PORC PORC PORC
01-4 01-4 01-4 01-4
Mgþ 0 wt% BaO Mgþ 4 wt% BaO Mgþ 9 wt% BaO Mgþ 14 wt% BaO
1120 1164 (þ 44) 1156 (þ 36) 1125 (þ 5)
Table 3 Phase composition of the SiO2–Al2O3–Na2O–K2O–MgOþ BaO glass–ceramic materials. Contents of barium oxide – BaO 0 wt%
4 wt%
9 wt%
14 wt%
Protoenstatite Quartz
Forsterite Hyalophane
Hyalophane Forsterite Celzjan
Hyalophane Forsterite Celzjan
of alkaline metal oxides. However, the increase observed in the other characteristic temperatures can be linked with the formation of new crystalline phases during heat treatment. Also, the phase composition of samples with BaO differed considerably in comparison to the input material, as shown in Table 3 and Fig. 1. The phase composition of the material without BaO shows the presence of protoenstatite Mg2(Si2O6) from the group of mono-ino-silicates, quartz, and the glassy phase. Protoenstatite was probably formed by crystallization from an aluminum-silicate melt. The composition point, which simplified to a three-component system of SiO2–Al2O3–MgO, was proximal to the protoenstatite primary crystallization field. In some areas, where the aluminum-silicate alloy was formed with progress in the heating, the oxide compositions differed. Hence, shifting the composition point to the crystallization field of this phase was possible, as confirmed by the SEM images (Fig. 2), which indicates areas with partly melted quartz crystals. In these areas, the SiO2 concentration was certainly higher than the average oxide content in the tested material. The presence of protoenstatite crystals can be observed in these areas as well. Barium oxide introduced in the lowest amount (4 wt%), in the presence alkali metal oxides, acted as a fluxing agent to a higher degree, which resulted in molten quartz grains. A higher SiO2 concentration in the alloy induced crystallization of forsterite (Mg2[SiO4]), instead of protoenstatite (Fig. 3). The presence of BaO also caused the occurrence of small clusters of crystals of barium feldspar, hyalophane, and tecto potassium–sodium–barium–aluminumsilicate ((Ba0.40K0.47Na0.13)(Si2.59Al1.41O8)) with structure similar to sanidine The clusters, which are not numerous, are evenly scattered within the volume of the material and are
separated by areas of vitreous phase and spots in which quartz and forsterite are present. With increase in the barium oxide amount (up to 9 wt%), further changes in the phase composition occurred. Apart from hyalophane and forsterite, barium feldspar–celsian–barium tectoaluminum-silicate: Ba0.8K0.2(Al1.77Si2.23)O8 (see Table 2 and Figs. 1 and 4) was also formed. The recalculated composition point resulting in a simplified three-component SiO2–Al2O3–BaO phase system showed that the point was located in the celsian crystallization field, indicating intensification of celsian crystallization. SEM images indicate a significantly higher concentration of the crystalline phases and a lower content of the vitreous phase. Forsterite crystals were still clustered in small areas of high SiO2 concentration in spots left by molten quartz (Fig. 4). In addition, the number and size of the hyalophane crystals increased (Figs. 4 and 5). Interestingly, the diversified hyalophane grain structure (Figs. 4 and 5) indicates the existence of two mechanisms i.e., dendrite and directional crystallization. The samples with the highest barium oxide content (14 wt%) showed a higher crystalline phase content (Figs. 6 and 7). The crystals have various sizes and structures, although disordered directional crystallization was predominant. The SEM images show a large number of hyalophane crystals K0.6Ba0.4(Al1.42Si2.58O8), lower amounts of celsian Ba0.8K0.2(Al1.77Si2.23)O8, while forsterite crystals were rare (Fig. 7). The type of bonds formed during the vitreous phase formation in the glass–crystalline materials exerts a major influence on the final properties of the materials. Silico-oxygen and/or silico-alumino-oxygen bonds are the basic elements of the vitreous phase structure. MIR and FIR analyses provide information on the silicon– aluminum–oxygen structure of the vitreous phase and can confirm the presence of crystalline phases [9,10]. In the MIR analysis, several characteristic (coordination number CK=4) silicon–oxygen bonding bands can be distinguished. The band at 1200 cm 1 is associated with the SiQO stretching vibrations, i.e., defects arising from the silico-oxygen tetrahedra connected with one another at the edges. The band at 1090 cm 1 is associated with the Si–O(Si) stretching vibrations and Si–O stretching vibrations at 980–1000 cm 1 [8–11]. These two bands represent the bridging oxygen atoms (BO, silico-oxygen bridges) and non-bridging oxygen atoms
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(NBO, broken silico-oxygen bridges). All the listed bands can be usually observed as one broad band within the wavenumber range 850–1400 cm 1, as shown in Fig. 8. From the shape of this band and the location of its maximum, the component bands (BO or NBO) predominant in a given material can be determined (Fig. 8). Moreover, the bands associated with bending vibrations of Si–O–Si at
790 cm 1 and bending vibrations of O–Si-O at 470 cm 1 can be identified in the MIR spectra. The bands within the 750–550 cm 1 range, which are associated with vibrations of the silico-oxygen rings and/or alumino-silico-oxygen rings with differing segmentations, are observed in the spectra of silicate materials [8–12]. Other bands may be related to bonds characteristic of some other crystalline phases and can be
Fig. 1. Phase composition of the SiO2–Al2O3–Na2O–K2O–MgO materials with a 4 wt%, 9 wt%, and 14 wt% BaO additive (Hy is Hyalophane, Cl is Celsian, Pr is Protoenstatite, and Q is quartz).
Fig. 2. SEM–EDS image of a SSiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composite sample without any BaO.
Fig. 3. SEM–EDS image of a SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composite with 4 wt% BaO.
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Fig. 4. SEM–EDS image of SiO2–Al2O3–Na2O–K2O–MgO samples with 9 wt% BaO.
Fig. 5. SEM image of a SiO2–Al2O3–Na2O–K2O–MgO sample with 9 wt% BaO.
Fig. 6. SEM image of a SiO2–Al2O3–Na2O–K2O–MgO sample with 14 wt% BaO.
interpreted by comparison with reference spectra of crystalline phases [12–14]. If MIR bands of materials without BaO are known, it is possible to define changes induced by BaO addition. MIR analysis (Fig. 8) was carried out within the 1600– 400 cm 1 range and bands with similar ranges of wavenumbers were observed for all the samples. The band at 608 cm 1 is an exception and occurs in samples with 4 wt% and 9 wt% BaO and can probably be attributed to forsterite [14–16]. There is also a band at 570 cm 1 for samples with 9 wt% and 14 wt% BaO. This band is could be attributed to sanidine, although the band could also originate from this phase because of its structural similarity to hyalophane (no hyalophane reference spectra were found in the MIR and FIR [20,21] structural databases). The disappearance of bands at 722– 724 cm 1 and 772–778 cm 1, which are associated with the bending vibrations of Si–O–Si [8,10–13,20,21], show the reduced order of the vitreous phase structure, i.e., a higher number of broken Si–O bonds (NBO). By observing the main bands within the 1400–840 cm 1 range, it can be noticed that there is a distinct shift in the maximum values of the bands towards lower wavenumbers, from 1059 cm 1 for the material without BaO to 1014 cm 1
for the sample with the highest (14 wt%) BaO content. This shift is clearly proportional to the barium oxide content. This band maximum shift indicates that the intensity of the component band within the 980–1000 cm 1 range increased. This band is associated with Si-O stretching vibrations, corresponding to broken silicon–oxygen bridges, i.e., the ones with an increased number of the NBOs. The presence of such bonds also reveals a higher degree of a disordered state of the silicon and oxygen structure in the vitreous phase in the sintered materials. Cations, which are located in the interstitial spaces, in this case Mg2 þ or Ba2 þ , may bond with unsaturated oxygen ions. Magnesium has a significantly lower ionic radius and therefore, preferentially occupies the interstitial positions in the silicon and oxygen structure and saturates the free oxygen bonds. The FIR spectra (Fig. 9) confirm the presence of crystalline phases with the sanidine structure (the bands at 97–111 cm 1) as well as protoenstatite and forsterite (bands at 291 cm 1 and 293–294 cm 1). To completely understand the structure and microstructure of the glass–ceramic materials dealt with in this study, it is essential to acquire information on the internal structure of the glassy phase, specifically by Raman spectroscopy. The Raman spectra
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Fig. 7. SEM–EDS image of a SiO2–Al2O3–Na2O–K2O–MgO sample with 14 wt% BaO.
Fig. 9. FIR spectra of SiO2–Al2O3–Na2O–K2O–MgO glass–crystalline composites with 4 wt%, 9 wt%, and 14 wt% BaO.
Fig. 8. MIR spectra of SiO2–Al2O3–Na2O–K2O–MgO glass–crystalline composites with 4 wt%, 9 wt%, and 14 wt% BaO.
of the glassy silico-oxygen or/and alumina–silico-oxygen materials shows a number of bands reflecting the multiple coordination of the Si–O bonds [18,19,22]. These bands can be linked to the three-dimensional structure of SiO2, indicated by Q0–Q4, the vibrational frequencies of which change with the polymerization degree of the silicate network, showing a distribution of SiO4 tetrahedra with 0, 1, 2, 3, or 4 BOs. Additionally, bands derived from the aluminum tetrahedra network with different configurations of NBO/BO ions can be observed. Fig. 10 shows the Raman spectra of the glassy phases of the tested samples. In all the spectra, the absence of the band at
710 cm 1 unambiguously indicates the absence of aluminooxygen octahedra [16–19,22] and thus, all the aluminum ions are arranged in a tetrahedral coordination, indicating that they take part in the formation of the glassy structure. In the sample without BaO, weak bands at 1135 cm 1 (corresponding to the Q4 group representing SiO2 and tectosilicate) and 821 cm 1 (linked to Q0 – monomeric SiO4) can be observed. The additional band at 770 cm 1 can be assigned to [AlO4] tetrahedra with three BOs and one NBO. The band at 583 cm 1 can be associated with the vibrations of the threemembered silico- and alumino-silico-oxygen rings. The addition of 4 wt% BaO led to the appearance of new bands in the 850–960 cm 1 range, which indicates the formation of the group Q1 – Si2O7 anion. The increase in the intensity of bands in the 800–850 cm 1 range shows an increase in the presence of monomeric SiO4 group Q0. The increase in the BaO content
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tectosilicate group, barium-potassium feldspar – hyalophane (K,Ba)[Al(Si,Al)Si2O8], barium feldspar–celsian (Ba0.8K0.2) (Al1.77Si2.23)O8, and forsterite-monosilicate Mg2(SiO4). 4) The presence of barium oxide promoted the formation of NBOs, resulting in the formation of a severely distorted silicon and oxygen structure with a vitreous phase in the tested materials. 5) The degree of the disordered state increased with increase in the amount of barium oxide added.
Acknowledgments This research has been carried out thanks to financing under the framework of NCBiR (Polish National Research and Development Committee) program Nos. N N508 477734 and PBS1/ B5/17/2012. References
Fig. 10. Raman spectrum of the glassy phase of the SiO2–Al2O3–Na2O–K2O– MgO samples with 4 wt%, 9 wt%, and 14 wt% BaO.
to 9 wt% did not cause significant changes in the Raman spectrum beyond the band shifts associated with the presence of the Si2O7 anion towards lower wavenumbers. Increase in the BaO content to 14 wt% significantly changed the structure of the glassy phase. The bands in the 800–850 cm 1 range completely disappeared, indicating the presence of SiO4 monomers, while the appearance of bands in the 1050– 1100 cm 1 and 1100–1200 cm 1 ranges may be linked to the presence of group Q2 with silicate chains and group Q4, comprising of SiO2 and tectosilicates. 4. Conclusion The key findings of our study are summarized below. 1) The addition of BaO to the SiO2–Al2O3–Na2O–K2O–MgO samples caused considerable changes with regard to the phase composition and an increase in the crystalline phase content. 2) With the lowest amount of barium oxide added, hyalophane, tecto potassium–sodium–barium–aluminum-silicate given by the formula (Ba0.40K0.47Na0.13)(Si2.59Al1.41O8), and small amounts of forsterite monosilicate were formed. 3) The crystalline phases formed with the addition of9 wt% and 14 wt% BaO were aluminum-silicate from the
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Please cite this article as: J. Partyka, Effect of BaO addition on the structure and microstructure of SiO2–Al2O3–Na2O–K2O–MgO glass–ceramic composites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.014