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Structure-property relations in calcium aluminate glasses containing different divalent cations and SiO2
Journal of Non-Crystalline Solids 427 (2015) 160–165
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Structure-property relations in calcium aluminate glasses containing different divalent cations and SiO2 Maryam M. Sebdani a,b, John C. Mauro c, Lars R. Jensen d, Morten M. Smedskjaer a,⁎ a
Department of Chemistry and Bioscience, Aalborg University, 9220 Aalborg, Denmark Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Science and Technology Division, Corning Incorporated, Corning, NY 14831, USA d Department of Mechanical and Manufacturing Engineering, Aalborg University, 9220 Aalborg, Denmark b c
a r t i c l e
i n f o
Article history: Received 12 May 2015 Received in revised form 30 June 2015 Accepted 29 July 2015 Available online xxxx Keywords: Calcium aluminate glass; Structure–property relation; Glass transition; Mechanical; Properties; Raman spectroscopy
a b s t r a c t Calcium aluminate glasses are transparent in the infrared region, exhibit ultralow optical losses, and possess excellent mechanical properties. However, these systems exhibit poor glass-forming ability, and it is therefore important to understand their composition-structure-property relations to enable the design of new compositions with optimized properties and glass-forming ability. In this paper, we study the structure-property relationships in CaO–Al2O3 based glasses with different additives (SiO2, MgO, SrO, BaO, and ZnO). Our results reveal a gradual change in all the measured properties (molar volume, hardness, crack resistance, glass transition temperature, and liquid fragility) upon the addition of SiO2 to the eutectic 64CaO–36Al2O3 composition and the partial substitution of CaO for different divalent cation oxides. The relation between the measured liquid fragility index and the glass-forming ability is also investigated. We discuss these phenomena based on structural information obtained by Raman spectroscopy and topological considerations. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Glasses in the CaO–Al2O3 system are of scientific interest, since they are glass-forming without containing any traditional network formers such as SiO2, B2O3, and P2O5 [1,2]. Their structural chemistry is interesting as aluminum can possess various coordination numbers (four-, five-, and six-fold) [3,4], which is related with the dual role of calcium as network modifier and/or charge compensator. In the binary calcium aluminate system, the dominant coordination number of Al is 4 for alumina concentrations less than 50 mol%, and in these glasses the network progressively depolymerizes with increasing CaO content. For glasses with more than 50 mol% Al2O3, aluminum is found in five- and sixfold coordination in addition to the dominant four-fold coordinated species [5]. The modifier-to-Al2O3 ratio also determines the variation of aluminum Qn speciation, where Q is a tetrahedral species and n is the number of bridging oxygens [3–6]. Due to their excellent optical and mechanical properties, calcium aluminate glasses are also of technological interest. For example, these glasses exhibit good infrared (IR) transmission and ultralow optical losses [7,8], with a high cut-off wavelength (6 μm) compared to silicate glasses [9]. However, the binary calcium aluminate glasses exhibit poor glass-forming ability (GFA), i.e., a high critical cooling rate is needed to produce a glassy solid, especially for systems not around the eutectic ⁎ Corresponding author. E-mail address: [email protected] (M.M. Smedskjaer).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.07.047 0022-3093/© 2015 Elsevier B.V. All rights reserved.
point (64CaO–36Al2O3, in mol%). This limits production of large geometry specimens, and partial crystallization can degrade their mechanical and optical properties. Therefore, several attempts have been done to identify more favorable compositions in terms of GFA [10]. Different additions, including SiO2 and alkaline earth oxides, have been investigated and it has been found that the GFA can be significantly improved [11–16]. For example, SiO2 increases the glass-forming region of calcium aluminate, but it has a negative effect on the IR transmission of such glasses. Therefore, it is important to clarify the effect of various compositional modifications of calcium aluminate glasses on the macroscopic properties. To enable quantitative design of calcium aluminate based glass compositions with tailored properties and acceptable GFA, the microscopic origins of the variations in the macroscopic properties should be established, i.e., the composition–structure–property relations should be established for calcium aluminate glasses. Glass-forming ability is a measure of the ease for producing a glass upon cooling from a liquid (i.e., related to the minimal critical cooling rate required for glass formation). Considering the binary CaO–Al2O3 system, the eutectic composition exhibits the best GFA due to its high liquidus viscosity. In this composition, the average Al coordination number has been found to be 4.2 [3,4], i.e., the structure is dominated by AlO4 tetrahedra with the Ca atoms (coordination number from 5 to 8) situated within the network voids [17]. In this work, we have investigated the effect of different additives (SiO2, MgO, SrO, BaO, and ZnO) to the base eutectic composition on the structure and properties. This is a follow-up study to our recent work, in which we investigated the
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crystallization behavior and thus glass stability of these glasses [18]. It was found that the glass stability increases upon the addition of up to 10 mol% SiO2 to the eutectic CaO–Al2O3 composition, but then decreases upon further addition of SiO2. Partial substitution of CaO for different divalent cation oxides also increases the glass stability, with the largest impact found for the cations with the greatest difference in ionic radius compared to Ca2+. Here, we have investigated the structural changes in the glasses using Raman spectroscopy. We have also determined density (ρ), molar volume (Vm), Vickers hardness (HV), crack resistance (CR), glass transition temperature (Tg), and liquid fragility index (m). 2. Experimental 2.1. Glass preparation We have prepared two series of CaO–Al2O3 based glasses using the melt-quenching technique as described in detail elsewhere [18]. In the first series, SiO2 was added in different concentrations (5, 10, 15, and 20 mol%) to the CaO–Al2O3 eutectic composition, while scaling the concentrations of CaO and Al2O3 proportionally. The eutectic composition is named as CaAl, whereas the glass containing SiO2 are named as CaAl– 5Si, CaAl–10Si, CaAl–15Si, and CaAl–20Si, respectively. In the second series, 10 mol% of CaO was substituted for different modifier oxides with divalent cations (MgO, SrO, BaO, and ZnO) in the eutectic composition, while keeping the total divalent oxide (RO) and Al2O3 concentrations constant. These glasses are named as CaAl–Mg, CaAl–Sr, CaAl–Ba, and CaAl–Zn, respectively. The glasses were annealed for 30 min at their respective glass transition temperatures to remove internal stresses and ensure uniform thermal history. The analyzed chemical compositions of samples were found using x-ray fluorescence to be within ±1 mol% of the batched compositions. Besides the eutectic composition (CaAl), which exhibits a few sharp, crystalline peaks, the other eight samples are fully x-ray amorphous [18].
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2.5. Differential scanning calorimetry Differential scanning calorimetry (DSC) experiments were carried out using a STA 449C Jupiter instrument (Netzsch) to determine glass transition temperature (Tg) and liquid fragility index (m). The measurements were conducted under a flow of argon at 40 ml/min. The isobaric heat capacity (Cp) curve for each measurement was calculated relative to the Cp curve of a sapphire reference material of comparable mass (56 mg). The samples were subjected to several runs of DSC upscans and downscans. The rates of the upscans and downscans were always equal, and the scan rate Q was varied (2, 5, 10, 20, and 30 K/min). At the upscan, the calorimetric fictive temperature (Tf) is defined as the cross point between the extrapolated straight line of the glass Cp curve before the transition zone and the tangent at the inflection point of the sharp rise curve of Cp in the transition zone. The value of Tf increases with increasing upscan rate, and the Tf value measured during an upscan rate of 10 K/min after a previous downscan at the same rate corresponds to the standard Tg value [19]. The liquid fragility index was determined from the change in Tf with Q [20], T f ;re f Q ¼ m−m log ; Tf Q re f
ð1Þ
where Q ref is the reference heating and cooling rate taken as 10 K/min, and Tf,ref is the fictive temperature corresponding to Qref and equivalent to our calorimetric definition of Tg. We note that this method is an indirect approach to determine m compared to direct viscosity measurements, but viscosity measurements were not possible due to the limited sample volumes (result of high cooling rates needed to avoid crystallization). However, the m values obtained from DSC are generally in agreement with those from viscosity [21]. 3. Results and discussion 3.1. Network structure
2.2. Raman spectroscopy Raman spectroscopy measurements were conducted at room temperature to obtain structural information. This was done using a Renishaw inVia micro-Raman spectrometer with 532 nm laser in the range from 200 to 1100 cm−1. All of the recorded spectra were subjected to a baseline correction and area normalization procedure. The data were analyzed using Fityk software. 2.3. Density Density (ρ) was determined using the Archimedes method by first weighing the sample in air and then weighing it when suspended in water at room temperature. This was repeated a minimum of ten times, and the samples were at least 5 g in weight. The standard deviation of the reported density values does not exceed ± 0.003 g/cm3. Molar volume (Vm) is calculated as the ratio between the molar mass of the glass and its density. 2.4. Vickers indentation A Vickers microindentor (Duramin 5, Struers) was used to determine the micromechanical properties of samples, i.e., crack resistance (CR) and Vickers hardness (HV). Measurements were performed in air at room temperature, performing at least twenty indentations with a dwell time of 10 s at each load for each sample. Hardness was determined at a load of 0.98 N. Crack resistance of glasses was determined as the load, where there is an average of two out of four radial cracks at the corners of the indent. These tests were performed by applying load in the range from 0.98 to 9.81 N (0.98, 1.96, 2.94, 4.91, and 9.81 N).
Raman spectra of the calcium aluminate glasses with different concentrations of SiO2 and different network modifiers are plotted in Fig. 1a and b, respectively. The spectrum of the binary eutectic CaO– Al2O3 composition (CaAl) is in good agreement with those already published [22–25]. This Raman spectrum consists of a band around 540 cm−l and another band near 780 cm−1 with a marked tail to higher frequency. The former band has been attributed to transverse motions of bridging oxygens within the Al–O–Al linkages [22–25], whereas the band at 780 cm−1 has been assigned to Al–O stretching vibrations of the tetrahedral aluminate groups [4]. Moreover, the CaAl glass exhibits a weak sharp peak at 1015 cm− 1, which is presumably because this composition is not completely amorphous [18]. Upon the addition of SiO2 to the binary calcium aluminate glass, the location of the lower frequency band is at slightly higher frequency. In addition, the intensity of the peak at 780 cm−1 decreases systematically with increasing SiO2 concentration, since the concentration of Al2O3 decreases in parallel (and thus the number of Al–O bonds). Simultaneously, a new band at 870 cm−1 appears upon the addition of 5 mol% SiO2. This band has been attributed to Si–O stretching modes with three non-bridging oxygens [22,23]. With increasing the SiO2 content, the band at 780 cm−1 gradually disappears and the band at 870 cm−1 increases in intensity and shifts towards higher frequency. These changes can be understood as follows. In all the studied composition, the concentration of CaO is larger than that of Al2O3 (peralkaline) and the excessive CaO plays the role of network-modifier that causes formation of non-bridging oxygens (NBOs). With increasing SiO2 concentration, the excess modifier concentration and consequently the amount of NBOs also decreases. Meanwhile, adding SiO2 to the calcium aluminate network gradually leads to replacement of Al in Q2 species by Si in Q1 species. These results are generally in accordance with the findings of
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Fig. 1. Raman spectra of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added.
Neuville and co-workers [22], who investigated the structure of Al2O3– SiO2–CaO–Na2O glasses with varying Si/Al ratio. Raman spectra of the glasses containing different divalent cations are plotted in Fig. 1b. The two bands around 540 and 780 cm−l are present in all of the samples. The 540 cm−l Raman band shifts to slightly lower frequencies with increasing size of the divalent cation, in agreement with a previous observation [24]. This shift to lower frequency results from a decrease in the Al–O force constant, suggesting that the different modifier cations are closely connected to the Al–O–Al linkages. The position of the broad band at 780 cm−1 exhibits a similar variation with the divalent cation size, since the peak frequencies of the CaAl–Mg and CaAl–Zn glasses are slightly larger than those of the CaAl, CaAl–Sr, and CaAl–Ba glasses. In addition to creating NBOs, the divalent cations play the role of charge compensators of the aluminum tetrahedral groups, which explains this shift in frequency due to the change in the modifier-oxygen–aluminum vibrational oscillator weight.
3.2. Molar volume Fig. 2 shows the composition dependence of molar volume (Vm), which is calculated as the ratio between the molar mass of the glass (based on analyzed chemical composition [18]) and its measured density. When SiO2 is substituted for CaO and Al2O3, the molar volume decreases (Fig. 2a), which is in parallel with the increase in connectivity
Fig. 2. Molar volume (Vm) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. Molar volumes are calculated based on measured densities and analyzed chemical compositions [18]. In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass, and the dashed line is a guide for the eye.
as the number of NBOs decreases. For the alkaline earth elements, the molar volume increases with increasing cation size (Fig. 2b). Although the coordination number of the modifier cations is complex in these glasses and composition dependent [26–28], we here assume octahedral coordination of the cations for simplicity and to enable plotting the different properties against the cation size. With this assumption the ionic radii of Ca2 +, Mg2 +, Sr2 +, Ba2 +, and Zn2 + are 1.00, 0.72, 1.18, 1.35, and 0.74 Å, respectively [29]. The trend in Fig. 2b reflects the tighter binding of oxygen to smaller cations, as expected from the larger field strength in the order Mg2 + N Ca2 + N Sr2 + N Ba2 +. The glass with ZnO does not follow this trend, but we note that this is based on the assumption of octahedral coordination. 3.3. Micromechanical properties Glass hardness is a measure of the mean contact stress for the formation of a permanent deformation and is given by the load divided by the project area. The composition dependence of Vickers hardness (HV) is illustrated in Fig. 3. Upon the addition of SiO2 to the eutectic composition, there is a slight (possibly negligible) increase in hardness (Fig. 3a). In terms of the network topology, hardness is governed by the number of rigid bong length and bond angular constraints at room temperature [30–32]. Both Si and Al are expected to be four-fold coordinated in these
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Fig. 3. Vickers hardness (HV) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. HV has been measured has at a load of 0.98 N. In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass.
peralkaline compositions and each of these atoms thus contributes 2 linear and 5 angular constraints. Since there are slightly more Si atoms entering the network than there are Al atoms leaving the network in this particular series, this could cause an increase in hardness. However, this effect is partly counterbalanced by the fewer number of NBOforming Ca atoms upon SiO2 addition, which could also contribute constraints to the network as it has been shown in recent work [33,34]. These two competing effects could cause hardness to be almost independent of the SiO2 content within this compositional regime. The calcium aluminate glasses containing different divalent cations exhibit an approximate linear decrease in hardness with increasing ionic radius of the divalent cation, which has been substituted into the glass (Fig. 3b). This is in agreement with previous findings [35] and could be due to the weakening of the network structure when the field strength of the divalent cations decreases [36]. Consequently, the attraction of the modifier cations to their surrounding structural groups of AlO4 tetrahedra is reduced. Crack resistance (CR) of the glasses has been determined using the method suggested by Kato et al. [37], in which the number of radial cracks emanating from the corners of the Vickers indent is counted as a function of the stepwise increasing load. CR is defined as the load at which an average of two radial crack (50% crack probability) form at corners of the Vickers indents. With increasing amount of SiO2, CR generally decreases (Fig. 4a), although the number of NBOs also decreases. This could instead be related with the decrease in molar volume with
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Fig. 4. Crack resistance (CR) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. CR is defined as the load at which an average of two radial crack (50% crack probability) form at corners of the Vickers indents. In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass.
increasing [SiO2]. With increasing contribution of densification to the indentation deformation, CR has been found to increase [38] and when the glass network is more compact, there is less open space available into which the glass can densify prior to cracking [39]. Fig. 4b shows the crack resistance of the glasses with different modifier oxides. Substituting both smaller and larger divalent cations compared to Ca2+ leads to a decrease of CR. Measurements to quantify the indentation deformation mechanism (contributions of densification vs. shear flow) are necessary to understand this effect. 3.4. Glass transition and fragility The glass transition temperature is determined as the onset temperature of the glass transition by DSC with a heating rate equal to the prior cooling of 10 K/min. An example of the recorded DSC scan is shown in Fig. 5 for the calcium aluminate glass containing 10 mol% SiO2. Tg is determined as the intercept of the extrapolated glass Cp and the inflection of the curve. With the addition of SiO2, Tg increases, especially upon the initial addition of 5 mol% SiO2 (Fig. 6a). This is presumably associated with the decrease in the number of NBOs in agreement with the Raman spectroscopy results. These results are also consistent with those of previous studies [40,41]. The partial substitution of various
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modifier oxides for CaO leads to a reduction of the glass transition temperature (Fig. 6b). We note that difference in the compositional scaling of HV (Fig. 3) and Tg (Fig. 5) could be due to the temperature dependence of the network constraints [30].
The liquid fragility index (m) of the investigated systems has been determined indirectly by DSC measurements from the change in fictive temperature with cooling rate. The composition dependence of m is shown in Fig. 7. The liquids become “stronger” (i.e., m decreases) when SiO2 is added to the binary calcium aluminate composition (Fig. 7a). The scaling of m with composition can be understood in terms of temperature-dependent constraint theory as the temperature derivative of the number of atomic constraints at Tg [42]. For boratecontaining liquids, we have previously found that the constraint onset temperature (i.e., temperature at which the constraints become rigid upon cooling) of O–B–O angular constraints is close to Tg [43]. The derivative of the O–B–O constraints is therefore large and these constraints have a large positive contribution to m. The partial substitution of O– Si–O for O–AlIV–O angular constraints could explain the measured decrease of m if the onset temperature of the O–AlIV–O angular constraints is lower and thus closer to Tg than that of the O–Si–O angular constraints. Fig. 7b shows the fragility of the compositions with different modifier oxides. Substituting both smaller and larger divalent cations compared to Ca2+ leads to a decrease of m. This composition dependence of m is in good agreement with the previously published composition dependence of glass stability [18]. Glass stability exhibits a minimum value for the eutectic composition and progressively increases as Ca2+ is partially substituted by cations, which have a difference size. The
Fig. 6. Glass transition temperature (Tg) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. Tg has been measured by DSC at 10 K/min heating rate. In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass. The uncertainty of Tg is approximately ±2 K.
Fig. 7. Fragility index (m) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. m has been determined by DSC using Eq. (1). In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass.
Fig. 5. Glass transition of the calcium aluminate glass containing 10 mol% SiO2 as determined by DSC at a heating rate of 10 K/min subsequent to a cooling rate of 10 K/min. The plot is shown as the isobaric heat capacity (Cp) against temperature (T).
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of CaO for different modifier oxides has a more subtle effect on the network structure, viz., the change in modifier field strength affects, e.g., the Al–O force constant. Molar volume increases and hardness decreases with decreasing modifier field strength, whereas crack resistance, glass transition temperature, and liquid fragility index exhibit a maximum value for the binary CaO–Al2O3 composition. By relating the fragility to the glass stability parameter from our recent study [18], we have found that fragility can be used as a measure of GFA within series of approximately constant network connectivity (varying modifier oxide). Acknowledgments We thank Mouritz N. Svenson (Aalborg University) for assistance with the Raman spectroscopy measurements. Fig. 8. Fragility index (m) as a function of the glass stability parameter KSP for the calcium aluminate glasses with varying SiO2 content and different modifier oxides added. The values of KSP are taken from Ref. [18].
presence of dissimilar modifier cation sites in the glassy networks increases the topological disorder and decreases fragility [44]. 3.5. Relation between fragility and glass stability The dependence of liquid viscosity on temperature is an important measure for the ease of glass formation since viscosity determines the magnitude of the kinetic barriers for nucleation and crystal growth. Fragile liquids with a strong dependence of viscosity on temperature exhibit a rapid decrease of viscosity and increase of diffusivity with increasing temperature [45], giving rise to favorable conditions for crystallization of the glass (i.e., low glass stability). Indeed for various metallic glass-forming liquids, fragility has been found to scale inversely with GFA [44,46,47], and an inverse relationship between fragility and glass stability has also been reported for calcium aluminosilicate glasses within systematic composition variations [48]. It is also well-known that SiO2 is an excellent glass former and at the same time the strongest known liquid. In our recent study, we have investigated the stability of the calcium aluminate glasses against crystallization [18], as quantified by the glass stability parameter proposed by Saad and Poulain (KSP) [49]. The inverse correlation among m and KSP is tested in Fig. 8. We cannot find a common trend for all investigated compositions, but for the compositions with different modifier oxides, m is found to decrease with increasing KSP. This behavior could suggest that the inverse correlation between fragility and GFA is at least valid within series of approximately constant network connectivity, whereas additional work is required to understand why the relation is not valid for the compositions with varying SiO2 content. However, we should note that some uncertainty might also arise from the fact that the determined glass stability parameter might not scale exactly with the GFA [50]. 4. Conclusions We have investigated the correlation between the structure and properties of CaO–Al2O3 based glasses containing different networkmodifier oxides (SrO, MgO, ZnO, and BaO) and SiO2 in different concentrations. With increasing SiO2 concentration, the excess modifier concentration decreases, and consequently the number of non-bridging oxygens also decreases. This structural change leads to an increase in Vickers hardness and glass transition temperature, whereas the decrease in molar volume with [SiO2] is presumably responsible for the decrease in crack resistance. These liquids also exhibit a gradual decrease in fragility index upon SiO2 addition. The partial substitution
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Structure-property relations in calcium aluminate glasses containing different divalent cations and SiO2 Maryam M. Sebdani a,b, John C. Mauro c, Lars R. Jensen d, Morten M. Smedskjaer a,⁎ a
Department of Chemistry and Bioscience, Aalborg University, 9220 Aalborg, Denmark Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Science and Technology Division, Corning Incorporated, Corning, NY 14831, USA d Department of Mechanical and Manufacturing Engineering, Aalborg University, 9220 Aalborg, Denmark b c
a r t i c l e
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Article history: Received 12 May 2015 Received in revised form 30 June 2015 Accepted 29 July 2015 Available online xxxx Keywords: Calcium aluminate glass; Structure–property relation; Glass transition; Mechanical; Properties; Raman spectroscopy
a b s t r a c t Calcium aluminate glasses are transparent in the infrared region, exhibit ultralow optical losses, and possess excellent mechanical properties. However, these systems exhibit poor glass-forming ability, and it is therefore important to understand their composition-structure-property relations to enable the design of new compositions with optimized properties and glass-forming ability. In this paper, we study the structure-property relationships in CaO–Al2O3 based glasses with different additives (SiO2, MgO, SrO, BaO, and ZnO). Our results reveal a gradual change in all the measured properties (molar volume, hardness, crack resistance, glass transition temperature, and liquid fragility) upon the addition of SiO2 to the eutectic 64CaO–36Al2O3 composition and the partial substitution of CaO for different divalent cation oxides. The relation between the measured liquid fragility index and the glass-forming ability is also investigated. We discuss these phenomena based on structural information obtained by Raman spectroscopy and topological considerations. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Glasses in the CaO–Al2O3 system are of scientific interest, since they are glass-forming without containing any traditional network formers such as SiO2, B2O3, and P2O5 [1,2]. Their structural chemistry is interesting as aluminum can possess various coordination numbers (four-, five-, and six-fold) [3,4], which is related with the dual role of calcium as network modifier and/or charge compensator. In the binary calcium aluminate system, the dominant coordination number of Al is 4 for alumina concentrations less than 50 mol%, and in these glasses the network progressively depolymerizes with increasing CaO content. For glasses with more than 50 mol% Al2O3, aluminum is found in five- and sixfold coordination in addition to the dominant four-fold coordinated species [5]. The modifier-to-Al2O3 ratio also determines the variation of aluminum Qn speciation, where Q is a tetrahedral species and n is the number of bridging oxygens [3–6]. Due to their excellent optical and mechanical properties, calcium aluminate glasses are also of technological interest. For example, these glasses exhibit good infrared (IR) transmission and ultralow optical losses [7,8], with a high cut-off wavelength (6 μm) compared to silicate glasses [9]. However, the binary calcium aluminate glasses exhibit poor glass-forming ability (GFA), i.e., a high critical cooling rate is needed to produce a glassy solid, especially for systems not around the eutectic ⁎ Corresponding author. E-mail address: [email protected] (M.M. Smedskjaer).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.07.047 0022-3093/© 2015 Elsevier B.V. All rights reserved.
point (64CaO–36Al2O3, in mol%). This limits production of large geometry specimens, and partial crystallization can degrade their mechanical and optical properties. Therefore, several attempts have been done to identify more favorable compositions in terms of GFA [10]. Different additions, including SiO2 and alkaline earth oxides, have been investigated and it has been found that the GFA can be significantly improved [11–16]. For example, SiO2 increases the glass-forming region of calcium aluminate, but it has a negative effect on the IR transmission of such glasses. Therefore, it is important to clarify the effect of various compositional modifications of calcium aluminate glasses on the macroscopic properties. To enable quantitative design of calcium aluminate based glass compositions with tailored properties and acceptable GFA, the microscopic origins of the variations in the macroscopic properties should be established, i.e., the composition–structure–property relations should be established for calcium aluminate glasses. Glass-forming ability is a measure of the ease for producing a glass upon cooling from a liquid (i.e., related to the minimal critical cooling rate required for glass formation). Considering the binary CaO–Al2O3 system, the eutectic composition exhibits the best GFA due to its high liquidus viscosity. In this composition, the average Al coordination number has been found to be 4.2 [3,4], i.e., the structure is dominated by AlO4 tetrahedra with the Ca atoms (coordination number from 5 to 8) situated within the network voids [17]. In this work, we have investigated the effect of different additives (SiO2, MgO, SrO, BaO, and ZnO) to the base eutectic composition on the structure and properties. This is a follow-up study to our recent work, in which we investigated the
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crystallization behavior and thus glass stability of these glasses [18]. It was found that the glass stability increases upon the addition of up to 10 mol% SiO2 to the eutectic CaO–Al2O3 composition, but then decreases upon further addition of SiO2. Partial substitution of CaO for different divalent cation oxides also increases the glass stability, with the largest impact found for the cations with the greatest difference in ionic radius compared to Ca2+. Here, we have investigated the structural changes in the glasses using Raman spectroscopy. We have also determined density (ρ), molar volume (Vm), Vickers hardness (HV), crack resistance (CR), glass transition temperature (Tg), and liquid fragility index (m). 2. Experimental 2.1. Glass preparation We have prepared two series of CaO–Al2O3 based glasses using the melt-quenching technique as described in detail elsewhere [18]. In the first series, SiO2 was added in different concentrations (5, 10, 15, and 20 mol%) to the CaO–Al2O3 eutectic composition, while scaling the concentrations of CaO and Al2O3 proportionally. The eutectic composition is named as CaAl, whereas the glass containing SiO2 are named as CaAl– 5Si, CaAl–10Si, CaAl–15Si, and CaAl–20Si, respectively. In the second series, 10 mol% of CaO was substituted for different modifier oxides with divalent cations (MgO, SrO, BaO, and ZnO) in the eutectic composition, while keeping the total divalent oxide (RO) and Al2O3 concentrations constant. These glasses are named as CaAl–Mg, CaAl–Sr, CaAl–Ba, and CaAl–Zn, respectively. The glasses were annealed for 30 min at their respective glass transition temperatures to remove internal stresses and ensure uniform thermal history. The analyzed chemical compositions of samples were found using x-ray fluorescence to be within ±1 mol% of the batched compositions. Besides the eutectic composition (CaAl), which exhibits a few sharp, crystalline peaks, the other eight samples are fully x-ray amorphous [18].
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2.5. Differential scanning calorimetry Differential scanning calorimetry (DSC) experiments were carried out using a STA 449C Jupiter instrument (Netzsch) to determine glass transition temperature (Tg) and liquid fragility index (m). The measurements were conducted under a flow of argon at 40 ml/min. The isobaric heat capacity (Cp) curve for each measurement was calculated relative to the Cp curve of a sapphire reference material of comparable mass (56 mg). The samples were subjected to several runs of DSC upscans and downscans. The rates of the upscans and downscans were always equal, and the scan rate Q was varied (2, 5, 10, 20, and 30 K/min). At the upscan, the calorimetric fictive temperature (Tf) is defined as the cross point between the extrapolated straight line of the glass Cp curve before the transition zone and the tangent at the inflection point of the sharp rise curve of Cp in the transition zone. The value of Tf increases with increasing upscan rate, and the Tf value measured during an upscan rate of 10 K/min after a previous downscan at the same rate corresponds to the standard Tg value [19]. The liquid fragility index was determined from the change in Tf with Q [20], T f ;re f Q ¼ m−m log ; Tf Q re f
ð1Þ
where Q ref is the reference heating and cooling rate taken as 10 K/min, and Tf,ref is the fictive temperature corresponding to Qref and equivalent to our calorimetric definition of Tg. We note that this method is an indirect approach to determine m compared to direct viscosity measurements, but viscosity measurements were not possible due to the limited sample volumes (result of high cooling rates needed to avoid crystallization). However, the m values obtained from DSC are generally in agreement with those from viscosity [21]. 3. Results and discussion 3.1. Network structure
2.2. Raman spectroscopy Raman spectroscopy measurements were conducted at room temperature to obtain structural information. This was done using a Renishaw inVia micro-Raman spectrometer with 532 nm laser in the range from 200 to 1100 cm−1. All of the recorded spectra were subjected to a baseline correction and area normalization procedure. The data were analyzed using Fityk software. 2.3. Density Density (ρ) was determined using the Archimedes method by first weighing the sample in air and then weighing it when suspended in water at room temperature. This was repeated a minimum of ten times, and the samples were at least 5 g in weight. The standard deviation of the reported density values does not exceed ± 0.003 g/cm3. Molar volume (Vm) is calculated as the ratio between the molar mass of the glass and its density. 2.4. Vickers indentation A Vickers microindentor (Duramin 5, Struers) was used to determine the micromechanical properties of samples, i.e., crack resistance (CR) and Vickers hardness (HV). Measurements were performed in air at room temperature, performing at least twenty indentations with a dwell time of 10 s at each load for each sample. Hardness was determined at a load of 0.98 N. Crack resistance of glasses was determined as the load, where there is an average of two out of four radial cracks at the corners of the indent. These tests were performed by applying load in the range from 0.98 to 9.81 N (0.98, 1.96, 2.94, 4.91, and 9.81 N).
Raman spectra of the calcium aluminate glasses with different concentrations of SiO2 and different network modifiers are plotted in Fig. 1a and b, respectively. The spectrum of the binary eutectic CaO– Al2O3 composition (CaAl) is in good agreement with those already published [22–25]. This Raman spectrum consists of a band around 540 cm−l and another band near 780 cm−1 with a marked tail to higher frequency. The former band has been attributed to transverse motions of bridging oxygens within the Al–O–Al linkages [22–25], whereas the band at 780 cm−1 has been assigned to Al–O stretching vibrations of the tetrahedral aluminate groups [4]. Moreover, the CaAl glass exhibits a weak sharp peak at 1015 cm− 1, which is presumably because this composition is not completely amorphous [18]. Upon the addition of SiO2 to the binary calcium aluminate glass, the location of the lower frequency band is at slightly higher frequency. In addition, the intensity of the peak at 780 cm−1 decreases systematically with increasing SiO2 concentration, since the concentration of Al2O3 decreases in parallel (and thus the number of Al–O bonds). Simultaneously, a new band at 870 cm−1 appears upon the addition of 5 mol% SiO2. This band has been attributed to Si–O stretching modes with three non-bridging oxygens [22,23]. With increasing the SiO2 content, the band at 780 cm−1 gradually disappears and the band at 870 cm−1 increases in intensity and shifts towards higher frequency. These changes can be understood as follows. In all the studied composition, the concentration of CaO is larger than that of Al2O3 (peralkaline) and the excessive CaO plays the role of network-modifier that causes formation of non-bridging oxygens (NBOs). With increasing SiO2 concentration, the excess modifier concentration and consequently the amount of NBOs also decreases. Meanwhile, adding SiO2 to the calcium aluminate network gradually leads to replacement of Al in Q2 species by Si in Q1 species. These results are generally in accordance with the findings of
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Fig. 1. Raman spectra of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added.
Neuville and co-workers [22], who investigated the structure of Al2O3– SiO2–CaO–Na2O glasses with varying Si/Al ratio. Raman spectra of the glasses containing different divalent cations are plotted in Fig. 1b. The two bands around 540 and 780 cm−l are present in all of the samples. The 540 cm−l Raman band shifts to slightly lower frequencies with increasing size of the divalent cation, in agreement with a previous observation [24]. This shift to lower frequency results from a decrease in the Al–O force constant, suggesting that the different modifier cations are closely connected to the Al–O–Al linkages. The position of the broad band at 780 cm−1 exhibits a similar variation with the divalent cation size, since the peak frequencies of the CaAl–Mg and CaAl–Zn glasses are slightly larger than those of the CaAl, CaAl–Sr, and CaAl–Ba glasses. In addition to creating NBOs, the divalent cations play the role of charge compensators of the aluminum tetrahedral groups, which explains this shift in frequency due to the change in the modifier-oxygen–aluminum vibrational oscillator weight.
3.2. Molar volume Fig. 2 shows the composition dependence of molar volume (Vm), which is calculated as the ratio between the molar mass of the glass (based on analyzed chemical composition [18]) and its measured density. When SiO2 is substituted for CaO and Al2O3, the molar volume decreases (Fig. 2a), which is in parallel with the increase in connectivity
Fig. 2. Molar volume (Vm) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. Molar volumes are calculated based on measured densities and analyzed chemical compositions [18]. In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass, and the dashed line is a guide for the eye.
as the number of NBOs decreases. For the alkaline earth elements, the molar volume increases with increasing cation size (Fig. 2b). Although the coordination number of the modifier cations is complex in these glasses and composition dependent [26–28], we here assume octahedral coordination of the cations for simplicity and to enable plotting the different properties against the cation size. With this assumption the ionic radii of Ca2 +, Mg2 +, Sr2 +, Ba2 +, and Zn2 + are 1.00, 0.72, 1.18, 1.35, and 0.74 Å, respectively [29]. The trend in Fig. 2b reflects the tighter binding of oxygen to smaller cations, as expected from the larger field strength in the order Mg2 + N Ca2 + N Sr2 + N Ba2 +. The glass with ZnO does not follow this trend, but we note that this is based on the assumption of octahedral coordination. 3.3. Micromechanical properties Glass hardness is a measure of the mean contact stress for the formation of a permanent deformation and is given by the load divided by the project area. The composition dependence of Vickers hardness (HV) is illustrated in Fig. 3. Upon the addition of SiO2 to the eutectic composition, there is a slight (possibly negligible) increase in hardness (Fig. 3a). In terms of the network topology, hardness is governed by the number of rigid bong length and bond angular constraints at room temperature [30–32]. Both Si and Al are expected to be four-fold coordinated in these
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Fig. 3. Vickers hardness (HV) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. HV has been measured has at a load of 0.98 N. In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass.
peralkaline compositions and each of these atoms thus contributes 2 linear and 5 angular constraints. Since there are slightly more Si atoms entering the network than there are Al atoms leaving the network in this particular series, this could cause an increase in hardness. However, this effect is partly counterbalanced by the fewer number of NBOforming Ca atoms upon SiO2 addition, which could also contribute constraints to the network as it has been shown in recent work [33,34]. These two competing effects could cause hardness to be almost independent of the SiO2 content within this compositional regime. The calcium aluminate glasses containing different divalent cations exhibit an approximate linear decrease in hardness with increasing ionic radius of the divalent cation, which has been substituted into the glass (Fig. 3b). This is in agreement with previous findings [35] and could be due to the weakening of the network structure when the field strength of the divalent cations decreases [36]. Consequently, the attraction of the modifier cations to their surrounding structural groups of AlO4 tetrahedra is reduced. Crack resistance (CR) of the glasses has been determined using the method suggested by Kato et al. [37], in which the number of radial cracks emanating from the corners of the Vickers indent is counted as a function of the stepwise increasing load. CR is defined as the load at which an average of two radial crack (50% crack probability) form at corners of the Vickers indents. With increasing amount of SiO2, CR generally decreases (Fig. 4a), although the number of NBOs also decreases. This could instead be related with the decrease in molar volume with
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Fig. 4. Crack resistance (CR) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. CR is defined as the load at which an average of two radial crack (50% crack probability) form at corners of the Vickers indents. In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass.
increasing [SiO2]. With increasing contribution of densification to the indentation deformation, CR has been found to increase [38] and when the glass network is more compact, there is less open space available into which the glass can densify prior to cracking [39]. Fig. 4b shows the crack resistance of the glasses with different modifier oxides. Substituting both smaller and larger divalent cations compared to Ca2+ leads to a decrease of CR. Measurements to quantify the indentation deformation mechanism (contributions of densification vs. shear flow) are necessary to understand this effect. 3.4. Glass transition and fragility The glass transition temperature is determined as the onset temperature of the glass transition by DSC with a heating rate equal to the prior cooling of 10 K/min. An example of the recorded DSC scan is shown in Fig. 5 for the calcium aluminate glass containing 10 mol% SiO2. Tg is determined as the intercept of the extrapolated glass Cp and the inflection of the curve. With the addition of SiO2, Tg increases, especially upon the initial addition of 5 mol% SiO2 (Fig. 6a). This is presumably associated with the decrease in the number of NBOs in agreement with the Raman spectroscopy results. These results are also consistent with those of previous studies [40,41]. The partial substitution of various
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modifier oxides for CaO leads to a reduction of the glass transition temperature (Fig. 6b). We note that difference in the compositional scaling of HV (Fig. 3) and Tg (Fig. 5) could be due to the temperature dependence of the network constraints [30].
The liquid fragility index (m) of the investigated systems has been determined indirectly by DSC measurements from the change in fictive temperature with cooling rate. The composition dependence of m is shown in Fig. 7. The liquids become “stronger” (i.e., m decreases) when SiO2 is added to the binary calcium aluminate composition (Fig. 7a). The scaling of m with composition can be understood in terms of temperature-dependent constraint theory as the temperature derivative of the number of atomic constraints at Tg [42]. For boratecontaining liquids, we have previously found that the constraint onset temperature (i.e., temperature at which the constraints become rigid upon cooling) of O–B–O angular constraints is close to Tg [43]. The derivative of the O–B–O constraints is therefore large and these constraints have a large positive contribution to m. The partial substitution of O– Si–O for O–AlIV–O angular constraints could explain the measured decrease of m if the onset temperature of the O–AlIV–O angular constraints is lower and thus closer to Tg than that of the O–Si–O angular constraints. Fig. 7b shows the fragility of the compositions with different modifier oxides. Substituting both smaller and larger divalent cations compared to Ca2+ leads to a decrease of m. This composition dependence of m is in good agreement with the previously published composition dependence of glass stability [18]. Glass stability exhibits a minimum value for the eutectic composition and progressively increases as Ca2+ is partially substituted by cations, which have a difference size. The
Fig. 6. Glass transition temperature (Tg) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. Tg has been measured by DSC at 10 K/min heating rate. In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass. The uncertainty of Tg is approximately ±2 K.
Fig. 7. Fragility index (m) of the calcium aluminate glasses with (a) varying SiO2 content and (b) different modifier oxides added. m has been determined by DSC using Eq. (1). In (b), r represents the ionic radius of the divalent cation (assuming octahedral coordination), which has been substituted into the glass.
Fig. 5. Glass transition of the calcium aluminate glass containing 10 mol% SiO2 as determined by DSC at a heating rate of 10 K/min subsequent to a cooling rate of 10 K/min. The plot is shown as the isobaric heat capacity (Cp) against temperature (T).
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of CaO for different modifier oxides has a more subtle effect on the network structure, viz., the change in modifier field strength affects, e.g., the Al–O force constant. Molar volume increases and hardness decreases with decreasing modifier field strength, whereas crack resistance, glass transition temperature, and liquid fragility index exhibit a maximum value for the binary CaO–Al2O3 composition. By relating the fragility to the glass stability parameter from our recent study [18], we have found that fragility can be used as a measure of GFA within series of approximately constant network connectivity (varying modifier oxide). Acknowledgments We thank Mouritz N. Svenson (Aalborg University) for assistance with the Raman spectroscopy measurements. Fig. 8. Fragility index (m) as a function of the glass stability parameter KSP for the calcium aluminate glasses with varying SiO2 content and different modifier oxides added. The values of KSP are taken from Ref. [18].
presence of dissimilar modifier cation sites in the glassy networks increases the topological disorder and decreases fragility [44]. 3.5. Relation between fragility and glass stability The dependence of liquid viscosity on temperature is an important measure for the ease of glass formation since viscosity determines the magnitude of the kinetic barriers for nucleation and crystal growth. Fragile liquids with a strong dependence of viscosity on temperature exhibit a rapid decrease of viscosity and increase of diffusivity with increasing temperature [45], giving rise to favorable conditions for crystallization of the glass (i.e., low glass stability). Indeed for various metallic glass-forming liquids, fragility has been found to scale inversely with GFA [44,46,47], and an inverse relationship between fragility and glass stability has also been reported for calcium aluminosilicate glasses within systematic composition variations [48]. It is also well-known that SiO2 is an excellent glass former and at the same time the strongest known liquid. In our recent study, we have investigated the stability of the calcium aluminate glasses against crystallization [18], as quantified by the glass stability parameter proposed by Saad and Poulain (KSP) [49]. The inverse correlation among m and KSP is tested in Fig. 8. We cannot find a common trend for all investigated compositions, but for the compositions with different modifier oxides, m is found to decrease with increasing KSP. This behavior could suggest that the inverse correlation between fragility and GFA is at least valid within series of approximately constant network connectivity, whereas additional work is required to understand why the relation is not valid for the compositions with varying SiO2 content. However, we should note that some uncertainty might also arise from the fact that the determined glass stability parameter might not scale exactly with the GFA [50]. 4. Conclusions We have investigated the correlation between the structure and properties of CaO–Al2O3 based glasses containing different networkmodifier oxides (SrO, MgO, ZnO, and BaO) and SiO2 in different concentrations. With increasing SiO2 concentration, the excess modifier concentration decreases, and consequently the number of non-bridging oxygens also decreases. This structural change leads to an increase in Vickers hardness and glass transition temperature, whereas the decrease in molar volume with [SiO2] is presumably responsible for the decrease in crack resistance. These liquids also exhibit a gradual decrease in fragility index upon SiO2 addition. The partial substitution
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