THE CATIONIC EFFECT ON PROPERTIES AND STRUCTURE OF CaO-MgO-SiO
2MELTS
Yong-Uk Han and Dong Joon Min
Yonsei University; Yonsei-ro 50, Seodaemun-gu, Seoul, 03722, Republic of Korea
Keywords: Electrical Conductivity, Viscosity, Slag Structure, Stability
Abstract
A study on the effect of cation species on the viscosity and electrical conductivity of CaO- MgO-SiO2 system is carried out. Rotating cylindrical and two-plate method is used for viscosity and electrical conductivity measurements, respectively. Raman spectroscopy is also carried out to understand the structure of the slags. Experimental results indicated that the cationic effect on viscosity and ionic conductivity is governed by classical Anderson-Stuart theory: The dominance of electrostatic interaction on steric hindrance is confirmed for depolymerized melts (NBO/T=2.0). For polymerized melts (NBO/T=0), however, the major cationic effect on transport properties are examined to be a strain field distortion energy. The viscosity and ionic electrical conductivity are in strong correlation and assumed as the structure-dependent property. The structure of polymerized melts is also affected by the transition in primary solidification phase: Abnormal changes in properties and structure are observed at diopside congruent composition. Such a change at the congruent composition is assured by the entropy calculation and the stability function, 𝜓
Introduction
Structure-composition-property relationship of silicate glasses and melts has long been of vast interest from geology, glass science, and metallurgy. Plenty of studies are devoted to elucidation of silicate melt structures with the aid of spectroscopic structural analysis including FT-IR[1],[2], Raman[3]-[7], and MAS-NMR[8]-[11] analysis. The effect of ionic distribution of silicate polymeric anion on properties of such melts were well reviewed by several pioneering works[5]-[7]. To the best of our knowledge, the effect of cations, however, has not been organized.
Mysen[6] once suggested the application of topological entropy as a measurement of the deviation of cations from random mixing. Topological entropy, calculated by subtracting random mixing entropy of Qn species from configurational entropy[12], indicates that the cation is unevenly distributed throughout the silicate melts. Although the idea of Mysen provides a glimpse into the cationic effect in molten silicates, the effect is only considered by the ionization potential, z/r2. Several works defies the z/r2 hierarchy in which the opposite trend in physical properties is experimentally observed.
The innovative aspect of present study is that it provides comprehensive understanding on the cationic effect on structure and property of molten silicate system, CaO-MgO-SiO2. To distinguish structural effect from ionic nature of cations, discussion on the fully polymerized melts (SiO2-saturated compositions) is performed. The classical Anderson-Stuart theory[13] is successfully applied for the explanation of the antithetic cationic effect on properties in polymerized and depolymerized melts. For polymerized melts, the effect of the transition in primary solidification phase is also discussed for its effect on properties and melt structure.
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
Experimental procedure
Reagent-grade MgO and SiO2 powders are used. CaO was obtained by calcining reagent- grade CaCO3 at 1273K for 10 hours. 100mg of powders are mixed and melted in a Pt crucible at 1873 K under high purity Ar atmosphere. To minimize segregation during the quenching procedure, a small amount of sample is melted in a halogen lamp installed high temperature confocal-laser microscope (Lasertec LV2000DX-SVF17SP). After a homogenized melt is obtained, He-quenching is applied to sustain initial cooling rates of 3000K/min. Fully- polymerized silica-saturated compositions (blue circles in Fig 1 (a)) are chosen as a control group for the ionic interactions and CaSiO3-MgSiO3 compositions (green diamonds in Fig 1 (a) are chosen to exclude the effect of the basicity on the properties.
The electrical conductivities of the melts were measured in a Pt–Rh crucible (ϕ 54 mm, length 65 mm). A pair of Pt-10Rh electrodes (10 × 40 × 1 mm) in contact with a Pt-Rh wire (ϕ 1 mm, length 1 m) was calibrated using a 1.0 N KCl aqueous solution and the cell factor was determined. A Wheatstone bridge was established to measure the electrical resistance between electrodes. The alternating current was made using a 500 mV electrical potential with a frequency of 1 kHz. A Keithley 2182A nanovoltmeter was used to measure the resistance in the Wheatstone bridge circuit. The validity of the measurements was confirmed by measuring CaO–CaCl2 and CaO–CaF2 molten salts.
The rotating cylinder method was applied in the viscosity measurements (Fig. 1). A Brookfield digital viscometer (RVDV-II+, Middleboro, MA) with a full-scale torque of 7.187
× 10-4 N∙m was used. To avoid slag contamination and eccentricity during the rotation, triple- joint Pt–Rh spindles were linked to the viscometer. The whole measuring system was calibrated using Brookfield standard oils with viscosities of 0.474, 0.967, 4.85, 9.25, and 49.8 dPa∙s at room temperature.
A Pt–Rh crucible was placed in the mullite reaction tube and heated up to 1923 K under an Ar atmosphere of 99.9999% purity. The electrodes and viscometer were inserted into the melts at 1923 K and the temperature was decreased in 50 K increments. A stabilizing interval of 20 min was carried out before every measurement.
The Raman spectra of the samples were investigated at room temperature in the range of 100–1900 cm-1. An Ar excitation laser was used with a wavelength of 514.532 nm at 2 mW for 300 s. A HORIBA Jobin Yvon Raman spectroscope (LabRam Aramis) was employed.
Quantitative peak deconvolution was carried out following the procedure described by Mysen et al6. PeakfitTM was applied to fit a Gaussian function in the range of 800–1200 cm-1 within a
±0.5% error limit.
Fig. 1. (a) The experimental compositions and phase diagram of CaO-MgO-SiO2 (b) Pseudo-binary phase diagram for CaSiO3-MgSiO3
Fig. 2. Experimental apparatus for viscosity and electrical conductivity measurements
Results and Discussion
1. Cationic Effect on Electrical Conductivity and Viscosity in Polymerized and Depolymerized Melts
The effect of the cationic exchange on electrical conductivity and viscosity of the polymerized (CaO-MgO-SiO2satd., NBO/T=0) and depolymerized(CaO-MgO-0.5∙SiO2, NBO/T=2.0) melts are described in Fig 3. (a) and (b), respectively. An antithetic trend on the cationic effect is observed for the sets: For polymerized melts, electrical conductivity increases and viscosity decreases with Ca to Mg substitution. For depolymerized melts, however, electrical conductivity decreases and viscosity increases with the substitution.
Fig 3. The effect of cationic substitution on (a) electrical conductivity and (b) viscosity for polymerized and depolymerized melts
To remedy this major discrepancy, Anderson-Stuart theory[13] is revisited for opposite dependence on the cationic radius. In the theory, the cationic radius has different effect on ionic interaction energy in strain field distortion energy as Eq (1):
𝐸𝑎=𝜀(𝑅+𝑅𝛽𝑧𝑧𝑂𝑒2
𝑂)+ 4𝜋𝐺𝑅𝐷(𝑅 − 𝑅𝐷)2 (1) z,zO and R, RO are the valences and ionic radii of the mobile ion and stationary oxygens, respectively; e is the electronic charge, and RD is the effective radius of the doorway. β, the Madelung constant, is the liquid lattice parameter, and G is the shear modulus. ε is the dielectric permittivity of the melt. It is notable that the effect of cationic radius, R, is opposite for two energy barriers. The electrostatic interaction energy, the former term, decreases with increasing cationic radius while the strain field distortion energy, the latter term, increases.
To ensure the difference in the effect of cationic radius on two activation energies, the apparent activation energy is calculated by the Arrhenian relationship from the temperature dependence of viscosity and electrical conductivity. The strain field distortion energy on the other hand, is calculated following McElfresh’s suggestion[13]. Radius of doorway passage for ion migration is taken to be 0.6 atomic unit from Norton’s approximation[13] and shear modulus of 3.05 × 1011dyne/cm2 is quoted from work of Anderson and Stuart. The calculated activation energies are described in Fig. 4.
Fig. 4. Apparent activation energy and strain field distortion energy of CaO-MgO-SiO2
system as a function of the arithmetic average cationic radius, <r>.
From the activation energy discussion, the electrostatic interaction energy, the subtraction of strain field distortion energy from apparent activation energy, is larger enough than strain field energy to take the strain field distortion energy to be negligible. As the cationic radius increases, the activation energy for the ionic migration consistently decreases allowing the ionic migration easier. Thus the electrical conductivity is enhanced and viscosity is decreased by enhanced ionic transfer. In fully-polymerized melts, in which silicate flow units are fully polymerized and electrochemically neutral, however, electrostatic interaction energy is assumed to lack hence the overall ionic migration is dependent only on the strain field distortion energy. Therefore, the ionic migration is easier for the cation of the smaller radius. Although the postulation successfully explains the antithetic trend of the cationic radius, the absence of electrostatic energy is yet to proven.
0.6 0.8 1.0 1.2 1.4
0 50 100 150 200 250
Apparent Activation Energies, Present Work Viscous Flow
Electrical Conduction
Apparent Activation Energies, Bockris et al Viscous Flow, MgO and BaO Electrical Conduction, MgO and BaO Strain Field Distortion Energy
Calculated by A-S Model
<r> (10-10 m)
Activation Energy (kJ/mol)
To prove the absence of ionic interaction in fully-polymerized melts, the Walden plot16-18 is exploited. From Nernst-Einstein equation, Harris19 modified the relationship between electrical conductivity and viscosity of ionic melt as following equation:
η𝑡× σ = (Constant) (2)
Angell et al.[16]-[18] discussed on the slope of logarithmic plot of Walden’s equation as an index of the intensity of ionic interaction: For ideal ionic solution, in which electrochemical neutrality is satisfied without electrostatic interaction, slope of the logarithmic Walden plot is unity.
The logarithmic Walden plot is established in Fig. 5. The Walden plot of depolymerized melts lies on the typical plots of alkaline-earth silicate of which slopes range of 0.65 to 0.8. The Walden plot of polymerized melts, however, is nearly unity, meaning the silica-saturated melts be an ideal ionic melts and the silicate units be the electrochemically neutral solvent.
Fig 5. Walden plot of (a) several alkali and alkaline-earth silicate melts including the results of present works (b) the results of present work only for convenience.
2. The Effect of the Transition in Primary Solid Phase on the Melt Properties and Structure
Further discussion lies on the elucidation of abrupt change in electrical conductivity and viscosity of the melt at the 50% of Ca to Mg substitution. (See Fig. 3 (a) and (b).) Together with the pseudo-binary phase diagram depicted in Fig 1 (b), the abrupt change in properties occurs at the vicinity of the diopside congruent composition.
The Raman spectra of CaSiO3-MgSiO3 melts are described in Fig. 6 (a). Notable changes in the Raman spectra by Mg2+ for Ca2+ substitution does not occur only at the high-frequency regions, but also at the low- and mid-range frequencies.
In the low-frequency range around 340cm-1 and 380 cm-1, the peaks shifts toward to lower wavenumbers with higher MgO. Such a blue shift of wollastonite characteristic peak with increasing MgO composition infers the decrease in local crystallinity of wollastonite for cationic substitution: A Ca2+ ion of 114 pm of radius has coordination numbers of 7 to 9. Hence, Ca2+ is located in a cubic packing site with oxygen. The maximum radius of cation in cubic site of Si-O is 115.73 pm. A Mg2+ ion, on the other hand, has coordinate number of 6, occupying an octahedral packing with oxygen. The ionic radius of Mg2+ is 86 pm, which is larger than that of geometric maximum radius of cation in octahedral site of 65.49 pm. Therefore, Mg2+ in
silicate necessarily leads local distortion of Si-O lattice. The increase in Si-O vibrational frequency with Ca2+ to Mg2+ substitution in Raman spectra is plausibly caused by such an energetic destabilization of Si-O bond. After the Mg2+ substitution for Ca2+ exceeds the congruent point(x=50), the low frequency peaks disappears
620 cm-1 and 720 cm-1 bands are regarded to be the bending motion of oxygen bonds in the Si-O-Si bonding and Si-O stretching, respectively. Combining the information from the high- frequency regions, the decrease in the 720 cm-1 peaks indicate a polymerization of the melt by Ca2+ to Mg2+ substitution, which corresponds well with previously measured viscosity values.
Description of the high-frequency 800~1200 cm-1 bands is well categorized by several researchers including Brawer[20], Mysen et al.[21], and McMillan et al[22].
Deconvoultion using a Gaussian function is employed to quantitatively assess the high- frequency Qn. The result of the deconvolution is expressed in Figure 6 (b). Notably, the main structural units of silicate melts is determined to be Q2, Q3, and Q0 (in order of abundance).
Virgo et al. [23] concluded equilibria of these structural units as following:
3𝑄2= 2𝑄3+ 𝑄0 (3)
It is notable that the population of each Qn species abruptly changes at the diopside congruent point. Q2 increases whereas Q3 and Q0 decreases, indicating a polymerization of the melt. The change in slope of the Q2, Q3, and Q0 is in the ratio of 3:2:1, supporting the equilibria expressed in eq. (3). The equilibrium constants in eq (3) can be calculated and correlated to the degree of depolymerization as Mysen suggested6.
K =[𝑄3]2[𝑄0]
[𝑄2]3 (4)
Degree of depolymerization, lnK decreases abruptly after the diopside congruent point implying polymerization of the melt.
Fig. 6. (a) Raman spectra of CaSiO3-MgSiO3 melts (b) Fig. 7 Qn unit population and calculated lnK in (100-x)CaSiO3-xMgSiO3 melts. It is notable that the slope of Q0 change is
1/3 of that of Q2 and half of that of Q3, corresponding with results of Virgo et al.[23,24]
The abrupt change in melt dynamic measurements of stability function, liquid fragility, and configurational and topological entropies are organized in Fig. 8[24]. Liquid fragility and entropies are acquired from the temperature dependence of viscosity whereas the stability function is calculated as Eq (5):
ψ = [
1 + 𝑋2𝜕 ln 𝛾𝜕𝑋22 ⋯ 𝑋𝑛𝜕 ln 𝛾2
𝜕𝑋𝑛
⋮ ⋱ ⋮
𝑋2𝜕 ln 𝛾𝜕𝑋𝑛
2 ⋯ 1 + 𝑋𝑛𝜕 ln 𝛾𝜕𝑋𝑛
𝑛 ]
(5)
Fig. 7. Configurational and topological entropies, degree of depolymerization, liquid fragility, and stability function of CaSiO3-MgSiO3 melts at 1873K [24]
Conclusions
In this work is performed the mutual measurement of the electrical conductivity and viscosity of CaO-MgO-SiO2 melts combined with Raman spectroscopic analysis to elucidate the cationic effect on the properties and structure of the melt. From the findings, followings are concluded:
(1) The effect of the cationic radius on the electrical conductivity and viscosity shows the antithetic trend in polymerized and depolymerized melts. In depolymerized melts, the electrostatic interaction energy dominates the strain field distortion energy leaving larger ions easier to migrate. In polymerized melts, however, the overall activation energy of ionic migration is solely dependent of strain field distortion energy thus the smaller ions are easier to move. Such a postulation is assured by the application of Walden plot.
(2) Structure and properties in depolymerized melts are concluded to be dependent of phase equilibria. The structural change at the congruent composition is confirmed by deconvolution of Raman spectra. The stability function is suggested to expect such a change from calcium silicate to magnesium silicate melt.
0 20 40 60 80
60 80 100 120 140
Configurational Entropy, Sconf Topologicla Entropy, Stopo Degree of Depolymerization, ln K Liquid Fragility, m
Magnesium Cationic Ratio, x
Configurational/Topological Entropies (J/mol K)
-5 -4 -3
ln K
-0.4 0.0 0.4
Stability Function,
2 4 6 8
Liquid Fragility, m
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