The MgO–TiO2–SiO2 system: Experiments and thermodynamic assessment
Abstract
Phase relations in the MgO–TiO2–SiO2 system have been investigated in air over a wide temperature range using the equilibration method. X-ray powder diffraction, scanning electron microscopy combined with wave length X-ray spectroscopy (SEM/EPMA), and differential thermal analysis (DTA) have been used for sample characterization. Based on the obtained experimental results, isothermal sections of the system at 1523, 1673, and 1773 K have been established. The solid-state invariant reaction MgTi2O5 + T-SiO2⇋P-MgSiO3 + TiO2 has been detected at 1625 ± 8 K by step-wise heat treatment. A partial liquidus projection has been suggested, and the temperatures and compositions of three eutectic invariant reactions have been experimentally measured by DTA and ex-situ analysis of the sample microstructures after melting using SEM/EPMA. Considering the newly obtained experimental data, thermodynamic parameters describing the system have been thermodynamically evaluated within the CALPHAD approach.
1 INTRODUCTION
The phases and the phase relations within the MgO–TiO2–SiO2 system are of interest for various application fields, including refractory materials, mineralogy, pyrometallurgy, and the development of ceramic filter materials. The aluminum industry nowadays demands strict metal quality standards, and filtration is the most feasible method for reducing the inclusions level entering a filter system before metal casting as the final inline treatment. Inclusions, primarily as Al2O3 or spinel MgAl2O4, arise from the oxidation of molten aluminum combined with dopants such as Mg or from furnace refractory. TiO2 coating deposited on corundum is supposed to filter actively spinel MgAl2O4 from Al-based molten alloy.1-3 For the Si,Mg-containing Al alloys, the reduction of TiO2 can then lead to the formation of MgTiO3, Al3Ti, or (Al,Si)3Ti.4 Therefore, to model the Al melt filtration process, in addition to the melt itself, one should consider the complex Al–Mg–Ti–Si–O system. Focusing on the oxide part of the complex system, the Al2O3–TiO2–MgO5, 6 and Al2O3–TiO2–SiO27, 8 subsystems were described in the authors preceding studies. As a part of the mentioned complex system and a linking bridge to combine the Al2O3-based systems, the MgO–TiO2–SiO2 subsystem is essential. Notably, the understanding of the phase relations in the MgO–TiO2–SiO2 system is of interest for the development of refractory materials technology, as well as for elucidating the geological processes that occur between the minerals that constitute this system (including both silicates and titanates).
Experimental information on phase relations in the MgO–TiO2–SiO2 system is available from the literature, however, this information is limited and mainly related to the melting behavior at 1 atm or to the effect of pressure on the melting behavior. Subsolidus phase relations have not been studied in detail so far. Nevertheless, those data have subsequently been used for thermodynamic assessments of the MgO–TiO2–SiO2 system. However, the thermodynamic models used for the phase descriptions are not consistent with those used in preceding studies of the present authors. Details are discussed in the next section. Therefore, the aim of the present work is an experimental study of phase relations in the MgO–TiO2–SiO2 system in a wide range of temperatures and compositions and a critical evaluation of available data on phase relations as well as thermodynamic assessment of thermodynamic parameters describing the system to derive a self-consistent thermodynamic database using the CALPHAD approach.
2 LITERATURE REVIEW
2.1 Binary systems
The phase relations in the MgO–TiO2, TiO2–SiO2, and MgO–SiO2 systems were repeatedly studied experimentally and their thermodynamic databases are available. The MgO–TiO2 system was recently assessed thermodynamically by Ilatovskaia et al.5 applying their own experimental data obtained in the range of 473−1710 K in air. This description, applying the ionic two-sublattice liquid model and considering the order-disorder phenomena in both Mg2TiO4 and MgTi2O5, is used in this study. The TiO2–SiO2 system was also recently assessed by Ilatovskaia and Fabrichnaya.8 The applied two-sublattice liquid model was reduced to the (SiO2,TiO2) substitutional model, which is consistent with the two-sublattice partially ionic model, and the corresponding interaction parameters were assessed.
The MgO–SiO2 system was first thermodynamically described by Hillert and Wang.9 The two-sublattice partially ionic model was applied for the liquid phase, while the solid phases were described as stoichiometric compounds. Huang et al.10 reoptimized the thermodynamic parameters describing the solid MgSiO3 phases (low-clinopyroxene, orthopyroxene, and protopyroxene), and thus a better fit to the available experimental data was achieved. No new data have been published so far. Since the thermodynamic models used to describe all three quasi-binary systems are consistent, the assessment for the MgO–SiO2 system by Huang et al.10 is accepted in this work without modifications.
2.2 Ternary MgO–TiO2–SiO2 system
Massazza and Sirchia11 constructed the liquidus projection of the system using the pyrometric cone method and microscopy investigation of the samples. Seven invariant reactions, including three ternary eutectics, L⇋MgTiO3+MgTi2O5+Mg2SiO4 at 1793 K, L⇋Mg2SiO4+MgSiO3+MgTi2O5 at 1713 K, and L⇋MgTi2O5+TiO2+MgSiO3 at 1663 K, and the monotectic LA+TiO2⇋LB+C-SiO2 at 1803 K, were indicated on the liquidus, as well as two maxima belong to the sections of Mg2SiO4–MgTi2O4 (1813 K) and MgSiO3–TiO2 (1693 K). This also revealed an information on phase fields on the solidus projection (triangulation). No ternary compound was found. MacGregor12 investigated the effect of pressure on the melting behavior along the MgSiO3–TiO2, Mg2SiO4–TiO2, and MgSiO3–MgTi2O5 sections at 1773−2173 K. The eutectic reaction in the MgSiO3–TiO2 section was suggested to be about 10 K lower than previously indicated.11 Two ternary eutectic reactions on the liquidus were verified at 1 atm: L⇋MgTi2O5+TiO2+MgSiO3 at 1663 K and L⇋Mg2SiO4+MgSiO3+MgTi2O5 at 1713 K. Moreover, the subsolidus reaction was found to occur at 1762 K and 15.2 kb, MgSiO3+MgTi2O5⇋Mg2SiO4+2TiO2. Using also the pyrometric cone method followed by microstructure investigation, Berezhnoi13 suggested the lowest crystallization temperature of 1688 K by the ternary eutectic reaction L⇋MgSiO3+TiO2+SiO2 rather than L⇋MgTi2O5+TiO2+MgSiO3 at 1663 K reported in Ref. [11] Panek et al.14 constructed the Mg2SiO4–Mg2TiO4 section indicating the peritectic decomposition L+MgO⇋Mg2SiO4+Mg2TiO4 at 1793 ± 10 K and 63.72MgO-24.94TiO2-11.34SiO2 (in mol.%). Later, Petrov et al.15 investigated the phase relations along the MgTiO3–Mg2SiO4, MgTi2O5–Mg2SiO4, MgTi2O5–MgSiO3, MgTiO3–MgSiO3, Mg2TiO4–MgSiO3, and Mg2TiO4–Mg2SiO4 sections at 1073−1773 K. No invariant reactions were observed within that temperature range. Sarver and Hummel16 pointed out the same phase relationships in the MgO–TiO2–SiO2 system at 1573 K as those presented on the solidus in Ref. [11]. An isothermal section at 1773 K with an emphasis on the liquid domain was recently reported.17 At 1883 K, the phase relations in the MgO–SiO2–TiOX were investigated under reducing conditions (p(O2) from 1.94∙10−9 to 2.75∙10−13 atm)18 and at high pressure of 10−24 GPa.19 Moreover, a wide area of the liquid immiscibility was investigated at 1883 K.20. Moreover, the cation substitutions and solubilities in MgTiO321, 22 and Mg2SiO423, 24 were observed.
The thermodynamic description of the MgO–TiO2–SiO2 system using the CALPHAD approach was first undertaken by Kaufman25 based on the experimental data of Massazza and Sirchia.11 The calculated isothermal sections in the temperature range of 800−2800 K indicated a joint extended miscibility gap in the liquid emanated from the MgO–SiO2 and TiO2–SiO2 systems. The liquidus projection of the MgO–TiO2–SiO2 system was also calculated by Kirschen and DeCapitani20 with an emphasis on the immiscibility in the liquid using the thermodynamic description derived on the basis of his own experimental data and those of Massazza and Sirchia.11 The liquid phase in Refs. [20, 25] was described by a substitutional model, which is incompatible with the two-sublattice partially ionic liquid model used in the present work. Also, the inversion in spinel and pseudobrookite was not modeled in Refs. [20, 25]. It should be noted that the calculations of the system at 1773 K using the FactSage software with corresponding oxide database were carried out by Chen et al.17 showing a rather good consistency between experimental and calculated data. However, a modified quasi-chemical model used to describe the liquid phase is not compatible with the model used in the present work and details of the thermodynamic description are not available. Therefore, a proper thermodynamic description of the MgO–TiO2–SiO2 system has not been suggested so far.
In summary, all published experimental data on phase relations in the MgO–TiO2–SiO2 system are mostly consistent to each other. Wherein, limited ranges of temperature and composition were only verified,12, 17 which in turn indicated good consistency with the original work11, while some data in their invariable and unverified form after Massazza and Sirchia11 were also widely used in further studies. Therefore, it turned out that the MgO–TiO2–SiO2 system has not been systematically investigated in a wide range of temperatures and compositions, and a new comprehensive study of phase equilibria in the MgO–TiO2–SiO2 system is necessary. Given this, the present study is devoted to an experimental study of phase relations in the MgO–TiO2–SiO2 system in air at 1500−2000 K. The obtained phase diagram data are the fundamental basis for the thermodynamic description of the system and, along with reliable published data, are taken into account in the thermodynamic evaluation.
3 MATERIALS AND METHODS
Samples within the MgO–TiO2–SiO2 system were prepared by coprecipitation followed by thermal decomposition of aqueous solutions, similarly to the procedure described by Fidancevska and Vassilev.26 The starting materials were magnesium nitrate Mg(NO3)2·6H2O (99.97%), titanium(IV) isopropoxide C12H28TiO4 (M = 284.23 g/mol, ρ = 0.955 g/cm3, 97%), and tetraethoxysilane C8H20SiO4 (M = 208.33 g/mol, ρ = 0.934 g/cm3, 98%), all produced by Alfa Aesar. The calculated volumes of the metal-organic precursors were first dissolved in absolute ethanol with magnetic stirring to obtain solutions 1A and 2A, respectively, while magnesium nitrate was dissolved in a small amount of distilled water to get solution 3A. The solution 1A consequently relates to the Ti-based precursor, 2A relates to Si, and 3A relates to Mg. Then, the solutions A1 and A2 were mixed with magnetic stirring at room temperature, and the solution A3 was added to the mixture (1A + 2A) with magnetic stirring and heating at 50−60°C in the presence of NH4OH to keep the pH above 9.0 during the process and to ensure complete precipitation of the corresponding hydroxides. The precipitate was visible as a cloudy white suspension. The precipitate was consistently evaporated at 80°C, dried at 90°C for 48 h, and two-step calcined at 500−800°C for 5 h to obtain the desired mixed oxide powder.
After, the obtained MgO–TiO2–SiO2 powders were ball milled, pressed into tablets at 300 MPa (tablet size: 8 mm in diameter and about 2−3 mm in height), and heat treated in a muffle furnace (Nabertherm) in air at chosen temperatures followed by furnace cooling. The temperature inside the muffle furnace was controlled (±3 K measuring accuracy) with the B-type thermocouple placed by the sample holder (PtRh crucible).
After heat treatment, the powdered samples were investigated at room temperature using an URD63 X-ray diffractometer (Seifert, FPM) with CuKα radiation (λ = 1.5418 Å). ICSD (Inorganic Crystal Structure Database, 2017, Karlsruhe, Germany) was used for the interpretation of the powder diffraction patterns. Qualitative and quantitative analyses of the XRD patterns were performed by Rietveld analysis using MAUD software.27, 28 To establish the phase distribution at the equilibrium state as well as invariant reactions, the microstructures of the samples after prolonged heat treatment and DTA observation were examined using JSM-7800 F (JEOL Ltd., Japan) equipped with an EDX detector. The phase compositions were accurately determined using JXA-8230 SuperProbe (JEOL Ltd., Japan) equipped with a wavelength-dispersive X-ray spectroscope (EPMA/WDX). The standards employed for EPMA were rutile for Ti Kα, periclase for Mg Kα, and quartz for Si Kα. The standard deviation of the EPMA measurements did not exceed 2−3 %.
Temperatures of the solid-state transformations and melting behavior were determined by TG-DTA SETSYS Evolution-1750 (SETARAM, France) using a B-type tricouple DTA rod (PtRh 6%/30% thermocouple). The ceramic specimens placed in open Pt crucibles were heated and cooled under a dynamic air atmosphere at rates of 10 and 30 K/min, respectively.
4 THERMODYNAMIC MODELING
The Thermo-Calc software29 and PARROT module were used to optimize the thermodynamic parameters applying for the description of the MgO–TiO2–SiO2 system. The thermodynamic descriptions of the binary subsystems of MgO–TiO2,5 MgO–SiO2,10 and TiO2–SiO28 are accepted without any alterations from the available thermodynamic descriptions. However, considering the ternary interactions in the studied MgO–TiO2–SiO2 system, the descriptions of the liquid phase and some solid phases were refined based on available experimental data (see details below). The list of the phases and the models used for their description are summarized in Table 1 and discussed in the following sections. The interaction parameters that are actually used in the present work including accepted and newly derived are included in Table 2.
Phase | Abbreviations | Model |
---|---|---|
Liquid | L (or Ionic) | (Mg+2,Ti+2,Ti+3)P(O−2,Va,SiO4−4,SiO2,O,TiO2)Q |
MgO Halite | Hal | (Mg+2)1(O−2)1 |
SiO2 Quartz | Q-SiO2 or Qua | SiO2 |
SiO2 Tridymite | T-SiO2 or Tr | SiO2 |
SiO2 Cristobalite | C-SiO2 or Cr | SiO2 |
TiO2 Rutile | Rut | |
Mg2TiO4 Spinel | Sp | |
MgTiO3 Ilmenite | Ilm | |
MgTi2O5 Pseudobrookite | Psbk | |
Mg2SiO4 Olivine | Ol | |
MgSiO3 Orthopyroxene | O-MgSiO3 or opx | |
MgSiO3 Protopyroxene | P-MgSiO3 or ppx | |
MgSiO3 Low-clinopyroxene | C-MgSiO3 or low-cpx |
Phase and Sublattice model | Thermodynamic parameter | References |
---|---|---|
Ionic Liquid | [30] | |
[31] | ||
[31] | ||
[32] | ||
[32] | ||
[31] | ||
[33] | ||
[10] | ||
[31] | ||
[10] | ||
[5] | ||
[10] | ||
[10] | ||
[10] | ||
[10] | ||
[10] | ||
[10] | ||
[10] | ||
[10] | ||
[5] | ||
[5] | ||
[31] | ||
[31] | ||
[31] | ||
[31] | ||
[31] | ||
[8] | ||
[8] | ||
[8] | ||
This work | ||
This work | ||
This work | ||
Mg2TiO4 (sp) | [5] | |
[5] | ||
[5] | ||
[5] | ||
MgTiO3 (ilm) | [5] | |
This work | ||
MgTi2O5 (psbk) | [5] | |
[5] | ||
[5] | ||
[5] | ||
[8] | ||
[8] | ||
[8] | ||
This work | ||
This work | ||
MgSiO3 (low-cpx) | [10] | |
MgSiO3 (opx) | [10] | |
MgSiO3 (ppx) | [10] | |
Mg2SiO4 (ol) | [10] | |
This work | ||
MgO (hal) (MgO)1 | [30] | |
TiO2 (rut) | [33] | |
[33] | ||
[33] | ||
[33] | ||
C-SiO2 (cr) (SiO2)1 | [10] | |
Q-SiO2 (qua) (SiO2)1 | [10] | |
T-SiO2 (tr) (SiO2)1 | [10] |
Function | Temperature range, K | |
---|---|---|
GMGOLIQ | (298.15−1700) | |
(1700−2450) | ||
(2450−3100) | ||
(3100−5100) | ||
GTI1O1 | (298.15−2500) | |
GTI2O3 | (298.15−470) | |
(470−2115) | ||
GHSERMG | (298.15−923) | |
(923−3000) | ||
GMGLIQ | (298.15−923) | |
(923−3000) | ||
GHSERTI | (298.15−900) | |
(900−1155) | ||
(1155−1940) | ||
(1940−6000) | ||
GLIQTI | (298.15−1300) | |
(1300−1940) | ||
(1940−6000) | ||
GSIO2LIQ | (298.15−2980) | |
(2980−4000) | ||
GHSERSI | (298.15−1687) | |
(1687−6000) | ||
GHSEROO | (298.15−1000) | |
(1000−3300) | ||
(3300−6000) | ||
GTIO2 | (298.15−4000) | |
GMGOSOL | (298.15−1700) | |
(1700−3100) | ||
(3100−5000) | ||
(5000−5100) | ||
GMGSIO4 | (298.15−6000) | |
LMGSIO0 | (298.15−6000) | |
LMGSIO1 | (298.15−6000) | −8402 |
LMGSIO2 | (298.15−6000) | |
LMGSIO3 | (298.15−6000) | −9405 |
SPINNORM | (298.15−6000) | |
SPININV | (298.15−6000) | |
INVSP | (298.15−6000) | |
GCORUND | (298.15−600) | |
(600−1500) | ||
(1500−3000) | ||
GALAL | (298.15−6000) | |
NSPINEL | (298.15−6000) | |
ISPINEL | (298.15−6000) | |
GMGMG | (298.15−6000) | |
GSIO2S | (298.15−540) | |
(540−770) | ||
(770−848) | ||
(848−1800) | ||
(1800−2960) | ||
(2960−4000) | ||
PSBNORM | (298.15−6000) | |
PSBINV | (298.15−6000) | |
INVPSB | (298.15−6000) | |
GTI3O5 | (298.15−6000) | |
GSI3O5 | (298.15−6000) | |
GAL3O5 | (298.15−6000) | |
GTIAL_NO | (298.15−6000) | |
GANDAL | (298.15−6000) | |
PSBNORMSI | (298.15−6000) | |
PSBINVSI | (298.15−6000) | |
GCLMGSIO | (298.15−6000) | |
GOLIVMG | (298.15−6000) | |
GCRISTOB | (298.15−373) | |
(373−453) | ||
(453−543) | ||
(543−3300) | ||
(3300−4000) | ||
GTRIDYM | (298.15−388) | |
(388−433) | ||
(433−900) | ||
(900−1668) | ||
(1668−3300) | ||
(3300−4000) |
- All values are given in SI units per mole of formula unit.
4.1 Liquid phase
The liquid phase is described by the two-sublattice model for ionic liquids35 with the formula (Mg+2,Ti+2,Ti+3)P(O−2,Va,SiO4−4,SiO2,O,TiO2)Q, where P and Q indicate the number of sites on each sublattice, which can vary with composition to maintain electroneutrality. The two-sublattice model for ionic liquids was also developed within the framework of CEF, Equations (1) to (4) are applied in their reduced form.
4.2 MgO–TiO2 phases
Mg2TiO4 has an inverse spinel structure (Strukturbericht H11). In Mg2TiO4, the degree of inversion is close to 1 at room temperature and decreases slightly with increasing temperature.36 Since there are no reports on any solubility of SiO2 in spinel Mg2TiO4, its description using the formula , where χ is the inversion degree, is accepted in this work after Ilatovskaia et al.5
where GSI3O5 (or ) is the Gibbs energy of a fictive compound Si3O5 with the pseudobrookite structure accepted after Ilatovskaia et al.8 and GANDAL is the Gibbs energy of andalusite,37 GAL3O5 (or ) is the Gibbs energy of a fictive compound Al3O5 with the pseudobrookite structure,6 and GTIO2 (or ) is the Gibbs energy of rutile TiO2.33 It may seem strange that the term involves parameters evaluated in the assessments both of Al3O5 and Ti3O5 although it does not contain any Al or Ti. This arises because there could only be one parameter acting as a reference in a phase, and the reference in the pseudobrookite phase in the complex Al2O3–MgO–TiO2–SiO2 system is agreed to be (or TI3O5). It would also be possible to adjust the values of Si3O5 and Al3O5 independently, and this would give different values for all parameters that have a net charge.
4.3 MgO–SiO2 phases
Enstatite MgSiO3 (or orthopyroxene) is of the pyroxene series, and it is stable at low temperatures. The other forms are protoenstatite (or protopyroxene; both enstatite and protoenstatite crystallize in the orthorhombic system, ), which occurs at high temperatures, and clinoenstatite (or low clinopyroxene; in the monoclinic system, ), which occurs in an unstable form at low temperatures. Since there are no reports on the solubility of TiO2 in MgSiO3, the descriptions of all three MgSiO3-based solid solutions are accepted after Huang et al.10
4.4 Rutile, halite, and SiO2-based phases
The rutile phase is tetragonal TiO2 crystallizing in the space group (Strukturbericht C4). Periclase MgO has the NaCl-type structure in the space group (Strukturbericht B1) with the common name halite. Generally, the halite or rutile phase is described using a model within CEF with two sublattices, one for metal ions and one for oxygen ions. Since there is no information on any solubility in TiO2 and MgO, TiO2 rutile and MgO halite are described as stoichiometric phases.
SiO2 exhibits polymorphism; there are forms of α-quartz1 (or Q-SiO2; in trigonal P3221 space group) at low temperatures and tridymite (or T-SiO2; in orthorhombic C2221 space group) and cristobalite2 (or C-SiO2; in tetragonal P41212 space group) at high temperatures. Regardless the experimentally observed solubility in T-SiO2 or C-SiO2 which could be caused by a nonequilibrium state due to high viscosity of SiO2, all SiO2-based phases are also described as stoichiometric. The description of TiO2 rutile is taken from Hampl and Schmidt-Fetzer,33 the description of MgO halite is from Hallstedt,30 and the descriptions of Q-SiO2, C-SiO2, and T-SiO2 are all from Huang et al.10
5 RESULTS AND DISCUSSION
5.1 Experimental results
The series of sample compositions were chosen to determine all possible phase relations occurring in the ternary system within the temperature range from 1523 K to melting. The chemical compositions of the heat-treated samples were measured using EDX and were found to be consisted with the nominal ones within an admissible error of 3%. The results of XRD and EPMA/WDX investigations of the heat-treated samples are summarized in Table 3.
Nominal sample composition, mole fraction | Volume fraction and lattice parameters, nm, by XRD | Phase composition by SEM/EPMA, mole fraction | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Sample | MgO | TiO2 | SiO2 | Annealing temperature, K/annealing time, hour | Equilibrium phases | Vol.% | a | b | c | beta | MgO | TiO2 | SiO2 |
MTS-1 | 0.6253 | 0.2498 | 0.1250 | 1523/192 | Mg2SiO4 | 37 | 0.4755 | 1.0231 | 0.5985 | ||||
MgTiO3 | 33 | 0.5053 | 1.3905 | ||||||||||
Mg2TiO4 | 30 | 0.8441 | |||||||||||
1673/120 | Mg2SiO4 | 38 | 0.4756 | 1.0228 | 0.5984 | ||||||||
MgTiO3 | 32 | 0.5052 | 1.3900 | ||||||||||
Mg2TiO4 | 30 | 0.8438 | |||||||||||
1873/10 min | Mg2SiO4 | 0.6617 | 0.0164 | 0.3219 | |||||||||
MgTiO3 | 0.4797 | 0.5194 | 0.0009 | ||||||||||
Mg2TiO4 | 0.6502 | 0.3480 | 0.0018 | ||||||||||
MgO | 0.9964 | 0.0033 | 0.0004 | ||||||||||
TiO2 | 0.0014 | 0.9976 | 0.0010 | ||||||||||
Eutectic 1 | 0.6453 | 0.1080 | 0.2468 | ||||||||||
Eutectic 2 | 0.5621 | 0.2918 | 0.1461 | ||||||||||
MTS-3a | 0.5334 | 0.0666 | 0.4000 | 1523/192 | Mg2SiO4 | 32 | 0.4760 | 1.0219 | 0.5989 | ||||
P-MgSiO3 | 55 | 0.9258 | 0.8765 | 0.5321 | |||||||||
MgTi2O5 | 13 | 0.9764 | 1.0033 | 0.3741 | |||||||||
1673/120 | Mg2SiO4 | 26 | 0.4759 | 1.0235 | 0.5984 | ||||||||
C-MgSiO3 | 60 | 0.9622 | 0.8830 | 0.5181 | 108.33 | ||||||||
MgTi2O5 | 14 | 0.9745 | 1.0007 | 0.3745 | |||||||||
1893a | Mg2SiO4 | 0.6545 | 0.0012 | 0.3440 | |||||||||
C-MgSiO3 | 0.4875 | 0.0122 | 0.5003 | ||||||||||
MgTi2O5b | – | – | – | ||||||||||
Liquid | 0.3651 | 0.2662 | 0.3687 | ||||||||||
MTS-4a | 0.5000 | 0.4000 | 0.1000 | 1523/192 | Mg2SiO4 | 32 | 0.4755 | 1.0205 | 0.5985 | ||||
MgTiO3 | 47 | 0.5054 | 1.3905 | ||||||||||
MgTi2O5 | 21 | 0.9729 | 1.0006 | 0.3742 | |||||||||
1673/120 | Mg2SiO4 | 32 | 0.4757 | 1.0207 | 0.5985 | ||||||||
MgTiO3 | 45 | 0.5055 | 1.3903 | ||||||||||
MgTi2O5 | 23 | 0.9748 | 0.9999 | 0.3740 | |||||||||
1873a | Mg2SiO4 | 0.6546 | 0.0125 | 0.3329 | |||||||||
MgTiO3 | 0.4930 | 0.5060 | 0.0011 | ||||||||||
MgTi2O5 | 0.3229 | 0.6762 | 0.0010 | ||||||||||
Liquid | 0.4839 | 0.3831 | 0.1330 | ||||||||||
MTS-5 | 0.5000 | 0.2500 | 0.2500 | 1523/192 | Mg2SiO4 | 59 | 0.4754 | 1.0207 | 0.5985 | ||||
MgTi2O5 | 41 | 0.9746 | 1.0014 | 0.3731 | |||||||||
1673/120 | Mg2SiO4 | 62 | 0.4756 | 1.0200 | 0.5983 | ||||||||
MgTi2O5 | 38 | 0.9738 | 1.0004 | 0.3741 | |||||||||
1773/24 | Mg2SiO4 | 0.6531 | 0.0045 | 0.3425 | |||||||||
MgTi2O5 | 0.3306 | 0.6637 | 0.0057 | ||||||||||
Liquidb | – | – | – | ||||||||||
1893a | Mg2SiO4 | 0.6534 | 0.0054 | 0.3412 | |||||||||
MgTi2O5 | 0.3252 | 0.6683 | 0.0055 | ||||||||||
Liquidb | – | – | – | ||||||||||
MTS-7 | 0.4202 | 0.2098 | 0.3700 | 1523/192 | Mg2SiO4 | 3 | 0.4759 | 1.0223 | 0.5960 | ||||
P-MgSiO3 | 60 | 0.9244 | 0.8751 | 0.5315 | |||||||||
MgTi2O5 | 37 | 0.9746 | 1.0013 | 0.3736 | |||||||||
1673/120 | Mg2SiO4 | 4 | 0.4773 | 1.0257 | 0.5964 | ||||||||
C-MgSiO3 | 66 | 0.9613 | 0.8822 | 0.5179 | 108.35 | ||||||||
MgTi2O5 | 30 | 0.9737 | 1.0003 | 0.3742 | |||||||||
1773/120 | Mg2SiO4 | 0.6743 | 0.0017 | 0.3239 | |||||||||
MgTi2O5 | 0.3326 | 0.6637 | 0.0036 | ||||||||||
Liquid | 0.4521 | 0.1929 | 0.3596 | ||||||||||
1923a | Liquid | 0.4480 | 0.2138 | 0.3383 | |||||||||
MTS-8 | 0.3669 | 0.1832 | 0.4500 | 1523/192 | P-MgSiO3 | 72 | 0.9243 | 0.8746 | 0.5314 | ||||
C-SiO2 | 4 | 0.4997 | 0.6952 | ||||||||||
TiO2 | 25 | 0.4592 | 0.2960 | ||||||||||
1616/240 | C-MgSiO3 | 52 | 0.9609 | 0.8818 | 0.5175 | 108.36 | |||||||
MgTi2O5 | 28 | 0.9728 | 0.9995 | 0.3739 | |||||||||
C-SiO2 | 20 | 0.4985 | 0.6943 | ||||||||||
TiO2 | <1 | 0.4594 | 0.2960 | ||||||||||
1633/120 | C-MgSiO3 | 59 | 0.9611 | 0.8821 | 0.5176 | 108.34 | |||||||
MgTi2O5 | 25 | 0.9734 | 1.0003 | 0.3742 | |||||||||
C-SiO2 | 16 | 0.4986 | 0.6949 | ||||||||||
1673/120 | C-MgSiO3 | 59 | 0.9610 | 0.8820 | 0.5177 | 108.34 | |||||||
MgTi2O5 | 23 | 0.9734 | 1.0002 | 0.3742 | |||||||||
C-SiO2 | 18 | 0.4986 | 0.6955 | ||||||||||
1773/120 | MgTi2O5b | – | – | – | |||||||||
C-SiO2b | – | – | – | ||||||||||
Liquid | 0.3884 | 0.2058 | 0.4058 | ||||||||||
1923a | C-SiO2 | 0.0026 | 0.0202 | 0.9772 | |||||||||
Liquid | 0.3827 | 0.1779 | 0.4395 | ||||||||||
MTS-9 | 0.1700 | 0.0900 | 0.7400 | 1598/120 | P-MgSiO3 | 28 | 0.9242 | 0.8743 | 0.5315 | ||||
C-SiO2 | 63 | 0.4998 | 0.6983 | ||||||||||
TiO2 | 9 | 0.4591 | 0.2958 | ||||||||||
1673/120 | C-MgSiO3 | 24 | 0.9615 | 0.8823 | 0.5179 | 108.27 | |||||||
MgTi2O5 | 11 | 0.9742 | 1.0010 | 0.3743 | |||||||||
C-SiO2 | 22 | 0.4991 | 0.6969 | ||||||||||
T-SiO2 | 43 | 0.5018 | 0.8209 | ||||||||||
1773/1 | C-SiO2 | 0.0002 | 0.0175 | 0.9823 | |||||||||
Liquid | 0.3725 | 0.1746 | 0.4529 | ||||||||||
MTS-10 | 0.1200 | 0.2400 | 0.6400 | 1673/120 | MgTi2O5 | 25 | 0.9736 | 1.0002 | 0.3742 | ||||
C-SiO2 | 70 | 0.4988 | 0.6953 | ||||||||||
T-SiO2 | 5 | 0.5012 | 0.8193 | ||||||||||
1773/24 | C-SiO2 | 0.0000 | 0.0256 | 0.9744 | |||||||||
TiO2 | 0.0001 | 0.9968 | 0.0031 | ||||||||||
Liquid | 0.3781 | 0.1833 | 0.4386 | ||||||||||
MTS-12 | 1673/120 | Mg2SiO4 | 33 | 0.4759 | 1.0224 | 0.5983 | |||||||
Mg2TiO4 | 32 | 0.8437 | |||||||||||
MgO | 35 | 0.4213 | |||||||||||
2003a | Mg2SiO4 | 0.6617 | 0.0164 | 0.3219 | |||||||||
Mg2TiO4 | 0.6502 | 0.3480 | 0.0018 | ||||||||||
MgO | 0.9820 | 0.0070 | 0.0110 | ||||||||||
MTS-13 | 0.150 | 0.600 | 0.250 | 1523/192 | P-MgSiO3 | 22 | 0.9243 | 0.8756 | 0.5318 | ||||
C-SiO2 | 14 | 0.5001 | 0.7010 | ||||||||||
TiO2 | 64 | 0.4593 | 0.2960 | ||||||||||
1673/120 | MgTi2O5 | 38 | 0.9735 | 1.0001 | 0.3741 | ||||||||
TiO2 | 25 | 0.4594 | 0.2959 | ||||||||||
C-SiO2 | 37 | 0.4988 | 0.6954 | ||||||||||
1773/120 | C-SiO2 | 0.0004 | 0.0251 | 0.9745 | |||||||||
TiO2 | 0.0003 | 0.9983 | 0.0013 | ||||||||||
Liquid | 0.3639 | 0.1792 | 0.4569 | ||||||||||
1923a | C-SiO2 | 0.0066 | 0.0901 | 0.9033 | |||||||||
TiO2 | 0.0002 | 0.9989 | 0.0009 | ||||||||||
Liquid | 0.3823 | 0.1755 | 0.4422 | ||||||||||
MTS-14 | 0.220 | 0.690 | 0.090 | 1523/192 | P-MgSiO3 | 14 | 0.9239 | 0.8764 | 0.5313 | ||||
MgTi2O5 | 36 | 0.9741 | 1.0008 | 0.3739 | |||||||||
TiO2 | 50 | 0.4593 | 0.2960 | ||||||||||
1673/120 | MgTi2O5 | 62 | 0.9736 | 1.0001 | 0.3741 | ||||||||
C-SiO2 | 16 | 0.4987 | 0.6950 | ||||||||||
TiO2 | 22 | 0.4594 | 0.2959 | ||||||||||
1773/24 | MgTi2O5 | 0.3258 | 0.6711 | 0.0031 | |||||||||
C-SiO2b | – | – | – | ||||||||||
TiO2 | 0.0002 | 0.9985 | 0.0013 | ||||||||||
Liquid 1 | 0.3476 | 0.0697 | 0.5827 | ||||||||||
Liquid 2 | 0.1973 | 0.3813 | 0.4215 | ||||||||||
1923a | Mg2SiO4 | 0.5869 | 0.0996 | 0.3135 | |||||||||
MgTiO3 | 0.4848 | 0.4359 | 0.0793 | ||||||||||
TiO2 | 0.0001 | 0.9989 | 0.0010 | ||||||||||
Liquid | 0.4665 | 0.2960 | 0.2375 | ||||||||||
MTS-15 | 0.280 | 0.420 | 0.300 | 1523/192 | P-MgSiO3 | 38 | 0.9243 | 0.8761 | 0.5317 | ||||
C-MgSiO3 | 17 | 0.9612 | 0.8816 | 0.5177 | 108.37 | ||||||||
MgTi2O5 | <1 | 0.9729 | 1.0006 | 0.3742 | |||||||||
TiO2 | 44 | 0.4593 | 0.2960 | ||||||||||
1673/120 | C-MgSiO3 | 16 | 0.9616 | 0.8823 | 0.5177 | ||||||||
MgTi2O5 | 53 | 0.9735 | 1.0005 | 0.3742 | |||||||||
C-SiO2 | 31 | 0.4988 | 0.6951 | ||||||||||
1773/2 | MgTi2O5 | 0.3439 | 0.6468 | 0.0093 | |||||||||
C-SiO2 | 0.0000 | 0.0189 | 0.9811 | ||||||||||
TiO2 | 0.0003 | 0.9969 | 0.0028 | ||||||||||
Liquid | 0.4353 | 0.1088 | 0.4559 | ||||||||||
1823a | TiO2 | 0.0004 | 0.9979 | 0.0017 | |||||||||
Liquid | 0.4331 | 0.1489 | 0.4180 |
- a Heated in air without holding at the reached temperature.
- b Areas are too small to be quantified by EPMA.
- Notably, P-MgSiO3 and C-MgSiO3 are reported as high- and low-temperature phases, respectively; however, in the present study, P-MgSiO3 is observed at 1523 K, while C-MgSiO3 is observed above 1616 K. A similar issue was reported by Song et al.,39 who indicated P-MgSiO3 at 1653 K and C-MgSiO3 above 1673 K.
Since a fine microstructure (with a grain size of less than 2−5 μm) is specific for the ceramic samples heat treated at low temperatures, it is impossible to accurately measure the phase compositions using EDX (or WDX). Therefore, XRD was used to identify the phases present in the samples heat treated at both 1523 and 1673 K. However, the microstructures of the samples heat treated at 1673 K were also investigated by EPMA/WDX, but they were only sufficient for the semiquantitative WDX analysis, so the EPMA/WDX data are omitted in this case. Considering the lack of relevant literature data on the solubility and stoichiometry of the intermediate compounds in the binary MgO–SiO2 and MgO–TiO2 systems, the extension of the intermediate compounds into the ternary system was neglected. Additionally, no new phases within the ternary system were observed. Thus, based on the XRD results obtained (see Table 3), the isothermal sections at 1523 and 1673 K are presented in Figure 1A,B, respectively. Four three-phase equilibria (MgO + Mg2SiO4+ Mg2TiO4, Mg2SiO4+ Mg2TiO4+ MgTiO3, Mg2SiO4+ MgTiO3+ MgTi2O5, and Mg2SiO4+ P-MgSiO3+ MgTi2O4) in the MgO–MgSiO3–MgTi2O5 part of the system remain stable in the range of 1523−1673 K, while the tie lines change in the TiO2–MgTi2O5–MgSiO3–SiO2 part was observed. These results indicate the occurrence of a solid-state reaction of a transitional type, MgTi2O5 + C-SiO2⇋P-MgSiO3 + TiO2. Consequently, a stepwise heat treatment followed by XRD examination was carried out for sample MTS-8 to clarify the reaction temperature. As shown in Figure 2 (see also Table 3), the reaction is completed at 1633 K, revealing the presence of a three-phase area (P-MgSiO3 + MgTi2O5 + C-SiO2), while traces of TiO2 were still present in MTS-8 after heat treatment at 1616 K. Thus, the reaction temperature was assumed to be 1625 ± 8 K.
At 1773 K, partial melting of the samples within the area of TiO2–MgTi2O5–Mg2SiO4–SiO2 was observed. It is worth noting that the experimental samples after heat treatment were not subjected to quenching, but were cooled down with the furnace. Therefore, the composition of the liquid phase could not be maintained with respect to 1773 K. Nonequilibrium solidification for the series of samples heat treated at 1773 K was subsequently observed by the microstructure investigation. The corresponding isothermal section, together with the EPMA/WDX data marked with open squares, is shown in Figure 1C. A three-phase equilibrium (Mg2SiO4 + MgTi2O5 + L) for samples MTS-5 and -7, a three-phase equilibrium (C-SiO2 + TiO2 + L) for samples MTS-10 and -13, and a two-phase equilibrium (C-SiO2 + L) for sample MTS-9 were established (Figure 3 and Table 3). Three other three-phase equilibria could thus tentatively exist: Mg2SiO4 + P-MgSiO3 + L, P-MgSiO3 + C-SiO2 + L, and MgTi2O5 + TiO2 + L. It was expected that the three-phase equilibrium (MgTi2O5 + TiO2 + L) could be observed for sample MTS-14. However, the equilibrium state at 1773 K could not be preserved due to slow cooling (furnace cooling) applied in this work. Thus, the crystallization of sample MTS-14 heat treated at 1773 K finished at a substantially lower temperature and the liquid composition shifted to a composition close to E3 on the liquidus (see below).
This is also a case for sample MTS-8. A three-phase equilibrium (C-SiO2 + MgTi2O5 + L) for sample MTS-8 heat treated at 1773 K was established (see Table 3), although the three phases of C-SiO2, MgTi2O5, and liquid could be in equilibrium below the temperature of the invariant reaction U3 (1715 K; see below) until crystallization at E3 (1690 K; see below). Considering that faster diffusion occurs in the liquid state than in solid and the sample was not quenched but subjected to furnace cooling, it can be assumed that the liquid composition could correspond to the temperature below than 1773 K (even below 1715 K). Thus, there is uncertainty regarding the liquid composition of MTS-8 that was heat treated at 1773 K, since the temperature corresponding to the measured composition of the liquid is not exactly known. On the other hand, the liquid composition of MTS-8 heat treated at 1773 K followed by furnace cooling is within the liquid area suggested for this isothermal section. Such a statement on nonequilibrium crystallization is quite relevant for all samples investigated in this work at 1773 K.
Nevertheless, although the results obtained at 1773 K in this work are limited and not enough to construct the complete equilibrium isothermal section, they are only qualitatively compared with those presented by Chen et al.17 The authors17 investigated samples of the MgO–TiO2–SiO2 system equilibrated at 1773 K and then quenched (their data are shown in red in Figure 1C). Considering all the above features of the nonequilibrium solidification of the samples in this work, the comparison of the data with the data of Chen et al.17 shows reasonable agreement.
DTA followed by SEM/EPMA investigations were performed in this work to get information about invariant reactions involving the liquid phase in the MgO–TiO2–SiO2 system. The experimental results obtained in this work are compared with the available literature data in Table 4. It should be mentioned that the temperatures and compositions of liquids of invariant reactions are in reasonable agreement with those in Ref. [11]. However, considering also the solid-state reaction discovered in this study, the character of the reactions in the TiO2–SiO2-rich side is different in comparison to those in Ref. [11]: eutectic L⇋P-MgSiO3 + SiO2 + MgTi2O5 in this work against eutectic L⇋MgSiO3 + TiO2 + MgTi2O5 in Ref. [11] and transitional-type L + TiO2⇋SiO2 + MgTi2O5 in this work against eutectic L⇋TiO2 + SiO2 + MgSiO3 in Ref. [11]. The liquidus projection of the MgO–TiO2–SiO2 system is shown in Figure 4A. Notably, an area of the liquid immiscibility and related monotectic reaction L1⇋L2 + TiO2 + C-SiO2 (shown tentatively by black dashed line in Figure 4A) were not studied in this work and can be a subject of a further study, while all other invariant reactions occurred in the MgO–TiO2–SiO2 system are discussed below.
Reaction | Type, definition | T, K | Composition of the liquid phase, mole fraction | References | ||
---|---|---|---|---|---|---|
MgO | TiO2 | SiO2 | ||||
L + MgO⇋Mg2SiO4 + Mg2TiO4 | Transitional, U1 | 1853 | 0.5899 | 0.2963 | 0.1138 | 11 |
1910 | 13 | |||||
1793 ± 10 | 0.6372 | 0.2494 | 0.1134 | 14 | ||
1921 | – | – | – | Experimenta | ||
1862 | 0.6087 | 0.2100 | 0.1791 | Calculateda | ||
L + Mg2TiO4⇋Mg2SiO4 + MgTiO3 | Transitional, U2 | 1813 | 0.5093 | 0.3705 | 0.1202 | 11 |
1835 | – | – | – | Experimenta | ||
1800 | 0.5548 | 0.2808 | 0.1622 | Calculateda | ||
L⇋Mg2TiO4 + Mg2SiO4 + MgTiO3 | Eutectic, E1 | 1793 | 0.4938 | 0.3810 | 0.1252 | 11 |
1822 | 0.4839 | 0.3831 | 0.1330 | Experimenta | ||
1783 | 0.5223 | 0.3066 | 0.1689 | Calculateda | ||
L⇋Mg2SiO4 + P-MgSiO3 + MgTi2O5 | Eutectic, E2 | 1713 | 0.4476 | 0.2427 | 0.3096 | 11 |
1713 | 0.4435 | 0.2367 | 0.2998 | 12 | ||
1704 | 0.4480 | 0.2138 | 0.3383 | Experimenta | ||
1717 | 0.4397 | 0.2246 | 0.3341 | Calculateda | ||
L⇋TiO2 + SiO2 + MgSiO3 | Eutectic | 1673 | 0.3846 | 0.2068 | 0.4086 | 11 |
1650 | – | – | – | 13 | ||
L + TiO2⇋T-SiO2 + MgTi2O5 | Transitional, U3 | 1715 | – | – | – | Experimenta |
1728 | 0.3652 | 0.2816 | 0.3518 | Calculateda | ||
L⇋MgTi2O5 + TiO2 + MgSiO3 | Eutectic | 1663 | 0.4369 | 0.2345 | 0.3286 | 11 |
1663 | 0.4375 | 0.2299 | 0.3326 | 12 | ||
C-SiO2⇋T-SiO2 + (TiO2 + L) | Degenerated, D1 | 1743 | 0.3091 | 0.3286 | 0.3623 | 11 |
1744 | 0.3477 | 3046 | 0.3462 | Calculateda | ||
C-SiO2⇋T-SiO2 + (P-MgSiO3 + L) | Degenerated, D2 | 1743 | 0.3956 | 0.1377 | 0.4667 | 11 |
1744 | 0.4219 | 0.1231 | 0.4539 | Calculateda | ||
L⇋P-MgSiO3 + T-SiO2 + MgTi2O5 | Eutectic, E3 | 1690 | 0.3827 | 0.1779 | 0.4395 | Experimenta |
1712 | 0.3394 | 0.2301 | 0.3746 | Calculateda | ||
L1⇋L2 + C-SiO2 + TiO2 | Monotectic, M-M’ | 1803 | 0.2503 | 0.3741 | 0.3756 | 11 |
0.0276 | 0.1145 | 0.8579 | ||||
1809 | 0.2803 | 0.3797 | 0.3385 | Calculateda | ||
0.0065 | 0.0492 | 0.9464 | ||||
L⇋Mg2SiO4 + MgTi2O5 | emax1 | 1813 | 0.4806 | 0.3747 | 0.1448 | 11 |
1827 | – | – | – | Experimenta | ||
Calculateda | ||||||
L⇋P-MgSiO3 + MgTi2O5 | emax2 | 1698 | 0.4423 | 0.2386 | 0.3191 | 11 |
1688 | – | – | – | 12 | ||
1715 | – | – | – | Experimenta | ||
Calculateda | ||||||
MgTi2O5 + T-SiO2⇋P-MgSiO3 + TiO2 | Transitional | 1625 ± 8 | – | – | – | Experimenta |
1626 | – | – | – | Calculateda |
- a This work.
The melting temperature of the three-phase assemblage (MgO + Mg2SiO4 + Mg2TiO4), as determined by sample MTS-12, was found to be 1918 K on heating (hereinafter, referred to as onset point as a transformation temperature), as shown in Figure 5A. The microstructure of this sample after melting in DTA (Figure 6A) indicated the coexistence of three phases with no traces of eutectic: primary MgO with Mg2SiO4 and Mg2TiO4. This indicates the occurrence of an invariant reaction of the transition type U1, L + MgO ⇋ Mg2SiO4 + Mg2TiO4. Even though the composition of the liquid phase at point U1 on the liquidus cannot be determined from the microstructural analysis, the thermal event at 1918 K is assigned to this U1 reaction. Notably, Berezhnoi13 reported a pseudobinary eutectic between Mg2SiO