CORROSION TESTING OF ZIRCONIA, BERYLLIA AND MAGNESIA CERAMICS IN MOLTEN ALKALI METAL CARBONATES
AT 900oC
Valery Kaplan, Igor Lubomirsky Department of Materials and Interfaces,
Weizmann Institute of Science, Rehovot 76100, Israel.
Keywords: CO2 electrolysis, molten carbonates, magnesia, zirconia, beryllia.
Abstract
An electrochemical cell containing molten Li2CO3-Li2O has been proposed for the conversion of the greenhouse gas CO2 to CO, which can then either be used to power gas turbines or converted to methanol. Since efficient electrolysis takes place at 900oC, the materials which can be used in such a cell must satisfy stringent requirements. In the current work, we have examined the static corrosion resistance of zirconia, beryllia and magnesia ceramics at 900 °C in the Li2CO3-Li2O mixture and in a Li-Na-K carbonate eutectic mixture with the ultimate objective of identifying suitable electrically insulating materials. Conclusions regarding material stability were based on elemental analysis of the melt, primarily via X-ray photoelectron spectroscopy, a particularly sensitive technique. It was found that magnesia is completely stable for at least 33 hrs in a Li2CO3-Li2O melt, while a combined lithium titanate/lithium zirconate layer forms on the zirconia ceramic as detected by XRD. Under the same melt conditions, beryllia shows considerable leaching into solution. In a Li-Na-K carbonate eutectic mixture containing 10.2 mol% oxide at 900°C under standard atmospheric conditions, magnesia showed no signs of degradation. Stabilization of the zirconia content of the eutectic mixture at 0.01-0.02 at% after 2 hrs is again explained by the formation of a lithium titanate/ lithium zirconate coating. On the basis of these results, we conclude that only magnesia can be satisfactorily used as an insulating material in electrolysis cells containing Li2CO3-Li2O melts.
Introduction
Electrochemical reduction of CO2 to CO is one of the very attractive routes for converting electrical energy from renewable sources into fuel which may be stored for later use [1-4]. CO can be used in any standard gas turbine to generate electricity, or it can be converted into methanol via a well-established chemical pathway. The technique for CO2 to CO conversion that we recently described is based on the electrolysis of molten Li2CO3 at 900 °C using a Ti cathode and a graphite or Ti\TiC\C [5-7] anode. Upon melting, Li2CO3 converts to a Li2CO3/Li2O mixture, in which the content of Li2O (up to 18 at %) is controlled by the pressure of CO2 in the environment [7]. Electrode reactions in carbonates are in general complex and multi-stage [8-
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
12]. The material-charge balance of the cathode reaction during the electrolysis of molten Li2CO3 at 900 °C with a Ti cathode is [6]:
CO23-2eCO2O2-. (1)
The mechanism of the anode reaction has not yet been identified: neither anode material, graphite or Ti\TiC\C, degrades during electrolysis and the overpotential does not exceed a few tens of mV even for very large current densities (>1.5 A/cm2) [5]. The material-charge balance of the anode reaction is:
O2-2e12O2. (2)
The reaction 12 2
2 -
2
3 2 CO O
CO e apparently does not take place because no CO2 was found in the anode gases even when the concentration of Li2O is only ~2at% [6]. Thus, a Li2CO3
electrolysis cell operates via the self-replenishing cycle:
) ( CO
Li CO O Li
(a) 2O CO 1 O Li 2 CO Li
3 2 2 2
2 2
3 2
b e
, (3)
The electrochemical conversion of CO2 to CO via electrolysis of molten Li2CO3 has a number of advantages compared to alternative CO2 reduction pathways [6,7]: (i) High thermodynamic efficiency (the ratio of the energy supplied with respect to the energy used to drive the reaction) is not uncommon for electrolysis with gaseous products. The entropy term TΔS is positive, thereby reducing the decomposition potential below that of the isothermal point: the decomposition potential of Li2CO3 is 0.87 V at 900 °C, which is approximately 30% lower than the isothermal voltage (1.28 V at 900 °C) (ΔG = 246 kJ/mol). This implies that a significant fraction of the heat of reaction can be converted to chemical energy, thereby compensating for ohmic losses. Thermodynamic efficiency close to 100% may result. (ii) Since the Li2CO3/Li2O melt is free of precipitate at 900 °C in the presence of ~ 2 at% CO2 [7], dilute sources of CO2 can be used. (iii) The CO2 to CO conversion is not affected by the presence of water in the incoming gas. (iv) Noble metals and solid electrolytes are not required, only welded Ti; therefore, process scale-up is more economically achieved than with a number of other technologies. (v) The electrolysis products are physically separated from each other, both within the cell and upon outflow. (vi) The method is tolerant to the presence of sulfur in the incoming gas: any sulfur entering the system is eventually converted into sulfate and then removed by electrolysis [13]:
) ( 2O ) 3 ( 2S ) 1 ( O Li )
( SO
Li2 4 meltelectrolysis 2 melt 2 cathode 2 anode, (4)
Consequently, sulfur-containing flue gas from power stations can be used as the source of CO2. However, despite all these advantages, the CO2 to CO conversion by electrolysis of molten Li2CO3 faces problems of material compatibility. Electrolysis proceeds efficiently only between 850-900 °C, and at these temperatures, the Li2CO3/Li2O melt is very corrosive (e.g., it dissolves alumina, Pt and Au) [6,14] . Although the melt is readily contained in a welded Ti container [13],
practical application requires construction of multi-cell devices in which insulating materials would play a dual role: (i) for the construction of high-temperature, feed-through electrical connectors and (ii) for the construction of bipolar electrodes (one surface anodic, the other surface, cathodic). Bipolar electrodes are the most common solution for connecting cells in series in order to reach the required voltage. In this way, improved matching to power sources may be accomplished. High melting point ceramics are obvious candidates and in the present study, we report the results of static corrosion testing of zirconia, beryllia, and magnesia ceramics in molten Li2CO3/Li2O.
Experimental
Ceramics were obtained from the following sources: partially yttria-stabilized zirconia (YSZ) from Retsch GmbH (5.2% Y2O3, other oxides < 0.3%, ~1at% Hf); beryllia (99.5 at%) and magnesia (>99.9 at%) both from The Institute of High Temperature Electrochemistry of the Ural Branch of the Russian Academy of Science. Of these, beryllia is the most costly and zirconia, the least costly. Beryllia and magnesia were in the form of thin, ceramic plates with a total surface area of 100-120 mm2 while the partially yttria-stabilized zirconia sample had the approximate shape of a short sphero-cylinder with diameter ~ 12 mm, length ~ 9 mm and surface area 550- 600 mm2. The crystal structure of the pristine zirconia sample is tetragonal; following immersion in the melt, a phase transition to monoclinic takes place. The cooled sample displays bi-phasic behavior, characteristic of weakly doped ZrO2. The ceramic samples were immersed in 100 (±5%) cm3 of the Li2CO3 melt (Acros Organics), or of a Li-K-Na carbonate eutectic, in a titanium container for 2 to 33 hrs at 900 °C. For preparation of the eutectic, Na2CO3 and K2CO3
were obtained from Alfa Aesar. Prior to the immersion of the ceramics, it was verified that the melt did not contain detectable amounts of Mg, Zr or Be. Mg may pose a particularly serious problem as some commercially available sources of Li2CO3 contain 0.5-100 mg/kg of Mg (depending on the purity [15]). Upon heating, Li2CO3 converts to a Li2CO3/Li2O mixture releasing CO2. In 1 atm CO2 the concentration ofLi2O in the Li2CO3/Li2O mixture is 0.2 mol%
[7]; in air, the concentration of Li2O reaches 18mol%. Probes (0.1-0.4 cm3) of the melt were taken using a cold rod and their content analyzed by energy-dispersive X-ray spectroscopy (EDS, qualitative analysis) and X-ray photoelectron spectroscopy (XPS, quantitative analysis) to determine the elemental composition (Table 1).
We found experimentally that this approach is far more sensitive than measuring weight loss after separating the ceramics from the solidified melt. EDS measurements were made on a Leo- Supra scanning electron microscope (SEM). XPS measurements of the Li(1s), C(1s), O(1s), Na(1s), K(2p), Mg(1s), Be(1s) and Zr(3d) core levels were carried out with the Kratos AXIS ULTRA XPS system using a monochromatic Al K X-ray source (h = 1486.6 eV) at 75W and detection pass energies ranging between 20 and 80 eV. A low-energy electron flood gun (eFG) was applied for charge neutralization. Depending on the shape of the background, either a linear or Shirley background [16] was used to extract the surface concentrations of the different elements. Curve fitting analysis was based on background subtraction and use of mixed Gaussian-Lorentzian line shapes [17,18]. Powder X-ray diffraction (XRD) patterns were measured on a Rigaku TTRAXIII theta-theta diffractometer at grazing incidence (fixed angle 4.5o) with Cu Kradiation. Phase identification and quantitative analysis was performed with Jade 9 (MDI, CA) with reference to the Powder Diffraction File (PDF-ICDD). X-ray fluorescence (XRF) was measured on a benchtop XRF ED instrument (model EX-Calibur, Xenemetrix, Migdal Haemek, Israel).
Table 1. Ceramic stability in Li2CO3+ Li2O and (Li, Na, K)2CO3 - eutectic melts at 900oC Material Melt
content (at%
metal)
Melt Atmosphere
(pressure 1 atm)
Duration (hours)
Detection Method
YSZ n.d.* Li2CO3+ Li2O (0.2 mol%) CO2 3,5,9,29,33 EDS, XPS YSZ n.d. Li2CO3+ Li2O (up to 18 mol%) Air 5,9,29,31,33 EDS, XPS YSZ <0.02 (Li, Na, K)2CO3 - eutectic with
(Li, Na, K) oxides (10.2 mol%)
Air 2,6 EDS, XPS
MgO n.d. Li2CO3+ Li2O (0.2 mol%) CO2 2,6 EDS, XPS
MgO n.d. Li2CO3+ Li2O (up to 18 mol%) Air 31 EDS, XPS MgO n.d (Li, Na, K)2CO3 - eutectic with
(Li, Na, K) oxides (10.2 mol%)
Air 6 EDS, XPS
BeO 0.47 Li2CO3+ Li2O (0.2 mol%) CO2 3 XPS
BeO 0.42 Li2CO3+ Li2O (0.2 mol%) CO2 5 XPS
BeO 1.44 Li2CO3+ Li2O (0.2 mol%) CO2 9 XPS
* n.d. = not detectable by either EDS or XPS
Results and discussion
Ceramic stability in molten Li2CO3/Li2O.
Figure 1a (red (lower) trace) shows the X-ray photoelectron spectroscopy (XPS) measurement of a cold rod probe of the Li2CO3+0.2 mol% Li2O melt taken after a YSZ (Tmp=2715oC) ceramic sphero-cylinder had been immersed for 9 hrs at 900°C under 1 atm of CO2. The 3d photoelectron peaks of Zr, if present, would appear at 182.5 eV (5/2) and 184.9 eV (3/2). It is clear that the spectrum shows no evidence of transfer to the melt of any detectable amount of Zr. Similar negative results were obtained when the YSZ sample had been in the melt for up to 33 hrs (data not shown).
We have also immersed YSZ cylinders in the Li2CO3- Li2O melt at 900°C in air for up to 33 hrs.
Under these conditions, the concentration of Li2O reaches 18 mol% and Li2O begins to precipitate. Also in this case XPS did not reveal any Zr in a cold rod probe of the melt (data not shown). This implies that lithium zirconates either do not form at all or that they do form but are not soluble in the melt and remain on the surface of zirconia. In order to verify which possibility is the correct one, grazing incidence XRD measurements (4.5o) were made on zirconia samples which had been in the melt for 6-48 hrs (under either 1 atm air or CO2). A thin grey colored material shell was observed to either fully or partially cover the light yellow core. After washing for 24 hrs to remove the melt residue, the diffraction patterns were shown to contain characteristic peaks of Li2ZrO3 (Figure 1b).
A reaction likely taking place on the surface of the zirconia ceramic:
ZrO2(s)+ Li2CO3(s)-> Li2ZrO3(s)+CO2(g) (5) has in fact been shown to proceed at temperatures above 700oC [19,20]. Following sample immersion for times longer than 6 hrs, the grazing incidence XRD patterns also showed evidence for the presence of LiTiO2 on the ceramic surface. Although XPS detected only 0.3 at% Ti in the
186 184 182 180 9 hrs in (Li,Na,K)2CO3 eutectic melt
9 hrs in Li2CO3-Li2O melt
Zr 3d
Intensity a.u.
Binding Energy (eV)
20 30 40 50 60
0.0 0.2 0.4 0.6 0.8 1.0
monoclinic ZrO2, ICSD Collection Code 41572
Li2ZrO3
ICSD Collection Code 94895 Li2TiO2
ICSD Collection Code 28323
Intensity, counts/sec
2, degrees
78.7
15.1 6.2 0
20 40 60 80
Li2ZrO3
LiTiO
2monoclinic ZrO2
Fraction, %
a) b) c)
Figure 1. a) XPS spectra of cold rod probes taken from a Li2CO3/Li2O melt ( under CO2, red trace) and from a (Li, Na, K)2CO3-eutectic melt ( in air, blue trace) following 9 hrs exposure of an yttria stabilized- ZrO2 sphero-cylinder in the former and 2-6 hrs in the latter. b) Grazing incidence (4.5o) XRD pattern of a washed YSZ spherocylinder after 31 hours of exposure to the Li2CO3/Li2O melt at 900°C in air. XRD peak positions and intensities of Li2ZrO3 (ICSD 94895) , LiTiO2 (ICSD 28323) and monoclinic ZrO2 (ICSD 41572) are shown. c) Quantitative analysis of the XRD pattern from the grey colored coating of the YSZ spherocylinder after 31 hours of exposure to the Li2CO3/Li2O melt at 900 °C in air.
melt (data not shown), quantitative analysis (Figure 1c) of the XRD pattern in Figure 1b showed five times as much lithium titanate as lithium zirconate. XRF spectroscopy gave definite evidence for the presence of Ti on the surface of YSZ samples, the amounts of which scaled with the intensity of the relevant XRD peaks. Interestingly, for the longer immersion times, diffraction from the YSZ core is weakened, indicating that the thickness of the lithium zirconate/lithiumtitanate layer increases with sample immersion time in the melt, thereby reducing the penetration of the grazing incidence X-ray beam. We tentatively attribute the presence of lithium titanate to the fact that the ceramic samples rest on the bottom surface of the titanium container, where it is known that both TiO2 and LiTiO2 layers are present [6]. Ti doping of lithium zirconate has been reported to take place upon mixing TiO2, Li2CO3 and ZrO2 under conditions similar to those which exist in the carbonate melt [21]. In the case of MgO (Tmp= 2852oC), cold rod probe XPS spectra were measured in the region of the Mg(1s) core level (1303 eV) following exposure of the MgO plate to the Li2CO3- Li2O melt under 1 atm CO2 for 6 hrs and in air for 31 hrs. No detectable amount of Mg leaching into the melt is observed for either sample (Figure 2a). Indeed, MgO is not known to form compounds with Li [22]. On the basis of these results, we were able to conclude that MgO ceramics are stable in the Li2CO3- Li2O melt at 900 °C. By contrast, the exposure of a BeO (Tmp= 2507oC) plate to the Li2CO3+0.2 mol% Li2O melt under 1 atm CO2 reveals a readily detectable amount of beryllium in the XPS spectrum (Figure 2b). The intensity of the peak due to the 1s core level increases with time of exposure up to 9 hrs. By normalizing to the XPS integrated peak intensities for the total probe elemental content, this increase is found to correspond to an increase in Be content of the melt from 0.47 at% at 3 hours to 1.44 at% at 9 hours, attesting to continuing BeO degradation. BeO is a highly amphoteric oxide with strong acidic tendencies, while Li2CO3-Li2O is strongly basic. Therefore, the reaction between BeO and Li2O is not particularly surprising, although no relevant
thermodynamic data are available. In view of the clear signs of degradation, beryllia was excluded from further measurement.
1300 1302 1304 1306 1308
Intensity a.u.
Binding Energy (eV) Mg 1s
31 hrs
6 hrs
118 116 114 112 110
9 hrs
3 hrs
Intensity, a.u.
Binding Energy(eV)
Be 1s
a) b)
Figure 2. a) XPS spectra of cold rod probes taken from a Li2CO3- Li2O melt after immersion of a MgO plate: 6 hrs under 1 atm CO2; and 31 hrs in air. A similar featureless spectrum is obtained for a cold rod probe taken from the alkali metal carbonate eutectic/oxide melt in air after 6 hrs exposure of the MgO plate (data not shown). b) XPS spectra of cold rod probes taken from a Li2CO3- Li2O (0.2 mol%) melt at 900oC after 3 (red trace) and 9 hrs (blue trace) of exposure of a BeO plate. The atmosphere was CO2.
Ceramic stability in molten Li-Na-K carbonate eutectic.
The flue gas from power stations often contains a few at% each of Na and K in the form of dust of various compositions, primarily silicates [23]. As an approximation to the long-term exposure of a ceramic insulator to such a dilute system, we have tested the stability of YSZ and MgO ceramics with respect to the eutectic mixture of (Li, Na, K)2CO3 for 2-6 hrs at 900 °C in air.
Under these conditions, the eutectic mixture also contains Li, Na and K oxides which form at 900oC due to the partial loss of CO2. To estimate the degree of conversion of the eutectic mixture of (Li, Na, K)2CO3 into a mixture of carbonates and oxides, we prepared a mixture of Li2CO3, Na2CO3 and K2CO3 from nominally anhydrous powders with the proportions of Li, Na, K corresponding to the eutectic. This mixture was heated to 500 °C to remove all traces of water, cooled and then weighed. The mixture was subsequently heated to 900 °C for 4 hrs and then cooled and weighed; followed by heating for 24 hrs, cooling, and weighing. A weight loss of 0.7±0.5% was detected after 4 hours heating ; after 24 hrs heating, the weight loss had stabilized at 4.5±0.1%. This corresponds to chemical decomposition of the carbonates to oxides of 1.6±1.1 mol% and 10.2±0.2 mol% respectively. Actually, if during industrial operation of the cell, the contents of Na and K were to increase beyond a few at%, both cations would precipitate in the form of silicates [23], recalling (as noted above) that silicates are introduced into the melt along with the flue gas particles.
XPS measurement of a cold rod probe of the eutectic carbonate-oxide melt at 900oC after a MgO ceramic had been immersed for up to 6 hours revealed a featureless spectrum in the energy
region of the 1s photoelectron peak (data not shown). This is consistent with the earlier finding that MgO is completely stable in molten K2CO3 at 980 °C [24]. On the other hand, XPS does detect leaching of a small but detectable amount (0.01-0.02 at%) of Zr from YSZ into the eutectic carbonate-oxide melt after 2 hours exposure. However, the amount of Zr in the melt does not further increase with exposure for up to 6 hrs (Figure 1a blue trace). This implies that between 2-6 hrs, interaction of YSZ with the melt has ceased. Although zirconia is known to be stable in a molten Li-K carbonate eutectic to at least 560 °C [25], it does react with Na2O- containing melts to form zirconates [26]. In order to determine the origin of this time-dependent stability, we studied the surface of the samples exposed to the melt using the EDS capability of the SEM as well as grazing incidence XRD. As described above, the YSZ samples extracted from the eutectic melt were washed, rinsed and dried. EDS found that the surface of the YSZ samples contained no statistically significant amounts of K or Na (<0.2±0.15 wt%, confidence level 98%). Grazing incidence XRD patterns of the ceramics exposed to the carbonate eutectic melt for 6 hrs were also measured. Of the several diffraction patterns measured from different locations on the sample surface, a subset did contain evidence for the presence of Li2ZrO3 (data not shown). As above, the presence of lithium zirconate can provide a plausible explanation for the stability of ZrO2with respect to long-term corrosion.
Conclusions
The results of XPS and EDS measurements of cold rod probes of the alkali metal carbonate/oxide melts following extended immersion of ceramic samples are summarized in Table 1. On the basis of these measurements, we conclude that MgO ceramics are suitable for use as a dielectric material in the electrochemical cell designed for reduction of CO2 to CO by electrolysis of a Li2CO3+Li2O melt at 900 °C. Yttria stabilized ZrO2 ceramics develop a Li zirconate/Li titanate surface coating, the increasing thickness of which may compromise long term structural stability, while BeO undergoes serious corrosion under melt conditions.
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