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Thermodynamic stability of Ca3TeO6 determined by a solid electrolyte EMF method
Thermochimica Acta 615 (2015) 38–42
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Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermodynamic stability of Ca3TeO6 determined by a solid electrolyte EMF method D. Sukhomlinova,* , F. Tesfayea,b , P. Taskinena a Aalto University School of Chemical Technology, Department of Materials Science and Engineering, Metallurgical Thermodynamics and Modeling Research Group, Vuorimiehentie 2 K, PO Box 16200, FI-00076 Aalto, Finland b Åbo Akademi University, Process Chemistry Centre, Piispankatu 8, FI-20500 Turku, Finland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 April 2015 Received in revised form 28 June 2015 Accepted 1 July 2015 Available online 13 July 2015
The standard thermodynamic properties of Ca3TeO6 were determined electrochemically utilizing fast O2 ion conducting solid electrolyte yttria-stabilized zirconia. The ternary phase was synthesized from the pure oxides CaO and TeO2 in excess of CaO. The electromotive force measurements were performed on two similar electrochemical cells of the type Te + CaO + Ca3TeO6|YSZ|O2, within the temperature range from 850 to 949 K. The standard Gibbs energy of formation for the ternary compound Ca3TeO6 was determined for the first time, based on the experimental data obtained. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Gibbs energy Electromotive force Calcium orthotellurate Thermodynamic properties Stabilized zirconia
1. Introduction The knowledge concerning the thermodynamic properties of compounds in the Ca–Te–O system is essential in the field of nuclear energy technology, pyrometallurgy, geochemistry, and other materials science applications [1,2]. New and accurate thermodynamic functions of the stoichiometric compound Ca3TeO6 presented in this article support process improvement as well as product development. Pure tellurium and tellurides are typical impurities occurring in base metal sulfide ores. During the base metals processing, tellurium is mostly distributed into the anode slimes forming different compounds, where its total concentration can reach several percent. The thermodynamic data concerning the Ca–Te–O system can support the modeling of chemical reactions and phase stabilities applied to industrial smelting and refining processes such as copper anode slime processing and the Doré smelting [3]. Recovery of tellurium has growing commercial interest because of its industrial applications. The major application of tellurium today is as an alloying additive in metallurgy. Also, tellurides of such elements as Sb, Bi and Cd have various commercial electronic and photovoltaic applications [2]. The partial phase diagram of the quasi-binary CaO–TeO2 system in the composition range 50–100 mol% TeO2 has been reported by
* Corresponding author. E-mail address: dmitry.sukhomlinov@aalto.fi (D. Sukhomlinov). http://dx.doi.org/10.1016/j.tca.2015.07.001 0040-6031/ ã 2015 Elsevier B.V. All rights reserved.
Malyutin et al. [4] and Mishra et al. [5]. The reported quasi-binary phase diagram indicates the presence of two stoichiometric compounds CaTeO3 and CaTe2O5. Mishra et al. [6] measured the Gibbs energy of formation of CaTeO3 and CaTe2O5 by the transpiration technique. Tripathi et al. [7] studied polymorphism of CaTeO3 and CaTe2O5 compounds by applying X-ray powder diffraction (XRPD), the lattice parameters and the space group of these phases were reported for the room temperature and for high temperature phases with respective phase-transition temperatures. However, Stöger et al.’s [8] crystallographic study shows disagreement with Tripathi et al. [7], and also validates the existence of a third ternary compound on the CaTeO3-TeO2 tie-line with a composition Ca4Te5O14, which has been discovered by Weil [9] from annealing reaction of CaCO3 and TeO2 in the molar ratio 1:2 at 1123 K. Concerning the phase relations on the CaO rich side, a disproportionation of Te6+ was reported [8,10] when an oxide mixture with a CaO/TeO2 ratio higher than 1 was annealed the reaction products appeared to be Te and calcium orthotellurate (Ca3TeO6). In contrast, Osinska et al.’s [11] TGA-DTA study claimed that in O2 free nitrogen atmosphere at 893 K CaTeO3 reacts with CaO to produce Ca3TeO6 and a CaTe intermetallic. Besides Ca3TeO6, the ternary compound CaTeO4 has been reported, which contain Te in 6+ oxidation state. Hottentot and Loopstra reported CaTeO4 as orthorhombic, space group Pbcn, with a = 5.231 (1), b = 12.676 (2), c = 4.977 (1) Å obtained with XRPD [12] and Ca3TeO6 as monoclinic, space group P21/n, with a = 5.5730 (6), b = 5.7964 (2), c = 8.0113 (3) Å, b = 90.24 (1) obtained from singlecrystal X-ray diffraction (SXRD) [13]. Also, Effenberger and Mayer
D. Sukhomlinov et al. / Thermochimica Acta 615 (2015) 38–42
Intensity
[14] reported crystal structure and lattice parameters of naturally occurring mineral carlfriesite CaTe3O8. The crystal structure is monoclinic, space group C2/c, with a = 12.576, b = 5.662, c = 9.994 Å, and b = 115.56 . It is a zeolite-like tellurite, where Te exists in both 4+ and 6+ oxidation states. The calcium ortotellurate Ca3TeO6 irradiated with neutrons was reported to be an iodine Mössbauer source, which allows application of 129I Mössbauer spectroscopy at room temperature [15,16]. The purpose of this experimental study is to enhance knowledge about the Ca–Te–O system and eliminate the existing discrepancies. This study presents the thermodynamic stability of Ca3TeO6 and observed phase relations in the Ca–Te–O system.
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2θ (°)
2. Experimental
Fig. 1. XRPD pattern of Ca3TeO6 saturated with CaO (solid line) measured at room temperature applying a Philips diffractometer equipped with CuKa radiation with a step size of 0.007 over a 2u range of 15–70 . CaO peaks are marked (*), the rest of the peaks corresponds to the major phase (Ca3TeO6).
2.1. Materials and sample preparation
(A) () Pt, Ir|Te(l) + CaO(s) + Ca3TeO6 (s)|YSZ|O2(g)|Pt (+)
The provenance and mass fraction purity of the materials used in this study are listed in Table 1. In order to synthesize Ca3TeO6 in excess of CaO, TeO2 was thoroughly mixed with 57 mol% of CaO both in a powder form (Table 1) in an agate mortar and pressed at a pressure of about 0.17 GPa to form a pellet with a thickness of 5 mm and diameter of 15 mm. The obtained pellet was inserted into a quartz glass crucible with a CaO protective pellet placed at the bottom of the crucible. The sample holder was placed on the bottom of a closedend alumina work tube of a vertical Lenton resistance furnace type LTF 16/-/610 and the sample was annealed under flowing argon gas (Table 1) protective atmosphere at 1000 K for 8 h. The sample obtained was ground into fine powder and mixed with additional 25 mol% of CaO and pressed to form a pellet. In the second heat treatment process, which was also conducted under the same conditions as in the initial annealing process, the sample was annealed at 1000 K for 21 h, at 1173 K for 0.25 h and at 973 K for 21 h. After the synthesized sample was ground and pressed into a pellet, the third heat treatment process was a repetition of the second annealing process under the same conditions. Dark-grey dust particles, due to the evaporation of Te-TeO2, were agglomerated on the inner surface of the furnace work tube, during all the three annealing cycles. The annealed material was analyzed after each intermediate cycle by SEM-EDS and XRPD techniques. As the result, the existence of two equilibrium phases CaO-Ca3TeO6 was confirmed. These observations agree well with information reported by Stöger et al. [8] and Trömel et al. [10]. No impurity phase was observed within 0.05 mass fraction XRPD detection limit in the obtained sample (Fig. 1). 2.2. Apparatus and assembly of the cell In order to determine the standard Gibbs energy of formation of calcium orthotellurate Ca3TeO6, the EMF values of the oxygen concentration galvanic cells (A) and (B) were measured in isothermal conditions in a temperature range from 850 to 949 K. Table 1 Purities and sources of materials used in the present study. Chemical
Source
Mass fraction purity
Te TeO2 CaO O2 Ar Pt ZrO2
Alfa Aesar (Germany) Alfa Aesar (Germany) Sigma–Aldrich (USA) AGA (Finland) AGA (Finland) Johnson–Matthey Noble Metals (UK) Friatec (Germany)
0.99999 0.9999 0.999 0.9995 0.99999 0.9999 doped by 0.085 of Y2O3
(B) () Pt, C|Te(l) + CaO(s) + Ca3TeO6 (s)|YSZ|O2(g)|Pt (+) The cells were examined by applying the experimental setup described in the previous study [17]. A vertical Lenton resistance tube furnace type LTF 16/-/450 was used in the experiments to achieve isothermal conditions. In order to minimize the temperature gradient over the galvanic cell, the temperature profile of the furnace was measured before the actual experiment applying two S-type Pt-Pt/Rh thermocouples (Johnson–Matthey Noble Metals) placed below and above the cell. The galvanic cell was placed in a hot zone with temperature gradient less than 1 K in the entire experimental temperature range. The thermocouples used were calibrated at the melting points of pure H2O, Sn and Cu. The experimental temperature measurement was performed at the bottom of the solid electrolyte zirconia tube by applying a thermocouple of the same type. The thermocouple was connected to a Keithley 2010 DMM multimeter, during the EMF measurements. The cold junction compensation was performed by a PT100 resistance thermometer (SKS-Group, Finland, tolerance class B 1/10), connected to a Keithley 2000 DMM multimeter. In a gas tight mullite work tube a steady oxygen flow (Table 1) was controlled by a digital mass flow controller, Aalborg DFC26 (USA). A solid electrolyte zirconia tube inserted in the work tube was used to separate the gaseous environments of the electrodes compartments. The outer surface of the solid-electrolyte zirconia tube was flushed by a steady flow of the pure oxygen, forming the reference electrode. In the test electrode compartment inside the zirconia tube, a nearly static protective argon atmosphere was arranged in order to prevent oxidation and to reduce volatilization of tellurium. The argon was purified from its original purity (Table 1) by passing it through a tube furnace heated up to 873 K and packed with titanium chips. The purified argon flow was controlled by a rotameter. In order to compensate a deviation of oxygen pressure in the reference electrode from the standard state, the atmospheric pressure was measured with a Vaisala PTU300 (Finland) pressure, humidity and temperature transmitter during the entire experiment. Also, the small hydrostatic pressure generated by a gas bubbler connected to the oxygen outlet was measured with a Beamex MC2 calibrator (Finland). A Keithley 6517B electrometer with a high input impedance of 2 1014 V was applied for the EMF measurements. Such high input impedance enables to conduct the EMF measurements in an open circuit and to provide the cell functioning reversibly [18]. Two identical, 0.25 mm in diameter Pt lead wires (Table 1) were connected to the electrometer. The cathode platinum lead wire in a shape of a coil was pressed toward the bottom of the solid electrolyte tube.
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The test electrode of the cell was comprised of three phases. The obtained biphasic mixture of Ca3TeO6-CaO was thoroughly mixed with Te powder and some additional CaO in such a way that molar ratio of Te:CaO:Ca3TeO6 was achieved to be 5:1:1. In order to minimize temperature gradient over the cell the amount of the sample was around 0.3 g. Pt lead wire cannot be directly utilized as a contact material to the anode, because of possible platinum tellurides formation [19]. In an attempt to avoid side reactions in the electrochemical cell a pure graphite [20–22] in a shape of a rod (10 mm long with 2 mm in diameter) was utilized as a contact material between the anode (the three-phase mixture) and the platinum lead wire. The graphite rod was hooked through a small hole with a tip of the anode platinum lead wire; also one coil of the wire was wrapped around the rod to provide reliable electric conductivity. In order to protect the Pt lead wire from the reaction with Te gas species, the wire was inserted into a thin (0.8/0.4 mm outer/inner diameter) alumina tube and then sheathed by a bigger (4/2 mm outer/inner diameter) alumina tube. The Pt–C connection point was sealed with alumina cement in the end of the bigger alumina tube. The sealing was arranged in such a way that only the tip of the graphite rod was uncovered (Fig. 2). An uncontrolled reduction process in the test electrode associated with utilized graphite contact is ruled out at the experimental temperature. The experimental temperature, pressure and EMF values were measured simultaneously at a rate of one data-point per 5 s and transferred to a computer for recording. The isothermal equilibrium was considered to be attained and the new measurement point was collected when the EMF values were constant or varied within less than 0.1 mV for several hours. To attain the equilibrium it required from few hours up to few days, depending on temperature. A grounded Faraday cage made of an alloy steel tube shielded the experimental cell from external electromagnetic fields. Two independent and similar galvanic cells were utilized in order to confirm repeatability of the measured EMF values. The EMF values were measured in both heating and cooling cycles. The test electrode composition was examined at the end of each experiment by XRPD to confirm the absence of any phase composition change.
The difference between cells (A) and (B) is the test electrode contact material utilized. A 20 mm long iridium wire tip spotwelded to the platinum lead wire was employed in the electrochemical cell (A). This contact material showed a poor durability and a short life time of the cell, due to the side reaction in the presence of metallic tellurium of formation of Ir3Te8 with a cubic unit cell [23,24]. Cell (B) was improved by utilizing a graphite rod (Fig. 2). 3. Results and Discussion The two half-cell reactions of the applied oxygen concentration cell are given below. The cathodic reaction is: 3/2O2(g) + 6e = 3O2,
(1)
and anodic reaction is: Te(l) + 3O2 + 3CaO(s) = Ca3TeO6(s) + 6e.
(2)
Therefore, the virtual cell reaction is: Te(l) + 3/2O2(g) + 3CaO(s) = Ca3TeO6(s).
(3)
3.1. Measured EMF values of the electrochemical cells Two cells altogether provided 15 measurement points as presented in Table 2. The median filter was applied to discriminate signal-noise of the experimental raw EMF data. The reported EMF values are averages of 200 data points (1000 s), which were recorded after equilibrium has been attained. The values reported for temperature and the prevailing pressure of the reference electrode are the respective average values recorded over the same time period. 3.2. Gibbs energy of formation of Ca3TeO6 from the elements Due to the fact that the solid electrolyte utilized in the measurements is a pure ionic oxygen conductor in the whole experimental temperature range [25], the Nernst Eq. (4) can be applied.
Table 2 The experimental values of EMF, temperature, prevailing total pressure of the reference electrode and calculated values of DrG . The values are listed in accordance with the date of measured data acquisition before temperature change.a
Fig. 2. Schematics of electrochemical cells (A) (left) and (B) (right): (1) YSZ tube, (2) upper alumina tube, (3) anode platinum lead wire shielded with a capillary alumina tube, (4) alumina cement, (5) Pt–Ir spot-welding point, (6) Pt–C connection point, (7) iridium contact, (8) graphite contact, (9) test electrode, (10) platinum coil, (11) alumina support, (12) lower alumina tube, (13) mullite work tube, (14) S-type thermocouple, (15) cathode platinum lead wire.
E (mV)
T (K)
P (hPa)
DrG (kJ mol1)
Cell A 767.62 776.76
889.5 869.7
1020.12 1020.97
444.32 449.60
Cell B 779.79 765.67 770.12 774.56 778.79 784.93 761.13 771.78 755.72 752.90 745.74 738.60 743.24
859.6 894.4 884.8 874.9 865.1 850.3 904.4 879.5 914.2 919.1 933.8 948.6 938.5
980.91 1011.91 1021.17 1027.31 1034.42 1036.52 1023.84 1012.51 1002.90 994.44 990.61 989.24 989.10
451.79 443.28 445.75 448.26 450.64 454.17 440.52 446.81 437.62 436.08 431.99 427.88 430.56
a Standard uncertainties u are u(E) = 0.10 mV, u(T) = 1.0 K, u(P) = 0.10 hPa, and expanded uncertainty U(DrG ) = 0.32 kJ mol1 (0.95 level of confidence).
D. Sukhomlinov et al. / Thermochimica Acta 615 (2015) 38–42
E¼
aO ðtÞ RT ln 2 ; ð4F Þ aO2 ðrÞ
(4)
where E is equilibrium EMF (V), R is the universal gas constant 8.3144 J mol1 K1 [26],T is absolute temperature (K), F is the Faraday constant 96,487 C mol1 [26], aO2 ðtÞ and aO2 ðrÞ are activities of oxygen in the test electrode and in the reference electrode, respectively. The standard Gibbs energy of the cell Reaction (3) can be expressed as: 2 3 aCa3 TeO6 4 5; Dr G ¼ RTln (5) 3=2 ðaTe aO2 ðtÞ a3CaO Þ where aCa3 TeO6 , aTe, and aCaO denote activities of the respective condensed phases. They can be assumed to be 1, due to the purity of the chemicals utilized (Table 1), an absence of deviations from the stoichiometry of the compounds and an absence of a significant mutual solubilities of the phases. The behavior of the oxygen gas phase can be assumed as ideal. The oxygen activity in the test electrode deduced from Eq. (4) was inserted into Eq. (5) and the assumptions described above were taken into account. Thus, the final equation for the calculation of the standard Gibbs energy change of the electrochemical cell Reaction (3) was obtained as: PO2 ðrÞ 3 Dr G ¼ 6FE þ (6) RTln ; 2 P where P is pressure of oxygen at the standard state 1013.25 hPa. Table 2 presents the summary of the EMF values, experimental temperature and prevailing total pressure of the reference electrode measured with electrochemical cells (A) and (B). The DrG values calculated with Eq. (6) are provided in Table 2 for each data point, respectively. Fig. 3 shows the calculated DrG values (Table 2) and the least squares fitting. Results obtained with electrochemical cells (A) and (B) are in a good agreement. Eq. (7) was derived by including experimental data obtained with both electrochemical cells (A) and (B). The determination coefficient R2 was calculated to be 0.9986.
DrG /(kJ mol1) 0.32 = 684.52 + 270.26 103 T/K, (850–949 K)
(7)
The standard errors for the intercept and for the slope in Eq. (7) are 2.55 and 2.84 103, respectively. The expanded uncertainty U (DrG ) in Eq. (7) is 0.32 kJ mol1 with 0.95 level of confidence calculated based on a statistical analysis of the experimental data. The standard Gibbs energy of formation of Ca3TeO6 from the pure elements was calculated according to Eq. (8) by combining Eq. (7) and the literature data concerning the thermodynamic
Δr G°/(kJ·mol -1 )
-426 -431 -436 -441 -446 -451 -456 840
860
880
900
920
940
960
Temperature (T/K) Fig. 3. The standard Gibbs energy of the virtual cell Reaction (3) as a function of temperature: (D) cell (A), (^) cell (B), (solid line) the least squares fitting for all experimental data points.
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stability of CaO.
Df GCa3 TeO6 ¼ Dr G þ 3Df GCaO :
(8)
The standard Gibbs energy of formation of CaO was calculated from Gibbs energies of the pure substances [27] to be:
DfG CaO/(kJ mol1) = 634.40 + 104.00 103 T/K. (750–1100 K)
(9)
The obtained Dt GCa3 TeO6 is expressed in Eq. (10). Its uncertainty can be calculated by applying the error propagation law. Due to an unknown contribution of u(DfG CaO) it was roughly estimated to be about one order of magnitude less accurate than the value reported for DrG , approximately 3 kJ mol1.
Df GCa3 TeO6 =ðkJ mol1 Þ 3 ¼ 2587:73 þ 582:27 103 T=K: ð850 949 KÞ This appears to be the first reported thermodynamic stability of calcium orthotellurate at elevated temperature. 3.3. Calculation of DfH 298, DfS 298 and S 298 of Ca3TeO6 Due to lack of heat capacity data of calcium orthotellurate, particularly from 298.15 to 949 K, it was estimated by applying Neumann–Kopp rule [29] from hypothetical Reaction (11) for further calculation of standard enthalpy and entropy of formation of calcium orthotellurate. TeO3(s) + 3CaO(s) = Ca3TeO6(s).
(11)
The heat capacity of CaO was taken from the literature [27]. For TeO3 the estimative heat capacity was calculated by Sahu et al. [28]. The Cp function of Ca3TeO6 calculated by Neumann–Kopp rule is expressed in Eq. (12): Cp/(J mol1 K1) = 261.336 19.527 103 T/K 60.5 105 (T/K)2 + 15.993 106 (T/K)2. (12) By applying the second law method, the standard enthalpy and entropy of formation of Ca3TeO6 at 298.15 K were calculated utilizing Eqs. (13) and (14): R P DfH 298 = DfH mean 298 TmeanDfCpdT + DH tr, (13) R Tmean P (DfCp/T) dT + (DH tr/T tr),
DfS 298 = DfS mean 298
(14)
where DfH mean and DfS mean are the corresponding values determined in the linear relationship (10), Tmean is appropriate average for all the temperature measurement data points (Table 2). DH tr and Ttr stand for enthalpy change of a phase transition of a substance involved into formation of Ca3TeO6 and respective temperature of the phase transition. The heat capacities of the elements (Ca, Te and O2) obtained from the literature [30] and the calculated heat capacity of Ca3TeO6 (expression (12)) were integrated from the standard temperature up to 895 K (Tmean), taking into account the stoichiometric coefficients of the formation reaction. The enthalpy of fusion of Te and a-b phase transition of Ca were considered in the calculation. The values calculated from Eqs. (13) and (14) are: DfH 298 = 2577.53 kJ mol1 and DfS 298 = 592.87 J mol1 K1. Sahu et al. [28] reported the estimative enthalpy of formation of Ca3TeO6 to be 2474.6 kJ mol1, which is in a good agreement with the value calculated in this study from primary results experimentally measured at elevated temperatures. The standard entropy of Ca3TeO6 at 298.15 K was calculated from DfS 298 and the entropy of the elements at standard
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temperature obtained from the literature [30] to be 1382.37 J mol1 K1. 4. Conclusions The EMF method is an experimental tool providing accurate thermodynamic data of alloys and compounds, which are necessary when modeling equilibria of chemical reactions and the phase stabilities. In this study, Ca3TeO6 was found to be the phase capable of coexisting with CaO and Te in the Ca–Te–O system; state-of-the-art equipment with carefully assembled oxygen concentration cells was employed to determine thermodynamic properties of Ca3TeO6 with high accuracy. Two similar experimental runs were carried out to prove the repeatability. Based on the obtained experimental data, the standard Gibbs energy of formation of calcium ortotellurate (Ca3TeO6) from the elements was determined over the temperature range from 850 to 949 K as:
Dr GCa3 TeO6 =ðkJ mol1 Þ 3 ¼ 2587:73 þ 582:27 103 T=K
.
The standard enthalpy and entropy of formation of Ca3TeO6 at 298.15 K were calculated as DfH 298 = 2577.53 kJ mol1 and DfS 298 = 592.87 J mol1 K1. The standard entropy of Ca3TeO6 at 298.15 K was calculated to be 1382.37 J mol1 K1. These data are the first report concerning the experimental determination of thermodynamic properties of calcium orthotellurate. Acknowledgments The authors are grateful to Boliden Harjavalta Oy, Outotec Oy and TEKES, theFinnish Funding Agency for Technology and Innovation for the financial support. This work was carried out as a sub task of Trace project. References [1] H. Kleykamp, The chemical state of the fission products in oxide fuels, J. Nucl. Mater. 131 (2–3) (1985) 221–246. [2] Critical raw materials for the EU Annexes to the report on Critical Raw Materials for the EU, 25 May 2014. Available at http://ec.europa.eu/enterprise/ policies/raw-materials/files/docs/crm-annexes_en.pdf (accessed 05.02.15). [3] M.E. Schlesinger, M.J. King, K.C. Sole, W.G.I. Davenport, Byproduct and waste streams, Extractive Metallurgy of Copper, fifth ed., Elsevier, Oxford, 2011415– 426 Chapter 21. [4] S.A. Malyutin, K.K. Samplavskaya, M.Kh. Karapet’yants, Thermographic Study of a Calcium Oxide–Tellurium Dioxide system, 62, Trudy Instituta-Moskovskii Khimiko-Tekhnologicheskii Institut imeni D.I. Mendeleeva, 196950–52. [5] R. Mishra, P.N. Namboodiri, S.N. Tripathi, S.R. Dharwadkar, Partial phase diagram of CaO–TeO2 system, J. Alloys Compd. 280 (1–2) (1998) 56–64.
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Thermodynamic stability of Ca3TeO6 determined by a solid electrolyte EMF method D. Sukhomlinova,* , F. Tesfayea,b , P. Taskinena a Aalto University School of Chemical Technology, Department of Materials Science and Engineering, Metallurgical Thermodynamics and Modeling Research Group, Vuorimiehentie 2 K, PO Box 16200, FI-00076 Aalto, Finland b Åbo Akademi University, Process Chemistry Centre, Piispankatu 8, FI-20500 Turku, Finland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 April 2015 Received in revised form 28 June 2015 Accepted 1 July 2015 Available online 13 July 2015
The standard thermodynamic properties of Ca3TeO6 were determined electrochemically utilizing fast O2 ion conducting solid electrolyte yttria-stabilized zirconia. The ternary phase was synthesized from the pure oxides CaO and TeO2 in excess of CaO. The electromotive force measurements were performed on two similar electrochemical cells of the type Te + CaO + Ca3TeO6|YSZ|O2, within the temperature range from 850 to 949 K. The standard Gibbs energy of formation for the ternary compound Ca3TeO6 was determined for the first time, based on the experimental data obtained. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Gibbs energy Electromotive force Calcium orthotellurate Thermodynamic properties Stabilized zirconia
1. Introduction The knowledge concerning the thermodynamic properties of compounds in the Ca–Te–O system is essential in the field of nuclear energy technology, pyrometallurgy, geochemistry, and other materials science applications [1,2]. New and accurate thermodynamic functions of the stoichiometric compound Ca3TeO6 presented in this article support process improvement as well as product development. Pure tellurium and tellurides are typical impurities occurring in base metal sulfide ores. During the base metals processing, tellurium is mostly distributed into the anode slimes forming different compounds, where its total concentration can reach several percent. The thermodynamic data concerning the Ca–Te–O system can support the modeling of chemical reactions and phase stabilities applied to industrial smelting and refining processes such as copper anode slime processing and the Doré smelting [3]. Recovery of tellurium has growing commercial interest because of its industrial applications. The major application of tellurium today is as an alloying additive in metallurgy. Also, tellurides of such elements as Sb, Bi and Cd have various commercial electronic and photovoltaic applications [2]. The partial phase diagram of the quasi-binary CaO–TeO2 system in the composition range 50–100 mol% TeO2 has been reported by
* Corresponding author. E-mail address: dmitry.sukhomlinov@aalto.fi (D. Sukhomlinov). http://dx.doi.org/10.1016/j.tca.2015.07.001 0040-6031/ ã 2015 Elsevier B.V. All rights reserved.
Malyutin et al. [4] and Mishra et al. [5]. The reported quasi-binary phase diagram indicates the presence of two stoichiometric compounds CaTeO3 and CaTe2O5. Mishra et al. [6] measured the Gibbs energy of formation of CaTeO3 and CaTe2O5 by the transpiration technique. Tripathi et al. [7] studied polymorphism of CaTeO3 and CaTe2O5 compounds by applying X-ray powder diffraction (XRPD), the lattice parameters and the space group of these phases were reported for the room temperature and for high temperature phases with respective phase-transition temperatures. However, Stöger et al.’s [8] crystallographic study shows disagreement with Tripathi et al. [7], and also validates the existence of a third ternary compound on the CaTeO3-TeO2 tie-line with a composition Ca4Te5O14, which has been discovered by Weil [9] from annealing reaction of CaCO3 and TeO2 in the molar ratio 1:2 at 1123 K. Concerning the phase relations on the CaO rich side, a disproportionation of Te6+ was reported [8,10] when an oxide mixture with a CaO/TeO2 ratio higher than 1 was annealed the reaction products appeared to be Te and calcium orthotellurate (Ca3TeO6). In contrast, Osinska et al.’s [11] TGA-DTA study claimed that in O2 free nitrogen atmosphere at 893 K CaTeO3 reacts with CaO to produce Ca3TeO6 and a CaTe intermetallic. Besides Ca3TeO6, the ternary compound CaTeO4 has been reported, which contain Te in 6+ oxidation state. Hottentot and Loopstra reported CaTeO4 as orthorhombic, space group Pbcn, with a = 5.231 (1), b = 12.676 (2), c = 4.977 (1) Å obtained with XRPD [12] and Ca3TeO6 as monoclinic, space group P21/n, with a = 5.5730 (6), b = 5.7964 (2), c = 8.0113 (3) Å, b = 90.24 (1) obtained from singlecrystal X-ray diffraction (SXRD) [13]. Also, Effenberger and Mayer
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Intensity
[14] reported crystal structure and lattice parameters of naturally occurring mineral carlfriesite CaTe3O8. The crystal structure is monoclinic, space group C2/c, with a = 12.576, b = 5.662, c = 9.994 Å, and b = 115.56 . It is a zeolite-like tellurite, where Te exists in both 4+ and 6+ oxidation states. The calcium ortotellurate Ca3TeO6 irradiated with neutrons was reported to be an iodine Mössbauer source, which allows application of 129I Mössbauer spectroscopy at room temperature [15,16]. The purpose of this experimental study is to enhance knowledge about the Ca–Te–O system and eliminate the existing discrepancies. This study presents the thermodynamic stability of Ca3TeO6 and observed phase relations in the Ca–Te–O system.
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15
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25
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35
40
45
50
55
60
65
70
2θ (°)
2. Experimental
Fig. 1. XRPD pattern of Ca3TeO6 saturated with CaO (solid line) measured at room temperature applying a Philips diffractometer equipped with CuKa radiation with a step size of 0.007 over a 2u range of 15–70 . CaO peaks are marked (*), the rest of the peaks corresponds to the major phase (Ca3TeO6).
2.1. Materials and sample preparation
(A) () Pt, Ir|Te(l) + CaO(s) + Ca3TeO6 (s)|YSZ|O2(g)|Pt (+)
The provenance and mass fraction purity of the materials used in this study are listed in Table 1. In order to synthesize Ca3TeO6 in excess of CaO, TeO2 was thoroughly mixed with 57 mol% of CaO both in a powder form (Table 1) in an agate mortar and pressed at a pressure of about 0.17 GPa to form a pellet with a thickness of 5 mm and diameter of 15 mm. The obtained pellet was inserted into a quartz glass crucible with a CaO protective pellet placed at the bottom of the crucible. The sample holder was placed on the bottom of a closedend alumina work tube of a vertical Lenton resistance furnace type LTF 16/-/610 and the sample was annealed under flowing argon gas (Table 1) protective atmosphere at 1000 K for 8 h. The sample obtained was ground into fine powder and mixed with additional 25 mol% of CaO and pressed to form a pellet. In the second heat treatment process, which was also conducted under the same conditions as in the initial annealing process, the sample was annealed at 1000 K for 21 h, at 1173 K for 0.25 h and at 973 K for 21 h. After the synthesized sample was ground and pressed into a pellet, the third heat treatment process was a repetition of the second annealing process under the same conditions. Dark-grey dust particles, due to the evaporation of Te-TeO2, were agglomerated on the inner surface of the furnace work tube, during all the three annealing cycles. The annealed material was analyzed after each intermediate cycle by SEM-EDS and XRPD techniques. As the result, the existence of two equilibrium phases CaO-Ca3TeO6 was confirmed. These observations agree well with information reported by Stöger et al. [8] and Trömel et al. [10]. No impurity phase was observed within 0.05 mass fraction XRPD detection limit in the obtained sample (Fig. 1). 2.2. Apparatus and assembly of the cell In order to determine the standard Gibbs energy of formation of calcium orthotellurate Ca3TeO6, the EMF values of the oxygen concentration galvanic cells (A) and (B) were measured in isothermal conditions in a temperature range from 850 to 949 K. Table 1 Purities and sources of materials used in the present study. Chemical
Source
Mass fraction purity
Te TeO2 CaO O2 Ar Pt ZrO2
Alfa Aesar (Germany) Alfa Aesar (Germany) Sigma–Aldrich (USA) AGA (Finland) AGA (Finland) Johnson–Matthey Noble Metals (UK) Friatec (Germany)
0.99999 0.9999 0.999 0.9995 0.99999 0.9999 doped by 0.085 of Y2O3
(B) () Pt, C|Te(l) + CaO(s) + Ca3TeO6 (s)|YSZ|O2(g)|Pt (+) The cells were examined by applying the experimental setup described in the previous study [17]. A vertical Lenton resistance tube furnace type LTF 16/-/450 was used in the experiments to achieve isothermal conditions. In order to minimize the temperature gradient over the galvanic cell, the temperature profile of the furnace was measured before the actual experiment applying two S-type Pt-Pt/Rh thermocouples (Johnson–Matthey Noble Metals) placed below and above the cell. The galvanic cell was placed in a hot zone with temperature gradient less than 1 K in the entire experimental temperature range. The thermocouples used were calibrated at the melting points of pure H2O, Sn and Cu. The experimental temperature measurement was performed at the bottom of the solid electrolyte zirconia tube by applying a thermocouple of the same type. The thermocouple was connected to a Keithley 2010 DMM multimeter, during the EMF measurements. The cold junction compensation was performed by a PT100 resistance thermometer (SKS-Group, Finland, tolerance class B 1/10), connected to a Keithley 2000 DMM multimeter. In a gas tight mullite work tube a steady oxygen flow (Table 1) was controlled by a digital mass flow controller, Aalborg DFC26 (USA). A solid electrolyte zirconia tube inserted in the work tube was used to separate the gaseous environments of the electrodes compartments. The outer surface of the solid-electrolyte zirconia tube was flushed by a steady flow of the pure oxygen, forming the reference electrode. In the test electrode compartment inside the zirconia tube, a nearly static protective argon atmosphere was arranged in order to prevent oxidation and to reduce volatilization of tellurium. The argon was purified from its original purity (Table 1) by passing it through a tube furnace heated up to 873 K and packed with titanium chips. The purified argon flow was controlled by a rotameter. In order to compensate a deviation of oxygen pressure in the reference electrode from the standard state, the atmospheric pressure was measured with a Vaisala PTU300 (Finland) pressure, humidity and temperature transmitter during the entire experiment. Also, the small hydrostatic pressure generated by a gas bubbler connected to the oxygen outlet was measured with a Beamex MC2 calibrator (Finland). A Keithley 6517B electrometer with a high input impedance of 2 1014 V was applied for the EMF measurements. Such high input impedance enables to conduct the EMF measurements in an open circuit and to provide the cell functioning reversibly [18]. Two identical, 0.25 mm in diameter Pt lead wires (Table 1) were connected to the electrometer. The cathode platinum lead wire in a shape of a coil was pressed toward the bottom of the solid electrolyte tube.
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The test electrode of the cell was comprised of three phases. The obtained biphasic mixture of Ca3TeO6-CaO was thoroughly mixed with Te powder and some additional CaO in such a way that molar ratio of Te:CaO:Ca3TeO6 was achieved to be 5:1:1. In order to minimize temperature gradient over the cell the amount of the sample was around 0.3 g. Pt lead wire cannot be directly utilized as a contact material to the anode, because of possible platinum tellurides formation [19]. In an attempt to avoid side reactions in the electrochemical cell a pure graphite [20–22] in a shape of a rod (10 mm long with 2 mm in diameter) was utilized as a contact material between the anode (the three-phase mixture) and the platinum lead wire. The graphite rod was hooked through a small hole with a tip of the anode platinum lead wire; also one coil of the wire was wrapped around the rod to provide reliable electric conductivity. In order to protect the Pt lead wire from the reaction with Te gas species, the wire was inserted into a thin (0.8/0.4 mm outer/inner diameter) alumina tube and then sheathed by a bigger (4/2 mm outer/inner diameter) alumina tube. The Pt–C connection point was sealed with alumina cement in the end of the bigger alumina tube. The sealing was arranged in such a way that only the tip of the graphite rod was uncovered (Fig. 2). An uncontrolled reduction process in the test electrode associated with utilized graphite contact is ruled out at the experimental temperature. The experimental temperature, pressure and EMF values were measured simultaneously at a rate of one data-point per 5 s and transferred to a computer for recording. The isothermal equilibrium was considered to be attained and the new measurement point was collected when the EMF values were constant or varied within less than 0.1 mV for several hours. To attain the equilibrium it required from few hours up to few days, depending on temperature. A grounded Faraday cage made of an alloy steel tube shielded the experimental cell from external electromagnetic fields. Two independent and similar galvanic cells were utilized in order to confirm repeatability of the measured EMF values. The EMF values were measured in both heating and cooling cycles. The test electrode composition was examined at the end of each experiment by XRPD to confirm the absence of any phase composition change.
The difference between cells (A) and (B) is the test electrode contact material utilized. A 20 mm long iridium wire tip spotwelded to the platinum lead wire was employed in the electrochemical cell (A). This contact material showed a poor durability and a short life time of the cell, due to the side reaction in the presence of metallic tellurium of formation of Ir3Te8 with a cubic unit cell [23,24]. Cell (B) was improved by utilizing a graphite rod (Fig. 2). 3. Results and Discussion The two half-cell reactions of the applied oxygen concentration cell are given below. The cathodic reaction is: 3/2O2(g) + 6e = 3O2,
(1)
and anodic reaction is: Te(l) + 3O2 + 3CaO(s) = Ca3TeO6(s) + 6e.
(2)
Therefore, the virtual cell reaction is: Te(l) + 3/2O2(g) + 3CaO(s) = Ca3TeO6(s).
(3)
3.1. Measured EMF values of the electrochemical cells Two cells altogether provided 15 measurement points as presented in Table 2. The median filter was applied to discriminate signal-noise of the experimental raw EMF data. The reported EMF values are averages of 200 data points (1000 s), which were recorded after equilibrium has been attained. The values reported for temperature and the prevailing pressure of the reference electrode are the respective average values recorded over the same time period. 3.2. Gibbs energy of formation of Ca3TeO6 from the elements Due to the fact that the solid electrolyte utilized in the measurements is a pure ionic oxygen conductor in the whole experimental temperature range [25], the Nernst Eq. (4) can be applied.
Table 2 The experimental values of EMF, temperature, prevailing total pressure of the reference electrode and calculated values of DrG . The values are listed in accordance with the date of measured data acquisition before temperature change.a
Fig. 2. Schematics of electrochemical cells (A) (left) and (B) (right): (1) YSZ tube, (2) upper alumina tube, (3) anode platinum lead wire shielded with a capillary alumina tube, (4) alumina cement, (5) Pt–Ir spot-welding point, (6) Pt–C connection point, (7) iridium contact, (8) graphite contact, (9) test electrode, (10) platinum coil, (11) alumina support, (12) lower alumina tube, (13) mullite work tube, (14) S-type thermocouple, (15) cathode platinum lead wire.
E (mV)
T (K)
P (hPa)
DrG (kJ mol1)
Cell A 767.62 776.76
889.5 869.7
1020.12 1020.97
444.32 449.60
Cell B 779.79 765.67 770.12 774.56 778.79 784.93 761.13 771.78 755.72 752.90 745.74 738.60 743.24
859.6 894.4 884.8 874.9 865.1 850.3 904.4 879.5 914.2 919.1 933.8 948.6 938.5
980.91 1011.91 1021.17 1027.31 1034.42 1036.52 1023.84 1012.51 1002.90 994.44 990.61 989.24 989.10
451.79 443.28 445.75 448.26 450.64 454.17 440.52 446.81 437.62 436.08 431.99 427.88 430.56
a Standard uncertainties u are u(E) = 0.10 mV, u(T) = 1.0 K, u(P) = 0.10 hPa, and expanded uncertainty U(DrG ) = 0.32 kJ mol1 (0.95 level of confidence).
D. Sukhomlinov et al. / Thermochimica Acta 615 (2015) 38–42
E¼
aO ðtÞ RT ln 2 ; ð4F Þ aO2 ðrÞ
(4)
where E is equilibrium EMF (V), R is the universal gas constant 8.3144 J mol1 K1 [26],T is absolute temperature (K), F is the Faraday constant 96,487 C mol1 [26], aO2 ðtÞ and aO2 ðrÞ are activities of oxygen in the test electrode and in the reference electrode, respectively. The standard Gibbs energy of the cell Reaction (3) can be expressed as: 2 3 aCa3 TeO6 4 5; Dr G ¼ RTln (5) 3=2 ðaTe aO2 ðtÞ a3CaO Þ where aCa3 TeO6 , aTe, and aCaO denote activities of the respective condensed phases. They can be assumed to be 1, due to the purity of the chemicals utilized (Table 1), an absence of deviations from the stoichiometry of the compounds and an absence of a significant mutual solubilities of the phases. The behavior of the oxygen gas phase can be assumed as ideal. The oxygen activity in the test electrode deduced from Eq. (4) was inserted into Eq. (5) and the assumptions described above were taken into account. Thus, the final equation for the calculation of the standard Gibbs energy change of the electrochemical cell Reaction (3) was obtained as: PO2 ðrÞ 3 Dr G ¼ 6FE þ (6) RTln ; 2 P where P is pressure of oxygen at the standard state 1013.25 hPa. Table 2 presents the summary of the EMF values, experimental temperature and prevailing total pressure of the reference electrode measured with electrochemical cells (A) and (B). The DrG values calculated with Eq. (6) are provided in Table 2 for each data point, respectively. Fig. 3 shows the calculated DrG values (Table 2) and the least squares fitting. Results obtained with electrochemical cells (A) and (B) are in a good agreement. Eq. (7) was derived by including experimental data obtained with both electrochemical cells (A) and (B). The determination coefficient R2 was calculated to be 0.9986.
DrG /(kJ mol1) 0.32 = 684.52 + 270.26 103 T/K, (850–949 K)
(7)
The standard errors for the intercept and for the slope in Eq. (7) are 2.55 and 2.84 103, respectively. The expanded uncertainty U (DrG ) in Eq. (7) is 0.32 kJ mol1 with 0.95 level of confidence calculated based on a statistical analysis of the experimental data. The standard Gibbs energy of formation of Ca3TeO6 from the pure elements was calculated according to Eq. (8) by combining Eq. (7) and the literature data concerning the thermodynamic
Δr G°/(kJ·mol -1 )
-426 -431 -436 -441 -446 -451 -456 840
860
880
900
920
940
960
Temperature (T/K) Fig. 3. The standard Gibbs energy of the virtual cell Reaction (3) as a function of temperature: (D) cell (A), (^) cell (B), (solid line) the least squares fitting for all experimental data points.
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stability of CaO.
Df GCa3 TeO6 ¼ Dr G þ 3Df GCaO :
(8)
The standard Gibbs energy of formation of CaO was calculated from Gibbs energies of the pure substances [27] to be:
DfG CaO/(kJ mol1) = 634.40 + 104.00 103 T/K. (750–1100 K)
(9)
The obtained Dt GCa3 TeO6 is expressed in Eq. (10). Its uncertainty can be calculated by applying the error propagation law. Due to an unknown contribution of u(DfG CaO) it was roughly estimated to be about one order of magnitude less accurate than the value reported for DrG , approximately 3 kJ mol1.
Df GCa3 TeO6 =ðkJ mol1 Þ 3 ¼ 2587:73 þ 582:27 103 T=K: ð850 949 KÞ This appears to be the first reported thermodynamic stability of calcium orthotellurate at elevated temperature. 3.3. Calculation of DfH 298, DfS 298 and S 298 of Ca3TeO6 Due to lack of heat capacity data of calcium orthotellurate, particularly from 298.15 to 949 K, it was estimated by applying Neumann–Kopp rule [29] from hypothetical Reaction (11) for further calculation of standard enthalpy and entropy of formation of calcium orthotellurate. TeO3(s) + 3CaO(s) = Ca3TeO6(s).
(11)
The heat capacity of CaO was taken from the literature [27]. For TeO3 the estimative heat capacity was calculated by Sahu et al. [28]. The Cp function of Ca3TeO6 calculated by Neumann–Kopp rule is expressed in Eq. (12): Cp/(J mol1 K1) = 261.336 19.527 103 T/K 60.5 105 (T/K)2 + 15.993 106 (T/K)2. (12) By applying the second law method, the standard enthalpy and entropy of formation of Ca3TeO6 at 298.15 K were calculated utilizing Eqs. (13) and (14): R P DfH 298 = DfH mean 298 TmeanDfCpdT + DH tr, (13) R Tmean P (DfCp/T) dT + (DH tr/T tr),
DfS 298 = DfS mean 298
(14)
where DfH mean and DfS mean are the corresponding values determined in the linear relationship (10), Tmean is appropriate average for all the temperature measurement data points (Table 2). DH tr and Ttr stand for enthalpy change of a phase transition of a substance involved into formation of Ca3TeO6 and respective temperature of the phase transition. The heat capacities of the elements (Ca, Te and O2) obtained from the literature [30] and the calculated heat capacity of Ca3TeO6 (expression (12)) were integrated from the standard temperature up to 895 K (Tmean), taking into account the stoichiometric coefficients of the formation reaction. The enthalpy of fusion of Te and a-b phase transition of Ca were considered in the calculation. The values calculated from Eqs. (13) and (14) are: DfH 298 = 2577.53 kJ mol1 and DfS 298 = 592.87 J mol1 K1. Sahu et al. [28] reported the estimative enthalpy of formation of Ca3TeO6 to be 2474.6 kJ mol1, which is in a good agreement with the value calculated in this study from primary results experimentally measured at elevated temperatures. The standard entropy of Ca3TeO6 at 298.15 K was calculated from DfS 298 and the entropy of the elements at standard
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temperature obtained from the literature [30] to be 1382.37 J mol1 K1. 4. Conclusions The EMF method is an experimental tool providing accurate thermodynamic data of alloys and compounds, which are necessary when modeling equilibria of chemical reactions and the phase stabilities. In this study, Ca3TeO6 was found to be the phase capable of coexisting with CaO and Te in the Ca–Te–O system; state-of-the-art equipment with carefully assembled oxygen concentration cells was employed to determine thermodynamic properties of Ca3TeO6 with high accuracy. Two similar experimental runs were carried out to prove the repeatability. Based on the obtained experimental data, the standard Gibbs energy of formation of calcium ortotellurate (Ca3TeO6) from the elements was determined over the temperature range from 850 to 949 K as:
Dr GCa3 TeO6 =ðkJ mol1 Þ 3 ¼ 2587:73 þ 582:27 103 T=K
.
The standard enthalpy and entropy of formation of Ca3TeO6 at 298.15 K were calculated as DfH 298 = 2577.53 kJ mol1 and DfS 298 = 592.87 J mol1 K1. The standard entropy of Ca3TeO6 at 298.15 K was calculated to be 1382.37 J mol1 K1. These data are the first report concerning the experimental determination of thermodynamic properties of calcium orthotellurate. Acknowledgments The authors are grateful to Boliden Harjavalta Oy, Outotec Oy and TEKES, theFinnish Funding Agency for Technology and Innovation for the financial support. This work was carried out as a sub task of Trace project. References [1] H. Kleykamp, The chemical state of the fission products in oxide fuels, J. Nucl. Mater. 131 (2–3) (1985) 221–246. [2] Critical raw materials for the EU Annexes to the report on Critical Raw Materials for the EU, 25 May 2014. Available at http://ec.europa.eu/enterprise/ policies/raw-materials/files/docs/crm-annexes_en.pdf (accessed 05.02.15). [3] M.E. Schlesinger, M.J. King, K.C. Sole, W.G.I. Davenport, Byproduct and waste streams, Extractive Metallurgy of Copper, fifth ed., Elsevier, Oxford, 2011415– 426 Chapter 21. [4] S.A. Malyutin, K.K. Samplavskaya, M.Kh. Karapet’yants, Thermographic Study of a Calcium Oxide–Tellurium Dioxide system, 62, Trudy Instituta-Moskovskii Khimiko-Tekhnologicheskii Institut imeni D.I. Mendeleeva, 196950–52. [5] R. Mishra, P.N. Namboodiri, S.N. Tripathi, S.R. Dharwadkar, Partial phase diagram of CaO–TeO2 system, J. Alloys Compd. 280 (1–2) (1998) 56–64.
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