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Cent. Eur. J. Chem. • 11(12) • 2013 • 2088-2097 DOI: 10.2478/s11532-013-0332-2 Central European Journal of Chemistry CaZrO3-based powders suitable for manufacturing electrochemical oxygen probes Research Article Magdalena Dudek1*, Alicja Rapacz-Kmita2 AGH-University of Science and Technology, Faculty of Fuels and Energy 30-059 Cracow, Poland 1 AGH-University of Science and Technology, Faculuty of Materials Science and Ceramics 30-059 Cracow, Poland, 2 Received 25 May 2013; Accepted 1 August 2013 Abstract: Calcium zirconate powders doped with a small amount of CaO were synthesised using the Pechini method. X-ray analysis revealed that solid solution was formed in the concentration up to 51.5% mol CaO. For synthesis of stoichiometric CaZrO3, the highest temperature was required (1150°C), but introduction of excess CaO from 50.5 to 51.5% mol enabled us to lower the synthesis temperature to 800°C. The sintering behaviour of such samples under non-isothermal conditions was studied by dilatometric methods. Deviations were found in stoichiometry; by increasing the CaO concentration in CaZrO3 sinterability improved in comparison to CaZrO3 with stoichiometric composition. The presence of CaO as second phase caused deterioration of the sinterability of the CaZrO3-based samples. Pellets sintered at 1500°C for 2 h reached 96-98% of theoretical density. SEM and TEM observations were used to characterise the microstructure of the prepared samples. The electrical properties of CaZrO3-based samples were investigated by the AC-impedance spectroscopy method. It was found that introduction of excess CaO into the CaZrO3 structure caused an increase in ionic conductivity up to the solubility limit. The possibility of using CaZrO3-based samples for constructing prototype electrochemical oxygen probes to determine activity of oxygen dissolved in molten copper is also demonstrated. Keywords: CaZrO3 • Electrochemical oxygen probe • Ionic conductivity • Electrolyte © Versita Sp. z o.o. 1. Introduction Calcium zirconate (CaZrO3) with a perovskite structure is one of many chemical compounds in the CaO-ZrO2 system. A high melting point (2340°C) and thermal and chemical stability make this compound a suitable component for refractory applications [1–4]. Perovskite structures (ABO3) are an important class of ceramics with a wide variety of derivative structure types (e.g. cubic, hexagonal, and orthorhombic). The physicochemical properties of ABO3 perovskite materials can be modified by doping cations with valencies different than those of the host cation B4+ or A2+ or variations of the A/B ratio, making them attractive functional materials for different areas in industry [5,6]. Orera et al. reported their characteristic infrared and Raman active modes of vibration, which were later confirmed by Zheng et al. 2088 [7,8]. They proposed the use of CaZrO3 doped with Er3+ as an optical material due to its luminescent properties [9]. Calcium-zirconate-based materials have also excellent catalytic and mechanical properties and as porous composites seem to be promising components for NOx fillers and lightweight structural components [10]. The material has a dielectric constant of ~30 and quality factor (Q=1/tan d) of 3000 at 13 GHz. The high dielectric constant combined with excellent thermal stability makes this material attractive for gate materials for MOSFET applications. Yu et al. determined a leakage current density of 9.5×10-8 (A cm-2) in an electrical field of 2.6 MV cm-1 [11]. CaZrO3 additions have also been observed to greatly increase the dielectric constant and temperature coefficient of capacitance (TCC) behaviour in BaTiO3based capacitors [12]. They are among the various * E-mail: [email protected] Unauthenticated Download Date | 8/12/17 12:14 AM M. Dudek, A. Rapacz-Kmita ceramic coatings currently in use for enhancing the biocompatibility of metallic implants. Ceramic proton-conducting materials based on CaZrO3 have been considered for potential applications in electrochemical probes to control the amount of hydrogen dissolved in aluminium alloy castings [13-16]. Janke [17,18] found that calcium zirconate modified by excess CaO could be applied as oxygen probes in metal alloys or steel castings. High-temperature ionic conductivity data suggested that solid solutions of calcia and fully-stabilized zirconia may be used for thermodynamic measurements at oxygen partial pressures as low as 10-23 atm at a temperature of 1273 K. So far, CaO-ZrO2 has been used in extensive studies of the thermodynamic properties of the majority of metal oxide systems. The Gibbs free energy of formation and thermodynamic activities for compounds and solid solutions formed in many oxide systems have been determined in this way. Calcium zirconate, however, enables measurement of thermodynamic properties in oxides at a low oxygen partial pressure limit, being several orders lower than that of the CaO-ZrO2 system, and therefore it has been successfully applied in oxygen activity measurements in molten metals. Our previous work [19–21] confirmed that modified CaZrO3 seems to be another promising electrolytic material for solid galvanic cells, designed to study thermodynamic data for oxide compounds and thermodynamic activity of compounds in binary oxide solid solutions. The present work is focused on the systematic study of the influence of chemical composition on the physicochemical properties of CaZrO3-based materials, which are crucial for preparing gas-tight components for the construction of prototypical electrochemical oxygen probes designed to study the activity of oxygen dissolved in non-ferrous molten metals. 2. Experimental procedure CaZrO3-based samples, involving CaO from 50 to 55% mol, were prepared by the Pechini method. Appropriate amounts of calcium carbonate were dissolved in zirconyl nitrate (1.1 mol dm-3). Citrate acid and ethylene glycol were added to the nitrate solution containing calcium and zirconium cations. The solutions were evaporated at 70°C to obtain hard gels; the dried gels were then calcinated at a temperature range of 650–1250°C for 1 hour and then a ground in rotaryvibratory mill with zirconia grinding media in dry ethanol. The phase composition of all powders and sintered samples were evaluated using X-ray diffraction analysis. The morphology of grounded CaZrO3-based powders was investigated by transmission electron microscopy (TEM). In order to determine the homogeneity of the chemical composition of monophase CaZrO3 powders synthesised using the Pechini method, the obtained preparations were washed with distilled water, which was later chemically analysed to determine the concentration of calcium ions washed out of the obtained powders. The chemical analysis was done by means of the inductively-coupled plasma atomic emission spectroscopy (ICP-AES) method. The experiment made it possible to estimate the amount of CaO dissolved in the CaZrO3 structure at the stage of synthesising the powder. The grounded powders were isostatically pressed under 200 MPa. The densification behaviour of the powder compacts was monitored via Dilatometry, using a constant rate of 5° min-1 at 25 to 1200°C. The samples were sintered at a temperature range of 1200–1600°C for 2 hours. Apparent density was measured by the Archimedes method. A scanning electron microscopy (Nova NanoSEM, FEI) was used to observe microstructures of the sintered samples. A numerical analysis of SEM microphotographs (Visilog 4 program, Noesis), taken from the polished and thermally etched surfaces, was applied to measure microstructural parameters quantitatively. Transmission Electron Microscopy (Tecnai G2 F20) was also used to study grain boundary structures of selected CaZrO3based samples. Ionic conductivity measurements were performed by the AC-impedance spectroscopy method in the temperature range 200–800°C in static air. Impedance spectra were recorded with an automated setup, suitable for measurements of large absolute values of impedance, which combined a Solatron 1260 Impedance/Gain Phase Analyser and a Keithley 428 Current Amplifier. Impedance was measured in the frequency range 10 MHz to 0.01Hz, with an applied AC signal of 30 mV rms. Measurements were made in air at constant temperatures from 200 to 800°C in two cooling and heating cycles (5 or 10°C temperature steps, 20 min stabilisation time before each measurement). The data acquisition program tested whether the impedance values changed with time and constant temperature. For a set of test frequencies, impedance was measured before and after acquisition of each spectrum. When the summed relative differences in measured impedance exceeded the assumed tolerance of 1%, the measurements were repeated. Impedance spectra were analysed by means of nonlinear leastsquares fitting of an equivalent circuit using the computer program Firdravn. 2089 Unauthenticated Download Date | 8/12/17 12:14 AM CaZrO3-based powders suitable for manufacturing electrochemical oxygen probes CaZrO3 tube ceramic seal EMF Pt-Rh wire Pt porous Figure 1. NiNiO Scheme of electrochemical non-stoichiometric CaZrO3 involving a 51% mol CaO oxygen probe for molten Cu-based melts. The AC four- probe method was also applied to study total electrical conductivity in a temperature range of 500–1150°C in air. The gas-tight elements prepared from CaZrO3 electrolytic material were tested with an electrochemical probe. The constructed cell (1) operated as an electrochemical oxygen probe to measure oxygen dissolved in metal melts. A schematic of this construction is presented in Fig. 1. Closed tubes made from non-stoichiometric CaZrO3 samples involving 51% mol CaO (CZ-51) were coated over their entire surface with a Pt electrode. The Ni-NiO mixture was used as a reference electrode. This metal/ metal oxide buffer had previously been tested as a reference electrode for measurements of oxygen activity in liquid copper. The CZ-51 oxygen sensor produces an EMF as a result of the differences in oxygen potential between liquid copper and the reference electrode, which can be related to temperature using the Nernst equation: (1) where pO2(ref) is the oxygen partial pressure established by the Ni/NiO buffer electrode. An electrochemical probe prepared in this way was applied to measure variations in oxygen activity in molten copper during the oxidation process. The effect of an amount of manganese on the variation in oxygen content in molten copper during the oxidising process was analysed. The temperature tested was 1100–1200°C. The conditions were similar to those described in paper [22], where an electrochemical probe with 8YSZ electrolyte was used to determine oxygen activity. Copper was melted in high alumina crucibles in an induction furnace. One added manganese pellet was used in each experiment. The mass of manganese was 0.2 and 0.8% of the mass of liquid copper respectively. The melting temperature was measured with an R-type thermocouple (Pt-Pt, 13% Rh). After the required temperature was attained, the oxygen sensor was introduced into the liquid copper. When the EMF signal of the electrochemical cell had been stabilised, the initial metal sample was withdrawn into a silica tube and a predetermined amount of manganese was added to start the deoxidising process. The working and reference electrodes of the electrochemical probe were covered with Pt electrodes. 3. Results and discussion 3.1. Preparation and physicochemical properties of non-stoichiometric CaZrO3 Only the orthorhombic CaZrO3 phase was detected by the XRD method for powders with stoichiometric composition (50% mol CaO) and including some excess CaO ranging from 50.5 to 51.5% mol. The highest temperature (1150°C) for the synthesis of the othorombic CaZrO3 phase was found for stoichiometric composition (50% mol CaO). Increasing CaO by 50.5 to 51.5% mol (excess CaO from 1 to 6% mol) relative to stoichiometric requirements led to a reduction in the synthesis temperature for the orthorhombic CaZrO3 phase to 800°C. The impact of CaO content on the falling temperature of the synthesis of monophase CaZrO3 has been previously mentioned for samples prepared by solid-state reactions. When solid-state synthesis was applied to the manufacture of CaZrO3 (50% mol CaO) powder, the monophase CaZrO3 samples were obtained at 1400°C for 1 h, whereas use of the Pechini method made it possible to obtain monophase powder with the same composition at a lower temperature, 1200°C. On the other hand, in the case of non-stoichiometric CaZrO3, a solid-state reaction made it possible to obtain monophase powder at 1200°C, but use of the Pechini method lowered the temperature of synthesis to 800°C. The highest particle sizes (about 90-115 nm) were found 2090 Unauthenticated Download Date | 8/12/17 12:14 AM M. Dudek, A. Rapacz-Kmita (a) Figure 2. Figure 3. (b) (a) TEM microphotograph of stoichiometric CaZrO3 powder (50% mol CaO). (b) TEM microphotograph of non-stoichiometric CaZrO3 powder (51% mol CaO). Dilatometric curves recorded for investigated CaZrO3based pellets. for pure CaZrO3, whereas non-stoichiometric CaZrO3based powders, due to a lower synthesis temperature, were characterised by crystallite sizes of 40 to 60 nm. A typical TEM microphotograph of CaZrO3 powder involving 50% mol CaO and non-stoichiometric CaZrO3 (51% mol) is presented in Figs. 2a, 2b. The uniform distribution of chemical elements in CaZrO3 powders is an additional crucial factor with a strong impact on the sinterability of the powders, next to manufacture of gas-tight sintered shapes and performance of electrolytic material. It has also been determined that washing with distilled water removes small amounts of calcium (ca. 0.6% wt) from the calcinated powders at 800°C. This shows that the assumed CaO dissolves into the CaZrO3 structures to reach solid-solution form as early as at the stage of powder synthesis. Contrary to CaZrO3-based powder prepared by solid-state reaction, after washing CaZrO3based powder with distilled water, considerable losses of CaO were detected. The results of TEM and EDS investigations as well as chemical analysis data clearly indicate that the applied synthesis conditions made it possible to obtain CaZrO3-based powder with high chemical homogeneity at rather low temperatures (below 1000°C) compared to solid-state reactions between ZrO2 and CaCO3 reagents, which were previously applied by Janke et al. for 70–80 years to manufacture CaZrO3based oxygen probes. Dilatometric studies were performed for the series of CaZrO3-based samples with different calcia contents. These curves (Fig. 3) show linear shrinkage of the samples during the sintering process in a dilatometer with a heating rate of 6°C min-1. As can be seen, intensive shrinkage of the CaZrO3 sample with stoichiometric compositions starts at 600°C, but shrinkage was achieved at values of about 9% of total shrinkage at 1200oC. Introduction of CaO in excess caused a small improvement in the sinterability of pellets prepared from such powders. The results indicate that the sintering curves of CaZrO3-samples with different calcia content show distinct differences. Shrinkage of non-stoichiometric CaZrO3 started at about 700°C and reached about 12% of total shrinkage. On the other hand, the diphase CaZrO3-based green samples (CaZrO3 with CaO precipitations) consolidate only above 1000°C, and the observed contractions are less than half the size of monophase samples. These results clearly indicated that stoichiometry and defect chemistry influences the sinterability of CaZrO3-based pellets. The impact of Ca/Zr molar ratio on CaZrO3-based samples was previously studied by Pollet et al. [23]. The analysis of shrinkage curves DL/Lo versus temperature also show that CaCO3 as a starting reagent excess 10% CaO addition to the CaZrO3 phase improves the sinterability of samples compared to stoichiometric composition. 2091 Unauthenticated Download Date | 8/12/17 12:14 AM CaZrO3-based powders suitable for manufacturing electrochemical oxygen probes Figure 4. (a) (b) (c) (d) (a) Microstructure of CaZrO3 involving a 50% mol CaO sintered sample at 1500°C for 2 h. (b) Microstructure of a CaZrO3 sample involving 50% mol CaO synthesised by solid state reaction and sintered at 1700°C for 2 h. (c) Microstructure of non-stoichiometric CaZrO3 (51% mol CaO) sample sintered at 1500°C for 2 h. (d) Microstructure of a non-stoichiometric CaZrO3-sintered sample (52% mol CaO) (the sample underwent additional heat treatment at 1200°C for 100 h). The relative density of monophase CaZrO3-based samples obtained from powders synthesised by the Pechini method and sintered at 1500°C was about 96–98% of theoretical density. On the other hand, the presence of CaO as a second phase in CaZrO3-based pellets led to a decrease in relative density to about 92– 93% of theoretical density. The microstructure of CaZrO3 sintered samples is presented in Figs. 4a-4d. The highest value of average grain size was detected for CaZrO3 with stoichiometric composition. The introduction of excess CaO into solid solution caused a decrease in grain sizes of CaZrO3 samples prepared by Pechini and solid state reaction methods. A comparable difference was detected in monophase non-stoichiometric CaZrO3 samples. An increase in CaO in CaZrO3 solid solution caused an increase in average grain sizes up to composition 51.5% mol CaO (excess 6% mol CaO) for solid solution, followed by a small decrease. Variations in grain sizes are higher for samples prepared using solid-state reactions. In contrast to the Pechini method, the solubility of CaO in CaZrO3 in samples prepared using solid-state synthesis increases with the rising temperatures of sintering pellets. This suggests that the solubility process takes place only during sintering, which is unfavorable for the densification of prepared powders, and for the electrical properties of the materials. Monophase CaZrO3-based samples heat-treated at 1700°C for 24 h were prepared using solid-state reactions. After these experiments the sintered samples collapsed into powders while being stored at room temperature. The (probably small) 2092 Unauthenticated Download Date | 8/12/17 12:14 AM M. Dudek, A. Rapacz-Kmita Figure 5. The variation of grain sizes of CaZrO3-based samples obtained by Pechini method as well as solid-state reaction. amount of free CaO remaining at grain junctions may be responsible for this; it may have reacted with H2O at room temperature and the subsequent volume expansion may have brought about the collapse. On the other hand, in the same experiment, CaZrO3based samples obtained by Pechini method did not collapse. Grain boundary chemical composition and structure are the main factors deciding the contribution of ionic conductivity in ceramic oxide electrolytes. According to the reported analyses, the resistivity of the grain boundary of ceramic oxide electrolytes such as fully yttria- stabilised zirconia or ceria-based solid solutions is typically two or three orders of magnitude higher than that of the grain interior [24,25]. In the case of cubic zirconia-yttria (c-YSZ) solid solutions, which were usually used as solid-oxide electrolytes, grain boundary properties were studied extensively. The summarised results of grain boundary studies and their influence on ionic conductivity suggested two main reasons for high temperature resistivity. One is a space-charge layer near the grain boundary resistivity layer, near the grain boundary formed by solute segregation. Conduction of oxygen ions decreases when the oxygen-vacancy concentration at the grain boundary drops below the concentration in the grain interior. This explains the intrinsic grainboundary resistivity of highly pure materials. The other is a siliceous phase or film at the grain boundary frequently described as a blocking layer of oxygen ions. Even a small addition of SiO2 has been known to segregate at grain boundary. Considering that SiO2 is a ubiquitous background impurity in ceramic processing, it is difficult to exclude extrinsic origin even in the relatively pure sample [26]. The grain boundary structure of CaZrO3-based samples with excess CaO has not yet been analysed through the use of transmission electron microscopy to correlate chemical segregation with the grain boundary with results of impedance spectroscopy. TEM investigations, performed for a CaZrO3 sample with 51.5% mol CaO (6% mol CaO excess), are presented in Fig. 6. The performed TEM observation on the thin film prepared from sintered samples did not detect the difference between the chemical composition of the grain boundary and grains. No presence of SiO2 was detected in the grain boundary. As can be seen, the nanometric amorphic separate presence on the surface grain CaO as precipitation was detected. In contrast, CaZrO3-based samples prepared from powders synthesised by reaction in solid phase, reaching the above-mentioned limit of solubility, are possible only at the temperature range of 1600–1800°C, depending on the properties of the starting reagent (CaCO3, ZrO2) powders. As it was proven earlier, the reaction method for solid phase between the ZrO2 and CaO or ZrO2 and CaO oxides did not lead to homogenous chemical composition at the stage of CaZrO3 powder synthesis. In order to visualise this statement, the changes of solubility limit were recorded for samples obtained by the Pechini method as well as solid-state reaction and results are shown in (Fig. 7). The impedance spectroscopy method enabled the study of ionic conductivity of CaZrO3-based samples. The electrical conductivity of CaZrO3 with stoichiometric composition was about 2×10-6 S cm-2 at 1000°C. This material seems to be a very poor oxide ion conductor. The lowest values of total electrical conductivity (4.8×10-7 S cm-2) were reached for CaZrO3 samples prepared by solid-state reaction and sintered at higher temperatures (1750°C). One reason for this behaviour might involve possible structural changes taking place at such high temperatures, is most likely due to a structural phase transformation from orthorhombic phase to cubic CaZrO3 phase. In the case of CaZrO3-based samples modified by excess CaO, impedance spectra were fitted by equivalent circuits, consisting of a series of 3 subcircuits of parallel resistor-CPA elements. The subcircuits were attributed to electrode dispersion (1) one or two grain boundary/additional phase (2) and bulk electrical resistance (3). Analysis of bulk conductivity values (Figs. 8a, 8b) for all investigated samples obtained by the Pechini method reveals that the introduction of CaO into CaZrO3 solidsolution form caused an increase in bulk conductivity as well as grain-boundary conductivity. This may have 2093 Unauthenticated Download Date | 8/12/17 12:14 AM CaZrO3-based powders suitable for manufacturing electrochemical oxygen probes Figure 6. TEM microphotograph of non-stoichiometric CaZrO3 (51.5% mol CaO) grain boundary structure. Figure 7. The changes of solubility limit of CaO in CaZrO3-based samples obtained through solid-state reaction as well as the Pechini method. 15 CSZ log s (S/cm) -4.2 (a) C A -6.3 B A -5.6 -7.0 Figure 8. C -4.9 -5.6 50 (b) 15CSZ -4.2 B -4.9 -3.5 log sg (S/cm) -3.5 contributed to an increase in the oxygen concentration of vacancies according to the proposed defect model [27]. For comparison, data for ionic conductivity was determined for samples prepared through solid-state reaction as well as other chemical methods. The data obtained for samples prepared with citrate and with coprecipitation calcination methods are close to the values obtained for samples sintered with the Pechini technique [28,29] The comparative analysis of impedance data for CaZrO3 samples prepared by solid-state reaction and the Pechini method enabled us to find some differences in the behaviour of such samples. In the case of CaZrO3-based samples (51.5% mol CaO) prepared through solid-state reaction (Fig. 9) the values of ionic conductivity (s at 700oC) depend on the temperature of the sintering sample as well as on the previouslymentioned homogeneity of CaO distribution in the samples. The maximum bulk conductivity depends on the temperature of the sintering samples. The increase up to 1700°C in the temperature of the sintering sample caused an increase in bulk conductivity due to the higher solubility of CaO in the CaZrO3 structure along with higher temperatures. On the other hand, CaZrO3 samples sintered at 1750°C have lower ionic conductivity values due to possible structural changes. This fact could be confirmed by the relation between electrical conductivity and solubility limit in the CaZrO3based samples. The dependence of activation energy of bulk conductivity on composition, shown in Fig. 10, seems to correlate with compositional dependence on conductivity: maximum conductivity corresponds to minimum energy activation. 51 52 53 54 % mol CaO 55 56 50 51 52 53 54 55 CaO, % mol (a) The dependence of bulk conductivity for CaZrO3-based samples recorded at 700°C; A – solid state reaction, B – citrate method, C – Pechini method; 15CSZ-cubic zirconia solid solution involving 15% mol CaO; (b) The dependence of grain boundary conductivity for CaZrO3-based samples recorded at 700°C; A – solid state reaction, B – citrate method, C – Pechini method; 15CSZ-cubic zirconia solid solution involving 15% mol CaO. 2094 Unauthenticated Download Date | 8/12/17 12:14 AM M. Dudek, A. Rapacz-Kmita 2.5x10-4 2.4x10-4 T =1200oC -4 ionic bulk conductivity 1.5x10-4 s (S/cm) partial pressure pO2, atm 2.0x10 1.0x10-4 5.0x10-5 total conductivity 0.0 1450 1500 1550 1600 1650 1700 1.8x10-4 data from Ref [22] 1.2x10-4 6.0x10-5 CaZrO3 electrolyte 0 Temperature of sintering, oC Figure 9. 2.4 bulk grain boundary Ea, eV 1.6 1.2 0.8 50 51 52 53 54 55 56 57 58 59 % mol CaO Figure 10. The changes of energy activation for bulk and grainboundary conductivity for investigated CaZrO3-based samples 4.0x10-2 T= 1000oC -2 st(S/cm) 3.2x10 2.4x10-2 1.6x10-2 0 5 10 15 20 25 30 35 40 45 time, h Figure 11. 4 6 8 10 time, min The variation of total electrical conductivity and bulk conductivity (s at 700oC) determined for CaZrO3-sintered samples involving 51.5% mol CaO; bulk conductivity determined by the AC impedance spectroscopy method, total conductivity determined by the DC four-probe method. 2.0 2 The time dependence of total electrical conductivity (st) vs. time for non-stoichiometric CaZrO3 sample (51 % mol CaO). Figure 12. The variation of pO2 in liquid copper during the deoxidation process; 0.2 mass % of initial manganese content was added. The most controversial problem is the interpretation of the impact of grain-boundary conductivity, as compared to that of chemical composition, on the electrical properties of CaZrO3-based samples [30,31]. In the case of CaZrO3-based samples synthesised by the Pechini method, the ionic conductivity of grain boundary increases to reach solubility limit CaO in CaZrO3. One way to explain such dependences is “the geometrical reason” – the grain size decreases with increasing CaO content and thus the geometrical factor of the g-b conductivity correlates with the activation energy of this conductivity. Time-dependent effects in the total electrical conductivity of nostoichiometric CaZrO3-based samples involving 51% mol CaO, prepared through solid- state reaction and the Pechini method at 1000°C were also presented in Fig.11. As can be seen in the case of the CZ-51 sample, stable values of electrical conductivity were observed for the investigated time up to 120 h in air at 1000°C. The obtained data for electrical conductivity values concerning their stability over time enabled us to test these materials as components for electrochemical probes to measure oxygen dissolved in molten nonferrous alloys. The constructed electrochemical oxygen probes were tested to measure oxygen dissolved in alloys. Fig. 12 shows the variation of pO2 (copper) in liquid copper during the deoxidation process when 0.2 mass % of initial manganese content was added. The presented data are in good agreement with results obtained by González-López [22] during measurements with an electrochemical probe manufactured from 8YSZ electrolyte and calculated through the interaction parameters and the oxygen and manganese content. These preliminary data indicated 2095 Unauthenticated Download Date | 8/12/17 12:14 AM CaZrO3-based powders suitable for manufacturing electrochemical oxygen probes Figure 13. The surface of nostoichiometric CaZrO3 (51 %mol CaO) -based electrochemical oxide probes after immersion in liquid copper. that a non-stoichiometric CaZrO3 gas-tight electrolyte obtained by a cost-effective method can be used as a component in the construction of electrochemical probes to measure oxygen activity in different nonferrous alloys. Further investigations need to focus on optimising of construction and performance of CaZrO3based probes. After tests, the cross-section of CaZrO3-gas tight ceramics (Fig. 13) was investigated by means of scanning electron microscopy. No cracks were identified on the surface of CaZrO3based electrochemical probes. 4. Conclusions The application of the Pechini method made it possible to obtain sinterable powders of monophase nonstoichiometric CaZrO3 in a temperature range of 1450– 1500°C for 2 h. It was found that introduction of excess CaO made it possible to lower the temperature of synthesis and then improve the sinterability of samples compared to stoichiometric CaZrO3 composition. The results presented in this paper indicate that CaZrO3-based samples sintered are characterised by considerably better ionic conductivity compared to solid-state synthesis. The main advantage of this proposed method of CaZrO3 synthesis is obtaining powder with high homogeneity, suitable for manufacturing gas-tight electrolytic shapes for electrochemical probes. Another valuable result of this method’s application is lack of precipitation of CaO as the second phase in the grain boundary of CaZrO3 sintered samples. The obtained CaZrO3-sintered components were successfully tested in electrochemical probes to determine oxygen activity in molten metals. Acknowledgment This paper was carried out under contract (11.11.210.217) AGH-Univeristy of Science and Technology, Faculty of Fuels and Energy, Cracow, Poland. 2096 Unauthenticated Download Date | 8/12/17 12:14 AM M. Dudek, A. Rapacz-Kmita References [1] J. Szczerba, Z. Pędzich, Ceramic International 36, 535 (2010) [2] A. Obregón, J. L. Rodríguez-Galicia, J. L.-Cuevas, P. Pena, C. Baudin, J. Eur. Ceram. Soc. 31, 61 (2011) [3] C. Gargori, S. Cerro, R. Galindo, A. García, M. Llusar, G. Monrós, Ceramic International 38, 4453 (2012) [4] J.E. Contreras, G.A. CastilloT, E.A. Rodrıguez, T.K. Das, A.M. Guzman, Materials Characterization 54, 354 (2005) [5] M.A. Pena, J.L.G. Fierro, Chem. Rev. 101, 1981 (2001) [6] J.G. Cheng, J.S. Zhou, J.B. Goodenough, Y. Sui, Y. Ren, M.R. Suchomel, Phys. Rev. B 83 644 (2011) [7] V.M. Orera, J.I.Pena, R.I. Merino, J.A. Lazaro, J.A.Valles, M.A. Rebolledo, Appl. Phys. Lett. 71, 2746 (1997) [8] R.I. Merino, R.A. Pardo, J.I. Pena, G.F. De la Fuente, A. 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