Materials Science Forum Vols. 798-799 (2014) pp 174-181 Online available since 2014/Jun/30 at www.scientific.net © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.798-799.174
Synthesis and Characterization of Zirconium Oxide Systems with Yttrium Rich Rare Earth Concentrate Additives P.Cajas1, R.Muñoz1, , A.C. Rodríguez2, J. E. Rodríguez-Páez3 C.R.M da Silva1 1
Universidade de Brasília- Brasília- DF- Brasil Universidade Federal de São Carlos- São Carlos-Brasil 3 Universidad del Cauca-Popayán Colômbia. Campus Universitário Darcy Ribeiro- Brasília DF. CEP: 70910-900
[email protected] 2
Key words: rare earth concentrate, controlled precipitation, ionic conductivity
Abstract In this work, the yttrium rich rare earth concentrate (Re2(CO3)3) was used as additive aiming stabilization of cubic an tetragonal phases at commercial zirconium oxide with 3% mol of yttrium oxide. The use of high purity rare earth oxide as additive is being commercially used and this work aims to demonstrate the potential use of lower cost additives to produce solid electrolyte for oxygen sensors and fuel cell applications. The powders for the additive production were synthesized by the controlled precipitation method. After synthesis, the powders were de-agglomerated using mechanical grinding and mixed to commercial zirconia to produce the compositions ZrO2:3% Mol Y2O3:ƞ % Mol Re2O3 (ƞ=3,4,5,6), followed by uniaxial press and sintering at 1500 0C in two hours. The obtained sintered densities were above 96% of theoretical. X-Ray diffractometric analysis and Rietweld refinement demonstrated the stabilization of cubic and tetragonal phases for all samples with yttrium rich rare earth concentrate additives. Finally the electric behavior of the evaluated samples was carried out with complex impedance spectroscopy, showing conductivity improvement for samples with the chosen additive. At 500 0C the sample A-9% had a conductivity of 1,11E-3 Ω1 .cm-1, well above of the sample without additive with conductivity 5,88E-4 Ω1.cm-1, indicative that use of yttrium rich rare earth concentrate as additive increases considerably the ionic conductivity of comercial zirconium oxide. Introduction Zirconium oxide has different properties, according to the existing crystalline phases. The monoclinic phase (m-ZrO2), stable at room temperature, is used on manufacturing of abrasive coatings and inorganic pigments1.Tetragonal (t-ZrO2) and cubic (c-ZrO2) phases are unstable at room temperature, being necessary the use of stabilizers oxides additives2, such as MgO, Y2O3, CeO2, CaO, Sc2O3, Yb2O3, Er2O3 and Gd2O33. Applications of stabilized tetragonal zirconia are based on thermo-mechanical properties4. The existence of total or partially stabilized cubic phase is necessary to produce solid electrolyte used in oxygen sensors and fuel cells, considering its adequate electric response5, subject of interest in this research. High purity stabilizers oxides, regularly used on solid electrolytes manufacturing are very expensive considering its complex synthesis processes. Aiming cost reductions on raw materials, the present work used alternative raw materials such as mixed rare earth oxides OTR (Re2O3) obtained from rare earth carbonates CTR (Re2(CO3)3) extracted from monazite. The synthesis of this mixed oxide is quite simple and less expensive than pure rare earth oxides.
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Considering the potential applications of ZrO2 stabilized in its cubic and tetragonal phases, in this work the powder synthesis was carried out with the controlled precipitation method using codoping of ZrO2:3%molY2O3 with yttrium rich rare earth carbonate CTR (Re2(CO3)3). Methods and Materials Commercial oxide ZrO2:3%mol Y2O3, (TOSHO) was co-doped with yttrium rich rare earth carbonate (Re2(CO3)3 (IPEN)6, with 3,4 and 6% mol content, using the controlled precipitation method. Aiming sample identifications, they were labeled in alphabetical order, A,B,C and D, followed by a number that indicates the total amount of the Y2O3 additive, as presented at table 1. Table 1. Rare Earth Y2O3 additive content at each sample. Sample % of Re2O3 content Total % of Y2O3 (moles) content(moles) A-9% B-8% C-7% D-6%
6% 5% 4% 3%
9% 8% 7% 6%
Synthesis precipitation method was used to synthesize the systems ZrO2:3 %mol Y2O3:ƞ %mol Re2O3 with ƞ = 3,4,5,6, (A-9%, B-8%, C-7% e D-6%, respectively). At first and addition of Re2(CO3)3 were carried out in a solution of 0,3 Mol of nitric acid, stirring continuously until complete powder solution, followed by ZrO2:3%mol Y2O3 powder addition. Afterwards the system PH was adjusted to 8.5 by controlled addition of ammonium hydroxide until complete precipitate formation. Finally the colloidal suspension was obtained, with complete precipitation after 3 hours. The resulting powder was dried and heat treated at 600 0C during two hours to eliminate remaining nitrates. A 100 ml friction mill was used to de-agglomerate synthesized powder in a citric acid medium. Zirconium oxide grinding ball were used during powder mixture with additives to avoid powder contamination. The samples were uniaxially pressed at 187 MPa at a press Marcon MPH-10, followed by sintering in a Nabertherm LHT 407GN6. The heating schedule included heating up to 1000 0C at a heating rate of 10 0C/min, followed by a heating up to 1500 0C with ramp of 3 0C/min, remaining at this temperature for two hours. X-ray diffratometry was carried out in a Bruker diffratometer, with copper anode, at Brasilia University. This technique was used to identify existing phases at synthesized powders and also at sintered samples. Tetragonal and cubic zirconium phases presents strong overlapping of diffraction peaks, hindering full identification of these phases. It is therefore necessary to use Rietveld refinement, which allows to identify each overlapping crystalline phase. The software used for Rietveld refinements in this work was the GSAS (General Structure Analysis System). The solid electrolyte used for oxygen sensor manufacturing should have density above 92% of theoretical, aiming to avoid gas leakage through voids8. Theoretical densities were calculated with the lever rule application, using the phase percentage of zirconium oxide of each sample, obtained with the Rietweld refinement8. Electric behavior of sintered samples was evaluated with complex impedance spectroscopy technique, which allows to measure separately the grain resistivity and grain boundary resistivity. Considering that the Zirconia- Ittrium Oxide system has thermally activated mechanism, measurements were made in a temperature range of 273-550 0C. The resistivity can be determined using equation (1): ρ=
(1)
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where A is the sample cross section, l the sample thickness and R the values of resistance found at the obtained Nyquist semicircles for each sample. The ionic conductivity (σ) of grain, grain boundary and total were calculated using reciprocal of equation (1). The Arrhenius graphics were obtained plotting the decimal logarithm of conductivity against the reciprocal of absolute temperature. Another interesting parameter to be calculated is the activation energy, which represents the minimum energy necessary for conduction occurrence. This parameter can be calculated using equation (2), using the slope of Arrhenius graphics: σ = σ0
(2)
where σ is the conductivity, σ0 the pre-exponential factor, K the Boltzman constant, T the absolute temperature and E tha activation energy. Results and Discussion Figure 1 shows difractograms of powder samples after heat treatment at 6000C, where can be identified a mixture of monoclinic and tetragonal phases in all samples. Moreover the peaks of yttrium oxide were not observed, localized at 2θ=29°( PDF 41-1105), indicative of possible solid solution of yttrium oxide at zirconium oxide crystalline structure. D-6%
Tetragonal Monoclinic
Intensity (u.a)
C-7%
B-8%
A-9%
20
25
30
35
40
45
50
55
60
65
70
2 Figure 1 – X-ray difractograms of powder samples, after heat treatment at 600 0C, in a 2θ range of 20 to 70 degrees. Crystalline phases of samples sintered at 1500 0C were also identified with X-Ray difractometry. At figure 2(A) is shown the experimental standard and calculated standard by the Rietweld refinement, for sample A-9%. In this case, cubic and tetragonal phases of zirconium oxide were identified. For better understanding of these phases overlapping, the magnification of calculated and experimental standard is presented at figure 2(B), with 2θ in a range of 59°- 60,5°. Peaks related to tetragonal phase is at 2θ= 59.75° e 60.45° (PDF 50-1089) and those concerning cubic phase is at 2θ= 60° (PDF 30-1468).
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A-9%
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100
50
0
0 20
40
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(A) (B) Figure 2. Rietveld refinement of sintered sample A-9%, showing experimental and observed standard (A) and crystalline phases at experimental standard in the range 2θ = 59°- 60,5° (B). The results of Rietweld refinement for all samples are presented at table 2
Samples A-9% B-8% C-7% D-6%
Table 2. Results of Rietveld refinement % of ZrO2 phases Density (g/cm3) c-ZrO2 t-ZrO2 c-ZrO2 t-ZrO2 85,134 14,866 6,030 6,062 80,855 19,145 6,018 6,078 75,701 24,299 6,034 6,119 66,166 33,834 6,029 6,057
According to results at Table 2, the yttrium rich additives promotes cubic phase formation at the commercial powder (ZrO2:3% mol Y2O3),increasing its percentage with increase on additive content. In this case, there are a higher percentage of cubic phases in sample A-9% in comparison with sample D-6%. Table 3 shows obtained densities and percentage of theoretical densities for sintered samples. Obtained values are above 96%, making this material strong candidate in applications related to solid electrolytes.
Sample A-9% B-8% C-7% D-6%
Table 3 – Samples densities Density (g/cm3) % of theoretical density 5,85 5,82 5,97 5,96
96,71 96,21 98,64 98,57
Figure 3 presents impedance diagrams for sample A-9%, showing resistivity variation with temperature increase. The same thermally activated behavior was observed for all samples. In this case is possible to highlight the existence of two semicircles, assigned to grain resistivity for high frequencies and to grain boundaries for intermediate frequencies.
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-Z|| (.cm)
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A-9%
0,0 0,0
5,0x10
4
1,0x10
5
1,5x10
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5
Figure 3 – Resistivity against temperature for sample A-9%. Figure 4 shows samples conductivity, with similar behavior for all samples between 273 and 400 0C. For temperatures above 400 0C the samples A-9% and B-8% show the worse conductivities. This effect can be attributed to the percentage of cubic phases in these samples.
-3
2,5x10
A-9% B-8%
A-9% B-8% C-7% D-6% ZrO2:3% molY2O3
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-1
cm
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5,0x10
0,0
200
250
300
350
400
450
500
550
600
650
T(°C)
Figure 4. Total conductivities with temperature variation for samples A-9%, B-8%, C-7%, D6% and ZrO2:3%mol Y2O3 without additive Arrhenius graphs are presented in figures 5 and 6, showing grain interior contribution (5(A)) and grain boundaries contributions (5(B)).
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(B)
Figure 5 – Arrenhius graphics of grain (A) and grain boundary (B) conductivities in temperature range between 276 e 550°C for samples A-9%, B-8%, C-7%, D-6% and ZrO2:3%mol Y2O3 without additives. In figure 5 (A), relative to grain interior, it is observed that in temperatures around 550 0C the samples A-9% and B-8% have better conductivities. In figure 5(B), related to grain boundary, the conductivities are also higher for samples A-9% and B-8%.
Figure 6. Arrenhius graphics for total conductivities for samples A-9%, B-8%, C-7%, D-6% and ZrO2:3%mol Y2O3 without additives The Arrenhius graphics for total conductivities in figure 6 indicate that the zirconium oxide systems A-9% and B-8% have similar electrical responses, both higher than all other compositions. This performance is related to cubic phase percentage. At 500 0C the composition A-9% has condutivity of 1,11E-3 Ω-1.cm-1 and the sample without additives, ZrO2:3% molY2O3, shows conductivity of 5,88E-4 Ω-1.cm-1. Therefore the use of additives in this work resulted conductitivity increase. Table 4 presents the calculated activation energies.
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Tabel 4. Activation energies at commertial zirconium oxide (ZrO2:3% molY2O3) and at sinterized samples A-9%,B-8%,C-7% e D-6%. Sample Grain Boundaries Grain Total (Ev) (Ev) (Ev) 1,036 1,087 0,967 A -9% 1,051 1,068 0,961 B-8% 0,957 1,015 0,957 C-7% 0,952 0,972 0,952 D-6% 1,015 0,864 0,903 ZrO2:3% molY2O3 Values of total activation energies found in Table 4 for samples A-9%, B-8%, C-7% e D-6% are analogous, with values whithin the range of energies oxygen conductor materials ( 1eV)10. Conclusions Difratomettric analisys for sintered samples were complemented with Rietweld refinement, considering diffraction peaks overlapping of cubic and tetragonal phases of zirconium oxides. A mixture of tetragonal and cubic phases was identified in all examined samples, with higher percentage of cubic phase in sample with higher amount of additives (A-9%). Densities values of sintered samples were above 96%, indicative of appropriate density for applications as solid electrolytes. With Arrhenius graphics examination, it was observed increase on conductivity for samples co-doped is this work, with better performance in terms of electric behavior for samples A-9% and B-8%, indicative of possible applications as solid electrolyte and oxygen sensors, at lower cost, compared to those manufactured with high purity rare earth. References [1]
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Synthesis and Characterization of Zirconium Oxide Systems with Yttrium Rich Rare Earth Concentrate Additives 10.4028/www.scientific.net/MSF.798-799.174
