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Home All issues Volume 11 (2024) Manufacturing Rev., 11 (2024) 1 Full HTML
Open Access
Issue
Manufacturing Rev.
Volume 11, 2024
Article Number 1
Number of page(s) 12
DOI https://doi.org/10.1051/mfreview/2023014
Published online 04 January 2024

© A.-N. Ghiță et al., Published by EDP Sciences 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

Nanomaterials based on zirconium oxide show considerable interest due to its properties, such as: low thermal conductivity, high temperature stability, corrosion resistance, chemical inertness, high ionic conductivity and biocompatibility [1–3].

Due to its unique combination of properties ZrO2 has an important role in the development of several applications: thermal barrier coatings [4], solid oxide fuel cells [5], gas sensors [6] catalysis [7], medical prostheses [8] and storage devices [9].

ZrO2, also known as zirconium oxide or zirconium, shows three polymorphic phases, depending on the temperature: monoclinic (m) (T < 1100 °C), tetragonal (t) (T = 1170–2370 °C) and cubic (c) (T = 2370–2706 °C) [10–12].

To avoid phase transitions during service, zirconia with the tetragonal and/or cubic phase is used by replacing Zr4+ with larger cations having a lower valence than such as Y2O3, MgO, CaO and Ln2O3 (Ln: all transition metals in the lanthanum series in the periodic table of elements) [13–15]. For example, the introduction of Y2O3 increases the concentration of oxygen vacancies according to the rule of charge neutrality [11]. This situation makes yttrium stabilized Zirconia useful as high ionic conductivity electrolyte in Solid Oxide Combustion Piles (SOFC) [16].

The solid oxide fuel cell (SOFC) converts chemical energy stored in a fuel (H2, CO, CH4, etc.) into electricity through an electrochemical reaction (by oxidizing a fuel) with low emissions [17]. SOFC is a sandwich structure composed of a porous oxide-based cathode, a dense ceramic electrolyte and a porous anode [18–21].

A key element for SOFCs is the electrolyte which performs two primary functions: conducting oxide, ions or protons and acting as a physical block (by isolating the fuel and air) [22].

Therefore, the main characteristics of the electrolyte are: high oxide, ion or proton conductivity; a low electronic conductivity, chemical stability at high temperatures; low porosity; good sinterability; sufficiently high mechanical resistance; low cost and easy cell manufacturing technology [18,23–30].

Several ionic conducting materials have been studied as electrolytes in SOFCs: (CeO2) and zirconia (ZrO2) based stabilized fluorites, LaGaO3-based perovskites and BCZY (BaCe0,2Zr0,6Y0,2O3−δ) [31,32].

For commercial SOFCs, ZrO2 is the most widely used electrolyte due to its high ionic conductivity, excellent stability, electrode compatibility, but also its negligible electronic conductivity and good chemical stability [33,34].

A current research subject is the development of electrolytes in the form of thin films for SOFCs operating at low temperatures (400–650 °C versus the conventional 650–800 °C). Reducing the size of the electrolyte by developing thin films is an effective strategy to reduce the operating temperature of SOFCs in order to improve performance, material stability, efficiency and cost [14,35].

Until now, several physical and chemical deposition techniques have been used to manufacture TF-SOFC: sputtering, chemical vapor deposition processes, such as: atomic layer deposition (ALD), pulsed laser deposition (LPG), physical vapor deposition (PVD), or RF sputtering [36–39].

From all these techniques, RF sputtering is considered to be a versatile technique by which both dense and porous films can be created by changing the deposition parameters. Thus, completely dense yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) films were successfully developed, along with the porous films Ni-YSZ and LSCF-YSZ with columnar nanostructures [40].

The present study is a first attempt to evaluate the possible use of natural occurring mixed REOs obtained by mineral processing of REOs concentrates as dopant for zirconium oxide sintered products and coatings with potential applications in electro-ceramics applications. With this aim, three types of ZrO2 powders doped by the hydrothermal method have been obtained. First, ZrO2 powders doped with 4% and 8% Y2O3, respectively, were obtained for use as standards obtained by the same method, ZrO2 materials doped with 8% Y2O3 being already used for the development of electrolytic materials intended for SOFC.

The next step was ZrO2 powder doping with an 8% mixture of rare earths (RE) obtained from monazite and having the natural proportions occurring in the concentrate after removal of Th and U. The powders obtained were characterized in terms of chemical composition, microstructure, morphology, thermal stability and specific surface. Based on the powder, three-step sintered pellets were obtained (at 1200 °C, 1300 °C, 1400 °C). The batch of pellets with the best density (sintering at 1400 °C) was selected in order to determine the electrochemical properties. RF-sputtering was used to obtain thin films based on 8ZrMZ, films that were structurally and morphologically characterized.

2 Materials and methods

2.1 Hydrothermal synthesis

Zirconium tetrachloride (ZrCl4 99% Merck) was used as a raw material for the preparation of a stock solution with a known concentration of Zr. Natural occurring mixed hydroxides with a total concentration of 31.06% REOs extracted from monazite concentrates according to method presented in [41] were used as dopant. Dissolution of precursors (Y2O3 > 99% −Merck and natural mixture of RE respectively) in ZrCl4 solution was made under vigorous mechanical mixing until a homogeneous clear solution was obtained. Ammonia solution (NH3 25% p.a., Chimreactiv srl) was added as a mineralizing agent until pH ∼ 9 was obtained. The pH measurement was performed continuously using a precision digital pH-meter. Doped powder was obtained by hydrothermal treatment in autoclave (Berghof, Germany, TEFLON vessel, capacity 5 L, maximum operating temperature 250 °C, maximum working pressure 200 atm., equipped with water cooling system). The solid precipitate obtained, was filtered and washed several times to remove soluble impurities, followed by drying in an oven at 110 °C to constant weight. The synthesis scheme is shown in Figure 1.

The next step was to determine the stability of the powder phases. Therefore, they were subjected to heat treatment at 1200 °C for 1 h and their phase compositions were analyzed.

thumbnail Fig. 1

Schematic flowsheet of the a) monazite concentrate processing stages and b) hydrothermal process used for obtaining ZrO2 powders doped.

2.2 Obtaining pellets

First, the powder, the solvent and the binder (2% APV with a concentration of 5%) and a few drops of distilled water are mixed in a container with the help of a spatula. For a uniform distribution this mixture was introduced in an ARE 250 THINKY centrifugal planar mixer, for 3 min at 2000 rpm. The suspension was then dried in an oven at a temperature of 110 °C, for approximately 24 h until it was completely dry. The obtained material is then mortared to obtain a fine powder. A manual press was used to obtain pellets of approximately 20 mm size. Thus, 3 types of pressed pills were obtained, named: P1-4ZrY, P2-8ZrY, P3-8ZrMZ.

It was proposed to follow the influence of the sintering temperature on the densification. Thus, Figure 2 shows the three sintering profiles followed.

The sintering addressed is a three-step sintering. The first two steps are attributed to the disintegration of the binder. According to literature studies, but also previous research, APV disintegrates in a temperature range between 300–500 °C [42].

thumbnail Fig. 2

Sintering profile of doped ZrO2-based pellets.

2.3 Characterization methods

The chemical composition was determined using inductively coupled plasma optical emission spectrometry (Agilent 725 ICP-OES) from Agilent Technologies Inc. The analysis was performed in accordance with ASTM E 1479-99 (2011).

The microstructure of the as-obtained Y2O3 and mixt RE doped ZrO2 powder and pellets was examined using a BRUKER D8 ADVANCE X-ray diffractometer (Bruker AXS Company, Germany) with monochromatic Cu Kα radiation, Bragg–Brentano diffraction method. Scans were obtained in the 2θ range 4–74°with a step size of 0.02° every 2.5 or 6s. For the identification of the phases contained in the samples, data processing was performed using software package DIFFRAC.SUITE.EVA release 2016 by Bruker AXS Company, Karlsruhe, Germany; SLEVE + 2020 and ICDD PDF-4 + 2020 database edited by International Centre for Diffraction Data (ICDD).

The morphology of the powder samples and pellets was investigated by scanning electron microscopy (SEM) using a high-resolution microscope Quanta 250 (FEI Company, Eindhoven, The Netherlands), incorporated with Energy Dispersive X-Ray Spectrometer, produced by EDAX (Mahwah, NJ, US), consisting of ELEMENT Silicon Drift Detector Fixed, Element EDS Analysis Software Suite APEX. 1.0, EDAX, Mahwah, NJ, USA. The analyzed samples were metallized by coating with a 10 nm thick Au layer.

Differential scanning calorimetry coupled with thermal gravimetry analysis (DSC-TG) has been performed on a SETARAM SETSYS Evolution equipment (SETARAM Instrumentation, Caluire, France) in inert atmosphere. Samples were introduced in alumina crucibles and heated with a heating rate of 10 K/min up to 1450 °C, then cooled to room temperature with the same rate. The experimental data were processed with the help of Calisto software v.1.097 (SETARAM Instrumentation, Caluire, France).

BET Specific Surface Area Measurements − The method used to determine specific surface area, pore volume or porosity, and pore shape and size is based on the physisorption of N2 gas at 77 K (−196 °C), with an adsorption-desorption isotherm. Measurements were performed using a Micromeritics TriStar II Plus analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). The specific surface area was obtained by the Brunauer–Emmett–Teller (BET) method, while the pore volume and pore size distribution were determined by the Barrett–Joyner– Halenda (BJH) method. Prior to each determination, the powder samples were subjected to a heat treatment at 300 °C for several hours to remove traces of liquids and impurities using the VacPrep 061 degassing stations.

The density of the sintered pellets was determined by the Archimedean method using distilled water as the immersion medium.

Impedance spectroscopy measurements of the pellets sintered at 1400° C were carried out in a Probostat equipment (NORECS, OSLO, Norway) placed into a vertical tubular furnace (ELITE, Leicestershire, United Kingdom). The samples had their surfaces prepared for measurements by obtaining parallel surfaces and by coating them with silver paste. The impedance analyzer an Array M3500A instrument (Array, Nanjing, China) was used in order to measure the dielectric constant in a frequency interval between 1kHz and 100 KHz. Temperature range was established between 500 °C and 800 °C at a 100 °C interval. For analysis and impedance data fitting NOVA 2.1 software from Methrom Autolab. (Metrohm Autolab B.V., Utrecht, The Netherlands) was used.

The structural information for thin films was obtained by X-ray diffraction (XRD) technique, carried out in air, at room temperature, with the help of PANalytical Empyrean (Almelo, The Netherlands) equipment provided with a characteristic Cu X-ray tube (λ CuKα1 = 1.541874 Å). The samples were scanned in the 2θ angle range of 10–80° using grazing incidence with a scan increment of 0.02° and a time of 100 s/step. Rietveld quantitative phase analysis was performed using the X′Pert High Score Plus 3.0 software (PANalytical, Almelo, The Netherlands). After Rietveld refining, values of 1.8885% for goodness of fit, 13.2104% for Rexpected, and 13.5669% for Rprofile were obtained.

The characterization of the films by SEM-EDS was performed using the QUANTA INSPECT F50 SEM (FEI Company, Eindhoven, The Netherlands) equipped with field emission gun electron (FEG) with 1.2 nm resolution and energy dispersive X-ray spectrometer (EDS), with the resolution at MnK of 133 eV.

3 Results and discussions

3.1 Hydrothermal synthesis

3.1.1 Chemical analysis

The chemical analysis of powders synthesized in hydrothermal conditions is presented in Table 1 and it is in accordance with the designed compositions.

Table 1

Chemical analysis of powders based on doped ZrO2.

3.1.2 XRD analysis

The XRD spectra of the initial and heat-treated powders are shown in Figure 3 and the quantitative phase analysis is shown in Table 2.

The 4ZrY and 8ZrMZ powders consist of tetragonal zirconium oxide as the main phase and cubic zirconium oxide as the secondary phase, while the 8ZrY sample consists only of the tetragonal phase.

The diffraction peaks for the initial 4ZrY, 8ZrY, 8ZrMZ powders are associated with (hkl) planes having tetragonal orientation along (101) (002) (110) (112) (200) (103) (211) and monoclinic orientation along (−111) (111). No Y/Y2O3 peaks were observed in the doped samples, emphasizing that the dopant was well dissolved in the ZrO2 crystal lattice.

The calcined 4ZrYC, 8ZrMZC powders consist of tetragonal Yttrium Zirconium Oxide as the main phase with (hkl) planes (10 1) (002) (110) (112) (200) (103) (211) (202) and a monoclinic secondary phase with planes (hkl) (−111) (111).

thumbnail Fig. 3

XRD pattern for a) the initial powders and b) calcined (1h, 1200 °C) powders.
Table 2

Quantitative phase analysis of as obtained and calcined doped ZrO2 powders.

3.1.3 SEM analysis

Figure 4 shows the morphology of the powders before and after calcination at 1200 °C.

Scanning electron microscopy images show that in all systems, both initial and after calcination, angular-shaped granular aggregates composed of fine particles are formed and EDS semi-quantitative chemical analyzes for the initial and calcined samples confirm the presence of doping elements (Fig. 5).

thumbnail Fig. 4

Representative SEM images at different magnifications for initial and after calcination 4ZrY, 8ZrY, 8ZrMZ powders.
thumbnail Fig. 5

EDS evaluation of 4ZrY, 8ZrY, 8ZrMZ powders a) initial and b) after calcination, with the marking of the areas of interest.

3.1.4 DSC-TG analysis

Thermal analysis was used to analyze the thermal stability and phase transformations during heat treatment of hydrothermally obtained 4ZrY, 8ZrY, 8ZrMZ powders.

The DSC-TG graph of the powders heated from room temperature to 1450 °C is shown in Figure 6.

DSC-TG measurement on all three powders shows a continuous mass loss up to temperatures of about 600 °C and an endothermic peak around about 85 °C, revealing a dehydration process of the material, which is in agreement with other literature studies [43].

thumbnail Fig. 6

DSC-TG analysis of powders based on doped ZrO2.

3.1.5 BET analysis

Figure 7 shows the adsorption-desorption isotherms for the 4ZrYC, 8ZrYC and 8ZrMZC powders calcined at 1200 °C.

According to the definition given by the International Union of Pure and Applied Chemistry (IUPAC), mesoporous materials, with pore sizes between 2–50 nm, are associated with a hysteresis loop, in which the lower branch represents measurements obtained by the progressive addition of gas to the adsorbent, and upper branch by progressive withdrawal.

The calculated Brunauer-Emmett-Teller (BET) surface areas for the 4ZrY, 8ZrY, and 8ZrMZ powders are 3.4910 m2/g, 16.1432 m2/g, and 0.5047 m2/g, respectively. The pore size distribution by the Barrett-Joyner-Helanda (BJH) method is 56.0085 nm (8ZrY), 26.4404 nm (8ZrY), 19.4368 nm (8ZrMZ).

thumbnail Fig. 7

Nitrogen adsorption ± desorption isotherms for a) 4ZrYC, b) 8ZrYC c) 8ZrMZc calcinated.

3.2 Pellets

3.2.1 SEM analysis

The morphology of the pellets sintered at different temperatures is shown in the Figure 8.

For the samples P1-4ZrY, P2-8ZrY sintered at a temperature of 1200 °C, the formation of grains with a size between 172–216 nm and 200–222 nm, respectively, is observed, as well as the presence of porosity. By increasing the sintering temperature, there is an increase in grains with sizes between 243 nm–425 nm (Tsint = 1300), 280–628 (Tsint = 1400) for the P1-4ZrY sample and 350 nm −1.4 µm (Tsint = 1300° C), 585 nm − 2 µm (Tsint = 1400 °C) for the P2-8ZrY sample.

If at sintering temperatures of 1200 °C the increase in dopant concentration does not have a significant effect on grain growth, at sintering temperatures of 1300 °C and 1400 °C the increase in the amount of dopant from 4% to 8% leads to the formation of grains with larger sizes.

The sample P3-8ZrMZ sintered at 1200 °C consists of agglomerations (no grains were formed) with a size between 328–432 nm. The presence of porosity is observed. Increasing the sintering temperature leads to the formation of grains with a size between 276–317 nm (Tsint = 1300 °C) and 550–700 nm (Tsint = 1400 °C).

The grain sizes of all samples are summarized in Table 3.

The EDS semi-quantitative chemical analyses are presented in Figure 9. Semi-quantitative EDS chemical analyzes for all sintered samples confirm the presence of doping elements.

thumbnail Fig. 8

Representative SEM images at different magnifications for P1-4ZrY, P2-8ZrY, P3-8ZrMZ pellets sintered at different temperatures.
Table 3

The grain size of all samples vs. sintering temperature.

thumbnail Fig. 9

EDS evaluation of the three types of pellets sintered at different temperatures, with the areas of interest marked.
thumbnail Fig. 10

Relative density for all samples sintered at 1200 °C, 1300 °C and 1400 °C.

3.2.2 Density

Figure 10 shows the density of the pellets at different sintering temperatures. The temperature of 1400 °C proved to be the most beneficial for obtaining a high density for the samples P1-4ZrY, P2-8ZrY. For sample P3-8zrMz, a slightly lower density was obtained, probably due to the impurities present in the doping material.

3.2.3 Electrochemical properties

Figure 11 present Nyquist plots for 4ZrY, 8ZrMz, 8ZrY. (500–800 °C).

An equivalent circuit with two resistors R1 and R2 and two capacitors C1 and C2 (Fig. 12 was used). This technique employs the electrical properties to separate the individual effects of the components (bulk and grain boundary). The complex impedance Z is composed of Z' (resistive component) the real component and Z” (capacitive component) the imaginary component of the impedance.

Z* = Z'+ Z(1)

The Nyquist diagram is a representation of impedance. In the case of 8ZrMz it is highlighted one semicircle in the interval 500C −700 °C., at 800 °C it starts to appears the second semicircle. Since only one semicircle appears, this is strictly related to the grain contribution, which leads to a distinct electronic property in conductors [44].

For 4ZrY, 8ZrY ceramics Nyquist diagram presents the appearance of two overlapping semicircular arcs what point out the contribution of the grain and the grain boundaries. When temperature increases it is obvious that semicircle becomes more and more complete. Once the temperature increases it can be observed the tendency of decreasing the impedance values towards high frequencies.

The NOVA 2.1 software was used to adjust the parameters of each element in the mixt circuit presented in Figure 12. The first parallel circuit correspond to resistance of the grain contribution R1 and C1 correspond to grain capacitance. In the second parallel circuit R2 and C2 were used as grain boundary resistance and grain boundary capacitance. The values of resistance of grain and grain boundary decrease with increasing temperature, for all ceramics. This may indicate the existence of a conduction mechanism that is thermally activated, within the grains and the grains boundary [45].

thumbnail Fig. 11

Nyquist plots for 4ZrY, 8ZrMz, 8ZrY (500–800 °C).
thumbnail Fig. 12

Mixt circuit used to fit the data.

4 Conclusions

The aim of the study was to analyze the possibility to develop a technology enabling the use mixtures of rare earth elements having the natural occurrence as extracted from monazite as dopant for zirconia ceramics, with potential applications in solid oxide fuel cells. The availability of dopants consisting from natural REEs mixtures could have a major impact on their efficient use by avoiding high costs and environmental problems during separation in individual elements. With this aim, ZrO2 doped with 8% natural mixture of rare earth elements (8ZrMZ, containing as major REOs: 0.78 La, 0.68 Nd, 0.08 Sm, 0.067 Y, 0.055 Gd according to chemical analysis) have been synthesized by a hydrothermal process at low temperatures (max. 250°C) and pressures (max. 200 atm.). ZrO2 doped with 4% Y2O3 (4ZrY) and 8%Y2O3 (8ZrY) used as standards in commercial SOFCs were synthesized in the same conditions.

The as-synthesized powders consist of tetragonal and monoclinic ZrO2. After calcination at 1200 °C, the formation of solid solutions Y0.043Zr0.957O1.979, Y0.16Zr0.84O1.96 and Ce0.08Zr0.92O2 type were observed for 4ZrY, 8ZrY and 8ZrMY powders respectively.

All powders, both initial and after calcination, consist of angular-shaped granular aggregates composed of fine particles. Their specific surface area was 3.4910 m2/g for 4ZrY, 16.1432 m2/g for 8ZrY and 0.5047 m2/g for 8ZrMZ calcined powders.

The study of the classical pressing and sintering of calcined powders was done in the temperature range 1200–1400 °C. The highest densities reported to the theoretical densities were 99.44% for 4ZrY, 99.18% for 8ZrY and 96.34% for 8ZrMZ for pellets sintered at 1400°C. For these sintered pellets the grain sizes were in the range 280–628 nm, 585–2000 nm and 550–700 nm respectively. EDS semi-quantitative analysis of sintered pellets confirm the presence of doping elements considered in the synthesis.

Pellets sintered at 1400 °C from the three compositions were used for electrochemical studies by impedance spectrometry. The comparison of Nyquist spectra for sintered mixed RE-doped zirconia (8ZrMZ) with traditional 4 and 8% yttria-doped zirconia (4ZrY and 8 ZrY) shows a clear difference between the conduction mechanisms in the temperature range 500–800 °C. The impedance spectra of 8ZrMZ samples show only one semicircle related to the grain contribution to ionic conduction in the range 500–700 °C, and the beginning of a second semicircle formation at 800 °C for the grain boundary contribution. For standard 4 and 8 Y-doped ZrO2 composition, both grain and grain boundary contributions to the ionic conduction are observed in the temperature range 500–800 °C and the impedance decrease with increasing temperatures toward high frequencies.

There may be two reasons for this lower ionic conduction properties of mixed RE-doped zirconia (8ZrMZ) compared to standard 4 and 8% yttria-doped zirconia: 1) the presence of some impurities following the mixed REE after Th and U removal from monazite concentrates, mainly Fe and Si (about 0.5% total) which are known as detrimental to ionic conductivity properties; 2) The ratio of REEs in the dopant composition affecting the ionic conductivity due to the association of structural defects in complex defects [27].

5 Future works

To improve the electrochemical properties of ZrO2 doped with mixed REOs, additional purification of the initial mixed REEs will be done to eliminate impurities detrimental for ionic conductivity detected (Si and Fe). The purified mixed REEs hydroxides will be further used as dopant using the procedure described in the paper and electrochemical properties will be measured in the temperature range 400–800 °C.

Further studies will be performed for mapping the influence of multiple REEs concentration on ionic conduction properties. An active factorial experimental plan will be proposed using purified REEs hydroxides to evaluate the ionic conductivity of each receipt and obtain a digital twin of materials based on ZrO2 doped with mixed REEs.

Author contributions

For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “Conceptualization, R.R.P.; methodology, A.E.S., L.L., and A.S.; validation, C.F.R. and L.L.; investigation, A.N.G. and B.S.V.; resources, R.R.P.; writing—A.E.S. and C.F.R.; writing—review and editing, R.R.P.; supervision, R.R.P and F.M; A.S. All authors have read and agreed to the published version of the manuscript”.

Funding

This work was carried out through the Core Program within the National Research Development and Innovation Plan 2022–2027, carried out with the support of MCID, project no 23250102.

Conflicts of interest

There is no conflict of interests.

Acknowledgments

This work was carried out through the Core Program within the National Research Development and Innovation Plan 2022–2027, carried out with the support of MCID, project no 23250102.

Thie research for obtaining mixed REOs hydroxides from concentrates was funded by UEFISCDI Romania and TUBITAK Turkey in the frame of ERAMIN III Program, Project ID 38 RETECH.

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Cite this article as: Andreea-Nicoleta Ghiță, Anca Elena Slobozeanu, Lidia Licu, Cristina Florentina Ciobota, Arcadii Sobetkii, Bogdan Stefan Vasile, Florin Miculescu, Radu Robert Piticescu, Hydrothermal synthesis of zirconia doped with naturally mixed rare earths oxides and their electrochemical properties for possible applications in solid oxide fuel cells, Manufacturing Rev. 11, 1 (2024)

All Tables

Table 1

Chemical analysis of powders based on doped ZrO2.

In the text
Table 2

Quantitative phase analysis of as obtained and calcined doped ZrO2 powders.

In the text
Table 3

The grain size of all samples vs. sintering temperature.

In the text

All Figures

thumbnail Fig. 1

Schematic flowsheet of the a) monazite concentrate processing stages and b) hydrothermal process used for obtaining ZrO2 powders doped.
In the text
thumbnail Fig. 2

Sintering profile of doped ZrO2-based pellets.
In the text
thumbnail Fig. 3

XRD pattern for a) the initial powders and b) calcined (1h, 1200 °C) powders.
In the text
thumbnail Fig. 4

Representative SEM images at different magnifications for initial and after calcination 4ZrY, 8ZrY, 8ZrMZ powders.
In the text
thumbnail Fig. 5

EDS evaluation of 4ZrY, 8ZrY, 8ZrMZ powders a) initial and b) after calcination, with the marking of the areas of interest.
In the text
thumbnail Fig. 6

DSC-TG analysis of powders based on doped ZrO2.
In the text
thumbnail Fig. 7

Nitrogen adsorption ± desorption isotherms for a) 4ZrYC, b) 8ZrYC c) 8ZrMZc calcinated.
In the text
thumbnail Fig. 8

Representative SEM images at different magnifications for P1-4ZrY, P2-8ZrY, P3-8ZrMZ pellets sintered at different temperatures.
In the text
thumbnail Fig. 9

EDS evaluation of the three types of pellets sintered at different temperatures, with the areas of interest marked.
In the text
thumbnail Fig. 10

Relative density for all samples sintered at 1200 °C, 1300 °C and 1400 °C.
In the text
thumbnail Fig. 11

Nyquist plots for 4ZrY, 8ZrMz, 8ZrY (500–800 °C).
In the text
thumbnail Fig. 12

Mixt circuit used to fit the data.
In the text

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