Hydrothermal synthesis of zirconia doped with naturally mixed rare earths oxides and their electrochemical properties for possible applications in solid oxide fuel cells

. Solid oxide fuel cells (SOFC) are electrochemical conversion devices that produces electricity directly from oxidizing a fuel and their development became of high importance to drastically reduce the greenhouse emission. Rare earth elements (REEs) are widely used as materials and dopants in controlling the ionic conductivity of solid electrolytes for SOFCs. Their criticality and high costs for separation to individual REEs lead to ﬁ rst studies aiming to search possible use of mixed REEs with natural occurrence as extracted from concentrates. This paper focused on obtaining sintered pellets based on zirconia doped with natural mixture of REEs extracted from monazite and study their microstructure, impedance spectra and dielectric properties vs. operating temperatures to assess their potential applications as solid electrolyte. ZrO 2 doped powders with 8% natural mixture of REEs (8ZrMZ) were synthesized by hydrothermal process. ZrO 2 doped with 4% Y 2 O 3 (4ZrY) and 8%Y 2 O 3 (8ZrY) were also obtained by the same route and used as standard materials already used in commercial SOFCs. All powders were uniaxially pressed and sintered in air, with highest densities obtained for 1400 ° C. The Niquist diagrams for 8ZrMZ samples show signi ﬁ cantly lower ionic conductivity compared to standards 4ZrY and 8 ZrY. This may be attributed to the presence of detrimental Fe and Si impurities following the mixed REE after Th and U removal from monazite concentrates and the ratio of REEs in the dopant composition affecting the ionic conductivity due to possible association of structural defects. Research works are further needed to improve the receipt for using naturally mixed REEs and asses their possible use as a competitive dopant for solid electrolytes.


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][2][3].
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 Y 2 O 3 , MgO, CaO and Ln 2 O 3 (Ln: all transition metals in the lanthanum series in the periodic table of elements) [13][14][15].For example, the introduction of Y 2 O 3 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 (H 2 , CO, CH 4 , 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][19][20][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].
For commercial SOFCs, ZrO 2 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].
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 ZrO 2 powders doped by the hydrothermal method have been obtained.First, ZrO 2 powders doped with 4% and 8% Y 2 O 3 , respectively, were obtained for use as standards obtained by the same method, ZrO 2 materials doped with 8% Y 2 O 3 being already used for the development of electrolytic materials intended for SOFC.
The next step was ZrO 2 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

Hydrothermal synthesis
Zirconium tetrachloride (ZrCl 4 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 (Y 2 O 3 > 99% ÀMerck and natural mixture of RE respectively) in ZrCl 4 solution was made under vigorous mechanical mixing until a homogeneous clear solution was obtained.Ammonia solution (NH 3 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 pHmeter.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.

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].

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 Y 2 O 3 and mixt RE doped ZrO 2 powder and pellets was examined using a BRUKER D8 ADVANCE X-ray diffractometer (Bruker AXS Company, Germany) with monochromatic Cu Ka radiation, Bragg-Brentano diffraction method.Scans were obtained in the 2u 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 À The method used to determine specific surface area, pore volume or porosity, and pore shape and size is based on the physisorption of N 2 gas at 77 K (À196 °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 (l CuKa1 = 1.541874A ).The samples were scanned in the 2u 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 X0Pert 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.

Results and discussions
3.1 Hydrothermal synthesis

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.

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.

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).

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].

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.

SEM analysis
The morphology of the pellets sintered at different temperatures is shown in the Figure 8.
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 (T sint = 1300 °C) and 550-700 nm (T sint = 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.

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.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.

Electrochemical properties
The Nyquist diagram is a representation of impedance.In the case of 8ZrMz it is highlighted one semicircle in the interval 500C À700 °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].All powders, both initial and after calcination, consist of angular-shaped granular aggregates composed of fine particles.Their specific surface area was 3.4910 m 2 /g for 4ZrY, 16.1432 m 2 /g for 8ZrY and 0.5047 m 2 /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 ZrO 2 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].

Future works
To improve the electrochemical properties of ZrO 2 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 ZrO 2 doped with mixed REEs.

Fig. 1 .
Fig. 1.Schematic flowsheet of the a) monazite concentrate processing stages and b) hydrothermal process used for obtaining ZrO 2 powders doped.

Fig. 5 .
Fig. 5. EDS evaluation of 4ZrY, 8ZrY, 8ZrMZ powders a) initial and b) after calcination, with the marking of the areas of interest.

Fig. 9 .
Fig. 9. EDS evaluation of the three types of pellets sintered at different temperatures, with the areas of interest marked.

Table 1 .
Chemical analysis of powders based on doped ZrO 2 .

Table 2 .
Quantitative phase analysis of as obtained and calcined doped ZrO 2 powders.

Table 3 .
The grain size of all samples vs. sintering temperature.