Structural , Morphological and Electrical Properties of Multi-Doped Calcium Phosphate Materials as Solid Electrolytes for Intermediate Temperature Solid Oxide Fuel Cells

Modified solution precipitation method was used to prepare pure and doped Mg, Sr and Na hydroxyapatite type materials (CaP, CaMgP and CaSrNaP). Modification consisted of partial substitution of nitrates by acetate solution in order to achieve a more soluble and cost effective synthesis. The obtained samples were calcined at 400 oC (CaP400, CaMgP400 and CaSrNaP400). All powders were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Calcined samples were densified at 1000 oC in an air for 3 h (CaP1000, CaMgP1000 and CaSrNaP1000). Sintered samples were characterized by XRPD, FTIR, SEM, EDS and complex impedance methods. The highest conductivity was found for the multi-doped phosphate sample (CaSrNaP1000) at 700 oC (1.90×10 -3 Ω -1 cm -1 ). The corresponding activation energies of conductivity amounted to 0.31 eV in the temperature range 500-700 oC.


Introduction
Due to the high efficiency and applicability of different types of fuels (biogas, natural gas, hydrogen and methane) [1][2][3][4], Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs) have become one of the most promising energy conversion devices and emphasis is placed on the development of micro-SOFCs as power sources [5], which could operate at reduced temperatures in porTab.electronic devices.To achieve this goal, it is necessary to synthesize material with corresponding properties.Generally, the conductivity of these systems depends on the processing and intrinsic properties of the material itself, where micro structural features and density have a key role [6].Recently, cerium (IV) oxide (CeO 2 ) solid solution materials became favored for potential use in IT-SOFCs, with appropriate doping methods may create fluorite-type crystal lattice which has a good influence at ionic conductivity but the use of CeO 2 and oxide rare-earth is very expensive [7][8][9][10].An adequate substitute for CeO 2 might be calcium phosphate hydroxyapatite HA ceramics -Ca 5 (PO 4 ) 3 OH, due to their physical and chemical properties and very low cost of synthesis [11,12].Various types of synthesis were used for preparation HA ceramics: sol-gel processes [13], hydrothermal synthesis [14] and solid state reaction method [15].In comparison to these methods, direct precipitation from aqueous solution provides an easier way of preparing HA ceramic and at the same time provides a large amount high purity material [16][17][18].
Furthermore, ion substituted calcium phosphate materials can be prepared using simple chemical precipitation method, which is of great importance when it comes to obtaining a solid ionic conductor for use in IT-SOFCs, the advantages of this method include simple synthesis, low cost and ability to obtain large quantity and high purity of solid electrolyte [18].
In recent years, ionic substitutions of Mg 2+ , Na + and Sr 2+ in HA [11,12] proved very interesting for their potential implementation in IT-SOFCs, owing to the morphology, thermal stability, and structural and mechanical properties [19].Low-temperature treatment (400-500 ºC) allows preserving the particle morphology and developing nanoscale structures i.e. grains [20,21].In order to obtain compact material densification process usually requires sintering at temperatures at or above 1000 ºC, however, this leads to the formation of high temperature calcium phosphate ceramics [22][23][24].Knowledge of thermal stability, ionic transport properties and possible mechanisms of conduction properties of different calcium phosphate materials provides a key for their application in IT-SOFCs [11,12].In this study, we show Mg 2+ , Na + and Sr 2+ HA materials as suiTab.candidates for ionic conductive materials.Materials were treated thermally in air atmosphere at 400 ºC (CaP 400 , MgCaP 400 and NaSrCaP 400 ).Additionally, behavior doped CaP after sintering at 1000 ºC during 3 h (CaP 1000 , MgCaP 1000 and NaSrCaP 1000 ) was studied.The main objective was to obtain a range of dense solid solution calcium phosphate materials, with suiTab.morphology and grain size for the application in Intermediate Temperature Solid Oxide Fuel Cells.

Synthesis of CaP, CaMgP and CaSrNaP samples
Samples were prepared by solution precipitation method.Pure hydroxyapatite (CaP) was prepared by drop wise addition of 250 ml 0.3 M NaH 2 PO 4 +H 2 O into 250 ml Ca(OH) 2 , 0.5 M solution which was magnetically stirred.The pH was adjusted to 11 using NH 4 OH, at temperature about 100 ºC.To synthesize Mg (CaMgP) and NaSr (NaSrCaP) substituted hydroxyapatite materials we used slightly modified precipitation method.To obtain MgHA, 250 ml NaH 2 PO 4 +H 2 O, 0.3 M was heated at 100 ºC and added drop wise into mechanically stirred mixture of 250 ml Ca(OH) 2 , 0.5 M and 250 ml MgCl 2 •6H 2 O, 0.25 M heated to about 100 ºC, pH was adjusted to 12 using NH 4 OH solution.For NaSrCaP synthesis, mixture of 250 ml of (CH 3 COOH) 2 Ca•H 2 O, 0.03 M and 250 ml Sr(CH 3 COO) 2 , 0.03 M was added drop wise into 500 ml NaH 2 PO 4 +H 2 O, 0.3 M at room temperature.The precipitates were washed, centrifuged at 3000 rpm and dried at 70 ºC for 17 h.Dry powders were thermally treated at 400 ºC during 3h in air atmosphere, and then uniaxially pressed at 105 MPa into compact pelets (8 mm diameter), and sintered at 1000 ºC for 3 h in air atmosphere.

Densification of CaP, CaMgP and CaSrNaP samples
Samples were uniaxially pressed at 105 MPa and thermally treated.Sample annealing was carried out at 1000 ºC for 3 h in an air at a heating rate of 5 ºC/min.The resulting density was determined via Archimedes' method.Densified samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and complex impedance methods.

Characterization of CaP, CaMgP and CaSrNaP samples
Phase composition before and after thermal treatment as well as crystallite size evolution were analyzed by X-ray diffraction (XRD) using Ultima IV Rigaku diffractometer, equipped with CuKα radiation, using a generator voltage 40.0 kV and a generator current 40.0 mA.The range of 5-80° 2θ was used in a continuous scan mode with a scanning step size of 0.02° and count time 2°/min.Analysis was done using PDXL2 software (version 2.0.3.0)[25], with reference to ICDD database [26].For quantitative phase analysis, RIR method was used.Structural modeling was done using VESTA program [27].Spectroscopic studies of the synthesized and thermally treated materials were carried out in the mid infrared (MIR) regions (4000-400 cm -1 ) using Fourier transforms infrared (FTIR) spectroscopy, in transmission mode by a Perkin Elmer Spectrum Two FT-IR spectrometer using the pressed KBr compacts (1:100) technique.
The morphology and chemical composition of samples were identified using a JEOL JSM-6610LV Scanning Electron Microscope with an X-Max Energy Dispersive Spectrometer.Samples were coated with gold using a BALTEC-SCD-005 sputter coating device, and the results were recorded under high vacuum conditions.The electrical conductivity of sintered samples was measured by complex impedance method, in a frequency range 1 Hz-0.1 MHz, using Interface 1000 Potentiostat/ Galvanostat/ ZRA and EIS300 Electrochemical Impedance Spectroscopy Software.The measurements were conducted in air, in the temperature range 500-700 ºC, with a 50 ºC increment.The amplitude of the applied sinusoidal voltage signal was 20 mV.Thin layer of high conductivity silver paste was applied onto both sides of the sample pellets in order to provide good electrical contact between electrolyte and electrodes.The samples were placed between the silver plates in a ceramic holder which was heated by vertical oven.A Pt-Rh thermocouple located just below the bottom silver plate was used for temperature monitoring.The impedance plots obtained experimentally were fitted by means of the software ZViews for Windows (Version 3.2b).The resistance values were determined from the impedance diagrams recorded at various temperatures.The specific conductance was calculated from the resistance data using the dimensions of the sample pellets.
XRD diffraction patterns of Mg doped HA are shown in Fig. 2. Analyses show almost identical patterns as pure HA powder, however, peaks are shifted compared to pure HA material (Fig. 1), suggesting that Mg 2+ cations were incorporated in the structure during thermal treatment at 400°C.XRD of CaMgP 1000 sample shows that heating to 1000 ºC leads to the formation of high temperature magnesium phosphate phases Mg 2 P 2 O 7 (ICDD: 00-008-0038, marked as open circles) and β-Ca 2 P 2 O 7 (ICDD: 01-071-2123, marked as solid circles).
It is evident that addition of magnesium to the HA structure leads to the formation of high temperature solid solution that includes high temperature CaMgP phases and hydroxyapatite.CaMgP 100 nominal phase composition calculated using PDXL2 software showed 55 % β-Ca 2 P 2 O 7 , 30 % Mg 2 P 2 O 7 and 15 % hydroxyapatite phase.The diffraction pattern of powder CaSrNaP (Fig. 3) reveals that the dried precipitate exhibits poor crystallinity, peaks around 10º and 18° 2θ (marked as inverted triangles) indicate secondary phase with chemical composition Sr 3 (P 3 O 9 ) 2 (H 2 O) 7 (ICDD:01-070-0007).The secondary phase existence is common during precipitation method of nano material synthesis [28].However, there is no evidence of the existence of secondary phase in thermally treated CaSrNaP 400 (Fig. 3).CaSrNaP XRD peaks are slightly shifted to the left implying a larger unit cell most likely due to the incorporation of Sr into the HA lattice (Fig. 3).Thermal treatment of hydroxylapatite materials below 700 ºC leads to surface reduction and single phase particle coalescence without densification [24,29].At 1000 °C, high temperature solid solution forms noted as CaSrNaP 1000 .Sintering at this temperature, with retention time of 3 h, high  Tab.I Unit cell parameters and crystallite sizes of obtained materials.Tab.I shows variation of lattice parameters due to structural substitution during precipitation and calcinations at 400 °C.Unit cell parameters and crystallite size calculations of were performed using cif.files from American Mineralogist Crystal Data Structure Base (AMCDSB) [31] (Tab.I).

Samples
There is no significant difference in the average crystallite size of CaP and CaP 400 samples (Tab.I).Unit cell parameters of both samples are in similar to pure hydroxyapatite (ICDD: 01-071-5048).With the increase of temperature unit cell parameter a increases while the parameter c decreases.Average crystallite size of CaMgP material increasing with temperature, which is consistent with peak shapes of obtained material.The unit cell parameters of CaMgP are higher than of CaP material, which is in correlation with published data [32].Average crystallite size of thermally treated samples is slightly higher than synthesized.Factors that influence peak broadening are the presence of amorphous phase during precipitation and a number of dislocations between crystallite boundaries [16,33].During calcination at 400 ºC the material becomes crystalline and thermodynamically sTab.allowing for better arrangement and growth of crystallites [34].The unit cell parameters of CaSrNaP and CaSrNaP 1000 have the highest value out of all examined materials, as the ionic radii of Sr 2+ (1.13 Å) and Na + (1.52 Å) are larger than ionic radii Ca 2+ (0.99 Å) and Mg 2+ (0.69 Å).Anisotropic change in unit cell dimensions could be attributed to the replacement of Ca 2+ with other cations, i.e.Mg 2+ , Na + and Sr 2+ in structure [35,36].

FTIR analysis
FTIR spectra of untreated (synthesized) samples: CaP, CaMgP and CaSrNaP (Fig. 4) showed the presence of characteristic apatite signals (around: 563, 610, 657, 1030, 1077 cm -1 ) Fig. 4b [36].The absorbed water band is around 3500 cm -1 , and the band at 1413 corresponds to CO 3 2-group vibration [18].In comparison with FTIR of stoichiometric HA, bands of CaSrNaP and CaMgP are red-shifted because of the presence of foreign ions incorporated into the structure [37][38][39].Absence of the 631 cm -1 band that belongs to vibration modes of the apatitic OH groups in all tree spectra most likely indicates a lack of hydroxyl content and a possible substitution of hydroxyl groups by carbonate groups [37].Furthermore, the FTIR spectrum of CaMgP shows a slight indication of 1410 cm -1 carbonate band (Fig. 4a) thus indicating no significant presence of CO 3 group in CaMgP powder [40].Weak band of MgHCO 3 can be noticed around 1634 cm -1 [36].CaSrNaP band at 1385 cm -1 indicates incorporation of Sr 2+ and Na + ions into the structure of hydroxylapatite (Fig. 4c) [35,38].A band that indicates carbonate group (coming from the reaction medium) in the structure of CaSrNaP is demonstrated by bands at 1417 and 1639 cm -1 [41].FTIR spectra confirm previously described XRD results, that using acetate solution multi doped hydroxyapatite material can be obtained.
FTIR spectra of thermally treated samples at 1000 ºC for 3 h in air are shown in Fig. 5, also shows presence of the characteristic signals of apatite [36].CaMgP 1000 (Fig. 5a) sample shows bands in spectral regions 1470-870 cm -1 and 605-560 cm -1 which are generally related to P-O species with different phosphorous configuration [42,43].Characteristic vibrations of Mg 2 P 2 O 7 phase are bands at 1050, 605 and 560 cm -1 [42].The bands at around 1050 cm -1 belong to triply degenerate asymmetric P-O stretching mode, while the band around 955 cm -1 belongs to P-O stretching mode, these bands belong to β-Ca 2 P 2 O 7 phase [43].There is no evidence of any characteristic hydroxyl group vibrations (631 and 3572 cm -1 ).CaP 1000 (Fig. 5b) bands are weaker than in untreated samples indicating lack of water vibrations in structure.The bands around 600 and 580 cm -1 are attributed to different types of P-O-P bending mode vibrations, which belong to β-Ca 2 P 2 O 7 [43].Figure 5c shows vibration spectrum of CaSrNaP 1000 material.Presence of previously described P-O and P-O-P vibrations which belong to β-Ca 2 P 2 O 7 phase also belong to Sr 2 P 2 O 7 [43,44].The band around 740 cm -1 belongs to P-O-P bridge stretching vibration, a striking feature of (P 2 O 7 ) 4-ions in Sr 2 P 2 O 7 .FTIR spectrum of CaSrNaP (Fig. 5c) shows no evidence of O-H stretching modes at around 3500 and 1600 cm -1 [44].Metal-oxygen vibrations in these types of systems are generally too low to be seen in the 400-4000 cm -1 region.Based on FTIR analysis, at 1000 °C high temperature CaMgP 1000 material is obtained.All of identified vibrations that belong to CaP 1000 material (Fig. 5b) confirm that β-Ca 2 P 2 O 7 is obtained by sintering hydroxyapatite for 3h on 1000°C in air atmosphere.The analysis of CaSrNaP 1000 spectrum (Fig. 5c) confirms absence of O-H vibrations and presence of (P 2 O 7 ) 4- vibrations which is unique for X 2 P 2 O 7 types of compounds [43,44].

Morphology (SEM analysis) and element composition (EDS analysis)
SEM analysis confirms that all obtained samples using precipitation method have crystallite size of nanometer dimensions.In addition, SEM images (Fig. 6) show that the particle size in all samples increases with the increase of temperature.Namely, nanoparticles have a natural tendency to agglomerate for two reasons: firstly−the agglomeration is a more sTab.configuration from an energetic point of view, and secondly−the agglomeration allows the crystallite growth [45,46].Sintering at 1000 ºC in air for 3 h led to a higher degree of densification of CaSrNaP 1000 (98 % TD) than of CaMgP 1000 (86 % TD).The main reason for the differences in density may be pore and particle size distribution, i.e. different morphology (Fig. 6).The samples after calcinations at 400 ºC during 3 h show more uniform particles and well ordered structure.Calcinations was done in order to allow better packing and lower inner activity, and it was expected that these samples may achieve higher density after sintering.This was achieved for CaSrNaP 1000 sample, where the grains are uniformly sintered and the contacts between grains show hexagonal forms and the inserted section clearly demonstrates that we have indeed obtained dense high temperature CaSrNaP material (Fig. 6c).On the other hand, CaMgP 1000 (Fig. 6b) exhibits grain growth due to formation of agglomerated grains.Densification retards or inhibits wide pore distribution, during densification the big pores became larger while small pores shrink and disappear [47,48].SEM micrograph of CaMgP 1000 (Fig. 6b) shows numerous agglomerates of hexagonal particles with plate like morphology bonded together, with average grain size between 2 and 5 µm with non-uniform particles with many vacancies, less compact particle packing, higher inner activity.To confirm the chemical composition, the EDS microanalysis was done on the CaP 1000 , CaMgP 1000 and CaSrNaP 1000 sintered samples (Fig. 6).It revealed that chemical composition very similar to nominal chemical composition.According to semi quantitative chemical analysis, the mean values for the ratios Ca/P/O for CaP 1000 sample determined from four runs are 2.1/2/7.5.These ratios correspond to empirically calculated compound with nominal composition-(Ca 2.1 P 2 O 7.5 ).The mean value of the chemical ratios Ca/Mg/P/O for CaMgP 1000 was also determined from four runs, and amounts to 1.2/0.7/2/7,which corresponds to empirical compound with following composition-(Ca 1.2 Mg 0.7 ) 1.9 P 2 O 7 .The mean value of the chemical ratios Ca/ Sr Na/ /P/O of the NaSrHA 1000 sample was determined from four runs, and amounts to 0.5/0.6/2/2/7,which corresponds to-Ca 0.5 Sr 0.6 Na 2 P 2 O 7 .These results (Fig. 6) confirm the assumption that sintered samples based on hydroxyapatite at high temperature (1000 ºC) remain sTab.and almost nominal chemical composition.The obtained EDS results are in good agreement with XRD results.

Electrical conductivity
Recently, ionic conductors with the apatite type structures attracted a great deal of attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate in IT-SOFCs, in the intermediate temperature range (600-800 ºC) [49][50][51][52].Namely, hydroxyapatite belongs to apatitic group of phosphate minerals with general formula Me 10 (XO 4 ) 6 Y 2 with ionic substitution ability [53].Based on XRD (Tab.I), unit cell parameters of CaMgP samples are slightly increased compared to pure CaP sample, the contraction of a and c parameter can be attributed to much smaller ionic radius of Mg 2+ than Ca 2+ [54].The change of unit cell parameters a and c of CaSrNaP indicate that Sr 2+ and Na + ions are incorporated in structure, and not just adhered to the crystal where Sr 2+ will firstly replace Ca 2+ ion in Ca2 site [55].On the other hand Na + ion as a smaller than Ca 2+ could possible replace Ca 2+ in Ca1 site while a vacancy may formed at the OH -site [56].At 1000°C structural transformation of hydroxyapatite occurs and it transforms to a high temperature mixed phosphate ceramics with M 2 X 2 O 7 structure where X ions are tetrahedral coordinated (Fig. 7.) The crystal structures of calcium, strontium and sodium phosphate materials are comprised of diphosphate ions which are linked to metallic ions to form a three dimensional network in β -Ca 2 P 2 O 7 , α-Sr 2 P 2 O 7 and Na 4 P 2 O 7 structures [57,58].A common characteristic of thermal behavior of all phosphates is that under thermal treatment they undergo dehydratation-condensation reaction which leads to phosphate tetrahedral bonded to mutual oxygen atom.The ion conductivity in these materials occurs mainly via O -interstitials with preferential c-axis conduction [50,51,56,59] in comparison to O 2-vacancy migration in fluorite and perovskite-based electrolytes [59][60][61][62][63].
Generally, the impedance spectra are presented as negative of imaginary component of impedance (-Z imag ) versus real component of impedance (Z real ) i.e., as Nyquist plot.The semicircles at high and intermediate frequency are ascribed to bulk and grain boundary processes, respectively, while semicircle at low frequency represents the electrode process contribution [56].In our case, for the potential application in IT-SOFCs the measurements of ionic conductivity of electrolytes in solid state of the CaMgP 1000 and CaSrNaP 1000 samples were done in intermediate temperature range of 500-700 ºC, with the increments of 50 ºC.The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are presented in Fig. 8.As it could be seen (Fig. 8a and 8b), with increasing the temperature the values of both resistance elements (R b and R gb ) obviously decrease, which causes an increase in ω max .Consequently, the whole region of the impedance points shifts towards the lowfrequency semicircle and at higher temperatures, instead of R b and R gb separately, only the whole sum R b +R gb became readable in the available frequency range.In this case, at higher temperatures, the time constants associated with the bulk and grain boundary impedances are much lower than those associated with the electrode interface.As a result, semicircles due to bulk and grain boundary disappear and only a single semicircle due to electrode interfacial processes can be observed [64,66].Therefore, only the whole sum R b +R ig became readable and the values of total resistance were estimated from the experimental cross section of obtained semicircles with the real component of impedance (Z real ), this intercept is marked by arrows in Fig. 8 (inset).New semicircles are observed to appear in a low-frequency region, being particularly visible in temperature range at 650-700 ºC (Fig. 8b).Almost doubtless, it originates from the oxygen electrode reactions, O 2 /O 2- [64], which does not belong to the scope of this study.The total ionic conductivity of the sintered CaSrNaP 1000 sample is shown in Tab.II.Obtained values (at 700 ºC amount to 4.18×10 -3 Ω -1 cm -1 ) are slightly higher than previously published ones [64,66].More specifically, values of ionic conductivity observed at 700 ºC in this work, are similar to values obtained at 800 and 900 ºC [67][68][69][70][71].This holds also if we compare the results we found at lower temperatures with the literature data [64][65][66][67].
Tab. II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP 1000 and CaSrNaP 1000 sample.
temperature calcium, and strontium and sodium phosphate materials developed.It is evident that peaks belongs to three different phosphate phases: β -Ca 2 P 2 O 7 (01-071-2123) α-Sr 2 P 2 O 7 (ICDD: 01-072-149) and Na 4 P 2 O 7 (ICDD: 01-073-5982).Strontium and sodium phosphate phases are orthorhombic while the calcium phosphate phase is tetragonal.Structural changes and phase transitions during sintering (temperatures above 700 °C) can be explained by the loss of hydroxyl ions from the hydroxyapatite structure at temperatures around and higher than 1000 °C and the formation of orthorhombic and tetragonal phosphate phases[24,30].The peaks on XRD diagram are narrow and well defined indicating high structural arrangement of sintered material.Quantitative phase analysis of CaSrNaP material showed 74% of α-Sr 2 P 2 O 7 , 16% of β -Ca 2 P 2 O 7 and 10% of Na 4 P 2 O 7 .Hydroxylapatite is still evident at high temperatures in the case of pure HA and HA doped with Mg.On the other hand, in the case high temperature treatment (1000 °C for 3h) of multi-doped HA leads to the formation of mixed CaSrNaP phase, with high structural arrangement.

Fig. 7 .
Fig. 7. Structures of M 2 X 2 O 7 : a) α-Sr 2 P 2 O 7 , b) β-Ca 2 P 2 O 7 and c) Na 4 P 2 O 7 ; (the green spheres are Sr atoms, blue spheres are Ca atoms, the yellow spheres are Na atoms, the red spheres are O atoms and the purple spheres are P atoms).

Fig. 8 .
Fig. 8. (a)(b) Complex impedance plots of the sintered samples CaSrNaP 1000 measured in the temperature range from 500 to 700 ºC, in air atmosphere.The arrows indicate the points on the real axis corresponding to the readings R b +R gb .