Colloidal Processing and Characterization of Aluminum-DopedLanthanum Oxyapatite, La10AlSi5O26.5

Isabel Santacruz,w,z Jose M. Porras-Vazquez,z Enrique R. Losilla,z M. Isabel Nieto,y Rodrigo Moreno,y

and Miguel A.G. Arandaz

zDepartamento de Quımica Inorganica, Cristalografıa y Mineralogıa, Universidad de Malaga, 29071 Malaga, Spain

yInstituto de Ceramica y Vidrio, CSIC, 28049 Madrid, Spain

Lanthanum apatite is one of the most promising materials aselectrolytes for intermediate-temperature solid oxide fuel cells(IT-SOFC), those operating close to 7001C. The complexity ofmicrostructures, composition, and geometries of SOFC devices,make it necessary to have a precise control of processing pa-rameters to obtain the desired combination of properties. Thiswork involves the preparation and optimization of La10AlSi5O26.5 materials by reaction sintering of the raw materials(La2O3, Al2O3, and SiO2) to be used as IT-SOFCs electrolytes.Homogeneous mixed suspensions of those ceramic oxides wereprepared. A heterocoagulation process was followed for ensuringa better reactivity during sintering. All the parameters involvedin the process, such as deflocculant content, particle size of theinitial powders, sonication time of the mixed suspensions, com-paction, and sintering cycle were optimized. The effect of allthese parameters were studied and discussed. Finally, La10AlSi5O26.5 electrolytes with density values of B88% of the the-oretical density and total conductivities of 1.7� 10

�2S/cm at

7001C were obtained. This study opens a path for the prepara-tion of concentrated suspensions for obtaining dense thin films.

I. Introduction

SOLID oxide fuel cells (SOFCs) electrolytes should meet nu-merous requirements, in particular a high ionic conductivity,

minimum electronic transport, moderate thermal expansion,and thermodynamic stability in wide ranges of oxygen partialpressure and temperature.1 One of the most used electrolytes isyttria-stabilized zirconia,2,3 which is well known as an excellentoxygen-conducting electrolyte at temperatures beyond 10001C.However, the costs associated with raw materials, processingtechnologies, and working temperatures should be as low aspossible. Because of this, the use of intermediate-temperaturesolid oxide fuel cell (IT-SOFC) components has a great impor-tance to decrease its cost by reducing the working temperature,the thermal stresses, to improve the lifetime of the system and toreduce the price of the interconnector materials. Hence, the de-velopment of new electrolytes with high ionic conductivities at7001C is required. Oxyapatites seem to be very promising to beused as oxide ions conductors at intermediate temperatures(7001–8001C) due to the presence of large conduction channelscontaining interstitial oxide ions.4–6 The general formula for anapatite-type material7 can be expressed as A10(TO4)6X2, whereA is generally a large cation (alkaline earth and/or rare earth

metals), TO4 is a tetrahedral group (Ge, P, V, As, etc.), and Xis an anion like O, OH, F, etc.8 The electrical properties of ox-yapatites have already been reported with detailed studies aboutthe electrical properties of substituted oxyapatites9,10; for exam-ple, aluminum doping on the Si site increases the conductivity ofthe material in comparison with undoped lanthanum silicates.11

The apatite structure is highly anisotropic and the interstitialoxygen migration along the c-axis is much larger than in the abplane. Electrical measurements in apatite single crystals showedconductivities along the c-axis three-times larger than in the abplane.9,12

One of the main limitations of oxyapatite-type materials isrelated to their poor sinterability.4 Many researchers have fo-cused their efforts in this key subject by obtaining apatite ma-terials through different routes, such as solid-state reaction,1,13

sol–gel,14,15 hot-pressing techniques,16 previous mechano-syntheses17,18 facilitating further processing (i.e., colloidalprocessing),19 or precipitate method combined with an azeotro-pic-distillation process.20

In colloidal processing, when two different species with op-posite charges coexist in a suspension,21,22 electrostatic attrac-tion between them occurs thus leading to heterocoagulation orcoagulation. However, heterocoagulation can be a suitable pro-cedure to obtain core-shell structures23 and to coat bigger par-ticles with smaller ones24; this has also been successfully used forcoating big nonspherical particles, such as carbon nanotubes,CNTs, of smaller particles.25

This work deals with the preparation and optimization ofLa10AlSi5O26.5 electrolytes by colloidal processing. Mixed sus-pensions of La2O3, Al2O3, and SiO2 were optimized, and aheterocoagulation process was followed to ensure a better reac-tivity during reaction sintering. Colloidal processing was usedfor improving the densification and reproducibility of the finalsamples, and hence, their electrical properties. All the involvedparameters, such as deflocculant content, particle size of theinitial powders, sonication time of the mixed suspensions, ap-plied pressure during compaction, and sintering cycle, were op-timized. The effect of these parameters on the final properties,such as composition, density, grain size, and electrical conduc-tivity was studied and discussed. The study of the colloid-chemistry of these suspensions opens a path for the preparationof more concentrated suspensions for obtaining dense thin-films(o100 mm) electrolytes by other colloidal routes to improve theconductivity of the electrolyte inside SOFCs. More work in thisway is being carried out.

II. Experimental Procedure

(1) Synthesis-Processing Conditions

La10AlSi5O26.5 samples were prepared by reaction sintering us-ing high-purity oxides: micrometer-sized powders of La2O3

(Sigma-Aldrich, Steinheim, Germany), g-Al2O3 (Alfa Aesar,Ward Hill, MA), and SiO2 (quartz, ABCR, Karlsruhe,

T. Gur—contributing editor

This work was supported by the Spanish Ministry of Science and Innovation andSpanishMinistry of Education through project MAT2009-07016, Ramon y Cajal fellowship(RYC-2008-03523), and AP2005-2462 studentship.

wAuthor to whom correspondence should be addressed. e-mail: isantacruz@uma.es

Manuscript No. 27602. Received February 24, 2010; approved July 14, 2010.

Journal

J. Am. Ceram. Soc., 94 [1] 117–123 (2011)

DOI: 10.1111/j.1551-2916.2010.04074.x

r 2010 The American Ceramic Society

117

Germany). Nanosized powders of SiO2 (Aeroxide 200, DegussaEvonik, Hanau, Germany), and g-Al2O3 (Aeroxide Alu C, Deg-ussa/Evonik, Hanau, Germany) were also used. Lanthanum ox-ide was precalcined at 10001C for 2 h for decarbonation. Thepowders were characterized in terms of surface area by onepoint nitrogen adsorption, density by He pycnometry, and par-ticle/agglomerate size by scanning electron microscopy (SEM),transmission electron microscopy (TEM), and by laser diffrac-tion. Suspensions of every powder were prepared separately indried absolute ethanol (99.7% grade, Panreac, Barcelona,Spain), and optimized using different quantities of a polymericdispersant that is soluble in ethanol (Hypermer KD6, Uniqema,Wirrall, U.K.), up to 3.0 wt% in dry solid based. The zeta po-tentials were measured on dilute suspensions, 0.1 wt% solidscontent, with and without dispersant, by microelectrophoresis(Zetasizer NanoZS, Malvern Instruments, Malvern, U.K.).Eight aliquots of every diluted suspension were taken for zetapotential measurements.

The suspensions with the stoichiometric mixtures of oxideswere prepared to a solids loading of 5 wt% (1 vol%) by mag-netic stirring for 15 min with the addition of the optimumamount of deflocculant. The effect of the particle size of thepowders was studied by comparing suspensions with microme-ter-sized Al2O3 and SiO2 powders (called Micro in the text), onlynanosized Al2O3 and SiO2 powders (called Nano in the text), ora combination of both particle sizes in a ratio of 50/50 for eachpowder (called Micro-Nano in the text). In all cases, micro-meter-sized calcined La2O3 powder was used.

The so-prepared suspensions (50 mL) were ultrasonically ho-mogenized (noted as U in the text) using a 400 W sonicationprobe (UP400s Hielscher Ultrasonics GmbH, Stuttgart, Ger-many) for 2 or 5 min, noted as U2 and U5. Suspensions werecooled in an ice-water bath during ultrasonication in order toavoid excessive heating. Then, the ethanol was evaporated for24 h in an oven at 651C. The dried powder was sieved through

Table I. Physical Properties of the Raw Materials

As-received

La2O3

Calcined La2O3

(10001C/2 h) g-Al2O3 SiO2 g- Al2O3 (Nano)w SiO2 (Nano)w

Purity (%) 99.999 99.999 99.997 99.8 99.6 99.8Particle/agglomerate size 4.8 mmz 4.8 mmz Large agglomeratesz

dv50 5 50 mmdv50 5 2.3 mmz

Bimodal: 0.2 and 3 mm13 nm 12 nm

Ss (m2/g) 1.9 0.9 51.2 8.4 100 200

Measured density(g/cm3)

5.6 5.6 3.5 2.6 — —

wValues provided by the supplier. zMeasured by laser diffraction.

Table II. Zeta Potential Values of the Diluted Suspensionswith Different Deflocculant Contents

Powder KD6 (wt%) Zeta potential (mV)

Calcined-La2O3 0 �12771.0 52731.5 60742.0 66742.5 64753.0 3976

Micrometersized-SiO2 0 �107761.0 �151762.0 �161743.0 �14873

Nanosized-SiO2 0 �41711.0 �42712.0 �42723.0 �4471

Micrometersized-Al2O3 0 12721.0 22712.0 23723.0 4171

Nanosized-Al2O3 0 �14711.0 �18722.0 �19713.0 �1571

I IIIII IV V

I II III IV V

−0.10

I IIIII IV V

I IIIII IV V

1600−0.30

−0.25

−0.20

−0.15

−0.10

−0.05

0.00

0.05

Temperature / °C

ΔL/L

−0.0008

−0.0006

−0.0004

−0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

I IIIII IV V

I II III IV V

−0.35

−0.30

−0.25

−0.20

−0.15

−0.10

−0.05

0.00

0.05

Temperature / °C

ΔL/L

−0.004

−0.003

−0.002

−0.001

0.000

0.001

0.002

d.ΔL

/L

I II III IV V

I IIIII IV V

−0.20

−0.15

−0.05

0.00

Temperature / °C

ΔL/L

−0.0010

−0.0008

−0.0006

−0.0004

−0.0002

0.0000

0.0002

0.0004

0.0006

d. Δ

L/L

d. Δ

L/L

I IIIII IV V

0 200 400 600 800 1000 1200 1400

16000 200 400 600 800 1000 1200 1400

16000 200 400 600 800 1000 1200 1400

a

b

c

Fig. 1. Dynamic sintering measurements for: (a) Micro, (b) Micro-Nano, and (c) Nano samples. Ultrasound time: 2 min, applied pressure:250 MPa.

118 Journal of the American Ceramic Society—Santacruz et al. Vol. 94, No. 1

a 100 mm mesh, and pressed at two different pressures: 250and 640 MPa into 10 mm diameter and B1 mm thicknesspellets.

Dynamic sintering studies were performed with a differentialdilatometer (Setsys 16/18, Setaram, Caluire, France) up to15001C. A two-step sintering cycle was followed, where T1

(10001 or 11201C) and T2 (14501, 15001, or 16001C) were opti-mized; t1 and t2 were fixed at 2 and 10 h, respectively. Compac-tion was calculated taking into account the pellet mass, volume,and the crystallographic density.

(2) Powder Diffraction

All compounds were characterized by high-resolution labora-tory X-ray powder diffraction (LXRPD) at room temperature.Powder patterns were collected using a Philips X’Pert Pro MPD(Almelo, the Netherlands) automated diffractometer equippedwith a Ge(111) primary monochromator (strictly monochro-matic CuKa1 radiation) and an X’Celerator detector (PANalyt-ical B.V., Almelo, the Netherlands). The studied 2y range was101–1301 with a 0.0171 step size. Rietveld26 analyses were carriedout using the GSAS27 suite of programs.

(3) Electrical Characterization

Electrodes were made by coating opposite sintered pellets faceswith METALOR

s

6082 platinum paste (Neuchatel, Switzer-land). The impedance data were collected using a Hewlett-Pack-ard 4284A impedance analyzer (Miami, FL) over the frequencyrange of 20 Hz–1 MHz from 3001 to 10001C. Measurementprocesses were electronically controlled by the winDETA pack-age of programs.28 The pellets were mounted in a home-made

alumina conductivity jig, with four Pt wires shielded in two al-umina tubes, which were placed in a tubular furnace. Electricaldata were taken every 501C. A delay time of 60 min at eachtemperature was selected to ensure thermal equilibrium. Tem-peratures were reproducible to 711C.

(4) Microstructural Characterization

The morphology of the sintered pellets was studied by SEM(JEOL SM 840, Tokyo, Japan). The ceramic surfaces were pol-ished and thermally etched. The grain sizes of the best sinteredpellets were estimated from SEM micrographs using the linearintercept method from at least 30 random lines with the help ofimage-analysis software.

III. Results and Discussion

The physical properties of the powders are shown in Table I.From SEM and TEM observations, it was seen that, in general,commercial micrometer-sized powders (La2O3, SiO2, and Al2O3)are agglomerated; micrometer-Al2O3 powders are also formingaggregates larger than 100 mm. The particle/agglomerate valueswere confirmed by laser diffraction (Table I). SiO2 and Al2O3

nanosized powders show a primary particle size of about10–15 nm, and a high value of specific surface due to their smallsize.

To obtain homogeneous suspensions, the deflocculant con-tent was optimized for every powder separately. During pro-cessing, La2O3 might transform into La(OH)3 in the presence ofwater,29 and then, a carbonation process of the surface ofLa(OH)3 might also occur through the formation of hydroxy-

Fig. 2. Scanning electron micrographs (cross section) of Micro-Nano samples prepared by applying 250 MPa (a and b), and 640 MPa (c and d) at twomagnifications. Ultrasound time: 2 min, sintering cycle: 10001C/2 h–15001C/10 h.

January 2011 Colloidal Processing and Characterization of Oxyapatites 119

carbonates.30 To avoid this effect or, at least to minimize itsconsequences, the humidity of ethanol was reduced by the ad-dition of zeolites into the ethanol bottle before use.

Zeta potential values (z) of the 0.1 wt% suspensions withdifferent deflocculant contents are shown in Table II. The differ-ences in zeta potential values for some powders after the addi-tion of deflocculant are indicative of its adsorption onto particlesurfaces; under these conditions, the best stabilization is ob-tained after the addition of 2.0 wt% surfactant for La2O3, wherethe maximum absolute value (6674 mV) was achieved. In thecase of micrometer-sized SiO2 and Al2O3 powders, maximumabsolute zeta potential values were achieved after the addition of2.0 and 3.0 wt% KD6, respectively. For nanosized SiO2 andAl2O3 powders, no relevant zeta potential improvements wereobserved after the deflocculant addition, and thus no extradispersant was added for the stabilization of the mixed suspen-sion. The apparent pH of these diluted suspensions lies between8 and 10.

Mixed suspensions (Micro, Micro-Nano, and Nano) wereprepared by the addition of the optimized amount of dispersantfor every powder and were ultrasonically homogenized (2 and5 min). Under these conditions, La2O3 and micrometer-sizedAl2O3 particles will be charged positively after the adsorption of2 and 3 wt% deflocculant, respectively. However, the restof the powders will be charged negatively under the selectedconditions (Table II), favoring the heterocoagulation process.23–25

It is expected that these attractive forces will help the bondingbetween particles with opposite charge and the subsequent re-action sintering. In the case of Nano suspensions, both nano-sized powders will be attracted by micrometer La2O3 particlesfacilitating the oxyapatite formation. In all the cases, theheterocoagulation process occurs, and samples with the desiredphase (La10AlSi5O26.5) were obtained. After the ethanol evapo-ration, the dried powder was compacted at two different pres-sures for obtaining the green samples.

Figure 1 shows the dynamic sintering measurements of thepressed samples (250 MPa) obtained from 2-min ultrasonicatedMicro, Micro-Nano, and Nano suspensions (Figs. 2(a)–(c), re-spectively). A five-step experimental shrinkage was observedduring heating. The first two steps (noted as I and II in thefigures) occurred around 3501 and 7001C, and comprise the

dehydratation process of lanthanum hydroxide and hydroxy-carbonate phases29; it is believed that the hydroxide phase wastotally dehydrated into La2O3 above 5001C. The third step(noted as III) corresponds to the decarbonation of a carbonateintermediate phase at about 7501C, resulting in lanthanum ox-ide, which can now react with SiO2 and Al2O3 to form the de-sired aluminum-doped lanthanum oxyapatite between 10001and 12001C (the fourth onset, IV).20,29 The last shrinkage,14501–15001C, might be attributed to the final densification ofthe samples, which occurs at a lower temperature for Nanosamples, as expected. The largest shrinkage values were obtainedfor IV and V onsets. No higher temperatures were measured dueto equipment specifications. Table III summarizes the effect ofultrasonication time, applied pressure during compaction, andthe sintering cycle onto the properties of the final samples, i.e.compaction (percentage of theoretical density (% TD)) and sec-ondary phases. As it can be observed, a short ultrasonicationtime (2 min) provides denser samples than longer times. Forexample, the density increases from B68% to B88% TD for

Table III. Density and Secondary Phases of Sintered Pellets Prepared Under Different Ultrasonication Times (U), Applied Pressures(P), and Sintering Cycle

Sample U (min) P (MPa)

Sintering cycle

Density (% TD) La2SiO5 (wt%) LaAlO3 (wt%)T1 (1C) T2 (1C)

Microw 2250 1000

1450 52 23.9 (7) 3.6 (4)1500 55 5.9 (6) 4.4 (3)

5 1450 Powder Not quantified Not quantified1500 Powder Not quantified Not quantified

Micro-Nano 2 250 10001450 52 3.9 (4) 0.5 (3)1500 53 — 1.2 (1)

6401000 1500 66 10.6 (9) 1.7 (5)

11201500 66 13.8 (7) 1.3 (2)1550 86 4.4 (8) 1.7 (3)1600 88 — 1.5 (1)

5 250 10001450 49 — 1.6 (4)1500 52 — 1.0 (2)

Nano2 250 1000

1450 61 — 6.6 (2)1500 83 — 4.4 (2)

640 11201500 62 — —1550 58 — —1600 88 — —

5 250 10001450 50 — 2.7 (2)1500 60 — 3.1 (2)

6401000 1500 55 — —

11201500 66 — —1600 68 — —

wThese samples also contain uncombined La2O3.

20 40 50 70 10090800603

I / a

.u.

2θ/°

Fig. 3. Observed (crosses), calculated (full line), and difference curve(bottom) of patterns for Nano La10AlSi5O26.5 sample sintered at11201C/2 h–16001C/10 h. Ultrasound time: 2 min, applied pressure:640 MPa.

120 Journal of the American Ceramic Society—Santacruz et al. Vol. 94, No. 1

Nano samples when 5 or 2 min U are applied, respectively. Thiswas observed in other systems, and was attributed to an ag-glomeration of the particles with long sonication times.31

Shorter times (0 min) produced inhomogeneous samples, andhence they were not further characterized. Pellet compactionimproved by increasing the applied pressure under the sameconditions, viz. from B53% to B66% TD for Micro-Nanosamples, U2, after 250 or 640 MPa, respectively. Figure 2 showsthe SEM micrographs (cross section) of these Micro-Nano sam-ples (2 min U) prepared by applying 250 MPa (Figs. 2(a) and(b)), and 640 MPa (Figs. 2(c) and (d)) at different magnifica-tions. In both cases, a large porosity is observed, which is inconcordance with the low measured compaction values.

The heterocoagulation process and the applied pressure havealso an influence on the presence of secondary phases. AllLa10AlSi5O26.5 pellets were ground and their powder patternswere analyzed by the Rietveld method using the structural de-scriptions reported previously32 for La9.5&0.5(Si5.5Al0.5O24)O2.The occupation factors for lanthanum, silicon, and aluminumsites were conveniently modified to describe the stoichiometry.Only the overall parameters (histogram scale factor, backgroundcoefficients, unit cell parameters, zero-shift error, and peakshape pseudo-Voigt coefficients) and weight fractions of sidephases (if necessary) were refined. Atomic parameters were notoptimized. Final Rietveld figures of merit were good with RF

ranging between 3% and 4%; as an example of this refinement

Fig. 4. Scanning electron micrographs (cross section) of Nano samples sintered at 11201C/2 h–15001C/10 h (a and b), and 11201C/2 h–16001C/10 h(c and d), and Micro-Nano samples (e and f) sintered at 11201C/2 h–16001C/10 h, at two magnifications. Ultrasound time: 2 min, applied pressure:640 MPa.

January 2011 Colloidal Processing and Characterization of Oxyapatites 121

quality, the fit to the X-ray pattern of La10AlSi5O26.5 for Nanosample (U2, 640 MPa, 11201C/16001C, 88% TD), called Nano-88 in the text, is shown in Fig. 3. The cell parameters (a, c, andvolume) for the Micro-Nano sample (U2, 640 MPa, 11201C/16001C, 88% TD), called Micro-Nano-88 in the text, were9.7269(1) A, 7.2208(1) A, and 591.66(1) A3, respectively. ForNano-88 sample, single crystalline phase, the corresponding val-ues were 9.7300(1) A, 7.2260(1) A, and 592.46(1) A3. It must benoted that the reported cell volumes, prepared by reaction sinte-ring, for La10AlSi5O26.5 and La10Al0.5Si5.5O26.75 were 593.0 and589.6 A3, respectively.33 The close agreement between the cellfor Nano-88 sample and the reported value ensures the stoic-hiometry of our colloidal processed sample. Furthermore, Mi-cro-Nano-88 sample shows a slightly smaller cell volume, whichis in full agreement with the small content of side phase, 1.5(1)wt% of LaAlO3 (see Table III). In any case, the cell volumeclearly indicates that the aluminum content is higher than0.80 per chemical formula.

Microsuspensions provided samples with secondary phases,and these samples transformed into powder within 15 days in adesiccator at room temperature. This is believed to be related tothe presence of some uncombined La2O3 that absorbs water,and promotes a large expansion and thus the destruction of thesamples into powder. These compositions were ruled out and nofurther studies were performed. Conversely, Micro-Nano andNano suspensions gave homogenous sintered pellets without orwith secondary phases (LaAlO3 and/or La2SiO5) without pow-der transformation (with no La2O3). Samples without secondaryphases were only obtained under a pressure of 640 MPa. Theabsence of secondary phases in samples obtained from Nanosuspensions is believed to be related with the heterocoagulationprocess, where all the La2O3 particles were coated by the nano-sized SiO2 and Al2O3; this was also helped by a better compac-tion, and hence, diffusion, by applying 640 MPa.

The sintering cycle has an important effect onto the sampledensification. The best results were obtained when T1 and T2

achieved 11201 and 16001C, respectively. It is reported in theliterature that apatite materials obtained by the solid-state re-action usually require high temperatures (15751–17001C) andlong dwelling times16,17; however, temperatures higher than16001C were not studied for economical reasons. Figure 4 showsthe SEM micrographs (cross section) of Nano samples (U2 and640 MPa) sintered at 11201C/15001C (Figs. 4(a) and (b)), and11201C/16001C (Nano-88, Figs. 4(c) and (d)), at different mag-nifications. Comparing these two samples, a higher compactiondegree (fromB62% toB88%TD) after increasing the sinteringtemperature by 1001C was observed. A similar microstructurewas obtained from the corresponding Micro-Nano-88 sample(Figs. 4(d) and (e)). Homogeneous microstructures were ob-tained for Nano and Micro-Nano samples with average grainsizes of 2.3 and 4.6 mm, respectively. In the case of theMicro-Nano-88 sample, it also shows some large grains homo-geneously distributed (15–20 mm large and 8 mm wide). Insummary, the sintering conditions for the pellets led to

specimens not fully dense with compactions ranging betweenB49% and B88% TD (see Table III). The best results wereobtained for Nano-88 and Micro-Nano-88 samples obtainedfrom suspensions after 2 min ultrasonication, pressed at640 MPa, and sintered by following a two step sintering cycleconsisting of a first step at 11201C (2 h) and a second stepat 16001C (10 h).

Impedance spectroscopy was used to determine the electricalconductivity data of pellets with a relative density larger than85% TD. Representative impedance data for Nano-88 at differ-ent temperatures are shown as impedance complex plane plotsin Fig. 5. Similar plots were obtained for the remaining com-positions. At low temperatures, i.e. 3001C, a set of overlappingsemicircles can be observed. The set of semicircles indicates thatseveral effects are involved in the electrical response of the pel-lets, likely due to the grain interior (bulk) and to internal inter-faces (grain boundary and/or porosity). Lower frequencyprocesses were observed, in the form of a spike with an associ-ated capacitance ofB0.03 mF/cm at 20 Hz. Because it is inclinedto the Z0 axis at roughly 451, it indicates a partial-blocking elec-trode response that allows limited diffusion. At higher temper-atures, i.e. 7501C, the spike collapses to a semicircular arc,indicating that oxygen molecules are able to diffuse throughthe entire thickness of the electrode. In summary, the conductingspecies appear to be oxide ions as for many other oxyapatites.

In order to quantitatively estimate bulk and grain-boundaryconductivities of the samples, complex impedance spectra wereanalyzed by nonlinear least squares fittings of equivalent circuitsusing the program Zview.34 Associated errors were high, andhence, the values are not reported. Overall conductivities (sT) ofdense pellets are obtained from the intercept (shown as RT inFig. 5) of the spikes (high frequency end) and/or the arc (lowfrequency end) on the Z0 axis. These values of conductivity wereplotted in a traditional Arrhenius format (Fig. 6), and they fallon a set of straight lines without evidence of proton contributiona low temperature. Total conductivity values at 7001C were1.7� 10�2 S/cm for Nano-88 and Micro-Nano-88 samples; and8.7� 10�3 S/cm for Micro-Nano-86 (B86% TD). These resultsare very promising compared with those reported in the litera-ture.33 Shaula and colleagues reported a total conductivity valueof 3� 10�2 S/cm at 7001C for the same composition (92% TD),

1000/T (K−1)

T (°C)

0.80 1.15 1.50

−4

−3

−2

−1

1000 875 750 625 500 375

logσ

T (S

·cm

−1)

Micro-Nano-1120°C/2h-1550°C/10h

Micro-Nano-1120°C/2h-1600°C/10h

Nano-1120°C/2h-1600°C/10h

Fig. 6. Arrhenius plot of the overall conductivity (logsT) for denseLa10AlSi5O26.5 pellets. Ultrasound time: 2 min, applied pressure:640 MPa. The overall conductivity values for La10AlSi5O26.5, fromShaula et al.,33 are shown as a solid line.

180

300°C

20 Hz 0.03 μF·cm

1 MHz 5.9 pF·cmR

20 Hz 3.2 μF·cm

330 kHz 0.1 nF·cmR

0

60

120

180

0

40

80

120)

-Z''(

kΩ·c

m

Z' (kΩ·cm ) ( 'Z ·cm )( 'Z Ω)

-Z''(

Ω·c

m

750°C

00 40 80 60 120120

Fig. 5. Complex impedance plane plot at two temperatures for NanoLa10AlSi5O26.5 sintered at 11201C/2 h–16001C/10 h (ultrasound time:2 min, applied pressure: 640 MPa). Selected frequency and capacitancepoints are highlighted. The Z0 axis intercept for obtaining the overallconductivity, RT, is highlighted.

122 Journal of the American Ceramic Society—Santacruz et al. Vol. 94, No. 1

using a dry reaction sintering methodology applying a final syn-thesis-sintering step of 16501C/10 h. Their curve is also shown(as a solid line) in Fig. 6 for the sake of comparison. Other ox-yapatites samples, i.e. La9.33(SiO4)6O2, prepared by hot-pressingtechniques at 14001C, show samples with densities of 92% TD16;however, their electrical conductivity values are low (2� 10�4 S/cm at 7001C). When oxyapatite powders are prepared in ad-vance, i.e. by previous mechano-syntheses,17,18 their electricalconductivity values are also generally low. Two regimes withdifferent activation energies are observed as for other apatite-doped materials,35,36 below 7501C, Ea range between 0.85(1) and0.68(1) eV. Above 7501C, Ea are much smaller, range between0.34(1) and 0.52(1) eV. These values are close to those calculatedby Shaula and colleagues, 0.58(1) at high temperature. This cur-vature is usually explained by the presence of a critical temper-ature, Tc,

37 below which the charge carriers are progressivelytrapped into clusters with a decreasing temperature.

IV. Conclusions

Homogeneous lanthanum aluminosilicate electrolytes were pre-pared through a colloidal process based on heterocoagulation,and the final oxyapatite composition was obtained by reactionsintering. Different processing parameters were optimized, suchas the initial powder size, the dispersant content, ultrasoundtime, applied pressure, and sintering cycle. Finally, samples withdensity values of up to B88% TD were obtained with the de-sired composition, which show high conductivities values,1.7� 10�2 S/cm at 7001C, potentially useful to be used as IT-SOFCs electrolytes. These conductivity results are very promis-ing, and it is expected they were increased by increasing thedensity of the samples. Furthermore, overall resistances may bereduced by decreasing the thickness of the electrolyte. This workopens a path for the preparation of more concentrated suspen-sions for obtaining dense thin films directly by different colloidalforming routes. More work in this way is under progress.

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