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Acta Materialia 56 (2008) 3065–3076

The effect of Cr2O3 addition on crystallization and propertiesof La2O3-containing diopside glass-ceramics

Ashutosh Goel a, Dilshat U. Tulyaganov a,b, Vladislav V. Kharton a,Aleksey A. Yaremchenko a, Jose M.F. Ferreira a,*

a Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193 Aveiro, Portugalb Scientific Research Institute of Space Engineering, 700128 Tashkent, Uzbekistan

Received 15 August 2007; received in revised form 31 January 2008; accepted 26 February 2008Available online 18 April 2008

Abstract

We report on the effect of La2O3 and Cr2O3 on the structural, thermal and crystallization behavior of diopside-based glasses and onthe processing, microstructure and properties of the sintered glass-ceramics (GCs). The structural features and thermal behavior of theglasses were investigated by Fourier transform infrared spectroscopy, density measurements, dilatometry and differential thermal anal-ysis. The sintering, crystallization, microstructure and properties of the GCs were investigated under different heat treatment conditions(800–1000 �C; 1–300 h). The good matching of thermal expansion coefficients and the strong, but not reactive, adhesion to yttria-stabi-lized zirconia and Crofer 22 APU alloy, in conjunction with a low level of electrical conductivity, indicates that the investigated GCs aresuitable candidates for further experimentation as solid oxide fuel cell sealants.� 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Non-metallic glasses (silicates); Sintering; Crystallization; Fuel cell materials; Electrical resistivity/conductivity

1. Introduction

Ceramics and glass-ceramics (GCs) based on pyroxeneshave attracted interest in several advanced fields [1–6] dueto the excellent controllability of their properties. Diopside(hereafter abbreviated Di) is an important member of theclinopyroxene group with composition CaMgSi2O6. Itforms a complete solid solution (s.s.) series with hedenberg-ite (CaFeSi2O6) and augite (hereafter abbreviated Aug),and partial solid solutions with orthopyroxenes, pigeoniteand a Ca–Al-bearing monoclinic pyroxene, knownas Ca–Tschermak (CaAl2SiO6, hereafter abbreviatedCa–Ts). Several papers have described the structuralfeatures of this family of compounds and the existence ofsolid solutions [5–14]. The solubility limit of Ca–Ts in Diwas found to be about 30 mol.% with crystallization

1359-6454/$34.00 � 2008 Acta Materialia Inc. Published by Elsevier Ltd. All

doi:10.1016/j.actamat.2008.02.036

* Corresponding author. Tel.: +351 234 370242; fax: +351 234 425300.E-mail address: jmf@ua.pt (J.M.F. Ferreira).

experiments in Di–Ca–Ts glasses between 850 and1000 �C for 1 h [5]. From the same study, monomineralGCs of Aug (CaMg0.70Al0.30Si1.70Al0.30O6) were developedfrom glasses with compositions of D/Ts = 80/20, 75/25 and70/30, indicating formation of Di–Ca–Ts s.s. Aug acted asan intermediary member between D and Ts, representing amineral midway between these two minerals (Di and Ca–Ts) along this series, where Al3+ occupies both octahedral(AlO6) and tetrahedral (AlO4) positions in the structure.

The effect of BaO on the crystallization, microstructureand properties of Di–Ca–Ts clinopyroxene-based GCs inthe CaO–MgO–Al2O3–SiO2–(B2O3) system was recentlyinvestigated [6]. The results showed that orthocelsian(OC) and hexacelsian (HC) were crystallized along withAug. The formation of monocelsian was observed in B-containing GCs after heat treatment at 900 �C for 300 h.The density of the GCs increased and the bending strengthdecreased with the increase of BaO content. The electricalconductivity was seen to increase with the BaO content;however, all the GCs exhibited good insulating properties.

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3066 A. Goel et al. / Acta Materialia 56 (2008) 3065–3076

In conjunction with the above-mentioned properties, thelow level of oxygen permeation fluxes under an air/(H2 + H2O + N2) gradient, the negligible interfacial reac-tion with 8YSZ (zirconia stabilized with 8 mol.% yttria)and the good matching of their coefficients of thermalexpansion (CTEs) indicated that the investigated GCshad the potential for further experimentation as sealantsin solid oxide fuel cells (SOFCs). In order to develop a suit-able glass-ceramic sealant it is, however, necessary tounderstand the crystallization kinetics, sealing and electri-cal behavior, and chemical interaction when in contactwith other SOFC components.

The aim of the present study is to investigate the effect ofLa2O3 and Cr2O3 on the thermal behavior, crystallizationand the properties of Di-based glasses and GCs. La2O3 isknown to control the viscosity and CTE of the silicateglasses and GCs [15], while Cr2O3 additions decrease thecrystallization temperature by acting as a nucleating agent[16–18] and reduce the surface tension of the glass [19].Furthermore, possible diffusion of chromium from stain-less steel or ceramic LaCrO3-based SOFC interconnectsinto the sealant may have a strong influence on adhesionand electrical properties [20]. Chromium poisoning ofSOFC electrodes is considered as one of the major degra-dation mechanisms in SOFCs [21], but is very difficult toavoid when Cr-containing interconnects are used. There-fore, La2O3 was added with the aim of altering the thermalcharacteristics of the glasses and the resultant GCs,whereas Cr2O3 was introduced with the primary aim ofassessing its influence on the physicochemical and electricalproperties. The latter information is necessary for potentialutilization of the expected positive effects of Cr2O3 addi-tions, namely reduced crystallization temperature andimproved flow behavior of the glasses, in the GC sealantsfor SOFCs where Cr-containing stainless steel or dopedLaCrO3 interconnects are used. Accordingly, Table 1 pre-sents four new glass compositions investigated in the pres-ent work. The composition 7A was derived by the complexsubstitution of 0.2CaO + 0.1SiO2 M 0.1BaO + 0.05-La2O3 + 0.05Al2O3, while glass 7B was derived by molarsubstitution 0.2CaO + 0.1MgO + 0.2SiO2 M 0.1BaO +0.1Al2O3 + 0.1La2O3 in the pure Di system. The glasses7A-Cr and 7B-Cr were derived by the addition of0.5 wt.% Cr2O3 in glasses 7A and 7B, respectively. NiOof (1 wt.%) NiO was added to all the glasses in order toimprove their adhesion behavior [19].

The experimental results address the potential qualifica-tion and feasibility of the new compositions as candidate

Table 1Batch compositions of parent glasses (wt.%)

Glass MgO CaO

7A: Ca0.8Ba0.1MgAl0.1La0.1Si1.9O6 16.90 18.827A-Cr 16.82 18.727B: Ca0.8Ba0.1Mg0.9Al0.2La0.2Si1.8O6 14.52 17.957B-Cr 14.44 17.86

1 wt.% Excess NiO was added to the batches.

SOFC-sealant materials. The structure, thermal and crys-tallization behavior of the glasses, the microstructure,and the properties of the sintered GCs produced afternon-isothermal (temperatures between 800 and 1000 �Cfor 1 h) and isothermal (900 �C for different times up to300 h) heat treatment are presented. The adhesion featuresof GCs to join SOFC electrolyte 8YSZ with the metallicinterconnect Crofer 22 APU (wt.%: Cr = 22.8, Mn =0.45, C = 0.005, Ti = 0.08, P = 0.016, S = 0.002, La =0.06, Fe = Bal.) are also discussed.

2. Experimental

Powders of technical grade SiO2 (purity >99.5%) andCaCO3 (>99.5%), and of reactive grade Al2O3, MgCO3,BaCO3, La2O3, Cr2O3 and NiO were used. Homogeneousmixtures of batches (�100 g), according to Table 1,obtained by ball milling, were preheated at 900 �C for 1 hfor decarbonization and then melted in Pt crucibles at1580 �C for 1 h, in air.

Glasses in bulk form were produced by pouring themelts into preheated bronze molds followed by annealingat 550 �C for 1 h. The samples of the glass-powder com-pacts were produced from glass frits, which were obtainedby quenching of glass melts in cold water. The frits weredried and then milled in a high-speed agate mill resultingin fine glass powders with mean particle sizes of 2.6–3.6 lm (determined by light scattering technique; CoulterLS 230, Beckman Coulter, Fullerton CA; Fraunhofer opti-cal model). Rectangular bars with dimensions of4 � 5 � 50 mm3 were prepared by uniaxial pressing(80 MPa). The bars were sintered under non-isothermalconditions for 1 h at 800, 900 and 1000 �C, and under iso-thermal conditions at 900 �C for 300 h. A slow heating rateof 1 K min�1 was used to prevent deformation of thesamples.

Infrared spectra for the glass powders were obtainedusing an infrared Fourier spectrometer (FT-IR, MattsonGalaxy S-7000, USA). For this purpose, each glass powderwas mixed with KBr in the proportion of 1/150 (by weight)and pressed into a pellet using a hand press.

Dilatometry measurements were done with prismaticsamples with a cross-section of 4 � 5 mm2 (Bahr ThermoAnalyse DIL 801 L, Hullhorst, Germany; heating rate5 K min�1). Differential thermal analysis of fine powderswas carried out in air (DTA-TG, Setaram Labsys, SetaramInstrumentation, Caluire, France). To calculate the activa-tion energy of crystallization, 50 mg of glass powder was

BaO SiO2 Al2O3 La2O3 Cr2O3

6.43 47.88 2.14 6.83 –6.40 47.64 2.13 6.80 0.506.14 43.28 4.08 13.04 –6.11 43.06 4.06 12.97 0.50

A. Goel et al. / Acta Materialia 56 (2008) 3065–3076 3067

heated up to 1000 �C with variable heating rates (b) of 5,20, 30 and 40 K min�1. The crystalline phases were deter-mined by X-ray diffraction (XRD) analysis (Rigaku Gei-gerflex D/Max, C Series, Tokyo, Japan; Cu Ka radiation;2h angle range 10�–80�; step 0.02� s�1). Microstructuralobservations were done on polished (mirror finished) sam-ples. The etching of GCs was done by immersion in 2 vol.%HF solution for 2 min while the unetched 8YSZ/GC andCrofer 22 APU/GC interfaces (cross-sectioned) wereobserved by field emission scanning electron microscopy(FE-SEM, Hitachi S-4100, Tokyo, Japan; 25 kV accelera-tion voltage, beam current 10 lA) in the secondary electronmode.

Archimedes’ method (i.e., immersion in diethyl phthal-ate) was employed to measure the apparent density of thebulk annealed glasses and sintered GCs. Molar volume(Vm), oxygen molar volume (Vo) and excess volume (Ve)were calculated using the density data for the bulk glassesvia following relations:

V m ¼Mq; ð1Þ

where M is the molar mass of the glass and q is the appar-ent density of the bulk glasses. Similarly, excess volume ofthe glasses can be expressed as

V e ¼ V m �X

i

xiV mðiÞ: ð2Þ

Here, xi is the molar concentration of every oxide andVm(i) is the molar volume of every oxide (Table 2). Themolar volume of the oxygen content of the glasses was cal-culated using the following relation:

V o ¼

Pi

xiMi

qP

inixi

; ð3Þ

where Mi is the molar weight of the oxide and ni is the oxy-gen content in the ith oxide.

The mechanical properties were evaluated by measuringthe three-point bending strength of rectified parallelepipedbars (3 � 4 � 50 mm3) of sintered GCs (Shimadzu Auto-graph AG 25 TA, Columbia, MD; 0.5 mm min�1 displace-ment). The linear shrinkage during sintering was calculatedfrom the difference in the dimensions between the green

Table 2Density and molar volume of the oxides

Oxide Density(g cm�3)

Molar weight(g mol�1)

Molar volume(cm3 mol�1)

MgO 3.60 40.30 11.19CaO 3.34 56.07 16.79BaO 5.72 153.33 26.81SiO2 2.53 60.07 23.74La2O3 6.50 325.81 50.12Al2O3 4.00 101.94 25.49NiO 6.70 74.69 11.15Cr2O3 5.20 151.99 29.22

and the sintered bars. The mean values and the standarddeviations (SD) presented for shrinkage, density and bend-ing strength have been obtained from at least 10 differentsamples.

To investigate the adhesion of the new compositions to8YSZ solid electrolyte, wetting experiments between glasspowders and fine polished flat (sintered at 1600 �C) pelletsof 8YSZ (Tosoh, Japan) were carried out at 900 �C for 1 hin air. In order to assess the adhesion and interaction of theglasses with Crofer 22 APU, a metallic interconnect mate-rial, the glass-interconnect diffusion couple was heated upto 850 �C in air at 2 �C min�1 and was maintained at thistemperature for 1 h. Finally, the temperature was broughtdown to SOFC operating temperature (i.e., 800 �C) andkept at this temperature for 10 h in air. Energy dispersivespectroscopy (EDS; Bruker Quantax, Germany) wasemployed to study the distribution of elements along theglass-interconnect diffusion couples.

The total conductivity (r) in air was studied by ACimpedance spectroscopy (HP4284A precision LCR meter,20 Hz�1 MHz, Agilent Technologies, Palo Alto, CA),using dense disk-shaped samples with porous Pt electrodes.The values of electrical resistance (RAC) were determinedfrom the impedance spectra using equivalent circuit soft-ware [22]. The conductivity was calculated as r = L/(S � RAC) where L and S are the thickness and cross-sec-tion area of the GC sample, respectively. The experimentalerror in the total conductivity values was less than 2%. Theion transference numbers were determined by the modifiedelectromotive force (EMF) technique, as described else-where [20,23]. In the course of measurements, the anodeof the Pt–GC–Pt cell was exposed to flowing 10% H2/90% N2 gas mixture, where the oxygen partial pressure(p1) was determined using an electrochemical sensor madeof 8YSZ electrolyte; the cathode was exposed to atmo-spheric air (p2 = 21 kPa). The EMF of the cell was mea-sured at 860–880 �C as a function of an externalresistance, RM. The ion transference numbers were calcu-lated using the regression model of Eq. (4) in combinationwith Eq. (5):

Etheor

Eobs

� 1 ¼ A1

RM

� �þ B; Re ¼

AB; ð4Þ

ti ¼ 1� Rbulk

Re

; ð5Þ

where A and B are regression parameters, Eobs the mea-sured EMF, Etheor the theoretical Nernst voltage, Rbulk

the bulk resistance determined from the impedance spectra,and Re the partial electronic resistance of the sample.

3. Results

3.1. Characterization of the glasses

For all the investigated compositions (Table 1), meltingat 1580 �C for 1 h was sufficient to obtain bubble-free,

Table 3Properties of the glasses

7A 7A-Cr 7B 7B-Cr

Density (g cm�3) 3.08 ± 0.001 3.08 ± 0.002 3.20 ± 0.002 3.21 ± 0.002Molar volume, Vm (cm3 mol�1) 19.71 ± 0.012 19.77 ± 0.017 20.39 ± 0.015 20.36 ± 0.015Oxygen molar volume, Vo (cm3 mol�1) 13.18 ± 0.006 13.21 ± 0.012 13.63 ± 0.012 13.61 ± 0.010Excess volume (Ve) 0.24 ± 0.012 0.28 ± 0.017 0.34 ± 0.015 0.29 ± 0.015Tg (±2) (�C) 685 675 630 672Ts (±5) (�C) 716 721 700 720Tp (�C) 912 895 951 916CTE (±0.1) (10�6 K�1) (200–500 �C) 8.69 8.48 8.50 8.78Ea (kJ mol�1) Ozawa (r2 to Eq. (4)) 501 (0.999) 363 (0.999) 450 (0.998) 342 (0.999)Kissinger (r2 to Eq. (5)) 507 (0.999) 362 (0.999) 471 (0.998) 345 (0.999)

aTg, Ts, CTE and Tp values are for b = 5 �C min�1.

7A

7A-Cr

7B

7B-Cr

507 731

1033

412

300 500 700 900 1100 1300

Wavenumber (cm-1)

Tra

nsm

ittan

ce (

%)

Fig. 1. FT-IR spectra of the investigated glasses, obtained from KBrpellets containing 5 wt.% fine glass powders. (The plots have beenarbitrarily shifted vertically for clarity.)

3068 A. Goel et al. / Acta Materialia 56 (2008) 3065–3076

homogeneous transparent glasses that had a dark honeycolor (due to the presence of NiO). The absence of crystal-line inclusions was confirmed by X-ray and SEM analyses.

Along the series of the four investigated compositions ofTable 1, the properties summarized in Table 3 reveal thefollowing general features.

3.1.1. Density measurementsGlass density, molar volume (Vm) and oxygen molar

volume (Vo) increase with La2O3 content; however, theaddition of Cr2O3 did not affect these parameters to anygreat extent. The density of all the glasses under investiga-tion is higher than the density of the Ba-containing D–Tsglasses investigated in our previous work [6]. The calcu-lated excess volume (Ve) for all the glasses was found tobe positive and increased with the increase in La2O3 andCr2O3 content.

3.1.2. FT-IR analysis

The room-temperature FT-IR transmission spectra ofall four glasses are shown in Fig. 1. All spectra exhibit threebroad transmittance bands in the region 300–1300 cm�1.The most intense transmission bands lie in the 800–1300 cm�1 region, the next one between 300 and600 cm�1, while the least intensive lies between 650 and800 cm�1. The broad bands in 800–1300 cm�1 region areassigned to the stretching vibrations of the SiO4 tetrahe-dron with different numbers of bridging oxygen atoms,while the bands in the 300–600 cm�1 region are due tobending vibrations of the Si–O–Si and Si–O–Al linkages[24,25]. The transmission bands in the 650–800 cm�1

regions are related to the stretching vibrations of theAl–O bonds with Al3+ ions in four-fold coordination[24]. A minute shoulder appeared at 412 cm�1, whichremains unresolved.

3.1.3. Dilatometry

The glass transition temperature (Tg) was determinedfrom the point where the slope of the dilatation curvestarted to deviate from its linear behavior, while the soften-ing temperature was determined from the point at whichdilatation stops in the dilatometer. In general, Tg and Ts,

as determined from dilatometric curves, decreased consid-erably with an increase in the content of La2O3, while noparticular trend could be observed due to the addition ofCr2O3. The Tg and Ts values for glass 7A, and the CTEsof all the four glasses are higher than those of all the pre-viously investigated glasses [5,6]. The highest CTE valuewas obtained for glass 7B-Cr. The mean values and thestandard deviations presented for Tg, Ts and CTE, pre-sented in Table 3, have been obtained from at least threedifferent samples for each composition.

3.1.4. Differential thermal analysisThe DTA plots of fine powders with a heating rate of

5 K min�1, shown in Fig. 2, feature a well-defined single

895

7A

7A-Cr

7B

7B-Cr916

951

912

775 825 875 925 975Temperature (ºC)

Hea

t Flo

w (

μV)

2μVExo

Endo

Fig. 2. Differential thermal analysis (DTA) of fine powders of theinvestigated glasses heated at a rate of 5 K min�1.

7A

7A-Cr

7B

7B-Cr

(0.999)

(0.999)

(0.998)

(0.999)10

11

12

13

0.76 0.78 0.8 0.82 0.84 0.86

ln (

Tp2 /β

)

7A

7A-Cr

7B

7B-Cr

(0.999)

(0.999)

(0.998)

(0.999)

1

2

3

4

0.76 0.78 0.80 0.82 0.84 0.861000/Tp

1000/Tp

lnβ

Fig. 3. Plots according to the modified (a) Kissinger and (b) Ozawaequations for determination of the activation energy for crystallization ofthe investigated glasses (Ea). The values or correlation coefficients (r2) arealso presented.

A. Goel et al. / Acta Materialia 56 (2008) 3065–3076 3069

exothermic crystallization curve. It was observed that thepeak temperature of crystallization (Tp) shifted to highertemperature with increasing heating rates (b). Tp increasedwith the La2O3 content as glass 7B showed the highest Tp

value while Cr2O3 addition decreased the crystallizationtemperature, with glass 7A-Cr showing the lowest valueof Tp. The decrease in Tp with the addition of Cr2O3 signi-fies that the latter might play the role of a nucleating agentby lowering the crystallization temperature of the glasses.However, from the experimental results, it was observedthat the addition of 0.5 wt.% Cr2O3 was not efficient tonucleate glasses 7A and 7B in bulk, because crystallizationwas induced mainly from the surface, resulting in a coarsestructure containing visible voids. Therefore, the activationenergy (Ea) of crystallization was calculated using the fol-lowing modified forms of the Kissinger (Eq. (6)) andOzawa (Eq. (7)) equations:

lnðT 2p=b

nÞ ¼ ðEa=RT pÞ þ constant; ð6Þln b ¼ �mEa=nRT p þ constant ð7Þwhere n is the Avrami constant, and m is the crystal growthdimensionality. These equations, proposed by Matusitaand Sakka [26], assume that nucleation does not occur dur-ing crystal growth, and crystal growth is interface con-trolled. In the present study, the values of n and m wereconsidered as both equal to 1, which is the case for surface

crystallization [26], for all the investigated glasses. The cal-culated Ea values for all the glasses are presented in Table 3and Fig. 3. The Ea value is highest for glass 7A while thelowest one was observed for glass 7B-Cr. The high valuesof correlation coefficients (r2) for the linear fitting of theexperimental data to the Eqs. (6) and (7) (least-squaresmethod) support these assumptions [27]. The negligible dif-ference between the Ea values determined by the two meth-ods is usual in cases of weak dependence of Tp on heatingrate [28].

3.2. Characterization of the sintered GCs

Fig. 4 shows the evolution of crystalline phases in glass-powder compacts between 800 and 1000 �C. The sampleswere still amorphous after heat treatment at 800 �C. Aug(ICDD card: 01-078-1392) developed after 900 �C for 1 h,as also confirmed by the SEM images (Fig. 5) [5,6]. Cal-cium lanthanum oxide silicate (ICDD: 01-071-1368), whosemost intensive peaks (2h = 30.917�) overlap with a peak ofAug (2h = 30.914�), was revealed in the 7B and 7B-Cr asthe secondary phase. It is worth noting that monomineralGCs of Aug were developed in both 7A and 7A-Cr(Fig. 4a and b) although in the former the low intensivepeak at 2h = 19.04� was not resolved.

800ºCAug

900ºC-1h

900ºC-300h

1000ºC

Calcium Lanthanum Oxide

Silicate

Orthocelsian (OC)

10 20 30 40 50 602θ (degrees)

Inte

nsity

(cp

s)

7A

800ºCAug

900ºC-1h

900ºC-300h

1000ºC

HC

10 20 30 40 50 60

2θ (degrees)

Inte

nsity

(cp

s)

7A-Cr

800ºCAug

900ºC-1h

900ºC-300h

1000ºC

OC

10 20 30 40 50 60

2θ (degrees)

Inte

nsity

(cp

s)

7B

800ºCAug

900ºC-1h

900ºC-300h

1000ºC

HC

10 20 30 40 50 60

2θ (degrees)

Inte

nsity

(cp

s)

7B-Cr

Fig. 4. X-ray diffractograms of glass-powder compacts of: (a) 7A, (b) 7A-Cr, (c) 7B and (d) 7B-Cr after heat treatment at different temperatures for 1 hand at 900 �C for 300 h. (The spectra have not been normalized. Full scale of intensity axes 7500 cps.)

3070 A. Goel et al. / Acta Materialia 56 (2008) 3065–3076

Prolonged heat treatment at 900 �C for 300 h did notsignificantly decrease the intensities of the Aug phase inthe diffractograms of the GCs (Fig. 4). Two polymorphsof BaAl2Si2O8, HC (ICDD card: 01-088-1048) and OC(ICDD card: 00-012-0725) were formed in Cr-containing(7A-Cr and 7B-Cr) and in Cr-free (7A and 7B) composi-tions, respectively. Calcium lanthanum oxide silicateappeared as a newly developed phase in 7A and was regis-tered in all the compositions except 7A-Cr. The peak at2h = 19.04 was not resolved in both 7A and 7B.

A similar crystalline regime is generally suggested fromthe XRD patterns at 1000 �C (Fig. 4), but the intensity ofthe crystalline phases in the Cr-free compositions decayedconsiderably (more in 7A) due to dissolution effect. More-over, the calcium lanthanum oxide silicate phase wasapparently more developed in 7B. From the overall analy-sis of XRD patterns, Cr-containing GCs featured the high-est intensity of the Aug, indicating improved crystallinitywith Cr2O3 doping, as was also demonstrated by theSEM images presented in Fig. 5c and d.

3.3. Properties of sintered GCs

Well-sintered dense glass-powder compacts wereobtained after heat treatment at 800 �C. There was no

evidence of detrimental effects, such as deformation or for-mation of open porosity, in the temperature interval 800–1000 �C. Shrinkage of the samples increased with anincrease in temperature as the highest values of shrinkage(with SD less than 5%) for all the compositions werereached in the temperature interval 900–1000 �C (Table4). In terms of composition, shrinkage systematicallydecreased for Cr2O3 containing GCs in comparison toCr-free analogues. In accordance with shrinkage measure-ments, all GCs showed the highest density in the tempera-ture interval 900–1000 �C (Table 4). In general, the densityof the GCs increased with the increase in La2O3 content;however, the Cr2O3-containing compositions had the lowerdensity values in comparison with their Cr-free counter-parts at any specific temperature. It is worth noting thatall the GCs showed good stability in terms of small densityvariations with temperature increasing in the range 900–1000 �C.

In general, the bending strength increased with theincrease in the temperature, irrespective of the composi-tions of the glasses or GCs. Nevertheless, La2O3 contenthad a mixed effect on the bending strength of the producedGCs (Table 4). For composition 7A, a maximum value(163 MPa) was observed at 1000 �C. However, for 7B,which maintained a higher strength than 7A at 800 and

Fig. 5. Microstructure (revealed via SEM imaging after chemical etching of polished surfaces with 2 vol.% HF solution) of the glass-ceramic 7A heattreated at (a) 900 �C and (b) 1000 �C; (c) GC 7B at 900 �C and (d) 1000 �C.

Table 5CTE (�10�6 K�1) of the GCs produced at different conditions togetherwith the CTE of 8YSZ

Composition 900 �C, 1 h 1000 �C, 1 h 900 �C, 300 h

7A 9.50 9.54 9.697A-Cr 9.62 9.62 9.627B 9.49 9.34 8.287B-Cr 10.28 9.68 9.418YSZ 10.01 – –

Standard deviation: ±0.1 � 10�6 K�1.

Table 4Properties of the GCs produced from glass-powder compacts after heattreatment at different temperatures for 1 h

Composition 800 �C 900 �C 1000 �C

Shrinkage (%)

7A 15.52 ± 0.13 16.58 ± 0.41 16.60 ± 0.437A-Cr 14.87 ± 0.25 15.30 ± 0.21 15.29 ± 0.317B 15.72 ± 0.24 16.90 ± 0.56 17.50 ± 0.587B-Cr 14.73 ± 0.37 15.55 ± 0.29 15.86 ± 0.27

Density (g cm�3)

7A 3.05 ± 0.002 3.19 ± 0.005 3.19 ± 0.0047A-Cr 3.07 ± 0.005 3.14 ± 0.004 3.15 ± 0.0037B 3.19 ± 0.005 3.28 ± 0.006 3.29 ± 0.0057B-Cr 3.19 ± 0.005 3.25 ± 0.008 3.25 ± 0.005

Bending strength (MPa)

7A 94.67 ± 15.90 136.31 ± 13.59 162.51 ± 8.447A-Cr 103.36 ± 18.59 148.73 ± 9.19 154.61 ± 12.527B 99.48 ± 17.07 145.35 ± 9.47 134.29 ± 2.077B-Cr 104.18 ± 16.76 149.27 ± 13.77 177.84 ± 13.78

A. Goel et al. / Acta Materialia 56 (2008) 3065–3076 3071

900 �C, the bending strength decreased at 1000 �C. TheCr2O3-containing compositions generally possessed higherbending strength in comparison with their Cr2O3-free ana-logues and GC 7B-Cr had the highest bending strength incomparison to other compositions: 178 MPa after sinteringat 1000 �C.

The CTE values of the GCs sintered at 900 and 1000 �Cfor 1 h are presented in Table 5. The GCs sintered at900 �C showed higher CTEs than those sintered at

1000 �C, except composition 7A-Cr which featured a stableCTE value at all investigated temperatures (9.62 � 10�6

K�1). La2O3 content did not affect significantly the CTEof the GCs; however, Cr2O3 addition increased the CTEof the GCs. The highest CTE was calculated for composi-tion 7B-Cr sintered at 900 �C. The CTE values of both 7Band 7B-Cr decreased considerably after prolonged isother-mal heat treatment at 900 �C for 300 h, while the CTE ofthe GC 7A slightly increased. Generally, CTE values of7A, 7A-Cr and 7B-Cr match fairly well the CTE of8YSZ, which varies in the range 9.8–10.4 � 10�6 K�1 atSOFC operating temperatures. In this work, the CTE(200–600 �C) of 8YSZ was determined to be 10.01 �10�6 K�1.

The experimental results of wetting experiments suggeststrong interfacial adhesion between the four investigatedcompositions and 8YSZ at 900 �C (1 h, air) (Fig. 6).

Fig. 6. Microstructure (SEM) of interface between solid zirconia (8YSZ)pellet and glass 7A, developed after heat treatment at 900 �C for 1 h.

3072 A. Goel et al. / Acta Materialia 56 (2008) 3065–3076

In particular, a good wetting regime was observed (i.e.,contact angle <90�) and continuous interfaces, with noreaction zones, cracks or gaps, were revealed after cross-section and polishing. No reaction products were alsodetermined by XRD analysis of compact samples madeof the powder mixtures of glasses and 8YSZ after heattreatment under similar conditions (i.e. 900 �C, 1 h). ASOFC interconnect alloy, Crofer 22 APU, was investi-

Fig. 7. Microstructure (SEM) and EDS element mapping of Cr and Ba at the irespectively, developed after heat treatment at 850 �C for 1 h and 800 �C for 10Crofer 22 APU into the glass while the particles in blue demonstrate the diffureferences to color in this figure legend, the reader is referred to the web versi

gated for its chemical compatibility with the investigatedglasses. Fig. 7 shows the SEM images of the interfaceof Crofer 22 APU/glass 7B and Crofer 22 APU/glass7B-Cr joins, respectively, after heat treatment at 850 �Cfor 1 h, followed by 800 �C for 10 h in air. The extentof diffusion of Cr from the alloy interconnect into theglass and of Ba from the glass into the interconnect mate-rial (Figs. 7 and 8) has been observed through EDS ele-ment mapping. Within an interfacial zone of about0.5 lm thickness the relative concentration of Cr decaysfrom about 85% to around 20%, while the relative con-centration profile of Ba is approximately constant acrossthe interface, suggesting the absence of diffusion barriersfor this element. All the Crofer 22 APU/glass interfacesshowed a homogeneous microstructure over the entirecross-section of the joint. A very thin layer can be seenat the interface; this resulted from the chemical reactionbetween the glass and Crofer 22 APU. However, the seal-ing GCs bonded well to the stainless steel and no gapswere observed even at the edges of the joints.

The electrical conductivity of 7A and 7B GCs is as lowas 1.1–1.3 � 10�7 S cm�1 at 780 �C and 2.2–2.5 � 10�7

S cm�1 at 880 �C, and tends to decrease with lanthanumadditions (Fig. 9). The introduction of 0.5 wt.% Cr2O3

results in 1.6–2.0 times higher conductivity. Nonetheless,the latter GCs exhibit better insulating properties ifcompared to the La- and Cr-free analog Ca0.8Ba0.2-Mg0.8Al0.4Si1.8O4 [6]. At the same time, the conductivity

nterface between Crofer 22 APU/glass 7B and Crofer 22 APU/glass 7B-Crh in air. The apparent small particles in red show the diffusion of Cr fromsion of Ba from the glass into Crofer 22 APU. (For interpretation of theon of this article.)

7B

0

20

40

60

80

100

0 1 2 3 4

Distance (μm)

Rel

ativ

e C

once

ntra

tion

(%)

Crofer22 APU Cr

Ba

Fig. 8. Cr and Ba EDS mapping at interface between glass 7B and Crofer22 APU alloy substrate after 10 h of exposure at 800 �C in air.

0 2 4 6 8 10

1/RM× 107, Ohm-1

0

0.4

0.8

1.2

Eth

eor/E

obs -

1

-1/Re

tg α = Ri + Rη

7B-Cr

860°Cair / 10%H2-90%N2 gradient

ti = 0.93 ± 0.02ρ = 0.9996

p1 = 1.1×10-12 atm

Fig. 10. Example of the ion transference number determination by themodified EMF technique. Solid line corresponds to the fitting results,using Eq. (5) as the linear regression model. Experimental conditions aregiven in the legend. q is the correlation coefficient. The error bars aresmaller than the data point symbols.

8.6 8.8 9.0 9.2 9.4 9.6

104/T (K-1)

-8.0

-7.6

-7.2

-6.8

-6.4

log

σ (S

× c

m-1

)

7A

7A-Cr

7B

7B-Cr

Ca0.8Ba0.2Mg0.8Al0.4Si1.8O6

Fig. 9. Total conductivity of dense GCs in air. The data on Ca0.8Ba0.2-Mg0.8Al0.4Si1.8O6 [6] are shown for comparison. The error bars are smallerthan the data point symbols.

A. Goel et al. / Acta Materialia 56 (2008) 3065–3076 3073

remains dominantly ionic, as for the BaO-doped materials[6]. For example, the ion transference numbers (ti) of 7B-CrGCs, selected for detailed characterization as the Cr-con-taining composition with relatively high electrical resistiv-ity, were found to be 0.91–0.93 at 860–880 �C under anair/10% H2 + 90% N2 gradient. In the course of transportnumber measurements by the modified EMF technique[23], the (Etheor/Eobs�1) vs. (1/RM) dependencies showedlinear behavior, in agreement with the regression model(Eq. (5)); one example is presented in Fig. 10. Due to verylow conductivity of the studied GCs, the contribution ofelectrode polarization resistance (Rg) to the overall cellresistance was small, lower than 5% with respect to thesample bulk. This indicates that deviations of the cellEMF from the theoretical Nernst voltage cannot be

attributed to electrode polarization. The incorporation ofvariable-valence Cr cations hence leads to a moderateincrease in the electronic conductivity. One should notethat the EMF measurements cannot indicate the exact typeof ionic charge carriers when an oxide membrane is placedunder an oxygen or metal chemical potential gradient andequilibrium is achieved at the electrodes (Ref. [20] and ref-erences therein). The measured transference numbers cor-respond to the total ionic transport, irrespective of thenature of mobile anions and/or cations. As the content ofalkaline metal impurities in the studied GCs is lower thanthe detection limits of the available analytical techniques,the mobile ionic charge carriers may include alkaline-earthcations such as Mg2+ and Ca2+, oxygen anions and pro-tons. Exact identification of the migrating ionic speciesby electrochemical methods, such as Faradaic efficiencymeasurements [20], is impossible due to very high resistivityof the studied GCs. However, previous studies of thesteady-state oxygen permeation through Ca0.95Ba0.05

Mg0.8Al0.4Si1.8O6 GCs under an air/(H2 + H2O + N2) gra-dient [6] indicated a significant oxygen-ion diffusion andpossible protonic contribution; similar mechanisms of theionic transport are expected in the studied GCs.

4. Discussion

In recent years, much effort has been devoted to devel-oping glass and GC sealants for SOFCs [29,30]. Glass sys-tems of BaO- and La2O3-containing silicates and borates,such as BaO–Al2O3–La2O3–B2O3–SiO2 [15,31,32], have

3074 A. Goel et al. / Acta Materialia 56 (2008) 3065–3076

attracted interest. The chemical compatibility of Ba-con-taining silicate glasses and GCs with YSZ electrolyte hasbeen reported as being generally good [6,28], but Yanget al. [33,34] have demonstrated that extensive reactiontakes place between Ba-containing glasses (particularlyBCAS glass) and metallic interconnect alloys, resulting inthe formation of the detrimental phase BaCrO4 at the edgesof the glass-interconnect joint in an air atmosphere. Cer-tainly, such reactions are of crucial importance to thelong-term stability and functionality of SOFC [20].

The present work investigated the effect of La2O3 andCr2O3 on the development of GCs in the Di-based systemwith the amount of BaO varying between 6.11 and6.43 wt.% (2.54–2.61 mol.%), which is significantly lowerthan those used in the systems mentioned above. Fromour previous study [6] this amount of BaO was foundappropriate to obtain GCs with good insulation, thermaland mechanical properties for potential application as seal-ants for SOFCs. The experimental results showed anincrease in the apparent density of the glasses with increas-ing La2O3 content, because the latter has almost twice thedensity of CaO (Table 2). The increase in the value of Vm

and Ve along with the density indicates that the structureof the glass tends to be more open with the addition ofLa2O3 and Cr2O3 [35].

In comparison with the La2O3-free glasses investigatedin our previous study [6], which had compositions very sim-ilar to the glasses presently under investigation, it is evidentthat the Tg increased with the incorporation of La2O3 inthe glasses (7A). This increase in the Tg of glass 7A canbe attributed to the greater strength of the La–O bond incomparison to the Ca–O bond [36]. However, the decreasein Tg with a further increase in the La2O3 content (7B) canbe attributed to the lower value of Vo [37,38]. The CTEs ofall four glasses was higher than the previously studiedglasses [6] which may be connected to the decrease in theconnectivity of the glass structure with the increase inLa2O3 content, as has also been observed in the case ofVe and FT-IR results (discussed below).

Glass 7A has a higher Ea in comparison to the La2O3-free GCs [6]. This may be due to the higher field strengthof the La3+ compared to Ca2+. However, the decrease inEa with the further increase in La2O3 content (7B) maybe due to the network-modifying effect and clustering ofLa3+ ions in the glass structure. Clustering is defined bythe existence of La–O–La linkages consistent with experi-mental interpretation of related systems [36,39,40]. Evi-dence for La clustering was found to occur in the systemssimulated over a range of 1–10 mol.% La2O3 [41], in agree-ment with experimental findings of related systems [42].The lowering of Ea for Cr2O3-containing glasses supportsthat the latter played an effective role of a nucleating agent.

It was observed that the FT-IR spectra of all the inves-tigated glasses exhibit broad transmission bands. This lackof sharp features is indicative of the general disorder in thesilicate network mainly due to a widely distribution of Qn

units occurring in these glasses. In the FT-IR spectra of

all the four glasses (Fig. 1) in the 1000–1300 cm�1 region,the main transmission band is centered at about1033 cm�1. These results indicate a distribution of Qn unitscentered around the Q3 unit, while the broadening of thistransmission band towards lower wavenumbers with theincrease in La2O3 (7B) and addition of Cr2O3 content indi-cates the presence of less polymerized units such as Q2 andQ1. This result is also supported by the increase in theexcess volume, Ve, of the glasses. The addition of Cr2O3

to the glasses made the Al–O transmission band between650 and 800 cm�1 broader and shifted towards lower wave-numbers in comparison to their Cr-free analogs. Generally,IR transmission bands of silicate glasses are related to thesymmetry of [SiO4] tetrahedron in the glasses. The symmet-ric and the antisymmetric stretching modes of the Si–O–Sibonds of the Qn (polymerization in the glass structure,where n denotes the number of bridging oxygens) unitsare IR active in the 800–1300 cm�1 region. The transmis-sion bands of the Qn units with n = 4,3,2,1 and 0 are cen-tered around 1200, 1100, 950, 900 and 850 cm�1,respectively [43,44]. In the structures of aluminosilicateglasses, because the atomic weights of Al and Si are almostequal, the vibrational mode of [SiO4] and [AlO4] tetrahedraare coupled with each other. The Al–O force constant issmaller than the Si–O force constant due to the lowervalence of Al3+. The vibrational frequency of [AlO4] tetra-hedra is lower than that of [SiO4] tetrahedra [45]. Thetransmission band near 1100 cm�1 might be attributed tothe stretching vibrations of [SiO4] tetrahedra sharing threecorners with neighboring [SiO4] tetrahedra and one cornerwith an Al–O polyhedron [45]. The transmission bandsshift to lower wavenumbers replacing the Si by Al atomas a consequence of the weaker Al–O bond.

Cr2O3 is a typical nucleating agent in pyroxene-basedglasses containing iron oxide [13,17]. It promotes fast spi-nel precipitation which becomes a nucleator for epitaxialpyroxene growth. Barbieri et al. [17] have reported that ifmore than 0.5 mol.% Cr2O3 is added to glasses of theCaO–MgO–SiO2–Al2O3 system, it is precipitated asMgCr2O4 spinel crystallites in the glass specimens actingas heterogeneous nucleation sites for crystallization of thefinal Di and anorthite phases. Their results also show thatin the case of fine grain glass particles, surface nucleation ispredominant even at 5 mol.% Cr2O3 content, but for coar-ser particles some degree of bulk nucleation may beobserved. Marghussian et al. [46] suggested that in theglasses of the SiO2–Al2O3–MgO–CaO(R2O) system, effec-tive nucleation may only occur by using a mixture ofCr2O3, Fe2O3 and TiO2. It is generally accepted that moreefficient bulk nucleation and crystallization processes occurin the presence of Fe2O3 additive along with Cr2O3 nucle-ant. In this case entrance of Fe3+ cations into the structureof MgO � Cr2O3 spinel increases the unit cell dimensions,and reduces the crystallographic mismatch between spinelphase and the final aluminum Di phase in the a and espe-cially the b directions, facilitating their nucleation [46,47].In the present study, 0.5 wt.% Cr2O3 addition was found

A. Goel et al. / Acta Materialia 56 (2008) 3065–3076 3075

to be inefficient at nucleating glasses 7A and 7B in the bulk,most probably because of these glasses are iron free. At thesame time, the role of Cr2O3 was quite evident in decreas-ing the crystallization temperature and improving thecrystallinity of the pyroxene phase. For this reason,Cr2O3-containing GCs featured high stability in terms ofsmall density variations in the temperature range 900–1000 �C and generally possessed higher bending strengthin comparison to their Cr2O3-free analogs. Another advan-tage was that La2O3-associated phases were not observedin composition 7A containing Cr2O3 (7A-Cr), even afterisothermal heat treatment at 900 �C for 300 h, and a mono-mineral GC of Aug was obtained. We presume that thehigh amount of lanthanides (about 6 wt.% La2O3) mightbe accommodated in the pyroxene structure when a smallamount of Cr2O3 is added to the glasses. This amount oflanthanide is comparable with the immobilization abilityof zirconolite GCs used as nuclear waste forms [48].

In comparison with the studies reported earlier [33,34],the present glasses showed an improved compatibility withthe Crofer 22 APU. In particular, barium–calcium–alumi-nosilicate-based glasses were reported to interact chemi-cally with chromia-forming alloys, forming BaCrO4,which leads to Ba depletion in the GCs and to the separa-tion of GCs from the alloy matrix due to thermal expan-sion mismatch [34]. In the present investigation, aconsiderable decrease in the diffusion of Cr from the inter-connect to the glass within an interfacial zone of about0.5 lm thickness along with constant CTE of the GCs,even after 300 h, demonstrates the feasibility of negligiblereaction at the interface and minimum thermal expansionmismatch. Moreover, all studied GCs exhibited good elec-trically insulating properties. The conductivity of thesematerials is significantly lower than that of the RO–BaO–SiO2 (R = Mg, Zn) GCs tested previously as potentialSOFC sealants [23].

The activation energy for the total conductivity (Ea) cal-culated from the standard Arrhenius equation,r ¼ r0

T expð� Ea

RTÞ, where r0 is the pre-exponential factor,has similar values for all the GCs studied, varying in thenarrow range 172–183 kJ mol�1 at 780–880 �C (Table 6).This fact indicates that, despite the moderate increase inthe electronic transport, the conductivity mechanismremains essentially unchanged for all compositions and issimilar to that in the BaO-doped GCs. One particular con-clusion is that slight diffusion of Cr from the SOFC inter-connects should not result in sealant degradation.

Table 6Activation energy for the total conductivity of GCs in air

Composition Ea, (kJ mol�1) (780–880 �C)

7A 172 ± 37A-Cr 180 ± 17B 175 ± 27B-Cr 183 ± 3Ca0.8Ba0.2Mg0.8Al0.4Si1.8O6 [6] 180 ± 2

5. Conclusions

The effect of La2O3 and Cr2O3 on the processing andcharacterization of four glasses and the derivative GCs inthe Di system were investigated. Both molar and excessvolume increased with the increase in La2O3 and Cr2O3

content. The CTE of all four glasses was observed to behigher than their La2O3- and Cr2O3-free analogs. The ther-mal parameters Tg, Ts and Ea are higher for glass 7A incomparison to composition 7B. The Ea values for Cr2O3-containing glasses were found to be lower than their Cr-free analogs. 0.5 wt.% of Cr2O3 was not found to be aneffective nucleating agent for inducing bulk nucleation inthe glasses. Well-sintered GCs comprised Aug as the pri-mary crystalline phase after sintering at 900–1000 �C for1 h. A pronounced increase in the intensity of Aug X-raypeaks was observed in Cr2O3-containing GCs. The absenceof monocelsian phase in all the GCs and formation ofmonomineral GCs for composition 7A and 7A-Cr are theessential features of the present work. The density of theGCs decreased and the bending strength increased withthe addition of Cr2O3. Although the conductivity of theGCs increased with the addition of Cr2O3, all the GCs stillexhibited good insulating properties. These properties, inconjunction with negligible interfacial reaction with8YSZ, and the good matching of CTEs and good adhesionwith Crofer 22 APU, qualify the investigated materials forfurther experimentation as candidate SOFC sealantmaterials.

Acknowledgments

A.G. is indebted to CICECO and University of Aveirofor the research grant. The authors are thankful to Dr.R.N. Basu, Head, Fuel Cell and Battery Section, CentralGlass and Ceramics Research Institute, Kolkata, Indiafor providing us with Crofer 22 APU alloy for this study.

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