Ionic conductivity behavior by activated hopping conductivity (AHC) of barium aluminoborosilicate glass–ceramic system designed for SOFC sealing

Journal of the European Ceramic Society 39 (2019) 3103–3111

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Original Article

Ionic conductivity behavior by activated hopping conductivity (AHC) of barium aluminoborosilicate glass–ceramic system designed for SOFC sealing

T

⁎

M.J. Da Silvaa, W.M. Pontuschkaa, J.F. Bartoloméb, , P. Jasinskic, J. Karczewskid, S.T. Reise a

Universidade de São Paulo, Instituto de Física, Departamento de Física Geral, Rua do Matão, 1371 Cidade Universitária-Butantã, 05508-090 São Paulo, SP, Brazil Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), C/Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain c Faculty of Electronics, Telecommunications and Informatics, Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdańsk, Poland d Faculty of Applied Physics and Mathematics, Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdańsk, Poland e Missouri University of Science and Technology, Missouri S&T, Department of Material Science & Engineering, MO 65409, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: SOFC Sealants Glass–ceramics Phase separation Impedance spectroscopy Activated hopping conduction

Non-conducting BaO-B2O3-Al2O3-SiO2 parent glasses designed for solid oxide fuel cell (SOFC) sealing applications were prepared using the melt-quenching technique. The glass formation region was determined according to phase equilibrium relations and was found to be in the composition range 70BaO-(x)Al2O3-(10−x)B2O320SiO2 where 3.0 < x < 6.0 wt%. The conductivity values obtained conductivity ranged from 10−5 to 10−10 S/cm at temperatures between 600 and 850 °C. The batch compositions presented a threshold of dc conductivity near 70BaO wt% with a quasi linear behavior with the decrease of the BaO content. Different values of conduction activation energy were observed at temperatures above the glass transition temperature (Tg) (up to 700 °C), which were attributed to the thermal bond-breaking of non-bridging oxygen (NBO) defects. The experimental results of the electrochemical characterization by impedance spectroscopy of glass–ceramic interfaces with yttria-stabilized zirconia (YSZ) acting as solid ionic conductor electrolyte are presented and discussed.

1. Introduction Solid oxide fuel cells (SOFCs) are solid-state electrochemical reactors based on a solid electrolyte. Compared to other types in its genre, the planar SOFC designs stand above all because they are simple, reliable, environmentally friendly, and with a power generation efficiency up to 70 wt% of possible fuel regeneration [1]. Unfortunately, the commercialization is not as quick and easy as it could be [2,3]. There is no debate that one of the drawbacks is the lack of sealing performance, because gas-tight seal is required to keep separated the fuel gas and air from each other to prevent direct combustion aside reactions and localized over-heating [4,5]. There are basically three sealing approaches for SOFC application: rigid, compressive, and compliant. The rigid glass–ceramic sealing approach has reached the most mature technological level after decades of research efforts [6–8]. The principle of use basically consists of the application of powdered glass over the edges of the single cell to produce a bond with the SOFC components stack. Then a thermal treatment to simulate the SOFC working condition was performed at the conditions determined according to the previous characterization [9]. The sealing temperature (Ts) means the maximum densification point

⁎

resulted from the viscous flow. A broad range of glass systems is available nowadays for sealing purpose, among them the chemical compositions based on alkaline-earth oxides. However, in the composition design of glasses for rigid sealing applications, the alkaline-earth modifier ion Ba2+ has been somewhat underestimated, probably due to the relative difficulty of obtaining such systems in glassy state. The binary BaO–SiO2 system demands temperatures high as 1500 °C to obtain a homogenous melt, and at the same time a high quenching rate is necessary to avoid crystallization [10]. On the other hand, it offers competitive advantages: (i) the high atomic weight of Ba, combining high resistivity with viscosity temperature dependence; (ii) high polarizability and density; (iii) the devitrification can be controlled with the aid of nucleating agents such as Al2O3; (iv) formation of the well desired phases containing barium silicate, barium borate and celsian with its well-known key parameters considered for a proper evaluation of the SOFC's glass–ceramic seals stability on heating [11]. Another important point to consider is that a sealant must behave as an electrical insulator, with ionic conductivity (σ) lower than 10−4 S/ cm, in order to avoid the occurrence of short-circuits [12]. For the majority of commercial glasses, the alkaline ions are the main charge

Corresponding author. E-mail address: [email protected] (J.F. Bartolomé).

https://doi.org/10.1016/j.jeurceramsoc.2019.03.027 Received 5 December 2018; Received in revised form 11 March 2019; Accepted 12 March 2019 Available online 20 March 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.

Journal of the European Ceramic Society 39 (2019) 3103–3111

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carriers. The conductivity behavior in those glassy systems is proportional to the concentration of the charge carrier ions and to their diffusion coefficients [13]. However, the conduction mechanism in various alkali-free silicate glasses containing earth-alkaline ions is still controversial [14–18]. It is well known that barium additions lead to a decrease in conductivity and increase of the corresponding activation energy, calculated by the Arrhenius equation [19]. In our glass compositions, a large Ba2+ ion content up to 70 wt% is present, whereas the anionic part of the glass can be varied. Throughout the glass lattice, the SiO4 tetrahedral units are linked by their vertices forming ring structures, and this complex structural environment plays an important role in ionic conduction [20–23]. Here we briefly question the current knowledge about rigid seals for SOFC applications and characteristic differences in the glass stability and ionic conductivity of so similar batch compositions. We assess the frontier of the available knowledge about amorphous solids: one of the contemporary science puzzles [24]. We use this information to address two general issues: (i) How the amorphous structure does affect the thermal properties and ionic transport behavior? (ii) What are the most likely explanations for ionic migration through glassy structures, and which can be ruled out? To summarize, the final impact of the ionic transport behavior on SOFC sealing under work conditions: temperatures up to 700 °C in reducing and/or oxidant environments, will depend on the interaction between the aforementioned factors and other fundamental questions.

Fig. 1. Ternary phase diagram of BaO–B2O3–SiO2 (4-coordinated glass former oxides: SiO2 + Al2O3) system and the indicated point of glass forming region for ternary BAS glasses. Glasses were quenched to room temperature in a stainless steel mold. Key: (+) BAS-4, (red) BAS-5, (white) BAS-6, and (○) BAS7. Invariant points at the vicinity of glass forming region: C = 875° ± 5 °C, D = 920° ± 10 °C, G = 980° ± 5 °C, H = 925° ± 10 °C [28]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Table 1 Batch glass compositions and the characteristic temperatures according to prior thermal analyses [9].

2. Materials and methods 2.1. Sealants design criteria and glass samples preparation

Batch compositions (wt%)

Glass composition calculation is generally based on (i) additive relationships and (ii) phase equilibrium relations approach. The additive approach obeys a linear relation based on assumptions for a chosen glass system:(i) additive relations work only in a narrow range of compositions; (ii) the additive coefficient of a specific oxide shall differ depending of the given system, and (iii) assumes the batch as an “ideal solution” [25] with no lack of linearity. However, we can appreciate that non linearity normally occurs among the glass batch components. As a result, computations governing the external inputs to a given batch compositions are subject to eventual errors because the underlying assumptions may be eventually not completely accurate. The approach that we have chosen is based on phase equilibrium diagrams analyses [26]. Consequently, we anticipate the crystalline phases which occurred during the firing of the melt. The study of phase relations is based on the assumption that the reactions in the system under consideration occurred at thermal equilibrium in the melt and frozen on quenching. If in one hand, the final amorphous state does not permit a condition of equilibrium to be established. In the other hand, it is known that the system is always approaching to equilibrium, so that the knowledge of the direction toward or away from equilibrium is relevant. In the isoplethal study we can determine the crystalline phases during the slow cooling of the melt under equilibrium conditions. The ternary phase diagram for the BaO-B2O3-SiO2 system [27,28] is shown in Fig. 1 along with the batch compositions located at the barium rich portion of the diagram, avoiding cristobalite (Crs) at the left upper side. It has been reported that cracking of the seals has occurred at interface with other cells components, and that two immiscible liquids (right upper side of the ternary diagram) were detected. Four parent glass compositions are presented in Table 1, labeled as BAS-4, BAS-5, BAS-6, and BAS-7 respectively, were designed for sealant purpose taking into account the BaO-B2O3-SiO2 ternary equilibrium diagrams [28], shown in Fig. 1. For all compositions of the system, the amount of oxides was distributed as (67.0–74.0)BaO-(7.0–16.0) (B2O3 + Al2O3)-(17.0–20.5)SiO2 wt%. Boron oxide acts as a flux agent decreasing the viscosity of the melt, decreasing the melting point of the

74.0BaO-3.0Al2O3-4.0B2O319.0SiO2 72.0BaO-4.5Al2O3-3.0B2O320.5SiO2 67.0BaO-6.0Al2O3-10.0B2O317SiO2 69.0BaO-4.5Al2O3-9.0B2O317.5SiO2

Sample ID

Characteristic temperatures (°C) Tg

Ts

Tx

BAS-4

643

745

850

BAS-5

678

828

857

BAS-6

670

734

756

BAS-7

680

753

758

batch compositions and the minor addition of aluminum oxide contributes to stabilize the glass structure (Table 1).

2.2. Material synthesis The starting raw materials analytical reagent graded Ba(OH)2 as source of BaO, Al2O3, SiO2, and H3BO3 from Sigma-Aldrich (purity ˃ 99.9%) were used to prepare the glasses. The starting weighted amounts were mixed for 60 min in a ball mill, transferred to an alumina crucible, molten in the temperature range of 1400 °C during a dwell time of 2 h, and quenched in a pre-heated metallic plate. The glasses obtained were crushed and milled in dry conditions in a rotating mill using agate containers and agate balls for 2 h to obtain a fine glass powder (∼50 to 70 μm).

3. Material characterization 3.1. X-ray powder diffraction (XRD) analyses The amorphous nature, after the melt quenching of the samples was confirmed by X-ray Diffraction (XRD) (Fig. 2).

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Fig. 2. Schematic drawing showing the break of the BO's covalent bond and an intermediary step producing NBOs and the pairs: [NBO−;Vo+], forming dipoles and quadrupoles.

plots (not shown) were modeled the circuit known as Randles cell, which can be used to fit process with a single capacitive arc [31–33]. The main drawback of this type of representation is that all frequency information is inherently lost. As described above, to avoid missing information, it is necessary to use another representation of the EIS called Bode plot, in which the impedance magnitude and phase angle are plotted versus frequency. The magnitude and phase angle are given, respectively, by the following expressions:

3.2. Thermal analyses The characteristic temperatures, namely the glass transition (Tg) and the onset of exothermic crystallization point (Tx), were determined by differential scanning calorimetry (DSC). The thermal expansion coefficient and shrinkage rate (maximum densification or sealing point) of glasses were determined in the temperature range RT to 850 °C. All measurements and the respective conditions were performed in air and described by Da Silva [9] in a previous work. The thermal stability of glass was calculated as the difference between Tx and Tg [29,30].

2 Zre2 + Zim

(3)

3.3. Ionic conductivity

and

To elucidate the dependence of the conductivity of the glass ceramics with temperature we used Arrhenius's type plot according to the expression:

Z θ = tan−1 ⎛ im ⎞ ⎝ Zre ⎠

(4)

E σT = σ0 exp ⎛− a ⎞ ⎝ KB T ⎠ ⎜

|Z| =

⎜

where Zim and Zre represent the imaginary and the real parts of the complex impedance, respectively. To obtain the Bode spectra through glass/YSZ interface, a symmetrical three-electrode cell configuration was employed [34]. The working electrode exposed area was 1.0 cm2 in air. The three-electrode symmetrical configuration cell was composed by a YSZ (nominal composition: Zr0.84Y0.16O2−x) disk 0.124 mm thick and 12.6 mm diameter with platinum wire fixed at the edge of the YSZ disk, and with glass coatings on both faces of the YSZ supports. The investigated glasses were built on YSZ supports. To prepare the supports, commercial YSZ powder (TZ-8Y, 8mol% Y2O3; Tosoh Corp., Tokyo, Japan) was axial pressed into disks under the pressure of 45 MPa, and sintered at 1400 °C for 2 h. The surface of the disk was polished down to 1 μm. Each glass composition was applied by brush painting (glass + binder paste) over a printable screen. After that a layer of platinum current collector was applied on each glass composition by brush painting of the ESL 5545 (Electroscience Laboratory) paste and finally annealing at sealing (Ts) temperature. The Bode representations were carried out using impedance spectroscopy at Ts estimated by DSC thermograms. This parameter was described previously [9,35].

⎟

(1)

where Ea is the activation energy for conduction, σ0 the pre-exponential factor, T the absolute temperature, and KB the Boltzmann's constant. The bulk resistance value (Rb) was obtained from complex impedance plots for all different samples and the conductivity values were determined with the equation:

l σ = Rb−1. ⎛ ⎞ ⎝ A⎠

⎟

(2)

where (1/Rb) is the conductance and (l/A) the geometric factor of the sample. 3.4. Impedance spectroscopy at interface YSZ/Glass Electrochemical Impedance Spectroscopy (EIS) measurements were carried out in the temperature range of 600–850 °C (working temperature range for intermediate SOFC) using Novocontrol Alpha A broadband dielectric spectrometer coupled to a high temperature digital controller Fuji PXR4. The frequency range was 1 Hz to 1 MHz and the ac signal was 1.5–2.5 Vrms. The samples were cut into pellets, 5 mm in diameter and 1 mm thick, and their faces were polished. Posteriorly gold electrodes were prepared by sputtering in vacuum. The Nyquist

3.5. Scanning electron microscopy (SEM) study The images of the glass/YSZ interface (cross-section) at Ts (Table 1) were obtained on surfaces polished with silica carbide and colloidal 3105

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provided adequate percolation conditions are satisfied, a hopping conduction process will be detected with corresponding Arrhenius activation energy equal to the depth of the corresponding potential well. As our samples were synthesized at oxidizing condition (in air), as well as the conductivity measurements (∼Tg in air), it is clear that the O20 molecule is present in the glass matrix interstices, together with the highly reactive O2− ions, which are readily oxidized as:

silica suspension by SEM (FEI QUANTA Mod. FEG 250). The microstructure near the interface was investigated to verify possible modifications on microstructure caused by phase transition, sintering and possible reactions toward thermal cycling. 4. Structural outline: the role of the non-bridging oxygen and oxygen vacancy defects

⇀

O 2 −⟵O− + e−

4.1. During the quenching of the melt

e− +

We found useful the application of a model based on the first precursor chemical reactions that occur in the melt during the preparation of an alkali-silicate glass [36]. The production of NBO− pairs can be directly inferred, considering that the melt-quenching method reactions occur at the liquid phase between the glass-former and glass-modifier oxides. In Fig. 2 it is sketched its application to a divalent Ba2+ modifier ion. It is assumed that the reaction of the previously formed tetrahedral SiO4 structural units linked by their vertices (Zachariasen rule) at the early formation phase of the continuous random network of the glass matrix. The further succession of collisions between the heavy Ba2+ against the covalent bonds of the neutral bridging oxygen (BO0), near the glass transition temperature, ∼Tg, causes the bond breaking (see Fig. 2a) and the subsequent reaction T ∼ Tg

(7)

and the overall reaction:

O2− +

1 0 ⇀ − 1 O2 ⟵O + O2− 2 2

(8)

Thus, at least two or more species of charge carriers are expected to contribute for the activated hopping conductivity (AHC) detected in this work. 4.3. Results and discussion In Fig. 3 we present the X-ray diffractograms of the glass samples BAS-4, BAS-5, BAS-6 and BAS-7. It is possible to verify that there are no crystalline peak patterns, the haloes are broad and diffuse, for all compositions. Therefore, it can be stated that the samples are amorphous according to XRD technique. In other words, all the compositions tested can be considered amorphous, or that the size of the crystallites eventually present in those compositions, previous to thermal treatment, is below the XRD detection limit (around 30 Å) [40]. The dependence of the conductivity of the glass ceramics with temperature in Arrhenius's type plot thermally activated according to the expression (Eq. (1)) over 600–850 °C is shown in Figs. 4 and 5. The macroscopic signature of the characteristics temperatures Tg, and Tx are present and in good agreement with previous thermal analyses [9,35]. However, measurements show that the behavior is more complicated than this simple scenario suggests. The analyzed glass–ceramics differs above transition point and below the onset of crystallization (BAS-6 and BAS-7). In some cases (BAS-4 and BAS-5 samples) there is an additional region in between glass transition and crystallization. For the studied compositions the range of conductivity is appropriate [5] for intermediate temperature SOFC applications, between 10−5 and 10−10 S/ cm. We can differentiate the investigated samples into two groups: in

(5)

BO0 + BaO ⥫⥬ = 2NBO−1 + Ba2 +

1 0 ⇀ 1 − O2 ⟵ O2 2 2

(6)

−

yielding the production of a pair of NBO s charge-compensated by the Ba2+cation, resulting the [NBO−,Ba2+,NBO−] electrical quadrupole shown in Fig. 2b. Conversely, the reverse equilibrium reaction would yield back the BaO and BO0, but at the temperature as high as ∼Tg, the recovery of the broken bond becomes improbable, whereas the BaO should appear dissociated as Ba2+ andO2− occupying the interstices of the silicate glass lattice leaving back the [NBO−,VO+] dipole in the place where it would be expected the original BO0 recovery (Fig. 2b), but at such high temperature, the Ba2+ and O2− ions remain occupying the interstitial places of the silicate glass continuous random network. The low mobility of the Ba2+ causes their permanent role of charge compensator of a pair of NBO−s, and the O2− ions diffuse through the available interstitial paths, always subject to the attractive coulombian forces with the positively charged oxygen vacancies VO+, as indicated in Fig. 2c. The VO+ are obviously fixed in their original positions, and are randomly distributed throughout the entire volume of the sample. 4.2. During the heating across Tg: a model of the activated hopping conductivity (AHC) On heating the sample above Tg, we suggest that the first broken covalent bonds are those of the NBOs, that persist until its complete elimination, leaving at their original sites the positively charged oxygen vacancies (VO+). The released O2− ions become free to move inside the glass structural interstices, thus contributing to the change of the local configuration in response to any temperature variation [37–39]. In order to analyze the local contribution to the onset of Tg, let us suppose that one of the BOs are broken on heating as shown in Fig. 6a. The Ba2+ ion remains about their original position, due to their high atomic weight and the electrostatic attraction of the opposite NBO−. The local effect of electrostatic repulsive force between VO+ and Ba2+ produces a small increase in the interstitial space in the vicinity of the VO+ (Fig. 6b), allowing the approximation of the O2− which is gradually attracted back to its original, being trapped by a succession of local potential wells of several depths, such as described in a recent work of J.M. Giehl et al on γ-irradiated tellurite glass heated during the measurements of thermally stimulated depolarization currents (TSDC) [37]. On applying an external electrical field on the O2− charge carrier,

Fig. 3. Diffractograms corresponding to the investigated glasses after melt and quenching and before crystallization, confirm the amorphous nature of all studied samples. 3106

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Fig. 4. DC conductivity vs. 1000/T and dependence of the activation energy (Ea) for bulk ionic conductivity (σ), as determined from Arrhenius plots after DC impedance spectroscopy assays between 600 and 850 °C for samples: BAS-4 and BAS-5. The square of the correlation coefficients (R2) after linear fit are presented inside parentheses.

the first one shown in Fig. 4a and b we identify three distinct linear regimes of the glasses BAS-4 and BAS-5, represented as a sequence of three linear intervals: (i) T < Tg, (ii) Tg < T < Tx and (iii) T > Tx, where Tx is the onset of crystallization point. In the second group, shown in Fig. 5a and b the regime (ii) has been collapsed for the samples of lower BaO content, remaining only the Tg at the crossing point of the straight lines of the former regimes (i) and (iii). The respective experimental Ea values are displayed in Table 2. As the temperature increases through the first crossing point shown in Fig. 4a and b, identified as Tg, as stated in Section 4.2, the AHC charge transport mechanism is directly related with the increase in the slope of the conductivity. It indicates the onset of the contribution to the configuration changes to maintain the oxygen “liquid” phase in thermal equilibrium on heating. The O2− ions, which originated from the thermal breaking of the NBO−s bonds, are now free to move by means of hopping steps or successive traps of potential wells of variable deepness in the neighborhood of the positively charged oxygen vacancies VO+ in the remaining glass matrix structure. As an external electric field is applied to the system, the thermally activated O2− carriers are drifted toward the cathode, generating thus the detected electrical conduction. According to the present model, notwithstanding the onset of the “liquid” phase behavior of the oxygen ions at Tg, the BOs remain fixed by their covalent bonds to the remaining glassforming CRN of the glass matrix. This behavior is maintained until the heating temperature reaches the value Tx, where a new slope is

Table 2 Temperature dependence of the bulk ionic conductivity (σ) as determined from AC impedance spectroscopy between 600 and 850 °C. The square of the correlation coefficients (R2) after linear fit are presented inside parentheses. Glass Sample ID

BAS-4 BAS-5 BAS-6 BAS-7

Ea (eV) Temperature range (°C) T < Tg

Tg < T < Tx

T > Tx

4.57 4.92 3.64 3.56

1.98 (.99) 2.16(.97) – –

1.65 1.54 2.98 3.06

(.98) (.98) (.99) (.99)

(.99) (.99) (.99) (.99)

observed in the conductivity. At this point a fraction of the glass composition was separated by the onset of crystallization, giving rise of the glass–ceramic system. The higher slope after T > Tx is a result of the contributions of additional mobility acquired by the oxygen ions which resulted from the breaking of BO bonds which belonged to the more stable glass-forming tetrahedral structural units. That means that the Tx is coincident with a new Tg point, as it results from the crossing point between two conductivity slopes. After the final liquid phase of the melt is reached, when the temperature has overcome all the ceramic crystalline component melting points, all the ions are free to move, including the Ba2+cations. Fig. 5. DC conductivity vs. 1000/T and dependence of the activation energy (Ea) for bulk ionic conductivity (σ), as determined from Arrhenius plots after DC impedance spectroscopy assays between 600 and 850 °C for samples: BAS-6 and BAS-7. The square of the correlation coefficients (R2) after linear fit are presented inside parentheses.

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The behavior of the conductivity shown in Fig. 5a and b indicates that for the glass chemical compositions of Ba2+ content less than 70 wt % the regime (ii) is absent because of the lesser content of the NBOs precursors of the oxygen vacancies generated near the first conductivity slope at Tg. Since glass–ceramics composites are formed by crystallites and an amorphous phase, the following scenario can be envisaged: after reaching the onset of crystallization mobile ions from the crystallites (barium silicates, borates, and celsian) clustered surround Ba2+ (modifier oxide ion) forming internal interphases. Such structural environments generate pathways extending at the vicinity of the clustered crystallites [41]. Depending of the composition, higher level of glass matrix, the ionic conductivity in the space charge regions is enhanced in relation to the bulk of the crystals. In this case, we can suggest that the percolation through interphases would be beneficial to the conductivity enhancement (BAS-4 and BAS-5 samples). In particular of the scenario envisaged, it has to be borne in mind that the population of Q2 units are prevailing in this glass network [35]. Consequently, decoupling of the stressed rigid regions improve the percolation through those less rigid or “floppy” units [42]. This is a useful simplification of the so-called percolation theory. Such generalization will be considered upon this work [24]. Figs. 7 and 8 show plots of conductivity vs. BaO and B2O3 content, respectively, at three characteristic temperatures Tg, Ts, and Tx previously determined and defined [9]. Among the glass–ceramic compositions studied, there is not a linear dependence between Ba2+ content and conductivity, as thermal treatment (Tg, Ts, Tx) may contribute to other changes in addition to ionic mobility. The two most possible explanations for the decrease in σ is the densification (Ts point) of the composite structure and that crystalline regions block the ionic motion consequently decreasing the conductivity. Previous results [9] report at T > Tg crystalline peaks attributed to hexacelsian (BAS-2), barium silicate (BS) and barium orthosilicate (B2S), whereas the ionic conductivity (σ) drops sharply as the crystallites begin to comprise at Ts point. In such compressed environment, small variations in the amorphous phase can trigger a discontinuity of the easy path of conduction. When the Ba2+content is 72wt% (Fig. 7), BAS-5 reaches its maximum in conductivity. After this point, the reverse trend occurs: the clusters of bonds which are conducting are randomly removed, and conductivity suddenly decreases. On the other hand, for BAS-4 sample (74wt% Ba2+), at this critical concentration there is a slight increase in conduction. The barium ion concentration can give us some clues. As the Ba2+ ion radius (1.35 Å) is 4 times larger thanB3+

Fig. 8. Conductivity as a function of B2O3 mass fraction (wt%) at characteristic temperatures: glass transition (Tg), sealing point (Ts), and onset of crystallization (Tx). The lines connecting the data were drawn only as a guide for the eye.

Fig. 6. The local effect produced by the electrostatic repulsive force between the positively charged oxygen vacancy (VO+) and ion Ba2+ triggering the migration of O2−.

(0.27 Å), the Ba2+ is the more likely ion to block the possible conduction paths. In Fig. 8, as the amount of glass former cation boron increases, the ionic conductivity (σ) gets progressively lower, reaching a minimum at 9 wt% of B2O3. These maxima and minima are explained in terms of the ability of a boron atom to change its coordination from 3 to 4 according to Beekenkamp observations [43]. When the concentration of boron exceeds 9 wt% the tendency of increasing the number of 4-coordinated boron atoms disappears. This implies that oxygen ions in the BAS-6 and BAS-7 glass samples are more highly coordinated and strongly bonded to the boron-oxygen structural units than in the remaining compositions (BAS-4 and BAS-5). Bode plots are shown in Fig. 9 for the studied compositions at sealing temperatures (see Table 1). In a previous section reference was made that impedance spectra were used to characterize ionic conductivity of the bulk glasses, as an indirect evidence of sintering and crystallization processes. Let us now consider a different scenario: fully stabilized crystals of ZrO2 containing Y2O3, whose migration vacancies give for the YSZ ceramics the status of a good ionic conductor at high temperatures, “sandwiched” by glass–ceramic. It is assumed that the impedance response obtained for glass/YSZ crystals/glass interface

Fig. 7. Conductivity as a function of BaO mass fraction (wt%) at characteristic temperatures: glass transition (Tg), sealing point (Ts), and onset of crystallization (Tx). The lines connecting the data were drawn only as a guide for the eye.

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(θ = −10°) fluctuations at low frequency (50 Hz) and the peak (θ = −75°) at high frequencies (50 kHz) for BAS-7 sample suggests the existence of a transport process through the interface that is best described in terms of impedance response to the grain properties [33]. The “hump” shape peak in the phase angle corresponds to a time constant of the system and an indication that the kinetics associated to this process becomes faster, the “hump” peak shifts to higher frequencies (e.g. BAS-7), in some cases becomes a hump or even a plateau (e.g. BAS-5), suggesting that the resistivity decrease because of increment of charge carriers with increasing Ts, which leads to the observed conductivity behavior (see Figs. 7 and 8). The SEM micrographs of the polished glass–ceramic/zirconia interface cross-section after thermal treatment at sealing temperatures (see Table 1) are shown in Fig. 10. The observation of the micrographs reveals that for all samples immiscibility can be verified by the presence of crystal phase, which are both solid and hollow, immersed in the glassy matrix. There is a distinct difference among the microstructures of the (a), (b), (c) and (d) images. Clearly the crystals in random orientation and the presence of residual glassy phase can be verified. The grown crystals are most evident and closely packed together in the sample BAS-7, than in the remaining ones under the same heating rate conditions (10 °C/min). The crystalline phase composed of sanbornite (BS) and/or celsian), as already evidenced by XRD [9], can reduce the flow of ionic charges probably induced by the chain formation of boron phase particles. An examination of the microstructure shown in Fig. 10d (BAS-7 sample), indicates that nucleation and growth of the second phase occurred as a needle-like structure. Some residual glassy phase can be verified predominantly between whiskers phases, in which an interlayer amorphous phase can be found. It is attributed to this type of morphology, presented mainly by the sample BAS-7, a strong influence on the transport properties such as ionic conductivity. Many studies have been established that the process of ion transport in amorphous materials is considerable and its mobility decreases with increasing crystalline phase [44].

Fig. 9. Bode plots of impedance spectra from glass–ceramic systems exposed to sealing temperatures: 745 °C (BAS-4), 825 °C (BAS-5), 734 °C (BAS-6), and 753 °C (BAS-7). Impedance magnitude (left axis), and phase angle (right axis) vs. frequency.

exhibits only one time constant [32]. The impedance magnitude remains constant over exposing time at lower to medium frequencies, over exposing time at sealing point (Ts) to each sample studied, suggesting few conductive paths through the structure and clearly dependent of the amorphous nature of the samples. The decrease of the resistance to polarization (Rp) with increasing Ts according to the following sequence: BAS-7, BAS-4, BAS-6 and BAS5, suggests a thermally activated conduction behavior and the development of channels of ionic conduction at some specific sites, as the charge carriers proceed at the glass/YSZ interface. Despite under harsh conditions of temperature, the BAS-5 glass ceramics sample shows remarkable stability (1–100 kHz), except for a slow decrease of the impedance magnitude at high frequencies (> 100 kHz) at expense of exposed to the highest sealing temperature (Ts = 825 °C). The phase angle plots became broader at this order: BAS-7, BAS-4, BAS-6, practically at the same temperature (around 730 °C), showing a shift of the phase angle peak toward higher frequency. The phase angle

Fig. 10. SEM images of the polished glass–ceramic/zirconia interface cross-section of glass BAS-4 (a), BAS-5 (b), BAS-6 (c), and BAS-7 (d), after thermal treatment at sealing temperatures (see Table 1). 3109

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5. Conclusions [12]

Four glass–ceramics batch compositions belonging to BaO-Al2O3B2O3-SiO2 quaternary system have been studied as potential glass ceramic SOFC sealants at intermediate temperatures using low cost ecofriendly source of oxides and designed according to phase equilibrium approach. The results for activation energy show that the ions move across the glassy phase and after that reach percolation pathways. There is a strong relationship between amorphous state, gaps of immiscibility and morphology, with ionic conductivity depending on the amount of residual glassy phase, clustered crystals and phase separation mechanisms (nucleation and growth). The transition from glassy to crystal state is related with changes on mobility of charge carries and to the accommodation of the structural units (relaxation). The electrochemical measurements (Bode plot) indicated that the level of glassy phase results in an enlargement of the phase angle peak to the middle to low frequency range related to structural relaxations process at transition temperature range. The maximum value of the impedance magnitude in the low frequency range remains constant during around 100 kΩ indicating stability at high temperature (> 800 °C). The four compositions labeled BAS-4, BAS-5, BAS-5 and BAS-6 can be tailored by proper use as rigid sealant intermediate temperatures SOFC. We found that these parent glasses, mainly the BAS-5 sample, are able to cope with the limiting factors by which this interface (YZS/glass) can affect the impedance spectrum response. The present study improves our previous methodologies by combining phase equilibrium relation with spectroscopy analyses. The investigated samples exhibit properly the range of ionic conductivity of 10−5 to 10−10 S/cm, in the temperature range (700–850 °C). Further investigations and measurements at the transition point will be achieved in near future.

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Acknowledgements [23]

The authors would like to acknowledge the Brazilian Research Agency – CNPq (www.cnpq.br) project No. 233658/2014-9 for the financial support and to acknowledge the Faculty of Electronics, Telecommunication and Informatics (eti.pg.edu;pl) at Gdansk University of Technology for the experimental support. The authors would like to acknowledge: Aleksander Chrzan, Dagmara Szymczewska, and Dr. Grzegorz Jasinski for all experimental assistance.

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