Microstructural study of Crofer 22 APU-glass interface for SOFC application

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 8 3 9 e3 8 4 7

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Microstructural study of Crofer 22 APU-glass interface for SOFC application Bhupinder Kaur, K. Singh, O.P. Pandey* School of Physics and Materials Science, Thapar University, Patiala, Punjab 147004, India

article info

abstract

Article history:

Glasses of composition (60
x)SiO2e15CaOe10Al2O3e(10 þ x)Na2Oe5TiO2 (x ¼ 0, 5, 10, 15)

Received 14 March 2011

were prepared. The crystallization kinetics of prepared glasses were investigated by

Received in revised form

differential thermal analyzer, dilatometer and X-ray diffraction techniques. In order to

22 April 2011

check the applicability of the prepared glasses as a sealant in solid oxide fuel cell, these

Accepted 23 April 2011

were deposited on Crofer 22 APU steel by slurry technique and thermally treated. The

Available online 11 June 2011

interface of the glassesteel composites was further analyzed under scanning electron microscope in conjunction with energy-dispersive X-ray spectroscopy and X-ray dot

Keywords:

mapping. All glass samples exhibit phase separation. The phase separation tendency

Crystallization kinetics

increases with increasing content of Na2O in glasses. The N-10 glass containing 10% Na2O

Hurby parameter

shows good adhesion with Crofer 22APU.

Microstructure

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

X-ray methods

reserved.

1.

Introduction

Solid oxide fuel cells (SOFCs) are energy conversion devices that can convert chemical energy of the fuel directly into electrical energy. This process is eco-friendly and efficient one [1e3]. SOFCs have received great attention as a promising new technology for power generation because they are able to utilize a wide variety of hydrocarbon fuels. A planar design of SOFC is simple, produces higher power densities with high efficiency than other designs. The typical design of planar SOFC requires hermetic seals to prevent fuel-oxidant mixing and also electrical insulation to the stack [4]. Glass and glass ceramics have been widely used for sealing different components of SOFC. First and the foremost requirements of a glass to be used as a seal is to have excellent thermo-chemical and thermo-mechanical stability in stringent oxidizing and reducing environments of fuel cell [5]. Lanthanum containing borosilicate glasses have been extensively studied as sealing materials for SOFC

[6e11]. Moreover, these reported compositions of sealing glasses are mostly confined to composition where silica acts as a glass former. Al2O3 in glasses acts as intermediate, which behaves as a network former and network modifier in the glass system. B2O3 is added in most of the glasses to decrease viscosity and increase the wettability with other components of SOFC. However, it also decreases the stability of seal as it leads to phase separation in borosilicate glasses. Furthermore, the presence of La2O3 additive in the glass matrix leads to formation of the devitrified phase at operating temperature of SOFC and the amount of that phase increases with high temperature aging [12]. Apart from B2O3, alkali metal oxide (Na2O) is used as a modifier to increase the wettability of glasses. It leads to decrease in glass transition temperature while the thermal expansion coefficient increases in hydrothermal conditions of SOFC [13]. This causes poor chemical stability of Na2O-SiO2 glasses in SOFC environment. However, addition of CaO in a Na2OSiO2 glass increases the chemical resistance to water

* Corresponding author. Tel.: þ919888401777. E-mail address: [email protected] (O.P. Pandey). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.160

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Table 1 e Glass compositions (mol %) with their label. SiO2

Al2O3

TiO2

CaO

Na2O

45 50 55 60

10 10 10 10

5 5 5 5

15 15 15 15

25 20 15 10

Materials and methods

2.1.

Glass preparation

Tp2

-10

Tm

Tp1

N-20

-20

induced phase separation [14,15]. Moreover, Al2O3 hinders the formation of the hydrated open network structure surface layer in a Na2OeCaOeSiO2 glasses [16]. Addition of polar oxides such as PbO and transition metal oxides such as ZrO2 and TiO2 in these glasses improves the chemical stability [15]. The stability of glass sealant depends on the inter-diffusion distance at the interface of glass-SOFC components, which should be less than 10 mm. The diffusion distance of Cr from ferritic steel into BaO-CaO borosilicate glasses increases from 10 to 15 mm with the thermal treatment time (from 10 h to 200 h) at 800  C [17]. Conversely, a Na2O-CaO-silicate glass reacts with pre-oxidized AISI 430 alloy and forms Cr, Mn, Fe and O enriched interface layer of 1e2 mm thickness in reducing atmosphere at 800  C after 200 h [18]. Smeacetto et al. [19] in their recent publication have also demonstrated that glasses containing a low amount of sodium oxide can be used as sealing material for SOFC. Based on these studies, soda lime aluminosilicate glass has been chosen in the present investigation as basic glass. TiO2 which acts as nucleating agent is added to improve the chemical stability [20]. In the present work, Na2OeCaOe Al2O3eSiO2eTiO2 glass system has been investigated in detail. To estimate the applicability of these glasses as sealants, their thermal properties, crystallization behavior, thermal expansion coefficient after heat treatment and the overall bonding characteristics of the glass with metallic (Crofer22APU) interconnect were investigated.

2.

N-15

V

N-25 N-20 N-15 N-10

Endo Down

Glass label

Tp2

Tp1

0

Tm

-30

Tp1

Tp1

Tp2 N-10

-40

Tp2 N-25

-50

Tm

Tm

400

600

800

1000

1200

Temperature ( C) Fig. 1 e DTA curves of prepared glasses at the heating rate of 10  C/min.

The glass ingots after casting were kept in a preheated furnace at 400  C for 12 h to remove the stresses of the quenched glasses.

2.2.

Phase identification

The amorphous nature of samples was characterized by X-ray diffraction (XRD) using X’Pert Pro, PAN alytical model of Philips, Netherlands. During the experiment, the scan speed was 0.02 /min. The crystalline phases obtained after heat treatment was identified by the Joint Committee on Powder Diffraction Standards (JCPDS-ICDD) files. Thermal behavior and crystallization kinetics of the glass was investigated by Differential Thermal Analysis (DTA) (DTA-TG, Perkin Elmer, USA) using powder samples with alumina as a reference material. For crystallization kinetics, 15 mg of glass samples were scanned by DTA in nitrogen at different heating rates (a) of 5, 10, 15, 20  C/min from 50  C to 1200  C. Thermal expansion coefficient of the glasses was measured using Netzsch DIL 402 PC, UK, at the heating rate of 5  C/min in the air environment.

Selected glass compositions for present study with their designation are given in Table 1. Glasses were synthesized using conventional splat quenching technique. The minimum purity of all the ingredients used to synthesize these glasses was 99.8%. For the synthesis of glasses, required amount of ingredients were taken. These ingredients were ground in ball mill (Retsch PM100) for 2 h in dry media. The ball milled mixtures were melted at 1500  C in a recrystallized alumina crucible.

2.3.

Activation energy calculations

The activation energy of the glass transition (Eg) and crystallization (Ec) can be obtained by the following relationship [21]: E ln a ¼
þ constant RT

(1)

Table 2 e Activation energy (kJ/mol) for the glass transition and crystallizations with various methods. Sample identity

N-25 N-20 N-15 N-10

Mahadevan method

Kissinger method

Augis and Bennett method

Eg

Ep1

Ep2

Eg

Ep1

Ep2

Eg

Ep1

Ep2

271.11 58.80 149.38 100.77

386.11 51.29 216.61 142.73

215.55 68.45 124.04 167.32

254.86 43.17 134.40 83.78

369.21 33.92 199.99 124.07

196.47 48.14 104.96 146.77

262.98 242.57 260.47 249.55

265.03 236.49 260.76 254.84

258.59 263.59 255.27 256.42

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1 .0 0

1 .0 1

1 .0 2

1 .0 3

ln ( /Tg)

-4 .0

3.5

1 .0 4

c

3.0

-4 .5

2.5 -5 .0

b

-1 1 .0

2.0

1.5

2

ln ( / Tg )

S (K)

-1-5 0 .5

-1 1 .5

1.0

-1 2 .0

0.5

ln ( )

3 .0

a

10

15

2 .5

20

25

Na2O (mol%)

2 .0

Fig. 4 e The graphical representation of S parameter with respect to Na2O (mol %) at heating rate 10  C/min.

1 .5 1 .0 0

1 .0 1

1 .0 2

1 .0 3

1 .0 4

-1

1 0 0 0 /T g (K )

The slope of the plot ln (a=T2 ) versus 1=T (K
1) gives the value of activation energy. Additionally, activation energy was also calculated using the method proposed by Augis and Bennett [23]:

Fig. 2 e Activation energy plot with (a) Mahadevan (b) Kissinger and (c) Augis and Bennett methods for N-25 glass.

ln where E is the activation energy and R is the gas constant. The second approach to evaluate the activation energy of the glass transition (Eg) and crystallization (Ec) is Kissinger equation as given below [22]:  ln

a T2



E ¼
þ constant RT

(2)

a T

Ec þ ln Ko ¼
RTp

(3)

The activation energy of the glass transition and the crystallization processes are calculated and compared with the values as obtained from the Kissinger and Mahadevan models. From the point of view of technological application, the glass should be thermally stable. A parameter usually employed to estimate the glass stability is the thermal stability (DT) [21], which is defined by the following equation: DT ¼ Tc
Tg

1.8 15 C/min 10 C/min 5 C/min

(4)

The larger difference between Tc and Tg, the higher is the kinetic resistance to crystallization. In other words, the glass is thermally stable.

N -25 N -20 N -15 N -10 N -1 0 G C C rofe r

0.006

1.2

dL/dLo X10

-6 -1 K

Hr

0.005

0.6

0.004

0.003

0.002

0.001

10

15

20

25

Na2O content (mol%) Fig. 3 e The graphical representation of Hruby parameter with respect to Na2O (mol %).

0.000 100

200

300

400

500

600

700

Tem perature ( C )

Fig. 5 e Thermal expansion curves of prepared glasses, N-10 glass ceramic and Crofer steel.

800

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Table 3 e Properties of glasses and steel. Properties Sample identity

DTA data

Dilatometric data

Glass transition Crystallization Glass transition Softening TEC (K
1) in the range temperature, Tg ( C) temperature, Tp ( C) temperature, Tg ( C) temperature, Ts ( C) of 200e550  C

N-25 N-20 N-15 N-10 Heat treated N-10 Crofer 22APU

692 616 607 708 e e

741, 705, 711, 831, e e

864 915 910 965

562 572 628 654 e e

Another thermal stability parameter, S, proposed by Saad and Poulain [24], is given by Eq. (5)

 S ¼ Tp
Tc Tc
Tg Tg

(5)

The thermal stability parameter, S, reflects the resistance to devitrification after the formation of the glass. (Tp-Tc) is

8.88 8.31 8.34 8.19 8.27 8.42

     

10
6 10
6 10
6 10
6 10
6 10
6

related to the rate of devitrification transformation of the glassy phases, while a high value of (Tc-Tg) delays the nucleation process. Hurby [25] has given a factor which combines both nucleation and growth aspects of glass transformation and is given by Hr ¼

1600

589 629 657 709 e e

Tc
Tg Tm
Tc

Where Tm is the melting temperature of glass.

N-10

Intensity (Counts)

1200

N-15 800

N-20 400

N-25 0 10

20

30

40

50

60

70

80

90

100

2 (Deg.)

Fig. 6 e XRD patterns of prepared glasses.

AlNa ( SiO4 ) SiO2

Intensity (Counts)

800

(c)

400

(b) (a)

0 20

40

60

80

2 (Deg.) Fig. 7 e XRD patterns of (a) N-10 glass, (b) N-10 heat treated glass at 900  C and (c) N-10 heat treated glass at 950  C for 1 h.

Fig. 8 e (a): SEM micrograph of interface between N-10 glass and Crofer steel showing the overall view of interface. (b): Back scattered electron micrograph of interface between N-10 glass and Crofer steel.

(6)

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Fig. 9 e (a): EDS spectrum analyses of the glass of the diffusion couple marked 1 in Fig. 8(b). (b): EDS spectrum analyses of the interface of the diffusion couple marked 2 in Fig. 8(b). (c): EDS spectrum analyses of the steel of the diffusion couple marked 3 in Fig. 8(c).

2.4.

Formation of the diffusion couple

The steel plates (Crofer) were cut in 1  1 cm2 to prepare the diffusion couple. This piece was degreased by ultrasonic cleaner in acetone media. After that, these pieces were dip in a solution of 5% HNO3 for 5 min to dissolve any oxide layer formation on the surface. The glass coating was applied onto the cleaned substrate surface by a slurry method. The thickness of the glass coating layer was 1 mm. For coating, the slurry was prepared from the mixture of the glass powder dispersed in 5% PVA. The diffusion couple was thermally treated at 900  C for 1 h, to check adhesion of the glass with Crofer 22APU. The heat treated diffusion couple was characterized by scanning electron microscopy (SEM, JEOL6400, Japan) to study the interface in conjunction with energy-dispersive X-ray spectroscopy (EDS). The sample was mounted in epoxy resin and ground flat by using 240, 400, 800 and 1200 grit abrasive papers consecutively and then polished with diamond paste of one mm to achieve a mirror-like surface finish. The sample was etched with 0.05 N HCl solution containing few drops of HF for

1 min before an examination under SEM. In order to check any variation in chemical composition before and after heat treatment, N-10 glass was further subjected to heat treatment at 900  C for 1 h. EDS analysis on polished surface of the heat treated glass was carried out.

3.

Results and discussion

3.1.

Thermal properties of glass and glasseceramic

Fig. 1 shows the DTA curves of all the glasses. DTA curves exhibit two exothermic peaks, which indicate phase separation in the glasses. Basically phase separation occurs due to the formation of a second glass matrix within the glass matrix. Higher modifier contained glasses, in general, exhibit phase separation tendency [26]. The glass transition temperature (Tg) of glasses is observed in the range of 600e800  C followed by two exothermic peaks, which belongs to crystallization of the glasses. Endothermic peaks

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Fig. 10 e X-ray dot mapping of Na, Al, Si, Cr and Fe of the interface of the diffusion couple.

at around 1050e1200  C are denoted for melting of the glasses (Fig. 1). As a modifier (Na2O) increases the Tg, Tc and Tm decrease in the present glasses. These observations are similar to that of reported in sodium silicate glasses [27]. Based on the DTA results, the theoretical calculations were made using different models as described in experimental section. Calculated activation energy of these glasses with various models is summarized in Table 2. Kissinger and Mahadevan methods give nearly same value of activation energy but Augis and Bennet’s method gives relatively large value as compared to others. The average value of activation energy of glasses except N-20 is large as compared to another borosilicate system [9]. From Table 2, it can be observed that N-20 glass shows very less value of activation energy. It is well reported in literature that with the increase of the modifier content the network weakens, which will decrease the value of activation energy [26]. But the value of activation energy follows the opposite trend in this system. This anomaly arises due to the presence of minor amount of SiO2 (Zeolite) phase in the microcrystalline glass matrix containing higher Na2O. The increase in Na2O content with a decrease of SiO2 content increases the phase separation tendency in glasses as observed in Fig. 1. It can be explained on the basis of the diffuse and broad exothermic/endothermic peaks in DTA curve. However, the trend observed for activation energy of all the glasses is same. Fig. 2 depicts such variation for N-25 glass. As mentioned in experimental part, the difference between Tg and Tc also indicate the thermal stability of the glasses. According to this approach, N-10 glass is more stable as compared to the other glasses as is evident from Fig. 1.

Fig. 3 shows the variation in Hr parameter for the second crystallization peak of different compositions at different heating rates. The Hr parameter of N-10 glass is higher than other glasses as shown in Fig. 3. In general, the modifier weakens the glass network and decreases the glass transition temperature so the less content of Na2O in glass composition shows better thermal stability [28]. Similarly, Saad and Poulain have suggested a criterion to check the stability of the glasses. According to this approach, N-10 glass exhibit higher S value which indicates the higher stability of this particular glass. The S parameter with respect to the Na2O is shown in Fig. 4. The dilatometer study was performed to know the variation in a thermal expansion coefficient of all the pristine glasses, which is shown in Fig. 5. N-10 glass shows a lower thermal expansion coefficient as compared to the other glasses. The thermal expansion coefficient originated due to asymmetric potential well of the solids [29]. In the present glasses, the observed TEC decreases as the modifier content decreases except N-20. But the increasing trend of Ts value is observed (Table 3). It is evident in XRD pattern of pristine glass as shown in Fig. 6. The XRD pattern of higher modifier containing glasses exhibits a sharp hump around 30 which is due to zeolite phase [30]. The presence of this phase may lead to decrease the TEC of the glasses since TEC of zeolite (SiO2) is very low (0.5  10
6/K). In addition to TEC measurement on pristine glasses, TEC of steel is also measured under a similar condition to check the compatibility between selected N-10 glass and steel. The difference in TEC values of glasses and steel is less than 1  10
6 which is necessary during SOFC operation (Table 3). During SOFC operation glass gets

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indicates that the glass may be useful for interfacial study with Crofer [32]. Based on the experimental and theoretical results, N-10 glass was selected to make the diffusion couple between Crofer 22APU steel and N-10 glass. The detailed microstructural study of the interface between N-10 glass and Crofer 22APU steel was done to get more insight about the adhesion and interfacial mechanism.

3.2.

Analysis of glassesteel interface

To study the interfacial phenomenon, glassesteel interface was analyzed under SEM. Cross-sectional microstructure of the interface between N-10 glass and Crofer 22APU is shown in Fig. 8 (a & b). The interface between glass and steel shows good adhesion (Fig. 8(a)). However, some voids and porosity are observed at the areas away from the interface (Fig. 8(b)). The EDS analysis at the marked area as shown in Fig. 8(b) was done to understand the diffusion and inter-diffusion of the elements. This is shown in Fig. 9(a)e(c) for the areas marked as 1, 2 and 3, respectively. As observed from the EDS analysis, the maximum diffusion of Cr has taken place from the steel side. On the other hand, only Al3þ has diffused from the glass side to the interface which is very less in amount. Basically, the interface

Fig. 11 e (a): Back scattered electron micrograph of the N-10 glass. (b): EDS spectrum analyses of the glass marked in Fig. 11(a).

converted to the glass ceramics. The presence of these crystalline phases in the glass matrix changes the TEC. Therefore, before the interfacial study, the selected glass was heat treated for different duration and temperature. The heat treatment is required to facilitate the maximum crystallization for diffusion to occur. Ultimately, this will lead to achieve good adhesion between steel and glass. Many researchers have reported that phase separation leads less activation energy of the crystallization, which is essential for good adhesion with other metallic materials [31]. The peak crystallization temperatures Tp1 and Tp2 of N-10 glass are observed at 780  C and 925  C, respectively. Based on the analysis of crystallization kinetics of the N-10 glass, the heat treatment temperature selected for present study was 900  C, which is just below the second crystallization temperature of this particular glass. The heat treated glass exhibits two different crystalline phases, i.e. aluminum sodium silicate (JCPDS #00-002e0625) and zeolite (JCPDS #01-073e3414) as shown in Fig. 7. The formation of zeolite (SiO2) phase in glass ceramic is harmful to SOFC application since it has low thermal expansion. However, overall thermal expansion of glass ceramic increases slightly as compared to pristine glass. As shown in Table 3, the variation in TEC values of glass and glass ceramic with steel is within the limit (<2%). This

Fig. 12 e (a): Back scattered electron micrograph of the N-10 heat treated glass at 900  C for 1 h (b): EDS spectrum analyses of the glasseceramic marked in Fig. 12(a).

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contained Cr3þ rich layer. Though its diffusion is within the permissible limit, but it has got a higher tendency as compared to other elements. The overall analysis indicates that layers of chromium oxide and a reaction layer of Cr diffusion into the glass identified by EDS were found at the interface between the glass and the steel. For confirmation of the diffusion from steel to glass and vice-versa, X-ray elemental dot mapping of the interface was done. These results are shown in Fig. 10. The SEM image of the interface provides good adhesion of glass with steel as can be seen in Fig. 10(a). The X-ray dot mapping of Na, Al, Si, Cr and Fe are shown in Fig. 10(bef) respectively. These results further confirmed the diffusion of Cr3þ which is more prominent than other elements. However, the interface is smooth and free from porosity, which is required for good sealing. EDS analysis of as prepared and heat treated glass are shown in Fig. 11(a and b) and Fig. 12(a and b), respectively. The weight percentage of Na content in both the cases is about 4%. There is no major change in the weight percentage of sodium content before and after thermal treatment i.e. sodium is present in the glass network.

4.

Conclusions

Differential Thermal Analysis of all the glasses showed two crystallization peaks due to phase separation in glasses. N-10 glass exhibit maximum thermal stability as compared to other glass compositions. The TEC of N-10 glass ceramic is higher than the pristine glass. Heat treated glass (N-10) exhibit sodium rich (aluminum sodium silicate) and sodium free (zeolite) phases which are not detrimental for SOFC applications. The diffusion couple between Crofer and glass (N-10) shows the good and smooth interface which is formed mainly by the diffusion of Cr3þ and Al3þ ions. Naþ ions are not present at the interface.

Acknowledgment The authors gratefully acknowledge the financial support provided by UGC for this research work.

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