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Chapter 66 Compatibility of Electrolyte and Electrode Materials with LTCC

6. Compatibility of Electrolyte and Electrode

Materials with LTCC

The earlier Chapters of this Thesis have given detailed description

about synthesis of nano-crystalline electrolyte and electrode materials

for Solid Oxide Fuel Cells (SOFC). We have seen optimization of

synthesis parameters for oxygen ion conductor (Gadolinium doped

Ceria), with respect to its structural, morphological and electrical

characterization in Chapter 3. Synthesis and characterization of two

proton ion conductors viz. Yttrium and Ytterbium doped Barium

Zirconate (BZYYbO) and Yttrium doped Barium Cerate is discussed in

Chapter 4. The experimental results for the preparation and

characterization of anode and cathode materials, viz. Copper Zinc oxide

(CuZnO) and Samarium and Strontium doped Cobaltite (SSC) is

presented in Chapter 5. The present Chapter is an important part of

this Thesis as it presents a detailed study on compatibility of the above

mentioned materials with standard LTCC materials, leading to device

fabrication.

6.1 Need for compatibility

Electrolyte and electrodes are important components of solid

oxide fuel cells. All these components in SOFC are made up of ceramic

(oxide) materials. As we have seen before, these materials require high

sintering temperature, which is usually above 1200°C. Their physical

properties, such as, shrinkage, temperature Co-efficient of Expansion

(TCE), thermal conductivities are dependent on constituent materials.

Especially, being a ceramic material they have high TCE values and low

thermal conductivity. On the hand, LTCC is a glass ceramic technology

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which has low sintering temperature, low TCE valuable 6.1 presents a

comparison of important properties of SOFC and LTCC materials.

Physical Properties SOFC materials LTCC materials (DuPont 951)

Materials type Ceramics Glass-ceramics

Sintering temperature (oC) >1200 875

Operating temperature (oC) 800-1000 600

Temperature Co-efficient of expansion (ppm/oC)

>12 5.8

Thermal conductivity (W/mK) 1-2 3.0

Density (gms.cm-3) >6 3.1

Shrinkage (%) 14% @1350°C 12.9% @ 875°C

Table 6.1: Overview of present status of physical properties of materials

used in SOFC and LTCC

It is clearly seen from Table 6.1 that the physical properties

of materials used in SOFC are quite different from that for LTCC. Their

chemical properties are also dissimilar. Electrodes of SOFC are

chemically active and are involved in catalytic reactions during SOFC

operation. On the contrary, LTCC materials are highly inactive in

sintered state. The electrical properties of SOFC and LTCC are also very

different from each other. The SOFC electrodes are mixed conductors,

the electrolytes are charge conductor at high temperature, while the

LTCC materials are essentially used as dielectric material having low

dielectric constant and low dielectric loss. It is clearly seen that most

physical, chemical and electrical properties of SOFC and LTCC

materials are in conflict with each other. Nevertheless, we have

undertaken the task of integrating these materials in view of enormous

advantage offered by the integrated and low temperature SOFC. This

Chapter presents details of our efforts to make these two different

materials technologies compatible with each other.

In order to overcome these dissimilarities in various properties of

SOFC and LTCC materials, there is a need to first tune up the physical

properties of SOFC materials by formulating new combinations of

materials or by adding some additional materials as sintering aid to

reduce sintering temperature, matching shrinkage and other physical

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properties. Tuning of the rest properties would follow. Note that the

sintering temperature of an electrolyte is very high (1350°C) compared

to anode and cathode materials (900-1100°), Further, the density of

electrolyte (must be >96%) is very important factor in ionic conductivity

properties. Therefore, study of electrolyte material becomes more

important than the electrode materials. Clearly there is a need of an

appropriate sintering aid that can lower the sintering temperature of

SOFC ceramic electrolyte to the range of LTCC firing temperatures,

while simultaneously achieving the required sintering density. A

carefully selected sintering aid, either in crystalline or in the form of

glass, may help in lowering sintering temperature of SOFC electrolyte

without compromising its ionic conductivity properties. The following

Section describes the detailed study about effect of chosen sintering

aids on physical, chemical and electrical properties of SOFC electrolytes

GDC and BCYO.

6.2 Effect of sintering aids on Gadolinium doped Ceria

To study effect of sintering aids on Gadolinium doped ceria, first

Bismuth oxide (Bi2O3) was selected as sintering aid in GDC. This is

because Bi2O3 is a well known high temperature oxygen ion conductor

in its δ-FCC phase below 825°C (727–824°C) and its crystal structure

matches well with GDC. It has high polarizability in cataion network

due to highly disordered anion structure. About 25% intrinsic vacancies

are present in Bi2O3, Bi2+ ions have good ability to accommodate

asymmetry present in lattice structure. Bi2O3 has low melting

temperature close to 850°C, and enormous densification properties due

liquid phase sintering capabilities (1). Due to these favourable

properties, Bi2O3 is good candidate as sintering aid for GDC, and was

chosen for experiments (2) (3). The experimental procedure and results

are described in following Sub-section.

6.2.1 Bi2O3 as sintering aid

The experiments were carried out by adding different weight

proportion of Bi2O3 (99.8% nanocrystalline Sigma Aldrich) in GDC nano

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powder synthesized using the optimized preparation condition given in

Chapter 3. These weight proportions are 1, 2, 5, 8 and 10% respectively.

These powders were first wet ground in agate pestle mortar for 1 hr. A

25% PVA solution prepared in DI water was then added as binder and

was mixed thoroughly. Pellets of this mixture were pressed by applying

280 MPa pressure for 10 min. These pellets were fired at 850°C for

2hrs. This temperature was chosen in view of the melting temperature

of Bi2O3. Density of fired pellets was measured by weight-volume

method. Platinum paste (Heraeus CL-11 7589, USA) was applied on

both surface and fired again at 800°C for 2hrs. Electrical

characterization was done impedance analysis using AUTOLAB PGSTAT

100 Potenstiostat/Galvanostat in the temperature range of 400-600°C

and in frequency perturbation of 1Hz-1MHz.

Bi2O3-GDC pellets fired at 850°C have shown shrinkage in the

range of 3-5%, which increases with Bi2O3 content. The density of these

pellets was above 80% and it was highest (~95%) for 2% Bi2O3.

Interestingly, the ionic conductivity of these pellets was found to be

higher than that recorded for GDC at 600°C. Table 6.2 presents

measured shrinkage, density and ionic conductivity of Bi2O3 added GDC

pellets.

Weight % of

Bi2O3

Shrinkage when

fired at 850°C

Density (%) Ionic conductivity at

600°C (× 10-3 S.cm-1)

1 2.2 81 6

2 5.3 95 11

5 4.6 94 6

8 3.4 93 3

10 3.1 87 2

Table 6.2: Effect of Bi2O3 added to GDC in different proportions along

with the shrinkage, density and ionic conductivity, for pellets fired at

850°C

Table 6.2 indicates that with increasing Bi2O3 content, the

properties of GDC pass through a peak. The shrinkage, density and

ionic conductivity properties were found to be highest at 2% Bi2O3

content and reduced as the Bi2O3 content increased further. Clearly,

higher weight percent of Bi2O3 found inappropriate in all respect. The

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highest conductivity at 2% Bi2O3 was recorded as 0.011 S.cm-1 at

600°C. In contrast, ionic conductivity of pure GDC sintered at 1350°C is

reported to be 0.014S.cm-1. Decrease in shrinkage, density and ionic

conductivity in case of 5-10% doped Bi2O3 may be attributed to longer

sintering dwell times and evaporation of Bi2O3. Hence, it can be

concluded that Bi2O3 may be useful with GDC as sintering aid and 2

wt%. Similar results 0.022S.cm-1 has been reported in literature;

however, sintering temperature was 1400°C (2). Present work improved

these results by achieving density and conductivity better than that

reported in literature (2).

Even though, Bi2O3 improved the results in terms of lower

sintering temperature, high density and improved ionic conductivity at

lower (600°C) operating temperature it is not a perfect choice as

sintering aid for LTCC applications, as maximum shrinkage obtained

was just 3.1%, which is far lower than 13-15% reported for LTCC. Even

an increase in sintering temperature to 1000°C for GDC+2%Bi2O3

pellets showed shrinkage of 5.4% which was low compared to LTCC.

This shrinkage mismatch would induce stresses during cooling part of

the firing cycle, causing warpage and even cracks in the LTCC

structures. In order to achieve LTCC equivalent shrinkage values, the

sintering aid must be in the form of glass as glass addition is expected

to increase shrinkage (4), (5). Low temperature melting recrystallized

glasses offer higher shrinkage values due to their higher agglomeration

mechanism (6). Hence, some low temperature melting glasses were

prepared by conventional solid state reaction and water quenching

method. Effect of different glasses on the ionic conductivity of GDC

pellets is discussed in the following Sub-section.

6.2.2 Glasses as sintering aid in case of GDC

Low temperature melting borate and phosphate glasses based on

Bismuth oxide are prepared to use as sintering aid in GDC electrolytes.

Boron oxide (B2O3) and Phosphorous Pentaoxide (P2O5) are low melting

temperature oxides having melting points 450°C and 360°C

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respectively. Also it is reported that P2O5 glasses are very good protonic

conductors (7), (8). Bi2O3 and V2O5 also have melting points around

817°C and 690°C respectively. These two oxides together form oxygen

ion conducting phases commonly known as BIMEVOX. BiVO4, Bi4V2O11,

Bi3.5V1.2O8.25, Bi23V2O44.5 and Bi8V2O17 are some of the oxygen ion

conducting phases of Bi2O3 and V2O5 (9). These phases are oxygen ion

conductors at high temperature (400-600°C). It is also reported that

Bi2O3-P2O5 glasses are exhibit phonon-assisted hopping conduction at

high temperatures (200-400°C) (10). Hence, by considering ionic

conductivity and low melting temperature of these oxides, B2O3, P2O5,

Bi2O3 and V2O5 were selected for synthesis of these glasses. The glasses

are synthesized by solid state reaction method. Four different

compositions were tried with GDC as reported in the following.

6.2.2.1 Synthesis of glasses

Row materials in the form of oxides were selected for synthesis of

glasses. Bismuth oxide (Bi2O3; 99.9% Sigma Aldrich), Vanadium

Pentaoxide (V2O5; >99.6% Sigma Aldrich), Boron oxide (B2O3; 99.9%

Sigma Aldrich) and Ammonium Phosphate (NH4H2PO4; >98% Sigma

Aldrich) were mixed together in different molar proportions as given in

Table 6.3.

Glass Name (Glass code)

Mole % Bi2O3

Mole % of V2O5

Mole % of B2O3

Mole % of NH4H2PO4

BBVP 63 7 15 15

BVP 40 40 0 20

BVO 66 33 0 0

Table 6.3: Composition of different glasses synthesized by conventional

mixing, melting and water quenching process.

Acetone (HPLC grade; Merck make) was added to mixture and

milled on three roll mill for 96 hrs using Zirconia balls in polythene

container. This oxide mixture was then dried and loaded in a 99%

Alumina crucible. This mixture was calcined at 900°C for 2hrs, at ramp

rate of 5°C/min and molten mass was poured into DI water. A yellow

colored glass frit was obtained in all cases. This glass frit was ground

again in an agate pestle-mortar to fine powder. Pellets were pressed of a

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mixture of GDC added with 10 wt% of each glass. These pellets were

fired at 1000°C for 40 min. The shrinkage was calculated by comparing

dimensions before and after firing and its density was measured by

weight-volume method. Platinum paste (Heraeus CL-11 7589, USA) was

applied on both surface and pellets were fired again at 850°C. These

pellets were used to measure ionic conductivity using impedance

analysis (AUTOLAB PGSTAT 100 otentiostat/Galvanostat) by employing

frequency perturbation in 1Hz to 1 MHz frequency range and at

different temperatures between 400°C-600°C. The selection of best glass

composition was done on the basis of sintering properties and ionic

conductivity measurements.

6.2.2.2 Impedance measurement of Glass doped GDC pellets

The fired pellet showed a shrinkage of the order of 7-10% when

fired at 1000°C. The density of the pellets was found to be in the range

of 60-70%. The impedance spectra for these pellets are presented in the

form of Nyquist plots in Figure 6.1.

The Nyquist plots, plotted for 10 wt% doped glass-GDC pellets

shows increase in impedance of the pellet compared to un-doped GDC.

The impedance is seen to be increase in order of un-doped GDC < BVP

< BBVP < BVO. It is seen that bulk conductivity is affected by glass

doping, this may be due to the distortion in lattice planes of host

Figure 6.1: Nyquist plot measured at 600°C for different glasses

doped at 10weight% in GDC

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material due to crystallization of glass. The addition of glass at 10wt% is

not only located at grain boundaries but also can diffuses in grains of

GDC and deteriorates the lattice structure of GDC. Grain boundary

impedance is also seen increased indicating poor grain boundary

structure due to presence of glasses. The grain boundary impedance is

found to be relatively lower for the pellets with BVP glass. Comparing

with the impedance of BVO glass it can be concluded that P2O5 in the

glass structure may be aiding ionic conduction across grain boundaries

(10). Higher impedance in case of BBVP glass is due to presence of

higher wt% of B2O3 in glass, addition of B2O3 above 0.4wt% and

sintering temperature above 850°C lower ionic conductivity is reported

in literature (11). The effect of glass doping on GDC in terms of

shrinkage, density and ionic conductivity is tabulated in Table 6.4.

Glass added

in GDC

Shrinkage at

1000°C (%) Density (%)

Total ionic conductivity at

600°C (x10-3 S.cm-1)

BBVP 10.22 69.5 4.0

BVP 7.22 63.4 2.9

BVO 10.44 68.7 0.2

Table 6.4: Shrinkage, density and total ionic conductivity measured for

10 wt% glass doped GDC pellets fired at 1000°C

From the results presented in Table 6.4 one can conclude that

using glass as sintering aid is much helpful with respect to the

shrinkage, especially in comparison with Bi2O3. However, the ionic

conductivity deteriorates. Clearly, the presently used glasses would not

be useful, and a different glass composition may be required.

One of the noticeable outcomes of the above experiments

conducted with glasses was the observation of lower grain boundary

impedance for BVP glass, although this glass composition showed

lowest shrinkage, density and ionic conductivity. It was, therefore,

decided to use this glass with the addition of some alkali oxide, since

alkali oxides are known ionic conductors. For example, Lithium and

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Sodium are well know alkali oxide used in batteries (12). Potassium

oxide (K2O) is an alkali oxide having anti-fluorite crystal structure, large

atomic radii of Potassium comparative with Phosphorous and

Vanadium. Hence it was decided to add K2O in Bi2O3-V2O5-P2O5 glass. A

glass composition used to study was x(Bi2O3-K2O)-y(V2O5-P2O5), wherein

both x and y vary from 0 to 0.51.

x(Bi2O3-K2O)-y(V2O5-P2O5) (abbreviated as BKVP) was synthesized

using conventional solid state reaction, melting and water quenching

method. Initially 10 wt% glass was added in GDC and pellets were

pressed and fired at 1000°C for 40 min. The platinum paste was applied

on both surfaces and fired again at 850°C for 40min. The ionic was

conductivity measured using impedance analysis as stated above.

The sintered pellet showed shrinkage of 12.78% and density

measured was found to be 6.49gms.cm-1, which is 92% of the

theoretical density. The increase in shrinkage and density was

attributed to lowered glass transition temperature of BKVP glass

compared to BVP glass. The glass transition temperature of BKVP was

found to be 650°C compared to BVP glass 730°C. This improvement

also brought about improvement in the electrical properties. The ionic

conductivity results indicated improvement in the conductivity due to

addition of K2O in glass. Figure 6.2 presents comparison of Nyquist

plots measured for all the glasses added to GDC at 10 wt%, the 2 wt%

Bi2O3 in GDC and undoped GDC measured at 600°C.

1 Exact glass composition is under patenting process.

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Nyquist plot presented in Figure 6.2 clearly indicates that,

addition of the K2O in BVP glass improve its bulk conductivity, although

this is less than the 2 wt% Bi2O3 added GDC and un-doped GDC. The

ionic conductivity of the 7.6 ×10-3 S.cm-1 was observed at 600°C. This

conductivity is highest amongst the glass-ceramic composite

electrolytes. The earlier reported highest glass-ceramic conductivity is

4×10-3 S.cm-1. This increase in bulk conductivity is due to addition of

alkali oxide (13). Addition of alkali oxides in the glass was reported to be

favorable in producing homogeneous melts at low temperature. Also,

addition of alkali oxides in glasses reduces dielectric losses of samples,

which found advantageous for ionic conductivity in glass ceramic

composite electrolytes (13).

Due to the improvement observed with 10 wt% addition of BKVP

glass, the BKVP doping percentage in GDC was increased up to 40 wt%

on steps of 5 wt% in the next set of experiments. The ionic conductivity

found to have increased enormously with increasing BKVP glass content

in GDC. Figure 6.3 presents effect of BKVP glass addition on ionic

conductivity of GDC based glass-ceramic electrolytes through Nyquist

plots and dependence of shrinkage and ionic conductivity of electrolyte.

Figure 6.2: Comparison of Nyquist plots measured for all glasses

with 10wt% addition to GDC, 2% Bi2O3 in GDC and un-doped GDC

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The Nyquist plots presented in Figure 6.3 indicate that the grain

and grain boundary impedance decreases with increasing glass content

in GDC. It is seen that grain and grain boundary impedance curves are

distinguishable up to 20 wt% addition of glass. Further increase in

glass doping caused very low grain boundary impedance. Low frequency

curves started vanishing from Nyquist plots with every 5 wt% increase

in glass content (25 wt% onwards) and the high frequency loop was

relatively dominant. This change is attributed to reduction in traps at

the grain boundaries due to glass diffusion in the grains and probably

into the lattice. Nyquist plots at the high frequency show presence of

Warburg in intra-grain regions, implying increase in ionic conductivity

due to increase in anion vacancies in lattice.

Glass wt %

Fitting parameter values

R1

(Ω)

R2

(Ω)

R3/L

(Ω)/H

W

(Ω)

CPE1

(F)

n CPE2

(F)

n Error

(%)

10 1 76 41 0 1×10-

4

0.3 2×10-4 0.9 0.02

15 2×10-

3

29 45 2×10-4 4×10-5 0.6 5×10-6 0.5 0.14

20 9 13 10 0 3×10-4 0.6 6×10-9 1 0.03

25 6 14 0 910 3×10-5 0.9 5×10-3 0.1 0.14

30 8×10-9 11 5 3.3 1×10-9 0.9 5×10-3 0.6 0.5

35 5.7 11 856 1753 1×10-5 0.2 5×10-4 0.2 7.0

40 9 7 1.5 1544 5×10-9 0.1 3×10-18 0.1 15

Table 6.5: Values of fitting parameter for each Nyquist plot presented in

Figure 6.3, for samples with wt% 10-40 of glass in GDC

Figure 6.3: Nyquist plots at 600°C for different wt% of BKVP glass added to GDC

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It is seen from the Table that, grain and grain boundary

contribution to ionic conductivity increases with increase in glass

weight addition. The fitting values of R2 lowered with increase in glass

addition. The R3 resistance is corresponding to grain boundary

resistance replaced by inductance. This implies very high ionic diffusion

across grain boundaries.

Figure 6.4 presents effect of glass wt% on shrinkage and ionic

conductivity. These pellets were fired at 1000°C and ionic conductivity

was measure at 600°C. It is observed that, increase in glass wt%

decreases shrinkage due to higher percentage of glass in molten state

cannot agglomerate GDC particles. Further increase in glass above 50%

shows negative shrinkage for the pellets (e.g. 60 wt% glass in GDC

showed shrinkage of -0.2%).

It can be seen from the figure that the ionic conductivity

increases continuously with the glass content and reaches 0.1 S.cm-1 at

40 wt% addition of BKVP glass. This is a significant increase in ionic

conductivity for the un-doped GDC powders showed 3.6 × 10-3S.cm-1

ionic conductivity at 600°C. The figure also indicates that shrinkage

decreased with glass content simultaneously. The two major

requirements for LTCC compatibility, therefore, are found to be moving

in opposite direction as the glass content increased. This implies that

there is a need to select a “golden mean” or an optimum values for the

both properties. Based upon the shrinkage requirements the 25 wt%

Figure 6.4: Effect of glass wt% on shrinkage and ionic conductivity

of glass-GDC composite

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glass content in GDC was chosen as an optimized condition for the time

being, where shrinkage was close to 9% and ionic conductivity 0.04

S.cm-1 at 600°C. It may be noted that this ionic conductivity is 10 times

higher than un-doped GDC at 600°C and is equivalent to the ionic

conductivity observed at 700°C operating temperature. This comparison

of results is presented by comparing the Nyquist plots in Figure 6.5.

Thus, it can be conclude from above discussion that using BKVP glass

is found to be useful in increasing both, the shrinkage and ionic

conductivity values of GDC.

To confirm that measured conductivity of glass ceramic composite

electrolyte is pure ionic conductivity, ionic transference number was

measured at 600°C for all samples. This ionic transference number

found close to 1 for all samples having glass content from 10-40 wt%.

Table 6.5 presents density, shrinkage, ionic conductivity and ionic

transference number for 10-40wt% glass added to GDC.

Figure 6.5: (a) Comparison of Nyquist plots measured at 600°C for

25 wt% BKVP glass in GDC and un-doped glass, (b) Nyquist plot

comparison of un-doped GDC and Glass doped GDC indicating

similarities at 100°C lower operating temperature using glass

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Weight% glass added

in GDC

Shrinkage (%)

Density of pellets

(gm.cm-3)

Ionic conductivity at 600°C

(×10-3 S.cm-1)

Ionic transference

number

10 12.78 5.25 7.6 0.99

15 12.44 6.20 11.0 0.99

20 9.78 5.90 30.7 0.99

25 8.67 5.82 35.4 0.99

30 7.00 5.74 40.6 0.99

35 3.89 5.32 63.3 0.98

40 2.11 5.30 98.8 0.98

Table 6.6: Effect of different glass weight % in GDC on shrinkage,

Density, ionic conductivity and ionic transeference number measured at

600°C

It is seen from Table 6.5 that ionic transference number for all

samples is ≥0.98. This confirms that this glass-ceramic composite

electrolyte has only ionic conductivity and the electronic component is

absent. Thus, it can be concluded that these novel glass-ceramics

composites show better performance as electrolytes for SOFC, and at

the same time these electrolyte materials shows better physical

compatibility with LTCC materials. The following Sub-section of this

Chapter provides detailed analysis of structural and morphological

studies of GDC-glass composite electrolyte.

6.2.3 Materials Characterization of GDC-glass composite electrolyte

GDC-glass composite electrolyte powders were characterized by

X-ray diffraction, Raman and TEM analysis. The surface morphology

was observed using FE-SEM (Field Emission-SEM) images. The results

are discussed in the following Sub-sections.

6.2.3.1 XRD analysis

GDC-glass composite powders were synthesized by adding

different proportions of glasses in GDC and milling together for 2 hr and

fired at 1000°C for 40 min. XRD patterns of these pellets are presented

in Figure 6.6.

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XRD patterns indicate that adding of glasses in GDC shifts

position of peaks towards higher 2θ values compared to pure GDC. This

implies decrease in the d-spacing of the structure. In case of 10 wt%

glass doping, glass gets accommodated in GDC structures and no

additional peaks was observed in XRD patterns compared to GDC. An

effect of glass doping becomes significant at 15 wt% glass content and

above. A small peak is appeared at 2θ of ~30.4° for glass content

between 15-25 wt%. This peak corresponds to the phase Bi10V2O20.

Further addition of glass in GDC increases impurity peaks around

<111> plane and peak at 30.4° split into two peaks. These peaks

correspond to Bi2O3, Bi2O4 and KO3. Peak intensities of all peaks are

also found diminishing with further increase in glass content in GDC,

attributed to deviation from pure phase of FCC structure to mixed

phase due to addition of recrystallized glass. All peaks corresponding to

FCC crystal structure of GDC remained unaffected by added impurities.

From these results it can be conclude that addition of glasses up to 10

wt% in GDC affects only the d-spacings in lattice; increase in glass

content above 10 wt% causes re-crystallization and some such peaks

are appear in XRD patterns without deteriorating GDC the matrix. It is

clear from above discussion that increase in glass content does affect

the lattice structure of GDC and the glass not only improves the grain

Figure 6.6: XRD patterns of pellets with glass in various weight

contents from 10-40 wt% in GDC and fired at 1000°C, along with

pattern of pure GDC powder

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boundary regions but may also be diffusing into lattice of GDC and

increase vacancies. This effect is already seen in the Nyquist plot where

intra-grain ionic conductivity also showed significant enhancement.

6.2.3.2 Raman spectroscopy

The GDC-glass composite powders were also characterized by

Raman analysis in energy range of 100-800cm-1, to find effect of glass

addition in GDC on bond lengths and symmetries of FCC crystal

structure. Figure 6.7 presents Raman spectra of GDC-glass composite

materials with different glass contents.

It is clearly seen from Figure that addition of glasses has

significant effect in Raman spectra of bond vibrations in the glass-GDC

composite. The comparison of Raman spectra for doped and undoped

glasses shows three major differences. These three major changes are

pointed out using arrows in red and blue color and a rectangle of red

color. A signature peak of F2g symmetry is observed in all cases,

showing that the symmetry of basic GDC lattice is unaffected by glass

doping. The peaks at 563cm-1 and 630cm-1 correspond to oxygen

vacancies. These peaks are seen shifting towards higher energy and

shoulder at higher energy becomes increasingly intense with increasing

glass content. This implies increase in oxygen vacancies with addition of

glasses in GDC. Raman active mode at lower energy of ~100cm-1 show

Figure 6.7: Raman spectra of pure GDC and with glass added in

different wt%; three significant effects are highlighted using arrows

and rectangle

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vibrational interactions between glass and GDC lattice due high glass

content. Intensity of F2g symmetry peak decreases due to polarizability

and lowering in concentration of Raman active groups. Peak at 120cm-1

to shifted at higher Raman frequency at 150cm-1 in case of 40 wt%

glass added in GDC shows incorporation of glass in GDC lattice

structure, and hence changing lattice parameters. This incorporation is

clearly seen in TEM images in next section.

It is clear from the Raman spectra that addition of glass in GDC

sets-in close lattice interactions between them. The increase in glass

doping does not affect symmetry of GDC, but it increases oxygen

vacancies in lattice.

6.2.3.3 TEM analysis

FE-TEM of GDC-glass composite powder was studied to study the

effect of glass content on lattice planes and crystallite size of

composites. Figure 6.8 presents FE-TEM images of GDC-glass

composite electrolyte powder with glass 40 wt%.

It can be very easily observed from the figure that addition of

glass in GDC distorts the lattice planes. The glass is seen partially

crystallized and amorphous. The highlighted area by a red oval shape

magnified in image (b). The crystallization of the glass is clearly

(a) (b)

Figure 6.8: FE-TEM images of GDC-glass composite powder

showing lattice planes and distortion of lattice plane due to

glass re-crystallization

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observed in this image. During the crystallization it distorted <111>

lattice plane of GDC. The area heighted by square indicated area where

glass is in amorphous form. Figure 6.9 presents TEM image and

corresponding SAED patterns. Presence of this amorphous phase is

observed in the Selected Area Electron Diffraction (SAED) pattern as a

broad and diffusive rings corresponding to amorphous glass could be

identified. Glass may not have fully crystallizedd due to lower sintering

dwell time. Increased bright spots compared to FCC crystal structure on

the periphery of each polycrystalline circle shows diffractions from

planes of the re-crystallized glass.

The above structural analysis of the GDC-glass composites

concludes that, glass doped in GDC partially crystallizes and the

composite becomes a mixture of polycrystalline and amorphous phases.

Crystalline glass distorts the <111> planes and creates its own planes

as confirmed as impurity peaks in the XRD.

6.2.3.4 SEM analysis of pellets

The surface microstructure of the pellets was observed under

SEM to understand the surface morphology of the pellets. Figure 6.10

presents, surface morphology of the pellets doped with 25 wt% BKVP

glass and sintered at 1000°C for 40 min and 3hr.

Figure 6.9: FEG-TEM image showing nanocrystallites of

compostite of GDC-glass and corresponding SAED pattern

confirming mixed polycrystalline and partial amarphous phases

Ph.D Thesis 2014

Chapter 6 Page 220

It is seen from the micrographs of 40min dwell time that some rod

and bush-like structure is developed over most of the pellet surface. The

EDAX of the pellet in this area shows all the elements content in the

glass as well as GDC are present in these structures, but those coming

from the glass elements were dominant. This implies these rods and

bushes are grown because of high glass content in the pellet. Glass

added in the pellet flows through voids present between the grains

along the grain boundaries, and the excess of glass may be forming

such structures on the pellet surface. Figure 6.10 (b) presents the pellet

surface sintered with dwell time of 3 hr. The surface of this pellet shows

void free surface throughout the pellet, and rods or bush like structures

are not found on the surface. This implies that with longer sintering

dwell time glass gets completely settled down on surface and between

the grains and there is complete grain growth. Such grain growth is not

observed in earlier case (Figure 6.10 (b)) where small pits were observed

on the surface. Hence, it can concluded from the SEM micrographs that

excess glass in pellets flow over the surface of the pellet and form grain

structure dependent on sintering dwell times. However, this grain

growth and change in surface morphology does not affect ionic

conductivity. Figure 6.11 presents ionic conductivity measurement for

pellets fired at different dwell times.

(a) (b) (c)

Figure 6.10: SEM micrographs of the surface of pellets prepared by

25 wt% glass added in GDC and fired at 1000°C (a) and (b) 40min

and (c) 3 hr dwell time

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The Nyquist plots of pellets fired at different dwell times at

1000°C shows that the impedance of the pellets is almost independent

of the sintering dwell times. There is very small change in ionic

conductivity observed for these pellets. However, small change in

sintering temperature made a huge difference in the ionic conductivity

and consequently the Nyquist plot of the GDC-glass doped pellet. Figure

6.12 (b) presents effect of sintering temperature on impedance of the

pellets. The ionic conductivity of the sample decreased with decrease in

sintering temperature from 1000°C to 875°C. The ionic conductivity as

measured at 600°C for samples sintered at 850°C, 950°C sintering

temperature was found to be 1.01× 10-2 S.cm-1, 1.23 × 10-2 S.cm-1

respectively and 3.5× 10-2 S.cm-1 for 1000°C sintering temperature.

However, unlike 1000oC sintering temperature, the Nyquist plot in case

of 875°C and 950oC sintering temperature showed two semicircles,

implying contribution from the bulk of the grain and grain boundaries

and the grain conductivity is found to be less than grain boundary

conductivity. This implies that the sintering temperature was

insufficient for glass diffusion in grain interior. It may be noted though,

that this ionic conductivity at 900°C is almost equal to the 1350oC fired

pure GDC pellet at 600°C. Thus, this study concludes that addition of

Figure 6.11: (a) Nyquist plot of pellets sintered at different dwell

times at 1000°C for 40, 120 and 180 min, (b) effect of sintering

temperature on the Nyquist plot when fired at 875°C, 950°C and

1000°C

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BKVP glass in GDC not only improves the sintering of GDC, but also

imparts LTCC compatibility without deteriorating the ionic conductivity

properties of electrolyte, instead it improved the conductivity. Table 6.6

presents modified physical properties of SOFC ceramic electrolyte after

addition of BKVP glass.

Property Un-doped

GDC Glass

doped GDC LTCC

DuPont

951

Remarks

Firing temperature (o

C) 1350 900 900 Close to LTCC firing temperature

Ionic conductivity

@600o

C (S.cm-1

) 0.014

2 0.012 - Ionic conductivity just

less than GDC TCE (0-300

o

C) (ppm/o

C) >12 6.8 5.8 Close to DuPont 951 LTCC tapes

Density (gms/cm3

) 7.24 6.55 3.1 Almost 90% of pure

GDC achieved at 400o

C lower temp

Shrinkage (%) 14 9 12.9 Shrinkage is less than DuPont 951 tapes but close to pellet prepared by LTCC powder

Table 6.7: Significant properties improvements in SOFC electrolyte and

its comparison with LTCC properties

6.2.3.5 Chemical compatibility with LTCC

To study chemical compatibility of these novel electrolyte GDC-

glass composite with LTCC, a screen printable paste of 25 wt% BKVP

glass doped in GDC was prepared by adding different binders,

plasticizers and dispersant organic / inorganic liquids and solids3 were

mixed together ground for 2 hrs in an agate pestle-mortar until paste

became viscous and flowing paste. This paste was printed on

commercially available LTCC green tapes (DuPont 951) with thicknesses

160 and 250µm (single tapes) and stacks of thickness 350 and 500µm,

both with X-Y dimensions 15 × 15mm. A screen having a square pattern

of 10 × 10mm was prepared and used for printing. This pattern was

printed and dried in normal atmosphere. The bottom surface of tape

2 Ionic conductivity obtained for GDC sample measured during synthesis parameters study. This powder is used to prepare GDC-glass composite electrolyte, GDC powder

optimized in Taguchi study showed ionic conductivity of the order of 0.11S.cm-1 at 600°C. If this powder is used for glass-ceramic composite, much better results can be expected. 3 This composition is being patented.

Ph.D Thesis 2014

Chapter 6 Page 223

was observed removing mylar to verify penetration of organic or

inorganic component of paste through tape. Such evidence was not

observed confirming that neither organics nor inorganic materials from

paste reacted with LTCC at room temperature and normal humidity

conditions. These printed tapes were kept under observation for two

days to verify inertness of the paste material with LTCC tapes. Later,

these tapes were fired using standard firing cycle recommended by the

manufacturer. The fired single tapes showed slight warpage only for

100µm thickness tape, while all other tapes and stacks showed good

flatness, confirming matching of shrinkage of paste and tape during

firing process. Figure 6.12 shows the line scans measured using

inducting gauge of thickness profiler (TALLYSURF CLI 2000, Talyor

Hobson).

It is observed from Figure 6.6 (a) that screen printed thick

films on 160µm tape shows slight warpage at the edges of thick film.

However, this may also be the result of thickness variation across

Thick film

(a)

Figure 6.12: Line scans recorded by inductive gauge showing

warpage and thickness of screen printed thick film printed on (a)

160µm thick and (b) 250µm thick commercial LTCC tapes (DuPont

951)

Thick film

Ph.D Thesis 2014

Chapter 6 Page 224

printing direction. The average fired thickness of thick film was 15µm.

Printing direction in this both cases was from left to right of the figure.

Figure 6.6 (b) shows surface scan of 250µm tape, the warpage on this

tape is slightly reduced than that for 160µm thick tape. The thickness

of thick film was 12µm and average warpage is observed in the range

±3µm across the ~8mm width. These warpage and thickness results are

comparable with the commercially available DuPont 6142D screen

printable thick film LTCC compatible paste (warpage ±1.8 µm over 8mm

width). The above description concludes that the prepared GDC-glass

composite thick film paste is physically compatible with LTCC.

In order to confirm chemical compatibility with LTCC, the

film surface and cross section of tape was first observed under

microscope. There was no unusual change in the appearance,

indicating less possibility of any chemical interaction. Thus, prima facie

it was concluded that LTCC tape and GDC-glass composite paste both

are compatible with each other. However, final confirmation of chemical

compatibility can be made only after analyzing the interface and the

whole sample using analytical techniques, such as, X-ray diffraction

and Raman spectroscopy etc. and through microstructural

observations. Presently, this analysis has not been completed.

Similar to GDC, different sintering aids and some proton

conducting glasses were tried for achieving low sintering temperature

for the proton ion conducting electrolyte, viz. Yttrium doped Barium

Cerate (BCYO). Following Sub-section report results of glass-BCYO

composites.

6.2.4 Effect of sintering aids on proton conducting BCYO

Bi2O3 was used as sintering aid for sol gel synthesized Yttrium

doped Barium Cerate. The pellets turned to black color after firing at

850°C. The electrical measurement by impedance brought out

occurrence of very high impedance for these pellets. The ionic

conductivity measured for these samples shows ionic conductivity in

the range of 10-6 to 10-7 S.cm-1 at 600°C. Such high impedance is

possibly a result of formation of insulating phase between Barium oxide

Ph.D Thesis 2014

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and Bismuth oxide due to chemical reaction (14). Due to repeated, high

impedance observations for such composite, this aspect of interaction

was not investigated further. It was concluded that Bi2O3 is an improper

choice as sintering aid for BCYO electrolyte. Some other low melting

temperature oxides, such as, Vanadium oxide (V2O5), Antimony oxide

(Sb2O5), Thallium oxide (Tl2O5) were also used as sintering aids for

BCYO. However, there were no significant improvement in the results

obtained using these sintering aids. Table 6.7 listed sintering aids and

corresponding shrinkage, density and ionic conductivity comparison

with BCYO pellet sintered at 1350°C.

Sintering aid in

BCYO

Shrinkage when

fired at 1000°C

Density (%) Ionic conductivity at

600°C(x10-3S.cm-1)

BCYO - - 14

2% V2O5 2.3 72 3.4

2% Sb2O5 1.8 68 0.5

2% Tl2O5 1.9 78 2.2

Table 6.8: Effect of different sintering aids on shrinkage, density and

ionic conductivity of BCYO

The results presented in this table indicate that low temperature

melting oxides including Bi2O3 are not useful as sintering aid for BCYO.

The ionic conductivity lowered due to addition of theses oxides due to

disorder produced in grain boundaries and produced insulating phases

with BCYO by these oxides. Therefore, instead of inorganic oxides as

sintering aid, known proton ion conducting glasses were tried as

sintering aid in BCYO. Synthesis of glasses and their effect on

shrinkage and ionic conductivity are presented the following Sub-

section.

6.2.4.1 Effect of glasses as sintering aid

This work was carried out selecting two different glasses based on

Silica, Zirconia and P2O5. These components were selected because

glasses of these oxides are reported to be good protonic ion conductors

at 200-400°C operating temperature (15). These glasses are well known

electrolytes for Proton Electrolyte Membrane Fuel Cell (PEMFC) (16).

Ph.D Thesis 2014

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Two different compositions were selected based on literature survey.

These glasses are synthesized by sol-gel method, obtained membranes

were fired at temperature range 30-600°C. A high proton conductivity of

9.3×10-3 S cm-1 was reported for the 83SiO2–5P2O5–2TiO2-10ZrO2

(mol%) composite at 80°C under 90% relative humidity (RH), while for

the 83SiO2–5P2O5–2ZrO2-10TiO2 (mol%) composite the conductivity was

only 7.1×10-3 S.cm-1 under the same conditions (16). Two

compositions of glasses were selected for this work, viz. 83SiO2–5P2O5–

2ZrO2-10TiO2 (SZPT) and 48SiO2-2ZrO2-25P2O5-25V2O5 (SZPV) (15),

(16), (17). SZPT glass was synthesized by sol-gel method and SZPV glass

was synthesized by standard solid state route.

For the synthesis of SZPT glass by sol gel method, the raw

materials used were tetraethyl orthosilicate (Si(OC2H5)4, TEOS, 99%

Merck), trimethyl phosphate (PO(OCH3)3), 99% Merck), titanium-iso-

propoxide (Ti(OC4H9)4, 99% Merck), Zirconium-iso-propoxide

(Zr(OC4H9)4, 99% Merck) and N-N- Dimethyl formamide (HCON(CH3)2,

99% Merck). The experimental procedure was followed as given in

literature (16). These synthesis steps are also presented in Figure 6.15.

The SZPT glass synthesized following the steps given in flow chart

was calcined at 600°C. A black colored glassy powder was obtained.

This powder was crushed to obtain fine powder. 10 wt% of this glass

was added to BCYO and its pellets were sintered at 1000°C. However, it

was seen that these pellets show an increase in dimensions up to 1%.

This would not be suitable for LTCC applications. Further, this glass

also caused increase in impedance of the glass-BCYO composite when

tested with 10 wt% glass content (Figure 6.14). Due to these results, it

was decided not to proceed with further experiments.

Ph.D Thesis 2014

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The other glass used for this investigation was having

composition 48SiO2-2ZrO2-25P2O5-25V2O5 (SZPV). In this glass TiO2

was replace by V2O5 considering its lower melting point, also molar % of

Silica was reduced to 48mole%. SZPV glass was synthesized by solid

state reaction using the raw materials as Silicon oxide (SiO2; 99.9%

Sigma Aldrich), Zirconium oxide (ZrO2; 99% Sigma Aldrich), Vanadium

Pentaoxide (V2O5; >99.6% Sigma Aldrich) and Ammonium Phosphate

(NH4H2PO4; >98% Sigma Aldrich). These raw materials were mixed

together in given molar percentage and wet-milled for 96 hr using

Acetone (HPLC grade; Merck) solvent. The milled powder was dried and

kept in furnace and fired at 800°C for 2hrs and molten mass was

transferred to water to obtain quenched glass. This glass was again ball

milled in polyurethane container to obtain a fine powder. 10 wt% of this

glass was added to BCYO and pellets were pressed. These pellets were

fired at 1000°C for 2hr. The sintered pellets show about 4.1 %

shrinkage. Platinum paste was applied on both the surfaces and was

fired again at 1000°C for 40 min. Impedance analysis these pellets was

carried out at 700°C in frequency perturbation of 1Hz to 1MHz. The

Figure 6.13: Flow chart for synthesis of SZPT glass by sol gel

method. Figure taken from reference (16)

Ph.D Thesis 2014

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impedance results for both glasses added to BCYO were compared with

the best results of pure BCYO. Figure 6.14 presents the Nyquist plots

for pure BCYO and for composites with 10wt% SZPT an SZPV glasses.

The Nyquist plots presented in Figure 6.14 indicate that doping of

glass is affecting the performance of BCYO resulting in high impedance

across pellet thickness. SZPT glass showed very high impedance

attributed to lower density and distortion in grain boundaries of the

pellets. The pellets with SZPV glass shows lower impedance compared

to SZPT glass, but its this impedance is still as high as 400 times higher

than pure BCYO. From the above results, it can concluded that the

SZPV as well as SZPT glasses are not compatible with BCYO. To make

BCYO electrolyte compatible with LTCC, more options of glass

compositions need to be explored. Due to lack of time this work could

not be undertaken for this Thesis.

Apart from the electrolytes, the electrode materials also need to be

made compatible with LTCC. The following Sub-section of this Chapter

presents details of compatibility studies for anode and cathode

materials.

6.3 Effect of sintering aids on electrode materials

Synthesis of electrode materials for SOFC was described in

Chapter 5. These electrode materials must be compatible with GDC-

Figure 6.14: Nyquist plots for composites of BCYO with 10 wt%

SZPT and SZPV glasses measured at 700°C and its comparison with

pure BCYO

Ph.D Thesis 2014

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glass composite electrolyte with respect to their physical properties

such as shrinkage, sintering temperature, Temperature co-efficient of

Expansion (TCE) and operating temperature. Amongst these matching

of shrinkage during sintering is an important parameter which must

tackled first.

As we have seen in earlier, Copper Zinc oxide (CuZnO) was

selected as anode material and Samarium and Strontium doped

Cobaltite (SSC) was selected as cathode material. CuZnO showed 14%

shrinkage when fired at 900°C, however, SSC showed such shrinkage

only when it was fired at 1100°C. Thus, these two electrode material

have higher shrinkage compared to GDC-glass composite electrolyte

and their firing temperatures are found varied from 900 to 1100°C. The

target of this study was to match their shrinkages when fired at 1000°C.

Initially, efforts were taken to make cathode material SSC

compatible with GDC-glass composite electrolyte material. This was

done by adding different weight proportion of BKVP glass in SSC and

fired at 1000°C for 40min. Shrinkage and density of the pellets were

measured after firing and found that shrinkage of SSC-glass composite

electrode was close to that of the composite electrolyte when 40% glass

was added to SSC. Table 6.8 presents results of adding glass with

different weight percentages to SSC with respect to shrinkage, density

and measured ionic conductivity at 600°C.

Weight % of BKVP glass

added in SSC

Shrinkage (%) when fired at

1000°C

Density (%) Area specific resistance (ASR)

(Ω.cm2) at 600°C

0 3.4 72 10

10 3.97 78 8

25 5.96 86 6

40 8.78 94 3

Table 6.9: Effect of different glass weight% added in SSC on shrinkage,

density and ASR

The results indicate that increase in glass content in SSC

increased the shrinkage of pellets as well as lowered Area Specific

resistance, which is a good sign for any cathode material. However,

increase in glass content increased the density of the pellets, reducing

Ph.D Thesis 2014

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their porosity. This may reduce catalytic activity of the cathode. It is

clear that ASR is lowered in presence of glass due to increase in ionic

conductivity of the pellet. TCE measured for 40 wt% glass added SSC

was measured at 600°C, which was found to be 7.2ppm/°C, which is

close to composite electrolyte. Initial results on LTCC tape by making

paste of SSC shows no evidence of chemical reaction, however, in case

of 250µm tape thickness diffusion was observed from backside of tape.

These results are better when a stack of 500µm is used as co-firing

substrate.

Hence, it can be conclude that 40 wt% BKVP glass addition in to

SSC has matching shrinkage properties with the GDC-glass composite

electrolyte and its ASR value also improves. However, increase in

density may reduce the catalytic performance. This aspect remains to

be verified.

The anode material selected for this work was Copper Zinc oxide

which shows 14% shrinkage at 900°C without any addition of sintering

aid. This oxide has melting point somewhere in between 950-1000°C

and hence its pellet melts when fired at 1000°C. Due to time limitation,

this anode material was applied after firing (post-fire process) for cell

fabrication discussed in the next Chapter.

6.4 Co-firing of SOFC material with LTCC

In this Chapter we have reported that, BKVP glass was useful for

making SOFC electrode and electrolyte materials compatible with the

LTCC materials. It is seen that the ionic conductivity results are

exceptionally good when sintered at 1000°C, when sintering

temperature lowered its ionic conductivity found lowered. On the other

hand, at 1000°C sintering temperature, anode material Copper Zinc

Oxide (CuZnO) melts and cathode material shows very high density.

Therefore, lower firing temperature is favorable for these electrode

materials. Further, LTCC firing temperature is around 875-900°C

considering silver metal is used as via filling conductor and for

conducting tracks. Increase in firing temperature in the range of 1000°C

for LTCC will require replacement of silver by gold and platinum. These

Ph.D Thesis 2014

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materials are costlier and will ultimately increase the cost of fabrication.

Clearly, the present LTCC co-firing study of SOFC electrolyte materials

bring up three different sintering or co-firing options where one has

compromise either properties of electrolyte or co-firing of the fuel cell.

Table 6.10 presents details of firing temperature options and

compromises forced for selecting any one of the.

Firing

temp.

Electrolyte

(GDC-glass composite)

Anode

(Copper Zinc oxide)

Cathode

(SSC-glass composite)

LTCC

(DuPont 951 tapes)

1000°C Good ionic conductivity properties

Cannot used as have low melting temperature

Porosity lowered, poor catalytic performance

Silver cannot be use, costlier gold and platinum

materials needed

950°C Ionic

conductivity slightly lowered

Cannot used

as have low melting temperature

Porosity

increases shrinkage matches with LTCC and

electrolyte

Silver cannot

be use, costlier gold and platinum materials

needed

875°C Ionic conductivity lowered to

35% of that at 1000oC

Can be used at optimum catalytic

activity

Can be used with optimum porosity and

catalytic activity

Silver can be used as conducting

metal, all components

can be co-fired

Table 6.10: Comparison of different sintering temperature possible in

co-firing of LTCC and SOFC and their effects on electrode, electrolyte

and LTCC material systems

It is clear from the table that, three different temperatures are

possible for co-firing of SOFC in LTCC depending on compromise

chosen. At 1000°C sintering/firing temperature electrolyte properties

are optimum, however, this temperature not suitable for electrode and

silver based LTCC packaging. At 950°C temperature, electrolyte

performance lowered in terms of conductivity, even though this

temperature is high for the electrodes and silver. Sintering temperature

875°C is suitable for electrode porosity, CuZnO can be use as anode

and silver can be used in LTCC structures, however, ionic conductivity

Ph.D Thesis 2014

Chapter 6 Page 232

properties of electrolyte lowered to 35% of optimum. This implies that,

sintering temperature in co-firing process can be defined by user by

compromising either of properties or by choosing golden mean. Lower

sintering temperature of 875°C is recommended as it provides optimum

electrode and LTCC properties by compromising ionic conductivity

properties of GDC-glass composite electrolyte. Thus, this can be

conclude that, present work provides flexibility of selection of sintering

temperature for co-firing process of SOFC integration in LTCC.

6.5 Conclusions

This Chapter describes compatibility studies of SOFC electrolyte

and electrode materials with LTCC. Different sintering aids viz. low

temperature melting oxides and low temperature melting glasses were

used to match the properties of synthesized electrode and electrolyte

materials with LTCC, physically as well as chemically. Mostly, this

compatibility was achieved without deteriorating ionic conductivity

properties of the host material. Gadolinium doped Ceria, an oxygen ion

conducting electrolyte shows close compatibility with LTCC when 25

wt% Bismuth based alkali phosphate glass (x(Bi2O3-K2O)-y(V2O5-P2O5))

was added to GDC. This addition of glass not only resolved compatibility

issue but also enhanced the conductivity and low temperature

sinterability of GDC. This GDC-glass composite was found compatible

with LTCC in all respects. The ionic conductivity of the order of 0.034

S.cm-1 was obtained at 600°C operating temperature and 1000°C

sintering temperature, which is at least double of ionic conductivity of

undoped GDC at same operating temperature and 1350°C sintering

temperature.

In case of BCYO proton conductor, different oxides and glasses

were used to achieve LTCC compatibility. However, the compatibility

could not be achieved due to lower shrinkages and deterioration of ionic

conductivity for all the such additions. Different glasses need to be

explored for improvement in these results.

In case of electrode materials, the cathode material SSC was

made compatible with GDC-glass composite electrolyte by adding 40

Ph.D Thesis 2014

Chapter 6 Page 233

wt% BKVP glass. Its shrinkage (8.78%) was close to GDC-glass

composite electrolyte and ASR values improved to 3Ωcm2, however,

density also increased to 94% due to this addition, which may lower its

catalytic action. This, however, remains to be proved. Copper Zinc oxide

was selected as anode material due to its better catalytic activity.

However, due to its lower melting point, it can be used only through a

co-firing process. More options need to be explored for the anode.

Depending of materials properties SOFC can co-fired LTCC at

three different temperatures, however, at each sintering temperature

some of the physical and chemical properties of either electrode or

electrolyte need to be compromise. This selection of sintering

temperature is as per user requirement. Lower firing temperature

From the results presented here, it is felt that the materials are

ready for fabrication of an SOFC on at least trial basis. The next

Chapter describes efforts in this direction.

6.6 References

1. Recent Material Developments in Fast Oxide Ion Conductors.

Mairesse, J. C. Boivin and G. 1998, Chemistry of Materials, Vol.

10, pp. 2870-2888.

2. Low-temperature densification and grain growth of Bi2O3-doped-

ceria gadolinia ceramics. Vanesa Gil, Carlos Moure, Pedro Durán,

Jesus Tartaj. 2007, Solid State Ionics 178, pp. 359–365.

3. Development of Oxygen Ion Conductors: One Relevant Tendency.

Kharton, F.M.B. Marques and V.V. 2001, Ionics 11, pp. 321-326.

4. Sintering and crystallization behavior of CaMgSi2O6–NaFeSi2O6

based glass-ceramics. Ashutosh Goel, Anna Maria Ferrari, Ishu

Kansal, Maria J. Pascual, Luisa Barbieri, Federica Bondioli,

Isabella Lancellotti, Manuel J. Ribeiro, and José M. F. Ferreira.

2009, Journal of Applied Physics, Vol. 106, pp. 093502-1-7.

5. Glass transition temperature and activation energy of sintering by

optical dilatometry. Alexander Karamanov, Boris Dzhantov,

Mariano Paganelli, Davide Sighinolfi. 2013, Thermochimica Acta ,

Vol. 553, pp. 1–7.

6. Influences of the Glass Phase on Densification, Microstructure,

and Properties of Low-Temperature Co-Fired Ceramics. Markus

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Eberstein, Torsten Rabe, and Wolfgang Arno Schiller. 6, 2006,

Int. Journal Applied Ceramic Technololgy, Vol. 3, pp. 428–436.

7. Metal Phosphates as Intermediate Temperature Proton Conducting

Electrolytes. Y. J. Huang, Q. F. Li, C. Pan, T. Anfimova, J. O.

Jensen and N. J. Bjerrum. 1, 2012, ECS Transactions, Vol. 45,

pp. 99-104.

8. Ion conducting phosphate glassy materials. S.S. Das, N.P. Singh,

P.K. Srivastava. 2009, Progress in Crystal Growth and

Characterization of Materials, Vol. 55, pp. 47-62.

9. Bi8V2O17 – a stable phase in the Bi2O3–V2O5 system. W.WROBEL,

F. KROK, I. ABRAHAMS, A. KOZANECKA-SZMIGIEL, M.MALYS,

S. C. M. CHAN, J. R. DYGAS. 1, 2006, Materials Science-Poland,

Vol. 24, pp. 23-30.

10. AC conduction in semiconducting CuO-Bi203-P20, glasses.

Chakravorty, Aswini Ghosh and D. 1990, Journal of Physics:

Condensed Matter, Vol. 2, pp. 5365-5372.

11. Influence of B2O3 addition on the ionic conductivity of

Li1.5Al0.5Ge1.5(PO4)3 glass ceramics. Harsharaj S. Jadhav, Min-

Seung Cho, Ramchandra. S. Kalubarme, Jong-Sook Lee, Kyu-

Nam Jung, Kyoung-Hee Shin, Chan-Jin Park. 2013, Journal of

Power Sources, Vol. 241, pp. 502-508.

12. A Novel Sodium-Ion Rechargeable Battery. Ralph, Manickam

Minakshi and David. 29, 2013, ECS Transactions, Vol. 45, pp.

95-102.

13. Microstructure and dielectric properties of Ca-Al-B-Si-O

glass/Al2O3 composites with various alkali oxides contents. Ming

Liu, Hong-qing Zhou, Hai-kui Zhu, Zhen-xing Yue, Jian-xin Zhao.

10, October 2012, Journal of Central South University, Vol. 19,

pp. 2733-2739.

14. Dielectric Properties of Bismuth Doped Barium Titanate (BaTiO3)

Ceramics. Vittayakorn, Naratip. 12, 2006, Journal of Applied

Sciences Research, Vol. 2, pp. 1319-1322.

15. Fast proton-conducting P2O5–ZrO2–SiO2 glasses. Masayuki

Nogami, Ritsuko Nagao, Kensuke Makita, and Yoshihiro Abe. 10,

1997, Applied Physics Letters, 1323 (1997);, Vol. 71, pp. 1323-

1325.

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16. Synthesis, characterization and electrochemical properties of

SiO2–P2O5–TiO2–ZrO2 glass membranes as proton conducting

electrolyte for low-temperature H2/O2 fuel cells. Nogami, G

Lakshminarayana and Masayuki. 2009, Journal of Physics D:

Applied Physics, Vol. 42, pp. 215501- 11.

17. Structural and proton conductivity study of P2O5-TiO2-SiO2

glasses. T. Uma, S. Izuhara, M. Nogami. 2006, Journal of the

European Ceramic Society , Vol. 26, pp. 2365–2372.

18. Oxygen Ion Conduction in Oxide Materials: Selected Examples

and Basic Mechanisms. L.S.M. TRAQUEIA, F.M.B. MARQUES,

V.V. KHARTON. 2006, Boletín de la Sociedad Española de

Cerámica V, 45 [3], pp. 115-121.