Enhanced electrical properties of Ba6K2(Ca4Nb4)O21 complex perovskite oxide through modified sintering techniques

Materials Science & Engineering B 246 (2019) 53–61

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Enhanced electrical properties of Ba6K2(Ca4Nb4)O21 complex perovskite oxide through modified sintering techniques Kavitha Karuppaiaha, Althaf Rajaa, P.K. Ojhab, Govindaraj Gurusamyc, Anuradha M. Ashoka,d,

T ⁎

a

Functional Materials Laboratory, PSG Institute of Advanced Studies, Coimbatore, Tamil Nadu 641 004, India Naval Materials Research Laboratory, Ambernath, Maharashtra 421 506, India c School of Physical, Chemical and Applied Sciences, Pondicherry University, Kalapet, Pondicherry 605014, India d Department of Physics, PSG College of Technology, Tamil Nadu 641 004, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Double perovskite Densification Sintering Conventional Microwave Spark plasma Electrical conductivity

In this study the oxygen deficient perovskite Ba0.75K0.25(Ca0.5Nb0.5)O2.75(BKCN 25) is synthesized by solid state method via ball milling. Improvement in total conductivity through reducing grain boundary blocking effect is attempted by performing densification using conventional, microwave and spark plasma sintering. Physical characterizations of the pellets reveal structural stability and variation in morphology and porosity among the pellets sintered using different techniques. XRD analysis confirm cubic double perovskite structure in all sintered pellets but each having different grain sizes. SEM images show densified pellets with well-defined grains separated with grain boundary. AC impedance studies reveal that oxide ionic conductivity of BKCN 25 is ∼1.08 × 10−3 S.cm−1 at 700 °C in air atmosphere similar to reported GDC15, BaCe0.5Zr0.3Y0.2O3-δand BaCe0.45Zr0.45Sc0.1O3-δSOFC electrolytes. Significant enhancement in the total conductivity can be observed for spark plasma sintered BKCN 25 sample with lowest grain boundary blocking contribution.

1. Introduction Complex perovskite oxides have potential applications in numerous solid-state devices such as solid oxide fuel cells (SOFC), H2 pumps, steam electrolyzers, gas sensors, etc. The presence of oxygen vacancies in such ceramic oxides is responsible for their proton and oxide ion conduction, which makes them suitable for these applications [1,2]. There are two methods to introduce vacancies to enhance the ionic conductivity; 1. Acceptor doping; 2. Stoichiometry shift (or creating non-stoichiometry) [3]. In Ba3Ca1+xNb2−xO9−δ (BCN), a set of good proton and oxide-ion conductors developed by Du and other researchers [1,2,4], the second method of oxygen vacancy creation is adapted. In BCN composition mentioned above x denotes the stoichiometric shift using which δ, the number of oxygen vacancies per formula unit can be estimated. Du et al., also reported that when Ca:Nb ratio is 1:2 the perovskite exhibits trigonal symmetry with no structural oxygen vacancies. However, when the ratio is 1:1, the perovskite adapts a face centered double perovskite crystal structure with two oxygen vacancies in each unit cell [5]. With these features, BCN (with x = 0.5 and 0.8) is reported to be suitable electrolyte for intermediate temperature (IT) SOFCs [4,6–9]. They are also proven to be chemically stable against CO2 and H2O

⁎

atmospheres [4]. However, there is a need for further improvement in electrical properties. In an effort to achieve this, a few research groups [8,10,11] have introduced potassium that can provide high basicity as dopant in the cuboctahedral Na sites. They studied the electrical conductivity of K doped 1:2 ordered (Ca:Nb = 1:2, x = 0) BCN and reported the highest total electrical conductivity of 2.4 × 10−4 S.cm−1 in air at 700 °C. Recently our attempt on increasing the electrical conductivity of 1:1 (Ca:Nb = 1:1 x = 0.5) ordered BCN with different percentages of potassium (K) showed enhanced conductivity with better structural stability for a dopant concentration of 25% (Ba0.75K0.25(Ca0.5Nb0.5)O2.75, BKCN 25) [12]. However, the obtained total electrical conductivity of 1.10 × 10−6 S.cm−1 in air at 700 °C for BKCN 25 is not as high as the reported value for other K doped 1:2 ordered BCN perovskites stated in the literature [10]. Though higher total conductivity is expected owing to additional oxygen vacancies created along with the inherent structural oxygen vacancies by monovalent potassium doping, there was a decrease in conductivity presumed to be due to segregation of oxygen vacancies at grain boundaries. This is mainly associated with the presence of larger portion of grain boundaries and pores, which obstruct the carrier movement. There is a need to reduce the grain boundary blocking contribution and make the material denser. This can be done by appropriate sintering

Corresponding author at: Functional Materials Laboratory, PSG Institute of Advanced Studies, Coimbatore, Tamil Nadu 641 004, India. E-mail address: [email protected] (A.M. Ashok).

https://doi.org/10.1016/j.mseb.2019.05.027 Received 6 December 2017; Received in revised form 11 May 2019; Accepted 28 May 2019 Available online 08 June 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Heat distribution in a. Conventional, b. Microwave and c. Spark plasma-sintering methods.

seen in the figure, the size and the compactness of different grains vary with different sintering techniques. Larger grain size with lower porosity can be achieved through spark plasma sintering. The main aim of this work is to enhance the total conductivity of BKCN 25 by reducing the resistance offered by grain boundaries. Likewise, BKCN 25 perovskite is densified using different sintering techniques such as conventional, microwave and spark plasma and its structural, morphological and electrical behaviors are studied. Suitable sintering method for BKCN 25, which provides highly dense material and thus decrease grain boundary blocking effect to the carrier is optimized. This could make BKCN 25 a more efficient denser material that can be used as an electrolyte for SOFCs.

process which is directly associated with densification. Also, BCN based ceramics are known to exhibit very large grain boundary and electrode contribution to the total conductivity, which limit their application in proton conducting fuel cells [8]. The grain boundary blocking contribution can be reduced by making the material denser and thus reducing the grain boundaries. Densification of materials is an important process that plays a major role in determining their structural and morphological characteristics. Densification strategies are essential in the fabrication of ceramic components to achieve better compactness and finer grain sizes that contribute to the electrical, thermal and mechanical properties [13]. Densification achieved by high temperature treatment is generally called as sintering process. This can be achieved by various methods. Suitable methods are chosen based on the required properties. Conventional sintering (CS) has been the most commonly used and traditional method. This sintering method requires high temperature for complete densification of ceramic materials that also results in larger grain size due to Ostwald ripening [14,15]. In conventional method, the surface of the material is first heated and then the interior part is heated as shown in Fig. 1a. This reveals that there is a temperature gradient from the surface to the center. For applications, which require higher densification with lower and uniform grain size, conventional method is not appropriate [14]. In addition, it is also a time consuming process. To overcome the issues several other sintering methods are used which can also maintain the grain size in nano-meter range with reduced sintering time. Microwave sintering (MS) is one such method, which provides fast volumetric heating, enhanced production rate, grain growth inhibition of ceramics and efficient uniform transfer of energy [14,16]. Unlike conventional sintering, in microwave sintering (Fig. 1b) heat is first distributed within the material and then the entire volume is heated [17,18]. MS is advantageous due to the following facts: enhanced diffusion processes, reduced energy consumption, high heating rates with reduced processing time and improved microstructure and electrical properties [17–19]. Another advanced technique is spark plasma sintering (SPS) which is also known as field assisted sintering technique (FAST). SPS has great potential due to its fast densification and slow crystallization during the consolidation of powders [15]. The enhanced densification is achieved by the simultaneous application of axial pressure and high temperature generated by high current flow (Fig. 1c) [20]. SPS takes only few minutes for complete densification when compared to other methods, which may take hours for the same [17]. When compared to conventional sintering, non-traditional techniques such as microwave and SPS sintering are more effective methods to obtain homogenous, dense and hard alloys in the case of metallic samples [21]. Due to the difference in heat distribution in each of these sintering methods, the microstructure of the material is altered greatly. The approximate distribution of grain and grain boundaries in materials sintered with different techniques is represented in Fig. 2. As can be

2. Experimental procedure 2.1. Sample preparation and thermal analysis Commercially available high purity (99.99% Sigma Aldrich) BaCO3, K2CO3, CaCO3 and Nb2O5 were used as starting precursors. According to the chemical composition of Ba6K2Ca4Nb4O21 (BKCN 25) stoichiometric amounts of preheated oxides were taken and milled in a planetary ball mill (Pulverisette P5, Fritch, Germany) for 20 h with ball to powder weight ratio of 10:1. Wet milling was carried out in a tungsten carbide (WC) bowl with toluene as process control agent (PCA). After 20 h of ball milling, the samples were collected and stored in an airtight container. Small amount of milled samples were subjected to thermal analysis using NETZSCH STA 449 F3 JUPITER thermo-gravimetry/differential scanning calorimetry analyzer (TG/DSC). The analysis was carried out from room temperature to 1200 °C with a heating rate of 20 °C/min in air atmosphere. 2.2. Sintering methods On confirmation of phase purity, the samples were pressed into pellets of 10 mm diameter and 2 mm thickness using PVA (Poly-Vinyl Alcohol) as binder in a hydraulic press with a pressure of 10 MPa. The pressed pellets were sintered in a muffle furnace (conventional sintering), some pellets were sintered using microwave furnace. For spark plasma sintering, the homogeneous powder was filled in graphite die, which was placed in a spark plasma chamber and sintered with an applied pressure of 30 MPa, with a heating rate of 200 °C/min. The experimental conditions at which the pellets were sintered using the above-mentioned methods are given in Table 1.The average thickness and area of each pellet was 0.2 cm and 0.6 cm2 respectively. 2.3. Microstructural analysis The pellets obtained through different sintering methods were 54

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Fig. 2. Schematic representation of the nature of grain boundaries of materials sintered using a. conventional, b. microwave and c. spark plasma sintering methods.

subjected to structural analysis and phase purity checkup by using Xray diffraction (using X’Pert PANalytical diffractometer) technique. The particle size and morphology of the sintered pellets were examined using high-resolution transmission electron microscopy (HRTEM) (JEOL-JEM 2100), fitted with an OXFORD energy dispersive X-ray spectrometer (EDS). The samples were viewed at an accelerating voltage of 200 KV. The selected area electron diffraction (SAED) patterns were processed and analyzed using the software Digital Micrograph1.85.1535 version. The morphology of the sintered pellets was examined by using Carl Zeiss EVO/18 scanning electron microscope.

2.4. Electrical measurements The total conductivity measurement of samples were carried out using impedance analyzer, AUTOLAB, ECO CHEMIE Netherlands from 10 Hz to 10 MHz between the temperature range 100–800 °C in air atmosphere. The data analysis was performed using Autolab software Frequency Response analyzer (FRA) 4.9 version. Equivalent circuit fitting of the experimental data are done using Electrochemical (EC lab) V10.40 and Z-view version 2.1b software.

Fig. 3. TG/DSC graph of ball milled precursors of BKCN 25.

3.2. XRD analysis The calcined sample is checked for its phase purity using XRD analysis. XRD data of BKCN 25 calcined at 800 °C is shown in Fig. 4. Calcined sample is indexed based on the PCPDF #490425. All diffraction peaks in the XRD pattern are indexed according to a face centered cubic structure (Fm3¯m #225) with cell parameter a ≈ 8.4 Å. The sintered samples are indexed based on the PCPDF#490425 which is in congruence with the reported structural data for similar type of perovskite oxides [10]. The calcined samples are later pressed into pellets and sintered separately by conventional, microwave and spark plasma methods. The sintered pellets are further subjected to XRD analysis to confirm the structural stability after sintering process. The X-ray diffractograms shown in Fig. 5 are taken from the crushed powders of individual pellets sintered using three different methods. The peaks in each diffractogram can be indexed based on the face centered cubic structure exhibited by the calcined powders of the same material. The refined unit cell parameters for different sintering methods of BKCN 25 sample is listed in Table 1. It is evident that no significant changes are observed in the crystal structure on sintering techniques. The results

3. Results and discussions 3.1. Thermal analysis Fig. 3 shows the TG/DSC plot of ball-milled precursors of BKCN 25 sample. From the TG curve, it is observed that weight loss occurs in several steps. Initial stage of continuous weight loss upto 600 °C occurs due to the removal of moisture content and other organic compounds in the sample (due to wet milling). The second stage of weight loss around 700 °C may be due to the removal of carbonates from the precursors due to the chemical reaction [12]. The exothermic peak around this temperature in DSC curve could also be due to the same process. The third stage of weight loss around 800 °C is attributed to the final stable phase formation that is also illustrated by an endothermic peak in the DSC plot and a horizontal curve in TG. Based on these observations; in order to obtain a single perovskite phase the ball-milled precursors were calcined at 800 °C. Table 1 Parameters obtained from structural analysis of BKCN 25. Sintering technique

PCPDF number

Space group

Bravais lattice

Refined cell parameter from XRD (Å)

Crystallite size (nm)

Conventional Sintering Microwave Sintering Spark Plasma Sintering

PCPDF#490425 PCPDF#490425 PCPDF#490425

Fm3¯m #225 Fm3¯m #225 Fm3¯m #225

FCC FCC FCC

8.340 8.454 8.482

54.50 34.92 26.32

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are seen with higher porosity. Though the calculated crystallite size from XRD data is the smallest for SPS sample, SEM image shows larger and denser grains with comparatively lower porosity than CS and MS samples. This indicates that each grain is polycrystalline the boundary of each individual grain and the porosity of the pellet could be the deciding factor for conductivity. Denser grains generally allow faster charge carrier mobility when compared to porous grains. Therefore, samples sintered using SPS and MS methods would exhibit better conductivity properties when compared to CS. The lesser porosity in SPS sample helps in lowering the hindrance (resistance) for carrier migration which will further enhance the conductivity. It is evident that at a given temperature (here 1200 °C) SPS is the most effective method for better densification. The higher density and larger grain size of SPS sample could reduce the resistance due to the segregation of oxygen vacancies at the grain boundaries which was a critical issue in 1:1 ordered BKCN 25 as discussed earlier in the introduction. The detailed impedance studies on the reduction of grain boundary blocking effect of the samples are discussed in later paragraphs. Fig. 7 shows the TEM images of BKCN 25 sintered by different methods. The morphology of the crushed particles of pellets appear as thin sheets. Spot patterns taken from powders of all three differently sintered pellets reveal the single crystalline nature of the individual crystallites in the pellets. The reflections in the SAED patterns are indexed based on the crystal structure obtained through XRD analysis. The SAED patterns taken from all sheets exhibited similar arrangement of reflections. This indicates that all the sheets have similar atomic arrangement and orientation. The interplanar distances in high resolution images are also interpreted based on the face centered cubic crystal structure obtained by XRD analysis (Fm3¯m #225 with cell parameter a ≈ 8.407 Å). The observation in TEM also indicate that the individual crystallites within the grains have sheet like morphology.

Fig. 4. XRD pattern of calcined powder of BKCN 25.

3.4. Electrochemical AC impedance analysis (EIS) EIS analysis in this study is used to understand the grain and grain boundary blocking contribution of BKCN 25 pellets sintered using different techniques. In the present study, equivalent circuit is modeled using Cole-Cole function. The semicircle in Cole-Cole type impedance plot is based on the following equation [23–25]; Fig. 5. XRD patterns of a. conventional, b. microwave and c. spark plasma sintered pellets of BKCN 25.

Z ∗ (ω) = Z′ (ω) − jZ″ (ω) = Rp /(1 + (jωτ )α )

(1)

where ω = 2πf is the angular frequency, Z′ is the real part of impedance, Z″ is the imaginary part of impedance, Rp is the resistance, τ(=RC) is the relaxation time and α is a parameter which characterizes the distribution of relaxation times with values ranging from 0 to 1. Fig. 8a-c shows the AC impedance plots of BKCN 25 sintered by conventional, microwave and spark plasma methods. The nature of semicircles in the impedance plots appear to be the resultant of two overlapped semicircles corresponding to grain interior (bulk) and grain boundary contributions to total conductivity. In this case, separation of individual bulk and grain boundary contribution is not possible. The low frequency intercept includes both grain boundary and bulk contribution in all the three plots. The evaluation of grain boundary blocking contribution is very essential in the present study to address the reduction in total conductivity due to possible oxygen vacancy segregation at grain boundaries. In order to distinguish grain and grain boundary contribution to the total conductivity, each Cole-Cole plot is fitted using an appropriate equivalent circuit indicating both grain (bulk) and grain boundary contribution. The grain and grain boundary resistance values obtained from the fitted data are used to calculate the conductivity due to grain (bulk) and grain boundary. The total conductivity is calculated by using the total resistance obtained through summing up the grain and grain boundary resistances. Bulk (σb), grain boundary(σgb) and total (σtot) conductivities of all the samples are calculated from the known thickness of the pellets and the resistance values obtained from the fitted data using the following

also show no diffraction peaks from impurity phases, indicating that the perovskite formed has a higher degree of purity and crystallinity. The average crystallite size as calculated from the XRD data is also listed in Table 1.

3.3. Electron microscopy analysis Fig. 6a-c shows the SEM images of pellets sintered by different methods. From the images, it is observed that in conventional (Fig. 6a) and microwave (Fig. 6b) sintered pellets the individual grains of few hundreds of nanometer size are separately visible along with large pores. A large variation in size of the particles can be observed on the surface of CS samples. Whereas in MS and SPS samples the particles seem to be of uniform size. SPS sintered (Fig. 6c) pellets appear to be well sintered with lower porosity. SPS, due to its rapidity has larger grain growth and exhibit homogeneous microstructure [22] with high density for BKCN 25. Porosity of the sintered pellets was measured by liquid absorption method. The percentage of porosity and density of each pellet is listed in Table 2. The values show that the relative density is about 77% (for CS), 90% (for MS) and 97% (for SPS) of the theoretical value (6.89 g cm−3). Conventionally sintered sample shows higher porosity when compared to MS and SPS samples. Particles with various sizes are observed in CS sample, whereas in MS sample larger particles 56

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Fig. 6. SEM images of BKCN 25 pellets sintered by a. conventional b. microwave and c. spark plasma methods.

size) SPS samples shows better total conductivity. The highest grain and grain boundary conductivity was exhibited by spark plasma sintered sample. In addition, the obtained σtot = 1.08 × 10−3 S.cm−1 for SPS sample is higher when compared to the previously reported value 2.4 × 10−4 S.cm−1 in air atmosphere at 700 °C for K doped 1:2 ordered BCN [10]. This enhancement in conductivity value is also associated with the additional oxygen vacancies created by 1:1 ordered B (octahedrally coordinated with oxygen) cations. The obtained oxide ion conductivity value is similar to that exhibited by GDC 15 [26], BaCe0.5Zr0.3Y0.2O3-δ [27] and BaCe0.45Zr0.45Sc0.1O3-δ [28]. In order to confirm the reduced grain boundary contribution to the bulk contribution, the grain boundary-blocking factor was calculated using following equation;

equations;

σb =

l Rb A

(2)

σgb =

l Rgb A

(3)

σtot =

l (Rb + Rgb) A

(4)

where l/A is the geometrical factor, l is the thickness, A is the cross sectional area of the sample and Rb and Rgb are the grain (bulk) and grain boundary resistances. The calculated bulk, grain boundary and total conductivities using Eq. (2) (3) and (4) at 700 °C are listed in Table 3. From Table 3, it can be observed that σgb is significantly lower when compared to σb for CS sample indicating least contribution from the grain boundaries to the conductivity. In the case of MS and SPS samples, σgb and σb have the same order of magnitude though the former has a slightly lower value. This confirms the larger resistance offered by the grain boundaries for the carriers in the case of CS sample. Among MS and SPS samples owing to better densification (lower porosity) and enhanced bulk conductivity (probably due to larger particle

αR =

Rgb Rb + Rgb

(5)

where Rb and Rgb are the grain (bulk) and grain boundary resistances respectively. The grain boundary-blocking factor provides the fraction of the electrical carriers being blocked at the impermeable internal surface to the total number of electrical carriers present in the sample [29]. The αR generally depends on dopant concentration, crystallite size

Table 2 Sintering conditions, porosity and density of BKCN 25. Sintering methods

Sintering Conditions

Porosity (%)

ρ (Density) g/cm3

Conventional (CS) Microwave (MS) Spark plasma (SPS)

1200 °C, 4 h holding time 1200 °C, 20 min holding time 1200 °C, 5 min holding time Pressure-30 MPa

35.2% 18% 10.7%

5.35 6.23 6.71

57

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Fig. 7. TEM images (left), diffraction patterns (middle) and high resolution images (right) of BKCN 25 pellets sintered by a. conventional, b. microwave and c. spark plasma methods.

be due to lesser blocking behavior of mobile ions at grain boundaries. In addition, almost uniform sized larger particles in MS and SPS samples have lesser area of grain boundaries. However, SPS sample requires higher activation, energy for σbulk when compared to other two samples. In the present study, significant improvement in total conductivity with reduced grain boundary blocking contribution of BKCN 25 is achieved by adapting SPS method. The reason may be the defects (transport property of the material is controlled by defects) present in higher concentration in grain boundaries as compared to grains. And the internal space charge created at the grain boundaries lead to increase in the concentration of charge effects. In the case of CS and MS samples, either the resistance offered by the grain boundaries or the higher blocking effect cause discouraged migrations of charge carriers across the grain boundaries. This may be the reason for high grain boundary blocking contribution in these samples compared to SPS sample. BKCN 25 sample sintered through conventional sintering method exhibits low conductivity due to large grain boundary blocking contribution and also due to high porosity. But denser and less porous SPS sample allows the comparatively easier migration of the carriers over grain boundaries and exhibit higher conductivity.

and density [29,30]. To understand the influence of densification on BKCN 25 pellets the grain boundary blocking factor, αR is calculated. As seen in Table 3 the lowest grain boundary-blocking factor is observed for SPS sintered sample, which also has the highest density. Fig. 9 shows the Arrhenius plots of BKCN 25 sintered with different methods. Here the calculated bulk, grain boundary and total conductivity values of CS, MS and SPS samples are plotted against 1000/T. The activation energies calculated using the slope of the Arrhenius plots (according to Eq. (6)) are listed in Table 3.

σ = σ0/ T exp (−Ea/ kB T )

(6)

where kB is the Boltzmann’s constant, T is the temperature, ΔEa is the activation energy for migration of O2− ions and σ0 is the pre-exponential factor. Arrhenius plot in Fig. 7c clearly shows the overall better performance of SPS sample at all measured temperatures. It is observed that activation energy for grain contribution is lower when compared to the grain boundary contributions for all the samples. The carriers need to overcome the barriers offered by the grain boundaries to be mobile through the sample. Therefore, as revealed by the SEM analysis owing to the presence of smaller particles along with larger particles and higher porosity, it is obvious that activation energy for ion migration is higher in CS sample. In the case of other two samples, slightly lower activation energy is exhibited for grain boundary conduction. This may

4. Conclusion This study reveals that better densification in BKCN 25 sample can 58

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Fig. 8. Cole-Cole plots of BKCN 25 pellets sintered using a. conventional, b. microwave c. spark plasma sintering and their enlarged view for higher temperatures and their equivalent circuits.

highest bulk conductivity of 8.63 × 10−3 S.cm−1 at 700 °C with reduced grain boundary blocking contribution. The total conductivity and density enhancement achieved is in the order of plasma sintering > microwave sintering > conventional sintering. The enhanced total conductivity and structural stability of SPS sintered BKCN 25 makes it more suitable for applications such as SOFCs, oxygen sensors, gas

be achieved by spark plasma sintering. In addition, lower grain boundary blocking behavior and larger particle size makes it better conducting material when compared to CS and MS samples. The ultimate aim of this work is to suppress the grain boundary blocking contribution of BKCN 25 sample to the total conductivity. From the obtained results, it is evident that spark plasma sintered sample exhibits

Table 3 Bulk (σb), grain boundary (σgb) and total (σtot) conductivity, activation energy and grain boundary-blocking factor (αR) at 700 °C. Sintering technique

σb, 700 °C S.cm−1

ΔEa eV

σgb, 700 °C S.cm−1

ΔEa eV

σ tot, 700 °C S.cm−1

ΔEa eV

αR

CS MS SPS

1.23 × 10−4 3.25 × 10−5 8.63 × 10−3

0.26 0.24 0.62

1.14 × 10−6 1.04 × 10−5 1.23 × 10−3

1.35 0.93 0.83

1.13 × 10−6 7.86 × 10−6 1.08 × 10−3

1.39 0.89 0.83

0.99 0.87 0.75

59

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Fig. 9. Arrhenius plot of BKCN 25 with grain, grain boundary and total conductivity.

separation membranes, etc. [9]

Acknowledgment

[10]

The authors acknowledge Naval Research Board, India (DNRD/05/ 4003/NRB/287) and UGC-RGNF for financial support and PSG Institute of Advanced studies and Naval Materials Research Laboratory (NMRL) for their infrastructure and intellectual support. The authors would like to thank Mr. T. Vijayaraghavan and Mr. N. Suresh Kumar for helping us in TEM and SEM analysis.

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