Microwave sintering of ZnO nanopowders and characterization for gas sensing

Materials Science and Engineering B 176 (2011) 181–186

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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Microwave sintering of ZnO nanopowders and characterization for gas sensing Zikui Bai a , Changsheng Xie b,∗ , Shunping Zhang b , Weilin Xu a , Jie Xu a a b

Key Lab for Green Processing and Functionalization of New Textile Materials, Ministry of Education, Wuhan Textile University, Wuhan 430073, PR China State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China

a r t i c l e

i n f o

Article history: Received 3 May 2010 Received in revised form 23 September 2010 Accepted 21 November 2010 Keywords: Microwave sintering ZnO Gas sensor Impedance spectroscopy

a b s t r a c t Thick film gas sensors based on ZnO nanopowders were fabricated by using microwave sintering. The surface and cross section morphologies were characterized by field-emission scanning electron microscopy (FE-SEM). The stability of the microstructure was studied by impedance spectroscopy. The results showed that the shape of the nanoparticles was not changed through microwave sintering, and the thick films had the more dense microstructures than that by muffle oven sintering. The resistance–temperature characteristic and the responses to toluene, methanol and formaldehyde revealed that the microwave sintering technique could effectively control the growth of ZnO nanoparticles, realize the uniform sintering of thick film, gain the stable microstructure and improve the response of sensor. In addition, the formative mechanism of the thick film microstructure was proposed according to microwave sintering mechanism. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Zinc oxide (ZnO) is a wide-band gap semiconductor that is desirable for many piezoelectric transducer, varistors, gas sensors and transparent conductive films [1–3]. By reducing the size of ZnO crystals to nanoscale dimensions or controlling the morphology of ZnO crystals to tetrapod-shaped nanopowders [4], nanowires and nanorods [5], researchers can tailor the properties of ZnO via quantum confinement and surface effect. ZnO gas sensor has attracted more and more researchers’ attention, especially the gas sensor based on ZnO nanopowders prepared by the method of vapourphase oxidation with an excellent sensing property to volatile organic compounds (VOCs)—benzene, toluene, xylene, ethanol and acetone [6,7]. The morphologies of ZnO particle and film play the key role to the response and stability of ZnO sensor. Moreover, the response and stability also depend on the ZnO sensing film preparation method, such as deposition method and post-processing treatment, which is related to some critical factors, such as surface state, morphology, surface-to-volume ratio and active center of the sensing film. The size of ZnO particles increases in the fabrication process of thick film to be far beyond the nanometer and the morphology of ZnO disappears [6]. It is not accurate if the effects of the grain size on the responses are discussed according to the size of raw powder [8,9].

∗ Corresponding author. Tel.: +86 27 87556544; fax: +86 27 87543776. E-mail address: [email protected] (C. Xie). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.11.005

Noble metal and transition-metal oxides dopants, such as Pt, Pd, TiO2 , CuO, Fe2 O3 , NiO, were all incorporated to enhance the sensitivity, selectivity and stability (3S) of sensor [7,10–14]. Besides, the fabrication techniques, such as the hydrothermal treatment of powders and sol suspension [15–17], post-deposition hydrogen doping and plasma treatment of ZnO micro/nano structured films [18,19], were adopted to improve the 3S of sensor. Although the great progress was obtained in the 3S of sensor, there is a sustained effort to improve the performance of sensor, especially the stability of sensor. For pure ZnO gas-sensing material, the smaller its grain size is, the higher its gas sensitivity is [8]. ZnO has an easy sintering characteristic [20]. So, it is a great challenge to keep the sintered thick film not only with the nanoparticle characteristic, but also with the stable microstructure. The morphology and microstructure of ZnO thick film are sensitive to the sintering temperature and time. High in sintering temperature and long in sintering time, the conventional thermal sintering technique in muffle oven usually results in the growth and the uneven agglomeration of ZnO nanoparticles to worsen the sensitivity and stability of the sensor. Microwave sintering has been extensively investigated in the fabrication process of structural, dielectric, magnetic, superconducting and piezoelectric materials, obtained a large number of technological breakthroughs [21–23]. However, it has not yet been reported to microwave application in the preparation of gas sensor. Compared to the conventional sintering, the microwave sintering has many advantages, such as the grain growth minimization, the uniform microstructure, the short sintering time and the low temperature [24–26]. Adopted the microwave sintering technique, it could effectively avoid the faults of the conventional thermal sintering method to

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a great extent and improve the sensitivity and stability of gas sensor. In this present study, we reported an approach to fabricate ZnO thick-film gas sensors by microwave sintering. The surface morphology and microstructure of the ZnO film were systematically investigated. To our knowledge, this is the first report related to fabricate ZnO gas sensors by using microwave sintering method. 2. Experimental procedures ZnO was prepared by the method of vapour-phase oxidation from metallic zinc as a raw material, which was described in detail in the literature [6]. The shape of ZnO nanoparticles is not uniform (sheet, rod and tetrapod-nanoparticles). The paste was prepared by mixing the ZnO with a given proportion of organic vehicle and then being milled for 6 h. ZnO thick films with the thickness of about 12 ␮m were prepared by a screen-printing technique onto alumina substrate attached with comb-type Au electrodes. Such thick films were heated at 250 ◦ C for 30 min to eliminate the organic vehicle, and then sintered in microwave oven (SAMSUNG CE107B-B.900W) for 20, 40 and 60 min, respectively. The size of the alumina substrate is 2 mm × 3 mm. The surface and cross section morphology of the films were observed by a Sirion II type field-emission scanning electron microscope (FE-SEM). Heater was an alumina substrate with RuO2 thick-film. The substrate with the ZnO film and the heater were together soldered onto a TO-8-003 support (Yixing City Jitai Electronics) to form gas-sensing devices by using gold wires and a welding machine. The devices were aged at 400 ◦ C for 6 days in the open air to enhance their stability. Electrical and gas-sensing properties were measured in a lab system (25 ◦ C, 80% relative humidity) as described in the literature [6]. The detected vapors were toluene, methanol and formaldehyde, and their concentrations were 100 ppm. The gas response (S) is given by S = Ra /Rg , where Ra and Rg are the resistance of the sensor in air and in detected gas, respectively. The response time is defined as the time taken by film to reach 90% of the saturation value after the surface is in contract with the detected gas; the recovery time is defined as the time taken by the film to come back to the 90% of ground state after the detected gas was removed. To evaluate the stability, the impedance measurements of samples were performed in dry air at 400 ◦ C for different times by using IM6ex electrochemical workstation in the frequency range of 1 Hz–3 MHz. In addition, the effects of different number of thermal cycles from room temperature to 400 ◦ C (heating and cooling times were 70 s and 80 s, respectively) on the impedance spectroscopy of samples were tested. The amplitude of sinusoidal voltage signal was 20 mV. The data analysis was performed with Zview 2.0 software.

In the conventional thermal sintering method, the sintering process generally takes place under isothermal condition, with the sintering time long and the heating rate low, where the heat conduction occurs from the heating elements of the furnace to the samples, and then from the surface to the interior. The driving force of sintering is a reduction of free energy by replacing highenergy free surfaces (solid–vapor) by low-energy grain-boundaries (solid–solid) and finally by minimizing also the grain boundary area (via grain growth). The number of nano-particles was reduced due to minimizing the grain boundary area between the large particles and the small particles (via grain growth). So, the bigger sphericity and ellipse particles were obtained in the surface of the thick film sintered in muffle oven at 700 ◦ C for 2.5 h, as shown in Fig. 1(d). If obtained the dense ZnO thick film, the high driving force would be needed, which was to increase the sintering temperature and duration. However, Microwave sintering is a complex process including the propagation and absorption of electromagnetic waves and the heat conduction in the ZnO thick film. The driving force of substance transmission and the sintering mechanism are different from those of the conventional thermal sintering. Birnboims’ studies showed that the local electric field was disproportionately intense close to grain boundaries and rough surfaces due to strong focusing, which could lead to a highly nonuniform energy aggregation and accelerated mass transfer rates via ponderomotive diffusion and plasma generation [27,28], suggesting that the microwave sintering had a series of non-thermal effects to enhance the sintering [29]. Moreover, the aggregation effect in grain boundaries decreased with the increase in the interparticles contact area to decelerate the mass transfer rates. This was the reason why the ZnO nanoparticles, which size and shape were not changed, were in well contact with each other, as shown in Fig. 1(b) and (c). There was also the same sintering characteristic in interior of the thick films by using microwave sintering, as shown in Fig. 2(a) Compared to Fig. 2(b), Fig. 2(a) exhibited that the ZnO nanoparticles in interior of the thick film by using microwave sintering were in better contact with each other than those by using conventional sintering method, suggesting that the thick films by using microwave sintering had the stable microstructure. On the other hand, it was noticeable that in spite of the highly inhomogeneous energy deposition during microwave sintering, significant differential thermal gradients still could not be maintained across micron size particles. The thermal time constants of micron-scale particles were very small, resulting in temperature gradient of no more than 10−6 K even for a heating rate of 100 K/min [30]. It could be deduced that the stress and strain in the ZnO thick film were very low, which was also benefit to improve the stability of the thick film.

3. Results and discussion

3.2. Resistance–temperature characteristics of the thick films

3.1. Characteristics of the microwave sintering thick-film

Fig. 3 shows the temperature dependence of resistance of ZnO sensors by using microwave sintering in air. It is observed that the resistance of the ZnO sensors with the increase of operating temperature exhibited three stages: a decrease in the range of 180–260 ◦ C, an increase in the range of 260–420 ◦ C and a decrease in the range of 420–470 ◦ C, respectively. The sintered ZnO thick film had a smaller resistance than the unsintered film, indicating that the effect of microwave sintering was evident. The intrinsic resistance of ZnO thick film decreases with an increase in operating temperature owing to more electrons entering into conductance band. Whereas, the absorbed oxygen on the surface of ZnO thick film gradually transform into oxygen ions (O2− and O− ) by extracting free electrons from the conductance band that leads to increase in resistance, which exist in an equilibrium

Fig. 1 shows surface morphology for ZnO thick films by microwave sintering (Fig. 1(a) 20 min, (b) 40 min and (c) 60 min) and by muffle oven at 700 ◦ C for 2.5 h (Fig. 1(d)). The dense thick films were achieved with the increase in microwave sintering time. It was noticeable that the size and shape of ZnO nanoparticles, such as sheet, rod, tetrapod- nanoparticles, was not changed. However, in the surface of the thick film sintered in muffle oven at 700 ◦ C for 2.5 h, there are the bigger sphericity and ellipse particles, which were not in well contact with each other to form the dense thick film, as shown in Fig. 1(d). It was related to the different sintering principles between a microwave sintering method and a conventional thermal sintering method.

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183

Fig. 1. The surface morphology of ZnO thick films by using microwave sintering for 20 min (a), 40 min (b), 60 min (c) and (d) by using muffle oven sintering at 700 ◦ C for 2.5 h.

Fig. 2. Cross-section image of ZnO thick films, (a) microwave sintering for 40 min; (b) muffle oven sintering at 700 ◦ C for 2.5 h.

state: [31] 50

WB 40 min

40

Resistance (MΩ)

O2 + 2e → 2O−

WB 20 min

−

O +e → O

WB 60 min WB 0 min

30

ZnO

20

5

0 150

200

250

300

350

400

450

500

Temperature (ºC) Fig. 3. The plot of resistance vs. temperature for ZnO thick films by using microwave sintering.

2−

(1) (2)

In the range of 180–260 ◦ C, the adsorption type of oxygen molecules belonged to physisorption and the adsorption attraction (i.e., Van der Waals attraction) was too weak. Therefore, the increase resulting from absorbed oxygen could not affect the dropping tendency of the resistance. This was the reason why the initial resistance decreased with an increase in operating temperature. In the higher temperature range of 260–420 ◦ C, the adsorption type was turned into chemisorption (i.e., chemical-bond attraction) and the concentration of oxygen molecules adsorbed on surface rose gradually with an increase in operating temperature. Therefore, more free electrons were extracted by the absorbed oxygen, which induced the increase of the resistance [32]. But the reaction (O2 + 2e → 2O− , O− + e → O2− ) was exothermic [33]. Once the operating temperature surpasses 420 ◦ C, the reaction would proceed to left, which led to the decrease of the trapped electrons from the

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a

16 14 12

Response

10

20 min 40 min 60 min 0 min 100 ppm toluene

8 6 4 2 0 150

200

250

300

350

400

450

Temperature (ºC)

b

16 14 12

Response

10 8

20 min 40 min 60 min 0 min 100 ppm methanol

6 4 2 0 150

200

250

300

350

400

450

500

450

500

Temperature (ºC)

c

14 12

Response

10 8

20 min 40 min 60 min 0 min 100 ppm formaldehyde

6 4 2 0 150

200

250

300

350

400

Temperature (ºC) Fig. 4. Response vs. operating temperature of ZnO thick films by using microwave sintering, (a) 100 ppm toluene, (b) 100 ppm methanol, (c) 100 ppm formaldehyde.

conductance band and the decrease of the resistance again, entering the third stage. In addition, the resistance–temperature characteristic curves indicated that the resistances of the thick films increased with an increase in microwave sintering time. It might be contributed to the decrease of defects in the ZnO thick films because of the sufficient oxidation reaction in the long sintering time, led to the carrier concentration low, hence the high resistance was obtained. 3.3. Gas-sensing properties of the thick films Fig. 4 shows the response as function of operating temperature of sensors by using microwave sintering. The responses

were greatly affected by the operating temperature and the sintering time. The sensor by using microwave sintering for 20 min obtained the highest response and that for 60 min was the lowest. The optimum response temperature of the same sensor to toluene, methanol and formaldehyde was different also. The optimum response of the sensor by using microwave sintering for 20 min to toluene and formaldehyde were obtained at about 380 ◦ C, whereas the optimum response temperature to methanol was higher, i.e., at about 425 ◦ C. To the sensor by using microwave sintering for 40 min, the optimum response temperature to toluene, methanol and formaldehyde were all at about 425 ◦ C. To the sensor by using microwave sintering for 60 min, the highest response temperature to toluene, formaldehyde and methanol were to increase gradually, followed by 380 ◦ C, 425 ◦ C and 460 ◦ C. Such results could be explained as follows: ZnO gas sensor belonged to surface-resistance-controlled type at high temperature condition, i.e., using the change of surface resistance to detect gas. The change in response, corresponding to the change in resistance, was mainly induced by the adsorption and desorption of oxygen ions (O− and O2− ) on the surface of the film. At high temperature condition, the oxygen molecule (O2 ) adsorbed on the surface of the thick film transformed into oxygen ions (O− and O2− ) in air environment, as the above formula (1) and (2). Some free electrons were extracted from the ZnO conductance band, which caused the decrease of carrier concentration and the increase of the film resistance [32,34]. When the film was exposed to the reducing gas (toluene, methanol and formaldehyde), the adsorbed oxygen ions (O− and O2− ) on the film surface reacted with the reducing gas to release free electrons, which led to the carrier concentration high and the surface resistance low. The amount of the adsorbed oxygen ions (O− and O2− ) was one of the key factors to affect the response of the ZnO thick film. The more the adsorbed oxygen ions (O− and O2− ) were, the higher the response was. The adsorbed oxygen ions (O− and O2− ) increased with the increase in the surface area of the film. As the aforementioned, the longer the microwave sintering time was, the denser the surface was. So, the surface area of the film decreased with the increase in the microwave sintering time. The result was that the amount of the adsorbed oxygen ions (O− and O2− ) was low. It was the reason why the sensor by using microwave sintering for 20 min obtained the highest response and that for 60 min was the lowest. However, the response of the sensor unsintered by microwave was very low and neglectable. It was because the ZnO nanoparticles were not fixated together to form the conduction pathways. The reaction of oxygen adsorption in the thick film (equilibrium (1) and (2)) is exothermic. Once the operating temperature preponderated over the optimum temperature, the concentration of the oxygen ions (O− and O2− ) on the surface was saturated and the resistance of sensor reached the maximum. The reaction would proceed to left, making the concentration of oxygen ions decrease, and thus the response would be decreased. Therefore, the optimum operating temperature was very important to receive the most perfect gas-sensing property. The response to toluene, methanol and formaldehyde were obviously different at the same operating temperature and their optimum response temperatures were also different, which might be related to the different chemical bonds (toluene, methanol and formaldehyde) [35]. The response-recovery characteristics of the ZnO thick films at operating temperature 400 ◦ C were listed in Table 1. The effect of microwave sintering time on the response-recovery characteristics was not obviously different. Both response and recovery time are in 10 s, in addition to toluene. It was noticeable that the recovery time to all the detected gas was longer than the response time, which was different from that of the thick film sintered by using muffle oven [6].

Z. Bai et al. / Materials Science and Engineering B 176 (2011) 181–186 Table 1 The response and recovery time of the ZnO thick films at operating temperature of 380 ◦ C. Vapors (100 ppm)

Time (min)

Toluene

Methanol

Formaldehyde

20 40 60

6(16) 9(24) 4(38)

8(11) 10(8) 9(10)

3a (6b ) 3(7) 7(9)

a

initial

-100

1h 3h

Microwave sintering

b

a

185

-80

6h

Z'' (KΩ)

10 h 26 h

-60

48 h WB,40 min,ZnO

-40

Response time (s). Recovery time (s).

-20

0

3.4. Microstructure and stability of the thick films

10

20

30

40

50

60

70

80

90 100 110

Z' (KΩ)

b

150

-60

200 250

-50

Z'' (KΩ)

350 400

-40

450 WB,40 min, ZnO

-30 -20 -10 0

0

10

20

30

40

50

60

Z' (KΩ)

c

-3500

initial 1h

-3000

3h -2500

Z'' (KΩ)

Impedance spectroscopy is a powerful tool for studying the microstructure characteristics of amorphous semiconductor, ionconductive glass and transition metal oxides. The change of impedance curve corresponds to the change of microstructure of the film, indicating the stability of the film. In the frequency range of 400 Hz-3 MHz, the impedance spectroscopy of the ZnO sensors sintered by using microwave for 40 min as a function of time (fixed temperature, 400 ◦ C) and as a function of the number of thermal cycles (from room temperature to 400 ◦ C) are shown in Fig. 5(a) and (b). In addition, the impedance spectroscopy of the ZnO sensor sintered by using muffle oven as a function of time (fixed temperature, 400 ◦ C) were tested (Fig. 5(c)). The impedance curves below 400 Hz, which were disordered or looped due to the so-called negative capacitance effect [36], could be ignored. The stabilities of the sensors at the fixed temperature (400 ◦ C) and under the thermal cycling condition (from room temperature to 400 ◦ C) were evaluated, respectively. Fig. 5(a) and (b) shows that the impedance curves were an approximate semicircular arc, in which the arc at high frequency side did not change with an increase in the high temperature time and in the number of thermal cycles, correspondingly that at low frequency side transferred to left. It was noticeable that the semicircular arc stopped transferring after 10 h at fixed temperature 400 ◦ C and 250 thermal cycles, whereas the impedance curves in Fig. 5(c) always fluctuated randomly during the fixed temperature 400 ◦ C. It is well known that the semicircular arcs at high and low frequencies sides may be assigned to charge transport through the grain interior and the grain boundary, respectively. In general, the grain boundary effect on electrical conductivity may originate from the grain boundary potential barrier or from the space charge layers which are depleted in majority charge carriers and which are localized along the grain boundaries [37]. The stability characteristic of the semicircular arc at low frequency side revealed the stability of the grain boundary, i.e. the connection characteristic between ZnO nanoparticles in thick film. The stable impedance curves of the thick film by using microwave sintering for 40 min in the states of the fixed temperature 400 ◦ C and the thermal cycles indicated the stable connection structure between ZnO nanoparticles in the thick film. The impedance curves of the thick film were the approximate semicircular arcs, indicating the homogeneous and stabile connection between ZnO nanoparticles. This could be attributed to the microwave sintering characteristics, the volumetric and uniform heating with high rates. In contrast, the anomalistic impedance curves of the thick film by using conventional thermal sintering revealed the unstable microstructure within the thick film. Based on the above description, it might be deduced that the thick films by using microwave sintering possessed the homogeneous and stable microstructure.

0

6h 20 h

-2000 -1500 -1000 -500 0 -500

0

500

1000 1500 2000 2500 3000 3500

Z' (KΩ) Fig. 5. The change of impedance spectroscopy curves of ZnO thick film sensors by using microwave sintering with the increase in the high temperature time (a) and the number of thermal cycles (b); by using muffle oven sintering with the increase in high temperature time (c).

4. Conclusion The ZnO thick film gas sensors were fabricated by using microwave sintering method. The shape of ZnO nanoparticles is not changed during microwave sintering, and the thick films have the more dense microstructures than that by using muffle oven sintering. The results of the resistance–temperature characteristics and the responses to VOCs indicated that that the microwave

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sintering technique could effectively control the growth of ZnO nanoparticles, realize the uniform sintering of thick film, gain the stable microstructure and improve the response of sensor. It could be deduced that the microwave sintering method is a promising approach to fabricate the gas sensor with high sensitivity and stability. Acknowledgement This work was supported by the National Basic Research Program of China (Grant nos. 2009CB939702 and 2009CB939705), Nature Science Foundation of China (no. 50927201), Nature Science Foundation of Hubei province (no. 2009CDB170), the Education Commission of Hubei Province (no. Q20101612), the Opening Research Foundation of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology). The authors are also grateful to Analytical and Testing Center of Huazhong University of Science and Technology References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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