Enhanced dielectric and ferroelectric properties of

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/259997566

Enhanced dielectric and ferroelectric properties of BaTiO 3 ceramics prepared by microwave assisted radiant hybrid sintering Article in Ceramics International · July 2014 DOI: 10.1016/j.ceramint.2014.01.039

CITATIONS

READS

4

152

5 authors, including: Sanjay Kumar Upadhyay

Ajay Gupta

Tata Institute of Fundamental Research

Amity University UP, Noida, India

15 PUBLICATIONS 59 CITATIONS

379 PUBLICATIONS 2,860 CITATIONS

SEE PROFILE

SEE PROFILE

Anand Mohan Awasthi

Shamima Hussain

UGC-DAE Consortium for Scientific Research

UGC-DAE Consortium for Scientific Research

98 PUBLICATIONS 1,345 CITATIONS

67 PUBLICATIONS 383 CITATIONS

SEE PROFILE

SEE PROFILE

Some of the authors of this publication are also working on these related projects:

Non-linear Magneto-electricity View project

Interlayer exchange-coupling in multilayers View project

All content following this page was uploaded by Sanjay Kumar Upadhyay on 03 February 2014.

The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately.

Available online at www.sciencedirect.com

CERAMICS INTERNATIONAL

Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint

Enhanced dielectric and ferroelectric properties of BaTiO3 ceramics prepared by microwave assisted radiant hybrid sintering V. Raghavendra Reddya,n, Sanjay Kumar Upadhyaya, Ajay Guptaa,1, Anand M. Awasthia, Shamima Hussainb a

UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452001, India b UGC-DAE Consortium for Scientific Research, Kalpakkam Node, Kokilamedu 603104, India Received 1 January 2014; accepted 8 January 2014

Abstract Structural, dielectric and ferroelectric properties of polycrystalline BaTiO3 (BTO) ceramics prepared with hybrid sintering i.e., microwave assisted radiant heating (MARH) are reported. It is observed that the permittivity (ε) and true switched ferroelectric charge density (QSW) of BTO ceramics can be enhanced by employing MARH. An enhancement of 58% in ε and 17% in QSW is observed for the BTO sample prepared with 30% microwave power applied during MARH as compared to the conventional radiant heating. The results are explained in terms of microstructure resulting from the microwave assisted sintering. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Microwave sintering; Barium titanate; Ferroelectricity; PUND measurements

1. Introduction In the recent past, considerable research has been focused on the aspects of improving dielectric and ferroelectric (FE) properties of lead free perovskite oxide ceramics such as BaTiO3 (BTO) for using in various applications viz., high permittivity capacitors, ferroelectric memories, pyroelectric sensors, piezoelectric/electrostrictive transducers etc. [1–9]. Barium titanate (BTO) is a well known and extensively studied FE material due to its fascinating and promising electrical, chemical and mechanical properties [4]. Most of the efforts and reports in the literature are towards improving the dielectric and FE properties of BTO based compounds. By employing different preparation routes the main aim has been to control various parameters like micro-structure, grain size, porosity etc., which are shown to affect the dielectric and FE properties of BTO based compounds [5–7,9]. The methods that are reported in the literature include the conventional radiant n

Corresponding author. Tel.: +91 0731 2463913; fax: +91 0731 2465437. E-mail addresses: [email protected], [email protected] (V.R. Reddy). 1 Present address: Amity Center for Spintronic Materials, Amity University, Noida 201303, India.

heating, soft-chemical routes and fast sintering methods such as spark plasma (SP) and microwave (MW) sintering. For example, SP sintering used to prepare BTO ceramics and permittivity (ε) as high as 104–105 at room temperature (RT) is reported [10,11]. Similarly, preparation of BTO with MW heating has been attempted and various reports are published in the literature [6,7]. Use of off-stoichiometric precursor TiO2 x is shown to enhance the microwave absorption resulting in the formation of tetragonal BTO phase at very low temperatures [12]. Sonia et al. have reported the low temperature synthesis of BTO with MW heating and found an ε of 2.5 103, 7.5 103 at room temperature (RT), close to the ferroelectric to paraelectric transition (TC) temperature, respectively [7]. Ying Ma et al. have found that the c/a ratio was larger for MW sintered samples of BTO as compared to samples prepared by conventional radiant heating [8]. Recently, Han et al. have reported an ε value of 3 105 and a relatively high value of dielectric loss values for BTO ceramics prepared by MW sintering [11]. However, it is to be noted that in fast sintering methods such as SP and MW sintering there will be accumulation of oxygen vacancies, charge carriers at the grain boundaries which essentially lead to the enhancement of ε. For example, about one order of magnitude reduction in the ε is observed by annealing the MW

0272-8842/$ - see front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2014.01.039 Please cite this article as: V.R. Reddy, et al., Enhanced dielectric and ferroelectric properties of BaTiO3 ceramics prepared by microwave assisted radiant hybrid sintering, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.039

2

V.R. Reddy et al. / Ceramics International ] (]]]]) ]]]–]]]

sintered BTO ceramics in flowing air [11]. Also, for some of the electrodes, the ε is found to increase from the extrinsic Schottky junction effect. For example, about 15% high ε values are observed in BTO ceramics because of Schottky effect [11]. Therefore, if the origin of such high ε values is because of space-charge carriers as mentioned above, the prepared BTO ceramics would not be useful in terms of FE applications in spite of high ε values. For applications based on FE properties, what one requires is basically the true switched FE charge density (QSW) i.e., switchable ferroelectric polarization coming from switching of permanent electric dipoles with application of external electric field. It is to be noted that even in the conventional P–E hysteresis loop measurements, one would get the contributions from such space-charge carriers apart from the hysteresis ferroelectric polarization variations. Therefore, methods, free from such artifacts, are being used in recent literature to overcome these difficulties. Positive Up Negative Down (PUND) is one such method, which uses successively two positive and two negative electrical pulses to directly measure the true switched charged density (QSW) i.e., switchable ferroelectric polarization [13–16]. The stoichiometric precursors in the preparation of BTO ceramics viz., BaCO3 and TiO2 are not readily susceptible to microwaves and hence the usual practice, as reported in most of the above mentioned work, is to place the samples in SiC susceptor and subjecting to microwave radiation. SiC, being an excellent microwave absorber gets heated up with the microwave absorption and heats the sample. This way one can achieve fast heating rates, microwave absorption by the samples at higher temperatures etc., but the quantification of microwave effects is not straight forward. However, on the other hand the use of hybrid sintering also known as microwave assisted radiant heating (MARH) technology, which is nothing but the application of either fixed or variable amounts of microwave energy simultaneously with the conventional radiant heating in a controlled way is a better way of preparing the samples which are not readily microwave susceptible. It is reported in the literature that at elevated temperatures (at about 0.5–0.6 TM, where TM is the melting temperature which is about 1600 1C for BTO) the absorption of microwaves by the materials increases due to the breaking of bonds between the ions [17]. Therefore, in principle, any material which is not readily susceptible to microwaves can be made microwave susceptible by using hybrid sintering. The applied small amount of microwave energy is expected to result in more uniform heating effects throughout the sample providing similar crystal structure and phase boundary properties at the surface and within the sample interior. Wang et al. have studied the effects of hybrid sintering in ZnO, yttriastabilized zirconia and Al2 O3 and reported that the microwave effect is greatest for the materials that absorbed microwaves most readily [18]. The preparation of BTO ceramics with hybrid sintering is not fully explored in the literature. In view of the above stated points, in the present work, a systematic study has been carried out to know the effects of MARH on the FE and dielectric properties of polycrystalline BaTiO3 ceramics. Series of BTO samples are prepared by varying the percentage of microwave power applied during the MARH i. e., the samples were prepared by subjecting to conventional

radiant heating with simultaneous application of microwaves. We have used PUND method to capture QSW. It is observed that hybrid sintering results in 17% and 58% enhancement in QSW and ε, respectively, for BTO ceramics as compared to conventional radiant sintering.

2. Experimental For the preparation of BaTiO3 ceramics, equimolar amount of stoichiometric BaCO3 (99.99%) and TiO2 (99.99%) were mixed thoroughly using acetone as a mixing medium. The mixed powder was calcined at 900 1C for 12 h. The calcined powder was then grinded for 4 h and pre-sintering was done at 1100 1C for 12 h. The resulting powder was made in to pellets of about 10 mm diameter and a thickness of 1 mm. A suitable amount of binder is added to the samples before making pellets. For final sintering the pellets were subjected to 1200 1C for 2 h with different percentage of microwave power (0, 15, 30, 50 and 75%), henceforth designated as BTO-0, BTO-15, BTO-30, BTO-50 and BTO-75, respectively. The samples were furnace cooled to room temperature after the heat treatment. Commercially purchased M/s Carbolite make MRF16/22-CMAT (Carbolite Microwave Assist Technology) hybrid furnace, having microwave power of 1.8 kW (operating at 2.45 GHz) in addition to molybdenum disilicide radiant heating elements (9 kW power) is used in the present study. Unlike devices which simply use microwaves to heat susceptible blocks which then radiate heat onto the sample, the CMAT furnace is able to heat the sample using both radiant heat and microwaves with independent control of radiant heating and either continuous or pulsed microwave energy from 0 to 100% of output level. The temperature of the sample was measured by the calibrated IR sensor during the hybrid sintering. The IR sensor was calibrated by adjusting the emissivity so as to get the same surface temperature of BaTiO3 sample measured with calibrated thermocouple when using only radiant heating. X-ray diffraction (XRD) measurements on the powder BTO (crushed pellets) were carried out using Bruker D8 Advance X-ray diffractometer, equipped with Cu-kα radiation. Field emission scanning electron microscopy (FESEM) images were recorded using AURIGA of CarlZeiss make. Silver electrodes were used for the dielectric constant and FE measurements. The ferroelectric (P–E) loop and PUND measurements were carried out by ferroelectric loop (P–E) tracer of M/s Radiant Instruments, USA. During the ferroelectric measurements, the samples were immersed in silicone oil to prevent the electric arcing, if any, at high applied voltages. The dielectric constant measurements (1 Hz–10 MHz) were carried out by Novo control Alpha-A impedance analyser and home-made sample holders.

3. Results and discussions The heating protocol used in the present study is shown in Fig. 1. First the sample was heated up to 400 1C at the rate of

Please cite this article as: V.R. Reddy, et al., Enhanced dielectric and ferroelectric properties of BaTiO3 ceramics prepared by microwave assisted radiant hybrid sintering, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.039


3

Fig. 1. (a) Typical time–temperature profile recorded during the sintering of BaTiO3 ceramics using microwave assisted radiant heating (MARH) with 30% microwave power applied along with conventional radiant heating. (b) The radiant heating element power levels required to heat the sample and maintain at the set point when subjected to different microwave power levels during MARH. Note the reduction in the power levels of heating elements with increase in microwave power.

3 1C/min with 120 min dwell time (to burn off the binder) and the set temperature was increased to 1200 1C (with dwell time of 2 h) at the rate of 5 1C/min. At this point, the microwave was switched on with fixed power (say 30%) for a period so as to cover the 2 h dwell time at 1200 1C and the time required to reach the 1200 1C temperature set point. This procedure was repeated with different microwave powers. It is to be noted that the input power to the radiant heating elements is controlled by the sample temperature, whereas the microwave power was maintained constant during the entire sintering. The set temperature, sample surface temperature, the percentage of power taken by the radiant heating elements of the furnace and the microwave power were recorded during each sintering. One such data is shown in Fig. 1(a). And Fig. 1(b) shows the power taken by the radiant heating elements to maintain the set temperature when the microwave power is varied. As one can see that the power of radiant heating elements gets reduced (as the microwave power increases) when the microwaves are applied indicating that the sample temperature is increased because of microwave absorption. This is similar to the study of Wang et al. in the preparation of ZnO with hybrid sintering [18] and depicts the fact that the heating is assisted with microwave energy. Fig. 2 shows the X-ray diffraction (XRD) patterns of all the prepared samples. The XRD data is refined with Rietveld refinement by using FullProf program considering the tetragonal P4mm space group for the estimation of lattice parameters [19]. No significant change is observed in the lattice parameters. As the microwave heating is expected to change the microstructure of the samples significantly, scanning electron microscopy (SEM) measurements are carried out to study the microstructure and the results are shown in Fig. 3. From the FESEM images, it is clear that the MARH results in

a considerable change of the microstructure. It is clear that while using only radiant heating (no microwave), there is unequal grain growth in the sample, which might be also responsible for the increased porosity in the sample. The microstructure of the sample which was prepared with MARH shows uniform grain growth as presented in Fig. 3. From the FESEM images, particle size and its distribution are calculated using ImageJ software [20] and the results are shown in Fig. 4. The observed grain sizes are in the range of 0.77–1:02 μm with a certain distribution in size. It is to be noted that the size distribution is narrow for the BTO-30 sample and roughly double the size distribution is observed for the remaining samples. The uniform grain growth is expected with microwave sintering because of the fact that the microwave heating is a volumetric heating. However, the present results indicate that one needs to optimize microwave power while preparing the samples by MARH. Because of excessive microwave heating there may be possibility of temperature gradient and thus minor cracks may be generated in the sample as observed for the BTO-50 sample. From the presented FESEM results, it is concluded that 30% of microwave energy (which corresponds to 0.54 kW) is an optimum value for the preparation of narrow size distributed BTO ceramics with MARH. Interestingly, better dielectric and FE properties are observed for the sample prepared with 30% of microwave energy as discussed below. The dielectric measurements of the all samples are carried out at room temperature in the frequency range of 1 Hz– 10 MHz and the data of permittivity (ε) and dielectric loss ð tan δÞ are shown in Fig. 5. It is to be noted that for all the samples, the dielectric loss is very small unlike the earlier reports of microwave sintered BTO ceramics. For example, Han et al. reported that microwave sintered BTO ceramics


4


Fig. 2. X-ray diffraction data of BaTiO3 ceramics prepared with microwave assisted radiant heating. Symbols represent the data, red color line represents the best fit and blue color line is the residual. Vertical symbols in the top frame show the allowed Bragg reflections. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

Fig. 3. Field emission scanning electron microscope images of BaTiO3 ceramics prepared with microwave assisted radiant heating with 0% (first column), 30% (second column) and 50%. (third column) of microwave power. Arrow in the last column shows the development of cracks in the sample.

exhibit extremely high dielectric loss ð tan δ ¼ 6:901Þ and it was necessary to further sinter with conventional heating in flowing air to recover back the insulating property of BTO [6].

Inset of Fig. 5(a) shows the variation of the observed ε at 1 kHz for all the samples and it is observed that the sample prepared with 30% microwave power exhibits an ε of 4 103,



5

Fig. 4. Particle size distribution obtained from the SEM data of Fig. 4. Average particle size (t) and the width of distribution (Δ) is mentioned.

Fig. 5. Room temperature frequency dependent (a) dielectric and (b) Tanδ data of BaTiO3 ceramics prepared with microwave assisted radiant heating. Inset shows the variation of dielectric constant value at 1 kHz for all the samples.

which is more than by 58% as compared to the BTO ceramic prepared with either no microwaves or excessive microwave energy.

The ferroelectric properties of all the prepared samples are studied both in a conventional way by recording P–E loops and by measuring the true polarization using PUND (Positive


6


Up Negative Down) technique. Fig. 6(a) shows the P–E loop data of all the samples and as mentioned above, the P–E loop may also contain the contribution of free charge carriers present in the sample apart from the intrinsic FE contribution. Therefore, to estimate the true FE signal and to illustrate the effects of MARH, we have carried out PUND measurements on the prepared samples. Fig. 6(b) shows the representative PUND data. PUND basically consists of applying five voltage pulses (Vmax) of certain pulse width and delay to the sample, in which the initial pulse is used to preset the sample to the polarization state opposite the sign of Vmax [13–16]. After this pre-set pulse, two pulses (second and third) of þ Vmax with a certain pulse delay are applied and the polarization is measured after each pulse. It is to be noted that after the second pulse what is measured is the total amount of polarization (P1) switched (which includes the true switched charged density due to FE and all other contributions such as space charges, leakage current etc.) and after the third pulse what is measured is the amount of polarization (P2) excluding the switched charged density due to FE. Therefore, the true switched FE charged density (QSW) is obtained as QSW=P1 P2 as shown in Fig. 6(b). Fourth and fifth pulses are similar to second and third pulses switching the sample in Vmax direction. The value of Vmax is selected so as to have a field of about 13 kV/cm, which corresponds to saturation field of P–E loop data (Fig. 6(a)). In this way, QSW for all the samples is measured and plotted in Fig. 6(c). The enhancement in the QSW is almost 17% for the BTO-30 sample. The observed enhancement of dielectric and FE properties of BTO-30 sample can be understood in terms of microstructure

and grain size uniformity. In fact, size effects on the FE properties of BTO ceramics are extensively studied in the literature and one can summarize the results in general terms as the following. The enhancement of FE properties for grain sizes in the range of 1:7 μm–0:5 μm, deterioration of FE below 0:5 μm and finally disappearance of FE for 10–30 nm [21–24]. Therefore, if one manages to prepare a ceramic with uniform grain size distribution, which was done in the present study by applying an optimum amount of microwave energy along with the conventional radiant heating, it is expected to have narrow distribution of tetragonality (c/a), Curie temperature etc., in a given ceramic resulting in better FE and dielectric properties. Methods such as the addition of seed grains, which are nothing but powder particles that have the same chemical composition as the powder to be sintered but considerably larger particle diameters, are employed to control the grain size distribution of BTO ceramics and hence obtaining high ε values [4]. However, since the microwave heating is an volumetric effect the MARH is a better method for controlling grain size distribution as shown in the present work. 4. Conclusions In conclusion in the present work microstructure, ferroelectric and dielectric properties of BaTiO3 (BTO) ceramic samples, prepared with microwave assisted radiant heating (MARH) are presented. The presented results demonstrate that one can enhance the functional properties of BTO ceramics with MARH. An enhancement of dielectric (58%) and ferroelectric properties (17%) of the BTO ceramics are observed

Fig. 6. (a) Ferroelectric (P–E) loops of all the prepared samples (b) PUND data of BaTiO3 ceramics prepared with 0% (BTO-0) and 30% (BTO-30) microwave power applied during microwave assisted radiant heating (MARH). The switched charge ferroelectric density (QSW) is determined from the data as shown and (c) the variation of QSW with the microwave power applied during MARH. Please cite this article as: V.R. Reddy, et al., Enhanced dielectric and ferroelectric properties of BaTiO3 ceramics prepared by microwave assisted radiant hybrid sintering, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.039


with MARH, which are explained in terms of microstructure. It is stated that, in general, one can use MARH for preparing various functional ceramic oxide materials with better properties as all the ceramics are expected to become microwave susceptible at high temperatures during MARH. Acknowledgments Authors thank Dr. Mukul Gupta for XRD measurements. References [1] B. Jaffe, W.R. Cook, Piezoelectric Ceramics, Academic Press, London, UK, 1971. [2] G.H. Heartling, Ferroelectric ceramics: history and technology, J. Am. Ceram. Soc. 82 (1999) 797–818. [3] B.D. Stojanovic, V.R. Mastelaro, C.O. Paiva, Santos, J.A. Varela, Structure study of donor doped barium titan ate prepared from citrate solutions, Sci. Sinter. 36 (2004) 179–188. [4] D. Hennings, Barium titanate based ceramic materials for dielectric use, Int. J. High Technol. Ceram. 3 (1987) 91. [5] M.P. McNeal, S.J. Jang, R.E. Newnham, The effect of grain and particle size on the microwave properties of barium titanate (BaTiO3), J. Appl. Phys. 83 (1998) 3288. [6] H. Han, D. Ghosh, J.L. Jones, J.C. Nino, Colossal permittivity in microwave-sintered barium titanate and effect of annealing on dielectric properties, J. Am. Ceram. Soc. 96 (2012) 485–490. [7] Sonia, R.K. Patel, P. Kumar, C. Prakash, D.K. Agrawal, Low temperature synthesis and dielectric, ferroelectric and piezoelectric study of microwave sintered BaTiO3 ceramics, Ceram. Int. 38 (2012) 1585–1589. [8] Ma. Ying, Elizabeth Vileno, Steven L. Suib, Prabir K. Dutta, Synthesis of tetragonal BaTiO3 by microwave heating and conventional heating, Chem. Mater. 9 (1997) 3023. [9] O.P. Thakur, Prakash Chandra, D.K. Agrawal, Structural and electrical properties of microwave processed barium titanate ceramics, Int. J. Ceram. Process. Res. 3 (2002) 75–79.

[10] Z. Valdez-Nava, S. Guillemet-Fritsch, Ch. Tenailleau, T. Lebey, B. Durand, J.Y. Chane-Ching, Colossal dielectric permittivity of BaTiO3 based nanocrystalline ceramics sintered by spark plasma sintering, J. Electroceram. 22 (2009) 238–242. [11] W. Heywang, Semiconducting barium titanate, J. Mater. Sci. 6 (1971) 1214–1226. [12] Dinesh Agrawal, Jiping Cheng, Hu Peng, Larry Hurt, Kuruvilla Cherian, Microwave energy applied to processing of high-temperature materials, Am. Ceram. Soc. Bull. 87 (2008) 39–44. [13] J.F. Scott, B. Pouligny, K. Dimmler, M. Parris, D. Butler, S. Eaton, Activation field, fatigue, and waiting-time effects in KNO3 thinfilm memories, J. Appl. Phys. 62 (1987) 4510–4513. [14] J.F. Scott, L. Kammerdiner, M. Parris, S. Traynor, V. Ottenbacher, et al., Switching kinetics of lead zirconate titanate submicron thinfilm memories, J. Appl. Phys. 64 (1988) 787–792. [15] J.F. Scott, C.A. Araujo, H.B. Meadows, L.D. McMillan, A. Shawabkeh, Radiation effects on ferroelectric thinfilm memories: retention failure mechanisms, J. Appl. Phys. 66 (1989) 1444–1453. [16] Y.S. Chai, Y.S. Oh, L.J. Wang, N. Manivannan, S.M. Feng, Y.S. Yang, et al., Intrinsic ferroelectric polarization of orthorhombic manganites with E-type spin order, Phys. Rev. B 85 (2012) 184406. [17] Y.V. Bykov, K.I. Rybakov, V.E. Semenov, High-temperature microwave processing of materials, J. Phys. D: Appl. Phys. 34 (2001) R55–R75. [18] J. Wang, J. Binner, B. Vaidhyanathan, N. Joomun, J. Kilner, G. Dimitrakis, T.E. Cross, Evidence for the microwave effect during hybrid sintering, J. Am. Ceram. Soc. 89 (2006) 1977–1984. [19] Juan Rodríguez-Carvajal, Recent advances in magnetic structure determination by neutron powder diffraction, Physica B 192 (1993) 55–69. [20] 〈http://imagej.nih.gov/ij/index.html〉. [21] Zhe Zhao, Vincenzo Buscaglia, M. Viviani, M.T. Buscaglia, L. Mitoseriu, A. Testino, et al., Grain-size effects on the ferroelectric behavior of dense nanocrystalline BaTiO3 ceramics, Phys. Rev. B 70 (2004) 024107. [22] M.H. Frey, D.A. Payne, Grain-size effect on structure and phase transformations for barium titanate, Phys. Rev. B 54 (1996) 3158–3168. [23] M.G. Frey, Z. Xu, P. Han, D.A. Payne, The role of interfaces on an apparent grain size effect on the dielectric properties for ferroelectric barium titanate ceramics, Ferroelectrics 206–207 (1998) 337–353. [24] G. Arlt, The influence of microstructure on the properties of ferroelectric ceramics, Ferroelectrics 104 (1990) 217–227.

Please cite this article as: V.R. Reddy, et al., Enhanced dielectric and ferroelectric properties of BaTiO3 ceramics prepared by microwave assisted radiant hybrid sintering, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.01.039 View publication stats

7