Journal of Microwave Power and Electromagnetic Energy, 45 (3), 2011, pp. 121-127 A Publication of the International Microwave Power Institute
Microwave Assisted Synthesis and Characterization of Barium Titanate Nanoparticles for Multi Layered Ceramic Capacitor Applications Sundararajan Thirumalai Department of Ceramic Technology, Alagappa College of Technology Campus, Anna University, Chennai-600025, India. Balasivanandha Prabu Shanmugavel Department of Mechanical Engineering, College of Engineering Guindy Campus, Anna University, Chennai-600025, India. Received: June 30, 2011 Accepted: August 30, 2011 ABSTRACT Barium titanate is a common ferroelectric electro-ceramic material having high dielectric constant, with photorefractive effect and piezoelectric properties. In this research work, nanoscale barium titanate powders were synthesized by microwave assisted mechano-chemical route. Suitable precursors were ball milled for 20 hours. TGA studies were performed to study the thermal stability of the powders. The powders were characterized by XRD, SEM and EDX Analysis. Microwave and Conventional heating were performed at 1000ºC. The overall heating schedule was reduced by 8 hours in microwave heating thereby reducing the energy and time requirement. The nano-scale, impurity-free and defect-free microstructure was clearly evident from the SEM micrograph and EDX patterns. LCR meter was used to measure the dielectric constant and dielectric loss values at various frequencies. Microwave heated powders showed superior dielectric constant value with low dielectric loss which is highly essential for the fabrication of Multi Layered Ceramic Capacitors. KEY WORDS: Barium titanate, dielectric constant, mechano-chemical route, microwave heating. INTRODUCTION The perovskite family includes many titanates used in various electro-ceramic applications, for example, electronic, electro-optical, and electromechanical applications of ceramics. Barium titanate (BT) is most widely utilized in the manufacturing of electronic components such as Multilayered Ceramic Capacitors (MLCCs), PTC thermistors, piezoelectric transducers, and a variety of electro-optic devices [Maison et al., 2003; Ohara et al., 2008]. In order to meet the consumer’s demands, improvements are made in the dielectric characteristics of BT at a rapid rate. Efforts are being made to reduce the size and weight of all communication devices as small and as light as possible. Recently MLCCs with thin dielectric BT layers are required due to the miniaturization of advanced electronic Journal of Microwave Power and Electromagnetic Energy, 45 (3), 2011 International Microwave Power Institute
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devices. According to the some recent reports the thickness of the BT layer will reach less than 500 nm in the near future. Therefore, particle size of BT smaller than 100 nm is eagerly desired [Kay et al., 1949; Stojanovic, and Mater, 2003]. After the synthesis of BT nanoparticles, the calcining protocol employed is very crucial in determining its dielectric constant. The calcining temperature can be reduced to a greater extent when the particle size distribution of the powders is in nano-scale region. Conventional synthesis methods that require high temperature at a slow heating and cooling rate and longer dwelling time lead to the undesired grain growth. Microwave application is gaining prominence in the calcining / sintering of various ceramics due to its versatile nature and multiple benefits [Clark and Folz, 2002]. Microwave kinetics depends on electric field intensity, frequency, loss factor and permittivity of the material. The ceramic materials with low dielectric loss at room temperature are difficult to heat in a microwave field. However above a critical temperature, Tc, such ceramic materials experience rapid coupling of microwave and are heated rapidly [Thostenson et al., 1999; Brosnan et al., 2003; Clark et al., 2000]. The dielectric ceramics like BT can easily be calcined by the application of microwaves because of its ability to absorb the microwaves even at room temperature. In this present work BT nanoparticles prepared by mechano-chemical process have been subjected to conventional and microwave heating. The properties of the BT nanoparticles calcined by two different source of heating methods were analyzed [Hench et al., 1990]. EXPERIMENTAL PROCEDURE Barium hydroxide (99% purity, Loba Chemicals Ltd.) and anatase grade titania (99% pure, Merck Laboratories) were used as the primary raw materials. These 122
precursors were mixed in stoichiometric quantities and subjected to high energy ball milling. The working principle involved in the planetary milling operation is reported elsewhere [Kong et al., 2008]. The stainless steel vile (200 ml capacity) was given tungsten carbide (WC) lining in the interior to prevent the iron contamination due to wear, thereby increasing the life of the vile and milling efficiency. Tungsten carbide balls of 10 mm diameter were used as the milling media. Powder to ball ratio (on weight basis) used in the experiment was 1:20. Toluene was used as the process control agent. Ten hours of non-intermittent wet milling was done at 280 r.p.m. After the milling was completed, the powders were dried in a hot air oven for 24 hours. After drying, the powders were subjected to microwave heating and conventional heating at 1000ºC. Conventional heating was performed at a heating rate of 5ºC/minute with 2 hours of soaking at the maximum temperature of 1000ºC. The total heating cycle time was 520 minutes. Microwave heating was performed at a rapid heating rate of 50ºC/minute with 30 minutes of soaking at 1000ºC. The total heating cycle time was just 70 minutes. CHARACTERIZATION The crystalline phase and size of the obtained powders were determined by X–ray powder diffraction (XRD) technique by using a Seifert (JSO-DEBYEFLEX 2002) diffractometer with Cu-Kα radiation (λ=0.1540 nm). A scan rate of 0.04º /s was employed to record the diffraction patterns. Characteristics peaks were indexed according to the JCPDS standards. TGA Analysis was carried out using TA Instruments Q50 Series equipments, respectively. Scanning Electron Microscopy (SEM) analysis and Energy Dispersive X-Ray (EDX) analysis were carried out using Variable Pressure SEM instrument (Hitachi,Model:S-3400N). Dielectric studies were carried out using LCZ Meter (N4L PSM 1700, HIOKI 3535).
Journal of Microwave Power and Electromagnetic Energy, 45 (3), 2011 International Microwave Power Institute
Sundararajan Thirumalai and Balasivanandha Prabu Shanmugavel, Microwave Assisted Synthesis and ...
RESULTS AND DISCUSSION X-Ray Diffraction Analysis X-Ray Diffraction (XRD) patterns of the conventionally heated and microwave heated BT are shown in Figures 1a and 1b, respectively. The characteristics peaks were indexed according to the JCPDS Standards (File No: 79-2265). The diffraction patterns of both the conventionally and microwave heated BT showed that the crystal structure was tetragonal in nature. Impurity peaks due to the wear of tungsten carbide milling media was anticipated due to longer milling periods. However, it was very clear from the XRD patterns that both powders showed highly crystalline single phase material without any secondary/impurity phases. The extent of tetragonality in the BT is an important factor, which directly influences its dielectric behavior [Yoon et al., 2002; Yoon, 2006]. In the XRD plot, the peak positions from 2θ = 44° to 46° play an important role in determining the tetragonal nature of the system which are shown in Figures 2a and 2b. For the microwave heated BT, the peak splitting was found clearly in the (002) plane, which shows its high degree of tetragonality. In the case of conventionally heated BT, there was a small splitting occurring at (002) plane, indicating less tetragonality in conventionally
Figure 1a. XRD pattern of the conventionally heated BT.
Figure 1b. XRD pattern of the microwave heated BT.
heated BT. Microwave heated BT showed a broader full width at half maximum value than that of the conventionally heated BT , which confirms the reduced particle size by microwave heating. The average crystallite size calculated using the Scherrer’s formula [Patterson, 1939] was found to be 89 nm for the microwave heated particles and 110 nm for the conventionally heated BT. For the conventionally heated BT, the c/a ratio was 1.0028. For the microwave heated BT, the c/a ratio was 1.0219. It can be inferred that the tetragonality was higher in the microwave heated BT; an important requirement for the fabrication of MLCC’s [Kwon and Yoon, 2006].
Figure 2a. Tetragonal peak splitting of the conventionally heated BT.
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Figure 2b. Tetragonal peak splitting of the microwave heated BT.
Thermogravimetric Analysis Thermograms of the conventionally and microwave heated BT are shown in Figures 3a and 3b, respectively. Controlled heating of the representative samples were done at the rate of 20ºC/min. Nitrogen gas was used for de-gassing the chamber, where the representative samples were placed, during the process of thermogravimetric analysis. Weight loss around 100ºC corresponds to the removal of moisture. Weight loss in the range of 300-500ºC was due to the elimination of hydroxyl defects, [Ohara et al., 2008] whereas the weight loss at higher temperatures may be due to the decomposition of residual
Figure 3a. Thermogram of the conventionally heated BT.
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Figure 3b. Thermogram of the microwave heated BT.
Ba(OH)2. Overall weight loss was 3.5 % for the microwave heated BT and 3.14 % for the conventionally heated BT. Larger weight loss in the microwave heated BT may be attributed to the presence of relatively larger amount of hydroxyl defects and residual Ba(OH)2, whose amount was so negligible that it was not detected in the XRD patterns. Due to the rapid microwave heating of the powders, the complete elimination of the hydroxide precursor had not taken place, which had resulted in the higher amount of residual Ba(OH)2, thereby causing higher weight loss. SEM Analysis The SEM micrographs of the conventionally and the microwave heated BT are shown in Figures 4a and 4b, respectively. From the SEM micrograph, the average grain size was found to be 107.4 nm for the conventionally heated BT and 87.3 nm for the microwave heated BT. The lower grain size of the microwave heated BT was attributed to the rapid heating rate, shorter soaking period and rapid cooling rate used in this process. Large numbers of agglomerates were noticed in the SEM image of the microwave heated BT, which may be due to the very high surface area of the nano-crystalline BT.
Journal of Microwave Power and Electromagnetic Energy, 45 (3), 2011 International Microwave Power Institute
Sundararajan Thirumalai and Balasivanandha Prabu Shanmugavel, Microwave Assisted Synthesis and ...
Figure 4a. SEM image of the conventionally heated BT. Figure 5b. EDX pattern of the microwave heated BT.
250 r.p.m. Figures 5a and 5b shows the EDX pattern of the BT heated by conventional and microwave heating technique. High intensity peaks corresponding to Ba and Ti elements were clearly noticed in the EDX patterns of both the samples. The relative intensities of the peaks corresponding to the Ba and Ti were higher in the case of microwave heated BT. Figure 4b. SEM image of the microwave heated BT.
EDX Analysis Energy Dispersive X-ray analysis (EDX) was carried out to identify the purity of the BT nanoparticles synthesized by high-energy ball milling. From the EDX analysis it was determined that BT did not contain any impurities even after milling for about 10 hours at constant speed of
Figure 5a. EDX pattern of the conventionally heated BT.
Dielectric Studies The variation of room temperature relative dielectric constant (εr) with applied frequency for the conventionally heated and microwave heated BT is shown in Figures 6a and 6b, respectively. Microwave heated BT nanoparticles showed higher value of room temperature relative dielectric constant value (εr = 2348 ) at the lowest frequency of 100 KHz than that of the conventionally heated BT. Microwave heated BT showed a 58% increase in the relative dielectric constant value. The dielectric loss value at the lowest frequency was less for the microwave heated BT (tanδ = 0.0853) than that of the conventionally heated BT (tanδ = 0.2346). The dielectric constant value exponentially decreased with increase in the applied frequency. Higher relative dielectric constant value showed by the microwave heated BT was attributed to the enhanced tetragonality and fine grain size of the particles. The internal stresses induced in the BT nanoparticles during the
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Figure 6a. Variation of εr with applied frequency for the conventionally heated BT.
Figure 6b. Variation of εr with applied frequency for the microwave heated BT.
powder processing and the lattice strain associated with the higher cooling rates in the microwave processing may also be a reason for increased dielectric constant in microwave heated BT [Ohara et al., 2008]. CONCLUSIONS Barium Titanate (BT) nanoparticles were prepared by the conventional planetary milling technique. The effect of microwave heating and conventional heating on the crystal structure, particle size, microstructure, thermal properties and dielectric properties has been investigated. It is found that the high energy ball milling using high density milling media like tungsten 126
carbide produced very fine impurity free tetragonal BT nanoparticles, which are confirmed by the XRD, SEM and EDX patterns. The tetragonal crystal structure was retained even at lower heating temperature of 1000ºC. The particle size is about 20 nm less for the microwave heated samples than the conventional heating. The relative dielectric constant values measured at room temperature was relatively higher than that of the conventionally heated product. The dielectric loss values were also relatively lesser for microwave heated samples. From the above discussed results, we can infer that microwave assisted heating is a promising technique to improve the properties of BT at the nano-scale range. This technique may soon get commercialized in the electroceramic industries. But the major drawback is the relatively higher percentage of residual hydroxyl ion in the microwave heated sample. This problem may be reduced by performing the dry milling of the precursors without the addition of process control agents. ACKNOWLEDGEMENT Sundararajan T., the corresponding author (
[email protected]), thanks the Centre for Technology Development and Transfer (CTDT), Anna University, Chennai, India for funding this project (File No:7672/CTDT-1/2010) under the “Students Innovative Research Projects Support Scheme” of Anna University. The support of Miss S. Jothimani and Mr. Arvind Joshua Jayadev of Materials Science Division, Department of Mechanical Engineering, Anna University, is gratefully acknowledged. REFERENCES Brosnan K.H., Messing G.L. and Agrawal D.K. (2003) “Microwave Sintering of Alumina at 2.45 GHz”, J. Am. Ceram. Soc., 86 (8) pp. 1307-1312. Clark D.E. and Folz D.C. (2002) “Microwave Processing of Materials”, Proceedings CIMTEC 2002, 10th International
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Ceramics Congress Part B, Florence, Italy, pp. 367-380, July 14-18, 2002. Clark D.E., Folz D.C. and West J.K. (2000) “Processing Materials with Microwave Energy”, Mater. Sci. Eng. A, 287 pp. 153– 158. Hench L.L. and West L.K. (1990) “Principles of Electronic Ceramics”, John Wiley & Sons,Inc. pp. 244-247. Kay H.F., Wellard H.J. and Vousden P. (1949) “Atomic Positions and Optical Properties of Barium Titanate”, Nature, 163 pp. 636-637. Kong L.B., Zhang T.S., Ma J., and Boey F. (2008) “Progress in Synthesis of Ferroelectric Ceramic Materials Via Highenergy Mechanochemical Technique”, Prog. Mater. Sci., 53 pp. 207-322. Kwon S.W. and Yoon Dang-Hyok (2007) “Effects of Heat Treatment and Particle Size on the Tetragonality of Nano-sized Barium Titanate Powder”, Ceram. Int., 33, pp. 13571362. Maison W., Kleeberg R., Heimann R.B. and Phanichphant S. (2003) “Phase Content, Tetragonality and Crystallite Size of Nanoscaled Barium Strontium Titanate
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