Materials Letters 12 ( 1992) 424-428 North-Holland
Microwave calcination and sintering of barium strontium titanate F. Selmi, F. Guerin, X.D. Yu, V.K. Varadan, V.V. Varadan and S. Komameni Centerfor the Engineering of Electronic and Acoustic Materials, The Pennsylvania State University,
University Park, PA 16802, USA
Received I 1 September 199 1; in final form 12 November 199 1
The calcination and sintering behavior of barium strontium titanate was investigated using microwave power absorption. With microwave power, BaCOj, SrCO, and Ti02 powders were calcined at 1100°C for 45 min, whereas conventional calcination requires typically more than 5 h at 1150°C. Pressed samples of microwave calcined Bao,ssSro.ssTiO, were sintered in 30 min at 1350°C using a microwave cavity. The permittivity and loss tangent of the sintered samples were also measured. It is shown that microwave calcination and sintering lead to an improvement in the dielectric properties.
1. Introduction High dielectric constant materials such as ferroelectric BaTiO, have a growing demand for different applications in electronics. The development of ceramic dielectrics technology has been influenced by the miniaturization of the equipment in which capacitors are used. The increase in available dielectric constant has been instrumental in this size reduction. Another property of barium titanate materials is their ability to change shape in the presence of an applied electric field. This conversion of electrical energy to mechanical energy is characteristic of piezoelectric materials which are used in microphones, sonars, hydrophones, linear or rotary motors, medical sonograms, etc. The third application is their potential use in microwave phase shifters for radar systems. The required properties of barium strontium titanate are a high dielectric constant and a low loss factor over a wide range of frequencies. It is well documented in the literature that these properties are strongly dependent upon the microstructure, in particular the grain size. Grain growth can be restricted, in principle, by establishing the conditions under which mass transport mechanisms favoring densification (e.g. lattice, and grain boundary diffusion) ’ Also with the Department of Agronomy. 424
are dominant over those favoring partial coarsening (e.g. surface diffusion). Like most ceramic materials a dense barium strontium titanate ceramic with a fine microstructure can be achieved either by the addition of solid solution forming additives which inhibit grain growth, or by the application of suitable process control, such as hot pressing or fast firing. The addition of dopants such as iron or manganese to barium strontium titanate is very effective in controlling the grain growth and therefore in producing a fine microstructure. In this case, the mechanism is either an increase in the lattice diffusion, or a reduction of grain boundary mobility by solid solution drag process. Hot pressing is another useful technique in which the driving force for densification is greatly enhanced, by the application of external pressure during firing. Fast firing is a relatively new technique which offers many advantages. It offers enhanced densification as well as reduced grain growth, since the time available for grain growth is severely shortened in this process. Borom and Lee [ 11, using a low thermal mass resistance heater showed that for alumina titanium carbide composites, the higher the heating rate, the higher the final density, for the same temperature. Ultrafine grain size and high density can be achieved in A1203 with rapid heating [ 21. This was explained by Harmer and Brook’s theory [ 3 ] as follows: when the heating rate is high, materials will pass
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more rapidly through the lower-temQerature regime where surface diffusion is more important, and the small grained microst~cture is raised to high temperature when grain boundary and lattice diffusion predominate over surface diffusion. However, the applicability of high heating rates in conventional processing is limited because of heat transfer and thermal shock limitations. In conventional heating, energy is absorbed only at the surface of the material and must be transferred into the bulk by conduction_ Temperature gradients exist in the fired sample until it achieves thermal equiIib~um~ As a consequence, it is difficult to obtain a uniform grain size. In the past few years, there has been a growing interest in heating and sintering of ceramics by microwave energy [ 4- 131. Microwave heating has the potential for overcoming problems encountered in conventional processes. IJniform and rapid heating can be achieved in a short time and at a temperature lower than in the conventional case, since the energy is absorbed in the volume of the heated object, rather than being conducted from the outside. A uniform, fine and dense microst~cture can therefore be achieved by using microwave power absorption. Moreover, energy consumption and processing time are reduced, and can further significantly be reduced by also using microwave power for the calcination stage. In this study, we determine the conditions for a satisfactory microwave calcination and describe several characteristics of barium strontium titanate samples processed by microwave sintering, such as microstructure, dielectric constant and loss tangent.
Fig. I. Temperature heating profile Bao.&ro.3sTi%.
of microwave calcination of
ducted in the 2.45 GHz microwave setup shown in fig. 2. Powders were placed in a quartz tube, heated to 1100°C and allowed to calcine for 45 min in air. A single-mode high-power microwave heating device was used in this experiment. The magnetron and associated control system can generate a maximum of 3 kW microwave power in a continuous or pulsed form. A circulator follows the magnetron to divert the reflected microwave to the dummy load to protect the magnetron. A section of quarter wavelength waveguide, which is equipped with two directional couplers for measuring forward and reflected power, is connected to the system next to circulator. The
2. Ex~rim~~taI procedure
BaCO,, SK03 and TiQz powders were mixed in the desired stoichiometry (0.65/0.35/l f and milled for 2 h. Milling was done in alcohol with zirconia grinding media. The alcohol was removed after drying in an oven, and the powder was microwave calcined at about 1100’ C for times ranging from 10 to 45 min. The heating profile is shown in fig. 1. The microwave calcination of powders was con-
Fig. 2. Schematic diagram of microwave sintering system.
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impedance analyzer, which has four detectors with fixed phase relationship, is used to monitor the working condition of the system through obtaining the reflection coefficient of the cavity. A 4-stub tuner is also placed before the microwave resonator for the tine tuning of the system. The microwave resonator is formed by an iris, a section of waveguide and a variable short-circuited waveguide. The sample is placed at the position where the electric lield has the maximum value. The diagnostic system has two main components, HP9826 PC and HP3497A data acquisition system. They collect and process the signal from the impedance analyzer to calculate the reflection coefficient, and from the temperature controller connected with a pyrometer to record temperature history. Upon turning on the microwave generator, the variable short is adjusted so that microwave energy can be absorbed by ceramics efficiently. The calcined powders corresponding to various calcination times were characterized by X-ray diffraction (XRD ).
3. Results and discussion Fig. 1 shows the microwave heating profile of calcination of barium strontium titanate. The heating starts with a sharp increase in the temperature due to the absorption of microwaves by the carbon from BaCOj and SrC03. The temperature peak reaches 1250°C where carbon starts reacting with oxygen from air, producing carbon dioxide. The temperature then decreases to around 1100 ‘C and was maintained at this value for 45 min for complete calcination. Figs. 3a, 3b and 3c show the X-ray diffraction patterns of the powder before calcination, and after it was microwave heated for 30 and 45 min respectively. Fig. 3b shows that carbon is still present after 30 min and that the formation of the Ba,,.,&o.~sTi03 phase is not complete yet. However fig. 3c shows the complete elimination of carbonates and the presence of the Bao.ssSro.~sTiO, phase only, as indicated by 4000 ‘BBC4
2.2. Microwave sintering The calcined powder was milled for 1 h and sieved through a 100 mesh screen. Polyvinyl alcohol (PVA) was added in the form of a 10% aqueous solution to the calcined powders and pellets were pressed at 36000 psi. The microwave sintering was conducted in the setup shown in fig. 2. The pressed samples were fired in the microwave cavity at 1350°C for 30 min. The density of microwave samples was determined by using the Archimedes method. Microstructure analysis of the polished surfaces was carried out using a scanning electron microscope. 2.3. Dielectric measurements The dielectric constant and loss tangent of microwave sintered Ba,,&Sr,,,TiOJ ceramics were measured as a function of temperature at 1 MHz. A silver paint was applied on the two sides of the sample for impedance measurements. Samples were encapsulated within a thin layer of silicone rubber and placed in an anti-freeze bath. The temperature was varied from 0 to 40°C. Impedance of the samples was measured by a HP 4 192A impedance analyzer.
2 THETA l”““,
2 THETA 4co0,
2 THETA Fig. 3. X-ray patterns of BaCO,, SrCOs and Ti02 mixed powders before calcination (A) and calcined at 1100°C for 30 min (B) and 45 min (C).
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the peaks, which are in very good agreement with those obtained for a conventionally calcined powder. The use of microwave energy for calcination has a big advantage in terms of time and energy saving. The conventional calcination of a barium strontium titanate powder usually takes more than 5 h at 115O”C, and the duration of the total cycle is greater than 12 h. Moreover, by calcining the powder in a short time using microwave energy, particle coarsening can be partly avoided and one can suppress the ball milling stage that follows conventional powder calcination. The densities of microwave sintered samples, expressed in percentage of the theoretical density, are greater than 92%. The samples show small and fairly uniform grains, as can be seen from the SEM micrographs of the polished surfaces (fig. 4). The typical dielectric constant and loss tangent at 1 MHz of microwave sintered Ba0.6SSro.35Ti03 samples are plotted in figs. 5 and 6, respectively, as a function of temperature. The samples have a Curie temperature T, of about 16’ C. The maximum relative permittivity, obtained at Tc,is 6000, while at the same temperature the loss tangent is 1.72~ 10e2.
Fig. 5. Dielectric constant as a function of temperature of microwave sintered BaO.&+,,,TiO~ samples,& 1 MHz. Results reported by Selmi et al. [ 141 show that for the same material processed by conventional calcination and sintering, the relative permittivity at 25”C, which in this case is the Curie temperature, is comprised between 5000 and 5500 and the loss tangent at 25 ‘C is about 4 x 10m2. In our case, the value of the relative permittivity exceeds 5000 in the tem-
Fig. 4. Microstructure as revealed by SEM of microwave sintered B~.6&0.95TiO~ at 1350°C for 30 min.
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microwave calcination and sintering allows significant time and energy savings. 0.016 -
This work was supported by US Army Communications Electronics Command, Fort ~onmouth, NJ, under contract number DAAB07-90-C-A033.
Fig. 6. Loss tangent as a function of temperature of microwave sintered Ba0.65Sr0.~STi03 samples,f= 1 MHz.
perature range of 12” to 22°C and the loss tangent is smaller than 2 X lo-’ for any temperature from - 8 o to 40°C. Therefore, the microwave calcination and sintering process gives a value of the dielectric constant which is slightly greater than the value obtained by conventional sintering, and a loss tangent more than two times smaller, which is the key element for many applications in electronics, such as phase shifters,
4. Conclusion The use of microwave absorption for the calcining and sintering of Ba0.65Sr0.35Ti03ceramics led to the development of a fairly uniform and fine microstructure, with a dielectric constant higher than the one obtained for conventionally processed sampIes and a smaller loss tangent. Besides, the combined
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