Novel Ceramic Ferroelectric Composites

Louise C. Sengupta

U.S. Army Research Laboratory

Aberdeen Proving Ground, Maryland

Electronic scanning antennas will be functionally important parts of future commercial and military communications and radar systems. Most current radar scanning is mechanical, owing to the high cost of currently available phased-array antennas, and it relies on a gimbal arrangement to physically rotate/elevate the radar antenna. Therefore, scanning is slow and subject to mechanical failure.

Present phased-array antennas are constructed from ferrite elements. These elements are ferromagnetic and current driven, where the phase shift is caused by a change in the permeability (related to the magnetism) of the material. Although the performance of this type of phase shifter is very good, the material cost alone for each element is roughly $3,000. Each element then must be hand wired, which further increases the cost by approximately $2,000 to $3,000 per element. Thus, in total each element costs almost $5,000 to $6,000. An array with roughly 1,000 elements, therefore, is approximately $5,000,000. This factor has exclusively limited the use of phased-array antennas to strategically dependent military applications.

Ferroelectric materials, however, require a voltage-driven circuit, where the phase shift is caused by a change in the dielectric constant (permittivity, related to the energy storage—that is, capacitance). These types of circuits can be electroded and processed using standard circuit board technology. The materials perform at least as well as the ferrite phase shifters and cost only $100 per element. The final packaging (including electroding and encapsulation) increases the cost per element by $100. Therefore, the same array (of 1,000 elements) will cost $200,000, which is only one-twenty-fifth the cost of the ferrite phased-array antenna.



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--> Novel Ceramic Ferroelectric Composites Louise C. Sengupta U.S. Army Research Laboratory Aberdeen Proving Ground, Maryland Electronic scanning antennas will be functionally important parts of future commercial and military communications and radar systems. Most current radar scanning is mechanical, owing to the high cost of currently available phased-array antennas, and it relies on a gimbal arrangement to physically rotate/elevate the radar antenna. Therefore, scanning is slow and subject to mechanical failure. Present phased-array antennas are constructed from ferrite elements. These elements are ferromagnetic and current driven, where the phase shift is caused by a change in the permeability (related to the magnetism) of the material. Although the performance of this type of phase shifter is very good, the material cost alone for each element is roughly $3,000. Each element then must be hand wired, which further increases the cost by approximately $2,000 to $3,000 per element. Thus, in total each element costs almost $5,000 to $6,000. An array with roughly 1,000 elements, therefore, is approximately $5,000,000. This factor has exclusively limited the use of phased-array antennas to strategically dependent military applications. Ferroelectric materials, however, require a voltage-driven circuit, where the phase shift is caused by a change in the dielectric constant (permittivity, related to the energy storage—that is, capacitance). These types of circuits can be electroded and processed using standard circuit board technology. The materials perform at least as well as the ferrite phase shifters and cost only $100 per element. The final packaging (including electroding and encapsulation) increases the cost per element by $100. Therefore, the same array (of 1,000 elements) will cost $200,000, which is only one-twenty-fifth the cost of the ferrite phased-array antenna.

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--> Other savings realized with the ferroelectric phase shifter concern size and weight. The size is reduced by 50 to 75 percent for the materials alone. Reduction also occurs for the wiring and control circuits, which are reduced in size by another 50 percent. The total electro-optic antenna may be less than one-tenth of the size of the large ferrite array. The weight of the actual materials used is probably similar for the two. However, the amount of ferroelectric material required for any particular application is substantially less. Multiple phase shifters can be produced on a single piece of ferroelectric material. Also, thin films of the ferroelectric material can be used in many of the antenna systems, further reducing size and weight. Circuit board technology can be used to fabricate the antennas, which, again as seen in the semiconductor industry, produces very small, lightweight components and systems. The problem until now has been that the ferroelectric materials that have produced substantial phase shift have too much loss (~20 dB) and, therefore, are not usable in phased-array antennas. However, the materials being investigated at our laboratory are low loss (~1 dB) yet highly tunable, thus offering excellent properties for use in phased-array antennas. These patented composites combine Ba1-XSr XTiO3 (BSTO) with other nonelectrically active oxide ceramics producing break-through electronic properties never previously attained (Sengupta et al., 1995). As shown in Table 1, the bulk ceramic composites have reduced real TABLE 1 Electronic Properties of Ba0.6Sr0.4TiO3, Ba0.55Sr0.45TiO3, Ba0.50Sr0.50TiO3, and Ba0.45Sr0.55TiO3/Oxide III Composite Bulk Ceramics, Measured at 1 kHz Barium Content Oxide Content (wt%) Dielectric Constant Loss Tangent Tunability Curie Temp (°C) Ba = 0.45 0 1,280.83 0.01184 15.20 -45   10 768.06 0.00068 3.90 -75   20 418.98 0.00064 2.47 -70   60 78.81 0.00049 3.67 -95 Ba = 0.50 0 1,907.99 0.05538 25.55 -25   10 928.01 0.00076 5.48 -55   20 592.20 0.00073 6.44 -55   60 77.52 0.00096 3.66 -95 Ba = 0.55 0 2,771.73 0.03904 33.40 -15   10 1,114.02 0.00094 8.88 -40   20 742.29 0.00085 8.77 -40   60 94.59 0.00034 6.46 -50 Ba = 0.60 0 5,160.64 0.00961 56.30 10   10 1,527.34 0.00162 16.60 -30   20 1,068.43 0.00194 15.80 -35   60 116.86 0.00148 9.99 -55   Source: Materials Directorate, U.S. Army Research Laboratory.

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--> dielectric constants, ε', where ε = ε' - iε", and loss tangents, tan δ, which reduce the overall impedance mismatch and insertion loss of the device. These composites include a variety of oxide additives, and they have been developed to include an entire family of materials having electronic properties that can be tailored to any given application. In addition, tunability—the change in the dielectric constant with applied voltage—is maintained at a relatively high level for the dielectric constants of interest (Sengupta et al., 1995). The temperature dependence of the electronic properties, such as aging and fatigue, also have been investigated. Microwave data were obtained at 10 GHz, as shown in Table 2, using a cylindrical TE01 mode-filtered X-band cavity. The minimum thickness limitation of the bulk materials is around 3 mils, which limits their usage to approximately 15 GHz. Initial results concerning the effect of ceramic processing, including the particle size and grain size of the ceramic, on the electronic properties will be presented in terms of additionally refining the ferroelectric/oxide compositions. To increase the operating frequencies of these phase shifters, we fabricated and electrically characterized films of these novel composites.. Initially, we developed single-layer composites via nonaqueous tape-casting. We then screen-printed the electrode patterns onto the tapes using a cofired ink. The processing parameters for the tape cast and electrode materials were optimized for structural integrity. Finally, we characterized the electrical properties and compared them to bulk ceramics. As shown in Table 3, the dielectric constants and loss tangents of the tape-cast specimens are similar to the bulk composites but decrease with an increase in oxide content and vary less than 2 percent with change in frequency (from 1 kHz to 1 MHz). The magnitudes of the dielectric constants and loss tangents are very similar to those of the bulk ceramics. Tunability is maintained at 12 percent (with a bias field of 2.00 V/μm), with up to 60 weight percent additive content. This trend was explained previously in the bulk ceramics by the position of the Curie temperatures and the size of the additive (O'Day et al., 1994). Also, laminated stacks with alternating layers of high and low dielectric constant were fabricated for use in a multilayer TABLE 2 Microwave Properties of Ba0.60Sr0.40TiO3/Oxide III Composites Oxide III Content (wt%) Frequency (GHz) Dielectric Constant Loss Tangent 30 2.139 646 0.0040 40 1.815 404 0.0042   3.304 401 0.0051 60 4.581 113 0.0065   10.02 106 0.012   Source: Materials Directorate, U.S. Army Research Laboratory.

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--> TABLE 3 Electronic Properties of BSTO (Ba = 0.60)/Oxide III Composite Single-Layer Tapes Measured at 1 Khz Oxide III Content Dielectric Constant Loss Tangent Tunability (%) Electric Field 0.0 3,192.2 0.0056 43.52 2.00 10.0 1,390.2 0.0015 15.03 2.00 20.0 616.44 0.0012 15.45 2.00 40.0 357.30 0.0041 14.00 2.00 60.0 91.16 0.0008 10.41 2.00   Source: Materials Directorate, U.S. Army Research Laboratory. ferroelectric composite waveguide. The microwave properties of these waveguide structures have been fully calculated at the 10 GHz frequency region. The thickness limitation of the tape cast specimens is approximately 0.5 mils, which limits their usage to approximately 35 Ghz. In order to further increase the operating frequency of these devices, we fabricated thin films using the pulsed-laser-deposition (PLD) method. The thin films were deposited on metallized single-crystal substrates, and the top electrodes then were deposited by a sputtering process to form the vertical capacitor structure. Electronic measurements of these structures have shown that the thin films follow similar trends in the electrical properties when compared to the bulk counterparts (Sengupta et al., 1994). It must be emphasized that high tunabilities were achieved in these thin films with much lower applied voltages than those applied to the bulk. It appears that certain specific additives can influence the lowering of the dielectric constant more than others even at a 1 weight percent level. To produce larger area films of these composites, we utilized the direct-liquid-injection Metallo-Organic Chemical Vapor Deposition (MOCVD) method. We then characterized the electrical properties of the MOCVD films and compared them to those of the bulk thick films and PLD films. A rigorous investigation of the material characteristics has been performed on all of the specimens included in this study. We have examined the microstructures, including grain size and phase analysis, using SEM and X-ray diffraction (glancing-angle X-ray diffraction was obtained for the thin-film materials), and we have investigated compositional topography using Raman and Fourier transform infrared microprobe spectroscopy, which identifies the reaction zones and provides a compositional topography of the specimens. The analysis of the phase formation and compositional variations has been related to connectivities and to the electronic properties of the materials. This paper has presented a complete evaluation of the processing, materials characteristics, and breakthrough performance levels of these novel ceramic composites for use in Army electronic devices.

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--> References O'Day, M. E., L. C. Sengupta, E. Ngo, S. Stowell, and R. Lancto. 1994. Processing and characterization of functionally gradient ceramic materials. Smart Structures and Materials Conference, May 1994. SPIE Proceedings 2189:388-399. Sengupta, L. C., S. Stowell, E. Ngo, M. E. O'Day, and R. Lancto. 1995. Barium strontium titanate and non-ferroelectric oxide ceramic composites for use in phased array antennas. Integrated Ferroelectrics 8:77-88. Sengupta, S., L. C. Sengupta, S. Stowell, D. P. Vijay, and S. B. Desu. 1994. Electrical characteristics of barium strontium titanate oxide composites. MRS'94 Symposium on Smart Materials 360:413.