The World of Ferroelectric Ceramics
Recent research and development efforts have focused on improving the dielectric properties of ferroelectric materials.
Ferroelectricity is the phenomenon by which spontaneous electric polarization of a material takes place. Reverse electric polarization is possible by applying an electric field (see Figure 1).
Ferroelectricity was discovered in 1920, when potassium sodium tartrate (also known as Rochelle salt) first exhibited the properties of sudden electric polarization. Since its discovery, various uses have been devised for ferroelectricity. Today, materials that exhibit this property—also called ferroelectric materials—are commonly used in the electronics sector. Surprisingly, neither ferroelectricity nor ferroelectric materials have any connection to iron (ferrite).
Ferroelectric materials, particularly polycrystalline ceramics, have attracted considerable interest for applications in a variety of fields, such as high dielectric constant capacitors, piezoelectric transducers, actuators, ferroelectric random access memories, electro-optic devices and more. Barium titanate (BaTiO3) was the first ferroelectric ceramic material to be discovered, and it is a good candidate for many applications due to its excellent dielectric, ferroelectric and piezoelectric properties. BaTiO3 is extensively used in high dielectric constant capacitors, multilayer ceramic capacitors (MLCCs) and energy storage devices.
BaTiO3 is a member of a large family of compounds with a perovskite structure (ABO3). The ideal perovskite structure of BaTiO3 is illustrated in Figure 2.
The structure of BaTiO3 spontaneously changes from cubic to tetragonal phase below its Curie temperature (1200°C). This phase transformation allows BaTiO3 to exhibit good dielectric and ferroelectric properties at room temperature.
Although pure BaTiO3 shows moderately high dielectric constant near the range of 1200-1500°C, it is still not enough to serve the dielectric property requirements of modern applications. As a result, pure BaTiO3 has limited applications. To overcome this challenge, recent research has focused on the development of BaTiO3-based ceramics with high dielectric properties using various dopants.
For BaTiO3, desirable dielectric properties are obtained by isovalent or aliovalent cation substitution into the perovskite lattice. In small concentrations, substitutions of zirconium oxide (ZrO2) and tantalum oxide (Ta2O5), for example, may also act as grain growth inhibitors. These oxides modify the ferroelectric properties by controlling grain size and by suppressing or broadening the Curie peak.
Dielectric properties and structural characteristics of BaTiO3 ceramics are influenced by a small addition (up to 2% wt) of ZrO2. ZrO2 modifies the ferroelectric properties by controlling grain size through a pinning effect, increasing density and suppressing or broadening the Curie peak.
According to literature data, for 1% ZrO2, around 96% of the theoretical density can be achieved at 13,100°C. Moreover, the dielectric constant can be increased up to 2500, and the Curie peak of the transition from tetragonal to cubic can be suppressed.1
Tantalum oxide (Ta2O5)
Similar to Zr4+, Ta5+ inhibits grain growth and promotes densification when added in a small amount. However, the effect of Ta2O5 doping is much more significant. According to literature data, the density of Ta5+-doped BaTiO3 (sintered at 13,000°C for four hours) can reach up to 85-88% of the theoretical density. The dielectric constant for 1.0 mole% Ta5+-doped BaTiO3 at room temperature can be as high as ~ 3000; near the Curie temperature, it can reach around 9800. The Curie temperature of the BaTiO3 ceramic with a Ta5+ doping concentration ranging from 0.5-1.0 mole% remains similar to that of the pure BaTiO3 ceramic.2
Working with single dopants has been encouraging; however, their effect is somewhat limited. Research and development is now focusing on doping BaTiO3 with double dopants in pursuit of achieving high dielectric properties.
Together, niobium oxide (Nb2O5) and yttrium oxide (Y2O3) have a prominent influence on the dielectric properties of pure BaTiO3. In lower concentrations, niobium enhances grain growth and improves the dielectric properties. Close to or above the limit of dopant solubility in barium titanate, however, Nb2O5 inhibits grain growth and changes electrical properties.
Similarly, yttrium has a prominent influence on grain growth and could enhance grain growth. In fact, the major role of both donor cations (Nb5+ and Y3+) is their ability to influence grain boundary mobility as the charge compensation has an important effect on grain boundary mobility.
According to literature data, a concentration of less than 0.6 mol% is close to the limit of solubility of niobium and yttrium dopants in barium titanate in the observed temperature regime. With double doping of Nb5+ and Y3+, a dielectric constant can be attained from 4500 to 10,700 at the Curie temperature and from 3000 up to 6300 at room temperature, thereby justifying the influence of niobium and yttrium doping on barium titanate.3
Over the past several years, entrepreneurs with a vision of developing excellent ferroelectric materials have invested heavily in research and development to produce BaTiO3-based compounds with the highest possible purities, fine grain size and high densities. Thus, it can be concluded that their efforts will help to drive electroceramic industries to new advances in the future.
- Timothy R. Armstrong, Laurie E. Morgens, Alena K. Maurice and Relva C. Buchanan, “Effects of Zirconia on Microstructure and Dielectric Properties of Barium Titanate Ceramics,” tr. Am. Cerum. Soc., 72  605-11, 1989.
- Yeon Jung Kim, June Won Hyun, Hee Soo Kim, Joo Ho Lee, Mi Young Yun, S. J. Noh, and Yong Hyun Ahn, “Microstructural Characterization and Dielectric Properties of Barium Titanate Solid Solutions with Donor Dopants,”Bull. Korean Chem. Soc., Vol. 30, No. 6, 2009.
- B.D. Stojanovic ,V.R. Mastelaro, C.O. Paiva Santos and J.A. Varela, “Structure Study of Donor Doped Barium Titanate Prepared From Citrate Solutions,” UDK 553.689:57.017.22:666.651.