Ceramic Industry

Understanding DC Bias Characteristics in High-Capacitance MLCCs

October 1, 2009
Advances in materials technology have lead to a mitigation of decreased capacitance in barium titanate-based MLCCs.

Electronic design engineers are always overcoming challenges, including choosing the correct capacitance values of LCR components. However, the change in capacitance with applied DC voltage (a phenomenon also known as DC bias) further complicates the task of choosing the right capacitance.

Because of their unique qualities, ceramics provide many important functions. However, ceramics’ ability for spontaneous polarization can make capacitance decrease in multi-layer ceramic capacitors (MLCCs). Recent advances in materials technology have led to a mitigation of this effect in barium titanate (BaTiO3)-based ceramics.

Ceramic Benefits

Regardless of manufacturer, ceramic-based components have been at the forefront of the miniaturization trend. Raw ceramics have been expertly manipulated to decrease capacitor size, thus allowing MLCCs to dominate the landscape. Coupled with fairly high volumetric capacitance, their very low impedance often makes MLCCs the logical choice over electrolytic capacitors (both solid state and with liquid electrolytes).

Ceramics are also in demand because of their piezoelectric capabilities, which allow for the production of electricity (when ceramic crystals are submitted to mechanical stress), as well as ferroelectricity. Ferroelectric ceramics offer piezoelectric constants many times higher than other natural materials. Further, the process leads to spontaneous polarization and reverse spontaneous polarization.

Figure 1. Crystalline structure of BaTiO3-type ceramics.

Ferroelectricity and Spontaneous Polarization

First discovered in 1921, ferroelectricity began to play a much larger role in electronic applications during the 1950s after the increased use of BaTiO3. This ferroelectric material is part of the corner-sharing oxygen octahedral structure, but ferroelectrics can also be grouped into three other categories: organic polymers, ceramic polymer composites and compounds containing hydrogen-bonded radicals.

Even within the corner-sharing oxygen octahedral structure, BaTiO3 is considered part of the perovskite family (see Figure 1). Specifically, BaTiO3 is ideal for MLCCs because of its large room temperature dielectric constant. For example, BaTiO3 ceramics with a perovskite structure are capable of dielectric constant values as high as 7000, compared to 20-70 for other ceramics like titanium dioxide (TiO2). And values as high as 15,000 are possible over a narrow temperature range, compared to less than 10 for most common ceramic and polymer materials.

Figure 2. Change in crystalline structure and relative dielectric constant on temperature change in pure BaTiO3.

The perovskite structure is cubic at temperatures over the Curie point (approximately 130°C, also referred to as the transition temperature for ferroelectric ceramics). When the temperature range is below the Curie point, one of the axes (C axis) stretches and another shrinks slightly to become tetragonal (see Figure 2). In this case, with the Ti4+ ion placed in the axial direction of the crystal unit away from the body center, polarization occurs. In other words, polarization is caused by asymmetry in the crystalline structure, which exists from the outset without an applied external electric field or pressure. This type of polarization is referred to as spontaneous polarization.

Figure 3. Micro-structure of BaTiO3-type ceramics.

BaTiO3-type ceramics are an aggregation of micro-crystallites (polycrystalline) having sub-µm diameters, as shown in Figure 3. These micro-crystallites are called grains, and their crystalline structures are neatly aligned. Those grains are divided into many randomly oriented domains at temperatures below the Curie point. Within each domain, the crystals share a common direction, also known as spontaneous polarization.

When BaTiO3-type ceramics are heated above the Curie point, the crystalline structure goes through a transition from tetragonal to cubic phase. Along with this transition, spontaneous polarization in the domains disappears. When cooled below the Curie point, the transition reverses from cubic to tetragonal, and grains simultaneously receive stress from the distortion of the surroundings. At this point, several small domains in the grains are generated, and spontaneous polarization of each domain can be easily reversed with a low electric field. Since relative dielectric constant corresponds with the reversal of spontaneous polarization per unit volume, it is measured as higher capacitance.

Figure 4. Capacitance and DC voltage characteristics.

DC Bias Characteristic

The challenge lies not with spontaneous polarization, but in reversing it. When spontaneous polarization is reversed under no voltage stress (no DC bias), MLCCs achieve a high capacitance. However, if an external bias is applied to the spontaneous polarization process, the free reversal of spontaneous polarization is much more difficult. As a result, the capacitance gained is lower compared to the capacitance before the application of the bias. This is why capacitance decreases when DC bias is applied-hence the term DC bias characteristic.

Even more so than spontaneous polarization, this unique phenomenon of DC bias in MLCC ferroelectric ceramics is little-known and often comes as a surprise to design engineers who are used to using tantalum or electrolytic materials. Electrolytic capacitors are non-ferroelectric with a very low dielectric constant. Their capacitance is derived from a very high surface area and nanometer-thick dielectric layers, and is not a function of applied voltage.

Figure 4 indicates types of temperature characteristics for the DC bias characteristics of MLCCs at normal temperature. The main component of temperature compensation type (C0G, U2J characteristics, etc.) is paraelectricity ceramics, where capacitance does not vary due to DC bias. Conversely, the capacitance of high-dielectric-constant BaTiO3-based ceramics (X7R, X5R characteristics, etc.) decreases under DC bias.

Advances in Ceramic Technology

So here lies the challenge. How can the effects of DC bias voltage on capacitance be reduced? Fortunately, new developments in BaTiO3 ceramic technology allow more control over this effect. Basically, the technology involves tailoring the BaTiO3-based crystals to soften the effect of polarization reversal. This lowers the effect of DC bias; however, it is often accompanied with a lower initial capacitance. A material has been developed that keeps the drop in zero bias capacitance to a minimum.

Better education and dissemination of information for DC bias characteristics have led to increased research activity, and the properties of advanced ceramics continue to improve as the molecular levels of this natural material are researched further. Innovative solutions, like MLCCs, are at the cutting edge of technology and are leading the electronics evolution toward smaller and more capable components.

For more information, contact Murata Electronics North America, Inc., 2200 Lake Park Dr., Smyrna, GA 30080-7604; (770) 436-1300; fax (770) 436-3030; or visit www.murata-northamerica.com.