Advances in materials technology have lead to a mitigation of decreased capacitance in barium titanate-based MLCCs.
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.
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
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
Figure 1. Crystalline structure of BaTiO3-type ceramics.
Ferroelectricity and Spontaneous Polarization
discovered in 1921, ferroelectricity began to play a much larger role in
electronic applications during the 1950s after the increased use of
. 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
). And values as high as 15,000 are possible over a
narrow temperature range, compared to less than 10 for most common ceramic and
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.
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
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
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.
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
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.
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
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.