• The temperature rise (ΔT) caused by the ripple current should not exceed 20°C above the ambient temperature of the capacitor before power is applied.
• Vp-p value and Vo-p value including DC bias for the applied voltage must be within the rated voltage (DC).
• The maximum operating temperature, including the amount of self-generated heat should not be exceeded.

Please refer to the appropriate technical data sheet for the self-heating data, from the sine wave ripple current, as reference data. Please also utilize the software tool (Murata Chip Capacitor Characteristics Data Library) to obtain reference values for circuit design simulation.

Also, when using capacitors with high frequency voltage, pulse voltage, etc. dielectric loss may cause self-heating. The load should be contained to the level such that when measuring at atmospheric temperature of 25°C, the product's self-heating remains below 20°C and surface temperature of the capacitor in the actual circuit remains within the maximum rated temperature.

Behavior of insulation resistance value

Directly after DC voltage is applied to a capacitor, the rush current, which is also called the charge current flows as shown in Figure 1. As the capacitor is gradually charged, the current decreases exponentially.

Figure 1

Current I(t) flowing after time t passes are categorized into three types as seen in the equation (1) below, namely, charge current Ic(t),

absorption current Ia(t) and leakage current Ir. I(t)=Ic(t)+Ia(t)+Ir equation (1)

Charge current indicates current flowing through an ideal capacitor. Absorption current flows with a delay compared with the charge current, accompanying dielectric loss at a low frequency and the reverse polarization for high dielectric constant type capacitors (ferroelectric) and the Schottky barrier which occurs at the interface between the ceramics and the metal electrodes.

Leakage current is a constant current flowing after a certain period of time when the influence of absorption current diminishes.

Therefore, the value of the flowing current varies depending on the amount of time voltage is applied to the capacitor. This means that the capacitor's insulation resistance value cannot be determined unless the timing of the measurement after voltage application is specified.

Insulation resistance value

Insulation resistance value is represented in units Meg Ohms [MΩ] or Ohm Farads [ΩF]. Its specified value varies depending on the capacitance value. The value is specified as the product of nominal capacitance value and insulation resistance (CR product), for example over 10,000MΩ for 0.047µF and lower, and over 500ΩF for higher than 0.047µF.

Guaranteed insulation resistance value [example]

As shown above, the higher capacitance value, the less its insulation resistance becomes. The reason is explained below. Insulation resistance can be figured out with the Ohm's law from applied voltage considering the monolithic ceramic capacitor as a conductor as well as electric current.

Resistance value R may be expressed with equation (2) with the length of the conductor as L, and the area of cross section as S and specific resistance as ρ.
R= ρ • L/S equation (2)

Likewise, capacitance C can be represented with equation (3) by expressing distance between electrodes for the monolithic ceramic capacitor (dielectric thickness) as L, the area of inner electrode as S and the dielectric constant as ε.
C ∝ ε • S/L equation (3)
Equation (4) can be derived from equation (2) and equation (3) indicating that R and C are inversely proportional.
R ∝ ρ • ε/C equation (4)

Insulation resistance being higher indicates that the leakage current under DC voltage is lower. Generally, circuits are supposed to have higher performance when the insulation resistance value is higher.

Spontaneous polarization and ferroelectricity of BaTiO3 type ceramics

As shown in Figure 1, BaTiO3 ceramics possess perovskite type crystalline structure. It is cubic at temperatures over the Curie point (approx. 130°C), and Ba is in peak, O is in face center and Ti is in body center.

Figure 1 Crystalline structure of BaTiO3 type ceramics

When within the normal temperature range is below the Curie point, one of the axes (C axis) stretches and other axes shrink slightly to become tetragonal (Figure 2). In this case, the Ti⁴+ ion will be positioned in the axial direction of the crystal unit away from the body center causing polarization to occur. In other words, polarization is caused by asymmetry in the crystalline structure, which exists from the outset without applying an external electric field or pressure. This type of polarization is called spontaneous polarization.

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

The direction of spontaneous polarization (position of Ti⁴+ ion) for BaTiO3 type ceramics can be easily reversed with application of an external electric field. The characteristics of having spontaneous polarization, and ability to reverse the direction of polarization with an external electric field is specifically called Ferroelectricity. BaTiO3 is a typical type of ferroelectric ceramics.

BaTiO3 type ceramics are an aggregation of micro crystallites (polycrystalline) having sub-µm diameter as shown in Figure 3. These micro crystallites are called grains, and their crystalline structures are neatly aligned as shown in Figures 1 and 2. Those grains are divided into many domains at temperatures below the Curie point. Within each domain, there is a common direction of crystals, therefore the direction of spontaneous polarization is the same as well.

Figure 3 Micro structure of BaTiO3 type ceramics

When BaTiO3 type ceramics is heated above the Curie point, the crystalline structure goes through a phase transition from tetragonal to cubic.

With this, spontaneous polarization the domains also disappear. When cooled below the Curie point, phase transition from cubic to tetragonal takes place near Curie point, and the C axis stretches in the axial direction. The other axes shrink slightly to form spontaneous polarization and domains. Simultaneously, grains receive stress from the distortion of its surroundings.

At this point, several small domains in 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.

DC bias characteristics

When spontaneous polarization in dielectric bodies can be easily reversed, higher capacitance can be gained. Spontaneous polarization is free without bias. When you apply an external bias a spontaneous polarization within a dielectric body is formed in the direction of the electric field, making the free reversal of spontaneous polarization more difficult. As a result, the capacitance gained is lower compared with the capacitance before the application of the bias.

This is the reason why capacitance decreases when DC bias is applied.

Figure 4 indicates types of temperature characteristics for the DC bias characteristics of Monolithic Ceramic Capacitors at normal temperature. The main component of temperature compensation type capacitors (C0G, U2J characteristics, etc.) is paraelectricity ceramics, and capacitance does not vary due to DC bias. Conversely, the capacitance of a high dielectric constant type capacitor (X5R, X7R, Z5U, Y5V characteristics, etc.) decreases due to DC bias, especially with Y5V characteristics.

Spontaneous polarization and ferroelectricity of BaTiO3 type ceramics

As seen in Figure 1, BaTiO3 type ceramics possesses perovskite type crystalline structure. It is cubic at temperature over Curie point, and Ba is in peak, O is in face center and Ti is in body center.

Figure 1 Crystalline structure of BaTiO3 type ceramics

When within the normal temperature range below Curie point, one of the axes (C axis) stretches about 1% and other axes shrink slightly to become tetragonal (Figure 2 on the next page). In this case, the Ti⁴+ ion will occupy the position near the O²-ion being displaced by 0.12 from the body center, in the direction of stretched axis. This displaces the center of gravity for negative and positive electric charges, causing polarization.

Polarization is caused by asymmetry of the crystalline structure, which exists from the outset without applying an external electric field or pressure. This type of polarization is called spontaneous polarization.

Figure 2 Change in crystalline structure and relative dielectric constant on temperature (pure BaTiO3)

The direction of spontaneous polarization (position of Ti⁴+ ions) for BaTiO3 type ceramics can be easily reversed with application of external electric field. The characteristics of having spontaneous polarization, and ability to reverse the direction of polarization with external electric field are specifically called ferroelectricity.

Mechanism of aging

BaTiO3 ceramics are an aggregation of polycrystalline having sub-µm diameter as shown in Figure 3. These crystalline are called grains, and their structures are neatly aligned as shown in Figures 1 and 2. Those grains are divided into many domains at temperatures below the Curie point. Within each domain, the direction of crystal axis is the same, therefore the direction of spontaneous polarization is the same as well.

Figure 3 Micro structure of BaTiO3 type ceramics

When BaTiO3 type ceramics is heated above the Curie point, the crystalline structure goes through a phase transition from tetragonal to cubic. With this, spontaneous polarization the domains also disappear.

When cooled below the Curie point, phase transition from cubic to tetragonal takes place near Curie point, and the C axis stretches by about 1% in the axial direction. The other axes shrink slightly to form spontaneous polarization and domains. Simultaneously, grains receive stress from the distortion of its surroundings.

At this point, several small domains in 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.

When the capacitor is left without load at the temperature below the Curie point, domains that faced random directions gradually realign themselves with the passage of time to become a larger and stable shape in terms of energy (Figure 3. 90° domain), releasing the stress caused by crystal distortion.

In addition, space charge of the boundary layer (slow moving ions and lattice vacancies) migrates, causing space charge polarization. Space charge polarization prevents spontaneous polarization from becoming reversed.

In other words, as time passes after spontaneous polarization occurs, it is realigned to a more stable condition, while space charge polarization occurs in the boundary layer preventing reversal of spontaneous polarization. In this state, we need a higher electric field to reverse spontaneous polarization of domains. This means fewer domains reverse under low electric field and capacitance decreases.

This is considered to be the mechanism for aging.

The Micro structure of crystals will return to the initial state when heated to temperature above Curie point, and when cooled, the aging process starts again.

Aging characteristics of Murata products

Generally, the capacitance of high dielectric constant type Monolithic Ceramic Capacitors decreases practically linearly on the logarithmic time graph with the value at 24 hours after the heat treatment over 125°C as the standard. Please refer to the appendix indicating typical examples of aging characteristics in the capacitance of Murata products.

Capacitance decreases from aging will recover by being heated during the soldering process, etc.

The capacitance of a ceramic capacitor is expected to maintain a value within spec when built onto the circuit. We have determined the capacitance range based on the reasons above.

No aging phenomenon is observed with temperature compensating type capacitors.

Handling care

Aging phenomenon is a basic characteristic of high dielectric constant type (BaTiO3) ceramic capacitors, and the degree of change in capacitance from aging varies depending on the type of ceramic material used. Also, when DC bias is applied on an actual circuit, the degree of capacitance aging varies depending on the level of DC bias voltage.

Therefore, when using high dielectric constant type ceramic capacitors, change in capacitance from the aging phenomenon should be taken into consideration, and especially when the stability of capacitance is important, it should be verified on the actual circuit.

Preservation condition: when stored after mounting on circuit boards.

Please store the capacitor within "usable temperature range" below, which is the specified temperature range intended for continuous use of the capacitor.

Usable temperature range