Among ceramic capacitors, the capacitance, especially of capacitors classified as a high dielectric constant (X5R,X7R characteristics etc.), decreases over time.
When using these products for time constant circuits, etc., please take time to fully understand their characteristics and check the actual conditions of use and actual equipment.
For example, as shown in the chart below, the longer the elapsed time, the more effective capacitance is reduced (it decreases almost linearly in a logarithmic time chart).
* In the following chart, the horizontal axis shows elapsed time (h), and the vertical axis shows the capacitance change ratio against the initial value.
Therefore, the nature (characteristic) of the decrease in capacitance over time is known as capacitance change over time (aging).
For your information, it is not only our products that have aging characteristics; it is a phenomenon commonly observed in high dielectric constant capacitors. Temperature compensating capacitors don't have aging characteristics.
In addition, even if the capacitance of a capacitor degrades due to aging, capacitance is recovered when heat above the Curie temperature (approx 125°C) is applied to the capacitor during a manufacturing process such as soldering.
When the capacitor cools down below the Curie point, aging starts again.
The mechanism of the aging characteristic
In high dielectric constant ceramic capacitors, at present BaTiO3 (barium titanate) is used as the principal component of the ceramic.
As shown below, BaTiO3 has a perovskite shaped crystal structure and above the Curie temperature it becomes cubic shape with Ba2+ ions to the vertices, O2- ion to face the center and Ti4+ ion in a body-centered position.
At the Curie temperature (approx 125°C) or more, it has a cubic crystal structure, and below the Curie temperature and within an ambient temperature range, one axis (axis C) stretches and the other axes shrink and turn into a tetragonal crystal structure.
In this case, polarization happens as a result of the unit shift of axially elongated Ti4+ ion crystal. This polarization occurs without applying an external electric field or pressure, and is known as "spontaneous polarization."
As explained above, a characteristic that has a spontaneous polarization and a property of changing orientation of spontaneous polarization by an external electric field to reverse is called "ferroelectricity."
Also, when BaTiO3 is heated above the Curie temperature, the crystal structure phase transits from tetragonal to cubic. Because of this, the spontaneous polarization is lost, and the domains are lost.
While the above structure is cooled down below the Curie temperature, the phase transits from tetragonal to cubic near the Curie temperature. With about 1% axial elongation in the direction of the C axis, other axes shrink a bit, then spontaneous polarization occurs and domains are generated. At the same time the crystal grains get stress from the surrounding distortion.
At this point, many fine domains are generated in the crystal grain and the spontaneous polarization of each domain tends to easily reverse even in a low electric field.
When the structure is left without load with a temperature below the Curie point, over time, domains that were randomly oriented become larger in size, and the sequence is gradually rearranged to the energetically stable form (as shown in the figure, 90° domain) and the stress caused by distortion of the crystal gets released.
In addition, the space charge of grain boundary layers (e.g., slow-moving ions or vacancies) move, and a partial region of space charge is generated. Space charge polarization acts upon spontaneous polarization and inhibits the reversal of spontaneous polarization.
In other words, over a period of time after the generation of spontaneous polarization, gradual spontaneous polarization is rearranged in a stable state. And in the boundary layer, space charge domains are generated and they inhibit spontaneous polarization reversal.
In this state, a higher electric field is required to reverse the spontaneous polarization of domains.
Since the dielectric constant is equivalent to the reversal of the spontaneous polarization per unit volume, if the number of domains with low-field reversal is reduced, the capacitance decreases.
This is considered as the mechanism of aging.