Class II MLCCs (such as X7R, Y5V, Z5U) are made with ferroelectric ceramics, typically barium titanate (BaTiO₃)-based dielectrics, which exhibit time-dependent aging of capacitance due to gradual realignment of ferroelectric domains. The effect is intrinsic to the material and predictable.

## 1. Mechanism of Aging

* Domain Relaxation: Ferroelectric ceramics contain microscopic polarization domains. Over time, these domains slowly shift toward a more thermodynamically stable configuration, reducing the net polarization that contributes to capacitance.
* Logarithmic Time Dependence: The capacitance decreases logarithmically with time, meaning the largest drop occurs shortly after manufacturing or first energization, and the rate slows over months to years.
* Depoling and Rejuvenation: If the capacitor is exposed to high voltage (DC bias) or elevated temperature, some domains may repolarize, partially restoring capacitance. This is called rejuvenation.

## 2. Typical Magnitude

* X7R: Capacitance decreases approximately 1–3% per decade hour at room temperature. After 1 year, total capacitance may drop 5–10%.
* Y5V/Z5U: Higher-permittivity, less stable dielectrics can lose 10–30% of initial capacitance over the same period.
* Class I MLCCs (C0G/NP0) show negligible aging because their ceramics are paraelectric and non-ferroelectric.

## 3. Temperature Dependence

* Aging Rate Increases with Temperature: Elevated storage or operating temperatures accelerate domain relaxation.
* Reversal by Heating: Some Class II MLCCs can be "rejuvenated" by heating above the Curie temperature, which realigns domains.

## 4. Design Implications

* Designers must account for initial capacitance drop when selecting Class II MLCCs for precision filtering or timing circuits.
* Use Class I (C0G/NP0) capacitors for applications requiring high stability over time.
* Aging is generally predictable and logarithmic, allowing engineers to derate initial capacitance to maintain long-term performance.

Summary:
Class II MLCCs experience gradual, logarithmic capacitance decrease over time due to ferroelectric domain relaxation. The effect is material-dependent, with higher-permittivity dielectrics showing larger drops. It is predictable, sometimes partially reversible, and must be considered in long-term circuit design.


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