Ceramic capacitors, particularly those with high capacitance such as B/X5R and R/X7R types, exhibit a gradual decrease in electrostatic capacitance over time. This phenomenon is especially important to consider when using these capacitors in critical applications like clock circuits, where stability and accuracy are essential. It’s crucial to evaluate their performance under actual operating conditions and equipment to ensure reliability.
For instance, the graph below illustrates that as time passes, the actual capacitance of the capacitor decreases. The rate of change is not linear on a logarithmic time scale, indicating a non-uniform aging process. The horizontal axis represents operating time in hours, while the vertical axis shows the percentage change in capacitance relative to its initial value.
This time-dependent reduction in capacitance is known as "aging" or "temporal change." As shown in the figure, this behavior is inherent to high-capacitance ceramic capacitors and is not exclusive to any specific manufacturer—such as Murata. However, temperature-compensating capacitors do not exhibit this aging effect.
Interestingly, if a capacitor that has lost some of its capacitance due to aging is exposed to temperatures above the Curie point (around 125°C), often during soldering or other thermal processes, the capacitance can be partially restored. But once the capacitor cools back below the Curie temperature, the aging process resumes, gradually reducing the capacitance again.
**Understanding the Aging Mechanism**
High-capacitance ceramic capacitors typically use barium titanate (BaTiO3) as the main dielectric material. BaTiO3 has a perovskite-like crystal structure. At temperatures above the Curie point (approximately 125°C), it adopts a cubic structure, with Ba²⺠ions at the corners, O²⻠ions at the centers of the faces, and Tiâ´âº ions at the center of the cube.
Below the Curie temperature, the structure changes to a tetragonal form, with elongation along the C-axis and slight contraction in the other axes. In this state, Tiâ´âº ions shift in the direction of elongation, creating spontaneous polarization—an internal electric field that exists without an external voltage.
This spontaneous polarization can be reoriented by an external electric field, which is a key characteristic of ferroelectric materials. When heated above the Curie temperature, the material undergoes a phase transition from tetragonal to cubic, causing the spontaneous polarization to disappear and the domains to vanish.
Upon cooling, the structure reverts to tetragonal, and spontaneous polarization and domains reform. During this process, the crystal grains experience stress. Over time, the randomly oriented domains tend to align into more stable configurations, such as 90° domains, releasing the internal stress caused by deformation.
In addition, charge carriers at the grain boundaries, such as mobile ions or vacancies, contribute to space charge polarization. This type of polarization interacts with the spontaneous polarization, making it harder for the domains to reorient under low electric fields.
As a result, the spontaneous polarization becomes more stable, and the ability of the domains to switch under weak electric fields diminishes. Since permittivity is related to the ease of domain switching, a reduction in the number of switchable domains leads to a decrease in overall capacitance. This explains why the electrostatic capacitance of high-capacitance ceramic capacitors tends to degrade over time.
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