Control of electromechanical properties of multilayer ceramic capacitors for vibration reduction

A multilayer ceramic capacitor (MLCC) contains layers of ceramics as the dielectric materials. It has been known that Class 2 MLCCs, made of ferroelectric ceramics such as barium titanate, tend to suffer from electromechanical coupling hence vibration, which leads to the generation of acoustic humming noise, a source of annoyance in many modern electronic devices. In this article, a repoling method to control the electromechanical properties and the resulting vibration of MLCCs is presented. The repoling protocol hinges on the understanding that two independent mechanisms are responsible for the electromechanical coupling in MLCCs: piezoelectricity and electrostriction of the ceramic layers. The vibration due to piezoelectricity is linearly proportional to the input voltage, whereas the vibration due to electrostriction shows a quadratic dependence. Given the DC bias and the AC input voltage under normal operating conditions, the vibration is composed of the fundamental component at the frequency of the AC input and the second harmonic component spawned by the quadratic nonlinearity of electrostriction. It is demonstrated that by engineering the coefficients of piezoelectricity and electrostriction of the ceramic layers through a carefully designed repoling treatment, vibration reduction can be achieved for both the fundamental and second harmonic components. Especially, the fundamental component of vibration can be reduced significantly, as the piezoelectric effect is made to offset the electrostrictive effect.