Compositionally Graded Multilayer Ceramic Capacitors

Abstract

Multilayer ceramic capacitors (MLCC) are widely used in consumer electronics. Here, we provide a transformative method for achieving high dielectric response and tunability over a wide temperature range through design of compositionally graded multilayer (CGML) architecture. Compositionally graded MLCCs were found to exhibit enhanced dielectric tunability (70%) along with small dielectric losses (<2.5%) over the required temperature ranges specified in the standard industrial classifications. The compositional grading resulted in generation of internal bias field which enhanced the tunability due to increased nonlinearity. The electric field tunability of MLCCs provides an important avenue for design of miniature filters and power converters.

Figure 1

Structural and dielectric properties of BTS-BCN composition with variation of Sn content. (a) X-ray diffraction patterns of BTS-BCN with variation in Sn content (Sn = 0.01~0.08). Right-hand side figure is magnified image of diffraction peaks in the vicinity of 2θ = 45°. The splitting of (2 0 0)/(0 0 2) peak gradually reduced and merged into a broad symmetrical peak with increasing Sn content. Tetragonal distortion and long range ordering in the system are reduced with increasing Sn content. (b) Loss tangent and (c) dielectric constants as function of temperature for various compositions of BTS-BCN. The Curie temperature of BTS-BCN is gradually decreased and the Curie peak (permittivity maximum, ε_max) becomes broader and shifts towards lower temperatures with increasing Sn content. The tanδ is less than 3% over the wide temperature range of −50 °C~125 °C for all the compositions.

Figure 2

Schematic structure of the compositionally graded multilayer ceramic capacitor. (a) Schematic representation of the cross-section of compositionally graded multilayer ceramic capacitor. Two layers of the eight individual compositions of BTS-BCN (Sn = 0.01~0.08) were stacked alternately. (b) Schematic description of inner electrode design of MLCC. The active area of the electrodes is 16 mm². (c) Sliced green samples of MLCCs containing 8 different compositions. (d) Final sintered samples at 1300 °C for 2 h.

Figure 3

Microstructures of the compositionally graded multilayer capacitor. (a) Cross-sectional SEM image of MLCC. Despite containing hetero-composition layers with different thermal expansion coefficients and Pt electrodes, MLCC does not exhibit any delaminations or mechanical defects after sintering process. (b) EDS mapping of Ba and Pt elements in the cross-section of MLCC. There is no interfacial elemental diffusion across the dielectric layer and Pt-electrode layers. (c) SEM micrograph of the dielectric layer. The grain size of BTS-BCN is in the range of 50~200 nm, of which small grain size can provide a higher dielectric strength and advantage to reduce the thickness of dielectric layers. (d) SEM micrograph depicting interfacial region of the Pt inner electrode and dielectric layer.

Figure 4

Dielectric properties of the compositionally graded multilayer capacitor. (a) Capacitance and (b) ielectric loss as a function of temperature at different frequencies for the MLCC with graded architecture. The maximum capacitance peak is significantly broadened due to compositional grading and the tan δ of the MLCC was below 2.5% over a wide temperature range of −55 °C~125 °C. (c) Temperature dependent capacitance of the graded MLCC under 0 V and 200 V DC bias conditions at 1 kHz. (d) Capacitance and dielectric loss of the graded MLCC with variation of applied DC bias at 1 kHz. The capacitance and tan δ are gradually decreased with increasing DC bias due to domain reorientation. The tunability of the graded MLCC is obtained to ~70% under 200 V DC bias. (e) Capacitance and dielectric loss as a function of frequency of the graded MLCC. The capacitance and tan δ are slowly varied with increasing frequency up to 50 kHz before changing rapidly at higher frequencies. The compositionally graded architecture does not exhibit the frequency dispersion over the phase transition temperature regime contrary to the core-shell architectures.

Figure 5

Grain microstructures and composition distributions in compositionally graded ferroelectric materials: layered structures in (a,c) series and (b,d) parallel configurations, and (e) random and (f) well-separated distributions of grains of different compositions as represented by different colors.

Figure 6

Phase field simulation of the compositionally graded multilayer capacitor. Simulated temperature dependence of the dielectric constants of the compositionally graded PZT polycrystals in (a) series and parallel layered structures in Fig. 5(a,b), (b) series and parallel layered structures of different layer thicknesses in Fig. 5(a~d), and (c) series and parallel layered structures in Fig. 5(a,b) and random and well-separated distributions of grains of different compositions in Fig. 5(e,f), respectively. For comparison, the dielectric constants of PZT polycrystals of the same grain microstructures but different homogeneous compositions are also simulated (black curves), from which the dielectric constants of compositionally graded polycrystals are calculated by using the rule of mixtures respectively for series and parallel configurations.