BCZT Ceramics with BNNWP Glass: Energy Storage

• The paper demonstrates that incorporating BNNWP glass into BCZT ceramics stabilizes the tetragonal phase and enhances dielectric breakdown strength.
• The study employs conventional solid-state sintering with optimized profiles to achieve significant grain size reduction and improved densification.
• Energy storage performance is notably enhanced, with BCZT4 achieving a recoverable energy density of 0.52 J/cm³ at 135 kV/cm, a 6.6-fold improvement over pure BCZT.

Lead-free Ba₀.₈₅Ca₀.₁₅Zr₀.₁₀Ti₀.₉₀O₃ (BCZT) ceramics, when modified by the addition of BaO–Na₂O–Nb₂O₅–WO₃–P₂O₅ (BNNWP) glass, exhibit significant advances in phase structure, microstructural refinement, and energy storage properties. These composites, designated as (1–x)BCZT–xBNNWP (abbreviated “BCZTx”; x = 0, 2, 4, 6, 8 wt%), are synthesized via conventional solid-state routes with optimized sintering profiles, targeting applications in high-power energy storage systems.

1. Material System and Synthesis Protocols

BCZT ceramics are modified by incorporating varying proportions of a specialized phosphate glass composed of BaO, Na₂O, Nb₂O₅, WO₃, and P₂O₅ (“BNNWP glass”). The solid-state process employed involves mixing stoichiometric amounts of raw materials, followed by calcination and sintering at temperatures selected to optimize density and phase purity. Five main compositions are explored: pristine BCZT (BCZT0), and BCZT with 2, 4, 6, and 8 wt% BNNWP glass (BCZT2–BCZT8). The addition of glass introduces a liquid phase during sintering, facilitating improved densification and grain boundary evolution.

2. Phase Evolution and Microstructural Modification

X-ray diffraction (XRD) reveals that pure BCZT exhibits simultaneous orthorhombic (O) and tetragonal (T) phases, indicative of its location near the morphotropic phase boundary (MPB), often linked to enhanced dielectric and piezoelectric behavior. Glass addition eliminates this coexistence, yielding a predominantly tetragonal phase in BCZTx (x = 2–8). The suppression of the orthorhombic phase is crucial for stabilizing dielectric performance and elevating breakdown strength.

Scanning Electron Microscopy (SEM) analysis demonstrates a marked decrease in average grain size as BNNWP glass content increases. The grain size decreases from approximately 6.4 µm (BCZT0) to ~1.25 µm (BCZT8). This fine-grained microstructure, in combination with higher density due to liquid phase sintering, directly enhances dielectric breakdown strength—an essential metric for energy storage functionality.

Composition	Avg. Grain Size (µm)	Observed Phase
BCZT0	6.4	O/T coexistence
BCZT2–8	1.25–~X	Tetragonal only

3. Dielectric Behavior and Phase Transition Dynamics

The dielectric constant ($\varepsilon_r$) of BCZTx diminishes as glass concentration increases, attributable to both the intrinsic low permittivity of BNNWP (ε ≈ 65) and the reduction in grain size. Dielectric spectroscopy reveals two sequential transitions: orthorhombic–tetragonal (O–T) and tetragonal–cubic (T–C). The Curie temperature (T$_c$) shifts as a function of glass content, while dielectric loss (tan $\delta$) remains low (<0.15) across measured frequencies—an indicator of minimal energy dissipation.

Thermal stability of $\varepsilon_r$ is enhanced with glass incorporation, which is advantageous for device reliability under variable thermal conditions. The loss-tangent values, staying consistently low, affirm the suitability of these ceramics for energy storage applications where minimal thermal and dielectric losses are imperative.

4. Ferroelectric Loop Characteristics and Energy Storage Efficiency

Ferroelectric polarization–electric field (P–E) hysteresis measurements demonstrate that pure BCZT exhibits classical ferroelectric loops with high maximum polarization (P$_\mathrm{max}$) and significant remnant polarization (P$_\mathrm{r}$). Upon addition of BNNWP glass, loops become slimmer, indicating reduced P$_r$, and the maximum sustainable electric field (E$_\mathrm{max}$) increases sharply.

A reduction in P$_r$ is desirable for energy storage materials, as it minimizes recoverable losses. The elevated breakdown field in glass-modified compositions allows application of higher operating voltages, thereby increasing the recoverable energy density.

5. Energy Storage Properties and Performance Enhancement

The most pronounced improvement occurs in the composition with 4 wt% BNNWP glass (BCZT4), which attains a recovered energy density ($W_\mathrm{rec}$) of 0.52 J/cm³ at an applied field of 135 kV/cm and an energy storage efficiency ($\eta$) of 62.4%. This constitutes a 6.6-fold enhancement over pure BCZT (BCZT0), whose $W_\mathrm{rec}$ is 0.075 J/cm³.

Sample	$W_\mathrm{rec}$ (J/cm³)	$E_\mathrm{max}$ (kV/cm)	Energy Storage Efficiency (%)
BCZT0	0.075	—	—
BCZT4	0.52	135	62.4

The substantial increase in energy density is linked to phase transition (elimination of O/T coexistence), grain refinement, and improved densification, which collectively raise the dielectric breakdown strength and thus the permitted operating field.

6. Landau–Ginzburg–Devonshire Theory Modeling

Application of the Landau–Ginzburg–Devonshire (LGD) phenomenological model provides theoretical underpinning for the observed energy density. The free energy expansion,

$F = aP^2 + bP^4$

with equilibrium condition,

$E = aP + bP^3$

allows calculation of the recoverable energy density via definite integration:

$$w = \int_{P_r}^{P_{max}} (aP + bP^3) dP = \frac{a}{2}(P_{max}^2 - P_r^2) + \frac{b}{4}(P_{max}^4 - P_r^4)$$

Experimental and LGD-theory-derived energy densities show substantial concordance, validating the reliability of the model parameters ($a$, $b$, $P_\mathrm{max}$) extracted by fitting P–E curves. This affirms the deterministic role of these quantities in governing energy storage performance in glass-modified BCZT.

7. Implications for Device Applications and Outlook

The transformation from mixed phase (O/T) to a stabilized tetragonal structure, coupled with marked grain size reduction, facilitates higher breakdown electric fields and lower remnant polarizations—cornerstones of high-performance energy storage ceramics. The BCZT4 composition’s six-fold improvement in recoverable energy density underscores the utility of glass addition in modulating both phase and microstructural properties.

The findings suggest potential deployment in high-power energy storage devices, including pulsed power systems. Enhanced thermal stability and improved breakdown strengths indicate that these glass-modified BCZT ceramics represent a promising route for robust, lead-free capacitor components. Future research should aim to further elucidate durability under cyclic and thermal load, as well as scale-up synthesis protocols for industrial realization.