Effect of TeO2 addition on the dielectric properties of CaCu3Ti4O12 ceramics derived from the oxalate precursor route

Abstract: CaCu3Ti4O12 (CCTO) ceramics which has perovskite structure gained considerable attention due to its giant permittivity. But, it has high tan {\delta} (0.1 at 1kHz) at room temperature, which needs to be minimised to the level of practical applications. Hence, TeO2 which is a good glass former has been deliberately added to CCTO nano ceramic (derived from the oxalate precursor route) to explore the possibility of reducing the dielectric loss while maintaining the high permittivity. The structural, morphological and dielectric properties of the pure CCTO and TeO2 added ceramics were studied using X-ray diffraction, Scanning Electron Microscope along with Energy Dispersive X-ray Analysis (EDX), spectroscopy and Impedance analyzer. For the 2.0 wt % TeO2 added ceramics, there is a remarkable difference in the microstructural features as compared to that of pure CCTO ceramics. This sample exhibited permittivity values as high as 7387 @ 10 KHz and low dielectric loss value of 0.037 @ 10 kHz, which can be exploited for the high frequency capacitors application.

• The paper shows that TeO2 addition significantly reduces dielectric loss by altering the Cu-rich phase at grain boundaries.
• The oxalate precursor route enabled controlled synthesis of CCTO ceramics, yielding lower AC conductivity and enhanced grain boundary resistance.
• The study demonstrates that although permittivity decreases slightly with TeO2, the values remain suitable for high-frequency capacitor applications.

Overview

The study investigates the effect of TeO2 additions on the dielectric properties of CaCu3Ti4O12 (CCTO) ceramics obtained via the oxalate precursor route. CCTO ceramics have been noted for their exceptionally high dielectric permittivity, though their application is limited by significant dielectric loss. This paper explores the potential for TeO2—recognizable as an effective network former with a low melting point—to mitigate dielectric losses in CCTO while preserving its high permittivity, suited particularly for high-frequency capacitor applications.

Experimental Methodology

The researchers employed an oxalate precursor route to derive CCTO ceramics, ensuring a controlled synthesis conducive to exploring modifications in material properties via TeO2 additions. The ceramic samples were synthesized with varying weight percentages of TeO2, namely 0.5%, 1.0%, 1.5%, and 2.0%. The resultant powders underwent a sequence of processes including ball milling, granulation, and sintering, specifically at high temperatures (1100°C and 1130°C). Structural and morphological analyses were conducted using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Dielectric properties, including permittivity and loss, were characterized using impedance spectroscopy.

Key Findings

1. Microstructural Alterations: The introduction of TeO2 significantly influenced the microstructure of the ceramics. The SEM analysis revealed a reduction in the Cu-rich phase at grain boundaries, which is believed to contribute to decreasing the dielectric loss.
2. Dielectric Properties: For the samples with a 2% TeO2 addition, a decrease in dielectric loss to 0.037 @ 10 kHz was observed, representing a substantial reduction compared to pure CCTO. This value showcases potential for practical applications in electronic devices requiring low-loss high-capacitance materials.
3. Permittivity Observations: The addition of TeO2 resulted in a reduced permittivity relative to pure CCTO, though the values remained commendable for applications. The permittivity at 1 kHz was recorded at 7725, with a notable finding being that permittivity values showed considerable dependence on both frequency and temperature.
4. Secondary Phases: XRD patterns suggested the emergence of secondary phases such as TiO2 and CaTiO3, potentially due to TeO2 interactions. However, secondary CuO phases were notably absent, indicating successful mitigation of conductive CuO at grain boundaries, a suspected contributor to losses.
5. AC Conductivity: Lower AC conductivity in TeO2-doped samples compared to pure CCTO suggests enhanced grain boundary resistance, associated with altered CuO segregation dynamics.

Implications and Future Directions

The study provides relevant insights into controlling dielectric losses in high-permittivity ceramics through minor compositional adjustments. By introducing TeO2, the electrical properties of CCTO were tuned favorably for high-frequency capacitor applications. The results support the notion that glass formers like TeO2 can effectively modulate the chemistry and microstructure at the grain boundary level.

Further investigations could explore the avenue of TeO2 addition in other dielectric materials with similar loss challenges or variations in sintering parameters to optimize the balance between permittivity and loss. Additional studies using advanced techniques such as TEM could offer deeper insights into the nature and interactions of these secondary phases and their broader implications on dielectric properties.

Conclusion

The integration of TeO2 into the CCTO matrix marks a viable approach to enhancing the dielectric performance by reducing losses without substantially compromising permittivity. The research underscores the importance of microstructural management and boundary engineering in the development of materials for high-frequency applications. This work contributes foundational knowledge potentially applicable to the design of next-generation electronic components requiring advanced dielectric materials.