Optimization of microstructure and electrical properties of ZnO-Cr2O3-based varistors by Tm2O3 doping

Technical Summary: Optimization of Microstructure and Electrical Properties of ZnO-Cr2O3-Based Varistors by Tm2O3 Doping

Problem Statement
Zinc oxide (ZnO) varistors are critical for overvoltage protection in power systems and electronic devices. However, the increasing demand for miniaturized electronic systems necessitates varistors with higher voltage gradients. While conventional ZnO-Bi2O3 systems face issues with Bi2O3 vaporization, and other rare-earth doped systems utilize expensive or toxic elements, ZnO-Cr2O3-based systems offer a promising alternative. Despite this, the specific impact of Thulium oxide (Tm2O3) doping on the microstructure and electrical performance of ZnO-Cr2O3-based varistors remains largely unexplored. The challenge lies in optimizing the grain size and grain boundary barrier characteristics to achieve a high voltage gradient, a high nonlinear coefficient, and low leakage current simultaneously.

Methodology
The study employed a conventional solid-state reaction route to synthesize ZnO-Cr2O3-based varistor ceramics with the nominal composition: (97.40-x) mol% ZnO + 0.10 mol% Cr2O3 + 0.50 mol% Co3O4 + 2.00 mol% CaCO3 + x mol% Tm2O3, where x ranged from 0.00 to 1.00 mol%.

• Processing: Powders were ball-milled, pressed into green bodies, and sintered at 1100°C for 3 hours. Silver electrodes were applied via firing at 600°C.
• Characterization: Phase composition was analyzed via X-ray diffraction (XRD). Microstructure and grain size distribution were examined using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS).
• Electrical Testing: Current-voltage (E-J) characteristics were measured to determine the nonlinear coefficient ($\alpha$), voltage gradient ($E_{1mA}$), and leakage current density ($J_L$).
• Barrier Analysis: Grain boundary barrier height ($\phi_b$), interface state density ($N_S$), and donor concentration ($N_D$) were calculated using the J-E-T method (measuring E-J characteristics across a temperature range of 313.15–423.15 K).
• Impedance and Dielectric Analysis: Impedance spectra (200 Hz–2 MHz) were analyzed to determine grain and grain boundary resistivity, while dielectric properties (relative permittivity $\epsilon'$ and loss tangent $\tan\delta$) were evaluated.

Key Contributions and Results
The research systematically elucidated the relationship between Tm2O3 doping content and the resulting microstructural and electrical properties:

1. Microstructural Evolution:

   • XRD analysis revealed that Tm³⁺ ions substitute for Zn²⁺ in the ZnO lattice up to a solid solubility limit of approximately 0.25 mol%, evidenced by a shift in ZnO diffraction peaks to lower angles. Excess Tm2O3 segregates as a secondary phase at grain triple junctions.
   • SEM analysis demonstrated that Tm2O3 doping induces a "Zener pinning" effect, inhibiting grain boundary migration. As doping increased from 0.00 to 0.50 mol%, the average grain size decreased significantly from 3.16 μm to 1.37 μm, with a more uniform distribution.
   • EDS mapping confirmed the co-enrichment of Cobalt (Co) and Thulium (Tm) at the grain boundaries. However, at doping levels exceeding 0.50 mol% (e.g., 1.00 mol%), excessive segregation led to uneven Tm2O3 clusters, causing abnormal grain growth (increasing to 1.71 μm).

2. Electrical Performance Optimization:

   • Voltage Gradient: The reduction in grain size increased the number of grain boundaries per unit thickness, raising the voltage gradient ($E_{1mA}$) from 428.37 V/mm (undoped) to a peak of 1778.33 V/mm at 0.50 mol% Tm2O3.
   • Nonlinearity and Leakage: The 0.50 mol% sample exhibited the optimal nonlinear coefficient ($\alpha$) of 54.36 and the lowest leakage current density ($J_L$) of 0.97 μA/cm². In contrast, the undoped sample had $\alpha$ = 6.22 and $J_L$ = 375.75 μA/cm².
   • Barrier Mechanism: The enhancement in electrical properties is attributed to an increase in the grain boundary barrier height ($\phi_b$), which rose from 0.23 eV to 0.66 eV at the optimal doping level. This increase was driven by a synergistic enrichment of the Tm2O3 phase and Co element at the boundaries, which increased the interface state density ($N_S$) from $1.55 \times 10^{16}$ to $5.69 \times 10^{16}$ m⁻².
   • Impedance and Dielectric Properties: The optimal sample showed the highest grain boundary resistivity ($\rho_{gb}$ = 18,832 MΩ·cm) and the lowest dielectric loss ($\tan\delta$) and relative permittivity ($\epsilon'$), indicating suppressed interfacial polarization and leakage conduction.

3. Degradation at Excessive Doping:

   • When Tm2O3 content exceeded 0.50 mol%, the inhomogeneous distribution of the secondary phase disrupted the uniformity of the grain boundaries. This led to a reduction in barrier height, a rebound in grain size, and a subsequent degradation of electrical performance (lower $\alpha$, higher $J_L$, and lower $E_{1mA}$).

Significance
The paper claims that the optimal doping of 0.50 mol% Tm2O3 represents an effective strategy for developing high-performance ZnO-Cr2O3-based varistors. The study demonstrates that a combination of grain refinement (to increase voltage gradient) and grain boundary engineering (to elevate barrier height and suppress leakage) can simultaneously achieve a high voltage gradient, a high nonlinear coefficient, and low leakage current. These findings provide valuable insights for the development of miniaturized varistors suitable for highly integrated electronic systems, offering a pathway to overcome limitations in existing varistor technologies without relying on volatile or toxic dopants.