1. Introduction
It is widely recognized that Zinc Oxide (ZnO) varistor ceramic discs often suffer from defects during the manufacturing and sintering processes, leading to high scrap rates. Common defects include pores, air pockets (interlayers), laminations, and mechanical damage. Visible pores, in particular, significantly impact the disc’s ability to withstand current surges; during rectangular wave current impact tests, dielectric breakdown often initiates at these pore sites. Therefore, addressing these defects is a top priority for manufacturers. Based on over two decades of practical experience, this article explores the causes of these defects and proposes effective preventive measures.
2. Causes and Prevention of Pores
Pores are categorized into “large pores” (>100μm) and “small pores” (<50μm). Large pores are the primary culprits for dielectric breakdown.
2.1 Causes of Sudden Outbreaks of Large Pores Large pores are generally caused by external organic or inorganic impurities:
- Contamination from Coal Dust: Many ZnO raw material suppliers use coal-fired furnaces. If environmental isolation is poor, coal dust can be absorbed into the ZnO. This dust leaves behind voids during sintering. Prevention: Rigorously audit suppliers and inspect slurry residues for coal particles.
- Ceramic Fiber Contamination: Insulation fibers from furnace repairs falling into powder containers. Prevention: Strictly manage workspace cleanliness during kiln maintenance.
- External Dust: Poor factory environmental control (e.g., proximity to chimneys, open windows, dusty packaging bags). Prevention: Seal production areas, maintain positive pressure, and clean all raw material packaging thoroughly before use.
- Silicon Carbide (SiC) Contamination: Cross-contamination in plants producing both SiC and ZnO components. Prevention: Separate production lines and maintain strict environmental hygiene.
2.2 Causes of Persistent Small Pore Defects
- Inadequate PVA Dissolution: Polyvinyl Alcohol (PVA) must be dissolved at 90°C–100°C. Undissolved particles lead to voids. Prevention: Ensure thorough dissolution and filter through a 200–250 mesh screen.
- Coarse or Hard Spray-Dried Granules: Granules larger than 130μm or those with hard shells that do not compress during molding cause inter-granular voids. Prevention: Target an average granule size of 80–100μm, add lubricants (e.g., glycerol), and optimize spray dryer parameters (inlet temperature <320°C).
- Non-Uniform Moisture Content: Improper atomization or insufficient mixing leads to uneven shrinkage. Prevention: Standardize mixing times and use 40–60 mesh screening for granulated powder.
3. Causes and Prevention of Air Pockets
Air pockets result from trapped air during the pressing process:
- Causes: Excessive compression speed, inappropriate pressure settings, or inadequate exhaustion intervals. If the exterior of the disc densifies too quickly, air becomes trapped in the center.
- Prevention: Implement a multi-stage pressing program with gradual pressure increases. Maintain a slow initial compression speed (3–5mm/s) and ensure at least a 2-second delay during exhaustion stages. Allowing the die sleeve to “float” by 3-5mm during the final compression can facilitate air release.
4. Laminations and Internal Cracks
- Causes: These occur due to “elastic after-effect” (rapid expansion upon pressure release), rough die surfaces, or excessively fast temperature ramps during binder burnout (exceeding 40°C/h).
- Prevention: Increase powder moisture slightly, optimize die surface finish, and ensure the binder burnout cycle is slow enough to allow gases to escape. Proper cooling and heating management in the kiln is critical.
5. Impact of Tooling and Equipment
- Causes: Misalignment, poor parallelism, or excessive clearance between the die and punch causes density variations and edge flashes.
- Prevention: Use high-precision dies with adjusted tolerances (0.8–0.9mm gap). Add lubricants to the powder to reduce friction, and ensure the press has well-regulated exhaustion and floating die capabilities.
6. Conclusion
To minimize defects and enhance the rectangular wave energy capacity of ZnO varistors:
- Strictly control raw material purity and eliminate external contamination.
- Optimize the granulation process to ensure uniform particle size and hardness.
- Refine the molding process by regulating compression speeds, exhaustion delays, and ensuring die precision to achieve uniform density throughout the ceramic body.
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