What is the primary function of aluminium foil in electrolytic capacitors?
Aluminium foil serves as the anode material in electrolytic capacitors, where its surface is chemically etched to increase surface area and then anodized to form a dielectric oxide layer (Al₂O₃). This oxide layer determines the capacitor's voltage rating and capacitance. The foil's purity (typically 99.99%) ensures minimal leakage current, while its thickness (5–100 μm) balances mechanical strength and energy density. The etched microstructure can achieve 100x geometric surface area expansion.
How does the etching process affect capacitor performance?
Etching creates microscopic tunnels/pores (1–10 μm diameter) using HCl or H₂SO₄ solutions, multiplying the effective surface area up to 200x. Deeper etching increases capacitance but reduces foil mechanical strength. The process requires precise temperature (60–80°C) and current density (0.1–1 A/cm²) control. Post-etching, a "formation" step grows the oxide layer under controlled voltage (e.g., 100V for high-voltage caps). Surface roughness (Ra 0.5–5 μm) is critical for adhesion.
What are the key differences between low-voltage and high-voltage capacitor foils?
Low-voltage foils (<50V) use aggressive etching for maximum surface area (capacitance), while high-voltage foils (>100V) prioritize thicker, more uniform oxide layers. HV foils are thicker (70–100 μm vs. 20–50 μm) with lower etch factors. They undergo multi-stage formation at progressively higher voltages (up to 600V). HV foils require higher purity (Fe/Si <50 ppm) to prevent dielectric defects. The oxide layer thickness scales with voltage (~1.4 nm/V).
Why is purity critical for capacitor-grade aluminium foil?
Impurities (Fe, Cu, Si >100 ppm) create weak spots in the oxide layer, leading to leakage current or short circuits. High-purity (>99.99%) aluminum ensures uniform anodization. Trace elements affect etching behavior-iron increases tunnel density but reduces uniformity. Special smelting (zone refining) and rolling (oil-lubricated mills) prevent contamination. Each 0.01% purity improvement can boost breakdown voltage by 5–10%.
How do foil parameters influence capacitor lifecycle and reliability?
Foil microstructure determines thermal stability-over-etched foils degrade faster at 105°C+. Oxide layer crystallinity (affected by formation temperature) impacts long-term dielectric losses. Edge burrs from slitting can initiate delamination. Modern foils use corrosion inhibitors (e.g., phosphate coatings) to extend humid environment performance. Accelerated aging tests (85°C/85% RH, 1000+ hours) validate foil durability.
