Tape Casting for Multilayer Piezoelectric Ceramics: Key Technologies for Ultra-Thin Green Tape Fabrication

Tape casting is the core process enabling thin-film and large-area production of multilayer piezoelectric ceramics. It supports the preparation of uniform ceramic green tapes with a thickness of 5–50μm and sizes ＞100 mm×100 mm, effectively overcoming the technical limitations of dry pressing, which tends to crack when thinning below 50 μm and deform when the area exceeds 50 mm×50 mm.

This solution outlines a fully implementable tape-casting process from four perspectives: fundamental principles, process workflow, and parameter optimization.

I. Fundamental Principles of the Tape-Casting Process

The tape-casting process forms thin ceramic sheets through three steps: slurry leveling → drying and film formation → carrier release.

Ceramic slurry is uniformly coated onto a flexible carrier film (e.g., PET release film) with thickness controlled by a doctor blade. The coated film then enters a controlled drying chamber where solvents evaporate, forming a mechanically stable ceramic green tape. Finally, the dried green tape is peeled from the carrier and used as the base unit for multilayer stacking.

• Key performance advantages include:
• Thickness tolerance ≤ ±2%
• Surface roughness Ra ≤ 0.2 μm
• Batch-to-batch stability ＞98%

These characteristics make tape casting highly suitable for the fabrication of high-layer-count multilayer piezoelectric ceramics (e.g., 200–500 layers).

II. Core Tape-Casting Processes and Critical Parameters

The tape-casting workflow consists of three critical stages:

slurry preparation → tape casting → green-tape post-treatment,

and each stage directly determines the final quality of the thin sheet. Strict parameter control is required.

1. Slurry Preparation: Ensuring Leveling Behavior and Film-Forming Capability

Tape-casting slurry must exhibit the following characteristics:

• Low viscosity
• High solid-loading
• No agglomeration

The formulation and preparation process are designed to ensure excellent rheology, dispersion, and drying behavior.

Slurry Composition and Requirements (Example: PZT-Based Tape-Casting Slurry)

Preparation Procedure

• Premixing:

Add the dispersant into the solvent and stir for 10–15 minutes until fully dissolved.

• Powder Addition:

Add the ceramic powder in 3–5 batches, stirring 5 minutes after each addition to prevent agglomeration.

• Ball Milling / Dispersion:

Perform ball milling to achieve uniform dispersion of the slurry.

• Binder / Plasticizer Addition:

After ball milling, add the binder and plasticizer. Stir at low speed (100–150 r/min) for 2–3 hours to avoid bubble formation.

• Slurry Filtration and Deaeration:

Filter the slurry through a 100–200 mesh nylon screen to remove large particles, then conduct vacuum deaeration at –0.095 MPa for 30–60 minutes to prevent pinholes during tape casting.

2. Tape Casting: Controlling Thickness and Uniformity

This step is the core of the tape-casting process and requires precise equipment control throughout the coating – drying – peeling sequence.

Base Film Pretreatment:

Use a 50–100 μm PET carrier film, ensuring the surface is free of oil and contaminants.

Casting Parameter Settings

•Doctor Blade Gap:

Adjust according to the target green tape thickness

(blade gap = target thickness × 1.2–1.5, considering 15–30% shrinkage after drying).

Example: For a 20 μm green tape, set the blade gap to 24–30 μm.

• Carrier Film Speed:

1–5 m/min

– Too fast: insufficient slurry leveling

– Too slow: localized thickness increase

Speed must match drying rate.

• Casting Pressure:

0.1–0.3 MPa, ensuring the slurry completely fills the gap between the blade and the carrier film without missing areas.

Multi-Stage Drying

A 3–5 zone drying chambers with controlled temperature gradients is required to avoid cracking caused by rapid solvent evaporation:

Zone 1 (Entry): 40–50°C

Slow evaporation of surface solvent to prevent pinholes.

Zone 2–4: 60–80°C

Gradual evaporation of internal solvent; total drying time 10–20 min.

Zone 5 (Exit): 50–60°C

Moisture equalization to avoid warpage.

Key metrics:

Residual solvent content after drying: ≤0.5%

Thickness variation: ≤ ±2%

Green Tape Peeling

Use an automatic peeling roller with a peeling angle of 30°–45°.

The peeling speed must synchronize with the carrier film speed (1–5 m/min) to prevent tape breakage caused by excessive peeling force.

3. Process Optimization and Common Issue Mitigation

1. Core Process Optimization (Improving Efficiency and Quality)

• Eco-friendly Slurry Replacement:

Adopt water-based solvents (deionized water + ethanol) to replace traditional organic solvents (toluene, xylene).

Use water-based binders (e.g., acrylic resins), reducing VOC emissions by over 80%.

Dispersants (e.g., sodium polyacrylate) and drying temperatures (70–90°C) must be adjusted accordingly.

• Automation Integration:

Integrate the tape-casting machine with the subsequent laminator through an automated conveyor system.

After green tape cutting, the tapes enter the laminator’s vision-alignment module directly, minimizing contamination and mechanical damage caused by manual handling.

This improves yield by 5–8%.

Common Issues and Corrective Actions