Plasma Techniques for Ceramic Powder Synthesis

Fundamentals of Ceramic Materials

• Course title: Fundamentals of Ceramic Materials
• Instructor: Prof. Dr. Filiz Şahin
• Academic year: 2024-2025
• Semester: Fall
• Department: I.T.U Department of Metallurgical & Materials Engineering

Ceramic Material Processing

• Mining: Ore preparation
• Powder synthesis: Powder precipitation, Vapor phase synthesis, Sol-gel processing, Carbothermal reduction
• Powder Processing: Milling, Mixing with additives, Granulation, Die pressing, Isostatic pressing, Extrusion, Injection Moulding, Slip Casting, Tape Casting, Solid state sintering, Liquid phase sintering, Pressure assisted sintering, Hot Pressing, Hot Isostatic pressing, Spark plasma sintering
• Forming: Die pressing, Isostatic pressing, Extrusion, Injection Moulding, Slip Casting, Tape Casting
• Sintering: Solid state sintering, Liquid phase sintering, Pressure assisted sintering, Hot Pressing, Hot Isostatic pressing, Spark plasma sintering
• Surface finishing: Quality control
• Ceramic Products: Final ceramic products

Raw Materials Selection Criteria

• Purity
• Particle size
• Reactivity

Purity

• Purity strongly influences high-temperature properties like strength, stress rupture life, and oxidation resistance.
• For example:
  • Si₃N₄ + MgO: Ca is concentrated at grain boundaries and decreases the softening point; decreasing creep resistance.
  • Si₃N₄ + Y₂O₃: Ca is absorbed into the solid solution, which does not reduce the refractoriness of the system.
• Impurities as inclusions do not affect creep or oxidation, but concentrate stress and decrease component strength, whose effect depends on inclusion size relative to grain size and the relative thermal expansion and elastic properties of the matrix and inclusion.
• WC inclusions in Si₃N₄ have little effect on strength.
• Fe and Si have a large effect on mechanical properties

Particle Size

• Particle size distribution is important for consolidation and shaping techniques aiming for maximum particle packing and uniformity, minimizing shrinkage and retained porosity during densification.
• A single particle size does not ensure proper packing, often leading to over 30% void space.
• Adding particles of a size equivalent to the largest voids reduces the void content to 26%, while adding a third, even smaller size, can reduce the pore volume to 23%.
• Therefore, a range of particle sizes is required for maximum particle packing and uniformity.

Reactivity

• The primary driving force for densification at high temperature is the change in surface free energy.
• Very small particles with high surface areas have high surface free energies, driving the thermodynamic drive to decrease the surface area via bonding together.
• Very small particles (approximately 1µm or less) can be compacted into a porous shape and sintered to near-theoretical density.
• Transparent polycrystalline alumina for sodium vapor lamps is an example, where pores larger than 0.4µm must be removed during sintering to achieve transparency.

Types of Powders

• Primary particles: The smallest identifiable unit, can be crystalline or amorphous.
• Agglomerates: Clusters of bonded primary particles (soft or hard, depending on the bonding strength).
• Granules: Large agglomerates (0.1-1mm in diameter) often formed by adding a granulating agent (polymer binder), and produced through tumbling for nearly spherical form and free flow.

1- Mechanical Sizing Methods

• Screening
• Air Classification
• Elutriation
• Ball Milling
• Attrition Milling
• Vibratory Milling
• Fluid Energy Milling
• Hammer Milling
• Roll Crushing

1. Screening

• A sorting method based on particle size.
• Powder is placed on a screen with openings of varying sizes.
• Larger particles remain on the screens, while smaller particles pass through.
• Screen sizes are classified by mesh sizes (number of openings per linear inch).

2. Air Classification

• Used to separate coarse and fine fractions of dry ceramic and ceramic powders.
• Separation is achieved by controlling horizontal centrifugal force and vertical air currents.
• Particles enter along the centerline and are accelerated outward by centrifugal force.
• Coarse particles lose velocity and settle into a collection cone, while finer particles are carried upward and away by air currents.
• Not suitable for powder below 10 µm.

3. Elutriation

• General term for particle size separation based on settling rate, where larger or high specific gravity particles settle faster than smaller ones in a suspension.
• Typically used in labs for very fine particle distributions.
• The powder is mixed with liquid (with wetting agent) to yield a dilute suspension.
• The liquid containing fine particles is then decanted.

4. Ball Milling

• A widely used method for particle size reduction.
• Grinding media (balls, cylinders, or rods) and ceramic particles are placed in a closed cylindrical container and rotated horizontally.

5. Attrition Milling

• Similar to ball milling (cylindrical with grinding media).
• Grinding media is agitated by stirring arms, resulting in quicker and more efficient fine particle size reduction, and lower contamination compared to ball milling.

6. Vibratory Milling

• Substantially different from ball and attrition milling.
• Energy for comminution is supplied through vibration (usually bottom center of the chamber), causing cascading/mixing and fracturing of particles.
• Typically lined with polyurethane or rubber, minimizing contamination.

7. Fluid Energy Milling

• Particle size reduction by particle-particle impact in a high-velocity fluid (compressed air, nitrogen, carbon dioxide, superheated steam, water, etc.)
• Powder is added in chamber and accelerated to sonic velocities.
• Ideally designed to maximize particle-particle and minimize particle-wall impact, minimizing contamination.

8. Hammer Milling

• A rigid bar or plate rotates rapidly.
• Particles are dropped in the path, fragmented by impact, and further fragmented by striking the mill walls.

9. Roll Crushing

• A coarse crushing method using two hardfaced rollers that rotate in opposite directions to pinch and crush ceramic chunks.

2- Chemical Sizing Methods

• Techniques aimed at producing high purity powder with controlled size distribution, overcoming limitations of mechanical sizing methods (contamination).
• Includes Precipitation, Freeze Drying, Hot Kerosene Drying, Sol-Gel Process, Spray Roasting, Decomposition, Hydrothermal, Plasma, and Laser methods.

5- Spray Roasting

• Spraying fine atomized droplets of a solution of precursors (in water or other liquid) into a heated chamber.
• Evaporation of the solvent and chemical reaction at high temperature yield fine crystallite powder.

6- Decomposition

• Uses decomposition reactions (nitrates, sulfates, carbonates, etc.) at high temperatures to yield the oxide.

7- Hydrothermal Process

• Crystallization of a composition in hot pressurized water (100-350 °C, 15 MPa). Wide variety of fine-particle ceramic compositions can be synthesized.

8- Plasma

• High-purity, small particle size (10-20 nm) ceramic powders produced through high-temperature plasma environments.
• Two types: dc arc jet and rf induction systems.

9- Laser

• Produces controlled particle sizes of silicon and SiC using CO₂ laser and gas mixtures (silane, methane).
• Controlled energy input to form SiC particles directly in the gas stream.

5.5 Preconsolidation

• Compacting sized powders into desired shapes using techniques like pressing, slip casting, and injection molding.
• Uniformity requirements typically necessitate special treatments or processing steps before compaction to minimize flaws.

Additives for Preconsolidation

• Additives are required for different purpose depending on the specific forming process, including supplying green strength, lubricating, activating densification, influencing rheological properties, controlling particle dispersion, etc.  

Drying and Firing

• Necessary to remove liquid and organic additives.
• Shrinkage and distortion can be significant.
• Proper drying and firing is critical for achieving proper density, strength, and dimensional accuracy.

Wet Forming

• Slip casting: Produces very thin or thin-walled parts with high homogeneity, using a slurry of ceramic powder and liquid.
• Tape casting: Similar to slip casting, involves the spreading of slip onto a surface to form a thin tape.
• Freeze casting: Ceramic particles are suspended in water, frozen, the liquid is sublimated before firing.
• Gelcasting: Slurry of ceramic particles in a monomer solution, with monomer polymerization that leads to the immobilization of the particles.

Plastic Forming

• Extrusion: Plastic (binder-fluid) mixture is forced through a shaped die.
• Injection molding: Molten or liquid plastic is injected into a mold.

Dive into the world of plasma synthesis techniques used for ceramic powders. This quiz explores various methods, challenges, and considerations related to the purity and scalability of ceramic powder production. Test your knowledge on the specifics of each technique and their applications in material science.