Ceramics

Examples of engineering, dielectric, optical ceramics

Engineering: oxide: Al2O3, non oxide: SiC Dielectric : strontium titanate (ST), calcium titanate (CT), magnesium titanate (MT), Optical ceramics: Al2O3, YAG, Nd: YAG, Yttrium oxide (Y2O3)

Examples of ceramics with ionic bonding

MgO, CaO, Al2O3, ZrO2

Examples of ceramics with covalent bonding

SiC, TiC, AlN, Si3N4

Give a basic differences between the monocrystalline oxide compound and ceramics

Monocrystalline oxide is not necessarily sintered, Ceramics are sintered material and we know the way of the uprising

Examples (3) of advanced ceramics

Si3N4, Zr2O3, SiC

Examples (3) of traditional ceramics

faience (quartz+ feldspar), porcelain (kaolin+quartz+feldspar), stoneware (stoneware products are produced from clay raw materials)

Main stages of ceramic technology

Raw materials → Chemical treatment → Milling → Batching → Mixing → Forming → Drying → Sintering → Final machining

What parameters are essential for characterization of the given powder?

particle size (granulometry), particle shape (morhpology), deegre of fragmentation, chemical composition, purity, degree of crystallinity and concentration of defects

How the surface energy of powder could be decreased?

Surface energy can be reduced by adding wetting agents

Difference between an aggregate and an agglomerate

Aggregate - a collection of crystallites of common borders, An agglomerate - a collection of crystallites or aggregates that contact each other pointwise

Difference between an agglomerate and a granule

The agglomerate is a spontaneously formed porous form composed of crystallites or aggregates contacting each other pointwise. Granula is a powder specially formulated in agglomerates with regular shapes, minimized porosity, created under conditions that allow slipping and dense packing.

Types of bonding in granules

Van der Waals or resulting from binder, electrostatic bonding

Types of bonding in agglomerates

Electrostatic or van der Waals, polymer

List examples (3) of natural raw materials for ceramic industry

Clay, Silica, Kaolinite

List the names (2) of natural hydroxides of alumina-silicates

Kaolinite (Al4[Si4O10](OH)8), Illite K0.6-0.85Al2(Si,Al)4O10(OH)2

Give examples (2) of equipment for crushing brittle raw materials

Hammer Crusher,, Jaw Crusher

Give examples (3) of mills used in ceramic industry

Ball mill, Roller mill, Atritor mill

Could you explain a term “calcination”

Calcination is a heat treatment in order to decompose the materials especially carbonate to its oxide state. It’s use only to simple oxides, is a endothermic reaction,

Give an example (1 reaction) of solid state synthesis of mixed oxides

CaO + TiO2 -&rt; CaTiO3 (perovskite, ferroelectric)

Give an example (1 reaction) of carbothermal synthesis of non-oxide ceramic powder

Si + C = SiC (???) Si3N4 + 3C ⇌ 3SiC + 2N2 (???)

Obligatory requirement for self-propagating high-temperature synthesis (SHS) powder

exothermic reaction - the rate of heat losses from the closed system is slower from the rate of the heat evolution: borides, carbides, nitrides, silicides - ignition

Give examples of chemical methods of ceramics powders manufacturing

Wet chemical methods: -spray frying; - freeze drying; - co-precipation; - pecchini method with critic acid; - hydrothermal methods; - SOL-GEL

Description of spray-drying method fo ceramic powder

Spray-drying is a process of producing a granulates ,flowable powder of uniform bulk density. It is process by which fluid feed material is transformed in a dry powderby spraying the feed into a hot drying medium (gas) EVAPORATION -&rt; PRECIPITATION -&rt; DRYINGE -&rt; DECOMPOSITION

Description of freeze drying method for ceramic powder

Freeze-drying is used to create a formable powder from a sprayed slurry mist. Freeze-drying creates softer particles with a more homogeneous chemical composition than traditional hot spray drying, but it is also more expensive.

.List the reagents (3) in sol-gel method for ceramic powder production

- water - alkoxides - acid

Principles of ceramic nanopowder production via gas-phase methods

EVAPORATION-CONDENSATION, DECOMPOSITION OF GASEOUS COMPOUNDS, REACTIONS BETWEEN GAS PHASES

What does it mean: “rheology”

RHEOLOGY: hydrodynamics of dispersed systems: solid+liquid

Parameters describing powder properties(3)

Particle size distribution, Apparent mean size, Specific Surface area (SSA)

Why powder particles from agglomerates?

Because it wants to reduce the surface energy

How water presence affect Hamaker’s constant of powder?

Water presence reducecs Hamaker's constant

Consider the role of water in solid powder particles behaviour

- Wetting the surface of ceramic particle and decrease of the friction coefficient - Introduction of liquid with high viscosity among powder particles - Dissolution of polymers - Dispersion of solid particles in the liquid

What is the meaning of “zero point charge” in suspension of powder particles

Isolectrical point = point of zero charge (pzc) occures at pH, where: [MO-] = [MOH2+]

Describe electrical double layer (EDL)

EDL is of a given thickness; the effective diameter of a particle is higher if measured with EDL EDL shows the effective chargé ζ Both, the thickness and the chargé of EDL can be changed via pH It is convenient to measure pH value for zeta potential ζ=0 (isoelectrical point)

How DLVO describes behaviour of particles in the liquid medium?

Particles could produce attractive and repulsive forces. If the repulsive forces are stronger then attractive a suspension is stable, if not particles coagulate

What is the origin of electrostatic repulsion of ceramic particles in the polar liquid?

electrostatic repulsion forces: repulsion of two identical charged ions

What are the means of suspension stabilization?

- pH-changing agents - polymer - sulfactants

Requirements (3) for polymer stabilization of suspension

-efficient thickness of polymer layer - total cover on particle surface by polymer - effective anchoring of polymer on particle Surface

What is the role of surfactants in stabilization of suspension?

Increase distance between particles suspension

How can you describe “a stable suspension”?

- Sedimentation is very slow - Sediment is well packed - The liquid over a sediment is „misty/cloudy” – it contains fine particles

List the types (4) of intergranular bonding of particles in water presence

a) bridged b) suspended c )capilary d) droplet

41. Give the stages of granules formation from the solution

Sprying solution -&rt; Powder -&rt; Wetting and absorbing -&rt; Solidified -&rt; Solid Briedge -&rt; Final Granules

List the main constituents (4) of suspension for casting

Solvent - controls viscosity, Binder - improves mechanical properties, Plasticizer- it gives elasticity, Defflocculant – it keeps dispersion of particles

Give the names of various casting (3)

Slip-casting a ceramic part, Centrifugal casting of ceramics, Tape casting

Describe the meaning of “slip” and “slurry”

Slip – water suspension of fine clay particles Slurry – complex suspension of fine powder in water or in solvent

Stages of slip casting to porous moulds

Making the slip -&rt; Pouring suspension (slip) into a mould of plaster -&rt; Water passes via capillary action into porous plaster -&rt; Sintering

Why the casted objects shrink during drying?

because of water/solvent removal

How water is removed from the casted object during casting to non-porous (plastic) moulds?

The water is removed from the casted thanks to rotation of the mould

Characterization of tape casting

Tape casting has been used to produce thin layers of ceramics. The Tape Casting process involves the casting the slurry onto a flat moving carrier surface. The slurry usually consists of a ceramic powder with the appropriate additions of solvents plasticisers and binders.

Why the thin ceramic foil can be produced from the viscous suspension?

Selection of the suitable composition of the suspension assures its good fl uidity and obtainment of the thin homogenous layers with the uniform structure

Which parameter control viscosity of ceramic suspension?

Amount of Solvent

Give an example of application of thin ceramic foil in industry

Fuel cells, Structural ceramics, Medical electronics

Give the principle of gel casting of ceramic slurries

Gel casting – the process is a method of forming a ceramic powder by in-situ polymerisation. Slurry is mixed with monomer, and then polymerisation begins.

Explain the term “ceramic additive manufacturing”

Ceramic component directly from CAD file. Components are produced in successive layers with a laser that polymerizes a paste made of photosensitive resin and ceramic. Time saving, cost effective (without moulds)

54. Describe stages (3) of injection moulding of ceramic slurries

• Design of the feedstock (ceramic powder, binder) • Injection forming • Debinding

Main components of ceramic slurry for injection moulding

• Main polymer • Plasticizer • Lubricant • Surfactants

Describe ceramic components made via injection moulding (examples, application)

• Al2O3; ZrO2 – Nozzles • Si3N4, sialon - Fuel injection valves, parts of shafts, rotors, valves • SiC, ZrO2 Si3N4, sialon - Parts of the fuel pumps,

List methods (3) of ceramic powder compaction

Uniaxial, isostatic, vibrational

Advantages (3) of powder uniaxial pressing in comparison to other methods of shaping

High yield, Simple, No shrinkage during drying

Disadvantages (3) of the powder uniaxial pressing in comparison to other methods of shaping

Shape limitations, Height limitations, Density non-homogeneity

List stages (3) of the powder uniaxial pressing

powder --&rt; perforation --&rt; green body

Give the reasons of granules supremacy over the loose powder during uniaxial pressing

Loose powder increases the risk of receiving defects like cracks or delamination during the uniaxial pressing. We can minimize those defect appearance by using granules that have wider porosity gauge than loose powder so they provide better way out for gas phase during the material densification.

How the stiff mould/die walls affect compaction behaviour of ceramic powder?

• Improves cohesion of product • Eliminates majority of internal pores • Improves mechanical properties (fatigue durability) • Helps to provide isotropic structure

COŚ TAM FRICTIONAL FORCES

Frictional forces between the compact and the die wall lead to characteristic stress gradients and density gradients in the compact. It is observed for single action compaction, using a particular die set and powder, that there is a critical compact depth below which no significant densificatnion occurs

What is the reason of nonhomogeneous distribution of density in the ceramic compact after uniaxial pressing?

Reason: friction between powder granules and the mould’s walls.

What kind of ceramic green bodies are preferentially shaped via uniaxial pressing?

Uniaxial pressing allows us to receive only simply shapes of green bodies such as cuboids, cylinders, or prisms with various simple shaped base. That shape of moulded pieces is determined by a way of pressure application used in that method of pressing.

Give the reasons (3) of advantage of isostatic pressing over the uniaxial one

• Isostatic pressing allows us to produce more sophisticated shapes of press moulds • Isostatic pressing gives us homogenous stress distribution that minimalizes friction between granules and mould walls and reduces the “density fade” effect. • In Isostatic pressuring we don’t use mould lubricants that could contaminate the products of pressing

Why cold isostatic pressing allows the trouble-free discharge of the green body from the mould?

Due to the elastic relaxation upon pressure relief the metal part shaping the inner contour of the hollow body can be pulled out.

Give the examples (2) of a dynamic compaction

• Dynamic consolidation of powders using a pulsed energy source (mechanical) • Shock compaction by use of high velocity gas stream

What is the meaning of “densification” of the green ceramic body

Elimination of porosity by mass transport mechanisms

List the most important macroscopic changes (3) of the green ceramic body after sintering

Dimensional changes - shrinkage, Property change eg. strength properties , Mass change.

List the most important microscopic changes (3) of the green ceramic body after sintering

Changing the shape of grain boundaries. Changing the shape and size of the grains. Elimination of porosity

Describe stages of sintering

Sintering = consolidation + compaction + recrystallization Consolidation - Increasing grain boundary surface area Compaction - Elimination of porosity by mass transport mechanisms Recrystallization - Grain growth

List reasons (2) of driving force during sintering

Reduction of enthalpy of free space taking into account volumes (v), limits of paragraph (b) and surface (s). The driving force of sintering is a reduction of Gibbs free energy (chemical potential or free enthalpy) by replacing high-energy free surfaces (solid-vapor) by low-energy grain-boundaries (solid-solid) and finally by minimizing also the grain boundary area (via grain growth)

What is the reason of pores elimination during sintering?

The cause of elimination of pores during sintering is mass transfer

Why the pore coordination number is significant in densification of the materials?

Properties such as compact strength or electrical conductivity depend on the coordination number

How Ostwald ripening effect modifies sintering process?

Oswald effect: the small grains become smaller, and the large larger, it leads to increase medium grains size

The role of liquid phase during sintering

Reduction of sintering rate

List the techniques of sintering methods

• Solid-state sintering • Liquid-state sintering • Reaction sintering • Constrained sintering

Give the advantages of hot press-sintering

• Low level of porosity in obtained ceramics, • Gives high dimensional precision of produced ceramics, • Low strains help to suppress grain growth in produced ceramics, • Allows to sinter powders with high melting temperatures (rougly up 2000oC), • Allows to produceMMC composites and cermetallics.

Why hot-press sintering is not a common industrial practice?

Hot-press sintering isn’t a common industrial practice because of lack of proper material for dies manufacturing. In common use are dies made of graphite,because it conducts heat/electricity and stands high temperatures used in process. Unfortunately mechanical properties of graphite are low and its wear resistance is even worse. Because of that there is big need for frequent exchange of dies and it makes cost of the process quite high.

Give examples (2) of new methods for fast sintering

- Spark plasma sintering - Microwave sintering