Kuliah_10_kxex 1110

CHAPTER

10 Applications and Processing of Ceramics 8-1

Fabrication and Processing of Ceramics

Glass •

Combination of transparency, strength, hardness and corrosion resistance. • Glass is an inorganic product of fusion that has cooled to a rigid condition without crystallization. • Glass does not crystallize up on cooling. • Up on cooling, it transforms from rubbery material to rigid glass. Figure 10.52

11-40

Glass Structure • Basic Unit: 4Si0 4 tetrahedron Si 4+ O2-

• Quartz is crystalline SiO2:

• Glass is amorphous • Amorphous structure occurs by adding impurities (Na+,Mg2+,Ca2+, Al3+)

• Impurities: interfere with formation of crystalline structure. Na + Si 4+ O2-

(soda glass)

•

In cristobalite, Si-O tetrahedron are joined corner to corner to form long range order. • In simple silica glass, tetrahedra are joined corner to corner to form loose network.

Composition of Glass •

Soda lime glass: Very common glass (90%).  71-73% SiO2, 12-14% Na2O, 10-12% CaO.

 Easier to form and used in flat glass and containers. • Borosilicate glass: Alkali oxides are replaced by boric oxide in silica glass network.  Known as Pyrex glass and is used for lab equipments and piping. • Lead glass: Lead oxide acts as network modifier and network former.  Low melting point – used for solder sealing.  Used in radiation shields, optical glass and TV bulbs.

Glass Modifying Oxides and Intermediate Oxides •

Network modifiers: Oxides that breakup the glass network.  Added to glass to increase workability.  Examples:- Na2O, K2O, CaO, MgO.

 Oxygen atom enters network and other ion stay in interstices. • Intermediate oxides: Cannot form glass network by themselves but can join into an existing network.  Added to obtain special properties.  Examples: Al2O3, Lead oxide.

Glass Properties • Specific volume (1/ρ) vs Temperature (T): • Crystalline materials:

Specific volume Liquid (disordered)

Supercooled Liquid

• Glasses:

Glass (amorphous solid) Crystalline (i.e., ordered)

Tg

-- crystallize at melting temp, Tm -- have abrupt change in spec. vol. at Tm

Tm

Adapted from Fig. 13.6, Callister, 7e.

solid

T

-- do not crystallize -- change in slope in spec. vol. curve at glass transition temperature, Tg -- transparent - no crystals to scatter light

Viscosity • Viscosity, η:

-- relates shear stress and velocity gradient: τ glass τ

dy dv

dv dy

dv τ=η dy

velocity gradient

η has units of (Pa-s)

Viscous Deformation of Glass • Viscosity decreases with T • Impurities lower Tdeform

Viscosity [Pa ⋅ s]

a ilic ds a se silic fu % x 96 yre e P -lim da so ss gla 10 14

10 10 10 6 10 2 1 200

• soda-lime glass: 70% SiO2 balance Na2O (soda) & CaO (lime) • borosilicate (Pyrex): 13% B2O3, 3.5% Na2O, 2.5% Al2O3 • Vycor: 96% SiO2, 4% B2O3 • fused silica: > 99.5 wt% SiO2

strain point annealing range Tdeform : soft enough to deform or “work”

Tmelt 600 1000 1400 1800 T(°C)

•

Viscous above Tg and viscosity decreases with increase in temperature.

η* = η0e+Q/RT

Q = Activation energy η* = Viscocity of glass (PaS) η0 = preexponential constant (PaS) • • • •

Figure 10.55

Working point: 103 PaS – glass fabrication can be carried out Softening point: 107 PaS – glass flows under its own weight. Annealing point: 1012 PaS – Internal stresses can be relieved.. Strain point: 10 13.5 PaS – glass is rigid below this point.

Forming Methods •

Forming sheet and plate glass: Ribbon of glass moves out of furnace and floats on a bath of molten tin. • Glass is cooled by molten tin. • After it is hard, it is removed and passed through a long annealing furnace.

Blowing •

Blowing: Air blown to force molten glass into molds. Compressed air

suspended Parison Finishing mold

Pressing

• Pressing: Optical and sealed beam lenses are pressed by a plunger into a mold containing molten glass. Gob

Parison mold

Pressing operation

Casting

• Casting: Molten glass is cast in open mold. • Centrifugal casting: Glass globs are dropped into spinning mold.  Glass first flows outward towards wall of mold and then upward against the mold wall.

Drawing

Continuous drawing – originally sheet glass was made by “floating” glass on a pool of mercury • Fiber drawing:

wind up plates, dishes, cheap glasses --mold is steel with graphite lining

Tempered Glass • • •

• •

Glass is heated into near softening point and rapidly cooled. Surface cools first and contracts. Interior cools next and contracts causing tensile stresses in the interior and compressive stress Figure 10.58 on the surface. Tempering strengthens the glass. Examples: Auto side windows and safety glasses.

Chemically Strengthened Glass

• Special treatment increases chemical resistance of glasses. • Example:- Sodium aluminosilicate glasses are immersed in a bath of potassium nitrate at 500C for 6 to 10 hours  Large potassium ions are induced into surface causing compressive stress.  Compressive layer is much thinner than that in thermal tempering.  Used for supersonic aircraft glazing and ophthalmic lenses.

Heat Treating Glass • Annealing: --removes internal stress caused by uneven cooling.

• Tempering: --puts surface of glass part into compression --suppresses growth of cracks from surface scratches. --sequence: before cooling

hot

surface cooling

further cooled

cooler hot cooler

--Result: surface crack growth is suppressed.

compression tension compression

2. Particulate Forming

Traditional Ceramics

A mixture of components used (50%) 1. Clay (25%) 2. Filler – e.g. quartz (finely ground) (25%) 3. Fluxing agent (Feldspar) binds it together aluminosilicates + K+, Na+, Ca+

Traditional Ceramics • • •

Clay: Provide workability and hardness. Silica: Provide better temperature resistance and MP. Potash Fledspar: Makes glass when ceramic is fired.

Quartz grain

High-silica glass

Engineering Ceramics •

Alumina (Al2O3): Aluminum oxide is doped with magnesium oxide, cold pressed and sintered.  Uniform structure. Used for electric applications.

•

Silicon Nitride (Si3N4): Compact of silicon powder is nitrided in a flow of nitrogen gas.  Moderate strength and used for parts of advanced engines.

•

Silicon Carbide (SiC): Very hard refractory carbide, sintered at 21000C.  Used as reinforcement in composite materials.

•

Zirconia (ZrO2): Polymorphic and is subject to cracking.  Combined with 9% MgO to produce ceramic with high fracture toughness.

Hydroplastic Forming • Milling and screening: desired particle size • Mixing particles & water: produces a "slip" Ao • Form a "green" component container --Hydroplastic forming:

force extrude the slip (e.g., into a pipe)

--Slip casting: pour slip into mold

absorb water into mold “green ceramic”

pour slip into mold

solid component

• Dry and fire the component

ram

bille t container

drain mold

hollow component

die holder extrusion

die “green ceramic”

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Adapted from Fig. 11.8 (c), Callister 7e.

Adapted from Fig. 13.12, Callister 7e. (Fig. 13.12 is from W.D. Kingery, Introduction to Ceramics, John Wiley and Sons, Inc., 1960.)

Feature of Slip Shear

• Clay is inexpensive • Adding water to clay -- allows material to shear easily along weak van der Waals bonds -- enables extrusion -- enables slip casting

• Structure of Kaolinite Clay:

charge neutral

weak van der Waals bonding 4+

charge neutral

Si 3+ Al OH 2O

Shear

Drying and Firing • Drying: layer size and spacing decrease.

wet slip

partially dry

Adapted from Fig. 13.13, Callister 7e. (Fig. 13.13 is from W.D. Kingery, Introduction to Ceramics, John Wiley and Sons, Inc., 1960.)

“green” ceramic

Drying too fast causes sample to warp or crack due to non-uniform shrinkage

• Firing: --T raised to (900-1400°C) --vitrification: liquid glass forms from clay and flows between SiO2 particles. Flux melts at lower T. Si02 particle (quartz) micrograph of porcelain

glass formed around the particle

70 µm

Adapted from Fig. 13.14, Callister 7e. (Fig. 13.14 is courtesy H.G. Brinkies, Swinburne University of Technology, Hawthorn Campus, Hawthorn, Victoria, Australia.)

Powder Pressing

Sintering - powder touches - forms neck & gradually neck thickens – add processing aids to help form neck – little or no plastic deformation Uniaxial compression - compacted in single direction

Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope

Hot pressing - pressure + heat

Sintering: useful for both clay and non-clay compositions. • Procedure: -- produce ceramic and/or glass particles by grinding -- place particles in mold -- press at elevated T to reduce pore size.

• Aluminum oxide powder: -- sintered at 1700°C for 6 minutes.

15 µm

Tape Casting

• thin sheets of green ceramic cast as flexible tape • used for integrated circuits and capacitors • cast from liquid slip (ceramic + organic solvent)

Cementation • Produced in extremely large quantities. • Portland cement: -- mix clay and lime bearing materials -- calcinate (heat to 1400°C) -- primary constituents: tri-calcium silicate di-calcium silicate

• Adding water -- produces a paste which hardens -- hardening occurs due to hydration (chemical reactions with the water). • Forming: done usually minutes after hydration begins.

Ceramics Glasses

Clay Refractories products

-optical -whiteware -bricks for high T -composite -bricks (furnaces) reinforce -containers/ household

Abrasives Cements

Advanced ceramics

-sandpaper -composites engine -cutting -structural -rotors -polishing -valves -bearings -sensors

8-2

Refractories • Need a material to use in high temperature furnaces. • Consider the Silica (SiO2) - Alumina (Al2O3) system. • Phase diagram shows: mullite, alumina, and crystobalite as candidate refractories. 2200 3Al2O3-2SiO2

T(°C) 2000

Liquid (L)

1800

mullite alumina + L mullite +L

crystobalite +L

1600 1400 0

mullite + crystobalite

20

alumina + mullite

40 60 80 100 Composition (wt% alumina)

• Acidic refractories: – Silica refractories have high mechanical strength and rigidity. – Fireclays: Mixture of plastic fireclay, flint clay and grog. Particles vary from coarse to very fine. – High aluminum refractories: Contain 50-90% alumina and have higher fusion temperature.

• Basic refractories: consists mainly of MgO and CaO. – Have high bulk densities, melting temperature and resistance to chemical attack. – used for lining in basic-oxygen steelmaking process.

Die • Die blanks: -- Need wear resistant properties!

die Ao die

• Die surface:

-- 4 µm polycrystalline diamond particles that are sintered onto a cemented tungsten carbide substrate. -- polycrystalline diamond helps control fracture and gives uniform hardness in all directions.

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tensile force

Cutting Tools • Tools: -- for grinding glass, tungsten, carbide, ceramics -- for cutting Si wafers -- for oil drilling

• Solutions: -- manufactured single crystal or polycrystalline diamonds in a metal or resin matrix. -- optional coatings (e.g., Ti to help diamonds bond to a Co matrix via alloying) -- polycrystalline diamonds resharpen by microfracturing along crystalline planes.

oil drill bits

blades coated single crystal diamonds polycrystalline diamonds in a resin matrix.

Sensors • Example: Oxygen sensor ZrO2 • Principle: Make diffusion of ions

Ca 2+

fast for rapid response.

• Approach:

A Ca 2+ impurity removes a Zr 4+ and a O2- ion.

Add Ca impurity to ZrO2: -- increases O2- vacancies -- increases O2- diffusion rate • Operation:

-- voltage difference produced when O2-

ions diffuse from the external surface of the sensor to the reference gas.

sensor gas with an unknown, higher oxygen content

O2diffusion

+

-

reference gas at fixed oxygen content

voltage difference produced!

Insulation •

About 70% of external surface is protected from heat by 24000 ceramic tiles. • Material: Silica fiber compound. • Density is 4kg/ft3 and withstands temperature up to 12600C.

Figure 10.51

Ceramic Insulator Materials •

Ionic and covalent bonding restricts the mobility of ions and electrons and hence ceramics are good insulators. • Electrical porcelain: 50% Clay + 25 % Fledspar.  Good plasticity, wider firing temperature range, cheap.  High power loss factor. • Steatite: 90% talc + 10 % clay  Good insulator, low power loss factor, impact strength no alkali ions • Fosterite: Mg2SiO4  Higher resistivity, low electrical loss • Alumina: Al2O3 Crystalline phase bounded to glassy matrix.  High dielectric strength, low dielectric loss

Capacitors

• Ceramics are used as dielectric materials for capacitors. • Example: Disk ceramic capacitors.  BaTiO3 + additive  Very high dielectric constant  Used in ceramic based thick film hybrid electronic circuit  Higher capacitance per unit area

Figure 10.38a

Semiconductors • • • •

Ceramics can be used as semiconducting materials. Thermistor: Thermally sensitive resistor. NTC thermistor: Conductivity raises with temperature. Solid solution oxides of Mn, Ni, Fe, Co and Cu are used to obtain necessary property ranges. • By combining low conducting metal oxide with low conducting oxides intermediate properties are obtained. • Example: Conductivity of Fe3O4 is reduced gradually by adding increasing amounts in solid solution of MgCr2O4

Electronic Packaging • Chosen to securely hold microelectronics & provide heat transfer • Must match the thermal expansion coefficient of the microelectronic chip & the electronic packaging material. Additional requirements include: – good heat transfer coefficient – poor electrical conductivity • Materials currently used include: • Boron nitride (BN) • Silicon Carbide (SiC) • Aluminum nitride (AlN) – thermal conductivity 10x that for Alumina – good expansion match with Si

6. Advanced Ceramics

Heat Engines • Advantages: – Run at higher temperature – Excellent wear & corrosion resistance – Low frictional losses – Ability to operate without a cooling system – Low density

• Disadvantages: – Brittle – Too easy to have voidsweaken the engine – Difficult to machine

• Possible parts – engine block, piston coatings, jet engines Ex: Si3N4, SiC, & ZrO2

• Ceramic Armor – Al2O3, B4C, SiC & TiB2 – Extremely hard materials • shatter the incoming projectile • energy absorbent material underneath