Processing of Ceramics, Glass, and SuperconductorsSuperconductors

Introduction

Ceramics are compounds of metallic and nonmetallic elements.

Derived from “Greek” word keromos – potter’s clay

Keramikos – clay products.

Properties of Ceramics:Properties of Ceramics:

• Low density

• high hardness

• high wear resistance

• low ductility

• High corrosion resistance

• Low thermal conductivity

• Low electrical conductuivityLow electrical conductuivity

C i d GlCeramic and Glass Parts

Figure 18.1 Photos of typical (a) ceramic and (b) glass parts. Source: Courtesy of Sanwa

(a) (b)

Figure 18.1 Photos of typical (a) ceramic and (b) glass parts. Source: Courtesy of Sanwa Components USA. (b) Courtesy of Corning Glass Works.

Shaping of Ceramics

1- First, the raw materials must be ground or crushed down into fine particlesparticles.

2- Next, the particles must be mixed with additives, which include:binder to hold particles togetherbinder- to hold particles togetherlubricant- to reduce friction and aid in removing from moldwetting agent- to improve mixing process (commonly water)plasticizer to improve ease of forming mixtureplasticizer- to improve ease of forming mixtureagents- control of foaming and sinteringdeflocculent- to create uniform mixture by applying like

h t ll ti l i th t lcharges to all particles, causing them to repel each other. Na2CO3, Na2SiO3

3 Fi ll h i l b h d d i d d fi d3- Finally, the material must be shaped, dried, and fired.

Steps in Making Ceramic PartsSteps a g Ce a c a ts

Figure 18.2 Processing steps involved in making ceramic parts.

Characteristics of Ceramics ProcessingC a acte st cs o Ce a cs ocess g

Slip CastingS p Cast g

• A slip is a suspension of ceramic particles in a liquid, generally water.

• In this process the slip is poured into a porous mold made of plaster p p p p pof paris.

• The slip must have sufficient fluidity and low viscosityThe slip must have sufficient fluidity and low viscosity.

• After the mold absorbed some of the water from the outer layers of the suspension it is inverted and the remaining suspension isthe suspension, it is inverted, and the remaining suspension is poured out.

Th th t f th t i t i d th ld i d d th• Then the top of the part is trimmed , the mold is opened, and the part is removed.

Slip Casting

Slip Casting (Drain Casting)-The crushed particles are first mixed with water, then are poured into a mold., p

Pouring must be done properly to avoid air pockets.

When some of the water has been absorbed, the remainder of the mixture is poured out of

the top of the mold.

The top of the part can then be trimmed.

Advantages- inexpensive components

Disadvantages- limited control of dimensions & low production rate

Slip-casting a Ceramic PartS p cast g a Ce a c a t

Figure 18.3 Sequence of operations in slip-casting a ceramic part. After the slip has been poured the part is dried and fired in an oven to give it strength and hardness Source:poured, the part is dried and fired in an oven to give it strength and hardness. Source: After F. H. Norton

Doctor-Blade Process

Figure 18.4 Production of ceramic sheets through the doctor bladethrough the doctor-blade process

Doctor-Blade Process- Used to produce ceramic sheets of thinner than 1.5mm.p

Ceramic mixture is forced under a blade to create a film, which is then dried in a drying chamber (usually attached to the same machine).

Extruding and Jiggeringt ud g a d J gge g

Figure 18.5 (a) Extruding and (b) jiggering operations. Source: After R F StoopsSource: After R. F. Stoops

Forming and Shaping of Glassg p gFlat Sheet and Plate Glass

Float Method- Molten glass is floated over a “bath” of molten tin before it is solidified in a separate chamber.

No additional finishing is necessary.No additional finishing is necessary.

Drawing Process- Molten glass is squeezed through two rolls, then

moves on o two smaller rollsmoves on o two smaller rolls.

Rolling Process- Similar to drawing process, but patterns are commonly imprinted from the rolls onto the glass, leaving a rough finish.

Molten Glass

Glass Tubing and RodsGlass Tubing and Rods

Tubing- Molten glass is wrapped around a mandrel and taken out by two rolls.g g pp y

Air is blown through the mandrel to prevent the tube from collapsing into itself.

Some machines manufacture 2000 fluorescent light bulbs per minute using this method.

Rods- Rods are made in exactly the same way,

but without the air blown through g

the mandrel.

This allows the glass to collapse and

become solid.

Discrete glass products

Processes used to make discrete glass objects• Blowing• Pressingg• Centrifugal casting• Sagging

Blowing

• Blowing process: Blown air expands a hollow gob of heated glass against the inner walls of a mold.

• A parting agent (such as oil or emulsion) is usually used to prevent the glass from sticking to the mold.

Steps in Manufacturing Glass Bottles

Figure 18.10 Stages in manufacturing an ordinary glass bottle.

Blowing

• Blow and blow process: After blowing a second blowing operation can be

d f fi li i d tused for finalizing product shape.

Blowing

• Applications: Hollow and thin-walled glass items (bottles, vases, and flasks)

• Surface finish: Acceptable for most applications

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Pros and cons of blowing

• Pros: Very economical for high-rate production.

Example: Highly-automated blowing machines can make around 2000 incandescent light bulbs per minute.

• Cons: Difficult to control the wall thickness of the productthe product

Float Methodoat et od

Figure 18.7 The float method of forming sheet glass. Source: Courtesy of Corning Glass Works.

Drawing Process and Rolling Processa g ocess a d o g ocess

Figure 18.8 (a) Drawing process for drawing sheet glass from a molten bath. (b) Rolling process. Source: After W. D. Kingery.

Glass Tubing ManufacturingG ass ub g a u actu g

Figure 18.9 Manufacturing process for glass tubing. Air is blown through the mandrel to keep the tube from collapsing. Glass tubes for fluorescent light bulbs are made by this method.

Manufacturing Glass by Pressing into Molda u actu g G ass by ess g to o d

fFigure 18.11 Manufacturing a glass item by pressing glass into a mold. Source: Courtesy of Corning Glass Works.

Pressing Glass into a Split Moldess g G ass to a Sp t o d

Figure 18.12 Pressing glass into a split mold. Source: After E. B. Shand.

Centrifugal casting process

• Centrifugal casting process: The centrifugal force pushes the molten glass against the wall.TV i t t b d i il• TV picture tubes and missile nose cones can be made with centrifugal casting.

Centrifugal Casting of GlassCe t uga Cast g o G ass

Figure 18.13 Centrifugal casting of glass. Large telescope lenses and television tube funnels are made by thislenses and television-tube funnels are made by this process. Source: Courtesy of Corning Glass Works.

Pressing

• Pressing process: A gob of molten glass is placed into a mold and pressed by a plunger into a confined shape.

• Molds may be one piece or split. Solidifying glass acquires the shape of the mold plunger cavitythe mold-plunger cavity.

• Similar to closed-die forging.

Pressing

• One-piece molds cannot be used in pressing if the plunger cannot be retracted.

• One-piece molds cannot be used for thin-walled itemsOne piece molds cannot be used for thin walled items• Split molds can accommodate thin-walled products

Pressing

• Pressing can produce higher dimensional accuracy than blowing.

Press and blow process

• After a part is pressed, it is blown to further expand the glass into the mold.

Sagging

• Sagging process: A sheet of glass is placed over a mold and heated. The glass sags by its own weight and takes the shape of the moldand takes the shape of the mold.

• Typical applications include dishes, sunglass lenses, mirrors for telescopes, and lighting panels.p g g p

Glass ceramics manufacture

• Trade names: Pyroceram, Corningware• Contain large proportions of several oxides.

Manufacturing involves a combination of methods• Manufacturing involves a combination of methods used for ceramics and glasses.

• Shaped into discrete products (such as dishes and baking pans) then heat treated.

• After heat treating glass is devitrified (recrystallized).

Glass Fibers

• Continuous glass fibers are drawn through multiple orifices (200 to 400 holes) in heated platinum plates at speeds as high as 500 m/s (1700ft/s).

• Fiber diameters as small as 2µm (80µin.)• Coated with chemicals to protect fiber surface.• Short fibers (chopped) are made as compressed air or steam

th fib it th h th ifipasses the fiber as it passes through the orifice.

Glass Fibers - Glass wool

• Glass wool is short glass fibers.• Glass wool is used for thermal and acoustic

insulationinsulation.• Made by a centrifugal spraying process. Molten glass

is ejected (spun) from a rotating head.• Glass wool fiber diameter is typically 20 to 30 µm

(800 to 1200 µin.)

Techniques for Strengthening and AnnealingTechniques for Strengthening and Annealing Glass

• Glass can be strengthened by thermal tempering, chemical tempering, and laminate strengthening.Fi i hi ti b d t i t d i d• Finishing operations can be used to impart desired properties and surface characteristics.

Thermal Tempering

• Surfaces of the hot glass are cooled rapidly by a blast of air. The surfaces solidify and are forced to contract as the bulk of the glass begins to coolas the bulk of the glass begins to cool.

• Surfaces develop residual compressive stresses.• The interior develops tensile stresses.The interior develops tensile stresses.• Compressive surface stresses improve the strength

of the glass.

Thermal Tempering

Chemical Tempering

• The glass is heated in a bath of molten KNO3, K2SO4, or NaNO3, depending on the type of glass.

• Ion exchanges take place and larger atoms replace smaller atoms on the surface of the glass.g

• Residual compressive stresses develop on the surface.

Laminated Glass

• Glass is strengthened through a method called laminate strengthening.

• Two pieces of flat glass have a thin sheet of tough plastic in between.

• When the glass is cracked, its pieces are held together by the plastic sheet.

Bulletproof Glass

• Bulletproof glass basically consists of glass laminated with a polymer sheet.

• Thickness ranges from 7 to 75 mm (.3 to 3 in.) Thinner glass isThickness ranges from 7 to 75 mm (.3 to 3 in.) Thinner glass is for handguns and the thicker glass is for rifles.

Finishing Operations

• Annealing removes residual stresses by heating the glass to a certain temperature and then cooling gradually.

• Annealing time ranges from a few seconds to 10 months.• Glass products may be cut, drilled, ground, and polished.• Care should be exercised in all finishing operations to ensure

there is no surface damage.

Design Considerations for Ceramics andDesign Considerations for Ceramics and Glasses

• Ceramic and glass products require careful selection of composition, processing methods, finishing operations and methods of assembly with otheroperations, and methods of assembly with other components.