Thermal Processing Flashcards
What are the three characteristics of sintering?
- shrinkage
- strengthening
- densification
What are the two different things you measure for sintering?
- density
- microstructure (grain size)
What does densification indicate? Coarsening?
densification —> shrinkage —>
On a grain size v. % of theoretical density graph, what do the lines for pure coarsening, coarsening + densification, and densifications followed by grain growth look like?
On a graph of grain size v. pore size, what do the lines for grain growth, densification, and coarsening look like?
What are the five different mechanisms by which sintering can occur?
- evaporation-condensation
- surface diffusion
- volume diffusion (from surface to neck or from grain boundary to neck)
- grain boundary diffusion
- viscous or creep flow
What kinds of processes does sintering occur by?
mass transfer process
What does the drying-burn out-sintering process look like?
What are some visuals for grain boundary diffusion?
What is sintering?
pore removal, mass transfer along grain boundaries to fill pores
What does a desirable pore look like? it’s characteristics?
- many mass transport paths
- pore too big to fit within grain
How should you think about a pore?
think about a pore as a roundabout and a car as mass being delivered to the pore
we want to increase the number of roads
What does a undesirable pore look like? its characteristics?
- few mass transport paths
- pore will fit within the grain
What is the equation you use to equate porosity?
What happens if grains grow too fast?
pores get trapped inside grains and are difficult to remove
What do desired homogenous microstructures look like?
How does sintering as a dynamic process start? What is diffusion?
diffusion becomes active at about 2/3 to 3/4 of the absolute melting temperature of the ceramic material.
Diffusion is like the movement of atoms within the material
What 7 things does the kinetics of sintering depend on?
- temperature
- time
- initial particle size and size distribution
- packing
- sintering atmosphere
- degree of agglomeration
- presence of impurities
TTiPSdi - tired tiggers involuntarily pounce slowly during invierno
Is full density difficult or easy to achieve?
difficult
How does the driving force of sintering compare to the driving force with chemical reactions?
sintering = small = few joules/mole
chemical reactions = large = few kJ / mol
Sintering is a process by which….
a powder compact is transformed to a strong, dense ceramic body upon heating
What are the two broad categories of sintering?
- solid-state sintering
- liquid-phase sintering
What are three benefits/characteristics of liquid-phase sintering?
1. reduces sintering temp by taking advantage of low melting eutectic composition
2. can sinter at temps where solid state diffusion is slow
3. material dissolves into liquid and precipitates out
What do the results of liquid-phase sintering look like?
What are the 6 different types of sintering?
- pressureless sintering
- hot pressing
- spark plasma sintering
- microwave sintering
- hot isostatic pressing
- reactive sintering
PHSMHR- please have some more hot ramen
What happens during pressure less sintering? (3 things)
- heating elements (graphite, MoSi2, ZrO2) radiation/resistance heat is transferred by thermal conductivity and convection to the sample
- no pressure is applied
- air/gas atmosphere/vacuum
What happens in hot pressing?
- heating elements (graphite, MoSi2, ZrO2) radiation/resistance heat is transferred by thermal conductivity and convection to sample (same as pressureless)
- pressure is applied (uniaxially)
- air/gas atmosphere/vacuum
What happens in spark plasma sintering?
- heat generated by high amperage pulse current
- pressure applied uniaxially
- gas atmosphere/vacuum
- very fast heating ramp and sintering cycle (minutes)
what happens during microwave sintering?
- heat generated by microwave radiation (conversion of electromagnetic energy to thermal energy) to provide volumetric heat of the whole piece
- air/ gas atmosphere/ vacuum
- energy savings and rapid sintering cycles
What happens during hot isostatic pressing?
- heating elements (graphite, MoSi2, ZrO2) radiation/resistance heat transferred by thermal conductivity and convection to sample
- pressure applied (isostatically)
- air/gas atmosphere/ vacuum
What happens during reactive sintering?
- new ceramic phase/composition is generated during the sintering process (chemical reaction)
- with or without applied pressure, by any of the other techniques mentioned before.
- air/gas atmosphere/vacuum
Which types of sintering include the step “heating elements (graphite, MoSi2, ZrO2) radiation/resistance heat transferred by thermal conductivity and convection to sample”?
- pressureless sintering
- hot pressing
- hot isostatic pressing
Which types of sintering don’t apply pressure?
- pressureless sintering
- reactive sintering (can be with or without)
Which types of sintering has pressure applied uniaxially?
- Hot pressing
- spark plasma sintering
Why is drying necessary?
to remove the solvent and moist from powder compact before sintering
Why does the drying rate have to be carefully controlled?
it can cause cracks, warpage, distortion, and flaws
What are pyrolysis and calcination?
steps required to remove binders, dispersants, surfactants, polymers, and any other organic compounds added to the ceramic powder mixture before sintering
What is the macroscopic driving force operating during sintering?
the reduction of the excess energy associated with surfaces?
What kind of furnace is this?
tube
what type of furnace is this?
pressureless furnace
What type of furnace is this?
hot pressing
(cards up to 21)
What are the three main steps in ceramic processing?
powders —> shaping —-> sintering
shaping: forming the green body
sintering: densification, dense body
At what temperature is the green body sintered at
2/3 of the absolute melting temp
What does sintering do?
Sintering densifies and strengthens the powder compact by diffusive mass transport which fills in the void space between the particels
What are the three steps you take before sintering?
- drying
- calcination
- pyrolysis
How are submicron ceramic powders produced?
sol-gel methods or milling of larger particles
How are shaping methods classified?
humidity content
What are 4 cons of drying green ceramic powder bodies?
- costly (requires energy) and time consuming
- drying results in shrinkage
- shrinkage can result in warpage (shape distortion)
- cracking
In general, how does drying of green body work?
Evaporation from the surface removes liquid
Capillary pressure drives densification and shrinkage
What defines slow drying?
the rate of evaporation is less than the rate of flow of liquid from the interior to the surface
No cracking because shrinkage is uniform and unconstrained
How does slow drying work?
- rate of evap < flow of liquid from interior to surface
- body reaches max consolidation and air/water interface enters the powder body
- evaporation removes the remaining water
- cracking can occur when the air/water interface enters the powder body, but cracking is unlikely
(extra) how do you check for the removal of organic material?
weight change
wt% v. T graph
DTA-TG machine
(extra) What do you need to check for burn-out?
- remove organic material (weight change)
- dimensions/shape/shrinkage
- microstructure
- composition
(extra) Why should you look at the microstructure during burn-out? How do you look for it?
you need to look for flaws, homogeneity, and the removal of organic things.
Use SEM
(extra) How do you check the composition?
XRD, EDS, XRF
(extra) What do you need to check for after sintering?
- shrinkage
- composition
- microstructure
- density
- mechanical/thermal/optical/electrical properties
(extra) What do you need to check for after drying and before sintering?
- shrinkage
- removal of organic material
- DTA-TG
(extra) how is shrinkage measured?
caliper, scanner, (~20%)
(extra) how is composition measured? What are you checking for?
XRD, EDS, XRF
Checking for: phases, secondary phases
(extra) how do you measure microstructure? What are you checking for?
SEM (microscopy- scanning), TEM (transmission electron microscopy)
Checking for: grain size, grain size distribution, grain boundaries, imperfections, porosity
(extra) What are you wanting when you measure density? how?
you want 100% of theoretical density / informs you about porosity
Archimedes
(extra) What do mechanical/thermal/electrical/ and optical properties need to be discussed in the context of?
density and grain size (distribution)
What is the qualification for fast drying? What does this mean for shrinkage?
Rate of evaporation > rate of flow liquid from interior to surface
Shrinkage is constrained
Explain how fast drying works?
- the air water interface enters the powder compact
- the top section wants to shrink but is constrained by the interior which can’t shrink since water can’t get out from the center to the surface
- tensile stresses develop in the surface as the top layer tries to shrink parallel to the surface
What forces appear in fast drying?
tensile forces (stretching/shrinking) happen in the surface as the top layer of inner water parallel to the surface results in mud cracks
What do the different sections of this diagram represent?
top layer (white & red) - drying surface that is shrinking
moist layer (blue & red) - not drying yet
What are the three stages of drying?
- constant rate period
- first falling rate period
- second falling rate period
What are the four driving forces for shrinkage?
- capillary pressure
- osmotic pressure
- disjoining pressure
- moisture stress
What are two transport processes for drying?
- darcy’s law
- diffusion
What is the constant rate period?
rate of evap per unit area is independent of time and there is shrinkage
what is the first falling rate period?
shrinkage stops. rate of evaporation decreases
What is the second falling rate period?
evaporation occurs inside the body
What are four different ways to avoid fracture in drying green ceramic powder bodies?
- slow drying
- humidity control
- solvents mixture
- additives
When do you need to use pyrolysis or calcination?
if green bodies have a certain amount of organic additives (dispersants, polymers, binders, solvents, etc.) you need to remove them
What is pyrolysis? What is it a typical stage for?
A thermochemical decomposition of organic material at elevated temperatures under a controlled atmosphere
typical stage for plastic routes
What is calcination? What is it a typical stage for?
A heat treatment normally in air or oxygen
typical stage for wet routes
At what temperature are heat treatments usually?
300-500C (the temp at which most organic compounds decompose)
What is sintering?
The process where a powder compact is transformed into a strong, dense ceramic body upon heating
What does sintering of ZrO2-Y2O3 look like with increasing temperature?
neck growth– clusters are bigger/ grain boundaries are formed
What is the macroscopic driving force operating during sintering?
the reduction of the excess energy associated with surfaces
What are the two ways excess energy associated with the surfaces can be reduced by? How do these two methods interact?
- Reduction of the total surface area by increasing the average size of the particles– leading to coarsening
- elimination of solid/vapor interfaces and the creation of grain boundary area and grain growth– leading to densification
they are in competition with each other
Which method to reduce the excess energy associated with surfaces is superior?
Densification – desirable
Coarsening – undesirable
What drives densification and shrinkage in drying?
Capillary pressure drives densification and shrinkage
