Powder Technology Flashcards
What are primary particles
individual objects of mass
What are three reasons why ceramic process is a powder processing?
- very high melting temp so casting isn’t economical
- casting makes large grains that reduce strength and ceramics are brittle
- fine grain microstructures can be produced by sintering
What is the ceramics processing scheme?
powders —> shaping (forming green body) —-> sintering (densification, dense body)
How are submicron ceramic powders produced?
sol-gel methods or milling of larger particles
At what temperature is the green body fired/sintered?
2/3 the absolute melting temperature
For the shaping step of the ceramic processing scheme how does humidity content behave?
humidity increases from dry route, plastic route, and wet route
What does sintering do in the ceramic processing scheme?
densifies and strengthens the powder compact by diffusive mass transport, which fills the void space between the particles
what do particles look like in solid, liquid, and gas
solid - typical particles
liquid - droplets (emulsion)
gas - foam
what is an agglomerate
mass of fine particles clustered together
what are granules
numerous particles forming a larger unit, more defined shape
what is a powder
a group of particles
define granular material
group of granules
What are three different particle characteristics that we care about
- composition
- shape
- size
What is “composition” for particle characteristics
- one chemical phase of multiphase
- determines density, conductivity, chemistry
- dense, porous
What is “shape” for particle characteristics?
- particle packing and properties
- regular: mathematical equations
- irregular: relation to regular particles through another parameter
what “size” is important for particle characteristics. what is large? small?
surface area to volume ratio
large (>10 microns) - body forces dominate: F=ma
small (<10 microns) - surface forces start to be more relevant
What number characterizes particle size
d
What is the volume of a sphere? area? circumference of a circle? area?
sphere
- volume : 4/3 pi r^3
- area: pi d^2
circle
- circumference: pi*d
- area: pi d^2/4
what is volume diameter?
d v , d of a sphere with the same volume as the particle
what is surface area diameter?
d s, d of a sphere with the same surface area as the particle
What is surface-volume diameter?
d sv, d of a sphere with the same surface area to volume ratio as the particle
What is sieve diameter
size of a sieve which the particle passes throgh
what is stokes diameter
d st, d of a sphere that settles a the same velocity as the particle
what is projected area?
d proj, d of a circle that has the same area as the 2D image of the particle
what is perimeter diameter?
d perimeter, d with the same perimeter in 2D image
How do you convey particle size distribution on a graph? (PSD)
Cumulative distribution? differential distribution?
% versus particle size (x) on a histogram
cum distribution is like a 3rd root graph going up
diff distrib is like the slope of the histogram
what is a monodisperse particle size distribution?
all particles are the same size
what is a monomodal particle size distribution?
only one peak, all particles are similar size
what is bimodal particle size distribution?
two peaks, particles of two different sizes
how do you identify the mode in a PSD
most frequent, peak in the distribution
how do you identify the median in PSD?
size where 50% particles are small size and 50% are larger
determine from the percentage of the cumulative distribution of particles
how do you identify the mean in PSD?
add them all up and divide
What are four different types of distributions that aren’t particle size and mass?
- number
- volume
- surface area
- intensity
how could you convert from one distribution to another?
shape factors
What are the 7 different methods for determining particle size (also agglomeration)?
- microscopy
- sieving
- sedimentation
- centrifugation
- permeation
- resistive pulse
- light scattering
Mari Said sarina can probably run long
What are the characteristics of microscopy?
- you observe and visually measure particle size/ have to sample thousands- tedious
- gives projected 2D image diameters
- optical (down to micro) or electron (down to nm)
***good to use with other techniques– to see agglomeration
What are the characteristics of sieving?
- good for particles >25micro
- particles passed through sieves w diff sized hole screens/each one is weighed to determine distribution
- cons: particles w/high aspect ratio can pass through (L/D or D/t)
How does sedimentation work?
- determines particle size by the rate at which particles settle in a fluid
- stokes law- the settling velocity of a particle falling in a fluid
- example: andreason pipette
How does centrifugation work?
- centrifugal acceleration
- stokes diameter
How does permeation work?
- fluid flows through a packed bed of particles
- mean of surface distribution found from the permeation
- Carman equation
How does the resistive pulse technique work?
- voltage pulse as a particle flows through an opening
- amplitude of pulse varies w particle volume
cons: limited by smallest orifice/errors if two particles pass at the same time
How does light scattering work?
- laser light used
- light scattered at different angles
- scattering intensity depends on the angle– function of the angle
What are most solids processed as?
particulates– like 60%
How do you achieve the right particle size/distribution?
- mechanical size reduction
- very energy intensive
- after they reach the grind limit, it’s furthered by the synthesis of particles
What properties are size separation and classification methods based on?
- shape
- size
- density
What is classification?
the separation of particles according to their settling rate in a fluid
What are the 5 different classification methods?
- screening (and sieving)
- sedimentation
- centrifugation
- dry (cyclones)
- wet (suspensions-hydrocyclones)
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