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Powder Characterizations

Characterizations

Physical

Characteristics

Chemical

Composition

Phase

Composition

Surface

Characteristics

1. Particle size anddistribution

2. Particle shape

3. Degree of

agglomeration

4. Surface area

5. Density andporosity

6. Flow properties

and granulation1. Major elements

2. Minor elements

3. Trace elements

(AAS, AES, XRF)

Crystal structure and

phase

composition

1. Surface structure

(LEED, STM, AFM)

2. Surface

composition

(AES, XPS, ESCA,

SIMS)

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Physical Characterizations of Powder

Types of Particles

Primary particleSmallest unit in

the powder with

a clearly defined

surface/isolated

porosity

Agglomerates

Cluster of

primary particles

held together by

surface force

Types: Soft and

Hard

ParticlesSmall units that

move as a

separate entity

(Coarse particle

1-100 mm)

Granules

Large

agglomerates(Dimension 100-

1000 mm)

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Types of Particles

Flocs

Cluster of

particles in a

liquid

suspension

Colloids

Finely divided

phase in a fluid

(Brownian

motion, negligible

sedimentation

under normalgravity (1 nm1

mm)

Aggregates

Coarse

constituent in a

mixture (pebbles

in concrete)

(1 mm)

Definition of Particle Size

Stokes diameter

Diameter of the sphere calculated fromStokes law

Projected area diameterprojected area of the particle under

microscope

Feretsdiameter/Martins diameterLinear dimension

measured parallel to some fixed direction

(Attached sheet)

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Methods for Measurement of Particle Sizes

Method Range (m)

Microscopy

Optical 1

SEM 0.1

TEM 0.001

Sieving 2010,000

Sedimentation 0.1100

Coulter counter 0.5400

Light Scattering

Scattering intensity 0.11000

Brownian motion 0.0051

X-ray line broadening 0.1

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Martins Diameter (XM )- length of a line that

bisects the area of the particle image

Ferets Diameter (XF )- distance between two

tangents on opposite sides of the particle,

parallel to some fixed direction

Projected Area Diameter (XPA )- diameter of a

circle having the same area as the two-

dimensional image of the particle

Perimeter Diameter (XC )- diameter of the circle

having the same circumference as the perimeterof the particle

The longest dimension is equal to the maximum

value of Ferets diameter

Measurement of Particle Size by Microscopy

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Method Range (m)

Microscopy

Optical 1

SEM 0.1

TEM 0.001

Sieving 2010,000



Light Scattering




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Measurement of Particle Size and Size Distribution by Sieving

Mesh size= the number of wires per

linear inch of the sieve screen, which is

the same as the number of square

apertures per inch.

If mesh number = M, aperture width = a,

wire diameter = w, and the open area =

A, then:

wa

M

1

wM

a 1

2

2

2

)()(

Mawa

aA

Problem: Find out the wire diameter

and open area of a 400 mesh sieve

with an aperture of 38 mm?

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Method Range (m)

Microscopy

Optical 1

SEM 0.1

TEM 0.001

Sieving 2010,000



Light Scattering




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Measurement of Particle Size and Size Distribution by Sedimentation

Stokess Law,

avF 6F = Frictional force

acting on the particle,

= Liquid viscosity,

a = particle radius

v = terminal velocity

Stokess Equation,

2/1

18

gddvxLs

x = diameter of the

particle with sphere

shape,

ds = particle densitydL = liquid density

For non spherical particle,

Limitations

1. Holds good for laminar (non-turbulent) flow

2. No inter-particle collision

3. No interactions between the particles

STKxx

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Laminar to turbulent flow occurs at some critical velocity,

2.0~

xd

N

v L

R

c

2.0

xvdL

2.0xvdL

NR= Reynolds Number

Laminar Flow

Turbulent Flow

The particle size distribution is determined by measuring

the change in concentration (or density) of the suspension as a function

of time, height along the suspension, or both.

A light beam or an x-ray beam is projected at a known height through a

glass cell containing the suspension.

The intensity of the transmitted beam is measured by a photocell or an

x-ray detector located at the opposite side.

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The intensity I of thetransmitted beam will increase as,

)exp(0 KACyII I0= Intensity of the incident beam

K = Extinction coefficient

A = Projected area per unit mass of the

particle

C = Concentration by mass of the particle

y = Optical path length through the

suspension

The particle size distribution [e.g., the cumulative mass percent finer

(CMPF) versus the Stokes diameter] is deconvoluted from the measured intensityratio, I/Io, coupled with Stokes Eq.

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Method Range (m)Microscopy

Optical 1

SEM 0.1

TEM 0.001

Sieving 2010,000



Light Scattering




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Measurement of Particle Size and Size Distribution by Coulter Counter

As a particle passes through the orifice,

it displaces an equivalent volume of the

electrolyte and causes a change in the

electrical resistance, the magnitude of which

is proportional to the volume of the particle.

The changes in resistance are converted to

voltage pulses, which are amplified, sized,

and counted to produce data for the particle

size distribution of

the suspended particles.

Since the number and volume of the

particles are measured in this technique, the

particle size distribution will consist of the

CNPF (or CNPL) versus the volume diameter

xV

Disadvantage

Blocking of the orifice by bigger particle

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