Powder Metallurgy: Particle Size Analysis and Sampling

What is a particle?

The smallest unit of a powder that cannot be subdivided.

What is the approximate size range of typical powder metallurgy particles shown in lecture?

Many are similar in size to human hair, roughly 25-100 μm.

What is one of the best tools for directly observing metal powders?

Scanning electron microscopy (SEM).

What powder characteristics are commonly measured?

Size/distribution, shape, surface area, friction, flow/packing, internal structure, composition, homogeneity, and contamination.

Why must both individual-particle and collective powder properties be considered?

Individual particles have their own properties, while interactions among many particles control bulk powder behavior.

What are important individual particle properties?

Size, shape, chemistry, microstructure, density, hardness, and surface attributes.

What are important collective powder properties?

Packing, flow, apparent density, tap density, friction, and particle-size distribution.

Why is particle size difficult to define for irregular particles?

Different dimensions or measurement techniques can give different equivalent particle sizes.

What must be specified when reporting particle size?

The measurement technique used.

What determines the particle size reported by an analyzer?

The technique used, parameter measured, and particle shape.

What is equivalent spherical volume diameter, Dv?

The diameter of a sphere having the same volume as the particle.

What is the equation for equivalent spherical volume diameter?

Dv = (6V/π)^(1/3).

What is equivalent spherical projected-area diameter, DA?

The diameter of a circle having the same projected area as the particle.

What is the equation for equivalent spherical projected-area diameter?

DA = (4A/π)^(1/2).

What is equivalent spherical surface diameter, Ds?

The diameter of a sphere corresponding to the particle's surface area.

What is the equation for equivalent spherical surface diameter?

Ds = (S/π)^(1/2).

Why can one irregular particle have several reported diameters?

Volume, projected area, surface area, maximum dimension, etc. produce different equivalent diameters.

Why is powder sampling important?

A small test sample must accurately represent a much larger powder lot.

What is the major difficulty when sampling a powder lot?

A very small laboratory sample may not represent the composition and size distribution of the entire lot.

Where should a representative powder sample preferably be taken?

From a moving powder stream rather than a stagnant lot.

Why should samples be collected from many points?

To reduce sampling bias and better represent the entire powder lot.

What device gives very low powder sampling error?

A rotary riffler.

Why can small particles be difficult to characterize?

They are cohesive and tend to form agglomerates.

What is an agglomerate?

A cluster of particles held by relatively weak forces that can be broken by stresses well below material strength.

What is an aggregate?

A cluster of particles or crystals held together by strong forces.

What can cause fine powder particles to stick together?

Adsorbed moisture and interparticle forces.

How does particle surface energy per unit volume change as diameter decreases?

It increases approximately with the inverse of particle diameter.

Why does agglomeration become more important as particle size decreases?

Smaller particles have much higher surface energy relative to their volume.

How can agglomeration affect measured particle size?

An instrument may measure a cluster as one large particle, giving an artificially large size.

How can particles larger than about 100 μm be treated to reduce moisture-related cohesion?

Evaporate the water.

Why can simply removing water be problematic for very fine particles?

As water is removed, capillary forces can increase and strengthen agglomerates.

How are very fine powders dispersed before particle-size measurement?

Use polar dispersants plus high-energy ultrasonic agitation or intensive mixing.

What is the purpose of a dispersant?

To help separate agglomerates and coat particles so they remain dispersed.

What is microscopy used for in particle-size analysis?

Direct measurement of particle dimensions or projected areas to generate a frequency distribution.

What is a disadvantage of microscopy for particle-size analysis?

It can be tedious because many particles must be measured and properly dispersed.

What is screening also called?

Sieve analysis.

What is screening?

Separating particles by passing powder through screens with specified opening sizes.

What is the approximate normal size range for sieve analysis?

About 20 μm to 125 mm; micromeshes can reach roughly 5 μm.

What does mesh number represent?

The number of wires per linear inch of screen.

What does a higher mesh number generally mean?

A smaller screen opening and therefore smaller particles passing through.

Approximately what opening corresponds to 400 mesh?

About 37 μm.

For mesh numbers 50 and above, what approximate relation can estimate particle diameter?

Particle diameter in mm ≈ 15/(mesh number).

What does -325 mesh mean?

Particles pass a 325-mesh screen, approximately

What does -100/+200 mesh mean?

Particles pass the 100-mesh screen but are retained on the 200-mesh screen.

What is subsieve powder?

Powder smaller than the practical size range of ordinary sieve analysis.

When is wet sieving commonly used?

For very fine powders, especially below about 325 mesh, where agglomeration becomes problematic.

What is a major limitation of sieve analysis?

It does not properly account for particle shape.

Why can two laboratories obtain different sieve sizes for an irregular powder?

Particle orientation, shape, screen variation, and operating conditions affect whether particles pass an opening.

What manufacturing variation exists in screen openings?

Average openings may vary about 3-7%, and individual openings can be much larger than specified.

What is sedimentation particle-size analysis based on?

Determining particle size from the terminal settling velocity of particles in a fluid.

What particle-size range is sedimentation most applicable to?

Approximately 0.02-100 μm under suitable conditions.

What three forces balance at terminal settling velocity?

Gravity, buoyancy, and drag.

What law is commonly used for sedimentation sizing?

Stokes' law.

What is the key concept of Stokes' law in powder sizing?

Terminal settling velocity depends on particle size, particle/fluid density difference, gravity, and fluid viscosity.

How does particle diameter affect Stokes settling velocity?

Velocity is proportional to particle diameter squared.

How does particle density affect settling velocity?

Greater particle-fluid density difference increases settling velocity.

How does fluid viscosity affect settling velocity?

Higher viscosity decreases settling velocity.

Why is sedimentation difficult for particles below about 1 μm?

Brownian motion becomes important.

What can be used for very small particles when normal gravitational sedimentation becomes too slow?

Centrifugal sedimentation.

What Reynolds-number condition is assumed for Stokes-law sedimentation in the slides?

Re < approximately 0.2.

What happens at high particle concentrations during sedimentation?

Settling becomes hindered by particle interactions and container-wall effects.

Why does internal particle porosity affect sedimentation results?

It lowers effective particle density and therefore changes settling velocity.

Why do irregular particles complicate sedimentation analysis?

Their shape changes drag, orientation, path, and settling velocity.

What is the approximate range of light-scattering particle-size analysis?

About 1-200 μm in the lecture.

How does scattering angle relate to particle size?

Scattering angle is inversely proportional to particle size.

How does scattering intensity relate to particle diameter in the lecture?

Intensity is directly proportional to the square of particle diameter.

What type of light is used in low-angle Fraunhofer scattering?

Monochromatic laser light.

What is dynamic ratio?

The ratio of the largest to smallest particle sizes measurable simultaneously.

What is a major advantage of light-scattering techniques?

Large numbers of particles can be analyzed automatically over a large dynamic range.

What is electrical zone sensing?

A technique that measures particle number and size from changes in electrical conductivity as particles pass through an aperture.

What happens electrically when a particle passes through the aperture?

Conductivity decreases, with the change related to particle volume.

What fluid is used in electrical zone sensing?

A conductive electrolyte, such as dilute salt water.

How should aperture size be selected for electrical zone sensing?

About 1.6 times the diameter of the largest particle.

What is the approximate dynamic ratio for electrical zone sensing?

About 30.

What is the approximate lower particle-size limit for electrical zone sensing?

About 0.5 μm.

Why can dense powders cause problems in electrical zone sensing?

They may settle in the chamber and skew the measured distribution.

Why must particle concentration be kept low in electrical zone sensing?

Multiple particles passing through or blocking the aperture simultaneously causes measurement errors.

What is a particle-size distribution?

The amount or frequency of powder occurring in different particle-size intervals.

What is the mode of a particle-size distribution?

The particle size corresponding to the peak frequency.

What is an arithmetic mean particle size?

A frequency-weighted average of particle sizes.

What is Di in particle-size distribution calculations?

The midpoint particle size of an interval.

What is Yi in particle-size distribution calculations?

The frequency of occurrence in an interval.

What is N in particle-size distribution calculations?

The total number of occurrences, ΣYi.

What does D50 represent?

The median size: 50% of the distribution lies below that size.

What does D10 represent?

The size below which 10% of the distribution lies.

What does D90 represent?

The size below which 90% of the distribution lies.

What distribution commonly describes many natural unscreened powders?

A log-normal type distribution.

What is a Rosin-Rammler distribution used for?

Describing cumulative particle-size distributions.

What does the Rosin-Rammler distribution include?

A characteristic size and a parameter describing distribution width.

What is the characteristic Rosin-Rammler size associated with in the lecture?

D37 on the plot.

What is the Andreasen distribution useful for?

Optimizing particle-size distributions for high packing density.

What does the Dinger-Funk distribution incorporate?

Both the largest and smallest particle sizes.

Why is particle shape important?

It influences packing, flow, compressibility, fabrication-route identification, and processing behavior.

What is aspect ratio?

Maximum particle dimension divided by minimum particle dimension.

What is the aspect ratio of a sphere?

1.

What is a typical aspect ratio for a ligament-shaped particle?

Approximately 3-5.

What aspect ratio is typical of a flake?

Greater than 10.

What can powder shape tell you?

It can provide clues about the powder fabrication method.

What are the two main surface-area measurement techniques in the slides?

Gas adsorption and gas permeability.

What does powder surface area help predict?

Chemical activity, catalysis, friction, adsorption, contamination, pressing, and sintering behavior.