ISO 10076:1991 pdf free download.Metallic powders — Determination of particle size distribution by gravitational sedimentation in a liquid and attenuation measurement.
The settling behaviour under gravity of a given mass of particles dispersed in an initially static liquid Is the basis of widely used sedimentation techniques for particle-size determination. The particle size is determined from the settling velocity by the use of Stokes equation. The particle diameter so determined, the Stokes diameter, is the diameter of a sphere having the same density and the same free-fall velocity as the particle in a fluid of a given density and viscosity, The particle concentration must be low so that interaction between particles Is negligible, and the Reynolds number must be low so that laminar flow conditions prevail.
Monitoring of the concentration of particles at a known depth below the surface of an Initially homogeneous suspension enables the particle- size distribution to be calculated as a function of the measured surface or mass.
in ISO 10076, two attenuation methods for the determination ol concentration are considered:
— absorption of a beam of light;
— absorption of a beam of X-rays.
Although they are indirect, these sedimentation-attenuation methods are frequently employed In powder metallurgy. They give reproducible results as long as precise conditions of preparation of the suspension and of measurement are followed.
ISO 10076 specifies methods for determining the particle size distribution of metallic powders by measuring the attenuation of an electromagnetic beam projected through a suspension of particles settling in a liquid under gravity.
The methods are suitable only where Stokes’ equation Is applicable, i.e. laminar flow corresponding to a Reynolds number less than 0.25. and for particles whose settUng rate is not affected by Brownian motion. They are therefore suitable for all metallic powders, including powders for hardmetals. containing particles In the size range I tm to 100 urn, They should not, however, be used for
a) powders containing particles whose shape Is far from equlaxial, I.e. flakes or fibres, except by special agreement;
b) mixtures of powders;
c) powders containing lubricant or binder,
d) powders which cannot be dispersed in a liquid.
These considerations set an upper and a lower limit on the size of particles to be tested by a sedimentation method (see 5.1).
If the largest particle present in the sample exceeds this limit, the viscosity of the liquid needs to be increased to meet this requirement.
Stokes’ law may be assumed to be valid for an initial concentration of the powder in the liquid of up to 0.5 % (V/k’). In some cases, higher concentrations of up to 1 % (V/k’) still give correct results, but validation tests are necessary for each material.
2 NormatIve references
The following standards contain provisions which, through reference in this text, constitute provisions of ISO 10076. At the time of publication. the editions indicated were valid All standards are subject to revision, and parties to agreements based on ISO 10076 are encoiraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.
ISO 32521982, Powder metallurgy — Vocabulary.
ISO 3g541977, Powders for powder metallurgical
purposes — Sampling.
3 Defir,Itions
For the purposes of ISO 10076, the following definitions apply.
3.1 Stokes diameter: Diameter or a sphere having
the same density and the same free-fall velocity as a powder particle in a fluid of a given density and viscosity.
3.2 effectIve density: Ratio of the mass of the powder to the volume as measured by pyknometry.
3.3 sedImentation height: Vertical distance between the upper surface of the suspension and the level of measurement of concentration.
3.4 cumulative undersize by mass: Mass of all the particles whose Stokes diameter is less than a given value. This is expressed as a percentage of the total mass of the partic?es
3.5 mass fraction: Mass of all the particles whose Stokes diameter lies between two given values. This
6.2 PreparatIon of the suspension
6.2.1 Selection of the suspending liquid
The liquid used to make the suspension shall satisfy the following requirements:
— the density or the liquid shall be lower than the density of the solid;
— the solid shall not dissolve in or react with the liquid;
— the liquid shall wet the solid so that agglomerates are not formed;
— the viscosity of the liquid shall be such that the test does not take too long and the coarsest particles do not settle too rapidly (see 6.1),
Frequently, a pure liquid does not produce a good dispersion of the particles. In such cases, a dispersing or welting agent shall be used, either dissolved in the liquid or added to the powder.
Annex C gives, as examples. a list of suspending liquids and dispersing agents. The suitability of a particular system shall be checked as indicated In 6.2.2 below.
6.2.2 Tests for dispersion
Several methods can be used to test whether a suspension Is free from agglomerates.
6.2.2.1 MIcroscopic examination
A drop of the prepared suspension is placed on a microscope slide, and a cover glass is lowered with extreme care over it. Examination of the preparation under a suitable magnification will show It the particles are entirely separate and form a good dispersion, or are assembled In chains or clusters.
6.2.2.2 QualItative sedimentation
The suspension is left to settle. A correctly dispersed suspension settles less rapidly than a flocculated suspension and shows no sharp boundary between the clear liquid and the turbid layer as settling proceeds. The sediment produced Is rigid and compact and has a minimum volume
6.2.2.3 Quantitative sedimentation with photometric measurement
Attenuation measurements are made immediately after agitation using different suspending liquids and the same powder concentration. A maximum value of optical density indicates an optimum dispersion.
6.2.2.4 QuantitatIve sedimentation with X-ray absorption
Quantitatke tests are made with suspensions In differeni liquids and having different concentrations, starting for instance with a low concentration or 0,1 % (V/V). The best system is the one which gives the highest proportion of line fractions.
62.3 Preparation of the suspension
6.2.3.1 The density and viscosity of the selected liquid at the test temperature shall be known or measured. The suspension may be prepared at the selected concentration, starting with a weighed test portion, or the suspending liquid may be added slowly arid the powder worked to a paste, then diluted to a suspension. A drop of wetting agent shall be added to the dry test portion or the suspending liquid if the liquid does not readily wet the powder. Make up to an appropriate voume.
6.2.3.2 The dispersion may be made by shaking the suspension or stirring it in the sedimentation vessel. If necessary, treat with ultrasonics or de-aerate under reduced pressure, either in a separate vessel or in the sedimentation vessel. The duration and intensity of dispersion Is selected so that unwanted agglomerates are destroyed but any aggregates or desired particles are not damaged.
6.3 SedImentatIon test
6.3.1 The general requirements for a satisfactory test are the following:
— the sedimentation cell shall be installed rigidly in a vertical position and without vIbration
— the cell shall be fitted with a thermostatic envelope. or the environment conditioned to maintain the cell at a known temperature, stable to within ± I C, with a rate of change of temperature of less than 0,01 C/min for prolonged analyses (longer than I h):
— care shall be taken to avoid convection currents in the liquid (due to evaporation or to external heat sources, for instance) and mass transfer currents (due to density inversion in the suspension).
6.3.2 Stir the suspension using either a stirring rod or a magnetic bar until the analysis Is started
(1= 0).
6.3.3 In most instruments, the initial optical density
or X-ray absorption of the suspension Is recorded. This corresponds to 100 % cumulative undersize. Before starting a test, the Instrument is zeroed so that the optical density or X-ray absorption of the clear liquid corresponds to 0 %.
NOTE I It Is recommended thai certified relerence powders be used for particle-size distribution deterrnlnation by sedlmentation In order to check the accuracy and the correct working of the Instrument.
7 ExpressIon of results
7.1 In photosedimentation, the optical density measurements give values of cumulative undersize by surface. These values are converted to cumulative undersize by mass for each Stokes diameter 4 corresponding to a time t1. (See annex A for a worked example.)
7.2 In X-ray sedimentation, the X-ray absorption value Is related directly to the mass concentration. I.e. to the cumulative undersize by mass
In some cases, It Is convenient to present the results as a graph plotting cumulative undersize by mass on a linear scale from 0 % to 100 % as a function of Stokes diameter on a logarithmic scale. Presentation of the results on special graphs, such as log- normal plots or Rosin-Rammler coordinates diagrams, is allowed by agreement between the Interested parties.