ASTM B922-02(2008)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: B922 – 02 (Reapproved 2008)
Standard Test Method for
Metal Powder Specific Surface Area by Physical Adsorption
This standard is issued under the fixed designation B922; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope B243 Terminology of Powder Metallurgy
E691 Practice for Conducting an Interlaboratory Study to
1.1 This test method covers determination of surface area of
Determine the Precision of a Test Method
metal powders. The test method specifies general procedures
that are applicable to many commercial physical adsorption
3. Terminology
instruments. The method provides specific sample outgassing
3.1 Definitions:
procedures for listed materials. It includes additional general
3.1.1 Refer to Terminology B243 for additional terms spe-
outgassing instructions for other metals. The multipoint equa-
2 cific to metal powders.
tion of Brunauer, Emmett and Teller (BET), along with the
3.2 Definitions of Terms Specific to This Standard:
single point approximation of the BET equation, forms the
3.2.1 adsorbate, n—material that has been retained by the
basis for all calculations.
process of adsorption.
1.2 This test method does not include all existing proce-
3.2.2 adsorbent, n—any solid having the ability to concen-
dures appropriate for outgassing metallic materials. The pro-
trate or collect significant quantities of other substances on its
cedures included provided acceptable results for samples
surface.
analyzed during interlaboratory testing. The investigator shall
3.2.3 adsorption, n—a process in which fluid molecules are
determine the appropriateness of listed procedures.
concentrated or collected on a surface by chemical or physical
1.3 The values stated in SI units are to be regarded as
forces, or both.
standard. No other units of measurement are included in this
3.2.4 adsorptive, n—any substance available for adsorption.
standard.
3.2.5 outgassing, n—the evolution of gas from a material in
1.3.1 State all numerical values in terms of SI units unless
a vacuum or inert gas flow, at or above ambient temperature.
specific instrumentation software reports surface area using
3.2.6 physical adsorption (van der Waals adsorption),
alternate units. In this case, present both reported and equiva-
n—the binding of an adsorbate to the surface of a solid by
lentSIunitsinthefinalwrittenreport.Manyinstrumentsreport
2 2
forces whose energy levels approximate those of condensation.
surface area as m /g, instead of using correct SI units (m /kg).
3.2.7 surface area, n—the total area of the surface of a
1.4 This standard does not purport to address all of the
powder or solid including both external and accessible internal
safety concerns, if any, associated with its use. It is the
surfaces (from voids, cracks, open porosity, and fissures). The
responsibility of the user of this standard to establish appro-
area may be calculated by the BET (Brunauer, Emmett, and
priate safety and health practices and determine the applica-
Teller) equation from gas adsorption data obtained under
bility of regulatory limitations prior to use.
specific conditions. It is useful to express this value as the
2. Referenced Documents specificsurfacearea,forexample,surfaceareaperunitmassof
sample (m /kg).
2.1 ASTM Standards:
3.2.8 surface area (BET), n—thetotalsurfaceareaofasolid
B215 Practices for Sampling Metal Powders
calculated by the BET (Brunauer, Emmett, Teller) equation,
from nitrogen adsorption or desorption data obtained under
This test method is under the jurisdiction of ASTM Committee B09 on Metal
specific conditions.
Powders and Metal Powder Products and is the direct responsibility of Subcom-
3.2.9 surface area, specific, n—the area, per unit mass of a
mittee B09.03 on Refractory Metal Powders.
granular or powdered or formed porous solid, of all external
Current edition approved April 1, 2008. Published April 2008. Originally
approved in 2002. Last previous edition approved in 2002 as B922–02. DOI:
plus internal surfaces that are accessible to a penetrating gas or
10.1520/B0922-02R08.
liquid.
Brunauer, S., Emmett, P. H., and Teller, E. “Adsorption of Gases in Multimo-
lecular Layers.” Journal of the American Chemical Society, Vol. 60, 1938, pp.
309-319.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on ASTM Dictionary of Engineering, Science, and Technology, 9th ed., ASTM
the ASTM website. International, West Conshohocken, PA, 2000.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B922 – 02 (2008)
4. Summary of Test Method versible sample changes occur. Typical minimum vacuum
-1
levels attained are 10 Pa. Typical flushing gases are helium,
4.1 An appropriately sized sample (to provide at least the
nitrogen, or a mixture of the two. Outgassing is complete when
minimum surface area required for reliable results for the
duplicate surface area analyses produce results within expected
instrument used) is outgassed under appropriate conditions
instrument repeatability limits, when a constant residual vapor
prior to analysis.
pressure is maintained upon isolation from the vacuum source,
4.2 Multipoint BET Analyses Only—Volume of gas ad-
or when flushing gas composition is unaffected while passing
sorbed,ordesorbed,isdeterminedascm correctedtostandard
over the sample.
temperature and pressure (STP) for a minimum of four relative
pressures within the linear BET transformation range of the
7. Apparatus
physical adsorption, or desorption, isotherm characteristic of
the metal. The linear range is that which results in a least
7.1 Commercial instruments are available from several
squares correlation coefficient of 0.9999 or greater for the manufacturers for the measurement of specific surface area by
relationshipbetweenBETtransformationandrelativepressure.
physical adsorption. Some are automated versions of the
Typically, the linear range includes relative pressures between classical vacuum apparatus. Others make use of balanced
0.05 and 0.30.
adsorption technology. Additionally, commercial instruments
4.3 Single Point BET Analyses Only—Volume of gas ad-
are available which measure physical adsorption based on the
sorbed,ordesorbed,isdeterminedascm correctedtostandard
dynamic flow method.
temperature and pressure (STP) at the highest known relative
7.2 Analytical Balance, capable of weighing to the nearest
pressure within the linear BET transformation range of the
0.1 mg.
physical adsorption, or desorption, isotherm. Typically, a
relative pressure of 0.30 is used. (It may be necessary to first
8. Reagents and Materials
perform a multipoint analysis of the material to determine the
8.1 Liquid Nitrogen.
optimum single point relative pressure.)
8.2 Nitrogen, 99.999 mole percent, with the sum of O ,
4.4 The sample is weighed to nearest 0.1 mg after analysis.
argon, CO , hydrocarbons (as CH ), and H O totaling less than
2 4 2
It is important to use an analytical balance to determine the
10 parts per million; dry and oil-free; cylinder, or other source
sample mass. The physical adsorption instrument measures the
of purified nitrogen.
total amount of gas adsorbed onto, or desorbed from, the
8.3 Helium, 99.999 mole percent, with the sum of N,O ,
2 2
sample under analysis. The sample mass is then used to
argon, CO , hydrocarbons (as CH ), and H O totaling less than
2 4 2
normalize the measured adsorption results. Any error in the
10 parts per million; dry and oil-free; cylinder, or other source
sample mass will affect the final BET surface area.
of purified helium, if needed for determination of void space
4.5 Calculations are based on the BET equation, as required
above sample.
by the instrument being used for the determination. The cross
8.4 Blended Nitrogen and Helium, dry and oil-free; cylin-
sectional area for the adsorbate is taken to be 0.162 nm if
der, or other source of blended gases. The actual composition
nitrogen is used as the adsorptive. Use the appropriate value
of the blend must be known. For use with dynamic flow
recommended by
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