ASTM B922-02
(Test Method)Standard Test Method for Metal Powder Specific Surface Area by Physical Adsorption
Standard Test Method for Metal Powder Specific Surface Area by Physical Adsorption
SIGNIFICANCE AND USE
Both suppliers and users of metals can benefit from knowledge of the surface area of these materials. Results of many intermediate and final processing steps are controlled by, or related to, specific surface area of the metal. The performance of many sintered or cast metal structures may be predicted from the specific surface area of the starting metal powder, or all or a portion of the finished piece.
SCOPE
1.1 This test method covers determination of surface area of metal powders. The test method specifies general procedures that are applicable to many commercial physical adsorption instruments. The method provides specific sample outgassing procedures for listed materials. It includes additional general outgassing instructions for other metals. The multipoint equation of Brunauer, Emmett and Teller (BET), along with the single point approximation of the BET equation, forms the basis for all calculations.
1.2 This test method does not include all existing procedures appropriate for outgassing metallic materials. The procedures included provided acceptable results for samples analyzed during interlaboratory testing. The investigator shall determine the appropriateness of listed procedures.
1.3 This method uses SI units as standard. State all numerical values in terms of SI units unless specific instrumentation software reports surface area using alternate units. In this case, present both reported and equivalent SI units in the final written report. Many instruments report surface area as m2/g, instead of using correct SI units (m 2/kg).
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
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Standards Content (Sample)
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Designation: B 922 – 02
Standard Test Method for
Metal Powder Specific Surface Area by Physical Adsorption
This standard is issued under the fixed designation B 922; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 This test method covers determination of surface area of 3.1 Definitions—Refer to Terminology B 243 for additional
metal powders. The test method specifies general procedures terms specific to metal powders.
that are applicable to many commercial physical adsorption 3.2 Definitions of Terms Specific to This Standard:
instruments. The method provides specific sample outgassing 3.2.1 adsorbate, n—material that has been retained by the
procedures for listed materials. It includes additional general process of adsorption.
outgassing instructions for other metals. The multipoint equa- 3.2.2 adsorbent, n—any solid having the ability to concen-
tion of Brunauer, Emmett and Teller (BET) , along with the trate or collect significant quantities of other substances on its
single point approximation of the BET equation, forms the surface.
basis for all calculations. 3.2.3 adsorption, n—a process in which fluid molecules are
1.2 This test method does not include all existing proce- concentrated or collected on a surface by chemical or physical
dures appropriate for outgassing metallic materials. The pro- forces, or both.
cedures included provided acceptable results for samples 3.2.4 adsorptive, n—any substance available for adsorption.
analyzed during interlaboratory testing. The investigator shall 3.2.5 outgassing, n—the evolution of gas from a material in
determine the appropriateness of listed procedures. a vacuum or inert gas flow, at or above ambient temperature.
1.3 This method uses SI units as standard. State all numeri- 3.2.6 physical adsorption (van der Waals adsorption),
cal values in terms of SI units unless specific instrumentation n—the binding of an adsorbate to the surface of a solid by
software reports surface area using alternate units. In this case, forces whose energy levels approximate those of condensation.
present both reported and equivalent SI units in the final 3.2.7 surface area, n—the total area of the surface of a
written report. Many instruments report surface area as m /g, powder or solid including both external and accessible internal
instead of using correct SI units (m /kg). surfaces (from voids, cracks, open porosity, and fissures). The
1.4 This standard does not purport to address all of the area may be calculated by the BET (Brunauer, Emmett, and
safety concerns, if any, associated with its use. It is the Teller) equation from gas adsorption data obtained under
responsibility of the user of this standard to establish appro- specific conditions. It is useful to express this value as the
priate safety and health practices and determine the applica- specific surface area, for example, surface area per unit mass of
bility of regulatory limitations prior to use. sample (m /kg).
3.2.8 surface area (BET), n—thetotalsurfaceareaofasolid
2. Referenced Documents
calculated by the BET (Brunauer, Emmett, Teller) equation,
2.1 ASTM Standards: from nitrogen adsorption or desorption data obtained under
B 215 Practices for Sampling Finished Lots of Metal Pow-
specific conditions.
ders 3.2.9 surface area, specific, n—the area, per unit mass of a
B 243 Definitions of Terms Used in Powder Metallurgy
granular or powdered or formed porous solid, of all external
E 691 Practice for Conducting an Interlaboratory Study to plus internal surfaces that are accessible to a penetrating gas or
Determine the Precision of a Test Method
liquid.
4. Summary of Test Method
This test method is under the jurisdiction of ASTM Committee B09 on Metal
4.1 An appropriately sized sample (to provide at least the
Powders and Metal Powder Products and is the direct responsibility of Subcom-
minimum surface area required for reliable results for the
mittee B09.03 on Refractory Metal Powders.
instrument used) is outgassed under appropriate conditions
Current edition approved Oct. 10, 2002. Published December 2002.
prior to analysis.
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.
3 5
Annual Book of ASTM Standards, Vol 02.05. ASTM. ASTM Dictionary of Engineering, Science, and Technology, 9th ed.
Annual Book of ASTM Standards, Vol 14.02. West Conshohocken, PA: ASTM, 2000.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B922–02
4.2 Multipoint BET Analyses only—Volume of gas ad- pressure is maintained upon isolation from the vacuum source,
sorbed, or desorbed, is determined as cm corrected to standard or when flushing gas composition is unaffected while passing
temperature and pressure (STP) for a minimum of four relative over the sample.
pressures within the linear BET transformation range of the
7. Apparatus
physical adsorption, or desorption, isotherm characteristic of
7.1 Commercial instruments are available from several
the metal. The linear range is that which results in a least
squares correlation coefficient of 0.9999 or greater for the manufacturers for the measurement of specific surface area by
physical adsorption. Some are automated versions of the
relationshipbetweenBETtransformationandrelativepressure.
Typically, the linear range includes relative pressures between classical vacuum apparatus. Others make use of balanced
adsorption technology. Additionally, commercial instruments
0.05 and 0.30.
are available which measure physical adsorption based on the
4.3 Single Point BET Analyses only—Volume of gas ad-
dynamic flow method.
sorbed, or desorbed, is determined as cm corrected to standard
7.2 Analytical Balance, capable of weighing to the nearest
temperature and pressure (STP) at the highest known relative
0.1 mg.
pressure within the linear BET transformation range of the
physical adsorption, or desorption, isotherm. Typically, a
8. Reagents and Materials
relative pressure of 0.30 is used. (It may be necessary to first
8.1 Liquid Nitrogen.
perform a multipoint analysis of the material to determine the
8.2 Nitrogen, 99.999 mole percent, with the sum of O ,
optimum single point relative pressure.)
argon, CO , hydrocarbons (as CH ), and H O totaling less than
4.4 The sample is weighed to nearest 0.1 mg after analysis.
2 4 2
10 parts per million; dry and oil-free; cylinder, or other source
It is important to use an analytical balance to determine the
of purified nitrogen.
sample mass. The physical adsorption instrument measures the
8.3 Helium, 99.999 mole percent, with the sum of N,O ,
total amount of gas adsorbed onto, or desorbed from, the
2 2
argon, CO , hydrocarbons (as CH ), and H O totaling less than
sample under analysis. The sample mass is then used to 2 4 2
10 parts per million; dry and oil-free; cylinder, or other source
normalize the measured adsorption results. Any error in the
of purified helium, if needed for determination of void space
sample mass will affect the final BET surface area.
above sample.
4.5 Calculations are based on the BET equation, as required
8.4 Blended Nitrogen and Helium, dry and oil-free; cylin-
by the instrument being used for the determination. The cross
der, or other source of blended gases. The actual composition
sectional area for the adsorbate is taken to be 0.162 nm if
of the blend must be known. For use with dynamic flow
nitrogen is used as the adsorptive. Use the appropriate value
instruments only.
recommended by the instrument manufacturer for adsorptives
other than nitrogen. Report this cross sectional area with the
9. Hazards
BET surface area results.
9.1 Precautions applying to the use of liquid nitrogen and
compressed gases should be observed.
5. Significance and Use
5.1 Both suppliers and users of metals can benefit from
10. Sampling, Test Specimens, and Test Units
knowledge of the surface area of these materials. Results of
10.1 It is important that t
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