1、Designation: B 922 02 (Reapproved 2008)Standard Test Method forMetal Powder Specific Surface Area by Physical Adsorption1This standard is issued under the fixed designation B 922; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、 year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers determination of surface area ofmetal powders. The test method specifies general pro
3、ceduresthat are applicable to many commercial physical adsorptioninstruments. The method provides specific sample outgassingprocedures for listed materials. It includes additional generaloutgassing instructions for other metals. The multipoint equa-tion of Brunauer, Emmett and Teller (BET),2along wi
4、th thesingle point approximation of the BET equation, forms thebasis for all calculations.1.2 This test method does not include all existing proce-dures appropriate for outgassing metallic materials. The pro-cedures included provided acceptable results for samplesanalyzed during interlaboratory test
5、ing. The investigator shalldetermine the appropriateness of listed procedures.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3.1 State all numerical values in terms of SI units unlessspecific instrumentation software repo
6、rts surface area usingalternate units. In this case, present both reported and equiva-lent SI units in the final written report. Many instruments reportsurface area as m2/g, instead of using correct SI units (m2/kg).1.4 This standard does not purport to address all of thesafety concerns, if any, ass
7、ociated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3B 215 Practices for Sampling Metal PowdersB 243 Terminol
8、ogy of Powder MetallurgyE 691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method3. Terminology3.1 Definitions:3.1.1 Refer to Terminology B 243 for additional termsspecific to metal powders.3.2 Definitions of Terms Specific to This Standard:43.2.1 adsorbate, n
9、material that has been retained by theprocess of adsorption.3.2.2 adsorbent, nany solid having the ability to concen-trate or collect significant quantities of other substances on itssurface.3.2.3 adsorption, na process in which fluid molecules areconcentrated or collected on a surface by chemical o
10、r physicalforces, or both.3.2.4 adsorptive, nany substance available for adsorption.3.2.5 outgassing, nthe evolution of gas from a material ina vacuum or inert gas flow, at or above ambient temperature.3.2.6 physical adsorption (van der Waals adsorption),nthe binding of an adsorbate to the surface o
11、f a solid byforces whose energy levels approximate those of condensation.3.2.7 surface area, nthe total area of the surface of apowder or solid including both external and accessible internalsurfaces (from voids, cracks, open porosity, and fissures). Thearea may be calculated by the BET (Brunauer, E
12、mmett, andTeller) equation from gas adsorption data obtained underspecific conditions. It is useful to express this value as thespecific surface area, for example, surface area per unit mass ofsample (m2/kg).3.2.8 surface area (BET), nthe total surface area of a solidcalculated by the BET (Brunauer,
13、 Emmett, Teller) equation,from nitrogen adsorption or desorption data obtained underspecific conditions.3.2.9 surface area, specific, nthe area, per unit mass of agranular or powdered or formed porous solid, of all externalplus internal surfaces that are accessible to a penetrating gas orliquid.1Thi
14、s test method is under the jurisdiction of ASTM Committee B09 on MetalPowders and Metal Powder Products and is the direct responsibility of Subcom-mittee B09.03 on Refractory Metal Powders.Current edition approved April 1, 2008. Published April 2008. Originallyapproved in 2002. Last previous edition
15、 approved in 2002 as B 92202.2Brunauer, S., Emmett, P. H., and Teller, E. “Adsorption of Gases in Multimo-lecular Layers.” Journal of the American Chemical Society, Vol. 60, 1938, pp.309-319.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serv
16、iceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4ASTM Dictionary of Engineering, Science, and Technology, 9th ed., ASTMInternational, West Conshohocken, PA, 2000.1Copyright ASTM International, 100 Barr Harbor Drive, PO
17、 Box C700, West Conshohocken, PA 19428-2959, United States.4. Summary of Test Method4.1 An appropriately sized sample (to provide at least theminimum surface area required for reliable results for theinstrument used) is outgassed under appropriate conditionsprior to analysis.4.2 Multipoint BET Analy
18、ses OnlyVolume of gas ad-sorbed, or desorbed, is determined as cm3corrected to standardtemperature and pressure (STP) for a minimum of four relativepressures within the linear BET transformation range of thephysical adsorption, or desorption, isotherm characteristic ofthe metal. The linear range is
19、that which results in a leastsquares correlation coefficient of 0.9999 or greater for therelationship between BET transformation and relative pressure.Typically, the linear range includes relative pressures between0.05 and 0.30.4.3 Single Point BET Analyses OnlyVolume of gas ad-sorbed, or desorbed,
20、is determined as cm3corrected to standardtemperature and pressure (STP) at the highest known relativepressure within the linear BET transformation range of thephysical adsorption, or desorption, isotherm. Typically, arelative pressure of 0.30 is used. (It may be necessary to firstperform a multipoin
21、t analysis of the material to determine theoptimum single point relative pressure.)4.4 The sample is weighed to nearest 0.1 mg after analysis.It is important to use an analytical balance to determine thesample mass. The physical adsorption instrument measures thetotal amount of gas adsorbed onto, or
22、 desorbed from, thesample under analysis. The sample mass is then used tonormalize the measured adsorption results. Any error in thesample mass will affect the final BET surface area.4.5 Calculations are based on the BET equation, as requiredby the instrument being used for the determination. The cr
23、osssectional area for the adsorbate is taken to be 0.162 nm2ifnitrogen is used as the adsorptive. Use the appropriate valuerecommended by the instrument manufacturer for adsorptivesother than nitrogen. Report this cross sectional area with theBET surface area results.5. Significance and Use5.1 Both
24、suppliers and users of metals can benefit fromknowledge of the surface area of these materials. Results ofmany intermediate and final processing steps are controlled by,or related to, specific surface area of the metal. The perfor-mance of many sintered or cast metal structures may bepredicted from
25、the specific surface area of the starting metalpowder, or all or a portion of the finished piece.6. Interferences6.1 This test method can be used to determine the internaland external surface of a powder or solid only after thesesurfaces have been cleaned of any physically adsorbed mol-ecules. Such
26、adsorbed species, for example water or volatileorganic compounds, prevent physical adsorption of the gasprobe molecules used to measure surface area. Therefore, it isnecessary to remove these adsorbed contaminants prior tosurface area analysis. Generally, such outgassing is performedby evacuating or
27、 flushing the sample. Outgassing can beaccelerated by using elevated temperatures, provided no irre-versible sample changes occur. Typical minimum vacuumlevels attained are 10-1Pa. Typical flushing gases are helium,nitrogen, or a mixture of the two. Outgassing is complete whenduplicate surface area
28、analyses produce results within expectedinstrument repeatability limits, when a constant residual vaporpressure is maintained upon isolation from the vacuum source,or when flushing gas composition is unaffected while passingover the sample.7. Apparatus7.1 Commercial instruments are available from se
29、veralmanufacturers for the measurement of specific surface area byphysical adsorption. Some are automated versions of theclassical vacuum apparatus. Others make use of balancedadsorption technology. Additionally, commercial instrumentsare available which measure physical adsorption based on thedynam
30、ic flow method.7.2 Analytical Balance, capable of weighing to the nearest0.1 mg.8. Reagents and Materials8.1 Liquid Nitrogen.8.2 Nitrogen, 99.999 mole percent, with the sum of O2,argon, CO2, hydrocarbons (as CH4), and H2O totaling less than10 parts per million; dry and oil-free; cylinder, or other s
31、ourceof purified nitrogen.8.3 Helium, 99.999 mole percent, with the sum of N2,O2,argon, CO2, hydrocarbons (as CH4), and H2O totaling less than10 parts per million; dry and oil-free; cylinder, or other sourceof purified helium, if needed for determination of void spaceabove sample.8.4 Blended Nitroge
32、n and Helium, dry and oil-free; cylin-der, or other source of blended gases. The actual compositionof the blend must be known. For use with dynamic flowinstruments only.9. Hazards9.1 Precautions applying to the use of liquid nitrogen andcompressed gases should be observed.10. Sampling, Test Specimen
33、s, and Test Units10.1 It is important that the test portion being analyzedrepresent the larger bulk sample from which it is taken. Thebulk sample should be homogenized before any sampling takesplace. Best results are obtained when a flowing bulk material istemporarily diverted into a collector for a
34、n appropriate time. Itis better to sample the entire flow for a short time than tosample a portion of the flow for a longer time. Collectingseveral small aliquants and combining them improves thereliability of the sampling process. Rotating rifflers are avail-able commercially which satisfy these sa
35、mpling requirements.Refer to Practices B 215 for information on the use of a chutesample splitter.11. Calibration and Standardization11.1 Follow manufacturers instructions for calibration andoperational verification of the instrument.B 922 02 (2008)212. Outgassing12.1 Weigh (to nearest 0.1 mg) a cle
36、an, empty sample tube,along with stopper or seal. Record the empty tube mass.12.2 Add test portion to empty sample tube. Sample quan-tity should be sufficient to satisfy minimum surface area asrequired by manufacturer.12.3 Attach prepared sample tube to outgassing port ofinstrument. Secure heating m
37、antle or oven around sample tubeat the time appropriate for sample.12.4 Initiate outgassing program according to manufactur-ers instructions. Program mantle or oven for initial outgassingtemperature. Increase temperature as appropriate for thesample. Allow sample to continue to outgas until prescrib
38、edvacuum level or detector signal is achieved, or for prescribedoutgassing time, or both. Samples analyzed during the inter-laboratory study were heated for2hat200C.12.5 Remove heating mantle or oven from sample tube.Allow sample tube to cool to ambient temperature. Removeand seal sample tube accord
39、ing to manufacturers instructions.12.6 Weigh sample tube (to nearest 0.1 mg) to obtainsample and tube mass. Record mass. Subtract empty sampletube mass determined in 12.1 to obtain outgassed sample mass.Record calculated mass.13. Procedure13.1 Attach appropriately prepared sample holder to analy-sis
40、 port according to manufacturers instructions. Include anyrequired hardware.13.1.1 Use nitrogen as adsorptive for all tests. Use blendednitrogen and helium with dynamic flow instruments. Use purenitrogen with volumetric instruments.13.1.2 Use helium to determine sample holder void spacewith volumetr
41、ic instruments as necessary.13.1.3 Use liquid nitrogen as cryogen for all tests.13.2 Automated Instruments OnlySelect, or input, desiredanalysis and report parameters.13.2.1 Multipoint BET Analyses OnlyUse at least fouranalysis points in the linear BET transformation range of theisotherm characteris
42、tic of the sample. If necessary, input theoutgassed sample mass. (The final mass should be determinedand entered after the analysis.)13.2.2 Single Point BET Analyses OnlyUse highest rela-tive pressure known to be in the linear BET transformationrange of the isotherm. If necessary, input the outgasse
43、d samplemass. (The final mass should be determined and entered afterthe analysis.)13.3 Dynamic Flow Instruments OnlyCollect data pointsas volume of gas desorbed versus relative pressure.13.3.1 Multipoint BET Analyses OnlyUse at least fouranalysis points in the linear BET transformation range of thei
44、sotherm characteristic of the sample.13.3.2 Single Point BET Analyses OnlyUse highest rela-tive pressure known to be in the linear BET transformationrange of the isotherm.13.4 Perform analysis using the specified conditions accord-ing to instrument manufacturers instructions.13.5 When the analysis h
45、as finished and the sample haswarmed to room temperature, remove and seal the sample tube.Dry tube and weigh (to nearest 0.1 mg). Record the final tubeand sample mass. Subtract the empty tube mass recorded in12.1 to obtain the final sample mass. Record final sample mass.13.6 Automated Instruments On
46、lyEdit the file containingsample information to include the final sample mass. Generatefinal sample report.14. Calculations14.1 Automated Instruments OnlySoftware automaticallycalculates results for the chosen reports using the final massinput in 13.6.14.2 Dynamic Flow Instruments OnlyFollow manufac
47、tur-ers instructions for multipoint, or single point, calculations.Use the final sample mass determined in 13.5 when calculatingthe specific surface area.15. Report15.1 Report the following information:15.1.1 Complete sample identification.15.1.2 Collected isotherm point(s) as volume adsorbed, ordes
48、orbed, versus relative pressure. Note whether adsorption ordesorption isotherm is used. Note any units used other thanstandard.15.1.3 Analysis gas used (with cross sectional area if otherthan nitrogen).15.1.4 BET specific surface area. Note any units used otherthan standard.15.1.5 Final sample mass.
49、 Note any units used other thanstandard.15.1.6 Sample outgassing method, including total time andoutgassing temperature(s).16. Precision and Bias16.1 An interlaboratory study is underway, conducted ac-cording to Practice E 691. The study includes iron, tungsten,nickel, cobalt, molybdenum, chromium carbide, and tungstencarbide powders. Expected precision will be determined forthese materials. No statement is given for other metal powders.16.2 PrecisionThe repeatability standard deviation andreproducibility standard deviation will be determined uponcompletion
