A proposed new standard currently being developed by ASTM International Committee B09 on Metal Powders and Metal Powder Products offers a solution to the challenge of handling and characterizing very fine powders. Subcommittee B09.02 on Base Metal Powders is developing the proposed standard, WK13023, Test Method for Determination of Tap Density of Metallic Powders by a Constant Volume Measuring Method.
There is an increasing demand within the powder metallurgy industry for metal powders smaller than one micrometre. These powders can now be produced at 0.2 micrometre and it is expected within a few years it will be possible to produce powders as small as 0.1 micrometre.
Tap density measurements (in which the powder has been tapped, to settle contents, in a container under specified conditions) are often used to control process parameters when working with fine powders. However, it can be difficult to use this type of measurement on extremely fine powders because the powder will adhere to the surfaces of weighing dishes and laboratory glassware.
According to Tom Villett, Subcommittee B09.02 member and manager, analytical resources group, Umicore Canada, Inc., measuring tap density by constant mass with fine “sticky” powders yields unreliable results for several reasons. Some of the mass of the powder is lost during the transfer from the weighing dish to the graduated cylinder. In addition, some of the powder may adhere to the cylinder wall and not be included in the measured volume. The relatively high aspect ratio of the graduated cylinders increases particle to cylinder wall friction, inhibiting the migration of the particles to their final packed position. Because these fine powders are not “free-flowing,” they do not pack evenly, often resulting in an uneven top surface of the packed column. This uneven packed surface means that visual interpretation of the volume is required.
Villet says that the constant volume measuring method that is described in the new proposed standard provides a way to do tap density measurements on very fine powders. “When measuring by constant volume these problems are reduced or eliminated,” says Villet. “The low aspect ratio of the density cup reduces friction. The mass of the sample is weighed after tapping with no loss of powder. Because the tapped height of the column is constant, there is no visual interpretation necessary.”
Subcommittee B09.02 welcomes participation in the continuing development of WK13023. //
Technical Information: Tom Villet,Umicore Canada Inc., Fort Saskatchewan, Alberta, Canada
ASTM Staff: Christine Sierk
Fort Lauderdale, Fla.