Introduction

The measurement of the permeability of a packed powder bed to a laminar gas flow is the basis of this document. The determination can be made either at constant pressure drop (steady-state flow) or at variable pressure drop (constant volume). This document deals only with determinations made under steady-state flow conditions. The permeability measured is influenced by the porosity of the powder bed. For a given particle shape, the values of permeability and porosity can be used to calculate a specific surface area of the powder by means of different formulae.

The surface area so calculated includes only those walls of the pores in the powder bed which are swept by the gas flow. It does not take into account closed or blind pores. It is known as the envelope-specific surface area. It can be very different from the total surface area of particles as measured, for instance, by gas adsorption methods.

A single equation is used in the standard methods described in this document. It entails certain limitations with respect to the type of powder (particle shape) and the porosity of the powder bed for which the method is applicable. Consequently, this is not an absolute method, and the value obtained depends upon the procedure used and the assumptions made.

The specific surface area determined can be converted into a mean equivalent spherical diameter (see Clause 3).

Scope

This document specifies a method of measuring the air permeability and the porosity of a packed bed of metal powder, and of deriving therefrom the value of the envelope-specific surface area. The permeability is determined under steady-state flow conditions, using a laminar flow of air at a pressure near atmospheric. This document does not include the measurement of permeability by a constant volume method.

Several different methods have been proposed for this determination, and several test devices are available commercially. They give similar, reproducible results, provided that the general instructions given in this document are respected, and the test parameters are identical.

This document does not specify a particular commercial test device and corresponding test procedure. However, for the convenience of the user, an informative annex has been included (see Annex A) which is intended to give some practical information on three specific methods:

— the Lea and Nurse method, involving a test device which can be built in a laboratory (see A.1);

— the Zhang Ruifu method, using a similar test device (see A.2);

— the Gooden and Smith method, involving a test device which can be built in a laboratory but for which a commercial test device also exists. (Two types of commercial test device exist; one of these is no longer available for purchase, but is still being used, see A.3.)

These methods are given as examples only. Other test devices available in various countries are acceptable within the scope of this document.

This testing method is applicable to all metallic powders, including powders for hardmetals, up to 1 000 µm in diameter, but it is generally used for particles having diameters between 0,2 µm and 75,0 µm. It is not intended to be used for powders composed of particles whose shape is far from equiaxial, i.e. flakes or fibres, unless specifically agreed upon between the parties concerned.

This testing method is not applicable to mixtures of different metallic powders or powders containing binders or lubricant.

If the powder contains agglomerates, the measured surface area can be affected by the degree of agglomeration. If the powder is subjected to a de-agglomeration treatment (see Annex B), the method used is to be agreed upon between the parties concerned.

The methods and test devices used in practice differ depending on the way in which the volume flow rate of the gas and the pressure drop are measured. Annex A describes three methods by way of example. The Kozeny-Carman relation applies only over a limited range of powder bed porosities, the range depending on the type of powder. It applies best to equiaxial powders. The Kozeny factor K varies with the particle shape and particle size distribution. In this document, the value of K is taken to be 5,0 but other values may be used by agreement between the parties concerned. Due to the limitations of the Kozeny-Carman relation, the variation of the specific surface area as a function of porosity shall first be determined experimentally for any particular type of powder.

For example, make several successive determinations of the permeability, using test portions of the same mass from the same laboratory sample, and packing the powder bed to give a decreasing series of porosities. Over a certain range of porosities, the specific surface area will be practically constant. Only determinations made within this range shall be taken as valid.

*Contact us (sales@goldapp.com.cn) to get this downloaded PDF*