Section 3.4.1.2.1. General

Informative accounts of the powder method of recording diffraction patterns have been given by Klug & Alexander (1954), D'Eye & Wait (1960) and Dent Glasser (1977). There are three principal methods of preparing polycrystalline specimens for mounting in powder cameras:

The most common method of preparing samples of polycrystalline materials is to encase them in thin-walled capillary tubes, for Debye–Scherrer camera work, or into sample holders, for Guinier camera and diffractometer measurements. This technique has the advantage that the sample can be readily protected from attack by oxygen, carbon dioxide and water vapour, and, if necessary, the sample preparation can be undertaken in an inert atmosphere (Lange & Haendler, 1972; D'Eye & Wait, 1960). The precise details of sample preparation and mounting will be dependent on the type of camera or diffractometer used, but the particle size should be generally less than 10 µm for stationary samples and diffractometer work. A slightly larger particle size, 45 µm, can be used for Debye–Scherrer camera work if the specimen is rotated. Foit (1982) has described a simple method of filling thin-walled capillaries using an ultrasonic vibrator. A frequent problem affecting intensity measurements from powder specimens is caused by preferred orientation when powder samples are packed or pressed. McMurdie, Morris, Evans, Paretzkin & Wong-Ng (1986) have described a method of sample preparation suitable for a diffractometer that minimizes this problem.

Capillaries made from lithium beryllium borate (Lindemann glass), borosilicate (e.g. Pyrex glass), or fused silica are commercially available in a variety of internal diameters. For very high temperatures, thin-walled ceramic or metal capillaries can be used. The diffraction pattern of the metal can be used as an internal standard. Capillaries that are suitable for materials that react with glass can be made from various organic polymers. Table 3.4.1.1 lists details of capillaries and other containers suitable for encasing powder specimens.

In the bonded method, the polycrystalline material is mixed with an adhesive such as gum tragacanth or ethyl cellulose, and the mixture is wetted with water or aqueous alcohol to form a viscous paste. The paste is then rolled between two glass slides or extruded through a glass capillary, using a glass or metal piston, to form a cylindrical sample. This can be cut to length and either glued, fixed with plasticine, or cemented (for high-temperature work) to the camera mounting pin. Alternatively, the sample can be compressed and compacted in a die to form a solid rod, or, for diffractometers, into a disc. In the case of very small quantities of material, the powder can be smeared with silicone vacuum grease over the surface of a disc-shaped silica crystal. The silica can then be used as an internal standard.

In the fibre-supported method, a silica, Lindemann, or borosilicate glass fibre moistened with adhesive (Canada balsam diluted with xylene, collodion, gum tragacanth and water, dilute fish glue) is dipped into the powder. Experience has shown that powder adhesion to the fibre is often improved if non-drying glues or viscous oils are employed. Hairs of fine organic filaments have been used in place of glass fibres, and for high temperature above 1270° C metal wires are useful. Once again, the metal diffraction patterns can act as internal standards. For extruded metal wires, the wire itself acts as the specimen, and the diameter can be reduced by etching if it is too large, or a glancing-angle diffraction technique can be employed. Various specialized holders for diffraction studies of polycrystalline samples can be found in annual conference proceedings such as EPDIC (European Powder Diffraction Conference, Switzerland: Trans Tech Publications) and Advances in X-ray Analysis (Proceedings of the Annual Conference on the Applications of X-ray Diffraction, New York/London: Plenum). The journals Reviews of Scientific Instruments (American Institute of Physics) and Nuclear Instruments and Methods (Elsevier, North-Holland) also provide useful sources of information.