Specimens and holders

Intensifying screens of one kind or another are routinely used for x-ray radiography but not for diffraction. Most film manufacturers make orfe or more kinds of film designed for radiography and another kind for diffraction, whether for the Laue or powder method (Chap. 6). Diffraction films are faster and have coarser grain. They are designed for use without an intensifying screen and are often named no screen to emphasize this fact. 

Obviously, a specimen for the transmission method must have low enough absorption to transmit the diffracted beams in practice, this means that relatively thick specimens of a light element like aluminum may be used but that the thickness of a fairly heavy element like copper must be reduced, by etching, for example, to a few thousandths of an inch. On the other hand, the specimen must not be too thin or the diffracted intensity will be too low, since the intensity of a diffracted beam is proportional to the volume of diffracting material. In the back-reflection method, there is no restriction on the specimen thickness and quite massive specimens may be examined, since the diffracted beams originate in only a thin surface layer of the specimen. This difference between the two methods may be stated in another way and one which is well worth remembering any information about a thick specimen obtained by the back-reflection method applies only to a thin surface layer of that specimen, whereas information recorded on a transmission pattern is representative of the complete thickness of the specimen, simply because the transmission specimen must necessarily be thin enough to transmit diffracted beams from all parts of its cross section. See Sec. 9-5 for further discussion of this point. 

There is a large variety of specimen holders in use, each suited to some particular purpose. The simplest consists of a fixed post to which the specimen is attached with wax or plasticine. A more elaborate holder is required when it is necessary to set a crystal in some particular orientation relative to the x-ray beam. In this case, a three-circle goniometer is used (Fig. 5-7) it has three mutually perpendicular axes of rotation, two horizontal and one vertical, and is so constructed that the crystal, cemented to the tip of the short metal rod at the top, is not displaced in space by any of the three possible rotations. 

After the orientation of a crystal has been determined by the Laue method, it is sometimes necessary to cut the crystal along some selected plane. A more massive goniometer-holder than that of Fig. 5-7 is then required such a holder can be rentoved from the track of the Laue camera and transferred to a similar track on the cutting device without disturbing the orientation of the crystal. 

In the examination of sheet specimens, it is frequently necessary to obtain diffraction patterns from various points on the surface, and this requires movement of the specimen, between exposures, in two directions at right angles in the plane of the specimen surface, this surface being perpendicular to the incident x-ray 

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The metal electrode to be studied must be carefully prepared, attached to an electrical lead and mounted so that a known surface area of one face is presented to the solution. Several procedures are used such as mounting in a cold setting resin (Araldite) or inserting into a close-fitting holder of p.t.f.e. In the case of metal-solution systems that have a propensity for pitting care must be taken to avoid a crevice at the interface between metal specimen and the mounting material, and this can be achieved effectively by mounting the... 

This technique can be applied to samples prepared for study by scanning electron microscopy (SEM). When subject to impact by electrons, atoms emit characteristic X-ray line spectra, which are almost completely independent of the physical or chemical state of the specimen (Reed, 1973). To analyse samples, they are prepared as required for SEM, that is they are mounted on an appropriate holder, sputter coated to provide an electrically conductive surface, generally using gold, and then examined under high vacuum. The electron beam is focussed to impinge upon a selected spot on the surface of the specimen and the resulting X-ray spectrum is analysed. 

The specimen stage usually holds several specimens and standards, with dimensions typically 20-30 mm, although for special purposes extra-large specimen stages are available - Matsuya et al. (1988) describe a holder with a working area of 300 x 300 mm. An airlock isolated from the main vacuum chamber reduces the time taken to change specimens. 

Powder cameras. A powder camera consists essentially of an aperture system to define the X-ray beam, a holder for the specimen, and a framework for holding the photographic film. For most identification purposes a camera 9-10 cm in diameter is found satisfactory an X-ray beam about 0 5 mm wide is generally used, the powder specimen being a little narrower than this—of the order of 0 3 mm. 

Fig. 2.47 View of DMA specimen holder, red arrow indicates specimen and blue arrow indicates environmental chamber that closes over the specimen grips during testing...

A horizontal line is drawn along the center of the 155 x 800 mm specimen and vertical marks are made on the line at 25 mm intervals. A marked specimen backed by a ceramic fiber insulation board is mounted in the specimen holder. The holder is inserted when the panel reaches equilibrium and the heat flux at the 50 mm position is at the desired level. After preheating to steady conditions (see Equation 14.5 and subsequent discussion), the specimen is ignited at the hot end... 

The PHRR in the cone calorimeter is strongly dependent on the test setup and the specimen, as well as the intrinsic fire properties of the materials. To obtain comparable results, it is essential for the specimen and sample holder to be used as defined in the standard. This is illustrated in Figure 15.7, where the cone calorimeter HRR curves are compared with results using a modified sample holder.80 81... 

The specimen holder must be robust enough to support fairly large and heavy specimens, and it must be possible to rotate the holder about the diffractometer axis independently of the counter rotation, in order to change the angle ij/. 

The design of sample assemblies used for the laser-moire approach is illustrated in Figures 5 and 6. Portions of the test surface are protected by Teflon bars that have machined recesses to avoid contact with reference (unexposed) areas of the sample. For some samples, the Teflon is supplemented with Parafilm M gaskets. Nylon fasteners are used where required near specimens, and stainless steel screws fasten together the parts of the Lucite holders. Although some exchange of liquid water between unprotected and protected zones is unavoidable for porous samples having unsmooth surfaces, the liquid is uncontaminated by materials of the holders and may be equilibrated with the specimens no difficulty has been apparent. 

Figure 2. Schematic of the notched-disk specimen and its holder.

The diameter of the frictional tracks on the disks was 5 cm. After the specimen was mounted to the holder, the specimen was initially rubbed against a 6/0 grade Emery paper placed on the disk. This pre-rubbing was useful for allowing a uniform contact between the specimen surface and the disk. After the pre-rubbing, the specimen and disk surfaces were made as clean as possible by rubbing with a soft cloth wetted with ethylalcohol. All experiments in this work were carried out in the room air of 20 2 C and of 55 10 % relative humidity. 

Other methods involve holding specimens in suitable fixtures so that they form the walls of channels through which the test solution can be passed at controlled rates of flow. Such devices have been used at the Harbor Island Test Station in North Carolina primarily for studying the electrode potential and polarisation characteristics of metals and alloys, but they are also suitable for observing effects of velocity on corrosion. This is illustrated in Fig. 19.5 in which the specimen and Pt electrode are of the same size and are placed parallel to one another in the holder. When required potentials are measured by inserting a capillary through the hole in the Pt it is then removed to avoid shielding effect. 

Capsule holders are typically located circumferentially around the core to provide assurance that test specimens and monitors duplicate, as closely as possible, the irradiation history of the reactor vessel, including neutron spectrum, temperature history and maximum neutron fluence. Holders must be designed to avoid interference with in-service inspections required by Section XI of the ASME Code (ASME, 2010a). Most surveillance programs have the capsules located between the thermal shield and the reactor vessel wall with the vertical center at the core mid-plane. C-E, General Electric and B W designs have the holders attached directly to... 

A typical capsule assembly, illustrated in Figure 5.3-1, consists of a series of three specimen compartments, connected by wedge couplings, and a lock assembly. Each compartment enclosure of the capsule assembly is internally supported by the surveillance specimens and is externally pressure tested to 3125 psi during final fabrication. The wedge couplings also serve as end caps for the specimen compartments and position the compartments within the capsule holders which are attached to the reactor vessel cladding. The lock assemblies fix the locations of the... 

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