Powder samples, Debye-Scherrer camera

X-Ray Powder Patterns. Samples for x-ray pattern determinations were sealed in 0.2-mm. glass capillary tubes under an atmosphere of argon. The samples were then exposed to nickel-filtered, CuKa radiation in an 11.459-cm. Debye-Scherrer camera for 18 to 20 hours. 

The X-ray powder data were measured by the large Debye-Scherrer camera at SPring-8 BL02B2 using solvent-free (Sc2C2) C84 powder sample. The exposure time on the IP was 80 min. The wavelength of incident X-rays was 0.75 A. The... 

Cylindrical samples, which are common in the Debye-Scherrer cameras Figure 3.2), are also used in powder diffractometry. Similar to flat transmission samples, small amounts of powder are required in the cylindrical specimen geometry. This form of the sample is least susceptible to the non-random distribution of particle orientations, i.e. to preferred orientation effects. 

XRD patterns were recorded on a Philips X-ray generator with a Debye-Scherrer camera. CuKa X-rays (X, = 1.5418 A) were used as the X-ray source. XRD measiuements were performed on powdered washcoat scraped fi om the comer region of exposed channels on the outer surface of the cores. The XRD apparatus used to analyze the samples have been described in detail previously [8]. Particle sizes were estimated using the Scherrer equation with correction for instrumental line broadening. 

Crystalline materials can be identified by rapid computerized powder diffraction techniques. The principle of this technique [6,7] is that the crystallites within a sample, placed in a collimated x-ray beam, reflect x-rays at specific angles and intensities. The diffraction pattern can be recorded photographically, using a camera, e.g. a Debye-Scherrer camera, or using a powder diffractometer. Chemical analysis depends on the fact that each chemical composition and crystallographic structure produces a unique angular distribution of diffracted intensity. Analysis is based on comparison of the diffractometer scan with known standards. Typical applications of the powder diffraction technique to polymers would be the identification of mineral fillers in engineering resins, the nature of crystalline contaminants and determination of crystalline phases in a material. 

The Debye - Scherrer camera is the most basic system with which measurements on a limited range of liquid crystalline materials can be made. The camera can only be used with powder samples, i. e., a sample with many microdomains covering all orientations with respect to the X-ray beam. 

Figure 5.8 A Debye-Scherrer powder camera for X-ray diffraction. The camera (a) consists of a long strip of photographic film fitted inside a disk. The sample (usually contained within a quartz capillary tube) is mounted vertically at the center of the camera and rotated slowly around its vertical axis. X-rays enter from the left, are scattered by the sample, and the undeflected part of the beam exits at the right. After about 24 hours the film is removed (b), and, following development, shows the diffraction pattern as a series of pairs of dark lines, symmetric about the exit slit. The diffraction angle (20) is measured from the film, and used to calculate the d spacings of the crystal from Bragg s law.

The x-ray diffraction patterns were obtained by mounting the sample particles on a glass filament in a 114.59-mm diameter powder camera (Debye-Scherrer) and irradiating with Cu-Ka x-rays at 30 kV and 15 mA for periods of time ranging from 8 to 24 h. 

A) Schematic of the Debye-Scherrer method, developed in 1916, for X-ray diffraction of powders (polycrystdlline samples). Each characteristic interplanar spacing in the crystal gives rise to a cone of diffracted X-rays, segments of which are captured on the film strip placed inside the camera. 

The classical photographic method for recording powder diffraction patterns is still used, particularly when the amount of sample is small. The most common instrument forthis purpose is the Debye-Scherrer pov/det camera, which is shown schematically in Figure 12-17a. Here, the beam from an X-ray tube is filtered to produce a nearly monochromatic beam (often the copper or molybdenum Ka line), which is collimated by passage through a narrow tube. 

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