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Transcript
Chapters 6 & 9 X-ray Diffractometry and Phase Identification Wenjea J. Tseng Department of Materials Science and Engineering National Chung Hsing University URL: http://audi.nchu.edu.tw/~wenjea/ Email: [email protected] Chapters 6 & 9 Introduction After the work of W.H. and W. L. Bragg on x-ray spectra and crystal structure, diffractometry passed in to a long period of relative disuse during which photographic recording in cameras was the most popular method of observing diffraction effects. In the late 1940s, commercial diffractometer instruments became available and rapidly became popular because they offered certain particular advantages over the film techniques. Bragg’s Law (1912) William H. Bragg William L. Bragg Chapters 6 & 9 Introduction (cont.) Debye-Scherrer camera method modify the Laue photography by adopting the Bragg Law into account. Debye-Scherrer Camera X-ray in Camera film is “taped” around the inner circle of the sampling chamber with sample being placed at the center. Sample position Chapters 6 & 9 Introduction (cont.) Problems associated with the camera method include primarily the lack of quantitative intensity information due to difficulties in distinguishing overlapping lines with close two theta values. Chapters 6 & 9 General Features A diffractometer is designed with a movable detector replaces the strip of film. The intensity of a diffracted beam is measured directly by the electronic x-ray detector, which converts incoming x-rays into surges or pulses of electric current for further computer processing. Diffractometers X-ray Source Sample Chamber Detector X-ray Source Detector Sample Chambe r Chapters 6 & 9 Basic Configuration Sample Chamber Detector X-ray Source Chapters 6 & 9 Example Chapters 6 & 9 Comparison Tungsten Five “rings” Five “peaks” Difficult to discern in camera film. Discernible in diffracted peaks. X-ray Source Chapters 6 & 9 Cross-sectional schematic Real cross-sectional photograph. Chapters 6 & 9 X-ray Source Chapters 6 & 9 X-ray Optics X-ray Source Sample Chambe r Detector How to “focus” the X-ray beam is a difficult issue. Use slit to get “parallel” rays. Sacrifice some x-ray intensity. Can adjust the slit size. Chapters 6 & 9 X-ray Optics (cont.) Slit is needed also on the detector side. X-ray Source Sample Chambe r Detector X-ray incoming slit X-ray diffracting slit Sacrifice some diffracted x-ray intensity. Can adjust the slit size. Chapters 6 & 9 X-ray Source X-ray Detection Sample Chambe r Detector Geiger counter Voltage Chapters 6 & 9 Sample Preparation Flat metal sheet or plate may be examined by diffractometer directly; however, such materials almost always exhibit preferred orientation so that relative intensity between diffracted peaks may vary. Whenever possible, powder samples are best choice for the diffractometry. Yet, packing of the powdered sample requires precautions. Use B-side for the incoming x-ray. Chapters 6 & 9 Example 1 A gel consisting of AgBr crystal is placed within a sample holder for x-ray examination. The gel surface was initially flat within the sample holder, while the surface of gel became more concave because of the drying shrinkage with time. Time increases Chapters 6 & 9 Example 2 Aluminum powder was packed level (0 displacement), above (+ displacement), and below (- displacement) the sample plane. The observed 2θ values are 38.423o, 38.665o (+0.242o), and 38.281o (-0.142o), respectively. This represents calculated displacement of +450 and -260 µm, respectively. Displacement of Al powder Use silicon powder as an internal standard for the 2θ precision. Chapters 6 & 9 Tips on the Internal Standard The most frequently used method to solve such a displacement problem is to include an internal standard in the sample tested. The selection of the internal standard material is, first of all, the diffracted peaks from the standard should NOT overlap with that from the tested sample. Second, the standard sample should NOT react with the tested sample. Pure materials such as Si, Ag in the powder form are frequently used. Chapters 6 & 9 Practice in Phase Identification A given crystalline substance (e.g., pure element, mixture, or compound) always produces a characteristic diffraction pattern. An unknown sample can hence in theory be identified by comparing its x-ray diffraction pattern with those in the databank. Qualitative analysis of unknown samples can be accomplished. Quantitative analysis is also possible because the intensities of the diffraction lines due to one phase of a mixture (or compound) depend on the proportion of that phase in the specimen. Any one powder pattern is characterized by a set of line positions 2θ and a set of relative line intensities I/Imax scaled relative to Imax. Chapters 6 & 9 XRD Databank Chapters 6 & 9 Typical JCPDS Card Chapters 6 & 9 Example JCPDS (or PDF) databank Experimentally obtained data Chapters 6 & 9 Choice of X-ray Source Chapters 6 & 9 Hanawalt Search Method Chapters 6 & 9 Hanawalt Search Method Chapters 6 & 9 Example: One Component 5-490 Chapters 6 & 9 Example: Multi-components Chapters 6 & 9 Example: Multi-components Chapters 6 & 9 Remarks Problems due to inadequate data treatment in finding the angular positions (i.e., 2θ) of the peak maxima, plus uncertainty in the value of the experimental wavelength, all conspire to add errors to the experimental d-values. In addition, the experimental intensities may be similarly distorted due to problems of preferred orientation, poor crystallinity, partially resolved diffraction wavelength multiplets, and line broadening due to particle size and/or strain considerations.