Materials Lab 1(MT344) X-ray Diffractometer Operation and Data Analysis. Instructor: Dr. Xueyan Wu ( 吴雪艳 )

Transcription

1 Materials Lab 1(MT344) X-ray Diffractometer Operation and Data Analysis Instructor: Dr. Xueyan Wu ( 吴雪艳 )

2 Goals To give students a practical introduction into the use of X-ray diffractometer and data collection.

3 How are X-rays produced? Hot-Cathode X-Ray Tube

4 Characteristic X-ray Two sharp peaks Right: Kα, X-rays produced by transitions from the n=2 to n=1 levels. Left: K β, X-rays produced by transitions from the n=3 to n=1 levels.

5 X-ray Diffractometer As the counter moves at constant angular velocity, a recorder automatically plots the diffracted bean intensity, which is monitored by the counter, as a function of 2θ. Schematic diagram of an X-ray diffractometer; T= x-ray source, S= specimen, C= detector, and O= the axis around which the specimen and detector rotate. 2θ: diffraction angle.

6 Filter: to filter K β, a suitable material is placed between the X-ray tube and the specimen. Filter materials should be chosen according to target materials. In general, the filter element has an atomic number which is 1 or 2 smaller than the target element. Z T < 40: Z F = Z T -1 Z T 40: Z F = Z T -2 T: target; F: filter

7 Widely-used Target and according Filter Target Atom No. K α Wavelength (nm) K ẞ Wavelength (nm) Filter Filter Atom No. λ k Cr V Fe Mn Co Fe Ni Co Cu Ni Mo Zr Ag Rh

8 The Bragg Law Suppose that the incident X-rays are reflected specularly from parallel planes of atoms in the crystal The path difference between beams should be an integer of the wavelength to have constructive interference: 2d sinθ = nλ The value of θ can be used to determine on which crystallographic plane the diffraction takes place. The relative intensities of the diffracted beams are determined by the composition of the material.

9 D V : volume weighted crystallite size; K :Scherrer constant describing the shape of the crystal (typical values fall in the range ; K=0.9 corresponds to "spherical crystals); λ : X-ray wavelength; B = integral breadth of a reflection (in radians) located at 2θ. B can be assumed to take a Lorentzian distribution; B : 1 2 π FWHM.

10 Positions, intensities and profile characteristics of Bragg peaks in an X-ray diffraction (XRD) can provide information of: atomic structure lattice constant atom position microstructure crystallite size microstrain preferred orientation

11 Thank you!

12 What are X-rays and Course How Description to Produce Them? Kinds of waves in the electromagnetic spectrum X-rays: energies ranging 100 ev to 100 KeV wavelength ranging around 0.01~10 nm

13 What is XRD X-ray diffraction (XRD) is a method of analyzing the diffraction pattern of a material by X-ray diffraction, obtaining the composition of the material, the structure or morphology of the atom or molecule inside the material.

14 2.2.Device for measurement Goniometer( 量角仪 ) change the incidence angle Counter ( 计数器 ) collect the phonons released by X-ray Monochromator( 单色仪 ) filter the light

15 2. Theory 2.3.Factors influencing the final choice Factors Reasons Target materials Background won't be too strong Mainly chosed by atomic number Filters Slits Scanning range Scanning speed Determined by the target material Ensure the X-ray in place 2 to 90 (normal condition) 2 /min or 4 /min

16 XRD: Course X-ray Description Diffraction Application: 1. To identify unknown crystalline materials 2. To characterize crystalline materials 3. To determine unit cell dimensions 4. To measure sample purity 5. To determine crystal structures using Rietveld refinement. 6. To determine of modal amounts of minerals (quantitative analysis) 7.

17 Data Collection The intensity of diffracted X-rays is continuously recorded as the sample and detector rotate through their respective angles. A peak in intensity occurs when the mineral contains lattice planes with d-spacings appropriate to diffract X-rays at that value of θ. Results are commonly presented as peak positions at 2θ and X-ray counts(intensity) in the form of a table or an x-y plot(shown in next slide page)

18 XRD pattern of α-iron polycrystal(cubic)

19 2.4.Phase Analysis 2. Theory PDF for SiO2 Qualitative phase analysis: 1.obtain X-ray diffraction spectrum 2.calculate planar spacing d&arrange them with according intensity I 3.compare the results with standard PDF Quantitative analysis: Acquire te fraction of different components in the material Figure 4 Powder Diffraction File of SiO 2

20 Determination of an unknown The d-spacing of each peak is obtained by solution of the Bragg equation for the appropriate value of λ. Automated search-match routines can be applied to compare the ds with those of known materials, a systematic procedure is used by ordering the d-spacings in terms of their intensity beginning with the most intense peak. Principle : Each minerals has a unique set of d-spacings.

21 Determination of an unknown Sources: 1) the International Centre for Diffraction Data(the Powder Diffraction File, PDF) 2) American Mineralogist Crystal Structure Database Note: 1) Phases as little as 1-3% sample weight can be identified 2) Samples must be crystalline.

22 References: Fundamentals Of Material Science and Engineering, 4 th Edition, Chapter 3, William D. Callister, JR. David G. Rethwisch. Elements of X-ray Diffraction, Cullity and Stock Introduction to X-ray Powder Diffractometry, Jenkins and Snyder Fundamentalls fo Powder Diffraction and Structural Characterization of Materials, Pecharsky and Vitalij