Engineering Assignment 代写 Powder X Ray Diffraction

Engineering Assignment 代写  Powder X Ray Diffraction

X-ray Powder Diffraction (XRD) Instrumentation - How Does It Work?

X-ray diffractometers consist of three basic elements: an X-ray tube, a sample holder, and an X-ray detector. X-rays are generated in a cathode ray tube by heating a filament to produce electrons, accelerating the electrons toward a target by applying a voltage, and bombarding the target material with electrons. When electrons have sufficient energy to dislodge inner shell electrons of the target material, characteristic X-ray spectra are produced. These spectra consist of several components, the most common being Kα and Kβ. Kα consists, in part, of Kα1 and Kα2. Kα1 has a slightly shorter wavelength and twice the intensity as Kα2. The specific wavelengths are characteristic of the target material (Cu, Fe, Mo, Cr). Filtering, by foils or crystal monochrometers, is required to produce monochromatic X-rays needed for diffraction. Kα1and Kα2 are sufficiently close in wavelength such that a weighted average of the two is used. Copper is the most common target material for single-crystal diffraction, with CuKα radiation = 1.5418Å. These X-rays are collimated and directed onto the sample. As the sample and detector are rotated, the intensity of the reflected X-rays is recorded. When the geometry of the incident X-rays impinging the sample satisfies the Bragg Equation, constructive interference occurs and a peak in intensity occurs. A detector records and processes this X-ray signal and converts the signal to a count rate which is then output to a device such as a printer or computer monitor.

Applications

One of the best methods of determining a crystal's structure is by X-ray diffraction.

X-ray powder diffraction is most widely used for the identification of unknown crystalline materials (e.g. minerals, inorganic compounds). Determination of unknown solids is critical to studies in geology, environmental science, material science, engineering and biology.

Other applications include:

characterization of crystalline materials

Identification of fine-grained minerals such as clays and mixed layer clays that are difficult to determine optically

determination of unit cell dimensions

measurement of sample purity

With specialized techniques, XRD can be used to:

determine crystal structures using Rietveld refinement

determine of modal amounts of minerals (quantitative analysis)

characterize thin films samples by:

determining lattice mismatch between film and substrate and to inferring stress and strain

determining dislocation density and quality of the film by rocking curve measurements

measuring superlattices in multilayered epitaxial structures

determining the thickness, roughness and density of the film using glancing incidence X-ray reflectivity measurements

Make textural measurements, such as the orientation of grains, in a polycrystalline sample.

Uses of X-Ray Powder Diffraction

The most widespread use of x-ray powder diffraction, and the one we focus on here, is for the identification of crystalline compounds by their diffraction pattern. Listed below are some specific uses that we will cover in this course:

Identification of single-phase materials - minerals, chemical compounds, ceramics or other engineered materials.

Identification of multiple phases in microcrystalline mixtures (i.e., rocks)

Determination of the crystal structure of identified materials

Identification and structural analysis of clay minerals

Recognition of amorphous materials in partially crystalline mixtures

Below are some more advanced techniques. Some of these will be addressed in an introductory fashion in this course. Many are left for more advanced individual study.

Crystallographic structural analysis and unit-cell calculations for crystalline materials.

Quantitative determination of amounts of different phases in multi-phase mixtures by peak-ratio calculations.

Quantitative determination of phases by whole-pattern refinement.

Determination of crystallite size from analysis of peak broadening.

Determine of crystallite shape from study of peak symmetry.

Study of thermal expansion in crystal structures using in-situ heating stage equipment.

Three major uses;