Uses And Application Of Powder Xray Diffraction Engineering Essay

From the research on my topic" Powder x ray diffraction" I studied the various uses and application of powder x ray diffraction. Present day, it has a great importance. In higher level industries its importance is very much. It helps to determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and diffracts into many specific directions. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, their disorder and various other information.

Since many materials can form crystal such as salts, metals, minerals, semiconductors, as well as various inorganic, organic and biological molecules. X-ray crystallography has been fundamental in the development of many scientific fields. In its first decades of use, this method determined the size of atoms, the lengths and types of chemical bonds, and the atomic-scale differences among various materials, especially minerals and alloys.

Contents

Introduction

What is X-ray Powder Diffraction (XRD)

Fundamental Principles of X-ray Powder Diffraction (XRD)

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

Applications

With specialized techniques, XRD can be used to:

Uses of X-Ray Powder Diffraction

Strengths of X-ray Powder Diffraction (XRD)

Limitations of X-ray Powder Diffraction (XRD)

Bibliography

INTRODUCTION

The discovery of X-rays in 1895 enabled scientists to probe crystalline structure at the atomic level. X-rays are electromagnetic radiation of wavelength about 1 Å (10-10 m), which is about the same size as an atom. They occur in that portion of the electromagnetic spectrum between gamma-rays and the ultraviolet.

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Powders of crystalline materials diffract x-rays. A beam of x-rays passing through a sample of randomly-oriented microcrystal produces a pattern of rings on a distant screen. Powder x-ray diffraction provides less information than single-crystal diffraction, however, it is much simpler and faster. Powder x-ray diffraction is useful for confirming the identity of a solid material and determining crystallinity and phase purity.

What is X-ray Powder Diffraction (XRD)

X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions. The analyzed material is finely ground, homogenized, and average bulk composition is determined.

X-ray diffraction has been in use in two main areas, for the fingerprint characterization of crystalline materials and the determination of their structure.

Each crystalline solid has its unique characteristic X-ray powder pattern which may be used as a "fingerprint" for its identification. Once the material has been identified, X-ray crystallography may be used to determine its structure, i.e. how the atoms pack together in the crystalline state and what the interatomic distance and angle are etc. X-ray diffraction is one of the most important characterization tools used in solid state chemistry and materials science.

We can determine the size and the shape of the unit cell for any compound most easily using the diffraction of x-rays.

Fundamental Principles of X-ray Powder Diffraction (XRD)

X-ray diffraction is now a common technique for the study of crystal structures and atomic spacing.

X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law (nλ=2d sin θ). This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed and counted. By scanning the sample through a range of 2θangles, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. Conversion of the diffraction peaks to d-spacing allows identification of the mineral because each mineral has a set of unique d-spacing. Typically, this is achieved by comparison of d-spacing with standard reference patterns.

All diffraction methods are based on generation of X-rays in an X-ray tube. These X-rays are directed at the sample, and the diffracted rays are collected. A key component of all diffraction is the angle between the incident and diffracted rays. Powder and single crystal diffraction vary in instrumentation beyond this.

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;-

X-ray radiography is used for creating images of light-opaque materials. It relies on the relationship between density of materials and absorption of x-rays. Applications include a variety of medical and industrial applications.

X-ray crystallography relies on the dual wave/particle nature of x-rays to discover information about the structure of crystalline materials.

X-ray fluorescence spectrometry relies on characteristic secondary radiation emitted by materials when excited by a high-energy x-ray source and is used primarily to determine amounts of particular elements in materials.

Strengths and Limitations of X-ray Powder Diffraction (XRD)

Strengths

Powerful and rapid (< 20 min) technique for identification of an unknown mineral

In most cases, it provides an unambiguous mineral determination

Minimal sample preparation is required

XRD units are widely available

Data interpretation is relatively straight forward

Limitations

Homogeneous and single phase material is best for identification of an unknown

Must have access to a standard reference file of inorganic compounds (d-spacing's, )

Requires tenths of a gram of material which must be ground into a powder

For mixed materials, detection limit is approximate 2% of sample.

For unit cell determinations, indexing of patterns for non-isometric crystal systems is complicated

Peak overlay may occur and worsens for high angle 'reflections'

Future Prospective:-

The method also revealed the structure and functioning of many biological molecules, including vitamins, drugs, proteins and nucleic acids such as DNA. X-ray crystallography is still the chief method for characterizing the atomic structure of new materials and in discerning materials that appear similar by other experiments. X-ray crystal structures can also account for unusual electronic or elastic properties of a material, shed light on chemical interactions and processes, or serve as the basis for designing pharmaceuticals against diseases.

It's future is bright because in nano technology, Solid Oxygen ε-Phase Crystal Structure Determined only with the help of powder x ray diffraction. All companies attempted powder X-ray diffraction experiments and structural analyses for the ε-phase in nano technology. In future, this non-destructive technique widely applied for the characterization of crystalline materials.