XRD and Residual stress measurement- lab demonstration

Key Concepts

• X-ray Diffraction (XRD)
• Crystal Structure Determination
• Stress Measurements
• Phase Identification
• Goniometer (Diffractometer Circle)
• X-ray Source
• Specimen Stage
• Detector
• Bragg-Brentano Diffractometer
• Focusing Geometry
• Para focusing
• Slits and Filters
• X-ray Tube
• Anode Material
• Beryllium Window
• Residual Stress Measurement

X-ray Diffraction (XRD) Applications Recap

The video begins by recapping the applications of X-ray diffraction discussed in the previous class, which include:

• Crystal structure determination: Determining the arrangement of atoms within a crystalline material.
• Stress measurements: Measuring the residual stresses present in a material.
• Phase identification: Identifying the different phases present in a phase mixture.

XRD Equipment Demonstration: Overview

The main focus of the video is a demonstration of XRD equipment in the lab. The presenter aims to provide a basic understanding of how the equipment looks, its primary components, and how an XRD experiment is performed on a polycrystalline material.

Components of the XRD Machine

The presenter introduces the Bruker X-ray diffractometer, highlighting the following components:

• Goniometer (Diffractometer Circle): A large circular disc that holds the X-ray source and detector.
• Specimen Stage: The platform where the sample (bulk or powder) is placed. Powder samples are typically mixed with a medium and placed on a glass slide.
• X-ray Source: The source of X-rays, which are directed towards the specimen.
• Detector: Detects the diffracted X-rays. It contains slits and filters.

Key Point: The X-ray source and detector are positioned on the goniometer circle.

X-ray Optics and Focusing Geometry

The presenter explains the importance of keeping the sample in the center of the goniometer circle by discussing X-ray optics and focusing geometry.

• Focusing Geometry: The presenter draws a diagram on the blackboard to illustrate the focusing geometry, where X-rays from the source are incident on the sample and the diffracted rays are focused at a point F. The angle between the incident and diffracted rays is 2θ.
• Bragg-Brentano Diffractometer: The type of diffractometer used, which employs a specific focusing geometry.
• Para focusing: The presenter draws two more figures to show how the Bragg-Brentano geometry and para focusing method are adopted in the diffractometer. There are two circles: the diffractometer circle (goniometer circle) and the focusing circle. The focusing circle varies depending on the 2θ angle. The specimen, detector, and source are positioned on the focusing circle. As the 2θ angle changes, the focusing circle also changes, maintaining the Bragg-Brentano geometry.

Slits and Filters

The presenter discusses the role of slits and filters in controlling the X-ray beam:

• Slits: Used to control the divergence of the X-ray beam, collimating it before it falls on the specimen.
• Filters: Used to suppress unwanted X-ray radiations, such as Kβ, while allowing the desired Kα radiation to pass through.

Specimen Stage Movement

The presenter demonstrates the movement of the specimen stage:

• Rotation: The stage can rotate 360 degrees.
• Semi-circular Movement: The stage can move in a semi-circular fashion.
• Purpose: These movements ensure that the sample is exposed to X-rays from all possible diffracting planes, providing statistically important data, especially for powder samples. These rotations are also exploited in texture measurements.

X-ray Tube Details

The presenter shows the X-ray tube, including:

• Target Material (Anode): Can be changed based on the specimen being analyzed (e.g., chromium, copper).
• Beryllium Window: A window transparent to X-rays, allowing them to exit the tube.

XRD Experiment Demonstration

The presenter demonstrates a complete XRD experiment using α-silicon oxide powder:

• Data Collection: The instrument collects data at various 2θ angles.
• Software Analysis: The collected data is analyzed by interface software, which filters the background and generates an X-ray diffraction pattern.
• Diffraction Pattern: A typical polycrystalline diffraction pattern is displayed, showing peaks corresponding to the characteristic peaks of α-silicon oxide.
• Phase Identification: The 2θ values can be compared to a JCPDS database to identify the crystal system.

Residual Stress Measurement System

The presenter introduces another XRD machine used for residual stress measurements:

• Parallel Beam: Unlike the Bragg-Brentano geometry, this machine uses a parallel X-ray beam.
• Detectors: Two detectors are positioned side by side to the X-ray source.
• Scanning: The X-ray source rotates in an off-circle manner, and the detectors scan for 2θ measurements.
• Advantage: This setup allows for the scanning of large components without size restrictions.

Conclusion

The video provides a basic overview of XRD equipment, including its components, operation, and applications. The presenter demonstrates both a powder diffractometer and a residual stress measurement system, highlighting the differences in their X-ray optics and scanning methods. The aim is to bridge the gap between theoretical knowledge and practical application, giving viewers a better understanding of how XRD experiments are conducted and how data is generated.