The next blog in the characterization miniseries was written by my colleague Anjali Singhal who started working in our organization about six weeks ago. She will be blogging about a new technique which we implemented in order to look inside of devices without disassembling the device!
I’m Anjali Singhal. I grew up in Jamshedpur, which is in the eastern part of India. I obtained my Bachelor’s degree in Metallurgical and Materials Engineering, from the Indian Institute of Technology at Kharagpur. Thereafter I joined the Department of Materials Science and Engineering at Northwestern University, in Evanston, Illinois, to pursue my doctorate degree. My dissertation work focused on understanding the mechanical properties of bones at the nanoscale using X-ray diffraction. I joined GE about a month and a half ago in the Chemical and Structural Analysis Laboratory. Here I use X-rays to image materials using the Micro/Nano-CT.
1. What is Micro-CT ?
Micro computed tomography or “micro-CT” is X-ray imaging in 3D, by the same method used in hospital CT (or “CAT”) scans, but on a small scale with much higher resolution. It is essentially a 3D microscopy technique, where the very fine scale internal structure of objects is imaged non-destructively. No special sample preparation is required for this technique.
2. How does the technique work?
A micro-focus X-ray source illuminates the object which is placed on a rotation stage as illustrated in the schematic.
The X-rays that pass through the object are scattered and/or absorbed. As the sample rotates, absorption (or “shadow” transmission) images of the sample are collected by an X-ray detector. An X-ray shadow image is essentially a two-dimensional projection from a three-dimensional object. Each point in the shadow image contains an integration of absorption information of the part of the object in the corresponding part of the X-ray beam. These absorption images contain information about the density of the materials through which the X-ray beam traverses. Therefore, a higher density material will have greater X-ray absorption. Once the rotation is complete, specialized algorithms are used to synthesize these shadow images to create a complete 3D representation of internal microstructure of the sample imaged. The 3D images consist of voxels (3D pixels), and with a visualization software, the 3D volume can be manipulated in real time.
3. Why do we use the instrument?
The main advantage of using Micro-CT is that a complete model of the internal and external structure of the object is obtained non-destructively. The CT works with any surface, geometry, color or material, up to a certain density and/or thickness penetrable with X-rays. The start-to-finish scan can take as little as half an hour, depending on the resolution requirements, the size and density of the object. The 3D volume generated from reconstruction can be manipulated with a visualization software. Because of this, it is possible to slice through anywhere inside the object, inspect and look for defects such as delaminations, cracks and voids, take accurate measurements of structural features, reconstruct a surface model to compare with CAD drawings etc.
Examples of CT images:
1. The following is an image of a section of a circuit board which was inspected by CT to determine if there are any joins in the solder balls which could have lead to failure of the circuit board. The solder balls are highlighted in yellow and the board is highlighted green. Near the top left corner of the image, within the red circle, the joining between the solder balls can be clearly seen. The CT is thus an easy way to inspect circuit boards for such defects.
2. The next example is a CT image of a carbon-epoxy composite shown in the bottom left figure. The voids which are less dense than the material are seen in black. The right figure shows a 3D rendering of a number of slices in the volume of the sample imaged, with the pores highlighted yellow and the matrix cyan. The volume fraction of pores and their spatial distribution in the sample can be calculated using the visualization software.
3. The next example of another carbon fiber composite illustrates that, samples with complex geometries can also be imaged using CT. The figure on the left is a 2D CT slice of the 3D object shown on the right. The architecture of the composite can be viewed from the 2D image, where delaminations between two fibers are evident within the red circle.
4. The last example is that of a cell. The different components of the cell can be clearly seen in the two CT images shown below. The CT data can then be used to select and visualize individual components of the cell without taking it apart.
From all the examples above, we have seen that the CT is a very useful tool to look “inside” objects of all kinds. All the images presented here are taken using our brand new V|tomex|M Nano-CT instrument.
Feel free to ask me any questions below!