Comparison and combination of imaging techniques for three dimensional analysis of electrical trees

Abstract

Imaging of electrical trees has been an important tool for studying the phenomenon. The authors have previously shown that electrical trees can be three-dimensionally (3D) imaged and virtual replicas generated using X-ray Computed Tomography (XCT) or Serial Block-Face Scanning Electron Microscopy (SBFSEM). Here these techniques are evaluated and compared for 3D analysis of electrical trees along with conventional optical methods. A number of types of laboratory created trees showing range of morphologies were grown and examined to delineate the capabilities of each technique. Cross-sectional images and virtual replicas of the electrical trees from XCT and SBFSEM techniques were compared both qualitatively and quantitatively. SBFSEM provides greater detail than XCT, evidenced by imaging smaller sub-branches and when comparing parameters such as the number of tree channels, tree length or tree volume captured. On average, SBFSEM captures almost double the number of tree channels per slice than XCT, and virtual replicas in most of the cases have larger volumes. However, SBFSEM is a destructive technique, which makes the imaging process less reliable than XCT and not suitable for multi-stage of tree growth experiments. For full analysis, a combination of imaging techniques is proposed. Optical methods are used first to monitor tree growth. Then, micro-XCT which provides pixel size down to ~0.4 µm with a field of view of around 1 mm × 1 mm, can be used to reveal the overall 3D structure of a normal/mature electrical tree. Nano-XCT can be used to explore in more detail regions of interest, with a pixel size of ~65 nm, but a limited field of view of 65 µm. Finally, sections of the tree can be analyzed in even greater detail using SBFSEM, which can provide resolutions below 50 nm. Using this approach, a deeper and more complete analysis of the structure of electrical trees can be achieved.

Three dimensional imaging of electrical trees in micro and nano-filled epoxy resin

“…Each sample was scanned over a period of ~2 h in which 1800 radiographs were taken with 3 s of exposure time per radiograph, using an X-ray energy of 11-13 keV. The pixel size was 0.45 µm which provided a field of view of 1.8 mm×1.2 mm per radiograph [8]. Reconstructed images were cropped maintaining the region of interest where the tree was located.…”

“…In both cases, then the region of interest containing the feature was selected for 3D model generation. Virtual replicas of electrical trees were generated, segmenting the tree from the stack of images resulting from the XCT and SBFSEM scans [8], using Avizo imaging processing software.…”

“…Recently, the authors have deployed X-ray computed tomography (XCT) and serial-block-face scanning electron microscopy (SBFSEM) techniques for 3D imaging of electrical trees in unfilled polymers [5,8]. In the past, 3D images of electrical trees in translucent XLPE have been generated using computed tomography and serial-sectioning methods combined with optical microscopy [9].…”

Three dimensional imaging of electrical trees in multiple stages

“…After the electrical tree initiated and propagated to a length of 115 μm, the voltage was removed. The micro-XCT system in the Manchester X-ray Imaging Facility (MXIF) has been introduced in [7]. The XCT Versa-520 system has a 0.2382 μm pixel size and the voltage was set at 110 KeV.…”

“…More recently, three high-resolution systems for visualization and generation of 3D electrical trees have been developed exploiting developments in X-ray techniques and electron microscopy [7,8]. Serial Block-Face Scanning Electron Microscopy (SBFSEM) has the same basic mechanism as the serial sectioning method (SSM) in [6] but can provide an improved resolution (~50 nm).…”