Letter pubs.acs.org/NanoLett
Three-Dimensional Nanometer Features of Direct Current Electrical Trees in Low-Density Polyethylene Love K. H. Pallon,† Fritjof Nilsson,† Shun Yu,† Dongming Liu,† Ana Diaz,‡ Mirko Holler,‡ Xiangrong R. Chen,§ Stanislaw Gubanski,§ Mikael S. Hedenqvist,† Richard T. Olsson,† and Ulf W. Gedde*,† †
School of Chemical Science and Engineering, Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden ‡ Paul Scherrer Institute, 5232 Villigen PSI, Switzerland § Department of Materials and Manufacturing Technology, High Voltage Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden S Supporting Information *
ABSTRACT: Electrical trees are one reason for the breakdown of insulating materials in electrical power systems. An understanding of the growth of electrical trees plays a crucial role in the development of reliable high voltage direct current (HVDC) power grid systems with transmission voltages up to 1 MV. A section that contained an electrical tree in low-density polyethylene (LDPE) has been visualized in three dimensions (3D) with a resolution of 92 nm by X-ray ptychographic tomography. The 3D imaging revealed prechannel-formations with a lower density with the width of a couple of hundred nanometers formed around the main branch of the electrical tree. The prechannel structures were partially connected with the main tree via paths through material with a lower density, proving that the tree had grown in a step-by-step manner via the prestep structures formed in front of the main channels. All the prechannel structures had a size well below the limit of the Paschen law and were thus not formed by partial discharges. Instead, it is suggested that the prechannel structures were formed by electro-mechanical stress and impact ionization, where the former was confirmed by simulations to be a potential explanation with electro-mechanical stress tensors being almost of the same order of magnitude as the short-term modulus of low-density polyethylene. KEYWORDS: Electrical tree, ptychography, DC-tree, HVDC, polyethylene
E
While trees induced by alternating current (AC) have been extensively studied,6−9 there is less information available on DC-trees, although DC-trees were observed already in the 1970s when polarity reversal occurred or when impulses were superimposed.10−13 DC-trees in LDPE can be generated in a controlled manner by prestressing the sample with a DCvoltage, followed by a sequence of opposite voltage impulses11 simulating an HVDC-cable polarity reversal. The tree grows instantaneously when a polarity shift occurs due to the high electric fields developed from the prestressed injected space charges and the reversed electrode polarity, where the rapid growth is similar to the formation and propagation of streamers in liquids, e.g., in transformer oils.14 The growth path of DC-trees has been attributed to the electro-mechanical stress that arises from the injected space charges and also to the detrapping and acceleration of injected charges to form avalanches that break molecular bonds.10 AC-
lectrical cable insulation failure is detrimental to society, leading in ill-fated situations not only to power losses but also to fires, wrecked machines, losses in production, and, in the worst case, casualties. One of the main polymeric cable insulating materials, low-density polyethylene (LDPE), ages over time and becomes more sensitive to electrical breakdown due to material degradation and electron avalanches.1 The latter may be promoted inside the materials, destroying the material structure and forming dendritic structures, known as electrical trees (like lightning in a thunderstorm).2 The electrical trees are initiated at points of a highly divergent electrical field, such as contaminants and voids, and they grow slowly over long periods of time until they initiate a breakdown.3−5 In the pursuit of an increased voltage for trans-continental power transmission (e.g., high-voltage direct current (HVDC) cables going from 320 kV to 1 MV) to meet the increasing demand for electricity and renewable energy, a greater electrical stress is imposed on the insulation, and thus, there is a greater risk of electrical breakdown, making it important to gain further knowledge about electrical direct current (DC) trees.5 © XXXX American Chemical Society
Received: October 14, 2016 Revised: February 2, 2017 Published: February 8, 2017 A
DOI: 10.1021/acs.nanolett.6b04303 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. (a) FIB-prepared pillar with the embedded electrical tree mounted on the sample mount with the scale bar corresponding to 20 μm. Note that the scale bar refers only to the sample, as the sketch of the beam is not to scale. Under acquisition the sample rotates around the z-axis (vertical axis). The green and blue axes correspond to x- and y-axis of the system. The X-ray beam interacts with the sample, and (b) a coherent diffraction pattern is recorded on a 2D detector in the far field. (c) Phase projection of the sample at 0° rotation angle, where a lower phase shift means less interaction due to a lower electron density. (d) A few selected 2D orthoslices of the 3D data set perpendicular to the rotation axis where the blue regions are low-density regions of the electrical tree.
we present the structure of a branched DC-electrical tree (see Supporting Information, Figure S1) in LDPE with a 92 nm resolution in 3D (see Supporting Information, Figure S2) obtained by employing ptychographic tomography in vacuum at 90 K. The low-density structures revealed in front of the electrical tree are well below the size required for partial discharges described by the Paschen law,8 and their formation is believed to be an important step in the growth of the DCelectrical tree. Figure 2a shows the tomogram of a middle section of the electrical tree, situated 200 μm from the root (wire) and 240 μm from the tip of the branched electrical tree, with the growth direction of the tree indicated by the arrow. The gray circular slice at the top (Figure 2a) shows the upper xy-plane of the LDPE-pillar. The features of the electrical tree are shown as a 3D iso-surface rendering in red with a density equal to 0.60 g (cm)−3 (Supporting Information: rotating video of the electrical tree), in contrast to the surrounding polyethylene with an average density of 0.90 g (cm)−3, although there was a minor gradient in the density through the pillar due to a metal cap surrounding the sample originating from the pillar fabrication. The inside of the main tree channels had a density close to 0 g cm−3; the channels were apparently hollow, as was also revealed by SEM (see Supporting Information, Figures S3 and S4). The tree splits at the top to form two branches diverging by an angle of 45° from each other and with an angle of 45° to the direction of the electrical field (arrow in Figure 2a), where the left channel deviates further from the right channel after a second splaying (Figure 2b). A single hole, with a diameter of ca. 2 μm (see Supporting Information, Figure S5), was revealed by SEM (see Supporting Information, Figure S3) and by optical microscopy (insert photomicrograph, Figure 2a). The channels had an elliptical cross-section with a rough surface, with both edgy and smooth features perpendicular to the major growth direction of the branches. The width and roundness of the channels vary, with diameters measured perpendicular to the major growth direction of the channel (orthoplane) to be between 800 and 1300 nm in both the channels. In Box I (Figure 2a,b), a string of four features with a density less than the threshold (0.60 g (cm)−3) can be seen. These features are not visibly connected to the main branch, but lie in the region from which the tree grew (Box I in Figure 2a; view perpendicular to E), as can also be seen in Figure 2b (Box I; note that this is a view along E). These low-density
trees in epoxy resins have recently been visualized in three dimensions (3D) using computed micro-X-ray tomography (CT) and serial focused ion beam SEM (FIB-SEM).6,15,16 The possibility of viewing the 3D structure of electrical trees leads to a better understanding of the development of electrical trees and makes it possible to tailor insulating materials designed to withstand high electrical fields.17,18 Nevertheless, the limited resolution of traditional CT and the similar electron densities of the polymeric materials may restrict the ability to reveal details. To gain further insight into the mechanism of electrical treeing in order to allow tailoring of breakdown-resistant nanocomposites, a resolution down to the nanoscale is required. Ptychographic nanotomography is a coherent diffraction imaging technique that has provided an effective method of approaching a nanometer resolution.19−21 X-ray ptychography makes use of a confined coherent illumination, across which the specimen is scanned so that illuminated areas partially overlap. At each scanning step, a coherent diffraction pattern is recorded in the far field (Figure 1a,b), and iterative phase retrieval algorithms are employed to reconstruct the complex-valued transmissivity of the specimen, providing both absorption and phase contrast (Figure 1c).22,23 This imaging technique can be extended to 3D (Figure 1d) by combining many such ptychographic 2D images acquired of the sample at different incidence angles with respect to the incoming beam in a tomographic reconstruction.20 In ptychographic tomography the phase images are used, to yield a 3D map of the electron density distribution of the specimen,24 which in turn can be transformed into the mass density distribution of the specimen.21 Since polyethylene is radiation-sensitive, the long-term irradiation exposure required to acquire a 3D map is challenging. To suppress the beam damage, cryogenic cooling and removal of the oxidizing atmosphere is helpful in quenching the movement of the free radicals and preventing photochemical reactions with additional reagents.25 To enable measurement without extensive sample movement and degradation, the polyethylene sample was measured with OMNY (tOMography Nano crYo stage), a cryogenic and ultra high vacuum version of the instrument presented by Holler et al.19 and developed at the Paul Scherrer Institute (see Supporting Information). This unique instrument made measurements on the LDPE samples possible at a temperature well below its glass transition temperature, in order to reduce the segmental mobility and the radiation damage. In this work, B
DOI: 10.1021/acs.nanolett.6b04303 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. Three-dimensional rendering of the iso-surface of the electrical tree in polyethylene where the red regions have a density value of 0.60 g (cm)−3. The diameters of the left (L) and right (R) branches are approximately 1 μm, and the gray circles are orthoslices (panel a, top; panel b, bottom) in the xy-plane of the measured LDPE-pillar. The growth of the electrical tree was downward in the direction of the electrical field (indicated by the white arrow marked with E in panel a) with a branching of the top of the pillar, as seen from the above in panel b. The dashed Box I shows four prestep structures with a neck area between the two middle voids. In Box II (d), just after the bifurcation, a necked prestep structure, marked with an arrow, is growing perpendicular to the larger branch. In Box III (c) a flat side branch deviates perpendicularly to the propagation, with four predischarge structures. The insert in panel a shows a photomicrograph of the tree.
structures grew in the direction of the electrical field and formed, beneath the upper structure, two regions connected by a necked region, to which a fourth structure just beneath that was not apparently connected. A similar growth pattern can be seen in Box II (Figure 2a,d), where a feature has grown perpendicular to the left branch (L) with a necked area between the feature and the L-branch. Box III (Figure 2a,c) shows another set of prestep structures that had grown in front of a large and flat side-branch that deviated by 90° from the main branch. Figure 2c, which shows Box III from beneath, shows the anisotropic growth of the tree structure, with a flat sidebranch extending within a single plane. The anisotropic extension may be related to mechanically weaker regions in LDPE. The four prestep structures were located in the same plane as the side-branch and had an orientation deviating by 90° from the upper to the lower prestep structure, when
observed from the side (the normal to the plane in which the side-branch has grown). Although these four prestep structures appeared to be freestanding features, this was not completely true. These structures were to various extents connected to the main electrical tree via paths of polyethylene with lower density (between 0.60 and 0.90 g (cm)−3). A 2D slice of the density of Box III is shown in Figure 3c where the two middle prestep structures (Box III in Figure 2a) are shown to be almost connected to the main branch. In Figure 3c the two middle structures display regions of marginally reduced density that stretch toward the side-branch (flat side-branch), where the density drops gradually by only a few percent (
