Analyses of local burnout in a sub-scale test coil for the 32 T magnet after spontaneous quenches during fast ramping

Industrial production of REBa2Cu3O7-δ (REBCO) coated conductors made it possible to construct the 32 T magnet, the first successful all-superconducting user magnet to exceed 30 T, which now serves users as SCM4 (Superconducting Magnet) at the NHMFL. Here we present an analysis of the damage that occurred in late-stage proof testing of the 32 T prototype coil after many essential facets of the design had been proven through more than 100 intentionally triggered quenches at fields up to 24 T. This prototype coil was then subjected to accelerated charge-discharge cycles at a rate 44 times faster than its design ramp rate to attempt to address its fatigue tolerance. The extra hysteresis loss of the fast ramps led to heating of the end pancakes which induced, after 55 fatigue cycles, 3 spontaneous quenches at progressively lower currents. Recognizing that the coil was damaged, the pancakes were then unwound and their REBCO tapes run through our continuous in-field transport Ic and remnant-field magnetization monitoring device, YateStar, which revealed 3 highly localized zones of low Ic in the end pancake that induced quench. Careful examination of these zones, especially the most intensely damaged one, revealed that the worst hot spot reached at least 779C during the quenches. Magneto-optical imaging showed that this damaged zone was about 5 mm in diameter and indeed the perpendicular damage length induced in neighboring turns by this localized quench heating was almost as great. Although there is much present concern about fatigue crack propagation from edge defects, we actually attribute this damage not to fatigue but to fluctuations in vortex pinning density due to imperfect BaZrO3 (BZO) nanorod growth that locally reduced the critical current Ic. These localized low-Ic regions then had to shed their excess current into the copper stabilizer, producing intense heating. We provide transmission and scanning electron microscopy evidence for local fluctuations of the BZO pinning structure and relate it to recent work that shows significant variations of 4 K, high field Ic values due to apparent production fluctuations of the growth conditions of the Zr-doped Metal-Organic Chemical Vapor Deposition (MOCVD) REBCO used for this test magnet.

Study on formation mechanism of edge defects of high-volume fraction SiCp/Al composites by longitudinal-torsional ultrasonic vibration-assisted milling

SiCp/Al composites are a kind of particle-reinforced composite material, which has been widely used in various fields due to its excellent performance. However, during the machining, the damage and failure of the SiC particles, the aluminum matrix, and the interface phase will cause many surface and edge defects. These defects will seriously affect the application of SiCp/Al composites. In this paper, a finite element model of randomly distributed multi-cell SiCp/Al composites with longitudinal-torsional ultrasonic vibration-assisted milling was established to analyze the formation mechanism of edge defects of the material. The simulation results showed that the main reason for the formation of edge defects was that the interface, SiC particles, and the Al matrix will produce cracks during machining, and these cracks will propagate and cause particle breakage, matrix tearing, and edge gaps. According to different machined parameters, the ultrasonic vibration-assisted milling experiment was carried out. The test results showed that the actual processed workpiece does have defects such as edge gaps. The depth of cut and the feed per tooth had a serious influence on the edge defect value, while the cutting speed had a small effect. Moreover, under the condition of applying appropriate ultrasonic amplitude, the phenomenon of edge defects and the surface quality were significantly improved. Therefore, the application of ultrasonic vibration-assisted milling can improve the surface and edge quality of SiCp/Al composites.

Structural Defects in Graphene Quantum Dots: A Review

Graphene quantum dots (GQDs) are known for their low toxicity, strong fluorescence, high surface area, large solubility and tunable band gaps. However, the change in their properties depends on the preparation processes of GQDs. Thus, certain types of preparation lead to certain defects, such as surface defect, edge defects, Stone-Wales defect. These structural defects are responsible for hindering GQDs to possess their regular shape that affects the morphological properties of GQDs. Thus, the optical and electrical properties get affected. The GQDs, which are synthesized via acidic methods are generally more vulnerable to defects compared to those synthesized using eco-friendly methods. Thereby, the aim of this review is to discuss the causes of structural defects. Moreover, it focuses on how they affect the properties of GQDs and to what extent they affect them. The processes of regulating defects have been elucidated so that more efficient applications can be designed using GQDs with controlled amounts of defects.

Role Played by Edge-Defects in the Optical Properties of Armchair Graphene Nanoribbons

We explore the implementation of specific optical properties of armchair graphene nanoribbons (AGNRs) through edge-defect manipulation. This technique employs the tight-binding model in conjunction with the calculated absorption spectral function. Modification of the edge states gives rise to the diverse electronic structures with striking changes in the band gap and special flat bands at low energy. The optical-absorption spectra exhibit unique excitation peaks, and they strongly depend on the type and period of the edge extension. Remarkably, there exist the unusual transition channels associated with the flat bands for selected edge-modified systems. We discovered the special rule governing how the edge-defect influences the electronic and optical properties in AGNRs. Our theoretical prediction demonstrates an efficient way to manipulate the optical properties of AGNRs. This might be of importance in the search for suitable materials designed to have possible technology applications in nano-optical, plasmonic and optoelectronic devices.

Development of a chemical-free floatation technology for the purification of vein graphite and characterization of the products

A novel and simple flotation technique has been developed to prepare high-purity graphite from impure graphite. In this method, a suspension of pristine powdered graphite (PG) is dispersed and stirred in water without adding froth formers or supportive chemicals. This makes fine particles of graphite move upwards and float on water. X-ray diffraction (XRD) analysis reveals that the floated graphite (FG) has a lower c-axis parameter, indicating the removal of interlayer impurities. A notable increase in the intensity ratio of the D band to G band in the Raman spectra indicates that the FG has more edge defects due to their smaller crystallite sizes. Transmission electron microscopic (TEM) analysis shows the number of layers in FG has been reduced to 16 from 68 in PG. The absence of C=O vibration of Fourier Transformed Infrared (FT-IR) spectroscopy in treated and untreated samples suggests that their layers are not significantly oxidized. However, X-ray photoelectron spectroscopic (XPS) analysis shows the presence of C–O–C ether functionalities, possibly on edge planes. Further, the product has higher purity with increased carbon content. Therefore, the technique is helpful for the value enhancement of graphite, the reduction of the chemical cost of the conventional techniques, environmental friendliness, and improvement of its applications.

Chip Recombination Method in Planar Deprocessing – A Solution for Failure Analysis on Chip Edge Defects

Planar deprocessing is a vital failure analysis (FA) technique for semiconductor chip reverse engineering. The basic concept of planar deprocessing is to remove all the “unnecessary” materials of a chip to expose an area of interest (AOI) and maintain the chip planarity and surface evenness. Finger deprocessing is one of the common techniques applied to this concept. This technique is essential in physical FA, especially for advanced bulk fin field-effect transistor (FinFET) devices. The success of finger deprocessing technique depends on certain factors, one of which is the location of AOI region. Application of finger deprocessing becomes incredibly challenging for AOI close to chip edge due to the chip edge effect, i. e. the chip edge is deprocessed much faster than the chip center. Plasma focused ion beam (PFIB) planar deprocessing is the primary solution to solve this problem. However, the PFIB capability is a luxury tool for most analysis labs. To overcome this challenge, a novel chip recombination method is introduced. With this method, planar deprocess can be achieved by conventional finger deprocessing technique and more importantly can be applied in general analysis labs. This paper will discuss the newly developed method in a step-by-step guide basis and show two cases with AOI(s) in the chip edge region to demonstrate its capability.

Role Played by Edge-Defects in the Optical Properties of Armchair Graphene Nanoribbons

We explore the implementation of specific optical properties of armchair graphene nanoribbons (AGNRs) through edge-defect manipulation. This technique employs the tight-binding model in conjunction with the calculated absorption spectral function. Modification of the edge states gives rise to the diverse electronic structures with striking changes in the band gap and special flat bands at low energy. The optical-absorption spectra exhibit exotic excitation peaks and they strongly depend on the type and period of the edge extension. Remarkably, there exist the unusual transition channels associated with the flat bands for selected edge-modified systems. We discover the special rule governing how the edge-defect influences the electronic and optical properties in AGNRs. Our theoretical prediction demonstrates an efficient way to manipulate the optical properties of AGNRs. This might be of importance in the search for suitable materials designed to have possible technology applications in nano-optical, plasmonic and optoelectronic devices.

Edge defects inducing the asymmetric transverse domain walls created in Ni80Fe20 nanowires

Polycrystalline permalloy nanowires with different widths were studied by means of a Lorentz microscope associated with micro-magnetic simulations. Each nanowire was designed to create a single head-to-head transverse domain wall. Edge defects at the long edges of those nanowires were incrementally controlled. Therein, a single pixel at different positions along the nanowire edges was removed. The horizontal nanowires were rotated with different angles, i.e. +/-5o, +/-10o, +/-30o and +/-45o, to produce a certain level of the edge roughness. Some curved nanowires with different widths were also designed, simulated and patterned. Lorentz images of those curved nanowires were recorded. The asymmetric levels of such created walls were measured and correlated to our wall phase diagram. The obtained results showed that the edge defects created along either side of a nanowire strongly induces the asymmetric level of a transverse domain wall.

Development of a Chemical-free Floatation Technology for the Purification of Vein Graphite and Characterization of the Products

A novel and simple flotation technique has been developed to prepare high-purity graphite from impure graphite. In this method, a suspension of powdered graphite (PG) is dispersed and stirred in water without adding froth formers or supportive chemicals. This makes fine particles of graphite to move upwards and float on water. X-ray diffraction (XRD) analysis reveals that the floated graphite (FG) has a lower c-axis parameter, indicating the removal of interlayer impurities. A notable increase in the intensity ratio of the D band to G band in the Raman spectra indicates that the FG has more edge defects due to their smaller crystallite sizes. Transmission electron microscopic (TEM) analysis shows the number of layers in FG has been reduced to 16 from 68 in PG. The absence of C = O vibration of Fourier Transformed Infrared (FT-IR) spectroscopy in treated and untreated samples suggests that layers of them are not significantly oxidized. However, X-ray photoelectron spectroscopic (XPS) analysis shows the presence of C-O-C ether functionalities, possibly on edge planes. Further, the product has higher purity with increased carbon content. Therefore, the technique is useful in the value enhancement of graphite, the reduction of the chemical cost of the conventional techniques, environmental friendliness, and improvement of its applications.