Fabrication and characterisation of complex shaped Si-Al-O-N ceramic components via additively manufactured moulds

This research programme was aimed at manufacturing complex-shaped components, such as gears and turbocharger rotors from SiAlON for use in engineering applications. SiAlONs are alloys of Si3N4 ceramics and offer the combination of good mechanical strength and chemical inertness together with the ability to perform at temperatures up to ~1400$^o$C. Although a lot of progress has been made for the fabrication of dense Si3N4 (and its alloys) components, research is still being conducted to find fabrication processes that are economical in terms of money and time and that yield high quality components. The manufacturing route evaluated in the current study was based on the use of complex-shaped additively manufactured polymeric moulds that could then be used during the green forming of the ceramic components, followed by their densification by sintering. The goal was to reduce (ideally eliminate) the need for any machining, leading to faster and lower cost production of complex-shaped ceramic components. The heart of the project was thus the additive manufacturing machine (Solidscape, UK & Ireland). Although intended for use in jewellery making, its ability to 3D-print fine details using high-grade synthetic polymer was exploited. Once each batch of moulds was designed and produced, the process involved mixing the precursor SiAlON powder with additives, pressing the powder into the moulds and subsequent mould removal, debinding and then sintering at 1750$^o$C. Dense (up to 99% of theoretical density) SiAlON ceramics were obtained. Components were characterised at every step of the process via visual inspection and physical and chemical analysis, including powder flowability, scanning electron microscopy, X-ray micro-computer tomography, X-ray diffraction, etc, for qualitative and quantitative analysis. Parameters affecting the resultant green and sintered body in terms of mechanical stability and surface finish were investigated. The study also included oxidation behaviour of the ceramic at temperatures from 1250 to 1450$^o$C. An organic binder was chosen from a select few based on its adhesion property and flowability when mixed with the ceramic powders. The moulds’ examination revealed irregular surfaces leading to non uniform surface profiles on the sintered parts. An interfacial surface on the moulds was introduced to improve the surface finish of the green body. Phase analysis coupled with microscopic examination of the sintered ceramic revealed the presence of two phases namely α – SiAlON and β - SiAlON. The Vickers hardness was measured to be HV10 = 2545 and the flexural strength was measured to be 87 MPa. Oxidation study revealed the onset of degradation of the ceramic from 1100$^o$C. With an increase in the oxidising time and temperature, an increase in the prominence of cristobalite, grain growth and phase separation was observed. Limitations of indirect additive manufacturing in the current study are identified. These include the part size limitation, the inefficiency of the printing unit to print uniform moulds without the presence of any debris, the strength of the moulds under stresses, failure in assessing the residual stresses in the sintered compacts and disadvantages of the powder compaction are discussed. The study concludes by suggesting future work to improve the fabrication process including the use of stronger mould material, pressureless green forming technique of consolidation such as gelcasting, development of simulations to predict the behaviour of the ceramic and its residual stresses

Measurements and review of failure mechanisms and reliability constraints of 4H-SiC Power MOSFETs under short circuit events

The reliability of the SiC MOSFET has always been a factor hindering the device application, especially under high voltage and high current conditions, such as in the short circuit events. This paper experimentally reviews the failure mechanisms caused by destructive short circuit impulses, and investigates the degradation patterns of key electrical parameters under repetitive short circuit events. The impact of test parameters on the short circuit reliability of SiC MOSFET has been analyzed. Approaches to characterize the electrical-thermal-mechanical stress during the short circuit period and advanced test methods are highlighted. Finally, the constraints from the standpoint of both manufacturers and users have been presented, including comparison of current SiC MOSFET devices, reliability evaluation of parallel SiC MOSFET devices, reliability improvement of the chip, performance improvement of protection circuits, and reliability assessment of SiC MOSFET devices under application-representative stress

Electrical conductivity of additively manufactured copper and silver for electrical winding applications

© 2022 The Authors. Published by MDPI. This is an open access article available under a Creative Commons licence. The published version can be accessed at the following link on the publisher’s website: https://doi.org/10.3390/ma15217563Efficient and power-dense electrical machines are critical in driving the next generation of green energy technologies for many industries including automotive, aerospace and energy. However, one of the primary requirements to enable this is the fabrication of compact custom windings with optimised materials and geometries. Electrical machine windings rely on highly electrically conductive materials, and therefore, the Additive Manufacturing (AM) of custom copper (Cu) and silver (Ag) windings offers opportunities to simultaneously improve efficiency through optimised materials, custom geometries and topology and thermal management through integrated cooling strategies. Laser Powder Bed Fusion (L-PBF) is the most mature AM technology for metals, however, laser processing highly reflective and conductive metals such as Cu and Ag is highly challenging due to insufficient energy absorption. In this regard, this study details the 400 W L-PBF processing of high-purity Cu, Ag and Cu–Ag alloys and the resultant electrical conductivity performance. Six Cu and Ag material variants are investigated in four comparative studies characterising the influence of material composition, powder recoating, laser exposure and electropolishing. The highest density and electrical conductivity achieved was 88% and 73% IACS, respectively. To aid in the application of electrical insulation coatings, electropolishing parameters are established to improve surface roughness. Finally, proof-of-concept electrical machine coils are fabricated, highlighting the potential for 400 W L-PBF processing of Cu and Ag, extending the current state of the art.This research was conducted with support from Innovate UK Knowledge Transfer Partnership KTP013117 (University of Wolverhampton/AceOn).Published onlin

FEM-based analysis of avalanche ruggedness of high voltage SiC Merged-PiN-Schottky and Junction-Barrier-Schottky diodes

Through comprehensive experimental measurements and TCAD simulation, it is shown that the avalanche ruggedness of SiC MPS & JBS diodes outperforms that of closely rated Silicon PiN diodes taking advantage of the wide-bandgap properties of SiC which leads to a high ionization and activation energy given the strong covalent bonds. Although the MPS diode structure favours a high reverse blocking voltage with small leakage current and a high current conduction, the localise current crowding caused by the multiple P+ implanted region leads to the avalanche breakdown at lower load currents than the SiC JBS diode. The results of Silvaco TCAD Finite Element modellings have a good agreement with the experimental measurements, indicating that SiC JBS diode can withstand the high junction temperature induced by avalanche in line with the calculated avalanche energy

Unclamped inductive stressing of GaN and SiC Cascode power devices to failure at elevated temperatures

In this paper, the ruggedness performance of GaN HEMT and SiC JFET devices in cascode configuration with a low voltage silicon power MOSFET has been evaluated experimentally. The impact of the bus voltage on the drain current and avalanche energy are investigated as well as the temperature sweep to enable analysis of the alternation of these parameters on the Unclamped Inductive Switching (UIS) ruggedness of cascode devices. The experimental measurements show that the GaN cascode devices have lower avalanche energy rating when compared with the closely rated SiC cascode devices just before the failure. SiC cascode devices can also withstand higher bus voltage in comparison to GaN cascode devices when under electrothermal stress by unclamped inductive switching. The analysis of transfer characteristics and leakage current of SiC JFET & GaN HEMT cascode structures following UIS stress have also been performed together with Computed Tomography (CT) Scan imaging to determine the per-area avalanche energy density