Significant temperature variation at 50 Hz was observed in SiC dies used in inverter applications, which can significantly reduce lifetime of SiC transistors. However, this important temperature variation at 50 Hz in GaN HEMT transistors was never observed before but they exist and may also reduce lifetime of GaN transistors used in PFC or inverter applications. This paper shows by simulation that junction temperature in GaN HEMT dies may have a temperature swing of up to 40 degrees C in ordinary applications. For that, precise instantaneous loss model is developed for a specific GaN transistor. Then, a precise method to estimate dynamic temperature and thermal impedance of different parts of GaN components is developed. This method uses Thermo-Sensitive Electrical Parameter (TSEP) to measure thermal impedance between the die and the heatsink. Experimental measurements of thermal cycle of GaN dies with representative instantaneous loss profile are shown in order to validate the dynamic junction temperature estimation model as well as to show that high frequency power cycle in GaN transistors is a strong stressor to the device aging process.
This work focuses on the design and the study of mechanical behavior of new adobe clay bricks material made from local ecological materials for use in construction. The hydraulic binder used is a clay mineral material from the Sibang district in Gabon, and the biomass consists of sawdust from tropical wood species (okoume) combined with additives such as sugar cane molasses and cassava starch. The sawdust comes from okoume, where the selected protocol is based on their availability and widespread use locally. Preleminary tests done on clay show detailed analyses using laser granulometry, chemical analysis of major elements in total rock, X-ray diffraction on total rock and oriented samples (normal and heated to 550°C and ethylene glycol), infrared spectroscopy, cation exchange capacities, and scanning electron microscopy. The mixture of sawdust from okoume, padouk, azobe specie and 85% Sibang clay allowed for the design of bricks with quality facades. The respective compression strength tests resulted in 6.44 MPa, 3.15 MPa and 3.13 MPa, where the mixture containing the okoume sawdust showed a resistance, of 6.44 MPa, two times higher than the others. The adobe bricks incorporating sawdust from okoume, padouk and azobe woods are in compliance with the French standard for compressed earth blocks. The sawdust-wood mixture combined with clay is an ecological material and an alternative to the use of traditional concrete blocks in Gabon.
This study investigates the long-term impact of dynamic overvoltage stress on GaN HEMTs using a newly designed test circuit, UIS3, a variant of classic UIS, which isolates key stress factors. Devices were subjected to short-duration repetitive overvoltage stress near and below their dynamic breakdown voltage. Characterization before and after stress reveals permanent degradation in CDS, IDSS and IGSS, suggesting deep-trapping or structural damage within the device. A distinct alteration in the CDS curve is observed, may indicate less spreading of the electric-field within the device. RDS,on degradation is also noted, likely due to trapping effects, with partial recovery at room temperature. Higher stress levels accelerate failure. Waveform analysis and post-failure characterization indicate a short-circuit failure mode, likely due to partial dielectric breakdown during overvoltage events. These results provide new insights into GaN HEMT degradation mechanisms under high-voltage stress.
Decarbonizing aviation aims to reduce greenhouse gas emissions from aircraft operations, paving the way for sustainable air travel. This endeavor requires adopting advanced technologies, alternative fuels, high-performance coatings and efficient engineering solutions that minimize environmental impact while maintaining performance and safety standards. Two of the most prevalent and cost-effective methods for applying protective and functional coatings to aerospace components are thermal spray and physical vapor deposition. These techniques enhance the durability and efficiency of several components, resulting in fuel savings and an extended service life across the aviation industry. This supports the broader aim of transitioning the aviation industry to net-zero emissions and sustainable growth. This roadmap explores how these two coating techniques can promote sustainable aviation and identifies the challenges and opportunities in the aerospace sector for researchers and manufacturers of thermal spray and physical vapor deposition (PVD) coatings. It also proposes research directions to address these challenges and discusses the role of AI, which is crucial for breakthrough technologies in process optimization and integration, new coating development and coating design optimization. The roadmap is organized into 20 concise subsections, each focusing on a specific topic. Renowned specialists in each area were invited to summarize the current status of their field, discuss the challenges it faces, and offer recommendations for necessary research and development to overcome these issues. Together, these contributions vividly highlight the essential elements of the field and the challenges that lie ahead. The innovative ideas and concepts outlined in the roadmap reveal that the future path is both expansive and far-reaching. A decade after the JTST released its roadmap on thermal spray, which emphasized the processes, coatings, and applications of thermal spray, the current roadmap shifts its focus to spray processes and vapor deposition methods aimed at decarbonizing aviation. Academic and industry experts are collaborating to share insights on how thermal spray and vapor deposition techniques and coatings can advance sustainable aviation and the necessary research to overcome associated challenges. This roadmap can serve as a valuable reference point for researchers aiming to understand the field’s trajectory and identify critical gaps to address.
SiC MOSFETs have higher thermal impedance compared to their Silicon counterparts for same rated power. For that reason, when used in AC/DC or DC/AC applications, they may suffer from temperature variation as high as 40 K at frequencies close to 50 Hz. This temperature variation, induced by Power Cycling, may reduce lifetime of power modules using SiC transistors, which was not the case for Silicon based power modules. These high frequency power cycles are indeed poorly modelled and rarely considered in lifetime estimation model of SiC power modules. This paper presents the procedure to take into account these high frequency power cycles when estimating SiC power module lifetime using automotive mission profile. The mission profile is used to create representative current waveforms flowing through the power module for the entire mission. Thus, instantaneous SiC die temperature (averaged in each switching period) is calculated based on precise instantaneous loss estimation coupled with accurate thermal impedance model. The result is a junction temperature profile which contains power cycles at the same frequency of the sinusoidal current flowing through the SiC die. The influence of such "high frequency" power cycles in the total lifetime of a SiC power module is then demonstrated using lifetime models found in literature. Results show that, using classical lifetime models, SiC power module lifetime can be overestimated by more than 10 times if such high frequency power cycles are not taken into account.
