High Temperature Field Research Articles

Hydrogeochemistry and isotopic assessment of a new geothermal prospect in rancho Nuevo (Guanajuato-Central Mexico)

Mexico has five high-temperature geothermal fields, three of which are located in an active volcanic arc known as the Trans-Mexican Volcanic Belt in central Mexico. In this geological province, there are numerous hot springs, many of which have not been studied yet. It is the case of the Rancho Nuevo area, which is composed of two hot springs and thermal groundwater wells. The present study focuses on the hydrogeochemical and isotopic characterization of the Rancho Nuevo geothermal system. According to the results cationic exchange, silicates and carbonates rock alteration processes are occurring. A cooling of thermal water is also carried out caused by a mixing process between deep Na-Cl type water and shallow Na-HCO3. In addition, the most relevant process identified in the study area is the water-rock interaction and corroborated by the stable isotope results showing enrichment of δ18O. All these processes are enhanced by the thermal activity in the region. The CO2 and N2 concentrations confirm the geothermal activity in the Rancho Nuevo area. The CH4 concentration is also typical of geothermal fluids and may represent a magma degassing.Finally, the reservoir temperature was estimated at 225 °C ± 15 °C based on silica, cation, and integrated multicomponent solute geothermometers. Therefore, the Rancho Nuevo geothermal area can be defined as a high-temperature system and as a very interesting prospect; however, for geothermal resource production, the calcium carbonate deposition in the wells shall be considered.

Urban heat islands and their effects on thermal comfort in the US: New York and New Jersey

In recent decades, urbanization and the dramatic increase in urban populations have exacerbated the urban heat island effect. At present, much attention has been paid to the causes and patterns of the urban heat island effect. However, there are few quantitative studies on the impact of the urban heat island effect. By extracting post-world climate data, DEM data, and land use data for 20 years between 2001 and 2020, this paper first studies the variation pattern and spatial distribution characteristics of urban heat islands in New York State, summarizes the seasonal distribution characteristics of temperature, and then uses Giles formula to calculate Tom's discomfort index and evaluate human thermal comfort, and evaluates the effect of heat island effect on human sensory thermal comfort. The results show that on the time scale, the surface temperature in the study area generally showed a slow upward trend over the past 20 years. For example, in July, the maximum and minimum temperatures increased by 3.2 °C and 4.1 °C, respectively. At the spatial scale, most of the heat island areas in the study area were distributed in the New York City agglomeration, especially from May to October, when the heat island effect was particularly obvious. The temperature map showed obvious high temperatures. Compared with 2001, the human discomfort index (DI) increased between June and August 2020. The land use map shows that as the city expands, people's DI index also increases, and the proportion of people who feel uncomfortable with heat increases to 50%. Except for the New York City cluster, other areas were mostly athermic-free. This result shows that excessive urban development concentration seriously affects residents' quality of life. We should pay attention to the superimposed impact of climate change and urban heat islands on the human discomfort index, and adjust the local high temperature and thermal field area through reasonable planning, strengthening greening, and using building technology to make cities more livable.

Geochemical and H–O–Sr–B isotope signatures of Yangyi geothermal fields: implications for the evolution of thermal fluids in fracture-controlled type geothermal system, Tibet, China

High-temperature hydrothermal systems are mainly distributed in the north–south graben systems of southern Tibet as an important part of the Mediterranean–Tethys Himalayan geothermal belt in mainland China. As the largest unit capacity and second stable operating geothermal power station in China, Yangyi is the fracture-controlled type geothermal field in the center of Yadong–Gulu Graben. In this paper, hydrogeological and hydrochemical characteristics, isotope composition (δD and δ18O, 87Sr/86Sr and δ11B) of borehole water, hot springs, and surface river samples were analyzed. From the conservative elements (such as Cl− and Li+) and δD and δ18O values, the geothermal water of the Yangyi high-temperature geothermal field is estimated to be of meteoric origin with the contributions of chemical components of the magmatic fluid, which is provided by partially molten granite as a shallow magmatic heat source. According to logging data, the geothermal gradient and terrestrial heat flow value of the Yangyi high-temperature geothermal field are 6.48 ℃/100 m and 158.37 mW m−2, respectively. Combining the hydrothermal tracer experiment, 87Sr/86Sr and δ11B ratios obtained with gradually decreasing reservoir temperatures from the Bujiemu stream geothermal zone to Qialagai stream geothermal zone, we suggested the deep geothermal waters were mixed with local cold groundwater and then flow northeastward, forming the shallow reservoir within the crushed zone and intersect spot of faults in the Himalayan granitoid. Furthermore, in the process of ascent, the geothermal water is enriched in K+, Na+, and HCO3− during the interaction with underlying Himalayan granitoid and pyroclastic rocks that occur as wall rocks. The detailed description and extensive discussion are of great significance for the further exploitation and utilization of north–south trending geothermal belts in Tibet.

Investigation on mechanical and thermal properties of MgAl2O4-Mg2TiO4 solid solutions with spinel-type structure

Magnesium aluminate (MA) spinel has shown potential as a refractory material under harsh environments, such as high temperatures and frequent temperature variations. However, understanding the mechanical and thermal properties of MA spinel is a key step towards further improvement of MA spinel refractory materials in high-temperature fields. The formation of solid solution represents a prominent strategy to improve mechanical and thermal properties. The present study develops novel (1-x)MgAl2O4-xMg2TiO4 (0≤ x ≤ 0.1) solid solutions by single-step reaction sintering method using light-burned magnesia, reactive alumina, and as-prepared Mg2TiO4 (M2T) powders. The effects of M2T content on the crystal structure, mechanical properties, and thermal properties of the MgAl2O4-Mg2TiO4 solid solutions (MATss) were studied. Based on XRD, TEM, and HRTEM results, MATss were highly crystallized with a cubic spinel structure. Besides, the M2T addition facilitated the flexural strength, hardness, and elastic modulus in the MA spinel. Among these solid solutions, the x = 0.08 sample exhibited the highest flexural strength, hardness, and elastic modulus of 219.47 MPa, 19.99 GPa, and 270.2 GPa, respectively. M2T additive played a solid-solution strengthening role, which enhanced the inherent resistance to microstructural damage. Additionally, the x = 0.08 sample had the highest thermal conductivity and thermal diffusivity of 12.45 W/(m·K) and 7.98 mm2/s, respectively. In comparison with pure MA spinel, the x = 0.08 sample showed a lower thermal expansion coefficient (8.63 × 10-6 K-1 at 1550 °C). These excellent mechanical properties and exceptional thermal behavior enabled its widespread application in refractory materials.

A Double Rotor Flux Switching Machine With HTS Field Coils for All Electric Aircraft Applications

The increasing demand for high-power density motors in electric transport industries opens a new research opportunity to develop motor topologies with less weight and high efficiency. In particular, all-electric aircraft applications require very high power density motors. This paper shows the design of a new 1 MW 20-pole/15-slot double rotor flux switching machine with high-temperature superconducting field coils and thermal management system. The proposed motor features an air-core stator. Aluminum Litz wire is used for the armature conductors, and the YBCO- high-temperature superconducting material is used for the field coils. The armature winding and field coils operate at 95K and 65K, respectively. The active part power density including rotors, armature windings and field coils obtained with the proposed design is 29.3kW/kg. The specific power density of proposed motor considering mechanical, support structure and TMS is about 18.5 kW/kg. The efficiency can be higher than 98.7% which satisfies the requirement of electric aviation.

Formation mechanism of Cube texture components ({001}) associated with dynamic recrystallization during compression in the single BCC phase region of Ti-22Al-25Nb alloy

The present study, employing EBSD as the primary method, investigates the formation mechanism of Cube texture components in Ti-22Al-25Nb alloy during uniaxial compression at high-temperature single-phase field. The research reveals that the formation of Cube components is closely related to texture evolution and dynamic recrystallization (DRX) behavior. Firstly, the formation of <001> and <111> dual fiber texture structures is a prerequisite for the development of Cube component, which mainly forms under large strains. Subsequently, continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) play distinct roles in the formation of Cube component. CDRX and DDRX occur in grains belonging to <001> and <111> fiber textures, respectively, exhibiting significant orientation dependence. <001> grains, characterized by a lower Taylor factor, undergo CDRX, causing sub-grains near the grain boundaries in <001> grains to rotate towards the Cube orientation. In contrast, <111> grains with a higher Taylor factor undergo DDRX, leading to the accumulation of high stored energy near the grain boundaries in <111> grains. When the two types of grains are adjacent, the significant orientation difference between them facilitates the rapid growth of Cube-oriented sub-grains through grain boundary migration. This results in a conspicuous Cube texture component on a macroscopic scale.

Cadomian Iron ore bodies in the Bafq-Saghand metallogenic province, Iran: genetic constraints from field observations and laser ablation ICP-MS mineral studies

ABSTRACT The volcano-sedimentary unit in the Bafq-Saghand metallogenic province (BSMP) of Central Iran hosts various types of iron ore bodies including Fe-oxides with jaspilite beds, Mn-dominated iron lenses, magnetite-apatite lenses, and massive magnetite-apatite deposits. The Fe-oxides with jaspilite beds and Mn-dominated iron lenses have limited outcrops and show volcano-sedimentary features in the field. The main features are the presence of syngenetic submarine rhyolite-hosted magnetite, lamination and bedding of iron ore body, jaspilite and dropstone. Volcano-sedimentary origin is testified by magnetite composition showing affinity to BIF and positive Eu and negative Ce anomaly. Magnetite-apatite lenses and massive magnetite-apatite deposits that having spatio-temporal correlation form the main economically important deposits. They have magmatic to high-temperature hydrothermal field characteristics (lava flow-like ore bodies, the presence of degassing tubes in ore bodies, occurrence of brecciated texture and developed Na-Ca metasomatism). Mineral composition (enrichment of LREE, magnetite and apatite negative Eu anomaly and Co/Ni < 1 in pyrites) is in agreement with magmatic and high temperature hydrothermal origin. We propose a hydrous and oxidizing mantle derived iron-rich basic magma that formed in an extensional post-collisional setting for origin of Kiruna-type IOA ore bodies of the BSMP. Fe-oxides with jaspilite beds and Mn-dominated iron lenses resulted from degassing or eruption of iron-rich magma during Ediacaran in a submarine environment. However, magnetite-apatite lenses and massive magnetite-apatite deposits formed from an Early Cambrian magmatic-hydrothermal transition system from a magnetite-crystal mush zone.

Oxide unlocking electron beam melting to separate boron for impurities co-removal of metallurgical grade silicon

Removing stubborn B from Si efficiently and simplifying the purification process are critical challenges that must be addressed in the manufacture of solar grade silicon (SoG-Si) via metallurgical routes. Herein, B is flexibly coupled with other impurities separation procedure, so that metallurgical grade silicon (MG-Si) is purified through simplified electron beam melting (EBM) unlocked by a few oxides (0.1 wt%). EBM fulfills three functions including promotion of oxidation, evaporation of impurities, and induction of directional solidification, giving high-temperature field, high vacuum field, and intense melt convection, which not only ensures the smooth process of B oxidation but also favors the co-removal of other impurities, obtaining high purity Si. As a result, purified Si ingot exhibits a low content of B (from 12.76 ppmw to 2.92 ppmw), and the total content of major impurities is reduced from 1217.24 ppmw to 5.66 ppmw. Significantly, pure SiO2 could more readily access the high-temperature region of Si melt during EBM, offering distinct advantages for the oxidation gas phase separation of B. This work provides an efficient and robust method for expanding ideas of B removal and simplifying metallurgical routes.

Design, construction and modeling of a small-scale high temperature field rotor test rig

In-field validation of radiation thermometer performance is time consuming and expensive, especially when prototype experiments are performed. This study focuses on the development of a novel small-scale high temperature field simulation test rig for measuring turbine blade surface temperatures under changing environmental conditions. When developing and testing a radiation thermometer, the testing is typically performed directly on a test rig that can simultaneously provide a standardized environment and compare temperature data. This apparatus allows for static and dynamic temperature testing in a hostile environment by combining temperature, particle, and atmospheric effects. In the present study, a test rig will be utilized to analyze and validate the radiation thermometer, which will be subjected to harsh combined environmental variables with a temperature range of up to 2000K. Notably, the rotating head structure of the two materials allows for the application of a vast range of temperature measurements (700–2000 K), which is a distinctive characteristic of the design. The requirements of the test rig are defined based on the specimens to be tested. The necessity for the combined complete test setup, as well as its design and functionality, are discussed in this paper. Finally, validation experiments are conducted to demonstrate the test rig's accuracy in measuring turbine blade surface temperature, which is 0.25%.

3D printing of high-temperature thick film platinum resistance temperature detector array

Dynamic high-temperature field measurements have garnered substantial interest for various promising applications, such as the health monitoring of aviation components. 3D printing offers unprecedented opportunities for the development of curved structure-function integrated sensors. However, most current resistance temperature detector (RTD) arrays based on 3D printing technology work well only within a narrow temperature range. Their application at high temperatures remains a significant challenge because of the lack of efficient manufacturing processes. Herein, a high-temperature thick film platinum (Pt) RTD array is presented based on 3D printing technology, suitable for high-temperature field measurement sensing applications at 50−800 °C. Three stages of Pt film sintering were revealed through the analysis of the microstructure, phase composition, and electrical properties during the sintering process. Compared to sputtered films, printed Pt films, which are confirmed to contain glass powder, exhibit excellent anti-aggregation and high-temperature properties. The resulting Pt RTD array shows outstanding stability (drift rate of 0.07%/h at 800 °C), high accuracy (0.75% full-scale), and can realize temperature gradient monitoring of a two-dimensional temperature field. The potential applications of the Pt RTD array, including bolt and rocket engine tester exhaust flame temperature monitoring, were demonstrated. This study paves the way for printing RTD arrays directly on curved parts.

Tribological investigations of boride layers on Ti6Al4V at room and elevated temperatures

PurposeThis paper aims to improve the wear resistance of titanium alloy using a high-hardness boride layer, which was fabricated on Ti6Al4V by a high-temperature boronizing process.Design/methodology/approachThe boride layers on Ti6Al4V were obtained at 1000°C for 5–15 h. Scanning electron microscopy, energy dispersive analysis and X-ray diffractometer were used to characterize the properties of the boride layer. The tribological performance of the boride layer at room and elevated temperatures was investigated.FindingsThe X-ray diffraction analysis showed that the boride layers were a dual-phase structure of TiB and TiB2. When the boronizing time increased from 5 h to 15 h, the microhardness increased from 1192 HV0.5 to 1619.8 HV0.5. At 25°C and elevated temperatures, the friction coefficients of the boride layers were higher than that of Ti6Al4V. The wear track areas of T-5 at 200°C and 400°C were 2.5 × 10–3 and 1.1 × 10–3 mm2, respectively, which were 6.1% and 2.6% of that of Ti6Al4V, indicating boride layer exhibited a significant wear resistance. The wear mechanisms of the boride layer transformed from slight peeling to oxidative wear and abrasive wear as the temperature was raised.Originality/valueThe findings provide an effective strategy for improving the wear resistance of Ti6Al4V and have important implications for the application of titanium alloy in a high-temperature field.

Thermochemical oxidation of methane by manganese oxides in hydrothermal sediments

Microbial anaerobic oxidation of methane coupled to the reduction of Mn(IV)-oxides, typically MnO2, derived from continental weathering and riverine transport has been proposed as a globally important sink of methane. However, the potential role of hydrothermal Mn(IV) oxide-rich sediments as a methane sink, and the mechanisms of underpinning methane oxidation at high-temperature hydrothermal fields remain poorly understood. Here, we report the occurrence of almost pure rhodochrosite with extremely negative δ13CPDB values (as low as –76.4‰) in direct association with hausmannite formed through hydrothermal activity in the Late Triassic Heqing Mn deposit in Southern China. Based on detailed petrography and geochemistry, this rhodochrosite is interpreted as the result of the thermochemical oxidation of methane by hausmannite during early diagenesis. Given high hydrothermal Mn2+ and CH4 release coupled to low sulfate concentration observed in the Archean oceans, we propose that hydrothermal Mn(IV) oxides may have been effective methane sinks in Earth’s ancient oceans.

A tapered multicore fiber sensor for measuring temperature and magnetic field

Tapered multicore fiber (TMCF) has been used to realize versatile optical fiber sensor for high sensitivity temperature and magnetic field sensing. The TMCF was melted to two multimode fibers (MMF) to sense the temperature, and the sensitivity reached up to 453.80 pm/℃ with linearity of ∼0.993. The MMF-TMCF-MMF structure was wrapped in polydimethylsiloxane (PDMS) to measure the response of the proposed sensor to temperature. The sensitivity increased up to −693.20 pm/℃, and the linearity was 0.997. These results show that the TMCF-based sensor has a very sensitive linear response to temperature, and the PDMS enhances the temperature sensitivity. An MMF-TMCF- MMF structure was wrapped in a mixture of PDMS and iron powder of the Titanium Doped Iron Oxide (Fe2O3-Ti) to measure the response characteristics of the magnetic field, showing sensitivity at the level of ∼-0.07 dB/mT, with linearity of 0.987. An MMF-TMCF-MMF structure was wrapped in magnetic fluid used to sense the magnetic field, giving the sensitivity of 172.60 pm/mT. The versatile sensor has small structure, stable performance, and simple fabrication, and it can be used in temperature/magnetic field sensing, respectively.

Adjusting the balance between strength and ductile of the tetragonal TiAl3 alloy: a first-principles investigation

ABSTRACT Although TiAl3 is widely used in various high-temperature fields, the adjustment of the balance between the strength and ductility of the tetragonal TiAl3 remains a challenge. To solve the key problem, here, we apply the first-principles method to study the influence of four transition metals (TM = V, Cr and Mo) on the mechanical properties of the tetragonal TiAl3. The structural stability of the TM-alloyed TiAl3 is examined by the impurity formation energy and phonon dispersion. The calculated results show that the TM-doped TiAl3 is stable at the ground state. In particular, the Mo-alloyed TiAl3 has better thermodynamic stability than the other TM-alloyed TiAl3. Importantly, the alloyed elements enhance the bulk modulus of TiAl3 because the alloyed elements improve the electronic hybridisation between the Ti atom and the Al atom. Here, the Mo-alloyed TiAl3 has the highest bulk modulus among the four TM-alloyed TiAl3. Furthermore, it is found that the alloyed elements improve the ductility of TiAl3.

Enhanced piezoelectric properties in coarse-grained 0.7Bi(Fe0.9985Mn0.0015)O3-0.3BaTiO3 ceramics

The lead-free BiFeO3-BaTiO3 (BF-BT) based ceramics with large piezoelectric coefficient (d33) have been desired for promising applications in high-temperature fields. Herein, the 0.7Bi(Fe0.9985Mn0.0015)O3-0.3BaTiO3 ceramics with different grain sizes are intentionally prepared by solid-state method, and a large d33 (232 pC/N) and high Curie temperature Tc (501 °C) are simultaneously achieved in the case of large grain size (>9 µm). X-ray diffraction and Rietveld refinement analyses indicate that these coarse-grained samples possess stronger rhombohedral distortion and enlarged lattice parameter compared to fine-grained samples, which make domain switching easier and give rise to the increase in strain. As a result, the piezoelectric and ferroelectric properties of such samples are significantly improved with increasing grain size from 3.05 µm to 9.07 µm. In addition, electric domain configuration and field-induced strain further prove that the strong piezoelectric response in coarse-grained samples is attributed mainly to enhanced non-180° domain mobility. This study reveals the origin of large piezoelectric response in coarse-grained BF-BT ceramics, and proposes an effective approach to improve the piezoelectric properties of BF-BT ceramics.

Stress-dependent sintering behavior of porous thermal barrier coatings

Thermal barrier coatings (TBCs) are subjected to high temperature and complex stress fields during service in gas turbines. In this process, densification and hardening take place as the result of sintering, which is sensitive to boundary condition/external load. The stress-dependent sintering behaviors of porous TBCs were investigated in this work using a customized four-point bending method. Furthermore, stress-dependent sintering model was developed and implemented in finite element analysis to elucidate sintering mechanisms. It was found that stress gradient induced nonlinear differential sintering behavior, due to the accelerating and retarding effects of compressive and tensile stresses, respectively. In addition, microstructure-mechanical property relation was determined following the exponential law and high-throughput method was proposed for the characterization of stress dependence. The in-depth understanding of stress-dependent sintering behavior could provide guidance to the design and failure analysis of TBCs applied on complex shaped components in the hot section of gas turbines.

The Growth and Evolution of Biomass Soot in Partial Oxidation-Assisted Hot Gas Filtration

At present, partial oxidation is applied in the filtration processes of biomass hot gas to aid in solving the blockage problems caused by tar and dust condensates. However, in the resulting high-temperature and oxygen-limited environment, the risk of tar polymerization forming soot is created during the purification processes. Thus, this work established a hardware-in-the-loop simulation model using the Lagrangian method coupled with the chemical reactions on the particle surface. The model was then used to simulate the entire evolution process of soot, including its formation, growth, and interception. The simulation results confirmed that under partial oxidation conditions, the increase in tar’s conversion rate promotes the formation of soot. Further analysis indicated that the high-temperature field formed as a result of oxidation and the increase in the naphthalene/oxygen ratio are the main reasons for the soot formation. On the other hand, the growth process of soot was inhibited by partial oxidation, which is mainly reflected in the relatively smaller increasing magnitude of soot particle mass and the decrease in the soot formation rate. Although the formation and growth of biomass soot cannot be completely avoided, the growth process is beneficial to interception and the soot escape rate can be minimized by varying the premixed oxygen content. On this basis, the potential of the partial oxidation-assisted hot gas filtration method can be further investigated and analyzed.

Process intensification of synthesis of metal organic framework particles assisted by ultrasound irradiation

This study synthesized UiO-66, a typical Zr-Metal Organic Framework (MOF), by using an ultrasound-assisted synthesis method to reduce the synthesis time. This method was short-time ultrasound irradiation at the initial stage of the reaction. As compared with average particle size of conventional solvothermal method (=192 nm), averaged particle size by the ultrasound-assisted synthesis method showed particle sizes that were smaller on average, ranging from 56 to 155 nm. In order to compare the relative reaction rates of the solvothermal method and the ultrasound-assisted synthesis method, the cloudiness of the reaction solution in the reactor was observed with a video camera, and the luminance was calculated from the images obtained by the video camera. It was found that the ultrasound-assisted synthesis method showed a faster increase in luminance and shorter induction time than the solvothermal method. The slope of the luminance increase during the transient period was also found to become increase with the addition of ultrasound, which also affects the growth of particles. Observation of the aliquoted reaction solution confirmed that particle growth was faster in the ultrasound-assisted synthesis method than in the solvothermal method. Numerical simulations were also performed using MATLAB ver. 5.5 to analyze the unique reaction field generated by ultrasound. Bubble radius and temperature inside a cavitation bubble was obtained using the Keller-Miksis equation, which reproduces the motion of a single bubble. The bubble radius expanded and contracted repeatedly according to the ultrasound sound pressure, and eventually collapsed. The temperature at the time of collapse was extremely high, exceeding 17,000 K. It was confirmed that the high-temperature reaction field generated by ultrasound irradiation promoted nucleation, leading to a reduction in particle size and induction time.

High-Temperature and High-Pressure Fiber Microsphere Fabry–Perot Sensor Based on Vernier Effect and FBG

A pressure and temperature sensor based on dual Fabry-Perot interferometers (F-PIs) using Vernier effect and fiber Bragg grating (FBG) in hollow silica microsphere-cavity (HSMC) structure is demonstrated theoretically and experimentally. The sensor consists of a F-P cavity formed by inserting the FBG into the HSMC and a reference F-P cavity. Two structures are connected by a 50% coupler to generate the Vernier effect. The change of external pressure will change the length of the microsphere F-P cavity. Utilizing the characteristics that FBG is temperature sensitive and not effected by pressure, the simultaneous detection of temperature and pressure can be realized by demodulating equation. With a pressure sensitivity of 49.5nm/MPa and a temperature sensitivity of 14.3pm/°C, the sensor is achieved pressure detection at 300°C ranging from 0 to 0.8 MPa. The sensor has the characteristics of temperature compensation, high production efficiency, low cost, strong robustness, self-encapsulation and so on. It has a broad prospect in the field of high temperature and high pressure detection.

Investigation of Off-State Stress Induced Degradation of SiC MOSFETs Under Short-Circuit Condition

Investigation of SiC MOSFETs short-circuit (SC) degradation mechanism is critical to improve the overall reliability of power converters. At present, research on the SiC MOSFETs degradation mechanism mainly focuses on the on-state phase of the SC period. While, ruggedness of SiC MOSFETs in the off-state phase of short-circuit tests (SCTs) were neglected. In this paper, SiC MOSFETs degradation mechanism in the off-state phase of SCTs are investigated comprehensively. Specifically, distribution and magnitude of electrothermal stress of devices under test (DUTs) in the off-state phase of SCTs are analyzed by TCAD simulation. Which reveals two important phenomena: i) the device will suffer from long-term high temperature and high electric field stress in the off-state phase when negative turn-off gate voltage (Vgs-off) is applied; ii) high hole current in the channel region will last for several microseconds after the device is turned off. The observed SiC MOSFETs degradation in the off-state phase of SCTs is then proved by experiments, through which, the excessively low Vgs-off can reduce the SC capability of SiC MOSFETs is concluded. Moreover, the degradation results of static parameters of SiC MOSFETs are presented and the degradation mechanism is analyzed in detail.