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The features of stationary random processes and the small parameter expansion approach are used in this work to examine the impact of random roughness on the electromagnetic flow in cylindrical micropipes. Utilizing the perturbation method, the analytical solution until second order velocity is achieved. The analytical expression of the roughness function ζ, which is defined as the deviation of the flow rate ratio with roughness to the case having no roughness in a smooth micropipe, is obtained by integrating the spectral density. The roughness function can be taken as the functions of the Hartmann number Ha and the dimensionless wave number λ. Two special corrugated walls of micropipes, i.e., sinusoidal and triangular corrugations, are analyzed in this work. The results reveal that the magnitude of the roughness function rises as the wave number increases for the same Ha. The magnitude of the roughness function decreases as the Ha increases for a prescribed wave number. In the case of sinusoidal corrugation, as the wave number λ increases, the Hartmann number Ha decreases, and the value of ζ increases. We consider the λ ranging from 0 to 15 and the Ha ranging from 0 to 5, with ζ ranging from −2.5 to 27.5. When the λ reaches 15, and the Ha is 0, ζ reaches the maximum value of 27.5. At this point, the impact of the roughness on the flow rate reaches its maximum. Similarly, in the case of triangular corrugation, when the λ reaches 15 and the Ha is 0, ζ reaches the maximum value of 18.7. In addition, the sinusoidal corrugation has a stronger influence on the flow rate under the same values of Ha and λ compared with triangular corrugation. [ABSTRACT FROM AUTHOR]

Silicon carbide is changing power electronics; it is enabling massive car electrification owing to its far more efficient operation with respect to mainstream silicon in a large variety of energy conversion systems like the main traction inverter of an electric vehicle (EV). Its superior performance depends upon unique properties such as lower switching and conduction losses, safer high-temperature operation and high-voltage capability. Starting briefly with a description of its physics, more detailed information is then given about some key manufacturing steps such as crystal growth and epitaxy. Afterwards, an overview of its inherent defects and how to mitigate them is presented. Finally, a typical EV's propulsion inverter is shown, proving the technology's effectiveness in meeting requirements for mass electrification. Foreword: In recent years, SiC has drawn the attention of a growing number of power electronics designers as the material has good prospects for reducing environmental impacts on a global basis. The goal of this paper, based on the author's contribution to the introduction of the technology at STMicroelectronics, is to show the potential of silicon carbide in enabling massive car electrification. The company's SiC MOSFETs, tailored to the automotive industry, are enabling visionary EV makers to pave the way for sustainable e-mobility. The intent of this paper is to describe, for a large crowd of readers, how SiC features can accelerate such a transition by quantifying the benefits they bring in terms of improved efficiency in an EV electric powertrain. The paper also has the ambition to highlight the material's physics and to give an overview of its production processes, starting from the crystal growth for realizing substrates to the main epitaxy techniques. Some space has been devoted to the analysis of the main crystal defects not present in silicon and whose nature poses new challenges in terms of manufacturing yields and screening. Finally, some insights into the market evolution and on the transition to 200 mm wafers are given. [ABSTRACT FROM AUTHOR]

The features of stationary random processes and the small parameter expansion approach are used in this work to examine the impact of random roughness on the electromagnetic flow in cylindrical micropipes. Utilizing the perturbation method, the analytical solution until second order velocity is achieved. The analytical expression of the roughness function ζ, which is defined as the deviation of the flow rate ratio with roughness to the case having no roughness in a smooth micropipe, is obtained by integrating the spectral density. The roughness function can be taken as the functions of the Hartmann number Ha and the dimensionless wave number λ. Two special corrugated walls of micropipes, i.e., sinusoidal and triangular corrugations, are analyzed in this work. The results reveal that the magnitude of the roughness function rises as the wave number increases for the same Ha. The magnitude of the roughness function decreases as the Ha increases for a prescribed wave number. In the case of sinusoidal corrugation, as the wave number λ increases, the Hartmann number Ha decreases, and the value of ζ increases. We consider the λ ranging from 0 to 15 and the Ha ranging from 0 to 5, with ζ ranging from −2.5 to 27.5. When the λ reaches 15, and the Ha is 0, ζ reaches the maximum value of 27.5. At this point, the impact of the roughness on the flow rate reaches its maximum. Similarly, in the case of triangular corrugation, when the λ reaches 15 and the Ha is 0, ζ reaches the maximum value of 18.7. In addition, the sinusoidal corrugation has a stronger influence on the flow rate under the same values of Ha and λ compared with triangular corrugation.

This study aims to analytically measure the fully developed laminar flow and heat transfer the water-based nanofluids, Cu, CuO, and Al2O3, within a micropipe with constant heat flux, under the temperature jump and slip rate boundary conditions. Knudsen number, nanoparticle volumes, and ratios of liquid layer thickness to particle radius are assumed, 0, 0.02, 0.04; 0%, 4%, %8, and 0.1, 0.2, 0.4, respectively. The findings suggest that adding nanoparticles to flow area has significant effect on both the velocity field and the heat transfer. There is a significant decline in the velocity both at the core and on the walls in the velocity area, due to the increase in the solid volume and the ratios of liquid layer thickness to particle radius after adding nanoparticles to flow area, and the increase of Nusselt number is significantly proportional to that of the solid volume and the ratios of liquid layer thickness to particle radius. Among the nanoparticles, Cu, CuO, and Al2O3, used as nanofluids within the micropipe, Cu is found to be the one with the highest heat transfer enhancement, followed by Al2O3, and CuO, respectively. [ABSTRACT FROM AUTHOR]

The precooling air-breathing technique has become a study focus in the aerospace field. Research on the internal flow and heat-transfer mechanism of the precooler is important for design and optimization. A large-eddy simulation was used to study the aerodynamic and thermal characteristics in a micropipe of the hypersonic engine precooler with supercritical methane as coolant and fuel. Under the effect of buoyancy, the high-temperature and low-density fluid near the wall in the circumferential direction gradually accumulate to the top wall. The accumulation of low-density fluid enhances the thermal acceleration effect at the top wall, which intensifies the local turbulent relaminarization and forms an M-shaped velocity distribution, resulting in the weakening of the heat transfer. On the other hand, the high-density fluid gathers to the bottom wall under the influence of gravity, the local thermal acceleration effect is weakened, and the flow heat transfer is enhanced. The influence of the relationship between the turbulent burst and the turbulent heat transfer under the effect of buoyancy is analyzed. It is found that the low-speed ejection events and high-speed sweep events are strengthened at the bottom wall, especially the low-speed ejection. However, the occurrence of these events at the top wall is restrained to a certain extent. [ABSTRACT FROM AUTHOR]

In this paper analytical expressions for time-dependent velocity profiles and pressure gradient are obtained for fully-developed laminar flows with given volume flow-rate conditions in circular pipe flows with slip boundary conditions. The governing equations are solved analytically using the traditional Laplace transform method together with Mellin’s inversion formula. The evolution of velocity profiles and pressure gradient for starting and pulsatile flow with slip boundary conditions are analyzed. New simplified expressions and perspectives on velocity and pressure gradient for no-slip and slip flows are obtained from the analytical results. New scalings in starting and pulsatile flows are proposed for pipe flows with no-slip and slip boundary conditions using non-dimensional numbers. Special attention is paid to the effect of slip factor and pulsatile flow frequency on the time-dependent skin-friction factor. Finally, by using the starting and pulsating flow results, analytical expressions of velocity and pressure for arbitrary inflow are obtained by approximating the arbitrary volume flow-rate by a Fourier series

The precooling air-breathing technique has become a study focus in the aerospace field. Research on the internal flow and heat-transfer mechanism of the precooler is important for design and optimization. A large-eddy simulation was used to study the aerodynamic and thermal characteristics in a micropipe of the hypersonic engine precooler with supercritical methane as coolant and fuel. Under the effect of buoyancy, the high-temperature and low-density fluid near the wall in the circumferential direction gradually accumulate to the top wall. The accumulation of low-density fluid enhances the thermal acceleration effect at the top wall, which intensifies the local turbulent relaminarization and forms an M-shaped velocity distribution, resulting in the weakening of the heat transfer. On the other hand, the high-density fluid gathers to the bottom wall under the influence of gravity, the local thermal acceleration effect is weakened, and the flow heat transfer is enhanced. The influence of the relationship between the turbulent burst and the turbulent heat transfer under the effect of buoyancy is analyzed. It is found that the low-speed ejection events and high-speed sweep events are strengthened at the bottom wall, especially the low-speed ejection. However, the occurrence of these events at the top wall is restrained to a certain extent.

In this paper analytical expressions for time-dependent velocity profiles and pressure gradient are obtained for fully-developed laminar flows with given volume flow-rate conditions in circular pipe flows with slip boundary conditions. The governing equations are solved analytically using the traditional Laplace transform method together with Mellin's inversion formula. The evolution of velocity profiles and pressure gradient for starting and pulsatile flow with slip boundary conditions are analyzed. New simplified expressions and perspectives on velocity and pressure gradient for no-slip and slip flows are obtained from the analytical results. New scalings in starting and pulsatile flows are proposed for pipe flows with no-slip and slip boundary conditions using nondimensional numbers. Special attention is paid to the effect of slip factor and pulsatile flow frequency on the time-dependent skin-friction factor. Finally, by using the starting and pulsating flow results, analytical expressions of velocity and pressure for arbitrary inflow are obtained by approximating the arbitrary volume flow-rate by a Fourier series. [ABSTRACT FROM AUTHOR]

In this article, derivation of a nonlinear model and nonlinear controller design for a micropipe conveying fluid, which is excited with a piezoelectric actuator, are accomplished. The governing equations are derived by using Hamilton's principle. The difference between the equations in micro-scales and macro-scales is established by using the modified couple stress theory. Unlike the classical Timoshenko beam theory, this new theorem includes a material length scale parameter which could help capturing the size effect. In addition, for thin members, so long as the deformation in the order of thickness, they do not remain in the elastic zone. Therefore, the linear theorem produces error in predicting inplane movement of the member. In this way, the nonlinear terms are considered in the equations by applying mid-plane stretching theory. After this derivation, a frequency analysis is performed on the model. In addition, the effective parameters on the peak value of the response are studied as well. Finally, a sliding mode controller for the input voltage of the system is designed. It has been observed that by using this type of nonlinear controller, the behavior of the system could be improved significantly. [ABSTRACT FROM AUTHOR]

Compact heat exchangers are of great technological importance in many industries such as automotive, submarine and spacecraft industry. Micro-channel cooling is an area which is providing a new platform for the development of these compact heat exchange devices. Micro-channel flows and heat transfer characteristics are different from what is usually observed in macrochannel flows. In the present work, computational fluid dynamics is used as a tool to study the influence of variable thermophysical properties on the Nusselt number, skin friction and pressure drop for different channel diameter, fluid inlet velocities, and heat fluxes. The Nusselt number and pressure drop is found to be higher for variable properties as compared to constant properties for an increase in diameters, inlet velocities, and heat fluxes. [ABSTRACT FROM AUTHOR]

Experimental and analytical investigation was carried out to show the effect of surface tension on the heat transfer coefficient of gas flow inside micropipe heat exchangers during the condensation process. An empirical correlation was formulated for the coefficient of the convective heat transfer. A comprehensive comparison between the experimental results with published correlations in the literature and that obtained by the developed correlation was conducted. The results showed that the predicted heat transfer coefficient of the proposed correlation was closer to the experimental results than any other correlation, a finding related to the consideration of the surface tension. [ABSTRACT FROM AUTHOR]

Abstract: As a single crystal SiC is grown, defects and dislocations occur due to many reasons. In particular, defects such as micropipes and micropores that are generated during the growth of single crystal SiC ingot have irregular locations and sizes. These defects continue to exist after the manufacturing process and undermine the properties of single crystal SiC wafer. Moreover, they lower the electrical properties of the wafers and can even cause detrimental damages after being applied in devices. We combined single crystal SiC wafer and graphene with a floating method in order to use graphene as a bridge to connect the SiC bonding that is broken due to defects such as micropipes and micropores in single crystal SiC wafer. In this process, we characterized the layers of graphene needed, ranging from monolayer to multilayer, to cover micropipes and micropores of various sizes. As a result of measuring the thermoelectrical conductivity of single crystal SiC wafer combined with monolayer graphene up to temperatures of 400°C, we observed electrical conductivity that was two or three orders higher than that of the SiC wafer alone. In addition, the connection between the SiC and the graphene was stable. [Copyright &y& Elsevier]

Synchrotron white-beam x-ray topography (SWBXT) studies of defects in 100-mm-diameter 4H-SiC wafers grown using physical vapor transport are presented. SWBXT enables nondestructive examination of thick and large-diameter SiC wafers, and defects can be imaged directly. Analysis of the contrast from these defects enables determination of their configuration, which, in turn, provides insight into their possible formation mechanisms. Apart from the usual defects present in the wafers, including micropipes, threading edge dislocations, threading screw dislocations, and basal plane dislocations, a new stacking fault with a peculiar configuration attracts our interest. This fault has the shape of a six-pointed star, comprising faults with three different fault vectors of Shockley type. Transmission and grazing topography of the fault area are carried out, and detailed contrast analysis reveals that the outline of the star is confined by 30° Shockley partial dislocations. A micropipe, which became the source of dislocations on both the basal plane slip system and the prismatic slip system, is found to be associated with the formation of the star fault. The postulated mechanism involves the reaction of 60° dislocations of a/3 〈 $$ \bar{2}110 $$〉 Burgers vector on basal plane and pure screw dislocations of a/3 〈 $$ 11\bar{2}0 $$〉 Burgers vector on prismatic plane and cross slip of the partial dislocation from prismatic plane to basal plane leading to expansion of the faults. [ABSTRACT FROM AUTHOR]

Abstract: The spectra of 6H–SiC crystals including micropipes have been examined for the Si face using Raman scattering. The first-order Raman features reveal that the intensity of the transverse optical phonon band centered at ∼796cm−1 is sensitive to the micropipes. And the second-order Raman features of the micropipes in bulk 6H–SiC are well-defined using the selection rules for second-order scattering in wurtzite structure. It is found that there are some second-order peaks missing for the micropipe-including sample, which may be induced by the reduction of the incident laser intensity at around the micropipe, especially the uneven surface in the inner wall of the micropipe. These features might also be employed to characterize other structural defects such as screw-dislocations and threading edge dislocations. [Copyright &y& Elsevier]

Abstract: Silicon Carbide (SiC) is an extremely attractive material for semiconductor power devices because of its electrical and physical characteristics. This paper describes the benefits of utilizing SiC Super Gate Turn-Off thyristors (SGTO) in pulsed power applications, reviews the current progress and development of SiC GTOs, and presents the static and pulsed characteristics of large area GTOs with high blocking capabilities. The wide pulsed evaluation of the 0.5cm2 SiC SGTOs has been demonstrated and reported by the Army Research Laboratory (ARL). This paper presents the wide pulsed capabilities of the 1cm2 SiC SGTOs. The 1cm2 SiC SGTO devices handled up to twice the peak current of the 0.5cm2 SiC SGTOs at a 1ms pulse width. The wide pulsed evaluation of these devices was demonstrated at ARL. ARL evaluated the static and pulsed characteristics of six of these devices. The devices had a forward blocking voltage rating of 9kV and a trigger requirement of a negative pulse of 1A to the gate for a millisecond pulse width. These devices were pulsed as high as 3.5kA at 1ms, equating to an action rate of 6×103 A2 s and a current density of 4.8kA/cm2, based on the device active area. The narrow pulsed evaluation of this device has been demonstrated by Cree Inc. A peak current of 12.8kA with a pulse width of 17μs (corresponding to 12.8kA/cm2 based on the chip size) was conducted with this device. [Copyright &y& Elsevier]

Detailed comparison has been drawn between synchrotron white beam X-ray topographic images of micropipes in 4H-SiC, recorded using pyramidal plane reflections in grazing-incidence geometry, and images simulated using the ray-tracing method. The simulations were carried out with and without the influence of surface relaxation effects. The images simulated in the absence of surface relaxation effects appear as white elliptical shaped features, canted to one side or other of the g-vector (depending on the dislocation sense), surrounded by a dark contrast perimeter which thickens at both ends of the major axis and which exhibits two fold symmetry axes parallel to the major and minor axes. On the other hand, on the images simulated taking into account the effects of surface relaxation, the features are again canted to one side or other of the g-vector (depending on the dislocation sense) but do not exhibit the same symmetry in that one of the sides of the oval shape is flattened (along the major axis) and the distribution of intensity around the perimeter no longer possesses the two fold symmetry axes parallel to the major and minor axes. While the details of the distribution of the dark perimeter contrast observed on the recorded images are not so easy to correlate with the simulated distributions, the sense of cant of the roughly elliptical white features on the observed images can be easily correlated with the simulations. Since the sense of cant has the same behavior as a function of dislocation sense for simulations carried out with and without surface relaxation, this provides a high level of confidence that the sense of cant of these features can be readily used to determine the senses of the micropipes. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a novel wafer level fabrication of hermetically sealed microcavity and related micro-to-macro interconnects for silicon-based microfluidic system applications. The interconnects are made of electroless plated Ni and strongly bonded with a micromachined silicon microcavity using Au/Sn bond. The technique has been successfully applied for the microreaction chamber fabrication of a low powered micro-cooler proposed for power management applications in which refrigerating capability and low operating power are essential. Experimental result indicates the metal interconnect is robust enough to stand for at least 100KPa stress. The technology can accompany not only the advantages of low manufacturing cost but also high hermeticity and mechanical reliability. [Copyright &y& Elsevier]

Micropipes are considered to be one of the most serious defects in silicon carbide (SIC) wafers affecting device yield. Developing a method to count and map micropipes accurately has been a challenging task. In this study, the different etching behavior of conductive and semi-insulating wafers in molten potassium oxide (KOH) is compared. Micropipes and closed-core screw dislocations exhibit different morphology after etching and can be easily distinguished with a polishing process. Based on a new sample preparation procedure and a digital imaging technique, a novel method of efficiently and reliably mapping and counting micropipes in both conductive and semi-insulating SiC wafers is developed. [ABSTRACT FROM AUTHOR]

Abstract: The micropipe formation in 6H-SiC single crystal fabricated by sublimation method has been investigated via a stacking fault with a core of inclusion model. Sizes, shapes and the density of micropipes were investigated and the inclusions in the micropipes and the stacking faults (SFs) were analyzed. SFs and inclusions are the two necessary conditions for the formation of the micropipes. The shapes of micropipes are determined by the SFs tracks, and their sizes and density are mainly done by the sizes and number of the inclusions. [Copyright &y& Elsevier]

This study presents a nondestructive and in-depth defect characterization method, based on the principle of polarized light microscopy (PLM), which can be used to quickly evaluate SiC substrates and epilayers. The developed PLM system has the capability to map, on a wafer scale, micropipes, elementary screw dislocations, and domain boundaries in SiC wafers. One unique feature of the PLM system is the ability to characterize the wafer with and without an epilayer, providing a newly found opportunity to investigate threading defect propagation in the overgrown epilayer. The correlation between SiC substrate defects and epilayer defects will be established. [ABSTRACT FROM AUTHOR]

A technique to reduce the micropipe density in SiC substrates by first filling in the defects and then growing an LPE layer on the filled material has been developed by TDI. LPE growth in SiC is known to result in poor surface morphology, namely step-bunching due to the off-axis substrate orientation. Chemical vapor deposition (CVD) growth experiments on SiC substrates with reduced micropipe density using a cold-wall CVD reactor resulted in a significant improvement in the surface morphology. Although preliminary device results are encouraging, the exact nature of the filled micropipes nor the impact of growing CVD epitaxial layers on LPE SiC had not been fully characterized. We have preformed transmission electron microscopy (TEM) measurements to evaluate the crystallographic properties of the CVD/LPE and LPE/substrate interface. It was observed that no new dislocations were nucleated at the LPE/CVD interface. Although a micropipe was not located in the samples characterized, a tilt of 1.5° was observed between the LPE layer and the substrate. In addition, dislocations were observed to propagate through the LPE layer from the substrate which are most likely the 1C close-core screw dislocations common to SiC hexagonal substrates. [ABSTRACT FROM AUTHOR]

Micropipe is a vital defect for fabricating SiC based devices. In order to understand the evolution of micropipe during growth process, the authors studied axial cuts sliced from different parts of the sublimation grown SiC single crystals. The cuts have been characterised using optical microscopy, etching in molten mixed KOH and K2CO3, scanning electron microscope and X-ray photoelectron spectroscopy. It is found that second phase inclusion (silicon droplet) is contributing not only to the formation of micropipe defect but also to its termination during SiC growth process. And X-ray photoelectron spectroscopy measurement result shows that growth interface can be demarcated obviously without intentionally doping any other impurity, which offers a good and simple method for observing crystal growth process. [ABSTRACT FROM AUTHOR]