A 4H-silicon carbide metal-insulator-semiconductor structure with ultra-thin Al2O3 as the gate dielectric, deposited by atomic layer deposition on tile epitaxial layer of a 4H-SiC （0001） 80N-/N＋ substrate, has been...A 4H-silicon carbide metal-insulator-semiconductor structure with ultra-thin Al2O3 as the gate dielectric, deposited by atomic layer deposition on tile epitaxial layer of a 4H-SiC （0001） 80N-/N＋ substrate, has been fabricated. The experimental results indicate that the prepared ultra-thin Al2O3 gate dielectric exhibits good physical and electrical characteristics, including a high breakdown electrical field of 25 MV/cm, excellent interface properties （1 × 10^14 cm^-2） and low gate-leakage current （IG = 1 × 10^-3 A/cm 2@Eox = 8 MV/cm）. Analysis of the current conduction mecha- nism on the deposited Al2O3 gate dielectric was also systematically performed. The confirmed conduction mechanisms consisted of Fowler-Nordheim （FN） tuaneling, the Frenkel-Poole mechanism, direct tunneling and Schottky emission, and the dominant current conduction mechanism depends on the applied electrical field. When the gate leakage current mechanism is dominated by FN tunneling, the barrier height of SiC/Al2O3 is 1.4 eV, which can meet the requirements of silicon carbide metal-insulator-semiconductor transistor devices.展开更多
4H-silicon carbides deposited by diamond films have wide applications in many fields such as semiconductor heterojunction,heat sink and mechanical sealing.Nucleation plays a critical role in the deposition of the diam...4H-silicon carbides deposited by diamond films have wide applications in many fields such as semiconductor heterojunction,heat sink and mechanical sealing.Nucleation plays a critical role in the deposition of the diamond film on 4H-silicon carbides.Nevertheless,as a typical polar material,the fundamental mechanism of diamond nucleation on different faces of 4H-silicon carbides has not been fully understood yet.In this contribution,nucleation of diamond was performed on the carbon-and silicon-faces of 4H-silicon carbides in a direct current chemical vapor deposition device.The nucleation density on the carbon-face is higher by 2-3 orders of magnitude compared to the silicon-face.Transmission electron microscopy verifies that there are high density diamond nuclei on the interface between the carbon-face and the diamond film,which is different from columnar diamond growth structure on the silicon-face.Transition state theory calculation reveals that the unprecedented distinction of the nucleation density between the carbon-face and the silicon-face is attributed to different desorption rates of the absorbed hydrocarbon radicals.In addition,kinetic model simulations demonstrate that it is more difficult to form CH2(s)-CH2(s)dimers on silicon-faces than carbon-faces,resulting in much lower nucleation densities on silicon-faces.展开更多
A new 4H silicon carbide metal semiconductor field-effect transistor （4H-SiC MESFET） structure with a buffer layer between the gate and the channel layer is proposed in this paper for high power microwave applicatio...A new 4H silicon carbide metal semiconductor field-effect transistor （4H-SiC MESFET） structure with a buffer layer between the gate and the channel layer is proposed in this paper for high power microwave applications. The physics-based analytical models for calculating the performance of the proposed device are obtained by solving one- and two-dimensional Poisson＇s equations. In the models, we take into account not only two regions under the gate but also a third high field region between the gate and the drain which is usually omitted. The direct-current and the alternating- current performances for the proposed 4H-SiC MESFET with a buffer layer of 0.2 ~tm are calculated. The calculated results are in good agreement with the experimental data. The current is larger than that of the conventional structure. The cutoff frequency （fT） and the maximum oscillation frequency （fmax） are 20.4 GHz and 101.6 GHz, respectively, which are higher than 7.8 GHz and 45.3 GHz of the conventional structure. Therefore, the proposed 4H-SiC MESFET structure has better power and microwave performances than the conventional structure.展开更多
Because of the high affinity of the same element Ti,cemented carbide tools containing Ti seem to be non⁃optimal in machining titanium alloys.However,in practice,cemented carbide tools containing Ti are still widely us...Because of the high affinity of the same element Ti,cemented carbide tools containing Ti seem to be non⁃optimal in machining titanium alloys.However,in practice,cemented carbide tools containing Ti are still widely used in machining titanium alloys.Cutting experiments were conducted in order to systematically explain the contradictions between the practice and theory.The diffusion process between titanium alloys and the cemented carbide tools was analyzed by auger electron spectroscopy detecting the cutting regions.It was also analyzed by Ti/Co diffusion behavior simulated by molecular thermodynamics.The experimental results and the simulation results showed that the mutual diffusion of Ti/Co atoms was the major reason for the diffusion wear.The dissolution⁃diffusion wear was one of the main wear mechanisms for the cemented carbide tools containing Ti in the coatings.Moreover,four types of cemented carbide tools and two other types of cermet tools were used to machine the Ti⁃6Al⁃4V alloys at different cutting speeds to further verify the high affinity of cutting tools containing Ti in the substrate/coating.The verification experiments results showed that the cemented carbide tools containing Ti generally cannot be used for machining titanium alloys,but could show less affinity in the cutting regions with reasonable cutting conditions.展开更多
Carbide slag was used as an activator to improve the activity of anhydrous phosphogypsum.Carbide slag could greatly improve the mechanical strength of anhydrous phosphogypsum than K_(2)SO_(4).The compressive strength ...Carbide slag was used as an activator to improve the activity of anhydrous phosphogypsum.Carbide slag could greatly improve the mechanical strength of anhydrous phosphogypsum than K_(2)SO_(4).The compressive strength of 11 wt%carbide slag and 1 wt%K_(2)SO_(4)activated anhydrous phosphogypsum increased greatly to 8.6 MPa at 3 d,and 11.9 MPa at 7 d,and 16.0 MPa at 28 d,respectively.The rate of hydration heat was accelerated and the total hydration heat was increased,and more calcium sulfate dihydrate was formed and cross-linked with other parts which improved the compressive strength of anhydrous phosphogypsum under the effects of different activators.It was indicated that carbide slag was a highly effective and cost-efficient activator.The result provides a highly effective and low-cost method which results in a novel and high value-added method for the utilization of phosphogypsum in the future.展开更多
Tianjin Institute of Hard Alloy,Tianjin 300222,China ABSTRACT The effects of solutes of wolfram and carbon on specific saturation magnetization(4πσ)of cobalt have been examined on the basis of the measurement of Ana...Tianjin Institute of Hard Alloy,Tianjin 300222,China ABSTRACT The effects of solutes of wolfram and carbon on specific saturation magnetization(4πσ)of cobalt have been examined on the basis of the measurement of Ana for hexagonal typeα-Co and cubic typeβ-Co.The Co-W-C solid solution simulative alloys have been fabricated according to binder composition in WC-Co cemented carbides and the effect of its component and structure on its Ana has been discussed too.It is pointed out that the Co phase structural factor must be considered as measuring the Ana of WC-Co cemented carbides.展开更多
High strength-to-weight ratio of non-ferrous alloys, such as aluminium, magnesium and titanium alloys, are considered to be possible replacement of widely accepted steels in transportation and automobile sectors. Amon...High strength-to-weight ratio of non-ferrous alloys, such as aluminium, magnesium and titanium alloys, are considered to be possible replacement of widely accepted steels in transportation and automobile sectors. Among these alloys, magnesium is self explosive and titanium is costlier, and aluminium is most likely to replace steels. Application of aluminium or its alloys is also thought of as an appropriate replacement in defence field, especially to enhance the easiness in mobility of combat vehicles while maintaining the same standard as that of conventional armour grade steels. Hence most of the investigations have been confined to aluminium or its alloys as base material and open an era of developing the newer composite materials to address the major limitation, i.e. tribological properties. The surface composites can be fabricated by incorporating the ceramic carbides like silicon carbide, carbides of transition metals and oxides of aluminium using surface modification techniques, such as high energy laser melt treatment, high energy electron beam irradiation and thermal spray process which are based on fusion route. These techniques yield the fusion related problems, such as interfacial reaction, pin holes, shrinkage cavities or voids and other casting related defects, and pave the way to need of an efficient technique which must be based on solid state. Recently developed friction stir processing technique was used in the present investigation for surface modification of AA7075 aluminum alloy, which is an alternative to steels. In the present investigation, 160 μm sized boron carbide powder was procured and was reduced to 60 μm and 30 μm using high energy ball mill. Subsequently these powders were used to fabricate the surface composites using friction stir processing.Ballistic performance testing as per the military standard(JIS.0108.01) was carried out. In the present work, an analytical method of predicting the ballistic behavior of surface composites was developed. This method was based on energy balance, i.e., the initial energy of impact is same as that of energy absorbed by multi layers. An attempt also has been made to validate the analytical results with the experimental findings. Variation between the analytical and experimental results may be accounted due to the assumptions considering such as isotropic behavior of target and shearing area of contact as cylindrical instead of conical interface As the analytical model yields the ballistic performance in the closer proximity of experimentally obtained, it can be considered to be an approximation to evaluate the ballistic performance of targets.展开更多
An optimized silicon carbide(SiC)trench metal-oxide-semiconductor field-effect transistor(MOSFET)structure with side-wall p-type pillar(p-pillar)and wrap n-type pillar(n-pillar)in the n-drain was investigated by utili...An optimized silicon carbide(SiC)trench metal-oxide-semiconductor field-effect transistor(MOSFET)structure with side-wall p-type pillar(p-pillar)and wrap n-type pillar(n-pillar)in the n-drain was investigated by utilizing Silvaco TCAD simulations.The optimized structure mainly includes a p+buried region,a light n-type current spreading layer(CSL),a p-type pillar region,and a wrapping n-type pillar region at the right and bottom of the p-pillar.The improved structure is named as SNPPT-MOS.The side-wall p-pillar region could better relieve the high electric field around the p+shielding region and the gate oxide in the off-state mode.The wrapping n-pillar region and CSL can also effectively reduce the specific on-resistance(Ron,sp).As a result,the SNPPT-MOS structure exhibits that the figure of merit(Fo M)related to the breakdown voltage(V_(BR))and Ron,sp(V_(BR)^2R_(on,sp))of the SNPPT-MOS is improved by 44.5%,in comparison to that of the conventional trench gate SJ MOSFET(full-SJ-MOS).In addition,the SNPPT-MOS structure achieves a much fasterwitching speed than the full-SJ-MOS,and the result indicates an appreciable reduction in the switching energy loss.展开更多
As a common impurity in 4 H silicon carbide(4 H-Si C),hydrogen(H)may play a role in tuning the electronic properties of 4 H-Si C.In this work,we systemically explore the effect of H on the electronic properties of bot...As a common impurity in 4 H silicon carbide(4 H-Si C),hydrogen(H)may play a role in tuning the electronic properties of 4 H-Si C.In this work,we systemically explore the effect of H on the electronic properties of both n-type and p-type4 H-Si C.The passivation of H on intrinsic defects such as carbon vacancies(V_(Si) )and silicon vacancies(V_(Si)) in 4 H-Si C is also evaluated.We find that interstitial H at the bonding center of the Si-C bond(H_(i)^(bc)) and interstitial H at the tetrahedral center of Si(H_(i)^(bc)) dominate the defect configurations of H in p-type and n-type 4 H-Si C,respectively.In n-type 4 H-Si C,the compensation of HSi-te iis found to pin the Fermi energy and hinder the increase of the electron concentration for highly N-doped 4 H-Si C.The compensation of Hbc iis negligible compared to that of V_(Si)on the p-type doping of Al-doped 4 H-Si C.We further examine whether H can passivate VCand improve the carrier lifetime in 4 H-Si C.It turns out that nonequilibrium passivation of VCby H is effective to eliminate the defect states of V_(Si),which enhances the carrier lifetime of moderately doped 4 H-Si C.Regarding the quantum-qubit applications of 4 H-Si C,we find that H can readily passivate V_(Si)during the creation of V_(Si)centers.Thermal annealing is needed to decompose the resulting V_(Si)-n H(n=1-4)complexes and promote the uniformity of the photoluminescence of V_(Si)arrays in 4 H-Si C.The current work may inspire the impurity engineering of H in 4 H-Si C.展开更多
The effect of cerium(Ce)on the solidification microstructure of Cr_(4)Mo_(4)V bearing steel was investigated via a combined experimental and theoretical method.With a trace amount(0.056 wt%)of Ce addition,the coarse c...The effect of cerium(Ce)on the solidification microstructure of Cr_(4)Mo_(4)V bearing steel was investigated via a combined experimental and theoretical method.With a trace amount(0.056 wt%)of Ce addition,the coarse columnar grains in as-cast microstructure transform into equiaxed ones,and the average diameter is reduced from 56 to 27μm.The network-like and bulky primary MC and M2C carbides at the interdendritic regions become disconnected and refined,and their volume percentage decreases from4.15 vol%to 2.1 vol%.Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries,both contribute to the refinement of grains.Thermodynamic calculations reveal that primary carbides are precipitated afterγ-austenite forms near the end of the solidification process.The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refinedγ-austenite grains in which the nucleation of carbides is promoted,while the growth is restrained owing to the less segregation of alloying elements.展开更多
The relationship between the microstructure transformation of type 17-4 PH stainless steel and the aging hardening behavior was investigated. The results showed that, when 17-4 PH stainless steel aging at 595℃, the b...The relationship between the microstructure transformation of type 17-4 PH stainless steel and the aging hardening behavior was investigated. The results showed that, when 17-4 PH stainless steel aging at 595℃, the bulk hardness of samples attains its peak value （42.5 HRC） for about 20 min, and then decreases at all time. TEM revealed the microstructure corresponding with peak hardness is that the fine spheroid-shape copper with the fcc crystal structure and the fiber-shape secondary carbide M23C6 precipitated from the lath martensite matrix. Both precipitations of copper and M23C6 are the reasons for strengthening of the alloy at this temperature. With the extension of holding time at this temperature, the copper and secondary carbide grow and lose the coherent relationship with the matrix, so the bulk hardness of samples decreases.展开更多
Thermal annealing effects on gamma irradiated Ni/4 H-SiC Schottky barrier diode(SBD) characteristics are analyzed over a wide range of temperatures(400–1100 °C). The annealing induced variations in the concentra...Thermal annealing effects on gamma irradiated Ni/4 H-SiC Schottky barrier diode(SBD) characteristics are analyzed over a wide range of temperatures(400–1100 °C). The annealing induced variations in the concentration of deep level traps in the SBDs are identified by thermally stimulated capacitance(TSCAP). A little decrease in the trap density at E_C – 0.63 eV and E_C –1.13 eV is observed up to the annealing temperature of 600 °C. Whereas, a gamma induced trap at E_C – 0.89 eV disappeared after annealing at 500 °C, revealing that its concentration(< 1013 cm-3) is reduced below the detection limit of the TSCAP technique.The electrical characteristics of irradiated SBDs are considerably changed at each annealing temperature. To understand the anomalous variations in the post-annealing characteristics, the interface state density distribution in the annealed SBDs is extracted.The electrical properties are improved at 400 °C due to the reduction in the interface trap density. However, from 500 °C, the electrical parameters are found to degrade with the annealing temperature because of the increase in the interface trap density.From the results, it is noted that the rectifying nature of the SBDs vanishes at or above 800 °C.展开更多
Although a high-quality homoepitaxial layer of 4H‑silicon carbide(4H-SiC)can be obtained on a 4°off-axis substrate using chemical vapor deposition,the reduction of defects is still a focus of research.In this stu...Although a high-quality homoepitaxial layer of 4H‑silicon carbide(4H-SiC)can be obtained on a 4°off-axis substrate using chemical vapor deposition,the reduction of defects is still a focus of research.In this study,several kinds of surface defects in the 4H-SiC homoepitaxial layer are systemically investigated,including triangles,carrots,surface pits,basal plane dislocations,and step bunching.Themorphologies and structures of surface defects are further discussed via optical microscopy and potassium hydroxide-based defect selective etching analysis.Through research and analysis,we found that the origin of surface defects in the 4H-SiC homoepitaxial layer can be attributed to two aspects:the propagation of substrate defects,such as scratches,dislocation,and inclusion,and improper process parameters during epitaxial growth,such as in-situ etch,C/Si ratio,and growth temperature.It is believed that the surface defects in the 4H-SiC homoepitaxial layer can be significantly decreased by precisely controlling the chemistry on the deposition surface during the growth process.展开更多
Geant4 based Monte Carlo study has been carried out to assess the improvement in efficiency of the planar structure of Silicon Carbide(SiC)-based semiconductor fast neutron detector with the stacked structure. A proto...Geant4 based Monte Carlo study has been carried out to assess the improvement in efficiency of the planar structure of Silicon Carbide(SiC)-based semiconductor fast neutron detector with the stacked structure. A proton recoil detector was simulated, which consists of hydrogenous converter, i.e., high-density polyethylene(HDPE) for generating recoil protons by means of neutron elastic scattering(n, p) reaction and semiconductor material SiC, for generating a detectable electrical signal upon transport of recoil protons through it. SiC is considered in order to overcome the various factors associated with conventional Si-based devices such as operability in a harsh radiation environment, as often encountered in nuclear facilities. Converter layer thickness is optimized by considering 10~9 neutron events of different monoenergetic neutron sources as well as ^(241)Am-Be neutron spectrum. It is found that the optimized thickness for neutron energy range of 1–10 MeV is ~400 μm. However, the efficiency of fast neutron detection is estimated to be only 0.112%,which is considered very low for meaningful and reliable detection of neutrons. To overcome this problem, a stacked juxtaposition of converter layer between SiC layers has been analyzed in order to achieve high efficiency. It is noted that a tenfold efficiency improvement has been obtained—1.04% for 10 layers stacked configuration vis-à-vis 0.112% of single converter layer detector. Further simulation of the stacked detector with respect to variable converter thickness has been performed to achieve the efficiency as high as ~3.85% with up to 50 stacks.展开更多
基金supported by the 2010 School Fundamental Scientific Research Fund of Xidian University (Grant No. K50510250008)
文摘A 4H-silicon carbide metal-insulator-semiconductor structure with ultra-thin Al2O3 as the gate dielectric, deposited by atomic layer deposition on tile epitaxial layer of a 4H-SiC （0001） 80N-/N＋ substrate, has been fabricated. The experimental results indicate that the prepared ultra-thin Al2O3 gate dielectric exhibits good physical and electrical characteristics, including a high breakdown electrical field of 25 MV/cm, excellent interface properties （1 × 10^14 cm^-2） and low gate-leakage current （IG = 1 × 10^-3 A/cm 2@Eox = 8 MV/cm）. Analysis of the current conduction mecha- nism on the deposited Al2O3 gate dielectric was also systematically performed. The confirmed conduction mechanisms consisted of Fowler-Nordheim （FN） tuaneling, the Frenkel-Poole mechanism, direct tunneling and Schottky emission, and the dominant current conduction mechanism depends on the applied electrical field. When the gate leakage current mechanism is dominated by FN tunneling, the barrier height of SiC/Al2O3 is 1.4 eV, which can meet the requirements of silicon carbide metal-insulator-semiconductor transistor devices.
基金the National Key Research and Development Project(No.2017YFE0128600)Ningbo 3315 Innovation Team(No.2019A-18-C)+9 种基金Science and Technology Innovation 2025 Major Project of Ningbo(No.2018023)National Defense Key Laboratory Fund(No.6142807180511)Innovation Funding of State Oceanic Administration(No.NBHY-2017-Z3)Ningbo Industrial Technology Innovation Project(No.2016B10038)‘13th Five-Year’Equipment Pre-research Sharing Project(No.E1710161)‘Key Talents’Senior Engineer Project of Ningbo Institute of Materials Technology and Engineeringthe financial support of CONICYT in the project Fondecyt 11180121the financial support given the VID in the framework of U-IniciaUI013/2018 and the academic direction of the University of Chilefinancial support from the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University(Faculty Grant SFO Mat LiU No.200900971)the Swedish Research Council(VR)。
文摘4H-silicon carbides deposited by diamond films have wide applications in many fields such as semiconductor heterojunction,heat sink and mechanical sealing.Nucleation plays a critical role in the deposition of the diamond film on 4H-silicon carbides.Nevertheless,as a typical polar material,the fundamental mechanism of diamond nucleation on different faces of 4H-silicon carbides has not been fully understood yet.In this contribution,nucleation of diamond was performed on the carbon-and silicon-faces of 4H-silicon carbides in a direct current chemical vapor deposition device.The nucleation density on the carbon-face is higher by 2-3 orders of magnitude compared to the silicon-face.Transmission electron microscopy verifies that there are high density diamond nuclei on the interface between the carbon-face and the diamond film,which is different from columnar diamond growth structure on the silicon-face.Transition state theory calculation reveals that the unprecedented distinction of the nucleation density between the carbon-face and the silicon-face is attributed to different desorption rates of the absorbed hydrocarbon radicals.In addition,kinetic model simulations demonstrate that it is more difficult to form CH2(s)-CH2(s)dimers on silicon-faces than carbon-faces,resulting in much lower nucleation densities on silicon-faces.
基金Project supported by the Pre-research Foundation from the National Ministries and Commissions of China(Grant No.51308030201)
文摘A new 4H silicon carbide metal semiconductor field-effect transistor （4H-SiC MESFET） structure with a buffer layer between the gate and the channel layer is proposed in this paper for high power microwave applications. The physics-based analytical models for calculating the performance of the proposed device are obtained by solving one- and two-dimensional Poisson＇s equations. In the models, we take into account not only two regions under the gate but also a third high field region between the gate and the drain which is usually omitted. The direct-current and the alternating- current performances for the proposed 4H-SiC MESFET with a buffer layer of 0.2 ~tm are calculated. The calculated results are in good agreement with the experimental data. The current is larger than that of the conventional structure. The cutoff frequency （fT） and the maximum oscillation frequency （fmax） are 20.4 GHz and 101.6 GHz, respectively, which are higher than 7.8 GHz and 45.3 GHz of the conventional structure. Therefore, the proposed 4H-SiC MESFET structure has better power and microwave performances than the conventional structure.
基金the National Science and Technology Major Project of China(Grant No.2019ZX04017001).
文摘Because of the high affinity of the same element Ti,cemented carbide tools containing Ti seem to be non⁃optimal in machining titanium alloys.However,in practice,cemented carbide tools containing Ti are still widely used in machining titanium alloys.Cutting experiments were conducted in order to systematically explain the contradictions between the practice and theory.The diffusion process between titanium alloys and the cemented carbide tools was analyzed by auger electron spectroscopy detecting the cutting regions.It was also analyzed by Ti/Co diffusion behavior simulated by molecular thermodynamics.The experimental results and the simulation results showed that the mutual diffusion of Ti/Co atoms was the major reason for the diffusion wear.The dissolution⁃diffusion wear was one of the main wear mechanisms for the cemented carbide tools containing Ti in the coatings.Moreover,four types of cemented carbide tools and two other types of cermet tools were used to machine the Ti⁃6Al⁃4V alloys at different cutting speeds to further verify the high affinity of cutting tools containing Ti in the substrate/coating.The verification experiments results showed that the cemented carbide tools containing Ti generally cannot be used for machining titanium alloys,but could show less affinity in the cutting regions with reasonable cutting conditions.
基金Funded by Innovation Group Fund Project of Hubei Province(No.2020CFA039)the Key Research and Development Program of Hubei Province(No.2020BCA077)。
文摘Carbide slag was used as an activator to improve the activity of anhydrous phosphogypsum.Carbide slag could greatly improve the mechanical strength of anhydrous phosphogypsum than K_(2)SO_(4).The compressive strength of 11 wt%carbide slag and 1 wt%K_(2)SO_(4)activated anhydrous phosphogypsum increased greatly to 8.6 MPa at 3 d,and 11.9 MPa at 7 d,and 16.0 MPa at 28 d,respectively.The rate of hydration heat was accelerated and the total hydration heat was increased,and more calcium sulfate dihydrate was formed and cross-linked with other parts which improved the compressive strength of anhydrous phosphogypsum under the effects of different activators.It was indicated that carbide slag was a highly effective and cost-efficient activator.The result provides a highly effective and low-cost method which results in a novel and high value-added method for the utilization of phosphogypsum in the future.
文摘Tianjin Institute of Hard Alloy,Tianjin 300222,China ABSTRACT The effects of solutes of wolfram and carbon on specific saturation magnetization(4πσ)of cobalt have been examined on the basis of the measurement of Ana for hexagonal typeα-Co and cubic typeβ-Co.The Co-W-C solid solution simulative alloys have been fabricated according to binder composition in WC-Co cemented carbides and the effect of its component and structure on its Ana has been discussed too.It is pointed out that the Co phase structural factor must be considered as measuring the Ana of WC-Co cemented carbides.
基金Financial assistance from Armament research board,New Delhi,India
文摘High strength-to-weight ratio of non-ferrous alloys, such as aluminium, magnesium and titanium alloys, are considered to be possible replacement of widely accepted steels in transportation and automobile sectors. Among these alloys, magnesium is self explosive and titanium is costlier, and aluminium is most likely to replace steels. Application of aluminium or its alloys is also thought of as an appropriate replacement in defence field, especially to enhance the easiness in mobility of combat vehicles while maintaining the same standard as that of conventional armour grade steels. Hence most of the investigations have been confined to aluminium or its alloys as base material and open an era of developing the newer composite materials to address the major limitation, i.e. tribological properties. The surface composites can be fabricated by incorporating the ceramic carbides like silicon carbide, carbides of transition metals and oxides of aluminium using surface modification techniques, such as high energy laser melt treatment, high energy electron beam irradiation and thermal spray process which are based on fusion route. These techniques yield the fusion related problems, such as interfacial reaction, pin holes, shrinkage cavities or voids and other casting related defects, and pave the way to need of an efficient technique which must be based on solid state. Recently developed friction stir processing technique was used in the present investigation for surface modification of AA7075 aluminum alloy, which is an alternative to steels. In the present investigation, 160 μm sized boron carbide powder was procured and was reduced to 60 μm and 30 μm using high energy ball mill. Subsequently these powders were used to fabricate the surface composites using friction stir processing.Ballistic performance testing as per the military standard(JIS.0108.01) was carried out. In the present work, an analytical method of predicting the ballistic behavior of surface composites was developed. This method was based on energy balance, i.e., the initial energy of impact is same as that of energy absorbed by multi layers. An attempt also has been made to validate the analytical results with the experimental findings. Variation between the analytical and experimental results may be accounted due to the assumptions considering such as isotropic behavior of target and shearing area of contact as cylindrical instead of conical interface As the analytical model yields the ballistic performance in the closer proximity of experimentally obtained, it can be considered to be an approximation to evaluate the ballistic performance of targets.
基金the National Natural Science Foundation of China(Grant Nos.61774052 and 61904045)the National Natural Science Foundation of Jiangxi Province of China(Grant No.20202BABL201021)the Education Department of Jiangxi Province of China for Youth Foundation(Grant No.GJJ191154)。
文摘An optimized silicon carbide(SiC)trench metal-oxide-semiconductor field-effect transistor(MOSFET)structure with side-wall p-type pillar(p-pillar)and wrap n-type pillar(n-pillar)in the n-drain was investigated by utilizing Silvaco TCAD simulations.The optimized structure mainly includes a p+buried region,a light n-type current spreading layer(CSL),a p-type pillar region,and a wrapping n-type pillar region at the right and bottom of the p-pillar.The improved structure is named as SNPPT-MOS.The side-wall p-pillar region could better relieve the high electric field around the p+shielding region and the gate oxide in the off-state mode.The wrapping n-pillar region and CSL can also effectively reduce the specific on-resistance(Ron,sp).As a result,the SNPPT-MOS structure exhibits that the figure of merit(Fo M)related to the breakdown voltage(V_(BR))and Ron,sp(V_(BR)^2R_(on,sp))of the SNPPT-MOS is improved by 44.5%,in comparison to that of the conventional trench gate SJ MOSFET(full-SJ-MOS).In addition,the SNPPT-MOS structure achieves a much fasterwitching speed than the full-SJ-MOS,and the result indicates an appreciable reduction in the switching energy loss.
基金Project supported by the National Key Research and Development Program of China(Grant No.2018YFB2200101)the National Natural Science Foundation of China(Grant Nos.91964107 and U20A20209)+2 种基金the“Pioneer”and“Leading Goose”Research and Development Program of Zhejiang Province,China(Grant No.2022C01021)partial support from the National Natural Science Foundation of China for Innovative Research Groups(Grant No.61721005)The National Supercomputer Center in Tianjin is acknowledged for computational support。
文摘As a common impurity in 4 H silicon carbide(4 H-Si C),hydrogen(H)may play a role in tuning the electronic properties of 4 H-Si C.In this work,we systemically explore the effect of H on the electronic properties of both n-type and p-type4 H-Si C.The passivation of H on intrinsic defects such as carbon vacancies(V_(Si) )and silicon vacancies(V_(Si)) in 4 H-Si C is also evaluated.We find that interstitial H at the bonding center of the Si-C bond(H_(i)^(bc)) and interstitial H at the tetrahedral center of Si(H_(i)^(bc)) dominate the defect configurations of H in p-type and n-type 4 H-Si C,respectively.In n-type 4 H-Si C,the compensation of HSi-te iis found to pin the Fermi energy and hinder the increase of the electron concentration for highly N-doped 4 H-Si C.The compensation of Hbc iis negligible compared to that of V_(Si)on the p-type doping of Al-doped 4 H-Si C.We further examine whether H can passivate VCand improve the carrier lifetime in 4 H-Si C.It turns out that nonequilibrium passivation of VCby H is effective to eliminate the defect states of V_(Si),which enhances the carrier lifetime of moderately doped 4 H-Si C.Regarding the quantum-qubit applications of 4 H-Si C,we find that H can readily passivate V_(Si)during the creation of V_(Si)centers.Thermal annealing is needed to decompose the resulting V_(Si)-n H(n=1-4)complexes and promote the uniformity of the photoluminescence of V_(Si)arrays in 4 H-Si C.The current work may inspire the impurity engineering of H in 4 H-Si C.
基金Project supported by the National Natural Science Foundation of China(52031013,52173305,52233017)the National Key Research and Development Program(2018YFA0702900)。
文摘The effect of cerium(Ce)on the solidification microstructure of Cr_(4)Mo_(4)V bearing steel was investigated via a combined experimental and theoretical method.With a trace amount(0.056 wt%)of Ce addition,the coarse columnar grains in as-cast microstructure transform into equiaxed ones,and the average diameter is reduced from 56 to 27μm.The network-like and bulky primary MC and M2C carbides at the interdendritic regions become disconnected and refined,and their volume percentage decreases from4.15 vol%to 2.1 vol%.Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries,both contribute to the refinement of grains.Thermodynamic calculations reveal that primary carbides are precipitated afterγ-austenite forms near the end of the solidification process.The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refinedγ-austenite grains in which the nucleation of carbides is promoted,while the growth is restrained owing to the less segregation of alloying elements.
基金This work was financially supported by the Key Nuclear Fuel and Nuclear Materials Laboratory of China(No.51481080104ZS8501).
文摘The relationship between the microstructure transformation of type 17-4 PH stainless steel and the aging hardening behavior was investigated. The results showed that, when 17-4 PH stainless steel aging at 595℃, the bulk hardness of samples attains its peak value （42.5 HRC） for about 20 min, and then decreases at all time. TEM revealed the microstructure corresponding with peak hardness is that the fine spheroid-shape copper with the fcc crystal structure and the fiber-shape secondary carbide M23C6 precipitated from the lath martensite matrix. Both precipitations of copper and M23C6 are the reasons for strengthening of the alloy at this temperature. With the extension of holding time at this temperature, the copper and secondary carbide grow and lose the coherent relationship with the matrix, so the bulk hardness of samples decreases.
文摘Thermal annealing effects on gamma irradiated Ni/4 H-SiC Schottky barrier diode(SBD) characteristics are analyzed over a wide range of temperatures(400–1100 °C). The annealing induced variations in the concentration of deep level traps in the SBDs are identified by thermally stimulated capacitance(TSCAP). A little decrease in the trap density at E_C – 0.63 eV and E_C –1.13 eV is observed up to the annealing temperature of 600 °C. Whereas, a gamma induced trap at E_C – 0.89 eV disappeared after annealing at 500 °C, revealing that its concentration(< 1013 cm-3) is reduced below the detection limit of the TSCAP technique.The electrical characteristics of irradiated SBDs are considerably changed at each annealing temperature. To understand the anomalous variations in the post-annealing characteristics, the interface state density distribution in the annealed SBDs is extracted.The electrical properties are improved at 400 °C due to the reduction in the interface trap density. However, from 500 °C, the electrical parameters are found to degrade with the annealing temperature because of the increase in the interface trap density.From the results, it is noted that the rectifying nature of the SBDs vanishes at or above 800 °C.
基金This work was supported by the Provincial Government of Shanxi[Grant No.20201102012].
文摘Although a high-quality homoepitaxial layer of 4H‑silicon carbide(4H-SiC)can be obtained on a 4°off-axis substrate using chemical vapor deposition,the reduction of defects is still a focus of research.In this study,several kinds of surface defects in the 4H-SiC homoepitaxial layer are systemically investigated,including triangles,carrots,surface pits,basal plane dislocations,and step bunching.Themorphologies and structures of surface defects are further discussed via optical microscopy and potassium hydroxide-based defect selective etching analysis.Through research and analysis,we found that the origin of surface defects in the 4H-SiC homoepitaxial layer can be attributed to two aspects:the propagation of substrate defects,such as scratches,dislocation,and inclusion,and improper process parameters during epitaxial growth,such as in-situ etch,C/Si ratio,and growth temperature.It is believed that the surface defects in the 4H-SiC homoepitaxial layer can be significantly decreased by precisely controlling the chemistry on the deposition surface during the growth process.
基金supported by the grant of a research fellowship from Indira Gandhi Centre for Atomic Research,Department of Atomic Energy,India
文摘Geant4 based Monte Carlo study has been carried out to assess the improvement in efficiency of the planar structure of Silicon Carbide(SiC)-based semiconductor fast neutron detector with the stacked structure. A proton recoil detector was simulated, which consists of hydrogenous converter, i.e., high-density polyethylene(HDPE) for generating recoil protons by means of neutron elastic scattering(n, p) reaction and semiconductor material SiC, for generating a detectable electrical signal upon transport of recoil protons through it. SiC is considered in order to overcome the various factors associated with conventional Si-based devices such as operability in a harsh radiation environment, as often encountered in nuclear facilities. Converter layer thickness is optimized by considering 10~9 neutron events of different monoenergetic neutron sources as well as ^(241)Am-Be neutron spectrum. It is found that the optimized thickness for neutron energy range of 1–10 MeV is ~400 μm. However, the efficiency of fast neutron detection is estimated to be only 0.112%,which is considered very low for meaningful and reliable detection of neutrons. To overcome this problem, a stacked juxtaposition of converter layer between SiC layers has been analyzed in order to achieve high efficiency. It is noted that a tenfold efficiency improvement has been obtained—1.04% for 10 layers stacked configuration vis-à-vis 0.112% of single converter layer detector. Further simulation of the stacked detector with respect to variable converter thickness has been performed to achieve the efficiency as high as ~3.85% with up to 50 stacks.
