bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Thermal Oxidation on Deep Levels Generated by Ion Implantation into n-Type and p-Type 4H-SiC
Authors: Kawahara Koutarou , Alfieri Giovanni , Hiyoshi Toru , Pensl Gerhard , Kimoto Tsunenobu
Published: May 2010

The authors have investigated effects of thermal oxidation on deep levels in the whole energy range of bandgap of 4H-SiC which are generated by ion implantation, by deep level transient spectroscopy (DLTS). The dominant defects in n-type samples after ion implantation and high-temperature annealing at 1700oC, IN3 (Z1/2: Ec – 0.63 eV) and IN9 (EH6/7: Ec – 1.5 eV) in low-dose-implanted samples, can be remarkably reduced by oxidation at 1150oC. However, in p-type samples, the IP8 (HK4: Ev + 1.4 eV) survives and additional defects, several defects such as IP4 (HK0: Ev + 0.72 eV) appear after thermal oxidation in low-dose-implanted samples. The defects except for the IP8 center can be reduced by subsequent annealing at 1400oC. These phenomena are explained by a model that excess interstitials are generated at the oxidizing interface and diffuse into the bulk region.
Comparison of the Threshold-Voltage Stability of SiC MOSFETs with Thermally Grown and Deposited Gate Oxides
Authors: Grieb Michael , Noborio Masato , Peters Dethard , Bauer Anton J. , Friedrichs Peter , Kimoto Tsunenobu , Ryssel Heiner
Published: May 2010

The electrical characteristics and the reliability of different oxides on the 4H-SiC Si-face for gate oxide application in MOS devices are compared under MOSFET operation conditions at room temperature, at 100°C and at 130°C. The oxides are either an 80nm thick deposited oxide annealed in NO or an 80nm thick grown oxide in diluted N2O. The deposited oxide shows significant higher QBD- and lower Dit-values as well as a stronger decrease of drain current under stress than the grown oxide. Although for the deposited oxide, the leakage current below subthreshold increases more than one order of magnitude during constant circuit stress at room temperature, for the thermal oxide it is quite constant, but at higher level for higher temperatures.
Ultra Fast High Temperature Microwave Annealing of Ion Implanted Large Bandgap Semiconductors
Authors: Rao Mulpuri V. , Tian Y.L. , Qadri Syed B. , Freitas Jaime A. , Nipoti Roberta
Published: May 2010

In this work, the surface, lattice and electrical properties of implanted 4H-SiC, GaN and ZnO, annealed by a novel ultra-fast microwave heating method, are compared to that of conventional annealing methods. In this new method, amplified and variable frequency microwaves from a signal generator are directly coupled to the semiconductor sample through a microwave head. Since, the microwaves are only absorbed by the sample, without heating of the ambient, ultra-high heating (> 2000°C/s) and cooling rates and very high (2100°C) annealing temperatures can be reached. For Al and P species implants into 4H-SiC, record low resistivity values were achieved with a lattice quality better than that of the virgin crystal. This annealing method improved the lattice quality of un-implanted region below the surface implanted region as well. Improved material characteristics were also obtained for GaN and ZnO.
Effect of Dopant Concentrations and Annealing Conditions on the Electrically Active Profiles and Lattice Damage in Al Implanted 4H-SiC
Authors: Weng Ming Hung , Roccaforte Fabrizio , Giannazzo Filippo , Di Franco Salvatore , Bongiorno Corrado , Saggio Mario , Raineri Vito
Published: May 2010

This paper reports on the electrical activation and structural analysis of Al implanted 4H-SiC. The evolution of the implant damage during high temperature (1650 – 1700 °C) annealing results in the presence of extended defects and precipitates, whose density and depth distribution in the implanted sheet was accurately studied for two different ion fluences (1.31014 and 1.31015 cm-2) by transmission electron microscopy. Furthermore, the profiles of electrically active Al were determined by scanning capacitance microscopy. Only a limited electrical activation (10%) was measured for both fluences in the samples annealed without a capping layer. The use of a graphite capping layer to protect the surface during annealing showed a beneficial effect, yielding both a reduced surface roughness and an increased electrical activation (20% for the highest fluence and 30% for the lowest one) with respect to samples annealed without the capping layer.
Novel Cap Annealing Process for SiC Crystal Using ECR-Sputtered Carbon Films and ECR Plasma Ashing
Authors: Hirono Shigeru , Torii Hironori , Tajima Tetsuya , Amazawa Takao , Umemura Shigeru , Kamata Tomoyuki , Hirabayashi Yasuo
Published: May 2010

A high dose impurity doping process for 4H-SiC crystals has been developed using electron cyclotron resonance (ECR) sputtered carbon cap film and ECR plasma ashing. ECR-sputtered carbon films are newly found crystalline carbon films of which the hardness is comparable to that of diamonds. Since this carbon film showed such a high thermal tolerance that the hardness did not change after 1900oC annealing, this carbon cap film worked well for suppressing roughening during annealing for aluminum-ion implanted 4H-SiC. Cap carbon film can be removed successfully by using high density ECR plasma ashing.
Comparative Study of Ohmic Contact Metallizations to Nanocrystalline Diamond Films
Authors: Tadjer Marko J. , Anderson Travis J. , Hobart Karl D. , Feygelson Tatyana I. , Butler James E. , Kub Fritz J.
Published: May 2010

Nanocrystalline diamond (NCD) films were deposited using plasma-enhanced chemical vapor deposition. The NCD films were Boron-doped for p-type conductivity, yielding sheet resistances from 6.17x1011 to 522.5 /. Four different metals were deposited as Ohmic contacts and investigated for contact resistance and thermal stability. Contact and film annealing was performed under different atmospheric conditions with variable N2 content.
Characteristics of Gold Wire Bonds with Ti- and Ni-Based Contact Metallization to n-SiC for High Temperature Applications
Authors: Guziewicz M. , Kisiel Ryszard , Gołaszewska Krystyna , Wzorek Marek , Stonert Anna , Piotrowska Anna , Szmidt Jan
Published: May 2010

The stability of Au wire connections to n-SiC/Ti ohmic contacts and to n-SiC/Ni ohmic contacts with top Au or Pt layers has been investigated. Long-term tests of the connections are performed in air at 400oC. Evaluation of electrical parameters, morphology and structure of the metallization as well as the strength of Au joint show stable Au wire bonds to the metallization with Ti-ohmic contacts.
Development of a Wire-Bond Technology for SiC High Temperature Applications
Authors: Heuck Nicolas , Baars F. , Bakin Andrey , Waag A.
Published: May 2010

In this paper a wire-bond technology for high temperatures (up to 500°C) based on silver-wire is presented. The mechanical properties of silver thick-wire wedge bonds are analyzed and compared to previously presented silver-stripes fastened onto the chip with the Low Temperature Joining Technique (LTJT) and to common aluminum thick-wire.
Morphology Improvement of Step Bunching on 4H-SiC Wafers by Polishing Technique
Authors: Kato Tomohisa , Kinoshita Akimasa , Wada Keisuke , Nishi Takashi , Hozomi Eiji , Taniguchi Hiroyoshi , Fukuda Kenji , Okumura Hajime
Published: May 2010

In this paper, we report a new polishing technique regarding the elimination of step bunching on the silicon carbide (SiC) surface. The step bunching generation is often observed as frequent phenomenon on the surface of SiC epilayers grown on low off-angle (0001) SiC wafers and on SiC devices after annealing to activate the dopants. We polished the step bunching surface using a chemical mechanical polishing (CMP) technique reported in a previous study, and we succeeded to improve the morphology with a flat and smooth surface which showed a small Rms value of around 0.1nm. We especially found an excellent polishing effect for the control of leakage current in reverse I-V characteristics of SiC Schottky barrier diodes (SBD).
Impact of CF4 Plasma Treatment on the Surface Roughness of Ion Implanted SiC Induced by High Temperature Annealing
Authors: Sugimoto Tatsunori , Satoh Masataka , Nakamura Tohru , Mashimo K. , Doi Hiroshi , Shibagaki Masami
Published: May 2010

The impact of CF4 plasma treatment on the surface roughening of SiC has been investigated for N ion implanted SiC(0001) which is implanted with the energy range from 15 to 120 keV at a dose of 9.2 x 1014/cm2. The N ion implanted sample, which is processed by CF4 plasma, shows small surface roughness of 1.6 nm after annealing at 1700 oC for 10 min, while the sample without CF4 plasma treatment shows the large surface roughness (6.6nm) and micro step structure. XPS measurements reveals that CF4 plasma treatment is effective to dissolved the residual oxide on the surface of SiC which is not removed by BHF acid of SiO2 layer on SiC. It is strongly suggested that the formation of micro step structure with the increase of the surface roughness is promoted by the residual oxide such as SiCOx, on SiC.
Model Calculations of SiC Oxide Growth Rate at Various Oxidation Temperatures Based on the Silicon and Carbon Emission Model
Authors: Hijikata Yasuto , Yaguchi Hiroyuki , Yoshida Sadafumi
Published: May 2010

We have tried to apply the oxidation model of SiC proposed previously, termed ‘Si-C emission model’, to the oxide growth rate at various oxidation temperatures. We have found that the model well reproduces the oxide thickness dependences of oxide growth rates for all of the temperatures measured for both of the SiC Si- and C-faces. We have estimated the temperature dependence of oxide depth profiles of Si and C interstitials by using the Si-C emission model, and discussed the structure at/near the SiC–oxide interface.
Direct Observation of Dielectric Breakdown Spot in Thermal Oxides on 4H-SiC(0001) Using Conductive Atomic Force Microscopy
Authors: Kozono Kohei , Hosoi Takuji , Kagei Yusuke , Kirino Takashi , Mitani Shuhei , Nakano Yuki , Nakamura Takashi , Shimura Takayoshi , Watanabe Heiji
Published: May 2010

The dielectric breakdown mechanism in 4H-SiC metal-oxide-semiconductor (MOS) devices was studied using conductive atomic force microscopy (C-AFM). We performed time-dependent dielectric breakdown (TDDB) measurements using a line scan mode of C-AFM, which can characterize nanoscale degradation of dielectrics. It was found that the Weibull slope () of time-to-breakdown (tBD) statistics in 7-nm-thick thermal oxides on SiC substrates was much larger for the C-AFM line scan than for the common constant voltage stress TDDB tests on MOS capacitors, suggesting the presence of some weak spots in the oxides. Superposition of simultaneously obtained C-AFM topographic and current map images of SiO2/SiC structure clearly demonstrated that most of breakdown spots were located at step bunching. These results indicate that preferential breakdown at step bunching due to local electric field concentration is the probable cause of poor gate oxide reliability of 4H-SiC MOS devices.
Electrical Characterization and Reliability of Nitrided-Gate Insulators for N- and P-Type 4H-SiC MIS Devices
Authors: Noborio Masato , Grieb Michael , Bauer Anton J. , Peters Dethard , Friedrichs Peter , Suda Jun , Kimoto Tsunenobu
Published: May 2010

In this paper, nitrided insulators such as N2O-grown oxides, deposited SiO2 annealed in N2O, and deposited SiNx/SiO2 annealed in N2O on thin-thermal oxides have been investigated for realization of high performance n- and p-type 4H-SiC MIS devices. The MIS capacitors were utilized to evaluate MIS interface characteristics and the insulator reliability. The channel mobility was determined by using the characteristics of planar MISFETs. Although the N2O-grown oxides are superior to the dry O2-grown oxides, the deposited SiO2 and the deposited SiNx/SiO2 exhibited lower interface state density (n-MIS: below 7x1011 cm-2eV-1 at EC-0.2 eV, p-MIS: below 6x1011 cm-2eV-1 at EV+0.2 eV) and higher channel mobility (n-MIS: over 25 cm2/Vs, p-MIS: over 10 cm2/Vs). In terms of reliability, the deposited SiO2 annealed in N2O exhibits a high charge-to-breakdown over 50 C/cm2 at room temperature and 15 C/cm2 at 200°C. The nitrided-gate insulators formed by deposition method have superior characteristics than the thermal oxides grown in N2O.
6.5 kV SiC PiN Diodes with Improved Forward Characteristics
Authors: Peters Dethard , Bartsch Wolfgang , Thomas Bernd , Sommer R.
Published: May 2010

The paper compares static and dynamic characteristics of 6.5 kV SiC PiN diodes fabricated with different p-emitters. The version with the thickest p-emitter (4 µm) showed the lowest forward voltage (3.4 V at 100 A/cm²) and the lowest (negative) temperature coefficient. Forward voltage DC stress tests revealed a stability within the measurement error of the test apparatus (<50 mV). The dynamic performance showed a soft recovery even at 4 kV. The reverse recovery charge Qrr is analyzed for different forward currents and junction temperatures. The dynamic losses of the SiC PiN diode are marginal with view to the application in industrial inverters.
Charge Collection Efficiency of 6H-SiC P+N Diodes Degraded by Low-Energy Electron Irradiation
Authors: Iwamoto Naoya , Onoda Shinobu , Ohshima Takeshi , Kojima Kazutoshi , Koizumi Atsushi , Uchida Kazuo , Nozaki Shinji
Published: May 2010

The effect of electron irradiation on the charge collection efficiency of a 6H-SiC p+n diode has been studied. The diodes were irradiated with electrons of energies from 100 keV to 1 MeV. The charge collection efficiencies of the samples were measured for alpha particles before and after the electron irradiation. The electron irradiation at 100 keV does not affect the charge collection efficiency, while the electron irradiation at 200 keV or higher decreases the charge collection efficiency. The degree of the degradation of the diodes correlates with the energy of the electron irradiation.
Physics of Hysteresis in MESFET Drain I-V Characteristics: Simulation Approach
Authors: Adjaye John , Mazzola Michael S.
Published: May 2010

The two-dimensional device simulator, MediciTM, was used to simulate 4H silicon carbide (4H-SiC) n-channel power metal semiconductor field effect transistors (MESFETs) with 0.5 µm gate length with and without p-type buffer layer between the n-channel and the semi-insulating (SI) substrate. The devices, which have previously been fabricated and characterized experimentally, have ion-implanted n+ source and drain ohmic contact regions. The simulations were performed with transient 30 V amplitude symmetrical triangular pulse with 30 s pulse width. Simulations show that hysteresis in drain I-V curves of MESFETs is due to substrate traps and source/drain implant damage traps. The hysteresis is caused by trapping and emission of channel electrons by the traps as VDS rises from 0 V to VDS(max) and as VDS falls from VDS(max) back to 0 V. This leads to difference in trap occupation, and hence difference in channel electron concentration as VDS rises and falls. This finally leads to difference in drain-source current (IDS) at a given VDS for a given VGS as VDS rises and falls, giving rise to the hysteresis in the I-V curves.
Design and Gate Drive Considerations for Epitaxial 1.2 kV Buried Grid N-on and N-off JFETs for Operation at 250°C
Authors: Lim Jang Kwon , Bakowski Mietek , Nee Hans Peter
Published: May 2010

The 1.2 kV 4H-SiC buried-grid vertical JFET structures with Normally-on (N-on) and
Normally-off (N-off) design were investigated by simulations. The conduction and switching
properties were determined in the temperature range from -50°C to 250°C. In this paper, the
characteristics of the N-on designs with threshold voltage (Vth) of -50 V and -10 V are compared
with the N-off design (Vth=0). The presented data are for devices with the same channel length at
250°C. The results show that the on-resistance (Ron) decreases with increasing channel doping
concentration and decreasing channel width. The presented turn-on, Eon, and turn-off, Eoff, energies
per pulse are calculated under the switching conditions 100 A/cm2 and 600 V with a gate resistance
of Rg=1 . For the two N-on designs the total switching losses, Esw=Eon+Eoff, differ less than 30%
with Wch 0.7 m. With Wch=0.5 m the switching losses of N-off design are almost one order of
magnitude higher than those of the N-on design with Vth = -50 V.
Performance, Reliability, and Robustness of 4H-SiC Power DMOSFETs
Authors: Ryu Sei Hyung , Hull Brett A. , Dhar Sarit , Cheng L. , Zhang Qing Chun Jon , Richmond Jim , Das Mrinal K. , Agarwal Anant K. , Palmour John W. , Lelis Aivars J. , Geil Bruce , Scozzie Charles
Published: May 2010

In this paper, we review the performance, reliability, and robustness of the current 4H-SiC power DMOSFETs. Due to advances in device and materials technology, high power, large area 4H-SiC power DMOSFETs (1200 V, 67 A and 3000 V, 30 A) can be fabricated with reasonable yields. The availability of large area devices has enabled the demonstration of the first MW class, all SiC power modules. Evaluations of 1200 V 4H-SiC DMOSFETs showed that the devices offer avalanche power exceeding those of commercially available silicon power MOSFETs, and have the sufficient short circuit robustness required in most motor drive applications. A recent TDDB study showed that the gate oxides in 4H-SiC MOSFETs have good reliability, with a 100-year lifetime at 375oC if Eox is limited to 3.9 MV/cm. Future work on MOS reliability should be focused on Vth shifts, instead of catastrophic failures of gate oxides.
Effect of ON-State Stress on SiC DMOSFET Subthreshold I-V Characteristics
Authors: Lelis Aivars J. , Green Ronald , Habersat Daniel B. , Goldsman Neil
Published: May 2010

We have observed a noticeable increase in the instability of the I-V characteristics following an ON-state current stress, especially in the subthreshold region. An increased stretch-out and negative shift can give rise to increased leakage current in the OFF-state if proper precautions are not met to provide a proper margin for the threshold voltage. State-of-the-art 50-A MOSFETs exhibit less instability than previous 20-A devices, and devices that run hotter show a larger degradation.
Monotectic Alloys and their Growth Morphologies
Authors: Ratke Lorenz , Müller Anja , Seifert Martin , Kapserovich Galina
Published: May 2010

Alloys of exact monotectic composition can decompose at the monotectic temperature into different two phase microstructures. The minority phase can be arranged as fibers similar to eutectics, as string of pearls or as irregularly distributed droplets. The directional solidification of three Al-base monotectic alloys was investigated: Al-Pb, Al-In and Al-Bi utilizing aerogel base furnace technology to ensure constant gradient and solidification velocity over the processing length. The solidified microstructures are characterized as a function of temperature gradient ahead of the solidification front and the solidification velocity. The experimental results are presented in form of stability diagrams for the three microstructures showing the regions in which fibers, string of pearls or irregular morphologies exist. The inter fiber spacing is analyzed and presented in from of Jackson and Hunt type relations and the transitions between the microstructures are discussed.
Influence of the Slag/Pool Interface on the Solidification in an Electro-Slag Remelting Process
Authors: Kharicha Abdellah , Schützenhöfer Wolfgang , Ludwig Andreas , Reiter Gerhard
Published: May 2010

The Electro-Slag-Remelting (ESR) is an advanced technology for the production of components of e.g. high quality steels. In the present study a comprehensive computational model using the VOF technique for the prediction of the slag/pool interface is presented for axisymmetric and steady state conditions. In this model the distribution of the electric current is not constant in time, but is dynamically computed according to the evolution of the slag and steel phase distribution. The turbulent flow, created by the Lorentz and buoyancy forces, is computed by solving the time-averaged mass and momentum conservation equations. The turbulence effect is modelled by using a k-model. Two numerical simulations were performed, one assuming a flat interface, and a second leaving the interface free to find an equilibrium shape. The results are then analysed and compared for both cases.
Combined Analytical and Numerical Front Tracking Approach to Modeling Directional Solidification of a TiAl-Based Intermetallic Alloy for Design of Microgravity Experiments
Authors: Rebow Marek , Browne David J. , Fautrelle Yves
Published: May 2010

A three-step combined analytical and numerical approach to thermal modelling of a two-heater power-down furnace for controlled directional solidification of an intermetallic alloy is proposed. An analytical sensitivity analysis of the thermal model is carried out to show the effect of adiabatic zone length, and both hot-zone and cold-zone heater temperatures, on the initial thermal gradient in the sample and on the length of melt in the adiabatic zone. The subsequent axisymmetric front tracking method (FTM) simulations of directional solidification of a binary intermetallic Ti-46at.%Al alloy show that temperature gradient in the melt declines and velocity of the solid-liquid front increases with time, thus promoting good conditions for a columnar to equiaxed transition. The proposed analytical calculations combined with full-scale numerical FTM simulations provide a convenient and predictive optimization tool for the two-heater power-down furnace design and growth conditions for the future microgravity experiments.
Nucleation and Grain Refinement
Authors: Dahle Arne K.
Published: May 2010

The nucleation dynamics and grain refinement during solidification are reviewed. Control of the nucleation of phases is an essential step in the manufacture of optimized materials. Development of inoculants is still based on trial-and-error. Using aluminium as an example, it is shown that there is much room for significant improvements. Areas for future research are identified and important materials with an urgent need for grain refiners are shown.
About Ultrasonic Melt Treatment during Solidification and its Influence on Grain Size and Mechanical Properties of Magnesium Alloy AZ 91
Authors: Pranke Katja , Eigenfeld Klaus
Published: May 2010

We investigated the influence of varying amplitudes at constant frequency on the grain size and mechanical properties of magnesium alloy AZ 91. Our experimental setup consists of an ultrasound generator connected to a steel gravity die by screw fitting. Magnesium alloy AZ 91 has been melted in a closed-argon-flooded steel crucible and poured into the die while ultrasound was generated. Cooling curves were recorded during the whole solidification process. The grain size and the mechanical properties were determined and analyzed. As a result, we achieved reduction in grain size as well as an increase in tensile strengths for amplitudes of up to 40% of the maximum amplitude. Yield strengths could be minimally improved in all experiments. The values of elongation and hardness (Brinell) could also be improved for nearly all investigated amplitudes. In summary it is possible to achieve both: grain refinement and increase in tensile strength, hardness and elongation.
CET by Fragmentation during the Solidification under Natural and Forced Convection of Non-Refined Al-Based Alloys
Authors: Jung H.J. , Mangelinck-Noël Nathalie , Nguyen-Thi Henri , Bergeon Nathalie , Billia Bernard , Buffet Adeline , Baruchel J.
Published: May 2010

The columnar to equiaxed transition (CET) has been widely studied for many years [1] because this phenomenon is observed in metallurgical applications like castings. In non refined alloys, detachment of dendrite fragments is the most probable mechanism responsible for the formation of an equiaxed microstructure [1]. In this frame, melt convection influences the grain structure evolution by playing a role in the fragmentation phenomena [2].
Columnar to Equiaxed Transition During Ingot Casting Using Ternary Alloy Composition
Authors: Könözsy Laszlo , Ishmurzin Anton , Grasser Monika , Wu Meng Huai , Ludwig Andreas , Tanzer Robert , Schützenhöfer Wolfgang
Published: May 2010

The present paper deals with the formation of macrosegregation in a benchmark ingot using (Fe-C-Cr) ternary alloy composition. The numerical investigation of complex multiphase phenomena is a difficult study, because the thermophysical properties depend strongly on the temperature, concentration, pressure and chemical composition as well. For the numerical modeling of solidification and melting processes different phases (e.g. liquid, equiaxed crystals and columnar dendrite trunks) have been considered. The mass, momentum, energy conservation and species conservation equations for each phase have been solved. The Eulerian-Eulerian model equations have been implemented in the commercial Finite Volume Method based FLUENT-ANSYS v6.3 CFD software using User-Defined Functions (UDF). The mass transfer has been modelled by diffusion controlled crystal growth by applying an advanced tip tracking algorithm for columnar solidification. The modeling of the grain density transport has been improved. The derivatives of the mass fraction quantities for each component appear in the nucleation rate term. It means that we obtain a new term of the right hand side of the grain density transport equation for using ternary alloy composition. This paper focuses on both the process and simulation parameters and their influence on the macrosegregation formation. The results show that the macrosegregation pattern does not change significantly above a well-chosen number of grid cells, and the computational time could be decreased, when the time step size has been increased.
Macroscopic Model for Predicting Columnar to Equiaxed Transitions Using Columnar Front Tracking and Average Equiaxed Growth
Authors: Mirihanage Wajira , McFadden Shaun , Browne David J.
Published: May 2010

A macroscopic model of Columnar-to-Equiaxed Transition (CET) formation is presented. The growth of a columnar zone and an equiaxed zone are treated separately and modeled on a fixed grid. The model uses a columnar Front Tracking (FT) formulation to compute the motion of the columnar front and the solidification of the dendritic columnar mushy zone. The model for the equiaxed zone calculates the average growth of equiaxed grains within the control volumes of undercooled liquid. The proposed model, which calculates the average equiaxed growth, is different from previous FT models which consider each equiaxed grain envelope separately. A lognormal size distribution model of seed particles is used for the equiaxed nucleation in the undercooled liquid zone. After nucleation, average equiaxed growth is computed by considering the equiaxed envelopes as spherical. The extended volume concept is used to deal with grain impingement. The Scheil equation is used to calculate the solid fraction and latent heat release. When the equiaxed fraction is great enough, advancement of the columnar front is halted and the CET position is determined. CET formation was simulated for directional solidification of an aluminium-silicon alloy. The results were compared with a previous FT-CET prediction model as well as with experimental data. Agreement was found in both cases.
Numerical and Experimental Investigation of NH4Cl Solidification
Authors: Könözsy Laszlo , Kharicha Mihaela Stefan , Eck Sven , Wu Meng Huai , Ludwig Andreas
Published: May 2010

This paper deals with the validation of a volume averaged multiphase solidification model based on a benchmark experiment using NH4Cl-H2O as model alloy and Particle Image Velocimetry (PIV) as optical measurement method. For the numerical modelling of the solidification, different phases (e.g. liquid, equiaxed grains and columnar dendrite trunks) have been considered. The mass, momentum, energy conservation and species transport equations for each phase have been solved. The Eulerian-Eulerian model equations have been implemented in the commercial Finite Volume Method based software FLUENT-ANSYS v6.3 using User-Defined Functions (UDF). The mass transfer has been modelled by diffusion controlled crystal growth. The simulation of the NH4Cl-H2O solidification has been numerically investigated as a two-dimensional unsteady process in the cross-section of a 100 x 80 x 10 (mm3) experimental benchmark. Since during the experiment both columnar and equiaxed growth of NH4Cl have been observed, both phenomena have been considered in the simulation. The predicted distribution of the solidification front and the measured thickness of the mushy zone have been compared with the measurements.
A Sensitivity Study of Grain Growth Model For Prediction of ECT and CET Transformations in Continuous Casting of Steel
Authors: Lorbiecka Agnieszka Zuzanna , Šarler Božidar
Published: May 2010

A two dimensional model was developed to predict the grain structure (Equiaxed to Columnar Transformation (ECT) and Columnar to Equiaxed Transformation (CET) in the continuous casting of steel. The processes of nucleation, growth and impingement of the grains are modelled as follows: (I) the nucleation is modeled through a continuous dependency of the nucleation density on temperature by the Gaussian distribution. Different nucleation parameters are used at the boundary and in the bulk region. (II) The growth and impingement are modeled by the Kurz, Giovanola, Trivedi (KGT) model. The Cellular Automata (CA) technique is used to solve the model. The CA method is based on the Nastac’s and simplified neighborhoods. Calculations are shown for square billets of the dimension 140 mm. Fixed input parameter of the model represents the macroscopic temperature field obtained from the Štore Steel billet simulation system [1]. All other grain structure physical model parameters are varied, such as: the surface and the bulk area, mean nucleation undercooling, standard deviation of undercooling, maximum density of nuclei. The computational parameters, such as the micro mesh size and the time step are varied as well. The influence of the variation of different parameters on calculated grain structure is shown. Finally, the model parameters are adjusted in order to obtain the experimentally determined actual billet ECT and CET positions for 51CrV4+Mo spring steel (Al: 0.02, Cr: 1.05, Cu: 0.125, Mn: 0.9,Mo: 0.025, Ni: 0.1, Si: 0.275, V: 0.155, C: 0.51, P: 0.0125, S: 0.0275 wt%). A systematic procedure is outlined for adjusting of the model data with the experiment.
Microstructural Response to Growth Rate and Mg Additions during Directional Growth of Al-Cu-Mg Alloys
Authors: Berkdemir Ayşe , Gündüz Mehmet
Published: May 2010

Al-rich Al-Cu-Mg alloys have wide range of applications for automotive and aerospace where the control of the original solidification microstructure is important to achieve desired properties. Under constant temperature gradient G (4.84±0.13 K.mm-1), a series of directional solidification experiments were performed at 6 different growth rates V (16.7 – 166.7 µm/s) and 6 different Mg contents C0-Mg (0 – 5 wt.% Mg) of Al – 5wt.% Cu – (0-5)wt.% Mg alloys. The solid-liquid interface was investigated in as-quenched specimens. Primary dendrite arm spacing λ1 was measured on longitudinal and transverse sections of the specimens. The variations of λ1 with respect to V and C0-Mg were determined and the results were compared to the related theoretical models and the published data.
On the Problems of a Migrating Hot Spot
Authors: Elmquist Lennart , Diószegi Attila
Published: May 2010

In this work the meaning of a migrating hot spot during solidification will be discussed. Initially, just after mould filling, global as well as local thermal centres, or hot spots, are established. This occurs at mass concentrations and heavy sections as the mould wall initially is at ambient temperature. However, these hot spots are found to be migrating during solidification. This migration influences any temperature measurement and subsequent thermal analysis, introducing uncertainties. It has also been found that common casting defects such as shrinkage porosity and metal expansion penetration can be associated with a migrating hot spot. In these cases the final position of the hot spot is located across the interface between the casting and the mould. The solidification starts at the mould walls and it was found that the columnar zone is affected by the migrating hot spot. The macrostructure was preserved using DAAS technique and it was revealed that an otherwise normal columnar zone because of the migrating hot spot had been affected. Based on these findings it is suggested that the migrating hot spot is making the columnar zone weaker and more susceptible for pressure differences arising during solidification.
Determining Thermal Properties of Insulating Sleeves
Authors: Krajewski Witold K. , Suchy Józef Szczepan
Published: May 2010

The presented work is aimed at determining thermal diffusivity, thermal conductivity and heat capacity of insulating sleeves used in Polish metallurgical/foundry practice. On basis of the theory elaborated in [1] the mean values of thermophysical properties for temperatures range of about 150-1000 oC were obtained. The results obtained during the examinations presented in the paper can be helpful when formulating boundary conditions during the computer aided simulation of the processes of heat and mass transfer in the system: casting (ingot) – mould riser (ingot head) – ambient, which uses the investigated insulating sleeves [2, 3]. The method of determining thermal properties can be also used for other foundry materials, e.g. sands or cores.
Effects of Carbon Content on the Ultimate Tensile Strength in Gray Cast Iron
Authors: Fourlakidis Vasilios , Diaconu Vasile Lucian , Diószegi Attila
Published: May 2010

This paper investigates the effect of different carbon contents and cooling rates on gray iron tensile properties as well on the formation of different microstructure features. Four heats with increasing amount of carbon were cast. Every heat constituted of three cylinders, each of them surrounded by different materials which provided a wide range of solidification rates. The casting specimens were subjected to tensile test measurements and to microstructure examination. The results indicate a clear correlation between cooling rates, ultimate tensile strength (UTS), carbon content and eutectic cell size. Microscopic analysis shows also a relation between the primary phase’s fraction and the number of the eutectic cells.
Effects of Ag Addition on High Temperature Thermoelectric Properties of (Ca0.9Gd0.1)3Co4O9 Ceramics
Authors: Lu Yan , Song Ying , Feng Jin , Wang Fu Ping
Published: May 2010

A series of (Ca0.9Gd0.1)3Co4O9/xAg(x=0, 0.1, 0.15, 0.2) ceramics were prepared by a polyacrylamide gel method and Spark Plasma Sintering. Scanning electron microscopy (SEM) revealed that small size Ag particles were dispersed into the (Ca0.9Gd0.1)3Co4O9(CGCO) matrix. The electrical conductivity of the composites was obviously higher than that of (Ca0.9Gd0.1)3Co4O9, and increased with increase of Ag content. However, the addition of Ag seemed to have a negative impact on the Seebeck coefficient (S) of the composites samples due to its poor S. Since the increase of electrical conductivity (σ) is more significant than the degradation of S, the power factor (P=σS2) was found to be improved by the addition of Ag. At 973 K, the P value of the sample with x =0.2 reached 3.17×10-4 W•m-1•K-2, which was 12.5% higher than that of Ca3Co4O9(CCO) bulk material.
Microwave-Assisted Synthesis of Antimony Oxide Nanostructures and their Electrochemical Properties
Authors: Wu Feng Dan , Wang Yong , Tang Jun Jun
Published: May 2010

A few antimony oxide-based nanostructures were successfully synthesized by a fast microwave irradiation method including orthorhombic Sb2O3 and Sb8O11Cl2 nanorods bundles, sheet-like orthorhombic and cubic Sb2O3 materials, pure orthorhombic Sb2O3 nanorod bundles. All these Sb-based nanostructures were characterized by XRD, SEM and TEM. It was found that precipitator agents and irradiation conditions played important roles in the formation of Sb2O3 based nanostructures. The electrochemical properties of Sb2O3 nanorod-bundles were also preliminarily investigated.
Hot Deformation Behavior and Mechanism of Alloy Inconel 625
Authors: Yu Q.J. , Sun Wen Ru , Cai M. , Wu X.J. , Guo Shou Ren , Yang H.C. , Hu Z.Q.
Published: May 2010

The hot deformation behavior and microstructure of rolled Inconel 625 alloy have been studied from 930°C to 1180°C, and at strain rate from 10 s-1 to 80 s-1, respectively. The results indicate that, as deformation temperature rises, both peak flow stress (PFS) and recrystallization critical strain (RCS) decrease; as the strain rate increases, the PFS is enhanced, but the RCS drops. When the deformation temperature is within 1100°C and 1180°C, the grain size coarsens markedly with the temperature increasing. When the deformation temperature is lower than 1100°C，a higher strain rate is helpful for grain refinement. However, when the temperature is beyond 1100°C,the effect of strain rate on the grain size is reduced.
Study on the Formation Mechanism of Micro-Arc Oxidation Coatings on Magnesium Alloys
Authors: Chen Xian Ming , Luo Cheng Ping , Liu Jiang Wen
Published: May 2010

This article concerned with the formation mechanism of the micro-arc oxidization(MAO) coating on magnesium alloys. The micro-arc oxidation ceramic coating on AZ91 magnesium alloy was prepared in a silicate alkaline solution (Na2SiO3:5-30g/l, NaOH: 1-5g/l, KF: 5-8g/l, Na3C6H5O7: 0.5-2g/l, EDTA: 0.5-2g/l). The experiments were carried out in AC current galvanostatic mode with a current density of 20-50A/dm2. The microstructure of the MAO coating was characterized by means of transmission electron microscopy(TEM). Microcrystallite and/or nanocrystallite were found in the MAO coatings, the size of the former was about 50-200nm and the latter was about 20-50nm. Cellular structure was observed in the coatings, which was composed of a lot of micro/nano grains. The grains grew preferentially in some directions during the MAO process, and the cellular structure is formed by the grains growing in a laminar way. The cellular structures also grew and aggregated by laminar way to form the whole MAO coatings.
SCC (Stress Corrosion Cracking) Behaviors of Mg-7Al Alloy
Authors: Zhang Ya , Zhou Xue Hua , Wei Zhong Ling , Huang Yuan Wei
Published: May 2010

Stress corrosion cracking (SCC) behaviors of Mg-7Al-0.3Mn alloy in 0.21M K2CrO4+0.6M NaCl solution and in air were studied using slow strain rate test (SSRT). The effect of electrode potential on SCC behaviors of the alloy was investigated in 0.21M K2CrO4+0.6M NaCl solution during SSRT. The fractographs of the alloy tested in air and in the solution were observed. These results indicated that SCC of the alloy was related to the hydrogen embrittlement and pitting corrosion. The pitting corrosion was the cause of crack initiation, and the hydrogen embrittlement leaded to crack propagation.
Preparation and Mechanical Property of Super-Light Aluminum Foam
Authors: Cao Zhuo Kun , Du Jin Jing , Yao Guang Chun
Published: May 2010

Aluminum foams with low relative density, especially less than 0.1, persist unique physical, mechanical and acoustic properties. There is a need of this modern material for special applications and scientific research. However, preparation of this super-light metal foam is quite difficult for the cell size of the foams would increase with its porosity. In this study, carbon fibers are used as novel stabilize additive for aluminum foams and the influence of alloying elements on cell wall thickness and area of Plateau border are studied. The results indicate that the cell wall thickness and bubble coarsening rate of the foam would decrease much when Mg is added into the melt and the relative density of prepared foam can be as low as 0.08. Results of compressive tests reveal the fact that these aluminum foams show a lower compressive strength. The plastic deformation region of this new foam is long and it performs good energy absorption capacity.
3D Dynamic Simulation for Random-Distributed Aggregate Model of Plain Concrete
Authors: Hong Jun , Guo Li , Qiao Ling , Guo Xiao Ming
Published: May 2010

Since meso-structure of plain concrete can not be observed directly, numerical simulation is the main approach to obtain the model coincident with the real structure on statistics. By applying the discrete element method, we have developed the 3D dynamic simulation for random aggregate model of plain concrete. According to the real ration of mass, the spatial positions of aggregate have been obtained, which is more close-grained compared with the random-distributed models based on Monte Carlo method. Compared with the geometrical generating algorithm for 2D random polyhedral aggregate, the algorithm for 2D or 3D random polyhedral aggregate is simpler. The results are the foundation for further studying the interface fracture and chloride diffused channels.
Mechanical Properties and Microstructure Characterization of Calcium Silicate Hydrate (C-S-H) Gel
Authors: Qiao Ling , Guo Li , Hong Jun , Guo Xiao Ming
Published: May 2010

The complex behavior of concrete is largely related to the mechanical response of calcium silicate hydrate (C-S-H) gel. During the past two decades, a great deal of efforts has been devoted to the structure of C-S-H at the nanoscale level, including both experiment investigation and molecular simulation. The smallest C-S-H unit has a layered crystal structure at atomic scale, with a lamella thickness in the nm range. And moreover, ordered stacks of up to several tens or even hundreds of nano-lamellae have been observed experimentally. The key features of structure and morphology is remarkably the ubiquitous presence of an orientational order. Now we focus on the physical understanding on such orientational ordered C-S-H structures at mesoscale. In respect that there exists a high nematic order in the stacking C-S-H of nano-lamellae, by introducing some physical parameters, such as the magnitude of nematic order of the nano-lamellae, we try to predict the mechanical properties of the crystalline in terms of its micro-structure.
Investigation on Toughness of Fibre Cocktail Reinforced Self Consolidating Concrete after High Temperature
Authors: Ding Yi Ning , Wang Yue Hua , Zhang Yu Lin
Published: May 2010

The effect of different fibres on the residual load-bearing capacity and the failure pattern of high-performance self consolidating concrete (HPSCC) after exposure to high temperature hass been studied in this work. The polypropylene fibers mitigate the spalling of HPSCC element clearly, but did not show clear effect on the mechanic properties of concrete. The macro steel fiber reinforced HPSCC showed higher flexural toughness and ultimate load before and after high temperatures. The mechanical properties of hybrid fibre reinforced HPSCC (HFHPSCC) after heating were better than that of mono-fibre reinforced HPSCC. The failure mode changed from pull-out of steel fibers at lower temperature to broken down of steel fibers at higher temperature. The use of hybrid fibre can be effective in providing the residual strength and failure pattern, and improving the toughness of HPSCC after high temperature.
Surfactant Protein-A as an Anti-Inflammatory Component in the Amnion: Implications for Human Pregnancy
Authors: Lee Deug-Chan , Romero Roberto , Kim Chong Jai , Chaiworapongsa Tinnakorn , Tarca Adi L. , Lee JoonHo , Suh Yeon-Lim , Mazaki-Tovi Shali , Vaisbuch Edi , Mittal Pooja , Draghici Sorin , Erez Offer , Kusanovic Juan Pedro , Hassan Sonia S. , Kim Jung-Sun
Published: May 2010

The mechanism of mouse parturition is thought to involve myometrial infiltration by amniotic fluid (AF) macrophages, activated by surfactant protein-A (SP-A). In humans, the concentration of AF SP-A decreases during labor, and no fetal macrophages are found in the myometrium after labor. Therefore, it appears that the mechanisms of labor in mice and humans are different. We investigated a potential role for SP-A in human pregnancy and parturition by examining SP-A expression patterns in AF and amnion. High molecular mass (>250 kDa) oligomeric SP-A was increased in AF with advancing gestation. Interestingly, these oligomers were more abundant in placental amnion before labor at term, while they increased primarily in reflected amnion during labor (p < 0.05). Immunoblotting showed a binding of high molecular mass SP-A in AF to amnion. In C57BL/6 mice, oligomeric SP-A was also readily detected in AF from E15 onwards, but not in amnion. Macrophage density in mice myometrium did not change with advancing gestational age. Microarray analysis of human amnion explants incubated with SP-A revealed a molecular signature of inhibited cytokine–cytokine receptor interaction with downregulation of IL-1β, CXCL2, and CXCL5 mRNA expression. The findings in this study strongly suggest that SP-A signals amniotic anti-inflammatory response via AF during pregnancy. We propose that an SP-A interaction among AF, placental amnion, and reflected amnion is a unique mechanism for immunoregulation in human pregnancy akin to that established in lung biology. However, AF SP-A and fetal macrophages by themselves do not seem to be exclusive effectors of parturition in humans.
Plasmodium falciparum Dihydrofolate Reductase and Dihyropteroate Synthase Mutations and the Use of Trimethoprim-Sulfamethoxazole Prophylaxis among Persons Infected with Human Immunodeficiency Virus
Authors: Malamba Samuel , Sandison Taylor , Lule John , Reingold Arthur , Walker Jordan , Dorsey Grant , Mermin Jonathan
Published: May 2010

A prospective cohort design was used to measure the association between daily cotrimoxazole-prophylaxis and infection with Plasmodium falciparum containing mutations associated with antifolate resistance among persons infected with human immunodeficiency virus (HIV) in Tororo and Busia District, in eastern Uganda. Of 149 cases of P. falciparum parasitemia diagnosed, 147 (99%) (smears from participants taking prophylaxis = 91 and smears from those not taking cotrimoxazole prophylaxis = 56) were successfully assessed for mutations in the dihydrofolate reductase (dhfr) and dihydropteroate synthase (dhps) mutations associated with antifolate resistance. Prevalences of the dhfr pure triple mutant (74% and 70%; P = 0.71), the dhps pure double mutant (95% and 88%; P = 0.21), and the dhfr/dhps pure quintuple mutant (73% and 64%; P = 0.36), were not significantly different between those taking and those not taking cotrimoxazole-prophylaxis, respectively. The overall prevalence of the pure quintuple mutant in this study was 69%, which is among the highest in Africa. Although resistance rates of P. falciparum to antifolate drugs are high, cotrimoxazole-prophylaxis in HIV-infected persons was not associated with a higher prevalence of mutations associated with antifolate resistance.