bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical, Dielectrical and Magnetic Properties of Zr-Mn Doped Nano-Ferrites
Authors: Khan K. , Maqsood Ashari
Published: May 2012

We measured the dc electrical resistivity as a function of temperature and dielectric parameters in the frequency range 100 Hz to 3 MHz of nanosized Zr-Mn spinel ferrites with nominal composition CoFe2-2xZrxMnxO4 (0.1 x 0.4). The dc electrical resistivity decreased with the rise in temperature for all the samples, showing a semiconductor like behavior. From the dc electrical resistivity the activation energy and drift mobility are determined. Both the drift mobility and activation energy increase with a rise in x. The dielectric constant, dielectric (losses) and ac electrical resistivity as a function of frequency are also reported. The low field ac magnetic susceptibility measurement showed that the ferrimagentic transition temperature is in the range of 4395 K to 6585 K.
Isotropic and Anisotropic Nanocrystalline NdFeB-Based Magnets Prepared by Spark Plasma Sintering and Hot Deformation
Authors: Liu Zhong Wu , Huang Y.L. , Huang H.Y. , Zhong X.C. , Yu Hong Ya , Zeng De Chang
Published: May 2012

Isotropic and anisotropic NdFeB permanent magnets were prepared by Spark Plasma Sintering (SPS) and SPS followed hot deformation (HD), respectively, using melt spun NdFeB ribbons with various compositions as starting materials. It is found that, based on RE-rich composition, SPSed magnets sintered at low temperatures (<700 C) almost maintained the uniform fine grain structure inherited from rapid quenching. At higher temperatures, a distinct two-zone (coarse grain and fine grain zones) structure was formed in the SPSed magnets. The SPS temperature and pressure have important effects on the grain structure, which led to the variations in the magnetic properties. By employing low SPS temperature and high pressure, high-density magnets with negligible coarse grain zone and an excellent combination of magnetic properties can be obtained. For single phase NdFeB alloy, because of the deficiency of Nd-rich phases, it is relatively difficult to consolidate micro-sized melt spun powders into high density bulk magnet, but generally a larger particle size is beneficial to achieve better magnetic properties. Anisotropic magnets with a maximum energy product of ~38 MGOe were produced by the SPS+HD process. HD did not lead to obvious grain growth and the two-zone structure still existed in the hot deformed magnets. The results indicated that nanocrystalline NdFeB magnets without significant grain growth and with excellent properties could be obtained by SPS and HD processes.
Anomalous Behavior of Chemically Synthesized Magnetoplumbite Strontium Ferrite Nano Particles
Authors: Asghar G. , Nasir S. , Awan M.S. , Tariq G.H. , Anis-ur-Rehman Muhammad
Published: May 2012

Strontium hexa-ferrite nanoparticles were prepared successfully by simple co-precipitation method. The XRD analysis confirmed the formation of single phase MFe12O19 (M=Sr). Parameters such as crystallite size, lattice constant, X-ray density and porosity were calculated from the X-ray diffraction data. The crystallite sizes were in the range 12-26 nm. The temperature dependent dc electrical resistivity measurements showed that the material was highly. Dielectric constant and dielectric loss factor (tanδ) were measured in the frequency range 20Hz-3MHz. The anomalous behavior of dielectric loss revealed a very important behavior of the prepared sample of SrFe12O19 in different frequency regions and that could be used for new applications of this material. The magnetic properties were determined from the hystersis loop obtained from vibrating sample magnetometer (VSM). The Curie temperature was determined by susceptometer. This material is potentially suitable for use as a recording medium in identification cards and credit cards and for the fabrication of permanent magnets.
Effect of Thermal Treatments on the Structural and Optical Properties of Thermally Deposited Polycrystalline Cu Thin Films
Authors: Shah Syed Khalid , Mansoor Muhammad , Asim M.M. , Zaman Sabah
Published: May 2012

Copper thin films are potentially used in optical and laser applications due to their intrinsic reflective indexes in visible and infrared region of the spectrum. The reflective properties of the thin films are mainly driven by their thickness, structure, and residual strain induced during the processing stages. Copper thin films of various thicknesses were deposited on glass slides using a thermal evaporation unit. The deposited substrates were thermally treated, in inert environment for 30 minutes, for various temperatures. Further, the substrates were characterized using various techniques. Structural studies of the thin films were carried out using XRD on the as deposited and heat treated films to study the phases, the crystallographic preferred orientation, residual strain and crystallite size. The polycrystalline Cu phase was revealed and no oxide phases were identified. The films were preferentially oriented along (111). The crystallite size increases while the residual strain decreases as the film thickness increases. The crystallite sizes were very small as compared to the film thickness. The optical properties of these coatings were investigated by double beam spectrophotometer. It was found that reflectance of these coatings strongly depends upon the film thickness and post deposition heat treatment. The optimum deposition procedure was established to obtain the enhanced reflecting power.
Effect of Zirconium Addition on Welding of Aluminum Grain Refined by Titanium
Authors: Zaid A.I.O.
Published: May 2012

Aluminum and its alloys solidify in large grains columnar structure which tends to reduce their mechanical behaviour and surface quality. Therefore, they are industrially grain refined by titanium or titanium + boron. Furthermore, aluminum oxidizes in ordinary atmosphere which makes its weldability difficult and weak. Therefore, it is anticipated that the effect of addition of zirconium at a weight percentages of 0.1% (which proved to be an effective grain refiner on the weldability of aluminum grain refined by Ti) is worthwhile investigating. This formed the objective of this research work.
In this paper, the effect of zirconium addition at a weight percentage of 0.1%, which corresponds to the peritctic limit on the aluminum-zirconium phase diagram, on the weldability of aluminum grain refined by Ti is investigated. Rolled sheets of commercially pure aluminum, Al grain refined Ti of 3 mm thickness were welded together using Gas Tungsten Arc Welding method (GTAW), formerly known as TIG. A constant air gap was maintained at a constant current level, 30 ampere AC, was used because it removes the oxides of the welding process under the same process parameters.
Metallographic examination of weldments of the different combinations of aluminum and its microalloys at the heat affected zone, HAZ, and base metal was carried out and examined for width, porosity, cracks and microhardness.
It was found that grain refining of commercially pure aluminum by Ti resulted in enhancement of its weldability. Similarly, addition of zirconium to Al grain refined by Ti resulted in further enhancement of the weldment. Photomicrographs of the HAZ regions are presented and discussed.
Effect of Thermal Treatment on Nano-Crystallites and Phase Transformation of CaZrO3Thermal Barrier Coatings
Authors: Bhatty Mustasim Billah , Khalid Fazal Ahmad , Nusair Khan Aamer
Published: May 2012

Thermal barrier coatings (TBCs) are multilayered coatings having the complex structure. In thermal spray coatings characteristic features of deposited molten particle and its microstructure is very important in defining coating properties. In this work stainless steel (AISI 316) was used as substrate material on which bond coat of Ni-20Cr and top coat of CaZrO3 were deposited by air plasma spraying method. Effect of isothermal treatment on the internal microstructure structure of lamella (splat) and phase changes in CaZrO3 coating was studied. The fractured surface of coatings was investigated to observe the splat morphology. It was observed that the nanocrystallites present within the splat grew with increase in temperature. Further, the monoclinic phase was formed as function of temperature and time.
Role of Heat Treatment in Magnetic Properties of Fe-Co-Ni-Al-Cu-Ti Alloy
Authors: Imran S. , Awan M.S. , Sarwar M.N. , Akhtar S. , Farooque M.
Published: May 2012

Anisotropic (Fe-24Co-15Ni-7.5Al-3Cu-0.3Ti) permanent magnets were prepared by alloy casting technique. Samples were characterized for structural, microstructural and magnetic properties using x-ray diffractrometer (XRD), Scanning electron microscope (SEM) equiped with energy dispersive x-ray spectrometer and DC magnetometer. There is a relation between metallurgical structure and magnetic properties. The magnetic properties of this type of alloy depend on heat-treatment that is controlling the cooling rate in the presence of magnetic field and double aging. How magnetic properties vary with heat treatment is discussed in this work. XRD studied revealed that the alloy was single phase (bcc) with (110) as the major crystallographic plane. Optical and SEM micrographs showed the presence of (Ti,S)-rich precipitates which are randomly distributed in the matrix. Their composition was analyzed by EDX analysis.
Resistance Pressure Sensor Based on Ag/Cu2O-PEPC-NiPc/Al Composite
Authors: Khan A. , Karimov Khasan S. , Shah M.
Published: May 2012

This work reports on the fabrication and investigation of pressure sensor based on Ag/Cu2O-PEPC-NiPc/Al composite. The active layer of the composite was deposited by drop-casting of the blend Cu2O-PEPC-NiPc on flexible substrate. The thin film of the blend consist of cuprous oxide (Cu2O) micropowder, (5 wt. %), poly-N-epoxypropylcarbazole (PEPC), (2 wt. %) and nickel phthalocyanine (NiPc) micropowder, (3 wt. %) in benzol (1 ml). The film thickness of the composite is in the range of 20-30 μm. It is found that the fabricated sensor is sensitive to pressure and showed good repeatability. The decrease in resistance of the sensor is observed 10 times by increasing the external uniaxial pressure up to 11.7 kNm-2. The experimentally obtained results are compared with the simulated results and showed reasonable agreement with each other.
Effect of Strain Rate on Cavity Closure during Compression between Flat Platens Using Superplastic Tin-Lead Alloy
Authors: Zaid A.I.O. , Al-Tamimi M.M.
Published: May 2012

Superplasticity is a feature of a material or alloy which allows the material to deform plastically to an extremely large strain at low values of stress under certain loading conditions of strain rate and temperature. Eutectic tin-lead alloy is a practical material for research investigations as it possesses a superplastic behavior at room temperature and low strain rate which makes it a useful tool in simulating the ordinary engineering materials at high strain rate and temperature. This alloy has been extensively used as a model material to simulate behavior of engineering materials at high strain rates and temperatures.
In this paper, superplastic tin-lead alloy was used at room temperature to simulate the closure of cavities in steels at high temperatures in the hot region under dynamic loading (high strain rate) under the effect of compressive loads using flat platens (open dies).
Hollow specimens having different values of bore diameter (Db) to outer diameter (Dout), of the same height and volume were investigated under different values of height reduction percentages ranging from 20% to 80% , and the percentage of cavity closure at each reduction percentage was determined. It was found that the cavity closure percentage increases or decreases at slow rate for reduction percentage in height less than 40% and increases more rapidly for reduction percentages in height above this value. Furthermore, specimens having smaller values of ratio (Db/Dout) resulted in higher percentage of cavity closure than specimens having higher ratios at the same value of reduction in height percentage. Complete cavity closure has occurred in specimens having the ratios of 0.1 and 0.2 at 75% reduction in height.
SEM Examination and Analysis of the Interface Character in Surface Modified Aramid-Epoxy Composite
Authors: Hussain S. , Khan Mohammad Bilal , Hussain R.
Published: May 2012

The surface of Kevlar fibers is chemically modified by treatment with Phthalic anhydride (PA) and the effect is examined by SEM for the laser cut, three point bending and interlaminar shear delaminated surfaces. The surface modification improved the adhesion to epoxy resin that clearly leads to cohesive fracture as opposed to interfacial failure in the untreated specimen. SEM reveals marginal surface roughening of fibers without compromising their strength. The interface modification technique described in this paper is attractive thermodynamically as it does not compromise surface free energy of the polymer matrix or that of the fiber itself to enhance wetability.
Variations in Structural and Electrical Properties of Ba-Hexaferrite due to Pb Substitution
Authors: Ul-Haq Anwar , Anis-ur-Rehman Muhammad
Published: May 2012

BaFe2-xPbxO9 (x=0.0 to 1.0) samples were prepared by co-precipitation method. All samples were sintered at 960±5°C for three hours. Structural properties were investigated by X-ray diffraction (XRD) and Scanning electron microscopy (SEM). Variation in structural parameters was calculated from XRD obtained data. SEM micrographs were used to study the morphology and grain growth. No regular increase or decrease in structural parameters and morphology is observed with increase in Pb content. For dc electrical characterizations, dc resistivity was measured as a function of temperature up to 750K. Data obtained was further analyzed and different dc parameters like activation energy and mobility were measured. For dielectric analysis, capacitance C was measured in the frequency range from 100 kHz to 3MHz. High dielectric constant for this material makes this material suitable for applications like multilayer capacitors (MLC), microwave devices and dynamic random access memory (DRAM).
Non-Destructive Evaluation of Plasma Sprayed CaZrO3 Coatings by Eddy Current Technique
Authors: Bhatty Mustasim Billah , Khalid Fazal Ahmad , Nusair Khan Aamer , Mehmood Khalid
Published: May 2012

Eddy current as a non-destructive technique has been used for evaluation of thermal barrier coatings (TBCs) system. In this study the TBCs system which consists of substrate material of stainless steel 316, Ni-20Cr as a bond coat and top coat of CaZrO3 was produced by air plasma spraying technique. Isothermal treatment of TBCs system was performed and its effect on the microstructures and oxide formation was investigated. The chromium oxide formation as a function of temperature and time was observed. The observed changes in microstructure and formation of chromium oxide were then correlated with variation in eddy current values.
Effect of Vanadium Addition to Aluminum and Aluminum Grain Refined by Titanium on their Chemical Corrosion Rate in Acidic Solution, HCL, at Different Temperatures
Authors: Zaid A.I.O. , Allawi G.T.A. , Al-Haj-Ali A.
Published: May 2012

The paper presents the effect of vanadium addition to aluminum and aluminum grain refined by titanium on the micron level, in the range from 0.005 wt % to 0.236 wt %, on their corrosion resistance in acidic solution, HCl, at three different temperatures namely: 25 °C, 40 °C and 60 °C. It was found that the corrosion rate was slightly increased by the addition of any percentage of vanadium at 25 °C. Furthermore, it was found that the corrosion rate increased with the increase of temperature at any percentage of vanadium addition in the case of both aluminum and aluminum grain refined by titanium. However it was found that addition of vanadium at any percentage to either aluminum or aluminum+ titanium, resulted in decrease of the corrosion rate i.e. improvement in their corrosion resistance in acidic solution at 40 °C and 60 °C. The maximum achieved reduction in corrosion rate was 77 % at 40 °C and 0.148 wt % vanadium addition.
Correlation of Microstructures with Electrical Transport Properties in Mn Doped Co Nanoferrite Particles
Authors: Akram M. , Anis-ur-Rehman Muhammad , Nasir S. , Asghar G.
Published: May 2012

Magnetic nanocomposites are offering a variety of novel features and tune able properties, mainly depending on particle size, cation distribution, morphology and porosity of the prepared materials. The aim of this research work is to understand the effects of Mn doping on the microstructures and hence consequences on the electrical transport properties with shift of cation distribution in CoFe2O4. Co1-xMnxFe2O4 nanocrystallite particles with stoichiometric proportion (x) varying from 0.0 to 1.0 were prepared by co-precipitation method. X-ray diffraction patterns confirmed the FCC spinel structure of synthesized particles. The crystal structure is found to be inverse cubic spinel with a space group Fd3m and the lattice constants ranges from 8.36 Å to 8.46 Å The crystallite sizes were calculated from the most intense peak (311) using the Debye-Scherrer formula for all the samples those were synthesized at reaction temperature of 70°C. Then samples were sintered at 600°C for 3 hours, characterized by X-ray diffraction at room temperature and DC electrical resistivity measurements were done as a function of temperature by two-probe method from 370 K to 690 K. The measurements showed that DC electrical resistivity decreased with increase in temperature ensuring the semiconductor nature of the material in this temperature range. DC electrical resistivity results were discussed in terms of polaron hopping model under the effects of cation distribution. AC electrical properties were also analyzed. All the observed properties were correlated with observed microstructures.
Synthesis of Laser Beam Rapidly Solidified Novel Surfaces on D2 Tool Steel
Authors: Ahmed B.A. , Rizwan K.F. , Minhas Jafar A. , Waheed-ul-Haq Syed , Shahid M.
Published: May 2012

Surface layer of D2 tool steel was subjected to laser surface melting using continuous wave 2.5kW CO2 laser in point source melting mode. The processing parameters were varied to achieve a uniform depth of around 2 mm. Microstructural study revealed epitaxial growth of fine dendritic structure with secondary dendrite arm spacing in the range of 20-25 µm. The phases in the parent annealed sample were BCC ferrite and chromium rich M7C3 carbide. The major phase after laser treatment was austenite and M7C3. The average hardness of annealed sample was 195 HV which increased to 410 HV after laser melting. Corrosion studies in 2% HCl solution exhibited a drastic improvement in corrosion resistance in laser treated samples. Improvement in properties is attributed to the refinement and uniformity of microstructure in the rapidly solidified surface. The case of a moving heat source was subjected to computer aided simulation to predict the melt depth at different processing conditions in point source melting mode. The calculated depths using the model, in ABAQUS software was found in good agreement with the experimental data.
Structural, Electrical and Dielectric Properties of Nanocrystalline Mg-Zn Ferrites
Authors: Anis-ur-Rehman Muhammad , Malik M.A. , Nasir S. , Mubeen M. , Khan K. , Maqsood Asghari
Published: May 2012

The nanocrystalline Mg-Zn ferrites having general formula Mg1-xZnxFe2O4 (x=0, 0.1, 0.2, 0.3, 0.4, 0. 5) were prepared by WOWS sol-gel route. All prepared samples were sintered at 700°C for 2 h. X-ray powder diffraction (XRD) technique was used to investigate structural properties of the samples. The crystal structure was found to be spinel. The crystallite size, lattice parameters and porosity of samples were calculated by XRD data analysis as function of zinc concentration. The crystallite size for each sample was calculated using the Scherrer formula considering the most intense (3 1 1) peak and the range obtained was 34-68 nm. The dielectric constant (ε), dielectric loss tangent () and AC electrical conductivity of nanocrystalline Mg-Zn ferrites are investigated as a function of frequency. The dielectric constant (ε), dielectric loss tangent () increased with increase of Zn concentration. All the electrical properties are explained in accordance with MaxwellWagner model and Koops phenomenological theory.
Effect of Carbon Black on Curing Behavior, Mechanical Properties and Viscoelastic Behavior of Natural Sponge Rubber-Based Nano-Composites
Authors: Bashir Arshad , Shahid M. , Alvi R.A. , Yahya A.G.
Published: May 2012

Natural rubber (polymer designation cis-1-4 polyisoprene, is obtained from the sap ("latex") of several rubber-yielding plants (e.g., HeveaBrasiliensis and Partheniaargentatum) by coagulation with chemicals, drying, electrical coagulation, and other processes. Foamy or sponge structure of natural rubber (NR) is very useful in aerospace and as auxetic materials (exhibiting negative poisons ratio) for use in application of homeland security.The main aim of this research to estimate the influence of carbon black on mechanical properties, curing and viscosity variation behaviors of the natural rubber based composites. Different amounts of carbon black were used along with other fillers. The influence of carbon black on curing behavior and mechanicalproperties of natural rubber foams was investigated at different feedingratios of the carbon black. The physical properties of the foamed NRs were then measured as a function of carbon blacks content. The mechanical properties of the foamed NRs such as tensile strength,strength at break and modulus,were gradually increased with increasingcarbon black content whereas elongation decreasedat break.
Plasma Sprayed Alumina Coating on Ti6Al4V Alloy for Orthopaedic Implants: Microstructure and Phase Analysis
Authors: Khalid Muhammad , Mujahid Mohammad , Nusair Khan Aamer , Rawat Rajdeep Singh , Salam Iftichar Us , Mehmood Khalid
Published: May 2012

Alumina was coated on Ti6Al4V alloy by atmospheric plasma spraying. Surface and cross sectional analysis of the coating by SEM showed that diameter and thickness of splat was greatly influenced by arc current and stand-off distance. In turn, this variation in splat morphology had effect on evolution of α-Al2O3 and γ-Al2O3 phases as revealed by XRD. Higher proportion of required γ-Al2O3 phase was achieved at stand-off distance of 130 mm and arc current of 500A.
Design of Axially Loaded Helical Spring Isolation Systems
Authors: Pasha Mustafa
Published: May 2012

Engineering metals have a desirable property of absorbing energy especially when they are manufactured into helical springs. This ability has been utilized favorably in the design of isolation systems. The material and spring behavior prolongs the time period of excitation thereby decreasing the frequency and acceleration of the shock into the structure. This paper presents the design and behavioral response of a steel spring isolation system subjected to shock excitation as experienced in real world applications. The isolation discussed is for the support motion where the shock excitation originates outside the structure. In particular the focus is on undamped single degree of freedom (SDOF) systems. The goal is to design a passive isolation system in the form of helical steel springs. The system must limit the instance of high acceleration energy transferred into the structure in a very short time frame. A transmissibility algorithm is developed and presented. A test case is designed and manufactured based upon the design data simulated from the algorithm. A comparative study is presented to validate the accuracy of the algorithm and isolation system against empirical results from the test case using a drop testing apparatus. The shock response spectra commonly used in problems of shock excitation to a half sine wave is presented.
Comparative Performance of Locally Made and the Foreign Made Dynamic Compression Plates
Authors: Bilal M. , Gul R.M. , Mujahid Mohammad , Askar Z.
Published: May 2012

Bone implants are widely used to treat patients due to trauma in different causalities. The major types of bone implants are plates known as Dynamic Compression Plates (DCP) and nails, both made of stainless steel (AISI 316L Grade). In Pakistan both local made and foreign made (DCP) are available. The unit price of foreign made DCP is about 8 to 10 times that of the local made, however, no comprehensive study has been done on the comparison of these plates.
An in-depth analysis was performed to compare the essential properties of six different brands of DCP including two foreign, two local and two unknown brands. These properties included mechanical properties, such as bending stiffness, yield strength, modulus of elasticity and hardness. Compositional analysis and various dimensions of plate important for bone healing process were also compared. The results show that all plates have similar mechanical properties. The compositional analysis showed some variations from the ASTM standards for most of the plates. The dimensional analysis of plates showed that Slot Width and Land were within range for most of the plates but the Spherical Radius was out of range for all the plates. Generally, all plates have no major differences in their properties, material and shape.
Investigation of Fiber-Reinforced Modified Epoxy Resin Composites
Authors: Quddos A. , Khan Mohammad Bilal , Khan R.N. , Ghauri M.K.K.
Published: May 2012

The impregnation of the fiber with a resin system, the polymeric matrix with the interface needs to be properly cured so that the dimensional stability of the matrix and the composite is ensured. A modified epoxy resin matrix was obtained with a reactive toughening agent and anhydride as a curing agent. The mechanical properties of the modified epoxy matrix and its fiber reinforced composites were investigated systematically.
The polymeric matrix possessed many good properties, including high strength, high elongation at break, low viscosity, long pot life at room temperature, and good water resistance. The special attentions are given to the matrix due to its low out gassing, low water absorption and radiation resistance. In addition, the fiber-reinforced composites showed a high strength conversion ratio of the fiber and good fatigue resistance. The dynamic and static of the composite material were studied by thermo gravimetric analysis (TGA), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM) with EDX. The influences of processing technique such as curing and proper mixing on the mechanical and interfacial properties were determined.
The results demonstrated that the modified epoxy resin matrix is very suitable for applications in products fabricated with fiber-reinforced composites.
An Inquisitive into the Indirect Measurement and Comparison of the Viscosity of Nanoconfined Polystyrene Nanoparticles
Authors: Nawaz Qamar , Rharbi Y.
Published: May 2012

The effects of nanoconfinement on the dynamic and the glass transition (Tg) of polymers remains the focus of a lot of research since over a decade. Particularly, the glass transition temperature (Tg) and the dynamic of polystyrene (PS) were found to be altered by nanoconfinement in thin films and on the bulk free-surface. However, the dynamic of polymer nanoconfined in nanoparticles has not been investigated, even though the close-packed nanoparticle geometry is commonly used in many applications such as waterborne coatings. We investigate the dynamic of polystyrene in nanoparticles by monitoring the closure of voids (interstices) between PS nanoparticles in the close-packed structure. Void-closure during the passage from the close-packed particles to bulk PS is monitored using small angle neutron scattering at the bulk Tg (100 °C). The relaxation time (τ) and the apparent viscosity (η) of nanoconfined polystyrene estimated from the void-closure decay is found to decrease only by ~2 times for particle diameters between 93 nm and 42 nm. These results infer that dynamic of nanoconfined PS in nanoparticles at the bulk Tg is not different from that of bulk polystyrene.
Effects of Sintering Temperature on Structural and Electrical Transport Properties of Zinc Ferrites Prepared by Sol-Gel Route
Authors: Anis-ur-Rehman Muhammad , Malik M.A. , Ahmad I. , Nasir S. , Mubeen M. , Abdullah A.
Published: May 2012

The effects of sintering temperature on the structural and electrical transport properties of nanocrystalline zinc ferrites are reported. The zinc ferrites were prepared by WOWS sol-gel synthesis route. The prepared sample was sintered at temperatures 500°C, 700°C and 900°C respectively for 2 h. X-ray Diffraction (XRD) technique was used to describe the structural properties. The crystallite size, lattice parameters and porosity of samples were measured from the analysis of XRD data. The average crystallite size for each sample was measured using the Scherrer formula by considering the most intense (3 1 1) peak. The dielectric constant (ε), dielectric loss tangent (tan ) and AC electrical conductivity of nanocrystalline Zn ferrites are investigated as a function of frequency and sintering temperature. All the electrical properties are explained in accordance with MaxwellWagner model and Koops phenomenological theory.
Use of Flexible Engineered Cementitious Composite in Buildings
Authors: Khitab Anwar , Nadeem M. , Hussain S.
Published: May 2012

This article describes the applications and benefits of a recently developed smart building material namely Engineered cementitious composite (ECC), also known as flexible or bendable concrete. Conventional concretes have a strain capacity of only 0.1 percent and are highly brittle and rigid. This lack of bendability is a major cause of failure under strain and has been a pushing factor in the development of an elegant material which is capable to exhibit an enhanced flexibility. An ECC has a strain capacity of more than 3 percent and thus acts more like a ductile metal rather than like a brittle glass. The aim of this paper is to highlight a probable success of ECC in terms of industrial and commercial use in Pakistan. With the introduction of flexible concrete in building technology, it is likely to have safer and more durable construction. The material is expected to display reduced detrimental impacts on the natural environment. A bendable concrete is composed of all the ingredients of a traditional concrete minus coarse aggregates or crushed stones and is reinforced with micromechanically designed polymer fibers. The mechanism of action of the micro-polymeric fibers in concrete has also been emphasized. The principles of mix designs of the mortar incorporating fibers to make an ECC have also been explained. It has also been mentioned in detail as how this technology can be used to enhance the flexibility of some modern concrete types like flowing concrete, self-compacting concrete, and lightweight concrete. ECC is a green construction material. The possible benefits like environment friendliness, cost effectiveness, and durability have been also been elucidated in the paper.
Permeability Measurement of some Barrier Materials as a Function of Temperature and Pressure
Authors: Maqsood M. , Faisal S. , Ali J. , Usman A. , Alamgir K. , Farooq M.
Published: May 2012

Barrier materials possess the ability to restrict the passage of gases, vapors, and organic liquids through their boundaries. These barrier materials have large number of applications in industry and scientific research. To measure the permeability of barrier materials, a specific gas flow system has been developed, pure helium gas is used to measure the back ground reading through SS-316. The permeability and break-through time has been measured through Inconel X-750, NBR and Viton below and above the atmospheric pressure and at different temperatures 20°C, 40°C and 70°C.
Effect of Grain Size and Load Ratio on the near Threshold Stress Intensity Factor during Fatigue Crack Propagation of Dual Phase Steel
Authors: Sarwar M. , Ahmad E. , Manzoor T.
Published: May 2012

Fatigue crack growth near-threshold stress intensity factor is affected by the microstructure of the material. A large portion of microstructural influence is due to the change in grain size of the material. Grain size in the dual phase steel was varied and found that the near-threshold stress intensity factor (rKth) increased as the grain size increased. Influence of load ratio nearthreshold fatigue crack propagation was also studied. It was observed that the near-threshold stress intensity range, rKth for fatigue growth decreased with increasing load ratio.
Cerium Doped Bismuth Based High Tc Superconductor: Synthesis and Study of Superconducting Properties
Authors: Mubeen M. , Anis-ur-Rehman Muhammad
Published: May 2012

High-Tc superconductivity has been an emerging field for researchers since its discovery. Bismith based superconductors commonly called BSCCO have great importance among the superconducting family. It is divided into three phases among them 2223 phase is highly studied in order to investigate its superconducting properties by substitution of different elements. We have studied the substitution of cerium (Ce) on calcium site of bismuth based Bi (Pb)Sr (Ba)-2223 high-Tc superconductor. The nominal compositions of Bi1.6Pb0.4Sr1.6Ba0.4(Ca1-xCex)2Cu3Ox ceramic superconductor were prepared by wet chemical method. Stoichiometric amounts of Bi2O3, PbO, Sr (NO3)2, BaCO3, CaCO3, CuO and CeO2 were used as starting materials. Structural analysis was done by X-ray diffraction (XRD) at room temperature and different parameters were calculated. DC resistivity measurements for the transition temperature of synthesized superconducting samples were taken by the standard four-probe method, apparatus for which was developed in our laboratory. It is observed that with the substitution of cerium on calcium site the single high-Tc 2223 phase is obtained.
Tempering Response to Different Morphologies of Martensite in Tensile Deformation of Dual-Phase Steel
Authors: Ahmad E. , Manzoor T. , Sarwar M. , Arif M. , Hussain N.
Published: May 2012

A low alloy steel containing 0.2%C was heat treated with three cycles of heat treatments with the aim to acquire different morphologies of martensite in dual phase microstructure. Microscopic examination revealed that the morphologies consisting of grain boundary growth, scattered laths and bulk form of martensite were obtained. These morphologies have their distinct patterns of distribution in the matrix (ferrite). In tensile properties observations the dual phase steel with bulk morphology of martensite showed minimum of ductility but high tensile strength as compared to other two morphologies. This may be due to poor alignments of bulk martensite particles along tensile axes during deformation. Tempering was employed with various holding times at 550°C to induce ductility in the heat treated material. The tempering progressively increased the ductility by increasing holding time. However, tempering response to strengths and ductilities was different to all three morphologies of martensite.
4H-SiC Epitaxial Growth on 2° Off-Axis Substrates using Trichlorosilane (TCS)
Authors: Aigo Takashi , Ito Wataru , Tsuge Hiroshi , Yashiro Hirokatsu , Katsuno Masakazu , Fujimoto Tatsuo , Ohashi Wataru
Published: May 2012

4H-SiC epitaxial growth on 2˚ off-axis substrates using trichlorosilane (TCS) is presented. Good surface morphology was obtained for epilayers with C/Si ratios of 0.6 and 0.8 at a growth temperature of 1600°C. The triangle defect density was reduced to a level below 5 cm-2 at 1600°C and below 1 cm-2 at 1625°C for a C/Si ratio of 0.8. Photoluminescence (PL) measurements were carried out with band-pass filters of 420 nm, 460 nm, and 480 nm to detect stacking faults. A stacking fault density of below 5 cm-2 was achieved at 1600°C and 1625°C with a C/Si ratio of 0.8. The optimal conditions for TCS growth were a C/Si ratio of 0.8 and a growth temperature of 1600°C. The evaluation of stacking faults and etch pit density indicated that the use of 2˚ off-axis substrates and TCS is effective for reducing basal plane dislocations. Comparing these results to those using silane (SiH4) with HCl added, it was demonstrated that TCS is much more suitable for obtaining high-quality epilayers on 2º off-axis substrates.
A Novel 4H-SiC Fault Isolation Device with Improved Trade-Off between On-State Voltage Drop and Short Circuit SOA
Authors: Sung Woong Je , Baliga B.J. , Huang Alex Q.
Published: May 2012

This paper aims to introduce a solid-state fault isolation device (FID) for the short circuit protection application in the power distribution systems. The key performance of a FID is to have a low on-state loss and a strong short circuit safe operating area (SCSOA). As a FID, a novel 15kV 4H-SiC field controlled diode (FCD) with a p+buried layer is proposed to provide an improved trade-off between the on-state forward voltage drop and the saturation current. Dynamic response to the fault and the application example of the proposed FCD are described in this paper.
Fabrication Issues of 4H-SiC Static Induction Transistors
Authors: Stavrinidis Antonis , Konstantinidis George , Kayambaki Maria , Cayrel Frédéric , Alquier Daniel , Gao Zhuo , Zekentes Konstantinos
Published: May 2012

The process technology for the fabrication of 4H-SiC Static Induction Transistors (SITs) has been developed. Conventional contact UV lithography and self-aligned techniques have been employed. Al-outdiffusion following Rapid Thermal Annealing (RTA) has been determined as the cause for the increased reverse leakage and early forward turn-on of the gate-source junction. The fabricated transistors, exhibited a specific RON value in the region of 2mΩ•cm2 and 80 V turn-off voltage.
Charge Trapping in Sic Power MOSFETs and its Consequences for Robust Reliability Testing
Authors: Green Ronald , Lelis Aivars J. , Habersat Daniel B.
Published: May 2012

Threshold voltage (VT) instability remains an important issue for the performance, reliability, and qualification of SiC power MOSFET devices. The direct application of existing reliability test standards to SiC power MOSFETs can in some cases result in an inconsistent pass/fail response for a given device. To ensure SiC MOSFET device reliability, some modifications to existing test methods may be necessary..
Carrot Defect Control in Chloride-Based CVD through Optimized Ramp up Conditions
Authors: Yazdanfar Milan , Leone Stefano , Pedersen Henrik , Kordina Olof , Henry Anne , Janzén Erik
Published: May 2012

Epitaxial growth of 4H-SiC on 8º off-axis substrates has been performed under different condition during the temperature ramp up in order to study the effect on the carrot defect. The study was done in a hot wall chemical vapor deposition reactor using the single molecule precursor methyltrichlorosilane (MTS). During the temperature ramp up, a small flow of HCl or C2H4 was added to the H2 ambient to study different surface etching conditions. The best result was obtained when HCl was added from 1175 to 1520 °C during the ramp up to growth temperature (1575 °C).
Integration of Temperature and Current Sensors in 4H-SiC VDMOS
Authors: Berthou Maxime , Godignon Philippe , Brosselard Pierre , Tournier Dominique , Millán José
Published: May 2012

Silicon Carbide VDMOS with integrated current and temperature sensors have been successfully fabricated without degradation of the chip forward or reverse characteristics due to the sensors. The temperature sensors show impedance correlated to the temperature, which permits to track the drift region’s temperature of the device. We have shown that the sensor current ratio can be influenced by the current spreading in the drift layer, especially when the channel resistance contribution is reduced. This aspect will be more critical on VDMOS with low channel resistance. Also, the sensor current ratio stability will be improved on devices with larger active area or thinner drift layer. Integration of such sensors will permit to monitor and protect innovative power electronic systems using SiC chips.
Calibration of Mobility and Interface Trap Parameters for High Temperature TCAD Simulation of 4H-SiC VDMOSFETs
Authors: Jaikumar M.G. , Karmalkar Shreepad
Published: May 2012

4H-Silicon Carbide VDMOSFET is simulated using the Sentaurus TCAD package of Synopsys. The simulator is calibrated against measured data for a wide range of bias conditions and temperature. Material parameters of 4H-SiC are taken from literature and used in the available silicon models of the simulator. The empirical parameters are adjusted to get a good fit between the simulated curves and measured data. The simulation incorporates the bias and temperature dependence of important physical mechanisms like interface trap density, coulombic interface trap scattering, surface roughness scattering and velocity saturation.
Enhanced Current Gain (>250) in 4H-SiC Bipolar Junction Transistors by a Deep-Level-Reduction Process
Authors: Miyake Hiroki , Kimoto Tsunenobu , Suda Jun
Published: May 2012

We demonstrate 4H-SiC bipolar junction transistors (BJTs) with an enhanced current gain over 250. High current gain was achieved by utilizing optimized device geometry as well as optimized surface passivation, continuous epitaxial growth of the emitter-base junction, combined with an intentional deep-level-reduction process based on thermal oxidation to improve the lifetime in p-SiC base. We achieved a maximum current gain (β) of 257 at room temperature and 127 at 250°C for 4H-SiC BJTs fabricated on the (0001)Si-face. The gain of 257 is twice as large as the previous record gain. We also demonstrate BJTs on the (000-1)C-face that showed the highest β of 439 among the SiC BJTs ever reported.
Improved Energy Efficiency Using an IGBT/SiC-Schottky Diode Pair
Authors: Parker-Allotey Nii Adotei , Hamilton Dean P. , Alatise Olayiwola , Jennings Michael R. , Mawby Philip  Andrew , Nash Rob , Magill Rob
Published: May 2012

This paper will demonstrate how the newer Silicon Carbide material semiconductor power devices can contribute to carbon emissions reduction and the speed of adoption of electric vehicles, including hybrids, by enabling significant increases in the driving range. Two IGBT inverter leg modules of identical power rating have been manufactured and tested. One module has silicon-carbide (SiC) Schottky diodes as anti-parallel diodes and the other silicon PiN diodes. The power modules have been tested and demonstrate the superior electrothermal performance of the SiC Schottky diode over the Si PiN diode leading to a reduction in the power module switching losses.
High dI/dt Pulse Switching of 1.0 cm2 SiC GTOs
Authors: O'Brien Heather , Shaheen William , Ogunniyi Aderinto , Scozzie Charles , Zhang Q. Jon , Agarwal Anant K. , Temple Victor
Published: May 2012

The Army Research Laboratory has collaborated with Cree, Inc. and Silicon Power Corp. to develop 9 kV-blocking, 1.0 cm2 Super-GTOs. In this study, several 1.0 cm2 GTOs were individually switched up to 6.0 kA in a low-inductance, high dI/dt (2.1 kA/µs) circuit to evaluate turn-on delay and optimize the gate control. Turn-on delay was evaluated relative to gate drive current, and the delay was reduced by 1.1 µs when gate amplitude was increased from 1 A to 8 A. Increasing gate current delivered to each GTO also successfully reduced variation in turn-on delay from device to device by at least 50%, and mitigated mismatch in turn-on between pairs of GTOs switched in parallel. As silicon carbide material processing and device development continue to evolve, the ultimate solution will be to reduce remaining material defects and to control minority carrier diffusion length through more uniform doping across the wafer. These steps will enable modules of parallel GTOs to perform at maximum capability.
Evaluation of High Power Experimental SiC SGTO Devices for Pulsed Power Applications
Authors: Lacouture Shelby , Lawson Kevin , Bayne Stephen , Giesslmann Michael , O'Brien Heather , Scozzie Charles J.
Published: May 2012

The development of new semiconductor designs requires that extensive testing be completed in order to fully understand the device’s characteristics and performance capabilities. This paper describes the evaluation of experimental Silicon Carbide high power Super Gate Turn Off Thyristors (SiC SGTOs) in a unique test bed that is capable of stressing the devices with very high energy/power levels while at the same time mimicking a realistic, real world application for such devices.
4H-SiC P+N UV Photodiodes : A Comparison between Beam and Plasma Doping Processes
Authors: Biondo Stéphane , Ottaviani Laurent , Lazar Mihai , Planson Dominique , Duchaine Julian , Le Borgne V. , El Khakani M.A. , Milesi Frédéric , Vervisch Wilfried , Palais Olivier , Torregrosa Frank
Published: May 2012

This paper presents a study of 4H-SiC UV photodetectors based on p+n thin junctions. Two kinds of p+ layers have been implemented, aiming at studying the influence of the junction elaborated by the ion implantation process (and the subsequent annealing) on the device characteristics. Aluminum and Boron dopants have been introduced by beam line and by plasma ion implantation, respectively. Dark currents are lower with Al-implanted diodes (2 pA/cm2 @ - 5 V). Accordingly to simulation results concerning the influence of the junction thickness and doping, plasma B-implanted diodes give rise to the best sensitivity values (1.5x10-1 A/W @ 330 nm).
Electrical and Optical Properties of High-Purity Epilayers Grown by the Low-Temperature Chloro-Carbon Growth Method
Authors: Kotamraju Siva Prasad , Krishnan Bharat , Beyer Franziska Christine , Henry Anne , Kordina Olle , Janzén Erik , Koshka Yaroslav
Published: May 2012

A reduced growth pressure (down to 10 Torr) was employed for the low-temperature chloro-carbon epitaxial growth. More than two times lower H2 flow rate became possible. The optimal input H2/Si and C/Si ratios were also lower. A significant reduction of the net free donor concentration resulted from the use of the low pressure, delivering partially compensated epilayers with the net free donor concentration below 7x1013 cm-3. Deep levels were characterized in the low-temperature epilayers for the first time. No Z1/2 or EH6/7 centers could be detected by DLTS. No strong D1 photoluminescence signature was observed. The high purity of the obtained epitaxial layers made it possible to use the low-temperature chloro-carbon epitaxial growth to fabricate drift regions of Schottky diodes for the first time. Promising values of the reverse breakdown voltage and the leakage current were obtained from the fabricated devices.
Improvement of Homoepitaxial Layer Quality Grown on 4H-SiС Si-Face Substrate Lower than 1 Degree Off Angle
Authors: Kojima Kazutoshi , Ito Satchiko , Nagata Akiyo , Okumura Hajime
Published: May 2012

In this study, we struck a balance between specular surface morphology and polytype homogeneity on an epitaxial layer grown on 4H-SiC Si-face substrate with off angle less than 1degree by controlling the C/Si ratio with the SiH4 flow rate. Schottky barrier diodes fabricated on a grown epitaxial layer exhibited a blocking of voltage over 1000 V and an n value of less than 1.1 with a high yield of more than 80%. A substrate with a low off angle was found to have an advantage as regareds the stress that generates the interfacial dislocations at the epitaxial layer/substrate interface during the epitaxial growth process.
Buried Selective Growth of p-Doped SiC by VLS Epitaxy
Authors: Carole Davy , Berckmans Stéphane , Vo-Ha Arthur , Lazar Mihai , Tournier Dominique , Brosselard Pierre , Soulière Véronique , Auvray Laurent , Ferro Gabriel , Brylinski Christian
Published: May 2012

Selective epitaxial growth in buried patterns was studied using the vapour-liquid-solid mechanism in Al-Si melt in order to obtain p+-doped SiC localized layers on 4H-SiC substrate. Homogeneous deposition with step bunched morphology was obtained by adding propane at room temperature before growth at 1100°C. Patterns as large as 800 µm and as narrow as 10 µm were completely filled in this way. The deposition kinetics demonstrates that the process is self limited and mainly depends on the initial amount of Si in the liquid. The deposit is highly p-type doped and the p-n junction is demonstrated.