M. A. Imam

Carnegie Mellon University, Pittsburgh, Pennsylvania, United States

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Publications (62)49.56 Total impact

  • Scripta Materialia 08/2013; 69(4):334-337. · 2.82 Impact Factor
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    ABSTRACT: Samples of rice husks were transformed to β (3C)-SiC by microwave processing in controlled conditions of temperature and vacuum. This simple and fast way of producing powdered samples of silicon carbide is technologically important if this material is to be used for electronics, sensors, biotechnology, and other applications. Using x-ray diffraction it was found that the microwave processed sample at 1900 °C consists of β (3C)-SiC phase. Raman scattering measurements confirmed the formation of β (3C)-SiC phase. Transmission electron microscopy revealed the presence of stacking faults along the [111] direction. The presence of 6H/4H stacking faults in 3C phase is explained in terms of their total energies. The presence of these stacking faults with a ∼1 eV band offset between the host 3C and hexagonal stacking faults implies that these stacking faults provide a conduction barrier, and the interfaces between the stacking faults and host lattice act as a heterojunction that may provide potential utility for various optoelectronic applications.
    Journal of Applied Physics 04/2012; 111(7). · 2.21 Impact Factor
  • Advanced Processing and Manufacturing Technologies for Structural and Multifunctional Materials III, 01/2010: pages 1 - 10; , ISBN: 9780470584392
  • Materials Science Forum - MATER SCI FORUM. 01/2010;
  • Materials Science Forum - MATER SCI FORUM. 01/2010;
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    ABSTRACT: The emerging reduction technologies for titanium from ore produce powder instead of sponge. The processing of titanium powder by conventional methods is energy intensive. Titanium acts as a getter for oxygen and tends to react with many materials at the high melt temperature (melting point: 1675°C). Most melts conventionally require high vacuum, 10-6 Torr or better. Plasma arcs have the additional problem of electrode consumption, and direct induction heating of the titanium powder is problematic. Microwave melting in an inert gas environment is potentially energy efficient due to the possibility of direct microwave heating of the titanium powder augmented by hybrid heating. A robust S-Band microwave system has been developed for melting titanium powder compacts up to few hundred grams in mass. The titanium powder compacts are heated to temperatures up to 1800°C in an over-moded cylindrical cavity with a flowing argon gas atmosphere. Microwaves from a 6 kW Cober S6F industrial microwave generator are injected along the cavity axis and a 3-stub tuner is used to optimize coupling efficiency. The compact is contained in a ceramic crucible surrounded by an insulating casket. Plasma formation during processing has been observed, produced by either breakdown of the argon atmosphere (blue light emission) or by titanium vapor reacting with residual oxygen (yellow light emission). The experimental setup and the results of melting and sintering experiments will be described.
    IEEE International Conference on Plasma Science 01/2010;
  • Key Engineering Materials - KEY ENG MAT. 01/2010; 436:131-140.
  • Processing and Properties of Advanced Ceramics and Composites, 05/2009: pages 1 - 14; , ISBN: 9780470522189
  • Chandra S. Pande, M. Ashraf Imam
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    ABSTRACT: Formation of annealing twins and its relation to grain growth is investigated. Our previous studies have established a relation between twin density, grain size, and temperature. A model of the mechanism of their formation based on the emergence of Shockley partial loops on consecutive {111} planes during grain migration has also been developed previously. This model can satisfactorily explain some experimental and theoretical results found over the years. In this paper we provide additional experimental evidence in support of our model by investigating twin formation in boron-doped nickel. We find that boron addition slows down grain growth, though the kinetics of grain growth remains parabolic. There is a drastic reduction in twin density due to boron addition especially around a boron content of ∼200ppm.
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing - MATER SCI ENG A-STRUCT MATER. 01/2009; 512(1):82-86.
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    ABSTRACT: Millimeter-wave sintering of ceramic laser host materials has been under investigation at the Naval Research Laboratory (NRL) for high-energy laser (HEL) applications. Potential advantages of polycrystalline, compared to single-crystal, laser host materials include lower processing temperature, higher gain from higher dopant concentration, cheaper fabrication, and larger devices. Successful production of laser quality polycrystalline laser host requires a high purity material and the sintering process must achieve nearly perfect micro structure as imperfections quickly result in unacceptable light absorption and scattering. Transparent, laser quality, polycrystalline Nd:YAG has recently been achieved in conventional vacuum furnaces using a reactive sintering process based on commercially available powders, and we have been trying to replicate these results in the NRL millimeter-wave materials processing facility. Compacts prepared from mixtures of the oxide precursors are placed in an open or closed crucible in a vacuum environment and are heated directly by the 83 GHz beam to temperatures of 1000- 1800degC. Over 99% theoretical density has been achieved at the highest temperatures with moderate grain growth. Fluorescence lifetime studies of the Nd+3 ion are in good agreement with published results for laser quality material. Our X-ray diffraction studies of the phases present at various temperatures are generally in agreement with previous work. However, full transparency has not been achieved and investigation of the sintered compact microstructure has revealed trapped pores and other imperfections. There has also been unexpected variability in the results. New modeling results for the open casket configuration, which was chosen for temperature measurement and heating efficiency considerations, indicate that large thermal gradients are likely. These gradients can produce non-uniform sintering and complicate temperature measurements. New closed casket configuration- s are being developed that are expected to result in smaller thermal gradients and more accurate temperature measurement while retaining good heating efficiency. The results of model calculations and available experimental results will be discussed.
    IEEE International Conference on Plasma Science 01/2009;
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    ABSTRACT: Millimeter-wave processing (sintering) of ceramic laser host materials has been under investigation at the Naval Research Laboratory (NRL) for high energy laser (HEL) applications. Advantages of polycrystalline, compared to single-crystal laser host materials, include lower processing temperature, higher gain from higher dopant concentrations, cheaper fabrication, and larger devices. Millimeter-wave processing has been shown to be an effective alternative to conventional vacuum furnaces for pressure-free sintering of low-loss oxide ceramic materials.
    Vacuum Electronics Conference, 2008. IVEC 2008. IEEE International; 05/2008
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    ABSTRACT: The cavity model and the dislocation mechanics were used to analyze the plastic energy dissipated in an indentation deformation. The plastic energy dissipated in an indentation cycle was proportional to the cube of the residual indentation depth. The experimental results supported the analysis for the indentation of commercially pure titanium by a Vickers indenter. Slip bands around the indentation were observed, suggesting that the indentation deformation was controlled by dislocation motion. The indentation hardness decreased with the indentation load, showing the indentation size effect. The ratio of the total energy to the plastic energy was found to be proportional to the ratio of the maximum indentation depth to the residual indentation depth. The effects of holding time were examined on the time-dependent plastic deformation of the commercially pure titanium at ambient temperature.
    Journal of Materials Research. 03/2008; 23(04):1068 - 1075.
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    ABSTRACT: Commercially pure titanium rods were processed by equal channel angular pressing (ECAP) to obtain an ultrafine-grained microstructure. The titanium rods were extruded for 2 passes at 623 K, using three different bending angles of 90°, 120° and 150°. For the bending angle of 90°, the titanium rods were extruded for 4 and 8 passes in addition to 2 passes. The microstructure of the ECAP-processed titanium was evaluated using the TEM technique. The bending angle had a strong effect on the grain refinement, while the number of pressing passes did not have a noteworthy effect on the evolution of the microstructure. Microindentations were conducted with the indentation load in the range 100-2000 mN. The indentation hardness decreased slightly with the increase in the indentation load. The titanium processed by the die with a bending angle of 90° had the highest indentation hardness. The energy ratio of the plastic energy to the total energy was found to be proportional to the ratio of the residual indentation depth to the maximum indentation depth.
    Journal of Physics D Applied Physics 01/2008; 41(10). · 2.53 Impact Factor
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    ABSTRACT: We report recent results of an investigation of millimeter-wave processing of yttria (Y 2 O 3 ) for fabrication of transparent, high strength polycrystalline ceramic laser hosts for High Energy Laser (HEL) applications.<sup>1,2</sup> The objective is to produce polycrystalline materials with optical quality comparable to that of a single crystal. It is difficult to produce yttria single crystals because of the phase transformation around 2000°C and the high melting temperature which is over 2400°C. While single crystals have high thermal conductivity and can operate at high powers, they are costly and limited in size and dopant concentration. Significant advantages of polycrystalline materials compared to single crystals, are lower processing temperature, higher gain as a result of higher dopant concentrations, faster and less expensive fabrication, and the possibility of larger devices. Millimeter-wave processing has been proposed as an alternative method to solve the problems of both conventional vacuum sintering and low frequency microwave sintering, such as low heating rates, poor coupling, and unfavorable thermal gradients. A major component of the NRL millimeter-wave processing facility is a 20-kW, continuous-wave (CW), 83-GHz gyrotron oscillator (GYCOM, Ltd.). Translucent yttria has been successfully sintered with millimeter-wave beams with up to 99% theoretical density. A partially transparent yttria ceramic sample has also been achieved using the millimeter-wave sintering process. Several factors impact the quality of the sintered material including the presence of agglomerates, impurities, processing atmosphere, sintering aids, and thermal gradients. Efforts to improve the transparency are in progress.
    Pulsed Power Conference, 2007 16th IEEE International; 07/2007
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    ABSTRACT: A prototype system is described for the large scale, continuous production of nanophase metals, metal oxides, and other nanophase materials using the polyol process. The polyol process employs an organic solvent such as ethylene glycol to reduce a metal oxide/metal salt at high temperature to the metal oxide or metal. The system employs a 6 kW, 2.45 GHz microwave source to rapidly heat the continuously flowing solution to a desired process temperature as it flows through a silica tube placed along the center line of a section of waveguide.
    Review of Scientific Instruments 03/2007; 78(2):023901. · 1.60 Impact Factor
  • Materials Science Forum - MATER SCI FORUM. 01/2007;
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    ABSTRACT: form only given. We report recent results of an investigation of millimeter-wave processing of yttria (Y2O3) for fabrication of transparent, high strength polycrystalline ceramic laser hosts for high energy laser (HEL) applications. The objective is to produce polycrystalline materials with optical quality comparable to that of a single crystal. It is difficult to produce yttria single crystals because of the phase transformation around 2000degC and the high melting temperature which is over 2400degC. While single crystals have high thermal conductivity and can operate at high powers, they are costly and limited in size and dopant concentration. Significant advantages of polycrystalline materials compared to single-crystals, are lower processing temperature, higher gain as a result of higher dopant concentrations, faster and less expensive fabrication, and the possibility of larger devices. Millimeter-wave processing has been proposed as an alternative method to solve the problems of both conventional vacuum sintering and low frequency microwave sintering, such as low heating rates, poor coupling, and unfavorable thermal gradients. A major component of the NRL millimeter-wave processing facility is a 20-kW, continuous-wave (CW), 83-GHz gyrotron oscillator (GYCOM, Ltd.). Translucent yttria has been successfully sintered with millimeter-wave beams with up to 99% theoretical density. A partially transparent yttria ceramic sample has also been achieved using the millimeter-wave sintering process. Several factors impact the quality of the sintered material including the presence of agglomerates, impurities, processing atmosphere, sintering aids, and thermal gradients. Efforts to improve the transparency are in progress.
    Plasma Science, 2007. ICOPS 2007. IEEE 34th International Conference on; 01/2007
  • Materials Science Forum - MATER SCI FORUM. 01/2007;
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    C S Pande, M A Imam
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    ABSTRACT: Nucleation and growth kinetics, and age hardening behavior in high strength, low carbon ferrous alloy containing 1.2 wt% copper and a model alloy, Fe–1.2%Cu, have been studied with and without the presence of carbides. It is well known that after austenization and quenching, Cu is retained in solid solution. During subsequent aging, Cu initially appears as a coherent nanometer size spherical precipitates. These precipitates, difficult to detect by conventional transmission electron microscopy (TEM), have been evaluated by small angle neutron scattering (SANS) measurements. Longer aging transforms the Cu precipitates into incoherent particles which are easily detected by conventional TEM. A combination of techniques such as TEM, SANS, and hardness measurements have been used to study nucleation and growth kinetics of copper in both alloy systems. Direct mea-surement from TEM micrographs and integral transform of the SANS data were used to calculate the size distribution for a variety of aging conditions. Published by Elsevier B.V.
    Materials Science and Engineering A. 01/2007; 457:69-76.
  • Nick E. Tran, S.G. Lambrakos, M. Ashraf Imam
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    ABSTRACT: The multi-component Mg–x wt.% Mm alloys are synthesized using the mechanical ball-milling technique and their hydrogen storage capacities, absorption/desorption kinetics, and thermodynamic parameters are quantified. The analysis of kinetic properties presented here is based on the method of exponential peeling. It is seen that the presence of misch metal (Mm) in the alloy samples dramatically decreases their rate of decrepitation and increases their cyclic stability. However, the increased concentration of misch metal in the samples has an adverse effect on their hydrogen storage capacity and their reaction rate. The hydrogen storage properties also vary with reaction temperature. The best hydrogen absorption kinetics are observed at temperatures around 300 °C and the desorption kinetics are quite fast at temperatures of 400 °C and above. The hydrogen desorption activation energy of Mg–x wt.% Mm hydride is much lower than that of MgH2. The pressure–composition–isotherm (P–C–T) plots of the samples at 300–420 °C indicate that the alloys possess good cyclic stability but very poor reversibility, making the study of their thermodynamic properties difficult. The P–C–T plots also indicate that the increase of the concentration of the misch metal's rare earths leads to an increase of the hydrogen equilibrium pressure and decrease of hydrogen storage capacity.
    Journal of Alloys and Compounds. 01/2006; 407(s 1–2):240–248.