@article{sloop_bumgardner_washington_loehle_sankar_lewis_2006, title={Keto-enol and enol-enol tautomerism in trifluoromethyl-beta-diketones}, volume={127}, ISSN={["0022-1139"]}, DOI={10.1016/j.jfluchem.2006.02.012}, abstractNote={The keto–enol (K⇌E) and enol–enol (E⇌E) equilibria of a variety of trifluoromethyl-β-diketones were investigated using 1H, 13C, 19F NMR spectroscopy, infrared spectroscopy and ultraviolet-visible spectrophotometry in nonpolar solvents. In general, NMR, IR and UV spectral evidence indicates that trifluoromethyl-β-diketones exist as mixtures of two chelated cis-enol forms in nonpolar media. Infrared spectroscopy and ultraviolet spectrophotometry show the E⇌E equilibrium lies in the direction of the enol form which maximizes conjugation in most cases. Exceptions are noted and discussed.}, number={6}, journal={JOURNAL OF FLUORINE CHEMISTRY}, author={Sloop, JC and Bumgardner, CL and Washington, G and Loehle, WD and Sankar, SS and Lewis, AB}, year={2006}, month={Jun}, pages={780–786} }
 @article{gilmore_chattopadhyay_kvit_sharma_lee_collis_sankar_narayan_2003, title={Growth, characterization,, and electrical properties of PbZr0.52Ti0.48O3 thin films on buffered silicon substrates using pulsed laser deposition}, volume={18}, ISSN={["0884-2914"]}, DOI={10.1557/JMR.2003.0016}, abstractNote={Epitaxial thin films of PbZr0.52Ti0.48O3 (PZT) were synthesized successfully on SrRuO3/SrTiO3/MgO/TiN/Si heterostructures by pulsed laser deposition. The films were single phase and had (001) orientation. The deposition parameters were varied to obtain the best epitaxial layer for each of the compounds. Transmission electron microscopy indicated good epitaxy for the entire heterostructure and sharp interfaces between the epilayers. Dielectric and P–E hysteresis loop measurements were carried out with evaporated Ag electrodes. The dielectric constant for the films was found to be between 400–450. The value of saturation polarization Ps was between 55–60 μC/cm2, and the coercive field Ec varied from 60–70 kV/cm. Integration of PZT films with silicon will be useful for future memory and micromechanical devices.}, number={1}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Gilmore, WM and Chattopadhyay, S and Kvit, A and Sharma, AK and Lee, CB and Collis, WJ and Sankar, J and Narayan, J}, year={2003}, month={Jan}, pages={111–114} }
 @article{kumar_sankar_narayan_singh_majumdar_2002, title={Low-temperature resistivity minima in colossal magnetoresistive La0.7Ca0.3MnO3 thin films}, volume={65}, number={9}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Kumar, D. and Sankar, J. and Narayan, J. and Singh, R. K. and Majumdar, A. K.}, year={2002}, pages={094407–1} }
 @article{wei_sankar_sharma_yamagata_narayan_2001, title={Effect of chamber pressure and atmosphere on the microstructure and nanomechanical properties of amorphous carbon films prepared by pulsed laser deposition}, volume={19}, ISSN={["0734-2101"]}, DOI={10.1116/1.1322641}, abstractNote={We have investigated the effect of chamber pressure and atmosphere on the microstructure and nanomechanical properties of amorphous carbon thin films prepared by pulsed laser deposition. The amorphous carbon films were deposited in various atmospheres such as nitrogen and argon at different chamber pressures. We used Raman spectroscopy to study the bonding characteristics of the deposited amorphous carbon films. Atomic force microscopy and optical microscopy were utilized to observe the surface conditions and the microstructures of the deposited films. Nanoindentation measurements were carried out on various samples prepared under different conditions to study the effect of chamber pressure and atmosphere on the elastic modulus and nanohardness of the films. It was found that reduced vacuum leads to formation of amorphous carbon films with reduced elastic modulus and nanohardness. Amorphous carbon films prepared under higher chamber pressures exhibit an increased density of particulates and significantly roughened surface. The results were understood in combination with the optical emission and electrostatic measurements of the laser plasma plume. It was found that the presence of atmosphere decreases the leading edge ionic energies of the species in the laser plasma plume and increases the thermalization of the laser plasma due to an increased possibility of collision.}, number={1}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A-VACUUM SURFACES AND FILMS}, author={Wei, Q and Sankar, J and Sharma, AK and Yamagata, Y and Narayan, J}, year={2001}, pages={311–316} }
 @article{kumar_narayan_kvit_sharma_sankar_2001, title={High coercivity and superparamagnetic behavior of nanocrystalline iron particles in alumina matrix}, volume={232}, ISSN={["1873-4766"]}, DOI={10.1016/S0304-8853(01)00191-3}, abstractNote={Single-domain nanoscale magnetic iron particles have been embedded uniformly in an amorphous matrix of alumina using a pulsed laser deposition technique. Structural characterization by transmission electron microscopy (TEM) reveals the presence of a crystalline iron and an amorphous alumina phase. Fine particle magnetism have been investigated by carrying out field and temperature dependence of magnetization measurements using superconducting quantum interference device magnetometer. The particle size of Fe in Al2O3 matrices prepared by changing the deposition time of Fe, have been found to be 9, 7 and 5 nm from TEM studies. At 10 K, the coercivities of these samples are found be 450, 350 and 150 Oe, respectively. At 300 K, the coercivity of Fe–Al2O3 sample decreases from 100 to 50 Oe as the particle size decreases from 9 to 7 nm and finally the sample turns superparamagnetic when the Fe particle size becomes around 5 nm. Based on the calculated value of blocking temperature, TB, (481 K), magnetic anisotropy K (4.8×105 erg/cm3) for Fe, and the Boltzmann constant kB (1.38×10−16 erg/K) from TB=KV/25kB, the mean radius of Fe particles is found to be 9.3 nm. in one of the samples. This is in good agreement with the particle size measured using TEM studies.}, number={3}, journal={JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS}, author={Kumar, D and Narayan, J and Kvit, AV and Sharma, AK and Sankar, J}, year={2001}, month={Jul}, pages={161–167} }
 @article{katiyar_kumar_nath_kvit_narayan_chattopadhyay_gilmore_coleman_lee_sankar_et al._2001, title={Magnetic properties of self-assembled nanoscale La2/3Ca1/3MnO3 particles in an alumina matrix}, volume={79}, ISSN={["0003-6951"]}, DOI={10.1063/1.1399001}, abstractNote={We have investigated the processing and properties of La2/3Ca1/3MnO3 self-assembled nanodots formed in a nonmagnetic alumina matrix, which were produced by a pulsed-laser deposition process. The size of the nanodots was found to be in the range of 10–15 nm using high-resolution transmission electron microcopy. The average interlayer separation between two dots has been found to be 2–5 nm, which is sufficient to decouple the magnetic grains. The decoupling of the grains is supported by the zero-field-cooled and field-cooled magnetization (M) data. The coercivity of the La2/3Ca1/3MnO3 nanodots has been measured using magnetization measurements as a function of field (H) at different temperatures above and below the blocking temperature of the samples. The coercivity is found to vary from 600 Oe at 10 K to 400 and 200 Oe at 20 and 50 K, respectively. Above the blocking temperature, the sample is found to transform to a superparamagnetic magnetic state, resulting in the disappearance of any hysteresis in the M–H loops.}, number={9}, journal={APPLIED PHYSICS LETTERS}, author={Katiyar, P and Kumar, D and Nath, TK and Kvit, AV and Narayan, J and Chattopadhyay, S and Gilmore, WM and Coleman, S and Lee, CB and Sankar, J and et al.}, year={2001}, month={Aug}, pages={1327–1329} }
 @article{wei_sankar_narayan_2001, title={Microstructural changes due to heat-treatment of annealing and their effect on the creep behavior of self-reinforced silicon nitride ceramics}, volume={299}, ISSN={["0921-5093"]}, DOI={10.1016/s0921-5093(00)01403-9}, abstractNote={In order to understand the improvement of creep resistance by furnace and microwave annealing, we have investigated the effect of heat-treatment on the microstructural characteristics of the crept self-reinforced silicon nitride (Si3N4) ceramic GS44. X-ray diffraction was performed on the as-sintered and heat-treated samples to study the phase changes due to annealing treatment. Optical microscopy and scanning electron microscopy (SEM) were used to study the fracture surface and to identify the creep mechanism. High resolution and analytical transmission electron microscopy (TEM) were employed to analyze the microstructures of the crept samples with as-received and heat-treated conditions. It has been reported that both conventional furnace and microwave annealing enhance the creep resistance of the material, and microwave annealing had the most significant effect. Fractography showed that the microwave annealed samples exhibits least creep damage. Furnace annealing also reduces the creep damage, but the effect is far less as compared to microwave annealing. Scanning electron microscopy (SEM) analysis showed significant amount of multiple-junction cavitation in the creep-tested samples. TEM observations showed significant devitrification of the amorphous phases in the microwave annealed specimens, as verified by micro-diffraction studies of the junction phases. This is also confirmed by X-ray diffraction and high-resolution lattice image of the triple junction phases. The microstructural observations were combined with a recent model of the effect of amorphous residues in ceramics on the creep behavior to explain the improvement in the creep resistance due to annealing.}, number={1-2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Wei, Q and Sankar, J and Narayan, J}, year={2001}, month={Feb}, pages={141–151} }
 @article{kumar_chattopadhyay_gilmore_lee_sankar_kvit_sharma_narayan_pietambaram_singh_et al._2001, title={Structural and magnetoresistance properties of La2/3Ca1/3MnO3 thin films on buffered silicon substrates}, volume={78}, ISSN={["0003-6951"]}, DOI={10.1063/1.1350603}, abstractNote={We report an epitaxial growth of LCMO (La2/3Ca1/3MnO3) film on Si by using a highly conducting diffusion barrier layer of TiN. In order to achieve epitaxial growth of LCMO films, MgO, and SrTiO3 films were used as intermediate layers between LCMO and TiN layers. The results have indicated that the properties of LCMO films on Si substrates, deposited under an optimized condition, are on par with the properties of LCMO films on conventional oxide substrates such as LaAlO3 and SrTiO3 in terms of paramagnetic to ferromagnetic transition temperature, insulator to metal transition temperature, and magnetoresistance ratio.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Kumar, D. and Chattopadhyay, S. and Gilmore, W. M. and Lee, C. B. and Sankar, J. and Kvit, A. and Sharma, A. K. and Narayan, Jagdish and Pietambaram, S. V. and Singh, R. K. and et al.}, year={2001}, month={Feb}, pages={1098–1100} }
 @article{wei_sankar_sharma_oktyabrsky_narayan_narayan_2000, title={Atomic structure, electrical properties, and infrared range optical properties of diamondlike carbon films containing foreign atoms prepared by pulsed laser deposition}, volume={15}, ISSN={["0884-2914"]}, DOI={10.1557/jmr.2000.0094}, abstractNote={We investigated the atomic structure, electrical, and infrared range optical properties of diamondlike carbon (DLC) films containing alloy atoms (Cu, Ti, or Si) prepared by pulsed laser deposition. Radial distribution function (RDF) analysis of these films showed that they are largely sp3 bonded. Both pure DLC and DLC + Cu films form a Schottky barrier with the measuring probe, whereas DLC + Ti films behave like a linear resistor. Pure DLC films and those containing Cu exhibit p-type conduction, and those containing Ti and Si have n-type conduction. Photon-induced conduction is observed for pure DLC, and the mechanism is discussed in terms of low-density gap states of highly tetrahedral DLC. Our results are consistent with relative absence of gap states in pure DLC, in accordance with theoretical prediction by Drabold et al.37 Temperature dependence of conductivity of DLC + Cu shows a behavior σ ∞ exp(−B/T1/2), instead of the T−1/4 law (Mott–Davis law). Contributions from band-to-band transitions, free carriers, and phonons to the emissivity spectrum are clearly identified in pure DLC films. The amorphous state introduces a large contribution from localized states. Incorporation of a small amount of Si in the DLC does not change the general feature of emissivity spectrum but enhances the contribution from the localized states. Cu and Ti both enhance the free carrier and the localized state contributions and make the films a black body.}, number={3}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Wei, Q and Sankar, J and Sharma, AK and Oktyabrsky, S and Narayan, J and Narayan, RJ}, year={2000}, month={Mar}, pages={633–641} }
 @article{godbole_narayan_xu_narayan_sankar_1999, title={Diamond films and composites on cobalt-chromium alloys}, volume={58}, ISSN={["0921-5107"]}, DOI={10.1016/s0921-5107(98)00430-9}, abstractNote={Abstract Silicon, aluminum nitride (AlN) and titanium carbide (TiC) layers were deposited on cobalt–chromium alloys to investigate their utility as buffer layers for the synthesis of diamond films and composites. Silicon and aluminum nitride were found to react with the substrate at diamond deposition temperatures, via out-diffusion of cobalt from the substrate. The layers of titanium carbide, on the other hand, were found to be useful as a diffusion barrier for outward diffusion of cobalt and inward diffusion of carbon during diamond deposition, thus making enhanced nucleation and growth of high quality diamond possible. The continuous diamond coatings thus formed, however, tend to bulge and then crack due to thermal expansion mismatches between the TiC coated substrate and the overlayer diamond film. Composite coatings of AlN–diamond and TiC–diamond have, therefore, been synthesized by sequential deposition of component materials. These composite coatings consist of randomly interconnected micron sized diamond crystallites which are primarily surrounded by buffer material (AlN or TiC) while the contact area between neighbouring diamond crystallites is minimum. The presence of such a discontinuous morphology of diamond crystallites renders more toughness and also leads to compensation of stresses in localized regions (local stress management) and thus reduces bulging and subsequent delamination effects.}, number={3}, journal={MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY}, author={Godbole, VP and Narayan, R and Xu, Z and Narayan, J and Sankar, J}, year={1999}, month={Mar}, pages={251–257} }
 @article{wei_sharma_sankar_narayan_1999, title={Mechanical properties of diamond-like carbon composite thin films prepared by pulsed laser deposition}, volume={30}, ISSN={["1879-1069"]}, DOI={10.1016/S1359-8368(99)00035-9}, abstractNote={We have investigated the mechanical properties of diamond-like carbon (DLC) thin films that contain foreign atoms. The DLC films were prepared by pulsed laser deposition. A novel target design was adopted to incorporate foreign atoms into the DLC films during film deposition. Copper, titanium and silicon are chosen as the dopants. The chemical composition of the doped films was determined using Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy and calibrated extrapolation. Experimental results of both visible and UV Raman are presented and discussed in terms of peak shape and position. The effect of dopants on the Raman spectrum is also analyzed. Optical microscopy of the pure DLC of a certain thickness showed severe buckling. A brief review of the theoretical background of adhesion is given and the possible mechanisms of adhesion that may work in DLC coatings are discussed. Qualitative scratch tests on the specimens show that pure DLC has quite poor adhesion due to the large compressive stress, while the doped DLC films exhibit much improved adhesion. Wear tests show improved wear resistance in the doped DLC coatings. Nanoindentation results give an average hardness above 40 GPa and effective Young's modulus above 200 GPa for pure DLC. The copper doped DLC films showed slightly decreased hardness and Young's modulus as compared to pure DLC films. Ti and Si can reduce the hardness and Young's modulus more than Cu. All these can be understood by analyzing the internal stress reduction as derived from Raman G-peak shift to lower wavenumbers. A preliminary model of the stress reduction mechanism is discussed.}, number={7}, journal={COMPOSITES PART B-ENGINEERING}, author={Wei, Q and Sharma, AK and Sankar, J and Narayan, J}, year={1999}, pages={675–684} }
 @article{wei_sankar_kelkar_narayan_1999, title={Microstructure evolution accompanying high temperature; uniaxial tensile creep of self-reinforced silicon nitride ceramics}, volume={272}, ISSN={["0921-5093"]}, DOI={10.1016/s0921-5093(99)00497-9}, abstractNote={Extensive transmission electron microscopy (TEM) has been performed to study the microstructure evolution of a self-reinforced silicon nitride associated with high temperature creep. A large population of strain whorls is observed in samples crept at relatively high temperatures and the strain whorls are not necessarily asymmetrical with respect to the grain boundary normal. Large angle convergent beam electron diffraction (LACBED) at the grain boundaries where strain whorl contrast is visible reveals severely curved Bragg lines, implying large residual strains. This indicates that grain boundary interlocking might be effective to enhance the creep resistance at high temperatures. Dislocation pile-ups, arrays and tangles are present in certain silicon nitride grains. However, a simple analysis rules out dislocations as the major creep mechanism. Most dislocations started from grain boundaries. The role of dislocations is to relieve the stress concentrations at the strain whorls. This adds to the diffusion mechanism of stress relaxation at the strain whorls and facilitates other creep mechanisms such as grain boundary sliding. A large density of multiple-junction cavities is observed in the samples crept at relatively high temperatures. It is proposed that grain boundary sliding and cavity formation, in addition to stress relaxation through nucleation of dislocations at the strain whorls act together to produce a much shorter life to failure at high temperatures. While at lower temperatures, the creep is more diffusion controlled which gives a stress exponent of unity.}, number={2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Wei, Q and Sankar, J and Kelkar, AD and Narayan, J}, year={1999}, month={Nov}, pages={380–388} }
 @article{wei_narayan_sharma_sankar_narayan_1999, title={Preparation and mechanical properties of composite diamond-like carbon thin films}, volume={17}, ISSN={["0734-2101"]}, DOI={10.1116/1.582074}, abstractNote={We have investigated mechanical properties of diamond-like carbon (DLC) thin films, particularly the internal compressive stress and ways to alleviate it. Foreign atoms such as copper, titanium, and silicon were incorporated into the DLC films during pulsed laser deposition. The chemical composition of the doped films was determined using Rutherford backscattering spectrometry (RBS) and x-ray photoelectron spectroscopy (XPS). Optical microscopy of the doped films showed that DLC films containing Cu exhibit much less particulate density as compared to the films containing Ti and Si. Visible Raman spectroscopy was used to characterize the films. The effect of dopants on the Raman spectrum was analyzed in terms of peak shape and position. Optical microscopy of the pure DLC of a certain thickness showed severe buckling. The mechanisms of adhesion associated with DLC coatings were discussed. Qualitative scratch tests on the specimens showed that pure DLC films have relatively poor adhesion due to a large compressive stress, while the doped DLC films exhibit much improved adhesion. Wear tests show improved wear resistance in the doped DLC coatings. Nanoindentation results suggest that pure DLC has an average hardness above 40 GPa and effective Young’s modulus above 200 GPa. The doped DLC films showed slightly decreased hardness and Young’s modulus as compared to pure DLC films. These results can be rationalized by analyzing the internal stress reduction as derived from Raman G-peak shift to lower wavenumbers. A preliminary interpretation of the stress reduction mechanism is discussed.}, number={6}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A-VACUUM SURFACES AND FILMS}, author={Wei, Q and Narayan, RJ and Sharma, AK and Sankar, J and Narayan, J}, year={1999}, pages={3406–3414} }
 @article{wei_narayan_narayan_sankar_sharma_1998, title={Improvement of wear resistance of pulsed laser deposited diamond-like carbon films through incorporation of metals}, volume={53}, ISSN={["0921-5107"]}, DOI={10.1016/s0921-5107(98)00150-0}, abstractNote={We have investigated the characteristics of diamond-like carbon (DLC), DLC doped with Cu, and DLC doped with Ti deposited by a sequential pulsed laser ablation of two targets. The composition of these films was determined by Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy and transmission electron microscopy studies showed typical features of DLC with a high fraction of sp3 bonded carbon in the doped films as well as in the undoped films. Wear resistance measurements made on the samples by means of the `crater grinding method' showed that DLC+2.75% Ti has the highest wear resistance, while that of pure DLC has the lowest amongst the samples. Careful analysis of the Raman data indicates a significant shift to shorter wavelength with the addition of metal, which means that the compressive stress in the DLC films has been reduced. We envisaged that the reduction in the compressive stress promotes the wear resistance of the coatings. The XPS studies showed evidence for the formation of Ti–C bonding in the Ti doped sample. Thus metal-doped DLC coatings are expected to improve the tribological properties and enhance the performance of components coated with metal-doped DLC.}, number={3}, journal={MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY}, author={Wei, Q and Narayan, RJ and Narayan, J and Sankar, J and Sharma, AK}, year={1998}, month={May}, pages={262–266} }
 @inproceedings{wei_sharma_narayan_ravindra_oktyabrsky_sankar_muth_kolbas_narayan_1998, title={Microstructure and IR range optical properties of pure DLC and DLC containing dopants prepared by pulsed laser deposition}, booktitle={Advances in laser ablation of materials: Symposium held April 13-16, 1998, San Francisco, California, U.S.A.  (Materials Research Society symposia proceedings ; v. 526).}, publisher={Warrendale, Pa.: Materials Research Society}, author={Wei, Q. and Sharma, A. K. and Narayan, R. J. and Ravindra, N. M. and Oktyabrsky, S. and Sankar, J. and Muth, J. F. and Kolbas, R. M. and Narayan, J.}, year={1998}, pages={331} }