237 results on '"Tyagi, A. K."'
Search Results
2. A review on evolution of airfoils and their characteristics in last three centuries. Part-1: Evolution of flights and shapes of wing sections before 1930 and NACA series.
- Author
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Gupta, S. B., Tyagi, Rajesh K., Pratiksha, Gairola, Ayush, Kumar, Vipin, Kumar, Sachin, Mishra, K.P., Sharma, Pramod Kumar, Seth, Deepti, Shukla, Sweta, and Bishnoi, Bhagwanti
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TECHNOLOGICAL innovations , *AEROFOILS , *LITERARY theory , *AERONAUTICS - Abstract
Airfoil is an integral part of an aircraft. For engineers and scientists in the field of Aeronautics and Aerospace, complete knowledge of characteristics of Airfoil is of utmost importance. This paper collects the information related to the development of flights and airfoil before 1930. This paper is an attempt to provide history of evolutions of flight by the efforts of the engineers without any clue of theory and literature. This paper exhibits the enthusiasm required to achieve new technology. It also elicits the time and patients required to achieve the new goal. This is the testimonial for the motto "nothing is impossible". It will encourage new generation to evolve breakthrough technologies in the field of Aeronautics and Aerospace. [ABSTRACT FROM AUTHOR]
- Published
- 2022
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3. Investigating the effect of material microstructure and irradiation temperature on the radiation tolerance of yttria stabilized zirconia against high energy heavy ions.
- Author
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Kalita, Parswajit, Ghosh, Santanu, Singh, Udai B., Kulriya, Pawan K., Grover, Vinita, Shukla, Rakesh, Tyagi, A. K., Sattonnay, Gaël, and Avasthi, Devesh K.
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YTTRIA stabilized zirconium oxide ,IRRADIATION ,RADIATION tolerance ,MICROSTRUCTURE ,HEAVY ions ,SPECTRUM analysis ,COUPLING reactions (Chemistry) - Abstract
Yttria stabilized zirconia pellets with different crystallite sizes were irradiated with 80 MeV Ag
6+ ions at room temperature and 1000 K to understand the effect of crystallite size/material microstructure and irradiation temperature on the radiation tolerance against high energy heavy ions [where electronic energy loss (Se ) dominates]. XRD and Raman spectroscopy measurements reveal that, irrespective of the irradiation temperature, the nano-crystalline samples suffered more damage when compared with the bulk-like sample. A reduction in the irradiation damage, i.e., improvement in the radiation tolerance, was observed for all the samples irradiated at 1000 K. The reduction in the damage, however, was remarkably higher for the nano-crystalline samples compared with the bulk-like sample, and hence the difference in the damage between the bulk-like and nano-crystalline samples was also significantly lower at 1000 K than that at room temperature. The irradiation damage, against Se , was thus found to be critically dependent on the interplay between the irradiation temperature and the crystallite size. These results are explained on the basis of the "in-elastic thermal spike" model by taking into consideration the combined effects of crystallite size and environmental (irradiation) temperature on the electron-phonon coupling strength and the lattice thermal conductivity and hence on the resulting thermal spike. These results, besides being crucial from the fundamental prospect of comprehending the size and temperature dependent radiation damage against Se , may also be important from the perspective of designing highly nano-crystalline materials for applications in various radiation environments. [ABSTRACT FROM AUTHOR]- Published
- 2019
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4. Role of temperature in the radiation stability of yttria stabilized zirconia under swift heavy ion irradiation: A study from the perspective of nuclear reactor applications.
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Kalita, Parswajit, Ghosh, Santanu, Sattonnay, Gael, Singh, Udai B., Grover, Vinita, Shukla, Rakesh, Amirthapandian, S., Meena, Ramcharan, Tyagi, A. K., and Avasthi, Devesh K.
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YTTRIA stabilized zirconium oxide ,IRRADIATION ,NUCLEAR reactors ,NUCLEAR industry ,NUCLEAR energy - Abstract
The search for materials that can withstand the harsh radiation environments of the nuclear industry has become an urgent challenge in the face of ever-increasing demands for nuclear energy. To this end, polycrystalline yttria stabilized zirconia (YSZ) pellets were irradiated with 80MeV Ag6þ ions to investigate their radiation tolerance against fission fragments. To better simulate a nuclear reactor environment, the irradiations were carried out at the typical nuclear reactor temperature (850μC). For comparison, irradiations were also performed at room temperature. Grazing incidence X-ray diffraction and Raman spectroscopy measurements reveal degradation in crystallinity for the room temperature irradiated samples. No bulk structural amorphization was however observed, whereas defect clusters were formed as indicated by transmission electron microscopy and supported by thermal spike simulation results. A significant reduction of the irradiation induced defects/damage, i.e., improvement in the radiation tolerance, was seen under irradiation at 850 μC. This is attributed to the fact that the rapid thermal quenching of the localized hot molten zones (arising from spike in the lattice temperature upon irradiation) is confined to 850μC (i.e., attributed to the resistance inflicted on the rapid thermal quenching of the localized hot molten zones by the high temperature of the environment) thereby resulting in the reduction of the defects/damage produced. Our results present strong evidence for the applicability of YSZ as an inert matrix fuel in nuclear reactors, where competitive effects of radiation damage and dynamic thermal healing mechanisms may lead to a strong reduction in the damage production and thus sustain its physical integrity. [ABSTRACT FROM AUTHOR]
- Published
- 2017
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5. Potential application of mono/bi-layer molybdenum disulfide (MoS2) sheet as an efficient transparent conducting electrode in silicon heterojunction solar cells.
- Author
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Chaudhary, Rimjhim, Patel, Kamlesh, Sinha, Ravindra K., Kumar, Sanjeev, and Tyagi, Pawan K.
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MOLYBDENUM disulfide ,MOLYBDENUM compounds ,ELECTRODE performance ,HETEROJUNCTIONS ,SOLAR cell efficiency - Abstract
In this paper, we have simulated the structure of n-type MoS
2 /silicon heterojunction solar cell and studied its function under different conditions. The optimization of parameters of the cell's layer has been carried out by using AFORS-HET software. In the present study, MoS2 has been considered as 3-D in nature instead of the reported 2-D nature. In order to ensure the formation of Schottky junction, electric contact has been made along the c-axis to collect the minority charge carriers. After optimizing the various parameters of n-type single layer MoS2 , power efficiency of 12.44% has been achieved at the room temperature, which has further decreased to 9.042% as the layer number has increased up to 40. Furthermore, after optimizing the parameters of silicon wafer maximum efficiency of 16.4% has been achieved. Temperature dependence of the cell performance has also been studied and the maximum efficiency has been achieved at 300 K. In the present study, we have demonstrated that n-type ultrathin layer of MoS2 can be used as an excellent transparent conducting electrode. [ABSTRACT FROM AUTHOR]- Published
- 2016
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6. Tuning of magnetic ordering by Y substitution onto Tb site in the nanocrystalline TbMnO3.
- Author
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Chakraborty, Keka R., Shukla, Rakesh, Kaushik, S. D., Mukadam, M. D., Siruguri, V., Tyagi, A. K., and Yusuf, S. M.
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CRYSTALLINITY ,FREQUENCY tuning ,ORTHORHOMBIC crystal system ,ANTIFERROMAGNETIC materials ,NEUTRON diffraction ,MAGNETIC moments - Abstract
We report the magnetic properties, of nano-crystalline powders Tb
1-x Yx MnO3 (x=0, 0.1, 0.2, 0.3 and 0.4), as perceived by neutron diffraction, and elucidate the effect of Tb site substitution on the magnetic structure of TbMnO3 . All samples crystallized in the orthorhombic structure conforming to space group Pnma, and exhibited an incommensurate collinear antiferromagnetic ordering of the Mn ions below ~40 K. Furthermore, at T≤20 K, all these samples showed a change in magnetic structure (of Mn moments) to a spiral ordering down to 2K, the lowest measured temperature. For the samples with x=0, 0.1, and 0.2, a short-ranged two dimensional (2D) ordering of Tb moments was also observed at 2K. However, for the other samples (x=0.3 and 0.4), no magnetic ordering of Tb moments was found down to 2K. So with Y substitution, a crossover from 2D ordering to a disordering of Tb moments was observed. The moments at the Mn site were found to be lower than the full Mn3+ (4µB ) moment for all the samples below 40 K. The magnetic properties of all the samples studied by us in nano form are more pronounced than those of the reported single crystals of same compositions [V. Yu. Ivanov et al., JETP Lett. 91, 392-397 (2010)]. [ABSTRACT FROM AUTHOR]- Published
- 2015
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7. Carrier mediated reduction of stiffness in nanoindented crystalline Si(100).
- Author
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Kataria, S., Dhara, Sandip, Dash, S., and Tyagi, A. K.
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NANOINDENTATION ,BAND gaps ,CRYSTAL structure ,NANOMECHANICS ,THERMODYNAMIC state variables ,PRESSURE drop (Fluid dynamics) ,FORCE & energy - Abstract
We report the observation of carrier mediated decrease in the stiffness of crystalline (c)-Si(100) under nanoindentation. The apparent elastic moduli of heavily doped (~1 × 10
21 cm-3 ) p- and n-type c-Si are observed to be lower by 5.3%-7.5% than the estimated value for intrinsic (~1 × 1014 cm-3 ) c-Si. The deviation observed with respect to elastic modulus remarkably matches with the estimated value while considering the electronic elastic strain effect on carrier concentration as an influence of negative pressure coefficient of band gap for Si (Γ-X). The value is predominantly higher than the reported value of a decrease of 1%-3% in stiffness as an effect of impurity in c-Si. [ABSTRACT FROM AUTHOR]- Published
- 2015
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8. Spin-phonon coupling in scandium doped gallium ferrite.
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Chakraborty, Keka R., Paul, Barnita, Grover, Vinita, Mukadam, M. D., Roy, Anushree, Basu, S., Tyagi, A. K., and Yusuf, S. M.
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SPIN-phonon interactions ,SCANDIUM compounds ,GALLIUM compounds ,FERRITES ,MAGNETIC properties - Abstract
We embarked on a study of Scandium (Sc) doped (onto Ga site) gallium ferrite (GaFeO
3 ) and found remarkable magnetic properties. In both doped as well as parent compounds, there were three types of Fe3+ ions (depending on the symmetry) with the structure conforming to space group Pna21 (Sp. Grp. No. 33) below room temperature down to 5 K. We also found that all Fe3+ ions occupy octahedral sites, and carry high spin moment. For the higher Sc substituted sample (Ga1-x Scx FeO3 :x = 0.3), a canted magnetic ordered state is found. Spin-phonon coupling below Néel temperature was observed in doped compounds. Our results indicated that Sc doping in octahedral site modifies spin-phonon interactions of the parent compound. The spin-phonon coupling strength was estimated for the first time in these Sc substituted compounds. [ABSTRACT FROM AUTHOR]- Published
- 2015
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9. Exploring the high-pressure behavior of the three known polymorphs of BiPO4: Discovery of a new polymorph.
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Errandonea, D., Gomis, O., Santamaría-Perez, D., García-Domene, B., Muñoz,, A., Rodríguez-Hernández, P., Achary, S. N., Tyagi, A. K., and Popescu, C.
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BISMUTH phosphate ,X-ray powder diffraction ,TEMPERATURE ,PHASE transitions ,POLYMORPHISM (Crystallography) ,ISOTHERMAL transformations - Abstract
We have studied the structural behavior of bismuth phosphate under compression. We performed x-ray powder diffraction measurements up to 31.5 GPa and ab initio calculations. Experiments were carried out on different polymorphs: trigonal (phase I) and monoclinic (phases II and III). Phases I and III, at low pressure (P<0.2-0.8 GPa), transform into phase II, which has a monazite-type structure. At room temperature, this polymorph is stable up to 31.5 GPa. Calculations support these findings and predict the occurrence of an additional transition from the monoclinic monazite-type to a tetragonal scheelite-type structure (phase IV). This transition was experimentally found after the simultaneous application of pressure (28 GPa) and temperature (1500 K), suggesting that at room temperature the transition might by hindered by kinetic barriers. Calculations also predict an additional phase transition at 52 GPa, which exceeds the maximum pressure achieved in the experiments. This transition is from phase IV to an orthorhombic barite-type structure (phase V). We also studied the axial and bulk compressibility of BiPO
4 . Room-temperature pressure-volume equations of state are reported. BiPO4 was found to be more compressible than isomorphic rare-earth phosphates. The discovered phase IV was determined to be the less compressible polymorph of BiPO4 . On the other hand, the theoretically predicted phase V has a bulk modulus comparable with that of monazite-type BiPO4 . Finally, the isothermal compressibility tensor for the monazite-type structure is reported at 2.4 GPa showing that the direction of maximum compressibility is in the (0 1 0) plane at approximately 15° (21°) to the a axis for the case of our experimental (theoretical) study. [ABSTRACT FROM AUTHOR]- Published
- 2015
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10. Modeling of planar high temperature solid oxide fuel cells.
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Bansal, Sahil, Tyagi, Anand K., and Sharma, Anuj Kumar
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SOLID oxide fuel cells , *NAVIER-Stokes equations , *DARCY'S law , *CHARGE transfer kinetics , *HIGH temperatures , *ELECTRIC power consumption , *FUEL cells - Abstract
Solid Oxide Fuel Cells (SOFCs) have the tremendous potential to be the future solution of our energy demands. This paper attempts to model a three dimensional, single, high temperature solid oxide fuel cell operating at 800°C. The model has been prepared using the butler Volmer equations for charge transfer kinetics. For the flow of the gases in the porous gas electrodes, Brinkman Equations have been used, which are combination of Darcy's Law and the Navier Stokes Equations. We have analysed the electrical properties of SOFC developed in the model. From the simulation we have plotted the polarization curve of SOFC, which is in good agreement with the experimental data. The power characteristics representing the output power variation with the current density of the SOFC is also studied and optimum current density is defined. Additionally, the polarization characteristics representing the variation of the average current density of the SOFC with SOFC voltage is also studied. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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11. Structural, morphological and dielectric studies of sol gel derived codoped SrTiO3-δ.
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Kaur, Ramanjeet, Tyagi, Anand K., and Kaur, Karamveer
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PERMITTIVITY , *DIELECTRICS , *DIELECTRIC properties , *COLLOIDS , *LATTICE constants , *CERAMICS - Abstract
Gadolinium and Thulium doped SrTiO3-δ i.e. Sr1-xGdxTi1-yTmyO3-δ for various values of 'x' and 'y' have been successfully synthesized using sol gel technique in the reported work. XRD patterns of the obtained ceramics are found to be sharp and well defined having no impurity phases for all the compositions. The lattice parameter 'a' decreases with increase in amount of Gadolinium. XRD, FESEM and EDX studies confirm the formation of Sr1-xGdxTi1-yTmyO3-δ ceramics with required cubic structure. FESEM/EDX analysis reveals that ceramics possess high density with marginal inter-granular porosity. The dielectric studies show that the synthesized samples possess high dielectric constants, high ac conductivity and low loss factors; and furthermore, dielectric properties improve on doping with gadolinium and thulium. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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- View/download PDF
12. Structural, morphological and dielectric studies of sol gel derived codoped SrTiO3-δ.
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Kaur, Ramanjeet, Tyagi, Anand K., and Kaur, Karamveer
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PERMITTIVITY ,DIELECTRICS ,DIELECTRIC properties ,COLLOIDS ,LATTICE constants ,CERAMICS - Abstract
Gadolinium and Thulium doped SrTiO
3-δ i.e. Sr1-x Gdx Ti1-y Tmy O3-δ for various values of 'x' and 'y' have been successfully synthesized using sol gel technique in the reported work. XRD patterns of the obtained ceramics are found to be sharp and well defined having no impurity phases for all the compositions. The lattice parameter 'a' decreases with increase in amount of Gadolinium. XRD, FESEM and EDX studies confirm the formation of Sr1-x Gdx Ti1-y Tmy O3-δ ceramics with required cubic structure. FESEM/EDX analysis reveals that ceramics possess high density with marginal inter-granular porosity. The dielectric studies show that the synthesized samples possess high dielectric constants, high ac conductivity and low loss factors; and furthermore, dielectric properties improve on doping with gadolinium and thulium. [ABSTRACT FROM AUTHOR]- Published
- 2020
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13. Effect of synthesis temperature on magnetization and properties of Y-Fe-garnet.
- Author
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Sen, Shashwati, Rao, K. Sandeep, Meena, Sher Singh, Sharma, R. K., Jagannath, Singh, S. G., Desai, D. G., Mandal, B., Mishra, J. K., Singh, Snigdha, Pande, Manjiri, Tyagi, A. K., Salunke, H. G., Sharma, Veerendra K., Prajapat, C. L., and Yusuf, S. M.
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MAGNETIZATION ,TEMPERATURE effect ,GARNET ,YTTRIUM oxides ,FERRIC oxide ,CHEMICAL elements - Abstract
The present work describes the synthesis and characterization of Y-Fe-Garnets for the development of RF circulators which requires disc of diameter 50 mm and density 5g/cc. For synthesis of the garnet, iron oxide (>99.5% purity) and yttrium oxide (>99.9% purity) were mixed in stoichiometric amount, cold pressed into pellets of 50 mm diameter and annealed at different temperatures. For the formation of disc and achieving the theoretical density of the garnet annealing under high vacuum was also carried. The fabricated disc was characterized by XRD, magnetization, XPS and Mössbauer spectra. The observed change in magnetization was correlated with change in the chemical state of the elements due to annealing condition and changes in the Mössbauer spectra. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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14. Phase transitions in delafossite CuLaO2 at high pressures.
- Author
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Salke, Nilesh P., Garg, Alka B., Rao, Rekha, Achary, S. N., Gupta, M. K., Mittal, R., and Tyagi, A. K.
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OPTICAL diffraction ,SPECTRUM analysis ,PHASE transitions ,STRUCTURAL stability ,RAMAN spectroscopy - Abstract
Structural stability of a transparent conducting oxide CuLaO
2 at high pressures is investigated using in-situ Raman spectroscopy, electrical resistance, and x-ray diffraction techniques. The present Raman investigations indicate a sequence of structural phase transitions at 1.8 GPa and 7GPa. The compound remains in the first high pressure phase when pressure is released. Electrical resistance measurements carried out at high pressures confirm the second phase transition. These observations are further supported by powder x-ray diffraction at high pressures which also showed that a-axis is more compressible than c-axis in this compound. Fitting the pressure dependence of unit cell volume to 3rd order Birch-Murnaghan equation of state, zero pressure bulk modulus of CuLaO2 is determined to be 154(25) GPa. The vibrational properties in the ambient delafossite phase of CuLaO2 are investigated using ab-initio calculations of phonon frequencies to complement the Raman spectroscopic measurements. Temperature dependence of the Raman modes of CuLaO2 is investigated to estimate the anharmonicity of Raman modes. © 2014 AIP Publishing LLC. [ABSTRACT FROM AUTHOR]- Published
- 2014
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15. Investigation of temperature dependent dielectric constant of a sputtered TiN thin film by spectroscopic ellipsometry.
- Author
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Sundari, S. Tripura, Ramaseshan, R., Jose, Feby, Dash, S., and Tyagi, A. K.
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ELLIPSOMETRY ,POLARIMETRY ,TITANIUM nitride films ,THIN film research ,PLASMONS (Physics) - Abstract
The temperature dependence of optical constants of titanium nitride thin film is investigated using Spectroscopic Ellipsometry (SE) between 1.4 and 5 eV in the temperature range of 300K to 650K in steps of 50K. The real and imaginary parts of the dielectric functions ε
1 (E) and ε2 (E) marginally increase with increase in temperature. A Drude Lorentz dielectric analysis based on free electron and oscillator model are carried out to describe the temperature behavior. With increase in temperature, the unscreened plasma frequency and broadening marginally decreased and increased, respectively. The parameters of the Lorentz oscillator model also showed that the relaxation time decreased with temperature while the oscillator energies increased. This study shows that owing to the marginal change in the refractive index with temperature, titanium nitride can be employed for surface plasmon sensor applications even in environments where rise in temperature is imminent. [ABSTRACT FROM AUTHOR]- Published
- 2014
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16. Size dependent magnetic and dielectric properties of nano CoFe2O4 prepared by a salt assisted gel-combustion method.
- Author
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Vasundhara, K., Achary, S. N., Deshpande, S. K, Babu, P. D., Meena, S. S., and Tyagi, A. K.
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MAGNETIC properties of nanostructured materials ,ELECTRIC properties of nanostructured materials ,MOSSBAUER spectroscopy ,COBALT compounds ,MAGNETIC properties of transition metal compounds ,X-ray diffraction - Abstract
In this communication, we report the preparation and properties of nano-CoFe2O4 by gel combustion in presence of KCl and subsequent heat treatments. The products were characterized by X-ray diffraction, Infrared spectroscopy, and Mössbauer Spectroscopy. Spinel type structure with all Fe in 3+ oxidation states was confirmed from the XRD and Mossbauer spectroscopy. The average crystallite sizes of the studied samples were about 6 and 50 nm. Low temperature magnetic and dielectric properties of the samples were studied by superconducting quantum interference device magnetometry and ac-impedance spectroscopy. The field and temperature dependent magnetization studies indicated superparamagnetic nature for 6 nm sample and ferromagnetic nature for 50 nm sample. The temperature-dependent dielectric properties measured over a wide range of frequencies indicated an increasing trend of dielectric permittivity with the decrease in crystallite size. Variable range polaron hopping conduction was observed in both samples. [ABSTRACT FROM AUTHOR]
- Published
- 2013
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17. High-pressure lattice dynamical study of bulk and nanocrystalline In2O3.
- Author
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Garcia-Domene, B., Ortiz, H. M., Gomis, O., Sans, J. A., Manjón, F. J., Muñoz, A., Rodríguez-Hernández, P., Achary, S. N., Errandonea, D., Martínez-García, D., Romero, A. H., Singhal, A., and Tyagi, A. K.
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NANOCRYSTALS ,RAMAN spectroscopy ,CORUNDUM ,INDIUM compounds ,CRYSTALLIZATION - Abstract
The effect of pressure on the vibrational properties of bulk and nanocrystalline powders of cubic bixbyite-type In2O3 has been investigated at room temperature by means of Raman spectroscopy up to 31.6 and 30 GPa, respectively. We have been able to follow the pressure dependence of up to sixteen and seven Raman modes in bulk and nanocrystalline cubic In2O3, respectively. The experimental frequencies and pressure coefficients of the Raman-active modes of bulk cubic In2O3 at ambient pressure are in good agreement with those predicted by our theoretical ab initio calculations. Furthermore, a comparison of our experimental data with our calculations for the Raman modes in rhombohedral corundum and orthorhombic Rh2O3-II structures and with already reported Raman modes of rhombohedral corundum-type In2O3 at room pressure indicate that Raman scattering measurements provide no experimental evidence of the cubic to rhombohedral or cubic to orthorhombic phase transitions either in bulk material or in nanocrystals up to 30 GPa. [ABSTRACT FROM AUTHOR]
- Published
- 2012
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18. New high-pressure phase and equation of state of Ce2Zr2O8.
- Author
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Errandonea, D., Kumar, R. S., Achary, S. N., Gomis, O., Manjón, F. J., Shukla, R., and Tyagi, A. K.
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HEMATITE ,X-ray diffraction ,RAMAN spectroscopy ,LATTICE theory ,HIGH pressure (Technology) - Abstract
In this paper we report a new high-pressure rhombohedral phase of Ce2Zr2O8 observed in high-pressure angle-dispersive x-ray diffraction and Raman spectroscopy studies up to nearly 12 GPa. The ambient-pressure cubic phase of Ce2Zr2O8 transforms to a rhombohedral structure beyond 5 GPa with a feeble distortion in the lattice. The pressure evolution of the unit-cell volume showed a change in compressibility above 5 GPa. The unit-cell parameters of the high-pressure rhombohedral phase at 12.1 GPa are ah = 14.6791(3) Å, ch = 17.9421(5) Å, and V = 3348.1(1) Å3. The structure relations between the parent cubic (P213) and rhombohedral (P32) phases were obtained via group-subgroup relations. All the Raman modes of the cubic phase showed linear evolution with pressure, with the hardest one at 197 cm-1. Some Raman modes of the high-pressure phase have a non-linear evolution with pressure, and softening of one low-frequency mode with pressure is found. The compressibility, equation of state, and pressure coefficients of Raman modes of Ce2Zr2O8 are also reported. [ABSTRACT FROM AUTHOR]
- Published
- 2012
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19. High pressure structural stability of BaLiF3.
- Author
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Mishra, A. K., Garg, Nandini, Shanavas, K. V., Achary, S. N., Tyagi, A. K., and Sharma, Surinder M.
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HIGH pressure (Science) ,X-ray diffraction ,PEROVSKITE ,BARIUM ,LITHIUM ,FLUORINE ,BAND gaps ,BRITTLENESS - Abstract
High pressure x-ray diffraction studies on inverse-perovskite BaLiF3 show that this compound is structurally stable up to ∼50 GPa. The bulk modulus of BaLiF3 is determined to be 75.9 GPa which is in close agreement with that determined from a semi-empirical formulation. Our ab initio calculations show that among the three alkaline earth fluoro perovskites (ALiF3, A = Ba, Ca, and Sr) which crystallize in the inverse-perovskite structure, BaLiF3 is the least brittle at ambient conditions and also that the degree of brittleness decreases at high pressures. The behavior of the elastic constants at high pressure accompanied by a reduction in the bandgap indicates a decrease in the directional nature of the bonding. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
- View/download PDF
20. High-pressure x-ray diffraction study of CdMoO4 and EuMoO4.
- Author
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Errandonea, D., Santamaria-Perez, D., Achary, S. N., Tyagi, A. K., Gall, P., and Gougeon, P.
- Subjects
HIGH pressure (Science) ,CADMIUM compounds ,MOLYBDATES ,X-ray diffraction ,CRYSTALS - Abstract
We studied the effects of high pressure on the crystalline structure of scheelite-type CdMoO4 and EuMoO4. We found that the compressibility of the materials is highly nonisotropic, with the c-axis being the most compressible one. We also observed clear evidence of a structural phase transition at 12 GPa (CdMoO4) and 8.8 GPa (EuMoO4). The high-pressure phase has a monoclinic structure similar to M-fergusonite. The transition is reversible, and no volume change is detected between the low- and high-pressure phases. The results contradict early x-ray diffraction studies carried out in CdMoO4 and are compared with those obtained previously in isomorphic molybdates. Finally, the equation of state for both compounds is also determined. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
- View/download PDF
21. High-pressure x-ray diffraction study of bulk and nanocrystalline PbMoO4.
- Author
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Errandonea, D., Santamaria-Perez, D., Grover, V., Achary, S. N., and Tyagi, A. K.
- Subjects
NANOCRYSTALS ,HYDROSTATICS ,SCHEELITE ,PRESSURE ,PROPERTIES of matter - Abstract
We studied the effects of high-pressure on the crystalline structure of bulk and nanocrystalline scheelite-type PbMoO
4 . We found that in both cases the compressibility of the materials is highly nonisotropic, being the c-axis the most compressible one. We also observed that the volume compressibility of nanocrystals becomes higher that the bulk one at 5 GPa. In addition, at 10.7(8) GPa we observed the onset of an structural phase transition in bulk PbMoO4 . The high-pressure phase has a monoclinic structure similar to M-fergusonite. The transition is reversible and not volume change is detected between the low-pressure and high-pressure phases. No additional structural changes or evidence of decomposition are found up to 21.1 GPa. In contrast nanocrystalline PbMoO4 remains in the scheelite structure at least up to 16.1 GPa. Finally, the equation of state for bulk and nanocrystalline PbMoO4 are also determined. [ABSTRACT FROM AUTHOR]- Published
- 2010
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22. Tailoring absorption and emission of light in dielectric polymer nanofluids of reinforced poly(vinylidene fluoride) with silver nanoparticles.
- Author
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Phule, A. D., Ram, S., and Tyagi, A. K.
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NANOFLUIDS ,POLYMER research ,FLUORIDES ,NANOPARTICLES ,DIELECTRICS research ,SILVER ,PERCOLATION theory ,ENERGY transfer - Abstract
Polymer nanofluids of poly(vinylidene fluoride) (PVF
2 ) containing 0–5 wt % Ag nanoparticles (Ag NPs) are synthesized via a simple in situ caloric Ag+ →Ag reaction in liquid N,N-dimethyl formamide by sonication in hot conditions. An upright dielectric field on formation of a percolation network over the electron donor Ag NPs and electron acceptor CF2 from an ordered β-PVF2 tailors light absorption 250–550 nm and green emission 520–620 nm of variable intensities, with a small percolation threshold of 0.1 wt % Ag NPs. On higher Ag contents, the intensity drops rapidly in both the absorption and emission roughly by an order of magnitude, following the percolation theory of a hybrid dielectric composite. Results open scope of developing newer materials for optical sensors, barcodes, energy transfer pairs, or light emitters. [ABSTRACT FROM AUTHOR]- Published
- 2010
- Full Text
- View/download PDF
23. High pressure behavior of ZrGeO4: A Raman spectroscopic and photoluminescence study.
- Author
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Rao, Rekha, Sakuntala, T., Achary, S. N., and Tyagi, A. K.
- Subjects
INDUSTRIAL chemistry ,HIGH pressure (Science) ,SPECTRUM analysis ,PHOTOLUMINESCENCE ,RAMAN effect ,HIGH pressure crystallography ,OPTICS - Abstract
High pressure behavior of ZrGeO
4 has been investigated using Raman and photoluminescence (PL) spectroscopies up to 25 GPa in a diamond anvil cell. Under the application of pressure, the GeO4 librational mode exhibits softening, suggesting dynamical instability of the scheelite structure. Qualitative changes are noted in the Raman spectrum above 12 GPa, suggesting a possible transition around this pressure. High pressure PL behavior of Eu3+ -related crystal field transitions indicates a clear change in the site symmetry of Eu3+ around 12 GPa, strongly supporting structural transition to a lower symmetry phase at this pressure. [ABSTRACT FROM AUTHOR]- Published
- 2009
- Full Text
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24. Study of the oxidation state and the structural aspects of the V-doped TiO2.
- Author
-
Bhattacharyya, K., Varma, S., Tripathi, A. K., Bhattacharyya, D., Mathon, O., and Tyagi, A. K.
- Subjects
CATALYST supports ,TITANIUM dioxide ,OXIDATION ,VANADIUM ,CRYSTAL lattices - Abstract
A modified sol-gel method for synthesizing vanadium doped titania is being reported. These materials were thoroughly characterized for their oxidation states by electron paramagnetic resonance and x-ray absorption near edge structure and the local environment of the V-atom were investigated by the x-ray absorption fine structure. V-doped titania was found to be more active than nanotitania for photo-oxidation of methane in air under ambient conditions using UV-visible irradiation. The vanadium doping in the crystal lattice of titania leads to a mixture of oxidation states of 4+ and 5+ in the crystal lattice sites of the TiO
2 , which is crucial for its catalytic activity. [ABSTRACT FROM AUTHOR]- Published
- 2009
- Full Text
- View/download PDF
25. Magnetic properties of sonochemically synthesized CoCr2O4 nanoparticles.
- Author
-
Dutta, Dimple P., Manjanna, J., and Tyagi, A. K.
- Subjects
COBALT chromite ,NANOPARTICLES ,MAGNETIC properties ,MAGNETISM ,SONOCHEMISTRY - Abstract
Cobalt chromite (CoCr
2 O4 ) is a potential multiferroic material. In order to understand the temperature dependent magnetic transitions on particle morphology, here we prepared CoCr2 O4 nanoparticles by sonochemical technique. We used powder x-ray diffraction, transmission electron microscopy, selected area electron diffraction, superconducting quantum interference device magnetometer, and ac susceptibility measurement techniques for characterization. The low-temperature magnetic behavior of CoCr2 O4 nanoparticles have been investigated in more detail. While the bulk CoCr2 O4 exhibits two magnetic transitions viz., Tc ≈98 K and Ts ≈26 K, the nanoparticles here showed a Tc ≈84 K and Ts ≈25 K. We tentatively attribute this shift in Tc to finite size effects. [ABSTRACT FROM AUTHOR]- Published
- 2009
- Full Text
- View/download PDF
26. Gd2O3:Eu3+ particles prepared by glycine-nitrate combustion: Phase, concentration, annealing, and luminescence studies.
- Author
-
Ningthoujam, R. S., Shukla, R., Vatsa, R. K., Duppel, V., Kienle, L., and Tyagi, A. K.
- Subjects
COMBUSTION ,GLYCINE ,NITRATES ,CHARGE transfer ,MAGNETIC dipoles ,LUMINESCENCE - Abstract
Eu
3+ doped Gd2 O3 particles have been prepared by glycine-nitrate combustion route. Eu–O charge transfer peak shifts to higher wavelength from 260 to 273 nm with increasing Eu3+ concentration, whereas the linewidth at half maximum intensity varies with Eu3+ concentrations and annealing temperatures as well. Lifetime for5 D0 level of Eu3+ increases with annealing temperature from 500 to 900 °C, and this is related to decreasing surface to volume atomic ratio of particles. Asymmetric ratio defined by the ratio of intensity of electrical dipole transition to that of magnetic dipole transition is found to be 10, which is more than the reported values (3–4) indicating enhancement in luminescence by this synthesis route. [ABSTRACT FROM AUTHOR]- Published
- 2009
- Full Text
- View/download PDF
27. Surface and interface reactions of sputtered TiNi/Si thin films.
- Author
-
Kumar, A. K. Nanda, Jayakumar, S., Kannan, M. D., Rajagopalan, S., Balamurugan, A. K., Tyagi, A. K., Kumar, Sanjeev, Reddy, G. L. N., Ramana, J. V., and Raju, V. S.
- Subjects
SPUTTERING (Physics) ,THIN films ,TITANIUM ,NICKEL ,SILICON ,HIGH temperatures ,SECONDARY ion mass spectrometry ,DIFFUSION - Abstract
We present the surface and interfacial chemical analysis of sputter deposited TiNi thin films over Si (100) substrates using Rutherford backscattering spectrometry (RBS), secondary ionization mass spectrometry (SIMS), and grazing incidence x-ray diffraction analysis. Upon annealing to high temperatures, significant diffusion of Ni into the Si substrate was observed. The analysis of the RBS and SIMS spectra confirm the net movement of the film species into the substrate. Further, we discuss the formation mechanism and the composition distribution surrounding the TiNi/Si interface that proves conducive for the nucleation and growth of the detected nickel silicide compound. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
28. Amorphization-decomposition behavior of HfW2O8 at high pressure.
- Author
-
Sakuntala, T., Rao, Rekha, Garg, Alka B., Achary, S. N., and Tyagi, A. K.
- Subjects
CHEMICAL decomposition ,HIGH pressure (Science) ,HAFNIUM ,TUNGSTEN oxides ,RAMAN spectroscopy ,RAMAN effect ,STRUCTURAL stability - Abstract
Structural stability of HfW
2 O8 is investigated at high pressure using Raman spectroscopy. Irreversible amorphization is found to occur when pressurized to above 3 GPa under hydrostatic conditions. The Raman spectrum of the pressure-amorphized sample closely resembles that of amorphous WO3 . On the other hand, spectrum of the recovered sample subjected to uniaxial compression of about 5 GPa showed bands characteristic of crystalline HfO2 besides the parent cubic phase and amorphous phase. Ex situ x-ray diffraction measurements on the pressure-cycled samples also indicated the presence of monoclinic HfO2 . These results suggest that the observed pressure-induced amorphization (PIA) under hydrostatic compression is indeed due to hindered decomposition, which is facilitated under uniaxial compression. Thus, the behavior of HfW2 O8 appears to be different from that of ZrW2 O8 , which exhibited only PIA and not pressure-induced decomposition at ambient temperature. [ABSTRACT FROM AUTHOR]- Published
- 2008
- Full Text
- View/download PDF
29. X-ray diffraction, μ-Raman spectroscopic studies on CeO2-RE2O3 (RE=Ho, Er) systems: Observation of parasitic phases.
- Author
-
Mandal, B. P., Roy, M., Grover, V., and Tyagi, A. K.
- Subjects
SPECTRUM analysis ,PHOTOLUMINESCENCE ,CRYSTALLIZATION ,RAMAN spectroscopy ,CERIUM oxides - Abstract
The phase relations in CeO
2 -Ho2 O3 and CeO2 -Er2 O3 systems have been established under the slow-cooled conditions. As per x-ray diffraction (XRD), in both the series a single-phasic solid solution forms up to the nominal composition Ce0.6 RE0.4 O1.8 (RE=Ho, Er) retaining the F-type structure of parent ceria. In Ce1-x Erx O2-x/2 system the presence of microdomains of C-type phase have been revealed by Raman spectroscopy for composition x=0.4, which has been identified as single phasic by XRD. Photoluminescence studies also show that biphasic region commences from x=0.4 for Ce1-x Hox O2-x/2 series. The biphasicity continues until x=0.7 for both the series. From x=0.8 the solid solutions exist as C-type single phasic, which is isotypic to another end member RE2 O3 (RE=Ho, Er) and as revealed by both XRD and Raman spectroscopy. High temperature XRD studies show that no temperature induced phase change has been observed in either of the series until 1273 K. In this work photoluminescence data was used to delineate the phase boundary. [ABSTRACT FROM AUTHOR]- Published
- 2008
- Full Text
- View/download PDF
30. Controlled in situ boron doping of diamond thin films using solution phase.
- Author
-
Roy, M., Dua, A. K., Nuwad, J., Girija, K. G., Tyagi, A. K., and Kulshreshtha, S. K.
- Subjects
DIAMOND thin films ,SEMICONDUCTOR doping ,ACID deposition ,PHOTOELECTRON spectroscopy ,RAMAN spectroscopy ,X-ray photoelectron spectroscopy ,X-ray diffraction - Abstract
Controlled boron doping of diamond film using nontoxic reagents is a challenge in itself. During the present study, attempts have been made to dope diamond films in situ with boron from a solution of boric acid (H
3 BO3 ) in methanol (CH3 OH) using a specially designed bubbler that ensured continuous and controlled flow of vapors of boron precursors during deposition. The samples are thoroughly characterized using a host of techniques comprising of x-ray photoelectron spectroscopy, Raman, x-ray diffraction, and current-voltage measurements (I-V). Cross-sectional micro-Raman spectroscopy has been used to obtain depth profile of boron in diamond films. Boron concentration ([B]) in the films is found to vary linearly on a semilog scale with molarity (M) of H3 BO3 in CH3 OH. Lattice constant of our samples is smaller than the reported American society for testing and materials (ASTM) values due to oxygen incorporation and it increases with [B] in the diamond samples. Heavily boron doped samples exhibit Fano deformation of the Raman line shape and negative and/zero activation barrier in temperature dependent I-V measurements that indicate the formation of metallic phase in the samples. The present study illustrates the feasibility of safe and controlled boron doping of diamond films using a solution of H3 BO3 in CH3 OH over a significant range of [B] from semiconductor to metallic regime but with a little adverse effect due to unintentional but unavoidable incorporation of oxygen. [ABSTRACT FROM AUTHOR]- Published
- 2006
- Full Text
- View/download PDF
31. Particle size dependence of magnetization and phase transition near TN in multiferroic BiFeO3.
- Author
-
Mazumder, R., Ghosh, S., Mondal, P., Bhattacharya, Dipten, Dasgupta, S., Das, N., Sen, A., Tyagi, A. K., Sivakumar, M., Takami, T., and Ikuta, H.
- Subjects
PHASE transitions ,PROPERTIES of matter ,MAGNETIC properties ,MAGNETIC fields ,X-ray diffraction ,MAGNETIC measurements - Abstract
We report results of a comprehensive study of the phase transition at T
N (∼643 K) as a function of particle size in multiferroic BiFeO3 system. We employed electrical, thermal, and temperature dependent x-ray diffraction studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime of 2–300 K under a magnetic field of 100–10 000 Oe both on bulk and nanocrystalline systems. While in the bulk system a sharp endothermic peak at TN together with a broad feature, ranging over nearly ∼100 K (ΔT), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at TN , is found to occur both at TO and TN across ΔT in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metastability increases in the nanoscale BiFeO3 with divergence between zero-field cooled and field cooled magnetizations below ∼100 K and faster magnetic relaxation. Interestingly, in nanoscale BiFeO3 one also observes finite coercivity at lower temperature, which points out that suitable design of particle size and shape may induce ferromagnetism. The inhomogeneous distribution of Bi/Fe ions and/or oxygen nonstoichiometry seems to be giving rise to broad features in thermal, magnetic as well as electrical responses. [ABSTRACT FROM AUTHOR]- Published
- 2006
- Full Text
- View/download PDF
32. Pressure evolution of resistance in framework structured materials α-ZrMo2O8 and α-HfMo2O8.
- Author
-
Karandikar, A. S., Mukherjee, G. D., Vijayakumar, V., Godwal, B. K., Achary, S. N., and Tyagi, A. K.
- Subjects
ELECTRIC conductivity ,FREE electron theory of metals ,ENERGY-band theory of solids ,PHASE transitions ,PHASE equilibrium ,STATISTICAL physics - Abstract
The ac resistance measurements on the trigonal (α-) phase of ZrMo
2 O8 and HfMo2 O8 carried out up to 5 GPa using the toroid anvil apparatus are reported. The ac resistance data in both the materials are found to be strongly frequency dependent and show a transition between two power law regimes with a crossover frequency (ωc ) near 1 kHz. For both the compounds slope changes and anomalies are observed in the ac resistance data at the structural transition pressures. The activation volumes at room temperature are obtained by analyzing the ac conductance data in terms of the pressure dependent activation volume model. The main conduction mechanism at 12 Hz is believed to be the diffusion of oxygens ions, whereas hopping of point defects is responsible for ac conductuance above 33 kHz. At 1 KHz, a crossover from hopping to diffusion of point defects is observed at the α-δ (monoclinic) phase transition pressure. [ABSTRACT FROM AUTHOR]- Published
- 2006
- Full Text
- View/download PDF
33. On the role of oxygen in the catalysis of C54 titanium disilicide by Ti5Si3 phase.
- Author
-
Ilango, S., Raghavan, G., Kalavathi, S., Panigrahi, B. K., and Tyagi, A. K.
- Subjects
OXYGEN ,TITANIUM ,SILICON oxide ,MASS spectrometry ,X-ray diffraction ,CATALYSIS - Abstract
The influence of the native SiO
2 in the phase formation sequence of Ti/Si system has been studied. Furnace annealing of Ti/Si couples has been investigated by secondary-ion-mass spectrometry, Rutherford backscattering, and glancing incidence x-ray diffraction. The influence of dilute concentration of oxygen at the interface in altering reaction kinetics was studied. These studies reveal that the highly metallic Ti5 Si3 phase forms during the intermediate stages of heat treatment. This phase is found to catalyze the direct formation of the low-resistive C54 TiSi2 phase, completely bypassing its polymorphic C49 counterpart. X-ray-diffraction line profile analysis of Bragg peaks indicates that the formation of the C54 phase is accompanied by a reduction in the microstrain. Subsequent annealing results in the complete transformation to TiSi2 with an increase in the average grain size. [ABSTRACT FROM AUTHOR]- Published
- 2005
- Full Text
- View/download PDF
34. Dielectric phase-transition and polarization studies in stepped and compositionally graded lead magnesium niobate–lead titanate relaxor thin films.
- Author
-
Ranjith, R., Sarkar, Asis, Laha, Apurba, Krupanidhi, S. B., Balamurugan, A. K., Rajagoplan, S., and Tyagi, A. K.
- Subjects
DIELECTRICS ,MAGNESIUM ,THIN films ,SURFACES (Technology) ,LASER ablation ,ELECTROMAGNETIC induction - Abstract
Ferroelectric properties of chemically heterogeneous multilayers and graded composition thin films of (1-x)Pb(Mg
1/3 Nb2/3 )O3 –xPbTiO3 , a well-known relaxor, were prepared using a multitarget pulsed laser ablation system. Target materials with different values of x (0.0, 0.1, 0.2, and 0.3) were used for ablation growth. Films with sharp interfaces and graded interfaces were prepared and were characterized for their electrical properties. The stepped films were found to have a phase-transition behavior dominated by the individual layer behavior with their corresponding local dielectric maxima. The graded films showed an overall averaged effect of the system. The graded films showed an enhanced diffusiveness in the phase transition and the γ value was found to be 2.43, which is a measure of diffusivity. The tunability of these graded films was found to be high around 60% and was twice that of any of the film of homogeneous composition films. The room-temperature polarization hysteresis curves exhibited slim loop characteristics with a saturation polarization of about 40 μC/cm2 in the case of both stepped and graded films. [ABSTRACT FROM AUTHOR]- Published
- 2005
- Full Text
- View/download PDF
35. Modifying the nanocrystalline characteristics—structure, size, and surface states of copper oxide thin films by high-energy heavy-ion irradiation.
- Author
-
Balamurugan, B., Mehta, B. R., Avasthi, D. K., Singh, Fouran, Arora, Akhilesh K., Rajalakshmi, M., Raghavan, G., Tyagi, A. K., and Shivaprasad, S. M.
- Subjects
COPPER oxide ,THIN films ,HEAVY ions ,RAMAN spectroscopy - Abstract
In the present study, x-ray diffraction, Raman spectroscopy, spectroscopic ellipsometry, photoluminescence, and x-ray photoelectron spectroscopy techniques were used to study the effect of 120 MeV [sup 107]Ag[sup 9+] ion irradiation on nanocrystalline Cu[sub 2]O thin films grown by the activated reactive evaporation technique. The influence of dense electronic excitations during ion irradiation on the structural and optical properties of the Cu[sub 2]O thin films was studied. Experimental results demonstrate that the phase and the size of nanocrystallites in the Cu[sub 2]O thin films as well as associated surface states can be tailored by controlling ion fluence. The Cu[sub 2]O higher symmetry cubic phase is observed to be quite stable under a higher temperature and irradiation-induced thermal spikes, which accompanies ion irradiation. [ABSTRACT FROM AUTHOR]
- Published
- 2002
- Full Text
- View/download PDF
36. Crystalline to amorphous transition and band structure evolution in ion-damaged silicon studied by spectroscopic ellipsometry.
- Author
-
Giri, P. K., Tripurasundari, S., Raghavan, G., Panigrahi, B. K., Magudapathy, P., Nair, K. G. M., and Tyagi, A. K.
- Subjects
PHASE transitions ,MICROSTRUCTURE ,SILICON ,CHEMICAL structure - Abstract
Crystalline to amorphous transition and subsequent microstructural evolution in silicon induced by Ar[sup +]-ion implantation over a wide range of ion fluences (6×10[sup 13]–1×10[sup 17] cm[sup -2]) have been investigated by spectroscopic ellipsometry. In the evaluation of the optical and microstructural properties of the damaged layer, the contribution of the surface overlayer to the measured dielectric spectra was separated by fitting a multilayer model with an effective medium approximation. The best fit to the dielectric spectra for disordered silicon could be obtained by taking our highest-fluence implanted (fluence=1×10[sup 17] ions/cm[sup 2]) amorphous silicon (a-Si) data as reference data instead of a-Si data available in the handbook. The derivative spectra as a function of fluence show a distinct and sharp transition from the crystalline to amorphous phase. The threshold fluence for this transition is derived from fitting. Evaluation of standard sum rules and optical moments for imaginary part of the pseudodielectric function reveals no substantial change in various physical parameters below the transition indicating their insensitivity to point defects, while it shows a large change with fluence above the threshold for amorphization. The disorder induced changes in the effective dielectric constant, number of valence electrons per atom participating in optical transition, Penn gap energy, average bond length, coordination number, effective dispersion oscillator energy, an average strength of the interband optical transition with fluence is discussed on the basis of microstructural evolution and corresponding band structure modification. It is also shown that the dielectric functions of damaged silicon are well represented by a sum of six classical Lorentz oscillators. With increasing fluences, each of the oscillator amplitude decreases and linewidth increases except for the 3.3 eV transition which shows increasing amplitude... [ABSTRACT FROM AUTHOR]
- Published
- 2001
- Full Text
- View/download PDF
37. Atomic force microscopy studies of homoepitaxial GaN layers grown on GaN template by laser MBE
- Author
-
Choudhary, B. S., primary, Singh, A., additional, Tanwar, S., additional, Tyagi, P. K., additional, Kumar, M. Senthil, additional, and Kushvaha, S. S., additional
- Published
- 2016
- Full Text
- View/download PDF
38. Structure, magnetism and electrical transport in epitaxial La0.23Pr0.41Ca0.36MnO3 thin films: Consequences of film thickness.
- Author
-
Singh, Sandeep, Tyagi, P. K., and Singh, H. K.
- Subjects
- *
THIN films , *FERROMAGNETIC materials , *MORPHOLOGY - Abstract
A systematic study of the consequences of in-plane tensile strain on the correlation between structure/microstructure and magneto-electrical properties of La0.23Pr0.41Ca0.36MnO3/SrTiO3 (001) thin films is performed. At lower film thickness (d≤27 nm) the dominant tensile strain supports layered morphology. At d=41 nm structural/microstructural crossover takes place and at d≥41 nm relaxation induced tilt and dilations of the lattice generate defects like dislocations, which in turn transforms the layer by layer growth into disordered brick type morphology. It appears that the relaxation even at higher thickness (∼81 nm) is partial only and the strain may have a self-sustained nature. The domination of the tensile strain suppresses the ferromagnetic-metallic phase due to the possible electronic reconstruction which could give rise to a magnetically disordered insulator 'dead' layer and a sizeable non-magnetic insulator state at d≤27 nm. In this thickness regime the possible selective orbital stabilization could also contribute to carrier localization. At d≥41 nm the severity of the impact of the interfacial electronic reconstruction is reduced with concomitant relaxation of the tensile strain which favors carrier delocalization and yields well defined metallic-ferromagnetic phase transitions. The hysteretic nature of the phase transitions reflects the thermal cycle dependent nature of the metallic-ferromagnetic and insulating antiferromagnetic phases in this strongly phase separated material. [ABSTRACT FROM AUTHOR]
- Published
- 2018
- Full Text
- View/download PDF
39. Effective relative permittivity and characteristic impedance of graphene loaded microstrip line by scalar S-parameters.
- Author
-
Patel, Kamlesh, Neha, and Tyagi, Pawan K.
- Subjects
GRAPHENE ,MICROSTRIP transmission lines ,PERMITTIVITY ,IMPEDANCE control ,ELECTRIC conductivity - Abstract
Graphene film has particular applications at the microwave frequencies because as the number of layers increases, the permittivity and conductivity are modulated. Particularly, an effective relative permittivity of graphene is of high interest. In the present work, effective relative permittivity and characteristic impedance for three kind of structures where microstrip line is either in air or loaded by glass or by graphene film on the glass substrate have been determined. The scalar S-parameters have been measured by adopting both transmission- reflection and short-open methods. The propagation constant and effective relative permittivity have been calculated and found to be nearly same for the both methods with maximum difference of 0.27% and 1.87%, respectively. However, the characteristic impedance values have significant variations as high as 12.13%. Enhanced effective relative permittivity in the range of 3.7 to 4.12 has been observed in the frequency range 1 to 8 GHz, if graphene film on the glass substrate has been loaded on top of the microstrip line. In this value the contribution of stripline substrate includes. Relative permittivity of only monolayer graphene film on the glass substrate is determined in the range of 1.62 to 4.05, and dispersive in nature. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
40. Localized tip enhanced Raman spectroscopic study of impurity incorporated single GaN nanowire in the sub-diffraction limit.
- Author
-
Patsha, Avinash, Dhara, Sandip, and Tyagi, A. K.
- Subjects
RAMAN spectroscopy ,GOLD nanoparticles ,GALLIUM nitride ,NANOWIRES ,CHEMICAL vapor deposition ,METAL inclusions - Abstract
The localized effect of impurities in single GaN nanowires in the sub-diffraction limit is reported using the study of lattice vibrational modes in the evanescent field of Au nanoparticle assisted tip enhanced Raman spectroscopy (TERS). GaN nanowires with the O impurity and the Mg dopants were grown by the chemical vapor deposition technique in the catalyst assisted vapor-liquid-solid process. Symmetry allowed Raman modes of wurtzite GaN are observed for undoped and doped nanowires. Unusually very strong intensity of the non-zone center zone boundary mode is observed for the TERS studies of both the undoped and the Mg doped GaN single nanowires. Surface optical mode of A
1 symmetry is also observed for both the undoped and the Mg doped GaN samples. A strong coupling of longitudinal optical (LO) phonons with free electrons, however, is reported only in the O rich single nanowires with the asymmetric A1 (LO) mode. Study of the local vibration mode shows the presence of Mg as dopant in the single GaN nanowires. [ABSTRACT FROM AUTHOR]- Published
- 2015
- Full Text
- View/download PDF
41. Multilayer graphene as a transparent conducting electrode in silicon heterojunction solar cells.
- Author
-
Patel, Kamlesh and Tyagi, Pawan K.
- Subjects
- *
GRAPHENE , *ELECTRODES , *SOLAR cells , *HETEROJUNCTIONS , *SCHOTTKY effect - Abstract
In this paper, the structure of a graphene/silicon heterojunction solar cell has been studied under simulated conditions. The parameters of the cell's layers have been optimized by using AFORS-HET software. Instead of reported 2D nature, we considered graphene as 3D in nature. To ensure the formation of Schottky junction, electrical contacts were made along c-axis to collect the minority carriers, which generate upon illumination. By optimizing the various parameters of n-type multilayer graphene, we achieved the best-simulated cell with the power conversion efficiency of 7.62 % at room temperature. Up to 40 layers of n-type graphene, the efficiency found to be constant and enhanced only to 7.623 %. After further optimization of the parameters of p-crystalline silicon wafer, a maximum efficiency of 11.23 % has been achieved. Temperature dependence on the cell performance has also been studied and an efficiency of 11.38 % has been achieved at 270 K. Finally, we have demonstrated that n-type multilayer graphene can act as an excellent transparent conducting electrode. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
- View/download PDF
42. Influence of in-plane and bridging oxygen vacancies of SnO2 nanostructures on CH4 sensing at low operating temperatures.
- Author
-
Bonu, Venkataramana, Das, A., Prasad, Arun K., Krishna, Nanda Gopala, Dhara, Sandip, and Tyagi, A. K.
- Subjects
NANOSTRUCTURED materials ,CHEMICAL vapor deposition ,PHOTOLUMINESCENCE ,METHANE ,POLLUTANTS - Abstract
Role of "O" defects in sensing pollutant with nanostructured SnO
2 is not well understood, especially at low temperatures. SnO2 nanoparticles were grown by soft chemistry route followed by subsequent annealing treatment under specific conditions. Nanowires were grown by chemical vapor deposition technique. A systematic photoluminescence (PL) investigation of "O" defects in SnO2 nanostructures revealed a strong correlation between shallow donors created by the in-plane and the bridging "O" vacancies and gas sensing at low temperatures. These SnO2 nanostructures detected methane (CH4 ), a reducing and green house gas at a low temperature of 50 °C. Response of CH4 was found to be strongly dependent on surface defect in comparison to surface to volume ratio. Control over "O" vacancies during the synthesis of SnO2 nanomaterials, as supported by X-ray photoelectron spectroscopy and subsequent elucidation for low temperature sensing are demonstrated. [ABSTRACT FROM AUTHOR]- Published
- 2014
- Full Text
- View/download PDF
43. Electronic, wetting, stability and magnetic properties of graphene.
- Author
-
Banerjee, S., Bhattacharya, D., Panigrahi, S., Bhattacharya, A., Sardar, M., Gayathri, N., Tyagi, A. K., and Raj, Baldev
- Subjects
GRAPHENE ,ATOMIC force microscopy ,STABILITY (Mechanics) ,MAGNETIC properties ,ELECTROMAGNETISM ,MOLECULAR orbitals ,DENSITY functionals - Abstract
We shall present extensive studies on electrical spectroscopy of graphene, wetting properties using frictional studies, ripple formation of graphene sheets at its edges using atomic force microscopy (AFM). We shall address several issues such as: (1) how does the electrical property of the graphene sheet changes when the graphene sheet is displaced by shear forces from over highly oriented pyrolytic graphite (HOPG) substrate? (2) how does the electrical property of the displaced graphene sheet change across it edges? (3) what is its wetting properties at the edges and (4) ripple formation on graphene sheet. We also report some theoretical understanding of graphene using molecular orbital calculations based on density functional theory (DFT). We shall show that ripple formation is the inherent nature of the graphene sheet and provide structural stability to graphene sheet, associated with unique mechanical and electronic properties. Electronic population analysis of the highest occupied molecular orbitals (HOMO) clearly shows that significantly more electronic crowd is located near the vacant site giving rise to magnetism in graphene. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
44. Magnetic Ordering in La1-xCexCrO3 (x = 0.2, 0.8, 1.0) Nanoparticles.
- Author
-
Manna, P. K., Yusuf, S. M., Shukla, R., and Tyagi, A. K.
- Subjects
CERIUM ,CHROMIUM compounds ,NANOPARTICLES ,NEUTRON diffraction ,ANTIFERROMAGNETISM ,DOPED semiconductors ,PEROVSKITE ,RIETVELD refinement - Abstract
We report the exact nature of magnetic ordering in La
1-x Cex CrO3 (x = 0.2, 0.8, 1.0) nanoparticles (diameter: ∼43 nm) using microscopic and macroscopic measurement techniques. The variable temperature neutron diffraction experiments revealed a G-type antiferromagnetic ordering of Cr3+ ions in an orthorhombic perovskite structure with magnetic moment oriented along the crystallographic b-axis. A decrease in Néel temperature has been observed with increasing x, which has been assigned to a decrease in the Cr-O-Cr bond angle obtained from the Rietveld refinement analysis of the neutron diffraction patterns. The observed decrease in lattice parameters with increasing x has been explained in terms lesser ionic radius of Ce3+ (1.34 Å) ion compared to that of La3+ (1.36 Å). [ABSTRACT FROM AUTHOR]- Published
- 2011
- Full Text
- View/download PDF
45. Thermo-analytical Investigations on High Temperature Superconductors of YBCO Family.
- Author
-
Tyagi, Anand K.
- Subjects
- *
HIGH temperature superconductors , *COPPER oxide , *SOLID state electronics , *MATERIALS at high temperatures , *ELECTRONIC materials - Abstract
The article focuses on the thermo-analytical investigations on high temperature superconductors of Yttrium Barium Copper Oxide (YBCO) family. The YBCO had been the most investigated high temperature superconductors (HTSC) system till date due to its application potential, low refrigeration cost and easy characteristics. There had been continuous efforts to improve to have a dense microstructure with large grain size and small pore fraction to ensure better intergrain connectivity.
- Published
- 2010
- Full Text
- View/download PDF
46. Effect of Thermal Treatment on Mechanical Properties of (A384.1)1-x[(Al2O3)p]x Particulate Composites.
- Author
-
Nripjit, Tyagi, A. K., Singh, Jagdev, and Singh, Nirmal
- Subjects
- *
METALLIC composites , *HEAT treatment of metals , *HEATING equipment , *MICROSTRUCTURE , *CONSTITUTION of matter - Abstract
Al and Al alloy based ceramic particulate composites have been identified as futuristic materials for a number of engineering applications. The present work reports on synthesis and characterization of Al2O3 reinforced particulate composites with A 384.1 Al alloy as matrix, keeping the particle size constant at 0.220 μm and the composition (A384.1)1-x[(Al2O3)p]x (x = 0.0 to x = 0.10). This investigation reveals the effect of reinforcement material and heat treatment i.e. ageing (thermal) on processing and characteristics of these metal matrix composites. The results have been explained in terms microstructural aspects and interfacial characteristics of reinforcement and matrix alloy. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
47. Crystal Structure and Cation Exchanging Properties of A Novel Open Framework Phosphate of Ce (IV).
- Author
-
Bevara, Samatha, Achary, S. N., Patwe, S. J., Sinha, A. K., Mishra, R. K., Kumar, Amar, Kaushik, C. P., and Tyagi, A. K.
- Subjects
PHOSPHATES ,CRYSTAL structure ,CATIONS ,ION exchange (Chemistry) ,CERIUM oxides ,SOLUTION (Chemistry) - Abstract
Herein we report preparation, crystal structure and ion exchanging properties of a new phosphate of tetravalent cerium, K
2 Ce(PO4 )2 . A monoclinic structure having framework type arrangement of Ce(PO4 )6 units formed by CeO8 square-antiprism and PO4 tetrahedra is assigned for K2 Ce(PO4 )2 . The K+ ions are occupied in the channels formed by the Ce(PO4 )6 and provide overall charge neutrality. The unique channel type arrangements of the K+ make them exchangeable with other cations. The ion exchanging properties of K2 Ce(PO4 )2 has been investigated by equilibrating with solution of 90Sr followed by radiometric analysis. In optimum conditions, significant exchange of K+ with Sr2+ with Kd ~ 8000 mL/g is observed. The details of crystal structure and ion exchange properties are explained and a plausible mechanism for ion exchange is presented. [ABSTRACT FROM AUTHOR]- Published
- 2016
- Full Text
- View/download PDF
48. Nd2±xZr2+xO7±x/2 (-0.2≤x≤0.4) Complex Oxides: Effect of Anion Disorder on Ionic Conductivity.
- Author
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Anithakumari, P., Grover, V., and Tyagi, A. K.
- Subjects
NEODYMIUM compounds ,HIGH temperatures ,SINTERING ,METAL complexes ,METALLIC oxides ,ANIONS ,IONIC conductivity - Abstract
In the present work, a series of Nd
2±x Zr2+x O7±x/2 (-0.2≤x≤0.4) was prepared by self assisted gel-combution method followed by high temperature sintering at 1673 K. Thorough structural characterizations were done by X-ray diffraction and Raman spectroscopic techniques. The nominal compositions Nd1.6 Zr2.4 O7.2 and Nd1.8 Zr2.2 O7.1 were found to possess single-phasic pyrochlore structure whereas Nd2.0 Zr2.0 O7 and Nd2.2 Zr1.8 O6.9 consisted of a pyrochlore phase and a small amount of hexagonal Nd2O3 as an impurity phase. Electrical behavior of the samples was examined by AC impedance analysis. Even though the activation energies of all the samples are not very different, a high pre-exponential factor for the Nd1.6 Zr2.4 O7.2 composition resulted in high ionic conductivity (3.37 × 10-3 Scm-1 at 773 K). This high ionic conductivity value makes it a superior candidate as an electrolyte material for SOFC applications. [ABSTRACT FROM AUTHOR]- Published
- 2016
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- View/download PDF
49. Raman Spectroscopic Investigation of H3[Co(CN)6]: An Anharmonicity Study.
- Author
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Mishra, K. K., Salke, Nilesh P., Achary, S. N., Tyagi, A. K., and Rao, Rekha
- Subjects
RAMAN spectroscopy ,THERMAL expansion ,TEMPERATURE effect ,ANHARMONIC motion ,TEMPERATURE measurements - Abstract
To investigate the anharmonicity of different phonons in the negative thermal expansion material H
3 [Co(CN)6 ], temperature dependent Raman spectroscopic studies have been carried out in the temperature range 80- 300 K. No discontinuous or slope changes of mode frequencies are observed with temperature implying that H3 [Co(CN)6 ] is stable in the entire temperature range. The temperature dependence of mode frequencies and their linewidths (FWHMs) are analyzed using the anharmonicity models, which indicate that the three-phonon decay process is dominant over the four-phonon process. [ABSTRACT FROM AUTHOR]- Published
- 2016
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- View/download PDF
50. Aluminum induced crystallization of amorphous Ge thin films on insulating substrate.
- Author
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Singh, Ch. Kishan, Tah, T., Sunitha, D. T., Polaki, S. R., Madapu, K. K., Ilango, S., Dash, S., and Tyagi, A. K.
- Subjects
ALUMINUM ,CRYSTALLIZATION ,AMORPHOUS alloys ,METALLIC thin films ,DENDRITIC cells ,RAMAN spectroscopy - Abstract
Aluminium (metal) induced crystallization of amorphous Ge in bilayer and multilayer Ge/Al thin films deposited on quartz substrate at temperature well below the crystallization temperature of bulk Ge is reported. The crystallization of poly-Ge proceeds via formations of dendritic crystalline Ge grains in the Al matrix. The observed phases were characterized by Raman spectroscopy and X-ray diffraction. The microstructure of Al thin film layer was found to have a profound influence on such crystallization process and formation of dendritic grains. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
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