Browsing by Author "P. Ramachandrarao"

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A model for lamellar eutectic solidification
(1987) L. Pandey; P. Ramachandrarao
Interface shape and interface solute concentrations have been obtained for lamellar eutectic solidification on the assumption that the latter is proportional to the deviation of the interface from a plane front. It has been shown that the well known constancy of λ2R and ΔTλ in lamellar eutectics is a consequence of conservation of solute during steady state growth and the need to maintain force balance at the triple points on the interface. The model is applied to the Al-CuAl2 eutectic system and the magnitudes of the Gibbs-Thompson coefficients for the two phases are found. These and the interfacial angles are shown to be in good agreement with experiment. © 1987.
An improved piston-and-Anvil technique for quenching liquid metals
(1970) P. Ramachandrarao; D. Banerjee; T.R. Anantharaman
[No abstract available]
Application of transformation diagrams to predict phase transformation in alloys
(2003) S. Ranganathan; P. Ramachandrarao
Equilibrium phase diagrams give information on the phases present and their compositions at equilibrium in alloy systems. Most metallurgical processes deviate significantly from equilibrium due to constraints imposed by kinetics of heat and mass transfer. The transformation diagrams can be used to study the non-equilibrium phase transformation processes as well as phase equilibria. They can be employed to address several questions which arise during the modeling of the processes but are analysed conventionally through empirical means. For example, these can be used to predict the interface composition in under-cooled melts and similar cases. These can be adopted to predict the conditions under which meta-stable phases are formed. The construction of these diagrams is similar to that of the conventional phase diagrams. They can be easily constructed using conventional data bases on the thermodynamics of alloys. The construction of these diagrams, their salient features and application are discussed.
Bamboo - A functionally graded composite-correlation between microstructure and mechanical strength
(2005) A.K. Ray; S. Mondal; S.K. Das; P. Ramachandrarao
Bamboo is supposed to be one of the best functionally gradient composite materials available. In a piece of bamboo, not only the number of fibres ('vascular bundles') but also the fibre quality varies from outer to inner-most periphery. It has been observed that near the outer periphery, within 1 mm 2 area, the number of fibres is approximately 8 whereas the same at the inner-most periphery is approximately 2. Again the cross-sectional shape of fibre at outer periphery is almost circular (diameter 0.14 mm) and compacted but at the inner-most periphery, a fibre (diameter of major axis 0.93 × diameter of minor axis 0.78 mm) has been sprayed, and contain matrix in it. This structural behaviour causes the variation of tensile strength, e.g., the strength of a fibre at the outer periphery is about 160 kg/mm2 and the same at the inner-most periphery is only 45 kg/mm2. It has also been observed that the matrix of bamboo can preferentially be removed from the fibre by alkali treatment. 10% NaOH can remove adhered matrix with little effect on fibres while 20% or stronger alkali reduces the strength of fibre. © 2005 Springer Science + Business Media, Inc.
Biomimetic synthesis of superparamagnetic iron oxide particles in proteins
(Materials Research Society, 2003) Arvind Sinha; Suprabha Nayar; G.V.S. Murthy; P.A. Joy; V. Rao; P. Ramachandrarao
Matrix-mediated in situ synthesis of monodispersed magnetite and maghemite nanoparticles (2-16 nm) was carried out using the cavities present in gels of globular proteins such as egg white and bovine serum albumin. Under stringent conditions, spatial-charge-distribution-assisted molecular recognition of proteins for inorganic ions led to the site- and polymorph-specific synthesis of superparamagnetic iron oxide particles. A transformation from magnetite to maghemite as a nucleating phase could be observed by partially denaturing the egg white protein, signifying the delicate role of quaternary structure of proteins under different reaction conditions, in determining the size and shape of the polymorph.
Characterization of antiplatelet properties of silver nanoparticles
(2009) Siddhartha Shrivastava; Tanmay Bera; Sunil K. Singh; Gajendra Singh; P. Ramachandrarao; Debabrata Dash
Thrombotic disorders have emerged as serious threat to society. As anticoagulant and thrombolytic therapies are usually associated with serious bleeding complications, the focus has now shifted to regulating and maintaining platelets in an inactive state. In the present study we show that nanosilver has an innate antiplatelet property and effectively prevents integrin-mediated platelet responses, both in vivo and in vitro, in a concentration-dependent manner. Ultrastructural studies show that nanosilver accumulates within platelet granules and reduces interplatelet proximity. Our findings further suggest that these nanoparticles do not confer any lytic effect on platelets and thus hold potential to be promoted as antiplatelet/antithrombotic agents after careful evaluation of toxic effects. © 2009 American Chemical Society.
Characterization of biomimetically synthesized Hap-Gel nanocomposites as bone substitute
(2008) Tanmay Bera; A.N. Vivek; S.K. Saraf; P. Ramachandrarao
There is an increasing demand for an affordable and easy-to-fabricate material to help patients having a long bone gap. In this paper, we describe the biomimetic synthesis of Hap-Gel in situ nanocomposite powders with varied proportions. Their biocompatibility and bone regeneration abilities were assessed on a rabbit model. The use of Hap crystals and Gel molecule, the soluble form of bone protein, makes the nanocomposites comparable to natural bone in constituents. The application of biomimetic principles improves crystal morphology and the interaction of Hap crystals with the Gel molecules as seen through in vitro characterizations. Out of the various compositions studied, one with 80:20 proportions of Hap to Gel proved to be closest to the characteristics of natural bone. The immunological response to this composite, assessed through intradermal inoculation, did not reveal any reaction. The in vivo implantation studies in the femoral condyle of the animals, as assessed by serial post-operative follow-up radiography and the histological evaluation, revealed a good biocompatibility and bone-regeneration ability of the material. Thus, nanocomposites of Hap-Gel have a great potential for serving as an effective and affordable biomaterial for bone grafting applications. © 2008 IOP Publishing Ltd.
Characterization of enhanced antibacterial effects of novel silver nanoparticles
(Institute of Physics Publishing, 2007) Siddhartha Shrivastava; Tanmay Bera; Arnab Roy; Gajendra Singh; P. Ramachandrarao; Debabrata Dash
In the present study, we report the preparation of silver nanoparticles in the range of 10-15 nm with increased stability and enhanced anti-bacterial potency. The morphology of the nanoparticles was characterized by transmission electron microscopy. The antibacterial effect of silver nanoparticles used in this study was found to be far more potent than that described in the earlier reports. This effect was dose dependent and was more pronounced against gram-negative bacteria than gram-positive organisms. Although bacterial cell lysis could be one of the reasons for the observed antibacterial property, nanoparticles also modulated the phosphotyrosine profile of putative bacterial peptides, which could thus affect bacterial signal transduction and inhibit the growth of the organisms. © IOP Publishing Ltd.
Diffuse Phase Transition in (Ba, Ca) TiO3Ceramics
(1987) Dhananjai Pandey; V.S. Tiwari; T.B. Singh; Pandey Lakshman; Om Parkash; P. Ramachandrarao
Dielectric behaviour of (Ba, Ca) TiO3ceramics prepared by conventional and a novel technique has been studied as function of temperature and frequency. It is shown that by ensuring uniform distribution of Ca2+ions in the BaTiO3matrix, the transition from the ferroelectric to the paraelectric phase becomes more diffuse and is free from any relaxational effects at least up to 10 KHz. It is suggested that the relaxor behaviour of ceramics exhibiting diffuse phase transitions is due to the limitations of the processing techniques and is not the intrinsic property of the material. © 1987, Taylor & Francis Group, LLC. All rights reserved.
Diffusion kinetics and some aspects of solidification under microgravity environment
(Springer India, 1982) P. Ramachandrarao
The paper surveys the available literature on the direct influence of microgravity on diffusion in liquid metals, rate of solidification, growth of dendrites, undercooling of liquid metals and alloys and monotectic solidification. Agreement between theoretical predictions and experimental observations is discussed critically and areas requiring further study are highlighted. © 1982 The Indian Academy of Sciences.
Entropy and free energy at the hcp-bcc transformation temperature in metals and alloys
(Springer-Verlag, 1980) P. Ramachandrarao; S. Lele
It has been shown that a plot of the Φ function, defined by - (G tr - Go)/Ttr where Gtr and G o are respectively the Gibb's free energies at the hcp → bcc allotropie transformation temperature (T tr ) and at absolute zero, vs the entropy (Str) of the hcp phase at Ttr is linear. This concept of the Φ function and its relation to S tr can be extended to alloys with the aid of the 'central atoms' solution model. Its use has been demonstrated with respect to the Ti-Zr system where the estimated transformation temperatures are shown to be within the (hcp + bcc) phase field. © 1980 American Society for Metals and The Metallurgical Society of AIME.
Estimation of complex concentration in a regular associated solution
(Springer-Verlag, 1981) S. Lele; P. Ramachandrarao
Novel procedures have been developed to estimate the concentration of A pB -type clusters in liquid alloys with the aid of regular associated solution model utilizing activity coefficients of the components at infinite dilution and activity data at any one other composition. The equilibrium constant for the dissociation of clusters as well as the nature and magnitude of pairwise interaction energies between unassociated atoms and clusters have been evaluated without making any assumptions whatsoever. These enable estimation of thermodynamic properties of the liquid alloys at any other desired composition. The utility of the procedures has been demonstrated with respect to the thermodynamics of association in molten Mg-Sn and In-Sb alloys. The calculated activities, enthalpies and free energies are shown to be in excellent agreement with experimental values. © 1981 American Society for Metals and the Metallurgical Society of AIME.
Estimation of the free energy of undercooled oxide melts
(1989) K.S. Dubey; P. Ramachandrarao; S. Lele
The difference in free energy between a liquid and a solid phase (ΔG) is an important parameter in nucleation theory. We show that a novel expression derived by a Taylor series expansion of ΔG around its value at the melting temperature describes accurately the ΔG of oxide melts as a function of degree of undercooling. This is demonstrated for a number of pure (SiO2, GeO2, and B2O3) and compound oxides.
Estimation of the heat of fusion of a eutectic mixture from phase diagram data
(1992) P. Ramachandrarao; K.S. Dubey; Mamta Saroch
Simple methods are presented for the estimation of the heat of fusion of a eutectic mixture from the entropy of formation of constituent phases with the aid of data obtainable from the binary phase diagrams. The suggested methods do not require any thermodynamic information concerning the terminal solid solutions if any one of them has negligible solid solubility. In such cases the present methods also do not require thermodynamic information concerning the liquid phase. The procedures are tested with respect to a number of eutectic systems based on simple metals and intermetallic compounds. Results obtained are in excellent agreement with values obtained from direct experiments.
ESTIMATION OF THE MODEL PARAMETERS OF A REGULAR ASSOCIATED SOLUTION: PART I THEORY.
(1985) S. Lele; G.V.S. Sastry; P. Ramachandrarao
The basic equations for the thermodynamic properties of regular associated solutions are presented. Equations for the partial excess free energies and enthalpies are considered at infinite dilution and shown to be related to the interchange energies and equilibrium constant for the formation of the complex. Procedures are indicated for evaluation of model parameters and complex concentration from known thermodynamic data. The detailed procedure for analysis of the data depends on stoichiometry and an accurate analysis is possible only for AB or A//2B associates.
EVALUATION OF THE IDEAL GLASS TRANSITION TEMPERATURE AND OTHER THERMODYNAMIC PROPERTIES OF GLASS FORMING LIQUIDS.
(1986) S.P. Pandey; K.S. Dubey; P. Ramachandrarao
An earlier hole theoretical model of the liquid state developed by the authors is modified to reduce the number of parameters required to describe the thermodynamic properties. It has been shown that good agreement can be achieved between observed and calculated Gibb's free energy change accompanying freezing at large undercooling for a variety of substances. Estimates of the Kauzmann glass transition temperatures of these substances also agree well with those extrapolated from relaxation and thermodynamic data. The present analysis simplifies the procedures for obtaining the viscous behavior and time-temperature transformation diagrams for glass forming substances.
Formation of faulted close-packed structures in silver-germanium alloys quenched from the Melt
(1969) P. Ramachandrarao; T.R. Anantharaman
A series of silver—germanium alloys containing up to 25 at. % Ge was rapidly cooled from the liquid state by the Duwez technique and subjected to x-ray examination. The Debye-Scherrer patterns of the liquid-quenched alloys provide the first experimental proof for the incidence of light to heavy faulting in f.c.c. and h.c.p. phases on solidification. © 1969 Taylor & Francis Group, LLC.
Free energies of undercooled phases
(1989) P. Ramachandrarao; K.S. Dubey; S. Lele
A novel approach for the estimation of the free energies of metastable liquid and allotropie phases has been presented. The method is based on the Taylor series expansion of the free energy différence between stable and metastable phases. The procedure has been applied to arrive at allotropie transformation temperatures in iron and also the isentropic temperature for any two coexisting phases. Results obtained strongly validate the expressions derived which involve only the use of equilibrium heat capacity and entropy data. © 1989.
HIGH-STRENGTH ALUMINIUM ALLOYS THROUGH RAPID SOLIDIFICATION PROCESSING.
(Trans Tech Publ, 1985) C. Suryanarayana; P. Ramachandrarao; T.R. Anantharaman
The techniques of rapid solidification are over two decades old and their beneficial effects, viz. , refinement of grain size, extension of solid solubility limits and formation of metastable intermediate phases as well as metallic glasses are now well established. After reviewing the various principles and techniques available to quench or rapidly solidify metallic melts at very high cooling rates, methods of consolidating the rapidly-solidified particulates are described. The types of microstructures developed and their control are discussed. A critical review of the literature reveals that the above-mentioned metastable effects of rapid solidification either singly or jointly lead to improved properties of the materials. New alloys can now be designed from first principles to achieve high specific strength and high specific modulus of elasticity. The development work done in aluminium alloys is reviewed with special reference to the Varanasi investigations.
Magnetic field-induced biomimetic synthesis of superparamagnetic poly (vinyl alcohol)-maghemite composite
(2004) Arvind Sinha; Jui Chakraborty; P.A. Joy; P. Ramachandrarao
Poly (vinyl alcohol)-mediated synthesis of monodisperse, self-assembled, superparamagnetic maghemite particles was carried out through a magnetic field-induced biomimetic route. Modifying the kinetics of precipitation, the magnetic field promoted the nucleation of the maghemite phase over magnetite and also induced a self-assembly-assisted shape anisotropy during the precipitation of the particles in the polymer matrix. © 2004 Materials Research Society.