Browsing by Author "Rajiv Kumar Mandal"

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Aggregation characteristics of Cu and Ag nanoparticles in presence of starch as the polymer stabilizer
(2010) Indrajit Sinha; Manjeet Singh; Rajiv Kumar Mandal
This presentation deals with the aggregation characteristics of Cu and Ag nanoparticles in presence of starch as the polymer stabilizer. Uncontrolled aggregation of the destabilized nanoparticles offers problem for applications based on surface plasmon activity. Polymer or small molecule surfactants are used to control nature of aggregation of nanoparticles produced by chemical reduction synthesis routes. Different growth models such as diffusion limited cluster aggregation (DLCA), reaction limited cluster aggregation (RLCA) proposed to explain the formation of fractal colloidal aggregates do not account for aggregate formation in presence of polymer or small molecule surfactants. We shall be discussing the role of starch on the aggregation characteristics of copper and silver nanoparticles formed by chemical reduction in aqueous conditions. The effect of NaOH concentration and consequently the pH on such aggregation kinetics during such synthesis is delineated. We use small angle x-ray scattering (SAXS) to quantitatively understand different aspects of aggregation behavior. © (2010) Trans Tech Publications.
Compositional dependence of glass-forming ability, mechanical and magnetic properties of Ce-Ni-Al (Ga) alloys
(Elsevier Ltd, 2024) Dharmendra Singh; Devinder Singh; Radhey Shyam Tiwari; Rajiv Kumar Mandal; Parthiban Ramasamy; Jürgen Eckert
The glass-forming ability, mechanical and magnetic properties of Ce60Ni25Al15-xGax (x=0, 4, 8 and 15) melt-spun alloys have been investigated. All the alloys show the formation of glassy phase. The supercooled liquid region increases with increase in Ga addition and becomes maximum for x=15 indicating its better glass-forming ability for this composition. A new metallic glass (MG) composition Ce60Ni25Ga15 has been found upon complete substitution of Al by Ga. The mechanical behavior of these alloys has been studied by determining various indentation parameters such as micro-hardness, yield strength and Meyer's exponent. The microhardness property of the Ce-Ni-Al alloy is improved by the addition of Ga. The difference in the formation of shear bands for x=0 and 15 alloys are discussed by estimating the pile-up parameter. The magnetization (M-H) curves of all the samples at 5 K show paramagnetic characteristics with no magnetic hysteresis. The findings reveal that the concentration of Ga has a significant impact on the properties of these alloys. © 2024 Elsevier B.V.
Diffraction from 1d quasicrystals
(1989) Rajiv Kumar Mandal; Shrikant Lele; Dhananjai Pandey
Levine and Steinhardt’s expression is. generalised for the structure factor of a binary quasicrystal with ATB composition. The atomic model used in this generalisation is based on the embedding of quasiperiodic chains of A and B atoms such that there is aB atom within every long interval of the A-chain. For the proposed embedding scheme, the range of interpenetration length as well as the ratio of long and short intervals in the two chains are shown to affect the diffracted intensities drastically. The effect of order-disorder on the motif, average quasilattice constant and diffracted intensities is also considered. Conditions for the composite chain to become self-similar, which for the most general embedding is not necessary, are obtained. The application of present decoration scheme to vacancy ordered phases is discussed. © 1989, Taylor & Francis Group, LLC. All rights reserved.
Glass based on a single proto-tile and inflation species without discrete diffractogram
(2001) Rajiv Kumar Mandal
The discovery of quasicrystals in the 80s has led to a qualitative change in our understanding about atomic arrangements, including definition of solids having discrete diffractograms. Owing to this, the emphasis has now been shifted to reciprocal space from direct space that was in vogue prior to the seminal finding of quasi-periodic atomic arrangements having non-crystallographic point group symmetry. The purpose of this communication is to cite two specific examples of two-dimensional tiling from literature that do not possess discrete diffractograms, but have subtle distinctions in direct space. It will be shown that one of the above constructs offers the first ever example of an isotropic glass based on a single proto-tile. The second example, on the other hand, displays characteristics which are akin to those of glasses in reciprocal space, but possesses finite rank in direct space. The consequence of these will be discussed in relation to Pisot-Vijayaraghavan (PV) number. It will be concluded that foundation of Copernican crystallography incorporating these is still missing and needs further substantiation.
Icosahedrally related 1D quasicrystals
(1990) Shrikant Lele; Rajiv Kumar Mandal
Taking a cut/section of a hyper lattice has proved a very elegant method of investigating the structure of incommensurate structures including the quasicrystals. We present here a method of generating a one dimensional incommensurate structure by distorting the six dimensional hypercubic lattice and the icosahedral basis vectors a long the 3 fold axis. Indexing of electron diffraction patterns of τ∞ phase has been accomplished on this model and a structure for this phase having 3 symmetry a long the incommensurate direction and periodicity in planes normal to it has been proposed. The requirement of alternating Al and transition metal/vacancy layers in such structures can be satisfied by ordering the hyper unit cell a long three of the six directions. © 1990.
Periodic pentagonal and decagonal quasicrystals
(1989) Rajiv Kumar Mandal; Shrikant Lele
A structure with quasiperiodicity in two dimensions and periodicity in the third has been generated by projection from 6D space. The effect of simultaneously distorting along one axis the 6D cubic cell as well as the icosahedron formed by the projection of the 6D unit vectors onto physical space has been examined for the first time. Some examples are considered to illustrate the generation of structures with P5mm, P105/mcm, and P10/mmm symmetry. © 1989 The American Physical Society.
Rational approximant pentagonal crystals based on icosa-hedral structures
(1989) Rajiv Kumar Mandal; Shrikant Lele
The generation of periodic quasicrystals by the projection method is considered. The effects of the simu-lteneous distortion of the projected icosahedron and 6D hyperlattice are presented. © 1989, Taylor & Francis Group, LLC. All rights reserved.
Simulation of microstructural evolution in electronic glass-ceramics
(Minerals, Metals & Materials Soc (TMS), 1998) Indrajit Sinha; Rajiv Kumar Mandal
It has been recognized that the presence of a connected network of the crystalline phase improves the performance of electronic glass-ceramics. Keeping this in view the theoretical simulation of microstructural evolution of the ceramic phase in a glassy matrix using the static Monte Carlo technique has been done in two and three dimensions. The problem has been approached from the perspective of the percolation theory. A normal distribution of probable sites of the ceramic phase in question has been done and the condition in commensurate with the physical realization that nucleation sites have a low probability of occurrence has been used to identify them.