Synthesis of single phase nano BiFeO3 particles and its irradiation studies
BiFeO3 (BFO) is a multiferroic material exhibiting both ferroelectric and antiferromagnetic ordering that has rendered it to a new dimension of multifunctionality. The remarkable property of multiferroic materials stems from the magnetoelectric effect i.e. the coupling between magnetic and electric ordering. Sol-gel technique has been adapted to get the nano BFO particles of ~30nm. BFO nanoparticles have been subjected to irradiation with 35MeV a-particles available at Variable Energy Cyclotron. Irradiation induced defects have been characterized by PAS. Magnetic & electrical polarization studies are going on. 

Understanding of the basic mechanisms of the plastic instability in Portevin-Le Chatelier effect
Many materials when subjected to deformation exhibits unstable plastic flow beyond the elastic limit. In certain range of strain rates and temperatures many solid solutions, both substitutional and interstitial, exhibit serrated yielding which is also referred as the Portevin-Le Chatelier (PLC) effect in literature. The origin of the PLC effect stems from the interaction of dislocations with solute atoms. Dislocation is the line defect within the crystal structure, along which some atoms are misaligned. In metallic alloys, the added solute atoms have their own strain fields. These strain fields interact with the strain field of the dislocations through various mechanisms and under certain experimental conditions the collective movement of the dislocations translates into PLC instabilities. The PLC effect is characterized by inhomogeneous deformation and results in serrated stress-strain characteristics as shown in Fig.1. The strains are localized as deformation bands and with increasing strain rate and/or decreasing temperature, the band character changes from static type C to hopping type B and finally to continuously propagating type A. 

Fig. 1

 

Fig. 2 

The intriguing spatio-temporal dynamics of the PLC effect has fascinated the researchers over several past decades. Moreover, it has some detrimental effects on the mechanical properties of the material. Hence, a huge amount of works are being done to understand the underlying dynamics of this phenomenon. In our section, several techniques of nonlinear dynamics and time series analyses have been adopted to study the dynamics of the PLC effect in different band regime indirectly from the stress-time series data. The tensile tests are performed on substitutional Al-2.5%Mg alloy and interstitial low carbon steel using the Universal tensile Testing Machine INSTRON over a wide range of strain rate. Analyses of the stress-time data reveals that the PLC dynamics in type A band regime has the memory less Markov property and the effective dimension of the dynamics is found to decrease with strain as shown in Fig.2. In the type B band regime, PLC dynamics exhibits deterministic chaos. The differences in relative positions of the solutes in substitutional and interstitial alloys are reflected in difference in the degrees of freedom of the dynamics. Moreover, a transition regime from type A to type B band could be identified and further analyses revealed that both type A and type B band does not exist simultaneously, instead a single band changes its character during deformation. Even though the band characters are different in different experimental conditions, the stress data recorded during the PLC effect is a macroscopic output of the dynamics. Thus the data should possess some general characteristics independent of the strain rate. This motivated  to search for a common feature in the PLC dynamics with imposed strain rate and two different techniques of time series analysis were adopted for this purpose: (1) Scaling analysis and (2) Quantification of complexity. The result of scaling analyses clearly suggests that the scaling behavior of the overall dynamics of the PLC effect at all strain rates follow Levy-walk property. Fig.3 exhibits the scaling nature of the Standard deviation Analysis (SDA) and Diffusion entropy Analysis (DEA) of the stress vs. time data obtained from Al-2.5%Mg alloy during tensile deformation at a strain rate of 3.85x10-4 Sec-1.

 

Fig. 3 

The complexity of the PLC dynamics is estimated through Multi Scale Entropy (MSE) analysis and it is observed that MSE analyses can be successfully employed to identify the type A and type B band regime. The transition regime could also be identified with this technique.

Study of the dynamics of dislocation by molecular dynamics simulation techniques

A. Lattice resistance at the nanoscale: Mechanical and carrier transport properties of thin films are extensively dependent on the structure, distribution and dynamics of dislocations present in them. Particularly in nanomaterials, the surface should play a vital role in dictating this dynamics as the dislocations are too close to the surfaces. A dislocation can move under the influence of a shear load, thereby causing the plastic strain in the crystal. Dynamics of dislocations are governed by drag forces including lattice resistance and other drags, although the effect of surfaces on mobility of dislocations has not been in focus of study so far. 

We propose a model [PRL 101, 115506 (2008)] that reveals a prominent change in the velocity of a dislocation due to the presence of a free surface in the proximity of the dislocation line in a finite nanoscale crystalline solid. This effect has been attributed to the altered lattice resistance to dislocation motion in different system configurations. To verify this finding, MD simulations for an edge dislocation in bcc Molybdenum (Mo) are performed and the results (Fig. 1) are found to be in agreement with the numerical implementations of this model. The reduction in this effect at higher stresses and temperatures, as revealed by the simulations, confirms the role of lattice resistance behind the observed change in the dislocation velocity. We think that such an alteration in the velocity of dislocation would play a significant role in deciding the mechanical properties in thin films.

Fig. 1 : MD simulation output with fitted profile  

B. Study of dynamics of dislocation pinning: The phenomenon of dislocation pinning is vital to the process of plastic deformation and dictates the mechanical strength of a crystalline solid. A moving dislocation can get pinned upon its interaction with obstacles like point defects, voids, precipitates and other dislocations. In this context, a substantial amount of research in dislocation science explicitly deals with the process of depinning and its relation with the nature of the obstacles. Owing to its effective mass, a moving dislocation is associated with certain momentum and kinetic energy, which is known to dissipate after it gets pinned. However, the dynamics of pinning at ultrafine scales of length and time are still unexplored. 

We harness the techniques of MD simulations to reveal the dynamics of dislocation pinning and report for the first time, observations of damped oscillations [Phys. Rev. B 82, 184113 (2010)] of pinned dislocation segments under static shear load (refer Figure 2). Studies are performed in two different systems, namely Molybdenum (Mo) and Tungsten (W). We have also used Koehler’s vibrating string model as a mathematical tool to analyze the simulation output. The combined approach of MD simulation and analytical solution reveal significant features of dislocation dynamics. Our MD results reveal that the relation between the oscillation frequency and the link length significantly differs from that predicted using the framework of the celebrated Koehler-Granato-Lücke theory, which leads us to modify the model assuming coupled oscillations of dislocation segments in contrast to the conventional idea of independent oscillations. We would like to point out that the reported oscillations may solve the longstanding mystery of electromagnetic emission occurring during the plastic deformation and earthquakes. 

 Fig. 2 : Snapshots of dislocation void interactions with oscillations  

C. Study on void induced dislocation climb (VIC) : The phenomenon of dislocation climb is important in context of irradiation creep and existing theories of climb are formulated in the framework of diffusion based kinetics. With advent of simulation techniques, a new kind of climb by the gliding dislocations at nanovoids has been recently observed in both molecular statics (MS) and molecular dynamics (MD) simulations and this process seems to be much faster. This is known as void induced climb (VIC) and we attempt to explore the underlying mechanism of this void-induced climb (refer Figure 3) in bcc systems.       

 Fig. 3 : A segment of dislocation if found to climb at a nano-void

In this work, we present a novel simulation strategy, which estimates the energies associated with the void-induced climb of dislocations. The results highlight that the curvature of the pinned dislocation segment plays a key role in mediating this climb. The lowering of critical depinning load and the effect of thermal assistance to void-induced climb is also explained. Our study reveals that the kinetics of this climb process is fundamentally distinct from the conventional diffusion-controlled climb. [Acta Materialia 60, 3789 (2012)] 

 

 Contact person : Dr. Mishreyee Bhattacharya

                        mishreyee[at]vecc.gov.in 

Developing irradiation facilities in the Materials Science beamline of the DAE Medical Cyclotron
The high intensity proton beams from the DAE Medical Cyclotron (Energy : 15 MeV to 30 MeV and Current: upto 350 micro-amp) provides a unique facility for radiation damage studies on nuclear materials, as energetic charged particles are useful for simulating the bulk damage induced by fast neutrons. For example 20 MeV protons with 350 microampere current will produce in stainless steel a damage of 2x10-5dpa/sec over a sample thickness of about 0.7mm. This is higher than the damage rate produced in fast reactors (~10-6dpa/sec). Due to the high energy available from this Medical cyclotron, thick samples of the order of 0.5 to 1 mm can be irradiated. This makes the post irradiation investigation of the samples by a variety of bulk techniques like X-ray, Positron lifetime, mechanical property measurements feasible. The main interest will be in studying the irradiation effects in Fast Breeder Reactor structural materials like D9, D9I and ferritic steels. Some of the important studies that will be carried out are (1) Ductile to Brittle Transition in Ferritic Steels, (2) Development of Void swelling resistant steels, (3) Phase Stability under irradiation in advanced austenitic steels etc. Apart from the above utilization for reactor materials, studying basic damage mechanisms will also be of importance for the better understanding of radiation effects in materials. 

Another major area of research that will be carried out in the materials science beam line is the induced radioactivity studies using the proton beam available at the DAE Medical Cyclotron. (1) Thin Layer Activation Analysis of nano coatings, (2) Production of Special Isotopes for use as sources in various experiments like PAS, PACS etc., (3) Study of Mass, Charge and Angular Momentum distribution of fission products in proton induced fission of actinides etc. are some of the experiments that are proposed to be carried out. 

The various experiments to be carried out will require different energies and currents. Taking into consideration the radiological safety, the Materials Science activities will be carried out in two different chambers depending on the energies and currents to be utilized for irradiation (as shown in the diagram). The chamber CH1 will be mainly used for high dose experiments where the currents will be of the order of 200 micro-amp. The low dose experiments up to about 50micro-amp beam current will be carried out at CH2. The development of the irradiation facilities include  

1. Design of general purpose target chamber for irradiation 

2. Development of target cooling facilities using Helium and water

3. Development of target handling facilities for handling high dose samples (including shielding wall arrangement and master-slave manipulator, Pb-cask for sample storage)

4. Safety and control of irradiation experiments 

 

Characterization of microstructure of deformed and irradiated nuclear structural materials by X-ray Diffraction technique
Microstructural characterization by X-ray diffraction technique are being carried out on nuclear structural materials such as Zirconium based alloys (Zircaloy-2, Zr-2.5%Nb and Zirlo), SS316L and D9 (Ti-modified austenitic stainless steel) in which defects have been introduced by either cold deformation (by hand-filing and cold-rolling) or by irradiation with charged particles (light ions and heavy ions from Variable Energy Cyclotron and ECR source).  

Nanotube Poly(3,4-ethylenedioxythiophene)
Nanotubular structure of poly(3,4-ethylenedioxythiophene)–NiFe2O4 nanocomposites have been synthesized in reverse microemulsion polymerization technique with the nanotube size ~20nm as characterized by Transmission Electron Microscope. The surface capacitance of this nanotubular material has been quite high. 

Magnetic Behavior of Template Grown 2-D Array of Cobalt Nanowire:
The arrays of cobalt nanowires with diameter 50, 150 and 275 nm were prepared by dc electrodeposition technique. Magnetization study showed the change in magnetic easy axis from axial to perpendicular direction as one increased the length of 50 and 150 nm wire.

Synthesis and Magnetic Properties of Template Grown Nickel Nanowires:
Nickel nanowires in the pores of the alumina membranes have been developed by electro-deposition technique. The magnetic properties of the nanowires are changing sensitively with the length and the diameter of the nanowires. Competition between the shape anisotropy, magneto-crystalline anisotropy and strong magneto-static interaction between the nanowires has been observed. 

Studies on Co-doped ZnO Nanoparticles:
Semiconducting oxides like ZnO display magnetism when doped (or, diluted) with magnetic transition elements. There is a phenomenon of polarization of charge carriers in the semiconductor by the spin of the magnetic elements doped. They are the candidates of the new generation Spintronics, i.e. spin coupled with electronics. ZnO doped with various atomic percentages of the transition metal Co (1-20 at.%) has been synthesized by wet chemical technique employing Zinc & Cobalt acetates. Particle sizes were in the range 60-100nm. 

Ion Irradiation on ZnO thin films:
Magnetism can be induced in nontransitional metal oxides through the generation of defects introduced by ion irradiation. we undertook the studies on Argon ion irradiation on nontransitional semiconducting ZnO thin films.grown by RF sputtering at VECC. Thickness of the films were in the range 80-90nm.150 keV Ar (9+) ions were used with a range of 90nm. An onset of magnetism has been observed for the film irradiated at the fluence of 1x1016ions/cm2 evident from magnetization versus temperature plot. 

Structural and electrical studies of low energy Ar9+ irradiated conducting polymer PANI-PVA using ECR Ion source:
Polyaniline (PANI) of high stability prepared in acidic aqueous solution using water soluble support polymer polyvinyl alcohol (PVA) was subjected to irradiation with 150keV Argon (Ar9+) ion. The dielectric constants are more for the Ar-implanted polymer than the unirradiated one. The dielectric loss is more for the implanted PANI rendering it useful for electromagnetic shielding.  

Structural and impedance spectroscopy of perovskite barium substituted lead zinc niobate
Relaxor ferroelectric materials with a broad transition in dielectric constant have enormous applications in the field of actuators, transformers etc. Lead Zinc Niobate [Pb(Zn1/3Nb2/3)O3] (PZN) is a well known relaxor. But it is Pb-based and is likely to be toxic. To remove Pb, the complex perovskite lead barium zinc niobate, (Pb1-xBax)(Zn1/3Nb2/3)O3 ceramic at x = 0.25 [(Pb0.75Ba0.25)(Zn1/3Nb2/3)O3] was prepared by a columbite precursor method.