Ratna Kumar Annabattula

Wrinkled-up Nanochannel Networks: Long-Range Ordering, Scalability, and X-ray Investigation

Highly ordered two-dimensional self-organized nanochannel networks as well as free-standing nanomembranes are produced on rigid substrates by means of III-V semiconductor compressively strained layers grown on top of an etchant-sensitive material. The releasing process is controlled by regularly spaced pits obtained from photolithography and a subsequent wet chemical etching. By tuning basic film parameters such as strain and thickness, one obtains periodic arrays of two-dimensional nanochannel networks with symmetries defined by the shape and periodicity of the photolithographic starting pits. Such nanochannel networks with a submicroscale lateral feature size exhibit a surprising flexibility with respect to the crystal lattice symmetry, retaining the original film crystalline quality as confirmed by X-ray grazing-incidence diffraction (GID) measurements. Finite element modeling helps in understanding the particular process of the cross-nanochannel formation.

Thermal Discrete Element Analysis of EU Solid Breeder Blanket Subjected to Neutron Irradiation

Abstract Due to neutron irradiation, solid breeder blankets are subjected to complex thermo-mechanical conditions. Within one breeder unit, the ceramic breeder bed is composed of spherical-shaped lithium orthosilicate pebbles, and as a type of granular material, it exhibits strong coupling between temperature and stress fields. In this paper, we study these thermo-mechanical problems by developing a thermal discrete element method (Thermal-DEM). This proposed simulation tool models each individual ceramic pebble as one element and considers grain-scale thermo-mechanical interactions between elements. A small section of solid breeder pebble bed in a helium-cooled pebble bed (HCPB) is modelled using thousands of individual pebbles and subjected to volumetric heating profiles calculated from neutronics under ITER-relevant conditions. We consider heat transfer at the grain scale between pebbles through both solid-to-solid contacts and the interstitial gas phase, and we calculate stresses arising from thermal expansion of pebbles. The overall effective conductivity of the bed depends on the resulting compressive stress state during the neutronic heating. The Thermal-DEM method proposed in this study provides access to the grain-scale information, which is beneficial for HCPB design and breeder material optimization, and a better understanding of overall thermo-mechanical responses of the breeder units under fusion-relevant conditions.

The effect of interface adhesion on buckling and cracking of hard thin films

The physics behind the strain-released buckling patterns including telephone cords and straight-sided wrinkles with and without cracks, as experimentally observed in sputter-deposited Ti-Si-N thin films on Si substrates, is investigated with model-based simulations by varying the mechanical properties of the interface. Our calculations reveal that the location of the cracks depends on the normal stiffness, the interfacial toughness, and the normal strength of the cohesive interface. These properties determine the geometrical shape of the buckles such as width, wavelength, and deflection, and hence the local bending-induced tensile stresses. Buckling patterns with cracks at the apexes occur for low-stiffness interfaces as well as for high-stiffness interfaces with high toughness. On the other hand, cracks at the bottom of the buckles are more likely to occur for interfaces with high stiffness and low toughness. By using an elastic material model with a fracture criterion for brittle behavior, we demonstrat...

Size-Dependent Crush Analysis of Lithium Orthosilicate Pebbles

Abstract The crushing strength of the breeder material [lithium orthosilicate (Li4SiO4 or OSi)] in the form of pebbles to be used for EU solid breeder concept is investigated. The pebbles are fabricated using a melt-spray method, and hence, a size variation in the pebbles produced is expected. Knowledge of the mechanical integrity (crush strength) of the pebbles is important for a successful design of a breeder blanket. In this paper, we present the experimental results of the crush (failure) loads for spherical OSi pebbles of different diameters ranging from 250um to 800um. The ultimate failure load for each size shows a Weibull distribution. Furthermore, the mean crush load increases with increase in pebble diameter. It is also observed that the level of opacity of the pebble influences the crush load significantly. The experimental data presented in this paper and the associated analysis could possibly help us to develop a framework for simulating a crushable polydisperse pebble assembly using discrete element method.

Micron-scale pattern formation in prestressed polygonal films

In this paper we explore the spontaneous formation of micropatterns in thin prestressed polygonal films using finite element simulations. We study films with different size, thickness, and shape, including square, rectangular, pentagonal, and hexagonal films. Patterns form when the films release the internal eigenstrain by buckling-up, after which the films bond-back to the substrate. After an initial symmetric evolution of the buckling profile, the symmetry of the deflection pattern breaks when the wavelength of wriggles near the film edges decreases. During bond back the deflection morphology converges to a fourfold, fivefold, and sixfold ridging pattern for the square, pentagonal and hexagonal films, respectively, showing a close resemblance with experimental film systems of similar size and shape. Rectangular films of large length to width ratio go through a transition in buckling shapes from the initial Euler mode, through the varicose mode into the antisymmetric telephone-cord mode. For all the film...

Finite Element Modelling of Stress-Induced Fracture in Ti-Si-N Films

Nanocomposite coating films have been increasingly used in industrial applications because of their unique mechanical and physical properties. Residual stresses generated during the growth of sputter-deposited thin films due to a strain mismatch between the film and the substrate may lead to significant failure problems. Large residual stresses may generate buckling, delamination and film fracture. Although buckles with cracks in thin films have been experimentally observed, their origins are still not well understood.