Anuradha Banerjee

Highly transparent and conducting indium-doped zinc oxide films by spray pyrolysis☆

Abstract Spray pyrolysis has been used to deposit highly transparent and conducting films of indium-doped zinc oxide. The roles of various deposition parameters have been investigated and the optimum deposition conditions have been outlined. Without any post-deposition heat treatment, as-deposited films with a resistivity of about (8 − 9) × 10 -4 Ω cm and an average visible transmittance of about 85% have been obtained. The structural, electrical and optical properties have been studied. The electron transport properties suggest that the films are degenerate and the mobility data can be understood in terms of a grain boundary trapping model.

Annealing studies of undoped and indium-doped films of zinc oxide

Annealing studies in vacuum, oxygen and air ambients were carried out on undoped and indium-doped films of zinc oxide deposited by spray pyrolysis. The effects of annealing on the electrical properties of these films are explained in terms of the chemisorption and desorption of oxygen at the grain boundaries, which in turn lead to the creation or annihilation of extrinsic trap states. The effect of the presence of these trap states on the electronic transport properties of different types of films is explained on the basis of the grain boundary carrier-trapping model.

Thickness-dependent properties of indium-doped ZnO films

The thickness dependence of microstructural, electrical and optical properties of transparent conducting indium-doped ZnO films was studied. The density of trap states due to chemisorbed oxygen at the grain boundaries was found to depend on the orientation of grains which, in turn, depended on the film thickness. The thickness dependence of the electrical parameters of oxygen-annealed films was explained on the basis of changes in the trap state density. The changes in the electrical parameters of vacuum-annealed films were attributed to a decrease in grain size. The shifts in optical gap on vacuum and oxygen annealing were attributed to changes in carrier concentration, considering two simultaneous effects on band gap widening and narrowing.

Optical and electronic properties of zinc oxide films prepared by spray pyrolysis

The optical properties of transparent conducting zinc oxide (ZnO) films prepared by spray pyrolysis were studied in the UV, visible and IR regions. The specular reflectance and transmittance data were used to determine the optical constants which correlate well with the data on single-crystal ZnO in the visible region. The optical behaviour in the IR region was explained on the basis of the Drude model. The electronic properties are found to be thickness dependent. The films doped with 3 at.% In exhibit thermal stability up to 650 K in vacuum and up to 450 K in oxygen ambients. The changes in the electrical properties of pure ZnO films on annealing in oxygen and vacuum are attributed to chemisorption and desorption of oxygen at grain boundaries.

Thermal fracture of interfaces in precracked thermal barrier coatings

Abstract Thermal barrier coatings (TBCs) make it possible to operate gas turbines, aircraft engines and diesel engines at higher temperatures, thus enabling significant improvements in the performance of these systems. In this paper, the effect of introducing surface cracks into plasma sprayed TBCs was studied. The resistance of these precracked coatings to interfacial fracture was determined both experimentally and analytically. In the experiments performed, beam-shaped specimens, with crack densities varying from 0 to 42 cracks inch −1 , were subjected to a high heat flux generated by a 1.5 kW CO 2 laser for a time interval of 4 s followed by natural convection cooling. In each case, the surface temperature which resulted in interface crack initiation was determined. Similarly, the final interfacial crack length corresponding to the different crack densities was measured as a function of maximum surface temperature. In all cases, it was shown that increasing crack density resulted in decreased interfacial cracking at the end of the thermal shock procedure. A numerical model of the experiment was developed using the finite element method and it was used to study the effect of the laser heating process on an interface crack in a precracked TBC system. The analytical results confirmed and explained the experimentally observed behavior.

Electrical and optical transport in undoped and indium-doped zinc oxide films

Electrical conduction in undoped and indium-doped ZnO films in as-deposited, vacuum-annealed and oxygen-annealed states has been studied. The as-deposited and oxygen-annealed films contain a large density (≥ 10 17 m −2 ) of trap states due to chemisorbed oxygen at the grain boundaries. The role of these trap states has been analyzed in terms of the grain boundary carrier trapping model. The vacuum-annealed films are free of chemisorbed oxygen, and the conduction in these films is controlled by scattering due to ionized impurities and grain boundary barriers. In the case of undoped ZnO films, intrinsic trap states at the grain boundaries also play a significant role. The optical behavior of all films in the UV and visible regions is dielectric-like and the optical bandgap shows a dependence on free carrier concentration that is controlled by a bandgap narrowing effect due to electron-electron and electron-impurity interactions as well as the Moss-Burstein effect of bandgap widening. In the IR region the optical behavior is metal-like due to free-electron effects and follows the Drude model.

Mammalian cortical bone in tension is non-Haversian

Cortical bone, found in the central part of long bones like femur, is known to adapt to local mechanical stresses. This adaptation has been linked exclusively with Haversian remodelling involving bone resorption and formation of secondary osteons. Compared to primary/plexiform bone, the Haversian bone has lower stiffness, fatigue strength and fracture toughness, raising the question why nature prefers an adaptation that is detrimental to bones primary function of bearing mechanical stresses. Here, we show that in the goat femur, Haversian remodelling occurs only at locations of high compressive stresses. At locations corresponding to high tensile stresses, we observe a microstructure that is non-Haversian. Compared with primary/plexiform bone, this microstructures mineralisation is significantly higher with a distinctly different spatial pattern. Thus, the Haversian structure is an adaptation only to high compressive stresses rendering its inferior tensile properties irrelevant as the regions with high tensile stresses have a non-Haversian, apparently primary microstructure.

Implementation and validation of a triaxiality dependent cohesive model: experiments and simulations

A recently formulated triaxiality dependent cohesive model for plane strain is implemented and its versatility is tested in simulation of ductile fracture of mild steel at different states of stress. The triaxiality dependent model was implemented as linear displacement formulation based elements whose constitutive behaviour was dependent on the stress-state of the neighbouring continuum element. By comparing the experimental data and predictions of corresponding plane strain simulations, the model parameters are estimated. The model is shown to be effective in reproducing characteristic features of the macroscopic response of both pre-cracked as well as geometries without a preexisting nominal defect. Since the model parameters are held constant for simulations at different stress-states, they are effectively material constants.

Haversian microstructure in bovine femoral cortices: An adaptation for improved compressive strength

Microstructural variations in bovine femoral cortices and its possible implications for the bones mechanical behavior are characterized for a mature and a young bovine femur. Histological examination at several locations shows the presence of Haversian systems to be largely confined to the posterior region of any cross-section. Haversian bone is shown to have higher compressive strength than the non-Haversian primary bone present in the corresponding anterior regions. The anatomical variation in the compressive strength along diaphysis is found to correlate strongly with the Haversian density. Based on the differences in the failure surfaces observed from compressive failure, it is argued that the presence of Haversian systems plays a role in deflection of crack path, leading to non-prismatic failure surfaces. As biomaterials, such as bone cement and implants, closely interact with bone material, the structure-property relation established here can provide a basis for better design of future biomaterials.

Splitting fracture in bovine bone using a porosity-based spring network model

We examine the specific role of the structure of the network of pores in plexiform bone in its fracture behaviour under compression. Computed tomography scan images of the sample pre- and post-compressive failure show the existence of weak planes formed by aligned thin long pores extending through the length. We show that the physics of the fracture process is captured by a two-dimensional random spring network model that reproduces well the macroscopic response and qualitative features of fracture paths obtained experimentally, as well as avalanche statistics seen in recent experiments on porcine bone.