Understanding crystallization in undoped and nitrogen doped GeTe thin films using substrate curvature measurements

Herein, we study the crystallization of undoped and nitrogen doped amorphous GeTe thin films (slightly rich in Ge) obtained by sputtering using substrate curvature measurements to understand the underlying mechanisms controlling stress evolution in the film throughout the phase transformation. At temperatures below crystallization temperature, amorphous films showed stress relaxation and the stress gradually became tensile with annealing time. The GeTe samples show a two-step crystallization wherein amorphous GeTe crystallized first (at the crystallization temperature Tx) followed by crystallization of excess Ge (Ge precipitation) at -Tx+50 °C. Upon GeTe crystallization, a sharp increase in the tensile stress is explained using a coalescence mechanism. This interpretation resolves the issue of the discrepancy between the measured stress buildup reported by several authors and the predicted stress jump from elastic accommodation of density change. The precipitation of excess Ge (from amorphous to crystalline) along grain boundaries in GeTe leads to compressive stress build-up. Nitrogen doping affects both the GeTe and Ge crystallization events leading to lesser tensile and compressive stress. The models for stress relaxation in the amorphous phase, stress build-up due to GeTe, and excess Ge crystallization are discussed

Sn Whisker Growth During Mechanical Loading and Unloading: Highlighting the Critical Role of Stress in Whisker Growth

Whisker formation in metallic coatings has been studied extensively over the past few decades. A wide consensus exists for stress as a driving force behind whisker growth. However, many of the details are not understood, and other theories have been proposed that challenge stress-based explanations. In this paper, we quantify the kinetics of stress-driven whisker growth by demonstrating that the whisker growth can be turned on and off by application and removal of mechanical load. We observed that (i) whiskers formed during the loading stage grow linearly with time, (ii) they stop growing soon after the load was removed, and (iii) they resume growth when the load was reapplied. The experimental results are explained using finite element analysis (FEA) of stress evolution due to applied load and average growth rate predictions. The results have implications for applications in which the coatings experience stresses, for example, due to fasteners or thermal cycling

Macro and Micro-Texture Study for Understanding Whisker Growth in Sn Coatings

The current work is aimed at analyzing the effects of crystallographic texture, both macro and micro-texture, on whisker growth. The macro-texture of the Sn coatings deposited on brass substrate was systematically studied by varying the process parameters used for electro-deposition. The combination of process parameters, such as deposition temperature and current density, and the resulting macro-texture most prone to whisker growth were identified by monitoring the whisker growth in Sn coatings. The electron back-scatter diffraction (EBSD) technique was then used to identify the grains where whiskers actually grow. Both macro-and micro-texture of Sn coatings revealed that coatings with pre-dominant low Miller index grain orientations, such as (100), were highly susceptible for whisker growth. Whisker propensity decreased as the texture transitioned from low to high Miller index grain orientation. The micro-texture mapping of the Sn coatings using EBSD technique confirmed that whisker grows from low Miller index grains with (100) or near (100) orientations, surrounded by grains of similar orientation followed by grains with high index plane grains, such as (211), (210) or (321). This particular double ring configuration along with local stress field can be used to predict the locations for whisker growth

Manipulating Crystallographic Texture of Sn Coatings by Optimization of Electrodeposition Process Conditions to Suppress Growth of Whiskers

The effects of two major electrodeposition process conditions, electrolyte bath temperature and current density, on the microstructure and crystallographic texture of pure tin coatings on brass and, ultimately, on the extent of whisker formation have been examined. The grain size of the deposited coatings increased with increasing electrolyte bath temperature and current density, which significantly affected the dominant texture: (211) or (420) was the dominant texture at low current densities whereas, depending on deposition temperature, (200) or (220) became the dominant texture at high current densities. After deposition, coatings were subjected to different environmental conditions, for example isothermal aging (room temperature, 50A degrees C, or 150A degrees C) for up to 90 days and thermal cycling between -25A degrees C and 85A degrees C for 100 cycles, and whisker growth was studied. The Sn coatings with low Miller index planes, for example (200) and (220), and with moderate aging temperature were more prone to whiskering than coating with high Miller index planes, for example (420), and high aging temperature. A processing route involving the optimum combination of current density and deposition temperature is proposed for suppressing whisker growth

Evaluating shock absorption behavior of small-sized systems under programmable electric field

A simple ball-drop impact tester is developed for studying the dynamic response of hierarchical, complex, small-sized systems and materials. The developed algorithm and set-up have provisions for applying programmable potential difference along the height of a test specimen during an impact loading; this enables us to conduct experiments on various materials and smart structures whose mechanical behavior is sensitive to electric field. The software-hardware system allows not only acquisition of dynamic force-time data at very fast sampling rate (up to 2 x 10(6) samples/s), but also application of a pre-set potential difference (up to +/- 10 V) across a test specimen for a duration determined by feedback from the force-time data. We illustrate the functioning of the set-up by studying the effect of electric field on the energy absorption capability of carbon nanotube foams of 5 x 5 x 1.2 mm(3) size under impact conditions. (C) 2014 AIP Publishing LLC

Effect of electric field on creep and stress-relaxation behavior of carbon nanotube forests

Carbon nanotube forests (CNTFs) are porous ensembles of vertically aligned carbon nanotubes, exhibiting excellent reversible compressibility and electric field tunable stress-strain response. Here, we report the effects of electric field on the time dependent mechanical behavior, namely creep and stress-relaxation, of CNTFs. Creep and stress-relaxation experiments were conducted under constant compressive stress and constant compressive strain, respectively, wherein variation of the strain and the stress, respectively, as functions of time were measured. Creep strain-time data of CNTFs showed a primary creep regime followed by a steady-state creep regime. The creep rate was substantially retarded upon application of electric field. The steady-state strain rate showed a power-law dependence on the stress; however, the stress exponent reduced when an electric field was applied. On other hand, electric field enhanced stress-relaxation in CNTFs, leading to a lower value of stress at a given time. However, the effect of electric field on the stress-relaxation reduced with compressive strain. Based on the Garofalo model of creep, a unified model for explaining the overall time dependent mechanical behavior of CNTFs and the observed experimental results was developed

A study on nanosized certium oxides systems for environmental catalysis

NR 2014080