Fracture of silicon at low length scales

At small length scales, perhaps no material is more industrially important than silicon. It enabled the information age, and micro-electro-mechanical systems (MEMS) made of silicon are increasingly integrated into our daily lives via smartphones. Classically, silicon is known as a brittle material, whose sharp brittle-ductile transition (BDT) occurs within a matter of one or two degrees Celsius at a temperature between 500 and 800 °C depending on the microstructure, strain rate, and crystal orientation [1]. However, recent advances in sample miniturization has revealed that plastic compressive deformation can occur in silicon at room temperature if the sample size is reduced below 400 nm [2]. This raised the question of whether silicon’s intrinsic fracture toughness also changed at reduced length scales. The development of many new micro-geometries for measurement of fracture toughness allowed this question to be comprehensively answered for the micron length scale – with the answer being no [3]. However, this didn’t necessitate that the BDT was unaffected. Please click Additional Files below to see the full abstract

Nanoindentation Under Dynamic Conditions

Nanoindentation has emerged as a leading technique for the investigation of mechanical properties on small volumes of material. Extensive progress has been made in the last 20 years in refining the nstrumentation of nanoindentation systems and in analysis of the resulting data. Recent development has enabled investigation of materials under several dynamic conditions. The palladium-hydrogen system has a large miscibility gap, where the palladium lattice rapidly expands to form a hydrogen-rich β phase upon hydrogenation. Nanoindentation was used to investigate the mechanical effects of these transformations on foils of palladium. Study of palladium foils, which had been cycled through hydrogenation and dehydrogenation, allowed the extent of the transformed region to be determined. Unstable palladium foils, which had been hydrogenated and were subject to dynamic hydrogen loss, displayed significant hardening in the regions which were not expected to have transformed. The reason for this remains unclear. Impact indentation, where the indenter encounters the sample at relatively high speeds, can be used to probe the strain rate dependence of materials. By combining impact indentation and elevated temperature indentation, the strain rate dependence of the superelasticity of nickel-titanium was probed over a range of temperatures. Similar trends in elastic energy ratios with temperature were observed with the largest elastic proportions occurring at the Austenite finish transformation temperature. Multiple impact and scratch indentation are two modes of indentation which are thought to approximate erosive and abrasive wear mechanisms, respectively. These were utilised to investigate the wear resistance of several novel coatings formed by plasma electrolytic oxidation (PEO) of Ti-6Al4-V. Multiple impact indentation results appear to subjectively rank the erosive wear performance of both ductile and brittle materials. Comparison of normalised performance of coating systems on aluminium in abrasive wear to scratch hardness showed similar degrees of resistance.This material is based upon work supported under a National Science Foundation Graduate Research Fellowship. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Additional support for this work was provided by funding from the Atomic Weapons Establishment

Economic Efficiency of Short-Term Versus Long-Term Water Rights Buyouts

Because of the decline of the Ogallala Aquifer, water districts, regional water managers, and state water officers are becoming increasingly interested in conservation policies. This study evaluates both short-term and long-term water rights buyout policies. This research develops dynamic production functions for the major crops in the Texas Panhandle. The production functions are incorporated into optimal temporal allocation models that project annual producer behavior, crop choices, water use, and aquifer declines over 60 years. Results suggest that long-term buyouts may be more economically efficient than short-term buyouts.dynamic production function, nonlinear optimization, Ogallala Aquifer, water rights buyout, Agribusiness, Environmental Economics and Policy, Q30, Q32, Q38,

Possible detection of singly-ionized oxygen in the Type Ia SN 2010kg

We present direct spectroscopic modeling of 11 high-S/N observed spectra of the Type Ia SN 2010kg, taken between -10 and +5 days with respect to B-maximum. The synthetic spectra, calculated with the SYN++ code, span the range between 4100 and 8500 \r{A}. Our results are in good agreement with previous findings for other Type Ia SNe. Most of the spectral features are formed at or close to the photosphere, but some ions, like Fe II and Mg II, also form features at ~2000 - 5000 km s$^{-1}$ above the photosphere. The well-known high-velocity features of the Ca II IR-triplet as well as Si II $\lambda$6355 are also detected. The single absorption feature at ~4400 \r{A}, which usually has been identified as due to Si III, is poorly fit with Si III in SN 2010kg. We find that the fit can be improved by assuming that this feature is due to either C III or O II, located in the outermost part of the ejecta, ~4000 - 5000 km s$^{-1}$ above the photosphere. Since the presence of C III is unlikely, because of the lack of the necessary excitation/ionization conditions in the outer ejecta, we identify this feature as due to O II. The simultaneous presence of O I and O II is in good agreement with the optical depth calculations and the temperature distribution in the ejecta of SN 2010kg. This could be the first identification of singly ionized oxygen in a Type Ia SN atmosphere.Comment: Submitted to MNRA

Probing the limits of strength in diamonds: From single- and nano-crystalline to diamond-like-carbon (DLC)

As the hardest known material, diamond represents the benchmark for the ultimate strength of materials. It is thus a very attractive material for a number of mechanical applications. Recent advances in synthesis techniques have enabled the fabrication of diamond in thin film form with various microstructures: single- and nano-crystalline and tetrahedral-amorphous or diamond-like carbon (DLC) [1, 2]. Microcompression has been demonstrated to enable the interrogation of even the strongest form of diamond - a -oriented single crystal - achieving the strength limit predicted by simulations (Figure 1) [3, 4]. Nowadays, these allotropes of carbon with high strength and low friction are used in microelectronics and micro-electromechanical systems (MEMS) as structure components [5]. However, the effects of these new nanostructures on the mechanical properties of these allotropes is mostly unknown especially at different service temperatures. In this study, the mechanical properties of single crystalline, nanocrystalline, and amorphous forms of diamond are systematically studied by conducting in situ microcompression at various temperatures in scanning electron microscope (SEM). This allows the investigation of thermally-activated defect behavior and activation energy for several different nanostructures of diamond. This is then correlated with the deformed structures using high resolution transmission electron microscope (HRTEM) and Raman spectroscopy to interpret the deformation mechanisms. Please click Additional Files below to see the full abstract

Plasticity and size effects in germanium: From cryogenic to elevated temperatures

Germanium is extensively used as a substrate in functional components of devices and microelectromechanical systems (MEMS) because of its tunable band structure and carrier mobility via. strain engineering [1]. The mechanical properties of Ge with a diamond-cubic structure at such small scales, i.e. in range of micro/nano-meter, are expected to be extraordinary since the improved strength and ductility of brittle crystals with minimized geometries [2]. Recent advances in nano-mechanical testing systems enable the investigation of the size- and temperature-dependent deformation behaviors and relevant parameters [3]. In the present study, micro-compression of FIB-machined micropillars is conducted to obtain a thorough understanding of the plasticity and size effects of Ge from cryogenic to elevated temperatures, i.e. in the range of -100°C to 600°C, shown in Figure 1(a). Dislocation motion in Ge is quantitatively evaluated as a function of sample size and crystalline orientation at the low temperature regime. Furthermore, the brittle-to-ductile transition is investigated to study the transition of deformation mechanisms, i.e. full to partial dislocation motion on the glide set, at the elevated temperatures regime [4]. Deformed regions in micropillars are subsequently characterized using HRTEM to track dislocations and microtwins. An unambiguous interpretation of dislocation processes in the diamond-cubic structure will be presented. Please click Additional Files below to see the full abstract

Portevin‐Le Chatelier effect studied at small scale

Portevin-Le Chatelier (PLC) effect [1] manifests itself as a serrated flow in the stress-strain curve associated with the phenomenon of dynamic strain aging (DSA), which arises from the interaction between solute atoms and matrix dislocations. The overwhelming majority of the data available in the literature about PLC effect is conducted at macro scale, often with a large and complicated microstructure. The PLC effect studies at small scale, the fundamental studies, could offer great insights to the dislocation theory of plasticity. Here we study the PLC effect in an Al-Cu diffusion couple using in situ strain rate jump micro-pillar-compression technique [2] facilitated with focused ion beam (FIB) machining. The deformed microstructures are characterize using high-resolution SEM images. Transmission electron microscopy (TEM) is used to study the atomistic origin of the DSA. References [1] A. Portevin, F. Le Chatelier, Comptes Rendus de l\u27Académie des Sciences Paris, 176 (1923) 507-510. [2] G. Mohanty, J.M. Wheeler, R. Raghavan, J. Wehrs, M. Hasegawa, S. Mischler, L. Philippe, J. Michler, Philosophical Magazine, 95 (2015) 1878-1895

Microscale additive manufacturing of metal – mechanical properties

Additive manufacturing (AM) is transforming the way we design and fabricate structures on many scales. A main driving force of this movement is the ability of AM to overcome geometrical constraints imposed by subtractive manufacturing techniques. Because such design restrictions become increasingly limiting at small length scales, microscale AM has the potential to significantly expand the capabilities of microfabrication. Yet, for AM to become a beneficial addition to current microfabrication techniques, the properties of materials fabricated by AM have to be determined and quality standards have to be established. Thus, a comparison was performed of the mechanical properties of metals deposited with most of the currently suggested microscale metal AM techniques [1]. The range of techniques studied includes well established approaches, e.g., focused electron beam induced deposition and laser forward transfer, as well as more novel methods, e.g., electrohydrodynamic printing and electrochemical deposition. The mechanical performance of structures deposited with these methods was evaluated using nanoindentation and microcompression (Fig. 1b), and the materials’ microstructure was analyzed using cross-sectional electron microscopy. Please click Additional Files below to see the full abstract