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Active sensing, which incorporates closed-loop behavioral selection of information during sensory acquisition, is an important feature of many sensory modalities. We used the rodent whisker tactile system as a platform for studying the role cortical sensory areas play in coordinating active sensing motions. We examined head and whisker motions of freely moving mice performing a tactile search for a randomly located reward, and found that mice select from a diverse range of available active sensing strategies. In particular, mice selectively employed a strategy we term contact maintenance, where whisking is modulated to counteract head motion and sustain repeated contacts, but only when doing so is likely to be useful for obtaining reward. The context dependent selection of sensing strategies, along with the observation of whisker repositioning prior to head motion, suggests the possibility of higher level control, beyond simple reflexive mechanisms. In order to further investigate a possible role for primary somatosensory cortex (SI) in coordinating whisk-by-whisk motion, we delivered closed-loop optogenetic feedback to SI, time locked to whisker motions estimated through facial electromyography. We found that stimulation regularized whisking (increasing overall periodicity), and shifted whisking frequency, changes that emulate behaviors of rodents actively contacting objects. Importantly, we observed changes to whisk timing only for stimulation locked to whisker protractions, possibly encoding that natural contacts are more likely during forward motion of the whiskers. Simultaneous neural recordings from SI show cyclic changes in excitability, specifically that responses to excitatory stimulation locked to whisker retractions appeared suppressed in contrast to stimulation during protractions that resulted in changes to whisk timing. Both effects are evident within single whisks. These findings support a role for sensory cortex in guiding whisk-by-whisk motor outputs, but suggest a coupling that depends on behavioral context, occurring on multiple timescales. Elucidating a role for sensory cortex in motor outputs is important to understanding active sensing, and may further provide novel insights to guide the design of sensory neuroprostheses that exploit active sensing context.

There are very few studies that have investigated directly the effect of an oxide film on tin whisker growth, since the cracked oxide theory was proposed by Tu in 1994. The current work has investigated the effect of using an electrochemically formed oxide and both a molybdate conversion coating and a tungstate conversion coating on tin whisker growth from Sn-Cu electrodeposits on Cu, and compared it with that from a native air-formed oxide. X-ray photoelectron spectroscopy (XPS) has been used to investigate the effect of coating parameters on the thickness and composition of the oxide film. The XPS studies show that the oxide film formed using either of the conversion coating baths was significantly thicker than that produced from the potassium bicarbonate-potassium carbonate bath. Initial observations suggest that both the tungstate-based conversion coatings and the molybdate-based conversion coatings significantly reduced whisker growth by over 80 %, compared with a native air-formed oxide, and provide improved whisker mitigation compared with the electrochemically formed oxides. The current work has also investigated the potential of using a dual-layer system, comprised of both an electrochemically formed oxide bottom layer and an acrylic conformal coating top layer, for the mitigation of tin whisker growth. The electrochemically formed oxide used in the dual-layer system was produced at 2 V vs. Ag/AgCl while passing a charge of 60 mC cm-2 and the thickness of the conformal coating was aimed to be between ~5 μm to ~6 μm. This thickness was chosen to enable the study of whisker growth on a shorter time scale and to study the effect the electrochemically formed oxide had when used in conjunction. Initial observations showed that the dual-layer system provided improved whisker mitigation compared with both the electrochemically formed oxides and acrylic conformal coatings when used singularly. As part of the self-healing work, nanocapsules filled with the reactive agent were needed to be synthesised and the compatibility of them with different solvents needed to be studied. Capsules filled with the reactive agent were successfully synthesised, however, it was found that the capsules agglomerated and the size of the capsules, in some instances, were too large to be incorporated into a thin conformal coating. Regardless, the capsules were still analysed to check the compatibility with different solvents, to identify a suitable conformal coating mixture that would not dissolve the polymer shell of the capsules. It was observed that the capsules were stable in three out of the five solvents that were analysed, them being isopropanol (IPA), butanone and methylcyclohexane.

<p>The phenomenon of spontaneous growth of metallic filaments, known as whisker growth has been studied. Until now the problem that Sn whisker growth could cause in electronics by making shorts has been partially prohibited as Pb and Sn have been used together in solders and coating. Regulations restricting Pb use in electronics has made the need to understand Sn whisker growth more current.</p><p>It is shown that whiskers are highly resilient towards vibrations and shocks. A Sn whisker is shown to withstand 55 mA.</p><p>Results show that reflowing of the Sn plated surface does not prevent extensive whisker growth. Results show that intermetallic compound growth can not be the sole reason behind whisker growth. Nickel and silver underlayer have been shown not to prevent whisker growth, but perhaps restrain whisker growth. Heat treatment damped whisker growth considerably. It is judged that base material CuSn6 is less prone to show whisker growth than CuSn0.15 and E-Cu58.</p>

The phenomenon of spontaneous growth of metallic filaments, known as whisker growth has been studied. Until now the problem that Sn whisker growth could cause in electronics by making shorts has been partially prohibited as Pb and Sn have been used together in solders and coating. Regulations restricting Pb use in electronics has made the need to understand Sn whisker growth more current. It is shown that whiskers are highly resilient towards vibrations and shocks. A Sn whisker is shown to withstand 55 mA. Results show that reflowing of the Sn plated surface does not prevent extensive whisker growth. Results show that intermetallic compound growth can not be the sole reason behind whisker growth. Nickel and silver underlayer have been shown not to prevent whisker growth, but perhaps restrain whisker growth. Heat treatment damped whisker growth considerably. It is judged that base material CuSn6 is less prone to show whisker growth than CuSn0.15 and E-Cu58.

Since their commercialization in 1990, the electrodes of the lithium-ion battery have remained fundamentally the same. While energy density improvements have come from reducing the cell packaging, higher capacity electrodes are needed to continue this trend. A lithium metal anode, where the negative electrode half reaction is the plating and stripping of metallic lithium, is explored as an alternative to current graphite anodes. The specific capacity of the lithium metal anode is over ten times that of the graphite anode, making it a serious candidate to further improve the energy density of lithium batteries. Electrodeposited lithium metal forms dendrites, sharp needles that can grow across the separator and short circuit the battery. Thus, a chief goal is to alter lithium’s plating morphology. This was achieved in two separate ionic liquid electrolytes by co-depositing lithium with sodium. The co-deposited sodium is thought to block dendritic sites, leading to a granular deposit. A nucleation study confirmed that metal deposits from the ionic liquid electrolyte containing sodium, prevented dendritic growth from nucleation on, and not after dendrites had already grown. A model based on the geometry of the nuclei was used to gain insight into the effect of the solid electrolyte interface (SEI) that forms on freshly deposited lithium metal. In addition to sodium, the effect of alkaline earth metals on the lithium deposit morphology was also explored. While these metals did not deposit from the ionic liquid electrolyte, their addition also resulted in granular, dendrite free, deposits. The alkaline earth additives generally increased the overpotential for nucleating on the substrate and lowered the current density achievable. Strontium and barium showed the least of these negative effects while still providing a dendrite free deposit. A second hurdle for lithium metal anodes is the instability between the electrolyte and lithium metal. A protective SEI layer that prevents undesired side reactions is difficult to form because of the large volume change associated with cycling. Formation of a better SEI on lithium metal was attempted through the addition vinylene carbonate, which greatly improved the coulombic efficiency of lithium metal plating and stripping. The effect of gases, such as oxygen, nitrogen and carbon dioxide, on the SEI layer was also investigated. It was found that the presence of nitrogen and oxygen improved the coulombic efficiency by facilitating a thinner SEI layer. This work presents attempts at improving the lithium metal anode both by increasing the coulombic efficiency of the redox process and by eliminating dendrite growth. The coulombic efficiency was improved through the bubbling of gases and addition of organic additives but work remains to increase this value further. Dendritic growth, which poses a safety hazard, was completely eliminated by two methods: 1) co-deposition and 2) adsorption of a foreign metal. Both methods could potentially be applied to different electrolytes, making them promising methods for preventing dendritic growth in future lithium metal anodes.

Pure Sn and Sn-alloys are widely used in electrical and microelectronic devices as protective layer to prevent oxidation of Cu conductors and also as a component of Pb-free, Sn-based solders. Sn coatings, typically 0.5-10 μm thick, deposited on substrates, e.g., Cu, brass, etc., are prone to spontaneous growth (i.e., without any external stimuli) of Sn whiskers under ambient conditions. The growth of whiskers from Sn plating has caused numerous failures in micro-electronic devices, mainly due to short-circuiting, leading to failure of components or devices. Whisker growth is, thus especially very critical in aviation, space and defines applications, where the electronic components are designed for longer life span. Furthermore, due to miniaturization of electronic devices, the spacing between adjacent conductors or interconnects can be as small as a few hundred nanometres to a few micrometres, making them more prone to whisker induced short-circuiting. Minor alloying of Sn with Pub was the principle way for mitigating the whisker growth in Sn plated components; however, due to the recent worldwide acceptance of European Union’s Restriction of Hazardous Substances (RoHS) act, enforcing Pub-free manufacturing, whisker growth has re-emerged as a reliability issue in Pub-free solders and the Sn plating finishes. Even after decades of research, a universal whisker growth mechanism and hence effective mitigation technique is still not available in the public domain. This is mainly due to the fact that large number of factors that affect the whisker growth directly or indirectly, making it difficult to devise an experimental procedure, which allows studying effect of one factor at a time while keeping other factors constant. Although many mechanistic models for Sn whispering have been proposed in the past, the experimental evidences to support them are lacking. For example, recrystallization of whisker grain was proposed by various researchers; however, a direct observation confirming whisker grain is indeed a recrystallized grain has never been reported. Nevertheless, it is well understood that whisker growth is a form of stress relaxation process and diffusion plays important role in the formation of whiskers. Since Sn is extremely anisotropic with tetragonal crystal structure, the stress state of Sn coatings, as well as the diffusion needed for mass transport of atoms, varies drastically depending upon the direction of interest. Therefore, it is important to study the role of crystallographic texture (both macroscopic and microscopic) on whisker propensity by systematically varying the crystallographic texture of Sn coating while keeping thickness, grain size, substrate material, and post-deposition storage conditions the same. Better understanding of role of macro- and micro- texture is very crucial before any whispering mechanism can be proposed. Furthermore, recent studies indicate that role of stresses in Sn coatings driving whisker growth is not fully understood. It is generally accepted that compressive stress in Sn coating is the main factor that drives the whisker growth. However, whiskers were also observed when Sn coating was under tensile stress, making the role of stress controversial. Again, the stresses in Sn have multiple origins and need a systematic approach to understand their origin, quantify them and then relate it to whisker growth. Such systematic approach was never adopted in previous works. Hence, the current thesis aims to address the role of macro- and micro- crystallographic texture, stress regeneration mechanism, nature (i.e., magnitude and sign) of stress and stress gradient in the Sn coatings via systematic variation of texture, post-deposition storage conditions and substrate composition, including deposition of an interlayer in between Sn coating and the brass or Cu substrate. Whisker growth was studied from electro-deposited Sn coatings. The deposition parameters were optimized for producing different thickness and grain orientations. X-Ray diffraction (XRD) techniques were used to extract macro-texture of the coatings. The macro-texture measurement using XRD and micro-texture measurement using electron backscatter diffraction (EBSD) showed the same dominant and the second dominant orientations. It was observed that current density and deposition temperature, which are the two main electro-deposition parameters, significantly influence the crystallographic orientation of the grains. Thus, the global or macro-texture can be manipulated by changing the deposition parameters systematically. It was observed that whisker propensity increases drastically by growth of low index planes, such as (100) and (110), during deposition. Hence, proper selection of deposition parameters that lead to growth of high index planes can be used to suppress the whisker growth. Furthermore, micro-texture surrounding whisker grain was studied using EBSD technique by observing the same set of grains surrounding a whisker grain before and after whispering. Orientation imaging microscopy (OIM) maps of several whisker regions clearly indicate that whiskers preferentially grow from low index planes, such as (100), etc. Furthermore, using orientation dependent stiffness mapping (in-plane and out-of-plane), it was noticed that whiskers preferentially grew from regions of soft oriented grains (low modulus) surrounded by hard orientations. In addition, grain boundary disorientation analysis revealed presence of high fraction of high angle grain boundaries (HAGBs) in the vicinity of whisker grain. It was observed that overall fraction of HAGBs in the whispering region was 0.7 while the fraction of HAGBs surrounding and leading to whisker grain was 0.85. In addition, it was observed that whisker grew from pre-existing grain and not from the recrystallized grain. Also, grain boundary sliding was not observed as a pre-requisite for whisker growth in Sn coatings on brass substrate. The local stress field around the whisker grain also plays a crucial role in whisker growth. Therefore, local stress field around whisker site was simulated using crystal plasticity simulation by incorporating grid resolved spatial description of orientation in terms of Euler’s angles. The crystal plasticity model included slip systems of Sn and other material parameters, such as anisotropic elastic stiffness constants, critical resolved shear stresses for different slip systems, etc. Thus, the slip in individual grain was accounted following homogenization to maintain compatibility at grain boundaries. The simulated stress field shows that both in-plane and out-of-plane stresses were highly inhomogeneous without any unique condition around whisker grain. It has been observed that high compressive hydrostatic stresses develop in the vicinity of the whisker grain, while whisker grain is slightly tensile. Therefore, the gradient of hydrostatic stress around the whisker suggests whisker growth is mainly controlled by vacancy transport phenomenon. The stress in Sn coatings may originate from many factors, such as residual stress inherent to electro-deposition, diffusion of substrate atoms (Cu, Zn, etc.) into the coating, formation of interfacial intermetallic compound (IMC) layer, segregation of impurities at Sn grain boundaries, formation of surface oxide layer, and coefficient of thermal expansion (CTE) mismatch between in Sn and substrate as well as between differently orientated grains of Sn. Therefore, it is important to understand the dominant stress regeneration mechanism responsible for whisker growth. To identify dominant mechanism, which can continuously regenerate the compressive stress in Sn, samples deposited under fixed electro-deposition conditions were exposed to different post-deposition storage conditions, such as isothermal aging at room temperature, 50 °C, 150 °C, and thermal cycling from -25 to 85 °C with and without hold time at the highest temperature. It has been observed that Cu6Sn5 IMC growth due to the inter-diffusion of Cu and Sn atoms is the dominant mechanism responsible for whisker growth. Both growth kinetics and morphology of IMC have a significant impact on whisker growth. The role of CTE mismatch in regenerating compressive stresses in Sn coatings on brass substrate for whisker growth is highly limited. The substrate composition as well as the under layer metallization affects the inter-diffusion between Sn and the substrate atoms and therefore IMC growth, which is mainly responsible for whisker growth in Sn coatings on brass or Cu substrates. The effects of substrate composition on whisker growth was studied by using pure Cu, brass (65 wt. % Cu 35 wt. % Zn) and Ni (bulk and electro-deposited under layer) as substrate. Whisker growth was more rapid if brass substrate was used instead of pure Cu. Whiskers were not observed when Sn was deposited on either bulk Ni or when Ni under layer was electro-deposited on brass or Cu substrates prior to Sn deposition. Ni under layer effectively stops the diffusion of Cu into Sn, thus avoiding the growth of Cu6Sn5 (which places Sn coatings under compressive stress). Thus, it is clear that continuous formation of Cu6Sn5 at the interface provides the long-term driving force for whisker growth. Since the whisker growth is a stress driven phenomenon, it is important to understand the stress evolution in Sn coatings. Stress state of the Sn coatings was studied using custom-built laser curvature set-up with multi-beam optical stress sensor (MOSS). This allowed monitoring of curvature change of the coating-substrate system in real time and the bulk average stress was calculated using Stoney’s equation. For multi-layer system such as Sn deposited on pre-deposited Ni under layer on brass substrate modified Stoney’s equation was used. In case of Sn deposited on brass without any under layer, it is known that the Cu6Sn5 IMC do not form a continuous layer at the interface between Sn and substrate under aging at ambient conditions, therefore, the curvature change due to IMC can be neglected. In addition, glancing angle X-ray diffraction was employed to analyse stress in the top surface region of the coating. The variation of glancing angle allowed probing strain at different penetration depths. Both the bulk stress and the stress in only near surface region evolve with time. The residual bulk stresses in Sn coatings are tensile immediately after deposition. The residual stresses relax very quickly upon room temperature aging and become compressive. The bulk of Sn coatings on brass substrate progressively become more compressive upon continued aging. However, stresses in Sn coatings deposited on brass substrate with Ni under layer saturate quickly at low compressive stress. Surprisingly, stress in the top-most region of Sn coating measured using XRD evolve differently. The surface of Sn coating deposited on brass substrate is compressive initially and progressively become more tensile (less compressive), while the initial compressive stress in the sample with Ni under layer saturated at a higher compressive stress than the bulk stress value recorded from curvature measurement. Therefore, the surface of the Sn coatings with Ni under layer is always more compressive than the bulk stress in the Sn coating. Therefore, a negative stress gradient for the diffusion of Sn atoms towards surface is never established and whiskers do not grow in these Sn coatings. Interestingly, through thickness voids are observed in the Sn coatings on Ni. Contrarily, in Sn coatings without Ni under layer after 170 h of aging, the surface stress becomes more tensile than the bulk of the Sn coating, favouring continuous migration of atoms from the highly compressed region near Cu6Sn5 IMC layer to the stress-free whisker root. Aforementioned observation indicates the crucial role of negative stress gradient in the mass transport of atoms required for whispering. The importance of stress and stress gradient was further studied by analysing the effect of externally imposing stress and stress gradient on whisker growth. The stresses were applied using a three-point bend setup. It has been observed that externally applied stress accelerates the whisker growth. This is mainly because applied stress alters the diffusion kinetics and growth of Cu6Sn5 IMC at the interface. However, the coating under tensile stress shows more whisker growth as compared to the coating under high compressive stress. This is attributed to the fact the coating under tensile stress is under higher negative stress gradient. Therefore, it is proposed that out-of-plane stress gradient is more important rather than the sign and the magnitude of stress in determining the propensity of whisker growth in Sn coatings.