Measuring nonlinear stresses generated by defects in 3D colloidal crystals

The mechanical, structural and functional properties of crystals are determined by their defects and the distribution of stresses surrounding these defects has broad implications for the understanding of transport phenomena. When the defect density rises to levels routinely found in real-world materials, transport is governed by local stresses that are predominantly nonlinear. Such stress fields however, cannot be measured using conventional bulk and local measurement techniques. Here, we report direct and spatially resolved experimental measurements of the nonlinear stresses surrounding colloidal crystalline defect cores, and show that the stresses at vacancy cores generate attractive interactions between them. We also directly visualize the softening of crystalline regions surrounding dislocation cores, and find that stress fluctuations in quiescent polycrystals are uniformly distributed rather than localized at grain boundaries, as is the case in strained atomic polycrystals. Nonlinear stress measurements have important implications for strain hardening, yield, and fatigue.Comment: in Nature Materials (2016

Spacings and pair correlations for finite Bernoulli convolutions

We consider finite Bernoulli convolutions with a parameter $1/2 < r < 1$ supported on a discrete point set, generically of size $2^N$. These sequences are uniformly distributed with respect to the infinite Bernoulli convolution measure $\nu_r$, as $N$ tends to infinity. Numerical evidence suggests that for a generic $r$, the distribution of spacings between appropriately rescaled points is Poissonian. We obtain some partial results in this direction; for instance, we show that, on average, the pair correlations do not exhibit attraction or repulsion in the limit. On the other hand, for certain algebraic $r$ the behavior is totally different.Comment: 17 pages, 6 figure

Aligned carbon nanotube carbon matrix nanocomposites

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 129-141).Materials comprising carbon nanotube (CNT) aligned nanowire (NW) polymer nanocomposites (A-PNCs) have emerged as promising architectures for next-generation multifunctional applications. Enhanced operating regimes, such as operating temperatures, motivate the study of CNT aligned NW ceramic matrix nanocomposites (A-CMNCs). Here we report the synthesis of CNT A-CMNCs through the pyrolysis of CNT A-PNC precursors, creating carbon matrix CNT A-CMNCs. The CNT A-CMNC processing parameters were evaluated using an apparent density measurement, polymer re-infusion modeling, and CNT quality analysis, which elucidate the limitations of the processing parameters currently used to fabricate CNT A-CMNCs. Theoretical tools developed to help quantify and analyze the morphology of the CNTs in the A-CMNCs, and NWs in general, show that morphological parameters, such as NW outer diameter and inter-wire spacing, that are usually overlooked may have significant effects on the physical properties of NW architectures. Mechanical characterization of the CNT A-CMNCs illustrates that the presence of aligned CNTs can lead to an enhancement of > 60% in microhardness, meaning that the fabrication of high strength, high temperature, lightweight next-generation material architectures may be possible using the presented method. Finally, factors that influence the physical properties of CNT A-CMNCs, such as CNT waviness and the porosity of the carbon matrix, are identified, and since their effects cannot be modeled using existing theory, future paths of study that could enable their quantification are recommended.by Itai Y. Stein.S.M

Diffusion Limited Aggregation on a Cylinder

We consider the DLA process on a cylinder G x N. It is shown that this process "grows arms", provided that the base graph G has small enough mixing time. Specifically, if the mixing time of G is at most (log|G|)^(2-\eps), the time it takes the cluster to reach the m-th layer of the cylinder is at most of order m |G|/loglog|G|. In particular we get examples of infinite Cayley graphs of degree 5, for which the DLA cluster on these graphs has arbitrarily small density. In addition, we provide an upper bound on the rate at which the "arms" grow. This bound is valid for a large class of base graphs G, including discrete tori of dimension at least 3. It is also shown that for any base graph G, the density of the DLA process on a G-cylinder is related to the rate at which the arms of the cluster grow. This implies, that for any vertex transitive G, the density of DLA on a G-cylinder is bounded by 2/3.Comment: 1 figur

Process-morphology scaling relations quantify self-organization in capillary densified nanofiber arrays

Capillary-mediated densification is an inexpensive and versatile approach to tune the application-specific properties and packing morphology of bulk nanofiber (NF) arrays, such as aligned carbon nanotubes. While NF length governs elasto-capillary self-assembly, the geometry of cellular patterns formed by capillary densified NFs cannot be precisely predicted by existing theories. This originates from the recently quantified orders of magnitude lower than expected NF array effective axial elastic modulus (E), and here we show via parametric experimentation and modeling that E determines the width, area, and wall thickness of the resulting cellular pattern. Both experiments and models show that further tuning of the cellular pattern is possible by altering the NF-substrate adhesion strength, which could enable the broad use of this facile approach to predictably pattern NF arrays for high value applications.United States. National Aeronautics and Space Administration (Grant NNX17AJ32G

Exohedral Physisorption of Ambient Moisture Scales Non-monotonically with Fiber Proximity in Aligned Carbon Nanotube Arrays

Here we present a study on the presence of physisorbed water on the surface of aligned carbon nanotubes (CNTs) in ambient conditions, where the wet CNT array mass can be more than 200% larger than that of dry CNTs, and modeling indicates that a water layer >5 nm thick can be present on the outer CNT surface. The experimentally observed nonlinear and non-monotonic dependence of the mass of adsorbed water on the CNT packing (volume fraction) originates from two competing modes. Physisorbed water cannot be neglected in the design and fabrication of materials and devices using nanowires/nanofibers, especially CNTs, and further experimental and ab initio studies on the influence of defects on the surface energies of CNTs, and nanowires/nanofibers in general, are necessary to understand the underlying physics and chemistry that govern this system.National Science Foundation (U.S.) (NSF Grant No. CMMI-1130437)National Science Foundation (U.S.) (NSF Award Number ECS-0335765)United States. Army Research Office (contract W911NF-07-D-0004

OUT-OF-OVEN CURING OF POLYMERIC COMPOSITES VIA RESISTIVE MICROHEATERS COMPRISED OF ALIGNED CARBON NANOTUBE NETWORKS

The broader adoption of composite materials in next-generation aerospace architectures is currently limited by the geometrical constraints and high energy costs of traditional manufacturing techniques of PMCs such as autoclave and vacuum-bag-only oven curing techniques. Here, an in situ curing technique for PMCs using a resistive heating film comprised of an aligned carbon nanotube (A-CNT) network is presented. A carbon fiber reinforced plastic (CFRP) prepreg system is effectively cured via a single-side CNT network heater incorporated on the outer surface of the laminate without using an autoclave. Evaluation of the curing efficacy shows that composites cured by A-CNT film heaters can achieve degrees of cure that are equivalent or better than composites cured by an autoclave. This manufacturing technique enables highly efficient curing of PMCs while adding multifunctionality to finished composites.United States. Army Research Office (contract W911NF-07-D-0004)United States. Army Research Office (contract W91NF-13-D-0001)Kwanjeong Educational Foundation (Korea)National Defense Science and Engineering Graduate (NDSEG) FellowshipConselho Nacional de Pesquisas (Brazil) (Science without Borders Program)United States. Naval Sea Systems Command (contract N00024-12-P-4069 for SBIR topic N121-058

Safety and efficacy of an intra-oral electrostimulator for the relief of dry mouth in patients with chronic graft versus host disease: case Series

Objectives: Patients with chronic graft-versus-host disease (cGVHD) often suffer from dry mouth and oral mu - cosal lesions. The primary objective of this study was to investigate the safety of an intra-oral electrostimulator (GenNarino) in symptomatic cGVHD patients. The secondary objective was to study the impact on the salivary gland involvement of cGVHD patients. Study Design: This paper presents a case series. The study included patients treated for 4 weeks, randomly as - signed to the active device and then crossed-over to a sham-device or vice versa. The patients and clinicians were blind to the treatment delivered. Data regarding oral mucosal and salivary gland involvement were collected. Results: Six patients were included in this series. Most of the intraoral areas with manifestations of cGVHD were not in contact with the GenNarino device. Two patients developed mild mucosal lesions in areas in contact with the GenNarino during the study. However, only one of them had a change in the National Institutes of Health (NIH) score for oral cGVHD. The unstimulated and stimulated salivary flow rate increased in 4 out of the 5 pa - tients included in this analysis. Symptoms of dry mouth and general oral comfort improved. Conclusion: This study suggests that GenNarino is safe in cGVHD patients with respect to oral tissues. Furthermore the use of GenNarino resulted in subjective and objective improvements in dry mouth symptoms. A large scale study is needed to confirm the impact and safety of GenNarino on systemic cGVHD