Ductile organic aerogels for multifunctional applications

Monolithic polyurea aerogels were prepared by controlling the relative isocyanate/water/catalyst (Et3N) ratio in acetone, acetonitrile, or DMSO to prevent precipitation, yielding polyurea (PUA) gels which can be subsequently dried to form highly porous (up to 98.6% v/v) aerogels over a very wide range of densities (0.016-0.55 g cm-3). The method has been implemented successfully with several aliphatic and aromatic di and triisocyanates. Polyurea aerogels were then studied at the molecular level, the elementary nanoparticle level, and the microscopic level. Their porous structure was probed with N2-sorption porosimetry. Despite that the nanomorphology varies with density from fibrous at the low density end to particulate at the high density end, all samples consist of similarly sized primary particles assembled differently, probably via a reaction-limited cluster-cluster aggregation mechanism at the low density end, which changes into diffusion-limited aggregation as the isocyanate concentration increases. Higher density PUA aerogels (\u3e0.05 g cm-3) are mechanically strong enough to tolerate the capillary forces of evaporating solvents and can be dried under ambient pressure; under compression, they can absorb mechanical energy (up to 90 J g-1 at 0.55 g cm-3) at levels observed only with polyurea-cross-linked silica and vanadia aerogels (50-190 J g-1 at similar densities). The airborne acoustic attenuation properties were found exceptional, and they do not follow mass law. These properties make the monolithic organic aerogels attractive multifunctional materials

Compressive Behavior of Crosslinked Mesoporous Silica Aerogels at High Strain Rates

Aerogels are low-density mesoporous materials with exceptionally low dielectric constants, low thermal conductivities (up to 40 times better thermal insulators than the best fiberglass) and high acoustic impedance. Their practical applications, however, have been slow due to their hydrophilicity and brittleness. The fragility problem was resolved by nanocasting a ~2-4 nm thick polymer layer on the skeletal silica nanoparticles that strengthens the weak inter-nanopartical necks. The thin polymer coats conformally the skeletal framework without clogging the mesopores and reinforces the structure by chemically crosslinking the nanoparticles. With a density increase by only three times, crosslinking aerogels have the flexural strength increased by 300 times. The method has been applied for crosslinking aerogels consisting of oxides of more than 30 different elements from the periodic table. The mechanical properties of crosslinked silica aerogels with different polymers were improved to a similar level since the polymer bond energy was similar among polymers.Therefore, to improve the mechanical properties further, we turn to the network morphology. For this study we turned to certain micelle-templated aerogels known to have a worm-like microstructure with a macro/mesoporous skeletal framework. For comparison, macroporous monolithic silica aerogels consisting of both random and ordered mesoporous walls have been synthesized via an acid-catalyzed sol-gel process from tetramethoxysilane (TMOS) using a tri-block copolymer (Pluronic P123) as a structure-directing agent and 1,3,5-trimethylbenzene (TMB) as a micelle-swelling reagent. Although those monoliths are more robust than base-catalyzed silica aerogels of similar density, the mechanical properties can be improved dramatically by letting di-isocyanate react with the silanols on the mesoporous surfaces. The compressive behavior of both crosslinked templated silica aerogels with/without ordered mesostructure and non-templated silica aerogels was characterized under high strain rates using a long split Hopkinson pressure bar (SHPB). Their mechanical properties at different strain rates are compared with those of engineering plastics polymethyl methacrylate (PMMA) and polycarbonate (PC). The effect of water absorption and of low temperatures on the compressive behavior was also investigated

Adherence to anti-tuberculosis treatment among pulmonary tuberculosis patients: a qualitative and quantitative study

BACKGROUND: Tuberculosis (TB) patients have difficulty following a long-term treatment regimen. Efforts to improve treatment outcomes require better understanding of adherence as a complex behavioral issue and of the particular barriers to and facilitators of patient adherence. METHODS: This study was carried out in Jiangsu Province of China with both quantitative and qualitative approaches. For the quantitative study, 780 sputum-smear positive TB patients consecutively registered since 2006 in 13 counties (districts) were queried with a structured questionnaire. Patients who had missed 10% of their total prescribed doses of TB drugs were deemed as non-adherent. Risks for non-adherence were estimated by computing odds ratios (ORs) and their 95% confidence intervals (95% CIs) using a logistic regression model. We also invited 20 TB patients and 10 local health workers for in-depth interviews. We then used content analysis based on this qualitative study to explore factors associated with non-adherence. RESULTS: The proportion of non-adherence among 670 patients was 12.2%. Univariate analysis showed that patients, who were illiterate, divorced/widowed, lacked health insurance and were migrants, were more likely to be non-adherent. The crude ORs(95%CIs) were 2.38(1.37-4.13), 2.42(1.30-4.52), 1.89(1.07-3.32) and 1.98(1.03-3.83), respectively. The risk of non-adherence was lower among patients whose treatment was given under direct observation by village doctors or regular home visits by health workers, with ORs (95% CIs) of 0.19(0.10-0.36) and 0.23(0.10-0.51), respectively. In multivariate analysis, factors associated with non-adherence included illiteracy (OR: 2.42; 95% CI: 1.25-4.67) and direct observation by village doctors (OR: 0.23; 95% CI: 0.11-0.45). The in-depth interviews indicated that financial burdens and extra medical expenditures, adverse drug reactions, and social stigma were additional potential factors accounted for non-adherence. CONCLUSION: More importance should be given to treatment adherence under the current TB control program. Heavy financial burdens, lack of social support, adverse drug reactions and personal factors are associated with non-adherence. Direct observation and regular home visits by health workers appear to reduce the risk of non-adherence. More patient-centered interventions and greater attention to structural barriers are needed to improve treatment adherence

Simulation of the Evolution of the Nanostructure of Crosslinked Silica-Aerogels under Compression

Silica-aerogels are ultra-low-density assemblies of silica nanoparticles, and possess superior acoustic, specific energy absorption and thermal insulation properties. A new class of aerogels encapsulated with polymer is classified as crosslinked silica-aerogels. Manufacturing of such crosslinked silica-aerogel structures, depending on the type and shape of the nanoparticles, the polymer cross-linker and the chemistry in use, yields structures with vastly different morphologies and a wide range of mechanical behavior. With this, it has become necessary to understand the nanostructure / macroscopic properties relationship. Modeling of the aerogel material properties from mesoscale and up approach is needed, which is not considered by the current phenomenological models based on continuum material assumption. Most of the existing simulation methodologies face difficulties mainly due to complex nanostructures, large distortions, and extensive contact. A relatively new numerical method called Material Point Method (MPM) can circumvent these problems. For example, MPM has been used effectively in modeling the microstructural evolution of the bulk metallic glass foam with 70% porosity, where 3D X-Ray microtomography was used first to obtain the representative volume element (RVE) of the closed-cell foam . Due to the particle description of matter, MPM is a very suitable for silica-aerogel simulations. In this regard, an approach based on X-Ray nano-computed tomography (n-CT) will be used to model cross-linked aerogel mesostructure. The voxel information from the 3D tomography will be used to generate material points in MPM. The parallel version (using Structured Adaptive Mesh Refinement Application Infrastructure) of MPM code will be used to simulate the response of the model under compression. In this paper, the MPM is used to model a crosslinked templated silicaaerogel (X-MP4-T045) in compression, and the simulation results are compared with the compressive stress-strain curve obtained experimentally. This work will focus on the deformation mechanisms in crosslinked templated silica-aerogel such as the elastic buckling, compaction and densification, as well as the dependence of mechanical properties on the porosity effect for this crosslinked templated silica-aerogel

Population Redistribution among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel ionized N2 molecules at various driver wavelengths in the near- and mid-infrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground state and the excited states occur in strong laser fields. The couplings result in the population inversion between the ground and the excited states at the wavelengths near 800 nm, which is verified by our experiment by observing the amplification of a seed at ~391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.Comment: 18 pages, 4 figure

Measurement of web surface profiles using fringe projection

In this paper a full-field measurement technique, namely fringe projection technique was implemented to determine the three-dimensional surface profile for situations where webs are opaque or translucent. In this method, a grating is projected to the surface of a web. The projected grating will be distorted due to the non-flat surface. The image of the distorted gratings will be shifted on a reference grating on a computer to form Moire fringes. The phase of the fringes is determined and converted to the surface profile of the web. Calibration was conducted, and validation was made on surfaces with known geometry. Surface profiles of Polyvinyl Fluoride (PVF), nonwoven and polyethylene webs, as well as non-flat side walls of a wound roll determined as examples illustrating the technique. The accuracy of the method is discussed. The method has potential for industrial scale applications due to its ease in setup and speed in three-dimensional reconstruction.Mechanical and Aerospace Engineerin

Shear slitting of aluminum webs using block knives

C Shear slitting of two aluminum webs, namely 1050 H18 of 0.28 mm thick and 5182 H19 of 0.20 mm thick, using block knives are investigated through experiments using a laboratory slitter. This investigation focused on two aspects of shear slitting using block knives, appropriate for relatively thick webs. They are: (1) tangential shear slitting at zero rake angle, i.e., traditional shear slitting with a pair of block knives. In this aspect, the effects of major slitting parameters on the burr height at the slit edge were investigated. These include the clearance, overlap, overdrive and cant angle. The critical clearances for both webs have been determined; and (2) slitting at a rake angle, a new method for edge trimming when the two blades are not necessarily in contact. The top blade geometry was modified for slitting with a rake angle of -15o to allow slitting of an aluminum web, up to 1 mm thick in this investigation. This new method of edge trimming using block knives was found to be very effective and robust over a (relatively) very wide range of slitting parameters. Very good slit edge was produced, and the burr height was found to be independent of slitting parameters over a relatively large range of slitting parameters. Because two blades do not have to be in contact in slitting so that the blade wear is much less than in the case of traditional shear slitting, this new method is expected to extend significantly the block knife service life while producing consistently high quality slit edges.Mechanical and Aerospace Engineerin

Decreased Triple Network Connectivity in Patients with Recent Onset Post-Traumatic Stress Disorder after a Single Prolonged Trauma Exposure

The triple network model provides a common framework for understanding affective and neurocognitive dysfunctions across multiple disorders, including central executive network (CEN), default mode network (DMN), and salience network (SN). Considering the effect of traumatic experience on post-traumatic stress disorder (PTSD), this study aims to explore the alteration of triple network connectivity in a specific PTSD induced by a single prolonged trauma exposure. With an arterial spin labeling sequence, three networks were first identified using independent component analysis among 10 PTSD patients and 10 healthy survivors, who experienced the same coal mining flood disaster. Then, the triple network connectivity was analyzed and compared between PTSD and non-PTSD groups. In PTSD patients, decreased connectivity was identified in left middle frontal gyrus of CEN, left precuneus and bilateral superior frontal gyrus of DMN, and right anterior insula of SN. The decreased connectivity in left middle frontal gyrus of CEN was associated with clinical severity. Furthermore, no significant connection of SN with CEN and DMN was found in PTSD patients. The decreased triple network connectivity was found in this study, which not only supports the triple network model, but also suggests a possible neurobiological mechanism for cognitive dysfunction of this type of PTSD