Spinning Continuous Fibers for Nanotechnology

Nanotubes of carbon and other materials are arguably the most fascinating materials playing an important role in nanotechnology today. Their unique mechanical, electronic, and other properties are expected to result in revolutionary new materials and devices. However, these nanomaterials, produced mostly by synthetic bottom-up methods, are discontinuous objects, and this leads to difficulties with their alignment, assembly, and processing into applications. Partly because of this, and despite considerable effort, a viable carbon nanotube–reinforced supernanocomposite is yet to be demonstrated. Advanced continuous fibers produced a revolution in the field of structural materials and composites in the last few decades as a result of their high strength, stiffness, and continuity, which, in turn, meant processing and alignment that were economically feasible. Fiber mechanical properties are known to substantially improve with a decrease in the fiber diameter. Hence, there is a considerable interest in the development of advanced continuous fibers with nanoscale diameters. However, conventional mechanical fiber spinning techniques cannot produce fibers with diameters smaller than about 2 μm robustly. Most commercial fibers are several times that diameter, owing to the trade-offs between the techno¬logical and economic factors

Structural Nanocomposites

Materials scientists predict that composites made with nanoscale reinforcing materials such as nanotubes, platelets, and nanofibers will have exceptional mechanical properties. However, the results obtained so far are disappointing, particularly when compared to advanced composites reinforced with high-performance continuous fibers (1–4). The reasons include inadequate dispersion and alignment of the nanoreinforcement, low nanoreinforcement volume fraction, and poor bonding and load transfer at interfaces. Intensive work is under way, but the prospect of bulk structural supernanocomposites appears more remote now than it did just a few years ago. However, recent work shows that some applications in reinforcement of small structures may have a near-term payoff that can foster longer-term work on nanocomposites. Most of the work on structural nanocomposites has relied on ultrastrong nanoreinforcement such as single- walled carbon nanotubes (SWCNTs) (1–3). However, the high SWCNT strength has not yet translated into bulk strength, and it is not even clear whether such translation is possible: Any attempt to create strong interfacial bonds will introduce defects into the SWCNTs that reduce their intrinsic strength. Still, multifunctional applications not relying solely on the mechanical superproperties will benefit (4, 5). Tailorability and controlled anisotropy are other useful special features of nanocomposites. Multiscale modeling (6) will help us achieve the desired balance between various functions

Analysis of the Effects of the Residual Charge and Gap Size on Electrospun Nanofiber Alignment in a Gap Method

In this paper, the effects of residual charges on nanofiber alignment in a gap method are studied and presented. The gap method was presented by Li and Xia (2003 Nano Lett. 3 1167); in it, a gap is introduced into a traditional collector. Due to the non-perfect conductivity of electrospun nanofibers, they carry residual charges after deposition across the gap. These residual charges will interact with the charges carried by the upcoming jet/fiber, that will also deposit across the gap. The effects of these charge interactions on nanofiber alignment were studied numerically at various gap sizes. Results showed that alignments of nanofibers improve substantially with the gap size increasing from 3 to 8 mm. Numerical studies on the effect of residual charges in already deposited nanofibers on the alignment of nanofibers deposited afterwards were also conducted. Studies showed that the residual charges result in worse alignment, with a 10%–25% decrease in orientation parameters

Antiplane surface acoustic waves propagating in elastic half-space coated with an anisotropic laminate

Dispersion relation of antiplane surface acoustic waves (SAWs) propagating in elastic half-space coated with an anisotropic laminate was determined explicitly by means of Stroh\u27s formalism within the subsonic range, where is no energy leakage into the substrate. During the procedure, the governing dynamic equation in each anisotropic layer and the displacement–traction continuity at interfaces were exactly satisfied by Stroh\u27s functions. Explicit algebraic equation was derived for determining the dispersion relation. As an example, the wave number vs. phase velocity diagram for steel half-space coated with a graphite-fiber/epoxy laminate with a [± 45°/0°2] lay-up was demonstrated. The given method can be used for the study of SAW properties of anisotropic coating systems and non-destructive evaluation based on surface/guided wave methods

Collapse analysis of nanofibers

Continuous nanofibers fabricated by the electrospinning technique have found increasing applications (e.g., nanofiber composites, nanofiber devices, bioengineering tissue scaffolding, etc.). For a nanofiber network subjected to a small external perturbation, the fiber segments within the network may deflect and stick to each other under the condition that their surface adhesion energy overcomes the elastic strain energy induced by fiber bending. Therefore, this paper aims to study adhesion-induced nanofiber collapse and relevant criteria. A simple fiber collapse model was proposed, which is based on the contact of two deflected elastic filaments under surface adhesion. Four fundamental fiber collapse modes (i.e., fiber-flat substrate, parallel fibers, orthogonal fibers and fibers at arbitrary angle) were considered, and corresponding collapse criteria were determined in explicit forms. Effects of fiber elasticity, surface adhesion and fiber geometries on the collapse criterion were explored in a numerical manner. Results show that for a fiber segment pair at a relatively large angle, the critical distance to induce the fiber collapse is independent of the fiber radius. This distance is a function of the fiber aspect ratio and the material intrinsic length (γ/E, where γ is the surface energy and E is Young’s modulus). The fiber collapse model developed in this study can be used as the theoretical basis for design and failure analysis of nanofiber networks and nanofiber devices, among others

Delamination Resistant Composites Prepared by Small Diameter Fiber Reinforcement at Ply Interfaces

A fiber reinforced composite material comprising a resin matrix and primary reinforcement fibers and further comprising secondary, smaller diameter, reinforcement fibers at one or more ply interfaces, or portion thereof, provides improved interlaminar toughness, strength, and delamination resistance and without substantial increase in weight. In one embodiment, the small fibers are attached to one side of a conventional prepreg prior to lamination. The small fibers are flexible and are expected to conform to the shape and distribution of the primary reinforcing fibers at the interface

Adhesive contact in filaments

This paper studies the elastic contact in filaments induced by surface adhesion, which plays an important role in the mechanical response of fibrous materials (e.g., fiber friction, sliding, compression hysteresis, etc.). During the process, a simple 3D elastic contact model was proposed. The filaments were assumed to be uniform, smooth elastic cylinders, and the adhesive force between filaments in contact was estimated according to Bradley’s approach (Bradley 1932 Phil. Mag. 13 853) that relies on the filament configurations before deformation. Under the action of fiber surface adhesion, the elastic deformation and the size of the contact zone were determined in closed-form based on the DMT theory (Derjaguin et al. 1975 J. Colloid Interface Sci. 53 314). Effects of filament radius and orientation, surface energies and elasticity on the elastic deformation and the size of the contact zone were explored numerically. The model developed in this work can be used for the study of the mechanisms of filament contacts, friction, sliding and compression hysteresis in fibrous materials subjected to external loading

Interfacial edge crack between two bonded dissimilar orthotropic strips under antiplane point loading

A closed-form solution is obtained for the interfacial edge crack between two bonded dissimilar orthotropic strips loaded by antiplane point loading in form of screw dislocation or line force. Conformal mapping and existing dislocation solutions are utilized for constructing the fundamental solution of the problem. The stress intensity factor (SIF) and the energy release rate(ERR) are obtained explicitly

Screw Dislocation Interacting with an Interfacial Edge Crack between Two Bonded Dissimilar Piezoelectric Wedges

This letter is concerned with an interface edge crack interacting with a screw dislocation under antiplane mechanical and in-plane electric loading in a piezoelectric bimaterial wedge. In addition with a discontinuous electric potential across the slip plane, the dislocation is subjected to a line-force and a line-charge at the core. The out-of-plane displacement and electric potentials are obtained in closed-form on the basis of the conformal mapping technique and the solution for a screw dislocation in an infinite piezoelectric bimaterial with a semi-infinite interface crack. The intensity factors (IFs) and energy release rate (ERR) are obtained explicitly

Intra-annual height growth dynamics of Scots and lodgepole pines and its relationship with meteorological parameters in central Latvia

The Scots pine (Pinus sylvestris L.) is the second-most widely used tree species in forestry in Latvia and is the only species used for afforestation on nutrient poor soils that cover considerable forest land in Latvia. Several studies have shown that, in such conditions, the lodgepole pine (Pinus contorta var. latifolia) may be more productive in terms of biomass and yield. It is important to consider climate change studies to assess the potential for a larger-scale use of the lodgepole pine in forestry. The aim was to assess the intra-annual height growth patterns of both species, the differences between them, and the influence of meteorological parameters on their height growth. Their height growth was monitored on a weekly basis in two sampling sites in central Latvia, and the height increment curves were described by Gompertz’s model. The height growth dynamics of individual trees and species differed notably, indicating the potential for the selection of the best-adapted genotypes. Our results indicate that the early onset of the active growth phase might be the most important factor determining the total height increment for both species. Temperature-related meteorological parameters were the only ones with a statistically significant influence on pines height growth and only when at least one of the variables were standardised prior to the analysis. A temperature increase had a slightly stronger positive effect on the growth of the lodgepole pine, indicating that it might be suitable for more intensive use in forestry under the climate change scenarios for Latvia