Damping Augmentation of Nanocomposites Using Carbon Nanofiber Paper

Vacuum-assisted resin transfer molding (VARTM) process was used to fabricate the nanocomposites through integrating carbon nanofiber paper into traditional glass fiber reinforced composites. The carbon nanofiber paper had a porous structure with highly entangled carbon nanofibers and short glass fibers. In this study, the carbon nanofiber paper was employed as an interlayer and surface layer of composite laminates to enhance the damping properties. Experiments conducted using the nanocomposite beam indicated up to 200–700% increase of the damping ratios at higher frequencies. The scanning electron microscopy (SEM) characterization of the carbon nanofiber paper and the nanocomposites was also conducted to investigate the impregnation of carbon nanofiber paper by the resin during the VARTM process and the mechanics of damping augmentation. The study showed a complete penetration of the resin through the carbon nanofiber paper. The connectivities between carbon nanofibers and short glass fibers within the carbon nanofiber paper were responsible for the significant energy dissipation in the nanocomposites during the damping tests.


INTRODUCTION
In recent years, nanoparticles have been attracting increasing attention in the composite community as they are capable of improving the mechanical and physical properties of traditional fiber reinforced composites [1][2][3][4].Their nanometer size, leading to high specific surface areas of up to more than 1000 m 2 /g and extraordinary mechanical, electrical, and thermal properties make them unique nano-fillers for structural and multifunctional composites.Commonly used nanoparticles in nanocomposites include multiwalled nanotubes (MWNTs), single-walled nanotubes (SWNTs), carbon nanofibers (CNFs), montmorillonite (MMT) nanoclays, and polyhedral oligomeric silsesquioxanes (POSS).Other nanoparticles, such as SiO 2 , Al 2 O 3 , TiO 2 , and nanosilica are also used in the nanocomposites.Compared to other particulate additives, carbon nanotubes and carbon nanofibers offer more advantages.The addition of small size and low loading of carbon nanotubes and carbon nanofibers can enhance the matrix-dominated properties of composites, such as stiffness, fracture toughness, and interlaminar shear strength [5][6][7][8][9].They have proven to be excellent additives to impart electrical conductivity in nanocomposites at lower loadings due to their high electrical conductivity and aspect ratio [10][11][12].In addition, they have better performance as flame retardant by reducing the heat release rate of polymer and conducting heat away from the flame zone [13,14].
While there are many reported benefits of carbon nanotubes and carbon nanofibers in composites, the potential of carbon nanotubes and carbon nanofibers to enhance the damping properties of composites has been less explored.Traditional damping enhancements of composites are based on viscoelastic polymer materials [15], carbon fiber prepregs [16], and magnetostrictive particles [17].The major limitations of the viscoelastic polymer materials are the structural integrity issue, the sacrifice of stiffness and strength of the composite system due to the resin penetration, and poor thermal stability.Kishi et al. [16] evaluated the damping properties of composite laminates with/without the interleaved films.The effects of the lay-up arrangements of carbon fiber prepregs on the damping properties of the interleaved laminates were examined.The viscoelastic properties of interleaved polymer films were reflected in the damping properties of the corresponding interleaved laminates.Magnetostrictive particles have been used in a polymer matrix as active transducer and passive damper, providing stiffness and strength while incorporating damping capabilities.Pulliam et al. [17] developed a novel manufacturing technique based on magnetic fields to distribute magnetostrictive particles in polymer resins and applied them in thin-layer on the surfaces for vibration damping.Recently, carbon nanotubes have been used in the composite system for structural damping and stiffness augmentation.Suhr et al. [18] conducted direct shear testing of epoxy thin films containing multiwalled carbon nanotubes and reported strong viscoelastic behavior with up to 1400% increase in loss factor (damping ratio) of the baseline epoxy resin.The great improvement in damping was achieved without sacrificing the mechanical strength and stiffness of the polymer, and with minimal weight penalty.Koratkar et al. [19,20] fabricated multiwalled nanotube thin films by using catalytic chemical vapor deposition of xylene-ferrocene mixture precursor.The nanotube films were employed as interlayers to reinforce the interfaces between composite plies, enhancing laminate stiffness and structural damping.The flatwise bending tests of a piezosilica composite beam with an embedded nano-film sublayer indicated up to 200% increase in the damping level and 30% increase in the baseline bending stiffness.
Traditionally, researchers fabricated composites by directly mixing carbon nanotubes and carbon nanofibers into polymers and then using casting and injection techniques to make nanocomposites.Gou et al. [21,22] have developed a new technique approach to fabricate nanocomposites using single-walled carbon nanotube bucky papers.The experimental details of fabrication of single-walled carbon nanotube bucky paper can be found in reference [23].The dynamic mechanical analysis (DMA) results indicated an enhancement of the thermomechanical properties of single-walled carbon nanotube bucky paper/epoxy resin nanocomposites.The present work describes the integration of carbon nanofiber paper as damping material into large structural level laminates-glass fiber reinforced composites.The very first time an example of carbon nanofiber paper-enabled nanocomposites in the dimension of a structural element is presented.The manufacturing via VARTM and the investigation of the damping properties and tensile properties of the fabricated nanocomposites are described.

Materials
The carbon nanofiber paper used in this study was obtained from Applied Sciences, Inc.The carbon nanofiber paper had good strength and flexibility to allow for handling like traditional glass fiber mat.The carbon nanofiber paper was composed of short glass fibers and vapor grown carbon nanofibers (Polygraf III) with diameter of 100-150 nm and length of 30-100 μm.The short glass fiber and carbon nanofibers appeared in an entangled and porous form within the paper.The unsaturated polyester resin (product code: 712-6117, Eastman Chemical Company) was used as matrix material for glass fiber reinforced composites.The polyester resin was used with the MEK peroxide hardener at a weight ratio of 100 : 1.

Manufacturing of carbon nanofiber paper-enabled nanocomposites
The VARTM process has been widely used to produce lowcost, high-quality, and geometrically complicated composite parts.In this study, the VARTM process was used to fabricate the carbon nanofiber paper-enabled nanocomposites, which was carried out in three steps.In the first step, glass fiber mats carbon nanofiber paper were placed on the bottom half of a mold.After the lay-up operation was completed, a peel ply, resin distribution media, and vacuum bag film were placed on the top of fiber mats.The vacuum film bag was then sealed around the perimeter of the mold and a vacuum pump was used to draw a vacuum within the mold cavity.The next step was the mold filling during which resin was sucked into the mold under atmospheric pressure.In the VARTM process, the distribution media provided a high permeability region in the mold cavity, which allowed the resin to quickly flow across the surface of the laminate and then wet the thickness of the laminate.Therefore, the dominant impregnation mechanism in the VARTM process was the through-thickness flow of resin.In the final step, the composite part was cured at room temperature for 24 hours and post-cured in the oven for another 2 hours at 100 AE C. In this study, the test laminates consisted of six plies of fiberglass with a single layer of carbon nanofiber paper embedded at the surface or the midplane.In the manufacturing of composite laminates with carbon nanofiber as an interlayer, one layer of carbon nanofiber paper was placed between the fiber mats.The peel ply and resin distribution media were used on both top and bottom sides to facilitate the resin flow through the thickness.

Damping test of carbon nanofiber paper-enabled nanocomposites
The regular composite beam without carbon nanofiber paper and the nanocomposite beam with carbon nanofiber paper were used as the specimens for damping test.For each beam, a PZT (lead zirconate titanate, a type of piezoceramic material) patch (20 mm ¢ 20 mm) was attached on one side as an actuator to excite the beam and a smaller PZT patch (10 mm ¢ 8 mm) was attached on the other side of the beam as a sensor to detect the beam's vibration, as shown in Figure 1.A micro laser sensor (NAIS-LM10-ANR12151) was also used to detect the beam's tip displacement.The micro laser sensor had a resolution of 20 μm (0.0008 inch).The testing specimen was clamped on an aluminum stand as shown in Figure 2.

Tensile test of carbon nanofiber paper-enabled nanocomposites
The tensile tests were performed using the VARTM manufactured composite laminates with and without carbon nanofiber paper.The tensile tests on the composite beams were conducted according to ASTM test standards.All these tests were performed on a Qualitest testing machine.

Electron microscopy
The SEM images were taken to study the porous structure of carbon nanofiber paper and the impregnation of carbon nanofiber paper by the resin.The interface between the carbon nanofiber paper and the resin was examined.The SEM specimens of the nanocomposites were obtained by the ultra microtome cutting.The carbon nanofibers within the paper have an average diameter about 100-150 nm. Figure 3(d) shows the SEM image of the fracture surface of the nanocomposites embedded with carbon nanofiber paper.This sample was fractured under tensile force.It can be clearly seen that the resin had completely penetrated the carbon nanofiber paper through the thickness direction during the VARTM process.

Damping properties of carbon nanofiber-enabled composite laminates
The damping test was conducted on the composite laminates with carbon nanofiber paper as midlayer and surface layer.During the damping test, the sweep sinusoidal signals were used as excitation source for the PZT actuator to get To further demonstrate the improved damping for the nanocomposite beam, the frequency responses of the regular composite beam and the nanocomposite beam are compared in Figure 6, which clearly shows that the peak magnitude of the first three modes has dropped dramatically.This means that the damping ratio values of the nanocomposite beam at these three natural frequencies are much larger than those of the regular composite beam.
To estimate the damping ratio for each mode, the halfpower bandwidth method was used.Corresponding to each natural frequency, there is a peak in the magnitudefrequency plot of the system.3 dB down from the peak, there are two points corresponding to half-power point.A larger frequency range between these two points means a larger damping ratio value.The damping ratio is calculated by using the following equation: where ω 1 , ω 1 are the frequencies corresponding to the halfpower point, ω n is the natural frequency corresponding to the peak value, and ζ is the damping ratio.Table 1 shows the first three modal frequencies and associated damping ratio of the two beams.From the damping ratio comparison, it is clear that the damping ratio of the nanocomposite beam has increased up to 200-700% at the 2nd mode and 3rd mode frequencies.However, there is little change in mode frequencies, which means that there is slight change in the stiffness of the composites.This demonstrates an advantage of nanocomposite over regular composite with viscoelastic layers.The regular composites with viscoelastic layers will sacrifice in reduced stiffness, though damping is improved.
For the nanocomposite beam with carbon nanofiber paper as surface layer, the analysis shows good agreement with the test data for the nanocomposite beam with carbon nanofiber paper as midlayer, as shown in Figure 7. Therefore, it is concluded that the incorporation of carbon nanofiber paper could result in a significant increase in structural damping of conventional fiber reinforced composites.

Tensile properties of carbon nanofiber paper-enabled nanocomposites
As stated earlier, the tensile properties of the composite laminates with and without carbon nanofiber paper were investigated.Table 2 shows the results from the tensile tests performed on the two sets of tensile specimens.It can be seen that the incorporation of carbon nanofiber paper had slight effects on the modulus and the strength of the composite laminates.

CONCLUSIONS
This paper presented the damping tests conducted using the nanocomposite beams with an embedded carbon nanofiber paper as interlayer or surface layer.The composite laminates without carbon nanofiber paper were also tested.The tests indicated up to 200-700% increase of the damping ratios at higher frequencies and slight change in tensile strength and Young's modulus of composite laminates due to the incorporation of carbon nanofiber paper.The SEM characterization of the carbon nanofiber paper and the nanocomposites showed the entanglement of carbon nanofibers and short glass fibers within the carbon nanofiber paper and the complete penetration of the resin through the carbon nanofiber paper.These cross-linkages within the carbon nanofiber paper are expected to be responsible for the energy dissipation in the nanocomposites due to the strong bonding and nonbonding interactions between carbon nanofibers and short glass fibers.

Call for Papers
Surface science has been always a matter of nanoscience even before the beginning of nanotechnology in modern science.
The surface science changes the scale of scientific research from micrometers to nanometers and makes the nanotechnology more complete.The surface analyses of semiconductors and metals are of interest for both basic scientific research and technological applications because their properties are highly depending on surface states of the material.
The surface science becomes more and more interesting in the nanodimensions.In the present scenario the basic nature and application of materials is dominantly determined by its surface behavior because of the high surface-to-volume ratio, in the nanomaterials.In addition, surface nanoscience has immense potential to declare the utility of materials for various applications.This special issue of Journal of Nanomaterials will be based on applied surface science, and openly call for new contributions in the field of surface nanoscience and catalysis.It intends to cover the entire range of basic and applied surface science focusing on synthesis, microscopy, and spectroscopy.The issue welcomes the contributions related to the fundamental understanding of catalytic and sensing mechanisms and spectroscopy in the nanoscale, and also the optical and electronic properties of various semiconductor and metals.We invite the manuscripts related to the size dependence of the phenomenon occurring at surface, as well as novel functions and applications of nanostructured materials, which will be the highlights of this special issue.The issue will be incomplete without the contributions from the field of carbon nanotubes.The studies related to interface and junctions will be a field of interest for this issue.
Topics of interest include (but are not limited to): • Surface analysis of materials and catalysis • Surface chemistry and wet chemical techniques • Gas-sensing mechanism in nanoscale and sizedependent gas-sensing properties • Microscopy/spectroscopy • Electrochromism • Thin films and polymer materials • Computer modeling and simulation of surface analysis Authors should follow the JNM manuscript format described at http://www.hindawi.com/GetJournal.aspx?journal=JNM.Prospective authors should submit an electronic copy of their complete manuscript through the JNM manuscript tracking system at http://www.hindawi.com/mts/,according to the following timetable:

Call for Papers
Architecture of crystallographic-oriented nanocrystals refers to design and fabrication of nanometer-size crystals having preferred crystallographic orientation.The main focus is on the crystallographic orientation of the nanocrystals, which has a major effect on properties and a consequent impact on performance of numerous devices.Nanocrystals refer to singlecrystals or polycrystals having at least one dimension in the nanometer scale.The nanocrystals can be made of any material (metal, semiconductor, ceramic, polymer), of any form (pure, solid solution or composite), of any type (powders or thin films), and of any shape.The nanocrystals can be grown using various techniques such as precipitation from a supersaturated solutions, ion implantation, diffusion, local phase transformation induced by external or internal forces, vapor phase deposition, liquid infiltration into pores, and more.The major difficulty is in controlling the crystallographic orientation of the crystals, especially under nonepitaxial conditions.This difficulty can be overcome by various state-of-the-art solutions such as nucleation and growth inside a highly dense array of nanopores while changing the electrical polarity of the nucleation sites by applied electric field or by molecular engineering, nucleation and growth at specific grain-boundaries sites having unique energies, precipitation from a solid solution of polar crystals under applied electric fields, and more.
The proposed issue combines multidisciplinary scientific fields needed for the design, fabrication, characterization, and modeling of crystallographic-oriented nanocrystals.
Topics of interest include (but are not limited to): • Design and fabrication processing of the nanocrystals (including nanowires and nanorod growth), with preferred crystallographic orientation

Call for Papers
Nanostructured materials represent a size limit of the miniaturization trend of current technology.Interest in nanophases has expanded as investigators have recognized that many of the properties of finely divided matter strongly depend on the interfacial properties of the constituents by virtue of the high fraction of the overall material which is in the vicinity of an interface as well as of the confinement of electrons, excitons, and photons in small volumes.One of the interesting and important issues in predicting and understanding nanostructures and their functional behaviors is whether the properties of matter evolve gradually from bulk, as system size is reduced, and what determines this evolution behavior.
The electromagnetic characterization of nanomaterials can be considered a major part of the emerging field of nanotechnology.The potentially profound implications both for the transport properties and optics are only beginning to be explored.In that respect, multicomponent magnetic nanophases are of significant technological interest, that is they can be considered as prospective granular magnetic films for tunable or nonreciprocal millimeter wave devices for monolithic microwave integrated circuit (MMIC) applications.This has also stimulated studies of the magnetoelectric effect, that is the polarization of a material in an applied field or an induced magnetization in an external electric field.
The practical importance and industrial interest in these materials demand optimization of several types of properties in these materials.These properties include: polarization, magnetization, and stability of the materials to mechanical, electrical, and magnetic fields applied during processing and operation.One of the fundamental goals of this field should be the understanding of the relationships of these properties on the composition, particle size and boundaries variations, defect structure and separation of the residual pores, but in most cases they are not wellunderstood.
In classical electrodynamics, the response of a material to electric and magnetic fields is characterized by two fundamental quantities: the permittivity ε and the magnetic permeability μ.In spite of the advances made, there is still no general agreement on interpretation of the experimental data of ε and μ of nanostructured materials since these quantities depend sensitively on the microstructural properties such as grain size, particle shape, and grain boundaries type.
Another related issue is the modeling of the polarization and magnetization mechanisms for these nanophases.In the effective medium approaches derived from continuum electromagnetism, only the volume fraction, or the particle number density, appears, while it is now well accepted that for dispersed two-phase nanostructures, appropriate descriptors of the interfaces should also appear.Therefore, collective magnetic and electromagnetic behaviors in nanosystems are challenging in terms of both experimental observation and development of theoretical analyses.
Papers are solicited in, but not limited to, the following areas: • Transport behavior in heterogeneous nanoscaled materials and composites • Effective medium modeling of nanoscale heterotructures • Electromagnetic response of clusters • Near-field microwave and optical spectroscopy on nanometer length scales • Measurement of material parameters spectra (permittivity and permeability) in nanocomposites and nanostructures • Ferromagnetic resonance (FMR), spin wave (SW) characterization of magnetic nanoparticles and nanostructures Authors should follow the JNM manuscript format described at http://www.hindawi.com/GetJournal.aspx?journal=JNM.Prospective authors should submit an electronic copy of their complete manuscript through the JNM manuscript tracking system at http://www.hindawi.com/mts/,according to the following timetable:

Figure 1 :
Figure 1: Regular composite beam and nanocomposite beam for damping test.

Figure 3 (
Figure 3(a)  shows the carbon nanofiber paper used in this research, which can be handled like traditional glass fiber mats.The SEM images of carbon nanofiber paper are shown in Figures 3(b) and 3(c).These images show the multiscale porous structure of carbon nanofiber paper formed by short glass fibers and carbon nanofibers.The pore size formed by short glass fibers was in the range of 100-200 μm and the pores formed by carbon nanofibers had an average opening around 1 μm.The carbon nanofibers within the paper have an average diameter about 100-150 nm.Figure3(d)shows the SEM image of the fracture surface of the nanocomposites embedded with carbon nanofiber paper.This sample was fractured under tensile force.It can be clearly seen that the resin had completely penetrated the carbon nanofiber paper through the thickness direction during the VARTM process.

Figure 3 :
Figure 3: Carbon nanofiber paper and nanocomposites: (a) carbon nanofiber paper in the dimension of a structural element, (b) the porous structure formed by short glass fiber within the paper, (c) the porous structure formed by carbon nanofibers within the paper, and (d) the fracture surface of the nanocomposites.

Figure 4 :Figure 5 :Figure 6 :
Figure 4: Sweep sine response (0.1 Hz to 100 Hz) of the composite beam without carbon nanofiber paper and the nanocomposite beam with carbon nanofiber paper as midlayer.

Figure 7 :
Figure7: Frequency response of the first three modes for the composite beam without carbon nanofiber paper and the nanocomposite beam with carbon nanofiber paper as surface layer.

Table 1 :
Damping ratio calculated by half-power bandwidth method.

Table 2 :
Results from the tensile tests performed on the laminates with/without carbon nanofiber paper.