Investigation on surface treatments of CFRP and aluminum to improve fracture toughness of adhesively-bonded CFRP–aluminum joints

In this study, the role of surface treatments of CFRP (graphite/epoxy composite) and aluminum (7075-T6) on the adhesively-bonded CFRP-aluminum joints has been investigated. The CFRP was surface-treated by Ar⁺ ion irradiation in an oxygen environment and the aluminum was surface-treated using a DC plasma. Ar⁺ ion irradiation treatment was carried out at Ar⁺ ion dose of 10¹⁶ ions/cm². Plasma treatment was carried out at a volume ratio of acetylene gas to nitrogen gas of 5:5 and the treatment time was 30 s. The effect of surface treatments on the fracture behavior CFRP-aluminum joints was determined from fracture tests using three different CLS (cracked lap shear) specimens: (1) untreated CFRP/untreated aluminum, (2) ion-irradiated CFRP/untreated aluminum and (3) untreated CFRP/plasma-treated aluminum. Fracture behaviors (fracture load, fracture toughness, fracture surfaces) of these three different specimens were compared. The results showed that both fracture load and fracture toughness of CFRP-aluminum joints were in the following order: ion-irradiated CFRP/untreated aluminum specimen > untreated CFRP/plasmatreated aluminum specimen > untreated CFRP/untreated aluminum specimen. SEM examination of fracture surfaces showed that fracture occurred as an interfacial failure for untreated specimens. On the other hand, a cohesive failure in the adhesive was the primary fracture mode for specimens surface-treated by ion irradiation or plasma.     

Improved graphitization and electrical conductivity of suspended carbon nanofibers derived from carbon nanotube/polyacrylonitrile composites by directed electrospinning

Single suspended carbon nanofibers on carbon micro-structures were fabricated by directed electrospinning and subsequent pyrolysis at 900 °C of carbon nanotube/polyacrylonitrile (CNT/PAN) composite material. The electrical conductivity of the nanofibers was measured at different weight fractions of CNTs. It was found that the conductivity increased almost two orders of magnitude upon adding 0.5 wt.% CNTs. The correlation between the extent of graphitization and electrical properties of the composite nanofiber was examined by various structural characterization techniques, and the presence of graphitic regions in pyrolyzed CNT/PAN nanofibers was observed that were not present in pure PAN-derived carbon. The influence of fabrication technique on the ordering of carbon sheets in electrospun nanofibers was examined and a templating effect by CNTs that leads to enhanced graphitization is suggested.     

Hot pressed and spark plasma sintered zirconia/carbon nanofiber composites

Zirconia/carbon nanofiber composites were prepared by hot pressing and spark plasma sintering with 2.0 and 3.3 vol.% of carbon nanofibers (CNFs). The effects of the sintering route and the carbon nanofiber additions on the microstructure, fracture/mechanical and electrical properties of the CNF/3Y-TZP composites were investigated. The microstructure of the ZrO₂ and ZrO₂–CNF composites consisted of a small grain sized matrix (approximately 120 nm), with relatively well dispersed carbon nanofibers in the composite. All of the composites showed significantly higher electrical conductivity (from 391 to 985 S/m) compared to the monolithic zirconia (approximately 1 × 10⁻¹⁰ S/m). The spark plasma sintered composites exhibited higher densities, hardness and indentation toughness but lower electrical conductivity compared to the hot pressed composites. The improved electrical conductivity of the composites is caused by CNFs network and by thin disordered graphite layers at the ZrO₂/ZrO₂ boundaries.     

The effect of Ne+ ion implantation on polyimide film

The fluence of Ne⁺ ion irradiation on the surface modification of polyimide (Kapton HN type) film was investigated. The irradiation of ion implantation onto a polyimide film was performed, and the surface chemical structure was analyzed in detail by X-ray photoelectron spectroscopy (XPS). An acceleration voltage of 100 keV was used in the ion implantation with different doses from 5 × 10¹⁴ to 5 × 10¹⁷ ion cm^?2 and a beam current density of 10 μA cm^?2. The elemental ratios of carbon, oxygen and nitrogen were calculated from 1s peaks of the corresponding elements. The results showed that the content of carbon in the surface layer increased after ion irradiation, while the ratios of oxygen decreased after irradiation, especially in the case of the polyimide film treated at ion fluence. The O1s spectra after ion irradiation are related to the rearrangement of those recoil atoms and the ion incorporated into the film and the formation of new types of bond, such as C–O and O–O.     

Room-temperature growth of ion-induced carbon nanofibers: Effects of ion species

Graphite surfaces were bombarded with Ne⁺, Ar⁺ and Xe⁺ ions at 450 eV–1 keV to induce the carbon nanofiber (CNF) growth at room temperature, and the dependence of size and numerical density of ion-induced CNFs on the ion species and ion energy was investigated in detail. The ion-sputtered surfaces were covered with densely distributed conical protrusions and aligned CNFs grew on the tips, except for the low-energy Xe⁺-sputtered surfaces. Longer CNFs grew by lighter-mass-ion irradiation, and finer CNFs formed by heavier-mass-ion bombardment. In addition, the higher the ion energy, the longer the length of the ion-induced CNFs. Because the size and numerical density were controllable by the ion-irradiation parameters, ion-induced CNFs were believed to be quite promising for myriad of applications such as high-resolution scanning probe microscope cantilevers, bio-cell manipulators and field emission source operating at low voltage.     

Electrospinning-derived ZnCo2O4@carbon nanofiber composite for high-performance lithium ion storage

The degradation of the cobalt-zinc oxide structure and its poor conductivity during the charge and discharge limit their further applications for lithium ion storage. Herein, ZnCo₂O₄@carbon nanofiber composite with nano-fibrous structure is obtained by electrospinning, annealing in argon and low-temperature oxidation to effectively overcome the above issue. The active sites of ZnCo₂O₄ are evenly dispersed inside the carbon nanofibers, which can effectively avoid its aggregation and improve electrical conductivity. Additionally, the stable nanofibrous structure can maintain structural stability. The composite exhibits superior lithium ion storage capacity when being served as anode electrode. The ZnCo₂O₄@carbon nanofiber electrode possesses a high capacity of 1071 mA h g⁻¹ at 0.1 A g⁻¹. Besides, the electrode shows an outstanding rate capability of 505 mA h g⁻¹ at 3 A g⁻¹ and maintain 714 mA h g⁻¹ after 250 cycles when current density is adjusted to 0.2 A g⁻¹ again. Additionally, the electrode has an outstanding long-cycle performance, which remains a capacity of 447.165 mA h g⁻¹ at 0.5 A g⁻¹ after 500 cycles and 421.477 mA h g⁻¹ at 1 A g⁻¹ after 518 cycles. This result demonstrates that ZnCo₂O₄@carbon nanofiber composite has potential application prospects in the fields of advanced energy storage.     

Encapsulating homogenous ultra-fine SnO2/TiO2 particles into carbon nanofibers through electrospinning as high-performance anodes for lithium-ion batteries

Homogenous ultra-fine SnO₂/TiO₂ particles encapsulated into carbon nanofibers (SnO₂/TiO₂@CNFs) with a uniform and ordered one-dimensional fibrous structure are fabricated through facile electrospinning technique and subsequent heat treatments, which are confirmed by XRD, Raman, TG, SEM, TEM, and XPS analyses. The battery performance reveals that the SnO₂/TiO₂@CNFs-1.5:1 (1.5:1 denotes the mole ratio of SnO₂ to TiO₂ in the carbon nanofibers) electrode displays the optimal electrochemical properties among the whole samples, which can deliver the initial charge and discharge specific capacity of 1061.2 and 1494.8 mAh/g with a coulombic efficiency of 71.0% at 100 mA/g, and exhibit a remarkable specific capacity of 766.1 mAh/g after 200 cycles. Moreover, the SnO₂/TiO₂@CNFs-1.5:1 electrode displays a high pseudocapacitive contribution of 73.9% at the scan rate of 2 mV/s and the lithium ion diffusion coefficient of approximately 1.20 × 10^?15 cm² s^?1. The excellent electrochemical performance of the SnO₂/TiO₂@CNFs-1.5:1 electrode is closely correlated with the synergetic effect of the proper amount of TiO₂ that enhances the electrochemical stability of the electrode and provides fractional capacity, and the flexible and conductive carbon nanofiber matrix that accommodates volume changes and increases overall electronic conductivity. The detailed investigations of the as-prepared electrode materials by a facile electrospinning process may pave possible instructions for the next generation SnO₂-based anodes and other related electrospun anodes for the energy storage device.     

Conductive polypyrrole nanofibers via electrospinning: Electrical and morphological properties

Conductive polypyrrole nanofibers with diameters in the range of about 70-300 nm were obtained using electrospinning processes. The conductive nanofibers had well-defined morphology and physical stability. Two methods were employed. Electrospun nanofibers were prepared from a solution mixture of polypyrrole (PPy), and poly(ethylene oxide) (PEO) acted as a carrier in order to improve PPy processability. Both the electrical conductivity and the average diameter of PPy nanofibers can be controlled with the ratio of PPy/PEO content. In addition, pure (without carrier) polypyrrole nanofibers were also able to be formed by electrospinning organic solvent soluble polypyrrole, [(PPy3)⁺ (DEHS)⁻]_x, prepared using the functional doping agent di(2-ethylhexyl) sulfosuccinate sodium salt (NaDEHS) [Jang KS, Lee H, Moon B. Synth Met 2004;143:289-94. [24]]. Electrospun blends of sulfonic acid (SO₃H)-bearing water soluble polypyrrole, [PPy(SO₃H)-DEHS], with PEO acting as a carrier, are also reported. The factors that facilitate the formation of electrical conduction paths through the electrospun nanofiber segments are discussed.     

Electrospun Ca3Co4−xO9+δ nanofibers and nanoribbons: Microstructure and thermoelectric properties

Oxide-based ceramics offer promising thermoelectric (TE) materials for recycling high-temperature waste heat, generated extensively from industrial sources. To further improve the functional performance of TE materials, their power factor should be increased. This can be achieved by nanostructuring and texturing the oxide-based ceramics creating multiple interphases and nanopores, which simultaneously increase the electrical conductivity and the Seebeck coefficient. The aim of this work is to achieve this goal by compacting electrospun nanofibers of calcium cobaltite Ca₃Co_4−xO_9+δ, known to be a promising p-type TE material with good functional properties and thermal stability up to 1200 K in air. For this purpose, polycrystalline Ca₃Co_4−xO_9+δ nanofibers and nanoribbons were fabricated by sol–gel electrospinning and calcination at intermediate temperatures to obtain small primary particle sizes. Bulk ceramics were formed by sintering pressed compacts of calcined nanofibers during TE measurements. The bulk nanofiber sample pre-calcined at 973 K exhibited an improved Seebeck coefficient of 176.5 S cm⁻¹ and a power factor of 2.47 μW cm⁻¹ K⁻² similar to an electrospun nanofiber-derived ceramic compacted by spark plasma sintering.     

The improved electrical conductivity of carbon nanofibers by fluorinated MWCNTs

In order to increase the conductivity of carbon nanofiber sheet, conductive multi wall carbon nanotubes (MWCNTs) was added into the carbon fibers. The dispersion of MWCNTs into the fibers and adhesion between carbon fibers and MWCNTs were improved through fluorine modification on surface of MWCNTs. By fluorination treatment, hydrophobic functional group was introduced on the surface of MWCNTs improving the affinity on interface between two carbon materials. These nanofibers made by electrospinning method were treated at different temperature in order to investigate the effect of temperature. According to the increment of temperature, the better conductivity of carbon nanofibers sheet was obtained due to the better oriented carbon structure. Eventually, the improved conductivity of carbon nanofiber sheet was resulted showing 27 S/cm.     

Degradation of polyimide by implantation with Ar+ ions

Polyimide (PI) samples were irradiated with 200 keV Ar⁺ ions to fluences from 5 × 10¹³−1 × 10¹⁷ cm⁻² and the concentration depth profiles of implanted Ar atoms as well as of carbon and oxygen atoms of the polymer matrix were determined using the Rutherford backscattering technique. The surface polarity, sheet resistivity, and thermoelectric power of PI samples were also determined as a function of the ion fluence and temperature. As a result of the ion irradiation, the polyimide surface layer is depleted of oxygen and enriched by carbon. The sheet resistivity exhibits a minimum at the ion fluence of 5 × 10¹⁶ cm⁻² and the temperature dependence of the sheet resistivity indicates the semiconducting character of irradiated PI and the variable range hopping mechanism of charge transport. The thermoelectric power of the PI samples irradiated to high fluences is small, of the order of μV/K, and independent of temperature. This behavior is typical for metals. The simultaneous appearance of metal and semiconducting properties is probably due to the complex structure of the PI surface layer modified by the ion irradiation. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 723–728, 1997     

Ion beam ablation of polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) was irradiated with 300 keV Ar⁺ ions to the fluences of 1 × 10¹⁴ to 1 × 10¹⁶ cm⁻²; the PTFE structural changes induced by the ion irradiation were studied by X-ray diffraction and UV–vis and IR spectroscopies. The electrical conductivity of the ion beam modified PTFE was also investigated using the standard technique, and the alterations of the surface polarity were determined by contact angle measurements. The ion irradiation leads to intensive PTFE ablation due to the breaking of the C—C bonds in the polymer molecular chains and due to the production and liberation of the molecular fragments C_xF_y. In contrast to other polymeric materials, the irradiated PTFE carbonizes to a lesser extent and the observed irradiation induced increase of the electrical conductivity is also not significant. In-coming ions cause a reduction of the crystalline phase content in the PTFE samples. © 1998 John Wiley & Sons, Inc. J Appl Polm Sci 69: 1257–1261, 1998     

Holistic insights on polyimide nanocomposite nanofiber

ABSTRACT

Polyimides form an important class of high-performance polymers. Polyamides with aromatic macromolecular architecture have revealed excellent conducting, mechanical, and thermal properties for aerospace, automotive, electronics, and other technical industries. Recently, polyimide nanofiber and polyimide nanocomposite nanofiber have been focused for fabrication, essential physical features, and subsequent performance. Due to diversity of dianhydrides, diamines, and reinforced nanoparticles, various polyimide nanocomposite nanofibers have been judiciously designed. This comprehensive review highlights indispensable perspectives of polyimide nanocomposite nanofiber particularly polyimide/graphene, polyimide/carbon nanotube, and polyimide/inorganic nanoparticle-based nanofibers. It is envisioned that nanocomposite nanofibers have various high-value applications such as membranes, battery separators, electrodes, etc.     

Surface treatment of CFRP composites by Ar+ irradiation to improve bonding strength between aluminum and CFRP composites

The effect of surface treatment of carbon fiber reinforced plastic (CFRP) composites on the T-peel strength and the shear strength between CFRP and aluminum panels was studied. The surface of the composite panel was treated with Ar⁺ irradiation under oxygen environment. The optimal Ar⁺ ion dose was determined by measuring the changes of contact angle and surface energy as a function of ion dose. T-peel tests and SLS tests were performed using irradiated CFRP/aluminum specimens and unirradiated CFRP/aluminum specimens to determine the treatment effect by Ar⁺ irradiation under oxygen environment on the T-peel strength and shear strength of CFRP/aluminum composites. The results showed that contact angle on the surface of the composite panel was reduced from ∼80° to ∼8° and the surface energy increased from 31 ergs/cm² to 72.4 ergs/cm² with an ion dose of 10¹⁷ ions/cm². T-peel strength and shear strength are significantly affected by the surface treatment of composite panel. T-peel strength and shear strength improved 650% and 56%, respectively, when the treatment was made with an ion dose of 10¹⁶ ions/cm². SEM examination showed that the improvement of bonding strength was attributed to the uniform spread and fracture of epoxy adhesive.     

Structural evolution in graphitization of nanofibers and mats from electrospun polyimide–mesophase pitch blends

The evolution of structure in multi-step thermal treatment of polyimide–mesophase pitch (PI–pitch) blend nanofiber mats obtained by an electrospinning process is described. The mats were thermally treated at a series of stages up to 3000 °C. The structural transformation of the nanofiber mats consisted of three regimes. First regime corresponds to the removal of the majority of non-carbon elements and the formation of initial residual carbon. Second regime involves slow growth of the graphitic layers and slow improvement of their stacking order. Progressive graphitization occurs in regime three when the fibers become highly graphitic. The addition of pitch was found to give rise to overall enhanced graphitic order in the PI–pitch blend nanofibers as reflected in the smaller inter-layer spacing d₀₀₂ approaching that of the perfect graphite crystal, and the larger crystal sizes, L_c and L_a, confirmed by XRD analysis, as well as the higher ratio of graphitic structure revealed by Raman spectroscopy. Development of highly localized oriented domains in these nanofibers were observed by dark field TEM. The addition of pitch led to enhancement of both electrical and thermal conductivity.     

Mass‐Production of Electrospun Carbon Nanofiber Containing SiOx for Lithium‐Ion Batteries with Enhanced Capacity

In this study, electrospun carbon nanofibers hybridized with silicon oxide (SiO_x) are prepared by using a syringeless electrospinning system of polyacrylonitrile (PAN) solution containing tetraethylorthosilicate (TEOS) via a sequential pyrolysis process. The syringeless electrospinning system provides a large number of composite nanofibers in a short time, and the obtained composite nanofibers exhibit uniform diameter and morphology. The composite nanofiber is converted into a carbon nanofiber containing SiO_x via a simple pyrolysis. The obtained SiO_x‐carbon nanofiber mat exhibits higher charge/discharge capacity than a general carbon nanofiber, and it provides more stable retention than single crystalline silicon materials. Thus, the mass‐production of a SiO_x‐carbon nanofiber from syringeless electrospinning is a promising method to produce anodic materials for Li‐ion batteries.     

Electrical Behavior of Nonstoichiometric TiN1+x Nanofibers by Electrospinning

Nonstoichiometric TiN_1+x(x = 0–0.26) nanofibers have been successfully fabricated by electrospinning versus ammonification process. Electrical properties of TiN_1+x nanofibers were investigated with respect to the stoichiometric deviation degree x. The electric conductivity of the TiN_1+x nanofiber reached as high as 121.1 S/cm with x = 0.03. When stoichiometric deviation degree increased, the electric conductivity decreased and nonlinear I–V character was shown in these TiN_1+x nanofibers. The nonohmic electric characters of the nonstoichiometric TiN_1+x nanofiber prepared by electrospinning combined with ammoniation demonstrated a promising candidate of one‐dimensional material applied in electric devices such as varistors or microswitch.     

Effects of poly (vinyl alcohol) (PVA) content on preparation of novel thiol-functionalized mesoporous PVA/SiO2 composite nanofiber membranes and their application for adsorption of heavy metal ions from aqueous solution

Thiol-functionalized mesoporous poly (vinyl alcohol)/SiO₂ composite nanofiber membranes and pure PVA nanofiber membranes were synthesized by electrospinning. The results of Fourier transform infrared (FTIR) indicated that the PVA/SiO₂ composite nanofibers were functionalized by mercapto groups via the hydrolysis polycondensation. The surface areas of the PVA/SiO₂ composite nanofiber membranes were >290 m²/g. The surface areas, pore diameters and pore volumes of PVA/SiO₂ composite nanofibers decreased as the PVA content increased. The adsorption capacities of the thiol-functionalized mesoporous PVA/SiO₂ composite nanofiber membranes were greater than the pure PVA nanofiber membranes. The largest adsorption capacity was 489.12 mg/g at 303 K. The mesoporous PVA/SiO₂ composite nanofiber membranes exhibited higher Cu²⁺ ion adsorption capacity than other reported nanofiber membranes. Furthermore, the adsorption capacity of the PVA/SiO₂ composite nanofiber membranes was maintained through six recycling processes. Consequently, these membranes can be promising materials for removing, and recovering, heavy metal ions in water.     

Electrochemical properties of electrospun PAN/MWCNT carbon nanofibers electrodes coated with polypyrrole

The multi-walled carbon nanotube (CNT)-embedded activated carbon nanofibers (ACNF/CNT) and activated carbon nanofibers (ACNF) were prepared by stabilizing and activating the non-woven web of polyacrilonitrile (PAN) or PAN/CNT prepared by electrospinning. Both ACNF and ACNF/CNT were partially aligned along the winding direction of the drum winder. The average diameter of ACNF was 330 nm, while that of ACNF/CNT was lowered to 230 nm with rough surface. This was attributed to the CNT-added polymer solution in the electrospinning process providing finer fibers by increasing the electrical conductivity compared with the CNT-free one. The specific surface area and electrical conductivity of ACNF were 984 m²/g and 0.42 S/cm, respectively, while those of ACNF/CNT were 1170 m²/g and 0.98 S/cm, respectively. PPy was coated on the electrospun ACNF/CNT (PPy/ACNF/CNT) by in situ chemical polymerization in order to improve the electrochemical performance. The capacitances of the ACNF and PPy/ACNF electrodes were 141 and 261 F/g at 1 mA/cm², respectively, whereas that of PPy/ACNF/CNT was 333 F/g. This improvement in capacitance was attributed to the following: (i) the preparation of aligned nano-sized ACNF/CNT by electrospinning and the addition of CNT and (ii) the formation of a good charge-transfer complex by the PPy coating on the surface of the aligned nano-sized ACNF/CNT. The former leads to a good morphology and superior properties, such as a higher surface area, the formation of mesopores and an increase in electrical conductivity. The latter offers a refined three-dimensional network due to the highly porous structure between ACNF/CNT and PPy.     

Antibacterial activity of photocatalytic electrospun titania nanofiber mats and solution-blown soy protein nanofiber mats decorated with silver nanoparticles

Highly porous photocatalytic titania nanoparticle decorated nanofibers were fabricated by electrospinning nylon 6 nanofibers onto flexible substrates and electrospraying TiO₂ nanoparticles onto them. Film morphology and crystalline phase were measured by SEM and XRD. The titania films showed excellent photokilling capabilities against E. coli colonies and photodegradation of methylene blue under moderately weak UV exposure (≤ 0.6 mW/cm² on a 15-cm illumination distance). In addition, solution blowing was used to form soy protein-containing nanofibers which were decorated with silver nanoparticles. These nanofibers demonstrated significant antibacterial activity against E. coli colonies without exposure to UV light. The nano-textured materials developed in this work can find economically viable applications in water purification technology and in biotechnology. The two methods of nanofiber production employed in this work differ in their rate with electrospinning being much slower than the solution blowing. The electrospun nanofiber mats are denser than the solution-blown ones due to a smaller inter-fiber pore size. The antibacterial activity of the two materials produced (electrospun titania nanoparticle decorated nanofibers and silver-nanoparticle-decorated solution-blown nanofibers) are complimentary, as the materials can be effective with and without UV light, respectively.