Low-Temperature Processing of ZrB2-ZrC Composites by Reactive Hot Pressing

Dense ZrB2-ZrC and ZrB2-ZrC x∼0.67 composites have been produced by reactive hot pressing (RHP) of stoichiometric and nonstoichiometric mixtures of Zr and B4C powders at 40 MPa and temperatures up to 1600 °C for 30 minutes. The role of Ni addition on reaction kinetics and densification of the composites has been studied. Composites of ∼97 pct relative density (RD) have been produced with the stoichiometric mixture at 1600 °C, while the composite with ∼99 pct RD has been obtained with excess Zr at 1200 °C, suggesting the formation of carbon deficient ZrC x that significantly aids densification by plastic flow and vacancy diffusion mechanism. Stoichiometric and nonstoichiometric composites have a hardness of ∼20 GPa. The grain sizes of ZrB2 and ZrC x∼0.67 are ∼0.6 and 0.4 μm, respectively, which are finer than those reported in the literature.

Synthesis of multiwall carbon nanotubes by chemical vapor deposition of ferrocene alone

Multiwall carbon nanotubes (MWNTs) filled with Fe nanoparticles (NPs) have been synthesized by thermal chemical vapor deposition of ferrocene alone as the precursor. The MWNTs were grown at different temperatures: 980 and 800 degrees C. Characterization of as-prepared MWNTs was done by scanning and transmission electron microscopy, and X-ray diffraction. The transmission electron microscopy study revealed that Fe NPs encapsulated in MWNTs grown at 980 and 800 degrees C are spherical and rod shaped, respectively. Room-temperature vibrating sample magnetometer studies were done on the two samples up to a field of 1T. The magnetization versus magnetic field loop reveals that the saturation magnetization for the two samples varies considerably, almost by a factor of 4.6. This indicates that Fe is present in different amounts in the MWNTs grown at the two different temperatures. (C) 2009 Elsevier Ltd. All rights reserved.

Environmental effects on fibre-Polymer composites

Moisture absorption characteristics and its effects on the mechanical properties and failure process of polymers (neat epoxy and polyester resins) and composites with simple (glass, carbon and kevlar) and hybrid (glass-carbon, carbon-kevlar and kevlar-glass) fibres were experimentally determined before and after immersion in water at 343 K for 20 days. The maximum moisture content (Mm) and diffusion coefficient (Dx) of these composites were determined. The degradation in ultimate tensile strength and Young's modulus due to the moisture content were experimentally determined and found to be quite significant. Acoustic emissions, from specimens before and after exposure, were monitored during the load cycle, and revealed a significant change in the failure process of these composites. Scanning Electron Microscope (SEM) studies on failed exposed and unexposed specimens revealed resin leach out and fibre prominence.

Effect of insertion of buffer-strip on the response of carbon-epoxy system in short beam tests

dThe work looks at the response to three-point loading of carbon-epoxy (CF-EP) composites with inserted buffer strip (BS) material. Short beam Shear tests were performed to study the load-deflection response as well as fracture features through macroscopy on the CF-EP system containing the interleaved PTFE-coated fabric material. Significant differences were noticed in the response of the CF-EP system to the bending process consequent to the architectural modification. It was inferred that introduction of small amounts of less adherent layers of material at specific locations causes a decrement in the load carrying capability. Further the number and the ease with which interface separation occurs is found to depend on the extent to which the inserted layer is present in either single or multiple layer positions.

Carbon nanotube reinforced supramolecular gels with electrically conducting, viscoelastic and near-infrared sensitive properties

Pristine and long-chain functionalized single-walled carbon nanotubes (SWNTs) were incorporated successfully in supramolecular organogels formed by an all-trans tri(p-phenylenevinylene) bis-aldoxime to give rise to new nanocomposites with interesting mechanical, thermal and electrical properties. Variable-temperature UV-vis and fluorescence spectra reveal both pristine and functionalized SWNTs promote aggregation of the gelator molecules and result in quenching of the UV-vis and fluorescence intensity. Electron microscopy and confocal microscopy show the existence of a densely packed and directionally aligned fibrous network in the resulting nanocomposites. Differential scanning calorimetry (DSC) of the composites shows that incorporation of SWNTs increases the gel formation temperature. The DSC of the xerogels of 1-SWNT composites indicates formation of different thermotropic mesophases which is also evident from polarized optical microscopy. The reinforced aggregation of the gelators on SWNT doping was reflected in the mechanical properties of the composites. Rheology of the composites demonstrates the formation of a rigid and viscoelastic solid-like assembly on SWNT incorporation. The composites from gel-SWNTs were found to be semiconducting in nature and showed enhanced electrical conductivity compared to that of the native organogel. Upon irradiation with a near IR laser at 1064 nm for 5 min it was possible to selectively induce a gel-to-sol phase transition of the nanocomposites, while irradiation for even 30 min of the native organogel under identical conditions did not cause any gel-to-sol conversion.

Synthesis of graphene oxide-intercalated alpha-hydroxides by metathesis and their decomposition to graphene/metal oxide composites

Graphene oxide-intercalated alpha-metal hydroxides were prepared using layers from the delaminated colloidal dispersions of cetyltrimethylammonium-intercalated graphene oxide and dodecylsulfate-intercalated alpha-hydroxide of nickel/cobalt as precursors. The reaction of the two dispersions leads to de-intercalation of the interlayer ions from both the layered solids and the intercalation of the negatively charged graphene oxide sheets between the positively charged layers of the alpha-hydroxide. Thermal decomposition of the intercalated solids yields graphene/nanocrystalline metal oxide composites. Electron microscopy analysis of the composites indicates that the nanoparticles are intercalated between graphene layers. (C) 2010 Elsevier Ltd. All rights reserved.

Structural, mechanical and biocompatibility studies of hydroxyapatite-derived composites toughened by zirconia addition

Hydroxyapatite(OHAp)-based ceramic composites with added ZrO2 have been prepared both by sintering at 1400 °C and by hot isostatic pressing (HIP) at 1450 °C and 140 MPa pressure (argon atmosphere). The development of the crystalline phases and the microstructure of the composites have been examined using X-ray diffraction, electron microscopy, infrared and magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopic techniques. The fracture toughness and biocompatibility of the composites have also been studied. The effect of the addition of CeO2- and Y2O3-stabilized ZrO2 and of simple monoclinic ZrO2 to the initial physical mixture, on the structure and properties of the resulting composites has been investigated. In most of the sintered or HIP samples, the OHAp decomposes into tricalcium phosphate (β-TCP). CaO, which forms as a product of decomposition, dissolves completely in ZrO2 and stabilizes the latter in its cubic/tetragonal phase. Presence of the β-TCP phase in the product seems to be the result of a structural synergistic effect of hexagonal OHAp. Two structurally distinct orthophosphate groups have been identified in the composites by MASNMR of 31P and attributed to decomposition products of OHAp at higher temperatures. The composites possess high KIC values (2–3 times higher than that of pure OHAp). Decomposition of hydroxyapatite gives rise to differences in microstructure between HIP and simply sintered composites although fracture toughness values are similar in magnitude indicating the presence of several toughening mechanisms. The in vitro SP2-O cell test suggests that these composites possess good biocompatibility. The combination of good biocompatibility, desirable microstructure and easy availability of initial reactants indicates that the simply sintered composite of OHAp and monoclinic ZrO2(ZAP-30) appears to be the most suitable for prosthetic applications.

Large negative magnetoresistance and metal-insulator transition observed in MnO/C composite coatings grown on ceramic alumina by metalorganic chemical vapor deposition

MnO/C composite coatings were grown by the metalorganic chemical vapor deposition process on ceramic alumina in argon ambient. Characterization by various techniques confirms that these coatings are homogeneous composites comprising nanometer-sized MnO particles embedded in a matrix of nanometer-sized graphite. Components of the MnO/C composite coating crystalline disordered, but are electrically quite conductive. Resistance vs. temperature measurements show that coating resistance increases exponentially from a few hundred ohms at room temperature to a few megaohms at 30 K. Logarithmic plots of reduced activation energy vs. temperature show that the coating material undergoes a metal-insulator transition. The reduced activation energy exponent for the film under zero magnetic field was 2.1, which is unusually high, implying that conduction is suppressed at much faster rate than the Mott or the Efros-Shklovskii hopping mechanism. Magnetoconductance us. magnetic field plots obtained at various temperatures show a high magnetoconductance (similar to 28.8%) at 100 K, which is unusually large for a disordered system, wherein magnetoresistance is attributed typically to weak localization. A plausible explanation for the unusual behavior observed in the carbonaceous disordered composite material is proposed. (C) 2010 Elsevier Ltd. All rights reserved.

Double Aromaticity in the 3,5-Dehydrophenyl Cation and in Cyclo[6]carbon

The presence of two (4n+2)-electron conjugated systems in perpendicular planes results in considerable aromatic stabilization. Despite having two fewer hydrogens, the 6 pi e-2 sigma e 3,5-dehydrophenyl cation (C6H3+, 1) is 32.7 (CCSD(T)/6-31G**) and 35.2 kcal/mol (RMP4sdtq/6-3iG*//RMP2(fu)/6-31G*) more stable than the phenyl cation (evaluated by an isodesmic reaction involving benzene and m-dehydrobenzene (4)). Cation 1, the global C6H3+ minimum, is 11.7,24.2, 11.8, and 30.4 kcal/mol lower in energy than the 2,6- (11) and 3,4-dehydrophenyl (12) cations as well as the open-chain isomers 13 and 14 (RMP4sdtq/6-31G*//RMP2(fu)/6-31G* + ZPE(RMP2(fu)/6-31G*)). The stability of 1 is increased hyperconjugatively by 2,4,6-trisilyl substitution. The double aromaticity of 1 is indicated by the computed magnetic susceptibility exaltations (IGLO/II//RMP2(fu)/6-31G*) of -5.2, -6.8, -15, and -23.2 relative to 11, 12, 13, and 14, respectively. Thus, 1 fulfills the geometric, energetic, and magnetic criteria of aromaticity. The double aromaticity of the D-6h cyclo[6]carbon is apparent from the same criteria

Probing the inter-tube transport in aligned and random multiwall carbon nanotubes

The temperature and magnetic field dependence of conductivity has been used to probe the inter-tube transport in multiwall carbon nanotubes (MWNTs). The scanning electron microscopy images show highly aligned and random distribution of MWNTs. The conductivity in aligned carbon nanotube (ACNT) and random carbon nanotube (RCNT) samples at low temperature follows T-1/2 (at T < 8 K) and T-3/4 (at T > 8 K) dependence in accordance with the weak localization and electron-electron (e-e) interaction model. The values of diffusion coefficient in ACNT and RCNT are 0.25 x 10(-2) and 0.71 x 10(-2) cm(2) s(-1), respectively, indicating that larger number of inter-tube junctions in later enhances the bulk transport. The positive magnetoconductance (MC) data in both samples show that the weak localization contribution is dominant. However, the saturation of MC at higher fields and lower temperatures indicate that e-e interaction is quite significant in RCNT. The T-3/4 and T-1/2 dependence of inelastic scattering length (l(in)) in ACNT and RCNT samples show that the inelastic e-e scattering is more important in aligned tubes. (C) 2011 American Institute of Physics. doi:10.1063/1.3552911]

An experimental approach for non-destructive evaluation of fatigue damage in CFRP composites

Study of fatigue phenomenon in composites requires a dynamic tool which can detect and identify different failure mechanisms involved. The tool should also be capable of monitoring the cumulative damage progression on-line. Acoustic Emission Technique has been utilized in the experimental investigations on unidirectional carbon fiber reinforced plastic (CFRP) composite specimens subjected to tension-tension fatigue. Amplitude as well as frequency distribution of Acoustic Emission (AE) signals have been studied to detect and characterize different failure mechanisms. For a quantitative measure of degradation of the material with fatigue load cycles, reduction in stiffness of the specimen has been measured intermittently. Ultrasonic imaging could give the information on the changes in the interior status of the material at different stages of fatigue life.

C-13 MAS NMR spectra of cis,cis-mucononitrile in liquid-crystalline media and in the solid state. Orientation of the carbon chemical-shift tensors

The spinning sidebands observed in the C-13 MAS NMR spectra of cis,cis-mucononitrile oriented in liquid-crystalline media and of the neat sample in the solid state are studied. There are differences in the sideband intensity patterns in the two cases. These differences arise because the order parameters which characterize the orientation of the solute in the liquid-crystalline media differ for different axes. It is shown that, in general, the relative intensities of the sidebands contain information on the sign and magnitude of an effective chemical-shift parameter which is a function of the sum of the products of the principal components of the chemical-shift tensor and the corresponding order parameters with respect to the director. A method for obtaining the orientation of the carbon chemical-shift tensor is proposed. The carbon chemical-shift tensors obtained from gauge-including atomic orbital calculations are also presented for comparison. (C) 1996 Academic Press, Inc.

Dynamic Behavior of Poly(2-vinylpyridine) in Solution Studied by 13C Nuclear Magnetic Relaxation

Abstract: The dynamics of poly(2-vinylpyridine) in chloroform solution has been examined by C-13 spin-lattice relaxation time and NOE measurements as a function of temperature. The experiments were performed at 50.3 and 100.6 MHz. The backbone carbon relaxation data have been analyzed in terms of six motional models. Among these models, the models which consider conformational transitions and bond librations for the backbone were found to be more successful. Pyridyl ring motion has been modeled as a restricted rotation with the rotational amplitude varying with temperature. The activation energy parameters obtained from the relaxation data of the pyridyl ring carbon have been compared with the energy barrier for ring rotation estimated from conformational energy calculations using the AM1 semiempirical quantum chemical method. The results of the conformational energy calculations support the description of pyridyl ring motion as a restricted rotation.

Discontinuous shear thickening in confined dilute carbon nanotube suspensions

A monotonic decrease in viscosity with increasing shear stress is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. Here we demonstrate a discontinuous shear-thickening transition on varying shear stress where the viscosity jumps sharply by four to six orders of magnitude in flocculated suspensions of multiwalled carbon nanotubes (MWNT) at very low weight fractions (approximately 0.5%). Rheooptical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shear-thinning and shear-thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear-thickening phenomena in confined geometries is pertinent for flow-controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues.

On the characterisation of syntactic foam core sandwich composites for compressive properties

Sandwich structures, especially those with honeycomb and grid structures as the core material, are very commonly employed in aircraft structures. There is an increasing use of closed-pore rigid syntactic foams as core materials in sandwich constructions because they possess a number of favourable properties. The syntactic foams, owing to their structure and formation, behave differently under compression compared to other traditionally used core materials. In the present study, therefore, syntactic foam core sandwich constructions are evaluated for their behaviour under compression in both edgewise and flatwise orientations. Further, the work characterises the relative performance of two sets of sandwich materials, one containing glass-epoxy and the other, glass/carbon hybrid-epoxy skins. As non-standard geometry test specimens were involved, only a comparative evaluation was contemplated in this approach. The experiments indicate that the nature of the reinforcement fabric in the skin has a bearing on the test results in edgewise orientation. Thus, the tendency towards initiation of vertical crack in the central plane of the core material, which is a typical fracture event in this kind of material, was found to occur after a delay for the specimens containing the glass fabric in the skin. Attempts are made to establish the correlation between observations made on the test specimen visually during the course of testing and the post-compression microscopic examinations of the fracture features.