Interfacial aspects of carbon composites

Abstract

Carbon-based composites bring great promise for various practical applications ranging from aviation industry to advanced biomedical sensors. The interface chemistry and the ultimate conductivity of these composites are responsible for their functional applicability. The interfaces can be modified by various chemical and physical techniques. This article reviews the synthesis methods of carbon composites and discusses how the interface properties dictate their applicability.     

Jahn-Teller effect induced phase transitions in CsCuCl3

Occurence of the three Jahn-Teller effect induced phase transitions of CsCuCl₃ at 423, 510 and 535 K has been established and the nature of the transitions examined by X-ray crystallography, far infrared spectroscopy and other techniques.     

Interfacial interaction in carbon fiber/thermoplastic composites

This work was undertaken in order to increase the understanding of the mechanism responsible for fiber/matrix interaction in carbon fiber/thermoplastic composite. From results of previous study on carbon fiber/PEEK composite, which suggested that the formation of the fiber/ matrix interaction was primarily related to a chemisorption mechanism, a study was done of the conditions required to obtain efficient fiber/matrix interaction in PA-12 and PP/carbon fiber composites. The interest in studying carbon fiber composite based on PP and PA-12 was that these two matrices are very different in terms of reactivity, polyamide having many more reactive groups than polypropylene. As expected, due to the non-reactive chemical structure of the polypropylene, fiber/matrix interaction in carbon fiber/PP composite occurred only when the matrix was thermally degraded, i.e. when the composite was molded at high temperature or under long residence time at the melt temperature. For the carbon fiber/PA-12 composite, strong fiber/matrix interaction occurred readily at relatively low molding temperature, i.e. well before thermal degradation of the matrix. It was also found that the short beam shear strength in these composites seems to evolve with molding temperature, and a maximum interfacial strength was observed at a molding temperature corresponding to the thermal degradation of the matrix. This indicates that although matrix degradation often results in strong reduction in the composite performance, some matrix degradation can be beneficial in terms of interfacial mechanical properties. Finally, this work demonstrated that while the formation of fiber/matrix interaction seems to be primarily related to a chemisorption mechanism, the contribution of interphase crystallinity to the interfacial strength is not negligible. In fact, interfacial crystallinity was found to be essential to ensure optimum interfacial strength.     

Interfacial bonding characteristic of nanoclay/polymer composites

Using a small amount of nanoclay (montmorillonite (MMT)) can significantly enhance the thermal and mechanical properties of polymer-based composites. Therefore, an in depth understanding of the bonding characteristic between the nanoclay and its surrounding matrix is essential. In this study, Fourier Transform Infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were conducted to analyze the chemical composition between epoxy matrix and nanocomposite. These experiments revealed that a chemical bonding at an interface between the matrix and nanoclay of the composites did exist. Thus, such bonding can enhance the mechanical and thermal properties of resultant polymer composites as reported in many literatures.     

Interfacial structure and mechanical properties of MgLi matrix composites

The study on interfacial structure and tensile properties of MgLi matrix composites. The results showed that there was a clear interface between the MgLi matrix and SiC whiskers. Calculation of thermodynamics confirmed that the clear interface between the matrix and SiC whiskers may contribute to the low reactionary potential or the low reactionary dynamics. However, some SiC whiskers were attacked. As a result, SiC whiskers connected with matrix in {111} and formed 70.5° or 109.5° stages on the whiskers surface in {111} face. The reason was the lower interfacial energy of {111} face. Tensile test confirmed that the SiC_w /MgLiAl composites showed higher tensile strength and higher modulus compared with MgLi matrix. Moreover, the specific strength and specific modulus were also increased obviously.     

Interfacial adhesion of lignocellulosic materials in polymer composites: an overview

Cellulosic materials have long been used as cost-cutting fillers in the plastic industry. Among the various factors which determine the final performance of the composite materials depend, to a large extent, on the adhesion between the polymer matrix and the reinforcements, and, therefore, on the quality of the interface. In fact, the majority of cellulosic raw materials are lignocellulosics of different polarity to plastics, and due to this divergent behavior, the adhesion between cellulosic materials and polymer matrices is very poor. However, a sufficient degree of interaction or adhesion between the surface of the cellulosic materials and matrix resin is usually desired to achieve an optimum performance of the end-product. In many cases surface modification of the cellulosics or the matrix resins, using various additives, vinyl monomers, or coupling agents, are considered to be essential to achieve this goal. The present paper surveys research work published in this field with special emphasis on the cellulosic materials' surface chemistry, morphology, as well as interfacial properties of the composites in order to elucidate the role of surface treatments. In order to elucidate the mechanism of interaction on molecular level, it is necessary to employ various techniques, such as spectroscopy that can measure surface events. In fact, the complexity of the interphase region requires a variety of characterization methods for the thorough understanding of the physical and chemical nature of this region. Moreover, a proper combination of different techniques is necessary to provide a true picture of the interphase.     

Spin incommensurability and two phase competition in cobaltites

The perovskite LaCoO3 evolves from a nonmagnetic Mott insulator to a spin cluster ferromagnet (FM) with the substitution of Sr2+ for La3+ in La1-xSrxCoO3. The clusters increase in size and number with x and the charge percolation through the clusters leads to a metallic state. Using elastic neutron scattering on La1-xSrxCoO3 single crystals, we show that an incommensurate spin superstructure coexists with the FM spin clusters. The incommensurability increases continuously with x, with the intensity rising in the insulating phase and dropping in the metallic phase as it directly competes with the commensurate FM, itinerant clusters. The spin incommensurability arises from local order of Co3+-Co4+ clusters but no long-range static or dynamic spin stripes develop. The coexistence and competition of the two magnetic phases explain the residual resistivity at low temperatures in samples with metalliclike transport.     

Surface induced anisotropy of metal-dielectric composites and the anomalous spin Hall effect

We show that optical anisotropy can exist in composite materials even when they consist of components that are isotropic in shape, spatial distribution, and optical properties. We demonstrate that the simple presence of a surface on a metal-dielectric composite induces an optical anisotropy that manifests itself in an unusual change of the state of polarization and spin Hall effect of reflected light.     

Interfacial conductivity enhancement in alkali oxy-salts by second phase dispersion

In the search for the improvement the conduction properties of composite ceramics, we have found a conductivity enhancement by two orders of magnitude when ferroelectric PZT particles are dispersed into an NZS particle matrix. Ferroelectric BaTiO₃ particle dispersion has shown that the enhancement does not depend on the particle size, but depends on the magnitude of dielectric constant. An application of solid super acid particles such as ZrO₂ treated with sulfuric acid is another example to enhance the conductivity. Lithium compounds also have the effect to enhance the conductivities in other lithium ion conductor systems. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Interfacial enhancement of flexible PVC-silica composites by silane coupling agents

Enhancement of a flexible poly(vinyl chloride) (PVC)-silica composite interface was studied by the application of γ-aminopropyltrimethoxysilane on silica. Composites containing silica and silanized silica up to 25.6 phr (per hundred resin) and prepared by sol-gel technology were subjected to water and water vapor attacks similar to that in their daily use. Silane application resulted in diminishing liquid water and water vapor sorption by about 24.0% and 11.9%, respectively. Equilibrium weight gain values of the composites having different amounts of silica correlated well with a peak at 3400 cm^-1 in the IR spectra which was attributed to the stretching vibration of the O-H group of water. Liquid water and water vapor diffusivities in composites determined by the evaluation of weight gain against time data were about 0.4 x 10^-13 and 0.4 x 10^-12 m² s^-1, respectively. Inhibition of dioctyl phthalate (DOP) migration from composites by silane application was also determined as 24% using UV measurements. The most impressive merit of silane enhancement was observed as the retention of ultimate tensile strength (UTS) under wet conditions. While the untreated silica composite reduced its UTS by about 21.2%, silanized silica composite reduced its UTS by only about 13.6%, on wetting.     

Interfacial morphology and domain configurations in 0-3 PZT-Portland cement composites

Cement-based piezoelectric composites have attracted great attention recently due to their promising applications as sensors in smart structures. Lead zirconate titanate (PZT) and Portland cement (PC) composite were fabricated using 60% of PZT by volume. Scanning Electron Microscope and piezoresponse force microscope were used to investigate the morphology and domain configurations at the interfacial zone of PZT-Portland cement composites. Angular PZT ceramic grains were found to bind well with the cement matrix. The submicro-scale domains were clearly observed by piezoresponse force microscope at the interfacial regions between the piezoelectric PZT phase and Portland cement phase, and are clearer than the images obtained for pure PZT. This is thought to be due to the applied internal stress of cement to the PZT ceramic particle which resulted to clearer images.     

Lignocellulosic materials and unsaturated polyester matrix composites: Interfacial modifications

The addition of particulate fillers (woodflour) and short fibers (sisal) into an unsaturated polyester matrix was performed and analyzed. The efficiency of the filler treatment was carefully investigated, in particular, esterification with two different anhydrides, maleic anhydride (MAN) and an alkenyl succinic anhydride (ASA). The reaction with MAN was performed under different experimental conditions in order to reach different degrees of esterification. The efficiency of the reactions was assessed by FTIR, titrimetric techniques and moisture absorption values. Esterification improves the wettability of the fillers by the resin, so that higher concentrations of filler could be incorporated in the composite. The reaction of the unsaturations of the attached anhydrides with the styrene co-monomer was confirmed by FTIR. This reaction was frequently invoked in the literature, but most times was not confirmed. Scanning electron microscopy study confirmed the improved adhesion of the fillers to the matrix. Surfaces of fractured esterified woodflour composites showed that the resin fills the hollow central regions of the woodflour cells (lumens) reducing capillarity effects during humidity or water sorption by the composites. Mechanical tests were performed on some of the composites to illustrate the effect of the covalent linkages created between the esterified lignocellulosics and the matrix. Tests in which the characteristics of the interphase played an important role were chosen in order to highlight differences resultant of the chemical co-reaction.     

Interfacial reinforcement in composites with PPT aramid fiber modified by network-intercalation method

—A novel surface treatment for poly(p-phenylene telephthalamide) (PPTA) fiber is performed with silanes and urethane binder that are usually used as sizes for glass fiber treatment. The PPTA used for the surface treatment is modified by a spinning process to make the gaps between PPTA crystallites open. In this treatment, supercritical carbon dioxide fluid method is used to impregnate the sizing molecules into open gaps in PPTA fiber. After the impregnation, the fiber is heated at 100–170°C to make the gaps close and turn open-gapped fiber to the normal type of PPTA modified with sizes. The interfacial shear strength of fiber to epoxy resin is measured by microdroplet method. The modified PPTA improves the interfacial shear strength by ca. 67% to the interfacial shear strength given by normal PPTA without treatment. Those improvements are 33% without heating, 18% with only silanes, and 12% with only urethane instead of the mixture of silane and urethane. In addition, the fiber strength shows no remarkable decrease after the treatment.     

Interfacial effects in short sisal fiber/maleated castor oil foam composites

In this study, bio-foam composites are produced using short sisal fiber as the reinforcement and modified castor oil as the matrix, respectively. The foam composites with an average cell size of 200 μm possess properties similar to those of commercial polyurethane foams. The effects of fiber loading, fiber length and foam density on the compressive properties of the foam composites are reported in relation to the interfacial interaction. It is found that the addition of chopped sisal alters cell structure of the foam. Surface pre-treatment of sisal by alkali or silane coupling agent helps to improve the mechanical properties and interfacial adhesion. The exposure of the fibers to the gas cells of the foam reduces the effectiveness of interfacial effect, which is different from the case of conventional bulk composites. As a result, the reinforcing ability of sisal fibers becomes a function of fiber length and so on.     

Ferromagnetism and colossal magnetoresistance from phase competition

We report a multicomponent theory for the coexistence of charge ordering (CO), and antiferromagnetic (AFM) and ferromagnetic (FM) spin ordering. This kind of state is invoked for manganites by Moreo et al., Science 283, 2034 (1999) and observed in recent experiments. We show that doping an AFM or CO state always generates a FM component. FM, AFM, and CO necessarily coexist in a particle-hole asymmetric system. Melting of large AFM-CO orders by small magnetic fields and colossal magnetoresistance (CMR) arise whenever the CO and AFM order parameters have similar magnitude and momentum structure. Hole doping favors FM metallic states while electron doping favors AFM-CO states, as in CMR manganites.     

Structural phase transition induced by the Jahn-Teller effect. Antiferrodistortive ordering

We study the phase transitions induced by the Jahn-Teller effect ofE-doublet ions in a cubic crystal with antiferrodistortive interactions. AnS=1 pseudospin model is constructed which takes the three lowest vibronic levels of the Jahn-Teller complexes into account. We find a second-order phase transition to a tetragonal phase with two inequivalent sublattices. The transitions between the vibronic levels give rise to bands of collective vibronic excitations with strongly temperature-dependent frequencies. The nature of the various modes is analyzed in detail. We also study the coupling to the elastic displacement field of the crystal. For a sufficiently large coupling constant, this coupling stabilizes a different low-temperature tetragonal phase with two equivalent sublattices. In a certain region of coupling constants, a transition occurs between the two tetragonal phases by second-order transitions to an intermediate phase of lower symmetry. The influence of the coupling on the dynamic behaviour is discussed.Supported by Schweizerischer Nationalfonds     

Structural phase transition induced by the Jahn-Teller effect. Antiferrodistortive ordering

We study the phase transitions induced by the Jahn-Teller effect ofE-doublet ions in a cubic crystal with antiferrodistortive interactions. AnS=1 pseudospin model is constructed which takes the three lowest vibronic levels of the Jahn-Teller complexes into account. We find a second-order phase transition to a tetragonal phase with two inequivalent sublattices. The transitions between the vibronic levels give rise to bands of collective vibronic excitations with strongly temperature-dependent frequencies. The nature of the various modes is analyzed in detail. We also study the coupling to the elastic displacement field of the crystal. For a sufficiently large coupling constant, this coupling stabilizes a different low-temperature tetragonal phase with two equivalent sublattices. In a certain region of coupling constants, a transition occurs between the two tetragonal phases by second-order transitions to an intermediate phase of lower symmetry. The influence of the coupling on the dynamic behaviour is discussed.     

Intrinsic spin hall effect induced by quantum phase transition in HgCdTe quantum wells

The spin Hall effect can be induced by both extrinsic impurity scattering and intrinsic spin-orbit coupling in the electronic structure. The HgTe/CdTe quantum well has a quantum phase transition where the electronic structure changes from normal to inverted. We show that the intrinsic spin Hall effect of the conduction band vanishes on the normal side, while it is finite on the inverted side. By tuning the Cd content, the well width, or the bias electric field across the quantum well, the intrinsic spin Hall effect can be switched on or off and tuned into resonance under experimentally accessible conditions.