Electrical Resistivity Of Composites Research Articles

Microstructure and properties of an electroconductive SiC-based composite

In this work, an SiC-based electroconductive composite is obtained through simultaneous addition of MoSi2 and ZrB2 particles. The composite material is fully densified by hot pressing at 1860 °C and the microstructure is investigated by SEM-EDS analysis. Microstructural features and mechanical properties are compared to those of a monolithic hot-pressed SiC material. The MoSi2 and ZrB2 particles, besides increasing the electrical conductivity of the silicon carbide matrix, also act as reinforcement for the material. Room-temperature strength reaches the value of 850 MPa and the fracture toughness is 4.2 MPa m0.5. The composite electrical resistivity is of the order of 10−3 Ω cm.

Use of polymeric emeraldine salt for conductive adhesive applications

The wide range of electrical, electrochemical, and optical properties associated with Polyaniline (PANI) and its composites has made them attractive for many industrial applications. In this study, Emeraldine Salt (ES), which is a doped conducting form of PANI, was chemically prepared in situ using the oxidizing agent ammonium persulphate in the presence of aqueous HCl solution. In order to gain insight into the efficiency of electrical conduction in relation to the chemical and viscoelastic behaviors of ES in homogeneous powder and as filler in composite adhesive forms, the interrelationship between their electrical resistivity and morphology was studied. The pressure-dependent electrical conduction behavior of ES powder shows, among other factors, the dependence of electrical resistivity on the intrinsic chemical and viscoelastic properties of powders. In order to obtain electrically conductive composite adhesive forms, a nonconducting nitrocellulose solution based adhesive was filled with as-synthesized ES in the amount 30%, 40%, and 50% by volume, and the effects of filler concentration on the composite's electrical resistivity were investigated. The results of our investigation revealed a typical percolation threshold behavior with a critical concentration of approximately 30% by volume. Finally, single lap joints were made using aluminum and zinc (plated on copper) as well as silver substrates bonded using the ES filled nitrocellulose adhesive developed, and the corresponding electrical and mechanical properties of these bonded interconnections were investigated. A complex redox-reaction mechanism catalyzed by ES filler is thought to be occurring at the boundary layer between the adhesive and the substrate for conversion from semiconductor to insulator of the joints in the cases of aluminum and zinc (plated) substrates.

Electromechanical behavior of hybrid carbon/glass fiber composites with tension and bending

AbstractTo understand the smart (i.e., good memory) characteristics of hybrid composites of carbon fibers (CFs) and glass fibers (GFs) with epoxy resin as a matrix, the changes in the electrical resistance of composites with tension and on bending were investigated. The electrical resistance behavior of composites under tension changed with the composition of the CF/GF, as well as with the applied strain. The fractional electrical resistance increased slowly with increasing strain within a relatively low strain region. However, with further loading it increased stepwise with the strain according to the fracture of the CF layers. The strain sensitivity of the samples increased with increasing CF weight percentage, and the samples incorporating more than 40 wt % CF showed a strain sensitivity higher than 1.54 for a single CF. The changes in the fractional electrical resistance with bending were not so dominant as those with tension. This difference was attributed to the action of two cancelling effects, which are the increasing and decreasing fractional electrical resistance due to tension and compression with bending, respectively. On recovery from a large applied bending, the fractional electrical resistance decreased slowly with unloading because of the increase of contacts between the fibers that resulted from the reorganization of ruptured CFs during the recovery. Even the composites incorporating a relatively small CF content showed an irreversible electrical resistance with both tension and bending. However, the strain sensitivity being larger with tension than with bending is ascribed to the difference in their mechanical behaviors. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2447–2453, 2002

Electrical and magnetic properties of BaTiO3−(Ni0.3Zn0.7)Fe2.1O4 composites

Ceramic composites consisting of ferroelectric \(BaTiO_3\) and ferrite (\(\left( {NiO_{0.3} Zn_{0.7} } \right)Fe_{2.1} O_4\) were synthesized by the mixed oxide route. The phase assemblage, electrical and magnetic properties of the samples were investigated. The results indicates that the \(BaTiO_3\) phase is compatible with (\(\left( {Ni_{0.3} Zn_{0.7} } \right)Fe_{2.1} O_4\) phase, and dense diphasic composite ceramics were obtained. Electrical resistivity of composites varies with increasing amounts of (\(\left( {Ni_{0.3} Zn_{0.7} } \right)Fe_{2.1} O_4\) phase, and shows a percolation-like drop. Magnetic hysteresis loops were observed in the composites.

Polymer composites prepared by compression molding of a mixture of carbon black and nylon 6 powder

AbstractThe results of an experimental study on the effect of processing variables and filler concentration on the electrical resistivity of conductive composites based on nylon 6 filled with carbon black are reported. A typical percolation behavior in the effect of electroconductive filler content on the resistivity was found. The electrical resistivity of the composites is > 1012 ohm°Cm unless the carbon black content reaches the percolation threshold at ∼9 wt%, beyond which the resistivity decreases markedly by as much as twelve orders of magnitude. Two parameters of molding process—temperature and time—were shown to have a notable effect on the resistivity of composites, whereas pressure has no influence on this property in the pressure range considered. There is no sharp variation in the density due to the onset of percolation, and the hardness of samples is not influenced by the presence of the filler.

Growth of crystals, composite crystal structures and electrical resistance of high-pressure phases of Mg 2B 1+ x (B=Sn,Ge)

Single crystals of high-pressure phases of Mg 2B 1+ x (B=Sn, Ge; x∼0.1) were grown at 5.8 GPa within the temperature range 600–1500 K. The composite crystal structures were investigated by the X-ray technique. Both structures are found to be closely related and each is composed of two incommensurate atomic subsystems. Two identically oriented hexagonal subcells are characterized by a common period a and an irrational value of the ratio γ= c I/ c II between incommensurate periods. The values of the cell parameters are, for Mg 2Sn 1+ x a=13.245(3) Å, c I=4.450(2) Å, c II=3.273(3) Å, γ∼1.36, and for Mg 2Ge 1+ x , a=12.53(1) Å, c I=4.175(3) Å, c II=3.037(3) Å, γ∼1.375. All Mg atoms and most of the Sn(Ge) atoms form subsystem I close to the α-PbCl 2 structure type (cotunnite). Both the structure type and the stoichiometry of the compounds are disturbed by Sn(Ge) atoms which form incommensurate subsystem II of the composite. Resistance measurements at low temperature confirm conduction of metallic character for both compounds.

Concrete as a new strain/stress sensor

Abstract A new strain/stress sensor technology was developed, based on the concept of short electrically conducting fiber pull-out that accompanies slight and reversible crack opening. The fiber pull-out reversibly increases the composite's electrical resistance, which is the signal provided by the sensor under static or cyclic loading. The new technology was manifested in concrete and mortar containing electrically conducting short fibers (e.g. carbon fibers and steel fibers), but not in those containing no fibers or those containing non-conducting (polyethylene) fibers. Carbon fibers worked best. They served to greatly decrease the crack height, so that reversible pull-out of the crack bridging fibers occurred. Even in the elastic regime, a part of the resistance change was irreversible, such that it provided memory of the first deformation; this is due to permanent damage, probably associated with the increase in fiber/matrix contact electrical resistivity due to the interface bond weakening. The stress at which this damage began was much lower under tension than compression. The ratio of the contribution to the fractional resistance increase by the reversible part to that by the reversible part was much higher under tension than compression. The irreversible part increased with increasing irreversible strain, which increased with increasing stress amplitude. The fractional increase in resistance at fracture was much larger under compression than under tension, was much larger for mortars than concretes at similar volume fractions, and was quite independent of the loading rate.

Electroconductive nylon 6 and copper composites

AbstractA study is reported on the effect of the filler size and concentration on the electrical resistivity, density, and hardness of composites made of copper powder embedded in nylon 6 matrix by means of compression molding. The electrical resistivity of the composites is > 1011 ohm·cm unless the metal content reached the percolation threshold, beyond which the resistivity decreased markedly by as much as 1012. The percolation concentration was found to decrease with a decrease in the average particle diameter. The density of the composites was measured and compared with values calculated assuming different void levels within the samples. However, there is no sharp variation in the density due to the onset of percolation. Furthermore, it is shown that a percolation concentration can be also defined in the hardness/metal concentration curves as the intercept of linear regression curves of the low and high metal content regimes, respectively.

Application of Electrical Resistance Method in study of Sic/Al Composite

In SiC/Al composite there are obvious correspondent relations between electrical resistivity and F/M interfaces change as well as Vf. Both the F/M reaction reaction and Vf can be determined by means of the composite electrical resistivity, if the resistivity is revised by a factor small than 1.

The formation of intermetallics in Cu/In thin films

The kinetics of the formation of intermetallics in the Cu–In bimetallic thin film couple have been studied from room temperature to 432 K by measuring the evolution of composite and contact electrical resistance with time and temperature. The resistivity measurements have been supplemented by x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Copper reacts with indium even at room temperature to form CuIn intermetallic and assuming a model of defect assisted diffusion into the grains, the activation energy averaged over five different samples is found to be 0.40 eV. The grain boundary diffusion is found to occur with an average activation energy of 0.55 eV. XRD confirms the growth of CuIn intermetallic and on annealing at higher temperature, for copper-rich films copper further reacts with CuIn to form Cu9In4. Further evidences of solid state reactions and grain boundary diffusion through Cu grain boundaries have been obtained from SEM study. TEM indicates the growth of the grain size on annealing and confirms the presence of the CuIn phase.

The kinetics of formation of intermetallics in Ag/In thin film couples

Silver and indium bimetallic thin film couples have been studied by monitoring the contact and composite electrical resistance behaviour with time for both as-deposited and annealed specimens. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies have also been performed on the samples. Silver reacts with indium at room temperature with the formation of ε-AgIn 2 phase, which is diffusion controlled, the activation energy being about 0.43 eV in the temperature range 318–401 K. The activation energy of grain boundary diffusion as determined from contact electrical resistance measurements is about 0.34 eV. Both XRD and TEM indicate the growth of ε phase, which gradually transforms to δ-Ag 2In phase for silver-rich films. Indium diffusion through silver grain boundaries has been observed by SEM. A tentative explanation of the first phase formation is proposed.

Electrical Resistivity of Composites

Percolation and Bruggeman's effective media theories, as they apply to the electrical conductivity of composites, are reviewed, and a general effective media (GEM) equation, which combines most aspects of both percolation and effective media theories, is introduced. It is then shown that the GEM equation quantitatively fits electrical resistivity (conductivity) as a function of the volume fraction data for binary composites. The parameters used to fit the experimental data are the electrical resistivities of the two phases, the percolation threshold for the lower resistivity phase (ôc), and an exponent t. Preliminary work, showing how the GEM equation can be used to model the piezoresistivity of composites by postulating that ôc is a function of the independent variable, is also presented.

A study of intermetallic compound formation in a copper–tin bimetallic couple

The formation of intermetallics in copper–tin bimetallic couples has been studied from room temperature to 183 °C by measuring the evolution of contact resistance and composite electrical resistance with time and temperature in order to assess the kinetic behavior of the system. X-ray diffractogram (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies have also been performed on the samples. As regards bulk diffusion, copper diffuses interstitially into tin rapidly at room temperature with the formation of η′-Cu6Sn5 intermetallic compound. Further diffusion through this phase as evaluated by composite electrical resistivity measurements is given by 0.40 eV, assuming a model of defect-assisted diffusion into the grains. The grain-boundary diffusion is found to occur with an activation energy of 0.78 eV as estimated from contact resistivity measurements. SEM confirms the presence of grain-boundary diffusion of tin in copper, whereas XRD and TEM measurements indicate the growth of η′ intermetallics at room temperature which exists up to 90 °C when it transforms to the ε-Cu3Sn phase. The presence of tin whiskers on the tin surface has also been occasionally observed.

Interfacial reactions in Ag [sbnd]Zn thin film couples

Interfacial reactions in Ag Zn thin film couples have been investigated by measuring the contact resistance and composite electrical resistance with time and temperature in order to understand the kinetic behaviour of the system. The resistivity measurements have been supplemented by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The activation energy of diffusion has been found to be 6.1 × 10 −20 J from electrical resistivity measurements where the model assumes rapid grain boundary diffusion followed by a defect-assisted path for diffusion into the grain and 8.3 × 10 −20 J from contact resistance measurements where the model is based on grain boundary diffusion. XRD indicates the growth of ß′-Ag Zn phase even at room temperature which changes to the ß phase above 250°C. SEM confirms the diffusion of silver through zinc grain boundaries, especially at the interface boundary. TEM indicates the growth of grain size with annealing and confirms the presence of the ß′ phase.

Electrical resistivity of an aluminium-graphite composite between 77 K and 300 K

The electrical resistivity, ϱ, of a composite consisting of 6061 aluminium alloy containing 28 vol. % graphite (Thornel 50) fibres has been studied using temperatures, T, between 77 K and 300 K as a function of thermal annealing. These studies have been made with the electrical current parallel to the fibres. The value of ϱ decreases linearly with increasing annealing temperature. The annealing process also changes the temperature dependence of ϱ. The values of ϱ of the matrix, which depend on the annealing temperature, have been calculated from the experimental data and are compared with the estimated values of pure 6061 alumimium alloy with different temper degrees. These results indicate that the fabrication process can cause the composite electrical resistivity to be different from the theoretical estimates based on values of components.

Electrical resistivity of copper-graphite powder composites

An investigation into the variation of the electrical resistivity of copper-graphite composites in the course of their compaction has shown that the electrical resistivity of such materials falls during pressing, fairly rapidly up to pressures of 1.2–1.6 tons/cm2 and more slowly at pressures above this level. The electrical resistivity of composites containing copper-clad graphite granules is much less than that of composites with bare graphite. The reason for this is that the former materials have continuous copper skeletons, while the latter have continuous graphite skeletons.