Fine Ti‐dispersed Al2O3 composites and their mechanical and electrical properties

Al₂O₃/Ti composites containing 0‐30 vol% dispersed fine Ti particles were fabricated using a hot‐press sintering method at 1500°C from mixtures of Al₂O₃ and TiH₂ powders. During sintering, TiH₂ decomposed to form metallic Ti. The effects of the Ti content on the mechanical and electrical properties of the composites were then investigated. No Ti‐Al intermetallic compounds were detected by X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy indicated the presence of Al‐Ti‐O solid solution and Ti‐O phases. The composites showed enhanced densification; the measured densities were higher than the calculated theoretical values. Microstructural observation revealed homogeneously distributed fine Ti particles dispersed in the Al₂O₃ matrix. The Ti particle size ranged from submicrometer to a few micrometers depending on the Ti content. The fracture mode of the composites was primarily transgranular, in contrast to the intergranular fracture mode of monolithic Al₂O₃. Although the flexural strength was decreased with increase in Ti content, the composite containing 20 vol% Ti displayed the maximum fracture toughness of 4.3 MPa·cm^1/2, which was 37% greater than that of monolithic Al₂O₃. The composites containing more than 15 vol% Ti exhibited drastic decreases in resistivity (~10^?1 Ωcm), which were attributed to the formation of interconnected Ti networks at these Ti contents. The percolation threshold volume for electrical conduction in the present system was calculated to be 13.8 vol%. The results indicate that dispersing fine Ti particles into Al₂O₃ increased the fracture toughness and improved the conductivity of Al₂O₃.     

A multiscale methodology quantifying the sintering temperature‐dependent mechanical properties of oxide matrix composites

A novel methodology combining multiscale mechanical testing and finite element modeling is proposed to quantify the sintering temperature‐dependent mechanical properties of oxide matrix composites, like aluminosilicate (AS) fiber reinforced Al₂O₃ matrix (AS_f/Al₂O₃) composite in this work. The results showed a high‐temperature sensitivity in the modulus/strength of AS fiber and Al₂O₃ matrix due to their phase transitions at 1200°C, as revealed by instrumented nanoindentation technique. The interfacial strength, as measured by a novel fiber push‐in technique, was also temperature‐dependent. Specially at 1200°C, an interfacial phase reaction was observed, which bonded the interface tightly, as a result, the interfacial shear strength was up to ≈450 MPa. Employing the measured micro‐mechanical parameters of the composite constituents enabled the prediction of deformation mechanism of the composite in microscale, which suggested a dominant role of interface on the ductile/brittle behavior of the composite in tension and shear. Accordingly, the AS_f/Al₂O₃ composite exhibited a ductile‐to‐brittle transition as the sintering temperature increased from 800 to 1200°C, due to the prohibition of interfacial debonding at higher temperatures, in good agreement with numerical predictions. The proposed multiscale methodology provides a powerful tool to study the mechanical properties of oxide matrix composites qualitatively and quantitatively.     

Manipulating the conductivity of carbon‐black‐filled immiscible polymer composites by insulating nanoparticles

The conductivity of an immiscible polymer blend system, microfibrillar conductive poly(ethylene terephthalate) (PET)/polyethylene (PE) composite (MCPC) containing carbon black (CB), was changed by the addition of insulating CaCO₃ nanoparticles. In MCPC, the PET forms microfibrils during processing and PE forms the matrix. The CB particles are selectively localized in the PET microfibrils. When the insulating CaCO₃ nanoparticles are added, they substitute for some of the conductive CB particles and obstruct the electron paths. As a result, the resistivity of the MCPC can be tailored depending on the insulating filler content. The resistivity‐insulating filler content curve displays a sluggish postpercolation region (the region immediately following the percolation region and in front of the equilibrium flat of the resistivity‐filler content curve), suggesting that the MCPC in the postpercolation region possesses an enhanced manufacturing reproducibility and a widened processing window. These features are of crucial importance in making sensor materials. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008.     

Degradation of Nextel™ 610‐based oxide‐oxide ceramic composites by aluminum oxychloride decomposition products

Nextel? 610 alumina fibers and alumina‐YAG (yttrium‐aluminum garnet) matrices were used to make oxide‐oxide ceramic matrix composites (CMCs) with and without monazite (LaPO₄) fiber‐matrix interfaces. Twelve sequential aluminum oxychloride (AlOCl) infiltrations with 1 hour heat treatments at 1100°C and a final 1 hour heat treatment at 1200°C were used for matrix densification. This matrix processing sequence severely degraded CMC mechanical properties. CMC tensile strengths and interlaminar tensile (ILT) strengths were less than 10 MPa and 1 MPa, respectively. Axial fracture of Nextel? 610 fibers was observed after ILT testing, highlighting the extreme degradation of fiber strength. Extensive characterization was done to attempt to determine the responsible degradation mechanisms. Changes in Nextel? 610 fiber microstructure after CMC processing were characterized by optical microscopy, SEM, and extensively by TEM. In AlOCl degraded fibers, grain boundaries near the fiber surface were wetted with a glass that contained Y₂O₃/SiO₂ or Y₂O₃/La₂O₃/P₂O₅/SiO₂, and near‐surface pores were partially filled with Al₂O₃. This glass must also contain some Al₂O₃ and initially some chlorine. AlOCl decomposition products were predicted using the FactSage^® Thermochemical code, and were characterized by mass spectrometry. Effects of AlOCl precursors on monazite coated and uncoated Nextel? 610 fibers tow and filament strength were evaluated. A mechanism for the severe degradation of the oxide‐oxide CMCs and Nextel? 610 fibers that involves subcritical crack growth promoted by release of chlorine containing species during breakdown of intergranular glasses in an anhydrous environment is proposed.     

Improvement in electrical properties of thermoelectric CuAlO2 ceramics aided by aluminosilicate glass

Delafossite CuAlO₂ (CAO) ceramics were fabricated by the solid‐state route, using aluminosilicate glass powders as a sintering aid to improve the sintering ability and electrical conductivity, at 1473 K for 3 hours. The CAO ceramics with glass addition obviously enhanced bulk density, grain size, and electrical conductivity. It is found that the conductivity of CAO ceramics increased with the increase in glass content under 9.5%, whereas it was over 9.5%, the conductivity went down. The glass coming up to 9.5% increased the sintering ability and the electrical conductivity which was increased by one order of magnitude, thus increasing the figure of merit ZT for thermoelectric performance of our CAO added with 9.5% glass up to 9.82 × 10^?3 at 773 K, which is a high value among the CAO ceramics. Besides, the impedance analysis shows that the impedance of the CAO ceramic was controlled by its grain boundary.     

Flash sintering of 3YSZ/Al2O3-platelet composites

Preparation of 3YSZ/Al₂O₃-platelet composites always requires high temperature, long duration, and/or high pressure. Herein, 3YSZ/Al₂O₃-platelet composites are prepared at low temperature of 492°C-645°C in 30 seconds by flash sintering under the electric field of 300-800 V/cm. The influence of electric field and current limit on the densification and grain growth of composites is investigated. The onset temperature for flash sintering is determined by electric field, which is decreased with increasing the electric field. Under the constant electric field, the current limit has a great effect on the density and grain size of composite. The flash-sintered 3YSZ/Al₂O₃-platelet composites exhibit relatively high hardness and elastic modulus. Both Joule heating and defects generation are proposed to be responsible for the rapid densification in flash sintering. This work demonstrates the feasibility of employing the flash sintering to prepare ceramic composites with fine grain size.     

Thermal and electrical conductivity of carbon–filled liquid crystal polymer composites

The thermal and electrical conductivity of resins can be increased by adding conductive carbon fillers. One emerging market for thermally and electrically conductive resins is for bipolar plates for use in fuel cells. In this study, varying amounts of five different types of carbon, one carbon black, two synthetic graphites, one natural flake graphite, and one calcined needle coke, were added to Vectra A950RX Liquid Crystal Polymer. The resulting composites containing only one type of filler were then tested for thermal and electrical conductivity. The objective of this work was to determine which carbon filler produced a composite with the highest thermal and electrical conductivity. The results showed that composites containing Thermocarb TC‐300 synthetic graphite particles had the highest thermal and electrical conductivity. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99; 1552–1558, 2006     

High‐temperature tensile strength and fracture behavior of ZrB2‐SiC‐graphite composite

The tensile behavior of ZrB₂‐SiC‐graphite composite was investigated from room temperature to 1800°C. Results showed that tensile strength was 134.18 MPa at room temperature, decreasing to 50.34 MPa at 1800°C. A brittle‐ductile transition temperature (1300°C) of ZrB₂‐SiC‐graphite composite was deduced from experimental results. Furthermore, the effect of temperature on the fracture behavior of ZrB₂‐SiC‐graphite composite was further discussed by microstructure observations, which showed that tensile strength was controlled by the relaxation of thermal residual stress below 1300°C, and was affected by the plastic flow during 1300°C and 1400°C. At higher temperature, the tensile strength was dominated by the changes of microstructures.     

Effect of thermal aging on electrical conductivity of the 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid‐doped polyaniline fiber

The thermal characteristics of inherently conductive polyaniline (PANi) fiber have been studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Fibers show five major weight losses at ∼100°C, 165°C, 215°C, 315°C, and 465°C, which are associated with the removal of moisture, residual solvent, decompositions of the sulfonic acid and degradation of PANi fiber, respectively. The 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid (AMPSA) that dopes the PANi (in fiber form) performs two‐stage decompositions. The conductivity of the drawn fibers aged at 50°C, 100°C, 150°C, and 190°C under vacuum for various periods of time decreases, particularly at temperatures higher than 100°C. The reduction in conductivity of the fiber aged at temperatures lower than 100°C is mainly due to the evaporation of the residual solvent (15–20% in the as‐spun fiber). Further decrease in conductivity of the fiber aged at temperatures higher than 100°C is caused by the decomposition of the dopant AMPSA. The temperature‐dependent conductivity of the fiber was measured at 15 K (−258.5°C) to 295 K (21.5°C). The conductivity of both aged and un‐aged fibers is all temperature activated, however, the conductivity of the un‐aged fibers is higher than that of the aged fibers. Although a negative temperature coefficient was observed in the temperature range from 240 K (–24.5°C) to 270 K (–3.5°C) for the un‐aged fibers, it was disappeared when the fibers were thermal aged at 100°C for 24 h in vacuum oven. These results indicate that the residual solvent trapped inside the fiber enhanced the electrical conductivity of the fibers and its “metallic” electrical conductivity at temperatures ∼263 K (–10°C). © 2001 John Wiley & Sons, Inc. † J Appl Polym Sci 79: 2503–2508, 2001     

Preparation and properties of an oxide fiber‐reinforced silica matrix composite

Oxide (Nextel? 440) fiber‐reinforced silica composites, with the density and porosity of 1.97 g/cm³ and 21.8%, were prepared through sol‐gel. Their average flexure strength, elastic modulus, shear strength, and fracture toughness at room temperature were 119.7 MPa, 25.6 GPa, 10.8 MPa, and 4.0 MPa·m^1/2, respectively. The composites showed typical toughened fracture behavior, and distinct pullout fibers were observed at the fracture surface. Their mechanical properties were performant up to 1000°C, with the maximum flexural strength of 132.2 MPa at 900°C. Moreover, the composites showed good thermal stability, even after thermal aging and thermal shock at elevated temperatures.     

Electrical Properties of Boron Nitride Matrix Composites: I, Analysis of McLachlan Equation and Modeling of the Conductivity of Boron Nitride–Boron Carbide and Boron Nitride–Silicon Carbide Composites

The McLachlan equation, which incorporates both effective medium models and percolation, was used to predict the volume fraction–conductivity relationships of insulator–conductor composites, and results were compared with experimental data. Two composite systems were investigated (BN–B₄C and BN–SiC). Both systems are anisotropic, because of the orientation of BN platelets perpendicular to the hot-pressing direction. For BN–B₄C composites, with increasing B₄C content, the ac and dc conductivities are relatively constant to ∼40% B₄C (the critical volume fraction). At this composition, the conductivity suddenly increases to a value closer to that of B₄C and then resumes a gradual increase. Little difference is seen for measurements made perpendicular or parallel to the hot-pressing direction, i.e., perpendicular or parallel to the BN platelets. Similar results are found for the BN–SiC composites, except that the critical volume fraction is ∼20% SiC in this case. The experimental curves are in good agreement with those predicted by the McLachlan equation. The parameters s and t of the McLachlan equation relate to the morphology of the phases present in the microstructure. The critical volume fraction relates to the connectivity of the phases in the composites.     

Influence of two structural phases of Fe3O4 and γ‐Fe2O3 on the properties of polyimide/iron oxide composites

Partially aliphatic polyimide/iron oxide composites based on the poly(amic acid) from 5‐(2,5‐dioxotetrahydro‐3‐furyl)‐3‐methyl‐3‐cyclohexene‐1,2‐dicarboxylic acid anhydride and 4,4′‐oxydianiline with iron oxide in different weight percentages were obtained. The structural phases of the transition of magnetite to maghemite occurring in these composites, at different temperatures, are discussed. The physical characteristics, including magnetic, thermal, structural and morphological properties, evaluated using X‐ray diffraction, scanning electron microscopy and thermal analysis, are influenced by the interplay of the filler content and the structural changes of the composite. The X‐ray diffraction patterns of all samples show a cubic structure indexed as magnetite (Fe₃O₄) or maghemite (γ‐Fe₂O₃). Quantification of these two phases was evidenced by the Rietveld method. The electrical properties analysed under different humidity conditions evidence the potential applicability of these polyimide/iron oxide materials as humidity sensors. © 2015 Society of Chemical Industry     

Microstructure and mechanical properties of B4C–TiB2–SiC composites toughened by composite structural toughening phases

B₄C–TiB₂–SiC composites toughened by composite structural toughening phases, which are the units of (TiB₂–SiC) composite, were fabricated through reactive hot pressing with B₄C, TiC, and Si as raw materials. The units of (TiB₂–SiC) composite with the size of 10‐20 μm are composed of interlocking TiB₂ and SiC with the size of 1‐5 μm. The addition of TiC and Si can effectively promote the sintering of B₄C ceramics. The relative densities of all the B₄C composites with different contents of TiB₂ and SiC are close to completely dense (98.9%‐99.4%), thereby resulting in superior hardness (33.1‐36.2 GPa). With the increase in the content of TiB₂ and SiC, the already improved fracture toughness of the B₄C composite continuously increases (5.3‐6.5 MPa·m^1/2), but the flexure strength initially increases and then decreases. When cracks cross the units of the (TiB₂–SiC) composite, the cracks deflect along the interior boundary of TiB₂ and SiC inside the units. As the crack growth path is lengthened, the crack propagation direction is changed, thereby consuming more crack extension energy. The cumulative contributions improve the fracture toughness of the B₄C composite. Therefore, the composite structural toughening units of the (TiB₂–SiC) composite play an important role in reinforcing the fracture toughness of the composites.     

Enhanced electrical properties by tuning the phase morphology of multiwalled carbon nanotube‐filled poly(ether ether ketone)/polyimide composites

A novel high‐performance material with enhanced electrical properties was obtained by tuning the phase morphology of poly(ether ether ketone) (PEEK)/thermoplastic polyimide (TPI)/multiwalled carbon nanotube (MWCNT) composites. MWCNTs were selectively located in the TPI phase due to discrepant affinity of MWCNTs between PEEK and TPI. The dependence of the electrical properties of the PEEK/TPI/MWCNT composites on the phase morphology was investigated by changing the PEEK/TPI ratio, and the maximum conductivity was achieved with a PEEK/TPI ratio of 50/50, which could be explained by the selective location of MWCNTs and the co‐continuous phase morphology of the composites. © 2015 Society of Chemical Industry     

Fabrication and evaluation of some mechanical and electrical properties of jute‐biomass based hybrid composites

Hybrid composites based on bisphenol‐C‐formaldehyde resin and jute mat with rice, wheat, sugar cane, and jamun husks have been fabricated at 150°C under 30.4 MPa pressure for 2 h. The resin content in composites was 50% of fibers. Tensile strength, flexural strength, electric strength, and volume resistivity of hybrid composites have been evaluated and compared with those of jute‐bisphenol‐C‐formaldehyde composites. It is observed that the tensile strength of composites is found to decrease by 53–72%, which is mainly due to random orientation of sandwiched fibers. Flexural strength has increased by 53–153% except jute–rice husk composite for which it is decreased by 26%. A little change in dielectric breakdown strength (1.89–2.11 kV/mm) is found but volume resistivity of Jute–wheat husk and Jute–jamun husk composites has improved by 437–197% and it is slightly decreased(2.3–25.2%) for the remaining two composites. Thus, hybrid composites possess good mechanical and electrical properties signifying their importance in low strength and light weight engineering applications as well as low cost housing units such as partition and hard boards. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1754–1758, 2006     

Characterization and electrical properties of methyl‐2‐hydroxyethyl cellulose doped with erbium nitrate salt

X‐ray diffraction, infrared (IR), and electrical properties for pure and Er (NO₃)₃‐doped methyl‐2‐hydroxyethyl cellulose (MHEC) with concentrations of 0.5, 1, 2, 5, 7, and 10 wt % were studied. X‐ray analysis indicates that the addition of Er (NO₃)₃, which is a crystalline material, to MHEC at concentrations 10 and 13 wt % leads to the formation of crystalline phases in the amorphous polymeric matrix. The appearance of the bending mode ν₂ and the combination mode (ν₁ + ν₄) of Er (NO₃)₃ in the IR spectra of composite samples indicates the coordination of nitro group in the chains of MHEC. From the I–V characteristics, it was found that the charge transport mechanism in MHEC appears to be essentially space charge limited conduction, while the predominant mechanism in the composite samples is Poole–Frenkel. Values of both drift mobility (μ) and the charge carrier density (n) has been reported. The temperature dependence conductivity data has been analyzed in terms of the Arrhenius and Mott's variable range hopping models. Different Mott's parameters such as the density of states, N(E_F), hopping distance (R), and average hopping energy (W) have been evaluated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:2352–2361, 2006     

Morphologies and electrical properties of electrospun poly[(R)‐3‐hydroxybutyrate‐co‐(R)‐3‐hydroxyvalerate]/ multiwalled carbon nanotubes fibers

Electrospinning process was used to fabricate fine fibers from poly[(R)‐3‐hydroxybutyrate‐co‐(R)‐3‐hydroxyvalerate] embedded with multiwalled carbon nanotubes (MWCNTs). Rotating disc collector was used to provide additional drawing force to stretch and align both the embedded MWCNTs and electrospun fibers themselves. Morphological observation revealed MWCNTs aligned to the fiber axis and protruding from the surface. To understand the electrical properties of the fiber, a single‐composite fiber has been deposited on a substrate, across multiple electrodes. Electrical conductivity of the single‐electrospun fiber with low MWCNT content of 0.2 wt % was calculated to be in a remarkable magnitude of about 2.07 Sm^?1. Electrical current flow spanning the fiber length of 1400 μm indicates that the presence of an interconnected network of MWCNTs exists within the fiber. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The use of electric potential drop techniques to detect delamination in a melt-infiltrated SiC-based composite

For composite systems where the matrix is electrically conductive, the possibility that the nature of electrical current flow in the composite can be used to detect defects such as out-of-plane delamination. Melt-infiltrated SiC-based composites are an ideal candidate material for such to verify this since the Si content of the matrix is the primary current carrier in the system. Two different potential drop techniques utilizing the four-point probe method were developed on a composite panel that had a large preexisting delamination defect that occurred during processing. The first technique incorporated current leads to supply current in a through-thickness manner and determine the nature of current spreading (voltage drop) some distance away from the current source to assess the location of the delamination. The other technique incorporated current leads in a more conventional manner to produce axial current flow and assessed the nature of current flow in between the current leads on both surfaces of the composite. In this way, in the presence of a delamination, current flow was forced to primarily flow on the side of the current leads. Both techniques were quite effective at locating the preexisting delamination in a panel of 2D woven Tyranno ZMI slurry-derived melt-infiltrated SiC composite.     

Temperature‐dependent tensile strength model for 2D woven fiber reinforced ceramic matrix composites

This paper presents a temperature‐dependent model for predicting the tensile strength of 2D woven fiber reinforced ceramic matrix composites. The model takes into account the combined effects of temperature, temperature‐dependent residual thermal stress, temperature‐dependent matrix strength, and fibers strength on the tensile strength of composites. To verify the model, the tensile strengths of 2D woven fiber reinforced ceramic matrix composites available are predicted at different temperatures. The model predictions agree well with the experimental data. This work could provide a practical technical means for predicting the temperature‐dependent tensile strength of 2D woven fiber reinforced ceramic matrix composites and uncovering the dominated mechanisms leading to the change of tensile strength and their evolution with temperature.     

Rheological,mechanical, and electrical properties of Sr0.7Bi0.2TiO3 modified BaTiO3 ceramic and its composites

Porous BaTiO₃-based relaxor ferroelectric ceramics with lamellar structure were achieved by ice templating method, and the rheological properties of ceramic slurry for freeze casting were deeply studied. Epoxy resin was then backfilled to generate ceramic–epoxy resin composites. Ceramic–epoxy composites with a lamellar structure were obtained when using a slurry with a ceramic content of 45 wt.%. The nanoindentation results showed that the introduction of ceramic materials into the epoxy resin can significantly improve the penetration resistance and hardness of the material. The dielectric and ferroelectric properties of the composites were also characterized. The interaction between the highly coupled dipoles in the polymers results in a decrease in the breakdown field strength of the composite. The dielectric constant reached up to ∼800. At 220 kV/cm, W_rec = 0.62 J/cm³, and η was ∼80%. At low frequencies, W_rec was ∼0.16 J/cm³, which indicated good stability.