Interfacial load transfer in carbon nanotube/ceramic microfiber hybrid polymer composites

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber–matrix interfacial adhesion, enhance the composite delamination resistance, and possibly improve its toughness and any matrix-dominated elastic property as well. In the present work aligned CNTs were grown upon ceramic fibers (silica and alumina) by chemical vapor deposition (CVD) at temperatures of 650 °C and 750 °C. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on pristine as well as on CNT-grown fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Significant increases (up to 50%) are observed in the fiber tensile strength and in the interfacial adhesion (which was sometimes doubled) with all fiber types upon which CNTs are CVD-grown at 750 °C. We discuss the likely sources of these improvements as well as their implications.     

Synthesis,microstructure and mechanical properties of Yttria Stabilized Zirconia (3YTZP) – Multi-Walled Nanotube (MWNTs) nanocomposite by direct in-situ growth of MWNTs on Zirconia particles

In this research, Yttria Stabilized Zirconia (3YTZP) – carbon nanotube (CNT) composites are fabricated by direct in-situ growth of CNTs on the Zirconia particles, followed by densification via the Spark Plasma Sintering (SPS) technique. Scanning electron microscopy analysis of the 3YTZP-CNT powders shows uniform distribution of CNTs without the formation of agglomerates frequently seen with the traditional ex-situ mixing of CNTs in ceramic compositions. The samples were sintered to nearly 100% theoretical density and with a finer grain size microstructure. High Resolution Transmission Electron Microscopy (HRTEM) and Raman Spectroscopy confirm CNT retention in the sintered nanocomposites up to 1600 °C. The flexural strength increases from ∼260 MPa for samples without CNTs sintered at 1600 °C to ∼312 MPa for samples with ∼4 wt.% CNTs sintered at the same temperature. A corresponding increase in the indentation fracture toughness is also observed for samples with ∼4 wt.% CNTs sintered at 1600 °C as compared to samples sintered at the same temperature without CNTs.     

A simple, low-cost approach to prepare flexible highly conductive polymer composites by in situ reduction of silver carboxylate for flexible electronic applications

In recent years, efforts to prepare flexible highly conductive polymer composites at low temperatures for flexible electronic applications have increased significantly. Here, we describe a novel approach for the preparation of flexible highly conductive polymer composites (resistivity: 2.5 × 10⁻⁵ Ω cm) at a low temperature (150 °C), enabling the wide use of low cost, flexible substrates such as paper and polyethylene terephthalate (PET). The approach involves (i) in situ reduction of silver carboxylate on the surface of silver flakes by a flexible epoxy (diglycidyl ether of polypropylene glycol) to form highly surface reactive nano/submicron-sized particles; (ii) the in situ formed nano/submicron-sized particles facilitate the sintering between silver flakes during curing. Morphology and Raman studies indicated that the improved electrical conductivity was the result of sintering and direct metal-metal contacts between silver flakes. This approach developed for the preparation of flexible highly conductive polymer composites offers significant advantages, including simple low temperature processing, low cost, low viscosity, suitability for low-cost jet dispensing technologies, flexibility while maintaining high conductivity, and tunable mechanical properties. The developed flexible highly conductive materials with these advantages are attractive for current and emerging flexible electronic applications.     

Fabrication of transparent conductive carbon nanotubes/polyurethane-urea composite films by solvent evaporation-induced self-assembly (EISA)

Transparent and conductive carbon nanotubes (CNTs)/polyurethane-urea (PUU) composite films were prepared by solvent evaporation-induced self-assembly (EISA). Pristine CNTs were treated with acids (H₂SO₄/HNO₃ = 3:1, v:v), acylated with thionyl chloride, and purified after filtration. These acylated CNTs (0.05 wt.% in dimethylformamide, DMF) were deposited onto the 3-aminopropyl triethoxysilane (APTES)-modified glass substrate by DMF EISA at 100 °C with the withdrawal rate of 3 cm/h. The CNT layers of 200–400 nm thicknesses were transferred to the PUU films by solution casting or resin transfer molding (RTM) at ambient temperature. Optical transmittances of the composite films were 60–75% at 550 nm wavelength and their sheet resistances were 5.2 × 10⁰–2.4 × 10³ kΩ/square, and which varied significantly with type of CNTs and the transferring methods of CNT layers.     

The influence of carbon nanotubes on the PVC glass transition temperature

The suspension poly(vinyl chloride) with the K number 70 (PVC S-70) compound was applied as a matrix of the nanocomposite, and the multi-wall carbon nanotubes (MWCNT) in the concentration range between 0.01 wt.% and 0.05 wt.% as a nanofiller. The T_g determination was realized by means of DMTA, dielectric losses measurements and DSC measurements. In all cases the PVC matrix revealed lower T_g comparing with corresponding composites with CNTs, and a clear relationship between the frequency, CNT contain and the T_g was found. With an increase of the charging frequency an increase of T_g of about 3 °C by DMTA measurements, realized by frequencies f = 1 Hz and f = 10 Hz, and of 9 °C by f = 1000 Hz, was noted. Independent on the frequency a maximum of the T_g increase was observed by the CNT concentration of about 0.01–0.02 wt.%.     

Dielectric and pyroelectric anisotropy in melt-quenched BaBi2(Nb1 − xVx)2O9 ceramics

The (0 0 l) textured BaBi₂(Nb_1 − xV_x)₂O₉ (where x = 0, 0.03, 0.07, 0.1 and 0.13) ceramics were fabricated via the conventional melt-quenching technique followed by high temperature heat-treatment (800-1000 °C range). The influence of vanadium content and sintering temperature on the texture development and relative density were investigated. The samples corresponding to the composition x = 0.1 sintered at 1000 °C for 10 h exhibited the maximum orientation of about 67%. The Scanning electron microscopic studies revealed the presence of platy grains having the a-b planes perpendicular the pressing axis. The dielectric constant and the pyroelectric co-efficient values in the direction perpendicular to the pressing axis were higher. The anisotropy in the dielectric constant is about 100 (at 100 kHz) at the dielectric maximum temperature and anisotropy in the pyroelectric co-efficient is about 50 μC cm⁻² °C⁻¹ in the vicinity of pyroelectric anomaly for the sample corresponding to the composition x = 0.1 sintered at 1000 °C. Higher values of the dielectric loss and electrical conductivity were observed in the direction perpendicular to the pressing axis which is attributed to the high oxygen ion conduction in the a-b planes.     

High-temperature properties of extruded titanium composites fabricated from carbon nanotubes coated titanium powder by spark plasma sintering and hot extrusion

Pure titanium matrix composite reinforced with carbon nanotubes (CNTs) was prepared by spark plasma sintering and hot extrusion via powder metallurgy process. Titanium (Ti) powders were coated with CNTs via a wet process using a zwitterionic surfactant solution containing 1.0, 2.0 and 3.0 wt.% of CNTs. In situ TiC formation via reaction of CNTs with titanium occurred during sintering, and TiC particles were uniformly dispersed in the matrix. As-extruded Ti/TiCs composite rods were annealed at 473 K for 3.6 ks to reduce the residual stress during processing. After annealing process, the tensile properties of the composites were evaluated at room temperature, 473, 573 and 673 K, respectively. Hardness test was also performed at room temperature up to 573 K with a step of 50 K. The mechanical properties of extruded Ti/CNTs composites at elevated temperature were remarkably improved by adding a small amount of CNTs, compared to extruded Ti matrix. These were due to the TiC dispersoids originated from CNTs effectively stabilized the microstructure of extruded Ti composites by their pinning effect. Moreover, the coarsening and growth of Ti grain never occurred even though they were annealed at 573, 673 K for 36 ks and 673 K for 360 ks, respectively.     

Mechanical behavior of self-assembled carbon nanotube reinforced nylon 6,6 fibers

The versatile electrospinning technique was used to successfully align and disperse multiwalled carbon nanotubes (MWCNT) in nylon 6,6 matrix to obtain composite fibers. The morphology of the composite fibers and the dispersion of the CNTs within the fibers were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. TEM analysis revealed that the CNTs were well-dispersed, separated and aligned along the fiber axis. The thermal and mechanical properties of the composite fibers were characterized as a function of weight fraction of the CNTs. Incorporation of the CNTs in the fibers resulted in an increase in glass-transition temperature (T_g) by ∼7 °C, indicating that the addition of CNTs has restricted the mobility of the polymer chains and provided confinement to neighboring molecular chains. Tensile and nanoindentation experiments were performed to investigate the mechanical deformation behavior of the composite fibers. The results suggested that incorporation of high strength and high aspect ratio CNTs into the fiber matrix enhanced significantly the stiffness and strength of nylon 6,6 fibers. An understanding of the structure–property relationships can provide fruitful insights to develop electrospun fibers with superior properties for miniaturized and load-bearing applications.     

Effect of size refinement and distribution of lubricants on friction coefficient of high temperature self-lubricating composites

IS304 high temperature self-lubricating composites were prepared by ball milling and induction sintering with embedded BaF₂/CaF₂ and Ag particles as lubricants. In comparison to PM304 composites with the same composition, the friction coefficient of IS304 is slight lower than that of PM304 in the temperature range from 20 to 250 °C. However, the friction coefficient of IS304 dramatically decreases at 260–280 °C while the friction coefficient of PM304 dramatically decreases at 330–350 °C under the same test condition. The improved friction coefficient of IS304 composites in the temperature range from 260 to 340 °C is due to the formation of self-lubricating fluoride films. The existence of fluoride film on the worn surface of IS304 is attributed to the refinement of lubricants from 30–70 μm to less than 2 μm and the appropriate distribution of fluorides, which can lead to a higher temperature rise (flash temperature) at the instantaneous contacting surface in comparison to that of the PM304 under the same test condition. It is this considerably higher superficial temperature rise at the contacting surface that can effectively activate the self-lubrication property of the fluoride phase and subsequently induce the formation the lubricating film.     

Influence of carbon nanotube-polymeric compatibilizer masterbatches on morphological, thermal, mechanical, and tribological properties of polyethylene

Study was made of the effect of multiwall carbon nanotubes (MWCNTs) and polymeric compatibilizer on thermal, mechanical, and tribological properties of high density polyethylene (HDPE). The composites were prepared by melt mixing in two steps. Carbon nanotubes (CNTs) were melt mixed with maleic anhydride grafted polyethylene (PEgMA) as polymeric compatibilizer to produce a PEgMA-CNT masterbatch containing 20 wt% of CNTs. The masterbatch was then added to HDPE to prepare HDPE nanocomposites with CNT content of 2 or 6 wt%. The unmodified and modified (hydroxyl or amine groups) CNTs had similar effects on the properties of HDPE-PEgMA indicating that only non-covalent interactions were achieved between CNTs and matrix. According to SEM studies, single nanotubes and CNT agglomerates (size up to 1 μm) were present in all nanocomposites regardless of content or modification of CNTs. Addition of CNTs to HDPE-PEgMA increased decomposition temperature, but only slight changes were observed in crystallization temperature, crystallinity, melting temperature, and coefficient of linear thermal expansion (CLTE). Young’s modulus and tensile strength of matrix clearly increased, while elongation at break decreased. Measured values of Young’s moduli of HDPE-PEgMA-CNT composites were between the values of Young’s moduli for longitudinal (E₁₁) and transverse (E₂₂) direction predicted by Mori-Tanaka and Halpin-Tsai composite theories. Addition of CNTs to HDPE-PEgMA did not change the tribological properties of the matrix. Because of its higher crystallinity, PEgMA possessed significantly different properties from HDPE matrix: better mechanical properties, lower friction and wear, and lower CLTE in normal direction. Interestingly, the mechanical and tribological properties and CLTEs of HDPE-PEgMA-CNT composites lie between those of PEgMA and HDPE.     

Optimization of the chemical vapor deposition process for fabrication of carbon nanotube/Al composite powders

In order to optimize the chemical vapor deposition process for fabrication of carbon nanotube/Al composite powders, the effect of different reaction conditions (such as reaction temperature, reaction time, and reaction gas ratio) on the morphological and structural development of the powder and dispersion of CNTs in Al powder was investigated using transmission electron microscope. The results showed that low temperatures (500-550 °C) give rise to herringbone-type carbon nanofibers and high temperatures (600-630 °C) lead to multi-walled CNTs. Long reaction times broaden the CNT size distribution and increase the CNT yield. Appropriate nitrogen flow is preferred for CNT growth, but high and low nitrogen flow result in carbon nanospheres and CNTs with coarse surfaces, respectively. Above results show that appropriate parameters are effective in dispersing the nanotubes in the Al powder which simultaneously protects the nanotubes from damage.     

The effect of molecular chain polarity on electric field-induced aligned conductive carbon nanotube network formation in polymer melt

Two ethylene–vinyl acetate (EVA) copolymers containing 10 and 25 wt.% vinyl acetate (EVA10 and EVA25) were utilized to explore the effect of molecular polarity on the formation of conductive carbon nanotube (CNT) network in EVA melt under an electric field. Because of the different interfacial energy, it was supposed to be stronger molecular chain-CNT interaction in CNT/EVA25 than that in CNT/EVA10. The critical time for conductive CNT network formation decreased with annealing temperature, filler loading and EVA polarity. The activation energy of conductive CNT network formation (93.9 kJ/mol) in CNT/EVA10 is lower than that (104.7 kJ/mol) in CNT/EVA25. By a thermodynamic percolation model, the percolation threshold at the equilibrium state was about 0.19 vol.% for CNT/EVA10, while it rose to 0.27 vol.% for CNT/EVA25. Morphological observation showed a high degree of CNT alignment in CNT/EVA10 compared to CNT/EVA25 after application of an electric field. The results suggested the strong CNT–EVA chain interaction and higher viscosity of polymer matrix limited the CNT alignment and the conductive network tended to form easily in EVA melt with a low chain polarity.     

Microwave dielectric properties and microstructures of CuO- and ZnO-doped LaAlO3 ceramics

The microwave dielectric properties and the microstructures of 0.25 wt.% CuO-doped LaAlO₃ ceramics with ZnO additions have been investigated. The sintered LaAlO₃ ceramics are characterized by X-ray diffraction spectra and scanning electron microscopy (SEM). Tremendous reduction in sintering temperature can be achieved with the addition of sintering aids CuO and ZnO. The ceramic samples show that dielectric constants (ε_r) of 22−24 and Q×f values of 33,000−57,000 (at 9.7 GHz) can be obtained at low sintering temperatures 1340−1460°C. The temperature coefficient of resonant frequency varies from −24 to −48 ppm/°C. At the level of 0.25 wt.% CuO and 1 wt.% ZnO additions, LaAlO₃ ceramics possesses a dielectric constant (ε_r) of 23.4, a Q×f value of 57,000 (at 9.7 GHz) and a τ_f value of −38 ppm/°C at 1400°C for 2 h.     

A novel gel polymer electrolyte based on polyacrylonitrile (PAN) and its application in a solar cell

A PAN-based gel polymer electrolyte with possible iodide ion conductivity was prepared by incorporating a mixture of Pr₄N⁺I⁻, iodine, EC and PC in PAN. Out of various compositions prepared and characterised, the sample with composition PAN (13%):EC (31%):PC (45%):Pr₄N⁺I⁻ (7%):I₂ (4%) by weight ratio, exhibited the maximum room temperature (25°C) conductivity of 2.95×10⁻³ S cm⁻¹. The predominantly ionic nature of the electrolyte was established by using the dc polarisation technique. The temperature dependence of ionic conductivity follows the VTF behaviour, indicating the amorphous nature of the electrolyte. Dye-sensitised photoelectrochemical solar cells prepared using this electrolyte exhibited an open circuit voltage (V_oc) of 0.69 V, a short circuit current (I_sc) of 3.73 mA cm⁻² for an incident light intensity of 600 W m⁻² yielding an overall quantum efficiency of 2.99%.     

Study on properties of CaO-SiO2-B2O3 system glass-ceramic

Low temperature co-fired ceramic (LTCC) is prepared by sintering a glass selected from CaO-SiO₂-B₂O₃ system, and its sintered bodies are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that the optimal sintering temperature for this glass-ceramic is 820 °C for 15 min, and the major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. The glass-ceramic possesses excellent dielectric properties: ?_r = 6.5, tan δ < 2 × 10⁻³ at 10 MHz, temperature coefficient of dielectric constant about −51 × 10⁻⁶ °C⁻¹ and coefficient of thermal expansion about 8 × 10⁻⁶ °C⁻¹ at 20-400 °C. Thus, this material is supposed to be suitable for the tape casting process and be compatible with Ag electrode, which could be used as the LTCC materials for the application in wireless communications.     

Hydro and thermal stability of die drawn wood polymer composites in comparison to solid wood

Both isotropic and oriented wood polymer composites (WPC) based on 40% w/w of a softwood powder/hardwood powder and polypropylene (PP), together with solid pieces of wood, were subjected to water immersion and thermal expansion tests. Although generally die drawing increased the amount of water absorbed by the WPC by about 2-fold when compare to isotropic WPC, the oriented WPC exhibited extremely high hydro-dimensional stability. The values of the longitudinal and transverse swelling/shrinkage of the WPC oscillated only between 0 and −2.3% compared to values of between 4 and 14% for the solid woods. Incorporation of soft/hard wood powders into PP also substantially decreased its thermal expansion coefficient α in both the isotropic and the oriented states. This extremely positive effect was enhanced by increasing the draw ratio. In the longitudinal direction, α decreased from about 80 × 10⁻⁶ °C⁻¹ (for the isotropic PP) to 5 × 10⁻⁶ °C⁻¹ for the highly drawn PP filled with softwood.     

Improved high-Q microwave dielectric resonator using CuO-doped MgNb2O6 ceramics

The microwave dielectric properties and the microstructures of MgNb₂O₆ ceramics with CuO additions (1-4 wt.%) prepared with conventional solid-state route have been investigated. The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. It is found that MgNb₂O₆ ceramics can be sintered at 1140 °C due to the liquid phase effect of CuO addition. At 1170 °C, MgNb₂O₆ ceramics with 2 wt.% CuO addition possesses a dielectric constant (ε_r) of 19.9, a Q×f value of 110,000 (at 10 GHz) and a temperature coefficient of resonant frequency (τ_f) of −44 ppm/°C. The CuO-doped MgNb₂O₆ ceramics can find applications in microwave devices requiring low sintering temperature.     

Ionic conductivity of a gel polymer electrolyte based on Mg(ClO4)2 and polyacrylonitrile (PAN)

Magnesium ion containing gel polymer electrolytes based on polyacrylonitrile (PAN) have been synthesized and characterized using ac impedance measurements. The electrolyte composition having the highest room temperature conductivity was found by varying the ratios propylene carbonate/ethylene carbonate (PC/EC) and PAN/Mg(ClO₄)₂. The corresponding composition was 18 mol% PAN:64 mol% EC:14 mol% PC:4 mol% Mg(ClO₄)₂. The ac conductivity measurements were carried out from room temperature upto 70 °C with blocking (stainless steel) electrodes. The room temperature conductivity is 3.2×10⁻³ S cm⁻¹ and the activation energy is 0.24 eV over the temperature range used. The high conductivity and the low activation energy of the material could possibly be due to the liquid electrolyte, Mg(ClO₄)₂ in EC/PC trapped in a matrix of PAN, as suggested by previous workers. According to dc polarization measurements, the gel electrolyte appears to be predominantly an anionic conductor.     

Evolution of microstructures and thermal properties with heat treatment for absorbing, emitting and scattering fibrous medium

The microstructure changes of a fibrous insulation for thermal protection system were examined before and after thermal exposures at different temperatures between 1000 °C and 1400 °C. The consequent thermal properties, i.e., thermal conductivity, extinction coefficient, albedo of scattering, and linear coefficient of phase function at different stages were measured by using a developed experimental device and data processing method. The effects of microstructure changes on the thermal properties degradation were discussed. It was found that the devitrification of mullite and the microstructure changes induced by heat treatment had a significant influence upon the thermal properties, and higher temperature treatment yielded a strong increase in thermal conductivity of fibrous insulation. The results also indicated that the relative contribution of conductive and radiative heat transfer would be re-regulated after high temperature thermal annealing.     

Preparation and properties of UV-cured acrylated silane intercalated polymer/LDH nanocomposite

A novel UV-cured polymer/layered double hydroxide (LDH) nanocomposite was prepared by modifying the LDH with sodium dodecyl sulfate (SDS) and [3-(methyl-acroloxy)propyl]trimethoxysilane (KH570) followed by UV irradiation after blended into a acrylate system. From the XRD analyses, the SDS-modified LDH-DS presented the basal spacing of 2.67 nm, whereas the further KH570-intercalated LDH-KH showed a slight decrease to 2.41 nm. After UV irradiated the exfoliated microstructure was formed, and observed by TEM and HR-TEM, showing the fine dispersion and random orientation of LDH in the polymer matrix. The storage modulus and glass transition temperature of the nanocomposite containing 5% LDH-KH increased to 47.5 MPa and 67.8 °C, respectively, from 39.7 MPa and 66 °C of the pure polymer from DMTA measurements. The tensile strength and Persoz hardness were enhanced to 10.6 MPa and 111 s, respectively, from 7.7 MPa and 85 s of the pure polymer.