Nanocomposite hybrid membranes containing polyvinyl alcohol or poly(tetramethylene oxide) for fuel cell applications

New hybrid membranes containing polyvinyl alcohol (PVA) and poly(tetramethylene oxide) (PTMO) with heteropolyacid (HPA) as a hydrophilic inorganic modifier in an organic/inorganic matrix were developed for low-temperature proton exchange membrane fuel cells (PEMFCs). A maximum conductivity of 4.8 × 10⁻³ S cm⁻¹ was obtained at 80 °C and 75% RH for PVA/PWA/PTMO/H₃PO₄ (10/15/70/5 wt%), whereas the PVA/SiWA/MPTS/H₃PO₄ (50/10/10/30 wt%) membrane demonstrated a maximum conductivity of 8.5 × 10⁻³ S cm⁻¹ under identical conditions. These hybrid composite membranes were subsequently tested in a fuel cell. A maximum current density of 240 mA cm⁻² was produced at 70 °C for the PVA/PWA/PTMO/H₃PO₄ membrane, and the corresponding value for the PVA/SiWA/MPTS/H₃PO₄ membrane under identical conditions was 230 mA cm⁻². The small deviations in cell performance can be explained in terms of the variations in thickness of the membranes as well as differences in their conductivities. The fuel cell performances of these membranes decreased drastically when the temperature was increased to 100 °C.     

Preparation and Characterization of Bioblends from Gelatin and Linear Low Density Polyethylene (LLDPE) by Extrusion Method

Abstract

Bioblends are composites of at least one biodegradable polymer with a non-biodegradable polymer. Successful development of bioblends requires that the biodegradable polymers be compatible with other component biodegradable/synthetic (non-biodegradable) polymers. Bioblends from LLDPE and gelatin were prepared by extrusion and hydraulic heat press technique. The gelatin content in the bioblends was varied from 5 to 20 wt%. Various physico-mechanical properties such as tensile, bending, impact strength (IS), thermal ageing and soil degradation properties of the LLDPE/gelatin bioblends with different gelatin contents were evaluated. The effect of thermal ageing on mechanical properties was studied. The mechanical properties such as tensile modulus (TM), bending strength (BS), bending modulus (BM) were found to increase with increasing gelatin content up to 20 wt%, however tensile strength (TS) and elongation at break (%E _b) were decreased with increasing gelatin content. Impact strength value increased with increasing gelatin content up to 10 wt% and then decreased slightly with increasing gelatin content. The blend containing 20 wt% gelatin showed relatively better mechanical properties than other blends. The values of TS, TM,%E _b, BS, BM and IS for the bioblend with 20 wt% gelatin content are 5.9MPa, 206.3MPa, 242.6%, 12.1MPa, 8 MPa and 13.7 J/cm², respectively. Water uptake increases with increasing soaking time in water and weight loss due to soil burial also increases with increasing gelatin content in the blends but both are significantly lower than that of pure gelatin sheet. Weight loss values after thermal ageing increase with time, temperature and increasing gelatin content in the blend but are much lower than pure gelatin. Mechanical properties such as TS, TM are increased and %E _b is decreased after thermal ageing at 60°C for 30 min. Consequently, among all of the bioblends prepared in this work the blend having 20% gelatin content yields properties such that it can be used as a semi-biodegradable material.     

Enhanced mechanical and electrical properties of polyimide film by graphene sheets via in situ polymerization

In this study, polyimide/graphene nanocomposite films which exhibited significant enhancements in mechanical properties and electrical conductivity were successfully fabricated. Graphene oxide (GO) synthesized by Hummer’s method was chemically modified with ethyl isocyanate to give ethyl isocyanate-treated graphene oxide (iGO), which is readily dispersed in N,N′-dimethylformamide (DMF). The iGO dispersion in DMF was then used as media for synthesis of polyimide/functionalized graphene composites (PI/FGS) by an in situ polymerization approach. It was shown that addition of only 0.38 wt% of FGS, Young’s modulus of the PI/FGS composite film was dramatically increased from 1.8 GPa to 2.3 GPa, which is approximately 30% of improvement compared to that of pure PI film, and the corresponding tensile strength was increased from 122 MPa to 131 MPa. In addition, the electrical conductivity of the PI/FGS with this graphene content was increased by more than eight orders of magnitude to 1.7 × 10⁻⁵ S m⁻¹.     

Fabrication and mechanical properties of Al2O3–TiC ceramic composites synergistically reinforced with multi-walled carbon nanotubes and graphene nanoplates

In this study, Al₂O₃–TiC composites synergistically reinforced with multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) were prepared via spark plasma sintering (SPS). The effects of the MWCNT and GNP contents on the phase composition, mechanical properties, fracture mode, and toughening mechanism of the composites were systematically investigated. The experimental results indicated that the composite grains became more refined with the addition of MWCNTs and GNPs. The nanocomposites presented high compactness and excellent mechanical properties. The composite with 0.8 wt% MWCNTs and 0.2 wt% GNPs presented the best properties of all analysed specimens, and its relative density, hardness, and fracture toughness were 97.3%, 18.38 ± 0.6 GPa, and 9.40 ± 1.6 MPa m^1/2, respectively. The crack deflection, bridging, branching, and drawing effects of MWCNTs and GNPs were the main toughening mechanisms of Al₂O₃–TiC composites synergistically reinforced with MWCNTs and GNPs.     

Fluorene oligomer with tetrathiafulvalenes as pendant groups: synthesis,electrochemical and spectroscopic properties

An oligomeric fluorene with pendant TTF units (OFT) was synthesized by the Yamamoto coupling reaction using Ni(COD)₂ as catalyst. The fluorescence spectra of fluorene-TTF (3a) and OFT displayed weak fluorescence intensity because of the photoinduced electron transfer (PET) interaction and energy transfer between the TTF and fluorene, while the fluorescence intensity would enhance significantly after chemical oxidation. The absorption spectra and cyclic voltammetry (CV) showed that an intramolecular interaction existed between the TTF and fluorene moieties in OFT, while in 3a such interaction could not be observed, due to the chain propagation of the fluorene backbone and an enhanced internal charge transfer interaction between the two electroactive moieties. Moreover, the conductivity showed that the doped OFT possessed a higher conductivity (1.4 × 10⁻³ S cm⁻¹) than the doped 3a (9.8 × 10⁻⁵ S cm⁻¹). These results demonstrated that OFT was a good candidate of fluorescence switches and conducting materials.     

Correlation of thermal properties and electrical conductivity of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>Cu<Subscript>0.2</Subscript>Fe<Subscript>0.8</Subscript>O<Subscript>3−δ</Subscript> material for solid oxide fuel cells

A copper-doped ferrite with the chemical composition La_0.7Sr_0.3Cu_0.2Fe_0.8O_3−δ (LaSrCuFe) was prepared using the classical ceramics method starting from the oxides. The linear thermal expansion coefficient in air was measured in the temperature range between 550 and 1,250 K to be between 10 × 10⁻⁶ and 15 × 10⁻⁶ K⁻¹. The electrical conductivity in air was found to be higher than 100 S cm⁻¹ for temperatures lower than 1,100 K. A change of oxygen stoichiometry was found above 650 K in an atmosphere of 20 vol% oxygen with argon. This change can be correlated with the electrical conductivity.     

Enhanced toughness and strength of boron carbide ceramics with reduced graphene oxide fabricated by hot pressing

Boron carbide (B₄C) hybrids with different contents of graphene oxide (GO) were prepared by a heterogeneous co-precipitation method using cetyltrimethyl ammonium bromide (CTAB) as the cationic surfactant. The as-obtained mixtures were further hot-pressed at 1950 °C for 60 min under 30 MPa, by which B₄C–reduced GO (rGO) composites were fabricated. It was found that the addition of only 0.5 wt% rGO could alter the predominance of trans-granular fracture in monolithic B₄C ceramic material to mixed trans-granular and inter-granular modes in B₄C–rGO composites. The flexural strength and fracture toughness of the B₄C–2 wt% rGO were increased by 31% (from 350 to 455 MPa) and 83% (from 3.20 to 5.85 MPa·m^1/2), respectively, compared with those of pure B₄C. The improved mechanical properties are attributed to the mechanisms of pull-out and bridging of rGO and crack deflection, as evidenced by microstructural observations. The energy dissipation in the present B₄C–rGO composites was further verified using two micromechanical models.     

Improved mechanical properties and toughening mechanism of mullite ceramics reinforced by introducing Ti3AlC2 particles

Ti₃AlC₂, as a toughening phase, was introduced into mullite ceramics for the first time by the pressureless sintering process aiming at improving the mechanical properties. Significant enhancement in density and mechanical performance of mullite ceramics was achieved through the introduction of Ti₃AlC₂ particles. The density of as-prepared mullite–Ti₃AlC₂ composites was increased by 23% (from 2.86 g/cm³ to 3.51 g/cm³) with Ti₃AlC₂ increasing from 0 wt% to 20 wt%. The formation of the liquid phase and decomposed particles from Ti₃AlC₂ are supposed to be responsible for the densification of mullite–Ti₃AlC₂ composites. The optimal mechanical properties were obtained in the mullite–Ti₃AlC₂ composites with 15 wt% Ti₃AlC₂. The bending strength, fracture toughness as well as Vickers hardness were reached 214.36 MPa, 4.84 MPa·m^1/2, and 9.21 GPa, which are 40%, 74%, and 113% higher than pure mullite ceramics, respectively. The improved mechanical performance was mainly attributed to the synergetic action of crack deflection, crack branching and bridging, and strengthened grain boundary.     

Fiber Reinforced Polyimide Aerogel Composites with High Mechanical Strength for High Temperature Insulation

Glass fiber/polyimide aerogel composites are prepared by adding glass fiber mat to a polyimide sol derived from diamine, 4,4′‐oxydianiline, p‐phenylene diamine, and dianhydride, 3,3′,4,4′‐biphenyltetracarboxylic dianhydride. The fiber felt acts as a skeleton for support and shaping, reduces aerogel shrinkage during the preparation process, and improves the mechanical strength and thermal stability of the composite materials. These composites possess a mesoporous structure with densities as low as 0.143–0.177 g cm^?3, with the glass fiber functioning to improve the overall mechanical properties of the polyimide aerogel, which results in its Young's modulus increasing from 42.7 to 113.5 MPa. These composites are found to retain their structure after heating at 500 °C, in contrast to pure aerogels which decompose into shrunken ball‐like structures. These composites maintain their thermal stability in air and N₂ atmospheres, exhibiting a low thermal conductivity range of 0.023 to 0.029 W m^?1 K^?1 at room temperature and 0.057to 0.082 W m^?1 K^?1 at 500 °C. The high mechanical strengths, excellent thermal stabilities, and low thermal conductivities of these aerogel composites should ensure that they are potentially useful materials for insulation applications at high temperature.     

The Microstructure and thermal diffusivity of carbon nanostructures/Si3N4 composites processed using spark plasma sintering

Graphene nanoribbons (GNRs) were obtained by unzipping multiwall carbon nanotubes (MWCNTs). Three different silicon nitride-carbon nanostructures were prepared by spark plasma sintering (SPS): ceramic composites that contained 1 wt% carbon nanofibers (CNFs), 1 wt% MWCNTs and 1 wt% GNRs respectively. The α to β-Si₃N₄ transformation ratio and thermal diffusivity of GNR/Si₃N₄ composites were higher than both CNF/Si₃N₄ composites and MWCNT/Si₃N₄ composites. Furthermore, the higher thermal diffusivities of GNR/Si₃N₄ composites can primarily be attributed to the higher number of elongate β-Si₃N₄ grains.     

Novel PEO-based solid composite polymer electrolytes with inorganic–organic hybrid polyphosphazene microspheres as fillers

Novel solid-state composite polymer electrolytes based on poly (ethylene oxide) (PEO) by using LiClO₄ as doping salts and inorganic–organic hybrid poly (cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) microspheres as fillers were prepared. Electrochemical and thermal properties of PEO-based polymer electrolytes incorporated with PZS microspheres were studied. Differential scanning calorimetry (DSC) results showed there was a decrease in the glass transition temperature of the electrolytes and the crystallinity of the samples in the presence of the fillers. Maximum ionic conductivity values of 1.2 × 10⁻⁵ S cm⁻¹ at ambient temperature and 7.5 × 10⁻⁴ S cm⁻¹ at 80° were obtained and lithium ion transference number was 0.29. Compared with traditional ceramic fillers such as SiO₂, the addition of PZS microspheres increased the ionic conductivity of the electrolytes slightly and led to remarkable enhancement in the lithium ion transference number.     

Mechanical,tribological and electrical properties of ZrB2 reinforced Cu processed via milling and high-pressure hot pressing

The present work investigates the effect of (0–10 wt%) ZrB₂ reinforcement on densification, mechanical, tribological and electrical properties of Cu. The consolidation of Cu–ZrB₂ samples was carried out using a hot press (temperature: 500 °C, pressure: 500 MPa, time: 30 min, vacuum pressure: 1.3 × 10^-2 mbar). The bulk density of the hot-pressed Cu composites decreased from 8.84 g/cc to 8.16 g/cc and the relative density of samples lowered from 98.6% to 92.1% with the addition of ZrB₂. The incorporation of hard ZrB₂ (up to 10 wt%) improved the hardness of Cu (1.32–2.55 GPa). However, the yield strength and compressive strength of Cu composites increased up to 5 wt% ZrB₂, and further addition of ZrB₂ lowered its strength. The yield strength of Cu samples varied from 602 to 672 MPa and the compressive strength between ~834 and 971 MPa. On the other hand, the coefficient of friction (COF) (from 0.49 to 0.18) and wear rate (from 49.3 × 10^-3 mm³/Nm to 9.1 × 10^-3 mm³/Nm) of Cu–ZrB₂ samples considerably decreased with the addition of ZrB₂. Significantly low wear was observed with Cu-10 wt% ZrB₂ (Cu-10Z) samples, which is 5.41 times less than pure Cu. As far as the wear mechanisms are concerned, in pure Cu, continuous chips (wear debris) were formed during sliding wear by plowing. Whereas the major amount of material loss was occurred due to the plowing mechanism with discontinuous and short chip formation for Cu–ZrB₂ composites. The electrical conductivity of Cu–ZrB₂ samples decreased from 75.7% IACS to 44.1% IACS. In particular, Cu with ZrB₂ (up to 3 wt%) could retain the conductivity of 66.8% IACS. This study reveals that the addition of ZrB₂ (up to 3 wt%) is advantageous to have a good combination of properties for Cu.     

Preparation and characterization of poly(ethylene glycol)/organo-vermiculite nanocomposite polymer electrolytes

Vermiculite (VMT) was readily intercalated by hexadecyl trimethyl ammonium bromide to yield organo-vermiculite (OVMT), which was confirmed by X-ray diffraction measurement and Fourier transform infrared spectroscopy. Poly(ethylene glycol)/organo-vermiculite (PEG/OVMT) nanocomposites were prepared by using the direct melt intercalation method, and its intercalation state was confirmed by transmission electron microscope. Thereafter, a lithium salt was dissolved in the PEG/OVMT nanocomposites to prepare composite polymer electrolytes. The highest conductivity was 2.1 × 10⁻⁵ S cm⁻¹ at room temperature, which was obtained by AC impedance analysis when the amount of OVMT based on PEG was 1 wt%.     

Conjugated opto/electroactive ethynylene-carbazole polymers with TTF as pendant group

Three conjugated ethynylene-carbazole polymers with Tetrathiafulvalene (TTF) as pendant group (P1–P3) were synthesized by using sonogashira coupling reaction and characterized by ¹H NMR, GPC, CV, UV–Vis, FL, and TGA. CV and UV–Vis spectra showed that an intramoleular interaction existed between the electron-rich moiety TTF and electron-deficient moiety polyethynylcarbazole of the polymers. A strong fluorescence quench (ca. 99%) could be observed, compared to the polyethynylene-carbazole without TTF units, which could be ascribed to the photo-induced electron transfer (PET) interaction from TTF moiety to the polyethynylene-carbazole backbone. The observed onset decomposition temperatures (T _d) for P1–P3 varied from 256 to 298 °C. The polymers mentioned above exhibited good thermal properties and higher conductivity (neutral conductivity ~7–11 × 10⁻⁷ S cm⁻¹; doped conductivity ~6–11 × 10⁻⁴ S cm⁻¹).     

Characterization of conductive multiwall carbon nanotube/polystyrene composites prepared by latex technology

Conductive multiwall carbon nanotube/polystyrene (MWCNT/PS) composites are prepared based on latex technology. MWCNTs are first dispersed in aqueous solution of sodium dodecyl sulfate (SDS) driven by sonication and then mixed with different amounts of PS latex. From these mixtures MWCNT/PS composites were prepared by freeze-drying and compression molding. The dispersion of MWCNTs in aqueous SDS solution and in the PS matrix is monitored by UV–vis, transmission electron microscopy, electron tomography and scanning electron microscopy. When applying adequate preparation conditions, MWCNTs are well dispersed and homogeneously incorporated in the PS matrix. The percolation threshold for conduction is about 1.5 wt% of MWCNTs in the composites, and a maximum conductivity of about 1 S m⁻¹ can be achieved. The approach presented can be adapted to other MWCNT/polymer latex systems.     

Eco-friendly tape casting of borosilicate glass/Al2O3 sheets for LTCC applications

This work reports the innovative development of a borosilicate glass/Al₂O₃ tape for LTCC applications using an eco-friendly aqueous tape casting slurry. Polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA) were the respective dispersants, while carboxymethyl cellulose (CMC) and styrene acrylic emulsion (SA) were the respective binders. The results showed that PVP was more suitable than PAA as the dispersant for the aqueous casting slurry, and that 1.5 wt% PVP would achieve well dispersion of CABS glass/Al₂O₃ powder in the aqueous slurry. Moreover, a small amount of 2.0 wt% CMC binder could yield smooth CABS glass/Al₂O₃ tapes crack free. A high-quality CABS glass/Al₂O₃ tape with a smooth surface was made from an aqueous slurry containing 1.5 wt% PVP dispersant, 2.0 wt% CMC binder, and 2.0 wt% PEG-400 plasticizer. The density, tensile strength, and surface roughness of the green tape were 2.05 g/cm³, 0.87 MPa, and 148 nm, respectively. The resulting CABS glass/Al₂O₃ composites sintered at 875 °C exhibited a bulk density of 3.14 g/cm³, a dielectric constant of 8.09, a dielectric loss of 1.0 × 10^?3, a flexural strength of 213 MPa, a thermal expansion coefficient of 5.30 ppm/^°C, and a thermal conductivity of 3.2 W m^?1 K^?1, thus demonstrating its broad prospects in LTCC applications.     

Properties of Hybrid-Reinforced Aluminum Matrix Composites for Precision Instruments

Hybrid-reinforcement SiC/Gr/Al composites were fabricated by squeeze-casting technology, to provide a novel solution to machinable materials for precision instruments. The effect of flake graphite particles on mechanical properties, machinability, dimensional stability and coefficient of thermal expansion was studied. With the addition of 5% graphite particles in the SiC/Al composites, the tool life during cutting is prolonged by 40% and the tensile strength, elastic modulus and specific modulus are 405 MPa, 150 GPa and 51 GPa cm³/g respectively. The micro-yield stress of SiC/5%Gr/Al composite is higher than 300 MPa and the linear CTE is 11.6 × 10⁻⁶ °C⁻¹. The properties of SiC/Gr/Al composite were in contrast to conventional materials of precision instruments and the advantages in application are discussed.     

Electrochemical investigation of Li–Al anodes in oligo(ethylene glycol) dimethyl ether/LiPF<Subscript>6</Subscript>

1 M LiPF₆ dissolved in oligo(ethylene glycol) dimethyl ether with a molecular weight 500 g mol⁻¹ was investigated as a new electrolyte (OEGDME500, 1 M LiPF₆) for metal deposition and battery applications. At 25 °C a conductivity of 0.48 × 10⁻³ S cm⁻¹ was obtained and at 85 °C, 3.78 × 10⁻³ S cm⁻¹. The apparent activation barrier for ionic transport was evaluated to be 30.7 kJ mol⁻¹. OEGDME500, 1 M LiPF₆ allows operating temperature above 100 °C with very attractive conductivity. The electrolyte shows excellent performance at negative and positive potentials. With this investigation, we report experimental results obtained with aluminum electrodes using this electrolyte. At low current densities lithium ion reduction and re-oxidation can be achieved on aluminum electrodes at potentials about 280 mV more positive than on lithium electrodes. In situ X-ray diffraction measurements collected during electrochemical lithium deposition on aluminum electrodes show that the shift to positive potentials is due to the negative Gibbs free energy change of the Li–Al alloy formation reaction.     

Fabrication of in-situ Ti-Cx-N1−x phase enhanced porous Si3N4 absorbing composites by gel casting

In view of the lack of a conformal, load-bearing, lightweight, and high-temperature resistant integrated microwave-absorbing composites for applications under extremely complex conditions, this study successfully applies gel casting craft to porous Si₃N₄ microwave-absorbing composites. The high-temperature decomposition reaction of Ti₃SiC₂ powder was utilized to generate uniform Ti-C_x-N_1−x grains in situ and to enhance the mechanical and microwave-absorption properties of porous Ti-C_x-N_1−x/Si₃N₄ composites. The bending strength, fracture toughness, density, maximum reflection loss value, absorption bandwidth and matched thickness in P-band of the composite with 30 wt% Ti₃SiC₂ addition were 164.87 ± 7.56 MPa, 2.61 ± 0.13 MPa·m^1/2, 2.077 g/cm³, 32.42 dB, 1.74 GHz and 4.2 mm, respectively. The fracture toughness and bending strength of the composite were increased by 71.71% and 58.13%, respectively, compared with monolithic porous Si₃N₄. The electromagnetic wave loss mechanisms of the composites are proposed as a combination of conductivity loss, multiple reflections and scattering, interfacial polarization and defect polarization.     

Enhanced irradiation-resistance of polymer-derived SiC by incorporation of multiwalled carbon nanotubes

In this work, multiwalled carbon nanotubes were introduced into polycarbosilane to fabricate carbon nanotubes reinforced nano-SiC composites (MWCNTs/ PCS-nano-SiC). Radiation effects on MWCNTs/ PCS-nano-SiC were studied by irradiation with 2 MeV Au²⁺ ions at room temperature and doses ranging from 2 × 10¹⁴ to 8 × 10¹⁴ ions/cm². The sample pyrolyzed at 1400 °C and containing 3 wt% carbon nanotubes exhibited excellent overall performances. The multiple graphite layers in the MWCNTs provided "absorption traps" for defects, improving the efficiency of defect recombination. Tightly combined MWCNTs/ PDC interface is required to enable the two phases to protect each other during irradiation. The critical amorphization dose was increased upon structural optimization, and the hardness degradation was significantly reduced. The rise of Young's modulus at a high damage dose was discovered because of the "interface-driven shrinkage" of nano-SiC. The present study provides insight into SiC_f/ SiC design for an advanced nuclear system.