Thermal properties of pure and doped (polyvinyl-alcohol) PVA

Films ≈350 μm of poly(vinyl-alcohol) composites, containing copper (Cu), aluminium (Al) and iron (Fe), metallic powder very fine, were prepared by a casting method. Thermal conductivity, phonon velocity, mean free path and specific heat were studied. The pure sample of PVA has a lower values of thermal conductivity than that which are doped with metals. For all samples the thermal conductivityK increases up to a certain temperatureT _gg (120–160°C) and then decreases with temperature. The specific heat increase with temperature up to ≈120°C and above 120°C is nearly independent on temperature. The pure sample of PVA has small values of mean free path (L)≈0.2 Å at room temperature, but for PVA⁺ metalsL≈2.0 Å. The phonon velocity of pure PVA is larger than that of PVA containing metals.     

Nanocomposites and high-modulus fibers based on ultrahigh-molecular-weight polyethylene and silicates: Synthesis, structure, and properties

Nanocomposites based on ultrahigh-molecular-weight polyethylene and inorganic fillers—such as organomodified layered aluminosilicates, aerosil, and diatomite—are prepared via polymerization filling. The polymerization of ethylene was conducted in the suspension mode with the use of a conventional Ziegler-Natta catalyst, TiCl₄ + Al(i-Bu)₃, under mild conditions (a temperature of 30°C and a pressure of 0.1MPa). The structure and properties of the composites are studied via X-ray diffraction analysis and DSC. The polyethylene matrix features a high enthalpy, a high melting temperature (up to 143°C), a crystallinity of 70–80%, a content of the monoclinic phase of 12–15%, and a bulk density of 0.05–0.15 g/cm³; the molecular mass is (1.5–1.6) × 10⁶. High-modulus, high-strength fibers with an elastic modulus of 25–28 GPa and a strength of 0.65–0.70 GPa are prepared via direct solvent-free molding of nascent reactor powders based on ultrahigh-molecular-weight polyethylene filled (7 wt %) with aerosol or montmorillonite modified with vinyltrimethoxysilane.     

Filler network structure in graphene nanoplatelet (GNP)-filled polymethyl methacrylate (PMMA) composites: From thermorheology to electrically and thermally conductive properties

In this work, graphene nanoplatelet (GNP) filled polymethyl methacrylate (PMMA) composites were prepared using solution method via a specially designed route and relatively high thermal conductivities of the composites were achieved at a low GNP loading. The effect of GNP content on rheological behavior, thermal and electrical conductivity of the composites was intensively investigated. Thermorheological complexity was displayed at elevated GNP loading, and the rheological percolation threshold of GNP in PMMA decreased from 7.96 wt% at 220 °C to 4.02 wt% at 260 °C according to Winter-Chambon method, suggesting that GNP was more likely to form a seepage network at higher temperature. The DMTA results showed that the increase in moduli of the composites should be ascribed to the formation of the GNP-GNP network structure. The electrical conductivity of the composites underwent a sudden jump by seven orders of magnitude, which also indicated the formation of a GNP conductive pathway in the matrix with an electrical percolation threshold of 2–4 wt%. The results of transient temperature measurement were in good consistent with the thermal conductivity versus GNP loading, which was compared with various thermal conduction models with a modified Agari model presenting an acceptable evaluation of the dispersion status of GNP in the matrix. The experimental electrical and thermal conductivities as a function of GNP content could well be interpreted by the filler network structure as observed in morphological studies.     

Conductivity of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-NiO composites

The conductivity and transport number of oxygen ions of Bi₂O₃-(10, 30, 50) vol % NiO composites are measured using the four-probe and coulomb-volumetric methods at various temperatures. It is shown that the Bi₂O₃-50 vol % NiO composite exhibits a high mixed ionic-electronic conductivity in the temperature range from 730 to 800°C.     

Wetting and conductivity of BiVO<Subscript>4</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript> ceramic composites

The conductivity and transport number of oxygen ions of BiVO₄-(5, 7, 10, 12) wt % V₂O₅ ceramic composites are measured using the four-probe and coulomb-volumetric methods, respectively, in the temperature range from 500 to 660°C. The phase transition of wetting of grain boundaries with eutectic melt at 640°C is discovered. It is shown that the grain boundary wetting significantly raises the ionic conductivity of composites.     

Electrical conductivity of titanium pyrophosphate between 100 and 400 °C: effect of sintering temperature and phosphorus content

The synthesis of titanium pyrophosphate is carried out, and the material is sintered at different temperatures between 370 and 970 °C. Yttrium is added during the synthesis to act as acceptor dopant, but it is mainly present in the material in secondary phases. The conductivity is studied systematically as a function of sintering temperature, pH₂O, pO₂, and temperature (100–400 °C). Loss of phosphorus upon sintering above 580–600 °C is confirmed by energy dispersive spectroscopy and combined thermogravimetry and mass spectrometry. The conductivity decreases with increasing sintering temperature and decreasing phosphorus content. The highest conductivity is 5.3?×?10^?4 S cm^?1 at 140 °C in wet air (pH₂O?=?0.22 atm) after sintering at 370 °C. The conductivity is higher in wet atmospheres than in dry atmospheres. The proton conduction mechanism is discussed, and the conductivity is attributed to an amorphous secondary phase at the grain boundaries, associated with the presence of excess phosphorus in the samples. A contribution to the conductivity by point defects in the bulk may explain the conductivity trend in dry air and the difference in conductivity between oxidizing and reducing atmospheres at 300–390 °C. Slow loss of phosphorus by evaporation over time and changes in the distribution of the amorphous phase during testing are suggested as causes of conductivity degradation above 220 °C.     

How xerogel carbonization conditions affect the reactivity of highly disperse SiO<Subscript>2</Subscript>–C composites in the sol–gel synthesis of nanocrystalline silicon carbide

A transparent silicon polymer gel was prepared by sol–gel technology to serve as the base in the preparation of highly disperse SiO₂–C composites at various temperatures (400, 600, 800, and 1000°C) and various exposure times (1, 3, and 6 h) via pyrolysis under a dynamic vacuum (at residual pressures of ~1 × 10^–1 to 1 × 10^–2 mmHg). These composites were X-ray amorphous; their thermal behavior in flowing air in the range 20–1200°C was studied. The encapsulation of nascent carbon, which kept it from oxidizing in air and reduced the reactivity of the system in SiC synthesis, was enhanced as the carbonization temperature and exposure time increased. How xerogel carbonization conditions affect the micro- and mesostructure of the xerogel was studied by ultra-small-angle neutron scattering (USANS). Both the carbonization temperature and the exposure time were found to considerably influence structure formation in highly disperse SiO₂–C composites. Dynamic DSC/DTA/TG experiments in an inert gas flow showed that the increasing xerogel pyrolysis temperatures significantly reduced silicon carbide yields upon subsequent heating of SiO₂–C systems to 1500°C, from 35–39 (400°C) to 10–21% (1000°C).     

Ionic conductivity in Na+, K+, and Ag+ β″-alumina

This paper presents measurements of the ionic conductivity in single crystals of β″-alumina (0.84 M₂O · 0.67 MgO · 5.2 Al₂O₃, M = Na, K, Ag). Single crystals of sodium β″-alumina were grown from a melt of Na₂O, MgO, and Al₂O₃ at 1660 to 1730°C. Selected crystals were converted to the other isomorphs by ion exchange. The conductivity of sodium β″-alumina varies from 0.18 to 0.01 (ohm · cm)^?1 at 25°C depending upon crystal growth conditions. Potassium β″-alumina has the unusually high room temperature conductivity of 0.13 (ohm · cm)^?1. Silver β″-alumina has a slightly lower conductivity, 4 × 10^?3 (ohm · cm)^?1 at 25°C. The activation energies of sodium and potassium β″-alumina decrease with increasing temperature, while that of silver β″-alumina is constant from ?80 to 450°C.     

New polymer electrolyte membrane for medium-temperature fuel cell applications based on cross-linked polyimide Matrimid and hydrophobic protic ionic liquid

New hydrophobic protic ionic liquid, 2-butylaminoimidazolinium bis(trifluoromethylsulfonyl)imide (BAIM-TFSI), has been synthesized. The ionic liquid showed good thermal stability to at least 350 °C. The conductivity of BAIM-TFSI determined by electrochemical impedance method was found to be 5.6 × 10^?2 S/cm at 140 °C. Homogeneous composite films based on commercial polyimide (PI) Matrimid and BAIM-TFSI containing 30–60 wt% of ionic liquid were prepared by casting from methylene chloride solutions. Thermogravimetric analysis data indicated an excellent thermal stability of PI/BAIM-TFSI composites and thermal degradation points in the temperature range 377 °C–397 °C. The addition of ionic liquid up to 50 wt% in PI films does not lead to any significant deterioration of the tensile strength of the polymer. The dynamic mechanical analysis results indicated both an increase of storage modulus E′ of PI/BAIM-TFSI composites at room temperature and a significant E′ decrease with temperature compared with the neat polymer. The cross-linking of the PI with polyetheramine Jeffamine D-400 allowed to prepare PI/Jeffamine/BAIM-TFSI (50%) membrane with E′ value of 300 MPa at 130 °C. The ionic conductivity of this cross-linked composite membrane reached the level of 10^?2 S/cm at 130 °C, suggesting, therefore, its potential use in medium-temperature fuel cells operating in water-free conditions.     

Performance improvement of EPDM and EPDM/Silicone rubber composites using modified fumed silica,titanium dioxide and graphene additives

In this work, a polymeric composite was prepared from ethylene propylene diene monomer (EPDM) and silicone rubber (S) with additives of modified fumed silica (MFS), titanium dioxide (TiO₂) and graphene. The dielectric and thermal performances of the EPDM-based composites were studied. An increase in the dielectric constant and AC dielectric breakdown strength was observed for the EPDM rubber composites containing MFS, TiO_2, and graphene additives. In addition, the incorporation of the additives resulted7in a significant increase in the thermal stability (~30–50 °C) and thermal conductivity (~7–35%) of the composites. The combination of these various improvements gives suitable performance advantage to the polymeric composite for use in insulating applications.     

High performance heteroaromatic poly(azo-ester)s and their miscible blends: Thermomechanical and conductivity profile

An interesting type of thermally stable and processable poly(azo-ester)s (PAEs) have been synthesized through the polycondensation of a novel diol—(E)-1-(5-(3-hydroxypyridylazo)thiocarbamoyl-aminonaphthyl)-3-(3-hydroxypyridylazo)thiourea, and various diacid chlorides. The dihydroxy compound containing azo group (?N=N?) was prepared via multi-step procedure in which the coupling of bisthiourea compound with diazonium chloride (in alkali) yielded the desired monomer. The polymeric materials were characterized in terms of FTIR, ¹H NMR, solubility, solution viscosity, molecular weight, electrical conductivity, glass transition and thermal degradation temperatures. PAEs possessed high inherent viscosity 1.19–1.23 dL/g and molar mass (8.3–8.5) × 10⁴. The polymers were thermally stable in the range 531–541°C (10% gravimetric loss T ₁₀) having T _g′ = 258–266°C. PAEs were, thus, melt blended with polyaniline resulting in high performance materials that potentially combined the fine thermal properties and processability of poly(azo-ester)s with electrical characteristics of polyaniline. The miscible blends exhibited good heat stability (T ₁₀ = 525–527°C, T _g = 246–252°C) and mechanical strength (61.81–63.19 MPa) compared with several polyaniline-based blends. FESEM showed nano-level homogeneity of the microstructure liable for better electrical conductivity (3.2–4.2 S/cm).     

Low percolation conductive graphite flakes-filled poly(urethane-imide) composites with high thermal stability via imidization self-foaming structure

In the study, the conductive graphite flakes filled poly(urethane-imide) composites (PUI/GFs) with high performance were constructed by the thermal imidization self-foaming reaction. It was found that the foaming action could promote the redistribution of GFs during curing process and the formation of stable linear conductive pathways. The percolation threshold of PUI/GFs composites was lowered from 1.26 wt% (2000 mesh GFs) or 0.86 wt% (1000 mesh GFs) to 0.79 wt% (500 mesh GFs), which were relatively low percolation thresholds for polymer/GFs composites so far. When the content of 500 mesh GFs was 4.0 wt%, the electrical conductivity of the composite was as high as 3.96 × 10^?1 S/m. Also, a poly(urethane-imide) (PUI) matrix with excellent thermal stability (T_d10%: 334.97 °C) and mechanical properties (elongation at break: 324.52%, tensile strength: 15.88 MPa) was obtained by introducing the rigid aromatic heterocycle into the polyurethane (PU) hard segments. Moreover, the zero temperature coefficient of resistivity for the composites was observed at the temperature range from 30 °C to 200 °C. Consequently, PUI/GFs composites may provide the novel strategy for considerable conductive materials with high thermal stability in electrical conductivity.     

Room temperature processable heat‐resistant epoxy‐oxazolidone‐based syntactic foams

Syntactic foams based on oxazolidone‐modified epoxy resin using glass microballoons as reinforcing filler with varying densities were processed. The influence of various grades of microballoons and their concentration on the mechanical, thermal, thermomechanical, and flammability characteristics were investigated. The effect of temperature on the compressive strength with density was monitored in detail. By incorporating the microballoons, T_g of the syntactic foam increased from 90 °C to 115 °C. Thermal conductivity was found to decrease from (0.064 to 0.056 W/(m·K)) in conjunction with decreasing resin to filler ratio. In the case of composites filled with K25 alone, the creation of large voids due to less effective packing between the microballoons led to lower thermal conductivity. The specific heat of the different composites was in the range of 0.32 to 0.44 cal/g/°C, and the coefficient of thermal expansion was in the range of 13.2 to 17.4 × 10⁻⁶/°C with limiting oxygen index of 28% to 33%.     

Preparation, structure, morphology, and dc and ac conductivity of the 88SiO2-6Li2O-6Nb2O5 (% mole) sol-gel derived glass-ceramics

The glass composition 88SiO₂-6Li₂O-6Nb₂O₅ (mole %) was successfully prepared by the sol-gel technique. The dried and translucent gel was heat-treated at temperatures between 500°C and 800°C. Lithium niobate crystallites, an important ferroelectric material, were detected in the gel derived glass-ceramics treated above 650°C. In the samples treated at 700 and 800°C the Li₂Si₂O₅ crystalline phase is present. The 800°C treated sample also presents the Li₃NbO₄ phase. The structure and morphology of the samples were studied by X-ray powder diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The SEM revealed that all the samples, heat-treated above 650°C, present crystallites embedded in the glass matrix. The particles detected in the 600°C treated sample are essentially amorphous, or with an incipient structure. The temperature dependence of the dc electrical conductivity (σ _dc ) shows two regions with different activation energies. The conductivity behaviour of the sample is mainly due to the mobile ion number. The ac conductivity (σ _ac ), measured at 1 kHz decreases with the rise of the treatment temperature due to the increase of the LiNbO₃ crystallites amount. The electrical behavior of the glass and glass-ceramics reflects the important role carried out by the treatment temperature in the gel-glass structure.     

Influence of MoSi2 content in suspension on the phase, microstructure and properties of hydrothermal electrophoretic deposited SiC n �CMoSi2 coating for SiC pre-coated C/C composites

To protect carbon/carbon (C/C) composites from oxidation at high temperature, a nano SiC?CMoSi₂ (SiC _n ?CMoSi₂) coating on SiC pre-coated C/C composites was prepared by hydrothermal electrophoretic deposition. The phase composition, surface and cross-section microstructures of the prepared SiC _n ?CMoSi₂ coating deposited with different MoSi₂/SiC _n mass ratio were characterized by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). The influence of MoSi₂ content in the hydrothermal electrophoretic deposition suspension on the phase composition, microstructure and high-temperature oxidation resistance of the multi-layer coatings were investigated. Results showed that the content of MoSi₂ phase in the prepared coating increases with the increase of MoSi₂ content in the suspension. The density and oxidation resistance of the SiC_n-MoSi₂ coating improve with the increase of MoSi₂ mass content from 20 to 60 wt% in the deposition suspension. However, micro-cracks and micro-holes in the coating are found when deposited with 80 wt% MoSi₂, and a decrease in oxidation resistance was also detected. The multi-layer coatings deposited with suspension of 60 wt% MoSi₂ exhibited the best anti-oxidation ability, which can effectively protect C/C composites from oxidation in air at 1,873 K for 90 h with weight loss of 2.08%.     

Phase composition and conductivity of La1 ? x Sr x ScO3 ? α (x = 0.01?0.20) under oxidative conditions

The phase composition was studied and overall conductivity of oxides La_{1 ? x} Sr _x ScO_{3 ? ??} (x = 0.01?0.20) was measured as dependent on air humidity (pH₂O = 0.04?2.35 kPa) in the temperature range from 100 to 900°C. The samples were synthesized in air at 1600°C. They are single-phase, with a perovskitetype structure with orthorhombic distortions and the density of 94?C99%. The conductivity measurements were carried out using the impedance technique and four-probe dc technique. The contributions of bulk and grain boundary resistances were determined, effective conductivity activation energies were calculated.     

Preparation and characterization of blended solid polymer electrolyte 49% poly(methyl methacrylate)-grafted natural rubber:poly(methyl methacrylate)–lithium tetrafluoroborate

The preparation and characterization of blended solid polymer electrolyte 49% poly(methyl methacrylate)-grafted natural rubber (MG49):poly(methyl methacrylate) (PMMA) (30:70) were carried out. The effect of lithium tetrafluoroborate (LiBF₄) concentration on the chemical interaction, structure, morphology, and room temperature conductivity of the electrolyte were investigated. The electrolyte samples with various weight percentages (wt.%) of LiBF₄ salt were prepared by solution casting technique and characterized by Fourier transform infrared spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy. Infrared analysis demonstrated that the interaction between lithium ions and oxygen atoms occurred at symmetrical stretching of carbonyl (C=O) (1,735 cm^?1) and asymmetric deformation of (O–CH₃) (1,456 cm^?1) via the formation of coordinate bond on MMA structure in MG49 and PMMA. The reduction of MMA peaks intensity at the diffraction angle, 2θ of 29.5° and 39.5° was due to the increase in weight percent of LiBF₄. The complexation occurred between the salt and polymer host had been confirmed by the XRD analysis. The semi-crystalline phase of polymer host was found to reduce with the increase in salt content and confirmed by XRD analysis. Morphological studies by SEM showed that MG49 blended with PMMA was compatible. The addition of salt into the blend has changed the topological order of the polymer host from dark surface to brighter surface. The SEM analyses supported the enhancement of conductivity with the addition of salt. The conductivity increased drastically from 2.0 to 3.4?×?10^?5 S cm^?1 with the addition of 25 wt.% of salt. The increase in the conductivity was due to the increasing of the number of charge carriers in the electrolyte. The conductivity obeys Arrhenius equation in higher temperature region from 333 to 373 K with the pre-exponential factor σ _o of 1.21?×?10^?7 S cm^?1 and the activation energy E _a of 0.46 eV. The conductivity is not Arrhenian in lower temperature region from 303 to 323 K.     

Electrical properties of the α, β, γ, and δ phases of bismuth sesquioxide

Conductivity measurements have been performed on compressed powder specimens of Bi₂O₃ in the temperature region 300–800°C. The conductivity in the β, γ, and δ phases is predominantly ionic. Oxide ions are the mobile charge carriers. The disorder in these phases is intrinsic even when the samples are contaminated with Au or Pt. The conductivity in the α phase is predominantly p type. The disorder in the α phase is extrinsic up to the α → δ transition at 729°C. From 650 to 729°C a rapidly increasing contribution of oxygen vacancies to the conductivity is apparent.     

Poly(vinyl alcohol)/melamine phosphate composites prepared through thermal processing: thermal stability and flame retardancy

Poly(vinyl alcohol)/melamine phosphate composites (PVA/MP) as a novel halogen‐free, flame‐retardant foam matrix were prepared through thermal processing, and then their thermal stability and flame retardancy were investigated by thermo‐gravimetric analysis, micro‐scale combustion calorimeter, cone calorimeter, vertical burning test, and limiting oxygen index (LOI) test. It was found that the thermal stability and combustion properties of the PVA/MP composites could be influenced by the addition of MP. Compared with the control PVA sample (B‐PVA), in the PVA/MP (75/25) composites, the temperature at 5% mass loss (T_5%) decreased about 10°C, the residual chars at 600°C increased by nearly 27%, the temperature at the maximum peak heat release rate (T_P) shifted from 292°C to 452°C, and the total heat released and the heat release capacity (HRC) decreased by 28% and 14%, respectively. Moreover, the PVA/MP composites could reach LOI value up to 35% and UL94 classification V‐0, showing good flame retardancy. At the same time, both Fourier transform infrared and X‐ray photoelectron spectroscopy spectra of the residual chars from the PVA/MP composites demonstrated that the catalytic effect of MP on the dehydration and decarboxylation reactions of PVA, and the chemical reactivity of MP during the chars‐forming reactions could be used to account for the changed thermal stability and flame retardancy of the PVA/MP composites. Copyright © 2012 John Wiley & Sons, Ltd.     

Physicochemical properties of (CH<Subscript>3</Subscript>)<Subscript>2</Subscript>NH<Subscript>2</Subscript>Cl–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites

Composite solid electrolytes were synthesized from the organic salt dimethylammonium chloride (1–x)C₂H₈NCl–xAl₂O₃. Their physicochemical properties were studied. In the starting C₂H₈NCl salt, there is a phase transition at 39°C accompanied by an increase in conductivity by two orders of magnitude. The conductivity of the high-temperature phase is 9.3 × 10^–6 S/cm at 160°C. A differential scanning calorimetry study showed that the salt in the composites spreads over the oxide surface and at x > 0.6 the salt melting enthalpy decreases to zero. The conductivity of the resulting composites was studied by impedance spectroscopy. It was shown that heterogeneous doping leads to a sharp increase in ion conductivity to 7.0 × 10^–3 S/cm at 160°C and a decrease in the activation energy to 0.55 eV.