Ac conductivity and dielectric permittivity of poly(ethylene glycol) during crystallization: Percolation picture

Dielectric conductivity and permittivity of poly(ethylene glycol) were measured in the frequency range between 10¹ and 10⁶ Hz during non-isothermal crystallization and melting with different cooling/heating rates (5, 10 and 20 K/h). The time development of the conductivity and permittivity spectra during crystallization and melting is discussed in terms of charge carrier diffusion in a percolation network formed by the amorphous phase of the semi-crystalline polymer.     

Influence of β nucleation on the mechanical properties of isotactic polypropylene and rubber modified isotactic polypropylene

The tensile and fracture behaviour of neat α and β nucleated isotactic polypropylene and rubber-modified α and β nucleated isotactic polypropylene has been investigated at test speeds of 0.0001-10 ms⁻¹ in the temperature range −30 to +60 °C. The presence of the β phase had little effect at low temperature. However, at +25 and +60 °C, it increased the speeds corresponding to the ductile-brittle transition in the neat polymer by more than three decades. This behaviour has been linked to changes in microdeformation mechanisms observed at the lamellar and spherulitic level, an increase in cavitational deformation in tensile tests and an increase in the strength of the β relaxation in dynamic mechanical spectra. In the blends, the presence of the β phase led to somewhat higher energy dissipation in regimes of ductile fracture. However, the ductile-brittle transitions were not significantly affected. The modifier phase was therefore inferred to control the initiation and propagation of the plastic zone ahead of the crack tip during fracture.     

Cocrystallization behavior of poly(hexamethylene terephthalate-co-hexamethylene 2,6-naphthalate) random copolymers

A series of poly(hexamethylene terephthalate-co-hexamethylene 2,6-naphthalate) (P(HT-co-HN)) random copolymers were synthesized by melt polycondensation and characterized using ¹H NMR spectroscopy and viscometry. Their cocrystallization behavior was investigated using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) method. Even though the P(HT-co-HN) copolymers synthesized are statistically random copolymers, they show a clear melting and a crystallization peak in DSC thermograms over the entire range of copolymer composition and have a minimum melting temperature in the plot of melting temperature versus copolymer composition. WAXD patterns of all the copolymer samples show sharp diffraction peaks and are largely divided into two groups, i.e. PHT type and PHN type crystals. In addition, WAXD patterns of the PHN type crystals are subdivided into two types of PHN α and PHN β according to the copolymer composition. These facts indicate that the P(HT-co-HN) copolymers show isodimorphic cocrystallization. The composition at which the crystal transition between PHT type and PHN type occurs is equivalent to the eutectic composition (22 mol% HN content) for the melting temperature. When the defect free energies were calculated by using the equilibrium inclusion model proposed by Wendling and Suter, the defect free energies in the case of incorporation of HT units in the PHN α and β crystals were higher than the case of incorporation of HN units in the PHT crystal lattice.     

Crystallization from the melt of α and β forms of syndiotactic polystyrene

The crystallization of trans-planar α and β forms of syndiotactic polystyrene is studied through X-ray diffraction and DSC analyses of melt-crystallized samples. The factors controlling the crystallization of the two forms are analyzed. Pure α and β forms of syndiotactic polystyrene can be easily obtained setting the maximum temperature at which the melt is heated and the permanence time of the melt at this temperature. The crystallization of the α and β forms does not depend on the crystallization temperature, at least in the range of accessible crystallization temperatures, between 240 and 270 °C, but only depends on the presence of the ‘memory’ of the α form in the melt. The most important factors are, indeed, the crystalline form of the starting material used in the melt crystallization experiments and the maximum temperature of the melt. Relevant recrystallization phenomena, occurring during the melting of the samples crystallized from the melt at low crystallization temperatures, are responsible for the complex melting behavior of the α and β forms. The recrystallization involves only lamellar thickening of the crystals of the same form (α or β) and not structural transformation.     

Transport properties of conducting amorphous carbon-poly(vinyl chloride) composite

The dc electrical conductivity of composites of poly(vinyl chloride) filled with amorphous carbon (a-C) flakes has been studied from room temperature to 1.2 K. The dc conductivity shows percolative behavior and as a function of a-C content, follows the scaling law σα(p − p_c)^t where p_c is percolation threshold and t the critical exponent with values 0.0165 and 3.1, respectively. The value of t shows evidence for non-universal value of critical exponent. In the temperature range 1.2 K to 50 K-60 K all the samples show the thermal fluctuation induced tunneling of the charge carriers through the insulating layer of polymer separating amorphous carbon aggregates. At high temperatures the differential thermal expansion of PVC and amorphous carbon plays a crucial role in conduction mechanism that leads to positive temperature coefficient of resistivity near to room temperature.     

Structure formation of polyamide 6 from the glassy state by fast scanning chip calorimetry

Fast scanning chip calorimetry has been employed to explore the kinetics of structure formation of polyamide 6 from the glassy state. First, the condition to obtain fully amorphous polyamide 6 has been evaluated. It was found that ordering processes are only permitted on cooling the relaxed melt slower than about 150 K s⁻¹; faster cooling was connected with complete vitrification of the supercooled liquid. Cold-ordering of fully amorphous, non-aged samples on continuous heating only takes place if the heating rate is lower than 500 K s⁻¹. Isothermal cold-ordering experiments of fully amorphous samples have been performed in a wide temperature range between 330 and 470 K, in order to obtain for the first time a complete relationship between half-time of cold-ordering/crystallization and temperature. The study was completed by an initial discussion of the temperature dependence of the mechanism of nucleation and of the effect of the crystal/mesophase polymorphism on the transition kinetics.     

Synthesis and electrical properties of the (PVA)0.7(KI)0.3·xH2SO4 (0 ≤ x ≤ 5) polymer electrolytes and their performance in a primary Zn/MnO2 battery

Solid acid polymer electrolytes (SAPE) were synthesised using polyvinyl alcohol, potassium iodide and sulphuric acid in different molar ratios by solution cast technique. The temperature dependent nature of electrical conductivity and the impedance of the polymer electrolytes were determined along with the associated activation energy. The electrical conductivity at room temperature was found to be strongly depended on the amorphous nature of the polymers and H₂SO₄ concentration. The ac (100 Hz to 10 MHz) and dc conductivities of the polymer electrolytes with different H₂SO₄ concentrations were analyzed. A maximum dc conductivity of 1.05 × 10⁻³ S cm⁻¹ has been achieved at ambient temperature for electrolytes containing 5 M H₂SO₄. The frequency and temperature dependent dielectric and electrical modulus properties of the SAPE were studied. The charge transport in the present polymer electrolyte was obtained using Wagner's polarization technique, which demonstrated the charge transport to be mainly due to ions. Using these solid acid polymer electrolytes novel Zn/SAPE/MnO₂ solid state batteries were fabricated and their discharge capacity was calculated. An open circuit voltage of 1.758 V was obtained for 5 M H₂SO₄ based Zn/SAPE/MnO₂ battery.     

Study of electrochemical hydrogen charge/discharge properties of FePO4 for application as negative electrodes in hydrogen batteries

We report the electrochemical hydrogen charge/discharge properties of electrodes containing crystalline and amorphous FePO₄ as active material in KOH electrolyte. Crystalline and amorphous FePO₄ were synthesized by an alcohol-assisted precipitation method, and the powders obtained were characterized by X-ray diffraction. X-ray photoelectron spectroscopy is used to investigate the mechanism of hydrogen charge/discharge behavior of FePO₄. The electrochemical hydrogen charge/discharge properties of electrodes containing crystalline and amorphous FePO₄ were investigated for potential application as negative electrodes in rechargeable hydrogen batteries. In galvanostatic discharge/charge mode at 25 °C, the crystalline FePO₄ showed a maximum discharge capacity of 109 mA h g⁻¹, while the amorphous FePO₄ showed a maximum discharge capacity of 81.4 mA h g⁻¹. The electrochemical kinetic properties, exchange current density, and proton diffusivity were calculated using linear polarization measurement and the potential-step method.     

DC conductivity and AC impedance of Mn doped magnesia alumina spinel (MgAl2-2xMn2xO4) over a large temperature range

MgAl_2-2xMn_2xO₄ (MAMO) with x = 0-0.12 was synthesized in a single-phase form by solid-state reaction. XRD analysis showed that the samples had the cubic center structure of the Fd-3 m space group. Electrical properties of the samples were studied over the temperature range of 300 K∼1073 K. The results showed that the DC conductivity (σ_DC) increased from 10⁻¹¹S/cm at 300 K (MAMO, x = 0) to 10^-3S/cm at 1073 K (MAMO, x = 0.12). The equivalent circuit of the complex impedance spectra suggested that the relaxation of charge carriers was of non-Debye type. The conduction was mainly caused by grain boundaries and the capacitance was mainly attributed to polarization. The complex permittivity values (ε’ and ε’’) were increased by two orders of magnitude with the increase in Mn content and temperature over the measured frequency range (1 Hz-1 MHz). Therefore, doping with Mn could be applied to modify the electrical properties of MAMO at high temperature.     

Melting and chemical behaviors of isothermally crystallized gamma-irradiated syndiotactic polystyrene

Melting and chemical behaviors of isothermally crystallized gamma-irradiated sPS have been investigated using differential scanning calorimetry, FTIR, and X-ray diffraction techniques. Amorphous sPS samples were subjected to gamma radiation in vacuum and in oxygen at different doses from 200 to 1000 kGy. Irradiated samples were heated to 310 °C, cooled to 220-260 °C range, held for 10 min, and re-melted. Three melting endothermic peaks observed for irradiated and non-irradiated samples isothermally crystallized at 220 °C were decomposed into individual Gaussian distributions, and enthalpies of the total melting endotherm and individual peaks were determined. Both α and β crystalline forms coexist in the crystallized irradiated sPS, regardless of the radiation treatment environment. Dose and irradiation environment have a great effect on the melting behaviors and chemical structures of the isothermally crystallized gamma-irradiated sPS. Crystallinity increases with increasing dose of irradiation in both vacuum and oxygen, with level of increase is greater in oxygen. The three melting peak temperatures decrease with increasing dose regardless of irradiation environments. Ketone and aldehyde oxidized products are formed in the isothermally crystallized gamma-irradiated sPS in oxygen through a combination of hydrogen abstraction or chain scission process and hydroxyl free radicals formation.     

High-pressure solution blending of poly(?-caprolactone) with poly(methyl methacrylate) in acetone + carbon dioxide

The miscibility of blends of poly(?-caprolactone) (PCL, M_w = 14,300) with poly(methyl methacrylate) (PMMA, M_w = 15K or 540K) in acetone + CO₂ mixed solvent has been explored. The liquid-liquid phase boundaries at different temperatures have been determined for mixtures containing 10 wt% total polymer blend, 50 wt% acetone and 40 wt% CO₂. The PCL and PMMA contents of the blends were varied while holding the total polymer concentration at 10 wt%. The polymer blend solutions all displayed LCST-type behavior and required higher pressures than individual polymer components for complete miscibility. Complete miscibilities were achieved at pressures within 40 MPa. The DSC scans show that the blends are microphase-separated. The blends display the melting transition of PCL and the glass transition temperature of the PMMA phases. The presence of PMMA is found to influence the crystallization and melting behavior of PCL in the blends. The DSC results on heat of melting and the FTIR spectra, specifically the changes at 1295 cm⁻¹ band show the changes (decrease) in overall crystallinity of the blend upon addition of PMMA.     

Kinetics of crystal nucleation of poly(L-lactic acid)

The kinetics of crystal nucleation of poly(L-lactic acid) (PLLA) has been analyzed by fast scanning chip calorimetry in a wide temperature range between 313 and 383 K, that is, between temperatures about 30 K below and 40 K above the glass transition temperature. The relaxed melt was rapidly cooled to the analysis temperature and subsequently aged between 10⁻¹ and 10⁴ s, to permit formation of nuclei. The number of formed crystal nuclei has been probed by analysis of the crystallization rate at 393 K. The nucleation rate is maximal at 370–375 K and decreases monotonously with decreasing temperature in the analyzed temperature range. The observation of a monomodal dependence of the nucleation rate on temperature points to predominance of a single nucleation mechanism in the analyzed temperature range, regardless nucleation occurs in the glassy or devitrified amorphous phase. The collected data suggest that nuclei formation at ambient temperature requires a waiting time longer than about 10⁸ s. The performed study is considered as a quantitative completion of nucleation-rate data available for PLLA only at temperatures higher than 360 K, suggesting that the nucleation mechanism is independent on temperature in the analyzed temperature range between 313 and 383 K.     

Anodic dissolution of amorphous Ni-P alloys

Anodic behaviour of amorphous Ni-P alloys containing 23 and 27 at.% P was compared to that of pure Ni. Measurements were performed in 0.1 M sulphate solutions of different pH by means of potentiodynamic polarization and impedance spectroscopy at selected anodic potentials. Significant discrepancies in the course of anodic polarisation curves and impedance spectra between amorphous Ni-P and pure Ni were seen. Amorphous Ni-P alloys exhibit a suppression of dissolution in the active range of Ni, whereas they show a faster dissolution in the passive domain of Ni. From the other side, there are minor modifications in impedance spectra taken for Ni-P over a wide range of anodic potentials, whereas Ni exhibit essential changes of impedance spectra within the same potential region, related to the active-passive transition of this metal. The charge transfer resistance of the anodic dissolution for amorphous Ni-P decreases gradually with increasing anodic potential in contrast to a sharp increase of this parameter for pure Ni in the region of its passivation. In contrast to Ni, the anodic dissolution of amorphous Ni-P exhibits a slight dependence on pH. These findings prove that the anodic dissolution suppression of amorphous Ni-P cannot be associated with the oxide passivity typical of pure Ni.     

Measurements of the melting point of graphite and the properties of liquid carbon (a review for 1963-2003)

Scientific literature on the melting temperature of graphite and its properties in melting is reviewed, beginning with the study by Bundy in 1963 and proceeding up to 2003. Data obtained by Pirani in 1930 that has been quoted in some recent publications is also considered. Successive experimental data and theoretical predictions on the melting point of carbon are summarized. The history of carbon studies, starting in 1963, is given, covering both laser and electrical heating of the graphite. The main divergence in the experimental results is in the value of the true melting temperature of graphite in the range of 4000 or 5000 K.The paper first describes laser heating. Pulsed laser heating of graphite usually shows the absence of a melting temperature plateau in the heating of a low-density graphite specimen (only a deflection point is observed on the increasing signal of the pyrometer). Carbon vapour, as a result of graphite sublimation, usually plays a leading role in the temperature measurements near the melting point under slow heating.The volume electrical heating of graphite is then discussed. Several electrical pulse investigations are listed: measurements of different properties; heating of low density graphite; and very slow pulse heating up to steady-state by alternating current. A separate section shows the data on spectral emissivity investigations, which are required in graphite temperature measurements.Reliable experimental data for the graphite melting point are presented: enthalpy of solid state under melting (10.5 kJ/g); enthalpy of liquid state under melting (20.5 kJ/g); heat of graphite melting (10 kJ/g); liquid-carbon resistivity (730 μΩ cm) near the melting point at a density of 1.8 g/cm³ under high pressures (several GPa); estimation of expansion (70%) during melting at 100 MPa pressure; and melting temperature T_m = 4800 ± 100 K at a pressure 10-100 MPa. Most of these data are obtained by electrical fast heating (1-5 μs), that are supported by the data of carefully executed laser-pulse heating.     

A carbon in molten carbonate anode model for a direct carbon fuel cell

The electrochemical oxidation of carbon at the anode of a direct carbon fuel cell (DCFC) includes charge transfer steps and chemical steps. A microstructural model of carbon particle is built, in which perfect graphene stacks are taken as the basic building blocks of carbon. A modified mechanism taking account of the irreversibility of the process and supposing that the electrochemical oxidation of carbon takes place only at the edges of the graphene sheets is proposed. A Tafel type overall rate equation is deduced along with expressions of exchange current density (j₀) and activation polarization (η_act). The performance of carbon black and graphite as the fuel of DCFC is examined. It has been found that j₀ is in the range of 0.10-6.12 mA cm⁻² at 923-1123 K and η_act is in the range of 0.024-0.28 V at 923-1123 K with current density in 10-120 mA cm⁻². Analysis of the j₀, η_act values and the product composition reveals that the charge transfer steps as well as the oxygen ion absorption steps are both important for the reaction rate. The activity of the carbon material with respect to atom location is introduced to the open circuit potential difference (OCP) calculation with Nernst equation.     

DEIS evaluation of the relative effective surface area of AISI 304 stainless steel dissolution process in conditions of intergranular corrosion

The results presented in this paper contribute further information concerning examinations of AISI 304 stainless steel dissolution process in conditions of proceeding intergranular corrosion (IG) which have been determined on the basis of dynamic electrochemical impedance spectroscopy (DEIS) measurements. For the first time changes of the relative effective surface area versus time of AISI 304 stainless steel dissolution process in conditions of proceeding intergranular corrosion (IG) in 0.5 M H₂SO₄ + 0.01 M KSCN solution within the range of reactivation polarization scan have been demonstrated. The assessment of the effective surface area of the investigated process was based on approximation to theory of iron dissolution in sulfuric acid medium according to the shape of instantaneous impedance spectra recorded by means of DEIS technique. As a result, it was possible to evaluate that initially changes of the employed equivalent circuit parameters are not only caused by the changes of the relative effective surface area but also by the changes of the AISI 304 SS dissolution process kinetics. Further progress of the examined process implied that changes of the equivalent circuit parameters are only affected by the changes of the relative effective surface area during proceeding IG. Moreover, it was found that the character of changes of the relative effective surface area in conditions of proceeding IG can be described by exponential function.     

Electrochemical behavior of centrifuged cast and heat treated Ti-Cu alloys for medical applications

The aim of this study was to evaluate the general electrochemical corrosion resistance of Ti-5, 7.1 and 15 wt.% Cu alloys with a view to medical applications. A centrifuged casting set-up and a solution heat treatment at 900 °C for 2 h were used to prepare the samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.15 M NaCl solution at 25 °C. An equivalent circuit analyses was also conducted. It was found that the corrosion rate increased with increasing Cu content. The results have shown that the addition of Cu has not stabilized the β phase. Martensite and Ti₂Cu intermetallic particles provided by casting and heat treatment processes, respectively, have important roles on the resulting impedance parameters and passive current densities of the Ti-Cu alloys.     

Growth of tetrahedral amorphous carbon film: Tight-binding molecular dynamics study

Molecular dynamics simulation using tight-binding potential has been performed to examine the growth and performance of tetrahedral amorphous carbon during ion deposition. The sp³ hybrid atom content, density, and compressive stress of the tetrahedral amorphous carbon film depend on the growing conditions such as substrate temperature, ion energy, ion dose, and annealing temperature. The critical temperature for sp³ transition to sp² decreases with ion energy (40, 80, and 120 eV). At low temperatures (<300 K) and low ion energies, the sp³ fraction increases up to 82%. At the annealing temperature less than 1200 K or with a few ions (<20) implanted into the film, its sp³ content and density have only slight changes while the compressive stress has a large reduction with the annealing temperature and the number of implanted ions. This large reduction in the compressive stress is due to a structural relaxation.     

Structural,dielectric properties and electrical conduction behaviour of Dy substituted CaCu3Ti4O12 ceramics

Dy substituted CCTO ceramics were synthesized using solid state reaction method. Effect of Dy on structural, microstructural, dielectric and electrical properties has been studied over a wide temperature (300–500 K) and frequency range (100 Hz–1 MHz). Rietveld refinement, carried out on the samples, confirmed single phase formation and indicated overall decrease in lattice constant. Microstructure showed bimodal distribution of grains in CCTO with bigger grains surrounded by smaller grains. Dy substitution reduced grain size. Dy substitution in CCTO reduces the dielectric constant which may be attributed to increase of the Schottky potential barrier. The dielectric constant remains nearly constant in temperature range 300–400 K. The AC conductivity obeys a power law, σ_ac=A fⁿ, where n is the temperature dependent frequency exponent. The AC conductivity behaviour can be divided into three regions, over entire temperature range, depending on conduction processes. The relevant charge transport mechanisms have been discussed.     

Innovative spectral investigations on the thermal-induced poly(aspartic acid)

Two-dimensional (2D) correlation FTIR spectroscopy was used to investigate the dynamic mechanism of poly(aspartic acid) during the heating process. According to the 2D asynchronous correlation spectra, the CO vibration band of Amide I was separated into three peaks at 1637, 1645 and 1677 cm⁻¹, assigned to α helical, random coil and β antiparallel conformation, respectively. And α helical structure would transform into the more ordered and stable conformation of β antiparallel in heating. In the region of 3700-2700 cm⁻¹, the H-contained groups were analyzed; the loosening of hydroxyl is found to change prior to the NH-related hydrogen bonds and the conformational reorganization of hydrocarbon chains.