The linear thermal expansion and the thermal diffusivity measurements for near-stoichiometric (U, Ce)O2 solid solutions

The thermal diffusivities of near-stoichiometric (U, Ce)O₂ solid solutions containing CeO₂ up to 22 mol% were investigated in the temperature range of 298-1273 K using the laser flash method. Also, linear thermal expansion measurements were performed in the temperature range of 298-1673 K using a thermomechanical analysis. The thermal conductivities were determined by a calculation of the thermal diffusivity, the density and the specific heat. The thermal conductivities of the tested samples could be expressed as a function of the temperature by the phonon conduction equation k = (A + BT)⁻¹. The thermal conductivity decreased gradually with an increasing Ce content. This was attributable to the increasing lattice defect thermal resistance caused by the U⁴⁺, Ce⁴⁺ and O²⁻ ions as phonon scattering centers.     

Effects of partial anion substitution on the thermoelectric properties of silver(I) chalcogenide halides in the system Ag5Q2X with Q=Te, Se and S and X=Br and Cl

A selection of mixed conducting silver chalcogenide halides of the general formula Ag₅Q₂X with Q=sulfur, selenium and tellurium and X=chlorine and bromine has been investigated due to their thermoelectric properties. Recently, the ternary counterpart Ag₅Te₂Cl showed a defined d¹⁰-d¹⁰ interaction in the disordered cation substructure at elevated temperatures where Ag₅Te₂Cl is present in its high temperature α-phase. A significant drop of the thermal diffusivity has been observed during the β−α phase transition reducing the values from 0.12 close to 0.08 mm² s⁻¹. At the same transition the thermopower reacts on the increasing silver mobility and jumps towards less negative values.Thermal conductivities, thermopower and thermal diffusivity of selected compounds with various grades of anion substitution in Ag₅Q₂X were determined around the silver-order/disorder β−α phase transition. A formation of attractive interactions could be observed for selenium substituted phases while no effect was detected for bromide and sulfide samples. Depending on the grade and type of substitution the thermopower changes significantly at and after the β−α phase transition. Thermal conductivities are low reaching values around 0.2-0.3 W m⁻¹ K⁻¹ at 299 K. Partial anion exchange can substantially tune the thermoelectric properties in Ag₅Q₂X phases.     

Oxidation of substituted pyridines by dimethyldioxirane: kinetics and solvent effects

The oxidation of a series of substituted pyridines by dimethyldioxirane (1) produced the expected N-oxides in quantitative yields. The second order rate constants (k₂) for the oxidation of a series of substituted pyridines (2a-g) by dimethyldioxirane were determined in dried acetone at 23 °C. An excellent correlation with Hammett sigma values was found (ρ = −2.91, r = 0.995). Kinetic studies for the oxidation of 4-trifluoromethylpyridine by 1 were carried out in the following dried solvent systems: acetone (k₂ = 0.017 M⁻¹ s⁻¹), carbon tetrachloride/acetone (7:3; k₂ = 0.014 M⁻¹ s⁻¹), acetonitrile/acetone (7:3; k₂ = 0.047 M⁻¹ s⁻¹), and methanol/acetone (7:3; k₂ = 0.68 M⁻¹ s⁻¹). Kinetic studies of the oxidation of pyridine by 1 versus mole fraction of water in acetone [k₂ = 0.78 M⁻¹ s⁻¹ (χ = 0) to k₂ = 11.1 M⁻¹ s⁻¹ (χ = 0.52)] were carried out. The results showed the reaction to be very sensitive to protic, polar solvents.     

Characterization and performance of proton exchange membranes for direct methanol fuel cell: Blending of sulfonated poly(ether ether ketone) with charged surface modifying macromolecule

Modification of sulfonated poly(ether ether ketone) (SPEEK) membrane was attempted by blending charged surface modifying macromolecule (cSMM). The modified membrane was tested for direct methanol fuel cell (DMFC) application; i.e. a SPEEK/cSMM blend membrane was compared to a SPEEK membrane and a Nafion 112 membrane for the thermal and mechanical stability, methanol permeability, and proton conductivity. Thermal and mechanical stability of the blended membrane were slightly reduced from the SPEEK membrane but still higher than the Nafion 112 membrane. The blend membrane was found to be promising for DMFC applications because of its lower methanol diffusivity (2.75 × 10⁻⁷ cm² s⁻¹) and higher proton conductivity (6.4 × 10⁻³ S cm⁻¹), than the SPEEK membrane. A plausible explanation was given for the favorable effect of cSMM blending.     

Structure and high-temperature thermoelectric properties of SrAl2Si2

Single crystals of SrAl₂Si₂ were synthesized by reaction of the elements in an aluminum flux at 1000 °C. SrAl₂Si₂ is isostructural to CaAl₂Si₂ and crystallizes in the hexagonal space group P-3m1 (90 K, a=4.1834 (2), c=7.4104 (2) Å, Z=1, R1=0.0156, wR2=0.0308). Thermal analysis shows that the compound melts at ∼1020 °C. Low-temperature resistivity on single crystals along the c-axis shows metallic behavior with room temperature resistivity value of ∼7.5 mΩ cm. High-temperature Seebeck, resistivity, and thermal conductivity measurements were made on hot-pressed pellets. The Seebeck coefficient shows negative values in entire temperature range decreasing from ∼−78 μV K⁻¹ at room temperature to −34 μV K⁻¹ at 1173 K. Seebeck coefficients are negative indicating n-type behavior; however, the temperature dependence is consistent with contribution from minority p-type carriers as well. The lattice contribution to the thermal conductivity is higher than for clathrate structures containing Al and Si, approximately 50 mW cm⁻¹ K, and contributes to the overall low zT for this compound.     

Thermophysical properties of SrHfO3 and SrRuO3

Polycrystalline samples of strontium series perovskite type oxides, SrHfO₃ and SrRuO₃ were prepared and the thermophysical properties were measured. The average linear thermal expansion coefficients are 1.13×10⁻⁵ K⁻¹ for SrHfO₃ and 1.03×10⁻⁵ K⁻¹ for SrRuO₃ in the temperature range between 423 and 1073 K. The melting temperatures T_m of SrHfO₃ and SrRuO₃ are 3200 and 2575 K, respectively. The longitudinal and shear sound velocities were measured by an ultrasonic pulse-echo method at room temperature in air, which enables to evaluate the elastic moduli and Debye temperature. The heat capacity was measured by using a differential scanning calorimeter, DSC in high-purity argon atmosphere. The thermal diffusivity was measured by a laser flash method in vacuum. The thermal conductivities of SrHfO₃ and SrRuO₃ at room temperature are 5.20 and 5.97 W m⁻¹ K⁻¹, respectively.     

Thermal properties of the pyrochlore, Y2Ti2O7

The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y₂Ti₂O_7, have been determined over the temperature range 2 K?T?300 K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θ_D, of ca. 970 K) and an assignment of the remaining 63 optic modes, as well as a correction for C_p−C_v. From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15 K)−H₀=34.69 kJ mol⁻¹ and S(T=298.15 K)−S₀=211.2 J K⁻¹ mol⁻¹. The thermal conductivity shows a peak at ca. θ_D/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10 μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y₂Ti₂O₇ is 2.8 W m⁻¹ K⁻¹, close to the calculated theoretical thermal conductivity, κ_min, for fully coupled phonons at high temperatures.     

Influence of hole mobility on the response characteristics of p-type nickel oxide thin film based glucose biosensor

RF sputtered p-type nickel oxide (NiO) thin film exhibiting tunable semiconductor character which in turns enhanced its functional properties. NiO thin film with high hole mobility is developed as a potential matrix for the realization of glucose biosensor. NiO thin film prepared under the optimized deposition conditions offer good electrical conductivity (1.5 × 10⁻³ Ω⁻¹-cm⁻¹) with high hole mobility (2.8 cm² V⁻¹ s⁻¹). The bioelectrode (GO_x/NiO/ITO/glass) exhibits a low value of Michaelis–Menten constant (K_m = 1.05 mM), indicating high affinity of the immobilized GO_x toward the analyte (glucose). Due to the high surface coverage (2.32 × 10⁻⁷ mol cm⁻²) of the immobilized enzyme on to the NiO matrix and its high electrocatalytic activity, the prepared biosensor exhibits a high sensitivity of 0.1 mA (mM⁻¹-cm⁻²) and a good linearity from 25 to 300 mg dL⁻¹ of glucose concentration with fast response time of 5 s. Various functional properties of the material (mobility, crystallinity and stress) are found to influence the charge communication feature of NiO thin film matrix to a great extent, resulting in enhanced sensing response characteristics.     

Thermal degradation kinetics of polyimide containing 2,6-benzobisoxazole units

A novel polyimide (PI) based on 2,6-bis(p-aminophenyl)-benzo[1,2-d;5,4-d′]bisoxazole has been synthesized via a conventional two-stage procedure with bis(ether anhydrides) (HQDPA). The intermediate poly(amic acid) had inherent viscosities of 1.70 dl/g and could be thermally converted into light yellow polyimide film. The resulted polyimide showed excellent thermal stability, and the glass transition temperatures (T_g) were above 283 °C, the 5% weight loss temperature of the polymer was at 572 °C in N₂. The thermal degradation of the polyimide was studied by thermogravimetric analysis (TGA) in order to determine the actual reaction mechanisms of the decomposition process. The activation energy of the solid-state process was determined using Flynn-Wall-Ozawa method, which does not require knowledge of the reaction mechanism, which resulted to be 361.36 kJ/mol. The activation energy of different mechanism models and pre-exponential factor (A) were determined by Coats-Redfern method. Compared with the value obtained from the Ozawa method, the actual reaction mechanism obeyed nucleation and growth model, Avrami-Erofeev function (A₃) with integral form g(X) = [−ln(1−X)]³.     

Enhanced thermal stability of poly(1,4-dioxan-2-one) in melt by adding a chelator

Thermal stability of poly(p-dioxanone) (PPDO) was investigated isothermally and non-isothermally under air atmosphere using thermogravimetry (TG). The addition of 4-benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-one (PMBP) could enhance successfully the thermal stability of PPDO compared with those of as-prepared and purified PPDO at temperature below about 230 °C. The activation energies for thermal degradation (ΔE_td) were evaluated at different weight loss values from TG data using the procedure recommended by MacCallum et al. The ΔE_td values of as-prepared PPDO, purified PPDO and PPDO containing 1.0 wt% PMBP were in the ranges of 20-50, 35-60, and 56-88 kJ mol⁻¹, respectively, when they were evaluated at weight loss values of 10-80%. The remaining weights increase with the amounts of PMBP added up to 1.5 wt%. The mechanism for the enhanced thermal stability of PPDO was discussed.     

Self-standing single lithium ion conductor polymer network with pendant trifluoromethanesulfonylimide groups: Li diffusion coefficients from PFGSTE NMR

Solid electrolyte materials have the potential to improve performance and safety characteristics of batteries by replacing conventional solvent-based electrolytes. For this purpose, new candidate single ion conductor self-standing networks were synthesized with trifluoromethane-sulfonylimide (TFSI) lithium salt based monomer using poly(ethyleneglycol) dimethacrylate (PEGDM 750) as crosslinker. The highest ionic conductivity was 3.4 × 10⁻⁷ S cm⁻¹ at 30 °C in the dry state. Thermal and mechanical analyses showed good thermal stability up to 190 °C and rubbery-like properties at ambient temperature. A direct relationship between ionic conductivity and glassy or rubbery state of the membranes was found. Vogel–Tammann–Fulcher behavior was observed in the dry state which is consistent with a lithium conductivity correlated with polymer chain mobility. By swelling the network in propylene carbonate, a self-standing electrolyte gel could be obtained with an ionic conductivity as high as 1 × 10⁻⁴ S cm⁻¹ at 30 °C. The individual diffusion coefficients of mobile species in the material (¹⁹F and ⁷Li) were measured and quantified using pulsed-field gradient nuclear magnetic resonance (PFG-NMR). Diffusion coefficients for the most mobile components of the lithium cations and fluorinated anions at 100 °C in dry membranes have been found to be 3.4 × 10⁻⁸ cm² s⁻¹ and 2.1 × 10⁻⁸ cm² s⁻¹ respectively.     

New ionic liquids with the bis[bis(pentafluoroethyl)phosphinyl]imide anion, [(C2F5)2P(O)]2N—Synthesis and characterization

A new series of low melting and hydrophobic ionic liquids (ILs) containing the bis[bis(pentafluoroethyl)phosphinyl]imide anion, [(C₂F₅)₂P(O)]₂N⁻ (FPI), and ammonium, phosphonium, imidazolium, pyridinium or pyrrolidinium cations were prepared and characterized. Their density, viscosity, melting point, glass transition temperature, decomposition temperature and conductivity are discussed. Many of these ionic liquids are liquids at room temperature with melting points below 15 °C, viscosities below 110 mm² s⁻¹ and thermal stabilities above 300 °C.     

Evaluation of kinetics parameters for poly(l-lactic acid) hydrolysis under high-pressure steam

In order to evaluate more precise kinetics parameters: rate constant k and E_a values for poly(l-lactic acid) hydrolysis, the reaction was carried out under high-pressure steam in a temperature range of 100-130 °C. Molecular weights of hydrolyzates were calculated by the universal calibration method without being influenced by any weight loss. The changes in molecular weight could be successfully explained according to the auto-catalytic hydrolysis mechanism, clearly indicating the critical point. Resulting k and E_a values were estimated as 8.4 × 10⁻⁵-7.2 × 10⁻⁴ s⁻¹ and 87.2 kJ mol⁻¹ with high R² values, respectively. Moreover, to determine the deviation of the parameter values, influences of four factors on the measurements and calculation: (1) use of number-average molecular weight value alone, (2) use of relative molecular weight based on polystyrene standards, (3) weight loss during the hydrolysis, and (4) selection of reaction mechanism were evaluated quantitatively.     

Kinetic study of the oxidative addition reaction between methyl iodide and [Rh(FcCOCHCOCF3)(CO)(PPh3)]: Structure of [Rh(FcCOCHCOCF3)(CO)(PPh3)(CH3)(I)]

The kinetics of oxidative addition of CH₃I to [Rh(FcCOCHCOCF₃)(CO)(PPh₃)], where Fc = ferrocenyl and (FcCOCHCOCF₃)⁻ = fctfa = ferrocenoylacetonato, have been studied utilizing UV/Vis, IR, ¹H and ³¹P NMR techniques. Three definite sets of reactions involving isomers of at least two distinctly different classes of Rh^III-alkyl and two different classes of Rh^III-acyl species were observed. Rate constants for this reaction in CHCl₃ at 25 °C, applicable to the reaction sequence below, were determined as k₁ = 0.00611(1) dm³ mol⁻¹ s⁻¹, k₋₁ = 0.0005(1) s⁻¹, k₃ = 0.00017(2) s⁻¹ and k₄ = 0.0000044(1) s⁻¹ while k₋₃ ? k₃ and k₋₄ ? k₄ but both ≠0. The indeterminable equilibrium K₂ was fast enough to be maintained during Rh^I depletion in the first set of reactions and during the Rh^IIIalkyl2 formation in the second set of reactions. From a ¹H and ³¹P NMR study in CDCl₃, K_c1 was found to be 0.68, K_c2 = 2.57, K_c3 = 1.00, K_c4 = 4.56 and K_c5 = 1.65.     

p-Type thermoelectric properties of the oxygen-deficient perovskite Ca2Fe2O5 in the brownmillerite structure

Brownmillerite calcium ferrite was synthesized in air at 1573 K and thermoelectric properties (direct current electrical conductivity σ, Seebeck coefficient α, thermal conductivity κ, thermal expansion α_L) were measured from 373 to 1050 K in air. Seebeck coefficient was positive over all temperatures indicating conduction by holes, and electrical properties were continuous through the Pnma-Imma phase transition. Based on the thermopower and conductivity activation energies as well as estimated mobility, polaron hopping conduction was found to dominate charge transport. The low electrical conductivity, <1 S/cm, limits the power factor (α²σ), and thus the figure of merit for thermoelectric applications. The thermal conductivity values of ∼2 W/mK and their similarity to Ruddlesden-Popper phase implies the potential of the alternating tetrahedral and octahedral layers to limit phonon propagation through brownmillerite structures. Bulk linear coefficient of thermal expansion (∼14×10⁻⁶ K⁻¹) was calculated from volume data based on high-temperature in situ X-ray powder diffraction, and shows the greatest expansion perpendicular to the alternating layers.     

Poly(arylene ether) ionomers containing sulfofluorenyl groups: Effect of electron-withdrawing groups on the properties

For polymer electrolyte membrane fuel membrane cell (PEMFC) applications, the effect of electron-withdrawing groups on the properties of sulfonated poly(arylene ether) (SPE) ionomer membranes was investigated. A series of poly(arylene ether)s containing fluorenyl groups and electron-withdrawing groups (sulfone, nitrile, or fluorine) was synthesized, which were sulfonated with chlorosulfonic acid using a flow reactor to obtain the title ionomers. The ionomers had high molecular weight (M_n > 77 kDa, M_w > 238 kDa) and gave tough, ductile membranes by solution casting. The ion exchange capacity (IEC) of the membranes ranged from 1.6 to 3.5 mequiv/g as determined by titration. The electron-withdrawing groups did not appear to affect the thermal properties (decomposition temperature higher than 200 °C). The presence of nitrile groups, especially at positions meta to the ether linkages, improved the oxidative stability of the SPE membranes, while it led to a deterioration of the hydrolytic stability. The perfluorinated biphenylene groups were effective in providing high mechanical strength with reasonable dimensional change, probably due to a somewhat decreased water absorbability. The SPE membrane containing sulfone groups showed the highest proton conductivity (10⁻³-10⁻¹ S/cm) at 20-93% RH (relative humidity) and 80 °C. The nitrile-containing SPE membrane showed smaller apparent activation energies for oxygen and hydrogen permeability and is thus considered to be a possible candidate for applications in PEMFCs.     

Li ion diffusion in Li4Ti5O12 thin film electrode prepared by PVP sol-gel method

Li₄Ti₅O₁₂ thin films for rechargeable lithium batteries were prepared by a sol-gel method with poly(vinylpyrrolidone). Interfacial properties of lithium insertion into Li₄Ti₅O₁₂ thin film were examined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and potentiostatic intermittent titration technique (PITT). Redox peaks in CV were very sharp even at a fast scan rate of 50 mV s⁻¹, indicating that Li₄Ti₅O₁₂ thin film had a fast electrochemical response, and that an apparent chemical diffusion coefficient of Li⁺ ion was estimated to be 6.8×10⁻¹¹ cm² s⁻¹ from a dependence of peak current on sweep rates. From EIS, it can be seen that Li⁺ ions become more mobile at 1.55 V vs. Li/Li⁺, corresponding to a two-phase region, and the chemical diffusion coefficients of Li⁺ ion ranged from 10⁻¹⁰ to 10⁻¹² cm² s⁻¹ at various potentials. The chemical diffusion coefficients of Li⁺ ion in Li₄Ti₅O₁₂ were also estimated from PITT. They were in a range of 10⁻¹¹-10⁻¹² cm² s⁻¹.     

The preparation and thermoelectric properties of molten salt electrodeposited boron wafers

We have prepared electrodeposited boron wafer by molten salts with KBF₄-KF at 680°C using graphite crucible for anode and silicon wafer and nickel plate for cathodes. Experiments were performed by various molar ratios KBF₄/KF and current densities. Amorphous p-type boron wafers with purity 87% was deposited on nickel plate for 1 h. Thermal diffusivity by ring-flash method and heat capacity by DSC method produced thermal conductivity showing amorphous behavior in the entire temperature range. The systematical results on thermoelectric properties were obtained for the wafers prepared with KBF₄-KF (66-34 mol%) under various current densities in the range 1-2 A/cm². The temperature dependencies of electrical conductivity showed thermal activated type with activation energy of 0.5 eV. Thermoelectric power tended to increase with increasing temperature up to high temperatures with high values of (1-10) mV/K. Thermoelectric figure-of-merit was 10⁻⁴/K at high temperatures. Estimated efficiency of thermoelectric energy conversion would be calculated to be 4-5%.     

Environmental assessment of CFC alternatives: Rate constants for the reactions of OH radicals with fluorinated compounds

The rate constants for the reactions of OH radicals with CF₃OCHFCF₃, and CF₃CHFCF₃ have been measured over the temperature range 250-430 K. Kinetic measurements have been carried out using the flash photolysis, and laser photolysis methods combined, respectively, with the laser induced fluorescence technique. The influence of impurities in the samples has been investigated by using gas chromatography. No sizable effect of impurities was found on the measured rate constants of these fluorinated compounds, if the purified samples were used in the measurements. The following Arrhenius expressions were determined: k(CF₃OCHFCF₃) = (4.39 ± 1.38) × 10⁻¹³ exp[−(1780 ± 100)/T] cm³ molecule⁻¹ s⁻¹, and k(CF₃CHFCF₃) = (6.19 ± 2.07) × 10⁻¹³ exp[−(1830 ± 100)/T] cm³ molecule⁻¹ s⁻¹.     

Comparison of pyrimethanil-imprinted beads and bulk polymer as stationary phase by non-linear chromatography

A pyrimethanil-imprinted polymer (P1) was prepared by iniferter-mediated photografting a mixture of methacrylic acid and ethylene dimethacrylate onto homemade near-monodispersed chloromethylated polydivinylbenzene beads. The chromatographic behaviour of a column packed with these imprinted beads was compared with another column packed with irregular particles obtained by grinding a bulk pyrimethanil-imprinted polymer (P2). The comparison was made using the kinetic model of non-linear chromatography, studying the elution of the template and of two related substances, cyprodinil and mepanipyrim. Extension of the region of linearity, capacity factors for the template and the related substances, column selectivity, binding site heterogeneity, apparent affinity constant (K) and lumped kinetic association (k_a) and dissociation rate constant (k_d) were studied during a large interval of solute concentration, ranging between 1 and 2000 μg/ml. From the experimental results obtained, in the linearity region of solute concentration column selectivity and binding site heterogeneity remained essentially the same for the two columns, while column capacity (at 20 μg/ml, P1 = 23.1, P2 = 11.5), K (at 20 μg/ml, P1 = 8.3 × 10⁶ M⁻¹, P2 = 2.5 × 10⁶ M⁻¹) and k_a (at 20 μg/ml, P1 = 3.5 μM⁻¹ s⁻¹, P2 = 0.47 μM⁻¹ s⁻¹) significantly increased and k_d (at 20 μg/ml, P1 = 0.42 s⁻¹, P2 = 0.67 s⁻¹) decreased for the column packed with the imprinted beads. These results are consistent with an influence of the polymerisation method on the morphology of the resulting polymer and not on the molecular recognition properties due to the molecular imprinting process.