Effect of AgNO3–MnCl2 mixed fillers on the physical properties of polystyrene films

This study deals with the effects of various filling levels of a mixture of two transition compounds [(X)AgNO₃(10 ? X)MnCl₂] on the structural, electrical, and magnetic properties of atactic polystyrene (PS) films. X‐ray diffraction (XRD) scans showed two main peaks for unfilled PS films. Crystalline peaks were unexpectedly detected with the filling and could be correlated to the formation of clusters. The IR transmission spectra revealed characteristic PS peaks. Certain IR peaks could be taken as evidence for the formation of polaron and bipolaron bound states in the polymeric matrix. The direct‐current (DC) electrical conduction measurements suggested that the conduction mechanism could be attributed to phonon‐assisted charge carrier hopping according to the interpolaron hopping model. The DC magnetic susceptibility results at 90–235 K obeyed the Curie–Weiss law. The negative values of the paramagnetic Curie temperature (θ_p) indicated the possibility of an antiferromagnetic exchange interaction, whereas the positive values of θ_p suggested a ferromagnetic exchange interaction at low temperatures. An electron spin resonance (ESR) spectrum at X = 0% revealed a broad Lorentzian signal. This suggested the presence of aggregated Mn²⁺ and was confirmation of cluster formation found in XRD studies. On the other hand, ESR spectra at higher values of X depicted hyperfine structures characterized by the six unresolved lines of the manganese nucleus, indicating the existence of isolated Mn²⁺. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95:1333–1341, 2005     

Crystal Structure and Physical Properties of PVDF Films Filled with CuCl2–MnCl2 Mixed Fillers

ABSTRACT

Poly(vinylidene fluoride) (PVDF) films filled with mixed fillers of CuCl₂–MnCl₂ were prepared. The differential thermal analysis (DTA) indicates the existence of two main endothermic peaks and the crystallization exothermic temperature. X-ray diffraction (XRD) evidenced the presence of a semicrystalline structure containing α, β, and γ crystalline phases. The optical absorption spectra depicted two shoulder-like bands as well as a strong valley. The IR spectra confirmed the XRD implications about the presence of α, β, and γ phases. The dc electrical resistivity results are discussed on the basis of Kuivalainen modified interpolaron hopping model. The temperature and filling level dependence of the hopping distance R₀ were studied. The dc magnetic susceptibility data follow the Curie-Weiss law. The electron spin resonance (ESR) investigation suggested the existence of aggregated Mn²⁺ for higher values of x where the spectra were characterized by Lorentzian signal. On the other hand, at lower values of x, the spectra were characterized by two unresolved sharp peaks.     

Effect of local structure of MnCl2-filled PVDF films on their optical,electrical, electron spin resonance,and magnetic properties

The filling level (W) dependence of the local structure of MnCl₂ through the PVDF matrix was explored. The presence of α- and β-PVDF crystalline phases were detected by X-ray diffraction, and it was confirmed by the infrared (IR) absorption spectra. A significant head-to-head content was implied by IR spectroscopy. Two filling, level-dependent optical energy gaps were found through the UV–VIS spectral investigation. An intrachain one-dimensional interpolaron hopping mechanism was assumed to proceed in the temperature range of 350–375 K. The calculated values of the charge carrier hopping distance were in the range of 6.5–9.7 nm. The temperature dependence of the direct current (dc) magnetic susceptibility exhibited a Curie–Weiss behavior. Positive values of the paramagnetic Curie temperature (θ_p) for W up to 14.5% indicated the presence of a ferromagnetic interaction, while negative θ_p obtained for higher W values suggested an antiferromagnetic interaction at lower temperature. The electron spin resonance (ESR) analysis revealed the existance of both isolated and aggregated Mn²⁺ ions within the PVDF matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1437–1445, 1998     

Magnetocaloric effect and magnetic properties of the isovalent Sr2+ substituted Ba2FeMoO6 double perovskite

Fine-tuning the charge distribution in Ba₂FeMoO₆ obtained via “isovalent” substitution at the A-site (i.e., Ba) is expected to bring about changes in the physical properties of the system that can be manipulated in magnetic refrigerants. With this motivation, the phase formation, crystal structure, microstructure, magnetic and magnetocaloric properties of the Ba_2−xSr_xFeMoO₆ (0≤x≤0.4) samples fabricated by solid state reaction method have been investigated. The X-ray diffraction analysis confirmed the formation of cubic structure with Fm3m space group in all the fabricated samples. The magnetization measurements and Arrott analysis revealed a second order of ferromagnetic phase transition in all the samples. An increase in magnetization and Curie temperature (T_C) was observed with the increase in Sr-content that was attributed to the increased orbital hybridization and exchange interaction between Fe and Mo ions. The magnitude of the maximum magnetic entropy change at the Curie temperature and the relative cooling power were observed to slightly decrease with the increased Sr doping. The excellent magnetocaloric features and convenient adjustment of Curie temperature make these materials useful for magnetic refrigeration in a wide range of temperature.     

Electrical and magnetic properties of conjugated schiff base polymers

Two new conjugated poly-Schiff bases (PPpP and PPmP) were synthesized by polycon-densation of p-phenylene diamine or m-phenylene diamine with 2,6-pyridine dicarboxal-dehyde. PPpP and PPmP can from charge transfer complexes with iodine. Maximum conductivity of PPpP-iodine complex at room temperature is 10⁻⁶ S/cm, which is 2 orders of magnitude higher than that of PPmP-iodine complex. Electronic spin resonance measurements discovered that there are stable radicals in both charge transfer complexes; and g value, line width, and spin concentration depend on doping degree. Magnetic susceptibility of charge transfer complexes of PPmP–iodine is composed of Curie magnetic susceptibility (χ_c) and Pauli magnetic susceptibility (χ_p). Its Curie constant (C), Curie spin concentration (N_c), and density of state at the Fermi level also depend on doping degree. © 1996 John Wiley & Sons, Inc.     

Thermal and NMR Studies of Polystyrene Blends

Abstract

The effect of addition of poly (propylene oxide) (PPO) and polystyrene with low molecular weight (LPS) to polystyrene (PS) was investigated blending these polymers in a Haake internal mixer. The PPO and LPS range was established up to 10% by weight. The blends were analysed by differential scanning calorimetry (DSC) and carbon-13 nuclear magnetic resonance spectroscopy at solid state (NMR), using conventional NMR techniques as cross-polarisation/magic angle spinning (CP/MAS) and proton spin-lattice relaxation time in the rotating frame (T ₁ ^H _p ). The addition of 1 and 5% of PPO and 5% of LPS to PS made the blends of PS/PPO and PS/LPS more rigid.     

Room temperature multiferroic properties of Ni-doped Aurivillus phase Bi5Ti3FeO15

Single-phase Aurivillius Bi₅Ti₃Fe_0.5Ni_0.5O₁₅ (BTFN) ceramics were synthesized by the solid-state reaction method. The substitution of Ni for half Fe ions does not introduce magnetic impurity phase but increases magnetic moment more than two orders. The ferroelectric and magnetic Curie temperatures are determined to be 1100 K and 726 K. The room-temperature multiferroic behavior of the BTFN ceramics were demonstrated by the ferroelectric (2P_r=8.5 μC/cm², 2E_c=74 kV/cm) and ferromagnetic (2M_r=27.86 m emu/g, 2H_c=553 Oe) measurements. The ferromagnetism can be ascribed to the aggregation of magnetic ions at the inner octahedra by Ni doping and the spin canting of magnetic-ion-based sublattices via the Dzyaloshinskii-Moriya interaction. The present work suggests the possibility of doped Bi₅Ti₃FeO₁₅ as a potential room-temperature multiferroic.     

Rapid and phase pure synthesis of microporous copper silicate (CuSH–1Na) with 12-ring channel system

Phase pure sample of the microporous copper silicate CuSH–1Na has been obtained by simplified hydrothermal method without using additives (H₂O₂ and Na₂HPO₄). Ion exchange of Na⁺ by Cs⁺, Ca²⁺ and Sr²⁺ ions showed that the structure can suffer partial replacement of the charge compensating cations. Ion exchange with Cs⁺ resulted in distinct dehydration while the ion exchange with Sr²⁺ increased the total amount of water. Water content in the Ca-exchanged sample is comparable to the as-synthesized sodium phase. Raman spectroscopy revealed that the divalent cations as Ca²⁺ and Sr²⁺ induce stronger local structural deformations than the monovalent Cs⁺. These structural changes have been also followed by the refined lattice distortions. Magnetic analyses showed that CuSH–1Na presents a very weak ferromagnetic interaction along the Cu²⁺ chains with a nearly vanishing Curie–Weiss temperature. This magnetic coupling is associated with super-super-exchange interactions through Cu–Na–O–Na–Cu paths. Antiferromagnetic coupling, attributed to inter-chains super-super-exchange interactions, competes with the ferromagnetic one and prevails at the lowest temperature.     

Magnetic Properties of Mixed Valence La(AgSr)MnO3 Manganites Obtained by Solid‐State Reaction Method

In this work, we present a systematic study on the effect of monovalent and divalent cation inclusion on the magnetic properties of the manganites series La_0.80(Ag_1?xSr_x)_0.20MnO₃ (x = 0.0–1.0) synthesized by the solid‐state reaction method. The decreasing Sr:Ag proportion across the compositional series was verified by X‐ray photoelectron spectroscopy. Concerning magnetic properties, the hysteresis curves manifested an initial paramagnetic response at x = 0.0, followed by a progressive ferromagnetic behavior with an optimum Ag:Sr ratio at x = 0.75, for which an enhanced saturation magnetization of 51 Am²/kg and a Curie temperature of 336 K were recorded. Results are explained on the basis of the effect of the increasing unit cell volume on the double exchange interaction between magnetic Mn³⁺– Mn⁴⁺ atoms.     

Origin of dielectric loss in Ba(Co1/3Nb2/3)O3 microwave ceramics

The microwave dielectric loss of stoichiometric and non‐stoichiometric Ba(Co_1/3Nb_2/3)O₃ ceramics have been measured from 2 to 300 K in magnetic fields ranging from 0 to 5 T using a dielectric resonator (DR) technique. The microwave absorption from spin excitations of unpaired d‐electrons in exchange coupled Co²⁺ ions dominate the loss of the Ba(Co_1/3Nb_2/3)O₃ ceramics at cryogenic temperatures. Two peaks in the loss tangent (tan δ) vs temperature relation from a distinctly different origin occur at 25‐30 K and 90 K, which increase in magnitude with increasing Co content in the bulk dielectric samples. Evidence that these peaks result from polaron conduction from hopping between Co²⁺ and Co³⁺ ions includes (i) the peak's observed temperature range; (ii) the decrease in peak intensity of approximately a factor of two in a large applied magnetic fields (5 T); and (iii) a strong correlation between the peak's magnitude and both the fraction of the minority Co³⁺ in the dominant Co²⁺ matrix and D.C. conductivity at elevated temperatures. A magnetic‐field independent high temperature peak with a maximum at 250 K dominates the room temperature microwave loss whose magnitude correlates with those of the low temperature peaks. This suggests that the defects responsible for carrier conduction play an important role in establishing the loss tangent at room temperature.     

Magnetic properties of hard (CoFe2O4)–soft (Fe3O4) composite ceramics

Composite ceramics of CoFe₂O₄/Fe₃O₄ with different weight ratios were synthesized by Spark Plasma Sintering (SPS) at a sintering temperature of 500 °C. The X-ray diffraction patterns demonstrate that all samples are composed of CoFe₂O₄ and Fe₃O₄ phases. The magnetization curves for all the composite ceramic are single-step loops indicating the existence of exchange spring effect. Due to the competition between the exchange interaction and the dipolar interaction, magnetic properties like coercivity (H_c) and remanence (M_r) are sensitive to the weight ratio of the soft phase.     

Curie temperature of a ferromagnetic polymer chain model

Phase transition of a self‐avoiding walking polymer chain with spatial nearest neighbor ferromagnetic Ising interaction on the simple cubic lattice is investigated using Metropolis dynamic Monte Carlo technique. Magnetic susceptibility χ is determined from the Brillouin function and from the linear response of magnetization to external magnetic field in small field region. We find the later is better, though both are in consistent with each other at high temperatures. The magnetic susceptibility χ can be expressed using the Curie law and thus the Curie temperature θ is estimated. The chain length dependent Curie temperature θ of the chain model can be well fitted as θ = 1.40 ? 4.0 × n^?0.619 based on the finite‐size scaling law. The transition temperature for an infinite long chain θ(∞) = 1.40J/k_B and the exponent ? = 0.619 are in agreement with our previous study ( 19 ). © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 969–973, 2006     

Solubility of CO2 in Glassy PMMA and PS over a Wide Pressure Range: The Effect of Carbonyl Groups

The equilibrium sorption isotherms of CO₂ in glassy PMMA and PS at 32, 42, and 52C over a wide pressure range from 20 to 340 atm were investigated. The normalized sorption concentration (C) isotherms for the polymers in terms of pressure (P) can be described fairly well by two power laws C=kP ⁿ for below and above critical pressure (P _c). The exponent n is a measure of sorption intensity and is closely associated with the interaction between CO₂ and the polymer. From the temperature variation of n values, a negligible interaction between CO₂ and the polymer is found in the sorption process below P _c whereas the interaction is significant above P _c. For a given temperature, the n value for PMMA is 12 times higher than that for PS as a result of the specific interaction of CO₂ and the carbonyl groups in PMMA. The pre-exponential constant k is a measure of sorption capacity and is closely associated with the heat of sorption. In sorption above P _c, k is found to decrease with increasing temperature due to the exothermic sorption process that leads to a decrease in sorption capacity with temperature. From the sorption isobars of PMMA and PS, we observe that the temperature necessary for chemisorption to occur is lower for PMMA than for PS; this results from the specific interaction of CO₂ and the carbonyl groups of PMMA.     

Solubility of hydrofluorocarbon (HFC‐134a,HFC‐152a) and hydrochlorofluorocarbon (HCFC‐142b) blowing agents in polystyrene

Solubilities of blowing agents (HFC‐134a, HCFC‐142b, and HFC‐152a) in polystyrene (PS) were measured at temperatures from 348 to 473 K and pressures up to 3.2 MPa with a volumetric method. For all conditions, the solubility of the blowing agents in PS decreased with increasing temperature. At a given temperature and pressure, the solubility of HFC‐134a, HFC‐152a, and HCFC‐142b in PS increased in that order. Solubilities could be correlated with the Sanchez‐Lacombe equation of state with temperature‐dependent binary interaction parameters to within 3.4% average relative percent deviation. The temperature dependence of the Henry's constants, K_p, for the blowing agents were found to have a linear relationship between ln(1/K_p) and (T_c/T)², where T_c is the critical temperature of the blowing agent.     

Competing magnetic interactions in the graphite-intercalation compound Li0.25Eu1.95C6

We present an extensive study of the magnetic properties of the new intercalation compound Li_0.25Eu_1.95C₆ performed by dc magnetization, ac susceptibility, muon-spin and Mössbauer spectroscopy measurements as a function of temperature. The experimental results indicate that the magnetic behaviour of this compound is determined by the competition between magnetic inter- and intra-cluster correlations. The former are mainly due to ferromagnetic-like exchange interactions between Eu spins located between two graphene planes (at high temperature) or surrounding them (at low temperature). The latter correlations, instead, seem to arise from low-temperature antiferromagnetic-like exchange interactions between small magnetic intercalated domains (SMIDs).     

Effect of different post-annealing durations on electromagnetic properties of La0.67Ca0.33MnO3:Ag0.2 thin films prepared by pulsed laser deposition

La_0.67Ca_0.33MnO₃:Ag_0.2 thin films were obtained by pulsed laser deposition followed by post-annealing at 1200 °C for varied durations. Surface morphologies, structures, and electrical and magnetic properties of films prepared with different annealing durations were significantly different. X-ray diffraction results showed that annealed films exhibited stronger diffraction peaks and C-axis preferred growth. Regarding their electrical properties, metal-insulator transition temperature (T_MI) and temperature coefficient of resistance (TCR) increased first and then decreased with the increase in annealing duration. In terms of their magnetic properties, thin films displayed ferromagnetic-paramagnetic transition. With the increase in annealing duration, Curie temperature increased first and then decreased. Specifically, the film annealed for 3 h showed excellent electromagnetic properties, with relatively high TCR of 20.3%·K^-1 and near room temperature T_MI of 285.7 °C. Owing to these excellent properties, as-prepared thin films have application potential in infrared detectors.     

Probing the early stages of polymerization of ethylene on a model chromium/silica catalyst by Fourier transform infrared spectroscopy

The nature of the interaction of monomer, and the early stages of growth of oligomers of ethylene on a rather more uniform surface of Cr/SiO₂ catalyst than hitherto studied has been investigated by differenceFTIR spectroscopy using C₂D₄ and C₂H₄ as reactants both with and without subsequent treatment of the catalyst with CO andTHF. The active catalyst was prepared by reaction of vapour phase CrO₂Cl₂ with the vicinal hydroxyls of the silica surface. Three distinct kinds of methylene groups were detected. Arguments are given for assigning the peaks at 2935 and 2860 cm^–1 to CH₂ groups directly bound to the active site and those at 2920 and 2850 to CH₂s in the growing chain well removed from the Cr. The peaks at 2160 and 2165 cm^–1 are attributed to CD₂ groups hydrogen bonded to surface hydroxyls.     

Synthesis of soluble oligsiloxane-end-capped hyperbranched polyazomethine and their application to CO2/N2 separation membranes

Three soluble hyperbranched polyazomethines containing oligosiloxane end group HBP-PAZ-SiO_n were successfully synthesized. HBP-PAZ-SiO_ns were used as modifiers of ethyl cellulose (EC) and polysulfone (PS) membranes. Blend membranes, HBP-PAZ-SiO_n/EC and HBP-PAZ-SiO_n/PS were prepared by blending the THF solution of HBP-PAZ-SiO_n with ethanol solution of EC and dichloromethane solution of PS, respectively. Surprisingly, the permeabilities for CO₂ of the blend membranes were more than 15–16 times higher than those of pure EC and PS membranes without any drop of pemselectivity to N₂. This unusual improvement has been achieved by both enhancement of diffusivity for carbon dioxide and nitrogen by the oligosiloxane groups and enhancement of affinity of the amino groups with carbon dioxide at the end groups of HBP-PAZ-SiO_n.     

A new room-temperature multiferroic material: Y2FeAlO6

In this paper, we report the finding and basic characterization of a new multiferroic material in the form of the Y₂FeAlO₆ (YFAO) double perovskite with orthorhombic structure. In order to study the structural and multiferroic properties in detail, we prepared the polycrystalline YFeO₃ (YFO) and Y₂FeAlO₆ ceramic samples through traditional high temperature solid-state reaction method. XRD analysis confirmed that these two samples are single-phase and all the peaks in the pattern can be indexed with orthorhombic symmetry of the Pnma space group. Compared to YFO, YFAO is more symmetric, and has a more stable crystalline structure and smaller grain size. The Fe ion in YFO and YFAO are in the mixed oxidation state of Fe²⁺ and Fe³⁺, while the relative amount of Fe²⁺ in YFAO is higher than that of YFO. Al³⁺ with filled 2p orbital and grain refinement may be the main reasons for the greatly enhanced multiferroicity of YFAO. The magnetic properties of YFO and YFAO are mainly dominant antiferromagnetism and weak ferromagnetism, and the exchange bias effect disappears in YFAO. The coexistence of ferroelectricity and ferromagnetic orders at room temperature in YFAO promises new opportunities and improvements in next generation applications for spintronic, information storage, and sensors etc.     

Enhancement of dielectric characteristics in donor doped Aurivillius SrBi2Ta2O9 ferroelectric ceramics

In this study, investigations have been made on the crystal structure, surface morphology, dielectric and electrical properties of tungsten doped SrBi₂(W_xTa_1−x)₂O₉ (0.0 ≤ x ≤ 0.20) ferroelectric ceramics. Dielectric measurements performed as a function of temperature at 1, 10 and 100 kHz show an increase in Curie temperature (T_c) over the composition range of x = 0.05–0.20. W⁶⁺ substitution in perovskite-like units results in a sharp dielectric transition at the ferroelectric Curie temperature with the dielectric constant at their respective Curie temperature increasing with tungsten doping. The dielectric loss reduces significantly with tungsten addition. The temperature dependence of ac and dc conductivity vis-à-vis tungsten content shows a decrease in conductivity, which is attributed to the suppression of oxygen vacancies. The activation energy calculated from the Arrhenius plots is found to increase with tungsten content.