Extended free radical network from condensed cyanuric chloride with p-phenylenediamine: An EPR/FMR study

A covalent layered network was obtained by condensation of cyanuric chloride with bridging paraphenylenediamine. The local chemical environment of the layered solid can be changed by a redox reaction to obtain new reconstructed derivatives. A blue product was obtained by treating an alcoholic dispersion of the layered solid with ferric nitrate or potassium persulfate, indicating the possible formation of an extended free radical. When iron nitrate was used as oxidant, the temperature-dependent magnetic resonance spectra were measured in the 290–4 K region. The magnetic resonance measurements showed the coexistence at room temperature of two spectra arising from two different magnetic centers: a narrow line centered at g = 2.0038(1) with linewidth of ΔH = 7.42(2) G (free radical) and a broad line centered at g = 2.254(1) with linewidth of ΔH = 1300(5) G (magnetic iron-oxide cluster). A new sample was prepared so that the broader line was more intense. The temperature dependence of the magnetic resonance lines was subject to intense changes in both cases. The integrated intensities decreased with decreasing temperatures in both spectra in the high temperature region. This type of behavior is similar to that of magnetic nanoparticles in non-magnetic matrices. Upon reducing the temperature with the gradient of ΔH_r/ΔT = 1.5(1) G/K, the resonance field of the broad line was shifted towards lower magnetic fields, while the narrow line was shifted towards higher magnetic fields with ΔH_r/ΔT = 0.020(1) G/K. The linewidth of the broader line increased with decreasing temperature, while the narrow line remained almost constant. The magnetic iron-oxide clusters could produce an internal magnetic field acting on free radicals. This field could compel free radicals to form a magnetic ordered state at high temperatures.     

FMR and DSC study of maghemite nanoparticles in PMMA polymer matrix

Nanocomposite samples of poly(methyl methacrylate) (PMMA) polymer with a γ-Fe₂O₃ (maghemite) filler have been synthesised and studied by ferromagnetic resonance (FMR) and differential scanning calorimetry (DSC) methods. Two types of samples have been investigated: containing 5 wt%, 10 wt% of a maghemite filler. DSC measurements have revealed an increase in the glass transition temperature T_g and a decrease in the heat capacity c_p with an increase in the concentration of nanoparticles. A FMR study in the 4–300 K temperature range has shown the presence of an asymmetric spectrum that has been analyzed in terms of two Gaussian-shaped components arising from the assumed magnetic anisotropy of the nanoparticles. The FMR investigation has exposed the temperature range of a superparamagnetic regime (60–290 K) and the blocking temperature of T_B ～ 60 K. In that range a shift in the resonance line δH_r and the linewidth ΔH is related by δH_r ～ (ΔH)ⁿ, where n = 2.79 indicates a fair amount of disorder in the maghemite system. An analysis of the FMR spectra in terms of two component lines has shown the importance of the dipole–dipole interaction for higher concentrations of maghemite and for T > 220 K.     

Dominance of magnetic scattering in the electrical-transport of Al70.5Pd22Mn7.5 icosahedral quasicrystal

Conductivity vs. temperature (σ–T) at zero and 8-T field, magneto-resistance (Δρ/ρ), magnetization vs. temperature (M–T) and magnetization vs. field (M–H) of Al_70.5Pd₂₂Mn_7.5 quasicrystal have been studied in the temperature range of 1.4 K to 300 K. The σ–T variations in both the field conditions show a σ–T minima. In addition to this the σ–T variation at 8 T shows a maxima also at ∼6 K. Comparative analysis shows that the observed σ–T minima arises due to competing inelastic scattering events in the presence of weak-localization effect. These events are e–ph scattering in the dirty-metallic limit (τ_i ∝ T⁻²), and the Kondo-type spin-flip scattering (τ_sf). The maxima observed for the σ–T variation at 8 T, has been attributed to suppression of the spin-flip scattering in the presence of field. The magneto-resistance is found to be large and positive. It was properly accounted only when the Stoner-enhancement, found in the case of spin fluctuating systems, was taken into consideration.     

Interplay between the magnetic field and the dipolar interaction on a magnetic nanoparticle system: A Monte Carlo study

We have studied the influence of the applied magnetic field on the blocking temperature (T_B) of a fine magnetic particle system. By means of a Monte Carlo technique we have simulated zero field cooling (ZFC) curves under different applied fields, obtaining the respective T_B as a function of H. We have focused our study on the limit H → H_K (where H_K is the anisotropy field), since the results found in the literature usually lack a detailed study of this range. The simulations were done at different sample concentration of the nanoparticles, with the purpose of observing how the magnetic dipolar interaction affects the field dependence of T_B. The classical expression predicts T_B to disappear for H ? H_K, independently of the dipolar interaction strength. Our simulations show that at strong interacting conditions T_B exists even for fields H > H_K.     

Influence of maghemite concentration on magnetic interactions in maghemite/PTT-block-PTMO nanocomposite

Temperature dependence of dc magnetization and ferromagnetic resonance (FMR) of two samples containing γ-Fe₂O₃ (maghemite) magnetic nanoparticles dispersed at low concentration (0.1 and 0.3 wt%) in a nanocomposite based on a poly(ether–ester) multiblock copolymer (PTT-block-PTMO) matrix was investigated. The polymer filler was in a powder form consisting of small-sized magnetic nanoparticles arranged in agglomerates 2–3 μm long and 100 nm thick. The studied samples were characterized by SEM spectroscopy. The SEM showed that the concentration of magnetic nanoparticles was homogenous in both samples The temperature dependence of the dc magnetization revealed that the blocking was about 100 K and the ZFC (zero-field cooling) mode at low magnetic fields uncovered the presence of magnetic interactions between magnetic nanoparticles depending on the properties of the matrix. FMR measurements were carried out in the temperature range 4.2–300 K. An intense resonance absorption line attributed to γ-Fe₂O₃ nanoparticles was recorded with a slightly asymmetric lineshape. At room temperature the resonance line was centered at H_r = 3241(2) and 3253(2) G, with linewidths of ΔH = 1069(1) and 1070(1) G for samples with concentrations of 0.1 and 0.3 wt%, respectively. All FMR parameters showed an anomalous behavior at matrix critical temperatures. It was shown that the difference in concentration of magnetic nanoparticles could be responsible for the observed differences in the thermal behavior of the FMR spectra.     

Glass formation of Ti–Ni–Sn ternary alloys correlated with TiNi–Ti3Sn pseudo binary eutectics

Glass-forming ability (GFA) of Ti–Ni–Sn ternary alloys was investigated. Applying recent models based on atomic size ratio and efficient packing, the composition favoring the glass formation is predicted. Our experiments indicate that the optimized glass-forming composition is located at Ti₅₆Ni₃₈Sn₆, with the critical thickness of complete glass formation approaching 100 μm for the melt-spun ribbons. The Ti₅₆Ni₃₈Sn₆ metallic glass exhibits a sizable supercooled liquid region (ΔT_x) of about 35 K and a reduced glass transition temperature (T_rg) of 0.52. We demonstrate that the glass formation of the Ti₅₆Ni₃₈Sn₆ alloy correlates with the (L → TiNi + Ti₃Sn) pseudo binary eutectic reaction in Ti–Ni–Sn ternary system, which has an invariant temperature and composition at ～1370 K and ～Ti₅₈Ni₃₄Sn₈, respectively. With respect to Sn-free Ti–Ni binary alloys, the GFA is enhanced for the Ti–Ni–Sn ternary alloys, but the improvement is limited possibly due to changes in the crystalline phases competing with glass formation.     

Effect of laser irradiation on thermal and optical properties of selenium–tellurium alloy

The crystallization parameters such as glass transition temperature (T_g), onset crystallization temperature (T_c), peak crystallization temperature (T_p) and enthalpy released (ΔH_C) of the bulk Se–Te chalcogenide glass has been studied by using Differential Scanning Calorimeter (DSC), under non-isothermal condition at a heating rate of 20 K/min. The values of T_g, T_c, T_p and ΔH_C with and without laser irradiation for different exposure time have been studied. The optical absorption of pristine and laser irradiated thermally evaporated Se–Te films has been measured. The films shows indirect allowed interband transition that is influenced by the laser irradiation. The optical energy gap has been found to decrease from 1.61 to 1.38 eV with increasing irradiation time from 5 to 20 min. The results have been analyzed on the basis of laser irradiation-induced defects in the film.     

A local structure change of bulk Pd40Ni40P20 glass during full relaxation

The change in the amorphous structure of bulk Pd₄₀Ni₄₀P₂₀ glass during structural relaxation was examined by an anomalous X-ray scattering (AXS) experiment with energies near the Ni K-absorption edge. It was confirmed by differential scanning calorimetry that the sample reached a meta-stable state (a fully relaxed state) with an equilibrium free volume concentration after annealing for about 1 × 10⁴ s at 563 K and 4 × 10⁴ s at 557 K just below the glass transition temperature T_g = 567 K. The structural changes on the progression toward a fully relaxed state were examined in samples annealed for 1 × 10³ and 2 × 10⁴ s at 563 K (glass A), and for 3.2 × 10³, 1 × 10⁴ and 7 × 10⁴ s at 557 K (glass B). The structural analysis revealed that the coordination number of Ni–Ni like atom pairs increased with annealing time and that of Ni–Pd, unlike atom pairs, decreased. Meanwhile, the coordination number N_PNi of P–Ni atom pairs and the nearest neighbor distance r_PNi did not show a remarkable variation. However, prolonged annealing of 7 × 10⁴ s at 557 K induced a remarkable change in N_PNi and r_PNi.     

Electrical properties of annealed a-SiC:H thin films

In the present work the dependence of electrical properties of a-SiC:H thin films on annealing temperature, T_a, has been extensively studied. From the measurements of dark dc electrical conductivity, σ_D, in the high temperature range (from 283 up to 493 K), was found that the conductivity activation energy, E_a, is invariant for T_a ⩽ 673 K and equal to 0.64 eV, whereas for T_a from 673 up to 873 K, E_a increases at about 0.2 eV reaching to a maximum value 0.85 eV at T_a = 873 K, suggesting the optimum material quality. This behavior of E_a as a function of T_a is mainly attributed to relaxation of the strain in the amorphous network, which is possibly combined with weak hydrogen emission for temperatures up to 873 K. For further increase of T_a (>873 K) the phenomenon of hydrogen emission, causes rapid decrease of E_a down to 0.24 eV at T_a = 998 K, deteriorating the material quality. These results are also supported by the measurements of dark dc electrical conductivity in the low temperature range (from 133 up to 283 K), where the dependence of the density of gap states at the Fermi level, N(E_F), on annealing temperature presents the minimum value at T_a = 873 K. The Meyer–Nelder rule was found to hold for the a-SiC:H thin films for annealing temperatures up to 873 K. Finally, the dependence of dark dc electrical conductivity at room temperature, σ_DRT, on T_a showed to reflect directly the dependence of E_a on T_a.     

NMR in soft materials: A study of DMPC/DHPC bicellar system

The DMPC/DHPC bicellar system at the molar ratio of 2.8:1 has been characterised by measurements of self-diffusion coefficient (using PFGSE and PFGSTE NMR sequences), differential scanning calorimetry (DSC) and small angle scattering of synchrotron radiation (SAXS). The DSC curve shows only one endothermic peak characterised by the peak temperature T_peak = 295.7 K and the onset temperature T_onset = 290.1 K. This peak can be assigned to the nematic to smectic phase transition. Below the phase transition temperature, NMR diffusion experiments indicate a two-exponential decay of the spin echo amplitude allowing two diffusion coefficients D₁ and D₂ to be extracted from the experimental data. The maximum size (D_max) of the bicelle determined from SAXS data using the pair distance distribution function p(r) is 11.2 nm and the bilayer thickness is 5 nm.     

Kinetics of the exchanges at the solid–liquid interface during the dissolution process of gallium orthophosphate

Gallium orthophosphate is a piezoelectric material with an α-quartz structure. In order to manufacture bulk acoustic wave devices (BAW), controlled chemical dissolution is often used to reduce the thickness of the piezoelectric membranes. This paper presents the kinetics of the chemical exchanges, which occur at the solid–liquid interface during the chemical dissolution of GaPO₄ in phosphoric acid. Based on chemical composition of phosphoric acid solvent, the pure dissolution rate is determined. A strong anisotropy of chemical reactivity is formed. The dissolution rate is the lowest for the crystallographic z-plane (0 0 0 1) but this orientation is the most sensitive with respect to the proton concentration and the temperature. In accordance with the crystal growth rates, the nucleation at the interface for the (1 0 2 0) plane, named X-plane, is also the most rapidly dissolved. Assuming the activation energies corresponding to dissolution and to nucleation are like standard activation energies, the different values of the standard enthalpy variation are calculated. The most important variation is obtained for the z-plane (Δ_rH=−14.3 kJ/mol) and the lowest for the X-plane (Δ_rH=−5.4 kJ/mol).     

Photo-crystallization in a-Se imaging targets: Raman studies of competing effects

Photo-induced crystallization of a-Se is investigated by Raman spectroscopy as a function of temperature (250–340 K) and exposure time in thin-film structures used as targets in high-gain avalanche rushing photoconductor (HARP) video cameras. The Stokes-to-Antistokes ratio is monitored to obtain the local temperature T_loc at the laser spot; fluxes (632 nm) of 17 and 10 W/cm² are used. We find a rich temperature behavior that reflects the competition of changes in viscosity and strain, and defines four distinct regimes. No photo-crystallization is seen for T_loc below 260 K, nor in a 15 K range around T_g ～ 310 K. For T_loc in the regime 260–302 K the initial rate of crystal growth after onset of photo-crystallization is temperature independent, whereas for T_loc > 318 K the growth rate is thermally enhanced. Our results are in qualitative accord with a theory by Stephens treating the effects of local strain on the secondary growth of crystalline nuclei in a-Se. We conclude that the observed growth rate between 260 and 302 K is driven by local strain, and that relaxation of this strain near T_g suppresses crystal growth until thermally assisted processes accelerate the photo-crystallization at higher temperatures.     

Temperature dependence of the generation and decay of E′ centers induced in silica by 4.7 eV laser radiation

We report a study of the generation of silicon dangling bonds (E′ centers) induced in fused silica by 4.7 eV laser irradiation in the 10 < T < 475 K temperature range, carried out by in situ optical absorption spectroscopy. The generation of the defects, occurring by transformation of pre-existing precursors, results to be a thermally activated process, quenched below 150 K and with a 0.044 eV activation energy. At T > 200 K the induced defects undergo a post-irradiation decay due to their reaction with mobile H₂. The interplay between generation and annealing gives rise to a bell-shaped temperature dependence of the concentration of induced E′ centers, peaking at 250 K.     

EPR study of crystalline and glassy ethanol

Pulsed X-band electron paramagnetic resonance (EPR) spectroscopy was applied in studying molecular dynamics in two different solid ethanol matrices. Nitroxyl radicals as paramagnetic reporter groups were embedded in crystalline and glassy ethanol and the phase memory time, T_m, was investigated at 5–80 K. Temperature variation revealed a maximum in 1/T_m centered around 50 K and a small linear decrease with temperature, below ca. 25 K. Faster phase memory time relaxation in crystalline ethanol than in ethanol glass was observed throughout the temperature range studied. This can be attributed to differences in spectral diffusion due to distinct molecular packing densities.     

Low-temperature electron paramagnetic resonance in silver-iron oxide nanoparticles

Water in oil microemulsion technique has been used to obtain silver nanoparticles with a mean size of 7 nm surrounded by a matrix of 2 nm γ-Fe₂O₃ nanoparticles. Magnetic measurements of the sample exhibit a cusp in the ZFC curve at 50 K, which corresponds to the blocking temperature. A detailed study of the thermal evolution of EPR spectra has been performed in the samples. It has been shown that the linewidth, the resonance field and the intensity of the EPR line show different behaviour near the blocking temperature.     

Glass forming ability criteria for La–Al–(Cu, Ni) alloys

Criteria for evaluating the glass forming ability (GFA) of La–Al–(Cu, Ni) alloys were studied. The GFA criteria can be categorized into two types, according to whether the liquidus temperature T_l is included in the formula or not. The T_l inclusive criteria generally predict a maximum in GFA at the eutectic composition at which T_l is a minimum. However, for La–Al–(Cu, Ni) alloys, the best GFA does not correspond to the eutectic. The paper discusses the suggestion that the solidification temperature T_s should be used instead of T_l for evaluating the GFA of La–Al–(Cu, Ni) alloys.     

Transport properties near the magneto/structural transition of Tb5Si2Ge2

We report high resolution measurements of the electrical resistivity (ρ, dρ/dT) and thermopower (S, dS/dT) measurements near the magnetic and structural transition of the layered Tb₅Si₂Ge₂ compound, which are fairly close but not fully coupled. The analysis of the transport properties confirms a split magneto/structural transition, with T_S ～ 97 K and T_S ～ 107 K for the structural transition (on cooling and heating respectively; 1st-order transition). The magnetic transition occurs only at T_C ～ 112 K and without hysteresis (2nd-order transition). The magnetic critical behavior of resistivity is analyzed, obtaining an almost classical mean field exponent (α ～ 0.59) for T > T_C. For the structural phase, and below T_S, we obtain a rather different exponent (α ～ 1.06).     

The shoving model for the glass-former LiCl·6H2O: A molecular dynamics simulation study

Molecular dynamics (MD) simulations of LiCl·6H₂O showed that the diffusion coefficient D, and also the structural relaxation time <τ>, follow a power law at high temperatures, D^?1 ∝ (T ? T_o)^?μ, with the same experimental parameters for viscosity (T_o = 207 K, μ = 2.08). Decoupling between D and <τ> occurs at T_x ～ 1.1T_o. High frequency acoustic excitations for the LiCl·6H₂O model were obtained by the calculation of time correlation functions of mass current fluctuations. The temperature dependence of the instantaneous shear modulus, G_∞(T), was considered in the shoving model for supercooled liquids [J.C. Dyre, T. Christensen, N.B. Olsen, J. Non-Cryst. Solids 352 (2006) 4635] resulting in a linear relationship log (D^?1) vs. G_∞/T.     

Effects of hydration,annealing, and melting on heat transport properties of fused quartz and fused silica from laser-flash analysis

Thermal diffusivity (D) at high temperature (T) was measured from 15 samples of commercial SiO₂ glasses (types I, II, and III with varying hydroxyl contents) using laser-flash analysis (LFA) to isolate vibrational transport, in order to determine effects of impurities, annealing, and melting. As T increases, D_glass decreases, approaching a constant (~ 0.69 mm²s^? 1) above ~ 700 K. From ~ 1000 K to the glass transition, the slope of D is small but variable. Increases of D with T of up to 6% correlate with either low water and/or low fictive temperature and are attributed to removal of strain and defects during annealing. Upon crossing the glass transition, D substantially decreases to 0.46 mm²s^? 1 for the anhydrous melt. Hydration decreases D_glass, makes the glass transition occur over wider temperature intervals and at lower T, and promotes nucleation of cristobalite from supercooled melt. Due to the importance of thermal history, a spread in D of about 5% occurs for any given chemical type. Combining prior steady-state, cryogenic data with our average results on type I glass provides thermal conductivity (k_lat = ρC_PD) for type I: k_lat increases from ~ 0 K, becoming nearly constant above 1500 K, and drops by ~ 30% at T_g. We find that D^? 1(T) correlates with thermal expansivity times temperature from ~ 0 K to melting due to both properties arising from anharmonicity.     

Formation of bulk Pt–Pd–Ni–P glassy alloys

The thermal behavior of (Pt_0.4Pd_0.3Ni_0.3)_100−xP_x (x = 16–25 at.%) glassy alloys has been investigated. It is found that the crystallization behavior of the (Pt_0.4Pd_0.3Ni_0.3)_100−xP_x glassy alloys changes from a single-stage exothermic reaction to a two-stage exothermic reaction depending on phosphorous content. When the phosphorous content is 23 at.%, the glassy alloy exhibits the largest supercooled liquid region (ΔT_x) and a sharp single exothermic peak. Fixing the phosphorous content at 23 at.%, the Pt_77−x−yPd_xNi_yP₂₃ (x = 7.7–61.6 at.%, y = 7.7–61.6 at.%) glassy alloys have a wide composition range in which the glassy alloys exhibit a large supercooled liquid region (ΔT_x beyond 60 K). In this range, the Pt_30.8Pd_23.1Ni_23.1P₂₃ glass has the largest ΔT_x (77 K) and a high reduced glass transition temperature (T_rg) of 0.60. This alloy can be cast into fully glassy rods with a diameter of 3 mm. Under uni-axial compression, bulk Pt_30.8Pd_23.1Ni_23.1P₂₃ glassy alloy has an elastic strain of ∼2%, an ultimate strain (to fracture) of ∼6.4%, a Young’s modulus of ∼106 GPa and a failure strength of ∼1390 MPa.