Controlled crystallization and ionic conductivity of a nanostructured LiAlGePO4 glass–ceramic

A Li_1.5[Al_0.5Ge_1.5(PO₄)₃] glass composition was subjected to several crystallization treatments to obtain glass–ceramics with controlled microstructures. The glass transition (T_g), crystallization onset (T_x) and melting (T_m) temperatures of the parent glass were characterized by differential scanning calorimetry (DSC). The glass has a reduced glass transition temperature T_gr = T_g/T_m = 0.57 indicating the possibility of internal nucleation. This assumption was corroborated by the similar DSC crystallization peaks from monolithic and powder samples. The temperature of the maximum nucleation rate was estimated by DSC. Different microstructures were produced by double heat treatments, in which crystal nucleation was processed at the estimated temperature of maximum nucleation rate for different lengths of time. Crystals were subsequently grown at an intermediate temperature between T_g and T_x. Single phase glass–ceramics with Nasicon structures and grain sizes ranging from 220 nm to 8 μm were then synthesized and the influence of the microstructure on the electrical conductivity was analysed. The results showed that the larger the average grain size, the higher the electrical conductivity. Controlled glass crystallization allowed for the synthesis of glass–ceramics with fine microstructures and higher electrical conductivity than those of ceramics with the same composition obtained by the classical sintering route and reported in literature.     

Semi-interpenetrating polymer networks based on polyurethane and poly(2-hydroxyethyl methacrylate): Dielectric study of relaxation behavior

The study of molecular dynamics by broadband dielectric spectroscopy (BDS) is presented for polyurethane (PU), poly(2-hydroxyethyl methacrylate) (PHEMA) and for semi-IPNs based on PU and PHEMA synthesized by photopolymerization. The dielectric properties were performed in wide range of frequencies and temperatures with the goal to establish the relation between the relaxations and the structure. Five relaxation phenomena were finally detected for PHEMA : γ-, βsw-, β-relaxations at low temperatures and α-relaxation at 150 °C at high frequencies plus ionic conductivity relaxation which starts at 0 °C. For semi-IPNs the overlapping of γ- and βsw-relaxations of PHEMA (?125/?75 °C), then with increasing the temperature α-relaxation in PU (?75/0 °C), next ionic conductivity relaxation which starts at 0 °C, and finally the α-relaxation of PHEMA (+125/+170 °C) were detected. The α-relaxation of PHEMA in semi-IPNs shifts to lower temperatures and became broader with increasing amount of PU due to incomplete phase separation in the system and formation of interphases. The dielectric relaxation phenomena were fitted with Havriliak–Negami equation. Activation energy, τ_o and α parameters were calculated. For α-relaxations corresponding dielectric characteristics have been determined from Vogel–Fulcher–Tammann equation. The relaxation map for investigated PU, PHEMA and semi-IPNs was built.     

Electrical properties of A2.6+xTi1.4−xCd(PO4)3.4−x (A = Li,K; x = 0.0–1.0) phosphate glasses

Electrical properties of A_2.6+xTi_1.4−xCd(PO₄)_3.4−x (A = Li, K; x = 0.0–1.0) phosphate glasses are investigated over a frequency range from 42 Hz to 1 MHz at different temperatures. Impedance spectroscopy is used to separate the bulk conductivity from electrode effect of electrical conductivity data. The bulk dc conductivity is Arrhenius activated, with activation energies and pre-exponential factors following the Meyer–Neldel rule. The real part of ac conductivity shows universal power law feature. The variation of dielectric constant with frequency is attributed to ion diffusion and polarization occurring in the phosphate glasses. The frequency dependent imaginary part of electric modulus M″(ω) plot shows non-Debye feature in conductivity relaxation. The Kohlrausch–Williams–Watts stretched exponential function was used to describe the modulus spectra and the stretching exponent β is found to be temperature independent. Scaling in M″(ω) shows that the electrical relaxation mechanisms are independent of temperature for given composition at different temperatures.     

Tracer diffusion of 22Na and 45Ca,ionic conduction and viscosity of two standard soda-lime glasses and their undercooled melts

The tracer diffusivities of ⁴⁵Ca in two different high purity standard soda-lime silica glasses have been measured by the radiotracer method below and above their calorimetric glass transition temperatures. Calorimetric glass transition temperatures (T_g) of 845 K and 867 K have been obtained for standard glasses I and II, respectively, using differential scanning calorimetry (DSC) at a heating rate of 20 K/min. In this paper, we focus on the results of ⁴⁵Ca diffusion and conductivity of the two standard soda-lime glasses and compare them with ²²Na diffusivities also obtained in our laboratory [E.M. Tanguep Njiokep, H. Mehrer, Solid State Ionics 177 (2006) 2839, E.M. Tanguep Njiokep, H. Mehrer, Defect Diffus Forum 237–240 (2005) 282]. The ⁴⁵Ca diffusion coefficients obtained are found to follow the Arrhenius law, both below (Tanguep Njiokep and Mehrer, 2006, 2005) and above T_g. In the Arrhenius diagram a change of slope of the ⁴⁵Ca diffusivities appears at 835 K for standard glass I and at 790 K for standard glass II. At the same time, the ionic conductivities display a change in slope at 790 K and 778 K for standard glasses I and II, respectively. These temperatures are somewhat smaller than the calorimetric glass transition temperatures obtained at a heating rate of 20 K/min. Rather, they appear to be close to values of T_g obtained by extrapolation to a vanishing heating rate (Tanguep Njiokep and Mehrer, 2006). The viscosity diffusion of standard glass I is considerably smaller than the conductivity diffusion coefficient and both tracer diffusivities. In both glasses the ionic conductivity is essentially due to the motion of Na ions. The contribution of Ca ions to the conductivity is negligible.     

Temperature dependency of impedance spectroscopy behaviors in side-chain liquid crystalline polymer

In this paper, the electrical properties of side-chain liquid crystalline polymer (SLCP) are investigated by impedance spectroscopy technique. We report the measurement of dielectric and conductivity for SLCP from 1 kHz to 10 MHz within the temperature range from 300 to 370 K. The DC conductivity obeys Arrhenius law and it gives a small deviation at 315 K. The activation energies are equal to 0.20 eV and 0.75 eV for high and low temperatures, respectively. The frequency dependence of conductivity satisfies the power law, σ_AC = Aw^s, with s = 0.50–0.57. The evaluated power law exponent s exhibits nearly linear decreasing behavior with temperature. This suggests that the Correlated Barrier Hopping (CBH) model is the operating mechanism.     

Acid catalyst concentration effect on structure and ion relaxation studies of Li2O–P2O5–B2O3–SiO2 glasses synthesized by sol–gel process

Lithium phosphoborosilicate (LPBS) glasses were synthesized through the sol–gel process by varying nitric acid concentrations as a catalyst. The sol–gel process was monitored through XRD and DSC to optimize the LPBS glass forming treatment. Characterization of LPBS glasses was conducted using XRD, FTIR and DSC techniques. Impedance measurements were carried out at different temperatures on LPBS samples synthesized by sol–gel process with various nitric acid concentrations and impedance data were analyzed using Boukamp equivalent circuit software. The conductivity of LPBS samples was calculated from analyzed impedance data and it was found that sample synthesized with 2.5 N nitric acid concentration showed the high conductivity σ = 2.28(±0.02) × 10⁻⁷ S cm⁻¹ at 443 K. Activation energy (E_a) is obtained from Arrhenius plots of dc conductivity and it is found to be 0.39 (±0.02) eV for the high conductance sample. Ac conductivity data were analyzed using Jonscher’s power law (JPL) and the power law exponent (s) exhibits a low s value for high conducting LPBS sample and a non-linear behavior with temperature. The electric modulus data were fitted with Kohlraush–William–Watts (KWW) stretched exponential function and modulus formalism is used to study the ionic relaxation behavior at different temperatures in LPBS glasses synthesized with varying nitric acid concentrations.     

Phase transformation and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy

The glass transition and crystallization kinetics of melt-spun Ni₆₀Nb₂₀Zr₂₀ amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka's relationship. The activation energies of crystallization, E_x, were determined to be 499.5 kJ/mol and 488.6 kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be E_x = 419.5 kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Crystallization of polymer-derived SiC/BN/C composites investigated by TEM

The crystallization behavior of two polymer-derived Si/B/C/N ceramics with similar compositions lying close to the three-phase field BN + SiC + C was investigated by (high-resolution) transmission electron microscopy. The materials were high-temperature mass stable up to T = 2000 °C. During thermolysis at 1050 °C a homogeneous amorphous solid formed. SiC crystallization started at about 1400 °C. Further annealing to higher temperatures up to 2000 °C led to formation of microstructures composed of SiC crystals embedded into a structured BNC_x matrix phase. With increasing temperature, both the size of the crystallites and the ordering of the matrix phase increased.     

Continuous compositional changes of crystal and liquid during crystallization of a sodium calcium silicate glass

This paper deals with a systematic study of crystal nucleation and growth kinetics in a 14.6Na₂O–34.0CaO–51.4SiO₂ mol% glass, which is close to the CaO · SiO₂–Na₂O · SiO₂ pseudo-binary section, just left of the stoichiometric Na₂O · 2CaO · 3SiO₂ (N₁C₂S₃) compound. We show that crystallization begins with nucleation of a Na_4+2xCa_4−x[Si₆O₁₈] (0 < x < 1) solid solution that is enriched in sodium as compared with both parent glass and the N₁C₂S₃ compound; while a fully crystallized sample is composed only by a solid solution that is stable at very high temperatures, but is metastable in the temperatures under investigation. We thus confirm a continuous compositional change of the crystals during the course of crystallization.     

Kinetics of crystal growth of germanium disulfide in Ge0.38S0.62 chalcogenide glass

The crystal growth kinetics of GeS₂ in Ge_0.38S_0.62 glass has been studied by Differential Scanning Calorimetry (DSC) and microsopy. The linear crystal growth kinetics of both high temperature α-GeS₂ and low temperature β-GeS₂ polymorphs has been observed over a relatively broad range of temperatures, i.e. 420 < T < 494 °C that correspond to viscosity of supercooled melt: 3 × 10⁹ > η > 8 × 10⁵ Pa s. It seems that 2D nucleated growth is the most probable mechanism of crystallization for high temperature α-GeS₂ under these conditions. However, there are significant deviations for this model for the crystallization of low-temperature β-GeS₂. This might indicate some changes in crystal-melt interfacial energy or break down of Stokes–Einstein relation in that particular case. At temperatures below 500 °C the temperature range of directly observed crystal growth overlaps with isothermal DSC measurements. In this case overall crystallization kinetics can be described by the Johnson–Mehl–Avrami (JMA) nucleation-growth model for kinetic exponent n ≅ 4. The value of activation energy of nucleation estimated from these experiments E_N = 434 kJ mol⁻¹ is comparable with the activation energy of viscous flow in supercooled Ge_0.38S_0.62 melt (E_η = 478 kJ mol⁻¹). A more complex eutectic crystallization involving both GeS₂ and GeS phases has been observed at higher temperatures. This process is probably associated with secondary nucleation and cannot be described by a simple JMA model.     

Thermal annealing of a-Si/Au superlattice thin films

The superlattice films, which consist of amorphous silicon (a-Si) and amorphous gold (Au), were prepared by ultra-high vacuum evaporation system. The first layer was grown a-Si with a thickness of 4.2 nm and the second layer was grown Au with a thickness of 0.8 nm. Thermal annealing was performed at 473, 673, and 873 K, respectively. The structural properties of the films were investigated using transmission electron microscope (TEM), X-ray diffraction (XRD), and Raman scattering spectroscopy. The electrical property was assessed by the temperature dependence of electrical conductivity. A crystallization of Si and a forming of Au nanoparticles were observed in all of the annealing films. The crystalline volume fraction reached 70% by annealing time for 15 min. An average diameter of the Au nanoparticles embedded in Si matrix also increased with increasing the annealing temperature. At annealing temperature above 873 K, Au atoms migrated toward the film surface. It was observed that the electrical conductivity changed in several temperatures.     

Effect of Nb in the nanocrystallization and magnetic properties of FeNbBCu amorphous alloys

The crystallization of melt-spun Fe_79?xNb_5+xB₁₅Cu₁ (x = 0, 2, 4) ribbons has been studied by differential scanning calorimetry and X-ray diffraction. A primary crystallization of bcc-Fe nanoparticles embedded in an amorphous matrix, followed by the precipitation of metastable borides from the residual matrix at higher temperatures is observed. The characteristic temperatures of crystallization events change with Nb concentration. The results obtained from thermal and structural characterization are related to the magnetic properties of the sample. A dependence of the magnetic behavior with the Fe/Nb content in the alloy is also unveiled. The decrease of Nb content in the alloy leads to an enhancement of both the saturation polarization and the Curie temperature due to variations in the exchange coupling between Fe atoms. However, the maximum values of magnetic entropy change do not vary appreciably among the three amorphous alloys. In nanocrystalline samples the amount of the nanocrystalline transformed fraction seems to be the main reason for the change in the saturation polarization of the sample.     

Influence of structural disorder on the dynamics of mobile oxygen ions in Dy2(Ti1−yZry)2O7

We report on the influence of structural disorder on the oxide-ion conductivity of Dy₂(Ti_1−yZr_y)₂O₇ (y = 0.55 and 0.90). A significant disorder is induced by mechanical milling synthesis of the samples, and, depending on the Zr/Ti, a partial and progressive structural ordering can be achieved by subsequent annealing at temperatures between 800 and 1500 °C. Ionic conductivity is relatively high for both compositions (up to 10⁻⁴ S/cm at 900 K), and the activation energies for dc conductivity (in the range 1.02–1.32 eV) are found to be larger in samples with more structural disorder. This result is quantitatively explained, by using Ngai’s coupling model, in terms of the enhancement of interactions between mobile oxygen vacancies in a more disordered structure.     

Effect of isothermal annealing and visible-light illumination on the AC-impedance behavior of undoped selenium thin films

The effect of post-deposition isothermal annealing (30 °C ? T_A ? 70 °C) and visible-light illumination on the complex AC-impedance of undoped selenium thin films deposited at the substrate temperatures T_S = 30, 50, 70 °C has been studied in the frequency range 0.2–12 kHz. The AC-impedance of amorphous selenium (a-Se) films (T_S, T_A < 50 °C) was mainly capacitive, with no loss peaks being observed in their Z″(ω)–ω curves, irrespective of illumination. This behavior was ascribed to a dominant charge-carrier trapping effect of bulk/surface charged defects usually present in a-Se. On the other hand, the measured Z″(ω)–Z′(ω) diagrams of illuminated polycrystalline Se samples (50 °C ? T_S, T_A ? 70 °C) exhibited almost full semicircles, whereas their Z″(ω)–ω curves revealed prominent loss peaks at well-defined frequencies. As the annealing temperature or light intensity is increased the loci of the points determined by intersections of these semicircles with the Z′-axis at the low-frequency side shift greatly towards the origin, while the loss-peak positions shift to higher frequencies. These experimental findings were explained in terms of a significant increase in electrical conductivity of selenium films due to thermally-induced crystallization at temperatures beyond glass-transformation region of undoped selenium and to creation of electron–hole pairs by visible-light illumination.     

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.     

Studies of lead–iron phosphate glasses by Raman,Mössbauer and impedance spectroscopy

The effect of Fe₂O₃ content on electrical conductivity and glass stability against crystallization in the system PbO–Fe₂O₃–P₂O₅ has been investigated using Raman, XRD, Mössbauer and impedance spectroscopy. Glasses of the molar composition (43.3 − x)PbO–(13.7 + x)Fe₂O₃–43P₂O₅ (0 ⩽ x ⩽ 30), were prepared by quenching melts in the air. With increasing Fe₂O₃ content and molar O/P ratio there is corresponding reduction in the length of phosphate units and an increase in the Fe(II) ion concentration, which causes a higher tendency for crystallization. Raman spectra of the glasses show that the interaction between Fe sites, which is essential for electron hopping, strongly depends on the cross-linking of the glass network. The electronic conduction of these glasses depends not only on the Fe(II)/Fe_tot ratio, but also on easy pathways for electron hopping in a non-disrupted pyrophosphate network. The Raman spectra of crystallized glasses indicate a much lower degree of cross-linking since more non-bridging oxygen atoms are present in the network. Despite the significant increase in the Fe₂O₃ content and Fe(II) ion concentration, there is a considerable weakening in the interactions between Fe sites in crystalline glasses. The impedance spectra reveal a decrease in conductivity, caused by poorly defined conduction pathways, which are result of the disruption and inhomogeneity of the crystalline phases that are formed during melting.     

Spatially localized current-induced crystallization of amorphous silicon films

Field-enhanced metal-induced solid phase crystallization (FE-MISPC) at room temperature is employed to create microscopic crystalline regions at predefined positions in hydrogen-rich amorphous silicon (a-Si:H) films. Electric field is applied locally using a sharp conductive tip in atomic force microscope (AFM) and nickel electrode below the a-Si:H film. The process is driven by a constant current of ?50 pA to ?500 pA while controlling the amount of transferred energy (1–300 nJ) as a function of time. Passing current leads to a formation of nanoscale pits in the a-Si:H films. Depending on the energy amount and rate the pits exhibit lower or orders of magnitude higher conductivity as detected by current-sensing AFM. High conductivity is attributed to a local crystallization of the films. This is confirmed by micro-Raman spectroscopy.     

Influence of composition in the crystallization process of Fe75−xNb10B15+x metallic glasses

The devitrification process of Fe_75−xNb₁₀B_15+x (x = 0, 5, 10) metallic glasses produced by melt-spinning has been analyzed by calorimetric, microstructural and magnetic measurements. The experimental results show large differences in the behavior of these alloys as a function of composition. The alloy x = 0 undergoes a primary crystallization process separated in two stages while only one is observed for alloys with x = 5 and x = 10. This difference, observed by DSC, is correlated with a microstructural change in the phases that precipitate. For alloys x = 5 and 10, Fe₂₃B₆ and bcc-Fe are identified after the first calorimetric peak. In the sample x = 0, bcc-Fe and an unidentified phase precipitate in the first peak but massive crystallization of bcc-Fe is observed after the second stage. Finally the thermal dependence of magnetization has been measured and the Curie temperatures for the metallic glasses are determined. The change of these quantities with heat treatment and composition is discussed.     

Thermal,vibrational and Ac impedance studies on proton conducting polymer electrolytes based on poly(vinyl acetate)

Proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO₄) have been prepared by solution casting technique with various compositions. The X-ray diffraction analysis confirms the polymer–HClO₄ complex formation. FTIR spectra analysis reveals the interaction between proton and ester oxygen of poly(vinyl acetate) (PVAc). The shift in T_g towards the lower temperature indicates that the polymer salt interaction takes places in the amorphous phase of the polymer matrix. Ac impedance spectroscopy reveals that 75 mol% PVAc:25 mol% HClO₄ exhibits maximum conductivity, 3.75 × 10^? 3 S cm^? 1 at room temperature (303 K). The increase in conductivity with increase in dopant concentration and temperature may be attributed to the enhanced mobility of the polymer chains, number of charge carriers and rotations of side chains. The temperature dependence of conductivity shows non-Arrhenius behavior at higher temperatures.