Mass fractal characteristics of sonogels prepared from sonohydrolysis of tetraethoxysilane with additions of dimethylformamide

Wet silica gels with ∼1.4 × 10⁻³ mol SiO₂/cm³ and ∼90 vol.% liquid phase were prepared from the sonohydrolysis of tetraethoxysilane (TEOS) with different additions of dimethylformamide (DMF). Aerogels were obtained by CO₂ supercritical extraction. The samples were studied mainly by small-angle X-ray scattering (SAXS) and nitrogen adsorption. Wet gels exhibit a mass fractal structure with fractal dimension D increasing from 2.23 to 2.35 and characteristic length ξ decreasing from ∼9.4 nm to ∼5.1 nm, as the DMF/TEOS molar ratio is increased from 0 to 4. The supercritical process apparently eliminates some porosity, shortening the fractality domain in the mesopore region and developing an apparent surface/mass fractal (with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3) in the micropore region. The fundamental role of the DMF addition on the structure of the aerogels is to diminish the porosity and the pore mean size, without, however, modify substantially the specific surface area and the average size of the silica particle of the solid network.     

Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS

Sonohydrolysis of mixtures of tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) with different TMOS/(TMOS + TEOS) molar ratio R was carried out to obtain ∼2.0 × 10⁻³ mol SiO₂/cm³ and ∼86%-volume liquid phase wet gels. Aerogels were obtained by supercritical CO₂ extraction in autoclave. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The structure of the wet gels can be described as a mass fractal structure with fractal dimension D ∼ 2.2 and characteristic length ξ increasing from ∼4.6 nm for pure TEOS to ∼6.4 nm for pure TMOS. A fraction of the porosity is eliminated with the supercritical process. The fundamental role of the TMOS/(TMOS + TEOS) molar ratio on the structure of the aerogels is to increase the porosity and the pore mean size as R changes from pure TEOS to pure TMOS. The supercritical process increases the mass fractal dimension and shortens the fractality domain in the mesopore region. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3.     

A SAXS study of the nanostructural characteristics of TEOS-derived sonogels upon heat treatment up to 1100 °C

Xerogels obtained from the acid-catalyzed and ultrasound stimulated hydrolysis of TEOS were submitted to heat treatment at temperatures ranging from 60 to 1100 °C and studied by small-angle X-ray scattering (SAXS). The SAXS intensity as a function of the modulus of the scattering vector q was obtained in the range from q₀=0.19 to q_m=4.4 nm⁻¹. At 60 °C the xerogels exhibit an apparent surface fractal structure with a fractal dimension D_S∼2.5 in a length scale ranging from 1/q₁∼1 to 1/q_m∼0.22 nm. This structure becomes extremely rough at 120 °C (D_S∼3) and at 150 °C, it apparently converts to a mass fractal with a fractal dimension D∼2.4. This may mean an emptying of the pores with preservation of a share of the original mass fractal structure of the wet aged gel, for it had presented a mass fractal dimension D∼2.2. A well characterized porous structure formed by 2.0 nm mean size pores with smooth surface of about 380 m²/g is formed at 300 °C and remains stable until approximately 800 °C. At 900 °C the SAXS intensity vanishes indicating the disappearance of the pores in the probed length scale. The elimination of the nanopores occurs by a mechanism in which the number of pores diminishes keeping constant their mean size. The xerogels exhibit a foaming phenomenon above 900 °C and scatter following Porod's law as does a surface formed by a coarse structure.     

Microstructure and optical properties of (Pb,Ca)TiO3 films grown on Si(1 0 0) substrates by a simple sol-gel process

Amorphous and polycrystalline (Pb_0.76Ca_0.24)TiO₃ (PCT) thin films deposited on an Si(1 0 0) substrate have been prepared by a simple sol-gel process. The microstructure and surface morphologies of the thin films have been studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The polycrystalline PCT film on the Si(1 0 0) substrate has a tetragonal perovskite structure with grain size from 60 to 110 nm. AFM reveals smooth surfaces and root mean square (rms) roughness of 0.17 and 4.4 nm for amorphous and polycrystalline films, respectively. The refractive index n and extinction coefficient k of the amorphous and polycrystalline thin films was obtained by spectroscopic ellipsometry as a function of the photon energy in the range from 2.0 to 5.4 eV. The maximum n and direct bandgap energies of amorphous and polycrystalline thin films were 2.66 and 4.11 eV, 2.64 and 3.84 eV, respectively.     

Erbium doped fluoride glass-ceramics waveguides fabricated by PVD

Transparent amorphous and glass-ceramics waveguides in the system ZrF₄-LaF₃-ErF₃-AlF₃ (ZELA) have been fabricated by physical vapor deposition (PVD). The ceramming process was studied by means of X-ray diffraction and transmission electron microscopy for different deposition temperatures. With increasing deposition temperature, formation of La_xEr_1−xF₃ nanocrystals with x ∼ 0.3 was observed. The decay curves of the ⁴I_13/2 level in the glass-ceramics with 14.5 mol% Er³⁺ gave evidence of the presence of erbium both in the amorphous matrix (τ = 8.6 ms) and in the crystal phase (τ = 2.2 ms). The decrease of lifetime was due to clustering of erbium incorporated in LaF₃ crystal lattice. No significant increase of attenuation loss was detected after waveguide cerammization (1.3 dB/cm at 1304 nm).     

Insulator-like electrical transport in structurally ordered Al65Cu20+xRu15−x (x = 1.5, 1.0, 0.5, 0.0 and −0.5) icosahedral quasicrystals

Low-temperature resistivities, in zero-field and 8 T field, and magnetoresistance have been measured down to 1.4-300 K for stable icosahedral quasicrystals Al₆₅Cu_20+xRu_15−x (x = 1.5, 1.0, 0.5, 0.0 and −0.5). The analysis of the magnetoresistance data shows an overwhelming presence of anti weak-localization effect (τ_so ∼ 10⁻¹² s). But the sample with x = −0.5 shows anomalous magnetoresistance and the anti weak-localization effect breaks down (τ_so to be 10⁻¹⁵ s). The in-field σ-T between 5 K and 20 K, for x = 1.0, 0.5, 0.0 and −0.5 samples, and between 1.4 K and 40 K for x = 1.5 sample, follow a power-law behavior with an exponent of 0.5 and above ∼30 K the exponent ranges from 1.17 to 1.58. The observed power-laws basically characterize the presence of critical regime of the metal-insulator (MI) transition, dominated by electron-electron and electron-phonon inelastic scattering events respectively. In samples with x = 1.0, 0.5, 0.0 and −0.5 the in field σ-T has been found to follow ln σ-vs-T^1/4 below 5 K, which indicates the presence of variable range hopping. The observed transport features indicate the occurrence of proximity of metal-insulator transition in these Al-Cu-Ru quasicrystal samples.     

Positron annihilation lifetime study of organic-inorganic hybrid materials prepared by irradiation

Silica based hybrid materials, some containing zirconia, prepared by gamma-irradiation, were studied by positron annihilation lifetime (PAL) spectroscopy, in the temperature range of 30-370 K. One long-lived component was observed in group of samples without zirconia. The behavior of this component as a function of temperature resembles that for polymers. This is explained by the elastomer like structure of the samples and the excellent linking between the inorganic and organic networks. The average radius, R, of the free-volume holes was in the range 0.26-0.42 nm depending on the temperature. In the group of samples containing zirconia two long-lived components were observed. A model with three different free-volume holes was suggested to explain the PAL results. Small closed holes (R ∼ 0.26 nm) and large closed holes introduced by Zr (R ∼ 0.5 nm), the sizes of both holes not changing with the temperature, together with intermediate polymer-like free-volume holes - the same behavior as those present in samples not containing zirconia.     

Molecular beam epitaxy of crystalline and amorphous GaN layers with high As content

We have studied the low-temperature growth of gallium nitride arsenide (GaN)As layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN_1−xAs_x alloys over a large composition range by growing the films much below the normal GaN growth temperatures with increasing the As₂ flux as well as Ga:N flux ratio. We found that alloys with high As content x>0.1 are amorphous and those with x<0.1 are crystalline. Optical absorption measurements reveal a continuous gradual decrease of band gap from ∼3.4 to ∼1.35 eV with increasing As content. The energy gap reaches its minimum of ∼1.35 eV at x∼0.6–0.7. The structural, optical and electrical properties of these crystalline/amorphous GaNAs layers were investigated. For x<0.3, the composition dependence of the band gap of the GaN_1−xAs_x alloys follows the prediction of the band anticrossing model developed for dilute alloys. This suggests that the amorphous GaN_1−xAs_x alloys have short-range ordering that resembles random crystalline GaN_1−xAs_x alloys.     

Influence of hydrogen on structural and optical properties of low temperature polycrystalline Ge films deposited by RF magnetron sputtering

To improve the properties of polycrystalline Ge thin films, which are a candidate material for the bottom cells of low cost monolithic tandem solar cells, ∼300 nm in situ hydrogenated Ge (Ge:H) thin films were deposited on silicon nitride coated glass by radio-frequency magnetron sputtering. The films were sputtered in a mixture of 15 sccm argon and 10 sccm hydrogen at a variety of low substrate temperatures (T_s)≤450 °C. Structural and optical properties of the Ge:H thin films were measured and compared to those of non-hydrogenated Ge thin films deduced in our previous work. Raman and X-ray diffraction spectra revealed a structural evolution from amorphous to crystalline phase with increase in T_s. It is found that the introduction of hydrogen gas benefits the structural properties of the polycrystalline Ge film, sputtered at 450 °C, although the onset crystallization temperature is ∼90 °C higher than in those sputtered without hydrogen. Compared with non-hydrogenated Ge thin films, hydrogen incorporated in the films leads to broadened band gaps of the films sputtered at different T_s.     

Effect of cooling rate on enthalpy and volume relaxation of polystyrene

This paper presents the results of measurements of physical aging on polystyrene with a narrow molecular weight distribution (M_w/M_n = 1.03). The evolution of the aging process was followed by recording the relaxed enthalpy and the accompanying decrease in volume, using differential scanning calorimetry and mercury-in-glass dilatometry, respectively. The measurements were carried out after cooling the sample at constant rate to the aging temperature. The cooling rate varied between 0.0037 and 1 °C/min. The aging data were fitted with the KWW and the TNM functions. The differences in the time scales of enthalpy, h, and volume, v, relaxation were relatively small. From the h(v) graphs the derivative dh/dv was found to amount to about 1.8 GPa, a value significantly exceeding the predictions of the thermodynamic model containing an additional internal variable.     

Casting vacuum effects on the precipitates and magnetic properties of Fe-based glassy alloys

The crystalline precipitates and magnetic properties of the Fe₆₁Co_9 − xZr₈Mo₅W_xB₁₇ (x = 0 and 2) cylinders prepared under various vacuum conditions were investigated in this paper. The fraction of the crystalline precipitates and liquidus temperature of the samples with tungsten content c_W = 0 decease with decreasing vacuum, while they increase in the samples with c_W = 2 at.%. For both tungsten contents, with decreasing vacuum, the magnetization at room temperature M_start and Curie temperature T^h_c of the samples decrease gradually, but the increment of magnetization ΔM_800 K at 800 K after heating to 973 K increases and the Curie temperature T^c_c of the residual amorphous matrix shows a decreasing tendency simultaneously. Meanwhile, with decreasing vacuum, Zr-B compounds tend to precipitate easily out from the melt or to remain undissolved during casting, and eutectic Fe-B compounds tend to be formed easily after heating to 973 K. The existence of Zr-B and Fe-B compounds changes the boron content in the residual melt/matrix and consequently affects the magnetic properties of the samples in the as-quenched state and after heating to 973 K.     

Structure and crystallization of potassium titanium phosphate glasses containing B2O3 and SiO2

Transparent glasses composition of which can be expressed by the formula: (100−x) · (K₂O · 2TiO₂ · P₂O₅) · x(K₂O · 2B₂O₃ · 7SiO₂), where x=5, 10, 15 and 20 mol% (KTP-xKBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO₃ units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP-5KBS and KTP-10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB₃(XO₄)₂(OH)₆ Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO₄ phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above T_g have allowed one to obtain transparent crystal-glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix.     

Lateral photovoltage measurements in hydrogenated amorphous silicon and silicon-oxygen thin films

Hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon-oxide alloy films (a-SiO_x:H) were investigated by temperature dependence of lateral photovoltage (LPV) measurements. The suboxide sample with [O] = 27 at.%, was found to exhibit larger LPV compared to the unalloyed sample. It is difficult to simply correlate LPV measurements to related diffusion length measurements, only. On the other hand, the observed magnitude of LPV in a-Si:H and its decrease with temperature, could be explained based on an internal electric field induced by diffusion electron and hole currents, and multiple trapping of the photocarriers.     

Structure and thermal stability of (Zr0.6Al0.4)O1.8 thin film on strained SiGe layer

Zr_0.6Al_0.4O_1.8 dielectric films were deposited directly on strained SiGe substrates at room temperature by ultra-high vacuum electron-beam evaporation (UHV-EBE) and then annealed in N₂ under various temperatures. X-ray diffraction (XRD) reveals that the onset crystallization temperature of the Zr_0.6Al_0.4O_1.8 film is about 900 °C, 400 °C higher than that of pure ZrO₂. The amorphous Zr_0.6Al_0.4O_1.8 film with a physical thickness of ∼12 nm and an amorphous interfacial layer (IL) with a physical thickness of ∼3 nm have been observed by high-resolution transmission electron microscopy (HRTEM). In addition, it is demonstrated there is no undesirable amorphous phase separation during annealing at temperatures below and equal to 800 °C in the Zr_0.6Al_0.4O_1.8 film. The chemical composition of the Zr_0.6Al_0.4O_1.8 film has been studied using secondary ion mass spectroscopy (SIMS).     

Superconducting state parameters of ternary amorphous superconductors

The theoretical investigations of the superconducting state parameters (SSP) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ^?, transition temperature T_C, isotope effect exponent α and effective interaction strength N_OV of (Ni₃₃Zr₆₇)_1 − xM_x (x = 0, 0.05, 0.1, and 0.15, M = Cu) ternary amorphous superconductors have been reported using Ashcroft's empty core (EMC) model potential. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used in the present investigation to study the screening influence on the aforesaid properties. The T_C obtained from Sarkar et al. (S) local field correction function is found in excellent agreement with available theoretical data. Quadratic T_C equation has been proposed, which provides successfully the T_C values of ternary amorphous alloys under consideration. Also, the present results are found in qualitative agreement with other such earlier reported data, which confirm the superconducting phase in the superconductors.     

Structure of potassium germanophosphate glasses by X-ray and neutron diffraction: 2. Medium-range order

Characteristics of the medium-range order (MRO) of K₂O-GeO₂-P₂O₅ (KGP) glasses are obtained from X-ray and neutron scattering data. The behavior of the MRO is expressed in changes of pre-peaks and smooth contributions in the structure factors, S(Q), for Q < 12 nm^-1. Peaks at Q = (7.5 ± 0.5) nm^-1 (Q - magnitude of the scattering vector) are the outstanding features and reach maximum intensity for a glass of ∼25/50/25 mol% K₂O/GeO₂/P₂O₅. The pre-peaks are explained by a structural model in which K-rich and Ge-rich regions are separated by PO₄ units. The distance of repetition of similar regions (∼1 nm) is responsible for the pre-peak at ∼7.5 nm^-1. The intensity of this pre-peak is reduced and finally eliminated for glass compositions approaching the binary GeO₂-P₂O₅ system. The pre-peak is changed to a smooth scattering contribution and finally shifted to ∼9 nm^-1 for glass compositions approaching the binary K₂O-GeO₂ system. The strong tendency of the PO₄ units to coordinate K⁺ and Ge neighbors at their four corners is the source for the special MRO of the KGP glasses.     

Structural transformation on Se0.8Te0.2 chalcogenide glass

Using X-ray diffraction and differential scanning calorimetry (DSC), the structure and the crystallization mechanism of Se_0.8Te_0.2 chalcogenide glass has been studied. The structure of the crystalline phase has been refined using the Rietveld technique. The crystal structure is hexagonal with lattice parameter a = 0.443 nm and c = 0.511 nm. The average crystallite size obtained using Scherrer equation is equal 16.2 nm, so it lies in the nano-range. From the radial distribution function, the short range order (SRO) of the amorphous phase has been discussed. The structure unit of the SRO is regular tetrahedron with (r₂/r₁) = 1.61. The Se_0.8Te_0.2 glassy sample obeys the chemical order network model, CONM. Some amorphous structural parameters have been deduced. The crystallization mechanism of the amorphous phase is one-dimensional growth. The calculated value of the glass transition activation energy (E_g) and the crystallization activation energy (E_c) are 159.8 ± 0.3 and 104.3 ± 0.51 kJ/mol, respectively.     

Investigation on dielectric relaxations of PVP-NH4SCN polymer electrolyte

Poly (N-vinyl pyrrolidone) (PVP) and ammonium thiocyanate (NH₄SCN) based polymer films with different compositions have been prepared by solution casting technique. The amorphous nature of the polymer electrolytes has been confirmed by XRD analysis. The FTIR analysis confirms the complex formation of the polymer with the salt. The conductivity analysis shows that the 20 mol% ammonium thiocyanate doped polymer electrolyte has high ionic conductivity and it has been found to be 1.7 × 10⁻⁴ S cm⁻¹, at room temperature. From the admittance plot, the activation energy has been found to be low for 20 mol% salt doped polymer electrolyte. The dielectric behavior has been analyzed using dielectric permittivity (ε^∗), dissipation factor (tan δ) and electric modulus (M^∗) of the samples.     

Synthesis and characterization of potassium, rubidium, and cesium thiogermanate glasses

Glass-forming regions were investigated for the binary xM₂S + (1 − x)GeS₂ (M=K, Rb, Cs) systems. Glasses were prepared from 0?x?0.20 mole fraction alkali sulfide using a novel preparation route involving the decomposition of the alkali hydrosulfides in situ. At higher alkali concentrations near x=0.33, the glass-forming regions are limited by the readily formed adamantane-like M₄Ge₄S₁₀ crystals. Structural characterization of the glasses and polycrystals for x?0.33 were performed using Raman scattering and IR absorption. Terminal Ge-S⁻ vibrational modes, observed between 473 and 479 cm⁻¹, increased in intensity and decreased in frequency with increasing alkali modifier content. Glass transition temperatures decreased with increasing alkali modifier, ranging from 250 to 215 °C. Corresponding crystallization onset temperatures were between 340 and 385 °C. DC conductivity values of the glasses ranged from 10⁻¹⁰ to 10⁻⁷ (Ω cm)⁻¹ with activation energies between 0.54 and 0.93 eV for the temperature range of ∼100-250 °C. Higher ionic conductivities were observed with increasing alkali concentration and decreasing alkali radii. Additionally, an increase in the activation energy was observed above the glass transition temperature.