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.     

Silica aerogels prepared via rapid supercritical extraction: Effect of process variables on aerogel properties

We have examined experimentally the effects of rapid supercritical extraction (RSCE) process variables and their resulting pressure and temperature characteristics on aerogel properties. We employ an RSCE process that uses a hydraulic hot press to seal and heat a contained mold until the aerogel precursors reach a supercritical state. After a short stabilization period the hot press restraining force is lowered and the supercritical fluid is allowed to escape, leaving behind an aerogel monolith. The entire process can be accomplished in fewer than 3 h. To control the process, we set the restraining force, the maximum temperature, the heating and cooling rates, the pressure release rate and the mold volume fill ratio (related to the amount of initial precursor material). To investigate the effects of these variables we made silica aerogels from a TMOS-based recipe. We varied the volume of precursor material from 10 to 15 mL (60-97% fill volume), the restraining force from 43 to 111 kN, the temperature heat rate from 0.7 to 4.2 °C/min, the maximum temperature from 288 to 371 °C and the pressure release rate from 0.23 to 0.66 MPa/min. The RSCE process is robust. We were able to make transparent, monolithic aerogels under almost all conditions with little effect on the resulting aerogel properties. Typical density measurements yielded values of approximately 0.065 g/mL (bulk) and 1.9 g/mL (skeletal). The samples were translucent and transmitted 70% of the light at 800 nm (for 5-mm thick samples). The BET surface areas ranged from 517 to 590 m²/g. Maximum temperature was the only variable found to have a significant effect on the aerogels’ properties. As the maximum temperature increased from 288 to 371 °C the surface area decreased from 560 to 395 m²/g and average pore diameter (BJH desorption) increased from 21 to 32 nm.     

Sol-gel synthesis of porous and dense silica microspheres

An acidic silica sol (35 ± 2 wt% equivalent SiO₂) having a gelling time of 9-10 min has been used as an aqueous phase for obtaining a w/o emulsion in CCl₄ as oil phase in presence of a surfactant, Tween 80. The silica sol was allowed to form gel at room temperature via polycondensation among the -Si-OH groups forming the porous silica gel microspheres. The surface area of the microspheres heated at 500°, 700° and 900 °C was found to be 227 m²/g, 167 m²/g and 81 m²/g indicating the gradual densification. The decreased surface area and unchanged -Si-O-Si- asymmetric stretching vibration at 1084 cm⁻¹ up to 700 °C probably indicate the formation of extensive cross-linked gel structure in the microsphere. The appearance of the -Si-O-Si- asymmetric stretching vibration at 1104 cm⁻¹ and the absence of porosity while heating at 1000 °C indicate the formation of dense silica glass microspheres.     

Tantalum based bulk metallic glasses

Ta-based bulk metallic glasses with high strength (2.7 GPa) and hardness (9.7 GPa), high elastic modulus (170 GPa) and high density (12.98 g/mm³) were developed. The best glass forming ability so far for a Ta-Ni-Co system reaches a critical diameter of 2 mm by the copper mold casting method. It shows an exceptionally high glass transition temperature of 983 K and a high crystallization temperature up to 1023 K. The unique mechanical and physical properties make them a promising high strength material.     

Silica xerogels of tailored porosity as support matrix for optical chemical sensors. Simultaneous effect of pH, ethanol:TEOS and water:TEOS molar ratios, and synthesis temperature on gelation time, and textural and structural properties

The sensing of a chemical environment is achieved mainly in the surface by interactions of the sensor material with its chemical surroundings. Therefore, porous structure control is key in developing good chemical sensors. The aim of this work was to obtain materials of tailored porosity to be used as support matrix for optical chemical sensors. We studied the simultaneous effect of pH, temperature, ethanol:TEOS, and water:TEOS molar ratios on gelation time, and textural and structural properties. We used a 2⁴ factorial design that evaluates the effect of each independent variable and their interactions. Samples were characterized by XRD, SEM, FTIR, and gas adsorption (N₂ at 77 K and CO₂ at 273 K). The gelation time decreased with increasing temperature, water:TEOS molar ratio, and pH; and with decreasing ethanol:TEOS molar ratios. Synthesis conditions also affected the xerogels porous textures. Xerogels obtained at pH 2.5 were ultramicroporous. In general, samples synthesized at pH 4.5 and ethanol:TEOS molar ratio of 2.25:1 were mesoporous, but the material is not appropriate for use as support in fiber optical sensors.     

The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition

During electric polarization charge is injected into the material. The structure is decorated with space charge and during the subsequent heating an apparent peak and the genuine peaks that are related to dipole randomization and charge detrapping are observed. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 623 K. Two weak relaxations have been observed around 337 K and around 402 K. The electrical conductivity changes with temperature in agreement with the Arrhenius law only below (W = (0.84 ± 0.03) eV ) and above ( W = (0.82 ± 0.03) eV) the temperature range where the β relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the β relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the β₁ sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the β₂ sub-glass relaxation is related to the rotation or oscillation of the imidic ring. At higher temperatures an apparent peak was observed. The relaxation time of the trapped charge, at 573 K, is high than 8895 s.     

Epitaxial growth of tin oxide film on TiO2(1 1 0) using molecular beam epitaxy

Growth of tin oxide thin films using molecular beam epitaxy in a pyrolyzed nitrogen dioxide atmosphere on a titanium dioxide (1 1 0) substrate was investigated using X-ray photoelectron spectroscopy (XPS), electron diffraction, and atomic force microscopy (AFM). Properties of deposited films were studied for their dependence on substrate temperature and oxidation gas pressure. Analyses using XPS data revealed that tin atoms were fully oxidized to Sn⁴⁺ and SnO₂ films were grown epitaxially in deposition conditions of substrate temperatures of 627 K or higher and NO₂ pressure greater than 3×10⁻³ Pa. At a substrate temperature of 773 K, a smooth surface with atomic steps was visible in the SnO₂ films, but above or below this temperature, fine grains with crystal facets or porous structures appeared. At pressures of 8×10⁻⁴ to 3×10⁻⁴ Pa, the randomly oriented SnO phase was dominantly grown. Further decreasing the pressure, the Sn metal phase, which was epitaxially crystallized at less than 500 K, was also grown.     

An investigation of the kinetic transformation mechanism of Ge12.5Te87.5 chalcogenide glass under non-isothermal regime

Differential scanning calorimetry (DSC) technique was used to study the kinetics of amorphous to crystalline transformation in Ge_12.5Te_87.5 chalcogenide glass. The kinetic parameters of glassy Ge_12.5Te_87.5 under non-isothermal conditions are analyzed by the model-free and model-fitting approaches from a series of experiments at different constant heating rates (5-50 K/min). The effective activation energy of crystallization was determined by analyzing the data using the isoconversional methods of Kissinger-Akahira-Sunose (KAS), Tang, Starink, Flynn-Wall-Ozawa (FWO) and Vyazovkin. The analysis of the present data shows that the effective activation energy of crystallization is constant throughout the entire interval of conversions and hence with temperature. The transformation mechanism examined using the local Avrami exponents indicates that one mechanism (three-dimensional growth) is responsible for the transformation process for all heating rates used. The reaction model that may describe the transformation process of the Ge_12.5Te_87.5 chalcogenide glass is the Avrami-Erofeev model (g(α) = [− ln(1 − α)]^1/n) with n = 3 for all heating range at the whole range of crystallized fraction (α = 0.05-0.95). A good agreement between the experimental and the reconstructed (α-T) curves has been achieved. The transformation from amorphous to crystalline phase in Ge_12.5Te_87.5 chalcogenide glass demonstrates a single-step mechanism.     

Diffusion of the S isotope in soda-lime-silica and sodium trisilicate glass melts

The diffusivity of sulphur in nominal 10 Na₂O-16 CaO-74 SiO₂ (NCS) and 26 Na₂O-74 SiO₂ (NS3) melts was investigated in the temperature range 1273-1473 K using the ³⁵S radioactive isotope in a sandwich setup. Samples were sealed in platinum capsules and run with vertical alignment at 100 MPa confining pressure in an internally heated gas pressure vessel. Using the lowest diffusion coefficient D (m² s^− 1) for each temperature the Arrhenian relations logD = −(4.6 ± 0.3) − (216 ± 7) kJ mol^− 1/RT for NCS and logD = −(6.3 ± 0.6) − (167 ± 17) kJ mol^− 1/RT for NS3 were determined. Viscosity of the melts was used to test the applicability of the Stokes-Einstein (SE) and the Eyring (EY) equations to sulphur diffusion. The SE equation yields unrealistically low radii of the diffusing particles, a consequent of the non-molecular structure of the silicate melts. On the other hand, the Eyring relation yields reasonable jump distances of 550 pm (NCS) and 750 pm (NS3) by fitting the diffusion data to the EY equation. These large values imply that sulphate ions (ionic diameter = 290 pm) migrate as large entities through the silicate network.     

Diffusion of chromium in chrysoberyl

Cr³⁺ diffusion in chrysoberyl (BeAl₂O₄) irradiated by H⁺ ions and electrons has been studied and compared with diffusion in non-irradiated samples. Chrysoberyl crystals were irradiated with 6 MeV H⁺ ions to fluencies of 1×10¹⁶ cm^–2 for 25 min and with 10 MeV electrons to fluencies of 2×10¹⁷ cm^–2 for 1 h. Three different types of samples, which were doped with Cr³⁺, were annealed in horizontal alumina tube furnaces by 50 K intervals in the temperature range from 1773 to 1923 K for 200 h. Scanning electron microscope–energy dispersive X-ray spectrometer (SEM–EDX) was used to measure the diffusion. Arrhenius equations for the diffusion coefficient for Cr³⁺ in the temperature range 1773–1923 K were developed:     

Effect of method of preparation and drying time on photophysical properties of coumarin 1 laser dye embedded in HCl catalysed sol-gel glasses

Coumarin 1 (C1) dye is impregnated in transparent sol-gel glass samples prepared by sol-gel process using three methods - (I) using HCl as catalyst and glycerol as a drying control chemical additive (DCCA), (II) using HCl as catalyst at 60 °C subsequent drying at room temperature, and (III) using HCl as catalyst at 60 °C and heated at 600 °C for 3 h. The sol-gel matrices prepared by Methods I and II are given dip treatment with methanol/distilled water (50/50 volume) for 16 h before dipping into dye solution. The effect of method and drying time of matrix on spectroscopic properties of C1 dye doped glass samples has been studied. The optical density (OD) at absorption maximum wavelength and fluorescence intensity (FI) at fluorescence maximum wavelength of all C1 dye doped samples prepared by Methods I and II decrease, where as there is no change in photophysical properties (OD/FI) is observed in samples prepared by Method III with the time of drying of the sol-gel samples. These absorption/fluorescence properties of C1 dye in sol-gel glass matrices are compared with its respective properties in methanolic solution in acidic environment.     

Stark splitting of the ground state of Er in fluorozirconate glass at low temperature

Optical properties of Er³⁺-doped ZBLAN glass matrix have been studied by luminescence spectroscopy under 488 nm excitation. The spectrum of the ⁴S_3/2⁴I_15/2 transition, carried out at temperature T = 2 K, shows a new line in the lowest energy region. This new line, centered at 17 996 cm⁻¹, was attributed to the lower transition between the Stark components of the ⁴S_3/2⁴I_15/2 transition. Measurements from T = 2 K to room temperature show the disappearance of this new line. From the results we estimate the splitting of 415 cm⁻¹ for the ground state and 100 cm⁻¹ for the ⁴S_3/2 excited multiplet. The experimental result allows us to assign the positions of the eight Stark components of the ground state multiplet of the Er³⁺ in the ZBLAN glass matrix.     

Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H₂ and NH₃, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10⁸ cm⁻².     

Conversion of batch to molten glass, II: Dissolution of quartz particles

Quartz dissolution during the batch-to-glass conversion influences the melt viscosity and ultimately the temperature at which the glass forms. Batches to make a high-alumina borosilicate glass (formulated for the vitrification of nuclear waste) were heated at 5 K min^− 1 and quenched from temperatures of 400 to 1200 °C at 100 K intervals. The batches contained quartz as a silica source, with particles ranging from 5 to 195 μm in diameter. The content of unreacted quartz in the samples was determined with X-ray diffraction. Most of the fine quartz dissolved during the early batch reactions (at temperatures < 800 °C), whereas coarser quartz dissolved mostly in a continuous glass phase via diffusion. The mass-transfer coefficients were assessed from the data as functions of the initial particle sizes and the temperature. A series of batches were also tested that contained nitrated components and additions of sucrose, known to accelerate melting. While sucrose addition had no discernible impact on quartz dissolution, nitrate batches melted somewhat more slowly than batches containing carbonates and hydroxides in addition to nitrates.     

Non-isothermal crystallization kinetics study on new amorphous Ga20Sb5S75 and Ga20Sb40S40 chalcogenide glasses

Bulk glasses of the system Ga₂₀Sb_xS_80−x (x = 5 and 40) were prepared for the first time by the known melt quenching technique. Non-isothermal differential scanning calorimetric (DSC) measurements of as-quenched Ga₂₀Sb_xS_80−x (x = 5 and 40) chalcogenide glasses reveal that the characteristic temperatures e.g. the glass transition temperature (T_g), the temperature corresponding to the maximum crystallization rate (T_p) recorded in the temperature range 400-650 K for x = 5 and 480-660 K for x = 40 are strongly dependent on heating rate and Sb content. Upon heating, these glasses show a single glass transition temperature (T_g) and double crystallization temperatures (T_p1 and T_p2) for x = 5 which overlapped and appear as a single crystallization peak (T_p) for x = 40. The activation energies of crystallization E_c were evaluated by three different methods. The crystallization data were examined in terms of recent analysis developed for non-isothermal conditions. The crystalline phases resulting from (DSC) have been identified using X-ray diffraction.     

Crystallization behavior and microstructure of powdered and bulk ZnO-Al2O3-SiO2 glass-ceramics

The crystallization behavior of glass with the composition: 55.6 mol% SiO₂, 22.8 mol% Al₂O₃, 17.7 mol% ZnO and 3.84 mol% of TiO₂ as nucleating agent and with different particle sizes has been studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and tranmission electron microscopy (TEM). In glass powders two crystalline phases: zinc-aluminosilicate s.s. with high-quartz structure, Zn_x/2Al_xSi_3−xO₆, (x varies dependent on heat-treatment temperature) and gahnite are formed. The ratio of these phases depends on particle sizes. In bulk glass, however, gahnite is the sole crystalline phase. The composition of initially formed zinc-aluminosilicate s.s. was determined by Rietveld refinement of XRD patterns to be Zn_0.69Al_1.38Si_1.62O₆. With temperature increase, the amount of zinc-aluminosilicate s.s decreased with simultaneous reduce of zinc and aluminum incorporated in the structure. Eventually at 1423 K almost pure high-quartz structure was formed. The activation energies of zinc-aluminosilicate s.s. and gahnite crystallization were determined by non-isothermal method to be 510 ± 18 and 344 ± 17 kJ mol⁻¹, respectively. The latter value matches well with those cited in literature for crystal growth of gahnite in similar glasses. That is attributed to the fact that the high-quartz structure acts as a precursor for gahnite crystallization.     

Development of optical nonlinearity, high dielectric constant and ferromagnetic behavior in a silicate glass nanocomposite by suitable heat treatment

We have explored the development of multifunctionalities viz, optical nonlinearity, high dielectric constant and ferromagnetic behavior in a nanostructured silica based glass of 14.0Na₂O, 26.0BaO, 26.0TiO₂, 16.0B₂O₃, 17.0SiO₂, 1.0NiO (mol%) composition. A heat treatment at 863 K for 4 h led to nonlinear refractive index and absorption coefficients at wavelength 800 nm of 0.11 × 10⁻¹⁹ m²/W and 1.15 × 10⁻³ cm/GW, respectively. A heat treatment at 1073 K for 2 h followed by 1113 K for 3 h increased the dielectric constant from 11 to 50, apparently due to the formation of nanocrystals of BaTiO₃ within the glass medium. Glass samples reduced at 923 K for 1 h exhibited ferromagnetic behavior due to the presence of nickel nanoparticles.     

Isobaric heating of polystyrene and evaluation of the Narayanaswamy parameter x from volume recovery data

Structure parameter x, defining the relative contributions of temperature and structure to the relaxation times, which is applied in Tool-Narayanaswamy-Moynihan (TNM) equation, was evaluated from volume heating isobars for polystyrene (PS) applying the peak shift method. The peak shifted with changing structural state of PS prior heating, which was reached by different period of aging time at 89 °C < T_g up to 2544 h. In this novelized procedure for volume data, originally tested only for enthalpic measurements, high sensitivity of peak temperature (position of inflection point of the heating isobar) was shown, together with high linearity of data. This allows to directly and sensitively calculate the shift and consequently structure parameter x. The value found in the research, x = 0.40 ± 0.02, is in good agreement with published results obtained by the peak-shift method from enthalpy data of PS.     

Effect of electron irradiation on Na-K silicate glass investigated using X-ray photoelectron spectroscopy and pattern recognition method

Alkali silicate glasses and melts play an important role in material science. Electron interaction with glasses is important for radioactive waste deposition, where electrons of various energies lead to irreversible changes. These changes are caused mainly by ionization and ballistic interaction of electrons with atoms, introducing structural disorder, changes in atomic composition and chemical state, accompanied by alkali ions diffusion. The Na-K silicate glass (5Na₂O · 10K₂O · 85SiO₂), pristine and electron irradiated (doses from 25 C m⁻² to 20 236 C m⁻²) are investigated using X-ray photoelectron spectroscopy (XPS) and the pattern recognition (PR) and fitting procedures. Changes of composition and chemical state of atoms dependent on electron dose are analyzed. At low doses (100-300 C m⁻²), decrease followed by increase of O and Si concentrations was observed. Surface segregation, probable desorption, and in-bulk diffusion of K and Na ions (doses of about 50 C m⁻² and 2000 C m⁻², respectively) were observed. This was accompanied by changes in the chemical state of K atom, where with an electron dose increasing content of elemental K form accompanied by decreasing potassium peroxide form were observed. No difference in chemical state of Si and O atoms was visible under electron irradiation dose to 20 236 C m⁻², within the sensitivity of the applied method.