Thermal and microstructural stability of the soft magnetic Fe60Co18Nb6B15Cu1 alloy

Temperature evolution of the coercivity of nanocrystalline samples of Fe₆₀Co₁₈Nb₆B₁₅Cu₁ alloy with different crystalline fractions was measured from room temperature up to 690 K. Although room temperature coercivity increases as the nanocrystallization progresses, the thermal stability of the magnetic properties of the system is clearly enhanced as the crystalline volume fraction increases. Microstructure was characterized using room temperature Mössbauer spectra which can be interpreted on the basis of the presence of three different regions: amorphous, crystalline and interface.     

Enhancement in electrical and stability properties of amorphous polymer based nanocomposite electrolyte

A novel combination of dispersed phase polymer nanocomposite electrolyte (PNCE) series based on an amorphous polymer host (PMMA)₄–LiClO₄ complex dispersed with nanocrystalline CeO₂ is reported. XRD analysis has confirmed the dispersed phase nanocomposite formation. Effect of nano CeO₂ dispersion on ion–ion and ion–polymer interactions has been analyzed. A drastic enhancement in electrical conductivity, by 2 orders of magnitude at 30 °C and 5 orders of magnitude at 100 °C, has occurred on nano CeO₂ dispersion when compared with room temperature conductivity of undispersed PS film. An excellent correlation between variation of d.c. conductivity and free mobile charge carriers has been observed. An ion conduction model is proposed. Strength of the model lies in the experimental evidences from FTIR, conductivity and TEM analyses. Thermal analysis indicates a strong dependence of thermodynamical parameters, e.g., glass transition temperature (T_g), crystalline melting temperature (T_m), enthalpy etc. on filler addition. Substantial improvement in voltage stability (~ 4.4 V), thermal stability and ion transport properties has been noticed on nano CeO₂ dispersion.     

Silica polymorphs,glass and melt: An in situ high temperature XAS study at the Si K-edge

High temperature X-ray absorption spectra at the Si K-edge were obtained for SiO₂ quartz from room temperature up to 2030 K. Important modifications are observed for the XANES spectra. These change are related to rearrangements of the SiO₄ tetrahedra beyond the short-range correlations. To interpret these spectral evolutions, SiO₂ polymorph samples were observed at room temperature and XANES calculations using FDMNES were performed. Very strong differences are shown between the different polymorphs even between α and β phases for which only small displacive angle rotations of the SiO₄ tetrahadra occurs. Therefore the quartz α to β transition could be identified at its expected temperature, 842 K. A badly defined transition toward β-cristobalite is observed between 1670 and 1940 K. The dynamics of this totally reconstructive transition was further investigated on heat treated cherts. Finally the liquid is reached around 2000 K. Many similarities were observed on SiO₂ between its glass at room temperature, β-cristobalite and liquid at high temperature.     

Optical constants and band gap expansion of size controlled silicon nanocrystals embedded in SiO2 matrix

Silicon nanocrystals (Si-NCs) with different sizes embedded in SiO₂ matrix were synthesized by phase separation and thermal crystallization of SiO_x/SiO₂ supperlattice approach. The optical constants and band gap expansion of Si-NCs have been investigated by spectroscopic ellipsometry, based on the Maxwell–Garnett effective medium approximation and the Forouhi–Bloomer optical dispersion model. Similar spectra shapes but smaller values of Si-NCs optical constants with respect to bulk crystalline Si is observed. With the size of Si-NCs decreasing from 6 nm to 2 nm, the band gap increases from 1.64 eV to 2.56 eV. The band gap expansion, as compared to bulk crystalline Si, which agrees with the prediction of first-principles calculations based on quantum confinement effect, is presented in this paper.     

Characterization of chromium silicide thin layer formed on amorphous silicon films

A detailed investigation of the compositional, optical and electrical properties of a chromium silicide layer grown at room temperature on top of doped amorphous silicon films is presented. The formation of the layer is promoted only when phosphorous atoms are present in the film. The deposition of a very thin n-type doped layer (around 5 nm) on top of a p-type doped film has allowed us to achieve the chromium silicide formation also on p-type material without changing its doping properties. Angle resolved X-ray photoelectron spectroscopy measurements demonstrate the presence of chromium-oxide, chromium silicide and metallic chromium in similar percentages for both p- and n-type doped layers. From the ellipsometric analysis, the refractive index spectra have been extracted, and the layer thickness has been estimated to be 5 nm for both p- and n-type doped layers. From planar conductivity measurements, we have found that the chromium silicide promotes an activation energy reduction from 0.24 eV down to 0.017 eV for the n-type layer and from 0.36 eV down to 0.14 eV for the p-type film.     

Iron environment in calcium-soda-phosphate glasses and vitroceramics

The structure of calcium-soda-phosphate glasses and vitroceramics with relatively high iron content was investigated by X-ray diffraction, electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. The X-ray diffraction analysis proves the vitreous state of the as prepared samples and the development of crystalline phases in the annealed samples. The ferric ions disposed in sites that give rise to the absorption line with g_ef ≈ 4.3 in the EPR spectra of vitreous samples are not more evidenced in the spectra of the annealed samples, from which only of a symmetric and narrowed line with g_ef ≈ 2 is recorded. The room temperature ⁵⁷Fe Mössbauer spectra both of glass and vitroceramic samples consist of two quadrupole doublets characteristic for octahedral sites of Fe²⁺ and Fe³⁺ ions. The isomer shift for glass samples decreases and for vitroceramic samples increases with the iron oxide content.     

Synthesis and characterization of M2O–MgO–WO3–P2O5 (M = K, Rb, Cs) glass system

Transparent glasses of the composition M₂O–MgO–WO₃–P₂O₅ (M = K, Rb, Cs), corresponding to the crystalline phases of M₂MgWO₂(PO₄)₂, have been prepared and studied by Raman and IR spectroscopy as well as DTA. Moreover, the thermally stimulated depolarization and dc conductivity have been measured. The glass transition temperature is 797, 795 and 773 K for the K-, Rb- and Cs-containing glass, respectively. Raman and IR studies have shown that these glasses have very similar structure. The main building blocks are pyrophosphate groups, WO₆ octahedra and magnesium–oxygen polyhedra. The dc conduction in these glasses is controlled by hopping of small polarons. The potassium containing glass was shown to be very stable whereas the rubidium and cesium glasses have significantly higher tendency for crystallization and phase separation. It seems, therefore, that the potassium containing glass is a suitable material for the preparation of samples containing non-linear and ferroelectric nanocrystals of the K₂MgWO₂(PO₄)₂ phosphate.     

Fictive temperature measurement of amorphous SiO2 films by IR method

The structure and properties of amorphous materials, in general, change with their thermal history. This is usually explained using the concept of fictive temperature, i.e., the temperature at which the super-cooled liquid state turned into a glassy state. In earlier studies, a simple IR method was used to determine the fictive temperature of silica glasses, both bulk and fiber. In the present study the applicability of the same technique for thin amorphous silica films on silicon was examined. It was found that the IR absorption as well as reflection peak wavenumber of the silica structural band can be used to determine the fictive temperature of amorphous silica films on silicon with an unknown thermal history. Specifically, IR absorbance spectra of an amorphous silica film of thickness greater than 0.5 μm grown on silicon can be taken before and after etching a thin surface layer of 20–30 nm and the peak wavenumber of the difference signal can be compared with the pre-determined calibration curve to convert the peak wavenumber to the fictive temperature. For a film thicker than ∼2 μm, IR reflection peak wavenumber can be converted directly to the fictive temperature of the film by using the calibration curve.     

Absorption and photoluminescence of a new thienylene–phenylene copolymer

This paper reports on the optical studies of the thin film of one thienylene–dialkoxyphenylene copolymer. The film for the optical measurements was prepared by spin coating on glass substrates. Absorption spectra were recorded at room temperature and photoluminescence spectra were recorded in the temperature range from 300 K to 20 K. The analysis of the relative intensity of the zero-phonon, and the 0–1 and 0–2 vibronic bands indicates a change in conjugation length with temperature in the PL spectra, and its evolution allows the determination of the Huang–Rhys factor.     

Amorphous to crystalline transition and optoelectronic properties of nanocrystalline indium tin oxide (ITO) films sputtered with high rf power at room temperature

ITO thin films were deposited on quartz substrates by the rf sputtering technique using various rf power keeping the substrates at room temperature. The influence of rf power on the structural, electrical, optical and morphological properties was studied by varying the rf power in the range 50–350 W. X-ray diffraction results show an amorphous – crystalline transition with nano grains. At a power of 250 W, the ITO film showed preferential orientation along (4 0 0) peak. It is observed from the optical transmission studies that the optical band gap increased from 3.57 to 3.69 eV when the rf power was increased from 50 to 250 W. The resistivity value is minimum and grain size is maximum for the ITO film deposited at 250 W. The X-ray photoelectron spectroscopy (XPS), Energy dispersive X-ray (EDX) and Atomic force microscopy AFM results confirm that the ITO films are stoichiometric and the surface contained nano-sized grains distributed uniformly all over the surface. It can be concluded that the ITO film deposited at room temperature with 250 W rf power, can provide the required optical and electrical properties useful for developing optoelectronic devices at lower temperatures.     

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.     

Preparation and properties of titanium silicide coating glass by CVD

Titanium silicide thin films were prepared on glass substrates by chemical vapor deposition using SiH₄ and TiCl₄ as the precursors. The phase structure of the thin films was identified by XRD. The surface morphology of the thin films was observed by FESEM. The sheet resistance and optical behaviors of the thin films were measured by the four point resistivity test system and FTIR spectrometer, respectively. Titanium disilicide (TiSi₂) thin films with the face-centered orthorhombic structure are formed. The suitable formation temperature of the TiSi₂ crystalline phase is about 710 °C. The formation of TiSi₂ crystalline phase is dependent on the thickness of thin films and a quantity of the crystalline phase of TiSi₂ in the thin film is directly related to mole ratio of SiH₄/TiCl₄. The sheet resistance of the TiSi₂ thin films is dependent on the formation of the TiSi₂ crystalline phase. With the mole ratio of SiH₄/TiCl₄ of 3, the lowest sheet resistance (0.7 Ω/□) of titanium silicide thin film is formed at 710 °C. The maximum reflectance of the TiSi₂ thin films is about 0.95 on the broad IR heat radiation. A related reaction mechanism was proposed.     

Generating ordered Si nanocrystals via atomic force microscopy

We describe two successful routes for generating ordered arrays of Si nanocrystals by using atomic force microscopy (AFM) and amorphous silicon thin films (200–400 nm) on Ti/Ni coated glass substrates. First, we show that field-enhanced metal-induced solid phase crystallization at room temperature can be miniaturized to achieve highly spatially localized (below 100 nm) current-induced crystallization of the amorphous silicon films using a sharp tip in AFM. In the second route, resistive nano-pits are formed at controlled positions in the amorphous silicon thin films by adjusting (lowering and/or stabilizing) the exposure currents in the AFM process. Such templated substrates are further used to induce localized growth of Si nanocrystals in plasma-enhanced chemical vapor deposition process. In both cases the crystalline phase is identified in situ as features of enhanced current in current-sensing AFM maps.     

Solid phase epitaxy of Na-ion irradiated α-quartz: Cathodoluminescence,kinetics and surface morphology

Even at low fluences doping of quartz by ion implantation results in amorphization. Here we report on the measurement of cathodoluminescence and surface structures during solid phase epitaxy in Na-implanted α-quartz annealed in ¹⁸O₂ atmosphere. Complete epitaxy was achieved under appropriate conditions of ion fluence, annealing temperature and time. The crystalline structure of the samples was studied by Rutherford backscattering channelling spectroscopy and the ¹⁸O–¹⁶O exchange between the matrix and ¹⁸O₂ annealing gas by elastic recoil detection analysis. In the cathodoluminescence spectra taken at room temperature, five bands were identified and assigned to various defect centres. Two of the bands in the violet region at 3.25 and 3.65 eV strongly vary in intensity at 843–1173 K annealing temperature and appear to be intimately correlated with the presence of Na-ions in the implanted region of the matrix. Finally, atomic force microscopy enabled, for the first time, the observation of the correlation of surface structures and epitaxy.     

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.     

ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE

High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm⁻¹ was found in Raman scattering spectra for all films with buffers, which is corresponding to the E₂ mode.     

Comparative study on photoluminescence of amorphous and nano-crystalline YAG:Tb phosphors prepared by a combustion method

Amorphous and nano-crystalline Y₃Al₅O₁₂:Tb phosphor samples were obtained via a facile combustion method by calcination at various temperatures, using yttrium oxide and aluminum nitrite as starting materials and citric acid as fuel. XRD, FT-IR and TEM results showed that the products were amorphous if prepared at 750 °C, well-crystalline when treated above 850 °C. In addition, partially crystalline YAG phase was observed at 800 °C (in air). The excitation spectra of the samples calcined at 750 °C and 800 °C exhibited some difference in the 230–255 nm range in comparison to those of nano-crystalline YAG:Tb, i.e. an extra band centered at 250 nm was detected via Gaussian curve-fitting. Furthermore, the photoluminescence intensity of as-synthesized samples decreased obviously with increasing the crystallinity under 250 nm excitation. Contrary, it increased monotonously when altering the excitation wavelength to 323 nm. The concentration-dependent emission spectra of samples calcined at 800 °C revealed that the strongest intensity could be obtained with 10% Tb doping. Red-shifts indicated changes of the inter-atomic distances within the Tb³⁺ coordination polyhedron with increasing Tb concentration. The low temperature photoluminescence of partially crystalline YAG:10% Tb was also investigated, displaying good-resolution but reduced intensity compared to the room-temperature photoluminescence.     

Zircon formation from amorphous powder and melt in the silica-rich region of the alumina–silica–zirconia system

Chemically homogeneous amorphous powders were prepared by sol–gel method from alkoxide mixtures in the silica-rich region of the alumina–silica–zirconia system. A glass with the same composition was obtained by quenching in water from the melt. The evolution with temperature in the range up to 1625 °C was studied by means of X-ray diffraction, infrared and UV-Raman spectroscopy, scanning electron microscopy and energy-dispersive X-ray analysis. Zircon crystallization and stability at temperatures higher than the liquidus temperature were confirmed in both melt-quenched and gel-glasses. Crystallization of cristobalite from the amorphous gel-glasses was observed above 1200 °C. The crystallization of zircon particles was observed after thermal treatment at 1550 °C. At 1625 °C zircon was the only stable crystalline phase due to the complete melting of the silica-rich matrix. The XRD pattern of the ternary gel treated at 1625 °C shows clearly the disappearance of the cristobalite phase, while at 1550 °C there is already partial melting. These results can help to determine the phase equilibrium in the ternary system.     

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.     

Carbon covered magnetic nickel nanoparticles embedded in PBT-PTMO polymer: Preparation and magnetic properties

Fine particles of a face-centered-cubic phase of Ni covered with a graphite layer were prepared and embedded in a PBT-block-PTMO polymer at a concentration of 0.1 wt%. The mean crystalline size of Ni varied from 8 to 30 nm. A magnetic resonance study of the obtained nanocomposites was carried out in the 4–300 K temperature range using an electron paramagnetic resonance spectrometer. An almost symmetrical and very intense magnetic resonance line was recorded for all the investigated samples. The resonance line was centered at g = 2.253(2) (the resonance field H_r = 3003(1) Gs) and had a peak-to-peak linewidth ΔH_pp = 693(2) Gs at room temperature. The amplitude of the resonance line increased with a temperature increase in the low temperature range (T < 40 K) and in the high temperature range (T > 100 K) but was constant at intermediate temperatures. The resonance field H_r decreased and linewidth ΔH_pp increased as the temperature decreased from room temperature what was similar to the changes observed for other systems of nanoparticles. The thermal gradient of the resonance field, ΔH_r/ΔT, strongly depended on the temperature range. The temperature shift of the resonance field and the linewidth were analyzed in terms of the demagnetizing fields of nonspherical agglomerates. A strong change of linewidth and resonance field was registered below 40 K due to the freezing of the spin system’s dynamical magnetic fluctuations. A comparison was made of the results obtained on the Ni/C with the previous measurements on γ-Fe₂O₃ nanoparticles embedded in a copolymer.