Size effects in the exciton energy and first-order phase transitions in CuCl nanocrystals in glass

The absorption spectra and the melting and crystallization kinetics of CuCl nanocrystals in glass are investigated in the range of particle radii 1–30 nm. Three discontinuities are found on the curves representing the size dependence of the melting point T _m(R) and the crystallization point T _c(R). As the particle radius gradually decreases from 30 nm in the range R⩽12.4 nm there is a sudden 60° drop in the temperature T _c in connection with the radius of the critical CuCl nucleus in the melt. A 30° drop in T _m is observed at R=2.1 nm, and a second drop of 16° in the temperature T _c is observed for CuCl particles of radius 1.8 nm. The last two drops are associated with changes in the equilibrium shape of the nanoparticles. In the range of smaller particles, R⩽1.34 nm the T _c(R) curve is observed to merge with the T _m(R) curve, owing to the disappearance of the work of formation of the crystal surface during crystallization of the melt as a result of the zero surface tension of CuCl particles of radii commensurate with the thickness of the effective surface layer. An increase in the size shift of the exciton energy is observed in this same range of CuCl particle radii (1–1.8 nm). The size dependence of the melting and crystallization temperatures of the nanoparticles is attributed to variation of the free energy in the surface layer of a particle. Fiz. Tverd. Tela (St. Petersburg) 41, 310–318 (February 1999)     

Phase segregation in the Fe90Zr10 amorphous alloy under heating

A study of the changes in the structure of melt-quenched Fe₉₀Zr₁₀ amorphous alloys by x-ray diffraction, Auger spectroscopy, and transmission electron microscopy is reported. The samples were subjected to isochronous (for 1 h) and isothermal anneals at 100–650 °C. It is shown that an amorphous alloy annealed for one hour at 300–500 °C crystallizes with formation of a supersaturated solid solution of Zr in α Fe and the intermetallic compound Fe₃Zr. Isothermal anneal at 100 °C for up to 7000 h produces nanocrystallites 110–30 nm in size, with fuzzy interfaces between the grains. An alloy subjected to such an anneal contains two solid solutions of Zr in Fe, having a cubic and a weakly tetragonal lattice. Crystallization taking place during low-temperature anneals is preceded by phase segregation of the alloy within the amorphous state. The lattice periods of the solid solutions have been determined. The possibility of the alloy crystallizing by spinodal decomposition during prolonged annealing is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1769–1772 (October 1998)     

Properties of reactively sputtered W–B–N thin film as a diffusion barrier for Cu metallization on Si

Thin films of W–B–N (10 nm) have been evaluated as diffusion barriers for Cu interconnects. The amorphous W–B–N thin films were prepared at room temperature via reactive magnetron sputtering using a W₂B target at various N₂/(Ar + N₂) flow ratios. Cu diffusion tests were performed after in-situ deposition of 200 nm Cu. Thermal annealing of the barrier stacks was carried out in vacuum at elevated temperatures for one hour. X-ray diffraction patterns, sheet resistance measurement, cross-section transmission electron microscopy images, and energy-dispersive spectrometer scans on the samples annealed at 500°C revealed no Cu diffusion through the barrier. The results indicate that amorphous W–B–N is a promising low resistivity diffusion barrier material for copper interconnects.     

A sonochemical-assisted synthesis and annealing temperature effect of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> nanoparticles

This paper reports on the sonochemical-assisted synthesis of La_0.7Sr_0.3MnO₃ (LSMO) nanoparticles (NPs) which have a single-crystalline perovskite structure. The average particle size of LSMO NPs was controlled from about 40 to 120 nm by changing the annealing temperatures from 750 to 1050°C. The particle size, electrical resistivity, and ferromagnetic transition temperature of LSMO NPs were strongly dependent on the annealing temperature. A substantial decrease in resistivity and an enhancement in the insulator–metal transition temperature were found on increasing the annealing temperature. Furthermore, the enhancement in magnetization and paramagnetic–ferromagnetic (PM–FM) transition temperatures was observed as the annealing temperature increases.     

Crystallization and electrochemical performance of La0.6Sr0.4Co0.2Fe0.8O3 − δ-Ce0.8Gd0.2O1.9 thin film cathodes processed by single solution spray pyrolysis

La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ-Ce_0.8Gd_0.2O_1.9 (LSCF-CGO) thin films obtained by spray pyrolysis of a single precursor solution were investigated by XRD, TEM and impedance spectroscopy at annealing temperatures ranging from 500 to 900 °C. Films annealed at 600 °C contained a mixture of amorphous regions and crystalline regions composed of fine crystallites (< 5 nm). Annealing above 600 °C increased the ratio of crystalline to amorphous material, led to the segregation of the films into distinct LSCF and CGO phases, and promoted grain growth. The electrical behavior of the films depended on annealing temperature. At testing temperatures of 400 °C and below, the polarization resistance of films with lower annealing temperatures was larger than the polarization resistance of films with higher annealing temperatures. However, at testing temperatures of 500 °C and above the polarization resistance of films with lower annealing temperatures was equal to or lower than the polarization resistance of films with higher annealing temperatures. This was reflected by the activation energy that decreased with increasing annealing temperature. The varying electrical behavior may be related to microstructural changes that caused bulk diffusion to be the rate-limiting step in films with lower annealing temperatures and oxygen dissociation to be the rate-limiting step in films with higher annealing temperatures.     

Amorphous linear polyethylene: Annealing effects

The effect of annealing amorphous linear polyethylene films prepared by an improved ultraquenching technique at temperatures just below and above a dynamic mechanical relaxation peak (torsion braid) observed at ∽190K has been characterized by electron microscopy and torsion braid analysis. Based on the results described, this peak is believed related to the lower glass transition temperature T_g(L), the T_g of wholly amorphous linear polyethylene, whereas the β peak at 260K is T_g (upper). Annealing just below T_g (L) results in a growth in size of the nodules observed in the as-quenched samples, whereas annealing above T_g (L) can result in the growth of single crystal-like structures, spherulites, and shish-kebobs. Storage of the crystallized samples at room temperatures results in a decrease in size of the relaxation peak during subsequent torsion braid spectroscopy measurements. The results indicate significant amounts of molecular motion can occur during crystallization even at T_g.     

Characterization of dip-coated ITO films derived from nanoparticles synthesized by␣low-pressure spray pyrolysis

In₂O₃:Sn (Indium Tin Oxide; ITO) films were prepared from a sol solution with highly crystalline ITO nanoparticles (less than 20 nm in size with 10 at.% Sn) which had been prepared by low-pressure spray pyrolysis (LPSP) in a single step. The ITO sol solution was prepared by dispersing LPSP-prepared ITO nanoparticles into ultra pure water. The nanoparticle ITO film was deposited on a glass substrate using a dip-coating method and then annealed in air at various temperatures. The optical transmittances of the ITO films were measured by UV–Vis spectrometry, and the films were found to have a high transparency to visible light (in the case of a film thickness of 250 nm annealed at 400°C, the transparency was in excess of 95% over the range λ=450–800 nm, with a maximum value near 100% at wavelengths above λ=700 nm). The optical transmittances of the films were influenced by the size of the ITO particle used, the film thickness and the annealing temperature. The ITO films showed a minimum resistivity of 9.5×10⁻² Ω cm, and their resistivity was affected by both the ITO particle size and the annealing temperature used.     

Initial stages in the decay of the amorphous phase in the bulk metallic glass Zr-Cu-Ti

Using methods of x-ray diffraction analysis, differential scanning calorimetry, dilatometry, and transmission electron microscopy, we have investigated the initial stages of decay of the amorphous phase in a bulk metallic glass based on zirconium. We found that crystallization of the bulk metallic glass proceeds in several stages, where in the first stage the bulk conversion effect is equal to more than 1.6%, or about 80% of the total bulk crystallization effect. We showed that the first stage of decay of the amorphous phase in the bulk metallic glass Zr₂₉Ti₁₁Cu₆₀ leads to the formation of a nanocrystalline structure with a grain size of 1–5 nm. We have analyzed the change in the shape of the diffraction maximum during the formation of the very fine nanocrystalline structure. Fiz. Tverd. Tela (St. Petersburg) 41, 1129–1133 (July 1999)     

Phase transformation of nanostructured titanium dioxide thin films grown by sol–gel method

Nanostructured TiO₂ thin films were deposited on quartz glass at room temperature by sol–gel dip coating method. The effects of annealing temperature between 200^∘C to 1100^∘C were investigated on the structural, morphological, and optical properties of these films. The X-ray diffraction results showed that nanostructured TiO₂ thin film annealed at between 200^∘C to 600^∘C was amorphous transformed into the anatase phase at 700^∘C, and further into rutile phase at 1000^∘C. The crystallite size of TiO₂ thin films was increased with increasing annealing temperature. From atomic force microscopy images it was confirmed that the microstructure of annealed thin films changed from column to nubbly. Besides, surface roughness of the thin films increases from 1.82 to 5.20 nm, and at the same time, average grain size as well grows up from about 39 to 313 nm with increase of the annealing temperature. The transmittance of the thin films annealed at 1000 and 1100^∘C was reduced significantly in the wavelength range of about 300–700 nm due to the change of crystallite phase. Refractive index and optical high dielectric constant of the n-TiO₂ thin films were increased with increasing annealing temperature, and the film thickness and the optical band gap of nanostructured TiO₂ thin films were decreased.     

Production, structure, and microhardness of nanocrystalline Ni-Mo-B alloys

Radiography, differential scanning calorimetry, luminescence and high-resolution electron microscopy are used to study the production, nanocrystalline structure, stability, and microhardness of alloys from the Ni-Mo-B system containing from 27 at. % to 31.5 at. % Mo and 10 at. % B. All studies of these alloys indicated that annealing at 600 °C leads to the creation of a granular phase consisting of FCC nanocrystallites with average grain sizes of 15–25 nm, depending on the chemical composition of the alloy. Annealing these nanocrystalline samples isothermally at a temperature of 600 °C has no appreciable effect on the grain size. Structurally, the nanocrystalline phase consists of grains of an FCC solid solution of Mo and B in Ni, dispersed in an amorphous matrix that isolates them from one another. The lattice parameters of the FCC nanocrystallites depend on the alloy composition and the duration of their isothermal anneal. Within this latter time, molybdenum and boron atoms diffuse from the FCC solid-solution lattice into the surrounding amorphous matrix. The stability of the nanocrystalline structure is determined by the thermal stability of the amorphous matrix, whose crystallization temperature increases with the isothermal annealing time due to enrichment by boron and molybdenum. As the structure forms, the alloy becomes harder as the nanocrystalline grains grow in size. This relation between hardness and grain size, which is opposite to the Hall-Petch law, is explained by hardening of the amorphous matrix due to changes in its chemical composition. Fiz. Tverd. Tela (St. Petersburg) 40, 10–16 (January 1998)     

Characteristics of electron beam evaporated nanocrystalline SnO2 thin films annealed in air

Tin oxide (SnO₂) thin films (about 200 nm thick) have been deposited by electron beam evaporation followed by annealing in air at 350-550 °C for two hours. Optical, electrical and structural properties were studied as a function of annealing temperature. The as-deposited film is amorphous, while all other annealed films are crystalline (having tetragonal structure). XRD suggest that the films are composed of nanoparticles of 5-10 nm. Raman analysis and optical measurements suggest quantum confinement effects that are enhanced with annealing temperature. For instance, Raman peaks of the as-deposited films are blue-shifted as compared to those for bulk SnO₂. Blue shift becomes more pronounced with annealing temperature. Optical band gap energy of amorphous SnO₂ film is 3.61 eV, which increases to about 4.22 eV after crystallization. Two orders of magnitude decrease in resistivity is observed after annealing at 350-400 °C due to structural ordering and crystallization. The resistivity, however, increases slightly with annealing temperature above 400 °C, possibly due to improvement in stoichiometry and associated decrease in charge carrier density.     

Near-ordering structural transitions in amorphous metal alloys

The temperature variations in the modulus of elasticity (Young’s modulus) E and internal friction Q ⁻¹ of the amorphous metal alloys Ti₅₀Cu_50−x Ni_x (5⩽x⩽20) are studied at temperatures of 300–800 K. There is an anomalous increase in E(T) at temperatures above T _x (which varies from 440 to 525 K, depending on the composition). When the amount of nickel in the alloy is high (x>12 at. %), a small peak shows up in Q ⁻¹(T). These effects are related to structural transitions in near-ordering regions (clusters). A model for structural relaxation of near ordering in amorphous alloys is proposed on the basis of these experiments. Fiz. Tverd. Tela (St. Petersburg) 40, 389–392 (March 1998)     

Structural transitions in ice samples at low temperatures and pressures

The structure of ice samples formed in the decay of a water impurity gel at temperatures above 4 K and atmospheric pressure has been examined. The X-ray diffraction analysis indicates that three phases coexist in the initial sample at temperatures of 85–110 K. These phases are amorphous ice occupying up to 30% of the sample volume, cubic-phase ice I _c metastable at low pressures (∼60%), and normal hexagonal ice I _h (≤6%). The characteristic sizes of crystals of the cubic and hexagonal phases are about 6 and 30 nm, respectively. The amorphous phase at annealing above 110 K is gradually transformed to the crystalline phase both cubic and hexagonal. This transition is accompanied by two processes, including a fast increase in the sizes of cubicphase nanocrystals and the partial transition of the cubic phase I _c to the hexagonal one I _h. Hexagonal ice I _h prevails in the bulk of the sample above 200 K.     

Atmospheric pressure chemical vapour synthesis of silicon–carbon nanoceramics from hexamethyldisilane in high temperature aerosol reactor

Silicon–carbon nanoceramics have been synthesised from hexamethyldisilane (HMDS) by the atmospheric pressure chemical vapour synthesis (APCVS). Direct aerosol phase synthesis enables continuous production of high purity materials in one-stage process. The particle formation is based on the decomposition of the precursor in a high temperature reactor. Reaction of the gas phase species leads to homogeneous nucleation and formation of the nanoparticles with a narrow size distribution (geometric mean diameter range of particle number size distribution 160–200 nm with 1.5–1.6 geometric standard deviation at reaction temperatures 800–1200 °C). A systematic investigation of the influence of the process temperature on the powder characteristics, including the particle size, crystallinity, chemical structure, surface and bulk composition and surface morphology, was carried out. At the reactor temperature of 800 °C, the synthesised nanoparticles were amorphous preceramics containing mostly SiC₄, Si–CH₂–Si and Si–H units. The composition of the powder turned towards nanocrystalline 3C–SiC (crystal size under 2 nm) when the reaction temperature was increased to 1200 °C. The reaction temperature appeared to be a key parameter controlling the structure and properties of the synthesised powders.     

Crystallization of polycarbonate from the glassy state. part II. Thin films melted and quenched

The crystallization of glassy amorphous thin polycarbonate films quenched from the melt has been followed. Structures similar to those observed during the crystallization of films containing small amount of adsorbed solvent are formed. However, crystallization, at the same temperatures, is much slower and more irregular. Preannealing of the samples below T_g affects the crystallization process, increasing the nucleation rate. Annealing above T_g does not destroy all order gained by annealing below T_g.     

Space-selective modification of the magnetic properties of transparent Fe3+-doped glass by femtosecond-laser irradiation

We have demonstrated spatially selective modification of the magnetic properties of transparent iron-oxide-doped glass by femtosecond- (fs-) laser irradiation and subsequent annealing. A near-infrared fs-laser beam with a wavelength of 775 nm was focused 1 mm below the surfaces of glass samples. This produces absorption peaks due to the formation of hole-trap centers in the irradiated region. Transparency was recovered after annealing at 450°C. A ferrimagnetic component was observed in the M–H curve even at room temperature, whereas the diamagnetic component dominated in the M–H curve of the as-prepared glass sample. This indicates that fs-laser irradiation enhanced the magnetization in the irradiated area. The irradiated and annealed glass sample also exhibited superparamagnetic blocking in the temperature dependence of the magnetization with a blocking temperature higher than room temperature. This change in magnetism is presumably due to local crystallization of ferrimagnetic nanoparticles, such as magnetite, induced by fs-laser irradiation and annealing. The magnetic and optical properties of glass that had been annealed but not irradiated by a fs-laser beam remained unchanged.     

Effects of implantation defects on the carrier concentration of 6H-SiC

The effects of ion irradiation defects on the carrier concentration of 6H-SiC epitaxial layer were studied by current–voltage (I–V), capacitance.-voltage (C–V) measurements, thermally stimulated capacitance and deep level transient spectroscopy. The defects were produced by irradiation with 10 MeV C⁺ at a fluence of 10¹¹ ions/cm² and subsequent thermal annealings were carried out in the temperature range 500–1700 K under N₂ flux. I–V and C–V measurements reveal the presence of a high defect concentration after irradiation and annealing at temperature lower than 1000 K. Thermally stimulated capacitance measurements show that some of the defects induce a deactivation of the nitrogen donor, while some of the generated defects, behaving as donor-like traps, contribute to increase the material free carrier concentration at temperatures above their freezing point. Deep level transient spectroscopy measurements performed in the temperature range 150–450 K show the presence of several overlapping traps after ion irradiation and annealing at 1000 K: these traps suffer a recovery and a transformation at higher temperatures. The annealing of all traps at temperatures as high as 1700 K allows one to completely restore the n-type conductivity. The defects mainly responsible of the observed change in the carrier concentration are identified. PACS 73.30.+y; 61.80.Jh; 61.82.Fk; 85.30.Hi     

Preparation and characteristics of?a?BaO–Al2O3–B2O3–SiO2–La2O3 glass ceramic via spray pyrolysis

The characteristics of a BaO–Al₂O₃–B₂O₃–SiO₂–La₂O₃ glass ceramic prepared by spray pyrolysis were studied. Glass powders with spherical shape and amorphous phase were prepared by complete melting at a preparation temperature of 1 500°C. The mean size and geometric standard deviation of the powders prepared at the temperature of 1 500°C were 0.6 μm and 1.3. The glass powders had similar composition to that of the spray solution. The glass transition temperature (T _g) of the glass powders was 600.3°C. Two crystallization exothermic peaks were observed at 769.3 and 837.8°C. Densification of the specimen started at a sintering temperature of 600°C, in which Ba₄La₆O(SiO₄)₆ as main crystal structure was observed. Complete densification of the specimen occurred at a sintering temperature of 800°C. The specimens sintered at temperatures above 800°C had main crystal structure of BaAl₂Si₂O₈.     

Control of the degree of long-range compositional order in ceramics of the compound perovskites Pb(B′1/2B″1/2)O3 using recrystallization

It is established that an ordered state forms in Pb(Yb_1/2Nb_1/2)O₃ at fairly low temperatures (650–800°C). Subsequent high-temperature heat treatment of the ceramic without additives (sintering or additional annealing) does not produce any significant change in the degree of long-range compositional order s because of the low diffusion rate of the Yb and Nb ions. The addition of Li₂CO₃, which forms a liquid phase, creates conditions for the dissolution of grains with a high value of s, the nucleation of new crystallization centers, and the growth of grains with a new equilibrium value of s at the sintering (annealing) temperature. Zh. Tekh. Fiz. 69, 24–30 (March 1999)     

Crystallization Behavior of Amorphous Poly(l-Lactide)

Amorphous poly(l-lactide) (PLLA) was annealed in two different ways: amorphous samples were heated at a given temperature to induce crystallization (one-step annealing); and amorphous samples were first crystallized at a low temperature and subsequently annealed at a higher temperature than the crystallization temperature. Samples thus prepared were measured by DSC. The original amorphous sample exhibited an exothermic peak at about 100°C (exothermic peak I), an exothermic peak just below the melting point (exothermic peak II), and an endothermic peak when it was melted. Exothermic peak I was caused by cold crystallization. When the melting points of PLLA samples, heat-treated in various ways, were plotted as a function of annealing temperature, there was discontinuity at about 120°C. From analyses of wide-angle X-ray diffraction patterns, it was found that when amorphous PLLA was crystallized at a temperature below 120°C, crystallites of the β-form formed, and when annealed at a temperature above 120°C, crystallites of the α-form grew. Thus, exothermic peak I was attributed to cold crystallization of the β-form, and peak II was caused by the phase transition of the β-form to a more stable form.