Kinetic Analysis of Calorimetric Measurements of Niobium–Aluminum Nb/Al Multifilament Strands

The formation of different phases for the reaction of Nb and Al in M/s Hitachi Ltd made wires using differential scanning calorimetry (DSC) has been investigated. The interfacial diffusion reaction has been studied in the temperature range 100–990 °C. Niobium (Nb) and aluminum (Al) react to form the phase NbAl₃, subsequently NbAl₃ reacts with the remaining Nb to form the A15 phase. The interfacial reactions play an important role. At higher heating rates, NbAl₃ and bcc‐structured NbAl are formed prior to the formation of Nb₃Al, but at lower heating rates the phase observed indicates σ phase Nb₂Al. DSC has been performed at different heating rates under nonisothermal conditions aimed to measure activation energy of crystallization for different phases: employing Kissinger's equation, Matusita–Sakka theory, and the Augis‐Bennett method. Activation values calculated from these three different methods are found to be in good agreement with each other. The activation energy for the first phase (NbAl₃) is lower than the other two phases (Nb₃Al) A15 phase and Nb₂Al, suggesting that NbAl₃ formation occurs easily. The value of n relating to the nucleation and growth mechanisms has also been calculated using the modified Kissinger method at different temperatures.     

Underpotential surface reduction of mesoporous CeO<Subscript>2</Subscript> nanoparticle films

The formation of variable-thickness CeO₂ nanoparticle mesoporous films from a colloidal nanoparticle solution (approximately 1–3-nm-diameter CeO₂) is demonstrated using a layer-by-layer deposition process with small organic binder molecules such as cyclohexanehexacarboxylate and phytate. Film growth is characterised by scanning and transmission electron microscopies, X-ray scattering and quartz crystal microbalance techniques. The surface electrochemistry of CeO₂ films before and after calcination at 500 °C in air is investigated. A well-defined Ce(IV/III) redox process confined to the oxide surface is observed. Beyond a threshold potential, a new phosphate phase, presumably CePO₄, is formed during electrochemical reduction of CeO₂ in aqueous phosphate buffer solution. The voltammetric signal is sensitive to (1) thermal pre-treatment, (2) film thickness, (3) phosphate concentration and (4) pH. The reversible ‘underpotential reduction’ of CeO₂ is demonstrated at potentials positive of the threshold. A transition occurs from the reversible ‘underpotential region’ in which no phosphate phase is formed to the irreversible ‘overpotential region’ in which the formation of the cerium(III) phosphate phase is observed. The experimental results are rationalised based on surface reactivity and nucleation effects.     

Influence of oxygen atmosphere on the photoluminescence properties of sol–gel derived ZrO2 thin films

Homogeneous and transparent ZrO₂ thin films were prepared by sol?Cgel dip coating method. The prepared ZrO₂ thin films were annealed in air and O₂ atmosphere at 500, 700 and 900?°C for 1, 5 and 10?h. X-Ray diffraction (XRD) pattern showed the formation of tetragonal phase with a change of stress in the films. Scanning electron microscope (SEM) revealed the nucleation and particle growth on the films. An average transmittance of >80?% (in UV?CVis region) was observed for all samples. The refractive index and direct energy band gap were found to vary as functions of annealing atmosphere, temperature and time. Photoluminescence (PL) revealed an intense emission peak at 379?nm weak emission peaks at 294, 586 and 754?nm. An enhancement of PL intensity was observed in films annealed in O₂ atmosphere. This is due to reconstruction of zirconium nanocrystals interfaces, which help passivate the non-radiative defects. At 900?°C, oxygen atoms react with Zr easily at the interface and destroy the interface states acting as emission centres and quench the PL intensity of the film. The enhancement of the luminescence properties of ZrO₂ by the passivation of non radiative defects presents in the films make it suitable for gas sensors development, tuneable lasers and compact disc (CD) read-heads.     

On the mechanism of the P2-Na0.70CoO2→O2-LiCoO2 exchange reaction—Part II: an in situ X-ray diffraction study

A model was proposed to describe the exchange reaction of sodium by lithium in P2 crystals, it was based first on the formation of nucleation centers and then on the growth of O2 domains in P2 crystals from these nucleation centers. This study has shown that depending on the ratio between the growing and nucleation speeds, O2, O6 or faulted structures are obtained and that this model allows a good analysis of the exchange process. XRD patterns simulation and their comparison with that of experimental O2-LiCoO₂ have shown that there was almost no defects in the O2-LiCoO₂ structure obtained by ion exchange in water. Therefore, this study has shown that the growth of the O2 domains in the P2-Na_0.7CoO₂ crystals is faster than the formation of nucleation centers.This P2-Na_0.7CoO₂→O2-LiCoO₂ exchange reaction was also studied in situ by X-ray diffraction; simulations of key XRD patterns by P2-O2 intergrowths were also achieved. It was shown, in good agreement with the simulations, that the growth of O2 domains was faster than the formation of the nucleation centers and kinetically activated by a P2-Na_0.70CoO₂→P2^*-Na_∼0.50CoO₂ phase transition. For those reasons, the P2-Na_0.70CoO₂→O2-LiCoO₂ exchange reaction in water leads to an O2 phase, with an almost ideal packing.     

Electric-field-induced antiferroelectric to ferroelectric and ferroelectric to paraelectric phase transition at various temperatures in (Pb,La)(Zr,Ti)O<Subscript>3</Subscript> antiferroelectric thick films

(Pb, La)(Zr, Ti)O₃ antiferroelectric thick films with (100)-preferred orientation were fabricated on Pt(111)/Ti/SiO₂/Si(100) substrates via a sol–gel method. The electric-field-induced antiferroelectric (AFE) to ferroelectric (FE) phase transition characteristics were studied by C (capacitance)–E (electric field) measurements at different temperature. The films were in AFE state under 0 kV/cm below 122 °C, and the switching field values decreased, with increasing temperature. The films were in FE state between 122 and 135 °C, and when the temperature above 135 °C, the films were in PE state. The temperature-dependent dielectric parameters were deconvoluted using a Gaussian fit multi-peaks showed that two typical phase transitions were discovered. The first peak is the AFE-to-FE phase transition and the second peak is the FE-to-PE phase transition which has been verified by C–E tests.     

Crystal growth of Ce2O(CO3)2·H2O in aqueous solutions: Film formation and samarium doping

Crystalline cerium oxide carbonate hydrate (Ce₂O(CO₃)₂·H₂O) was grown in aqueous solutions at a low temperature of 80 °C under ambient pressure. When cerium nitrate was used as a starting material, large Ce₂O(CO₃)₂·H₂O particles were precipitated through homogeneous nucleation and subsequent fast crystal growth. In contrast, the usage of cerium chloride was found to promote the preferential precipitation of Ce₂O(CO₃)₂·H₂O on foreign substrates through heterogeneous nucleation and slow crystal growth. This phenomenon was applied to a chemical bath deposition of Ce₂O(CO₃)₂·H₂O films. Immersion of glass substrates in the solution at 80 °C for typically 24 h resulted in formation of solid films with a unique morphology like a micrometer-scale brush. It was also found that samarium could be incorporated into Ce₂O(CO₃)₂·H₂O during the crystal growth in the solutions, as evidenced by characteristic photoluminescence of Sm³⁺ in heating products of CeO₂. These results suggest that rare-earth oxide carbonate hydrates with a variety of compositions and morphologies can be synthesized from the aqueous solutions.     

Evolution of Structure of the Precursor During Sol-Gel Processing of ZnO and Low Temperature Formation of Thin Films

ZnO thin films were prepared on silicon substrate with Pt electrode by the sol-gel processing using Zn alkoxide solution prepared from Zn(NO₃)₂·6H₂O and 2-methoxyethanol. FT-IR spectroscopy showed the presence of Zn species in the alkoxide, with methoxyethoxide and nitrato groups as coordination ligands, indicating formation of Zn(NO₃)(OCH₂CH₂OCH₃). Smooth and homogeneous thin films were obtained by heat treating coating gel films in the temperature range from 250 to 500°C. The ZnO thin films exhibited a preferred growth of crystals with c-axis perpendicular to the Si substrate surface when fired at 250°C. It was discussed that the presence of nitrogen atoms in precursors had affected the phase development of crystals and was the basis of the structural relaxation for crystallization at low temperature.     

Electrochemical formation of a low-dimensional phase on the boundary between the LaF<Subscript>3</Subscript>:Eu<Superscript>2+</Superscript> single crystal and the metal

The kinetics of the formation of a new phase at the interface between the LaF₃:Eu²⁺ single crystal and the (Sn, Bi, or Sb) metallic electrodes was studied using potentiostatic transient measurements and voltammetry with a linear variation of voltage. A comparison of the theoretical and experimental reduced transients that define two-dimensional instantaneous nucleation on a plot of I/I _m vs. t/t _m and three-dimensional growth of the instantaneous and progressive types of nucleation on a plot of I ²/I _m² vs. t/t _m showed that the model was not fully consistent with the experiment. The dependence of the stationary current logI _A(max) of the potentiostatic transients on 1/η during the formation of the intermediate phase on the boundaries of LaF₃:Eu²⁺|Sn and LaF₃:Eu²⁺|Bi was found to be linear, which corresponds to two-dimensional nucleation and growth of the new phase.     

Step-like change of the growth rate of primarily nucleated centres in the film formation of camphor

The theoretical time dependence of the surface coverage θ_f is derived for the case of film formation according to two-dimensional instantaneous nucleation and growth with a step-like change in the growth rate. By means of double potential step experiments it is shown that the experimental time dependence of θ_f for the formation of a camphor film can be described by the model considered.     

One-step approach for nano-crystalline hydroxyapatite coating on titanium via micro-arc oxidation

Nano-crystalline hydroxyapatite (HAp) films were formed at the surface of Ti by a single-step micro-arc oxidation (MAO) using Ca²⁺ and P⁵⁺ ion-containing electrolytes. The HAp films were 10–25 μm thick, showing strong crystallinity dependence on the CaCl₂ concentration in the electrolytes. Also, the formation of an amorphous CaTiO₃ interlayer was identified to exist between the HAp and Ti substrates. In contrast to the previous researches using K₂HPO₄ for the electrolytes, KH₂PO₄ was used in this study, and this could allow the formation of the crystalline HAp layer. It is suggested as the most probable mechanism for the HAp formation that the high-density hydroxyl groups of TiO(OH)₂, formed by the reactions between the amorphous CaTiO₃ interlayer and the H⁺ ions from the dissolution of the KH₂PO₄, can play a key role in the nucleation and crystal growth of HAp by attracting Ca²⁺ and P⁵⁺ ions in the electrolytes.     

Preparation and phase transition characterization of VO2 thin film on single crystal Si (100) substrate by sol–gel process

Vanadium dioxide (VO₂) thin films were fabricated on single crystal Si (100) substrates by sol–gel method, including a process of annealing a vanadium pentoxide (V₂O₅) gel precursor at different temperatures. The crystalline structure and morphology of the films were investigated by XRD, FE-SEM and AFM, indicating that the films underwent the grain growth, agglomeration and grain refinement process with increased annealing temperatures. The film annealed at 500 °C exhibits the formation of VO₂ phase with a strong (011) preferred orientation and high crystallinity, the surface of the film is uniform and compact with a grain size of about 120 nm. Meanwhile, the film exhibits excellent phase transition properties, with a decrease of transmittance from 35.5 to 2.5% at λ = 25 μm and more than 3 orders of resistivity magnitude variation bellow and above the phase transition temperature. The phase transition temperature is evaluated at 60.4 °C in the heating transition and 55.8 °C in the cooling transition. Furthermore, the phase transition property of the VO₂ film appears to be able to remain stable over repetitive cycles 100 times.     

Influence of Vapor Phase Pretreatments of Epoxy Surfaces on the Nucleation of MOCVD Copper Thin Films

Vapor phase pretreatments of epoxy composite material reinforced with carbon fibers were carried out prior to the growth of Cu thin films by metal-organic chemical vapor deposition (MOCVD) using Cu (hfa)(COD) as copper precursor. These dry surface oxidation processes include H₂O/UV, O₂/UV and O₂/Plasma treatments. Oxygen plasma method is the most efficient to oxidize the surface and it has the greatest effect to improve the wettability of epoxy samples. As a consequence, the higher hydrophilicity of the plasma-modified epoxy surface induces a higher nucleation density in the Cu film. Furthermore, this treatment reduces drastically the induction period observed for the growth of the metal. Even though the O₂/UV pretreatment incorporates almost the same amount of oxygen in the epoxy surface than the plasma treatment, the functional groups are different, as revealed by XPS analyses, and the surface is less hydrophilic. Correlations between oxidation, wettability and nucleation density of the Cu films are discussed.     

Formation of cadmium sulfide (CdS) nanofilm on a Cd(OH)2/SiO2 precursor layer

Dense thin nanostructured films of cadmium sulfide CdS obtained by chemical deposition from aqueous solutions are strongly bound to a substrate due to the formation of cadmium hydroxide Cd(OH)₂ in the system. By X-ray reflectometry and grazing incidence diffraction it is found that at the beginning of the deposition a dense Cd(OH)₂/SiO₂ layer is produced on the surface of a silicon or glass substrate. This layer is formed through the atomic-layer adsorption of crystalline 1–3 nm thick Cd(OH)₂ film by the oxygencontaining substrate surface. During sulfidation of this cadmium-containing substrate layer, a surface nucleation layer of CdS/Cd(OH)₂/SiO₂ forms, which provides the growth, denseness, and strong adhesion to the substrate of nanostructured CdS film with a disordered structure. According to the obtained experimental data, a “hydroxide scheme” of film deposition is confirmed and refined, and the stages of CdS nanofilm formation are determined.     

Formation and performances of porous InVO4 films

Porous complex oxide films consisting of preferentially orientated orthorhombic phase of InVO₄ have been prepared using a novel simple method by pyrolysis of amorphous complex precursor. The formation and controlling of porous InVO₄ films can be easily obtained by modifying the calcination temperature. The pure orthorhombic InVO₄ phase can be obtained at a relatively lower temperature (500 °C), and the films are preferential orientation of the (200) face parallel to the substrate. The phase separation mechanism was suggested for the formation of porous films. Under visible light irradiation (λ>400 nm), porous InVO₄ films have shown the photocatalytic activity for photodegradation of gaseous formaldehyde, and can generate photocurrent. The electrochemical properties of the films with different crystal structure and pore structure were also investigated.     

The electrochemical behaviour of polycrystalline silver electrodes in Na2CO3 solution and the effect of ClO− 4 ions

The electrochemical behaviour of polycrystalline silver electrodes in Na₂CO₃ solutions was studied under potentiodynamic and potentiostatic conditions and complemented with X-ray diffraction analysis. Potentiodynamic E/i anodic curves exhibit active passive transition prior to an oxygen evolution reaction. The active region involves a small peak AI followed by a major peak AII before the passive region. Peak AI is assigned to the formation of an Ag₂O layer while peak AII is due to the formation of an Ag₂CO₃ layer. The height of the anodic peaks increases with increasing Na₂CO₃ concentration, scan rate and temperature. The effect of increasing additions of NaClO₄ on the electrochemical behaviour of Ag in Na₂CO₃ solutions was investigated. The perchlorate ions stimulate the active dissolution of Ag, presumably as a result of the formation of soluble AgClO₄ salt. In the passive region, ClO⁻ ₄ ions tend to break down the dual passive film, leading to pitting corrosion at a certain critical pitting potential. The pitting potential decreases with ClO⁻ ₄ concentration. Potentiostatic current/time transients showed that the formation of Ag₂O and Ag₂CO₃ layers involves a nucleation and growth mechanism under diffusion control. However, in the presence of ClO⁻ ₄ ions, the incubation time for pit initiation decreases on increasing the anodic potential step. Received: 3 July 1998 / Accepted: 10 March 1999     

Crystal growth in alumina/poly(ethylene terephthalate) nanocomposite films

The crystal growth and morphology in 150‐nm‐thick PET nanocomposite thin films with alumina (Al₂O₃) nanoparticle fillers (38 nm size) were investigated for nanoparticle loadings from 0 to 5 wt %. Transmission electron microscopy of the films showed that at 1 wt % Al₂O₃, the nanoparticles were well dispersed in the film and the average size was close to the reported 38 nm. Above 2 wt % Al₂O₃, the nanoparticles started to agglomerate. The crystal growth and morphological evolution in the PET nanocomposite films kept at an isothermal temperature of 217 °C were monitored as a function of the holding time using in situ atomic force microscopy. It was found that the crystal nucleation and growth of PET was strongly dependent on the dispersed particles in the films. At 1 wt % Al₂O₃, the overall crystal growth rate of PET lamellae was slower than that of the PET homopolymer films. Above 2 wt % Al₂O₃, the crystal growth rate increased with nanoparticle loading because of heterogeneous nucleation. In addition, in these PET nanocomposite thin films, the Al₂O₃ nanoparticles induced preferentially oriented edge‐on lamellae with respect to the surface, which was not the case in unfilled PET as determined by grazing‐incidence X‐ray diffraction. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 747–757, 2007     

Kinetics of precipitation and growth of thin indium(III) sulfide films

Thin films of indium(III) sulfide have been prepared by chemical precipitation from aqueous solutions containing indium(III) nitrate, thioacetamide, tartaric acid, and hydroxylamine hydrochloride at 333–368 K. Kinetics of In₂S₃ precipitation and the films growth under conditions of spontaneous formation of the solid phase in the solution has been studied. Formal rate law of indium(III) sulfide formation accounting for the partial orders of In₂S₃ precipitation with respect to the system components and the process activation energy has been derived. The effects of the reaction mixture composition, temperature, and the synthesis duration of In2S3 films growth have been studied.     

Effect of silver on phase separation and crystallization of niobium oxide containing glasses

Effect of silver introduction in sodium phosphate and sodium borophosphate glasses containing large amount of niobium oxide have been investigated using differential scanning calorimetry and XRD. Same sodium niobate phase in the Nb₂O₅-NaNbO₃ based solid solution have been observed following two heat treatments designed for nucleation and growth of the crystalline phase. Silver introduction in the glass composition is clearly responsible for increasing the crystallization rate. Its effect after nucleation and crystallization treatments has been shown. Phase metastable separation is occurring during heat treatment with formation of a phosphate rich and niobium rich phase. Crystallization effect on optical transparency of glasses and on Raman scattering response have been investigated.     

Voltammetric,chronoamperometric and chronoabsorptometric studies of the nucleation of n-heptyl viologen films on SnO2, silane-modified SnO2 and ion-beam-treated ito-metallized polymer films

Nucleation of the first monolayers of the n-heptyl viologen-bromide or-biphthalate films on SnO₂ electrodes was found to be accelerated by the addition of one molecular layer of dimethyldiethoxy (dmde)-silane to the SnO₂ surface. Partitioning of the reactant viologen into the non-polar surface layer and lowering of the surface tension against nucleation site growth in the silane layer may both be responsible for the enhanced nucleation rates. Similar acceleration of the nucleation of n-HV⁺ was found on ion-beam-etched indium-tin oxide-metallized polymer electrodes (MPOTE). A second nucleation site with a much lower, and distinguishable rate of nucleation, was produced on this type of electrode surface. Chronoamperometry and chronoabsorptometry were used to confirm that the nucleation of n-heptyl viologen cation-radical salts follows an instantaneous rather than progressive nucleation mechanism, on any of the surfaces studied.     

Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures

In order to examine whether monolayer or sub-monolayer extents of surface oxidation can be realized experimentally at Ni prior to onset of bulk-phase oxide formation (as they can for example at Pt, Ru or Au already at room temperature), cyclic voltammetric experiments down to low temperature (−90° C) have been conducted on Ni in solutions of NaOH in 80 mol% methanol with water. The cyclic voltammograms for the first stage of Ni oxidation to α-Ni(OH)₂, and its reduction, show that extents of surface oxidation down to an equivalent monolayer, or less, of Ni(OH)₂ can be realized at sufficiently low temperatures. However, even at these low levels of oxidation of the metal, irreversibility between the processes of Ni oxide formation and reduction is maintained in a way characteristic of the behavior of three-dimensional oxide films. It therefore appears that even at low levels of surface oxidation of Ni which are attainable at low temperature, the oxidation mechanism involves nucleation and growth of the oxide in islands rather than an initial surface-chemical process of OH or O array formation, as at Pt or Au. However, no indications of a dissolution-and precipitation type of oxide formation process, which would involve mass-transport in solution, are given by the present results obtained from experiments in dilute alkali at low temperatures, and at the rotating Ni disc electrode.