Kinetics Of Phase Formation Research Articles

Phase formation and densification peculiarities of Y3Al5O12:Nd3+ during reactive sintering

Processes of phase formation and densification during reactive sintering of YAG:Nd3+ laser ceramics have been studied through the 2.88Y2O3–0.12Nd2O3–5Al2O3 model powder system with a bimodal yttria particle size distribution. It has been found that utilization of submicron yttria powders can provide a competitive advantage of shrinkage over swelling processes by changing the kinetics of YAG phase formation. Excluding the swelling effect during densification allows one to produce YAG:Nd3+ (4 at%) laser ceramics by reactive sintering with in-line optical transmission of about 80% at λ=650 for a 1mm thick sample.

Features of the formation of cubic BCN phases in comparison with natural and synthetic polycrystalline diamonds

A study of the boron-carbon-nitrogen system was carried out to synthesize new materials and examine phase relations. Crystalline graphite- and diamond-like BCN phases were synthesized by thermobaric treatment of mixtures of boron with carbon nitride or melamine powders. These BCN phases crystallized in the basic lattice of boron nitride with statistically distributed carbon atoms. New structural modifications were discovered by studying the kinetics of phase formation. The hexagonal lattice of a graphite-like phase is transformed by a diffusionless mechanism into a cubic structure at pressures of around 6.5 GPa. A diamond-like BCN phase was synthesized in the form of individual crystals or polycrystalline aggregates. The microstructure of these aggregates is comparable to natural and synthetic carbonado-type diamond. The obtained results argue in favour of the hypothesis that natural carbonados formed in the Earth’s mantle.

Effect of BN-Added Precursors on Phase Formation and Transport Properties of (Bi,Pb)-2223 HTS

(Bi,Pb)-2223 HTSs (high temperature superconductors) were synthesized from nominally pure (reference) and BN-added Bi1.7Pb0.3Ca2Sr2Cu3O y (BN) x precursors (x=0,0.10,0.15, and 0.20) by the solid state reaction method using alumina crucibles. The influence of boron nitride addition on the phase formation kinetics and transport properties of (Bi,Pb)-2223 HTSs was studied using X-ray diffraction (XRD), resistivity and critical current density measurements. BN-added compounds reveal a significant enhancement in both the high-T c 2223 phase formation and critical current density compared to the reference specimen.

Preparation and Characterization of Mg1−x B2 Bulk Samples and Cu/Nb Sheathed Wires with Low Grade Amorphous Boron Powder

MgB2 bulk and wire samples were prepared using cheap, low grade amorphous boron powders. Based on chemical analysis performed on the starting reagents, three nominal stoichiometries were studied. It was found that the structural and superconducting properties of the bulk samples were not affected by the composition, but that residual Mg was left in the wires for the nominal MgB2 composition. In contrast, slightly Mg-deficient compositions were free from residual Mg and exhibited higher critical current densities. The MgB2 phase formation kinetics was not influenced by the variations in the nominal powder composition.

Formation of Pd2Si on single-crystalline Si (100) at ultrafast heating rates: An in-situ analysis by nanocalorimetry

The kinetics of intermediate phase formation between ultrathin films of Pd (12 nm) and single-crystalline Si (100) is monitored by in-situ nanocalorimetry at ultrafast heating rates. The heat capacity curves show an exothermic peak related to the formation of Pd2Si. A kinetic model which goes beyond the conventional linear-parabolic growth to consider independent nucleation and lateral growth of Pd2Si along the interface and vertical growth mechanisms is developed to fit the calorimetric curves. The model is used to extract the effective interfacial nucleation/growth and diffusion coefficients at the unusually high temperatures of silicide formation achieved at very fast heating rates.

Growth kinetics of the Cu3Sn phase and void formation of sub-micrometre solder layers in Sn–Cu binary and Cu–Sn–Cu sandwich structures

Ultra-thin (sub-micrometre thickness) solder layer samples: Sn–Cu binary and Cu–Sn–Cu sandwich structures (for chip-stacking interconnections) were fabricated by electroplating. Reflow was conducted with a peak temperature of 265°C and the aging was conducted with a temperature of 120°C for up to 500h. The special interfacial phenomena were investigated and the evolution of the intermetallic compounds (IMCs) was studied. The experimental results showed that: solder materials were totally converted into IMCs when the solder thickness was under 1.0μm. After the depletion of Sn, the Cu3Sn phase kept developing based on the consumption of the Cu6Sn5 phase. Thus a drop of the Cu6Sn5 layer thickness was observed. Microvoids formed in IMC layers, at the grain boundaries, especially at the triple junctions. For samples without a proper cleaning process, the morphology of the IMC changed into prism-like crystallites.

Effect of Mixed Thermodynamic and Kinetic Hydrate Promoters on Methane Hydrate Phase Boundary and Formation Kinetics

In this communication, formation kinetics and stability of methane hydrate in mixtures of (propanone + sodium lauryl sulfate (SDS)), as thermodynamic and kinetic hydrate promoter, have been studied. Results show that both the induction time and initial formation rate have been increased in the presence of propanone and the mixture of (propanone + SDS). Propanone and mixtures of (propanone + SDS) solutions decrease the self-preservation at low pressures and temperatures below the ice point. Moreover, the presence of propanone decreases the storage capacity of methane hydrate. The addition of SDS, however, increases the experimental storage capacity in comparison with the theoretical value.

Phase Transition Analysis between LiFePO4 and FePO4 by In-Situ Time-Resolved X-ray Absorption and X-ray Diffraction

In-situ time-resolved X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are applied to investigate the phase transition mechanism between LiFePO4 and FePO4. Phase formation kinetics is discussed by using the phase fractions of LiFePO4 and FePO4 directly obtained from the XRD measurement and the amount of the passed current synchronized by the XRD measurement with Kolmogorov-Johnson-Mehl-Avrami (KJMA) analysis. For both of the lithium extraction and insertion process, the KJMA analysis from the XRD and amount of the passed current data indicate that the phase formation of LiFePO4 and FePO4 proceeds via one-dimensional phase boundary movement. For the lithium extraction process, the two phase coexistence of LiFePO4 and FePO4 is observed under the potential step conditions, and the KJMA plot based on the passed current data is relatively in good agreement with the one based on the XRD data. On the other hand, for the lithium insertion process, considerable peak boarding which implies intermediate state formation during the phase transition occurs, and the LiFePO4 phase formation is delayed compared with the amount of the passed current, especially in the case of small particle size. These results indicate that the phase transition behavior during lithium insertion is different from that of the typical two phase reaction.

Kinetics of anatase phase formation in TiO2 films during atomic layer deposition and post-deposition annealing

Anatase phase formation in TiO2 films obtained by atomic layer deposition (ALD) is investigated. At growth temperature close to 200 °C, the anatase phase of TiO2 originates from crystalline seeds formed in an amorphous layer. These seeds, formed in the initial stages of a film growth, allow an expansion of the anatase phase in the amorphous parts and their transformation. This expansion occurs either during a further growth process or during a post-deposition annealing at relatively low temperatures (160–220 °C). The process of a lateral expansion of the anatase phase within the amorphous one was found to be thermally activated with an activation energy of 1.5 eV.

Kinetics of stabilized cubic zirconia formation from MnO2–ZrO2 diffusion couple

Microstructure and kinetics of the cubic phase formation are studied in ZrO2–MnO2 diffusion couple. During annealing in Ar, no other compounds are detected except the zirconia solution. With diffusion of Mn ions from manganese oxide to ZrO2, the cubic zirconia are found at the phase interface. The growth kinetics of the cubic zirconia solution layer in the submicrometer scale are analyzed. The growth behavior of the cubic zirconia layer is better described by the grain boundary diffusion model, and our experiment data exhibit a t 1/4 dependence on time. The deviation from the experiment data to the theoretical value is also explained.

Carbon sphere influence on textural properties and bioactivity of mesoporous bioactive glass/hydroxyapatite nanocomposite

Mesoporous bioactive glass/hydroxyapatite nanocomposite (MBG-HA) synthesis was conducted through evaporation-induced self-assembly (EISA) method followed by in situ carbonization, with non-ionic block co-polymer as mesoporous template and glucose-derived carbon sphere as co-template. The mixture of different carbon sphere contents into a glass network in the CaO–SiO2–P2O5 system tailored the structural, morphological, and textural properties of MBG–HAs. Based on the preliminary results, the carbon sphere content and textural and structural parameters of as synthesized MBG–HAs showed a negative trend. The MBG–HA 0.5 with additional low carbon sphere content, showed a high surface area and large pore volume. The inclusion of HA nanoparticles inside the channels strongly influenced the high carbon sphere content of the MBG–HA8 mesoporous structure. For in vitro bioactivity tests, MBG–HAs with higher textural parameters possessed a faster apatite phase formation kinetics following the sequence MBG–HA0.5>MBG–HA2>MBG–HA5>MBG–HA8.

The role of Laves phase on microstructure evolution and creep strength of novel 9%Cr heat resistant steels

The influence of Laves phase (type Fe2W) formation and growth kinetics on the creep strength at 650 °C of two different 9%Cr heat resistant steels of the authors' design was investigated. The microstructure evolution was characterized using transmission electron microscopy in the scanning mode (STEM). Kinetic modeling was carried out using the software DICTRA. STEM investigations revealed that the Laves phase precipitates tend to form clusters, have an irregular shape and are often located close to M23C6 carbides, along martensite lath boundaries or sub-grain boundaries. DICTRA simulations showed that the growth kinetics of Laves phase was high in the first thousand hours of creep, reaching its equilibrium volume fraction after 7000–10,000 h. Simultaneous competitive growth of M23C6 carbides and Laves phase was simulated showing that Laves phase grows very slowly to reach the final equilibrium phase fraction only after almost 13,000 h, while M23C6 reached the equilibrium phase fraction already during tempering. Best creep results were obtained for the 9%Cr alloy with low carbon content, reduced sub-grain growth, very slow coarsening of MX carbonitrides and slow growth of Laves phase.

Structure and Composition Analysis of Cu(In,Al)Se2 Thin Films Prepared by Rapid Thermal Selenization

Thin films of CuIn1-xAlxSe2 (CIAS) were prepared by stacked elemental precursor layers in an inert ambient. The stacking sequence of precursor layers may affect the kinetics of phase formation. The soda lime glass (SLG)/Cu/Al/In/Se sample heated at 750 °C for 30 s with a temperature ramp rate of 15 °C/s may react to form single-phase CIAS thin films with a chemical composition fairly close to the predetermined value. Transmission electron microscopy (TEM) analysis of a selenized film prepared at 600 °C revealed the segregation of the content of Al to the substrate side and a relatively large variation of Al distributed in a lateral direction as compared with those of other elements. Changing the Cu and Al layer sequence may affect the reaction paths and lead to the formation of a mixture of two CIAS quaternary phases with different compositions.

Kinetics of Phase Formation in Solid‐Gas Reactions: Iron and Arsenic

Kinetics of Phase Formation in Solid‐Gas Reactions: Iron and Arsenic

High performance yttrium-doped BSCF hollow fibre membranes

Oxygen production from BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3−δ) and yttrium-doped BSCF (Ba0.5Sr0.5Co0.8Fe0.175Y0.025O3−δ) hollow fibres was investigated, and the role of yttrium in the crystal structure was further explored using high-temperature X-ray diffraction. Yttrium substitution acted to increase the oxygen flux significantly, from 4.9 to 7.0mlcm−2min−1 at 900°C for the BSCF and the BSCFY membranes, respectively. Permeation was particularly enhanced at lower temperatures, between 66% and 92% over the range 650–800°C. The lattice expansion determined from high temperature X-ray diffraction measurements in air was similar for both compositions, suggesting that the higher oxygen fluxes obtained for BSCFY hollow fibres could be attributed to the higher non-stoichiometry due to yttrium addition to the BSCF crystal structure. In addition, the improvement of oxygen fluxes for small wall thickness (∼0.3mm) hollow fibres operating below the critical length (i.e. limited surface kinetics regime) indicates that yttrium has enhanced the surface exchange rates. XRD patterns showed split peaks around 2θ 31° and 56° above 200°C, likely corresponding to a coexisting hexagonal perovskite phase. This peak-splitting was more pronounced for BSCFY, suggesting that the kinetics of the hexagonal phase formation may be faster for the yttrium-doped perovskite. The lattice volume of BSCFY expanded more than BSCF when exposed to nitrogen at 900°C, confirming a higher release of oxygen and enhanced oxygen non-stoichiometry.

Kinetics of aging of the Cu-Be alloy with different beryllium concentrations in an external constant magnetic field

The data are reported on the microhardness and small-angle neutron scattering of samples from specially melted copper-berillium alloys (the Be concentrations are 0.5, 1.0, and 1.6 wt %; the rest is Cu) aged at a temperature of ∼580 K in the dc magnetic fields ∼5.3 and 7.0 kOe and with no field. It has been established that magnetic field substantially influences the kinetics of phase formation during aging.

Synthesis of a hydrogen producing nanocrystalline ZnFe2O4 visible light photocatalyst using a rapid microwave irradiation method

A rapid microwave solid-state synthesis method is systematically investigated to achieve a H2 producing visible light active spinel photocatalyst. ZnFe2O4 nanocrystallites were obtained by microwave irradiation of precursor compacts under optimized conditions. This investigation led to a uniform sized nanocrystalline photocatalyst that yielded a quantum-yield of H2 evolution ∼3.8 times higher than that of conventionally synthesized ZnFe2O4. The synthesis parameters – microwave power, synthesis temperature, and time, were found to control the physico-chemical properties viz phase formation kinetics, phase purity, crystallinity, specific surface area and photochemical efficiency, of the synthesized photocatalyst. The study reveals that the threshold microwave power of ≥3 kW was necessary to obtain a spinel phase structure, while lower power (<3 kW) could not induce the crystallization even after prolonged low-power irradiation of 180 min. At the threshold power, a minimum of 10 min. synthesis time was enough to obtain uniform sized nanocrystallites, indicating that the synthesis method is ∼24 times faster than the solid state reaction method, which needs nearly 4 h. The particle morphology evolution with irradiation time from 10–150 min. exhibited de-crystallization phenomena. Longer irradiation displayed a morphological crystallization probably induced due to the simultaneous area and volumetric heating effect. The possible “formation mechanism” of these uniform nanocrystallites has been presented here for qualitative understanding. Thus synthesized photocatalysts generated hydrogen from a water–methanol mixture even without the co-catalyst loading. The ferrite photocatalyst was found to decolorize methylene blue dye with a maximum decay constant of 0.232 h−1, thereby demonstrating its capability in the pollutant decomposition applications, all under visible light photons.

Microstructural Characterisation of Copper-Stainless Steel Brazed Joint with Copper Based Filler Metal

Copper to stainless steel brazed components are used in the thrust chamber of cryogenic propulsion system of Indian Space Programme. Temperature and time play a major role in the optimization of brazing cycle. An effort has been made to study the influence of the above parameters and correlate with the microstructure and shear strength achieved. In the present work, brazing was performed using Cu based filler metal. Temperature selected was within the range of 1010-1045 °C for two different time durations (20 minutes and 30 minutes) under high vacuum (10-5 Torr). Extensive scanning electron microscopy with energy dispersive spectrometry was carried out to understand the kinetics of elemental diffusion and phase formation at and near the brazed joint. Microhardness was evaluated across the joints to facilitate further understanding of the joint chemistry. The optimized shear strength of the joints was as high as 155 MPa for the joint brazed at 1030 °C for 30 minutes against the required strength of 150 MPa. The joint could retain strength at cryo temperature and meet the minimum expected value of 320 MPa. At the same time, it exhibited good mechanical properties at elevated temperatures. This paper brings out details of the investigation carried out.

Kinetics of phase formation upon the treatment of La2(SO4)3 and La2O2SO4 in a hydrogen flow

The sequence of phases occurring during treatment of lanthanum sulfate, La2(SO4)3 and lanthanum oxysulfate, La2O2SO4 in a hydrogen flow is established. The temperature ranges in which homogeneous La2O2S is produced are revealed: when La2(SO4)3 is a precursor, the range is 770–1220 K; in the case of La2O2SO4, the interval is 950–1220 K. The kinetic curves showing the time dependence of the yield of La2O2S is constructed and treated using the Avrami-Erofeev and contracting volume equations. The activation energies of the reactions are determined.

Anatase phase formation kinetics in Ti and TiOx nanoparticles produced by gas-phase condensation

Anatase TiO2 nanoparticles were successfully synthesized by post-heat treatments of partially crystalline Ti and amorphous TiOx nanoparticles, respectively produced by inert gas condensation and subsequent oxidation. The nanoparticles condensed on a liquid-nitrogen containing cooling finger (sample LN) were identified to be partially crystalline Ti phase with ~10–20vol.% amorphous TiOx. On the other hand, those condensed on a room-temperature cooling finger (sample RT) were almost completely amorphous TiOx phase. Differential scanning calorimetry scan curves of as-oxidized samples were interpreted using Kissinger analysis, the non-isothermal kinetics, and activation energy for the anatase formation was determined as ~455 and 865kJ/mol for samples LN and RT, respectively. As-oxidized samples LN and RT were heat treated at 400°C for 2h, respectively (samples LN-H and RT-H). Samples LN-H and RT-H showed the onset of UV–visible light absorption near 400nm and the optical band gap of 3.12 and 3.21eV, respectively, corresponding to anatase. The sample LN-H showed faster photocatalytic decomposition of methylene blue and rhodamine B dyes compared to the sample RT-H due to high crystallinity of anatase and rutile phases.