Sequence Of Phase Evolution Research Articles

Optimization of precursor structure by dispersants to promote nitrogen reduction process to prepare high-quality VN

In this study, three dispersants of polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG) and citric acid (CA) are used to optimize the structure of the precursor in order to prepare high quality VN. Under optimum condition, the VN with nitrogen content of 17.94 % is prepared by adding 5 % PVP. The sequence of phase evolution from the precursor to VN is: APV → V2O5 → V6O13 → V7O13 → VO2 → V3O5 → V2O3 → (VC) → VN. The precursors of adding 5 % PVP have lower Ea of 0.17 kJ mol−1, 20.51 kJ mol−1 and 62.70 kJ mol−1 during reduction and nitration process, which is easier to be reduced and nitridated. PVP not only promotes the uniform dispersion of the carbon powders in the vanadium rich solution but also enhance the hydrogen bonding of APV ions to promote their nucleation and growth. The precursor with uniform and concentrated particle size distribution, complete and stable encapsulation of shell of APV and core of carbon powder, and uniform and moderate thickness of APV shell is successfully prepared by adding PVP. The precursor has a larger phase reaction interface and more reactive sites during the nitriding reduction process, which results in a faster reduction and nitriding rate. In comparison with the current process, the nitriding reaction temperature has been reduced from 1400 ∼ 1500 °C to 1150 ∼ 1200 °C, the reaction time has been shortened by 75 %, and the N2 consumption has been reduced by 40 %.

Combined effects of the deposition temperature and metal electrodes on ferroelectric properties of atomic-layer-deposited Hf0.5Zr0.5O2 films

The combined effects of the atomic-layer-deposition (ALD) temperature (220 °C–280 °C) and metal electrodes (TiN and Mo) on the ferroelectric properties of Hf0.5Zr0.5O2 films were studied. Regardless of the metal electrode, a tetragonal–orthorhombic–monoclinic phase evolution sequence was observed with increasing ALD temperature after post-metallization annealing. However, the phase transition temperature slightly changed depending on the metal electrode, which was predetermined based on the as-deposited states. Additionally, the out-of-plane orientation of the final orthorhombic grains was highly dependent on the crystallographic alignment of the metal electrode grains, resulting in different values of the maximum remanent polarization. In terms of long-term reliability, the fatigue characteristics were highly dependent on the electrode characteristics, and the wake-up characteristics were mainly affected by the initial phase distribution determined by both the ALD temperature and metal electrode.

Hierarchical phase evolution during direct laser deposition of an in-situ Ni-NbC composite

Direct laser deposition of a mixture of 90 wt.% nickel (Ni) and 10 wt.% niobium-carbide (NbC) using the laser engineered net shaping process resulted in the formation of an in-situ Ni-NbC composite with a complex hierarchical multi-phase microstructure. Detailed microstructural characterization at multiple length scales revealed that the in-situ composite consisted of a primary dendritic Ni-rich γ phase with an interdendritic γ + NbC eutectic. The interdendritic eutectic γ phase further decomposes via discontinuous precipitation into a lamellar mixture of γ + δ phase (Ni3Nb). The sequence of phase evolution in this complex hierarchical in-situ composite system has also been established in the present study.

Revealing complex microstructure formation mechanisms in the Al[formula omitted]Cr[formula omitted]Fe[formula omitted]Ni[formula omitted]Ti[formula omitted] compositionally complex alloy

The complex phase evolution sequence of a promising new high temperature structural alloy with composition Al20Cr20Fe35Ni20Ti5 (at.%) has been studied in situ using an electromagnetic levitation (EML) setup combined with time-resolved high energy X-ray diffraction (XRD), as well by scanning transmission electron microscopy (STEM) observations during in situ heating. These experiments were supplemented by both scanning electron microscopy (SEM) and high resolution STEM observations of the as-cast alloy microstructure, as well as by energy dispersive X-ray spectroscopy (EDS) composition measurements. Differential scanning calorimetry (DSC) was used to identify liquidus, solidus, A2 ↔ B2, and B2 ↔ L21 transition temperatures. The results obtained by these complementary techniques indicate that the alloy forms primary dendrites of A2-structured body-centered cubic (bcc) phase initially from the melt, followed by ordered bcc of B2-structure in interdendritic regions once compositional segregation has sufficiently enriched the remaining liquid. Ordered B2 precipitates form within the primary A2-structured dendrites and the interdendritic B2 regions undergo a continuous decomposition reaction into a maze-like arrangement of A2 and B2/L21 phases below the solidus. Below approximately 1380 K, the B2 domains in both dendritic and interdendritic regions likely undergo a continuous ordering transition to the L21 phase. The experimental observations of Al20Cr20Fe35Ni20Ti5 are found to be in good agreement with CALPHAD predictions but suggest that the solubility limits of the L21-Heusler phase in the database should be relaxed to include greater Fe solubility for the Ni sublattice and to consider deviations in order parameter from stoichiometric ordering.

Electric field‐assisted solid‐state reaction of BaCO3‐TiO2 system

AbstractWe report phase transition process during the solid‐state reaction of BaCO3‐TiO2 system under the assistance of electric field. Experiments were conducted at a constant heating rate with preset field strength and current limit. Solid‐state reaction was completed upon reactive flash sintering taking place at ~1002℃ under 200 V/cm. Hexagonal BaTiO3 phase, which rarely occurs at such temperatures, was obtained after flash sintering at a current density of 23.5 mA/mm2. It is speculated that oxygen deficiency during flash sintering triggered cubic‐to‐hexagonal transition of BaTiO3. Furthermore, X‐ray diffraction results show that solid‐state reaction takes place prior to flash sintering. Electric field could accelerate the reaction but did not alter the sequence of phase evolution.

In-situ synchrotron investigation of the phases- and their morphology-development in Mg–Nd–Zn alloys

Abstract The addition of Zn to the Mg–Nd system improves the yield strength and creep resistance, however its influence on the intermetallic phases in the ternary system is not yet fully understood. Understanding the sequence of phase-formation and phase-evolution during solidification and processing is essential to microstructure design. The solidification was investigated with in-situ synchrotron radiation-diffraction and tomography during cooling from the molten state to 200°C to investigate the phase-formation and transformation characteristics. The solidification starts with α-Mg followed by two distinct intermetallic phases T2 and T3. The results suggest that Zn stabilizes the Mg3Nd phase and accelerates precipitate formation. The dendritic morphology changes during solidification towards coarser shapes, thus impedes feeding and promotes hot tearing.

Thermal stability and crystallisation behaviour of amorphous alumina-yttria synthesised by co-precipitation and combustion synthesis

Amorphous alumina-yttria powders with 20, 25, 30 and 37.5 mol% yttria were synthesised by a co-precipitation method using the respective nitrates as precursors for identifying a composition with the best amorphous phase stability. Thermal analysis showed that calcination to yield amorphous oxides completed at around 800 °C. Crystallisation behaviour investigated by x-ray diffraction (XRD) showed that Al2O3–25 mol% Y2O3 (25Y) had the highest crystallisation temperature of 900 °C. Crystallisation involved the appearance and disappearance of metastable hexagonal yttrium aluminate (H-YAlO3) below 1000 °C. The phase evolution sequences for all the compositions were studied in detail. All the phases were nanocrystalline, establishing the efficacy of amorphous phase crystallisation as a viable route for the synthesis of nanocrystalline ceramics. Solution combustion synthesis (SCS) was selected to synthesis 25Y with enhanced amorphous phase stability using different proportions of citric acid and ethylene glycol. Thermal analysis revealed that the powder was free of organic residue after heat treating at 650 °C. The powder produced with 10 vol% of fuel (citric acid + ethylene glycol) yielded the highest crystallisation temperature of 925 °C and the appearance and disappearance of H-YAlO3 between 950 and 1100 °C. Fourier transform infrared (FT-IR) spectroscopy revealed that the co-precipitation synthesis might have residual NH4+ and NO3- radicals whereas SCS did not contain any residual carbonate above 700 °C.

In-situ synthesis and characterization of Fe – TiC based cermet produced from enhanced carbothermally reduced ilmenite

In-situ synthesis of Fe-TiC based cermet by carbothermic reduction of South African ilmenite ore has been successfully achieved. This was carried out in an argon containing atmosphere in the temperature range of 850–1350 °C. Powder mixtures of ilmenite concentrate and graphite in molar ratio of 1:4 respectively, were milled for 2 h in a planetary mill (PM 100) to obtain intimate mixtures. The mixtures were then synthesized in a laboratory high temperature furnace at different reduction temperatures. The obtained X-ray diffraction results showed that Fe-TiC has been successfully formed at 1350 °C with 15 min holding time. The sequence of phase evolution showed a stepwise reduction of ilmenite FeTiO3 initially to form a mixture of Fe and TiO2, which was successively further reduced to Ti4O7, Ti3O5, Ti2O3 and finally TiC. Three or more major peaks were observed from thermo-analyses (TG, DTG and DTA) corresponding to the initial reduction of FeTiO3 at 983.3 °C, Ti4O7 at 1050 °C, Ti3O5 at 1200 °C and finally TiC at 1350 °C. Field Emission SEM coupled with elemental analysis showed a clear phase boundary between the metallic phase (Fe) and the ceramic –TiC in the cermet synthesized at 1350 °C.

Synthesis and sintering of Ti3GeC2 MAX phase powders

Ti3GeC2 is one of the potential MAX phase compounds, for which, production and processing methods are yet to be optimized. There are very few reports on the synthesis of this compound, which are mostly in bulk form and contain large impurities. Present investigation attempts to synthesize Ti3GeC2 powder with improved phase purity and to understand phase evolution sequences. Synthesis was characterized as a liquid-solid reaction process; initiated after melting of Ge particles. At low temperature (~1200°C), yield was found to be very low, and the product had several intermetallics and carbides. At relatively higher temperature (at 1500°C), the conversion was found to be very high and particles were mostly finer than 10µm. Phase purity was further increased (over 96wt%) during sintering stage, as observed through x-ray diffraction analysis where impurity peaks were very small. Ti3GeC2 powder was found to exhibit poor densification behavior during pressureless sintering; however, it showed slow grain growth characteristics too, a highly desired feature for structural applications.

Yellow-emitting (Tb1−xCex)3Al5O12 phosphor powder and ceramic (0≤x≤0.05): Phase evolution, photoluminescence, and the process of energy transfer

(Tb1-xCex)3Al5O12 yellow phosphors (0≤x≤0.05) were calcined from their coprecipitated carbonate precursors, and the effects of the temperature and atmosphere (air and H2) of calcination on the sequence of phase evolution and the characteristics of the powders were investigated in detail. The activation energy for the crystallite growth during calcination was estimated to be ~39kJ/mol. The powder calcined at 1000°C showed good reactivity and was sintered into a ceramic plate of ~97% dense (average grain size: ~1.3µm) in a H2/Ar gas mixture at the relatively low temperature of 1500°C. The phosphors simultaneously exhibit the 4f8→4f75d1, 7F6→5D3 and 7F6→5D4 excitations of Tb3+ and the 4f1→5d1 excitation of Ce3+ when monitoring the yellow emission of Ce3+ at 560nm, suggesting the presence of efficient Tb3+→Ce3+ energy migration. The optimal Ce3+ content for luminescence was found to be x=0.015 and 0.01 under the direct excitation of Ce3+ and through Tb3+→Ce3+ energy transfer, respectively, and concentration quenching of luminescence was analyzed to be resulted from exchange interaction. Luminescence features of the phosphors, including excitation, emission, quantum yield, fluorescence lifetime, color coordinates and color temperature, were thoroughly investigated against the processing temperature and Ce3+ content, with an in-depth discussion on the process of energy transfer among the optically active Tb3+ and Ce3+ ions. The materials may find application in blue-light excited white LEDs.

Sequence of phase evolution during mechanically induced self-propagating reaction synthesis of TiB and TiB2 via magnetically controlled ball milling of titanium and boron powders

Mechanically induced self-propagating reaction synthesis of titanium boride and diboride was investigated for elemental mixtures of titanium and amorphous boron in starting atomic ratios of Ti50B50 and Ti34B66. Samples were milled for different periods leading up to and after an ignition point which was determined by in-situ monitoring of the mill temperature. Methods of X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy were used to determine product evolution.For both compositions, a partial crystallisation of amorphous boron commences during the early stages of milling and progresses through to the ignition point. At later stages and prior to ignition, HRTEM combined with electron diffraction revealed the formation of nanocrystalline titanium diboride, which was confirmed by Raman spectroscopy to be of an off-stoichiometric, boron-lean composition; TiB2-x. This phase is believed to have formed by solid-state reaction at the interface of the heavily deformed Ti and amorphous B via a mechanism which parallels the formation of nanocrystalline off-stoichiometric Ti1+xC1-x prior to the self-propagating synthesis of TiC from Ti and graphite. After ignition, the product for Ti34B66 comprised relatively large facetted grains of TiB2, liquid phase sintered by thin regions of unreacted Ti which had melted due to high heat of reaction. In contrast, the product of Ti50B50 comprised platelike TiB particles, and spheroidal dispersions of TiB2 in unreacted Ti and B. Likely mechanisms for the different product evolution routes for the two compositions are proposed and discussed.

Preparation of high quality ferrovanadium nitride by carbothermal reduction nitridation process

High quality ferrovanadium nitride was prepared successfully by carbothermal reduction nitridation process by using V2O5 and Fe as the raw materials. The effects of reaction temperature and C/O molar ratio (content of O is defined as the oxygen in V2O5) on the quality of ferrovanadium nitride were investigated in detail. It was demonstrated that C/O molar ratio had a great influence on the residue carbon, as well as the nitrogen content in the final products. It was found that the nitrogen content of the final product achieved 11.52% when the C/O molar ratio was 90% of the stoichiometric value. However, the nitrogen contents of the final products decreased with the further increase of C/O molar ratio. For the temperature higher than 1773 K, Fe became liquid phase and surrounded the solid V(C, N, O) particles tightly. The formation of liquid Fe was beneficial for the further removal of oxygen atoms left in the V(C, N, O) lattice, since it provided a liquid tunnel for carbon diffusion which greatly enhanced the reaction kinetics. From the results of X-ray diffraction (XRD) and thermodynamic calculations, phase evolution sequence of V2O5 can be obtained as: V2O5?VO2?V2O3?V(C, N, O). Moreover, the kinetic process from V2O3 to VN was studied and the results shown that the reaction rate can be described by Jander diffusion model with the extracted activation energy of 188.173 kJ/mol.

A novel method to synthesize submicrometer vanadium carbide by temperature programmed reaction from vanadium pentoxide and phenolic resin

Submicrometer vanadium carbide (V8C7) can be synthesized from V2O5 encapsulated in phenol resin. The precursor powders were produced by coating V2O5 powders with phenolic resin shells. Then the precursor powders were roasted at various temperatures from 1173K to 1473K for 2h in a flowing Ar atmosphere. The decomposed carbon from phenolic resin could be used to reduce V2O5. The samples obtained with different conditions were characterized by X-ray diffraction (XRD), Field-emission scanning electron microscope (FE-SEM), Transmission Electron Microscope (TEM), and Energy Disperse Spectroscopy (EDS). The results show that phase evolution sequences from V2O5 are V2O5→VO2→V2O3→V6C5→V8C7. The single phase of V8C7 can be prepared as the molar ratio of V2O5 to phenol resin is 1:1.2 at the temperature higher than 1373K. The prepared V8C7 particles are about 300nm in size.

The formation of crystalline phases in metakaolin-based geopolymers in the presence of sodium nitrate

Geopolymers generated by alkali-activation of amorphous aluminosilicate sources are considered as an alternative immobilizing matrix for hazardous and nuclear wastes. Low and intermediate level nuclear waste streams are often highly alkaline saline solutions containing various concentrations of nitrate salts. The aim of the research project presented here was to study the effect of nitrate ions on the formation and evolution of metakaolin-based geopolymeric systems at moderate temperatures, i.e., at 40 °C. Metakaolin was alkali-activated using NaOH solutions of varying concentrations, yielding H2O:OH- ratios of 5.50, 9.15, 13.75, and 27.50. Sodium nitrate was added to the activation solutions at a constant [NO3 −]: [OH−] ratio of 0.25. Most geopolymeric mixtures were designed to obtain a Na2O:Al2O3 ratio of 1.00 for nitrate-free mixtures, or 1.25 for those including sodium nitrate. In addition, the effect of deviation from these values (Na2O:Al2O3 ratios of 0.84 and 1.20 for nitrate-free samples, 1.04 and 1.50 for nitrate-bearing samples) was also studied. The samples were cured in sealed containers at 40 °C for periods ranging from 1 day to 3 months. The products were characterized by X-ray diffractometry, Fourier transform mid-infrared spectroscopy, and scanning electron microscopy as well as by compressive strength measurements. The results demonstrate the influence of composition, alkalinity, SiO2:Al2O3 ratio, Na2O:Al2O3 ratio, nitrate concentration, and curing times on the mineralogy of the geopolymeric matrix. Various crystalline phases such as zeolite A, zeolite X, and nitrate-bearing phases, namely nitrate sodalite and nitrate cancrinite, were identified among the reaction products. The sequence of phase evolution in these geopolymeric systems was elucidated.

Investigation of fabrication of Ti4O7 by carbothermal reduction in argon atmosphere and vacuum

High quality Ti4O7 powder was synthesized by carbothermal reduction in flowing argon atmosphere and in vacuum. Phase evolution sequences during reaction and influence of main technological parameters such as compacting pressure, different heating temperature and reducing time and carbon black mole ratio in flowing argon atmosphere were investigated by X-ray diffraction and scanning electron microscope. Concluding the effect of main technological parameters, the carbothermal reaction was mainly controlled by solid phase-boundary reaction between the TiO2 and carbon black particles at the first stage. As the reduction progresses and the particle size increased, the contacting area between TiO2 and carbon particles decreased. The indirect reactions reduced by CO gas proceeding through gaseous intermediates subsequently accelerate the rate of the whole reaction at the second stage. The Rietveld (R) plot result showed that high quality of 98.5 % content Ti4O7 powders were obtained at 1100 °C in flowing argon atmosphere for 2.5 h and they were mainly in regular cylindrical morphology with an average diameter of 5 μm. The high quality of 87.5 % content Ti4O7 powders were obtained by reduced at 1025 °C in vacuum for 2 h and they were mainly in irregular spherical morphology with an average particle size of 2.10 μm. The resistance of the Ti4O7 powders and ceramics prepared by SPS showed that the good dispersed cylindrical Ti4O7 particles produced in argon atmosphere was good to fabricate Ti4O7 electrode.

The synthesis and gas sensitivity of the β-Ti3O5 powder: Experimental and DFT study

The β-Ti3O5 powder was synthesized using carbothermal reduction technique in vacuum. Phase evolution sequences during reaction and influence of main technological parameters in vacuum were investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM). The results showed that high quality β-Ti3O5 powders were obtained by at 1300 °C for 3 h and they were mainly in irregular polyhedron shape with an average particle size of 5.5 μm. Thermal analysis showed that β-Ti3O5 powder has a good temperature stability in resistance and specific gas sensitivity to 20% O2/80% N2 and 20% H2/80% N2. Beside this, first principle calculation based on DFT was used to investigate the electronic structure of O2 adsorption on the β-Ti3O5 (110) surface.

Microstructural Evolution in Novel Suction Cast Multicomponent Ti-Fe-Co Alloys

The present work is aimed at understanding the solidification pathways of the Ti-rich Ti-Fe-Co in situ composites consisting of ultrafine eutectic with micron-scale dendrites. The effect of addition of Co in the Ti-rich binary Ti-Fe alloys has been systematically investigated. The series of Ti-Fe-Co ternary alloys i.e., Ti75Fe25−x Co x (x = 0, 5, 10, 12.5, 15, 20), (Ti70.5Fe29.5)100−x Co x (x = 0, 2, 4, 6, 8, and 10), Ti70Fe30−x Co x (x = 0, 5, 10, 15, 20, 25), Ti65Fe35−x Co x (x = 0, 10, 15, 17.5, 20, 25), and Ti60Fe40−x Co x (x = 0, 16, 18, 20, 22, 24) are synthesized by arc melting cum suction casting technique under high purity Ar atmosphere to obtain alloy cylinders having diameter (ϕ) of 3 mm. Detailed X-ray diffraction and electron microscopic (SEM and TEM) study are carried out to identify the phases as well as to monitor the sequence of phase evolution in the ternary alloys. The present study conclusively proves that the Ti65Fe10Co25, Ti70Fe30−x Co x (x = 10, 20, 25), and Ti75Fe15Co10 alloys show the ternary quasi-peritectic reaction of L + Ti(Co,Fe) → (β-Ti)ss + Ti2(Co,Fe) at invariant point P (=Ti75.5±0.8Fe6.3±2.1Co18.2±2.7) which is cooperated by means of eutectic reaction of L → (β-Ti)ss + Ti2(Co,Fe) below P and peritectic reaction L + Ti(Co,Fe) → Ti2(Co,Fe) for Ti65Fe10Co25,Ti70Fe10Co20, and Ti70Fe5Co25 alloys or eutectic reaction L → (β-Ti)ss +Ti(Co,Fe) for Ti70Fe20Co10, and Ti75Fe15Co10 alloys above the point P. In addition, the peritectic reaction L + Ti(Co,Fe) → Ti2(Co,Fe) plays a significant role in the phase evolution. The microstructural evolution, phase equilibria and solidification pathways have been explained by generating liquidus projection of the investigated alloys. Interestingly, the Co addition leads to the formation of complex Ti2Co phase and significantly affects the compositional stability of TiFe phase. This is found to have significant influence on the microstructural development during suction casting of the investigated alloys.

Nucleation, growth and evolution of calcium phosphate films on calcite

Marble, a stone composed of the mineral calcite, is subject to chemically induced weathering in nature due to its relatively high dissolution rate in acid rain. To protect monuments and sculpture from corrosion, we are investigating the application of thin layers of hydroxyapatite (HAP) onto marble. The motivation for using HAP is its low dissolution rate and crystal and lattice compatibility with calcite. A mild, wet chemical synthesis route, in which diammonium hydrogen phosphate salt was reacted with marble, alone and with cationic and anionic precursors under different reaction conditions, was used to produce inorganic HAP layers on marble. Nucleation and growth on the calcite substrate was studied, as well as metastable phase evolution, using scanning electron microscopy, grazing incidence X-ray diffraction, and atomic force microscopy. Film nucleation was enhanced by surface roughness. The rate of nucleation and the growth rate of the film increased with cationic (calcium) and anionic (carbonate) precursor additions. Calcium additions also influenced phase formation, introducing a metastable phase (octacalcium phosphate) and a different phase evolution sequence.

Effects of Al content on microstructure of as-cast Mg–3·5Ca alloy

Mg–xAl–3·5Ca (x = 0, 1, 2·5, 5 and 7·5, wt-%) alloys were prepared, and the effects of Al content on the microstructures of the as cast Mg–3·5Ca were investigated. The results showed that with increasing of the Al content, second phase evolution sequences are Mg2Ca→(Mg,Al)2Ca→Al2Ca+Mg17Al12. And the morphology transformation process of the phases formed along the interdendritic regions in as cast alloys was from a fine granular shape, a lamella type to thick bone with Al addition. These changes associate with an increase in the Al content and the Ca/Al ratio. With increasing of Al content in Mg–3·5Ca alloy, the mean grain size has a trend of first decrease and then increase, which is mainly caused by the constitutional undercooling as well as the changes of the distribution and morphology of the phases at as cast state.

In Situ Phase Evolution of Ni/Ti Reactive Multilayers

Joining shape-memory alloys (SMA) to other materials is strongly required in order to enlarge their fields of application. Fusion welding induces strong compositional and microstructural changes that significantly affect the shape-memory effect and the superelastic behavior of these alloys. The exothermic and in some cases self-propagating character of some nano-multilayer reactions is explored in this study as an alternative for joining SMA. To follow these very fast reactions, high brilliance sources, such as synchrotron radiation, are required. In situ high-resolution x-ray diffraction data, giving the phase evolution sequence with temperature of the Ni/Ti multilayer thin films under study, are presented. A correlation between the multilayer design and the tendency for the sequence of phase formation is established.