Microstructure Of Precursors Research Articles

Eliminating bimodal structures of W-Y2O3 composite nanopowders synthesized by wet chemical method via controlling reaction conditions

The bimodal distribution of tungsten grain size, which is also called bimodal structure, is a universal phenomenon in synthesizing tungsten based nanopowders by wet chemical method, and could significantly deteriorate further sintering characteristics of the powders. In this paper, W-Y2O3 composite nanopowders with small grain size and no bimodal structure were synthesized by an improved wet chemical method. It was found that reaction conditions, especially the hydrogen ion concentration(ρ(H+)) could significantly influence the microstructure and morphology of the prepared composite nanopowders and has an optimum value of approximately 0.16 mol L−1. W-Y2O3 composite nanopowders synthesized under this optimum value got not only the minimum average grain size, which is about 17 nm, but also the best grain size uniformity with no bimodal structure. The microstructure of co-deposited precursors and prepared powders was analyzed in detail to figure out the specific influence mechanism of ρ(H+), and it was indicated that ρ(H+) could influence the microstructure and the Y2O3 content of the reduced powders through affecting the fraction of precursor that has little yttrium content, which may lead to the bimodal distribution of tungsten grain size of the prepared powders. Furthermore, the formation mechanism of bimodal structure was systematically analyzed and a feasible method was proposed to eliminate the bimodal structure. It is suggested that the bimodal distribution of tungsten grain size is attributed to the distribution of Y2O3 during hydrogen reduction process and can be well avoided by controlling ρ(H+).

A novel method to synthesize vanadium nitride nanopowders by ammonia reduction from combustion precursors

Vanadium nitride (VN) has many applications ranging from structural materials to electrode material owing to its high hardness, excellent wear resistance, high electrical and thermal conductivity and high-temperature stability. The conventional synthesis methods are often plagued by high temperature (above 1000 °C) and long reaction times. In the paper, VN nanopowders are firstly synthesized by a large-scale, low-temperature and short-time solution combustion synthesis-based method. Firstly, vanadium dioxide (VO2) precursors are prepared through ultrafast (within one minutes) solution combustion synthesis method. During the subsequent ammonia reduction process, the VN nanopowders are successfully obtained at 700 °C for 2 h. In the investigation, the phase transformations and microstructures of precursors during ammonia reduction procedure have been studied in detail. The as-prepared VN nanopowders are composed of nanoparticles with particles sizes ranging from 30 to 40 nm and possess mesoporous structures. Upon evaluating the VN nanopowder as an anode material for lithium ion batteries, it displays good electrochemical performances and exhibits high reversible capacity of 634 mAh g−1 at 0.1 A g−1 after 250 cycles.

Research on the multi-scale microstructure of polyacrylonitrile precursors prepared by a dry-jet wet spinning process

An understanding of the properties of polyacrylonitrile (PAN) precursors is an essential precondition for manufacturing high-performance carbon fibres, and the structure of the precursors has a direct and profound effect on the performance of carbon fibres. In this study, PAN precursors, formed in a multistage coagulation bath, were spun by a dry-jet wet spinning process, and the multi-scale microstructure and morphology of the precursors were investigated by separating the fibrils from the precursors. Scanning electron microscopy and high-resolution transmission electron microscopy were employed to examine the surface morphology, cross-sectional morphology and microstructure of the precursors. X-ray diffraction was used to characterize the crystal structure. The micropore sizes of the precursors were determined with nitrogen adsorption experiments; the adsorption increased after ultrasonic etching and decreased with an increase in the treated concentration. All the results demonstrated that the PAN precursors had a multi-scale microstructure, the precursors consisted of fibrils with diameters of 80–200 nm and the fibrils consisted of some microfibrils with diameters of 20–40 nm, including the periodic tissues with thicknesses of 16–30 nm perpendicular to the fibre axis.

Low-temperature synthesis of high-purity boron carbide via an aromatic polymer precursor

Abstract

Phytic acid derived LiFePO4 beyond theoretical capacity as high-energy density cathode for lithium ion battery

A glucose-derived carbon-encapsulated (GC) LiFePO4 (LFP) with internal carbon sheets (IC) composite, GC/IC/LFP, is synthesized by using phytic acid (PhyA) as a special phosphorus source. PhyA plays an important role as a structure-director, complexing agent and antioxidant, and the resulted IC contributes to improving electrochemical properties. The special morphology and microstructure of precursors and final materials are characterized and discussed in detail. The prepared GC/IC/LFP shows not only an excellent cycle performance, but an ultra-high specific capacity of 192mAhg−1 at 0.1C, which exceeds the theoretical capacity of LFP. A remarkable rate performance of 140mAhg−1 at 10C was also achieved. The energy efficiency of GC/IC/LFP can reach nearly 96% at 0.1C.

A precursor-derived morphology-controlled synthesis method for mesoporous Co3O4nanostructures towards supercapacitor application

The controlled synthesis of multifarious nanostructures is valuable for understanding the properties associated with the surface structure and for exploring potential applications. Herein, various morphological Co3O4 precursors were synthesized hydrothermally for fabricating mesoporous Co3O4 nanostructures. Importantly, the morphology, size, and crystalline phase of precursors were independently tuned by controlling the components in solution (e.g. anions, cations, and hydrolysis agents). The microstructure of Co3O4 precursors has obviously changed from porous hexagons to nanoneedles by changing the anions in solution. After calcination, as-prepared precursors were converted into mesoporous Co3O4 nanostructures, and their morphology and size were well preserved. The effect of experimental conditions on the growth of Co3O4 precursors was explored, and the corresponding growth mechanism was proposed. Moreover, the electrochemical properties of Co3O4 with different morphologies were tested. The results showed that the 3D hierarchical flower-like Co3O4 assembled by hexagonal nanosheets exhibited higher specific capacitance (327.3 F g−1 at 0.5 A g−1) with excellent retention (96.07% at 5 A g−1) after 10 000 cycles. It is believed that this anion-assisted synthetic approach can be extended to the preparation of other transition metal oxides.

Investigation on preparation and performance of spinel LiNi0.5Mn1.5O4 with different microstructures for lithium-ion batteries.

The high voltage spinel LiNi0.5Mn1.5O4 is a promising cathode material in next generation of lithium ion batteries. In this study, LiNi0.5Mn1.5O4 with various particle microstructures are prepared by controlling the microstructures of precursors. LiNi0.5Mn1.5O4 spinel samples with solid, hollow and hierarchical microstructures are prepared with solid MnCO3, hollow MnO2 and hierarchical Mn2O3 as precursor, respectively. The homemade spinel materials are investigated and the results show that the content of Mn3+ and impurity phase differ much in these three spinel samples obtained under the same calcining and annealing conditions. It is revealed for the first time that an inhomogeneous migration of atoms may introduce Mn3+ and impurity phase in the spinel. The hierarchical microstructure with the primary particles interconnected is optimal for electrode materials because this microstructure has a higher conductivity between the interconnected primary particles and appropriate specific surface area. LiNi0.5Mn1.5O4 in this microstructure has the best rate capability and also the best long-term cycling stability.

Selective dissolution of amorphous Zr–Cu–Ni–Al alloys

Amorphous Zr–Ni–Cu–Al ribbons were dealloyed by electrochemical selective dissolution. As a result three-dimensional microporous structures were obtained at different conditions (electrochemical potential, temperature, dissolution time). The smallest porous were obtained at low temperatures and potentials. To study the effect of precursor microstructure on the porous structure the as-cast alloys were annealed at two different temperatures – above Tg to cause structural relaxation of the amorphous material and at temperature above Tx to produce crystalline alloy. It is shown that only the entirely amorphous (as-cast and low-temperature annealed) alloys form homogeneous microporous structures as a result of the selective dissolution.

Effect of lanthanide on the microstructure and structure of LnMn0.5Fe0.5O3 nanoparticles with Ln=La, Pr, Nd, Sm and Gd prepared by the polymer precursor method

The synthesis of LnMn0.5Fe0.5O3 perovskite nanoparticles by the polymer precursor method showed a strong intrinsic dependence with different lanthanides (Ln=La, Pr, Nd, Sm and Gd). The polymerization level reached in the polymer precursor was proportional to the atomic number of lanthanide with exception of samarium, which showed the formation of a different precursor based in a citrate chelate with ethyleneglycol bonded as adduct. The increasing level of polymerization of the polymer precursors showed the formation of large-size perovskite nanoparticles after its calcination. SAXS and TEM analyses suggested that nanoparticles obtained, using this method, have a squared-like microstructure in connection with the polymer precursor microstructure. Structural analysis showed an orthorhombic structure with a slight decline in the Jahn–Teller distortion when the atomic number of lanthanide increases. Mössbauer spectroscopy showed the presence of a majority site in agreement with the Pbnm orthorhombic structure best fitted with Rietveld refinements and in some cases, a more distorted site attributed to local inhomogeneities and oxygen vacancies.

A hydrothermal assisted method to prepare Samarium Tungstate sheets at lowered reaction temperature

In the present work, a hydrothermal-assisted synthesis of sheet-like Samarium Tungstate powders is developed. The structures and morphology of these powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results show that a new phase of SmWO4(OH) was obtained during the hydrothermal reaction. This phase was further used as precursor to obtain Sm2WO6 crystallites with much lowered heating temperature during the calcination. Furthermore, both of the SmWO4(OH) and Sm2WO6 products were found to exhibit sheet-like microstructures without obvious morphological differences, suggesting that controlling the precursor microstructure is an effective approach to maintain the calcined Sm2WO6 morphology. Finally, the schematic comparison between hydrothermal assisted method and conventional solid state reaction method was presented.

Fabrication of Porous and Nanoporous Aluminum via Selective Dissolution of Al-Zn Alloys

Porous and nanoporous aluminum have been fabricated via selective dissolution. Al-Zn alloys were dealloyed in an aqueous solution of nitric acid to selectively dissolve zinc. Fast solidification methods permitted to adjust the precursor microstructure of the parent alloy in order to induce supersaturation of zinc in aluminum. An electric potential applied during corrosion affected the final morphology of porosity. Electronic imaging evinced the presence of regions with less than 100 nm diameter pores. This nanoporosity was only present in electrochemically dealloyed samples. We observed two types of porosity in dealloyed samples: a primary porosity resulting from the selective removal of zinc-rich interdendritic phases and a secondary porosity resulting from nanoporosity evolution inside zinc-supersaturated dendrites.

Quasi-periodic layer structure of die-upset NdFeB magnets

Isotropic magnets were prepared from melt-spun powders at different hot pressing temperatures from 550 to 700 °C, then upset into fully dense anisotropic magnets at the same die-upsetting temperature of 850 °C. Die-upset magnets had the characteristics of inhomogeneous microstructure, including well-aligned grains structure and nonaligned grains layers transverse to press direction, which was quasi-periodic layer structure with a total length of 5–15 μm. Nonaligned grains layers were mainly made of large grains and had higher Nd content. To clearly understand the formation of layer structure, the microstructure of isotropic precursors with different hot pressing temperatures and their subsequent die-upset magnets was investigated. A new interpretation for the formation of layer structure was proposed in this paper: the layer structure was correlated to the original ribbon interface which was divided into three types based on the contact forms. Because of the incomplete contact of neighboring ribbons, concentration of stress occurred in the contacted points and the Nd-rich phase was squeezed into interspaces at high temperature under stress. Due to the release of interfacial energy and the fluidity of enough Nd-rich liquid phases, the nonaligned layers with large grains formed both in hot compaction and subsequent hot deformation process. The layer structure affected the magnetic properties of die-upset magnets. With increase of the hot pressing temperature, the nonaligned grains layers became thicker, and the magnetic performance of die-upset magnets decreased. It was necessary to reduce the thickness of large grains layers for the preparation of high-performance die-upset magnets.

EFFECT OF PRECURSOR MICROSTRUCTURE ON MORPHOLOGY FEATURE AND MECHANICAL PROPERTY OF C-Mn-Si STEEL

The effects of different precursor microstructure on the morphology and mechanical properties of the 0.22C-1.9Mn-1.32Si multiphase steel which was obtained by the treatment of in- tercritical reheating-quenching and partitioning (IQP while multiphase steel which has a bainite-ferrite (B-F) precursor can obtain a microstructure of equiaxed-like ferrite matrix and particale like retained austenite. After the IQP while multiphase steel which has a M precursor has realized the combined properties of high strength and excellent ductility, product of strength and elongation of this kind of steel reaches 31 GPa·%. As for the work hardening behavior of the uniform elongation stage, although B-F pre- cursor multiphase steel has a higher work hardening index n than the M precursor multiphase steel, stability of the retained austenite in this kind of steel is relatively poor, variation behavior curve of the instantaneous n value with true strain for this kind of steel shows a notched-like shape; as for the

Precursor Nanoscale Textures in Ferroelastics: Interplay between Anisotropy and Disorder

We study the effect of anisotropy and disorder on the morphology of precursor microstructures within a Ginzburg-Landau free energy framework. In addition, with increasing disorder at low anisotropy we find a crossover from a twinned state to a non-transforming frozen state. Results are compared with available experimental data.

Microstructure of nano precursors of La-Mg hydrogen storage alloy synthesized by sol-gel technology at different pH values

Sol-gel technology was employed to synthesize nanosized precursors of La-Mg hydrogen storage alloy at different pH values (0.5, 1.5, 8.0 and 9.0) of reaction solution. The effect of pH value on microstructure of the nano precursors of La-Mg hydrogen storage alloy was studied by infrared radiation (IR), thermo-gravimetric and differential thermal analysis (TG/DTA), X-ray diffraction analyzer (XRD) and transmission electron microscopy (TEM). IR results indicate that the chelating agent, citric acid, is not fully ionized, and carboxyl groups are not entirely used to complex metal ions in acidic solutions. The efficiency of complexing metal ions is enhanced in basic solutions. TG/DTA results show that the combustion may take place with low rate of the flame propagation that causes the longer combustion time when pH<1.5. On the contrary, the dry gel synthesized in basic solution combusts at low ignition temperature and combustion reaction is violent; it is easy to form fine particles. XRD and TEM results reveal that the precursor powders are mainly two-phase mixture of La2O3 and MgO. The morphology of the particles is almost flake with the size of ∼30 nm when pH is 8.0.

Self-assembled 3D flower-shaped NaY(WO4)2:Eu3+ microarchitectures: Microwave-assisted hydrothermal synthesis, growth mechanism and luminescent properties

Three dimensional (3D) flower-shaped microarchitectures of NaY(WO4)2 were synthesized via a microwave-assisted hydrothermal process in the presence of trisodium citrate (Na3Cit) and a post-calcination process. The effects of reaction conditions on the morphology of precursor microstructures were studied. It was found that Na3Cit, as the chelating agent and shape modifier, plays a key role in the microstructure growth. A possible growth mechanism for the flower-shaped microarchitectures was proposed. The as-formed precursor can completely transform into NaY(WO4)2 with its original flower-shaped morphology via a heat treatment process. The concentration and temperature quenching behaviors of Eu3+ fluorescence in the flower-shaped NaY(WO4)2 were studied, and the optimal doping concentration was confirmed, meanwhile the activation energy was obtained. Judd-Ofelt parameters Ωλ (λ = 2, 4 and 6) of Eu3+ in the flower-shaped NaY(WO4)2 phosphor were obtained by using the emission spectrum of Eu3+, moreover the radiative transition properties were analyzed.

Microstructures and Photoluminescence Properties of Three-Dimensional Multi-Layered ZnO Flowers by Surfactant-Free Hydrothermal Method

In this paper, a large-scale of three-dimensional multi-layered ZnO flowers constructed by porous nanosheets were fabricated on Zn foils through a surfactant-free hydrothermal method. The morphology and microstructures of precursors and the mesoporous ZnO flowers were revealed by scanning electron microscopy. Based on the urea hydrolyzation, a possible growth mechanism of etching-deposition was proposed. The quantities and sizes of ZnO petals can be turned by urea concentration variations. The optical properties of the multi-layered ZnO flowers prepared with variety urea contents were also investigated by room temperature photoluminescence spectroscopy and showed the lower urea added sample has better crystalline quality.

Influence of Microwave Irradiation on Precursor Microstructure and Catalytic Performance of Cu/ZnO/Al&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt; for Slurry Methanol Synthe-sis

Influence of Microwave Irradiation on Precursor Microstructure and Catalytic Performance of Cu/ZnO/Al&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt; for Slurry Methanol Synthe-sis

Self-Assembled 3D Urchin-Like NaY(MoO4)2:Eu3+/Tb3+ Microarchitectures: Hydrothermal Synthesis and Tunable Emission Colors

Novel 3D urchin-like NaY(MoO4)2 microarchitectures have been successfully synthesized by a complexing-agent-assisted hydrothermal process followed by a subsequent heat treatment process. The shape and size of the double alkaline rare earth molybdates precursor microstructures can be tuned effectively by controlling the reaction conditions, such as reaction time, the amount of organic additive trisodium citrate (Cit3−), and the kinds of organic additives. The possible formation mechanism for urchin-like microarchitectures has been presented. It is found that the Cit3− organic molecule, acting as the chelating agent and shape modifier, plays a key role in fine-tuning the precursor microstructures. The as-formed precursor could transform into NaY(MoO4)2 with their original urchin-like morphology and slight shrinkage in the size after postannealing process. Under UV and low-voltage electron beam excitation, 5 mol % Eu3+ and 5 mol % Tb3+ doped NaY(MoO4)2 samples exhibit strong red and green emission, corresponding to the characteristic lines of Eu3+ and Tb3+, respectively. Moreover, the luminescence colors of the Eu3+ and Tb3+ codoped NaY(MoO4)2 samples can be tuned from red, yellow, and green-yellow to green by simply adjusting the relative doping concentrations of the activator ions under a single wavelength excitation, which might find potential applications in the fields such as light display systems and optoelectronic devices.

Effects of carbonization temperatures on microstructure of carbon fiber precursors prepared from liquefied wood

In order to enlarge the utilization field of wood and decrease the costs of carbon fibers, carbon fiber precursors from liquefied wood were prepared by soaking liquefied wood in a solution containing hydrochloric acid and formaldehyde, after melt-spinning by adding hexamethylenetetramine. The microstructure evolution of the precursor during carbonization was studied by FTIR, X-ray analysis and Raman spectroscopy. The results show that precursors from liquefied wood above 400°C had diffraction peaks corresponding to the (100) crystal plane. When the carbonization temperature reached 500°C, Raman spectroscopy showed the D peak at wave number of 1360 cm−1 and the G peak at 1595 cm−1. By increasing the carbonization temperature, the microstructure of the precursors became more ordered. Although the structure of the precursor changed at 500 and 800°C, the peaks at 1632 and 1454 cm−1 corresponding to the characteristic vibrations of aromatic rings, remained during carbonization. This implies that the precursor from liquefied wood cannot be easily formed into graphite.