Yttria stabilized zirconia synthesis in supercritical CO2: Understanding of particle formation mechanisms in CO2/co-solvent systems

The increasing interest of supercritical (SC) fluids for inorganic materials synthesis recently stimulated the development of innovative synthesis processes and strategies. The supercritical CO₂ aided sol–gel process, developed for preparing various ceramic oxide powders with attractive applications in cosmetics, chromatography, catalysis or solid oxide fuel cells, usually suffer from both reproducibility problems and poor knowledge of the key parameters defining the final powder characteristics. In the present work a specific effort has been put on the understanding of reaction mechanisms and process parameters like co-solvent polarity and ageing time of the starting solution, which appeared to play a crucial role for the control of powder characteristics. Two different reaction mechanisms have been proposed to explain the formation of tetragonal yttria-doped zirconia powders by a batch process in either CO₂/pentane or CO₂/isopropanol mixtures. The first mechanism corresponds to a CO₂ anti-solvent precipitation process while the other one is based on a condensation reaction as in the conventional sol–gel process. This improved understanding in particle formation allows better control of powder characteristics and reproducibility.     

Spherical AlN particles synthesized by the carbothermal method: Effects of reaction parameters and growth mechanism

In this work, AlN particles with spherical morphology, smooth appearance and controllable particle size were purposely synthesized through an efficient carbothermal strategy at 1800 °C aiding with the CaF₂ additive. The influences of typical synthesis parameters, such as carbon content, CaF₂ particle size and reaction time on the formation rate, particles size and surface morphology of AlN particles were deeply and comprehensively studied. It was indicated that the intermediate Ca-aluminates was extremely essential to enhance the nitridation rate, promote the AlN growth and form the spherical morphology. More importantly, based on the systematic investigations and intensive analyses, the underlying growth mechanism of spherical AlN particles in the carbothermal process was rationally proposed and elaborated herein.     

Electroless deposition of Au around electrochemically evolved hydrogen bubbles

In this report, we use stripe-patterned substrates and TEM grids as working electrodes to demonstrate that electrochemically evolved hydrogen bubbles can serve as templates and reducing agents for electroless deposition of Au from a Na₃Au(SO₃)₂ electrolyte to form hollow particles. This new hollow nanoparticle formation mechanism, which is termed as bubble template synthesis, has the potential to become a general route to the fabrication of various metallic hollow nanoparticles. We have also found that the addition of Ni²⁺ ions in the electrolyte has an effect on the particle formation process. Electrodeposited metallic Ni enhances the hydrogen evolution and therefore the hollow particle formation. It also leads to an improved size distribution of the hollow particles.     

Formation of sodium bismuth titanate—barium titanate during solid‐state synthesis

Phase formation of sodium bismuth titanate (Na_0.5Bi_0.5TiO₃ or NBT) and its solid solution with barium titanate (BaTiO₃ or BT) during the calcination process is studied using in situ high‐temperature diffraction. The reactant powders were mixed and heated to 1000°C, while X‐ray diffraction patterns were recorded continuously. Phase evolutions from starting materials to final perovskite products are observed, and different transient phases are identified. The formation mechanism of NBT and NBT–xBT perovskite structures is discussed, and a reaction sequence is suggested based on the observations. The in situ study leads to a new processing approach, which is the use of nano‐TiO₂, and gives insights to the particle size effect for solid‐state synthesis products. It was found that the use of nano‐TiO₂ as reactant powder accelerates the synthesis process, decreases the formation of transient phases, and helps to obtain phase‐pure products using a lower thermal budget.     

Controllable crystallite and particle sizes of WO3 particles prepared by a spray‐pyrolysis method and their photocatalytic activity

Information about correlation of material properties parameters (i.e., crystallite and particle sizes) and photocatalytic activity of tungsten trioxide (WO₃) particles are still lacking. For this reason, the purpose of this study was to synthesize WO₃ particles with controllable crystallite (from 18 to 50 nm) and particle sizes (from 58 to 677 nm) using a spray‐pyrolysis method and to investigate correlation of crystallite/particle size and photocatalytic activity. To gain control of crystallite/particle size, synthesis temperature (120–1300^°C) and initial precursor concentration (2.5–15 mmol/L) were investigated, which were then compared with the proposal of the particle formation mechanism. The results showed that both crystallite and particle sizes played an important role in photocatalytic activity. In this research, the optimum condition to produce the highest photocatalytic performance of WO₃ particles was at the temperature of 1200^°C (crystallite size: 25 nm), and initial concentration of 10 mmol/L (particle size: 105 nm). © 2013 American Institute of Chemical Engineers AIChE J, 60: 41–49, 2014     

Kinetic Modeling of the Gas Antisolvent Process for Synthesis of 5‐Fluorouracil Nanoparticles

Mathematical modeling for 5‐fluorouracil (5‐FU) nanoparticle synthesis via gas antisolvent (GAS) process was investigated. 5‐FU was precipitated from a dimethyl sulfoxide (DMSO) solution using CO₂ as antisolvent. The particle size was controlled by nucleation and growth rates, therefore, the kinetic modeling study is essential. Thermodynamic modeling was applied to determine optimal operating conditions for experimental 5‐FU synthesis. Kinetic parameters were evaluated by fitting the particle size distribution predicted by the model to experimental data. The experimental and modeling results indicated that the particle size decreased with increasing the antisolvent addition rate.     

Synthesis and characterization of large, pure mordenite crystals

Large, pure mordenite (MOR) crystals were synthesized hydrothermally using silicic acid powder as the silica source, and the synthesized MOR samples were characterized by XRD and SEM techniques. The effects of several synthesis parameters, such as particle size and heat pre-treatment temperature of the used silicic acid powder as well as SiO₂/Al₂O₃ molar ratio and alkalinity in the synthesis solution, on the formation of MOR crystals were systematically investigated. The results indicate that the low solubility of the silicic acid powder, resulting from the use of the powder with large particle sizes or its heat pre-treatment at high temperatures, leads to the formation of unwanted ??-SiO₂ phase. Additionally, a low ratio of SiO₂ to Al₂O₃ and a high alkalinity in the synthesis solution can result in the formation of analcime-type (ANA) zeolite. Large, pure prismatic MOR crystals with uniform size and shape can be synthesized hydrothermally at a high ratio of SiO₂ to Al₂O₃ and a low alkalinity in the synthesis solution using silicic acid powder with fine particle sizes without heat pre-treatment as the silica source. The synthesized MOR crystals with acid treatment can significantly enhance their micropore accessibility.     

Kinetics and mechanism of the formation of hollandites in the BaO(Cs<Subscript>2</Subscript>O)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> system from initial mixtures prepared by different methods

The mechanism and kinetics of formation of solid solutions based on hollandite in the BaO(Cs₂O)-Al₂O₃-TiO₂ system are investigated using the initial mixtures prepared by two methods: (i) mechanical grinding and mixing of the initial components and (ii) coprecipitation from aqueous solutions of the salts. It is established that the mechanism of formation of hollandite in the system under investigation depends on the degree of dispersion of the initial mixtures used in the synthesis. When the synthesis is performed with the initial mixture prepared by mechanical grinding and mixing of the initial reactants (the particle size is equal to 50–300 nm), hollandite is formed at temperatures in the range 1100–1250°C in the presence of the accompanying phase Cs₂Al₂Ti₂O₈. When the synthesis is performed with the initial mixture prepared by coprecipitation from aqueous solutions of salts (the particle size is equal to 10–12 nm), hollandite is formed at temperatures in the range 850–1050°C. The investigation into the kinetics of formation of the hollandite phase from the above mixtures made it possible to determine the temperature-time conditions for the synthesis of this titanate in the form of a powder with a particle size of approximately 50 nm or in the form of a dense ceramic material with a particle size of ～200 nm.     

Plasma dynamic synthesis of highly defective fine titanium dioxide with tunable phase composition

Titanium dioxide is currently one of the most known promising photocatalysts. However, its use in the visible light range is limited due to its high energy gap. In this work, to solve the mentioned problem, it is proposed to obtain highly defect structures of titanium dioxide by means of a high-energy plasma dynamic synthesis method. It possible to synthesize TiO₂ Titanium dioxide powders with a tunable ratio of rutile and anatase modifications, as well as a particle size distribution were synthesized, by optimizing the synthesis conditions, including the process energy and parameters of the gaseous medium. The formation of shock-wave structures in the pulsed synthesis process results in obtaining fine particles of rutile and anatase with a highly defective crystal structure. The final product was revealed to have an extended working absorption spectrum region and a reduced band gap (2.74 eV). A possibility of photocatalytic applications for the synthesized TiO₂ powder was demonstrated in measurements of photocurrent density with time (j-t) under intermittent visible light irradiation.     

Kinetics of fluid–solid reaction with an insoluble product: zinc borate by the reaction of boric acid and zinc oxide

Mixing parameters influencing the final particle size and conversion of zinc oxide were studied for the formation of zinc borate. Formation of zinc borate was via a fluid–solid reaction. The process was kinetically controlled above the minimum speed for particle suspension, N_s. The reaction kinetics was developed and the rate constant was estimated. Copyright © 2004 Society of Chemical Industry     

Pre-treatment of raw materials for the hydrothermal synthesis of hydrotalcite-like compounds

Hydrotalcite-like compounds are layered materials with anionic exchange and adsorption properties. Alternatively, this material is synthesized by hydrothermal treatment of magnesium oxide (MgO) and aluminium trihydroxide (ATH). Since this synthesis deals with solid raw materials, and the reaction proceeds through a solution mediated dissolution–precipitation process, it is a challenge to enhance the reactivity of the raw materials. This research aims to investigate the effect of four different pre-treatments: blending, ultrasound treatment, and wet grinding on the conversion rate. In experiments at 80 °C, only 3R₁ polytype HTlc was formed, while a mixture of 3R₁ and 3R₂ polytypes was found in the experiments at 170 °C. All the pre-treatment techniques cause a decrease in particle size of the raw materials whereby wet grinding induced the strongest effect. Experimentally the fastest conversions at both synthesis temperatures were achieved by wet grinding. A reaction mechanism is proposed which explains the observed conversions as a function of the particles size of the reactants, the exposure time of the MgO to water and the reaction temperature. Two important parameters to be taken into account in the synthesis are thus exposure to water prior to the reaction and the particle size of the raw materials.     

Influence of precursor morphology on the microstructure of silicon carbide nanopowder produced by combustion syntheses

Silicon carbide nanopowder was synthesized using the combustion-based approach. Combustion synthesis was performed in reduction type SiO₂–Mg–C system. Silicon oxide powders with different morphologies and average particle size were used as starting powders. It was shown that even micro-size silica allows formation of nano-size silicon carbide powder. However, the specific surface area of synthesized SiC particles increases with the decrease the size of the silicon oxide precursor. The mechanism of silicon carbide formation in the combustion wave is also discussed.     

Controllable preparation of boron nitride microspheres and their epoxy-based thermoconductive composites

How to prepare spherical boron nitride (BN) particle with different size is an extremely challenging work. In this paper, the controllable preparation of spherical BN particle from nanospheres to microsphere was realized by changing the synthesis temperature of trimethyl borate (B(OMe)₃) and ammonia. The spherical precursor (SP) with high oxygen content was obtained first, and then it was heated under flowing ammonia atmosphere to form stable boron nitride microspheres (BNMS). The BNMS exhibits onion-like cavitation structure with a diameter of 0.8–3.4 μm. The effects of the lower reaction temperature (700–825 °C) and gas flow rate on the spherical precursor are discussed. A possible mechanism is proposed to explain the formation of precursors and the appearance of onion-like structure. It is believed that the formation of microsphere is due to the deposition and growth of BO species during the flow process of nanosphere. In addition, the effect of the addition of BNMS on the thermal conductivity of epoxy resin (EP) composites was investigated.     

A Trend to the Production of Calcium Hydroxide and Precipitated Calcium Carbonate with Defined Properties

In this paper, production of precipitated calcium carbonate (PCC) with required particle size and morphological structure, along with its dependence on technological parameters and the properties of Ca(OH)₂, is discussed. The effect of the reaction environment on the kinetics of CaO hydration and the formation of crystals in water suspension was established. A remarkable difference in the system's restoration ability after stirring was observed. The hydration process is initially controlled by a kinetic mechanism, followed by a diffusion‐controlled process. The dissolution speed of lime hydrated to suspension is eight times higher than that of lime hydrated to powder. Particles of hydrated lime appeared in various forms.     

Effect of particle size on the formation of Ti2AlC using combustion synthesis

This paper provides an insight into the effect of particle size of elemental metal powders and carbon source on the formation mechanism of Ti₂AlC MAX-phase ceramic produced by self-propagating high-temperature synthesis (SHS). The effect of titanium, aluminium and carbon particle size on the 2Ti+Al+C→Ti₂AlC reaction, the phase evolution of the final product and the porosity in both the green body and product has been examined. The effect of the carbon source in the form of graphite, carbon black and short carbon fibres on the reaction mechanism is explained. It is found that the particle size of the titanium and aluminium reactants had little effect on the phases formed but affected the green density of the reactants and the porosity in the final product. The carbon source used in the combustion reaction had an influence on the phases formed by the SHS reaction and was influenced by the dispersion of carbon particles and the titanium–aluminium particle contact.     

Synthesis of TiB2 from a carbon‐coated precursors method

This article deals with the synthesis of TiB₂ from carbon‐coated TiO₂ precursors with the addition of B₄C. The carbon‐coated precursors method alters the reaction process, compared with the conventional mixing of reactants, to produce high‐quality TiB₂ powders. The produced powders have a single phase, a submicron particle size (~0.3 to 0.8 μm), regular shape, loose agglomeration, and low level of contaminations (less than 0.5 wt% carbon and 0.6 wt% oxygen). The formation mechanism proposed is based on experimental results and thermodynamic evaluations. For comparison, the powders obtained from the mixture of reactants show higher agglomeration, a large particle size (>1 μm), high level of contaminations (0.7 wt% carbon and 1.1 wt% oxygen), and difficulty to control the reaction process (formation of TiBO₃ and Ti₂O₃ as the intermediate phases). The synthesized powders from the precursors method can be hot pressed to a relative density of ~94.5% with the formation of platelike grains at 1800°C under a pressure of 35 MPa without additives.     

Two-step synthesis process for high-entropy diboride powders

High-entropy diboride powders were produced by a two-step synthesis process consisting of boro/carbothermal reduction followed by solid solution formation. Nominally phase-pure (Hf,Zr,Ti,Ta,Nb)B₂ in a single-phase hexagonal structure had an average particle size of just over 400 nm and contained 0.3 wt% carbon and 0.3 wt% oxygen. The fine particle size was due to the use of high-energy ball milling prior to boro/carbothermal reduction, which led to a relatively low synthesis temperature of 1650°C. Oxygen and carbon contents were minimized by completion of the boro/carbothermal reduction reactions under vacuum. This is the first report of synthesis of a nominally phase pure high-entropy diboride powder from oxides using a two-step process.     

铁黄合成体系流变特性

针对碱法铁黄制备过程,研究了体系的流变特性．在反应初期铁黄合成体系粘度较小,随反应进行体系粘度显著增大,在反应中后期反应液为满足幂指数规律的假塑性流体,并具有很强的剪切稀化行为．反应温度、碱比、搅拌转速和通气量等通过改变粒子的生成过程和粒子的形态来改变铁黄合成体系的流变特征．在铁黄合成体系中加人硅酸纳会使体系粘度显著下降,流变特性随之发生变化．     

Effects of Al particle size and nitrogen pressure on AlN combustion synthesis

This study investigates the combustion synthesis of AlN fibers using an NH₄Cl additive and reports the effects of Al particle size (3, 30, and 180 µm) and N₂ pressure (0.10, 0.25, and 0.50 MPa) on the purity and morphology of AlN fibers. The combustion temperature was directly measured during the synthesis to elucidate the formation mechanism of the AlN fibers. The phase purity and morphology of the products were studied using X-ray diffraction and scanning electron microscopy, respectively. When the particle size of Al was reduced from 180 to 3 µm, the purity of the AlN product increased significantly owing to the large reaction area, which increased the combustion temperature. Furthermore, lower N₂ pressures enhanced the formation of AlN nanofibers due to the accelerated gasification of Al. The optimum values of the particle size of Al and the N₂ pressure for the formation of high-purity AlN nanofibers were found to be 3 µm and 0.10 MPa, respectively.     

Magnetite nanoparticles: Electrochemical synthesis and characterization

Magnetite (Fe₃O₄) nanoparticles (NP) with sizes between 20 and 30 nm have been obtained by Fe electrooxidation in the presence of an amine surfactant, which acted as a supporting electrolyte and coating agent, controlling particle size and aggregation during the synthesis. The effect of different parameters on the nature and size of the particles as well as the mechanism of formation of the particles have been studied by different techniques. It was concluded that, under the electrochemical conditions used in this work, the NP mean size was found to be constant at around 20 nm when the electrooxidation current density is increased from 10 to 200 mA cm⁻². However, when the potential is over 6 V, particle size decreases from 30 to 20 nm and metallic iron appears as an impurity. The mechanism of particles formation has being clarified and the critical effect of the distance between electrodes for obtaining magnetic iron oxide nanoparticles has been understood. Finally, the presence of an electrostatic adsorbed surfactant coating the particles allows the functionalization of the particles easily by exchange reaction with biomolecules of interest, which makes this material very promising for future application in biotechnology.