Simultaneous high-frequency induction heated combustion synthesis and consolidation of nanostructured HfSi2–SiC composite

Dense nanostructured HfSi₂–SiC composite was synthesized by high-frequency induction heated combustion synthesis (HFIHCS) method in one step from powders of mechanically activated HfC and Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of an induced current and mechanical pressure. Highly dense HfSi₂–SiC was produced under simultaneous application of a 60 MPa pressure and the induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.     

Synthesis and formation mechanism of high-purity Ti3AlC2 powders by microwave sintering

High-purity titanium aluminum carbide (Ti₃AlC₂) powders were synthesized by a microwave sintering method using different titanium sources as raw materials. The prepared products were characterized by differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicated that the synthesized Ti₃AlC₂ powders have high purity (97.5%) and even distribution of the grain size when using a 3TiH₂/1.2Al/2C mixture as raw materials when the microwave sintering temperature and time were 1300°C and 30 minutes, respectively. The formation mechanism of the Ti₃AlC₂ is described as proceeding via four stages. The solid-phase reaction between titanium and aluminum occurs below the melting point of aluminum and the main product is a Ti₃Al phase, which is an observed intermediate compound for the formation of Ti₂AlC and Ti₃AlC₂. Thus, this study provides a beneficial approach to low-temperature synthesis of high-purity Ti₃AlC₂ materials.     

Synthesis of high-purity high-entropy metal diboride powders by boro/carbothermal reduction

Synthesis of high-purity high-entropy metal diboride powders is critical to implementing their extensive applications. However, the related studies are rarely reported. Herein we first theoretically studied the synthesis possibility of high-purity high-entropy diboride powders, namely (Hf_0.25Ta_0.25Nb_0.25Ti_0.25)B₂ (HTNTB), via boro/carbothermal reduction by analyzing the thermodynamics of the possible chemical reactions and then successfully synthesized the high-purity and superfine HTNTB powders via boro/carbothermal reduction for the first time. The as-prepared powders exhibited low-oxygen impurity content of 0.49 wt% and small average particle size of 260 nm. Meanwhile, they possessed good single-crystal hexagonal structure of metal diborides and high-compositional uniformity from nanoscale to microscale. This work will open up a new research field on the synthesis of high-purity high-entropy metal diboride powders.     

Crystallization behaviour and microstructural development in ZrSiO4 and V-ZrSiO4 solid solutions from colloidal gels

Zircon and vanadium-doped zircon blue pigments were prepared by heat treatment of gel precursors. Gels with nominal compositions V_x-ZrSiO₄ with x=0.0, 0.002, 0.004, 0.02 and 0.2 were prepared by formation of a silica coating on zirconia colloidal particles previously obtained. The crystallization behavior and microstructural evolution were studied using X-ray powder diffraction (XRD), energy dispersive X-rays microanalysis (EDX), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The results indicated that the vanadia loading in the precursor gels speeds up the crystallization of the vanadium-containing tetragonal zirconia solid solutions and their transformation to the monoclinic form. The overall conversion rate of gel precursors to pigmenting powders increased when the vanadia content was higher. Microstructural data revealed that the used procedure for the preparation of vanadium-zircon pigments allowed high-purity and non-agglomerated powders with controlled particle size to be obtained.     

Preparation,characterization and catalytic application of Barium molybdate (BaMoO4) and Barium tungstate (BaWO4) in the gas-phase oxidation of toluene

Barium molybdate and Barium tungstate are important materials due their photoluminescent properties and they also have catalysis and photocatalysis applications. In this work, powders of these compounds were prepared by microwave-assisted hydrothermal (MAH) method and polymeric precursor method (PPM) and their structural and optical properties were studied. Furthermore, these materials were employed as solid catalysts towards gas phase toluene oxidation reactions. X-ray diffraction confirms the purity of materials at both preparation methods and reveals a preferential growth when the powders are prepared by MAH due polymeric agents and processing using microwave, which was confirmed by Field emission scanning electron microscopy. Photoluminesce emission was attributed to the charge-transfer transitions within the [WO₄]^2- and [MoO₄]^2- complexes. The H₂ Temperature-Programmed Reduction (H₂-TPR), O₂-chemisorption and extended X-ray absorption fine structure (EXAFS) results indicated that BaWO₄ samples, compared with BaMoO₄ samples, have higher oxygen mobility and oxygen vacancies that appear as key factors for the achievement of better catalytic performances.     

Enhanced high-temperature oxidation resistance of HfSi2-modified silicon based multilayer ceramic coating on Nb alloy prepared by a novel strategy

NbSi₂/SiO₂-Nb₂O₅/HfSi₂-HfO₂ multilayer ceramic coating containing Hf-rich out layers is designed and prepared on dense NbSi₂ surface via pack cementation and subsequent liquid plasma-assisted particle deposition and sintering of HfSi₂ particles to enhance oxidation resistance. The obtained ceramic layer contains the SiO₂-Nb₂O₅ intermediate bonding-layer and the oxygen barrier outer-layer of HfSi₂-HfO₂. The mass gain of NbSi₂/SiO₂-Nb₂O₅/HfSi₂-HfO₂ coating (5.583 mg/cm²) is only 40 % compared to the NbSi₂ coating (13.702 mg/cm²) after 100 h at 1200 °C. The HfO₂ and HfSiO₄ phases formed at high-temperatures are anchored in SiO₂ scale, enhancing the structural stability of ceramic phase and blocking oxygen penetration. Namely, the "Hf nail" fixed on SiO₂ scale effectively inhibits Nb-oxide growth and crack initiation.     

A study of the oxidation of ZrB2 powders during high-energy ball-milling in air

Oxide impurities in ZrB₂ powders promote coarsening, resulting in lower sinterability. Given its importance for sintering, we studied the oxidation of ZrB₂ powders during high-energy ball-milling (the form of milling able to refine ZrB₂ powders to the nanoscale) with the aid of various characterization techniques, namely fusion in inert gas, X-ray diffractometry, helium pycnometry, transmission electron microscopy, and Fourier-transform infrared, Raman, and X-ray photoemission spectroscopy. It was found that high-energy ball-milling in air introduces twice as much oxygen into the ZrB₂ powders as the more conventional attrition milling. Also, this oxygen does not form solid-solutions with ZrB₂, but amorphous oxides (i.e., ZrO₂ and B₂O₃) that locate preferentially on the surface of the ultra-fine agglomerates resulting from cold-welding of the primary nano-particles that form during the ball-milling.     

Synthesis of lanthanum beta-alumina powders by the polymeric precursor technique

La-β-Al₂O₃ (LaAl₁₁O₁₈) powders were synthesized by the polymeric precursor technique using lanthanum nitrate and aluminum nitrate. The transformations during thermal treatment of the precursor solution with ethylene glycol and citric acid were evaluated by thermal analysis. Fourier transform infrared spectroscopy analysis was performed after calcinations of the polymeric resin for determination of residual carbon. The specific surface area was evaluated by the BET method. Fine powders with ∼121 m²/g specific surface area and 20 nm average particle size were obtained and observed by scanning and transmission electron microscopy. Nearly single phase LaAl₁₁O₁₈ was obtained after pressing and sintering these powders at 1600 °C with small additions of MgO. The sintered pellets were characterized by X-ray diffraction and scanning electron microscopy. Impedance spectroscopy measurements carried out in the 1000–1200 °C range show the electrolytic behavior of the La-β-Al₂O₃ pellets, suggesting their application as solid electrolytes in high temperature potentiometric oxygen sensors.     

Photocatalytic decomposition of benzene with plasma sprayed TiO2-based coatings on foamed aluminum

Plasma spraying technique was used to deposit thin TiO₂-based photocatalytic coatings on foamed aluminum. Before plasma spraying, the composites of nano-TiO₂ powder (P25) and nano-ZnO/CeO₂/SnO₂ powders were agglomerated into microsized powders by spray-drying process. X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photocatalytic activity evaluation by the decomposition of gas-phase benzene (C₆H₆) were applied to characterize the starting powders and the coatings, respectively. The results showed that all the three plasma sprayed TiO₂-based coatings were the mixture phases of anatase and rutile. On the splats’ surfaces of the as-sprayed coatings, fine nano-crystalline particles were observed. However, grain growth occurred on the surface of plasma sprayed 90%TiO₂–10%ZnO coating. The XPS spectra revealed that the Ti, Zn, Ce and Sn elements existed on the surfaces of plasma sprayed TiO₂-based coatings as the chemical states of Ti⁴⁺, Zn²⁺, Ce⁴⁺ and Sn⁴⁺, respectively, whilst, the oxygen element was composed of three kinds of chemical states, i.e. crystal lattice oxygen, hydroxyl oxygen and physical-adsorbed oxygen. It was found that plasma sprayed 90%TiO₂–10%CeO₂ coating and 90%TiO₂–10%SnO₂ coating exhibited similar photocatalytic activity, which was higher than that of plasma sprayed 90%TiO₂–10%ZnO coating. The photocatalytic activity is not only dependent on the anatase content but also on the surface morphology and the hydroxyl content formed on the surface of plasma sprayed TiO₂-based coatings as well as the additive character.     

Battery-like behavior of Ni-ceria based systems: Synthesis,surface defects and electrochemical assessment

NiO, CeO₂ and respective composites are extensively used in energy storage devices due to mostly their high electrochemical activity. However, the assessment of battery-like behavior of Ni-ceria based systems comprising (Ni or Gd)-doped ceria combined with NiO seems to be neglected in the literature. In this work, NiO and ceria-based solid solutions composite powders were obtained by a co-precipitation synthesis method. The structure and particle size of the calcined powders were investigated by X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM), respectively. Oxidative states of composites were inspected by X-ray photoelectron spectroscopy (XPS). The electrochemical performance of powders was evaluated by cyclic voltammetry, galvanostatic charge-discharge and impedance spectroscopy. Refinement of the XRD patterns showed that powders have nanosized crystallites and mean size of particles within 20 – 70?nm were revealed by FESEM. The improved specific capacity of the NiO-CeO₂ electrode material (about 2.5 times higher than that of NiO-CGO at 5?mV?s^?1) is due to an increase in Faradic reactions taken place on its surface with a higher fraction of defects (namely Ni³⁺, Ce³⁺ and oxygen vacancies), as determined by XPS. The superior electrochemical performance of the NiO-CeO₂ electrode is also confirmed by electrochemical impedance spectroscopy.     

Multicatalytic properties of nanoparticle CaO2 synthesized by a novel,simple and economical method for wastewater treatment

The synergistic effect of oxygen releasing and oxidizing power of calcium peroxide (CaO₂) provides considerable advantages to environmental issues. In this research, a novel productive method to synthesize high-purity nanoparticle was reported. Ca(NO₃)₂ was used as an effective precursor under mild conditions. High-purity CaO₂ with nanosized spherical particle was observed. Disinfection effect of the CaO₂ powders was investigated using Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Surprisingly, photoactivity of CaO₂ was discovered. The CaO₂ exhibited intriguing photoactivity on dye oxidation and reduction under visible light. This method produces high-purity nanoparticulate CaO₂ for pilot-scale testing.     

Fabrication of Si3N4 preforms from Si3N4 produced via CRN technique

Ceramic preforms with randomly distributed particles as reticulated porous structure which are generally used for metal infiltration as reinforcement, membranes, catalyst supports etc. Preforms are characterized by open porosity making possible their infiltration by liquid metal alloys. In this work, quartz powders using carbon black as a reducing agent were used for alpha Si₃N₄ powders synthesis through a carbothermal reduction and nitridation (CRN) process. The CRN process was carried out under nitrogen flow at 1,450 °C for 4 h. At high temperatures, carbon as reducing agent reacts with the oxygen of SiO₂, and the resulting metallic silicon compounds with nitrogen gas to obtain silicon nitride powder. The reacted powders were used to obtain reticulated ceramic by replica method. The powders containing various bentonite ratios were mixed in water to prepare slurry. The slurry was infiltrated into a polyurethane sponge. A high porous ceramic foam (preform) structure was achieved after burn out of the sponge. All ceramic preforms were sintered to increase stiffness (in the temperature range 900–1,350 °C). The sintered ceramic foams were subjected to compressive tests. The scanning electron microscopy was used to examine the reticulated ceramic foam structure, and X-ray diffraction analysis was performed to determine phases.     

Growth behavior of nanosized ceria powders prepared by coprecipitation routes

Nanosized ceria (CeO₂) powders were obtained by coprecipitation routes of cerium nitrate hexahydrate [Ce(NO₃)₃·6H₂O]. The growth behavior of the nanosized CeO₂ powders was investigated by X-ray diffraction, transmission electron microscopy and selected area electron diffraction. The XRD results showed that the dried precursor powders contained a single crystalline phase of CeO₂, and only a single phase of CeO₂ appeared when the dried precursor powders were calcined at different temperatures for various durations. Moreover, the crystallite size of CeO₂ increased on increasing the calcination temperature and duration. The kinetics equation of the nanosized CeO₂ powders grown between 673 and 1273 K for various durations is described as     

Influence of the size of spraying powders on the microstructure and corrosion resistance of Fe-based amorphous coating

The Fe-based amorphous coatings with the composition of Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ were successfully sprayed on mild steel substrate by the high velocity oxygen fuel (HVOF) spraying process with different feedstock powder sizes (i.e., powder A: −33 + 20 μm, powder B: −45 + 33 μm, powder C: −55 + 45 μm). The coatings were characterized for its morphology, microstructure and thermal stability by using X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The corrosion behavior of the coatings in 3.5 wt% NaCl solution was studied with potentiodynamic and potentiostatic polarization test. It was found that the particle size of the feedstock powders had a significant influence on microstructure and corrosion resistance of the resultant coatings. The coatings sprayed with the finest powders show the most compact structure; while the coating with the coarser powders exhibits a better corrosion resistance. It is found that the corrosion resistance of the coatings is closely related to the wetting behavior which is affected by the oxygen content and the roughness of coatings. The coatings with hydrophobicity exhibit a better corrosion. The present result demonstrates that the amorphous coatings with hydrophobicity and excellent corrosion resistant are promising for industrial application in marine environment.     

Pt–calcium cobaltate enables sorption-enhanced steam reforming of glycerol coupled with chemical-looping CH4 combustion

Sorption-enhanced steam reforming (SESR) process is usually highly energy intensive in producing high-purity hydrogen. Herein, the sorbent decarbonation was conducted in the presence of O₂ to enable the exothermic reaction between CaO and CoO_x to form calcium cobaltate (CCO). By utilizing CCO as oxygen carrier (OC), the chemical-looping methane combustion was employed prior to the SESR of glycerol (SESRG), by which CCO was prereduced to Co catalysts and CaO sorbent, thereby significantly improving the H₂ yield from SESRG. With a Pt-doped CCO acting as precatalyst, CO₂ sorbent, and OC, we realized 70% CH₄ conversion and 96 vol% H₂ with 120% yield (based on glycerol) for 20 cycles, and the excess H₂ was due to steam gasification of coke. The promoting effects of Pt toward CH₄ conversion and H₂ production were rationalized by CH₄ temperature-programmed reduction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. Our results demonstrate the feasibility of process integration enabled by multifunctional materials.     

Nafion-TiO2 composite DMFC membranes: physico-chemical properties of the filler versus electrochemical performance

TiO₂ nanometric powders were prepared via a sol-gel procedure and calcined at various temperatures to obtain different surface and bulk properties. The calcined powders were used as fillers in composite Nafion membranes for application in high temperature direct methanol fuel cells (DMFCs). The powder physico-chemical properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and pH measurements. The observed characteristics were correlated to the DMFC electrochemical behaviour. Analysis of the high temperature conductivity and DMFC performance reveals a significant influence of the surface characteristics of the ceramic oxide, such as oxygen functional groups and surface area, on the membrane electrochemical behaviour. A maximum DMFC power density of 350 mW cm⁻² was achieved under oxygen feed at 145 °C in a pressurized DMFC (2.5 bar, anode and cathode) equipped with TiO₂ nano-particles based composite membranes.     

Oxygen Interaction with Hexagonal OsB2 at High Temperature

The stability of ReB₂‐type hexagonal OsB₂ powder at high temperature with oxygen presence has been studied by thermogravimetric analysis, differential scanning calorimetry, SEM, EDS, and high‐temperature scanning transmission electron microscopy and XRD. Results of the study revealed that OsB₂ ceramics interact readily with oxygen present in reducing atmosphere, especially at high temperature and produces boric acid, which decomposes on the surface of the powder resulting in the formation of boron vacancies in the hexagonal OsB₂ lattice as well as changes in the stoichiometry of the compound. It was also found that under low oxygen partial pressure, sintering of OsB₂ powders occurred at a relatively low temperature (900°C). Hexagonal OsB₂ ceramic is prone to oxidation and it is very sensitive to oxygen partial pressures, especially at high temperatures.     

Effect of Nano-ZrO2 on the Properties of Al-Al2O3 Nanocomposites Prepared by Mechanical Alloying

In the present work, mechanical alloying was used to prepare Al-20wt.% Al₂O₃ metal-matrix nanocomposites having up to 4wt.% ZrO₂ at the expense of Al₂O₃. The powders were milled for different time intervals. To characterize the powders after milling, x-ray diffraction and transmission electron microscopy were used to identify the phase composition, crystallite size and morphology. In order to study the sinterability, the milled powders were cold pressed and sintered in argon atmosphere at different firing temperatures up to 470 °C for 1 h. The relative density and apparent porosity of the sintered composites were determined according to Archimedes principle. Moreover, the microstructure was examined by a scanning electron microscope attached with an energy dispersive spectrometer (EDS). Microhardness and AC conductivity of sintered composites were also measured. The results pointed out that the increasing of milling time is responsible for uniform distribution of Al₂O₃-ZrO₂ particles in the Al matrix as well as remarkable increases in relative density, microhardness and AC conductivity of the sintered specimens. Also, the relative density was affected considerably by the increasing of sintering temperature. Moreover, increasing of ZrO₂ content led to a significant decrease in the crystal size of the milled powders and increase in the microhardness of the sintered compacts. No changes were observed on the conductivity after addition of ZrO₂.

     

Synthesis,densification and characterization of Ag doped ceria nanopowders

Nanosized Ag-doped ceria (Ce_1-xAl_xO_2-δ)powders (0.1 ≤ x ≤ 0.04) were obtained by self-propagating room temperature reaction. The solid solubility of Ag into ceria lattice was the highest reported so far. X-ray diffraction analysis and field emission scanning microscopy results showed that the doped samples are single phase solid solutions with fluorite-type structure and all prepared powders were nanometric in size. The average size of Ce1-xAgxO2-▯ particles lies at about 4 nm. Raman spectra revealed an increase in the amount of oxygen vacancies with the increase of Ag concentration, such as is foreseen. The thermal stability of solid solution was followed by XRD. Microstructure development was studied by scanning electron microscopy. By controlling the processing variables, it was possible to obtain high density samples with homogeneous microstructure at low sintering temperature.     

Synthesis of magnesium silicon nitride nanopowder by employing two-step method

In this work, a new method for the preparation of magnesium silicon nitride (MgSiN₂) nanopowder was studied using a two-step process in nitrogen gas from Mg/Si starting mixtures. This method is known as mechanical alloying followed by heat treatment. The results showed that the compositions of the combustion products depended on the starting mixtures. In addition, the content of magnesium and silicon in the starting powder should fulfil the condition Mg/Si?=?2 to obtain single phase MgSiN₂.Single phase MgSiN₂ nanopowder can be fabricated from Mg and Si powders with mole ratio of 2:1 at 1350°C in N₂ atmosphere. Composition and structure of reactants and products were examined by X-ray diffraction, field emission scanning electron microscopy and high resolution transmission electron microscopy.