As-obtained Powders Research Articles

Dense high-entropy boride ceramics with ultra-high hardness

Highly dense high-entropy boride ceramics with ultra-high hardness were prepared by spark plasma sintering of the self-synthesized high-entropy boride powders. High-entropy boride compounds of (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2, (Hf0.2Mo0.2Zr0.2Nb0.2Ti0.2)B2 and (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 with some oxide impurities were obtained by borothermal reduction at 1600 °C. The yield of the samples after sintering was improved to be 97.2 wt%, 92.8 wt% and 97.6 wt%. The densification and hardness of the high-entropy boride ceramics were remarkably enhanced after sintering the as-obtained powders at 2000 °C. The relative density of Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2 ceramics was 99.2% and the hardness achieved a value up to 28.3 ± 1.6 GPa, which was significantly higher than reported values.

Polycrystalline to preferred-(100) single crystal texture phase transformation of yttrium iron garnet nanoparticles.

Nanocrystalline Ce-substituted yttrium iron garnet (YIG) powders of different compositions, Y3−xCexFe5O12 (0 ≤ x ≤ 2.0), were synthesized by a combination of sol–gel auto-combustion and solid-state synthesis techniques. The as-obtained powder samples were sintered at 1150 °C for 10 h. The garnet structure formation is confirmed by the X-ray diffraction pattern, which shows that the calculated lattice parameter increased for x = 1.0 and shows a decreasing trend for x ≥ 1.0 with the addition of cerium ions. The lattice parameter increased from 12.38 Å to 12.41 Å for x ≤ 1.0 whereas it decreased from 12.412 Å to 12.405 Å with the cerium composition for x > 1.0. The average particle size determined by high resolution transmission electron microscopy is in the range of 50 to 90 nm and found to increase with the substitution of cerium ions in YIG. The room temperature magnetic parameters such as saturation magnetization, coercivity and remanence magnetization are greatly affected by the substitution of cerium ions. The values of saturation magnetization decrease from 25.5 to 15 emu g−1 whereas coercivity increases from 1 to 28 Oe with the substitution of cerium ions. The pure YIG sample shows polycrystalline nature that changed towards a single-crystal structure leading to a preferred-(100) orientation with the Ce substitution. The change from a ring to a spotty pattern observed in SAED confirmed the crystalline phase transformation and is well supported by HRTEM and magnetic measurements. The behavior of magnetic and electrical properties is well supported by the poly- and single-crystalline nature of YIG and Ce-YIG, respectively. The crystal structure transformation in YIG brought about by Ce substitution could unveil enormous opportunities in the preparation of single-crystal materials from their polycrystalline counterparts.

Towards understanding the enhancement of antibacterial activity in manganese doped ZnO nanoparticles

In this work we focus on enhancing the antibacterial activity of ZnO nanoparticles by Mn doping, synthesized using a wet-chemical method. The as-obtained precursor powders were deeply investigated by thermal analyses correlated with the evolved gas analysis (TG-DTA-FT-IR) and by in situ high-temperature XRD to elucidate the thermally induced processes and to understand the manganese doped ZnO nanoparticles formation. The hexagonal wurtzite-type structure and the morphological characteristics of the thermally treated samples have been investigated by X-ray diffraction, and HRTEM. An average particle size ranging between 10 and 29 nm and a polyhedral and spherical morphology with a tendency to form aggregates were evidenced by TEM images. Optical absorption measurements reveal that the band gap of ZnO decreased from 3.19 to 2.99 eV, which confirmed the existence of Zn-O-Mn interaction. The incorporation of the Mn ions into the ZnO lattice has been studied by EPR spectroscopy and also, the generation of reactive oxygen species (ROS) has been evidenced by using the EPR coupled with the spin trapping probe technique. Here, we report that in addition to altering the crystallite size, morphology and optical absorption characteristics of ZnO, the introduction of Mn dopant also improves the antibacterial efficiency against pathogenic microorganisms, namely Escherichia coli and Bacillus cereus.

Networked mesoporous SnO2 nanostructures templated by Brij® 35 with enhanced H2S selective performance

Sensors based networked mesoporous SnO2 nanostructures templated by non-ionic surfactant - Brij® 35 were prepared via hydrothermal chemistry route. Specific patterns of the structure, morphology, surface chemistry and sensing properties were obtained by pH and autogenous pressure tuning. Consequently, the as-obtained SnO2 powders were subjected to extensive BET, Raman, TEM, HRTEM and XPS complementary investigations. The sensitive films were obtained by screen printing deposition of the powders onto commercial Al2O3 substrates. The gas sensing properties were assessed towards different hazardous gas species: CO, CH4, NH3, NO2, SO2 and H2S over a wide range of operating temperatures. Our particular SnO2 – HP sensor synthesized at high autogenous pressure showed the highest selective-sensitivity to H2S (<5 ppm) under 50% relative humidity (RH).The enhancement in the H2S sensitivity at low operating temperature under infield conditions was found to be closely connected to the morphological aspects and surface chemistry, peculiarities that can be assessed as consequences of the chemical tuning.

Novel silicon–tungsten oxide–carbon composite as advanced negative electrode for lithium-ion batteries

A cheap Si powder byproduct from solar cell production was applied as a negative electrode for lithium-ion batteries. To improve the cycle and rate performances, the as-obtained Si powder was composited with tungsten oxide and coated with carbon, in sequence. After preparing the composite material, its electrochemical performance is evaluated, which exhibits a high reversible capacity of 975mAhg−1 with improved cycleability. Especially, the rate capability is significantly improved to 390mAhg−1 under 1000mAg−1 applied current. This advanced electrochemical performance is attributed to the matrix effect of the formed Li2O and W metal with carbon coating, as identified using ex-situ X-ray diffraction experiments, and associated with the low polarization resistance observed in the galvanostatic charge intermittent technique.

Synthesis of SiO2-Coated Core–Shell ZnO Composites for Preparing High-Voltage Varistors

Monodispersed ZnO composite microspheres were successfully prepared by a facile ultrasound irradiation method. Then, the uniform core–shell structured composites were synthesized through the hydrolysis of tetraethyl orthosilicate on the surface of the ZnO composite microspheres. Microstructural studies of the as-obtained powders were carried out using the techniques of the x-ray powder diffraction, field emission scanning electron microscopy and transmission electron microscopy with energy dispersive x-ray spectroscopy. The results show that the pink ZnO composite powders as the core were spherical structures with the size of approximately 100 nm, and the SiO2 shell was fully coated on the surface of the core. On the basis of these results, the effect of SiO2 content on the thickness of the synthesized composites and microstructure, as well as the electrical properties of the ZnO varistors sintered in air at 1150°C for 2 h, were fully studied. In particular, the ZnO varistor prepared with the appropriate amount of the SiO2 coating (∼40 nm) leads to a superior electrical performance with the high breakdown voltage of 418 V mm−1 and an excellent nonlinear coefficient of 70.7, compared with the varistors obtained without the SiO2 coating. The high performance is attributed to the smaller and more homogeneous ZnO grains obtained via the SiO2 coating.

In-situ fabrication of nano-sized TiO2 reinforced Cu matrix composites with well-balanced mechanical properties and electrical conductivity

In this work, Cu matrix composites reinforced with rutile-TiO2 nanoparticles were fabricated by a facile in-situ method, which involved the in-situ synthesis of TiO2-Cu composite powders by molecular-level mixing process and subsequent vacuum hot-pressing sintering of the as-obtained composite powders, leading to dense bulk composites with homogeneously distributed TiO2 nanoparticles. The influences of in-situ route on the microstructures, TiO2 distribution and mechanical properties of the composites were investigated. The results showed that the composite with 1.72vol% TiO2 not only exhibited a high yield strength of 290MPa which is about 1.6 times larger than that of pure Cu (110MPa), but also possessed a high fracture elongation of 32% and an outstanding electrical conductivity of 97% IACS, indicating the as-obtained TiO2/Cu composites possessing well-balanced mechanical properties and electron conductivity. Moreover, the strengthening mechanism of TiO2 nanoparticles for the Cu matrix composites was analyzed based on the experimental results.

Raman-spectroscopic investigation of sulfate ion concentration change in a continuously stirred tank reactor during the preparation of a cathode material precursor

The concentration of sulfate ions was continuously monitored by Raman spectroscopy during the preparation of cathode material precursors. The obtained spectra were used for the quantitative analysis of sulfate ions by comparison with a reference solution, and a gradual increase of sulfate ion concentration was detected. Using a mathematical approach, a time-dependent equation describing sulfate ion concentration was theoretically derived, and the calculated concentrations were compared with the measured ones, showing good agreement between these values. Due to the increased sulfate ion concentration, the metal hydroxide cathode precursor became more contaminated with elemental sulfur. Two as-obtained hydroxide powders with different sulfur impurity contents were calcined with a lithium source, producing cathode materials. Electrochemical analysis of the prepared cathode materials showed that morphological aspects (such as particle size) more highly affected the cathode performance more than the sulfur impurity content.

Influence of different pore-forming agents on wollastonite microstructures and adsorption capacities

In this study, porous macro- and micro-cellular wollastonite-based ceramics was synthesized. A ceramic precursor, methylhydrocyclosiloxane, together with micro-sized CaCO3, was used as a starting material. After 20min of ultrasound treatment, and calcination at 250°C for 30min, different pore-forming agents were added to the as-obtained powders. Differential thermal analysis was used to determine characteristic temperatures of processes occurring within powders during heating. Based on the obtained results, the sintering regime was set up. The prepared mixtures were pressed into pallets and sintered at 900°C. During the sintering regime, porous wollastonite-based ceramics was obtained. The phase composition of the sintered samples, as well as the microstructures, were analyzed by using X-ray diffraction and SEM. A two-phase system was detected in all samples, CaSiO3 wollastonite and Ca2SiO4 larnite, and their ratio varied with each pore-forming agent. It was observed that the addition of different pore-forming agents resulted in significantly different microstructures. In a batch test, the influence of pH, the contact time and the initial ion concentration on the adsorption efficiency of As+5, Cr+6and phosphate ions on the synthesized adsorbents were studied. Time-dependent adsorption is best described by the pseudo-second-order kinetic model and the Weber-Morris model, which predict intra-particle diffusion as a rate-controlling step of the overall process. High adsorption capacities, 21.93, 23.88, and 27.29mgg–1, were obtained for the CaCO3-siloxane-nanocellulose sorbent, and similar/lower capacities were obtained for the CaCO3-siloxane-PMMA and CaCO3-siloxane-cotton wool adsorbents.

Microstructures and properties of equiatomic CuZr and CuZrAlTiNi bulk alloys fabricated by mechanical alloying and spark plasma sintering

Equiatomic CuZr and CuZrAlTiNi bulk alloys were designed and fabricated by mechanical (MA) alloying and spark plasma sintering (SPS). The results show that during the MA process, the phases of as-milled samples transform from solid solutions to amorphous phases with increasing milling time. Moreover, the as-milled CuZrAlTiNi amorphous high entropy powders possess higher thermal stability and smaller particle sizes than the as-obtained CuZr powders. After SPS, the bulk CuZrAlTiNi samples are comprised of three solid solutions and a AlNi2Zr phase, and have good phase stability. The CuZrAlTiNi sample sintered at 950 °C shows the higher microhardness value of 1007 HV0.2 due to the fine precipitates, the existence of AlNi2Zr phase and solid solution strengthening. In addition, the chemical corrosion behavior of the as-sintered samples has been investigated in the seawater solution. The 950 °C-sintered CuZrAlTiNi sample exhibits excellent passivity and higher pitting resistance as compared to other sintered samples. All the CuZr and CuZrAlTiNi bulk samples show better corrosion resistance than 304 stainless steel revealed by the reduced corrosion rate.

Synthesis of Carbon-Coated ZnO Composite and Varistor Properties Study

In this article, monodisperse ZnO composite nanoparticles were successfully prepared by sol–gel mixed precursor method. Subsequently, carbon as the shell was homogeneously coated on the surface of the ZnO composite nanoparticles via a simple adsorption and calcination process. Microstructural studies of the as-obtained powders were carried out using the techniques of the x-ray powder diffraction, scanning electron microscopy, field emission scanning electron microscopy, transmission electron microscopy with energy dispersive x-ray spectroscopy, and Fourier transform infrared spectroscopy. The results show that the pink ZnO composite powders were fully coated by carbon. Based on the results, the effect of glucose content on the microstructure of the synthesized composites and the electrical properties of the ZnO varistors sintered in air at 1150°C for 2 h were also fully studied. As the amount of glucose increased, the thickness of carbon can be increased from 2.5 nm to 5 nm. In particular, the ZnO varistor fabricated with the appropriate thickness of the carbon coating (5 nm) leads to the superior electrical performance, with present high breakdown voltage (Vb = 420 V/mm) and excellent nonlinear coefficient (α = 61.7), compared with the varistors obtained without carbon coating.

Formation of porous wollastonite-based ceramics after sintering with yeast as the pore-forming agent

In this paper, synthesis of porous wollastonite-based ceramics was reported. Ceramic precursor, methylhydrocyclosiloxane, together with micro-sized CaCO3, was used as starting material. After 20 min of ultrasound treatment, and calcination at 250 oC for 30 min, yeast as a pore-forming agent was added to the as-obtained powders. Sintering regime was set up based on the results obtained by differential thermal analysis. Prepared mixture was pressed into pallets and sintered at 900 oC for 1 h. After the sintering regime, porous wollastonite-based ceramics was obtained. The phase composition of the sintered samples as well as microstructures was analyzed by X-ray diffraction method and SEM. In a batch test, the influence of pH, contact time and initial ion concentration on adsorption efficiency of As+5, Cr+6, and phosphate ions on synthesized wollastonite-based ceramics were studied. Time-dependent adsorption was best described by pseudo-second-order kinetic model and Weber-Morris model that predicted intra-particle diffusion as a rate-controlling step of overall process. High adsorption capacities 39.97, 21.87, and 15.29 mgg-1 were obtained for As+5, Cr+6, and phosphate ions, respectively.

Phase composition and morphology characteristics of ceria-stabilized zirconia powders obtained via sol-gel method with various pH conditions.

High purity, fine crystalline, degradation-free at low temperature powders have attracted special interest in CAD/CAM prosthetic dentistry full ceramic restorations. This study reports the preparation and characterisation of zirconia-ceria (0.9 ZrO2 0.1 CeO2) powders. Materials were obtained from zirconium-n-alkoxide and cerium nitrate hexahydrate in the pH 2-4 and 8-10. Zirconia- ceria powders were obtained with the sol-gel method in a humid-free environment. Thermal analysis (TGA/DTA) of the as-prepared materials was made for an assessment of its behaviour at elevated temperatures. Specimens were dried at 80 °C and calcinated in two stages: at 300 °C with soaking time 2.5 h and 850 °C with holding time 2.5 h, in order to evaluate the phase transformations. Thermal analyses of the as-dried powders were made for an assessment of its thermal behaviour during heat treatment up to 1000 °C. By X-ray diffraction (XRD), polymorphs of ZrO2 were identified. Additionally, scanning electron microscopy (SEM) and laser particle size distribution (PSD) were involved for characterisation of morphology of the powders. A correlation between the pH of the colloidal system and the morphology of the as-obtained powders were found. Based on analysis (SEM, PSD), structures were identified known as soft and hard agglomerates. It can be stated that differences found between powder morphology were dependent on the value pH used, which can be crucial for powder densification during sintering and compacting green bodies which, as a consequence, may be crucial for the lifetime of zirconia prostheses. Correlations between phase composition and pH are difficult to grasp, and require further, more sophisticated, studies.

Hydrothermal preparation of nickel-manganese oxide with microsphere structure grown on Ni foam and supercapacitive performance

Nickel-manganese oxide film with unique morphology was synthesized and its electrochemical performances were also investigated. The addition of nickel element affects significantly capacitive behaviors of the as-obtained powder. As an electrode material in supercapacitor, nickel-manganese oxide exhibited high specific capacitance of 407Fg−1 at 1Ag−1 and excellent cycling stability of 98.4% retention after 1000 cycles. Its outstanding electrochemical properties are mainly attributed to the synergistic effect between nickel and manganese elements and its novel architecture.

Investigating the Influence of Transition Metal Composition on High-Voltage Cathodes Performance By Using Magnetic Susceptibility

High-voltage cathodes are indispensable to enable the high-energy-density batteries. This work demonstrates the effect of Ni/Mn composition on structure of pristine and cycled layered Li-Ni-Mn/Al-Co oxide (NMC and NCA) during high-voltage operation (4.75V vs. Li). Particle morphology and elemental composition of all material was investigated by using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) analysis. Among the various materials NMC622 had the best uniformity in particle size at ~10um while the other materials ranged from ~5-10um. X-ray diffraction (XRD) spectra of the as-obtained powders showed little variation in the structure of layered oxide with different transition metal ion compositions. Temperature dependent magnetic susceptibility results show that NMC622 (with 60% Ni) is layered with less magnetic ordering which was very similar to that of NCA811 (80% Ni) while NMC442 (40% Ni) showed strong magnetic ordering, which suggests exchange of strong magnetic interaction among transition metal ions in NMC442 material. The structural differences that were observed from magnetic susceptibility results were correlated with the electrochemical performance. The detailed pristine structure and structural evolution in layered oxides varying the Ni concentration in the light of magnetic susceptibility will be presented.

A Direct Ultrasound Irradiation Method Synthesis of Monodisperse ZnO Microspheres for Varistors Ceramics Application

Monodisperse and uniform ZnO microspheres were synthesized via an ultrasound irradiation method. The microstructure and morphology of the as-prepared sample were characterized by x-ray powder diffraction, Fourier transformation infrared spectra and scanning electron microscopy. The results indicate that the size of ZnO microspheres was strongly affected by the Zn(NO3)2·6H2O. As the amount of the precursor increased, the diameters of the ZnO microspheres can be turned from ∼500 nm to ∼2 μm. The electrical properties of the varistors ceramics prepared from the as-obtained ZnO powders were investigated. The results show that the varistors ceramics made from ZnO with a size of ∼500 nm and sintered in air at 1150°C for 2 h possess a density of 5.50 g/cm3 corresponding to 95.1% of the theoretical density, with breakdown voltage of 280.9 V/mm and nonlinear coefficient of ∼61.3.

Facile wick-and-oil flame synthesis of high-quality hydrophilic onion-like carbon nanoparticles

Because of their unique 0-D structure, small (<10 nm) diameter, high electrical conductivity and relatively easy dispersion, compared to 1-D nanotubes and 2-D graphene, onion-like carbon (OLC) has been shown to be ideal as an active material for supercapacitor electrodes. However, its implementation is fraught with lack of convenient methods for their high yield preparation and purification. Here we report a facile scalable and one-step “wick-and-oil” flame synthesis of OLC nanopowder from ‘clarified butter’ without contaminated by any other forms of carbon; thus eliminating the post processing purification procedure. Brunauer–Emmett–Teller (BET) specific surface area of as-prepared OLC was 218 m2/g, which is higher as compared to other reported flame synthesis methods. The as-obtained OLC powder is highly dispersible and the suspension is stable for months indicating the presence of surface functional groups which is confirmed by Fourier transform infrared spectroscopy (FTIR). It is demonstrated that by suitable choice of a precursor material, very pure and hydrophilic OLCs can be prepared in bulk quantities by flame synthesis method. An effective and simple strategy was developed to prepare flexible, binder-free OLC electrodes on cotton fabrics and demonstrated their application in a flexible supercapacitor device. The capacitance of OLC is 102 F/g with a measured power density of 1224 W/kg and an energy density of 3.5 W h/kg in 1 M Na2SO4 as the electrolyte.

Novel synthesis of monodisperse ZnO-based core/shell ceramic powders and applications in low-voltage varistors

In this article, monodisperse ZnO microspheres were successfully prepared by a facile ultrasound irradiation method. Then, by using the microspheres as the templates, the novel uniform ZnO/Co2O3-Bi2O3 core/shell composites were successfully synthesized via a situ precipitation–coating route due to the electrostatic interaction of [BiEDTA]− and [Co(NH3)6]3+. Microstructural studies of the as-obtained powders were carried out using the techniques of the XRD, FTIR, SEM, TEM, TGA and EDS. The results show that the white ZnO powders as the core were spherical structure with the size of approximate 500nm and the thickness of the additive metal oxide shell was about 80nm, respectively. On the basis of the results, the spherical ZnO microspheres fully coated with additional transition metal oxides were identified and Bi12Co0.60O18.80 phase was observed after the ZnO composites were calcinated at 420°C for 4h. The formation mechanism of monodisperse ZnO microspheres and the ZnO-based core/shell structured composites were also brief introduced. The varistors ceramics sintered in air at 1250°C for 2h possesses a density of 5.64g/cm3 corresponding to 97.7% of the theoretical density, with breakdown voltage of 78.6V/mm and nonlinear coefficient of ~7.1.

Experimental studies from FT-IR with TG-DTA analysis of ferrites

Mg0.40Mn0.60-xNixFe2O4 spinel nanoferrites were obtained by the co-precipitation method. The as-obtained powders were characterised by XRD analysis. All nanoferrite powders obtained from different organic precursors, after synthesising, were pure spinel phase, without secondary phases. TEM and AFM analyses confirmed that the average crystallite size, estimated from Scherrer equation, was the smallest for nanoferrites obtained from a mixture of the compounds and showing their particle size ranging from 20 nm to 50 nm. The prepared nanoferrites were characterised by Fourier transform-infrared spectroscopy (FT-IR), thermogravimetry (TG), differential thermal analysis (DTA) and derivative thermogravity (DTG). The results of these observations are described and discussed.

L-cystine-assisted synthesis of Cu2ZnGeS4 nanocrystal powder via solvothermal process

Cu2ZnGeS4 powders were successfully synthesized via a mild solvothermal method using CuCl, ZnCl2, GeCl4, l-cystine, and N, N-dimethylformamide as the precursors to realize the process at 200°C for 24h. XRD, FE-SEM, XPS, and TEM were employed to characterize the microstructures and constituents of the as-obtained Cu2ZnGeS4 powders. The results showed that the crystalline phase of the as-obtained powders was Cu2ZnGeS4, and the size of the Cu2ZnGeS4 powders ranged from 100 to 200nm. UV–vis absorption spectrum showed that the Cu2ZnGeS4 powders exhibited a broad absorption band with a band gap of 2.15eV in the whole visible range.