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.     

One-step synthesis of coral-like high-entropy metal carbide powders

The synthesis of high-entropy metal carbide powders is critical for implementing their extensive applications. However, the one-step synthesis of high-entropy metal carbide powders is rarely studied. Herein, the synthesis possibility of high-entropy metal carbide powders, namely (Zr_0.25Ta_0.25Nb_0.25Ti_0.25)C (ZTNTC), via one-step carbothermal reduction was first investigated theoretically by analyzing chemical thermodynamics and lattice size difference based on the first-principle calculations, and then the ZTNTC powders with particle size of 0.5-2 μm were successfully synthesized experimentally. The as-synthesized powders not only had a single rock-salt crystal structure of metal carbides, but also possessed high-compositional uniformity from nanoscale to microscale. More interestingly, they exhibited the distinguished coral-like morphology with the hexagonal step surface, whose growth was governed by a classical screw dislocation growth mechanism.     

Preparation of uniformly dispersed YAG ultrafine powders by co-precipitation method with SDS treatment

Uniformly dispersed yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) ultrafine powders were synthesized by co-precipitating a mixed solution of aluminum and yttrium nitrates with ammonium hydrogen carbonate in the presence of sodium dodecyl sulfate (SDS) as dispersing agent. The primary purpose of introducing SDS was to protect YAG particles from agglomeration. The evolution of phase composition and micro-structure of the as-synthesized YAG powders were characterized by thermogravimetry/differential scanning calorimetry, X-ray diffraction, infrared spectra and scanning electron microscopy. The results showed that phase-pure YAG powders could be achieved by calcination of the precursor at 900 °C for 2 h. Uniformly dispersed YAG powders with a particle size of approximately 90-100 nm were obtained with optimum molar ratio of Al³⁺ to SDS at 2. And excessive SDS restrained good dispersion of the YAG powders. The dispersion mechanism of SDS in the preparation process was discussed.     

Synthesis of Hydroxyapatite Nanopowders via Sucrose-Templated Sol–Gel Method

The present research describes synthesis of hydroxyapatite (HAp) nanopowders using a sol–gel route with calcium nitrate and ammonium hydrogen phosphate as calcium and phosphorous precursors, respectively. Sucrose is used as template material, and alumina is added as a dopant to study its effects on particle size and surface area. Synthesized powders are characterized using X-ray diffractometry, BET surface-area analysis, and transmission electron microscopy. Results show that alumina stabilizes the HAp crystalline phase. Average particle size of mesoporous HAp samples is between 30 and 50 nm with surface area of 51–60 m²/g.     

微通道反应器中反应沉淀过程的工艺研究

采用Y型和线型微通道反应器,成功制备出平均粒径为35～110nm、无因次方差为0.2～0.3的纳米BaSO4颗粒:同时利用TEM、BET及XRD分别对微反应器和普通反应釜合成的硫酸钡粉体性质进行了表征.实验结果表明,反应物流量增大,混合效率提高,平均粒径及方差下降;初始浓度或体积流量比增加,粒径下降:在相同的工艺条件下,通过较大尺寸Y型微反应器制备的颗粒粒径及方差略大于小尺寸Y型合成的,而线型微反应器合成的产物粒子粒径最小.     

Influence of Tiron concentration on dispersability and sintering behaviors of hydroxyapatite in an aqueous system

Rheological and sintering behaviors of hydroxyapatite slurries with different Tiron concentrations were investigated. It was shown that HAp slurries could be stabilized at pH 11 with 2.25 wt% Tiron concentrations. Results confirmed that well dispersed slurries could be obtained with solid content as high as 50 wt%. Zeta potential measurements were conducted on HAp powders evaluated the influence of dispersant on surface charge properties of the particles. X-ray diffraction technique was used to study the crystalline phase evolution; scanning electron microscopy was used to estimate the particle size of the powder and fracture surfaces of bodies sintered at various temperatures. After sintering process at 1200 and 1350 °C, relative density and bending strength of HAp samples reached 76.67% and 92.18% and 57.77 and 97.36 MPa, respectively.     

Molten salt synthesis,characterization, and formation mechanism of superfine (HfxZr1-x)B2 solid-solution powders

High-purity superfine (Hf_xZr_1-x)B₂ solid-solution powders with various chemical compositions (x = 0.2, 0.5, and 0.8) were successfully synthesized via molten salt synthesis technique at 1423 K using ZrO₂, HfO₂, and 20% excessive B as precursors within a KCl/NaCl molten salt for the first time. The results showed that the as-synthesized solid-solution powders exhibited the irregular polyhedral morphology with the average particle sizes of 170–185 nm and simultaneously possessed a single-crystalline hexagonal structure and good compositional uniformity from nanoscale to microscale. In addition, their formation mechanisms were well interpreted by analyzing the thermodynamics of the possible chemical reactions in the molten salt.     

Sintering behavior and growth mechanism of β-TCP in nanocrystalline hydroxyapatite synthesized by mechanical alloying

Nanocrystalline carbonated HAp powder has been synthesized successfully within 2 h by mechanical alloying the stoichiometric mixture of CaCO₃, CaHPO₄·2H₂O at room temperature under open air. To observe the sintering behavior of HAp the as-milled sample is sintered at different temperatures. The amorphous HAp phase (~14 vol%) in as-synthesized sample transforms completely to crystalline HAp after sintering at 700 °C and after sintering the sample at 800 °C, the crystalline HAp partially transforms to β-TCP phase. Presence of low content of β-TCP phase in HAp powder could be useful in artificial hard tissue applications. Increase in sintering temperature up to 1000 °C results in enhancement of decomposition rate of HAp into β-TCP phase. Microstructure characterization in terms of lattice imperfections and relative phase abundances in non-sintered and all sintered samples are made both by analyzing the respective XRD patterns using Rietveld's structure refinement method as well as TEM images. The growth mechanism of β-TCP from crystalline HAp phase has been proposed based on structure and microstructure characterizations of sintered samples.     

Fully dense ZrB2 ceramics by borothermal reduction with ultra-fine ZrO2 and solid solution

The effects of ZrO₂ particle size (55 nm and 113 nm) and borothermal reduction routes (borothermal reduction with water-washing (BRW) and in situ 5 mol% TaB₂ solid solution, BRS) on synthesis and densification of ZrB₂ were investigated. Irrespective of reduction routes, the use of finer ZrO₂ powders as raw materials resulted in finer ZrB₂ powders. Compared to the powders derived from BRS, the powders derived from BRW had smaller particle size with higher oxygen content, especially the powders synthesized with finer ZrO₂. Irrespective of ZrO₂ particle size, the oxygen contents of ZrB₂ powders prepared by the BRS route were similar. Because of the high oxygen content, the ZrB₂ ceramics synthesized by BRW with finer ZrO₂ demonstrated the lowest relative density (90.5%), which resulted in the lowest Vickers’ hardness (14.2 ± 0.9 GPa). Due to the low oxygen content and small particle size of ZrB₂ powders, fully dense ZrB₂ ceramics (relative density: 99.6%) with highest Vickers’ hardness (16.0 ± 0.2 GPa) were achieved by BRS with finer ZrO₂ powders.     

Atomic layer deposition of ZnO layers on Bi2Te3 powders: Comparison of gas fluidization and rotary reactors

Interface engineering of thermoelectric powder materials via atomic layer deposition (ALD) has attracted significant research interest owing to the dramatic improvement in energy conversion efficiency. Using ALD to uniformly coat ultrathin (a few nanometers) ZnO layers on the microscale irregular shape of bismuth telluride-based powders is a challenge. An ALD reactor that fluidizes or agitates the powders can be adapted for this purpose. In this study, two types of ALD reactors, a gas fluidization reactor and a rotary reactor, were used to coat selenium-doped bismuth telluride powders with ZnO. Uniform and conformal ZnO layers were successfully grown using both the ALD reactors. However, the crystalline structure, particle size distribution, and chemical bonding states of ZnO were affected by reactor type. Pelletization of the ALD-coated powders was performed by spark plasma sintering at a high temperature (500 °C) and pressure (60 MPa). The morphologies of the powders did not change with palletization; however, differences in the chemical states of the ZnO layers on the BTS powders were observed. It was observed that the remnant water molecules and mobile ion species might compensate for the carrier mobility in pellets made of ALD-coated powders.     

Microwave assisted combustion synthesis of nanocrystalline yttria and its powder characteristics

Microwave assisted combustion synthesis is used for fast and controlled processing of advanced ceramics. Single phase and sinter active nanocrystalline cubic yttria powders were successfully synthesized by microwave assisted combustion using the organic fuels urea, citric acid and glycine as reducing agents. The precursor powders were investigated by thermogravimetry (TG) and differential scanning colorimetry (DSC) analyses. The as-prepared precursors and the resulting oxide powders calcined at 1100 °C in oxygen atmosphere were characterized for their structure, particle size and morphology. The thermal analyses (TG/DSC), X-ray diffraction (XRD) and Fourier transform infra red (FT-IR) results demonstrate the effectiveness of the microwave assisted combustion synthesis. The scanning electron microscopy (SEM) observations show the different morphologies of as-prepared powders and transmission electron microscopy (TEM) shows the particle sizes in the range of 30-100 nm for calcined powders for different fuels. The results confirm that the homogeneous, nano scale yttria powders derived by microwave assisted combustion have high crystalline quality and the morphology of the as-prepared precursor powders depends on the nature of organic fuel used.     

Preparation and properties of bimodal porous apatite ceramics through slip casting using different hydroxyapatite powders

A bimodal porous hydroxyapatite (HAp) body with high flexural strength was prepared through slip casting. The effect of different particle sizes on the flexural strength and microstructure of three different types of hydroxyapatite (HAp) powders was studied. The powder characteristic of laboratory-synthesized HAp powder (L-HAp) was obtained through a wet-milling method, drying and heating of a mixture of calcium hydrogen phosphate di-hydrate and calcium carbonate. The median particle size of L-HAp was 0.34 μm, and the specific surface area was 38.01 m²/g. The commercial HAp had median particle sizes for the K-HAp (Kishida chemical Co. Ltd., K-HAp) and T-HAp (Taihei chemical Co. Ltd., T-HAp) of 1.13 and 3.65 μm, and specific surface areas of 11.62 and 6.23 m²/g, respectively. The different powder characteristics affected the slip characteristics, and the flexural strength and microstructure of the sintered porous HAp bodies were also different. The flexural strengths of the porous HAp ceramics prepared by heating at 1200 °C for 3 h in air were 17.59 MPa for L-HAp with a porosity of 60.48%, 3.92 MPa for commercial K-HAp with a porosity of 79.37%, and 4.55 MPa for commercial T-HAp with a porosity of 76.46%.     

Effect of calcination temperature on the microstructure,composition and properties of nanometer agglomerated 8YSZ powders for plasma spray-physical vapor deposition (PS-PVD) and coatings thereof

Homemade nano-agglomerated powders 8YSZ powders for PS-PVD were prepared by the spray drying, then calcination processes at four different temperatures (500 °C, 700 °C, 900 °C and 1100 °C) were carried out on the spray-dried powders. Checked by laser particle sizer, scanning electron microscope (SEM) and X-ray diffraction (XRD), the physical properties, microstructure and phase constitutions of the calcined powders were investigated. The results show that the size of powders calcined at 500 °C is increased relative to the spray-dried powder, whereas the powders calcined at 700 °C, 900 °C and 1100 °C possess smaller size. The binding force of the primary particles tend to rise with the increase of calcination temperature. When the temperature was up to 900 °C and above, it was found that the sintering neck indicating with strong binding was formed between the primary particles. In parallel, the powders underwent an m-ZrO₂ to t-ZrO₂ transition as the calcination temperature rose. It is also found that the PS-PVD prepared coatings which were obtained by using the above powders undergo a transformation from a feather-like to a dense laminate structure as the calcination temperature rises. It is noteworthy that the coating obtained by the powders calcined at 700 °C have a special three-layer composite structure of near dense surface layer, columnar intermediate layer and dense sub-layer. The composite structural coating has excellent adhesion and thermal shock resistance, with a bonding strength of 81MPa and no major spalling when water quenched 100 cycles at 1100 °C.     

Synthesis of boron nitride nanotubes by catalytic pyrolysis of organic-inorganic hybrid precursor

Large quantities of hexagonal boron nitride (h-BN) nanotubes (BNNTs) with high purity have been successfully synthesized under ammonia gas flow at 1200 °C via catalytic pyrolysis of organic-inorganic hybrid precursor which was pre-prepared through a wet chemistry method at 95 °C. Several characterizations, such as SEM, TEM, XRD, FTIR, EDS, XPS and SAED measurements, were used to confirm the morphology, composition and crystalline structure of the as-synthesized powders. It was observed that the obtained product was a kind of nanotubes (NTs) in hexagonal BN phase with a curved shape and smooth surface. The diameter of BNNTs was distributed in a range of 60–200 nm while its length was about tens of microns. The possible growth mechanism of BNNTs was also proposed in this paper.     

Dual role of activated carbon as fuel and template for solution combustion synthesis of porous zinc oxide powders

In this work, nanosized zinc oxide (ZnO) powders were fabricated by urea–nitrate solution combustion synthesis using activated carbon as a structure-directing template and secondary fuel at different fuel–oxidant ratios. The as-synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption–desorption measurements, UV–Vis diffuse reflectance spectroscopy, and photoluminescence. The effect of fuel amount on photocatalytic activity of ZnO powders was evaluated by the degradation of an azo dye Orange G. It was observed that combustion synthesis with activated carbon as a secondary fuel had a profound effect on reducing crystallite size and enhancement of specific surface area. The crystallite size of the as-synthesized powders varied from 46 to 26 nm. The ZnO powder prepared at a fuel–oxidant ratio of 1.8 possessed the small crystallite size and high specific surface area of 69 m²/g. It correspondingly resulted in the highest dye removal percentage of 99% with a rate constant of 0.027 min⁻¹. The improvement in dye degradation can be due to the synergistic interaction and interplay of enhanced surface area and catalytic ability of the photocatalyst. This study provides a simple single-step synthesis methodology to produce metal oxide nanopowders with tunable surface properties for high potential applications in catalysis, optoelectronics, and gas sensors.     

Synthesis of europium-doped calcium silicate hydrate via hydrothermal and coprecipitation method

In this paper, calcium silicate hydrate doped with various amounts of Eu (Eu–CSH) via hydrothermal and coprecipitation methods have been systematically investigated. The hydrothermal method produced xonotlite while coprecipitation gave 11 Å tobermorite. Regardless of the synthesis method, incorporation of Eu inhibited the crystallite growth and particle size of the as-synthesized powders, while increasing their thermal stability. In both phases, Eu³⁺ was found to occupy the cation sites in preference to Ca²⁺. Comparing both synthesis methods, the hydrothermally synthesized powders were of higher crystallinity and thermal stability than the powders prepared by coprecipitation. Moreover, Eu–CSH powder by hydrothermal exhibited stronger photoluminescence intensity than the one by coprecipitation, primarily attributed to its higher degree of crystallinity. Thus, they showed higher potential for nanomedicine application where a combination of biocompatibility and light emission is desired.     

Extrusion-based 3D printing of gelatin methacryloyl with nanocrystalline hydroxyapatite

The particle shape and size distribution of inorganic fillers play a crucial role in the scaffold buildability when those are incorporated in the viscoelastic polymers. In order to address this issue, the phase pure rod-shaped nanocrystalline hydroxyapatite (HAp) powders with varying particle sizes and shapes were synthesized by a one-pot hydrothermal method without any regulatory surfactant at an initial solution pH of 9. As-synthesized nanocrystalline HAp particles (0–5 wt%) were incorporated into 15 wt% pre-cross-linked gelatin methacryloyl (GelMA) hydrogel matrix to fabricate a predesigned scaffold architecture using a custom-made 3D bioprinter. The printing parameters (nozzle diameter, extrusion pressure, and printing speed) were optimized for each composition. The biophysical properties (uniaxial compression behavior, swelling ratio, and in vitro degradation) of the composite hydrogel scaffolds were critically analyzed to unravel the role of nano-sized HAp addition. The compression strength and modulus were substantially improved, while the rate of water uptake and bio-enzymatic degradation significantly reduced with HAp content. We propose that the inorganic–organic nanocomposite hydrogel could be efficiently assembled to formulate a potential bioink for 3D bioprinting applications toward tissue regeneration.     

Combined Crystallization and Drying in a Pilot-Scale Spray Dryer

Combined crystallization and drying of lactose solutions was performed in a pilot-scale spray dryer over a wide range of operating conditions. The effect of different parameters, including temperature, moisture content, atomizing air flow rate, liquid feed rate, main drying air flow rate, and particle size, on the degree of crystallinity of the spray-dried powders was analyzed. Water-induced crystallization (WIC) and modulated differential scanning calorimetry (MDSC) were used to assess the effect of these parameters on the degree of crystallinity of the spray-dried powders. The particles were characterized in terms of the final moisture content using WIC and distinctive differences in the peak heights, which are indicative of the particle crystallinity, were found for spray-dried particles using different drying conditions, supporting the results from MDSC. MDSC showed that decreasing the inlet air temperature by 40°C increased the degree of crystallinity in the particles threefold from 22 to 72%. A decrease in the inlet air temperature may decrease the particle temperature, resulting in wetter particles, and a lower temperature meant a longer particle drying time and allowed the particles to rearrange themselves into a more crystalline form. Up to 72% crystallinity is achievable in a pilot-scale spray dryer by suitable adjustment of the operating conditions. The results suggest differences in the rate of crystallization and particle size between small and pilot-scale spray dryers.     

Synthesis of triclinic and hexagonal SmBO3 powders by mechanically activated annealing of Sm2O3 and B2O3 blends

Samarium borate (SmBO₃) powders were fabricated from oxide raw materials by a two-step solid-state synthesis method including mechanical activation and annealing. Blends containing stoichiometric amounts of samarium oxide (Sm₂O₃) and boron oxide (B₂O₃) were mechanically activated in a high-energy ball mill and subsequently annealed in air. Afterwards, mechanically activated and annealed powders were washed with distilled water in order to remove probable unreacted B₂O₃ phase. The effects of mechanical activation duration (15 min, 1 h, 3 h and 9 h) and annealing temperature (700–1250 °C) on the resultant powders were investigated. Compositional, microstructural, physical, thermal and optical properties of the powders obtained throughout the different process steps were characterized by using an X-ray diffractometry (XRD), particle size analysis (PSA), stereomicroscopy (SM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), gas pycnometry, differential scanning calorimetry (DSC), heating stage microscopy (HSM), atomic absorption spectrometry (AAS), Fourier transform infrared (FTIR) spectrometry and ultraviolet-visible spectrophotometry (UV–vis) techniques. Fine-grained and pure SmBO₃ powders were successfully synthesized via a simple, feasible and scalable route, yielding both triclinic and hexagonal crystal structures. Triclinic SmBO₃ powders were synthesized after mechanical activation for 1 h and annealing at 700 °C for 2 h. The polymorphic transformation temperature of SmBO₃ powders from triclinic to hexagonal is about 1080 °C. Due to the effect of mechanical activation, the synthesis of triclinic SmBO₃ phase and its transformation to hexagonal form were found to take place at ∼50–100 °C lower temperatures than those reported in other methods. Mainly hexagonal SmBO₃ powders were obtained after annealing at 1150 °C in the presence of a very small amount of triclinic SmBO₃. The resultant powders showed intense UV absorptions in the range between 1025 and 1150 nm with minimum reflectivity of 0.57% (triclinic SmBO₃ phase) and 0.68% (hexagonal SmBO₃ phase) depending on their crystal structures.