Effect of synthesis parameters on the size of cobalt ferrite crystallite

Nanoparticles of cobalt ferrite (CoFe₂O₄) were synthesized by the EDTA/Citrate complexing method and hydrothermal method without addition of surfactant. The influence of the pH of the reaction medium (8, 9 or 10), the temperature of the thermal treatment (600 °C, 800 °C or 1000 °C for the EDTA/Citrate method, and 120 °C, 140 °C or 160 °C for the hydrothermal method), and the duration of the thermal treatment (2, 4 or 6 h for the EDTA/Citrate complexing method, and 6, 15 or 24 h for the hydrothermal method) on the average crystallite size was studied by means of an experimental design based on the results obtained by XRD. Statistical analysis led to quantification of the influence of the synthesis parameters on the crystallite size of the powders. Results showed that the pH of the reaction medium is the parameter that shows the greatest influence on the growth of the crystallites of the powders obtained by the hydrothermal method, while calcination temperature is the most significant one for the powders produced by the EDTA/Citrate complexing method.     

Evaluating the photocatalytic activity of Pt/TNT film catalyst

TiO₂ nanotubes (TNT) were prepared by a hydrothermal method from the commercially available TiO₂-P25. Five types of TNT were produced at different temperatures (120 °C, 130 °C, and 150 °C) and by using different reaction times (12 h, 24 h, and 30 h). The photocatalytic reactor that was used is a film catalytic reactor, in which the height of the catalyst is 1.0 mm. The BET and FESEM analysis results showed that TNT130-24 (130 °C, 24 h) and TNT150-12 (150 °C, 12 h) possessed well-formed tubular structures with a high specific surface area (282.9–316.7 m² g⁻¹) and large pore volumes (0.62–0.70 cm³ g⁻¹). However, TNT120-30 (120 °C, 30 h) presented the best photocatalytic activity upon CO removal due to the synergistic effect of TiO₂ nanotubes and TiO₂ particles. After the TNT catalysts were modified with Pt particles, the removal efficiency was in the order of Pt/TNT120-30>Pt/TNT130-24>Pt/P25. Pt/TNT120-30 showed 99% removal efficiency in a continuous photoreactor with a high space velocity of 1.79×10⁴ h⁻¹. The results of the TEM and DRS analyses confirmed that the Pt particles enhanced the photocatalytic reaction, which was attributed to the well-dispersed nature of the 1 nm nanoscaled Pt particles on the surfaces of the TNT catalysts, and narrowed the band gap from 3.22 eV to 3.01 eV.     

Preferential occupancy of metal ions in the hydroxyapatite solid solutions synthesized by hydrothermal method

The solid solutions in the systems of Ca-Cd HAp [Ca_10−xCd_xHAp (x = 0–10)], Ca-Sr HAp [Ca_10−xSr_xHAp (x = 0–10)] and Ca-Pb HAp [Ca_10−xPb_xHAp (x = 0–10)], were successfully synthesized at 200 °C for 12 h under hydrothermal conditions. The site of the metal ions in the solid solutions was analyzed by the Rietveld method. The results of the Rietveld analysis indicated that the metal ions of Pb²⁺, Sr²⁺, and Cd²⁺ all preferentially occupied M (2) sites in the apatite structure. The preferential occupancy of the metal ions in M (2) sites were explained mainly by their ionic radius and electronegativity.     

Hydrothermal synthesis of titanate nanostructures with high UV absorption characteristics

The titanate nanostructures with high UV absorption characteristics could be fabricated by hydrothermal method within a temperature range of 90–150 °C. TEM, XRD, BET analyses, and UV–vis spectroscopy were employed to elucidate the synthesized titanate nanostructure characteristics which were microstructure, phase transformation, specific surface area, and band gap energy, respectively. With an increase in the hydrothermal treating temperature from 90 to 120 °C, the specific surface area of titanate nanostructures was increased from 83 to 258 m²/g, while the band gap energy of titanate nanostructures was increased from 3.44 to 3.84 eV and then slightly decreased to 3.81 eV at 150 °C. The fabricated titanate nanostructures could exhibit higher UV adsorption capability but lower photocatalytic activity when compared with that of commercial TiO₂ powders.     

Hydrothermal synthesis of 3D TiO2 nanostructures using nitric acid: Characterization and evolution mechanism

Various morphologies of TiO₂ nanostructures were synthesized by HNO₃ assisted hydrothermal treatment with respect to the acid molarity (1 M, 3 M, and 8 M), temperature (110, 140, and 180 °C), and time (1, 3, and 6 h). An additional sample was synthesized inside the protonated titanate nanoribbon coated vessel with the acid molarity of 8M at 140 °C for 3 h. The crystal structure and morphology of the nanostructures synthesized were investigated using X-Ray diffractometer, scanning electron microscope, and transmission electron microscope. The results revealed that lower acid concentrations, longer synthesis durations and higher temperatures favored anatase phase formation. Meanwhile, a phase pure 3D lotus structure rutile TiO₂ could be obtained by hydrothermal synthesis at 8M HNO₃ concentration at 140 °C for 3 h using protonated H-titanate nanoribbons. A probable mechanism for the evolution of 3D rutile lotus structure was highlighted.     

Characteristics of nano-sized perovskite structured LaSrMn derived from hydrothermally synthesized amorphous LaSrMn oxide powder

Nano-sized and spherical shaped LaSrMn oxide particle was synthesized by thermal treatment at 900 °C for 3 h after hydrothermally treatment at 200 °C for 8 h, and characterized to be applied as a cathode material in solid oxide fuel cells. A spherical LSM nano-particle with sizes ranging from 100 to 120 nm was shown in SEM image and particle size distribution. The pellets of five types composed of x-wt% LSM synthesized and commercial y-wt% YSZ were manufactured and pre-sintered. Their ionic conductivities were enhanced responding to LSM concentrations, and it was the highest in 8LSM:2YSZ showing 74.0 S cm⁻¹.     

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.     

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%.     

Preparation of zinc oxide ceramics with a sustainable antibacterial activity under dark conditions

Fabrication of ZnO ceramics with a sustainable antibacterial activity even in the dark has been conducted. Fine ZnO powders were hydrothermally treated in 0.5–3 mol ml^?1 Zn(NO₃)₂ aqueous solutions at 110–180 °C for 3–20 h. After an uniaxial pressing of the ZnO powders thus prepared, they were sintered at 400–600 °C for 1 h in air. Sustainability in antibacterial activity was evaluated using a colony count method with Escherichia coli bacteria on nutrient agar medium (36 °C/24 h) in a Na–P-buffer solution. The best data was attained for the ZnO ceramics prepared from the following conditions: a 3 mol ml^?1 zinc nitrate solution for the hydrothermal treatment at 120 °C for 7 h and sintering in air with a step-by-step pattern (470 °C/1 h–485 °C/1 h–500 °C/1 h). ESR and chemical photoluminescence analyses have cleared that radical oxygen of super-oxide (O₂^?) originated from the surface of ZnO might exhibit an antibacterial activity even under the dark condition.     

Preparation of dense β-CaSiO3 ceramic with high mechanical strength and HAp formation ability in simulated body fluid

In present study, dense CaSiO₃ (CS) ceramics have been fabricated through spark plasma sintering (SPS) technique using β-CS powder prepared by chemical precipitation method. The β-CS ceramic sintered at 950 °C has a relative density of about 95% and shows a fine microstructure with an average grain size of 0.6 μm, thus expresses good bending strength of about 294 MPa. The simulated body fluid (SBF) immersion tests show that the dense β-CS ceramic has a high hydroxyapatite (HAp) formation rate on its surface. The HAp layer formed on the CS ceramic surface has a granular structure consisting of silkworm-like HAp grains, and the thickness of HAp and Si-rich layer are 70 and 120 μm, respectively.     

From nanometric zirconia powder to transparent polycrystal

Coprecipitated zirconia-yttria (8 mol%) gel subjected to hydrothermal treatment at 240 °C resulted in the solid solution powder of 8 nm particle sizes and specific surface area 132.7 m²/g. Uniaxial compaction followed by cold isostatic pressing under 300 MPa resulted in samples of the extremely small and narrow pore size distribution. Such samples start to shrink at about 200 °C which is related to the desorption of water layers surrounding particles. The state of closed porosity is achieved at 1150 °C. Pore closing was performed in air or oxygen atmosphere. Hot isostatic pressing at 1150 °C for 2 h under 250 MPa argon pressure led to transparent materials. Some pores remained in the material whose preliminary pore closure was performed in air. The samples initially sintered in oxygen atmosphere show no porosity and higher light transmittance than the former ones.     

Temperature-sensitive hydrogel modified by polymerizable liquid crystal AAc-Brij-58: Optical and protein adsorption/desorption behaviors

To avoid template-like action of polyoxyethylene 20 cetyl ether (Brij-58) in hydrogel matrix, a polymerizable liquid crystal AAc-Brij-58 based acrylic acid (AAc) and Brij-58, and poly(NIPAm-co-AAc-Brij-58) (ACHX) hydrogels based on N-isopropylacrylamide (NIPAm) and AAc-Brij-58 were synthesized by esterification and free-radical polymerization. The chemical structures of resulting ACHX hydrogels were confirmed by FTIR and ¹H NMR. The optical property of ACHX pre-polymerization solution was monitored by ultraviolet–visible spectrophotometry (UV/Vis). The pore structure was observed by scanning electron microscopy (SEM). The adsorption and desorption behaviors of Bovine Serum Albumin (BSA) were investigated by temperature-oscillating between 37 °C and 25 °C. The result shows that AAc-Brij-58 and ACHX hydrogels have been successfully synthesized. With increasing mass percent of AAc-Brij-58/NIPAm from 0 to 10 wt%, the transparency of ACH01-10 hydrogel decreases to 0 at 20, 17, 14, 12 and 16 min, respectively. The interior of ACHX hydrogels presents honeycomb structure with thick pore wall with pore size from 120 to 600 μm, but shows vent-like structure on the outer surface after copolymerization by AAc-Brij-58. The adsorption and desorption of ACHX hydrogels exhibit reversible temperature oscillation responsibility between 37 °C and 25 °C. As mass percent of AAc-Brij-58/NIPAm is 1 wt%, ACH01 hydrogel presents highest adsorbed BSA amount, about 1090 ± 24 mg/g at 37 °C. After desorption at 25 °C, columniform BSA aggregates on the surface of ACHX hydrogels were not observed.     

Continuous synthesis of lithium iron phosphate (LiFePO4) nanoparticles in supercritical water: Effect of mixing tee

Continuous supercritical hydrothermal synthesis of olivine (LiFePO₄) nanoparticles was carried out using mixing tees of three different geometries; a 90° tee (a conventional Swagelok^® T-union), a 50° tee, and a swirling tee. The effects of mixing tee geometry and flow rates on the properties of the synthesized LiFePO₄, including particle size, surface area, crystalline structure, morphology, and electrochemical performance, were examined. It was found that, when the flow rate increased, the particle size decreased; however, the discharge capacity of the particles synthesized at the high flow rate was lower due to the enhanced formation of Fe³⁺ impurities. The use of a swirling tee led to smaller-sized LiFePO₄ particles with fewer impurities. As a result, a higher discharge capacity was observed with particles synthesized with a swirling tee when compared with discharge capacities of those synthesized using the 90° and 50° tees. After carbon coating, the order of initial discharge capacity of LiFePO₄ at a current density of 17 mA/g (0.1C) and at 25 °C was swirling tee (149 mAh/g) > 50° tee (141 mAh/g) > 90° tee (135 mAh/g). The carbon-coated LiFePO₄ synthesized using the swirling tee delivered 85 mAh/g at 20C-rate and at 55 °C.     

Thermal cycling induced capacity enhancement of graphite anodes in lithium-ion cells

Graphite electrodes were electrochemically cycled in Li-ion cells at 50 and 60 °C in order to determine the changes in their surface properties in comparison to the electrodes tested at 25 °C. A 17% drop in planar capacity occurred during the first cycle at 60 °C compared to a 40% at 25 °C and reduced the amount of damage that occurred to graphite due to a rapidly formed solid electrolyte interphase (SEI). During the following cycles, a planar capacity of 3.11 ± 0.12 mAh cm⁻² was attained at 60 °C rather than 0.53 ± 0.03 mAh cm⁻² at 25 °C. The SEI layer formed at 60 °C predominantly consisted of Li₂CO₃ and was devoid of residual LiClO₄ detected at 25 °C. At 25 °C, the diffusion coefficient of Li⁺ (D_Li⁺) was calculated as 1.07 × 10⁻⁸ cm² s⁻¹, whereas at 60 °C, D_Li⁺ increased to 3.25 × 10⁻⁸ cm² s⁻¹. A pre-treatment conducted at 60 °C enhanced the cyclic performance of graphite subsequently cycled at 25 °C; a Li₂CO₃-enriched SEI, generated during the 60 °C pre-treatment, covered the graphite surface uniformly and resulted in a 28% increase in battery capacity at 25 °C.     

Biomimetic synthesis and characterization of carbon nanofiber/hydroxyapatite composite scaffolds

Three dimensional electrospun carbon nanofiber (CNF)/hydroxyapatite (HAp) composites were biomimetically synthesized in simulated body fluid (SBF). The CNFs with diameter of ∼250 nm were first fabricated from electrospun polyacrylonitrile precursor nanofibers by stabilization at 280 °C for 2 h, followed by carbonization at 1200 °C. The morphology, structure and water contact angle (WCA) of the CNFs and CNF/HAp composites were characterized. The pristine CNFs were hydrophobic with a WCA of 139.6°, resulting in the HAp growth only on the very outer layer fibers of the CNF mat. Treatment in NaOH aq. solutions introduced carboxylic groups onto the CNFs surfaces, and hence making the CNFs hydrophilic. In the SBF, the surface activated CNFs bonded with Ca²⁺ to form nuclei, which then easily induced the growth of HAp crystals on the CNFs throughout the CNF mat. The fracture strength of the CNF/HAp composite with a CNF content of 41.3% reached 67.3 MPa. Such CNF/HAp composites with strong interfacial bondings and high mechanical strength can be potentially useful in the field of bone tissue engineering.     

The densification of zeolite/apatite composites using a pulse electric current sintering method: A long-term assurance material for the disposal of radioactive waste

Pulse electric current sintering (PECS) method was applied to the fabrication of zeolite, hydroxyapatite (HAp) and fluorapatite (FAp) sintered bodies that should be long-term assurance materials for the disposal of radioactive waste. The weight ratio of zeolite and apatite was ca. 3/7. Zeolite powder evenly covered with HAp thin layers prepared by a hydrothermal method and spherical FAp powder by spray dryer were employed for the PECS; the sample was sintered at 900 °C for 10 min at a rate of 50 °C/min under a uniaxial pressure of 50 MPa and then cooled to 600 °C at 5 °C/min in vacuum. The powder X-ray diffractions indicated that the structure of zeolite changed to the amorphous. The zeolite powder was well dispersed in FAp matrix as the results of element mapping analyses by energy dispersive X-ray spectrometer. The observations by a scanning transmission electron microscope indicated that amorphous zeolites were covered with needle-like HAp crystals of which layer completely coupled with sintered FAp grains. The HAp thin layers thus play an important role for improving the affinity between FAp matrix and the zeolite. The microhardness and three-point bending strength of the sintered bodies were also elucidated by a dynamic-ultra microhardness tester and a universal tester, respectively.     

Application of Eu3+ as a fluorescence probe for phase transformation of hydrothermally prepared YSZ nanopowder

In this contribution, well-size-distributed nanopowder of 8YSZ:Eu³⁺ composite oxide was synthesized by hydrothermal method. TG-DSC, SEM, TEM and XRD were applied to characterize the thermal decomposition, morphology and crystal structure of nanopowder. The thermal stability of the powder was investigated by thermal treated at temperatures ranging from 500 °C to 1300 °C for 2 h. Fluorescence properties variation and phase composition variation of the powder were investigated after thermal exposure at 1300 °C for 192 h. The relationship between the phase composition variation and fluorescence properties variation was further examined. The results show that when monoclinic phase is absent, the intensity ratio I₅₉₃/I₆₀₈ of fluorescence spectroscopy can be used to preliminary assess the extent of the phase transformation.     

Preparation and characterization of LnMgAl11O19 (Ln=La,Nd, Gd) ceramic powders

Lanthanide hexaaluminate powders of LaMgAl₁₁O₁₉ (LMA), NdMgAl₁₁O₁₉ (NMA) and GdMgAl₁₁O₁₉ (GMA) were synthesized via the solid state reaction or sol–gel and calcination method. The LMA and NMA powders synthesized by the sol–gel and calcination method at 1600 °C for 8 h exhibit a single hexaaluminate phase with magnetoplumbite structure; however, the GMA powder synthesized by the sol–gel and calcination method at 1600 °C for 8 h contains both a hexaaluminate phase and a small amount of second phase GdAlO₃ with a perovskite structure. The powders synthesized by the solid state reaction method at 1500 °C for 4 h have a small particle size of 1–3 μm, and a large specific surface area and a good uniformity. The powders synthesized by the sol–gel and calcination method at 1600 °C for 8 h have a particle size of 5–20 μm, and exhibit to a certain extent agglomeration.     

Spark plasma sintering of transparent Y2O3 ceramic using hydrothermal synthesized nanopowders

Y₂O₃ nanopowders were synthesized by the hydrothermal treatment of Y(NO₃)₃·6H₂O and citric acid (CA) as Y⁺³ and the capping agent, respectively. The effect of different CA:Y⁺³ mol ratios, heat treatment time, and calcination temperature was investigated in order to determine their influence on the morphology, particle size and phase of Y₂O₃ nanopowders. The narrow size distribution of particles was obtained with CA:Y⁺³ mol ratio=1.6, heat treatment time of 6 h, and a calcination temperature at 900 °C for 90 min. Then, the synthesized Y₂O₃ nanopowder was consolidated by the spark plasma sintering technique at 1500 °C with a heating rate of 100 °C/min and held for 8 min before turning off the power. As a result, the ceramic prepared with 3 mm thickness got the highest transmission of 80% at 2.5–6 µm wavelength. The highest density and the grain size of yttria ceramic were 99.58% and 1–1.2 µm at 1500 °C, respectively.     

Application of hydrophilic ionic liquid treatment to the morphological observations of hydrated porous ceramic green bodies

In this study, a simple and convenient method for observing the surface morphology of hydrated porous ceramic green bodies is proposed. The porous hyrdoxyapatite (HAp) green body was prepared by a gelcasting process and was dried in a humid chamber from 90 to 50% relative humidity at 25 °C before subsequent treatment with a hydrophilic ionic liquid (IL). The surface morphology of the IL-treated porous HAp green body was observed using FE-SEM. The results showed that the pore morphology and microstructure of the HAp green body was readily observable without evidence of charging. The as-prepared sample showed pores approximately 300–600 μm in diameter, which gradually contracted to approximately 200–400 μm upon drying in the humid chamber. Following sintering at 1000 °C, the pores had further contracted to approximately 100–300 μm. The IL binds with the surrounding water to prevent the sample from drying in vacuum and acts as a conductive media, allowing the HAp ceramics to be observed in the electron microscope. In comparison to the micro-focused X-ray CT analysis, the fine pore structure (less than 100 μm) could only be observed using FE-SEM when the porous body had also been subjected to the IL treatment.