Preparation and characterization of tetragonal-ZrO2 nanopowders by a molten hydroxides method

Pure tetragonal-ZrO₂ nanopowders are prepared by a molten hydroxides method, using hydrated zirconium nitrate as the starting material at 200 °C. X-ray diffraction analysis and transmission electron microscopy observation reveal that the nanopowders exhibit poor crystalline quality. After heat treated at 400 °C for 10 h in air, the nanopowders are crystalline with size range of ∼10–12 nm and most of them are agglomerated. The formation mechanism of the ZrO₂ nanopowders has been proposed. The heat treated nanopowders have a BET surface area of ∼27.3 m²/g due to agglomeration. The photoluminescence of the heat treated nanopowders has been investigated at room temperature.     

Improving sintering behavior of MWCNT/BaTiO3 ceramic nanocomposite with Bi2O3-B2O3 addition

The present research systematically investigated the novel low-temperature fabrication of a multi-walled carbon nanotube (MWCNT)/barium titanate nanocomposite using a two-step mixing technique. The synthesis was conducted using different amounts of MWCNT (0.25%, 0.5%, 1%, 2%, 4%, and 8% wt) with different compositions of (Bi₂O₃ + B₂O₃) as a sintering aid. Scanning and transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, three-point bending strength, Vickers hardness indentation and Archimedean technique were used to characterize the as-synthesized specimens. It was found that the appropriate content of sintering aid (Bi₂O₃+B₂O₃) strongly decreased the sintering temperature from 1200 °C to 950 °C. The results also revealed that any sintering aid with the optimum composition that included 30% (mol) Bi₂O₃ was optimal for a sintering aid content of 6% (wt). Consequently, the highest values of the flexural strength and fracture toughness were achieved by applying the optimal amounts of MWCNT (1% wt) and sintering aid (6% wt).     

Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures

In this work we successfully obtained freeze-cast alumina (Al₂O₃) and magnesium aluminate spinel (MgAl₂O₄) samples. Camphene was used as the freezing vehicle in this study. The specimens prepared herein were examined by Archimedes tests, scanning electron microscopy, and X-ray powder diffraction. Cold crushing tests were also carried out at room temperature. It was observed that the pore structure of Al₂O₃ samples can be tailored by changing the solid loading and freezing rate; the higher the solid loading and freezing rate, the finer the pore structure of the freeze-cast sample. MgAl₂O₄-based specimens were fabricated by keeping the solid loading in the starting slurry at 30 vol% and using liquid nitrogen as the cooling agent. The material obtained from a 60 Al₂O₃?40 MgO slurry showed a spinel amount of about 90%, an expressive total porosity (63 ± 3%), and a significant cold crushing strength (58 ± 6 MPa). In addition, this material exhibited the finest pore structure among the composition studied herein, showing a mean pore size of about 4 µm.     

Microstructure evolution of Al2O3/Al2O3 joint brazed with Ag-Cu-Ti + B + TiH2 composite filler

Al₂O₃/Al₂O₃ joint was achieved using Ag-Cu-Ti + B + TiH₂ composite fillers at 900 °C for 10 min. The evolution mechanism of interface during brazing was discussed. Effects of Ti and B atoms content on microstructure of joints were investigated. Results show that a continuous and compact reaction layer Ti₃(Cu,Al)₃O forms at Al₂O₃/brazing alloy interface. Ti(Cu,Al) precipitates near Ti₃(Cu,Al)₃O layer. In situ synthesized TiB whiskers evenly distribute in Ag and Cu based solid solution. The higher content of B powders in composite fillers increases TiB whiskers content, but decreases the thickness of Ti₃(Cu,Al)₃O layer, while the higher TiH₂ powders content thickens Ti₃(Cu,Al)₃O layer. Ag and Cu based solid solutions become uniform and fine with the increasing of TiB whiskers content. Ti(Cu,Al) intermetallics content increase and they gradually distribute from Al₂O₃ side to the central of brazing alloy, but the content of Cu based solid solution decreases when the TiH₂ content increases.     

CVD synthesis of Al2O3 nanotubular structures using a powder source

Alumina (Al₂O₃) 1D nanotubular structures were synthesized by chemical vapor deposition (CVD) using aluminum (Al) and Al₂O₃ powder sources at a temperature of 1300 °C and a pressure of 100 Pa. At present, no research has been published regarding the synthesis of α-phase Al₂O₃ nanotubes using only a powder source. In this work, we attempted to grow α-phase Al₂O₃ nanotubes without catalysts. As-deposited nanotubular structures had an irregular and ragged surface morphology that was related to the boundary layer thickness. Moreover, complementary nanotubular structures can be obtained via an annealing process. A model to describe the irregular nanotubular structure formation was suggested, and the effect of the boundary layer thickness was demonstrated in our experimental conditions.     

Low-temperature pressureless sintering of Al2O3-SiC-Ni nanocermets in air environment

This paper introduces a simplified method for low-temperature pressureless sintering of Al₂O₃-Ni-SiC nanocermets in air environment. In this method, a thin and continuous Ni shell was coated on the surface of Al₂O₃ particles using electroless deposition method. The composite powders were subsequently compressed to prepare bulk specimens. By preventing the ceramic particles from direct contact during the densification of green specimens, sintering temperature of cermet materials was reduced from that of Al₂O₃ (>?1400?°C) to the range of Ni solid-phase sintering temperature. Furthermore, dissolution of a low amount of phosphorus in the composition of Ni coatings caused the further decrease of the sintering temperature to 800?°C. At such low temperatures, pressureless sintering of the cermets in the air environment was successfully performed instead of the common hot pressing process in a reducing atmosphere. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD) characterizations indicated that the microstructure of such sintered samples consists of a continuous Ni network surrounding Al₂O₃ grains, without any structural defects or Ni oxidation. Furthermore, mechanical properties of the cermet materials were improved through reinforcement of the continuous Ni network by different amounts of SiC nanoparticles. The results showed that Al₂O₃-Ni-5?wt% SiC nanocermets sintered at 800?°C obtain the highest compressive strength of 242.5?MPa, hardness of 56.8 RA, and the lowest wear weight loss of 0.04?mg/m.     

Synergistic effect in structural and supercapacitor performance of well dispersed CoFe2O4/Co3O4 nano-hetrostructures

Herein, we report a facile homogeneous urea – assisted hydrothermal approach for the design of CoFe₂O₄/Co₃O₄ nano hetrostructure. A variation in Co concentration leads to smartly designed composite material namely CFC-11 and CFC-12 where CFC-12 appreciates the benefits of both CoFe₂O₄ and Co₃O₄ nanoparticles. The physico – chemical properties of as developed materials were investigated by X-ray diffraction (XRD), field emission electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron microscopy (XPS) and Raman spectroscopy. The specific surface area and pore size distribution was determined by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halendo (BJH) respectively. Magnetic measurements via. vibrating sample magnetometer (VSM) demonstrate that saturation magnetization decreases with the incorporation of Co₃O₄ antiferromagnetic nanoparticles. To explore the utility of as designed nano-hetrostructures as supercapacitor electrodes, we employed cyclic voltammetry (CV) and electrochemical impedence spectroscopy (EIS) measurements. A high specific capacitance of 761.1?F?g^?1 at 10?mV?s^?1, admirable cyclic durability of 92.2% and a low resistance value obtained from impedence measurements was observed for CFC-12. The favorable performance demonstrates the synergistic effect of CoFe₂O₄ and Co₃O₄ nanoparticles and thus promise an excellent material for energy storage devices.     

Synthesis of LaB6 powders from La2O3, B2O3 and Mg blends via a mechanochemical route

This study reports a room temperature mechanochemical route for the synthesis of LaB₆ powders from La₂O₃–B₂O₃–Mg blends. The synthesis reaction was driven by high-energy ball milling and was gradually examined in terms of milling duration and process control agent. Following the mechanochemical synthesis, unwanted MgO phase and Fe contamination worn off from the milling vial/balls were removed with HCl acid leaching under the effect of ultrasonic stirring. Pure LaB₆ powders were obtained after repeated centrifuging, repeated washing and drying. Subsequent annealing was performed in a tube furnace at 800 °C for 5 h under Ar atmosphere in order to reveal residual elements. Phase and microstructural characterizations of the milled, leached and annealed powders were performed using X-ray diffractometry (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. A novel route for producing fine-grained LaB₆ powders was accomplished with shorter reaction times resulting in higher purity.     

HF/HCl acid resistance mechanisms of alumina ceramics in the Al2O3-MgO-CaO-SiO2-Y2O3 system

Silicon compounds in raw materials are the main reason for the low HF/HCl acid resistance of alumina ceramics. Y₂O₃ can improve the acid resistance of alumina ceramics. This work aimed to reveal the mechanisms of the effects of Y₂O₃ on the form of Si and the durability of the ceramic. An experiment on a high-temperature reaction between Y₃Al₅O₁₂ and a polycrystalline alumina ceramic was designed. The effect of corrosion time on the acid solubility of the alumina ceramic was investigated. The results show that Si can dissolve in Y₃Al₅O₁₂ to generate solid solutions, impeding the generation of Si-containing compounds with bad acid resistance, and decreasing the content of amorphous Si. The acid solubility of the ceramic was only 0.95%, even when the corrosion time was extended to 60 times the industry standard. This revelation of the acid resistance mechanisms can provide a new idea for designing corrosion-resistant ceramics.     

Synthesis of Al2O3 whisker-reinforced yttria-stabilized-zirconia (YSZ) nanocomposites through in situ formation of alumina whiskers

Enhanced mechanical properties of yttria-stabilized zirconia based composites were obtained by introducing alumina whiskers as reinforcement. The alumina whiskers were formed in situ by decomposition of ammonium aluminum carbonate hydroxide (AACH) whiskers during calcination. The whiskers thus formed were amorphous and were converted to α-alumina during sintering at 1450 °C. The AACH whiskers were produced by hydrothermal treatment of an aqueous solution of urea and aluminum nitrate at 120 °C for 24 h. The Vickers hardness of the sintered composite sample increased with an increase in the alumina content up to 10 wt% and then decreased. The maximum hardness achieved at 10 wt% of alumina whiskers was 13.8 GPa, which further increased to 14.4 GPa with the addition of 1.0 wt% of cetyl-trimethyl-ammonium bromide (CTAB). The improved mechanical strength of the composites was attributed to the enhanced dispersion of alumina whiskers due to four times volume decrease during transformation of AACH to alumina and relatively lower aspect ratio of AACH whiskers as well as the deflocculating effect of CTAB.     

Effects of phase and chemical composition of precursor on structural and morphological properties of (Lu0.95Eu0.05)2O3 nanopowders

Europium-doped lutetium oxide nanopowders have been synthesized by the co-precipitation technique using ammonium hydrogen carbonate as a precipitant. Effects of chemical and phase composition of carbonate precursors on the morphology and sinterability of (Lu_0.95Eu_0.05)₂O₃ nanopowders have been studied. Two types of precursors have been obtained differing by the molar ratio R=NH₄HCO₃/Lu³⁺: a mixture of crystalline Lu_0.95Eu_0.05(OH)(CO₃)·4H₂O and unidentified amorphous phases at R=4–7 and crystalline Lu_0.95Eu_0.05(H₂O)_x(HCO₃)₃·nH₂O precursor at R=8–20. The two-phase precursor consists of spherulite-like aggregates, while the crystalline one is characterized by plate-like morphology. Calcination of Lu_0.95Eu_0.05(H₂O)_x(HCO₃)₃·nH₂O leads to formation of (Lu_0.95Eu_0.05)₂O₃ nanopowders that inherit the precursor morphology, while no morphology succession is observed for (Lu_0.95Eu_0.05)₂O₃ nanopowders obtained by heat treatment of the two-phase precursor. Calcination of the two-phase mixture leads to breakdown of the spherulites and to formation of equiaxed particles with an average diameter of 40 nm with the standard deviation of particle size distribution of about 15%. The obtained low-agglomerated nanopowders were used in vacuum sintering to produce (Lu_0.95Eu_0.05)₂O₃ optical ceramics with in-line transmittance of 41% at 611 nm.     

Effect of Al2O3 on mechanical properties of Ti3SiC2/Al2O3 composite

The effect of Al₂O₃ on mechanical properties of Ti₃SiC₂/Al₂O₃ composite fabricated by SPS was studied systematically. The results show that the hardness of the Ti₃SiC₂/Al₂O₃ composite can reach 10.28 GPa, 50% higher than that of pure Ti₃SiC₂. However, slight decrease in the other mechanical properties was observed with Al₂O₃ addition higher than 5–10 vol.%, which is believed to be due to the agglomeration of Al₂O₃ in the composite.     

The nucleation and crystallization of MgO-B2O3-SiO2 glass

The nucleation and crystallization of MgO-B₂O₃-SiO₂ (MBS) glass were studied by means of a non-isothermal, thermal analysis technique, X-ray diffraction and scanning electron microscopy. The temperature range of the nucleation and the temperature of the maximum nucleation rate for MBS glass were determined from the dependences of the inverse temperature at the DSC peak (1/T_p) and the maximum intensity of the exothermic DSC crystallization peak ((δT)_p) on the nucleation temperature (T_n). For MBS glass the nucleation occurred at 600-750 °C, with the maximum nucleation rate at 700 °C, whereas the nucleation and crystal growth processes overlapped at 700 °C < T ≤ 750 °C. The analyses of the non-isothermal data for the bulk MBS glass using the most common models (Ozawa, Kissinger, modified Kissinger, Ozawa-Chen, etc.) revealed that the crystallization of Mg₂B₂O₅ was three-dimensional bulk with a diffusion-controlled crystal growth rate, that n = m = 1.5 and that the activation energy for the crystallization was 410-440 kJ/mol.     

Effect of surface-modified Al2O3 on the thermomechanical properties of polybutylene succinate/Al2O3 composites

This study investigates the effect of the incorporation of alumina particles on the thermomechanical properties of polybutylene succinate (PBS)/Al₂O₃ composites. The alumina surface was modified with the carboxylic groups of maleic acid through simple acid-base and in situ polymerization reactions. Scanning electron microscope (SEM) results revealed the introduction of maleic acid treated alumina significantly affect the morphology of the PBS/Al₂O₃ composites as compared to the neat PBS. The thermal conductivity of the composite (0.411?W?m^?1 K^?1) was more than twice that of neat PBS. The composite containing polymerization-modified alumina showed a 50% increase in storage modulus compared with that of neat PBS. In addition, universal testing machine (UTM) and differential scanning calorimetry (DSC) measurements indicated an increase in the tensile strength and degree of crystallinity after the incorporation of modified alumina in the PBS/Al₂O₃ composite.     

Synthesis and characterization of multiferroic BiFeO3 powders fabricated by hydrothermal method

The influence of processing parameters on phase formation and particle size of hydrothermally synthesized BiFeO₃ powders was investigated. BiFeO₃ powder was synthesized by dissolving bismuth nitrate and iron nitrate in KOH solution at temperatures ranging from 150 to 225 °C. X-ray diffraction patterns and scanning electron microscopy observation indicated that rod-like α-Bi₂O₃ phase was formed at initial stage of reaction and dissolved into ions to form thermodynamically stable BiFeO₃ phase. Single-phase perovskite BiFeO₃ has been formed using a KOH concentration of 8 M at a temperature of ≥175 °C in a 6 h reaction period. BiFeO₃ particle growth was promoted by lowering the KOH concentration, or increasing the duration time or reaction temperature. The effects of processing conditions on the formation of crystalline BiFeO₃ powders were discussed in terms of a dissolution–precipitation mechanism. The magnetization of the BiFeO₃ powders at room temperature showed a weak a ferromagnetic nature.     

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 °C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al₂O₃/Y₂O₃ liquid was done at 1850 °C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS.     

The controllable microstructure of porous Al2O3 ceramics prepared via a novel freeze casting route

In traditional aqueous slurry freezing casting processing, the growth method of ice crystals is hard to control, resulting in the uncontrollable pore's morphologies of the porous ceramics. In the experimental, the pure Al₂O₃ sol was used to substitute water as a medium for preparing ceramic slurry. With Al₂O₃ sol addition, it becomes easy to control the microstructure and pore's morphologies of the porous Al₂O₃ ceramics via adjusting of the solid loading, composition of the ceramic slurries, as well as the cooling methods. The SEM micrographs showed that the sol-contained ceramic slurry combined with freeze casting processing can easily prepare the porous Al₂O₃ ceramics with different pore sizes and different morphologies. The porous Al₂O₃ ceramics prepared from 70 wt.% to 90 wt.% solid loading sol-contained Al₂O₃ slurries and sintered at 1500 °C for 2 h have open porosities from 81.7% to 64.6%.     

Synthesis of mesoporous nanocrystalline MgAl2O4 spinel via surfactant assisted precipitation route

In this research, mesoporous nanocrystalline MgAl₂O₄ spinel powders were synthesized with a facile synthesis method by co-precipitation route using CTAB as surfactant. The prepared samples were characterized by X-ray diffraction, N₂ adsorption, thermal gravimetric and differential thermal analyses, Fourier transform infrared spectroscopy and transmission electron microscope. The effects of surfactant to metal molar ratio on the structural properties of the samples were investigated. The obtained results showed that the sample prepared without addition of surfactant presented a lower specific surface area and bigger crystallite size compared with those obtained for the samples prepared by CTAB/metal molar ratio of 0.3. The results showed that the sample prepared by CTAB/metal molar ratio of 0.3 has the highest specific surface area and the smallest crystallite size. Moreover, the CTAB/metal molar ratio higher than 0.3 led to a decrease in the specific surface area, due to destruction of the pore walls.     

Fabrication of Al2O3@BaFe12O19 core-shell powder by a modified heterogeneous precipitation method

Core-shell structural composites are promising to quench the desire for low-cost, lightweight and multifunctional magnetic materials, but the fabrication of highly pure shell nanoparticles with a desired morphology on a heterogeneous nuclear is still a big challenge. This study gives a successful illustration by in-situ synthesizing BaFe₁₂O₁₉ nanoparticles as a shell on Al₂O₃ core powders using BaCl₂ and FeCl₃ as precursors, instead of commonly used Ba(NO₃)₂ and Fe(NO₃)₃, by a modified heterogeneous precipitation process followed by calcination for crystallization. After calcination of the precipitants from the raw precursor mixture of BaCl₂:FeCl₃ for 1:10 at 800?°C, and for 1:8.5 at 900?°C, respectively, pure BaFe₁₂O₁₉ formed on pristine Al₂O₃, mostly with a hexagonal plane, about 100–200?nm in basal diameter and 20–100?nm in thickness. The highest saturation magnetization and coercivity of the coated Al₂O₃ @?BaFe₁₂O₁₉ powders was 23.3?emu/g and 5149?Oe, quite higher than that of the direct-mixed Al₂O₃-BaFe₁₂O₁₉ composite powder, and BaFe₁₂O₁₉-containing composites reported in literature, which could be attributed to the hexagonal shape of BaFe₁₂O₁₉ and uniform shell dispersion on Al₂O₃ core. The formation mechanism for the Al₂O₃@BaFe₁₂O₁₉ powders was discussed in detail.