Hydrothermal synthesis of nanocrystalline BaSnO3 using a SnO2·xH2O sol

A BaSnO₃ powder with a crystallite size of 27.6 nm has been prepared through a hydrothermal reaction of a peptised SnO₂·xH₂O and Ba(OH)₂ at 250 °C and the following crystallization of this hydrothermal product at 330 °C. The peptisation of the SnO₂·xH₂O gel is dependent on the pH value. Through peptisation the mean particle size of SnO₂·xH₂O in the aqueous solution has been decreased by a factor of 100 to 8 nm. A limited agglomeration in the sol-prepared powder has been observed under the microscope. The structure evolution and crystallisation behaviours of the sol-prepared powders were investigated by TG-DTA, IR and XRD. The BaSn(OH)₆ phase in the as-prepared powder transforms into an amorphous phase at 260 °C, from which the BaSnO₃ particles nucleate and grow with an increase in temperature. The single-phase BaSnO₃ powder has been obtained at a temperature as low as 330 °C. This sol-prepared powder is more sinter-reactive than the gel-prepared powder and can be sintered to a ceramic with 90.7% of the theoretic density.     

Characterization of nanocrystalline manganese oxide powder prepared by inert gas condensation

Nanocrystalline manganese oxide powders were synthesized by an inert gas condensation technique. The manganese oxide powders, prepared from vaporized metallic manganese in a helium pressure of 10 mbar, then oxidized under the oxygen pressures of 50 and 100 mbar, exhibited a mixture of β-Mn, MnO and Mn₃O₄ phases. X-ray diffractometry (XRD) analysis identified the predominant oxide formation of the as-prepared powder to be the phase of MnO. In situ synchrotron XRD and differential scanning calorimetry (DSC) data showed that synthesized manganese powder converts from β-Mn to MnO in the temperature range of 150–250 °C and subsequently converts to Mn₃O₄ above 250 °C. Transmission electron microscopy examinations confirmed that oxidation starts from the surface of the condensed β-Mn particles. Detailed valance variations of manganese of the resulting powders were investigated by synchrotron X-ray absorption spectroscopy techniques.     

Synthesis,sintering and characterisation of TaON materials

Theoretical investigations predict that TaON is likely to possess a relatively high hardness, thus making it a candidate for application as an abrasive or cutting tool material [J.E. Lowther, Theoretical study of potential high pressure phases of TaON and quaternary ZrTaO₃N, in press]. TaON powder was produced by nitridation of amorphousTa₂O₅ powder in flowing ammonia in the 700–900 °C temperature range and an ammonia flow rate range of 40–50 cm³/min. The resulting powders were characterised in oxidation resistance by thermo-gravimetric analysis (TGA), phase purity by X-ray diffraction (XRD) and surface area by the BET method. The materials were densified under pressure using a belt type high pressure apparatus at 3–5.5 GPa in the temperature range of 920–1200 °C. The sintered samples were characterised in phase purity, Vickers macro-hardness and fracture toughness.     

Microwave dielectric properties of SnO2-doped CaSiO3 ceramics

SnO₂-doped CaSiO₃ ceramics were successfully synthesized by a solid-state method. Effects of different SnO₂ additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn_1−xSi_x)O₃ (x=0.5–1.0) ceramics have been investigated. SnO₂ improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn⁴⁺ substituting for Si⁴⁺ sites leads to the emergence of Ca₃SnSi₂O₉ phase, which has a positive effect on the dielectric properties of CaO–SiO₂–SnO₂ materials, especially the Qf value. The Ca(Sn_0.1Si_0.9)O₃ ceramics sintered at 1375 °C possessed good microwave dielectric properties: ε_r =7.92, Qf =58,000 GHz and τ_f=−42 ppm/°C. The Ca(Sn_0.4Si_0.6)O₃ ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of ε_r=9.27, Qf=63,000 GHz, and τ_f=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices.     

Synthesis of plate-like α-Al2O3 single-crystal particles in NaCl–KCl flux using Al(OH)3 powders as starting materials

Plate-like α-Al₂O₃ single-crystal particles were successfully synthesized in NaCl–KCl flux using Al(OH)₃ powders as starting materials, and the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders was focused. When Al(OH)₃ powders are used as starting materials, the synthesized product at 900 °C is mainly composed of α-Al₂O₃ and κ-Al₂O₃, and most synthesized particles show alveolate morphology. At 1100 °C, single-phase α-Al₂O₃ powders are developed, in which there are many aggregations of intensively bound plate-like particles. In contrast, using porous amorphous Al₂O₃ powders obtained by pre-calcining Al(OH)₃ powders at 550 °C for 3 h as the starting material, plate-like α-Al₂O₃ single-crystal particles can be well developed above 900 °C. The reason of the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders is also discussed in the paper.     

Mechanochemical synthesis of MgTa2O6 ceramic

MgTa₂O₆ powders were prepared by mechanochemical synthesis from MgO and Ta₂O₅ in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa₂O₆ after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa₂O₆ from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.     

Synthesis of Zn2SnO4 anode material by using supercritical water in a batch reactor

Zn₂SnO₄ anode powders were successfully synthesized using supercritical water (SCW) and metal salt solutions with 10 min reaction time. Effect of NaOH concentration, Zn to Sn ratio, and synthesis temperature were studied with a SCW batch reactor. X-ray diffraction (XRD), scanning electron microscopy (SEM), and charge/discharge cycling tests were employed to characterize the physical properties and electrochemical performance of the as-prepared samples. Alkaline solution concentration and synthesis temperature played a key role in the production of single-phase Zn₂SnO₄ powders. At a solution concentration of 0.3 M NaOH and a molar ratio of Zn:Sn = 2:1 at 400 °C and 30 MPa, the average size range of the pure Zn₂SnO₄ powders was 0.5-1.0 μm, and the morphology was nearly uniform and cubic-like in shape. The initial specific discharge capacity of the Zn₂SnO₄ powders prepared at this condition was 1526 mAh/g at a current density of 0.75 mA/cm² in 0.05-3.0 V, and their irreversible capacity loss was 433 mAh/g. The discharge capacities of the Zn₂SnO₄ powders decreased with cycling and remained at 856 mAh/g after 50 cycles, which was 56% of the initial capacity.     

Synthesis of nanocrystalline ytterbium-doped yttria by citrate-gel combustion method and fabrication of ceramic materials

Nanosized ytterbium doped yttria powders were prepared by citrate-gel combustion techniques. As-synthesized precursor and calcined powders were characterized for their crystalline structure, particle size and morphologies. Nanocrystalline Yb³⁺:Y₂O₃ powders with pure cubic yttria crystal structure were obtained by calcination of as-prepared precursors at 1100 °C for 3 h. Powders obtained were well dispersed with an average particle size of 60 nm. By using the obtained powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were produced by vacuum sintering at 1800 °C for 12 h. The emission spectrum of the sintered ceramics under the excitation wavelength of 905 nm illustrates that there are three fluorescence peaks locating at 976 nm, 1030 nm and 1075 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of ytterbium ion.     

Synthesis of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 powders through the molten salt method

The Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ powder synthesis through molten salt method was investigated in the temperature range of 650–700 °C for 2–4 h. The XRD results indicated that the optimal synthesizing temperature for molten salt method was 700 °C, significantly lower than that for conventional processing route of solid state reaction method, where a calcining temperature of 850 °C was needed. The SEM results revealed better crystallization of the powders obtained through molten salt method, compared with those through the conventional processing route of solid state reaction method.     

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

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

Agglomeration of α-Al2O3 powders prepared by gel combustion

Ultrafine α-Al₂O₃ powders were prepared by a gel combustion method and the agglomeration characteristic of the resultant powders was studied. A variety of fine crystallite α-Al₂O₃ powders with different agglomeration structures could be obtained by altering the citrate-to-nitrate ratio γ and calcining the precursors at 1050 °C for 2 h. All the powders were of nearly equivalent crystallite size (60–80 nm) except for the P1 powder (113 nm) from the gel with γ = 0.033. The primary crystallites of the obtained α-Al₂O₃ powders were formed into large secondary particles with different degree of agglomeration. Except for the powder P1, the mean particle sizes from specific surface area and particle size distribution measurement increase with increasing citrate-to-nitrate ratio in the fuel-lean condition and decrease in the fuel-rich condition. Densities of alumina ceramics from powders P4 and P5 sintered at different temperatures were relatively low due to the wide particle size distribution.     

Formation of LiNi0.5Co0.5VO4 nano-crystals by solvothermal reaction

LiNi_0.5Co_0.5VO₄ nano-crystals were solvothermally prepared using a mixture of LiOH·H₂O, Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O and NH₄VO₃ in isopropanol at 150–200 °C followed by 300–600 °C calcination to form powders. TGA curves of the solvothermal products show weight losses due to evaporation and decomposition processes. The purified products seem to form at 500 °C and above. The products analyzed by XRD, selected area electron diffraction (SAED), energy dispersive X-ray (EDX) and atomic absorption spectrophotometer (AAS) correspond to LiNi_0.5Co_0.5VO₄. V–O stretching vibrations of VO₄ tetrahedrons analyzed using FTIR and Raman spectrometer are in the range of 620–900 cm⁻¹. A solvothermal reaction at 150 °C for 10 h followed by calcination at 600 °C for 6 h yields crystals with lattice parameter of 0.8252 ± 0.0008 nm. Transmission electron microscope (TEM) images clearly show that the solvothermal temperatures play a more important role in the size formation than the reaction times.     

Low temperature synthesis of nanocrystalline magnesium aluminate spinel by a soft chemical method

A new simple soft chemical method – synthesizing nanocrystalline MgAl₂O₄ spinel powder with oxalic acid as organic template and nitric acid as an oxidizing agent – is described. The method was developed with the objective of obtaining phase pure nanocrystalline MgAl₂O₄ spinel powder with uniform particle size and morphology at a much lower temperature than that used by conventional methods. The synthesized powders were characterized by X-ray diffractometry (XRD), thermogravimetry (TGA), Fourier transform infrared spectroscopy (FTIR), surface area analysis (BET) and field emission scanning electron microscopy (FE-SEM). The average crystallite size of the single phase material was 30 nm. Through this method, porous MgAl₂O₄ powder with a high surface area of 162.2 m²g⁻¹ and 141 m²g⁻¹ was obtained at 600 °C and 700 °C, respectively.     

Pressureless sintered Al2O3–SiC nanocomposites

Al₂O₃ + 5 vol% SiC composite ceramics were prepared via a conventional powder processing route followed by pressureless sintering. Commercially available Al₂O₃ and SiC powders were milled together in an aqueous suspension. The slurry was freeze granulated, and green bodies were obtained by cold isostatic pressing of the granules. Pressureless sintering was carried out in a nitrogen atmosphere at 1750 and 1780 °C. Near full density (>99%) was achieved at 1780 °C. Densification at the lower sintering temperature was promoted by smaller additions of MgO. Vickers hardness and indentation fracture toughness varied around 18 GPa and 2.3 MPa m^1/2 after sintering at 1780 °C. Transmission electron microscopy revealed that the SiC particles were located predominantly to the interior of the matrix grains and well distributed throughout the composite microstructures. The intragranular particles had sizes in the range 50–200 nm while the intergranular particles were larger, typically 200–500 nm in diameter.     

Development of metal oxide nanoparticles by soft chemical method

An extensive work for the study of SnO₂ samples doped with x-mol% of Sb (x = 0, 6, 10, 14 and 18) is reported. The materials were prepared by the polymeric precursor method (Pechini method), calcined for 4 h between 800 °C and 1200 °C. The Rietveld method with X-ray diffraction data (XRD) was used to analyze the unit cell dimensions, crystallite size and microstrain. It was observed the crystallite size increasing and decrease of the microstrain with the increase of the calcining temperature. The synthesis of tin oxide nanoparticles with high thermal stability against particle growth rate was achieved by doping SnO₂ particles with Sb₂O₃. All the phases tend to have the same dimension when the temperature increases, although its values varies with x and reaches the maximum value when fired at 1100 °C. These variations seem to be an indication that the oxidation state of the antimony changes with the amount of Sb added to the material.     

Microstructure and dielectric properties of Dy/Mn doped BaTiO3 ceramics

Dy/Mn doped BaTiO₃ with different Dy₂O₃ contents, ranging from 0.1 to 5.0 at% Dy, were investigated regarding their microstructural and dielectric characteristics. The content of 0.05 at% Mn was constant in all the investigated samples. The samples were prepared by the conventional solid state reaction and sintered at 1290°, and 1350 °C in air atmosphere for 2 h. The low doped samples (0.1 and 0.5 at% Dy) exhibit mainly fairly uniform and homogeneous microstructure with average grain sizes ranged from 0.3 μm to 3.0 μm. At 1350 °C, the appearance of secondary, abnormal, grains in the fine grain matrix and core–shell structure were observed in highly doped Dy/BaTiO₃. Dielectric measurements were carried out as a function of temperature up to 180 °C. The low doped samples sintered at 1350 °C, display the high value of dielectric permittivity at room temperature, 5600 for 0.1Dy/BaTiO₃. A nearly flat permittivity–temperature response was obtained in specimens with 2.0 and 5.0 at% additive content. Using a Curie–Weiss and modified Curie–Weiss low, the Curie constant (C), Curie like constant (C′), Curie temperature (T_C) and a critical exponent (γ) were calculated. The obtained values of γ pointed out the diffuse phase transformation in highly doped BaTiO₃ samples.     

Mechanochemical synthesis and characterisation of BaTa2O6 ceramic powders

Mechanochemical synthesis was used to prepare BaTa₂O₆ powders from BaCO₃ and Ta₂O₅ precursors in a planetary ball mill. Effect of milling time and heat treatment temperature on the formation of BaTa₂O₆ and on the microstructure was investigated. Intensive milling of starting materials resulted in crystallization of BaTa₂O₆ even after 1 h of milling time and single phase BaTa₂O₆ was obtained after 10 h of milling under optimal conditions. The powder derived from 10 h of mechanical activation had crystallite size of 22 nm. But the increase in milling time did not decrease the crystallite size further. High energy milling activated the powders that although 1 h of milling led to formation of single phase BaTa₂O₆ at 1200 °C, this temperature decreased to 900 °C after 5 h of milling. No significant grain growth was observed when the milled powders were heat treated below 900 °C. However, annealing at 1100 and 1200 °C gave an average BaTa₂O₆ grain size of 180 and 650 nm, respectively. An unidentified phase started to form at 1100 °C increasing to high amounts at 1200 °C and they had different shapes and sizes than BaTa₂O₆ grains. These elongated large grains were thought to be due to liquid phase formation caused by iron contamination.     

Low temperature sintering and microwave dielectric properties of CaSiO3–Al2O3 ceramics for LTCC applications

The effects of CuO, Li₂CO₃ and CaTiO₃ additives on the densification, microstructure and microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics for low-temperature co-fired applications were investigated. With a single addition of 1 wt% Li₂CO₃, the CaSiO₃–1 wt% Al₂O₃ ceramic required a temperature of at least 975 °C to be dense enough. Large amount addition of Li₂CO₃ into the CaSiO₃–1 wt% Al₂O₃ ceramics led to the visible presence of Li₂Ca₃Si₆O₁₆ and Li₂Ca₄Si₄O₁₃ second phases. Fixing the Li₂CO₃ content at 1 wt%, a small amount of CuO addition significantly promoted the sintering process and lowered the densification temperature to 900 °C whereas its addition deteriorated the microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics. Based on 10 wt% CaTiO₃ compensation in temperature coefficient, good microwave dielectric properties of ε_r=8.92, Q×f=19,763 GHz and τ_f=−1.22 ppm/°C could be obtained for the 0.2 wt% CuO and 1.5 wt% Li₂CO₃ doped CaSiO₃–1 wt% Al₂O₃ ceramics sintered at 900 °C. The chemical compatibility of the above ceramics with silver during the cofiring process has also been investigated, and the result showed that there was no chemical reaction between silver and ceramics, indicating that the as-prepared composite ceramics were suitable for low-temperature co-fired ceramics applications.     

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.     

3-D micro-ceramic components from hydrothermally processed carbon nanotube–boehmite powders by electrophoretic deposition

Boehmite/multi-wall carbon nanotube (MWCNT) composite powders were prepared by hydrothermal processing. Starting chemical of aluminum acetate powders (2Al(OH)(C₂H₃O₂)₂) and MWCNTs were mixed for the formation of stoichiometric boehmite powders in an attempt to synthesize MWCNT-reinforced boehmite nano-powders via hydrothermal synthesis at 200 °C for 2 h. Kinetically stable suspensions of MWCNT–boehmite composite powders were prepared and subsequently electrophoretic deposition (EPD) was applied to obtain complex shape products in the form of micro-gears. It is shown that the EPD technique is a powerful tool to manufacture small components in a short time. Detail TEM observations also indicated that hydrothermal processing provides an ideal environment to obtain homogeneous mixtures of MWCNT–boehmite powders due to effective surface functionalization of MWCNTs under hydrothermal conditions.