Microwave-assisted hydrothermal synthesis of fine BaZrO<Subscript>3</Subscript> and BaHfO<Subscript>3</Subscript> powders

Fine BaZrO₃ and BaHfO₃ powders have been prepared by a microwave-assisted hydrothermal (MWHT) process. The powders have been characterized by x-ray diffraction and scanning electron microscopy, and their particle size distribution has been assessed from dynamic light scattering data. The results indicate that microwave processing during hydrothermal synthesis notably reduces the average particle size of the resulting powder and ensures a narrower particle size distribution. BaHfO₃ particles prepared under the optimal MWHT synthesis conditions are predominantly spherical in shape and uniform in size, with an average size (1.2 μm) a factor of 2.5 smaller in comparison with particles prepared by a conventional hydrothermal process (2.9 μm).     

Preparation and characterization of BaSnO<Subscript>3</Subscript> powders by hydrothermal synthesis from tin oxide hydrate gel

BaSnO₃ powders have been prepared from the tin oxide hydrate gel and the Ba(OH)₂ solution via hydrothermal synthesis route. The influence of the process parameters on the characteristics of BaSnO₃ has been studied. A powder with the single-phase of BaSnO₃ can be obtained only when the concentration of Ba(OH)₂ solution is no less than 0.2 M and the ratio of Ba:Sn lies between 1.0 and 1.2. At a hydrothermal temperature of 330 °C or higher, uniform BaSnO₃ powders can be directly prepared through hydrothermal reaction. When the hydrothermal temperature is lower than 250 °C, the as-prepared powder consists of BaSn(OH)₆ that transforms through an amorphous phase into BaSnO₃ by calcination at 260 °C. In the hydrothermal temperature range of 130–250 °C, a higher temperature can promote the crystallization of BaSnO₃, increases its specific surface area and decreases the average particle size. The duration of the hydrothermal reaction affects the morphology of the powder particles. The effects of the nonaqueous solvents on the properties of powders have also been investigated.     

Modification of solid solutions of the NaNbO<Subscript>3</Subscript>-LiNbO<Subscript>3</Subscript> system by different ions

Solid solution ceramics of the NaNbO₃-LiNbO₃ systems modified by Ag, Cd, Cr, Fe, La, Ti, Si, Mn, V, U, and W ions inserted overstoichiometrically are obtained. The crystal structure and piezoelectric properties of the modified ceramics are studied. The properties of the systems modified stoichiometrically and overstoichiometrically are compared. The regularities in the changes in the properties of the (Na, Li)NbO solid solutions (SSs) under modification are corrected.     

Phase composition and dielectric properties of NaNbO<Subscript>3</Subscript> ceramics

The phase composition and dielectric properties of NaNbO₃ ceramics ranging in closed porosity from 2.0 to 13.6% have been studied in wide temperature and frequency ranges. The phase composition of the ceramics has been shown to depend on their porosity and temperature. The temperature and frequency dependences of the dielectric properties of the NaNbO₃ ceramics correlate with their phase composition and porosity.     

Self-propagating high-temperature synthesis of Lu<Subscript>2</Subscript>O<Subscript>3</Subscript> powders for optical ceramics

A technique has been developed for the self-propagating high-temperature synthesis of lutetium oxide (Lu₂O₃) powders using citric acid, glycine, and lutetium acetate as fuels. We have carried out thermodynamic analysis of synthesis conditions and examined the effect of the nature of the fuel on the properties of the resultant powders. Using vacuum sintering at a temperature of 1780°C and powders prepared with glycine as a fuel and containing 25 mol % yttrium oxide and 5 mol % lanthanum oxide as sintering aids, we have obtained transparent lutetium oxide-based ceramics.     

Solid-state reaction synthesis of sodium niobate (NaNbO<Subscript>3</Subscript>) powder at low temperature

A modified solid-state reaction was applied to produce lead-free piezoelectric sodium niobate (NaNbO₃) powders. The mixture of Na₂C₂O₄ and Nb₂O₅ was identified by thermo gravimetric analysis (TGA) and differential thermal analysis (DTA). The powders were characterized using a scanning electron microscope (SEM) and the X-ray diffraction technique (XRD). The SEM image suggested that the particle size of the powders obtained ranged from 180 to 360 nm. The XRD pattern showed that the pure perovskite phase of NaNbO₃ could be synthesized at the low temperature of 475 °C for 1 h, with an average crystallite size of 31.45 ± 5.28 nm. This temperature was about 300 °C lower than that when using the conventional solid-state method with Na₂CO₃ as reactant, which resulted in a cost-, energy-, and time-saving method.     

Effect of hydrothermal duration on synthesis of WO<Subscript>3</Subscript> nanorods

Among different type of transition metal oxides, tungsten trioxide (WO₃) is a suitable candidate for electronic device fabrication due to its n-type property and wide band gap. Herein, one-dimensional tungsten trioxide (WO₃) nanorods were achieved from an aqueous solution of sodium tungstate dihydrate (Na₂WO₄·2H₂O) and sodium chloride (NaCl) in an acidic media by a time-optimized hydrothermal synthesis in autoclave at 180°C or different synthesis durations. For studying morphology and size of obtained powder, X-ray diffraction (XRD), scanning electron microscope (SEM), and high resolution transmission electron microscope (HRTEM) were applied. Finally, WO₃ nanorods of about 2–3 μm in length and 100–200 nm in diameter were obtained during 3 h hydrothermal process.     

Formation and structural phase transitions of mesoporous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and TiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> xerogels under hydrothermal conditions

We have studied the effect of the hydrothermal synthesis temperature on Al₂O₃ structure formation and examined the role of the phase composition of the precursor gel and surfactant in the formation of the pore structure of Al₂O₃. A technique has been proposed for the synthesis of TiO₂/Al₂O₃ binary xerogels, and the effect of TiO₂ content on the pore structure parameters and adsorption properties of TiO₂/Al₂O₃ has been investigated.     

Oxalic acid assisted hydrothermal synthesis and optical absorption property of WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanocomposites

The WO₃/TiO₂ nanocomposites were successfully prepared via a facile oxalic acid assisted hydrothermal process. The oxalic acid played a vital role on the preparation of WO₃/TiO₂ nanocomposites. Notably, it has been observed that the nanocomposites exhibited the wider absorption edge, and the higher photocatalytic activity, compared with pure TiO₂. In addition, the photocatalytic mechanism was proposed, and it elaborated that WO₃/TiO₂ nanocomposite promoted the separation of the photoproduction carriers, and improved photocatalytic activity. The WO₃/TiO₂ nanocomposite may have a potential application as a UV–visible photocatalyst.     

Synthesis of anisotropic NaNbO<Subscript>3</Subscript> seed crystals and fabrication of textured (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript>-based ceramics

High aspect ratio patelike NaNbO₃ particles with pure perovskite structure have been successfully synthesized by topochemical microcrystal conversion (TMC) from plate-like precursor particles of the layer-structured Bi_2.5Na_3.5Nb₅O₁₈. By changing the Bi_2.5Na_3.5Nb₅O₁₈/Na₂CO₃ ratio, large and thin NaNbO₃ particles with a thickness of approximately 0.5 μm and a width of approximately 20 μm were obtained. The obtained NaNbO₃ particles is quite suitable for fabricating textured (K_0.5Na_0.5)NbO₃-based ceramics. Using the fine platelike NaNbO₃ particles as templates, dense <001> -oriented (K_0.5Na_0.5)NbO₃-0.5 mol %MnO₂ ceramics with high texture quality (Lotgering factor F ₀₀₁ = 87 %) and excellent piezoelectric properties were produced by templated grain growth. Compared with randomly oriented ceramics, textured samples show greatly enhanced properties. The room-temperature strain S, the piezoelectric coefficient d ₃₃ ^* and d ₃₃ reach up to 0.093 %, 233 pm/V and 195pC/N, respectively, which are all about 1.5 times larger than those of non-textured ceramics.     

Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/TiN composites produced from TiO<Subscript>2</Subscript>-Modified Si<Subscript>3</Subscript>N<Subscript>4</Subscript> powders

Si₃N₄/TiN composites have been produced by hot pressing at temperatures from 1600 to 1800°C in a nitrogen atmosphere, using silicon nitride powders prepared by self-propagating high-temperature synthesis and surface-modified with titanium dioxide nanoparticles. We examined the effect of TiO₂ content on the microstructure, phase composition, and mechanical strength of the ceramics. It is shown that titanium nitride can be formed by the reaction Si₃N₄ + TiO₂ → TiN + NO + N₂O + 3Si. The Si₃N₄/TiN composites containing 5–20% TiN have a low density, high porosity, and a bending strength of 60 MPa or lower. In Si₃N₄/TiN ceramics produced using calcium aluminates as sintering aids, the silicon nitride grains are densely packed, which ensures an increase in strength to 650 MPa.     

Synthesis and sintering of nano-sized BaSnO<Subscript>3</Subscript> powders containing BaGeO<Subscript>3</Subscript>

The formation of solid solutions of the type [Ba(HOC₂H₄OH)₄][Sn_1−x Ge _x (OC₂H₄O)₃] as BaSn_1−x /Ge _x O₃ precursor and the phase evolution during its thermal decomposition are described in this paper. The 1,2-ethanediolato complexes can be decomposed to nano-sized BaSn_1−x /Ge _x O₃ preceramic powders. Samples with x = 0.05 consist of only a Ba(Sn,Ge)O₃ phase, whereas powders with x = 0.15 and 0.25 show diffraction patterns of both the Ba(Sn,Ge)O₃ and BaGeO₃ phase. The sintering behaviour was investigated on powders with a BaGeO₃ content of 5 and 15 mol%. These powders show a specific surface area of 15.4–15.9 m²/g and were obtained from calcination above 800 °C. The addition of BaGeO₃ reduced the sintering temperature of the ceramics drastically. BaSn_0.95Ge_0.05O₃ ceramics with a relative density of at least 90% can be obtained by sintering at 1150 °C for 1 h. The ceramic bodies reveal a fine microstructure with cubical-shaped grains between 0.25 and 0.6 μm. For dense ceramics, the sintering temperature could be reduced down to 1090 °C, when the soaking time was extended up to 10 h.     

Properties of nanocrystalline ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript>-CoO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

We have studied the properties of nanocrystalline ZrO₂-Y₂O₃-CeO₂-CoO-Al₂O₃ powders prepared via hydrothermal treatment of a mixture of coprecipitated hydroxides at 210°C. A number of general trends are identified in the variation of the properties of the synthesized powders during heat treatment at temperatures from 500 to 1200°C. Our results demonstrate that the addition of 0.3 mol % CoO to nanocrystalline ZrO₂-based powders containing 1 to 5 mol % Al₂O₃ allows one to obtain composites with good sinterability at a reduced temperature (1200°C).     

Spindlelike Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> nanorod bundles: hydrothermal synthesis and photoluminescence properties

Uniform spindlelike Y(OH)₃ nanorod bundles were successfully prepared for the first time via a simple hydrothermal method at 200 °C for 12 h with the presence of Na₂H₂EDTA · 2H₂O (EDTA). Scanning electron microscope (SEM) images show that the obtained Y(OH)₃ spindlelike nanorod bundles have a length of about 11 μm and a diameter of about 2 μm in the middle part. The nanorod bundles are composed of numerous nanorods, and all these nanorods are orientationally aligned and grow uniformly along the bundles. The individual nanorod is with typical width of about 100 nm, thickness of about 40 nm, and length longer than 1 μm. The effects of reaction temperature, reaction time, and the concentration of NaOH and EDTA on the sizes and morphologies of the products have been investigated. The possible formation mechanism of the nanorod bundles was suggested. Spindlelike Y₂O₃ nanorod bundles were obtained after thermal treatment of the as-obtained Y(OH)₃ nanorod bundles at 700 °C for 4 h. X-ray powder diffraction (XRD) results demonstrate that the as-prepared Y(OH)₃ and Y₂O₃ are attributed to hexagonal phase and cubic phase, respectively. Eu³⁺ doped Y₂O₃ nanorod bundles were also prepared and their photoluminescence (PL) properties were investigated.     

Synthesis and microstructure of nanocrystalline Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

Nanocrystalline ytterbia powders have been synthesized using different precursors prepared by precipitation from nitrate solutions: ytterbium carbonates, oxalates, and hydroxides. The powders have been characterized by X-ray diffraction and scanning electron microscopy. The nature of the precursor has no effect on the crystallization temperature of ytterbia but influences its microstructure. The particles range in shape from spherical to platelike. The average crystallite size of the Yb₂O₃ powders is 20–25 nm. Raising the heat-treatment temperature from 600 to 1000°C increases the crystallite size to 33–46 nm. The highest thermal stability is offered by the ytterbia powders prepared through carbonate decomposition.     

Formation of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> from Ti-Si-TiC powders by pulse discharge sintering (PDS) technique

Ti/Si/TiC powder mixture with molar ratios of 2:2:3 were sintered at various temperatures from 700–1300 °C for 15 min by PDS technique. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the evaluation of phase composition in different samples for the understanding of the sintering mechanism for this system. Results showed that Ti₅Si₃ formed as the intermediate phase during sintering. The reaction between Ti₅Si₃ and TiC as well as Si induces the formation of Ti₃SiC₂, and TiSi₂ appears as the byproduct in this process. At temperature above 1000 °C, TiSi₂ reacts with TiC to form Ti₃SiC₂. High Ti₃SiC₂ phase content bulk material can be synthesized at 1300 °C for 15 min.     

Improvement in synthesis of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> powders by Ge<Superscript>4+</Superscript> acceptor doping

In this paper, the effects of doping with GeO₂ on the synthesis temperature, phase structure and morphology of (K_0.5Na_0.5)NbO₃ (KNN) ceramic powders were studied using XRD and SEM. The results show that KNN powders with good crystallinity and compositional homogeneity can be obtained after calcination at up to 900°C for 2 h. Introducing 0.5 mol.% GeO₂ into the starting mixture improved the synthesis of the KNN powders and allowed the calcination temperature to be decreased to 800°C, which can be ascribed to the formation of the liquid phase during the synthesis.     

Carbothermal synthesis of TiB<Subscript>2</Subscript> powders of micron size

The characteristic details of the carbothermal synthesis of TiB₂ powders from the stoichiometric mixture TiO₂–H₃BO₃–C at temperatures lower 1700 K are investigated using thermal analysis (ТG—thermogravimetry and DSC—differential scanning calorimetry), as well as X-ray diffraction and scanning electron microscopy. In the temperature interval 300 K → 1673 K → 1273 K and at a heating rate of 10 K/min, the reaction in the powder mixture begins at approximately 1300 K and ends at 1470 K during cooling. After 3 h of isothermal synthesis at 1473 K, the TiB₂ yield is more than 90%. The resulting products are hexagonal plate-like crystals 5–10 μm across with thickness of 3 to 4 μm. Kinetic analysis showed that in the temperature range of 1330 to 1673 K the TiB₂ synthesis reaction is of the first-order, and the calculated activation energy of the process is 315 ± 24 kJ/mol.     

High-temperature liquid-phase synthesis and sintering of MoSi<Subscript>2</Subscript> powders

Regularities of synthesis and sintering of molybdenum disilicide are studied. The synthesis is carried out in the combustion mode (self-propagating high-temperature synthesis) with the use of high-caloric mixtures of molybdenum oxide with aluminum and silicon. The cast synthesis product (MoSi₂) is subjected to grinding and subsequent sintering. The studies show that the basic parameters that influence the process of synthesis and its regularities are gas pressure in the reaction vessel and sizes of silicon particles. The sintering process, microstructure, and characteristics of sintered samples may be controlled through varying the sintering temperature and ratios of the ZrO₂ and Y₂O₃ additives.     

Formation behavior of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> from CaTiO<Subscript>3</Subscript>, CuO and TiO<Subscript>2</Subscript>

The formation behavior of CaCu₃Ti₄O₁₂ (CCTO) had been investigated via solid state reaction from CaTiO₃, CuO and TiO₂ powders. In the temperature range from 750 to 1,200 °C, the reaction sequence was traced by XRD, and the microstructure evolution of calcined powders was also investigated by SEM. CCTO began to form owing to the reaction between CaTiO₃, CuO and TiO₂ at around 850 °C, and became the major phase at 1,000 °C. Finally, the single phase CCTO was obtained at 1,150 °C. However, CCTO was decomposed at CaTiO₃, CuO and TiO₂ when the temperature increased to 1,200 °C. In addition, no other intermediate phases occurred in the synthesized process. The formation behaviors indicated that CaTiO₃ prevented the formation and growth of CCTO.