Synthesis of Nanocrystalline KTiOPO4 Powder by Chemical Method

Nanocrystalline potassium titanyl phosphate (KTiOPO₄, KTP) powder with average particle size of 25–50 nm has been synthesized from aqueous solution of titanyl chloride through coprecipitation method, using aqueous solution of potassium dihydrogen phosphate and potassium carbonate in the presence of triethanolamine (TEA) as a capping agent. In the presence of tetrapotassium pyrophosphate as a capping agent it produces the particles having average size around 70–80 nm. The same sample with average particle size ∼250 nm has also been synthesized without any capping agent. Initial amorphous phase after precipitation requires heat treatment at 500–550°C for 2 h to generate nanocrystalline powders of KTP. Formation of the KTP phase in the heat-treated samples has been confirmed through X-ray powder diffraction, thermogravimetric and differential thermal analysis, ³¹P Magic angle spinning-nuclear magnetic resonance and Fourier transform infrared spectroscopy studies, and surface area measurements.     

Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by pyrolysis of complex compounds of aluminum and magnesium with triethanolamine (TEA). The soluble metal ion–TEA complexes formed the precursor material on complete dehydration of the complexes of aluminum–TEA and magnesium–TEA. Single-phase MgAl₂O₄ spinel powder resulted after heat treatment of the precursor material at 675°C. The precursor and the heat-treated powders were characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis, and transmission electron microscopy (TEM). The average crystallite size as measured from the X-ray line broadening was around 14 nm and the average particle size from TEM studies was around 20 nm.     

Synthesis of Blue-Emitting CaMgSi2O6:Eu2+ Phosphor Using an Electrostatic Self-Assembly Deposition Method

Eu²⁺-doped CaMgSi₂O₆ phosphor was prepared by depositing mixed hydroxides of Ca, Mg, and Eu over spherical SiO₂ particles (300 nm) pre-coated with polycations (polyethyleneimine), followed by calcination at 1200°C in a reducing atmosphere. The prepared phosphor showed intense blue emission, ascribable to the 4f⁷-4f⁶5d transition of Eu²⁺. In contrast, the luminescence intensity of the phosphor was considerably decreased when prepared without polycations. It was suggested that negatively charged hydroxides are deposited on positively charged SiO₂ surfaces pre-coated with polycations through electrostatic self-assembly interaction. On calcination, the hydroxide shells react with the SiO₂ cores to produce Eu²⁺:CaMgSi₂O₆.     

Pressureless Sintering of α-Si3N4 Porous Ceramics Using a H3PO4 Pore-Forming Agent

A new method for preparing high bending strength porous silicon nitride (Si₃N₄) ceramics with controlled porosity has been developed by using pressureless sintering techniques and phosphoric acid (H₃PO₄) as the pore-forming agent. The fabrication process is described in detail and the sintering mechanism of porous ceramics is analyzed by the X-ray diffraction method and thermal analysis. The microstructure and mechanical properties of the porous Si₃N₄ ceramics are investigated, as a function of the content of H₃PO₄. The resultant high porous Si₃N₄ ceramics sintered at 1000°–1200°C show a fine porous structure and a relative high bending strength. The porous structure is caused mainly by the volatilization of the H₃PO₄ and by the continous reaction of SiP₂O₇ binder, which could bond on to the Si₃N₄ grains. Porous Si₃N₄ ceramics with a porosity of 42%–63%, the bending strength of 50–120 MPa are obtained.     

Formation Mechanism and Synthesis of Apatite-Type Structure Ba2+xLa8−x(SiO4)6O2−δ

The formation process of Ba₂La₈(SiO₄)₆O₂ was clarified using thermogravimetry–differential thermal analysis (TG-DTA) and a high-temperature powder X-ray diffraction (HT-XRD) method. Phase changes identified from the HT-XRD data surprisingly corresponded to the weight loss and/or endothermic peaks observed in the TG-DTA curves. Raw material with the composition Ba₂La₈(SiO₄)₆O₂ was completely reacted at 1400°C and produced only an apatite-type compound without a secondary phase. Moreover, the synthesis of Ba_{2+ x} La_{8− x} (SiO₄)₆O_2−δ crystals with x = 0–2 was attempted using a solid-state reaction.     

Microwave-Hydrothermal Processing of BiFeO3 and CsAl2PO6

Microwave-hydrothermal (M-H) processing was compared with conventional-hydrothermal (C-H) processing in the crystallization of BiFeO₃ and CsAl₂PO₆ phases. The presence of the microwave field led to accelerated kinetics of the crystallization of both these phases as detected by powder X-ray diffraction. The acceleration of reaction rates under microwave field is expected to lead to energy savings during ceramic processing.     

Synthesis of Compositionally Homogeneous Sr(CuxZn1−x)1/2W1/2O3

A complex perovskite of Sr(Cu _x Zn_{1- x} )_1/2 W_1/2O₃ (SCZW) is synthesized by a new combination of wet and dry processess. Mixed oxides containing Cu²⁺ and Zn²⁺ (CZ) are prepared by the wet process (coprecipitate method). SCZW is obtained by the dry process (mixed-oxide method) from a mixture of CZ, SrCO₃, and WO₃. SCZW has practically no compositional, unlike solid solutions prepared by the conventional dry method. The wet–dry process method is useful because the wet process is applied to only B-site cations having the same valence.     

Structural Characterization and Luminescent Properties of a Red Phosphor Series: Y2−xEux(MoO4)3 (x=0.4–2.0)

A series of rare earth molybdates, Y_{2− x} Eu _x (MoO₄)₃ for x =0.4, 0.8, 1.2, 1.6 and 2.0 were prepared by solid-state method and their crystal structures, photo luminescent characteristics were investigated. The powders are mainly studied for their red light emission efficiency under near UV excitation. The crystal structures of the powders were found to depend on annealing temperature and the yttrium concentration. Mixtures of monoclinic ( C 2 /c ) and orthorhombic ( Pba 2, Pbna ) structures were formed in varying proportions depending on the value of x and annealing temperatures (700°–800°C). The luminescence behavior depended on the resultant composition of the crystal phase and the Eu³⁺ concentration. The excitation spectra showed the characteristic and broad O→Mo charge transfer (CT) band of the MoO₄ tetrahedra and the sharp intra-configurational 4 f –4 f transitions of Eu³⁺ in the host lattice. The integrated emission ratio (⁵D₀→⁷F₂/⁵D₀→⁷F₁) of Eu³⁺ depends on the annealing temperature and reveals that the local site symmetry of Eu³⁺ ions decreases with increasing concentration of Eu³⁺. The emission spectra obtained by exciting at 396 nm, gave highest red emission intensity for Y_0.4Eu_1.6(MoO₄)₃ annealed at 700°C/6 h among this series of samples.     

Superconductivity in the (Yba2Cu3)1−xNaxO7−δ System

The (YBa₂Cu₃)_1−xNa_xO_7–δ system in the range of x = 0–0.8 was investigated. Experimental data suggest that the sodium doping with x 0.26 does not affect the critical transition temperature T_c, and the crystal structure maintains the orthorhombic lattice with a slightly smaller unit cell. However, sodium doping increases the sintering and grain growth kinetics, resulting in a higher superconducting phase volume and an enhanced Meissner effect. It also lowers the processing temperaturel. The experimental data also suggest that the sodium atoms diffuse into the superconducting YBa₂Cu₃O_7−δ crystallites, which stabilizes the orthorhombic phase. The transition temperature (ortho-rhombic to tetragonal) in sodium-doped materials increases with the increasing concentration of sodium.     

Preparation and Cu+-Ion-Conducting Properties of the Glasses in the System CuBr–Cu2MoO4–Cu3PO4

Cu⁺-ion-conducting glasses were prepared in the pseudoternary system CuBr–Cu₂MoO₄–Cu₃PO₄, and the ion-conducting properties of the glasses obtained were compared to those of the glasses in the system CuI—Cu₂MoO₄—Cu₃PO₄, which contain CuI instead of CuBr. The CuBr—Cu₂MoO₄—Cu₃PO₄ glasses showed high ion conductivities in the range of 10⁰ to 10⁻² S · m⁻¹ at room temperature. The change in ion-conducting properties caused by the substitution of PO^3-₄ anions for MoO²⁻₄ anions was larger than the change caused by the substitution of Br⁻ anions for I⁻ anions in the glasses containing a constant amount of the cuprous halides.     

Synthesis of β-Si3N4 by Chemical Vapor Deposition

Beta-type CVD-Si₃N₄ plates (up to 1.1 mm thick) have been prepared by adding TiCl₄ vapor to the system SiCl₄-NH₃-H₂ at deposition temperatures of 1350° to 1450°C, while α-type or amorphous CVD-Si₃N₄ was obtained without TiCl₄ vapor at the same deposition temperature. Three to four wt % 777V was included in the β-type CVD-Si₃N₄ matrix. The density, preferred orientation, and lattice parameters of β-type CVD-Si₃N₄ were examined.     

Structure and Microwave Dielectric Characteristics of Ca1+xNd1−xAl1−xTixO4 Ceramics

CaNdAlO₄ microwave dielectric ceramics were modified by Ca/Ti co-substitution, and their dielectric characteristics were evaluated along with their structure and microstructures. Ca_{1+ x} Nd_{1− x} Al_{1− x} Ti _x O₄ ( x =0, 0.025, 0.05, 0.10, 0.15, 0.20) ceramics with the relative density of over 95% theoretical density were obtained by sintering at 1400°–1450°C in air for 3 h, where the K₂NiF₄-type solid solution single phase was determined from the compositions of x <0.20, while a small amount of CaTiO₃ secondary phase was detected for x =0.20. With Ca/Ti co-substitution in CaNdAlO₄ ceramics, the dielectric constant (ɛ_r) increased with increasing x , and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative to positive, while the Q × f ₀ value increased significantly at first and reached an extreme value at x =0.025 and the maximum at x =0.15. The best combination of microwave dielectric characteristics were achieved at x =0.15 (ɛ_r=19.5, Q × f ₀=93 400 GHz, τ_f=−2 ppm/°C). The improvement of the Q × f ₀ value primarily originated from the reduced interlayer polarization with Ca/Ti co-substitution, while the decreased tolerance factor, the subsequent increased interlayer stress, and the appearance of CaTiO₃ secondary phase brought negative effects upon the Q × f ₀ value.     

Molten Salt Synthesis of Bi4(V0.85Co0.15)2O11−δ (BICOVOX) Ceramic Powders

Cobalt (15 at.%)-doped bismuth vanadate, Bi₄(V_0.85Co_0.15)₂O_11−δ (BICOVOX) is known to have high oxygen ion conduction. However, the conductivity is highly anisotropic due to its layered structure. In this paper, we report the synthesis of BICOVOX powders with platelet habit by molten salt synthesis using a NaCl–KCl eutectic mixture as the reaction medium. The effects of treatment temperature and time on the morphology and phase content of the BICOVOX powders were investigated. By this method, it is possible to synthesize powders with platelets of major face dimension ∼50 μm and aspect ratio (major face dimension to thickness) as high as 15.     

Synthesis of Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

Cement Formulations in the Calcium Phosphate H2O–H3PO4–H4P2O7 System

Despite numerous reports of calcium phosphate cement materials, a calcium cement that sets to form a matrix consisting of a pyrophosphate phase has not been reported. The formulation of such a material from the mixture of α-tricalcium phosphate (TCP), β-TCP, or tetracalcium phosphate with a solution containing pyro- and orthophosphoric acid is reported in this study. The effects of liquid and solid compositions on the setting times, compressive strengths and phase compositions of the resultant cements were investigated. It was found that cements could be produced that set to form up to 28 wt% dicalcium pyrophosphate, which appeared by comparison with Rietveld refinement and chemical methods to be entirely amorphous in nature. The solubilities of the different solid components were shown to have a marked effect on the composition of the cements. The strongest cement formulations exhibited compressive strengths comparable with those previously reported in the literature for brushite cements and set within clinically relevant time scales. This class of cement would appear to demonstrate potential as a bone replacement material.     

Hydrothermal Synthesis and Characterization of KxNa(1−x)NbO3 Powders

NaNbO₃, KNbO₃, and K _x Na_{(1− x )}NbO₃ powders were successfully prepared by the hydrothermal method. The phase of the products was identified to be orthorhombic structure by X-ray diffraction (XRD) technique, and the XRD results revealed that the x value of the K _x Na_{(1− x )}NbO₃ gradually increased with the increase in the ratio of K⁺ to Na⁺ in alkaline solution. The morphology and the microstructure were investigated by scanning electron microscopy, energy-dispersive spectroscopy, and transmission electron microscopy, and the results indicated that the ratio of K⁺ to Na⁺ in the solution had a great effect on the morphology and the size of products. Na_0.5K_0.5NbO₃ with morphotropic phase boundary composition could be synthesized when the molar ratio of K⁺ to Na⁺ was between 4:1 and 6:1 in the solution. A possible formation mechanism of the K _x Na_{(1− x )}NbO₃ crystal was also proposed based on the experimental results.     

Parallel Solution-Based Synthesis Approach for Search of Lanthanoid-Activated Ca2SnO4 Phosphor Materials

In the search for new fluorescent materials among lanthanoid-activated Ca₂SnO₄ compounds, a parallel solution-based synthesis approach was applied and a new blue-emission Ca₂SnO₄:Ce phosphor was discovered; its emission intensity reaches 80% of the intensity of one of the best commercial Y₃Al₅O₁₂:Ce³⁺ phosphors. Among 14 lanthanoid-activated materials, three categories could be distinguished with respect to the effect of the dopant ion on the excitation and emission properties of the phosphor. In addition, potential phosphors in the A–Sn–O (A=Mg, Ca, Sr, and Ba) systems were investigated and Ca₂SnO₄ was found to be the most suitable host compound for the lanthanoid-activated phosphors.