Synthesis of nanocrystalline GaN from Ga2O3 nanoparticles derived from salt-assisted spray pyrolysis

Gallium nitride (GaN) nanoparticles were successfully produced from nano-sized gallium oxide (Ga₂O₃) particles under a flow of ammonia gas. The gallium oxide nanoparticles were prepared by salt-assisted spray pyrolysis (SASP). Highly crystalline Ga₂O₃ nanoparticles with an average diameter of approximately 10 nm were obtained at various temperatures when a flux salt (LiCl, 5 mol/l) was added to the precursor solution. The effects of the crystallinity of the Ga₂O₃ particles and nitridation time on transformation to GaN were characterized using X-ray diffraction and scanning/transmission electron microscopy. Highly crystalline GaN nanoparticles with a mean size of 23.4 nm and a geometric standard deviation of 1.68 nm were obtained when Ga₂O₃ nanoparticles with relatively low crystallinity were used as the starting material. The resulting GaN nanoparticles showed a photoluminescence peak at 364 nm under UV excitation at 254 nm.     

Synthesis of neodymium-doped yttrium aluminum garnet (YAG) nanocrystalline powders leading to transparent ceramics

We report on the synthesis of Nd:YAG nanoparticles by the coprecipitation technique and the optimum conditions for the processing of transparent ceramics. The powders prepared by the coprecipitation technique display significantly less agglomeration of crystallites, indicating higher sinterability. The crystallite size dependence on the calcinations temperature suggests the optimum temperature of 1100 °C, at which phase purity of the YAG nanopowder with the highest sinterability can be obtained. We have demonstrated that the optimum temperature for the vacuum-sintering is about 1785 °C for the uniaxially pressed samples to obtain transparent ceramic with uniform particle size of about 5-7 μm, and above this temperature, enormously large grain growth occurs, and facilitates large pores formation in the intergrain region that makes the ceramic fragile at the grain boundary.     

Synthesis of polycrystalline yttrium iron garnet and yttrium aluminium garnet from organic precursors

Polycrystalline Y₃Fe₅O₁₂ (YIG) and Y₃Al₅O₁₂ (YAG) garnets have been prepared by the organic solution technique using a novel organic precursor. The thermal decomposition of the precursor and subsequent formation of the garnet phases was studied by thermal analysis, X-ray diffraction and Fourier transform infra red spectroscopy (FTIR) including diffuse reflectance FTIR (DRIFT). The precursor of YIG decomposes to give the garnet phase as the main component at 800 °C. YFeO₃ and Fe₂O₃ are also present, and react to give YIG at higher temperatures. Single-phase YAG can be obtained from its precursor at 1100 °C. The reaction proceeds via a hexagonal YAlO₃ intermediate which is formed at 850 °C.     

Synthesis and characterization of nanocrystalline Nd3+-doped gadolinium scandium aluminum garnet powders by a gel-combustion method

Nd³⁺-doped gadolinium scandium aluminum garnet (Nd:GSAG) precursor was synthesized by a gel combustion method using metal nitrates and citric acid as raw materials. The structure and morphology of the precursor and the sintered powders were studied by means of X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscopy (TEM). The results showed that the precursor transformed into pure GSAG polycrystalline phase at about 800 °C, and the powders sintered at 800–1000 °C were well-dispersed with average particle sizes in the range of 30–80 nm. Optical properties of Nd:GSAG nano-powders were characterized by using photoluminescence spectroscopy. The highest photoluminescence intensity was achieved for the powder sintered at 900 °C.     

Synthesis of nanocrystalline chromium nitride from ammonolysis of chromium chloride

Nanocrystalline CrN was synthesized from ammonolysis of CrCl₃. The structure and morphology of the products were investigated by XRD, TEM and SAED. The influence of temperature and time on size of particles was also studied by XRD, TEM, SAED. Elemental analysis was carried out to establish the Cr:N atomic ratio of 1.04:1. XPS results further revealed that pure phase were obtained with the Cr:N atomic ratio of 1.05:1.     

Crystallinity and luminescent properties of citrate sol-gel derived BAM phosphor

By adopting a facile citrate sol-gel process, pure well-crystallized BaMgAl₁₀O₁₇:Eu²⁺(BAM) blue phosphors were successfully prepared. The crystallinity, particle size, morphology, chemical composition and luminescent properties were characterized by XRD, FE-SEM, EDS and spectrofluorometer respectively. The results indicated the precursor was amorphous below 800 °C, and begun to crystallize at 1100 °C. The synthesis temperature decreased by about 300 °C compared to solid-state method to prepare BAM. FE-SEM images showed a well-crystallized, platelet morphology with a particle size of 1-3 μm. The obtained BAM phosphors were fine powders and had higher luminescent intensity compared to solid-state derived BAM phosphors due to high purity and perfect crystalline morphology.     

Synthesis of nanocrystalline MoN from a new precursor by TPR method

Molybdenum nitride (MoN) nanoparticles with high BET surface area (S _BET = 351.0352 m²/g) have been successfully obtained from ammonium polymolybdate, (NH₄)₆Mo₇O₂₄. Differential thermal analysis (DTA), thermogravimetric analysis (TGA) and X-ray powder diffraction (XRD) analysis were used to investigate the effect of water in the precursor. X-ray photoelectron spectroscopy (XPS) was used to determine the empirical formula of the product. Also, MoN nanorods with high aspect ratio have been prepared using Al₂O₃ template. XRD, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were used to characterize the structure of the nanorods.     

Synthesis and properties of nanocrystalline CsI

The structure and properties of fine CsI powders prepared by spray pyrolysis and synthetic opal (SiO₂) based composites containing CsI in their pores have been studied by electron microscopy, X-ray diffraction, and luminescence spectrometry. The powders consisted of nanocrystalline particles which increased in size from 35 to 55 nm with increasing pyrolysis temperature. In the opal/CsI composites, the CsI nanocrystallites were about 45–50 nm in size and formed a single three-dimensional network between the SiO₂ spheres. In going from bulk crystals to their nanocrystalline analogs, the size effect on CsI luminescence leads to a reduction in luminescence yield, a redshift of the emission bands of the on-center and off-center self-trapped excitons (STEs), and an increase in the contribution of the off-center STEs to the net intrinsic emission yield. The emission decay kinetics of the off-center STEs is insensitive to the crystallite size.     

Synthesis of high-purity nanocrystalline BiFeO3

The kinetics of BiFeO₃ formation from xerogel prepared by reverse coprecipitation were studied at different synthesis temperatures. We examined the influence of annealing temperature on the morphology of the reaction products and optimized synthesis conditions, which allowed us to obtain 99.7 wt % pure BiFeO₃ consisting of crystalline grains less than 50 nm in size. Its magnetic properties were investigated.     

双肼基硫代乙二胺双Schiff碱的合成与表征

采用双肼基硫代乙二胺分别与HPMBP、对二甲氨基苯甲醛、香草醛、间硝基苯甲醛、对羟基苯甲醛、苯甲醛双缩合成了相应的双Schiff碱。利用元素分析法,红外光谱法,核磁共振对其进行了表征。     

Synthesis of nanocrystalline sodalite with organic additives

Ethanol, urea and acetone were used as organic additives in the synthesis of nanocrystalline sodalite. The sodalite crystals with sizes of less than 100 nm were produced when ethanol was introduced in the synthesis solution. When both ethanol and urea were present, the resultant sodalites exhibited even smaller crystal sizes, and showed a nitrogen sorption capability as high as around 170-180 cm³/g and a BET surface area of over 78.3 m²/g. However, the addition of acetone in the synthesis solution yielded sodalite with sizes of ca. 200 nm and a trace amount of zeolite A.     

纳米级水氯铁镁石的制备及颗粒团聚控制

水氯铁镁石是具有Mg/Fe结构层的一类特殊阴离子粘土,具有典型的阴离子粘土层状结构、较大的比表面积和热分解特性,是吸附和催化领域内性能优异的新型粘土材料,而纳米水氯铁镁石颗粒的团聚是阻碍其性能开发的关键问题,本文从材料合成、超声分散和悬浮液干燥3个方面阐述了纳米水氯铁镁石制备过程中颗粒团聚的机理和控制措施,并利用XRD、透射电子显微镜、扫描电镜、粒径分布和比表面积等检测手段对合成产物进行表征.研究表明,通过快速沉淀-水热法合成的水氯铁镁石悬浮液颗粒呈薄片状,具有完整的正六边形晶型,粒径大多在166~675 nm,选用有机溶剂洗涤,并缩短晶化时间,采用最低限度的超声分散,冷冻干燥有助于制备颗粒尺寸小、表面细腻、排列整齐的水氯铁镁石纳米材料,最终达到理想的分散效果.     

Synthesis and crystallization of yttrium-aluminium garnet and related compounds

Amorphous oxide combustion products with compositions corresponding to Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ were synthesized by the glycine-nitrate process and heat-treated to induce crystallization. The crystalline structure of the resulting powders was determined by powder X-ray diffraction techniques. The phase stabilities of the crystalline phases were investigated as functions of the glycine-to-nitrate ratio, the yttrium-to-aluminium ratio, and the heat-treatment conditions. Heat treatment for short durations resulted in incompletely crystalline powders that consisted of a mixture of Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ phases, regardless of the chemical composition of the amorphous combustion product. However, heat treatment for longer durations or higher temperature generated both pure-phase, monoclinic Y₄Al₂O₉ and Y₃Al₅O₁₂ with the garnet structure. Prolonged heat treatment at high temperature failed to generate pure-phase orthorhombic YAlO₃. Subsequent analysis revealed a sluggish, complex crystallization process involving the formation and decomposition of several phases.     

Synthesis and characterization of photoluminescent cerium-doped yttrium aluminum garnet

Powder phosphor yttrium aluminum garnet (YAG), doped with trivalent cerium (Ce³⁺) is synthesized by sol-gel method. The formation of YAG and YAG:Ce (cerium-doped yttrium aluminum garnet) was investigated by means of X-ray diffraction (XRD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also used. The purified crystalline phases of YAG and YAG:Ce were obtained at 1000 °C. The maximum average grain size is about 20-23 nm for undoped samples and 28-34 nm for doped samples. The crystalline YAG:Ce emission shows one peak in the range 480-535 nm with the maximum near 520 nm. Photoluminescence (PL) intensity of 5d → 4f transition of Ce³⁺ increased with increasing annealing temperature. With increasing the concentration of Ce³⁺, the photoluminescence peak shifts towards the red region.     

Low-temperature synthesis and microstructure-property study of single-phase yttrium iron garnet (YIG) nanocrystals via a rapid chemical coprecipitation

Single-phase yttrium iron garnet (Y₃Fe₅O₁₂, YIG) nanocrystals have been synthesized via a rapid chemical coprecipitation process with reverse strike operations, followed by calcining the precipitates at the temperature around 750 °C. The formation of YIG nanocrystals from the amorphous precipitates and their microstructural features and magnetic properties were investigated by FT-IR, XRD, TG-DSC, FESEM, TEM and VSM. It has been found that the as-obtained precipitates could be thermally activated to directly form the crystalline phases of garnet structure around 650 °C, including cubic YIG and minor tetragonal YIG but no trace of YFeO₃, which was often involved during the synthesis of YIG or doped-YIG when a chemical coprecipitation method was used. The calcinations could make the tetragonal YIG entirely transform into the cubic phase at 750 °C and allow the crystallites of the latter to grow from ∼22 nm to ∼50 nm in size almost linearly as a function of the temperature ranging from 650 °C to 900 °C. Moreover, the room temperature saturation magnetization of the samples after calcinations at various temperatures showed a nonlinear increase from 0.24 emu g⁻¹ to 24.54 emu g⁻¹, which should be associated with the alignments of atomic magnetic moments in the materials from completely-disordered to partially-ordered firstly and further to completely-ordered and, in the last stage, mainly with the growing YIG nanocrystals.