Obtaining nanodisperse powders of neodymium-activated yttrium aluminum garnet by self-propagating high-temperature synthesis method

Nanosized slightly agglomerated powders of neodymium-activated yttrium aluminum garnet (YAG:Nd) were obtained by a self-propagating high-temperature synthesis method. An increase in content of reducing substituents in metal-organic complexes as compared to oxidizing substituents leads to an increase in average size of the forming grains from 23 to 70 nm; thereupon, a decrease in powder agglomeration degree is observed. Firing of x-ray amorphous powders results in formation of polycrystalline YAG:Nd without forming intermediate phases.     

Preparation of weakly agglomerated yttrium aluminum garnet powders by burning a mixture of yttrium aluminum hydroxynitrates, urea, and acetic acid

We have optimized the composition of a mixture of aluminum yttrium hydroxynitrates, urea, and acetic acid for the self-propagating high-temperature synthesis of yttrium aluminum garnet (YAG) powder. The powder prepared in this way offers a low degree of agglomeration, and the ceramic produced from it has a low carbon content. A technique has been proposed for carbon determination in YAG ceramics through gas chromatographic analysis of the gaseous products of carbide hydrolysis after the dissolution of the ceramic in pyrophosphoric acid.     

Phase formation in yttrium aluminum garnet powders synthesized by chemical methods

Yttrium aluminum garnet (YAG) powders were synthesized by precipitation of hydroxides using three types of precursors: nitrates (nitrate process), isopropoxides (alkoxide process), and isopropoxides chelated with ethyl acetoacetate (modified alkoxide process). The phase development in the powders during heat treatments was investigated with DTA and XRD. An intermediate hexagonal YAlO₃ (YAH) phase was formed at 800°C in all powders regardless of the synthesis processes, but its complete transformation to YAG at higher temperatures (1000°C) occurred only in the powders prepared by the nitrate and modified alkoxide processes. The alkoxide process led to the largest deviation from the bulk composition, producing a single phase of YAH that transformed into YAG plus a stable YAM (Y₄Al₂O₉) phase. The modified alkoxide process led to the most homogeneous bulk composition, resulting in the least amount of YAH in the powder. The poor chemical homogeneity in the powders prepared by the nitrate and alkoxide processes was attributed to the segregation of the hydroxides and to the presence of the double alkoxide, respectively.     

钇铝硝酸盐共熔制备YAG微粉

以Al(NO3）3和Y(NO3)3为原料，按Y:aL=3:5原子摩尔比配成溶液，蒸发去水获得Al(NO3)3和Y(NO3)3共熔体，经1000℃煅烧得到YAG粉体，比通常YAG形成温度降低约600℃，粉体平均粒径为300nm；加入2.0%(质量分数)100nm a-Al2O3籽晶可使最终相变温度降低为900℃，粉体平均粒径为150nm。     

Synthesis of fine Pb(Fe0.5Nb0.5)O3 perovskite powders by coprecipitation method

Lead iron niobate, Pb(Fe_0.5Nb_0.5)O₃ (PFN) is an optimal material for multilayered ceramic capacitors (MLCCs). Fine lead iron niobate powders with average size of 500 nm were prepared from the precipitation precursor. In the experiment, Nb₂O₅ was used as Nb source to substitute Nb alkyloxide or oxalate and successfully converted into aqueous Nb⁵⁺ solution. The results shows that the perovskite PFN was emerged at a temperature as low as 400 °C, and pure perovskite was obtained at the temperature of 800 °C. The formation of perovskite at a low temperature was attributed to the core–shell structure in the precipitates of hydroxide. Besides, the particles of perovskite PFN show the good chemical stoichiometry and small size of 300–500 nm.     

Effect of the composition of starting yttrium aluminum hydroxide sols on the properties of yttrium aluminum garnet powders

A technique has been developed for the synthesis of yttrium aluminum garnet sols aggregationstable in water as a dispersion medium. Depending on the type of precursor used, the temperature of yttrium aluminum garnet formation varies from 900°C to 1100°C. We have obtained yttrium aluminum garnet nanopowders with an average particle size from 40 to 300 nm, depending on the aluminum yttrium hydroxide hydrosol synthesis procedure and annealing temperature.     

Preparation of fine powders with perovskite structure from metal alkoxides

The preparation of perovskite powders was investigated by the metal alkoxide methods. The complex alkoxide as a precursor was hydrolysed under specific conditions, and submicrometre powders of perovskite compounds differing in composition, BaTiO3, Ba(Mg1/3Nb2/3)O3, PbTiO3, and Pb(Mg1/3Nb2/3)O3, could be obtained. The simultaneous use of a dipolar aprotic solvent, such as acetone, etc., with ethanol was found to be effective for the preparation of the spherical or nearly spherical powders with submicrometre diameter. The submicrometre powders were formed through nucleation–aggregation, but not by the usual nucleation–grain growth. Such powder formation, and the affinity of the solvent for the diffusion layer surrounding the colloidal particle, are important. The powders obtained here could be crystallized to the desired crystalline phase at lower temperatures.     

均相共沉淀法制备钇铝石榴石(YAG)纳米粉末

以Y2O3、Al(NO3)3·9H2O、(NH4)2SO4为原料,尿素为沉淀剂,正硅酸乙酯作为添加剂,采用均相共沉淀法制备出YAG前驱体粉末,在沉淀过程中采用静电稳定、聚合物位阻以及共沸方法相结合有效地防止了纳米颗粒硬团聚的形成,前驱体颗粒尺寸小于100nm.采用DTA/TG、IR、XRD和TEM测试手段对粉末材料进行了表征,根据Scherrer公式计算出晶粒大小,研究了晶粒尺寸分布及其变化情况.研究结果表明前驱体粉末经过1200℃温度烧结后,全部转化YAG相,其晶粒尺寸小于50nm,随着烧结温度提高,晶粒平均尺寸增大.     

Sol-gel processing and sintering of yttrium aluminum garnet (YAG) powders

Gels of yttrium aluminum garnet (YAG) with the stoichiometric composition 3Y₂O₃·5AI₂O₃, were prepared by a sol-gel technique and dried by supercritical extraction with CO₂. Powders were produced by lightly grinding the dried gels. Crystallization of the powder occurred at 900°C and within the limits of detection, the X-ray diffraction pattern of the crystallized material was identical to that of the stoichiometric composition. Powder compacts with a green density of 0.50 of the theoretical were sintered to nearly full density in O₂ during constant heating rate sintering at 5 °C min^–1 to 1600 °C. This is better than the density obtained with powders from a similar gel dried conventionally (by evaporation of the liquid) and considerably better than that obtained with powders prepared by solid state reaction. The room temperature flexural strength and fracture toughness of the material fabricated from the supercritically dried gels were 190 MPa and 2.2 MPa.m^1/2, respectively. These strength and fracture toughness values are higher than those reported in other studies for YAG produced by the sintering route.     

Hydrothermal synthesis of fine perovskite PbTiO3 powders with a simple mode of size distribution

Perovskite phase PbTiO₃ powders were hydrothermally synthesized so as to be fine and homogeneous in physico-chemical properties. Such powders are currently used in the fabrication of electronic ceramic components. The powders were successfully synthesized using a strong alkali as catalyst. Borderline synthesizing conditions, such as temperature and time, were established and their effects on geometrical properties, like mean particle size were determined. Polyacrylamide as an additive was found to be effective for the synthesis of the powders with a simple mode of particle-size distribution.     

Preparation of CaTiO3 fine powders by the hydrothermal method

Fine powders consisting of 0.1–0.5 μm size crystallites of CaTiO₃ are prepared at 150–200°C by the hydrothermal method starting from hydrated titania gel and reactive calcium oxide suspended as an aqueous slurry in an autoclave. The resulting high-purity CaTiO₃ is characterised by TEM, X-ray powder diffraction, chemical analyses and sintering characteristics. The hydrothermally prepared CaTiO₃ powders are sinterable to high-density ceramics below 1400°C. The dc conductivity behaviour of the chemically reduced ceramics is presented.     

Synthesis and characterization of yttrium aluminum garnet nanostructures by cathodic electrodeposition method

Pure nanostructures of yttrium aluminum garnet (YAG) was prepared based on the cathodic electrodeposition method from the mixture of YCl₃ and AlCl₃ dissolved in water/ethanol solution. At first, hydroxide precursors cathodically were grown on the steel substrates then, the hydroxide powders heat treated at 850 °C for 4 h in dry air atmosphere. The formation of crystalline YAG nanopowder was confirmed by X-ray diffraction (XRD), thermogravimetric analysis (DSC-TGA), scanning electron microscopy (SEM) and Fourier transformed infrared spectroscopy (FT-IR). The results of the SEM showed that applied current density and bath temperature have the prominent effect on the morphology and particle size of the products. The results revealed that cathodic electrodeposition followed by heat-treatment can be used as a facile method for preparation of YAG nanostructures with different morphology.     

Preparation and characterization of cobalt ferrite by the polymerized complex method

Magnetic nanoparticles of Co-ferrite were prepared by the polymerized complex method. Heating in vacuum of a precursor solution containing citric acid (CA), ethylene glycol (EG) and cobalt and iron salts with a molar ratio of Co/Fe/CA/EG=1/2/9/22.5 at 130 °C produced a brownish transparent polymeric gel, which have been characterized by IR and NMR spectroscopy. The results of both techniques suggest two types of reactions: the formation of metal-CA complexes and successive esterification reactions between CA and EG. The organic fraction was removed by controlled thermal treatments (200–800 °C) whereby the bimetallic oxide was formed. The powders obtained were characterized by X-ray diffraction (XRD), vibrational sample magnetometry (VSM) and transmission electron microscopy (TEM). XRD analysis showed the presence of CoFe₂O₄ at 400 °C. The saturation magnetization values of the samples increased as a function of calcination temperature and reached a maximum of 79.8 emu/g at 800 °C. The TEM images showed spherical nanoparticles with sizes between 20 and 40 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.     

Preparation and characterization of perovskite ceramic powders by gelcasting

Perovskite La0.6Sr0.4Co0.8Fe0.2O3– (LSCF) powders have been successfully synthesized from oxide and carbonates based on the principle of gelcasting. Phase-forming temperature is very dependent on the ball-milling process during the suspension preparation. As the ball-milling time is increased, the temperature of phase formation decreases, therefore the perovskite powder obtained has a larger Brunaver–Emmett–Teller (BET) specific surface area. The grain sizes were around 1 m at 1000°C and 2 m at 1100°C from scanning electron microscopy (SEM) photographs. The perovskite powders have good sinterability: the sintering densities of ceramic bodies shaped with as-prepared powders were investigated. SEM photos show that sintered ceramics exhibit a well defined morphology in the packing and sintering of particles. The oxygen permeance of disc shaped samples, with a thickness ranging from 1.02 to 1.98 mm was 6.39 × 10–8–1.99 × 10–8 mol cm–2 s–1 at 900°C indicating that LSCF ceramics have high oxygen permeation. It can be concluded that gelcasting is a simple and effective method for preparing practical multicomponent perovskite powders.     

Preparation of M-Ba-ferrite fine powders by sugar-nitrates process

In this paper, fine M-type barium hexaferrite (M-Ba-ferrite) particles were synthesized from sugar and nitrates by simple route, which revealed the feasibility of using sugar as chelating agent in forming solid precursors of BaFe₁₂O₁₉. The effects of factors, such as the molar ratio of Fe/Ba, calcination temperature and time, on the morphology, the phase component and the magnetic properties of M-type barium hexaferrite particles were studied by means of X-ray diffraction, infrared spectroscopy, transmission electron microscopy and physical property measurement system. The results showed that the molar ratio of Ba²⁺ to Fe³⁺ influenced significantly on the formation of the single phase barium ferrite. The hexagonal platelet barium ferrite particles with a specific saturation magnetization of 64.48 emu/g, remanence magnetization (Mr) of 33.84 emu/g, and coercive force (Hc) of 1848.85 Oe were obtained when the molar ratio of Fe/Ba was 11.5 and the calcination temperature was 1100 °C for 2 h.     

Synthesis of BF–PT perovskite powders by high-energy ball milling

0.7BiFeO₃–0.3PbTiO₃ (BF–PT) powders were synthesized from a mixture of the oxides Bi₂O₃, Fe₂O₃, PbO and TiO₂ using a Fritsch P4™ vario-planetary ball milling system. The perovskite structure of the BF–PT powder can be obtained well and the crystallite size of the powders was greatly reduced to 20–35 nm after milling for 8 h. The pre-calcined course shows a rhombohedral–tetragonal phase transition with the increasing temperature and shows the structure transition near the Curie temperature T_c.