Synthesis of nanosized (Pb,Sr)TiO3 perovskite powders by coprecipitation processing

Nanosized perovskite lead strontium titanate (Pb_xSr_1−x)TiO₃ (PST) powders were successfully prepared by a simple coprecipitation method. Lead-strontium titanyl oxalate (PSTO) precursor was first synthesized at room temperature, and the precursor was then calcined at 600 °C for 1 h to produce the single phase perovskite PST powders. Characterization studies were carried out on the as-dried precursor and the calcined PST powders by various techniques. The results showed a strong dependence of the chemical composition of final PST powders on pH value in the coprecipitation reaction. PST powders with desired composition could be synthesized by adding 25 mol% excess Sr. PST particles were found to be spherical in nature with an average size of 10 nm.     

Red ceramic pigments of terbium-doped ceria prepared through classical and non-conventional coprecipitation routes

Reddish ceramic pigments based on cerianite compositions Ce_1−xTb_xO₂ (x = 0.1, 0.2 and 0.3) and Ce_0.9Tb_0.05Y_0.05O₂ have been prepared. Firstly, the optimal composition giving the best red colour was selected (Ce_0.9Tb_0.1O₂) from ceramic and chlorides coprecipitation samples fired at 1100–1500 °C. Secondly, the effect of using different classical and non-conventional coprecipitation routes on the synthesis and colouring performance of CeO₂-Tb red pigments was analyzed for the first time. In addition to classical coprecipitation we tested also the use of hydrothermal treatment and other more homogeneous coprecipitation methods with oxalates and urea. Homogeneous coprecipitation was also combined with ultrasonic or microwave irradiation. Samples were characterized by thermal analysis, XRD, SEM/EDX, optical spectroscopy and colour measurements. Interestingly, the optimization of synthesis methods enabled to obtain more homogeneous (single phase) CeO₂-Tb solid solutions at lower temperatures (400–1100 °C), exhibiting very nice reddish colourations associated to lower energy bandgaps (E_g below 2.30 eV). The obtained optimal compositions could be therefore alternative candidates as environmentally friendly reddish ceramic pigments.     

Growth and optical properties of cerium dioxide nanocrystallites prepared by coprecipitation routes

Nanosized cerium dioxide (CeO₂) powders have been synthesized using coprecipitation methods and cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) as the starting material. The growth and optical properties of nanosized CeO₂ powders were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), nano-beam electron diffraction (NBED), high resolution TEM (HRTEM), and ultraviolet–visible (UV–vis) absorption spectrophotometry. The XRD result shows that the dried CeO₂ precursor powders (both before and after calcination at various temperatures and times) contained a single crystalline phase of CeO₂. In the dried precursor powders, the crystallites of CeO₂ measured 10.4 nm and 66.8 nm before and after calcination at 1273 K for 240 min, respectively. The indirect band gap energy (_Ei$E_i$) of CeO₂ decreased from 3.03 eV to 2.68 eV as the crystallite size increased from 10.4 nm to 66.8 nm, whereas the direct band gap energy (_Ed$E_d$) of CeO₂ also decreased from 3.79 eV to 3.38 eV.     

共沉淀法制备纳米镁铬尖晶石粉体

镁铬尖晶石具有较高的熔点而被广泛应用于耐火材料中。而常规镁铬尖晶石粉体因烧结温度较高，铬蒸发而使得耐火材料的致密度难以提高。为改善镁铬耐火材料的烧结性能，进行了纳米镁铬尖晶石粉体的研究：以Cr(NO3)3，Mg(NO3)2，PEG-400和CO(NH2)2为原料，采用共沉淀法获得了铬酸镁纳米粉(nc2o3：nMgo=1：1)的前驱体，经过超临界流体干燥(SCFD)制备出镁铬二元纳米气凝胶，经600℃煅烧后获得了纳米级立方相镁铬尖晶石粉体。经XRD，DSC-TG，BET和透射电镜分析，结果表明：前驱体纳米粉的比表面积为350～780m^2／g，平均粒径为1～2nm，煅烧后的纳米粉的比表面积为36～65m^2／g，平均粒径为5～10nm。     

水热法制备氧化锆微粉的进展

二氧化锆纳米粉是制造高科技陶瓷的重要原料，水热法是目前生产纳米粉体最具发展潜力的工艺方法之一。综述了水热法（包括水热沉淀、水热氧化、水热电埋弧、微波水热等）在生产纳米二氧化锆系粉体中的应用，以及水热法的发展情况。     

Synthesis of nanosized gadolinium doped ceria solid solution by high energy ball milling

This paper reports the development of a new process for the synthesis of gadolinium-doped ceria (20GDC) solid solution nanoparticles, as a solid electrolyte for use in solid oxide fuel cells. It is based on high energy milling (mechanical alloying or MA) of CeO₂ and Gd₂O₃ powders containing 10 mol% Gd₂O₃. The samples, obtained after different milling times of 10 to 60 h, were characterized by X-ray diffraction (XRD), BET method of the specific surface area measurement, electron microscopy and Raman spectroscopy. The fluorite-structured Ce_0.8Gd_0.2O_1.9 solid solution nanopowder with the surface area of 16.86 m²/g and particle sizes below 50 nm was obtained by milling the oxide mixture for 30 h. Increase in the milling time beyond 30 h led to the smaller crystallite sizes and more agglomeration. The structural changes, as evidence for the formation of solid solution, which occurred during the course of the milling process, could be appropriately followed and discussed by Raman spectroscopy.     

共沉淀法制备三元系铋基焦绿石型陶瓷粉体

采用改进的Mergen A．方案，用氧化锌、三氧化二铋、三氧化二锑为原料，氨水、草酸铵和碳酸铵分别为沉淀剂，盐酸为溶剂，用共沉淀法成功制备了亚微米级颗粒、高活性的铋基Bi1.5ZnSb1.5O7（BZS）焦绿石型介电陶瓷粉体。探讨了用不同的沉淀剂、在不同的烧结温度所得产物的形态。结果表明氨水做沉淀剂在570℃就可生成纯的焦绿石结构。与Mergen A方法相比成本大大降低，可以使瓷片组织的均匀性提高，电性能得到改善，电压梯度下降。     

Doped ceria powders prepared by spray pyrolysis for gas sensing applications

In the present study, ceria powder with and without gadolinium (Gd) or zirconium (Zr) dopants were synthesized by a spray pyrolysis (SP) process. The resulting powders (undoped CeO₂, Gd-doped ceria and Zr-doped ceria) were first mixed with organic binders, screen printed on alumina substrates, and then heat treated at 1200 °C for 2 h in air. Experimental results showed that the as-pyrolyzed powders were nanocrystalline and spherical in shape with uneven surfaces. After heat treatment, the powder coatings showed a 3-D network structure with interconnected pores exhibiting a high surface area. The electrical conductivity of CeO₂ was increased by the dope of Zr. The increasing rate of conductivity of ZDC (Zr-doped ceria) with decreasing oxygen pressure was also higher than that of undoped CeO₂. The CeO₂ and ZDC exhibited an n-type semiconductance in all the oxygen pressure regions, showing promise as candidates for sensor applications. The GDC (Gd-doped ceria) revealed a p-type and an n-type semiconductance in high- and low-oxygen pressure regions, respectively. The sensor applications of GDC are thus limited. Meanwhile, the ZDC exhibited a shorter response time due to its smaller grain size, showing a better oxygen sensing behavior.     

Synthesis of ceria nanoparticles by flame electrospray pyrolysis

A flame electrospray pyrolysis is presented for synthesizing CeO₂ nanoparticles with a dense morphology, high crystallinity and nanometer size. Hydrated cerium nitrate precursor dissolved in an ethanol/diethylene glycol butyl ether mixture was injected into a CH₄/air premixed flame using an electrospray method. The number size distributions of the as-prepared particles were trimodal. It is suggested that the particles for the fine mode were formed by a Rayleigh disintegration of the charged precursor droplets during the droplet evaporation. The particles for the coarse and middle modes are surmised to come from primary and secondary droplets, respectively, which were formed simultaneously during the atomization processes. The CeO₂ nanoparticles for the coarse mode were nonspherical and composed of few crystallites. The nanoparticles for the fine and middle modes were nearly spherical and nonagglomerated. The as-prepared CeO₂ nanoparticles showed highly crystallinity.     

Compression behavior of ceramic powders by inductive plasma sphero process

Strength of ceramic powders can be affected by the powders’ structure. In this study, two typical structured yttria-stabilized zirconia (YSZ) powders, agglomerated and sintered, were tested in a micro compression machine. The results showed that the sintered powders exhibited much higher compressive strength than the agglomerated powders. It was also found that the strength of the agglomerated powders was independent on the particle size due to their uniform microstructure, while the sintered powders showed decreased strength with the increasing of particle size because they had lower level of sintering. Finite element analysis was performed which indicated that thinner shells trended to sustain higher tensile stress/strain than the thicker shells, so the larger particles with thinner shells were easier to break.     

Effects of pH on formation and morphology of barium titanate powders prepared by coprecipitation process

Abstract

The effects of pH on the formation and microstructure of barium titanate (BaTiO₃) prepared by a coprecipitation process have been investigated. After calcination of the precipitates at 800°C, pure BaTiO₃ was formed in samples prepared from oxalic solutions with pH in the range 5-7, and the powders obtained exhibited spherical shape with a size range of 5-10 μm. In contrast, small particles (around 0·1 μm) with Ti enriched phases (BaTi₃O₇, BaTi₄O₉) were produced at pH 1, and BaCO₃ with irregular shape appeared at pH 10. The phases and morphologies of the calcined powders markedly influenced the sintering behaviour and dielectric properties of BaTiO₃. After sintering at 1350°C, densified ceramics with high dielectric constants were derived from the calcined powders containing pure BaTiO₃ phase. These results show that close control of the pH of the oxalic solutions is required to produce BaTiO₃ powders with high sinterability and high dielectric constant.     

Physical and electrical properties of yttria stabilised zirconia prepared from nanosized powders

Abstract

Nanosized powders of ZrO₂-10 mol-%Y₂O₃ were prepared by a wet chemical process, with the main purpose of verifying changes in the physical properties of powders, and to study their influence on the electrical resistivity of the sintered ceramic. Optimisation of some parameters involved in the synthesis process resulted in a powder with high specific surface area (> 150 m² g^-1) and with monomodal pore size distribution. The rate of shrinkage was a maximum at 1040°C and high density was obtained on optimising the firing schedule. Impedance spectroscopy results show that the intragrain resistivity was approximately equal to that of a single crystal of the same chemical composition.     

Determination of kinetic parameters of the oxidehydrogenation of ethane with CO2 on nanosized calcium-doped ceria under fast deactivation processes

Kinetic parameters of the oxidehydrogenation of ethane with CO₂ on nanosized Ca-doped CeO₂ have been investigated. During reaction, interaction of the reactants with the oxide catalyst causes a fast deactivation process. Overlapping of this fast deactivation with catalytic reaction makes quite difficult the reliable determination of kinetic parameters. This handicap can be overcome by getting sufficient data in a short testing time, thus reducing the degree of deactivation. The kinetics of catalytic reaction and of catalyst deactivation have been studied by conducting a series of consecutive tests at increasing temperatures in steps of 20 °C and monitoring the evolution of the reaction for periods of 30 min on stream at each temperature, with full product analysis every 3 min. At temperatures above 680 °C, catalytic rates decreased linearly with run time at isothermal operation, while deactivation rates increased with increasing temperature. Analysis of the results allows to uncouple catalyst deactivation and catalytic reaction and to obtain the kinetic parameters of both processes (i.e., steady-state and deactivation rates, and their apparent activation energies). Deactivation rate of CO formation is one order of magnitude faster than of ethene formation but both processes show the same apparent activation energy, ca. 47 kcal/mol. The apparent activation energy values for the catalytic reaction are 32 ± 4 and 26 ± 2 kcal/mol for the rates of formation of ethene and CO, respectively.     

Ultrafast high-temperature sintering of gadolinia-doped ceria

Ultrafast high-temperature sintering (UHS) is a rapidly growing research area of material science and engineering. Herein we present UHS of gadolinia-doped ceria (GDC) powders in single and multi-step approaches. The sintered ceramics were characterized from a physical and electrochemical point of view. When the power is applied gradually during the multistep UHS process crack-free GDC ceramics can be obtained with 95 % bulk density using commercial powder. Oxalate converted GDC powder gave 86 % bulk density with the same multistep sintering process. Additionally, it is shown that multistep UHS is also suitable for multilayer co-sintering necessary for solid oxide fuel cells (SOFC), as demonstrated by the production of dense GDC electrolyte in tight contact with porous electrodes.     

Characteristics of nanosized Bi-based glass powders prepared by flame spray pyrolysis as transparent dielectric layer material

Bi-based glass powders with particle size of 34 nm were prepared by high-temperature flame spray pyrolysis. The glass transition temperature (T_g) of the powders was 442 °C. Dielectric layers fired at temperatures of 480 and 500 °C contained voids, while those fired at temperatures above 540 °C had clean surfaces and no voids. The dielectric layers sintered at temperatures of 560 and 580 °C had transmittances of 70% in the visible range. Further, it was observed that the dielectric layers formed from the nanosized glass powders obtained from spray solutions containing excess boron had higher transmittances (80% in the visible range at a sintering temperature of 580 °C) than the layers formed from spray solutions containing stoichiometric amounts of boron.     

Co/CeO2-ZrO2 catalysts prepared by impregnation and coprecipitation for ethanol steam reforming

Co/CeO₂-ZrO₂ catalysts for the ethanol steam reforming were prepared by wet incipient impregnation and coprecipitation methods. These catalysts were characterized by nitrogen adsorption, TEM-EDX, XRD, H₂-TPR, and CO chemisorption techniques. It was found that the catalyst reducibility was influenced by the preparation methods; catalysts with different reduction behaviors in the pre-reduction showed different catalytic activities toward hydrogen production. The H₂-TPR studies suggested the presence of metal–support interactions in Co/CeO₂-ZrO₂ catalysts during their hydrogen pre-reduction, a necessary treatment process for catalysts activation. These interactions were influenced by the preparation methods, and the impregnation method is a favorable method to induce a proper metal–support effect that allows only partial reduction of the cobalt species and leads to a superior catalytic activity for the hydrogen production through ethanol steam reforming. At 450 °C, the impregnated catalyst gives a hydrogen production rate of 147.3 mmol/g-s at a WHSV of 6.3 h⁻¹ (ethanol) and a steam-to-carbon ratio of 6.5.     

Rotenone coprecipitation with biodegradable polymers by supercritical assisted atomization

The aim of this study was to coprecipitate rotenone, a biopesticide, with biodegradable polymers using supercritical assisted atomization (SAA) process. Rotenone encapsulation can protect the pesticide from degradation thus reducing the quantity to be used. Different water soluble polymers were tested, such as polyethylene glycol (PEG), polyvinylpyrrolidone (PVP) and sodium alginate. Solutions with one or two solvents and rotenone suspensions were tested to perform the experiments. Different biodegradable polymer/pesticide ratios were tested: 2 and 8 in the case of PEG⿿rotenone coprecipitates, 25, 50 and 100 in the cases of PVP⿿rotenone and alginate⿿rotenone coprecipitates. The particles obtained were spherical with the mean diameter ranging between 0.6 and 1.5 μm. The effective loading of the particles was evaluated by HPLC and the best results were obtained in the case of alginate⿿rotenone coprecipitates that show a coprecipitation efficiency close to 100% and for PEG⿿rotenone particles the efficiency was about 98%. PVP⿿rotenone composite microparticles showed the lowest coprecipitation efficiency that ranged between 30% and 50%.     

N-doped TiO2 nanoparticles obtained by a facile coprecipitation method at low temperature

N-doped TiO₂ nanoparticles (NPs) were synthesized using a facile synthesis route by coprecipitation method. The effect of the HNO₃ volume and calcination temperature on the structural, morphological, optical and surface properties of the N-doped TiO₂ NPs was studied. X-ray diffraction analysis showed particles of nanometric size (< 16 nm), which are consistent with HR-TEM micrographs. A slight shift of the absorption edge to higher wavelengths is observed as the HNO₃ volume and calcination temperature increases. Both X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) show the presence and stability of nitrogen in the N-doped TiO₂ structure. The photocatalytic activity of the N-doped TiO₂ NPs was assessed by testing the degradation of rhodamine B (RhB) under ultraviolet (UV) and visible light.     

Synthesis of europium-doped calcium silicate hydrate via hydrothermal and coprecipitation method

In this paper, calcium silicate hydrate doped with various amounts of Eu (Eu–CSH) via hydrothermal and coprecipitation methods have been systematically investigated. The hydrothermal method produced xonotlite while coprecipitation gave 11 Å tobermorite. Regardless of the synthesis method, incorporation of Eu inhibited the crystallite growth and particle size of the as-synthesized powders, while increasing their thermal stability. In both phases, Eu³⁺ was found to occupy the cation sites in preference to Ca²⁺. Comparing both synthesis methods, the hydrothermally synthesized powders were of higher crystallinity and thermal stability than the powders prepared by coprecipitation. Moreover, Eu–CSH powder by hydrothermal exhibited stronger photoluminescence intensity than the one by coprecipitation, primarily attributed to its higher degree of crystallinity. Thus, they showed higher potential for nanomedicine application where a combination of biocompatibility and light emission is desired.     

Conventional and field-assisted sintering of nanosized Gd-doped ceria synthesized by co-precipitation

Gadolinium-doped ceria is an attractive electrolyte for potential application in SOFCs operating at intermediate temperature; for such use, the fundamental compositions typically contain 10–20 mol% Gd₂O₃. In this work, we produced nanosized 10 mol% gadolinium-doped ceria powder by co-precipitation, starting from Ce and Gd nitrate solutions and using ammonia solution as precipitating agent. The co-precipitate was characterized by DTA-TG, TEM, XRD and nitrogen adsorption analyses. We studied the behavior of the nanopowder under both conventional and Flash sintering. Very different behavior was seen: the conventional sintering cycle produced a poorly densified material, while Flash sintering allowed production of a perfectly densified material, with uniform sub-micrometric grain size.