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.     

Characteristics of Ag-doped BaTiO3 nanopowders prepared by spray pyrolysis

Pure and Ag-doped BaTiO₃ nanopowders were prepared by spray pyrolysis. Precursor powders, prepared from a spray solution with citric acid and ethylenediaminetetraacetic acid (EDTA) as chelating agents, had large, hollow particles irrespective of Ag doping. Both pure and Ag-doped powders had partially aggregated particles after post-treatment at 900 °C that could be easily milled to nanoparticles. The mean sizes of the pure and Ag-doped BaTiO₃ particles were 75 and 91 nm, respectively. The Ag-doped particles were mainly of cubic BaTiO₃ crystal structure, with small Ag phases observed. High-density BaTiO₃ pellets were formed by sintering the powders at the low temperature of 1000 °C. The silver was uniformly distributed in a tetragonal BaTiO₃ phase without phase separation in the doped pellet. The dielectric constants of the pellets formed from the pure and Ag-doped BaTiO₃ powders were 1826 and 2400, respectively.     

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.     

Characteristics of Bi-based glass powders with various glass transition temperatures prepared by spray pyrolysis

Spherical shape, submicron, and non-aggregated bismuth-based glass powders were prepared. Glass powders with low glass transition temperature melted the silver powders at firing temperatures of as low as 400 °C. After firing at 400 °C, the specific resistances of the silver conducting films obtained from glass powders with glass transition temperatures of 498 and 373 °C were 21.6 and 5.8 μΩ cm, respectively. After firing at 450 and 500 °C, the specific resistances of the silver conducting films obtained from glass powders with glass transition temperature of 425 °C were the lowest, i.e., 3.0 and 3.1 μΩ cm, respectively.     

Additives content in ZnO films prepared by spray pyrolysis

ZnO films containing additives like bismuth, manganese, copper or cobalt have been deposited by spray pyrolysis of precursors dissolved in water. The additive content in the final material has been measured and it can be different from its value in the starting solution. These differences are discussed by looking at the chemistry of the starting solution.     

Characteristics of BaNd2Ti5O14 powders directly prepared by high-temperature spray pyrolysis

BaNd₂Ti₅O₁₄ powders were directly prepared by high-temperature spray pyrolysis. The powders prepared at temperatures of 1300 and 1500 °C exhibited a pure BaNd₂Ti₅O₁₄ phase. The powders prepared at 1300 °C were spherical in shape. However, the powders prepared at 1500 °C showed non-spherical shapes. The BaNd₂Ti₅O₁₄ powders had a composition similar to that of the spray solution. The mean sizes of the BaNd₂Ti₅O₁₄ powders increased from 0.23 to 0.60 μm when the concentration of the spray solution was increased from 0.01 to 0.2 M. At a sintering temperature of 1100 °C, bridge-like structures were formed between the powders. Pellets sintered at 1300 °C exhibited a dense structure comprising rod-like crystals.     

Highly-dispersible boron nitride nanoparticles by spray drying and pyrolysis

A spray drying and pyrolysis synthesis route was developed and it successfully prepared boron nitride (BN) nanoparticles with high dispersivity. During the experiment, the extremely rapid drying of the boric acid/urea solution during the spray-drying process resulted in the formation of homogeneous precursors, which was the key for the final pyrolysis synthesis of BN nanoparticles with high dispersibility and uniform diameters (~20 nm). Compared with boron nitride synthesized without using spray drying, the as-prepared BN nanoparticles possess higher specific surface area (145.01 m² g⁻¹) and larger pore volume (0.41 cm³ g⁻¹). Especially, we used the BN nanoparticles as lubricant and incorporated it into the liquid paraffin (LP). The experiment results show that the LP presents outstanding antifriction properties for a BN content of 1.5 wt%. These results suggest that the h-BN nanoparticles have significant potential applications in the field of tribology.     

喷雾热解法合成高性能球形钴蓝的研究

研究了喷雾热解法合成高性能球形钻蓝。试验对比了两种不同喷雾热解前驱体溶液对钻蓝形貌及反射性能的影响，一种是硝酸盐溶液体系；另一种是聚合物溶液体系，聚合物溶液是由硝酸铝溶液加入氨水水解合成的。由X射线荧光光谱分析可知，喷雾热解合成的钻蓝较商品钻蓝的纯度高。喷雾热解产物再经1000℃煅烧2h，由硝酸盐溶液体系合成的钻蓝的形貌是不规则的，而由聚合物体系合成出的是实心球形的钻蓝，且比不规则外形的及商品的钻蓝有较高的反射性能。     

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.     

Firing characteristics of size-controlled silver-glass composite powders prepared by spray pyrolysis

Size-controlled spherical silver-glass composite powders were directly prepared by using spray pyrolysis. The mean sizes of the composite powders changed from 0.34 to 0.78 μm when the concentration of the spray solution was changed from 0.05 to 2 M. The firing characteristics of composite powders formed from the spray solutions with a glass content equal to 3 wt.% of the silver component were affected by the mean sizes of the powders. Silver-conducting films formed from large-sized composite powders had a denser structure than those formed from small-sized composite powders. Further, silver-conducting films formed from composite powders with a mean size of 0.78 μm had specific resistances of 3 and 2 μΩ cm at firing temperatures of 450 and 500 °C. However, silver-conducting films formed from composite powders with a mean size of 0.34 μm had specific resistances of 8.2 and 6.9 μΩ cm at firing temperatures of 450 and 500 °C.     

Electrical properties of samaria-doped ceria electrolytes from highly active powders

The correlations of the microstructures and the electrical properties of high reactive Ce_0.8Sm_0.2O_1.9 (SDC) powders, synthesized via an optimal carbonate coprecipitation method, were investigated. Microstructure of the SDC ceramics sintered at 900-1400 °C showed uniform grain and small grain size, compared with those prepared with various methods under similar sintering conditions. These features may be related to high conductivity (σ_600 °C = 0.022 S cm⁻¹) and low activation energy for conduction (0.66 eV). AC impedance spectra were involved to resolve grain interior and grain boundary resistance. Grain boundary contribution to the total resistance showed the values below 1/2 at 200-450 °C, suggesting low grain boundary effect. The motion enthalpy for the grain interior conduction decreased while the association enthalpy increased with sintering temperature up to 1300 °C, which might be possibly originated from the increase in lattice parameters with the sintering temperature.     

Characteristics of Ag powders coated with Pb-based glass material prepared by spray pyrolysis under various gas environments

Ag powders coated with Pb-based glass material for Si solar cell application are directly prepared by spray pyrolysis in various gas environments. Pb-based glass is successfully formed in the composite powders irrespective of gas environment. The composite powders have bimodal size distributions of nanometer and submicron sizes. However, the number of nano-sized powders decreases when the reducing gas was used as the carrier gas. The silver-conducting films fired at 700 and 800 °C have dense structures without pores irrespective of the gas environment in the preparation of the composite powders. Glass materials are uniformly segregated between micron-sized silver grains. The conducting film formed from the composite powders prepared under 20% H₂/Ar atmosphere have sheet resistance of 7.8, 6.8, 5.1 and 5.9 mΩ/sq at firing temperatures of 500, 600, 700 and 800 °C, respectively.     

Nanosized barium ferrite powders prepared by spray pyrolysis from citric acid solution

Highly crystalline nanosized barium ferrite (BaFe₁₂O₁₉) powders were prepared by spray pyrolysis from a spray solution containing a high concentration of the metal components. The precursor powders obtained from the spray solution containing citric acid were amorphous with a porous and hollow structure. Purely crystalline and fine BaFe₁₂O₁₉ powders were obtained after post-treatment between 700 and 1000 °C and subsequent mechanical grinding in an agate mortar. The mean sizes of the powders post-treated at 700 and 1000 °C were 125 and 550 nm, respectively. The specific magnetization of the powders prepared from the spray solution containing citric acid was 57 emu/g.     

Green-emitting yttrium silicate phosphor particles prepared by large scale ultrasonic spray pyrolysis

Tb doped Y₂SiO₅ phosphor particles with spherical morphology, fine size, high crystallinity and good photoluminescence intensity were prepared by spray pyrolysis. The colloidal solution obtained by adding the fumed silica particles was introduced to control the characteristics of Y₂SiO₅: Tb phosphor particles. The particles prepared from the colloidal solution had a spherical and filled morphology even after post-treatment. The particles post-treated below 1,200 ‡C had X1 type crystal structure but the crystal structure changed from X1 to X2 after post-treatment above 1,300 ‡C. When crystal structure was changed from X1 to X2, the PL intensity greatly increased. The maximum PL intensity of particles, which were prepared from the solution with 120% excess of stoichiometric fumed silica, was about 4 times higher than that of the particles prepared from the stoichiometric solution. The particles prepared from the stoichiometric solution of yttrium nitrate and fumed silica had mixed phases of X1 and X2 type and had impurity as Y₂O₃. On the other hand, the particles prepared from the solution with 120% excess of stoichiometric fumed silica had high crystallinity of X2 type.     

Effect of precursor characteristics on zirconia and ceria particle morphology in spray pyrolysis

Ultrasonic spray pyrolysis of acetate-based precursors with precisely measured precursor drop size was employed to produce ZrO₂ and CeO₂ particles. A bimodal size distribution of the product particles indicates a significant influence of the gas-to-particle conversion mechanism in addition to the conventionally accepted one-particle-per-drop mechanism. Due to the differences in solubility of the precursors, ZrO₂ particles are spherical in shape and smooth on their surfaces while the CeO₂ particles are bowl-like in shape with uneven surfaces. Spherical and monodispersed particles with a peak diameter <100 nm can be obtained by reducing the precursor concentrations to 0.01 wt.% in both the different precursor system.     

Synthesis and characterization of electrodeposited samaria and samaria-doped ceria thin films

Samaria (Sm₂O₃) and samaria-doped ceria (SDC) films are electrochemically deposited on stainless steel in view of a potential use in solid oxide fuel cells. As it is possible to deposit separately pure ceria (CeO₂) and pure samaria (Sm₂O₃) in similar conditions, SDC films were successfully obtained in one electrochemical conditions set. Thin films have been fabricated at low-temperature (30 °C) by applying a cathodic potential of −0.8 V/SCE, for 2 h. Structural and morphological properties of electrodeposited films have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), techniques and Raman spectroscopy. Special attention has been focused on the Raman spectroscopy study to emphasize the effect of heat treatment and samarium doping. Despite cracks, single SDC phase was obtained crystallizing in a cubic symmetry.     

Synthesis of spherical shape borate-based bioactive glass powders prepared by ultrasonic spray pyrolysis

Spherical shape borate-based bioactive glass powders with fine size were directly prepared by high temperature spray pyrolysis. The powders prepared at temperatures between 1200 and 1400 °C had mixed phase with small amounts of fine crystal and an amorphous rich phase. Glass powders with amorphous phase were prepared at a temperature of 1500 °C. The mean size of the glass powders prepared by spray pyrolysis was 0.76 μm. The glass powders prepared at a temperature of 1200 °C had two distinct exothermic peaks (T_c1 and T_c2) at temperatures of 588 and 695 °C indicating crystallization. The glass transition temperature (T_g) of the powders prepared at a temperature of 1200 °C was 480 °C. Phase-separated crystalline phases with spherical shape were observed from the surface of the pellet sintered at a temperature of 550 °C. Crystallization of the pellet was completely occurred at temperatures of 750 and 800 °C. The pellets sintered at temperatures below 700 °C had single crystalline phase of CaNa₃B₅O₁₀. The pellet sintered at a temperature of 800 °C had two crystalline phases of CaNa₃B₅O₁₀ and CaB₂O₄.     

Characterization of silver and silver/nickel composite particles prepared by spray pyrolysis

Pure silver and silver/nickel composite particles were prepared by spray pyrolysis of aqueous solutions of AgNO₃, and mixed salts of AgNO₃ and Ni(NO₃)₂·6H₂O, respectively. In the case of pure silver, reduction to metallic silver and subsequent sintering to highly spherical and dense particles took place immediately and almost simultaneously once favorable conditions for the former were imposed, irrespective of the nature of the carrier gas. For the composite particles, the high rates of reduction and sintering of the silver were still maintained, while crystallization of the silver, and the reduction and sintering of the nickel were considerably retarded, compared to the spray pyrolysis of each pure salt. Once the counterpart salt was added, the size of the composite particles increased compared to that of each pure metallic particles, but it was little affected by the furnace set temperature, the residence time and the molar ratio of the two precursor salts. Within single particles, an increase in either the temperature or the residence time caused segregation—silver in the shells and nickel in the cores—and improved the particles' surface smoothness and sphericity accordingly.     

Synthesis and characterization of iron-impregnated porous carbon spheres prepared by ultrasonic spray pyrolysis

Porous carbon microspheres impregnated with iron-based nanoparticles are prepared in a single step, continuous process using ultrasonic spray pyrolysis (USP). Precursor solutions containing a carbon source, an inorganic salt, and an iron salt are ultrasonically aerosolized and pyrolyzed. Solutions containing nitrate or chloride salts are examined. During pyrolysis, sucrose is dehydrated to carbon, and the metal salt is converted to crystalline or non-crystalline iron species, depending on processing conditions. The product’s porosity is generated from: (1) aromatization of carbon around an in situ template, (2) in situ gasification of isolated carbon, or (3) in situ chemical activation of the carbon precursor. Porous carbon spheres (0.5–3 μm diameter) containing well-dispersed iron oxide nanoparticles (4–90 nm diameter), referred to here as Fe–C, are prepared. Iron loadings between 1 and 35 wt.% are achieved while maintaining well-dispersed Fe nanoparticles with as-produced surface areas up to 800 m²/g. Post-pyrolysis heat and hydrogen treatments increase the surface area of the materials while reducing iron species. USP Fe–C materials may have useful catalytic applications due to their potential for high-loading of well-dispersed metal nanoparticles. Despite negligible surface Fe content, chromium reduction tests indicate that internal Fe sites are catalytically active.     

Synthesis and sintering of Gd-doped CeO2 nanopowders prepared by ultrasonic spray pyrolysis

Ultrasonic spray pyrolysis (USP) of nitrate-based precursors was used to fabricate fine and polycrystalline Ce_0.8Gd_0.2O_1.9 (CGO) powders with controlled microstructure in a one-step process varying four spray-pyrolysis parameters: precursor solution concentration, atomizing frequency, carrier gas flow rate, and pyrolysis temperature. A design of experiment (DOE) was used to understand the independent and combined influences of these process parameters on the particle size, particle size distribution and crystallite size. In this study, smaller CGO particles with narrower particle size distribution and finer grain size showed a full densification starting at a low temperature. CGO densification was found to be strongly dependent on CGO powder microstructural properties.