Year 2017. Volume 37

Abstract

Protonic conductors of BaZr_0.5Ce_0.3Ln_0.2O_3-δ (BZCLn532, Ln=Y, Sm, Gd, Dy) were successfully synthesized by using a cost-effective solid state reactive sintering (SSRS) method with 1 wt.% NiO as a sintering aid. The pellets of the BZCLn532 were obtained at sintering temperatures between 1300 - 1600 ℃. The results show that the morphologies and the final relative densities of the obtained BZCLn532 pellets are influenced significantly when different sintering temperatures were applied. Dense pellets of the BZCLn532 can be obtained at sintering temperatures of 1600 ℃ for BaZr_0.5Ce_0.3Y_0.2O_3-δ and 1400 ℃ for BaZr_0.5Ce_0.3Sm_0.2O_3-δ, BaZr_0.5Ce_0.3Gd_0.2O_3-δ and BaZr_0.5Ce_0.3Dy_0.2O_3-δ. The ionic conductivity results show that the BaZr_0.5Ce_0.3Y_0.2O_3-δ (BZCY532) and BaZr_0.5Ce_0.3Dy_0.2O_3-δ (BZCD532) ceramics are demonstrated to be good candidates of oxygen ion conductor and proton conductor materials for intermediate temperature solid oxide fuel cells (ITSOFCs) applications.

Abstract

The phase transformation of zirconia from monoclinic to tetragonal polymorph at room temperature under mechanical processing has been a subject of a great interest due to technological importance of this material. Mechanism of this transformation has been widely investigated and plenty of explanation theories of zirconia stabilisation have been developed as well. This article critically reviews the systematic development regarding this transformation under mechanical processing and includes the summarised results of key-publications on this topic.

Abstract

The phase structure, microstructure and electrical properties of (1-x)Pb (Zr_y Ti_1-y)O₃-xSm(Fe³⁺_0.5, Nb⁵⁺_0.5)O₃ (PZT–SFN) (with x = 2 %, 41%≤ y ≤57 %) piezoelectric ceramics were prepared by the conventional solid state method, and effects of SFN and the Zr/Ti ratio content on the piezoelectric properties of PZT ceramics were mainly investigated. A stable solid solution has been formed between PZT and SFN, and a morphotropic phase boundary of PZT–SFN ceramics is identified in the range of 51% ≤ y ≤55 %. The Curie temperature of PZT–SFN ceramics decreases with increasing at Zr/Ti ratio content. A higher ε_r value and a lower tanδ value are demonstrated for the PZT–SFN ceramics with y = 53 %. The PZT–SFN ceramics with y = 53 % has an enhanced electrical behavior of k_p ~ 61.2 %, Q_m ~ 104, ε_r ~ 566, tanδ ~ 2.02 % and T_C ~ 370 OC. As a result, PZT–SFN ceramics are promising candidate materials for the field of lead piezoelectric materials and piezoelectric device.

Abstract

Pb_1-xCa_x [(Zr_0.52Ti_0.48)_0,98(Cr³⁺_0.5, Ta⁵⁺_0.5)_0,02]_0,96P_0,04O₃ (x = 0,00, 0.02, 0.04, 0.06) ceramics were prepared using the conventional mixed-oxide route. The resultant samples were sintered at different temperatures and subsequently characterized in terms of both microstructure and dielectric properties to study the effects of sintering behavior. X-ray diffraction analysis reveals that all specimens are a pure perovskite phase without pyrochlore phase and exhibits a phase transition from a rhombohedral phase to the coexistence of rhombohedral and tetragonal phases with an increase of sintering temperature. The grain size first increases up to x = 0.02 and then decreases. Comparing with the undoped ceramics, the dielectric properties of the Ca-doped PZT–PCTP specimens are significantly improved. The results show that the ceramics sintered at 1180 °C have optimum electrical properties for x= 0.02: a high dielectric constant (ε_r = 16800) at T_c, a low dissipation factor (tanδ = 0.009) and a low resistivity (ρ= 0.09 ×10⁺⁴) (Ω.cm) at 1 kHz, which indicates that the PZT–CCTP ceramics are promising for lead practical applications.

Abstract

An alternative strategy to the conventional seeded-assisted synthesis of Faujasite (FAU) nanozeolites and their assembling into thin films on ceramic substrates are reported. The method, which can be easily extended to other zeolite topologies, offers good opportunities for industrial applications.
The molar composition of the precursor gel used for the synthesis of FAU nanozeolites and membranes was conceived to achieve a high degree of supersaturation and trigger a uniform and abundant nucleation. Tubular alumina supports seeded with NaX particles of 2μm have been used to address the crystallization process. The synthesis was carried out in “soft condition” at near ambient temperature (30°C). Pure-phase FAU zeolites having uniform particle size dimensions in the range of 35-56 nm have been produced in high yield. Moreover, a uniform gel layer containing precursor entities has been concomitantly formed on the inner surface of the support after immersion in the synthesis gel. Thin and dense FAU membranes have been prepared through drygel conversion at higher temperatures. SEM images showed that the FAU layers have a thickness of ca. 2 μm and are constituted by closely packed, well intergrown nanocrystals, whose dimension was ca. 20-30 nm. The mass transport properties of the prepared membranes were probed by feeding dry single gases (N₂ and C_O2) at ambient temperature, obtaining low permeance (ca. 3.8·10^-9 molm^-2s^-1Pa^-1 for N₂) and ideal selectivity higher than the corresponding Knudsen value.

Abstract

The characteristics and sinterability of the Al₂O₃-ZrO₂(Y₂O₃) nanoparticles produced by simple and effective microwave and molten salts methods and processed by using spark plasma sintering were studied and compared. The crystalline powders with the specific surface area in the range of 72–108 m²/g and crystallite size of 5–13 nm were obtained by calcination of samples prepared by both methods at 800°C. The content of t-ZrO₂ phase depends on concentration of Al₂O₃, Y₂O₃ and on calcination temperature but the impact of the preparation method is insignificant. The phase transition of tetragonal ZrO₂ to monoclinic for the samples without Y₂O₃ started at 1000°C though it was incomplete in the case of high content of Al₂O₃. The bulk materials with relative density of 86.1–98.7% were fabricated by the spark plasma sintering method at 1500–1600°C depending on the content of Al₂O₃ and Y₂O₃.

Abstract

The technologically acceptable modified sol-gel technique was developed for obtaining the ultradispersed powdery composition Y₂O₃-Bi₂O₃-ZnO, which is promising new generation material for photocatalysts and solar cells. The solution for preparing a sintering composition with a high chemical homogeneity was proposed to provide using ultradispersed powders consisting of ZnO nanoparticles (40-70 nm), decorating with Bi₂O₃ and Y₂O₃ particles with average size of 4-20 nm. All synthesized powders were characterized by XRD and TEM methods. It was found that ZnO surface differently affects the dispersity of Bi₂O₃ and Y₂O₃ in the composites. TEM microphotographs show that the Bi₂O₃ particles uniformly distributed over ZnO surface. The Bi₂O₃ particle size on ZnO surface was 5-11 nm, which is significantly less than in the single powder of Bi₂O₃ (110-130 nm). However, in the case of Y₂O₃ similar effect was not observed, and average size of its crystallites was 5-12 nm, which agrees well with particle sizes in single powder.

Abstract

The modified sol-gel technique for the preparation of Со, Ni, Mo and W oxides nanoand submicron powders as well as binary composites based on them was developed. The optimal parameters of synthesis of metal oxide nanopowders with a particle size less than 100 nm were determined. The developed approach allows to obtain the nanoscale metal oxide particles with the given sizes, varying their within range 10-1000 nm. Mesoporous composites containing Mo-, Co-, Ni-oxides, Al2O3 and Ti-silicate were synthesized, characterized and tested in the catalytic alcohol synthesis from CO and H2. It was found that selectivity to C1-C4-alcohols formed over model catalytic systems reaches 50-85% in temperature range 280-360°С.

Abstract

A hybrid sol-gel method was employed to develop a uniform and highly dispersed alumina nanopowder in the presence of hyperbranched dendritic poly(ethylene)imine (PEI) acting as template material and complexation agent for aluminium ions. For this purpose, the hydrolysis and polycondensation reactions followed the complexation reaction between the Al(NO₃)₃ precursor and PEI, whereas ammonium polymethacrylate was added to improve the powder dispersion. The as-formed nanopowder was characterized before and after calcination studies carried out in the temperature range 100-1200°C. For this purpose Scanning Electron Microscopy (SEM), Field Emission SEM, Transmission Electron Microscopy (TEM), X Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric and Differential Thermal Analysis (TG-DTA), N₂ porosimetry and ζ-potential measurements at different pH were carried out. The analysis confirmed the successful formation of a boehmite-PEI hybrid material of uniform tiny spheroid crystals (~ 1-2 nm) and small agglomerates. The boehmite phase is kept up to 300°C, whereas after calcination at 600°C a stabilized γ-alumina powder of high surface area and crystal sizes around 2-5 nm results. This phase is quite stable being kept even after calcination at 1000°C. The transformation to the stable α-alumina phase is completed at 1100°C leading to an easily dispersed nanopowder with crystal sizes ranging between 5-25 nm.

Abstract

Different samples of pseudoboehmite (PB) were synthesized through the sol-gel process, using aluminum nitrate as precursor. The influence of variables on the synthesis and calcinations of the PB on the specific area of the obtained gamma-alumina were studied. The variables were the ageing temperature (25 and 130°C), addition or not of polyvinyl alcohol to the precursor solution and the ageing time of the PB. The pH adjustment of the precursor solution was made by using ammonium carbonate. The products, which were obtained on different conditions, were then characterized by x-ray diffraction, specific area measurements through the BET process, and afterwards by thermal analysis (DTA and TG). After characterization, the synthesis products were calcined at 500ºC; during this process the gamma-Alumina transformation was observed. The calcination products were characterized by the same methods (x-ray diffraction, DTA and TG) and the desorption-absorption curves were obtained as well, in order to measure the pore volume of the samples and the specific surface area through B.E.T. method. Finally, the results were analyzed through an experimental factorial planning, which showed that high specific surface area gamma-Al₂O₃ (around 330m²/g) can be obtained through this process.

Abstract

The calcium phosphate nanostructured bioceramics and nanocomposites with calcium phosphate matrix are subjects of research and have raised scientific, political, industrial and commercial interest because these biomaterials present differentiated properties from microporosity, bioactivity and cell adhesion on the surface of grain and micropores compared to conventional biomaterials. This work aimed at the wet synthesis of a nanostructured hydroxyapatite bone matrix for subsequent preparation of nanocomposite hydroxyapatite/α-Al₂O₃ sol-gel powders, at concentrations 1, 2, 3 and 5% in volume of nanocrystalline α-Al₂O₃ dispersed in inter-intragranular position in the hydroxyapatite matrix. The method used for the preparation of HA/α-Al₂O₃ nanocomposite powders was high-energy attrition milling. This method allows obtaining nanocomposite powders formed by fine particles with sizes smaller than 100nm. The technique of scanning electron microscopy served for observation of morphology, microstructure and nanostructure. The X-ray diffractometry, laser particle analysis method and differential thermal analysis technique were used to identify the phases, particle sizes and thermal behavior for nanostructured powders retrieved from the attrition milling.

Abstract

As known fabrication of fully transparent YAG ceramics using solid-reaction method needs highly sinterable Y₂O₃ and Al₂O₃ powders. In the work reported, as a source of alumina Taimicron TM-DR nanopowder was used, but Y₂O₃ nanopowder with controlled morphology was prepared by AHC precipitation starting from nitrates. The effect of ammonium sulfate was intensively studied throughout the precipitation process. As found, sinterability of final Y₂O₃ powders were strongly affected by the addition of (NH₄)₂SO₄. Transmittance of sintered YAG ceramics based on commercial alumina and yttria powders prepared showed that an optimum of ammonium sulfate addition exists. Mechanism of Y₂O₃ nanopowders properties regulation by presence of ammonium sulfate in precipitation process was studied and discussed.