Synthesis of Y-doped BaCeO3 nanopowders by a modified solid-state process and conductivity of dense fine-grained ceramics

Powders of BaY_xCe_1 ? xO_3 ? δ (x = 0, 0.1 and 0.15) with specific surface area of 6–8 m²g^? 1 (BET equivalent particle size of 130–160 nm) were prepared by a modified solid-state route using nanocrystalline BaCO₃ and CeO₂ raw materials. These powders showed excellent densification at relatively low temperatures. Dense (96–97% relative density) ceramics with submicron grain size (0–4–0.6 µm) were obtained after sintering at 1250–1280 °C. Ceramics sintered at 1450 °C revealed only a moderate grain growth (grain size ≤ 2 µm), uniform microstructure and very high density (≥ 98%). The total conductivity of the submicron ceramics at 600 °C was comparable with the reference values reported in the literature, meaning that the high number of grain boundaries was not a limiting factor. On lowering temperature, the contribution of the blocking grain boundaries becomes progressively more important and the conductivity decreases in comparison to coarse-grained ceramics. Microscopic conductivities of grain interior and grain boundary are the same irrespective of grain size meaning that the different macroscopic behaviour is only determined by a geometric factor (a trivial size effect).     

Size-dependent luminescence in Y2Si2O7 nanoparticles doped with Ce3+ ions

Powders and thin films of nanocrystalline yttrium disilicate (Y₂Si₂O₇) doped with Ce³⁺ have been prepared by a sol–gel method. Structure and morphology of the synthesised samples have been determined and spectroscopic properties compared. The triclinic α-Y₂Si₂O₇ form (space group P 1-) for the powders annealed between 1000°C and 1200°C has been found. A total conversion into a thortveitite-type monoclinic β-Y₂Si₂O₇ polymorph after annealing of powder samples at 1400°C (space group C2/m) has been observed. In the case of films even at 1300°C the basically pure triclinic α-Y₂Si₂O₇ phase was observed with luminescent spectroscopy. The influence of grain size, controlled by thermal treatment, on the structure and luminescence properties of the fabricated materials are presented and discussed.     

Influence of sintering conditions on the electrical properties of 10% ZrO2–10% Y2O3–CeO2 (mol%)

Yttria–zirconia doped ceria, 10% ZrO₂–10% Y₂O₃–CeO₂ (mol%) (CZY) and 0.5 mol% alumina-doped CZY (CZYA), prepared through oxide mixture process, were sintered by isothermal sintering (IS) and two-step sintering (TSS) having as variable the temperature and soaking time. The electrical conductivity of sintered samples was investigated in the 250 to 600 °C temperature range by impedance spectroscopy in air atmosphere. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Alumina, as additive, improves the grain boundary conductivity of samples sintered at temperatures lower than 1500 °C. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (> 95%) at relatively low sintering temperature and long soaking time.     

Effects of sintering temperature and atmosphere on magnetism in Mn-doped TiO2 bulk samples

Magnetic properties have been investigated on Mn doped TiO₂(Ti_0.98Mn_0.02O₂) bulk samples prepared by solid state reaction, which were sintered at different temperature ranging from 450 ^°C to 900 ^°C in air and argon atmosphere, respectively. The results show that the magnetic properties were strongly dependent on the sintering temperature and atmosphere. For samples sintered in air, the magnetization initially increase with the increase of sintering temperature up to 600 ^°C and thereafter it decrease. While the magnetization of samples sintered in argon atmosphere decreases monotonically with the increase of sintering temperature. Furthermore, for samples sintered at 600 ^°C in air, the magnetic susceptibility exhibits a dominant Curie-Weiss behaviour and no magnetic transition is observed over the temperature range from 10 to 300 K. In contrast, for samples sintered in argon atmosphere, besides the magnetic transition near 45 K perhaps caused by Mn₃O₄, another magnetic transition appears near room temperature.     

Low-temperature sintering effects on NASICON-structured LiSn<Subscript>2</Subscript>P<Subscript>3</Subscript>O<Subscript>12</Subscript> solid electrolytes prepared via citric acid-assisted sol-gel method

LiSn₂P₃O₁₂ with sodium (Na) super ionic conductor (NASICON)-type rhombohedral structure was successfully obtained at low sintering temperature, 600 °C via citric acid-assisted sol-gel method. However, when the sintering temperature increased to 650 °C, triclinic structure coexisted with the rhombohedral structure as confirmed by X-ray diffraction analysis. Conductivity–temperature dependence of all samples were studied using impedance spectroscopy in the temperature range 30 to 500 °C, and bulk, grain boundary and total conductivity increased as the temperature increased. The highest bulk conductivity found was 3.64?×?10^?5 S/cm at 500 °C for LiSn₂P₃O₁₂ sample sintered at 650 °C, and the lowest bulk activation energy at low temperature was 0.008 eV, showing that sintering temperature affect the conductivity value. The voltage stability window for LiSn₂P₃O₁₂ sample sintered at 600 °C at ambient temperature was up to 4.4 V. These results indicated the suitability of the LiSn₂P₃O₁₂ to be exploiting further for potential applications as solid electrolytes in electrochemical devices.     

Study of mechanochemically prepared nanocrystalline La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript>

The nanocrystalline La_0.8Sr_0.2MnO₃ (LSM) is prepared by varying the revolutions per minute and milling time of planetary monomill during the mechanochemical method. The LSM forms in a relatively shorter milling time with an increase in the milling speed from 250 to 600 rpm. The structural phase transition from orthorhombic to rhombohedral phase in the LSM prepared by ball milling at the speed 250 rpm for 36 h is seen due to sintering it at 700 °C for 4 h. The crystallite size reduces with the increase in both the milling speed and the milling time individually or combined. The microhardness (HV) and sintered density increase with the reduction in the crystallite size. The temperature-activated transition temperature is suppressed by reducing the grain size in the nanometer range. The electrical dc conductivity increases with the reduction in the grain/crystallite size.     

Nanocrystalline CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> powders prepared by egg white solution route: synthesis,characterization and its giant dielectric properties

Nanocrystalline CaCu₃Ti₄O₁₂ powders with particle sizes of 50–90 nm were synthesized by a simple method using Ca(NO₃)₂·4H₂O, Cu(NO₃)₂·4H₂O, titanium(diisoproproxide) bis(2,4-pentanedionate) and freshly extracted egg white (ovalbumin) in aqueous medium. The synthesized precursor was characterized by TG-DTA to determine the thermal decomposition and crystallization temperature which was found to be at above 400 °C. The precursor was calcined at 700 and 800 °C in air for 8 h to obtain nanocrystalline powders of CaCu₃Ti₄O₁₂. The calcined CaCu₃Ti₄O₁₂ powders were characterized by XRD, FTIR, SEM and TEM. Sintering of the powders was conducted in air at 1100 °C for 16 h. The XRD results indicated that all sintered samples have a typical perovskite CaCu₃Ti₄O₁₂ structure and a small amount of CuO, although the sintered sample of the 700 °C calcined powders contained some amount of CaTiO₃. SEM micrographs showed the average grain sizes of 12.0±7.8 and 15.5±8.9 μm for the sintered CaCu₃Ti₄O₁₂ ceramics prepared using the CaCu₃Ti₄O₁₂ powders calcined at 700 and 800 °C, respectively. The sintered samples exhibit a giant dielectric constant, ε^′ of ∼ 1.5–5×10⁴. The dielectric behavior of both samples exhibits Debye-like relaxation, and can be explained based on a Maxwell–Wagner model. PACS 77.22.Gm; 81.05.Je; 81.07.Wx; 81.20.Ev     

Ionic conductivity of Ce1−xNdxO2−x / 2

The electrical conductivity of sintered samples of Ce_1−xNd_xO_2−x / 2 (0.01 ≤ x ≤ 0.2) was investigated in air as a function of temperature between 150 and 600 °C using AC impedance spectroscopy. The individual contribution of the bulk and grain boundary conductivities has been discussed in detail. In the low temperature range (< 350 °C), the activation enthalpy for bulk conductivity exhibited a shallow minimum at 3 mol% Nd, with a value of 0.68 eV. The activation enthalpy also produced a shallow minimum at 5 mol% Nd in the high temperature range (> 350 °C), with a value of 0.56 eV. It was shown that Ce_1−xNd_xO_2−x / 2 is an electrolyte that obeys the Meyer Neldel rule. The bulk conductivity data measured by others for the same system has also been recalculated and re-evaluated to facilitate easier comparison with our own data.     

Pulsed laser deposition of high temperature protonic films

Pulsed laser deposition has been used to fabricate nanostructured BaCe_0.85Y_0.15O_3−δ films. Protonic conduction of the fabricated BaCe_0.85Y_0.15O_3−δ films was compared to the sintered BaCe_0.85Y_0.15O_3−δ. Sintered samples and laser targets were prepared by sintering BaCe_0.85Y_0.15O_3−δ powders derived by solid state synthesis. Films 1 to 8 μm thick were deposited by KrF excimer laser on porous Al₂O₃ substrates. Thin films were fabricated at deposition temperatures of 700 to 950 °C at O₂ pressures up to 200 mTorr using laser pulse energy densities of 1.4–3 J/cm². Fabricated films were characterized by X-ray diffraction, electron microscopy and electrical impedance spectroscopy. Single phase BaCe_0.85Y_0.15O_3−δ films with a columnar growth morphology are observed with preferred crystal growth along the [100] or [001] direction. Results indicate [100] growth dependence upon laser pulse energy. Electrical conductivity of bulk samples produced by solid state sintering and thin film samples were measured over a temperature range of 100 to 900 °C. Electrical conduction behavior was dependent upon film deposition temperature. Maximum conductivity occurs at deposition temperature of 900 °C; the electrical conductivity exceeds the sintered specimen. All other deposited films exhibit a lower electrical conductivity than the sintered specimen. Activation energy for electrical conduction showed dependence upon deposition temperature, it varied from 115 to 54 kJ. Film microstructure was attributed to the difference in electrical conductivity of the BaCe_0.85Y_0.15O_3−δ films.     

Study of the formation of Co3O4 thin films using sol–gel method

Sol–gel derived coatings containing cobalt have been analyzed using impedance and reflection measurements. It is found that during the thermal treatments in air at temperatures in the range of 35–400 °C, cobalt migrates to the front surface of the coating where it is oxidized by the atmospheric oxygen and forms a top layer rich in nanocrystalline Co₃O₄. In samples heated above 260 °C, the current flows entirely through this top layer because it has higher conductivity than the rest of the coating. For these samples, the impedance spectra show two semicircles, related with the electrical properties of grain and grain boundaries of the cobalt oxide layer. Using the brick model, the grain boundary volume fraction was calculated as a function of the heating temperature.     

Preparation of Y<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore using high-energy ball milling and their structural,thermal and conducting properties

Pyrochlore-structured materials are very important materials due to their structural and conducting properties. These properties can be further modified by changing processing conditions. In the present study, pyrochlore (Y₂Ti₂O₇) is synthesized using high-energy ball milling. During various stages of ball milling, the ball-milled powder is taken for investigating the structural and thermal properties. The replacement of Ti₂O₃ by TiO₂ in nominal composition leads to lower ball milling duration to form Y₂Ti₂O₇. Differential thermal analysis showed the single exothermic peak below 800 °C, which indicates formation of disordered pyrochlore phase. The as prepared powders (40-h ball milled) were compacted and heat treated at 1,450 °C for 12 h. The conductivity of sintered sample is found to be one order higher than earlier reported pure Y₂Ti₂O₇ pyrochlore.     

Nanocrystalline CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> powder by PVA sol–gel route: synthesis,characterization and its giant dielectric constant

Nanocrystalline CaCu₃Ti₄O₁₂ powders were synthesized by a simple PVA sol–gel route and calcined at 700 and 800°C in air for 8 h. The diameter of the powders ranges from 40–100 nm. The calcined CaCu₃Ti₄O₁₂ powders were characterized by TG-DTA, XRD, FTIR, SEM, and TEM. Sintering of the powders was conducted in air at 1100°C for 16 h. The XRD results indicated that all sintered samples had a typical perovskite CaCu₃Ti₄O₁₂ structure although the sintered samples contained some amount of CaTiO₃. SEM of the sintered CaCu₃Ti₄O₁₂ ceramics showed the average grain sizes of 13–15 μm. The samples exhibit a giant dielectric constant, ε′∼10⁵ at 150 to 200°C with weak temperature dependence below 1 kHz in the sample sintered using the powders calcined at 700°C. The Maxwell–Wagner polarization mechanism is used to explain the high permittivity in these ceramics. It is also found that all sintered samples have the same activation energy of grains, which is ∼0.122 eV.     

Crystal microstructure of annealed nanocrystalline Chromium studied by synchrotron radiation diffraction

The microstructure of electrodeposited nanocrystalline chromium (n-Cr) was studied by using synchrotron radiation (SR) diffraction, SEM, TEM, and EDX techniques. The as-prepared n-Cr samples show the standard bcc crystal structure of Cr with volume-averaged column lengths varying from 25 to 30 nm. The grain growth kinetics and the oxidation kinetics were studied by time resolved SR diffraction measurements with n-Cr samples annealed at 400, 600, and 800 °C. The grain growth process is relatively fast and it occurs within the first 10 min of annealing. The final crystallite size depends only on the annealing temperature and not on the initial grain size or on the oxygen content. The final volume-averaged column lengths observed after 50 min annealing are 40(4), 80(1), and 120(2) nm for temperatures 400, 600, and 800 °C, respectively. It is shown that annealing ex situ of n-Cr at 800 °C both under vacuum and in air gives a grain growth process with the same final crystallite sizes. The formation of the Cr₂O₃ and CrH phases is observed during annealing.     

Sintering kinetics and oxide ion conduction in Sr-doped apatite-type lanthanum silicates,La9Sr1Si6O26.5

These last past years, a major interest has been devoted to decrease the working temperature of solid oxide fuel cells (SOFCs) down to about 700 °C.Apatite materials (La_10 ? xSr_xSi₆O_27?x/2) are attractive candidates for solid electrolytes, with a high ionic conductivity at these intermediate temperatures. An apatite powder (x = 1) with a 0.75 µm mean particle size, produced by solid state reaction, was tape cast to obtain green sheets with a thickness of about 260 µm.On one hand, the densification mechanism of the apatite ceramic during the intermediate solid state sintering has been approached. It appeared from the kinetical tests performed under isothermal conditions between 1250 and 1550 °C, that densification could be controlled by the diffusion at grain boundaries of the rare-earth element, La, with an activation energy of 470 kJ/mol.On the other hand, conductivity measurements were performed on apatite samples sintered at 1400 and 1500 °C. The ionic conductivity was mainly sensitive to the presence of secondary phases at 1400 °C. The ionic conductivity of the apatite sintered at 1500 °C (mean grain size = 3.9 µm) is equal to 1.2 × 10^? 2 S/cm at 700 °C.     

Synthesis of SmAlO<Subscript>3</Subscript> nanocrystalline powders by polymeric precursor method

SmAlO₃ nanocrystalline powders were successfully synthesized by the polymeric precursor method using ethylenediaminetetraacetic acid as a chelating agent. The precursor and the derived powders were characterized by thermogravimetry analysis (TG) and differential scanning calorimetry analysis (DSC), infrared spectroscopy (IR), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The results showed that pure SmAlO₃ powder with orthorhombic perovskite structure could be synthesized at 800°C for 2 h without formation of any intermediate phase. The average particle size of the powder synthesized at 900°C was as low as 28 nm. Subsequently, the bulk SmAlO₃ ceramics were prepared at various sintering temperatures using the synthesized powders calcined at 900°C for 2 h as starting materials. The sintering experiments indicated that the sample sintered at 1550°C for 2 h exhibited the highest relative density of 97.2% and possessed the best microwave dielectric properties of ε _r=20.94, Q×f=78600 GHz and τ _f=−71.8 ppm/°C.     

Influence of Gd3+ and Dy3+ co-doping and sintering regime on enhancement of electrical conductivity of ceria-based solid electrolyte

In order to improve the conductivity of ceria-based solid electrolytes, effect of co-doped Gd³⁺ and Dy³⁺ was evaluated. For this purpose, nano-crystalline Gd_0.2???x Dy _x Ce_0.8O_1.9 powders with various composition ranges (x?=?0.05, 0.1, 0.15, 0.2) were initially synthesized by high-energy milling method. The effect of micro-structural evolution and co-doping on electrical properties of the dense sintered samples fabricated by two-step sintering and conventional sintering of the synthesized powders were investigated. Electrical conductivity of the samples was discussed based on the results obtained by AC impedance spectroscopy at temperatures in the range of 300–700 °C. The co-doping and sintering regime were found to significantly influence the conductivity of the electrolytes. The electrical conductivity of the co-doped samples depends on Dy³⁺ content and the maximum conductivity obtained by 0.15 mol% Dy and 0.05 mol% Gd. The conductivity of Gd_0.2???x Dy _x Ce_0.8O_1.9 (x?=?0.15) was 0.03 S/cm at 700 °C. A thorough discussion was made, based on the present experimental data.     

Electrical behaviour of LSGM–LSM composite cathode materials

Composite cathode materials were prepared by mixing La_0.83Sr_0.17Ga_0.83Mg_0.17O_2.83 (LSGM) and La_0.8Sr_0.2MnO₃ (LSM) powders fired at 1300 °C. Several compositions were set up containing 1, 5, 25, 50, 75 wt.% of LSM. Their microstructure and electrical behaviour were investigated by XRPD, SEM/EDS and EIS. In composites containing 50 and 75 wt.% of LSM, the electronic contribution to conductivity is predominant, then there is only a single point at the low frequency end of the Nyquist plot. On the contrary, in the composites with up to 25 wt.% of LSM, there is a significant amount of ionic transport, then the IS spectra show complex features: at least three different arcs can be devised and their interpretation depends upon temperature. LSGM bulk and grain boundary conductivity, as well as interface polarization between the ionic (LSGM) and electronic (LSM) phases can be separated at temperatures below 600 °C; total LSGM contribution, i.e. bulk plus grain boundary, LSGM–LSM interface and electrode polarizations are attributed above 600 °C.     

氧离子导体La1.9Y0.1Mo2O9细晶粒陶瓷的制备和电学性质研究

采用溶胶凝胶法合成的La_1.9Y_0.1Mo₂O₉纳米晶粉体, 结合微波烧结技术制备出不同晶粒度的La_1.9Y_0.1Mo₂O₉块体样品. 利用X射线衍射仪(XRD)、高分辨透射显微镜(HRTEM)、场扫描显微镜(SEM)对粉体及陶瓷块体的物相、 形貌进行了表征, 利用交流阻抗谱仪测试了样品不同温度下的电导率. 实验结果表明, 掺Y的La_1.9Y_0.1Mo₂O₉能将高温立方β 相稳定到室温; 块体样品致密均匀, 平均晶粒度范围在60 nm–4 μm之间; 致密度高的样品表现出高的电导率, 其中900 ℃烧结样品的电导率600 ℃时高达0.026 S/cm, 比固相反应法制备的La_1.9Y_0.1Mo₂O₉样品高出约1倍. 总结认为样品的致密性对电导率影响较大, 是通过影响晶界电导率来影响总电导率的, 样品的晶粒度(在60 nm–4 μm范围内)对电导率的影响还不能确定. 关键词： 氧离子导体 1.9Y_0.1Mo₂O₉')" href="#">La_1.9Y_0.1Mo₂O₉ 细晶粒陶瓷 微波烧结     

Preparation and characterization of Yb-doped Ba(Ce,Zr)O<Subscript>3</Subscript> nanopowders with high sinterability

The Ba(Ce_0.8Zr_0.2)_0.95Yb_0.05O_2.975 ceramics electrolyte was prepared via a Pechini method using metal nitrate salts as starting materials. An optimum annealing temperature of 1,400 °C was needed to obtain a pure perovskite-like phase with orthorhombic structure. Particle size distribution showed a bimodal distribution that corresponds to the loose powders and agglomerates size. Scanning electron micrograph revealed that the loose powders were in the nanosize range (70–200 nm). These ultrafine loose powders enhanced the densification of a pellet with relative density ∼95% obtained at 1,400 °C. The sample formed clear and compact grains with submicron sizes. Impedance results showed that the impedance semicircle of the grain was observed only at T ≤ 250 °C. The introduction of 20 mol% Zr improved the chemical stability of BaCe_0.95Yb_0.05O_2.975 sample in atmosphere containing carbon dioxide at 600 °C. The sample also exhibited high proton conductivity in wet hydrogen.     

Synthesis and characterization of nanocrystalline MgAl2O4 spinel by polymerized complex method

Ultrafine MgAl₂O₄ powder has been synthesized by a polymerized complex method. Heating of a precursor solution containing citric acid (CA), ethylene glycol (EG) and Mg and Al salts with a molar ratio of Mg/Al/CA/EG=1/2/8/32 at 180°C produced a transparent polymeric gel, which have been characterized by FT-IR spectroscopy and TG/DTA. The organic fraction was removed by controlled thermal treatments (350–1200°C) whereby the bimetallic oxide was formed. XRD analysis showed the presence of MgAl₂O₄ at 600°C. TEM observation showed that the spherical nanosized powders with good uniformity was obtained. Furthermore, these powders showed excellent sinterability, relative density up to 99.8% was achieved when sintered at 1550°C for 3 h in air without any sintering additive.