The influence of ageing on consolidation and sinterability of a sub-micron alumina powder

A commercial sub-micron alumina powder was used for investigation of the influence of exposure to atmospheric humidity on powder characteristics, consolidation behaviour, densification and final microstructure of alumina ceramics. A significant uptake of atmospheric water by inadequate storage confirmed by thermal analysis, XPS, and FTIR, resulted in decreased sinterability of the powder, although no significant influence on the mean size of alumina grains was observed. In addition, the effect of low temperature heat treatment (drying at 120 °C and calcination at 700 °C) was also studied. The sinterability of pre-dried powder increased if a wet consolidation method (pressure filtration) was used, but a negative effect of pre-drying was observed in case of dry forming (axial pressing). The calcination decreased the ability of the powder to adsorb water. The presence of aggregates formed by calcination markedly decreased the green and sintered densities in compacts consolidated by axial pressing.     

Microstructure and ionic conductivity properties of gadolinia doped ceria (GdxCe1−xO2−x/2) electrolytes for intermediate temperature SOFCs prepared by the polyol method

Gd_0.1Ce_0.9O_1.95 and Gd_0.2Ce_0.8O_1.9 powders were prepared through the polyol process without using any protective agent. Microstructural and physical properties of the samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG) and impedance analysis methods. The results of the thermogravimetry/differential thermal analysis (TG/DTA) and XRD indicated that a single-phase fluorite structure formed at the relatively low calcination temperature of 500 °C. The XRD patterns of the samples revealed that the crystallite size of the samples increased as calcination temperatures increased. The sintering behavior and ionic conductivity of pellets prepared from gadolinia doped ceria (GDC) powders, which were calcined at 500 °C, were also investigated. The relative densities of the pellets, which were sintered at temperatures above 1300 °C, were higher than 95%. The results of the impedance spectroscopy revealed that the GDC-20 sample that was sintered at 1400 °C exhibited an ionic conductivity of 3.25×10⁻² S cm⁻¹ at 800 °C in air. This result clearly indicates that GDC powder with adequate ionic conductivity can be prepared through the polyol process at low temperatures.     

LiNi1/3Mn1/3Co1/3O2 synthesized by the Pechini method for the positive electrode in Li-ion batteries: Material characteristics and electrochemical behaviour

The feasibility of the Pechini method for the preparation of LiNi_1/3Mn_1/3Co_1/3O₂ with characteristics appropriate for Li-ion battery positive electrodes was investigated. The method involves formation of one single chemically homogenous precursor material and thus permits reduced calcination times and minimal lithium evaporation. The physical and electrochemical properties of the materials were investigated with variation in final calcination temperature. Chemical analysis showed that the materials could be prepared with high crystallinity and yet little or no loss of lithium. The material calcined at 1000 °C showed the highest specific capacity—180 mAh g⁻¹ when cycled between 4.5 and 3 V, and it maintained its capacity over 50 cycles. The advantage of this material over those prepared at 800 and 900 °C can probably be attributed to the fact that the degree of crystallinity, crystallite size and size of the primary particles increase with calcination temperature, and that the powder attains a more suitable morphology which promotes electronic connectivity to all of the oxide material. A temperature above 1000 °C should however not be used as indicated by an abrupt change in lattice parameters and decrease in electronic conductivity when going from 1000 to 1050 °C. The Pechini method presents an attractive option for the preparation of LiNi_1/3Mn_1/3Co_1/3O₂ positive electrode material.     

Preparation and characterization of the lead-free piezoelectric ceramic of Bi0.5Na0.5TiO3 doped with CuO

This study investigates the effects of copper oxide (CuO) addition, calcining temperature, and sintering temperature on the microstructure and the electrical properties (such as dielectric constant and loss tangent) of lead-free piezoelectric ceramic of bismuth sodium titanate (Bi_0.5Na_0.5TiO₃), BNT, which was prepared using the mixed oxide method. Three kinds of starting powders (Bi₂O₃, Na₂CO₃ and TiO₂) were mixed and calcined. This calcined BNT powder and a certain weight percentage of CuO were mixed, calcined, and compressed into a green compact of BNT-CuO. This green compact of BNT-CuO was sintered to be a disk doped with CuO, and its characteristics were measured. In this study, the calcining temperature ranged from 700 to 1000 °C, the sintering temperature ranged from 950 to 1050 °C, and the weight percentages of CuO doping included 2, 4, 6, and 8 wt.%. The largest relative density of the BNT-CuO disk obtained in this study was 96.7% at the calcining temperature of 700 °C, the sintering temperature of 950 °C, and 4 wt.% of CuO addition. The corresponding dielectric constant and loss tangent were 494 and 0.181%, respectively. This study shows that adding CuO to the BNT not only improves the relative density and the dielectric constant of the BNT disk, but it also lowers the sintering temperature.     

Effect of 1 wt% LiF additive on the densification of nanocrystalline Y2O3 ceramics by spark plasma sintering

Densification of nanocrystalline cubic yttria (nc-Y₂O₃) powder, with 18 nm crystal size and 1 wt% LiF as a sintering additive was investigated. Specimens were fabricated by spark plasma sintering at 100 MPa, within the temperature range of 700-1500 °C. Sintering at 700 °C for 5 and 20 min resulted in 95% and 99.7% dense specimens, with an average grain size of 84 and 130 nm, respectively. nc-Y₂O₃ without additive was only 65% dense at 700 °C for 5 min. The presence of LiF at low sintering temperatures facilitated rapid densification by particle sliding and jamming release. Sintering at high temperatures resulted in segregation of LiF to the grain boundaries and its entrapment as globular phase within the fast growing Y₂O₃ grains. The sintering enhancement advantage of LiF was lost at high SPS temperatures.     

Pressure infiltration of boron nitride preforms with molten aluminum

The infiltration of compacted cubic BN (cBN) with molten aluminum has been investigated as a potential route for a cheap and easy method of manufacturing cBN/metal composites. CBN compacts have been infiltrated with molten Al at a temperature between 670 and 800 °C and pressure of 15 MPa in vacuum. At these temperatures no pronounced interactions between hexagonal and cubic BN with Al was observed, allowing the complete infiltration of cBN with 12 μm mean grain size. After infiltration at 800 °C, the temperature was increased without pressure to convert aluminum into borides and AlN. The hardness of the resulting materials depends on the content of hexagonal, cubic BN and the rate of conversion of Al into borides and AlN. The infiltration height of less than 1 mm obtained from infiltrating the 3 μm cBN powder green compacts gave a hardness of 22.0 ± 0.6 GPa after heat treatment.     

Mechanical behavior of La0.8Sr0.2Ga0.8Mg0.2O3 perovskites

This paper examines the important mechanical properties of commercially purchased La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ at room temperature and 800 °C. Sr and Mg-doped lanthanum gallates (LSGM) are strong candidates for use as solid electrolytes in lower temperature solid oxide fuel cells operating at or below 800 °C. The material was found to be phase pure with a Young's modulus value of ∼175 GPa. The four point bending strength of the LSGM samples remained almost constant from 121 ± 35 MPa at room temperature to 126 ± 20 MPa at 800 °C. The fracture toughness, as measured by the single edge V notch beam (SEVNB) method, was 1.22 ± 0.06 MPa√m at room temperature, 1.04 ± 0.09 MPa√m at 700 °C followed by a small increase 1.31 ± 0.16 MPa√m at 800 °C. We also report, for the first time, the static subcritical (or slow) crack-growth (SCG) behavior of natural cracks in LSGM performed in four point bending tests at room temperature. The exponent of a power-law representation in the SCG tests was found to be n = 15, a rather low value showing LSGM to be highly susceptible to room temperature SCG.     

Synthesis of CeAlO3/CeO2-Al2O3 for use as a solid oxide fuel cell functional anode material

A composite electrocatalyst was developed to be fitted for the purpose of satisfying the features required for use as a solid oxide fuel cell functional anode material. The main functionality searched for was the ability to make the direct oxidation of carbon containing fuels in an SOFC without being severely coked. The present paper deals with the synthesis and characterization of such material. Therefore, ceramic electrocatalysts composed of CeAlO₃, CeO₂ and Al₂O₃ were synthesized by the amorphous citrate method and calcined at temperatures ranging from 300 °C to 900 °C. The synthesis procedures were designed to produce nanometric sized powders for which the calcination conditions were selected in order to fulfill requirements such as ease to be sintered; formation of selected phases upon calcinations at different temperatures; particle size control; surface area and morphology well suited for the production of ceramic suspensions to be processed into an SOFC functional anode. The main results have shown that increasing the calcination temperature under an oxidizing atmosphere induces the CeAlO₃ phase with a tetragonal perovskite type structure to undergo a phase transformation to CeO₂ (and Al₂O₃) with cubic fluorite type structure. However, the structure is able to be reversed and reduced back to the CeAlO₃ phase if calcined under a hydrogen atmosphere. The increase in the calcination temperature increases the particle average size, reduces the surface area and increases the material density, considering the same phase and crystalline structure. It was shown that the synthesis and calcinations procedures hinder the crystallographic identification of the presence of alumina.     

Solid oxide derived from waste shells of Turbonilla striatula as a renewable catalyst for biodiesel production

Biodiesel production via transesterification of mustard oil with methanol using solid oxide catalyst derived from waste shell of Turbonilla striatula was investigated. The shells were calcined at different temperatures for 4 h and catalyst characterizations were carried out by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Fourier transform infrared spectrometer (FT-IR), thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) and Brunauer-Emmett-Teller (BET) surface area measurements . Formation of solid oxide i.e. CaO was confirmed at calcination temperature of 800 °C. The effect of the molar ratio of methanol to oil, the reaction temperature, catalyst calcination temperature and catalyst amount used for transesterification were studied to optimize the reaction conditions. Biodiesel yield of 93.3% was achieved when transesterification was carried out at 65 ± 5 °C by employing 3.0 wt.% catalyst and 9:1 methanol to oil molar ratio. BET surface area indicated that the shells calcined in the temperature range of 700 °C-900 °C exhibited enhanced surface area and higher pore volume than the shells calcined at 600 °C. Reusability of the catalysts prepared in different temperatures was also investigated.     

Processing of dense nanostructured HAP ceramics by sintering and hot pressing

A nanosized HAP powder was sintered and hot pressed, in order to obtain dense HAP ceramics. In a first series of experiments, the powder was isostatically pressed into uniform green compacts and sintered at temperatures ranging from 1000 °C to 1200 °C in air atmosphere for different times. In a second series, the isostatically pressed green compacts were hot pressed in argon atmosphere at 900 °C, 950 °C and 1000 °C. The SEM micrograph of the sample sintered at 1200 °C for 2 h showed a uniform 3 μm mean grain size dense microstructure. In the case of hot pressed HAP compacts, full dense, translucent nanostructures were obtained having mean grain size below 100 nm and improved mechanical properties. With the grain size decreasing from 3 μm to 50 nm, the fracture toughness of pure HAP ceramics increased from 0.28 MPa m^1/2 to 1.52 MPa m^1/2.     

Synthesis and characterization of Al2O3 nanopowders by a simple chitosan-polymer complex solution route

Al₂O₃ nanopowders were synthesized by a simple chitosan-polymer complex solution route. The precursors were calcined at 800–1200 °C for 2 h in air. The prepared samples were characterized by XRD, FTIR and TEM. The results showed that for the precursors prepared with pH 3–9 γ-Al₂O₃ and δ-Al₂O₃ are the two main phases formed after calcination at 800–1000 °C. Interestingly, when the precursor prepared with pH 2 was used, α-Al₂O₃ was formed after calcination at 1000 °C, and pure α-Al₂O₃ was obtained after calcination at 1200 °C. The crystallite sizes of the prepared powders were found to be in the range of 4–49 nm, as evaluated by the XRD line broadening method. TEM investigation revealed that the Al₂O₃ nanopowders consisted of rod-like shaped particles and nanospheres with particle sizes in the range of 10–300 nm. The corresponding selected-area electron diffraction (SAED) analysis confirmed the formation of γ- and α-Al₂O₃ phases in the samples.     

Synthesis of thermally stable, high surface area, nanocrystalline mesoporous tetragonal zirconium dioxide (ZrO2): Effects of different process parameters

Nanocrystalline mesoporous zirconium dioxide powder with high surface area and remarkable thermal stability was synthesized using ethylene diamine and zirconyl chloride octahydrate. Ethylene diamine used as precipitating agent also acted as a colloidal protecting agent. The material retained high surface area (193.1 m²/g) mesoporous structure even after calcination at 900 °C with the surface area of the as-prepared material exceeding 440 m²/g depending upon the preparation conditions. Effects of different process parameters such as digestion time, pH, precursor concentration and calcination temperature on structural properties of the material were studied. These preparation conditions significantly affected the structural stability, crystal size and the crystal phase of the final material. The material was characterized by nitrogen adsorption-desorption, X-ray diffraction, scanning electron microscopy, thermogravimetric and differential scanning calorimetry, energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The final material has very high tetragonal phase stability even after calcination at 1000 °C temperature.     

Analysis of structural and thermal properties of Li2TiO3 ceramic powders

Li₂TiO₃ ceramic powders have been developed by a solid state reaction method and those have been sintered at four different temperatures (600 °C, 700 °C, 800 °C and 900 °C) towards the optimization of sintering temperature that has been found to be at 800 °C based on the nature of the XRD profiles. The sample sintered at 800 °C has shown a good crystallinity situation from its XRD peaks and the sample is found to be in monoclinic structure which is in accordance with the reported data of JCPDS 33-0831. The SEM images for samples sintered at 600 °C, 700 °C, 800 and 900 °C, EDAX peaks, FTIR profile have been measured for the temperature optimized (800 °C) sample for understanding the structural details of Li₂TiO₃ ceramic powders. Besides these, dielectric constant, dielectric loss and a.c. conductivities have been measured for the temperature optimized sample. In order to strengthen the observations made in the XRD profiles at four different temperatures, Raman spectra of those four sintered ceramic powders have also been studied. In respect of the thermal properties, only for the as synthesized (precursor) sample, simultaneous measurement of TG-DTA profiles has been carried out for analysis.     

Synthesis and sintering of Y2O3-doped ZrO2 powders using two Pechini-type gel routes

Yttria-tetragonal zirconia polycrystal (ZrO₂ + 4.5 mol% Y₂O₃) nanocrystalline powder was synthesized by two Pechini-type gel routes, the in situ polymerized complex (IPC) method and the PEG/AF method. FTIR spectra confirmed coordination of metal ions with the polymer by different routes, depending on the method used. The crystallite size of the powder increased from 5 nm to 8 nm when the temperature was increased from 450 °C to 600 °C and calcination times increased from 2 h to 24 h. The morphology of the powders differed only when the organic impurities were not completely eliminated. After calcination, the morphology of the powders produced by the two methods showed porous agglomerates composed of smaller particles. All the resulting microstructures were very similar, regardless of the method employed to obtain the powder or the calcination times and temperatures.     

Transesterification of palm oil and esterification of palm fatty acid in near- and super-critical methanol with SO4-ZrO2 catalysts

Sulfated zirconia (SO₄-ZrO₂) catalysts, prepared with three different sulfur loading contents (0.75%, 1.8% and 2.5%) at two calcination temperatures (500 °C and 700 °C), were tested for use in the transesterification of purified palm oil (PPO) and the esterification of palm fatty acid (PFA) in near-critical and super-critical methanol. Techniques including BET, XRD, NH₃- and CO₂-TPD revealed that the sulfur content and calcination temperature strongly affects the catalyst base-acid site, specific surface area, average pore size, phase structure, and thus the catalytic reactivity. The most suitable sulfur loading content was found to be 1.8% and the optimum calcination temperature 500 °C. The results show that the use of SO₄-ZrO₂ reduces esterification reaction times, the amount of methanol necessary and the required reaction temperature. The reactions at 250 °C in the presence of the SO₄-ZrO₂ catalyst at 0.5 w/w% catalyst to PPO or PFA were found to give the highest FAMEs conversions. Under these conditions, 90% and 75% conversions were achieved within 10 and 1 min from PPO (at 25:1 MeOH:PPO molar ratio) and PFA (at 6:1 MeOH:PFA molar ratio), respectively.     

Ni0.5Zn0.5Fe2O4 nanoparticles and their magnetic properties and adsorption of bovine serum albumin

Ni_0.5Zn_0.5Fe₂O₄ nanoparticles were synthesized by the facile citrate-gel process and the preliminary measurement for adsorption of bovine serum albumin (BSA) protein on these nanoparticles was carried out. The gel precursor and resultant nanoparticles were characterized by TG-DSC, FTIR, XRD, TEM and VSM techniques and the BSA adsorption on the nanoparticles was analyzed by UV spectrophotometer at room temperature. The results show that the single phase of spinel Ni_0.5Zn_0.5Fe₂O₄ is formed at 400 °C. With increasing calcination temperature from 400 to 700 °C, the average grain size increases from about 14 to 45 nm and consequently, the specific saturation magnetization of Ni_0.5Zn_0.5Fe₂O₄ nanoparticles increases from about 46 to 68 Am²/kg. The coercivity initially increases and then decreases with increasing calcination temperature, with a maximum value 9.2 kA/m at 500 °C. The as-prepared Ni_0.5Zn_0.5Fe₂O₄ nanoparticles exhibit a good adsorbing ability for BSA and the optimized adsorption is achieved for the Ni_0.5Zn_0.5Fe₂O₄ nanoparticles calcined at 500 °C with grain size about 24 nm.     

Mechanochemical synthesis of MgTa2O6 ceramic

MgTa₂O₆ powders were prepared by mechanochemical synthesis from MgO and Ta₂O₅ in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa₂O₆ after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa₂O₆ from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.     

Effect of Na2CO3 on hexagonal boron nitride prepared from urea and boric acid

Formation of hexagonal boron nitride (hBN) from a precursor obtained by the reaction of urea and boric acid was studied in nitrogen, ammonia and argon atmospheres in 700-1200 °C temperature range. Effect of sodium carbonate (Na₂CO₃) addition on this process was investigated. Reaction products were subjected to X-ray diffraction, particle size distribution, gravimetric and Fourier transformed infrared spectroscopy analyses. Particle size and crystallite thickness of the formed hBN were seen to increase from about 60 nm and 5 nm at 700 °C to 230 nm and 19 nm at 1200 °C, respectively in NH₃ atmosphere with Na₂CO₃ addition. Highest conversion of boron in the precursor into hBN was achieved as 73.6% when Na₂CO₃ added precursor was reacted at 1200 °C in NH₃. hBN powder with high yield and relatively large particle size was obtained at low temperature such as 1200 °C with Na₂CO₃ addition. Role of Na₂CO₃ addition was suggested to be formation of a sodium borate melt from which hBN crystallized via the reaction of borate and nitrogen ions in the melt. Obtained hBN has the potential for utilization as a clean starting material for synthesis of B or N containing compounds.     

Synthesis of high surface area Al2O3-CeO2 composite nanopowder via inverse co-precipitation method

In the present work, Al₂O₃-CeO₂ composite nanopowder was synthesized by inverse co-precipitation method using metal chlorides, aluminum powder and NH₄OH as precipitant agent. The thermal decomposition of the precipitate and subsequent formation of Al₂O₃-CeO₂ were investigated by X-ray diffractometery, scanning electron microscopy, thermogravimetric and differential thermal analysis, Brunauer-Emmett-Teller surface area measurement and Fourier transform infrared spectroscopy. The results showed that the presence of ceria suppressed the formation of α-Al₂O₃. The BET-specific surface area was 173 m²/g for powders calcined at 800 °C. The particle size examined by using scanning electron microscopy was in the range 30-70 nm. The activation energy of Al₂O₃-15 wt.% CeO₂ nanocrystallite growth during calcination was measured to be 32.4 kJ/mol whereas that of Al₂O₃ was about 23.8 kJ/mol.     

Modified CaO-based sorbent looping cycle for CO2 mitigation

CaO-based sorbent looping cycle, i.e. cyclic calcination/carbonation, is one of the most interesting technologies for CO₂ capture during coal combustion and gasification processes. In order to improve the durability of limestone during the multiple calcination/carbonation cycles, modified limestone with acetic acid solution was proposed as an CO₂ sorbent. The cyclic carbonation conversions of modified limestone and original one were investigated in a twin fixed bed reactor system. The modified limestone shows the optimum carbonation conversion at the carbonation temperature of 650 °C and achieves a conversion of 0.5 after 20 cycles. The original limestone exhibits the maximum carbonation conversion of 0.15 after 20 cycles. Conversion of the modified limestone decreases slightly as the calcination temperature increases from 920 °C to 1100 °C with the number of cycles, while conversion of the original one displays a sharp decay at the same reaction conditions. The durability of the modified limestone is significantly better than the original one during the multiple cycles because mean grain size of CaO derived from the modified limestone is lower than that from the original one at the same reaction conditions. The calcined modified limestone shows higher surface area and pore volume than the calcined original one with the number of cycles, and pore size distribution of the modified limestone is superior to the original one after the same number of calcinations.