Effect of alkali roasting and sulfatization on the extraction of different metal oxides from waste alum sludge

Water treatment plants (WTP) generate a significant amount of sludge as byproducts with environmentally harmful elements. Thus, this work focused on the recycling of alum sludge through the extraction of different metal oxides, i.e., Al₂O₃, Fe₂O₃ and SiO₂, for use in different applications, such as ceramics, cement, and agriculture. The extraction of Al₂O₃, Fe₂O₃, and SiO₂ from alum sludge was performed using sulfatization and roasting to compare which of the two processes could produce the metal oxides of the highest purity. Precipitated powders were calcined at 700°, 900° and 1100 °C. Moreover, the obtained prepared and calcined powders were characterized by studying their phase compositions, microstructure, particle size, and surface area. Results indicated that roasting achieved the highest yield of alumina. Iron oxide was extracted mostly in maghemite form through roasting after calcination at 1100 °C. Further, silica was obtained in cristobalite and quartz phases after calcination at 1100 °C for the samples prepared through sulfatization. However, these phases of silica were combined with albite and obtained after calcination at 1100 °C for the samples prepared through roasting method.     

Characterization of Yb2SiO5-based environmental barrier coating prepared by plasma spray–physical vapor deposition

Due to the high-input power compared to atmospheric plasma spraying (APS), plasma spray-physical vapor deposition (PS-PVD) can primarily achieve a splat-like deposition, allowing for the preparation of high-density environmental barrier coatings (EBCs). In this paper, dense Yb₂SiO₅-based coatings are prepared by PS-PVD at different substrate temperatures. It was found that the coating deposited at the substrate temperature of 700 °C contained a large amount of silicon-rich amorphous phase. When the substrate temperature increased to 1100 °C and a slow cooling process after deposition was involved, a coating with high crystallinity of ～77% and low porosity of less than ～2% was achieved. Phase evolution of the coatings was studied by a semi-in-situ high-temperature X-ray diffractometer. During the heating process, metastable phases X1-Yb₂SiO₅ and α-Yb₂Si₂O₇ emerged and transformed into stable phases following high-temperature treatment. Furthermore, the effects of long-term thermal aging at 1300 °C on the microstructure, phase composition, thermal conductivity, and hardness of the coating prepared at the substrate temperature of 1100 °C were found to be limited.     

Direct synthesis of PMN samples by spray-drying

The processing and characterisation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) materials, obtained either by spray-drying the solution of the precursors or by the conventional “columbite” method, were investigated and the morphological and micro-structural characteristics were compared. The acid solution of ammonium-peroxo-niobium complex, magnesium and lead nitrates was spray-dried and the precursor powder obtained was calcined at different temperatures ranging from 350 to 900 °C. The morphologies and the XRD patterns of the powders were compared. The calcined powders exhibited a pyrochlore phase above 400 °C converting into an almost pure perovskite phase at 800 °C. The powder calcined at 350, 500 and 800 °C were sintered at different temperatures, ranging from 950 to 1150 °C, always resulting in a pure perovskite PMN material. The XRD patterns of as-fired surfaces of samples sintered at 950 and 1050 °C showed an unwanted PbO phase together with the main PMN, nevertheless this secondary phase is not present in the ground surfaces. The high reactivity of sprayed powder is reflected in the formation and densification of pure perovskite PMN material with a faster process as regards the conventional one; in particular samples of about 96% theoretical density were obtained starting from the amorphous powder calcined at low temperature (350 °C) through a reaction sintering process. Furthermore, due to the better flowability of the spray-dried powder, the cold consolidation process is highly improved and no binder addition to powder is necessary.     

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu₂Ti₂O₇) bodies were fabricated by spark plasma sintering using Lu₂O₃ and TiO₂ powders calcined from 700 °C to 1200 °C. No solid-state reaction was identified after calcination at 700 °C, whereas single-phase Lu₂Ti₂O₇ powder was prepared at 1100 and 1200 °C. The calcination at 700 °C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu₂Ti₂O₇ bodies with low transparency. Low-density and opaque Lu₂Ti₂O₇ bodies formed owing to the coarsening of the powder calcined at 1200 °C. The Lu₂Ti₂O₇ body sintered using the powder calcined at the moderate temperature of 1100 °C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550 nm and 2000 nm, respectively.     

Densification of zirconia doped yttria transparent ceramics using co-precipitated powders

Ho:Y₂O₃ ceramics were prepared using co-precipitated powders, with ammonium sulfate as dispersant. Y³⁺ was co-precipitated together with Ho³⁺ and Zr⁴⁺ to produce precursors, which were calcined at 1100–1400 °C to produce yttria-based powders. At calcination temperatures of ≤1300 °C, agglomeration of powders was not observed. When the temperature was increased to 1400 °C, severe agglomeration was detected. Densification was closely related to the calcination temperature: a lower calcination temperature resulted in a faster densification of ceramics to the relative density of 99.7%. The ultimate densification to ~100% was also closely related to powders' impurity level and agglomeration. Grain growth was mainly determined by sintering temperature, but not by the initial crystallite size of powders. The optimal calcination temperature was 1300 °C, at which the obtained Ho:Y₂O₃ powder was free from agglomeration. Using this powder, the resultant Ho:Y₂O₃ ceramics showed pore-free microstructure and good optical transparency.     

Synthesis of La0.9Sr0.1Al0.85Mg0.1Co0.05O2.875 using a polymeric method

Nanocrystalline La_0.9Sr_0.1Al_0.85Mg_0.1Co_0.05O_2.875 (LSAMC) powders were synthesized via a polymeric method using poly(vinyl alcohol) (PVA). The effect of PVA content on the synthesized powders was studied. When the ratio of positively charged valences (Mⁿ⁺) to hydroxyl groups (OH) is 1.5:1, crystalline LaAlO₃ could be obtained at such a low calcination temperature as 700 °C. While at 900 °C the ratio is of less importance, since pure LaAlO₃ perovskite could be formed for all powders after calcination at 900 °C. Thermal analysis (TG/DTA) was utilized to characterize the thermal decomposition behaviour of precursor powders. The chemical structure of the calcined powder was studied by Fourier transform infrared (FTIR) spectroscopy. The powder morphology and microstructure were examined by SEM. Dense pellets with well-developed submicron microstructures could be formed after sintering at 1450 °C for 5 h. Compared with the solid-state reaction method, the sintering temperature is substantially lower for powder prepared by the PVA method. This is due to the ultrafine and highly reactive powder produced.     

Magnesium Ferrite (MgFe2O4) Nanostructures Fabricated by Electrospinning

Magnesium ferrite (MgFe₂O₄) nanostructures were successfully fabricated by electrospinning method. X-ray diffraction, FT-IR, scanning electron microscopy, and transmission electron microscopy revealed that calcination of the as-spun MgFe₂O₄/poly(vinyl pyrrolidone) (PVP) composite nanofibers at 500–800 °C in air for 2 h resulted in well-developed spinel MgFe₂O₄ nanostuctures. The crystal structure and morphology of the nanofibers were influenced by the calcination temperature. Crystallite size of the nanoparticles contained in nanofibers increased from 15 ± 4 to 24 ± 3 nm when calcination temperature was increased from 500 to 800 °C. Room temperature magnetization results showed a ferromagnetic behavior of the calcined MgFe₂O₄/PVP composite nanofibers, having their specific saturation magnetization (M _s) values of 17.0, 20.7, 25.7, and 31.1 emu/g at 10 Oe for the samples calcined at 500, 600, 700, and 800 °C, respectively. It is found that the increase in the tendency of M _s is consistent with the enhancement of crystallinity, and the values of M _s for the MgFe₂O₄ samples were observed to increase with increasing crystallite size.     

Compositional inhomogeneity and segregation in (K0.5Na0.5)NbO3 ceramics

In this report, the effects of the calcination temperature of (K_0.5Na_0.5)NbO₃ (KNN) powder on the sintering and piezoelectric properties of KNN ceramics have been investigated. KNN powders are synthesized via the solid-state approach. Scanning electron microscopy and X-ray diffraction characterizations indicate that the incomplete reaction at 700 °C and 750 °C calcination results in the compositional inhomogeneity of the K-rich and Na-rich phases while the orthorhombic single phase is obtained after calcination at 900 °C. During the sintering, the presence of the liquid K-rich phase due to the lower melting point has a significant impact on the densification, the abnormal grain growth and the deteriorated piezoelectric properties. From the standpoint of piezoelectric properties, the optimal calcination temperature obtained for KNN ceramics calcined at this temperature is determined to be 800 °C, with piezoelectric constant d₃₃=128.3 pC/N, planar electromechanical coupling coefficient k_p=32.2%, mechanical quality factor Q_m=88, and dielectric loss tan δ=2.1%.     

Dielectric properties of nano-sized Ba0.7Sr0.3TiO3 powders prepared by spray pyrolysis

Nano-sized Ba_0.7Sr_0.3TiO₃ powders are prepared by post-treatment of the precursor powders with hollow and thin wall structure at temperatures between 900 and 1100 °C. Ethylenediaminetetraacetic acid and citric acid improve the hollowness of the precursor powders prepared by spray pyrolysis. The mean sizes of the powders post-treated at temperatures of 900, 1000 and 1100 °C are 42, 51 and 66 nm, respectively. The densities of the Ba_0.7Sr_0.3TiO₃ pellets obtained from the powders post-treated at 900, 1000 and 1100 °C are each 5.36, 5.55 and 5.38 g cm^?3 at a sintering temperature of 1300 °C. The pellet obtained from the powders post-treated at 1000 °C has higher maximum dielectric constant than those obtained from the powders post-treated at 900 and 1100 °C.     

Temperature-controlled synthesis of spinel lithium nickel manganese oxide cathode materials for lithium-ion batteries

In this work, we successfully synthesized series of LiNi_0.5Mn_1.5O₄ (LNMO) cathode materials with spinel structure by using a facile sol-gel method and then calcined at various temperature ranging from 600 to 1000 °C. The application of different calcination temperatures significantly influenced the surface morphology, stoichiometry and crystalline nature of the as-synthesized LNMO material. According to the results of physical characterizations, the LNMO materials calcined at various temperatures mainly revealed the stoichiometric disordered Fd-3m structure with a small amount of well-ordered P4₃32 phase. The structural analysis also exhibited that the control of the calcination temperature contributed to the higher crystalline nature. Moreover, the morphological investigations indicated that the increasing calcination temperatures caused the formation of large micron-sized LNMO material. In turn, the electrochemical evaluations revealed the impact of the calcination temperatures on enhancing the electrochemical performances of the LNMO electrode materials up to 900 °C. The LNMO electrode calcined at 900 °C exhibited an impressive initial discharge specific capacity of ca. 142 mAh g⁻¹ between 3.5 and 4.9 V vs. Li/Li⁺, and remarkably improved capacity retention of 97% over 50 cycles. Those excellent electrochemical properties were associated with the presence of the dominant Fd-3m phase over the P4₃32 phase. Additionally, the results of the corrosion and dissolution tests which were performed for all calcined LNMO materials in order to estimate the amount of manganese and nickel ions leached from them, proved that the micro-sized LNMO calcined at 900 °C was the most stable.     

Synthesis of Fe:ZnSe nanopowders via the co-precipitation method for processing transparent ceramics

Fe:ZnSe nanopowders were synthesized via the co-precipitation method for fabricating transparent ceramics. Fe_xZn_1−xSe (0.00 ≤ x ≤ 0.06) powders that were calcined at 400°C yielded a single-phased cubic ZnSe, but when the calcination temperature was raised to 500-600°C, ZnO phase was created. Introduction of pressure could avoid appearance of ZnO. XRD Scherrer analysis revealed a monotonic increase in lattice parameter with increasing Fe²⁺ content. The average powder particle size increased with calcination temperature from several nanometers at 80°C to hundreds of nanometers at 600°C. Attempts to pressurelessly sinter ZnSe powders resulted in the partial decomposition of ZnSe, thus spark plasma sintering was employed to sinter Fe_0.01Zn_0.99Se transparent ceramics with pure ZnSe phase composition, which could be well sintered at 950°C for 30 minutes under an applied pressure of 60 MPa. SEM observations of the polished and thermally etched microstructure of the ceramic revealed a dense microstructure with average grain size of approximately 35 μm, and a few micropores were observed at the grain boundaries. The transparent ceramic exhibited good transmittance in the mid-far infrared range, with the highest transmittance 57% at 12 μm. This paper confirmed the scheme of synthesis of Fe:ZnSe nanopowders by liquid-phase co-precipitation method for sintering transparent ceramics.     

The preparation and characterization of HA/β-TCP biphasic ceramics from fish bones

The aim of this research is to observe the physicochemical characterization and evaluate the biocompatibility of the HA/β-TCP biphasic calcium phosphate ceramics (BCP) produced from fish bones. In addition, the mechanism of the formation of BCP after calcination of fish bones was discussed. Three kinds of fish bones (Salmo salar, Anoplopoma fimbria and Sardine) were prepared and calcined for one hour at different temperatures ranging from 600 °C to 1100 °C in a muffle furnace. The calcined bones were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (GTA), inductively coupled plasma optical atom emissions spectroscopy (ICP-OES), X-ray fluorescence (XRF) and scanning electron microscopy (SEM). The in vitro cytotoxicity assessment was used to evaluate the biocompatibility of the biphasic ceramics. BCP materials were produced from all kinds of fish bones by calcination above 700 °C, the carbonated hydroxyapatite and multiple trace element were also found in the calcined bones. With the increase of temperature, the ratio of HA/β-TCP varied and the major organic components were progressively removed. The carbonated hydroxyapatite disappeared when temperature rises above 900 °C. Rising temperature also caused crystal growth that eventually gave rise to the increase of the BCP grain size and influenced the mesoporous structure. The BCP materials were confirmed to have no obvious cytotoxicity to mesenchymal stem cells (MSC) in the in vitro cytotoxicity assessment. Calcium-deficient hydroxyapatite(CDHA) may make up the major inorganic constituent of fish bones that could decompose to HA and β-TCP when calcined above 700 °C. 800–900 °C is considered to be the optimal temperature to fabricate BCP materials which contain more β-TCP, carbonated hydroxyapatite and retain distinct mesoporous structure while has good biocompatibility. With the unique composition and structure, these three kinds of fish-bone-derived BCP materials can be further applied to fabricate bioceramic scaffolds for biomedical applications.     

Synthesis and applications of composite cathode based on Sm0.5Sr0.5Co3−δ for solid oxide fuel cell

Cobaltite based perovskites, such as Sm_0.5Sr_0.5Co_3?δ (SSC), are attractive solid oxide fuel cell (SOFC) cathodes due to their high electrochemical activity and electrical conductivity. To obtain higher fuel cell performance with smaller particles, nano-sized SSC powders were synthesized by a complex method with/without carbon black, HB170. However, during synthesis, carbon black reacted with Sr, and unfortunately formed SrCO₃. To obtain pure perovskite SSC, a calcination temperature of 900 °C is needed. At 680 °C, an SOFC with SSC (calcined at 700 °C and synthesized without HB170) exhibited a higher fuel cell performance, of 0.68W·cm^?2, than that with SSCHB (calcined at 900 °C and synthesized with HB170), of 0.58W·cm^?2. Adding GDC for composite cathode is more effective in SSCHB porous cathodes than in SSC porous cathodes. At 680 °C, the composite cathode of SSCHB6-GDC4 exhibited the highest maximum power density of 0.72W·cm^?2 which results from the combined effects of lowered charge transfer polarization and mass transfer polarization. To obtain higher fuel cell performance, optimum composition and processes are necessary.     

Annealing effects for calcination of tin oxide powder prepared via homogeneous precipitation

Tin oxide powders were prepared from a homogeneous precipitation using the aqueous solution of SnCl₄ with urea as a precipitator at 90 °C and followed by a calcination process. The calcination was performed using two different methods; conventional furnace annealing (CFA) and rapid thermal annealing (RTA). The crystallization of the tin oxide finished at 600 °C regardless of the calcination method used. The crystallite size increased with as the calcination temperature increased due to the crystal growth and agglomeration. The tin oxide calcined using RTA has a relative smaller crystallite size than CFA at the same temperature. The tin oxide powders calcined with RTA showed higher specific surface areas than those that used CFA over a wide range of temperatures.     

Characteristics of samaria-doped ceria nanoparticles prepared by spray pyrolysis

Samaria-doped ceria (SDC) nanoparticles were prepared by spray pyrolysis. The means sizes of the samaria-doped ceria nanoparticles were controlled from 21 to 150 nm by changing the calcination temperatures between 700 and 1200 °C. The pellets formed from the SDC particles calcined at temperatures between 700 and 1000 °C had similar grain sizes between 0.75 and 0.82 μm. However, pellet formed from the SDC particles calcined at a temperature of 1200 °C had large grain size of 1.22 μm. The pellet formed from the SDC particles calcined at a temperature of 1000 °C had slightly smaller resistance of grain-boundary than those of the pellets formed from the SDC particles calcined at temperatures between 700 and 900 °C. However, the pellet formed from the SDC particles calcined at a temperature of 1200 °C had low resistance of grain-boundary. The pellet formed from the SDC particles calcined at a temperature of 1200 °C had conductivity of 44.65 × 10^?3 S cm^?1 at a measuring temperature of 700 °C that more twice than those of the pellets formed from the SDC calcined below 1000 °C.     

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.     

Improvement of high-temperature mechanical properties of heat treated Cf/geopolymer composites by Sol-SiO2 impregnation

Unidirectional carbon fiber reinforced geopolymer composite was prepared by ultrasonic-assisted slurry infiltration method and heat treated at 1100 °C. Then it was impregnated with Sol-SiO₂ to seal the cracks and pores formed during heat treatment. The ambient strength of composite after impregnation was enhanced by 35.6% due to the increase relative density from the starting 79% to 93.6%. Composites both before and after impregnation fractured in a non-brittle manner at both ambient and high temperatures. Over an elevated temperature range from 700 to 900 °C, the strength of the two composites showed anomalous gains and reached their maximum values at 900 °C, 322.1 and 425.1 MPa, respectively. These values were 19.8% and 16.8% higher than their ambient ones. When the temperature was further increased to 1100 °C, the impregnated composite showed superior high-temperature properties, which was attributed to the improved fiber integrity due to the Sol-SiO₂ sealing effect.     

In-vitro evaluation of wollastonite nanopowder produced by a facile process using cheap precursors for biomedical applications

Wollastonite nanopowder (β-CaSiO₃) is the most nanoceramic powder that is most frequently applied in biomedical applications due to its good bioactivity and biocompatibility. Although the preparation of wollastonite in a solid-state is distinguished as a simple and cheap method with large-scale production, it requires high temperatures (=1400 °C) and consumes quite a long time. The wet methods are considered the best when it comes to preparing the wollastonite nanopowders. However, it has some drawbacks such as its extravagant raw materials and its shorting in preparation which inhibits successful coverage for large-scale production. Herein facile, one-pot modified co-precipitation approach with an easy procedure, shorter reaction time, and in-expensive precursor sodium meta-silicate-pentahydrate and CaCO₃ has been utilized for large-scale production of wollastonite nano-powders (76–150 nm). The precipitated product was calcined at different temperatures (800, 900, 1000, and 1100 °C). The phase composition and microstructure of the calcined powders were investigated. They were analyzed by XRD, FTIR, FESEM, and HRTEM. The in-vitro bioactivities of the calcined powders at 1000 &1100 °C were investigated by analyzing their abilities to form apatite on their surface after 21 days in SBF. The apatite mineralization of the powder surfaces was examined through FESEM, EDX, and Raman spectra. The results show that a single-phase wollastonite got formed at all calcined temperatures with a unique silkworm texture. SBF in-vitro test states the formation of HA on the powder surface. Therefore, these powders are expected to be valuable and promising for biomedical applications such as coating and bio cement.     

Effect of Calcination Conditions on Crystalline Structure and Pore Size Distribution for a Mesoporous Alumina

In this article, a mesoporous commercial alumina was calcined in the temperature range of 600°C–1200°C. The effect of several parameters such as calcination temperature, calcination time, heating rate, and calcination steps on phase transformation and crystal size was experimentally investigated. The characterization of the commercial mesoporous alumina and samples calcined at 1000°C, 1040°C, 1070°C, 1100°C, and 1200°C by single-step and multi-step calcination was performed using XRD and N₂ adsorption/desorption techniques. For the commercial mesoporous alumina, TG/DTA analysis was also performed. Experimental results showed that mostly pure α-Al₂O₃ was obtained at 1100°C.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.