Effect of heat treatment conditions on the growth of MgAl2O4 nanoparticles obtained by sol-gel method

MgAl₂O₄ nanoparticles (NPs) were prepared by sol–gel method using aluminium nitrate, magnesium nitrate and citric acid as starting materials, phenolic formaldehyde resin and carbon black as additives. Growth of MgAl₂O₄ NPs in different heat treatment conditions (temperature, atmosphere, carbon additives and in Al₂O₃-C system) was investigated. MgAl₂O₄ NPs were formed at 600 °C in air atmosphere with serious agglomeration of nanoparticles having diameter of approximate 30 nm. The size of MgAl₂O₄ NPs increased greatly from 40 to 50 nm to several hundreds of nanometres as the temperature was raised from 800 °C to 1400 °C. Partial sintering of NPs was observed upon heating at temperatures higher than 1200 °C in air. In reducing atmosphere, the size of MgAl₂O₄ NPs (about 30–50 nm) changed slightly with increasing temperature. This was attributed to the dispersion of carbon inclusions in the MgAl₂O₄ grain boundaries, inducing a steric hindrance effect and inhibiting the growth of particles. MgAl₂O₄ NPs (30–50 nm) in the Al₂O₃-C system were in-situ formed at high temperatures with the use of dried precursor gels. MgAl₂O₄ NPs can contribute to improving the thermal shock resistance of Al₂O₃-C materials.     

Chemically induced order disorder transition in magnesium aluminium spinel

Magnesium aluminate spinel (MgAl₂O₄) spinel powder was synthesized by nitrate citrate auto-ignition route taking different ratios of nitrate and citrate solution. The ‘as prepared’ black ash was calcined at different temperatures in the range 650–1250 °C for 9 h. Phase evolution of calcined powder samples as studied by X-ray diffraction indicates the presence of disorder at lower calcination temperatures, which transforms to an ordered structure at higher calcination temperatures. Finally, Raman spectroscopy confirms the order–disorder phase transition in spinel sample.     

Fabrication and properties of Co:MgAl2O4 transparent ceramics for a saturable absorber from coprecipitated nanopowder

The 0.05 at.% Co:MgAl₂O₄ precursor was synthesized by the coprecipitation method from a mixed solution of magnesium, aluminum, and cobalt nitrates using ammonium carbonate as the precipitant. 0.05 at.% Co:MgAl₂O₄ transparent ceramics were successfully obtained via vacuum sintering and hot isostatic pressing (HIP) of 0.05 at.% Co:MgAl₂O₄ nanopowder calcined at 1100°C for 4 hours. The properties of powder and ceramics were comprehensively investigated. X-ray diffraction (XRD) results showed that Co:MgAl₂O₄ nanopowder had a pure spinel phase. Also, the in-line transmittances of the HIP posttreated Co:MgAl₂O₄ ceramics with the thickness of 1.2 mm were 82% at 400 nm and 84.7% at 900 nm. The average grain sizes of 0.05 at.% Co:MgAl₂O₄ ceramics before and after the HIP posttreatment were 11 and 28 μm, respectively. The calculated ground state absorption cross section of 0.05 at.% Co:MgAl₂O₄ ceramics was 2.9 × 10⁻¹⁹ cm², indicating that this ceramics is a promising material applied as a saturable absorber for passive laser Q-switches in the 1.3-1.7 μm domain.     

Effects of calcination temperature on structure and magnetic properties of pure FeCo2O4 powders

A series of FeCo₂O₄ powders was initially synthesized using a hydrothermal method and subsequently calcined at various temperatures to produce the final product. Pure phase FeCo₂O₄ powders can only be formed in the temperature range of 950–1050 °C. In this work, we study the cation occupation, cation valence, bond length and bond angle changes of the pure phase FeCo₂O₄ powders formed in such a narrow temperature range. Octahedral lattice distortion in the pure phase FeCo₂O₄ samples has been observed. More tetrahedral Fe³⁺ and octahedral Co²⁺ are excited and exchanged their sites as the calcination temperature increases from 950 °C to 1000 °C, and part of Co³⁺ ions are reduced to Co²⁺ in the sample calcined at 1050 °C. The structure of the sample calcined at 1000 °C is close to that of the ideal FeCo₂O₄ spinel. Magnetic measurements show that ferrimagnetism and anti-ferromagnetism coexist in the pure phase FeCo₂O₄ samples. Interaction changes between ferrimagnetism and antiferromagnetism caused by the structural changes of the samples have been studied. Due to the pinning of the local anti-ferromagnetism to ferrimagnetism in the sample, the sample shows a Barkhausen jump below 150 K. As the measurement temperature increases further, the system enters into a reentrant spin glass state.     

Microstructure,mechanical and optical properties of MgAl2O4/MgAl2O4 joints bonded using CaO-Al2O3-SiO2 glass filler

Transparent MgAl₂O₄ ceramics were bonded by using CaO-Al₂O₃-SiO₂ (CAS) glass filler. The CAS glass filler exhibited the same thermal expansion behavior as MgAl₂O₄ ceramic and excellent wetting ability on the surface of MgAl₂O₄ ceramic. When the cooling rate of 15 °C/min was used, no interfacial reaction was observed and the amorphous brazing seam could be obtained. However, low joining temperature (1250 °C) led to the formation of pores and high joining temperature (1400 °C) resulted in the formation of cracks. Furthermore, the slow cooling rate of 5–10 °C/min induced the crystallization of CaAl₂Si₂O₈ and Mg₂Al₄Si₅O₁₈ due to the dissolution of MgAl₂O₄ substrate. The optimal flexural strength of 181–189 MPa was obtained when the joining temperature and cooling rate were 1300–1350 °C and 15 °C/min respectively. Moreover, the in-line transmittance of the joint at 1000 nm was 82.1%, which was slightly lower than that of MgAl₂O₄ ceramic (85.6%).     

Solid Solution Effects on the Thermal Properties in the MgAl2O4–MgGa2O4 System

Solid solution effects on thermal conductivity within the MgO–Al₂O₃–Ga₂O₃ system were studied. Samples with systematically varied additions of MgGa₂O₄–MgAl₂O₄ were prepared and the laser flash technique was used to determine thermal diffusivity at temperatures between 200°C and 1300°C. Heat capacity as a function of temperature from room temperature to 800°C was also determined using differential scanning calorimetry (DSC). Solid solution in the MgAl₂O₄–MgGa₂O₄ system decreases the thermal conductivity up to 1000°C. At 200°C thermal conductivity decreased 24% with a 5 mol% addition of MgGa₂O₄ to the system. At 1000°C, the thermal conductivity decreased 13% with a 5 mol% addition. Steady‐state calculations showed a 12.5% decrease in heat flux with 5 mol% MgGa₂O₄ considered across a 12 inch thickness.     

Synthesis andcharacterisation of nanocrystalline MgAl2O4 ceramic powders by use of molten salts

Abstract

Nanocrystalline MgAl₂O₄ powders were prepared by a thermal decomposition method, i.e. by use of molten salts. This method involves co-melting stoichiometric amounts of magnesium nitrate hydrate Mg(NO₃)₂.6H ₂O and aluminium nitrate hydrate Al(N O₃) ₃.9H₂O at 500°C. The spinel content of the co-melted and calcined powders at different firing temperatures up to 1000°C was determined by chemical analysis and the powders were characterised with respect to spinel formation, crystallite and particle sizes by X RD, T EM , and IR spectroscopy. The results obtained revealed that the co-melted materials were amorphous. After heat treatment of the amorphous materials at up to 1000°C, pure spinel powder was obtained, reaching over 98% spinel content. During calcination at different firing temperatures up to 1000°C the amorphous material progressively crystallised, forming nanocrystalline spinel with a maximum crystallite size of about 10 nm and particle size of around 300 nm. Bands in IR spectra were observed corresponding to the ex istence of AlO₆ groups prior to magnesium spinel formation, which was the only crystalline phase at 1000°C.     

Poly(N-isopropylacrylamide) assisted microwave synthesis of magnesium aluminate spinel oxide for production of highly transparent films by hot compression

Magnesium aluminate spinel oxides have been prepared via poly(N-isopropylacrylamide) assisted microwave technique. The prepared MgAl₂O₄ powders showed a crystalline cubic structure with spinel phase after calcination at 600 °C only. The poly(N-isopropylacrylamide) amount showed a high effect on the crystallite size and the densification behavior of MgAl₂O₄. The increase of the amount of poly(N-isopropylacrylamide) reduced the sintering temperature of MgAl₂O₄ from 1400 °C to 1050 °C. The hot-pressed of MgAl₂O₄ powders in the presence of 3 wt% of poly(N-isopropylacrylamide) exhibited a full density at sintering temperature 1100 °C in 15 min only. The sintered films showed high transparency (81 ± 2%) in the wavelength range 500–1000 nm.     

Transparent polycrystalline Gd2Hf2O7 ceramics

We have successfully developed transparent polycrystalline Gd₂Hf₂O₇ ceramics with high in‐line transparency. A sol–gel process was used to synthesize the Gd₂Hf₂O₇ powder. Simultaneous thermal gravimetric analysis and differential thermal analysis (TGA/DTA) was used to identify the decomposition sequence as a function of temperature for the as‐synthesized sol–gel powders. The calcined powder is single phase and was formed with an estimated average particle size of 120 nm. Crystallization was confirmed by x‐ray diffraction (XRD) and a single phase was achieved by calcining at 1000°C. The calcined powders were hot‐pressed at 1500°C to achieve >95% theoretical density with closed pore structure followed by a hot isostatic pressing at 1500°C at 207 MPa to achieve a fully dense structure. Microstructural characterization shows a uniform grain size distribution with an average grain size of about 11 μm. In‐line transmission measurements revealed high transparency in the red and infrared. Dielectric properties remain stable with relative permittivity values around 180 and loss tangents less than 0.005 up to 350°C. Thermal conductivity was measured to be ~1.8 W/m°K at room temperature, decreasing to ~1.5 W/m°K by 500°C.     

Gel-casting of MgAl2O4 transparent ceramics using a common dispersant

The ethanolaminic salt of citric acid (commercial name Dolapix CE 64) has commonly been used as a dispersant for colloidal based ceramic forming process. In this paper, a surprise was presented that MgAl₂O₄ spinel slurries consisting of MgAl₂O₄ spinel nanoparticles and Dolapix CE 64 gelled in air at room temperature spontaneously. The MgAl₂O₄ spinel slurries with high solid loading (54 vol%) were prepared with Dolapix CE 64 and the green body with up to 57% relative density was obtained. MgAl₂O₄ transparent ceramics with small grain size (0.92 μm) and high transmittance (81.7% at 600 nm) were fabricated after pre-sintering at 1500°C and hot-isostatic sintering at 1550°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.     

Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

Rare earth-doped Ni/MgAl2O4 catalysts prepared via a one-step sol–gel method for steam reforming of volatile organic solvents at low temperatures

To develop efficient steam reforming catalysts that are widely applicable to various volatile organic solvents under low temperatures, rare earth (La, Ce or Pr)-doped Ni/MgAl₂O₄ were prepared via a novel one-step sol–gel method. Through various characterizations and DFT calculations, it demonstrated that the rare earths not only improved the catalyst structure, but also promoted the dispersion, electron density and d-band center of Ni. Consequently, all the rare earth-doped Ni/MgAl₂O₄ catalysts exhibited higher catalytic activity in steam reforming of different volatile organic solvents than the Ni/MgAl₂O₄ in 500–550 °C. Thereinto, the 10Ni-2La/MgAl₂O₄ performed the best, achieving a conversion efficiency of 97.3% at 550 °C for the mixture of toluene, acetone, tetrahydrofuran, and n-hexane. The outstanding performance of Ni-La/MgAl₂O₄ catalyst arises from the structural compatibility between low-spin La atom and Ni cluster, which enables La to modify Ni dispersion and electronic structure more significantly than Ce or Pr.     

White‐Light and Yellow/Blue Photoluminescence Emission Based on Dy3+‐Doped SiO2–Gd2O3 Composites

This work attempts to obtain Dy³⁺‐doped SiO₂–Gd₂O₃ by sol–gel process_, with a molar ratio of 70Si⁴⁺–30Gd³⁺ and Dy³⁺ concentrations of 0.1, 0.3, 0.5, and 1 mol%. Heat treatment at temperatures of 1000°C, 1100°C, 1200°C, and 1300°C have been performed. From XRD, the Gd₂O₃ cubic phase was observed at 1000°C and 1100°C, at 1200°C also were observed Gd₂O₃ monoclinic phase, predominant at 1300°C. The band‐gap values vary between 4.4 and 5.3 eV, showing dependence on the crystalline phase. Under UV excitation, emission spectra show bands assigned to the Dy³⁺ transitions: ⁴F_9/2→⁶H_15/2 (484 nm), ⁴F_9/2→⁶H_13/2 (572 nm), and ⁴F_9/2→⁶H_11/2 (668 nm). The excitation at 275 nm has shown more effective. The ratio between the most intense emission bands (yellow/blue) show values around 0.84 and 1.63. CIE chromaticity diagrams show color coordinates at blue, yellow, and white regions, as a function of Dy³⁺ concentration and heat treatment. The lifetime values of excited state ⁴F_9/2 were around 0.20 and 0.69 ms. The morphology of particles changed from spherical to coral‐like shape as a function of heat treatment are observed. The sol–gel process showed to be an interesting route to obtain Dy³⁺‐doped binary system materials.     

Impact on aggregate/matrix bonding when a refractory contains zinc aluminate instead of spinel and magnesia-chrome

The high hot strength of MgO–Cr₂O₃ refractory is often ascribed to its intimate aggregate/matrix bonding. For a fundamental comparison with it, ∼2 mm aggregates of MgO and Al₂O₃ were separately embedded in ZnAl₂O₄ and MgAl₂O₄ matrices, sintered at 1600°C, and examined. It was found that similarity of thermal expansion coefficient (TEC) between the aggregate and the matrix is critical to achieve good bonding and this is more important than the extent of interdiffusion. The TEC mismatch of ≥5.7 × 10⁻⁶ K⁻¹ caused significant undesirable debonding in MgO aggregate/MgAl₂O₄ matrix sample and MgO/ZnAl₂O₄ despite >736 μm Zn²⁺ diffusion depth in the latter. Direct bonding, as inferred from a thicker interfacial reaction layer and a greater shift of the aggregate/matrix interface before and after firing, was better in MgAl₂O₄/ZnAl₂O₄ combination, followed by tabular Al₂O₃/ZnAl₂O₄ and Al₂O₃/MgAl₂O₄. Powder X-ray diffraction indicated that the volatilization of ZnAl₂O₄ at 1600°C in air was negligible compared to MgO–Cr₂O₃.     

Preparation and catalytic activity in ethanol combustion reaction of cobalt–iron spinel catalysts

Iron–cobalt spinel catalysts were prepared via the coprecipitation method. The effect of different parameters on textural, structural and catalytic properties, in ethanol combustion, was investigated. The CoFe₂O₄ phase was obtained at calcination temperatures as low as 500 °C and the usage of ammonia as precipitating agent, results in the formation of Fe₂O₃ in addition to the spinel phase. The catalyst prepared using nitrate salts, NaOH as precipitation agent and calcined at 600 °C had the best catalytic performance achieving ethanol complete oxidation at 271 °C.     

Synthesis and characterization of bioactive zirconia toughened alumina doped with HAp and fluoride compounds

Alumina–zirconia nanostructured composites (ZrO₂ addition by 20 wt%) were prepared using combined gelation–precipitation process. A modified sol–gel process has been developed to prepare nano structured spinel [MgAl₂O₄], Al₂O₃, ZrO₂ and their composite materials. This process is useful in retaining tetragonal phase of zirconia at room temperature, which provides transformation toughening in the nano composites. Dried gels powders were calcined up to 1250 °C. Similarly, hydroxyapatite powders were produced by wet-chemical method and calcined at different temperatures. All the dried gel and calcined powders were characterized by using X-ray diffraction, DTA/TGA and SEM. Samples were prepared by uniaxial pressing the composites powders using ZTA, HAp, MgF₂ and CaF₂ in different ratio. Incorporation of CaF₂ and MgF₂ as a source for fluorine was also done to improve the sinterability of composites. The samples were sintered at 1400 °C for three hours. Densification and mechanical behaviour of sintered samples were observed. Bioactivities of all compositions were tested using SBF solution and then characterizing by FTIR. The main objective of work was to dope ZTA nano composites with HAp and fluoride compounds to obtain better sinterability at lower temperatures. Then evaluate the obtained ZTA based bioactive composite ceramics that have high mechanical strengths. This study verifies the bioactivities of HAp-added ZTA composites.     

Fabrication and characterization of IR-transparent Fe2+ doped MgAl2O4 ceramics

Initial investigations on the preparation of highly transparent Fe²⁺:MgAl₂O₄ ceramics using nanopowders synthesized in a laser plume were carried out. For the first time, dense Fe²⁺:MgAl₂O₄ ceramics with high transmission in the mid-IR range were fabricated at a temperature as low as 1300°C and with a short sintering time (1 hour). The obtained Fe²⁺:MgAl₂O₄ ceramics contain a secondary (MgO)_0.91(FeO)_0.09 phase with a low wt% content, causing a substantial decrease in transmittance in the visible range. The transmittance increases with an increase in wavelength due to a decrease in Rayleigh scattering and reaches 85.6% at λ = 4 μm, which is close to the theoretical value. The absorption cross section of divalent iron ions was estimated to be σ = (1.66 ± 0.14) × 10⁻²⁰ cm².     

Synthesis of Nanofiber‐like Mesoporous γ‐Al2O3 toward Its Adsorption Efficiency for Congo Red

Nanofiber‐like mesoporous γ‐Al₂O₃ was synthesized using freshly prepared boehmite sol in the presence of triblock copolymer, P123 following evaporation‐induced self‐assembly (EISA) process followed by calcinations at 400°C–1000°C. The samples were characterized by thermogravimetry (TG), differential thermal analysis (DTA), X‐ray diffraction (XRD), N₂ adsorption–desorption, and transmission electron microscopy (TEM). The adsorption efficiency of the samples with Congo red (CR) was studied by UV – vis spectroscopy. XRD results showed boehmite phase in the as‐prepared sample while γ‐Al₂O₃ phase obtained at 400°C was stable up to 900°C, a little transformation of θ‐Al₂O₃ resulted at 1000°C. The Brunauer‐Emmett‐Teller surface area of the 400°C‐treated sample was found to be 175.5 m²g ^{? 1}. The TEM micrograph showed nanofiber‐like morphology of γ‐Al₂O_3. The 400°C‐treated sample showed about 100% CR adsorption within 60 min.     

Synthesis of TiO2–Ag nanocomposite with sol–gel method and investigation of its antibacterial activity against E. coli

TiO₂–Ag nanocomposite was prepared by the sol–gel method and an azeotropic distillation with benzene was used for dehydration of the gel. Because of gel dehydration by distillation method a nanopowder with a surface area of 230 m²/g was produced which decreased to 80 m²/g after calcination. TEM micrographs and XRD patterns showed that spherical nanosized Ag particles (≈ 10 nm) were deposited among TiO₂ particles. The antibacterial activity of calcined powder at 300 and 500 °C was studied in the presence and in the absence of UV irradiation against Escherichia coli as a model for Gram-negative bacteria. The antibacterial tests confirmed the powder calcined at 300 °C possessed more antibacterial activity than the pure TiO₂, amorphous powder and the powder calcined at 500 °C under UV irradiation. In the absence of UV, the reduction in viable cells was observed only with calcinated powder at 300 °C.