Solution combustion synthesis of CeFeO3 under ambient atmosphere

Polycrystalline CeFeO₃ was prepared with almost single phase purity of 93% by a solution combustion synthesis (SCS), but contrarily to all other reports, CeFeO₃ could be obtained under ambient atmosphere. The perovskite-type phase was gained at 400 °C from a solution containing stoichiometric amounts of metal nitrates and by using glycine as fuel. For comparison, CeFeO₃ was also prepared under inert atmosphere by a conventional, oxalate-based high-temperature solid-state reaction. The products were structurally characterised by X-ray powder diffraction (XRD), and the lattice parameters were determined from subsequent Rietveld refinements. Since solution combustion is often referred as the method delivering nano-sized ceramics, the morphological characteristics of the products obtained through the two different synthesis methods were compared by scanning electron microscopy (SEM). Furthermore, the particle sizes were determined by laser diffraction analysis.     

Solution combustion synthesis of iron-deficient Sc2-xFexO3 (x = 0.17-0.47) nanocrystals with bixbyite structure: The effect of spatial constraints

In this paper, nanopowders based on iron-deficient Sc_2-xFe_xO₃ (x = 0.17–0.47) nanocrystals with bixbyite structure and crystallite size of 3.7–38.9 nm were successfully synthesized via solution combustion. Variable glycine-to-nitrate (G/N) ratio was the main controlling factor. A wide range of experimental and computational methods were used to analyze the impact of spatial constraints on the resulting solid-state products. It was found that solution combustion mode greatly influenced on the temperature and gaseous products in the reaction zone. Volume (G/N = 0.4–0.8, T_max = 1179–1511 °C), self-propagating (G/N = 1.0–1.4, T_max = 614–957 °C) or smoldering (G/N = 0.2, T_max = 443 °C) combustion modes were acquired during the synthesis depending on G/N ratio. It was shown that the formation of impurity phases of am-Fe₂O₃ (T_max < 850 °C), c-Fe₃O₄ (900 °C < T_max < 1500 °C) or c-FeO (T_max > 1500 °C) was possible, depending on the combustion temperature. Besides, the combustion mode affected the porous and surfacial structure of resulting mesoporous nanopowders – specific surface area and total pore volume varied in ranges of 1.7–82.8 m²/g and 0.0088–0.1538 cm³/g, consequently. Chemical composition and unit cell parameters of Sc_2-xFe_xO₃ showed the positive deviation from Vegard's law. The average sizes of the interpore thickness (h) depending on G/N ratio were found from values of specific surface area and pycnometric density of nanopowders, which made it possible to establish the presence of spatial constraints for the crystals' growth of Sc_2-xFe_xO₃ at h values below 10 nm. Analysis of aspect (h/D) ratio allowed to determine synthetic parameters which led to mono- or polycrystalline structure of interpore space in resulting Sc_2-xFe_xO₃-based nanopowders. The results and patterns established in this paper allowed to synthesize a new type of foam-like functional materials based on rare-earth ferrites.     

Solution combustion synthesis of ZnO powders using CTAB as fuel

Zinc oxide (ZnO) powders have been prepared by solution combustion synthesis method using cetyltrimethylammonium bromide (CTAB) as fuel. The effects of fuel to oxidant ratios (? = 0.5, 0.75, 1 and 1.5) on the combustion behavior, phase evolution, microstructure, optical properties and photocatalytic performance were investigated by thermal analysis, X-ray diffractometry, electron microscopy, and diffuse reflectance spectrometry techniques. The slow decomposition rate of CTAB guaranteed the direct formation of single phase and well-crystalline ZnO powders regardless of fuel content. The specific surface area of the as-combusted ZnO powders with platelet particles increased from 21 ± 1 to 35 ± 2?m²/g with fuel content. The band gap energy also increased from 2.99 to 3.13?eV due to the decrease of particle size. The as-combusted ZnO powders at ? = 1.5 exhibited the highest photodegradation (~69%) of methylene blue under ultraviolet light irradiation, due to their good crystallinity and smaller particle size.     

Solution combustion synthesis of ZnO powders using mixture of fuels in closed system

Single phase ZnO powders with wurtzite structure were synthesized by solution combustion method using various amounts of mixed glycine-citric acid fuel in the presence (open system) and absence (closed system) of air oxygen. Phase evolution, microstructure and optical properties were investigated by thermal analysis, X-ray diffractometry, electron microscopy and Raman, photoluminescence (PL) and diffuse reflectance spectrometry techniques. Rapid combustion reaction in closed system led to weak crystallinity, as confirmed by deep-level emissions in PL spectra. Larger spherical particles (~200?nm) were synthesized in open system at ? =?1. The as-combusted ZnO powders in closed system exhibited higher photocatalytic activity under ultraviolet irradiation, due to their higher adsorption capacity of methylene blue on ZnO surface. Photodegradation rate increased with the increase of fuel content in as-combusted ZnO powders produced by open route as a result of the reduction of particle size and band gap energy.     

Solution combustion synthesis (SCS) of chrome alumina as a high temperature pink pigment

In this article, one step synthesis of the chrome alumina pink pigments is investigated. Results indicated that adjusting ignition parameters such as the fuel type (glycine, citric acid, and urea) and the oxidizer to fuel ratio, is the essential factor to obtain the straight corundum structure of the pigment. According to this, chrome alumina pink pigment was just synthesized by the urea fuel in 1.8 oxidizer to fuel (O/F) ratio. The effect of ignition parameters on the morphology was also studied. The microstructure was changed from ultra‐fine irregular agglomerates to sponge‐like flakes by the change in the fuel type from citric acid to urea. Colorimetric characteristics confirmed the relationship between the formation of the corundum structure and one step synthesis of the pink pigment. The pink color of synthesized pigments was comparable with commercial purplish pink pigments of tiles.     

Solution combustion synthesis of MnFeCoNiCu and (MnFeCoNiCu)3O4 high entropy materials and sintering thereof

Single-phase, fine-grained high entropy MnFeCoNiCu alloy and (MnFeCoNiCu)₃O₄ oxide powders have been pioneered by an in-situ solution combustion synthesis without post-treatment, utilizing alanine and glycine as organic reducers and ammonium nitrate as an auxiliary oxidizing agent. In order to regulate the porosity, homogeneity, phase composition and morphology of the obtained powders, multivariate studies on the influence of the following parameters were performed: type and quantity of reducers, pH of solution, ignition temperature, presence of oxidizing agent, etc.. It has been shown that the amount of the reducer holds the key for governing combustion temperature and speculation of lower and upper combustion limits in the systems under study. In turn, according to the temperature-time profiles of combustion, product microstructure, as well as phase composition of the product, the combustion area turned out to be divided into two sub-domains: volume combustion (with vigorous flaming) and self-propagation combustion. After thorough characterization, high entropy MnFeCoNiCu alloy and high entropy (MnFeCoNiCu)₃O₄ oxide powders were subjected to consolidation by spark plasma sintering and mechanical properties were evaluated. The influence of sintering temperature on the phase transformation of the products was revealed.     

A study of salt-assisted solution combustion synthesis of magnesium aluminate and sintering behaviour

We synthesized nano MgAl₂O₄ with a ~80?nm particle size by salt-assisted solution combustion synthesis using LiCl as salt. Nano MgAl₂O₄ produced by conventional solution combustion synthesis commonly exhibits poor uniformity in terms of size with partially sintered particles and a high degree of agglomeration, leading to poor sinterability. It was found in this study that the use of the salt-assisted solution combustion method has successfully lowers the degree of agglomeration with uniform particle size and morphology, demonstrating superior sinterability. Conventional sintering in air atmosphere at 1550?℃ of MgAl₂O₄ obtained by salt-assisted solution combustion followed by calcination at 700–1100?℃ yielded up to 94% of relative density, while the conventional solution combustion method could not match this. In addition, using the spark plasma sintering technique, fully dense (over 99%) submicrometer (~340?nm) transparent polycrystalline MgAl₂O₄ with elevated mechanical properties (~16.6?GPa) was achieved. The salt-assisted solution combustion method could be effectively used for fully dense material, but can be further developed for various nano oxide materials where high dispersion with a low degree of agglomeration is preferred.     

Morphological, structural and electrochemical properties of lithium iron phosphates synthesized by Spray Pyrolysis

In the field of materials for lithium ion batteries, the lithium iron phosphate LiFePO₄ has been proven for use as a positive electrode due to its good resistance to thermal degradation and overcharge, safety and low cost. The use of nanostructured materials would improve its efficiency. This work shows the results of the synthesis of nanostructured materials with functional properties for lithium batteries through aerosol techniques. The Spray Pyrolysis method allows synthesizing nanostructured particles with spherical geometry, not agglomerates, with narrow distribution of particle size and homogeneous composition in respect to a precursor solution. Experimental techniques were focused on the morphological (SEM and TEM), structural (XRD and HRTEM-SAED), chemical (EDS) and electrochemical characterization.     

Combinatorial synthesis and characterization of mixed metal oxides for soot combustion

A polymerizable-complex method (PCM) was used with automated solution deposition to generate combinatorial libraries of oxide powders that were evaluated for their ability to catalyze the combustion of soot. Chemical compositions were examined for catalytic activity in the soot combustion reaction in parallel using infrared thermography and serially using automated thermogravimetric analysis. Although infrared analysis is much faster, it is not easily suited to analyzing soot combustion in the configuration employed in the present study. Instead, the slower automated thermogravimetric analysis (TGA) was found to be much more reliable and reproducible. The results of characterizing simple oxide systems (Ce–K–O, Cu–K–O, Ce–Na–O and Cu–Na–O) are reported.     

Low‐temperature molten salt synthesis of forsterite powders with controllable morphology

Forsterite powders with controllable morphology were synthesized using oxides as raw materials in NaCl–KCl molten salt media. The effects of MgO/SiO₂ ratio, calcining temperature, and salt/oxide ratio on the phase composition and morphology of the powders are investigated. The results indicate that single‐phase forsterite powders can be synthesized from a mixture of MgO and SiO₂ with a MgO/SiO₂ molar ratio of 2:1.3 at 700°C. With the increase in calcining temperature, the powders obtained changes from an irregular to a columnar morphology. In addition, the morphology of the forsterite powders produced can also be controlled by altering the salt/oxide ratio.     

Electrochemical synthesis of bismuth molybdates with controllable composition and their photocatalytic performance

Bismuth molybdate photocatalysts were controllably prepared via an electrochemical approach at room temperature. The composition and optical property of each product were determined according to the NaOH and Na₂MoO₄ quantities in the electrolyte. Pure Bi_3.64Mo_0.36O_6.55 was prepared at a NaOH concentration range of 0.2‐0.8 mol L^?1, whereas the Bi_3.64Mo_0.36O_6.55/Bi₁₄MoO₂₄ composite was obtained in an electrolyte containing 0.4 mol L^?1 NaOH and 0.5 mol L^?1 Na₂MoO₄. The results of rhodamine B degradation under visible light indicated that Bi_3.64Mo_0.36O_6.55 nanoparticles with a size of 10‐50 nm displayed the best photocatalytic efficiency, which was considerably higher than that of composite one and bulk Bi_3.64Mo_0.36O_6.55.     

Solution combustion synthesis of FeCr2O4 powders for pigment applications: Effect of fuel type

Solution combustion synthesis of iron chromite was reported using iron(III) nitrate nonahydrate and chromium(III) nitrate nonahydrate as starting materials, as well as glycine, urea, citric acid, and ethylene glycol as fuels. The influence of fuel type on the structure, molecular, microstructure as well as chromatic properties of samples was investigated. The X-ray diffraction (XRD) patterns showed that unlike themodynamical prediction, glycine fuel led to strongest combustion and consequent highest XRD peak intensities and lower lattice parameters. Moreover, the change of fuel type and mixing of fuels affected XRD data. Fourier transform infrared analysis showed that the band position of Cr–O and Fe–O bonds were shifted to higher frequencies by using of fuels with weaker combustion reactions. In addition, scanning electron micrographs showed that different morphologies of FeCr₂O₄ particles were obtained depending on the fuel type and ratios. Energy-dispersive X-ray spectroscopy analysis of the samples showed that oxygen concentration of samples was less than that of stoichiometric ratio of FeCr₂O₄ due to local reducing atmosphere. Furthermore, chromatic properties of the powders showed that the pigment synthesized with glycine fuel has a better and lighter grayish brown color than the other ones and can be used as a suitable industrial candidate to create a brown color in the ceramic glaze.     

Solution combustion synthesis of metal nanopowders: Nickel—Reaction pathways

Nanopowders of pure nickel were directly synthesized for the first time by conventional solution combustion synthesis (SCS) method. In this article, a specific reaction pathway is suggested to describe the metallic phase formation during SCS. It is proposed that the exothermic reaction between NH₃ and HNO₃ species formed during the decomposition of glycine and nickel nitrate acts as the source of energy required to achieve the self‐sustained reaction regime. A thermodynamic analysis of the combustion synthesis reaction indicates that increasing glycine concentration leads to establishing a hydrogen rich reducing environment in the combustion wave that in turn results in the formation of pure metals and metal alloys. TGA of reaction systems and XRD analysis of products in the quenched combustion wave show that the formation of oxide phases occurs in the reaction front, followed by gradual reduction of oxide to pure metallic phases in the postcombustion zone. A methodology for SCS of pure metals and metal alloys nanoparticles can be inferred from the results presented. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Influence of fuel on phase formation,morphology, electric and dielectric properties of iron oxides obtained by SCS method

Nanopowders of iron oxides were obtained by Solution Combustion Synthesis (SCS) method from sol-gel compositions containing iron nitrates and soluble organic reducing agents (glycine, urea, citric acid). The synthesis processes and their intensity depending on the type of fuel and fuel/oxidizer ratio (?) were investigated. It was established that the combustion regime affects the phase formation of the obtained powders, their morphology, the color of the final Fe₂O₃ powders, dielectric properties and etc. It was shown that iron oxides with a preferred morphology and high dielectric properties (ε = 44.5 at ? = 0.6 vs ε = 4.0 at ? = 1.4 using urea as fuel) could be produced by the SCS method.     

Enhancement of electrochemical performance of lithium iron phosphate by controlled sol–gel synthesis

The porous phase pure lithium iron phosphate (LiFePO₄/C) composite particles with a few nanometers thick layer of carbon were synthesized by sol–gel method. The in situ coating of carbon on the LiFePO₄ particles was achieved by the pyrolysis of carbon source during the thermal treatment. The synthetic conditions were observed to affect physical, morphological and electrochemical properties of the composites. The composite synthesized via a single-step thermal treatment at 700 °C in the presence of a mixture of citric acid and sucrose possesses a large surface area and porous structure. The structure of the residual carbon coated in this sample is observed to be graphene-rich with the lowest D/G (disordered/graphene) ratio in the Raman spectra. When the three LiFePO₄/C composites were evaluated as cathode materials in lithium cells at room temperature, the composite prepared in the presence of sucrose as an additional carbon source showed the highest electrochemical performance exhibiting high discharge capacities of 153 (corresponding to 90% of the theoretical capacity), 120, 112, and 94 mAh/g at 0.1, 1, 3, and 5 C-rates, respectively.     

Molten-salt-assisted combustion synthesis of B4C powders: Synthesis mechanism and dielectric and electromagnetic wave absorbing properties

A novel electromagnetic wave (EMW) absorber was prepared by combustion synthesis. Boron carbide (B₄C) powders with different grain sizes using a molten-salt-assisted combustion technique with B₂O₃, CB (carbon black), and Mg powders as starting materials, and NaCl as an additives. The effects of the NaCl content on the phase compositions and the microstructure of the products were characterized. A combustion front quenching method was used to elucidate the mechanism for the B₄C powders synthesis. The dielectric, and EMW absorbing properties in the X-band were also investigated. The results showed that the addition of NaCl significantly reduced the grain size of B₄C powders. Nanoscale B₄C powders with cubic polyhedral structures were synthesized using 6 wt% NaCl (labeled as N-6). According to the quenching test results can be obtained that the first step in the combustion synthesis was melting B₂O₃ into a glassy substance. At the same time, Mg melted and formed a liquid pool into which the NaCl dissolved, followed reduction of the B₂O₃ to B. The formed B eventually reacted with CB to form B₄C, and the B₄C particles precipitated from the matrices. The N-6 sample exhibits optimal dielectric and EMW absorbing properties, because of a high specific surface area that enhances interfacial and space charge polarization.     

The roles of lime and iron oxide on the formation of ash and deposits in PF combustion

This paper presents the results of a study to assess the slagging propensities of a suite of UK, Spanish and South African coals, ranging from lignites to anthracites. Laboratory deposits were collected on ceramic deposition probes at gas temperatures of ∼1250°C, using an entrained flow reactor that simulates the time-temperature conditions experienced by pulverised coal particles in a large utility boiler. The degree of sintering and consolidation of the deposits would not have been predicted from bulk ash chemistry, indicating the importance of mineral matter distributions in the pulverised coal. Deposits with similar base to acid ratios and Fe₂O₃ contents displayed a range of slagging propensities on CCSEM analysis, consistent with the visual ranking. CCSEM analysis of the fly ashes collected from the combustion gases revealed a similar chemical composition to the coal ash and ash collected at the base of the EFR. CaO was observed to have readily assimilated into the aluminosilicate fly ash particles. On deposition, the CaO distribution largely remained unchanged. Fe₂O₃ was redistributed on forming a deposit possibly aided by CaO already dissolved in the aluminosilicates. The study provides an insight into the observations made by boiler operators burning coals with high CaO and Fe₂O₃ ashes.     

Solution combustion synthesis of metal nanopowders: Copper and copper/nickel alloys

Based on a general methodology for the preparation of metal–nanopowders by solution combustion synthesis (SCS), the reaction pathways for SCS of pure copper and copper–nickel alloy nanopowders are investigated. It is confirmed that the necessary condition for SCS of metals in a metal‐nitrate oxidizer–glycine system is the property of the oxidizer to decompose with formation of HNO₃ species. In this case, for compositions with excess of glycine, a hydrogen reducing atmosphere develops in the reaction front, leading to the formation of reduced metals. The proposed reaction pathways are supported by X‐ray diffraction analysis of the quenched samples and DTA–TGA studies of the Cu(NO₃)₂·6H₂O–glycine and Ni(NO₃)₂·6H₂O/Cu(NO₃)₂·6H₂O–glycine systems. The results show that the formation of Cu₂O and CuO oxide phases takes place at early stages in the reaction front followed by their reduction to pure Cu phase in the postcombustion zones. However, in a Cu–Ni alloy, a fraction of intermetallic Cu–Ni phase appeared directly in the combustion front, whereas the rest of the oxygen‐free alloy formed through reduction of oxide phases. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Influence of different lithium sources on the morphology,structure and electrochemical performances of lithium-rich layered oxides

Lithium-rich layered oxides were synthesized via co-precipitation by using different lithium sources (LiOH, Li₂CO₃ and CH₃COOLi). Scanning electron microscope (SEM), Thermo gravimetric analysis (TGA), Brunauer-Emmett-Teller (BET), Inductively coupled plasma atomic emission spectrometry (ICP-AES), X-ray diffraction (XRD) and electrochemical measurements were used to investigate the morphology, reaction process, specific surface area, composition, structure and electrochemical performance of the lithium-rich oxides, respectively. The use of different lithium sources mainly affects the primary particle size and secondary particle morphology of the final product. Using LiOH as the lithium source, the maximum discharge capacity of sample can reach to 272.1 mA h g^–1 in the voltage range of 2.0–4.6 V at room temperature, even after 50 cycles, the retention rate is still reach 91.4%. The electrochemical impedance spectroscopy (EIS) results show that lithium-rich oxides using LiOH as the lithium source have the minimum value of impedance after 50 cycles. Therefore, the choice of appropriate lithium source is an effective way to improve the electrochemical properties of lithium-rich layered oxides.     

Hydrothermal synthesis and electrochemical properties of cobalt–carbon nanotubes nanocomposite

Cobalt–carbon nanotubes composite (Co–CNTs) is synthesized through a facile hydrothermal route. SEM and TEM characterizations reveal that the Co–CNTs composite contains abundance of carbon nanotubes connected by cobalt spheres and some of the CNTs are filled with metallic nanoparticles or nanorods. A series of electrochemical measurements show that the adding CNTs can remarkably enhance the electrochemical activity of the Co, leading to a notable improvement of the discharge capacity and the cycle performance. The practical maximum discharge capacity of the active Co is 495 mAh g⁻¹ after deducting the weight contribution of CNTs, which is about 280 mAh g⁻¹ higher than that of pure Co. The electrochemical reaction mechanism can be attributed to the dissolution–precipitation mechanism of Co in alkaline solution. And the functions of the CNTs are to improve dispersion of Co particles, increase contact area between Co and alkaline solution and promote the charge-transfer reaction.