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Charge generation during the synthesis of doped lanthanum manganites via combustion of organo-inorganic precursors
Abstract
Complex oxides on the basis of LaMnO3±y were obtained via combustion of precursors containing nitrates and soluble organic agents (polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylamide, polyethylene glycol, cellulose; glycine, glycerine and citric acid). The phenomenon of charge generation processes during the synthesis was analysed by measuring the potential difference between the ground and precursor. It was proposed that one of the main factors determining the processes of charge generation is the release of gaseous molecular charged particles into the environment. The maximum value of generated charges was fixed for polyvinyl alcohol containing precursors and the lower for glycine containing ones. The composition of the initial precursors (used organic component and the φ value) effects the maximum temperature achieved during synthesis (300°C - 1100°C), concentration of released gases (50 – 600 ppm (CO), 250 – 2800 ppm (NO)) and the potential difference between the ground and precursor. The dependence of specific surface area (7.5 – 21 m²/g), the temperature of the beginning of the intense sintering (750°C – 1150°C) and the maximum achievable shrinkage (5 – 21%) on the value of measured potential difference in precursors was shown. The established regularities allow expanding ability for the production of complex oxide materials with the specified properties.
... The main reasons for the appearance of potential differences ranging from a few hundredths to several volts include the spatial separation of charges of different signs due to their different mobility when passing the combustion front. The phenomenon of generation of electric charges during heating and combustion of organic-inorganic (mainly nitrate-containing) precursors [33,[44][45][46][47][48][49][50] has been established in the processes of obtaining complex oxides such as solution combustion synthesis. ...
... The generation of charges has been found to begin at the stage of heating the initial compositions even before the start of intense combustion (pyrolysis) [44,45]. The generation also continues during combustion, and the potential difference measured between the precursor and the ground reaches tens and even hundreds of volts [33,[44][45][46][47][48][49][50]. As shown earlier on samples of precursors containing film polymers [44,45], the surface charge density can be more than 1 µC cm −2 , and the volume density can be 2 × 10 15 of charges per cm 3 . ...
... As shown earlier on samples of precursors containing film polymers [44,45], the surface charge density can be more than 1 µC cm −2 , and the volume density can be 2 × 10 15 of charges per cm 3 . Charges in precursors appear due to the removal of charged molecular groups into the environment, which have the opposite sign of charge to the generated one [33,[44][45][46][47][48][49][50]. It can be assumed that water molecules and carbonate-like groups that carry away an electron, i.e., are negatively charged, or nitrogen monoxide molecules, on the contrary, positively charged, can be released into the gaseous environment [44,45]. ...
The development and characterization of synthesis techniques for oxide materials based on ceria is a subject of extensive study with the objective of their wide-ranging applications in pursuit of sustainable development. The present study demonstrates the feasibility of controlled synthesis of Ce1−xMxO2−δ (M = Fe, Ni, Co, Mn, Cu, Ag, Sm, Cs, x = 0.0–0.3) in combustion reactions from precursors comprising glycine, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, and cellulose as organic components. Controlled synthesis is achieved by varying the composition of the precursor, the type of organic component, and the amount of organic component, which allows for the influence of the generation of high-density electrical charges and outgassing during synthesis. The intensity of charge generation is quantified by measuring the value of the precursor–ground potential difference. It has been demonstrated that an increase in the intensity of charge generation results in a more developed morphology, which is essential for the practical implementation of ceria as a catalyst to enhance contact with gases and solid particles. The maximum value of the potential difference, equal to 68 V, is obtained during the synthesis of Ce0.7Ni0.3O2−δ with polyvinyl alcohol in stoichiometric relations, which corresponds to a specific surface area of 21.7 m² g⁻¹. A correlation is established between the intensity of gas release for systems with different organic components, the intensity of charge generation, morphology, and the value of the specific surface area of the samples.
... The studies of the phenomenon of charge generation during the solution combustion process, provided by Ostroushko et al. [44], showed that the reaction mixture with the maximum charges reached the maximum combustion temperature, and the final oxide powder, obtained from the precursor with the maximum charges, had the highest specific surface area. Moreover, the experiments with different fuels allowed to establish the relationship between the type of fuels and the number of generated charges, which could optimize the synthesis conditions of oxide materials. ...
... As shown in Ref. [44], the formation of charged particles on the surface of complex oxide materials during precursor combustion causes the particles to repel each other, and such powders will subsequently have a higher specific surface area. Therefore, by varying the fuel, the important characteristics of the precursor combustion process, such as the maximum developed temperature, the content of the exhaust gases, and the potential difference between the precursor and the ground, were studied. ...
... The electrode was electrically connected to a metal shield on which the polarity of the charges generated in the solution was determined using the IPEP-1device (Minsk Research Instrument-making Institute, Belarus). The results were recorded automatically on a computer using a program implemented in Python [44]. ...
The aim of this work is to establish the relationship between the electrochemical performance of the Pr1.6Cа0.4Ni0.6Cu0.4O4+δ-based electrodes and the properties of the electrode powders, conditioned by their synthesis history, as well as the electrode design and the sintering conditions of the electrode layers. The Pr1.6Cа0.4Ni0.6Cu0.4O4+δ (PCNCO) powders are synthesized by combustion of salt compositions using different fuels: glycine, polyvinyl alcohol and citric acid. The influence of the composition of the redox mixture on the synthesis process, the phase composition of the obtained powders and their properties have been studied. The mi-crostructure of the PCNCO electrodes formed from the powders with different dispersions is studied by electron microscopy. The electrochemical performance of the electrodes in contact with the Ce0.8Sm0.2O1.9 (SDC) elec-trolyte is studied by impedance spectroscopy. Based on the correlations established between the chemical stability and dispersion of the powders and the microstructure and polarization resistance of the corresponding electrodes , the optimal parameters for the synthesis of the PCNCO complex oxide for the use as a cathode material have been determined. The lowest polarization resistance equal to 0.38 Ω cm 2 at 700°C is obtained for the bi-layer electrode with the PCNCO functional layer synthesized by the citrate-nitrate combustion and sintered at 1050°C, and the LaNi0.6Fe0.4O3-δ oxide collector sintered at 900°C. The developed synthesis procedure and electrode design can be recommended as promising for the fabrication of air electrodes in the intermediate-temperature electrochemical devices.
... Как показывают исследования в области синтеза сложнооксидных материалов, при горении прекурсоров в них могут возникать электрические заряды высокой плотности [21][22][23][24] за счет уноса в газовую среду заряженных (ионизированных) молекулярных группировок. К таким частицам относятся ионизированные молекулы воды, ассоциированные с дополнительным электроном, карбонатоподобные группировки [25,26]. ...
... К таким частицам относятся ионизированные молекулы воды, ассоциированные с дополнительным электроном, карбонатоподобные группировки [25,26]. В таком случае прекурсор приобретает противоположный положительный заряд [21][22][23][24]27]. Если в газовую среду уносятся преимущественно молекулы оксида азота NO, то формируется отрицательный заряд прекурсора [21-24, 27, 28], так как молекулы оксида азота достаточно СИНТЕЗ И СВОЙСТВА НЕОРГАНИЧЕСКИХ СОЕДИНЕНИЙ ЖУРНАЛ НЕОРГАНИЧЕСКОЙ ХИМИИ, 2024, том 69, № 2, с. 143-154 легко ионизируются, образуя NO + , отдавая электрон частицам прекурсора. ...
... Для создания вполне ощутимых зарядов достаточно возникновения одной ионизированной частицы на 10 5 -10 6 выделяющихся молекул газообразных продуктов горения [27]. Исследования процессов синтеза сложных оксидов в реакциях горения [21,23] указывают на возможность управления интенсивностью генерирования зарядов за счет выбора органического компонента прекурсоров, что связано с составом выделяющихся газов, наличием в них ионизированных молекулярных частиц, их количеством и природой. Так, в частности, использование поливинилового спирта приводит к значительно более интенсивному генерированию зарядов по сравнению с глицином [21]. ...
... The properties of complex oxide materials obtained via combustion reactions are influenced by such conditions as process temperature, composition and concentration of the gases formed. One of the important phenomena accompanying the synthesis processes of complex oxides by combustion of nitrate-organic precursors [5,16,19, is the generation of high-density electric charges in the precursors and nanoparticles [45,[56][57][58][59][60][61][62][63]. The possibility of charge formation is associated with the release of ionized molecular groups into the gaseous environment and also with the lability of the oxidation state of transition metal ions which are the part of the precursors and the resulting material. ...
... In many cases, the presence of charges during SCS determines the morphology of the synthesized complex oxides and has a significant impact on powder sintering processes. The temperature interval of intense sintering can be reduced by hundreds of degrees in the presence of high charges [57,59]. The presence of high charges leads to mutual repulsion of nanoparticles, formation of small contact area between them, which significantly increases the reserve of excess surface energy of particles. ...
... Mixing the solutions resulted in the formation of precursors. The ratio of the organic part to the nitrates was stoichiometric (ϕ = 1) according to the combustion reaction [59] with the formation of water, carbon dioxide and nitrogen as the main gaseous products, or exceeded it (ϕ > 1). The following starting reagents (analytical grade) were used: lanthanum (III) nitrate hexahydrate La(NO 3 ) 3 · 6H 2 O, strontium nitrate Sr(NO 3 ) 2 , manganese (II) nitrate tetrahydrate Mn(NO 3 ) 2 · 4H 2 O, glycine (Gly) NH 2 CH 2 COOH, polyvinyl alcohol (PVA) (C 2 H 4 O) n (viscosity of 4 % aqueous solution at room temperature 11 cP, content of residual acetate groups not exceeding 2 %) and polyvinyl pyrrolidone (PVP) (C 6 H 9 NO) n (molecular weight 40000, Sigma-Aldrich, PVP40). ...
... The identically charged precursor particles experience electrostatic repulsion, which minimizes the contact between them, loosens the sample and increases the value of the specific surface area. The increase in the specific surface area, in turn, provides a significant reduction in the temperature of intensive sintering during the production of functional ceramics from compacted complex oxide powders [52]. Based on the obtained characteristics of the synthesis of lanthanum manganites doped with alkali metal ions, it is possible to explain that the samples LaMnO 3 (PVA, ϕ = 4), La 0.9 Li 0.1 MnO 3 (PVA, ϕ = 1, 2 and 4), La 0.9 K 0.1 MnO 3 (PVA, ϕ = 4), La 0.9 Cs 0.1 MnO 3 (PVA, ϕ = 4) have sufficiently high values of specific surface area due to sufficiently high values of maximum potential difference and values of potential difference maintained after synthesis. ...
... Calcium-doped Lanthanum manganite and strontium-doped lanthanum manganite are the most efficient cathode materials reported for SOFCs due to their compatibility with the zirconia electrolyte. To reduce the operating temperature and improve the electroactivity, several researches have reported the A-or B-site partial substitution of lanthanum manganites [26,[34][35][36][37][38]. Particularly, A-site partial substitution of La by other rare earth ions has shown an improvement in the overall performance of the cell by reducing cathode polarization, operating temperature and electrical and ionic resistivity [39,40]. ...
In this work, the Pechini method was used to synthesize La0.7−xLnxCa0.3MnO3,(Ln=ProrSm)-La1−xLnxCa0.3MnO3 type perovskites, evaluating the effect of the type of cation and composition on the structural, morphological and textural properties. The use of similar rare earth cations was studied to promote weak bonds between the reactive surface and adsorbed oxygen species that could facilitate the oxygen reduction reaction and thus improve electrochemical performance of SOFC devices. To achieve this goal, different compositions of Pr or Sm ions (x = 0.1, 0.3, 0.5 or 0.6) were used for the partial substitution of lanthanum at the A site. The results indicated that the substitution of La by Pr or Sm did not modify the original orthorhombic perovskite structure of Ca-doped lanthanum manganites. However, a shift in the main reflections can be obtained as the content of both cations increases due to cell distortion. Rietveld refinement confirms that the crystal structure belongs to the Pnma space group with a large distortion along the b-axis but no secondary phase formation. The compensated charge neutrality of the Mn³⁺/Mn⁴⁺ ratio influences the octahedral sites of MnO6 and cell volume. Orthorhombic distortion (c2<a<b) occurs through Jahn-Teller mechanism promoting a hole-doped system for electrical conductivity. The adsorption-desorption isotherms reveal that in any composition of Pr, a mesoporous isotherm (type IV) is obtained. In contrast, the isotherms changed from micro- to meso-porous features depending on the amount of Sm substitution at the A-site. All the prepared samples showed a soft granular structure with agglomerates ranging between 200 and 300 nm with well-interconnected pores. Comparing the La substitution by Pr or Sm, it was found that Sm can form perovskites that can better promote oxygen vacancies and triple boundary phase formation, which is essential in SOFC devices.
... Among such techniques one can highlight hydrothermal synthesis [28], various types of the solution combustion method [29,30] and sol-gel synthesis [31,32], the co-reduction method [33], spray pyrolysis synthesis [34], micro-emulsion method [35] and others. Particular attention shall be paid to the solution combustion method implemented on the basis of a homogeneous solution of initial reagents and allowing the synthesis of a large number of nanostructured powders, which include simple oxides [36], spinel ferrites [37] and orthoferrites [38], manganites [39], metals [40] etc. An important feature of this technique for produing nanomaterials is the ability to control a wide range of synthesis parameters that ensure the production of pre-ceramic powders with the specified structural, morphological and functional characteristics. ...
Soft LiZnMn ferrites with low coercive force values and small grain sizes were developed by the solution combustion synthesis and low temperature sintering technique for microwave applications at a high frequencies. Bismuth oxide was used as an additive to lower the sintering temperature. The examination covered the influence of Bi2O3 on the crystal structure, microstructure, primary magnetic and dielectric characteristics of lithium-zinc-manganese ferrites. The most promising sample designed for using in microwave devices was produced by sintering at 1075 ℃ temperature for 8 hours with added 1.5 wt. % Bi2O3. These production conditions have yielded 2.98 μm average grain size in a ceramic product, the density is 4.84 cu cm/g, and the coercive force, residual induction, and saturation induction are 58 A/m, 2,078.4 G, and 3,439.1 G, respectively. In addition, this sample demonstrates a high initial magnetic permeability (μi = 168), Curie temperature (Tc = 437.5 ℃), high values of the dielectric loss tangent (tan δɛ = 6.32⸱10-3), ferromagnetic resonance line width (ΔH = 280 Oe) and the resonance line of spin waves (ΔHk = 1.87 Oe). Further increase in the bismuth oxide content allows observing a change in the ceramics microstructure, accompanied by a deterioration in the magnetic and electromagnetic characteristics. Here, the discussion covers the mechanism of change in the functional properties of lithium-zinc-manganese ferrites produced in the conditions of solution combustion with added bismuth oxide. Thus, synthesizing of initial pre-ceramic powder by glycine-nitrate combustion and subsequent low-temperature sintering with added bismuth oxide is a novel efficient technique of producing advanced soft high-frequency LiZnMn ferrites.
... suitable for further sintering of final ceramic samples was successfully obtained. which is a characteristic feature of polycrystalline systems of similar composition [51,52]. ...
Self-propagating high-temperature (SPHT) assisted method was successfully used to obtain soft magnetic ceramics based on the nanostructured powder of multicomponent lithium ferrite Li0.45Zn0.08Mn0.06Fe2.41O4. The influence of various temperature modes of sintering (900-1075 ℃) on the microstructure, morphology, phase composition, and magnetic and electromagnetic properties of the initial powder and final ceramic products was analyzed in detail by methods of scanning electron microscopy, transmission electron microscopy, energy-dispersive and atomic absorption spectroscopy, thermogravimetry, calorimetry, X-ray diffractometry, induction magnetometry and measurement of electromagnetic parameters. It was shown that the particle size of the initial powder is ⁓ 40 nm and depending on the sintering modes, it is possible to obtain lithium-zinc-manganese ferrite ceramics with an average grain size from 0.35 to 2.96 μm and porosity varying from 3.8 to 23.3%. The coercive force, residual induction, and saturation induction of the obtained samples were 51.3-816.6 A/m, 761.40-2131.30 G, and 1234.28-3431.71 G, respectively. It was found that the sample sintered at a temperature of 1075 ℃ for 8 hours demonstrated the most pronounced soft magnetic behavior. The electromagnetic characteristics of the obtained samples also depended on the selected sintering mode and reached their maximum for ceramics sintered at a temperature of 1075 ℃ (ɛ' = 11.52; ɛ" = 6.32⸱10-3 ; tan δɛ = 5.49⸱10-4 ; ΔH = 280 Oe; ΔHk = 1.87 Oe).
The perovskite-type structure has attracted interest in several applied and fundamental areas of solid-state science and advanced materials research. Perovskite ceramics exhibit, among others, a plethora of intriguing optical and electronic properties, where different techniques can characterize their properties. The fabrication process is essential for any device superimposed onto the physical structure. The device characterization deals directly with the finished product and is, thus, directly related to the final goal of all research efforts, namely, to produce a device efficiently. The challenge of the fabrication process lies not only in the synthesis and processing of the samples but also in the characterization methods applied for ceramics, which go hand in hand with the interpretation of data. This chapter reports on the synthesis, processing, characterization, and advanced applications of perovskite ceramics.
Lanthanum manganite doped with strontium samples were synthesized by the combustion method of nitrate-organic precursors of different composition. The combustion process was realized under the influence of an external alternating electromagnetic field and in its absence. It was found that thermochemical generation of charges occurs during precursor’s combustion, recorded as a potential difference of precursor-earth. It is shown that the composition of the initial precursor (organic component and its quantity), as well as the presence of an external alternating electromagnetic field, affect the magnitude of the potential difference that occurs, varying from -7 to 125 V. The relationship between the studied electromagnetic properties (saturation magnetization, coercive force, Curie temperature) of the obtained samples and the precursor–earth potential difference arising during synthesis is shown. The Curie temperature values varied in the range of 38-79°C for samples obtained under the influence of an alternating electro-magnetic field, and 49-104°C in its absence.
Nickel-zinc ferrites are important industrial materials in the production of various types of microwave devices; therefore, studies of new methods of obtaining functional ceramic on their basis are of great interest at present. In this work, soft magnetic ceramics based on Ni0.4Zn0.6Fe2O4 spinel ferrite with low values of the coercive force were successfully obtained under various sintering modes (1000 and 1100 °C, holding time – 16 hours) based on nanostructured pre-ceramic ferrite powder synthesized by the solution combustion method. The initial powder and sintered ceramics were investigated by EDX, SEM and PXRD methods. The electromagnetic parameters of the final product were investigated by vibration magnetometry and using the method of rectangular waveguide transmissions in the X-band. It was shown that, depending on the selected sintering mode, it is possible to obtain magnetic ceramics with an average grain size in the range from 1 to 3 µm and with values of the coercive force (Hc) from 16.32 to 19.41 Oe, remanent magnetization (Mr) from 3.39 to 4.31 emu/g and saturation magnetization (Ms) from 67.90 to 78.42 emu/g. After the preparation of a ferrite-polymer composite with different content of Ni0.4Zn0.6Fe2O4 (0–50 wt%), it was found that the highest absorption characteristics of electromagnetic waves were observed for the sample with 40 wt% of spinel ferrite obtained at 1100 °C, 16 hours sintering mode.
Nanocrystalline LiZnMn-ferrite powders were synthesized by solution combustion at various glycine-nitrate ratios (G/N = 0.2, 0.4, ..., 1.4). Elemental analysis showed that the composition of the obtained samples corresponds to the ferrite-spinel Li0.45Zn0.05Mn0.06Fe 2.43O4. Scanning electron microscopy showed that the compositions possess a porous microstructure and a developed surface; x-ray phase analysis attests that the synthesized LiZnMnFe-spinel is single-phase and possesses a high degree of crystallinity (to 99%). It was shown that the magnetic and dielectric characteristics of the obtained samples systematically change depending on the redox ratio of the reaction mixture and reach maximum values at stoichiometric value G/N = 0.4-0.6.
Abstract: The presence of the generation of electric charges in precursors during the synthesis of nanosized complex oxides in the combustion reactions of compositions including metal nitrates, as
well as glycine and glycerin as organic components, is shown. To measure the resulting potential difference, an automated system based on the IPEP-1 electrostatic field parameter meter and a signal amplification system were used.
Keywords: Nanosized complex-oxide materials, combustion synthesis, organic-inorganic compositions, charge generation, measurements.
A strong electric field formed during the initial stage of the combustion of single Zr, Ti, Fe, and Ni particles. The electric
field lasted for 20-400 ms and decayed before the particle temperature reached its maximum. A low voltage was generated during
the combustion of particles initially surrounded by a thick oxide film or when the ambient oxygen concentration was low. A
decrease in the rate of oxygen transport to the reaction zone generated a bipolar signal during the combustion of Zr and Ti
and/or electric oscillations of 0.5-10 Hz. Melting of either the reactants, or intermediate or final products annihilated
the electrical field. The maximum voltage and current were attained for particles of ∼0.8 mm diam. The largest unipolar electric
voltage and current were produced during the combustion of either a Zr or a Ti particle (∼2 V and ∼100 mA). A rapid increase
in the rate of temperature rise in Zr and Ti particles followed the annihilation of the electric field. It may have been caused
by Joule heating following electric breakdown through the oxide film. A shift from homogeneous to relay-race combustion occurred
upon increasing the distance between particles in a row. This shift affected the qualitative features of the generated electric
field as well as the temperature at which it formed. © 2003 The Electrochemical Society. All rights reserved.
The temperature behavior of bismuth orthoferrite nanoparticles obtained by the glycine-nitrate combustion method was studied by high-temperature X-ray diffractometry and complex thermal analysis. The region of stability of the material in a single-phase state was found. It was shown that the nanocrystalline BiFeO3 did not undergo decay in the temperature interval 550–780 °С. In this temperature interval, we have obtained the nanocrystalline material with average crystallite sizes 40–90 nm and average particle sizes 100–150 nm the sizes of which depend on temperature. The features of formation of BiFeO3 and the process of their sintering were studied. Results show that the crystallite growth slowed down after the amorphous phase disappeared. The sintering of nanopowder became more intense in the temperature interval 600–700 °С, but no noticeable increase in the crystallite sizes occurred. The magnetic behavior of obtained material was also discussed. It was found to be consistent with the concept of violation of the cycloidal magnetic order in bismuth orthoferrite.
The paper presents the results of the experimental study of the influence of an external electrostatic field on the mass burning rate and flame temperature of PMMA, SKD-2, SKN-26, SKMS-30, SKEPT localized in different parts of the combustion zone.
Glycine-nitrate combustion method was used to obtain powders based on CeFeO3 nanocrystals with average crystallite size in the range from 33 ± 3 to 51 ± 5 nm. The influence exerted by parameters of the glycine-nitrate combustion process and, in particular, by the glycine-nitrate ratio (G/N) on the composition and crystallite size of the synthesis products was determined. The optimal G/N ratio at which nanocrystalline cerium orthoferrite is formed with the minimum amount of impurity phases Fe2O3 and CeO2 was found to be 0.8. It was demonstrated that the composition of the starting solution affects the nature of the phase heterogeneity in the resulting product, crystallite size, and porosity of the nanocrystalline powders being formed. The patterns determined in the study make it possible to optimize the technology of nanocrystalline powders based on CeFeO3 in order to obtain powders with prescribed phase composition and crystallite sizes to enable their use as a basis for photocatalytic materials.
Yttrium orthoferrite nanocrystallites with hexagonal and orthorhombic structures were obtained directly by the glycine-nitrate synthesis. The nanocrystallites have plate-like morphology and are strongly agglomerated in highly porous structures, as was shown by the TEM investigation. The influences of the synthesis conditions on the yttrium orthoferrite crystallization, its nanocrystallite size and morphology are discussed.
Nanosized yttrium orthoferrite was obtained by the heat treatment of an X_ray amorphous precursor prepared by the glycine–nitrate combustion process in orthorhombic and hexagonal phases with average crystallite sizes of 27–33 nm and 6–11 nm, respectively. The processes controlling YFeO3 formation under the specified conditions were shown to be the decomposition of yttrium carbonate and yttrium oxycarbonate. The transition from the metastable hexagonal phase to the stable orthorhombic phase in YFeO3 was shown to occur when the hexagonal YFeO3 crystallites reach sizes of 15 ± 5 nm. The YFeO3 nanoparticle size distribution was analyzed to suggest a phase transition scheme.
Nanopowders based on MgFe2 + δO4 + 1.5δ (δ = 0.1–8.0) Mg-Fe spinels with an average crystallite size from 45 to 60 nm have been prepared by glycine-nitrate combustion synthesis. As a result of the high combustion temperature (near 1330°C), the phase composition of the synthesized powder corresponds to the equilibrium one.
Glycine-nitrate combustion method was used to obtain nanopowders of orthorhombic and hexagonal yttrium orthoferrite with particles sizes in the ranges from 30 to 53 nm and from 6 to 14 nm, respectively. It is shown that the structure and size of the particles are determined by the process temperature, which depends on the relative amounts of glycine and nitrates in the initial mixture. It is shown that yttrium orthoferrite nanopowders with a certain crystal structure and particle size can be obtained by varying the glycine/nitrate ratio. The nanopowders and materials on their basis can be used as photocatalysts.
Single phase BiFeO3 nanoparticles have been successfully synthesized for the first time by a novel citrate combustion method without using any solvent. Well mixed metal nitrates along with citric acid which is used as fuel combust to give BiFeO3 nanoparticles after annealing. These particles are single phase in nature and crystallize in the rhombohedral distorted perovskite structure (space group-R3c) which has been confirmed by the Rietveld refinement of the room temperature powder x-ray diffraction data. Nearly spherical particles of average particle size 47 nm have been seen from transmission electron micrograph. Room temperature magnetic hysteresis measurement shows weak ferromagnetism though the magnetization does not saturate upto 1.75 T applied field. The coercive field value is calculated to be 180 Oe which is 3 times higher than that prepared by solvent free combustion method using glycine. 57Fe Mössbauer spectrum can be fitted with a sextet corresponding to single magnetic state of hyperfine field about 49.5 T corresponding to Fe3+ state of the iron atom. The dielectric relaxation and ac conductivity as a function of frequency have been discussed. High dielectric permittivity has not been found in these nanoparticles like other reported BiFeO3 ceramics.
Recent experimental studies on generation of electric voltage during combustion synthesis of zinc sulfide (ZnS) have revealed high-voltage signals (4 V) with a duration of around 1 s, which is at least four times higher than those produced by the gas-solid and solid-solid combustion reactions studied previously. These data have raised the question about mechanism of the phenomenon. In this work, we developed a novel (distributed) model describing the generation electric potential during combustion synthesis of sulfides (CSS). Simulations of the evolution in the behavior of charge carriers and voltage profiles suggest that it is the emission of negatively charged sulfur ions as well as electrons that has a strong impact on the amplitude and temporal evolution of combustion-induced voltage.
The temperature and magnetic-field dependences of the magnetocaloric effect (MCE) in silverdoped manganites La0.9Ag0.1MnO3, La0.8Ag0.2MnO3, and La0.8Ag0.15MnO3 have been studied. Giant variations in the temperature of samples have been observed in response to a magnetic field charge by δH = 26 kOe. The MCE peaks in these compounds are observed in the vicinity of room temperature. A relationship between the sample preparation conditions and the magnetocaloric characteristics of samples has been established.
A model explaining and predicting generation of a temporal electric potential during nitridation of a single metal pellet has been developed. The model takes into account the kinetics of defects formation and assumes that the rate of the chemical reaction can be described by the shrinking-core process. The model simulations have shown that time scale of the generated electric potential depends on both the initial nitride shell thickness and heat removal from the particle surface. At thin initial shell and low rate of heat removal the maximum of the surface electric potential is attained before the temperature and surface nitrogen concentration have reached their maximums but after the maximum of nitridation has appeared. Quasi-neutral distributions of metal vacancies and electron holes are formed at the maximum temperature. At thick initial shell and/or high rates of heat removal from the particle surface the potential maximum may be observed much later: after the maximum temperature has been achieved. Correspondingly, non-equilibrium concentrations of the charged defects exist till the end of nitridation. In contrast to oxidation the nitrogen adsorption rate constants (the activation energy and pre-exponent) have negligible effect on the surface potential form and amplitude. At the ignition limit the rate of nitridation is proportional to the power of −1/2 for the ambient nitrogen pressure in the proposed scheme of defects formation. Metal vacancies and electron holes are the main charged defects in nitrides during nitrogen combustion. The nitride formation is limited by transfer of the vacancies in nitride.
The low-temperature minimum of the resistivity of La0.85Ag0.15MnO3 is investigated in detail. Analysis of the experimental data shows that the observed low-temperature minimum of the zero-field resistivity and the large magnetoresistive effect, which increases with decreasing temperature, can be explained in a model of spin-polarized tunneling of charge carriers through grain boundaries.
An investigation of the critical behavior of the specific heat of the manganites La1-xAgxMnO3 ×(x=0.1,0.15,0.2) near the Curie temperature is carried out. The behavior of the universal critical parameters near the phase transition point is established. All of the samples studied correspond to the ferromagnetic Heisenberg 3D universality class of critical behavior, with the critical exponent alpha=-0.115, -0.106, and -0.106 for La0.9Ag0.1MnO3, La0.85Ag0.15MnO3, and La0.8Ag0.2MnO3, respectively. It is shown that the universality class of the critical behavior of the specific heat of the manganites La1-xAgxMnO3 is independent of the silver concentration.
The Sr2Ni0.7Mg0.3MoO6-δ double perovskites synthesis has been performed by a pyrolysis of organic-nitrate compositions with various ratios of ϕ (glycerol- or glycine content) and R (ammonium nitrate content). The solution combustion study revealed that the redox mixture composition for the Sr2Ni0.7Mg0.3MoO6-δ synthesis influenced the pyrolysis temperature, thermochemical charge generation, particles size, phase composition and physicochemical properties of the final oxide material. Investigations of thermodynamic stability, thermal expansion and electrical conductivity demonstrated that the Sr2Ni0.7Mg0.3MoO6-δ sample, obtained via pyrolysis synthesis with glycine triple excess without exothermic additive, may be considered as a prospective anode material for intermediate-temperature solid oxide fuel cells. It was confirmed by the sample stability in oxidative and reducing atmospheres, optimal values of linear coefficient of thermal expansion (14.4·10⁻⁶ K⁻¹ in air; 13.1·10⁻⁶ K⁻¹ in 50% H2/Ar) and the highest electrical conductivity value (0.53 S cm⁻¹ at 800 °C in air) among glycine-samples.
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 (φ) 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 φ = 0.6 vs ε = 4.0 at φ = 1.4 using urea as fuel) could be produced by the SCS method.
A high dielectric permittivity perovskite (Na1/3Ca1/3 Bi1/3Cu3Ti4O12) or NCBCTO was successfully synthesized by three different routes: solid state (SSS), solution combustion (SCS) and polymeric precursor (PPS). Wide angle x-rays (WAXD) and scanning electron microscopy (SEM) were used as control characterization techniques. A pure NCBCTO phase was obtained straightforward by the SSS method; by the SCS synthesis, however, only after a calcination at 800 °C was the resultant powder converted into the pure NCBCTO. The PPS samples maintained the same initial level of impurities (probably CuO), even after the calcination step. Further sintering of all the samples at 1000 and 1080 °C and characterization of their microstructures by SEM, their density and porosity by immersion tests and dielectric permittivity by impedance spectroscopy were done. Grain size increase and porosity decrease with the increase of the sintering temperature were observed. The samples produced by the PPS method, calcined at 800 °C and sintered at 1080 °C displayed the highest dielectric permittivity (above 20 000), but also the highest dielectric loss (tan δ = 0.35) at 1 kHz. However, the highest ε’/tan δ ratio (160 400 at 1 kHz, with ε’ = 9500 and tan δ = 0.059) was observed in samples produced by the SCS route, calcined at 800 °C and sintered at 1080 °C; these samples had also the smallest and sharpest grain size of all the samples. Thus, this last material was considered the best perovskite to be used as a high k ceramic in dielectric nanocomposites.
A series of Fe-doped ZnO (Zn1-xFexO, x =0, 0.01, 0.03, 0.05, 0.07 and 0.10) samples were synthesized by solution combustion synthesis method using metal nitrates as oxidizer and glycine as fuel. A spongy network-type microstructure is observed by the SEM micrographs of all the synthesized samples. For Fe concentration lower than 0.05 (x ≤ 0.05), formation of phasepure wurtzite (hexagonal) structured Zn1-xFexO powder with P63mc space group was confirmed from the X-ray diffraction results. However, for x 0.07, precipitation of ZnFe2O4 impurity phase was observed. Hence, the solubility limit for Fe in ZnO lattice is about x = 0.05 for the samples synthesized by solution combustion method. Two quadruple doublets observed in the 57Fe-Mössbauer spectra for each of the samples suggest that paramagnetic Fe3+ cations occupy at two different lattice sites in the ZnO structure: (1) the substitutional sites without distortion of the surrounding structure and (2) with distortion due to the defects present in the surrounding structure. In the DRS spectra, an Urbach-like tail was observed in the band gap region, indicating that Fe-doping in the ZnO lattice modifies the electronic structure and enhances the absorption of visible light. Furthermore, the Kubelka-Munk plots suggest the presence of two different local structures validating the Mössbauer results. We studied the photocatalytic degradation of the methylene blue dye using these Fe doped ZnO samples as catalysts and revealed that presence of Fe could lead to formation of carbonaceous material on the surface the solution combustion synthesized Zn1-xFexO samples. Overall, our results demonstrate the structural characteristics of Fe in Fe-doped ZnO samples synthesized by solution combustion method.
Di-phase 0.66BaTiO3–0.33Co0.8Zn0.2Fe2O4 ceramic composites with randomly mixed phases have been obtained by conventional sintering method (CM) and Spark Plasma Sintering (SPS). The impact of sintering method on the structural, microstructural and functional properties is comparatively analysed. The dielectric properties are dominated by the BaTiO3 phase, with contributions from the interfaces mostly on the dielectric relaxation properties, while the ferrimagnetic character of the composite is derived from the ferrite phase as a sum property. Both CM and SPS ceramics show bi-tunable character, i.e. their permittivity can be modified by the application of both electric and magnetic fields. A high dielectric nonlinearity is mostly characteristic to the ceramics with larger grains. The electrical tunability tends to reduce by the application of a magnetic field an almost is cancelled in the SPS ceramic. The two types of ceramics show opposite signs of their magnetocapacitance properties, indicating that they have different origins. A microwave filter realised with these two types of ceramics have been tested. With increasing magnetic field from 0 to 1.9 kOe (above the magnetic coercive field), the resonance frequency can be up/down shifted, as result of the positive/negative magnetoelectric coupling.
Polycrystalline La0.85Sr0.15MnO3:Agx (LSMO:Agx,0≤x≤0.5) ceramics were synthesized using sol-gel procedure. All specimens showed rhombohedral perovskite structures in Rc3‾
space group. As the amount of Ag increased, grain size of ceramics increased, and resistivity accordingly decreased. Large temperature coefficient of resistivity (TCR) of 22.9% K−1 was obtained at room temperature (289.2 K) for x=0.4. Interestingly, analyses of resistivity-temperature (ρ-T) experimental data showed that conduction mechanism over entire temperature region perfectly matched theoretical percolation model. This reveals that electrical transport properties of LSMO:Agx ceramics are mainly affected by domains, grain boundaries (GBs), and Kondo-like spin-dependent scattering. It also demonstrates the coexistence of paramagnetic (FM) and ferromagnetic (PM) phases in LSMO:Agx ceramics. The ρ-T and scanning electron microscopy (SEM) results confirmed that the introduction of Ag weakened these scattering effects.
A series of cubic perovskite phase BaCoxFe0.7-xZr0.3O3–δ(0.2 ≤ x ≤ 0.5) (BCxFZ) cathode powders are synthesized and their cobalt to iron ratios is varied to explore the best performance on the cell. The experimental results show that as the cobalt content increases, the conductivity and thermal expansion of the cathode samples increase. The density functional theory (DFT) calculation suggests the Oxygen vacancy formation energy continue to decrease but the hydration energy decreases at first and then gets worse. As a result, in BaZr0.3Ce0.5Y0.2O3–δ (BZCY)-based cell applications, the maximum power output of the cell reaches 679 mW/cm² when the cobalt content is 0.4, and the impedance value is 0.067 Ω cm² in this ratio.
The present work reports the effects Mn site doping on the structural, magnetic and magnetotransport properties of Nd0.67Sr0.33Mn0.9T0.1O3, where T = Mn, Fe, Cr and Ni. Structural analysis based on Rietveld refinement confirms that the samples crystallize in orthorhombic structure with Pbnm space group. A detailed investigation on the FTIR spectra hints to the improved metallic character of Nd0.67Sr0.33Mn0.9Mn0.1O3 and Nd0.67Sr0.33Mn0.9Ni0.1O3 samples. Strong ferromagnetic character of Nd0.67Sr0.33Mn0.9Mn0.1O3 and Nd0.67Sr0.33Mn0.9Cr0.1O3 is revealed from ZFC-FC magnetization studies, while the Fe and Ni doped samples possess strong AFM ground state. An irreversible metamagnetic transition is discernible from the hysteresis curve of Nd0.67Sr0.33Mn0.9Fe0.1O3. Transport studies reveal a charge ordered state in Fe doped sample close to its spin glass transition temperature, Tg = 65 K. Nd0.67Sr0.33Mn0.9Mn0.1O3 behave like a typical double exchange material with nearly same TC and TP values, whereas for all other samples TP lie much below TC. Low resistive state materialized in a strong AFM material as observed in Nd0.67Sr0.33Mn0.9Ni0.1O3, is rarely observed in manganites. Colossal MR values exhibited by Fe and Ni doped samples are understood based on phase separation scenario of manganites. Large dissimilarities evident in the magnetic and electronic properties of the compounds, though the dopant ions possess nearly same ionic size, are attributed to differences in their electronic configuration. Resistivity behavior of samples in different temperature regimes are comprehended based on existing models of conduction.
The key features and challenges of the use of electrophoretic deposition for the formation of functional layers of solid oxide
fuel cells are considered. Theoretical models and experimental results of the studies of electrophoretic deposition are
presented. The analysis covers the physicochemical deposition mechanisms, methods for preparing suspensions and
conditions necessary for obtaining thin-film electrode and protective single- and multi-layers with both dense and porous
structure for solid oxide fuel cells. The prospects of theoretical simulations of the method and its potential practical
applications are evaluated.
This work elaborates the effects of monovalent doping on the structural, magnetic and magnetotransport properties of La0.833R0.167MnO3, where R = Li⁺, Na⁺, Ag⁺ and K⁺. Rietveld refinement of XRD profiles reveal that the compounds crystallize in rhombohedral structure, with the rhombohedral distortion angle (α) exhibiting an inverse relation with A-site ionic radius (˂ rA˃). A detailed investigation on room temperature Fourier transform infrared spectra (FTIR) of samples comprehends the evolution of Mn-O bond vibrations across the metal-insulator transition temperature (Tp). Surface morphological studies done by field emission scanning electron microscopy (FESEM) confirm the presence of well grown grains in the samples. The magnetization studies on La0.833Li0.167MnO3 indicate that the sample exhibits a magnetically frustrated spin glass state with irreversible metamagnetic transition at high magnetic fields. On the other hand, Na Ag and K doped samples exhibit a soft ferromagnetic nature, with faster saturation at lower fields. Magnetotransport studies indicate that Li doped sample possess a charge ordered state close to its spin glass transition temperature, Tg = 113 K. Double resistivity peaks exhibited by La0.833Na0.167MnO3, La0.833Ag0.167MnO3 and La0.833K0.167MnO3 are discussed based on magnetic phase separation inherent in these samples. The transport and magnetic studies of the present samples contradict the widely accepted notion that Curie temperature (Tc) and Tp in manganites closely follow ˂rA˃ values. Fitting the resistivity data of samples to various theoretical models reveal the nature of conduction mechanisms realized in different temperature regimes. High resistivity of La0.833Li0.167MnO3 signals that the charge carriers are in a highly localized state. This is attributed to its low ˂ rA˃ and the consequent structural distortions.
Using glucose (C6H12O6) and copper nitrate (Cu(NO3)2) as fuel and oxidizing agent respectively, CuO/Cu2O/C composites with different carbon contents were successfully prepared by the solution combustion synthesis method. The as-obtained CuO/Cu2O nanoparticles exhibited uniform spherical morphology and, by changing the amount of fuel and ambient temperature, carbon was synthesized in-situ with content ranging from 3 to 36 wt%. The electrochemical performance of the CuO/Cu2O/C anode in Li-ion batteries was investigated systematically, demonstrating >400 mAh g⁻¹ capacity at 20 mA g⁻¹ current density and highly stable cycling performance with capacity 260 mAh g⁻¹ after 600 cycles at current density 0.2 A g⁻¹. This performance is attributed to the synergistic effect of anodes porous structure, conducting carbon coating and two-component CuO/Cu2O structure. Owing to the inexpensive and facile solution combustion synthesis method and the resulting high electrochemical performance, the CuO/Cu2O/Carbon composite is a promising anode material for application in Li-ion batteries.
The perovskite-type LaFe0.5Ni0.5O3 belonging to the rhombohedral (space group R-3c) crystal structure has been synthesized for which we have identified a magnetic transition at T1 = 8 K corresponding to the minimum observed in the derivative of temperature dependent magnetization. A bifurcation in the ZFC and FC curves is observed below T1 that suggests a frustrated magnetic behavior. The non-zero moment above T1 hints the possibility of the presence of a high-temperature magnetic transition in the material. The resistivity of LaFe0.5Ni0.5O3 evolves as a function of temperature similar to that of a semiconductor. Mott's variable range hopping governs the conduction mechanism of the material. Presence of various anisotropy terms and inhomogeneous magnetic interactions lead to the presence of antiferromagnetic and ferromagnetic interfaces, which eventually causes a magnetic exchange bias and magnetic hysteresis in resistivity. We have also observed direction dependent magnetoresistance in the material.
Defect spinel Fe3-δO4 samples doped with high-valent Mo and V cations have been prepared by sol-gel auto-combustion rout with increasing concentrations of Mo and V, respectively. Energy Dispersive X-ray fluorescence (EDXRF) and Attenuated Total Reflection Fourier transform Infrared (ATR-FTIR) spectroscopies have verified the compositions of the samples and the oxidation states of the metal cations. Synchrotron Radiation X-ray Powder Diffraction (SR-XRPD) patterns were measured at MCX beamline of ELETTRA synchrotron. Structure and microstructure of the samples were studied by the Rietveld method. Increasing densities of vacancies were found to be generated by the high-valent Mo and V cations which were correlated to the lattice parameter, crystallite size and microstrain of each sample. The hysteresis loops were measured at room temperature using the Vibrating Sample Magnetometer (VSM) and good agreement was found between the Fe-cation distributions revealed from Rietveld adjustments and those predicted by the saturation magnetization (Ms) measurements. The High-Resolution Transmission Electron Microscopy (HRTEM) confirmed the proposed isotropic crystallite size model implemented in the Rietveld adjustments. Finally, the results presented herein show agreement with previous works on iron oxide materials used as intercalation cathode for Li-ion batteries and predict better performance for the current Mo- and V-doped defect spinel.
Double perovskite Sr2Ni0.75Mg0.25MoO6-δ powders were synthesized by the combustion method using systems containing nitrates and varying amounts of the following organic components: glycine, glycerol, and polyvinyl alcohol. The characteristics (temperature, gas composition, etc.) of the synthesis process were studied and the optimal conditions for obtaining single-phase samples were determined. It was established that the usage of one-and-a-half excess of glycerol as organic component provides the formation single-phase complex oxide. The crystal structure of the obtained complex oxide, which is of interest as an anode material for solid oxide fuel cells, was refined and its physico-chemical properties (specific surface area, particle size, electrical conductivity, and catalytic activity in the reaction of methane oxidation) were investigated.
Magnetite (Fe3O4) nanoparticles have been readily prepared via a one-step solution combustion synthesis (SCS) method by designing a simple airless device: a beaker with perforated rubber plug could not only separate the outside air but also release the gases generated during the combustion reaction, which could ensure an air free condition in the SCS process. The whole process did not involve any toxic or unavailable reagents, and could be finished in a few minutes by its self-generated energy derived from the redox reaction between glycine (fuel) and ferric nitrate (oxidizer). Innovatively, the combustion reaction mechanism, morphology and microstructure, phase composition and magnetic properties of SCS products in relation to the glycine have been systematically investigated. The results revealed that with the increasing molar ratio (ϕ) of glycine to ferric nitrate, the combustion mode varied from self-propagating combustion to smouldering combustion and the average grain size of SCS products increased in nanometer scale. On the contrary, the iron oxidation state of SCS products decreased with the increase of ϕ value, and the oxide phase changed from α-Fe2O3 to Fe3O4 and then to FeO sequentially. It was noteworthy that when ϕ = 0.7, we could easily obtain pure phase Fe3O4 nanoparticles with the highest saturation magnetization of 89.17 emu g⁻¹ and small average grain size of 57.3 nm, which would have great potential for various applications, such as magnetic drug delivery, magnetic data storage and novel ferrofluids.
La1–xAgxMnO3 ± y (x = 0-0.3) mixed oxides have been synthesized by the pyrolysis of polymer–salt compositions using different organic compounds and different salt: organic compound ratios. The correlation between the reaction medium temperature during pyrolysis, the composition of the resulting oxide, and synthesis conditions has been investigated. The effect of these conditions on the character of the pyrolysis process, on the phase composition and microstructure of the resulting oxide particles and metallic silver, and on their mutual distribution is reported. The catalytic properties of the synthesized oxides in methane and soot oxidation are considered, and a correlation is established between the catalytic activity of the oxides and the synthesis conditions.
The gel formed in the synthesis of Mg(Fe0.8Ga0.2)2O4 powder from corresponding metal nitrates and glycine was studied by thermal and IR spectral analyses. Being a bidentate ligand, glycine was established to initiate the gel combustion reaction resulting in the oxide of the above composition. Based on the data on ΔH298° and Cp of the reagents and reaction products, the thermodynamic parameters of this process were calculated.
Solution-combustion synthesis (SCS) of nanoparticles was characterized by the temperature effect (ΔTad) calculated upon neglect by the temperature dependence of heat capacity. Thus calculated ΔTad values were found to be a linear function of the inverse radius of metal ions. Our calculations have shown that SCS reactions may yield not only oxides but also hydroxides and carbonates. Suggested was a simple formula for evaluating the ΔTad values attained in SCS of complex oxides.
Heterogeneous combustion (or self-propagating high-temperature synthesis (SHS)) is widely used for making
inorganic materials. SHS in such systems is accompanied by generation of electric potentials, which appear between the combustion wave front and condensed products of combustion as a result of chemical ionization of starting materials and intermediate products (so-called Electro Motive Force of combustion - EMF). Maximal registered EMF signals in SHS now having amplitude more than 2 Volts. Experiments were carried out for ions of the starting reagents as well as ions of intermediate and final products. Control of the processes may be carried out by using external electric and magnetic fields. During SHS-process intermediate reaction products arise in an ionic form. In condensed media SHS processes involve an electrochemical stage, which can be considered to be a concentration cell, that moves together with the combustion wave front and result in origin of EMF of combustion. As a result of that, the full ion-diagrams of combustion processes have been constructed for many complex oxide systems with the elements of I-VIII groups of Periodic Table. It is opening the wide horizons for the diagnostics of heterogeneous combustion processes at micro and nano-level. Besides that, as a result of our experiments, preconditions for the new directions in combustion science have been formed, such as ionic chemistry of heterogeneous combustion and dynamic ionography of heterogeneous combustion processes. External electrical and magnetic fields modify combustion parameters of the SHS systems (including EMF).
The methods of X-ray diffraction (XRD) and scanning electron microscopy (SEM) are applied to study the processes of the formation, phase composition, morphology of particles, and phase distribution in composite layers of a nanostructured catalyst system consisting of highly porous cellular metal (nickel foam), an intermediate layer, and a complex oxide coating. The optimal conditions are selected for the formation of an intermediate layer of NiO and a catalytic coating of La0.75Me0.25MnO3 (Me = Ag, Cs, or Sr). Complex oxides are synthesized by the pyrolysis of polymer–salt compositions. The chemical composition of the coatings is monitored using inductively coupled plasma spectrometry; the specific surface area of samples is measured by low-temperature nitrogen adsorption.
A finely dispersed powder of composition Mg(Fe0.8Ga0.2)2O4 was prepared by glycine-nitrate method. The use of glycine as a reducing agent allows preparation of the product without carbon-containing admixtures with unimodal particle size distribution and lower crystallization temperature (as compared with nitrate-citrate method of Mg(Fe0.8Ga0.2)2O4 synthesis) immediately after reaction completion.
It is found that nano- and microcrystals of a number of compounds grown in ion-free vapor of high supersaturation acquire electric charge. The electrophoretic mobility of these crystals changes, as is demonstrated by the data concerning aerosols formed in cooling the vapor above boiling melts of a number of substances (boiling temperature, 500–1800 K) placed into a 300 K carrier gas. Nano- and microparticles assembled into aggregates are found in flows of the cooled vapor. It is found that the application of an electric field with an intensity of 150–1000 V/cm induces the movement of particles toward the negatively charged electrode at a rate of 5–15 cm/s, the speed of their movement being related linearly to the field intensity and corresponding to the presence of ∼103 charge carriers per every particle. It is established that the experimental data are in accordance with the assumption that the growth of each particle results in the decomposition of vapor molecules adsorbed by the particle into ions with the nonequivalent liberation of cations and anions into vapor; i.e., an adsorption-ionization-desorption route for the charging of growing particles is discovered.
The effect is considered exerted by synthesis conditions and composition of complex oxide materials ABO3±y based on d metals (Mn, Co, etc.) with perovskite structure on their catalytic activity in CO and n-heptane oxidation. The possibility of using such an empirical parameter as the content of active oxygen for prognosticating catalytic properties is evaluated. Neutron diffraction analysis was applied to determine the structure of the complex oxides and characteristics of particles of these materials.
The processes involved in the solution combustion synthesis of α-Al2O3 using urea as an organic fuel were investigated. The data describing the influence of the relative urea content on the characteristic features of the combustion process, the crystalline structure and the morphology of the aluminium oxide are presented herein. Our data demonstrate that the combustion of stable aluminium nitrate and urea complexes leads to the formation of α-alumina at temperatures of approximately 600–800 °C. Our results, obtained using differential thermal analysis and IR spectroscopy methods, reveal that the low-temperature formation of α-alumina is associated with the thermal decomposition of an α-AlO(OH) intermediate, which was crystallised in the crystal structure of the diaspore.
The glycine nitrate process has been successfully employed to prepare nanosized, porous, stoichiometric, homogeneous CuCrO2 powders without ambient control. In this method, a precursor solution was prepared by mixing glycine with an aqueous solution of blended (Cu–Cr) metal–nitrates in their stoichiometric ratios. The glycine-mixed precursor solution was first heated in a beaker to evaporate excess water for forming a viscous bluish semi-transparent gel. The beaker was then covered with a metallic mesh, and the temperature increased slowly to 170°C to auto-ignite the material. The combustion was self-sustaining and very rapid, producing gray colored powders. The as-prepared powders were nanosized (∼20nm) into a spherical shape and crystallized in a delafossite structure. The powders showed a very large surface area of 30.92m2/g, as determined by BET surface area measurements. The SEM/TEM studies on these powders confirmed their nanosized nature and porous structure.
Ce1 − x
Lnx
O2 − d
(Ln = Sm, Pr; x = 0–0.30) oxides have been prepared through pyrolysis of polymer-salt systems, and their particle size and specific surface area have been determined. Their sintering behavior and thermal expansion have been studied using dilatometry. The thermal expansion of the Pr-containing materials is shown to vary considerably with temperature. X-ray diffraction data (obtained at various temperatures and oxygen pressures) suggest that the Ce1 − x
Prx
O2 − d
oxides undergo a low-temperature phase transition, presumably a reversible decomposition of one solid solution to two others. The structural parameters of the phases involved and the oxygen nonstoichiometry of the samples are determined.