Modified Pechini Process Research Articles

Synthesis and Electrochemical Performance of High‐Entropy Spinel‐Type Oxides Derived from Multimetallic Polymeric Precursors

High‐entropy spinel‐type oxides are synthesized by a modified Pechini process, wet chemistry approach, and solid‐state synthesis method and characterized as anode materials for Li‐ion batteries. The Pechini process that involves chelation and polyesterification reactions facilitates the formation of high‐entropy spinel‐type oxides without compositional segregation at ≈600 °C as confirmed by in situ and ex situ XRD. XAFS analysis and the Rietveld refinement of room‐temperature neutron diffraction data suggest the composition (Mn0.05Fe0.48Co0.47, tetrahedral)(Cr0.61Mn0.52Fe0.11Co0.09Ni0.68, octahedral)O4 for phase‐pure specimens. Compared to high‐entropy spinel‐type oxides synthesized by the solid‐state method, the precursor‐derived materials demonstrate higher specific capacity as anodes, in which the materials without citric acid addition exhibit low capacity fading at high current densities and maintained a capacity of ≈200 mAh g−1 after 1000 cycles. The generation of a rock‐salt‐type phase during cycling is confirmed for the first time by in situ charging–discharging XRD. The charging–discharging of this anode material is achieved mainly through the embedding–disembedding of lithium ions in the lattice of the generated rock‐salt‐type phase.

Synthesis of high-entropy carbides from multi-metal polymer precursors

Three compositions of high-entropy carbides, (TiHfVNbTa)C, (TiZrHfNbTa)C and (TiZrNbTaW)C were synthesized via a modified Pechini process, in which citric acid served as both a cation chelating agent and a carbon source. Through pyrolysis and spark plasma sintering, single phase high-entropy carbides were formed from the homogeneous precursors at a relatively low temperature of 1800 °C. The dispersion of cations in the polymer precursor facilitated shorter diffusion distances in polymer-derived materials, and thus compositional homogeneity was significantly improved relative to materials produced by a solid-state method, as quantified by a defined coefficient of variation applied to energy-dispersive X-ray spectroscopy. A finer microstructure in polymer-derived materials results in improved fracture toughness with a KIC value of 4.29 MPa·m1/2 achieved for (TiHfVNbTa)C.

Structure, optical properties and preferential site substitution of Eu3+ activated Ca8NaBi(PO4)6F2 red emitting phosphors prepared by modified Pechini process

Eu3+-doped and undoped Ca8NaBi(PO4)6F2:xEu (x = 0.01–0.5) apatite phosphors have been synthesized at 900 °C by the modified Pechini process. Their structural and morphological characteristics were determined by XRD, TEM and Raman spectroscopy. The structure of the host matrix was refined by the Rietveld method: Ca8NaBi(PO4)6F2 crystallizes in hexagonal class, sp. gr. P63/m, a = b = 9.3980 (6) Å, c = 6.9039 (9) Å and γ = 120°). The electronic structure was simulated revealing an indirect bandgap of 3.46 eV Eu3+ ions in the host matrix replace for the Bi3+ ones in two types of sites: M1 (C3 symmetry and IX-fold oxygen coordination) and M2 (CS symmetry and VII-fold coordination by one fluoride and six oxygens). The doping induces a redistribution of Eu3+ ions over the M1 and M2 sites in favor of the former species. The transition probabilities for Eu3+ ions in M2 sites were determined using the Judd-Ofelt theory yielding a radiative lifetime of 2.21 ms. It was found that phosphors exhibit relatively weak luminescence quenching. The CIE 1931 color coordinates for the compound with x = 0.35 Eu are (0.648,0.352) falling in the red region and corresponding to a correlated color temperature of 2919 K and a color purity of >96%. The thermal stability of the phosphor was studied until 400 °C (the determined activation energy is 0.70 eV) indicating a redistribution of ions in favor of the M2 sites at high temperatures. Ca8NaBi(PO4)6F2:xEu apatites are promising as red phosphors for white-light LEDs.

Luminescence of Bi3+ and Bi2+ ions in novel Bi-doped SrAl4O7 phosphor

Bismuth-doped SrAl4O7 powders were synthesized via modified Pechini process and their photoluminescence, radioluminescence and thermoluminescence properties were studied and discussed. Obtained results showed that in studied materials bismuth is present in two valence states, Bi3+ and Bi2+. Upon UV excitation SrAl4O7:Bi powders yield broad multicomponent luminescence in the 350–670 nm range, associated with the 3P0,1 → 1S0 transition of Bi3+ ions, and the most intense component located in green (λmax = 515 nm) part of spectrum. It was found that the emission spectral distribution is excitation wavelength-dependent. By changing the excitation wavelength in the 270–340 nm range the emission colour can be modulated from blue, through bluish-cyan to green. The studied phosphors also showed broad-band 2P3/2 → 2P1/2 Bi2+ emission in deep red region centered at around 710 nm. This luminescence was found predominant in radioluminescence and thermoluminescence spectra, which is deemed unique for bismuth-doped materials. The Bi2+ 630–830 nm emission might be attractive for bioimaging as it appears in the Ist biological window and may be utilized for deep red scintillation.

Effect of the Synthesis Method on the Functional Properties of Lithium-Rich Complex Oxides Li1.2Mn0.54Ni0.13Co0.13O2

Layered Li-rich transition metal oxides are considered among the most promising cathode materials for high energy density lithium-ion batteries. It was studied how the method and conditions of synthesis of Li-rich oxides Li1.2Mn0.54Ni0.13Co0.13O2 affect their electrochemical properties. Coprecipitation methods and modified Pechini process were used. It was shown that it is necessary to carefully choose the synthesis conditions when using the modified Pechini method because of their significant effect on the morphology of Li-rich oxides. Samples were obtained with high electrochemical characteristics: capacity discharge of 260–270 mAh/g (16 mA/g) and 60–70 mAh/g (988 mA/g) within the voltage range of 2.5–4.8 V.

Magnetization reversal in Bi5Ti3Co0.5Fe0.5O15 ceramics

Here we report the structural, magnetic and dielectric properties of polycrystalline Bi5Ti3Co0.5Fe0.5O15 ceramic compounds. The samples annealed at 1073, 1173, and 1273 K were prepared by a modified Pechini process. XRD patterns of the samples show a single phase with a four-layered Aurivillius structure. The XPS analysis reveals that the oxygen vacancies increase with the sintering temperature. Temperature-induced magnetization reversal is observed around 133 and 127 K in a field-cooled mode with an applied field of 50 Oe in the samples sintered at 1173 and 1273 K, respectively. We identify that the magnetization reversal in these oxides originates from the competition between single-ion magnetic anisotropy and antisymmetric Dzyaloshinsky-Moriya interaction. In addition, we have also observed that the dielectric constant and loss tangent are enhanced by increasing sintering temperature. Both the appearance of magnetic reversal and the rising dielectric loss tangent are induced by the intensified distortion of the lattice structure due to the increment of oxygen vacancies as the increase in the sintering temperature.

Surface morphology effects on Li ion diffusion toward CeO 2 :Cu nanostructured thin films incorporated in PEG matrix

Undoped and Cu-doped CeO2 (2.5, 5, 7.5 mol%) thin films were prepared by modified Pechini process. Grazing incidence X-ray diffraction investigations of the films revealed that the ceria thin film has a cubic structure and Cu doping inhibited the crystal growth. The presence of predominant of Ce4+ oxidation state (CeO2) and Cu2+ in the films was approved by X-ray photoelectron spectroscopy. The fourier transform infrared spectra confirmed the metal-oxygen (Ce–O) bonding. Scanning electron microscopy images of the films displayed a crack free surface and the nano-grains regularly distributed on the surface. Atomic force microscopy analyses indicated that roughness parameters changed by Cu incorporation and 5 mol% Cu-doped CeO2 is the smoothest surface. Fractal dimensions of surface were calculated by box counting and triangulation method. The fractal dimension increased by Cu doping up to 5 mol% and then reduced. The optical transmittance was enhanced and the absorption edge was shifted to lower wavelength by Cu doping. The refractive indices and extinction coefficients which were determined using theoretical approach were decreased by Cu doping. Photoluminescence emission intensity of CeO2 thin film was quenched by Cu doping. The electrochemical behavior of the films was examined in 1 M LiClO4/propylene carbonate electrolyte. The total charge density of ceria thin film increased by Cu doping and 5 mol% Cu-doped CeO2 film had the highest ion storage capacitance. The fractal analyses showed that the surface morphology affected the Li ion diffusion behavior of CeO2:Cu thin films.

Wet chemical synthesis and characterisation of Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3 − δ proton conductor

Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3−δ (BSCZGY) proton conducting electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs) has been synthesized by a sol-gel modified Pechini process and its sinterability, thermal expansion, microstructure, ionic conductivity and chemical stability have been investigated. Ionic conductivity at 700°C was measured to be ~8×10−3Scm−1 in wet 5vol.% H2/Ar atmospheres. Chemical stability test in pure CO2 up to 1200°C shows that the material is highly stable; better than the stability of BaZr0.3Ce0.5Y0.1Yb0.1O3−δ.

Controlled preparation of aluminum borate powders for the development of defect-related phosphors for warm white LED lighting

The optimization of the elaboration conditions of a new family of highly emissive white phosphors based on glassy yttrium aluminum borates (g-YAB) compositions is presented. Their preparation from solutions is based on the polymeric precursor method (modified Pechini process), involving non-toxic and low cost precursors. The resulting resins were first dried at moderate temperatures followed by two-step annealing treatments of the obtain powders under controlled atmospheres: a first pyrolysis under nitrogen followed by a calcination under oxygen. This favored the gradual oxidation of organic moieties coming from starting materials, avoiding uncontrolled self-combustion reactions, which generate localized hot spots. This prevented phase segregations and the formation of pyrolytic carbon or carbonates, which are strongly detrimental to the luminescence properties. Thus, coupled chemical analyses and luminescence characterizations showed the high chemical homogeneity of the resulting powders and their intense emissions in the whole visible range. These emissions can be tuned from blue to warm white by adjusting the calcination temperature that is an important advantage for the development of LED devices. We showed that impurities of monovalent and divalent cations act as quenching emission centers for these phosphors. Therefore, by increasing the purity grade, we significantly enhanced the PL emissions leading to high internal quantum yields (80–90%). Finally, cathodoluminescence emissions showed the homogeneous dispersion of emitting centers in the g-YAB matrix.

Pechini synthesis of high ionic conductivity Li1.3Al0.3Ti1.7 (PO4)3 solid electrolytes: The effect of dispersant

A NASICON-type structure lithium ion conducting solid electrolyte with the composition of Li1.3Al0.3Ti1.7(PO4)3 (LATP) has been successfully synthesized via a modified pechini process with ethylene glycol or glucose as dispersant. The influences of dispersant, calcination temperature for the precursor powders, sintering temperature and holding period for the electrolyte pellets on electrical properties of the LATP electrolytes were investigated. The produced LATP sample using glucose as dispersant has a higher electrical conductivity than those samples with ethylene glycol as dispersant or no dispersant. The highest total conductivity of 6.0 × 10−4 S/cm at 303 K and the lowest activation energy of 0.31 eV were obtained for the LATP electrolyte sample sintered in 900 °C for 3 h and prepared using the precursor powders calcined at 850 °C for 5 h in air. Additionally, this electrolyte sample has a negligible electronic conductivity. These results imply that the LATP electrolytes obtained in this work can be considered as candidates for solid state electrolytes applied in Lithium ion batteries.

Influence of Microstructure and Surface Activation of Dual‐Phase Membrane Ce0.8Gd0.2O2−δ–FeCo2O4 on Oxygen Permeation

Dual‐phase oxygen transport membranes are fast‐growing research interest for application in oxyfuel combustion process. One such potential candidate is CGO‐FCO (60 wt% Ce0.8Gd0.2O2−δ–40 wt% FeCo2O4) identified to provide good oxygen permeation flux with substantial stability in harsh atmosphere. Dense CGO‐FCO membranes of 1 mm thickness were fabricated by sintering dry pellets pressed from powders synthesized by one‐pot method (modified Pechini process) at 1200°C for 10 h. Microstructure analysis indicates presence of a third orthorhombic perovskite phase in the sintered composite. It was also identified that the spinel phase tends to form an oxygen deficient phase at the grain boundary of spinel and CGO phases. Surface exchange limitation of the membranes was overcome by La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) porous layer coating over the composite. The oxygen permeation flux of the CGO‐FCO screen printed with a porous layer of 10 μm thick LSCF is 0.11 mL/cm2 per minute at 850°C with argon as sweep and air as feed gas at the rates of 50 and 250 mL/min.

Donor–acceptor joint effect in barium titanate systems

Modified Pechini process was used to prepare nanopowders of barium titanate (BT) co-doped with different concentrations of niobium and manganese. Influence of dopants on the grain growth inhibition and microstructure development was analyzed. The addition of dopants significantly influenced the dielectric properties modification. The obtained ɛ–T curves of all ceramics have the shape of typical ferroelectric material but the increase of dielectric permittivity value and movement of phase transitions with different dopant concentrations were clearly noticed. Grain and grain boundary resistivities for all co-doped ceramics were extracted from detailed impedance analysis and the grain boundary effect was indicated as a dominant effect in the total conduction of the materials. The well-defined and saturated P–E hysteresis loops of the ceramics confirmed the existence of ferroelectric properties.

Nonlinear properties of antimony-doped BaTiO3 ceramics

Barium titanate nanopowders doped with various concentrations of antimony were synthesized by polymeric precursors method based on modified Pechini process. Obtained powders were pressed and sintered at 1,573 K for 8 h. The temperature dependence of dielectric permittivity pointed on the significant influence of added antimony on dielectric properties of obtained materials. The analysis of impedance spectrum in the temperature range 250–445 K has shown the presence of one semicircle that corresponds to the grain contribution but impedance spectrum at 773 K showed possible overlapping of two semicircles indicating also grain boundary resistivity. The total specific resistivity obtained from impedance analysis showed modest PTCR jump in the heavily doped barium titanate ceramics. The electric field dependence of permittivity of the ceramics was investigated in detail. The experimental data were analyzed using Johnson model.

Fe doped Li1.2Mn0.6-x/2Ni0.2-x/2FexO2 (x≤0.1) as cathode materials for lithium-ion batteries

A series of Li1.2Mn0.6-x/2Ni0.2-x/2FexO2 (x=0, 0.03, 0.06, 0.10) cathode materials has been prepared by a modified Pechini process. The structure and morphology of the obtained materials have been examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques. The electrochemical properties of Li1.2Mn0.6-x/2Ni0.2-x/2FexO2 have been characterized systematically and Li1.2Mn0.585Ni0.185Fe0.03O2 exhibits the best cyclic stability and high-rate capability. The Li1.2Mn0.585Ni0.185Fe0.03O2 electrode delivers a large reversible discharge capacity of 261.6 mAh g−1 at 0.1C-rate with a high capacity retention rate of 90.9% after 80 cycles. Meanwhile it displays obviously improved rate capability with the capacity of 187.3 mAh g−1 at the 1C-rate and stable cycling performance of 165 mAh g−1 after 150 cycles. The effects of a small amount of Fe doping are explored through ex-situ XRD, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscope (HRTEM), differential capacity vs. voltage (dQ/dV) plots and electrochemical impedance spectroscopy (EIS). The results show stable layered structure and well-crystallized particle surface for Li1.2Mn0.585Ni0.185Fe0.03O2 electrode in contrast with spinel nanodonmains distribution and amorphous surface region for Li1.2Mn0.6Ni0.2O2 electrode upon cycling, which account for the outstanding electrochemical performance of Fe doped Li1.2Mn0.6Ni0.2O2 cathode material.

Electrical properties of antimony doped barium titanate ceramics

Antimony doped barium titanate powders were synthesized by polymeric precursors method based on modified Pechini process. Obtained powders were pressed and sintered at 1300°C for 8h. XRD analysis showed the formation of tetragonal crystal structure in barium titanate ceramics. The decrease of tetragonality was noticed with antimony concentration increase. The influence of antimony concentration on structure change, grain size reduction and microstructure development was analyzed. Dielectric behavior of pure and antimony doped ceramics showed the change from classical to diffuse ferroelectric material. A modified Curie–Weiss law was used to explore the connection between doping level and the degree of diffuseness of phase transition. The analysis of impedance at higher temperatures showed the presence of both grain interior and grain boundary effects. Modest PTCR jump was observed in the doped barium titanate ceramics.

The electrical properties of chemically obtained barium titanate improved by attrition milling

Barium titanate ceramics were prepared using the nanopowder resulting from a polymeric precursor method, a type of modified Pechini process. The obtained nanopowder was observed to agglomerate and in order to de-agglomerate the powder and enhance the properties of the barium titanate the material was attrition milled. The impact of this attrition milling on the electrical properties of the barium titanate was analysed. The temperature dependence of the relative dielectric permittivity showed three structural phase transitions that are characteristic for ferroelectric barium titanate ceramics. The relative dielectric permittivity at the Curie temperature was higher for the attrition-treated sample than for the non-treated barium titanate. The dielectric losses were below 0.04 in both barium titanate ceramics. The grain and grain-boundary contributions to the total resistivity were observed using impedance analyses for both ceramics. A well-defined ferroelectric hysteresis loop and piezoelectric coefficient d33 = 150 pC/N were obtained for the ceramics prepared from the de-agglomerated powder. In this way we were able to demonstrate that by attrition milling of chemically obtained powders the ferroelectric and piezoelectric properties of the ceramics could be enhanced.

Preparation and photocatalytic properties of ilmenite NiTiO3 powders for degradation of humic acid in water

The powders of ilmenite structure NiTiO3 were prepared by a modified Pechini process using tetrabutyl titanate and nickel acetate as raw materials, and using citric acid and ethanol as a chelating agent and a solvent respectively. The powder samples were characterized by thermogravimetric and differential thermal analysis (TG-DTA) and X-ray diffraction (XRD). The photocatalytic activity of NiTiO3 under the irradiation of ultraviolet rays (UV) light was evaluated by degrading humic acid (HA) in water as a probe reaction. The possible photodegradation mechanism was studied by the examination of active species ·OH, ·O2−, and holes (h+) through adding scavengers. The TG-DTA and XRD results indicated that the good crystal structure of ilmenite phase NiTiO3 could be obtained when the Ni-Ti citrate complex was calcined at 600°C. The photocatalytic activity experiments indicated that NiTiO3 had favourable photocatalytic activity under the irradiation of UV light, and the photocatalytic degradation rate of HA reached 95.3% after a 2.5 h reaction with the photocatalyst calcined at 600°C and a photocatalyst dosage of 0.4 g/L. The possible photocatalytic mechanism was deduced that holes (h+) and ·OH radicals are the major reactive active species in the photocatalytic reaction, and dissolved oxygen plays a weak role in the degradation of HA.

Improvement of barium titanate properties induced by attrition milling

Barium titanate powder was prepared by soft chemical process from polymeric precursors (modified Pechini process). The synthesized barium titanate (BT) powder was nanosized and the factor of agglomeration (Fagg) pointed the existence of agglomerates. In order to de-agglomerate nanopowder and to enhance BT properties the attrition milling was performed. The milled powder (BTA) possessed smaller particles and the size and number of agglomerates was significantly reduced. To investigate the effect of milling on improvement of ceramics electrical properties, both BT and BTA powders were uniaxially pressed and sintered at 1300°C for 8h in air. The temperature dependence of relative permittivity showed three structural phase transitions for ferroelectric barium titanate ceramics. The dielectric constant at Curie temperature was ∼6700 for BTA which is much higher than 1340, obtained for non-treated BT. The dielectric losses were below 0.04 in both BT ceramics. At higher temperatures the analysis of impedance measurements showed the presence of both grain interior and grain boundary effects. Much higher grain and grain boundary resistivities were obtained for the BTA ceramics.

Synthesis and properties of ZnO nanorods by modified Pechini process

Zinc oxide (ZnO) nanorods have been successfully synthesized by modified Pechini process. The as-synthesized ZnO nanorods were characterized by X-ray diffraction, scanning electron microscope, Raman spectrum, ultraviolet–visible (UV–vis) spectrum. X-ray diffraction result shows that the ZnO nanorods are oriented in wurtzite phase. Raman spectroscopy measurements revealed the presence of E2 (high) mode at 437 cm−1 indicating the high crystallinity of the as-synthesized ZnO nanorods. The optical property of the ZnO nanorods were studied from their UV–vis spectroscopy analysis which exhibits the absorbance at 373 nm corresponds to the Zn–O absorption. A mechanism for modified Pechini process for the synthesis of ZnO nanorods using citric acid–ethylene glycol precursor has also been proposed.

Development of Zirconia Based Oxygen Sensor for Lead and Lead-Bismuth Eutectic

Nanocrystalline yttria stabilized zirconia (YSZ) solid electrolyte powders of composition ZrO2-5 mol% Y2O3 were prepared through modified Pechini process and were characterized. Sintered compacts of these powders in form of pellets had 96% of the theoretical density and helium leak rate <10-9 S cc /s. Using a pellet of this electrolyte in a galvanic cell with different oxygen partial pressures across it, emf was measured as a function of temperature and thus qualified for sensor application. Towards the fabrication of the oxygen sensor for Pb and LBE loop, a compact design wherein a YSZ pellet, glass soldered to a metallic component had been chosen for the oxygen meter.