Pechini Technique Research Articles

Structure Evolution and Up‐Conversion Studies of ZnX2O4:Er3+/Yb3+(X = Al3+, Ga3+, In3+) Nanoparticles

AbstractA series of ZnX2O4:Er3+/Yb3+(X = Al3+, Ga3+, In3+) nanoparticles were obtained by using a modified Pechini technique. The crystal structure, morphology, and chemical composition of the engineered compounds were characterized in detail by means of X‐ray diffraction, Micro‐Raman spectroscopy, transmission electron microscopy (TEM) and selected area electron diffraction, and energy‐dispersive X‐ray spectroscopy. Particle sizes were estimated by using theoretical calculations (Scherrer and Rietveld methods) as well as experimental techniques [TEM and dynamic light scattering (DLS)] and were, furthermore, cross‐compared. The calculated average grain size, for dry powders, is in the range 20–45 nm. The hydrodynamic size was measured by using the DLS technique on nonmodified, surfactant‐free particles of the whole ZnX2O4series. The up‐conversion properties of the particles were discussed in detail. The low value of the green‐to‐red ratio points to a significant contribution of non‐radiative pathways, such as multiphonon relaxation or cross relaxation processes, to the depopulation of the green band, which feeds and favors the red emission of Er3+in the up‐conversion process.

Synthesis and Sintering Behavior of Ultrafine (<10 nm) Magnesium Aluminate Spinel Nanoparticles

This article reports a comparative characterization of ultrafine MgAl2O4 spinel nanoparticles synthesized by polymeric precursor (Pechini) and coprecipitation methods. The nanoparticles were evaluated in terms of purity and surface cleanliness, size distribution, state of agglomeration, and sintering behavior. Powders synthesized by the Pechini technique were highly agglomerated and revealed a bimodal particle size distribution centered around 12 and 27 nm. Thermal analysis and infrared spectroscopy measurements indicated that carbon species remained on the surface of the powders only to be released when temperatures exceeded 1000°C. Isothermal sintering of such nanopowders at 1300°C showed a maximum relative density of only 54%. MgAl2O4 synthesized via coprecipitation created small nanoparticles, around 5–6 nm after calcination at 800°C, with significantly less agglomeration. Compared with the precursor‐derived powders, excellent sinterability of the coprecipitated powders was obtained under the same sintering conditions. Relative densities above 90% were obtained after only 10 min, which further increased to greater than 95% after 20 min with no sintering aids or dopants. The results highlight the importance of purity and processing control to exploit the beneficial high sinterability of nanoparticles.

Effect of Microwave Sintering on the Structural and Electrochemical Behavior of Yttria Substituted BaCeO3 Nanocomposites for Solid Oxide Fuel Cell (SOFC) Applications

Abstract. The Yttria substituted BaCeO3 nanocomposites were prepared by modified Pechini technique followed by novel microwave sintering at 1400°C. The powder XRD results show that the material exhibit orthorhombic crystalline structure. The average grain size was calculated to be as 55 nm. SEM results show that the homogeneous distribution of particles in the lower nanometer range leads to dense microstructure. DTA peaks at 575 and 648°C indicate that the crystallization of the material and the associated weight losses were observed in the TG curve. The 96% of theoretical density was measured for the sintered sample through Archimedes principle. The maximum power density of 788mWcm2 and the maximum open voltage (OCV) of about 0.991 V was measured from the I-V and I-P results. The results confirm that the microwave sintering technique enhances the material properties in the nanometer scale with large potential for SOFC applications.

Effect of heterogeneous interface on the microwave dielectric properties of Ca(Mg1/3Nb2/3)O3/CaTiO3 thin films

Effects of heterogeneous interface on the dielectric properties of Ca(Mg1/3Nb2/3)O3/CaTiO3 (CMN/CT) thin films fabricated by Pechini technique were investigated in this work. As-prepared CMN/CT thin films were uniform, smooth and dense with the thickness of around 190nm. AFM results showed that the root mean square (RMS) surface roughness of CMN/CT thin films decreased as the heterogeneous interface numbers increases and the NI-11 sample possesses minimal RMS. Compared with measured results, the calculated dielectric constant was slightly lower and with increasing heterogeneous interface numbers, the measured dielectric constant of the films increased gradually while the dielectric loss decreased. These results implied that heterogeneous interfaces were favorable for the improvement of dielectric properties, which was also attributed to the formation and accumulation of depletion layers at the CMN/CT interfaces that reduced lattice relaxation. Being indexed by XRD patterns and EDS spectrum, the inter-diffusion between the CMN and CT layer was further confirmed. Moreover, the CMN/CT thin films had single phase in their own layers independently regardless of interface number.

Effect of Y-doping on optical properties of multiferroics BiFeO3 nanoparticles

We have synthesized yttrium-doped bismuth ferrite nanoparticles through a modified Pechini technique. X-ray diffractometer, transmission electron microscope (TEM) and ultraviolet-visible spectrophotometer (UV- Vis) probes have been utilized to characterize the nano- particles. Average particle size estimated from TEM found to be 29 nm for Bi0.99Y0.01FeO3 samples. The band gap of the prepared BFO and BYFO nanoparticles varies from 1.97 to 2.29 eV, that is, within the visible range of the sunlight. This property of these nanoparticles can be uti- lized in photo catalytic decomposition of organic contam- inants, such as Rhodamine-B (RhB) under visible light irradiation. We have explored and observed that RhB degrade up to 8 % while mixed with Bi0.90Y0.1FeO3 for 1 h under 40 W lamp due to photo catalysis together with sensitization.

Hydroxyapatite formation on surface of calcium aluminate cements

In this study, we report the formation and characterisation of a calciumaluminate cement for potential application as a hard tissue replacement. Phasepure monocalcium aluminate powder was synthesised using the Pechini technique,using a relatively low temperature (1000°C) and sintering time (3h). Characterisation of the hardened material showed a strong relationshipbetween the liquid/powder ratio to which the cement was mixed and the mechanicalperformance of the hardened materials with little influence on cement microstructure.Interestingly, following immersion in phosphate buffered saline, there wasextensive precipitation on the surface of the cement sample. X-ray diffractionshowed that the precipitate was hydroxyapatite, which on evaluation usingscanning electron microscopy was shown to be of porous structure.

On-board H2 generation by a catalytic hollow fibre microreactor for portable device applications

Abstract A novel catalytic hollow fibre microreactor (CHFMR) that uses an asymmetric YSZ hollow fibre has been developed for on-board H 2 generation to be used in portable device applications. In the development of CHFMR, the 10 wt.%Ni/MgO–CeO 2 catalyst that was selected as a catalyst for the ethanol steam reforming (ESR) was impregnated into the inner surface of YSZ hollow fibres using the sol–gel Pechini technique. The catalytic activity tests were performed in the YSZ hollow fibres and the results were compared with fixed-bed reactors. An excellent catalyst utilisation in the CHFMR enables a small quantity of catalyst to be used for the ESR. The result shows that 8 units of CHFMR that only has an 18 mg catalyst produced a relatively similar amount of H 2 compared to a fixed-bed reactor that used 100 mg.

Synthesis and structural characterization of a new yttrium phosphate: Na 2.5Y 0.5Mg 7(PO 4) 6 with fillowite-type structure

A new phosphate, Na 2.5Y 0.5Mg 7(PO 4) 6 was synthesized as single crystals by the flux method and as a powdered sample by the Pechini technique, and investigated by single crystal X-ray diffraction, charge distribution (CD) and 31P NMR spectroscopy. This compound crystallized in the rhombohedral space group R ▪ and its equivalent hexagonal cell had the following parameters: a=1.4976(6) nm, c= 4.2599(7) nm, Z= 18. The structure consisted of MgO 5, MgO 6, YO 6, (Na,Y)O 8 and NaO x ( x=6, 7 and 9) polyhedra which were linked either directly through common corners, edges and faces and by means of the PO 4 tetrahedra via common corners and edges, giving rise to a complex three-dimensional framework, similar to that of the fillowite-like structure. The cation distribution was confirmed by charge distribution (CD) calculations and 31P NMR spectroscopy study.

Modified Pechini synthesis of Na3Ce(PO4)2 and thermochemistry of its phase transition

Effect of the synthesis conditions of Pechini technique on crystallinity and purity of Na3Ce(PO4)2 compound was investigated. Nano-sized cerium-sodium phosphate obtained when EDTA was used as an additional chelating agent for Ln3+. The total enthalpy change of Na3Ce(PO4)2 phase transition was determined as 14.2±0.7 kJ mol−1 for sample synthesized by conventional solid-solid reaction. The phase transition process was confirmed to occur at 1060°C or in temperature range 920–1060°C depending on thermal treatment of powders.

Analysis of Ca-PZT powder obtained by the Pechini and partial oxalate methods

Ca-Pb(Zr,Ti)O3 (Ca-PZT) powders were prepared by a combined method, Pechini technique for the intermediate ZrTiO4 (ZT) particles and oxalate route for the final powder. The intermediate and final products were characterized by X-ray diffraction and BET for phase identification and granulometric analysis, respectively. The surface area of ZT powder reduced remarkably from 70 to 7.4 m²/g when the calcining temperature increased from 600 to 800 °C. Incremental pore volume and average pore diameters of the powder calcined at 700 °C for 3 h were 0.026 cm³/g and 70 Å, respectively. Ca-PZT powder calcined at 800 ºC for 3 h, with agglomeration factor (AF) 2.8, showed no hysteresis in BET analysis, which indicate small agglomeration without micropores among particles. The powder calcined at 750 ºC for 3 h, however, exhibited small AF and high sinterability.

Synthesis and properties of multiferroic BiFeO3 ceramics

BiFeO3 was synthesized using a modified sol–gel process, i.e., so-called Pechini method, as evidenced by X-ray diffraction pattern. Optimal conditions for the synthesis of single-phase BiFeO3 ceramics were obtained. This Pechini technique developed in this work is expected to be useful in synthesis of doped BiFeO3 or BiFeO3-based solid solution. Conventional sintering and spark plasma sintering processes were used to fabricate BiFeO3 ceramics. Ferroelectric and magnetic loops were measured at room temperature. The ceramic sample shows a stable dielectric constant and low loss tangent between 100 Hz and 10 MHz.

Spectroscopic properties of Y3TaO7 powders activated with Eu, Er and co-activated with Yb

A new synthesis method of Y3TaO7 is reported. Pure phase powders were prepared by modified Pechini technique. Precursors obtained through that route were heated at the temperature about 1000°C with or without use of Li2SO4 flux. That implicates in different morphology of specimens and also their spectroscopic properties. Eu ions were used as a spectroscopic probe to prove inhomogeneous broadening of excitation and emission spectra. Samples with Er and Yb doping are also presented. Photoluminescence and up-conversion emissions, under 378 nm and 980 nm excitation respectively, strongly depend on the method of powders synthesis.

Variation of emission color of Y3Al5O12:Ce induced by thermal treatment at reducing atmosphere

Nanocrystalline powders of Y3Al5O12:Ce (YAG:Ce) were prepared using Pechini technique and precipitation with urea at 80–90 °C. It was shown that material obtained using Pechini procedure, after heating at 900–1000 °C is crystallographically pure, while that made with urea is contaminated with Y4Al2O9 and YAlO3. For all powders, heating at reducing atmosphere of nitrogen–hydrogen mixture at 1000 °C caused a red shift of the Ce3+ luminescence by about 20–25 nm. The effect was stronger for the powder prepared with urea. Also this powder was characterized by a longer decay time of the Ce3+ emission indicating the treatment at reducing atmosphere diminished nonradiative relaxation probability of the excited electrons.

Wet chemical synthesis of monocalcium aluminate

Monocalcium aluminate, CaAl 2O 4, is the main constituent of calcium aluminate cements (CACs), which are used in a wide range of applications. Conventionally, monocalcium aluminate is prepared by long lasting sintering of the solid oxide mixture in temperatures exceeding 1400 °C. The Pechini process is an alternative, low temperature polymeric precursor route for the synthesis of high purity oxides. In the present work, CaAl 2O 4 is prepared through the Pechini technique. Conventional solid-state synthesis is used in order to compare the final products with those obtained through the Pechini technique. As it is concluded, the Pechini technique can be successfully applied for the synthesis of CaAl 2O 4. Its formation requires a 3-h sintering at 900 °C. The final product is crystalline CaAl 2O 4 of high purity, without any impurities of reactants or secondary phases.

Chemical synthesis of hydraulic calcium aluminate compounds using the Pechini technique

3CaO·Al 2O 3 and 4CaO·Al 2O 3·Fe 2O 3 are two of the main compounds in portland cement and are conventionally prepared by sintering the solid oxide mixture above 1350 °C for many hours. The Pechini technique is a polymeric precursor route for the synthesis of ceramics. In this work, an equimolar mixture of C 3A and C 4AF was prepared by the Pechini technique. The preparation of the precursor mixture and the formation of the ceramic product were monitored using TG/DTA, XRD, FTIR and SEM. The combination of these techniques led to the recording of all the transformations occurring during the processing of the precursors and the formation of the final products. As it was concluded, the Pechini technique can be successfully applied for the preparation of 3CaO·Al 2O 3 and 4CaO·Al 2O 3·Fe 2O 3 mixtures. The final products consists of fine and well-formed grains of 3CaO·Al 2O 3 and 4CaO·Al 2O 3·Fe 2O 3. Their formation requires a 3-h sintering at 1000 °C.

Laser Sintering of BaTiO<sub>3</sub> Ceramics Obtained from Nanometric Powders

We have studied the synthesis and sintering of BaTiO3 (BTO) nanometric powders produced by Pechini technique. A laser sintering procedure was applied to the BTO and its major advantage was the quickness of the processing. We have used the radiation of a CO2 laser (CW – 100 W) as the heating source. A linear rate of irradiation was applied to avoid macroscopic defects. The calcined powder at 800°C presented a single crystalline phase confirmed by the XRD and a crystallite size of 20 nm. BTO ceramics sintered at Pmax = 7.0 W/mm2 for 10 min of irradiation showed a high relative density (98 ± 1) % with an average grain size of 400 nm for a total time of sintering of 40 minutes.

LiMn 2− xTi xO 4 spinel-type compounds ( x ≤ 1): Structural, electrical and magnetic properties

LiMn 2− x Ti x O 4 compounds with 0 ≤ x ≤ 1 were prepared by solid state reaction and Pechini technique. Powder X-ray diffraction showed that all samples crystallize with the spinel crystal structure (S.G. Fd3¯m). The cubic unit-cell parameter increases with the Ti content. The influence of the Ti content and cationic distribution on the magnetic properties of the compounds was studied by measuring the temperature and magnetic field dependences of the magnetization: substitution by non-magnetic d 0 Ti 4+ ions appeared to weaken the magnetic interactions between the manganese ions. The electrical properties of LiMnTiO 4 were studied by AC impedance spectroscopy and DC polarisation measurements, which revealed the electronic character of the conduction process.

Preparation of Oxygen Ion Conducting Ceramic Hollow-Fiber Membranes

Hollow-fiber membranes of mixed conducting perovskite Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ) were prepared using a sequence of extrusion, gelation, and sintering steps. For this purpose, a finely divided mixture of the component oxides was prepared by the modified Pechini technique and suspended in a polymer solution. The suspension was extruded through a spinneret and gelled in water. The resulting fiber was first heated at 800 °C to remove the polymer and then at 1100 °C to form the perovskite and simultaneously sinter the particles to a gastight membrane. The fibers were characterized by scanning electron microscopy and tested for air separation at ambient pressure and temperature between 700 and 950 °C. The maximum oxygen flux measured was 3.9 mL/min/cm^2 at 950 °C and 0.022 atm of average permeate oxygen partial pressure.

Microstructure and Spectroscopy of Lu<sub>2</sub>O<sub>3</sub>:Eu Prepared Using Various Synthesis Techniques

Nanocrystalline powders of Lu2O3:Eu with activator content varying between 0.2%-10% were prepared using four different methods of synthesis. The products differed in their microstructure and crystallites sizes. Combustion of Lu(NO3)3 with urea produced strongly agglomerated material, most probably with significantly non-uniform distribution of the Eu3+ dopant. Replacing urea with glycine for the combustion produced only slightly agglomerated, voluminous, fluffy powder. Applying the Pechini technique resulted in significantly agglomerated powder while the homogeneous precipitation of Lu(OH)3 with urea at 90 °C and its subsequent decomposition to Lu2O3 at 650 °C resulted in a powder of perfectly spherical particles with a uniform size of about 130 nm with very low agglomeration. The efficiency of X-ray excited luminescence of our nanocrystalline Lu2O3:5%Eu was compared to that of the commercial microcrystalline Gd2O2S:Eu. It was found that the commercial phosphor performed four times more efficiently than our nanocrystalline powder. We consider this to be rather encouraging as the fabrication of our powder is not optimized yet. It seems that Lu2O3:Eu, even in nanocrystalline form, can perform much more efficiently which would make it a promising X-ray phosphor.

LaSrMnCoO 6: a new cubic double perovskite oxide

Single-phase polycrystalline powder samples of the double perovskite oxide LaSrMnCoO 6 were synthesized by the Pechini (citrate-gel) technique. The structural, magnetic and electrical properties of the obtained powders were investigated by X-ray diffraction, electron microscopy, dc magnetization, ac susceptibility and dc resistivity measurements. The crystal structure of the new compound was found to be cubic of space group Fm 3 ̄ m at room temperature. Below 225 K, the samples exhibit ferrimagnetic behavior with a spin-glass-like character. Resistivity measurements indicate semiconducting behavior with two different conductivity mechanisms: thermally activated behavior below 190 K and variable range hopping above 190 K.