Citrate-nitrate Process Research Articles

Shape control and associated magnetic and dielectric properties of MFe12O19 (M = Ba, Pb, Sr) hexaferrites

Single-phase MFe12O19 (M = Ba, Pb, Sr) powders composed of hexagonal grains were prepared by the citrate-nitrate process by adjustment of the pH to 7 and with an M-to-iron molar ratio of 1:11, a nitrate-to-citrate ratio of 1:1.5, and a heating temperature of 850 °C. The physical properties were comprehensively investigated by different techniques. Rietveld refinement shows that the samples can be indexed in hexagonal symmetry (space group P63/mmc), which was confirmed by Fourier-transform infrared spectroscopy. The variation of the saturation magnetization of the MFe12O19 hexaferrite is explained mainly by the formation of a magnetically dead layer in the grains. The coercive field of the samples studied increases with increase of the average particle size, which originates mainly from the sum of all the changes in anisotropy energies. Dielectric results for the samples studied show the presence of short-range conduction of the localized charge carriers in the relaxation process and thus a departure from an ideal Debye-type behavior, was also confirmed by the results of ac conductivity studies.

Synthesis of BaCe0.9-xZrxY0.1O3-δ nanopowders and the study of proton conductors fabricated on their basis by low-temperature spark plasma sintering

Using a citrate-nitrate process, BaCe0.9-xZrxY0.1O3-δ (x = 0, 0.5, 0.6, 0.7 and 0.8) nanopowders were synthesized, on the basis of which proton-conducting solid electrolytes (average size of coherent scattering region (CSR) - 15–53 nm) were obtained by spark plasma sintering (SPS) at a low temperature (900°С). The dependence of phase composition, microstructure and electrophysical properties of the obtained samples on ZrO2 content and consolidation conditions was established. It was found that the BaCe0.4Zr0,5Y0.1O3-δ solid solution had the highest electrical conductivity among zirconium-containing ceramic materials in the studied concentration range. It was shown that SPS is a promising method for obtaining solid electrolytes for proton-conducting solid oxide fuel cells (PCFC) at lower temperatures (by 400–500°С), compared to traditionally-used temperature conditions (1400–1800 °C).

Electrical properties of proton-conducting BaCe0.8Y0.2O3-δ and the effects of bromine addition

A detailed analysis of the electrical properties of the proton-conducting BaCe0.8Y0.2O3-δ phase (BCY20) and the effects of Br doping has been undertaken. Members of the system of nominal stoichiometry BaCe0.8Y0.2O2.9-(x/2)±δBrx (x = 0, 0.05, 0.1 and 0.2) were synthesised by a citrate-nitrate process and high-temperature annealing up to 1500 °C. X-ray diffraction revealed the formation of single-phase material with a monoclinically distorted perovskite structure (space group I2/m) with greater distortion for Br-synthesised phase. Significant contrasts in electrical behaviour and stability between BCY20 and Br-synthesised series members were also found. However, chemical analysis by total X-ray fluorescence spectroscopy indicated that only trace amounts of Br remain after synthesis, indicating that Br addition influences stoichiometry but not directly the physicochemical properties. Higher conductivity is observed for the Br-synthesised compositions in wet and dry oxidising conditions in the temperature range 300–900 °C, reaching a value of 5.8 S m−1 at 800 °C for the x = 0.2 member in wet O2 (pH2O ∼ 0.022 atm). Determination of partial-conductivity components indicated that, in humid conditions and low temperature, the conductivity of 20% Br-synthesised material is principally protonic (tH = 0.91 at 600 °C and pO2 = 0.2 atm) and superior to that of BCY20. Mixed oxide-ionic-electronic conductivity dominates at high pO2 (1 atm) and oxide-ionic conductivity at low pO2 (∼10−4 atm) in the temperature range 600–900 °C for both BCY and Br-doped samples. Stability was found to be poorer in CO2 for the Br-synthesised phases as determined by thermogravimetry and prolonged conductivity measurements.

Characterization and microstructure investigation of novel ternary ZrO2–Al2O3–TiO2 composites synthesized by citrate–nitrate process

The citrate–nitrate, sol–gel technique was used to prepare two different composites in the ZrO2–Al2O3–TiO2 system. The influence of two various compositions and calcination temperatures on the phase transformation and microstructure was investigated by the simultaneous thermal analysis, X-ray diffraction (XRD), Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). Results showed that while the combustion intensity was vigorous for the stoichiometric sample (ZAT-1) at 346 °C, the combustion occurs at a temperature range (270–500 °C) for the alumina-rich sample (ZAT-2). FTIR spectra confirmed the formation of complex compounds with citrate ligands. The XRD patterns revealed ZrTiO4 formation at 900 °C in both samples due to its thermodynamic stability; surprisingly, in stoichiometric samples calcined at 1100 °C, Al2TiO5 and monoclinic ZrO2 phases were detected, which indicated ZrTiO4 decomposition, whereas in alumina-rich samples, no ZrTiO4 decomposition was observed. The SEM micrographs also showed orthorhombic particles of ZrTiO4 with average particle size around 100 nm for the stoichiometric sample calcined at 1100 °C.

High-performance anode-supported solid oxide fuel cells based on nickel-based cathode and Ba(Zr0.1Ce0.7Y0.2)O3−δ electrolyte

Iron-stabilized lanthanum nickelate oxide with a perovskite structure, LaNi0.6Fe0.403−δ(LNF), was prepared by a citrate–nitrate process and evaluated as the cathode component of a proton-conducting SOFC based on a Ba(Zr0.1Ce0.7Y0.2)O3−δ (BZCY) electrolyte. XRD analysis shows that LNF reacts with BZCY to form a K2NiF4-type phase in the course of cathode calcination, but remains chemically well compatible with BZCY at the operating temperature. A single cell was assembled and tested using humidified hydrogen as fuel gas and atmospheric air as the oxidant. The matching thermal expansion coefficient (TEC), high conductivity and good catalytic activity of the LNF cathode enable the cell to generate high power densities and low polarization resistances. At 700°C, the peak power density is 668mWcm−2 and the polarization resistance is 0.046Ωcm2. This work confirms that LNF may be a prospective cathode material for BZCY proton-conducting electrolytes.

Nanoparticles of antimony doped tin dioxide as a liquid petroleum gas sensor: effect of sizeon sensitivity

The gas sensitivity exhibited by nanoparticles of 1 wt% Pd catalysed antimony doped tin dioxide(ATO) prepared by a citrate–nitrate process is reported here. The reduction of particle size to < 3 nm, a dimension smaller than double the thickness of the charge depletion layer,has resulted in an exceptionally high butane sensitivity and selectivity. Thesensitivity and selectivity of ATO particles of different sizes unequivocally provedthat reducing the size of particles to below twice the Debye length dimensionproduces materials with exceptionally high sensitivity and selectivity for sensorapplications. The sensitivity of the samples towards 1000 ppm butane varied in the order98% > 55% > 47%, forCNP > SP > CP samples having crystallite sizes of the order of 2.4 nm to 18 nm to 25 nm,respectively. The ATO nanoparticles exhibited not only a remarkableincrease in gas sensitivity of around 98% towards 1000 ppm butane at350 °C, but also a preferential selectivity to butane compared to other gases such as CO,CO2,SO2,CH4 andH2. In addition to the exceptionally high sensitivity and selectivity, the developed sensors alsoexhibited an improved response time and long term stability, which are of paramountimportance for practical device development.

Preparation of nanoparticles of oxides by the citrate–nitrate process

Abstract This paper reports preparation of nanoparticles of oxides by the citrate–nitrate process and the effect of metal ions on the thermal decomposition characteristics of the corresponding citrate–nitrate gel precursors. In order to understand the effect of metal ions on the thermal decomposition characteristics of the precursors, we have prepared a series of single component oxides such as MO, where M = Zn, MO2, where M = Sn, Ce, Zr, and M2O3 where M = Al, Fe, Bi. In all the cases the citrate to nitrate ratio was fixed at 0.3. In order to ascertain the decomposition characteristics of the gel samples, TG/DTA studies were performed on the dried gel samples. After complete physico-chemical characterization of the precursors and the calcined products, it could be concluded that the nature of decomposition of the precursors depends largely on the nature of the metal ions. Finally, the advantages of the citrate–nitrate process such as its high degree of reproducibility, its potential for large-scale produ...

Synthesis and characterization of zirconia doped alumina nanopowder by citrate–nitrate process

Al 2O 3–ZrO 2 nanopowders containing 5–20 wt.% ZrO 2 was synthesized by a simple aqueous sol–gel method using a citrate polymeric precursor derived from aluminium nitrate, zirconium oxychloride and citric acid mixed solution. The thermal decomposition of the precursor and subsequent formation of α-Al 2O 3 was investigated by differential thermal analysis. X-ray diffraction analysis shows that the phases present at 1200 °C are α-Al 2O 3 and t-ZrO 2 irrespective of zirconia content. The t-ZrO 2 is found to increase with increase in zirconia content. FTIR also confirms the formation of α-Al 2O 3 incorroboration with X-ray studies. TEM micrograph of calcined powder revealed nanosize spherical particle of ∼75 nm. EDAX confirmed the presence of alumina and zirconia particle in the nanopowder calcined at 1200 °C.

Synthesis of BaCeO3 powders by a fast aqueous citrate–nitrate process

A fast aqueous citrate–nitrate process has been successfully used to prepare the stoichiometric BaCeO3 powders. It was found that an optimal amount of water added in the mixing process of barium nitrate, cerium nitrate, and citric acid is helpful on the formation of a clear gel that was subsequently formed by the condensation of added diethylene glycol monomers at room temperature. The gel was heated to an easily handled precursor in a powder form at 400°C for 1h. The precursors were characterized by X-ray powder diffraction (XRD), differential thermal analysis (DTA), thermal gravimetric analysis (TGA), thermal mechanical analysis (TMA), Fourier transform infrared spectrometry (FTIR), and scanning electron microscopy (SEM). The phase evolution from the precursor to the desired BaCeO3 powders by XRD and FTIR revealed that the presence of BaCO3 is a complex problem in this process. A high-temperature trigonal BaCO3 intermediate was found between 800°C and 1000°C by the in-situ X-ray powder diffraction experiments. The phase purity of the formed BaCeO3 powders may be more properly identified by FTIR instead of XRD because of the amorphous feature of the carbonate phase. The TMA revealed that at 1100°C part of the precursor crystallized to needle-shaped BaCeO3 crystals accompanied by a small expansion, which is also related to the presence of BaCO3 in the precursor, and this can be removed by a preheating treatment of the precursor at 900°C for 4h.