M. Rajendran

Low-temperature formation of alpha alumina powders from carboxylate and mixed carboxylate precursors

Formation of alpha alumina fine powders from aluminum carboxylate and mixed carboxylate precursors have been studied using thermoanalytical and powder X-ray diffraction techniques. Among these carboxylates, aluminum acetate and formoacetate produce alpha alumina below 1000°C, whereas, the other precursors require about 1200°C. The nature of the precursor has been found to influence the course of thermal decomposition and the sequence of phase evolution. The resultant alpha alumina powders have been have been characterised employing surface area and porosity measurements.

Combustion synthesis, powder characteristics and crystal structure of phases in Ce-Pr-O system

The combustion method has been employed to produce homogeneous, single phased mixed rare-earth oxides in Ce1 − xPrxO2−y system for x ranging from 0 to 0.7. A cubic fluorite structure is formed for the compositions 0 ≤ x ≤ 0.7, while for x > 0.7 mixed phases are obtained. The mixed oxides are formed at the furnace temperature of 500 °C in a short duration of 10 min. In view of the importance of these powders in catalysis, crystallite size, surface area and porosity measurements have been carried out. The crystallite size of the powders increases with x while the surface area decreases. As the temperature is increased to 850 °C, the surface area decreases and the effect is much pronounced in cerium rich oxides. The powders on calcination above 900 °C in air results in the demixing of Ce and Pr to give two fluorite phases.

Low temperature preparation of orthoferrite thin-films by an inorganic sol–gel process

Abstract Aqueous precursor sols of orthoferrites were prepared by room temperature processing of inexpensive commercially available metal salts. Crystalline thin-films of orthoferrites of general formula, LnFeO3(Ln=La, Nd, Sm, Gd, Dy, Er, Yb and Y) were prepared by dip-coating with the precursor sol and heating at 650°C. All the films showed >80% transmittance independent of the rare earth ion. The optical band-gap was 3.1 eV and the refractive index was 1.61±0.02 independent of the rare earth ion. The films exhibited uniformity in thickness, mechanical and thermal integrity upon heating.

Preparation and characterization of LnPrO3+y (Ln=Y and lanthanide)—a series of mixed lanthanide oxides

Abstract Citrate gel method has been employed to produce a series of praseodymium based homogeneous mixed rare-earth oxides, LnPrO 3+ y (Ln=Y and lanthanide ion). The ionic size of Ln determines the crystal structure of the mixed oxides. LnPrO 3+ y and CePrO 3+ y crystallise in the fluorite structure, whereas the remaining oxides adopt the C -type structure. The crystal structure and the unit cell parameters of the oxides obtained at 1173 K in static air have been reported. Wet-chemical analysis suggests that the compositions adopting the fluorite structure are non-stoichiometric, whilst the compositions crystallising in the C -type structure are stoichiometric. The surface area has been measured and the adsorption–desorption isotherms obtained for the compositions. The BET surface area of the oxides are in the range 1.5 to 3.5 m 2 g −1 and the isotherms reveal that the oxides are free from porosity.

A process for the production of sub-micron to millimetre sized thermally stable α-alumina spheres

Abstract The sol-emulsion-gel method has been employed to produce alumina gel spheres from unseeded as well as seeded alumina sol. Controlled heating of the unseeded gel spheres leads to the formation of α-alumina phase at 1200°C. Various seeds have been tried to determine the effect of seeding on the formation of α-alumina and it is found that seeding with 0.5 wt.% Fe 2 O 3 leads to the formation of α-alumina at a relatively low temperature of 950°C. The sizes of the microspheres can be varied in the range between 0.5 μm and 1 mm by suitable choice of the process parameters. The resultant gel spheres on controlled heating transform into thermally stable α-alumina spheres.

FABRICATION AND CHARACTERIZATION OF AQUEOUS SOL–GEL-DERIVED LaFeO3 THIN FILMS

A novel all-inorganic aqueous sol–gel process has been developed to fabricate LaFeO3 thin films by dip-coating. Stable, positively charged colloidal sol particles of hydrous lanthanum ferrite with an average particle size (Zav) of 7 nm were prepared and coated onto quartz plates under controlled conditions. The sols have been characterized using photon correlation spectroscopy (PCS) for Zav and size distribution. The redispersible gel was characterized by thermogravimetric and differential thermal analysis (TG-DTA) and also by isothermal heating followed by X-ray diffraction to identify the reaction sequence to form LaFeO3. The sol–gel films as deposited were X-ray amorphous on heating up to 500°C, partially crystalline at 600°C, fully crystalline and single phase at 650°C and above. These films were continuous, polycrystalline, single phase, had uniform thickness in the range between 180 to 1000 nm, depending on deposition conditions, and showed about 80% optical transmittance. The optical band gap varied from 2.7 to 3.3 eV as a function of the annealing temperature. The refractive index increased with increase in annealing temperature from 1.55 at 500°C to 1.86 at 800°C.

Electrical transport studies and temperature-programmed oxygen evolution of PrO1.83

The phases in the homologous series PrnO2n−2 (n=11, 10, 9 and 7) have been studied by temperature-programmed reduction (TPR), electrical conductivity and Seebeck coefficient measurements using PrO1.83 as the starting material. TPR measurements in helium indicate that oxygen evolution from PrO1.83 occurs in three distinct steps and quantification of the oxygen evolution shows the formation of the phases PrnO2n−2 (n=10, 9 and 7). Temperature-dependent electrical conductivity measurements for p(O2)=0 show breaks in the conductivity which occur at 635, 714 and 797 K, whereas in air the breaks occur only at 720 and 953 K. These correspond to the compositionally controlled phase transitions. The Arrhenius conductivity expression has been used to calculate the activation energies and pre-exponential factors in the stoichiometric regions. Results from TPR and conductivity experiments indicate that PrO1.83 and PrO1.71 have easily established temperature ranges of composition while PrO1.80 and PrO1.78 have stability ranges which are very much smaller. Seebeck coefficient measurements (thermopower) as a function of oxygen partial pressure and temperature indicate that the conduction changes from n to p type for the composition PrO1.71. The approximate independence of the Seebeck coefficient with temperature fits the Heikes theory for a hopping conductor. The discrepancies in the earlier reports on the conductivity of PrO1.83 are attributed to the variations in the p(O2) employed, rate of heating and also to a certain extent the partial hydroxylation and carbonation of the samples used. The results of the present experiments point out these aspects and clarify the discrepancies between previously published data.

STRUCTURE AND THICKNESS DEPENDENT OPTICAL PROPERTIES OF NANOCRYSTALLINE HAEMATITE THIN FILMS

An aqueous sol-gel process has been developed to deposit nanocrystalline haematite (α-Fe2O3) thin films by dip-coating. Stable, positively charged colloidal sol particles of hydrous iron (III) oxide with an average particle size (Zav) of 6 nm were prepared and coated onto quartz plates under controlled conditions. The sols were characterised using photon correlation spectroscopy (PCS) for Zav and size distribution. The re-dispersible gel was characterised by thermoanalytical techniques (TG-DTA) and also by isothermal heating followed by X-ray diffraction to identify the reaction pathway to form α-Fe2O3. The films as deposited were X-ray amorphous up to 450°C, nan°Crystalline in the range 450 to 600°C, crystalline and single phase above 600°C. The films were optically transparent, continuous and had uniform thickness in the range between 180 nm to 1000 nm depending on deposition conditions. The optical band gap varied from 2.1 to 2.6 eV as a function of film thickness. The refractive index increased with i...

Structure, oxygen stoichiometry and electrical conductivity of LnPrO3+y (Ln=Y and Lanthanide) oxides

Abstract The crystal structure and d.c. electrical conductivity of a series of homogeneous mixed rare-earth oxides in LnPrO 3+y (Ln=Y and lanthanide ion) system are reported. It is found that the ionic size of Ln determines the crystal structure of the oxides in this series. LaPrO 3+y and CePrO 3+y crystallise in the fluorite structure(fcc), whereas the remaining oxides adopt the C-type rare-earth oxide structure. The electrical conductivity measurements show that conductivity is structure type dependent with the fluorites (wherein Ln=La) having a lower conductivity than the C-type mixed oxides (wherein Ln=Gd, Sm, Ho, Er and Y). The conductivity of the mixed oxides at 1020 K, is of the order of 10 −2 Ω −1 cm −1 which is about five orders of magnitude higher compared to their room temperature conductivity. Oxygen excess in these compositions has been determined from wet-chemical analysis and temperature programmed oxygen evolution studies. The mixed oxides evolve oxygen in two distinct stages and the conductivity–temperature plot shows discontinuities corresponding to the onset of oxygen evolution.

Structure and oxygen stoichiometry of phases in La-Pr-O system

Citrate gel method has been employed to produce homogeneous mixed rare-earth oxides in La2 xPrxO3 y (05 x52) system. The phases obtained at 1473 K in static air have been characterised for the entire range of compositions for their crystal structure. Hexagonal structure is stable for 05x50.6 whereas cubic fluorite phase is stable for 15 x5 2. The unit cell parameters are reported for the range of single phase compositions. Wet-chemical analysis reveals that the compositions adopting hexagonal structure are stoichiometric (y0) whereas the compositions crystallising in the fluorite structure have the y increasing from 0.33 to 0.66 as x is varied from 1 to 2.