A. Hartridge

Phase transition in BiVO4

Abstract Monophasic zircon-type tetragonal BiVO 4 was prepared by a new co-precipitation method that employs accurate control of pH and temperature. The highly crystalline phase converts irreversibly to its monoclinic isomorph when heated between 350 and 400 °C. Mechanical grinding at room temperature also resulted in the tetragonal BiVO 4 to transform irreversibly to the monoclinic form. The amount transformed was found to be dependant on the duration of grinding.

Optical constants of nanocrystalline lanthanide-doped ceria thin films with the fluorite structure

Abstract The optical constants and structure of yttria-doped ceria and ceria doped with Nd, Sm, Gd, Dy, Er and Yb oxides are reported. These films were prepared using an inorganic sol-gel route and characterized for their refractive index, optical absorption edge, structure and crystallite size. All the films crystallized into the fluorite structure well below 450 °C, with cell constants altering as a function of ionic radius. The films were also nanocrystalline and continuous, with crystallite sizes in the range of 10–16nm after 600 °C. They have high transparency in the region 350 to 1500 nm and a reasonably high refractive index (1.7–1.79 at a wavelength of 800 nm) in the dispersion-free region. The dispersion behaviour in the refractive index has been fitted to the single oscillator model and is also shown to be dependent on the ionic size of the dopant material. It is therefore proposed that they should be suitable materials for application as transparent ion intercalation films and optoionic smart windows.

Temperature and ionic size dependence of the properties of ceria based optionic thin films

Thin films of CexLn1−xOy (Ln=La, Nd, Gd, and Yb and x=0.85) were deposited by an inorganic sol–gel technique and the dependence of the refractive index, optical absorption and absorption edge of these films on post deposition annealing temperature has been studied. It has been found that the refractive index of all the films increases with increase in temperature up to 1050°C. Significantly, the value of refractive index obtained is dopant ionic size dependent. After annealing at 1050°C for 1 h, the undoped ceria films showed the highest value of 2.13 at 800 nm while the highest value obtained in La doped films was 1.97 at the same wavelength and temperature. The values for Nd, Gd and Yb were 1.94, 1.97 and 1.85, respectively at the same wavelength and temperature. Annealing at 1050°C for 16 h increased the refractive index further but reduced the transmittance in the short wavelength region (<500 nm) for all the materials. It has also been found that both the optical absorption and absorption edge are temperature dependent.

Sol gel preparation, structure and thermal stability of crystalline zirconium titanate microspheres

Very pure and crystalline ZrTiO4 microspheres (15–50 μm) were prepared by two sol gel methods using a zirconia sol and two types of titania sols and characterized by scanning electron microscopy, X-ray diffraction and simultaneous differential thermal and thermogravimetric analysis. On gelation of the mixed sols, microspheres of an amorphous material with Zr/Ti ratio of ∼ 1.0 were obtained by each route. The amorphous materials obtained by the two routes transformed to fully crystalline ZrTiO4 at 500 and 600 °C, respectively. The high temperature thermal stabilities of these materials were also studied.

An X-ray diffraction and Mossbauer study of nano-crystalline Fe2O3–Cr2O3 solid solutions

A series of gels with nominal composition Fe2−xCrxO3 (x=0–2) was prepared at room temperature by an inorganic sol–gel route and studied by X-ray diffraction and Mossbauer spectroscopy. The gels dried at 105°C were found to be X-ray amorphous, but Mossbauer studies revealed the gels to be nano-crystalline solid-solution particles of the composition above, with super-paramagnetic properties. Further heating to 600°C gave crystalline X-ray patterns which allowed lattice parameter and crystallite size calculations to be made. It was found that lattice parameters and crystallite sizes decreased with increasing chromia content, and that at the higher chromia/iron ratios, a partially collapsed Mossbauer pattern results, indicating reductions in crystallite size and hyperfine field with increasing chromia content.

Low-temperature synthesis and characterization of ceria-based oxide ion conductors

Solid solutions of the general formula Ce1−xLnxO2−x/2□x/2 (Ln = lanthanide (III) and □ = anion vacancy), were prepared by a novel sol-gel route. These materials were characterized by powder diffraction and scanning electron microscopy. The gels formed on sol evaporation were found to be solid solutions with the fluorite structure and a crystallite size of approximately 6 nm. This is the lowest temperature of formation to date. The gels densified readily at 700 °C and the lattice parameter of these materials was found to be directly proportional to the ionic radius of the dopant.

Preparation and characterization of Ln2Zr2O7 microspheres by an inorganic sol-gel route

Free-flowing Ln2Zr2O7 microspheres (Ln=lanthanide) were prepared by an aqueous inorganic sol-gel route without any intermediate phase formation. The gel spheres obtained at room temperature were shown by X-ray diffraction to be amorphous but calcination to 750 °C produced fully crystalline fluorite phases. On calcination to 850 °C, pyrochlore phases were formed with suitable lanthanides. The microspheres were characterized by X-ray diffraction and scanning electron microscopy with energy dispersive analysis of X-rays to give accurate determination of structure, composition and crystallite size.

Environment sensitive impedance spectroscopy and dc conductivity measurements on NiWO4

Measurements of impedance spectroscopy and dc conductivity over the temperature range 300–1000K in atmospheres of dry and wet argon on antiferromagnetic NiWO4 are presented. The measured polycrystalline material was produced by a co-precipitation route. The conductivity was found to be sensitive to moisture over the temperature range 300–450 K. Impedance spectroscopy measurements separated the contributions of the bulk lattice and grain boundaries to the conductivity. These were separated over the temperature range 615–833 K by equivalent circuit modelling and an activation energy for the bulk lattice conductivity obtained. Conductivity measurements indicated that the compound is an intrinsic semiconductor and obeys the Arrhenius conductivity equation; σ=492.7 exp(−1.15 eV/kT)Ω-1 m-1, above 625 K where K and T have their usual meaning.

A study of nanocrystalline CeO2/PrOx optoionic thin films: temperature and oxygen vacancy dependence

Abstract The structure and optical constants of praseodymia doped ceria thin films are reported with and without oxygen annealing. These films were prepared using an inorganic sol-gel route and crystallized into the cubic fluorite structure well below 400°C, with cell constants altering as a function of dopant concentration and oxygen treatment/vacancy concentration. The films were also nanocrystalline, continuous and highly transparent in the region between 350–1500 nm. Optical transmission decreased with increased crystallite size due to scattering, below 600 nm. The refractive indices of the films at 800 nm in the dispersion free region were shown to vary in direct proportion to the lattice parameter/vacancy concentration and temperature treatment, and varied between 1.77 and 2.26. It is therefore proposed that they should be suitable materials for application as transparent ion intercalation films and optoionic smart windows.

Selective oxidation of methane to carbon monoxide on supported palladium catalyst

Abstract Partial oxidation of methane by oxygen has been studied on a number of supported palladium catalysts in the temperature range of 373–1023 K at GHSVs ranging from 5000 to 20,000 h −1 and with CH 4 :O 2 ratio range from 4:1 to 8:1. The supports studied included oxides of metals belonging to Group IIIa and IVa of the periodic table, oxides of representative metals belonging to the lanthanide series, γ-Al 2 O 3 and SiO 2 . CO, CO 2 , H 2 O and H 2 were the only detectable reaction products on all the catalysts. The CO selectivity on all catalysts increases with an increase in reaction temperature, a decrease in GHSV and an increase in the CH 4 :O 2 ratio. Carbon monoxide is formed with greater than 90% selectivity on palladium support on all the oxides, excepting SiO 2 , studied here at a temperature of 1023 K, GHSV of 5000 h −1 and with a feed composition of 40% CH 4 , 5% O 2 and 55% Ar.