Studies on transparent conducting oxides and hydroxyapatites

At ID15B, González et al. have conducted in-situ, high-temperature, diffraction experiments on transparent conducting oxides and hydroxyapatites powder materials, in both bulk and nano-crystalline form. Complete data can be measured in short time scales that correspond to relevant material processing conditions. This is achieved with high-energy x-rays because hundreds of reflections are accessible and entire Debye-cones are collected with an area detector. This experimental set-up allows for in situ phase transformations and grain growth kinetics to be studied.

Indium oxide, tin oxide and indium-tin oxide (ITO) are transparent conductors widely used in commercial applications, such as, flat-panel displays. The effects of defect structure, micro-structure and defect population on the desirable electrical properties of these materials were studied using x-ray diffraction. The maximum solubilities of tin in indium oxide at temperatures greater than 800°C have been reported in literature, but the results varied extensively. Using two different starting compositions of overdoped-Sn nano-ITO powders, the equilibrium solubility limits of Sn in indium oxide in the temperature range of 900°C to 1400°C were determined. The x-ray diffraction and x-ray fluorescence results suggest that the Sn solubility limits in ITO are lower than 3 atomic percent in the temperature range studied. The higher solubilities reported in literature may be attributed to a lack of high-resolution measurements so as to differentiate between similar phases, to over-doped and non-equilibrium tin solutions in thin films and nano-powders, and to incorrect Sn/In concentrations from EDX chemical analysis.

The kinetics of tin oxide precipitation and the kinetics of ITO and tin oxide grain growth were also studied at ID15B. Activation energy of 450kJ/mole suggests that the diffusion of Sn is the slowest kinetic mechanism for tin oxide precipitation from over-doped Sn ITO nano-powders. The conductivity at room temperature was measured for these materials and correlated to the diffraction results. The conductivity is dependent on grain size and the tin-doping level in ITO. The highest conductivity values were obtained for samples with the large grains (~1µm) and with tin concentrations close to the solubility limit.

Using a similar experimental set-up, the in situ formation temperature and crystal structure of the rhombohedral phase In4Sn3O12 was also investigated. While this phase was reported 20 years ago, its crystal structure, formation temperature and stoichiometry remained unclear. This phase is a promising transparent conductor for commercial applications due to its good optical and electrical properties, and reduced material cost compared to that of ITO. The kinetics of phase transformation were investigated during isothermal annealing treatments ranging from 1335°C up to 1400°C. One example of the phase transformation kinetics at 1400°C is shown in the figure below. The kinetics deviated from the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model after 75% of the transformation was reached. Because the entire Debye-cone is captured with an area detector, one of the advantages of this set-up over traditional θ-2θ point counting is an improved sensitivity to detecting minute amounts of a new phase, which led to the in situ observation of the first grains of In4Sn3O12 at 1345°C. Structural results obtained from Rietveld analysis included atomic positions, lattice parameters and phase analysis compositions of the samples during heating, cooling and isothermal conditions. The after-annealing indium and tin compositions of the samples were also measured with x-ray fluorescence to corroborate the stoichiometry of the transformed phase.

Calcium hydroxyapatite (CaHA) is a bone-analog ceramic used in clinical practice, such as in bone and dental reconstructions. In addition to its medical use, CaHA has also environmental and chemical applications due to its capability to trap and retain toxic heavy metals in ground water and soil. The substitutions of metals like zinc, cadmium, strontium and lead into the two Ca sites of CaHA were studied with powders prepared by a co-precipitation process. The x-ray diffraction results suggest that these metals prefer to occupy the Ca II site at the doping levels investigated (up to 35% substitution), but the degree of preference is metal-dependent. The lattice parameters for this hexagonal phase were dependent on the dopant concentrations. The unit cell decreased with increasing cadmium and zinc doping due to the smaller ionic radii of these metals, compared to calcium. On the other hand, the lattice parameters of CaHA increased with increasing lead and strontium concentrations due to the larger radii of these dopants. In order to better understand the precipitation mechanism and to follow the evolution and phase formations of CaHA precursors, high-energy x-ray diffraction experiments were performed in situ at high temperatures. The kinetics of crystallization for both liquid and solid amorphous hydroxyapatite precursors and their phase transitions and stability have been investigated at ID15B as a function of temperature and different chemical synthesis conditions.

Evolution of phases during in situ annealing of indium oxide and tin oxide.

D.E. Ellis, J. Terra, O. Warschkow, M. Jiang, G.B. González, J.S. Okasinski, M.J. Bedzyk, A.M. Rossi, and J.G. Eon - A theoretical and experimental study of lead substitution in calcium hydroxyapatite - Phys. Chem. Phys. 8:(8), 967-976, (2006).
G.B. González, J.S. Okasinski, T.O. Mason, V. Honkimäki, and T. Buslaps - In situ studies on the kinetics of formation and crystal structure of the phase In4Sn3O12 using high-energy, x-ray diffraction - J. Appl. Phys. 104, 043520 (2008).