Most materials in their nanocrystalline state exhibit unusual properties. The tremendous enhancement of mechanical, physical, and chemical properties caused by the fine admixture of order (inside the crystallites) and disorder (in the grain boundary regions) is being exploited in a number of frontier technologies. A concern in the use of nanocrystalline materials is their stability during high temperature exposure, either while processing or in service. There is no experimental data in the literature on the Gibbs energy of formation of any nanocrystalline material and no standard technique is available for measurement, especially at elevated temperatures. Quantitative information on the thermodynamic driving force for degradation to the microcystalline state and mechanistic understanding of the degradation processes would be useful in the design and processing of nanocrystalline materials for new applications. Reliable information on surface and interfacial properties of ceramic materials, which provide important guidelines for understanding nanocrystalline materials, are not available in the literature, even for model systems. Magnesium aluminate, which is used as an adsorbent for removing surfactants from aqueous solutions, as a support material for transition metal and lanthanide phosphors, and as a catalyst, was selected as a model compound in this study.Reported in this communication is a novel application of the solid state electrochemical technique for probing the thermodynamic state of nanocrystalline MgAl 2 O 4 at high temperatures. The method has been used recently for the measurement of the Gibbs energy of microcrystalline MgAl 2 O 4 .[1] Although the principle of this method for thermodynamic measurements was outlined in the classic papers of Kiukkola and Wagner, [2,3] the technique has not been applied to nanocrystalline materials. The reversible electromotive force (emf) of the cell was measured as a function of temperature in the range 900 to 1250 K. The cell is written such that the right-hand electrode is positive. Single crystal CaF 2 was used as the solid electrolyte and the cell was operated under dry oxygen. The reference electrode consisted of an intimate equimolar mixture of MgO and MgF 2 powders, compacted at a pressure of 250 MPa. Similarly, the working electrode was a compacted equimolar mixture of MgAl 2 O 4 , a-Al 2 O 3 , and MgF 2 . The particle size of the powders used to prepare the reference and working electrodes usually vary from 2 to 10 mm. In a recent study of the Gibbs energy of formation of MgAl 2 O 4 , [1] the average size of the aluminate particles used, as determined by scanning electron microscopy (SEM), was 3 mm. In the present study, the measurements were made using nanocrystalline MgAl 2 O 4 powders, keeping all other conditions the same.To obtain constant and reversible emf at any given temperature, it was necessary to remove moisture completely in the ceramic enclosure surrounding the cell. Formation of CaO on the surface of CaF 2 , by the reaction of water vapor wi...

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The one-dimensional (1-D) nanostructures of cadmium chalcogenides (Il-VI: CdSe, CdTe), InP and GaAs (III-V), and the ternary chalcopyrites CulnS2, CulnSe2, and CulnTe2 (I-III-VI2) are the candidate semiconductors of interest as absorber layers in solar cells. In the confinement regime (approximately 1-10 nm) of these 1-D nanostructures, the electronic energy levels are quantized so that the oscillator strength and the resultant absorption of solar energy are enhanced. Moreover, the discrete energy levels effectively separate the electrons and holes at the two electrodes or at the interfaces with a polymer in a hybrid structure, so that an oriented and 1-D nanostructured absorber layer is expected to improve the conversion efficiency of solar cells. The intrinsic anisotropy of Il-VI and l-lll-VI2 crystal lattices and the progress in various growth processes are assessed to derive suitable morphological features of these 1-D semiconductor nanostructures. The present status of research in nanorod-based solar cells is reviewed and possible routes are identified to improve the performance of nanorod-based solar cells. Finally, the characteristics of nanorod-based solar cells are compared with the dye-sensitized and organic solar cells.

The standard Gibbs energy of formation of the spinel MgAl 2 O 4 from component oxides, MgO and ␣-Al 2 O 3 , has been determined in the temperature range 900 to 1250 K using a solid-state cell incorporating single-crystal CaF 2 as the solid electrolyte. The cell can be represented as The standard Gibbs energy of formation from binary oxides, computed from the reversible emf, can be represented by the expression ⌬G°f ,ox = −23600 − 5.91T (±150) J/mol The ''second-law'' enthalpy of formation of MgAl 2 O 4 obtained in this study is in good agreement with hightemperature solution calorimetric studies reported in the literature.

The wide-gap semiconductor ZnO with nanostructures such as nanoparticle, nanorod, nanowire, nanobelt, nanotube has high potential for a variety of applications. This article reviews the fundamentals of one-dimensional ZnO nanostructures, including processing, structure, property, application and their processing-microstructure-property correlation. Various fabrication methods of the ZnO nanostructures including vapor-liquid-solid process, vapor-solid growth, solution growth, solvothermal growth, template-assisted growth and self-assembly are introduced. The characterization and properties of the ZnO nanostructures are described. The possible applications of these nanostructures are also discussed.