Thermal design of air cooled condenser of a solar adsorption refrigerator

The objective of this paper is to study the design of a condenser of a solar adsorption refrigerator which will be tested in the region of Biskra (Algeria). The LMTD (log mean temperature difference) method is used to calculate the size of the condenser applying experimental data obtained from the literature. For this purpose, a calculation code has been developed to determine the total heat transfer area of the heat exchanger. Therefore, we present a comparison between calculated and experimental results obtained from the literature. This comparison allowed the validation of the calculation method by applying the same experimental conditions. The discussion of the results indicates that we cannot use the ambient air in free convection mode as a cooling fluid if its temperature exceeds 30°C. This problem presents the greatest obstacle especially in the Saharan regions, such as in Biskra, where the average ambient air temperature during the summer exceeds 35°C. As a solution, we propose in this article the improvement of the heat transfer by the air-forced convection mode. Thus, it is established that the use of the air fan can extend the operating temperature limits of the condenser above 35°C

Y0.76Ho0.24FeGe2O7: a new member of thortveitite-like layered compounds

Y0.76Ho0.24FeGe2O7 (yttrium holmium iron digermanate) was synthesized by solid-state reaction at 1573 K. This thortveitite-like compound presents a crystallographic group–subgroup isotranslational (klassengleiche) relation with some other pyrogermanates, such as FeInGe2O7, In1.08Gd0.92Ge2O7 and InYGe2O7, which are configurationally isotypic with the Sc2Si2O7 thortveitite structure first reported by Zachariasen [(1930 ▶). Z. Kristallogr. 73, 1–6]. Holmium cations share with yttrium the 4f Wyckoff position at the center of a seven-coordinated pentagonal bipyramid, while Fe atoms also occupy one site with Wyckoff position 4f at the center of the octahedron. All these sites have the point symmetry C 1. Two types of Ge2O7 diorthogroups with point symmetry C 1h are present in the structure, each one of them defining a layer type which alternates with the other. These diorthogroups have their tetrahedral groups in an eclipsed conformation

Rietveld refinement of Y2GeO5

Y2GeO5 (yttrium germanium penta­oxide) was synthesized by solid-state reaction at 1443 K. The arrangement, which has monoclinic symmetry, is isostructural with Dy2GeO5 and presents two independent sites for the Y atoms. Around these atoms there are distorted six-coordinated YO6 octa­hedra and seven-coordinated YO7 penta­gonal bipyramids. The YO7 polyhedra are linked together, sharing their edges along a surface parallel to ab, forming a sheet. Each of these parallel sheets is inter­connected by means of GeO4 tetra­hedra, sharing an edge (or vertex) on one side and a vertex (or edge) on the other adjacent side. Parallel sheets of YO7 polyhedra are also inter­connected by undulating chains of YO6 octa­hedra along the c axis. These octa­hedra are joined together, sharing a common edge, to form the chain and share edges with the YO7 polyhedra of the sheets

Superconducting tantalum disulfide nanotapes; growth, structure and stoichiometry

Superconducting tantalum disulfide nanowires have been synthesised by surface-assisted chemical vapour transport (SACVT) methods and their crystal structure, morphology and stoichiometry studied by powder X-ray diffraction (PXD), scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and nanodiffraction. The evolution of morphology, stoichiometry and structure of materials grown by SACVT methods in the Ta-S system with reaction temperature was investigated systematically. High-aspect-ratio, superconducting disulfide nanowires are produced at intermediate reaction temperatures (650 degrees C). The superconducting wires are single crystalline, adopt the 2H polytypic structure (hexagonal space group P6(3)/mmc: a = 3.32(2) angstrom, c = 12.159(2) angstrom; c/a = 3.66) and grow in the <2<(1)over bar>(1) over bar0> direction. The nanowires are of rectangular cross-section forming nanotapes composed of bundles of much smaller fibres that grow cooperatively. At lower reaction temperatures nanowires close to a composition of TaS3 are produced whereas elevated temperatures yield platelets of 1T TaS2

A combined XAS and XRD Study of the High-Pressure Behaviour of GaAsO4 Berlinite

Combined X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) experiments have been carried out on GaAsO4 (berlinite structure) at high pressure and room temperature. XAS measurements indicate four-fold to six-fold coordination changes for both cations. The two local coordination transformations occur at different rates but appear to be coupled. A reversible transition to a high pressure crystalline form occurs around 8 GPa. At a pressure of about 12 GPa, the system mainly consists of octahedral gallium atoms and a mixture of arsenic in four-fold and six-fold coordinations. A second transition to a highly disordered material with both cations in six-fold coordination occurs at higher pressures and is irreversible.Comment: 8 pages, 5 figures, LaTeX2

A synchrotron study of Na2.27Ho7.73(SiO4)6O0.72

A well crystallized powder sample of sodium holmium orthosilicate oxyapatite, Na2.27Ho7.73(SiO4)6O0.72, was obtained after mechanical milling and thermal treatment at 1123 K. Crystal structure analysis was performed from the results of Rietveld refinement of the synchrotron diffraction data. As in other rare-earth orthosilicate apatites, sodium cations appear located sharing with holmium the 4f Wyckoff position at the center of a tricapped trigonal prism. In its turn, holmium almost fully occupies the 6h position at the center of a seven-coordinated penta­gonal bipyramid. A small quantity of Na atoms was found at this site. No vacancies are present in the two independent crystallographic sites available for Ho and Na atoms

NaIrO3 - A pentavalent post-perovskite

Sodium iridium(V) oxide, NaIrO3, was synthesized by a high pressure solid state method and recovered to ambient conditions. It is found to be isostructural with CaIrO3, the much-studied structural analogue of the high-pressure post-perovskite phase of MgSiO3. Among the oxide post-perovskites, NaIrO3 is the first example with a pentavalent cation. The structure consists of layers of corner- and edge-sharing IrO6 octahedra separated by layers of NaO8 bicapped trigonal prisms. NaIrO3 shows no magnetic ordering and resistivity measurements show non-metallic behavior. The crystal structure, electrical and magnetic properties are discussed and compared to known post-perovskites and pentavalent perovskite metal oxides.Comment: 22 pages, 5 figures. Submitted to Journal of Solid State Chemistr

Rietveld refinement of langbeinite-type K2YHf(PO4)3

Potassium yttrium hafnium tris­(orthophosphate) belongs to the langbeinite-family and is built up from [MO6] octa­hedra [in which the positions of the two independent M sites are mutually occupied by Y and Hf in a 0.605 (10):0.395 (10) ratio] and [PO4] tetra­hedra connected via vertices into a three-dimensional framework. This framework is penetrated by large closed cavities in which the two independent K atoms are located; one of the K atoms is nine-coordinated and the other is 12-coordinated by surrounding O atoms. The K, Y and Hf atoms lie on threefold rotation axes, whereas the P and O atoms are located in general positions

Poly[[μ-(1-ammonio­ethane-1,1-di­yl)bis­(hydrogenphospho­nato)]diaquachloridodisodium]: a powder X-ray diffraction study

The title compound, [Na2(C2H8NO6P2)Cl(H2O)2]n, has a polymeric two-dimensional structure extending parallel to (001). The asymmetric unit contains two Na+ cations located on a centre of symmetry and on a mirror plane, respectively, one half of a bis-phospho­nate anion (the entire anion is completed by mirror symmetry), one chloride anion on a mirror plane and one water mol­ecule in general positions. The two Na+ cations exhibit distorted octa­hedral NaCl2O4 coordination polyhedra, each consisting of two deprotonated O atoms of the bis-phospho­nate anion, of two water mol­ecules and of two chloride anions. Strong O—H⋯O hydrogen bonds between the –OH group and one of the free O atoms of the bis-phospho­nate anion connect adjacent layers along [100], supported by N—H⋯Cl inter­actions. Intra­layer O—H⋯O and N—H⋯O hydrogen bonds are also observed