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Abstract

The reversibility of the dehydration of monoclinic CaZn2(PO4)2·2H2O to the corresponding monohydrate is proven and the dehydration enthalpy determined by combination of thermal analysis and X-ray diffraction. The mechanism of the monohydrate formation is investigated by scanning electron microscopy (SEM) and temperature dependent in situ X-ray powder diffraction. Mechanistic impact on the powder bulk properties are deduced. As for the orthorhombic dimorph, a crystalline monohydrate species is identified and firstly indexed. Thermal induced changes of the lattice parameters up to the formation of the monohydrate are analyzed. A linear elongation of the lattice parallel to the directions [100] and [001] is found. The corresponding elongation coefficients and the volumetric expansion of the unit cell are determined. With regard to the option of synthetic production of the compound, a possible application of the material for thermochemical energy storage and conversion is conceivable. The relevance of fundamental mechanistic features on the bulk behaviour upon cyclic de- and rehydration is discussed.

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The observed stacking disorder in scholzite can be explained by a stacking ambiguity of layers parallel to (100). This is due to equivalent surroundings of the calcium atoms on the boundary planes of two neighbouring layers in two possible relative positions to each other. Consequently, a series of possible polytypes of this compound was derived, among which the structure of parascholzite is a likely candidate, from which the space group and the lattice parameters could be found in agreement with these predictions. It was demonstrated that the OD structure does not consist of layers of one kind but of two kinds.
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Thermal energy storage and conversion aims to improve the high inefficiency of the industrial processes and renewable energy systems (supply versus demand). Chemical sorption processes and chemical reactions based on solid–gas systems are a promising way to store and convert thermal energy for heating or cooling applications and, thereby to increase the efficiency of the processes and to reduce the greenhouse effect. Although more efforts are required to bring this technology to the market, some important breakthrough have been made regarding to system efficiency. Over the last two decades, the experimental research in this field has increased largely to validate these advances under practical conditions. Therefore, this paper gives a state-of-art review of performances obtained under practical conditions by the different prototypes built over the last two decades. In addition, the main advantages and disadvantages of solid–gas chemical sorption processes and chemical reactions are summarized.
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After introducing the notion of order-disorder (OD) structures, three basic ideas of the theory of OD structures are presented: (1) explanation of the phenomena of polytypism and stacking disorder on the basis of partial symmetries, (2) construction of a symmetry theory for families of crystal structures consisting of layers and (3) definition of a small number of outstanding structures among the infinite number of stacking sequences in a family of OD structures. The relations between OD structures and polytypes and the contents of a database for OD structures are described.
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Simple thermal decomposition reactions have been investigated for the purpose of solar thermal energy storage. Ten criteria regarding the thermodynamics and kinetics of the reaction and the physical properties of the components of the reaction have been established. One particular reaction, the decomposition of ammonium hydrogen sulfate, has been evaluated in a preliminary manner and appears to satisfy all of the established criteria. The efficiency of storage is high and the decomposition occurs in the vicinity of 500°C. Other compounds such as ammonium halides, alkali and alkaline earth metal hydroxides, carbonates, sulfates and oxides have also been examined.ResumenHan sido investigadas reacciones térimicas de descomposición simples con el propósito de acumular energía térmica solar. Han sido establecidos diez criterios observando la thermodinámica y cinética de la reacción, y las propiedades físicas de los componentes. En forma preliminar ha sido evaluada un reacción particular, la descomposición del sulfato ácido de amonio, y aparece satisfaciendo todos los criterios establecidos. El rendimiento de acumulación es alto y la descomposición occurre en la vecindad de los 500°C. También han sido examinados otros compuestos como haluros de amonio, álcalis e hidróxidos, carbonatos, sulfatos y óxidos.RésuméOn a recherché des réactions décomposition thermique simples au sujet du stockage de l'énergie thermique solaire. On a établi dix critères relatifs à la thermodynamique, à la cinétique de la réaction, et aux propriétés physiques des composants de cette réaction. Une réaction particulière, la décomposition de sulfate Hydrogène d'Ammonium a étéévaluée en premier lieu et elle paraît satisfaire tous les critères étabilis. L'efficacité d'un tel stockage est grande et la décomposition se produit au voisinage de 500°C. D'autres composés tel que des hydroxydes, Carbonates, Sulfates et Oxydes de métaux alcalins et alcalinoterreux ont également été étudiés.
Article
Parascholzite (SG: I2/c, a = 17.186(6) Å, b = = 7.413(3) Å, c = 6.663(2) Å, β = 95.39(3)°, Z = 4, ϱ = = 3.12g/cm3, ϱ = 3.118g/cm3, V = 845(5) Å3, R(F) = = 3.0%) and the idealized substructure of scholzite, SG: Pbcn, a = 17.149(3) Å, b = 7.421(1) Å, c = 6.667(1) Å, Z = 4, are “building unit structures” with layer compositions of [ZnPO4]−2n and [Ca · 2H2O]2+n. The respective phase of CaZn2[PO4]2 · 2H2O originates from both the conformation of the crystal water together with the oxygens of the zinc phosphate and two alternatives of octahedrally coordinated interstices for the Ca2+-ions. The prediction of the parascholzite structure, which had been made earlier with the aid of order-disorder considerations (Taxer, 1992), was confirmed experimentally.
Article
Thermal energy storage (TES) is an advanced technology for storing thermal energy that can mitigate environmental impacts and facilitate more efficient and clean energy systems. Thermochemical TES is an emerging method with the potential for high energy density storage. Where space is limited, therefore, thermochemical TES has the highest potential to achieve the required compact thermal energy storage. Thermochemical TES is presently undergoing research and experimentation. In order to develop an understanding of thermochemical TES systems and to improve their implementation, comprehensive analyses and investigations are required. Here, principles of thermochemical TES are presented and thermochemical TES is critically assessed and compared with other TES types. Recent advances are discussed.
Article
The new mineral parascholzite, CaZn2(POo)2.2H2O, from Hagendorf, Bavaria, is mono- clinic, Cc ot A/c, with a : 17.864(5), b :7.422(2), c : 6.67aQ)A, and B: 106'27(l)'. Com- parison with scholzite from Reaphook Hill, South Australia shows that the two minerals are dimorphous, scholzite being orthorhombic, Pbc2,,with a: 17.178(8), b:22.24(l), and c : 6.681(3)A. Aside from its monoclinic distortion, the unit cell of parascholzite corresponds to the subcell of scholzite, which has B : b/3 : 7 .4134 and symmetry Pbcn. Both scholzite and parascholzite are derived from this basic structure. Weak diffuse nonrational superstructur€ reflections from parascholzite indicate a supercell with c' = 3c. Parascholzite is invariably twinned with {100} as both the twin and composition plane. Parascholzite is white to colorless with a white streak. The speci-frc gravity is 3.12 (obs.) and density 3.10 g/cm3 (calc.). Hardness (Mohs) is 4. Optically it is biaxial (+) with 2Y": 25o (obs.), r ) v, 4 : 1.587, P: 1.588, y : 1.603; the orientation of the indicatrix is bllX and cAZ : l3o in the acute angle between c and a. Physical, optical, and morphological properties of the two minerals are similar, but para- scholzite may be distinguished by its inclined extinction and unique powder diffraction lines at 4.158 and2.779A. Scholzite and parascholzite commonly form syntaxial intergrowths with (100) the composition plane. Much previously published scholzite data were obtained from these intergrowths or from parascholzite itsele which explains some of the confusion in the literature on the former mineral. A new set of physical optical and crystallographic data for Australian scholzite is presented. Worldwide occurrences of the two minerals are also re- viewed.
Article
THE thermal decomposition, in vacuo, of potassium permanganate1 and silver permanganate2 crystals is markedly affected by pre-irradiation in Bepo or in a cobalt-60 hot-spot. The induction periods are shortened and the maximum velocities are increased. A mechanism has been suggested involving the growth of decomposition spikes and the accumulation of strain in the crystal over the induction period.
Article
Two kinds of phosphate conversion coatings, including zinc phosphate coating and zinc–calcium phosphate coating, were prepared on the surface of AZ31 alloy in phosphate baths. The morphologies of these coatings were observed using scanning electron microscopy. Their chemical compositions and structures were characterized using energy-dispersive X-ray spectrum, X-ray photoelectron spectroscopy and X-ray diffraction. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization technique. The results show that the flowerlike Zn–Ca phosphate conversion coatings are mainly composed of hopeite (Zn 3 (PO 4) 2 ·4H 2 O). They have a quite different morphology from the dry-riverbed-like Zn phosphate coatings that consist of MgO, MgF 2 , Zn or ZnO and hopeite. Both of the zinc and zinc–calcium phosphate coatings can remarkably reduce the corrosion current density of the substrates. The Zn–Ca coating exhibits better corrosion resistance than the Zn coating. Introduction of calcium into the phosphate baths leads to the full crystallinity of the Zn–Ca coating.
Article
As an alternative to storage of sensible heat in liquids or solids or as latent heat of fusion, heat storage by means of reversible chemical reactions is under investigation. By this method, a chemical is separated into two components by heating and heat absorption, following which the components are stored in separate vessels and are recombined to generate heat when it is needed. The attractiveness of this concept of heat storage is not only higher energy density, but the capability to store energy as long as desired at ambient temperature, the option of transporting the chemicals to generate heat at another location, and the high temperatures characteristic of some of the reactions which result in high efficiency when the stored heat is used to generate electricity. Many reactions have been proposed and analyzed. Experimental work is in progress on inorganic hydroxide/oxide reactions, the decomposition of ammoniated salts, sulfur trioxide decomposition, ammonium sulfate decomposition, and others. The problems to be solved and potential applications are illustrated by the results of work in progress on Mg(OH)2 and Ca(OH)2 decomposition.
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