Exergy maximization of cascade refrigeration cycles and its numerical verification for a transcritical CO2–C3H8 system

Analysis of an endoreversible two-stage cascade cycle has been implemented and optimum intermediate temperature for maximum exergy and refrigeration effect have been obtained analytically. Further, the heat reservoir temperatures has been optimised independently. A comprehensive numerical model of a transcritical CO₂–C₃H₈ cascade system was developed with intent to verify the theoretical results. It is seen that the simulation results agree well for optimal T_L but deviate modestly from the theoretical optimum of T_H. It has also been observed that system performance improves as T_H increases and unlike theoretical predictions, no optimal T_H is present within feasible working temperatures.     

N2 decomposition by hot wire and N2 post-deposition treatment on hydrogenated microcrystalline silicon thin films

Post-deposition treatment of hydrogenated microcrystalline silicon (µc-Si:H) was carried out using a hot wire in atmospheres of N₂, N₂/H₂ or H₂ and the states of the bonds in the µc-Si:H films were investigated using X-ray photoelectron spectroscopy. For the µc-Si:H film treated in N₂ at the filament temperature (T_f) of 1600 °C, a weak N1s peak was observed. It increased slightly with increasing T_f from 1600 to 1900 °C and increased dramatically with increasing T_f from 1900 to 2000 °C. The N1s peak of the µc-Si:H film treated in N₂/H₂ at T_f = 2000  °C was one order of magnitude lower than that in N₂ at T_f = 2000 °C. These findings indicate that N₂ molecules decompose on the heated filament and that the addition of H₂ prevents N₂ decomposition.     

Study on structural phase transitions and relaxation processes of Cs2Co(SO4)2·6H2O and Cs2Zn(SO4)2·6H2O crystals

The structural phase transitions and relaxation processes of Cs₂Co(SO₄)₂·6H₂O and Cs₂Zn(SO₄)₂·6H₂O single crystals were investigated, with the phase transitions of both crystals being determined from NMR data. The spin–lattice relaxation time, T₁, of the ¹³³Cs nucleus in two crystals undergoes a significant change near the phase transition temperature, T_C, and these changes coincide with the changes in the splitting of the ¹³³Cs resonance lines. The variations in the temperature dependence for the splitting of the ¹³³Cs resonance lines and T₁ near T_C are related to changes in the symmetry of surrounding Cs⁺. In addition, the ¹³³Cs T₁ of Cs₂Co(SO₄)₂·6H₂O, which contains paramagnetic ions, was found to be shorter than that of Cs₂Zn(SO₄)₂·6H₂O. This relaxation time is inversely proportional to the square of the magnetic moment of the paramagnetic ions. The differences between the ¹³³Cs T₁ of these compounds are probably due to the differences between the electronic structures of their metal ions.     

Synthesis of New Crystalline Pu(V) Compounds from Solutions: IV. Double Pu(V) Malonates of the Composition Co(NH3)6[PuO2C3H2O4]2A ? nH2O, A = NO3, ClO4, Cl, and Br

New crystalline compounds of the composition Co(NH₃)₆[PuO₂L]₂A ⋅ nH₂O, where L = C₃H₂O₄ and A = NO₃, ClO₄, Cl, and Br, were obtained by addition of a small excess of Co(NH₃) ₆ ³⁺ ions to freshly prepared neutral malonate solutions of Pu(V). They are fairly stable in storage in air and isostructural to the corresponding Np(V) compounds. Thermal behavior of the compounds synthesized was studied, and their IR spectra were measured.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 224–227.Original Russian Text Copyright © 2005 by Krot, Bessonov, Charushnikova, Grigor’ev, Makarenkov.     

Synthesis and crystal sructure of a new dihydrogenomonophosphate (4-C2H5C6H4NH3)H2PO4

Chemical preparation, crystal structure, calorimetric, and spectroscopic investigations are given for a new organic-cation dihydrogenomonophosphate, (4-C₂H₅C₆H₄NH₃)H₂PO₄ in the solid state. This compound crystallizes in the orthorhombic space group Pbca with the following unit cell parameters: a=8.286(3) Å, b=9.660(2) Å, c=24.876(4) Å, Z=8, V=1991.2(7) Å³, and D_X=1.442 g cm⁻³. Crystal structure was solved with a final R=0.054 for 3305 independent reflections. The atomic arrangement coaled described as H₂PO₄⁻ layers between which are located the 4-ethylanilinium cations.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac (1–T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c (1 – T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^–8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency () and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 – T/T _p]H _ac ^–0.17.     

Thermodynamic analysis of optimal condensing temperature of cascade-condenser in CO2/NH3 cascade refrigeration systems

This study thermodynamically analyzed a cascade refrigeration system that uses carbon dioxide and ammonia as refrigerants, to determine the optimal condensing temperature of the cascade-condenser given various design parameters, to maximize the COP and minimize the exergy destruction of the system. The design parameters include: the evaporating temperature, the condensing temperature and the temperature difference in the cascade-condenser. The results agreed closely with the reported experimental data. The optimal condensing temperature of the cascade-condenser increases with T_C, T_E and ΔT. The maximum COP increases with T_E, but decreases as T_C or ΔT increases. Two useful correlations that yield the optimal condensing temperature of the cascade-condenser and the corresponding maximum COP are presented.     

Hydrothermal synthesis, characterization and magnetic properties of (N4C6H21)·(Co(H2PO4)(HPO4)2)

The title compound, (N₄C₆H₂₁)·(Co(H₂PO₄)(HPO₄)₂), was prepared hydrothermally (473 K, 10 days, autogenous pressure), in the presence of the tris(2-aminoethyl)amine as organic template. Its structure is built up from a network of four membered-rings, formed by the vertex linkages between [CoO₄] and [H₂PO₄] tetrahedra with [HPO₄] moieties hanging from the Co center. Hydrogen bonds involving the cobalt phosphate units and the triply protonated amine molecule, contribute to the stability of the structure. The IR spectrum shows bands characteristic of the (N₄C₆H₂₁)³⁺ cations and the (H₂PO₄)⁻ and (HPO₄)²⁻ phosphate anions. The UV-Visible-NIR spectrum confirms the tetrahedral coordination of Co²⁺ ions. The TGA analysis indicates that the dehydration of (N₄C₆H₂₁)·(Co(H₂PO₄)(HPO₄)₂) occurs in one step. Magnetic measurements from 4.5 to 305 K show a weak antiferromagnetic character of this compound.     

Two Pseudogap Behavior in La2?xSrxCuO4: Thermoelectric Power at High Temperature

Thermoelectric power (TEP) of high-T _C superconductors has been investigated in a wide range of temperature (T _C < T < 700 K) for La_2–x Sr _x CuO₄. TEP of La_2–x Sr _x CuO₄ shows different temperature dependences in three temperature regions. In the low-temperature region, a positive broad TEP peak is observed near T _p, which shifts to lower temperature upon doping. As temperature increases, TEP decreases linearly at intermediate temperature. In the high-temperature region, TEP deviates from the linear temperature dependence at a certain temperature, T _h showing a saturation behavior. As the doping concentration increases, the characteristic temperatures, T _C, T _p, and T _h, show systematic changes. In comparison with pseudogap temperature estimated from other experiments, the large pseudogap behavior in TEP at high temperature has been discussed and distinguished from the small pseudogap observed at lower temperature. A possibility of bound pairs formation in the normal state opening the pseudogap at high temperature is discussed briefly. The coexistence of bound pairs and the normal independent carriers for T _C < T < T_h could be the origin of the intrinsic inhomogeneity.     

Synthesis and crystal structure of new U(VI) and Np(VI) benzoates, K11(AnO2)23(OOCC6H5)57(H2O)18+x

The structure of new double benzoates of hexavalent actinides, K₁₁(AnO₂)₂₃(O₂C₇H₅)₅₇(H₂O)_18+x (An = U, Np), was studied. There are five crystallographically independent actinide atoms in the crystals. The coordination polyhedra of An(1), An(3), An(4), and An(5) are distorted hexagonal bipyramids, and that of An(2) is a distorted pentagonal bipyramid. Ten crystallographically independent benzoate ions have been found in the crystals. All but one anions are bidentate chelating and form with AnO₂²⁺ cations either electrically neutral [AnO₂(O₂C₇H₅)₂(H₂O)₂][An(1)O₂²⁺ cations] or negatively charged [AnO₂(O₂C₇H₅)₃]⁻[An(3)O₂²⁺, An(4)O₂²⁺, An(5)O₂²⁺ cations] complexes. The bidentate bridging benzoate ion links two adjacent An(2)O₂²⁺ cations with the formation of peculiar electrically neutral cyclic fragments [AnO₂(O₂C₇H₅)₂(H₂O)]₆ arranged perpendicular to the c-axis. There are three independent K⁺ cations in the structure. The coordination surrounding of K(1)⁺ (coordination number CN 6) is formed by oxygen atoms of two electrically neutral [AnO₂(O₂C₇H₅)₂(H₂O)₂] fragments and of the complex anion [AnO₂(O₂C₇H₅)₃]⁻. The coordination surrounding of K(2)⁺ (CN 6) consists of oxygen atoms of three complex anions [AnO₂(O₂C₇H₅)₃]⁻. The disordered K(3)⁺ cation is arranged near the sixfold axis between the cyclic fragments [AnO₂(O₂C₇H₅)₂(H₂O)]₆. An important structure-forming factor in the crystals is hydrogen bonding involving coordinated water molecules. In the coordination polyhedra of actinides, the An-O bond lengths regularly decrease in going from U to Np owing to actinide contraction.     

Crystal structure of a new organic condensed phosphate (C6H9N3O)H2P2O7

Crystals of a new organic compound, the isonicotinic acid hydrazide dihydrogendiphosphate, (C₆H₉N₃O)H₂P₂O₇ (denoted INHDP) were prepared and grown at room temperature. The INHDP crystallizes in the triclinic system with the space group. Its unit cell dimensions are: a = 7.316(3) Å, b = 7.783(3) Å, c = 10.802(4) Å, α = 82.41(3)°, β = 75.19(3)°, γ = 72.57(3)°, with V = 566.3(4) Å³ and Z = 2. Crystal structure has been determined and refined to a reliability R factor of 0.0389. The atomic arrangement can be described as inorganic infinite ribbons of H₂P₂O₇²⁻ anions spreading parallel to the b-axis. These ribbons are themselves interconnected by the organic (C₆H₉N₃O)⁺ cations so as to build a three dimensional arrangement. In the present work, we describe the crystal structure, thermal behaviour and IR analysis of this new compound.     

Pinning Energy of High-Tc (Bi, Pb)2Sr2Ca2Cu3Oy

The flux pinning potential energy of silver (Ag) sheathed high-T _c Bi-2223 tapes determined from measured current densities, J(T,H), is reported in this paper. Measurements of the transport current density, J, of the Ag sheathed Bi-2223 tapes were performed with an applied magnetic field directions parallel (H‖ab) and perpendicular (H‖c) to the tape’s board surface from 0 to 3 T, at temperatures from 23 to 77 K. To obtain the pinning potential energy of the Bi-2223 superconductor, an empirical scaling of J(T,H) is introduced in this study. Under the flux dynamics, the effective pinning energy, U _eff(T,H,J), can be formulated using the measured current density, J(T,H). Furthermore, the pinning potential energies, U(T,H), for both H‖ab and H‖c were quantitatively determined by the scaling of J(T,H). The two energies, U(T,H), exhibit as a function that is inversely proportional to both temperature and logarithmic magnetic field. The magnitude of U(T,H) for H‖ab is three or more times larger than that for H‖c under the considered conditions. The U(T,H) is sensitive to the temperature, but insensitive to the magnetic field. This characteristic of insensitiveness towards magnetic field is an advantage for applications on the conditions of high current density, high magnetic field, and low temperature.     

Upper critical and irreversibility fields in selectively doped YBa2Cu3O7

The electrical resistivity of (Y_1–xPr_x)Ba₂Cu₃O_7–gd and YBa₂(Cu_1–xZn_x)₃O_7– thin films and (Y_1–x. Tb_x)Ba₂Cu₃O_7– thin films and single crystals has been measured as a function of 0xx_crit, 2KT300K and OTH20T. The samples were oriented withc-axis parallel to applied fields. Upper critical field H_c2 and irreversibility field H_irr values have been determined from these measurements. Increased Tb doping appears to shift H_irr to higher temperatures. This coupled with observed twin peaks in magnetization measurements reflects an enhancement of flux pinning. Unlike Tb which does not appear to alterT _c , Pr and Zn doping of this system tends to depress bothT _c and the slope of the mean field normal phase-mixed phase boundary line (dH _c2/dT).     

Synthesis of New Crystalline Pu(V) Compounds from Solutions: VI. Malonates Co(NH3)6[PuO2C3H2O4]2A · nH2O, A = HCOO, CH3COO, C2H5COO, and C3H3O4

New crystalline compounds of the general composition Co(NH₃)₆[PuO₂L]₂A · nH₂O, where L = CH₂(COO)₂ and A = HCOO, CH₃COO, C₂H₅COO, and HL, were prepared by adding a small excess of Co(NH₃) ₆ ³⁺ ions to freshly prepared neutral Pu(V) malonate solutions containing single-charged organic anions. The complexes are sparingly soluble in water and fairly stable when stored in air; they have a similar structure. Previously unknown double Np(V) malonates Co(NH₃)₆[NpO₂L]₂A · nH₂O, where A = HCOO, CH₃COO, and C₂H₅COO, isostructural with the corresponding Pu(V) compounds, were also prepared. The IR spectra of the new compounds were measured, and their behavior at heating was examined. __________ Translated from Radiokhimiya, Vol. 47, No. 5, 2005, pp. 423–426. Original Russian Text Copyright ? 2005 by Krot, Bessonov, Charushnikova, Grigor'ev, Makarenkov     

Low-energy electron interaction with nitrobenzene: C6H5NO2

Low-energy electron attachment to C₆H₅NO₂ (nitrobenzene) in the gas phase is reported in the electron energy range from about 0 up to 10 eV with an energy resolution of 120 meV. Dissociative and nondissociative electron attachment to nitrobenzene were observed. From the numerous ions observed, the two most abundant were NO₂⁻ and C₆H₅NO₂⁻. Based on comparison of the abundance of studied ions with Cl⁻ in the dissociative electron attachment to CCl₄ at 0.8 eV, estimates of cross sections for the all observed ions were obtained for the first time (e.g. σ(NO₂⁻)=4.6×10⁻²⁰ m² and σ(C₆H₅NO₂⁻)=3.8×10⁻²¹ m²).     

Inorganic-organic hybrid semiconductor nanomaterials: (ZnSe)(N2H4)x(C5H5N)y

In this paper, inorganic-organic hybrid semiconductor (ZnSe)(N₂H₄)_x(C₅H₅N)_y nanosheets as well as (ZnSe)(C₅H₅N)_y nanoparticles were first synthesized by a solvothermal method in a ternary solution and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectra, ultraviolet-visible (UV-vis) absorption spectra, thermogravimetric analysis (TGA), differential scanning calorimeter (DSC) and photoluminescence (PL) spectra. The results indicated that the morphology and composition of the products were largely influenced by the reaction temperature and the volume ratio of water. When the reaction temperature and the water content were lower, (ZnSe)(N₂H₄)_x(C₅H₅N)_y nanosheets were formed. As the reaction temperature or the content of water was high enough, (ZnSe)(C₅H₅N)_y nanoparticles were formed. Pure hexagonal wurtzite ZnSe nanosheets or zinc blende ZnSe nanoparticles were obtained by extracting the corresponding hybrids. The bandgap absorption of ZnSe nanocrystals blue shifted in comparison with the bulk. The photoluminescent intensity of (ZnSe)(N₂H₄)_x(C₅H₅N)_y nanosheets was much stronger than that of (ZnSe)(C₅H₅N)_y nanoparticles.     

Synthesis and characterization of a new 2,4,6-triaminopyridinium dihydrogendiphosphate dihydrate (C4H8N5)2H2P2O7·2H2O

Structural properties of the 2,4,6-triaminopyridinium dihydrogendiphosphate dihydrate are discussed on the basis of an X-ray structure investigation. (C₄H₈N₅)₂H₂P₂O₇·2H₂O is monoclinic, C2/c, with a = 10.414(1) Å, b = 13.365(1) Å, c = 13.736(2) Å, β = 98.39(4)°, and Z = 4. The structure has been solved by a direct method and refined to a reliability R factor of 0.0375 (R_w = 0.0961) using 2751 independent reflections. The structural arrangement can be described as inorganic infinite ribbons, , spreading along the c direction; the organic groups, [C₄H₈N₅]⁺, link the precedent ribbons, via their hydrogen bonds, to form a three-dimensional network. The present work, deals with crystal structure, thermal behavior and IR analysis of this new compound.     

Metastable C3H5 produced by electron impact of propane

Metastability of C₃H₅²⁺ ions formed by electron impact ionization of propane was monitored with the help of a magnetic mass spectrometer of reversed geometry (BEE geometry). In the present paper, we report decay reactions resulting in C₃H₄⁺+H⁺, and C₃H₃⁺+H₂⁺. We observed fragment ions which are formed with high kinetic energy. Mass analyzed ion kinetic energy (MIKE) scan technique in the third field free region of the mass spectrometer was applied to identify the fragment ions and determine their kinetic energy release (KER). An average KER of 1.6±0.3 eV for the first reaction and 0.67±0.15 eV for the second reaction are reported.     

Hc2 of MgCNi3 Determined from the Specific Heat and Resistivity Measurements

H _c2 of MgCNi₃ has been determined from the specific heat C and resistivity measurements in the same sample. The results from are nearly identical with those determined from the anomaly in C. Furthermore, utilizing the relation (H) H and the value of d/dH, the obtained value of H _c2 is the same as that by the WHH model, if the spin paramagnetic effect and the spin–orbit interaction are taken into account. The results of this comparison have strong implications on the order parameter of MgCNi₃.     

A New Method for the Estimation of Hc2 Anisotropy in Polycrystalline MgB2 Samples

By using isothermal magnetization measurements in polycrystalline MgB₂ samples, we estimate the H ^c _c2 in the interval [0, T _c]. By combining these measurements to the estimated H ^ab _c2 from the onset of the diamagnetic transition in isofield and isothermal magnetic measurements, an estimation of the anisotropy parameter can be achieved. The H ^c _c2 values coming from high quality polycrystalline samples agrees nicely to those obtained on single crystals. Our results show a temperature variation of the (T ) = H ^ab _c2/H^c2 with (T _c) 3.