A small helium-3 pulse-tube refrigerator

A new three-stage pulse-tube refrigerator (PTR) is developed by scaling down a previous PTR by 50%. The new system is small in size and weight, capable of operating using little input power, and uses a small amount of working gas and regenerator material. In addition to that the system is flexible and convenient for modifications. The volume of the low-temperature part of the new PTR (pulse tubes + regenerator) is as small as 0.28 l. With ³He as a working fluid a no-load temperature of 1.73 K is reached and a cooling power of 124 mW at 4.2 K is realized.     

Regenerator performance improvement of a single-stage pulse tube cooler reached 11.1 K

In order to improve the cooling performance of pulse tube cooler (PTC) at 20-40 K, hybrid regenerators are often employed. In this paper a three-layer regenerator, which consists of woven wire screen, lead sphere and Er₃Ni is optimized to enhance the cooling performance and explore the lowest attainable refrigeration temperature for a single-stage PTC. The efforts focus on the temperature range of 80-300 K, where woven wire screens are used. Theoretical and experimental studies are carried out to study the metal material and the mesh size effect of woven wire screens on the performance of the single-stage G-M type PTC. A lowest no-load refrigeration temperature of 11.1 K was obtained with an input power of 6 kW. The PTC can supply 17.8 W at 20 K and 39.4 W at 30 K, respectively.     

Synthesis, structure refinement at 296 K and physico-chemical characterizations of KMnHP3O10

Potassium manganese(III) monohydrogentriphosphate KMnHP₃O₁₀ was synthesized by flux method and characterized by single-crystal X-ray diffraction, crystallizes in the monoclinic system with centric space group C2/c. The parameters of the unit cell are a = 12.104(1), b = 8.287(1). c = 9.150(1) Å, β = 110.97(1)° and Z = 4. The structure was solved at 296 K using 893 independent reflections and refined until R(F) = 0.022; wR(F²) = 0.045. The atomic arrangement of the title compound consists of MnO₆ octahedra linked by hydrogentriphosphate anions to form a three-dimensional framework containing tunnels parallel to the c-axis where the K⁺ cations are inserted. The structure of KMnHP₃O₁₀ contains a single Mn site which is surrounded by typical Jahn-Teller [2 + 2 + 2] distorted octahedron. The title material has been also characterized by different physico-chemical techniques: powder X-ray diffraction, IR, NMR and CI spectroscopies and DTA-TGA-DSC thermal analysis.     

Synthesis of TiC-TiB2-Ni cermets by thermal explosion under pressure

TiC/TiB₂-based cermets were fabricated in situ by means of the thermal explosion under pressure technique starting from Ti-B₄C powders with the addition of varying contents of Ni metal binder to achieve near-net-shape bulks. The combustion reaction was ignited in a graphite die heated by current. Full conversion of the reactants was obtained by thermal explosion and the process yielded TiC-TiB₂-Ni materials characterised by a fine microstructure. Appreciable differences in terms of microstructure, hardness and fracture toughness by indentation were observed between core and external surface of the products due to fast cooling caused by heat transfer to the die walls. Cermets with a high content of Ni showing high hardness and fracture toughness were obtained, with values of HV₅ = 2182 and K_Ic = 8.8 MPa m^1/2 for 30 wt.% Ni and of HV₅ = 1684 and K_Ic = 12.7 MPa m^1/2 for 47 wt.% Ni.     

High temperature-induced phase transitions in Sr2GdRuO6 complex perovskite

The crystal structure behavior of the Sr₂GdRuO₆ complex perovskite at high-temperature has been investigated over a wide temperature range between 298 K ≤ T ≤ 1273 K. Measurements of X-ray diffraction at room-temperature and Rietveld analysis of the experimental patterns show that this compound crystallizes in a monoclinic perovskite-like structure, which belongs to the P2₁/n (#14) space group and 1:1 ordered arrangement of Ru⁵⁺ and Gd³⁺ cations over the six-coordinate M sites. Experimental lattice parameters were obtained to be a =5.8103(5) Å, b =5.8234(1) Å, c =8.2193(9) Å, V = 278.11(2) Å³ and angle β = 90.310(5)°. The high-temperature analysis shows the occurrence of two-phase transitions on this material. First, at 573 K it adopts a monoclinic perovskite-type structure with I2/m (#12) space group with lattice parameters a = 5.8275(6) Å, b = 5.8326(3) Å, c = 8.2449(2) Å, V = 280.31(3) Å³ and angle β = 90.251(3)°. Close to 1273 K it undergoes a complete phase-transition from monoclinic I2/m (#12) to tetragonal I4/m (#87), with lattice parameters a = 5.8726(1) Å, c = 8.3051(4) Å, V = 286.39(8) Å³ and angle β = 90.0°. The high-temperature phase transition from monoclinic I2/m (#12) to tetragonal I4/m (#87) is characterized by strongly anisotropic displacements of the anions.     

Crystal structure and physicochemical properties of a new organic monohydrogen monophosphate hydrate

Crystals of 2(2-ammonium ethyl ammonium) ethanol monohydrogenmonophosphate monohydrate: (C₄H₁₄N₂O)HPO₄·H₂O abbreviated as AEEHP, were prepared and grown at room temperature. The AEEHP crystallizes in the monoclinic system with the P2₁/a space group. Its unit cell dimensions are: a = 4.8236(2), b = 28.429(2), c = 7.0711(6) Å, and β = 94.881(4)° with V = 966.14(11) ų and Z = 4. The structure of this compound was determined by using X-ray data collection on single-crystal. The AEEHP structure is built up from infinite inorganic HPO₄²⁻ chains parallel to the a-axis, alternating with infinite chains of H₂O molecules. These chains are interconnected by organic groups so as to build a three dimensional arrangement. In the present work, we describe the crystal structure, thermal behavior and IR analysis of this new compound.     

Diffuse scattering in the cesium pyrochlore CsTi0.5W1.5O6

The structure of the defect pyrochlore CsTi_0.5W_1.5O₆ has been investigated using electron, synchrotron X-ray and neutron diffraction methods. The material is cubic a = 10.2773 Å with displacive disorder of the Cs cations along the 〈1 1 1〉 direction. The local structure, revealed by the diffuse structure in the electron diffraction patterns shows there is correlated displacement of the heavy Cs atoms along the 〈1 1 0〉 directions. The thermal expansion of the material is also described.     

Synthesis and characterization of a new monophosphate (5-Cl-2,4-(OCH3)2C6H2NH3)H2PO4

Chemical preparation, crystal structure and NMR spectroscopy of a new organic cation 5-chloro(2,4-dimethoxy)anilinium monophosphate H₂PO₄ are given. This new compound crystallizes in the monoclinic system, with the space group P2₁/c and the following parameters: a = 5.524(2) Å, b = 9.303(2) Å, c = 23.388(2) Å, β = 90.66(4), V = 1201.8(2) Å³, Z = 4 and D_x = 1.573 g cm⁻³. Crystal structure has been determined and refined to R = 0.031 and R_w = 0.080 using 1702 independent reflections. Structure can be described as an infinite (H₂PO₄)_nⁿ⁻ corrugated chains in the a-direction. The organic groups (5-Cl-2,4-(OCH₃)₂C₆H₂NH₃)⁺ are anchored between adjacent polyanions through multiple hydrogen bonds. This compound is also investigated by IR, thermal, and solid-state, ¹³C, ³¹P MAS NMR spectroscopies.     

Hygroscopicity and bulk thermal expansion in Y2W3O12

Negative thermal expansion material, Y₂W₃O₁₂ has been synthesized by the solid-state method and bulk thermal expansion of the material has been investigated from 300 to 1100 K. The material reversibly forms a trihydrate composition whose X-ray diffraction pattern can be indexed to an orthorhombic unit cell with a = 10.098(1) Å, b = 13.315(3) Å, c = 9.691(4) Å. The cell volume of the hydrated pattern is 7% smaller than the unhydrated cell volume. According to the dilatometric studies, the material shows a 3-6% increase in the linear strain at about 400 K, which can be attributed to the removal of water. Sintering the material at 1473 K leads to large grain size of >100 μm, which results in a large hysteresis in the bulk thermal expansion behavior. Hot pressing at 1273 K under a uniaxial pressure of 25 MPa results in a fine-grained (2-5 μm) ceramic. Glazing the ceramic prevents moisture pick up and a linear thermal expansion over the entire temperature range 1100-300 K and an average linear thermal expansion co-efficient of −9.65 × 10⁻⁶/K is observed. The effect of water on the thermal expansion behavior of this system is discussed.     

Crystal structure, thermal analysis and IR spectrometric investigation of bis(o-anisidinium) sulfate (C7H10NO)2SO4

Chemical preparation, X-ray single-crystal, thermal behaviour, and IR spectroscopy investigations are given for a new organic cation sulfate (C₇H₁₀NO)₂SO₄ (denoted BOAS) in the solid state. This compound crystallizes in the monoclinic space group P2₁/c. The unit cell dimensions are: a = 7.010(3) Å, b = 11.142(5) Å, c = 20.770(8) Å, β = 95.27(3)° with V = 1615.4(12) Å³ and Z = 4. The structure has been solved using a direct method and refined to a reliability R factor of 0.047. The title compound consists of a framework of isolated SO₄ tetrahedral interleaved with organic molecules, so as to build isolated ribbons parallel to a-axis. In the present work, we describe the crystal structure, thermal behaviour and IR analysis of this new compound.     

6H hexagonal structure of low loss Ba3MTiWO9 (M = Mg, Zn) dielectrics

The crystal structure of Ba₃MTiWO₉ (M = Mg, Zn) oxides has been found to be 6H hexagonal, space group P6₃/mmc, parameters of unit cell: a = 5.7943(1) Å, c = 14.1642(1) Å and a = 5.7993(1) Å, c = 14.1626(1) Å for M = Mg and Zn, respectively. The tungsten and titanium atoms are randomly distributed in pairs of face-sharing octahedra separated by octahedral layers containing magnesium or zinc. It was revealed that the structures are well ordered which is believed to result in a low dielectric loss at a microwave frequency reported for these materials.     

Preparation and characterization of In(I)-β″-alumina

In(I)-β″-alumina has been prepared by ion exchange from Na-β″-alumina. It is stable on heating in air up to 400 °C. Its structure was determined by Rietveld analysis using high-resolution X-ray powder diffraction data [R-3m, a = 5.6044(1) Å, c = 34.4807(1) Å, Z = 3]. The coordination of In(I) in this material is asymmetric, as indicated by the distortion index for this cation, suggesting some stereochemical activity for the lone pair on the cation. However, the distortion is not as pronounced as that seen in the Ga(I) analogue.     

Specific heat, electrical resistivity and thermoelectric power of YbNi4Si

The studies of the specific heat, electrical resistivity and thermoelectric power of YbNi₄Si are reported. These studies are supported by magnetic susceptibility and X-ray photoemission spectroscopy (XPS) measurements. YbNi₄Si does not order magnetically down to 4 K. Nearly in the whole temperature range studied the magnetic susceptibility follows a Curie law with μ_eff = 4.15 μ_B/f.u. This effective magnetic moment is close to the value expected for the 4f¹³ configuration (4.54 μ_B). The Yb²⁺ and Yb³⁺ peaks observed by XPS in the valence band region confirm the domination of the Yb³⁺ valence state. Based on the specific heat measurements, the electronic specific heat coefficient γ = 25 mJ/mol/K² and the Debye temperature θ_D = 320 K were derived. A quadratic dependence of electrical resistivity at low temperatures has been observed. The Kadowaki-Woods ratio has been discussed. The thermoelectric power has been analyzed in the framework of the two band model.     

Intermediate structural phases in rare-earth substituted BiFeO3

The room-temperature crystal structure and local ferroelectric properties of Bi_0.85RE_0.15FeO₃ (RE = Sm, Gd, Dy) polycrystalline samples have been investigated by X-ray diffraction (XRD) and piezoresponse force microscopy (PFM) techniques. XRD measurements show that the rare-earth substitution causes the transformation of R3c structure typical of BiFeO₃ to yield orthorhombic phases with √2a × 2√2a × 2a and √2a × 2a × √2a superlattices (a is the parameter of the cubic perovskite subcell). Results of PFM investigations imply that both the orthorhombic phases are polar. The models describing crystal structure of the phases are discussed.     

Synthesis and Li ion conductivity of Li2O-Nb2O5-P2O5 glasses and glass-ceramics

Glasses with the compositions of xLi₂O-(70 − x)Nb₂O₅-30P₂O₅, x = 30-60, and their glass-ceramics are synthesized using a conventional melt-quenching method and heat treatments in an electric furnace, and Li⁺ ion conductivities of glasses and glass-ceramics are examined to clarify whether the glasses and glass-ceramics prepared have a potential as Li⁺ conductive electrolytes or not. The electrical conductivity (σ) of the glasses increases monotonously with increasing Li₂O content, and the glass of 60Li₂O-10Nb₂O₅-30P₂O₅ shows the value of σ = 2.35 × 10⁻⁶ S/cm at room temperature and the activation energy (E_a) of 0.48 eV for Li⁺ ion mobility in the temperature range of 25-200 °C. It is found that two kinds of the crystalline phases of Li₃PO₄ and NbPO₅ are formed in the crystallization of the glasses and the crystallization results in the decrease in Li⁺ ion conductivity in all samples, indicating that any high Li⁺ ion conducting crystalline phases have not been formed in the present glasses. 60Li₂O-10Nb₂O₅-30P₂O₅ glass shows a bulk nanocrystallization (Li₃PO₄ nanocrystals with a diameter of ∼70 nm) and the glass-ceramic obtained by a heat treatment at 544 °C for 3 h in air exhibits the values of σ = 1.23 × 10⁻⁷ S/cm at room temperature and E_a = 0.49 eV.     

Heat transfer characteristics of oscillating flow regenerators in cryogenic temperature range below 20 K

Linearized thermoacoustic model considering temperature oscillation in the solid wall is applied to analyze the heat transfer characteristics of compressible oscillating flow in parallel-plate and circular-tube regenerators. In particular, the study focus results of heat transfer analysis are applicable in lower cryogenic temperature ranges (<20 K). Complete expression for Nusselt number is derived and it is shown to be the function of six nondimensional parameters when the shape of the regenerator is fixed. These parameters are discussed, respectively. Simplified expressions of the Nusselt numbers for both parallel plates and circular tubes structured regenerators are derived. Heat transfer characteristics can be evaluated via these simple expressions. Possible approaches of enhancing heat transfer in a thermoacoustic regenerator are discussed.     

Synthesis and characterization of a layered chlorozincophosphate Zn(HPO4)Cl·[C4H10NO]

A chlorozincophosphate of the composition Zn(HPO₄)Cl·[C₄H₁₀NO] has been synthesised under mild condition water medium in the presence of morpholine as organic template. Its unit cell is monoclinic P2₁/a with parameters a = 8.655(6) Å, b = 9.302(5) Å, c = 12.180(5) Å, β = 101.10(4)°, Z = 4 and V = 962.1(9) Å³. The structure was determinated by single crystal X-ray diffraction. The structure involves a network of ZnO₃Cl and PO₃(OH) tetrahedra forming macroanionic inorganic layers with four- and eight-membered rings. Charge balance is achieved by the protonated amine which is trapped in the interlayers space and interacts with the organic framework through hydrogen bonding. Solid state ³¹P and ¹³C MAS-NMR spectroscopies are in agreement with the X-ray structure.     

Crystal structure of rhombohedral MCd(NO2)3 [M = K, Rb, Cs, Tl] from X-ray powder diffraction data

Crystal structure of double nitrites MCd(NO₂)₃ [M=K, Rb, Cs, Tl] at room temperature was ab initio determined from X-ray powder diffraction data. The compounds are isostructural and can be described as rhombohedrally distorted perovskites: space group R3 (no. 146); Z=1; M=K: a=7.5887(1), c=9.1285(2) Å; M=Rb: a=7.6290(1), c=9.2355(2) Å; M=Cs: a=7.7305(1), c=9.4170(3) Å; M=Tl: a=7.6219(1), c=9.2300(2) Å. The coordination polyhedra of cadmium and M cations are the distorted tricapped trigonal prism [(3O+3N)+3O] and cuboctahedron, respectively.     

Strontium ordering, structural modulation in layered hexagonal SrxCoO2 and physical properties of Sr0.35CoO2

Layered Sr_0.35CoO₂ has been synthesized by means of an ion exchange reaction from Na_0.7CoO₂. Resistivity measurements show that this material can be either metallic or semiconducting depending on the annealing conditions. The magnetic susceptibility of the sample increases with decreasing temperature, showing a Curie-Weiss behavior in high temperatures. Transmission electron microscopy observations reveal the presence of two superstructures arising respectively from the intercalated Sr-ordering (a compositional modulation) with q₁ = a*/3 + b*/3 and a periodic structural distortion (a transverse structural modulation) with q₂ = a*/2.     

Structure and magnetism in the oxygen-deficient perovskites Ce1−xSrxCoO3−δ (x ≥ 0.90)

We have examined the structure and phase behaviour of strontium-doped Ce_1−xSr_xCoO_3−δ and found that the perovskite form is stabilised over a relatively narrow solid solution range (x > 0.85). A combination of electron, powder X-ray and neutron diffraction has revealed tetragonal superstructures of the basic perovskite unit; (I4/mmm) 2a_p × 2a_p × 4a_p (x = 0.90) and (P4/mmm) a_p × a_p × 2a_p (x = 0.95). Magnetisation measurements show ferromagnetic behaviour under applied magnetic fields. Low temperature neutron diffraction of Ce_0.10Sr_0.90CoO_2.80 in zero field reveals a magnetic cell of dimension 2a_p × 2a_p × 4a_p with an ordered cobalt moment of 1.7 B.M. at 25 K.