Thermal conduction measurement of miniature coaxial cables between 0.3 and 4.5 K for the wiring of superconducting detectors

Thermal conduction of thin coaxial cables made of 70-30 CuNi and stainless steel 304 with a PTFE insulator was measured in a temperature range between 0.3 and 4.5 K. Thermal conductivity of the 70-30 CuNi and PTFE was measured independently and their contribution to the thermal conduction of the coaxial cables was investigated. The thermal conductivity of the 70-30 CuNi differed from the literature by 25% at 0.36 K and 40% at 4.2 K, and the alloy exhibited a weak temperature dependence, which indicated the effects of mechanical treatment. It has been confirmed that the thermal conduction of the coaxial cables are low enough to keep a cold stage of ³He cryostats at a temperature below 0.3 K, even when one hundred cables are installed between 0.3 and 3 K for the read-out of superconducting tunnel junction arrays. The cables were installed in a cryogen-free ³He cryostat, and the operation below 0.3 K was successful.     

Electrical properties of LiNi3/5Fe1/5Cu1/5VO4 ceramics

In this work, LiNi_3/5Fe_1/5Cu_1/5VO₄ ceramic was prepared by solution-based chemical route whose X-ray diffraction results reveal an orthorhombic unit cell structure with lattice parameters of a = 9.6329 (23) Å, b = 6.8527 (23) Å, c = 5.4667 (23) Å. Complex electrical impedance and electrical conductivity were studied using complex impedance spectroscopy technique. The analysis of complex electrical impedance shows: (i) grain interior, grain boundary and electrode-material interface contributions to electrical response and (ii) the presence of temperature dependent electrical relaxation phenomena in the material. A detailed study of electrical conductivity indicates that electrical conduction in the material is a thermally activated process.     

Impedance spectroscopy studies of bulk electrical conduction in A-site acceptor (K)-doped BaTiO3

BaTiO₃ ceramics with different acceptor concentrations of potassium at A-site Ba_1?x K _x TiO_3?x/2 (BKT) were prepared by solid-state processes. The solution limit of potassium in BKT is determined to be 1 mol%. Furthermore, electrical conduction behaviours were investigated in the temperature range of 300–800 °C by analysing impedance spectra and were found extremely depending on processing and measurement conditions. When annealed at 800 °C and measured in an atmosphere of dry N₂, BKT is a mixed oxide ion/electron conductor and its ionic transference number decreases with increasing temperature. But when annealed at 800 °C and measured in an atmosphere of dry air and O₂, BKT is a p-type semiconductor with a transition from a trapped holes’ conduction to activated holes’ conduction as temperature increases. On exposure of BKT to an atmosphere of wet N₂, BKT is a proton conductor in the temperature of 300–550 °C; however, it reverts to a p-type semiconductor above 550 °C as water is lost from the sample.     

Modulus spectroscopy of lead potassium titanium niobate (Pb<Subscript>0.95</Subscript>K<Subscript>0.1</Subscript>Ti<Subscript>0.25</Subscript>Nb<Subscript>1.8</Subscript>O<Subscript>6</Subscript>) ceramics

The modulus Spectroscopy of Lead Potassium Titanium Niobate (Pb_0.95K_0.1Ti_0.25Nb_1.8O₆, PKTN) Ceramics was investigated in the frequency range from 45 Hz to 5 MHz and the temperature, from 30 to 600 °C. XRD analysis in PKTN indicated a orthorhombic structure with lattice parameters a = 18.0809 Å, b = 18.1909 Å and c = 3.6002 Å. The dielectric anomaly with a peak was observed at 510 °C. Variation of ε^I and ε^II with frequency at different temperatures exhibit high values, which reflects the effect of space charge polarization and/or conduction ion motion. The electrical relaxation in ionically conducting PKTN ceramic analyzed in terms of Impedance and Modulus formalism. The Cole–Cole plots of impedance were drawn at different temperatures. The dielectric modulus, which describes the dielectric relaxation behaviour is fitted to the Kohlrausch exponential function. Near the phase transition temperature, a stretched exponential parameter β indicating the degree of distribution of the relaxation time has a small value. From the AC conductivity measurements the activation energy near phase transition temperature (T _C°C) has been found to different from that of the above and below T _C. The temperature dependence of electrical modulus has been studied and results are discussed.     

Thermoelectric properties of GdBaCo2−x Fe x O5+δ ceramics

The influence of Fe doping on the lattice structure and thermoelectric properties of GdBaCo_2?x Fe _x O_5+δ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ceramics was studied from room temperature to 525 K. The results show that presence of Fe will increase both electrical resistivity and Seebeck coefficients of the samples. Due to the lattice misfit and carrier concentration reduction caused by Fe, the thermal conductivity of the sample decreases. The activation energy of conductivity is larger than that of thermopower calculated in the semiconductive region, which means that the conduction mechanism may be determined as a small polaron hopping model. The optimum Fe doping amount is x = 0.6, which results in a ZT value 0.02 at 373 K.     

The Superconducting Ferromagnet UCoGe

The correlated metal UCoGe is a weak itinerant ferromagnet with a Curie temperature T _C=3 K and a superconductor with a transition temperature T _s=0.6 K. We review its basic thermal, magnetic—on the macro and microscopic scale—and transport properties, as well as the response to high pressure. The data unambiguously show that superconductivity and ferromagnetism coexist below T _s=0.6 K and are carried by the same 5f electrons. We present evidence that UCoGe is a p-wave superconductor and argue that superconductivity is mediated by critical ferromagnetic spin fluctuations.     

A capacitance-graded cryogenic high voltage bushing for vertical or horizontal mounting

The cryogenic high voltage bushing for the terminations of the 138 kV, 4100 A superconducting cable prototype at the Brookhaven National Laboratory (BNL) is described. The bushing is energized with the line-to-ground voltage between the coaxial centre and outer surrounding conductors, in the axial direction there is a temperature difference from ambient to about 6 K.The capacitively graded bushing can be mounted in any position. It is vacuum tight and withstands pressures of more than 20 bar. The heat leak of the insulation body in axial direction is only 8.5 W.The electrical specifications of the bushing are: 825 kV, 1.2 × 50 μs impulse voltage; 240 kV rms ac 2h test voltage: and > 160 kVs partial discharge inception voltage.This bushing concept may easily be modified to meet most conceivable thermal, mechanical and electrical requirements of cryogenic high voltage engineering.     

Electronic behavior and impedance analysis of microcrystalline LiFePO4

We report on the structure, DC electrical conductivity, dielectric, and impedance spectroscopic characterization of microcrystalline LiFePO₄ cathode material. Frequency variation of the dielectric constant (ε′) exhibits the dispersion that can be modeled with a modified Debye’s function, which considers the possibility of more than one ion contributing to the relaxation. At a constant frequency, the dielectric constant value increases with increasing temperature. At 100 kHz, the measured values of ε′ at 433 and 473 K are 4.6 and 5.7, respectively. The real (Z′) and imaginary part (Z″) of impedance as a function of frequency at different temperatures indicate the existence of relaxation processes and their distribution in LiFePO₄. Cole–Cole plots at different temperatures indicate that the conductivity is predominantly due to the intrinsic bulk grains. Temperature variation of DC electrical conductivity (σ _dc) (273–573 K) follows the Arrhenius relationship. Activation energy (E _a) calculated from the ln σ _dc versus 1000/T plot is 0.44 eV, which indicates the predominant electronic conduction mechanism in LiFePO₄. The AC conductivity increases with increasing frequency and temperature.     

Properties of trivalent-ion doped tungsten bronzes

Measurements of the electrical conductivity and the Hall coefficient have been made on polycrystalline yttrium-doped tungsten oxide, Y _x WO₃, for 0.05 ≤ x ≤ 0.20. This range of x includes the predicted metal–insulator transition. The results are compared with findings for La _x WO₃. Samples were prepared by solid-state reaction. Powder x-ray diffraction results for Y _x WO₃ show a single-phase perovskite cubic structure for 0.08 < x < 0.13, with additional phases occurring outside this range. Room temperature diffuse reflectance properties in the visible and infrared are dominated by the x-dependent metallic carrier concentration. The electrical conductivity and Hall effect results are similar to those of La _x WO₃ for x < 0.13, but show differences at higher x linked to the appearance of non-cubic crystallographic phases. The transport properties are typical of metal–insulator systems, with a change from metallic to activated behavior as x is reduced. The Hall results suggest that the trivalent ions donate their valence electron to the WO₃ conduction band.     

Effect of calcination temperature on structure and thermoelectric properties of CuAlO<Subscript>2</Subscript> powders

Copper aluminum oxide (CuAlO₂) with delafossite phase was synthesized by the Pechini method using different calcination temperatures to evaluate its influence on the structure and thermoelectric material properties. X-ray diffraction and Raman spectroscopy confirm that delafossite phase was formed at 1100 °C with the presence of 2H-CuAlO₂ and Al₂O₃ impurities, while at lower calcination temperatures (900 and 1000 °C), a mixture of CuO + CuAl₂O₄ (spinel phase) was observed. Energy-dispersive X-ray elemental maps display an even distribution of copper, aluminum and oxygen in the sample calcined at 1100 °C. Direct optical band gap, E _g = 3.6 eV, was calculated from reflectance diffuse spectra by Kubelka–Munk and Tauc methods. An absorption band at 1.7 eV accounts for defect levels, masking the characteristic indirect transition. The thermoelectric properties, such as Seebeck coefficient, and thermal and electrical conductivities of the sample calcined at 1100 °C were measured at different temperatures. Hall voltage and positive values of the Seebeck coefficient (425.8–434.4 µV K^?1) confirm the material’s p-type character. The independence of the Seebeck coefficient on the operation temperature indicates a small polaron electrical conduction mechanism. Thermal conductivity decreases exponentially with the temperature from 43.45 to 23.9 W m^?1 K^?1, where the principal contribution is due to phonons. Figure of merit ZT of sample calcined at 1100 °C between 100 and 800 °C increases from 1.42 × 10^?8 to 4.94 × 10^?4 in the order of the literature reports. From the Arrhenius plot ln(σT) versus 1000/T, an activation energy E _a = 0.32 eV for the electrical conductivity was calculated.     

Quasi-static Thermal Deflection of a Thin Clamped Hollow Circular Disk Due to Heat Generation

This paper deals with the determination of the thermal deflection in a thin clamped hollow circular disk defined as a ≤ r ≤ b; 0 ≤ z ≤ h under an unsteady temperature field due to internal heat generation within it. A thin hollow circular disk is considered having an arbitrary initial temperature and subjected to heat flux at the outer circular boundary (r = b) where an inner circular boundary (r = a) is at zero heat flux. Also, the upper surface (z = h) and the lower surface (z = 0) of the disk are at zero temperature. The governing heat conduction equation has been solved by using an integral transform technique. The inner and outer edges of the disk are clamped ${\frac{\partial \omega }{\partial r}=0}$ at r = a, r = b. The results are obtained in a series form in terms of Bessel’s functions and are illustrated graphically.     

Nanocrystalline copper sulfide and copper selenide thin films with p-type metallic behavior

Copper chalcogenide materials are interesting for multiple applications due to the feasibility of suiting their optical absorption and electrical conduction by the creation of copper vacancies. Here, Cu _x S and Cu _x Se nanocrystalline films with p-type conductivity have been obtained by heating evaporated copper layers of various thicknesses with elemental sulfur or selenium, at temperatures ranging from 250 to 400 °C. These preparation parameters determine the composition and the crystalline structure of the samples, which in turn control their morphology, optical and electrical properties. Thus, the surface roughness increases with the mean crystallite size, whereas the hole concentration increases as the copper atomic proportion (or x value) decreases. Owing to the high carrier densities achieved, around 10²² cm^?3, the samples show a metallic behavior with plasmonic absorption in the near infrared and electrical transport dominated by phonon scattering. Apart from such common behavior, some differences have been established between the sulfide and selenide films. One is the superior thermal stability of hexagonal CuS, present in all the temperature range, with respect to hexagonal CuSe, which evolves to cubic Cu_1.8Se above 300 °C. Other is about the bandgap, wider for the sulfide than selenide samples.     

Development of Superconducting Coaxial Cables for Cryogenic Detectors

Fast readout with low noise is essential to apply superconducting detectors to such experiments as time-of-flight mass spectrometry or X-ray spectroscopy, where high counting efficiency is required. We have developed a thin seamless superconducting coaxial cable employing NbTi alloy in both of the outer and inner electrical conductors. The outer diameter is 1.60 mm, and both conductors are separated by dielectric material of PTFE. The coaxial cable revealed to become superconductive below about 10 K. The thermal conductance was measured between 0.4 and 6 K and consistent with literature. Performance at high frequency was also measured at 4.2 K. The attenuation was very small and less than 1 dB up to about 7 GHz. The effect of mechanical treatment to thermal and electrical properties of NbTi alloy seems to be small in the present forming process of coaxial cable.      

Anisotropic Magnetocaloric Effect in Single-crystalline Pr0.52Sr0.48MnO3

We report the magnetocaloric effect (MCE) in a Pr_0.52Sr_0.48MnO₃ single crystal estimated from the isothermal magnetization curve using the Maxwell relation. Isothermal magnetization curves are measured over the range 20 K to 320 K where the field was applied parallel (??) and perpendicular (??) to the [110] direction of the perovskite structure with Pbnm space group. A peak in the temperature (T) dependence of magnetic entropy change (??S _M) with a fairly large negative value (???3.3 J/kg?K) is observed at 275 K close to the Curie temperature (T _C) for a change in field of ??H=40 kOe. The ?? and ?? components of ??S _M deviate from each other below ??260 K and an inverse MCE is observed below ??150 K. We note that the Landau theory of phase transitions satisfactorily explains the ??S _M vs T plot around the second-order transition at T _C.     

Magnetic, Electrical and Thermodynamic Properties of UCuT x Al11−x Alloys Where T = Mn, Fe and x=4 and 5

The structure, magnetic, electrical, and thermodynamic properties of UCuT _x Al_11?x alloys, where T = Mn or Fe and x=4 or 5 are presented. The behavior of the Fe alloys is ferromagnetic-like with the Curie points amounting to 180 and 230 K, and the saturation magnetic moments under magnetic field of 5 T equal to 4.75 and 6.02 μ_B/f.u., respectively, whereas under a magnetic field of about 34 T the magnetic moments amount to 6.9 and 9.0 μ_B/f.u. for the alloys with x=4 and 5, respectively. The Curie points are reflected in the temperature dependence of the specific heat in which the anomalies are found at 180–200 and 230 K for alloys with x=4 and 5, respectively, however, it shows no reflection in the temperature dependence of the electrical resistivity. The field dependence of the magnetization at T=1.9 K for both compounds exhibits considerable hysteresis. There is a pronounced difference between ZFC and FC magnetization in its temperature dependence below the Curie point for materials with x=4 and 5. The Mn alloys exhibit ferrimagnetic-like character for which, supposedly, the interplay of the uranium and manganize sublattices is responsible. Magnetic transitions are determined at T _N =300 (x=4) and 380 K (x=5). However, those anomalies do not find confirmation in the temperature dependence of the specific heat and the electrical resistivity. Magnetic moments determined at T=1.9 K and in a magnetic field of 5 T are very low and in both cases amount to about 0.35 μ_B/f.u. and these values are slightly higher in a magnetic field of 34 T reaching a value of about 1.5 μ_B/f.u. Also for the Mn alloys the clear difference between ZFC and FC magnetization in its temperature dependence below the Curie point is observed.     

Heat capacity, thermopower and magnetoresistance effects in multiferroic La0.5Bi0.5Mn0.5Fe0.5O3

We have carried out systematic investigations in perovskite multiferroic La_0.5Bi_0.5Mn_0.5Fe_0.5O₃ by means of X-ray diffraction, magnetisation, electrical resistivity, thermoelectric and heat capacity measurements. The magnetic behaviour of this composition is rather complex, though the magnetisation curve seems to be like a weak ferromagnetic material. However, there is no clear evidence of λ-anomaly in the heat capacity data down to 2 K, yet this behaviour corroborate the trends of semiconducting silicon below room temperature. The sensitivity of magnetic behaviour to the iron-manganese ratio is also demonstrated. In presence of an external field of 7 T, it exhibits a magnetoresistance of ?5 % at 130 K. The thermoelectric value increases linearly with decreasing temperature, and at room temperature the value is +85 μV/K, which is associated with the p-type polaronic conductivity.     

Synthesis and characterization of nanocrystalline TiO<Subscript>2</Subscript> thin films

Nanocrystalline thin films of TiO₂ have been synthesized by sol gel spin coating technique Thin films of TiO₂ annealed at 700 °C were characterized by X-ray diffraction(XRD), Atomic Force Microscopy, High resolution TEM and Scanning Electron Microscopy (SEM), The XRD shows formation of tetragonal anatase and rutile phases with lattice parameters a = 3.7837 Å and c = 9.5087 Å. The surface morphology of the TiO₂ films showed that the nanoparticles are fine with an average grain size of about 60 nm. Optical studies revealed a high absorption coefficient (10⁴ cm^?1) with a direct band gap of 3.24 eV. The films are of the n type conduction with room temperature electrical conductivity of 10^?6 (Ω cm)^?1.     

High temperature thermoelectric properties of CoSb3 skutterudites with PbTe inclusions

Nanostructured thermoelectric materials generally exhibit enhanced properties. PbTe–CoSb₃ thermoelectric composites (0–8 wt% of PbTe) have been successfully prepared by freeze-drying nanoparticles of PbTe (6 nm in diameter, synthesized by laser fragmentation of micron-sized particles in water) with micron-sized skutterudite CoSb₃ powders (~5 μm in diameter, synthesized by powder metallurgy), followed by spark plasma sintering. X-ray diffraction analyses and scanning electron microscopy observations have been performed. Microstructures reveal an agglomeration of the PbTe particles at the grain boundaries of CoSb₃. Electrical resistivity, thermopower, and thermal conductivity measurements have been performed in the 300–800 K temperature range. The composites exhibit n-type conduction whereas the reference CoSb₃ skutterudite is p-type. This change of conduction mode is attributed to substitution of Sb for a minute amount of Te in the composites. The influence of both PbTe nanoparticules and Te on the thermoelectric properties is discussed in detail.     

Structural and Electrical Properties of Fluorine Doped (Bi, Pb)-1212 Ceramics

In the aim to test the effect of fluorine doping on structural and electrical properties of semiconducting Bi(Pb)-1212 phase, bulk samples with nominal composition of Bi_0.4Pb_0.35Sr₂Y_0.8Ca_0.2Cu_2.05O _y F _x with x=0,0.2,0.4 and 0.6 are prepared in air by solid state reaction method and characterized. Similarly to other Bi-based cuprates, the scanning electron microscopy shows that fluorine doping increases the grain size of the samples. The obtained X-ray diffraction patterns are indexed in P4/mmm space group. A slight increase of cell volume by doping is revealed, a result which is in line with that seen in Y-123 compounds. Even if the resistivity decreases with fluorine content, all compounds exhibit a semiconducting behavior. The conduction mechanism obeys well to the two dimensional variable range hopping (2D-VRH) model from room temperature down to approximately 40 K. The analysis of the fitting results suggests that the improvement of electrical conduction could be due to a delocalization of charge carriers.     

Dielectric and impedance characteristics of Ba(Bi0.5Nb0.5)O3 ceramics

A lead free polycrystalline material Ba(Bi_0.5Nb_0.5)O₃ was prepared using a high-temperature mixed oxide technique using high purity ingredients. The formation of the material in monoclinic crystal structure was confirmed by an X-ray structural analysis at room temperature. The nature and texture of microstructure by scanning electron microscopy show that the compound has well defined grains uniformly distributed throughout the surface of the sample. Detailed studies of dielectric and impedance properties of the material, carried out in the frequency range of (1 kHz–1 MHz) at different temperatures (30 °C to 475 °C), have shown many interesting properties. Dielectric study showed an existence of diffuse phase transition around 317 °C. The temperature dependence of impedance parameters (impedance, modulus etc.) of the material exhibits a strong correlation of its micro-structure (i.e., bulk, grain boundary, etc.) with the electrical parameters. An existence of negative temperature coefficient of resistance (NTCR) type behavior in the material similar to that of semiconductors was also observed. The complex electric modulus analysis indicates the existence of hopping conduction mechanism in the system with non-exponential type of conductivity relaxation. The nature of variation of dc conductivity with temperature confirms the Arrhenius behavior of the material. The ac conductivity spectra show a typical signature of an ionic conducting system, and are found to obey Jonscher’s universal power law. The temperature dependent pre-exponential factor (A) shows peak and frequency exponent (n) possesses a minimum at transition temperature.