Low-temperature Conductivity Research Articles

Effect of the substrate on the electrical transport and fluctuation processes in NbRe and NbReN ultrathin films for superconducting electronics applications

NbRe-based superconducting thin films recently received relevant interest in the field of low-temperature electronics. However, for these materials the electrical conduction mechanisms, in particular in the normal state, still need to be investigated in more detail. Here, NbRe and NbReN films of different thicknesses have been deposited on two different substrates, namely monocrystalline Si and text {SiO}_2 buffered Si. The films were characterized by DC electrical transport measurements. Moreover, a connection with the charge carriers fluctuation processes has been made by analyzing the electrical noise generated in the normal state region. Despite the films morphology seems not to be affected by the substrate used, a lower noise level has been found for the ones grown on text {SiO}_2, in particular for NbReN. From this study it emerges that both NbRe and NbReN ultrathin films are of very good quality, as far as the low-temperature electrical noise and conduction are concerned, with noise levels competitive with NbN. These results may further support the proposal of using these materials in a nanowire form in the field of superconducting electronics.

Особенности низкотемпературной проводимости пленок металлоорганических перовскитов при введении в них частиц оксида графена

The influence of the introduction of graphene oxide (GO) particles on the low-temperature conductivity of composite films based on organometallic perovskites CH3NH3PbBr3 with GO particles with a concentration of 0–5 wt % has been studied. It has been established that the introduction of GO particles into ITO/CH3NH3PbBr3:GO/ITO/glass films manifests itself in a decrease in the activation energy of the temperature dependence of the conductance. A sharp increase by 5–6 orders of magnitude of the resistance of the films at temperatures below 150 K was found. It is assumed that in the studied CH3NH3PbBr3:GO systems at T>150 K, the hopping mechanism of transport predominates, associated with the capture and accumulation of charge carriers in GO particles, and the increase in resistance at T<150 K may be due to the structural phase transition characteristic of organometallic perovskites in this temperature range.

Features of the low-temperature conductivity of organometallic perovskite films with the introduction of graphene oxide particles into them

The influence of the introduction of graphene oxide (GO) particles on the low-temperature conductivity of composite films based on organometallic perovskites CH3NH3PbBr3 with GO particles with a concentration of 0-5 wt.% has been studied. It has been established that the introduction of GO particles into ITO/CH3NH3PbBr3 : GO/ITO/glass films manifests itself in a decrease in the activation energy of the temperature dependence of the conductance. A sharp increase by 5-6 orders of magnitude of the resistance of the films at temperatures below 150 K was found. It is assumed that in the studied CH3NH3PbBr3 : GO systems at T>150 K, the hopping mechanism of transport predominates, associated with the capture and accumulation of charge carriers in GO particles, and the increase in resistance at T<150 K may be due to the structural phase transition characteristic of organometallic perovskites in this temperature range. Keywords: organometallic perovskites, graphene oxide, electrical conductivity, low-temperature transport.

Study on Low Temperature Conduction Mechanism of Al Doped ZnO/SiO<sub>2</sub>/ P-Si Heterojunction

The 3 at% Al doped ZnO thin films were deposited on p-Si substrate with a native SiO2 layer by spray pyrolysis method. Low temperature conduction behaviors were studied by analysis of impedance spectroscopy and low temperature ac conductivity. The results of impedance spectroscopy showed that the grain boundaries contributed to the resistivity of Al doped ZnO/SiO2/p-Si heterojunction. The calculated activation energy was 0.073 eV for grain boundaries. The equivalent circuit to demonstrate the electrical properties of Al doped ZnO/SiO2/p-Si heterojunction was a series connection of two parallel combination circuits of a resistor and a universal capacitor. Low temperature ac conductivity measurements indicated that the conductivity increased with temperature. Low temperature conductivity mechanism was electron conductivity, and the activation energy was 0.086 eV.

Influence of Electrochemically Exfoliated Graphite Addition on the Dielectric Properties of Epoxy/Montmorillonite Nanocomposites

The influence of hydrophilic electrochemically exfoliated graphite (EEG) and hydrophobic reduced EEG (rEEG) on the electrical conductivity, dielectric properties, and high-frequency dielectric losses of epoxy-based composites with montmorillonite was described. It was confirmed, that the addition of EEG changes the low-temperature conduction mechanism. The electrical conductivity in composite with EEG and montmorillonite was described by correlated barrier hopping model, whereas for composites with montmorillonite and rEEG two models were used: non-overlapping small polaron tunneling and correlated barrier hopping. The addition of EEG drastically changes the activation energy of charge carriers motions from 2.68 to 0.83 eV, whereas the addition of rEEG only to 2.43 eV. Also composite with EEG was characterized by highest high-frequency dielectric losses.

Conduction and stability performance of CsH2PO4 with NaH2PO4/ZrO2 for fuel cell

Solid acid composites of CsH2PO4/NaH2PO4/ZrO2 with different weight ratios of CsH2PO4 (CDP), NaH2PO4 (SDP), and ZrO2 were synthesized and characterized. The characterizations of the solid acid composite were carried out for determining structure, thermal stability, and conductivity, utilizing x-ray Diffraction, Fourier Transform Infrared Spectroscopy, Thermogravimetric analysis, Differential Thermal Analysis, and conductivity measurements. The composite powders were in the form of pellets of 4 mm thickness for conductivity measurement. SDP in the composites has a large effect on the conductivity measurement and this material increased the low-temperature conductivity of CDP by up to 1.5 orders of magnitude. The superprotonic transition was identified at temperatures from 220 to 270 °C in CDP/SDP/ZrO2 composites under atmospheric pressure. The thermal stability of the solid acid composites for dehydration, melting, and decomposition was investigated in the temperature range of 50 to 300 °C under environments with atmospheric humidification.

Synthesis and Characterisation of Zinc-doped Antimony Selenide

Antimony selenide substituted with Sb0.4Se0.6 and doped with zinc at three doping ratios (x=0, 0.01 and 0.03) was prepared via the solid state reaction method. The three prepared compositions were reacted thermally at 400 °C for 3 h. The structure of specimens was characterised via X-ray powder diffractometer to obtain the type of crystalline structure and lattice parameters of the prepared specimens, which showed a polycrystalline, orthorhombic structure. Optical characterisation was then achieved via UV-visible spectroscopy to exhibit the transmittance and reflectance spectra and estimate the band gap values of the prepared compositions. The samples showed high absorption spectra at low wavelengths (from 60% to 90%) and low reflectance values (from nearly zero to 17%). The band gap measurement showed an indirect transition, with values ranging from 1.2 eV to 1.23 eV. The electrical characteristics were represented by DC resistivity measurement at low temperature and AC conductivity measurement against frequency. The compositions showed a semiconducting behaviour in DC resistivity and compatible results in AC conductivity.

Construction of Stable Wide-Temperature-Range Proton Exchange Membranes by Incorporating a Carbonized Metal-Organic Frame into Polybenzimidazoles and Polyacrylamide Hydrogels.

Proton exchange membrane fuel cells (PEMFCs) are promising devices for clean power generation in fuel cell electric vehicles applications. The further request of high-efficiency and cost competitive technology make high-temperature proton exchange membranes utilizing phosphoric acid-doped polybenzimidazole be favored because they can work well up to 180°C without extra humidifier. However, they face quick loss of phosphoric acid below 120°C and resulting in the limits of commercialization. Herein UiO-66 derived carbon (porous carbon-ZrO2 ), comprising branched poly(4,4'-diphenylether-5,5'-bibenzimidazole) and polyacrylamide hydrogels self-assembly (BHC1-4) membranes for wide-temperature-range operation (80-160°C) is presented. These two-phase membranes contained the hygroscopicity of polyacrylamide hydrogels improve the low-temperature proton conductivity, relatively enable the membrane to function at 80°C. An excellent cell performance of BHC2 membrane with high peak power density of 265 and 656mWcm-2 at both 80 and 160°C can be achieved. Furthermore, this membrane exhibits high stability of frequency cold start-ups (from room temperature to 80°C) and long-term cell test at 160°C. The improvement of cell performance and stability of BHC2 membrane indicate a progress of breaking operated temperature limit in existing PEMFCs systems.

Flexibility of Room-Temperature-Synthesized Amorphous CdO-In2O3 Alloy Films and Their Application as Transparent Conductors in Solar Cells.

Due to their low-temperature deposition, high mobility (>10 cm2/V·s), and electrical conductivity, amorphous ionic oxide semiconductors (AIOSs) have received much attention for their applications in flexible and/or organic electro-optical devices. Here, we report on a study of the flexibility of CdO-In2O3 alloy thin films, deposited on a polyethylene terephthalate (PET) substrate by radio frequency magnetron sputtering at room temperature. Cd1-xInxO1+δ alloys with the composition of x > 0.6 are amorphous, exhibiting a high electron mobility of 40-50 cm2/V·s, a low resistivity of ∼3 × 10-4 Ω·cm, and high transmittance over a wide spectral window of 350 to >1600 nm. The flexibility of both crystalline and amorphous Cd1-xInxO1+δ films on the PET substrate was investigated by measuring their electrical resistivity after both compressive and tensile bending with a range of bending radii and repeated bending cycles. Under both compressive and tensile bending with Rb = 16.5 mm, no significant degradation was observed for both the crystalline and amorphous films up to 300 bending cycles. For a smaller bending radius, the amorphous film shows much less electrical degradation than the crystalline films under compressive bending due to less film delamination at the bending sites. On the other hand, for a small bending radius (<16 mm), both crystalline and amorphous films degrade after repeated tensile bending, most likely due to the development of microcracks in the films. To demonstrate the application of amorphous Cd1-xInxO1+δ alloy in photovoltaics, we fabricated perovskite and bulk-heterojunction organic solar cells (OSCs) on glass and flexible PET utilizing amorphous Cd1-xInxO1+δ layers as transparent electrodes. The organic-inorganic hybrid perovskite solar cells (PSCs) exhibit a power conversion efficiency (PCE) of ∼11 to 12% under both front and back illumination, demonstrating good bifacial performance with bifaciality factor >90%. The OSCs fabricated on an amorphous Cd1-xInxO1+δ-coated flexible PET substrate achieve a promising PCE of 12.06%. Our results strongly suggest the technological potentials of amorphous Cd1-xInxO1+δ as a reliable and effective transparent conducting material for flexible and organic optoelectronic devices.

Reduction in low-temperature thermal conductivity of Cu2Se via substitution of Se by Te atoms

We report the structural and thermoelectric properties of the Cu2Se compound with a systematic substitution of Se by Te atom. The bulk polycrystalline Cu2Se1-xTex samples for the Te concentrations of x = 0.00, 0.02, 0.04, 0.06, and 0.08 were prepared by the solid-state reaction method. The room-temperature XRD studies revealed that the studied samples possess a monoclinic crystal structure. The oxidation state and electronic structure are confirmed by employing X-ray photoemission spectroscopy (XPS). The hardness of the samples increased systematically with an increase in Te doping concentration. The electrical resistivity of the doped samples was found to be increased in comparison to the pure sample. The Seebeck coefficient was positive throughout the temperature range under investigation, indicating that holes are the majority charge carriers in the studied compounds. The thermal conductivity of the doped samples decreased, which is presumably attributed to the point defect scattering. The systematic evolution of thermoelectric properties via substitution of Se by Te over the range (0.00 ≤ x ≤ 0.08) concludes that the overall thermoelectric power factor (PF) and figure of merit (ZT) decrease with Te substitution compared to the pristine Cu2Se compound.

Dopant-segregation to grain boundaries controls electrical conductivity of n-type NbCo(Pt)Sn half-Heusler alloy mediating thermoelectric performance

Science-driven design of future thermoelectric materials requires a deep understanding of the fundamental relationships between microstructure and transport properties. Grain boundaries in polycrystalline materials influence the thermoelectric performance through the scattering of phonons or the trapping of electrons due to space-charge effects. Yet, the current lack of careful investigations on grain boundary-associated features hinders further optimization of properties. Here, we study n-type NbCo1-xPtxSn half-Heusler alloys, which were synthesized by ball milling and spark plasma sintering (SPS). Post-SPS annealing was performed on one sample, leading to improved low-temperature electrical conductivity. The microstructure of both samples was examined by electron microscopy and atom probe tomography. The grain size increases from ~230 nm to ~2.38 μm upon annealing. Pt is found within grains and at grain boundaries, where it locally reduces the resistivity, as assessed by in situ four-point-probe electrical conductivity measurement. Our work showcases the correlation between microstructure and electrical conductivity, providing opportunities for future microstructural optimization by tuning the chemical composition at grain boundaries.

Bioinspired thermochromic transparent hydrogel wood with advanced optical regulation abilities and mechanical properties for windows

The huge heat loss/gain through windows is the cause of great energy consumption in buildings. In addition, the traditional fabrication method for glass causes many environmental problems. Recently, transparent wood has emerged as a promising alternative to traditional glass because of its high transmittance, strong mechanical properties, excellent thermal insulation ability and sustainability. In this study, inspired by jellyfish, a thermochromic transparent hydrogel wood that can smartly regulate solar irradiation is proposed as a smart window material by impregnating Poly(N-isopropylacrylamide)-polyacrylamide hydrogel into delignified wood. The novel thermochromic transparent hydrogel wood inherits the excellent thermochromic properties of PNIPAM and strong mechanical properties of wood, showing advanced optical regulation ability (i.e. Tlum = 82.7% and 39.8% at the cold and hot states &ΔTsol = 38.1%), low transition temperature (i.e. Tc = 22.9 °C), mechanically robust (i.e. σ = 11.6 MPa along the axial direction) and low thermal conductivity (i.e. K = 0.37 W m−1 K−1 along the perpendicular direction of the wood growth). A field test conducted in October in Hong Kong shows that thermochromic transparent hydrogel wood can reduce the indoor air temperature by 4.3 °C. Furthermore, a computational simulation for an office building proves that 2.6–10.2% energy could be saved by thermochromic transparent hydrogel wood in four different climate-zone cities. Besides, thanks to the flexibility, thermochromic transparent hydrogel wood can be easily fitted on existing windows, demonstrating the great potential for use in energy-efficient buildings.

Separated transport relaxation scales and interband scattering in thin films of SrRuO3 , CaRuO3 , and Sr2RuO4

The anomalous charge transport observed in some strongly correlated metals raises questions as to the universal applicability of Landau Fermi-liquid theory. The coherence temperature ${T}_{\text{FL}}$ for normal metals is usually taken to be the temperature below which ${T}^{2}$ is observed in the resistivity. Below this temperature, a Fermi liquid with well-defined quasiparticles is expected. However, metallic ruthenates in the Ruddlesden-Popper family frequently show non-Drude low-energy optical conductivity and unusual $\ensuremath{\omega}/T$ scaling, despite the frequent observation of ${T}^{2}$ dc resistivity. Herein we report time-domain THz spectroscopy measurements of several different high-quality metallic ruthenate thin films and show that the optical conductivity can be interpreted in more conventional terms. In all materials, the conductivity has a two Lorentzian line shape at low temperature and a crossover to a one Drude peak line shape at higher temperatures. The two component low-temperature conductivity is indicative of two well-separated current relaxation rates for different conduction channels. In ${\mathrm{SrRuO}}_{3}$ and ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, both relaxation rates scale as ${T}^{2}$, while in ${\mathrm{CaRuO}}_{3}$ the slow relaxation rate shows ${T}^{2}$, and the fast relaxation rate generates a constant background in conductivity. We discuss three particular possibilities for the separation of rates: (a) strongly energy-dependent inelastic scattering; (b) an almost conserved pseudomomentum operator that overlaps with the current, giving rise to the narrower Drude peak; and (c) the presence of multiple conduction channels that undergoes a crossover to stronger interband scattering at higher temperatures. None of these scenarios requires the existence of exotic quasiparticles. However, the interpretation in terms of multiple conduction channels in particular is consistent with the existence of multiple Fermi surfaces in these compounds and with the expected relative weakness of ${\ensuremath{\omega}}^{2}$ dependent effects in the scattering as compared to ${T}^{2}$ dependent effects in the usual Fermi-liquid treatment. The results may give insight into the possible significance of Hund's coupling in determining interband coupling in these materials. Our results also show a route towards understanding the violation of Matthiessen's rule in this class of materials and deviations from the ``Gurzhi'' scaling relations in Fermi liquids.

Genetic analysis and resource evaluation of Dazhuang geothermal reservoir in the Minle Basin

To understand the characteristics of geothermal resources of the Dazhuang geothermal reservoir in the Minle Basin, regional geothermal geological data of the Minle Basin were collected. Combined with the Dazhuang geothermal reservoir survey, drilling and logging data, the main thermal control factors of Dazhuang geothermal reservoir “source-reservoir-pass-cover” are analysed, and a geothermal model of Dazhuang geothermal reservoir is established. The Dazhuang geothermal reservoir is based on the geothermal flow of the pull-apart basin formed on the basis of a right-handed strike-slip fault. It uses atmospheric precipitation as well as melted ice and snow in the southern Qilian Mountains as replenishment sources and flows into the thermal reservoir of the basin through the Neogene pore strata. The heat flow generated by the asthenosphere in the longitudinal direction of the Earth’s crust normally increases to form a medium–low-temperature conduction geothermal system. The Dazhuang geothermal reservoir has the characteristics of large caprock thickness, many thermal reservoir sections and rapid replenishment. The thermal reservoir caprock is mainly Quaternary and Neogene Pliocene, with a thickness of 1798 m. The thermal reservoir is composed of sandstone in the Quanzi member of the Baiyanghe Formation in the Miocene, and the cumulative thickness of the aquifer can reach 390m. The resource evaluation of the Dazhuang geothermal reservoir research area is performed using the heat storage method. The total geothermal resources in the study area are 2.08 × 108 GJ, equivalent to 612.1 × 104 t of standard coal, which has a significant development value.

Numerical Modeling of the Anomalous Low-Temperature Dependence of the Tunneling Conductance of a "Dirty" N-I-N Junction

A numerical study of the low-temperature tunneling conductance G(T) of the “dirty” (low concentrations of the same non-magnetic impurities in the I layer) N-I-N junction (N is a normal metal; I is an insulator) with random quantum jumpers penetrating the disordered I-layer is performed. In a wide range of low impurity concentrations, the dependence G(T) anomalously, both qualitatively and quantitatively, differs from the corresponding dependence G0(T) in the “pure” (without impurities in the I layer) N-I-N junction. The numerical analysis of the dependence G(T) shows the possibility of the experimental manifestation of these anomalies.

Measurement of apparent thermal conductivity of regenerator materials in 4–20 K temperature range

The axial heat leakage between the cold and hot ends of the regenerator caused by the huge temperature difference is an important part of the cryocooler loss, where the apparent thermal conductivity of regenerator materials is the key parameter that determines the heat leakage. At present, only the apparent thermal conductivity of stainless steel wire mesh, lead and copper sphere above 80 K can be obtained, while measurement results for lower temperatures or other materials are currently not available. A low-temperature apparent thermal conductivity measurement apparatus for regenerator materials has been developed, and the apparent thermal conductivities of sphere-type material (GOS, HoCu2, Er3Ni, and lead), mesh-type material (stainless steel wire mesh), and helium-4 under various pressures have been measured. When the regenerator is in a vacuum state (below 10-4 Pa), the experimental results show that the thermal conduction factors (the ratio of the apparent thermal conductivity of the filled regenerator to that of the material itself) are 0.02 for GOS in 4–10 K temperature range, 0.28 for HoCu2, 0.43 for Er3Ni in 4–20 K temperature range, 0.005 for lead sphere and 0.13 for stainless steel wire mesh in 10–40 K temperature range, respectively. However, when the regenerator is filled with helium-4, the thermal conduction factor values increase by one order of magnitude.

A benzoate coprecipitation route for synthesizing nanocrystalline GDC powder with lowered sintering temperature

Electrolyte powders with low sintering temperature and high-ionic conductivity can considerably facilitate the fabrication and performance of solid oxide fuel cells (SOFCs). Gadolinia-doped ceria (GDC) is a promising electrolyte for developing intermediate- and low-temperature (IT and LT) SOFCs. However, the conventional sintering temperature for GDC is usually above 1200 °C unless additives are used. In this work, a nanocrystalline powder of GDC, (10 mol% Gd dopant, Gd0.1Ce0.9O1.95) with low-sintering temperature has been synthesized using ammonium benzoate as a novel, environmentally friendly and cost-effective precursor/precipitant. The synthesized benzoate powders (termed washed- and non-washed samples) were calcined at a relatively low temperature of 500 °C for 6 h. Physicochemical characteristics were determined using thermal analysis (TG/DTA), Raman spectroscopy, FT-IR, SEM/EDX, XRD, nitrogen absorptiometry, and dilatometry. Dilatometry showed that the newly synthesized GDC samples (washed and non-washed routes) start to shrink at temperatures of 500 and 600 °C (respectively), reaching their maximum sintering rate at 650 and 750 °C. Sintering of pelletized electrolyte substrates at the sintering onset temperature for commercial GDC powder (950 °C) for 6 h, showed densification of washed- and non-washed samples, obtaining 97.48 and 98.43% respectively, relative to theoretical density. The electrochemical impedance spectroscopy (EIS) analysis for the electrolyte pellets sintered at 950 °C showed a total electrical conductivity of 3.83 × 10−2 and 5.90 × 10−2 S cm−1 (under air atmosphere at 750 °C) for washed- and non-washed samples, respectively. This is the first report of a GDC synthesis, where a considerable improvement in sinterability and electrical conductivity of the product GDC is observed at 950 °C without additives addition.

Critical anomalous metals near superconductivity in models with random interactions

Anomalous metals are observed in numerous experiments on disordered two-dimensional systems proximate to superconductivity. A characteristic feature of an anomalous metal is that its low temperature conductivity has a weakly temperature dependent value, significantly higher than that of a disordered Fermi liquid. We propose a dynamical mean-field model of an anomalous metal: interacting electrons similar in structure to that of the well-studied universal Hamiltonian of mesoscopic metallic grains, but with independent random interactions between pairs of sites, involving Cooper pair hopping and spin exchange. We find evidence for critical anomalous phases or points between a superconducting phase and a disordered Fermi liquid phase in this model. Our results are obtained by a renormalization group analysis in a weak coupling limit, and a complementary solution at large $M$ when the spin symmetry is generalized to USp($M$). The large $M$ limit describes the anomalous metal by fractionalization of the electron into spinons, holons, and doublons, with these partons forming critical non-Fermi liquids in the Sachdev-Ye-Kitaev class. We compute the low temperature conductivity in the large $M$ limit, and find temperature-independent values moderately enhanced from that in the disordered metal.

Estimation of density of charged defect states in some glasses of SeTeSnPb system using low-temperature d.c. conductivity measurements

The charged defect states in chalcogenide glasses (ChGs) are responsible for the hopping conduction. The estimation of the density of defect states (i.e., DDS) is a crucial task when we choose a specific ChG as electronic material for its utilization in industry-level applications. ChGs are also famous as electronic materials since they possess exceptional electrical properties (e.g., thermally governed ac/dc conduction, resistive switching, dielectric dispersion). The present study reports the results of the low-temperature dc conductivity measurements in some multicomponent glasses of SeTeSnPb. For this, the dc conductivity has been measured in the low-temperature range. The detailed analysis of obtained data indicates that the dc conductivity obeys Mott’s T−1/4 law in this region. The DDS has been determined and its composition dependence is also discussed.

Low-temperature thermal conductivity of thermoelectric Co1−xMxSi (M = Fe, Ni) alloys

We study the low-temperature electrical and thermal conductivity of CoSi and Co1−xMxSi alloys (M = Fe, Ni; x ≤ 0.06). Measurements show that the low-temperature electrical conductivity of Co1−xFexSi alloys decreases at x > 0.01 by an order of magnitude compared with that of pure CoSi. It was expected that both the lattice and electronic contributions to thermal conductivity would decrease in the alloys. However, our experimental results revealed that at temperatures below 20 K, the thermal conductivity of Fe- and Ni-containing alloys is several times larger than that of pure CoSi. We discuss possible mechanisms of the thermal conductivity enhancement. The most probable one is the dominant scattering of phonons by charge carriers. We propose a simple theoretical model that takes into account the complex semimetallic electronic structure of CoSi with nonequivalent charge carrier pockets. This model explains well the increase of the lattice thermal conductivity with increasing disorder and the linear temperature dependence of thermal conductivity in the Co1−xFexSi alloys below 20 K.