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At least 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Temperature measurement in a gas turbine engine combustor

A method and system for determining a temperature of a working gas passing through a passage to a turbine section of a gas turbine engine. The method includes identifying an acoustic frequency at a first location in the engine upstream from the turbine section, and using the acoustic frequency for determining a first temperature value at the first location that is directly proportional to the acoustic frequency and a calculated constant value. A second temperature of the working gas is determined at a second location in the engine and, using the second temperature, a back calculation is performed to determine a temperature value for the working gas at the first location. The first temperature value is compared to the back calculated temperature value to change the calculated constant value to a recalculated constant value. Subsequent first temperature values at the first location may be determined based on the recalculated constant value.

42 ENGINEERING↗
DOE OSTI2020

Development and Test of High Temperature Surface Acoustic Wave Gas Sensors

The demand for sensors in hostile environments, such as power plant environments, aerospace environments, oil and gas extraction, and high-temperature metallurgy environments, has risen over the past decades in a continuous attempt to increase process control, improve energy and process efficiency in production, reduce operational and maintenance costs, increase safety, and perform condition-based maintenance in equipment and structures operating in high-temperature harsh-environment conditions. The increased reliability, improved performance, and development of new sensors and networks with a multitude of components, especially wireless networks, are the target for operation in harsh environments. Gas sensors, in particular H2 sensors, operating above 200 C are required in the instrumentation, process control and general safety of a number of industries including coal, natural gas, and nuclear power generation facilities, the aerospace and automotive industries, metallurgical production and defense-related applications. The surface acoustic wave (SAW) platform is a particularly promising option for high-temperature, harsh-environment gas sensing applications since the platform exhibits advantages, such as battery-free and wireless operation, small size, possibility for scale production using well-developed technologies from the semiconductor industry, and low cost of installation and operation. In this work, one-port SAW resonators (SAWRs) operating along five different orientations on a commercially available langasite (LGS) wafer employing Pt-Al2O3 electrodes and reflectors were designed, fabricated, and used as high-temperature H2 sensors. Two of the selected orientations were predicted and confirmed to have temperature-compensated operation above 150 C. A gas sensor test setup was developed, capable of gas cycling between N2, O2 and N2/H2 mixtures under extended high-temperature periods (up to 650 C for over 20 hours). Thin film Pt-Al2O3 was used as the electrode material for the transducers and reflectors capable of high-temperature operation, and also as H2 sensing film. In addition, yttria-stabilized zirconia (YSZ) thin films with Pt decoration were tested as sensing films aimed to enhance the SAWR sensor response to H2. The SAW devices were monitored in excess of 1700 hours in real-time during gas cycling sequences up to 600 C, leading to the following findings: i) the Pt-Al2O3 electrodes performed better for H2 sensing than the Pt-decorated YSZ sensing film, showing as much as 50% higher frequency variation response in the 200 C to 400 C range; ii) different crystallographic orientations operating on the same LGS wafer experienced different responses to H2 exposures up to 500 C; iii) the surface oxidation state of the SAWR sensors was shown to have an important impact on subsequent H2 exposure responses. In addition, a sensor system employing two LGS SAWRs, aligned along two different orientations, has been developed to simultaneously determine H2 presence and temperature. Finally, wireless interrogation of a SAWR sensor was successful within the gas cycling test fixture, and successful wireless H2 detection was achieved above 400 C.

Ayes Moncada, Armando↗
DOE OSTI2016

Alloying effect of Nb and Sn on the Zirconium alloy fuel claddings behavior at high temperature oxidation in steam - 6.1 STP-2016-0072

As the material of fuel claddings for water cooled reactors most commonly used binary and multicomponent zirconium alloys Zr-Nb (E110,M5), Zr-Sn-Fe (Zircaloy-2,-4) and Zr-Nb-Sn-Fe (E635, Zirlo) are used, which improvement extends, in particular, by varying their composition by Nb, Sn and Fe. At the same time, the importance is given to ensuring the safe operation of the fuel rods not only in normal conditions but also in emergency situations such as LOCA. In this paper the researches results of the studies of influence of alloying elements Nb, Sn (and Fe) on corrosion resistance and embrittlement at high temperature steam oxidation of fuel claddings based on alloys types Zr-xNb (x=1.0/2.5) and Zr-xNb-ySn-zFe (x=0.6/2.4; y=0.24/1.1; z=0.18/0.34) made with using zirconium sponge are presented. High temperature steam oxidation tests were carried out at temperatures of 1000, 1100 and 1200 deg. C with continuous measurement of the weight gain during the experiment and subsequent cooling in steam with rate ∼ 20 deg. C/s. The studies of kinetics of high-temperature oxidation in steam, structural-phase state changes, alloying elements distribution, absorbed hydrogen content and residual ductility after rapid cooling of the oxidized specimens were made. The difference in the oxidation kinetics of the materials studied was revealed, it decreases with the oxidation temperature increase. At the same time, an increase of the Sn content in the alloy influences on specimens 'breakaway' oxidation intensification at 1000 deg. C. It is shown that the mechanical properties of fuel claddings oxidized under the same conditions depend on their alloying composition, the hydrogen fraction absorbed by the specimen and formed by the oxidation ZrO2, a-Zr(O) and 'ex-β' layers structure, which in turn varies depending on the alloy composition. The increase of Nb, Sn and Fe content in the zirconium alloy leads to decrease in residual ductility of fuel claddings after high temperature steam oxidation. According to obtained results the most resistant to high-temperature steam oxidation and embrittlement at temperatures of 1000-1200 deg. C among the materials studied is Zr-1Nb alloy. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2019

Autoclave Corrosion Experiment of Zircaloy-4 at Temperatures Between 180 deg. C and 270 deg. C - 19167

Corrosion of zirconium alloys is a key issue for the overall nuclear industry that has been studied under various temperatures for plant operation, severe accidents, storage, and final disposal of spent cladding. The present work aims to investigate the corrosion kinetics of zirconium alloys at intermediate temperature regions between the operation and storage/disposal conditions, to examine if the extrapolation of high temperature data to lower temperatures is valid. To achieve this, an autoclave experiment, which is a common way to study corrosion behavior, was applied for up to 485 days at 4 different temperatures: 180 deg. C, 210 deg. C, 240 deg. C and 270 deg. C. The weight gain changes due to the oxide growth of the PWR cladding tube (Zircaloy-4) showed that the corrosion kinetics in temperatures above 210 deg. C follow the cubic rate law. The activation energy evaluated by the Arrhenius relationship of the cubic rate constants was determined and found to be 132.8 ± 0.6 kJ/mol, which corresponds to that generally reported under the operation temperatures. However, the kinetics at 180 deg. C was found to be close to the parabolic rate law, which suggests the kinetics changes from cubic to parabolic around 200 deg. C. Thus, it is unlikely to predict accurately long-term corrosion behavior under storage and disposal conditions by simply extrapolating the high temperature corrosion data. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2016

Distributed Temperature Sensing within a 19-pin Wire-Wrapped Fuel Bundle Model

It has always been difficult to measure local temperature and flow velocity within fuel bundle geometries because of their complex and compact layouts. High-resolution mapping of the flow field is especially challenging. High-resolution data is typically obtained using optical instruments but optical access to the interior regions of a bundle is severely limited. Velocity fields within fuel bundle models have been mapped with particle image velocimetry (PIV), an optical technique that employs lasers and cameras. Optical access to the bundle interior is obtained using transparent materials and matching the refractive index of the physical structures with that of the fluid. Testing is necessarily isothermal because refractive indices vary with temperature. However, the ultimate aim of such studies is understanding fluid dynamics and heat transfer within real reactors and so flow field mapping under nonisothermal conditions is of particular interest. Temperature fields can be mapped at high-resolution using laser-induced fluorescence (LIF). However, as an optical technique it is unsuitable for nonisothermal liquid flows. This is due to the same sort of optical distortion that restricts PIV. It is also unsuitable for opaque fluids and setups with restricted optical access. Fiber optic distributed temperature sensing (DTS) offers a high-resolution alternative that does not rely upon imaging. A single sensor can map temperature fields at resolutions far exceeding that achievable with conventional sensors such as thermocouples. Though fiber optic sensors cannot match the spatial resolution and bandwidth of LIF, they are suitable for nonisothermal and opaque flows. And though fibers are physical probes that perturb the flow, they can be strung across complex geometries to provide data in regions lacking the optical access necessary for techniques like LIF. Distributed temperature sensing has the potential to provide fuel bundle temperature data of a type that cannot be obtained with optical methods or conventional sensors. This paper reports on preliminary results of efforts to use fiber optic distributed temperature sensors in a fuel bundle model. We are conducting a feasibility study exploring implementation issues such as strain control in the mounting configuration and signal noise associated with flow-induced vibration. We describe here the setup and present preliminary data from our first phase testing with stagnant flow. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2023

Temperature Rise on Liner Surfaces of Fuel Cell Electric Vehicle Tanks during Fueling Process

Herein, the temperature of the inner tank walls, or plastic liners, of composite pressure tanks in fuel cell electric vehicles during fueling using three‐dimensional computational fluid dynamics (CFD) models is evaluated. The liner materials are limited to 85.0 °C to prevent thermal stress causing material failure that would result in a hydrogen leak. Therefore, the temperature of hydrogen gas must be limited to below 85.0 °C during the fueling process as dictated by the current fueling protocol. However, there are limited experimental or simulation data confirming that the temperature changes do not exceed the threshold. Herein, the liner temperatures with CFD tank models for two sizes of type IV tanks representative of the upper and lower system bounds that are close to the SAE J2601 fueling protocol (36.0 and 244.0 L) are evaluated. First, each model's reliability is validated with experimental data and then analyzed, and the data are used to evaluate the maximum hydrogen and liner temperatures under real‐world fueling conditions. The evaluation shows that the maximum liner surface temperature of each tank model is at least 7 °C lower than that of the hydrogen. Additionally, there is at least 12 °C difference found between the upper limit and actual liner temperatures.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2018

Fluid–Solid Coupling Simulation on the Temperature Distribution of Tuyere Used for Oxygen Bottom Blowing Converter

For an oxygen bottom blowing converter, a high-temperature fire spot zone forms above the tuyere owing to oxygen reaction. The bottom blowing tuyere and surrounding brick are simultaneously heated by high-temperature fire spot and flowing molten steel. High-speed fluids flowing in the inner tube and the annular gap of the tuyere can remove part of the heat transferred from the fire spot zone and molten steel, playing a vital role in cooling the tuyere. To determine the temperature distributions of the tuyere and surrounding brick under the condition of oxygen bottom blowing, a fluid–solid coupling heat transfer numerical model that considers both the radiation of the fire spot and convection of the molten steel was established in this study. The simulation results agree well with previous experimental results; thus, the numerical model was validated. Utilizing the validated model, the influences of the tuyere inner tube material and fire spot zone temperature on the tuyere temperature distribution were studied in detail. Along with an increase in the material thermal conductivity, the maximum temperature of the inner tube at the hot face decreased, verifying the superiority of copper as the inner tube. More importantly, it was found that reducing the fire spot zone temperature by mixing CO{sub 2} in the bottom blowing oxygen fundamentally alleviated the burning loss of the tuyere and the surrounding brick.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of Temperature on Reaction Mechanism of Ilmenite Ore Smelting for Titanium Production

The carbothermic smelting reduction process of ilmenite ore at high temperature was investigated by thermodynamic calculations in conjunction with smelting experiments. Based on thermodynamic calculations, conducting the smelting process at a higher temperature was recommended to achieve a larger amount of FeO reduction, i.e., higher Ti-enrichment, as less precipitate and thus large amounts of a liquid slag were predicted. However, even though the reduction of FeO in ilmenite ore at the initial stage seemed to be faster as the temperature increased, no significant difference in the TiO{sub 2} or FeO concentration was observed after the reaction was complete, regardless of the temperature. This was caused by the precipitation of pseudobrookite due to the local depletion of FeO during reaction at higher temperatures, by which further reduction reaction was prohibited. The apparent rate constant increased with increasing temperature and the activation energy of the reduction process was estimated to be 144 kJ/mol, from which it was concluded that the reduction reaction of FeO in ilmenite slag by carbonaceous reductant was generally controlled through the mass transfer in the slag phase. Additionally, the formation of TiC also occurred in the iron bath. At 1923 K (1650 °C), approx. 20 pct more TiC was generated as compared to TiC formation at 1823 K (1550 °C), which also prevented further reduction of Fe at higher temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Temperature Effect on the Compressive Behavior of Closed-Cell Aluminum-Alloy Foams

In this research, the mechanical behavior of closed-cell aluminum (Al)-alloy foams was investigated at different temperatures in the range of 25-450 °C. The main mechanical properties of porous Al-alloy foams are affected by the testing temperature, and they decrease with the increase in the temperature during uniaxial compression. From both the constant/serrated character of stress–strain curves and macro/microstructural morphology of deformed cellular structure, it was found that Al foams present a transition temperature from brittle to ductile behavior around 192 °C. Due to the softening of the cellular structure at higher temperatures, linear correlations of the stress amplitude and that of the absorbed energy with the temperature were proposed. Also, it was observed that the presence of inherent defects like micropores in the foam cell walls induced further local stress concentration which weakens the cellular structure’s strength and crack propagation and cell-wall plastic deformation are the dominant collapse mechanisms. Finally, an energy absorption study was performed and an optimum temperature was proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Shock Response of Commercial Purity Polycrystalline Magnesium Under Uniaxial Strain at Elevated Temperatures

Here, results of plate impact experiments designed to investigate the onset of incipient plasticity in commercial purity polycrystalline magnesium (99.9%) under weak uniaxial strain compression and elevated temperatures up to melt are presented. The dynamic stress at yield and post yield of magnesium, as inferred from the measured normal component of the particle velocity histories at the free (rear) surface of the target plate, are observed to decrease progressively with increasing test temperatures in the range from 23 to 500 °C. At (higher) test temperatures in the range 500–610 °C, the rate of decrease of dynamic stress with temperature at yield and post-yield in the sample is observed to weaken. At still higher test temperatures (617 and 630 °C), a dramatic increase in dynamic yield as well as flow stress is observed indicating a change in dominant mechanism of plastic deformation as the sample approaches the melt point of magnesium at strain rates of ~105/s. In addition to these measurements at the wavefront, the plateau region of the free surface particle velocity profiles indicates that the longitudinal (plastic) impedance of the magnesium samples decreases continuously as the sample temperatures are increased from room to 610 °C, and then reverses trend (indicating increasing material longitudinal impedance/ strength) as the sample temperatures are increased to 617 and 630 °C. Lastly, electron back scattered diffraction analysis of the as-received and annealed pre-test magnesium samples reveal grain coarsening as well as grain re-orientation to a different texture during the heating process of the samples.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effects of cryogenic temperature and grain size on fatigue-crack propagation in the medium-entropy CrCoNi alloy

CrCoNi-based high-entropy alloys have demonstrated outstanding mechanical properties, particularly at cryogenic temperatures. Here we investigate the fatigue-crack propagation properties of the equiatomic, single-phase, face-centered cubic, medium-entropy alloy (MEA), CrCoNi, that displays exceptional strength, ductility and toughness, all of which are enhanced at cryogenic temperatures. Fatigue-crack growth is examined, at a load ratio of 0.1 over a wide range of growth rates, from ~10 –11 to >10 –7 m/cycle, at room (293 K) and cryogenic (198 K, 77 K) temperatures for two grain sizes (~7 and 68 µm), with emphasis on near-threshold behavior. We find that the ΔK th fatigue thresholds are increased with decreasing temperature and increasing grain size: from 5.7 MPa√m at 293 K to 8 MPa√m at 77 K in the fine-grained alloy, and from 9.4 MPa√m at 293 K to 13.7 MPa√m at 77 K in the coarse-grained alloy. Mechanistically, transgranular cracking at 293 K transitions to a mixture of intergranular and transgranular at cryogenic temperatures, where the increased propensity of nano-twins appears to inhibit growth rates by deflecting the crack path. However, the main factor affecting near-threshold behavior is roughness-induced crack closure from interference between the crack flanks, which is enhanced by the rougher fracture surfaces at low temperatures, particularly in the coarser-grained microstructure. Here, fatigue-crack propagation behavior in CrCoNi is comparable to nickel-based superalloys but is superior to that of the high-entropy CrMnFeCoNi (Cantor) alloy and many high-strength steels, making the CrCoNi alloy an excellent candidate material for safety-critical applications, particularly involving low temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Machine learning based simultaneous control of air handling unit discharge air and condenser water temperatures set-point for minimized cooling energy in an office building

In this study, an artificial intelligence based real-time prediction and control model to optimize condenser water temperature and discharge air temperature (DAT) set-points in water-cooled air handling unit (AHU) system has been developed. EnergyPlus-MATLAB co-simulation has been conducted to analyze the developed model's effectiveness. Here, to develop artificial neural networks (ANN) model, embedded neural network objects in MATLAB was utilized. The developed model could decide an optimal temperature set-points based on outdoor air wet-bulb temperature to reflect the Korean climate context. As a result, the developed ANN prediction model showed the predictive performance of Cv(RMSE) of approximately 21%. Compared to the conventional fixed temperature algorithm, which fixes AHU DAT at 14°C and condenser water temperature at 32°C, the ANN based optimized control showed a 22% total cooling energy reduction. These results show that significant energy savings can be achieved by simultaneously controlling condenser water temperature and AHU DAT set-points considering Korean climatic characteristics using AI technologies such as ANN models.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2017

Mechanical response of 99.999% purity aluminum under dynamic uniaxial strain and near melting temperatures

In this study, a series of reverse geometry normal plate impact experiments are employed to investigate the incipient plastic behavior of commercial purity polycrystalline aluminum under ultra-high strain-rates (~105 /s) and test temperatures up to melt (i.e. 643 °C). The applied stress at the aluminum/target interface, as inferred from the measured normal free-surface particle velocity record, shows progressive weakening with increasing test temperatures ranging from 23–620 °C; at higher temperatures (470–620 °C) this rate of weakening is observed to decrease, and at the highest test temperatures employed in the study, i.e. temperatures approaching the melt point of aluminum (643 °C), a net increase in the longitudinal stress is observed. Additionally, estimates of longitudinal elastic impedance of the sample material, as inferred via impedance matching from the particle velocity states at shock plateau show similar monotonic decrease with temperature, except for the highest test temperature case, in which reversal of this trend is observed. Though slight, these noticeable changes in the impedance, and stress/velocity states likely indicate variances in the strengthening/hardening response of the material, which can be linked to transition in the dominant plastic flow mechanisms as the sample approaches the melting point. Scanning electron microscopy of the impact surface of the post-test samples reveal coarsening of the sample grains due to static recrystallization during the heating phase of the experiment. Additional experiments are performed on annealed specimens to probe the effects of microstructural changes, i.e. grain size, and/or relief of residual strains in the interpretation of the measured response of aluminum. Finally, the results, which show a decrease in the longitudinal acoustic impedance, and decrease in the levels of stress/particle velocity under shock loading, indicate that the effects of annealing may play a role in the observed response by promoting softening of the dynamic strength, and/or lessening of the rate of hardening in a post-yield regime.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of sintering temperature on structural, magnetic and electrical transport properties of La0.67Ca0.33MnO3 ceramics prepared by Plasma Activated Sintering

Highlights: • La{sub 0.67}Ca{sub 0.33}MnO{sub 3} were prepared by using Plasma Activated Sintering technology. • Detailed studies of sintering temperature effect on structural, magnetic and electrical transport behaviors were investigated. • The observed behaviors have been explained by oxygen vacancy effect in combination with grain size effect. - Abstract: La{sub 0.67}Ca{sub 0.33}MnO{sub 3} (LCMO) ceramics were prepared by Plasma Activated Sintering (PAS). The effect of sintering temperature on structural, magnetic and electrical transport properties of the LCMO ceramics were investigated. XRD and SEM characterizations of the LCMO ceramics indicated that all samples exhibited single orthorhombic phase. Also, through PAS dense LCMO ceramics were obtained. Certain interesting phenomena were observed from the PAS sintering temperature effect on the magnetic and transport behaviors of LCMO ceramics, which were significantly different from traditional sintering methods The magnetization decreased and the resistivity increased as the sintering temperature increased, whereas the T{sub C} and T{sub IM} shifted towards the lower temperature side. The conduction mechanism of all samples was electron-magnon scattering process in the ferromagnetic low-temperature region, while the paramagnetic high-temperature region mechanism was explained with the variable range hopping (VRH) model. All these behaviors were explained through the oxygen vacancy effect in combination with the grain size effect.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Wide-Temperature Electrolytes for Lithium-Ion Batteries

Formulating electrolytes with solvents of low freezing points and high dielectric constants is a direct approach to extend the service temperature range of lithium (Li)-ion batteries (LIBs), for which propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl butyrate (MB) are excellent candidates. In this work, we report such low temperature electrolyte formulations by optimizing the content of ethylene carbonate (EC) in the EC-PC-EMC ternary solvent system with LiPF6 salt and CsPF6 additive. An extended service temperature range from 40°C to 60°C was obtained in LIBs with lithium nickel cobalt aluminum mixed oxide (LiNi0.80Co0.15Al0.05O2, NCA) as cathode and graphite as anode. The discharge capacities at low temperatures and the cycle life at room and elevated temperatures were systematically investigated in association with the ionic conductivity and phase transition behaviors. The most promising electrolyte formulation was identified as 1.0 M LiPF6 in EC-PC-EMC (1:1:8 by wt.) with 0.05 M CsPF6, which was demonstrated in both coin cells of graphite||NCA and 1 Ah pouch cells of graphite||LiNi1/3Mn1/3Co1/3O2. This optimized electrolyte enables excellent wide-temperature performances, as evidenced by the 68% capacity retention at 40C and C/5 rate, and nearly identical stable cycle life at room and elevated temperatures up to 60C.

25 ENERGY STORAGE↗
DOE OSTI2025

Deep Learning Advances Arctic River Water Temperature Predictions

The accelerated warming in the Arctic poses serious risks to freshwater ecosystems by altering streamflow and river thermal regimes. However, limited research on Arctic River water temperatures exists due to data scarcity and the absence of robust methodologies, which often focus on large, major river basins. To address this, we leveraged the newly released, extensive AKTEMP data set and advanced machine learning techniques to develop a Long Short-Term Memory (LSTM) model. By incorporating ERA5-Land reanalysis data and integrating physical understanding into data-driven processes, our model advanced river water temperature predictions in ungauged, snow- and permafrost-affected basins in Alaska. Our model outperformed existing approaches in high-latitude regions, achieving a median Nash-Sutcliffe Efficiency of 0.95 and root mean squared error of 1.0°C. The LSTM model learned air temperature, soil temperature, solar radiation, and thermal radiation—factors associated with energy balance—were the most important drivers of river temperature dynamics. Soil moisture and snow water equivalent were highlighted as critical factors representing key processes such as thawing, melting, and groundwater contributions. Glaciers and permafrost were also identified as important covariates, particularly in seasonal river water temperature predictions. Our LSTM model successfully captured the complex relationships between hydrometeorological factors and river water temperatures across varying timescales and hydrological conditions. This scalable and transferable approach can be potentially applied across the Arctic, offering valuable insights for future conservation and management efforts.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Low temperature dielectric and conductivity relaxation studies on magnetoelectric Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3}

The single phase perovskite Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3} [PFW] was synthesized by modified low – temperature (sintering at 850°C) solid-state reaction. Rietveld refinement ofroom temperature (RT) X-ray diffraction (XRD) and neutron diffraction (ND) patterns of the samples confirm the single phase formation with cubic structure (Pm-3m). Surface morphology of the compounds was studied by Scanning electron microscope (SEM) and average grain size was estimated to be ∼2 µm. The RT dielectric properties of PFW ceramic are studied as a function of frequency from 100 - 1MHz. The temperature dependent (120 – 293K) dielectric properties were studied at few selected frequencies. We found the frequency dependent dielectric constant shows increasing trend with increase in temperature from 120 – 293K, with minimum dielectric loss. The frequency dependence of dielectric loss shows a maximum in between 10 Hz and 1 kHz, confirms the extrinsic phenomena like interfacial polarization due to space charge accumulation at grain boundaries. Impedance spectroscopy is used to study the electrical behaviour of PFW in the frequency range from 100 to 1MHz and in the temperature range from 120 - 293 K. The frequency-dependent electrical data are analysed by impedance formalisms and shows the relaxation (conduction) mechanism in the sample. We suggest this low temperature sintered PFW is a suitable candidate for the multilayer ceramic capacitorsandrelated negative temperature coefficient of resistance type (NTCR) behavior like that of semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Temperature Dependence of the Upper Critical Field in Disordered Hubbard Model with Attraction

We study disorder effects upon the temperature behavior of the upper critical magnetic field in an attractive Hubbard model within the generalized DMFT+Σ approach. We consider the wide range of attraction potentials U—from the weak coupling limit, where superconductivity is described by BCS model, up to the strong coupling limit, where superconducting transition is related to Bose–Einstein condensation (BEC) of compact Cooper pairs, formed at temperatures significantly higher than superconducting transition temperature, as well as the wide range of disorder—from weak to strong, when the system is in the vicinity of Anderson transition. The growth of coupling strength leads to the rapid growth of H{sub c2}(T), especially at low temperatures. In BEC limit and in the region of BCS–BEC crossover H{sub c2}(T), dependence becomes practically linear. Disordering also leads to the general growth of H{sub c2}(T). In BCS limit of weak coupling increasing disorder lead both to the growth of the slope of the upper critical field in the vicinity of the transition point and to the increase of H{sub c2}(T) in the low temperature region. In the limit of strong disorder in the vicinity of the Anderson transition localization corrections lead to the additional growth of H{sub c2}(T) at low temperatures, so that the H{sub c2}(T) dependence becomes concave. In BCS–BEC crossover region and in BEC limit disorder only slightly influences the slope of the upper critical field close to T{sub c}. However, in the low temperature region H{sub c2} (T may significantly grow with disorder in the vicinity of the Anderson transition, where localization corrections notably increase H{sub c2} (T = 0) also making H{sub c2}(T) dependence concave.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗