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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

SE2-ANL Subchannel Analysis with New Heating Calculation Based on VARIANT Transport Code

The multi-assembly, steady-state sub-channel analysis code SE2-ANL has been widely used in thermal-hydraulic analysis of sodium cooled fast reactors (SFR). It is used to compute the core-wide temperature profiles for a given flow distribution and to determine the assembly flow rates yielding a desired (assembly-wise) peak temperature distribution. It is a modified version of SUPERENERGY-2 coupled with the DIF3D/GAMSOR/DIF3D procedure for coupled neutron and gamma heating calculations, developed at Argonne National Laboratory (ANL). In the current version of SE2-ANL, the neutron and gamma fluxes are calculated using the finite-difference diffusion theory option of the DIF3D code with six triangular meshes per hexagonal assembly. In addition, the pin power distributions are estimated by assuming that the power distribution within an assembly is separable in the radial and axial directions. Furthermore, the total heat generation rate in an assembly duct wall is simply determined by a user-specified fraction of the total gamma heating within that assembly. In order to determine the pin power distributions more accurately by eliminating these limitations, a new coupled heating calculation procedure has recently developed based on the neutron and gamma transport calculations with the VARIANT code. For this, the GAMSOR code has been extended to generate the intra-nodal gamma source distribution to be consistent with the trial functions of VARIANT. Using the computed neutron and gamma flux distributions, the power distribution in each fuel pin and duct wall is determined using a new utility code CURVE, which has been developed to evaluate the VARIANT flux solution at the center of each fuel pin and along the duct mid-wall. For each pin segment contained in a VARIANT node, the axial power shape is represented as a quadratic profile. The SE2-ANL code has been modified to interface with the new, coupled neutron and gamma heating calculation scheme. In addition, several improvements have been made on the computational methods and models of SE2-ANL. To eliminate the limit on the axial mesh size due to the numerical instability problem of the current explicit scheme, the numerical scheme for discretizing the energy equation in the axial direction has been changed to the {theta}-method of the SLTHEN code. An automated flow allocation scheme has also been implemented in SE2-ANL to determine assembly flow rates such that the peak cladding mid-wall temperatures of individual orifice zones are equalized over the burn cycle. The impacts of the new heating calculation scheme based on VARIANT transport solutions on the cladding and fuel temperatures have been tested against the existing method of SE2-ANL. Using the existing and new heating calculation schemes, full-core subchannel analyses were performed for the metal core design of the 1000 MWt Advanced Burner Reactor (ABR-1000), and the resulting coolant, cladding and fuel temperatures were compared. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2019

Tailoring the Pore Size of a Polypropylene Separator with a Polymer Having Intrinsic Nanoporosity for Suppressing the Polysulfide Shuttle in Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are being considered as one of the most promising candidates for the development of next-generation energy storage technologies. Although much progress has been made over the past decade, the development of Li–S batteries is still held back by a crucial polysulfide-shuttle problem. To address this critical issue, an approach to reduce the pore size of the separator is presented here, to prevent the penetration of soluble polysulfide species. A polymer with intrinsic nanoporosity (PIN) is developed within the micrometer-scale pores of a polypropylene separator. The framework of polypropylene acts as a skeleton to sustain reliable mechanical properties with the thin membrane. Upon the formation of PIN in the pores, the polypropylene separator maintains its thickness. With the thin PIN–polypropylene membrane, the Li–S cells can be operated with a relatively high sulfur loading. The PIN allows the transport of Li + ions, but suppresses the penetration of the polysulfide species. The Li–S batteries with the PIN-modified polypropylene separator exhibit enhanced cycling performance.

25 ENERGY STORAGE↗
DOE OSTI2019

A Review of Magneto-Elastic Interaction and Its Theoretical Descriptions in Type-II Superconductors

It is well known that the interaction between defect-induced strain field and vortex lattice (VL) gives rise to vortex pinning behaviors. Likewise, magneto-elastic (ME) interaction is caused by the difference between specific volumes of normal phase and superconducting (SC) phase. Acting as inhomogeneous strain sources, the normal-state vortex cores produce local deformation in the surrounding SC matter and thus affect VL energy. ME interaction belongs to the fundamental physics of interdependences between vortex matter and crystal lattice. The review starts with the effects of ME interaction on magnetization, vortex morphology and intervortex interaction. In Sect. 2, theoretical descriptions of ME interaction are classified in terms of different simplifications. The core ideas and outstanding results of these theories are given. It is followed by Sect. 3, where the correlation and difference between ME and pinning interaction are outlined. Although pinning differs from ME interaction in their formation mechanisms, the pinning interaction is established upon the vortex-induced strain field from Ginzburg-Landau (GL) viewpoint. Section 4 goes further with the GL treatment for solving ME and pinning problem; the general thoughts and steps can be found in this section. The last section gives some outlooks which involve pressure dependence of T{sub c} and anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effect of Material Properties and Process Parameters on Morphology and Strength of Friction-Stir-Welded Dissimilar T-Joints

The influence of the material properties and the process parameters on the mechanical behavior of friction-stir-welded dissimilar T-joints of AA5083-H111 and AA6082-T6 aluminum alloys was investigated. The welds were produced in 3-mm-thick plates, in a T-joint configuration, but using each material either as the skin or as the stringer. Two different tools, with the same design of the shoulder and the initial part of the progressive pin, cylindrical threaded, but with different pin tip geometry, one conical threaded and the other pyramidal, were used. The transverse speed was varied between 30 and 220 mm/min. The results show that it is possible to produce welds without any defects and or reduction in the skin thickness as well as with an excellent filling of the fillets. The flow of material in the welds is very much conditioned by the geometry of the progressive pin of the tools. The welds produced with a higher traverse speed and using the threaded pin tool presented higher tensile strength than those performed with the pyramidal pin tool. The weld series in which the AA5083 alloy was used as the skin exhibited greater tensile and fatigue strength than the welds where the AA6082 alloy was used as the skin.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A study on the impact of using a subchannel resolution for modeling of large break loss of coolant accidents

The nuclear industry is investigating the feasibility of transitioning from 18- to 24-month fuel cycles because of the positive impact it would have on the operational costs for the current fleet of light-water reactors. A challenge to making this change is the increased risk of fuel fragmentation, relocation, and dispersal (FFRD) due to the known potential for ceramic fuel to pulverize into fine particles at the higher discharge burnups. Previous work has been performed by the Nuclear Energy Advanced Modeling and Simulation program to assess FFRD risk in high-burnup cores using the BISON fuel performance code and a coarse mesh thermal hydraulics (T/H) solution for a loss-of-coolant accident (LOCA) using the TRACE system T/H code. Because of the importance of the T/H solution for FFRD assessment, this study seeks to investigate the impact of using higher-fidelity subchannel techniques for modeling of the LOCA transient. CTF was used to model a subregion of a high-burnup core that was depleted by the Virtual Environment for Reactor Applications (VERA) multiphysics core simulator. Both coarse-mesh and pin-resolved models were created in CTF, and a consistent coarse-mesh TRACE model was also developed to allow for benchmarking the code results. Further, a large-break loss-of-coolant accident (LBLOCA) reflood transient was simulated using these three models, and results were compared. Results showed some consistent differences between the CTF and TRACE coarse models, including a higher peak cladding temperature (PCT) prediction in CTF and later quenching in CTF; however, the transient clad temperature behavior was similar, and these differences are likely due to post-critical heat flux heat transfer modeling differences and minimum film boiling temperature model differences. The pin-resolved results indicate that the PCT in the lumped model is often under-predicted by as much as 70 °C and that PCT occurs at a different location than the high-power pin in the assembly. The lumped model predicts a difference of 10 °C or less between the average and hot pins in the assembly, whereas the pin-resolved model predicts a range of over 100 °C. These results indicate that higher-fidelity T/H results may have an impact on predicted core behavior during LOCA, which may be important to consider when assessing FFRD risk.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2018

A compositional gradient Ce1−xZrxO2 buffer architecture for producing high-performance YBCO film

Highlights: • The compositional gradient CZO buffer architecture can help improve the biaxial texture of YBCO film. • The high density stacking faults are directly observed via a spherical aberration corrected transmission electron microscope. • The enhanced flux pinning force and current-carrying capacity of YBCO films are achieved as a result of lattice strains. - Abstract: In this work, YBCO films were epitaxially grown on Ce{sub 1−x}Zr{sub x}O{sub 2} (CZO) buffered yttria stabilized zirconia (YSZ) single crystal substrates through a fluorine-free sol-gel process. The influence of two different CZO buffer architectures on the microstructure and superconducting properties of YBCO films was investigated. According to XRD results in combination with (S)TEM analysis, it was confirmed that YBCO film grown on the gradient CZO buffer layer (CZO-G) exhibits better in-plane and out-of-plane texture than YBCO film deposited on the uniform CZO buffer layer (CZO-20). And besides, the superconducting performances especially the current-carrying capacity under magnetic field of the former was much higher than that of the latter, which means that the enhanced flux pinning force will be presented in YBCO film on CZO-G buffer layer. The maximum pinning force of the YBCO film grown on CZO-G buffer architecture is up to 17.67 GN/m{sup 3} @ 65 K. HR-STEM analysis shows that the stacking faults within YBCO matrix gave rise to the considerable nanoscale lattice-strains which may suppress the formation of Copper pairs and produce nanoscale non-superconducting regions served as pinning centers to significantly improve the flux pinning performance of YBCO film. Given the above, it has been implied that high-performance YBCO coated conductor can be developed by using a compositional gradient Ce{sub 1−x}Zr{sub x}O{sub 2} films as buffer architecture.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

A whole-core steady-state thermal-hydraulic model for annular fuel type fluoride-salt-cooled reactors

A whole-core, steady-state thermal-hydraulic model is developed for the fluoride-salt-cooled small modular advanced high-temperature reactor (SmAHTR) that employs an annular fuel configuration. This pre-conceptual reactor design by Oak Ridge National Laboratory (ORNL) has the annular fuel and moderator pins arranged in a hexagonal layout. The FLiBe coolant flows from the bottom to the top of the core, parallel to the hexagonal bundle. The fuel and moderator pins in the core are discretized into finite volumes and the 3-D heat conduction equation is solved to obtain the temperature profile. Inter-fuel assembly conduction is also addressed. For this fuel assembly configuration, the coolant flows through two distinct regions – the hexagonal pin bundle and the annulus between the fuel pin and the tie rod. The fluid flow through the hexagonal bundles is modeled using the subchannel approach, in which the coolant region is discretized into corner, edge and interior subchannels and the resulting conservation equations are systematically solved. The 1-D mass, momentum and energy equations are solved for the annulus channels between the fuel pin and the tie rod. Pertinent closure models from the literature are employed to close the system of equations. We also performed a preliminary code-to-code comparison between the present model and a CFD model.. The resulting thermal-hydraulic model can provide temperature, flow rate and pressure drop profiles for the different solid and fluid regions throughout the entire core. Whole-core thermal-hydraulic results for a representative power profile are presented and discussed.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2014

Elevated temperature tribology of cobalt and tantalum-based alloys

This paper describes the friction and wear behavior of a Co–Cr alloy sliding on a Ta–W alloy. Measurements were performed in a pin-on-flat configuration with a hemispherically tipped Co-base alloy pin sliding on a Ta–W alloy flat from ambient to 430°C. Focused ion beam-scanning electron microscopy (FIB-SEM) and cross-sectional transmission electron microscopy (TEM) were used to identify the friction-induced changes to the chemistry and crystal structure in the subsurface regions of wear tracks. During sliding contact, transfer of material varied as a function of the test temperature, either from pin-to-flat, flat-to-pin, or both, resulting in either wear loss and/or volume gain. Friction coefficients (μ) and wear rates also varied as a function of test temperature. The lowest friction coefficient (μ=0.25) and wear rate (1×10 –4 mm 3 /N•m) were observed at 430°C in argon atmosphere. This was attributed to the formation of a Co-base metal oxide layer (glaze), predominantly (Co, Cr)O with Rocksalt crystal structure, on the pin surface. Part of this oxide film transferred to the wear track on Ta–W, providing a self-mated oxide-on-oxide contact. Once the oxide glaze is formed, it is able to provide friction reduction for the entire temperature range of this study, ambient to 430°C. Furthermore, the results of this study indicate that glazing the surfaces of Haynes alloys with continuous layers of cobalt chrome oxide prior to wear could protect the cladded surfaces from damage.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Enhancement in the transport critical current density J{sub c} in YBa{sub 2}Cu{sub 3}O{sub 7−δ} added with an insulating nano crystalline YBa{sub 2}HfO{sub 5.5} perovskite

When a magnetic field is applied to type II superconductors, such as YBa{sub 2}Cu{sub 3}0{sub 7−δ} (YBCO), the flux quanta penetrate the material as a regular array of vortices. However when transport currents are applied, they act to move these vortices, thus lowers the critical current density (J{sub c}) as well as destroying superconductivity. The development of microstructures made of YBCO materials has enabled engineers to increase the critical current density, within Type II materials by introducing flux pinning centres into the material. The microstructure and flux pinning properties of YBa{sub 2}Cu{sub 3}O{sub 7−δ} system with varying levels (0-5 wt. %) of a nano perovskite ceramic insulator; YBa{sub 2}HfO{sub 5.5} addition was studied in detail. Orthorhombic YBa{sub 2}Cu{sub 3}O{sub 7−δ} powder was prepared through conventional solid state route and a modified combustion method was used for synthesizing nanocrystalline YBa{sub 2}HfO{sub 5.5}. The structure and microstructure of the samples examined by X-ray diffraction and scanning electron microscopy showed that YBa{sub 2}HfO{sub 5.5} and YBCO remained unreacted even at higher processing temperature without deteriorating the superconducting properties. The scanning electron microscope image shows that YBa{sub 2}HfO{sub 5.5} forms an electrical-network between grains. These observations suggest that the YBa{sub 2}HfO{sub 5.5} addition to the Y-123-compounds improve the electrical connection between superconducting grains and substantial improvements in the relative electrical transport properties of the composites. The variation of sintering temperature, density, critical transition temperature (T{sub c}) and magnetic field dependence of critical current density (J{sub c}) of YBa{sub 2}Cu{sub 3}O{sub 7−δ} having different proportions of YBa{sub 2}HfO{sub 5.5} in the matrix were also studied in detail. It is found that the addition of these elements considerably enhances the flux pinning strength of the system, and there is also an increase of critical temperature (T{sub c}) and critical current density (J{sub c}) up to an optimum value of 8.76 × 10{sup 4} A/cm{sup 2} for a concentration of 2 wt % addition of nano YBa{sub 2}HfO{sub 5.5}. The enhancement of the critical current density in the YBa{sub 2}Cu{sub 3}O{sub 7−δ} - YBa{sub 2}HfO{sub 5.5} samples is attributed to the sustained formation of the insulating and non-reacting YBa{sub 2}HfO{sub 5.5} phase acting as pinning center. The addition of YBa{sub 2}HfO{sub 5.5} in YBa{sub 2}Cu{sub 3}O{sub 7−δ} bulk superconductor also enhances the pinning force density from 3 ×10{sup 6} N/m{sup 3} to 69.6 × 10{sup 6}N/m{sup 3} at 77 K.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Field-induced domain wall motion of amorphous [CoSiB/Pt]{sub N} multilayers with perpendicular anisotropy

Amorphous CoSiB/Pt multilayer is a perpendicular magnetic anisotropy material to achieve high squareness, low coercivity, strong anisotropy, and smooth domain wall (DW) motion, because of the smoother interface compared with crystalline multilayers. For [CoSiB(6 Å)/Pt (14 Å)]{sub N} multilayers with N = 3, 6, and 9, we studied the field-induced DW dynamics. The effective anisotropy constant K{sub 1}{sup eff} is 1.5 × 10{sup 6} erg/cm{sup 3} for all the N values, and the linear increment of coercive field H{sub c} with N gives constant exchange coupling J. By analyzing the field dependence of DW images at room temperature, a clear creep motion with the exponent μ = 1/4 could be observed. Even though the pinning field H{sub dep} slightly increases with N, the pinning potential energy U{sub c} is constant (=35 k{sub B}T) for all the N values. These results imply that the amorphous [CoSiB/Pt]{sub N} multilayers are inherently homogeneous compared to crystalline multilayers. For N ≤ 6, the pinning site density ρ{sub pin} is less than 1000/μm{sup 2}, which is about 1 pinning site per the typical device junction size of 30 × 30 nm{sup 2}. Also, the exchange stiffness constant A{sub ex} is obtained to be 0.48 × 10{sup −6} erg/cm, and the domain wall width is expected to be smaller than 5.5 nm. These results may be applicable for spin-transfer-torque magnetic random access memory and DW logic device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Doping effect of nano-Ho{sub 2}O{sub 3} and naphthalene in MgB{sub 2} superconductor prepared by powder-in-sealed-tube method

The effect on crystal structure, critical temperature (T{sub C}), and critical current density (J{sub C}) of bulk MgB{sub 2} doped with nano-Ho{sub 2}O{sub 3} and naphthalene was studied. Among all the samples studied, the sample doped with 2.5 wt. % nano-Ho{sub 2}O{sub 3} have shown the best field dependent critical current density [J{sub C}(H)], i.e., 0.77 × 10{sup 5 }A/cm{sup 2} at 2 T and 10 K. While naphthalene doped MgB{sub 2} sample has shown the least J{sub C}(H) characteristics. The improved J{sub C}(H) characteristics in the nano-Ho{sub 2}O{sub 3} doped MgB{sub 2} samples are attributed to improved flux pinning properties due to the formation of HoB{sub 4} and in naphthalene doped MgB{sub 2} samples. The slight lower T{sub C} value (37.01 K) in naphthalene doped samples is attributed to the occurrence of lattice defect by the substitution of carbon at boron site of MgB{sub 2} superconductor. Lower ΔT{sub C} value implies the lesser anisotropy in all the synthesized samples. The flux pinning force density (F{sub P}/F{sub Pmax}) curves are theoretically analyzed using Dew-Hughes model. The result revealed that point pinning is the dominant pinning mechanism for nano-Ho{sub 2}O{sub 3} doped MgB{sub 2} samples, while, surface and grain boundary pinning become dominant with increasing naphthalene addition in nano-Ho{sub 2}O{sub 3} doped MgB{sub 2} samples.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Transient Modeling and Simulation of a Generic Stable Salt Reactor

A SAM system-level model of a generic stable salt reactor has been developed to investigate thermal-hydraulic behavior and safety performance under steady and transient conditions. The model integrates information generated from a reactor physics analysis using PROTEUS and PERSENT, and a computation fluid dynamics (CFD) analysis using STAR-CCM+. A loose, iterative coupling scheme between PROTEUS and SAM is implemented to calculate the equilibrium power and temperature distributions in the steady-state critical core condition. The converged steady-state model is then used in PERSENT to calculate the four reactivity feedback temperature coefficients (Doppler, fuel density, coolant density, and core radial expansion) and kinetic parameters that are needed in SAM to model the temperature feedback effects in transient simulations. Within the fully enclosed liquid fuel pins, natural convection is the dominant heat transfer mechanism. The STAR-CCM+ model of the fuel pin considers conjugate heat transfer from the liquid fuel salt to the pin cladding and external reactor coolant. The CFD results of the axial and radial temperature profiles are used to empirically determine an effective fuel salt thermal conductivity in the SAM fuel pin model so that the temperatures predicted by the SAM model match as closely as possible the CFD results. In the central region of the fuel pin, the effective thermal conductivity is as high as similar to 60 times the physical fuel salt thermal conductivity. The whole-plant SAM model is then used to simulate an unprotected station blackout transient. The results of this simulation showed that the large negative fuel axial expansion reactivity feedback reduces fission power to similar to 2.4% nominal power. The core is cooled by natural circulation, which removes heat in the core to the emergency heat removal system, and ultimately, to the ambient. However, peak fuel salt and cladding temperatures can potentially reach as high as 1500 K, albeit briefly, if the shutdown mechanism fails to operate.

stable salt reactor; transient simulations; system↗
DOE OSTI2021

Magnetic order of tetragonal CuO ultrathin films

Here, we present a detailed low-energy muon spin rotation and x-ray magnetic circular dichroism (XMCD) investigation of the magnetic structure in ultrathin tetragonal (T)-CuO films. The measured muon-spin polarization decay indicates an antiferromagnetic (AFM) order with a transition temperature higher than 200 K. The XMCD signal obtained around the CuL 2,3 edges indicates the presence of pinned Cu 2+ moments that are parallel to the sample surface, and additionally, isotropic paramagnetic moments. The pinning of some of the Cu moments is caused by an AFM ordering consisting of moments that lie most likely in plane of the film. Moreover, pinned moments show a larger orbital magnetic moment contribution with an approximate ratio of m orb /m spin =1.5, indicating that these spins are located at sites with reduced symmetry. Some fractions of the pinned moments remain pinned from an AFM background even at 360 K, indicating that T N >360K. We propose a simple model that explains these experimental findings, showing that the magnetic order of the T-CuO ultrathin films differs from the theoretical predictions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Nonlinear dynamics, avalanches, and noise for driven Wigner crystals

Here we consider the driven dynamics of Wigner crystals interacting with random disorder. Using numerical simulations, we find a rich variety of transport phenomena as a function of charge density, drive, and pinning strength. For weak pinning, the system forms a defect-free crystal that depins elastically. When the pinning is stronger, a pinned glass phase appears that depins into a filamentary flow state, transitions at higher drives into a disordered flow phase, and finally forms a moving smectic. Within the filamentary flow phase, the conduction curves can show switching dynamics as well as negative differential conductivity in which switching events cause some flow channels to be blocked. The velocity noise in the filamentary flow regime exhibits narrow band characteristics due to the one-dimensional nature of the motion, while the moving smectic has narrow band velocity noise with a washboard frequency. In the disordered flow state, the noise power reaches a peak value and the noise has a 1/$\mathcal{f}$ character. Our transport results are consistent with recent experimental transport studies in systems where Wigner crystal states are believed to be occurring. Below the conduction threshold, we find that avalanches with a power-law size distribution appear when there are sudden local rearrangements of charges from one pinned configuration to another.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Effect of hexagonal patterned arrays and defect geometry on the critical current of superconducting films

Understanding the effect of pinning on the vortex dynamics in superconductors is a key factor towards controlling critical current values. Large-scale simulations of vortex dynamics can provide a rational approach to achieve this goal. Here, we use the time-dependent Ginzburg-Landau equations to study thin superconducting films with artificially created pinning centers arranged periodically in hexagonal lattices. We calculate the critical current density for various geometries of the pinning centers—varying their size, strength, and density. Furthermore, we shed light upon the influence of pattern distortion on the magnetic-field-dependent critical current. We compare our result directly with available experimental measurements on patterned molybdenum-germanium films, obtaining good agreement. In conclusion, our results give important systematic insights into the mechanisms of pinning in these artificial pinning landscapes and open a path for tailoring superconducting films with desired critical current behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Doping evolution of the second magnetization peak and magnetic relaxation in ( B a 1 - x K x ) F e 2 A s 2 single crystals

Here, we present a thorough study of doping dependent magnetic hysteresis and relaxation characteristics in single crystals of (Ba 1-x K x ) Fe 2 As 2 (0.18 ≤ x ≤ 1). The critical current density J c reaches maximum in the underdoped sample x = 0.26 and then decreases in the optimally doped and overdoped samples. Meanwhile, the magnetic relaxation rate S rapidly increases and the flux creep activation barrier U 0 sharply decreases in the overdoped sample x = 0.70. These results suggest that vortex pinning is very strong in the underdoped regime, but it is greatly reduced in the optimally doped and overdoped regime. Transmission electron microscope (TEM) measurements reveal the existence of dislocations and inclusions in all three studied samples x = 0.38, 0.46, and 0.65. An investigation of the paramagnetic Meissner effect (PME) suggests that spatial variations in T c become small in the samples x = 0.43 and 0.46, slightly above the optimal doping levels. Our results support that two types of pinning sources dominate the (Ba 1-x K x ) Fe 2 As 2 crystals: (i) strong δl pinning, which results from the fluctuations in the mean free path l and δT c pinning from the spatial variations in T c in the underdoped regime, and (ii) weak δT c pinning in the optimally doped and overdoped regime.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Peak effect and dynamics of stripe- and pattern-forming systems on a periodic one-dimensional substrate

Here we examine the ordering, pinning, and dynamics of two-dimensional pattern-forming systems interacting with a periodic one-dimensional substrate. In the absence of the substrate, particles with competing long-range repulsion and short-range attraction form anisotropic crystal, stripe, and bubble states. When the system is tuned across the stripe transition in the presence of a substrate, we find that there is a peak effect in the critical depinning force when the stripes align and become commensurate with the substrate. Under an applied drive, the anisotropic crystal and stripe states can exhibit soliton depinning and plastic flow. When the stripes depin plastically, they dynamically reorder into a moving stripe state that is perpendicular to the substrate trough direction. We also find that when the substrate spacing is smaller than the widths of the bubbles or stripes, the system forms pinned stripe states that are perpendicular to the substrate trough direction. The system exhibits multiple reentrant pinning effects as a function of increasing attraction, with the anisotropic crystal and large bubble states experiencing weak pinning but the stripe and smaller bubble states showing stronger pinning. We map out the different dynamic phases as a function of filling, the strength of the attractive interaction term, the substrate strength, and the drive, and demonstrate that the different phases produce identifiable features in the transport curves and particle orderings.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2015

Structural transitions in Pb(In 1/2 Nb 1/2 )O 3 under pressure

Here, Raman scattering, x-ray diffraction, and first-principles calculations were employed to investigate the behavior of disordered Pb(In 1/2 Nb 1/2 )O 3 (PIN) under pressure up to 50 GPa. The experiments were performed at room temperature while ab-initio calculations were carried out at 0 K. Raman spectra for disordered PIN show broad bands but a peak near the 380 cm -1 increases its intensity with pressure. The linewidth of the band at 550 cm -1 also increases with pressure, while two of the Raman peaks merge above 6 GPa. Above 16 GPa, we observe additional splitting of the band at 50 cm -1 . The pressure evolution of the diffraction patterns for PIN shows obvious splitting for Bragg peaks above 16 GPa; the results indicate a possible lowering symmetry transition. We identified that the transition at 0.5 GPa is from pseudo-cubic to orthorhombic (Pbam), the transitions at 16 GPa are isostructure transitions due to changes in linear compressibility and octahedral titling, and the transition at 30 GPa is from orthorhombic to monoclinic. First-principles calculations results indicated that ground state is Pbam with antiferrodisdortion consistent with experiment.

36 MATERIALS SCIENCE↗