SEARCH · Search NASA
Results for “doping effect”
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Effect of Ti and Zn Doping on the Structural, Electric, and Magnetic Properties of GaFeO{sub 3}
Multidoped (Zn,Ti) gallium orthoferrite [GaFeO{sub 3} and Ga{sub 0.98}Zn{sub 0.02}Fe{sub 0.98}Ti{sub 0.02}O{sub 3}] ceramic was synthesized through a solid-state reaction. X-ray analysis of the prepared material provides its basic crystal data of a single-phase orthorhombic system. The scattered crystallite size and lattice strain of the material were estimated. Analysis of the micrograph of field emission scanning electron microscope shows uniform grain distribution in the sample, suggesting the formation of good quality of sample. Founded on the magnetic measurements, it is concluded that (Zn,Ti)-modified gallium ferrite has provided a reduction in the Curie temperature (T{sub C}); however, no significant changes in the magnetization values with Zn and Ti integration in GaFeO{sub 3} lattice. A noteworthy effect of substitution of multiple elements at the Ga and Fe sites on dielectric constant and tangent loss of GaFeO{sub 3} has been observed. Complete studies of temperature (180–400 K) and frequency (10–10{sup 7} Hz) dependence of dielectric constant and impedance have provided the effect of grains and grain boundaries on the conduction mechanism and dielectric relaxation of the material.
Identification of dopant site and its effect on electrochemical activity in Mn-doped lithium titanate
Doped metal oxide materials are commonly used for applications in energy storage and conversion, such as batteries and solid oxide fuel cells. The knowledge of the electronic properties of dopants and their local environment is essential for understanding the effects of doping on the electrochemical properties. Using a combination of x-ray absorption near-edge structure spectroscopy (XANES) experiment and theoretical modeling we demonstrate that in the dilute (1 at .%) Mn-doped lithium titanate (Li 4/3 Ti 5/3 O 4 , or LTO) the dopant Mn 2+ ions reside on tetrahedral (8a) sites. First-principles Mn K-edge XANES calculations revealed the spectral signature of the tetrahedrally coordinated Mn as a sharp peak in the middle of the absorption edge rise, caused by the 1s → 4 p transition, and it is important to include the effective electron-core hole Coulomb interaction in order to calculate the intenisty of this peak accurately. Finally, this dopant location can explain the impedance of Li migration through the LTO lattice during the charge-discharge process, and, as a result, the observed remarkable 20% decrease in electrochemical capacity of the 1% Mn-doped LTO compared to pristine LTO.
Reducing Dark Conductivity of Cesium Tin Halide Perovskites with Donor Doping
The inorganic tin halide perovskites (such as Cs Sn Br 3 and Cs Sn I 3 ) exhibit high hole concentrations, which have been attributed to the presence of intrinsic defects such as cation vacancies. Despite their promise as photovoltaic materials, this high intrinsic conductivity limits deployment. Using hybrid density functional theory, we examine the impact of these intrinsic defects on the electronic properties of Cs Sn Br 3 and Cs Sn I 3 . We find that tin and cesium vacancies have low formation energies, especially under Sn -poor conditions, in agreement with prior work, and that the presence of these native acceptor defects can lead to high hole concentrations. However, effective donor doping of these systems can be obtained via Sc or Y incorporation. These impurities substitute on the Sn site, where they act as single donors. By examining the formation energy of these dopants versus the native defects, we show that Sc or Y doping should lead to a strong compensation, reducing hole concentrations under Sn -rich conditions. Published by the American Physical Society 2025
Simultaneous activation and N-doping of hydrothermal carbons by NaNH 2 : An effective approach to CO 2 adsorbents
Synthesis of carbon materials with both large surface area and abundant heteroatoms is an important task in scientific research. Traditional approaches mostly proceed at extremely high temperature, and suffer from complex routes and/or expensive raw materials. NaNH 2 has been recently found to be able to not only etch C atoms to create nanochannels, but also substitute O species to introduce N species. In this work, an effective approach to N-doped porous carbons was realized by treating hydrothermal carbons (HTCs) with sodium amide (NaNH 2 ). The considerable amount of O species preserved in pristine HTCs enables the achievement in simultaneous activation and N-doping of HTCs by NaNH 2 at relatively moderate temperature (400~600 °C). The porous and chemical structure of NaNH 2 -treated HTCs were characterized systematically. Although the pristine HTCs are almost non-porous and N-free, the specific surface areas and total N contents of prepared NaNH 2 -treated HTCs reach 190~2430 m 2 /g and 0.78~6.57 wt.% respectively. Furthermore, the CO 2 capture performance of NaNH 2 -treated HTCs was also examined considering their highly porous and N-doped nature. Interestingly, NaNH2-treated HTCs exhibit high CO 2 capacities, large CO 2 /N 2 selectivities, fast CO 2 adsorption rate and excellent recyclability, endowing them with potential application as solid adsorbents for CO 2 capture.
Synthesis, Structure, and Magnetic Properties of Ni{sub 1−x}Zn{sub x}Fe{sub 2}O{sub 4} Nanoparticles
Ni{sub 1−x}Zn{sub x}Fe{sub 2}O{sub 4} precursor solutions with different ratios (where x = 0.0, 0.5, and 1.0) were prepared by sol–gel synthesis using Ni-, Zn-, and Fe-based alkoxides which were dissolved into solvent and chelating agent. Ni{sub 1−x}Zn{sub x}Fe{sub 2}O{sub 4} nanoparticles, annealed at various temperatures, were observes to determine their doping ratio effects on magnetic and structural properties. The particle size and crystal structure of the nanoparticles were characterized by X-ray diffraction and Size Malvern Instruments Zetasizer Nano-ZS. Magnetic properties of Ni{sub 1−x}Zn{sub x}Fe{sub 2}O{sub 4} nanoparticles were investigated by a Quantum Design Physical Property Measurement System measurement system. The magnetization and structure of the Ni{sub 1−x}Zn{sub x}Fe{sub 2}O{sub 4} nanoparticles with different doping ratios are presented in this study.
Pyroelectric and electrocaloric effects in ferroelectric silicon-doped hafnium oxide thin films
The emergent ferroelectricity in HfO 2 -based systems has attracted significant attention as this simple binary high- k dielectric now offers the possibility of nonvolatile function. Here we employ zero-field and field-dependent pyroelectricity to show that thin films of silicon-doped HfO 2 do exhibit a broken-inversion symmetry and are indeed ferroelectric. In addition, the pyroelectric response is found to exhibit a wake-up behavior akin to the wake-up phenomenon observed in the ferroelectric polarization with electric-field cycling. Using polarization-electric field hysteresis measurements, this wake-up phenomenon is attributed to the presence of defect dipoles which explains the measured pyroelectric response. Finally, direct electrocaloric measurements are performed on these silicon-doped HfO 2 thin films, revealing an electrocaloric coefficient ~ 4 times larger in magnitude than that expected for the measured pyroelectric coefficient. This enhancement is explained using the plausible role played by defect dipoles that contribute to additional configurational or dipolar entropy.
Observation of Weak Counterion Size Dependence of Thermoelectric Transport in Ion Exchange Doped Conducting Polymers Across a Wide Range of Conductivities
Conducting polymers are of interest for a broad range of applications from bioelectronics to thermoelectrics. The factors that govern their complex charge transport physics include the structural disorder present in these highly doped polymer films and the Coulombic interactions between the electronic charge carriers and the dopant counterions. Previous studies have shown that at low doping levels carriers are strongly trapped in the vicinity of the counterions, while at high doping levels charge transport is not limited by Coulombic trapping, which manifests itself in the conductivity being independent of the size of the dopant counterion. Here a recently developed ion exchange doping method is used to investigate the ion size dependence of a semicrystalline polythiophene–based model system across a wide range of conductivities. It is found that the regime in which the charge and thermoelectric transport is not or only weakly dependent on ion size, extends to surprisingly low conductivities. This surprising observation is explained by a heterogeneous doping that involves doping of the amorphous domains to high doping levels first before doping of the ordered, crystalline domains occurs. The study provides new insights into how the thermoelectric physics of conducting polymers evolves as a function of doping level.
Stability and Mobility of Disconnections in Solute Atmospheres: Insights from Interfacial Defect Diagrams
This Letter explores the stability of disconnections (step-dislocation defects) at grain boundaries in binary alloys. We introduce interfacial defect diagrams, derived from atomistic simulations and segregation theory, to predict the stability of disconnections in the temperature-solute concentration phase space and relate it to governing segregation mechanisms. These diagrams reveal multiple stability regimes influenced by solute-induced clustering and pinning effects impacting the thermal migration of disconnections and offering insights into their thermodynamics and kinetic properties.
Electron–Hole Dimers in the Parent Phase of Quasi–2D Cuprates
The key feature of parent cuprates of the La{sub 2}CuO{sub 4} type, in addition to their high ionic polarizability and closeness to polarization catastrophe, is identified as their instability against charge transfer that is accompanied by the formation of a system of metastable dipole-active Mott–Hubbard excitons, i.e., electron–hole (EH) dimers. This feature determines the behavior of cuprates upon nonisovalent substitution. Within the simplest model equivalent to a system of composite bosons, nonisovalent substitution shifts the phase equilibrium toward condensation of EH dimers and the formation of inhomogeneous EH liquid. To describe the electronic state of doped cuprates effectively, we propose to use the pseudospin S = 1 formalism. It enables us to treat cardinally new charged states such as Anderson’s RVB phases. Recombination of EH dimers at a critically low energy of local and nonlocal correlations drives the system into the state of a Fermi liquid.
Probing putative orbital differentiation effects via Eu 2+ spin dynamics in Sr 1-x Eu x Fe 2 As 2
Here, in this work, we report x-ray powder diffraction, elemental analysis, electrical resistivity, magnetic susceptibility, specific heat, and electron spin resonance (ESR) in single crystals of Sr 1-x Eu x Fe 2 As 2 . We observed a breakdown of the previously reported scaling between the Eu 2+ Korringa relaxation rate obtained from ESR and the spin density wave temperature evolution for Sr-rich samples. This result suggests a distinct evolution of the orbital differentiation of the Fe 3d bands along the Sr-based series when compared to the Ba counterpart. We argue that this difference is related to a larger splitting between the structural (tetragonal-to-orthorhombic) and the Fe-driven spin density wave transitions induced by Eu doping in this series. In fact, our results indicate that the two transitions follow an opposite x-Eu dependence for Sr-concentrated samples. Our work shows that Sr 1-x Eu x Fe 2 As 2 series and the comparison with their Ba-based counterparts are exciting platforms to be explored for understanding the interplay among orbital differentiation, magnetism, and structural distortions in the iron pnictides
Probing the electronic properties of gap states near the surface of n -SrTiO 3-δ / i -Si (001) heterojunctions with high sensitivity
We have measured soft x-ray resonant photoemission for the n–SrTiO 3–δ /i–Si(001) (δ=~0.0003) hybrid semiconductor heterojunction with the goal of probing occupied electronic states in the band gap near the surface. By utilizing both swept energy, angle-integrated and fixed-energy, and angle-resolved spectroscopies, we identify the atomic and orbital character of the in-gap state as well as the different electronic phases that contribute to it. Specifically, we isolate the state associated with trapped charge on the n–SrTiO 3–δ surface that causes surface depletion from the localized gap state resulting from electron correlation effects in the complex oxide. Using an x-ray energy close to select dipole allowed TiL 3 → 3d x-ray absorption resonances results in a 40-fold increase in sensitivity. Furthermore, this feature makes photoemission possible when probing electrons from dopants at concentrations as low as a few hundredths of a percent.
Insight into the oxidation mechanism of Nb3Si(111) surface: First-principles calculations
Highlights: • Oxygen adsorption behavior on Nb{sub 3}Si(111) is studied by using first-principles. • O atom tends to be adsorbed on the NbSi bridge site. • The capacity of oxidation follow the order of Cr > Al > Si. • Cr can improve the Nb{sub 3}Si’s oxidation resistance because of Cr d-orbital effect. - Abstract: The oxidation resistance plays an important role in high-temperature ceramics. Although transition metal silicides exhibit the better oxidation behavior, the improvement of oxidation resistance is still a great challenge for their applications. Here, we apply first-principles calculations to investigate the oxidation mechanism and the influence of alloying elements (Cr and Al) on the oxidation resistance of Nb{sub 3}Si(111) surface. The calculated results show that the O atom is adsorbed on the NbSi-bridge site because of the charge interaction between O-2p state, Si-3p state and Nb-4d state. We further find that Cr-doping is more thermodynamically stable than that of Al-doping. In particular, the capacity of oxidation resistance follows the order of Cr > Al > Si. Finally, we conclude that Cr-doping can effectively improve the oxidation resistance of Nb{sub 3}Si(111) surface. The calculated electronic structure shows that Cr d-orbital plays a crucial role in oxidation resistance.
Influence of Ga doping on magnetic properties, magnetocaloric effect, and electronic structure of pseudobinary GdZn 1 - x Ga x ( x = 0 - 0.1 )
Here, we explore the impact of introducing IIIA-group element Ga in place of IIB-group element Zn in binary intermetallic GdZn on its magnetic and magnetocaloric properties, as well as explicate the modified electronic band structure of the compound. The magnetic transition temperature of the compound decreases with the increase of Ga concentration in GdZn 1-x Ga x (x = 0–0.1) while the crystal structure (CsCl prototype) and lattice parameters remain unchanged. Our detailed analysis of magnetization and magnetocaloric data conclusively proves that long-ranged magnetic ordering exists in the sample, despite the magnetic interaction considerably weakening with the increase of Ga. The experimental data are rationalized using both a theoretical machine-learning model and first-principle density functional theory. The electronic band structure of GdZn is manifested with some unusual complex features which gradually diminish with Ga doping and the conventional sinusoidal feature of Ruderman-Kittel-Kasuya-Yosida-type interactions also disappears. A mean-field theory model is developed and can successfully describe the overall magnetocaloric behavior of the GdZn 1-x Ga x series of samples.
3D Porous Graphitic Nanocarbon for Enhancing Performance and Durability of Pt Catalysts: Balance between Graphitization and Hierarchical Porosity
Carbon supports used in oxygen-reduction cathode catalysts for proton exchange membrane fuel cells (PEMFCs) are vulnerable to corrosion under harsh operating conditions, leading to poor performance durability. To address this issue, we have developed a highly stable porous graphitic carbon (PGC) produced through pyrolysis of a 3D polymer hydrogel in combination with Mn. The resulting PGC features multilayer carbon sheets assembled in porous and flower-like morphologies. In-situ high-temperature electron microscopy was employed to dynamically monitor the carbonization process up to 1100°C, suggesting that 3D polymer hydrogel provides high porosity at multiple scales, and that Mn catalyzes the graphitization process more effectively than other metals. Compared to conventional carbons such as Vulcan, Ketjenblack, and graphitized carbon, the PGC provides an improved balance between high graphitization and hierachical porosity, which is favorable for uniform Pt nanoparticle dispersion and enhanced corrosion resistance. As a result, Pt supported on PGC exhibits remarkably enhanced stability. In addition to thorough testing in aqueous electrolytes, we also conducted fuel cell testing using durability protocols recommended by the U.S. Department of Energy (DOE). After 5000 voltage cycles from 1.0 to 1.5 V, the Pt/PGC catalyst only lost 9 mV at a current density of 1.5 A/cm 2 , dramatically exceeding the DOE support durability target (<30 mV), and surpassing commercial Pt/C catalysts. Along with the enhanced carbon corrosion resistance of the PGC support, the enhanced catalyst-support interactions are beneficial for stability improvement, likely due to nitrogen doping into carbon, which was further elucidated through X-ray absorption spectroscopy and density functional theory (DFT) calculations. Thus, the high stability and activity of PGC-based Pt catalysts are attributed to the combination of high graphitization degree, favorable surface areas and porosity, and nitrogen doping, which effectively stabilize highly dispersed Pt nanoparticles.
Anomalous elastic softening in ferroelectric hafnia under pressure
his study employs first-principles density-functional theory (DFT) calculations to explore the elastic and mechanical properties of ferroelectric hafnia (HfO 2 ) in its polar orthorhombic 𝑃𝑐𝑎2 1 phase under varying hydrostatic pressure conditions up to 30 GPa. Utilizing a plane-wave basis set and Perdew-Burke-Ernzerhof generalized-gradient approximation for solids in our DFT calculations, we investigate both pure and yttrium-substituted HfO 2 . Our findings reveal an anomalous reduction in the 𝐶 33 component of the elastic tensor with increasing pressure, which becomes significant above 15 GPa and signals a potential pressure-driven structural phase transition at higher pressure. The analysis of atomic displacements under pressure sheds light on the unusual mechanical behavior and phase stability of this material. Additionally, we observe a transition from an indirect band gap to a direct band gap with increasing pressure, which could have significant implications for optical applications. Here, the effects of yttrium substitution on the mechanical and electronic properties are further examined, revealing that yttrium substitution softens the elastic response of this material and reduces the electronic band gap. These results enhance our understanding of elastic and mechanical responses of ferroelectric hafnia and its potential for applications in microelectronics, piezoelectric devices, and nonvolatile ferroelectric random-access memories. Further experimental validation is recommended to confirm our predictions and explore the practical implications of the observed phase transitions and electronic behavior of the ferroelectric hafnia under high-pressure conditions.
Phase-field model of coupled insulator-metal transitions and oxygen vacancy redox reactions
A predominant ubiquitous feature of strongly correlated oxides is the possible presence of oxygen vacancies, which has recently been shown to have profound effects on their electronic phase transitions. Here, we formulate a comprehensive phase-field model of intercoupled insulator-metal transitions and oxygen vacancy redox reactions, taking into account the valence electron state of oxygen vacancies. We use the model to study the voltage self-oscillation phenomenon in a prototypical strongly correlated oxide, VO 2 , and discover the mutual activation of the insulator-metal transition and oxygen vacancy redox reactions leading to systematic enhancement of the oscillation frequency. Furthermore, the established methodology and the mutual activation mechanism are generally applicable to understanding any insulator-metal transition dynamics in oxygen-deficient correlated oxides and improving the performance of voltage self-oscillation-based artificial neurons.
Depinning, melting, and sliding of generalized Wigner crystals in Moiré systems
We numerically examine the depinning, sliding, and melting of commensurate and incommensurate Wigner crystals on two-dimensional hexagonal periodic substrates near fillings of 1 / 3 , 1 / 2 , and 2 / 3 to model the dynamics of generalized Wigner crystals in moiré heterostructures. At low temperatures where thermal fluctuations are irrelevant, for commensurate fillings we find a strongly pinned state that depins into a sliding crystal, while at incommensurate fillings the depinning threshold is strongly reduced. For fillings below 1 / 2 , the depinning occurs in a two-step process. Above the first depinning threshold, there is an extended range of drives where the conduction occurs via the sliding of antikinks along the charge stripes, followed by a second threshold where all the charges begin to slide. At finite temperatures, the external driving reduces the effective melting temperature at commensurate fillings and enhances creep at incommensurate fillings. We show that depinning into different sliding states, such as moving fluids or moving crystals, produces nonlinear features in the transport curves. We also show that transport is asymmetric on either side of a commensurate filling due to the different dynamics for interstitials versus holes in the commensurate structure. A variety of sliding states should be accessible for generalized Wigner crystals at finite temperatures even for low drives. If experiments can realize sliding dynamics in moiré systems, it would open another class of commensurate and incommensurate phases for study. Additionally, the sliding dynamics and antikink flow could provide unique functionalities for charge transport in moiré heterostructures. Published by the American Physical Society 2025