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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Shifting and tilting towards enhanced piezoelectricity in high-temperature ceramics: an average structure study of (1 − x )BiFe 2/8 Ti 3/8 Mg 3/8 O 3 – x PbTiO 3 through X-ray and neutron diffraction

A comprehensive report is provided of the average structure of a high Curie temperature ferroelectric ceramic, (1 − x)BiFe 2/8 Ti 3/8 Mg 3/8 O 3 –xPbTiO 3 (BFTM–xPT) when x = 0.25–0.375 at room temperature. Both neutron and synchrotron X-ray powder diffraction data were collected and combined Rietveld refinements were completed. These data revealed that, of the compositions studied, BFTM–xPT crystallizes as mixed phase material. At x = 0.25–0.275, the average structure of each phase is best described by space groups R3c and Cc. At 0.30 ≥ x ≥ 0.35, a morphotropic phase boundary is present and is best modelled by space groups P4mm and Cc, where space group Cc is the dominant phase. As the concentration of PbTiO 3 increases, the amount of octahedral tilting decreases and x = 0.375 exhibits a loss of octahedral tilting and crystallizes in space groups P4mm and Cm. In conclusion, an updated phase diagram at room temperature of this novel system is given and structural intricacies responsible for its piezoelectric properties are revealed.

high Curie temperature↗
DOE OSTI2025

Enhanced domain dynamics in alternating current poled rhombohedral Pb(Mg 1/3 Nb 2/3 )O 3 –PbTiO 3 single crystals

Abstract The understanding of domain dynamics in ferroelectric materials is crucial for optimizing their performance in piezoelectric and electro‐optic applications. Although previous studies have focused on static domain structures and macroscopic characteristics, the time‐resolved approach of domains remains largely unexplored. In this study, we compare the dynamic responses of direct current (DC) and alternating current (AC) poled [001]‐oriented rhombohedral Pb(Mg 1/3 Nb 2/3 )O 3 –PbTiO 3 (PMN–PT) single crystals using X‐ray photon correlation spectroscopy (XPCS) during the application of external electric fields. Our results demonstrate that the AC‐poled sample exhibit enhanced reconfiguration of domain variants in response to driving fields compared to the DC‐poled counterpart, as evidenced by accelerated correlation decay and faster relaxation time. This phenomenon is attributed to enhanced reversible domain wall motion achieved through AC poling, which facilitates field‐induced domain realignment. These findings provide insight into the relationship between dynamics and macroscopic properties in relaxor‐PT single crystals for high‐performance applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Analysis of useful ion yield for the Mg dopant in GaN by quadrupole—SIMS

The authors present a quantitative secondary ion mass spectrometry (SIMS) analysis of the useful ion yield of magnesium dopant in a gallium nitride matrix. A quadrupole SIMS instrument was used to analyze an Mg-doped GaN sample grown by metal organic chemical vapor deposition. Oxygen (O2+) was used as the primary ion beam and its energy was varied in the range from 0.5 to 5 kV with and without oxygen flooding near the sample. The results of the analysis can be used to determine the primary beam energies for optimal magnesium sensitivity.

Senevirathna, M. K. Indika (ORCID:0000000243575029↗
DOE OSTI2020

Investigation of the electrical behavior of AlGaN/GaN high electron mobility transistors grown with underlying GaN:Mg layer

In the current study, the electrical behavior of the AlGaN/GaN high electron mobility transistors (HEMTs) grown with an underlying GaN:Mg layer is detailed. It is shown that the activation of the buried p-GaN layer is achieved without hydrogen diffusion out of the layer. Reversal in the electrical behavior of the two-dimensional electron gas (2DEG) is also observed in the as-grown structure based on the p-GaN activation sequence. This behavior is attributed to the complex role played by hydrogen in the overgrown HEMT layers. The results of this study provide new insights into the development of metal organic chemical vapor deposition grown HEMTs with activated buried p-GaN films.

Mahaboob, Isra (ORCID:0000000171558130)↗
DOE OSTI2020

3D printed Mg-NiTi interpenetrating-phase composites with high strength, damping capacity, and energy absorption efficiency

It is of significance, but still remains a key challenge, to simultaneously enhance the strength and damping capacities in metals, as these two properties are often mutually exclusive. Here, we provide a multidesign strategy for defeating such a conflict by developing a Mg-NiTi composite with a bicontinuous interpenetrating-phase architecture through infiltration of magnesium melt into three-dimensionally printed Nitinol scaffold. The composite exhibits a unique combination of mechanical properties with improved strengths at ambient to elevated temperatures, remarkable damage tolerance, good damping capacities at differing amplitudes, and exceptional energy absorption efficiency, which is unprecedented for magnesium materials. The shape and strength after deformation can even be largely recovered by heat treatment. This study offers a new perspective for the structural and biomedical applications of magnesium.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Redox Potential Measurements of Cr(II)/Cr Ni(II)/Ni and Mg(II)/Mg in Molten MgCl 2 –KCl–NaCl Mixture

The redox potential of Ni(II)/Ni, Cr(II)/Cr, and Mg(II)/Mg in MgCl 2 –KCl–NaCl (Magnak chloride) were studied utilizing chronopotentiometry (CP) method in a three-electrode electrochemical cell. Four different CrCl 2 concentrations from 873 K to 1073 K, three NiCl 2 concentrations from 773 K to 1073 K, and two MgCl 2 concentrations from 823 K to 1073 K have been measured. The activity coefficients were calculated referring to a supercooled potential accordingly. The empirical equation of formal potential will be of great significance for the fundamental thermodynamic prediction in high-temperature Magnak chloride systems where corrosion product impurities are present.

14 SOLAR ENERGY↗
DOE OSTI2021

Communication—Mg 2+/0 as a Reliable Reference Electrode for Molten Chloride Salts

In this report, we describe the use of the Mg 2+/0 couple as a reference in molten MgCl 2 -NaCl. We demonstrate that this redox couple provides a robust and reliable reference potential over a range of melt temperatures (500 °C–675 °C). Finally, we demonstrate the construction of a simple molten magnesium reference electrode, for operation at temperatures greater than the melting point of magnesium (650 °C).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Understanding the Effect of Electrochemical Properties and Microstructure on the Microgalvanic Corrosion of Mg Alloys via Phase-Field Simulations

In this work, we apply a phase-field model to gain insights into the corrosion behavior of Mg alloys. Specifically, we study the effect of electrochemical properties of the materials and their environment and the spatial distribution of second phases in a magnesium alloy. To this end, we perform sensitivity analyses in which we separately vary the exchange current density and corrosion potential of the second phase, as well as the electrolyte conductivity. We describe the effects of these parameters on the corrosion current and the resulting corrosion-front morphology. We also confirm that the ratio between the Wagner length, calculated for the anodic phase, and the initial width of the anodic domain is a good predictor of the morphological characteristic of the corroding surface. In addition, we examine the effect of the microstructure on the corrosion behavior in both 2D and 3D systems by varying the shape, size, and distribution of the second-phase regions. Our findings provide insights into the influence of the two-phase microstructure on the corrosion behavior in a magnesium alloy.

Electrochemistry↗
DOE OSTI2020

Exploring the Charge Compensation Mechanism of P2-Type Na 0.6 Mg 0.3 Mn 0.7 O 2 Cathode Materials for Advanced Sodium-Ion Batteries

P2-type sodium layered transition metal oxides have been intensively investigated as promising cathode materials for sodium-ion batteries (SIBs) by virtue of their high specific capacity and high operating voltage. However, they suffer from problems of voltage decay, capacity fading, and structural deterioration, which hinder their practical application. Therefore, a mechanistic understanding of the cationic/anionic redox activity and capacity fading is indispensable for the further improvement of electrochemical performance. Here, a prototype cathode material of P2-type Na 0.6 Mg 0.3 Mn 0.7 O 2 is comprehensively investigated, which presents both cationic and anionic redox behaviors during the cycling process. By a combination of soft X-ray absorption spectroscopy and electroanalytical methods, we unambiguously reveal that only oxygen redox reaction is involved in the initial charge process, then both oxygen and manganese participate in the charge compensation in the following discharge process. In addition, a gradient distribution of Mn valence state from surface to bulk is disclosed, which could be mainly related to the irreversible oxygen activity during the charge process. Furthermore, we find that the average oxidation state of Mn is reduced upon extended cycles, leading to the noticeable capacity fading. Our results provide deeper insights into the intrinsic cationic/anionic redox mechanism of P2-type materials, which is vital for the rational design and optimization of advanced cathode materials for SIBs.

25 ENERGY STORAGE↗
DOE OSTI2021

Spin-flop Phase in a Honeycomb Antiferromagnet Mn 0.84 Mg 0.16 TiO 3

The spin-flop phase of a honeycomb antiferromagnet of Mn 0.84 Mg 0.16 TiO 3 has been studied under magnetic field along normal direction to the honeycomb plane by neutron scattering. With increasing a magnetic field from 0 to 5 T, the inelastic scattering intensity increased by 1.5 times at (1 0 0), where the magnetic structure was found to be collinear-antiferromagnet with the spin parallel to [1 1 0] direction. The spin direction is consistent with a 1.5 times increase of the inelastic intensity. Furthermore, the negligible energy difference between K and K' points suggests small values for both Dzyaloshinskii–Moriya interaction and bond-dependent symmetric anisotropic exchange interaction in this material.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗