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At least 1,495 records · Page 83
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Iodine Capture Studies of Copper- and Bismuth-Based Sorbents

The release of radioiodine, one of several radionuclides of concern when recycling used nuclear fuel (UNF), is an important consideration in the fuel cycle. In this study, two sorbent materials, Cu 0 -polyacrylonitrile (PAN) and Bi 0 -PAN, were tested as solid sorbent candidates for iodine capture. Experiments using a thin bed of sorbent material were exposed to vaporized iodine for over 300 hours (~2 weeks) under varied conditions in a dynamic flow environment. The overall performance was monitored in real-time using thermogravimetric analysis, and the materials were subsequently characterized for surface and bulk analysis using scanning electron microscopy – energy-dispersive spectroscopy (SEM-EDS) and powder x-ray diffraction (pXRD), respectively. Iodine (in the form of I 2 ) is expected to be released primarily in the dissolver off-gas (DOG) stream; therefore, this study demonstrates the effects of elemental iodine (I 2 ), water vapor (H 2 O), and nitrogen dioxide (NO 2 ). When exposed to ‘ideal’ conditions (in which I 2 is carried by dry air), Cu 0 -PAN and Bi 0 -PAN behave differently, with TGA analysis indicating that Cu 0 sorption performance is higher than that of Bi 0 , as evidenced by a larger mass change. Under ‘harsh’ conditions—such as a gaseous feed containing I 2 , H 2 O, and NO 2 vapors carried by air,—iodine capture performance for both Cu 0 - and Bi 0 -PAN are affected. SEM-EDS and pXRD analysis of these materials is discussed herein.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
NASA NTRS1969

Two devices for analysis of nystagmus

Electromechanical Slope Computer /ESC/ and Electronic Summation Device /ESD/ facilitates rapid analysis of nystagmus records. The ESC reads out the slope and time of each nystagmus wave form. The ESD provides much faster analysis than the ESC. It provides an immediate analog display and digital display of analyzed nystagmus.

Guedry, F. E., Jr.↗
DOE OSTI2016

Synthesis of silver nano-materials from Grevillea robusta A Cunn (Silver-oak tree) leaves extract and shape directing role of cetyltrimethylammonium bromide

Grevillea robusta (Silver-oak tree) tree is a medicinal tree. Conventional UV-visible spectrophotometric and transmission electron microscopic technique were used to determine the morphology of silver nanoplates (AgNP) using Grevillea robusta (Silver-oak tree) aqueous leaves extract for the first time. The visible spectra showed the presence of three well defined surface plasmon absorption (SPR) bands at 500, 550 and 675 nm which was attributed to the anisotropic growth of Ag-nanoplates. Transmission electron microscopic (TEM) analysis of AgNP showed formation of truncated triangular, polyhedral with some irregular shapes nanoplates in the size range 8-20 nm. Cetyltrimethylammonium bromide (CTAB) has no significant effect on the shape of the spectra, position of SPR bands, size and size distribution of AgNP.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Synthesis of Hybrid Materials Based on Iron Nanoparticle-Decorated Multiwalled Carbon Nanotubes

Fe-containing nanoparticles have been grown for the first time on the surface of multiwalled carbon nanotubes by metalorganic chemical vapor deposition using iron acetylacetonate, Fe(acac){sub 3}, as a precursor. The resultant hybrid nanomaterial has been characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and thermogravimetric analysis. The results demonstrate that the synthesized material consists of multiwalled carbon nanotubes whose surface is decorated with iron nanoparticles.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Iron Oxides

Abstract: Fe oxides are common clay-sized oxide, oxyhydroxide and hydroxide soil minerals. They are compounds of Fe, O, and H that have structures based on close-packed arrays of O. The octahedral and tetrahedral cavities within these arrays are filled with either Fe3+ or Fe2+ to form Fe(O/OH)6, FeO6, or FeO4 structural units. All of the naturally occurring Fe oxide minerals usually undergo some degree of isomorphous substitution of other metal ions for Fe in their structures. Relatively simple techniques may be used to identify Fe oxides in the field based on their typical colors and magnetic properties. In the laboratory, a variety of instrumental techniques can be used to confirm phase identity and to quantify amount. Of these, X-ray diffraction, infrared spectroscopy, electron microscopy, thermal analysis, and Mössbauer spectroscopy are the most commonly used techniques. As oxides, the functional groups on their surfaces may have positive, negative, or no charge depending on pH and on the concentration and nature of other ions in the contact solution. A net positive surface charge usually is observed in soils because Fe oxides have a point-of-zero-charge in the neutral or slightly basic pHs. The functional groups on the surface form complexes with cations and anions from the aqueous phase. Their sorption and electron-buffering properties significantly affect the geochemical cycles of almost all elements having agronomic or environmental significance.

electron-transfer chemistry↗
DOE OSTI2021

Structure-property relationships and mobility optimization in sputtered La-doped BaSn O 3 films: Toward 100cm 2 V -1 s -1

he wide band gap semiconducting perovskite BaSnO 3 is of high current interest due to outstanding room temperature mobility at high electron density, fueled by potential applications in oxide, transparent, and power electronics. Due in part to a lack of lattice-matched substrates, BaSnO 3 thin films suffer from high defect densities, however, limiting electron mobility. Additionally, the vast majority of BaSnO 3 thin film research has focused on pulsed laser deposition or molecular beam epitaxy. In this work, we present an exhaustive optimization of the mobility of Ba 0.98 La 0.02 SnO 3 films grown by a scalable, high-throughput method: high-pressure-oxygen sputter deposition. Considering target synthesis conditions, substrate selection, buffer layer structure, deposition temperature, deposition rate, thickness, and postdeposition annealing conditions, and by combining high-resolution x-ray diffraction, reciprocal space mapping, rocking curve analysis, scanning transmission electron microscopy, atomic force microscopy, and temperature-dependent electronic transport measurements, detailed understanding of synthesis-structure-property relationships is attained. Optimized room temperature mobility of 96 cm 2 V –1 s –1 is achieved in vacuum-annealed GdScO 3 (110)/BaSnO 3 (120 nm)/Ba 0.98 La 0.02 SnO 3 (200 nm) heterostructures, as well as 92 cm 2 V –1 s –1 on unbuffered substrates and 87 2 V –1 s –1 without postdeposition annealing. These results, including important trends in defect densities and a surprising dependence of mobility on lattice mismatch, substantially expand the understanding of the interplay between deposition conditions, microstructure, and transport in doped BaSnO 3 films, establishing competitive mobilities in films fabricated via a scalable, high-throughput, industry-standard technique.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Microstructure analysis of laser beam weldments performed on neutron-irradiated 304L steel containing 3 and 8 appm helium

We report that AISI 304 L austenitic stainless steel is one of the major structural materials used in light water reactors (LWRs). In the future, 304 L components may require repair, and several welding techniques have been proposed as candidates. In this study, laser beam welding using the low-energy contribution approach was performed in ~2016 on neutron-irradiated AISI 304 L steel with nominal values of 3 and 8 appm helium (He). The goal was to investigate the impact of helium on the irradiated material weldability and evaluate helium-associated damage. The weld and heat-affected zone (HAZ) microstructure was studied using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy (TEM). Analysis of the weldment cross-sections did not reveal severe cracking. Still, evidence of incipient helium-associated damage was observed in the HAZ, manifesting as degraded grain boundaries (GBs) “decorated” by pore chains and mostly small (i.e., ≤ 25-30 µm) scattered cracks. The largest observed crack was ~50 µm in length. Helium-associated damage was localized within ~200-300 µm of the weld pool boundary, and the fraction of the compromised GBs, as a rule, was below ~9-11% of the total GB network. TEM analysis showed significant annealing of the radiation-induced defects at distances up to ~300 µm from the weld pool boundary. Inside the HAZ, the cavities (likely, helium bubbles) tended to form at GBs and inclusions, such as manganese sulfide precipitates in the grain interior. Crystallography analysis of the HAZ damage showed that random high-angle boundaries were most susceptible to helium-associated damage. In contrast, low-angle random boundaries and twin boundaries appeared to be strongly resistant to degradation from the combined effects of helium and welding.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Microstructure and transformation behavior of Ni{sub 24.7}Ti{sub 50.3}Pd{sub 25} high temperature shape-memory alloy with Sc micro-addition

NiTiPd shape-memory alloys (SMAs) are potential functional materials for use as solid-state actuators in the temperature range 100–250 °C. The present study investigates the effect of 1.0 at.% Sc micro-addition to Ni{sub 24.7}Ti{sub 50.3}Pd{sub 25} alloy, Sc replacing either Ti or Ni. Results show that all the three alloys studied have stable transformation behavior on stress-free thermal cycling and hence, are suitable for cyclic actuation applications. However, the addition of Sc to NiTiPd alloy leads to decrease of transformation temperatures, the magnitude of decrease being greater for the alloy with Sc replacing Ni. The martensite finish (M{sub f}) temperature of 181 °C for the NiTiPd alloy decreased to 139 °C for Sc replacing Ti and 83 °C for Sc replacing Ni. Also, the indentation modulus of NiTiPdSc (Sc replacing Ni) alloy is found to be significantly low compared to the other alloys. Analysis indicates that the observed differences in the alloy properties are related to the solubility of Sc in the NiTiPd matrix. While the quaternary NiTiPdSc alloy, Sc replacing Ti, has a single phase microstructure, the alloy with Sc replacing Ni shows the presence of Sc-rich and TiPd-type second phases in the microstructure. TEM examination revealed that the TiPd-type phase has a distinct rod-like morphology (30–50 nm) arranged in a grid-like structure. The transformation and indentation behavior of the alloys is elucidated using thermodynamic calculations of frictional energy and an electronic structure based analysis. - Highlights: • TEM of Ni{sub 23.7}Ti{sub 50.3}Pd{sub 25}Sc{sub 1} showed distinct grid of TiPd-type phase nanorods < 50 nm. • Stress-free thermal cycling of all the three alloys showed stable transformation behavior. • Ni{sub 24.7}Ti{sub 49.3}Pd{sub 25}Sc{sub 1} and Ni{sub 23.7}Ti{sub 50.3}Pd{sub 25}Sc{sub 1} showed single and multiphase structures. • Sc micro-addition (1 at.%) to Ni{sub 24.7}Ti{sub 50.3}Pd{sub 25} alloy decreased TTs significantly. • Ni{sub 23.7}Ti{sub 50.3}Pd{sub 25}Sc{sub 1} exhibited lower modulus of 67 GPa to 85 GPa of Ni{sub 24.7}Ti{sub 50.3}Pd{sub 25}.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Electrical, thermal and electrochemical properties of disordered carbon prepared from palygorskite and cane molasses

We have synthesized and electrochemically tested a carbon sample that was suitable as anode for lithium secondary battery. The synthesis was based on the use of the palygorskite clay as template and sugar cane molasses as carbon source. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, Brunauer–Emmett–Teller (BET) measurements and High Resolution Transmission Electron Microscope (HRTEM) analysis showed that the nanometric carbon material has a highly disordered graphene-like wrinkled structure and large specific surface area (467 m{sup 2} g{sup −1}). The compositional characterization revealed a 14% of heteroatoms-containing groups (O, H, N, S) doping the as-prepared carbon. Thermophysical measurements revealed the good thermal stability and an acceptable thermal diffusivity (9·10{sup −7} m{sup 2} s{sup −1}) and conductivity (1.1 W m{sup −1} K{sup −1}) of this carbon. The electrical properties showed an electronic conductivity of hole-like carriers of approximately one S/cm in a 173–293 K range. The testing of this material as anodes in a secondary lithium battery displayed a high specific capacity and excellent performance in terms of number of cycles. A high reversible capacity of 356 mA h g{sup −1} was reached. - Graphical abstract: TEM image and electrochemistry behavior of a new graphene oxide-like carbon. - Highlights: • A high disordered graphene oxide-like conducting carbon is reported. • The synthesis was based on palygorskite and sugar cane molasses as precursors. • The disordered conducting carbon is composed of doped- graphene heterogeneous domains. • This material combines a large specific surface area and high electric conductivity. • The thermophysical and electrochemical properties of this material reveal adequate behavior.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2014

A Predictive Bubble Point Pressure Model for Porous Liquid Acquisition Device Screens

This article presents a simplified model for porous screen channel liquid acquisition devices based on a maximum bubble point pressure method from Adamson and Gast (1997). To validate the model, three 304 stainless steel (325 × 2300, 450 × 2750, and 510 × 3600) mesh samples were tested in methanol, acetone, isopropyl alcohol, and water. Screen pores are estimated based on analysis from scanning electron microscopy, historical data, and current test data. Results show that the bubble point pressure is proportional to the surface tension of the fluid only when accounting for nonzero contact angles. The previous assumption that bubble point pressure scales inversely with effective pore diameter is shown to be invalid, as the second finest 450 × 2750 produced the highest bubble point of the three screens. The simplified bubble point model can be used to make predictions for any pure fluid when pore diameters are based on bubble point tests and not SEM analysis.

Liquid Acquisition Device↗
DOE OSTI2018

Catalytic Aquathermolysis of High-Viscosity Oil Using Iron, Cobalt, and Copper Tallates

The results of a study of the composition of active forms of the catalyst formed upon degradation of the precursor, based on the results of physical modeling of a sample of high-viscosity oil having high asphaltene and resin contents, are presented. Oil-soluble iron, cobalt, and copper tallates were used as the objects of the study. The composition of the separated powder of the active form of the catalyst was determined by X-ray diffraction analysis, and the catalyst particle size was determined by scanning electron microscopy. The SARA (saturate, aromatic, resin and asphaltene) analysis data revealed a marked decrease in high-molecular-weight oil components due to thermocatalytic cracking. The basic transformation mechanism is breakdown of the high-molecular-weight compounds along the sulfur-bearing bonds, as indicated by elemental CHNS (carbon, hydrogen, nitrogen and sulfur) analysis data. It is shown that the cobalt- and copper-based oil-soluble complexes turn are converted to sulfide forms and the iron-based complex is converted to the oxide form. According to the results of scanning electron microscopic analysis of the catalyst, the particle size is about 60 nm.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Quantum Hall effect and hopping conductivity in n-InGaAs/InAlAs nanoheterostructures

The longitudinal and Hall magnetoresistances are measured in the quantum Hall effect regime in the n-InGaAs/InAlAs heterostructures at temperatures of T = (1.8–30) K in magnetic fields up to B = 9 T. Temperature-induced transport in the region of the longitudinal resistance minima, corresponding to the plateau regions at Hall resistance, is investigated within the framework of the concept of hopping conductivity in a strongly localized electron system. The analysis of variable-range hopping conductivity in the region of the second, third, and fourth plateau of the quantum Hall effect provides the possibility of determining the localization length exponent.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Interfacial Microstructural Evolution and Metallurgical Bonding Mechanisms for IN718 Superalloy Joint Produced by Hot Compressive Bonding

The novel metallurgical joining process for bonding IN718 superalloy was investigated by hot compressive bonding (HCB) process under the deformation temperature range of 1000 °C to 1150 °C and true strains ranging from 0 to 0.5 at a strain rate of 0.001 s{sup −1}. The effect of HCB process parameters on the tensile strength was analyzed. Both the as-deformed and the interfacial microstructures were characterized using the optical microscope, electron backscattered diffraction and transmission electron microscope (TEM) analysis. The results of tensile property revealed that the degree of metallurgical bonding is promoted by increasing deformation temperature and strain. The evolution of the interfacial microstructure showed that the migration of interfacial grain boundary (IGB), which is characterized by discontinuous dynamic recrystallization, is the dominant metallurgical bonding mechanism in the early stages of bonding. TEM analysis indicated that the dislocation density is distributed heterogeneously over both sides of IGB, which is the significant reason for the migration of IGB, during the initial stage of HCB process.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Amorphous TiCu-Based Additives for Improving Hydrogen Storage Properties of Magnesium Hydride

Magnesium hydride has long been regarded as a promising candidate material for hydrogen and heat storage due to its high hydrogen capacity, reversibility, and low cost. Catalytic doping has been demonstrated as one of the most effective methods to improve hydrogen storage properties of MgH 2 . In this study, amorphous Ti 45 Cu 41 Ni 9 Zr 5 and Ti 40 Cu 47 Zr 10 Sn 3 alloys are used as additives for MgH 2 . Nanostructured MgH 2 doped with amorphous or crystalline TiCu-based alloys are prepared by using a high-energy mechanochemical synthesis method. Results show that the amorphous TiCu additives provide enhanced catalytic effects compared to crystalline alloys of the same composition. Doping MgH 2 using an amorphous Ti 45 Cu 41 Ni 9 Zr 5 alloy yielded improved dehydrogenation kinetics compared to using crystalline Ti 40 Cu 47 Zr 10 Sn 3 alloy. The analysis using transmission electron microscopy reveals that there are nanostructured catalytic particles uniformly distributed in the amorphous TiCu-catalyzed MgH 2 . The MgH 2 system catalyzed by amorphous TiCu-based alloy shows little degradation during hydrogenation and dehydrogenation cycling at 300 °C. The amorphous TiCu-based catalysts are thermally stable at temperatures up to 360 °C. Heating the amorphous Ti 45 Cu 41 Ni 9 Zr 5 -catalyzed MgH 2 to temperatures above 360 °C led to disproportionation of the catalyst alloy and the formation of MgCu 2 and Ti 2 Cu. In addition, PCI analysis of the amorphous Ti 45 Cu 41 Ni 9 Zr 5 -catalyzed MgH 2 shows a slight increase in hydrogen equilibrium pressure, resulting in a reaction enthalpy of -78.7 kJ/mol • H 2 and an entropy of 145.0 J/K•mol•H 2 . As a result, the entropy calculated from this study is approximately 10 J/K•mol•H 2 higher than values previously reported for undoped and catalyzed Mg-H systems.

08 HYDROGEN↗
DOE OSTI2018

Er3+-Yb3+/Er3+-Yb3+-Li+/Er3+-Yb3+-Zn2+:Gd2O3 nanophosphors for efficient frequency upconverter and temperature sensing applications

Highlights: • The Er{sup 3+}-Yb{sup 3+}:Gd{sub 2}O{sub 3} nanophosphors infer the enhancement in UC emission intensity via incorporation of Li{sup +} and Zn{sup 2+} ions. • The average particles size of the developed nanophosphors is found to be lie in nanometer range by XRD and TEM analysis. • The maximum sensitivity ∼ 0.0116 K{sup −1} has been reported in the Er{sup 3+}-Yb{sup 3+}-Zn{sup 2+}:Gd{sub 2}O{sub 3} nanophosphors. • The colour coordinate for the developed Er{sup 3+}-Yb{sup 3+}-Zn{sup 2+}:Gd{sub 2}O{sub 3} nanophosphors lie in the greenish yellow colour region. - Abstract: Er{sup 3+}/Yb{sup 3+}/Li{sup +}/Zn{sup 2+}:Gd{sub 2}O{sub 3} nanophosphors synthesized via combustion synthesis method have been structurally and optically characterized. The average particles size of the developed nanophosphors has been found to be ∼ 25 nm by Transmission Electron Microscopy (TEM) analysis which is in good agreement with the X-rays Diffraction (XRD) results. The significant enhancement in upconversion (UC) emission intensity for green (∼ 20 and 70 times) and red (∼ 7 and 20 times) band have been observed after codoping with Li{sup +} and Zn{sup 2+} ions respectively in Er{sup 3+}-Yb{sup 3+}:Gd{sub 2}O{sub 3} system. The temperature sensing capability of the optimized nanophosphors has been studied corresponding to two thermally coupled {sup 2}H{sub 11/2} and {sup 4}S{sub 3/2} levels of the Er{sup 3+} ions. The sensitivity ∼ 0.0096 K{sup -1}, 0.00109 K{sup -1} and 0.0116 K{sup -1} have been reported in the Er{sup 3+}-Yb{sup 3+}:Gd{sub 2}O{sub 3}, Er{sup 3+}-Yb{sup 3+}- Li{sup +}:Gd{sub 2}O{sub 3} and Er{sup 3+}-Yb{sup 3+}- Zn{sup 2+}:Gd{sub 2}O{sub 3} nanophosphors respectively. The photometric characterizations of the developed nanophosphors have also been reported.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Measurement of G-phase volume fraction and number density in duplex stainless steels using transmission electron microscopy

Duplex stainless steels (DSS) have a high toughness and strength due the presence of both austenitic and ferritic phases. These alloys have had limited use in power production applications due to thermal embrittlement caused by spinodal decomposition and development of G-phase precipitates in the ferrite. Lean grade DSS alloys (e.g., 2101, 2003) may offer improved thermal stability due to the reduction of Cr- and Ni-equivalent elements when compared to standard grade compositions (e.g., 2205, 2209). The abundance of the G-phase was measured in five duplex stainless steels, three wrought alloys (2101, 2003, 2205) and their matching filler metals (2101-w, 2209-w), after aging at 427 °C for 1000 h and 10,000 h. The G-phase volume fraction, number density, size, and precipitate spacing were found using quantitative analysis of transmission electron microscopy dark field images and the composition of G-phase precipitates on other clusters were characterized with atom probe tomography (APT). In the welded alloys, the G-phase was found to develop rapidly, relative to the wrought material. A positive correlation was found between the nickel equivalent composition of the alloy and the G-phase volume fraction. The alloys 2205, 2209, and 2101-w, which are higher in Cr and Ni, all showed significant G-phase precipitation, further strengthening the hypothesis that lean grade DSS alloys are more thermally stable against precipitation in the ferrite. Electron diffraction showed a secondary phase present in the 2101 wrought alloy at 10,000 h, but it was not crystallographically consistent with the G-phase; APT showed the presence of nanoclusters rich in both nickel and copper for this alloy. In conclusion, no secondary phases or clusters were found in 2003 after 10,000 h of aging, so it may be a candidate alloy for applications that require long-life times at high operating temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Tollen’s reagent assisted synthesis of hollow polyaniline microsphere/Ag nanocomposite and its applications in sugar sensing and electromagnetic shielding

Graphical abstract: Probable scheme to demonstrate the mechanism of PnHMAg showing enhanced EMI shielding compared to PnHM. - Highlights: • Hollow polyaniline microsphere (PnHM) exhibits superior properties due to its enhanced surface to volume ratio. • PnHMAg has been used in developing efficient sensor for the detection of sugar. • Presence of Ag nanoparticles enhances the electrical conductivity of PnHMAg resulting in the improvement of electromagnetic interference shielding in both X- and S-band regions. • Such properties could be harnessed effectively for development of devices for commercial as well as national purposes. - Abstract: The present study is focused on synthesis of polyaniline hollow microspheres (PnHM) nanocomposites of silver (Ag) i.e., PnHMAg by emulsion polymerization of aniline and Tollen’s reagent as a source for Ag nanoparticles. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis of PnHMAg indicated presence of silver nanoparticles dispersed on polyaniline surface. The electrical conductivity of PnHMAg is increased by ∼6 times compared to PnHM. Cyclic voltammogram of PnHM in sugar sensing exhibits characteristics redox peaks at ∼0.09 (sugar) and ∼0.53 V (polyaniline). Interestingly, PnHMAg showed a single peak at ∼−0.18 V with increased intensity (∼5 times) indicating its high sugar sensing ability. PnHMAg also exhibits high shielding efficiency of 19.5 dB (11.2 GHz) due to the presence of highly conducting Ag nanoparticles. TEM studies confirmed that Ag nanoparticles are well distributed on PnHM. As a result, a continuous electronic path is developed due to enhanced interconnectivity of PnHM.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Local and long-range atomic/magnetic structure of non-stoichiometric spinel iron oxide nanocrystallites

Spinel iron oxide nanoparticles of different mean sizes in the range 10–25 nm have been prepared by surfactant-free up-scalable near- and super-critical hydrothermal synthesis pathways and characterized using a wide range of advanced structural characterization methods to provide a highly detailed structural description. The atomic structure is examined by combined Rietveld analysis of synchrotron powder X-ray diffraction (PXRD) data and time-of-flight neutron powder-diffraction (NPD) data. The local atomic ordering is further analysed by pair distribution function (PDF) analysis of both X-ray and neutron total-scattering data. It is observed that a non-stoichiometric structural model based on a tetragonal γ-Fe 2 O 3 phase with vacancy ordering in the structure (space group P 4 3 2 1 2) yields the best fit to the PXRD and total-scattering data. Detailed peak-profile analysis reveals a shorter coherence length for the superstructure, which may be attributed to the vacancy-ordered domains being smaller than the size of the crystallites and/or the presence of anti-phase boundaries, faulting or other disorder effects. The intermediate stoichiometry between that of γ-Fe 2 O 3 and Fe 3 O 4 is confirmed by refinement of the Fe/O stoichiometry in the scattering data and quantitative analysis of Mössbauer spectra. The structural characterization is complemented by nano/micro-structural analysis using transmission electron microscopy (TEM), elemental mapping using scanning TEM, energy-dispersive X-ray spectroscopy and the measurement of macroscopic magnetic properties using vibrating sample magnetometry. Notably, no evidence is found of a Fe 3 O 4 /γ-Fe 2 O 3 core-shell nanostructure being present, which had previously been suggested for non-stoichiometric spinel iron oxide nanoparticles. Finally, the study is concluded using the magnetic PDF (mPDF) method to model the neutron total-scattering data and determine the local magnetic ordering and magnetic domain sizes in the iron oxide nanoparticles. The mPDF data analysis reveals ferrimagnetic collinear ordering of the spins in the structure and the magnetic domain sizes to be ∼60–70% of the total nanoparticle sizes. The present study is the first in which mPDF analysis has been applied to magnetic nanoparticles, establishing a successful precedent for future studies of magnetic nanoparticles using this technique.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗