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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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6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

A method for mapping submicron-scale crystallographic order/disorder applied to human tooth enamel

Tooth enamel, the outermost layer of human teeth, is a complex, hierarchically structured biocomposite. The details of this structure are important in multiple human health contexts, from understanding the progression of dental caries (tooth decay) to understanding the process of amelogenesis and related developmental defects. Enamel is composed primarily of long, nanoscale crystallites of hydroxyapatite that are bundled by the thousands to form micron-scale rods. Studies with transmission electron microscopy show the relationships between small groups of crystallites and X-ray diffraction characterize averages over many rods, but the direct measurement of variations in local crystallographic structure across and between enamel rods has been missing. Here, we describe a synchrotron X-ray-based experimental approach and a novel analysis method developed to address this gap in knowledge. A ~500-nm-wide beam of monochromatic X-rays in conjunction with a sample section only 1 μm in thickness enables 2D diffraction patterns to be collected from small well-separated volumes within the enamel microstructure but still probes enough crystallites (~300 per pattern) to extract population-level statistics on crystallographic features like lattice parameter, crystallite size, and orientation distributions. Furthermore, the development of a quantitative metric to characterize relative order and disorder based on the azimuthal autocorrelation of diffracted intensity enables these crystallographic measurements to be correlated with their location within the enamel microstructure (e.g., between rod and interrod regions). These methods represent a step forward in the characterization of human enamel and will elucidate the variation of the crystallographic structure across and between enamel rods for the first time.

crystallographic order/disorder↗
NASA NTRS1996

The Cooling History and Structure of the Ordinary Chondrite Parent Bodies

Most major meteorite classes exhibit significant ranges of metamorphism. The effects of metamorphism have been extensively characterized, but the heat source(s) and the metamorphic environment are unknown. Proposed beat sources include Al-26, Fe-60, electromagnetic induction, and impact. It is typically assumed that metamorphism occurred in parent bodies of some sort, but it uncertain whether these bodies were highly structured ("onion skins") or were chaotic mixes of material ("rubble piles"). The lack of simple trends of metallographic cooling rates with petrologic type has been considered supportive of both concepts. In this study, we use induced thermoluminescence (TL) as an indicator of thermal history. The TL of ordinary chondrites is produced by sodic feldspar, and the induced TL peak temperature is related to its crystallographic order/disorder. Ordered feldspar has TL peak temperatures of approx. 120 C, and disordered feldspar has TL peak temperatures of approx. 220 C. While ordered feldspar can be easily disordered in the laboratory by heating above 650 C and is easily quenched in the disordered form, producing ordered feldspar requires cooling at geologic cooling rates. We have measured the induced TL properties of 101 equilibrated ordinary chondrites, including 49 H, 29 L, and 23 LL chondrites. For the H chondrites there is an apparent trend of decreasing induced TL peak temperature with increasing petrologic type. H4 chondrites exhibit a tight range of TL peak temperatures, 190 C - 200 C, while H6 chondrites exhibit TL peak temperatures between 180 C and 190 C. H5 chondrites cover the range between H4 and H6, and also extend up to 210 C. Similar results are obtained for LL chondfiles and most L6 chondrites have lower induced TL peak temperatures than L5 chondrites.

Benoit, P. H.↗
DOE OSTI2021

Theory and application of the vector pair correlation function for real-space crystallographic analysis of order/disorder correlations from STEM images

Deviations of local structure and chemistry from the average crystalline unit cell are increasingly recognized to have a significant influence on the properties of many technologically important materials. Here, we present the vector pair correlation function (vPCF) as a new real-space crystallographic analysis method, which can be applied to atomic-resolution scanning transmission electron microscopy (STEM) images to quantify and analyze structural order/disorder correlations. Our STEM-based vPCFs have several advantages over radial PCFs and/or 3D pair distribution functions from x-ray total scattering: vPCFs explicitly retain crystallographic orientation information, are spatially resolved, can be applied directly on a sublattice basis, and are suitable for any material that can be imaged with STEM. To show the utility of our approach, we measure partial vPCFs in Ba 5 SmSn 3 Nb 7 O 30 (BSSN), a tetragonal tungsten bronze (TTB) structured complex oxide. Many TTBs are known to be classical or relaxor ferroelectrics, and these properties have been correlated with the presence of superlattice ordering. BSSN, specifically, exhibits relaxor behavior and an incommensurate structural modulation. From the vPCF data, we observe that, of the cation sites, only the Ba (A2) sublattice is structurally modulated. We then infer the local modulation vector and reveal a marked anisotropy in its correlation length. Finally, short-range correlated polar displacements on the B2 cation sites are observed. This work introduces the vPCF as a powerful real-space crystallography technique, which enables direct, robust quantification of short-to-long range order on a sublattice-specific basis and is applicable to a wide range of complex material types.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Applications of plasma-enhanced metalorganic chemical vapor deposition

The use of plasma enhancement for growth of III-V compound semiconductor materials by metalorganic chemical vapor deposition (MOCVD) is examined, to improve control of microstructure, develop understanding of the underlying growth mechanisms, and expand the range of materials combinations for photovoltaic and solid-state lighting applications. Whereas plasma is commonly used with group-IV materials, such as silicon, few studies have examined the impact of plasma-enhanced MOCVD (PE-MOCVD) for III-V materials growth. PE-MOCVD provides improved decomposition of metalorganic precursors, which is driven by hydrogen plasma to augment hydride reactions and thermal pyrolysis, but plasma generation requires low reactor pressure. We have demonstrated elemental Al films grown by PE-MOCVD, which show distinct crystallographic texturing, and assume epitaxial microstructure upon post-growth annealing. We also demonstrate PE-MOCVD of GaAs at temperatures as low as 300 °C. With increasing radio-frequency power, the GaAs growth rate shows a transition from power limited to mass-transport limited. PE-MOCVD grown Ga 0.5 In 0.5 P shows no detectable spontaneous atomic ordering, which offers a mechanism to form order/disorder unicompositional heterostructures, without temperature change or impurity incorporation. In its current implementation, PE-MOCVD grown films have shown microstructural degradation, that we attribute to direct plasma exposure during growth. Improvements in the system design and growth sequences are at the focus of on-going efforts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Phase transformation and growth mechanism of RF sputtered ferroelectric lead scandium tantalate (PbSc 0.5 Ta 0.5 O 3 ) films

Lead scandium tantalate (PbSc 0.5 Ta 0.5 O 3 , PST), an order/disorder ferroelectric, is a potential candidate for electrocaloric cooling and pyroelectric infrared (IR) detector. In this work, we report the phase transformation kinetics from two series of samples containing pure amorphous and mixture of amorphous and pyrochlore to desired perovskite phase using postdeposition rapid thermal processing (RTP) as well as growth mechanism of RF sputtered PST thin films using excess lead target on platinized silicon (Pt/Ti/SiO 2 /Si) substrates. We find that small changes in the temperature ramp have a large effect on the degree of perovskite conversion (ferroelectric phase), orientation (crystallographic texture), and long-range order parameter (< S 111 >). Through isothermal annealing, we obtained optimal perovskite phase at ≥700°C temperature. The phase transformation is characterized by spontaneous formation of center-type in-plane radial rosette-like structures revealed by scanning electron microscopy. The PST perovskite crystallites were found to coexist with pyrochlore in RTP annealed films. The volume fractions for perovskite and pyrochlore phase were obtained from the analysis of “rosettes” and respective X-ray diffraction intensities which helped to determine various parameters associated with phase kinetics (n, k, and activation energy, Ea) and accompanying growth. The effective activation energies of perovskite transition and growth were found to be 332 ± 11 kJ/mol (345 ± 11 kJ/mol) and 114 ± 10 kJ/mol (122 ± 10 kJ/mol), respectively, for pure amorphous only (and mixed amorphous and pyrochlore) phase following nucleation-growth controlled Avrami's equation. A linear growth rate (n~1) for the perovskite phase indicates predominant interface-controlled process and diffusion-limited phenomena thus inhibiting rosette size owing to reactant depletion and soft impingement at the grain boundary. However, the growth behavior is isotropic in two-dimension parallel to the plane of the substrates for both sample series. Furthermore, lead loss was severe for in-situ growth and RTP combined with conventional furnace annealing than those of RTP only films, which were closer to stoichiometric albeit with excess lead and marginal oxygen vacancies (V o ).

36 MATERIALS SCIENCE↗
DOE OSTI2020

Competing Interactions between Mesoscale Length-Scales, Order-Disorder, and Martensitic Transformation in Ferromagnetic Shape Memory Alloys

In the present study, the effects of composition and heat treatments and the resulting microstructural changes on the martensitic transformation and ferromagnetic transition have been investigated in NiCoMnIn metamagnetic shape memory alloys. In this shape memory alloy system, it is observed that upon heat treatments at a wide temperature range, the onset temperature of the martensitic transformation follows a non-monotonic behavior with respect to heat treatment time, and the nature of the non-monotonic behavior is also a function of composition. This behavior cannot be attributed to well-known factors such as precipitation, change in local composition due to the precipitation and/or global degree of order. In this work, a systematic investigation through synthesis, thermal processing and characterization via thermo-physical measurements, transmission electron microscopy and in-situ synchrotron x-ray diffraction experiments has been used to correlate the non-monotonic dependence of the martensitic transformation and ferromagnetic transition temperatures to the evolution of L2 1 domains arising from a order/disorder phase transition. A thermodynamic model for the magneto-structural transition is combined with classical nucleation theory to further ascertain the role of microstructural length-scales on the onset of the martensitic transformation. This work thus provides understanding of the thermodynamic and kinetic factors that can be controlled to tune the coupled magneto-structural transformations in NiCoMnIn metamagnetic shape memory alloys.

36 MATERIALS SCIENCE↗