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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 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Self-organization of S adatoms on Au(111): √3R30° rows at low coverage

Using scanning tunneling microscopy, we observe an adlayer structure that is dominated by short rows of S atoms, on unreconstructed regions of a Au(111) surface. This structure forms upon adsorption of low S coverage (less than 0.1 monolayer) on a fully reconstructed clean surface at 300 K, then cooling to 5 K for observation. The rows adopt one of three orientations that are rotated by 30° from the close-packed directions of the Au(111) substrate, and adjacent S atoms in the rows are separated by √3 times the surface lattice constant, a. Monte Carlo simulations are performed on lattice-gas models, derived using a limited cluster expansion based on density functional theory energetics. Models which include long-range pairwise interactions (extending to 5a), plus selected trio interactions, successfully reproduce the linear rows of S atoms at reasonable temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Ambidextrous Helical Nanotubes from Self-Assembly of Designed Helical Hairpin Motifs

Tandem repeat proteins exhibit native designability and represent potentially useful scaffolds for the construction of synthetic biomimetic assemblies. We have designed two synthetic peptides, HEAT R1 and LRV M3Δ1, based on the consensus sequences of single repeats of thermophilic HEAT (PBS HEAT) and Leucine- Rich Variant (LRV) structural motifs, respectively. Self-assembly of the peptides afforded high aspect-ratio helical helical nanotubes. Cryo-electron microscopy with direct electron detection was employed to analyze the structures of the solvated filaments. Three dimensional reconstructions from the cryo-EM maps led to atomic models for the HEAT R1 and LRV M3Δ1 filaments at resolutions of 6.0 Å and 4.4 Å, respectively. Surprisingly, despite sequence similarity at the lateral packing interface, HEAT R1 and LRV M3Δ1 filaments adopt the opposite helical hand and differ significantly in helical geometry, while retaining a local conformation similar to previously characterized repeat proteins of the same class. The differences in the two filaments could be rationalized on the basis of differences in cohesive interactions at the lateral and axial interfaces. These structural data reinforce previous observations regarding the structural plasticity of helical protein assemblies and the need for high resolution structural analysis. Despite these observations, the native designability of tandem repeat proteins offers the opportunity to engineer novel helical nanotubes. Furthermore, the resultant nanotubes have independently addressable and chemically distinguishable interior and exterior surfaces that would facilitate applications in selective recognition, transport, and release.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Crystal structure of hexachlorothallate within a caesium chloride–phosphotungstate lattice Cs 9 (TlCl 6 )(PW 12 O 40 ) 2 ·9CsCl

Crystal formation of caesium thallium chloride phosphotungstates, Cs 9 (TlCl 6 )(PW 12 O 40 ) 2 ·9CsCl showcases the ability to capture and crystallize octahedral complexes via the use of polyoxometalates (POMs). The large number of caesium chlorides allows for the POM [α-PW 12 O 40 ] 3- to arrange itself in a cubic close-packing lattice extended framework, in which the voids created enable the capture of the [TlCl 6 ] 3- complex.

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

The role of collective elasticity on activated structural relaxation, yielding, and steady state flow in hard sphere fluids and colloidal suspensions under strong deformation

In this work, we theoretically study the effect of external deformation on activated structural relaxation and aspects of the nonlinear mechanical response of glassy hard sphere fluids in the context of elastically collective nonlinear Langevin equation theory. This microscopic force-based approach describes activated relaxation as a coupled local–nonlocal event involving caging and longer range collective elasticity, with the latter becoming more important and ultimately dominant with increasing packing fraction under equilibrium conditions. The central new question we address is how this physical picture of activated relaxation, and the relative importance of local caging vs collective elasticity physics, depends on external deformation. Theoretical predictions are presented for deformation-induced enhancement of mobility, the onset of relaxation speed up at remarkably low values of stress, strain, or shear rate, apparent power law thinning of the steady state structural relaxation time and viscosity, a non-vanishing activation barrier in the shear thinning regime, an apparent Herschel–Bulkley form of the rate dependence of the steady state shear stress, exponential growth of different measures of a dynamic yield or flow stress with the packing fraction, and reduced fragility and dynamic heterogeneity under deformation. The results are contrasted with experiments and simulations, and qualitative or better agreement is found. An overarching conclusion is that deformation strongly reduces the importance of longer range collective elastic effects relative to the local caging aspect for most, but not all, physical questions, with deformation-dependent fragility and dynamic heterogeneity phenomena being qualitatively sensitive to collective elasticity. Overall, nonlinear rheology is predicted to be a more local problem than quiescent structural relaxation, albeit with deformation-modified activated processes still important.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Anion effect on the topological frameworks of a series of manganese coordination polymers based on 1,4-bis(imidazol-1-yl)-benzene: Syntheses, crystal structures and magnetic properties

In this work, the rod-like ligand 1,4-bis(imidazol-1-yl)-benzene (bib) has been utilized as a building block to perform counterion researches on the structural diversities of coordination polymers. A series of new manganese compounds, ([Mn(bib){sub 3}(ClO{sub 4}){sub 2}](CHCl{sub 3}){sub 2}){sub n} (1), [Mn(bib){sub 2}(N{sub 3}){sub 2}]{sub n} (2), [Mn(bib){sub 2}(HCO{sub 2}){sub 2}]{sub n} (3), [Mn(bib){sub 2}(Ac){sub 2}]{sub n} (4), ([Mn(bib){sub 2}(CF{sub 3}SO{sub 3}){sub 2}](CH{sub 2}Cl{sub 2}){sub 4}){sub n} (5), and [Mn(bib){sub 2}(SO{sub 4})]{sub n} (6) have been successfully synthesized. Compound 1 shows a 3D interpenetrating α-Po network only based on the bib linker. Compounds 2 and 3 exhibit a 2D (4,4) layer with parallel and incline interpenetration, respectively. Compounds 4 and 5 display a parallel-packing 2D (4,4) layer with the porosity of 23.4% and 61.4%, respectively. Compound 6 furnishes a 3D α-Po framework with a 2D (4,4) layer pillared by the μ{sub 2}-SO{sub 4}{sup 2−} ion. The structural diversities among 1–6 have been carefully discussed, and the roles of counterions (from coordination affinity and molecular size) in the self-assembly of coordination polymers have also been well documented. Furthermore, magnetic properties of 6 have been carefully studied. - Graphical abstract: Six new compounds have been successfully synthesized. Structural studies reveal that the topology, entanglement and porosity are tunable by the counterion. Furthermore, the weak ferromagnetic coupling is conducted in 6. - Highlights: • Six new Mn{sup II} compounds have been synthesized. • Compounds 1–3 exhibit diverse interpenetrating frameworks. • Compounds 4 and 5 exhibit 3D packing porous architectures. • The tunable effect of counterion has been documented. • Magnetic properties of 6 have been studied by fitting the data.

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

On-device lead sequestration for perovskite solar cells

Perovskite solar cells, as an emerging high-efficiency and low-cost photovoltaic technology face challenges on their way towards commercialization. Substantial improvements have been made to device stability but potential issues with lead toxicity and leaching from devices remain relatively unexplored. The potential for lead leakage could be perceived as an environmental and public health risk when using perovskite solar cells in building-integrated photovoltaics. Here we present a chemical approach for on-device sequestration of more than 96 per cent of lead leakage caused by severe device damage. A coating of lead-absorbing material is applied to the front and back sides of the device stack. On the glass side of the front transparent conducting electrode, we use a transparent lead-absorbing molecular film containing phosphonic acid groups that bind strongly to lead. On the back (metal) electrode side, we place a polymer film blended with lead-chelating agents between the metal electrode and a standard photovoltaic packing film. The lead-absorbing films on both sides swell to absorb the lead, rather than dissolve, when subjected to water soaking, thus retaining structural integrity for easy collection of lead after damage.

14 SOLAR ENERGY↗
DOE OSTI2018

Tenfold increase in the photostability of an azobenzene guest in vapor-deposited glass mixtures

Improvements to the photostability of organic glasses for use in electronic applications have generally relied on the modification of the chemical structure. We show here that the photostability of a guest molecule can also be significantly improved—without chemical modification—by using physical vapor deposition to pack molecules more densely. Photoisomerization of the substituted azobenzene, 4,4'-diphenyl azobenzene, was studied in a vapor-deposited glass matrix of celecoxib. We directly measure photoisomerization of trans- to cis-states via Ultraviolet-visible (UV-Vis) spectroscopy and show that the rate of photoisomerization depends upon the substrate temperature used during co-deposition of the glass. Photostability correlates reasonably with the density of the glass, where the optimum glass is about tenfold more photostable than the liquid-cooled glass. Molecular simulations, which mimic photoisomerization, also demonstrate that photoreaction of a guest molecule can be suppressed in vapor-deposited glasses. Finally, from the simulations, we estimate that the region that is disrupted by a single photoisomerization event encompasses approximately 5 molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Be:B Amorphous Coatings and Order-Disorder Transitions

Here Be:B films were explored as a possible ablator material for use in inertial confinement fusion target capsules. It was found that Be:B forms an amorphous structure near the eutectic composition of 11 to 12 at. % B. It is believed that having an amorphous ablator should be useful in suppressing Rayleigh-Taylor instabilities during compression of the target. As the composition is moved away from the eutectic, an amorphous-to–columnar structure transition was more likely to be observed after some finite thickness of amorphous material had been deposited. Microstructural analysis indicated that this transition involved the nucleation of nanocrystal structures within the amorphous matrix. This nanocrystal nucleation is believed to be due to supersaturation of the dopant atom in the host. Finally, an efficient packing analysis is also presented in an effort to explain the most favorable amorphous composition of 11 to 12 at. % B doping observed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Sound speed and refractive index of amorphous CaSiO3 upon pressure cycling to 40 GPa

Abstract Brillouin spectroscopy at room temperature and pressures up to 40 GPa documents nearly identical elasticity and refractive index of amorphous CaSiO3 created by two different methods: temperature-quenching the melt at ambient pressure and pressure-amorphizing crystalline wollastonite at room temperature. We find reproducible hysteresis of 0 to 8% on pressure cycling that is small relative to the 30 to 60% changes in shear and longitudinal wave velocities over this pressure range. Together with observed changes in refractive index and previous results from Raman spectroscopy, these measurements reveal a continuous and reversible change in atomic packing induced by pressure. Unlike many other silicate glasses, amorphous CaSiO3 exhibits highly reproducible properties, behaving smoothly and reversibly under pressure cycling and possessing similar structure and elasticity regardless of synthesis paths for the starting material, which suggests that the amorphous solid may mimic the liquid over the pressure range investigated.

Geochemistry & Geophysics↗
DOE OSTI2017

Ceramic Proppant Design for In-situ Microbially Enhanced Methane Recovery

This project designed a new type of multi-functional lightweight proppant. The proppant is utilized as the conventional lightweight proppant but also transports microorganisms to coalbed reservoirs. The proppant is coated with a polymer which protects the methanogenic microorganisms and serves as a time-release delivery for methane generation. To produce the multifunctional proppant, we assigned five tasks: 1) culturing methanogenic microbes from natural carbon sources; 2) identifying optimized growth and methanogenesis conditions for the microbial consortia; 3) synthesizing the lightweight ceramic proppant; 4) encapsulating the consortia and proppant; and 5) demonstrating lab scale simulated performance by monitoring in-situ methane generation and hydraulic conductivity. Task 1) To evaluate the feasibility of ex-situ cultivation, natural microbial populations were collected from various hydrocarbon-rich environments and locations characterized by natural methanogenesis. Different rank coals, complex hydrocarbon sources, hydrocarbon seeps, and natural biogenic environments were incorporated in the sampling. Three levels of screening allowed selection of microbial populations, favorable nutrient amendments, sources of the microbial community, and quantification of methane produced from various coal types. Incubation periods of up to 24 weeks were evaluated at 23°C. Headspace concentrations of CH 4 and CO 2 were analyzed by gas chromatography. After a two-week incubation period of the most promising microbes, generated headspace gas concentrations reached 873,400 ppm for methane and 176,370 ppm for carbon dioxide. Task 2) A central composite design (CCD) was used to explore a broad range of operational conditions, examine the effects of the important environmental factors, such as temperature, pH and salt concentration, and query a feasible region of operation to maximize methane production from coal. Coal biogasification was optimal for this consortium at an initial pH value of 5.5, at 30 °C, and at a NaCl concentration 3.7 mg/cm 3 (i.e., 145,165 ppm). Task 3) To create the lightweight ceramic proppant, the kaolinite and iron oxide (Fe 2 O 3 ) mixture were sintered under reducing atmosphere varying partial pressures of oxygen from 1.7×10 -13 atm to 1.8×10-11 atm. The Fe 2 O 3 reduces to FeO and reacts with kaolinite decomposed to mullite to form Fe 2 SiO 4 , FeSiO 3 , and FeAl 2 O 4 . As a result the proppant develops large pores (~100 µm), giving it a low bulk density (1.43 g/cm 3 ), and high porosity (45.2 vol%) at P_(O_2 )of 1.8×10 -11 atm. The proppant sintered at P_(O_2 )of 1.7×10 -13 atm is characterized by smaller pores (26 µm), higher density (1.72 g/cm3) and lower porosity (37.5 vol%). Crush resistance testing at 9,000 psi yields 6.8 wt% fine particles increasing to 17.7 wt% in porous samples. Acid solubility varies from 5.5 wt% loss increasing to 12.9 wt% in porous samples. Task 4 and 5) The microbial consortia obtained from the optimized condition and lightweight proppant were encapsulated by calcium-alginate polymer. The encapsulation system, including microbes and a proppant, was stored at 20°C and 36°C. The sample stored at 36°C generated 7.1×[10] 3 ppm of methane at 670 h, whereas the sample incubated at 20°C generated 1.1×[10] 3 ppm of methane. Benchtop lab-scale bioreactors were constructed and showed with real-time measurements that, as the microbes produced methane, the pH decreased, and hydrogen and carbon dioxide gases were consumed. A calcium ion-selective sensor revealed the release mechanism of the microbes from encapsulation. The calcium ions interconnecting the alginate monomers were disassociated by counter cations such as protons and potassium ions. The counter cations bound to the carboxyl group on the alginate monomer, which thus lost its ionic interactions with the calcium ions, resulting in decomposition of the hydrogel structure. Furthermore, the conductivity was increased to ~700 mD-ft from ~530 mD-ft once the encapsulated sample was applied to the coal pack. The particle size of coal was reduced when the encapsulated sample was added to the coal. The reduced coal size implied that the microbes would consume the hydrocarbon in coal.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2018

Time-resolved PIV measurements in a low-aspect ratio facility of randomly packed spheres and flow analysis using modal decomposition

This work experimentally investigated the flow characteristics in a facility with randomly packed spheres at a low-aspect ratio of 4.4. Velocity fields in the near-wall region and in the pores between spheres were obtained by employing the matched-index-of-refraction (MIR) and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers of 340, 520, and 720. From the obtained TR-PIV velocity vector fields, flow characteristics including first- and second-order statistics, such as mean velocity, root-mean-square fluctuating velocity, and Reynolds stress profiles, were computed. The effects of the wall enclosure and Reynolds numbers on the flow patterns were investigated by comparing the computed flow statistics and evaluating two-point cross correlations of the velocities measured adjacent to and far from the wall. Comparisons of the mean velocities, root-mean-square fluctuating velocities, and Reynolds stress component showed an increase in flow mixing and turbulent intensity in the gaps between spheres in the packed bed. The re-circulation-region sizes, however, were found to be independent from an increase in Reynolds numbers. Finally, flow modal decompositions, such as the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), were applied to the vorticity fields extracted from sub-regions located near and far from the wall to reveal the most dominant POD and DMD flow structures.

42 ENGINEERING↗
DOE OSTI2021

Improving Gas Selectivity in Membranes Using Polymer-Grafted Silica Nanoparticles

Herein, we demonstrate that tuning of structural parameters and material chemistry enables control of the permeability and selectivity of gases (CO 2 , He, and CH 4 ) in membranes based on poly(butyl methacrylate)-grafted nanoparticles (PGNs). Our data show that the presence of nanoparticles and the overall dense packing of grafted chains noticeable in PGNs with low grafting density have an adverse effect on the diffusivity of gases. This effect is compensated by an improvement in the solubility of CO 2 gas promoted by the silica nanoparticle surface, yielding a substantial improvement in the permeability of CO 2 versus CH 4 . Furthermore, in membranes with high grafting density, changes in the structural arrangements and alterations in the membrane porosity, evident from small-angle X-ray scattering and positron annihilation lifetime spectroscopy, positively influence the permeability of He and CO 2 gases. In contrast, CH 4 permeability in the same membranes is significantly suppressed, suggesting the formation of a unique, highly selective environment for gas separation. As a result, an improvement of up to 50% in selectivity for gas pairs containing large CH 4 molecules is observed. Our studies provide fundamental insights into the role that structural parameters play in gas transport through polymer membranes, laying a foundation for the rational design of membranes with improved permeability and selectivity.

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

X‐ray characterization of three possible edible oleogelators: Experiment and theory

Abstract Three oleogelator molecules (Triacontane (TC), Stearic acid (SA), and Behenyl Lignocerate (BL)) were studied individually, in pairs, or all together to make an oleogel using triolein as the oil. WAXS, SAXS and USAXS were used to elucidate the solid structures from angstroms to a few micrometers. A two‐dimensional mapping of atomic positions for each molecule was carried out to understand the crystalline multilayer structures formed. We assumed that the molecules were rigidly extended and that they underwent no significant (hindered) rotations so that the free energy is determined by the Lennard‐Jones interactions of closely packed multilayers. TC molecules were predicted to form a tilt angle of , yielding a SAXS line at Å ─1 , in acceptable agreement with the measured . For SA crystals (predicted) yielding a SAXS line at compared to (observed). No mixed crystals were observed for any pair of molecules or when all three were used. USAXS data showed that SA forms large nanocrystals compared to TC and BL. All three combinations of molecular pairs showed basic scatterers smaller or similar to those of individual molecules. The theory presented here, together with the experimental results, showed why no mixed crystals are formed from two or all three molecules. Data from the USAXS region suggested that, when using all three molecules, a more compact fractal structure was obtained, compared with those if one or two of the molecules were used.

Chemistry↗
DOE OSTI2017

A computational study of yttria-stabilized zirconia: I. Using crystal chemistry to search for the ground state on a glassy energy landscape

Yttria-stabilized zirconia (YSZ), a ZrO 2 -Y 2 O 3 solid solution that contains a large population of oxygen vacancies, is widely used in energy and industrial applications. Past computational studies correctly predicted the anion diffusivity but not the cation diffusivity, which is important for material processing and stability. One of the challenges lies in identifying a plausible configuration akin to the ground state in a glassy landscape. This is unlikely to come from random sampling of even a very large sample space, but the odds are much improved by incorporating packing preferences revealed by a modest sized configurational library established from empirical potential calculations. Ab initio calculations corroborated these preferences, which prove remarkably robust extending to the fifth cation-oxygen shell about 8 Å away. Yet because of frustration there are still rampant violations of packing preferences and charge neutrality in the ground state, and the approach toward it bears a close analogy to glass relaxations. Fast relaxations proceed by fast oxygen movement around cations, while slow relaxations require slow cation diffusion. The latter is necessarily cooperative because of strong coupling imposed by the long-range packing preferences.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A neutron tomography study: probing the spontaneous crystallization of randomly packed granular assemblies

We study the spontaneous crystallization of an assembly of highly monodisperse steel spheres under shaking, as it evolves from localized icosahedral ordering towards a packing reaching crystalline ordering. Towards this end, real space neutron tomography measurements on the granular assembly are carried out, as it is systematically subjected to a variation of frequency and amplitude. As expected, we see a presence of localized icosahedral ordering in the disordered initial state (packing fraction ≈ 0.62). As the frequency is increased for both the shaking amplitudes (0.2 and 0.6 mm) studied here, there is a rise in packing fraction, accompanied by an evolution to crystallinity. The extent of crystallinity is found to depend on both the amplitude and frequency of shaking. We find that the icosahedral ordering remains localized and its extent does not grow significantly, while the crystalline ordering grows rapidly as an ordering transition point is approached. In the ordered state, crystalline clusters of both face centered cubic (FCC) and hexagonal close packed (HCP) types are identified, the latter of which grows from stacking faults. Our study shows that an earlier domination of FCC gives way to HCP ordering at higher shaking frequencies, suggesting that despite their coexistence, there is a subtle dynamical competition at play. This competition depends on both shaking amplitude and frequency, as our results as well as those of earlier theoretical simulations demonstrate. It is likely that this involves the very small free energy difference between the two structures.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Sluggish atomic dynamics in a Y-Sc-Co-Al high entropy bulk metallic glass

We present how 20 at% Sc addition affects the atomic packing and dynamics in a Y-Co-Al metallic glass (MG) and find that it greatly suppresses the dynamics of Co atoms by reducing the atomic packing difference in their surroundings. The Co atoms tending to be mobile or static depend on their nearest neighbors, i.e., possessing more Y/Y+Sc atoms but fewer Co atoms or vice versa. The X-ray absorption fine structure results confirm that Sc addition mainly changes the local environment around Co atoms, forming more Co-Co and Co-Sc pairs but significantly fewer Co-Y pairs than in the ternary counterpart, which can seriously slow the atomic dynamics and stabilize the competing phases. Our findings shed new light on the understanding of strong glass forming ability of the Sc-added Y-based high entropy MG from its local structure and dynamics and will be helpful in developing new bulk MGs.

36 MATERIALS SCIENCE↗
DOE OSTI2017

BECN2 interacts with ATG14 through a metastable coiled-coil to mediate autophagy: BECN2 CCD Structure and Interaction with ATG14

ATG14 binding to BECN/Beclin homologs is essential for autophagy, a critical catabolic homeostasis pathway. Here, we show that the α-helical, coiled-coil domain (CCD) of BECN2, a recently identified mammalian BECN1 paralog, forms an antiparallel, curved homodimer with seven pairs of nonideal packing interactions, while the BECN2 CCD and ATG14 CCD form a parallel, curved heterodimer stabilized by multiple, conserved polar interactions. Compared to BECN1, the BECN2 CCD forms a weaker homodimer, but binds more tightly to the ATG14 CCD. Mutation of nonideal BECN2 interface residues to more ideal pairs improves homodimer self-association and thermal stability. Unlike BECN1, all BECN2 CCD mutants bind ATG14, although more weakly than wild type. Thus, polar BECN2 CCD interface residues result in a metastable homodimer, facilitating dissociation, but enable better interactions with polar ATG14 residues stabilizing the BECN2:ATG14 heterodimer. These structure-based mechanistic differences in BECN1 and BECN2 homodimerization and heterodimerization likely dictate competitive ATG14 recruitment.

59 BASIC BIOLOGICAL SCIENCES↗
NASA NTRS1999

Fabrication of Pop-up Detector Arrays on Si Wafers

High sensitivity is a basic requirement for a new generation of thermal detectors. To meet the requirement, close-packed, two-dimensional silicon detector arrays have been developed in NASA Goddard Space Flight Center. The goal of the task is to fabricate detector arrays configured with thermal detectors such as infrared bolometers and x-ray calorimeters to use in space fliGht missions. This paper focuses on the fabrication and the mechanical testing of detector arrays in a 0.2 mm pixel size, the smallest pop-up detectors being developed so far. These array structures, nicknamed "PUDS" for "Pop-Up Detectors", are fabricated on I pm thick, single-crystal, silicon membranes. Their designs have been refined so we can utilize the flexibility of thin silicon films by actually folding the silicon membranes to 90 degrees in order to obtain close-packed two-dimensional arrays. The PUD elements consist of a detector platform and two legs for mechanical support while also serving as electrical and thermal paths. Torsion bars and cantilevers connecting the detector platform to the legs provide additional flexures for strain relief. Using micro-electromechanical structure (MEMS) fabrication techniques, including photolithography, anisotropic chemical etching, reactive-ion etching, and laser dicing, we have fabricated PLTD detector arrays of fourteen designs with a variation of four parameters including cantilever length, torsion bar length and width, and leg length. Folding tests were conducted to test mechanical stress distribution for the array structures. We obtained folding yields and selected optimum design parameters to reach minimal stress levels. Computer simulation was also employed to verify mechanical behaviors of PUDs in the folding process. In addition, scanning electron microscopy was utilized to examine the flatness of detectors and the alignment of detector pixels in arrays. The fabrication of thermistors and heaters on the pop-up detectors is under way, preparing us for the next step of the experiment, the thermal test.

Li, Mary J.↗