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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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The addition of 17 OH 2 to silica at room temperature leads to spontaneous hydroxyl oxygen exchange and the selective isotopic enrichment of silanols, as observed using in and ex situ 17 O NMR. Furthermore, the discovery that silanols are labile alters our understanding of the chemistry of silica surfaces.
Single-stranded DNA molecules modified with cholesterol functional groups are physically tethered to silica nanoparticles (diameter 25 nm) that are encapsulated in a lipid bilayer. Such tethering increases the azimuthal mobility of the DNA molecules across the nanoparticle surface and enables nonspecific bonding, eliminating the need for specialized surface chemistries (such as silane or thiol ligands). To induce assembly, double-stranded DNA ‘bridge’ molecules are then added with complementary nucleotides to the DNA ‘anchor’ molecules that are physically tethered to the lipids on the surface of the particles. Assembly is observed to occur at room temperature and without the need for temperature annealing. Using automated liquid handling tools, assemblies are created in high throughput and rapidly characterized using SAXS. It is determined that the relative concentration of DNA-to-silica and the ionic strength of the solution are important parameters that affect the resulting assembly. Analysis of SAXS data is performed using coarse-grained particle dynamics simulations. The results support the spontaneous formation of semi-crystalline particle assemblies by particle condensation, where the interparticle distance is tuned by the sequence of the DNA ‘bridge’ used to link the particles. Crystallinity analysis performed on the resulting simulations, optimized to match SAXS observations, suggest that particle clusters display increased crystallinity in the center of the clusters, but their maximum size remains relatively small (sub-micron) before settling occurs, which limits the extent of crystallization.
The fracture of particles has a significant influence on the engineering behavior of granular materials. There are no reported conclusive answers in the literature to explain the influence of pore water on the fracture of sand, gravel, railroad ballast rock, aggregates, rockfill, and so on. Here, in this paper, two novel techniques were adopted to investigate the influence of pore water on the fracture of natural silica sand at the particle scale. The three-dimensional (3D) synchrotron microcomputed tomography (SMT) imaging technique was used to acquire and analyze multiple 3D images of specimens composed of wetted silica sand that were subjected to confined one-dimensional (1D) compression loading up to the fracture stage. A few representative particles were identified and digitally separated from the liquid water and gas phases. The 3D SMT images offer clear experimental evidence of the phase transition of water from liquid to gas within the opening cracks. In addition, a computational fluid dynamics numerical simulation framework for water flow into an opening crack was adopted and the results show that the fast opening of the crack induced cavitation, water phase transition, and water hammer effects. The results of the numerical simulations agree with the confined 1D compression experiments where the onset of the bubbles (gas phase) within the crack is mainly generated during the secondary cavitation stage, and the generated bubble near the crack mouth opening resulted from the primary cavitation, which was directly caused by the crack expansion. The results reported in this paper present a new cavitation phenomenon that occurs during particle fracture, which offers a physics-based explanation of why water-wetted sand fractures at smaller stresses than dry and can pave the way for more in-depth future studies on the fracture of water-wetted sand.
Fundamental understanding of the subcritical/supercritical behavior of key hydrocarbon species inside nano-porous matrices at elevated pressure and temperature is less developed compared to bulk fluids, but this knowledge is of great importance for chemical and energy engineering industries. This study explores in detail the structure and dynamics of ethane (C 2 H 6 ) fluid confined in silica nanopores, with a focus on the effects of pressure and different ratios of C 2 H 6 and CO 2 at non-ambient temperature. Quasi-elastic neutron scattering (QENS) experiments were carried out for the pure C 2 H 6 , C 2 H 6 :CO 2 = 3:1, and 1:3 mixed fluids confined in 4-nm cylindrical silica pores at three different pressures (30 bars, 65 bars, and 100 bars) at 323 K. Two Lorentzian functions were required to fit the spectra, corresponding to fast and slow translational motions. No localized motions (rotations and vibrations) were detected. Higher pressures resulted in hindrances of the diffusivity of C 2 H 6 molecules in all systems investigated. Pore size was found to be an important factor, i.e., the dynamics of confined C 2 H 6 is more restricted in smaller pores compared to the larger pores used in previous studies. Molecular dynamics simulations were performed to complement the QENS experiment at 65 bars, providing supportive structure information and comparable dynamic information. The simulations indicate that CO 2 molecules are more strongly attracted to the pore surface compared to C 2 H 6 . The C 2 H 6 molecules interacting with or near the pore surface form a dense first layer (L1) close to the pore surface and a second less dense layer (L2) extending into the pore center. Both the experiments and simulations revealed the role that CO 2 molecules play in enhancing C 2 H 6 diffusion (“molecular lubrication”) at high CO 2 :C 2 H 6 ratios. The energy scales of the two dynamic components, fast and slow, quantified by both techniques, are in very good agreement. Herein, the simulations identified the fast component as the main contributor to the dynamics. Molecule motions in the L2 region are mostly responsible for the dynamics (fast and slow) that can be detected by the instrument.
A semi-analytical model for deterministic surface smoothing of fused silica below the evaporation temperature threshold is presented. Using this model, a method for calculating deterministic track depths for laser polishing fused silica is derived and validated with experiments. In this work, model predictions were in good agreement with experiment across varying laser preheating temperature and power. With the semi-analytical method presented here, deterministic laser smoothing can be achieved in the densification regime to close the process loop of laser polishing.
The structural and dynamical properties of nanoconfined solutions can differ dramatically from those of the corresponding bulk systems. Understanding the changes induced by confinement is central to controlling the behavior of synthetic nanostructured materials and predicting the characteristics of biological and geochemical systems. A key outstanding issue is how the molecular-level behavior of nanoconfined electrolyte solutions is reflected in different experimental, particularly spectroscopic, measurements. This is addressed here through molecular dynamics simulations of the OH stretching infrared (IR) spectroscopy of NaCl, NaBr, and NaI solutions in isotopically dilute HOD/D 2 O confined in hydroxylated amorphous silica slit pores of width 1–6 nm and pH ~2. In addition, the water reorientation dynamics and spectral diffusion, accessible by pump–probe anisotropy and two-dimensional IR measurements, are investigated. In this work, the aim is to elucidate the effect of salt identity, confinement, and salt concentration on the vibrational spectra. It is found that the IR spectra of the electrolyte solutions are only modestly blue-shifted upon confinement in amorphous silica slit pores, with both the size of the shift and linewidth increasing with the halide size, but these effects are suppressed as the salt concentration is increased. This indicates the limitations of linear IR spectroscopy as a probe of confined water. However, the OH reorientational and spectral diffusion dynamics are significantly slowed by confinement even at the lowest concentrations. The retardation of the dynamics eases with increasing salt concentration and pore width, but it exhibits a more complex behavior as a function of halide.
Silica (SiO 2 ) aerogel is widely used in high-energy-density shock experiments due to its low and adjustable density. Reported here are measurements of the shock velocity, optical radiance, and reflectivity of shocked SiO 2 aerogel with initial densities of 0.1, 0.2, and 0.3 g/cm 3 . These results are compared with similar data from three solid polymorphs of SiO 2 , silica, quartz, and stishovite with initial densities 2.2, 2.65, and 4.3 g/cm 3 , respectively. Interestingly, below a brightness temperature of T bright ≈ 35,000 K, the slope of the radiance vs shock velocity is the same for each of the SiO 2 aerogels and solid polymorphs. At T bright ≈ 35000 K, there is an abrupt change in the radiance vs shock velocity slope for aerogels, but not seen in the solid polymorphs over the pressures and temperatures explored here. An empirical model of shock front radiance as a function of SiO 2 density and laser drive parameters is reported to aid in the design of experiments requiring maximum shock front radiance.
Supported metallic nanoparticles play a central role in catalysis. However, predictive modeling is particularly challenging due to the structural and dynamic complexity of the nanoparticle and its interface with the support, given that the sizes of interest are often well beyond those accessible via traditional ab initio methods. With recent advances in machine learning, it is now feasible to perform MD simulations with potentials retaining near-density-functional theory (DFT) accuracy, which can elucidate the growth and relaxation of supported metal nanoparticles, as well as reactions on those catalysts, at temperatures and time scales approaching those relevant to experiments. Furthermore, the surfaces of the support materials can also be modeled realistically through simulated annealing to include effects such as defects and amorphous structures. We study the adsorption of fluorine atoms on ceria and silica supported palladium nanoparticles using machine learning potential trained by DFT data using the DeePMD framework. We show defects on ceria and Pd/ceria interfaces are crucial for the initial adsorption of fluorine, while the interplay between Pd and ceria and the reverse oxygen migration from ceria to Pd control spillover of fluorine from Pd to ceria at later stages. In contrast, silica supports do not induce fluorine spillover from Pd particles.
An understanding of carbon dioxide fluid properties within geological mesopores is important in applications ranging from carbon sequestration to shale oil recovery. Here, a molecular dynamics study is presented that aims to shed light on these systems by simulation of CO 2 fluid confined in β-cristobalite silica slit pores with different pore widths and pressure conditions. The weakly associating nature of carbon dioxide leads to little difference in structural and interfacial dynamical properties for different pore sizes and pressures. Rather, the behavior is found to be dominated by the entropic effects, namely, how the CO 2 organizes next to the silica surface. The CO 2 self-diffusion coefficient shows the strongest pore size dependence. It is strongly diminished in small pores and does not reach the bulk fluid value even in 6 nm pores. It also decreases with pressure, yielding an activation volume that increases with pore size. These results provide new insight into the behavior of a compressible fluid in nanoscale confinement.
Stereolithography has been used to create ceramic parts with complex geometry that is difficult to achieve with conventional fabrication techniques. Here this study used stereolithography to print silica honeycomb structures with a commercial Formlabs Form2 printer. The printed samples were sintered at different temperatures, and the print shape was retained up to 1300°C, but significant distortion from partial melting occurred at 1400°C. Higher sintering temperatures lead to more shrinkage, but it is non-uniform among directions, with the open cell plane shrinking more than the dense plane of the sample. As expected, the density of samples also increases with the sintering temperature. At higher sintering temperatures, there is an increase in cristobalite and a decrease in quartz, tridymite, and amorphous silica. Regarding mechanical properties, the out-of-plane compressive strength is approximately one order of magnitude higher than the in-plane compressive strength. When compressed along the out-of-plane direction, the samples sintered at lower temperatures surprisingly exhibit higher strength, which is explained by the micro-cracking mechanism. As expected, the samples sintered at higher temperatures display higher strength when compressed along the in-plane direction.
Vibrational spectroscopy has been widely used to investigate various structural aspects of the glass network, and there are a plethora of papers reporting subtle but consistent changes in infrared and Raman spectral features of glass upon alterations of glass compositions, thermal histories, mechanical stresses, or surface treatments. However, interpretations of such spectral features are still obscured due to the lack of well-established physical principles accurately describing vibrational modes of the non-crystalline glass network. Due to the non-equilibrium nature of the glass network, three-dimensionally connected without any long-range orders, vibrational spectral features of glass cannot be interpreted using the analogy with those of isolated molecular moieties or crystalline counterparts. Here, this feature article explains why such comparisons are outdated and describes the recent advances made from theoretical calculations of vibrational spectral features of amorphous networks or comparisons of computational results with experimental data. For the interpretation of vibrational spectral features of silica and silicate glasses, the following empirical relationships are suggested: (i) the intensity-weighted peak position of the Si-O-Si stretch mode negatively correlates with the weighted average of the Si-O bond length distribution, and (ii) the broad band of the Si-O-Si bending mode negatively correlates with the Si-O-Si bond angle distribution. Selected examples of vibrational spectroscopic imaging of surface defects are discussed to deliberate the implication of these findings in the structure-property relationship of silica and silicate glass materials. Unanswered questions and continuing research challenges are identified.
The National Ignition Facility (NIF) is intentionally operated with the final fused silica glass (SiO2) optics exposed to fluences and intensities with the potential to induce damage that will grow with additional laser exposure. Therefore, the NIF operates a recycle loop to refurbish final optics by mitigating any initiated surface damage to arrest its growth. However, the morphology of filamentary damage, caused by local self-focusing in the bulk of a silica optic, adds complexity to optics mitigation and provides a limitation to optic reusability. Here, we evaluate techniques for mitigating isolated and clustered filamentary damage. Optical microscopy before and after installation on NIF was used to determine the efficacy of filamentary mitigation after a series of high fluence and intensity laser exposures. The challenges and success rate of the methods are compared for various filamentary damage mitigation strategies.
Glass is increasingly desired as a material for manufacturing complex microscopic geometries, from the micro-optics in compact consumer products to microfluidic systems for chemical synthesis and biological analyses. As the size, geometric, surface roughness, and mechanical strength requirements of glass evolve, conventional processing methods are challenged. We introduce microscale computed axial lithography (micro-CAL) of fused silica components, by tomographically illuminating a photopolymer-silica nanocomposite that is then sintered. We fabricated three-dimensional microfluidics with internal diameters of 150 micrometers, free-form micro-optical elements with a surface roughness of 6 nanometers, and complex high-strength trusses and lattice structures with minimum feature sizes of 50 micrometers. As a high-speed, layer-free digital light manufacturing process, micro-CAL can process nanocomposites with high solids content and high geometric freedom, enabling new device structures and applications.
Structured conical blockers manufactured on optical input surfaces are being used to shadow, and thereby, arrest growth of exit surface damage on large optics for applications in high-energy laser systems to extend their lifetime usage. However, as the size of the Shadow Cone Blocker (SCB) is increased, the deflected light must be carefully managed to prevent consequential damage from downstream intensification. Here, we design and fabricate a distinct input SCB geometry to alleviate this problem, enabling larger damage site mitigation. The demonstrated ~1 mm diameter SCB was manufactured with a convergent approach using a CO 2 pulsed laser ablative scanning sequence on a fused silica window. The SCB design was refined to maximize resistance to both input and exit surface damage initiations on 1 cm thick fused silica windows when exposed to 351 nm irradiation and validated with laser damage testing. The design showed to prevent damage onset to the exit surface for incident fluences on the SCB of 10.7 ± 1.3 J·cm -2 and is resistant to damage on the input surface exceeding 30 J·cm -2 input fluence.
Monolayers containing subnanometer striations of silica and hafnia to form composite materials at varying ratios are explored as a method to develop high-index dielectric layers with increased laser-induced–damage thresholds (LIDTs). These layers can then be used in multilayer dielectric coatings for short-pulse, high-peak-power laser applications, particularly in regions of the highest electric-field intensity. Fabrication is achieved by means of exposure to two different evaporant vapor plumes, where local exposure to each plume is controlled via shielding to prevent simultaneous exposure. The LIDT of the resulting layers has been evaluated at 1053 nm with 600-fs pulses. The results indicate that such hafnia/silica layers exhibit LIDTs similar to silica for a refractive index of ≤1.65. These results suggest that the use of these layers in locations subjected to high electric-field intensity within multilayer dielectric coatings may significantly improve the LIDT, with this deposition process providing particular benefit for scaling to large-aperture, high-fluence components.
We use photoluminescence (PL) imaging to study damage growth precursors within laser damage sites on the surface of silica. Damage site evolution is induced by multiple shots of UV nanosecond pulsed laser at various energy densities and monitored throughout the early stages of growth. Wide-field PL imaging rapidly locates microscopic light absorption centers within the silica damage site. Our quantitative analysis shows that damage sites with strong local PL intensity show a higher probability of growth upon subsequent laser pulses. Scanning electron microscopy (SEM) paired with a study of PL spectrum shows that the strong PL intensity appears from the subsurface fractures with high defect density, which provides a local light absorption center leading to significant damage growth. We believe that this result offers an efficient optical damage mitigation strategy by providing a rapid and non-destructive optical inspection approach.
A method for fabrication of large-aperture waveplate suitable for operation in the near-ultraviolet spectral region is demonstrated. The waveplate was fabricated using femtosecond direct laser writing of bulk fused silica. This method enables the precise control of the induced birefringence, which is introduced via the generation of nanopores with geometrical features that are controlled by the laser processing parameters. The resulting structures also introduce a small loss of transmission that is on the order of 4% at 351 nm. The laser-induced–damage threshold in the nanosecond regime was found to approach that of the native fused-silica substrate, which makes it a suitable candidate for polarization control in laser fusion research applications.
Here, we describe a wide-field approach to probe transient changes in photoluminescence (PL) of defects on silica surfaces. This technique allows simultaneous capture of spatially resolved PL with spontaneous quenching behavior. We attribute the quenching of PL intensity to photochemical reactions of surface defects and/or subsurface fractures with ambient molecules. Such quenching curves can be accurately reproduced by our theoretical model using two quenchable defect populations with different reaction rates. The fitting parameters of our model are spatially correlated to fractures in silica where point defects and mechanical stresses are known to be present, potentially indicating regions prone to laser-induced damage growth. We believe that our approach allows rapid spatial resolved identification of damage prone morphology, providing a new pathway to fast, non-destructive predictions of laser-induced damage growth.