The Next Big Thing: Nanomaterials Development for Space Technology Applications
The ability to modify material properties and composite structures on the nanoscale offers intriguing possibilities for space system designers. Aerospace is evaluating nanomaterials and processing techniques that will directly affect future space capabilities.
Advances in space capabilities are often driven by advances in materials science. Satellite structures require durable, lightweight materials that can withstand the physical stresses of launch and operate for extended periods in the extreme environment of space. System capability is largely defined by individual components such as sensors and circuits that combine various materials with different processing requirements. Recent years have seen a growing interest in the ability to manipulate compounds on the nanoscale to increase functionality, versatility, and performance; however, many challenges associated with nanomaterial synthesis, nanostructure fabrication, and device manufacture must be addressed to fully realize the potential of nanomaterials for space systems enhancement. Aerospace has been leading innovative efforts to solve these problems, drawing upon broad expertise and specialized resources not found anywhere else.
Nanostructured Thin Films for Infrared Sensors
Space systems rely on sophisticated sensors for numerous military and intelligence applications, including surveillance, target tracking and discrimination, fire control, and infrared and hyperspectral imaging. Current sensor systems generally must be cooled to ultralow temperatures to enhance operating stability and resolution and to minimize thermal noise and dark current. However, cryogenically cooled devices present significant design challenges related to size, weight, power, and cost, as they are expensive to manufacture, require frequent calibration and maintenance, and consume appreciable amounts of power.
Infrared detectors made from pyroelectric materials (which generate a temporary electrical current in response to a change in temperature) have received significant attention as a result of their stability, sensitivity, wide spectral response, and low amount of dark current. In a pyroelectric detector, a ferroelectric absorbing layer is used to capture radiant energy, which heats up the pyroelectric material, causing a spontaneous and reversible electric polarization and a measurable variation in the surface charge. When integrated with the appropriate electronic circuitry, the output current can be correlated to the rate of temperature change. These capacitive detectors have particularly benefited from the inclusion of a thermally sensitive dielectric layer made from multimetallic oxides and perovskites (a class of crystalline mineral characterized by high pyroelectric coefficients and low dielectric loss tangents).
Several fundamental barriers must be overcome before perovskites and other promising nanomaterials can be incorporated into next-generation uncooled infrared imaging and detection devices. For example, perovskites generally must be processed under harsh, high-temperature conditions. Thus, the fabrication of well-defined perovskite nanostructures is not compatible with direct monolithic integration of detector elements onto commercially fabricated circuits. Moreover, traditional fabrication methods do not readily support the processing and activation of integrated homogeneous thin films to create functional, thermally isolated pixel arrays.
Aerospace has been leading cross-disciplinary efforts to overcome these barriers, drawing upon recent advances in the synthesis and deposition of biomimetic nanomaterials, laser processing of nanomaterials, and design and performance testing of nanomaterial sensors. In particular, Aerospace has been building upon work conducted at the Institute for Collaborative Biotechnologies at UC Santa Barbara, which developed a catalytic process to synthesize nanostructured multimetallic oxides and perovskites such as barium titanate (BaTiO3) and barium strontium titanate (BaSrTiO3). Unlike traditional processes that require high temperatures and extreme pH conditions—and provide limited control of composition, size, and shape—this new technique mimics enzyme-mediated biomineralization and relies on temporal and vectorial catalyst gradients to provide high-purity crystalline materials with complex nanostructures under benign conditions, i.e., low temperature, ambient pressure, and nearly neutral pH. The kinetically controlled vapor-diffusion method produces exceptionally small (2–5 nanometers [nm] diameter) and stable BaTiO3 and BaSrTiO3 nanocrystals that can be dispersed and spin-cast to form high-quality, homogeneous, crack-free thin films on prefabricated infrared sensor circuitry. The films are mechanically robust, and the nanoscale particles provide high surface-to-volume ratios for enhanced sensitivity and rapid heating and cooling for extremely fast response times and high spatial resolution.
Nonetheless, these perovskite thin films still present challenges that limit their usefulness in infrared focal-plane arrays. In bulk form, BaTiO3 and BaSrTiO3 perovskites are room-temperature pyroelectric materials having a tetragonal crystal structure; however, as particle dimensions approach the nanoscale (less than 30 nm), the tetragonal crystal structure gives way to a cubic crystal structure, and the ferroelectric nature of the material vanishes. At this scale, a high-temperature annealing step is necessary to convert the material to the tetragonal phase and restore pyroelectricity. This has typically been accomplished using conventional resistive and furnace heating; however, these techniques lead to global phase transformations, where the entire film undergoes structural conversion. The ideal technique would permit site-selective activation of pyroelectric regions in the perovskite film. Fortunately, lasers provide an alternative to furnace heating that is physically nonintrusive and allows for precise phase transformations and patterned annealing at selective locations.
Aerospace has developed a revolutionary laser processing technique and micro/nanoengineering workstation to fully exploit the ferroelectric properties of perovskite nanoparticle thin films for infrared focal-plane arrays. Specifically, Aerospace is using digitally scripted pulse-modulation techniques to induce patterned phase transformation of micro- and nanoscale aggregates of the perovskite nanoparticles. This novel approach offers several key benefits: precise photon flux control for high-fidelity pyroelectric phase transformation; spatially localized phase conversion for proper thermal isolation; and 2-D and 3-D patterning capability with computer-assisted design and manufacturing (CAD/CAM) systems to support rapid prototyping, component uniformity, and low-cost manufacture. The laser processing station pushes current technology limits on motion control, active vibration damping, photonics, optical pulse modulation, and synchronization of tool path motion and photon delivery, with complete 3-D motion control on the 20–50 nm scale while traveling at velocities in excess of 200 mm/sec. The specialized capabilities of the Aerospace laser workstation cannot be achieved with traditional laser processing approaches and do not currently exist on any commercial instruments.
The key to laser material processing is the ability to precisely vary the laser parameters and system controls for the specific material under irradiation at the optimal time. Precise real-time modulation of the intensity, polarization, wavelength, and temporal and spatial characteristics of the laser pulses permits the control of energy flow (e.g., heat) into a material during patterning. Consequently, the energy flux can be regulated to express specific chemical and physical properties in the material on a highly localized scale. Precise light modulation can affect both thermal and nonthermal processes and can facilitate the desired type of materials processing and alteration, such as the phase transformation of cubic nonpyroelectric to tetragonal pyroelectric BaTiO3. This approach is ideal for a moving substrate under constant laser irradiation and essential for a variegated substrate that consists of heterogeneous interconnected materials or phases that require different processing conditions (e.g., BaTiO3 thin-film overlayers interfaced with semiconductor or fanout architectures). Conventional laser processing techniques suffer from limited power control and do not permit synchronized laser-pulse modulation during patterning. This limitation can lead to problems such as inconsistent photon exposure, thermal energy transfer outside the irradiated region, material removal from nonirradiated regions, defect formation, and residual stress and fracture, all of which can significantly degrade component functionality and device performance.
The Aerospace approach, known as “laser genotype direct-write processing,” overcomes these problems by carefully tailoring and modulating the photon delivery during patterning. By analogy with the genotype function and trait expression via base pairing in biology, the Aerospace technique involves merging or pairing a sequence of concatenated laser-pulse scripts with the Cartesian tool path. Each laser-pulse script is designed to induce a specific material transformation and express multiple functionalities (traits) on a common substrate. Because the laser-pulse modulation is synchronized with the tool path in a line-by-line fashion, each laser-irradiated spot receives the appropriate photon exposure to achieve the desired outcome.
The laser-pulse scripts can be adapted in real time—via integrated ultrafast spectroscopic detection and intelligent feedback algorithms—to compensate for the chemical and physical changes that occur in a material as a result of the laser process itself. Each predefined pulse script is delivered on a per-spot basis, so the photon flux is spatially localized to a high degree, which increases thermal isolation and decreases dark current. The development of user-defined patterns by the CAD/CAM system enables the 2-D and 3-D processing of existing architectures and facilitates ultimate integration into commercial devices.
Prior to the Aerospace efforts, laser-induced phase transformations in multimetallic oxides and perovskite thin films remained elusive and largely unexplored. In fact, the first attempts to use laser direct-write processing to induce patterned pyroelectric phase conversion and activate an infrared response in perovskite thin films were conducted at Aerospace, but these conventional approaches proved unsuccessful. Despite modifying a wide range of laser processing parameters—including wavelength, power, pulse repetition rate, and pulse length—the efforts either failed to provide the thermal transients in the crystal lattice needed to induce phase conversion or resulted in a small extent of conversion that was accompanied by appreciable disruption to the perovskite film (e.g., delamination and desorption, surface roughening, and sintering).
The Aerospace team then used a different approach, applying patented laser genotype direct-write processing techniques for high-fidelity, position-synchronized photon modulation—and the results were striking. Using one-temperature modeling studies of the interaction of ultrashort laser pulses with perovskite nanoparticles, researchers were able to quantitatively examine the electron thermalization and cooling rates in the thin films. The resulting lattice temperature distributions were used to devise preliminary laser-pulse scripts, which were used to carefully control the delivery of energy (heat) into the phonon subsystem during patterning to rapidly increase the temperature of the perovskite thin film above the phase-transition temperature and to maintain a constant heating and cooling rate—all without disruption or damage to the thin film or underlying electronics. Subsequent Raman spectroscopy, which probes the local molecular structure and dynamic vibrational symmetry, showed that the extent of laser-induced phase conversion was consistent with that attained via bulk furnace heating; structural analysis via atomic-force microscopy revealed no damage to the perovskite thin film.
Aerospace has also developed and applied piezoresponse-force microscopy techniques for nanoscale phase contrast characterization of the nonferroelectric and ferroelectric domains in thin-film and bulk dielectric substrates. Piezoresponse-force microscopy permits direct imaging of ferroelectric structures and is well suited for examining perovskite films following laser processing because of its relative insensitivity to topography and ease of implementation (see sidebar, Piezoresponse-Force Microscopy). This technique revealed appreciable ferroelectric phase contrast in the perovskite films that were patterned via laser genotype direct-write processing and further confirmed successful pyroelectric conversion to the infrared-active phase. The laser-structured and pyroelectrically activated regions were highly uniform, with domain sizes ranging from 50 to 500 nm. Infrared pixel arrays—including prototype 8 × 8, 32 × 32, and 256 × 256 (100 × 100 micron) detector pixel arrays—were successfully fabricated on pure and doped BaTiO3 and BaSrTiO3 thin films deposited on various silicon substrates. These prototype arrays are now being analyzed by defense researchers to assess infrared responsiveness and detector performance.
Aerospace’s advances in the laser processing and activation of nanostructured perovskites and other nanoscale films are enabling new strategies for the fabrication of infrared detectors. Standard fabrication approaches comprise numerous steps that are lengthy and expensive. In contrast, laser direct-write infrared activation is a maskless, single-step phase-conversion process, and thus does not rely on photolithography or etching for pixel structure formation or pattern transfer. Because the thin films are directly integrated onto prefabricated sensor circuitry, there is no need for solder bump formation or flip-chip bonding. The thin-film element is a freestanding pyroelectric membrane, where each infrared-activated pixel is isolated within a pyroelectrically inactive matrix; the detector array is less susceptible to thermal mismatch and pixel-to-pixel optical crosstalk and does not require substrate thinning. Integrated spectroscopic detection permits phase transformation control and process optimization, and CAD/CAM linking allows for rapid changes in sensor design and pixel geometry. These features promote rapid prototyping and batch fabrication of new affordable infrared sensor devices, and are now being considered for commercial transfer.
Aerospace has been investigating potential applications for another class of engineered materials known as carbon nanotubes. These remarkable materials possess the chemical properties of carbon, the thermal conductivity of diamond, and the electrical conductivity of copper or silicon. However, significant challenges still exist in translating these properties into the macrostructures required for future space vehicles.
For example, carbon nanotubes have strength-to-mass ratios about 50 times greater than that of typical composites reinforced with carbon fiber and about 600 times greater than aluminum alloys.. Because some spacecraft and launch vehicle components could, in principle, be constructed from carbon nanotubes, the weight of a spacecraft could decrease by an order of magnitude or more with no change in size or function. For example, large satellites derive approximately 10 percent of their mass from just bulky copper wiring harnesses. A weight savings of nearly 50 percent could be achieved by swapping out copper for electrically conducting carbon nanotubes. The technology to do so is probably still far off; more moderate increases in strength-to-mass ratios could be attained much earlier by using new composites infused with carbon nanotubes and nanotube-infused fibers. While a number of researchers in government and private labs are working in these composites areas, few have successfully produced high-strength polymer composites with nanotube infusion, and few (if any) are addressing how to manufacture usable structures with them once the materials are available. Furthermore, stronger and lighter structures that take as long to manufacture as current ones will probably be just as expensive. Thus, automated production techniques that can yield stronger materials rapidly and inexpensively must be perfected before the full potential of carbon nanotubes can be realized.
One promising technique is ink-jet printing; it is fast and efficient, and it is currently one of the best direct-patterning techniques. It relies on piezo-driven nozzles to release tiny droplets of ink from the printer head. This noncontact technique is advantageous for depositing nanomaterials such as carbon nanotubes directly onto substrates because it does not require masks or patterns. Aerospace researchers have demonstrated a simple, reliable method for forming uniform carbon nanotube arrays directly onto a surface via ink-jet printing. The key to the invention is a unique nanotube-infused liquid-matrix “ink” that has low enough viscosity to be extruded efficiently though ink-jet sprayers, and yet hardens on contact with the substrate. In addition, researchers have formulated a nanocomposite ink that consists of a low-viscosity polyimide/POSS material.
Both of the inks developed at Aerospace consist of mostly unaligned tangles of carbon nanotubes. The goal is to somehow align these nanotubes to increase the directional strength of the deposited layer. To do so, Aerospace researchers tried placing organic molecular marks on substrates of gold and glass to help guide the initial self-assembly of the nanotubes.
Researchers also evaluated the use of self-assembled monolayer boundaries for controlling the absorption of single-wall nanotubes on the gold substrate. Single-wall nanotubes are known to be attracted to the boundary between hydrophilic and hydrophobic features made by using thiols (organic compounds similar to alcohols and phenols). A separate cartridge was used to deposit patterns of thiols approximately 10 microns wide on the functionalized gold. Using this technique, researchers were able to control the position and shape of the monolayer and construct various geometries.
Choosing the appropriate combination of solvent and substrate is crucial to prevent clogging of the ink cartridge and to obtain homogenous films. Aerospace has demonstrated the production of nanotube ink formulas that promoted polyelectrolyte exchange reactions, which produced homogeneous fluids over a broad compositional range that possessed the viscosities required for flow through the microcapillary nozzles of the ink-jet printer. These results will be used toward higher-ordered structures made of single-wall nanotubes by a combined process of self-assembly and drop-on-demand printing techniques.
Qualities such as electrical conductivity, high aspect ratio, high modulus, and high strength make carbon nanotubes a natural candidate for use as fillers in polymer composites for spacecraft structures. Aerospace has been investigating ways to exploit these characteristics in polymer blends.
For example, polymer films in the thermal blankets used on most satellites are typically coated with a conductive layer of indium tin oxide to prevent the buildup of electrostatic charges that could lead to potentially harmful discharges; however, these coatings can crack and oxidize, which reduces their conductivity. Aerospace has developed a transparent polymer blend with sufficient bulk conductivity and environmental stability to mitigate surface charging on satellites. The material—a carbon-nanotube/polyimide blend—could eliminate hundreds of straps used to ground the conductive front surface of the blankets to the spacecraft. Researchers have been able to increase the optical transmittance of the material by using fluorinated polyaniline in the polyimide. A target surface conductivity of 1 × 10−6 to 1 × 10−8 Siemens per centimeter was chosen as a compromise between the competing goals of optical clarity and electrical conductivity. Moreover, recent results show that carbon nanotube concentrations of less than 0.5 percent by weight in the polyimide base provide the necessary high optical clarity (or low solar absorption) at the benchmark peak of 500 nm in the ultraviolet/visible spectrum.
In a related effort, Aerospace studied the effect of adding carbon nanotubes to polycyanurate, a space-grade epoxy which, when cured, displays a highly branched backbone that forms a semi-interpenetrating network with a correspondingly large free volume. Researchers sought to determine whether the rigid open structure of the polycyanurate would wrap around the single-wall nanotubes. If the load could be effectively transferred to the individual nanotubes through intimate bonding between the nanotubes and the polymeric resin, then theoretically, the modulus of the composite should be similar to that of a randomly oriented short-fiber composite containing fibers of extremely high modulus.
The polycyanurate matrix was based on a bisphenol-A cyanate ester monomer. The rigid molecules in this material have an internal structure that would lend itself to breaking up large nanotube ropes and help disperse them into the polymer matrix. This noncovalent approach was preferred because it would not alter the structure or strength of the carbon nanotubes and would not introduce a surfactant into the resin matrix.
The addition of carbon nanotubes (0.54 percent by volume) to polycyanurate composite thin films increased the elastic modulus from 303,400 to 690,000 pounds per square inch (psi)—a 127 percent increase. Other research groups outside of Aerospace had reported an increase in modulus with fractional loading of carbon nanotubes, but none had reported an increase of more than 100 percent. Using the rule of mixtures, the predicted elasticity of this carbon-nanotube composite was 1.1 × 106 psi. The fact that this predicted value was higher than the measured value indicated that dispersion was not optimized. Decreasing the amount of carbon nanotubes to 0.01 percent by volume gave a lower value of 313,000 psi, which was close to the 317,800 psi value predicted for that concentration. Evidently, as nanotube concentration increases, so does tube bundling, which yields a modulus of elasticity much lower than the predicted maximum.
Nanostructured Chemical Sensors
The presence of large quantities of toxic or explosive chemical compounds is an unfortunate reality in the business of launching satellites. The risk to civilian populations from exposure to toxic plumes can limit access to space. If the accuracy of plume dispersion models can be improved with real-time data on actual exhaust plume concentrations around launch sites, the resulting confidence could greatly increase launch opportunities. In addition, improved chemical sensors for hazardous propellants are needed to ensure the safety of personnel working around launch vehicles and payloads. The development of small, low-power but sensitive chemical sensors might also allow the instrumentation of spacecraft, launch vehicle, and tanking areas to monitor exposure to hazardous gases. Such devices might allow mission managers to rapidly assess the extent and consequences of a small leak and reduce the resources spent on failure investigations.
Polyaniline is a unique conducting polymer that can be tailored for specific applications through a reversible acid-base doping. Its conductivity can be increased from less than 10−10 ohm−1cm−1 in the undoped form to more than 1 ohm−1cm−1 in the doped form. Anything that alters either the charge transport along the polymer backbone or hopping of carriers between polymer chains will affect its conductivity. This principle enables polyaniline to be used as a simple resistance chemical sensor.
A polyaniline sensor typically consists of a planar interdigitated electrode and the polymer selective layer. Conductivity changes in the polymer film upon gas exposure can be monitored with an ohmmeter or electrometer. Much sensor research has focused on maximizing the interaction between vapor molecules and the polymer by modifying the polymer backbone. However, limited diffusion through the sensor material reduces sensitivity and increases response time and can dominate any attempts to modify the material because only a limited number of surface sites are available for gas interaction.
The use of nanostructured sensor layers, such as polyaniline nanofibers, can greatly improve diffusion, because nanostructured materials have a much greater exposed surface area and permit greater penetration for gas molecules, as compared with the bulk material. Even when the thickness of an ultrathin film is similar to the diameter of the nanofibers, the fibers are expected to outperform a thin film because their shape presents a higher surface-to-volume ratio. The small diameter of the nanofibers (less than 100 nm) coupled with the possibility of gas approaching from all sides should result in sensors with improved performance.
To test this hypothesis, Aerospace researchers examined thin films of conventional polyaniline and polyaniline nanofibers. The two films were coated onto electrode arrays consisting of pairs of interdigitated gold electrodes with gaps of 10 to 40 microns fabricated on an insulating substrate. The researchers compared resistance changes in the two films after exposure to hydrochloric acid vapor (which dopes the material and thereby affects its conductivity). The nanofiber film (about 2.5 microns thick) gave a much greater response than a conventional polyaniline film (about 1 micron thick), even though it was considerably thicker. This was probably due to the higher surface area of the nanofiber film, which allowed more interaction between vapor molecules and the polyaniline.
As for the conventional polyaniline films, the researchers determined that their response time and sensitivity to vapors depended strongly on thickness, with thinner films typically performing better. When the thickness was decreased from 1 to 0.3 microns, the response time decreased significantly. In addition, the response magnitude at a fixed time increased by more than five orders of magnitude. One explanation is that because only the outermost surface interacts with the vapor molecules, a thicker film has more inactive material that does not immediately contribute to the sensor response. Therefore, the performance of conventional polyaniline thin film sensors is limited by the thickness of the film.
The sensor performance when nanofibers were used as the selective layer was essentially unaffected by thickness. The porosity of the nanofiber films allows vapor molecules to penetrate through the entire film and interact with all the fibers. Even in thicker films, all the fibers can contribute to the sensing process.
Polyaniline nanofibers can be synthesized in water, but conventional polyaniline is insoluble and requires the use of aggressive organic solvents. Not only is the aqueous process better environmentally, it opens up simple routes to produce nanocomposites with other materials (see sidebar, Synthesis of Polyaniline Nanostructures). This has enabled Aerospace to produce sensors for new molecules that are not possible with plain polyaniline nanofibers. For example, excellent sensors for hydrogen sulfide result from composites containing certain metal salts such as copper because the reaction between the metal salt and the hydrogen sulfide gas produces a strong acid that dopes the polyaniline. Using a similar strategy, the sensitivity of polyaniline can be tailored to produce sensors for hydrazine, arsine, and phosgene. Aerospace researchers also discovered a novel mechanism that enables polyaniline nanofibers to be used as a hydrogen sensor.
Aerospace has also begun to investigate graphene as a material for advanced applications. Graphene is a two-dimensional sheet of graphite (a carbon allotrope) with a thickness of one atomic layer. Until recently, graphene was only a theoretical material used as a model for other materials such as buckyballs and carbon nanotubes, which are three-dimensional carbon species derived from it. Many labs have now isolated graphene flakes, and measurements show the material has phenomenal electrical properties. Graphene’s quality clearly reveals itself in a pronounced electric field effect such that charge carriers can be tuned continuously between electrons and holes in high concentrations with mobilities that can exceed 100,000 cm2/V • sec even under ambient conditions. The high mobility of charge carriers has many potential applications for molecular electronic devices such as room-temperature ballistic transistors and solar cells. The extremely high conductivity has also led to applications in the realm of chemical sensors—a device based on graphene was recently shown to achieve single molecule sensitivity.
One of the limitations of working with graphene is the preparation method. Studies to date have relied on samples obtained by micromechanical cleavage of bulk graphite. The technique looks no more sophisticated than drawing with a piece of graphite and repeatedly peeling it with adhesive tape until the thinnest flakes are found. This is a tedious and unreliable method for preparing devices based on graphene. Aerospace has started investigating alternative methods such as reduction of graphite oxide and chemical vapor deposition of single layers on copper foil. Aerospace researchers have succeeded in producing single layers of graphene and transferring them to silicon wafers for characterization and device fabrication. Applications could include sensors, electronic devices, batteries, and solar cells.
Novel nanomaterials synthesis and nanostructure fabrication, laser-induced material activation and transformation, and integration of nanomaterials into devices on the micron and submicron scale have opened new avenues for the design and development of space systems technology. Any new material or manufacturing technique requires rigorous testing and evaluation to verify its suitability for space applications, so it may take time before the latest advances can influence space system design. In the meantime, Aerospace research in materials science will help identify and develop new and emerging technologies with the greatest potential for enhancing space capabilities.
The authors thank Paul Adams, William Hansen, Henry Helvajian, Hyun Kim, and Lee Steffeney for their contributions to this article. Special gratitude is extended to Daniel Morse and Krisztian Niesz from UC Santa Barbara and to Wendy Sarney from the U.S. Army Research Laboratory for their efforts on an Army-funded research program aimed at the development of new uncooled infrared sensor technology. The authors would also like to acknowledge the work of Jesse Fowler, Shabnam Virji, and our collaboration over the past ten years with Richard Kaner and his research group in the chemistry department at UCLA. Special thanks go to Jiaxing Huang, now a professor at Northwestern University. Many of Kaner’s graduate students have worked as interns at Aerospace, namely Robert Kojima, Matthew Allen, Jonathan Wassei, Emil Song, and Jaime Torres. Thanks also to David Straw, Russell Lipeles, and Ivan Bekey for their helpful discussions and insight of printing carbon nanotubes. In addition, the authors appreciate the technical support of FujiFilm (materials printer) and Stephen Turner of Brewer Science, Inc. (carbon nanotube solutions). Lastly, the authors would like to thank Gary Hawkins for his support of this nanotube research and his efforts in obtaining funding for the materials printer.
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