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Summary Saturated steam (>121°C and >205 kPa) is widely used in the medical sterilization process known as autoclaving. However, solar-driven steam generation at such high temperature and pressure requires expensive optical concentrators. We demonstrate a passive solar thermal device mostly built from low-cost off-the-shelf components capable of delivering saturated and pressurized steam to drive sterilization cycles even under hazy and partly cloudy weather. Enabled by an optimized ultra-transparent silica aerogel, the device utilizes an efficient thermal concentration strategy to locally increase the heat flux and temperature obviating the need for active optical concentrators. With almost 2× higher energy efficiency (47%) than those previously reported at 100°C, the device demonstrated successful sterilization in a field test performed in Mumbai, India. In addition to enabling passive sterilization, this work promises the development of solar thermal energy systems for saturated steam generation in energy conversion, storage, and transport applications.

Heat at intermediate temperatures (120-220 °C) is in significant demand in both industrial and domestic sectors for applications such as water and space heating, steam generation, sterilization, and other industrial processes. Harnessing heat from solar energy at these temperatures, however, requires costly optical and mechanical components to concentrate the dilute solar flux and suppress heat losses. Thus, achieving high temperatures under unconcentrated sunlight remains a technological challenge as well as an opportunity for utilizing solar thermal energy. In this work, we demonstrate a solar receiver capable of reaching over 265 °C under ambient conditions without optical concentration. The high temperatures are achieved by leveraging an artificial greenhouse effect within an optimized monolithic silica aerogel to reduce heat losses while maintaining high solar transparency. This study demonstrates a viable path to promote cost-effective solar thermal energy at intermediate temperatures.

The antioxidant toughening agent for the hydrocarbon based proton exchange polymer electrolyte membranes, cerium/organosiloxane polymer network (Ce/OSPN), is synthesized via sol-gel reaction. Ce/OSPN is introduced to the sulfonated poly(ether ether ketone) (SPEEK), a typical hydrocarbon type polymer electrolyte membrane, by formation of a semi-interpenetrating polymer network (semi-IPN) structure. As Ce/OSPN possesses superior properties to SPEEK in mechanical flexibility, proton conductivity, and oxidation stability than SPEEK, it resolves 3 inherent drawbacks of the pristine SPEEK membrane including (i) brittleness, (ii) low proton conductivity, and (iii) poor durability. Addition of 20 wt% Ce/OSPN (at Ce/silicon mol ratio = 0.10) enhances the elongation at break of the SPEEK membrane about twice. The power density of the MEA fabricated with the semi-IPN membrane is 208 mW cm−2, which is much higher than that of the pristine SPEEK membrane, 165 mW cm−2. The power density loss of the same semi-IPN membranes as determined by the Fenton's test is 4.8%, whereas those of pristine and semi-IPN membrane without cerium are 33.9% and 34.0%, respectively. This Ce/OSPN agent is expected to be applied to a variety of hydrocarbon based polymer electrolyte membranes.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, February 2017.

We present functionalized three-dimensional graphene as a promising thermal additive to increase the effective thermal conductivity of porous crystal adsorbents. Due to the percolation and high porosity of three-dimensional graphene, the thermal conductivity of the adsorbent-graphene composite was enhanced up to 500 % with a minimal reduction in adsorption capacity of approximately 2 %. The functionalization demonstrated to be an effective way to implement a hydrophobic carbon network with hydrophilic porous crystals. The functionalized three-dimensional graphene can be applicable to numerous types of porous crystals including zeolites and metal organic frameworks to overcome their intrinsic low thermal conductivity. This work provides insights for the development of binders for enhanced thermal performance of porous materials in various adsorption systems., United States. Advanced Research Projects Agency-Energy

We present a systematic study of carbon nanomaterials with different geometries and thermal properties, including few-layer graphene (FLG), graphene oxide (GO), and functionalized carbon nanotubes (fCNT) as additives to enhance the thermal conductivity of microporous adsorbent materials. The dimensionality and intrinsic thermal conductivity of the additives were found to be critical for both maximizing the thermal conductivity enhancement, and minimizing the reduction in the adsorption capacity of the active materials. We demonstrated that two-dimensional (2D) FLG was the most effective thermal additive for zeolite (ZT) adsorbents due to its high thermal conductivity and preferential 2D geometry. Meanwhile, negligible enhancement was observed from one-dimensional (1D) additives such as fCNTs, which is consistent with the predictions from a modified effective medium analysis (EMA). Our work provides insights for the development of additives to enhance the thermal performance of porous materials in applications such as adsorption heat pumps, gas storage, and separation processes. Keywords: Thermal conductivity, Microporous adsorbent, Zeolite, Metal organic frameworks, Graphene, Graphene oxide, Carbon nanotube

Adsorption based heat-pumps have received significant interest owing to their promise of higher efficiencies and energy savings when coupled with waste heat and solar energy compared to conventional heating and cooling systems. While adsorption systems have been widely studied through computational analysis and experiments, general design guidelines to enhance their overall performance have not been proposed. In this work, we identified conditions suitable for the maximum utilization of the adsorbent to enhance the performance of both intermittent as well as continuously operating adsorption systems. A detailed computational model was developed based on a general framework governing adsorption dynamics in a single adsorption layer and pellet. We then validated the computational analysis using experiments with a model system of zeolite 13X-water for different operating conditions. A dimensional analysis was subsequently carried out to optimize adsorption performance for any desired operating condition, which is determined by the choice of adsorbent–vapor pair, adsorption duration, operational pressure, intercrystalline porosity, adsorbent crystal size, and intracrystalline vapor diffusivity. The scaling analysis identifies the critical dimensionless parameters and provides a simple guideline to determine the most suitable geometry for the adsorbent particles. Based on this selection criterion, the computational model was used to demonstrate maximum utilization of the adsorbent for any given operational condition. By considering a wide range of parametric variations for performance optimization, these results offer important insights for designing adsorption beds for heating and cooling systems.

Solar thermal energy systems combined with low-cost thermal storage provide a sustainable, dispatchable source of renewable energy. One approach to increase the attractiveness of these systems is to use high-performing solar transparent, thermally insulating silica aerogel to significantly increase efficiency. Several past works have proposed using these ultra-nanoporous materials to reduce thermal losses in the receiver, but only recently have aerogels reached the high solar transparency necessary to be considered for concentrated solar applications (>97%). However, the durability and stability of optically transparent silica aerogels at the operating conditions of solar-thermal receivers has not been examined. Here, we investigate the high temperature stability of transparent silica aerogel for use in concentrated solar thermal energy applications. Transparent samples (visible transmission >95% at 4 mm thickness) were annealed for several months at 400, 600, and 800 °C to investigate the relative change in nanostructure, solar transparency, and effective thermal conductivity. Results showed that at 400 and 600 °C, the temperature-dependent changes reach a plateau within 30 days of continuous annealing, but at 800 °C, samples are structurally unstable. A simple receiver efficiency model was used to show stable performance at 400 and 600 °C temperatures, even after months of exposure. This work validates that transparent silica aerogels can be used in solar thermal receivers below 800 °C, yielding appreciable increases in efficiency for solar energy harvesting operation.

Conductive fillers, such as amorphous carbon, carbon nanotube and graphene etc., are generally mixed with nanostructured metal oxide materials to improve the performance of electrode materials in energy storage devices. However, the conductive framework that provides path for electric conduction does not normally form a well-connected and robust 3-D network to ensure optimized ions transport. Here, we report a convenient, inexpensive and scalable method for synthesizing hybrid carbon and titanium dioxide co-gels and co-aerogels to improve the electrochemical capacity by combining both the lithium insertion and the surface storage mechanisms in Li ion batteries (LIBs) anodes. A monolithic piece of a hybrid C/TiO2 co-aerogel can be directly used as an active electrode without the addition of binders, such as polyvinylidene fluoride (PVDF). As a result, the performance of LIB anodes using the hybrid co-aerogel is significantly improved over current LIBs based on carbon/titanium oxide composites. The reversible discharge capacity was stabilized at ∼400 mAh g−1 at a 168 mA g−1 scan rate and an operating voltage between 3.0 and 0.05 V vs. Li+/Li with excellent cyclic capacity retention. This approach, however, is not limited to only C/TiO2 system but can be extended to other metal oxides to form co-gels with carbon to improve their potential use in numerous electrochemical, photocatalytic, and photoelectronic devices.

We present a bottom-up synthesis, spectroscopic characterization, and ab initio simulations of star-shaped hexagonal zinc oxide (ZnO) nanowires. The ZnO nanostructures were synthesized by a low-temperature hydrothermal growth method. The cross-section of the ZnO nanowires transformed from a hexagon to a hexagram when sulfur dopants from thiourea [SC(NH2)2] were added into the growth solution, but no transformation occurred when urea (OC(NH2)2) was added. Comparison of the X-ray photoemission and photoluminescence spectra of undoped and sulfur-doped ZnO confirmed that sulfur is responsible for the novel morphology. Large-scale theoretical calculations were conducted to understand the role of sulfur doping in the growth process. The ab initio simulations demonstrated that the addition of sulfur causes a local change in charge distribution that is stronger at the vertices than at the edges, leading to the observed transformation from hexagon to hexagram nanostructures.

Haze in optically transparent aerogels severely degrades the visual experience, which has prevented their adoption in windows despite their outstanding thermal insulation property. Previous studies have primarily relied on experiments to characterize haze in aerogels, however, a theoretical framework to systematically investigate haze in porous media is lacking. In this work, we present a radiative transfer model that can predict haze in aerogels based on their physical properties. The model is validated using optical characterization of custom-fabricated, highly-transparent monolithic silica aerogels. The fundamental relationships between the aerogel structure and haze highlighted in this study could lead to a better understanding of light-matter interaction in a wide range of transparent porous materials and assist in the development of low-haze silica aerogels for high-performance glazing units to reduce building energy consumption.

Silica aerogel has been known as a promising candidate for high performance transparent insulation material (TIM). Optical transparency is a crucial metric for silica aerogels in many solar related applications. Both scattering and absorption can reduce the amount of light transmitted through an aerogel slab. Due to multiple scattering, the transmittance deviates from the Beer-Lambert law (exponential attenuation). To better understand its optical performance, we decoupled and quantified the extinction contributions of absorption and scattering separately by identifying two sets of radiative properties. The radiative properties are deduced from the measured total transmittance and reflectance spectra (from 250 nm to 2500 nm) of synthesized aerogel samples by solving the inverse problem of the 1-D Radiative Transfer Equation (RTE). The obtained radiative properties are found to be independent of the sample geometry and can be considered intrinsic material properties, which originate from the aerogel's microstructure. This finding allows for these properties to be directly compared between different samples. We also demonstrate that by using the obtained radiative properties, we can model the photon transport in aerogels of arbitrary shapes, where an analytical solution is difficult to obtain., National Science Foundation (U.S.) (Grant DMR-1419807)

This study presents a simple method to reproducibly obtain well-aligned vertical ZnO nanowire arrays on silicon oxide (SiOx) substrates using seed crystals made from a mixture of ammonium hydroxide (NH4OH) and zinc acetate (Zn(O2CCH3)2) solution. In comparison, high levels of OH(-) concentration obtained using NaOH or KOH solutions lead to incorporation of Na or K atoms into the seed crystals, destroying the c-axis alignment of the seeds and resulting in the growth of misaligned nanowires. The use of NH4OH eliminates the metallic impurities and ensures aligned nanowire growth in a wide range of OH(-) concentrations in the seed solution. The difference of crystalline orientations between NH4OH- and NaOH-based seeds is directly observed by lattice-resolved images and electron diffraction patterns using a transmission electron microscope (TEM). This study obviously suggests that metallic impurities incorporated into the ZnO nanocrystal seeds are one of the factors that generates the misaligned ZnO nanowires. This method also enables the use of silicon oxide substrates for the growth of vertically aligned nanowires, making ZnO nanostructures compatible with widely used silicon fabrication technology.

We have characterized the physical and electrical properties of a W–TiN stacked gate in metal–oxide–semiconductor devices. The degree of tungsten crystallization was enhanced when the N2/Ar ratio was increased during TiN sputtering deposition and/or if the annealing temperature for W–TiN films was raised over 600 °C. However, the lowest resistivity was observed from W–TiN films annealed at 600 °C. We suggest that the TiOx intermediate layer was formed between the TiN gate electrode and SiO2 if the annealing temperature increases to more than 600 °C. In addition, we found that TiN effectively suppresses the fluorine diffusion into SiO2, and that the N2/Ar ratio determines the flatband voltage shift of the W–TiN/SiO2/Si capacitors.

Double-walled carbon nanotubes (DWNTs) have recently been recognized as important members in the carbon nanotube family because they are expected to have certain unique properties. For example, DWNTs are expected to replace single-walled carbon nanotubes (SWNTs) in biomarker applications and optoelectronics if the observed luminescence from DWNTs can be verified. However, due to unavoidable byproducts, such as SWNTs, optical properties of DWNTs still remain controversial. There is an ongoing debate concerning the ability of DWNTs to exhibit photoluminescence (PL). In this report, we aim to clearly resolve this debate through the study of carefully separated DWNTs. DWNTs were successfully separated from SWNTs using density gradient ultracentrifugation. Here we clearly show that light is emitted from the inner wall of DWNTs; however, the intensity of the emission is significantly quenched. Interestingly, it was found that a very narrow range of diameters of the inner walls of DWNTs is required for PL to be observable. All other diameters led to complete PL quenching in DWNTs. In short, we have shown that both sides of the debate are correct under certain situations. The real answer to the question is that some DWNTs do emit light but most DWNTs do not.

A dense array of parallel single-walled carbon nanotubes (SWNTs) as the device channel can carry higher current, be more robust, and have smaller device-to-device variation, thus is more desirable for and compatible with applications in future highly integrated circuits when compared with single-tube devices. The density of the parallel SWNT arrays and the diameter of SWNTs both are key factors in the determination of the device performance. In this paper, we present a new multiple-cycle chemical vapor deposition (CVD) method to synthesize horizontally aligned arrays of SWNTs with densities of 20-40 SWNT/μm over large area and a diameter distribution of 2.4 ± 0.5 nm on the quartz surface based on a methanol/ethanol CVD method. The high nucleation efficiency of catalyst particles in multiple-cycle CVD processes has been demonstrated to be the main reason for the improvements in the density of SWNT arrays. More interestingly, we confirmed the existence of an etching effect, which strongly affects the final products in the multiple-cycle growth. This etching effect is likely the reason that only large-diameter SWNTs were obtained in the multiple-cycle CVD growth. Using these high-density and large-diameter nanotube arrays, two-terminal devices with back-gates were fabricated. The performances of these devices have been greatly improved in overall resistance and on-state current, which indicates these SWNT arrays have high potential for applications such as interconnects, high-frequency devices, and high-current transistors in future micro- or nanoelectronics.

Purification of high-quality few-walled carbon nanotubes (fWNTs) was developed by slow but selective oxidation in hydrogen peroxide (H(2)O(2)) at room temperature. The purity, nanotubes' structure, and thermal stability of purified fWNTs were characterized by transmission electron microscopy (TEM), Raman spectroscopy, and thermogravimetric analysis (TGA), respectively. The results showed that fWNTs could be selectively purified by prolonging the stirring time in 30 wt % H(2)O(2) solution. Highly purified fWNTs were obtained, having a high G/D ratio in Raman spectra and good thermal stability indicating the good quality of the purified fWNTs. The approach provides a simple low cost method for purification that also has higher nanotube yield than other purification methods.