Xiaolin Zheng, Postdoctoral Faculty Sponsor
High thermoelectric figure of merit of porous Si nanowires from 300 to 700K.
2021; 12 (1): 3926
Thermoelectrics operating at high temperature can cost-effectively convert waste heat and compete with other zero-carbon technologies. Among different high-temperature thermoelectrics materials, silicon nanowires possess the combined attributes of cost effectiveness and mature manufacturing infrastructures. Despite significant breakthroughs in silicon nanowires based thermoelectrics for waste heat conversion, the figure of merit (ZT) or operating temperature has remained low. Here, we report the synthesis of large-area, wafer-scale arrays of porous silicon nanowires with ultra-thin Si crystallite size of ~4nm. Concurrent measurements of thermal conductivity (kappa), electrical conductivity (sigma), and Seebeck coefficient (S) on the same nanowire show a ZT of 0.71 at 700K, which is more than ~18 times higher than bulk Si. This ZT value is more than two times higher than any nanostructured Si-based thermoelectrics reported in the literature at 700K. Experimental data and theoretical modeling demonstrate that this work has the potential to achieve a ZT of ~1 at 1000K.
View details for DOI 10.1038/s41467-021-24208-3
View details for PubMedID 34168136
Enhancing Mechanical and Combustion Performance of Boron/Polymer Composites via Boron Particle Functionalization.
ACS applied materials & interfaces
High-speed air-breathing propulsion systems, such as solid fuel ramjets (SFRJ), are important for space exploration and national security. The development of SFRJ requires high-performance solid fuels with excellent mechanical and combustion properties. One of the current solid fuel candidates is composed of high-energy particles (e.g., boron (B)) and polymeric binder (e.g., hydroxyl-terminated polybutadiene (HTPB)). However, the opposite polarities of the boron surface and HTPB lead to poor B particle dispersion and distribution within HTPB. Herein, we demonstrate that the surface functionalization of B particles with nonpolar oleoyl chloride greatly improves the dispersion and distribution of B particles within HTPB. The improved particle dispersion is quantitatively visualized through X-ray computed tomography imaging, and the particle/matrix interaction is evaluated by dynamic mechanical analysis. The surface-functionalized B particles can be uniformly dispersed up to 40 wt % in HTPB, the highest mass loading reported to date. The surface-functionalized B (40 wt %)/HTPB composite exhibits a 63.3% higher Young's modulus, 87.5% higher tensile strength, 16.2% higher toughness, and 16.8% higher heat of combustion than pristine B (40 wt %)/HTPB. The surface functionalization of B particles provides an effective strategy for improving the efficacy and safety of B/HTPB solid fuels for future high-speed air-breathing vehicles.
View details for DOI 10.1021/acsami.1c06727
View details for PubMedID 34110148
- Ultrahigh Doping of Graphene Using Flame-Deposited MoO3 IEEE ELECTRON DEVICE LETTERS 2020; 41 (10): 1592–95
- Enhancing combustion performance of nano-Al/PVDF composites with beta-PVDF COMBUSTION AND FLAME 2020; 219: 467–77
- On-demand production of hydrogen by reacting porous silicon nanowires with water NANO RESEARCH 2020
Synergistically Chemical and Thermal Coupling between Graphene Oxide and Graphene Fluoride for Enhancing Aluminum Combustion.
ACS applied materials & interfaces
Metal combustion reaction is highly exothermic and is used in energetic applications, such as propulsion, pyrotechnics, powering micro- and nano-devices, and nanomaterials synthesis. Aluminum (Al) is attracting great interest in those applications because of its high energy density, earth abundance, and low toxicity. Nevertheless, Al combustion is hard to initiate and progresses slowly and incompletely. On the other hand, ultrathin carbon nanomaterials, such as graphene, graphene oxide (GO), and graphene fluoride (GF), can also undergo exothermic reactions. Herein, we demonstrate that the mixture of GO and GF significantly improves the performance of Al combustion as interactions between GO and GF provide heat and radicals to accelerate Al oxidation. Our experiments and reactive molecular dynamics simulation reveal that GO and GF have strong chemical and thermal couplings through radical reactions and heat released from their oxidation reactions. GO facilitates the dissociation of GF, and GF accelerates the disproportionation and oxidation of GO. When the mixture of GO and GF is added to micron-sized Al particles, their synergistic couplings generate reactive oxidative species, such as CF x and CF x O y , and heat, which greatly accelerates Al combustion. This work demonstrates a new area of using synergistic couplings between ultrathin carbon nanomaterials to accelerate metal combustion and potentially oxidation reactions of other materials.
View details for DOI 10.1021/acsami.9b20397
View details for PubMedID 31950820
- Experimental effective metal oxides to enhance boron combustion COMBUSTION AND FLAME 2019; 205: 278–85
- Modified Micro-Emulsion Synthesis of Highly Dispersed Al/PVDF Composites with Enhanced Combustion Properties ADVANCED ENGINEERING MATERIALS 2019; 21 (5)
- Tuning the morphological, ignition and combustion properties of micron-Al/CuO thermites through different synthesis approaches COMBUSTION AND FLAME 2018; 195: 303–10
Enhanced interfacial bonding and mechanical properties in CNT/Al composites fabricated by flake powder metallurgy
2018; 130: 333-339
View details for DOI 10.1016/j.carbon.2018.01.037
Energetic Performance of Optically Activated Aluminum/Graphene Oxide Composites.
Optical ignition of solid energetic materials, which can rapidly release heat, gas, and thrust, is still challenging due to the limited light absorption and high ignition energy of typical energetic materials ( e.g., aluminum, Al). Here, we demonstrated that the optical ignition and combustion properties of micron-sized Al particles were greatly enhanced by adding only 20 wt % of graphene oxide (GO). These enhancements are attributed to the optically activated disproportionation and oxidation reactions of GO, which release heat to initiate the oxidization of Al by air and generate gaseous products to reduce the agglomeration of the composites and promote the pressure rise during combustion. More importantly, compared to conventional additives such as metal oxides nanoparticles ( e.g., WO3 and Bi2O3), GO has much lower density and therefore could improve energetic properties without sacrificing Al content. The results from Xe flash ignition and laser-based excitation experiments demonstrate that GO is an efficient additive to improve the energetic performance of micron-sized Al particles, enabling micron-sized Al to be ignited by optical activation and promoting the combustion of Al in air.
View details for DOI 10.1021/acsnano.8b06217
View details for PubMedID 30335365
Electroless Deposition and Ignition Properties of Si/Fe2O3 Core/Shell Nanothermites.
2017; 2 (7): 3596–3600
Thermite, a composite of metal and metal oxide, finds wide applications in power and thermal generation systems that require high-energy density. Most of the researches on thermites have focused on using aluminum (Al) particles as the fuel. However, Al particles are sensitive to electrostatic discharge, friction, and mechanical impact, imposing a challenge for the safe handling and storage of Al-based thermites. Silicon (Si) is another attractive fuel for thermites because of its high-energy content, thin native oxide layer, and facile surface functionality. Several studies showed that the combustion properties of Si-based thermites are comparable to those of Al-based thermites. However, little is known about the ignition properties of Si-based thermites. In this work, we determined the reaction onset temperatures of mechanically mixed (MM) Si/Fe2O3 nanothermites and Si/Fe2O3 core/shell (CS) nanothermites using differential scanning calorimetry. The Si/Fe2O3 CS nanothermites were prepared by an electroless deposition method. We found that the Si/Fe2O3 CS nanoparticles (NPs) had a lower reaction onset temperature (∼550 °C) than the MM Si/Fe2O3 nanothermites (>650 °C). The onset temperature of the Si/Fe2O3 CS nanothermites is also insensitive to the size of the Si core NP. These results indicate that the interfacial contact quality between Si and Fe2O3 is the dominant factor for determining the ignition properties of thermites. Finally, the reaction onset temperature of the Si/Fe2O3 CS NPs is comparable to that of the commonly used Al-based nanothermites, suggesting that Si is an attractive fuel for thermites.
View details for DOI 10.1021/acsomega.7b00652
View details for PubMedID 31457677
View details for PubMedCentralID PMC6641388