An official website of the United States government
Colloidal quantum dots, with their size-tunable optoelectronic properties and scalable synthesis, enable applications in which inexpensive high-performance semiconductors are needed. Synthesis science breakthroughs have been key to the realization of quantum dot technologies, but important group III–group V semiconductors, including colloidal gallium arsenide (GaAs), still cannot be synthesized with existing approaches. The high-temperature molten salt colloidal synthesis introduced in this work enables the preparation of previously intractable colloidal materials. We directly nucleated and grew colloidal quantum dots in molten inorganic salts by harnessing molten salt redox chemistry and using surfactant additives for nanocrystal shape control. Synthesis temperatures above 425°C are critical for realizing photoluminescent GaAs quantum dots, which emphasizes the importance of high temperatures enabled by molten salt solvents. We generalize the methodology and demonstrate nearly a dozen III-V solid-solution nanocrystal compositions that have not been previously reported.
more »
« less
Low Temperature Molten Salt Production of Silicon Nanowires by Electrochemical Reduction of CaSiO₃
Silicon is an extremely important technological material, but the current industrial production of silicon by carbothermic reduction of SiO₂ is energy intensive and generates CO₂ emission. Here we developed a new and more sustainable method to produce silicon nanowires in bulk quantities via direct electrochemical reduction of CaSiO₃, an abundant and inexpensive silicon source soluble in molten salts, at a low temperature of 650 ⁰C by using low melting point ternary molten salts CaCl₂-MgCl₂-NaCl, which still retains high CaSiO₃ solubility, and a supporting electrolyte of CaO, which facilitates the transport of O²¯ anions, drastically improves the reaction kinetics and enables the electrolysis at low temperatures. The Si nanowire product can be used as high-capacity Li-ion battery anode materials with excellent cycling performance. This practical strategy at lower temperatures can be applied to other molten salt systems and also promising for waste glass and coal ash recycling.
more »
« less
- Award ID(s):
- 1313968
- PAR ID:
- 10044102
- Date Published:
- Journal Name:
- Angewandte Chemie International Edition
- ISSN:
- 1433-7851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Molten salts are under consideration as the working fluid in thermal power generation. Nitrate molten salts store vast amounts of energy at high temperature and are an efficient energy production medium. Nitrate molten salts are corrosive to structural materials in these applications. Static corrosion studies may neglect the effects of fluid flow on corrosion and flowing test loops can be expensive and complex. A rotating cylinder electrode (RCE) can simulate the effects of fluid flow on the corrosion of structural materials and are more compact and economical then flow loops. We have developed a rotating cylinder electrode apparatus to study the corrosion of structural metals in flowing molten salts using accelerated electrochemical corrosion testing. In this study, we have evaluated the corrosion behavior in molten nitrate salts and used various surface characterization techniques to compare the results from static corrosion tests. Results and analysis of these studies will be presented.more » « less
-
Nanocrystalline silicon can have unique thermal transport and mechanical properties governed by its constituent grain microstructure. Here, we use phonon ray-tracing and molecular dynamics simulations to demonstrate the largely tunable thermomechanical behaviors with varying grain sizes (a0) and aspect ratios (ξ). Our work shows that, by selectively increasing the grain size along the heat transfer direction while keeping the grain area constant, the in-plane lattice thermal conductivity (kx) increases more significantly than the cross-plane lattice thermal conductivity (ky) due to anisotropic phonon–grain boundary scattering. While kx generally increases with increasing ξ, a critical value exists for ξ at which kx reaches its maximum. Beyond this transition point, further increases in ξ result in a decrease in kx due to substantial scattering of low-frequency phonons with anisotropic grain boundaries. Moreover, we observe reductions in the elastic and shear modulus with decreasing grain size, and this lattice softening leads to significant reductions in phonon group velocity and thermal conductivity. By considering both thermal and mechanical size effects, we identify two distinct regimes of thermal transport, in which anisotropic phonon–grain boundary scattering becomes more appreciable at low temperatures and lattice softening becomes more pronounced at high temperatures. Through phonon spectral analysis, we attribute the significant thermal conductivity anisotropy in nanograined silicon to grain boundary scattering of low-frequency phonons and the softening-driven thermal conductivity reduction to Umklapp scattering of high-frequency phonons. These findings offer insights into the manipulation of thermomechanical properties of nanocrystalline silicon via microstructure engineering, carrying profound implications for the development of future nanomaterials.more » « less
-
Pyroprocessing is a potential route to close the nuclear fuel cycle. Used nuclear fuel (UNF) is electrolytically reduced from UO2 to U0 at a stainless-steel cathode while oxygen evolution occurs at a platinum anode in a molten LiCl-Li2O environment. Platinum is consumed during this process as a result of the formation and spallation of lithium platinate. To increase the economic viability of pyroprocessing, alternative low-cost, electrochemically efficient materials are needed to replace platinum. In this study, metal-oxide coated 316L stainless streel rods were explored as potential replacements. The characteristics of these coatings in molten LiCl-Li2O was evaluated through electrochemical techniques. The surface chemistry of the coatings was explored through X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy and scanning electron microscopy before and after exposure to molten salts to understand the degradation of the coatings. Results detailing the performance of the coatings will be presented.more » « less
-
The demand for clean energy production and storage has increased interest in molten salt technologies, including Molten Salt Reactors (MSR). Understanding of how molten salts properties change with respect to temperature and structure is vital to establishing efficient, cost effective MSR systems. Research into these materials however has been limited due to the difficulty in accurately measuring properties of these reactive materials at elevated temperatures and controlled environment in a time efficient way. Much research has turned to molecular dynamic (MD) modeling to alleviate these issues. This research presents a custom fabricated falling ball viscometer system for measuring molten salt viscosity quickly. A model for correlating velocity to viscosity for Re < 300 was also developed for use with this system. The viscometer is demonstrated on eutectic FLiNaK and NaF-ZrF4 (53–47 mol%) up to 150 K above the respective melting points. The results are compared to MD simulations to verify their effectiveness for predicting viscosity and previously reported measurements.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/anie.201707064
