HIGH TEMPERATURE NANOMANUFACTURING FOR EMERGING TECHNOLOGIES

High temperature processing can provide sufficient activation energy for materials’ compositional, structural, and morphological evolutions, and is essential for various kinds of reactions, synthesis, and post-treatment. However, the current high temperature heating sources, mostly furnaces, are far from satisfying for nanomaterials processing owing to their bulky size and limited temperature and ramp range (~1300 K, ~10 K/min). In this thesis, we have focused on the study of electrical triggered Joule heating as a new route for high temperature engineering of nanomaterials toward nanomanufacturing. We developed facile, highly stable and controllable heating strategies for micro/nanoscale high temperature engineering. Ultrahigh temperature annealing (>2500 K) is applied to carbon nanomaterials to address the defects and poor interfacial problems. In the carbon nanofibers (CNFs), the high temperature graphitizes the carbon nanomaterials with significantly improved crystallinity and less defects. Importantly, the rapid heating (~100 K/min) leads to junction welding at fiber intersections. Similarly in carbon nanotubes (CNTs), welded CNTs is achieved by incorporating a thin polymer coating, followed by high temperature annealing to form 3D interconnected structures, defined as an “epitaxial welding” process. Ultrafast thermal shock (~2000 K in 55 ms) is applied to metal salt loaded carbon substrates for in-situ synthesis of ultrasmall, well-dispersed nanoparticles. Metal salts decompose rapidly at high temperatures and nucleate into well-dispersed nanoparticles during the rapid cooling (rate of ~10E5 K/s). By varying the composition in salt mixtures, we synthesized bimetallic, multimetallic and high entropy alloy nanoparticles (HEA-NPs) containing up to 8 different and immiscible elements. The high temperature leads to atomic mixing in the liquid alloy state, while rapid quenching freezes the completely mixed state to form solid solution nanoparticles with a narrow size distribution. This is for the first time HEA-NPs were synthesized, enabled by the unique thermal shock method. We also developed scalable approaches such as employing non-contact radiative heating for large scale substrates (either conductive or non-conductive) and continuous roll-to-roll production. The high temperature engineering on nanomaterials are highly facile, energy-efficient, and reliable toward scalable nanomanufacturing. More exciting results and products are expected for various nanomaterials during/after the unique high temperature engineering

Evolution of the tetragonal to rhombohedral transition in (1 − x)(Bi1/2Na1/2)TiO3 − xBaTiO3 (x ≤ 7%)

(1 − x)(Bi1/2Na1/2)TiO3 − xBaTiO3 has been the most studied Pb-free piezoelectric material in the last decade; however, puzzles still remain about its phase transitions, especially around the important morphotropic phase boundary (MPB). By introducing the strain glass transition concept from the ferroelastic field, it was found that the phase transition from tetragonal (T, P4bm) to rhombohedral (R, R3c) was affected by a strain glass transition at higher temperature for x ≥ 4%. In these compositions, the T–R transition was delayed or even totally suppressed and displayed huge thermal hysteresis upon cooling and heating. Also, isothermal phase transitions were predicted and realized successfully in the crossover region, where the interaction between the T–R transition and the strain glass transition was strong. Our results revealed the strain glass nature in compositions around the MPB in this important material, and also provide new clues for understanding the transition complexity in other (Bi1/2Na1/2)TiO3-based Pb-free piezoelectric materials

Direct observation of the formation and stabilization of metallic nanoparticles on carbon supports

Direct formation of ultra-small nanoparticles on carbon supports by rapid high temperature synthesis method offers new opportunities for scalable nanomanufacturing and the synthesis of stable multi-elemental nanoparticles. However, the underlying mechanisms affecting the dispersion and stability of nanoparticles on the supports during high temperature processing remain enigmatic. In this work, we report the observation of metallic nanoparticles formation and stabilization on carbon supports through in situ Joule heating method. We find that the formation of metallic nanoparticles is associated with the simultaneous phase transition of amorphous carbon to a highly defective turbostratic graphite (T-graphite). Molecular dynamic (MD) simulations suggest that the defective T-graphite provide numerous nucleation sites for the nanoparticles to form. Furthermore, the nanoparticles partially intercalate and take root on edge planes, leading to high binding energy on support. This interaction between nanoparticles and T-graphite substrate strengthens the anchoring and provides excellent thermal stability to the nanoparticles. These findings provide mechanistic understanding of rapid high temperature synthesis of metal nanoparticles on carbon supports and the origin of their stability

Spatial distribution of the persistent organic pollutants across the Tibetan Plateau and its linkage with the climate systems: a 5-year air monitoring study

The Tibetan Plateau (TP) has been contaminated by persistent organic pollutants (POPs), including legacy organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) through atmospheric transport. The exact source regions, transport pathways and time trends of POPs to the TP are not well understood. Here polystyrene–divinylbenzene copolymer resin (XAD)-based passive air samplers (PASs) were deployed at 16 Tibetan background sites from 2007 to 2012 to gain further insight into spatial patterns and temporal trends of OCPs and PCBs. The southeastern TP was characterized by dichlorodiphenyltrichloroethane (DDT)-related chemicals delivered by Indian monsoon air masses. The northern and northwestern TP displayed the greatest absolute concentration and relative abundance of hexachlorobenzene (HCB) in the atmosphere, caused by the westerly-driven European air masses. The interactions between the DDT polluted Indian monsoon air and the clean westerly winds formed a transition zone in central Tibet, where both DDT and HCB were the dominant chemicals. Based on 5 years of continuous sampling, our data indicated declining concentrations of HCB and hexachlorocyclohexanes (HCHs) across the Tibetan region. Inter-annual trends of DDT class chemicals, however, showed less variation during this 5-year sampling period, which may be due to the ongoing usage of DDT in India. This paper demonstrates the possibility of using POP fingerprints to investigate the climate interactions and the validity of using PAS to derive inter-annual atmospheric POP time trends

Ultra-fast self-assembly and stabilization of reactive nanoparticles in reduced graphene oxide films.

Nanoparticles hosted in conductive matrices are ubiquitous in electrochemical energy storage, catalysis and energetic devices. However, agglomeration and surface oxidation remain as two major challenges towards their ultimate utility, especially for highly reactive materials. Here we report uniformly distributed nanoparticles with diameters around 10 nm can be self-assembled within a reduced graphene oxide matrix in 10 ms. Microsized particles in reduced graphene oxide are Joule heated to high temperature (∼1,700 K) and rapidly quenched to preserve the resultant nano-architecture. A possible formation mechanism is that microsized particles melt under high temperature, are separated by defects in reduced graphene oxide and self-assemble into nanoparticles on cooling. The ultra-fast manufacturing approach can be applied to a wide range of materials, including aluminium, silicon, tin and so on. One unique application of this technique is the stabilization of aluminium nanoparticles in reduced graphene oxide film, which we demonstrate to have excellent performance as a switchable energetic material

Hydrothermal Preparation of Visible-Light-Driven N-Br-Codoped TiO

Using a facile hydrothermal method, N-Br-codoped TiO2 photocatalyst that had intense absorption in visible region was prepared at low temperature (100°C), through a direct reaction between nanocrystalline anatase TiO2 solution and cetyltrimethylammonium bromide (CTAB). The results of X-ray photoelectron spectroscopy (XPS) showed the existence of N-Ti-N, O-Ti-N-R, Ti3+ (attribute to the doped Br atoms by charge compensation), and TiOxNy species, indicating the successful codoping of N and Br atoms, which were substituted for lattice oxygen without any influence on the crystalline phase of TiO2. In contrast to the N-doped sample, the N-Br-codoped TiO2 photocatalyst could more readily photodegrade methylene blue (MB) under visible-light irradiation. The visible-light catalytic activity of thus-prepared photocatalyst resulted from the synergetic effect of the doped nitrogen and bromine, which not only gave high absorbance in the visible-light range, but also reduced electron-hole recombination rate