Yanli Shi

Fabrication of nanocrystalline λ-Ti3O5 with tunable terahertz wave transmission properties across a temperature induced phase transition

λ-Ti3O5 was a newly discovered material with intriguing phase transition characteristics, which exhibits huge potential in the application of memory and tunable optoelectronic devices. However, the fabrication of λ-Ti3O5 still presents a great challenge and its application needs further investigation. In this work, we developed a novel method to fabricate nanocrystalline λ-Ti3O5 by carbothermal reduction of nano-TiO2, and explored its terahertz transmission properties through a temperature induced phase transition. A second phase was introduced to inhibit the grain growth of titanium oxide during the carbothermal reduction, by performing a surface modification of the precursor nano-TiO2 particles with Al2O3. This process was proved to be critical for the formation of nanocrystalline λ-Ti3O5. An in situ XRD analysis combined with a first-principles calculation based on plane wave DFT indicated that the nanocrystalline λ-Ti3O5 exhibited a semimetallic λ phase to metallic α phase transition across a large temperature range. The phase transition was accompanied by continuous, slow and reversible tuning of the terahertz transmission amplitude. This work provides considerable insights into the synthesis of λ-Ti3O5 and opens up studies on the applications of λ-Ti3O5 in the THz range, such as but not limited to sensors and smart windows.

Stabilization of microcrystal λ-Ti3O5 at room temperature by aluminum-ion doping

λ-Ti3O5 is an intriguing phase-transition material that has been proposed to be metastable and has emerged at room temperature only in the form of nanocrystals. In this work, λ-Ti3O5 was stabilized to room temperature in the form of microcrystals by aluminum (Al)-ion doping. Al entered the Ti3O5 lattice in the substitutional mode, which reduced the threshold temperature (Tc) of the β-λ phase transition in Ti3O5 and maintained a λ-phase Ti3O5 at room temperature. Al doping caused a significant decrease in resistivity of Ti3O5, which corresponds to a semiconductor-metal transition that is induced by Al-ion doping. We have developed a mechanism to fabricate λ-Ti3O5 by ion doping and have provided a fundamental foundation for a more available application of λ-Ti3O5 in smart optoelectronic devices.

Density functional theory calculations of point defects and hydrogen isotopes in Li4SiO4

The Li4SiO4 is a promising breeder material for future fusion reactors. Radiation induced vacancies and hydrogen isotope related impurities are the major types of point defects in this breeder material. In present study, various kinds of vacancies and hydrogen isotopes related point defects in Li4SiO4 are investigated through density functional theory (DFT) calculations. The band gap of Li4SiO4 is determined by UV-Vis diffuse reflectance spectroscopy experiments. Formation energies of all possible charge states of Li, Si and O vacancies are calculated using DFT methods. Formation energies of possible charge states of hydrogen isotopes substitution for Li and O are also calculated. We found that Li-vacancies will dominate among all vacancies in neutral charge state under radiation conditions and the O, Li, and Si vacancies (VO,VLi,VSi) are stable in charge states +2, -1, -4 for most of the range of Fermi level, respectively. The interstitial hydrogen isotopes (Hi) and substitutional HLi are stable in the c...

Low-temperature synthesis of Li4SiO4 pebbles as tritium breeding material by SiO2 coating approach

ABSTRACT Li4SiO4 pebbles are widely selected as attractive ceramic tritium breeding materials for the test blanket modules (TBM) of international thermonuclear experimental reactor (ITER). In this work, SiO2 coating approach was first employed to synthesize precursor powders for fabricating the Li4SiO4 pebbles. Lithium source and silicon source were mixed by hydrolyzing tetraethyl orthosilicate (TEOS) in the Li2CO3 solution. Compared with the traditional solid state method, the precursor powders synthesized by SiO2 coating approach displayed distinct coating structure, which was able to effectively lower the phase formation temperature and the sintering temperature of Li4SiO4 pebbles, and then decrease the grain size of Li4SiO4 pebbles. Thermogravimetry and different scanning calorimetry (TG-DSC) and X-ray diffraction (XRD) analyses indicate that the phase formation temperature of Li4SiO4 synthesized by SiO2 coating approach is far below that of the conventional solid state reaction. Then, wet method was employed to prepare green spheres with the as-prepared precursor powders. Finally, Li4SiO4 pebbles with small grain size (average value 1.12 µm), high phase purity, good sphericity, and uniform microstructure could be obtained by sintering the green spheres at a low temperature of 625°C, which are expected to show favorable tritium release behavior and be used as tritium breeding materials for blankets.