Jianqi Qi

Transparent Ce:Y3Al5O12 ceramic phosphors for white light-emitting diodes

We present our recent achievement of a transparent ceramic able to produce white light when directly combined with commercially available blue light emitting diodes. The photoluminescence properties of ceramic phosphor (Y1-xCex)(3)Al5O12 are studied as a function of doping fraction (x = 0.0005-0.0020). The emission color is tunable by variations of Ce3+ concentration and ceramic phosphor thickness. A maximum luminous efficacy exceeding 93 lm/W at a low correlated color temperature of similar to 4600 K is obtained, which is superior to samples made from commercial phosphor powders. Hence, the present transparent ceramic phosphor is expected to be an ideal candidate for generating white light

Light extinction by pores in AlON ceramics: the transmission properties

The transmission properties of aluminium oxynitride (AlON) ceramics were studied. AlON samples with different transmittances were prepared. SEM detected two types of pores with a different range of diameters in the samples; the porosity was obtained by counting the number of pores in a certain area. Mie theory was applied to illuminate the effects of pores on the transmittances. The calculated transmittances were compared with the experiments. The existence of pores with sizes in the 1.1-1.6 mu m range and porosities above 10(-4) accounted for the degradation of the transmittance spectra at 2.5 mu m. Discrepancies of the spectra between the samples were caused mainly by different densities of the A-type pores with sizes above 1 mu m. The concepts of critical size and critical concentration were proposed and were quantitatively characterized by critical curves, which divided the regions of translucency and transparency into three parts: R(1), R(2) and R(3) depending on porosity. New approaches on how to control the pore sizes and porosities quantitatively for transparent ceramics design were presented.

Shock-induced cation disorder in magnesium aluminate spinel

An increase in lattice constants and an order-disorder phase transition were observed in the magnesium aluminate spinel (MgAl(2)O(4)) powders after shock compression. Theoretical calculations on the basis of density functional theory confirm that the remarkable volume expansion in shocked MgAl(2)O(4) powders is closely related to the substantial site disorder in the MgAl(2)O(4) lattice. The calculations also show that the partially inverse MgAl(2)O(4) spinel with an inversion index of 0.7 represents the greatest disordered metastable phase and the most unstable structure

Effects of Al2O3 phase composition on AlON powder synthesis via aluminothermic reduction and nitridation

Abstract Using micro-sized aluminum powder (11 wt.%) and nano-sized Al2O3 powder (89 wt.%) with five different phase compositions, AlON powders were synthesized at different temperatures in flowing-nitrogen atmosphere. Our results suggest that for starting materials with low γ-Al2O3/total Al2O3 ratio R (0 and 0.15), a calcination temperature of 1 750 °C is required to obtain single-phase AlON. This temperature is about 50 °C higher than for other batches starting with larger R values (0.5, 0.85, and 1.0). The AlON powders fabricated from reactants with a high R value in this work show narrower particle size distribution and better particle homogeneity than those prepared from batches with lower R values.

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.

Fast crystallization of amorphous Gd2Zr2O7 induced by thermally activated electron-beam irradiation

We investigate the ionization and displacement effects of an electron-beam (e-beam) on amorphous Gd2Zr2O7 synthesized by the co-precipitation and calcination methods. The as-received amorphous specimens were irradiated under electron beams at different energies (80 keV, 120 keV, and 2 MeV) and then characterized by X-ray diffraction and transmission electron microscopy. A metastable fluorite phase was observed in nanocrystalline Gd2Zr2O7 and is proposed to arise from the relatively lower surface and interface energy compared with the pyrochlore phase. Fast crystallization could be induced by 120 keV e-beam irradiation (beam current = 0.47 mA/cm2). The crystallization occurred on the nanoscale upon ionization irradiation at 400 °C after a dose of less than 1017 electrons/cm2. Under e-beam irradiation, the activation energy for the grain growth process was approximately 10 kJ/mol, but the activation energy was 135 kJ/mol by calcination in a furnace. The thermally activated ionization process was considered the fast crystallization mechanism.

Yield Strength of Transparent MgAl2O4 Nano-Ceramic at High Pressure and Temperature

We report here experimental results of yield strength and stress relaxation measurements of transparent MgAl2O4 nano-ceramics at high pressure and temperature. During compression at ambient temperature, the differential strain deduced from peak broadening increased significantly with pressure up to 2 GPa, with no clear indication of strain saturation. However, by then, warming the sample above 400°C under 4 GPa, stress relaxation was obviously observed, and all subsequent plastic deformation cycles are characterized again by peak broadening. Our results reveal a remarkable reduction in yield strength as the sintering temperature increases from 400 to 900°C. The low temperature for the onset of stress relaxation has attracted attention regarding the performance of transparent MgAl2O4 nano-ceramics as an engineering material.

Unique mechanical properties of nanostructured transparent MgAl2O4 ceramics

Nanoindentation tests were performed on nanostructured transparent magnesium aluminate (MgAl2O4) ceramics to determine their mechanical properties. These tests were carried out on samples at different applied loads ranging from 300 to 9,000 μN. The elastic recovery for nanostructured transparent MgAl2O4 ceramics at different applied loads was derived from the force-depth data. The results reveal a remarkable enhancement in plastic deformation as the applied load increases from 300 to 9,000 μN. After the nanoindetation tests, scanning probe microscope images show no cracking in nanostructured transparent MgAl2O4 ceramics, which confirms the absence of any cracks and fractures around the indentation. Interestingly, the flow of the material along the edges of indent impressions is clearly presented, which is attributed to the dislocation introduced. High-resolution transmission electron microscopy observation indicates the presence of dislocations along the grain boundary, suggesting that the generation and interaction of dislocations play an important role in the plastic deformation of nanostructured transparent ceramics. Finally, the experimentally measured hardness and Young’s modulus, as derived from the load–displacement data, are as high as 31.7 and 314 GPa, respectively.

Synthesis and characterization of Gd2Zr2O7 defect-fluorite oxide nanoparticles via a homogeneous precipitation-solvothermal method

Defect-fluorite structured Gd2Zr2O7 nanoparticles were successfully synthesized via a homogeneous precipitation-solvothermal method using urea as a precipitant. The obtained nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis and transmission electron microscopy (TEM). Compared to the traditional solvothermal method, this homogeneous precipitation-solvothermal method has the advantage of producing nanoparticles with small grain sizes, a narrow size-distribution, high surface areas and little agglomeration. Particularly, the mean crystallite size of Gd2Zr2O7 obtained by this method is 20–30 nm, providing a great opportunity of using these nanoparticles as starting nano-sized building blocks for low temperature preparation of homogeneous and dense ceramics.