Latest Articles(Sr1-xZnx)Al2Si2O8 (x=0, 0.005, 0.010, 0.030, 0.060) microwave dielectric ceramic was prepared by a solid-phase sintering method. The effect of Zn2+ doping content on the crystal structure and microwave dielectric properties of SrAl2Si2O8 (SAS) ceramic was investigated. The site (Sr2+ or Al3+) was determined by the first-principle calculation to confirm the site occupied by Zn2+. The result shows that Zn2+ substitution can increase SAS Q×f value (from 37 502 GHz to the maximum value of 48 252 GHz), and improve the density and τf value. The (Sr0.99Zn0.01)Al2Si2O8 ceramic sintered at 1 500℃ for 5 h has excellent microwave dielectric properties(i.e., εr=7.0, Q×f=48 252 GHz, τf= -36.35×10-6/℃).
Magnesium gallate spinel ceramic is a promising candidate material for communication system devices due to its unique microwave dielectric properties. In this work, a pure phase MgO·1.15Ga2O3 ceramic powder with the uniform and fine particle size was synthesized via solid-state reaction. The crystal structure of synthesized powder was determined by X-ray diffraction, showing that the structure is (Mg0.118Ga0.882)IV[Mg0.781Ga1.185□0.033]VIO4. The ceramic was fabricated by pressureless sintering and subsequent hot isostatic pressing sintering with the powder. Furthermore, the correlations among crystal structure, phonon mode parameters and intrinsic dielectric properties were investigated by Raman scattering spectroscopy and Fourier transform far-infrared reflection spectroscopy. The results show that MgO·1.15Ga2O3 ceramic has good microwave dielectric properties (i.e., dielectric constant εr of 13.21 and quality factor Qu×f of 166 THz). Besides, the [Mg—O] polyhedra vibrational group yields a larger contribution to infrared active intensity and dielectric loss rather than other vibration modes. The lattice vibration modes at low frequencies also provide a considerable contribution to intrinsic dielectric loss.
Ceramics with tetragonal tungsten bronze (TTB)-type structures undoped with sintering aids are difficult to densify due to the high anisotropy of their crystal structures. In this paper, TTB-structured NaSr2Nb5O15 (NSN) was selected as the research object to prepare dense ceramics by the design of raw materials. Two powders synthesized by conventional solid-state sintering and molten salt methods were used. The former powder consisted of three phases as the matrix, NSN and intermediate phases (NaNbO3 and SrNb2O6). The latter powder, with a large particle size, acted as a seed. The effects of seed content on the sintering behavior and microstructure of NSN ceramics were investigated. Dense NSN ceramics were prepared by reaction sintering using seed addition into the matrix. The seed content had an obvious influence on the microstructures of the NSN ceramics.
The structures and electrical transport properties of Fe3O4 nano-powders under high pressure were studied by means of high pressure X-ray diffraction and high pressure in-situ Hall effect measurement. The results indicate that Fe3O4 nano-powders undergoes a structural phase transition from inverse spinel to high-pressure phase at about 17.2 GPa. The discontinuous changes of the resistivity occurred at 6.0 GPa, which is related to the metallization phase transition of Fe3O4 nano-powders. Below 6.0 GPa, the decreasing resistivity is due to the rapid increase of the carrier concentration. Above 6.0 GPa, the carrier concentration increases gently and reaches saturation at 11.0 GPa. Further increasing pressure, the increased carrier mobility is the dominant factor for the decreasing resistivity.
The chemical compositions and macrostructures of black-glazed tea bowls, white-rim black porcelains and dual-colored porcelains from Jizhou in the Song dynasty, China, were determined by energy-dispersive X-ray fluorescence and optic microscopy to investigate the manufacture techniques for different composite decorated porcelains. The results show that there are both consistency and difference in porcelain making materials, recipes and glazing techniques of the Jizhou black-glazed tea bowls, white-rim black porcelains and dual-colored porcelains. In addition to the reasonable selection of raw materials and recipes, the control of the glaze thickness and height difference formed by the slight trimming of the body in glaze scraping process is a key to the sharp contrast and clear boundaries between black and white of the white-rim black porcelains.
In order to analyze the growth mechanism of silicon faceted dendrite and reproduce its growth realistically, a 3D model based on cellular automaton-lattice Boltzmann method was proposed. The growth of silicon dendrite was simulated via the cellular automaton. The temperature field of solidifying silicon crystal was calculated by the lattice Boltzmann method. The cellular automaton model and the lattice Boltzmann method were coupled together by the latent heat of solidification released during the growth of silicon crystals and a high anisotropy function was introduced, thus simulating the growth of 3D silicon faceted dendrite. The effects of interfacial energy anisotropy and initial undercooling on the morphology of 3D silicon faceted dendrite were analyzed. The results show that 3D silicon faceted dendrites have a strong anisotropy. The 3D model can simulate the growth of silicon faceted dendrites more realistically and accurately.
The In-rich Al1-xInxN films were grown on Si and quartz substrates by radio-frequency magnetron sputtering. The structures, morphologies, and optical bandgap of as-grown Al1-xInxN films were characterized by X-ray diffraction, scanning electron microscopy, and ultra-violate-visible-near infrared spectroscopy. The photoluminescence (PL) spectra of Al1-xInxN films were measured at room temperature. The Al1-xInxN films have single-phase wurtzite structures with (002) preferred orientation. The In content x was tuned in the range of 0.64-0.76. With increasing the In content x, the Al1-xInxN films exhibit smoother surface morphology and the optical bandgap shows a red-shifted from 1.93 eV to 1.87 eV. The PL emission is dominated by the band edge emission at x = 0.64. The additional emission could be explained by the Burstein-Moss effect when the In content x was increased.
Fe-doped TiO2 with rich surface oxygen vacancies was prepared by a facile solvothermal method combined with HF etching. The doping of Fe extends the light absorption range of TiO2 from ultraviolet to visible light. The abundant oxygen vacancies on the TiO2 surface cooperate with the d orbital of Fe3+ to provide a large number of active sites for activating nitrogen molecules, and inhibit the recombination of electrons and holes. In ambient condition, without any sacrificial agent, the ammonia production of 6% Fe-TiO2 reaches 94.3 µmol·g-1·h-1, compared to that of pure TiO2. In visible light illumination, the ammonia production of 6% Fe-TiO2 reaches 45.6 µmol·g-1·h-1.
A novel red-emitting phosphor CaAl12-x-y(PO4)0.1O18.85: xMn4+, yMg2+ was synthesized by a high-temperature solid-state method. The crystal structure and luminescence properties of the samples were characterized by X-ray diffraction, scanning electron microscopy and fluorescence spectroscopy. The results show that the synthesized phosphor has a hexagonal crystal structure with the space group of P63/mmc. The optimal doping amounts of Mn4+ and Mg2+ are x of 0.01 and y of 0.1, respectively. Under excitation at 396 nm, CaAl11.89(PO4)0.1O18.85: 0.01Mn4+, 0.1Mg2+ phosphor shows the maximum red light emission at 654 nm, corresponding to 2E→4A2g transition of Mn4+. The luminescence intensity and lifetime of phosphor are improved due to the co-doping with Mg2+ charge compensator. The absolute quantum efficiency of phosphor is 60%, the fluorescence lifetime is 0.97 ms, the color coordinate is in the deep-red region, and the color purity is 97%. The luminescence intensity is still 60% of the initial intensity when the temperature increases to 413K, indicating that this red phosphor with a superior thermal stability is a novel red luminescent material with broad application prospects.
Long afterglow phosphors are widely used in fields such as optical storage and photocatalysis. Aluminosilicate has the advantages of good physical and chemical stability, thermal stability, high light conversion rate, low cost and emission bandwidth. The layered SrAl2Si2O8:Eu2+, Dy3+ phosphors with strontium feldspar structure were synthesized by high-temperature solid-state reaction method. The phosphor can be excited by 276 nm ultraviolet light to produce a broad emission band with a peak at 426 nm. Co-doped Dy3+ ions do not affect the crystal structure and emission peak position. Dy3+ is imported into the trap center to cause continuous light emission. The phosphor has excellent thermal stability, and it can maintain 90.5% of the initial luminous intensity at 200℃. The addition of H3BO3 flux does not affect the crystal structure. It improves the luminous intensity and prolongs the afterglow lifetime. After adding the flux, the lifetime can reach 122.3 s, and the quantum efficiency is increased by 30.2%. The luminous characteristics and afterglow lifetime can indicate that SrAl2Si2O8: Eu2+, Dy3+ phosphors have development potential in the field of long afterglow light emitting diodes (LEDs) in the future.
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