Materials Science Forum Vol. 848

Abstract: A series of zeolite-derived silica glasses have been fabricated through the order-disorder transition process of zeolites using spark plasma sintering (SPS). The samples were characterized by UV-VIS-NIR transmittance, Raman spectra and Vicker’s hardness. Results showed that the transmittance of zeolite-derived silica glass samples increased with the sintering temperature increasing. The transmittance of the sample prepared at 1300°C was lower than 5%. The highly transparent samples could be obtained when the sintering temperature was above 1350°C. The transmittance of these samples was higher than 80% in the range of 780~1700nm and 60% in the UV-vis range of 250~780nm. One absorption band at 300nm can also be observed in the spectrum. The reason of this phenomenon could be ascribed to a few left crystallite fragments of ZSM-5. The Raman spectra showed that the bands at 378 and 292cm^-1of ZSM-5 were significantly weakened and then disappeared with increasing the sintering temperature and the bands at 489 and 600cm^-1 of silica glass became more apparent. The Raman spectra of the samples obtained above 1350°C were the same as that of the fused silica glass. The Vicker’s hardness of as-prepared samples increased with the rising of the sintering temperature. The maximum hardness of 7.31 GPa was achieved for the sample sintered at 1400°C. The fracture toughness of samples increased from 0.83 MPa·m^1/2 to 2.37 MPa·m^1/2 with the sintering temperature increasing.

Abstract: Silica glass was prepared via the spark plasma sintering (SPS) method by using SBA-15 as the starting material. Five temperatures (1000, 1010, 1020, 1030 and 1040°C) were selected as the final sintering temperature above 600°C. The transparent silica glass was prepared by SPS sintering of mesoporous silica SBA-15 at 1040°C. The impact of temperature structure collapse of SBA-15, the structure and transmittance of the glass were studied using X ray diffraction, SEM, TEM, Infrared analysis, Raman analysis and UV-VIS spectrophotometer. Results show that SBA-15 collapsed completely at 1040°C, the sintered glass had high transmittance of above 90% in visible spectra, and its hardness was 6.96 Gpa, and that the sintered transparent sample was silica glass. The work demonstrated a novel strategy to use SPS to prepare highly transparent silica glass by sintering of SBA-15.

Abstract: Magnesium-stabilized aluminum titanate powder was prepared via non-hydrolytic sol-gel method using titanium tetrachloride and anhydrous aluminium chloride as precursors, anhydrous ethanol as the oxygen donor, magnesium powder, magnesium fluoride, magnesium ethoxide and anhydrous magnesium acetate as stabilizers. The effect of magnesium stabilizers on low temperature synthesis of aluminum titanate was investigated, and their role and mechanism in stabilizing aluminum titanate were also studied by XRD, FT-IR and thermal expansion dilatometer. The results show that introducing magnesium powder or magnesium fluoride can’t stabilize aluminum titanate, they also lead to the failure of aluminum titanate low-temperature synthesis at 750 °C due to its promotion of non-hydrolytic homogeneous condensation. Anhydrous magnesium acetate and magnesium ethoxide can react with aluminum alkoxide and titanium alkoxide in the precursor mixture to form heterogeneous condensation bonds, which promotes magnesium ion to dope into aluminum titanate lattice at 750 °C, and hence to improve its thermal stability.

Abstract: Aluminum titanate (AT) ultrafine powder with high productivity ratio was prepared via hydrolytic sol-gel (HSG) method at 1250 °C, using aluminum nitrate (Al (NO₃)₃·9H₂O) and titanium tetrachloride (TiCl₄) as precursors, ethanol as solvent through solvothermal treatment process. Water required for hydrolysis reaction was supplied by the crystal water of aluminum nitrate itself. The effect of process parameters including filling ratio, solvothermal temperature and soaking time on the synthesis of AT powder was investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The cause of low temperature synthesis of AT was investigated. The result showed that excessive or insufficient filling ratio, too high or low solvothermal temperature, too long soaking time was not beneficial for the preparation of AT. The optimized ultrafine AT powder appeared at the decomposition temperature of 1250 °C with average particle size less than 1μm at a relatively high productivity ratio of 98 % with filling ratios of 60 %, solvothermal temperatures of 130 °C and soaking time of 30 min. The refinement of particles in the AT xerogel through solvothermal treatment process, and increasing reactivity of precursor, plays the key role in the low temperature synthesis of AT.

Abstract: In-situ growth of mullite whisker from waste coal gangue and different aluminium source mixtures by dry pressing were investigated aiming at the preparation of mullite - corundum lightweight refractory by controlling mullite whisker precursor pseudo particle size and quantity in corundum matrix. The phase composition and microstructure were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Apparent porosity and bending strength were also measured. The samples exhibited characteristic alumina and mullite phases. The formation mechanism and influence factors for mullite whiskers materials were discussed. The experimental results showed that the whisker of the obtained sample changed from the accumulated short column to mesh cross acicular whisker shape, and the length to diameter ratio increases. When the aluminium source was aluminium hydroxide, the length to diameter ratio and apparent porosity reached the maximum of 17.6 and the minimum of 2.15%. The bending strength of all the samples was similar, that of the sample prepared by calcined bauxite was the largest (84 MPa), that of the sample prepared by calcined alumina was inferior, and the sample prepared by aluminum hydroxide was the lowest.

Abstract: Barium ferrite micro/nanofibers were successfully prepared via the electrospinning by using dimethyl formamide (DMF) as the solvent, poly vinyl pyrrolidone (PVP) as the spinning auxiliaries and iron nitrate and barium nitrate as raw materials. The effect of poly vinyl pyrrolidone on the structure, morphology, magnetic and microwave absorbing properties were investigated by scanning electron microscope (SEM), X-ray diffraction analysis (XRD), vibration sample magnetometer (VSM) and vector network analyzer (VNA). XRD patterns of the samples confirmed that when the additive content of PVP was up to 10%, (wt%) pure barium ferrite fibers formed under the condition of the same heat treatment. Also, the FE-SEM images showed that the morphology of the fibers improved with the increase of PVP content. Moreover, the VSM results demonstrated that the saturation magnetization can reach 54.7 emu/g when the PVP dosage is 14% (wt%) in the precursor solution.

Abstract: The modified behavior of the phase transition temperatures (T_O-T and/or T_C) between orthorhombic (O), tetragonal (T) and cubic (C) that caused by doping Sb⁵⁺ in (Li_0.052Na_0.493K_0.455)(Nb_1-xSb_x)O₃ (LNKNS_x) ceramics was reported in the present investigation. The results show that differing from the insensitive T_O-T to the Sb⁵⁺ content, T_C splits into two peaks T_C^I and T_C^II when doping Sb⁵⁺. The decreased T_C^I by raising x may be ascribed to the Sb-rich grains and the settled T_C^II round 480 °C resulting from the Sb-lack ones. The enhanced piezoelectric coefficient d₃₃ value of 263 pC/N and planar mode electromechanical coupling coefficient k_p value of 42.5% at x=0.052 can be attributed to the polymorphic phase boundary (PPB) behavior with an appropriate ratio between T and O phases without any second phase.

Abstract: This paper presents an approach to investigate the influence of metamaterial to radio-frequency (RF) magnetic field in magnetic resonance imaging (MRI) at 3T. The variety of magnetic fields of RF receiving coil was calculated using the commercial electromagnetic simulation software (CST). The simulation results demonstrate that the transmitting and receiving magnetic field (B₁+ and B₁-) can be enhanced when the metamaterial is inserted into the RF coil, suggesting that the metamaterial has potential in MRI applications at 3T.

Abstract: A dual-band linear polarization transformer with diode-like asymmetric transmission using a three-layer composite metamaterial was proposed and investigated numerically. The proposed three-layer composite metamaterial was comprised of two layers of asymmetric split-ring resonators and a sub-wavelength cross structures sandwiched symmetrically between these layers. By introducing the sub-wavelength cross structure into the asymmetric chiral metamaterial, an electromagnetic wave tunneling effect formed for the incident waves. Thus, the transmissions in the composite structures can be strongly modified, one of the two off-diagonal cross-polarized transmission (t_xy) of the Jones matrix is enhanced while all the others (t_xx, t_yy, and t_yx) are suppressed extremely at resonances. This present design for enhancing asymmetric transmission via polarization conversion can be used as many novel devices, such as optical isolators, asymmetric wave splitters, and circulators.

Abstract: A non-magnetic microwave absorber was proposed, for which the-10dB absorbing bandwidth would operate over all the X-band with a total thickness of 4 mm. The designed structure was composed by resistively single square-loop FSS on the surface of metal-backed dielectric substrate. The influence of the substrate and FSS was discussed for absorption optimization. Besides, the current distribution of the FSS was investigated to explain the principle of resonance. Good accordance of the experimental results and the simulated results has been achieved, indicating the feasibility of our designed structure and the reliability of the simulated method.