Applied Mechanics and Materials Vols. 670-671

Abstract: To avoid the effect of the error of dielectric volume fraction brought by fabricating process on band gap, the diamond crystals with different the error of dielectric volume fraction were designed and fabricated to investigate the fluctuation of band gaps. Theses photonic crystals with a diamond structure were fabricated using alumina by stereolithography, gel-casting and sintering. The photonic band gaps were observed along <100> direction and the photonic band gaps were formed in different frequency range. Increasing the radius from 4.2mm to 4.32mm and changing band width from 0.8 to 1.39GHz, the radius is increased by 2.86%, and corresponding band width increased by 73.75%. Therefore, the fabrication error in mold and fabrication process including injecting mold, sintering should be maintained under the same conditions, which can retain the error of band gap.

Abstract: Looking for photonic crystals with wide absolute bandgaps is always a challenging, attractive and significant task for optics scientists and optics engineers. In this paper, a new square-lattice photonic crystal structure is proposed based on a unit cell with a hollow cylinder and cross connecting plates. It is demonstrated through plane wave expansion method to have wider absolute photonic bandgaps compared with others reported.

Abstract: A new type of two-dimensional square-lattice photonic crystal structure made of rotated square cylinders and cross thin plates is presented and numerically investigated using the plane-wave expansion method. The largest absolute photonic bandgap (APBG) reaches 14.3%. The proposed structure can have promising application for its large APBG and convenience in designing optical circuits.

Abstract: Multiferroic BiFeO₃ powders were synthesized by two methods: sol-gel process and hydrothermal method. The synthesized powders were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (SEM) and particle size distribution analysis. The results obtained by XRD, which is consistent with 86-1518 JCPDS card, show that powders prepared by hydrothermal method are composed of the single phase with the trigonal structure (perovskite-type). It can be found by particle size distribution analysis that the particle size of the samples prepared by sol-gel process is finer and more uniform than that of the samples synthesized by the hydrothermal method. The SEM images of samples depicts that the synthesized BiFeO₃ powders are united and the average grain size of hydrothermal processed samples is significantly large as compared to the sol-gel derived sample.

Abstract: The Mo thin films have been used as the back electrodes of Cu (In,Ga)Se₂ and Cu₂ZnSn (S,Se)₄ thin film solar cells. In this study, the Mo thin films were sputtered on flexible polyimide, polyethylene terephthalate, stainless steel foil, Ti foil, and Al foil substrates. The experimental results indicate that the Mo thin films with promising properties can grow on different flexible substrates. The favourable substrate material is stainless steel foil. By post-annealing, the average sheet resistance of Mo thin film on stainless steel foil substrate reduces to 2.05 Ω/sq.

Abstract: (1-x)SnO₂-xZn₂SnO₄ composite ceramics were prepared by traditional ceramic processing and the varistor, dielectric properties were investigated. With increasing Zn₂SnO₄ content, the breakdown electrical field E_B and nonlinear coefficient α reaches the minimum of 6.9 V/mm and 2.5 at x=0.206, respectively. In the dielectric spectra, the relative dielectric constant ε_r exhibits strong frequency dependent character and at 40 Hz, ε_r for the sample of x=0.206 reaches a maximum as high as 3×10⁴. In the frequency region lower than 1 kHz, accompanied by the sharp increase of dielectric loss at 40 Hz, ε_r is depressed and a dielectric peak is presented in the spectra with increasing bias voltage. In the low electrical current range of 1.37-20 μA, The barrier height φ_B about 1.0 eV are obtained and it is found that φ_B decreases with increasing measuring current for each sample. Based on the results, the varistor behavior with high dielectric constant is explained by the Schottky barriers at grain boundaries.

Abstract: Graphene is a kind of new carbon material with isomer. Its basic structure is composed of six carbon atoms in closed loop structure. In order to make graphene with excellent properties in practical application, people have proposed various methods of preparing grapheme. Graphene shows promising applications in solar cell. This paper introduced preparation and applications of graphene in the high-tech fields.

Abstract: Linear and kinked bulky monomers were incorporated into the main chain of a polyimide in order to investigate the effect of kinked versus linear polymers on membrane properties such as water uptake and proton conductivity. Polymers prepared using linear 1,4-bis (4-aminophenoxy)-naphthyl-2,7-disulfonic acid (BAPNDS), SPI-N, and using kinked 2,2’-bis (p-aminophenoxy)-1,1’-binaphthyl-6,6’-disulfonic acid (BNDADS), SPI-BN, were cast into membranes. All the copolymers showed excellent solubility and good film-forming capability. Membranes are thermally stable up to 300 °C under air. For SPI-BN, the nonplanar binaphthyl group result in polymer chain relaxation and produce large water uptake. However, the conductivity of kinked, SPI-BN membranes is lower than those prepared from SPI-N for a given IEC but water uptakes are higher. This might be related to substitution position of the sulfonic acid groups and the microstructure. Sulfonic acid group were located at the same side of the main chain will be favorable for forming hydrophilic clusters, thus better proton conductivity performance would be achieved.

Abstract: For achieving high-temperature resistance and a broadband of microwave transmission, ceramic metamaterials consisting of fused silica ceramic substrates and electrically conductive networks/ arrays are desirable. A new strategy of fabricating the fused silica metamaterials is to combine the low temperature co-fired ceramic (LTCC) technique with a method of ceramic joining via green tapes. The important part of the new strategy lies in the preparation of suitable green tapes that are co-firable with a conductive silver-based film/strip and have a strong affinity to the fused silica substrates. Therefore, in this paper, three green tape materials were prepared and intensively characterised using scanning electron microscopy, x-ray diffraction, dilatometry, dielectric measurement, etc. It was found that the tape materials were based on dielectric glasses and crystalline phases of major eulytite and minor cristobalite, leading to rather low levels of dielectric constant (<6) and loss tangent (in the order of 10^-3). The three tape materials also had different levels of thermal expansion coefficients, co-firability with a conductive silver-based paste, and bondability to the fused silica substrates. These findings suggest that one can achieve desirable ceramic matematerials with well-controlled shapes and dimensions of the condutive networks/arrays after properly laminating the green tapes between the fused silica substrates.

Abstract: Fluorine and silicon resin has excellent thermal stability properties. The thermal stability properties of polymers modified by fluorine and silicon resin can be improved. In this paper, fluorine and silicon resin has been prepared by (1,3,5-tris(trifluoropropylmethyl)-cyclotrisiloxane and 3-aminopropyltriethoxysilane. The FTIR spectra and the 1H NMR spectrum showed the structure of fluorine silicon resin. The thermo gravimetric traces indicated that fluorine silicon resin had improved the thermal stability properties of epoxy matrix resin significantly. The temperature of decomposition velocity of unmodified epoxy matrix resin and modified epoxy matrix resin began to increase rapidly were 356oC, 375oC respectively. The final weight fraction of unmodified epoxy matrix resin and modified epoxy matrix resin were 4.6%, 6.5% , respectively. The temperature of the maximum rate of degradation were 398oC, 420oC, respectively.