Materials Science Forum Vols. 546-549

Abstract: In this work, MgO thin films were prepared by rf magnetron sputtering technique on two different substrates of Si (100) wafers and amorphous glasses. The influence of different deposition conditions such as substrate temperature, Ar pressure, film thicknesses on the crystal structure of MgO thin films were studied. BaTiO3 ferroelectric thin films were subsequently deposited on the MgO films. The XRD results showed that the orientation of MgO films was dependent greatly on the substrate temperature. A highly (100) oriented MgO thin films were obtained at the temperature of 800°C. The crystallographic texture has been deteriorated rapidly as the argon pressure decreased to 1.0 Pa. It has been also found that the film thickness has a great influence on the film orientation. High substrate temperature, high argon pressure and a certain thickness appear to be favorable for formation of a good texture for the MgO films. The structure and microstructure of the BaTiO3 films were various both with deposition conditions and with the crystallographic texture of the MgO. A highly (001) oriented ferroelectric BTO film was obtained on the MgO films with an optimized deposition conditions.

Abstract: Sintering of the Self-Propagating High-Temperature Synthesis (SHS) of β-Si3N4 powder with 6.67 wt.% Y2O3 and 3.33 wt.% Al2O3 as sintering additives has been emphatically investigated using hot-press sintering process. The relative density of hot-pressed β-Si3N4 reached near to the full densification (99.43%) at 1700°C. The similar micrographs with self-reinforcing rod-like β-Si3N4 grains forming an interlocking structure were observed. The better mechanical properties of hot-pressed Si3N4, such as the hardness (16.73GPa), fracture toughness (5.72 MPa·m1/2) and bending strength (611.72MPa) values, were obtained at 1700°C. The results indicate that good sinter ability can be obtained with the cheaply SHS of silicon nitride powder for preparing silicon nitride materials, which will make the cost of silicon nitride materials lowered.

Abstract: In this paper, an intelligent modeling method, which is based on the Tree-Structured fuzzy inference system, is proposed for building a fuzzy dynamic model of a giant magnetostrictive actuator. In this new method, a series-parallel frame is introduced to transform the multi-valued mapping of the hysteresis into a one-to-one mapping so that the fuzzy technique can be used to approximate the characteristic of the hysteresis. The proposed model has a simple architecture, which is of low computational load and can adapt to different operating conditions. Compared the calculating results with the experimental ones, it is found that the fuzzy dynamic model is able to well reproduce the dynamic behavior of the giant magnetostrictive actuator across a wide frequency band (below 30Hz).

Abstract: Vitreous silica, as high temperature resistant material, has not been completely studied with the influence of extreme working conditions due to experimental limitations. In this work, the structure correlations of vitreous silica were investigated by molecular dynamics method at elevated temperatures from 0 K to 4000 K. COMPASS force field was firstly used in simulating vitreous silica. The temperature dependence of volume for vitreous silica was studied and a maximum of volume was found. The calculated density and the thermal expansion coefficient are close to experimental results. The evolutions of structure in thermal history were discussed in detail. The correlations between the average Si-O bond length and the Si-O-Si bond angle is shown in agreement with the studies used other potentials in literatures. It is proved that the COMPASS force field is appropriate for simulating vitreous silica in some extent, especially in depicting the Si-O interaction and the [SiO4] tetrahedron. Finally, the origin of the volume maxima was discussed based on the analysis of the structure.

Abstract: A ceramic thermocouple (CTC) was assembled using SiC as the cathode and MoSi2 as the anode for the temperature test of the aluminum electrolysis. The NiFe2O4 was sprayed on the surface of SiC cathode for increasing the corrosion resistance ability in the cryolite. The corrosion behavior of NiFe2O4 in the cryolite was investigated. The results of the corrosion tests show that the NiFe2O4 has a very low corrosion rate in the cryolite, and a corrosion layer was formed during the corrosion test. The mechanisms of the NiFe2O4 corrosion are reactive corrosion and particles flaking. The thermo emf of SiC-MoSi2 CTC was tested from 25 °C to 1300 °C in air. The results show that thermo emf has a linear relationship with the temperature dependence, and reached 350 mV at 1300 °C. The CTC which consists of SiC-MoSi2 prepared in the same batch has a preferable interchangeability. By holding temperature at 1300 °C, the deviation of thermo emf is 0.5 %, which is better than the criterion of industry thermocouple, 0.75 %. The CTC response time was also analyzed.

Abstract: Multiple degree-of-freedom (DOF) vibration isolation is essential for precision control of space-borne structures and weapon systems. A new design and analysis of actuators employing magnetostrictive material TbDyFe is presented. Then, this paper studies the design and control problems of a six DOF Stewart platform using the concept of cubic configuration. Optimal geometry for the sensor configuration to get best signal is designed. To control the smart structure, a real time computer control system is built. Improved robust adaptive filtering algorithm based on nonlinear constitutive relation proposed in this paper and used in the computer control system. More than 20 dB of vibration attenuation is achieved in real-time experiments.

Abstract: The Tb/Dy ratio of Tb0.30Dy0.70Fe1.95 giant magnetostrictive alloy was shifted away from 0.30/0.70 to 0.36/0.64 to enlarge the operating temperature range. However, the magnetostriction was reduced under low magnetic field and its hysteresis was enlarged with the shift of Tb/Dy ratio. Ho addition improves the magnetostriction under low magnetic field and reduces the hysteresis of TbxDyyHozFe1.95 (x+y+z=1). The crystal structure, lattice constant, Curie temperature, spin reorientation temperature, magnetization, magnetic anisotropy and magnetostriction of TbxDyyHozFe1.95 alloys were investigated. X-ray diffraction patterns demonstrate that all the samples possess MgCu2-type cubic Laves structure. The lattice parameter, Curie temperature and magnetic anisotropy constant K1 decreased with increasing Ho concentration. The hysteresis represented by the width of magnetostriction versus the applied field was reduced due to the addition of Ho. At a compressive stress of 10 MPa and a magnetic field of 1000Oe, the addition of Ho to the ternary TbxDyyFe1.95 compound reduced the width of magnetostriction by 25% for Tb0.296Dy0.472Ho0.232Fe1.95 and by 27% for Tb0.296Dy0.472Ho0.232Fe1.95 compared to the Tb0.36Dy0.64Fe1.95 alloy, while the strains were reduced by only 3% and 9%.

Abstract: In order to obtain quasi one-dimensional nanostructures, various methods have been developed for the preparation and many mechanisms have been put forward such as VS (vapor-solid), VLS (vapor-liquid-solid), SLS (solid-liquid-solid), and so on. In present study, the zinc oxide one-dimensional nanostructures synthesized by carbon reduction route without catalyst isn’t controlled by the conventional VLS mechanism because that there is no droplet at the growth end of the nanostructures. By using a variety of growth times and the ratios of the zinc oxide and the activated carbon, it can be deduced that the liquid zinc island formed firstly and then the ZnO nanostructures growth on them. The morphologies and microstructures of samples were characterized by scanning electron microscope (SEM) observations, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The structure characterization confirms that all the grown one-dimensional materials are hexagonal wurtzite ZnO.

Abstract: The dielectric ceramics comprised of (Zn1-xMgx)TiO3 (x=0.1~0.5, abbreviated to ZMT) were developed by conventional mixed-oxide method combined with a semi-chemical processing. V2O5 was used as sintering aids to lower the sintering temperature of ceramics. The results showed that the hexagonal phase stability region extended from 900 to 1150°C as the amount of magnesium increased. The densification temperature of ZMT ceramics could be reduced from 1200 to 875°C by V2O5 additions and using semi-chemical processing, the relative density of ZMT ceramics with 1.00wt% V2O5 addition reach 95% at 875°C. The dielectric properties of ZMT were measured at different frequencies and the results showed that there existed a decreasing tendency of the loss tangents (tg δ) with increasing measuring frequency. It is concluded that εr=22 and tg δ= 5.7×10-4 can be obtained for ZMT ceramics at sintering temperature 875°C.

Abstract: A modified α-LMS algorithm is presented and is applied to the controller design of active vibration control. A magnetostrictive material actuator is used as experimental equipment for active vibration control in this paper. The merit of the presented method is that it is not necessary to build the mathematical model of magnetostrictive actuator. The experimental result shows that the controller designed is efficient for active vibration control.