Microstructure and erosion resistance of in-situ SiAlON reinforced BN-SiO<Subscript>2</Subscript> composite ceramics

BN-SiO₂-SiAlON composite ceramics were successfully prepared by the means of hot pressed sintering. Xe plasma flow generated by Hall Thruster was used for sputtering the surface of the samples in order to evaluate the plasma erosion resistance. XRD, TEM, SEM, and LSCM were used to characterize the phase composition and morphologies of as-made composite ceramics before and after Xe plasma erosion. The ceramics were composed of h-BN, fused silica, and SiAlON, which maintained structural stability during the process of Xe plasma sputtering. In conclusion, comparing with BN-SiO₂ composite ceramics, the plasma erosion rate of BN-SiO₂-SiAlON composite ceramics decreases significantly at first then rises with the increase of AlN addition. Erosion pits can be observed by using SEM on the surface after plasma sputtering, which demonstrates that the BN grains have dropped off the surface. In addition, mechanical denudation by high-speed Xe ions is recognized as the injury mechanism for the BN-matrix composite materials.     

Morphology study of oriented SmBCO film deposited by MOCVD

c-axis-oriented SmBa_2Cu_3O_7(SmBCO) films have been deposited on(100)- LaA1O_3(LAO)substrate by metal organic chemical vapor deposition(MOCVD) technique.The effects of deposition temperature(T_(dep)) and total pressure(P_(tot)) on the orientation and microstructure of SmBCO films were investigated.The orientation of SmBCO films transformed from α-axis to c-axis with increasing of T_(dep) from 900 to 1 100℃.At T_(dep)=1 050℃,SmBCO films had c-axis orientation and tetragon surface.At P_(tot)~(dep)=400-800 Pa and T_(dep)=1 050 ℃,totally c-axis-oriented SmBCO films were obtained.The R_(dep) of SmBCO films increased firstly and then decreased with increasing P_(tot).The surface of SmBCO films exhibited tetragon morphology at 1 050 ℃ and400 Pa.Maximum thickness of SmBCO film deposited was 1.2μm at P_(tot)= 600 Pa,and the corresponding R_(dep)was 7.2 μm·h~(-1).     

Influencing mechanism and interaction of muscovite on thermal decomposition of ammonium polyphosphate

The interaction mechanism and phase evolution of ammonium polyphosphate(APP)mixed with muscovite(APP/muscovite)were studied by TG,XRD and SEM,respectively,during heating.When the temperature is not higher than 300 ℃,muscovite has no effect on the thermaldecomposition of APP,and the initialdecomposition temperature of APP/muscovite at 283 ℃ is basically the same as the APP at 295 ℃,and the main thermaldecomposition products are polyphosphoric acid and NH_4H_2PO_4 at 300 ℃.The polyphosphoric acid,the decomposition products of APP,can enable K and Siout of muscovite and interact with muscovite chemically to generate Al_2O_3·2SiO_2,α-SiO_2 and phosphates(AlPO_4 and K_5P_3O_(10))compounds during 400 ℃-800 ℃,which own obvious adhesive phenomenon and porous structure with the apparent porosity of 58.4%.Further reactions between phosphates other than reactions among Al_2O_3·2SiO_2 and α-SiO_2 can generate KAlP_2O_7 at 1 000 ℃ and the density of residualproduct is improved by low melting point phosphate filling pore to form relatively dense structure and decrease the apparent porosity to 44.4%.The flame resistant and self-supported ceramic materials are expected to enhance the fire-retarding synergistic effect between APP and muscovite.     

Use of different barium salts to inhibit the thaumasite form of sulfate attack in cement-based materials

We investigated the effects of different barium compounds on the thaumasite form of sulphate attack(TSA)resistance of cement-based materials when they were used as admixtures in mortars.Moreover,we analyzed the inhibition mechanisms within different types of barium salts,namely BaCO_3 and Ba(OH)_2,on the thaumasite formation.The controlcement mortar and mortars with barium salts to cement and limestone weight ratios of 0.5%,1.0%,and 1.5% were immersed in 5%(by weight)MgSO_4 solution at 5 ℃ to mimic TSA.Appearance,mass,and compressive strength of the mortar samples were monitored and measured to assess the generaldegradation extent of these samples.The products of sulphate attack were further analyzed by XRD,FTIR,and SEM,respectively.Experimentalresults show that different degradation extent is evident in allmortars cured in MgSO_4 solution.However,barium salts can greatly inhibit such degradation.Barium in hydroxide form has better effectiveness in protection against TSA than carbonate form,which may be due to their solubility difference in alkaline cement pore solution,and the presence of these barium compounds can reduce the degree of TSA by comparison with the almost completely decomposed controlsamples.     

Controllable preparation and superior rate performance of spinel LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> hollow microspheres as cathode material for lithium-ion batteries

Spinel LiMn₂O₄ microspheres and hollow microspheres with adjustable wall thickness have been prepared using controllable oxidation of MnCO₃ microspheres precursors and following solid reactions with lithium salts. Scanning electron microscopy (SEM) investigations demonstrate that the microsphere morphology and hollow structure of precursors are inherited. The effect of hollow structure properties of as-prepared LiMn₂O₄ on their performance as cathode materials for lithium-ion batteries has been studied. Electrochemical performance tests show that LiMn₂O₄ hollow microspheres with small wall thickness exhibit both superior rate capability and better cycle performance than LiMn₂O₄ solid microspheres and LiMn₂O₄ hollow microspheres with thick wall. The LiMn₂O₄ hollow microspheres with thin wall have discharge capacity of 132.7 mA·h·g^-1 at C/10 (14.8 mA·g^-1) in the first cycle, 94.1% capacity retention at C/10 after 40 cycles and discharge capacity of 116.5 mAh·g^-1 at a high rate of 5C. The apparent lithium-ion diffusion coefficient (D _app) of as-prepared LiMn₂O₄ determined by capacity intermittent titration technique (CITT) varies from 10^-11 to 10^-8.5 cm²·s^-1 showing a regular “W” shape curve plotted with test voltages. The Dapp of LiMn₂O₄ hollow microspheres with thin wall has the largest value among all the prepared samples. Both the superior rate capability and cycle stability of LiMn₂O₄ hollow microspheres with thin wall can be ascribed to the facile ion diffusion in the hollow structures and the robust of hollow structures during repeated cycling.     

<Emphasis Type="Italic">Ab initio</Emphasis> study of doping mechanisms in BaTiO<Subscript>3</Subscript>-BiYO<Subscript>3</Subscript>

A density functional plane-wave pseudopotential method is used to study the doping mechanisms of impurity defects(Bi_Ba, Y_Ti) in BaTiO₃-BiYO₃. Single Bi_Ba and Y_Ti impurities have little structure distortion. Bi forms ionic bond with nearby O atom in single Bi impurity, Y formed [YO₆] octahedral in single Y impurity. However, in the co-doped Bi_Ba and Y_Ti structure, Bi formed three valence bonds with nearby O atom, which causes the large structure distortion. The doped ion makes the mobile of Ti⁴⁺ difficult and loss local ferroelectricity, which will broaden the dielectric constant temperature curve and increase the temperature stability of BaTiO₃ ceramic matrix.     

Effect of pore structures on the electrochemical performance of porous silicon synthesized from magnesiothermic reduction of biosilica

Two kinds of porous silicon(PS) were synthesized by magnesiothermic reduction of rice husk silica(RHS) derived from the oxidization of rice husks(RHs). One was obtained from oxidization/reduction at 500 ℃ of the unleached RHs, the other was synthesized from oxidization/reduction at 650 ℃ of the acidleached RHs. The structural difference of the above PS was compared: the former had a high pore volume(PV, 0.31 cm3/g) and a large specific surface area(SSA, 45.2 m~2/g), 138 % and 17 % higher than the latter, respectively. As anode materials for lithium ion batteries, the former had reversible capacity of 1 400.7 m Ah/g, 987 m Ah/g lower than the latter; however, after 50 cycles, the former had 64.5 % capacity retention(907 m Ah/g), which was 41.2 % higher than the latter(555.7 m Ah/g). These results showed that the electrochemical performance of PS was significantly affected by its pore structures, and low reduction temperature played the key role in increasing its porosity, and therefore improving its cycling performance.     

Influence of deposition pressure on properties of ZnO: Al films fabricated by RF magnetron sputtering

Transparent conductive aluminum doped zinc oxide(ZnO:Al,AZO) films were prepared on glass substrates by rf(radio frequency) magnetron sputtering from ZnO: 3wt% Al_2O_3 ceramic target. The effect of argon gas pressure(PAr) was investigated with small variations to understand the influence on the electrical, optical and structural properties of the films. Structural examinations using X-ray diffraction(XRD) and scanning electron microscopy(SEM) showed that the ZnO:Al thin films were(002) oriented. The resistivity values were measured by four-point probe with the lowest resistivity of 5.76×10~(-4) Ω?cm(sheet resistance=9.6 Ω/sq. for a thickness=600 nm) obtained at the PAr of 0.3 Pa. The transmittance was achieved from ultravioletvisible(UV-VIS) spectrophotometer, 84% higher than that in the visible region for all AZO thin films. The properties of deposited thin films showed a significant dependence on the PAr.     

Synthesis,characterization and thermal stability of CeO<Subscript>2</Subscript> stabilized ZrO<Subscript>2</Subscript> ultra fine nanoparticles via a sol-gel route

CeO₂ stabilized ZrO₂ ultra fine nanoparticles were successfully synthesized via a simple and effective sol-gel synthetic approach by using zirconylchloride octahydrate, cerium nitrate hexahydrate, and citric acid as starting materials. A series of techniques, including X-ray diffraction (XRD), thermogravimetry (TG), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and N₂-sorption analysis, were used to characterize the structure and morphology of the as-prepared samples. XRD studies indicate that the as-synthesized sample is of well crystallized tetragonal phase of CeO₂ stabilized ZrO₂ with high purity. TEM images show that the as-synthesized sample is composed of a large number of fine dispersive nanoparticles with an average size about 10 nm. The as-synthesized tetragonal CeO₂ stabilized ZrO₂ sample was heated at different temperatures in order to evaluate its thermal stability. The exprimental results reveal that the as-synthesized tetragonal CeO₂ stabilized ZrO₂ sample exhibits excellent stability without the occurrence of phase transformation.     

Aging behaviors of silicone rubber composite materials under outdoor environment

We investigated the aging effect on the chemical structure of silicone rubber composite materials under outdoor environment. The variations of low molecular weight siloxanes in silicone rubber were probed by gas chromatography-mass spectrometry during the degradation process. The experimental results indicate that a series of cyclic siloxanes exist in both the virgin and aged silicone rubber samples, while the additional low molecular weight siloxanes (hexamethyl cyclotrisiloxane) only appear in the aged samples. Meanwhile, the total amounts of low molecular weight siloxanes in the aged samples are much less than those in the virgin ones. The loss of low molecular weight siloxanes is induced by the chain scission and depolymerization.