First-Principles Study of the High-Temperature Behaviors of the Willemite-Ⅱ and Post-Phenacite Phases of Silicon Nitride

The structural and elastic properties of the recently-discovered wⅡ- and δ-Si_3N_4 are investigated through the plane-wave pseudo-potential method within ultrasoft pseudopotentials.The elastic constants show that wⅡ- and δ-Si_3N_4 are mechanically stable in the pressure ranges of 0-50 GPa and 40-50 GPa,respectively.The α→wⅡ phase transition can be observed at 18.6 GPa and 300 K.The β→δ phase transformation occurs at pressures of 29.6,32.1,35.9,39.6,41.8,and 44.1 GPa when the temperatures are100,200,300,400,500,and 600 K,respectively.The results show that the interactions among the N-2s,Si-3s,3p bands(lower valence band) and the Si-3p,N-2p bands(upper valence band) play an important role in the stabilities of the wⅡ and S phases.Moreover,several thermodynamic parameters(thermal expansion,free energy,bulk modulus and heat capacity) of δ-Si_3N_4 are also obtained.Some interesting features are found in these properties.δ-Si_3N_4 is predicted to be a negative thermal expansion material.The adiabatic bulk modulus decreases with applied pressure,but a majority of materials show the opposite trend.Further experimental investigations with higher precisions may be required to determine the fundamental properties of wⅡ- andδ-Si_3N_4.     

Effect of wafer size on the film internal stress measurement by wafer curvature method

Wafer curvature method has been applied to determine the internal stress in the films using Stoney’s equation. During the film deposition, the wafer fixation on the sample holder will restrict the deformation of the rectangle-shaped wafer, which may result in the stress datum difference along length and width direction. In this paper, the effect of wafer size and the wafer fixation on the TiN film internal stress measured by wafer curvature method was discussed. The rectangle-shaped wafers with different length/width ratios (L/W=1:1, 2:1, 3:1 and 4:1) were fixed as a cantilever beam. After the TiN films deposition, the profiles of the film/wafer were measured using a stylus profilometer and then the internal stress was calculated using the Stoney equation in the film. The results showed that the fixed end of the wafers limited to some degree the curvature of the wafers along the width direction. For film internal stress measured by wafer curvature method, the wafer profile should be scanned along the length direction and the scan distance should be greater than or equal to half of wafer length. When the length/width ratio of the wafer reached 3:1, the wafer curvature and the calculated stress were basically the same at different positions along the length direction. For film internal stress measured by wafer curvature method, it was recommended that the length/width ratio of wafer should be considered to be greater than or equal to 3:1, and the deformed profile was scanned along the length direction.     

Characterization and methanol adsorption of walnut-shell activated carbon prepared by KOH activation

Walnut-shellactivated carbons(WSACs)were prepared by the KOH chemicalactivation.The effects of carbonization temperature,activation temperature,and ratio of KOH to chars on the pore development of WSACs were investigated.Fourier transform infrared spectroscopy(FTIR),X-ray powder diffraction(XRD),and scanning electron microscopy(SEM)were employed to characterize the microstructure and morphology of WSACs.Methanoladsorption performance onto the optimalWSAC and the coal-based AC were also investigated.The results show that the optimalpreparation conditions are a carbonization temperature of 700 ℃,an activation temperature of 700 ℃,and a mass ratio of 3.The BET surface area,the micropore volume,and the micropore volume percentage of the optimalWASC are 1636 m~2/g,0.641 cm~3/g and 81.97%,respectively.There are a lot of micropores and a certain amount of meso-and macropores.The characteristics of the amorphous state are identified.The results show that the optimalWSAC is favorable for methanoladsorption.The equilibrium adsorption capacity of the optimalWSAC is 248.02mg/g.It is shown that the equilibrium adsorption capacity of the optimalWSAC is almost equivalent to that of the common activated carbon.Therefore the optimalWSAC could be a potentialadsorbent for the solar energy adsorption refrigeration cycle.     

Rheological behaviors of fresh cement pastes with polycarboxylate superplasticizer

The rheologicalbehaviors of fresh cement paste with polycarboxylate superplasticizer were systematically investigated.Influentialfactors including superplasticizer to cement ratio(Sp/C),water to cement ratio(w/c),temperature,and time were discussed.Fresh cement pastes with Sp/Cs in the range of 0 to 2.0% and varied W/Cs from 0.25 to 0.5 were prepared and tested at 0,20 and 40 °C,respectively.Flowability and rheologicaltests on cement pastes were conducted to characterize the development of the rheologicalbehavior of fresh cement pastes over time.The exprimentalresults indicate that the initialflowability and flowability retention over shelf time increase with the growth in superplasticizer dosage due to the plasticizing effect and retardation effect of superplasticizer.Higher temperature usually leads to a sharper drop in initialflowability and flowability retention.However,for the cement paste with high Sp/C or w/c,the flowability is slightly affected by temperature.Yield stress and plastic viscosity show similar variation trends to the flowability under the abovementioned influentialfactors at low Sp/C.In the case of high Sp/C,yield stress and plastic viscosity start to decline over shelf time and the decreasing rate descends at elevated temperature.Moreover,two equations to roughly predict yield stress and plastic viscosity of the fresh cement pastes incorporating Sp/C,w/c,temperature and time are developed on the basis of the existing models,in which experimentalconstants can be extracted from a database created by the rheologicaltest results.     

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.