SCALING LAWS FOR PARTICLE BREAKUP IN NOZZLE GENERATED SHOCKS

Conditions for the onset of particle breakup in normal shock waves have been investigated. A normalized particle drag behind the shock has been determined in terms of gas stagnation conditions and particle diameter for a range of gas Mach numbers 1 ≤ M₁ ≤ 5 by introducing appropriately defined particle Knudsen and Reynolds numbers into analytical expressions for the drag coefficient. Numerical computations of the particle drag, normalized with gas stagnation pressure and particle area, indicate a peak at a gas Mach number M₁ ≳ 2.2, The magnitude of the peak was found to decrease with increasing particle diameter and reservoir gas density.

Criteria for the onset of agglomerate breakup were defined in terms of a modified Weber number for the adhesion mechanisms due to Van der Waals forces, electrostatic attraction and adsorbed surface films. These results indicate that larger more closely packed agglomerates made up of smaller constituent particles have a greater tendency to resist breakup for a given set of stagnation conditions and shock Mach number.     

Magnetic properties of 3d-doped TiSe2 and TiTe2 intercalate compounds

The ground-state structures of Ti dichalcogenides are optimized from first principles using the full-potential linearized augmented-plane-wave method. The electronic structure and magnetization in several M_1/3TiSe₂ and M_1/3TiTe₂ (M = Cr, Mn, Fe) systems are studied in the supercell approach. Calculated exchange splitting of Cr2p_3/2 core level in Cr_1/3TiSe₂ agrees well with that observed by X-ray photoelectron spectroscopy; calculated local magnetic moments are consistent with the magnetic susceptibility data. An evidence is given in favour of localized polaron-type states being created in M_xTiSe₂ systems, that accounts for an observed correlation between structure distortion, magnetic moments, and electrical conductivity data.     

A Metallographic Study of the High-Temperature Decomposition of Austenite in Alloy Steels Containing Mo and Cr

Optical and electron microscopy have been used to study the complex microstructures developed during the isothermal decomposition of austenite above 550°C in Fe-4Mo-0.2C and Fe-10Cr-0.4C alloy steels. As the transformation temperature is decreased, the decomposition products change from the disordered growth of nodular alloy pearlites to blocky ferrite structures containing fine dispersions of alloy carbide, and finally to acicular ferrite structures also containing alloy carbide. The branched M₆C and M₂₃C₆ of the high-temperature pearlite is replaced by Mo₂C and M₇C₃ with a fibrous or lath morphology in the lower temperature structures. The decomposition microstructures are explained in terms of a model which takes account of the growth of particular alloy carbides at the interfaces of ferrite allotriomorphs, where the growth mechanism, and hence the morphology, is sensitive to transformation temperature.     

Sur les coefficients de Hall et l'aimantation spontanée des films minces de nickel déposés et étudiés sous ultra-vide

A phenomonological theory of the Hall effect in ferromagnetics shows that, in contrast to the usual interpretation, the study of the experimental curve which represents the Hall resistivity versus the applied field does not permit the Hall coefficients R₀ and R₁ to be determined unless the exact value of the spontaneous magnetization M_s is known.

This theory is shown to fit qualitatively very well with the experimental results and assuming that M_s has the same value as in bulk nickel, the calculated values of R₀ and R₁ for thick films are in good agreement with the tabulated ones for bulk nickel.     

Radiation of X-Rays Using Polarized LiNbO3 Single Crystal in Low-Pressure Ambient Gas

The dependence of X-ray intensity on the pressure and type of ambient gas was investigated for LiNbO₃ single crystals polarized in the c-axis direction at pressures of approximately 1 to 30 Pa. Ionization of surrounding gas molecules by the electric field generated by the crystal led to the production of both positive ions and free electrons. The electrons were accelerated toward a Cu target, radiating both white X-rays and X-rays specific to the crystal or target material by bremsstrahlung. The integrated X-ray intensity per cycle in the energy range 1 to 20 keV showed a local maximum value at a pressure P_max. The logarithm of P_max was proportional to the Boltzmann factor using the first ionization energy of each ambient gas molecule. The value of P_max was found to be independent of the electrical surface area of the crystal. The integrated X-ray intensity was approximated qualitatively by a quadratic function with pressure, which was upwardly convex. It was found that one of the causes of the reduction in X-ray intensity at pressures P > P_max is the adsorption of positive ions generated by the ionization of gas molecules on the negative electric surface. It was also discovered that the lifetime of the X-ray radiation device could be improved when the X-ray radiation case was covered with another hermetically sealed decompression case. The gas with the smallest first ionization energy, with a partial pressure of P_max, was enclosed inside the X-ray radiation case (inner case) and the gas with the largest first ionization energy was enclosed at a suitable pressure between the inner and outer cases.     

EPR and SQUID studies on magnetic properties of SiO2-doped Ni–Zn ferrite nanocomposites

Effects of the silica content and temperature on the magnetic properties of Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites have been studied by electron paramagnetic resonance (EPR) and superconducting quantum interference device (SQUID) technique. The content of TEOS in the starting solution affects the interaction between NiZn ferrite and silica, and then determines the particle size and the EPR properties. In addition, the SQUID properties of the composites were sensitive to particle size. It was observed that the saturation magnetization (M_s), remnant magnetization (M_r) and initial susceptibility (χ_i) decreased with increasing SiO₂ content.     

Inconel 600合金中不同类型晶界处铬的浓度

应用TEM、EDS和EBSD等技术研究了Inconel 600合金在715℃时效过程中不同类型晶界处碳化物的结构、形貌和晶界附近Cr浓度的分布。结果表明,不同类型晶界处碳化物的结构和形貌有较大的不同：在Σ3_c晶界析出的碳化物很少,在Σ3_i晶界析出不规则形状的M₂₃C₆碳化物,在Σ9晶界析出较大的M₂₃C₆碳化物颗粒,在Σ27晶界随机析出粗大的M₇C₃碳化物颗粒。富Cr碳化物在晶界的析出使晶界附近贫铬,在相同的时效条件下晶界的Σ值越高其附近贫铬越严重。随着时效时间的延长各晶界附近贫铬区的宽度不同程度地增大,时效15 h贫铬区的深度最大,时效50 h后深度不同程度地减小。     

Physics and diagnostics of laser ablation plume propagation for high-Tc superconductor film growth

The formation, composition and propagation of laser-produced plasmas used for pulsed laser deposition (PLD) of Y₁Ba₂Cu₃O_7−x have been studied under film growth conditions. Four complementary spatially and temporally resolved in situ diagnostic techniques are applied to characterize the expansion of the laser plume into both vacuum and ambient gases: optical emission and absorption spectroscopy, fast ion probe measurements, and fast photography with a gated, image-intensified charge-coupled detector-array (ICCD) camera system. Transient optical absorption spectroscopy reveals large densities of ground state atoms, ions, and molecules in the plume as well as a slower component to the plume transport than is indicated by the plasma fluorescence and ion current.

Ablation into background gases results in scattering and attenuation of the laser plume. The exponential attenuation of the positive ion flux transmitted through 50–300 mTorr background oxygen is measured and used to define an overall ion-oxygen reaction cross-section σ_i−O2 = 2.3 × 10⁻¹⁶ cm² under the described film growth conditions.

The slowing of the laser plasma and formation of shock structures due to collisions with the ambient gas are described using ion probe measurements and ICCD photographic comparisons of expansion into vacuum and background oxygen. At the pressures used for PLD, distance-time R−t plots derived from the photographs and ion probe waveforms indicate that the higher pressure plume initially expands through the ambient gas in accordance with a drag model (where R = x_f[1 − exp( − βt)]), experiencing little slowing until a visible shock structure forms. Following a transition period, in which the plume appears to have two components, a single-component shock structure propagates in better agreement with a shock, or blast wave (R = ξ₀(E/₀)^1/5t^2/5) model.     

Powder X-ray diffraction, infrared and conductivity studies of AgSbMP3O12 (M = Al, Ga, Fe and Cr)

New Nasicon type of compounds of composition AgSbMP₃O₁₂ (M = Al, Ga, Fe and Cr) are synthesized by solid-state method. All the compounds crystallize in the hexagonal lattice with space group . The infrared spectra of these compounds show characteristic bands due to PO₄ group. The frequency independent conductivity of these compounds shows Arrhenius type behavior and the activation energy for conduction is in the range 0.40–0.55 eV. Frequency independent conductivity (σ_dc) studies and frequency dependent (σ_ac) impedance measurements correlate well. The Cole–Cole plots do not show any spikes on the lower frequency side indicating negligible electrode effects. The activation energies obtained from the plots of log σ_dcT versus 1/T, log σ_ac(0) versus 1/T and log τ versus 1/T are approximately the same. The peak width at half height for electric modulus (M″) plot is 1.24 decades for all samples, which is close to 1.14 decades observed for Debye solid. The height of electric modulus (M″) obtained from the experimental plots are close to that of M″ (max) = C₀/2C indicating the Debye nature of the samples.     

D.c. properties of ZnO thin films prepared by r.f. magnetron sputtering

In the development of ZnO-based varistors the electrical properties of ZnO/Bi₂O₃ junctions and of the two individual oxides are being investigated. Following our recent work on a.c. conductivity in Al---ZnO---Al sandwich structures we currently report d.c. measurements. The structures were prepared by r.f. magnetron sputtering in an argon/oxygen mixture in the ratio 4:1. Capacitance-voltage data confirm that the Al/ZnO interface does not form a Schottky barrier and measurements of the dependence of capacitance on film thickness indicate that the relative permittivity of the films is approximately 9.7. With increasing voltage the current density changed from an ohmic to a power-law dependence with exponent n≈3. Furthermore measurements of current density as a function of reciprocal temperature showed a linear dependence above about 240 K, with a very low activation energy below this temperature consistent with a hopping process. The higher temperature results may be explained assuming a room-temperature electron concentration n₀ and space-charge-limited conductivity, dominated by traps exponentially distributed with energy E below the conduction band edge according to N = N₀exp(−E/kT_t), where k is Boltzmann's constant. Typical derived values of these parameters are: n₀ = 7.2 × 10¹⁶ m⁻³, N₀ = 1.31 × 10⁴⁵ J⁻¹ m⁻³ and T_t = 623 K. The total trap concentration and the electron mobility were estimated to be 1.13 × 10²⁵ m⁻³ and (5.7−13.1) ×10⁻³m²V⁻¹s⁻¹ respectively.     

Synthesis and Characterization of Methacrylic Derivatives as Drug Carriers

p-Methoxyphenyl methacrylate (ESMAA) was copolymerized with methacrylic acid (MAA) in dioxane solutions using azobisisobutyronitrile (AIBN) as an initiator. The compositions of the copolymers were assayed for the aromatic ester content by measuring the ultraviolet (UV)absorbance at 240 nm in dioxane. The monomer reactivity ratios r₁and r₂ for copolymerization of MAA(M₁)and ESMAA (M₂)are 1.52 and 3.01. The glass transition temperatures of MAA-ESMAA copolymers are almost equal to the weight-average values. The drug released from copolymers increases with the composition of methacrylic acid units. The releases of piroxicam from the MAA-co-ESMAA matrices containing 55 mole%MAA in pH 7.4 and 10 buffer solutions are steady for the whole release period, and release profiles matches closely to the disintegration profiles of the MAA-55 copolymer.     

微量Ce元素对高铬高钴型马氏体耐热钢力学性能的影响

在不同条件下对X20Co高钴高铬型马氏体耐热钢进行热处理,用光学显微镜、扫描电镜、X射线衍射仪以及拉伸实验等手段进行表征,研究了微量Ce元素对其微观组织和力学性能的影响。结果表明,在X20Co钢的淬火过程中,添加质量分数为50×10^-6 Ce元素能促进M₆C型碳化物沿晶析出,阻碍晶界迁移,使奥氏体晶粒细化;在回火过程中能抑制M₂₃C₆型碳化物沿晶界聚集长大。同时,添加50×10^-6 Ce元素对X20Co高钴高铬型马氏体耐热钢的室温硬度、室温强度、高温瞬时拉伸强度没有显著的影响,但是使其室温韧性、塑性和高温塑性显著改善。     

Mechanical and electronic properties of C60 under structure distortion studied with density functional theory

B3PW91/6-31G density functional method was employed to investigate the elastic, strength and electronic properties of C₆₀(I_h) in its ground electronic state (X ¹A_g). Most of the properties were examined for larger structure distortions. The over-all elastic constant were derived from the near-equilibrium potential energy curves (PECs) in five independent directions [according to symmetries by 1. D_5d, 2. D_3d, 3. D_2h, 4. C_2h(1), 5. C_2h(2)]. By extension of the distortions as large as the structure of C₆₀ was destroyed, the necessary energies were obtained, which quantitatively illuminated the stability of C₆₀ theoretically. Analytical polynomial fit of the full PECs reproduced the energy data accurately. Time-dependent B3PW91/6-31G analysis showed significant electronic spectra changes associated with the structure distortions. Elongation in the direction of D_5d and compression in that of D_2h encountered potential energy surface cross-linkages, which might be considered as a single electron pump for further application in designs of single electron devices.     

Total energy calculation for the uniform electron gas in the GW approximation

Expressing the self-energy operator, Σ, in the GW approximation as GW (where G is the Green function of a free electron and W is the screened interaction potential), we have calculated the total energy of a uniform electron gas in terms of one-particle state energies and normalization factors describing the one-particle state contributions to the ground state of the system, at different densities r_s. The results of our calculations are in rough coincidence with the RPA (random phase approximation) total energies for r_s = 0.5, 0.7, 1.5, 2.5, 3.0, 4.0 and are in good coincidence for r_s = 1.0, despite differences in basic assumptions. r_s is determined according to 4πr_s³a₀³/3 = 1/n where a₀ is the Bohr radius and n is the number of particles per unit volume. At low densities, r_s > 1, we find there are unphysical solutions of Dyson's equation, with negative normalization factors. They were ignored in our total energy calculations. Such solutions were also obtained by Hedin, et al in 1967 but were not correctly interpreted. Our results have implications for the anticipated application of this methology to real materials.     

共挤出法制备双层中空纤维陶瓷复合膜

中空纤维陶瓷膜具有装填密度高, 传质阻力低, 使用寿命长等优点, 被广泛用于膜分离领域。高度非对称结构的中空纤维膜有利于同时实现高通量与高截留率, 本研究采用共挤出法制备双层中空纤维陶瓷复合膜, 内外层纺丝液分别掺杂平均粒径为1 μm和300 nm的α-Al₂O₃粉体。系统考察了内层纺丝液TiO₂掺杂量、外层纺丝液Al₂O₃/聚醚砜(PESf)质量比和煅烧温度对膜的结构与性能的影响。结果表明, 在内层纺丝液TiO₂掺杂量为2wt%, 外层纺丝液Al₂O₃/PESf质量比为5.60, 烧结温度为1350 ℃的最优条件下, 中空纤维膜断裂负荷为24 N、平均孔径为0.15 μm、去油率为97.5%。     

Comparative study of the elastic properties of polycrystalline aluminum nitride films on silicon by Brillouin light scattering

Brillouin light scattering (BLS) has been used to investigate the elastic properties of polycrystalline AIN films, about 1 μm thick, grown by DC-reactive magnetron sputtering on Si₃N₄ coated (100)-Si substrates. Taking advantage from the detection of a number of different acoustic modes, a complete elastic characterization of the films has been achieved. The elastic constants c₁₁ and c₆₆ have been selectively determined from detection of the longitudinal and of the shear horizontal bulk modes, respectively, travelling parallel to the film surface. The three remaining elastic constants, namely c₄₄, c₃₃ and c₁₃, have been obtained from detection of the Rayleigh surface wave and of the longitudinal bulk wave propagating at different angles from the surface normal. The values of the elastic constants of these sputtered AIN films depend on the deposition conditions and on the microstructural properties of the films, especially oxygen contamination and quality of texture. In the case of the films with the best degree of texture and the lowest oxygen content, the values of the elastic constants are rather close to those previously determined in epitaxial A1N films grown at high temperature by MOCVD. This demonstrates that sputter deposition at relatively low temperature can be used to grow high quality A1N films on Si and is of great importance in view of the integration of these films in the technology of IC semiconductors.     

Rotation numbers for quasi-periodically forced monotone circle maps

Rotation numbers have played a central role in the study of (unforced) monotone circle maps. In such a case it is possible to obtain a priori bounds of the form „ - 1/n ≤(1/n)(y_n - y₀) ≤„ + 1/n, where (1/n)(y_n - y₀) is an estimate of the rotation number obtained from an orbit of length n with initial condition y₀, and „ is the true rotation number. This allows rotation numbers to be computed reliably and efficiently. Although Herman has proved that quasi-periodically forced circle maps also possess a well-defined rotation number, independent of initial condition, the analogous bound does not appear to hold. In particular, two of the authors have recently given numerical evidence that there exist quasi-periodically forced circle maps for which y_n - y₀ - „n is not bounded. This renders the estimation of rotation numbers for quasi-periodically forced circle maps much more problematical. In this paper, a new characterization of the rotation number is derived for quasiperiodically forced circle maps based upon integrating iterates of an arbitrary smooth curve. This satisfies analogous bounds to above and permits us to develop improved numerical techniques for computing the rotation number. Additionally, the boundedness of y_n - y₀ - „n is considered. It is shown that if this quantity is bounded (both above and below) for one orbit, then it is bounded for all orbits. Conversely, if for any orbit y_n - y₀ - „n is unbounded either above or below, then there is a residual set of orbits for which y_n - y₀ - „n is unbounded both above and below. In proving these results a min-max characterization of the rotation number is also presented. The performance of an algorithm based on this is evaluated, and on the whole it is found to be inferior to the integral based method.     

FTIR spectroscopy of hydrogen, carbon monoxide, and methane adsorbed and co-adsorbed on zinc oxide

Adsorption of dihydrogen, carbon monoxide and methane, and co-adsorption of H₂/CO, H₂/CH₄ and CO/CH₄ on zinc oxide was studied by means of Fourier transform infrared spectroscopy. Besides the already known dissociation of dihydrogen and molecular adsorption of CO, methane was found to be adsorbed molecularly on coordinatively unsaturated Zn²⁺ ions. Adsorption lowers the CH₄ symmetry from T_d to C_3v, which is reflected in activation of the v₁ (symmetric stretching) mode and discrete frequency shifts of the v₃ (antisymmetric stretching) and ν₄ (bending) modes. Co-adsorption of the above gases on ZnO having pre-adsorbed hydrogen results, in all cases, in a bathochromic shift of the v(Zn–H) band and a hypsochromic shift of the v(O–H) band, which originally appear at 1710 and 3492 cm⁻¹, respectively. The magnitude of these shifts depends upon the nature of the co-adsorbed gas.