A prepared pattern with wavelength selection in directional solidification

As crystal growth is a vital link in the long chain of processes leading to state-of-the-art technological devices, a great deal is known about patterns formed at the solid-liquid interface of a growing crystal. However, some basic questions are still unanswered concerning macroscopic features exhibited by a moving solid-liquid interface. Even for the first instability, the cellular instability, a unique steady-state wavelength does not emerge from theory. Furthermore, while wavelength selection is observed in many different materials, its origin is still to be discovered. By breaking continuous rotational symmetry of the flat solid-liquid interface about the pulling direction v, we prepared a cellular pattern with a well-defined wavelength by front propagation into the unstable uniform state. The material is succinonitrile and the rectangular interface geometry is formed by loading it into a flat capillary. The capillaries are chosen to provide a sample thicknessy ₀ = 100n , and width 10y ₀ and 20y ₀. We use a high-resolution directional solidification apparatus and grow the crystal from grain-boundary-free seed crystals. Surprisingly, the shape of the groove next to the uniform state is initially well-described by nearly self-similar Gaussians. This suggests that the initial perturbation of the interface is localized to a region /2 around a groove. A pattern with a well-defined wavelength is established when the half-width of the Gaussians ₀16m is small compared to 80m so there is little overlap between a groove and its predecessor or successor. When overlap is significant, the pattern is time-dependent. These results suggest that wavelength selection in this prepared pattern is a consequence of front propagation of a localized perturbation.     

Copper and Copper Oxide Nanoparticle Formation by Chemical Vapor Nucleation From Copper (II) Acetylacetonate

Crystalline nanometer-size copper and copper (I) oxide particle formation was studied by thermal decomposition of copper acetylacetonate Cu(acac)₂ vapor using a vertical flow reactor at ambient nitrogen pressure. The experiments were performed in the precursor vapor pressure range of P _prec = 0.06 to 44 Pa at furnace temperatures of 431.5°C, 596.0°C, and 705.0°C. Agglomerates of primary particles were formed at P _prec0.1 Pa at all temperatures. At 431.5°C the number mean size of the primary particles increased from D _p = 3.7 nm (with geometric standard deviation _g = 1.42) to D _p = 7.2 nm (_g = 1.33) with the increasing precursor vapor particle pressure from 1.8 to 16 Pa. At 705.0°C the primary particle size decreased from D _p = 24.0 nm (_g=1.57) to D _p = 7.6 nm (_g = 1.54), respectively.At furnace temperatures of 431.5°C and 596.0°C only crystalline copper particles were produced. At 705.0°C the crystalline product of the decomposition depended on the precursor vapor pressure: copper particles were formed at P _prec>10 Pa, copper (I) oxide at P _precleq 1 Pa, and a mixture of the metal and its oxide at intermediate vapor pressures. A kinetic restriction on copper particle growth was shown, which leads to the main role of Cu₂ molecule participation in the particle formation. The formation of copper (I) oxide particles occurs due to the surface reaction of the decomposition products (mainly carbon dioxide). For the explanation of the experimental results, a model is proposed to build a semiempirical phase diagram of the precursor decomposition products.     

Void formation in Nickel during high temperature proton irradiation

Pure Ni foils, doped with He from 0 to 28 appm, were irradiated with protons at temperatures in the range 0.3–0.6 T_m (T_m = melting point in °K) and void formation was studied. The influence of He doping, irradiation temperature and alloying were investigated. For constant He content and proton fluence, void number density and swelling are maximum at about 400°C, while the void size increases with temperature. Most voids are octahedral in shape with no sign of truncation. Helium is required to nucleate voids, and lowering the stacking fault energy by alloying suppresses void formation completely. Present results suggest that void nucleation is inhomogeneous. Some implications of these findings are discussed.     

Generation of microcellular poly(methyl methacrylate) by compressed gases

Abstract

Generation of microcellular poly(methy1 methacrylate) (PMMA) was studied in CO₂ and N₂O at pressures from 2 to 15MPa at three temperatures, 293.2K, 308.2K, and 323.2 K. The average diameter d and average number density N of voids generated by a rapid expansion of compressed gases in PMMA were measured by use of an optical microscope. Effects of gases, temperature, and pressure on the d and N values were examined. Even at pressure below glass transition pressure of PMMA with both gases, voids of diameter being as small as those found at high pressure, 15MPa, were obtained at each temperature. However, the void density of PMMA at lower pressure by both gases was not so good as those obtained at high pressures.     

Voids at the tunnel-soil interface for calculation of ground vibration from underground railways

Voids at the tunnel-soil interface are not normally considered when predicting ground vibration from underground railways. The soil is generally assumed to be continuously bonded to the outer surface of the tunnel to simplify the modelling process. Evidence of voids around underground railways motivated the study presented herein to quantify the level of uncertainty in ground vibration predictions associated with neglecting to include such voids at the tunnel-soil interface. A semi-analytical method is developed which derives discrete transfers for the coupled tunnel-soil model based on the continuous Pipe-in-Pipe method. The void is simulated by uncoupling the appropriate nodes at the interface to prevent force transfer between the systems. The results from this investigation show that relatively small voids () can significantly affect the rms velocity predictions in the near-field and moderately affect predictions in the far-field. Sensitivity of the predictions to void length and void sector angle are both deemed to be significant. The findings from this study suggest that the uncertainty associated with assuming a perfect bond at the tunnel-soil interface in an area with known voidage can reasonably reach and thus should be considered in the design process.     

Void content and interfacial properties of composite laminates under different autoclave cure pressure

With an identical temperature profile, various cure pressures were applied to determine the effect of cure pressure on void distribution and interlaminar shear strength (ILSS) of [0]₁₀ T800/X850 composite laminate. Void shape, distribution, and void content within the composite laminates were characterized using optical microscope. The ILSS was evaluated using short-beam three-point bending tests and their interface debonding failure and fracture surfaces were determined using scanning electron microscope. The experimental results indicated that long strips voids are generated in the low-pressure curing stage. The voids mainly exist in the two forms of rod-like shape and spherical shape, and their number and size decrease as the cure pressure increases. The influence of cure pressure on the void content and ILSS shows a different trend in two stages: when the cure pressure is lower than 0.4 MPa, the ILSS decrease by 5.21% with 1% increase in void content, and their relationship is inversely linear. However, when the cure pressure is higher than 0.4 MPa, the void content is less than 1% and ILSS increase slightly by 1.69% when the cure pressure goes from 0.4 to 0.6 MPa, the influence of cure pressure on void content and ILSS is clearly presented.     

Low-temperature thermal conductivity of an opal + epoxy-resin nanocomposite

The thermal conductivity of an opal + epoxy-resin nanocomposite under 100% filling of first-order opal voids by epoxy resin was measured in the range 5–100 K. For T < T₀ (T₀ is the temperature at which the thermal conductivity of epoxy resin becomes equal to that of amorphous SiO₂ opal spheres, with inclusion of their porosity associated with second-and third-order voids), the thermal conductivity of the opal + epoxy-resin nanocomposite undergoes a sharp decrease, which is qualitatively accounted for by the appearance of Kapitsa heat resistance at the contacts between the amorphous opal spheres and epoxy resin.     

Evaluation of copper Chemical-Vapor-Deposition films on glass and Si(100) substrates

Thin films of pure copper have been deposited on glass and Si(100) substrates using copper acetylacetonate [Cu(acac)₂] and copper HexaFluoroAcetylacetonate [Cu(HFA)₂] sources. A thermal, cold-wall, reduced pressure (3325–5985 Pa) Metal-Organic Chemical Vapor Deposition (MOCVD) process was employed. The effect of H₂O vapor on the grain size, deposition rate, and resistivity was examined. Electrical resistivities of 2.4 cm for copper films deposited on Si(100) and 3.44 cm for copper films deposited on glass at substrate temperatures of 265° C and a [Cu(acac)₂] source temperature of 147° C with the use of H₂O vapor were measured. When [Cu(HFA)₂] was used, the substrate temperature was 385° C and the source temperature was 85° C. An activation energy for the copper film deposition process was calculated to be 22.2 kJ/mol in the case of the [Cu(acac)₂] source. A deposition rate of 11 nm/min was obtained with Cu(acac)₂ as the source and the rate was 44.4 nm/min with the Cu(HFA)₂ source; both were obtained with the use of H₂O vapor. No selectivity was observed with either source for either substrate. The deposited films were fully characterized using XRD, LVSEM, SAXPS, and RBS.     

Integration in the GHP Formalism IV: A New Lie Derivative Operator Leading to an Efficient Treatment of Killing Vectors

In order to achieve efficient calculations and easy interpretations of symmetries, a strategy for investigations in tetrad formalisms is outlined: work in an intrinsic tetrad using intrinsic coordinates. The key result is that a vector field is a Killing vector field if and only if there exists a tetrad which is Lie derived with respect to ; this result is translated into the GHP formalism using a new generalised Lie derivative operator with respect to a vector field . We identify a class of it intrinsic GHP tetrads, which belongs to the class of GHP tetrads which is generalised Lie derived by this new generalised Lie derivative operator in the presence of a Killing vector field . This new operator also has the important property that, with respect to an intrinsic GHP tetrad, it commutes with the usual GHP operators if and only if is a Killing vector field. Practically, this means, for any spacetime obtained by integration in the GHP formalism using an intrinsic GHP tetrad, that the Killing vector properties can be deduced from the tetrad or metric using the Lie-GHP commutator equations, without a detailed additional analysis. Killing vectors are found in this manner for a number of special spaces.     

Photophysical Processes in the Vapor of Carbazole

The characteristics of the rapid and retarded fluorescence of the vapor of carbazole excited by the radiation of a nitrogen laser are studied. The dependences of the intensity and rate of decay of the retarded fluorescence on the exciting-radiation intensity, the temperature, and the pressure of the vapor and foreign gases are used to determine the predominant mechanisms of relaxation of triplet molecules. It is shown that the decay of the retarded luminescence is governed by the competition between the processes of triplet-triplet annihilation that lead to the emission of annihilation retarded fluorescence and intercombination conversion of the triplet molecules to the ground electronic state. The characteristic times of these processes and the lifetimes of the triplet state in the vapor _T are evaluated; the temperature dependence of _T is analyzed in a wide interval of temperatures (77–573 K). It is inferred that in all aggregative states, the reduction in _T with increase in the temperature has a common nature and reflects an increase in the rate of intercombination transition T ₁ — S ₀ with increase in the content of vibrational energy.     

Positron lifetime study of molybdenum neutron-irradiated at various temperatures

Positron Lifetime measurements reveal that neutron irradiation at 350 and 400°C can produce sub-electron microscopic voids in Mo. The results show that the lifetime in voids reaches a saturation value with increasing void size to a dimension less than 20 Å. It is suggested that the difference among known data for τ₂ ? 450 ps can be attributed to differences in the metal-void interface and gaseous conditions within the void.     

面心立方铜晶体中空洞生长与贯通的尺寸效应

利用分子动力学方法模拟沿拉伸方向排布的两个空洞在单轴拉伸作用下的动力学行为.着重研究不同尺寸空洞对其拉伸贯通过程的影响.结果表明,不同尺度的空洞都是通过空洞表面发射位错环长大与贯通的.空洞在弹性阶段沿加载方向缓慢长大,在塑性阶段沿垂直方向生长后形成类八面体形状.随空洞尺寸的减小,临界屈服应力逐渐增大.当半径较大时,位错对称成核、迁移,空洞沿加载方向被拉长,演化过程相似;当半径较小时,位错不对称成核,空洞沿垂直方向被拉长.空洞生长分为弹性变形、独立长大、融合贯通和平稳生长四个阶段.独立生长阶段随尺寸的减小逐渐缩短甚至消失.     

Density Oscillations at the Interface Between Vapor and Liquid

We study the interface between liquid and vapor in the context of the van der Waals theory, considering the non-local free energy functional recently derived by Lebowitz, Mazel, and Presutti from a system of particles in the continuum with Kac potentials. We prove that the density profile between vapor and liquid is monotone when the inverse temperature is between the critical value _c and a second critical value *> _c , becoming oscillatory after * and overshooting the equilibrium density of the liquid phase infinitely often.     

Formation of Ohmic contacts to n-GaAs using ion beam mixing of Tellurium

Ohmic contacts were formed on n-GaAs using thin evaporated layers of Te followed by bombardment of 100 keV Ar⁺ ions. The specific contact resistance _c showed a strong dependence on the ion dose in the range 10¹⁴ to 10¹⁶ ions cm^–2, with higher doses leading to progressively lower specific contact resistance. The substrate temperature during ion bombardment was varied in the range from 25 to 200° C and was found to have only a minor effect on the resultant values of _c. Elevated temperature aging of the Ohmic contacts at 200° C resulted in a progressive increase in the specific contact resistance, independent of either the ion dose or the substrate temperature used for ion beam mixing. Rutherford backscattering studies (RBS) indicate that the Ohmic contact behaviour was due to the in-diffusion of Te and subsequent formation of a heavily doped n ⁺ layer at the Te-GaAs interface.     

Asymptotic degeneracy of the transfer matrix spectrum for systems with interfaces: Relation to surface stiffness and step free energy

Two- and three-dimensional Ising-type systems are considered in the finite-cross-section cylindrical geometry. An interface is forced along the cylinder (strip in 2d) by the antiperiodic or +– boundary conditions. Detailed predictions are presented for the largest asymptotically degenerate set of the transfer matrix eigenvalues. For rough interfaces, i.e., for 0<T<T _c in 2d,T _R<T<T _c in 3d, the eigenvalues are split algebraically, and the spectral gaps are governed by thesurface stiffness coefficient. For rigid interfaces, i.e., 0<T<T _R in 3d, the eigenvalues are split exponentially, with the gaps determined by thestep free energy.     

Investigation on dislocation-based mechanisms of void growth and coalescence in single crystal and nanotwinned nickels by molecular dynamics simulation

Abstract

Molecular dynamics simulations were conducted to elucidate dislocation mechanisms of the void growth and coalescence in single crystal and nanotwinned nickels subjected to uniaxial tension. The simulation results reveal that twin boundary is capable of decreasing the critical stress, suppressing the emission of dislocations and reducing the overall stiffness of the crystal. A size-scale dependence of critical stress is definitely illustrated through stress–strain response, where the larger void size leads to the lower critical stress and strain. It is the successive emissions of leading partials and the subsequent trailing partials that cause the atoms on the void surfaces to escape from the void surfaces continually, and consequently the voids grow to be larger and larger with increasing strain. The voids in the nanotwinned nickel coalesce earlier than those in the single crystal nickel even though the initiation of dislocations in the former is later than that in the latter. Void fraction remains a constant during elastic deformation, while it presents a linear increase with increasing strain during plastic deformation. Evolution of void fraction during void growth and coalescence is independent on void size.     

Neutron radiation damage in zirconium and its alloys

There has been much research and speculation recently on the nature of radiation induced defects in zirconium and its alloys, and in particular on the absence of voids at high fluences and temperatures in the range 0.3 to 0.5 T _m (T _m is the absolute melting temperature). Wolfenden and Farrell¹ have reviewed the evidence and suggest that α-Zr has so far resisted void formation during neutron irradiation because of: (a) the absence of a dislocation (loop or tangle) structure and/or (b) a low insoluble gas (e.g. helium) content.     

Production rate and decay lifetime measurements ofBs0 mesons at LEP usingDs and? mesons

The study of the properties of inclusive production ofD _s mesons and of events in which a and a muon are present in the same jet provides two independent measurements of the probability,f _s ^w , for a heavy quark to hadronize into a strangeB orD meson. The data sample analysed corresponds to 243,000 hadronicZ ⁰ decays. The combined value of these measurements isf _s ^w =0.19±0.06±0.08. From the flight distance distributions ofD _s and of (-lepton) secondary vertices, with the lepton emitted at high transverse momentum relative to the jet axis, two values are obtained for theB _s ⁰ meson lifetime. Combining these measurements with a previous result based on the study ofD _s- events, theB _s ⁰ meson lifetime is measured to be: 0.96±0.37 ps.     

Muonium formation and the “missing fraction” in vapors

The vapor phase fractional polarizations of positive muons thermalizing as the muonium atom (P _M) and in diamagnetic environments (P _D) has been measured in H₂O, CH₃OH, C₆H₁₄, C₆H₁₂, CCl₄, CHCl₃, CH₂Cl₂ and TMS, in order to compare with the corresponding fractions measured in the condensed phases. There is a marked contrast in every case, with the vapor phase results being largely understandable in terms of a charge exchange/hot atom model. Unlike the situation in the corresponding liquids, there is no permanent lost fraction in the vapor phase in the limit of even moderately high pressures (1 atm); at lower pressures, depolarization is due to hyperfine mixing and is believed to be well understood. For vapor phase CH₃OH, C₆H₁₄, C₆H₁₂, and TMS therelative fractions are found to be pressure dependent, suggesting the importance of termolecular hot atom (or ion) reactions in the slowing down process. For vapor phase H₂O and the chloromethanes, the relative fractions are pressure independent. For CCl₄,P _M=P _D0.5 in the vapor phase vs.P _D=1.0 in the liquid phase; fast thermal reactions of Mu likely contribute significantly to this difference in the liquid phase. For H₂O,P _M 0.9 andP _D0.1 in the vapor phase vs.P _D 0.6 andP _M0.2 in the liquid phase. Water appears to be the one unequivocal case where the basic charge exchange/hot atom model is inappropriate in the condensed phase, suggesting, therefore, that radiation induced spur effects play a major role.