Phase behavior and shear alignment in SWNT-surfactant dispersions

The effect of single-walled carbon nanotubes (SWNT) on the phase behavior of the cationic surfactant cetyltrimethylammonium bromide (CTAB) in aqueous solutions is investigated at room temperature. Small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) are used for characterization of bulk dispersions and nanometrically thin films. Additional carbonaceous additives (fullerenes, multi-walled carbon nanotubes, and carbon black) serve as reference systems. It is found that dispersions of carbonaceous additive (excluding fullerenes) at intermediate surfactant concentrations (below the liquid-crystalline region of the native surfactant) induce demixing and macroscopic phase separation in otherwise homogeneous solutions of CTAB. Two coexisting liquid phases of similar CTAB concentrations are observed, with the carbonaceous species residing within the lower phase. At high CTAB concentrations (liquid-crystal region) the SWNTs are found to incorporate into the ordered lyotropic liquid-crystalline phase while preserving the native d-spacing. Investigation of nanometrically thin films at intermediate surfactant concentrations under external shear reveals shear-induced structure (SIS) in the presence of minute amounts of SWNTs. The effect is found to be exclusive to SWNT and does not occur in dispersions of other carbonaceous additives.     

Order causes secondary Bragg peaks in soft materials

Highly ordered soft materials exhibit Bragg peaks that cannot be indexed assuming homogeneous crystal structures. Their origin has been attributed to changes in the crystal structure that are induced by the ordering process such as by application of external fields. This would restrict the use for the generation of highly ordered nano- and microstructured materials where a homogeneous crystal structure is a key requirement. Here, we demonstrate that these Bragg peaks are an inherent property of homogeneous ordered soft materials related to the finite coherence of their crystalline lattice. Their consideration allows a detailed and quantitative analysis of the diffraction patterns of seemingly unrelated materials such as lyotropic liquid-crystalline phases, mesoporous materials, colloidal dispersions, block copolymers, electrorheological fluids and photonic crystals. It further enables us to develop a concise picture of order, line density, field-induced orientation and epitaxial relations for soft-material lattices.     

Determination of the concentration of single-walled carbon nanotubes in aqueous dispersions using UV-visible absorption spectroscopy

Stable, homogeneous, aqueous dispersions of single-walled carbon nanotubes (SWNTs) are prepared by nonspecific physical adsorption of surfactants enhanced by sonication. Upon centrifugation, supernatant and precipitate phases are obtained. The initial weights of the SWNTs and the surfactant are divided between these two phases, and the respective SWNT concentration in each phase is unknown. The focus of this work is on the determination of the true concentration of raw, exfoliated HiPCO SWNTs in the supernatant phase. A UV-visible absorption-based approach is suggested for a direct measurement of the SWNT and the surfactant concentration in the supernatant. UV-visible absorbance spectra of SWNTs-surfactant dispersions and surfactants alone reveal that the intensity of a certain peak, attributed to the pi-plasmon resonance absorption, is unaffected by the presence of most surfactants. A calibration plot is then made by monitoring the intensity of the peak as a function of the true concentration of the exfoliated SWNTs. Thus, we are able to determine the unknown concentration of surfactant-dispersed HiPCO SWNTs in the supernatant solution, simply by measuring its optical absorbance. Moreover, we can now calculate the surfactant efficiency in dispersing SWNTs. Cryogenic-transmission electron microscopy and thermogravimetric analysis techniques are used for the characterization of these dispersions and to complement the UV-visible measurements.     

Finding new phases for precipitate-hardening in platinum and palladium alloys

Precipitate hardening (via ordered phases rather than phase separation) of platinum and palladium can be effective even with a small volume-fraction of the ordered phase [M. Carelse, C.I. Lang, Scripta Materialia 54 (7) (2006) 1311]. The approach is particularly well suited to jewelry alloys which must be 95 wt.% pure and where ordered phases of 7:1 or 8:1 stoichiometries can be formed. We examined eight systems where this approach may lead to new applications: Pt-Al, Pd-Al, Pd-Cu, Pd-Mg, Pd-Nb, Pt-Mo, Pt-V, and Pd-V. In each system, using first-principles-based cluster expansion modeling, we have identified high stoichiometric-ratio phases that are stable. Furthermore, using Monte Carlo simulations, we have estimated the order-disorder transition temperatures to identify experimentally feasible phases. In three cases, the computational results are verified by experiment, suggesting that the remaining predictions are likely to be useful as well.     

Precipitation of an ordered intermetallic phase in WC-10% Ni

The cause of the variability of the intensity of magnetization of the nickel binder phase in cemented carbides was sought by electron diffraction and dark-field microscopy studies of fractured specimens, one of which was partially magnetic and the other almost non-magnetic. A coherent phase in the nickel binder having either a b c c or a CsCI-type structure was found on the boundaries of crack paths. A coherent non-randomly arranged CsCI-type phase was found in an epitaxial relationship with (12.0) planes of WC. The lattice parametera=0.295 nm for both coherent phases. This phase or phases occurs more frequently in non-magnetic than in magnetic nickel binders of hardmetals. When seen near WC grains the ordered phase is often accompanied by randomly arranged tungsten trioxide particles. The implication is that the ordered phase occurs more readily in carbon-deficient surroundings. The phase is postulated to be NiW because large quantities of tungsten are known to form solid solutions with nickel.     

Molecular-level comparison of alkylsilane and polar-embedded reversed-phase liquid chromatography systems

Stationary phases with embedded polar groups possess several advantages over conventional alkylsilane phases, such as reduced peak tailing, enhanced selectivity for specific functional groups, and the ability to use a highly aqueous mobile phase. To gain a deeper understanding of the retentive properties of these reversed-phase packings, molecular simulations were carried out for three different stationary phases in contact with mobile phases of various water/methanol ratios. Two polar-embedded phases were modeled, namely, amide and ether containing, and compared to a conventional octadecylsilane phase. The simulations show that, due to specific hydrogen bond interactions, the polar-embedded phases take up significantly more solvent and are more ordered than their alkyl counterparts. Alkane and alcohol probe solutes indicate that the polar-embedded phases are less retentive than alkyl phases for nonpolar species, whereas polar species are more retained by them due to hydrogen bonding with the embedded groups and the increased amount of solvent within the stationary phase. This leads to a significant reduction of the free-energy barrier for the transfer of polar species from the mobile phase to residual silanols, and this reduced barrier provides a possible explanation for reduced peak tailing.     

The effect of local ordering on the structure phase transition in disordered KTa1/2Nb1/2O3 from first principles studies

First principles density functional theory (DFT) was applied to investigate the effect of B site cations ordering on the structure phase transition in disordered KTa_1/2Nb_1/2O₃ (KTN) solid solution. Several 20- and 40-atom ordered supercells with composition KTN have been studied. The calculated results are in good agreement with the available experimental and theoretical data. The stable ferroelectric and antiferroelectric states are observed in the relaxed ordered KTN supercells, which are caused by the off-center displacements of B site cations. It is found that different chemical order of B site cations result in different phases in the ordered KTN cells. In conclusion, the local ordering of B site cations significantly influence the structure phase transition of disordered KTN.     

A computational study of kinetic phase diagrams for CoPt alloy films during epitaxial growth

We have studied the kinetic processes of the epitaxial growth for CoPt alloy films using the master equation method. The kinetic phase diagrams of CoPt alloy films which show the phase formation conditions during the epitaxial growth are determined. From the kinetic phase diagrams, we find that the [001] ordered structure is much easy to be grown at high temperature while the [100] ordered structure is easy to be grown at low temperature although both the [001] and [100] ordering could be the equilibrium ground states. The atomic deposition, ordering and surface segregation lead to a rich variety of phases in epitaxial growth. The surface segregation is found to enhance the [001] ordering and leads to the formation of the [001] ordered phase at high temperature.     

Annealing behaviour of vapour-deposited Ni-Te thin films

Vapour-deposited phases in the Ni-Te system show interesting properties after annealing. Pulse-annealed thin films of NiTe₂ alloy show the presence of an ordered cubic phase with the lattice parameter a = 0.74 nm (temperature approximately 473 K). On further annealing (temperature approximately 513 K) this ordered structure transforms to a long-period phase and finally a highly disordered structure results. All these transformations are of an irreversible nature. The mechanism of these transformations and the nature of the disorder will be discussed.     

The microstructure of Al-8 wt% Fe-based alloys prepared by rapid quenching from the liquid state

The results of a transmission electron microscope study of the microstructure of splat-quenched Al-8% Fe are described in detail. Two distinct structures, zone A and zone B, are examined in as-quenched samples, and each characterized in terms of the dispersions and types of phases present. The decomposition behaviours of zone A and zone B during annealing at temperatures between 573 and 823 K (300 and 550°C) are investigated and the associated phase transformations determined. The effect on the as-quenched, and annealed, microstructures of adding either 3% Mn or 1% Zr to the alloy is described. The observed microstructures and phase transformations are correlated with micro-hardness measurements.     

Long-Range Ordered Amorphous Atomic Chains as Building Blocks of a Superconducting Quasi-One-Dimensional Crystal

Crystalline and amorphous structures are two of the most common solid-state phases. Crystals having orientational and periodic translation symmetries are usually both short-range and long-range ordered, while amorphous materials have no long-range order. Short-range ordered but long-range disordered materials are generally categorized into amorphous phases. In contrast to the extensively studied crystalline and amorphous phases, the combination of short-range disordered and long-range ordered structures at the atomic level is extremely rare and so far has only been reported for solvated fullerenes under compression. Here, a report on the creation and investigation of a superconducting quasi-1D material with long-range ordered amorphous building blocks is presented. Using a diamond anvil cell, monocrystalline (TaSe₄)₂I is compressed and a system is created where the TaSe₄ atomic chains are in amorphous state without breaking the orientational and periodic translation symmetries of the chain lattice. Strikingly, along with the amorphization of the atomic chains, the insulating (TaSe₄)₂I becomes a superconductor. The data provide critical insight into a new phase of solid-state materials. The findings demonstrate a first ever case where superconductivity is hosted by a lattice with periodic but amorphous constituent atomic chains.     

Phase Transitions in Spin-1/2 Falicov–Kimball Model on a Two-dimensional Triangular Lattice

Phase transitions from low-temperature (ordered) phases to high-temperature (disordered/homogeneous) phases for different fillings are studied on a triangular lattice using the spin-dependent Falicov–Kimball model. Numerical diagonalization and Monte Carlo simulation methods are used to study thermodynamic properties of the system. It has been observed that low-temperature ordered phases persist up to a finite temperature and after reaching a critical temperature (\(T_c\)), homogeneous phases are observed for all parameter space. We have also calculated the temperature dependence of specific heat and observed a sharp jump at \(T_c\) indicating the phase transition, and this \(T_c\) increases with increase in on-site Coulomb correlation U and electron fillings.     

Kinetics of phase transitions in block copolymers

A rich variety of equilibrium phases have been identified in block copolymer melts. In this paper we study the kinetics of phase transitions from one equilibrium state to another, from both experimental and theoretical points of view. The formation of ordered phases is often mediated by non-equilibrium intermediate states. In addition the phase transitions can proceed by either conventional nucleation, or by spinodal decomposition.     

Annealing-induced ordering of bulk nonstoichiometric vanadium carbide

The ordering of bulk nonstoichiometric vanadium carbide is studied by optical microscopy and x-ray diffraction. The results indicate that the formation of V₆C₅ and V₈C₇ superstructures in vanadium carbide is accompanied by the breakdown of the grains of the parent, disordered cubic phase into nanodomains of the ordered phase. The nanodomains are formed on the surface of the grains of the disordered phase. The domain size depends on the heat treatment conditions under which ordered phases are produced in vanadium carbide.     

Alkyl chain conformation of octadecylsilane stationary phases by Raman spectroscopy. 1. Temperature dependence

Raman spectroscopy is used for the first time to probe the effect of temperature on the conformational order of polymeric and monomeric octadecylsilane stationary phases. Spectral data in the ν(C-C) and ν(C-H) regions are interpreted in terms of alkyl chain conformational state and its dependence on temperature. In contrast to the liquidlike disordered state characteristic of these stationary phases at room temperature, at liquid N(2) temperatures, the alkyl chains exist in a more ordered state with a residual level of gauche conformational defects. Systematic studies between -15 and 95 °C reveal more subtle changes in conformational order as ascertained from empirical spectral indicators including the intensity ratios I[ν(a)(CH(2))]/I[ν(s)(CH(2))] and I[ν(C-C)(T)]/I[ν(C-C)(G)]. Plots of these ratios as a function of temperature reveal two distinct regimes of behavior. By extrapolating the linear regions of these plots, a surface "phase transition" temperature of ～20 °C for both surface-confined octadecylsilane stationary phases is estimated that represents subtle changes in alkyl chain conformational order from a more ordered phase to a slightly more disordered phase. The similarity in behavior between the polymeric and monomeric octadecylsilane stationary phases is interpreted as evidence for similar interchain spacing of the alkylsilanes on these silica surfaces.     

Microstructure Evolution of Rapidly Solidified Ti_3Al-2Nb Alloy

It has been found from the analysis of X-ray diffraction and the observation of TEM that the metastable ordered orthorhombic martensite phase in the rapidly solidified Ti3Al-2Nb alloy transforms to various phases at different aging temperatures. When the alloy aged at temperature of 500℃ for 20 min, the phase decomposition into B2 and O phases is related to the stressin the alloy. When the aged temperature above 600℃, the phase transforms to α2 phase by both shearing and chemical ordering     

The mechanism of order-induced intrinsic embrittlement in a stoichiometric Ni4Mo alloy by TEM and 3DAP characterization

To illustrate the mechanism of order-induced intrinsic embrittlement in a stoichiometric Ni₄Mo alloy, TEM and 3DAP were employed to investigate the phase separation during ordering in this paper. It showed that the atomic ordering initiated homogeneously, but some oriented ordered domains can grow preferentially later. Therefore, with atomic ordering, the average ordered domain size continues to increase, which improves the yield strength and ultimate strength due to increasing the critical shear stresses. However, except the growth of ordered phase, different phases with enriched molybdenum and depleted molybdenum were formed after ordering. The depleted molybdenum phase gradually reduces the Mo composition, which deteriorates the ultimate strength, and meantime the strength of grain boundary does not enhance, or even weakens. Hence, the atomic ordering induces embrittlement.     

In Situ Characterization of a Nb and Mo Containing γ‐TiAl Based Alloy Using Neutron Diffraction and High‐Temperature Microscopy

In recent times, novel titanium aluminides containing the bcc β‐phase at high temperatures are being developed for improved hot‐working capabilities, however, predictions of the phase diagrams are merely uncertain. Here we present in‐situ neutron studies, which are particularly sensitive to the atomic disorder in the ordered phases. Complementary laser scanning confocal microscopy is employed for in‐situ microstructural investigations.     

Self‐Assembled Pb Nanostructures on Si(111) Surfaces: From Nanowires to Nanorings

A template‐directed growth method for metals is described in which ordered arrays of super‐long single‐crystalline metal nanowires with atomic‐level‐controlled width, thickness (height), and surface location are prepared by molecular beam epitaxy. Their subsequent examination by in situ scanning tunneling microscopy is also outlined. A phase‐separated stripe pattern composed of alternately a Ge‐rich incommensurate phase and a √3 × √3 phase is first obtained by Ge deposition on Si(111) substrates. Further deposition of Pb on this patterned surface leads to a well‐ordered array of super‐long Pb nanowires. Using the same mechanism, superconducting Pb nanorings can also be fabricated. In this review of our recent work, these Pb single‐crystalline nanowires and nanorings are shown to serve as an ideal platform for the study of superconductivity in reduced dimensionalities. Furthermore, because the widths and spatial distributions of two phases can be precisely controlled by the Ge coverage and substrate temperature, and because a metal will always selectively nucleate on one of two phases, this template‐directed growth method can be applied to a wide range of metals.     

Realization of Regular-Mixed Quasi-1D Borophene Chains with Long-Range Order

The polymorphism of borophene makes it a promising system to realize tunable physical or chemical properties. Various pure borophene phases consisting of quasi-1D boron chains with different widths have been commonly obtained in experimental studies. Here, it is shown that, due to a substrate mediation effect, artificial long-range ordered phases of borophene consisting of different combinations of boron chains seamlessly joined together can be achieved on Ag(100). Scanning tunneling microscopy measurements and theoretical calculations reveal that mixed-chain phases are more stable than the pure phase, and interact only weakly with the substrate. The mixed-chain phases with various proportions of different chains can be well separated based on the crystal direction of the substrate. The successful growth of mixed-chain phases is expected to deepen the impact of substrate tailored synthesis of borophene.