Showing papers on "Phase (matter) published in 2006"

Experimental Observation of Quantum Hall Effect and Berry's Phase in Graphene

Abstract: When electrons are confined in two-dimensional materials, quantum-mechanically enhanced transport phenomena such as the quantum Hall effect can be observed. Graphene, consisting of an isolated single atomic layer of graphite, is an ideal realization of such a two-dimensional system. However, its behaviour is expected to differ markedly from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron–hole degeneracy and vanishing carrier mass near the point of charge neutrality. Indeed, a distinctive half-integer quantum Hall effect has been predicted theoretically, as has the existence of a non-zero Berry's phase (a geometric quantum phase) of the electron wavefunction—a consequence of the exceptional topology of the graphene band structure. Recent advances in micromechanical extraction and fabrication techniques for graphite structures now permit such exotic two-dimensional electron systems to be probed experimentally. Here we report an experimental investigation of magneto-transport in a high-mobility single layer of graphene. Adjusting the chemical potential with the use of the electric field effect, we observe an unusual half-integer quantum Hall effect for both electron and hole carriers in graphene. The relevance of Berry's phase to these experiments is confirmed by magneto-oscillations. In addition to their purely scientific interest, these unusual quantum transport phenomena may lead to new applications in carbon-based electronic and magneto-electronic devices.

Critical features of colossal magnetoresistive manganites

Abstract: Colossal magnetoresistance (CMR) phenomena are observed in the perovskite-type hole-doped manganites in which the double-exchange ferromagnetic metal phase and the charge–orbital ordered antiferromagnetic phase compete with each other. The quenched disorder arising from the inherent chemical randomness or the intentional impurity doping may cause major modifications in the electronic phase diagram as well as in the magnetoelectronic properties near the bicritical point that is formed by such a competition of the two phases. One is the phase separation phenomenon on various time-scales (from static to dynamic) and on various length-scales (from glass-like nano to grain-like micron). The other is the enhanced phase fluctuation with anomalous reduction in the transition temperatures of the competing phases (and hence in the bicritical-point temperature). The highly effective suppression of such a phase fluctuation by an external magnetic field is assigned here to the most essential ingredient of the CMR physics. Such profound and dramatic features as appearing in the bicritical region are extensively discussed in this paper with ample examples of the material systems specially designed for this purpose. The unconventional phase-controls over the competing phases in terms of magnetic/electric fields and photo-excitations are also exemplified.

UV Raman spectroscopic study on TiO2. I. Phase transformation at the surface and in the bulk

Abstract: Phase transformation of TiO2 from anatase to rutile is studied by UV Raman spectroscopy excited by 325 and 244 nm lasers, visible Raman spectroscopy excited by 532 nm laser, X-ray diffraction (XRD), and transmission electron microscopy (TEM). UV Raman spectroscopy is found to be more sensitive to the surface region of TiO2 than visible Raman spectroscopy and XRD because TiO2 strongly absorbs UV light. The anatase phase is detected by UV Raman spectroscopy for the sample calcined at higher temperatures than when it is detected by visible Raman spectroscopy and XRD. The inconsistency in the results from the above three techniques suggests that the anatase phase of TiO2 at the surface region can remain at relatively higher calcination temperatures than that in the bulk during the phase transformation. The TEM results show that small particles agglomerate into big particles when the TiO2 sample is calcined at elevated temperatures and the agglomeration of the TiO2 particles is along with the phase transformat...

Protic Ionic Liquids: Solvents with Tunable Phase Behavior and Physicochemical Properties

Abstract: The phase behavior, including glass, devitrification, solid crystal melting, and liquid boiling transitions, and physicochemical properties, including density, refractive index, viscosity, conductivity, and air−liquid surface tension, of a series of 25 protic ionic liquids and protic fused salts are presented along with structure−property comparisons. The protic fused salts were mostly liquid at room temperature, and many exhibited a glass transition occurring at low temperatures between −114 and −44 °C, and high fragility, with many having low viscosities, down to as low as 17 mPa·s at 25 °C, and ionic conductivities up to 43.8 S/cm at 25 °C. These protic solvents are easily prepared through the stoichiometric combination of a primary amine and Bronsted acid. They have poor ionic behavior when compared to the far more studied aprotic ionic liquids. However, some of the other physicochemical properties possessed by these solvents are highly promising and it is anticipated that these, or analogous protic s...

Synthesis of Nonspherical Colloidal Particles with Anisotropic Properties

Abstract: We describe a promising and flexible technique for fabricating uniform nonspherical particles with anisotropic phase and surface properties. Our approach is based on the seeded polymerization technique in which monomer-swollen particles are polymerized. The polymerization causes a phase separation to occur, giving rise to two-phase nonspherical particles. We show that the elastic contraction of the swollen polymer particles induced by elevated polymerization temperatures plays an important role in the phase separation. Moreover, chemical anisotropy of nonspherical particles can be obtained by using immiscible polymer pairs and by employing surface treatments. Furthermore, we are able to produce amphiphilic dumbbell particles consisting of two different bulbs: hydrophilic poly (ethylene imine)-coated polystyrene and hydrophobic polystyrene. Controlled geometries of these amphiphilic nonspherical particles will allow a wide range of potential applications, such as engineered colloid surfactants.

Substitution-induced phase transition and enhanced multiferroic properties of Bi1−xLaxFeO3 ceramics

Abstract: Single-phase, insulating Bi1−xLaxFeO3 (BLFOx, x=0.05, 0.10, 0.15, 0.20, 0.30, and 0.40) ceramics were prepared. An obvious phase transition from rhombohedral to orthorhombic phase was observed near x=0.30. It is found that the phase transition destructs the spin cycloid of BiFeO3 (BFO), and therefore, releases the locked magnetization and enhances magnetoelectric interaction. As a result, improved multiferroic properties of the BLFO0.30 ceramics with remnant polarization and magnetization (2Pr and 2Mr) of 22.4μC∕cm2 and 0.041emu∕g, respectively, were established.

Microstructure evolution in a Mg–15Gd–0.5Zr (wt.%) alloy during isothermal aging at 250°C

Abstract: The microstructural evolution in a Mg–15Gd–0.5Zr (wt.%) alloy during isothermal aging at 250 °C, has been investigated using transmission electron microscopy. The decomposition of α-Mg supersaturated solid solution (S.S.S.S., cph) in the alloy with increasing aging time is as follows: β″ (D019) → β′(cbco) → β1(fcc) → β(fcc), which is similar to that of Mg–Gd–Y, Mg–Gd–Nd and Mg–Y–Nd alloys, but different from previously reported three stage sequence: S.S.S.S. → β″ (D019) → β′(cbco) → β(fcc). It is found that the metastable β″ and β′ phases coexist in the matrix at the very early stage of aging. Peak age-hardening is attributed to the precipitation of prismatic β′ plates in a triangular arrangement. At the over-aged stage, β1 phase appears to take place via an in situ transformation from a decomposed β′ phase but grows in a direction different from the previous one of β′ phase. Continued aging makes the β1 phase transform in situ to the equilibrium β phase and the orientation relationship between the precipitate and matrix phases is retained through the in situ transformation of the β1 phase.

Catalytic growth of nanowires: Vapor–liquid–solid, vapor–solid–solid, solution–liquid–solid and solid–liquid–solid growth

Abstract: Catalytic growth is a powerful tool to form a variety of wire (whisker) like structures with diameters ranging from just a few nanometres to the millimetre range. A range of phases (gas, solid, liquid, solution and supercritical fluid) have been used for the feeder phase, i.e. the source of material to be incorporated into the nanowire. Solid, liquid, eutectic, alloy and metastable phases have all been invoked to explain the structure of the catalytic particle. Rather than focussing on the differences that lead to the proliferation of an alphabet soup of names for the various growth techniques, this review attempts to focus on the similarities between all of these catalytic growth processes in an attempt to help stimulate a more universal understanding of the phenomenon. The review begins with a precis of the materials from which nanowires have been formed and then proceeds to a discussion of mechanistic aspects.

Bulk and dispersed aqueous phase behavior of phytantriol: effect of vitamin E acetate and F127 polymer on liquid crystal nanostructure.

Abstract: Phytantriol (3,7,11,15-tetramethylhexadecane-1,2,3-triol, PHYT) is a cosmetic ingredient that exhibits similar lyotropic phase behavior to monoolein (GMO), forming bicontinuous cubic liquid crystalline structures (Q(II)) at low temperatures and reversed hexagonal phase (H(II)) at higher temperatures in excess water. Despite these similarities, phytantriol has received little attention in the scientific community. In this study, the thermal phase behavior of the binary PHYT-water and ternary PHYT-vitamin E acetate (VitEA)-water systems have been studied and compared with the behavior of the dispersed cubosomes and hexosomes formed with the aid of a stabilizer (Pluronic F127). The phase behavior and nanostructure were studied using crossed polarized light microscopy (CPLM), differential scanning calorimetry (DSC), and small-angle X-ray scattering (SAXS) techniques. The presence of lipophilic VitEA in the PHYT-water system suppressed the temperature of the Q(II)-to-H(II)-to-L2 transitions, indicating that lipophilic compounds, in relatively small amounts, may have a significant impact on the phase behavior. Increasing the F127 concentration in the phytantriol-based cubosome system did not induce the Q(II)(Pn3m) to Q(II)(Im3m) transition known for the GMO-water system. This indicates a different mode of interaction between F127 and the lipid domains of phytantriol-water systems. Taken together, these results indicate that phytantriol may not only provide an alternative lipid for preparation of liquid crystalline systems in excess water but may also provide access to properties not available when using GMO.

Effect of polymorphic phase transformations in alumina layer on ignition of aluminium particles

Abstract: The mechanism of aluminium oxidation is quantified and a simplified ignition model is developed. The model describes ignition of an aluminium particle inserted in a hot oxygenated gas environment: a scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particle's temperature exceeds the alumina melting point. The model analyses processes of simultaneous growth and phase transformations in the oxide scale. Kinetic parameters for both direct oxidative growth and phase transformations are determined from thermal analysis. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at ...

Theory of Phase Separation in Polymer Nanocomposites

Abstract: The microscopic polymer reference interaction site model theory of polymer nanocomposites composed of flexible chains and spherical nanoparticles has been employed to study second virial coefficients and spinodal demixing over a wide range of interfacial chemistry, chain length, and particle size conditions. For hard fillers, two distinct phase separation behaviors, separated by a miscibility window, are generically predicted. One demixing curve occurs at relatively low monomer−particle attraction strength and corresponds to a very abrupt transition from an entropic depletion attraction-induced phase separated state to an enthalpically stabilized miscible fluid. The homogeneous mixture arises via a steric stabilization mechanism associated with the formation of thin, thermodynamically stable bound polymer layers around fillers. The second demixing transition occurs at relatively high monomer−particle adsorption energy and is inferred to involve the formation of an equilibrium physical network phase with l...

Analysis of amorphous and nanocrystalline solids from their X-ray diffraction patterns.

Abstract: The purpose of this paper is to provide a physical description of the amorphous state for pharmaceutical materials and to investigate the pharmaceutical implications. Techniques to elucidate structural differences in pharmaceutical solids exhibiting characteristic X-ray amorphous powder patterns are also presented. The X-ray amorphous powder diffraction patterns of microcrystalline cellulose, indomethacin, and piroxicam were measured with laboratory XRPD instrumentation. Analysis of the data were carried out using a combination of direct methods, such as pair distribution functions (PDF), and indirect material modeling techniques including Rietveld, total scattering, and amorphous packing. The observation of X-ray amorphous powder patterns may indicate the presence of amorphous, glassy or disordered nanocrystalline material in the sample. Rietveld modeling of microcrystalline cellulose (Avicel® PH102) indicates that it is predominantly disordered crystalline cellulose Form Iβ with some amorphous contribution. The average crystallite size of the disordered nanocrystalline cellulose was determined to be 10.9 nm. Total scattering modeling of ground samples of α, γ, and δ crystal forms of indomethacin in combination with analysis of the PDFs provided a quantitative picture of the local structure during various stages of grinding. For all three polymorphs, with increased grinding time, a two-phase system, consisting of amorphous and crystalline material, continually transformed to a completely random close packed (RCP) amorphous structure. The same pattern of transformation was detected for the Form I polymorph of piroxicam. However, grinding of Form II of piroxicam initially produced a disordered phase that maintained the local packing of Form II but over a very short nanometer length scale. The initial disordered phase is consistent with continuous random network (CRN) glass material. This initial disordered phase was maintained to a critical point when a transition to a completely amorphous RCP structure occurred. Treating X-ray amorphous powder patterns with different solid-state models, ranging from disordered nanocrystalline to glassy and amorphous, resulted in the assignment of structures in each of the systems examined. The pharmaceutical implications with respect to the stability of the solid are discussed.

Crystallization, morphology, and mechanical behavior of polylactide/poly(ε‐caprolactone) blends

Abstract: Optically pure polylactides, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), were blended across the range of compositions with poly(e-caprolactone) (PCL) to study their crystallization, morphology, and mechanical behavior. Differential scanning calorimetry and dynamic mechanical analysis (DMA) of the PLA/PCL blends showed two Tgs at positions close to the pure components revealing phase separation. However, a shift in the tan δ peak position by DMA from 64 to 57°C suggests a partial solubility of PCL in the PLA-rich phase. Scanning electron microscopy reveals phase separation and a transition in the phase morphology from spherical to interconnected domains as the equimolar blend approaches from the outermost compositions. The spherulitic growth of both PLA and PCL in the blends was followed by polarized optical microscopy at 140 and 37°C. From tensile tests at speed of 50 mm/min Young's modulus values between 5.2 and 0.4 GPa, strength values between 56 and 12 MPa, and strain at break values between 1 and 400% were obtained varying the blend composition. The viscoelastic properties (E′ and tan δ) obtained at frequency of 1 Hz by DMA are discussed and are found consistent with composition, phase separation, and crystallization behavior of the blends. POLYM. ENG. SCI., 46:1299–1308, 2006. © 2006 Society of Plastics Engineers

Electric-field-, temperature-, and stress-induced phase transitions in relaxor ferroelectric single crystals

Abstract: Electric-field-induced phase transitions have been evidenced by macroscopic strain measurements at temperatures between 25 degrees C and 100 degrees C in [001](C)-poled (1-x)Pb(Mg1/3Nb2/3)O-3-xPbTiO(3) [(PMN-xPT);x=0.25,0.305,0.31] and (1-x)Pb(Zn1/3Nb2/3)O-3-xPbTiO(3) [(PZN-xPT);x=0.05,0.065,0.085] single crystals. Such measurements provide a convenient way of ascertaining thermal and electrical phase stabilities over a range of compositions and give direct evidence for first-order phase transitions. A pseudorhombohedral (M-A)-pseudo-orthorhombic (M-C)-tetragonal (T) polarization rotation path is evidenced by two first-order-like, hysteretic discontinuities in strain within the same unipolar electric field cycle for PZN-5PT, PMN-30.5PT, and PMN-31PT whereas, in PMN-25PT, a single first-order-like M-A-T transition is observed. This agrees well with in situ structural studies reported elsewhere. Electric-field-temperature (E-T) phase diagrams are constructed showing general trends for M-A, M-C, and T phase stabilities for varying temperatures and electric fields in poled samples over the given range of compositions. The complex question of whether the M-A and M-C states constitute true phases, or rather piezoelectrically distorted versions of their rhombohedral (R) and orthorhombic (O) parents, is discussed. Finally, stress-induced phase transitions are evidenced in [001](C)-poled PZN-4.5PT by application of a moderate compressive stress (< 100 MPa) both along and perpendicularly to the poling direction (longitudinal and transverse modes, respectively). The rotation path is likely R-M-B-O, via a first-order, hysteretic rotation within the M-B monoclinic plane. The results are presented alongside a thorough review of previously reported electric-field-induced and stress-induced phase transitions in PMN-xPT and PZN-xPT.

Excitation of nanoscale vapor bubbles at the surface of gold nanoparticles in water.

Abstract: Intense nonequilibrium femtosecond laser excitation of gold nanoparticles in water leads to a transient heating of the nanoparticles, which decays via heat transfer to the water phase. It is shown that the water temperature rises to near the critical temperature and the water undergoes an explosive evaporation in the subnanosecond range. The formation of vapor bubbles shows a threshold dependence on laser fluence. The nascent nanoscale vapor bubbles change the heat dissipation drastically. The nanoscale structure is resolved directly with a combination of x-ray scattering methods sensitive to the particle lattice expansion and the change in the water structure factor.

Heterogeneous nucleation in and out of pores.

Abstract: We study the nucleation of a new thermodynamic phase in pores and find that the nucleation often proceeds via two steps: nucleation of pore filling, and nucleation out of the pore These two rates have opposing dependencies on pore size, resulting in a pore size at which the nucleation rate of the new phase is maximal This finding is relevant to attempts to design and use porous media to crystallize proteins

Thermal conductivity of diamond-like carbon films

Abstract: The authors report the thermal conductivity (K) of a variety of carbon films ranging from polymeric hydrogenated amorphous carbons (a-C:H) to tetrahedral amorphous carbon (ta-C). The measurements are performed using the 3ω method. They show that thermal conduction is governed by the amount and structural disorder of the sp3 phase. If the sp3 phase is amorphous, K scales linearly with the C–C sp3 content, density, and elastic constants. Polymeric and graphitic films have the lowest K (0.2–0.3W∕mK), hydrogenated ta-C:H has K∼1W∕mK, and ta-C has the highest K (3.5W∕mK). If the sp3 phase orders, even in small grains such as in micro- or nanodiamond, a strong K increase occurs for a given density, Young’s modulus, and sp3 content.

Tungsten carbides and W-C phase diagram

Abstract: The crystal structures of the tungsten monocarbide δ-WC and the disordered lower carbide β-W2C are studied. Using magnetic susceptibility measurements, the hexagonal carbide δ-WC is shown to be stable from 300 to 1200 K. The sequence of phase transformations associated with β-W2C ordering is analyzed. The temperature and composition stability limits of the cubic carbide γ-WC1−x are evaluated, and the first data are presented on the variation of its lattice parameter with composition. An optimized W-C phase diagram is proposed which takes into account detailed structural and phase-equilibrium data for tungsten carbides.

Phase Transition Temperatures and Piezoelectric Properties of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3–BaTiO3 Lead-Free Piezoelectric Ceramics

Abstract: The phase transition temperatures of x(Bi1/2Na1/2)TiO3–y(Bi1/2K1/2)TiO3–zBaTiO3) [x+y+z=1, y:z=2:1] [abbreviate to BNBK2:1(x)] ceramics were investigated using electrical measurements. We discussed the determination of the depolarization temperature, Td, and defined the Td for (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions. We also determined the rhombohedral–tetragonal phase transition temperatures, TR–T, for BNBK2:1(x), and verified them using dielectric and piezoelectric measurements. It was demonstrated that TR–T corresponded with Td at x=0.94. The existence of an intermediate phase with ferroelectric and antiferroelectric properties at temperatures higher than the Td around the morphotropic phase boundary (MPB) was also revealed.

Ab initio prediction of half-metallic properties for the ferromagnetic heusler alloys Co2MSi (M=Ti,V,Cr)

Abstract: By means of density functional calculations, the magnetic and electronic properties and phase stabilities of the Heusler compounds Co2MSi (with M=Ti,V,Cr,Mn,Fe,Co,Ni) were investigated. Based on the calculated results, we predict the ferromagnetic phases of the compounds Co2TiSi, Co2VSi, and Co2CrSi to be half metals. Of particular interest is Co2CrSi because of its high density of majority-spin states at Fermi energy in combination with a reasonably high estimated Curie temperature of 747K. The compounds Co2TiSi and Co2VSi are thermodynamically stable, whereas Co2CrSi is of a metastable phase which might be stabilized by suitable experimental techniques.

Understanding the self-assembly of charged nanoparticles at the water/oil interface

Abstract: We present a thermodynamic evaluation of the self-assembly of charged nanometer-sized particles at the water/oil interface. The chemical potentials of the nanoparticles in the bulk (aqueous) phase and at the water/oil interface are calculated taking into account interfacial energies, van der Waals interactions, and electrostatic repulsions. An isotherm of the interfacial particle density as a function of the surface charge density on the particles is obtained and compared with experimental results on gold and CdTe nanoparticles self-assembled at the water/heptane interface. Our model provides a semi-quantitative explanation for the spontaneous self-assembly of several types of metallic and semiconducting charged nanoparticles upon reduction of their surface charge.