Effect of Fe content on the thermal stability and dynamic mechanical behavior of NdAlNiCu bulk metallic glasses

The dependence of microstructure and thermal stability on Fe content of bulk Nd60Al10Ni10Cu20-xFex (0 less than or equal to x less than or equal to 20) metallic glasses is investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and high-resolution transmission electron micrograph (HRTEM). All samples exhibit typical amorphous feature under the detect limit of XRD, however, HRTEM results show that the microstructure of Nd60Al10Ni10Cu20-xFex alloys changes from a homogeneous amorphous phase to a composite structure consisting of clusters dispersed in amorphous matrix by increasing Fe content. Dynamic mechanical properties of these alloys with controllable microstructure are studied, expressed via storage modulus, the loss modulus and the mechanical damping. The results reveal that the storage modulus of the alloy without Fe added shows a distinct decrease due to the main a relaxation. This decrease weakens and begins at a higher temperature with increasing Fe content. The mechanism of the effect of Fe addition on the microstructure and thermal stability in this system is discussed in terms of thermodynamics viewpoints. (C) 2004 Elsevier B.V. All rights reserved.

Displacive To Order-Disorder Two-Step Phase Transition Model For Para-Ferroelectric Transition

Two-step phase transition model, displacive to order-disorder, is proposed. The driving forces for these two transitions are fundamentally different. The displacive phase transition is one type of the structural phase transitions. We clearly define the structural phase transition as the symmetry broking of the unit cell and the electric dipole starts to form in the unit cell. Then the dipole-dipole interaction takes place as soon as the dipoles in unit cells are formed. We believe that the dipole-dipole interaction may cause an order-disorder phase transition following the displacive phase transition. Both structural and order-disorder phase transition can be first-order or second-order or in between. We found that the structural transition temperatures can be lower or equal or higher than the order-disorder transition temperature. The para-ferroelectric phase transition is the combination of the displacive and order-disorder phase transitions. It generates a variety of transition configurations along with confusions. In this paper, we discuss all these configurations using our displacive to order-disorder two-step phase transition model and clarified all the confusions.

Scanning tunneling spectroscopic studies on high-temperature superconductors and Dirac materials

This thesis details the investigations of the unconventional low-energy quasiparticle excitations in electron-type cuprate superconductors and electron-type ferrous superconductors as well as the electronic properties of Dirac fermions in graphene and three-dimensional strong topological insulators through experimental studies using spatially resolved scanning tunneling spectroscopy (STS) experiments.

Magnetic-field- and temperature-dependent evolution of the spatially resolved quasiparticle spectra in the electron-type cuprate La_0.1Sr_0.9CuO₂ (La-112) T_C = 43 K, are investigated experimentally. For temperature (T) less than the superconducting transition temperature (TC), and in zero field, the quasiparticle spectra of La-112 exhibits gapped behavior with two coherence peaks and no satellite features. For magnetic field measurements at T < TC, first ever observation of vortices in La-112 are reported. Moreover, pseudogap-like spectra are revealed inside the core of vortices, where superconductivity is suppressed. The intra-vortex pseudogap-like spectra are characterized by an energy gap of V_PG = 8.5 ± 0.6 meV, while the inter-vortex quasiparticle spectra shows larger peak-to-peak gap values characterized by Δ_pk-pk(H) >V_PG, and Δ_pk-pk (0)=12.2 ± 0.8 meV > Δ_pk-pk (H > 0). The quasiparticle spectra are found to be gapped at all locations up to the highest magnetic field examined (H = 6T) and reveal an apparent low-energy cutoff at the V_PG energy scale.

Magnetic-field- and temperature-dependent evolution of the spatially resolved quasiparticle spectra in the electron-type "122" iron-based Ba(Fe_1-xCo_x)₂A_s2 are investigated for multiple doping levels (x = 0.06, 0.08, 0.12 with T_C= 14 K, 24 K, and 20 K). For all doping levels and the T < T_C, two-gap superconductivity is observed. Both superconducting gaps decrease monotonically in size with increasing temperature and disappear for temperatures above the superconducting transition temperature, T_C. Magnetic resonant modes that follow the temperature dependence of the superconducting gaps have been identified in the tunneling quasiparticle spectra. Together with quasiparticle interference (QPI) analysis and magnetic field studies, this provides strong evidence for two-gap sign-changing s-wave superconductivity.

Additionally spatial scanning tunneling spectroscopic studies are performed on mechanically exfoliated graphene and chemical vapor deposition grown graphene. In all cases lattice strain exerts a strong influence on the electronic properties of the sample. In particular topological defects give rise to pseudomagnetic fields (B ~ 50 Tesla) and charging effects resulting in quantized conductance peaks associated with the integer and fractional Quantum Hall States.

Finally, spectroscopic studies on the 3D-STI, Bi₂Se₃ found evidence of impurity resonance in the surface state. The impurities are in the unitary limit and the spectral resonances are localized spatially to within ~ 0.2 nm of the impurity. The spectral weight of the impurity resonance diverges as the Fermi energy approaches the Dirac point and the rapid recovery of the surface state suggests robust topological protection against perturbations that preserve time reversal symmetry.

Investigations of noise and of quantum interference in proximity effect bridges

This work reports investigations upon weakly superconducting proximity effect bridges. These bridges, which exhibit the Josephson effects, are produced by bisecting a superconductor with a short (<1µ) region of material whose superconducting transition temperature is below that of the adjacent superconductors. These bridges are fabricated from layered refractory metal thin films whose transition temperature will depend upon the thickness ratio of the materials involved. The thickness ratio is changed in the area of the bridge to lower its transition temperature. This is done through novel photolithographic techniques described in the text, Chapter 2.

If two such proximity effect bridges are connected in parallel, they form a quantum interferometer. The maximum zero voltage current through this circuit is periodically modulated by the magnetic flux through the circuit. At a constant bias current, the modulation of the critical current produces a modulation in the dc voltage across the bridge. This change in dc voltage has been found to be the result of a change in the internal dissipation in the device. A simple model using lumped circuit theory and treating the bridges as quantum oscillators of frequency ω = 2eV/h, where V is the time average voltage across the device, has been found to adequately describe the observed voltage modulation.

The quantum interferometers have been converted to a galvanometer through the inclusion of an integral thin film current path which couples magnetic flux through the interferometer. Thus a change in signal current produces a change in the voltage across the interferometer at a constant bias current. This work is described in Chapter 3 of the text.

The sensitivity of any device incorporating proximity effect bridges will ultimately be determined by the fluctuations in their electrical parameters. He have measured the spectral power density of the voltage fluctuations in proximity effect bridges using a room temperature electronics and a liquid helium temperature transformer to match the very low (~ 0.1 Ω) impedances characteristic of these devices.

We find the voltage noise to agree quite well with that predicted by phonon noise in the normal conduction through the bridge plus a contribution from the superconducting pair current through the bridge which is proportional to the ratios of this current to the time average voltage across the bridge. The total voltage fluctuations are given by <V^2(f ) > = 4kTR^2_d I/V where R_d is the dynamic resistance, I the total current, and V the voltage across the bridge . An additional noise source appears with a strong 1/f^(n) dependence , 1.5 < n < 2, if the bridges are fabricated upon a glass substrate. This excess noise, attributed to thermodynamic temperature fluctuations in the volume of the bridge, increases dramatically on a glass substrate due to the greatly diminished thermal diffusivity of the glass as compared to sapphire.

Stability against crystallization and spectroscopic properties of Tm3+ doped fluorophosphate glasses

Fluorophosphate glasses with various content of Al(PO3)(3) were prepared. With the increment of Al(PO3)(3) content, density decreases while refractive index increases, and transition temperature, crystallization peak temperature and melt temperature increase which were suggested by differential scanning calorimetry. These glasses exhibit the best stability against crystallization with 7-9 mol'Yo Al(PO3)(3) content. Normalized Raman spectra were used to analyze structure and phonon state. The increment of Al(PO3)(3) content does not affect phonon energy but results in the augment of phonon density. Absorption spectra were measured. H-3(6) -> F-3(4) transition exhibits absorption at L band of the third communication window. Compared with the energy of Tm3+ excited states in other glass system, F-3(4) energy of Tm3+ in these glasses is considerable higher and H-3(4) energy is considerable lower, and it can be predicted that emission band of H-3(4) -> F-3(4) transition is close to the amplified band of gain-shift Tm3+ doped fiber amplifier. Analyses of Judd-Ofelt theory suggest when Al(PO3)(3) content is no more than 7 mol%, Judd-Ofelt parameters Omega(t) and the lifetime of H-3(4) energy level of TM3+ vary little with the increment of Al(PO3)(3) content, and when Al(PO3)(3) content is more than 7 mol%, Omega(2) and Omega(6) increase and radiative lifetime of H-3(4) energy level of Tm3+ drops sharply with the increment of Al(PO3)(3) content. (c) 2006 Elsevier B.V. All rights reserved.

Preparação, caracterização e avaliação de nanocompósitos de PBAT/amido de milho e vermiculita organofilizada

Nos últimos 20-30 anos polímeros biodegradáveis vêm sendo estudados e desenvolvidos e atualmente já são comercializados. Contudo, o custo, a processabilidade e algumas propriedades ainda dificultam a penetração desses polímeros no mercado e a competição com as chamadas commodities. Não são poucos os autores que se dedicam a desenvolver aditivos e formulações para superar essas limitações. Desta forma, esta Tese se dedicou ao desenvolvimento de compósitos de Ecobras, fabricado pela Basf e comercializado pela Corn Products, utilizando como carga mineral resíduo da extração da bauxita, no município de Santa Luzia/PB, o qual consiste em sua totalidade de vermiculita. Esta vermiculita foi quimicamente modificada com sais de alquil fosfônio para melhorar a compatibilidade com a matriz polimérica e também espaçar as camadas de aluminossilicato. De fato, a modificação com o brometo de hexadecil tributil fosfônio resultou na esfoliação da vermiculita tornando-a potencialmente apropriada para a obtenção de nanocompósitos. A preparação dos compósitos foi realizada pelo método de intercalação no estado fundido e foram comparadas a utilização da câmara interna de mistura e da mini extrusora de dupla rosca, sendo esta última mais eficaz na dispersão da vermiculita, conforme revelado pela microscopia eletrônica de varredura, difração de raios-X e reometria de placas. O grau de dispersão também foi influenciado pela estrutura química do modificador da vermiculita e pelo teor dessa carga incorporada à matriz. Teores mais elevados levaram a formação de aglomerados, enquanto a modificação da carga implicou na formação de micro e nanocompósitos. Ainda houve alterações das propriedades térmicas com aumento dos valores da temperatura de transição vítrea, de cristalização e fusão, embora o grau de cristalinidade tenha sido mantido. Nitidamente, foram obtidos materiais mais rígidos, com maior módulo e menor capacidade de deformação. Cerca de 58% de perda de massa foi observada para os micro e nanocompósitos obtidos após 17 semanas de enterro em solo simulado para avaliação da biodegradabilidade, valor bem próximo ao Ecobras puro. De modo geral, a incorporação das diferentes vermiculitas retardou nas primeiras semanas a biodegradação, provavelmente em função de modificações na estrutura cristalina, conforme sugerido pelos maiores valores de temperatura de fusão observados durante o acompanhamento do processo de biodegradação. No entanto, após 7 semanas os perfis de biodegradação dos micro e nanocompósitos se aproximaram bastante do Ecobras puro. Desta forma, foi possível nesta Tese obter um nanocompósito de Ecobras com vermiculita modificada com brometo de hexadecil fosfônio utilizando ferramentas comuns de processamento no estado fundido com biodegradabilidade próxima ao polímero de partida, porém mais rígido e menos deformável

Spin-cluster effect and lattice-deformation-induced Kondo effect, spin-glass freezing, and strong phonon scattering in La 0.7 Ca 0.3Mn 1-xCr xO 3

Besides the Kondo effect observed in dilute magnetic alloys, the Cr-doped perovskite manganate compounds La0.7 Ca0.3 Mn1-x Crx O3 also exhibit Kondo effect and spin-glass freezing in a certain composition range. An extensive investigation for the La0.7 Ca0.3 Mn1-x Crx O3 (x=0.01, 0.05, 0.10, 0.3, 0.6, and 1.0) system on the magnetization and ac susceptibility, the resistivity and magnetoresistance, as well as the thermal conductivity is done at low temperature. The spin-glass behavior has been confirmed for these compounds with x=0.05, 0.1, and 0.3. For temperatures above Tf (the spin-glass freezing temperature) a Curie-Weiss law is obeyed. The paramagnetic Curie temperature θ is dependent on Cr doping. Below Tf there exists a Kondo minimum in the resistivity. Colossal magnetoresistance has been observed in this system with Cr concentration up to x=0.6. We suppose that the substitution of Mn with Cr dilutes Mn ions and changes the long-range ferromagnetic order of La0.7 Ca0.3 MnO3. These behaviors demonstrate that short-range ferromagnetic correlation and fluctuation exist among Mn spins far above Tf. Furthermore, these interactions are a precursor of the cooperative freezing at Tf. The "double bumps" feature in the resistivity-temperature curve is observed in compounds with x=0.05 and 0.1. The phonon scattering is enhanced at low temperatures, where the second peak of double bumps comes out. The results indicate that the spin-cluster effect and lattice deformation induce Kondo effect, spin-glass freezing, and strong phonon scattering in mixed perovskite La0.7 Ca0.3 Mn1-x Crx O3. © 2005 American Institute of Physics.

Anomalous temperature dependence of photoluminescence from a-C : H film deposited by energetic hydrocarbon ion beam

The temperature dependence of photoluminescence (PL) from a-C:H film deposited by CH3+ ion beam has been performed and an anomalous behavior has been reported. A transition temperature at which the PL intensity, peak position and full width at the half maximum change sharply was observed. It is proposed that different structure units. at least three, are responsible for such behavior. Above the transition point. increasing temperature will lead to the dominance of non-radiative recombination process, which quenches the PL overall and preferentially the red part, Possible emission mechanisms have been discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.

Temperature dependence of the optical properties of InAs/GaAs self-organized quantum dots with bimodal size distribution

In this work we report the optical and microscopic properties of self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy on (1 0 0) oriented GaAs substrates. A distinctive double-peak feature of the PL spectra from quantum dots has been observed, and a bimodal distribution of dot sizes has also been confirmed by scanning tunneling microscopy (STM) image for uncapped sample. The power-dependent photoluminescence (PL) study demonstrates that the distinctive PL emission peaks are associated with the ground-state emission of islands in different size branches. The temperature-dependent PL study shows that the PL quenching temperature for different dot families is different. It is shown that the coupling between quantum dots plays a key role in unusual temperature dependence of QD photoluminescence. In addition, we have tuned the emission wavelength of InAs QDs to 1.3 mu m at room temperature. (C) 2000 Elsevier Science B.V. All rights reserved.

Electron injection assisted phase transition in a nano-Au-VO2 junction

The semiconductor-metal transition of vanadium dioxide (VO2) thin films epitaxially grown on C-plane sapphire is studied by depositing Au nanoparticles onto the thermochromic films forming a metal-semiconductor contact, namely, a nano-Au-VO2 junction. It reveals that Au nanoparticles have a marked effect on the reduction in the phase transition temperature of VO2. A process of electron injection in which electrons flow from Au to VO2 due to the lower work function of the metal is believed to be the mechanism. The result may support the Mott-Hubbard phase transition model for VO2.

Thermodynamic investigation on properties of gasohol

The heat capacities (C-p) of three types of gasohol (which consisted of 20 wt % ethanol and 80 wt % unleaded gasoline 93(#) (system S1), 30 wt % ethanol and 70 wt % unleaded gasoline 931 (system S2), 40 wt % ethanol and 60 wt % unleaded gasoline 930 (system S3), where "93(#)" denotes the octane number) were measured by adiabatic calorimetry in the temperature range of 80320 K. A glass transition was observed at 94.24, 95.15, and 95.44 K for system S1, S2, and S3, respectively. A solid-solid phase transition and solid-liquid phase transition were observed at 135.18 and 151.30 K for system S1, 131.82 and 152.10 K for system S2, and 121.29 and 155.09 K for S3, respectively. The polynomial equations for C, with respect to the thermodynamic temperature (T), and with respect to the content of ethanol (x), were established through the least-squares fitting. The thermodynamic functions and the excess thermodynamic functions of the three samples were derived using these thermodynamic relationships and equations.

Critical behavior of brittle-ductile transition of polymers

We report that the brittle-ductile transition of polymers induced by temperature exhibits critical behavior. When t close to 0, the critical surface to surface interparticle distance (IDc) follows the scaling law: IDc proportional to t(-v) where t = 1 - T/T-BD(m) (T and T-BD(m) are the test temperature and brittle-ductile transition temperature of matrix polymer, respectively) and v = 2/D. It is clear that the scaling exponent v only depends on dimension (D). For 2, 3, and 4 dimension, v = 1, 2/3, and 1/2 respectively. The result indicates that the ID, follows the same scaling law as that of the correlation length (xi), when t approach to zero.

Synthesis and properties of soluble poly[bis(benzimidazobenzisoquinolinones)] based on novel aromatic tetraamine monomers

Four aromatic tetraamine monomers possessing flexible ether linkages were successfully synthesized by nucleophilic aromatic substitution of hydroquinone, 4,4'-dihydroxybiphenyl, 2,2'-bis(4-hydroxyphenyl)propane, and 2,7-dihydroxynaphthalene with 5-chloro-2-nitroaniline, followed by reduction, respectively. With these monomers, a new class of soluble poly[ bis(benzimidazobenzisoquinolinones)] was prepared by a one-step, high-temperature solution polycondensation. The resulting polymers were completely soluble in phenolic solvents and had high inherent viscosities ranging from 1.2 to 1.5 g dL(-1). These polymers had glass transition temperatures in the range of 427-449 degrees C. Thermogravimetric analysis showed that all polymers were thermally stable, with 5% weight loss recorded above 510 degrees C in nitrogen.

Polyimides Derived from 1,4-Bis [3-oxy-(N-aminophthalimide)] Benzene

A novel diamine, 1,4-bis [3-oxy-(N-aminophthalimide)] benzene (BOAPIB), was synthesized from 1,4-bis [3-oxy-(N-phenylphthalimide)] benzene and hydrazine. Its structure was determined via IR, H-1 NMR, and elemental analysis. A series of five-member ring, hydrazine-based polyimides were prepared from this diamine and various aromatic dianhydrides via one-step polycondensation in p-chlorophenol. The inherent viscosities of these polyimides were in the range of 0.17-0.61 dL/g. These polymers were soluble in polar aprotic solvents and phenols at room temperature. Thermogravimetric analysis (TGA) showed that the 5% weight-loss temperatures of the polyimides were near 450 degrees C in air and 500 degrees C in nitrogen. Dynamic mechanical thermal analysis (DMTA) indicated that the glass-transition temperatures (T(g)s) of these polymers were in the range of 265-360 degrees C. The wide-angle X-ray diffraction showed that all the polyimides were amorphous.

Crystallization behavior of poly(epsilon-caprolactone) in poly(epsilon-caprolactone) and poly(vinyl methyl ether) mixtures

The morphological development and crystallization behavior of poly(epsilon-caprolactone) (PCL) in miscible mixtures of PCL and poly(vinyl methyl ether) (PVME) were investigated by optical microscopy as a function of the mixture composition and crystallization temperature. The results indicated that the degree of crystallinity of PCL was independent of the mixture composition upon melt crystallization because the glass-transition temperatures of the mixtures were much lower than the crystallization temperature of PCL. The radii of the PCL spherulites increased linearly with time at crystallization temperatures ranging from 42 to 49 degrees C. The isothermal growth rates of PCL spherulites decreased with the amount of the amorphous PVME components in the mixtures. Accounting for the miscibility of PCL/PVME mixtures, the radial growth rates of PCL spherulites were well described by a kinetic equation involving the Flory-Huggins interaction parameter and the free energy for the nuclei formation in such a way that the theoretical calculations were in good agreement with the experimental data. From the analysis of the equilibrium melting point depression, the interaction energy density of the PVME/PCL system was calculated to be -3.95 J/cm(3).