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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 3554-3561 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A low thermal gradient furnace design is described which utilizes Debye–Scherrer geometry for performing high temperature x-ray powder diffraction with synchrotron radiation at medium and high energies (35–100 keV). The furnace has a maximum operating temperature of 1800 K with a variety of atmospheres including oxidizing, inert, and reducing. The capability for sample rotation, to ensure powder averaging, has been built into the design without compromising thermal stability or atmosphere control. The ability to perform high-resolution Rietveld refinement on data obtained at high temperatures has been demonstrated, and data collected on standard Al2O3 powder is presented. Time-resolved data on the orthorhombic to rhombohedral solid state phase transformation of SrCO3 is demonstrated using image plates. Rietveld refinable spectra, collected in as little as 8 s, opens the possibility of performing time-resolved structural refinements of phase transformations. © 1999 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of medicinal chemistry 23 (1980), S. 852-857 
    ISSN: 1520-4804
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6317-6319 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Low-temperature specific-heat measurements were performed between 0.4 and 4 K on Nd–Ba–Cu–O-samples doped with Ca and additional Nd. In these samples, the specific-heat anomaly due to magnetic ordering of the Nd3+ ions as a function of hole concentration behaves similarly to that of oxygen-deficient NdBa2Cu3Ox. In samples with 0〈Tc〈70 K, a broad anomaly due to short range magnetic interaction was found but no indication for long range magnetic ordering could be observed. In the semiconducting samples, 3D-Ising-like anomalies with TN around 1.6 K were measured. In order to explain the behavior of the magnetic anomaly in the oxygen-deficient undoped samples, one could assume that the magnetic interaction depends on lattice parameters or interatomic distances. Our newest results clearly show, however, that the position and shape of the anomaly is mainly controlled by the density of carriers in the superconducting CuO2 planes, and not by structural properties. Thus, the magnetic rare-earth ions are isolated enough from the CuO2 planes not to influence superconductivity (except for Pr), but are not totally decoupled from the rest of the unit cell and the carrier concentration.
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  • 4
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In bulk permanent magnets, the extrinsic properties relating to the microstructure determined the level of hard magnetic properties that is actually achieved. Many researchers have worked on the microstructure property relationships in the Nd2Fe14B (2-14-1) system but, in general, the approach has been to analyze the microstructure of materials which have been shown to have good magnetic properties rather than to study the effect of alloy additions on the metallurgical properties of the system. The microstructure which is obtained under a given set of processing conditions is highly dependent on the solidification behavior of the alloy, as well as grain growth phenomena. Alloy additions which effect the solidification behavior of the melt and then form precipitates which pin grain boundaries, and therefore control grain size, should be valuable in producing the uniform microstructure required for good magnetic properties. A number of refractory metal carbides perform both of these functions. The carbides have a reasonable degree of solubility in the 2-14-1 melt and, under conditions of reasonable rapid solidification, a degree of metastable solubility in the 2-14-1 phase. However, there is very limited equilibrium solid solubility in the 2-14-1 phase so that the intrinsic magnetic properties are not effected. The addition of these materials to 2-14-1 results in a factor or three reduction of the quench rate required to produce amorphous material. In addition, the crystallization temperature of the glass is enhanced leading to enhance nucleation and finer grain size during crystallization. Finally refractory metal carbide precipitates decorate the grain boundaries of the crystallized material inhibiting grain growth. Transition metal carbide formation, solid and liquid solubility, effects on solidification, nucleation, and grain growth will be discussed. © 1996 American Institute of Physics.
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  • 5
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Rapidly solidified samples of Nd9.5Fe84.5B6 with and without 3 at. % TiC were prepared by melt spinning and melt extraction and then annealed in vacuum (3×10−6 Torr) at temperatures from 600 to 750 °C. For alloys melt spun under similar conditions, the overquenched state was achieved at wheel speeds 〉10 m/s for the TiC added alloy while 〉20 m/s was necessary without TiC. The overquenched samples contained a smaller fraction of α-Fe in smaller grains than the undercooled samples where Fe dendrites formed near the free surface during solidification. These Fe dendrites were not removed by annealing. In addition, large orientated 2-14-1 grains nucleated on the Fe dendrites. This combination is detrimental to the magnetic properties. The addition of TiC results in improved control of the microstructure over a larger fraction of the ribbon volume enhancing the magnetic properties. © 1998 American Institute of Physics.
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  • 6
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: For the Nd2Fe14B (2-14-1) compound, the optimal grain size should be smaller than the size of the single domain size of 150 nm. Transition metal carbides (TMC) also reduce the quench rate necessary to achieve the optimal or overquenched condition. This allows inert gas atomization (IGA) to produce viable magnetic materials. In this article we will demonstrate that optimal microstructure for the 2-14-1 can be produced by IGA with the addition of TiC. Moreover, a solidification model will be presented to show (1) how recalescence is a critical feature to the evolution of the microstructure in rapidly solidified materials and (2) the role TMC and other solute phases have on inhibiting grain growth so that lower quench rates can be employed. © 1997 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 556-558 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the growth of Bi2Sr2Ca2Cu3O10+δ from mixed powders of Pb-doped Bi2Sr2Ca1Cu2O8+δ and other oxides, it has been discovered that a dense array of hillocks or mesas grow at the interface between a Ag overlay and Pb-doped Bi2Sr2Ca1Cu2O8+δ grains during the ramp up to the reaction temperature. As viewed in an environmental scanning electron microscope, the Ag coated grains develop a texture that looks like "chicken pox'' growing on the grains at about 700 °C. These hillocks are about 100 nm across and are spaced at about 500 to 1000 nm. If there is no Ag, this texture does not develop. Preliminary measurements indicate that the hillocks are a recrystallization of (Bi,Pb)2Sr2Ca1Cu2O8+δ, and are definitely not a Pb rich phase. © 1996 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 56 (1990), S. 2042-2044 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetic flux relaxation data on a composite specimen of YBa2Cu3O7−δ +Ag shock compacted at 167 kbar show that the flux-pinning energy at 70 K and 10 kOe is enhanced by a factor of 2–3. This enhancement persists after annealing at 890 °C in oxygen for 24 h. The increased pinning energy is apparently caused by shock-induced 〈100〉 and 〈110〉 line defects having a density up to 1012 cm−2. Annealing replaces line defects with (001) extrinsic stacking faults having displacement vectors of 1/6[031]. The average separation of the stacking faults in the [001] direction is 0.04 μm, which is comparable to the separation between fluxoids at 10 kOe. These results indicate that intragranular critical current densities can be increased significantly in bulk materials by shock processing. High densities of flux-pinning sites could be induced efficiently in industrial-scale quantities of bulk high Tc oxides by explosives.
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 3572-3574 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: When NdBa2Cu3O7−δ (Nd123) is melt-textured in low oxygen partial pressure (Po2) and subsequently annealed in oxygen to produce the superconducting phase, there is a considerable enhancement of flux pinning compared to Y123. This enhancement is accompanied by a precipitation mechanism due to a change of the solubility limits of Nd1+xBa2−xCu3O7−δ between the high temperature low Po2 conditions used in melt-textured growth and the low temperature oxygen annealing. Transmission electron microscopy reveals a coherent intermediate precipitate structure with a composition of BaCuO2 and a high density of dislocations. We believe these defects are responsible for the strong flux pinning in this material. © 1997 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 1086-1088 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: There has been considerable interest in inserting controlled defects in high-temperature superconductors to enhance their flux pinning potential. One means of obtaining controlled defects in the YBa2Cu3O7−δ (Y123) structure is to decompose YBa2Cu4O8 (Y124). However, not only is the decomposition of the Y124 to optimize pinning difficult to control, sintering dense compacts of Y124 is very difficult. It is shown that the addition of 1.2 wt% CuO to Y123 sintered at 830 °C results in evenly spaced 1/6[031] extrinsic stacking faults. These stacking faults have also been shown to increase pinning energy, and are stable to at least to 900 °C. Intragranular Jc, as calculated from the hystersis loops, shows a significantly reduced temperature dependence at high fields. Jc dropped only a factor of 30 from 2×106 at 10 K to 6×104 A/cm2 at 70 K at 1 T.
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