F. M. Mueller

Los Alamos National Laboratory, Los Alamos, NM, United States

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Publications (118)221.68 Total impact

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    ABSTRACT: The transverse Jc distribution in YBCO coated conductors was measured non-destructively with high resolution using a 'magnetic knife' made of permanent magnets. The method utilizes the strong depression of Jc in applied magnetic fields. A narrow region of low (including zero) magnetic field, in a surrounding higher field, is moved transversely across the sample in order to reveal the critical-current density distribution. The net resolution of this device is approximately 65 µm, and the Jc resolution is better than 0.5%. A Fourier series inversion process was used to determine the transverse Jc distribution in the sample. The Jc profile was correlated with other sample properties of coated conductors prepared by pulsed laser deposition. Because of its straightforward and inexpensive design, this Jc imaging technique can be a powerful tool for quality control in coated-conductor production.
    Superconductor Science and Technology 09/2008; 21(11):115021. · 2.76 Impact Factor
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    ABSTRACT: Magnesium diboride (M g B 2 ) is a superconducting material that has a transition temperature (T c ) of ∼40 K, which is ∼30 K higher than niobium (Nb) that has been used for most superconducting RF cavities in the past decades. Last year, it was demonstrated that the RF surface resistance of M g B 2 can be lower than Nb at 4 K. One of the problems with other high-Tc materials such as YBCO was its rapid increase in RF surface resistance with higher surface magnetic fields. Recently, we have shown that M g B 2 shows little increase in the surface resistance up to ∼120 Oe, equivalent of an accelerating field of ∼3 MV/m. The highest field tested was limited by available power. This result is encouraging and has made us consider fabrication of a cavity coated with M g B 2 and test it. Also, there is a potential that this material has a higher critical magnetic field that enables the cavity to run at a higher gradient than Nb cavities in addition to the possibility of operation at higher temperatures.
    Particle Accelerator Conference, 2005. PAC 2005. Proceedings of the; 06/2005
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    ABSTRACT: MgB2 samples synthesized by solid-state reaction were found to contain several types of crystalline defects, including Mg deficiency, coherent precipitates, screw dislocations, and oxygen impurity, depending on the synthesis conditions. The Mg deficiency introduced significant strain in the crystalline lattice of MgB2. A high density of screw dislocations lying on (0001) planes with a Burgers vector of partially relieved the lattice strain. The remnant strain in the MgB2 lattice directly determines the critical superconducting temperature, which decreases with strain. A small amount of oxygen dissolved in the MgB2 matrix is not responsible for lowering Tc observed in our samples.
    Superconductor Science and Technology 06/2004; 17(8):1026. · 2.76 Impact Factor
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    ABSTRACT: We present the fabrication and test results of hot-isostatic-pressed (HIPed) powder-in-tube (PIT) MgB2 coils. The coils' properties were measured by transport and magnetization at different applied fields (H) and temperatures (T). The engineering critical current (Je) value is the largest reported in PIT MgB2 wires or tapes. At 25 K our champion six-layer coil was able to generate a field of 1 T at zero external field (Ic>220 A, J_\rme \sim 2.8 \times 10^4~{\mathrm {A~cm^{-2}}} ). At 4 K this coil generated 1.6 T under an applied field of 1.25 T (I_{\mathrm {c}} \sim 350 A, J_\rme \sim 4.5 \times 10^4~{\mathrm {A~cm^{-2}}} ). These magnetic fields are high enough for a superconducting transformer or magnet applications such as MRI. An SiC doped MgB2 single layer coil shows a promising improvement at high fields and exhibits Jc> 104 A cm-2 at 7 T.
    Superconductor Science and Technology 01/2004; 17(10). · 2.76 Impact Factor
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    ABSTRACT: We present a detailed analysis of the effect of heat treatments on the microstructure, magnetization, and transport properties of MgB2 wires produced by the powder-in-tube method. We have used commercial MgB2 powder with 5 at. % Mg powder added as an additional source of magnesium and stainless steel as sheath material. We measure the dc transport critical current that can be increased or decreased by more than one order of magnitude as compared with the as-drawn wire, depending on the annealing parameters. We correlate the changes in the critical current with changes in the microstructure, as determined from scanning and transmission electron microscopy analysis. We show through magnetization measurements of short annealed wires that inappropriate annealing conditions result in a deterioration of the connectivity due to the loss of Mg and in inhomogeneous weak-link limited current flow, rendering the critical state model inapplicable. We discuss the optimization of the annealing conditions that strongly improve the connectivity by eliminating most of the microcracks present in the unannealed wires, where excess Mg promotes the recrystallization. The loss of Mg during the heat treatment may be precluded by annealing long wire lengths with a high heating rate. © 2003 American Institute of Physics.
    Journal of Applied Physics 09/2003; 94(6):4024-4031. · 2.21 Impact Factor
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    ABSTRACT: Bulk MgB<sub>2</sub> samples were prepared under different synthesis conditions and analyzed by scanning and transmission electron microscopy. The critical current densities were determined from the magnetization versus magnetic field curves of bulk and powder-dispersed-in-epoxy samples. Results show that through a slow cooling process, the oxygen dissolved in bulk MgB<sub>2</sub> at high synthesis temperatures can segregate and form nanometer-sized coherent precipitates of Mg(B,O)<sub>2</sub> in the MgB<sub>2</sub> matrix. Magnetization measurements indicate that these precipitates act as effective flux pinning centers and therefore significantly improve the intra-grain critical current density and its field dependence.
    IEEE Transactions on Applied Superconductivity 07/2003; · 1.20 Impact Factor
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    ABSTRACT: MgB<sub>2</sub> wires were produced by the powder-in-tube method, using commercial MgB<sub>2</sub> powder with 5%at Mg powder added as an additional source of magnesium, and stainless steel as sheath material. Depending on the parameters of the heat treatments, the dc transport critical current can be increased or decreased by more than one order of magnitude as compared with the as-drawn wire. Analysis of the zero-field-cooling dc magnetization demonstrates that these large variations are due to changes in the connectivity of the wires. We discuss the optimization of the annealing conditions in terms of the changes in the microstructure, as determined from TEM and SEM analysis. We show that annealing can strongly improve the connectivity by eliminating most of the micro-cracks present in the un-annealed wires. In contrast, inappropriate annealing conditions result in a deterioration of the connectivity due to the loss of Mg.
    IEEE Transactions on Applied Superconductivity 07/2003; · 1.20 Impact Factor
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    ABSTRACT: We have performed microwave measurements on superconducting hot-isostatically- pressed (HIPed) bulk MgB2 using a parallel-plate resonator technique. The high density and strength of the HIPed material allowed preparation of samples with mirror-like surfaces for microwave measurements. The microwave surface resistance decreased by about 40% at 20 K when the root-mean-square surface roughness was reduced from 220 nm to 110 nm through surface-polishing and ion-milling. The surface resistance was independent of surface microwave magnetic field at least up to 4 Oe and below 30 K. We attribute this behavior, and the overall low surface resistance (~0.8 mOhms at 10 GHz and 20 K), to the high density of our samples and the absence of weak links between grains.
    Applied Physics Letters 03/2003; · 3.79 Impact Factor
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    ABSTRACT: The microstructures of MgB2 wires prepared by the powder-in-tube technique and subsequent hot isostatic pressing were investigated using transmission electron microscopy. Large amount of crystalline defects including small angle twisting, tilting, and bending boundaries, in which high densities of dislocations reside, were found forming sub-grains within MgB2 grains. It is believed that these defects resulted from particle deformation during the hot isostatic pressing process and are effective flux pinning centers that contribute to the high critical current densities of the wires at high temperatures and at high fields. Comment: 16 pages, 6 figures
    Superconductor Science and Technology 03/2003; · 2.76 Impact Factor
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    ABSTRACT: We report dc transport and magnetization measurements of J c in MgB 2 wires fabricated by the powder-in-tube method, using commercial MgB 2 powder with 5 %at Mg powder added as an additional source of magnesium, and stainless steel as sheath material. By appropriate heat treatments, we have been able to increase J c by more than one order of magnitude from that of the as-drawn wire. We show that one beneficial effect of the annealing is the elimination of most of the micro-cracks, and we correlate the increase in J c with the disappearance of the weak-link-type behavior.
    Applied Physics Letters 01/2003; 82. · 3.79 Impact Factor
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    ABSTRACT: The critical current density (J c) of hot isostatic pressed (HIPed) MgB 2 wires, measured by d.c. transport and magnetization, is compared with that of similar wires annealed at ambient pressure. The HIPed wires have a higher J c than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field (H irr). The HIPed wires are promising for applications, with J c >10 6 A/cm 2 at 5 K and zero field and >10 4 A/cm 2 at 1.5 T and 26.5 K, and H irr ~ 17 T at 4 K. The improvement is attributed to a high density of structural defects, which are the likely source of vortex pinning. These defects, observed by transmission electron microscopy, include small angle twisting, tilting, and bending boundaries, resulting in the formation of sub-grains within MgB 2 crystallites.
    Applied Physics Letters 01/2003; 82. · 3.79 Impact Factor
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    ABSTRACT: A process of preparing superconducting magnesium diboride powder by heating an admixture of solid magnesium and amorphous boron powder or pellet under an inert atmosphere in a Mg:B ratio of greater than about 0.6:1 at temperatures and for time sufficient to form said superconducting magnesium diboride. The process can further include exposure to residual oxygen at high synthesis temperatures followed by slow cooling. In the cooling process oxygen atoms dissolved into MgB.sub.2 segregated to form nanometer-sized coherent Mg(B,O) precipitates in the MgB.sub.2 matrix, which can act as flux pinning centers.
    01/2003;
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    ABSTRACT: MgB2 samples prepared by solid-state reaction were investigated using high-resolution transmission electron microscopy (HREM), X-ray energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS), and energy-filtered imaging. Large amounts of coherent precipitates with a size range from about 5 nm up to about 100 nm were found in the MgB2 crystallite matrices. The precipitates are of different shapes including sphere, ellipsoid, and faceted polyhedron depending on the size of the precipitates. EDX and EELS analyses confirm that smaller precipitates contain magnesium, boron and oxygen while larger faceted precipitates contain mainly magnesium and oxygen, implying that the oxygen content increases with precipitate size. HREM and electron diffraction investigations found that the precipitates have the same crystal lattice structure as that of MgB2 but with various composition modulations depending on the composition of the precipitates. The precipitates transform to the MgO phase after long exposure to residual oxygen in flowing Ar gas at high temperatures. The effect of the precipitates in different size ranges on flux pinning is discussed. Comment: 29 pages, 9 Figures, to be published in J. Applied Physics
    Journal of Applied Physics 12/2002; · 2.21 Impact Factor
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    ABSTRACT: This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB2, with the objective to improve its flux pinning. A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (Jc) at higher fields than its un-HIPed counterpart. The HIPed sample had a Jc of 10 000 A/cm2 in 50 000 Oe (5 T) at 5 K. This was 20 times higher than that of the un-HIPed sample, and the same as the best Jc reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep Jc drop with magnetic field H that occurred in the un-HIPed MgB2 pellet at H>45 000 Oe(4.5 T) and T=5 K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB2 pellet and to generate more dislocations in the pellets. These dispersed MgO particles and dislocations improved flux pinning also at H<45 000 Oe. The HIPing process was also found to lower the resistivity at room temperature. © 2002 American Institute of Physics.
    Journal of Applied Physics 06/2002; 92(1):351-356. · 2.21 Impact Factor
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    ABSTRACT: The superconductivities of samples prepared by several procedures were found to degrade under ambient environment. The degradation mechanism was studied by measuring the change of surface chemical composition of dense MgB2 pellets (prepared by hot isostatic pressure, HIPed) under atmospheric exposure using X-ray Photoelectron Spectroscopy (XPS). Results showed that samples with poor connectivity between grains and with smaller grain sizes degrade with time when exposed to ambient conditions. In these samples, the Tc did not change with time, but the superconducting transition became broader and the Meissner fraction decreased. In contrast, our well-sintered and the HIPed samples remained stable for several months under ambient condition. The degradation was found to be related to surface decomposition as observed by XPS. We observed the formation of oxidized Mg, primarily in the form of a Mg hydroxide, the increase of C and O contents, and the reduction of B concentration in the surface layer of MgB2 samples. Comment: 15 pages, 3 figures
    Applied Physics Letters 06/2002; · 3.79 Impact Factor
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    ABSTRACT: MgB2 samples prepared by three different sets of synthesis parameters were investigated using transmission electron microscopy. Results suggest that oxygen dissolved in bulk MgB2 at high synthesis temperatures when the MgB2 samples were exposed to trace amount of oxygen from flowing ultrahigh purity Ar gas. The lower solubility of oxygen in MgB2 at a lower temperature led to the precipitation of nanometer-sized coherent Mg(B,O) in the interior of MgB2 grains during subsequent cooling. The precipitates, which act as effective flux pinning centers, are of composition-modulated structures with the same basic crystal lattice and orientation as the MgB2 matrix. This study has demonstrated the potential of tailoring the size and distribution of Mg(B,O) precipitates through optimizing synthesis parameters for optimum flux pinning. © 2002 American Institute of Physics.
    Applied Physics Letters 06/2002; 80(23):4398-4400. · 3.79 Impact Factor
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    ABSTRACT: Here we describe the results of an atomic resolution study of oxygen incorporation into bulk MgB2. We find that ~20-100 nm sized precipitates are formed by ordered substitution of oxygen atoms onto boron lattice sites, while the basic bulk MgB2 crystal structure and orientation is preserved. The periodicity of the oxygen ordering is dictated by the oxygen concentration in the precipitates and primarily occurs in the (010) plane. The presence of these precipitates correlates well with an improved critical current density and superconducting transition behavior, implying that they act as pinning centers. Comment: Submitted to Applied Physics Letters, 6 pages, 3 figures
    Applied Physics Letters 03/2002; · 3.79 Impact Factor
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    ABSTRACT: We studied the influence of sample preparation and defects in the superconducting properties samples using atomic ratios of Mg:B=1:1 and Mg:B=1:2. Samples were characterized by SEM, and XRD, and the magnetization properties were examined in a SQUID magnetometer. The presence of Mg vacancies was determined by Rietveld analysis. Most of the samples exhibited sharp superconducting transitions with Tcs between 37- 39 K. We found a strong correlation between the crystal strain and the Tc. This strain was related to the presence of Mg vacancies. In addition, results showed that some samples degraded with time when exposed to ambient conditions. In these samples the Tc did not change with time, but the superconducting transition became broader and the Meissner fraction decreased. This effect was only present in samples with poor connectivity between grains and smaller grain sizes. The degradation was related to a surface decomposition as observed by X-ray Photoelectron Spectroscopy. No correlation was found between this effect and the presence of Mg vacancies.
    01/2002;
  • Journal of Physics and Chemistry of Solids 01/2002; · 1.53 Impact Factor
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    ABSTRACT: Rietveld refinements of X-ray powder diffraction data were performed on RBa4Cu3O8.5+δ (R143) compounds, where R=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb, to determine their oxygen content and distribution, lattice parameter, and space group. The total oxygen content was found to oscillate with the atomic number of the rare earth elements. The compounds with R having even atomic numbers have higher total oxygen content than those with R having odd atomic numbers. With increasing atomic number of the rare earth element, the oxygen distribution on the O3 and O4 sites changes from uniform (e.g. Sm143) to primarily on the O3 sites. The lattice parameter decreases smoothly with increasing atomic number, which is caused by the monotonic decease in the ionic radii of the rare earth elements. However, the ionic radius does not have significant influence of the total oxygen contents. The refinement also revealed that the correct space group of Sm143 and Eu143 and P23̄ and Pm3̄, respectively.
    Journal of Physics and Chemistry of Solids - J PHYS CHEM SOLIDS. 01/2002; 63(1):23-29.

Publication Stats

1k Citations
221.68 Total Impact Points

Institutions

  • 1987–2008
    • Los Alamos National Laboratory
      • • Superconductivity Technology Center
      • • Materials Science and Technology Division
      Los Alamos, NM, United States
  • 2002
    • Boston College, USA
      Boston, Massachusetts, United States
  • 2000
    • Hebrew University of Jerusalem
      • Racah Institute of Physics
      Jerusalem, Jerusalem District, Israel
  • 1994
    • Boston University
      • Department of Physics
      Boston, MA, United States
  • 1977–1978
    • Radboud University Nijmegen
      Nymegen, Gelderland, Netherlands
  • 1973–1975
    • Northwestern University
      • Department of Physics and Astronomy
      Evanston, Illinois, United States
  • 1968–1975
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States
  • 1971
    • Northern Illinois University
      • Department of Physics
      DeKalb, IL, United States