Focused Research GroupTwo-gap Superconductivity in Bulk Form Magnesium Diboride

and its Implications for ApplicationsDavid C. Larbalestier, University of Wisconsin-Madison, DMR 0514592

& FSU

A collaboration between groups at Arizona State University (Nate Newman and John Rowell), , Pennsylvania State University (Zi-Kui Liu and Xiaoxing Xi), the University of Puerto Rico at Mayaguez, and the

University of Wisconsin (Eric Hellstrom*, David Larbalestier* and Paul Voyles)

*now at the National High Magnetic Field Laboratory, Florida State University

5 two page nuggets of the recent FRG work follow

Tuning the superconducting properties of MgB2 by doping using mechanical alloying

MgO Platelets in Doped MgB2 Thin Films

Thermodynamic Modeling of Mg-B-C system

Effects of oxygen incorporation on Tc, Jc, & Hc2 of MgB2 films

Accomplishments for Science is Fun - magnetismASU Outreach Progress : Science is Fun

Tuning the superconducting properties of MgB2 by doping using mechanical alloying

E. Hellstrom, D. Larbalestier (FSU) and O. Uwakweh (UPRM) DMR-05-14592 (FRG)

The two-gap superconductivity in MgB2 offers the possibility to tune MgB2’s superconducting properties by doping on the Mg and B sites. We used high-energy ball milling to mechanically alloy MgB2 with C and other metal diborides (MeB2). We found that milling has the added advantages of reducing the MgB2 grain size, which can increase pinning, and creating strain in the grains, which increases Hc2. C milled into MgB2 in <60 min. Milling C into MgB2 increased Hc2 to ~37 T at 0K. The maximum C content with HIPping at 1000C is ~ x = 0.04 (in Mg(B1-xCx)2). Our first exploratory milling of MeB2 (Me = Al, Ta, V, Zr) into MgB2 showed that AlB2 mills into MgB2. In contrast, TaB2, VB2, and ZrB2 do not alloy to an appreciable extent even when consolidating the milled powder in a HIP at 1000C. The other commercially-available MeB2s (Me = Cr, Nb, Ti) are currently being investigated in the ongoing collaboration between UPRM and FSU to evaluate how they alloy with MgB2.

Jc highest for 1200 min milling at > 5T (15K). Nominal composition Mg(B0.96C0.04)2)

1 2 3 4 5 6 7 8

103

104

105

1069877654

3

60min 120min 300min 600min 2400min

MgB2 unmilled

J c (A

/cm

2 )

0H (T)

2

SPEX - 5 at. %

Jc highest for 600 min milling at > 5T (4.2K). Overall composition is 0.95 MgB2 + 0.05 TaB2.

C doping

TaB2 doping

Tuning the superconducting properties of MgB2 by doping using mechanical alloying

E. Hellstrom, D. Larbalestier (FSU) and O. Uwakweh (UPRM) DMR-05-14592 (FRG)

A strength of the ongoing collaboration between UPRM and FSU on MgB2 is the student exchange. This past year three students from UPRM came to FSU (and UW-Madison) to work on MgB2: Richard Perez and Yenny Cardona who are graduate students, and Pedro Vargas who is an undergraduate student. Richard Perez also overlapped with Prof. Durval Rodrigues from the University of Sao Paulo-Lorena, Brazil, who spent an 18 month sabbatical leave at UW-Madison working on MgB2. There was an interesting interflow of knowledge and expertise as a result of our student exchanges. Two years ago Perez helped refurbish an old glove box while at UW-Madison that was subsequently sent to UPRM for MgB2 work. This past summer, Pedro Vargas, the undergrad who helps maintain the box at UPRM, spent part of his summer at FSU helping refurbish an old glove box for MgB2 work at FSU.

Pedro Vargas, a UPRM undergrad who works with Uwakweh spent summer 2007 as an REU student working on MgB2 with Hellstrom and Larbalestier at FSU.

Yenny Cardona (UPRM), Jian Zhou (FSU), and Richard Perez (UPRM) in front of the hot isostatic press (HIP) at FSU. These graduate students have worked together on the high-energy milling experiments.

MgO Platelets in Doped MgB2 Thin Films

Paul M. Voyles (U. Wisconsin-Madison), DMR-0514592

Superconducting magnesium diboride (MgB2) has the potential for applications in NMR, MRI, and high-field magnets, provided that the origins of the exceptional properties of some carbon-doped thin films can be understood. We have studied MgB2 thin films deposited using methane gas as the source of carbon. In addition to carbon doping, these films contain nanoscale MgO platelets within the MgB2 layer. These platelets are associated both with increased upper critical magnetic field enhanced vortex pinning. A plausible cause in both cases is the strain introduced into the boron planes.

Figure: (top left) Z-contrast STEM image, (top right) High-resolution TEM image, and (bottom) EELS spectrum from a C-doped MgB2 thin film with MgO platelet inclusions.

O

B

10 nm 10 nmSiC

MgB2

MgO

SiC

MgB2

MgO

International Collaboration on Superconducting MgB2

Paul M. Voyles (U. Wisconsin-Madison), DMR-0514592

As part of an international collaboration between the FRG on MgB2 and the University of Genoa, the University of Naples, and the Italian national laboratory CNR-INFM, we have studied the defects introduced into high quality thin films of MgB2 by neutron irradiation. The Italian groups have shown that irradiation causes merging of the two superconducting gaps. We showed it destroys the crystal lattice in pockets of the MgB2 a few nanometers in diameter, rendering them amorphous. Voyles visited Genoa in June to promote this collaboration.

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SiC

MgB2

amorphous pocket

Thermodynamic Modeling of Mg-B-C system Prof. Zi-Kui Liu, The Pennsylvania State University

DMR-0514592

Using CALculation of PHAse Diagram (CALPHAD) approach, we developed thermodynamic database for Mg-B-C system to understand thin films produced by Hybrid Physical-Chemical Vapor Deposition process (HPCVD). From isothermal section at 1000 K and ~1 torr (condition for growth thin films with HPCVD process) of the Mg-B-C ternary phase diagram, we predicted that the maximum solubility of carbon in MgB2 is less than 7%. The predicted carbon solubility is in agreement with the experimental value mention in App. Phys. Lett., 91 (082513) 2007.

Based on the predicted phase equilibrium between MgB2 and C, we used the elastic constants from our first-principles calculations to predict the lattice parameter behavior of carbon-doped MgB2 thin films.

Isothermal section of ternary phase diagram of Mg-B-C system at 1000 K and ~1 torr.

Preliminary result from the prediction of lattice parameter of carbon-doped MgB2 thin films with the elastic effect.

Future work:Using current work as a guideline, we can focus on the elements that might have significant effects on MgB2 and develop the thermodynamic database for those potential system.

Phases Research Lab:Our group have three undergraduate students, seven graduate students and Three research fellows working on various projects such as magnesium alloys, aluminum alloys, boride, Ni-based superalloys, perovskite oxide and diffusion in intermetallic.

Pure MgB2 have a high Tc (39 K) but other superconducting properties such as Hc2 or Jc values are still small. Doping MgB2 with various different elements help improve those properties. Our current work is focus on creating a guideline for possible effects upon doping MgB2 with those elements from the first-principles calculations.

Current work:Explore the effects of doping MgB2 with various

elements ( > 40 elements) from first-principles calculation.

Enthalpy of formation for doped MgB2 with various elements

Thermodynamic Modeling of Mg-B-C system Prof. Zi-Kui Liu, The Pennsylvania State University

DMR-0514592

• Novel route to obtain high Hc2s by depositing films on low temperature substrates & then annealing at low temperatures.

• slopes dHc2/dT are often larger than 1.5T/K (i.e. Hc2s potentially >50 T)

• This work suggests that to increase Hc2 in MgB2, maximum scattering must be achieved in materials with Tc of say 30 to 33K.

• Results suggest that today the MgB2 wire manufacturers are using reaction temperatures that are far too high to achieve necessary carrier scattering in MgB2.

Achieving high Hc2 by thermal processing of MgB2 deposited on low temperature substrates

N. Newman and J.M. Rowell, Arizona State University - DMR-0514592

Effects of oxygen incorporation on Tc, Jc, & Hc2 of MgB2 films

N. Newman and J.M. Rowell, Arizona State University

DMR-0514592

Research impact:• Oxygen incorporated in situ and ex situ

• Very high Jc & Hc2 (~ 3x105 A/cm2 at 8 T and 4.2 K )

• dHc2/dT!! increases w/ oxygen concentration– as high as 1.5 T/K for dHc2/dT & ~ 50 T for Hc2(0)

• oxygen distributed both in & between the grains. – While Tc reduces, intra-grain resistivity increases w/

increased oxygen in sample. – Low inter-grain connectivity obtained by ex situ

oxygen incorporation suggests grain boundaries preferentially oxidized

Education:7 graduate students & 4 undergraduates involved in MgB2 research at ASU. R. Gandikota and L. Yu graduated in 2007. Jihoon Kim graduated in 2005.

Societal impact:The high Hc2 and Jc values may enable production of high-

field magnets for wide range of practical applications.

● Two fast-paced, minds-on Science is Fun - Magnetism presentations have

been developed with multiple explorations (15 and 45 minute presentations). The associated training materials have also been developed.

● The 15 minute magnetism presentation was so successful that components of both the 15 minute module and the 45 minute module have been incorporated into the daily science is fun standard demonstration that provides services to over 12000 students each year.

● Components of the presentations have been piloted with a diverse population of over 2,000 students with positive feedback:

65% Hispanic13% Native American6% Black

● The full 45 minute module was presented to over 2500 students between fall 2007 and spring 2008.

Two-Gap Superconductivity in MgB2 & its Implications for Applications ASU Outreach Progress : Science is Fun

N. Newman (co-PI), David Wright & Michael McKelvy, Arizona State University, - DMR-0514592

• The magnetism pilot, previously piloted to 4th grade students, has been expanded to include grades K-8 and is designed to meet Arizona State standards for those grades. Additionally, classroom materials have been developed for teachers to incorporate further activities on energy, magnetism and electricity.

• Minority groups that experience “Science is Fun” make up more than half of the student population served. Hispanic, Black/African, Asian/Pacific Islanders, and American Indian populations comprise 42.1%, 6.4%, 3.3%, and 2.6% respectively of the population served by the Science is Fun program. 48.8% of the student population served is comprised of female students.

• Collaborated with FRG partners to enhance outreach impact.

Accomplishments for Science is Fun - magnetismASU Outreach Progress : Science is Fun

N. Newman (co-PI), David Wright & Michael McKelvy, Arizona State University, DMR-0514592